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-11111558-B2
Application number	US-201816619814-A
Country	US
Kind code	B2
Filing date	Mar 30, 2018
Priority date	Jun 30, 2017
Publication date	Sep 7, 2021
Grant date	Sep 7, 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 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.

First claim

The invention claimed is:

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, N, 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 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 μm 2 of a cross section parallel to a thickness direction of the member within a range of 100 μm 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.

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, N, 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 μm or less, and at least 20 Nb-based and Ti-based precipitates having a grain size of less than 0.10 μm 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.

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, N, 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 Ac 3 transformation temperature to 1000° C.; and
then hot pressing the steel sheet.

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.

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 allo

Assignees

• Jfe Steel Corp

Inventors

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

Classifications

• C21D8/04

  to produce plates or strips for drawing, e.g. for deep-drawing · CPC title

• C25D7/0614

  Strips or foils · CPC title

• C22C38/04

  containing manganese · CPC title

• C21D6/002

  containing Cr (C21D6/004 takes precedence) · CPC title

• C22C38/60

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