High-strength cold-rolled steel sheet excellent in coating adhesion, workability and hydrogen embrittlement resistance, and steel component for automobile

A cold rolled steel sheet,

• wherein said cold rolled steel sheet has a chemical composition comprising, on the basis of percent by mass:
• C ranging from 0.06 to 0.6%;
• Si ranging from 0.1 to 2%;
• Al ranging from 0.07 to 3%;
• Si+Al ranging from 1 to 4%;
• Mn ranging from 1 to 6%;
• S of 0.02% or less;
• N of 0.01% or less;
• O of 0.01% or less;
• and
• the remainder being Fe and unavoidable impurities;
• wherein:
• and
• Si/Mn≦0.40;
• said cold rolled steel sheet has in the surface of said steel sheet, 10 or more pieces/100 μm2 of Mn—Si composite oxide of which a Mn—Si atom ratio (Mn/Si) is 0.5 or over and a major axis is from 0.01 μm to 5 μm, and has a covering ratio of 10% or below wherein the surface of said steel sheet is covered with oxide comprising Si as a main component;
• and said cold rolled steel sheet comprises, on the basis of the total volume fraction of the steel:
• 50% or over tempered martensite;
• from 4 to 40% ferrite;
• and
• from 3 to 25% retained austenite
• wherein a combined fraction of tempered martensite and ferrite is 75% or more and the tensile strength of the steel sheet is 780 MPa or higher.

A cold rolled steel sheet, wherein

• said cold rolled steel sheet has a chemical composition comprising, on the basis of percent by mass:
• C ranging from 0.06 to 0.6%;
• Si ranging from 0.1 to 2%;
• Al ranging from 0.07 to 3%;
• Si+Al ranging from 1 to 4%;
• Mn ranging from 1 to 6%;
• S of 0.02% or less;
• N of 0.01% or less;
• O of 0.01% or less;
• and
• the remainder being Fe and unavoidable impurities;
• wherein:
• Si/Mn≦0.40;
• said cold rolled steel sheet comprises, on the basis of the total volume fraction of the steel:
• 40% or over bainitic ferrite;
• from 1 to 50% polygonal ferrite;
• and
• from 3% to 25% retained austenite,
• wherein a combined fraction of bainitic ferrite and polygonal ferrite is 75% or more, the tensile strength of the steel sheet is 780 MPa or higher;
• and there exists no crack of 3 μm or less in width and 5 μm or over in depth in any 10 fields of view when observation at 2000× magnification by SEM is made of a cross section in the proximity of the surface of said steel sheet.

A cold rolled steel sheet, wherein

• said cold rolled steel sheet has a chemical composition comprising, on the basis of percent by mass:
• C ranging from 0.06 to 0.6%;
• Si ranging from 0.1 to 2%;
• Al ranging from 0.07 to 3%;
• Si+Al ranging from 1 to 4%;
• Mn ranging from 1 to 6%;
• S of 0.02% or less;
• N of 0.01% or less;
• O of 0.01% or less;
• and
• the remainder being Fe and unavoidable impurities;
• wherein:
• Si/Mn≦0.40;
• said cold rolled steel sheet comprises, on the basis of the total volume fraction of the steel:
• 50% or over tempered martensite;
• from 4 to 40% ferrite;
• and
• from 3 to 25% retained austenite;
• wherein a combined fraction of tempered martensite and ferrite is 75% or more, the tensile strength of the steel sheet is 780 MPa or higher;
• and there exists no crack of 3 μm or less in width and 5 μm or over in depth in any 10 fields of view when observation at 2000× magnification by SEM is made of a cross section in the proximity of the surface of said steel sheet.