FR2838137A1 - STEEL FOR THE MANUFACTURE OF MOLDS FOR INJECTION MOLDING OF PLASTIC MATERIALS OR FOR THE MANUFACTURE OF TOOLS FOR THE WORKING OF METALS - Google Patents

Abstract

Steel whose chemical composition comprises, in% by weight: 0, 12% ≤ C ≤ 0, 40%; If ≤ 0, 8%; Mn ≤ 2.5%; Ni ≤ 3%; Cr ≤ 3.5%; Mo + W / 2 ≤ 2, 2%; V + Nb / 2 + Ta / 4 ≤ 0.5%; Al ≤ 0.4%; Ti + Zr / 2 ≤ 0, 1%; Cu ≤ 1.5%; optionally 0, 0005% <B <0.015%, S + Se + Te <0.2%, Pb + Bi <0.2%, Ca <0.1%, the remainder being iron and impurities resulting from development. The chemical composition further satisfies the following relationships: 3.2 ≤ Tr ≤ 6.5; 77 ≤ Dr ≤ 85; U ≤ 850; in which, for contents expressed in%, Tr = 1, 8xC + 1, 1xMn +0.7xNi + 0.6xCr + 1, 6xMo * + K, with K = 0 if the steel does not contain boron and = 0, 5 if the steel contains boron; Δ = 54xC0.25 + 24.5x (Mo + 3xV *) 0.30 + 1, 58xMn + 0.74xNi + 1.8xSi + 12.5x (Cr) 0.20; U = 1600xC + 100x (0, 25xCr + Mo * + 4.5xV *); R = 3.8xC + 10xSi + 3.3xMn + 2, 4xNi + 1, 4x (Cr + Mo *); Mo * = Mo + W / 2; V * = V + Nb / 2 + Ta / 4. Steel blocks for the manufacture of injection molds of plastics or metals or for the manufacture of tools for metalworking.

Description

The present invention relates to a steel that can be used, in particular

  for the manufacture of molds for injection molding of plastics or for

  manufacture of tools for metalworking.

  Molds for injection molding of plastics are, in general, made of steels whose hardness is close to 300HB. However, when these molds are used for molding plastics such as engineering plastics or thermosetting plastics, it is conceivable to use harder steels that are more resistant to wear. A type 55 NCDV 7 steel containing about 0.55% carbon, 1.75% nickel, chromium, molybdenum and vanadium is then used. This steel makes molds with a hardness close to 400 HB. This steel however has several disadvantages: it is difficult to machine and difficult to weld. In addition, this steel often contains localized segregations which constitute hard points detrimental to the ability to polish or chemical graining. These two disadvantages are particularly troublesome because the manufacture of molds requires extensive machining and the molds are generally repaired by soldering and polished or grained. In addition, these molds must be surface-hardenable, for example by nitriding, without losing their hardness. These steels are also used to make molds for molding metals such as light alloys or to manufacture metalworking parts, for example by stamping or stamping or forging. For these applications, the known steels have the same disadvantages

  only for the molding of plastics.

  The object of the present invention is to remedy these drawbacks by proposing a steel for molds or for metalworking which is easier to weld, easier to machine, to polish and to grain than the steels according to the prior art, and which makes it possible to manufacture parts having a hardness of the order of 400HB, including after surface hardening by nitriding, which imposes that the required characteristics, in particular hardness, are compatible with an income

at least 530 C.

  For this purpose, the subject of the invention is a steel making it possible to manufacture a block of thickness greater than 20 mm and up to 1500 mm, whose structure is martensitic or martensite-bainitic, whose hardness is between 370 HB and about 450 HB at any point, for the manufacture of mold parts or for metal working, the chemical composition of which comprises, in% by weight:

0.120% <C <0.400%

  If <0.8% Mn <2.5% Ni <3% Cr <3.5% Mo + W / 2 <2.2% V + Nb / 2 + Ta / 4 <0.5% Al <0, 4% Ti + Zr / 2 <0.1% 0 Cu <1.5% - optionally boron in a content of between 0.0005% and 0.015%, - optionally one or more elements selected from sulfur, selenium and tellurium, the sum of the contents of these elements being less than or equal to 0.2%, - optionally one or more elements taken from lead and bismuth, the sum of the contents of these elements being less than or equal to 0.2% - optionally calclum in a content less than or equal to 0.1%, the rest being iron and impurities resulting from the preparation, the chemical composition further satisfying the following relations: 3.2 <Tr <6.5 77 <Dr <85

U <850

  Mo * + 3xV *> 0.1% in which, for contents expressed in%,: Tr = 1.8xC + 1.1xMn + 0.7xNi + 0.6xCr + 1.6xMo * + K 2s with K = 0 if steel does not contain boron and K = 0,5 if steel contains boron Dr = 54xC025 + 24,5x (Mo * + 3xV *) 030 + 1,58xMn + 0,74xNi + 1,8xSi + 12.5x (Cr) 2 U = 1600xC + 100x (0.25xCr + Mo * + 4.5xV *) R = 3.8xC + 10xSi + 3.3xMn + 2.4xNi + 1.4x (Cr + Mo *) Mo * = Mo + W / 2 V * = V + Nb / 2 + Ta / 4 Preferably, the chemical composition is such that:

R> 11

  Also preferably, the chemical composition is such that: R <2.7xTr When the steel contains boron, it is preferable that the boron, aluminum, titanium, zirconium and nitrogen contents, expressed in thousandths of a% by weight, are such that: B 2 1 xK1 + 0.5 with K1 = Min (I *; J *) I * = Max (O; I) and J * = Max (O; J) I = Min (N; N-0 , 29 (Ti + Zr / 2-5))

  J = Min N; 0.5 (N - 0.52 Al + j (N - 0.52 Al) 2 + 283)).

  It is then preferable that the silicon content remains strictly lower than

0.45% by weight.

  Preferably, the composition is such that: U <750, more preferably

U <700, and more preferably U <650.

  In addition, and more particularly when U <750, it is preferable that the chemical composition of the steel is such that:

0.200% <C <0.320%

  If <0.30% 0.3% <Mn <1.8% Ni <2% 0.5% <Cr <2.5% o Mo + W / 2 <1.8% V + Nb / 2 + Ta / 4 <0.3% Mo * + 3xV *> 0.3% and better still, such as U <650 and:

0.220% <C <0.280%

  If <0.15% 0.8% <Mn <1.6% Ni <2% 1% <Cr <2.5% Mo + W / 2 <1.5% o V + Nb / 2 + Ta / 4 <0, 2% Mo * + 3xV *> 0.4%

  It is then preferable that the composition is such that Tr> 4.5.

  The invention also relates to a steel block according to the invention, intended for the manufacture of molds for the injection of plastics material, of thickness greater than mm, the structure of which is entirely martensitic or martensito-bainitic, and

  whose hardness in all points is between 370 HB and 450 HB.

  Finally, the invention relates to a piece of machined steel in a block according to the invention, at least a part of the surface is hardened by

  nitriding and whose hardness in all points is between 370HB and 450HB.

  The steel according to the invention is also intended for manufacturing parts for the

metalwork.

  The invention will now be described more precisely but not

  limiting and illustrated by examples.

  The mold parts or metalworking parts are machined from hardened solid steel blocks to obtain a homogeneous martensite-bainitic structure and returned to achieve the desired hardness and ductility properties. It is then necessary to use a steel having a high quenchability and a high hardenability. But these hardened steels must have the best possible machinability and the highest possible thermal conductivity. This last property is useful for improving the productivity of molding operations. A priori, the combination of these different properties is contradictory. Indeed, it is known that steel is all the easier to machine because it is hard and it is known to improve machinability by adding addition elements such as sulfur, calclum, selenium , tellurium or lead. But in mold steels these additions must be limited because, although they are acceptable when the surface of the mussel impressions is rough, they are harmful when

  the surfaces are polished. In any case, such additions are insufficient.

  It is also known that the thermal conductivity of the steel and its quenchability vary inversely as a function of its composition. These requirements are therefore contradictory. However, the inventors have found in a new way that it is possible to find compositional domains making it possible to obtain

  o combinations of properties significantly better than those of known steels.

  These composition domains are defined on the one hand by ranges of contents in each of the elements of the composition, and on the other hand by formulas to be respected. To obtain such combinations of properties, the steel must contain: from 0.120% to 0.400% of carbon to form hardening carbides without, however, greatly deteriorating the weldability, the toughness and the machinability, preferably this content must be included between 0.200% and 0.320%, and better, between

0.220% and 0.280%.

  less than 0.8%, preferably less than 0.45% and better still, less than 0.30%, and more preferably less than 0.15% of silicium. This element commonly used to deoxidize steel during processing has an adverse effect on thermal conductivity. However, it is always present at least in the state of traces. o - less than 2.5% of manganese, and preferably from 0.3% to 1.8%, and better still, from 0.80% to 1.60%, to obtain good hardenability without, however, generating too much segregations that would reduce the ability to obtain good surface conditions on mussels. This element is always present at least in the state of traces. In addition, it is preferable that its content is greater than 0.1% in order to trap the sulfur always present in the state of impurities. If sulfur has been added to improve machinability, the minimum manganese content should preferably be adjusted accordingly and be at least 5 times, and

  preferably 7 times the sulfur content.

  less than 3% of nickel, preferably less than 2%. This element allows o to increase the quenchability but it is very expensive. It can be present in the state of traces. However, in applications requiring a high toughness and a very good homogeneity of hardness, it may be advantageous to reduce the manganese content in favor of nickel on the one hand manganese substitution of two parts of nickel. Such substitution has the advantage of reducing segregations. less than 3.5% of chromium, and preferably of 0.5% to 2.5% of chromium, and still more preferably of 1% to 2.5%. This element makes it possible to increase the hardenability, but in excessive quantity, it tends to enrich the carbides in chromium to the detriment of other more favorable elements such as molybdenum, tungsten, vanadium, nlobium and tantalum. It can be present in the state of traces. - molybdenum eVou tungsten in contents such that the sum Mo * = Mo

  W / 2 is less than 2.2%, and preferably less than 1.8%, and more preferably 1.5%.

  These elements are strongly soaking. In addition, they significantly reduce the softening of the income, which is desirable when the impressions of the molds are subjected to surface treatments such as nitriding at temperatures of at least 500 C. However, too much, they

  deteriorate the machinability.

  optionally at least one element selected from vanadium, niobium and tantalum in such quantities that the sum V * = V + Nb / 2 + Ta / 4 is less than 0.5%, and better still less than 0, 3%, and better still, less than 0.2%. These elements make it possible to increase the resistance to the softening with the income, in particular when the income is carried out above 550 C. They also make it possible to increase the wear resistance of the imprints of the molds. But in

  Too much, they deteriorate the machinability and weldability.

  optionally from 0.0005% to 0.015% of boron. This element substantially increases the quenchability without impairing the thermal conductivity. In addition, its effect disappearing at high austenitization temperatures encountered in welding, it is favorable to good weld repairability. Below 0.0005%, which is practically the limit of detection by the means of analysis, it has no significant effect. Above 0.015% it generates a

  embrittlement of steel without increasing quenchability.

  optionally up to 0.4% of aluminum, and optionally one or more elements selected from titanium and zirconium, the sum Ti + Zr / 2 of up to 0.1%. These elements are strong deoxidizers. In addition, they fix the nitrogen always present at least as an impurity in contents generally between 0.004% and 0.025%, but may be less than 0.004% or greater than 0.025%. However, when the steel contains boron, the nitrogen content should preferably be less than 0.025%. The presence of aluminum,

  titanium or zirconium is desirable for the boron to be fully effective.

  So that aluminum, titanium and zirconium, taken alone or in combination of two or three of these elements, protect boron against nitrogen and thus give it its full efficiency, the boron, aluminum, titanium, zirconium contents and nitrogen, expressed in thousandths of wt.%, should preferably be such that: B 2 3 xK1 + 0.5 with K1 = Min (I *; J *) I * = Max (O; I) and J * = Max (O; J) At I = Min (N; N-0.29 (Ti + Zr / 2-5 ")

  J = Min N; 0.5 (N - 0.52 Al + :( N - 0.52 Al) 2 + 283).

  - possibly up to 1.5% copper. This element can exist as a trace

  or impurity, but at higher levels it has a hardening effect.

  However, beyond 1.5%, it excessively deteriorates the ability to deform

hot plastic steel.

  optionally one or more elements selected from sulfur, selenium and tellurium in a small quantity, the sum of the contents of these elements having to remain less than 0.200%. However, when the steel is intended for the manufacture of 0 molds whose surface is polished and chemically grained, the sum of the contents

  in these elements must remain below 0.025%, or better, less than 0.005%.

  optionally one or more elements selected from lead and bismuth, the sum of the contents of these elements being less than 0.2%. However, when the steel is intended for the manufacture of molds whose surface is polished and chemically grained, it is preferable that the steel does not contain such elements. - possibly calclum in a content less than 0.100%. However, when the steel is intended for the manufacture of molds whose surface is polished and chemically grained, it is preferable that the steel does not contain this element because its positive action on the machinability is realized in conjunction with the sulfur

  whose addition is preferentially limited when the steel is to be polished or grained.

  the rest of the composition consists of iron and impurities resulting from the preparation. It should be noted that, for all addition elements whose minimum content is not imposed, when these elements are not added, they can always be found at least in the form of residuals or impurities, in particular

very low levels.

  Within the limits that have just been defined, the composition of the steel must be chosen in order to obtain the desired characteristics of use. For this purpose, the composition must be such that: - the quantity Tr = 1.8xC + 1.1xMn + 0.7xNi + 0.6xCr + 1.6xMo * + K, with K = 0 if the steel does not contain any boron, and K = 0.5 if the steel contains boron, that is to say when boron has been added in a content greater than or equal to 0.0005%, represents the hardenability of the steel and must be greater than 3, 2 and better than 4.5 to obtain sufficient quenchability. In particular, Tr must be greater than 4.5 to obtain a martensito-bainitic structure without trace of pearlitic structure on parts whose thickness may exceed

1000 mm and reach 1500mm.

  - The size Dr = 54xC025 + 24.5x (Mo * + 3xV *) 030 + 1.58xMn + 0,74xNi + 1, 8xSi + 12.5x (Cr) 2 must be between 77 and 85 in order to obtain a curing

  carbides sufficient without, however, greatly impair machinability.

  - the magnitude U = 1600xC + 100x (0.25xCr + Mo * + 4.5xV *) which is an indicator of machinability (the lower U is, the better the machinability), must remain lower than

  850, and preferably 750, and more preferably 700 or 650.

  the magnitude R = 3.8xC + 10xSi + 3.3xMn + 2.4xNi + 1.4x (Cr + Mo *) which varies as the thermal resistivity, ie the inverse of the thermal conductivity, must, preferably, stay below 2.7xTr. However, taking into account all the desired characteristics requirements for steel, this quantity can not, in general, fall below 11, but preferably it must remain below 20, or better still 15, also , The invention relates more particularly to steels for which R> 11, while being the

weaker possible.

  in view of the various constraints, the composition must furthermore satisfy, at o the relationship Mo * + 3 × V *> 0.1%; when U <750 and the analysis corresponds to the preferential domain:

0.200% <C <0.320%

  If <0.30% 0.3% <Mn c 1.8% z Ni <2% 0.5% <Cr <2.5% Mo + W / 2 <1.8% V + Nb / 2 + Ta / 4 <0.30% the analysis should be such that: Mo * + 3xV *> 0, 3%; when U <650 and the analysis o corresponds to the even more preferential domain:

0.220% <C <0.280%

  If <0.15% 0.8% <Mn <1.6% Ni <2% 1% <Cr <2.5% Mo + W / 2 <1.5% V + Nb / 2 + Ta / 4 < 0.2%

  The analysis must satisfy the relationship: Mo * + 3xV *> 0.4%.

  In order to manufacture a mold with this steel, the steel is made, cast, hot rolled or forged in a known manner and cut to obtain blocks having a thickness greater than 20 mm and can exceed 100mm and reach 400mm, even 600mm, and even 1500mm. It should be noted that, for the least important thicknesses, the blocks may be sheets or large flat, and that for

  the highest thicknesses are, in general, forged blocks.

  The blocks are austenitized, optionally in the hot forging or rolling, at a temperature above AC3 and preferably below 950 C, especially when the steel contains boron, and then they are soaked in air, the oil or the water according to the thickness and the hardenability of the steel, so as to obtain a martensitic or martensite-bainitic structure throughout the mass. Finally, they are returned to a temperature above 500 C and preferably at least equal to 550 C, but less than AC. A hardness of between 370HB and 450HB is thus obtained. In such blocks, mold parts are made in known manner, with imprints which are polished and optionally grained. Optionally, these parts are cured on the surface, for example by gaseous nitriding. After nitriding gas, except the extreme surface of the parts which is nitrided, the hardness of

  the steel remains between 370 HB and 450 HB approximately.

  By way of example, for comparison, we consider the analyzes reported in

  Table 1, whose quantities Tr, Dr. U. R and R / 2.7Tr are reported in Table 2.

  Examples 1 to 20 are in accordance with the invention and Examples 21 to 26 are given for comparison. These steels do not contain additions of sulfur, selenium, tellurium, lead, bismuth or calclum. However, they contain a little

sulfur, between 0.010% and 0.020%.

  For all these steels, the HB hardness was determined in the quenched tempering state, that is to say for a martensitic or martensite-bainitic structure returned to 550 ° C. in all cases, except for Examples 7, 20 and 26 of which the income was made to

625 C.

Table 1

  C ISI | Mn | Nj | Cr IMO IW IV | B * | AI * ITI * N *

  1: 1 0.20 1.2 0.95 2.1 0.65 O 0.15 2 65 O 6

  2 1, 1.4 1.0 2.5 0.65 0 O.

  3 1 0.15 1.2 0.95 2.1 0.8 O 0.1 7 2.5 57 O 6

  4 0.10 1.0 0.8 2.2 0.6 O 0.06 2 62 O 5

  0.14 1.5 0.2 1.5 0.65 O 0.17 2 45 O 3

  6: 0.12 1.1 0.2 1.7 1.6 O 0.15 2.5 55 O 6

  7 1 0.14 1.3 0.9 2.2 0.8 O 0.12 O 5 O 12

  8 0.17 0.12 1.1 0.2 1.7 1.6 O 0.15 O 12 O 8

  9 0.28 O. 13 1.4 0.85 2.05 0.95 O 0.02 1.5 25 15 3

  0.30 O, 09 1.3 O, 75 1.9 0.6 O O 19 1 54 O 1

  1 1 0.24 0.05 1.25 0.3 1.8 0.32 0.67 0.16 3 3 15 7

  12 0.17 0.07 1.3 0.9 2.7 0.6 0.7 0.17 2 52 O 7

  13 0.23 0.04 0.3 0.1 1.5 0.7 O 0.19 2 60 O 5

  14 0.27 O. 15 1.7 1.8 1.9 O, 5 0.4 O 3 64 O 9

  0.28 0, 11 0.3 0.7 2.9 1.5 0.5 O 2 57 O 5

  16 0.38 O, 03 0.25 2.9 O, 3 O, 55 0.4 0, 16 4 65 O 11

  17 0.24 0.27 1.3 0.6 1.9 0.67 O 0.15 3 63 O 8

  18 0.20 0.42 1.2 0.7 2.7 0.2 1.0 D, 1 2 85 O 8

  19 0.26 0.35 1.5 2.2 2.0 0.45 O D, 2 O 20 O 8

  O, 30 O, 30.0 3.0 1.5 0.8 O 3.3 O 45 O 12

  21 0.43 0.33 1.25 0.3 1.9 0.5 O O 24 O 11

  22 10,42 1, 3 | 1,2 10,2 | 2,1 10,25 1 1OI1 1 | 18 1,115

  23 1, 43 1 14 11 3 1O 1 12.9 1, 5 1 10.2 1 12 1 11O

  24 1, 49 1, 35 | 1,2 10,2 | 1,8 1OI15 1 | 0,12 1 12 1 18

  1 55 1 3 1 8 117 | 1,1 1 45 1 1O 1 1 125 1 18

  26 1 39 1 30 1 57 1 1 13.2 1 94 1 1 19 1 115O 1 19

  boron, nitrogen, titanium and aluminum are expressed in thousandths of a%.

e

Table 2

É: 31 Tr | Dr. | U | R I R / 2.7Tr

/, 80.9 585 13 0.92

tom 5.23 81.6 595 13.9 0.98

, 37 81 549 12.5 0.86

4 5.07 81.3 702 111.5 0.84

4.72 80.9 611 10.9 0.85

, 74 81.1 542 10.6 0.68

7 5.25 84.2 717 13.3 0.94

, 24 81.1 542 10.6 0.75

, 89 81.4 603 13.2 0.83

5.1 82.8 673 1 1.6 0.85

11 79.6 567 9.69 0.77

12 5.97 79.1 479 12.8 0.8 I

13 77.9 561 5.58 0.62

14 83 617 16.1 0.93

1 84.9 696 11.3 0.66

16 [I 60.9 763 11 0.84

17 4.99 81.3 589 12.9 0.96 I

18 5.41 81.9 570 15.4 1.05

19 5.58 82.4 601 18.1 1.2

5.04 85.3 733 15.2 1.12

21 4.3 80.6 786 13.1 1.13 I

22 1 81 795 12.3 1.18

23 1 83.1 856 11.7 1.06

24 81.9 898 12.5 1.27

1; 84.8 998 14 1.17

26 14 :: 884 12.4 0.95

  The HVZAT hardness was also determined in the heat affected area in the vicinity of a weld compared to the base metal HVbase hardness.

  not affected by heat. These results are reported in Table 3.

at

Table 3

Ex HB HV ZAT HVZAT / HVbase

1 1 393 550 1.27

2,402,558 1.26

3,395,492 1.13

4,402,640 1.45

402,555 1.26

6 1 403 458 1.03

7,403,627 1.41

8,404,458 1.03

9,402,577 1.31

420,591 1.28

11 1,390,527 1.23

12,375,462 1.12

13,500 1.23

14 420 577 1.25

435,577 1.21

16,410,645 1.43

17,393,538 1.24

18 402 516 1.17

19,410,575 1.28

420,596 1.29

21,393,717 1.66

22,708 1.6

23 1,425 726 1.55

24: 751 1.67

1,795 1.7

26,689 1.58

  In view of these two tables, it can be seen that, at comparable hardness (HB) and comparable gold coefficient of performance, the Solon steels of the invention have better machinability (lower U-value) than the steels given as comparison. In

28381 37

  in addition, they have a better weld repair ability and above all a much better after-repair polishing ability than the comparison steels since the hardness in ZAT is lower and the HVZAT / HVbase ratio is lower. For the steels according to the invention, the hardness in ZAT HVZAT is always lower than 670 HV, and lower than 600HV when the carbon is less than or equal to 0.3%, and the HVZAT / HVbase ratio is always less than 1.5. , and

  less than 1.3 when the carbon is less than 0.3%.

  In addition, for most of the examples, the Tr quenchability is greater than that of the steels given for comparison and can exceed 4.5, or even 5 or even 6, while retaining good properties elsewhere, which allows to manufacture blocks whose thickness can reach 1500mm and whose properties are in accordance with what is desired for the manufacture of molds or parts for

metalworking.

  These steels are suitable for the manufacture of plastic injection mold parts or metal molds such as light alloys. They are

  also suitable for the manufacture of tooling parts for metalworking.

28381 37

Claims (9)

  1 - Steel whose chemical composition comprises, in% by weight: 0.12% <C <0.40%   If <0.8% Mn <2.5% Ni <3% Cr <3.5% o Mo + W / 2 <2.2% V + Nb / 2 + Ta / 40.5% Al <0.4 % Ti + Zr / 2 <0.1% Cu <1.5% - optionally boron in a content of between 0.0005% and 0.015%, - optionally one or more elements selected from sulfur, selenium and tellurium , the sum of the contents of these elements being less than or equal to 0.2%, - optionally one or more elements taken from lead and bismuth, the sum of the contents of these elements being less than or equal to 0.2%, zo optionally the calclum in a content less than or equal to 0.1%, the rest being iron and impurities resulting from the preparation, the chemical composition also satisfying the following relations: 3.2 <Tr <6.5 < Dr <85 U <850   Mo * + 3xV *> 0.1% in which, for contents expressed in%,: Tr = 1.8xC + 1.1xMn + 0.7xNi + 0.6xCr + 1.6xMo * + K with K = 0 if steel does not contain boron and K = 0,5 if steel contains boron Dr = 54xC025 + 24,5x (Mo * + 3xV *) 030 + 1,58xMn + 0,74xNi + 1,8xSi + 12 , 5x (Cr) 2 U = 1600xC + 100x (0.25xCr + Mo * + 4.5xV *) R = 3.8xC + 10xSi + 3.3xMn + 2.4xNi + 1.4x (Cr + Mo *) Mo * = Mo + W / 2 V * = V + Nb / 2 + Ta / 4 2 -Act according to claim 1, the chemical composition of which is such that: R> 11   3 - Steel according to claim 1 or claim 2 characterized in that R <20 4 - Steel according to claim 1, claim 2 or claim 3 characterized in that: o R <2,7xTr   - Steel according to any one of claims 1 to 4 characterized in that   that the contents of boron, aluminum, titanium, zirconium and nitrogen, expressed in thousandths of a% by weight, are such that: B 2 3 xK1 + 0.5 with K1 = Min (I *; J *) I * = Max ( O; I) and J * = Max (O; J) I = Min (N; N-0.29 (Ti + Zr / 2-5)) J = Min (N; 0.5 (N-0.52) Al + (N-0.52Al) 2+ 283):   6 - Steel according to any one of claims 1 to 5 characterized in that   that the silicon content is strictly less than 0.45% by weight.   7 - steel according to any one of claims 1 to 6 characterized in that that U <750.   8 - Steel according to claim 7 characterized in that U <700.   9 - Steel according to claim 8 characterized in that U <650.   10 - Steel according to any one of claims 7 to 9 characterized in that that its composition is such that: 0.200% <C <0.320% 28381 37   If <0.30% _ 0.3% <Mn <1.8% Ni <2% 0.5% _ Cr <2.5% Mo + W / 2 <1.8% V + Nb / 2 + Ta / 4 <0.3% Mo * + 3xV *> 0.3% 11 - Steel according to Claim 9, characterized in that its composition is such that: 0.220% <C <0.280%   If _ 0.15% 0.8% <Mn <1.6% Ni <2% 1% <Cr <2.5% Mo + W / 2 <1.5% V + Nb / 2 + Ta / 4 < 0.2% Mo * + 3xV *> 0.4%   12 - Steel according to claim 10 or 11 characterized in that Tr> 4.5.   13 - Steel block according to any one of claims 1 to 12, for   the manufacture of molds for the injection of plastics or for the molding of metals or metalworking workpieces, of a thickness of more than 20 mm, the structure of which is wholly martensitic or martensito-bainitic, and the   Hardness at any point is between 370 HB and 450 HB.   14 - Steel mold part machined in a block conforming to the   claims 12, at least a part of the surface of which is hardened by nitriding