High-strength flat steel product having a bainitic-martensitic microstructure and method for producing such a flat steel product
The disclosed flat steel products have optimal mechanical properties such as high strength and good toughness, but are also well-suited for welding and other manufacturing processes. The steel products may have a ferrite-free microstructure with at least 95% by volume martensite and bainite, with a martensite content of at least 5% by volume, and no more than 5% by volume residual austenite and unavoidable microstructure constituents from the production process. In addition to iron and unavoidable impurities, a composition of the flat steel products may include in percent by weight: 0.08%-0.10% C, 0.015%-0.50% Si, 1.20%-2.00% Mn, 0.020%-0.040% Al, 0.30%-1.00% Cr, 0.20-0.30% Mo, 0.020-0.030% Nb, 0.0015-0.0025% B, up to 0.025% P, up to 0.010% S, and up to 0.006% N. Impurities can include up to 0.12% Cu, up to 0.090% Ni, up to 0.0030% Ti, up to 0.009% V, up to 0.0090% Co, up to 0.004% Sb, and up to 0.0009% W.
1. A flat steel product having a thickness of 4-12 mm and comprising:
a ferrite-free bainitic-martensitic microstructure having at least 95 percent by volume martensite and bainite, with a martensite content of at least 5 percent by volume, wherein a remainder of the ferrite-free microstructure has up to 5 percent by volume residual austenite and unavoidable microstructure constituents from a production process; and
a composition that includes the following in addition to iron and unavoidable impurities:
0.08%-0.10% by weight C;
0.015%-0.50% by weight Si;
1.20%-2.00% by weight Mn;
0.020%-0.040% by weight Al;
0.30%-1.00% by weight Cr;
0.20%-0.30% by weight Mo;
0.020%-0.030% by weight Nb;
0.0015%-0.0025% by weight B;
up to 0.025% by weight P;
up to 0.010% by weight S; and
up to 0.006% by weight N,
wherein the impurities include
up to 0.12% by weight Cu,
up to 0.090% by weight Ni,
up to 0.0030% by weight Ti,
up to 0.009% by weight V,
up to 0.0090% by weight Co,
up to 0.004% by weight Sb, and
up to 0.0009% by weight W, and
wherein the flat steel product in an as-hot-rolled state has a yield strength of at least 900 MPa, a notch impact energy at −20° C. of 65-132 J, and at −40° C. of 40-128 J.
2. The flat steel product of claim 1 wherein for a carbon equivalent CE ∥W of the composition:
CE ∥W ≤0.5 and
CE ∥W =% C+% Mn/6+(% Cr+% Mo+% V)/5+(% Cu+% Ni)/15 wherein
% C is a respective C content in % by weight,
% Mn is a respective Mn content in % by weight,
% Cr is a respective Cr content in % by weight,
% Mo is a respective Mo content in % by weight,
% V is a respective V content in % by weight,
% Cu is a respective Cu content in % by weight, and
% Ni is a respective Ni content in % by weight.
3. The flat steel product of claim 1 wherein the composition comprises less than or equal to 0.25% by weight Si.
4. The flat steel product of claim 1 wherein the composition comprises at least 0.001% by weight N.
5. A method for producing a flat steel product having a thickness of 2-12 mm and, the method comprising:
casting a steel melt to form a slab, with the steel melt comprising in addition to iron and unavoidable impurities:
0.08%-0.10% by weight C,
0.015%-0.50% by weight Si,
1.20%-2.00% by weight Mn,
0.020%-0.040% by weight Al,
0.30%-1.00% by weight Cr,
0.20%-0.30% by weight Mo,
0.020%-0.030% by weight Nb,
0.0015%-0.0025% by weight B,
up to 0.025% by weight P,
up to 0.010% by weight S, and
up to 0.006% by weight N,
wherein the impurities include
up to 0.12% by weight Cu,
up to 0.090% by weight Ni,
up to 0.0030% by weight Ti,
up to 0.009% by weight V,
up to 0.0090% by weight Co,
up to 0.004% by weight Sb, and
up to 0.0009% by weight W;
heating the slab to an austenitization temperature of 1200-1300 degrees Celsius;
rough rolling the slab heated at a rough rolling temperature of 950-1250 degrees Celsius, wherein a total deformation achieved by the rough rolling is at least 50%;
hot rolling the rough-rolled slab to form a hot strip, wherein a final rolling temperature in the hot rolling is 810-875 degrees Celsius, wherein a total deformation achieved by the hot rolling is at least 70%, wherein the hot rolling occurs without wetting rolling material with lubricant;
cooling the hot-rolled hot strip at a cooling rate of at least 40 K/s to a coiling temperature of 200-500 degrees Celsius, wherein cooling sets in within 10 seconds following the hot rolling; and
coiling the hot strip that has been cooled down to the coiling temperature.
6. The method of claim 5 wherein the steel melt comprises at least 0.001% by weight N.
7. The method of claim 5 wherein, after coiling, the flat steel product has ferrite-free, bainitic-martensitic microstructure having at least 95 percent by volume martensite and bainite, with a martensite content of at least 5 percent by volume, wherein a remainder of the ferrite-free microstructure has up to 5 percent by volume residual austenite and unavoidable microstructure constituents resulting from processing.