JP2013521411A - Tool steel for extrusion - Google Patents

Abstract

The present invention is low cost compared to conventional H13 steel, has high tempering resistance, has a chemical composition (in weight percent) of 0.4 to 0.6 carbon, less than 1 silicon, 0.03 Less than phosphorus, 2.5 to 4.5 chromium, tungsten optionally substituted in a ratio of 2W to 1Mo by 0.5 to 0.7 molybdenum, 0.1 to 1 vanadium, less than 1 manganese Further, the present invention relates to a steel for an extrusion tool whose balance is substantially Fe and inevitable impurities. As an option to promote high surface hardness after nitriding, the steel of the present invention can have an aluminum content of up to 1.0, but for high toughness, the aluminum content must be kept below 0.10. I must.

Description

  The present invention relates to steels intended for use in various hot forming tools and dies, especially hot forming tools and dies for extrusion of aluminum alloys or other non-ferrous metals. This material was originally designed for the extrusion process, but can also be employed in other hot forming processes where the metal to be formed can withstand temperatures in excess of 600 ° C. However, this steel can also be employed in processes at lower or ambient temperatures. The composition of the steel in question is that the steel can be classified as a hot work tool steel, the main feature being low in high-cost alloying elements such as molybdenum and vanadium, but against tempering resistance (or hardness reduction) Resistance) is greater than the conventional steel of the prior art concept. An alternative to the steel of the present invention is provided to increase the hardness after nitriding, which can result in a higher level of performance than that of the conventional steel, while at the same time reducing costs with a simpler chemical composition. Can be kept low. This effect is possible by carefully designing the alloy and setting optimal ranges for the elements, ie carbon, chromium, molybdenum and aluminum.

  The term hot working tool applies to a number of hot forming processes and has been adopted in the industry and its focus is on the manufacture of parts for machine applications, especially automotive parts. The best known hot forming processes are forging steel and extruding or casting non-ferrous alloys. Other applications carried out at high temperatures, typically above 500/600 ° C., can also be classified as hot working. In these applications, molds, molds, punches, inserts and other forming devices are categorized by the generic term hot working tools. These tools are usually made of steel that requires special properties to withstand the high temperatures and mechanical stresses of the processes employed.

  Among these main properties of hot-worked steel, the following: physical resistance such as resistance after high-temperature tempering, resistance to hardness reduction called tempering resistance, toughness, hardenability and thermal conductivity and specific heat The characteristic characteristics are outstanding.

  Extrusion dies used for non-ferrous alloys, especially aluminum alloys, are the main hot working object for applying the steel of the present invention. These typical dies occupy an important position in the tool steel market both in Brazil and abroad. In this application, the steel is based on steels such as ABNT H13 (see Table 1), with emphasis on lower manufacturing costs, but with less stringent quality requirements than other applications, such as those of die cast casting. Is well standardized.

  The cost of alloys, especially Mo and V, has greatly diminished this sector, and therefore anxious for low-cost alternatives. Low alloy steels such as DIN 1.2714 (chemical composition shown in Table 1) are employed. However, their low wear resistance due to reduced hot strength and low hardness after nitriding hinders their use.

  The focus of recent developments, such as US Patent Application Publication No. 2009/0191086, has been on the reduction of alloying elements by reducing Cr, Mo and V contents. However, reducing the amount of Cr has a negative effect. First, the alloy composition is not sufficient to achieve high hardness after tempering (at least 45 HRC after tempering at 600 ° C.). Second, the reduced amount of Cr may result in lower hardness after nitriding, which is a clear gain caused by nitriding in extrusion applications (substantially all extrusion dies are now nitrided) Is not appropriate for these applications.

Table 1 Typical chemical composition of steel of the prior art concept. Mo, V, and Co have the highest cost and are closely related to the final cost of the tool steel, so the sum of Mo + V + Co is shown. Ingredients are percentages by mass with the balance being Fe. For all extrusion applications, the amount of W is low, usually less than 0.1%.

  A third problem of the invention of US 2009/0191086 relates to the possibility of lower mold core hardness due to reduced hardenability as a result of reduced Cr and Mo content. doing. In order to avoid this, the invention alloy of US Patent Application Publication No. 2009/0191086 has a higher Mn content, but this increases the hardenability and may cause segregation problems (band formation) and excess. Austenite residue is obtained. Both effects can compromise the ultimate hardness and toughness, and thus the tool life. The last aspect can also be stated with respect to high Mn content. That is, scrap from this steel can hardly be incorporated into the production of conventional hot-worked steel with low Mn content.

US Patent Application Publication No. 2009/0191086

  Given all these drawbacks, the authors consider that US Patent Application Publication No. 2009/0191086 is a cost-cutting solution but inferior in characteristics. In this patent document, the authors determine that the expected efficiency loss is about 20-30% lower than that of steel H13. Considering the machining and heat treatment costs associated with the die, this efficiency loss is considered critical. Therefore, material costs need to be reduced by over 30% to compensate for replacement. For example, given that only 60% of the final die cost is associated with the tool steel used, only 30% lower life is possible when the cost of the new material is half that of the conventional material. This is true from 2005 to 2008 when raw material costs were at their peak (but this is still difficult to happen because the required cost difference is too high). However, in the current scenario, this cost reduction can hardly be achieved for steel H13, considering only the reduction of Mo and Cr. Thus, the cost reduction associated with the loss of efficiency of the invention alloy of US 2009/0191086 is now considered impractical for this application.

  Considering this scenario, there is a clear need for a tool steel that has performance equivalent to that of steel H13, is less costly than steel H13, and has a positive effect on tool life efficiently. . This is because the steel in question has the same tempering resistance as steel H13 and hardness after tempering at 600 ° C. (typical heat treatment conditions), but with a lower alloy element content and suitable after nitriding. Only possible if it has hardness. Furthermore, the materials used have a high hardenability, but there should be no problems associated with high amounts of Mn. In this way, it can be applied to a tool larger than the extrusion die.

  As such, the steel of the present invention meets all these needs.

  In order to achieve the goal of cost reduction / zero quality loss, the effects of the main elements Cr and Mo related to hot strength were studied separately. Apart from important findings, this study also showed that changes in the content of these elements were not sufficient to promote the required hot strength. In this way, the amount of C could be increased while reducing the amount of P and the amount of Si until reaching a level that does not affect toughness. Finally, the effect of Al was used to compensate for the possible Cr reduction and hence the reduction in hardness after nitriding. This study also noted this point because the nitrided layer is essential to impart wear resistance to various hot forming tools, particularly extrusion and hot forging tools.

Therefore, in order to satisfy the above conditions, the steel of the present invention has the following alloy composition in mass percentage.
0.40 to 0.60 C, preferably 0.45 to 0.55 C, typically 0.50 C
2.5-4.5 Cr, preferably 3.0-4.2 Cr, typically 3.8 Cr
0.30 to 0.90 Mo, preferably 0.50 to 0.70 Mo, typically 0.60 Mo. Considering its chemical similarity with W, Mo may be replaced with W in a mass ratio of 2W: 1Mo.
V of 0.1 to 1.0, preferably 0.3 to 0.8 V, typically 0.4. V may be partially or completely replaced with Nb according to a mass ratio of 1Nb: 0.5V.
Up to 1.0 Si, preferably up to 0.50 Si, typically 0.30 Si,
Up to 1.0 Mn, preferably up to 0.80 Mn, typically up to 0.50 Mn.

As described below, Al can be added simultaneously to the alloys of the present invention because of its advantage with respect to hardness after nitriding, but it also has a negative effect on toughness and complexity of the steel manufacturing process. Therefore, the amount of Al must be added as a percentage by mass as follows.
Up to 1.0 Al, preferably up to 0.80 Al, typically up to 0.60 Al. For compositions where the effects of Al are not targeted, this element should be treated as a residual impurity limited to 0.10, typically less than 0.05.

This composition should be characterized by a balance consisting of Fe (iron) and metal or non-metal toxics unavoidable in the steel manufacturing process. Here, examples of the non-metallic harmful substances include, but are not limited to, the following elements by mass percentage.
A maximum of 0.030 P, preferably a maximum of 0.015 P, typically a maximum of 0.010 P.
Up to 0.10 S, preferably up to 0.030 S, typically up to 0.008 S.
Ni or Co up to 1.5, preferably up to 1.0 Ni or Co, typically less than 0.5 Ni and Co.

  The applicant will then explain the ratio of the new material composition specifications. The percentages listed refer to mass percent.

  C: Carbon is mainly involved in martensite quenching under low temperature conditions. However, together with the alloying elements, carbon plays a role in secondary quenching, which is important for high temperature quenching. In these cases, the amount of C is more important for hardness at temperatures below 600 ° C. At this time, the hardness still depends on the martensite hardness or the formation of cementite or Cr carbide. In addition, carbon is an important hardenability promoting element and does not increase costs. Increasing the hardness to 45 HRC or more is equally important, and a carbon content of at least 0.40%, preferably greater than 0.45% is recommended. On the other hand, when the amount of C is very large, excessively precipitated carbide particles are precipitated during quenching (especially when the amounts of Mo and V are high). In addition, the hardness and amount of secondary carbide increases. Therefore, toughness is usually impaired. The amount of C should be limited to a maximum of 0.60%, preferably less than 0.55%. This limitation also serves to reduce the amount of retained austenite and prevents problems associated with dimensional instability and embrittlement.

Since Cr: chromium favors the hardenability which is important for large tool applications, the chromium content should be greater than 2.5% and preferably greater than 3.0%. However, the chromium content should be limited. The present invention incorporates the concept of reducing Cr content to improve tempering resistance. The mechanism of this effect is not fully understood, but may be related to the formation of secondary Cr carbides of the M ₇ C ₃ type, which is the first carbide formed by dissolving Mo and V. Therefore, the lower the amount of Cr, the smaller the amount of M ₇ C ₃ carbide. Therefore, the amount of Mo and V available for forming fine carbides M ₂ C and MC, which are also important for secondary quenching, is increased. The end result is that the tempering resistance is significantly higher in steels with lower Cr content, thus allowing a reduction in Mo content when compared to prior art concept steels.

Mo and W: Low concentration of Mo in the present invention is not only for cost reduction, but also tempering equivalent to or greater than that of the highest secondary hardness and steel H13 in relation to Cr and C content. Adopted to promote resistance. To that end, the alloy of the present invention must contain at least 0.30%, preferably more than 0.50%. On the other hand, if the amount of Mo is remarkably high, the toughness may be impaired by precipitation of primary crystal carbides during the rapid cooling stage, which may significantly increase the alloy cost in the direction opposite to the cost reduction target of the present invention. Therefore, the Mo content should be limited to 0.90%, preferably less than 0.70%. Tungsten and molybdenum produce a similar effect in the tool steel of the present invention to form M ₂ C or M ₆ C secondary carbides. They can therefore both be specified together by the tungsten equivalent relationship (W _eq ) given by the sum of W + 2Mo which normalizes the difference in atomic weight between the two elements.

  V: Vanadium is mainly important for the formation of MC secondary carbide. Since they are very thin, these carbides prevent dislocation line migration and increase mechanical strength. V also improves grain growth and makes it possible to increase the austenitizing temperature (above 1000 ° C.). For such an effect, V must be greater than 0.1%, preferably greater than 0.3%. However, an excessively high degree of V can generate primary carbides that are difficult to solubilize, thus reducing toughness and further promoting a significant increase in cost. Therefore, the V content should be less than 1.0%, preferably less than 0.6%.

  Si: Silicon has a strong effect on secondary quenching and toughness. If the Si concentration is low, the secondary carbide is better distributed and the toughness is improved. Therefore, the Si content of the material of the present invention should be less than 1.0%, typically less than 0.5%.

  Mn: A high amount of Mn promotes intense microsegregation that produces stripes of varying degrees of hardness, which is considered undesirable for further increasing the amount of retained austenite. Therefore, Mn is regarded as a harmful element in the present invention. Therefore, the amount of Mn should be limited to 1.0%, preferably less than 0.8%, typically less than 0.50%.

  Al: The amount of Al in the alloy can be high to promote greater hardness of the nitrided layer. However, the Al content should be limited to 1.0% as it results in reduced toughness under these conditions. Therefore, an Al content of 0.40% to 0.60% can be an interesting content for this purpose. However, for applications where the hardness of the nitrided layer is slightly lower than that of steel H13 but high toughness is required, the Al content of the alloys of the present invention is less than 0.1%, typically 0.00. It can be less than 05%.

  Residual elements: Other elements such as Ni and Co should be considered as hazardous materials associated with the deoxidation process of steel production or specific to the production process. Therefore, the Ni content and Co content should be limited to 1.5%, preferably less than 1.0%. Regarding the formation of inclusions, sulfur should be controlled because inclusions can cause cracking during operation. Therefore, the amount of S should remain below 0.050%, preferably below 0.020%. Furthermore, because of increased toughness, brittle elements such as P should be avoided, P <0.030%, preferably P <0.015%, typically P <0.010%. It is desirable. Indeed, the low Cr content also helps to reduce the P content in the electric arc furnace steel manufacturing process, thus leading to a conclusion consistent with the desired cost reduction philosophy.

  The above alloys can be produced as rolled products or forged products such as wires, rods, steel wires, sheets and steel strips by conventional or special processes such as powder metallurgy, thermal spray molding or continuous casting.

  The experiments performed are described below and reference is made to the following accompanying figures.

The figure which shows the effect with respect to the hardness after tempering in 600 degreeC of Mo amount. The figure which shows the effect of the amount of Cr in 0.60% Mo and normal C amount. The figure which shows the effect of Cr amount in 0.60% Mo and higher C amount. The horizontal dashed lines in FIGS. 1A, 1B and 1C indicate the minimum hardness desired for the application. The figure which shows the effect with respect to the tempering tolerance of molybdenum similarly to FIG. The figure which shows the effect with respect to the tempering tolerance of chromium. The figure which shows the effect with respect to the tempering tolerance of chromium. The higher the hardness at high temperatures, the greater the tempering resistance of the alloy. In all cases (FIGS. 2A-2C), the alloy was initially annealed at 600 ° C. CCT curve for 0.50% C, 3.00% Cr composition. Quantitative hardenability results can be obtained from the number of phases formed (pearlite and bainite), most importantly from the final hardness obtained for speed. The compositions are summarized in Table 1, Base 3, based on 3% and 4% Cr content for comparison. CCT curve for 0.50% C, 4.00% Cr composition. The figure which shows the CCT curve of H13 steel of a prior art concept. The data can be compared with the results for the steel of the present invention. The same data regarding the number and hardness of the phases shown in FIG. 3 can be evaluated for various cooling rates. Comparison of hardness after tempering of alloys PI1 to PI3 having the final composition of the present invention. Comparison of hardness loss at 600 ° C. vs. time (referred to tempering resistance text) for alloys PI1-PI3 having the final composition of the present invention. FIG. 4 compares the results of impact toughness tests performed on Charpy V with two types of laterally folded specimens: no notch (7 mm × 100 mm cross section by NADCA) or 10 mm × 10 mm cross section and V notch. All materials processed to a hardness of 45 HRC according to the parameters of FIG. 5a. The figure which shows the hardness profile of the nitride layer of alloy PI1, PI2, and PI3 versus steel H13. A plasma nitriding process was performed on steel H13. Prior to nitriding, all sample alloys were quenched and tempered to reach 45 HRC.

Example 1 Effect of Molybdenum, Chromium and Carbon For this study, approximately 200 g samples of the same dissolution and different composition were collected in a laboratory VIM furnace. As shown in Table 1 below, three steels were produced with varying amounts of Cr, Mo and C (Details: Appendix 1). Steel H11 already had half of the V content and was therefore the base for these alloys. The material was always characterized after a special heat treatment (austenitizing at 1010 ° C., oil-solubilizing and over-annealing at 810 ° C.). In this process, Applicants used annealing at 1020 ° C and tempering at 400-650 ° C. A typical industrial composition steel H13 was used as a base.

The hardness after tempering at 600 ° C. is shown in FIG. The effect of reducing the amount of Mo and Cr is shown, and the effect of increasing the amount of C is also shown. Regarding the amount of Mo, when the Mo concentration is low, the hardness after tempering becomes lower. However, when the Cr content decreases, the hardness after tempering increases. If the amount of Cr is low, the amount of M ₇ C ₃ decreases, which can then dissolve Mo. Therefore, a high content of free Mo should be present in low Cr content alloys, which explains a stronger response to tempering.

  Despite this important Cr effect, reducing the amount alone is not sufficient to promote the required hardness (about 45 HRC). In some cases, the required hardness can be obtained by tempering at a lower temperature. However, this practice may not be feasible for hot working because the ideal tempering temperature should be 50-80 ° C. higher than the working temperature to provide adequate tempering resistance. Thus, for hot working including extruded and cast aluminum, a typical tempering temperature should be 600 ° C.

Table 1. Chemical composition used for samples from the same dissolution where one element changes. The asterisks in the Cr and Mo columns in the table below indicate that several compositions using this base are manufactured from the same melt, increasing the content of this element while maintaining the base composition of the molten steel. It has been shown.

  In order to increase the hardness after tempering at 600 ° C., the applicant increased the amount of C. As shown in FIG. 1, the result was effective and a hardness higher than that of H13 was obtained. In this case, the effect of C is related to an increase in the formation of secondary carbides, which, when associated with lower Cr content, initiates this study even in alloys with lower Mo content (half of steel H13). To provide the necessary hardness. Similar Cr effects can be observed in higher C content alloys.

  In addition to hardness after tempering, loss of hardness is also a major factor in promoting the proper response to high temperatures experienced by the alloy in question. The results shown in FIG. 2 demonstrate an important Mo effect on this (FIG. 2A), and Cr content reduction is also an interesting option to reduce hardness loss (this makes the curve more It also demonstrates that it means replotting to a higher hardness level (see FIG. 2B)). This effect is even greater in alloys with higher C content (FIG. 2C). Thus, the low Cr / high C combination seems interesting.

  On the other hand, the Cr content cannot be made too low so that the hardenability is not reduced. This effect was examined in the curve of FIG. 3 and compared with steel H13 in FIG. Quantitatively, the hardness reached after 0.3 and 0.1 ° C./s corresponds to steel H13 with 635 HV and 521 HV (FIG. 4). In contrast, the 3% Cr alloy corresponds to 595 HV and 464 HV under the same conditions (FIG. 3A). For a 4% Cr alloy this reaches a hardness of H13 or higher, ie 696 HV and 523 HV for speeds of 0.3 and 0.1 ° C./s (FIG. 3B). Therefore, the Cr amount close to 4% Cr seems more interesting. If the Cr is significantly lower than this value, that is, 3% or less, the amount of bainite and the hardness after tempering may hinder the use. Therefore, a 3.8% Cr content was chosen for all other tests, pilot scale billet production and mechanical property evaluation.

Example 2 Effect of Al amount After the target of the alloy was determined, four steel materials (50 kg cast billet, 140 mm average cross section) were manufactured and forged into a steel plate having a size of 65 mm × 165 mm (Table 2). The materials were then annealed according to the same process described in Example 1 and their properties were evaluated as discussed below.

  This result confirmed the initial results shown in FIGS. 1 and 2 as shown in FIG. Thus, the new alloy can reach a similar result with respect to hardness at 600 ° C. (FIG. 5A) or even better with respect to tempering resistance when compared to steel H13 (FIG. 5A). FIG. 5B).

Table 2 Experimental 50 kg billet (PI) and steel H13 produced for the alloys of the present invention

  Another important point can be compared in FIG. 6 regarding toughness. The toughness of the alloy of the present invention is equivalent to that of steel H13 when the Al content is low. This demonstrates that the low Si and P content of alloy PI1 compensates for the loss of toughness that is likely to occur as the C content increases relative to steel H13. FIG. 6 also shows that toughness is inversely proportional to the amount of Al.

  As shown in FIG. 7, the amount of Al is involved in a significant increase in hardness after nitriding. Therefore, for applications where the high hardness of the nitrided layer is considered more relevant than toughness (eg, solid form extrusion), alloy PI2 has a toughness exceeding 200 J and a significantly higher hardness of the nitrided layer (almost It is interesting to have 1400HV). Alloy PI3 shows no increase with respect to the nitrided layer, but the toughness is much lower.

  On the other hand, in applications that are highly susceptible to cracking, such as pipe extrusion dies, toughness can be considered a major characteristic. For these cases, alloy PI1 appears to be more suitable and also exhibits a post-nitridation hardness similar to that of steel H13, which reaches over 1000 HV at the surface, which is typical for extrusion tools. Moreover, as already shown in FIG. 5, alloy PI1 also exhibits improved hot strength properties.

  Thus, in light of the properties required for hot working applications, the alloys of the present invention show results that are equal to or better than those of steel H13. Such results are of considerable relevance to non-ferrous alloys, such as Al alloy extrusion dies, or hot forging dies. Alloy PI1 has improved tempering resistance, but has post-nitridation hardness and toughness similar to steel H13, while alloy PI2 has lower toughness but significantly higher tempering resistance and steel H13. Hardness after nitriding. The alloy should be selected based on the most critical properties required for the application. However, in all cases, significant cost savings can be obtained with the low Mo and V contents of the alloys of the present invention.

Claims (16)

  In mass percent, 0.40 to 0.60 C, less than 1.0 Si, less than 0.030 P, 2.5 to 4.5 Cr, 0.5 to 0.7 Mo,. A steel for an extrusion tool characterized by an alloy composition consisting essentially of V of 10 to 1.0, Mn of less than 1.0, a maximum of Al, the balance being Fe and inevitable harmful substances.   In weight percent, 0.40 to 0.60 C, less than 0.50 Si, less than 0.030 P, 3.0 to 4.2 Cr, 0.55 to 0.65 Mo,. Steel for extrusion tools characterized by an alloy composition consisting essentially of V of 30 to 0.8, Mn of less than 0.8, Al of maximum 0.80, the balance being Fe and inevitable harmful substances.   In weight percent, 0.45 to 0.55 C, less than 0.5 Si, less than 0.030 P, 3.5 to 4.2 Cr, 0.55 to 0.65 Mo, 0. Steel for extrusion tools characterized by an alloy composition consisting essentially of V of 30 to 0.50, Mn of less than 0.50, Al of maximum 0.60, the balance being Fe and inevitable harmful substances.   The steel for an extrusion tool according to any one of claims 1 to 3, wherein the amount of Al is limited to 0.10 mass percent.   The steel for an extrusion tool according to any one of claims 1 to 3, wherein the aluminum content is 0.05 mass percent or less.   The steel for an extrusion tool according to any one of claims 1 to 5, wherein the amount of cobalt and the amount of nickel are less than 1.0 mass percent.   The steel for an extrusion tool according to any one of claims 1 to 6, wherein the amount of phosphorus and the amount of sulfur are less than 0.030 mass percent.   The steel for an extrusion tool according to any one of claims 1 to 7, wherein the amount of phosphorus is less than 0.010 mass percent.   The steel for extrusion tools according to any one of claims 1 to 8, wherein Mo is substituted with W in a ratio of 1Mo = 2W. V is
1V = 2Nb or 1Ti
The steel for an extrusion tool according to any one of claims 1 to 9, wherein Nb or Ti is substituted at a ratio of   It is steel for use after quenching and tempering in order to obtain a hardness of 30-60 HRC, and then performing a nitriding heat treatment to obtain a surface hardness of 800-1500 HV for a maximum thickness of 0.50 mm. The steel for extrusion tools according to any one of claims 1 to 10, wherein the steel is for extrusion tools.   12. The method according to claim 1, wherein the solid material and the liquid material are applied to a mold, a die and a general use tool for forming a solid material and a liquid material at room temperature or a temperature of a maximum of 1300 ° C. The steel for extrusion tools described.   It is applied to the tool for shape | molding a metal at the temperature of 300-1300 degreeC in the use of forging, extrusion, or casting of a ferrous alloy or a nonferrous alloy, The any one of Claim 1-11 characterized by the above-mentioned. Steel for extrusion tools described in 1.   12. Extrusion tool steel according to any one of claims 1 to 11, characterized in that it is applied to hot extrusion tools of non-ferrous alloys, especially aluminum alloys, and extrusion dies of solid shapes or pipes. .   15. Any one of claims 1 to 14, characterized in that it is manufactured for processes including billet casting and hot forming and cold forming, or is used in conjunction with the overall composition of melting. The steel for extrusion tools described in the item.   15. Manufactured for a process involving fragmentation of a liquid metal, such as powder metallurgy, powder injection or thermal spray forming processes, according to any one of claims 1 to 14 Steel for extrusion tools.