MX2012010394A - Steel for extrusion tools. - Google Patents

Abstract

The present invention relates to a steel for extrusion tools which is lower in cost and has enhanced temper resistance compared to conventional H13 steel, the chemical composition being (in weight per cent): from 0.4 to 0.6 carbon; less than 1 silicon; less than 0.03 phosphorous; from 2.5 to 4.5 chromium; from 0.5 to 0.7 molybdenum, which can be substituted by tungsten in a 2 W to 1 Mo ratio; from 0.1 to 1 vanadium; less than 1 manganese; Fe and inevitable impurities substantially making up the remainder. As an option for promoting high surface hardness after nitriding, the steel of the present invention can have an aluminium content of up to 1.0, but for high toughness the aluminium content must be kept less than 0.10.

Description

STEEL FOR EXTRUSION TOOL Description of the invention The present invention relates to a steel intended for use in various tools and hot forming dies, particularly for the extrusion of aluminum alloys or other non-ferrous metals. Although initially designed for extrusion processes, the material can also be used in other hot forming processes, in which the metal to be formed supports temperatures above 600 ° C, although steel can be used in processes at temperatures lower or even at room temperature. The composition of the steel in question allows it to be classified as hot working tool steel, whose primary characteristic is the lower content of high cost alloying elements, such as molybdenum and vanadium, but with resistance to tempering (or resistance). to the loss of hardness) greater than that of conventional steels of the prior art concept. A further alternative to the steel of the present invention is provided to increase the hardness after nitriding, and may result in even higher operating levels than conventional steels, while the cost remains low due to a simpler chemical composition . Such an effect is possible by designing Ref.:235198 carefully alloy, and fixing the optimal intervals of the elements: carbon, chromium, molybdenum and aluminum.

The term hot work tools is applied to a large number of hot forming operations, used in industries and focuses on the production of parts for mechanical applications, especially automotive parts. The most popular hot forming processes are steel forging, and the extrusion or casting of non-ferrous alloys. Other applications carried out at high temperature, typically above 500/600 ° C, can also be classified as 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 high temperatures and the mechanical stresses of the processes in which these tools are used.

Among its most important properties of hot working steels, the following stand out: resistance after tempering at high temperature, resistance to loss of hardness called tempering resistance, toughness, hardenability and physical properties such as thermal conductivity and specific heat.

Extrusion dies used for non-ferrous alloys, especially aluminum alloys, are primarily the hot working target for applying the steel of the present invention. These typical matrices comprise an important segment of the tool steel market both in Brazil and abroad. In this application, the steels are highly standardized, based on steels such as ABNT H13 (see Table 1), with quality requirements not as strict as those of other applications, for example die-cast die but with emphasis on lower production costs.

The increased cost of metal alloys, especially Mo and V, significantly impaired this segment, making it desirous of low-cost alternatives. Low alloy steels have been used, such as DIN 1.2714 (chemical composition given in Table 1). However, their low wear resistance due to reduced thermal resistance and a hardness after lower nitriding prevents them from being applied.

Recent developments, such as US 2009/0191086, were focused on the reduction of the alloying elements, by means of reduced Cr, Mo and V content. However, the negative effects are produced by reducing the Cr content. First, the composition of the alloys is not sufficient to reach high hardness after tempering (at least 45 HRC after tempering at 600 ° C). Second, a reduced Cr content can also generate a lower hardness after nitriding, which is not appropriate for extrusion applications, considering the obvious increase produced by nitridation in these applications (virtually all extrusion dies are currently nitrified) .

Table 1: Typical chemical composition of the steels of the prior art concept. The sum of Mo + V + Co is shown because these elements have the highest cost, and are closely related to the final cost of tool steel. Content in percentage by mass and Fe balance. For all extrusion applications the content of W is low, generally < 0.1%.

A third problem of the invention US 2009/0191086 relates to the hardness of the core of the matrix, which may be lower due to the decreased hardenability as a result of reduced Cr and Mo content. To avoid this, the alloys of the invention US 2009/0191086 have a higher Mn content, which lead to a higher hardenability, to problems of potential segregation (stratification) and to the retention of excessive austenite. Both effects can deteriorate the hardness and final tenacity and, thus, the life of the tool. A final aspect can also be mentioned, with respect to the high content of Mn: The waste of this steel can be incorporated sparingly in the production of hot working steels, low conventional Mn content.

Given all these disadvantages, the invention US 2009/0191086 is considered by the authors as a cost-reducing solution, but with inferior properties. In the text of the patent, the authors quantify the predicted loss of efficacy, from approximately 20 to 30% lower than that of steel H13. Considering the costs of machining and thermal treatment associated with the matrices, this loss of efficiency can be considered absolutely significant, thus requiring a reduction of the cost of the material by more than 30% to compensate the substitution. For example, considering that only 60% of the final cost of the matrix is associated with the tool steel used, a life of less than 30% can only be viable if the cost of the new material is half the cost of conventional material. From 2005 to 2008, when the cost of raw materials reached its peak, this could be true (although still difficult to happen, due to the cost difference required is too high). However, for the current scenario, such cost reduction can hardly be achieved for H13 steel, considering only the reduction of the Mo and Cr content. Thus, the reduction in cost associated with the loss of efficiency of the alloy of the patent US 2009/0191086 can currently be considered impractical for such an application.

Given this scenario, the need for a tool steel that effectively has a positive effect on tool life through equivalent operation, but at a lower cost than steel H13, is evident. This is only possible if the steel in question has resistance to hardening and hardness after hardening at 600 ° C (typical heat treatment condition) equivalent to those of steel H13, but with a content of the elements of lower alloy and appropriate hardness after nitriding. In addition, the material used must have high hardenability, but free of problems associated with high Mn content, thus allowing it to be applied to larger tools than extrusion dies.

Therefore, the steel of the present invention will fulfill all these needs.

To achieve cost reduction / goal of zero quality loss, the effect of the main elements related to heat resistance, Cr and Mo, was studied separately. Apart from significant results, this study also showed that the variation of the content of these elements is not sufficient to promote the required heat resistance. Thus, the content of C could be increased to levels that do not affect the hardness, especially with the accompaniment of the content of P and Si under. Finally, the effect of Al was used to compensate for the reduction of Cr, therefore, a potentially lower hardness after nitriding. This work also focused on this problem because the nitrified layer is critical to provide wear resistance to the different hot forming tools, especially extrusion and hot forging tools.

Therefore, to satisfy the above conditions, the steel of the present invention has a composition of the alloying elements, which, in percentage by mass consists of: • 0.40 to 0.60 C, preferably 0.45 to 0.55 C, typically 0.50 C · 2.5 to 4.5 Cr, preferably 3.0 to 4.2 Cr, typically 3.8 Cr • 0.30 to 0.90 Mo, preferably 0.50 to 0.70 Mo, typically 0.60 Mo.

Given its chemical similarity to W, Mo can be replaced with the ratio by mass W, 2W: lMo. • 0.1 to 1.0 V, preferably 0.3 to 0.8 V, typically 0.4 V; V can be partially or completely replaced by Nb, following a ratio by mass of lNb: 0.5 V.

· Up to 1.0 of Si, preferably up to 0.50 of Si, typically 0.30 of Si, maximum 1.0 of Mn, preferably maximum of 0.80 of Mn, typically maximum of 0.50 of Mn.

As described below, Al can be added simultaneously to the alloys of the present invention to provide increases in terms of hardness after nitriding, but also negative effects in terms of toughness and complexity of the steelmaking process. Thus, the content of Al must be dosed as follows, in percentage by mass: - maximum 1.0 of Al, preferably maximum 0.80 of Al, typically maximum 0.60 of Al, thus avoiding considerable losses in the tenacity of steel (in the concept of the previous art, for compositions in which the effects of Al are not foreseen, this element is treated as residual impurity with content = 0.10, typically below 0.05). Minimum 0.1 of Al; preferably minimum 0.2 of Al, typically minimum 0.4 of Al, thus making sure that the steel surface is properly hardened during the nitriding treatment phase.

The compositions must be characterized by the balance by Fe (iron) and deleterious metallic or non-metallic substances unavoidable to the steelmaking process, in which non-metallic deleterious substances include but are not limited to the following elements, in percentage by weight. dough: • Maximum 0.030 of P, preferably maximum 0.015 of P, typically maximum 0.010 of P.

• Maximum 0.10 of S, preferably maximum 0.030 of S, typically maximum 0.008 of S.

• Maximum 1.5 of Ni or Co, preferably up to 1.0 of Ni or Co, typically below 0.5 Ni and Co.

Next, we describe the proportions of the invention of the composition of the new material. The percentages listed refer to percentage by mass.

C: Carbon is mainly responsible for the hardening of martensite under low temperature conditions. However, along with alloying elements, carbon also plays a role in secondary hardening, important for high temperature hardening. In these cases, the content of C is more important for the hardness at temperatures below 600 ° C, when the hardness still depends on the hardness of martensite or cementite formation or retained, high content of Mn can be considered undesirable to promote micro -intensive segregation generating stratification in different degrees of hardness, and to increase the content of retained austenite; therefore Mn is considered a deleterious element in the present invention. Thus, the Mn content should be limited to 1.0%, preferably below 0.8%, typically below 0.50%.

Al: to promote greater hardness of the nitrified layer, the Al content of the alloys can be high. However, the content of Al, under these conditions, should be limited to 1.0% because they lead to decreased tenacity. Thus, the content of Al between 0.40% and 0.60% may be of interest for this purpose.

Residual elements: Other elements such as Ni and · Co should be considered as deleterious substances associated with the processes of deoxidation of steel fabrication or inherent to manufacturing processes. Therefore, the content of Ni and Co should be limited to 1.5%, preferably below 1.0%. In terms of inclusions formation, the sulfur content must be controlled, because such inclusions can lead to cracking during the operation; therefore the content of S must remain below 0.050%, preferably below 0.020%. Also, for high tenacity purposes, fragile elements such as P should be avoided, P < 0.030%, preferably P < 0.015%, typically P < 0.010% In fact, a low Cr content also helps reduce the P content in electric arc furnace steel making processes, thus leading to conclusions that are not contradictory to the philosophy of cost reduction desired.

The alloy, as described above, can be produced as rolled or forged products through conventional or special processes such as powder metallurgy, aerosol formation or continuous casting, such as wire rod, bars, wires, sheets and strips. It can be applied to molds, dies and tools of general use, to form liquid and solid materials or at temperatures up to 1300 ° C, particularly between 300 and 1300 ° C, in forging applications, ferrous and non-ferrous alloys.

In the following description of experiments carried out, reference is made to the following appended figures: - Figure 1A shows the effect of the hardness Mo content after tempering at 600 ° C, while figures IB and 1C show the effect of the Cr content on 0.60% Mo in the usual C content (Figure IB) and a higher content of C (Figure 1C); the dashed horizontal line of Figures 1A, IB and 1C indicates the minimum hardness desirable for the application.

- Likewise, Figures 1A-1C, Figures 2A, Figure 2B and Figure 2C show the effect of molybdenum (Figure 2A) and chromium (Figure 2B and Figure 2C) on tempering resistance. The greater the hardness at high temperatures, the greater the tempering resistance of the alloy. In all cases, the alloys were first annealed at 600 ° C.

- Figures 3A and Figure 3B show the CCT curve of the compositions of the present invention, considering two Cr contents. The quantitative hardenability results can be obtained from the number of phases formed (pearlite and bainite) and, most importantly, of the final hardness obtained by rate. The compositions are summarized in Table 1, base 3, considering the Cr content of 3% and 4% for comparison purposes. Figure 3A illustrates the CCT curve for 0.50% of C, composition of Cr of 3.00%, and Figure 3B shows the CCT curve for 0.50% of C, composition of 4.00% of Cr.

- Figure 4 shows the curve CCT of steel H13 of the prior art concept, whose data can be compared to the results of the steel of the present invention. The same data regarding the number of phases and hardness shown in Figures 3A-3B can be determined for different cooling rates.

- In Figures 5A and Figure 5B, the alloys with the final composition of the present invention, PI 1 to PI 3, are compared in terms of hardness after tempering (Figure 5A) and loss in hardness versus time (Figure 5B) to 600 ° C (mentioned in the tempering resistance text).

- Figure 6 compares the results of conducted impact toughness tests for two types of transverse test specimens: without notch (7 mm x 10 mm section, according to NADCA) or Charpy in V, with 10 mm x section 10 mm and notch V. All materials treated at a hardness of 45 HRC according to the parameters of Figure 5A.

- Figure 7 shows the hardness profile of the nitrified layer of the alloys PI 1, PI 2 and PI 3 against steel H13. A plasma nitriding process was conducted for H13 steel. Prior to nitriding, all alloys in the sample were quenched and tempered to reach 45 HRC.

Examples EXAMPLE 1: Effect of Molybdenum, Chromium and Carbon For this work, samples of approximately 200 g were collected in an experimental VIM furnace with varied composition for the same heating. Therefore, three warm-ups were produced by varying the content of Cr, Mo and C, as shown in table 1 below (details: Annex 1). The Hll steel served as a base for these alloys since it already has half the content of V. The materials were always characterized after special annealing (austenitization at 1010 ° C, oil solubility and over-annealing at 810 ° C). In this process we use annealing at 1020 ° C and annealing between 400 and 650 ° C. H13 steel, of typical industrial composition, was used as a base.

The hardness after annealing at 600 ° C is shown in Figures 1A-1C, highlighting the effects of reduced Mo and Cr content, and also the effect of a higher C content. With respect to the Mo content, a lower concentration of Mo results in a lower hardness after quenching. However, if the Cr content decreases, the hardness after tempering increases. One possibility is that a lower Cr content reduces the amount of M7C3 which, in turn, dissolves the Mo. Thus, a higher content of free Mo must be present in alloys of a lower Cr content, which explains a more intense response to tempering.

Despite this important effect of Cr, only the reduction of its content is not enough to promote the required hardness (approximately 45 HRC). Possibly, the required hardness can be obtained by tempering at lower temperatures. However, this practice is sometimes not feasible for hot work because the ideal tempering temperature should be 50 to 80 ° C above the working temperature to provide resistance to proper tempering. Thus, hot work involves extruded and molded aluminum, the typical tempering temperature must be 600 ° C.

Table 1: Chemical compositions adopted for the samples of the same heating with the variation of a single element. The asterisks used in the Cr and Mo fields in the table below indicate that several compositions using this base were produced for the same heating, increasing the content of this element, but maintaining the base composition of the heating.

* Variation of Mo: 0.05; 0.30; 0.60; 0.90; 1.22; 1.51; ** variation of Cr, considering 0.36% of C: 2.0; 3.0; 4.0; 5.1; 6.2; 7.1, *** Cr variation, considering 0.48% of C: 2.0; 3.0; 4.0; 5. 1; 6.1; 7.0; Therefore, to increase hardness after tempering at 600 ° C, we increased the C content. As shown in Figures 1A-1C, the result was effective and the hardness was even greater than that of the H13 that were obtained. In this case, the effect of C is related to the increased formation of secondary carbides and, when associated with a lower Cr content, it provides the hardness required to start the work, even in alloys with a lower content of Mo ( half of steel H13). In alloys with a higher content of C, a similar Cr effect can be observed.

In addition to the hardness after hardening, the loss of hardness is also an important factor in promoting the proper response by the alloys in question to the high temperatures at which they were subjected. The results shown in Figures 2A-2C show the effect of Mo important in this regard (Figure 2A), and also that the reduction of the Cr content is also an interesting option to reduce the loss of hardness, which means to redraw the curves at higher hardness levels (Figure 2B). In alloys with a higher C content (Figure 2C), this effect is even stronger. Thus, the combination Cr low / C high seems interesting.

On the other hand, the Cr content can not be too low, since the hardenability is not reduced. This effect was studied in the curves of Figures 3A-3B and compared to steel H13 in Figure 4. Quantitatively, the hardness reached after 0.3 and 0.1 ° C / s corresponds to steel H13 with high 635 HV and 521 HV ( Figure 4), while the Cr alloy of 3% corresponds to 595 HV and 464 HV under the same conditions (Figure 3A). The scenario changes for the Cr alloy of 4%, which reaches hardness = H13, in this case, 696 HV and 523 HV for speeds of 0.3 and 0.1 ° C / s (Figure 3B). Therefore, the Cr content near Cr of 4% seems to be more interesting. Extremely below this value, in this case, 3% Cr or less, bainite volume and hardness after tempering can avoid application. Thus, a Cr content of 3.8% was selected for the rest of the tests, the production of ingots at pilot scale and the evaluation of mechanical properties.

EXAMPLE 2: Effect of Al content After defining an objective alloy, four warmings (50 kg of molded ingots, average section of 140 mm) were produced and forged as plates (Table 2) with dimensions of 65mm x 165mm. The materials were then annealed after the same process described in Example 1 and their properties were evaluated as discussed below.

The results confirmed the initial results shown in Figures 1A-1C and 2A-2C, as shown in Figures 5A-5B. Thus, new alloys can achieve similar results in terms of hardness at 600 ° C (Figure 5A), or even improve them, in terms of tempering resistance, when compared to steel H13 (Figure 5B).

Table 2: Experimental 50 kg ingots produced the alloys of the present invention (PI) and H13 steel.

Another important point can be compared in Figure 6, in terms of tenacity. The toughness of the alloy of the present invention, when it is low in Al content, is equivalent to that of H13 steel. This shows that the low Si and P content of the PI 1 alloy compensates for the tenacity loss likely to occur while the C content increases in relation to the H 13 steel. Figure 6 also shows that tenacity 'is inversely proportional to the content of Al.

The content of Al is responsible for a significant increase in hardness after nitriding, as shown in Figure 7. Thus, for applications in which the high hardness of the nitrified layer is considered more relevant than toughness (for example, extrusion of solid forms), the alloy PI 2 becomes interesting for tenacity > 200J and extremely high hardness of the nitrified layer (almost 1400 HV). The alloy PI 3 shows no increases in terms of the nitrified layer, but the toughness is much lower.

On the other hand, in applications highly susceptible to cracking, such as pipe extrusion dies, tenacity can be considered a main property. For these cases, the PI1 alloy seems more appropriate, also showing hardness after nitriding similar to that of H13 steel, reaching more than 1000 HV on the surface, which is the typical specification for extrusion tools. In addition, as previously shown in Figures 5A-5B, the PI 1 alloy also exhibits improved thermal resistance properties.

Therefore, considering that the properties required for hot working applications, the alloys of the present invention show results equivalent to or better than those of steel H13. Such results are absolutely relevant for extrusion matrices of non-ferrous alloy, for example, Al alloys, or hot forging matrices. The PI 1 alloy has improved the tempering strength, but the hardness after nitriding and the toughness equivalent to the H13 steel, while the PI 2 alloy has lower toughness, but hardening resistance and hardness after significantly higher nitriding than steel H13. The alloy should be selected based on the most critical properties required for the application. However, in all cases, significant cost reductions can be obtained due to the low Mo and V content of the alloys of the present invention.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (10)

REIVI DICACIONES Having described the invention as above, the content of the following claims is claimed as property:

1. Steel for extrusion tools, characterized in that a composition of alloying elements comprises, in percentage by mass: carbon between 0.40 and 0.60, silicon below 1.0, phosphorus below 0.030, chromium between 2.5 and 4.5, molybdenum between 0.5 and 0.7, vanadium between 0.10 and 1.0, manganese under 1.0, aluminum between 0.1 and 1.0, the rest is composed of Fe and unavoidable deleterious substances.

2. Steel for extrusion tools, characterized in that a composition of alloy elements comprising, in percentage by mass: carbon between 0.40 and 0.60, silicon below 0.50, phosphorus below 0.030, chromium between 3.0 and 4.2, molybdenum between 0.55 and 0.65, vanadium between 0.30 and 0.8, manganese below 0.8, aluminum between 0.2 and 0.80, the rest is composed of unavoidable deleterious substances.

3. Steel for extrusion tools, characterized in that a composition of alloy elements consists essentially of, in mass percentage, carbon between 0.45 and 0.55, silicon below 0.5, phosphorus below 0.030, chromium between 3.5 and 4.2, molybdenum between 0.55 and 0.65 , vanadium between 0.30 and 0.50, manganese below 0.50, aluminum between 0.4 and 0.60, the rest is composed of unavoidable deleterious substances.

4. Steel for extrusion tools, according to any of claims 1 to 3, characterized in that it has molybdenum substituted with tungsten in a proportion of lMo = 2W.

5. Steel for extrusion tools, according to any of claims 1 to 4, characterized in that it has vanadium substituted with niobium in a ratio of lV = 2Nb or lTi.

6. Steel for extrusion tools, according to any of claims 1 to 5, characterized in that it is applied to molds, dies and tools of general use to form solid and liquid materials, at room temperature or temperatures at 1300 ° C.

7. Steel for extrusion tools, according to any of claims 1 to 6, characterized in that it is applied to tools for forming metals at temperatures between 300 and 1300 ° C, in forging, extrusion or melting applications of ferrous or non-ferrous alloys.

8. Steel for extrusion tools, according to any of claims 1 to 7, characterized in that it is applied to non-ferrous hot extrusion tools, particularly aluminum alloys, and to pipe extrusion dies or solid form.

9. Steel for extrusion tools, according to any of claims 1 to 8, characterized in that it is produced for processes involving ingot melting and hot and cold forming, or even used with the coarse melting structure.

10. Steel for extrusion tools, according to any of claims 1 to 9, characterized in that it is produced for processes involving liquid metal fragmentation, such as powder metallurgy, powder injection or aerosol forming processes.