Steel sheet

Patent metadata

Field	Value
Publication number	US-12084740-B2
Application number	US-202017432330-A
Country	US
Kind code	B2
Filing date	Apr 24, 2020
Priority date	Apr 24, 2019
Publication date	Sep 10, 2024
Grant date	Sep 10, 2024

Abstract

The present invention relates to steel sheet realizing both formability and weldability. The steel sheet of the present invention is characterized in that at a surface layer part of a region down to 30 μm from the surface of the steel sheet in the sheet thickness direction, Al oxide grains are present in 3000 to 6000/mm 2 in number density, the natural logarithms of the particle sizes of the Al oxide grains measured in μm units are on the average −5.0 to −3.5, the standard error is 0.6 or less, and the number of Al oxide grains with deviations of the natural logarithms of the particle sizes from the average larger than 2 times the standard error is 5% or less of the total number of Al oxide grains and at a position of ½ of the thickness of the steel sheet, the number density of the Al oxide grains is 1000/mm 2 or less.

First claim

The invention claimed is:

1. A steel sheet with a tensile strength of 780 MPa or more,
a region down to 30 μm from a surface of the steel sheet in a sheet thickness direction being defined as a “surface layer part”,
at the surface layer part of at least one side,
particle size 20 nm or more Al oxide grains being present in 3000 to 6000/mm 2 in number density,
an average of natural logarithms of the particle sizes of the Al oxide grains measured in μm units being −5.0 to − 3 . 5 ,
a standard error of the natural logarithms of the particle sizes being 0.8 or less,
a number of Al oxide grains with deviations of the natural logarithms of the particle sizes from the average of larger than 2 times the standard error being 5% or less of the total number of Al oxide grains,
a chemical composition at a position of ¼ of the thickness from the surface of the steel sheet comprising, by mass %,
C: 0.050 to 0.800%,
Si: 0.01 to 1.20%,
Mn: 0.01 to 8.00%,
P: 0.100% or less,
S: 0.050% or less,
Al: 0 to 3.000%,
N: 0.010% or less,
O: 0 to 0.020%,
Cr: 0 to 3.00%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Ti: 0 to 0.200%,
Nb: 0 to 0.200%,
V: 0 to 0.20%,
Cu: 0 to 1.000%,
Ni: 0 to 1.000%, and
bal.: Fe and impurities,
a number density of Al oxide grains at a position of ½ of the thickness of the steel sheet being 1000/mm 2 or less.

2. The steel sheet according to claim 1 , wherein the chemical composition further comprises at least one element of, by mass %,
Cr: 0.01 to 3.00%,
Mo: 0.01 to 1.00%,
B: 0.001 to 0.0100%,
Ti: 0.010 to 0.200%,
Nb: 0.010 to 0.200%,
V: 0.01 to 0.20%,
Cu: 0.010 to 1.000%, and
Ni: 0.010 to 1.000%.

3. The steel sheet according to claim 1 wherein metallic structures at a position of ¼ of the thickness from the surface of the steel sheet include, by area ratio, retained austenite in 10% or more.

4. The steel sheet according to claim 1 , wherein the steel sheet further comprises a hot dip galvanized layer, a hot dip galvannealed layer, or an electrogalvanized layer at the surface of the surface layer part.

Assignees

• Nippon Steel Corp

Inventors

• Suzuki Yuya

Classifications

• C23C2/28

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

• C23C2/40

  Plates; Strips · CPC title

• C22C38/60

  containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur · CPC title

• C22C38/54

  with boron · CPC title

• C22C38/50

  with titanium or zirconium · CPC title