High-strength steel sheet and production method thereof

Patent metadata

Field	Value
Publication number	US-2021108282-A1
Application number	US-201917043514-A
Country	US
Kind code	A1
Filing date	Mar 20, 2019
Priority date	Mar 30, 2018
Publication date	Apr 15, 2021
Grant date	—

Abstract

A high-strength steel includes a steel structure with: in area fraction, 60.0% to less than 90.0% of ferrite, 0% to less than 5.0% of unrecrystallized ferrite, 2.0% to 25.0% of martensite, 0% to 5.0% of carbide, and 0% to 3.0% of bainite; in volume fraction, more than 7.0% of retained austenite; in a cross-sectional view of 100 μm×100 μm, a value obtained by dividing number of retained austenite that are not adjacent to retained austenite whose crystal orientations are different by a total number of retained austenite being less than 0.80, an average crystal grain size of the ferrite being 6.0 μm or less, an average crystal grain size of the retained austenite being 3.0 μm or less, and a value obtained by dividing, by mass %, an average content of Mn in the retained austenite by an average content of Mn in steel being 1.50 or more.

First claim

1 - 9 . (canceled)

10 . A high-strength steel sheet comprising:
a component composition including: by mass %, 0.030% to 0.250% of C; 0.01% to 2.00% of Si; 2.00% or more and less than 3.10% of Mn; 0.001% to 0.100% of P; 0.0001% to 0.0200% of S; 0.0005% to 0.0100% of N; 0.001% to 1.200% of Al; and the balance Fe and inevitable impurities; and a steel structure with:
in terms of area fraction, 60.0% or more and less than 90.0% of ferrite, 0% or more and less than 5.0% of unrecrystallized ferrite, 2.0% to 25.0% of martensite, 0% to 5.0% of carbide, and 0% to 3.0% of bainite;
in terms of volume fraction, more than 7.0% of retained austenite;
in a cross-sectional view of 100 μm×100 μm, a value obtained by dividing number of retained austenite that are not adjacent to retained austenite whose crystal orientations are different by a total number of retained austenite being less than 0.80,
an average crystal grain size of the ferrite being 6.0 μm or less,
an average crystal grain size of the retained austenite being 3.0 μm or less, and
a value obtained by dividing an average content, by mass %, of Mn in the retained austenite by an average content, by mass %, of Mn in steel being 1.50 or more.

11 . The high-strength steel sheet according to claim 10 , wherein the component composition further includes: by mass %, at least one kind of element selected from: 0.002% to 0.200% of Ti; 0.005% to 0.200% of Nb; 0.005% to 0.500% of V; 0.0005% to 0.500% of W; 0.0003% to 0.0050% of B; 0.005% to 1.000% of Cr; 0.005% to 1.000% of Ni; 0.005% to 1.000% of Mo; 0.005% to 1.000% of Cu; 0.002% to 0.200% of Sn; 0.002% to 0.200% of Sb; 0.001% to 0.100% of Ta; 0.0005% to 0.0050% of Zr; 0.0005% to 0.0050% of Ca; 0.0005% to 0.0050% of Mg; and 0.0005% to 0.0050% of REM; and the balance Fe and inevitable impurities.

12 . The high-strength steel sheet according to claim 10 , wherein diffusible hydrogen in steel is 0.50 ppm by mass or less.

13 . The high-strength steel sheet according to claim 11 , wherein diffusible hydrogen in steel is 0.50 ppm by mass or less.

14 . A method of producing a high-strength steel sheet, the method comprising:
reheating a steel slab having a component composition including: by mass %, 0.030% to 0.250% of C; 0.01% to 2.00% of Si; 2.00% or more and less than 3.10% of Mn; 0.001% to 0.100% of P; 0.0001% to 0.0200% of S; 0.0005% to 0.0100% of N; 0.001% to 1.200% of Al; and the balance Fe and inevitable impurities; hot rolling the steel slab with a finishing delivery temperature in a temperature range of 750° C. to 1,000° C., such that the steel slab becomes a hot rolled steel sheet; coiling the hot rolled steel sheet in a temperature range of 300° C. to 750° C.; performing acid pickling on the hot rolled steel sheet; holding the hot rolled steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+150° C. for more than 21,600 seconds and 259,200 seconds or less; cooling the hot rolled steel sheet in a temperature range from 550° C. to 400° C. at an average cooling rate of 5° C./hour to 200° C./hour; cold rolling the hot rolled steel sheet; holding the hot rolled steel sheet in a temperature range of Ac 1 transformation temperature+30° C. to Ac 1 transformation temperature+130° C. for 20 seconds to 900 seconds; cooling the hot rolled steel sheet to room temperature; performing acid pickling on the hot rolled steel sheet; increasing temperature of the hot rolled steel sheet in a temperature range from 400° C. to Ac 1 transformation temperature at an average temperature increase rate of 0.5° C./second to 50° C./second; holding the hot rolled steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+150° C. for 20 seconds to 900 seconds; and cooling the hot rolled steel sheet to room temperature.

15 . The method according to claim 14 , wherein the component composition further includes: by mass %, at least one kind of element selected from: 0.002% to 0.200% of Ti; 0.005% to 0.200% of Nb; 0.005% to 0.500% of V; 0.0005% to 0.500% of W; 0.0003% to 0.0050% of B; 0.005% to 1.000% of Cr; 0.005% to 1.000% of Ni; 0.005% to 1.000% of Mo; 0.005% to 1.000% of Cu; 0.002% to 0.200% of Sn; 0.002% to 0.200% of Sb; 0.001% to 0.100% of Ta; 0.0005% to 0.0050% of Zr; 0.0005% to 0.0050% of Ca; 0.0005% to 0.0050% of Mg; and 0.0005% to 0.0050% of REM; and the balance Fe and inevitable impurities.

16 . The method according to claim 14 , wherein diffusible hydrogen in steel is 0.50 ppm by mass or less.

17 . The method according to claim 15 , wherein diffusible hydrogen in steel is 0.50 ppm by mass or less.

18 . The method according to claim 14 , further comprising:
holding the hot rolled steel sheet in a temperature range of 50° C. to 300° C. for 1,800 seconds to 259,200 seconds; and cooling the hot rolled steel sheet to room temperature.

19 . The method according to claim 15 , further comprising:
holding the hot rolled steel sheet in a temperature range of 50° C. to 300° C. for 1,800 seconds to 259,200 seconds; and cooling the hot rolled steel sheet to room temperature.

20 . The method according to claim 16 , further comprising:
holding the hot rolled steel sheet in a temperature range of 50° C. to 300° C. for 1,800 seconds to 259,200 seconds; and cooling the hot rolled steel sheet to room temperature.

21 . The method according to claim 17 , further comprising:
holding the hot rolled steel sheet in a temperature range of 50° C. to 300° C. for 1,800 seconds to 259,200 seconds; and cooling the hot rolled steel sheet to room temperature.

22 . A method of producing a high-strength steel sheet, the method comprising:
reheating a steel slab having a component composition including: by mass %, 0.030% to 0.250% of C; 0.01% to 2.00% of Si; 2.00% or more and less than 3.10% of Mn; 0.001% to 0.100% of P; 0.0001% to 0.0200% of S; 0.0005% to 0.0100% of N; 0.001% to 1.200% of Al; and the balance Fe and inevitable impurities; hot rolling the steel slab with a finishing delivery temperature in a temperature range of 750° C. to 1,000° C., such that the steel slab becomes a hot rolled steel sheet; coiling the hot rolled steel sheet in a temperature range of 300° C. to 750° C.; performing acid pickling on the hot rolled steel sheet; holding the hot rolled steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+150° C. for more than 21,600 seconds and 259,200 seconds or less; cooling the hot rolled steel sheet in a temperature range from 550° C. to 400° C. at an average cooling rate of 5° C./hour to 200° C./hour; cold rolling the hot rolled steel sheet; holding the hot rolled steel sheet in a temperature range of Ac 1 transformation temperature+30° C. to Ac 1 transformation temperature+130° C. for 20 seconds to 900 seconds; cooling the hot rolled steel sheet to room temperature; performing acid pickling on the hot rolled steel sheet; increasing temperature of the hot rolled steel sheet in a temperature range from 400° C. to Ac 1 transformation temperature at an average temperature increase rate of 0.5° C./second to 50° C./second; holding the hot rolled steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+150° C. for 20 seconds to 900 seconds; cooling the hot rolled steel sheet; performing galvanization on the hot rolled steel sheet; and cooling the hot rolled steel sheet to room temperature.

23 . The method according to claim 22 , wherein the component composition further includes: by mass %, a

Assignees

• Jfe Steel Corp

Inventors

• Kawasaki Yoshiyasu
• Endo Kazuki
• Toji Yuki
• Funakawa Yoshimasa
• Maeda Satoshi
• Aoyama Mai

Classifications

• C21D8/02

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

• Y02P10/20

  Recycling · CPC title

• C21D9/46 Primary

  for sheet metals · CPC title

• C21D2211/001

  Austenite · CPC title

• C22C38/58

  with more than 1.5% by weight of manganese · CPC title