Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc
  • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• Patent Document 2 also discloses a high-strength hot-dip galvanized steel sheet with excellent delayed fracture resistance, which has a volume fraction of 40 to 90% ferrite phase and 5% or less retained austenite phase, the volume fraction of unrecrystallized ferrite in the entire ferrite phase is 50% or less, the grain size ratio, which is the value obtained by dividing the average grain size in the rolling direction of the ferrite phase by the average grain size in the sheet width direction, is 0.75 to 1.33, the length ratio, which is the value obtained by dividing the average length in the rolling direction of the hard structure dispersed in island shapes by the average length in the sheet width direction, is 0.75 to 1.33, and the average aspect ratio of inclusions is 5.0 or less.
• Patent Document 4 also discloses a high-strength hot-dip galvanized steel sheet with excellent workability and a high TS-El balance, excellent stretch flangeability, and low YR, characterized by a composition containing, by mass%, 0.05-0.3% C, 0.01-2.5% Si, 0.5-3.5% Mn, 0.003-0.100% P, 0.02% or less S, 0.010-1.5% Al, 0.007% or less N, with the balance being Fe and unavoidable impurities, and a microstructure containing, by area ratio, 20-87% ferrite, 3-10% martensite and retained austenite in total, and 10-60% tempered martensite, and a method for manufacturing the same.
• Patent Document 2 discloses a high-strength hot-dip galvanized steel sheet in which the main structure inside the steel sheet is soft ferrite and unrecrystallized ferrite is limited to a small amount to improve ductility, and delayed fracture resistance and its anisotropy are improved by forming a decarburized layer on the surface of the steel sheet.
• no consideration is given at all to the improvement in bendability and fracture resistance during vehicle collision due to the formation of a surface soft layer (decarburized layer), and the press formability of the steel sheet end portions.
• Patent Document 4 discloses a high-strength hot-dip galvanized steel sheet having improved ductility, which is the press formability inside the steel sheet, and stretch flangeability, which is the press formability at the ends of the steel sheet, but does not take into consideration at all the improvement of bendability by forming a soft surface layer (decarburized layer) or the improvement of fracture resistance during a vehicle collision.
• the steel sheet referred to here includes zinc-plated steel sheet, which can be hot-dip galvanized steel sheet (hereinafter also referred to as GI) or alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).
• GI hot-dip galvanized steel sheet
• GA alloyed hot-dip galvanized steel sheet
• the bendability of a steel plate is excellent when a 90-degree V-bend test with a bending radius of 0.5 mm is performed in accordance with JIS Z 2248 (2022), and the length of a crack that propagates along a bending ridge formed other than the end of the bending ridge (crack length other than the V-bend end surface) is 200 ⁇ m or less;
• a close contact bending test is performed, and the spacer plate thickness at the crack limit where cracks of 0.5 mm or more do not occur along the bending ridge is 3.0 mm or less;
• This refers to a condition in which a contact bending test with a 3.0 mm spacer is performed, the crack depth (contact bending internal crack depth) that progresses in the plate thickness direction at the bending ridge subjected to compressive stress is 200 ⁇ m or less, and when a contact bending + orthogonal 90 degree V bending test is performed, the bending radius at which cracks of 0.5 mm or more do not occur along the
• the composition of the base steel sheet of the steel sheet is appropriately adjusted, and the base steel sheet of the steel sheet has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1/4 position of the sheet thickness, and the surface soft layer satisfies the following formula (1):
• the structure in the superficial soft layer is as follows: Area ratio of ferrite: 50.0% or more and 100.0% or less, Among structures other than ferrite, the area ratio of fresh martensite divided by the total area ratio of bainite, fresh martensite, and tempered martensite (excluding retained austenite) is 0.5 or less,
• the structure at 1/4 of the thickness of the base steel sheet is as follows: an area ratio of ferrite: 76.5% or less (including 0.0%); a total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less; a volume ratio of
• the present invention was completed based on the above findings and through further investigation.
• the gist and configuration of the present invention are as follows.
• C 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%; P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%) and the balance being Fe and unavoidable impurities,
• the steel sheet has a surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1/4 sheet thickness position from the surface of the steel sheet, The surface soft layer satisfies the following formula (1):
• the structure of the surface soft layer is The area ratio of ferrite is 50.0% or more and 100.0% or less, When the area ratio of ferrite is less than
• a steel plate having a tensile strength TS of 780 MPa or more and less than 1180, high yield stress YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability of the steel plate end (bendability of the steel plate end (shear cross section)) can be obtained.
• members made from the steel plate of the present invention have high strength and can be used extremely advantageously as impact energy absorbing members for automobiles.
• the steel sheet of the present invention has a base steel sheet having a component composition containing, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.005% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), with the balance being Fe and unavoidable impurities;
• the steel sheet has a soft surface layer having a Vickers hardness of 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet, The surface soft layer satisfies the following formula (1):
• the structure in the superficial soft layer is as follows: The area ratio of ferrite is 50.0% or more and 100.0% or less
• X is the thickness ( ⁇ m) of the soft surface layer
• [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
• composition of the steel sheet according to the embodiment of the present invention will be described. Note that the unit of the composition is always “mass%”, but hereinafter, unless otherwise specified, it will be simply shown as “%”.
• C 0.050% or more and 0.400% or less C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite and retained austenite to ensure a TS of 780 MPa or more and less than 1180 MPa and a high YS.
• the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to achieve a TS of 780 MPa or more. In addition, this also leads to a decrease in YS.
• the C content exceeds 0.400%, the area ratio of fresh martensite increases excessively, making it difficult to make TS less than 1180 MPa.
• Mn 1.00% or more and less than 3.50%
• Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc.
• the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 780 MPa or more. In addition, this also leads to a decrease in YS.
• Ms point or Ms the martensite transformation start temperature
• S 0.0001% or more and 0.0200% or less S exists as sulfides in steel.
• the S content is set to 0.0200% or less, preferably 0.0080% or less, and more preferably 0.0050% or less. Due to restrictions on production technology, the S content is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.
• N 0.0100% or less N exists as nitrides in steel.
• the N content is set to 0.0100% or less, and preferably to 0.0050% or less.
• the N content is preferably 0.0005% or more, more preferably 0.0010% or more, and further preferably 0.0020% or more.
• Sb 0.200% or less (including 0%)
• Sb is a useful element that can improve plating and chemical conversion treatment properties by segregating on the steel sheet surface during annealing. Therefore, the Sb content is preferably 0.002% or more.
• the Sb content is more preferably 0.005% or more.
• the Sb content is more preferably 0.007% or more, and further preferably 0.009% or more.
• the Sb content exceeds 0.200%, the effect of improving the plating property and the chemical conversion property is saturated, and there is a possibility that the press formability (bendability of the steel sheet) and the crack propagation resistance inside the steel sheet are deteriorated. Therefore, when Sb is contained, the Sb content is set to 0.200% or less.
• the Sb content is more preferably 0.020% or less. Further preferably, it is 0.018% or less.
• the Sb content is more preferably 0.016% or less, and further preferably 0.014% or less.
• Sn 0.200% or less (including 0%)
• Sn is a useful element that can improve plating and chemical conversion treatment properties by segregating on the steel sheet surface during annealing. Therefore, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. The Sn content is more preferably 0.007% or more, and further preferably 0.009% or more. On the other hand, if the Sn content exceeds 0.200%, the effect of improving the plating property and the chemical conversion property will be saturated, and there is a possibility that the press formability (bendability inside the steel sheet) and the crack propagation resistance inside the steel sheet will be deteriorated.
• the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the above basic components, with the balance other than the above basic components including Fe (iron) and unavoidable impurities.
• the base steel sheet of the steel sheet according to one embodiment of the present invention has a composition that contains the above basic components, with the balance consisting of Fe and unavoidable impurities.
• the base steel sheet of the steel sheet according to one embodiment of the present invention may contain at least one selected from the optional components shown below. Note that the effects of the present invention can be obtained so long as the optional components shown below are contained in amounts below the upper limit amounts, so no lower limit is set. Note that when the optional elements listed below are contained in amounts below the preferred lower limit values described below, the elements are considered to be included as unavoidable impurities.
• Nb 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less , Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.100 At least one selected from the following: 0% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less
• Ti 0.200% or less Like Nb, Ti increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. In order to obtain such an effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. The Ti content is more preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated.
• the coarse precipitates and inclusions may become the starting point of cracks during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel sheet and the bendability of the shear end surface may not be achieved. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less. The Ti content is more preferably 0.050% or less, and even more preferably 0.030% or less.
• V 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
• the V content is preferably 0.001% or more.
• the V content is more preferably 0.005% or more.
• the V content is further preferably 0.010% or more, and even more preferably 0.020% or more.
• the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated.
• the coarse precipitates and inclusions may become the starting point of cracks during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface may not be achieved. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.
• Cr 1.000% or less
• Cr is an element that enhances hardenability, so the addition of Cr produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS.
• the Cr content is preferably 0.0005% or more.
• the Cr content is more preferably 0.010% or more.
• Cr is more preferably 0.030% or more, and even more preferably 0.040% or more.
• the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, and the fresh martensite becomes the origin of void generation in the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test.
• the Cr content is preferably 1.000% or less. Moreover, the Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less. The Cr content is more preferably 0.100% or less, and even more preferably 0.080% or less.
• Mo 1.000% or less
• Mo is an element that enhances hardenability, so the addition of Mo produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS.
• the Mo content is preferably 0.010% or more.
• the Mo content is more preferably 0.030% or more.
• the Mo content exceeds 1.000%, the area ratio of fresh martensite increases excessively, and fresh martensite becomes the origin of void generation in the 90 degree V bending test, the close bending test, and the close bending + orthogonal 90 degree V bending test. As a result, there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end face cannot be achieved.
• the Mo content when Mo is contained, it is preferable that the Mo content is 1.000% or less.
• the Mo content is more preferably 0.500% or less, further preferably 0.450% or less, and further preferably 0.400% or less.
• the Mo content is more preferably 0.350% or less, and further more preferably 0.300% or less.
• the Mo content is more preferably 0.100% or less, and further preferably 0.080% or less.
• the fresh martensite and the coarse precipitates and inclusions may become the starting point of void generation during the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test, and there is a risk that the desired bendability of the steel sheet and the bendability of the sheared end surface cannot be achieved. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.
• W 0.500% or less W is an element that enhances hardenability, so the addition of W produces a large amount of tempered martensite, ensuring a TS of 780 MPa or more and a high YS.
• the W content is preferably 0.001% or more.
• the W content is more preferably 0.020% or more.
• the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, and fresh martensite becomes the origin of void generation in the 90 degree V-bend test, the close bending test, and the close bending + orthogonal 90 degree V-bend test.
• the Mg content is preferably 0.0200% or less.
• the Mg content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Mg content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.
• the Zn content is preferably 0.0200% or less.
• the Zn content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Zn content is more preferably 0.0100% or less, and even more preferably 0.0080% or less.
• the Co content is preferably 0.0200% or less.
• the Co content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are each 0.0200% or less.
• the As content is preferably 0.0500% or less.
• the Se content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Se content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Se content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Te content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Te content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Te content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Ge content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Ge content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Ge content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the As content is more preferably 0.0010% or more, and even more preferably 0.0015% or more.
• the As content is more preferably 0.0100% or more, and even more preferably 0.0150% or more.
• the As content is more preferably 0.0400% or less, and even more preferably 0.0300% or less.
• the Sr content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Sr content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Sr content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Cs content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Cs content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Cs content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Hf content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Hf content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Hf content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Pb content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Pb content is more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Pb content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Bi content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Bi content is more preferably 0.0100% or less, and even more preferably 0.0050% or less.
• the REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the base steel sheet of the steel sheet of the present invention has, in mass%, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, Sb: 0.200% or less (including 0%), and Sn: 0.200% or less (including 0%), and optionally Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less It has a composition containing at least one selected from the following: Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn:
• the steel structure of the steel plate according to one embodiment of the present invention will be described.
• the area ratio of ferrite is 76.5% or less (including 0.0%)
• the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 20.0% or more and 90.0% or less
• the area ratio of retained austenite is 3.5% or more and 10.0% or less
• the area ratio of fresh martensite is 10.0% or less (including 0.0%).
• the area ratio of ferrite is set to 76.5% or less.
• the area ratio of ferrite is preferably 60.0% or less.
• the lower limit of the area ratio of ferrite is not particularly limited and may be 0.0%.
• the area ratio of ferrite may be 5.0% or more, or 10.0% or more.
• Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more and 90.0% or less
• Bainitic ferrite and tempered martensite have intermediate hardness between soft ferrite and hard fresh martensite, and are important phases for ensuring good bending properties of steel sheets and bending properties of sheared end faces.
• Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite.
• Tempered martensite is effective for improving TS. Therefore, the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): 20.0% or more. Preferably, it is 30.0% or more.
• the total area ratio of bainitic ferrite and tempered martensite is set to 90.0% or less.
• the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is preferably 87.0% or less. It is more preferable that the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) is 80.0% or less.
• Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.
• Area fraction of retained austenite 3.5% or more and 10.0% or less
• the area fraction of retained austenite is set to 3.5% or more.
• the area fraction of retained austenite is preferably more than 3.5%.
• the area fraction of the retained austenite is set to 10.0% or less.
• the area fraction of the retained austenite is preferably 9.0% or less, and more preferably 8.0% or less.
• Area ratio of fresh martensite 10.0% or less (including 0.0%) If the area ratio of fresh martensite increases excessively, the fresh martensite may become the starting point for void generation in the 90 degree V-bend test, the contact bending test, and the contact bending + orthogonal 90 degree V-bend test, and the desired bendability of the steel plate and the bendability of the sheared end face may not be achieved. From the viewpoint of ensuring good bendability of the steel sheet and bendability of the sheared end surface, the area ratio of fresh martensite is set to 10.0% or less, and preferably 5.0% or less. The lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0.0%. It should be noted that fresh martensite is martensite that has not been quenched (i.e., has not been tempered).
• the area ratio of the remaining structure other than the above is preferably 10.0% or less.
• the area ratio of the remaining structure is more preferably 7.0% or less, and further preferably 5.0% or less.
• the area ratio of the remaining structure may be 0.0%.
• the remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of the remaining structure can be confirmed by observation using, for example, a scanning electron microscope (SEM).
• X is the thickness ( ⁇ m) of the soft surface layer
• [Sb] and [Sn] are the contents (mass%) of Sb and Sn in the steel, respectively.
• the soft surface layer in the present invention refers to a region in which the Vickers hardness is 84% or less of the Vickers hardness at a position 1/4 of the sheet thickness from the surface of the base steel sheet.
• the thickness (X) of the soft surface layer must satisfy the formula (1). If the thickness (X) of the surface soft layer is less than 20 ⁇ m, the desired bendability intended by the present invention may not be obtained.
• the surface soft layer thickness (X) exceeds (120-3800 ⁇ [Sb]-1900 ⁇ [Sn]) ⁇ m, it is not possible to achieve both high strength and excellent press formability as intended by the present invention. Therefore, the surface soft layer thickness (X) is specified to be 20 ⁇ m or more and (120-3800 ⁇ [Sb]-1900 ⁇ [Sn]) ⁇ m or less.
• Sb and Sn are added as necessary to improve plating property and chemical conversion property, but when Sb and Sn are added, the allowable upper limit of the soft surface layer thickness (X) that affects bending cracks is lowered due to the surface segregation of these elements as described above.
• the upper limit of the soft surface layer that provides good bending property is (120-3800 x [Sb]-1900 x [Sn]) ⁇ m.
• the thickness of the surface soft layer is preferably 25 ⁇ m or more, and more preferably 30 ⁇ m or more.
• the thickness of the surface soft layer is preferably 100 ⁇ m or less, and more preferably 90 ⁇ m or less.
• the area ratio of ferrite in the soft surface layer is preferably 60.0% or more.
• the area ratio of ferrite may be 100.0%.
• the area ratio of ferrite may be 99.9% or less, 95.0% or less, or 90.0% or less.
• the value obtained by dividing the area ratio of martensite in the soft surface layer by the area ratio of hard phases other than ferrite is set to 0.5 or less.
• the hard phase other than ferrite refers to bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite).
• the lower limit of the value obtained by dividing the area ratio of martensite in the soft surface layer by the area ratio of the hard phase other than ferrite is not particularly limited, and may be 0.00.
• the area ratio of fresh martensite in the surface soft layer divided by the area ratio of hard phases other than ferrite can be suppressed to 0.5 or less.
• a tension of 2.0 kgf/ mm2 or more once or more after the first holding step untransformed austenite undergoes processing-induced transformation to become fresh martensite, which is then tempered during the subsequent second holding step, and finally becomes tempered martensite.
• the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard phase (hard second phase (retained austenite + fresh martensite)) in the 1/4 position of the sheet thickness of the base steel sheet and in the soft surface layer are measured as follows.
• the structure of the soft surface layer is measured at a position of 1/2 of the thickness of the soft surface layer.
• a sample is cut out from the base steel sheet so that the plate thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet becomes the observation surface.
• the observation surface of the sample is then mirror-polished using diamond paste.
• the observation surface of the sample is then finish-polished using colloidal silica, and etched with 3 vol. % nital to reveal the structure.
• Hard second phase (retained austenite + fresh martensite): This is a region that is white to light gray in color and has an amorphous form. It does not contain iron-based carbides. If the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray.
• Carbides These are white areas that are dot-like or linear in shape and are included in tempered martensite, bainitic ferrite, and ferrite.
• Remaining structure The above-mentioned lower bainite, pearlite, inner oxides, etc. are included, and the forms thereof are as known in the art.
• the area of each phase identified in the structural image is calculated using the following method.
• An equally spaced 20 x 20 grid is placed on an area of 25.6 ⁇ m x 19.2 ⁇ m in actual length on the 5000x magnification SEM image, and the area ratios of ferrite, bainitic ferrite, tempered martensite, and other hard phases (hard second phases) are investigated using the point counting method, which counts the number of points on each phase.
• the area ratio is the average value of three area ratios determined on separate 5000x magnification SEM images.
• the area ratio of retained austenite is measured as follows.
• the base steel sheet is mechanically ground in the thickness direction (depth direction) to a position of 1/4 of the thickness, and then chemically polished with oxalic acid to obtain an observation surface.
• the observation surface is then observed by X-ray diffraction.
• MoK ⁇ rays are used as the incident X-rays, and the ratio of the diffraction intensity of each of the (200), (220) and (311) faces of fcc iron (austenite) to the diffraction intensity of each of the (200), (211) and (220) faces of bcc iron is obtained, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensity of each face.
• the retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of the retained austenite is taken as the area fraction of the retained austenite.
• the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of the hard second phase determined as described above.
• [Area ratio of fresh martensite (%)] [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]
• the area ratio of the remaining structure is determined by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of other hard phases (hard second phases) determined as described above from 100.0%.
• [Area ratio of remaining structure (%)] 100.0 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]
• the tensile strength TS of a steel plate according to one embodiment of the present invention is 780 MPa or more and less than 1180 MPa.
• the specified yield stress (YS), yield ratio (YR), internal stretch formability (total elongation (El)), bendability of the steel plate, and bendability of the sheared end surface of the steel plate according to one embodiment of the present invention are as described above. It is preferable that the ratio YR (yield ratio) of the yield stress YS to the tensile strength TS satisfies 0.70 ⁇ YR.
• TS tensile strength
• Yield ratio YR
• yield stress YS
• El total elongation
• the bendability of the steel plate is measured by a close bending test and a close bending + orthogonal 90 degree V-bend test described later in the Examples.
• the bendability of the sheared end surface is measured by a 90 degree V-bend test described later in the Examples.
• a steel sheet according to one embodiment of the present invention may have a plating layer formed on the base steel sheet (on the surface of the base steel sheet), and this plating layer may be provided on only one surface of the base steel sheet, or on both surfaces.
• the plating layer (zinc plating layer) referred to here refers to a plating layer whose main component is Zn (Zn content is 50.0% or more), and examples of this include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.
• the hot-dip galvanized layer is composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al.
• the hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%.
• the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.
• the galvannealed layer is preferably composed of, for example, 20% by mass or less Fe and 0.001% by mass or more and 1.0% by mass or less Al.
• the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM in a total amount of 0% by mass or more and 3.5% by mass or less.
• the Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more.
• the Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.
• the coating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g/ m2 or more. Also, the coating weight of the plating layer (zinc plating layer) per side is preferably 80 g/ m2 or less.
• the plating weight of the plating layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% aqueous hydrochloric acid solution. Next, a test steel sheet (galvanized steel sheet) is immersed in the treatment solution to dissolve the plating layer (galvanized layer). The mass loss of the test material before and after dissolution is then measured, and the value is divided by the surface area of the base steel sheet (the surface area of the part that was covered with plating) to calculate the plating coverage (g/ m2 ).
• a corrosion inhibitor for Fe Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.
• the thickness of the steel plate according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more, more preferably 0.6 mm or more, and even more preferably 0.8 mm or more.
• the thickness of the steel plate is preferably 2.3 mm or less, more preferably 1.6 mm or less, and even more preferably 1.2 mm or less.
• t2 is the annealing time (s)
• Ac1 is Ac1 (° C.).
• the above temperatures refer to the surface temperatures of the steel slab and the steel plate.
• a steel slab having the above-mentioned composition is prepared.
• a steel material is melted to obtain molten steel having the above-mentioned composition.
• the melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used.
• the obtained molten steel is then solidified to obtain a steel slab.
• the method for obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, a thin slab casting method, etc. can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferred.
• the steel slab is then hot rolled to produce a hot rolled steel sheet.
• the hot rolling may be performed by applying an energy-saving process, such as direct rolling (a method in which a steel slab is not cooled to room temperature, but is charged as a hot piece into a heating furnace and hot rolled) or direct rolling (a method in which a steel slab is briefly kept at a certain temperature and then immediately rolled).
• the hot rolling conditions are not particularly limited, and the hot rolling can be performed, for example, under the following conditions. That is, the steel slab is once cooled to room temperature, and then reheated and rolled.
• the slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoints of dissolving carbides and reducing the rolling load.
• the slab heating temperature is preferably 1300°C or lower.
• the slab heating temperature is based on the temperature of the steel slab surface.
• the steel slab is subjected to rough rolling according to the usual method to obtain a rough rolled plate (hereinafter also referred to as a sheet bar).
• the sheet bar is then subjected to finish rolling to obtain a hot rolled steel plate.
• the slab heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling in order to prevent problems during the finish rolling.
• the finish rolling temperature is preferably 800°C or higher in order to reduce the rolling load.
• the reduction rate in the unrecrystallized state of austenite becomes high, an abnormal structure elongated in the rolling direction may develop, which may reduce the workability of the annealed sheet.
• the finish rolling temperature be in the range of 800°C or higher. It is also preferable that the finish rolling temperature be in the range of 950°C or lower.
• the hot-rolled steel sheet is coiled.
• the coiling temperature is preferably 450°C or higher. It is also preferable that the coiling temperature is 750°C or lower.
• the sheet bars may be joined together during hot rolling and continuous finish rolling may be performed.
• the sheet bar may be wound once before the finish rolling.
• a part or all of the finish rolling may be lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material.
• the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.
• hot rolling process including rough rolling and finish rolling
• steel slabs are made into sheet bars by rough rolling and then made into hot rolled steel sheets by finish rolling.
• such division is not important and it is not a problem as long as the specified size is achieved.
• the hot-rolled steel sheet is pickled.
• pickling oxides on the surface of the steel sheet can be removed, and good chemical conversion treatment properties and plating quality are ensured.
• Pickling may be performed once or multiple times. There are no particular limitations on the pickling conditions, and the usual methods may be followed.
• the cold rolling is carried out by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
• the reduction ratio (cumulative reduction ratio) of the cold rolling is not particularly limited, but is set to 20% or more and 80% or less. If the reduction ratio of the cold rolling is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and there is a risk that the TS and bendability of the final product will decrease. On the other hand, if the reduction rate in cold rolling exceeds 80%, the steel sheet is likely to have defective shape and the coating weight may become non-uniform. Furthermore, the cold-rolled steel sheet obtained after cold rolling may be optionally subjected to pickling.
• the steel sheet obtained as described above is heated and annealed in an atmosphere having an annealing temperature of Ac1 (° C.) or more and 900° C. or less, an annealing time of 20 seconds or more, and a dew point (annealing dew point) of ⁇ 10° C. or more.
• the number of annealing steps may be two or more, but one annealing step is preferable from the viewpoint of energy efficiency.
• Annealing temperature Ac1 (°C) to 900°C
• the annealing temperature is lower than Ac1 (°C)
• the ratio of austenite generated during heating in the two-phase region of ferrite and austenite becomes insufficient, and the area ratio of ferrite increases excessively after annealing, lowering TS and YS.
• the annealing temperature exceeds 900°C, excessive grain growth of austenite occurs, the Ms point increases, and a large amount of tempered martensite containing carbides is formed, making it difficult to obtain a retained austenite area ratio of 3.5% or more, and reducing ductility.
• the annealing temperature is set to be equal to or higher than the Ac1 point (° C.) and equal to or lower than 900° C.
• the annealing temperature is preferably equal to or lower than 880° C.
• the annealing temperature is the maximum temperature (soaking temperature) reached in the annealing process.
• [%C] is the C content of the steel plate (steel slab)
• [%Si] is the Si content of the steel plate (steel slab)
• [%Mn] is the Mn content of the steel plate (steel slab).
• Annealing time (soaking time): 20 seconds or more If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and TS and YS decrease. In addition, the C concentration in austenite during annealing increases excessively, and the desired bendability of the sheared end surface cannot be achieved. Furthermore, a surface soft layer thickness of 20 ⁇ m or more cannot be formed during annealing, and the desired bendability of the steel sheet cannot be achieved. Therefore, the annealing time is 20 seconds or more. The annealing time is preferably 40 seconds or more.
• the annealing time is a holding time in a temperature range of (annealing temperature -40°C) or more and (annealing temperature) or less.
• the annealing time includes not only the holding time at the annealing temperature, but also the residence time in a temperature range of (annealing temperature -40°C) or more and (annealing temperature) or less during heating and cooling before and after reaching the annealing temperature.
• T is the annealing temperature (° C.)
• t1 is the time (s) from 650° C. to the annealing temperature T during the temperature rise in the annealing process
• t2 is the annealing time (s)
• Ac1 is Ac1 (° C.).
• Y in formula (3) is less than 2400, the soft surface layer defined in the present invention is less than 20 ⁇ m.
• the soft surface layer defined in the present invention is more than (120-3800 ⁇ [Sb]-1900 ⁇ [Sn]) ⁇ m. Therefore, Y in formula (3) is set to be 2400 or more and 20000 or less. It is preferable that t1 is 30 s or more. Also, it is preferable that t1 is 80 s or less.
• Dew point of the atmosphere (annealing atmosphere) in the annealing process (annealing dew point): -10°C or higher
• the dew point of the atmosphere (annealing atmosphere) in the annealing process is preferably -10°C or higher.
• the dew point of the annealing atmosphere in the annealing process is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably +10°C or higher.
• the dew point of the annealing atmosphere in the annealing process is preferably 30°C or lower.
• an isothermal holding step may be performed, if necessary, at 400° C. or more and 600° C. or less (hereinafter also referred to as an isothermal holding temperature range) for less than 80 seconds in order to promote bainite transformation.
• bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite, thereby ensuring a predetermined area ratio of retained austenite and improving elongation.
• Isothermal holding temperature range 400°C or higher and 600°C or lower If the isothermal holding temperature is less than 400°C, lower bainite and martensite containing a large amount of carbides are generated, and the diffusion of C into the untransformed austenite is suppressed, so that it may not be possible to ensure the area ratio of the specified amount of retained austenite. On the other hand, if the isothermal holding temperature exceeds 600° C., untransformed austenite may be transformed into pearlite, and TS and ductility may not be ensured. Therefore, the isothermal holding temperature is preferably set to 400° C. or higher and 600° C. or lower.
• Holding time in isothermal holding temperature range less than 80 seconds If the holding time in the isothermal holding temperature range is 80 seconds or more, the area ratio of bainitic ferrite increases excessively, the C concentration in the untransformed austenite increases excessively, and there is a risk that the desired bendability of the sheared edge cannot be achieved. Therefore, it is preferable that the holding time in the isothermal holding temperature range is less than 80 seconds.
• the steel sheet after the annealing step is cooled to a cooling stop temperature of 100° C. or more and 300° C. or less.
• the average cooling rate is preferably 10° C./s or more and 50° C./s or less, and the dew point of the atmosphere is preferably ⁇ 20° C. or less.
• Cooling stop temperature 100°C or higher and 300°C or lower Average cooling rate: 10°C/s or higher and 50°C/s or lower Atmospheric dew point: -20°C or lower (preferable conditions)
• the cooling start temperature can be Ac1 (° C.) or more and 900° C. or less, and can be 400° C. or more and 600° C. or less when an isothermal holding step is performed.
• the cooling step is a step necessary for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the subsequent first holding step (reheating and holding step) within a predetermined range. If the cooling stop temperature is less than 100°C, the untransformed austenite present in the steel in the cooling step is almost entirely transformed into martensite. As a result, the area ratio of tempered martensite ultimately increases excessively, making it difficult to obtain an area ratio of retained austenite of 3.5% or more, and reducing ductility. On the other hand, when the cooling stop temperature exceeds 300°C, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases.
• the cooling stop temperature is 100°C or more and 300°C or less.
• the cooling stop temperature is preferably 120°C or more.
• the cooling stop temperature is preferably 280°C or less.
• the average cooling rate during this cooling step is preferably 10° C./s or more.
• the average cooling rate during this cooling step is preferably 50° C./s or less.
• the metal phase defined in the present invention can be obtained by this cooling step.
• the average cooling rate is less than 10° C./s, the amount of untransformed austenite that is entirely transformed into martensite during the cooling step increases, making it difficult to finally obtain retained austenite at an area ratio of 3.5% or more, and ductility may decrease.
• the average cooling rate exceeds 50° C./s, self-relaxation during martensite transformation may be suppressed, and the plate shape may deteriorate.
• the dew point of the atmosphere in this cooling step is preferably -20°C or less. If the dew point of the atmosphere exceeds -20°C, the thickness of the soft surface layer in the in-plane direction of the steel sheet becomes more uneven, and the tensile strength specified in the present invention may not be obtained.
• the dew point of the atmosphere in this cooling step is preferably -20°C or less.
• the above average cooling rate (° C./s) is obtained by dividing the difference between the cooling start temperature (° C.) and the cooling end temperature (° C.) in the cooling step by the cooling time (s).
• first reheating holding process Next, in the first holding step (first reheating holding process), the steel sheet is reheated to a temperature range of 370°C or more and 460°C or less (also referred to as reheating holding temperature range, but hereinafter also referred to as first reheating holding temperature range in order to distinguish it from the reheating holding temperature range of the second holding step), and held for 10 seconds or more.
• first reheating holding temperature range also referred to as reheating holding temperature range, but hereinafter also referred to as first reheating holding temperature range in order to distinguish it from the reheating holding temperature range of the second holding step
• Reheating holding temperature (first reheating holding temperature): 370°C or higher and 460°C or lower
• C is concentrated in the austenite remaining after the cooling step, thereby reducing the area ratio of fresh martensite in the final structure while ensuring the area ratio of a predetermined amount of retained austenite. If the reheating holding temperature (first reheating holding temperature) is less than 370°C, the C concentration in the austenite remaining after the cooling step is insufficient, making it difficult to obtain retained austenite at an area ratio of 3.5% or more, and ductility is reduced.
• the reheating temperature (first reheating temperature) exceeds 460°C, C is excessively concentrated in the untransformed austenite, and the untransformed austenite in the surface layer does not undergo processing-induced transformation in the surface strain introduction step described below, and becomes retained austenite or fresh martensite.
• the reheating temperature (first reheating temperature) is set to 370°C or higher and 460°C or lower.
• Holding time in the reheating holding temperature range (first reheating holding temperature range): 10 seconds or more If the holding time in the reheating holding temperature range is less than 10 seconds, the C concentration in the austenite remaining after the cooling step is insufficient, making it difficult to obtain retained austenite of 3.5% or more in area ratio, and ductility may decrease. Therefore, the holding time in the first reheating holding temperature range is set to 10 seconds or more.
• a tension of 2.0 kgf/ mm2 or more is applied between the first holding step (reheating and holding step) and the second holding step, thereby introducing strain into the surface layer.
• a tension of 2.0 kgf/ mm2 or more is applied between the first holding step (reheating and holding step) and the second holding step, thereby introducing strain into the surface layer.
• the load cells must be arranged parallel to the tension direction.
• the load cells are preferably disposed at positions 200 mm from both ends of the roll, and the body length of the roll used is preferably 1500 to 2500 mm.
• this tension is preferably 2.2 kgf/ mm2 or more, and more preferably 2.4 kgf/ mm2 or more.
• this tension is preferably 15.0 kgf/mm2 or less , and more preferably 10.0 kgf/ mm2 or less.
• the steel sheet is held at 300° C. or more and 460° C. or less for 10 seconds or more.
• the term "holding” as used herein also includes cooling (slow cooling) within a range of 300° C. to 460° C. for 10 seconds or more.
• Second holding temperature (reheating holding temperature range (second reheating holding temperature range): 300° C. or higher and 460° C. or lower
• the martensite formed in the surface layer in the surface strain introduction step is tempered.
• the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) becomes 0.5 or less, and the desired bendability of the steel sheet is obtained.
• the second holding temperature is less than 300°C, the martensite formed in the surface layer in the surface strain introduction process is not tempered, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite exceeds 0.5.
• the second holding temperature exceeds 460°C, the retained austenite inside the steel sheet decomposes, and the desired El cannot be obtained. Therefore, the second holding temperature (second reheating holding temperature range) is set to 300°C or higher and 460°C or lower.
• Second reheating holding temperature range 10 seconds or more If the holding time at the second holding temperature (reheating holding temperature range: 300°C or more and 460°C or less) is less than 10 seconds, the martensite generated in the surface layer in the surface strain introduction step will not be tempered sufficiently, and the value obtained by dividing the area ratio of fresh martensite in the surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite will exceed 0.5. Therefore, the holding time in the reheating holding temperature range is set to 10 seconds or more.
• the steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet.
• the galvanizing treatment include a hot-dip galvanizing treatment and a galvannealing treatment.
• the galvanizing treatment in the plating step is performed after the annealing step.
• the galvanizing treatment may be performed, for example, during the cooling step, during the first holding step, after the first holding step and before the surface strain introducing step, after the surface strain introducing step and before the second holding step, during the second holding step, or after the second holding step.
• hot-dip galvanizing it is preferable to immerse the steel sheet in a zinc plating bath (hot-dip galvanizing bath) at 440°C to 500°C, and then adjust the coating weight by gas wiping or the like.
• a zinc plating bath hot-dip galvanizing bath
• the hot-dip galvanizing bath there are no particular limitations on the hot-dip galvanizing bath as long as it has the composition of the zinc plating layer described above, but it is preferable to use, for example, a plating bath with an Al content of 0.10 mass% to 0.23 mass%, with the balance consisting of Zn and unavoidable impurities.
• alloying hot-dip galvanizing treatment it is preferable to carry out the hot-dip galvanizing treatment as described above, and then to carry out alloying treatment by heating the hot-dip galvanized steel sheet to an alloying temperature of 450° C. or more and 600° C. or less. If the alloying temperature is less than 450° C., the Zn—Fe alloying rate becomes slow, and alloying may become difficult. On the other hand, when the alloying temperature exceeds 600° C., untransformed austenite is transformed into pearlite, and ductility is reduced.
• the alloying temperature is more preferably 480° C. or higher.
• the alloying temperature is more preferably 550° C. or lower.
• the steel sheet obtained as described above may be further subjected to temper rolling. If the reduction rate of temper rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when the steel sheet is formed into a component may decrease. Therefore, the reduction rate of temper rolling is preferably 2.00% or less.
• the lower limit of the reduction rate of temper rolling is not particularly limited, but it is preferably 0.05% or more from the viewpoint of productivity.
• Temper rolling may be performed on a device connected to the annealing device for performing each of the above-mentioned processes (online), or on a device not connected to the annealing device for performing each of the processes (offline).
• the number of rolling times of temper rolling may be one or more than two. As long as the same elongation rate as that of temper rolling can be imparted, rolling using a leveler or the like may be used.
• Conditions other than those mentioned above are not particularly limited and may be in accordance with conventional methods. From the viewpoint of productivity, it is preferable to carry out a series of processes such as the above-mentioned annealing, hot-dip galvanizing, and alloying treatment of the zinc plating in a continuous galvanizing line (CGL). After hot-dip galvanizing, wiping is possible to adjust the coating weight. Note that plating conditions other than those mentioned above may be in accordance with conventional methods for hot-dip galvanizing.
• a member according to an embodiment of the present invention is a member made using the above-mentioned steel plate (as a raw material).
• the raw material steel plate is subjected to at least one of forming and joining to form a member.
• the above steel plate has a TS of 780 MPa or more and less than 1180 MPa, and has a high YS, excellent press formability inside the steel plate (bendability and stretch formability of the steel plate), and excellent press formability at the steel plate end (bendability of the steel plate end (shear cross section)). Therefore, the member according to one embodiment of the present invention has high strength and excellent press formability. Therefore, the member according to one embodiment of the present invention is particularly preferably applied to an impact energy absorbing member used in the automotive field.
• a method for manufacturing a component according to one embodiment of the present invention includes a step of subjecting the above-mentioned steel plate (e.g., a steel plate manufactured by the above-mentioned steel plate manufacturing method) to at least one of forming and joining to form a component.
• the molding method is not particularly limited, and for example, a general processing method such as press processing can be used.
• the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, crimp joining, etc.
• the molding conditions and joining conditions are not particularly limited, and may be in accordance with ordinary methods.
• the galvanizing bath temperature was 470° C. for both GI and GA production.
• the zinc plating coverage was 45 to 72 g/m2 per side when producing GI, and 45 g/ m2 per side when producing GA.
• the composition of the finally obtained plating layer (zinc plating layer) of the steel sheet was as follows: GI: 0.1-1.0 mass% Fe, 0.2-0.33 mass% Al, and the balance being Zn and unavoidable impurities, whereas GA: 8.0-12.0 mass% Fe, 0.1-0.23 mass% Al, and the balance being Zn and unavoidable impurities.
• the plating layers were formed on both sides of the base steel sheets in each case.
• the steel structure of the base steel plate was identified using the obtained steel plate in the manner described above.
• the measurement results are shown in Table 3.
• F is ferrite
• BF is bainitic ferrite
• TM is tempered martensite
• RA is retained austenite
• FM is fresh martensite.
• LB lower bainite
• ⁇ is carbide.
• the method for measuring the surface soft layer is as follows. After smoothing the thickness cross section (L cross section) parallel to the rolling direction of the steel sheet by wet polishing, measurements were performed at 1 ⁇ m intervals using a Vickers hardness tester with a load of 10 gf (9.8 ⁇ 10 ⁇ 2 N) from a position 1 ⁇ m from the steel sheet surface in the thickness direction to a position 100 ⁇ m in the thickness direction. Thereafter, measurements were performed at 20 ⁇ m intervals to the center of the sheet thickness. The region where the Vickers hardness is reduced to 84% or less compared to the hardness at the 1/4 position of the sheet thickness is defined as the soft layer (surface soft layer), and the thickness of the region in the thickness direction is defined as the thickness of the soft layer.
• the structure of the soft surface layer was identified at a position halfway through the thickness of the soft surface layer using a method similar to that used to identify the steel structure of the base steel plate.
• tensile tests 90 degree V-bend tests, contact bending tests, and contact bending + orthogonal 90 degree V-bend tests were conducted according to the following procedures, and the tensile strength (TS), yield stress (YS), yield ratio (YR), total elongation (El), bendability of the steel plate, and bendability of the sheared edge were evaluated according to the following criteria.
• V-bend end crack length The length of the crack on the V-bend end surface is 200 ⁇ m or less.
• FIG. 2(b) is an overhead view of the sample viewed from the Z direction shown in Figure 2(a). If the section from the bend apex along the steel plate surface with a total width of 5 mm in the C direction (2.5 mm on both sides from the bend apex) is defined as the bend ridgeline, then the section (area o) with a width of 5 mm in the L direction from the very end of the bend ridgeline is defined as the bend ridgeline end.
• the crack length Y1 that propagates from the bend ridgeline end in the ridgeline direction (L direction) and the crack length Y2 that propagates in the L direction along the bend ridgeline formed other than the bend ridgeline end are each measured using the following methods.
• the symbol y shown in FIG. 3-1(a) corresponds to the symbol Y1 (crack length Y1) shown in FIG. 2(b).
• the shear surface a of the bent sample after a 90-degree V-bend test with a bending radius of 0.5 mm was placed on top, and the end of the bend ridgeline was photographed at a magnification of 40 times using a one-shot 3D shape measuring machine (Keyence Corporation, VR6000 series or newer models).
• the obtained height data was analyzed using the analysis software attached to the one-shot 3D shape measuring machine.
• an arc-shaped measurement line i was drawn as close as possible to the outside of the bend that was subjected to tensile stress in accordance with the bend ridgeline.
• An example of the obtained profile waveform j is as shown in Figure 3-2 (b), and the length of each crack (y1 + y2) / 2 was obtained using a measurement tool in the software, and the length of the longest crack was taken as the crack length that propagated from the end of the bend ridgeline in the ridgeline direction after a 90-degree V-bend test with a bending radius of 0.5 mm.
• the length of the cracks that propagate along the bend ridge formed other than at the end of the bend ridge was measured by visual observation at 25x magnification using a stereo microscope.
• the observation surface of the sample was mirror-polished using diamond paste.
• a scanning electron microscope (SEM) was used to photograph a field of view of 2560.0 ⁇ m ⁇ 1920.0 ⁇ m (m in FIG. 4-2(d)) at the position m in FIG. 4-2(d) which is the bending apex of the observation surface of the sample, under conditions of an acceleration voltage of 15 kV and a magnification of 50 times, and the entire crack was observed.
• the distance X between the start point and the end point of the crack was taken as the crack depth.
• This X A close contact bending test was carried out using a 3.0 mm spacer, and the crack depth was evaluated as the depth of cracks that progressed in the plate thickness direction at the bent ridge line subjected to compressive stress.
• At least one of the tensile strength (TS), yield stress (YS), total elongation (El), crack length other than at the V-bend end face, crack length at the V-bend end face, tight bending spacer thickness, tight bending internal crack depth, and handkerchief bending boundary bending radius was insufficient.

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  • 2024-10-03 WO PCT/JP2024/035463 patent/WO2025075094A1/ja unknown
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