Precoated sheets for manufacturing press-hardened coated steel parts

1. A precoated, cold-rolled and annealed sheet with a thickness between 0.5 and 2.6 mm for the fabrication of press hardened parts, comprising:

a steel substrate for heat treatment with a carbon content C 0 between 0.07% and 0.5%, expressed by weight;

a metal pre-coating on at least the two principal faces of the steel substrate, the metal pre-coating being aluminum, an aluminum alloy, zinc or a zinc alloy, or is composed of a layer of aluminum or an aluminum-based alloy topped by a layer of zinc or a zinc alloy;

a composition of the steel substrate contains, expressed in percent by weight:

0.07%≤C≤0.5%;

0.5%≤Mn≤3%;

0.02%≤Si≤0.5%;

0.01%≤Cr≤1%;

Ti≤0.2%;

Al≤0.25%;

S≤0.05%;

P≤0.1%;

0.0005%≤B≤0.010%; and

a remainder of the composition being iron and the unavoidable impurities resulting from processing;

the steel substrate including a decarburized area on the surface of each of the two principal faces, the depth p 50% of the decarburized area being between 6 and 30 micrometers, p 50% being the depth at which the carbon content is equal to 50% of the content C 0 ,

the sheet not including a layer of iron oxide between the substrate and the metal pre-coating, and

the steel substrate having an oxygen content O 0 and having, underneath an interface between the pre-coating and the substrate, at a depth located between 0 and 5 micrometers starting from the interface, an average oxygen content O m , whereby O m /O 0 is greater than 15.

2. The pre-coated sheet as recited in claim 1 , wherein the composition of the steel substrate contains, expressed in percent by weight:

0.09%≤C≤0.38%;

0.8%≤Mn≤1.5%;

0.1%≤Si≤0.35%;

0.01%≤Cr≤0.3%;

0.02%≤Ti≤0.1%;

0.001%≤Al≤0.25%;

S≤0.05%;

P≤0.1%;

0.002%≤B≤0.005%; and

the remainder of the composition being iron and the unavoidable impurities resulting from processing.

3. The pre-coated sheet as recited in claim 1 , wherein the composition of the steel substrate contains, expressed in percent by weight, 0.15%≤C≤0.25%.

4. The pre-coated sheet as recited in claim 1 , wherein the substrate contains, in an area located between 0 and 5 micrometers below the interface between the pre-coating and the substrate, oxides that include at least one element selected from a group consisting of titanium, silicon, manganese, aluminum, chromium, a density of the oxides with a diameter greater than one micrometer in this area is greater than 50/mm 2 .

5. The pre-coated sheet as recited in claim 1 , wherein the composition of the steel substrate includes 0.0005%≤Ca≤0.005%.

6. The pre-coated sheet as recited in claim 2 , wherein the composition of the steel substrate includes 0.0005%≤Ca≤0.005%.