COLD ROLLED STEEL SHEET AND METHOD OF MANUFACTURE OF COLD ROLLED STEEL SHEET Russian patent published in 2016 - IPC C22C38/06 C21D8/02 

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, in particular to cold-rolled steel sheet for hot stamping that is used in the automotive industry. Sheet contains wt.%: C of 0.030 to 0.150, Si: 0.010 to 1.000, Mn from 2.70 to 1.50, P 0.001 to 0.060, S 0.001 to 0.010, N of 0.0005 to 0.0100, Al: 0.010 to 0.050 and optionally one or more of the following elements: B of 0.0005 to 0.0020, Mo of 0.01 to 0,50, Cr from 0.01 to 0,50, V of 0.001 to 0.100, Ti 0.001 to 0.100, Nb 0.001 to 0.050, by Ni 0,01 to 1,00, Cu of 0.01 to 1,00, Ca of 0.0005 to 0.0050, and REM: 0.0005 to 0.0050, the balance being Fe and unavoidable impurities. With that, the following ratio is executed: (5 × [Si] + [Mn]) / [C] > 11 wherein [C] represents the C content, expressed in weight percentages, [Si] represents the Si content, expressed in weight percent, and [Mn] represents the Mn content, expressed in weight percent. Metallographic structure before and after the hot forming comprises from 40 % to 90 % ferrite and 10 % to 60 % martensite by the relative area. Sum of relative area of ferrite and martensite is relative area of 60 % or more. Metallographic structure optionally further comprises one or more of the following phases: 10 % or less of the relative area of pearlite, 5 % or less of retained austenite on the relative volume and less than 40 % bainite constituent remains relatively square. Hardness of martensite, measured by nanoindenter meets hot forming the following relationships before and after: H2 / H1 < 1.10 σHM < 20, where H1 is an average hardness of martensite in the surface portion of the sheet thickness before and after the hot stamping, H2 represents the average hardness of the martensite in the central portion of the sheet thickness, is a region in which the width is 200 m in the thickness direction in the middle of the thickness of the sheet prior to and after the hot stamping, and σHM is a change in the hardness of the martensite in the central portion of the thickness of the sheet before and after hot stamping. Multiplication of TS×λ tensile strength TS and hole expansion ratio λ is 50,000 MPa •% or more.

EFFECT: provides high resistance and sheet moldability and hand holes.

20 cl, 11 dwg, 9 tbl, 1 ex