HOT-PRESSED MEMBER AND METHOD FOR MANUFACTURING SAME, AND COLD-ROLLED STEEL SHEET FOR HOT PRESSING AND METHOD FOR MANUFACTURING SAME

• US 20200131597A1
• Filed: 03/30/2018
• Published: 04/30/2020
• Est. Priority Date: 06/30/2017
• Status: Active Grant

• Current Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Original Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Inventors: TAKASHIMA, Katsutoshi, KOBAYASHI, Takashi, FUNAKAWA, Yoshimasa, NAKAJIMA, Seiji
• Granted Patent: US 11,111,558 B2
• Time in Patent Office: Days
• CPC Class Codes: B21D 22/022 by heating the blank or stamping associated with heat treatment C21D takes precedence
• CPC Class Codes: B21D 22/208 by heating the blank or deep-drawing associated with heat treatment C21D takes precedence
• CPC Class Codes: B21D 35/005 characterized by the material of the blank or the workpiece B21D26/053 takes precedence
• CPC Class Codes: B21D 53/88 other parts for vehicles, e.g. cowlings, mudguards
• CPC Class Codes: B32B 15/012 one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
• CPC Class Codes: B32B 15/013 one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
• CPC Class Codes: C21D 1/18 Hardening C21D1/02 takes precedence; Quenching with or without subsequent tempering quenching devices C21D1/62
• CPC Class Codes: C21D 1/25 Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
• CPC Class Codes: C21D 1/84 Controlled slow cooling cooling-beds for metal rolling B21B43/00
• CPC Class Codes: C21D 2211/001 Austenite
• CPC Class Codes: C21D 2211/004 Dispersions; Precipitations
• CPC Class Codes: C21D 2211/005 Ferrite
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 6/002 containing Cr C21D6/004 takes precedence
• CPC Class Codes: C21D 6/005 containing Mn
• CPC Class Codes: C21D 6/007 containing Co
• CPC Class Codes: C21D 6/008 containing Si
• CPC Class Codes: C21D 7/13 by hot working
• CPC Class Codes: C21D 8/0405 of ferrous alloys
• CPC Class Codes: C21D 8/0426 Hot rolling
• CPC Class Codes: C21D 8/0436 : Cold rolling
• CPC Class Codes: C21D 8/0463 : following hot rolling
• CPC Class Codes: C21D 9/48 : deep-drawing sheets
• CPC Class Codes: C22C 38/001 : containing N
• CPC Class Codes: C22C 38/002 : containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/005 : containing rare earths, i.e. Sc, Y, Lanthanides
• CPC Class Codes: C22C 38/008 : containing tin
• CPC Class Codes: C22C 38/02 : containing silicon
• CPC Class Codes: C22C 38/04 : containing manganese
• CPC Class Codes: C22C 38/06 : containing aluminium
• CPC Class Codes: C22C 38/08 : containing nickel C22C38/105 takes precedence
• CPC Class Codes: C22C 38/10 : containing cobalt
• CPC Class Codes: C22C 38/12 : containing tungsten, tantalum, molybdenum, vanadium, or niobium
• CPC Class Codes: C22C 38/14 : containing titanium or zirconium
• CPC Class Codes: C22C 38/22 : with molybdenum or tungsten
• CPC Class Codes: C22C 38/24 : with vanadium
• CPC Class Codes: C22C 38/26 : with niobium or tantalum
• CPC Class Codes: C22C 38/28 : with titanium or zirconium
• CPC Class Codes: C22C 38/32 : with boron
• CPC Class Codes: C22C 38/38 : with more than 1.5% by weight of manganese
• CPC Class Codes: C22C 38/60 : containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• CPC Class Codes: C23C 10/28 : using solids, e.g. powders, pastes
• CPC Class Codes: C23C 2/06 : Zinc or cadmium or alloys based thereon
• CPC Class Codes: C23C 2/12 : Aluminium or alloys based thereon
• CPC Class Codes: C23C 2/28 : Thermal after-treatment, e.g. treatment in oil bath
• CPC Class Codes: C23C 2/29 : Cooling or quenching
• CPC Class Codes: C23C 2/40 : Plates; Strips
• CPC Class Codes: C25D 5/50 : by heat-treatment
• CPC Class Codes: C25D 7/0614 : Strips or foils

Abstract

Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, and excellent resistance to resistance welding cracking by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 7.5 μm or less, a volume fraction of martensite is 95% or more, and at least 10 Nb-based and Ti-based precipitates having a grain size of less than 0.10 μm are present on average per 100 μm² of a cross section parallel to a thickness direction of the member within a range of 100 μm or less in the thickness direction from the surface of the member, and such that a B concentration in prior austenite grain boundaries is at least 3.0 times a B concentration at a position 5 nm away from the grain boundaries.

Claims

1. A hot-pressed member comprising:

a steel chemical composition containing, by mass %, C;

0.28% or more and less than 0.42%, Si;

1.5% or less, Mn;

1.0% or more and 2.4% or less, P;

0.05% or less, S;

0.005% or less, Al;

0.01% or more and 0.50% or less, N;

0.005% or less, Nb;

0.005% or more and 0.15% or less, Ti;

0.005% or more and 0.15% or less, and B;

0.0005% or more and 0.0050% or less, where among the components of the member, C, Si, Nb, Ti, and B satisfy the following Expression (1),

((Nb+(Ti−

3.4N)×

100B)/((C/8)+Si)≥

0.25 

 

(1)

where each element symbol represents the content by mass % of the corresponding element, with the balance being Fe and inevitable impurities;

a microstructure in which

a prior austenite average grain size is 7.5 μ

m or less, a volume fraction of martensite is 95% or more, and at least 10 Nb-based and Ti-based precipitates having a grain size of less than 0.10 μ

m are present on average per 100 μ

m² of a cross section parallel to a thickness direction of the member within a range of 100 μ

m in the thickness direction from a surface of the member;

a B concentration in prior austenite grain boundaries being at least 3.0 times a B concentration at a position 5 nm away from the grain boundaries; and

a tensile strength of 1780 MPa or more.

2. The hot-pressed member according to claim 1, wherein the steel chemical composition further contains, by mass %, at least one selected from the group consisting of Mo:

0.50% or less, Cr;

0.50% or less, Sb;

0.001% or more and 0.020% or less, Ca;

0.005% or less, Mg;

0.005% or less, REM;

0.005% or less, V;

0.15% or less, Cu;

0.50% or less, Ni;

0.50% or less, Sn;

0.50% or less, Zn;

0.10% or less, Co;

0.10% or less, Zr;

0.10% or less, Ta;

0.10% or less, and W;

0.10% or less.

3. The hot-pressed member according to claim 1, comprising, on a surface layer thereof, an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

11. The hot-pressed member according to claim 2, comprising, on a surface layer thereof, an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

4. A cold-rolled steel sheet for hot pressing, comprising:

a chemical composition containing, by mass %, C;

0.28% or more and less than 0.42%, Si;

1.5% or less, Mn;

1.0% or more and 2.4% or less, P;

0.05% or less, S;

0.005% or less, Al;

0.01% or more and 0.50% or less, N;

0.005% or less, Nb;

0.005% or more and 0.15% or less, Ti;

0.005% or more and 0.15% or less, and B;

0.0005% or more and 0.0050% or less, where among the components of the member, C, Si, Nb, Ti, and B satisfy the following Expression (1),

((Nb+(Ti−

3.4N)×

100B)/((C/8)+Si)≥

0.25 

 

(1)

where each element symbol represents the content by mass % of the corresponding element, with the balance being Fe and inevitable impurities,

a microstructure which contains 10% or more by volume fraction of martensite having an average grain aspect ratio of 2.5 or less and 5% or more by volume fraction of bainite having an average grain size of 6 μ

m or less, and at least 20 Nb-based and Ti-based precipitates having a grain size of less than 0.10 μ

m present on average per 100 μ

m² of a cross section parallel to a thickness direction of the steel sheet within a range of 100 μ

m in the thickness direction from a surface of the steel sheet.

5. The cold-rolled steel sheet for hot pressing according to claim 4, wherein the chemical composition further contains, by mass %, at least one selected from Mo:

0.50% or less, Cr;

0.50% or less, Sb;

0.001% or more and 0.020% or less, Ca;

0.005% or less, Mg;

0.005% or less, REM;

0.005% or less, V;

0.15% or less, Cu;

0.50% or less, Ni;

0.50% or less, Sn;

0.50% or less, Zn;

0.10% or less, Co;

0.10% or less, Zr;

0.10% or less, Ta;

0.10% or less, and W;

0.10% or less.

6. The cold-rolled steel sheet for hot pressing according to claim 4, wherein the steel sheet comprises on a surface thereof an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

7. A method for manufacturing the cold-rolled steel sheet for hot pressing as recited in claim 4, the method comprising:

preparing a steel raw material comprising a chemical composition containing, by mass %, C;

0.28% or more and less than 0.42%, Si;

1.5% or less, Mn;

1.0% or more and 2.4% or less, P;

0.05% or less, S;

0.005% or less, Al;

0.01% or more and 0.50% or less, N;

0.005% or less, Nb;

0.005% or more and 0.15% or less, Ti;

0.005% or more and 0.15% or less, and B;

0.0005% or more and 0.0050% or less, where among the components of the member, C, Si, Nb, Ti, and B satisfy the following Expression (1),

((Nb+(Ti−

3.4N)×

100B)/((C/8)+Si)≥

0.25 

 

(1)

where each element symbol represents the content by mass % of the corresponding element, with the balance being Fe and inevitable impurities;

hot rolling the steel raw material to obtain a hot-rolled steel sheet;

after the hot rolling, subjecting the hot-rolled steel sheet to primary cooling whereby the hot-rolled steel sheet is cooled to a cooling end temperature of 700°

C. or lower at a first average cooling rate of 70°

C./s or higher up to the cooling end temperature;

after the primary cooling, subjecting the steel sheet to secondary cooling whereby the steel sheet is coiled at a coiling temperature of 550°

C. or lower at a second average cooling rate of 5°

C./s to 50°

C./s up to the coiling temperature;

then pickling and cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and

then subjecting the steel sheet to an annealing treatment whereby the steel sheet is heated to a first soaking temperature range of 800°

C. to 900°

C. at an average heating rate of 3°

C./s to 30°

C./s, held in the soaking temperature range for 15 seconds or longer, cooled to a cooling end temperature range of 200°

C. to 350°

C. at a third average cooling rate of 3°

C./s or higher up to a cooling end temperature, then heated to a second soaking temperature range of 350°

C. to 450°

C., held in the second soaking temperature range for 120 seconds or longer, and then cooled to room temperature.

8. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 7, wherein

the chemical composition further contains, by mass %, at least one selected from the group consisting of Mo:

0.50% or less, Cr;

0.50% or less, Sb;

0.001% or more and 0.020% or less, Ca;

0.005% or less, Mg;

0.005% or less, REM;

0.005% or less, V;

0.15% or less, Cu;

0.50% or less, Ni;

0.50% or less, Sn;

0.50% or less, Zn;

0.10% or less, Co;

0.10% or less, Zr;

0.10% or less, Ta;

0.10% or less, and W;

0.10% or less.

9. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 7, the method further comprising:

after the annealing treatment, applying an Al or Al alloy coating or plating treatment or a Zn or Zn alloy coating or plating treatment to a surface of the cold-rolled steel sheet.

10. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 4 in a temperature range of an Ac₃ transformation temperature to 1000°

C.; and

then hot pressing the steel sheet.

12. The cold-rolled steel sheet for hot pressing according to claim 5, wherein the steel sheet comprises on a surface thereof an Al or Al alloy coating or plating layer or a Zn or Zn alloy coating or plating layer.

13. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 8, the method further comprising:

after the annealing treatment, applying an Al or Al alloy coating or plating treatment or a Zn or Zn alloy coating or plating treatment to a surface of the cold-rolled steel sheet.

14. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 5 in a temperature range of an Ac₃ transformation temperature to 1000°

C.; and

then hot pressing the steel sheet.

15. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 6 in a temperature range of an Acs transformation temperature to 1000°

C.; and

then hot pressing the steel sheet.

16. A method for manufacturing a hot-pressed member, comprising:

heating the cold-rolled steel sheet for hot pressing as recited in claim 12 in a temperature range of an Acs transformation temperature to 1000°

C.; and

then hot pressing the steel sheet.