Steel having high mechanical strength and wear resistance

1. Method for reducing the disadvantageous effect of the segregated seams of a steel which has high mechanical strength and high wear resistance and whose composition consists of, in % by weight:

0.30%≦C≦0.85%

0.05%≦Si≦1.5%

Mn≦1.95%

Ni≦2.9%

1.1%≦Cr≦7.9%

0.61%≦Mo≦4.4%

optionally one or more elements selected from vanadium, niobium and tantalum at contents such that V≦1.45%, Nb≦1.45%, Ta≦1.45% and V+Nb/2+Ta/4≦1.45%,

optionally up to 0.1% of boron,

less than 0.005% of sulphur,

optionally up to 0.01% of calcium ,

optionally up to 0.5% of rare earths,

optionally up to 1% of aluminum,

optionally up to 1% of copper,

the balance being iron and impurities resulting from the production operation,

the composition further complying with:

800≦D≦1150

with:

D=540(C) 0.25 +245(Mo+3 V+1.5 Nb+0.75 Ta) 0.30 +125 Cr 0.20 +15.8 Mn+7.4 Ni+18 Si

according to which method:

the molybdenum is completely or partially replaced with double the proportion of tungsten so that the content of tungsten is greater than or equal to 0.21%,

the titanium and/or zirconium are added such that the contents of titanium and/or zirconium and the content of carbon are adjusted so that, after adjustment, the contents are such that, if C′ is the content of carbon after adjustment and C is the content of carbon before adjustment:

Ti+Zr/2≧0.20×W

C′=C+Ti/4+Zr/8

(Ti+Zr/2)×C′≧0.07

and Ti+Zr/2≦1.49%,

the precision for carrying out those analytical adjustments at the steel works being such that:

0.95×D before adjustment≦D after adjustment≦1.05×D before adjustment,

with:

D after adjustment=540(C′−Ti/4 −Zr/8) 0.25 +245(Mo after adjustment+W/2+3V+1.5 Nb+0.75 Ta) 0.30 +125 Cr 0.20 +15.8 Mn+7.4 Ni+18 Si.

2. Method according to claim 1 , characterized in that, when the content of chromium is from 2.5% to 3.5%, if the content of carbon, before adjustment of the composition is such that C≧0.51%, then W≦0.85% if Mo after adjustment <1.21% and W/Mo≦0.7 if Mo after adjustment ≧1.21%.

3. Method according to claim 1 , characterized in that:

D after adjustment=D before adjustment.

4. Steel which has high mechanical strength and high wear resistance and whose chemical composition consists of in % by weight:

0.35%≦C≦0.85%

0.05%≦Si≦1.5%,

Mn≦1.95%,

Ni≦2.9%,

1.1%≦Cr≦7.9%,

0%≦Mo≦4.29%,

0.21%≦W≦4.9%

0.61%≦Mo+W/2≦4.4%

0%≦Ti≦1.49%

0%≦Zr≦2.9%

0.21%≦Ti+Zr/2≦1.49%

optionally one or more elements selected from vanadium, niobium and tantalum, at contents such that V≦1.45%, Nb≦1.45%, Ta≦1.45% and V+Nb/2+Ta/4≦1.45%,

optionally up to 0.1% of boron,

less than 0.005% of sulphur,

optionally up to 0.01% of calcium ,

optionally up to 0.5% of rare earths,

optionally up to 1% of aluminum,

optionally up to 1% of copper,

the balance being iron and impurities resulting from the production operation,

the composition complying with the following conditions:

(Ti+Zr/2)/W≧0.20

(Ti+Zr/2)×C≧0.07

0.3%≦C*≦1.42%

800≦D≦1150

with

D=540(C*) 0.25 +245 (Mo+W/2+3V+1.5 Nb+0.75 Ta) 0.3 +125 Cr 0.20 +15.8 Mn+7.4 Ni+18 Si

and

C*=C−Ti/4−Zr/8,

and, furthermore, when C*≧0.51% and 2.5%≦Cr≦3.5%, then W≦0.85% if Mo<1.21% and W/Mo≦0.7 if Mo≧1.21%.

5. Steel according to claim 4 , characterized in that:

C*≦1.1%.

6. Steel according to claim 4 , characterized in that:

W≦0.85%.

7. Steel according to claim 4 , characterized in that:

Si≧0.45%.

8. Steel according to claim 4 , characterized in that:

Si<0.45%.

9. Steel according to claim 4 , characterized in that:

Mo+W/2≧2.2%.

10. Steel according to claim 4 , characterized in that:

Cr≧3.5%.

11. Steel according to claim 4 , characterized in that:

Ti+Zr/2<0.7%.

12. Steel according to claim 4 , characterized in that:

Ti+Zr/2≧0.7%.

13. Method for producing a steel workpiece according to claim 4 , characterized in that:

a liquid steel is produced having the desired composition with the contents of titanium and/or zirconium in the bath of molten steel being adjusted, with local excess concentrations of titanium and/or zirconium in the bath of molten steel being prevented at all times;

the steel being cast in order to obtain a semi-finished product;

then, the semi-finished product is subjected to a forming processing operation by means of plastic deformation in the hot state and, optionally, a thermal processing operation in order to obtain the workpiece.

14. Method according to claim 13 , characterized in that the addition of titanium and/or zirconium is carried out by progressively adding titanium and/or zirconium to a slag which covers the bath of liquid steel and by allowing the titanium and/or zirconium to diffuse slowly in the bath of liquid steel.

15. Method according to claim 13 , characterized in that the addition of titanium and/or zirconium is carried out by a wire comprising titanium and/or zirconium being introduced into the bath of liquid steel, with the bath being agitated.

16. A workpiece of the steel according to claim 4 , which workpiece is obtained by a method wherein:

a liquid steel is produced having the desired composition with the contents of titanium and/or zirconium in the bath of molten steel being adjusted, with local excess concentrations of titanium and/or zirconium in the bath of molten steel being prevented at all times;

the steel being cast in order to obtain a semi-finished product;

then, the semi-finished product is subjected to a forming processing operation by means of plastic deformation in the hot state and, optionally, a thermal processing operation in order to obtain the workpiece.