US-12624410-B2 - Steel sheet, member, and method for producing them

US12624410B2US 12624410 B2US12624410 B2US 12624410B2US-12624410-B2

Abstract

A steel sheet with a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a member, and a method for producing them. In a region of the steel sheet within 4.9 μm in the thickness direction, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more. The lowest Si concentration L Si and the lowest Mn concentration L Mn in the region within 4.9 μm from the surface of the steel sheet and a Si concentration T Si and a Mn concentration T Mn at a quarter thickness position of the steel sheet satisfy the following formula (1): L Si +L Mn ≤( T Si +T Mn )/4 (1).

Inventors

Yoshiyasu Kawasaki
Lingling Yang
Shotaro TERASHIMA
Hidekazu Minami
Tatsuya Nakagaito
Shunsuke Yamamoto
Katsuya Hoshino
Yuki Takeda

Assignees

JFE STEEL CORPORATION

Dates

Publication Date: 20260512
Application Date: 20210325
Priority Date: 20200331

Claims (20)

1 . A steel sheet having a chemical composition comprising, by mass %: Si: 0.20% to 2.00%; Mn: 1.00% or more and less than 2.70%; C: 0.120% to 0.400%; P: 0.001% to 0.100%; S: 0.0200% or less; Al: 0.010% to 2.000%; N: 0.0100% or less; optionally at least one selected from the group consisting of: Sb: 0.200% or less, and Sn: 0.200% or less; optionally at least one selected from the group consisting of: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% 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 (except Ce): 0.0200% or less; optionally an equivalent carbon content Ceq in a range of 0.490% or more and less than 0.697%, wherein the equivalent carbon content Ceq is determined by the following formula: Ceq =[C %]+([Si %]/24)+([Mn %]/6)+([Ni %]/40)+([Cr %]/5)+([Mo %]/4)+([V %]/14), where the [element symbol %] in the formula represents the element content as % by mass; the remainder being Fe and incidental impurities, wherein: the steel sheet has a microstructure including a ferrite area fraction in a range of 15% to 70%, a bainitic ferrite area fraction in a range of 3% to 25%, a tempered martensite area fraction in a range of 1% to 15%, and a retained austenite volume fraction in a range of 5% to 30%, in a region within 4.9 μm in a thickness direction from a surface of the steel sheet, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more, the lowest Si concentration L Si and the lowest Mn concentration L Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T Si and a Mn concentration T Mn at a quarter thickness position of the steel sheet satisfy the following formula (1): L Si +L Mn ≤( T Si +T Mn )/4 (1), the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa, and optionally, an amount of diffusible hydrogen in the steel sheet is 0.50 ppm or less by mass.
2 . The steel sheet according to claim 1 , wherein: the steel sheet comprises a soft layer with a thickness in a range of 1.0 to 50.0 μm in the thickness direction, the soft layer being a region with hardness corresponding to 65% or less of the hardness at a quarter thickness position from the surface of the steel sheet, optionally crystal grains containing an oxide of Si and/or Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet have an average grain size in a range of 1 to 15 μm, and optionally the Mn concentration L Mn and the Mn concentration T Mn satisfy the following formula (2): L Mn ≤T Mn/ 3 (2).
3 . The steel sheet according to claim 1 , wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.
4 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 1 .
5 . The steel sheet according to claim 2 , wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.
6 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 2 .
7 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 3 .
8 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 5 .
9 . A method for producing the steel sheet according to claim 1 , the method comprising: a hot-rolling step of hot-rolling a steel slab with the chemical composition followed by coiling at a coiling temperature in a range of 450° C. to 750° C.; a cold-rolling step of holding the steel sheet after the hot-rolling step in a temperature range of 400° C. or more for 3600 seconds or more, pickling the steel sheet, and cold-rolling the steel sheet at a rolling reduction of 30% or more; a first annealing step of holding the steel sheet after the cold-rolling step in a first annealing temperature range of 820° C. or more for 20 seconds or more; a second annealing step of holding the steel sheet after the first annealing step in an atmosphere with a dew-point temperature of −35° C. or more in a second annealing temperature range of 740° C. to 900° C. for 20 seconds or more, cooling the steel sheet, at an average cooling rate of 8° C./s or more from the second annealing temperature range to 550° C., to a cooling stop temperature in a range of 150° C. to 300° C., and bending and unbending the steel sheet 3 to 15 times in total using a roller having a radius in a range of 100 to 1000 mm during the cooling from 740° C. to the cooling stop temperature; a reheating step of reheating the steel sheet after the second annealing step to a reheating temperature range of (the cooling stop temperature+50° C.) to 500° C. and holding the steel sheet in the reheating temperature range for 10 seconds or more; optionally a plating step of performing hot-dip galvanizing on the steel sheet after the reheating step or performing the hot-dip galvanizing followed by reheating to a temperature in a range of 450° C. to 600° C. and performing alloying treatment; and optionally a dehydrogenation step of holding the steel sheet at a temperature in a range of 50° C. to 300° C. for 0.5 to 72.0 hours after the reheating step.
10 . A method for producing a member, the method comprising performing at least one of forming and welding on the steel sheet produced by the method for producing the steel sheet according claim 9 .
11 . A method for producing the steel sheet according to claim 1 , the method comprising: a hot-rolling step of hot-rolling a steel slab with the chemical composition followed by coiling at a coiling temperature in a range of 450° C. to 750° C.; a cold-rolling step of holding the steel sheet after the hot-rolling step in a temperature range of 400° C. or more for 3600 seconds or more, pickling the steel sheet, and cold-rolling the steel sheet at a rolling reduction of 30% or more; a first annealing step of holding the steel sheet after the cold-rolling step in a first annealing temperature range of 820° C. or more for 20 seconds or more; a second annealing step of holding the steel sheet after the first annealing step in an atmosphere with a dew-point temperature of −35° C. or more in a second annealing temperature range of 740° C. to 900° C. for 20 seconds or more, cooling the steel sheet, at an average cooling rate of 8° C./s or more from the second annealing temperature range to 550° C., to a first cooling stop temperature in a range of 350° C. to 500° C., and bending and unbending the steel sheet 3 to 15 times in total using a roller having a radius in a range of 100 to 1000 mm during the cooling from 740° C. to the first cooling stop temperature; a plating step of performing hot-dip galvanizing on the steel sheet after the second annealing step or performing the hot-dip galvanizing followed by reheating to a temperature in a range of 450° C. to 600° C. and performing alloying treatment; a reheating step of cooling the steel sheet after the plating step to a second cooling stop temperature in a range of 50° C. to 350° C., reheating the steel sheet to a reheating temperature exceeding the second cooling stop temperature and in a range of 300° C. to 500° C., and holding the reheating temperature for 10 seconds or more; and optionally a dehydrogenation step of holding the steel sheet at a temperature in a range of 50° C. to 300° C. for 0.5 to 72.0 hours after the reheating step.
12 . A method for producing a member, the method comprising performing at least one of forming and welding on the steel sheet produced by the method for producing the steel sheet according to claim 11 .
13 . A steel sheet having a chemical composition comprising, by mass %: Si: 0.20% to 2.00%; Mn: 1.00% or more and less than 2.70%; C: 0.120% to 0.400%; P: 0.001% to 0.100%; S: 0.0200% or less; Al: 0.010% to 2.000%; N: 0.0100% or less; optionally at least one selected from the group consisting of: Sb: 0.200% or less, and Sn: 0.200% or less; optionally at least one selected from the group consisting of: Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% 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 (except Ce): 0.0200% or less; optionally an equivalent carbon content Ceq in a range of 0.490% or more and less than 0.697%, wherein the equivalent carbon content Ceq is determined by the following formula: Ceq =[C %]+([Si %]/24)+([Mn %]/6)+([Ni %]/40)+([Cr %]/5)+([Mo %]/4)+([V %]/14), where the [element symbol %] in the formula represents the element content as % by mass; the remainder being Fe and incidental impurities, wherein: the steel sheet has a steel microstructure including a ferrite area fraction in a range of 15% to 70%, a bainitic ferrite area fraction in a range of 3% to 25%, a tempered martensite area fraction in a range of 1% to 15%, and a retained austenite volume fraction in a range of 5% to 30%, in a region within 15.0 μm in a thickness direction from a surface of the steel sheet, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more, the lowest Si concentration L Si and the lowest Mn concentration L Mn in the region within 15.0 μm in the thickness direction from the surface of the steel sheet and a Si concentration T Si and a Mn concentration T Mn at a quarter thickness position of the steel sheet satisfy the following formula (1): L Si +L Mn ≤( T Si +T Mn )/4 (1), and the steel sheet has a tensile strength of 780 MPa or more and less than 1180 MPa, and optionally, an amount of diffusible hydrogen in the steel sheet is 0.50 ppm or less by mass.
14 . The steel sheet according to claim 13 , wherein: the steel sheet comprises a soft layer with a thickness in a range of 1.0 to 50.0 μm in the thickness direction, the soft layer being a region with hardness corresponding to 65% or less of the hardness at a quarter thickness position from the surface of the steel sheet, optionally crystal grains containing an oxide of Si and/or Mn in the region within 4.9 μm in the thickness direction from the surface of the steel sheet have an average grain size in a range of 1 to 15 μm, and optionally the Mn concentration L Mn and the Mn concentration T Mn satisfy the following formula (2): L Mn ≤T Mn/ 3 (2).
15 . The steel sheet according to claim 13 , wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.
16 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 13 .
17 . The steel sheet according to claim 14 , wherein the steel sheet comprises a hot-dip galvanized layer or a hot-dip galvannealed layer on a surface of the steel sheet.
18 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 14 .
19 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 15 .
20 . A member produced by performing at least one of forming and welding on the steel sheet according to claim 17 .

Description

TECHNICAL FIELD This application relates to a steel sheet, a member, and a method for producing them. More particularly, the application relates to a steel sheet with a tensile strength (TS) of 780 MPa or more and less than 1180 MPa, a high yield stress (YS), high ductility, high stretch-flangeability (hole expandability), good fatigue properties, and high LME resistance, a member, and a method for producing them. A steel sheet according to this application is suitable for an impact energy absorbing member used in the automotive field. BACKGROUND In recent years, from the viewpoint of global environmental conservation, improvement of fuel efficiency in automobiles has been an important issue. Thus, there is a strong movement under way to strengthen body materials in order to decrease the thicknesses of the body materials and thereby decrease the weight of automobile bodies. On the other hand, social demands for improvement in crash safety of automobiles have become even higher, and it is desired not only to strengthen steel sheets but also to develop steel sheets and members thereof with a good anti-crash property in case of collision while driving a vehicle. However, only steel sheets with a tensile strength (hereinafter also referred to simply as TS) up to 590 MPa are used for impact energy absorbing members exemplified by front side members and rear side members. This is because high strength reduces formability, such as ductility and stretch-flangeability (hole expandability), and causes cracking in a crushing test or an axial crushing test simulating a collision test, thus resulting in insufficient absorption of impact energy. Although it is effective to improve yield stress (hereinafter also referred to simply as YS) in order to increase impact absorbed energy, it is also difficult to improve YS due to the fear of poor formability as described above. Furthermore, it has recently been confirmed that spot welding of a high-strength hot-dip galvanized steel sheet and a high-strength hot-dip galvannealed steel sheet or spot welding of a high-strength cold-rolled steel sheet and a galvanized steel sheet causes liquid metal embrittlement cracking (LMEC, hereinafter also referred to as LME cracking) at a weld when assembling automobile bodies and parts. LME cracking is caused by melting of zinc in a galvanized layer during spot welding, penetration of molten zinc into a grain boundary of a steel microstructure of a weld, and the action of stress generated when a welding electrode is opened. Even for an ungalvanized high-strength cold-rolled steel sheet, spot welding with a galvanized steel sheet may cause LME cracking due to contact between zinc melted in the galvanized steel sheet and the high-strength cold-rolled steel sheet. Due to high C, Si, and Mn contents, high-strength steel sheets with a TS of 780 MPa or more may cause LME cracking. Various high-strength steel sheets have been developed for automotive parts. For example, Patent Literature 1 discloses a high-strength steel sheet that contains 40% or more by volume of ferrite and 5% or more by volume of tempered martensite, has a ferrite hardness (DHTF) and martensite hardness (DHTM) ratio (DHTM/DHTF) in the range of 1.5 to 3.0, has the remainder microstructure composed of ferrite and bainite microstructures, and has a tensile strength (TS) of 590 MPa or more and high flangeability and formability, and a method for producing the high-strength steel sheet. Patent Literature 2 discloses a coated steel sheet that has a microstructure containing, on a volume fraction basis, tempered martensite: 3.0% or more, ferrite: 4.0% or more, and retained austenite: 5.0% or more at a quarter thickness position of a steel sheet from a surface of the steel sheet, wherein the tempered martensite in a base material has an average hardness in the range of 5 to 10 GPa, and part or all of the tempered martensite and the retained austenite in the base material form a martensite-austenite constituent (MA), the volume fraction of the ferrite in a decarburized ferrite layer is 120% or more of the volume fraction of the ferrite in the base material at the quarter thickness position of the steel sheet from the surface of the steel sheet, the ferrite in the decarburized ferrite layer has an average grain size of 20 μm or less, the decarburized ferrite layer has a thickness in the range of 5 μm to 200 μm, tempered martensite in the decarburized ferrite layer has a volume fraction of 1.0% or more by volume, the tempered martensite in the decarburized ferrite layer has a number density of 0.01/μm2 or more, the tempered martensite in the decarburized ferrite layer has an average hardness of 8 GPa or less, and the coated steel sheet can have high strength and improved ductility and bendability, and discloses a method for producing the coated steel sheet. Patent Literature 3 discloses a steel sheet, a hot-dip galvanized steel sheet, and a hot-dip galvannealed steel sheet that have an in