HIGH-STRENGTH COLD-ROLLED SHEET STEEL AND METHOD OF ITS PRODUCTION Russian patent published in 2014 - IPC C21D8/02 C22C38/00 

FIELD: metallurgy.

SUBSTANCE: roughing is performed at reduction of 80% or higher and finishing is performed at steel finishing feed temperature of 800-950°C. Proposed composition contains the following substances, in wt %: C 0.05-0.15, Si 0.2-1.2, Mn 1.0-2.0, Al 0.005-0.10, N 0.006 or less, and at least one element selected from Ti 0.03-0.13, Nb 0.02-0.10 and V 0.02-0.15, iron and unavoidable impurities making the rest. After finishing, sheet is cooled in two steps. Note here that at first step, this sheet is cooled from finishing feed temperature to 550-610°C at mean cooling rate of 25°C/s or higher. At second step, it is cooled from cooling temperature of previous step to winding temperature of 350-550°C or higher to wind the sheet. It features the content of fine bainite phase makes 50% or higher in microstructure of surface 500 mcm deep layer from surface and inside the sheet. It features the content of fine bainite phase makes 90% or higher in microstructure of the sheet depth central part, somewhere between 1/4 and 1/3 of sheet depth. Tensile strength makes 780 MPa and higher.

EFFECT: higher fatigue strength.

10 cl, 3 tbl, 1 dwg