High-strength flat steel product and method for producing same

1. A flat steel product which has a tensile strength R m of at least 1200 MPa and which consists of a steel that contains (in wt %)

C: 0.10-0.50%,

Si: 0.1-2.5%,

Mn: 1.0-3.5%

Al: up to 2.5%,

P: up to 0.020%,

S: up to 0.003%,

N: up to 0.02%,

and optionally one or more of the elements “Cr, Mo, V, Ti, Nb, B and Ca” in the following quantities:

Cr: 0.1-0.5%,

Mo: 0.1-0.3%,

V: 0.01-0.1%,

Ti: 0.001-0.15%,

Nb: 0.02-0.05%,

wherein Σ(V, Ti, Nb) ≦0.2% for the sum Σ(V, Ti, Nb) of the quantities of V, Ti and Nb,

B: 0.0005-0.005%, and

Ca: up to 0.01% in addition to Fe and unavoidable impurities,

and a microstructure with (in surface percent) less than 5% ferrite, less than 5% bainite, 5-70% untempered martensite, 5-30% residual austenite and 25-80% tempered martensite, at least 99% of the iron carbide contained in the tempered martensite having a size of less than 500 nm.

2. The flat steel product according to claim 1 , wherein (in wt %) the Al content is 0.01-1.5%, the Cr content is 0.20-0.35 wt %, the V content is 0.04-0.08%, the Ti content is 0.008-0.14%, the B content is 0.002-0.004% or the Ca content is 0.0001-0.006%.

3. The flat steel product according to claim 1 , wherein for the carbon equivalent CE of its steel the following is valid:

0.35 wt % ≦CE ≦1.2 wt %

wherein CE =% C +(% Mn+% Si)/6 +(% Cr+% Mo+% V)/5 +(% Ni+% Cu)/15,

% C: C content of the steel,

% Mn: Mn content of the steel,

% Si: Si content of the steel,

% Cr: Cr content of the steel,

% Mo: Mo content of the steel,

% V: V content of the steel,

% Ni: Ni content of the steel,

% Cu: Cu content of the steel.

4. The flat steel product according to claim 3 , wherein for the carbon equivalent CE the following is valid:

0.5 wt % ≦CE ≦1.0 wt %

5. The flat steel product according to claim 1 , wherein it is provided with a metallic protective layer applied by hot-dip coating.

6. A method for producing a high-strength flat steel product, according to claim 1 comprising the following work steps:

providing an uncoated flat steel product of a steel that contains (in wt %)

C: 0.10-0.50%,

Si: 0.1-2.5%,

Mn: 1.0-3.5%,

Al: up to 2.5%,

P: up to 0.020%,

S: up to 0.003%,

N: up to 0.02%,

and optionally one or more of the elements “Cr, Mo, V, Ti, Nb, B and Ca” in the following quantities:

Cr: 0.1-0.5%,

Mo: 0.1-0.3%, 0

V: 0.01-0.1%,

Ti: 0.001-0.15%.

Nb: 0.02-0.05%.

wherein Σ(V,Ti,Nb) ≦0.2% for the sum Σ(V,Ti,Nb) of the quantities of V, Ti and Nb,

B: 0.0005-0.005%,

Ca: up to 0.01% in addition to Fe and unavoidable impurities;

heating the flat steel product to an austenitisation temperature T HZ above the A c3 temperature of the steel of the flat steel product and with a maximum of 960 ° C. at a heating speed θ H1 , θ H2 of at least 3° C./s;

holding the flat steel product at the austenitisation temperature for an austenitisation period t Hz of 20-180 seconds;

cooling of the flat steel product to a cooling stop temperature T Q , greater than the martensite stop temperature T Mf and less than the martensite start temperature T Ms (T Mf <T Q <T MS ), at a cooling speed θ Q for which the following is valid:

θ Q ≦θ Q(min)

where θ Q(min) [° C./s]=−314.35° C./s +(268.74% C +56.27% Si+58.50% Al+43.40% Mn+195.02% Mo+166.60% Ti+199.19% Nb)° C./(wt % ·s),

% C: C content of the steel,

% Si: Si content of the steel,

% Al: Al content of the steel,

% Mn: Mn content of the steel,

% Mo: Mo content of the steel,

% Ti: Ti content of the steel,

% Nb: Nb content of the steel;

holding the flat steel product at the cooling stop temperature T Q for a holding time t Q of 10-60 seconds;

starting from the cooling stop temperature T Q , heating the flat steel product at a heating speed θ P1 of 2-80° C./s to a partitioning temperature T P of 400-500° C.;

optionally holding the flat steel product isothermally at the partitioning temperature T P for a holding time t P1 of up to 500 seconds;

starting from the partitioning temperature T P cooling the flat steel product at a cooling speed θ P2 of between −3° C./s and −25° C./s.

7. The method according to claim 6 , wherein in the cooling starting from the partitioning temperature T P at a cooling speed θ P2

the flat steel product is initially cooled to a molten bath entry temperature T B of 400 to <500° C.;

then the flat steel product cooled to the molten bath entry temperature T B is hot-dip coated by being passed through a molten bath and the thickness of the protective layer created on the flat steel product is set;

and finally the flat steel product leaving the molten bath with the protective layer is cooled to ambient temperature at a cooling speed θ P2 .

8. The method according to claim 6 , wherein to the austenitisation temperature T HZ takes place in two consecutive stages without interruption at different heating speeds θ H1 , θ H2 .

9. The method according to claim 6 , wherein the heating speed θ Hl of the first stage is 5-25° C./s and the heating speed θ H2 of the second stage is 3-10° C.

10. The method according to claim 6 , wherein the flat steel product is heated at the first heating speed θ H1 to an intermediate temperature T W of 200-500° C. and in that the heating is then continued at the second heating speed θ H2 to the austenitisation temperature T HZ .

11. The method according to claim 6 , wherein the cooling speed θ Q is −20° C./s to −120° C./s.

12. The method according to claim 6 , wherein the cooling stop temperature T Q is at least 200° C.

13. The method according to claim 6 , wherein the holding time t Q , for which the flat steel product is held at the cooling stop temperature T Q is 12-40 seconds.

14. The method according to claim 6 , wherein the heating speed θ P1 at which the heating takes place from the cooling stop temperature T Q is 2-80° C./s.

15. The method according to claim 6 , wherein heating to the partitioning temperature T P takes place within a heating time t A of 1-150 seconds.

16. The method according to claim 15 , wherein for the time t Pr of partitioning during heating to partitioning temperature T P the following is valid:

t Pr [s]=0−t A .

17. The method according to claim 6 , wherein for a diffusion length X D the following is valid:

X D ≧1.0 μm

where X D =X Di +X Dr

x Di : the contribution obtained in the course of isothermic holding to the diffusion length x D , calculated according to the formula

x Di =6*√{square root over ( D*t Pi )}

where t Pi =time for which isothermal holding is performed, in seconds,

D=D 0 * exp (−Q/RT), D 0 =3.72 * 10 −5 m 2 /S

Q=148 kJ/mol, R=8.314 J/(mol·K)

T =partitioning temperature T P in Kelvin and

X Dr : the contribution obtained in the course of heating to the partitioning temperature to the diffusion length X D , calculated according to the formula

X Dr =Σ j (6*√{square root over ( D j *Δt Pr,j )})

where Δt Pr,j =is the time step between two calculations in seconds,

D j =D 0 *exp(−Q/RT j ), D 0 =3.72*10 −5 m 2 /s,

Q=148 kJ/mol, R=8.314 J/(mol·K)

T j =current partitioning temperature T P in each case in Kelvin.

wherein x Di or x Dr can also be 0.