WO2020174805A1 - 高強度鋼板およびその製造方法 - Google Patents
高強度鋼板およびその製造方法 Download PDFInfo
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- WO2020174805A1 WO2020174805A1 PCT/JP2019/048186 JP2019048186W WO2020174805A1 WO 2020174805 A1 WO2020174805 A1 WO 2020174805A1 JP 2019048186 W JP2019048186 W JP 2019048186W WO 2020174805 A1 WO2020174805 A1 WO 2020174805A1
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- WIPO (PCT)
- Prior art keywords
- less
- retained austenite
- steel sheet
- area ratio
- transformation point
- Prior art date
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- 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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- 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
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
- B21C47/00—Winding-up, coiling or winding-off metal wire, metal band or other flexible metal material characterised by features relevant to metal processing only
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
- C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
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- C21D1/18—Hardening; Quenching with or without subsequent tempering
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- C21D1/22—Martempering
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- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
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- C21D8/0436—Cold rolling
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- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/001—Austenite
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Definitions
- the present invention relates to a high-strength steel sheet excellent in formability, which is suitable as a member used in the industrial fields such as automobiles and electricity, and a manufacturing method thereof, and particularly, has a TS (tensile strength) of 980 MPa or more and is ductile.
- TS tensile strength
- a high-strength steel sheet that utilizes the work-induced transformation of retained austenite has been proposed.
- Such a steel sheet has a structure having retained austenite and is easily formed by the retained austenite at the time of forming the steel sheet, but has high strength because the retained austenite becomes martensite after the formation.
- Patent Document 1 0.2 to 2.5% by weight of Mn is contained, tensile strength is 1000 MPa or more, and EL (total elongation) is 30% or more.
- High strength steel sheets having high ductility have been proposed. Such a steel sheet is produced by subjecting a steel sheet containing C, Si, and Mn as basic components to austenite, and then performing so-called austempering treatment in which the steel is quenched in the bainite transformation temperature range and kept isothermal. Although the retained austenite is generated by the concentration of C in the austenite by this austempering treatment, a large amount of C exceeding 0.3% must be added to obtain a large amount of retained austenite.
- Patent Document 2 a steel containing 4 to 6% by weight of Mn is used, and a high strength-ductility balance is obtained by performing heat treatment in the two-phase region of ferrite and austenite.
- Patent Document 2 does not consider improvement of ductility by Mn concentration in untransformed austenite, and there is room for improvement of workability.
- Patent Document 3 a steel containing 3.0 to 7.0 mass% of Mn is used to perform heat treatment in a two-phase region of ferrite and austenite, whereby Mn is concentrated in untransformed austenite. By doing so, stable retained austenite is formed and the total elongation is improved. However, since the heat treatment time is short and the diffusion rate of Mn is slow, it is presumed that the concentration is insufficient.
- Patent Document 4 by using a steel containing 0.50 to 12.00 mass% of Mn, a hot rolled sheet is subjected to a heat treatment for a long time in a two-phase region of ferrite and austenite, and thereby, the untransformed austenite is formed.
- the formation of retained austenite with a large aspect ratio that promotes the Mn enrichment of No. 3 improves uniform elongation and hole expandability.
- the above-mentioned document is only a study of improvement of ductility and hole expandability of high strength steel sheet, and improvement of hole expandability and bendability by controlling the dispersed state of the second phase composed of retained austenite and martensite are studied. Not not.
- the present invention has been made in view of the current situation as described above, and an object thereof is TS (tensile strength) of 980 MPa or more, excellent formability, particularly excellent ductility as well as excellent hole expandability and bending. To provide a high-strength steel sheet having excellent properties and a manufacturing method thereof.
- TS tensile strength
- the inventors of the present invention have made extensive studies from the viewpoint of the composition of the steel sheet and the manufacturing method in order to solve the above problems, and have found the following.
- the steel structure has an area ratio of 35% or more and 80% or less of ferrite, 5% or more and 35% or less of quenched martensite, and 0.1% or more and less than 3.0% of tempered martensite.
- the area ratio of retained austenite is 8% or more, the average crystal grain size of the ferrite is 6 ⁇ m or less, the average crystal grain size of the retained austenite is 3 ⁇ m or less, and the average Mn amount in the retained austenite (% by mass).
- the value obtained by dividing the sum of the area ratios of austenite by the sum of the area ratios of fully quenched martensite and total retained austenite is less than 0.4, and the area ratio of retained austenite in contact with three or more ferrites having different crystal orientations. It was found that it becomes possible to manufacture a high-strength steel sheet having excellent formability, in which the value obtained by dividing by the area ratio of the total retained austenite is less than 0.6.
- the present invention has been made based on the above findings, and its gist is as follows. [1]% by mass, C: 0.030% to 0.250%, Si: 0.01% to 3.00%, Mn: 2.50% to 8.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, N: 0.0005% to 0.0100%, Al: 0.001% to 2.000%, balance Fe and unavoidable Component composition consisting of impurities and area ratio, ferrite is 35% or more and 80% or less, quenched martensite is 5% or more and 35% or less, and tempered martensite is 0.1% or more and less than 3.0%,
- the area ratio of retained austenite is 8% or more, the average crystal grain size of the ferrite is 6 ⁇ m or less, the average crystal grain size of the retained austenite is 3 ⁇ m or less, and the average Mn amount in the retained austenite (mass%) ) Is divided by the average Mn amount (% by mass)
- the composition of the components is, in mass %, Ti: 0.005% to 0.200%, Nb: 0.005% to 0.200%, V: 0.005% to 0.500%, W: 0.005% to 0.500%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.
- a steel slab having the composition as described in [1] or [2] is heated and hot-rolled at a finish rolling outlet temperature of 750°C or higher and 1000°C or lower, and 300°C or higher and 750°C or lower. After winding and cold rolling, after holding for 20 to 1800 s in the temperature range of Ac 3 transformation point or higher, it is cooled to a cooling stop temperature in the range of 50° C. to 350° C. for 2 s to the cooling stop temperature.
- a method for producing a high-strength steel sheet which comprises cooling after holding for 600 s, then holding for 20 to 1800 s in a temperature range of Ac 1 transformation point or higher and Ac 1 transformation point +150° C. or lower, and then cooling.
- a steel slab having the composition as described in [1] or [2] is heated and hot-rolled at a finish rolling outlet temperature of 750°C or higher and 1000°C or lower, and 300°C or higher and 750°C or lower. After winding and cold rolling, after holding for 20 to 1800 s in the temperature range of Ac 3 transformation point or higher, it is cooled to a cooling stop temperature in the range of 50° C. to 350° C. for 2 s to the cooling stop temperature.
- a method for producing a high-strength steel sheet which is cooled for 20 to 1800 s at room temperature.
- a high-strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent in formability, particularly ductility as well as hole expandability and bendability can be obtained.
- C 0.030% or more and 0.250% or less C is an element necessary for forming martensite and increasing the strength.
- C is an element effective for improving the stability of retained austenite and improving the ductility of steel. If the C content is less than 0.030%, it is difficult to secure the desired area ratio of martensite, and the desired strength cannot be obtained. Further, it is difficult to secure a sufficient area ratio of retained austenite, and good ductility cannot be obtained.
- C is excessively contained in an amount exceeding 0.250%, the area ratio of hard martensite becomes excessively large, and during the hole expanding test, microvoids at the crystal grain boundaries of martensite increase, and further cracking occurs. Is further propagated and the hole expandability is deteriorated.
- the C content is set to 0.030% or more and 0.250% or less.
- the C content is preferably 0.080% or more.
- the C content is preferably 0.200% or less.
- hard martensite refers to quenched martensite (as-quenched martensite).
- Si 0.01% or more and 3.00% or less Si improves the work-hardening ability of ferrite, and is effective for ensuring good ductility. If the amount of Si is less than 0.01%, the effect of inclusion becomes poor, so the lower limit was made 0.01%. However, an excessive Si content exceeding 3.00% not only causes the embrittlement of the steel but also causes the deterioration of the surface properties due to the generation of red scale. Furthermore, the chemical conversion processability and the plating quality are deteriorated. Therefore, the Si content is set to 0.01% or more and 3.00% or less. The Si content is preferably 0.20% or more. The Si content is preferably 2.00% or less, more preferably less than 0.70%.
- Mn 2.50% or more and 8.00% or less Mn is a very important contained element in the present invention.
- Mn is an element that stabilizes retained austenite, is effective in ensuring good ductility, and is an element that increases the strength of steel by solid solution strengthening. Such an action is recognized when the Mn content of steel is 2.50% or more. However, an excessive Mn content exceeding 8.00% deteriorates the chemical conversion treatment property and the plating quality. From this viewpoint, the Mn content is set to 2.50% or more and 8.00% or less.
- the Mn content is preferably 3.10% or more, more preferably 3.20% or more.
- the Mn content is preferably 6.00% or less, more preferably 4.20% or less.
- P 0.001% or more and 0.100% or less
- P is an element that has a function of solid solution strengthening and can be contained according to desired strength. Further, P is an element effective for forming a composite structure because it promotes ferrite transformation. In order to obtain such effects, the P content needs to be 0.001% or more.
- the P content is set to 0.001% or more and 0.100% or less.
- the P content is preferably 0.005% or more.
- the P content is preferably 0.050% or less.
- S 0.0001% or more and 0.0200% or less S segregates at grain boundaries to embrittle the steel during hot working, and also exists as a sulfide to reduce local deformability. Therefore, the S content needs to be 0.0200% or less, preferably 0.0100% or less, and more preferably 0.0050% or less. However, the S content needs to be 0.0001% or more because of the limitation in production technology. Therefore, the S content is 0.0001 or more and 0.0200% or less. The S content is preferably 0.0001% or more. The S content is preferably 0.0100% or less, more preferably 0.0050% or less.
- N 0.0005% or more and 0.0100% or less
- N is an element that deteriorates the aging resistance of steel. Particularly, when the N content exceeds 0.0100%, the deterioration of the aging resistance becomes remarkable.
- the N content is preferably 0.0010% or more.
- the N content is preferably 0.0070% or less.
- Al 0.001% or more and 2.000% or less
- Al is an element effective for expanding the two-phase region of ferrite and austenite and reducing the annealing temperature dependency of mechanical properties, that is, for material stability. If the Al content is less than 0.001%, the effect of containing Al becomes poor, so the lower limit was made 0.001%.
- Al acts as a deoxidizing agent and is an element effective for the cleanliness of steel, and is preferably added in the deoxidizing step. However, a large content exceeding 2.000% increases the risk of occurrence of billet cracking during continuous casting and reduces manufacturability. From this viewpoint, the Al content is 0.001% or more and 2.000% or less.
- the Al content is preferably 0.200% or more.
- the Al content is preferably 1.200% or less.
- Ti 0.005% or more and 0.200% or less
- Nb 0.005% or more and 0.200% or less
- V 0.005% or more and 0.500% or less.
- W 0.005% to 0.500%
- B 0.0003% to 0.0050%
- Ni 0.005% to 1.000%
- Cr 0.005% to 1.000 % Or less
- Mo 0.005% or more and 1.000% or less
- Cu 0.005% or more and 1.000% or less
- Sn 0.002% or more and 0.200% or less
- Sb 0.002% or more 0.
- REM abbreviation of Rare Earth Metal: at least one element selected from 0.0005% or more and 0.0050% or less can be arbitrarily contained.
- Components other than the above components are Fe and inevitable impurities. When the above components are contained below the lower limit, the components contained below the lower limit shall be contained as unavoidable impurities.
- Ti 0.005% or more and 0.200% or less Ti is effective for precipitation strengthening of steel, and can improve the strength of ferrite to reduce the hardness difference from the hard second phase (martensite or retained austenite). It is possible to secure good hole expandability. The effect is obtained at 0.005% or more. However, if it exceeds 0.200%, the area ratio of hard martensite becomes excessive, microvoids at the grain boundaries of martensite increase during the hole expansion test, and further crack propagation proceeds. The hole expandability (punching) may be reduced. Therefore, when Ti is contained, its content is set to 0.005% or more and 0.200% or less. The Ti content is preferably 0.010% or more. The Ti content is preferably 0.100% or less.
- Nb, V, and W are for precipitation strengthening of steel. It is effective, and each effect is obtained at 0.005% or more. Further, similar to the effect of containing Ti, by improving the strength of ferrite, the hardness difference from the hard second phase (martensite or retained austenite) can be reduced, and good hole expandability can be secured. The effect is obtained when each of Nb, V, and W is 0.005% or more.
- Nb exceeds 0.200% and V and W exceed 0.500%
- the area ratio of hard martensite becomes excessive, and microvoids at the grain boundaries of martensite increase during the hole expanding test.
- crack propagation may progress and the hole expansibility may decrease. Therefore, when Nb is contained, its content is set to 0.005% or more and 0.200% or less.
- the Nb content is preferably 0.010% or more.
- the Nb content is preferably 0.100% or less.
- V and W are contained, the content is 0.005% or more and 0.500% or less.
- the V and W contents are preferably 0.010% or more.
- the V and W contents are preferably 0.300% or less.
- B 0.0003% or more and 0.0050% or less B has an action of suppressing the generation and growth of ferrite from the austenite grain boundaries, and improves the strength of the ferrite, whereby the hard second phase (martensite or The hardness difference from the retained austenite) can be reduced, and good hole expandability can be ensured.
- the effect is obtained at 0.0003% or more. However, if it exceeds 0.0050%, the formability may decrease. Therefore, when B is contained, its content is set to 0.0003% or more and 0.0050% or less.
- the B content is preferably 0.0005% or more.
- the B content is preferably 0.0030% or less.
- Ni 0.005% or more and 1.000% or less
- Ni is an element that stabilizes retained austenite, is effective in ensuring good ductility, and is an element that increases the strength of steel by solid solution strengthening. The effect is obtained at 0.005% or more.
- the content exceeds 1.000%, the area ratio of hard martensite becomes excessive, the microvoids in the grain boundaries of martensite increase during the hole expanding test, and further the propagation of cracks progresses. Therefore, the hole expandability may be deteriorated. Therefore, when Ni is contained, the content is set to 0.005% or more and 1.000% or less.
- the Ni content is preferably 0.010% or more.
- the Ni content is preferably 0.500% or less.
- Cr 0.005% or more and 1.000% or less
- Mo 0.005% or more and 1.000% or less
- Cr and Mo have an effect of improving the balance between strength and ductility, and thus may be contained if necessary. it can.
- the effect is obtained when Cr: 0.005% or more and Mo: 0.005% or more.
- Cr is contained in excess of 1.000% and Mo in excess of 1.000%
- the area ratio of hard martensite becomes excessively large, and microvoids at crystal grain boundaries of martensite during a hole expanding test. May increase, and further, the propagation of cracks may progress and the hole expandability may decrease.
- the amounts thereof are respectively Cr: 0.005% or more and 1.000% or less and Mo: 0.005% or more and 1.000% or less.
- the Cr content is preferably 0.010% or more.
- the Cr content is preferably 0.500% or less.
- the Mo content is preferably 0.010% or more.
- the Mo content is preferably 0.500% or less.
- Cu 0.005% or more and 1.000% or less
- Cu is an element effective for strengthening steel. The effect is obtained at 0.005% or more.
- the content exceeds 1.000%, the area ratio of hard martensite becomes excessive, the microvoids in the grain boundaries of martensite increase during the hole expanding test, and further the propagation of cracks progresses. Therefore, the hole expandability may be deteriorated. Therefore, when Cu is contained, the amount is set to be 0.005% or more and 1.000% or less.
- the Cu content is preferably 0.010% or more.
- the Cu content is preferably 0.500% or less.
- Sn 0.002% or more and 0.200% or less
- Sb 0.002% or more and 0.200% or less
- Sn and Sb are decarburized in a region of several tens of ⁇ m of the steel sheet surface layer caused by nitriding or oxidation of the steel sheet surface. From the viewpoint of suppressing, it is contained as necessary.
- Sn in an amount of 0.002% or more or Sb in an amount of 0.002% or more, such nitriding and oxidation are suppressed, and the area ratio of martensite on the steel plate surface is prevented from decreasing. As a result, it is effective for ensuring strength and material stability.
- any of these elements is contained in excess of 0.200%, the toughness is lowered.
- Sn and Sb when Sn and Sb are contained, their contents should be 0.002% or more and 0.200% or less, respectively.
- the Sn and Sb contents are each preferably 0.004% or more.
- the Sn and Sb contents are each preferably 0.050% or less.
- martensite refers to quenched martensite.
- Ta 0.001% or more and 0.100% or less Ta, like Ti and Nb, forms alloy carbides and alloy carbonitrides, and contributes to strengthening.
- a solid solution with Nb carbide or Nb carbonitride to form a composite precipitate such as (Nb, Ta)(C, N)
- the coarsening of the precipitate is significantly suppressed and precipitation strengthening is achieved. It is considered that this has an effect of stabilizing the contribution to strength by. Therefore, it is preferable to contain Ta.
- the above-described effect of stabilizing the precipitate is obtained by setting the content of Ta to 0.001% or more.
- the Ta content is preferably 0.005% or more.
- the Ta content is preferably 0.050% or less.
- Ca, Mg, Zr, and REM are effective elements for making the shape of the sulfide spherical and further improving the adverse effect of the sulfide on the hole expandability.
- the content of each is 0.0005% or more.
- each excessive content exceeding 0.0050% causes an increase in inclusions and causes surface and internal defects. Therefore, when Ca, Mg, Zr, and REM are contained, their contents are 0.0005% or more and 0.0050% or less, respectively.
- the Ca, Mg, Zr and REM contents are each preferably 0.0010% or more.
- the Ca, Mg, Zr and REM contents are each preferably 0.0040% or less.
- Area ratio of ferrite 35% or more and 80% or less In order to secure sufficient ductility, the area ratio of ferrite needs to be 35% or more. Further, in order to secure the tensile strength of 980 MPa or more, the area ratio of soft ferrite needs to be 80% or less.
- the term "ferrite” as used herein refers to polygonal ferrite, granular ferrite, or acicular ferrite, and is relatively soft and highly ductile.
- the area ratio of ferrite is preferably 40% or more.
- the area ratio of ferrite is preferably 75% or less.
- Area ratio of quenched martensite 5% or more and 35% or less
- the area ratio of quenched martensite needs to be 5% or more. Further, in order to secure good ductility, the area ratio of quenched martensite needs to be 35% or less.
- the area ratio of quenched martensite is preferably 5% or more.
- the area ratio of quenched martensite is preferably 30% or less.
- Tempered martensite 0.1% or more and less than 3.0% Tempered martensite is required to be 0.1% or more in order to secure good hole expandability. Further, in order to achieve a TS of 980 MPa or more, the area ratio of tempered martensite needs to be less than 3.0%.
- the area ratio of tempered martensite is preferably 0.1% or more.
- the area ratio of tempered martensite is preferably 2.0% or less.
- the area ratio of ferrite, quenched martensite, and tempered martensite is 3 vol.% after polishing the plate thickness cross section (L cross section) parallel to the rolling direction of the steel plate. % Corroded with Nital, and at a plate thickness 1/4 position (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) using a SEM (scanning electron microscope), 10 fields of view at a magnification of 2000 times. Observed, using the obtained microstructure image, the area ratio of each microstructure (ferrite, martensite, tempered martensite) was calculated for 10 visual fields using Image-Pro of Media Cybernetics, and those values were averaged. You can ask for it. Further, in the above-mentioned structure image, ferrite has a gray structure (base structure), martensite has a white structure, and tempered martensite has a structure having a gray internal structure inside white martensite.
- Area ratio of retained austenite 8% or more In order to secure sufficient ductility, the area ratio of retained austenite needs to be 8% or more. It is preferably at least 12%.
- the area ratio of the retained austenite was determined by polishing the steel plate from the 1/4 position of the plate thickness to a surface of 0.1 mm, and then further polishing the surface 0.1 mm by chemical polishing using a CoK ⁇ ray with an X-ray diffractometer.
- the ratio was measured, the obtained nine integrated intensity ratios were averaged to obtain the volume ratio, and the value of the volume ratio was used as the area ratio.
- Average crystal grain size of ferrite 6 ⁇ m or less Refining the crystal grains of ferrite contributes to the improvement of TS. Therefore, in order to secure a desired TS, the average crystal grain size of ferrite needs to be 6 ⁇ m or less. It is preferably 5 ⁇ m or less.
- Average crystal grain size of retained austenite 3 ⁇ m or less Refining the crystal grains of retained austenite contributes to improvement of ductility and hole expandability. Therefore, in order to secure good ductility and hole expandability, the average crystal grain size of retained austenite needs to be 3 ⁇ m or less. It is preferably 2.5 ⁇ m or less.
- the area of stable retained austenite in which Mn is concentrated needs to be high. It is preferably 2.0 or more.
- the amount of Mn in the retained austenite was determined by using FE-EPMA (Field Emission-Electron Probe Micro Analyzer; field emission electron probe microanalyzer), and the distribution of Mn in each phase in the rolling direction cross section at the 1/4 position of the sheet thickness.
- the state can be quantified, and can be determined by the average value of the quantitative analysis results of the Mn amounts of 30 random retained austenite grains and 30 ferrite grains in the measurement visual field.
- the sum of the area ratios of quenched martensite with a circle equivalent diameter of 3 ⁇ m or more and retained austenite with a circle equivalent diameter of 3 ⁇ m or more divided by the sum of area percentages of fully quenched martensite and total retained austenite is less than 0.4 yen
- the sum of the area ratios of quenched martensite with an equivalent diameter of 3 ⁇ m or more and retained austenite with a circle equivalent diameter of 3 ⁇ m or more divided by the sum of the area ratios of fully quenched martensite and total retained austenite is less than 0.4. This is an important constituent feature in the present invention.
- the refinement of the crystal grain size of quenched martensite and retained austenite contributes to the improvement of hole expandability.
- the area ratio of fine quenched martensite and retained austenite I need to do a lot. It is preferably less than 0.3%, more preferably less than 0.2.
- the average crystal grain sizes of ferrite, martensite, and retained austenite are obtained by calculating the area of each of ferrite grains, martensite grains, and retained austenite grains using the above-mentioned Image-Pro, and calculating the equivalent circle diameters thereof. The value of was averaged and calculated. Martensite and retained austenite were identified by Phase Map of EBSD (Electron Backscattered Diffraction).
- the value obtained by dividing the area ratio of retained austenite in contact with three or more ferrites with different crystal orientations by the area ratio of total retained austenite is less than 0.6.
- the value obtained by dividing the area ratio of the retained austenite located at the triple points of the boundary by the area ratio of the total retained austenite is less than 0.6. If the number of ferrite grains having different crystal orientations in contact with the retained austenite is small, stress concentration during hole expanding deformation and bending deformation is relaxed, which contributes to improvement of hole expandability and bendability. Therefore, in order to secure good hole expandability, the area ratio of retained austenite in contact with three or more ferrites having different crystal orientations needs to be low. It is preferably less than 0.5.
- ferrites having different crystal orientations mean that the Euler angles of the ferrite grains that can be obtained by EBSD measurement differ by 1 degree or more. Further, the retained austenite in contact with three or more ferrites having different crystal orientations is identified by the IPF map obtained by EBSD measurement.
- the value obtained by dividing the area ratio of massive austenite by the sum of the area ratios of lath austenite and massive austenite is preferably less than 0.6. If the area ratio of massive austenite is too large, the hole expandability of steel may be reduced. Therefore, in order to secure better hole expandability, it is preferable that the value obtained by dividing the area ratio of massive austenite by the sum of the area ratios of lath austenite and massive austenite is within 0.6. The value obtained by dividing the area ratio of massive austenite by the sum of the area ratios of lath austenite and massive austenite is preferably within the range of less than 0.4.
- the massive austenite as used herein has an aspect ratio of the major axis and the minor axis of less than 2.0, and the lath austenite has an aspect ratio of the major axis and the minor axis of 2.0 or more.
- the aspect ratio of retained austenite was calculated by drawing an ellipse circumscribing the retained austenite grains using Photoshop elements 13 and dividing the major axis length by the minor axis length.
- the microstructure of the present invention includes a total of carbides such as bainite, pearlite, and cementite in an area ratio of 10% or less. The effect of the invention is not impaired.
- the heating temperature of the steel slab is not particularly limited, but the heating temperature of the steel slab is preferably 1100°C or higher and 1300°C or lower.
- the precipitates existing in the heating stage of the steel slab exist as coarse precipitates in the finally obtained steel sheet and do not contribute to the strength. Therefore, Ti and Nb-based precipitates precipitated during casting are redissolved. It is possible.
- the heating temperature of the steel slab should be 1100° C. or higher from the viewpoint of scaling off air bubbles and segregation of the steel slab surface layer to further reduce cracks and irregularities on the steel plate surface and achieve a smoother steel plate surface. Is preferred.
- the heating temperature of the steel slab is preferably set to 1300° C. or lower from the viewpoint of reducing scale loss due to the increase in the amount of oxidation.
- the heating temperature of the steel slab is more preferably 1150°C or higher.
- the heating temperature of the steel slab is more preferably 1250° C. or lower.
- ⁇ Steel slabs are preferably manufactured by continuous casting to prevent macro segregation, but they can also be manufactured by ingot casting or thin slab casting. Further, after manufacturing the steel slab, in addition to the conventional method of once cooling to room temperature and then heating again, it was charged into the heating furnace as it was without cooling to room temperature, or a slight heat retention was performed. Energy-saving processes such as direct feed rolling and direct rolling, which are immediately rolled later, can be applied without any problems. Also, steel slabs are made into sheet bars by rough rolling under normal conditions, but if the heating temperature is made low, use a bar heater or the like before finish rolling from the viewpoint of preventing problems during hot rolling. It is preferable to heat the sheet bar.
- Finish rolling end temperature of hot rolling 750°C or higher and 1000°C or lower
- the steel slab after heating is hot rolled by rough rolling and finish rolling to be a hot rolled steel sheet.
- the finishing temperature exceeds 1000° C.
- the amount of oxide (scale) generated sharply increases, the interface between the base iron and oxide becomes rough, and the surface quality after pickling and cold rolling tends to deteriorate. It is in.
- the ductility and hole expandability are adversely affected. Further, the crystal grain size becomes excessively large, and the surface of the pressed product may be roughened during processing.
- the finishing temperature is less than 750° C.
- the rolling load increases, the rolling load increases, and the rolling reduction in the unrecrystallized austenite state increases.
- the desired crystal grain size is not obtained, an abnormal texture develops, the in-plane anisotropy in the final product becomes remarkable, and not only the uniformity of the material (material stability) is impaired, but also the ductility It also drops. Therefore, it is necessary to set the finish rolling outlet temperature of the hot rolling to 750°C or higher and 1000°C or lower.
- the finish rolling exit temperature of hot rolling is preferably 800° C. or higher.
- the finish rolling exit temperature of hot rolling is preferably 950°C or lower.
- Winding temperature after hot rolling 300° C. or higher and 750° C. or lower
- the winding temperature after hot rolling exceeds 750° C.
- the strength of the hot-rolled sheet increases, the rolling load in cold rolling increases, and defective sheet shape occurs, resulting in reduced productivity. To do. Therefore, it is necessary to set the winding temperature after hot rolling to 300°C or higher and 750°C or lower.
- the winding temperature after hot rolling is preferably 400° C. or higher.
- the winding temperature after hot rolling is preferably 650°C or lower.
- rough rolling plates may be joined together during hot rolling and continuous finish rolling may be performed. Further, the rough rolled plate may be once wound. Further, in order to reduce the rolling load during hot rolling, part or all of finish rolling may be lubrication rolling. Lubrication rolling is also preferable from the viewpoint of uniformizing the shape of the steel sheet and the material. When performing lubrication rolling, the friction coefficient during lubrication rolling is preferably 0.10 or more and 0.25 or less.
- Optional pickling of the hot-rolled steel sheet manufactured in this way It is preferable to carry out pickling, because it enables removal of oxides on the surface of the steel sheet and further improves chemical conversion treatability and plating quality.
- pickling When heat-holding the hot-rolled steel sheet, in order to remove oxides on the surface of the steel sheet, it may be pickled once after cooling after heating and held, or may be pickled in multiple steps. good.
- the pickling When the pickling is performed plural times, it is preferable to carry out the pickling after cooling after heating and holding, because the oxide on the surface of the steel sheet can be removed more.
- pickling may be performed after cooling after each heating and holding.
- the heat treatment method may be either continuous annealing or batch annealing. Further, after the above heat treatment, it is cooled to room temperature, but its cooling method and cooling rate are not particularly specified, and any cooling such as furnace cooling in batch annealing, gas jet cooling in air cooling and continuous annealing, mist cooling, water cooling, etc. I do not care. Further, when the pickling treatment is performed, a conventional method may be used.
- the obtained steel sheet is cold rolled.
- the cold rolling rate is not limited, but is preferably 15 to 80%. By performing cold rolling in this range, a sufficiently recrystallized desired structure can be obtained and ductility is improved.
- the material is cooled to 50° C. or higher and 350° C. or lower, and is kept at the cooling stop temperature for 2 to 600 seconds.
- the technical idea of the present invention is to generate thin film austenite (nucleus of austenite that rarely contacts ferrite) in the structure before annealing.
- the film-shaped austenite is changed to lath-shaped austenite (austenite that is less in contact with ferrite), and Mn is concentrated in the lath-shaped austenite.
- film-like austenite which becomes lath-like austenite in the subsequent annealing is generated.
- the temperature is kept below 50° C., the martensitic transformation is completed, so that film-like austenite does not remain, and as a result, lath-like austenite cannot be obtained. Further, if the temperature is kept above 350° C., the film-form austenite is decomposed, and as a result, the lath-form austenite cannot be obtained. Therefore, when the temperature is kept in a temperature range lower than 50° C. and higher than 350° C., retained austenite, which is less likely to come into contact with ferrite, cannot be obtained, and in the subsequent annealing step, three or more grains are selected from the grain boundaries and the triple boundaries. A large amount of retained austenite having different crystal orientations is formed.
- the retained austenite in contact with the ferrite having three or more different crystal orientations increases with respect to the retained austenite less in contact with the ferrite having three or more different crystal orientations, and a desired structure is obtained.
- the holding time is less than 2 s
- retained austenite, which is less likely to come into contact with ferrite cannot be obtained, and a desired structure cannot be obtained.
- the retained austenite, which is less likely to come into contact with ferrite is decomposed, and the retained austenite in contact with ferrite having three or more different crystal orientations is also increased, so that a desired structure cannot be obtained.
- Ac 1 in transformation point or Ac 1 transformation point + 150 °C the following temperature range 20 ⁇ 1800 s holding Ac 1 transformation point or above Ac 1 transformation point + 150 °C below the temperature range 20 ⁇ 1800 s holding be, in the present invention, critical It is a feature of the invention.
- the carbide formed during the temperature rise remains undissolved, and it becomes difficult to secure a sufficient area ratio of martensite and retained austenite, and the strength decreases.
- the concentration of Mn in austenite is saturated, the retained austenite having a sufficient area ratio cannot be obtained, and the ductility decreases.
- the Ac 1 transformation point is preferably +100° C. or lower. Further, when it is retained for more than 1800 s, martensite increases, the strength is increased, and retained austenite having an area ratio sufficient to secure ductility cannot be obtained.
- Galvanizing treatment When performing the hot dip galvanizing treatment, the steel sheet subjected to the annealing treatment is dipped in a galvanizing bath at 440° C. or higher and 500° C. or lower to perform the hot dip galvanizing treatment, and then by gas wiping or the like. Adjust the coating weight. Note that it is preferable to use a zinc plating bath having an Al content of 0.08% or more and 0.30% or less for zinc plating.
- the conditions of other manufacturing methods are not particularly limited, but from the viewpoint of productivity, it is preferable to perform the above-mentioned annealing (heating and holding) in a continuous annealing facility. Further, it is preferable to perform a series of treatments such as annealing, hot dip galvanizing, and galvanizing alloying treatment by CGL (Continuous Galvanizing Line) which is a hot dip galvanizing line.
- CGL Continuous Galvanizing Line
- the above-mentioned "high-strength steel sheet” and “high-strength galvanized steel sheet” can be skin-pass rolled for the purpose of shape correction and surface roughness adjustment.
- the reduction rate of skin pass rolling is preferably in the range of 0.1% to 2.0%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit of the good range. Further, if it exceeds 2.0%, the productivity is remarkably reduced, so this is made the upper limit of the good range.
- the skin pass rolling may be performed online or offline. Further, the skin pass having a desired reduction rate may be performed at once, or may be performed in several times. Further, various coating treatments such as resin and oil coating can be applied.
- the hot dip galvanizing bath uses a zinc bath containing Al: 0.19 mass% in the hot dip galvanized steel sheet (GI), and the zinc bath containing Al: 0.14 mass% in the alloyed hot dip galvanized steel sheet (GA).
- GI hot dip galvanized steel sheet
- GA alloyed hot dip galvanized steel sheet
- the tensile test is performed according to JISZ 2241 (2011) using a JIS No. 5 test piece that is sampled so that the tensile direction is perpendicular to the rolling direction of the steel sheet, and TS (tensile strength), EL ( The total elongation) was measured.
- the mechanical properties were judged to be good in the following cases.
- TS 980 MPa or more and less than 1080 MPa
- EL 20% or more
- EL 16% or more
- TS: 1180 MPa or more less than 1270 MPa
- EL 12% or more
- plate thickness 1.0 ⁇ 1.8 mm.
- the hole expandability was measured according to JIS Z2256 (2010). After cutting each obtained steel plate into 100 mm x 100 mm, punch out a hole with a diameter of 10 mm with a clearance of 12% ⁇ 1%, or carve it into a hole with a diameter of 10 mm by reaming and then wrinkle it using a die with an inner diameter of 75 mm. With a pressing force of 9 tons, a 60° conical punch is pushed into the hole to measure the hole diameter at the crack initiation limit, and the limit hole expansion rate ⁇ (%) is calculated from the following formula. The hole expandability was evaluated from the value of.
- reaming is to finish the inner diameter machined with a drill by cutting and widening it with a cutting edge to a predetermined hole size and then rubbing the machined surface with a margin.
- Limit hole expansion rate ⁇ (%) ⁇ (D f ⁇ D 0 )/D 0 ⁇ 100
- D f is the hole diameter (mm) at the time of crack generation
- D 0 is the initial hole diameter (mm). In the present invention, the following cases were judged to be good for each TS range.
- the bending direction is a bending axis (Bending direction) from each annealed steel sheet.
- a bending test piece having a width of 30 mm and a length of 100 mm was sampled so that the measurement was carried out based on the V-block method of JISZ 2248 (1996).
- the bending property of the steel sheet was determined to be good when the limit bending R/t at 90° V bending of 2.5 or less (t: thickness of steel sheet (mm)) was satisfied.
- the chemical conversion treatability is obtained by performing a chemical conversion treatment on the obtained cold-rolled steel sheet using a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) manufactured by Nippon Parkerizing Co., Ltd. by the following method to form a chemical conversion coating. The processability was evaluated.
- a chemical conversion treatment liquid Palbond L3080 (registered trademark) manufactured by Nippon Parkerizing Co., Ltd.
- the cold-rolled steel sheet obtained is degreased using a degreasing liquid Fine Cleaner (registered trademark) manufactured by Nippon Parkerizing Co., Ltd., and then washed with water, and then a surface conditioning liquid prepalene Z (registered trademark) manufactured by Nippon Parkerizing Co. Was used for surface conditioning for 30 seconds.
- the cold-rolled steel sheet whose surface was adjusted was immersed in a chemical conversion treatment liquid (Palbond L3080 (registered trademark)) at 43° C. for 120 seconds, then washed with water and dried with warm air.
- a chemical conversion treatment liquid Palbond L3080 (registered trademark)
- Platability was evaluated by appearance. There is no appearance defect such as non-plating, uneven alloying, or other defects that impair the surface quality, and the appropriate surface quality is secured ⁇ , especially when it has an excellent appearance without uneven color tone, ⁇ , When a slight defect was observed, it was determined as ⁇ , and when many surface defects were observed, it was determined as ⁇ .
- the high-strength steel sheets of the examples of the present invention each have a TS of 980 MPa or more, and high-strength steel sheets with excellent formability are obtained.
- TS 980 MPa
- EL EL
- ⁇ bendability
- chemical conversion treatment property chemical conversion treatment property
- plating property is inferior.
- a high-strength steel sheet having a TS (tensile strength) of 980 MPa or more and excellent formability can be obtained.
- TS tensile strength
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Abstract
Description
[1]質量%で、C:0.030%~0.250%、Si:0.01%~3.00%、Mn:2.50%~8.00%、P:0.001%~0.100%、S:0.0001%~0.0200%、N:0.0005%~0.0100%、Al:0.001%~2.000%を含有し、残部がFeおよび不可避的不純物からなる成分組成と、面積率で、フェライトが35%以上80%以下、焼入れマルテンサイトが5%以上35%以下であり、焼戻しマルテンサイトが0.1%以上3.0%未満であり、面積率で残留オーステナイトが8%以上であり、さらに、前記フェライトの平均結晶粒径が6μm以下、前記残留オーステナイトの平均結晶粒径が3μm以下であり、前記残留オーステナイト中の平均Mn量(質量%)を前記フェライト中の平均Mn量(質量%)で除した値が1.5以上であり、さらに、粒径が円相当径で3μm以上の焼入れマルテンサイトと、円相当径で3μm以上の残留オーステナイトの面積率との和を全焼入れマルテンサイトと全残留オーステナイトとの面積率の和で除した値が0.4未満であり、異なる結晶方位をもつ三つ以上のフェライトと接する、残留オーステナイトの面積率を全残留オーステナイトの面積率で除した値が0.6未満である鋼組織と、を有する、高強度鋼板。
[2]前記成分組成が、さらに、質量%で、Ti:0.005%~0.200%、Nb:0.005%~0.200%、V:0.005%~0.500%、W: 0.005%~0.500%、B:0.0003%~0.0050%、Ni:0.005%~1.000%、Cr:0.005%~1.000%、Mo:0.005%~1.000%、Cu:0.005%~1.000%、Sn:0.002%~0.200%、Sb: 0.002%~0.200%、Ta:0.001%~0.100%、Ca:0.0005%~0.0050%、Mg:0.0005%~0.0050%、Zr:0.0005%~0.0050%、REM:0.0005%~0.0050%のうちから選ばれる少なくとも1種の元素を含有する、[1]に記載の高強度鋼板。
[3]表面に亜鉛めっき層を有する、[1]または[2]に記載の高強度鋼板。
[4]前記亜鉛めっき層が、合金化溶融亜鉛めっき層である、[3]に記載の高強度鋼板。
[5][1]、または[2]に記載の成分組成を有する鋼スラブを、加熱し、仕上げ圧延出側温度を750℃以上1000℃以下で熱間圧延し、300℃以上750℃以下で巻き取り、冷間圧延を施し、その後、Ac3変態点以上の温度域で20~1800s保持後、50℃以上350℃以下の範囲内の冷却停止温度まで冷却し、前記冷却停止温度で2s~600s保持後冷却し、その後、Ac1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持後、冷却する、高強度鋼板の製造方法。
[6][1]、または[2]に記載の成分組成を有する鋼スラブを、加熱し、仕上げ圧延出側温度を750℃以上1000℃以下で熱間圧延し、300℃以上750℃以下で巻き取り、冷間圧延を施し、その後、Ac3変態点以上の温度域で20~1800s保持後、50℃以上350℃以下の範囲内の冷却停止温度まで冷却し、前記冷却停止温度で2s~600s保持後冷却し、その後、Ac1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持後、冷却した後、更にAc1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持後、冷却する、高強度鋼板の製造方法。
[7]巻き取り後、Ac1変態点以下の温度域で1800s超保持する、[5]又は[6]に記載の高強度鋼板の製造方法。
[8]更に亜鉛めっき処理を施す、[5]~[7]のいずれかに記載の高強度鋼板の製造方法。
[9] 前記亜鉛めっき処理後、450℃~600℃で合金化処理を施す、[8]に記載の高強度鋼板の製造方法。
Cは、マルテンサイトを生成させて、強度を上昇させるために必要な元素である。また、Cは、残留オーステナイトの安定性を向上させ、鋼の延性を向上させるのに有効な元素である。C含有量が0.030%未満では所望のマルテンサイトの面積率を確保することが難しく、所望の強度が得られない。また、十分な残留オーステナイトの面積率を確保することが難しく、良好な延性が得られない。一方、Cを、0.250%を超えて過剰に含有すると、硬質なマルテンサイトの面積率が過大となり、穴広げ試験時に、マルテンサイトの結晶粒界でのマイクロボイドが増加し、さらに、亀裂の伝播が進行してしまい、穴広げ性が低下する。また、溶接部および熱影響部の硬化が著しく、溶接部の機械的特性が低下するため、スポット溶接性、アーク溶接性などが劣化する。こうした観点からC量を、0.030%以上0.250%以下とする。C含有量は、好ましくは、0.080%以上である。C含有量は、好ましくは、0.200%以下である。以下、硬質なマルテンサイトとは、焼入れマルテンサイト(焼入れたままのマルテンサイト)を指す。
Siは、フェライトの加工硬化能を向上させるため、良好な延性の確保に有効である。Si量が0.01%に満たないとその含有効果が乏しくなるため、下限を0.01%とした。しかしながら、3.00%を超えるSiの過剰な含有は、鋼の脆化を引き起こすばかりか赤スケールなどの発生による表面性状の劣化を引き起こす。さらに、化成処理性および、めっき品質の低下を招く。そのため、Si含有量は0.01%以上3.00%以下とする。Si含有量は、好ましくは、0.20%以上である。Si含有量は、好ましくは、2.00%以下、より好ましくは、0.70%未満である。
Mnは、本発明において極めて重要な含有元素である。Mnは、残留オーステナイトを安定化させる元素で、良好な延性の確保に有効であり、さらに、固溶強化により鋼の強度を上昇させる元素である。このような作用は、鋼のMn含有量が2.50%以上で認められる。ただし、Mn含有量が8.00%を超える過剰な含有は、化成処理性およびめっき品質を悪化させる。こうした観点からMn含有量を、2.50%以上8.00%以下とする。Mn含有量は、好ましくは、3.10%以上、より好ましくは、3.20%以上である。Mn含有量は、好ましくは、6.00%以下、より好ましくは、4.20%以下である。
Pは、固溶強化の作用を有し、所望の強度に応じて含有できる元素である。また、Pは、フェライト変態を促進するために複合組織化にも有効な元素である。こうした効果を得るためには、P含有量を0.001%以上にする必要がある。一方、P含有量が0.100%を超えると、溶接性の劣化を招くとともに、亜鉛めっきを合金化処理する場合には、合金化速度を低下させ、亜鉛めっきの品質を損なう。したがって、P含有量は0.001%以上0.100%以下とする。P含有量は、好ましくは0.005%以上とする。P含有量は、好ましくは0.050%以下とする。
Sは、粒界に偏析して熱間加工時に鋼を脆化させるとともに、硫化物として存在して局部変形能を低下させる。そのため、S含有量は0.0200%以下、好ましくは0.0100%以下、より好ましくは0.0050%以下とする必要がある。しかし、生産技術上の制約から、S含有量は0.0001%以上にする必要がある。したがって、S含有量は0.0001以上0.0200%以下とする。S含有量は、好ましくは0.0001%以上とする。S含有量は、好ましくは0.0100%以下、より好ましくは0.0050%以下とする。
Nは、鋼の耐時効性を劣化させる元素である。特に、N含有量が0.0100%を超えると、耐時効性の劣化が顕著となる。N含有量は少ないほど好ましいが、生産技術上の制約から、N含有量は0.0005%以上にする必要がある。したがって、N含有量は0.0005%以上0.0100%とする。N含有量は、好ましくは0.0010%以上とする。N含有量は、好ましくは0.0070%以下とする。
Alは、フェライトとオーステナイトの二相域を拡大させ、機械的特性の焼鈍温度依存性の低減、つまり、材質安定性に有効な元素である。Al含有量が0.001%に満たないとその含有効果に乏しくなるので、下限を0.001%とした。また、Alは、脱酸剤として作用し、鋼の清浄度に有効な元素であり、脱酸工程で添加することが好ましい。しかし、2.000%を超える多量の含有は、連続鋳造時の鋼片割れ発生の危険性が高まり、製造性を低下させる。こうした観点からAl含有量は、0.001%以上2.000%以下とする。Al含有量は、好ましくは、0.200%以上である。Al含有量は、好ましくは、1.200%以下である。
上記成分以外の成分は、Feおよび不可避的不純物である。なお、上記成分を下限値未満で含む場合、下限値未満で含まれる成分は不可避的不純物として含まれるものとする。
Tiは、鋼の析出強化に有効であり、フェライトの強度を向上させることで硬質第2相(マルテンサイトもしくは残留オーステナイト)との硬度差を低減でき、良好な穴広げ性を確保可能である。その効果は0.005%以上で得られる。しかし、0.200%を超えると、硬質なマルテンサイトの面積率が過大となり、穴広げ試験時に、マルテンサイトの結晶粒界でのマイクロボイドが増加し、さらに、亀裂の伝播が進行してしまい、穴広げ性(打ち抜き)が低下する場合がある。従って、Tiを含有する場合には、その含有量を0.005%以上0.200%以下とする。Ti含有量は、好ましくは0.010%以上とする。Ti含有量は、好ましくは0.100%以下とする。
Nb、V、Wは、鋼の析出強化に有効で、その効果はそれぞれ0.005%以上で得られる。また、Ti含有の効果と同様に、フェライトの強度を向上させることで、硬質第2相(マルテンサイトもしくは残留オーステナイト)との硬度差を低減でき、良好な穴広げ性を確保可能である。その効果は、Nb、V、Wのそれぞれが0.005%以上で得られる。しかし、Nbは0.200%、V、Wは0.500%を超えると、硬質なマルテンサイトの面積率が過大となり、穴広げ試験時に、マルテンサイトの結晶粒界でのマイクロボイドが増加し、さらに、亀裂の伝播が進行してしまい、穴広げ性が低下する場合がある。従って、Nbを含有する場合には、その含有量は0.005%以上0.200%以下とする。Nb含有量は、好ましくは0.010%以上とする。Nb含有量は、好ましくは0.100%以下とする。V、Wを含有する場合は、その含有量は0.005%以上0.500%以下とする。V、W含有量は、好ましくは,0.010%以上とする。V、W含有量は、好ましくは0.300%以下とする。
Bは、オーステナイト粒界からのフェライトの生成および成長を抑制する作用を有し、フェライトの強度を向上させることで、硬質第2相(マルテンサイトもしくは残留オーステナイト)との硬度差を低減でき、良好な穴広げ性を確保可能である。その効果は、0.0003%以上で得られる。しかし、0.0050%を超えると成形性が低下する場合がある。従って、Bを含有する場合には、その含有量は0.0003%以上0.0050%以下とする。B含有量は、好ましくは0.0005%以上とする。B含有量は、好ましくは0.0030%以下とする。
Niは、残留オーステナイトを安定化させる元素で、良好な延性の確保に有効であり、さらに、固溶強化により鋼の強度を上昇させる元素である。その効果は、0.005%以上で得られる。一方、1.000%を超えて含有すると、硬質なマルテンサイトの面積率が過大となり、穴広げ試験時に、マルテンサイトの結晶粒界でのマイクロボイドが増加し、さらに、亀裂の伝播が進行してしまい、穴広げ性が低下する場合がある。従って、Niを含有する場合には、その含有量は、0.005%以上1.000%以下とする。Ni含有量は、好ましくは0.010%以上とする。Ni含有量は、好ましくは0.500%以下とする。
Cr、Moは、強度と延性のバランスを向上させる作用を有するので必要に応じて含有することができる。その効果は、Cr:0.005%以上、Mo:0.005%以上で得られる。しかしながら、それぞれCr:1.000%、Mo:1.000%を超えて過剰に含有すると、硬質なマルテンサイトの面積率が過大となり、穴広げ試験時に、マルテンサイトの結晶粒界でのマイクロボイドが増加し、さらに、亀裂の伝播が進行してしまい、穴広げ性が低下する場合がある。従って、これらの元素を含有する場合には、その量をそれぞれCr:0.005%以上1.000%以下、Mo:0.005%以上1.000%以下とする。Cr含有量は、好ましくは0.010%以上とする。Cr含有量は、好ましくは0.500%以下とする。Mo含有量は、好ましくは0.010%以上とする。Mo含有量は、好ましくは0.500%以下とする。
Cuは、鋼の強化に有効な元素である。その効果は、0.005%以上で得られる。一方、1.000%を超えて含有すると、硬質なマルテンサイトの面積率が過大となり、穴広げ試験時に、マルテンサイトの結晶粒界でのマイクロボイドが増加し、さらに、亀裂の伝播が進行してしまい、穴広げ性が低下する場合がある。従って、Cuを含有する場合には、その量を:0.005%以上1.000%以下とする。Cu含有量は、好ましくは0.010%以上とする。Cu含有量は、好ましくは0.500%以下とする。
SnおよびSbは、鋼板表面の窒化や酸化によって生じる鋼板表層の数十μm程度の領域の脱炭を抑制する観点から、必要に応じて含有する。Snを0.002%以上、もしくはSbを0.002%以上含有することにより、このような窒化や酸化を抑制し、鋼板表面においてマルテンサイトの面積率が減少するのを防止する。その結果、強度や材質安定性の確保に有効である。一方で、これらいずれの元素についても、0.200%を超えて過剰に含有すると靭性の低下を招く。従って、SnおよびSbを含有する場合には、その含有量は、それぞれ0.002%以上0.200%以下とする。SnおよびSb含有量は、それぞれ、好ましくは0.004%以上とする。SnおよびSb含有量は、それぞれ、好ましくは0.050%以下とする。ここで、マルテンサイトとは、焼入れマルテンサイトを示す。
Taは、TiやNbと同様に、合金炭化物や合金炭窒化物を生成して高強度化に寄与する。加えて、Nb炭化物やNb炭窒化物に一部固溶し、(Nb、Ta)(C、N)のような複合析出物を生成することで析出物の粗大化を著しく抑制し、析出強化による強度への寄与を安定化させる効果があると考えられる。このため、Taを含有することが好ましい。ここで、前述の析出物安定化の効果は、Taの含有量を0.001%以上とすることで得られる。一方で、Taを過剰に含有しても析出物安定化効果が飽和する上、合金コストも増加する。従って、Taを含有する場合には、その含有量は、0.001%以上0.100%以下とする。Ta含有量は、好ましくは0.005%以上とする。Ta含有量は、好ましくは0.050%以下とする。
Ca、Mg、ZrおよびREMは、硫化物の形状を球状化し、穴広げ性への硫化物の悪影響をさらに改善するために有効な元素である。この効果を得るためには、それぞれ0.0005%以上含有することが好ましい。しかしながら、それぞれ0.0050%を超える過剰な含有は、介在物等の増加を引き起こし表面および内部欠陥などを引き起こす。従って、Ca、Mg、ZrおよびREMを含有する場合は、その含有量はそれぞれ0.0005%以上0.0050%以下とする。Ca、Mg、ZrおよびREM含有量は、それぞれ、好ましくは0.0010%以上とする。Ca、Mg、ZrおよびREM含有量は、それぞれ、好ましくは0.0040%以下とする。
十分な延性を確保するため、フェライトの面積率を35%以上にする必要がある。また、980MPa以上の引張強度確保のため、軟質なフェライトの面積率を80%以下にする必要がある。なお、ここで云うフェライトとは、ポリゴナルフェライトやグラニュラーフェライトやアシキュラーフェライトを指し、比較的軟質で延性に富むフェライトのことである。フェライトの面積率は、好ましくは、40%以上である。フェライトの面積率は、好ましくは、75%以下である。
980MPa以上のTSを達成するためには、焼入れマルテンサイトの面積率を5%以上にする必要がある。また、良好な延性の確保のため、焼入れマルテンサイトの面積率を35%以下にする必要がある。焼入れマルテンサイトの面積率は好ましくは5%以上である。焼入れマルテンサイトの面積率は、好ましくは、30%以下である。
焼戻しマルテンサイトは良好な穴広げ性を確保するために0.1%以上必要である。また、980MPa以上のTSを達成するためには焼戻しマルテンサイトの面積率を3.0%未満にする必要がある。焼戻しマルテンサイトの面積率は好ましくは、0.1%以上である。焼戻しマルテンサイトの面積率は、好ましくは、2.0%以下である。
十分な延性を確保するため、残留オーステナイトの面積率を8%以上にする必要がある。好ましくは12%以上である。
フェライトの結晶粒の微細化は、TSの向上に寄与する。そのため、所望のTSを確保するため、フェライトの平均結晶粒径を6μm以下にする必要がある。好ましくは、5μm以下である。
残留オーステナイトの結晶粒の微細化は延性と穴広げ性の向上に寄与する。そのため、良好な延性、穴広げ性を確保するためには、残留オーステナイトの平均結晶粒径を3μm以下にする必要がある。好ましくは、2.5μm以下である。
残留オーステナイト中の平均Mn量(質量%)をフェライト中の平均Mn量(質量%)で除した値が1.5以上であることは、本発明において極めて重要な構成要件である。良好な延性を確保するためには、Mnが濃化した安定な残留オーステナイトの面積が高い必要がある。好ましくは2.0以上である。
円相当径で3μm以上の焼入れマルテンサイトと、円相当径で3μm以上の残留オーステナイトの面積率の和を全焼入れマルテンサイトと全残留オーステナイトの面積率の和で除した値が0.4未満であることは本発明において重要な構成要件である。焼入れマルテンサイトと残留オーステナイトの結晶粒径の微細化は穴広げ性の向上に寄与する。良好な穴広げ性を確保するためには、高強度と高延性を得るのに十分な焼入れマルテンサイトと残留オーステナイトの面積率を確保した上で、微細な焼入れマルテンサイトと残留オーステナイトの面積率を多くする必要がある。好ましくは0.3%未満、より好ましくは0.2未満である。
異なる結晶方位をもつ三つ以上のフェライトと接する、つまり、フェライト粒界の3重点に位置する残留オーステナイトの面積率を全残留オーステナイトの面積率で除した値が0.6未満であることは本発明において重要な構成要件である。残留オーステナイトに接する異なる結晶方位をもつフェライト粒の数が少なければ、穴広げ変形および曲げ変形時の応力集中が緩和されるため穴広げ性と曲げ性の向上に寄与する。そのため、良好な穴広げ性を確保するためには異なる結晶方位を持つ三つ以上のフェライトと接する残留オーステナイトの面積率は低い必要がある。好ましくは0.5未満である。
特に限定はしないが、鋼スラブの加熱温度は1100℃以上1300℃以下にすることが好ましい。鋼スラブの加熱段階で存在している析出物は、最終的にえられる鋼板内では粗大な析出物として存在し、強度に寄与しないため、鋳造時に析出したTi、Nb系析出物を再溶解させることが可能である。さらに、鋼スラブ表層の気泡、偏析などをスケールオフし、鋼板表面の亀裂、凹凸をより減少し、より平滑な鋼板表面を達成する観点からも、鋼スラブの加熱温度は1100℃以上にすることが好ましい。一方、酸化量の増加に伴うスケールロスを減じる観点から、鋼スラブの加熱温度は1300℃以下にすることが好ましい。鋼スラブの加熱温度は、より好ましくは、1150℃以上とする。鋼スラブの加熱温度は、より好ましくは、1250℃以下とする。
加熱後の鋼スラブは、粗圧延および仕上げ圧延により熱間圧延され熱延鋼板となる。このとき、仕上げ温度が1000℃を超えると、酸化物(スケール)の生成量が急激に増大し、地鉄と酸化物の界面が荒れ、酸洗、冷間圧延後の表面品質が劣化する傾向にある。また、酸洗後に熱延スケールの取れ残りなどが一部に存在すると、延性や穴広げ性に悪影響を及ぼす。さらに、結晶粒径が過度に粗大となり、加工時にプレス品表面荒れを生じる場合がある。一方、仕上げ温度が750℃未満では圧延荷重が増大し、圧延負荷が大きくなることや、オーステナイトが未再結晶状態での圧下率が高くなる。その結果、所望の結晶粒径が得られず、異常な集合組織が発達し、最終製品における面内異方性が顕著となり、材質の均一性(材質安定性)が損なわれるだけでなく、延性そのものも低下する。従って、熱間圧延の仕上げ圧延出側温度を750℃以上1000℃以下にする必要がある。熱間圧延の仕上げ圧延出側温度は好ましくは800℃以上とする。熱間圧延の仕上げ圧延出側温度は好ましくは950℃以下とする。
熱間圧延後の巻き取り温度が750℃を超えると、熱延板組織のフェライトの結晶粒径が大きくなり、最終焼鈍板の所望の強度確保が困難となる。一方、熱間圧延後の巻き取り温度が300℃未満では、熱延板強度が上昇し、冷間圧延における圧延負荷が増大したり、板形状の不良が発生したりするため、生産性が低下する。従って、熱間圧延後の巻き取り温度を300℃以上750℃以下にする必要がある。熱間圧延後の巻き取り温度は好ましくは400℃以上とする。熱間圧延後の巻き取り温度は好ましくは650℃以下とする。
Ac1変態点以下の温度域で、1800s超保持することは、続く冷間圧延を施すための鋼板を軟質化させるので好ましい。
Ac1変態点以下の温度域で保持する場合、オーステナイト中にMnが濃化し、冷却後、硬質なマルテンサイトと残留オーステナイトが生成する。その結果、円相当径で3μm以上の焼入れマルテンサイトと、円相当径で3μm以上の残留オーステナイトの面積率の和を全焼入れマルテンサイトと全残留オーステナイトの面積率の和で除した値が0.30未満の好適条件が得られる。また、1800s未満で保持する場合、熱間圧延後のひずみが除去できず、鋼板の軟質化がなされない場合がある。
得られた鋼板に冷間圧延を施す。冷間圧延率に制限はないが、15~80%が好ましい。この範囲にて冷間圧延を施すことにより、十分に再結晶した所望の組織が得られ、延性が向上する。
Ac3変態点未満の温度域および20s未満で保持する場合、十分な再結晶が行われず、所望の組織が得られないため、λ(打ち抜き)および曲げ性が低下する。また、その後のめっき品質確保のためのMn表面濃化が十分に行われない。一方、1800sを超えて保持する場合、Mn表面濃化が飽和する。
本発明の技術思想は、焼鈍前組織に薄いフィルム状オーステナイト(フェライトと接することの少ないオーステナイトの核)を生成させることで、続く焼鈍工程で当該フィルム状オーステナイトをラス状オーステナイト(フェライトと接することの少ないオーステナイト)にし、当該ラス状オーステナイト中にMnを濃化させるというものである。50℃以上350℃以下に冷却・保持することで、その後の焼鈍でラス状オーステナイトとなるフィルム状オーステナイトを生成させる。50℃未満の保持ではマルテンサイト変態が完了してしまうため、フィルム状オーステナイトが残らず、結果としてラス状オーステナイトも得られないことになる。また、350℃超の保持では、フィルム状オーステナイトが分解してしまうため、結果としてラス状オーステナイトも得られないことになる。したがって、50℃未満および350℃超の温度域で保持する場合、フェライトと接することが少なくなる残留オーステナイトを得ることが出来ず、その後の焼鈍工程において、粒界や粒界3重点から三つ以上の異なる結晶方位を持つ残留オーステナイトが多く形成されてしまう。その結果、三つ以上の異なる結晶方位をもつフェライトに接する残留オーステナイトが、上述の三つ以上の異なる結晶方位をもつフェライトに接することが少なくなる残留オーステナイトに対して増加し、所望の組織が得られない。また、2s未満で保持する場合も同じく、フェライトと接することが少なくなる残留オーステナイトを得ることが出来ず、所望の組織が得られない。さらに600sを超えて保持する場合、フェライトと接することが少なくなる残留オーステナイトが分解し、同様に三つ以上の異なる結晶方位をもつフェライトに接する残留オーステナイトが増加し、所望の組織が得られない。
Ac1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持することは、本発明において、極めて重要な発明構成要件である。Ac1変態点未満の温度域および20s未満で保持する場合、昇温中に形成される炭化物が溶け残り、十分な面積率のマルテンサイトと残留オーステナイトの確保が困難となり、強度が低下する。また、Ac1変態点+150℃を超える温度域ではオーステナイト中へのMn濃化が飽和し、十分な面積率の残留オーステナイトを得られず延性が低下する。好ましくは、Ac1変態点+100℃以下である。さらに、1800sを超えて保持する場合、マルテンサイトが増加し高強度化、また、延性確保のための十分な面積率の残留オーステナイトを得ることができない。
溶融亜鉛めっき処理を施す場合には、前記焼鈍処理を施した鋼板を440℃以上500℃以下の亜鉛めっき浴中に浸漬し、溶融亜鉛めっき処理を施し、その後、ガスワイピング等によって、めっき付着量を調整する。なお、亜鉛めっきにはAl量が0.08%以上0.30%以下である亜鉛めっき浴を用いることが好ましい。
Ac3変態点(℃)=910-203√(%C)+45×(%Si)-30×(%Mn)-20×(%Cu)-15×(%Ni)+11×(%Cr)+32×(%Mo)+104×(%V)+400×(%Ti)+200×(%Al)
ここで、(%C)、(%Si)、(%Mn)、(%Ni)、(%Cu)、(%Cr)、(%Mo)、(%V)、(%Ti)、(%Al)は、それぞれの元素の含有量(質量%)である。
TS:980MPa以上1080MPa未満の場合、EL:20%以上
TS:1080MPa以上1180MPa未満の場合、EL:16%以上
TS:1180MPa以上1270MPa未満の場合、EL:12%以上
但し、板厚:1.0~1.8mm。
ただし、Dfは亀裂発生時の穴径(mm)、D0は初期穴径(mm)である。なお、本発明では、TS範囲ごとに下記の場合を良好と判断した。
TS:980MPa以上1080MPa未満の場合、(打ち抜き)λ:15%以上、(リーマ加工)λ:40%以上
TS:1080MPa以上1180MPa未満の場合、(打ち抜き)λ:12%以上、(リーマ加工)λ:35%以上
TS:1180MPa以上1270MPa未満の場合、(打ち抜き)λ:10%以上、(リーマ加工)λ:30%以上
曲げ試験は、各焼鈍鋼板から、圧延方向が曲げ軸(Bending direction)となるように幅30mm、長さ100mmの曲げ試験片を採取し、JISZ 2248(1996年)のVブロック法に基づき測定を実施した。押し込み速度100mm/秒、各曲げ半径でn=3の試験を実施し、曲げ部外側について実体顕微鏡で亀裂の有無を判定し、亀裂が発生していない最小の曲げ半径を限界曲げ半径R(mm)とした。なお、本発明では、90°V曲げでの限界曲げR/t:2.5以下(t:鋼板の板厚(mm))を満足する場合を、鋼板の曲げ性が良好と判定した。
評点5:5%以下
評点4:5%超10%以下
評点3:10%超25%以下
評点2:25%超40%以下
評点1:40%超
評点4または評点5であれば化成処理性が良好と言える。なかでも、評点5であることが好ましい。
Claims (9)
- 質量%で、
C:0.030%~0.250%、
Si:0.01%~3.00%、
Mn:2.50%~8.00%、
P:0.001%~0.100%、
S:0.0001%~0.0200%、
N:0.0005%~0.0100%、
Al:0.001%~2.000%を含有し、残部がFeおよび不可避的不純物からなる成分組成と、
面積率で、フェライトが35%以上80%以下、焼入れマルテンサイトが5%以上35%以下、焼戻しマルテンサイトが0.1%以上3.0%未満であり、面積率で残留オーステナイトが8%以上であり、さらに、前記フェライトの平均結晶粒径が6μm以下、前記残留オーステナイトの平均結晶粒径が3μm以下であり、
前記残留オーステナイト中の平均Mn量(質量%)を前記フェライト中の平均Mn量(質量%)で除した値が1.5以上であり、
さらに、粒径が円相当径で3μm以上の焼入れマルテンサイトと、円相当径で3μm以上の残留オーステナイトの面積率との和を全焼入れマルテンサイトと全残留オーステナイトとの面積率の和で除した値が0.4未満であり、
異なる結晶方位をもつ三つ以上のフェライトと接する、残留オーステナイトの面積率を全残留オーステナイトの面積率で除した値が0.6未満である鋼組織と、を有する、高強度鋼板。 - 前記成分組成が、さらに、質量%で、
Ti:0.005%~0.200%、
Nb:0.005%~0.200%、
V:0.005%~0.500%、
W:0.005%~0.500%、
B:0.0003%~0.0050%、
Ni:0.005%~1.000%、
Cr:0.005%~1.000%、
Mo:0.005%~1.000%、
Cu:0.005%~1.000%、
Sn:0.002%~0.200%、
Sb: 0.002%~0.200%、
Ta:0.001%~0.100%、
Ca:0.0005%~0.0050%、
Mg:0.0005%~0.0050%、
Zr:0.0005%~0.0050%、
REM:0.0005%~0.0050%のうちから選ばれる少なくとも1種の元素を含有する、請求項1に記載の高強度鋼板。 - 表面に亜鉛めっき層を有する、請求項1または2に記載の高強度鋼板。
- 前記亜鉛めっき層が、合金化溶融亜鉛めっき層である、請求項3に記載の高強度鋼板。
- 請求項1、または2に記載の成分組成を有する鋼スラブを、加熱し、仕上げ圧延出側温度を750℃以上1000℃以下で熱間圧延し、300℃以上750℃以下で巻き取り、冷間圧延を施し、その後、Ac3変態点以上の温度域で20~1800s保持後、50℃以上350℃以下の範囲内の冷却停止温度まで冷却し、前記冷却停止温度で2s~600s保持後冷却し、その後、Ac1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持後、冷却する、高強度鋼板の製造方法。
- 請求項1、または2に記載の成分組成を有する鋼スラブを、加熱し、仕上げ圧延出側温度を750℃以上1000℃以下で熱間圧延し、300℃以上750℃以下で巻き取り、冷間圧延を施し、その後、Ac3変態点以上の温度域で20~1800s保持後、50℃以上350℃以下の範囲内の冷却停止温度まで冷却し、前記冷却停止温度で2s~600s保持後冷却し、その後、Ac1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持後、冷却した後、更にAc1変態点以上Ac1変態点+150℃以下の温度域で20~1800s保持後、冷却する、高強度鋼板の製造方法。
- 巻き取り後、Ac1変態点以下の温度域で1800s超保持する、請求項5又は6に記載の高強度鋼板の製造方法。
- 更に亜鉛めっき処理を施す、請求項5~7のいずれかに記載の高強度鋼板の製造方法。
- 前記亜鉛めっき処理後、450℃~600℃で合金化処理を施す、請求項8に記載の高強度鋼板の製造方法。
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