Method for Fabricating Steel Sheet for Press Hardening, and Parts Obtained by this Method
The present invention provides a rolled steel sheet, for press hardening, for which the chemical composition includes, with contents 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%, 0.0001%≦P≦0.025%, it being understood that the titanium and nitrogen content satisfy: Ti/N>3.42, and that the carbon, manganese, chromium and silicon content satisfy: 2.6 C + Mn 5.3 + Cr 13 + Si 15 ≥ 1.1 % , with the chemical composition optionally including one or more of the following elements: 0.05%≦Mo≦0.65%, 0.001%≦W≦0.30%, 0.0005%≦Ca≦0.005%, with the remainder made up of iron and inevitable impurities coming from preparation. The sheet includes a nickel content Ni surf at any point of the steel near the surface of said sheet over a depth Δ, such that Ni surf >Ni nom , where Ni nom designates the nominal nickel content of the steel, and such that Ni max designates the maximum nickel content within Δ: ( Ni max + Ni nom ) 2 × ( Δ ) ≥ 0.6 and such that: ( Ni max + Ni nom ) Δ ≥ 0.01 , with the depth Δ expressed in microns and the Ni max and Ni nom contents expressed in percentages by weight.
1 - 54 . (canceled)
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 θ 1 between 620° C. and 680° C. for a holding time t 1 between 6 and 15 hours, such that the pre-coating no longer contains free aluminum of phase τ 5 of type Fe 3 Si 2 Al 12 , and τ 6 of type Fe 2 Si 2 Al 9 , 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.
60 : The fabrication method according to claim 59 , wherein the steps are performed successively.
61 : The fabrication method according to claim 59 , further comprising the step of deforming the blank by cold stamping after the step of cutting the sheet.
62 - 63 . (canceled)