COLD ROLLED STEEL SHEET AND MANUFACTURING METHOD THEREOF

摘要

When the amount of C, the amount of Si and the amount of Mn are respectively represented by [C], [Si] and [Mn] in unit mass %, the cold rolled steel sheet satisfies a relationship of (5×[Si]+[Mn])/[C]>10, the metallographic structure contains, by area ratio, 40% to 90% of a ferrite and 10% to 60% of a martensite, further contains one or more of 10% or less of a pearlite by area ratio, 5% or less of a retained austenite by volume ratio and 20% or less of a bainite by area ratio, the hardness of the martensite measured using a nanoindenter satisfies H20/H10<1.10 and σHM0<20, and TS×λ representing the product of TS that is a tensile strength and λ that is a hole expansion ratio is 50000 MPa·% or more.

主权项

1. A cold rolled steel sheet comprising, by mass %:
C: more than 0.150% to 0.300%; Si: 0.010% to 1.000%; Mn: 1.50% to 2.70%; P: 0.001% to 0.060%; S: 0.001% to 0.010%; N: 0.0005% to 0.0100%; and Al: 0.010% to 0.050%, and optionally one or more of: B: 0.0005% to 0.0020%; Mo: 0.01% to 0.50%; Cr: 0.01% to 0.50%; V: 0.001% to 0.100%; Ti: 0.001% to 0.100%; Nb: 0.001% to 0.050%; Ni: 0.01% to 1.00%; Cu: 0.01% to 1.00%; Ca: 0.0005% to 0.0050%; and REM: 0.0005% to 0.0050%, and a balance including Fe and unavoidable impurities, wherein, when an amount of C, an amount of Si and an amount of Mn are respectively represented by [C], [Si] and [Mn] in unit mass %, a relationship of the following formula 1 is satisfied, a metallographic structure contains, by area ratio, 40% to 90% of a ferrite and 10% to 60% of a martensite, and further contains one or more of 10% or less of a pearlite by area ratio, 5% or less of a retained austenite by volume ratio and 20% or less of a bainite by area ratio, a hardness of the martensite measured using a nanoindenter satisfies the following formulae 2a and 3a, and TS×λ representing a product of TS that is a tensile strength and λ that is a hole expansion ratio is 50000 MPa·% or more,
(5×[Si]+[Mn])/[C]>10  (1)H20/H10<1.10  (2a)σHM0<20  (3a) here, the H10 represents an average hardness of the martensite at the surface part of the cold rolled steel sheet, the H20 represents an average hardness of the martensite at a center portion of a sheet thickness that occupies a ±100 μm range from a sheet thickness center of the cold rolled steel sheet in a thickness direction, and the σHM0 represents a variance of the hardness of the martensite present in the center portion of the sheet thickness.