Steel sheet and high strength press hardened steel part and method of manufacturing the same
A steel sheet made of a steel having a composition including, C: 0.3-0.4%, Mn: 0.5-1.0%, Si: 0.4-0.8%, Cr: 0.1-1.0%, Mo: 0.1-0.5%, Nb: 0.01-0.1%, Al: 0.01-0.1%, Ti: 0.008-0.03%, B: 0.0005-0.003%, P≤0.020%, Ca≤0.001%, S≤0.004%, N≤0.005% and including optionally Ni<0.5%, having a microstructure including, in surface fraction, from 60% to 95% of ferrite, the rest being martensite-austenite islands, pearlite or bainite, and including a bulk and a skin layer occupying the outermost 10% of the thickness on either sides of the bulk, the skin layer having a skin layer inclusion population wherein the cumulated surface fraction of oxides, MnS and TiNbCN is equal to or below 75*10 −6 .
1 . A steel sheet made of a steel having a composition comprising, by weight percent:
C: 0.3-0.4%
Mn: 0.5-1.0%
Si: 0.4-0.8%
Cr: 0.1-1.0%
Mo: 0.1-0.5%
Nb: 0.01-0.1%
Al: 0.01-0.1%
Ti: 0.008-0.03%
B: 0.0005-0.003%
P≤0.020%
Ca≤0.0010%
S≤0.004%
N≤0.005%
and comprising optionally:
Ni<0.5%
a remainder of the composition being iron and unavoidable impurities resulting from processing,
the steel sheet having a microstructure comprising, in surface fraction, from 60% to 95% of ferrite, the rest being martensite-austenite islands, pearlite or bainite,
the steel sheet comprising from the bulk to the surface of the coated steel sheet:
the bulk, the bulk being topped by a skin layer occupying the outermost 10% of the thickness on either side of the bulk, the skin layer having a skin layer inclusion population wherein the cumulated surface fraction of oxides, MnS and TiNbCN is equal to or less than 75*10 −6 .
2 . The steel sheet as recited in claim 1 , further comprising a metallic coating on at least one side of the steel sheet.
3 . A press hardened steel part, the steel part having a composition comprising, by weight percent:
C: 0.3-0.4%
Mn: 0.5-1.0%
Si: 0.4-0.8%
Cr: 0.1-0.4%
Mo: 0.1-0.5%
Nb: 0.01-0.1%
Al: 0.01-0.1%
Ti: 0.008-0.03%
B: 0.0005-0.003%
P≤0.020%
Ca≤0.001%
S≤0.004%
N≤0.005%
and comprising optionally:
Ni<0.5%
a remainder of the composition being iron and unavoidable impurities resulting from processing,
the steel part having a microstructure comprising, in surface fraction, more than 95% of martensite and up to 5% of bainite or ferrite
the steel part comprising from a bulk to the surface of the steel part:
the bulk, the bulk being topped by a skin layer occupying the outermost 10% of the thickness on either side of the bulk, said skin layer having a skin layer inclusion population wherein the cumulated surface fraction of oxides, MnS and TiNbCN is equal to or less than 75*10 −6 .
4 . The press hardened steel part as recited in claim 3 wherein the press hardened steel part has a tensile strength TS of at least 1800 MPa and a bending angle in the rolling direction normalized to 1.5 mm of at least 50°.
5 . A process for manufacturing a press hardened steel part, the process comprising the following successive steps:
providing the steel sheet as recited in claim 1 ;
cutting the steel sheet to a predetermined shape, so as to obtain a steel blank;
heating the steel blank to a temperature from 880° C. to 950° C. for 10 seconds to 15 minutes to obtain a heated steel blank;
transferring the heated 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.