0.35%≤C≤0.8%

0%≤Si≤2%

0%≤Al≤2%

0.35%≤Si+Al≤2%

0%≤Mn≤2.5%

0%≤Ni≤5%

0%≤Cr≤5%

0%≤Mo≤0.50%

0%≤W≤1.00%

0.1%≤Mo+W/2≤0.50%

0%≤B≤0.02%

0%≤Ti≤2%

0%≤Zr≤4%

0.05%≤Ti+Zr/2≤2%

0%≤S≤0.15%

N<0.03%

optionally from 0% to 1.5% of copper,

optionally at least one element selected from Nb, Ta and V at contents such that Nb/2+Ta/4+V≤0.5%,

optionally at least one element selected from Se, Te, Ca, Bi, Pb at contents which are less than or equal to 0.1%,

the balance being iron and impurities resulting from the production operation, the chemical composition further complying with the following relationships:

0.1%≤C−Ti/4−Zr/8+7×N/8≤0.55%

and:

Ti+Zr/2−7×N/2≧0.05%

and:

1.05×Mn+0.54×Ni+0.50×Cr+0.3×(Mo+W/2)^1/2+K>1.8

with K=0.5 if B≧0.0005% and K=0 if B<0.0005%, according to which the workpiece is subjected to a thermal quenching processing operation which is carried out in the heat for forming in the hot state, or after austenitization by reheating in a furnace in order to carry out the quenching, the process comprising:

cooling the workpiece at a mean cooling rate greater than 0.5° C./s between a temperature greater than AC₃ and a temperature of from T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2), with C*=C−Ti/4−Zr/8+7×N/8, to T−50° C.,

then cooling the workpiece at a core cooling rate Vr<1150×ep^−1.7 and greater than 0.1° C./s between the temperature T and 100° C., ep being the thickness of the plate expressed in mm,

cooling the workpiece as far as ambient temperature.