Steel sheet and method for producing the same

Patent metadata

Field	Value
Publication number	US-11268164-B2
Application number	US-201716329504-A
Country	US
Kind code	B2
Filing date	Sep 28, 2017
Priority date	Sep 28, 2016
Publication date	Mar 8, 2022
Grant date	Mar 8, 2022

Abstract

A steel sheet having a specified chemical composition and a method for producing the steel sheet. The steel sheet has a microstructure including martensite and bainite. The total area fraction of the martensite and the bainite to the entirety of the microstructure is 95% or more and 100% or less. The balance of the microstructure is at least one of ferrite and retained austenite. The microstructure includes specific inclusion clusters, the content of the inclusion clusters in the microstructure being 5 clusters/mm2 or less. The microstructure includes prior-austenite grains having an average size of more than 5 μm. The steel sheet has a tensile strength of 1320 MPa or more.

First claim

The invention claimed is:

1. A steel sheet having a chemical composition comprising, by mass %:
C: 0.13% or more and 0.40% or less,
Si: 1.5% or less,
Mn: 1.7% or less,
P: 0.030% or less,
S: less than 0.0010%,
sol. Al: 0.20% or less,
N: 0.0055% or less,
O: 0.0025% or less,
Nb: 0.002% or more and 0.035% or less,
Ti: 0.002% or more and 0.040% or less, and
the balance being Fe and inevitable impurities,
wherein the steel sheet has a microstructure including martensite and bainite, the total area fraction of the martensite and the bainite to the entirety of the microstructure being in a range of 95% or more and 100% or less, and the balance, if any, being at least one of ferrite and retained austenite, the microstructure including (i) prior-austenite grains having an average grain size of more than 5 μm , and (ii) inclusion clusters having a major axis of 20 to 80 μm, the content of the inclusion clusters in the microstructure being 5 clusters/mm2 or less, each of the inclusion clusters constituted by at least one inclusion particle, where:
in the case where each of the inclusion clusters is constituted by one inclusion particle, the inclusion particle has a major axis of 20 to 80 μm, and
in the case where the inclusion clusters are constituted by two or more inclusion particles, the inclusion particles have a major axis of 0.3 μm or more, and the shortest distance between the inclusion particles is 10 μm or less,

Formula (1) and Formula (2) are both satisfied:
[% Ti]+[% Nb]>0.007   (1)

[% Ti]×[% Nb] 2 ≤7.5×10 −6   (2)

where [%Nb] and [%Ti] represent the contents (%) of Nb and Ti, respectively, and

the steel sheet has a tensile strength of 1320 MPa or more.

2. The steel sheet according to claim 1 , the chemical composition further comprising, by mass %, at least one group selected from Groups A-E:
Group A: B: 0.0002% or more and less than 0.0035%,
Group B: at least one element selected from the group consisting of Cu: 0.005% or more and 1% or less, and Ni: 0.01% or more and 1% or less,
Group C: at least one element selected from the group consisting of:
Cr: 0.01% or more and 1.0% or less.
Mo: 0.01% or more and less than 0.3%,
V: 0.003% or more and 0.45% or less,
Zr: 0.005% or more and 0.2% or less, and
W: 0.005% or more and 0.2% or less,

Group D: at least one element selected from the group consisting of:
Ca: 0.0002% or more and 0.0030% or less,
Ce: 0.0002% or more and 0.0030% or less.
La: 0.0002% or more and 0.0030% or less, and
Mg: 0.0002% or more and 0.0030% or less, and

Group E: at least one element selected from the group consisting of Sb: 0.002% or more and 0.1% or less, and Sn: 0.002% or more and 0.1% or less.

3. The steel sheet according to claim 2 , wherein the steel sheet has a coating layer disposed on a surface thereof.

4. The steel sheet according to claim 1 , wherein the steel sheet has a coating layer disposed on a surface thereof.

5. A method for producing the steel sheet according to claim 1 , the method comprising:
holding a steel slab having the chemical composition for 100 minutes or more with a slab-surface temperature of 1220° C. or more and subsequently hot-rolling the steel slab into a hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet into a cold-rolled steel sheet at a cold-rolling ratio of 40% or more; and
performing continuous annealing of the cold-rolled steel sheet, the continuous annealing including treating the cold-rolled steel sheet for 240 seconds or more with an annealing temperature higher than 850° C., subsequently reducing the temperature from 680° C. or more to 260° C. or less at an average cooling rate of 70° C/s or more, then performing reheating as needed, and subsequently performing holding at a temperature in a range of 150° C. to 260° C. for in a range of 20 to 1500 seconds.

6. The method for producing a steel sheet according to claim 5 , further comprising performing coating of the steel sheet subsequent to the continuous annealing.

7. A method for producing the steel sheet according to claim 2 , the method comprising:
holding a steel slab having the chemical composition for 100 minutes or more with a slab-surface temperature of 1220° C. or more and subsequently hot-rolling the steel slab into a hot-rolled steel sheet;
cold-rolling the hot-rolled steel sheet into a cold-rolled steel sheet at a cold-rolling ratio of 40% or more; and
performing continuous annealing of the cold-rolled steel sheet, the continuous annealing including treating the cold-rolled steel sheet for 240 seconds or more with an annealing temperature higher than 850° C., subsequently reducing the temperature from 680° C. or more to 260° C. or less at an average cooling rate of 70° C/s or more, then performing reheating as needed, and subsequently performing holding at a temperature in a range of 150° C. to 260° C. for in a range of 20 to 1500 seconds.

8. The method for producing a steel sheet according to claim 7 , further comprising performing coating of the steel sheet subsequent to the continuous annealing.

Assignees

• Jfe Steel Corp

Inventors

• Yoshioka Shimpei
• Ono Yoshihiko
• Hidaka Kenta
• Yamaguchi Ryosuke

Classifications

• C21D8/02

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

• Y02P10/20

  Recycling · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/005

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

• C22C38/04 Primary

  containing manganese · CPC title