WO2020017609A1 - Steel sheet - Google Patents
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- WO2020017609A1 WO2020017609A1 PCT/JP2019/028357 JP2019028357W WO2020017609A1 WO 2020017609 A1 WO2020017609 A1 WO 2020017609A1 JP 2019028357 W JP2019028357 W JP 2019028357W WO 2020017609 A1 WO2020017609 A1 WO 2020017609A1
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- steel sheet
- retained austenite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- the present disclosure relates to a steel sheet having excellent formability and ultra-high strength characteristics, and specifically relates to a steel sheet having excellent local ductility and high tensile strength and having a high content of Mn.
- Patent Document 1 a so-called TRIP steel utilizing the transformation-induced plasticity of retained austenite has been proposed (for example, Patent Document 1).
- Retained austenite is obtained by enriching C in austenite so that austenite does not transform into another phase even at room temperature.
- a technique for stabilizing austenite it has been proposed to include a carbide precipitation suppressing element such as Si and Al in a steel sheet and to enrich C in austenite during bainite transformation occurring in the steel sheet in a steel sheet manufacturing stage. I have.
- the austenite is further stabilized, the amount of retained austenite can be increased, and as a result, a steel sheet excellent in both strength and elongation can be produced.
- welding is often performed on the steel sheet.
- Non-Patent Document 1 As a steel sheet having a C content lower than that of the TRIP steel, a retained austenite amount larger than that of the TRIP steel, and a strength and ductility exceeding the TRIP steel, a steel to which Mn of more than 4.0% by mass is added is proposed.
- the steel requires a long heating process such as box annealing. For this reason, material design in a short-time heating process such as continuous annealing suitable for manufacturing a high-strength steel sheet to be used as a member for an automobile has not been sufficiently studied, and a requirement for increasing local ductility in that case was not clear.
- Patent Document 2 discloses a steel sheet to which 3.5% by mass or more of Mn is added and which has excellent tensile strength and elongation by controlling ferrite to 30% to 80%.
- Patent Document 3 discloses that in a steel containing 3 to 7% by mass of Mn, local ductility is improved by suppressing the generation of retained austenite to less than 20% by volume. Specifically, local elongation is 8%. The above steel sheet is disclosed.
- Patent Document 4 discloses a high-strength steel sheet containing 2.0 to 6.0% by mass of Mn and having a retained austenite amount of 20% by volume or more.
- Patent Document 5 discloses a high-strength steel sheet containing 1.68 to 3.8% by mass of Mn, having excellent local ductility, and having a local elongation of 4% or more.
- a steel sheet having a high content of Mn as disclosed in, for example, Patent Document 2 and Non-Patent Document 1 contains a large amount of unrecrystallized ferrite and thus does not work harden and has low workability and formability. That is, a steel sheet having a high content of Mn having such a structure containing a large amount of unrecrystallized ferrite cannot have both the tensile strength and the workability and formability required for a steel sheet for automobiles.
- the steel sheet as an example of the invention described in Patent Document 3 has a low C content of less than 0.2% by mass, and thus has a tensile strength of 1090 MPa or less and maintains the C content at less than 0.2% by mass.
- a comparative example in which the tensile strength is 1233 MPa and the local elongation is 1.3% is shown, and the local ductility is reduced.
- the steel sheet described in Patent Document 4 is obtained by preheating a hot-rolled sheet or a cold-rolled sheet to 800 ° C. or higher, austenitizing and cooling, and then performing annealing twice at an annealing temperature of 650 to 750 ° C. This is to promote the concentration of alloy elements therein to generate retained austenite of 20% by volume or more.
- sufficient local ductility was not obtained.
- the present inventors have set the metal structure of the steel sheet to have an area ratio (area%) of 25% or more and 90% or more as a matrix.
- the following tempered martensite and 3% or less of ferrite, and 10% or more and 50% or less of retained austenite and 25% or less of fresh martensite are contained as other phases. It has been found that it is effective to set the interface density between retained austenite and fresh martensite to 2.7 ⁇ m ⁇ 1 or more.
- Local ductility is a property that a material can withstand crack propagation generated in a local deformation region of stretch flange forming or bending, and its index is local elongation obtained by a uniaxial tensile test.
- voids may be generated in a constricted region after the start of local deformation.
- the voids are connected and propagated in the subsequent deformation process, thereby generating and propagating a crack, leading to fracture.
- the present inventors believe that it is effective to improve the local ductility to have a microstructure with little variation with respect to crack propagation from any direction, and as a result of earnest study, the present inventors have found that the following microstructure By doing so, a steel sheet having high strength and excellent local ductility can be obtained.
- the matrix of the metal structure of the steel sheet is tempered martensite.
- Martensite is a low-temperature transformation phase containing many dislocations in its phase, and is a useful phase for increasing the strength of a steel sheet. Further, by using tempered martensite, the balance between strength and local ductility is improved.
- the ductility is improved by containing retained austenite in the microstructure.
- the ductility improving effect of soft austenite but also the TRIP effect is exhibited by the work-induced transformation to martensite during working, and the balance of strength-ductility-local ductility can be further improved.
- Fresh martensite partially transformed during quenching may be adjacent to the retained austenite.
- Fresh martensite and martensite formed by process-induced transformation are harder phases than the tempered martensite of the parent phase, and not only increase the strength of the steel, but also reduce the propagation of cracks that propagate from the soft phase. It also has the role of stopping at the interface.
- the metal structure at a position 1/4 of the thickness from the surface is tempered martensite having an area ratio of 25% or more and 90% or less, 3% or less ferrite, and 10% or more and 50% or less. Contains retained austenite and up to 25% fresh martensite.
- the L section refers to a surface cut parallel to the sheet thickness direction and the rolling direction so as to pass through the central axis in the rolling direction of the steel sheet.
- FIG. 1 is a scanning electron microscope (SEM) image obtained by observing the L section of a steel sheet according to an embodiment of the present invention after mirror polishing and nital treatment.
- FIG. 2 is a two-tone image obtained by binarizing the SEM image of FIG.
- the metal structure of the steel sheet is controlled such that the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is 2.7 ⁇ m ⁇ 1 or more.
- the interface between the retained austenite and the fresh martensite with respect to the parent phase which is an obstacle to any crack propagation direction, exists at a fixed ratio, so that the interface strength is improved and excellent local ductility is obtained.
- the steel plate of the present disclosure has been made based on the above findings, and the gist is as follows. (1) In mass%, C: more than 0.20% and less than 0.55%, Si: 0.001% or more and less than 3.50%, Mn: more than 4.00% and less than 9.00%, sol.
- the interface density which is a value obtained by dividing the total length of the boundary between the second region and the second region by the total area of the first region and the second region, is 2.7 ⁇ m ⁇ 1 or more.
- steel plate (2) In mass%, B: The steel sheet according to the above (1), which contains 0.0003% or more and less than 0.010%.
- a steel sheet according to claim 1. In mass%, Cr: 0.01% or more and less than 2.00%, Mo: 0.01% or more and 2.00% or less, W: 0.01% or more and 2.00% or less, (1) or (2), characterized by containing one or more of Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less.
- a steel sheet according to claim 1. (4) In mass%, Ti: 0.005% or more and 0.300% or less, (1) to (3), wherein one or more of Nb: 0.005% to 0.300%, and V: 0.005% to 0.300% are contained. The steel sheet according to any one of the above.
- FIG. 1 is a scanning electron microscope (SEM) image obtained by mirror-polishing and nital-treating the L section of the steel sheet obtained in the example.
- FIG. 2 is a two-tone image obtained by binarizing the SEM image of FIG.
- FIG. 3 is an SEM image obtained by observing the L section of the steel sheet obtained in the comparative example after mirror polishing and nital treatment.
- C is a very important element for increasing the tensile strength of steel.
- a C content of more than 0.20% is required.
- the upper limit of the C content is set to less than 0.55%.
- the lower limit of the C content is preferably 0.24% or more, more preferably 0.28% or more.
- the upper limit of the C content is preferably 0.40% or less, more preferably 0.35% or less.
- Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving local ductility. Further, Si also has an effect of suppressing precipitation and coarsening of cementite and facilitating control of austenite generated during annealing. In order to obtain the above effects, a Si content of 0.001% or more is required.
- the lower limit of the Si content is preferably 0.01% or more, more preferably 0.30% or more, and still more preferably 0.50% or more. By setting the lower limit of the Si content to the above range, the local ductility of the steel sheet can be further improved.
- the upper limit of the Si content is set to less than 3.50%. Further, the upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.
- Mn is an element that stabilizes austenite and enhances hardenability. Further, in the steel sheet of the present disclosure, Mn is distributed in austenite to further stabilize austenite. To stabilize austenite at room temperature, more than 4.00% Mn is required. On the other hand, if the steel sheet excessively contains Mn, ductility and local ductility are impaired, so the upper limit of the Mn content is set to less than 9.00%.
- the lower limit of the Mn content is preferably at least 4.30%, more preferably at least 4.80%.
- the upper limit of the Mn content is preferably 8.00% or less, more preferably 7.50% or less.
- Al is a deoxidizing agent and needs to be contained at 0.001% or more.
- Al also has the effect of increasing the material stability in order to widen the two-phase temperature range during annealing. The effect increases as the content of Al increases, but if the content of Al is excessive, deterioration of surface properties, paintability, weldability, and the like is caused.
- the upper limit of Al was set to less than 1.00%. sol.
- the lower limit of the Al content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more. sol.
- the upper limit of the Al content is preferably 0.80% or less, more preferably 0.60% or less. sol.
- sold. Al means “acid-soluble Al”.
- P 0.100% or less
- the upper limit of the P content is set to 0.100% or less.
- the upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Since the steel sheet according to the present embodiment does not require P, it may not substantially contain P, and the lower limit of the P content is 0.000%.
- the lower limit of the P content may be more than 0.000% or 0.001% or more, but the smaller the P content, the more preferable.
- S (S: 0.010% or less) S is an impurity, and if the steel sheet excessively contains S, MnS elongated by hot rolling is generated, which causes deterioration of toughness. Therefore, the upper limit of the S content is set to 0.010% or less.
- the upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require S, it does not need to substantially contain S, and the lower limit of the S content is 0.000%.
- the lower limit of the S content may be more than 0.000% or 0.001% or more, but the lower the S content, the more preferable.
- N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is impaired. Therefore, the upper limit of the N content is set to less than 0.050%.
- the upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to the present embodiment does not require N, it need not substantially contain N, and the lower limit of the N content is 0.000%.
- the lower limit of the N content may be more than 0.000% or 0.001% or more, but the smaller the N content, the more preferable.
- O is an impurity, and when the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is set to less than 0.020%.
- the upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require O, it does not need to substantially contain O, and the lower limit of the O content is 0.000%.
- the lower limit of the O content may be more than 0.000% or 0.001% or more, but the lower the O content, the more preferable.
- B is not an essential element in the steel sheet according to the present embodiment, and thus may not be included.
- the content is 0.000% or more.
- B has a large effect of strengthening the interface between retained austenite and fresh martensite with respect to the matrix mainly composed of tempered martensite and further improving local ductility. Therefore, the steel sheet of the present embodiment preferably contains B. .
- a B content of more than 0.0003% is required.
- the upper limit of the B content is set to 0.010% or less.
- the lower limit of the B content is preferably 0.0005% or more, more preferably 0.0008% or more, and the upper limit of the B content is preferably 0.0050% or less, more preferably 0.0030%. It is as follows.
- the steel sheet of the present embodiment further includes one or two selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi.
- the above may be contained.
- the steel sheet according to the present embodiment does not require Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi, Cr, Mo, W, It may not contain Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content may be 0%.
- REM refers to a total of 17 elements of Sc, Y, and lanthanoid
- the REM content refers to the content of REM when one kind is used and the total content of REM when two or more kinds are used.
- Point to. REM is also generally supplied as misch metal, which is an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added so that the REM content falls within the above range, or, for example, the REM content may be added in the form of a misch metal so that the REM content falls within the above range. May be contained.
- Cr 0.00% or more and less than 2.00%
- Mo 0.00% or more and 2.00% or less
- W 0.00% or more and 2.00% or less
- Cu 0.00% or more and 2.00% or less
- Ni 0.00% or more and 2.00% or less
- Cr, Mo, W, Cu, and Ni may be included because they are elements that improve the strength of the steel sheet.
- the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni.
- the upper limit of the content of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to 2.00% or less.
- Ti, Nb, and V are not essential elements in the steel sheet according to the present embodiment, and thus may not be included, and each content is 0.000% or more. However, since Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of a steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V.
- the lower limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is preferably set to 0.005% or more.
- the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less.
- Ca, Mg, Zr, and REM are not essential elements in the steel sheet of the present disclosure, they may not be included, and their contents are 0.000% or more.
- Ca, Mg, Zr, and REM improve the toughness of the steel sheet.
- the lower limit of the content of each of one or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0.001% or more.
- the upper limit of the content of each of these elements is set to 0.010% or less, and 1 selected from the group consisting of Ca, Mg, Zr, and REM is used. It is preferred that the total content of the species or two or more elements be 0.030% or less.
- Sb, Sn, and Bi are not essential elements in the steel sheet of the present disclosure, and thus may not be included, and each content is 0.000% or more.
- Sb, Sn, and Bi suppress oxidizable elements such as Mn, Si, and / or Al in the steel sheet from diffusing to the steel sheet surface to form oxides, and reduce the surface properties and plating properties of the steel sheet.
- the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001% or more. % Or more.
- the upper limit of the content of each of these elements is set to 0.050% or less.
- the balance is Fe and impurities.
- Impurities are components that are mixed in due to various factors in the ore, scrap, and other raw materials and manufacturing processes when steel products are manufactured industrially, and are acceptable within a range that does not adversely affect the present invention.
- % representing a fraction of each phase means an area ratio (%) unless otherwise specified.
- the metal structure at a quarter position (also referred to as a 1/4 t portion) of the thickness from the surface is 25% or more and 90% or less of tempered martensite and 3% or less as a matrix. It contains 10% to 50% of retained austenite and 25% or less of fresh martensite as ferrite and other phases. Further, the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is 2.7 ⁇ m ⁇ 1 or more.
- the fraction of each phase changes depending on the annealing conditions, and affects the material such as strength and local ductility.
- the L section refers to a plane cut in such a manner as to pass through the central axis in the rolling direction of the steel sheet in parallel with the sheet thickness direction and the rolling direction.
- the area ratio of tempered martensite and ferrite is calculated from the observation of the structure with a scanning electron microscope (SEM). After the L section of the steel plate is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the metallographic structure at a quarter position from the surface is observed with a scanning electron microscope at a magnification of 5000 times. The area ratio of each of the returned martensite and the ferrite is measured.
- the area ratio of retained austenite is measured by the X-ray diffraction method. Since it is difficult to distinguish between retained austenite and fresh martensite (that is, untempered martensite) in the observation with a scanning electron microscope, the area ratio of tempered martensite and fresh martensite is measured by the following method. After the L section of the steel sheet is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the microstructure at a quarter position of the thickness from the surface of the steel sheet is observed with a scanning electron microscope with a magnification of 5000 times. Then, the total area ratio of retained austenite and fresh martensite is measured. Next, the area ratio of fresh martensite is calculated by subtracting the area ratio of retained austenite measured by X-ray diffraction from the total area ratio of retained austenite and fresh martensite.
- the metal structure at a 1 / 4t part of the thickness from the surface includes tempered martensite having an area ratio of 25% or more and 90% or less.
- Tempered martensite is a parent phase of the steel sheet according to the present embodiment that improves local ductility. Therefore, in order to improve local ductility, tempered martensite of 25% or more is required.
- the lower limit of the area ratio of tempered martensite is preferably 40% or more.
- the upper limit of the area ratio of tempered martensite is preferably 80% or less.
- the metal structure at a 1 / 4t part of the thickness from the surface includes retained austenite in an area ratio of 10% or more and 50% or less.
- Retained austenite is a phase that enhances the tensile strength and ductility of a steel sheet by transformation induced plasticity. Retained austenite can be transformed into martensite by overhanging, drawing, stretch-flanging, or bending with tensile deformation, and thus contributes to an improvement in the strength of a steel material obtained by processing a steel sheet. To obtain these effects, the area ratio of retained austenite needs to be 10% or more. The lower limit of the area ratio of retained austenite is preferably 15% or more, more preferably 20% or more, and further preferably 30% or more.
- the area ratio of retained austenite is preferably 45% or less, and more preferably 40% or less, in view of hydrogen embrittlement.
- the metal structure at a 1 / 4t portion of the thickness from the surface includes fresh martensite having an area ratio of 25% or less.
- the fresh martensite in the metal structure is martensite that has not been tempered, and is a phase that strengthens the steel sheet according to the present embodiment and increases the tensile strength of the steel sheet.
- fresh martensite itself is a hard phase, it is also a phase having an effect of deteriorating local ductility.
- the area ratio of fresh martensite in the metal structure is set to 25% or less, preferably 15% or less, and more preferably 10% or less.
- the metallographic structure at a 1 / 4t part of the thickness from the surface includes ferrite of 3% or less in area ratio.
- the content of ferrite in the metal structure is small. This is because when the ferrite content in the metal structure increases, the ductility decreases.
- the area ratio of ferrite in the metal structure is set to 3% or less, more preferably 1% or less, and further preferably substantially 0%.
- Interface density of retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite in a metal structure of a tt portion of a steel sheet is 2.7 ⁇ m ⁇ 1 or more
- the first region which is either retained austenite or fresh martensite
- the interface density which is a value obtained by dividing the sum of the lengths of the boundaries between the first region and the second region by the total area of the first region and the second region, is 2. 7 ⁇ m ⁇ 1 or more. More preferably, the interface density is 3.15 ⁇ m ⁇ 1 or more.
- the retained austenite and the fresh martensite form an interface with a matrix mainly composed of tempered martensite.
- the steel sheet according to the present embodiment has an interface density between retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite, that is, an interface density of 2.7 ⁇ m ⁇ 1 or more.
- the measurement of the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite can be carried out based on the microstructure image by the above-mentioned scanning electron microscope.
- the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite greatly affects the local ductility.
- the density is 2.7 ⁇ m ⁇ 1 or more, the retained austenite and the fresh martensite are fine.
- the interface distribution becomes uniform, and good local ductility is obtained.
- the interface density between the retained austenite and the fresh martensite with respect to the parent phase mainly composed of tempered martensite is less than 2.7 ⁇ m ⁇ 1
- the retained austenite and the fresh martensite have a coarse and interconnected structure, and therefore have a homogeneous structure. And the effect of suppressing crack propagation is impaired.
- the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is derived as follows.
- the distinction between the first region and the second region is based on the fact that retained austenite and fresh martensite are regions that are relatively brighter with respect to other phases in a microstructure image by a scanning electron microscope. Do it.
- the interface density between the retained austenite and the fresh martensite with respect to the parent phase shows, for example, an area of about 24 ⁇ m ⁇ 18 ⁇ m in a metal structure at a 1 / 4t portion from the surface in the L cross section of the steel sheet at a magnification of 5000 times with a scanning electron microscope (SEM) photograph. There can be.
- the interface density is derived as follows.
- the interface density between the retained austenite and the fresh martensite with respect to the parent phase mainly composed of tempered martensite is measured using image analysis software ImageJ.
- a target tissue is observed at a magnification of 5000 times using an SEM to obtain an SEM image (24 ⁇ m ⁇ 18 ⁇ m).
- 1280 ⁇ 960 divided regions are formed on the SEM image using ImageJ.
- binarization processing is performed so that a region that is either retained austenite or fresh martensite is black, and the other regions are white, to obtain a two-tone image.
- the measurement of the interface density between the retained austenite and the fresh martensite with respect to the tempered martensite was performed by measuring the length of the interface with respect to the tempered martensite between all the isolated residual austenite and the fresh martensite in the obtained two-tone image.
- the analysis is performed sequentially, and the total value is divided by the area (24 ⁇ m ⁇ 18 ⁇ m) of the entire area of the image.
- the sequential analysis refers to measuring the interface density of each of retained austenite and fresh martensite without overlapping the interface.
- the tensile strength (TS) of the steel sheet according to the present embodiment is preferably 1200 MPa or more, more preferably 1320 MPa or more. This is because when a steel sheet is used as a material for an automobile, the sheet thickness is reduced by increasing the strength, thereby contributing to weight reduction.
- the upper limit of the tensile strength of the steel sheet is not particularly defined, but may be, for example, 1600 MPa.
- the upper limit of the total elongation is not particularly limited, but may be, for example, 35% or less.
- the local elongation is preferably at least 2%, more preferably at least 3%, further preferably at least 4%, still more preferably at least 5%.
- the upper limit of the local elongation is not particularly defined, but may be, for example, 6% or less.
- the steel sheet of the present disclosure has high strength, good local ductility, and excellent formability, and is therefore most suitable for structural parts of automobiles such as cross members. Furthermore, since the steel sheet of the present disclosure has a high content of Mn, which contributes to the reduction of the weight of automobiles, the industrial contribution is extremely remarkable.
- the steel sheet according to the present embodiment is prepared by melting a steel having the above-described chemical composition by a conventional method, casting to produce a slab or a steel ingot, and heating and hot rolling the obtained steel sheet to obtain a hot-rolled steel sheet. Is pickled, cold rolled, and annealed to produce.
- Hot rolling may be performed in a normal continuous hot rolling line.
- Annealing may be performed in either an annealing furnace or a continuous annealing line as long as the conditions described later are satisfied, but preferably, the first annealing and the second annealing described below are both performed using the continuous annealing line. Therefore, productivity can be improved.
- the first annealing and the second annealing are preferably performed in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen. By performing the heat treatment in a reducing atmosphere, the scale can be prevented from adhering to the surface of the steel sheet, and can be directly sent to the plating step without requiring acid cleaning. Further, skin pass rolling may be performed on the steel sheet after cold rolling.
- the molten steel may be one produced by the ordinary blast furnace method, and the raw material may generate a large amount of scrap, such as steel produced by the electric furnace method. May be included.
- the slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
- the above-mentioned slab or steel ingot is heated and hot-rolled. It is preferable that the temperature of the steel material to be subjected to hot rolling is 1100 ° C. or more and 1300 ° C. or less. By setting the temperature of the steel material subjected to hot rolling to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. In this specification, the temperature is a temperature measured at a central position on the surface of the steel sheet.
- the time for maintaining the temperature in the temperature range of 1100 ° C. or more and 1300 ° C. before hot rolling is not particularly limited, but is preferably 30 minutes or more, and more preferably 1 hour or more, in order to improve the bendability. . Further, in order to suppress excessive scale loss, the time for maintaining the temperature in a temperature range of 1100 ° C. to 1300 ° C. is preferably 10 hours or less, more preferably 5 hours or less. When direct rolling or direct rolling is performed, hot rolling may be performed as it is without performing heat treatment.
- the finish rolling start temperature is preferably set to 700 ° C. or more and 1000 ° C. or less. By setting the finish rolling start temperature to 700 ° C. or higher, it becomes possible to reduce deformation resistance during rolling. On the other hand, by setting the finish rolling start temperature to 1000 ° C. or lower, deterioration of the surface properties of the steel sheet due to grain boundary oxidation can be suppressed.
- the hot-rolled steel sheet obtained by performing finish rolling can be cooled, wound, and made into a coil.
- the winding temperature after cooling is preferably set to 700 ° C. or lower. By setting the winding temperature to 700 ° C. or lower, internal oxidation is suppressed, and subsequent pickling is facilitated.
- the winding temperature is more preferably 650 ° C or lower, and further preferably 600 ° C or lower.
- the lower limit of the winding temperature is not particularly specified, but may be room temperature, for example.
- the hot rolled sheet may be tempered at a temperature of 300 ° C. or more and 600 ° C. or less before being cold-rolled after being cooled to room temperature.
- cold rolling is performed to obtain a cold-rolled steel sheet.
- the shape is corrected by performing light rolling of more than 0% to about 5% before or after pickling and before or after cold rolling because it is advantageous in terms of securing flatness. Further, by performing light rolling before pickling, pickling properties are improved, removal of surface-concentrating elements is promoted, and there is an effect of improving chemical conversion property and plating property.
- the structure after annealing can be homogenized, that is, the interface distribution between the retained austenite and the fresh martensite in the matrix mainly composed of tempered martensite is homogenized. Is done. As a result, more interfaces between the retained austenite and the fresh martensite can be present in the metal structure.
- the upper limit of the rolling reduction of the cold rolling is 50% or less, preferably 20% or less, more preferably 18% or less, and even more preferably 15% or less.
- the lower limit of the rolling reduction of the cold rolling is 0% or more, and preferably 5% or more. Making the rolling reduction of the cold rolling 50% or less is an important requirement for satisfying the interface density condition specified in the present invention. By setting the rolling reduction of the cold rolling to preferably 18% or less, it is possible to obtain a larger interface density of 3.15 ⁇ m ⁇ 1 or more and obtain a larger local elongation of 3.0% or more. Can be.
- the cold-rolled steel sheet obtained through the hot rolling step and the cold rolling step is heated to 740 ° C. or more and less than 800 ° C. to perform the first annealing.
- the temperature is held at a temperature range of 740 ° C or more and less than 800 ° C for 10 seconds or more, and thereafter, from the above-mentioned holding temperature of 740 ° C or more and less than 800 ° C, a temperature of 500 ° C or less at an average cooling rate of 2 ° C / sec or more and 2000 ° C / sec or less. It is preferable to cool to a region and cool to room temperature.
- a second annealing is performed by heating again to 600 ° C. or higher and lower than the Ac 3 point.
- the first annealing temperature after cold rolling is 740 ° C. or more and less than 800 ° C.
- the annealing temperature is preferably Ac 3 or more.
- austenite grain growth is suppressed, and the interface density between residual austenite and fresh martensite with respect to tempered martensite increases due to the refining effect of old austenite grains, and local ductility increases. Contribute to improvement.
- the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. be 10 seconds or more.
- the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. is preferably 300 seconds or less.
- the steel material is cooled at a rate higher than the critical cooling rate, and the entire steel material after cooling can have a structure mainly composed of martensite.
- the water quenching cooling method or the mist injection cooling method it is difficult to control the average cooling rate to 2000 ° C./sec or more, so the practical upper limit of the average cooling rate is 2000 ° C./sec.
- the steel is cooled to the martensite transformation start temperature or lower, and the entire steel material after cooling can have a martensite-based structure.
- the steel sheet is preferably cooled to room temperature.
- the tempering treatment may be performed by holding the material in a temperature range of 100 ° C. or more and 500 ° C. or less for 10 seconds or more and 1000 seconds or less.
- the second annealing temperature after the first annealing is 600 ° C. or more and less than Ac 3 points.
- the second annealing temperature below 3 points 600 ° C. or higher Ac, martensite tempering a can control a desired area ratio, it is possible to increase the tensile strength and the local ductility.
- the holding time in a temperature range of 600 ° C. or more and less than the Ac 3 point is preferably 5 seconds or more. Further, from the viewpoint of productivity, the holding time in a temperature range of 600 ° C. or more and less than the three Ac points is preferably 300 seconds or less.
- cooling is performed from the above-mentioned holding temperature of 600 ° C. or more and less than Ac 3 points to a temperature range of 300 ° C. or less at an average cooling rate of 10 ° C./sec or more.
- it is kept at a temperature range of 200 ° C. or more and 450 ° C. or less for 30 seconds or more.
- the practical upper limit of the average cooling rate in the cooling after the second annealing is 2000 ° C./sec.
- diffusion of supersaturated carbon in fresh martensite into retained austenite is promoted, and the generation of retained austenite is promoted and the interface density is high.
- C diffusion is promoted, the effect of uniforming C inside retained austenite is obtained, and local ductility is enhanced.
- the cooling after holding for 30 seconds or more in a temperature range of 200 ° C. or more and 450 ° C. or less may be performed as it is to room temperature if the steel sheet is not plated. Moreover, when plating on a steel plate, it is manufactured as follows.
- the steel sheet held at a temperature range of 200 ° C. to 450 ° C. for 30 seconds or more is heated again to a temperature range of 430 to 500 ° C.
- the steel sheet is dipped in a hot-dip galvanizing bath to perform hot-dip galvanizing.
- the conditions of the plating bath may be within a normal range. After the plating process, it may be cooled to room temperature.
- An alloying process of hot-dip galvanizing is performed at a temperature. Alloying treatment conditions may be within a normal range.
- the steel sheet according to the present embodiment can be obtained.
- the obtained slab was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 2.6 mm.
- the obtained hot-rolled steel sheet was pickled and cold-rolled at a cold-rolling rate shown in Table 2 to produce cold-rolled steel sheets having various thicknesses shown in Table 2.
- the obtained cold-rolled steel sheet was subjected to a heat treatment under the conditions shown in Table 3 to produce an annealed cold-rolled steel sheet.
- the heat treatment of the cold rolled steel sheet was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
- Some examples of the annealed cold-rolled steel sheets were subjected to a hot dip galvanizing treatment, and then to an alloying treatment without cooling to room temperature.
- the alloy was heated to 520 ° C. and held at 520 ° C. for 5 seconds to perform an alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./sec.
- the area ratios of tempered martensite and ferrite were calculated from observation of the structure by a scanning electron microscope.
- the area ratio of the retained austenite and fresh martensite was calculated from the structure observation by a scanning electron microscope and the X-ray diffraction measurement.
- the L section of the steel sheet cut in parallel to the rolling direction is mirror-polished, and then the microstructure is revealed with 3% nital.
- the microstructure at a quarter position from the surface is obtained with a scanning electron microscope at a magnification of 5000 times.
- the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite was measured using image analysis software ImageJ.
- a target tissue was observed at a magnification of 5000 times using an SEM to obtain an SEM image (24 ⁇ m ⁇ 18 ⁇ m).
- 1280 ⁇ 960 divided regions were formed in the SEM image using ImageJ. Each of the divided regions was subjected to a binarization process so that a region that was either retained austenite or fresh martensite was black, and the other regions were white, to obtain a two-tone image.
- the measurement of the interface density between the retained austenite and the fresh martensite with respect to the tempered martensite was performed by measuring the length of the interface with respect to the tempered martensite between all the isolated residual austenite and the fresh martensite in the obtained two-tone image.
- the analysis was performed sequentially, and the total value was divided by the area (24 ⁇ m ⁇ 18 ⁇ m) of the entire area of the image.
- the sequential analysis refers to measuring the interface density of each of retained austenite and fresh martensite without overlapping the interface.
- JIS No. 5 tensile test pieces were sampled from a direction perpendicular to the rolling direction of the steel sheet, and tensile strength (TS), total elongation (EL), and local elongation (LEL) were measured.
- the tensile test was carried out using a JIS No. 5 tensile test piece by a method specified in JIS Z2241: 2011.
- the total elongation was measured using a JIS No. 5 test piece according to the method specified in JIS Z2241: 2011.
- the local elongation was calculated by subtracting the value of the elongation at the maximum load point (uniform elongation) from the value of the total elongation when the broken test pieces were butted.
- Evaluation results Table 4 shows the results of the above evaluations.
- a steel sheet exhibiting an interface density of 2.7 ⁇ m ⁇ 1 or more, a tensile strength of 1200 MPa or more, and a local elongation of 2.0% or more was evaluated as a steel sheet having excellent local ductility and high strength.
- FIG. 2 shows a two-tone image obtained by binarizing the SEM image of FIG.
- FIG. 4 shows an SEM image of the L section of the steel sheet No. 4 observed by mirror polishing and nital treatment.
- the interface density measured from the two-tone image of FIG. 2 was 3.35 ⁇ m ⁇ 1 .
- the interface density measured from the binary image obtained by binarizing the SEM image in FIG. 3 was 1.50 ⁇ m ⁇ 1 .
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Abstract
Provided is a steel sheet that has a high Mn concentration, and has excellent local ductility and high strength. This steel sheet is characterized by containing predetermined amounts of C, Si, and acid-soluble Al and in mass%, more than 4% but less than 9% of Mn, limited amounts of P, S, N, and O, and an arbitrarily selected element, the balance being iron and impurities, wherein the metal structure at a 1/4t portion in an L cross-section thereof contains, in an area proportion, 25-90% of tempered martensite, 3% or less of ferrite, 10-50% of residual austenite, and 25% or less of fresh martensite, and has an interface density of 2.7 μm-1 or more determined by dividing, by the total area of the first and second regions, the total sum of the lengths of boundaries between first regions that contain either residual austenite or fresh martensite, and second regions which are regions obtained by removing the first regions from the metal structure at the 1/4 position.
Description
本開示は、優れた成形性と超高強度特性とを有する鋼板に関係し、具体的には優れた局部延性と高い引張強度とを有する含有Mn濃度の高い鋼板に関係する。
The present disclosure relates to a steel sheet having excellent formability and ultra-high strength characteristics, and specifically relates to a steel sheet having excellent local ductility and high tensile strength and having a high content of Mn.
自動車の車体及び部品等の、軽量化と安全性との両方を達成するために、これらの素材である鋼板の高強度化が進められている。一般に、鋼板を高強度化すると、伸びが低下し、鋼板の成形性が損なわれる。鋼板を素材とする自動車の車体用部品の多くがプレス加工により成形されるため、車体部品用として使用される高強度鋼板には、優れたプレス成形性を有することが要求される。特に、伸びフランジ成形や穴広げ加工が主体となる車体用部材に対しては、鋼板の機械的特性として、高い強度を有しながら、高い局部延性を有することが求められている。
鋼板 In order to achieve both weight reduction and safety of automobile bodies and parts, the strength of steel sheets, which are these materials, is being enhanced. In general, when the strength of a steel sheet is increased, elongation is reduced, and formability of the steel sheet is impaired. Since many automotive body parts made of steel sheets are formed by press working, high-strength steel sheets used for body parts are required to have excellent press formability. In particular, a steel plate is required to have high locality and high local ductility as a mechanical property of a steel sheet for a vehicle body member mainly formed by stretch flange forming and hole expanding.
加工・成形性を向上させるために、これまでに、残留オーステナイトの変態誘起塑性を利用した、いわゆるTRIP鋼が提案されている(例えば、特許文献1)。
い わ ゆ る In order to improve workability and formability, a so-called TRIP steel utilizing the transformation-induced plasticity of retained austenite has been proposed (for example, Patent Document 1).
残留オーステナイトは、Cをオーステナイト中に濃化させることによって、オーステナイトが室温でも他の相に変態しないようにすることによって得られる。オーステナイトを安定化させる技術として、Si及びAl等の炭化物析出抑制元素を鋼板に含有させて、鋼板の製造段階において鋼板に生じるベイナイト変態の間にオーステナイト中にCを濃化させることが提案されている。この技術では、鋼板に含有させるC含有量が多ければ、オーステナイトがさらに安定化し、残留オーステナイト量を増やすことができ、その結果、強度と伸びとの両方が優れた鋼板を造ることができる。しかしながら、鋼板が構造部材に使用される場合、鋼板に溶接が行われることが多いが、鋼板中のC含有量が多いと溶接性を十分確保することが困難となり、構造部材として使用することに制限がかかる。したがって、より少ないC含有量で、鋼板の強度と伸びとの両方を向上することが望まれている。
(4) Retained austenite is obtained by enriching C in austenite so that austenite does not transform into another phase even at room temperature. As a technique for stabilizing austenite, it has been proposed to include a carbide precipitation suppressing element such as Si and Al in a steel sheet and to enrich C in austenite during bainite transformation occurring in the steel sheet in a steel sheet manufacturing stage. I have. In this technique, when the C content in the steel sheet is large, the austenite is further stabilized, the amount of retained austenite can be increased, and as a result, a steel sheet excellent in both strength and elongation can be produced. However, when a steel sheet is used for a structural member, welding is often performed on the steel sheet. However, if the C content in the steel sheet is large, it is difficult to secure sufficient weldability, and the steel sheet is used as a structural member. There are restrictions. Therefore, it is desired to improve both the strength and the elongation of the steel sheet with a smaller C content.
C含有量が上記TRIP鋼よりも少なく、さらに、残留オーステナイト量が上記TRIP鋼よりも多く、強度と延性が上記TRIP鋼を超える鋼板として、4.0質量%超のMnを添加した鋼が提案されている(例えば、非特許文献1)。しかしながら、上記鋼は箱焼鈍のような長時間加熱プロセスを要件としている。そのため、自動車用の部材に供する高強度鋼板の製造に適する連続焼鈍のような短時間加熱プロセスにおける材料設計は十分に検討されておらず、その場合の局部延性を高める要件は明らかでなかった。
As a steel sheet having a C content lower than that of the TRIP steel, a retained austenite amount larger than that of the TRIP steel, and a strength and ductility exceeding the TRIP steel, a steel to which Mn of more than 4.0% by mass is added is proposed. (For example, Non-Patent Document 1). However, the steel requires a long heating process such as box annealing. For this reason, material design in a short-time heating process such as continuous annealing suitable for manufacturing a high-strength steel sheet to be used as a member for an automobile has not been sufficiently studied, and a requirement for increasing local ductility in that case was not clear.
特許文献2には、3.5質量%以上のMnを添加した鋼板であって、フェライトを30%~80%に制御することによって、引張強度及び伸び性が優れる鋼板が開示されている。
Patent Document 2 discloses a steel sheet to which 3.5% by mass or more of Mn is added and which has excellent tensile strength and elongation by controlling ferrite to 30% to 80%.
特許文献3には、3~7質量%のMnを添加した鋼について、残留オーステナイトの生成を20体積%未満に抑制することによって局部延性を向上させた、具体的には、局部伸びが8%以上の鋼板が開示されている。
Patent Document 3 discloses that in a steel containing 3 to 7% by mass of Mn, local ductility is improved by suppressing the generation of retained austenite to less than 20% by volume. Specifically, local elongation is 8%. The above steel sheet is disclosed.
特許文献4には2.0~6.0質量%のMnを含有し、残留オーステナイト量を20体積%以上有する高強度鋼板が開示されている。
Patent Document 4 discloses a high-strength steel sheet containing 2.0 to 6.0% by mass of Mn and having a retained austenite amount of 20% by volume or more.
特許文献5には、1.68~3.8質量%のMnを含有し、局部延性に優れ、局部伸びが4%以上の高強度鋼板が開示されている。
Patent Document 5 discloses a high-strength steel sheet containing 1.68 to 3.8% by mass of Mn, having excellent local ductility, and having a local elongation of 4% or more.
自動車用の部材として高強度鋼板を使用するためには、溶接性を低下させずに、相反する特性である強度と成形性とを確保することが望まれる。具体的には、優れた伸び特性及び高強度を有することが望まれる。
In order to use a high-strength steel sheet as a member for automobiles, it is desired to ensure strength and formability, which are contradictory properties, without reducing weldability. Specifically, it is desired to have excellent elongation characteristics and high strength.
しかしながら、例えば上述した特許文献2及び非特許文献1に開示されているような含有Mn濃度が高い鋼板においては、未再結晶フェライトを多く含むので加工硬化せず、加工・成形性が低い。すなわち、このような未再結晶フェライトを多く含む組織を有する含有Mn濃度の高い鋼板は、自動車用鋼板に求められる引張強度と加工・成形性とを兼備し得るものではない。
However, a steel sheet having a high content of Mn as disclosed in, for example, Patent Document 2 and Non-Patent Document 1 contains a large amount of unrecrystallized ferrite and thus does not work harden and has low workability and formability. That is, a steel sheet having a high content of Mn having such a structure containing a large amount of unrecrystallized ferrite cannot have both the tensile strength and the workability and formability required for a steel sheet for automobiles.
特許文献3に記載の発明例となる鋼板は、Cの含有量が0.2質量%未満と低いため、引張強度が1090MPa以下であり、Cの含有量を0.2質量%未満に維持したまま引張強度を1200MPa以上の高強度としたものについては、引張強度が1233MPaで、局部伸びが1.3%である比較例が示されており、局部延性が低下している。
The steel sheet as an example of the invention described in Patent Document 3 has a low C content of less than 0.2% by mass, and thus has a tensile strength of 1090 MPa or less and maintains the C content at less than 0.2% by mass. With respect to those having a high tensile strength of 1200 MPa or more, a comparative example in which the tensile strength is 1233 MPa and the local elongation is 1.3% is shown, and the local ductility is reduced.
特許文献4に記載の鋼板は、熱延板あるいは冷延板を800℃以上に予備熱処理してオーステナイト化して冷却し、その後650~750℃の焼鈍温度にて焼鈍を2回行うことによって、オーステナイト中への合金元素の濃縮を促進して20体積%以上の残留オーステナイトを生成させたものである。しかし、特許文献4に記載の発明となる鋼板では、十分な局部延性が得られていなかった。
The steel sheet described in Patent Document 4 is obtained by preheating a hot-rolled sheet or a cold-rolled sheet to 800 ° C. or higher, austenitizing and cooling, and then performing annealing twice at an annealing temperature of 650 to 750 ° C. This is to promote the concentration of alloy elements therein to generate retained austenite of 20% by volume or more. However, in the steel sheet according to the invention described in Patent Literature 4, sufficient local ductility was not obtained.
特許文献5に記載の含有Mn濃度を高めた鋼板については、Mnの含有量が6.0質量%で、局部伸びが0.9%である比較例が示されており、局部延性は高くない。
Regarding the steel sheet with an increased content Mn concentration described in Patent Document 5, a comparative example in which the Mn content is 6.0% by mass and the local elongation is 0.9% is shown, and the local ductility is not high. .
したがって、優れた局部延性と高い引張強度とを有する含有Mn濃度の高い鋼板が望まれている。
Therefore, there is a demand for a steel sheet having an excellent local ductility and a high tensile strength and a high content of Mn.
含有Mn濃度の高い鋼板において、優れた局部延性と高強度とを確保するために、本発明者らは、鋼板の金属組織に、面積率(面積%)で、母相として25%以上90%以下の焼き戻しマルテンサイト及び3%以下のフェライト、並びにその他の相として10%以上50%以下の残留オーステナイト及び25%以下のフレッシュマルテンサイトを含ませ、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度が2.7μm-1以上とすることが有効であることを見出した。
In order to ensure excellent local ductility and high strength in a steel sheet having a high content of Mn, the present inventors have set the metal structure of the steel sheet to have an area ratio (area%) of 25% or more and 90% or more as a matrix. The following tempered martensite and 3% or less of ferrite, and 10% or more and 50% or less of retained austenite and 25% or less of fresh martensite are contained as other phases. It has been found that it is effective to set the interface density between retained austenite and fresh martensite to 2.7 μm −1 or more.
局部延性は、伸びフランジ成形や曲げ加工の局所変形領域で生じる亀裂進展に材料が耐えうる特性であり、その指標は、単軸引張試験によって得られる局部伸びである。
Local ductility is a property that a material can withstand crack propagation generated in a local deformation region of stretch flange forming or bending, and its index is local elongation obtained by a uniaxial tensile test.
単軸引張試験において、ミクロ組織中に高硬度を有するフレッシュマルテンサイト又は残留オーステナイトが存在した場合、局部変形開始後のくびれ領域においてボイドが発生し得る。ボイドが発生すると、その後の変形過程でボイド同士が連結、進展することで、き裂が発生・伝播し破断に至る。
In the uniaxial tensile test, when fresh martensite or retained austenite having high hardness is present in the microstructure, voids may be generated in a constricted region after the start of local deformation. When a void is generated, the voids are connected and propagated in the subsequent deformation process, thereby generating and propagating a crack, leading to fracture.
本発明者等は、任意の方向からの亀裂伝播に対してばらつきの少ないミクロ組織とすることが局部延性向上に有効であると考え、鋭意検討の結果、本発明者らは以下のミクロ組織とすることで高強度且つ優れた局部延性を有する鋼板が得られることを見出した。
The present inventors believe that it is effective to improve the local ductility to have a microstructure with little variation with respect to crack propagation from any direction, and as a result of earnest study, the present inventors have found that the following microstructure By doing so, a steel sheet having high strength and excellent local ductility can be obtained.
鋼板の金属組織の母相を焼き戻しマルテンサイトとする。マルテンサイトはその相中に転位を多く含む低温変態相であり、鋼板の強度を増加させるのに有用な相である。さらに焼き戻しマルテンサイトにすることで、強度-局部延性のバランスが向上する。
。The matrix of the metal structure of the steel sheet is tempered martensite. Martensite is a low-temperature transformation phase containing many dislocations in its phase, and is a useful phase for increasing the strength of a steel sheet. Further, by using tempered martensite, the balance between strength and local ductility is improved.
ミクロ組織中に残留オーステナイトを含有することで延性を向上させる。これによって、軟質なオーステナイトによる延性向上効果だけでなく、加工時にマルテンサイトに加工誘起変態することでTRIP効果が発現して、強度-延性-局部延性のバランスをさらに向上させることができる。
(4) The ductility is improved by containing retained austenite in the microstructure. As a result, not only the ductility improving effect of soft austenite but also the TRIP effect is exhibited by the work-induced transformation to martensite during working, and the balance of strength-ductility-local ductility can be further improved.
残留オーステナイトの周囲には焼入れ時に一部変態したフレッシュマルテンサイトが隣接し得る。フレッシュマルテンサイト及び加工誘起変態により生じたマルテンサイトは、母相の焼き戻しマルテンサイトよりも硬質な相であり、鋼の強度を増加させるだけでなく、軟質相から伝播してくる亀裂の伝播を界面で止める役割も有する。
(4) Fresh martensite partially transformed during quenching may be adjacent to the retained austenite. Fresh martensite and martensite formed by process-induced transformation are harder phases than the tempered martensite of the parent phase, and not only increase the strength of the steel, but also reduce the propagation of cracks that propagate from the soft phase. It also has the role of stopping at the interface.
具体的には、L断面において表面から厚みの1/4位置における金属組織が、面積率で、25%以上90%以下の焼き戻しマルテンサイト、3%以下のフェライト、10%以上50%以下の残留オーステナイト、及び25%以下のフレッシュマルテンサイトを含む。なお、ここで、L断面とは、板厚方向と圧延方向に平行に鋼板の圧延方向の中心軸を通るように切断した面をいう。
Specifically, in the L section, the metal structure at a position 1/4 of the thickness from the surface is tempered martensite having an area ratio of 25% or more and 90% or less, 3% or less ferrite, and 10% or more and 50% or less. Contains retained austenite and up to 25% fresh martensite. Here, the L section refers to a surface cut parallel to the sheet thickness direction and the rolling direction so as to pass through the central axis in the rolling direction of the steel sheet.
ただし、残留オーステナイトとフレッシュマルテンサイトが互いに連結した粗大な組織形態を有している場合、焼き戻しマルテンサイトの母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面が少なくなることによって亀裂伝播の障害が減少し、早期に破断に至る、すなわち局部延性が劣化し得ることを知見した。
However, when the retained austenite and the fresh martensite have a coarse structure morphology connected to each other, the interface between the retained austenite and the fresh martensite with respect to the parent phase of the tempered martensite is reduced, so that the obstacle of the crack propagation is reduced. It has been found that it decreases and leads to early fracture, that is, local ductility may deteriorate.
そこで、本発明者等は、残留オーステナイトとフレッシュマルテンサイトを微細かつ複雑な形状とすることで局部延性が著しく向上することをさらに見出した。具体的には、図1及び図2に示すように、残留オーステナイトとフレッシュマルテンサイトとからなる組織が焼き戻しマルテンサイトに対して細かく複雑に入り組んだ組織とすることで、軟質層から伝播してくる亀裂伝播を、焼き戻しマルテンサイトの母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面で止めることができる。図1は、本発明の一実施形態の鋼板のL断面を鏡面研磨及びナイタール処理して観察した走査型電子顕微鏡(SEM)画像である。図2は、図1のSEM画像を2値化処理した2階調画像である。焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度が2.7μm-1以上となるように、鋼板の金属組織を制御する。この金属組織には、任意の亀裂伝播方向に対して障害となる母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面が一定の割合で存在するため、界面強度が向上し、優れた局部延性が得られる。
Then, the present inventors have further found that local ductility is remarkably improved by forming retained austenite and fresh martensite into a fine and complicated shape. Specifically, as shown in FIG. 1 and FIG. 2, the structure composed of retained austenite and fresh martensite is finely and intricately intertwined with tempered martensite, and is transmitted from the soft layer. Upcoming crack propagation can be stopped at the interface between the retained austenite and the fresh martensite for the matrix of tempered martensite. FIG. 1 is a scanning electron microscope (SEM) image obtained by observing the L section of a steel sheet according to an embodiment of the present invention after mirror polishing and nital treatment. FIG. 2 is a two-tone image obtained by binarizing the SEM image of FIG. The metal structure of the steel sheet is controlled such that the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is 2.7 μm −1 or more. In this metallographic structure, the interface between the retained austenite and the fresh martensite with respect to the parent phase, which is an obstacle to any crack propagation direction, exists at a fixed ratio, so that the interface strength is improved and excellent local ductility is obtained. Can be
本開示の鋼板は上記知見に基づいてなされたものであり、その要旨は以下のとおりである。
(1)質量%で、
C:0.20%超0.55%未満、
Si:0.001%以上3.50%未満、
Mn:4.00%超9.00%未満、
sol.Al:0.001%以上3.00%未満、
P:0.100%以下、
S:0.010%以下、
N:0.050%未満、
O:0.020%未満、
B:0.0000%以上0.010%未満
Cr:0.00%以上2.00%未満、
Mo:0.00%以上2.00%以下、
W:0.00%以上2.00%以下、
Cu:0.00%以上2.00%以下、
Ni:0.00%以上2.00%以下、
Ti:0.00%以上0.300%以下、
Nb:0.00%以上0.300%以下、
V:0.00%以上0.300%以下、
Ca:0.00%以上0.010%以下、
Mg:0.00%以上0.010%以下、
Zr:0.00%以上0.010%以下、
REM:0.00%以上0.010%以下、
Sb:0.00%以上0.050%以下、
Sn:0.00%以上0.050%以下、及び
Bi:0.00%以上0.050%以下を含有し、
残部が鉄及び不純物であり、
L断面において表面から厚みの1/4位置における金属組織が、面積率で、25%以上90%以下の焼き戻しマルテンサイト、10%以上50%以下の残留オーステナイト、25%以下のフレッシュマルテンサイトライトを含み、
前記1/4位置における金属組織において、残留オーステナイトまたはフレッシュマルテンサイトのいずれかである領域である第1の領域と、前記1/4位置における金属組織から前記第1の領域を除いた領域である第2の領域と、の境界の長さの総和を、前記第1の領域と前記第2の領域との合計面積で除した値である界面密度が2.7μm-1以上であること
を特徴とする鋼板。
(2)質量%で、
B:0.0003%以上0.010%未満
を含有することを特徴とする、上記(1)に記載の鋼板。
(3)質量%で、
Cr:0.01%以上2.00%未満、
Mo:0.01%以上2.00%以下、
W:0.01%以上2.00%以下、
Cu:0.01%以上2.00%以下、及び
Ni:0.01%以上2.00%以下
の1種又は2種以上を含有することを特徴とする、上記(1)又は(2)に記載の鋼板。
(4)質量%で、
Ti:0.005%以上0.300%以下、
Nb:0.005%以上0.300%以下、及び
V:0.005%以上0.300%以下
の1種又は2種以上を含有することを特徴とする、上記(1)~(3)のいずれかに記載の鋼板。
(5)質量%で、
Ca:0.0001%以上0.0100%以下、
Mg:0.0001%以上0.0100%以下、
Zr:0.0001%以上0.0100%以下、及び
REM:0.0001%以上0.0100%以下
の1種又は2種以上を含有することを特徴とする、上記(1)~(4)のいずれかに記載の鋼板。
(6)質量%で、
Sb:0.0005%以上0.0500%以下、
Sn:0.0005%以上0.0500%以下、及び
Bi:0.0005%以上0.0500%以下
の1種又は2種以上をさらに含有することを特徴とする、上記(1)~(5)のいずれかに記載の鋼板。
(7)前記鋼板の表面に溶融亜鉛めっき層を有することを特徴とする、上記(1)~(6)のいずれかに記載の鋼板。
(8)前記鋼板の表面に合金化溶融亜鉛めっき層を有することを特徴とする、上記(1)~(6)のいずれかに記載の鋼板。 The steel plate of the present disclosure has been made based on the above findings, and the gist is as follows.
(1) In mass%,
C: more than 0.20% and less than 0.55%,
Si: 0.001% or more and less than 3.50%,
Mn: more than 4.00% and less than 9.00%,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
B: 0.0000% or more and less than 0.010% Cr: 0.00% or more and less than 2.00%,
Mo: 0.00% or more and 2.00% or less,
W: 0.00% or more and 2.00% or less,
Cu: 0.00% or more and 2.00% or less,
Ni: 0.00% or more and 2.00% or less,
Ti: 0.00% or more and 0.300% or less,
Nb: 0.00% or more and 0.300% or less,
V: 0.00% or more and 0.300% or less,
Ca: 0.00% or more and 0.010% or less,
Mg: 0.00% or more and 0.010% or less,
Zr: 0.00% or more and 0.010% or less,
REM: 0.00% or more and 0.010% or less,
Sb: 0.00% or more and 0.050% or less,
Sn: 0.00% or more and 0.050% or less, and Bi: 0.00% or more and 0.050% or less;
The balance is iron and impurities,
In the L cross section, the metal structure at a quarter of the thickness from the surface is 25% or more and 90% or less of tempered martensite, 10% or more and 50% or less of retained austenite, and 25% or less of fresh martensite in terms of area ratio. Including
In the metal structure at the 1/4 position, a first region that is either retained austenite or fresh martensite, and a region obtained by removing the first region from the metal structure at the 1/4 position. The interface density, which is a value obtained by dividing the total length of the boundary between the second region and the second region by the total area of the first region and the second region, is 2.7 μm −1 or more. And steel plate.
(2) In mass%,
B: The steel sheet according to the above (1), which contains 0.0003% or more and less than 0.010%.
(3) In mass%,
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
(1) or (2), characterized by containing one or more of Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less. A steel sheet according to claim 1.
(4) In mass%,
Ti: 0.005% or more and 0.300% or less,
(1) to (3), wherein one or more of Nb: 0.005% to 0.300%, and V: 0.005% to 0.300% are contained. The steel sheet according to any one of the above.
(5) In mass%,
Ca: 0.0001% or more and 0.0100% or less,
Mg: 0.0001% or more and 0.0100% or less,
(1) to (4), characterized by containing one or more of Zr: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.0100% or less. The steel sheet according to any one of the above.
(6) In mass%,
Sb: 0.0005% or more and 0.0500% or less,
(1) to (5), further comprising one or more of Sn: 0.0005% to 0.0500%, and Bi: 0.0005% to 0.0500%. ).
(7) The steel sheet according to any one of the above (1) to (6), wherein the steel sheet has a hot-dip galvanized layer on a surface thereof.
(8) The steel sheet according to any one of the above (1) to (6), wherein the steel sheet has a galvannealed layer on a surface thereof.
(1)質量%で、
C:0.20%超0.55%未満、
Si:0.001%以上3.50%未満、
Mn:4.00%超9.00%未満、
sol.Al:0.001%以上3.00%未満、
P:0.100%以下、
S:0.010%以下、
N:0.050%未満、
O:0.020%未満、
B:0.0000%以上0.010%未満
Cr:0.00%以上2.00%未満、
Mo:0.00%以上2.00%以下、
W:0.00%以上2.00%以下、
Cu:0.00%以上2.00%以下、
Ni:0.00%以上2.00%以下、
Ti:0.00%以上0.300%以下、
Nb:0.00%以上0.300%以下、
V:0.00%以上0.300%以下、
Ca:0.00%以上0.010%以下、
Mg:0.00%以上0.010%以下、
Zr:0.00%以上0.010%以下、
REM:0.00%以上0.010%以下、
Sb:0.00%以上0.050%以下、
Sn:0.00%以上0.050%以下、及び
Bi:0.00%以上0.050%以下を含有し、
残部が鉄及び不純物であり、
L断面において表面から厚みの1/4位置における金属組織が、面積率で、25%以上90%以下の焼き戻しマルテンサイト、10%以上50%以下の残留オーステナイト、25%以下のフレッシュマルテンサイトライトを含み、
前記1/4位置における金属組織において、残留オーステナイトまたはフレッシュマルテンサイトのいずれかである領域である第1の領域と、前記1/4位置における金属組織から前記第1の領域を除いた領域である第2の領域と、の境界の長さの総和を、前記第1の領域と前記第2の領域との合計面積で除した値である界面密度が2.7μm-1以上であること
を特徴とする鋼板。
(2)質量%で、
B:0.0003%以上0.010%未満
を含有することを特徴とする、上記(1)に記載の鋼板。
(3)質量%で、
Cr:0.01%以上2.00%未満、
Mo:0.01%以上2.00%以下、
W:0.01%以上2.00%以下、
Cu:0.01%以上2.00%以下、及び
Ni:0.01%以上2.00%以下
の1種又は2種以上を含有することを特徴とする、上記(1)又は(2)に記載の鋼板。
(4)質量%で、
Ti:0.005%以上0.300%以下、
Nb:0.005%以上0.300%以下、及び
V:0.005%以上0.300%以下
の1種又は2種以上を含有することを特徴とする、上記(1)~(3)のいずれかに記載の鋼板。
(5)質量%で、
Ca:0.0001%以上0.0100%以下、
Mg:0.0001%以上0.0100%以下、
Zr:0.0001%以上0.0100%以下、及び
REM:0.0001%以上0.0100%以下
の1種又は2種以上を含有することを特徴とする、上記(1)~(4)のいずれかに記載の鋼板。
(6)質量%で、
Sb:0.0005%以上0.0500%以下、
Sn:0.0005%以上0.0500%以下、及び
Bi:0.0005%以上0.0500%以下
の1種又は2種以上をさらに含有することを特徴とする、上記(1)~(5)のいずれかに記載の鋼板。
(7)前記鋼板の表面に溶融亜鉛めっき層を有することを特徴とする、上記(1)~(6)のいずれかに記載の鋼板。
(8)前記鋼板の表面に合金化溶融亜鉛めっき層を有することを特徴とする、上記(1)~(6)のいずれかに記載の鋼板。 The steel plate of the present disclosure has been made based on the above findings, and the gist is as follows.
(1) In mass%,
C: more than 0.20% and less than 0.55%,
Si: 0.001% or more and less than 3.50%,
Mn: more than 4.00% and less than 9.00%,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
B: 0.0000% or more and less than 0.010% Cr: 0.00% or more and less than 2.00%,
Mo: 0.00% or more and 2.00% or less,
W: 0.00% or more and 2.00% or less,
Cu: 0.00% or more and 2.00% or less,
Ni: 0.00% or more and 2.00% or less,
Ti: 0.00% or more and 0.300% or less,
Nb: 0.00% or more and 0.300% or less,
V: 0.00% or more and 0.300% or less,
Ca: 0.00% or more and 0.010% or less,
Mg: 0.00% or more and 0.010% or less,
Zr: 0.00% or more and 0.010% or less,
REM: 0.00% or more and 0.010% or less,
Sb: 0.00% or more and 0.050% or less,
Sn: 0.00% or more and 0.050% or less, and Bi: 0.00% or more and 0.050% or less;
The balance is iron and impurities,
In the L cross section, the metal structure at a quarter of the thickness from the surface is 25% or more and 90% or less of tempered martensite, 10% or more and 50% or less of retained austenite, and 25% or less of fresh martensite in terms of area ratio. Including
In the metal structure at the 1/4 position, a first region that is either retained austenite or fresh martensite, and a region obtained by removing the first region from the metal structure at the 1/4 position. The interface density, which is a value obtained by dividing the total length of the boundary between the second region and the second region by the total area of the first region and the second region, is 2.7 μm −1 or more. And steel plate.
(2) In mass%,
B: The steel sheet according to the above (1), which contains 0.0003% or more and less than 0.010%.
(3) In mass%,
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
(1) or (2), characterized by containing one or more of Cu: 0.01% or more and 2.00% or less, and Ni: 0.01% or more and 2.00% or less. A steel sheet according to claim 1.
(4) In mass%,
Ti: 0.005% or more and 0.300% or less,
(1) to (3), wherein one or more of Nb: 0.005% to 0.300%, and V: 0.005% to 0.300% are contained. The steel sheet according to any one of the above.
(5) In mass%,
Ca: 0.0001% or more and 0.0100% or less,
Mg: 0.0001% or more and 0.0100% or less,
(1) to (4), characterized by containing one or more of Zr: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.0100% or less. The steel sheet according to any one of the above.
(6) In mass%,
Sb: 0.0005% or more and 0.0500% or less,
(1) to (5), further comprising one or more of Sn: 0.0005% to 0.0500%, and Bi: 0.0005% to 0.0500%. ).
(7) The steel sheet according to any one of the above (1) to (6), wherein the steel sheet has a hot-dip galvanized layer on a surface thereof.
(8) The steel sheet according to any one of the above (1) to (6), wherein the steel sheet has a galvannealed layer on a surface thereof.
本開示によれば、優れた局部延性及び高強度を有する含有Mn濃度の高い鋼板を提供することができる。
According to the present disclosure, it is possible to provide a steel sheet having excellent local ductility and high strength and having a high Mn content.
以下、本開示の鋼板の実施形態の例を説明する。
1.化学組成 Hereinafter, an example of the embodiment of the steel plate of the present disclosure will be described.
1. Chemical composition
1.化学組成 Hereinafter, an example of the embodiment of the steel plate of the present disclosure will be described.
1. Chemical composition
本開示の鋼板の化学組成を上述のように規定した理由を説明する。以下の説明において、各元素の含有量を表す「%」は特に断りがない限り質量%を意味する。
(4) The reason why the chemical composition of the steel sheet of the present disclosure is defined as described above will be described. In the following description, “%” representing the content of each element means mass% unless otherwise specified.
(C:0.20%超0.55%未満)
Cは、鋼の引張強度を高めるために、極めて重要な元素である。十分な残留オーステナイト量を得るためには、0.20%超のC含有量が必要となる。一方、Cを過剰に含有すると鋼板の溶接性を損なうので、C含有量の上限を0.55%未満とした。引張強度と全伸びとを高める点から、C含有量の下限値は、好ましくは0.24%以上、より好ましくは0.28%以上である。C含有量の上限値は、好ましくは0.40%以下、より好ましくは0.35%以下であり、C含有量の上限値を上記範囲にすることによって、鋼板の溶接性をより高めることができる。 (C: more than 0.20% and less than 0.55%)
C is a very important element for increasing the tensile strength of steel. In order to obtain a sufficient amount of retained austenite, a C content of more than 0.20% is required. On the other hand, if C is excessively contained, the weldability of the steel sheet is impaired. Therefore, the upper limit of the C content is set to less than 0.55%. From the viewpoint of increasing the tensile strength and the total elongation, the lower limit of the C content is preferably 0.24% or more, more preferably 0.28% or more. The upper limit of the C content is preferably 0.40% or less, more preferably 0.35% or less. By setting the upper limit of the C content to the above range, the weldability of the steel sheet can be further increased. it can.
Cは、鋼の引張強度を高めるために、極めて重要な元素である。十分な残留オーステナイト量を得るためには、0.20%超のC含有量が必要となる。一方、Cを過剰に含有すると鋼板の溶接性を損なうので、C含有量の上限を0.55%未満とした。引張強度と全伸びとを高める点から、C含有量の下限値は、好ましくは0.24%以上、より好ましくは0.28%以上である。C含有量の上限値は、好ましくは0.40%以下、より好ましくは0.35%以下であり、C含有量の上限値を上記範囲にすることによって、鋼板の溶接性をより高めることができる。 (C: more than 0.20% and less than 0.55%)
C is a very important element for increasing the tensile strength of steel. In order to obtain a sufficient amount of retained austenite, a C content of more than 0.20% is required. On the other hand, if C is excessively contained, the weldability of the steel sheet is impaired. Therefore, the upper limit of the C content is set to less than 0.55%. From the viewpoint of increasing the tensile strength and the total elongation, the lower limit of the C content is preferably 0.24% or more, more preferably 0.28% or more. The upper limit of the C content is preferably 0.40% or less, more preferably 0.35% or less. By setting the upper limit of the C content to the above range, the weldability of the steel sheet can be further increased. it can.
(Si:0.001%以上3.50%未満)
Siは、焼き戻しマルテンサイトを強化し、組織を均一化し、局部延性を改善するのに有効な元素である。また、Siは、セメンタイトの析出と粗大化とを抑制し、焼鈍中に生成するオーステナイトを制御しやすくする作用も有する。上記効果を得るために、0.001%以上のSi含有量が必要となる。Si含有量の下限値は、好ましくは0.01%以上、より好ましくは0.30%以上、さらに好ましくは0.50%以上である。Si含有量の下限値を上記範囲にすることによって、鋼板の局部延性をさらに向上させることができる。一方、Siを過剰に含有すると鋼板のめっき性や化成処理性を損なうので、Si含有量の上限値を3.50%未満とした。さらに、Si含有量の上限値は、好ましくは3.00%以下、より好ましくは2.50%以下である。 (Si: 0.001% or more and less than 3.50%)
Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving local ductility. Further, Si also has an effect of suppressing precipitation and coarsening of cementite and facilitating control of austenite generated during annealing. In order to obtain the above effects, a Si content of 0.001% or more is required. The lower limit of the Si content is preferably 0.01% or more, more preferably 0.30% or more, and still more preferably 0.50% or more. By setting the lower limit of the Si content to the above range, the local ductility of the steel sheet can be further improved. On the other hand, if Si is excessively contained, the plating property and chemical conversion treatment property of the steel sheet are impaired, so the upper limit of the Si content is set to less than 3.50%. Further, the upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.
Siは、焼き戻しマルテンサイトを強化し、組織を均一化し、局部延性を改善するのに有効な元素である。また、Siは、セメンタイトの析出と粗大化とを抑制し、焼鈍中に生成するオーステナイトを制御しやすくする作用も有する。上記効果を得るために、0.001%以上のSi含有量が必要となる。Si含有量の下限値は、好ましくは0.01%以上、より好ましくは0.30%以上、さらに好ましくは0.50%以上である。Si含有量の下限値を上記範囲にすることによって、鋼板の局部延性をさらに向上させることができる。一方、Siを過剰に含有すると鋼板のめっき性や化成処理性を損なうので、Si含有量の上限値を3.50%未満とした。さらに、Si含有量の上限値は、好ましくは3.00%以下、より好ましくは2.50%以下である。 (Si: 0.001% or more and less than 3.50%)
Si is an element effective for strengthening tempered martensite, homogenizing the structure, and improving local ductility. Further, Si also has an effect of suppressing precipitation and coarsening of cementite and facilitating control of austenite generated during annealing. In order to obtain the above effects, a Si content of 0.001% or more is required. The lower limit of the Si content is preferably 0.01% or more, more preferably 0.30% or more, and still more preferably 0.50% or more. By setting the lower limit of the Si content to the above range, the local ductility of the steel sheet can be further improved. On the other hand, if Si is excessively contained, the plating property and chemical conversion treatment property of the steel sheet are impaired, so the upper limit of the Si content is set to less than 3.50%. Further, the upper limit of the Si content is preferably 3.00% or less, more preferably 2.50% or less.
(Mn:4.00%超9.00%未満)
Mnは、オーステナイトを安定化させ、焼入れ性を高める元素である。また、本開示の鋼板においては、Mnをオーステナイト中に分配させ、よりオーステナイトを安定化させる。室温でオーステナイトを安定化させるためには、4.00%超のMnが必要である。一方、鋼板がMnを過剰に含有すると延性及び局部延性を損なうので、Mn含有量の上限を9.00%未満とした。Mn含有量の下限値は、好ましくは4.30%以上、より好ましくは4.80%以上である。Mn含有量の上限値は、好ましくは8.00%以下、より好ましくは7.50%以下である。Mn含有量の下限値及び上限値を上記範囲にすることによって、さらにオーステナイトを安定化させることができる。 (Mn: more than 4.00% and less than 9.00%)
Mn is an element that stabilizes austenite and enhances hardenability. Further, in the steel sheet of the present disclosure, Mn is distributed in austenite to further stabilize austenite. To stabilize austenite at room temperature, more than 4.00% Mn is required. On the other hand, if the steel sheet excessively contains Mn, ductility and local ductility are impaired, so the upper limit of the Mn content is set to less than 9.00%. The lower limit of the Mn content is preferably at least 4.30%, more preferably at least 4.80%. The upper limit of the Mn content is preferably 8.00% or less, more preferably 7.50% or less. By setting the lower limit and the upper limit of the Mn content in the above ranges, austenite can be further stabilized.
Mnは、オーステナイトを安定化させ、焼入れ性を高める元素である。また、本開示の鋼板においては、Mnをオーステナイト中に分配させ、よりオーステナイトを安定化させる。室温でオーステナイトを安定化させるためには、4.00%超のMnが必要である。一方、鋼板がMnを過剰に含有すると延性及び局部延性を損なうので、Mn含有量の上限を9.00%未満とした。Mn含有量の下限値は、好ましくは4.30%以上、より好ましくは4.80%以上である。Mn含有量の上限値は、好ましくは8.00%以下、より好ましくは7.50%以下である。Mn含有量の下限値及び上限値を上記範囲にすることによって、さらにオーステナイトを安定化させることができる。 (Mn: more than 4.00% and less than 9.00%)
Mn is an element that stabilizes austenite and enhances hardenability. Further, in the steel sheet of the present disclosure, Mn is distributed in austenite to further stabilize austenite. To stabilize austenite at room temperature, more than 4.00% Mn is required. On the other hand, if the steel sheet excessively contains Mn, ductility and local ductility are impaired, so the upper limit of the Mn content is set to less than 9.00%. The lower limit of the Mn content is preferably at least 4.30%, more preferably at least 4.80%. The upper limit of the Mn content is preferably 8.00% or less, more preferably 7.50% or less. By setting the lower limit and the upper limit of the Mn content in the above ranges, austenite can be further stabilized.
(sol.Al:0.001%以上1.00%未満)
Alは、脱酸剤であり、0.001%以上含有させる必要がある。また、Alは、焼鈍時の二相温度域を広げるため、材質安定性を高める作用も有する。Alの含有量が多いほどその効果は大きくなるが、Alを過剰に含有させると、表面性状、塗装性、及び溶接性などの劣化を招くので、sol.Alの上限を1.00%未満とした。sol.Al含有量の下限値は、好ましくは0.005%以上、より好ましくは0.010%以上、さらに好ましくは0.020%以上である。sol.Al含有量の上限値は、好ましくは0.80%以下、より好ましくは0.60%以下である。sol.Al含有量の下限値及び上限値を上記範囲にすることによって、脱酸効果及び材質安定向上効果と、表面性状、塗装性、及び溶接性とのバランスがより良好になる。本明細書にいう「sol.Al」は、「酸可溶性Al」を意味する。 (Sol. Al: 0.001% or more and less than 1.00%)
Al is a deoxidizing agent and needs to be contained at 0.001% or more. Al also has the effect of increasing the material stability in order to widen the two-phase temperature range during annealing. The effect increases as the content of Al increases, but if the content of Al is excessive, deterioration of surface properties, paintability, weldability, and the like is caused. The upper limit of Al was set to less than 1.00%. sol. The lower limit of the Al content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more. sol. The upper limit of the Al content is preferably 0.80% or less, more preferably 0.60% or less. sol. By setting the lower limit and the upper limit of the Al content in the above ranges, the balance between the deoxidizing effect and the material stability improving effect and the surface properties, coating properties, and weldability are further improved. As used herein, “sol. Al” means “acid-soluble Al”.
Alは、脱酸剤であり、0.001%以上含有させる必要がある。また、Alは、焼鈍時の二相温度域を広げるため、材質安定性を高める作用も有する。Alの含有量が多いほどその効果は大きくなるが、Alを過剰に含有させると、表面性状、塗装性、及び溶接性などの劣化を招くので、sol.Alの上限を1.00%未満とした。sol.Al含有量の下限値は、好ましくは0.005%以上、より好ましくは0.010%以上、さらに好ましくは0.020%以上である。sol.Al含有量の上限値は、好ましくは0.80%以下、より好ましくは0.60%以下である。sol.Al含有量の下限値及び上限値を上記範囲にすることによって、脱酸効果及び材質安定向上効果と、表面性状、塗装性、及び溶接性とのバランスがより良好になる。本明細書にいう「sol.Al」は、「酸可溶性Al」を意味する。 (Sol. Al: 0.001% or more and less than 1.00%)
Al is a deoxidizing agent and needs to be contained at 0.001% or more. Al also has the effect of increasing the material stability in order to widen the two-phase temperature range during annealing. The effect increases as the content of Al increases, but if the content of Al is excessive, deterioration of surface properties, paintability, weldability, and the like is caused. The upper limit of Al was set to less than 1.00%. sol. The lower limit of the Al content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.020% or more. sol. The upper limit of the Al content is preferably 0.80% or less, more preferably 0.60% or less. sol. By setting the lower limit and the upper limit of the Al content in the above ranges, the balance between the deoxidizing effect and the material stability improving effect and the surface properties, coating properties, and weldability are further improved. As used herein, “sol. Al” means “acid-soluble Al”.
(P:0.100%以下)
Pは不純物であり、鋼板がPを過剰に含有すると靭性や溶接性を損なう。したがって、P含有量の上限を0.100%以下とする。P含有量の上限値は、好ましくは0.050%以下、より好ましくは0.030%以下、さらに好ましくは0.020%以下である。本実施形態に係る鋼板はPを必要としないので、Pを実質的に含有しなくてもよく、P含有量の下限値は0.000%である。P含有量の下限値は0.000%超または0.001%以上でもよいが、P含有量は少ないほど好ましい。 (P: 0.100% or less)
P is an impurity, and if the steel sheet contains excessive P, toughness and weldability are impaired. Therefore, the upper limit of the P content is set to 0.100% or less. The upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Since the steel sheet according to the present embodiment does not require P, it may not substantially contain P, and the lower limit of the P content is 0.000%. The lower limit of the P content may be more than 0.000% or 0.001% or more, but the smaller the P content, the more preferable.
Pは不純物であり、鋼板がPを過剰に含有すると靭性や溶接性を損なう。したがって、P含有量の上限を0.100%以下とする。P含有量の上限値は、好ましくは0.050%以下、より好ましくは0.030%以下、さらに好ましくは0.020%以下である。本実施形態に係る鋼板はPを必要としないので、Pを実質的に含有しなくてもよく、P含有量の下限値は0.000%である。P含有量の下限値は0.000%超または0.001%以上でもよいが、P含有量は少ないほど好ましい。 (P: 0.100% or less)
P is an impurity, and if the steel sheet contains excessive P, toughness and weldability are impaired. Therefore, the upper limit of the P content is set to 0.100% or less. The upper limit of the P content is preferably 0.050% or less, more preferably 0.030% or less, and still more preferably 0.020% or less. Since the steel sheet according to the present embodiment does not require P, it may not substantially contain P, and the lower limit of the P content is 0.000%. The lower limit of the P content may be more than 0.000% or 0.001% or more, but the smaller the P content, the more preferable.
(S:0.010%以下)
Sは不純物であり、鋼板がSを過剰に含有すると、熱間圧延によって伸張したMnSが生成し、靭性の劣化を招く。したがって、S含有量の上限を0.010%以下とする。S含有量の上限値は、好ましくは0.007%以下、より好ましくは0.003%以下である。本実施形態に係る鋼板はSを必要としないので、Sを実質的に含有しなくてもよく、S含有量の下限値は0.000%である。S含有量の下限値を0.000%超または0.001%以上としてもよいが、S含有量は少ないほど好ましい。 (S: 0.010% or less)
S is an impurity, and if the steel sheet excessively contains S, MnS elongated by hot rolling is generated, which causes deterioration of toughness. Therefore, the upper limit of the S content is set to 0.010% or less. The upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require S, it does not need to substantially contain S, and the lower limit of the S content is 0.000%. The lower limit of the S content may be more than 0.000% or 0.001% or more, but the lower the S content, the more preferable.
Sは不純物であり、鋼板がSを過剰に含有すると、熱間圧延によって伸張したMnSが生成し、靭性の劣化を招く。したがって、S含有量の上限を0.010%以下とする。S含有量の上限値は、好ましくは0.007%以下、より好ましくは0.003%以下である。本実施形態に係る鋼板はSを必要としないので、Sを実質的に含有しなくてもよく、S含有量の下限値は0.000%である。S含有量の下限値を0.000%超または0.001%以上としてもよいが、S含有量は少ないほど好ましい。 (S: 0.010% or less)
S is an impurity, and if the steel sheet excessively contains S, MnS elongated by hot rolling is generated, which causes deterioration of toughness. Therefore, the upper limit of the S content is set to 0.010% or less. The upper limit of the S content is preferably 0.007% or less, more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require S, it does not need to substantially contain S, and the lower limit of the S content is 0.000%. The lower limit of the S content may be more than 0.000% or 0.001% or more, but the lower the S content, the more preferable.
(N:0.050%未満)
Nは不純物であり、鋼板が0.050%以上のNを含有すると靭性を損なう。したがって、N含有量の上限を0.050%未満とする。N含有量の上限値は、好ましくは0.010%以下、より好ましくは0.006%以下である。本実施形態に係る鋼板はNを必要としないので、Nを実質的に含有しなくてもよく、N含有量の下限値は0.000%である。N含有量の下限値を0.000%超または0.001%以上としてもよいが、N含有量は少ないほど好ましい。 (N: less than 0.050%)
N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is impaired. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to the present embodiment does not require N, it need not substantially contain N, and the lower limit of the N content is 0.000%. The lower limit of the N content may be more than 0.000% or 0.001% or more, but the smaller the N content, the more preferable.
Nは不純物であり、鋼板が0.050%以上のNを含有すると靭性を損なう。したがって、N含有量の上限を0.050%未満とする。N含有量の上限値は、好ましくは0.010%以下、より好ましくは0.006%以下である。本実施形態に係る鋼板はNを必要としないので、Nを実質的に含有しなくてもよく、N含有量の下限値は0.000%である。N含有量の下限値を0.000%超または0.001%以上としてもよいが、N含有量は少ないほど好ましい。 (N: less than 0.050%)
N is an impurity, and if the steel sheet contains 0.050% or more of N, the toughness is impaired. Therefore, the upper limit of the N content is set to less than 0.050%. The upper limit of the N content is preferably 0.010% or less, more preferably 0.006% or less. Since the steel sheet according to the present embodiment does not require N, it need not substantially contain N, and the lower limit of the N content is 0.000%. The lower limit of the N content may be more than 0.000% or 0.001% or more, but the smaller the N content, the more preferable.
(O:0.020%未満)
Oは不純物であり、鋼板が0.020%以上のOを含有すると延性の劣化を招く。したがって、O含有量の上限を0.020%未満とする。O含有量の上限値は、好ましくは0.010%以下、より好ましくは0.005%以下、さらに好ましくは0.003%以下である。本実施形態に係る鋼板はOを必要としないので、Oを実質的に含有しなくてもよく、O含有量の下限値は0.000%である。O含有量の下限値を0.000%超または0.001%以上としてもよいが、O含有量は少ないほど好ましい。 (O: less than 0.020%)
O is an impurity, and when the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is set to less than 0.020%. The upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require O, it does not need to substantially contain O, and the lower limit of the O content is 0.000%. The lower limit of the O content may be more than 0.000% or 0.001% or more, but the lower the O content, the more preferable.
Oは不純物であり、鋼板が0.020%以上のOを含有すると延性の劣化を招く。したがって、O含有量の上限を0.020%未満とする。O含有量の上限値は、好ましくは0.010%以下、より好ましくは0.005%以下、さらに好ましくは0.003%以下である。本実施形態に係る鋼板はOを必要としないので、Oを実質的に含有しなくてもよく、O含有量の下限値は0.000%である。O含有量の下限値を0.000%超または0.001%以上としてもよいが、O含有量は少ないほど好ましい。 (O: less than 0.020%)
O is an impurity, and when the steel sheet contains 0.020% or more of O, ductility is deteriorated. Therefore, the upper limit of the O content is set to less than 0.020%. The upper limit of the O content is preferably 0.010% or less, more preferably 0.005% or less, and still more preferably 0.003% or less. Since the steel sheet according to the present embodiment does not require O, it does not need to substantially contain O, and the lower limit of the O content is 0.000%. The lower limit of the O content may be more than 0.000% or 0.001% or more, but the lower the O content, the more preferable.
(B:0.0000%以上0.010%以下)
Bは本実施形態に係る鋼板に必須の元素ではないので含まれなくてもよく、含有量は0.0000%以上である。しかしながら、Bは、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトの界面を強化し、局部延性をさらに向上させる効果が大きいので、本実施形態の鋼板は好ましくはBを含む。B添加による局部延性の向上効果を得るためには、0.0003%超のB含有量が必要となる。一方、Bを過剰に含有すると靭性を損なうので、B含有量の上限を0.010%以下とした。B含有量の下限値は、好ましくは0.0005%以上、より好ましくは0.0008%以上であり、B含有量の上限値は、好ましくは0.0050%以下、より好ましくは0.0030%以下である。 (B: 0.0000% or more and 0.010% or less)
B is not an essential element in the steel sheet according to the present embodiment, and thus may not be included. The content is 0.000% or more. However, B has a large effect of strengthening the interface between retained austenite and fresh martensite with respect to the matrix mainly composed of tempered martensite and further improving local ductility. Therefore, the steel sheet of the present embodiment preferably contains B. . In order to obtain the effect of improving local ductility by adding B, a B content of more than 0.0003% is required. On the other hand, if B is contained excessively, toughness is impaired, so the upper limit of the B content is set to 0.010% or less. The lower limit of the B content is preferably 0.0005% or more, more preferably 0.0008% or more, and the upper limit of the B content is preferably 0.0050% or less, more preferably 0.0030%. It is as follows.
Bは本実施形態に係る鋼板に必須の元素ではないので含まれなくてもよく、含有量は0.0000%以上である。しかしながら、Bは、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトの界面を強化し、局部延性をさらに向上させる効果が大きいので、本実施形態の鋼板は好ましくはBを含む。B添加による局部延性の向上効果を得るためには、0.0003%超のB含有量が必要となる。一方、Bを過剰に含有すると靭性を損なうので、B含有量の上限を0.010%以下とした。B含有量の下限値は、好ましくは0.0005%以上、より好ましくは0.0008%以上であり、B含有量の上限値は、好ましくは0.0050%以下、より好ましくは0.0030%以下である。 (B: 0.0000% or more and 0.010% or less)
B is not an essential element in the steel sheet according to the present embodiment, and thus may not be included. The content is 0.000% or more. However, B has a large effect of strengthening the interface between retained austenite and fresh martensite with respect to the matrix mainly composed of tempered martensite and further improving local ductility. Therefore, the steel sheet of the present embodiment preferably contains B. . In order to obtain the effect of improving local ductility by adding B, a B content of more than 0.0003% is required. On the other hand, if B is contained excessively, toughness is impaired, so the upper limit of the B content is set to 0.010% or less. The lower limit of the B content is preferably 0.0005% or more, more preferably 0.0008% or more, and the upper limit of the B content is preferably 0.0050% or less, more preferably 0.0030%. It is as follows.
本実施形態の鋼板は、更に、Cr、Mo、W、Cu、Ni、Ti、Nb、V、Ca、Mg、Zr、REM、Sb、Sn及びBiからなる群から選択される1種又は2種以上を含有してもよい。しかしながら、本実施形態に係る鋼板はCr、Mo、W、Cu、Ni、Ti、Nb、V、Ca、Mg、Zr、REM、Sb、Sn及びBiを必要としないので、Cr、Mo、W、Cu、Ni、Ti、Nb、V、Ca、Mg、Zr、REM、Sb、Sn及びBiを含有しなくてもよい、すなわち含有量の下限値は0%であってもよい。本明細書にいうREMとは、Sc、Y、およびランタノイドの合計17元素を指し、REM含有量とは、REMが1種の場合はその含有量、2種以上の場合はそれらの合計含有量を指す。また、REMは一般的には複数種のREMの合金であるミッシュメタルとしても供給されている。このため、個別の元素を1種または2種以上添加してREM含有量が上記の範囲となるように含有させてもよいし、例えば、ミッシュメタルの形で添加して、REM含有量が上記の範囲となるように含有させてもよい。
The steel sheet of the present embodiment further includes one or two selected from the group consisting of Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi. The above may be contained. However, since the steel sheet according to the present embodiment does not require Cr, Mo, W, Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn, and Bi, Cr, Mo, W, It may not contain Cu, Ni, Ti, Nb, V, Ca, Mg, Zr, REM, Sb, Sn and Bi, that is, the lower limit of the content may be 0%. REM as used herein refers to a total of 17 elements of Sc, Y, and lanthanoid, and the REM content refers to the content of REM when one kind is used and the total content of REM when two or more kinds are used. Point to. REM is also generally supplied as misch metal, which is an alloy of a plurality of types of REM. For this reason, one or more individual elements may be added so that the REM content falls within the above range, or, for example, the REM content may be added in the form of a misch metal so that the REM content falls within the above range. May be contained.
(Cr:0.00%以上2.00%未満)
(Mo:0.00%以上2.00%以下)
(W:0.00%以上2.00%以下)
(Cu:0.00%以上2.00%以下)
(Ni:0.00%以上2.00%以下)
Cr、Mo、W、Cu、及びNiはそれぞれ、本実施形態に係る鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.00%以上である。しかしながら、Cr、Mo、W、Cu、及びNiは、鋼板の強度を向上させる元素であるので、含有されてもよい。鋼板の強度向上効果を得るために、鋼板は、Cr、Mo、W、Cu、及びNiからなる群から選択された1種又は2種以上の元素それぞれを0.01%以上含有してもよい。しかしながら、鋼板がこれらの元素を過剰に含有すると、熱延時の表面傷が生成しやすくなり、さらには、熱延鋼板の強度が高くなりすぎて、冷間圧延性が低下する場合がある。したがって、Cr、Mo、W、Cu、及びNiからなる群から選択された1種又は2種以上の元素それぞれの含有量の上限値を2.00%以下とする。 (Cr: 0.00% or more and less than 2.00%)
(Mo: 0.00% or more and 2.00% or less)
(W: 0.00% or more and 2.00% or less)
(Cu: 0.00% or more and 2.00% or less)
(Ni: 0.00% or more and 2.00% or less)
Since Cr, Mo, W, Cu, and Ni are not essential elements in the steel sheet according to the present embodiment, they may not be included, and their contents are 0.00% or more. However, Cr, Mo, W, Cu, and Ni may be included because they are elements that improve the strength of the steel sheet. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. . However, if the steel sheet contains these elements in excess, surface flaws are likely to be generated during hot rolling, and the strength of the hot rolled steel sheet may be too high, and the cold rollability may be reduced. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to 2.00% or less.
(Mo:0.00%以上2.00%以下)
(W:0.00%以上2.00%以下)
(Cu:0.00%以上2.00%以下)
(Ni:0.00%以上2.00%以下)
Cr、Mo、W、Cu、及びNiはそれぞれ、本実施形態に係る鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.00%以上である。しかしながら、Cr、Mo、W、Cu、及びNiは、鋼板の強度を向上させる元素であるので、含有されてもよい。鋼板の強度向上効果を得るために、鋼板は、Cr、Mo、W、Cu、及びNiからなる群から選択された1種又は2種以上の元素それぞれを0.01%以上含有してもよい。しかしながら、鋼板がこれらの元素を過剰に含有すると、熱延時の表面傷が生成しやすくなり、さらには、熱延鋼板の強度が高くなりすぎて、冷間圧延性が低下する場合がある。したがって、Cr、Mo、W、Cu、及びNiからなる群から選択された1種又は2種以上の元素それぞれの含有量の上限値を2.00%以下とする。 (Cr: 0.00% or more and less than 2.00%)
(Mo: 0.00% or more and 2.00% or less)
(W: 0.00% or more and 2.00% or less)
(Cu: 0.00% or more and 2.00% or less)
(Ni: 0.00% or more and 2.00% or less)
Since Cr, Mo, W, Cu, and Ni are not essential elements in the steel sheet according to the present embodiment, they may not be included, and their contents are 0.00% or more. However, Cr, Mo, W, Cu, and Ni may be included because they are elements that improve the strength of the steel sheet. In order to obtain the effect of improving the strength of the steel sheet, the steel sheet may contain 0.01% or more of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni. . However, if the steel sheet contains these elements in excess, surface flaws are likely to be generated during hot rolling, and the strength of the hot rolled steel sheet may be too high, and the cold rollability may be reduced. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Cr, Mo, W, Cu, and Ni is set to 2.00% or less.
(Ti:0.000%以上0.300%以下)
(Nb:0.000%以上0.300%以下)
(V:0.000%以上0.300%以下)
Ti、Nb、及びVは、本実施形態に係る鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.000%以上である。しかし、Ti、Nb、及びVは、微細な炭化物、窒化物または炭窒化物を生成する元素であるので、鋼板の強度向上に有効である。したがって、鋼板は、Ti、Nb、及びVからなる群から選択される1種または2種以上の元素を含有してもよい。鋼板の強度向上効果を得るためには、Ti、Nb、及びVからなる群から選択される1種または2種以上の元素それぞれの含有量の下限値を0.005%以上とすることが好ましい。一方で、これらの元素を過剰に含有させると、熱延鋼板の強度が上昇しすぎて、冷間圧延性が低下する場合がある。したがって、Ti、Nb、及びVからなる群から選択される1種または2種以上の元素それぞれの含有量の上限値を0.300%以下とする。 (Ti: 0.000% or more and 0.300% or less)
(Nb: 0.000% to 0.300%)
(V: 0.000% to 0.300%)
Ti, Nb, and V are not essential elements in the steel sheet according to the present embodiment, and thus may not be included, and each content is 0.000% or more. However, since Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of a steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V. In order to obtain the effect of improving the strength of the steel sheet, the lower limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is preferably set to 0.005% or more. . On the other hand, when these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased, and the cold-rollability may be reduced. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less.
(Nb:0.000%以上0.300%以下)
(V:0.000%以上0.300%以下)
Ti、Nb、及びVは、本実施形態に係る鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.000%以上である。しかし、Ti、Nb、及びVは、微細な炭化物、窒化物または炭窒化物を生成する元素であるので、鋼板の強度向上に有効である。したがって、鋼板は、Ti、Nb、及びVからなる群から選択される1種または2種以上の元素を含有してもよい。鋼板の強度向上効果を得るためには、Ti、Nb、及びVからなる群から選択される1種または2種以上の元素それぞれの含有量の下限値を0.005%以上とすることが好ましい。一方で、これらの元素を過剰に含有させると、熱延鋼板の強度が上昇しすぎて、冷間圧延性が低下する場合がある。したがって、Ti、Nb、及びVからなる群から選択される1種または2種以上の元素それぞれの含有量の上限値を0.300%以下とする。 (Ti: 0.000% or more and 0.300% or less)
(Nb: 0.000% to 0.300%)
(V: 0.000% to 0.300%)
Ti, Nb, and V are not essential elements in the steel sheet according to the present embodiment, and thus may not be included, and each content is 0.000% or more. However, since Ti, Nb, and V are elements that generate fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of a steel sheet. Therefore, the steel sheet may contain one or more elements selected from the group consisting of Ti, Nb, and V. In order to obtain the effect of improving the strength of the steel sheet, the lower limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is preferably set to 0.005% or more. . On the other hand, when these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased, and the cold-rollability may be reduced. Therefore, the upper limit of the content of each of one or more elements selected from the group consisting of Ti, Nb, and V is set to 0.300% or less.
(Ca:0.000%以上0.010%以下)
(Mg:0.000%以上0.010%以下)
(Zr:0.000%以上0.010%以下)
(REM:0.000%以上0.010%以下)
Ca、Mg、Zr、及びREM(希土類金属)は、本開示の鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.000%以上である。しかしながら、Ca、Mg、Zr、及びREMは、鋼板の靭性を向上させる。この効果を得るためには、Ca、Mg、Zr、及びREMからなる群から選択される1種または2種以上の元素それぞれの含有量の下限値を好ましくは0.0001%以上、より好ましくは0.001%以上とする。しかし、過剰量のこれら元素は、鋼板の加工性を劣化させるので、これら元素それぞれの含有量の上限を0.010%以下とし、Ca、Mg、Zr、及びREMからなる群から選択される1種または2種以上の元素の含有量の合計を0.030%以下とすることが好ましい。 (Ca: 0.000% or more and 0.010% or less)
(Mg: 0.000% or more and 0.010% or less)
(Zr: 0.000% or more and 0.010% or less)
(REM: 0.000% or more and 0.010% or less)
Since Ca, Mg, Zr, and REM (rare earth metal) are not essential elements in the steel sheet of the present disclosure, they may not be included, and their contents are 0.000% or more. However, Ca, Mg, Zr, and REM improve the toughness of the steel sheet. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0.001% or more. However, since an excessive amount of these elements deteriorates the workability of the steel sheet, the upper limit of the content of each of these elements is set to 0.010% or less, and 1 selected from the group consisting of Ca, Mg, Zr, and REM is used. It is preferred that the total content of the species or two or more elements be 0.030% or less.
(Mg:0.000%以上0.010%以下)
(Zr:0.000%以上0.010%以下)
(REM:0.000%以上0.010%以下)
Ca、Mg、Zr、及びREM(希土類金属)は、本開示の鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.000%以上である。しかしながら、Ca、Mg、Zr、及びREMは、鋼板の靭性を向上させる。この効果を得るためには、Ca、Mg、Zr、及びREMからなる群から選択される1種または2種以上の元素それぞれの含有量の下限値を好ましくは0.0001%以上、より好ましくは0.001%以上とする。しかし、過剰量のこれら元素は、鋼板の加工性を劣化させるので、これら元素それぞれの含有量の上限を0.010%以下とし、Ca、Mg、Zr、及びREMからなる群から選択される1種または2種以上の元素の含有量の合計を0.030%以下とすることが好ましい。 (Ca: 0.000% or more and 0.010% or less)
(Mg: 0.000% or more and 0.010% or less)
(Zr: 0.000% or more and 0.010% or less)
(REM: 0.000% or more and 0.010% or less)
Since Ca, Mg, Zr, and REM (rare earth metal) are not essential elements in the steel sheet of the present disclosure, they may not be included, and their contents are 0.000% or more. However, Ca, Mg, Zr, and REM improve the toughness of the steel sheet. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Ca, Mg, Zr, and REM is preferably 0.0001% or more, more preferably 0.001% or more. However, since an excessive amount of these elements deteriorates the workability of the steel sheet, the upper limit of the content of each of these elements is set to 0.010% or less, and 1 selected from the group consisting of Ca, Mg, Zr, and REM is used. It is preferred that the total content of the species or two or more elements be 0.030% or less.
(Sb:0.000%以上0.050%以下)
(Sn:0.000%以上0.050%以下)
(Bi:0.000%以上0.050%以下)
Sb、Sn、及びBiは、本開示の鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.000%以上である。しかしながら、Sb、Sn、及びBiは、鋼板中のMn、Si、及び/又はAl等の易酸化性元素が鋼板表面に拡散され酸化物を形成することを抑え、鋼板の表面性状やめっき性を高める。この効果を得るために、Sb、Sn、及びBiからなる群から選択される1種又は2種以上の元素それぞれの含有量の下限値を好ましくは0.0005%以上、より好ましくは0.001%以上とする。一方、これら元素それぞれの含有量が0.050%を超えると、その効果が飽和するので、これら元素それぞれの含有量の上限値を0.050%以下とした。 (Sb: 0.000% to 0.050%)
(Sn: 0.000% to 0.050%)
(Bi: 0.000% to 0.050%)
Sb, Sn, and Bi are not essential elements in the steel sheet of the present disclosure, and thus may not be included, and each content is 0.000% or more. However, Sb, Sn, and Bi suppress oxidizable elements such as Mn, Si, and / or Al in the steel sheet from diffusing to the steel sheet surface to form oxides, and reduce the surface properties and plating properties of the steel sheet. Enhance. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001% or more. % Or more. On the other hand, when the content of each of these elements exceeds 0.050%, the effect is saturated. Therefore, the upper limit of the content of each of these elements is set to 0.050% or less.
(Sn:0.000%以上0.050%以下)
(Bi:0.000%以上0.050%以下)
Sb、Sn、及びBiは、本開示の鋼板に必須の元素ではないので含まれなくてもよく、それぞれの含有量は0.000%以上である。しかしながら、Sb、Sn、及びBiは、鋼板中のMn、Si、及び/又はAl等の易酸化性元素が鋼板表面に拡散され酸化物を形成することを抑え、鋼板の表面性状やめっき性を高める。この効果を得るために、Sb、Sn、及びBiからなる群から選択される1種又は2種以上の元素それぞれの含有量の下限値を好ましくは0.0005%以上、より好ましくは0.001%以上とする。一方、これら元素それぞれの含有量が0.050%を超えると、その効果が飽和するので、これら元素それぞれの含有量の上限値を0.050%以下とした。 (Sb: 0.000% to 0.050%)
(Sn: 0.000% to 0.050%)
(Bi: 0.000% to 0.050%)
Sb, Sn, and Bi are not essential elements in the steel sheet of the present disclosure, and thus may not be included, and each content is 0.000% or more. However, Sb, Sn, and Bi suppress oxidizable elements such as Mn, Si, and / or Al in the steel sheet from diffusing to the steel sheet surface to form oxides, and reduce the surface properties and plating properties of the steel sheet. Enhance. In order to obtain this effect, the lower limit of the content of each of one or more elements selected from the group consisting of Sb, Sn, and Bi is preferably 0.0005% or more, more preferably 0.001% or more. % Or more. On the other hand, when the content of each of these elements exceeds 0.050%, the effect is saturated. Therefore, the upper limit of the content of each of these elements is set to 0.050% or less.
本実施形態に係る鋼板の化学組成において、残部はFeおよび不純物である。
に お い て In the chemical composition of the steel sheet according to the present embodiment, the balance is Fe and impurities.
「不純物」とは、鋼材を工業的に製造する際に、鉱石、スクラップ等の原料、製造工程の種々の要因によって混入する成分であって、本発明に悪影響を与えない範囲で許容されるものを意味する。
“Impurities” are components that are mixed in due to various factors in the ore, scrap, and other raw materials and manufacturing processes when steel products are manufactured industrially, and are acceptable within a range that does not adversely affect the present invention. Means
2.金属組織
次に、本実施形態に係る鋼板の金属組織について説明する。以下の説明において、各相の分率を表す「%」は特に断りが無い限り面積率(%)を意味する。 2. Next, the metal structure of the steel sheet according to the present embodiment will be described. In the following description, “%” representing a fraction of each phase means an area ratio (%) unless otherwise specified.
次に、本実施形態に係る鋼板の金属組織について説明する。以下の説明において、各相の分率を表す「%」は特に断りが無い限り面積率(%)を意味する。 2. Next, the metal structure of the steel sheet according to the present embodiment will be described. In the following description, “%” representing a fraction of each phase means an area ratio (%) unless otherwise specified.
本実施形態に係る鋼板のL断面において表面から厚みの1/4位置(1/4t部ともいう)における金属組織は、母相として25%以上90%以下の焼き戻しマルテンサイト及び3%以下のフェライト、並びにその他の相として10%以上50%以下の残留オーステナイト及び25%以下のフレッシュマルテンサイトを含む。さらに、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度が2.7μm-1以上である。ここで、各相の分率は、焼鈍の条件によって変化し、強度、局部延性などの材質に影響を与える。L断面とは、板厚方向と圧延方向に平行に鋼板の圧延方向の中心軸を通るように切断した面をいう。
In the L section of the steel sheet according to the present embodiment, the metal structure at a quarter position (also referred to as a 1/4 t portion) of the thickness from the surface is 25% or more and 90% or less of tempered martensite and 3% or less as a matrix. It contains 10% to 50% of retained austenite and 25% or less of fresh martensite as ferrite and other phases. Further, the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is 2.7 μm −1 or more. Here, the fraction of each phase changes depending on the annealing conditions, and affects the material such as strength and local ductility. The L section refers to a plane cut in such a manner as to pass through the central axis in the rolling direction of the steel sheet in parallel with the sheet thickness direction and the rolling direction.
焼き戻しマルテンサイト及びフェライトの面積率は、走査型電子顕微鏡(SEM)による組織観察から算出される。鋼板のL断面を鏡面研磨した後に、3%ナイタール(3%硝酸―エタノール溶液)で腐食し、倍率5000倍の走査型電子顕微鏡で、表面から1/4位置における金属組織を観察して、焼き戻しマルテンサイト及びフェライトのそれぞれの面積率を測定する。
面積 The area ratio of tempered martensite and ferrite is calculated from the observation of the structure with a scanning electron microscope (SEM). After the L section of the steel plate is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the metallographic structure at a quarter position from the surface is observed with a scanning electron microscope at a magnification of 5000 times. The area ratio of each of the returned martensite and the ferrite is measured.
残留オーステナイトの面積率はX線回折法により測定される。走査型電子顕微鏡の観察において残留オーステナイトとフレッシュマルテンサイト(即ち、焼き戻しされていないマルテンサイト)との区別が難しいので、焼き戻しマルテンサイト及びフレッシュマルテンサイトの面積率は次の方法で測定する。鋼板のL断面を鏡面研磨した後に、3%ナイタール(3%硝酸―エタノール溶液)で腐食し、倍率5000倍の走査型電子顕微鏡で、鋼板の表面から厚みの1/4位置のミクロ組織を観察して、残留オーステナイト及びフレッシュマルテンサイトの合計の面積率を測定する。次いで、残留オーステナイト及びフレッシュマルテンサイトの合計の面積率からX線回折法で測定された残留オーステナイトの面積率を差し引いて、フレッシュマルテンサイトの面積率を算出する。
面積 The area ratio of retained austenite is measured by the X-ray diffraction method. Since it is difficult to distinguish between retained austenite and fresh martensite (that is, untempered martensite) in the observation with a scanning electron microscope, the area ratio of tempered martensite and fresh martensite is measured by the following method. After the L section of the steel sheet is mirror-polished, it is corroded with 3% nital (3% nitric acid-ethanol solution), and the microstructure at a quarter position of the thickness from the surface of the steel sheet is observed with a scanning electron microscope with a magnification of 5000 times. Then, the total area ratio of retained austenite and fresh martensite is measured. Next, the area ratio of fresh martensite is calculated by subtracting the area ratio of retained austenite measured by X-ray diffraction from the total area ratio of retained austenite and fresh martensite.
(鋼板の1/4t部の金属組織中の焼き戻しマルテンサイトの面積率:25%以上90%以下)
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、25%以上90%以下の焼き戻しマルテンサイトを含む。焼き戻しマルテンサイトは、局部延性を高める本実施形態に係る鋼板の母相である。したがって、局部延性を向上させるためには、25%以上の焼き戻しマルテンサイトが必要である。一方、焼き戻しマルテンサイトを過剰に含有すると、残留オーステナイトとフレッシュマルテンサイトが過少となるため、引張強度が低くなるだけでなく、延性が劣化するので、焼き戻しマルテンサイトの面積率の上限を90%以下とした。焼き戻しマルテンサイトの面積率の下限値は好ましくは40%以上である。焼き戻しマルテンサイトの面積率の上限値は、好ましくは80%以下である。 (Area ratio of tempered martensite in metal structure of 1 / 4t part of steel sheet: 25% or more and 90% or less)
In the L section of the steel sheet according to the present embodiment, the metal structure at a 1 / 4t part of the thickness from the surface includes tempered martensite having an area ratio of 25% or more and 90% or less. Tempered martensite is a parent phase of the steel sheet according to the present embodiment that improves local ductility. Therefore, in order to improve local ductility, tempered martensite of 25% or more is required. On the other hand, if the tempered martensite is excessively contained, the residual austenite and the fresh martensite are too small, so that not only the tensile strength is lowered but also the ductility is deteriorated. % Or less. The lower limit of the area ratio of tempered martensite is preferably 40% or more. The upper limit of the area ratio of tempered martensite is preferably 80% or less.
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、25%以上90%以下の焼き戻しマルテンサイトを含む。焼き戻しマルテンサイトは、局部延性を高める本実施形態に係る鋼板の母相である。したがって、局部延性を向上させるためには、25%以上の焼き戻しマルテンサイトが必要である。一方、焼き戻しマルテンサイトを過剰に含有すると、残留オーステナイトとフレッシュマルテンサイトが過少となるため、引張強度が低くなるだけでなく、延性が劣化するので、焼き戻しマルテンサイトの面積率の上限を90%以下とした。焼き戻しマルテンサイトの面積率の下限値は好ましくは40%以上である。焼き戻しマルテンサイトの面積率の上限値は、好ましくは80%以下である。 (Area ratio of tempered martensite in metal structure of 1 / 4t part of steel sheet: 25% or more and 90% or less)
In the L section of the steel sheet according to the present embodiment, the metal structure at a 1 / 4t part of the thickness from the surface includes tempered martensite having an area ratio of 25% or more and 90% or less. Tempered martensite is a parent phase of the steel sheet according to the present embodiment that improves local ductility. Therefore, in order to improve local ductility, tempered martensite of 25% or more is required. On the other hand, if the tempered martensite is excessively contained, the residual austenite and the fresh martensite are too small, so that not only the tensile strength is lowered but also the ductility is deteriorated. % Or less. The lower limit of the area ratio of tempered martensite is preferably 40% or more. The upper limit of the area ratio of tempered martensite is preferably 80% or less.
(鋼板の1/4t部の金属組織中の残留オーステナイトの面積率:10%以上50%以下)
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、10%以上50%以下の残留オーステナイトを含む。 (Area ratio of retained austenite in metal structure of 1 / 4t part of steel sheet: 10% or more and 50% or less)
In the L section of the steel sheet according to the present embodiment, the metal structure at a 1 / 4t part of the thickness from the surface includes retained austenite in an area ratio of 10% or more and 50% or less.
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、10%以上50%以下の残留オーステナイトを含む。 (Area ratio of retained austenite in metal structure of 1 / 4t part of steel sheet: 10% or more and 50% or less)
In the L section of the steel sheet according to the present embodiment, the metal structure at a 1 / 4t part of the thickness from the surface includes retained austenite in an area ratio of 10% or more and 50% or less.
残留オーステナイトは、変態誘起塑性によって、鋼板の引張強度と延性を高める相である。残留オーステナイトは、引張変形を伴う張出し加工、絞り加工、伸びフランジ加工、または曲げ加工によってマルテンサイトに変態し得るので、鋼板を加工することによって得られる鋼材の強度の向上にも寄与する。これら効果を得るために、残留オーステナイトの面積率を10%以上にすることが必要である。残留オーステナイトの面積率の下限値は好ましくは15%以上、より好ましくは20%以上、さらに好ましくは30%以上である。
Retained austenite is a phase that enhances the tensile strength and ductility of a steel sheet by transformation induced plasticity. Retained austenite can be transformed into martensite by overhanging, drawing, stretch-flanging, or bending with tensile deformation, and thus contributes to an improvement in the strength of a steel material obtained by processing a steel sheet. To obtain these effects, the area ratio of retained austenite needs to be 10% or more. The lower limit of the area ratio of retained austenite is preferably 15% or more, more preferably 20% or more, and further preferably 30% or more.
残留オーステナイトの面積率は高いほど好ましい。しかしながら、上述した化学成分を有する合金を以下で述べる方法で製造した場合、50%が残留オーステナイトの面積率の上限となる。9.0%超のMnを含有させれば、残留オーステナイトを50%超にすることができるが、この場合、熱間加工性や鋳造性が損なわれる。残留オーステナイトの面積率は、水素脆性の観点を考慮すると、45%以下が好ましく、40%以下がより好ましい。
高 い The higher the area ratio of retained austenite, the better. However, when an alloy having the above-described chemical components is manufactured by the method described below, 50% is the upper limit of the area ratio of retained austenite. If more than 9.0% of Mn is contained, retained austenite can be made more than 50%, but in this case, hot workability and castability are impaired. The area ratio of retained austenite is preferably 45% or less, and more preferably 40% or less, in view of hydrogen embrittlement.
(鋼板の1/4t部の金属組織中のフレッシュマルテンサイトの面積率:25%以下)
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、25%以下のフレッシュマルテンサイトを含む。 (Area ratio of fresh martensite in 1 / 4t metal structure of steel sheet: 25% or less)
In the L section of the steel sheet according to the present embodiment, the metal structure at a 1 / 4t portion of the thickness from the surface includes fresh martensite having an area ratio of 25% or less.
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、25%以下のフレッシュマルテンサイトを含む。 (Area ratio of fresh martensite in 1 / 4t metal structure of steel sheet: 25% or less)
In the L section of the steel sheet according to the present embodiment, the metal structure at a 1 / 4t portion of the thickness from the surface includes fresh martensite having an area ratio of 25% or less.
本実施形態に係る鋼板においては、金属組織中のフレッシュマルテンサイトは、焼き戻しされていないマルテンサイトであり、本実施形態に係る鋼板を強化し、鋼板の引張強度を高める相である。しかし、フレッシュマルテンサイト自体は硬質の相であるので、局部延性を劣化させる作用を有する相でもある。局部延性を低下させないために、金属組織中のフレッシュマルテンサイトの面積率を25%以下、好ましくは15%以下とし、さらに好ましくは10%以下とする。
鋼板 In the steel sheet according to the present embodiment, the fresh martensite in the metal structure is martensite that has not been tempered, and is a phase that strengthens the steel sheet according to the present embodiment and increases the tensile strength of the steel sheet. However, since fresh martensite itself is a hard phase, it is also a phase having an effect of deteriorating local ductility. In order not to lower the local ductility, the area ratio of fresh martensite in the metal structure is set to 25% or less, preferably 15% or less, and more preferably 10% or less.
(鋼板の1/4t部の金属組織中のフェライトの面積率:3%以下)
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、3%以下のフェライトを含む。 (Area ratio of ferrite in metal structure of 1 / 4t part of steel sheet: 3% or less)
In the L section of the steel sheet according to the present embodiment, the metallographic structure at a 1 / 4t part of the thickness from the surface includes ferrite of 3% or less in area ratio.
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織は、面積率で、3%以下のフェライトを含む。 (Area ratio of ferrite in metal structure of 1 / 4t part of steel sheet: 3% or less)
In the L section of the steel sheet according to the present embodiment, the metallographic structure at a 1 / 4t part of the thickness from the surface includes ferrite of 3% or less in area ratio.
実施形態に係る鋼板においては、金属組織中のフェライトの含有量が少ないことが重要である。金属組織中のフェライト含有量が多くなると、延性が低下するためである。延性を低下させないために、金属組織中のフェライトの面積率を3%以下とし、より好ましくは1%以下とし、さらに好ましくは実質的に0%とする。
鋼板 In the steel sheet according to the embodiment, it is important that the content of ferrite in the metal structure is small. This is because when the ferrite content in the metal structure increases, the ductility decreases. In order not to lower the ductility, the area ratio of ferrite in the metal structure is set to 3% or less, more preferably 1% or less, and further preferably substantially 0%.
(鋼板の1/4t部の金属組織中における焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトの界面密度が2.7μm-1以上)
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織において、残留オーステナイトまたはフレッシュマルテンサイトのいずれかである領域である第1の領域と、1/4位置における金属組織から第1の領域を除いた領域である第2の領域と、の境界の長さの総和を、第1の領域と第2の領域との合計面積で除した値である界面密度が2.7μm-1以上である。より好ましくは、界面密度は3.15μm-1以上である。 (Interface density of retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite in a metal structure of a tt portion of a steel sheet is 2.7 μm −1 or more)
In the metallographic structure at the 1 / 4t part of the thickness from the surface in the L cross section of the steel sheet according to the present embodiment, the first region, which is either retained austenite or fresh martensite, and the metallographic structure at the 1/4 position The interface density, which is a value obtained by dividing the sum of the lengths of the boundaries between the first region and the second region by the total area of the first region and the second region, is 2. 7 μm −1 or more. More preferably, the interface density is 3.15 μm −1 or more.
本実施形態に係る鋼板のL断面において表面から厚みの1/4t部における金属組織において、残留オーステナイトまたはフレッシュマルテンサイトのいずれかである領域である第1の領域と、1/4位置における金属組織から第1の領域を除いた領域である第2の領域と、の境界の長さの総和を、第1の領域と第2の領域との合計面積で除した値である界面密度が2.7μm-1以上である。より好ましくは、界面密度は3.15μm-1以上である。 (Interface density of retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite in a metal structure of a tt portion of a steel sheet is 2.7 μm −1 or more)
In the metallographic structure at the 1 / 4t part of the thickness from the surface in the L cross section of the steel sheet according to the present embodiment, the first region, which is either retained austenite or fresh martensite, and the metallographic structure at the 1/4 position The interface density, which is a value obtained by dividing the sum of the lengths of the boundaries between the first region and the second region by the total area of the first region and the second region, is 2. 7 μm −1 or more. More preferably, the interface density is 3.15 μm −1 or more.
本実施形態に係る鋼板においては、残留オーステナイトとフレッシュマルテンサイトとが、焼き戻しマルテンサイトを主体とする母相との間に界面を生成する。残留オーステナイトまたはフレッシュマルテンサイトのいずれかの領域である第1の領域と、第1の領域以外の焼き戻しマルテンサイトを主体とする母相である第2の領域と、の境界の長さの総和を、第1の領域と第2の領域との合計面積で除した値である界面密度は2.7μm-1以上である。このように、本実施形態に係る鋼板は、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面の密度、(すなわち、界面密度)が2.7μm-1以上である組織を有する。焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度の測定は、上記の走査型電子顕微鏡によるミクロ組織画像に基づいて行うことができる。
In the steel sheet according to the present embodiment, the retained austenite and the fresh martensite form an interface with a matrix mainly composed of tempered martensite. Sum of the lengths of the boundaries between the first region, which is either retained austenite or fresh martensite, and the second region, other than the first region, which is a matrix mainly composed of tempered martensite. Is divided by the total area of the first region and the second region, and the interface density is 2.7 μm −1 or more. As described above, the steel sheet according to the present embodiment has an interface density between retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite, that is, an interface density of 2.7 μm −1 or more. Have an organization. The measurement of the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite can be carried out based on the microstructure image by the above-mentioned scanning electron microscope.
焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度は局部延性に大きく影響し、その密度が2.7μm-1以上の場合、残留オーステナイトとフレッシュマルテンサイトとは微細かつ互いに独立した構造を呈する為、均質な界面分布となり、良好な局部延性が得られる。
The interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite greatly affects the local ductility. When the density is 2.7 μm −1 or more, the retained austenite and the fresh martensite are fine. In addition, since they have structures independent from each other, the interface distribution becomes uniform, and good local ductility is obtained.
焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度が2.7μm-1未満の場合、残留オーステナイトとフレッシュマルテンサイトとが粗大且つ互いに連結した構造を呈する為、均質な界面分布とならず、き裂進展を抑制する効果を損なう。
When the interface density between the retained austenite and the fresh martensite with respect to the parent phase mainly composed of tempered martensite is less than 2.7 μm −1 , the retained austenite and the fresh martensite have a coarse and interconnected structure, and therefore have a homogeneous structure. And the effect of suppressing crack propagation is impaired.
走査型電子顕微鏡によるミクロ組織画像から、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度の導出は、以下のようにする。先ず、ミクロ組織画像において、残留オーステナイトまたはフレッシュマルテンサイトのいずれかの領域を第1の領域とし、前記ミクロ組織画像の全領域から第1の領域を除いた領域を第2の領域とする。第1の領域と第2の領域との区別は、残留オーステナイトとフレッシュマルテンサイトとが、走査型電子顕微鏡によるミクロ組織画像では他の相に対して相対的に輝度の高い領域となることを利用して行う。そして、第1の領域と第2の領域との境界の長さの総和を導出し、第1の領域と第2の領域との合計面積で除した値を、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度とする。上記のミクロ組織画像は、鋼板のL断面において表面から厚みの1/4t部における金属組織において、例えば、走査型電子顕微鏡(SEM)写真により5000倍の倍率で観察した24μm×18μm程度の領域であることができる。
界面 From the microstructure image by the scanning electron microscope, the interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite is derived as follows. First, in the microstructure image, either the region of retained austenite or fresh martensite is defined as a first region, and a region obtained by removing the first region from the entire region of the microstructure image is defined as a second region. The distinction between the first region and the second region is based on the fact that retained austenite and fresh martensite are regions that are relatively brighter with respect to other phases in a microstructure image by a scanning electron microscope. Do it. Then, the sum of the lengths of the boundaries between the first region and the second region is derived, and the value obtained by dividing by the total area of the first region and the second region is mainly made of tempered martensite. The interface density between the retained austenite and the fresh martensite with respect to the parent phase. The above microstructure image shows, for example, an area of about 24 μm × 18 μm in a metal structure at a 1 / 4t portion from the surface in the L cross section of the steel sheet at a magnification of 5000 times with a scanning electron microscope (SEM) photograph. There can be.
より具体的には、以下のようにして界面密度の導出を行う。焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度は、画像解析ソフトウェアImageJを用いて測定される。SEMを用いて対象とする組織を5000倍の倍率で観察してSEM画像(24μm×18μm)を得る。次いで、ImageJを用いて、SEM画像に1280×960個の分割領域を形成する。各分割領域について、残留オーステナイトまたはフレッシュマルテンサイトの何れかである領域を黒、その他の領域を白となるように2値化処理を施して2階調画像を得る。2値化の閾値は、「Glasbey, CA (1993), "An analysis of histogram-based thresholding algorithms", CVGIP: Graphical Models and Image Processing 55: 532-537」に記載されている輝度値の平均値を閾値として採用する手法を用いて決定する。このアルゴリズムはImageJに実装されており、Auto threshold機能を利用して閾値の決定方法をMethod=Meanとすることで自動的に2値化する。すなわち、2値化の閾値は、ImageJにてMethod=Mean、radius=15として、各ピクセル値を、着目したピクセルを中心として半径15ピクセル以内のピクセル値の平均と置き換えて、スムージングした後のヒストグラムから自動的に決定する。焼き戻しマルテンサイトに対する残留オーステナイトとフレッシュマルテンサイトとの界面密度の測定は、得られた2階調画像において、孤立したすべての残留オーステナイトとフレッシュマルテンサイトとの焼き戻しマルテンサイトに対する界面の長さを逐次解析し、その合計値を画像の全領域の面積(24μm×18μm)で除することで行う。残留オーステナイトとフレッシュマルテンサイトとが隣接する場合、上記2値化処理過程で両者は一体組織となるため、残留オーステナイトとフレッシュマルテンサイトとの界面の長さは除かれる。逐次解析とは、残留オーステナイトとフレッシュマルテンサイトのそれぞれについて、界面を重複することなく、界面密度を測定することをいう。
More specifically, the interface density is derived as follows. The interface density between the retained austenite and the fresh martensite with respect to the parent phase mainly composed of tempered martensite is measured using image analysis software ImageJ. A target tissue is observed at a magnification of 5000 times using an SEM to obtain an SEM image (24 μm × 18 μm). Next, 1280 × 960 divided regions are formed on the SEM image using ImageJ. For each of the divided regions, binarization processing is performed so that a region that is either retained austenite or fresh martensite is black, and the other regions are white, to obtain a two-tone image. The threshold for binarization is the average value of the luminance values described in "Glasbey, CA (1993)," An analysis of histogram-based thresholding algorithm ", CVGIP: Graphic Model and image Processing 55: 532-537. It is determined using a method adopted as a threshold. This algorithm is implemented in ImageJ, and is automatically binarized by setting the method of determining a threshold to Method = Mean using the Auto @ threshold function. That is, the binarization threshold is set to Method = Mean and radius = 15 in ImageJ, and each pixel value is replaced with the average of the pixel values within a radius of 15 pixels around the pixel of interest, and the histogram after smoothing is performed. Automatically determined from The measurement of the interface density between the retained austenite and the fresh martensite with respect to the tempered martensite was performed by measuring the length of the interface with respect to the tempered martensite between all the isolated residual austenite and the fresh martensite in the obtained two-tone image. The analysis is performed sequentially, and the total value is divided by the area (24 μm × 18 μm) of the entire area of the image. When the residual austenite and the fresh martensite are adjacent to each other, the two have an integrated structure in the binarization process, and thus the length of the interface between the residual austenite and the fresh martensite is excluded. The sequential analysis refers to measuring the interface density of each of retained austenite and fresh martensite without overlapping the interface.
3.機械特性
次に、本実施形態に係る鋼板の機械特性について説明する。 3. Next, mechanical properties of the steel sheet according to the present embodiment will be described.
次に、本実施形態に係る鋼板の機械特性について説明する。 3. Next, mechanical properties of the steel sheet according to the present embodiment will be described.
本実施形態に係る鋼板の引張強度(TS)は、好ましくは1200MPa以上、より好ましくは1320MPa以上である。これは、鋼板を自動車の素材として使用する際、高強度化によって板厚を減少させ、軽量化に寄与するためである。鋼板の引張強度の上限は特に規定されないが、例えば1600MPaであってもよい。また、本実施形態に係る鋼板をプレス成形に供するためには、延性と局部延性が優れることが望ましい。延性に関して、引張試験の全伸びは、好ましくは15%以上である。全伸びの上限は特に規定されないが、例えば35%以下であってもよい。局部延性に関して、局部伸びは、好ましくは2%以上、より好ましくは3%以上、さらに好ましくは4%以上、さらにより好ましくは5%以上である。局部伸びの上限は特に規定されないが、例えば6%以下であってもよい。
鋼板 The tensile strength (TS) of the steel sheet according to the present embodiment is preferably 1200 MPa or more, more preferably 1320 MPa or more. This is because when a steel sheet is used as a material for an automobile, the sheet thickness is reduced by increasing the strength, thereby contributing to weight reduction. The upper limit of the tensile strength of the steel sheet is not particularly defined, but may be, for example, 1600 MPa. Further, in order to provide the steel sheet according to the present embodiment for press forming, it is desirable that the ductility and the local ductility be excellent. With respect to ductility, the total elongation in the tensile test is preferably at least 15%. The upper limit of the total elongation is not particularly limited, but may be, for example, 35% or less. With respect to local ductility, the local elongation is preferably at least 2%, more preferably at least 3%, further preferably at least 4%, still more preferably at least 5%. The upper limit of the local elongation is not particularly defined, but may be, for example, 6% or less.
本開示の鋼板は上記のように、高強度を有し、さらに局部延性も良好であり成形性に優れているので、クロスメンバーなどの自動車の構造部品用途に最適である。さらに、本開示の鋼板は含有Mn濃度が高いので、自動車の軽量化にも寄与するので、産業上の貢献が極めて顕著である。
鋼板 As described above, the steel sheet of the present disclosure has high strength, good local ductility, and excellent formability, and is therefore most suitable for structural parts of automobiles such as cross members. Furthermore, since the steel sheet of the present disclosure has a high content of Mn, which contributes to the reduction of the weight of automobiles, the industrial contribution is extremely remarkable.
4.製造方法
次に、本実施形態に係る鋼板の製造方法について説明する。 4. Manufacturing Method Next, a method for manufacturing the steel sheet according to the present embodiment will be described.
次に、本実施形態に係る鋼板の製造方法について説明する。 4. Manufacturing Method Next, a method for manufacturing the steel sheet according to the present embodiment will be described.
本実施形態に係る鋼板は、上述の化学組成を有する鋼を常法で溶製し、鋳造してスラブまたは鋼塊を作製し、これを加熱して熱間圧延し、得られた熱延鋼板を酸洗した後、冷間圧延し、焼鈍を施して製造する。
The steel sheet according to the present embodiment is prepared by melting a steel having the above-described chemical composition by a conventional method, casting to produce a slab or a steel ingot, and heating and hot rolling the obtained steel sheet to obtain a hot-rolled steel sheet. Is pickled, cold rolled, and annealed to produce.
熱間圧延は、通常の連続熱間圧延ラインで行えばよい。焼鈍は、後で述べる条件を満たせば、焼鈍炉及び連続焼鈍ラインのどちらで行ってもよいが、好ましくは後述する1回目の焼鈍及び2回目の焼鈍はいずれも、連続焼鈍ラインを用いて行うことができるので、生産性を向上することができる。1回目の焼鈍及び2回目の焼鈍は、好ましくは還元雰囲気で行われ、例えば窒素98%及び水素2%の還元雰囲気で行ってもよい。還元雰囲気で熱処理することにより、鋼板の表面にスケールが付着するのを防ぐことができ、酸洗浄を要せずにめっき工程にそのまま送ることができる。更に、冷延圧延後の鋼板に、スキンパス圧延を行ってもよい。
Hot rolling may be performed in a normal continuous hot rolling line. Annealing may be performed in either an annealing furnace or a continuous annealing line as long as the conditions described later are satisfied, but preferably, the first annealing and the second annealing described below are both performed using the continuous annealing line. Therefore, productivity can be improved. The first annealing and the second annealing are preferably performed in a reducing atmosphere, for example, in a reducing atmosphere of 98% nitrogen and 2% hydrogen. By performing the heat treatment in a reducing atmosphere, the scale can be prevented from adhering to the surface of the steel sheet, and can be directly sent to the plating step without requiring acid cleaning. Further, skin pass rolling may be performed on the steel sheet after cold rolling.
本開示の鋼板の機械特性を得るためには、熱延条件、特に焼鈍条件を、以下に示す範囲内で行うことが好ましい。
を 得 In order to obtain the mechanical properties of the steel sheet of the present disclosure, it is preferable to perform hot rolling conditions, particularly annealing conditions, within the following ranges.
本実施形態に係る鋼板が上述の化学組成を有する限り、溶鋼は、通常の高炉法で溶製されたものであってもよく、電炉法で作成された鋼のように、原材料がスクラップを多量に含むものでもよい。スラブは、通常の連続鋳造プロセスで製造されたものでもよいし、薄スラブ鋳造で製造されたものでもよい。
As long as the steel sheet according to the present embodiment has the above-described chemical composition, the molten steel may be one produced by the ordinary blast furnace method, and the raw material may generate a large amount of scrap, such as steel produced by the electric furnace method. May be included. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.
上述のスラブまたは鋼塊を加熱し、熱間圧延を行う。熱間圧延に供する鋼材の温度は、1100℃以上1300℃以下とすることが好ましい。熱間圧延に供する鋼材の温度を1100℃以上にすることにより、熱間圧延時の変形抵抗をより小さくすることができる。一方、熱間圧延に供する鋼材の温度を1300℃以下にすることにより、スケールロス増加による歩留まりの低下を抑制することができる。本明細書において、温度は、鋼板表面の中央位置で測定される温度である。
加熱 The above-mentioned slab or steel ingot is heated and hot-rolled. It is preferable that the temperature of the steel material to be subjected to hot rolling is 1100 ° C. or more and 1300 ° C. or less. By setting the temperature of the steel material subjected to hot rolling to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the temperature of the steel material to be subjected to hot rolling to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. In this specification, the temperature is a temperature measured at a central position on the surface of the steel sheet.
熱間圧延前に1100℃以上1300℃の温度域に保持する時間は特に規定しないが、曲げ性を向上させるためには、30分間以上とすることが好ましく、1時間以上にすることがさらに好ましい。また、過度のスケールロスを抑制するために、1100℃以上1300℃以下の温度域に保持する時間は、10時間以下とすることが好ましく、5時間以下とすることがさらに好ましい。なお、直送圧延または直接圧延を行う場合には、加熱処理を施さずにそのまま熱間圧延に供してもよい。
The time for maintaining the temperature in the temperature range of 1100 ° C. or more and 1300 ° C. before hot rolling is not particularly limited, but is preferably 30 minutes or more, and more preferably 1 hour or more, in order to improve the bendability. . Further, in order to suppress excessive scale loss, the time for maintaining the temperature in a temperature range of 1100 ° C. to 1300 ° C. is preferably 10 hours or less, more preferably 5 hours or less. When direct rolling or direct rolling is performed, hot rolling may be performed as it is without performing heat treatment.
仕上圧延開始温度は700℃以上1000℃以下とすることが好ましい。仕上圧延開始温度を700℃以上とすることにより、圧延時の変形抵抗を小さくすることが可能になる。一方、仕上圧延開始温度を1000℃以下にすることにより、粒界酸化による鋼板の表面性状の劣化を抑制することができる。
The finish rolling start temperature is preferably set to 700 ° C. or more and 1000 ° C. or less. By setting the finish rolling start temperature to 700 ° C. or higher, it becomes possible to reduce deformation resistance during rolling. On the other hand, by setting the finish rolling start temperature to 1000 ° C. or lower, deterioration of the surface properties of the steel sheet due to grain boundary oxidation can be suppressed.
仕上圧延を行って得られる熱延鋼板を冷却し、巻取り、コイルにすることができる。冷却後の巻取温度を700℃以下とすることが好ましい。巻取温度を700℃以下にすることによって、内部酸化が抑制され、その後の酸洗が容易になる。巻取温度は、より好ましくは650℃以下であり、さらに好ましくは600℃以下である。巻取温度の下限は特に規定しないが、例えば室温でもよい。冷間圧延時の破断を抑制するために、室温まで冷却された後、冷間圧延前に300℃以上600℃以下で熱延板を焼き戻してもよい。
熱 The hot-rolled steel sheet obtained by performing finish rolling can be cooled, wound, and made into a coil. The winding temperature after cooling is preferably set to 700 ° C. or lower. By setting the winding temperature to 700 ° C. or lower, internal oxidation is suppressed, and subsequent pickling is facilitated. The winding temperature is more preferably 650 ° C or lower, and further preferably 600 ° C or lower. The lower limit of the winding temperature is not particularly specified, but may be room temperature, for example. In order to suppress breakage during cold rolling, the hot rolled sheet may be tempered at a temperature of 300 ° C. or more and 600 ° C. or less before being cold-rolled after being cooled to room temperature.
熱延鋼板は、常法により酸洗を施された後に、冷間圧延が行われ、冷延鋼板とされる。
After hot-rolled steel sheet is subjected to pickling by a conventional method, cold rolling is performed to obtain a cold-rolled steel sheet.
冷間圧延の前であって酸洗の前または後に0%超~5%程度の軽度の圧延を行って形状を修正すると、平坦確保の点で有利となるので好ましい。また、酸洗前に軽度の圧延を行うことより酸洗性が向上し、表面濃化元素の除去が促進され、化成処理性やめっき処理性を向上させる効果がある。
(4) It is preferable that the shape is corrected by performing light rolling of more than 0% to about 5% before or after pickling and before or after cold rolling because it is advantageous in terms of securing flatness. Further, by performing light rolling before pickling, pickling properties are improved, removal of surface-concentrating elements is promoted, and there is an effect of improving chemical conversion property and plating property.
焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトの界面密度を制御する観点から、冷間圧延の圧下率を低減することが重要である。冷間圧延の圧下率を低く抑えることで、焼鈍後の組織が均質化させることができ、すなわち、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面分布が均質化される。その結果、残留オーステナイトとフレッシュマルテンサイトの界面がより多く金属組織中に存在することが可能となる。この効果を得るために、冷間圧延の圧下率の上限値は50%以下であり、好ましくは20%以下、より好ましくは18%以下、さらにより好ましくは15%以下である。冷間圧延の圧下率の下限値は0%以上であり、好ましくは5%以上である。冷間圧延の圧下率を50%以下とすることは、本発明で規定する界面密度の条件を満足するための重要な要件である。冷間圧延の圧下率を、より好ましくは18%以下とすることにより、3.15μm-1以上のより大きな界面密度を得ることができ、且つ3.0%以上のより大きな局部伸びを得ることができる。
From the viewpoint of controlling the interface density of retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite, it is important to reduce the rolling reduction of cold rolling. By keeping the rolling reduction of the cold rolling low, the structure after annealing can be homogenized, that is, the interface distribution between the retained austenite and the fresh martensite in the matrix mainly composed of tempered martensite is homogenized. Is done. As a result, more interfaces between the retained austenite and the fresh martensite can be present in the metal structure. In order to obtain this effect, the upper limit of the rolling reduction of the cold rolling is 50% or less, preferably 20% or less, more preferably 18% or less, and even more preferably 15% or less. The lower limit of the rolling reduction of the cold rolling is 0% or more, and preferably 5% or more. Making the rolling reduction of the cold rolling 50% or less is an important requirement for satisfying the interface density condition specified in the present invention. By setting the rolling reduction of the cold rolling to preferably 18% or less, it is possible to obtain a larger interface density of 3.15 μm −1 or more and obtain a larger local elongation of 3.0% or more. Can be.
上記熱間圧延工程及び冷間圧延工程を経て得られた冷延鋼板を740℃以上800℃未満に加熱して1回目の焼鈍を行う。740℃以上800℃未満の温度域で10秒以上保持し、その後に、740℃以上800℃未満の上記保持温度から、平均冷却速度2℃/秒以上2000℃/秒以下で500℃以下の温度域まで冷却し、室温まで冷却することが好ましい。次いで、600℃以上Ac3点未満に再度加熱して2回目の焼鈍を行う。600℃以上Ac3点未満の温度域で5秒以上保持し、その後に、600℃以上Ac3点未満の上記保持温度から、平均冷却速度10℃/秒以上で300℃以下の温度域まで冷却する。次いで、200℃以上450℃以下の温度域で30秒以上保持し、その後に、室温まで冷却する。
The cold-rolled steel sheet obtained through the hot rolling step and the cold rolling step is heated to 740 ° C. or more and less than 800 ° C. to perform the first annealing. The temperature is held at a temperature range of 740 ° C or more and less than 800 ° C for 10 seconds or more, and thereafter, from the above-mentioned holding temperature of 740 ° C or more and less than 800 ° C, a temperature of 500 ° C or less at an average cooling rate of 2 ° C / sec or more and 2000 ° C / sec or less. It is preferable to cool to a region and cool to room temperature. Next, a second annealing is performed by heating again to 600 ° C. or higher and lower than the Ac 3 point. Hold at a temperature range of 600 ° C. or more and less than Ac 3 points for 5 seconds or more, and then cool from the above holding temperature of 600 ° C. or more and less than Ac 3 points to a temperature range of 300 ° C. or less at an average cooling rate of 10 ° C./sec or more. I do. Next, it is kept in a temperature range of 200 ° C. or more and 450 ° C. or less for 30 seconds or more, and then cooled to room temperature.
冷間圧延後の1回目の焼鈍温度は740℃以上800℃未満である。上記焼鈍温度を740℃以上とすることにより、再結晶を著しく促進することができ、さらに、鋼板中のフェライトの面積率を少なくすることができ、延性を向上することができる。フェライトの面積率を0%とするために、1回目の焼鈍温度はAc3以上であることが好ましい。一方、焼鈍温度を800℃未満とすることで、オーステナイト粒成長が抑制され、旧オーステナイト粒の微細化効果により、焼き戻しマルテンサイトに対する残留オーステナイトとフレッシュマルテンサイトの界面密度が増加し、局部延性の向上に寄与する。未再結晶を完全に除去し、良好な靭性を安定して確保するには、740℃以上800℃未満の温度域での保持時間を10秒以上とすることが好ましい。生産性の観点からは、740℃以上800℃未満の温度域での保持時間を300秒以内とすることが好ましい。ここで、C:0.05%~0.5%、Si:0%~3.5%、Mn:0~9.0%、及びAl:0~2.0%を含有する複数種類の冷延鋼板について加熱速度0.5~50℃/秒においてAc3点を計測し検討した結果、Ac3点として以下の式:
Ac3=910-200√C+44Si-25Mn+44Al
が得られ、この式を用いてAc3点を算出することができる。上記の式中の元素記号には、対応する元素の含有量(質量%)が代入される。 The first annealing temperature after cold rolling is 740 ° C. or more and less than 800 ° C. By setting the annealing temperature to 740 ° C. or higher, recrystallization can be remarkably promoted, the area ratio of ferrite in the steel sheet can be reduced, and the ductility can be improved. In order to reduce the area ratio of ferrite to 0%, the first annealing temperature is preferably Ac 3 or more. On the other hand, by setting the annealing temperature to less than 800 ° C., austenite grain growth is suppressed, and the interface density between residual austenite and fresh martensite with respect to tempered martensite increases due to the refining effect of old austenite grains, and local ductility increases. Contribute to improvement. In order to completely remove unrecrystallized crystals and stably secure good toughness, it is preferable that the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. be 10 seconds or more. From the viewpoint of productivity, the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. is preferably 300 seconds or less. Here, a plurality of types of cold containing C: 0.05% to 0.5%, Si: 0% to 3.5%, Mn: 0 to 9.0%, and Al: 0 to 2.0%. As a result of measuring and examining three Ac points at a heating rate of 0.5 to 50 ° C./sec for the rolled steel sheet, the following equation was obtained as the three Ac points:
Ac 3 = 910-200 ° C. + 44Si-25Mn + 44Al
Is obtained, and the Ac 3 point can be calculated using this equation. The content (% by mass) of the corresponding element is substituted for the element symbol in the above formula.
Ac3=910-200√C+44Si-25Mn+44Al
が得られ、この式を用いてAc3点を算出することができる。上記の式中の元素記号には、対応する元素の含有量(質量%)が代入される。 The first annealing temperature after cold rolling is 740 ° C. or more and less than 800 ° C. By setting the annealing temperature to 740 ° C. or higher, recrystallization can be remarkably promoted, the area ratio of ferrite in the steel sheet can be reduced, and the ductility can be improved. In order to reduce the area ratio of ferrite to 0%, the first annealing temperature is preferably Ac 3 or more. On the other hand, by setting the annealing temperature to less than 800 ° C., austenite grain growth is suppressed, and the interface density between residual austenite and fresh martensite with respect to tempered martensite increases due to the refining effect of old austenite grains, and local ductility increases. Contribute to improvement. In order to completely remove unrecrystallized crystals and stably secure good toughness, it is preferable that the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. be 10 seconds or more. From the viewpoint of productivity, the holding time in a temperature range of 740 ° C. or more and less than 800 ° C. is preferably 300 seconds or less. Here, a plurality of types of cold containing C: 0.05% to 0.5%, Si: 0% to 3.5%, Mn: 0 to 9.0%, and Al: 0 to 2.0%. As a result of measuring and examining three Ac points at a heating rate of 0.5 to 50 ° C./sec for the rolled steel sheet, the following equation was obtained as the three Ac points:
Ac 3 = 910-200 ° C. + 44Si-25Mn + 44Al
Is obtained, and the Ac 3 point can be calculated using this equation. The content (% by mass) of the corresponding element is substituted for the element symbol in the above formula.
1回目の焼鈍後、740℃以上800℃未満の上記保持温度から、平均冷却速度2℃/秒以上で500℃以下の温度域まで冷却することが好ましい。平均冷却速度を2℃/秒以上とすることによって臨界冷却速度以上で冷却され、冷却後の鋼材全体をマルテンサイト主体の組織にすることができる。一方、水焼入れ冷却法やミスト噴射冷却法を用いても、平均冷却速度を2000℃/秒以上に制御することすることは難しいので、平均冷却速度の実質的上限は2000℃/秒になる。また、冷却停止温度を500℃以下にすることにより、マルテンサイト変態開始温度以下まで冷却され、冷却後の鋼材全体をマルテンサイト主体の組織にすることができる。上記冷却の後、鋼板を室温まで冷却することが好ましい。上記冷却後2回目の焼鈍の前に、100℃以上500℃以下の温度域で10秒以上1000秒以下の時間で保持して焼戻し処理を行ってもよい。
(4) After the first annealing, it is preferable to cool from the above holding temperature of 740 ° C. or more and less than 800 ° C. to a temperature range of 500 ° C. or less at an average cooling rate of 2 ° C./sec or more. By setting the average cooling rate to 2 ° C./sec or more, the steel material is cooled at a rate higher than the critical cooling rate, and the entire steel material after cooling can have a structure mainly composed of martensite. On the other hand, even if the water quenching cooling method or the mist injection cooling method is used, it is difficult to control the average cooling rate to 2000 ° C./sec or more, so the practical upper limit of the average cooling rate is 2000 ° C./sec. Further, by setting the cooling stop temperature to 500 ° C. or lower, the steel is cooled to the martensite transformation start temperature or lower, and the entire steel material after cooling can have a martensite-based structure. After the cooling, the steel sheet is preferably cooled to room temperature. Before the second annealing after the cooling, the tempering treatment may be performed by holding the material in a temperature range of 100 ° C. or more and 500 ° C. or less for 10 seconds or more and 1000 seconds or less.
1回目の焼鈍の後の2回目の焼鈍温度は600℃以上Ac3点未満である。2回目の焼鈍温度を600℃以上Ac3点未満にすることにより、焼き戻しマルテンサイトを所望の面積率に制御でき、引張強度と局部延性とを高めることができる。セメンタイトを溶解させ、良好な靭性を安定して確保する観点から、600℃以上Ac3点未満の温度域での保持時間を5秒以上とすることが好ましい。また、生産性の観点からは、600℃以上Ac3点未満の温度域での保持時間を300秒以内とすることが好ましい。
The second annealing temperature after the first annealing is 600 ° C. or more and less than Ac 3 points. By the second annealing temperature below 3 points 600 ° C. or higher Ac, martensite tempering a can control a desired area ratio, it is possible to increase the tensile strength and the local ductility. From the viewpoint of dissolving cementite and stably securing good toughness, the holding time in a temperature range of 600 ° C. or more and less than the Ac 3 point is preferably 5 seconds or more. Further, from the viewpoint of productivity, the holding time in a temperature range of 600 ° C. or more and less than the three Ac points is preferably 300 seconds or less.
2回目の焼鈍後、600℃以上Ac3点未満の上記保持温度から、平均冷却速度10℃/秒以上で300℃以下の温度域まで冷却する。次いで、200℃以上450℃以下の温度域で30秒以上保持する。
After the second annealing, cooling is performed from the above-mentioned holding temperature of 600 ° C. or more and less than Ac 3 points to a temperature range of 300 ° C. or less at an average cooling rate of 10 ° C./sec or more. Next, it is kept at a temperature range of 200 ° C. or more and 450 ° C. or less for 30 seconds or more.
平均冷却速度10℃/秒以上で300℃以下の温度域まで冷却することで、残留オーステナイト組織の粗大化を抑制できるので、焼き戻しマルテンサイトの母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面分布を固定し、所望の界面密度にすることが可能になる。すなわち、平均冷却速度10℃/秒以上という速度で300℃以下の温度域まで冷却することで、1回目の焼鈍で生成した界面分布を維持して界面密度の減少を避けることができ、局部延性の向上に寄与する。なお、上述したように、平均冷却速度を2000℃/秒以上に制御することすることは難しいので、2回目の焼鈍後の冷却における平均冷却速度の実質的上限も2000℃/秒になる。次いで、200℃以上450℃以下の温度域で30秒以上保持することでフレッシュマルテンサイト中の過飽和炭素の残留オーステナイト中への拡散が助長され、残留オーステナイトの生成を促進するとともに、界面密度が高いことに起因してC拡散が促進され、残留オーステナイト内部のCが均一になる効果が得られ、局部延性が高まる。
By cooling to a temperature range of 300 ° C. or less at an average cooling rate of 10 ° C./sec or more, coarsening of the retained austenite structure can be suppressed, so that the interface distribution between the retained austenite and the fresh martensite with respect to the parent phase of the tempered martensite. Is fixed, and a desired interface density can be obtained. That is, by cooling to a temperature range of 300 ° C. or lower at an average cooling rate of 10 ° C./second or higher, the interface distribution generated in the first annealing can be maintained, and a decrease in interface density can be avoided. Contribute to the improvement of As described above, since it is difficult to control the average cooling rate to 2000 ° C./sec or more, the practical upper limit of the average cooling rate in the cooling after the second annealing is 2000 ° C./sec. Next, by maintaining the temperature in a temperature range of 200 ° C. or more and 450 ° C. or less for 30 seconds or more, diffusion of supersaturated carbon in fresh martensite into retained austenite is promoted, and the generation of retained austenite is promoted and the interface density is high. As a result, C diffusion is promoted, the effect of uniforming C inside retained austenite is obtained, and local ductility is enhanced.
200℃以上450℃以下の温度域で30秒以上保持した後の冷却は、鋼板にめっきしない場合には、そのまま室温まで行われればよい。また、鋼板にめっきする場合には、以下のようにして製造する。
The cooling after holding for 30 seconds or more in a temperature range of 200 ° C. or more and 450 ° C. or less may be performed as it is to room temperature if the steel sheet is not plated. Moreover, when plating on a steel plate, it is manufactured as follows.
鋼板の表面に溶融亜鉛めっきを施して溶融亜鉛めっき鋼板を製造する場合には、200℃以上450℃以下の温度域で30秒以上保持した鋼板を、430~500℃の温度範囲に再び加熱し、次いで鋼板を溶融亜鉛のめっき浴に浸漬して溶融亜鉛めっき処理を行う。めっき浴の条件は通常の範囲内とすればよい。めっき処理後は室温まで冷却すればよい。
In the case of producing a hot-dip galvanized steel sheet by performing hot-dip galvanizing on the surface of the steel sheet, the steel sheet held at a temperature range of 200 ° C. to 450 ° C. for 30 seconds or more is heated again to a temperature range of 430 to 500 ° C. Then, the steel sheet is dipped in a hot-dip galvanizing bath to perform hot-dip galvanizing. The conditions of the plating bath may be within a normal range. After the plating process, it may be cooled to room temperature.
鋼板の表面に合金化溶融亜鉛めっきを施して合金化溶融亜鉛めっき鋼板を製造する場合には、鋼板に溶融亜鉛めっき処理を施した後、鋼板を室温まで冷却する前に、450~620℃の温度で溶融亜鉛めっきの合金化処理を行う。合金化処理条件は、通常の範囲内とすればよい。
In the case of producing a galvannealed steel sheet by subjecting the surface of the steel sheet to galvannealing, after subjecting the steel sheet to a hot-dip galvanizing treatment, before cooling the steel sheet to room temperature, a temperature of 450 to 620 ° C. An alloying process of hot-dip galvanizing is performed at a temperature. Alloying treatment conditions may be within a normal range.
以上のように鋼板を製造することによって、本実施形態に係る鋼板を得ることができる。
鋼板 By manufacturing the steel sheet as described above, the steel sheet according to the present embodiment can be obtained.
本開示の鋼板を、例を参照しながらより具体的に説明する。ただし、以下の例は本開示の鋼板の例であり、本開示の鋼板は以下の例の態様に限定されるものではない。
鋼板 The steel sheet of the present disclosure will be described more specifically with reference to examples. However, the following example is an example of the steel sheet of the present disclosure, and the steel sheet of the present disclosure is not limited to the embodiments of the following examples.
1.評価用鋼板の製造
表1に示す化学成分を有する鋼を転炉で溶製し、連続鋳造により245mm厚のスラブを得た。 1. Production of steel sheet for evaluation Steel having the chemical components shown in Table 1 was melted in a converter, and a slab having a thickness of 245 mm was obtained by continuous casting.
表1に示す化学成分を有する鋼を転炉で溶製し、連続鋳造により245mm厚のスラブを得た。 1. Production of steel sheet for evaluation Steel having the chemical components shown in Table 1 was melted in a converter, and a slab having a thickness of 245 mm was obtained by continuous casting.
得られたスラブを表2に示す条件にて熱間圧延し、2.6mm厚の熱延鋼板を製板した。次いで、得られた熱延鋼板を酸洗し、表2に示す冷間圧延率で冷間圧延を施して、表2に示す種々の板厚の冷延鋼板を製板した。
The obtained slab was hot-rolled under the conditions shown in Table 2 to produce a hot-rolled steel sheet having a thickness of 2.6 mm. Next, the obtained hot-rolled steel sheet was pickled and cold-rolled at a cold-rolling rate shown in Table 2 to produce cold-rolled steel sheets having various thicknesses shown in Table 2.
得られた冷延鋼板について、表3に示す条件の熱処理を施して焼鈍冷延鋼板を作製した。冷延鋼板の熱処理は、窒素98%及び水素2%の還元雰囲気で行った。
熱処理 The obtained cold-rolled steel sheet was subjected to a heat treatment under the conditions shown in Table 3 to produce an annealed cold-rolled steel sheet. The heat treatment of the cold rolled steel sheet was performed in a reducing atmosphere of 98% nitrogen and 2% hydrogen.
一部の焼鈍冷延鋼板例については、最終の焼鈍を行った後、冷却を460℃で停止し、冷延鋼板を460℃の溶融亜鉛のめっき浴に2秒間浸漬して、溶融亜鉛めっき処理を行った。めっき浴の条件は従来のものと同じである。後で述べる合金化処理を施さない場合、460℃の保持後に、平均冷却速度10℃/秒で室温まで冷却した。
For some annealed cold-rolled steel sheets, after the final annealing, cooling was stopped at 460 ° C, and the cold-rolled steel sheets were immersed in a 460 ° C hot-dip galvanizing bath for 2 seconds to perform hot-dip galvanizing. Was done. The conditions of the plating bath are the same as the conventional one. When the alloying treatment described later was not performed, the temperature was maintained at 460 ° C. and then cooled to room temperature at an average cooling rate of 10 ° C./sec.
一部の焼鈍冷延鋼板例については、溶融亜鉛めっき処理を行った後に、室温に冷却せずに、続いて合金化処理を施した。520℃まで加熱し、520℃で5秒間保持して合金化処理を行い、その後、平均冷却速度10℃/秒で室温まで冷却した。
Some examples of the annealed cold-rolled steel sheets were subjected to a hot dip galvanizing treatment, and then to an alloying treatment without cooling to room temperature. The alloy was heated to 520 ° C. and held at 520 ° C. for 5 seconds to perform an alloying treatment, and then cooled to room temperature at an average cooling rate of 10 ° C./sec.
このようにして得られた焼鈍冷延鋼板を伸び率0.1%で調質圧延し、各種評価用鋼板を準備した。
を The annealed cold-rolled steel sheet thus obtained was subjected to temper rolling at an elongation of 0.1% to prepare various evaluation steel sheets.
2.評価方法
各例で得られた焼鈍冷延鋼板について、焼き戻しマルテンサイト、残留オーステナイト、フレッシュマルテンサイト、及びフェライトの面積率、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度、引張強度、全伸び、並びに局部伸びを評価した。各評価の方法は次のとおりである。 2. Evaluation method For the annealed cold-rolled steel sheet obtained in each example, tempered martensite, retained austenite, fresh martensite, and the area ratio of ferrite, retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite. Were evaluated for interface density, tensile strength, total elongation, and local elongation. The method of each evaluation is as follows.
各例で得られた焼鈍冷延鋼板について、焼き戻しマルテンサイト、残留オーステナイト、フレッシュマルテンサイト、及びフェライトの面積率、焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度、引張強度、全伸び、並びに局部伸びを評価した。各評価の方法は次のとおりである。 2. Evaluation method For the annealed cold-rolled steel sheet obtained in each example, tempered martensite, retained austenite, fresh martensite, and the area ratio of ferrite, retained austenite and fresh martensite with respect to a matrix mainly composed of tempered martensite. Were evaluated for interface density, tensile strength, total elongation, and local elongation. The method of each evaluation is as follows.
焼き戻しマルテンサイト及びフェライトの面積率は走査型電子顕微鏡による組織観察から算出した。残留オーステナイト及びフレッシュマルテンサイトの面積率は、走査型電子顕微鏡による組織観察及びX線回折測定から算出した。鋼板を圧延方向に平行に切断したL断面について、鏡面研磨を行い、次いで3%ナイタールによりミクロ組織を現出させて、倍率5000倍の走査型電子顕微鏡で、表面から1/4位置におけるミクロ組織を観察し、0.1mm×0.3mmの範囲について画像解析(Photoshоp(登録商標))により、焼き戻しマルテンサイトの面積率、フェライトの面積率、及び残留オーステナイトとフレッシュマルテンサイトとの合計の面積率を算出した。さらに、得られた鋼板から幅25mm、長さ25mmの試験片を切り出し、この試験片に化学研磨を施して板厚1/4分を減厚し、化学研磨後の試験片の表面に対して、Co管球を用いたX線回折分析を3回実施し、得られたプロファイルを解析し、それぞれを平均して残留オーステナイトの面積率を算出した。走査型電子顕微鏡観察で算出した残留オーステナイトとフレッシュマルテンサイトとの合計の面積率から、X線回折測定で算出した残留オーステナイトの面積率を差し引いて、フレッシュマルテンサイトの面積率を算出した。
面積 The area ratios of tempered martensite and ferrite were calculated from observation of the structure by a scanning electron microscope. The area ratio of the retained austenite and fresh martensite was calculated from the structure observation by a scanning electron microscope and the X-ray diffraction measurement. The L section of the steel sheet cut in parallel to the rolling direction is mirror-polished, and then the microstructure is revealed with 3% nital. The microstructure at a quarter position from the surface is obtained with a scanning electron microscope at a magnification of 5000 times. Was observed, and image analysis (Photoshop (registered trademark)) was performed on an area of 0.1 mm × 0.3 mm to determine the area ratio of tempered martensite, the area ratio of ferrite, and the total area of retained austenite and fresh martensite. The rate was calculated. Furthermore, a test piece having a width of 25 mm and a length of 25 mm was cut out from the obtained steel sheet, and the test piece was subjected to chemical polishing to reduce the thickness of the test piece by 4 minute. X-ray diffraction analysis using a Co tube was performed three times, the obtained profiles were analyzed, and the respective profiles were averaged to calculate the area ratio of retained austenite. The area ratio of fresh martensite was calculated by subtracting the area ratio of retained austenite calculated by X-ray diffraction measurement from the total area ratio of retained austenite and fresh martensite calculated by scanning electron microscope observation.
焼き戻しマルテンサイトを主体とする母相に対する残留オーステナイトとフレッシュマルテンサイトとの界面密度は、画像解析ソフトウェアImageJを用いて測定した。まず、SEMを用いて対象とする組織を5000倍の倍率で観察してSEM画像(24μm×18μm)を得た。次いで、ImageJを用いて、SEM画像に1280×960個の分割領域を形成した。各分割領域について、残留オーステナイトまたはフレッシュマルテンサイトの何れかである領域を黒、その他の領域を白となるように2値化処理を施して2階調画像を得た。2値化の閾値は、「Glasbey, CA (1993), "An analysis of histogram-based thresholding algorithms", CVGIP: Graphical Models and Image Processing 55: 532-537」に記載されている輝度値の平均値を閾値として採用する手法を用いて決定した。このアルゴリズムはImageJに実装されており、Auto threshold機能を利用して閾値の決定方法をMethod=Meanとすることで自動的に2値化した。すなわち、2値化の閾値は、ImageJにてMethod=Mean、radius=15として、各ピクセル値を、着目したピクセルを中心として半径15ピクセル以内のピクセル値の平均と置き換えて、スムージングした後のヒストグラムから自動的に決定した。焼き戻しマルテンサイトに対する残留オーステナイトとフレッシュマルテンサイトとの界面密度の測定は、得られた2階調画像において、孤立したすべての残留オーステナイトとフレッシュマルテンサイトとの焼き戻しマルテンサイトに対する界面の長さを逐次解析し、その合計値を画像の全領域の面積(24μm×18μm)で除することで行った。残留オーステナイトとフレッシュマルテンサイトとが隣接する場合、上記2値化処理過程で両者は一体組織となるため、残留オーステナイトとフレッシュマルテンサイトとの界面の長さは除かれている。逐次解析とは、残留オーステナイトとフレッシュマルテンサイトのそれぞれについて、界面を重複することなく、界面密度を測定することをいう。
(4) The interface density between the retained austenite and the fresh martensite with respect to the matrix mainly composed of tempered martensite was measured using image analysis software ImageJ. First, a target tissue was observed at a magnification of 5000 times using an SEM to obtain an SEM image (24 μm × 18 μm). Next, 1280 × 960 divided regions were formed in the SEM image using ImageJ. Each of the divided regions was subjected to a binarization process so that a region that was either retained austenite or fresh martensite was black, and the other regions were white, to obtain a two-tone image. The threshold for binarization is the average value of the luminance values described in "Glasbey, CA (1993)," An analysis of histogram-based thresholding algorithm ", CVGIP: Graphic Model and image Processing 55: 532-537. It was determined using the method adopted as the threshold. This algorithm is implemented in ImageJ, and was automatically binarized by setting the method of determining a threshold to Method = Mean using the Auto @ threshold function. That is, the binarization threshold is set to Method = Mean and radius = 15 in ImageJ, and each pixel value is replaced with the average of the pixel values within a radius of 15 pixels around the pixel of interest, and the histogram after smoothing is performed. Automatically determined from. The measurement of the interface density between the retained austenite and the fresh martensite with respect to the tempered martensite was performed by measuring the length of the interface with respect to the tempered martensite between all the isolated residual austenite and the fresh martensite in the obtained two-tone image. The analysis was performed sequentially, and the total value was divided by the area (24 μm × 18 μm) of the entire area of the image. When the residual austenite and the fresh martensite are adjacent to each other, the two have an integrated structure in the binarization process, and thus the length of the interface between the residual austenite and the fresh martensite is excluded. The sequential analysis refers to measuring the interface density of each of retained austenite and fresh martensite without overlapping the interface.
(機械的性質)
鋼板の圧延方向に直角方向からJIS5号引張試験片を採取し、引張強度(TS)、全伸び(EL)、及び局部伸び(LEL)を測定した。引張試験は、JIS5号引張試験片を用いて、JIS Z2241:2011に規定される方法で行った。全伸びの測定は、JIS5号試験片を用いて、JIS Z2241:2011に規定される方法で行った。局部伸びの測定は、破断した試験片を突き合せた際の全伸びの値から最大荷重点の伸び(均一伸び)の値を引くことにより算出した。 (mechanical nature)
JIS No. 5 tensile test pieces were sampled from a direction perpendicular to the rolling direction of the steel sheet, and tensile strength (TS), total elongation (EL), and local elongation (LEL) were measured. The tensile test was carried out using a JIS No. 5 tensile test piece by a method specified in JIS Z2241: 2011. The total elongation was measured using a JIS No. 5 test piece according to the method specified in JIS Z2241: 2011. The local elongation was calculated by subtracting the value of the elongation at the maximum load point (uniform elongation) from the value of the total elongation when the broken test pieces were butted.
鋼板の圧延方向に直角方向からJIS5号引張試験片を採取し、引張強度(TS)、全伸び(EL)、及び局部伸び(LEL)を測定した。引張試験は、JIS5号引張試験片を用いて、JIS Z2241:2011に規定される方法で行った。全伸びの測定は、JIS5号試験片を用いて、JIS Z2241:2011に規定される方法で行った。局部伸びの測定は、破断した試験片を突き合せた際の全伸びの値から最大荷重点の伸び(均一伸び)の値を引くことにより算出した。 (mechanical nature)
JIS No. 5 tensile test pieces were sampled from a direction perpendicular to the rolling direction of the steel sheet, and tensile strength (TS), total elongation (EL), and local elongation (LEL) were measured. The tensile test was carried out using a JIS No. 5 tensile test piece by a method specified in JIS Z2241: 2011. The total elongation was measured using a JIS No. 5 test piece according to the method specified in JIS Z2241: 2011. The local elongation was calculated by subtracting the value of the elongation at the maximum load point (uniform elongation) from the value of the total elongation when the broken test pieces were butted.
3.評価結果
上記の評価の結果を表4に示す。2.7μm-1以上の界面密度、1200MPa以上の引張強度、2.0%以上の局部伸びを示す鋼板を、優れた局部延性及び高強度を有する鋼板として評価した。 3. Evaluation results Table 4 shows the results of the above evaluations. A steel sheet exhibiting an interface density of 2.7 μm −1 or more, a tensile strength of 1200 MPa or more, and a local elongation of 2.0% or more was evaluated as a steel sheet having excellent local ductility and high strength.
上記の評価の結果を表4に示す。2.7μm-1以上の界面密度、1200MPa以上の引張強度、2.0%以上の局部伸びを示す鋼板を、優れた局部延性及び高強度を有する鋼板として評価した。 3. Evaluation results Table 4 shows the results of the above evaluations. A steel sheet exhibiting an interface density of 2.7 μm −1 or more, a tensile strength of 1200 MPa or more, and a local elongation of 2.0% or more was evaluated as a steel sheet having excellent local ductility and high strength.
図1に、試料No.31の鋼板のL断面を、上記鏡面研磨及びナイタール処理して観察したSEM画像を示す。図2に、図1のSEM画像を2値化処理した2階調画像を示す。図3に、試料No.4の鋼板のL断面を、上記鏡面研磨及びナイタール処理して観察したSEM画像を示す。図2の2階調画像から測定された界面密度は3.35μm-1であった。図3のSEM画像を2値化処理した2階調画像から測定された界面密度は1.50μm-1であった。
FIG. The SEM image which observed the L section of the 31 steel plate after performing said mirror-polishing and the nital processing is shown. FIG. 2 shows a two-tone image obtained by binarizing the SEM image of FIG. FIG. 4 shows an SEM image of the L section of the steel sheet No. 4 observed by mirror polishing and nital treatment. The interface density measured from the two-tone image of FIG. 2 was 3.35 μm −1 . The interface density measured from the binary image obtained by binarizing the SEM image in FIG. 3 was 1.50 μm −1 .
Claims (8)
- 質量%で、
C:0.20%超0.55%未満、
Si:0.001%以上3.50%未満、
Mn:4.00%超9.00%未満、
sol.Al:0.001%以上3.00%未満、
P:0.100%以下、
S:0.010%以下、
N:0.050%未満、
O:0.020%未満、
B:0.0000%以上0.010%未満
Cr:0.00%以上2.00%未満、
Mo:0.00%以上2.00%以下、
W:0.00%以上2.00%以下、
Cu:0.00%以上2.00%以下、
Ni:0.00%以上2.00%以下、
Ti:0.000%以上0.300%以下、
Nb:0.000%以上0.300%以下、
V:0.000%以上0.300%以下、
Ca:0.000%以上0.010%以下、
Mg:0.000%以上0.010%以下、
Zr:0.000%以上0.010%以下、
REM:0.000%以上0.010%以下、
Sb:0.000%以上0.050%以下、
Sn:0.000%以上0.050%以下、及び
Bi:0.000%以上0.050%以下を含有し、
残部が鉄及び不純物であり、
L断面において表面から厚みの1/4位置における金属組織が、面積率で、25%以上90%以下の焼き戻しマルテンサイト、3%以下のフェライト、10%以上50%以下の残留オーステナイト、及び25%以下のフレッシュマルテンサイトを含み、
前記1/4位置における金属組織において、残留オーステナイトまたはフレッシュマルテンサイトのいずれかである領域である第1の領域と、前記1/4位置における金属組織から前記第1の領域を除いた領域である第2の領域と、の境界の長さの総和を、前記第1の領域と前記第2の領域との合計面積で除した値である界面密度が2.7μm-1以上であること
を特徴とする鋼板。 In mass%,
C: more than 0.20% and less than 0.55%,
Si: 0.001% or more and less than 3.50%,
Mn: more than 4.00% and less than 9.00%,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
B: 0.0000% or more and less than 0.010% Cr: 0.00% or more and less than 2.00%,
Mo: 0.00% or more and 2.00% or less,
W: 0.00% or more and 2.00% or less,
Cu: 0.00% or more and 2.00% or less,
Ni: 0.00% or more and 2.00% or less,
Ti: 0.000% to 0.300%,
Nb: 0.000% to 0.300%,
V: 0.000% to 0.300%,
Ca: 0.000% or more and 0.010% or less,
Mg: 0.000% or more and 0.010% or less,
Zr: 0.000% or more and 0.010% or less,
REM: 0.000% or more and 0.010% or less,
Sb: 0.000% to 0.050%,
Sn: 0.000% to 0.050%, and Bi: 0.000% to 0.050%,
The balance is iron and impurities,
In the L section, the metal structure at a quarter position of the thickness from the surface is 25% or more and 90% or less of tempered martensite, 3% or less of ferrite, 10% or more and 50% or less of retained austenite, and % Or less of fresh martensite,
In the metal structure at the 1/4 position, a first region that is either retained austenite or fresh martensite, and a region obtained by removing the first region from the metal structure at the 1/4 position. The interface density, which is a value obtained by dividing the total length of the boundary between the second region and the second region by the total area of the first region and the second region, is 2.7 μm −1 or more. And steel plate. - 質量%で、
B:0.0003%以上0.010%未満
を含有することを特徴とする、請求項1に記載の鋼板。 In mass%,
B: The steel sheet according to claim 1, comprising 0.0003% or more and less than 0.010%. - 質量%で、
Cr:0.01%以上2.00%未満、
Mo:0.01%以上2.00%以下、
W:0.01%以上2.00%以下、
Cu:0.01%以上2.00%以下、及び
Ni:0.01%以上2.00%以下
の1種又は2種以上を含有することを特徴とする、請求項1又は2に記載の鋼板。 In mass%,
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01% or more and 2.00% or less,
W: 0.01% or more and 2.00% or less,
3. The method according to claim 1, wherein one or more of Cu: 0.01% to 2.00% and Ni: 0.01% to 2.00% are contained. 4. steel sheet. - 質量%で、
Ti:0.005%以上0.300%以下、
Nb:0.005%以上0.300%以下、及び
V:0.005%以上0.300%以下
の1種又は2種以上を含有することを特徴とする、請求項1~3のいずれか一項に記載の鋼板。 In mass%,
Ti: 0.005% or more and 0.300% or less,
Nb: 0.005% or more and 0.300% or less, and V: 0.005% or more and 0.300% or less. A steel sheet according to one of the preceding claims. - 質量%で、
Ca:0.0001%以上0.0100%以下、
Mg:0.0001%以上0.0100%以下、
Zr:0.0001%以上0.0100%以下、及び
REM:0.0001%以上0.0100%以下
の1種又は2種以上を含有することを特徴とする、請求項1~4のいずれか一項に記載の鋼板。 In mass%,
Ca: 0.0001% or more and 0.0100% or less,
Mg: 0.0001% or more and 0.0100% or less,
5. The method according to claim 1, wherein one or more of Zr: 0.0001% or more and 0.0100% or less and REM: 0.0001% or more and 0.0100% or less are contained. A steel sheet according to one of the preceding claims. - 質量%で、
Sb:0.0005%以上0.0500%以下、
Sn:0.0005%以上0.0500%以下、及び
Bi:0.0005%以上0.0500%以下
の1種又は2種以上を含有することを特徴とする、請求項1~5のいずれか一項に記載の鋼板。 In mass%,
Sb: 0.0005% or more and 0.0500% or less,
6. The method according to claim 1, wherein one or more of Sn: 0.0005% to 0.0500%, and Bi: 0.0005% to 0.0500% are contained. A steel sheet according to one of the preceding claims. - 前記鋼板の表面に溶融亜鉛めっき層を有することを特徴とする、請求項1~6のいずれか一項に記載の鋼板。 The steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a hot-dip galvanized layer on a surface thereof.
- 前記鋼板の表面に合金化溶融亜鉛めっき層を有することを特徴とする、請求項1~6のいずれか一項に記載の鋼板。 The steel sheet according to any one of claims 1 to 6, wherein the steel sheet has a galvannealed layer on a surface of the steel sheet.
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