High strength, cold rolled steel with reduced sensitivity to hydrogen embrittlement and method for the manufacture thereof

1 . A high strength, cold rolled steel flat product, the steel flat product comprising a steel substrate consisting of, in % by mass,

C: 0.20 to 0.40%,

Mn: 1.50 to 3.00%,

Si: 0.90 to 1.50%,

A1 0.005 to 1.00%,

V: 0.07 to 0.30%,

optionally Cr: 0.01 to 1.00%,

optionally Mo: 0.005 to 0.20%,

optionally B: 0.00001 to 0.002%,

optionally Nb and Ti the total content of Nb and Ti being 0.005 to 0.2%,

P: up to 0.020%,

S: up to 0.005%,

N: up to 0.008%,

and as the remainder Fe and unavoidable impurities, the sum of the shares of the impurities being ≤0.8%,

and

exhibiting a microstructure determined in accordance with ISO 9042 comprising, in % by area,

65 to 92% primary (tempered) martensite and

at least 8% retained austenite (RA),

the remainder being filled by:

up to 27% of secondary (un-tempered) martensite,

up to 10% of bainite or bainitic ferrite,

and/or up to ≤5% polygonal ferrite, the sum of the shares of the secondary (untempered) martensite, the bainite or bainitic ferrite and the polygonal ferrite being ≤27%,

wherein the microstructure of the steel substrate has a precipitate density of ≥1000 per μm 2 of V-based-precipitates with a diameter of less than 10 nm determined by measuring the precipitates in carbon extraction replicas of a longitudinal section of the steel flat product by transmission electron microscope images in combination with X-ray microanalysis in 5 measuring fields and averaging the results of the 5 measurement fields.

2 . The steel flat product according to claim 1 ,

wherein the primary martensite contained in the microstructure of the steel substrate has a fine lath structure which laths have a maximum length of 1000 nm determined by scanning electron microscopy using sections treated with a 3% Nital etch which were taken at one third of the plate thickness of the steel substrate and averaging the results of five measurements.

3 . The steel flat product according to claim 2 , wherein the maximum length of the laths is 500 nm determined by scanning electron microscopy using sections treated with a 3% Nital etch which were taken at one third of the plate thickness of the steel substrate, and averaging the results of five measurements.

4 . The steel flat product according to claim 1 , wherein the C content of the steel of the steel substrate is 0.22 to 0.3% by mass.

5 . The steel flat product according to claim 1 , wherein the Mn content of the steel of the steel substrate is 1.9 to 2.8% by mass.

6 . The steel flat product according to claim 1 , wherein the V content of the steel of the steel substrate is 0.07 to 0.20% by mass.

7 . The steel flat product according to claim 1 , wherein the steel flat product has a yield strength of at least 1000 MPa, a tensile strength of at least 1300 MPa and an elongation A80 of at least 10% each determined according to DIN EN ISO 6892 (sample form 2 ).

8 . The steel flat product according to claim 1 , wherein the steel flat product has a hole-expansion determined according to ISO 16630 of at least 20%.

9 . The steel flat product according to claim 1 , wherein an anti-corrosion coating is provided on at least one of the surfaces.

10 . The steel flat product according to claim 9 , wherein the coating is applied by electrolytically coated, hot-dip galvanizing or galvannealing.

11 . A method for the manufacture of the steel flat product according to claim 1 , comprising the following working steps:

a) providing a steel melt consisting of, in % by mass, C: 0.2 to 0.4%, Mn: 1.5 to 3.0%, Si: 0.9 to 1.5%, Al: 0.005 to 1.0%, V: 0.07 to 0.3%, optionally Cr: 0.01 to 1.00%, optionally Mo: 0.005 to 0.20%, optionally B: 0.00001 to 0.002%, optionally Nb and Ti the total content of Nb and Ti being 0.005 to 0.2%, P: up to 0.020%, S: up to 0.005%, N: up to 0.008%, and as the remainder Fe and unavoidable impurities, the sum of the shares of the impurities being ≤0.8%;

b) casting the steel melt into a slab;

c) heating through the slab to a reheating temperature of 1000 to 1300° C.;

d) hot rolling the reheated slab into a hot strip, wherein the hot rolling is finished at a hot rolling finish temperature of 850 to 980° C.;

e) cooling the hot strip to a coiling temperature of 400 to 600° C., the cooling being finished within a maximum of 25 s after the finish of the hot rolling, and coiling the hot strip into a coil;

f) optionally pickling the hot strip;

g) cold rolling the hot strip into a cold strip with cold reduction rates of 20 to 80%,

h) final annealing of the cold strip by:

heating the cold strip to a soaking temperature TS, which is at least 50° C. higher than the Ac3 temperature of the respective steel and 950° C. at most, with a heating rate ΘS of 2 to 10° C./s, wherein the Ac3 temperature of the respective steel is determined using dilatometry,

immediately followed by holding the cold strip at the soaking temperature TS for a soaking time tS of more than 40 s and less than 200 s;

immediately followed by quenching the cold strip with a quenching rate ΘQ of 20 to 100° C./s to a quenching stop temperature TQ which is lower than the martensite start temperature T_MS of the steel and at least equal to that temperature TQ_min at which in the microstructure of the cold strip 65 to 92% by area primary martensite is present, wherein the T_MS temperature of the steel is determined using dilatometry according to September 1681-1998-06;

and holding the annealed cold strip at the quenching stop temperature TQ for 4 to 20 s;

i) over-aging the final annealed cold strip with the over-aging treatment comprising of:

heating the annealed cold strip to an over-aging temperature TP of 380 to 460° C., then

holding the annealed cold strip at the over-aging temperature for 50 to 200 s,

and, then

cooling the annealed cold strip to less than 100° C. with a cooling rate ΘC of 0.5 to 20° C./s.