High-strength steel sheet and method for manufacturing same

Patent metadata

Field	Value
Publication number	US-2021115541-A1
Application number	US-201917042250-A
Country	US
Kind code	A1
Filing date	Mar 20, 2019
Priority date	Mar 30, 2018
Publication date	Apr 22, 2021
Grant date	—

Abstract

A high-strength steel sheet includes a steel structure with: ferrite being 35% to 80%, martensite being 5% to 35%, and tempered martensite being 0% to 5% in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of: the ferrite being 6 μm or less; and the retained austenite being 3 μm or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing, by mass %, an average Mn content in the retained austenite by an average Mn content in the ferrite being 1.5 or more; and a value obtained by dividing, by mass %, an average C content in the retained austenite by an average C content in the ferrite being 3.0 or more.

First claim

1 - 7 . (canceled)

8 . A high-strength steel sheet comprising:
a component composition including: by mass %, C: 0.030% to 0.250%; Si: 0.01% to 3.00%; Mn: 3.10% to 4.20%; P: 0.001% to 0.100%; S: 0.0001% to 0.0200%; N: 0.0005% to 0.0100%; Al: 0.001% to 1.200%; and balance Fe and inevitable impurities; and a steel structure with: ferrite being 35% to 80%, martensite being 5% to 35%, and tempered martensite being 0% to 5% in terms of area fraction; retained austenite being 8% or more in terms of volume fraction; an average grain size of the ferrite being 6 μm or less; an average grain size of the retained austenite being 3 μm or less; a value obtained by dividing an area fraction of blocky austenite by a sum of area fractions of lath-like austenite and the blocky austenite being 0.6 or more; a value obtained by dividing an average Mn content, by mass %, in the retained austenite by an average Mn content, by mass %, in the ferrite being 1.5 or more; and a value obtained by dividing an average C content, by mass %, in the retained austenite by an average C content, by mass %, in the ferrite being 3.0 or more.

9 . The high-strength steel sheet according to claim 8 , wherein the component composition of the high-strength steel sheet further includes: by mass %, at least one element selected from Ti: 0.005% to 0.200%; Nb: 0.005% to 0.200%; V: 0.005% to 0.500%; W: 0.005% to 0.500%; B: 0.0003% to 0.0050%; Ni: 0.005% to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.100%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0050%; and balance Fe and inevitable impurities.

10 . A method of manufacturing a high-strength steel sheet, the method comprising:
heating a steel slab having a component composition including: by mass %, C: 0.030% to 0.250%; Si: 0.01% to 3.00%; Mn: 3.10% to 4.20%; P: 0.001% to 0.100%; S: 0.0001% to 0.0200%; N: 0.0005% to 0.0100%; Al: 0.001% to 1.200%; and balance Fe and inevitable impurities; hot rolling the steel slab with a finishing delivery temperature in hot rolling within a temperature range of 750° C. to 1,000° C., such that the steel slab becomes a hot rolled steel sheet; coiling up the hot rolled steel sheet within a temperature range of 300° C. to 750° C.; holding the hot rolled steel sheet for more than 21,600 s within a temperature range of an Ac 1 transformation temperature to the Ac 1 transformation temperature+150° C.; cold rolling the hot rolled steel sheet; holding the hot rolled steel sheet for 20 s to 900 s within a temperature range of the Ac 1 transformation temperature to the Ac 1 transformation temperature+150° C.; and cooling the hot rolled steel sheet.

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

12 . The method according to claim 10 , further comprising:
performing galvanization treatment on the hot rolled steel sheet.

13 . The method according to claim 11 , further comprising:
performing galvanization treatment on the hot rolled steel sheet.

14 . The method according to claim 12 , further comprising:
performing galvannealing treatment on the hot rolled steel sheet within a temperature range of 450° C. to 600° C.

15 . The method according to claim 13 , further comprising:
performing galvannealing treatment on the hot rolled steel sheet within a temperature range of 450° C. to 600° C.

16 . A method of manufacturing a high-strength steel sheet, the method comprising:
heating a steel slab having a component composition including: by mass %, C: 0.030% to 0.250%; Si: 0.01% to 3.00%; Mn: 3.10% to 4.20%; P: 0.001% to 0.100%; S: 0.0001% to 0.0200%; N: 0.0005% to 0.0100%; Al: 0.001% to 1.200%; and balance Fe and inevitable impurities; hot rolling the steel slab with a finishing delivery temperature in hot rolling within a temperature range of 750° C. to 1,000° C., such that the steel slab becomes a hot rolled steel sheet; coiling up the hot rolled steel sheet within a temperature range of 300° C. to 750° C.; holding the hot rolled steel sheet for more than 21,600 s within a temperature range of an Ac 1 transformation temperature to the Ac 1 transformation temperature+150° C.; cold rolling the hot rolled steel sheet; holding the hot rolled steel sheet for 20 s to 900 s within a temperature range of the Ac 1 transformation temperature or more; cooling the hot rolled steel sheet; performing pickling treatment on the hot rolled steel sheet; holding the hot rolled steel sheet for 20 s to 900 s within a temperature range of the Ac 1 transformation temperature to the Ac 1 transformation temperature+150° C.; cooling the hot rolled steel sheet; and performing galvanization treatment the hot rolled steel sheet.

17 . The method according to claim 16 , wherein the component composition further includes: by mass %, at least one element selected from Ti: 0.005% to 0.200%; Nb: 0.005% to 0.200%; V: 0.005% to 0.500%; W: 0.005% to 0.500%; B: 0.0003% to 0.0050%; Ni: 0.005% to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.002% to 0.200%, Ta: 0.001% to 0.100%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, Zr: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0050%; and balance Fe and inevitable impurities.

18 . The method according to claim 16 , further comprising:
performing galvannealing treatment on the hot rolled steel sheet within a temperature range of 450° C. to 600° C.

19 . The method according to claim 17 , further comprising:
performing galvannealing treatment on the hot rolled steel sheet within a temperature range of 450° C. to 600° C.

Assignees

• Jfe Steel Corp

Inventors

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

Classifications

• Y02P10/20

  Recycling · CPC title

• C23C2/0224

  Two or more thermal pretreatments · CPC title

• C23C2/02

  Pretreatment of the material to be coated, e.g. for coating on selected surface areas (C23C2/30 takes precedence) · CPC title

• C21D6/008 Primary

  containing Si · CPC title

• C22C38/06

  containing aluminium · CPC title