BRPI0620491A2 - hot tool steel, one-piece steelmaking process, one-piece steel and one-part uses - Google Patents

Abstract

STEEL FOR HOT TOOL, PROCESS OF MANUFACTURING A PIECE OF STEEL, PIECE OF STEEL AND USES OF A PIECE. The present invention relates to steel for hot tooling, composed by weight percentages: - 0.30% <243> C <243> 0.39%, 4.00% <243> Cr <243> 6.00 %, strokes <243> Si <243> 0.50%, strokes <243> Mn <243> 0.80%, strokes <243> W <243> 1.45% - strokes <243> Co <243> 2 , 75%, 0.80% <243> Ni <243> 2.80%, 1.50% <243> Mo <243> 2.60% with 1.50% <243> Mo + 0.65W <243 > 3.20%, 0.55% <243> V <243> 0.80%, with -0.65 <243> K <243> 0.65, where K = K2 - K1 and K2 = 0, 75 x (Ni - 0.60), K1 = 1.43 x (V - 0.40) + 0.63 x [(Mo + 0.65W) -1.20], dashes <243> AI <243> 0.080%, <243> S <243> 0.0040%, <243> P <243> 0.0200%, <243> Ti <243> 0.05%, <243> Zr <243> 0.05%, strokes <243> Nb <243> 0.08%, strokes <243> N <243> 0.040%, 10 P + As + 5 Sb + 4 Sn <243> 0.21%, strokes <243 > The <243> 30ppm, the rest being iron and unavoidable impurities. The present invention also refers to the part produced from this steel, its manufacturing process and the use of these parts.

Description

"HOT TOOL STEEL, STEEL PIECE MANUFACTURING PROCESS, STEEL PIECE AND ONE PIECE USES"

Field of the Invention

The present invention relates to the field of hot mold tooling steels usable in casting and molding, forging, spinning or extrusion.

Background of the Invention

A preferred but not exclusive field of application of the present invention is the manufacture of large molds for the die casting of light aluminum or magnesium based alloys or cuprous alloys.

During use, hot molding tools are subjected to cylindrical stresses that damage them.

These requests come from:

- mechanical due to the direct efforts applied by machines such as presses;

- thermal: sudden changes in temperature due to alternating contacts with the hot matter to be transformed, and cooling by projections of lubricants or coatings, which cause swelling gradients, which are sources of local mechanical stresses.

Damage is sometimes caused by sudden breakages that instantly destroy tooling when the toughness of the material is not sufficient. It is usually produced by cracking that begins during the first hundreds of cycles of use and progressively develops to effective ruin after a few tens or hundreds of thousands of cycles. This process is called the generic term "thermal fatigue". Resistance to thermal fatigue damage requires sufficient toughness at the temperature corresponding to the coldest point of the thermal cycle. This quality is conventionally measured by the impact bending energy of standard specimens, specimens tested at ambient temperatures to 150 ° C. It also requires sufficient hardness and break-in properties during operation at higher cycle temperatures.

The manufacture of large size molds or tooling (for example, thickness greater than 200 mm) requires improved properties of the steel forming it. During quenching, as the cooling rate is naturally moderated by surface-limited thermal fluxes and the industrialist's concern not to deform or rupture parts, reference steels do not generate martensitically dominant quench structures that would be favorable for use properties. great. Continuous quenching (TRC) diagrams describe for each composition the nature of the phases formed at the cooling rates, but are notoriously insufficient to account for the temper-tempered tempera- ture loss caused by the quenching speed reduction.

Among the known steels for this use, we can mention:

- AISI H11 steel containing approximately C = 0.40%, Si = 0.90%, Mn = 0.40%, Cr = 5%, Mo = 1.30%, V = 0.5%;

- AISI H13 steel identical to the above except that it contains V = 0,95%;

- steel W - 1.2367 which contains approximately C = 0.40%, Si = 0.30%, Mn = 0.40%, Cr = 5%, Mo = 2.9%, V = 0.65%; - a steel that is comparable to AISI H11 but contains Si = 0.3% and accepts Ni = 0.2% (see EP-B1-0 663 018); Its nominal composition is C = 0.3-0.4%, Si <0.8%, Mn <0.8%, Cr = 4.5-5.8%, Mo = 0.75-1.75% , V <1,3%, W <1,5%, Ni ≤ 0,5%, P <0,008%, Sb ≤ 0,002%, Sn <0,003%, As <0,005%, with 10P + 5Sb + 4Sn + As <0.10%.

In order to improve the properties of these known steels in order to allow a better compromise between hardness, toughness and stability of working properties, in particular hardness, studies have been carried out. This could increase the hot strength against H11 steel by increasing the Mo and V contents as in H13 and W-1.2367 above, but this results in a degradation of toughness. On the other hand, toughness is increased when Si content is increased or Ni is added which also improves its temperability. But Ni degrades the hardness and the hot elastic limit.

Description of the Invention

The purpose of the present invention is to propose a new specification of hot mold tooling steel that achieves an excellent compromise between the various properties just mentioned.

To this end, the present invention relates to a hot tool steel of weight percent composition:

- 0.30% ≤ C ≤ 0.39%

- 4.00% ≤ Cr ≤ 6.00%

- dashes ≤ Si ≤ 0,50%

- dashes ≤ Mn ≤ 0,80%

- dashes ≤ W ≤ 1,45%

- dashes ≤ Co ≤ 2,75%

- 0.80% ≤ Ni ≤ 2.80% - 1.50% <Mo <2.60% with 1.50% <Mo + 0.65W <3.20%

- 0.55% <V <0.80%

- with -0.65 <K <0.65 where K = K2 - K1

and K2 = 0.75 χ (Ni - 0.60)

K1 = 1.43 χ (V - 0.40) + 0.63 χ [(Mo + 0.65W) - 1.20]

- traits <Al <0.080%

- dashes <S <0.0040%

- dashes <P <0.0200%

- dashes <Ti <0.05%

- dashes <Zr <0.05%

- traits <Nb <0.08%

- dashes <N <0.040%

- 10 P + As + 5 Sb + 4 Sn <0.21%

- traces ^ O <30ppm,

the rest being iron and unavoidable impurities. Preferably 0.33% <C <0.38%. Preferably dashes <Si £ 0.40%. Preferably, dashes <Mn <0.60%. Preferably 4.6% <Cr <6.0%.

Preferably 1.60% <Mo <2.00% and 1.60% <Mo + 0.65 W

Preferably, dashes <Al <0.030%. Preferably, dashes <S are 0.0010%. Preferably, dashes <P <0.0080%. Preferably, dashes <Ti <0.01%. Preferably, dash Zr <0.02%. Preferably, dashes <Nb <0.01%. Preferably, dashes ≤ N ≤ 0.01%. Preferably 10 P + As + 5 Sb + 4 Sn ≤ 0.10%. Preferably dashes ≤ O ≤ 15ppm. Preferably, - 0.35 ≤ K ≤ 0.35. Preferably:

- 0.335% ≤ C ≤ 0.375%

- 1.50% ≤ Ni ≤ 2.10%

-1.60% ≤ Mo + 0.65 W ≤ 2.20% with 1.60% ≤ Mo ≤ 2.00%

- 0,62% ≤ V ≤ 0,75%.

Preferably:

- 0.335% ≤ C ≤ 0.375%

- 2.00% ≤ Ni ≤ 2.40%

- 1.80% ≤ Mo + 0.65W ≤ 2.90% with 1.80% ≤ Mo ≤ 2.40% and W ≤ 0.90%

- 0,66% ≤ V ≤ 0,76%.

Preferably:

- 0.335% ≤ C ≤ 0.375%

- 0,90% ≤ Ni ≤ 1,50%

-1.50% ≤ Mo + 0.6 W ≤ 1.90% with W ≤ 0.40%

- 0,55% ≤ V ≤ 0,63%.

Preferably: 0.335% ≤ C ≤ 0.375%, 4.60% ≤ Cr ≤ 6.00%, dashes ≤ Si ≤ 0.40%, dashes ≤ Mn ≤, 60%, dashes ≤ W ≤ 1, 45%, dashes ≤ Co ≤ 2.75%, 1.50% ≤ Ni ≤ 2.10%, 1.60% ≤ 6 002607 Mo + 0.65 W ≤ 2.20% with 1.60% ≤ Mo ≤ 2.00% 0.62% ≤ V ≤ 0.75%, with -0.35 ≤ K ≤ 0.35, dashes ≤ Al ≤ 0.030%, dashes ≤ S ≤ 0.0010%, dashes ≤ P ≤ 0.0080%, dashes ≤ Ti ≤ 0.011%, dashes ≤ Zr ≤ 0.02%, dashes ≤ Nb ≤ 0.01%, dashes ≤ N ≤ 0.01%, dashes ≤ O ≤ 15ppm.

The present invention also relates to a process of fabricating a steel part, characterized in that said steel part of the above type is prepared and subjected to an austenization in the temperature range 1000-1050 ° C, followed by of a temper.

Preferably, austenization occurs in the range 1015-1040 ° C.

Preferably, after tempering, the part is subjected to at least two tempering in the temperature range 550-650 ° C, giving the part a hardness of 42 to 52HRC.

The present invention also relates to a steel part obtained by the above process, characterized in that it is a hot molding tooling part.

Said part may have a thickness greater than or equal to 200 mm.

It may be a mold or die for pressure function of light or cuprous alloys.

Said part may be a forging tool.

Said piece may be a forge die.

Said part may be a steel pipe drilling or rolling tool.

Said part may be a glass shaping instrument.

Said part may be a plastics molding tool.

Said part may be made of a steel in which 0.335% = C = 0.375%, 2.00% = Ni = 2.40%, 1.80% = Mo + 0.65W = 2.90% with 1.80% = Mo = 3.40% and W = 0.90%, 0.66% = V = 0.76%, and when it comes to an aluminum alloy extrusion die or die casting.

The present invention also relates to the use of a hot tooling part, characterized in that said part is of a steel in which 0.335% ≤ C ≤ 0.375%, 2.00% ≤ Ni ≤ 2.40%, 1 80%

≤ Mo + 0.65W ≤ 2.90% with 1.80% ≤ 6 Mo ≤ 3.40% and W ≤ 0.90%, 0.66%

≤ V ≤ 0.76% and its surface temperature is below 680 ° C.

The present invention also relates to the use of a hot tooling part, characterized in that said part is of a steel in which 0.335% ≤ C ≤ 0.375%, 0.90% ≤ Ni ≤1.50%, 1 , 50% ≤ Mo + 0.6 W ≤ 1.90% with W ≤ 0.40%, 0.55% ≤ V ≤ 0.63% and its operating surface temperature is below 770 ° C.

As can be seen, with respect to the aforementioned known steels, in particular that described in EP-B1-0 663 018, the present invention relies in particular on a simultaneous adaptation of the hardening and stabilizing elements which are Mo and V, and that neutralizes its weakening effects. Coupling the assembly produces improved temperability, so improves the ability to reproduce on large parts the properties that were previously only accessible to smaller tools.

The optimization of the steel composition according to the present invention was possible because the inventors first concerned themselves with effectively measuring the instantaneous heat fluxes that pass through the surface of the hot molding tool during its use. They then deduced from the calculation the transient mechanical stresses induced by the thermal impacts that develop the cracks. This allowed a better understanding of the mechanical behavior of the material in use. They were able to establish, thanks to experimental measures that reconstitute industrial quenching speeds in test samples, and thanks to thermodynamic simulations, the links between the steel composition, the heat treatment parameters prior to its use and the microstructure thus obtained. In particular, they could highlight the importance of the interdependence between composition and tempering temperature in order to obtain the desired compromise between the various important mechanical properties in hot tool steels.

The present invention will be better understood with the following description given in relation to the accompanying drawings:

- Figure 1 shows the evolution of the undissolved carbide fraction according to the authorization temperature for

(fig. 1a) to (1e)) for a composition according to the present invention (fig. (1b)),

Figure 2 showing the TRC curves of a reference steel (Fig. 2a) and a steel according to the present invention (Fig. 2b).

Fig. 3 showing the comparison, for the different reference samples and the samples according to the present invention, between the rupture energies after a tempering performed under laboratory conditions and a tempering performed under industrial conditions.

The tests to which reference will be made in the following description were performed on samples whose compositions are given in Table 1. In this table, the coefficients K2, K1 and K correspond to the following quantities in which the contents are expressed as weight percentages:

K2 = 0.75 χ (Ni - 0.60)

K1 = 1.43 χ (V - 0.40) + 0.63 χ [(Mo + 0.65W) - 1.20]

K = K2 - K1 <table> table see original document page 10 </column> </row> <table> <table> table See original document page 11 </column> </row> <table> The present invention essentially rests in the study of the actions of the elements Carbon, Chromium, Molybdenum, Vanadium and Nickel and the influence of the pre-tempering austenization temperature on the mechanical properties of the studied steels.

Influence of Austenization Temperature

The austenization temperature governs the sharing of the alloying elements between the undissolved carbides and the matrix. Carbide dissolution increases as the temperature rises.

Undissolved carbides must remain in adequate quantity in the final product to control grain size. A fine grain is required to ensure toughness and fatigue resistance properties.

The alloying elements dissolved in the matrix govern the temperability, tempering resistance and generally all mechanical properties.

Table 2 illustrates, for one of the compositions studied (reference casting 10), the effect of quench temperature on the microstructure and properties.

<table> table see original document page 12 </column> </row> <table> Table 2 - Experimental Casting Reference 10 Effect of Austenization Temperature on Microstructure (Distribution of C ε V Elements) as Mechanical Properties

In a context in which vanadium carbides dissolve well progressively, the increase in austenization temperature in this case causes at the same time an improvement in the heat softening resistance and a loss of toughness.

It should be noted that the definition of an optimal material for the considered applications must imperatively associate the composition and the austenization conditions. The thermodynamic simulation, by describing the phase balances with the Thermocalc® calculation code currently used by metallurgists, gives concrete information about the undissolved carbide rate for each type VC, M23C6 and possibly MeC1 Fe3C, M2C . Figure 1 was performed using this simulation. It shows the evolution of the undissolved carbide fraction according to the austenization temperature for five reference compositions (Fig. (1a) to (1e)) and a composition according to the present invention (Fig. (1f)).

The competition between the Mo and V elements to fix carbon according to their preferred carbide types is well known. Eventually added nickel has only a second-order effect on these mechanisms.

Experimental microstructural observations on the raw quenching state confirm the trends indicated by the simulation. Austenization temperatures are optimized according to the following principles:

- At an appropriate temperature, M6C and M23C6 carbides, which are not effective for grain size control, must be dissolved so that the released Meo carbon metal elements give the matrix maximum potential for temperability.

- A minimum percentage of 0.20% molar fraction in undissolved vanadium carbides according to the thermodynamic estimate is necessary to ensure grain homogeneity and fineness; the austenization temperature must remain below the corresponding threshold.

- the reference temperature must take into account a tolerance of about 10 to 15 degrees from that reference which corresponds to the usual temperature dispersion in the load of industrial batches.

The austenization temperatures of the various compositions thus defined are recapitulated in Table 3.

<table> table see original document page 14 </column> </row> <table>

Table 3: Definition of Ideal Austenization Temperatures for the Various Experimented Foundries

Definition of Optimized Conditions ε Measurement of Key Properties

As has been said, an essential challenge of the present invention is to strike a balance between:

- On the one hand, the elements Molybdenum, Vanadium and optionally Tungsten, which are favorable to hardening and softening resistance in operation, but which are brittle,

- On the other hand, Nickel, which is favorable to toughness but harmful to hot hardness.

Knowing that the field steels of the present invention must have sufficient hot hardness to prevent sinking and resist fatigue, and that at first glance they show the same relationship between hardness at 20 ° C and hot hardness. were compared in temperate and tempering thermal states that give them the same hardness at 20 ° C. The levels shown are 47, 45, 42 HRC.

In accordance with an original and innovative procedure, the measurements were carried out systematically and in parallel at the same time on bars used as high-temperature tempered laboratory specimens and on tempered specimens in an experimental device that reconstituted the specimen. quenching speed representative of the treatment of industrial parts and chosen at 22 ° C per minute on average in the 900/400 ° C range.

These measures include:

- a description of the hardness evolution according to tempering temperature to a 2 hour double reversal to define the tempering to be applied to achieve the desired hardnesses;

the break-in resistance measured by the hardness loss caused by an 80 hour maintenance at 560 ° C from an initial hardness state of 47 HRC;

- the toughness measured by the impact bending energy on Charpy V specimens, broken at staggered temperatures between +20 and 200 ° C.

Replenishment Point Ac1

In operation, this point should not be exceeded as the resulting structural changes in the workpiece material would result in a noticeable change in mechanical properties.

According to table 4 which gathers the most representative results obtained in several samples, it is confirmed that the results Mo and V do not have a clear influence; In contrast, the Ac1 point decreases the higher the nickel content. Consequently, high nickel compositions should be avoided for applications where the working surface temperature is very high (for certain forging tools), but remain compatible with multiple applications such as light alloy die castings which are subjected to more moderate surface temperatures.

Table 4

Evolution of Re-Austenization Point Ac1 according to Composition

<table> table see original document page 16 </column> </row> <table>

Table 4: Evolution of Re-Austenization Point Ac1 by Composition.

Ε Tempering Resistance to Running-In Softening

Table 5 illustrates the effect of alloying elements on hardness lowering resistance during high temperature maintenance.

Hardnesses of 47 and 42 HRC are obtained after two tempering two hours each, the first at 550 ° C, the second having the characteristic temperature shown in the table.

Hardness loss is measured from an initial state of 47 HRC.

Table 5A presents the results obtained in a reference sample 1 and two samples 12, 13 which have a higher nickel content than hers. Table 5B presents the results obtained in sample 1 and samples 3, 5, 6, 8 which present Mo contents and possibly V higher than hers. Table 5C presents the results obtained in samples 8 and 22, on one side, 6 and 26 on the other, which have higher N, Mo and V contents than sample 1.

Table 5

<table> table see original document page 17 </column> </row> <table> <table> table see original document page 18 </column> </row> <table>

Table 5-A highlights the detrimental effect of a simple addition of nickel that slowly lowers the tempering temperature for a duration objective and increases hardness loss in case of prolonged hot maintenance. A lowering of tempering temperature is detrimental because steel must offer the highest possible operating temperature, at least between 600 and 630 ° C, otherwise you will soften it excessively.

Since surface temperatures are often close to 520-560 ° C in aluminum injection and even higher during forging, this criterion will be important to consider in determining whether or not the given composition is fit for a given application. .

Table 5-B shows the beneficial effect of simple addition of molybdenum and vanadium to increase tempering and break-in resistance in operation. In contrast, the lowering of quenching speed between laboratory conditions and industrial conditions is detrimental to these characteristics, which is due to insufficient material temperability.

The comparison of composition pairs (8, 22) and (6.26) in Table 5-C illustrates that under laboratory conditions than under laboratory conditions, nickel foundries offer lower hardness-lowering resistance than low-grade foundries. corresponding nickel content, but with an industrial temper, its properties become very close.

In summary, under conditions of an industrial heat treatment, the combined and balanced addition of Nickel, Molybdenum and Vanadium provides long-lasting tempering and softening resistance properties equivalent to the nickel-free specifications.

These favorable results are explained by the significant gain in temperability illustrated in the accompanying figure 2 comparing the TRC continuous cooling diagrams of reference composition 1 (fig.2a) which was subjected to austenization at 990 ° C for 30 minutes and of composition 22 according to the present invention (fig.2b) which was subjected to austenization at 1030 ° C for 30 minutes.

The composition according to the present invention has clearly lagged perlite and bainitic zones near the low cooling rates relative to the reference composition. Consequently, knowing that the usual industrial temperings (whose paths are in bold in Figures 2a and 2b) allow to reach, in the tools to be treated, a temperature of 400 ° C in 1000 to 5000 seconds depending on the part sizes and the In the workpiece situation, the composition according to the present invention allows a unique martensitic transformation. In contrast, the reference composition imposes the formation of a significant proportion of bainite, which may be favorable for obtaining the desired properties.

Tenacity

The unfavorable effect of decreasing quenching speed between laboratory and industrial conditions is even more pronounced on the breaking energy of Charpy V impact bending specimens.

Table VI illustrates representative trends in a selection of results; The combined addition of Ni, Mo, V practiced in foundry 21 according to the present invention is favorable to obtain at the same time the highest resilience values after treatment under industrial conditions and the lower deduction caused by the decrease in tempering speed.

Table 6

<table> table see original document page 20 </column> </row> <table>

A: fast quenching speed (bar oil quenching) L: slow quenching speed (laboratory reconstituted industrial speed).

The attached figure 3 compares, for all foundries, the values obtained with a quench according to the industrial speed and those obtained from a quick quench for the same metal composition, and the pairs of specimens are subjected. after tempering to reach hardnesses of 42, 45 and 47 HRC and the specimens ruptured at 20 ° C and 100 ° C. Each point is representative of a hardness and a rupture temperature of the specimen. The results demonstrate that the hardness loss due to quenching speed is much more generally limited for the compositions according to the present invention.

The trends expressed for laboratory testing are confirmed by testing with tool blocks treated under the following conditions:

- Dimension blocks 570 χ 450 χ 228 mm

- identical oven positioning

- Quench in the same industrial oven under 500 kPa (5 bars) gas pressure, with same gas flow

- Double tempering with individual temperature adjustment to achieve hardness level of 46 ± 0.5 HRC.

- Removal of C harpy V impact bending specimens in transverse direction: in the center of the largest face near the surface and core of the block.

The average values of shock bending energies grouped in table 7 confirm that steel 22 according to the present invention has superior properties, in particular in block core position, representative position of even larger part sizes.

Table 7

Results of Impact Bending Tests on Industrially Treated Tool Blocks.

<table> table see original document page 21 </column> </row> <table> All these mechanical results illustrate the harmful effects of tempera speed lowering, in particular:

- the lowering of the bending energy of shock of equal hardness

- increased hardness loss by prolonged maintenance at 560 ° C

However, the amplitude of these changes is not identical for all compositions, and it is found that the simultaneous and balanced addition of alloying elements according to the rules specified below reduces it significantly.

Alloy Element Effects

The effects of the various alloy elements and their interactions could be evaluated by comparative experimental study of the properties of the experimental foundries and interpreted by thermodynamic simulation. By observing the following principles regarding tempering conditions, the trends were confirmed.

Carbon favors temperability, increases the ideal austenization temperature and determines the maximum hardness obtained after tempering around 550 ° C. But, it has a harmful effect on toughness. Associated with high levels of molybdenum or vanadium, it can lead to the formation of eutectic carbides, harmful to the microstructure and toughness. Its level must be within a range of at least 0,30% required to obtain sufficient hardness and not more than 0,39% in order to avoid irreparable brittleness. The optimum range is 0.33-0.38%.

Chromium has a favorable effect on temperability. It participates in temper hardening, and for the privileged applications of the present invention, ie large size parts requiring high hardness (42 to 52 HRC), this feature is advantageous. However, the carbides it generates rapidly evolve into more stable forms and prove to be ineffective for resistance to hardness lowering at elevated temperature. It is therefore indispensable to complete the addition of Cr by other carburigenic elements such as Mo and V. The content of this element must be limited to a minimum of 4,0% required for temperability and a maximum of 6,0% above which its action. partially inhibits that of vanadium and molybdenum. Preferably, a Cr content of 4.6 to 6% is imposed.

Molybdenum improves temperability. It associates with Chromium in the same Chromium based carbides, which contributes to increase its number. At high levels, it forms species specific M2C, M6C. Given its macroscopic properties, it increases hardness and tempering resistance and decreases toughness. Its content is between 1.50 and 2.60%. Consider the possible presence of tungsten as will be seen later. Preferably Mo is from 1.60 to 2.00% with Mo + 0.65 W from 1.60 to 2.20%.

Vanadium forms VC-specific carbides which, in the field covered by experimental foundries, are predominant among undissolved precipitates at austenization temperature and thus ensure no degree increase. During quenching after quenching, new generations of micro- and nanometric carbides precipitate and, through their interaction with martensite crystalline defects, actively participate in secondary hardening and break-in strength under the influence of temperature and cyclic stresses. An excess of carbides formed in the quench, on the other hand, causes a marked weakening. According to the studied compositions, and following the principles established for the choice of austenization temperature, the vanadium content must be between 0.55% and 0.75%.

Nickel has an effect on hardness in the treated state; It decreases the tempering temperature to be applied to obtain a desired hardness, and the break-in resistance during maintenance at service temperatures. In addition, an excessive content of around 3% greatly reduces the austenization point in the field of use temperatures, which must be imperatively avoided. On the other hand, nickel increases temperability, in particular at 1 to 3% and significantly improves toughness. It is considered that in the present invention the Ni content is between 0.80 and 2.80%. The negative effects on the hardness of a strong Ni addition can be offset by the addition of Cr, Mo, VeW within the prescribed limits.

Tungsten may be an optional addition element, up to a maximum of 1.45% and under conditions such that Mo + 0.65W is between 1.50 and 3.20% with Mo between 1.50 and 2.60 %, preferably from 1.60 to 2.20% with Mo of from 1.60 to 2.00%. In fact, tungsten completes the action of Molybdenum in an equivalence ratio of 1% to 0.65% Mo. This addition of tungsten causes limited negative effects on toughness and temperability, and positive effects on heat softening resistance, in particular at test temperatures above 560 ° C, for example 600 ° C.

Cobalt may be added up to an upper limit of 2.75%. It has a favorable effect on softening resistance, in particular for residence temperatures of the order of 600 ° C, but its action is detrimental to temperability. Considering the high cost price of this addition element, its use does not seem to be particularly recommended.

In addition, achieving an optimal compromise of use properties requires that simultaneous additions of Molybdenum, Vanadium, Nickel, and eventually Tungsten be balanced and observe the following relationships:

K is from -0.65 to +0.65, preferably from -0.35 to +0.35, optimally as close to zero as possible, with:

K = K2 - K1

K2 = 0.75 χ (% Ni - 0.60)

K1 = 1.43 χ (% V - 0.40) + 0.63 χ [% Mo + (0.65 χ% W) -1.20]

Table 1 shows the values of the coefficients Κ1, K2, K for all foundries.

Best results are obtained when you simultaneously have the following conditions:

0.335% <C <0.375%; and 1.50% <Ni <2.10%; and 1.60% <Mo + 10 0.65W <2.00%, with Mo> 1.60%; and 0.62% <V <0.75%.

For more particular applications, the following simultaneous conditions may also be recommended:

- 0.335% <C <0.375% and 2.00% <Ni <2.40%

and 1.80% <Mo + 0.65W <2.90% with 1.80 <Mo <2.40% and W <0.90%

and 0.66% <V <0.76%

when a remarkable temperability is desired for the manufacture of large size parts reserved for applications where, considering the lowering of the transformation point A1 by Ni, the surface working temperature is below 680 ° C, for example applications of extrusion die or Al alloy die casting molds;

- 0.335% <C <0.375% and 0.90% <Ni <1.50%

and 1.50% <Mo + 0.65 W <1.90% with W <0.40%

and 0.55% <V <0.63%

when notable properties are desired for medium sized parts and fit for applications for which the operating surface temperature is below 770 ° C.

On the other hand, other elements to be cited must or may be present within precise limits.

Silicon, by its effect on toughness, should be kept at a low level compatible with economic industrial design conditions; a limit of 0.50 and preferably 0.40 should not be exceeded.

Manganese, which is favorable to temperability but harmful to toughness, should not be present in a content higher than 0.80%, preferably 0.60%.

The elements Sulfur, Phosphorus, Arsenic, Tin1 Antimony, Titanium, Zirconium, Niobium, Nitrogen unfavorable to tenacity and likely to cause a weakening in operation should be limited to the lowest levels compatible with industrial and economic needs.

The maximum permissible contents are:

- for S: 0.0040%, preferably 0.0010%

- for P: 0.0200%, preferably 0.0080%

- for Ti: 0.05%, preferably 0.01%

- for Zr: 0.05%, preferably 0.02%

- for Nb: 0.08%, preferably 0.01%

- for N: 0.0400%, preferably 0.0100%.

In addition, the contents of P, As, Sb, Sn shall be as follows:

10 P + As + 5 Sb + 4 Sn ≤ 0.21%, preferably ≤ 0.10%.

The aluminum content should be between dashes and 0.080%, preferably between dashes and 0.030%. Its function is to deoxidize steel, thereby limiting the amount of susceptible oxide inclusions, in particular of degrading the steel's fatigue strength. From this perspective and simultaneously, the oxygen content should not exceed 30 ppm, preferably 15 ppm. A high Al content decreases the dissolved O content of liquid steel, but it also makes liquid steel more sensitive to atmospheric reoxidation during casting and therefore increases the risk of forming harmful oxidized inclusions.

In general, the steels of the present invention may have two quality levels.

A "standard" quality level is reached when the composition is not imperatively in the optimal ranges that have been defined above for all elements. The improvement over the prior art then lies mainly in the properties of temperability. These properties allow the manufacture of large size products with a high and homogeneous hardness across the entire product section.

A "higher" quality level is achieved when all elements are in the optimum range defined above. Under these conditions, in addition to improved temperability, a toughness is obtained which provides, in conjunction with high hardness, a great resistance to thermal fatigue and sudden breakage.

To achieve these results, it is necessary to use a design that includes, after primary tuning in the electric furnace and crucible, a reflowable electrode reflow by vacuum arc reflow (VAR) or electroconductive slag (ESR) reflow, which make the very low values of O targeted accessible. In addition, as is usual with these types of steel, it is necessary to provide for cast steel a thermomechanical rolling process and annealing which give the steel a compact, coalesced, thin and homogeneous structure in conjunction with solidification conditions that generate small dendrites and little segregated.

Parts that can be fabricated from steel according to the present invention made as described include in particular hot molding tooling parts in general, and in particular:

- molds or dies for die casting of light alloys or cuprous alloys;

- the forging dies;

- steel pipe drilling and rolling tools;

- glass and plastics molding tools.

The present invention finds a privileged application in the manufacture of such parts whose thickness is 200 mm and more.

Claims (34)

1. HOT TOOL STEEL, characterized by its composition in weight percentages: - 0.30% ≤ C ≤ 0.39% - 4.00% ≤ Cr ≤ 6.00% - Traces ≤ Si ≤ 0.50 % - dashes ≤ Mn ≤ 0.80% - dashes ≤ W ≤ 1.45% - dashes ≤ Co ≤ 2.75% - 0.80% ≤ Ni ≤ 2.80% - 1.50% ≤ Mo ≤ 2, 60% with 1.50% ≤ Mo + 0.65W ≤ 3.20% - 0.55% ≤ V ≤ 0.80% - with -0.65 ≤ K ≤ 0.65 where K = K2 - K1 and K2 = 0.75 χ (Ni - 0.60) K1 = 1.43 χ (V - 0.40) + 0.63 χ [(Mo + 0.65W) -1.20] - dashes ≤ Al ≤ 0.080 % - dashes ≤ S ≤ 0,0040% - dashes ≤ P ≤ 0,0200% - dashes ≤ Ti ≤ 0,05% - dashes ≤ Zr ≤ 0,05% - dashes ≤ Nb ≤ 0,08% - dashes ≤ N ≤ 0.040% - 10P + As + 5 Sb + 4 Sn ≤ 0.21% - Traces ≤ O ≤ 30ppm, with the remaining iron and unavoidable impurities. Steel according to claim 1, characterized in that it has: 0.33% ≤ C ≤ 0.38%. Steel according to Claim 1 or 2, characterized in that it has: traces <Si ^ 0,40%. Steel according to one of Claims 1 to 3, characterized in that it has: traces <Mn <0,60%. Steel according to one of Claims 1 to 4, characterized in that it has: 4,6% <Cr <6,0%. STEEL according to one of Claims 1 to 5, characterized in that it has: 1.60% <Mo <2.00% and 1.60% <Mo + -0.65 W <2.20%. Steel according to one of Claims 1 to 6, characterized in that it has: traces <Al <0.030%. Steel according to one of Claims 1 to 7, characterized in that it has: traces <S <0.0010%. Steel according to one of Claims 1 to 8, characterized in that it has: traces <P <0.0080%. STEEL according to one of Claims 1 to 9, characterized in that it has: traces <Ti <0.01%. STEEL according to one of Claims 1 to 10, characterized in that it has: traces <Zr <0.02%. STEEL according to one of Claims 1 to 11, characterized in that it has: traces <Nb ^ 0.01%. Steel according to one of Claims 1 to 12, characterized in that it has: traces <N <0.01%. STEEL according to one of Claims 1 to 13, characterized in that it has: 10 P + As + 5 Sb + 4 Sn <0.10%. STEEL according to one of Claims 1 to 14, characterized in that it has: traces <O <15ppm. Steel according to one of Claims 1 to 15, characterized in that it has: - 0,35 <K <0,35. Steel according to one of Claims 1 to 16, characterized in that it has: - 0,335% <C <0,375% - 1,50% <Ni <2,10% - 1,60% <Mo + 0, 65 W <2.20% with 1.60% <Mo <2.00% - 0.62% <V <0.75%. Steel according to one of Claims 1 to 5, 7 to 16, characterized in that it has: - 0,335% <C <0,375% - 2,00% <Ni <2,40% - 1,80% < Mo + 0.65W <2.90% with 1.80% <Mo <2.40% and W <. -0.90% - 0.66% <V <0.76%. STEEL according to one of Claims 1 to 5, 7 to 16, characterized in that it has: - 0,335% <C <0,375% - 0,90% <Ni <1,50% - 1,50% < Mo + 0.6 W <1.90% with W <0.40% - 0.55% <V <0.63%. Steel according to one of Claims 1 to 17, characterized in that it has: 0.335% <C <0.375%, 4.60% <Cr <-6.00%, traces <Si <0.40% , dashes <Mn <0.60%, dashes <W <1.45%, dashes <Co <2.75%, 1.50% <Ni <2.10%, 1.60% <Mo + 0.65 W <2.20% with 1.60% <Mo <2.00%, 0.62% <V <0.75%, with - 0.35 <K <0.35, dashes <Al <0.030%, dashes <S <0.0010%, dashes <P <0.0080%, dashes <Ti <0.011%, dashes <Zr ≤ 0.02%, dashes <Nb <0.01%, dashes <N <0.01 %, dashes <O <15ppm. STEEL PIECE MANUFACTURING PROCESS, characterized in that said steel part is prepared as described in one of claims 1 to 20 and subjected to an austenization in the temperature range of 1000-1050 °. C, followed by a temper. Process according to Claim 21, characterized in that the austenization occurs in the range 1015-1040 ° C. Process according to Claim 21 or 22, characterized in that after tempering the part is subjected to at least two tempering in the temperature range 550-650 ° C, giving said part a hardness of 42 to 52HRC. STEEL PIECE, obtained by the process as described in one of claims 21 to 23, characterized in that it is a hot molding tooling part. Piece according to claim 24, characterized in that said piece has a thickness greater than or equal to 200 mm. Part according to claim 24 or 25, characterized in that it is a mold or die for the pressure casting of light or cuprous alloys. Part according to claim 24 or 25, characterized in that it is a forging tool. Piece according to claim 24 or 25, characterized in that it is a forge die. Part according to claim 24 or 25, characterized in that it is a steel pipe drilling or rolling tool. Part according to claim 24 or 25, characterized in that it is a glass molding tool. Part according to claim 24 or 25, characterized in that it is a plastics molding tool. 32. Part according to claim 24 or 25, characterized in that it is made of a steel as described in claim 18 and that it is an extrusion die or an aluminum alloy die cast. Use of a part as described in claim -24 or 25, characterized in that said part is a steel as described in claim 18 and that the surface working temperature is below 680 ° C. Use of a part as described in claim -24 or 25, characterized in that said part is a steel as described in claim 19 and that the operating surface temperature is below 770 ° C.