Method for manufacturing flat steel products and flat steel product

A method for manufacturing flat steel products which, following a five percent biaxial deformation, exhibit, on at least one of their surfaces, a Wsa(1-5) value of less than 0.35 μm, a planar anisotropy Δr of −0.5 to +0.5 and, in a region which extends from the surface of the respective flat steel product to a depth of less than 200 μm, a nanohardness H of more than 0.1 GPa and less than 3.0 GPa, comprising the following working steps:

• a) providing a slab which is made of a steel which comprises (in % by weight)
• C:
• 0.0003-0.050%
• Si:
• 0.0001-0.20%
• Mn:
• 0.01-1.5%
• P:
• 0.001-0.10%
• S:
• 0.0005-0.030%
• Al:
• 0.001-0.12%
• N:
• 0.0001-0.01%
• as well as, optionally, one or more elements from the group “Ti, Nb, B, Cu, Cr, Ni, Mo, Sn”, provided that
• Ti:
• 0.0001-0.15%
• Nb:
• 0.0001-0.05%
• B:
• ≤0.005%
• Cu:
• ≤0.15%
• Cr:
• ≤0.15%
• Ni:
• ≤0.15%
• Mo:
• ≤0.10%
• Sn:
• ≤0.05%
• the remainder being Fe and unavoidable impurities;
• b) heating the slab in a furnace to a slab drawing temperature Bzt of 1200-1270° C. at which the slab is drawn out of the furnace;
• c) hot-rolling the slab into a 3-5 mm thick hot-rolled flat steel product, wherein the hot-rolling comprises a rough rolling with a thickness reduction of 80-90% and a finishing rolling with a thickness reduction of 85-95%, wherein the overall degree of deformation achieved during the hot-rolling is 95-99.5%, wherein a thickness reduction ΔdF of 1-25% is achieved in the last roll pass and wherein the final hot-rolling temperature is 850-950° C.;
• d) cooling the obtained hot-rolled flat steel product to a coiling temperature of 620-780° C., wherein the cooling rate is 4-30 K/s;
• e) coiling the hot-rolled flat steel product into a coil;
• f) pickling the hot rolled flat steel product to remove scale;
• g) cold-rolling the pickled hot-rolled flat steel product into a cold-rolled flat steel product, wherein the total degree of deformation achieved through the cold-rolling is 70-90%;
• and
• h) recrystallisation annealing the cold-rolled flat steel product at an annealing temperature of 650-900° C.
• wherein the slab drawing temperature Bzt, a total residence time GLZ which the slab spends in the furnace between being pushed in and drawn out, the thickness reduction in the last hot roll pass ΔdF and the coiling temperature HT satisfy the following condition:
• −0.529653*Q+0.944372*HT_t+0.711559* ΔdF _t<−0.1889459
• where Q=((Bzt/GLZ) −5.55281° C./min)/(1.777359° C./min)
• Bzt:
• slab drawing temperature in ° C.
• GLZ:
• total residence time in min
• HT _t=(HT-728.13030° C.)/42.300114° C.
• HT:
• coiling temperature in ° C.
• ΔdF _t=(ΔdF −12.43384%)/2.306328% ΔdF:
• thickness reduction in the last hot roll pass in %.