Steel sheet, coated steel sheet, and methods for manufacturing same

Patent metadata

Field	Value
Publication number	US-2019106760-A1
Application number	US-201716090892-A
Country	US
Kind code	A1
Filing date	Mar 8, 2017
Priority date	Apr 19, 2016
Publication date	Apr 11, 2019
Grant date	—

Abstract

A steel sheet having TS of 980 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a predetermined steel microstructure, where an average aspect ratio of crystal grains of each phase (polygonal ferrite, martensite, and retained austenite) is 2.0 or more and 20.0 or less, wherein the polygonal ferrite has an average grain size of 4 μm or less, the martensite has an average grain size of 2 μm or less, the retained austenite has an average grain size of 2 μm or less, and a value obtained by dividing a Mn content in the retained austenite in mass % by a Mn content in the polygonal ferrite in mass % equals 2.0 or more.

First claim

1 - 7 . (canceled)

8 . A steel sheet comprising:
a chemical composition containing, in mass %,
C: 0.030% or more and 0.250% or less,
Si: 0.01% or more and 3.00% or less,
Mn: more than 4.20% and 6.00% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
N: 0.0100% or less, and
Ti: 0.005% or more and 0.200% or less, and
optionally further containing, in mass %, at least one selected from the group consisting of
Al: 0.01% or more and 2.00% or less,
Nb: 0.005% or more and 0.200% or less,
B: 0.0003% or more and 0.0050% or less,
Ni: 0.005% or more and 1.000% or less,
Cr: 0.005% or more and 1.000% or less,
V: 0.005% or more and 0.500% or less,
Mo: 0.005% or more and 1.000% or less,
Cu: 0.005% or more and 1.000% or less,
Sn: 0.002% or more and 0.200% or less,
Sb: 0.002% or more and 0.200% or less,
Ta: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less, and
REM: 0.0005% or more and 0.0050% or less,

with the balance consisting of Fe and inevitable impurities; and
a steel microstructure that contains, in area ratio,
5% or more and 50% or less of polygonal ferrite,
10% or more of non-recrystallized ferrite, and
15% or more and 30% or less of martensite, and
that contains, in volume fraction, 12% or more of retained austenite, where an average aspect ratio of crystal grains of each of the polygonal ferrite, the martensite, and the retained austenite is 2.0 or more and 20.0 or less,
wherein the polygonal ferrite has an average grain size of 4 μm or less, the martensite has an average grain size of 2 μm or less, the retained austenite has an average grain size of 2 μm or less, and a value obtained by dividing a Mn content in the retained austenite in mass % by a Mn content in the polygonal ferrite in mass % equals 2.0 or more.

9 . The steel sheet according to claim 8 , wherein the steel microstructure further contains, in area ratio, 2% or more of ε phase with an hcp structure.

10 . The steel sheet according to claim 8 , wherein the retained austenite has a C content that satisfies the following formula in relation to the Mn content in the retained austenite:
0.04*[Mn]+0.056−0.180≤[C]≤0.04*[Mn]+0.056+0.180
where
[C] is the C content in the retained austenite in mass %, and
[Mn] is the Mn content in the retained austenite in mass %.

11 . The steel sheet according to claim 9 , wherein the retained austenite has a C content that satisfies the following formula in relation to the Mn content in the retained austenite:
0.04*[Mn]+0.056−0.180≤[C]≤0.04*[Mn]+0.056+0.180
where
[C] is the C content in the retained austenite in mass %, and
[Mn] is the Mn content in the retained austenite in mass %.

12 . A coated steel sheet comprising:
the steel sheet according to claim 8 ; and one selected from a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer, and an electrogalvanized layer provided on the steel sheet.

13 . A coated steel sheet comprising:
the steel sheet according to claim 9 ; and one selected from a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer, and an electrogalvanized layer provided on the steel sheet.

14 . A coated steel sheet comprising:
the steel sheet according to claim 10 ; and one selected from a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer, and an electrogalvanized layer provided on the steel sheet.

15 . A coated steel sheet comprising:
the steel sheet according to claim 11 ; and one selected from a hot-dip galvanized layer, a galvannealed layer, a hot-dip aluminum-coated layer, and an electrogalvanized layer provided on the steel sheet.

16 . A method for manufacturing the steel sheet according to claim 8 , the method comprising:
(i) heating a steel slab having a chemical composition containing, in mass %,
C: 0.030% or more and 0.250% or less,
Si: 0.01% or more and 3.00% or less,
Mn: more than 4.20% and 6.00% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
N: 0.0100% or less, and
Ti: 0.005% or more and 0.200% or less, and
optionally further containing, in mass %, at least one selected from the group consisting of
Al: 0.01% or more and 2.00% or less,
Nb: 0.005% or more and 0.200% or less,
B: 0.0003% or more and 0.0050% or less,
Ni: 0.005% or more and 1.000% or less,
Cr: 0.005% or more and 1.000% or less,
V: 0.005% or more and 0.500% or less,
Mo: 0.005% or more and 1.000% or less,
Cu: 0.005% or more and 1.000% or less,
Sn: 0.002% or more and 0.200% or less,
Sb: 0.002% or more and 0.200% or less,
Ta: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less, and
REM: 0.0005% or more and 0.0050% or less,

with the balance consisting of Fe and inevitable impurities;
(ii) hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet; (iii) coiling the steel sheet; (iv) then subjecting the steel sheet to pickling to remove scales; (v) retaining the steel sheet in a temperature range of Ac 1 [transformation temperature+20° C.] to [Ac 1 transformation temperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rolling the steel sheet at a rolling reduction of less than 30%; and (vii) then retaining the steel sheet in a temperature range of [Ac 1 transformation temperature+10° C.] to [Ac 1 transformation temperature+100° C.] for 20 s to 900 s and subsequently cooling the steel sheet.

17 . A method for manufacturing the steel sheet according to claim 9 , the method comprising:
(i) heating a steel slab having a chemical composition containing, in mass %,
C: 0.030% or more and 0.250% or less,
Si: 0.01% or more and 3.00% or less,
Mn: more than 4.20% and 6.00% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
N: 0.0100% or less, and
Ti: 0.005% or more and 0.200% or less, and
optionally further containing, in mass %, at least one selected from the group consisting of
Al: 0.01% or more and 2.00% or less,
Nb: 0.005% or more and 0.200% or less,
B: 0.0003% or more and 0.0050% or less,
Ni: 0.005% or more and 1.000% or less,
Cr: 0.005% or more and 1.000% or less,
V: 0.005% or more and 0.500% or less,
Mo: 0.005% or more and 1.000% or less,
Cu: 0.005% or more and 1.000% or less,
Sn: 0.002% or more and 0.200% or less,
Sb: 0.002% or more and 0.200% or less,
Ta: 0.001% or more and 0.010% or less,
Ca: 0.0005% or more and 0.0050% or less,
Mg: 0.0005% or more and 0.0050% or less, and
REM: 0.0005% or more and 0.0050% or less,

with the balance consisting of Fe and inevitable impurities;
(ii) hot rolling the steel slab with a finisher delivery temperature of 750° C. or higher and 1000° C. or lower to obtain a steel sheet; (iii) coiling the steel sheet; (iv) then subjecting the steel sheet to pickling to remove scales; (v) retaining the steel sheet in a temperature range of Ac 1 [transformation temperature+20° C.] to [Ac 1 transformation temperature+120° C.] for 600 s to 21,600 s; (vi) optionally cold rolling the steel sheet at a rolling reduction of less than 30%; and (vii) then retaining the steel sheet in a temperature range of [Ac 1 transformation temperature+10° C.] to [Ac 1 transformation temperature+100° C.] for 20 s to 900 s and subsequently cooling the steel sheet.

18 . A method for manufacturing the steel sheet according to claim 10 , the method comprising:
(i) heating a steel slab having
a chemical composition containing, in mass %,
C: 0.030% or m

Assignees

• Jfe Steel Corp

Inventors

• Yamashita Takako
• Kawasaki Yoshiyasu
• Kobayashi Takashi
• Ueno Masayasu
• Nagataki Yasunobu

Classifications

• C21D8/0273

  Final recrystallisation annealing · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C22C38/02

  containing silicon · CPC title

• C22C38/04 Primary

  containing manganese · CPC title

• C21D9/46

  for sheet metals · CPC title