Method for making an abrasion resistant steel plate and plate obtained

1. Method for producing a plate of steel which is resistant to abrasion and whose chemical composition comprises, by weight:

0.24%≦C<0.35%

0%≦Si≦2%

0%≦Al≦2%

0.5%≦Si+Al≦2%

0%≦Mn≦2.5%

0%≦Ni≦5%

0%≦Cr≦5%

0%≦Mo≦1%

0%≦W≦2%

0.1%≦Mo+W/2≦1%

0%≦B≦0.02%

0%≦Ti≦1.1%

0%≦Zr≦2.2%

0.5%<Ti+Zr/2≦1.1%

0%≦S≦0.15%

N<0.03%

optionally up 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:

C*=C−Ti/4−Zr/8+7×N/8≧0.095%

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 plate is subjected to a thermal quenching processing operation which is carried out in the heat for rolling in the hot state or after austenitization by reheating in a furnace, in order to carry out the quenching:

the plate is cooled at a mean cooling rate greater than 0.5° C./s between a temperature greater than AC 3 and a temperature of from approximately T=800−270×C*−90×Mn−37×Ni−70×Cr−83×(Mo+W/2), to T-50° C.,

the plate is then cooled at a mean 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,

the plate is cooled as far as ambient temperature and optionally planishing is carried out.

2. Method according to claim 1 , wherein:

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

3. Method according to claim 1 , wherein:

C*≧0.12%.

4. Method according to claim 1 , wherein:

Si+Al≧0.7%.

5. Method according to claim 1 , wherein tempering is further carried out at a temperature which is less than or equal to 350° C.

6. Method according to claim 1 , wherein, the chemical composition of steel is obtained by a melting process during which or after the steel is placed in contact with a slag containing titanium and the titanium of the slag is caused to diffuse in the steel which is in a liguid state.

7. A plate of steelwhich is resistant to abrasion and whose chemical composition comprises, by weight:

0.24%≦C<0.35%

0%≦Si≦2%

0%≦Al≦2%

0.5%≦Si+Al≦2%

0%≦Mn≦2.5%

0%≦Ni≦5%

0%≦Cr≦5%

0%≦Mo≦1%

0%≦W≦2%

0.1%≦Mo+W/2≦1%

0%≦B≦0.02%

0%≦Ti≦1.1%

0%≦Zr≦2.2%

0.5%<Ti+Zr/2≦1.1%

0%≦S≦0.15%

N<0.03%

optionally up 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:

C*=C−Ti/4−Zr/8+7×N/8≦0.095%

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%,

the steel having a martensitic or martensitic/bainitic structure, the structure containing from 5% to 20% of retained austenite and carbides.

8. Workpiece according to claim 7 , characterized in that:

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

9. A plate according to claim 7 , wherein:

C*≧0.12%.

10. A plate according to claim 7 , wherein:

Si+Al≧0.7%.

11. A plate according to claim 7 , wherein the plate has a thickness of from 2 mm to 150 mm.