Method for Fabricating Steel Sheet for Press Hardening, and Parts Obtained by this Method

55:

• A fabrication method for a hot rolled steel sheet, comprising the steps of:
• casting an intermediate product with a chemical composition including, with contents being expressed by weight:
• 0.24%≦C≦0.38%;
• 0.40%≦Mn≦3%;
• 0.10%≦Si≦0.70%;
• 0.015%≦Al≦0.070%;
• 0%≦Cr≦2%;
• 0.25%≦Ni≦2%;
• 0.015%≦Ti≦0.10%;
• 0%≦Nb≦0.060%;
• 0.0005%≦B≦0.0040%;
• 0.003%≦N≦0.010%;
• 0.0001%≦S≦0.005%;
• and
• 0.0001%≦P≦0.025%;
• the titanium and nitrogen contents satisfying Ti/N>3.42;
• the carbon, manganese, chromium and silicon contents satisfying:
• 2.6
• 
• C
• +
• Mn
• 5.3
• +
• Cr
• 13
• +
• Si
• 15
• ≥
• 1.1
• 
• %
• ;and a remainder of the chemical composition being made up of iron and inevitable impurities resulting from processing;
• reheating the intermediate product to a temperature between 1250° C. and 1300° C. for a hold time between 20 and 45 minutes;
• hot rolling the intermediate product until an end of rolling temperature, ERT, between 825° C. and 950° C. to obtain a heat rolled sheet;
• coiling the hot rolled sheet at a temperature between 500° C. and 750° C. to obtain a hot rolled and coiled sheet;
• and
• pickling an oxide layer formed during the preceding steps.

56:

• A fabrication method for a cold rolled and annealed sheet, comprising the steps of:
• supplying a hot rolled sheet, coiled and pickled, fabricated by the method according to claim 55;
• and
• cold rolling the hot rolled, coiled and pickled sheet to obtain a cold rolled sheet;
• and
• annealing the cold rolled sheet at a temperature between 740° C. and 820° to obtain a cold rolled and annealed sheet.

57:

• A fabrication method for a precoated sheet, comprising the steps of:
• supplying a rolled sheet fabricated according claim 55;
• and
• performing a continuous hot-dip pre-coating, the precoating being aluminum or an aluminum or aluminum-based alloy, or zinc or a zinc or zinc-based alloy.

58:

• A fabrication method for a precoated and pre-alloyed sheet comprising:
• supplying a rolled sheet according to claim 55;
• performing a continuous hot-dip precoating with an aluminum or aluminum-based alloy;
• and
• performing a thermal pretreatment of the precoated sheet at a temperature θ1 between 620° C. and 680° C. for a holding time t1 between 6 and 15 hours, such that the pre-coating no longer contains free aluminum of phase τ5 of type Fe3Si2Al12, and τ6 of type Fe2Si2Al9, and such that an austenitic transformation does not occur in the steel substrate, the pretreatment being done in a furnace under hydrogen and nitrogen atmosphere.

59:

• A fabrication method for a press hardened part comprising the steps of:
• supplying a sheet fabricated by a method according to claim 55;
• cutting the sheet to obtain a blank;
• heating the blank to a temperature between 810° C. and 950° C. to obtain a fully austenitic structure in the steel;
• transferring the blank inside a press;
• hot stamping the blank to obtain a part;
• holding the part inside the press to obtain a hardening by martensitic transformation of the austenitic structure.