Patent MX-2017000177-A (MX)
Title: Method for producing a high strength steel sheet having improved strength, ductility and formability.
Inventor: DONGWEI FAN
IPC: C21D1/18
Filed: 2015-07-03
Abstract: A method for producing a high strength steel sheet having a yield strength YS of at least 850 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%. The chemical composition of the steel contains: 0.15% = C = 0.25%, 1.2% = Si = 1.8%, 2% = Mn = 2.4%, 0.1% = Cr = 0.25%, Nb = 0.05 %, Ti = 0.05 %, Al = 0.50%, the remainder being Fe and unavoidable impurities. The sheet is annealed at an annealing temperature TA higher than Ac3 but less than 1000°C for more than 30 s, by cooling it to a quenching temperature QT between 275°C and 325°C, at a cooling speed sufficient to have, just after quenching, a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85 and 97% of the sum of martensite and bainite, without ferrite, heated to a partitioning temperature PT between 420°C and 470°C and maintained at this temperature for time between 50 s and 150 s and cooled to the room temperature. A method for producing a high strength steel sheet having a yield strength YS of at least 850 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%. The chemical composition of the steel contains: 0.15% = C = 0.25%, 1.2% = Si = 1.8%, 2% = Mn = 2.4%, 0.1% = Cr = 0.25%, Nb = 0.05 %, Ti = 0.05 %, Al = 0.50%, the remainder being Fe and unavoidable impurities. The sheet is annealed at an annealing temperature TA higher than Ac3 but less than 1000°C for more than 30 s, by cooling it to a quenching temperature QT between 275°C and 325°C, at a cooling speed sufficient to have, just after quenching, a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85 and 97% of the sum of martensite and bainite, without ferrite, heated to a partitioning temperature PT between 420°C and 470°C and maintained at this temperature for time between 50 s and 150 s and cooled to the room temperature. A method for producing a high strength coated steel sheet having a yield stress YS > 800 MPa, a tensile strength TS > 1180 MPa, and improved formability and ductility. The steel contains: 15% = C = 0.25%, 1.2% = Si = 1.8%, 2% = Mn = 2.4%, 0.1% = Cr = 0.25%, Al = 0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000°C for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250°C and 350°C, to obtain a structure consisting of at least 60 % of martensite and a sufficient austenite content such that the final structure contains 3 % to 15% of residual austenite and 85 % to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430°C and 480°C and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature. Obtained coated sheet.