Coated steel sheet and high strength press hardened steel part and method of manufacturing the same
A coated steel sheet and press hardened steel part having a composition including, by weight percent: C 0.15-0.25%, Mn 0.5-1.8%, Si 0.1-1.25%, Al 0.01-0.1%, Cr 0.1-1.0%, Ti 0.01-0.1%, B 0.001-0.004%, P≤0.020%, S≤0.010%, N≤0.010% the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The press hardened steel part includes a bulk having a microstructure including, in surface fraction, more than 95% of martensite and less than 5% of bainite, a coating layer at the surface of the steel part, a ferritic interdiffusion layer between the coating layer and the bulk, and a ratio between the ferritic grain width in the interdiffusion layer GW int over prior austenite grain size in the bulk PAGS bulk , satisfying following equation (GW int /PAGS bulk )−1≥30%.
1. A coated steel sheet made of a steel having a composition comprising, by weight percent:
C: 0.15-0.25%
Mn: 0.5-1.8%
Si: 0.1-1.25%
Al: 0.01-0.1%
Cr: 0.1-1.0%
Ti: 0.01-0.1%
B: 0.001-0.004%
P≤0.020%
S≤0.010%
N≤0.010%
and comprising optionally one or more of the following elements, by weight percent:
Mo≤0.40%
Nb≤0.08%
Ca≤0.1%
a remainder of the composition being iron and unavoidable impurities resulting from processing;
the coated steel sheet comprising from a bulk to the surface of the coated steel sheet:
the bulk with a microstructure comprising, in surface fraction, from 60% to 90% of ferrite,
a rest being martensite-austenite islands, pearlite or bainite,
a decarburized layer topping the bulk and comprising in upper part a ferrite layer having a thickness from 1 μm to 100 μm; and
a coating layer made of aluminum or aluminum alloy.
2. A method for producing a coated steel sheet, the method comprising the following successive steps:
casting a steel to obtain a slab, the steel having a composition comprising, by weight percent:
C: 0.15-0.25%
Mn: 0.5-1.8%
Si: 0.1-1.25%
Al: 0.01-0.1%
Cr: 0.1-1.0%
Ti: 0.01-0.1%
B: 0.001-0.004%
P≤0.020%
S≤0.010%
N≤0.010%
and comprising optionally one or more of the following elements, by weight percent:
Mo≤0.40%
Nb≤0.08%
Ca≤0.1%
a remainder of the composition being iron and unavoidable impurities resulting from processing;
reheating the slab at a temperature T rehea t of 1100° C. to 1300° C.;
hot rolling the reheated slab at a finish hot rolling temperature of 800° C. to 950° C.;
coiling the hot rolled steel sheet at a coiling temperature T coil lower than 670° C. to obtain a coiled steel sheet;
optionally pickling the coiled steel sheet;
optionally cold rolling the coiled steel sheet to obtain a cold rolled steel sheet;
heating the hot rolled steel sheet or the cold rolled steel sheet to an annealing temperature T A of 700° C. to 850° C. and maintaining the steel sheet at the temperature T A for a holding time t A of 10 s to 1200 s, to obtain an annealed steel sheet, the atmosphere comprising from 0% to 15% of H2 and having a dew point Top1 strictly higher than −10° C. and below or equal to +20° C.;
cooling the annealed steel sheet to a temperature range from 560° C. to 700° C.;
coating the annealed steel sheet with aluminum or with an aluminum alloy coating; and
cooling the coated steel sheet to room temperature.
3. A press hardened steel part, the steel part having a composition comprising, by weight percent:
C: 0.15-0.25%
Mn: 0.5-1.8%
Si: 0.1-1.25%
Al: 0.01-0.1%
Cr: 0.1-1.0%
Ti: 0.01-0.1%
B: 0.001-0.004%
P≤0.020%
S≤0.010%
N≤0.010%
and comprising optionally one or more of the following elements, by weight percent:
Mo≤0.40%
Nb≤0.08%
Ca≤0.1%
the remainder of the composition being iron and unavoidable impurities resulting from processing;
the steel part comprising successively from the bulk to the surface of the steel part:
a bulk having a microstructure comprising, in surface fraction, more than 95% of martensite and less than 5% of bainite,
a ferritic interdiffusion layer,
a coating layer based on aluminum,
wherein a ratio between the ferritic grain width in the interdiffusion layer GW int over prior austenite grain size in the bulk PAGS bulk , satisfies the following equation:
( GW int /PAGS bulk )−1≥30%.
4. The press hardened steel part as recited in claim 3 wherein the press hardened steel part includes a layer of martensite with a carbon gradient between the bulk and the ferritic interdiffusion layer.
5. The press hardened steel part as recited in claim 3 wherein the press hardened steel part has a tensile strength TS above or equal to 1350 MPa and a bending angle higher than 70°.
6. The press hardened steel part as recited in claim 5 wherein the press hardened steel part has a yield strength YS above or equal to 1000 MPa.
7. A process for manufacturing the press hardened steel part as recited in claim 5 , the process comprising the following successive steps:
providing a steel sheet having the composition;
cutting the steel sheet to a predetermined shape, so as to obtain a steel blank;
heating the steel blank to a temperature of 880° C. to 950° C. for 10 s to 900 s to obtain a heated steel blank;
transferring the heated steel blank to a forming press;
hot-forming the heated blank in the forming press to obtain a formed part; and
die-quenching the formed part.