Hot-pressed member and method for manufacturing same, and cold-rolled steel sheet for hot pressing and method for manufacturing same

Patent metadata

Field	Value
Publication number	US-11085101-B2
Application number	US-201816618887-A
Country	US
Kind code	B2
Filing date	Mar 30, 2018
Priority date	Jun 30, 2017
Publication date	Aug 10, 2021
Grant date	Aug 10, 2021

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.

First claim

The invention claimed is:

1. 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 0.005 μm or more and 0.10 μm or less are present on average per 100 μm 2 of a cross section parallel to a thickness direction of the steel sheet within a range of 100 μm 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 μm in the thickness direction within a range from a depth position of 100 μm from the surface of the steel sheet to a mid-thickness part.

2. The cold-rolled steel sheet for hot pressing according to claim 1 , 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.

3. The cold-rolled steel sheet for hot pressing according to claim 1 , 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.

4. The cold-rolled steel sheet for hot pressing according to claim 2 , 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.

5. 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 0.005 μm or more and 0.10 μm or less are present on average per 100 μm 2 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; and

a tensile strength of 1780 MPa or more.

6. The hot-pressed member according to claim 5 , 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.

7. The hot-pressed member according to claim 5 , 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.

8. The hot-pressed member according to claim 6 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.

9. A method for manufacturing the cold-rolled steel sheet for hot pressing as recited in claim 1 , 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.

10. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 9 , 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.

11. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 9 , 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.

12. A method for manufacturing a hot-pressed member, comprising:
heating the cold-rolled steel sheet for hot pressing as recited in claim 1 in a temperature range of an Ac 3 transformation temperature to 1000° C.; and
then hot pressing the steel sheet.

13. The method for manufacturing the cold-rolled steel sheet for hot pressing according to claim 10 , 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 2 in a temperature range of an Ac 3 transformation temperature to 1000° C.; and
then hot pressing the steel sheet.

15. A method for manuf

Assignees

• Jfe Steel Corp

Inventors

• Takashima Katsutoshi
• Kobayashi Takashi
• Funakawa Yoshimasa
• Nakajima Seiji

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D1/18 Primary

  Hardening (C21D1/02 takes precedence); Quenching with or without subsequent tempering (quenching devices C21D1/62) · CPC title

• C23C2/12

  Aluminium or alloys based thereon · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/60

  containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur · CPC title