High strength steel sheet and method for manufacturing the same

Patent metadata

Field	Value
Publication number	US-12378624-B2
Application number	US-202017770118-A
Country	US
Kind code	B2
Filing date	Oct 8, 2020
Priority date	Oct 23, 2019
Publication date	Aug 5, 2025
Grant date	Aug 5, 2025

Abstract

Provided is a high strength steel sheet that has a predetermined chemical composition and is manufactured under optimum conditions, the high strength steel sheet having a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more, wherein the quotient of the area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by the total area fraction of the retained austenite is 0.3 or more, wherein the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite is 1.5 or more.

First claim

The invention claimed is:

1. A high strength steel sheet comprising:
a chemical composition containing, by mass,
C: 0.030% or more and 0.250% or less,
Si: 0.01% or more and 3.00% or less,
Mn: 2.50% or more and 8.00% 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
Al: 0.001% or more and 2.000% or less,
with the balance being Fe and incidental impurities;
a steel microstructure including, by area, ferrite: 30% or more and 80% or less, tempered martensite: 3.0% or more and 35% or less, and retained austenite: 8% or more,
wherein a quotient of an area fraction of grains of the retained austenite, the grains having an aspect ratio of 2.0 or more and a minor axis length of 1 μm or less, divided by a total area fraction of the retained austenite is 0.3 or more,
wherein a quotient of an average Mn content (mass %) in the retained austenite divided by an average Mn content (mass %) in the ferrite is 1.5 or more, and a product of the quotient of the average Mn content (mass %) in the retained austenite divided by the average Mn content (mass %) in the ferrite and an average aspect ratio of the retained austenite is 3.0 or more,
wherein a quotient of an average C content (mass %) in the retained austenite divided by an average C content (mass %) in the ferrite is 3.0 or more,
wherein a quotient of the average C content (mass %) in the retained austenite divided by the average Mn content (mass %) in the retained austenite is 0.05 or more, and
wherein a content of diffusible hydrogen in steel is 0.3 ppm by mass or less.

2. The high strength steel sheet according to claim 1 , wherein the chemical composition further contains at least one element selected from, by mass,
Ti: 0.200% or less,
Nb: 0.200% or less,
V: 0.500% or less,
W: 0.500% or less,
B: 0.0050% or less,
Ni: 1.000% or less,
Cr: 1.000% or less,
Mo: 1.000% or less,
Cu: 1.000% or less,
Sn: 0.200% or less,
Sb: 0.200% or less,
Ta: 0.100% or less,
Ca: 0.0050% or less,
Mg: 0.0050% or less,
Zr: 0.0050% or less, and
REM: 0.0050% or less.

3. The high strength steel sheet according to claim 2 , the high strength steel sheet further comprising a galvanized layer disposed on a surface of the high strength steel sheet.

4. The high strength steel sheet according to claim 3 , wherein the galvanized layer is a galvannealed layer.

5. The high strength steel sheet according to claim 1 , the high strength steel sheet further comprising a galvanized layer disposed on a surface of the high strength steel sheet.

6. The high strength steel sheet according to claim 5 , wherein the galvanized layer is a galvannealed layer.

7. A method for manufacturing a high strength steel sheet according to claim 1 , the method comprising heating a steel slab having the chemical composition, hot rolling the steel slab at a finish rolling delivery temperature of 750° C. or more and 1000° C. or less, then performing coiling at 300° C. or more and 750° C. or less, subsequently performing cold rolling, then performing holding at a temperature equal to or higher than an Ac 3 transformation temperature−50° C. for 20 s or more and 1800 s or less, subsequently performing cooling to a cooling stop temperature equal to or lower than a martensite transformation start temperature, then performing reheating to a reheating temperature of 120° C. or more and 450° C. or less, subsequently performing holding at the reheating temperature for 2 s or more and 600 s or less, then performing cooling to room temperature, subsequently performing holding at a temperature equal to or higher than an Ac 1 transformation temperature and equal to or lower than the Ac 1 transformation temperature+150° C. for 20 s or more and 600 s or less, then performing cooling to 100° C. or less, then performing holding at higher than 100° C. and 400° C. or less for 10 s or more, and subsequently performing cooling.

8. The method for manufacturing a high strength steel sheet according to claim 7 , the method further comprising, subsequent to the coiling and prior to the cold rolling, performing holding at a temperature equal to or lower than the Ac 1 transformation temperature for more than 1800 s.

9. The method for manufacturing a high strength steel sheet according to claim 8 , the method further comprising, subsequent to the holding at the temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 1 transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.

10. The method for manufacturing a high strength steel sheet according to claim 9 , the method further comprising, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less.

11. The method for manufacturing a high strength steel sheet according to claim 7 , the method further comprising, subsequent to the holding at the temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 1 transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.

12. The method for manufacturing a high strength steel sheet according to claim 11 , the method further comprising, subsequent to the galvanizing treatment, performing an alloying treatment at 450° C. or more and 600° C. or less.

13. A method for manufacturing a high strength steel sheet according to claim 2 , the method comprising heating a steel slab having the chemical composition, hot rolling the steel slab at a finish rolling delivery temperature of 750° C. or more and 1000° C. or less, then performing coiling at 300° C. or more and 750° C. or less, subsequently performing cold rolling, then performing holding at a temperature equal to or higher than an Ac 3 transformation temperature−50° C. for 20 s or more and 1800 s or less, subsequently performing cooling to a cooling stop temperature equal to or lower than a martensite transformation start temperature, then performing reheating to a reheating temperature of 120° C. or more and 450° C. or less, subsequently performing holding at the reheating temperature for 2 s or more and 600 s or less, then performing cooling to room temperature, subsequently performing holding at a temperature equal to or higher than an Ac 1 transformation temperature and equal to or lower than the Ac 1 transformation temperature+150° C. for 20 s or more and 600 s or less, then performing cooling to 100° C. or less, then performing holding at higher than 100° C. and 400° C. or less for 10 s or more, and subsequently performing cooling.

14. The method for manufacturing a high strength steel sheet according to claim 13 , the method further comprising, subsequent to the coiling and prior to the cold rolling, performing holding at a temperature equal to or lower than the Ac 1 transformation temperature for more than 1800 s.

15. The method for manufacturing a high strength steel sheet according to claim 14 , the method further comprising, subsequent to the holding at the temperature equal to or higher than the Ac 1 transformation temperature and equal to or lower than the Ac 1 transformation temperature+150° C. for 20 s or more and 600 s or less, performing cooling, then performing a galvanizing treatment, and subsequently performing the cooling to 100° C. or less.

16. The method for m

Assignees

• Jfe Steel Corp

Inventors

• ENDOH Kazuki
• Kawasaki Yoshiyasu
• Toji Yuki

Classifications

• C21D8/02

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

• C23C2/40

  Plates; Strips · CPC title

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C22C38/38

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

• C22C38/28

  with titanium or zirconium · CPC title