High-strength steel sheet and method for producing same

Patent metadata

Field	Value
Publication number	US-11136643-B2
Application number	US-201716320864-A
Country	US
Kind code	B2
Filing date	Aug 10, 2017
Priority date	Aug 10, 2016
Publication date	Oct 5, 2021
Grant date	Oct 5, 2021

Abstract

A high-strength steel sheet having a specified chemical composition and a method for producing the same. The steel sheet including a multi-phase structure including, by volume fraction, ferrite of 35% to 65%, residual austenite of 7% or less (including 0%), martensite of 20% to 60%, bainite of 20% or less (including 0%), the ferrite having an average crystal grain size of 8 μm or less and an average aspect ratio of 3.5 or less, the martensite having an average crystal grain size of 3 μm or less, and the bainite having an average crystal grain size of 3 μm or less when contained. A standard deviation of Vickers hardness of the steel sheet being 30 or less when measured at 200 μm intervals from a position of 100 μm from a surface to a central portion in a thickness direction of the sheet.

First claim

The invention claimed is:

1. A steel sheet having a chemical composition comprising, by mass %:
C: 0.05% to 0.15%,
Si: 1.6% or less,
Mn: 1.3% to 2.4%,
P: 0.05% or less,
S: 0.0050% or less,
Al: 0.01% to 0.10%,
N: 0.010% or less,
Ti: 0.005% to 0.10%,
B: 0.0002% to 0.004%, and
at least one of Mo: 0.005% to 0.50% and Cr: 0.005% to 1.0%,
the balance being Fe and unavoidable impurities; and
a multi-phase structure comprising, by volume fraction, ferrite in a range of 35% to 65%, residual austenite in a range of 7% or less, inclusive of 0%, martensite in a range of 20% to 60%, and bainite in a range of 20% or less, inclusive of 0%,
wherein the ferrite has an average crystal grain size of 8 μm or less and an average aspect ratio of 3.5 or less,
the martensite has an average crystal grain size of 3 μm or less,
the bainite has an average crystal grain sizes of 3 μm or less when contained, and
a standard deviation of Vickers hardness of the steel sheet is 30 or less when measured at 200 μm intervals from a position of 100 μm from a surface to a central position in a thickness direction of the steel sheet.

2. The steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of V: 0.05% or less, Nb: 0.50% or less, Cu: 0.30% or less, Ni: 0.05% or less, and at least one of Ca and REM: 0.0050% or less in total.

3. The steel sheet according to claim 1 , further comprising a plated layer disposed on the surface.

4. The steel sheet according to claim 3 , wherein the plated layer is a hot-dip galvanized layer or a hot-dip galvannealed layer.

5. The steel sheet according to claim 2 , further comprising a plated layer disposed on the surface.

6. The steel sheet according to claim 5 , wherein the plated layer is a hot-dip galvanized layer or a hot-dip galvannealed layer.

7. The steel sheet according to claim 1 , wherein the chemical composition includes Mn: 1.3% to 2.1% by mass %.

8. A method for producing the steel sheet according to claim 1 , comprising:
hot-rolling a steel material having the chemical composition under a condition in which a rolling reduction rate of a final pass of a finish rolling is 12% or more, the rolling reduction rate of a pass before the final pass is 15% or more, a finish rolling starting temperature is in a range of 1000° C. to 1150° C., and a finish rolling finishing temperature is in a range of 850° C. to 950° C.;
primary cooling the hot-rolled steel sheet obtained in the hot rolling step at a first average cooling rate of 75° C./s or more to a cooling stop temperature of 700° C. or lower;
secondary cooling the steel sheet obtained in the primary cooling step under a condition of a second average cooling rate in a range of 5° C./s to 50° C./s down to a coiling temperature of 600° C. or lower, and a coiling step of coiling the steel sheet at the coiling temperature;
pickling the hot-rolled steel sheet after the coiling step, and cold-rolling the pickled steel sheet; and
annealing the cold-rolled steel sheet obtained in the cold rolling step at an average heating rate in a range of 3° C./s to 30° C./s up to a temperature in a range of 760° C. to 900° C., holding the steel sheet at a soaking temperature in the temperature range of 760° C. to 900° C. for 15 seconds or longer, and then cooling the steel sheet under a condition of an average cooling rate in a range of 3° C./s to 30° C./s down to a cooling stop temperature of 600° C. or lower.

9. The method according to claim 8 , further comprising subjecting the steel sheet obtained after the cooling in the annealing step to a plating treatment.

10. The method according to claim 9 , wherein the plating treatment is a treatment of hot-dip galvanizing and alloying at a temperature in a range of 450° C. to 600° C.

11. A method for producing the steel sheet according to claim 2 , comprising:
hot-rolling a steel material having the chemical composition under a condition in which a rolling reduction rate of a final pass of a finish rolling is 12% or more, the rolling reduction rate of a pass before the final pass is 15% or more, a finish rolling starting temperature is in a range of 1000° C. to 1150° C., and a finish rolling finishing temperature is in a range of 850° C. to 950° C.;
primary cooling the hot-rolled steel sheet obtained in the hot rolling step at a first average cooling rate of 75° C./s or more to a cooling stop temperature of 700° C. or lower;
secondary cooling the steel sheet obtained in the primary cooling step under a condition of a second average cooling rate in a range of 5° C./s to 50° C./s down to a coiling temperature of 600° C. or lower, and a coiling step of coiling the steel sheet at the coiling temperature;
pickling the hot-rolled steel sheet after the coiling step, and cold-rolling the pickled steel sheet; and
annealing the cold-rolled steel sheet obtained in the cold rolling step at an average heating rate in a range of 3° C./s to 30° C./s up to a temperature in a range of 760° C. to 900° C., holding the steel sheet at a soaking temperature in the temperature range of 760° C. to 900° C. for 15 seconds or longer, and then cooling the steel sheet under a condition of an average cooling rate in a range of 3° C./s to 30° C./s down to a cooling stop temperature of 600° C. or lower.

12. The method according to claim 11 , further comprising subjecting the steel sheet obtained after the cooling step in the annealing step to a plating treatment.

13. The method according to claim 12 , wherein the plating treatment is a treatment of hot-dip galvanizing and alloying at a temperature in a range of 450° C. to 600° C.

Assignees

• Jfe Steel Corp

Inventors

• Takashima Katsutoshi
• Taniguchi Koichi
• Kobayashi Takashi
• Matsuda Hiroshi
• Ikeda Rinsei

Classifications

• C21D8/02

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

• C22C38/38 Primary

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

• C21D6/005

  containing Mn · CPC title

• C22C38/08

  containing nickel {(C22C38/105 takes precedence)} · CPC title

• C21D6/008

  containing Si · CPC title