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

• US 20200131596A1
• 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 11,085,101 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: 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 2211/004 Dispersions; Precipitations
• CPC Class Codes: C21D 2211/008 Martensite
• CPC Class Codes: C21D 6/001 containing Ni C21D6/004 takes precedence
• CPC Class Codes: C21D 6/002 containing Cr C21D6/004 takes precedence
• CPC Class Codes: C21D 6/005 containing Mn
• CPC Class Codes: C21D 6/008 containing Si
• CPC Class Codes: C21D 6/02 Hardening by precipitation
• CPC Class Codes: C21D 7/13 by hot working
• CPC Class Codes: C21D 8/0205 of ferrous alloys
• CPC Class Codes: C21D 8/0226 Hot rolling
• CPC Class Codes: C21D 8/0236 Cold rolling
• CPC Class Codes: C21D 8/0263 following hot rolling
• 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 9/46 : for sheet metals
• 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 38/16 : containing copper
• 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 38/60 : containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• 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
• CPC Class Codes: Y10T 428/12799 : Next to Fe-base component [e.g., galvanized]
• CPC Class Codes: Y10T 428/12958 : Next to Fe-base component

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 delayed fracture resistance after projection welding by properly adjusting its chemical composition and its microstructure such that at least 5 Ti-based precipitates having a grain size of 0.10 μm or less 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 in a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μm to 100 μm in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μm are present on average in a prior austenite grain.

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 2.2% 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, and Ti;

0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities;

a microstructure in which

at least 5 Ti-based precipitates having a grain size of 0.10 μ

m or less 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 a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μ

m to 100 μ

m in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μ

m are present on average in a prior austenite grain; 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 Nb:

0.15% or less, B;

0.0050% or less, 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 2.2% 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, and Ti;

0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities,

a microstructure in which at least 10 Ti-based precipitates having a grain size of 0.10 μ

m or less are 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; and

a standard deviation of Vickers hardness of 40 or less, the Vickers hardness being measured every 200 μ

m in the thickness direction within a range from a depth position of 100 μ

m from the surface of the steel sheet to a mid-thickness part.

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 Nb:

0.15% or less, B;

0.0050% or less, 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 2.2% 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, and Ti;

0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities;

hot rolling the steel raw material to obtain a hot-rolled steel sheet under a set of conditions including a rolling reduction of 12% or more at a final pass of finish rolling, a rolling reduction of 15% or more at a pass immediately before the final pass, and a finisher delivery temperature of 860°

C. to 950°

C.;

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 520°

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

C./s to 50°

C./s up to the coiling temperature;

then pickling the coiled hot-rolled steel sheet, and then subjecting the steel sheet to a first heat treatment whereby the steel sheet is heated in a temperature range of 830°

C. to 930°

C. for 600 seconds or shorter;

then cold rolling the steel sheet to obtain a cold-rolled steel sheet, then subjecting the cold-rolled steel sheet to a second heat treatment whereby the cold-rolled steel sheet is heated to a temperature range of 720°

C. to 850°

C. at an average heating rate of 5°

C./s to 20°

C./s and subjected to 15 seconds to 600 seconds of soaking in the temperature range of 720°

C. to 850°

C.; and

after the second heat treatment, subjecting the cold-rolled steel sheet to a tertiary cooling whereby the cold-rolled steel sheet is cooled to a cooling end temperature of 600°

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

C./s or higher.

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 Nb:

0.15% or less, B;

0.0050% or less, 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 tertiary cooling, 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 tertiary cooling, 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 Ac₃ 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 Ac₃ transformation temperature to 1000°

C.; and

then hot pressing the steel sheet.