Patent ID: 7559998
Filing Date: 2009-07-14
Classification: C21D,C22C

Abstract:
1. A hot forming method comprising heating a steel sheet having a steel composition consisting essentially of, in mass percent, C: 0.15-0.45%; Mn: 0.5-3.0%; Cr: 0.1-0.5%; Ti: 0.01-0.1%; B: 0.0002-0.004%; Si: at most 0.5%; P: at most 0.050%; S: at most 0.05%; Al: at most 1%; N: at most 0.01%; one or more of Ni: at most 2%, Cu: at most 1%, Mo: at most 1%, V: at most 1%, and Nb: at most 1%; and a remainder of Fe and unavoidable impurities to a temperature of the Ac 3 point or higher, holding it at that temperature, and then forming the heated steel sheet to the shape of a finished member, wherein the formed member is quenched by cooling from the forming temperature during forming or after forming in such a manner that the cooling rate to the Ms point is at least the critical cooling rate, and that the average cooling rate from the Ms point to 200° C. is in the range of 25-150° C./s, wherein control of the average cooling rate from the Ms point to 200° C. obtains an auto-tempered martensite structure and a hardness expressed in Vickers hardness being less than the value of (maximum quenching hardness—10) and at least the value of (maximum quenching hardness—100).