Patent MX-2016012490-A (MX)
Title: Cold-rolled flat steel product and method for the production thereof.
Inventor: ANDREAS BONGARDS
IPC: C22C38/18
Filed: 2015-03-18
Abstract: The invention relates to a cold-rolled flat steel product and to a method for the production thereof. The flat steel product has a proof stress Rp0.2 of = 320 MPa, an elongation at break A80 of = 20% and a microstructure comprising (in % by unit area) 62-82% ferrite, 10-30% martensite, 1.5-8% residual austenite and in total = 10% other microstructural constituents. The flat steel product in this case consists of a steel alloy containing (in % by weight) C: 0.06-0.1%, Si: 0.15-0.4%, Mn: 1.5-2%, Cr: 0.2-0.5%, Al: = 0.1%, the sum total of the contents of C, Si, Mn and Cr being = 2.3% and = 2.8% and the sum total of the contents of Si and Al being = 0.4%, P: = 0.03%, S: = 0.006%, N: = 0.008%, and further technically unavoidable impurities, including = 0.0006% B, = 0.02% V and also in each case = 0.01% Nb, Ti and also in each case = 0.1% Mo, Ni and Cu, remainder iron. To produce such a flat steel product, firstly a cold-rolled flat steel product is produced, and this is then brought in a continuous pass to 760 - 860°C, at which it is held for an annealing period Gt, for which the following holds true depending on the thickness D of the flat steel product: Gtu = Gt = Gto, where Gtu [s] = 3.56*D2 [s/mm2] - 5.1*D [s/mm] + 9.8 s Gto [s] = -21.4*D2 [s/mm2] + 132.8*D [s/mm] + 47 s. The invention relates to a cold-rolled flat steel product and to a method for the production thereof. The flat steel product has a proof stress Rp0.2 of = 320 MPa, an elongation at break A80 of = 20% and a microstructure comprising (in % by unit area) 62-82% ferrite, 10-30% martensite, 1.5-8% residual austenite and in total = 10% other microstructural constituents. The flat steel product in this case consists of a steel alloy containing (in % by weight) C: 0.06-0.1%, Si: 0.15-0.4%, Mn: 1.5-2%, Cr: 0.2-0.5%, Al: = 0.1%, the sum total of the contents of C, Si, Mn and Cr being = 2.3% and = 2.8% and the sum total of the contents of Si and Al being = 0.4%, P: = 0.03%, S: = 0.006%, N: = 0.008%, and further technically unavoidable impurities, including = 0.0006% B, = 0.02% V and also in each case = 0.01% Nb, Ti and also in each case = 0.1% Mo, Ni and Cu, remainder iron. To produce such a flat steel product, firstly a cold-rolled flat steel product is produced, and this is then brought in a continuous pass to 760 - 860°C, at which it is held for an annealing period Gt, for which the following holds true depending on the thickness D of the flat steel product: Gtu = Gt = Gto, where Gtu [s] = 3.56*D2 [s/mm2] - 5.1*D [s/mm] + 9.8 s Gto [s] = -21.4*D2 [s/mm2] + 132.8*D [s/mm] + 47 s. To produce a flat steel product having a yield strength of = 700 MPa and a microstructure which is bainitic to an extent of = 70% by volume, the following working steps are carried out according to the invention: a) melting of a steel melt consisting of (in % by weight) C: 0.05-0.08%, Si: 0.015-0.500%, Mn: 1.60-2.00%, P: = 0.025%, S: = 0.010%, Al: 0.020-0.050%, N: = 0.006%, Cr: = 0.40%, Nb: 0.060-0.070%, B: 0.0005-0.0025%, Ti: 0.090- 0.130% and unavoidable impurities, remainder Fe; b) casting of the melt to form a slab; c) reheating of the slab to 1200-1300°C; d) rough rolling of the slab at 950-1250°C and with an overall pass reduction of = 50%; e) hot finish-rolling of the rough-rolled slab with a hot-rolling end temperature of 800-880°C; f) cooling of the hot finish-rolled flat steel product within = 10 s after the hot finish-rolling to 50-620°C at a cooling rate of = 40 K/s; g) coiling of the hot finish-rolled flat steel product.