Patent Application
20130243639
The present invention relates to a steel intended for use in various hot form tools and dies, particularly for extrusion of aluminum alloys or other non-ferrous metals. Although initially designed for extrusion processes, the material can also be employed in other hot forming processes, in which the metal to be formed withstands temperatures above 600° C., although the said steel can be employed in processes at lower temperatures or even at ambient temperature. The composition of the steel in question allows it to be classified as hot work tool steel, whose primary characteristic the lower content of high-cost alloying elements, such as molybdenum and vanadium, but with tempering resistance (or resistance to loss of hardness) greater than that of conventional steels of prior art concept. An additional alternative to the steel of the present invention is provided to increase hardness after nitriding, and may result in performance levels even greater than those of conventional steels, at the same time that the cost is kept low due to a simpler chemical composition. Such an effect is possible by carefully designing the alloy, and setting the optimum ranges of the elements: carbon, chromium, molybdenum and aluminum.
The term hot work tools is applied to a large number of hot-forming operations, employed in industries and focused on the production of parts for mechanical applications, especially automotive parts. The most popular hot-forming processes are forging of steel, and the extrusion or casting of non-ferrous alloys. Other applications performed 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 efforts of the processes in which those tools are employed.
Among their key properties of hot work steels, the following stand out: resistance after high temperature tempering, the resistance to the loss of hardness called tempering resistance, the toughness, the hardenability and physical properties such as thermal conductivity and specific heat.
The extrusion dies used for non-ferrous alloys, especially aluminum alloys are main hot work target for applying the steel of the present invention. These typical dies comprise an important segment of the tool steel market both in Brazil and abroad. In this application, the steels are very standardized, based on steels such as ABNT H13 (see Table 1), with quality requirements not as strict as those of other applications, e.g., pressure die casting, but with emphasis on lower production costs.
The increased cost of metal alloys, especially Mo and V, significantly impaired this segment, making it eager for low-cost alternatives. Low-alloy steels have been employed, such as DIN 1.2714 (chemical composition given in Table 1). However, their low wear resistance due to reduced hot strength and lower post-nitriding hardness prevents them from being applied.
Recent developments, such as US 2009/0191086, were focused on the reduction of alloying elements, by means of reduced Cr, Mo and V content. However, negative effects are produced by reducing the Cr content. First, the alloys' composition is not sufficient to achieve high hardness after tempering (at least 45 HRC after tempering at 600° C.). Second, a reduced Cr content can also generate lower hardness after nitriding, which is not suitable for extrusion applications, considering the apparent gain produced by nitridation in these applications (virtually all extrusion dies are currently nitrified).
A third problem of invention US 2009/0191086 relates to the hardness of the die core, which may be lower due to decreased hardenability as a result of reduced Cr and Mo contents. To avoid this, the alloys of invention US 2009/0191086 have higher Mn content, which lead to higher hardenability, potential segregation problems (banding) and excessive austenite retention. Both effects may impair the final hardness and toughness and, thus, the tool life. A final aspect can also be mentioned, with regard to the high Mn content: Scrap from this steel can hardly be incorporated into the production of conventional, low-Mn-content hot work steels.
Given all these drawbacks, 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 expected efficiency loss, of about 20 to 30% lower than that of steel H13. Considering the machining and heat treatment costs associated with dies, this efficiency loss can be considered quite 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 die cost is associated with the tool steel used, a 30% lower life can only be viable if the cost of the new material is half the cost of the conventional material. From 2005 to 2008, when the cost of raw materials peaked, this could be true (though still difficult to occur, because the cost difference required is too high). However, for the current scenario, such cost reduction can hardly be achieved for steel H13, considering only the reduction of the Mo and Cr contents. Thus, reduction in cost associated with efficiency loss of the alloy of patent US 2009/0191086 can be currently considered impractical for such application.
Given this scenario, it is evident the need for a tool steel which effectively has a positive effect on tool life by means of an equivalent performance, but at a cost lower than that of steel H13. This is only possible if the steel in question has tempering resistance and hardness after tempering at 600° C. (typical heat treatment condition) equivalent to those of steel H13, but with lower content of alloying elements and suitable 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 tools larger than extrusion dies.
Therefore, the steel of the present invention will fulfill all these needs.
To achieve the cost reduction/zero quality loss goal, the effect of the key elements related to hot strength, Cr and Mo, was studied separately. Apart from significant findings, this study also showed that the variation of the content of these elements is not sufficient to promote the hot strength required. Thus, the C content could be increased up to levels that did not impact toughness, especially whit the accompaniment of low P and Si contents. Finally, the Al effect was used to compensate for the reduction of Cr, hence, a potential lower hardness after nitriding. This work also focused on this issue because the nitrified layer is critical to providing wear resistance to various hot forming tools, especially extrusion and hot forging tools.
Therefore, in order to satisfy the above conditions, the steel of the present invention has a composition of alloying elements, which, in percentage by mass consists of:
As described below, Al can be added simultaneously to the alloys of the present invention to provide gains in terms of hardness after nitriding, but also negative effects in terms of toughness and complexity of the steel-making process. Thus, the Al content must be dosed as follows, in percentage by mass:
The compositions should be characterized by balance by Fe (iron) and metallic or non-metallic deleterious substances inevitable to the steelmaking process, in which said non-metallic deleterious substances include but are not limited to the following elements, in percentage by mass:
Max 0.030 P, preferably max 0.015 P, typically max 0.010 P.
Max 0.10 S, preferably max 0.030 S, typically max 0.008 S.
Max 1.5 Ni or Co, preferably up to 1.0 Ni or Co, typically below 0.5 Ni and Co.
Next, we describe the ratios of the specification of the composition of the new material. The percentages listed refer to percent by mass.
C: Carbon is primarily responsible for martensite hardening under low temperature conditions. However, together with the alloying elements, carbon also plays a role in the secondary hardening, important for the hardening at high temperature. In these cases, the C content is more important for hardness at temperatures below 600° C., when hardness still depends on the martensite hardness or formation of cementite or Cr carbides. Furthermore, carbon is an important hardenability-promoting element, and causes no increase in cost. It is also considered important to increase hardness to 45 HRC and up, carbon contents of at least 0.40% are recommended, preferably above 0.45%. On the other hand, very high C contents, cause excessive precipitation of grain-shaped carbides at the time of quenching (especially when Mo and V contents are high), as well as lead to increased hardness and volume of secondary carbides. Thus, toughness is generally impaired, the C content should be limited to a maximum value of 0.60%, preferably below 0.55%. This limitation also plays a role in the reduction of the amount of retained austenite, preventing problems associated with dimensional instability and embrittlement.
Cr: The chromium content should be higher than 2.5%, preferably greater than 3.0%, because this element favors hardenability, which is important for application in large tools. However, the Cr content should be limited. The present invention has incorporated the concept of reducing the Cr content to improve tempering resistance. The mechanisms of this effect are not fully understood but they may be related to the formation of secondary Cr carbides, M7C3-type, which dissolve Mo and V are the first carbides to be formed. Therefore, the lower the Cr content, the lower the amount of M7C3 carbides and, thus, the greater the amount of Mo and V available for the formation of fine carbides M2C and MC, which are also important for secondary hardening. The end result is a significantly higher tempering resistance in steels with lower Cr content, thus enabling the reduction of the Mo content when compared to steels of prior art concept.
Si: silicon produces a strong effect on secondary hardening and toughness. When a low Si concentration is used, toughness improves due to a better distribution of secondary carbides. Therefore, the Si content of the material of the present invention must be lower than 1.0%, typically below 0.5%.
Mn: high Mn contents may be considered undesirable for promoting intense micro-segregation generating banding at different degrees of hardness, and for increasing the retained austenite content; 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 alloys' Al content can be high. However, the Al content, under these conditions, should be limited to 1.0% because they lead to decreased toughness. Thus, Al contents between 0.40% and 0.60% may be of interest for this purpose. However, for applications in which the hardness of the nitrified layer is slightly lower than that of steel H13, but high toughness is required, the Al content of the alloy of the present invention can be <0.1%, typically below 0.05%.
Residual Elements: Other elements such as Ni and Co should be considered as deleterious substances associated with the steelmaking deoxidation processes or inherent to the manufacturing processes. Hence, the Ni and Co content should be limited to 1.5%, preferably below 1.0%. In terms of formation of inclusions, the sulfur content should be controlled, because such inclusions may lead to cracking during operation; therefore the S content should remain below 0.050%, preferably below 0.020%. Also, for high toughness purposes, embrittling elements such as P should be avoided, being desirable P<0.030%, preferably P<0.015%, typically P<0.010%. Indeed, a low Cr content also helps to reduce the P content in electric arc furnace steelmaking processes, thus leading to conclusions that are not contradictory to the cost reduction philosophy desired.
The alloy, as described above, can be produced as rolled or forged products through conventional or special processes such as powder metallurgy, spray forming or continuous casting, such as wire rods, bars, wires, sheets and strips.
The experiments carried out are described below, and reference is made to the following attached figures:
Likewise
In
For this work, samples of approximately 200 g were collected in an experimental VIM furnace with varied composition for the same heat. Therefore, three heats were produced by varying the Cr, Mo and C contents, as shown in Table 1 below (details: Annex 1). Steel H11 served as a base for these alloys since it already has half of V content. The materials were always characterized after special annealing (austenitizing at 1010° C., oil solubilization and over-annealing at 810° C.). In this process we used annealing at 1020° C. and tempering between 400 and 650° C. Steel H13, of typical industrial composition, was used as a base.
Hardness after tempering at 600° C. is shown in
Despite this important Cr effect, just reducing its content is not sufficient to promote the required hardness (about 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 proper tempering resistance. Thus, for hot work involving extruded and cast aluminum, the typical tempering temperature should be 600° C.
Table 1: Chemical compositions adopted for samples from the same heat with variation of a single element. The asterisks used in the Cr and Mo fields of the table below indicate that several compositions using this base were produced for the same heat, increasing the content of this element, but keeping the base composition of the heat.
Therefore, to increase hardness after tempering at 600° C., we increased the C content. As shown in
Besides hardness after tempering, loss of hardness is also a key factor to promote adequate response by the alloys in question to the high temperatures they are subjected to. The results shown in
On the other hand, the Cr content cannot be too low, such that hardenability is not reduced. This effect was studied in the curves of
After defining an alloy target, four heats (50 kg cast billets, 140 mm average section) were produced and forged as plates (Table 2) with dimensions of 65 mm×165 mm. The materials were then annealed following the same process described in Example 1 and their properties were evaluated as discussed below.
The results confirmed the initial results shown in
Another important point can be compared in
Al contents are responsible for a significant increase of hardness after nitriding, as shown in
On the other hand, in applications highly susceptible to cracking, such as pipe extrusion dies, toughness can be considered a key property. For these cases, alloy PI1 seems more appropriate, also showing hardness after nitriding similar to that of steel H13, reaching more than 1000 HV on surface, which is the typical specification for extrusion tools. Furthermore, as previously shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| PI 1003185-5 | Mar 2010 | BR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/BR11/00059 | 3/4/2011 | WO | 00 | 12/13/2012 |
