High-strength steel sheet, high-strength hot-dip galvanized steel sheet, high-strength hot-dip aluminum-coated steel sheet, and high-strength electrogalvanized steel sheet, and methods for manufacturing same

Patent metadata

Field	Value
Publication number	US-10550446-B2
Application number	US-201515520531-A
Country	US
Kind code	B2
Filing date	Oct 29, 2015
Priority date	Oct 30, 2014
Publication date	Feb 4, 2020
Grant date	Feb 4, 2020

Abstract

A high-strength steel sheet with excellent formability and high yield ratio that has TS of 980 MPa or more and YR of 68% or more is obtained by providing a predetermined chemical composition and a steel microstructure that contains, in area ratio, 15 to 55% of polygonal ferrite, 8% or more of non-recrystallized ferrite, and 15 to 30% of martensite, and that contains, in volume fraction, 12% or more of retained austenite, in which the polygonal ferrite has a mean grain size of 4 μm or less, the martensite has a mean grain size of 2 μm or less, the retained austenite has a mean grain size of 2 μm or less, and a value obtained by dividing an Mn content in the retained austenite (in mass %) by an Mn content in the polygonal ferrite (in mass %) equals 2.0 or more.

First claim

The invention claimed is:

1. A high-strength steel sheet, having a tensile strength of 980 MPa or more, 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: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and 0.200% or less, and the balance being Fe and incidental impurities; and
a steel microstructure that contains, in area ratio, 15% or more and 55% or less of polygonal ferrite, 8% 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,
wherein the polygonal ferrite has a mean grain size of 4 μm or less, the martensite has a mean grain size of 2 μm or less, the retained austenite has a mean grain size of 2 μm or less, and a value obtained by dividing an Mn content in the retained austenite in mass % by an Mn content in the polygonal ferrite in mass % equals 2.0 or more.

2. The high-strength steel sheet according to claim 1 , wherein the chemical composition further contains, 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.

3. The high-strength steel sheet according to claim 2 , wherein the steel microstructure further contains, in area ratio, 2% or more of c phase with an hcp structure.

4. The high-strength steel sheet according to claim 2 , 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 content]+0.056−0.180[C content]≤0.04*[Mn content]+0.056+0.180

where
[C content] is the C content in the retained austenite in mass %, and
[Mn content] is the Mn content in the retained austenite in mass %.

5. The high-strength steel sheet according to claim 2 , wherein when the steel sheet is subjected to tensile working with an elongation value of 10%, a value obtained by dividing a volume fraction of the retained austenite after the tensile working by a volume fraction of the retained austenite before the tensile working equals 0.3 or more.

6. The high-strength steel sheet according to claim 2 , wherein the high-strength steel sheet is a high-strength hot-dip galvanized steel sheet comprising a hot-dip galvanized layer, a high-strength hot-dip aluminum-coated steel sheet comprising a hot-dip aluminum-coated layer or a high-strength electrogalvanized steel sheet comprising an electrogalvanized layer.

7. The high-strength steel sheet according to claim 1 , wherein the steel microstructure further contains, in area ratio, 2% or more of c phase with an hcp structure.

8. The high-strength steel sheet according to claim 1 , 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 content]+0.056−0.180[C content]≤0.04*[Mn content]+0.056+0.180

where
[C content] is the C content in the retained austenite in mass %, and
[Mn content] is the Mn content in the retained austenite in mass %.

9. The high-strength steel sheet according to claim 1 , wherein when the steel sheet is subjected to tensile working with an elongation value of 10%, a value obtained by dividing a volume fraction of the retained austenite after the tensile working by a volume fraction of the retained austenite before the tensile working equals 0.3 or more.

10. The high-strength steel sheet according to claim 1 , wherein the high-strength steel sheet is a high-strength hot-dip galvanized steel sheet comprising a hot-dip galvanized layer, a high-strength hot-dip aluminum-coated steel sheet comprising a hot-dip aluminum-coated layer or a high-strength electrogalvanized steel sheet comprising an electrogalvanized layer.

11. A method for manufacturing the high-strength steel sheet as recited in claim 1 , the method comprising:
heating a steel slab having the 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: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and 0.200% or less, and the balance being Fe and incidental impurities, to 1100° C. or higher and 1300° C. or lower;
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;
coiling the steel sheet at 300° C. or higher and 750° C. or lower;
then subjecting the steel sheet to pickling to remove scales;
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;
cold rolling the steel sheet at a rolling reduction of 30% or more; and
then retaining the steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+100° C. for 20 s to 900 s, and subsequently cooling the steel sheet.

12. A method for manufacturing the high-strength steel sheet as recited in claim 10 , the method comprising:
heating a steel slab having the 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: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and 0.200% or less, and the balance being Fe and incidental impurities, to 1100° C. or higher and 1300° C. or lower;
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;
coiling the steel sheet at 300° C. or higher and 750° C. or lower;
then subjecting the steel sheet to pickling to remove scales;
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;
cold rolling the steel sheet at a rolling reduction of 30% or more;
then retaining the steel sheet in a temperature range of Ac 1 transformation temperature to Ac 1 transformation temperature+100° C. for 20 s to 900 s, and subsequently cooling the steel sheet; and
then subjecting the steel sheet to any one of the following:
galvanizing treatment, either alone or followed by alloying treatment at 450° C. or higher and 600° C. or lower,
hot-dip aluminum-coating treatment, or
electrogalvanizing treatment.

13. A method for manufacturing the high-strength steel sheet as recited in claim 2 , the method comprising:
heating a steel slab having the 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: 0.0001% or more and 0.0200% or less, N: 0.0005% or more and 0.0100% or less, and Ti: 0.005% or more and 0.200% or less, and at least one selected from the group consisting of Al: 0

Assignees

• Jfe Steel Corp

Inventors

• Kawasaki Yoshiyasu
• Matsuda Hiroshi
• Yokota Takeshi
• Yamashita Takako
• Seto Kazuhiro

Classifications

• C21D8/02

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

• C22C38/001

  containing N · CPC title

• C22C38/005

  containing rare earths, i.e. Sc, Y, Lanthanides · CPC title

• C21D8/0236

  Cold rolling · CPC title

• C21D6/001

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