HOT-PRESSED MEMBER AND METHOD FOR MANUFACTURING SAME, AND COLD-ROLLED STEEL SHEET FOR HOT PRESSING AND METHOD FOR MANUFACTURING SAME
First Claim
1. A hot-pressed member comprising:
- a steel chemical composition containing, by mass %, C;
0.28% or more and less than 0.42%, Si;
1.5% or less, Mn;
1.0% or more and 2.2% or less, P;
0.05% or less, S;
0.005% or less, Al;
0.01% or more and 0.50% or less, N;
0.005% or less, and Ti;
0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities;
a microstructure in whichat least 5 Ti-based precipitates having a grain size of 0.10 μ
m or less are present on average per 100 μ
m2 of a cross section parallel to a thickness direction of the member within a range of 100 μ
m in a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μ
m to 100 μ
m in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μ
m are present on average in a prior austenite grain; and
a tensile strength of 1780 MPa or more.
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Accused Products
Abstract
Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more and excellent delayed fracture resistance after projection welding by properly adjusting its chemical composition and its microstructure such that at least 5 Ti-based precipitates having a grain size of 0.10 μm or less are present on average per 100 μm2 of a cross section parallel to a thickness direction of the member within a range of 100 μm in a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μm to 100 μm in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μm are present on average in a prior austenite grain.
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Citations
16 Claims
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1. A hot-pressed member comprising:
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a steel chemical composition containing, by mass %, C;
0.28% or more and less than 0.42%, Si;
1.5% or less, Mn;
1.0% or more and 2.2% or less, P;
0.05% or less, S;
0.005% or less, Al;
0.01% or more and 0.50% or less, N;
0.005% or less, and Ti;
0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities;a microstructure in which at least 5 Ti-based precipitates having a grain size of 0.10 μ
m or less are present on average per 100 μ
m2 of a cross section parallel to a thickness direction of the member within a range of 100 μ
m in a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μ
m to 100 μ
m in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μ
m are present on average in a prior austenite grain; anda tensile strength of 1780 MPa or more. - View Dependent Claims (2, 3, 11)
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4. A cold-rolled steel sheet for hot pressing, comprising:
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a chemical composition containing, by mass %, C;
0.28% or more and less than 0.42%, Si;
1.5% or less, Mn;
1.0% or more and 2.2% or less, P;
0.05% or less, S;
0.005% or less, Al;
0.01% or more and 0.50% or less, N;
0.005% or less, and Ti;
0.005% or more and 0.15% or less, with the balance being Fe and inevitable impurities,a microstructure in which at least 10 Ti-based precipitates having a grain size of 0.10 μ
m or less are present on average per 100 μ
m2 of a cross section parallel to a thickness direction of the steel sheet within a range of 100 μ
m in the thickness direction from a surface of the steel sheet; anda standard deviation of Vickers hardness of 40 or less, the Vickers hardness being measured every 200 μ
m in the thickness direction within a range from a depth position of 100 μ
m from the surface of the steel sheet to a mid-thickness part. - View Dependent Claims (5, 6, 7, 8, 9, 10, 12, 13, 14, 15, 16)
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Specification