EP0709481A1 - Low alloy steel for the manufacture of moulds for plastic materials or for rubber articles - Google Patents

Abstract

The low alloy steel comprises (wt.%): 0.24-0.35 C; 1-2.5 Mn; 0.3-2.5 Cr; 0.2-1.6 W; 0.1-0.8 (Mo+W/2); 0-25 Ni; 0-0.3 V; 0-0.5 Si; 0.002-0.005 B; 0.005-0.1 Al; 0-0.1 Ti; 0-0.02 P; and 0-2 Cu. There may also be present less than 0.1% of at least one of the following: Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and rare earth elements. The remainder is Fe plus possible impurities. The compsn. also satisfies the following equations: U = 409(%C) + 19.3(%Cr + %Mo + %W/2) + %V) + 29.4(%Si) + 10(%Mn) + 7.2(%Ni) < 200 and R = 3.82(%C) + 9.79(%Si) + 3.34(%Mn) + 11.94(%P) + 2.39(%Ni) + 1.43(%Cr) + 1.43(%Mo + W/2) < 11.14.

Description

The present invention relates to a low alloy steel used in particular for the manufacture of molds for plastics or for rubber.

The molds for plastics or for rubber are produced by machining massive metal blocks whose thickness can exceed 500mm. The surface of the impression obtained by machining is most often either polished or chemically grained in order to give the objects obtained by molding the desired surface appearance. In order to minimize mold wear, any point on their surface must have a high hardness of between 250HB and 400HB and most often between 270HB and 350HB. They must also have the highest possible elastic limit and good resilience to resist shock and deformation.

The machining operation being very important, since it commonly represents 70% of the total cost of manufacturing the mold, the metal must be as machinable as possible and, very often, the aptitude for machining cannot be obtained by conventional additions which are too large, such as sulfur or lead, since these additions deteriorate the ability to polish or to emboss.

As the molds are quite often repaired by welding, the metal used must also be as weldable as possible.

Finally, the molding of plastics or rubber being hot, the metal used must have the highest possible thermal conductivity in order to facilitate heat transfers which limit the productivity of the production of molded objects.

In order to manufacture the molds, blocks of low-alloy steel which are sufficiently quenching are generally used to obtain, after quenching and tempering, a martensitic or martensitic-bainitic structure having sufficient hardness, a high elastic limit, good toughness.

The most used steel is steel P20 according to the AISI standard or steels W1.2311 or W1.2738 according to the German standard WERKSTOFF.

Steel P20 contains, by weight, from 0.28% to 0.4% of Carbon, from 0.2% to 0.8% of Silicon, from 0.6% to 1% of Manganese, from 1.4 % at 2% chromium, from 0.3% to 0.55% molybdenum, the rest being iron and impurities linked to the production.

W1.2311 and W1.2738 steels contain, by weight, from 0.35% to 0.45% of Carbon, from 0.2% to 0.4% of Silicon, from 1.3% to 1.6% Manganese, 1.8% to 2.10% Chromium and 0.15% to 0.25% Molybdenum; W1.2738 steel also contains 0.9% to 1.2% Nickel, the rest being iron and impurities linked to the production.

These steels have good wear resistance, but they have insufficient weldability, machinability, toughness and thermal conductivity.

In order to improve the weldability, it has been proposed, in patent application EP 0 431 557, a steel containing, by weight, from 0.1% to 0.3% of carbon, less than 0.25% of Silicon, 0.5% to 3.5% Manganese, less than 2% Nickel, 1% to 3% Chromium, 0.03% to 2% Molybdenum, 0.01% to 1% Vanadium, less than 0.002% Boron, an element considered to be a harmful impurity, the rest being substantially iron; the composition must also satisfy the relationship: $$\text{BH = 326 + 847.3 (\% C) +18.3 (\% Si) -8.6 (\% Mn) -12.5 (\% Cr) ≤460}$$

Given this relationship, the Carbon content must remain below 0.238%.

This steel which certainly has good weldability and acceptable machinability, however has insufficient thermal conductivity.

In fact, a person skilled in the art always chooses an analysis located within the ranges indicated so as to obtain sufficient quenchability to be able to produce pieces of thickness which may exceed 400 mm; in particular the different elements can never be simultaneously at the bottom of the forks. Therefore all these steels have a thermal conductivity less than 35W / m / K and when, in some molds, it is necessary to have certain parts whose thermal conductivity is significantly higher, the corresponding parts are made of copper alloy / Aluminum / Iron with thermal conductivity greater than 40W / m / K. But this technique has the disadvantage of complicating the manufacture of molds since they are then composite objects, moreover the alloys used are much more expensive than steel.

The object of the invention is to provide a steel for the manufacture of molds for plastics or for rubber which, while having at least the same mechanical properties and the ability to machine known steels, has a higher thermal conductivity. at 40W / m / K to allow in particular to manufacture molds entirely of steel.

$$\text{1\% ≤ Mn ≤ 2,5\%}$$

$$\text{0,3\% ≤ Cr ≤ 2,5\%}$$

$$\text{0,1\% ≤ Mo + W/2 ≤ 0,8\%}$$

$$\text{0\% ≤ Ni ≤ 2,5\%}$$

$$\text{0\% ≤ V ≤ 0,3\%}$$

$$\text{Si ≤ 0,5\%}$$

$$\text{0,002\% ≤ B ≤ 0,005\%}$$

$$\text{0,005\% ≤ Al ≤ 0,1\%}$$

$$\text{0\% ≤ Ti ≤ 0,1\%}$$

$$\text{P ≤ 0,02\%}$$

To this end, the subject of the invention is a low-alloy steel intended for the manufacture of molds for plastics or for rubber, the chemical composition of which comprises by weight: $$\text{0.24\% ≤ C ≤ 0.35\%}$$

$$\text{1\% ≤ Mn ≤ 2.5\%}$$

$$\text{0.3\% ≤ Cr ≤ 2.5\%}$$

$$\text{0.1\% ≤ Mo + W / 2 ≤ 0.8\%}$$

$$\text{0\% ≤ Ni ≤ 2.5\%}$$

$$\text{0\% ≤ V ≤ 0.3\%}$$

$$\text{If ≤ 0.5\%}$$

$$\text{0.002\% ≤ B ≤ 0.005\%}$$

$$\text{0.005\% ≤ Al ≤ 0.1\%}$$

$$\text{0\% ≤ Ti ≤ 0.1\%}$$

$$\text{P ≤ 0.02\%}$$

et, $$\text{R=3,82(\%C)+9,79(\%Si)+3,34(\%Mn)+11,94(\%P)+2,39(\%Ni)+1,43(\%Cr)+1,43(\%Mo+\%W/2)<11,14}$$

This analysis satisfies, in addition, the following relationships: $$\text{U = 409 (\% C) +19.3 [\% Cr +\% Mo +\% W / 2 +\% V] +29.4 (\% Si) +10 (\% Mn) +7.2 (\% Ni) <200}$$

and, $$\text{R = 3.82 (\% C) +9.79 (\% Si) +3.34 (\% Mn) +11.94 (\% P) +2.39 (\% Ni) +1.43 (\% Cr) + 1.43 (\% Mo +\% W / 2) <11.14}$$

$$\text{1\% ≤ Mn ≤ 1,3\%}$$

$$\text{1\% ≤ Cr ≤ 1,5\%}$$

$$\text{0,3\% ≤ Mo+W/2 ≤ 0,4\%}$$

$$\text{0,03\% ≤ V ≤ 0,1\%}$$

Preferably the steel contains, $$\text{0.24\% ≤ C ≤ 0.28\%}$$

$$\text{1\% ≤ Mn ≤ 1.3\%}$$

$$\text{1\% ≤ Cr ≤ 1.5\%}$$

$$\text{0.3\% ≤ Mo + W / 2 ≤ 0.4\%}$$

$$\text{0.03\% ≤ V ≤ 0.1\%}$$

The steel, preferably, should contain less than 0.1% silicon.

Copper can also be added in order to obtain additional hardening during tempering, the steel must then contain from 0.8% to 2% of Nickel and from 0.5% to 2.5% of Copper.

The hardness can be improved by additions of Niobium, in contents lower than 0.1% and the machinability can be improved by additions of Sulfur, Tellurium, Selenium, Bismuth, Calcium, Antimony, Lead, Indium, Zirconium or Earths rare in contents lower than 0.1%.

The invention also relates to the use of a steel according to the invention for the manufacture by machining of quenched quenched steel blocks whose hardness is between 270HB and 350HB.

The invention will now be described with reference to FIG. 1 which represents a diagram for measuring machinability in drilling according to the Taylor method.

• plus de 0,24%C pour obtenir après trempe et revenu à plus de 500°C, une dureté supérieure à 270HB, et moins de 0,35%C pour ne pas trop détériorer la soudabilité et pour limiter l'importance des ségrégations défavorables à l'usinabilité, à la polissabilité et à la grainabilité ; de préférence, la teneur en Carbone doit être comprise entre 0,24% et 0,28%.
• plus de 1% de Manganèse pour augmenter la trempabilité de l'acier et moins de 2,5% et de préférence moins de 1,3% pour éviter de trop diminuer la conductibilité thermique de l'acier.
• plus de 0,3% de Chrome également pour augmenter la trempabilité et notamment éviter la formation de phases ferrito-perlitiques défavorables à la polissabilité et moins de 2,5% afin de ne pas détériorer la soudabilité et d'éviter la formation d'une quantité trop importante de carbures de Chrome défavorables notamment à l'usinabilité ; de préférence la teneur en Chrome doit être comprise entre 1% et 1,5% .
• plus de 0,1% et de préférence plus de 0,3% de Molybdène pour augmenter la trempabilité et pour ralentir l'adoucissement au revenu, mais moins de 0,8% et de préférence moins de 0,4% car, en trop grande quantité le Molybdène forme des carbures très durs défavorables à l'usinabilité, et il ségrège fortement en veines ce qui est défavorable à la polissabilité, à la grainabilité et peut également engendrer des ruptures d'outils au cours de l'usinage. Le Molybdène peut être remplacé totalement ou partiellement par du Tungstène à raison de 2% de Tungstène pour 1% de Molybdène, si bien que la teneur à prendre en compte est Mo +W/2.
• entre 0% et 0,3% et de préférence entre 0,03% et 0,1% de Vanadium afin de produire un durcissement secondaire au cours du revenu.
• entre 0,002% et 0,005% de Bore accompagné de 0,005% à 0,1% d'Aluminium et de 0% à 0,1% de Titane de façon à augmenter significativement la trempabilité sans détériorer les autres propriétés. L'aluminium et le Titane servent à éviter que le Bore ne se combine à l'Azote presque toujours en quantité telle qu'il faut protéger le Bore.

The steel according to the invention is a low-alloy steel containing mainly, by weight:

• more than 0.24% C to obtain, after quenching and tempering at more than 500 ° C, a hardness greater than 270HB, and less than 0.35% C so as not to deteriorate the weldability too much and to limit the extent of unfavorable segregations machinability, polishability and grainability; preferably, the carbon content must be between 0.24% and 0.28%.
• more than 1% of manganese to increase the hardenability of the steel and less than 2.5% and preferably less than 1.3% to avoid reducing the thermal conductivity of the steel too much.
• more than 0.3% of chromium also to increase the hardenability and in particular to avoid the formation of ferrito-pearlitic phases unfavorable for polishability and less than 2.5% in order not to deteriorate the weldability and to avoid the formation of a too large a quantity of chromium carbides unfavorable in particular to machinability; preferably the chromium content should be between 1% and 1.5%.
• more than 0.1% and preferably more than 0.3% of molybdenum to increase the hardenability and to slow the softening on tempering, but less than 0.8% and preferably less than 0.4% because, in excess large quantity Molybdenum forms very hard carbides unfavorable for machinability, and it segregates strongly in veins which is unfavorable for polishability, grainability and can also cause tool breaks during machining. Molybdenum can be completely or partially replaced by Tungsten at the rate of 2% of Tungsten for 1% of Molybdenum, so that the content to be taken into account is Mo + W / 2.
• between 0% and 0.3% and preferably between 0.03% and 0.1% of Vanadium in order to produce a secondary hardening during tempering.
• between 0.002% and 0.005% of Boron accompanied by 0.005% to 0.1% of Aluminum and from 0% to 0.1% of Titanium so as to significantly increase the quenchability without deteriorating the other properties. Aluminum and Titanium are used to prevent the Boron from combining with the Nitrogen almost always in such a quantity that it is necessary to protect the Boron.

• moins de 0,02% de Phosphore qui est une impureté fragilisante.

For these additions to be effective, when the nitrogen content is greater than 50 ppm the aluminum content must be greater than 0.05% when the titanium content is less than 0.005%; when the titanium content is greater than 0.015%, the aluminum content may be less than 0.03% and preferably be between 0.020% and 0.030%.

• less than 0.02% of Phosphorus which is an embrittling impurity.

In addition to these main elements of chemical composition, the steel contains or may contain elements such as Silicon, Copper, Nickel either as impurities or as elements of complementary alloy.

Steel, especially when made from scrap contains a little copper and nickel. When the Nickel is in small quantity, the Copper in too high contents creates defects during hot rolling or hot forging because it weakens the grain boundaries. In the absence of specific additions, the nickel and copper contents remain below 0.5% each

Up to 2.5% Nickel can be added to increase the hardenability.

Copper can also be added to produce a structural hardening effect. In this case, the copper content must be between 0.5% and 2% and be accompanied by a nickel content between 0.8% and 2.5%.

The hardness can also be adjusted by additions of Niobium in contents of less than 0.1%.

When the requirements of aptitude for polishing or for graining allow it, the machinability can be improved by adding Sulfur, Tellurium, Selenium, Bismuth, Calcium, Antimony, Lead, Indium, Zirconium or Rare earths in contents lower than 0 , 1%.

l'usinabilité est très sensiblement meilleure que pour les aciers de type P20.The inventors have found that, in this area of chemical composition, when: $$\text{U = 409 (\% C) +19.3 [\% Cr + (\% Mo +\% W / 2) +\% V] +29.4 (\% Si) +10 (\% Mn) +7.2 (\% Ni) <200}$$

the machinability is very significantly better than for P20 type steels.

Finally, for the thermal conductivity to be sufficient, it is necessary that: $$\text{R = 3.82 (\% C) +9.79 (\% Si) +3.34 (\% Mn) +11.94 (\% P) +2.39 (\% Ni) +1.43 (\% Cr) + 1.43 (\% Mo +\% W / 2) <11.14}$$

Also, the chemical composition must be chosen so that U <200 and R <25. The thermal conductivity is then greater than 40W / m / K

To make a mold, a steel is produced according to the invention, optionally by pre-oxidation with silicon, then deoxidation with aluminum, then titanium and boron are added.

The liquid metal thus obtained is poured in the form of a semi-finished product such as an ingot, a slab or a billet.

The semi-finished product is then reheated to a temperature preferably below 1300 ° C. and either forged or rolled to obtain a bar or a sheet.

The bar or the sheet is then quenched to obtain a martensitic or martensito-bainitic structure in all its mass.

The quenching can be done either directly in the hot rolling or forging if the end of rolling or end of forging temperature is less than 1000 ° C, or after austenitization at a temperature above the Ac₃ point and preferably less than 1000 ° C.

After quenching in air, oil or water depending on the dimensions, the bars or sheets are subjected to tempering above 500 ° C and preferably above 550 ° C so as to obtain a hardness between 270HB and 350HB, and preferably close to 300HB, at all points of the bars or sheets and so that the internal stresses generated by the quenching are relaxed.

Then blocks of the desired size are cut which are machined so as in particular to form the imprint of the object which it is desired to obtain by molding.

The surface of the impression can then be subjected to a surface treatment such as polishing or embossing to give it the desired surface appearance and possibly be nitrided or chromed.

$$\text{Si = 0,25\%}$$

$$\text{Mn = 1,1\%}$$

$$\text{Cr = 1,3\%}$$

$$\text{Mo = 0,35\%}$$

$$\text{Ni = 0,25\%}$$

$$\text{V = 0,04\%}$$

$$\text{Cu = 0,3 \%}$$

$$\text{B = 0,0027\%}$$

$$\text{Al = 0,025\%}$$

$$\text{Ti = 0,020\%}$$

$$\text{S = 0,001\%}$$

$$\text{P = 0,010\%}$$

For example, molds were made with steel A of composition: (% by weight) $$\text{C = 0.25\%}$$

$$\text{If = 0.25\%}$$

$$\text{Mn = 1.1\%}$$

$$\text{Cr = 1.3\%}$$

$$\text{Mo = 0.35\%}$$

$$\text{Ni = 0.25\%}$$

$$\text{V = 0.04\%}$$

$$\text{Cu = 0.3\%}$$

$$\text{B = 0.0027\%}$$

$$\text{Al = 0.025\%}$$

$$\text{Ti = 0.020\%}$$

$$\text{S = 0.001\%}$$

$$\text{P = 0.010\%}$$

400 mm thick blocks were produced, austenitized at 900 ° C. for 1 hour, soaked in water and then returned to 550 ° C. for 1 hour and cooled in air. There was thus obtained a martensito-bainitic structure of hardness between 300HB and 318HB at all points of the product. The elastic limit Re is 883MPa and the breaking strength Rm is 970MPa, ie a Re / Rm ratio close to 0.91; the resilience KCV at + 20 ° C is around 60J / cm.

est : $$\text{Ceq = 0,808}$$

l'indice BH est : $$\text{BH = 508}$$

l'indice d'usinabilité est : $$\text{U = 151}$$

la conductibilité thermique est : $${\text{λ= 41Wm}}^{\text{-1}}{\text{K}}^{\text{-1}}$$

The equivalent carbon of this steel calculated according to the IIW formula. $$\text{Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15}$$

East : $$\text{Ceq = 0.808}$$

the BH index is: $$\text{BH = 508}$$

the machinability index is: $$\text{U = 151}$$

the thermal conductivity is: $${\text{λ = 41Wm}}^{\text{-1}}{\text{K}}^{\text{-1}}$$

$$\text{Si = 0,45\%}$$

$$\text{Mn = 0,95\%}$$

$$\text{Cr = 1,85\%}$$

$$\text{Ni = 0,3\%}$$

$$\text{Mo = 0,38\%}$$

après austénitisation à 900°C, trempe à l'eau et revenu à 580°C pendant 1 heure, la dureté était comparable et centrée autour de 300HB. La limite d'élasticité Re était de 825 MPa et la résistance à la rupture Rm de 1010 MPa soit un rapport Re/Rm voisin de 0,82. La résilience KCV à +20°C était de l'ordre de 20J/cm.By way of comparison, a block of the same dimension produced in a P20 type steel of composition, $$\text{C = 0.34\%}$$

$$\text{If = 0.45\%}$$

$$\text{Mn = 0.95\%}$$

$$\text{Cr = 1.85\%}$$

$$\text{Ni = 0.3\%}$$

$$\text{Mo = 0.38\%}$$

after austenitization at 900 ° C, quenching with water and returning to 580 ° C for 1 hour, the hardness was comparable and centered around 300HB. The elastic limit Re was 825 MPa and the breaking strength Rm of 1010 MPa, ie a Re / Rm ratio close to 0.82. The KCV resilience at + 20 ° C was of the order of 20 J / cm.

The equivalent Carbon was: $$\text{Ceq = 0.964}$$

The BH coefficient: $$\text{BH = 591}$$

The machinability index: $$\text{U = 207}$$

Thermal conductivity: $${\text{λ = 35Wm}}^{\text{-1}}{\text{K}}^{\text{-1}}$$

The difference in machinability index U results in a difference in machinability as shown in fig. 1 which represents Taylor lines in drilling for steel A and for steel P20 taken example. It can be seen in this figure that at equal cutting speed, the length that can be drilled in steel A is approximately 10 times greater than in steel P20, or, that with equal drilled length, the speed of permissible cut is 25% greater in steel A than in steel P20.

The weldability being all the better when the equivalent carbon or the BH coefficient is low, it is found that the steel according to the invention has better weldability than the P20 steel.

It is found that steel A has a thermal conductivity 17% higher than that of steel P20, moreover it has an elastic limit and a resilience markedly higher than that of steel P20.

$$\text{Si = 0,09\%}$$

$$\text{Mn = 2,15\%}$$

$$\text{Cr = 1,45\%}$$

$$\text{Mo = 1,08\%}$$

$$\text{V = 0,55\%}$$

$$\text{B = 0,0007\%}$$

Also for comparison, a block of comparable size was made of steel of composition: $$\text{C = 0.17\%}$$

$$\text{If = 0.09\%}$$

$$\text{Mn = 2.15\%}$$

$$\text{Cr = 1.45\%}$$

$$\text{Mo = 1.08\%}$$

$$\text{V = 0.55\%}$$

$$\text{B = 0.0007\%}$$

After austenitization at 900 ° C, quenching with water, and returned to 570 ° C, the block had a hardness close to 300HB throughout the mass and:

The equivalent Carbon was: $$\text{Ceq = 1.144}$$

The BH coefficient was: $$\text{BH = 435}$$

Machinability index U $$\text{U = 153}$$

Thermal conductivity: $${\text{λ = 35Wm}}^{\text{-1}}{\text{K}}^{\text{-1}}$$

This steel has a BH index better than that of steel A but it has a worse equivalent Carbon. Its machinability index is comparable to that of steel A but its thermal conductivity is 15% lower.

Blocks of 400mm thick made of steel B according to the invention were also made, austenitized at 920 ° C., quenched with water and returned to 560 ° C. then cooled in air. The hardness at all points was between 300HB and 315HB. The elastic limit Re was 878MPa, and the breaking strength Rm was 969MPa, ie a Re / Rm ratio of 0.91.

$$\text{Si = 0,1\%}$$

$$\text{Mn = 1,3 \%}$$

$$\text{Cr = 1,3\%}$$

$$\text{Mo = 0,4\%}$$

$$\text{V = 0,01\%}$$

$$\text{B = 0,0025\%}$$

$$\text{Al = 0,055\%}$$

$$\text{S = 0,002\%}$$

$$\text{P = 0,015\%}$$

$$\text{Ni = 0,8\%}$$

$$\text{Cu = 0,35\%}$$

The composition of the steel was: $$\text{C = 0.25\%}$$

$$\text{If = 0.1\%}$$

$$\text{Mn = 1.3\%}$$

$$\text{Cr = 1.3\%}$$

$$\text{Mo = 0.4\%}$$

$$\text{V = 0.01\%}$$

$$\text{B = 0.0025\%}$$

$$\text{Al = 0.055\%}$$

$$\text{S = 0.002\%}$$

$$\text{P = 0.015\%}$$

$$\text{Ni = 0.8\%}$$

$$\text{Cu = 0.35\%}$$

The equivalent carbon was: $$\text{Ceq = 0.83}$$

The BH coefficient was: $$\text{BH = 512}$$

The machinability index was: $$\text{U = 153}$$

Thermal conductivity: $${\text{λ = 44Wm}}^{\text{-1}}{\text{K}}^{\text{-1}}$$

This steel, whose analysis differs from that of steel A mainly by the silicon and nickel content, has the same advantages as steel A and moreover, it has a much better thermal conductivity.

Claims (11)

Low alloy steel whose chemical composition includes, by weight: $$\text{0.24\% ≤ C ≤ 0.35\%}$$

$$\text{1\% ≤ Mn ≤ 2.5\%}$$

$$\text{0.3\% ≤ Cr ≤ 2.5\%}$$

$$\text{0.1\% ≤ Mo + W / 2 ≤ 0.8\%}$$

$$\text{0.1\% ≤ W / 2 ≤ 0.8\%}$$

$$\text{Ni ≤ 2.5\%}$$

$$\text{0\% ≤ V ≤ 0.3\%}$$

$$\text{If ≤ 0.5\%}$$

$$\text{0.002\% ≤ B ≤ 0.005\%}$$

$$\text{0.005\% ≤ Al ≤ 0.1\%}$$

$$\text{0\% ≤ Ti ≤ 0.1\%}$$

$$\text{P ≤ 0.02\%}$$

$$\text{Cu ≤ 2\%}$$

optionally, at least one element chosen from Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and Rare earths, in contents of less than 0.1%, the rest being iron and bound impurities in the development,
the chemical composition satisfying, moreover, the relationships: $$\text{U = 409 (\% C) +19.3 [\% Cr + (\% Mo +\% W / 2) +\% V] +29.4 (\% Si) +10 (\% Mn) +7.2 (\% Ni) <200}$$

and, $$\text{R = 3.82 (\% C) +9.79 (\% Si) +3.34 (\% Mn) +11.94 (\% P) +2.39 (\% Ni) +1.43 (\% Cr) + 1.43 (\% Mo +\% W / 2) <11.14}$$

Low alloy steel according to Claim 1, characterized in that the chemical composition of the low alloy steel comprises, by weight: $$\text{0.24\% ≤ C ≤ 0.28\%}$$

$$\text{1\% ≤ Mn ≤ 1.3\%}$$

$$\text{1\% ≤ Cr ≤ 1.5\%}$$

$$\text{0.3\% ≤ Mo + W / 2 ≤ 0.4\%}$$

$$\text{0.03\% ≤ V ≤ 0.1\%}$$

Low alloy steel according to claim 1 or claim 2 characterized in that Si ≤ 0.1% Low alloy steel according to any one of Claims 1 to 3, characterized in that: $$\text{0.5\% ≤ Ni ≤ 2.5\%}$$

$$\text{0.5\% ≤ Cu ≤ 2\%}$$

Use for the manufacture of a mold for plastics or for rubber by machining at least one block of tempered tempered steel, of low-alloy steel whose chemical composition comprises, by weight: $$\text{0.24\% ≤ C ≤ 0.35\%}$$

$$\text{1\% ≤ Mn ≤ 2.5\%}$$

$$\text{0.3\% ≤ Cr ≤ 2.5\%}$$

$$\text{0.1\% ≤ Mo + W / 2 ≤ 0.8\%}$$

$$\text{Ni ≤ 2.5\%}$$

$$\text{0\% ≤ V ≤ 0.3\%}$$

$$\text{If ≤ 0.5\%}$$

$$\text{0.002\% ≤ B ≤ 0.005\%}$$

$$\text{0.005\% ≤ Al ≤ 0.1\%}$$

$$\text{0\% ≤ Ti ≤ 0.1\%}$$

$$\text{P ≤ 0.02\%}$$

$$\text{Cu ≤ 2\%}$$

optionally, at least one element chosen from Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and Rare earths, in contents of less than 0.1%, the rest being iron and bound impurities in the development,
the chemical composition satisfying, moreover, the relationships: $$\text{U = 409 (\% C) +19.3 [\% Cr + (\% Mo +\% W / 2) +\% V] +29.4 (\% Si) +10 (\% Mn) +7.2 (\% Ni) <200}$$

and, $$\text{R = 3.82 (\% C) +9.79 (\% Si) +3.34 (\% Mn) +11.94 (\% P) +2.39 (\% Ni) +1.43 (\% Cr) + 1.43 (\% Mo +\% W / 2) <11.14}$$

Use of a steel according to claim 5 for the manufacture of a mold for plastics or for rubber, by machining at least one block of steel, quenched tempered, of hardness between 270HB and 350HB. Low alloy steel whose chemical composition includes, by weight: $$\text{0.24\% ≤ C ≤ 0.28\%}$$

$$\text{1\% ≤ Mn ≤ 1.3\%}$$

$$\text{0.3\% ≤ Cr ≤ 1.5\%}$$

$$\text{0.3\% ≤ Mo + W / 2 ≤ 0.4\%}$$

$$\text{Ni ≤ 2.5\%}$$

$$\text{0\% ≤ V ≤ 0.3\%}$$

$$\text{If ≤ 0.5\%}$$

$$\text{0.002\% ≤ B ≤ 0.005\%}$$

$$\text{0.005\% ≤ Al ≤ 0.1\%}$$

$$\text{0\% ≤ Ti ≤ 0.1\%}$$

$$\text{P ≤ 0.02\%}$$

$$\text{Cu ≤ 2\%}$$

optionally, at least one element chosen from Nb, Zr, S, Se, Te, Bi, Ca, Sb, Pb, In and Rare earths, in contents of less than 0.1%, the rest being iron and bound impurities in the development,
the chemical composition satisfying, moreover, the relationships: $$\text{U = 409 (\% C) +19.3 [\% Cr + (\% Mo +\% W / 2) +\% V] +29.4 (\% Si) +10 (\% Mn) +7.2 (\% Ni) <200}$$

and, $$\text{R = 3.82 (\% C) +9.79 (\% Si) +3.34 (\% Mn) +11.94 (\% P) +2.39 (\% Ni) +1.43 (\% Cr) + 1.43 (\% Mo +\% W / 2) <11.14}$$

Low alloy steel according to Claim 7, characterized in that the chemical composition of the low alloy steel comprises, by weight: Si ≤ 0.1% Low-alloy steel according to either of Claims 7 and 8, characterized in that: $$\text{0.5\% ≤ Ni ≤ 2.5\%}$$

$$\text{0.5\% ≤ Cu ≤ 2\%}$$

Use of a steel according to any one of Claims 7 to 9 for the manufacture of a mold for plastics or for rubber, characterized in that the mold is produced by machining at least one block of steel, tempered income. Use of a steel according to claim 10 for the manufacture of a mold for plastics or for rubber, by machining at least one block of steel, quenched tempered, and of hardness between 270HB and 350HB.

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