BRPI0904607A2 - high resistance to tempering action - Google Patents

Abstract

HIGH STRENGTH STEEL FOR THE REVENUE, featuring a composition of alloying elements consisting essentially of by weight of carbon between 0.20 and 0.50; Silicon below 1.0; Phosphorus below 0.030; Chromium between 3.0 and 4.0; Molybdenum between 1.5 and 4.0; Vanadium between 0.1 and 2.0, Cobalt less than 1.5, the remainder substantially Fe and unavoidable impurities. Can be produced by processes involving ingot casting and hot or cold forming, or even used with the raw melt structure; or by processes involving atomization or dispersion of the liquid metal, such as powder metallurgy, powder injection or the "spray forming" process

Description

"HIGH STRENGTH OF STEEL FOR RESIDENTIAL" This report deals with a steel intended for use in hot forming metals tools, typically in processes where the metal to be formed has temperatures above 600 ° C, although processes at lower or lower temperatures. even at room temperature they can be used with said steel. The steel in question has a composition that classifies it as a hot working tool steel, having as its main feature the higher resistance to hardness loss at high temperature - called tempering resistance, but also maintaining high toughness and adequate temperability and thermal conductivity. Such an effect is possible thanks to careful alloy design, establishing the optimum ranges of P, Si, Mo and Cr elements.

The term hot work tools is used for a large number of hot forming operations, employed in various industrial branches and focused on the production of parts intended for mechanical applications and especially automotive parts. The best known hot forming processes are forging, extruding or casting non-ferrous alloys. Other high temperature applications, typically above 500/600 0C, can also be classified as 20 hot work. In these applications, molds, dies, punches, inserts and other forming devices are classified by the generic term: hot work tools. These tools are usually made of steels, which require special properties to withstand the high temperatures and mechanical characteristics of the processes in which they are employed.

Among its main properties, the following stand out in hot work steels: hot strength, more specifically tempering resistance, toughness, temperability and physical properties such as VCk ^ fc ■ Χ

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thermal conductivity, specific heat, both correlated by thermal diffusivity, and the coefficient of thermal expansion.

In forging applications, the hot forging of steels, especially of mechanical steels applied to automotive parts, stands out. In such operations, forged billet has temperatures above 1100 ° C. During forming, it heats up the surface of the tool, the temperature reached the higher the contact time. The heat generated therefore requires high heat resistance from the steel employed. The hardening mechanism of steels for hot forming dies is mainly the precipitation of fine carbides. Noteworthy are carbides of Mo or W, type M2C, or carbides of V, type MC. High-Cr steels also include Cr-rich M7C3 carbides, but also Mo and V in solid solution. Despite their high stability, these carbides tend to coalesce after long periods of time at high temperatures, typically above 550 ° C - conditions easily reached in the working regions of the tool. As a result, the hardness of the region decreases, causing hot wear or plastic deformation and causing the tool to fail.

Improving the material's resistance to hardness loss or tempering resistance is thus critical to improving the performance of tools with high heat. Examples of such applications are hot forging tools for steel parts or other alloys, non-ferrous alloy extrusion and non-ferrous alloy casting dies (the latter two applications being more important for Al alloys). The same is true for other applications such as extrusion or non-ferrous alloy casting. In steel forging, for example, the temperatures of the preforms to be forged are in the order of 1200 ° C. Even with the short tool contact time (of the order of seconds), the heating 3/17 ..α ^

of its surface regions is expressive, causing loss in hardness due to tempering of these regions. In extrusion of aluminum alloys or other non-earth alloys, the billet temperature is lower, in the range of 400 to 600 ° C. However, in these applications, the contact time is significantly longer (from tens of minutes to hours of operation). In addition, the local friction generated by tool / aluminum contact intensifies heating, increasing the loss of hardness in the tool steel and consequently causing its wear. In die casting die casting, the liquid metal is injected at high pressure and temperature (around 700 ° C), also promoting the die surface heating. In this case, the failure occurs mainly by thermal fatigue cracks, caused by the successive heating and cooling of the matrix working region. However, the high heat exchange of the liquid aluminum and the matrix surface facilitate the heating of the superficial regions, generating, as in other applications, loss in hardness and, therefore, facilitating the initiation of fatigue cracks.

This mechanism of hardness loss after heating is therefore notoriously important in hot working tool steels, thus increasing the material's resistance to this phenomenon. In terms of steel employed, the improvement in hot strength is usually achieved by employing higher contents of secondary carbide forming elements such as Mo, W and V, or by solid solution hardening. Although effective in increasing heat resistance, excessive increase in the content of such elements implies reduced toughness, worsening diffusivity and thermal conductivity or causes intense material cost increase. This latter economic implication is especially important today, given the high cost of raw materials used as the source of Mo, W and V alloy elements.

In order to situate the present invention, below are described some of the steels used, present in the prior art (whose chemical composition is summarized in Table 1). Firstly, the H11 and H13 steels stand out, currently the most widely used tool steels in hot work. These materials have 5% Cr to ensure adequate temperability and aid in hot strength, 0.9% V and 1.2% Mo for improved hot strength, and usually low P and S content to promote adequate toughness. However, to improve tempering resistance, higher Mo contents would be required. DIN 1.2365 and DIN 1.2367 steels are used in this regard. They have high Mo content, to promote improvement of hot strength. However, increasing the content of this element in DIN 1.2367 tends to reduce its toughness and thermal conductivity and diffusivity. In steel 1,2365, this reduction in thermal conductivity by increasing Mo content is offset by the reduction in Cr content. However, the reduced Cr content promotes lower temperability, limiting applications in large tools.

It is important to explain at this time the concern with the properties of thermal conductivity and toughness. During work, increasing thermal conductivity is important so that the tool steel can more quickly homogenize the temperature difference between the shaped material and the tool core, reducing the stresses and the incidence of thermal cracks. And in the occurrence of these cracks, the toughness of the material is also fundamental, since it delays its propagation and, consequently, the damage due to thermal fatigue. Thus, it is clear that the isolated increase in Mo content as achieved in DIN 1.2367 and DIN 1.2365 steel is not sufficient for the overall improvement of the properties of hot work steels.

Table 1: Typical chemical composition of state of the art steels. The sum Mo + V + Co is entered because these elements have the highest cost and are closely related to the final cost of the tool steel. Contents ν "';.

in mass percentage and balance in Fe. The sum Mo + W + V + Co is put,

Description C Si Cr Mo W V Co Mo + W + V + Co H11 0.36 1.0 5.0 1.2 - 0.5 - 1.7 H13 0.38 1.0 5.0 1.2 - 1 .0 - 2.2 DIN 1.2365 0.36 0.3 2.8 2.8 - 0.5 - 3.3 DIN 1.2367 0.38 0.3 5.0 3.0 - 0.5 - 3.5 Pl 9909160-7 0.36 0.2 5.0 2.3 - 0.5 - 2.8

A steel was most recently developed, described in Pl 9909160-7. As in DIN 1.2367, this material uses an increase in Mo content, but with a reduction in Si and P content to improve toughness. In this case, the cost increase is avoided by using a not very high Mo1 content but the gain in hot strength is not significant in relation to H13 steel.

In this scenario, it is evident the need for a tool steel with superior hot strength than the state-of-the-art H13 steel, but without the use of excess alloying elements that impair thermal conductivity and material cost. Also, the material employed must have high temperability and thus be able to be applied to large tools. The steel of the present invention therefore fulfills all

these needs.

It was the object of the invention, firstly, to study the influence of Cr and Mo content on hot work tool steels, identifying a synergy of the two elements in hot strength. More specifically, when the increase in Mo content is accompanied by the reduction in Cr content, a more significant effect on hot strength is observed. In addition, reducing Cr content promotes improved thermal conductivity, thus reversing the negative effect of increasing Mo content. On the other hand, the. Jyr / -> / ·, cf '9

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Cr content should be carefully balanced, as very low values "as commented above, impair the temperability and limit the application on tools of dimensions above 100 mm of average section. Therefore, the material of the present invention shows an arrangement chemical composition Cr and Mo content, able to surpass the temper strength properties of state-of-the-art steels, without significant cost increase and with adequate thermal conductivity, toughness and temperability.

In order to satisfy the aforementioned conditions, the steel of the present invention has an alloying composition which, by weight, consists of:

• 0.20 to 0.50 C, preferably 0.3 to 0.45 C, typically

0.36 C.

• 3.0 to 4.0 Cr, preferably 3.5 to 3.9 Cr, typically 3.8 Cr.

· 1.5 to 4.0 Mo, preferably 2.0 to 3.0 Mo, typically 2.5

Mo. Because of its chemical similarity to W, Mo can be substituted for W in a ratio in which 2 parts by mass of W equals 1 part of Mo.

• 0.1 to 2.0 V, preferably 0.3-1.0 V, typically 0.5 V, where V may be partially or fully replaced by Nb in a

ratio where 1.0% Nb corresponds to 0.5% V.

Up to 1.0 Si, preferably up to 0.5 Si, typically 0.3 Si.

• Maximum 0.030 P, preferably maximum 0.015 P, typically maximum 0.010 P.

Iron balance and metallic or non-metallic impurities unavoidable to the steelmaking process, wherein said non-metallic impurities include, but are not limited to the following elements, by weight:

• Maximum 0.10 S, preferably maximum 0.020 S, typically maximum 0.008 S. • Maximum 1.5 Al, Mn or Co, preferably up to 1.0 Al, Mn or Co, typically below 0.5 Mn, Al and Co.

The following are the reasons for specifying the composition of the new material. The percentages given refer to the percentage by mass.

C: Carbon is primarily responsible for hardening martensite at low temperatures. Along with the alloying elements, carbon acts on secondary hardening, which is important for high temperature hardening. For these purposes, carbon contents above at least 0.20% are indicated, preferably above 0.30%. Very high grades, on the other hand, promote excessive carbide precipitation in grain boundaries at the time of quenching (especially when Mo and V levels are high), and promote higher hardness and increased secondary carbide volume. In this way the toughness in general is impaired. Thus, the C content should be limited to a maximum of 0.50%, preferably below 0.40%. This limitation also contributes to reducing the amount of retained austenite, avoiding problems of dimensional instability and brittleness.

Cr: Chromium content should be higher than 3.0%, preferably higher than 3.5%, because this element contributes to temperability, which is important for large tool applications. However, the content should be limited. The concept of reducing the Cr content to promote a greater effect of tempering resistance has been developed in the present invention. This effect is important and significantly superior temper strength is obtained than prior art steels. The mechanisms of this effect must be related to the formation of secondary carbides of Cr, type M7C3, which dissolve Mo and V and are the first carbides to form. Therefore, the lower the Cr content, the lower the carbide volume &

M7C3 and thus the greater the amount of Mo and V available for secondary hardening. The end result is significantly higher tempering resistance when the Cr content of the alloy is lower than that of prior art alloys. And even in relation to the alloy of Pl 9909160-7, there is a significant gain in tempering resistance. This is significant because the alloys have equivalent Mo contents (a high cost alloying element), showing that the present invention is capable of assuming high heat strength values without overly increasing the Mo content. For all these effects, Cr content must be below 5.0% of conventional steels, with Cr content lower than 4.0%. Finally, adjusting the Cr content in the present invention identifies the ideal Cr value for maximizing tempering resistance - between 3.0% and 4.0%. In addition to the hot strength property, the lower Cr content helps to increase thermal conductivity, avoiding the damage of this property with the increase of Mo content. This lower Cr range is therefore carefully adjusted for maximum tempering resistance and adequate thermal conductivity.

Mo and W: High Mo contents are employed in the alloy of the present invention to increase tempering resistance properties. This occurs by the formation of stable Mo-rich carbides in tempering, mainly M2C carbide. Thus, the alloy of the present invention should be at least 1.5%, preferably above 2.0%. On the other hand, very high Mo contents may impair toughness due to precipitation of pro-eutectic carbides at the time of quenching, and may significantly increase the cost of the alloy, making it impractical for many tools. Therefore, the Mo content should be limited to 4.0%, preferably below 3.0%. As tungsten and molybdenum have similar effects on the tool steel of the present invention, forming secondary carbides type M2C or M6C. Thus, can be jointly specified λ ', Γ

through the tungsten equivalent ratio (Weq) 1 given by the sum W + 2Mo, which normalizes the atomic weight differences of the two elements.

V: Vanadium is of prime importance for the formation of MC type secondary carbides. Because they are very thin, these carbides act as barriers to the movement of lines of disagreement, increasing the mechanical resistance. It also aids in contouring grain growth, allowing for high austenitization temperatures (above 1000 ° C). For these effects the V should be above 0.1%, preferably above 0.3%. However, excessively high levels of V can generate difficult to solubilize primary carbides, reducing toughness. Thus, the V content should be below 2.0%, preferably below 1.0%.

Si: Silicon has a strong effect on secondary hardening and toughness. At high levels, Si increases secondary hardness to tempering temperatures of 600 ° C. However, in the study of the present invention, the reduction of Si content proved important to reduce hardness loss at high temperature, thereby increasing tempering resistance. The reduction in Si content also showed a significant increase in toughness, this effect being applied in the present invention. Therefore, the material of the present invention should have Si content below 1.0%, typically below 0.5%. P: The reduction of phosphorus also promotes significant increase

of tenacity, because this element segregates into grain contours and thus promotes loss of cohesion in these regions. Therefore, the P content should be below 0.030%, typically below 0.015%.

Residuals: Other elements, such as Mn and Al, should be understood as impurities, related to steelmaking deoxidation processes or inherent to manufacturing processes. Therefore, the Mn and Al content is limited to 1.5%, preferably below 1.0%. The Co content should also be limited to the same values, as despite the beneficial effect on resistance to 10/17.

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hot, focuses very heavily on the cost of the alloy. In terms of forming "" "" inclusions, you should control the S content, as such inclusions facilitate fracture during work; thus, the S content should be below 0.050%, preferably below 0.020%.

The alloy as described may be produced in the form of

rolled or forged products by conventional or special processes such as powder metallurgy, spray forming or continuous casting such as wire rod, bars, wires, sheets and strips.

In the following description of experiments performed, reference is made to the attached figures, in which:

Figure 1 shows the effects of Si and P on alloys 1 to 8 in terms of hardness after tempering and toughness.

Figure 2 shows the comparison of alloys 1 to 8, but showing the effect of P on toughness as a function of tempering temperature.

Figure 3 shows the distribution of carbides in the high and low Si alloys, showing the best distribution in the latter and thus justifying their superior toughness.

Figure 4 compares the hardness reduction as a function of time at 600 ° C, indicating tempering resistance. The farther to the right, the greater the resistance to tempering the alloy.

Figure 5 represents the comparison of the thermal conductivity values for some of the studied alloys.

Figure 6 shows the comparative toughness of alloys 9 to 12 and alloys P1 and P1; data obtained by the notched impact test (7 χ 10 mm2 specimens) and Charpy V.

Figure 7 is a complete and detailed photo of the hot forging punch in which industrially produced P 2 steel was applied and compared with prior art H13 steel. Observe in (a) 1a ^ v wear and crack failures and in (b) the hardness profile showing the fall in the working regions (surface distance = zero).

EXAMPLE 1: Effect of Silicon and Phosphorus: Eight experimental ingots were initially produced,

to evaluate the effect of Si and P on H11 steel present in the state of the art. The compositions are shown in Table 2. The hardness and impact results are shown in Figure 1. The strong effect of Si on raising hardness can be observed at tempering temperatures below 500 ° C, but the same effect does not occur at tempered above 600 ° C, being equivalent to the hardness of the high and low Si alloys. The effect of P is compared in Figure 2 for various tempering temperatures. In this case, it is observed that the reduction of P promotes significant improvement in the toughness of high Si alloys, but a lesser effect on low Si alloys. Therefore, the results show that the best combination,

in terms of toughness, they would be low P and Si alloys. High Si alloys are only warranted in situations where hardness above 52 HRC is employed and tempering temperatures below 600 ° C are employed. . In these cases, the reduction of P is even more important. The reasons for these important effects of Si and P are not

are fully defined, but preliminary scientific results conducted by the inventors of this patent have shown to be related to the formation of secondary carbides. In high Si alloys, secondary carbides tend to be concentrated in regions of high diffusion (clapboard or grain boundaries), due to the difficulty imposed by Si on the formation of cementite. In low Si alloys, on the other hand, cementite is rapidly formed, leading to a better distribution of secondary alloy carbides formed at higher temperatures. The transmission electron microscopy images of Figure 3 exemplify these observations.

Table 2: Chemical compositions of the different Si and P contents studied in the state-of-the-art H11 alloy.

Alloy: 1 2 3 4 5 6 7 8 C 0.36 0.34 0.36 0.36 0.36 0.35 0.36 0.36 0.35 Si 0.05 0.32 0.98 1.92 0, 05 0.33 1.01 1.90 Mn 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 P 0.023 0.028 0.024 0.012 0.012 0.012 0.011 0.008 S 0.004 0.004 0.004 0.003 0.004 0.004 0.005 0.003 Co 0.06 0.05 0.05 0.05 0.05 0.05 0.05 0.06 Cr 5.09 5.13 5.06 5.08 5.08 5.03 5.10 5.05 Mo 1.28 1.31 1.33 1.24 1.32 1.32 1.33 1.23 Ni 0.20 0.19 0.19 0.20 0.19 0.20 0.19 0.20 V 0.44 0.44 0.42 0.41 0.44 0.44 0.45 0.45 0.43 W 0.10 0.11 0.11 0.10 0.11 0.10 0.11 0.10 Nb <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Al 0.029 0.020 0.023 0.036 0.024 0.022 0.036 0.043 Weq (= W + 2Mo) 1.48 1.53 1.55 1.44 1.54 1.52 1.55 1.43

EXAMPLE 2: Effect of Cr and Mo:

For the definition of the effect of Cr and Mo1 were produced more

Experimental ingots, comprising 4 state-of-the-art steels: H11, H13 and the steel described in Pl 9909160-7 and two alloys proposed for the present invention (see Table 3). These two compositions have the reduction advantages of Si and P described in Example 1, but different Cr and Mo balances. As commented, the goal of Pl 1 and PI 2 alloys is to obtain

greater resistance to hardness loss, ie tempering resistance. Therefore, the hardness reduction after various exposure times at 600 ° C was evaluated, and the results obtained are shown in Table 4. The time increments were performed logarithmically, as can be seen from the graph in Figure 4b. These results show that, comparing Pl 1 and H11 alloys, there are 13/17

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also an increase in heat resistance only with modification of Si and P contents (but the effect should probably be related only to Si, since P does not act on carbide formation).

However, this gain in hot strength with the reduction of Si content is not sufficient to obtain results significantly higher than H13. Therefore, the increase of Mo content was applied, but accompanied by the concomitant reduction of Cr content in Pl 2 alloy. In this case, there was a significant variation in hot strength, providing greater hardness after the same exposure time. And, as shown in Figure 4a, the same hardness drop in H13 steel occurs after much longer times in the case of Pl 2 alloy. For example, the hardness reduction from 45 HRc to 35 HRC at a temperature of 600 ° C, occurs after 25 hours, while the same phenomenon occurs in the Pl 2 alloy only after 60 hours.

This significant improvement in tempering resistance is related to the increase of Mo content but also the reduction of Cr content. This effect is clear when comparing the differences of alloy PI2 and alloy 12 (patent P 9909160-7). It also explains the high alloy tempering resistance results 11.

Table 3: Chemical compositions of the state steels

technique and proposals

of the present invention.

Alloy: 9 10 11 12 Pl 1 Pl 2 NOTE: H11 H13 DIN 1.2365 Patent 9909160-7 Present invention C 0.36 0.4 0.31 0.35 0.35 0.35 Si 1.02 0.96 0, 3 0.13 0.3 0.31 Mn 0.48 0.34 0.3 0.49 0.27 0.3 P 0.025 0.023 0.023 0.009 0.007 0.01 S 0.005 0.006 0.005 0.005 0.005 0.006 Co 0.02 0 0.01 0.02 0.01 0.02 Cr 5.03 5.23 2.85 4.99 4.96 3.78

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! "Λ Alloy: 9 10 11 12 Pl 1 Pl 2 Mo 1.4 1.31 2.8 2.28 1.39 2.49 Ni 0.19 0.20 0.20 0.19 0.20 0, 19 V 0.34 0.85 0.5 0.57 0.42 0.52 W 0.03 0.02 0.02 0.02 0.02 0.02 0.02 Nb <0.010 0.02 0.02 0, 01 <0.010 0.005 Al <0.005 0.014 0.019 0.009 <0.005 0.005 Weq (= W + 2Mo) 2.8 2.6 5.6 4.6 2.8 5.0 W + Mo + Co + V 1.79 2.2 3 .33 2.89 1.84 3.05

Table 4: Hardness loss after exposure at 600 ° C for

several times. Initial hardness around 45

Alloy 9 Alloy 10 Alloy 11 Alloy 12 Pl 1 Pl 2% Cr e Mo: 5Cr 1.4Mo 5.2Cr 1.3Mo 2.8Cr 2.8Mo 5, OCr 2.3Mo 5, OCr 1.4Mo 3.8Cr 2, 5Mo Initial 45.5 44.6 45.3 45.7 44.9 45.4 3h 43.3 43.4 44.7 43.3 44.8 10h 39.6 41.3 43.8 41 , 40.8 43.7 30h 34.4 36.1 41.0 37.6 36.9 40.5 100h 31.1 30.7 36.0 32.0 32.1 34.5

HRC.

Despite the interesting effect on tempering resistance, the Cr contents cannot be too low, so as not to impair temperability and thus limit application to large tools. This can be considered the main drawback of state-of-the-art DIN 1.2365 (Alloy 11) steel, with excellent tempering resistance but low temperability. Table 5 exemplifies this Cr and temperability problem based on dilatometry assay results. The composition P 2 can be considered ideal in this respect, with lower Cr content than H13 (alloy 10) steel, to provide better tempering resistance, but not as low as DIN 1.2365 (alloy 11) steel. Highest Mo content of alloy Pl 2 15/17> r '·' _ η

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It also assists in achieving adequate temperability levels, compensating q, Cr effect reducing effect and ensuring application in large tools.

Another advantage of using lower Cr content than alloy 12 and other prior art alloys is the ability to maintain adequate thermal conductivity. As shown in Figure 5, this property is decreased with increasing Mo content (comparing alloys 12 and 10), but increasing with decreasing Cr content (alloys 11 and P1). Therefore, in addition to being ideal for heat resistance, the combination of Cr and Mo contents of Pl 2 alloy allows the maintenance of thermal conductivity to levels even higher than traditional H13 (alloy 10) steel.

Table 5: Results of the TRC curve of the steels studied, used for temperability evaluation. The lower the critical rate and the higher the hardness obtained after cooling to 0.1 ° C, the higher the temperability.

15

Alloy 10 Alloy 11 Alloy 12 PM Pl 2 H13 DIN 1.2365 Pl 9909160-7 Critical rate for onset of bainite formation (° C / s) 0.2 8.0 0.3 0.5 0.5 Hardness obtained after cooling a 0.1 ° C / s (HV) 538 389 534 512 486

Another gain of Pl 1 and Pl 2 alloys over the prior art alloys H11, H13eDIN 1.2365 (alloys 9-11) is the superior toughness. These results can be compared in Figure 6. In particular, we highlight the gain of alloy P 2 in relation to alloy 11, which also has high tempering resistance. That is, in addition to the superior temperability, the chemical composition balance of P 2 makes this alloy significantly more tenacious than the alloy 11.0 effect, in which case it is mainly associated with reduced Si and P contents as discussed in example 1. EXAMPLE 3: field test

The following is a detailed field study in which Pff jyv. was compared to steel H13 in forging tools. Results were analyzed based on failure modes and material properties. The process in question deals with a warm forging of

high speed (see Figure 7a). Although forged billets have a lower than usual hot forging temperature, the high process speed makes the contact between the heated billet and the die high by heating its surface.

The process also employs high cooling,

thus promoting thermal shock in the surface region. Process data: Product: shaft end.

Tool: Precision warm forging punch. Forged material: modified SAE 1045 and 1050 steel. Billet temperature: approximately 900 0C. Cooling: High, with water. Stroke application speed: high. Previously used tool steel: AISI H13 (hardness 53 HRC). Steel tested: Pl 2, same hardness.

Figure 7a shows the puncture analyzed after end of life. As this type of forging produces parts with high dimensional accuracy, deviations of tenths of millimeters compromise the part produced. End of life is caused by wear on the rounded protruding regions and the appearance of thermal cracks (see Fig. 7a). After end of life, the matrix was destroyed and analyzed. Figure 7b shows the hardness data as a function of distance from the contact surface, observing its decrease for near surface regions. Wear is actually related to this loss in hardness Λ *

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during work, being little affected by the hardness of the core. The emergence of thermal cracks is also related to the loss in hardness, as regions of lower hardness become more sensitive to thermal crack initiation. Increasing the tempering resistance of the material is therefore essential to increase the tool life.

The steel of the present invention, Pl 2, was then tested and approved for application, promoting a 56% increase in tool life. In numerical values, about 5,000 pieces were forged with H13 until the end of the tool life and this number increased to 7,500 pieces with Pl 2 steel; Comparative analysis of tempering curves and hardness drop as a function of time for H13 (Alloy 10) and Pl 2 steels, Figures 4 and 6, allows a good understanding of the phenomenon. In both there is loss in hardness when the steel is subjected to high temperatures, the greater the time and temperature employed. However, higher stability is observed at high temperature of Pl 2 alloy. Thus, during the forging process, the failure will occur after a greater number of strokes, producing the observed yield gain.

Claims (10)

1. HIGH RESISTANCE STEEL, characterized in that it has a composition of alloying elements consisting essentially of by weight of carbon between 0.20 and 0.50; Silicon below 1.0; Phosphorus below 0.030; Chromium between 3.0 and 4.0; Molybdenum between 1.5 and 4.0; Vanadium between 0.1 and 2.0, Cobalt less than 1.5, the remainder substantially Fe and unavoidable impurities. HIGH RESIDENTIAL STEEL OF THE INVENTION according to claim 1, characterized in that it has a composition of alloying elements consisting essentially, by weight, of carbon between 0.30 and 0.50; Silicon below 0.8; Phosphorus below 0.020; Chromium between 3.0 and 4.0; Molybdenum between 2.0 and 3.0; Vanadium between 0.1 and 1.0, Cobalt less than 1.0, the remainder substantially Fe and unavoidable impurities. HIGH STRENGTH OF THE INVENTION according to claim 2, characterized in that it has a composition of alloying elements consisting essentially, by weight, of carbon between 0.30 and 0.45; Silicon below 0.5; Phosphorus below 0.015; Chromium between 3.2 and 3.9; Molybdenum between 2.0 and 3.0; Vanadium between 0.3 and 1.0, Cobalt less than 1.0, the remainder substantially Fe and unavoidable impurities. HIGH RESIDENTIAL STEEL OF THE INVENTION according to claim 3, characterized in that it has a composition of alloying elements consisting essentially, by weight, of carbon between 0.30 and 0.40; Silicon below 0.4; Phosphorus below 0.010; Chromium between 3.5 and 3.9; Molybdenum between 2.2 and 2.8; Vanadium between 0.3 and 0.8, Cobalt less than 0.5, the remainder substantially Fe and unavoidable impurities. 5.- STEEL OF HIGH RESISTANCE TO THE INVESTED, AFFECTION; Claim 1 or 2 or 3 or 4, characterized in that it has ρ tungsten-substituted molybdenum, in a proportion in which 1 part Mo equals 2 parts W. A high strength steel according to one of claims 1 or 2 or 3 or 4, characterized in that the vanadium is replaced by Niobium or Titanium, in a proportion where 1 part of V equals 2 parts of Nb or 1 part of you. HIGH-RESISTANT STEEL OF THE INVENTION according to one of claims 1 or 2 or 3 or 4, characterized in that they are applied in molds, dies and general purpose tools for forming solid or liquid materials at room temperature or at temperatures up to 1300 0C. 8. HIGH RESISTANT STEEL OF THE INVENTION according to one of claims 1 or 2 or 3 or 4, characterized in that they are applied in metal forming tools, which are at temperatures between 300 and 1300 ° C, forging applications. , extrusion or casting of ferrous or non-ferrous alloys. 9.- HIGH-RESISTANT STEEL FOR THE INVENTION, characterized by being produced by processes involving ingot casting and hot or cold forming, or even used with the raw melting structure. 10.- HIGH-RESISTANT STEEL FOR THE REVENUE, characterized by being produced by processes involving atomization or dispersion of liquid metal, such as powder metallurgy, powder injection or the "spray forming" process.