Steel sheet and high strength press hardened steel part having excellent bending anisotropy and method of manufacturing the same
A steel sheet has a chemical composition including in wt % C: 0.2-0.4%, Mn: 0.8-2.0%, Si: 0.1-0.5%, Al: 0.01-0.1%, Ti: 0.01-0.1%, B: 0.0005-0.005%, P≤0.040%, Ca≤0.01%, S≤0.006%, N≤0.01%. The steel sheet includes from the bulk to the surface of the coated steel sheet a bulk and a skin layer occupying the outermost 10% of the thickness on either side of the bulk. The bulk includes an inclusion population in which the sum of clustering indexes of MnS and TiN/Ti(C,N) inclusions is less than or equal to 300 μm/mm 2 . This allows to manufacture hot pressed parts having a tensile strength equal to or greater than 1300 MPa and a bending anisotropy equal to or lower than 7°.
1. A process for manufacturing a steel sheet, comprising the following successive steps:
providing a liquid steel having a chemical composition comprising, by weight percent:
C: 0.2-0.3%;
Mn: 0.8-2.0%;
Si: 0.1-0.5%;
Al: 0.01-0.1%;
Ti: 0.01-0.1%;
B: 0.0005-0.005%;
P≤0.040%;
Ca≤0.01%;
S≤0.006%;
N≤0.01%;
and comprising optionally:
Cr≤0.4%;
Mo≤0.3%;
Nb≤0.1%;
V≤0.3%;
wherein Cr+Mo+Nb+V≤0.5%,
the remainder of the composition being iron and unavoidable impurities,
casting said liquid steel to obtain a semi-product able to be hot-rolled;
hot rolling the semi-product at a finish hot rolling temperature comprised from 800° C. to 950° C.; and
coiling the hot rolled steel sheet at a coiling temperature T coil lower than 670° C. to obtain a coiled steel sheet,
the obtained steel sheet having a microstructure in surface fraction comprising from 75% to 90% of ferrite, a rest being comprised of Fe 3 C and hard phases made of martensite and/or bainite,
said steel sheet comprising from a bulk to the surface of the steel sheet:
the bulk representing 80% of a thickness of the steel sheet; and
the bulk being topped by a top skin layer and a bottom skin layer occupying an outermost 10% of the thickness on either sides of the bulk, said bulk comprising an inclusion population wherein a sum of clustering indexes of MnS and TiN/Ti(C,N) inclusions is less than or equal to 300 μm/mm 2 .
2. The process according to claim 1 , wherein:
C: 0.2-0.25%; and/or
Mn: 1.0-1.4%; and/or
Si: 0.1-0.4%; and/or
Al: 0.02-0.06%; and/or
Ti: 0.02-0.06%; and/or
B: 0.002-0.004%; and/or
P≤0.020%; and/or
Ca≤0.005%; and/or
S≤0.005%; and/or
N≤0.008%.
3. The process according to claim 1 , in which the step of providing the liquid steel comprises a phase of refining the liquid steel, during which levels of measured sulfur at a beginning of the refining, Al addition at the beginning of the refining, Ca addition during the refining and volume of O 2 blowing are controlled to verify that a combination C1=Al_added+0.1953*(S_start*1000+O 2 _inj)−9.367*Ca_added stays below a pre-determined cut-off value,
Al_added being the Al added at the beginning of the refining, in kg of aluminium per ton of liquid steel,
S_start being the sulfur content before the refining, in weight %,
O 2 _inj being an amount of O 2 injected during an optional step of aluminothermic heating, expressed in Normal cubic meters of O 2 per ton of liquid steel,
Ca_added being an amount of Ca added in the liquid steel, measured in weight % within the liquid steel.
4. The process according to claim 3 , wherein the cut-off value is equal to 1.80.
5. The process according to claim 1 , wherein the semi product is reheated at a temperature comprised from 1150° C. to 1300° C. before hot-rolling.
6. The process according to claim 1 , wherein the steel sheet is pickled to remove oxidation after coiling.
7. The process according to claim 1 , wherein the coiled steel sheet is cold rolled to obtain a cold rolled steel sheet.
8. The process according to claim 7 , wherein the cold-rolling reduction ratio ranges from 20% to 80%.
9. The process according to claim 1 , wherein the steel sheet is heated in an annealing furnace to a soaking temperature comprised from 700° C. to 850° C. and maintained at said soaking temperature for a soaking time comprised from 10 seconds to 20 minutes.
10. The process according to claim 9 , wherein the annealed steel sheet is cooled to a temperature range from 400° C. to 700° C.
11. The process according to claim 9 , wherein the annealed steel sheet is coated with a metallic coating.
12. The process according to claim 1 , wherein the steel sheet is coated with a metallic coating comprising at least 50% of Al in weight.
13. The process according to claim 1 , wherein the steel sheet is coated with a metallic coating comprising at least 50% of Zn in weight.
14. The process according to claim 1 , wherein the composition further respects the following condition, all elements being expressed in weight %:
(S—Ca*32/40)+(30*Ti*N)≤0.0045.
15. The steel sheet according to claim 2 , wherein the composition comprises, by weight percent:
Si: 0.15−0.35%.
16. The steel sheet according to claim 2 , wherein the composition comprises, by weight percent:
N≤0.005%.