WO2021106936A1 - ホットスタンプ成形品及びホットスタンプ用鋼板 - Google Patents
ホットスタンプ成形品及びホットスタンプ用鋼板 Download PDFInfo
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
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- C25D7/00—Electroplating characterised by the article coated
- C25D7/06—Wires; Strips; Foils
- C25D7/0614—Strips or foils
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- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
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- B32B15/00—Layered products comprising a layer of metal
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- C21D2211/00—Microstructure comprising significant phases
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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- C25D5/34—Pretreatment of metallic surfaces to be electroplated
- C25D5/36—Pretreatment of metallic surfaces to be electroplated of iron or steel
Definitions
- the present invention relates to a hot stamped molded product and a steel plate for hot stamping.
- the present application claims priority based on Japanese Patent Application No. 2019-213593 filed in Japan on November 26, 2019, the contents of which are incorporated herein by reference.
- Hot stamping parts are used for skeletal parts of the vehicle body, and are often used as reinforcing parts around the cabin to secure a living space for occupants and as a route for transmitting a load at the time of a collision. In order to respond to the weight reduction of the vehicle body and the sophistication of collision tests for fuel efficiency regulation, hot stamping parts are also required to have higher strength. However, since the risk of hydrogen embrittlement cracking increases as the strength increases, measures for suppressing hydrogen embrittlement cracking are required.
- the microstructure is controlled and the cleanliness of inclusions is defined in order to achieve both hardness stability and delayed fracture resistance. Further, in the technique of Patent Document 2, the amount of precipitation of microstructure and inclusions is controlled in order to obtain tensile strength and toughness.
- Ca is an element added to steel to control the morphology of sulfides. Stretched MnS is suppressed by the formation of spherical CaS. It is said that toughness improves when stretched MnS is suppressed.
- the black portion is an inclusion containing CaO-Al 2 O 3
- the white portion is a steel matrix.
- the present invention has been made in view of such a situation, and an object of the present invention is to prevent hydrogen embrittlement cracking due to coarse inclusions containing CaO-Al 2 O 3 while maintaining a desired tensile strength. It is to provide a hot stamping molded product and a steel plate for hot stamping that can be used.
- the hot stamped product according to one aspect of the present invention has a mass% of C: 0.25% or more and 0.55% or less. Si: 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5 1 or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less.
- a hot stamped product having a chemical composition consisting of the balance Fe and impurities having a chemical composition consisting of the balance Fe and impurities.
- the tensile strength of the hot stamped molded product left in the air at room temperature of 20 to 30 ° C. for 48 hours or more exceeds 1600 MPa.
- the feature is that the major axis of the inclusions containing CaO-Al 2 O 3 contained in the hot stamped product is 50 ⁇ m or less.
- the hot stamp molded product according to one aspect of the present invention has a mass% of C: 0.25% or more and 0.55% or less.
- Si 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5 1 or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less.
- a hot stamped product having a chemical composition consisting of the balance Fe and impurities having a chemical composition consisting of the balance Fe and impurities.
- the tensile strength of the hot stamped molded product left in the air at room temperature of 20 to 30 ° C. for 48 hours or more exceeds 1600 MPa.
- Total elongation measured using a tensile test piece of a hot stamped product charged with hydrogen so that the uniform elongation is El 1 (%) and the amount of diffusible hydrogen is 0.5 ⁇ 0.1 wt ppm. When is El 2 (%), (El 2 / El 1 ) ⁇ 100 ⁇ 100 (%) is satisfied.
- the maximum length of MnS existing in the central portion of the plate thickness may be 300 ⁇ m or less.
- By mass% Ni + Cu + Sn: 0.005% or more and 2% or less may be contained.
- a plating layer may be provided on the surface.
- the steel sheet for hot stamping has a mass% of C: 0.25% or more and 0.55% or less. Si: 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5 1 or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less.
- a hot stamping steel sheet having a chemical composition consisting of the balance Fe and impurities After heating the above steel sheet for hot stamping at 950 ° C. for 1 minute, using a tensile test piece cooled to 200 ° C. or lower at an average cooling rate of 30 to 100 ° C./s, the atmosphere at room temperature of 20 to 30 ° C. for 48 hours or more.
- the tensile strength measured after being left in exceeds 1600 MPa,
- the feature is that the major axis of the inclusions containing CaO-Al 2 O 3 contained in the hot stamping steel sheet is 50 ⁇ m or less.
- the steel sheet for hot stamping according to one aspect of the present invention has a mass% of C: 0.25% or more and 0.55% or less.
- Si 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5 1 or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less.
- a hot stamping steel sheet having a chemical composition consisting of the balance Fe and impurities is heated at 950 ° C. for 1 minute and then left at room temperature of 20 to 30 ° C. for 48 hours or more using a tensile test piece cooled to 200 ° C. or lower at an average cooling rate of 30 to 100 ° C./s.
- the maximum length of MnS existing in the central portion of the plate thickness may be 300 ⁇ m or less.
- By mass% Ni + Cu + Sn: 0.005% or more and 2% or less may be contained.
- a plating layer may be provided on the surface.
- a hot stamped molded product and a hot stamping steel sheet that can prevent hydrogen embrittlement cracking while maintaining a desired tensile strength.
- the present inventors have found that when a material containing hydrogen is subjected to a tensile test, fracture occurs due to hydrogen embrittlement, and coarse inclusions containing CaO-Al 2 O 3 are present at the starting point. It was. Then, hydrogen embrittlement cracks originating from the above inclusions were confirmed.
- the present inventors conducted a tensile test on a material containing hydrogen, observed the fracture surface, and evaluated the size of inclusions that are the starting points of hydrogen embrittlement cracks. As a result, the major axis of such inclusions was 50 ⁇ m. I found it to be super. From this result, the present inventors have found that hydrogen embrittlement cracking can be prevented by defining the major axis of such inclusions to 50 ⁇ m or less.
- the hot stamp molded product according to the first embodiment has a mass% of C: 0.25% or more and 0.55% or less. Si: 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5 % Or less, one or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less, and having a chemical composition consisting of the balance Fe and
- the hot stamped product according to the first embodiment has a tensile strength of more than 1600 MPa and CaO contained in the hot stamped product, which has been left in the air at room temperature of 20 to 30 ° C. for 48 hours or more.
- the major axis of the inclusions containing Al 2 O 3 is 50 ⁇ m or less.
- C (C: 0.25% or more and 0.55% or less) C is an element added to secure the material with the structure after cooling as martensite, and it is necessary to add 0.25% or more in order to secure the strength of 1600 MPa or more. If the amount added is too large, it becomes difficult to secure the strength at the time of impact deformation, so the upper limit is set to 0.55%.
- Si 0.001% or more and 2.0% or less
- Si is a solid solution strengthened alloy element and is necessary for ensuring strength, but if it exceeds 2.0%, a problem of surface scale occurs. Therefore, Si is specified to be 2.0% or less.
- the upper limit is preferably 1.0%.
- the lower limit of the Si addition amount is set to 0.001%. More preferably, the amount of Si added is in the range of 0.01 to 0.5%.
- Mn 0.3% or more and 3.0% or less
- Mn is an element that improves strength and hardenability. If it is less than 0.2%, sufficient strength at the time of quenching cannot be obtained, and even if it is added in excess of 3.0%, the effect is saturated. Was specified in the range of 0.3 to 3.0%.
- the amount of Mn added is more preferably 2.2% or less.
- Al 0.005% or more and 1.0% or less
- Al is a necessary element used as a deoxidizing material for molten steel, and is also an element that fixes N, and its amount has a great influence on the crystal grain size and mechanical properties. In order to have such an effect, a content of 0.005% or more is required, but if it exceeds 1.0%, non-metal inclusions increase and surface defects are likely to occur in the product. Therefore, Al is specified in the range of 0.005 to 1.0%.
- the amount of Al added is more preferably 0.01% or more, and more preferably 0.5% or less.
- S affects non-metallic inclusions in steel, deteriorating toughness and hydrogen embrittlement resistance. Therefore, S is specified to be 0.003% or less. In addition, S is preferably 0.001% or less.
- P 0.02% or less Since P is an element that adversely affects toughness and hydrogen embrittlement resistance, P was restricted to 0.02% or less. In addition, P is preferably 0.015% or less. Further, more preferably, P is 0.010% or less.
- the chemical composition of the hot stamped product according to the present embodiment may further contain at least one selective element of Cr, Mo, Ca, Ni, Cu, and Sn instead of a part of Fe. ..
- These selective elements may be contained according to the purpose. Therefore, it is not necessary to limit the lower limit of these selective elements, and the lower limit may be 0%. Further, even if these selective elements are contained as impurities, the effects of these selective elements are not impaired.
- Cr 0% or more and 1.0% or less
- Cr is an element that improves hardenability. If it exceeds 1.0%, the carbides present after hot rolling, cold rolling or annealing (including after plating) are stabilized, and melting by heating with hot stamping is delayed to reduce hardenability. It is conceivable to do. Therefore, the upper limit of Cr is defined as 1.0%. The lower limit is not specified because it is possible to secure the hardenability required for hot stamping by adding elements that bring about hardenability such as Mn, Mo, and Ni.
- the amount of Cr added is more preferably 0.05% or more, and more preferably 1.0% or less.
- Mo 0% or more and 1.0% or less
- Mo is an element that improves hardenability. If it exceeds 1.0%, the carbides present after hot rolling, cold rolling or annealing (including after plating) are stabilized, and melting by heating with hot stamping is delayed to reduce hardenability. It is conceivable to do. Therefore, the upper limit of Mo is defined as 1.0%. The lower limit is not specified because it is possible to secure the hardenability required for hot stamping by adding an element that provides hardenability such as Mn and Ni.
- the amount of Mo added is more preferably 0.05% or more, and more preferably 0.5% or less.
- Ca 0% or more and 0.0010% or less
- Ca is not added in order to suppress the formation of CaO-Al 2 O 3 which can be the starting point of hydrogen embrittlement cracking.
- it may be mixed from the ladle slag during smelting or the powder used during casting. Therefore, it is necessary to suppress this, and the upper limit of Ca is limited to 0.0010%. Within this range, the possibility of forming coarse CaO-Al 2 O 3 that can be the starting point of hydrogen embrittlement cracking is low.
- a small amount of CaO-Al 2 O 3 may aggregate and coarsen, so it is important to use a refining method that prevents coarsening.
- B (B: 0.0005% or more and 0.01% or less) B is added in order to improve the hardenability during press molding or in cooling after press molding, but it is necessary to add 0.0005% or more in order to exert this effect. However, if the amount of this addition is increased unnecessarily, there is a concern of cracking in the heat and the effect is saturated. Therefore, the upper limit of B is preferably 0.01%.
- Ti, Nb, V, and Zr are added for the purpose of fixing B and N that forms a compound in order to effectively exert the effect of B.
- Ti, Nb, V, and Zr also have the effect of improving toughness and hydrogen embrittlement by forming carbides and nitrides and suppressing grain growth of austenite during heating to make them finer.
- the upper limit of these addition amounts is set to 0.5%.
- the amount of these elements added is 0.005% or more, respectively, from the viewpoint of the amount of addition required to exert the effect.
- N (N: 0.02% or less) If N exceeds 0.02%, the toughness tends to deteriorate due to the coarsening of the nitride. Therefore, N is preferably defined as 0.02% or less. N is more preferably 0.01% or less.
- the hot stamped article according to the first embodiment may contain Ni + Cu + Sn (total of Ni, Cu and Sn): 0% or more and 2% or less in mass% as a chemical composition.
- Ni, Cu and Sn can improve the plating adhesion by adding these elements. Furthermore, improvement in corrosion resistance can be expected. In order to obtain this effect, it is preferable to add 0.005% or more in the total amount of Ni, Cu and Sn. However, since excessive addition increases the alloy cost, the upper limit of the sum of the addition amounts of these elements is set to 2%. More preferably, Ni + Cu + Sn has an upper limit of 1.0% and a lower limit of 0.05%.
- Mg, Y, As, Sb, and REM are considered to change the shape of MnS, which is a main sulfide, to improve impact characteristics and delayed fracture characteristics, and may be added.
- the upper limit is preferably 0.1% or less. Further, even when an element not specified above such as Se or W is contained as another element, there is no big problem in the characteristics as long as it does not preferably exceed 0.1%.
- the hot stamped product according to this embodiment contains impurities.
- impurity refers to ore, scrap, or a substance mixed from the manufacturing environment or the like as a raw material when steel is industrially manufactured.
- P, S, O, and N are preferably limited as described above.
- the content of impurities is preferably small, it is not necessary to limit the lower limit value, and the lower limit value of impurities may be 0%.
- CaO-Al 2 O 3 is an inclusion that becomes a starting point in hydrogen embrittlement cracking that occurs in a tensile test when a certain amount or more of hydrogen is contained. This is confirmed by observing the fracture surface of the test piece by a tensile test.
- the stress applied to the inclusions depends on the major axis of the inclusions, and it is presumed that the larger the major axis, the higher the applied stress and the fractures of the inclusions, which becomes the starting point of hydrogen embrittlement cracking. That is, the length of the major axis of the inclusions is important, and it is considered that by limiting this, it is possible to suppress the fracture of the inclusions and suppress the hydrogen embrittlement cracking.
- the major axis of the inclusions is 50 ⁇ m or less, hydrogen embrittlement cracking does not occur, so it is preferable to specify the major axis of the inclusions as an upper limit of 50 ⁇ m. Further, the major axis of the inclusions is more preferably 30 ⁇ m or less in order to ensure the above effect.
- the major axis of the inclusions is obtained by observing the fracture surface of the test piece that has been fractured by the tensile test with an SEM (scanning electron microscope). Since the inclusions existing on the fracture surface broken in the tensile test are often broken and not all remain, the iron dents around the inclusions are regarded as the parts where the inclusions were present. The length of the inclusion can be regarded as the major axis of the inclusion. Further, when the thickness of the inclusions in the plate thickness direction is small, the major axis thereof can be defined by setting the range in which the inclusions are clogged in the recess as the inclusions.
- the major axis of inclusions can be measured directly from images and photographs taken using SEM.
- FIG. 2 is a schematic cross-sectional view for explaining inclusions that are the starting points of hydrogen embrittlement cracks, and represents a cross-sectional view of a plane perpendicular to the rolling direction in the steel material S of a hot stamped product. ing.
- the starting point of fracture by the tensile test is an inclusion in the center of the grain boundary fracture surface formed in an elliptical shape.
- the longer major axis of the inclusion is the starting point of fracture, and this major axis is the major axis of the inclusion.
- an inclusion A is present at the center of the grain boundary fracture surface SA
- an inclusion B is present at the center of the grain boundary fracture surface SB
- an inclusion C is present at the center of the grain boundary fracture surface SC.
- the hot stamped product according to the first embodiment has a tensile strength of 1600 MPa measured using a tensile test piece of the hot stamped product left in the air at room temperature of 20 to 30 ° C. for 48 hours or more. Exceed. By leaving the hardened hot stamped product in the air at room temperature of 20 to 30 ° C. for 48 hours or more, sufficient hydrogen is released from the inside of the hot stamped product.
- the maximum length of MnS existing in the central portion of the plate thickness may be 300 ⁇ m or less.
- the central portion of the plate thickness means a range from the center of the plate thickness to 1/3 of the plate thickness in the plate thickness direction of the hot stamped molded product.
- the maximum length of the MnS is more preferably 200 ⁇ m or less.
- MnS Since MnS is stretched for a long time during rolling, it is considered that when stress is applied, stress concentration occurs at the end thereof and becomes the starting point of hydrogen embrittlement cracking. As the MnS becomes longer, the stress generated at the end becomes larger, which promotes the occurrence of hydrogen embrittlement cracks. Further, since MnS is likely to occur in the central segregation portion, the precipitate having the largest size is present in the central portion of the plate thickness. Therefore, it is preferable to specify the maximum length of MnS existing in the central portion of the plate thickness to be 300 ⁇ m or less.
- the length of MnS can be obtained by mirror-polishing the cross section of the target steel material in the rolling direction and observing MnS at the center of the plate thickness with a metallurgical microscope at a magnification of 200 to 500 times. It can be confirmed that it is MnS by performing elemental analysis with SEM-EDS (scanning electron microscope equipped with EDS :) or EPMA (electron probe microanalyzer: Electron probe Micro Analyzer). It is desirable to carry out observation in about 10 fields of view.
- central segregation occurs in which solute elements are macro-concentrated.
- the central segregation is a collection of the surrounding concentrated molten steel in the central part due to the flow of the liquid phase generated by bulging, solidification shrinkage, and the like.
- solute elements are concentrated as compared with the normal solidification section, and coarse MnS is likely to be generated even if the S concentration is lowered.
- a method is known in which the slab is reduced to the extent that the solidification shrinkage is compensated, or a flow is generated in the molten steel by electromagnetic agitation to cause equiaxed crystallization.
- Either method can reduce central segregation and suppress the formation of coarse MnS. Further, if the S concentration is reduced to 0.0030% by mass or less by implementing such a central segregation countermeasure, coarse MnS that causes a problem is hardly generated. In addition, if the S concentration can be reduced to 0.0010% by mass or less, the formation of MnS itself can be suppressed.
- the hot stamped molded product according to the first embodiment may have a plating layer on the surface.
- Aluminum plating, aluminum-zinc plating, or zinc plating may be applied to the steel sheet for the hot stamped product as described above.
- the plating composition is mainly composed of aluminum and zinc, an element such as Ni may be added to improve the characteristics. Further, an element such as Fe may be contained as an impurity.
- pickling and cold rolling may be carried out by a conventional method, and the subsequent aluminum plating step, aluminum-zinc plating step, and zinc plating step may also be carried out by a conventional method. It's fine.
- a Si concentration in the bath of 5 to 12% is suitable, and for aluminum-zinc plating, a Zn concentration in the bath is 40 to 50%.
- a steel sheet having the same characteristics can be manufactured without any particular problem.
- plating is possible under normal conditions in both continuous plating equipment with a non-oxidizing furnace and continuous plating equipment without an oxidation-free furnace, and special control is required only for this steel sheet. Since it does not, it does not hinder productivity. Further, as long as it is a zinc plating method, any method such as hot-dip galvanizing, electrogalvanizing, and alloyed hot-dip galvanizing may be adopted.
- the surface of the steel sheet is not pre-plated with metal before plating, but there is no particular problem even if Ni pre-plating, Fe pre-plating, or other metal pre-plating to improve the plating property is applied. Further, a different kind of metal plating, a film of an inorganic compound, an organic compound, or the like may be applied to the surface of the plating layer.
- the hot stamp molded product according to the second embodiment has a mass% of C: 0.25% or more and 0.55% or less. Si: 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5 % Or less, one or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less, and having a chemical composition consisting of the
- the hot stamped product left in the air at room temperature of 20 to 30 ° C. for 48 hours or more has a tensile strength of more than 1600 MPa and a uniform elongation of El 1 (%).
- the total elongation measured using a tensile test piece of a hot stamped product charged with hydrogen so that the amount of diffusible hydrogen is 0.5 ⁇ 0.1 wt ppm is El 2 (%).
- (El 2 / El 1 ) ⁇ 100 ⁇ 100 (%) is satisfied.
- the chemical composition of the hot stamped product according to the second embodiment is the same as that of the hot stamped product according to the first embodiment.
- the hot stamped product according to the second embodiment is the same as the hot stamped product according to the first embodiment, which is a tension of the hot stamped product left in the air at room temperature of 20 to 30 ° C. for 48 hours or more.
- the tensile strength measured using the test piece exceeds 1600 MPa, and the uniform elongation is El 1 (%).
- the hot stamped product according to the second embodiment was measured using a tensile test piece of the hot stamped product charged with hydrogen so that the amount of diffusible hydrogen was 0.5 ⁇ 0.1 wt ppm.
- the total elongation is El 2 (%), (El 2 / El 1 ) ⁇ 100 ⁇ 100 (%) is satisfied.
- the tensile properties are the tensile test piece of the hot stamped product in a normal state in which hydrogen is released, and the amount of diffusible hydrogen is 0.4 to 0.6 wt ppm (0). It is measured using a tensile test piece of a hot stamped product charged with hydrogen so as to have a concentration of .5 ⁇ 0.1 wt ppm). The amount of diffusible hydrogen can be measured by the thermal desorption method.
- the steel sheet for hot stamping according to the third embodiment has a mass% of C: 0.25% or more and 0.55% or less. Si: 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5
- a steel sheet for hot stamping containing 1 or 2 or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less, and having a
- a tensile test piece obtained by heating the hot stamping steel sheet at 950 ° C. for 1 minute and then cooling to 200 ° C. or lower at an average cooling rate of 30 to 100 ° C./s is used.
- the tensile strength measured after being left in the air at room temperature of 20 to 30 ° C. for 48 hours or more exceeds 1600 MPa, and the major axis of inclusions containing CaO-Al 2 O 3 contained in the hot stamped product is 50 ⁇ m or less. Is.
- the chemical composition of the hot stamping steel sheet according to the third embodiment is the same as that of the hot stamping molded product according to the first embodiment.
- a tensile test piece obtained by heating the hot stamping steel sheet at 950 ° C. for 1 minute and then cooling to 200 ° C. or lower at an average cooling rate of 30 to 100 ° C./s is used.
- the tensile strength measured after being left in the air for 48 hours or more at room temperature of 20 to 30 ° C. exceeds 1600 MPa. Even if the hot stamping steel sheet is not hot stamped, if the tensile test is performed by heating and quenching under the above conditions, the strength level and hydrogen embrittlement resistance when processed into a hot stamped product can be simulated. Can be predicted and evaluated.
- Hot stamping steel sheet according to the third embodiment similar to the hot stamping molded article according to the first embodiment, the major diameter of inclusions containing CaO-Al 2 O 3 contained in the hot stamp steel sheet is 50 ⁇ m or less.
- the steel sheet for hot stamping according to the fourth embodiment has a mass% of C: 0.25% or more and 0.55% or less. Si: 0.001% or more and 2.0% or less, Mn: 0.3% or more and 3.0% or less, P: 0.02% or less, S: 0.003% or less, Al: 0.005% or more and 1.0% or less, Cr: 0% or more and 1.0% or less, Mo: 0% or more and 1.0% or less, N: 0.02% or less, Ca: 0% or more and 0.0010% or less, B: 0.0005% or more and 0.01% or less, Ti: 0.005% or more and 0.5% or less, Nb: 0.005% or more and 0.5% or less, V: 0.005% or more and 0.5% or less, and Zr: 0.005% or more and 0.5
- a steel sheet for hot stamping containing 1 or 2 or more selected from the group consisting of% or less, and Ni + Cu + Sn: 0% or more and 2% or less, and having a
- a tensile test piece obtained by heating the hot stamping steel plate at 950 ° C. for 1 minute and then cooling to 200 ° C. or lower at an average cooling rate of 30 to 100 ° C./s is used. Measured after being left in the air at room temperature of 20 to 30 ° C. for 48 hours or more, the tensile strength exceeds 1600 MPa, the uniform elongation is El 3 (%), and the amount of diffusible hydrogen is 0.5 ⁇ . When the total elongation measured using the tensile test piece charged with hydrogen so as to be 0.1 wt ppm is El 4 (%), (El 4 / El 3 ) ⁇ 100 ⁇ 100 (%). I am satisfied.
- the chemical composition of the hot stamping steel sheet according to the fourth embodiment is the same as that of the hot stamping molded product according to the first embodiment.
- the hot stamping steel sheet according to the fourth embodiment is the same as the hot stamping molded product according to the first embodiment, and is subjected to a tensile test of the hot stamped product left in the air at room temperature of 20 to 30 ° C. for 48 hours or more.
- the tensile strength measured using the piece exceeds 1600 MPa, and the uniform elongation is El 3 (%).
- the steel sheet for hot stamping according to the fourth embodiment was hydrogen-charged so that the amount of diffusible hydrogen was 0.5 ⁇ 0.1 wt ppm, as in the case of the hot stamped product according to the second embodiment.
- the total elongation measured using the tensile test piece of the hot stamped product is El 4 (%), (El 4 / El 3 ) ⁇ 100 ⁇ 100 (%) is satisfied.
- a heating / quenching process equivalent to hot stamping is performed in a simulated manner, and a tensile test is performed using a hydrogen-released or hydrogen-charged tensile test piece. If the elongation ratio, that is, (El 4 / El 3 ) ⁇ 100 is 100% or more, hydrogen embrittlement does not occur.
- the molten steel discharged from the converter is used.
- a method of performing ladle refining in a ladle refining facility (LF: Ladle Furnace), then performing vacuum degassing refining in a vacuum degassing device (DH, RH, etc.), and further casting can be preferably adopted.
- the desulfurization effect and the amount of CaO-Al 2 O 3 inclusions produced depend on the method of adding Ca or CaO to the molten steel.
- the following three types of methods for adding Ca and the like, which are carried out in the secondary refining after the operation of the converter, are mainly exemplified.
- the slag composition (CaO (wt%)) / (Al 2 O 3 (mass%)) is set to 1.0 to less than 1.1.
- the slag composition (CaO (mass%)) / (Al 2 O 3 (mass%)) is set to more than 1.0 to 1.2 in the first half of the LF treatment, and the slag composition (CaO (mass%)) in the latter half of the LF treatment CaO (% by mass)) / (Al 2 O 3 (% by mass)) is set to 0.9 to less than 1.0.
- (CaO (wt%)) of the slag composition in the first half of the LF process / (Al 2 O 3 (wt%)) was 1.2 to 1.5
- the slag composition in the second half of the LF process (CaO (Mass%)) / (Al 2 O 3 (mass%)) is set to 0.7 to 0.9 in the latter half.
- the first half of the LF treatment can be defined as a desulfurization period (until the S concentration becomes less than 0.003% by mass). Further, the latter half of the LF treatment can be defined as a stirring period (the purpose of improving cleanliness such as floating removal of non-metal inclusions) following the desulfurization period. Further, CaO concentration (slag composition of (CaO (wt%)) / (Al 2 O 3 (wt%))) can be measured by a fluorescent X-ray method.
- the hot stamped product according to the first embodiment or the second embodiment and the hot stamping steel plate according to the third embodiment or the fourth embodiment are subjected to slag refining using LF to control the slag composition. Therefore, CaO-based flux and CaO-containing slag are added to the slag instead of the molten steel, and in the first half of the LF treatment, the slag composition (CaO (mass%)) / (Al 2 O 3 (mass%)) is 1 In order to control the range from 0 to 1.5 and reduce CaO-Al 2 O 3 system inclusions in the latter half of the LF treatment, (CaO (mass%)) / (Al 2 O 3 (mass%)) ) In the range of 0.7 to 1.1 (preferably, in the first half of the LF treatment, the slag composition (CaO (% by mass)) / (Al 2 O 3 (% by mass)) is more than 1.1 to 1.2.
- the slag composition (CaO (mass%)) / (Al 2 O 3 (mass%)) is 1.0 to less than 1.1, or in the first half of the LF treatment, the slag composition (CaO (mass%)) (% by mass)) / (Al 2 O 3 (% by mass)) is set to more than 1.0 to 1.2, and in the latter half of the LF treatment, the slag composition (CaO (% by mass)) / (Al 2 O 3 (% by mass)) ))) Is 0.9 to less than 1.0, more preferably 1.2 to 1.5 of the slag composition (CaO (% by mass)) / (Al 2 O 3 (% by mass)) in the first half of the LF treatment. and then, (CaO (wt%)) of the slag composition in the second half of the LF process / (Al 2 O 3 (wt%)) by controlling the second half 0.7-0.9), and to obtain
- the time from the end of secondary refining to the start of casting is preferably in the range of 20 to 120 minutes. If the time exceeds 120 minutes, the agglomeration and coarsening of inclusions progresses remarkably, so the time is preferably 120 minutes or less. On the other hand, it is technically difficult to start casting in less than 20 minutes.
- the slab is manufactured by casting after the above-mentioned secondary refining.
- continuous casting which is generally popular, can be adopted.
- the produced slab is heated to 1000 to 1400 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a finishing temperature of 800 to 1000 ° C. and a winding temperature of 300 to 800 ° C. and a plate thickness of 1.5 to 6.0 mm. Is preferable. After that, it is preferable to carry out cold rolling or warm rolling at a rolling reduction of 30 to 85% to obtain a rolled steel sheet.
- the rolled steel sheet may be subjected to a surface treatment such as the plating treatment described above.
- the hot stamping steel plate according to the third embodiment or the fourth embodiment may be obtained by the above procedure.
- the heating conditions are preferably maintained at a temperature of 850 to 1000 ° C. for a time of 10 seconds to 20 minutes.
- stamping is started at an Ar 3 transformation temperature or higher, a load of a surface pressure of 3 MPa or higher, more preferably 10 MPa or higher is applied in the mold to hold the bottom dead point for 3 to 30 seconds, and cooling is performed.
- the completion is preferably at a temperature below the martensitic transformation completion temperature, preferably at 200 ° C. or lower.
- the hot stamped molded product and the hot stamping steel plate of the present invention will be specifically described with reference to examples.
- the examples shown below are merely examples of the present invention, and the hot stamped molded product and the hot stamping steel sheet of the present invention are not limited to the following examples.
- Example 1 Steel type No. shown in Table 1.
- the following three types of secondary refining methods A, B and C were carried out and the results were obtained.
- "-" of Table 1 or Table 3 means that the content of these elements was less than the detection limit.
- S 0.0004 to 0.0015% by mass
- Ca 0.0001 to 0.0008% by mass changed, but this is a change derived from the secondary refining method.
- S and Ca in Table 1 the results by the secondary refining method A are shown.
- slag CaO (mass%) / Al 2 O 3
- LF energized type ladle refining device
- a desulfurization agent containing CaO as a main component was blown into the molten steel together with an inert gas by an injection type ladle refining apparatus to perform desulfurization.
- the basic unit of the desulfurizing agent used at this time is 2.0 to 5.0 kg / steel-ton.
- Table 2 shows the method of secondary refining with symbols A, B and C.
- Steel type No. in Table 2 Is the steel type No. in Table 1. Corresponds to.
- a slab having the chemical components shown in Table 1 (mass%, the balance consisting of Fe and impurities) was produced by continuous casting.
- the casting was performed with a vertical curved continuous casting machine (vertical portion: 2.5 m) at a casting speed in the range of 1.0 to 1.2 m / min.
- the time from the end of the secondary refining to the start of casting was in the range of 20 to 120 minutes.
- Each manufactured slab was heated to 1050 to 1350 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a finishing temperature of 830 to 900 ° C. and a winding temperature of 450 to 730 ° C. and a plate thickness of 3.3 mm. Then, it was cold-rolled to a plate thickness of 1.6 mm to obtain a cold-rolled steel sheet.
- these cold-rolled steel sheets and surface-treated steel sheets are heated to an austenite region of 950 ° C. having 3 points or more of Ac by furnace heating, and then from 900 ° C. having 3 points or more of Ac in a press machine having a water-cooled die. Hardened.
- the shape of the mold was flat for the purpose of evaluating the characteristics after quenching.
- the bottom dead center holding time was set to 10 seconds, and cooling was performed with a mold.
- the temperature after cooling was 200 ° C. or lower.
- a part was cut out from the molded product formed by the above hot stamping method to prepare a tensile test piece.
- the plating layer was removed by chemical treatment or mechanical grinding before collecting the tensile test piece.
- the plating was removed by immersing in a solution containing an inhibitor (Hibiron manufactured by Sugimura Chemical Industrial Co., Ltd.) that prevents the dissolution of Fe in 5% hydrochloric acid for 24 hours.
- an inhibitor Hibiron manufactured by Sugimura Chemical Industrial Co., Ltd.
- the plating layer was removed using a milling cutter or the like.
- the sampling direction of the tensile test piece was a direction perpendicular to the rolling direction of the steel material, and the shape was a JIS No. 5 tensile test piece.
- the shape of the tensile test piece may be collected.
- the shape of the tensile test piece be the test piece described in JIS or ISO if possible, but if the shape cannot be collected, a shape different from the standard may be used.
- test piece was evaluated as follows.
- Tensile strength the tensile strength of the test piece after quenching was determined by a tensile test based on JIS Z 2241 (2011). A JIS No. 5 test piece was used as the test piece, and the crosshead displacement speed was fixed at 2 mm / min. Other conditions were carried out by the method described in JIS Z 2241 (2011). Before carrying out this test, hydrogen was released by leaving it in the air at room temperature of 20 to 30 ° C. for 48 hours or more. A case where a tensile strength of 1600 MPa or more was obtained in the tensile test was regarded as acceptable.
- the amount of diffusible hydrogen was measured under the following conditions using the thermal desorption method.
- a material obtained by removing the plating layer by chemical treatment or mechanical grinding before collecting tensile test pieces is used in accordance with the above method.
- a sample for measuring hydrogen is ultrasonically cleaned in acetone for 120 seconds, then placed in a quartz tube installed in a small heating furnace to raise the temperature, and hydrogen released by gas chromatography using Ar as a carrier gas.
- the temperature rise rate was 100 ° C./hour, and sampling was performed for 5 minutes / time.
- the cumulative amount of hydrogen released up to 250 ° C. in this measurement was defined as diffusible hydrogen. This measurement was performed using JTF-20A manufactured by J-Science Lab.
- a tensile test piece that had been charged with hydrogen was subjected to a tensile test at a tensile speed of 2 mm / min, and it was confirmed whether or not fracture would occur due to hydrogen embrittlement before reaching the tensile strength. Samples that did not break were accepted. Five tensile tests were performed, and the one with 0 breaks was regarded as "pass ( ⁇ : GOOD)", and the one with even one break was regarded as "fail (x: BAD)".
- the starting point of fracture is the center of the grain boundary fracture surface formed in an elliptical shape.
- the longer the major axis of the inclusions was the starting point. For example, in the example of FIG. 2, it is determined that the inclusion of A having the longest major axis lA of the inclusion is the starting point.
- the composition of inclusions present at the starting point of fracture was analyzed by EDS (Energy Dispersive X-ray Spectroscopy) or EPMA. It was confirmed that the composition contained Ca, Al, and O, and CaO-Al 2 O 3 was contained. Here, it was confirmed that Ca and Al were contained in an amount of 1% or more and O was contained in an amount of 5% or more by weight. In addition, other elements were also confirmed.
- composition analysis result of the inclusions existing at the starting point of fracture in Table 2 or Table 4 those containing 1% or more of Ca, Al, or O are marked with “ ⁇ ”, and Ca, Al, or O is 1%. Those containing less than or not detected were marked with "-”.
- the elements of inclusions are measured low because the signal of the base metal (steel matrix) is included in the observation of the precipitate composition on the fracture surface. Often done. Therefore, even an analytical value of this degree can be evaluated as being contained in inclusions.
- the major axis of the inclusions was measured as follows. First, the inclusions that are the starting points of hydrogen embrittlement cracks as described above were confirmed by SEM. Since the inclusions existing on the fracture surface are often destroyed and not all remain, the length (major axis) of the iron dents around the inclusions is used as the part where the inclusions existed. It was measured. When the thickness of the inclusions in the plate thickness direction was small, the major axis was measured with the size of the inclusions up to the point where the inclusions were clogged in the recess.
- MnS The length of MnS was measured by the following method. The cross section of the steel material in the rolling direction was mirror-polished, and MnS at the center of the plate thickness was observed with a metallurgical microscope at a magnification of 200 to 500 times, the length was measured, and the longest one was recorded. Observation was performed in 10 or more fields of view. The central portion of the plate thickness is in the range from the center of the plate thickness to the thickness of 1/3 of the plate thickness.
- the inclusions observed by such stretching are MnS.
- elemental analysis may be performed by SEM-EDS or EPMA.
- the length of MnS was accepted up to 300 ⁇ m. MnS may not be observed in this evaluation, in which case the length is described as 0 ⁇ m.
- Table 2 also shows the tensile strength, the fracture state in the tensile test after hydrogen charging, the composition and major axis of inclusions that are the starting point for observing the fracture surface, and the length of MnS.
- Experiment No. 2 performed in the secondary refining method B. In 2, 5, 8 and 11, fracture occurred in the tensile test after hydrogen charging, and it was confirmed from the fracture surface observation that inclusions containing CaO-Al 2 O 3 having a major axis of 50 ⁇ m or more existed as the starting point.
- a current-type ladle refining device (LF) is used to control the desulfurization and inclusion composition, respectively, in the first half and the second half of the treatment of slag (CaO (mass%) / Al 2 O). 3 (mass%)) was adjusted to be lower in the second half than in the first half.
- slag CaO (mass%) / Al 2 O 3 (mass%)
- Example 2 A slab of the chemical components shown in Table 3 (mass%, the balance consisting of Fe and impurities) was produced.
- the slab was manufactured by continuous casting.
- the casting was performed with a vertical curved continuous casting machine (vertical portion: 2.5 m) at a casting speed in the range of 1.0 to 1.2 m / min.
- the time from the end of the secondary refining to the start of casting was in the range of 20 to 120 minutes.
- Each manufactured slab was heated to 1050 to 1350 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a finishing temperature of 830 to 900 ° C. and a winding temperature of 450 to 730 ° C. and a plate thickness of 3.3 mm. Then, it was cold-rolled to a plate thickness of 1.6 mm to obtain a cold-rolled steel sheet.
- hot-dip aluminum plating, alloyed hot-dip galvanizing, or hot-dip galvanizing was performed.
- SAface treatment is an experimental example of alloying hot-dip galvanizing
- GI is an experimental example of hot-dip galvanizing
- AL is an experimental example of hot-dip galvanizing
- CR is a plating treatment. It means each experimental example with the cold-rolled steel sheet as it is.
- those cold-rolled steel sheets and surface-treated steel sheets are heated to an austenite region of 950 ° C. having 3 points or more of Ac by furnace heating, and then from 900 ° C. having 3 points or more of Ac in a press machine having a water-cooled die. Hardened.
- the shape of the mold was a flat mold for the purpose of evaluating the characteristics after quenching.
- the bottom dead center holding time was set to 10 seconds, and cooling was performed with a mold.
- the temperature after cooling was 200 ° C. or lower.
- Example 1 A part was cut out from the hot stamped molded product to create a tensile test piece. As this method, the same method as in Example 1 was used. After that, the tensile strength after quenching was determined by a tensile test in the same manner as in Example 1.
- Example 1 the hydrogen embrittlement resistance was evaluated in the same manner as in Example 1.
- the composition and major axis of the inclusions that were the starting points of fracture were measured by the same method as in Example 1.
- the uniform elongation and the total elongation after hydrogen charging were measured by the same method as in Example 1.
- the size of MnS was measured by the same method as in Example 1.
- Table 4 shows the tensile strength, the fracture state in the tensile test after hydrogen charging, the composition / major axis of the inclusions that are the starting points for observing the fracture surface, and the length of MnS.
- Experiment No. 17 and 25 are experimental examples in which metal Ca was added afterwards for comparison, although they were produced by a method of suppressing the formation of CaO-Al 2 O 3 at the time of secondary refining. Therefore, Experiment No. At 17 and 25, coarse CaO-Al 2 O 3 was generated and the hydrogen embrittlement resistance was lowered.
- Experiment No. 29 is an example in which the strength was excessively increased because the amount of C was more than the limit. Under this condition, brittle fracture occurred before the tensile strength was measured during the tensile test. For reference, when the Vickers hardness was measured after mirror-polishing the cross section of the steel sheet, a value of 681 was shown. In addition, since the strength of this material was excessively increased, the hydrogen embrittlement resistance was also inferior, and although no coarse inclusions were observed on the fracture surface, the grain boundary fracture surface peculiar to hydrogen embrittlement was exhibited.
- Example 3 Steel type No. shown in Table 1.
- the three types of secondary refining methods A, B and C of Example 1 were carried out and the results were compared. The results are shown in Table 5.
- the slab was manufactured by continuous casting after the above secondary refining.
- the casting was performed with a vertical curved continuous casting machine (vertical portion: 2.5 m) at a casting speed in the range of 1.0 to 1.2 m / min.
- the time from the end of the secondary refining to the start of casting was in the range of 20 to 120 minutes.
- Each manufactured slab was heated to 1050 to 1350 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a finishing temperature of 830 to 900 ° C. and a winding temperature of 450 to 730 ° C. and a plate thickness of 3.3 mm. Then, it was cold-rolled to a plate thickness of 1.6 mm to obtain a cold-rolled steel sheet.
- a part was cut out from the cold-rolled steel sheet to prepare a tensile test piece.
- the plating layer was removed by chemical treatment or mechanical grinding before collecting the tensile test piece.
- the plating was removed by immersing in a solution containing an inhibitor (Hibiron manufactured by Sugimura Chemical Industrial Co., Ltd.) that prevents the dissolution of Fe in 5% hydrochloric acid for 24 hours.
- an inhibitor Hibiron manufactured by Sugimura Chemical Industrial Co., Ltd.
- the plating layer was removed using a milling cutter or the like.
- the sampling direction of the tensile test piece was a direction perpendicular to the rolling direction of the steel material, and the shape was a JIS No. 5 tensile test piece.
- the shape of the tensile test piece be the test piece described in JIS or ISO if possible, but if the shape cannot be collected, a shape different from the standard may be used.
- Experiment No. 2 performed in the secondary refining method C. Fracture also occurred in 55, 58, 61, and 64 in the tensile test after hydrogen charging, and it was confirmed from the fracture surface observation that inclusions containing CaO-Al 2 O 3 having a major axis of 50 ⁇ m or more existed as the starting point.
- a current-type ladle refining device (LF) is used to control the desulfurization and inclusion composition, respectively, in the first half and the second half of the treatment of slag (CaO (mass%) / Al 2 O). 3 (mass%)) was adjusted to be lower in the second half than in the first half.
- slag CaO (mass%) / Al 2 O 3 (mass%)
- Example 3 the results of the experimental example relating to the steel sheet for hot stamping showed the same tendency as the results of the experimental example of the hot stamped molded product of Example 1.
- Example 4 In order to examine the secondary refining method A of Example 1 in more detail, the first half (CaO (mass%) / Al 2 O 3 (mass%)) and the second half (CaO (mass%) / Al) of the LF treatment 2 O 3 (mass%)) was changed from the following condition A-1 to condition A-3 to perform secondary refining, and the results were compared.
- Condition A-1 (CaO (mass%) / Al 2 O 3 (mass%)) in the first half of the LF treatment is 1.2 or more and 1.5 or less, and (CaO (mass%) / Al 2) in the second half of the LF treatment. O 3 (mass%)) is 0.7 or more and 0.9 or less
- Condition A-2 The first half (CaO (mass%) / Al 2 O 3 (mass%)) of the LF treatment is more than 1.0 in the first half.
- (CaO (mass%) / Al 2 O 3 (mass%)) in the latter half of the LF treatment is 0.9 or more and less than 1.0
- Condition A-3 (CaO (mass%) / in the first half of the LF treatment Al 2 O 3 (mass%)) is more than 1.1 in the first half and 1.2 or less, and (CaO (mass%) / Al 2 O 3 (mass%)) in the second half of LF treatment is 1.0 or more and less than 1.1.
- the target chemical composition is the steel type No. shown in Table 1. Although it was set to 1 to 4, the chemical composition of the steel sheet fluctuated depending on the conditions of the secondary refining. Table 6 shows the actual values of the chemical components. The test results are shown in Table 7.
- the slab was manufactured by continuous casting after the above secondary refining.
- the casting was performed with a vertical curved continuous casting machine (vertical portion: 2.5 m) at a casting speed in the range of 1.0 to 1.2 m / min.
- the time from the end of the secondary refining to the start of casting was in the range of 20 to 120 minutes.
- Each manufactured slab was heated to 1050 to 1350 ° C. and hot-rolled to obtain a hot-rolled steel sheet having a finishing temperature of 830 to 900 ° C. and a winding temperature of 450 to 730 ° C. and a plate thickness of 3.3 mm. Then, it was cold-rolled to a plate thickness of 1.6 mm to obtain a cold-rolled steel sheet.
- the cold-rolled steel sheets 65 to 73 were subjected to hot-dip aluminum plating or alloyed hot-dip galvanizing.
- these cold-rolled steel sheets and surface-treated steel sheets were heated in an austenite region at 950 ° C., which has 3 or more Acc points, for 1 minute by furnace heating, and then quenched so that the average cooling rate was 30 to 100 ° C./s. It was.
- the temperature after cooling was 200 ° C. or lower.
- a thermodynamic cycle tester was used to perform such heating and quenching.
- the plating layer was removed by chemical treatment or mechanical grinding before collecting the tensile test piece.
- the plating was removed by immersing in a solution containing an inhibitor (Hibiron manufactured by Sugimura Chemical Industrial Co., Ltd.) that prevents the dissolution of Fe in 5% hydrochloric acid for 24 hours.
- an inhibitor Hibiron manufactured by Sugimura Chemical Industrial Co., Ltd.
- the plating layer was removed using a milling cutter or the like.
- the sampling direction of the tensile test piece was a direction perpendicular to the rolling direction of the steel material, and the shape was a JIS No. 5 tensile test piece.
- the shape of the tensile test piece be the test piece described in JIS or ISO if possible, but if the shape cannot be collected, a shape different from the standard may be used.
- the present invention it is extremely useful in industry because it is possible to provide a hot stamped molded product and a steel plate for hot stamping that can prevent hydrogen embrittlement cracking while maintaining a desired tensile strength.
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Abstract
Description
しかし、高強度化すると水素脆化割れ発生のリスクが高くなるため、水素脆化割れを抑制するための対策が求められている。
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ成形品であって、
20~30℃の室温にて48時間以上大気中で放置された上記ホットスタンプ成形品の引張強度が、1600MPaを超え、
ホットスタンプ成形品に含まれるCaO-Al2O3を含む介在物の長径が50μm以下であることを特徴とする。
(2)本発明の一態様に係るホットスタンプ成形品は、質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ成形品であって、
20~30℃の室温にて48時間以上大気中で放置された上記ホットスタンプ成形品の引張強度が1600MPaを超え、
均一伸びがEl1(%)であって、拡散性水素量が0.5±0.1重量ppmとなるように水素チャージされたホットスタンプ成形品の引張試験片を用いて測定された全伸びがEl2(%)であるとき、(El2/El1)×100≧100(%)を満足することを特徴とする。
板厚中央部に存在するMnSの最大長さが300μm以下であってもよい。
(4)上記(1)から(3)のいずれか1項に記載のホットスタンプ成形品では、
質量%で、
Ni+Cu+Sn:0.005%以上2%以下、を含有してもよい。
(5)上記(1)から(4)のいずれか1項に記載のホットスタンプ成形品では、
表面にめっき層を有してもよい。
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ用鋼板であって、
上記ホットスタンプ用鋼板を950℃で1分加熱後、平均冷却速度30~100℃/sで200℃以下まで冷却された引張試験片を用いて、20~30℃の室温にて48時間以上大気中で放置された後に測定された引張強度が1600MPaを超え、
ホットスタンプ用鋼板に含まれるCaO-Al2O3を含む介在物の長径が50μm以下であることを特徴とする。
(7)本発明の一態様に係るホットスタンプ用鋼板は、質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ用鋼板であって、
上記ホットスタンプ用鋼板を950℃で1分加熱後、平均冷却速度30~100℃/sで200℃以下まで冷却された引張試験片を用いて、20~30℃の室温にて48時間以上放置された後に測定された引張強度が1600MPaを超え、
均一伸びがEl3(%)であって、拡散性水素量が0.5±0.1重量ppmとなるように水素チャージされた上記引張試験片を用いて測定された全伸びがEl4(%)であるとき、(El4/El3)×100≧100(%)を満足することを特徴とする。
板厚中央部に存在するMnSの最大長さが300μm以下であってもよい。
(9)上記(6)から(8)のいずれか1項に記載のホットスタンプ用鋼板では、
質量%で、
Ni+Cu+Sn:0.005%以上2%以下、を含有してもよい。
(10)上記(6)から(9)のいずれか1項に記載のホットスタンプ用鋼板では、
表面にめっき層を有してもよい。
第1実施形態に係るホットスタンプ成形品は、質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、残部Fe及び不純物からなる化学組成を有するホットスタンプ成形品である。
第1実施形態に係るホットスタンプ成形品は、20~30℃の室温にて48時間以上大気中で放置された前記ホットスタンプ成形品の引張強度が1600MPaを超え、ホットスタンプ成形品に含まれるCaO-Al2O3を含む介在物の長径が50μm以下である。
Cは冷却後の組織をマルテンサイトとして材質を確保するために添加する元素であり、強度1600MPa以上を確保するためには0.25%以上添加する必要がある。添加量が多すぎると、衝撃変形時の強度確保が困難となるため、その上限を0.55%とする。
Siは固溶強化型の合金元素であり、強度を確保するために必要であるが、2.0%を超えると、表面スケールの問題が生じる。このため、Siは2.0%以下に規定した。また、鋼板表面にメッキ処理を行う場合は、Siの添加量が多いとめっき性が劣化するため、上限を1.0%とすることが好ましい。添加量を減少させた場合には材質上大きな問題はないものの、添加量が少ないと製鋼コストが増加するためSiの添加量の下限を0.001%とする。なお、更に好ましくは、Siの添加量は0.01~0.5%の範囲である。
Mnは強度及び焼入れ性を向上させる元素であり、0.2%未満では焼入れ時の強度を十分に得られず、また、3.0%を超えて添加しても効果が飽和するため、Mnは0.3~3.0%の範囲に規定した。Mnの添加量は、2.2%以下がより好ましい。
Alは溶鋼の脱酸材として使われる必要な元素であり、またNを固定する元素でもあり、その量は結晶粒径や機械的性質に大きな影響を及ぼす。このような効果を有するためには0.005%以上の含有量が必要であるが、1.0%を超えると非金属介在物が多くなり製品に表面疵が発生しやすくなる。このため、Alは0.005~1.0%の範囲に規定した。Alの添加量は0.01%以上がより好ましく、0.5%以下がより好ましい。
Sは鋼中の非金属介在物に影響し、靱性と耐水素脆性を悪化させる。このため、Sは0.003%以下に規定した。なお、好ましくは、Sは0.001%以下である。
Pは靱性や耐水素脆性に悪影響を及ぼす元素であるため、Pは0.02%以下に規制した。なお、好ましくは、Pは0.015%以下である。また、更に好ましくはPは0.010%以下である。
Crは焼入れ性を向上させる元素である。1.0%を超えると熱間圧延後、冷間圧延後または焼鈍後(めっき処理後も含む)に存在する炭化物を安定化させ、ホットスタンプでの加熱での溶解を遅らせて焼入れ性が低下することが考えられる。そこでCrの上限を1.0%と規定した。下限についてはMn、Mo、Niといった焼入れ性をもたらす元素を添加することでホットスタンピングに必要な焼入れ性の確保が可能なことから特に規定しない。Crの添加量は、0.05%以上がより好ましく、1.0%以下がより好ましい。
Moは焼入れ性を向上させる元素である。1.0%を超えると熱間圧延後、冷間圧延後または焼鈍後(めっき処理後も含む)に存在する炭化物を安定化させ、ホットスタンプでの加熱での溶解を遅らせて焼入れ性が低下することが考えられる。そこでMoの上限を1.0%と規定した。下限についてはMnやNiといった焼入れ性をもたらす元素を添加することでホットスタンピングに必要な焼入れ性の確保が可能なことから特に規定しない。Moの添加量は0.05%以上がより好ましく、0.5%以下がより好ましい。
Caは水素脆化割れの起点となりうるCaO-Al2O3の生成を抑制するためには添加しないことが望ましい。ただし無添加の場合においても製錬時の取鍋スラグや鋳造時で用いられるパウダーから混入する可能性がある。そのため、これを抑制する必要がありCaの上限値を0.0010%に限定した。この範囲であれば、水素脆化割れの起点となりうる粗大なCaO-Al2O3が生成する可能性は低くなる。ただし、精錬条件によっては微量に存在するCaO-Al2O3が凝集して粗大化する可能性があり、粗大化を防止する精錬方法を用いることが重要である。
Bはプレス成形中あるいはプレス成形後の冷却での焼入れ性を向上させるために添加するが、この効果を発揮させるためには0.0005%以上の添加が必要である。しかしながら、この添加量がむやみに増加すると熱間での割れの懸念があることや、その効果が飽和するため、Bの上限は0.01%とすることが好ましい。
Ti,Nb,V,ZrはBの効果を有効に発揮させるため、Bと化合物を生成するNを固着する目的で添加する。Ti,Nb,V,Zrは、炭化物・窒化物を形成して加熱中のオーステナイトの粒成長を抑制して細粒化することにより、靱性や耐水素脆性を改善する効果もある。しかし、過度の添加は靱性を劣化させるため、これらの添加量の上限を0.5%に規定した。これらの元素の添加量は、効果を発現させるために必要な添加量の観点から、それぞれ0.005%以上である。
Nは0.02%を超えると窒化物の粗大化により靱性が劣化する傾向がみられる。このため、Nは0.02%以下に規定することが好ましい。Nは、より好ましくは0.01%以下である。
第1実施形態に係るホットスタンプ成形品は、化学組成として、質量%で、Ni+Cu+Sn(Ni、Cu及びSnの合計):0%以上2%以下を含有してもよい。
またその他の元素としてSe、W等、上記に規定していない元素を含有した場合でも、好ましくは0.1%を超えなければ特性に大きな問題は無い。
すなわち、破面の観察結果からは、起点となったと考えられるCaO-Al2O3には亀裂が生じていることが多く、他の析出物・介在物と比較して脆いことが推察される。そのため、引張応力が介在物に付与された際に破壊が生じやすく、その破壊により鋼材の水素脆化割れが引き起こされるものと推察される。
すなわち、介在物の長径の長さは重要であり、これを制限することにより介在物の破壊を抑制して水素脆化割れを抑制することが可能となると考えらえる。
また、介在物の板厚方向の厚みが小さい場合には、凹み中に介在物が詰まっている範囲を介在物として、その長径を規定することができる。
引張試験による破断の起点は、楕円状に生じた粒界破面の中心の介在物である。楕円状の粒界破面が2か所以上ある場合には、介在物の長径が長い方を破断の起点として、この長径を介在物の長径とする。図2の例では、粒界破面SAの中心に介在物Aが、粒界破面SBの中心に介在物Bが粒界破面SCの中心に介在物Cがそれぞれ存在する。
焼入れ後のホットスタンプ成形品を、20~30℃の室温にて48時間以上大気中で放置することで、ホットスタンプ成形品の内部から十分に水素が放出される。
アルミニウムめっきであれば、浴中Si濃度は5~12%が適しており、アルミニウム-亜鉛めっきでは、浴中Zn濃度は40~50%が適している。
なお、めっき工程における雰囲気については、無酸化炉を有する連続式めっき設備でも無酸化炉を有しない連続式めっき設備でも通常の条件とすることでめっき可能であり、本鋼板だけ特別な制御を必要としないことから生産性を阻害することもない。また、亜鉛めっき方法であれば、溶融亜鉛めっき、電気亜鉛めっき、合金化溶融亜鉛めっきなどいかなる方法を採用しても良い。
第2実施形態に係るホットスタンプ成形品は、質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、残部Fe及び不純物からなる化学組成を有するホットスタンプ成形品である。
第2実施形態に係るホットスタンプ成形品は、20~30℃の室温にて48時間以上大気中で放置されたホットスタンプ成形品の引張強度が1600MPaを超え、均一伸びがEl1(%)であって、拡散性水素量が0.5±0.1重量ppmとなるように水素チャージされたホットスタンプ成形品の引張試験片を用いて測定された全伸びがEl2(%)であるとき、(El2/El1)×100≧100(%)を満足する。
本来の引張特性(引張強度及び伸び)は、水素を放出した状態の引張試験片を用いて引張試験を行うことにより確認できる。
このような全伸びによる評価は、均一伸びの場合よりも厳格な評価になる。
拡散性水素量は、昇温脱離法により測定することができる。
第3実施形態に係るホットスタンプ用鋼板は、質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、残部Fe及び不純物からなる化学組成を有するホットスタンプ用鋼板である。
第3実施形態に係るホットスタンプ用鋼板は、前記ホットスタンプ用鋼板を950℃で1分加熱後、平均冷却速度30~100℃/sで200℃以下まで冷却された引張試験片を用いて、20~30℃の室温にて48時間以上大気中で放置された後に測定された引張強度が1600MPaを超え、ホットスタンプ成形品に含まれるCaO-Al2O3を含む介在物の長径が50μm以下である。
第4実施形態に係るホットスタンプ用鋼板は、質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、残部Fe及び不純物からなる化学組成を有するホットスタンプ用鋼板である。
第4実施形態に係るホットスタンプ用鋼板は、前記ホットスタンプ用鋼板を950℃で1分加熱後、平均冷却速度30~100℃/sで200℃以下まで冷却された引張試験片を用いて、20~30℃の室温にて48時間以上大気中で放置された後に測定された、引張強度が1600MPaを超え、均一伸びがEl3(%)であって、拡散性水素量が0.5±0.1重量ppmとなるように水素チャージされた前記引張試験片を用いて測定された全伸びがEl4(%)であるとき、(El4/El3)×100≧100(%)を満足する。
本来の引張特性(引張強度及び伸び)は、水素を放出した状態の引張試験片を用いて引張試験を行うことにより確認できる。
このような全伸びによる評価は、均一伸びの場合よりも厳格な評価になる。
第1実施形態又は第2実施形態に係るホットスタンプ成形品、ならびに第3実施形態又は第4実施形態に係るホットスタンプ用鋼板を製造する方法としては、転炉から出鋼された溶鋼に対して取鍋精錬設備(LF:Ladle Furnace)で取鍋精錬を行い、その後、真空脱ガス装置(DH、RHなど)で真空脱ガス精錬を行い、さらに鋳造する方法が好ましく採用できる。
介在物の形態を制御する目的、例えば、硫化物を球状のCaS介在物とする目的で、真空脱ガス装置(DH、RH等)で精錬中の溶鋼に直接、金属Caを添加する方法がある。この場合、CaO-Al2O3系介在物を多量に生成するリスクが、次に記載する方法に比べて高くなる。よって、この方法は好ましく用いることができない。
CaOを主成分とする脱硫剤を溶鋼中に吹き込む場合には、脱硫剤が全てスラグ側に吸収されて溶鋼中から除去されれば、鋼の清浄性が保たれて問題にならない。しかし実際には、脱硫剤が溶鋼中に一部残存して、溶鋼中のAl2O3との合体やAlとの反応により、CaO-Al2O3系介在物が生成する。CaO-Al2O3系介在物を多量に生成するリスクは、上記の金属Ca添加の場合と同等、もしくは少し低い程度となる。よって、この方法も好ましく用いることができない。
LFを用いてスラグ精錬を行う場合は、スラグ組成を制御するために、CaO系フラックスやCaO含有スラグを、溶鋼中ではなくスラグ中に添加する方法がある。スラグ中のCaO濃度(スラグ組成の(CaO(質量%))/(Al2O3(質量%)))が高くなると、脱硫反応が進んで有利になる。一方、CaO濃度が高くなり過ぎると、スラグとメタル(溶鋼)間の反応によって、Ca成分が溶鋼へ少量溶解してCaO-Al2O3系介在物が生成してしまう。
表1に示す鋼種No.1~4の化学成分(質量%、残部がFe及び不純物からなる)において介在物を適正に制御するために、以下の3種類の二次精錬方法A、B及びCを実施してその結果を比較した。
なお、表1又は表3の「-」は、これらの元素の含有量が検出限界未満であったことを意味する。また、精錬方法によって、S:0.0004~0.0015質量%、Ca:0.0001~0.0008質量%の範囲で変化したが、これは、二次精錬方法に由来する変化である。表1のS及びCaについては、二次精錬方法Aによる結果を示している。
二次精錬方法Aでは、まず、スラグ中の(CaO(質量%)/Al2O3(質量%))=1.0~1.5のスラグにより脱硫を行った後、CaO-Al2O3を含有する介在物生成量を抑制するためにスラグ中の(CaO(質量%)/Al2O3(質量%))を0.7~1.1の範囲に制御した。
表2の「表面処理」の項目において、GAは合金化溶融亜鉛めっきを施した実験例、ALは溶融アルミニウムめっきを施した実験例、CRはめっき処理をしない冷延鋼板のままの実験例をそれぞれ意味する。
めっき層を有する供試材の場合は、引張試験片採取の前にめっき層を化学的処理もしくは機械研削により除去した。
(引張強度)
まず、焼入れ後の試験片の引張強度をJIS Z 2241(2011)に準拠した引張試験により求めた。試験片はJIS5号試験片を用い、クロスヘッド変位速度は2mm/分と固定した。他の条件はJIS Z 2241(2011)に記載の方法で行った。この試験を実施する前には20~30℃の室温にて48時間以上大気中で放置して水素を放出させた。引張試験にて引張強度が1600MPa以上得られた場合を合格とした。
次に水素脆性の評価を行った。
3%の食塩水+チオシアン酸アンモニウム(3g/l)中で、評価対象となる引張試験片を陰極として水素チャージした。その後、電気亜鉛めっきを片面の厚みが10μmとなるように試験片の両面に施して水素の放出を抑制した。水素チャージで導入する水素は拡散性水素量0.5±0.1重量ppmとした。
温度上昇率は100℃/時間とし、サンプリングは5分/回行った。この測定で250℃までに放出された累積水素量を拡散性水素とした。この測定は、ジェイ・サイエンス・ラボ製、JTF-20Aを用いて行った。
たとえば、図2の例では、介在物の長径lAが最も長いAの介在物が起点であると判断する。
JIS Z 2241(2011)に記載の方法にて、各試験片について、均一伸びEl1と水素チャージ後の全伸びEl2とを測定して、El2/El1を求めた。(El2/El1)×100が100%以上であれば水素脆化を起こさないため、これを良好な結果とした。ここで全伸び、均一伸びとは、それぞれJIS Z 2241 (2011)に記載の破断伸びと最大試験力全伸びである。
また介在物の長径が50μm超であるかを確認した。
介在物の長径の測定は下記のように行った。まずSEMにより上記のような水素脆化割れの起点となる介在物を確認した。破面に存在する介在物は破壊されて、すべてが残存していない場合が多いことから、介在物の周囲に存在する鉄の凹みを介在物が存在した部位として、その長さ(長径)を測定した。介在物の板厚方向の厚みが小さい場合には、凹み中に介在物が詰まっているところまでを介在物の大きさとして、その長径を測定した。
また、MnSの長さは下記の方法で測定した。
鋼材の圧延方向の断面を鏡面研磨して200~500倍の倍率にて金属顕微鏡にて板厚中央部のMnSを観察して、その長さを測定し、一番長いものを記録した。観察は10視野以上行った。板厚中央部は、板厚中心から板厚の1/3の厚みまでの範囲とした。
表3に示す化学成分(質量%、残部がFe及び不純物からなる)の鋳片(スラブ)を製造した。
二次精錬の前半では(CaO(質量%)/Al2O3(質量%))を1.0~1.5の範囲で制御して脱硫を効率的に行い、二次精錬の後半では(CaO(質量%)/Al2O3(質量%))を0.7~1.1の範囲に制御した。二次精錬の後半の(CaO(質量%)/Al2O3(質量%))は前半よりも低くなるように調整した。
表4の「表面処理」の項目において、GAは合金化溶融亜鉛めっきを施した実験例、GIは溶融亜鉛めっきを施した実験例、ALは溶融アルミニウムめっきを施した実験例、CRはめっき処理をしない冷延鋼板のままの実験例をそれぞれ意味する。
また、均一伸び及び水素チャージ後の全伸びを実施例1と同様の方法で測定した。またMnSのサイズを実施例1と同様の方法で測定した。引張強度、水素チャージ後の引張試験での破断状況、破面の観察での起点となる介在物の組成・長径、MnSの長さを表4に示した。
そのため、実験No.17,25では、粗大なCaO-Al2O3が生成して耐水素脆性が低下した。
またこの材料は過度に強度が上昇したため耐水素脆性も劣位であり、破面に粗大な介在物が見られないにも関わらず、水素脆性特有の粒界破面を呈していた。
実験No.39はP添加量が本発明の規定以上であったため、耐水素脆性が低下して破断した。
表1に示す鋼種No.1~4の化学成分において介在物を適正に制御するために、実施例1の3種類の二次精錬方法A、B及びCを実施してその結果を比較した。その結果を表5に示す。
めっき層を有する供試材の場合は、引張試験片採取の前にめっき層を化学的処理もしくは機械研削により除去した。
実施例1の二次精錬方法Aについてさらに詳細に検討するために、LF処理の前半の(CaO(質量%)/Al2O3(質量%))と後半の(CaO(質量%)/Al2O3(質量%))を下記の条件A-1から条件A-3のように変化させて二次精錬を行い、その結果を比較した。
条件A-2:LF処理の前半の(CaO(質量%)/Al2O3(質量%))が前半1.0超1.2以下、LF処理の後半の(CaO(質量%)/Al2O3(質量%))が0.9以上1.0未満
条件A-3:LF処理の前半の(CaO(質量%)/Al2O3(質量%))が前半1.1超1.2以下、LF処理の後半の(CaO(質量%)/Al2O3(質量%))が1.0以上1.1未満
Claims (10)
- 質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ成形品であって、
20~30℃の室温にて48時間以上大気中で放置された前記ホットスタンプ成形品の引張強度が、1600MPaを超え、
前記ホットスタンプ成形品に含まれるCaO-Al2O3を含む介在物の長径が50μm以下である
ことを特徴とするホットスタンプ成形品。 - 質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ成形品であって、
20~30℃の室温にて48時間以上大気中で放置された前記ホットスタンプ成形品の引張強度が1600MPaを超え、
均一伸びがEl1(%)であって、拡散性水素量が0.5±0.1重量ppmとなるように水素チャージされた前記ホットスタンプ成形品の引張試験片を用いて測定された全伸びがEl2(%)であるとき、(El2/El1)×100≧100(%)を満足する
ことを特徴とするホットスタンプ成形品。 - 板厚中央部に存在するMnSの最大長さが300μm以下である
ことを特徴とする請求項1又は2に記載のホットスタンプ成形品。 - 質量%で、
Ni+Cu+Sn:0.005%以上2%以下、を含有する
ことを特徴とする請求項1から3のいずれか1項に記載のホットスタンプ成形品。 - 表面にめっき層を有する
ことを特徴とする請求項1から4のいずれか1項に記載のホットスタンプ成形品。 - 質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ用鋼板であって、
前記ホットスタンプ用鋼板を950℃で1分加熱後、平均冷却速度30~100℃/sで200℃以下まで冷却された引張試験片を用いて、20~30℃の室温にて48時間以上大気中で放置された後に測定された引張強度が1600MPaを超え、
前記ホットスタンプ用鋼板に含まれるCaO-Al2O3を含む介在物の長径が50μm以下である
ことを特徴とするホットスタンプ用鋼板。 - 質量%で
C:0.25%以上0.55%以下、
Si:0.001%以上2.0%以下、
Mn:0.3%以上3.0%以下、
P:0.02%以下、
S:0.003%以下、
Al:0.005%以上1.0%以下、
Cr:0%以上1.0%以下、
Mo:0%以上1.0%以下、
N:0.02%以下、
Ca:0%以上0.0010%以下、
B:0.0005%以上0.01%以下、
Ti:0.005%以上0.5%以下、Nb:0.005%以上0.5%以下、V:0.005%以上0.5%以下、及びZr:0.005%以上0.5%以下、からなる群から選択される1種又は2種以上、及び
Ni+Cu+Sn:0%以上2%以下、を含有し、
残部Fe及び不純物からなる化学組成を有するホットスタンプ用鋼板であって、
前記ホットスタンプ用鋼板を950℃で1分加熱後、平均冷却速度30~100℃/sで200℃以下まで冷却された引張試験片を用いて、20~30℃の室温にて48時間以上大気中で放置された後に測定された、引張強度が1600MPaを超え、
均一伸びがEl3(%)であって、拡散性水素量が0.5±0.1重量ppmとなるように水素チャージされた前記引張試験片を用いて測定された全伸びがEl4(%)であるとき、(El4/El3)×100≧100(%)を満足する
ことを特徴とするホットスタンプ用鋼板。 - 板厚中央部に存在するMnSの最大長さが300μm以下である
ことを特徴とする請求項6又は7に記載のホットスタンプ用鋼板。 - 質量%で、
Ni+Cu+Sn:0.005%以上2%以下、を含有する
ことを特徴とする請求項6から8のいずれか1項に記載のホットスタンプ用鋼板。 - 表面にめっき層を有する
ことを特徴とする請求項6から9のいずれか1項に記載のホットスタンプ用鋼板。
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JP2021561454A JP7364935B2 (ja) | 2019-11-26 | 2020-11-25 | ホットスタンプ成形品及びホットスタンプ用鋼板 |
KR1020227016855A KR20220084382A (ko) | 2019-11-26 | 2020-11-25 | 핫 스탬프 성형품 및 핫 스탬프용 강판 |
MX2022006042A MX2022006042A (es) | 2019-11-26 | 2020-11-25 | Miembro estampado en caliente y lamina de acero para estampado en caliente. |
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WO2023162893A1 (ja) * | 2022-02-25 | 2023-08-31 | 日本製鉄株式会社 | 鋼板、および鋼板の製造方法 |
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WO2015147216A1 (ja) | 2014-03-26 | 2015-10-01 | 新日鐵住金株式会社 | 高強度熱間成形鋼板部材 |
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WO2023162893A1 (ja) * | 2022-02-25 | 2023-08-31 | 日本製鉄株式会社 | 鋼板、および鋼板の製造方法 |
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