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

• the present invention relates to steel plates, components made from the steel plates, and methods for manufacturing them.
• high-strength steel sheets used in automobile skeleton structural members, etc. are required to have high member strength when press-formed.
• YR yield ratio
• YS yield stress
• impact absorption energy the impact absorption energy
• the automobile skeleton structural members for example, crash boxes have a bent portion. Therefore, from the viewpoint of press formability, it is preferable to apply a steel sheet having high bendability to such parts.
• the steel sheet used as the material of the automobile parts is often zinc-plated. Therefore, there is a demand for the development of a hot-dip galvanized steel sheet having excellent press formability and impact resistance in addition to having high strength.
• Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet having a thickness of 0.6 to 5.0 mm and a plating layer on the surface of the steel sheet, the steel sheet structure includes a ferrite phase with a volume fraction of 40 to 90% and a retained austenite phase with a volume fraction of 3 to 25%, the retained austenite phase has a solute carbon content of 0.70 to 1.00%, an average particle size of 2.0 ⁇ m or less, an average distance between particles of 0.1 to 5.0 ⁇ m, a decarburized layer in the surface layer of the steel sheet with a thickness of 0.01 to 10.0 ⁇ m, an average particle size of oxides contained in the surface layer of the steel sheet of 30 to 120 nm, and an average density of 1.0 ⁇ 10 12 particles/m
• the present invention discloses a high- strength hot-dip galvanized steel sheet having excellent mechanical cutting properties, characterized in that the steel sheet has a maximum
• 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 3 discloses a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having good elongation characteristics and bendability, the hot-dip galvanized steel sheet comprising a steel sheet and a hot-dip galvanized layer, the steel sheet including a base material and a decarburized ferrite layer, a structure at a depth position of 1 ⁇ 4 of the sheet thickness of the steel sheet containing 5.0 vol% or more of tempered martensite and 0.5 vol% or more and less than 7.0 vol% of retained austenite, the balance mainly consisting of 4 to 70 vol% of ferrite and bainite, a part or all of the tempered martensite and the retained austenite forming M-A, the decarburized ferrite layer containing 120% or more of ferrite with respect to the content of ferrite at a depth of 1 ⁇ 4 of the sheet thickness, an average ferrite grain size of 20 ⁇ m or less, a thickness of 5 ⁇ m or more and 200 ⁇
• 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.
• improving the yield stress YS (hereinafter also referred to as YS) is effective in increasing the energy absorbed during a collision (hereinafter also referred to as impact absorption energy).
• YS yield stress YS
• increasing the YS of a steel plate generally reduces press formability, particularly properties such as ductility, hole expandability, and bendability. Therefore, when it is assumed that such a steel plate with increased TS and YS is used in the above-mentioned impact energy absorbing component of an automobile, not only does press forming become difficult, but the component will crack in an axial crushing test that simulates a crash test. In other words, the actual impact absorption energy is not as high as expected from the YS value. Therefore, the current situation is that the above-mentioned impact energy absorbing component is limited to the use of steel plates with a TS of 590 to 980 MPa.
• Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet in which ductility is improved by the generation of retained austenite inside the steel sheet and mechanical cuttability is improved by the formation of a decarburized layer on the surface of the steel sheet.
• ductility is improved by the generation of retained austenite inside the steel sheet
• mechanical cuttability is improved by the formation of 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 portion.
• 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 improvement in bendability and fracture resistance during vehicle collision due to the formation of a surface soft layer (decarburized layer), and press formability of the steel sheet end portion.
• Patent Document 3 discloses a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet in which ductility is improved by the generation of M-A inside the steel sheet and bendability is improved by the formation of a soft layer (decarburized ferrite layer) on the surface layer of the steel sheet, but gives no consideration at all to the press formability of the end portions of the steel sheet.
• 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. From the above, it cannot be said that the steel plates disclosed in Patent Documents 1 to 4 have a TS of 1180 MPa or more, a high YS, excellent press formability inside the steel plate (bendability inside the steel plate), and excellent press formability at the end of the steel plate (bendability of the steel plate end (shear cross section)).
• the present invention was developed in consideration of the above-mentioned current situation, and aims to provide a steel plate, component, and manufacturing method thereof, which has a tensile strength (TS) of 1180 MPa or more and less than 1470 MPa, and which has a high yield stress (YS), excellent ductility, excellent press formability inside the steel plate (bendability inside the steel plate), excellent press formability at the end of the steel plate (bendability at the end of the steel plate (shear cross section)), and high crack propagation resistance.
• TS tensile strength
• YS high yield stress
• the steel sheet referred to here also includes zinc-plated steel sheet, which is either 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 tensile strength (TS) is measured by a tensile test in accordance with JIS Z 2241 (2011).
• having a high yield stress (YS) means satisfying the following.
• a high YS means that the YS measured by a tensile test in accordance with JIS Z 2241 (2011) satisfies the following formula (A) or (B) depending on the TS measured by the tensile test.
• A) In the case of 1180 MPa ⁇ TS ⁇ 1320 MPa, 820 MPa ⁇ YS
• B In the case of 1320 MPa ⁇ TS ⁇ 1470 MPa, 920 MPa ⁇ YS
• excellent press formability (bendability) inside the steel sheet means that R (limit bending radius)/t (sheet thickness) measured in a 90 degree V-bending test in accordance with JIS Z 2248 (2022) satisfies the following formula (A) or (B) depending on TS.
• excellent press formability of the steel plate end means that in a 90-degree V-bend test with a bending radius R of 0.5 mm conforming to JIS Z 2248 (2022), the crack length progressing from the end of the bent ridge in the ridge direction is 200 ⁇ m or less (the V-bend end face crack length is 200 ⁇ m or less).
• having high crack propagation resistance means that the formula (A) or (B) is satisfied in the following test (i) and the formula (C) or (D) is satisfied in the following test (ii).
• (i) Notched tensile test after heat treatment at 170°C for 20 minutes A) When 1180MPa ⁇ TS ⁇ 1320MPa, notched El ⁇ 6.5%
• C If 1180MPa ⁇ TS ⁇ 1320MPa, notched El ⁇ 5.5%
• El of the notched tensile test after the above-mentioned heat treatment at 170°C for 20 minutes is an index showing the resistance of cracks to propagation in the vertical wall of the axial crushing component (impact energy absorbing component).
• El of the notched tensile test after the above-mentioned 2% nominal tensile strain is introduced and heat treatment at 170°C for 20 minutes is an index showing the resistance of cracks to propagation in the bending ridge of the axial crushing component (impact energy absorbing component).
• a TS of 1,180 MPa or more can be ensured by setting the area ratio of ferrite to 55.0% or less (including 0.0%) and the combined area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) to more than 40.0% and not more than 100.0% in the structure at the 1/4 position of the plate thickness of the base steel plate.
• a high YS can be ensured by setting the total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite) at the 1/4 position of the plate thickness of the base steel plate to more than 40.0% and not more than 100.0%, the area ratio of retained austenite to less than 3.5%, and the area ratio of fresh martensite to 10.0% or less (including 0.0%).
• the steel sheet has a specified chemical composition and a soft surface layer whose 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, and the soft surface layer satisfies the following formula (1).
• 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 area ratio of ferrite in the soft surface layer is set to 60.0% or more and 100.0% or less
• the area ratio of fresh martensite in the structures other than ferrite is set to 0.5 or less when the area ratio is divided by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite
• the area ratio of retained austenite is set to 3.0% or less, thereby realizing excellent press formability (bendability of the steel sheet inside) and high crack propagation resistance.
• the present disclosure has been made based on the above findings. That is, the gist of the present disclosure is as follows. [1] In mass%, C: 0.050% or more and 0.400% or less, Si: 0.02% or more and 3.00% or less, Mn: 1.50% 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 60.0% or more and 100.
• 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 composition further includes, in mass%, 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: 0.1000% 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, The steel sheet according to the above [1], containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
• An annealing process in which the steel sheet after the hot rolling process is heated and annealed under conditions of an annealing temperature of 750° C. or more and 900° C. or less, an annealing time of 20 seconds or more, and a dew point of ⁇ 10° C.
• the method for producing a steel sheet includes a cold rolling step of performing cold rolling at a rolling reduction rate of 20% or more and 80% or less on the steel sheet after the hot rolling step and before the annealing step to obtain a cold-rolled steel sheet.
• 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.).
• FIG. 1 is an example of a tissue image taken by SEM used for identifying the tissue.
• FIG. 2(a) is a schematic diagram of the sample after being bent at 90 degrees in a V-shape
• FIG. 2(b) is a diagram of the sample shown in FIG. 2(a) as viewed in the Z direction (negative direction).
• FIG. 13 is a schematic diagram of a sample after being bent at 90 degrees.
• FIG. 1 is a schematic diagram of an end face crack caused by a 90-degree V-bend.
• 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: 0.02% or more and 3.00% or less, Mn: 1.50% 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 surface soft layer having a Vickers hardness of 84% or less of the Vickers hardness at a 1/4 position of the sheet thickness from the surface of the base steel sheet, and the surface soft layer satisfies the following formula (1): As a structure in the soft surface layer, the area ratio of ferrite is 60.0% 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 represented as "%”.
• C 0.050% or more and 0.400% or less C is an effective element for generating an appropriate amount of tempered martensite, bainitic ferrite, etc., to ensure a TS of 1180 MPa or more 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 1180 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, TS becomes excessively high, and El decreases.
• the C content is set to 0.050% or more and 0.400% or less.
• the C content is preferably 0.070% or more.
• the C content is more preferably 0.080% or more, and even more preferably 0.090% or more.
• the C content is preferably 0.300% or less.
• the C content is more preferably 0.280% or less, and further preferably 0.250% or less.
• Si 0.02% or more and 3.00% or less Si is an element that suppresses excessive softening of tempered martensite. If the Si content is less than 0.02%, the tempered martensite becomes excessively soft, making it difficult to ensure a TS of 1180 MPa or more. On the other hand, if the Si content exceeds 3.00%, the C concentration in the austenite during annealing increases excessively with an excessive increase in the area ratio of ferrite, and the desired bendability of the sheared edge cannot be achieved. Therefore, the Si content is set to 0.02% or more and 3.00% or less.
• the Si content is preferably 0.10% or more.
• the Si content is more preferably 0.20% or more, and further preferably 0.30% or more.
• the Si content is preferably 1.80% or less.
• the Si content is more preferably 1.70% or less, and further preferably 1.60% or less.
• Mn 1.50% or more and less than 3.50%
• Mn is an element that adjusts the area ratio of bainitic ferrite and tempered martensite. If the Mn content is less than 1.50%, the area ratio of ferrite increases, making it difficult to achieve a TS of 1180 MPa or more. This also leads to a decrease in YS. On the other hand, when the Mn content is 3.50% or more, the martensite transformation start temperature Ms (hereinafter also simply referred to as Ms point or Ms) decreases, and the martensite generated in the cooling process decreases.
• Ms point or Ms the martensite transformation start temperature
• the Mn content is 1.50% or more and less than 3.50%.
• the Mn content is preferably 2.00% or more, more preferably 2.20% or more.
• the Mn content is preferably 3.20% or less.
• the Mn content is more preferably 3.10% or less, and even more preferably 3.00% or less.
• P 0.001% or more and 0.100% or less
• P is an element that has a solid solution strengthening effect and increases the TS and YS of a steel sheet. To obtain such an effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates to the prior austenite grain boundaries and embrittles the grain boundaries.
• the P content is set to 0.001% or more and 0.100% or less.
• the P content is preferably 0.002% or more, more preferably 0.004% or more.
• the P content is preferably 0.030% or less.
• the P content is preferably 0.025% or less, more preferably 0.020% or less.
• S 0.0001% or more and 0.0200% or less S exists as sulfides in steel.
• the S content exceeds 0.0200%, voids are generated and cracks develop from the sulfides during a V-bend test, making it difficult to ensure the press formability of the steel plate end (bendability of the steel plate end (shear cross section)).
• the S content is set to 0.0200% or less.
• the S content is preferably 0.0080% or less.
• the S content is more preferably 0.0050% or less, and even more preferably 0.0030% or less.
• the S content is set to 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more.
• Al 0.005% to 2.000%
• Al promotes ferrite transformation during annealing and in the cooling process after annealing. That is, Al is an element that affects the area ratio of ferrite.
• the Al content is less than 0.005%, the area ratio of ferrite decreases, and ductility decreases.
• the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 1180 MPa or more. In addition, this also leads to a decrease in YS. Therefore, the Al content is set to 0.005% or more and 2.000% or less.
• the Al content is preferably 0.010% or more.
• the Al content is more preferably 0.020% or more, and further preferably 0.030% or more.
• the Al content is preferably 1.000% or less, more preferably 0.800% or less, and further preferably 0.500% or less.
• N 0.0100% or less N exists as nitrides in steel.
• the N content exceeds 0.0100%, voids are generated and cracks develop from the nitrides during a V-bend test, making it difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the crack length developing from the bend ridge end to the ridge line direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 ⁇ m or less. Therefore, the N content is set to 0.0100% or less. In addition, the N content is preferably 0.0050% or less.
• the N content is more preferably 0.0045% or less, and even more preferably 0.0040% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more, more preferably 0.0010% or more, and further preferably 0.0015% or more.
• Sb 0.200% or less (including 0%)
• Sb is a useful element that segregates on the steel sheet surface during annealing to improve plating and chemical conversion treatment properties. Therefore, the Sb content may be 0%, but 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 0.008% or more. On the other hand, if the Sb content exceeds 0.200%, the effect of improving the plating property and chemical conversion property is saturated, and there is a risk of causing a decrease in the press formability (bendability inside the steel sheet) and crack propagation resistance inside the steel sheet. 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. The Sb content is further preferably 0.015% or less. The Sb content is even more preferably 0.012% or less, and even more preferably 0.011% or less.
• Sn 0.200% or less (including 0%)
• Sn is a useful element that segregates on the steel sheet surface during annealing to improve plating and chemical conversion treatment properties. Therefore, the Sn content may be 0%, but is preferably 0.002% or more. The Sn content is more preferably 0.003% or more.
• the Sn content exceeds 0.200%, the effect of improving the plating property and chemical conversion property will be saturated, and there is a risk of causing a decrease in the press formability (bendability inside the steel sheet) and crack propagation resistance inside the steel sheet. Therefore, when Sn is contained, it is necessary to make the Sn content 0.200% or less.
• the Sn content is preferably 0.008% or less, and more preferably 0.004% or less.
• 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
• Nb 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
• the Nb content is preferably 0.001% or more.
• the Nb content is more preferably 0.005% or more.
• the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated.
• the coarse precipitates and inclusions become the starting points of voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the crack length that progresses from the bending ridge end to the ridge line direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 ⁇ m or less. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.
• Ti 0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. 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. 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 become the starting points of voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the crack length that progresses from the bend ridge end to the ridge line direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 ⁇ m or less. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% 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 0.030% or more.
• the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated.
• the coarse precipitates and inclusions become the starting points of voids and cracks during a V-bend test, making it difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the crack length that progresses from the bend ridge end to the ridge line direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 ⁇ m or less. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.
• B 0.0100% or less
• B is an element that enhances hardenability by segregating at the austenite grain boundaries.
• B is an element that controls the generation and grain growth of ferrite during cooling after annealing.
• the B content is preferably 0.0001% or more.
• the B content is more preferably 0.0002% or more.
• the B content is further preferably 0.0005% or more, and even more preferably 0.0007% or more.
• the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling.
• the B content is preferably 0.0100% or less.
• the B content is more preferably 0.0050% or less.
• Cr 1.000% or less Cr is an element that enhances hardenability, so that the addition of Cr produces an appropriate amount of tempered martensite, thereby increasing TS and YS.
• the Cr content is preferably 0.0005% or more.
• the Cr content is more preferably 0.100% or more, and further preferably 0.150% or more.
• the Cr content is preferably 1.000% or less.
• the Cr content is more preferably 0.800% or less, and even more preferably 0.700% or less.
• Ni 1.000% or less
• Ni is an element that enhances hardenability, so the addition of Ni produces a large amount of tempered martensite, increasing TS and YS.
• the Ni content is more preferably 0.020% or more.
• the Ni content is further preferably 0.040% or more, and even more preferably 0.060% or more.
• the Ni content exceeds 1.000%, the area ratio of fresh martensite increases, the bendability in the V-bend test decreases, and it becomes difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)).
• the Ni content is preferably 1.000% or less.
• the Ni content is more preferably 0.800% or less.
• the Ni content is more preferably 0.600% or less, and even more preferably 0.400% or less.
• Mo 1.000% or less Mo is an element that enhances hardenability, and thus the addition of Mo produces a large amount of tempered martensite, thereby increasing TS and YS. To obtain such an effect, the Mo content is preferably 0.100% or more, and more preferably 0.150% or more.
• the Mo content exceeds 1.000%, the area ratio of fresh martensite increases, the bendability in the V-bend test decreases, and it becomes difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the crack length propagating from the bend ridge end to the ridge line direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 ⁇ m or less. Therefore, 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, even more preferably 0.450% or less, and even more preferably 0.400% or less.
• the Mo content is more preferably 0.350% or less, and even more preferably 0.300% or less.
• Cu 1.000% or less
• Cu is an element that enhances hardenability, so the addition of Cu produces a large amount of tempered martensite, increasing TS and YS.
• the Cu content is more preferably 0.008% or more, and even more preferably 0.010% or more.
• the Cu content is more preferably 0.020% or more.
• the Cu content is even more preferably 0.100% or more, and even more preferably 0.150% or more.
• the area ratio of fresh martensite may increase excessively.
• a large amount of coarse precipitates and inclusions may be generated.
• the excessively generated fresh martensite and the coarse precipitates and inclusions become the starting points of voids and cracks during the V-bend test, making it difficult to ensure the press formability of the steel sheet end (bendability of the steel sheet end (shear cross section)), and the crack length that progresses from the bending ridge end to the ridge line direction in a 90-degree V-bend test with a bending radius of 0.5 mm cannot be controlled to 200 ⁇ m or less. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.
• Ta 0.100% or less Ta, like Ti, Nb and V, increases TS and YS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing.
• Ta partially dissolves in Nb carbides or Nb carbonitrides to generate composite precipitates such as (Nb, Ta) (C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS and YS.
• the Ta content is 0.001% or more. It is more preferable that the Ta content is 0.002% or more, and even more preferable that it is 0.004% or more.
• the Ta content is preferably 0.100% or less.
• the Ta content is more preferably 0.030% or less, and even more preferably 0.010% or less.
• W 0.500% or less W is an element that enhances hardenability, and thus the addition of W produces a large amount of tempered martensite, thereby increasing TS and YS.
• the W content is preferably 0.001% or more.
• the W content is more preferably 0.010% or more, and even more preferably 0.030% or more.
• the W content is preferably 0.500% or less.
• the W content is more preferably 0.450% or less, and even more preferably 0.400% or less. It is even more preferable that the W content is 0.300% or less.
• Mg 0.0200% or less
• Mg is an element that is effective in making the shape of inclusions such as sulfides and oxides spherical and improving the hole expandability and bendability of steel sheets.
• the Mg content is 0.0001% or more.
• the Mg content is more preferably 0.0010% or more, and even more preferably 0.0030% or more.
• the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated.
• the Mg content is preferably 0.0200% or less.
• the Mg content is more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Zn 0.0200% or less
• Zn is an effective element for making the shape of inclusions spherical and improving the hole expandability and bendability of the steel sheet.
• the Zn content is preferably 0.0010% or more.
• the Zn content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
• the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated.
• 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.
• Co 0.0200% or less
• Co is an effective element for making the shape of inclusions spherical and improving the hole expandability and bendability of the steel sheet.
• the Co content is preferably 0.0010% or more.
• the Co content is more preferably 0.0020% or more, and even more preferably 0.0030% or more.
• the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the excessively coarse precipitates and inclusions may become the starting points of voids and cracks during a V-bend test, and the desired R/t may not be obtained. Therefore, when Co is contained, 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.
• Zr 0.1000% or less
• Zr is an element that is effective in making the shape of inclusions spherical and improving the hole expandability and bendability of the steel sheet.
• the Zr content is preferably 0.0010% or more.
• the Zr content is preferably 0.1000% or less.
• the Zr content is more preferably 0.0300% or less, even more preferably 0.0150% or less, and even more preferably 0.0100% or less.
• Ca 0.0200% or less Ca exists as inclusions in steel.
• the Ca content is preferably 0.0200% or less.
• the Ca content is preferably 0.0020% or less.
• the Ca content is more preferably 0.0019% or less, and even more preferably 0.0018% or less.
• the lower limit of the Ca content is not particularly limited, but the Ca content is preferably 0.0005% or more.
• the Ca content is more preferably 0.0010% or more.
• the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM exceed 0.0200% and/or the content of As exceeds 0.0500%, a large amount of coarse precipitates and inclusions may be generated. In such a case, the excessively coarse precipitates and inclusions may become the starting points of voids and cracks during a V-bend test, so that the desired R/t may not be obtained. Therefore, when at least one of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM is contained, the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi and REM are preferably 0.0200% or less. When As is contained, the As content is preferably 0.0500% or less.
• the Se content is preferably 0.0010% or more, more preferably 0.0050% or more.
• the Se content is more preferably 0.0180% or less, and further 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 even 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 even more preferably 0.0050% 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 even 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 even 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 even 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 even 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 even more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Bi content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the Bi content is even more preferably 0.0100% or less.
• the REM content is more preferably 0.0005% or more, and even more preferably 0.0008% or more.
• the REM content is more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• the REM content is more preferably 0.0180% or less, and even more preferably 0.0150% or less.
• the REM content is even more preferably 0.0100% or less.
• REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanoids ranging from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71.
• the REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REMs.
• the REM is not particularly limited, but is preferably at least one of Sc, Y, Ce and La.
• the area ratio of ferrite is set to 55.0% or less.
• the area ratio of ferrite is preferably set to 45.0% or less, and more preferably set to 30.0% 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 1.0% or more, or 2.0% or more.
• Total area ratio of bainitic ferrite and tempered martensite (excluding retained austenite): more than 40.0% and not more than 100.0%
• 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, bending properties of shear end faces, and axial crushing properties.
• 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): more than 40.0%.
• the total area ratio of bainitic ferrite and tempered martensite is preferably 65.0% or more, more preferably 80.0% or more.
• the upper limit of the total area ratio of bainitic ferrite and tempered martensite may be 100.0%.
• the total area ratio of bainitic ferrite and tempered martensite may be 96.0% or less, or 92.0% or less.
• Bainitic ferrite is upper bainite with little carbide that is formed in a relatively high temperature range.
• Area ratio of retained austenite less than 3.5% (including 0.0%)
• the area ratio of the retained austenite is set to less than 3.5%.
• the area ratio of the retained austenite is preferably 3.0% or less, more preferably 2.5% or less, and even more preferably 2.0% or less.
• the lower limit of the area ratio of the retained austenite is not particularly limited, and the area ratio of the retained austenite may be 0.0%, is preferably 0.1% or more, and is more preferably 0.2% or more.
• the term "retained austenite" as used herein also includes island-like (isolated) retained austenite within ferrite grains.
• Area ratio of fresh martensite 10.0% or less (including 0.0%)
• the area ratio of fresh martensite is set to 10.0% or less.
• the area ratio of fresh martensite is preferably set to 8.0% or less, and more preferably set to 6.0% or less.
• the lower limit of the area ratio of fresh martensite is not particularly limited and may be 0.0%.
• the area ratio of fresh martensite may be 1.0% or more, or 2.0% or more.
• fresh martensite used here refers to as-quenched (untempered) martensite, and also includes islands of fresh martensite (isolated) within ferrite grains.
• 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 even more 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 include carbides such as pearlite and cementite.
• the type of remaining structure can be confirmed, for example, by observation with a SEM (Scanning Electron Microscope).
• the area ratios of ferrite, bainitic ferrite, tempered martensite and hard second phase are measured at a 1/4 position in the sheet thickness of the base steel sheet as follows. That is, a sample is cut out 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 polished with diamond paste, and then finish polished with alumina. The observation surface of the sample is then etched with 1 vol. % nital to reveal the structure. Next, the observation position is set to 1/4 of the sheet thickness of the steel sheet, and five fields of view are observed at a magnification of 3000 times by SEM.
• the field of view to be observed is selected within the range of 1/4 of the sheet thickness of the steel sheet ⁇ 100 ⁇ m, and one field of view is 38 ⁇ m ⁇ 30 ⁇ m.
• the area ratio of each constituent structure ferrite, bainitic ferrite, tempered martensite, and hard second phase (fresh martensite + retained austenite)) divided by the measured area is calculated for five fields of view using Adobe Photoshop from Adobe Systems, and the area ratio of each structure is calculated by averaging these values.
• the zinc plating layer is excluded, and the image is taken so as to include the internal oxide layer.
• Ferrite A black region that is lumpy in shape. It contains almost no carbides. Also, isolated islands of fresh martensite and isolated islands of retained austenite within the ferrite grains are not included in the area ratio of ferrite. Bainitic ferrite: This is a region that is black to dark gray in color and has a blocky or amorphous shape. It also contains a relatively small number of carbides. Tempered martensite: This is a gray area with an amorphous morphology. It also contains a relatively large number of carbides. Hard second phase (retained austenite + fresh martensite): This is a region that is white to light gray in color and has an amorphous morphology. It also does not contain carbides.
• Carbides These are white areas that are dot-like or linear in shape and are included in bainite, tempered bainite, and tempered martensite.
• Remaining structure The above-mentioned pearlite and cementite are included, and the forms thereof are as known.
• 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 the hard second phase 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 base steel sheet of the steel sheet according to one embodiment of the present invention it is preferable that the base steel sheet has a soft surface layer on its surface.
• the soft surface layer contributes to suppressing bending crack propagation during press forming and vehicle body collision, thereby further improving bending fracture resistance.
• the soft surface layer means a decarburized layer, and is a surface region having a Vickers hardness of 84% or less of the Vickers hardness of the cross section at 1/4 of the plate thickness. The Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.
• 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, it is not possible to achieve both high strength and excellent bendability as intended by the present invention.
• the thickness (X) of the surface soft layer 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 30 ⁇ m or more, and more preferably 40 ⁇ m or more.
• the thickness of the surface soft layer is preferably 120 ⁇ m or less, and more preferably 100 ⁇ m or less.
• the area ratio of ferrite is set to 60.0% or more.
• the area ratio of ferrite is preferably set to 80.0% or more, and more preferably set to 90.0% or more.
• the area ratio of ferrite may be 100.0%.
• the area ratio of ferrite may be less than 100.0%.
• the area ratio of ferrite may be 98.0% or less, or 96.0% or less.
• the value obtained by dividing the area ratio of fresh martensite by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is set to 0.5 or less. This value may be 0.4 or less, or 0.35 or less.
• the lower limit of the value obtained by dividing the area ratio of fresh martensite in the soft surface layer by the total area ratio of bainitic ferrite, fresh martensite, and tempered martensite (excluding retained austenite) is not particularly limited, and may be 0.0. This value may be 0.1 or more, or 0.15 or more.
• Area ratio of retained austenite 3.0% or less
• the untransformed austenite in the surface soft layer is transformed into hard martensite by processing-induced transformation
• the subsequent second holding step in which the holding is performed for 10 seconds or more
• the hard martensite is tempered to produce tempered martensite
• the area ratio of retained austenite is finally controlled to 3.0% or less, so that the crack length propagating from the bent ridge end in the ridge direction in a 90-degree V-bending test with a bending radius R of 0.5 mm can be reduced to 200 ⁇ m or less.
• the area ratio of the retained austenite is set to 3.0% or less.
• the area ratio of the retained austenite is preferably set to 2.8% or less, more preferably 2.5% or less.
• the lower limit of the area ratio of the retained austenite is not particularly limited, but is preferably 0.2% or more, and more preferably 0.5% or more.
• a zinc-plated layer is formed on the steel sheet, first peel off the zinc-plated layer and measure the structure at a position half the thickness of the soft surface layer in the same way as at a position 1/4 of the thickness of the base steel sheet.
• the tensile strength (TS) of a steel plate according to one embodiment of the present invention is 1180 MPa or more and less than 1470 MPa.
• the yield stress (YS) and total elongation (El) of the steel plate according to one embodiment of the present invention are as described above.
• the tensile strength (TS), yield stress (YS) and total elongation (El) are measured by a tensile test in accordance with JIS Z 2241 (2011) described later in the examples.
• the ratio YR (yield ratio) of the yield stress (YS) to the tensile strength (TS) preferably satisfies 0.70 ⁇ YR.
• R critical bending radius
• t plate thickness
• El when a notched tensile test was performed after heat treatment at 170° C. for 20 minutes with a 2% nominal tensile strain are introduced are as described above. These are measured by the method 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, 20.0% 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 0.0% 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 plating weight of the plating layer (zinc plating layer) per side is not particularly limited, but is preferably 20 g/ m2 or more and 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, it is 0.6 mm or more. Even more preferably, it is 0.8 mm or more.
• the thickness of the steel plate is preferably 2.3 mm or less. More preferably, it is 1.6 mm or less. Even more preferably, it is 1.2 mm or less.
• the method for producing a steel sheet of the present invention includes a hot rolling step in which a steel slab having the above-mentioned composition is hot-rolled under a finish rolling temperature of 820°C or higher, and an annealing step in which the steel sheet after the hot rolling step is heated and annealed under conditions of an annealing temperature of 750°C to 900°C, an annealing time of 20 seconds or longer, and a dew point of -10°C or higher in an atmosphere, and further, under conditions that satisfy formulas (2) and (3).
• 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 (soaking time) (s)
• Ac1 is Ac1 (° C.).
• the above temperatures refer to the surface temperatures of the steel slab and the steel plate.
• the method of smelting the steel material is not particularly limited, and any of the known smelting methods, such as converters and electric furnaces, are suitable.
• any of the known smelting methods such as converters and electric furnaces
• energy-saving processes such as direct rolling and direct rolling, in which the steel slab is not cooled to room temperature but is still loaded into the heating furnace, or is rolled immediately after a short period of heat retention, can also be applied without any problems.
• the slab heating temperature is preferably 1100° C. or higher from the viewpoint of dissolving carbides and reducing the rolling load.
• the slab heating temperature is preferably 1300° C. or lower.
• the slab heating temperature is the temperature of the slab surface.
• the slab is made into a sheet bar by rough rolling under normal conditions, but when the heating temperature is low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing trouble during hot rolling.
• Finish rolling temperature 820°C or higher Finish rolling increases the rolling load and the reduction ratio in the unrecrystallized state of austenite, which leads to the development of abnormal structures elongated in the rolling direction, resulting in a decrease in the ductility, hole expandability, and bendability of the final material.
• the finish rolling temperature is set to 820°C or higher.
• the finish rolling temperature is preferably 830°C or higher, more preferably 850°C or higher.
• the finish rolling temperature is preferably 1080°C or lower, more preferably 1050°C or lower.
• the coiling temperature after hot rolling is not particularly limited, but consideration must be given to the possibility of reducing the ductility, hole expandability, and bendability of the final material. For this reason, it is preferable that the coiling temperature after hot rolling be 300°C or higher. It is also preferable that the coiling temperature after hot rolling be 700°C or lower.
• the rough rolled sheets may be joined together during hot rolling and then continuously finished rolling may be performed.
• the rough rolled sheets may also be wound up once.
• some or all of the finish rolling may be performed as lubricated rolling.
• Performing lubricated rolling is also effective from the standpoint 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.
• the hot-rolled steel sheet produced as described above may be subjected to pickling.
• Pickling can remove oxides from the steel sheet surface, and therefore can be performed to ensure good chemical conversion treatability and plating quality in the final high-strength steel sheet product.
• Pickling may be performed once or multiple times.
• the hot-rolled pickled sheet or hot-rolled steel sheet obtained as described above is subjected to cold rolling as necessary.
• the pickled sheet may be subjected to cold rolling as it is after hot rolling, or may be subjected to cold rolling after heat treatment.
• the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling.
• 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 cold rolling reduction ratio is 20% or more and 80% or less.
• the cold rolling reduction ratio (cumulative reduction ratio) is not particularly limited, but is preferably 20% or more and 80% or less. If the cold rolling reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, and the TS and bendability of the final product may decrease. On the other hand, if the cold rolling reduction ratio exceeds 80%, the shape of the steel sheet is likely to be defective, and the amount of zinc coating may become non-uniform.
• the steel sheet after the hot rolling process (after the cold rolling process in the case of performing cold rolling) is heated and annealed under the conditions of an annealing temperature of 750° C. or more and 900° C. or less, an annealing time of 20 seconds or more, and an atmosphere with a dew point of ⁇ 10° C. or more, further satisfying formulas (2) and (3).
• Y [ ⁇ (T-Ac1) ⁇ t1 ⁇ /2 ⁇ ]+ ⁇ (T-Ac1) ⁇ t2 ⁇ ...
• 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.).
• Annealing temperature 750° C. to 900° C.
• the annealing temperature is set to 750° C. or higher and 900° C. or lower.
• the annealing temperature is preferably 880° C. or lower.
• the annealing temperature is more preferably 870° C. or lower.
• the annealing temperature is preferably 780° C. or higher, more preferably 800° C. or higher.
• the annealing temperature is the maximum temperature (soaking temperature) reached in the annealing process.
• Annealing time (soaking time): 20 seconds or more If the annealing time is less than 20 seconds, the generation rate of austenite 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 cannot be obtained. Therefore, the annealing time is set to 20 seconds or more.
• the annealing time is preferably 30 seconds or more, more preferably 50 seconds or more.
• the upper limit of the annealing time is not particularly limited, but the annealing time is preferably 600 seconds or less, more preferably 250 seconds or less.
• 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.
• the number of annealing steps may be two or more, but is preferably one from the viewpoint of energy efficiency.
• Dew point of the atmosphere in the annealing step -10°C or higher
• the dew point of the atmosphere in the annealing step is preferably -10°C or higher.
• the dew point of the annealing atmosphere in the annealing step is preferably -5°C or higher, more preferably 0°C or higher, and even more preferably +5°C or higher.
• the dew point of the annealing atmosphere in the annealing step is preferably 30°C or lower.
• the dew point of the annealing atmosphere in the annealing step is more preferably 25°C or lower, and even more preferably 20°C or lower.
• T is the annealing temperature (° C.)
• t1 is the time (s) from 650° C. to the annealing temperature (soaking 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.
• Y in formula (3) is set to be 2400 or more and 20000 or less.
• Y is preferably 9,000 or more, and more preferably 12,000 or more.
• Y is preferably 19,000 or less, and more preferably 18,000 or less.
• t1 is 30 s or more. Also, it is preferable that t1 is 80 s or less.
• Ac1 (°C) 727.0-32.7 ⁇ [%C]+14.9 ⁇ [%Si]+2.0 ⁇ [%Mn]
• [%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).
• Cooling to a cooling stop temperature of less than 100°C Average cooling rate: 10°C/s or more, 50°C/s or less, atmospheric dew point: -20°C or less (preferred conditions)
• the steel sheet after the annealing step is cooled to a cooling stop temperature of less than 100° C.
• the cooling start temperature can be 750° C. or more and 900° C. or less.
• the cooling stop temperature is preferably 80° C. or lower, and more preferably 60° C. or lower.
• the cooling stop temperature is preferably 5° C. or higher, and more preferably 15° C. or higher.
• the average cooling rate during this cooling step is preferably 10° C./s or more and 50° C./s or less.
• 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. Therefore, 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).
• the steel sheet is reheated to a reheating holding temperature range of not less than the cooling stop temperature and not more than 440° C., and held for not less than 10 seconds.
• the bainitic ferrite and tempered martensite defined in the present invention can be formed by reheating the steel sheet to a reheating holding temperature range of from the cooling stop temperature to 440°C and holding the temperature range for 10 seconds or more.
• the reheating holding temperature range is preferably 420°C or less, and more preferably 400°C or less.
• the retention time is preferably 20 seconds or more, and more preferably 30 seconds or more.
• the retention time is preferably 100 seconds or less, more preferably 80 seconds or less.
• a tension of 2.0 kgf/ mm2 or more is applied to the steel sheet after the first holding step in the reheating holding temperature range. This makes it possible to control El to the value specified in the present invention or higher when a notched tensile test is performed after a heat treatment at 170°C for 20 minutes.
• 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 more preferably 10.0 kgf/ mm2 or less, and more preferably 4.0 kgf/ mm2 or less.
• the steel sheet after the surface layer strain introduction step is held in the reheating holding temperature range for 10 seconds or more.
• the holding time in the second holding step is preferably 15 seconds or more, more preferably 20 seconds or more.
• the holding time is preferably 60 seconds or less, more preferably 50 seconds or less.
• the steel sheet is subjected to a zinc plating treatment in the plating step, thereby obtaining a zinc-plated steel sheet.
• the galvanizing treatment include hot-dip galvanizing treatment and hot-dip galvannealing treatment.
• the zinc plating treatment may be carried out, 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.
• the galvanizing treatment may be carried out, for example, after the annealing step and before the first holding step, during the cooling treatment in the cooling 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 is slow, and alloying may be difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite is transformed into pearlite, and it becomes difficult to achieve a TS of 1180 MPa or more.
• the alloying temperature is more preferably 500°C or more, and further preferably 510°C or more.
• the alloying temperature is more preferably 570°C or less.
• the coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is preferably 20 to 80 g/ m2 per side.
• the coating weight can be adjusted by gas wiping or the like.
• 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.
• Other manufacturing method conditions are not particularly limited, but from the viewpoint of productivity, it is preferable to carry out the above-mentioned series of processes such as annealing, hot-dip galvanizing, and alloying of 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 can be based on standard hot-dip galvanizing methods.
• 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 1180 MPa or more and less than 1470 MPa, and has a high yield stress (YS), excellent ductility, excellent press formability inside the steel plate (bendability inside the steel plate), excellent press formability at the steel plate end (bendability at the steel plate end (shear cross section)), and high crack propagation resistance.
• the member according to one embodiment of the present invention has a tensile strength (TS): 1180 MPa or more and less than 1470 MPa, and has a high yield stress (YS), excellent ductility, excellent press formability inside the steel plate (bendability inside the steel plate), excellent press formability at the steel plate end (bendability at the steel plate end (shear cross section)), and high crack propagation resistance. 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.
• TS tensile strength
• YS high yield stress
• 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 (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 obtained steel slab was heated to 1200°C, and after heating, the steel slab was subjected to rough rolling and hot rolling (finish rolling temperature: 900°C) to obtain a hot-rolled steel sheet.
• the obtained hot-rolled steel sheets No. 1 to No. 70 were subjected to pickling and cold rolling to obtain cold-rolled steel sheets having the thicknesses shown in Table 3.
• the obtained cold-rolled steel sheet was subjected to an annealing process, a cooling process, a plating process (hot-dip galvanizing process or alloyed hot-dip galvanizing process) during the cooling process, a reheating and holding process (first holding process), a surface strain introduction process, and a second holding process under the conditions shown in Table 2 to obtain a steel sheet.
• the average cooling rate was 10°C/s and the dew point of the atmosphere was -30°C.
• hot-dip galvanizing or alloyed hot-dip galvanizing was performed to obtain 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 type of plating process is also indicated as "GI” or "GA”.
• alloying process is not performed, so the alloying temperature is indicated as -.
• steel sheet that is not treated in the plating process is indicated as "CR”.
• 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 zinc plating layers were formed on both sides of the base steel sheet 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
• M is fresh martensite
• RA is retained austenite
• BF is bainitic ferrite
• TM is tempered martensite.
• ⁇ 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 Vickers 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.
• ⁇ TS tensile strength
• ⁇ (Pass) 1180 MPa or more and less than 1470 MPa
• ⁇ (Fail) Less than 1180 MPa or 1470 MPa or more
• 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 length of the crack propagating from the end of the bent ridge in the ridge direction is measured as follows.
• the crack at the end of the bending ridge of the sample after the V-bend test is shown in Figure 3-1 (a).
• Figure 3-1 (b) When measuring the length of the crack at the center of the bending ridge, it is common to observe the plate surface (b-side) from the Z direction. Since the sample after the actual V-bend test has a saddle shape as shown in Figure 3-1 (b), the b-side is significantly deformed, which reduces the measurement accuracy of the crack length and may make it difficult to accurately evaluate the bendability of the sheared end surface.
• the following measurement method is used to enable accurate measurement.
• 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.
• Figure 3-2 (a) a circular 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.
• Notched tensile test The obtained steel plate was subjected to a heat treatment at 170° C. for 20 minutes. Then, a JIS No. 5 tensile test specimen was prepared so that the longitudinal direction of the test specimen was aligned perpendicular to the rolling direction, and a notch (V notch: depth 2 mm, V interior angle 60 degrees) was prepared at both width ends at the center position of the longitudinal parallel part of the test specimen. The notched tensile test was performed according to JIS Z 2241 (2011) with a gauge length of 15 mm, and the total elongation (El) was measured. The obtained steel sheet was also subjected to a 2% nominal tensile strain and heat treatment at 170° C. for 20 minutes.
• a JIS No. 5 tensile test piece was prepared so that the longitudinal direction of the test piece was aligned perpendicular to the rolling direction, and a notch (V notch: depth 2 mm, V interior angle 60 degrees) was made at both width ends at the center position of the longitudinal parallel part of the test piece.
• the notched tensile test was performed according to JIS Z 2241 (2011) with a gauge length of 15 mm to measure the total elongation (El).
• the components obtained by forming or joining using the steel plate of the present invention have the excellent characteristics characteristic of the present invention in terms of tensile strength (TS), yield stress (YS), total elongation (El), R/t in a V-bend test, crack length propagating from the end of the bent ridge in the ridge direction, and notch El.

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