STEEL WITH HIGH TEMPERING RESISTANCE
STEEL WITH HIGH TEMPERING RESISTANCE comprising a composition of alloying elements consisting essentially of, in percent by mass, C between 0.20 and 0.50, Si lower than 1.0, P lower than 0.030, Cr between 3.0 and 4.0, Mo between 1.5 and 4.0, V between 0.1 and 2.0, Co lower than 1.5, being the remaining composed of Fe and inevitable deleterious substances. The steel is produced by processes involving ingot casting and hot/cold forming, or used with the cast structure; or by processes involving atomization or dispersion of the molten metal, such as powder metallurgy, powder injection or spray forming.
This report deals with a steel designed for hot metal forming tools, typically in cases where the metal to be formed withstands temperatures above 600° C., even though processes at lower temperatures or even at room temperature can be used with the said steel. The steel in question has a composition that allows ranking it as hot work tool steel, whose main characteristic is increased resistance to loss of hardness at high temperature—called tempering resistance, while retaining high toughness and adequate thermal conductivity and hardenability. Such an effect is possible by carefully designing the alloy, and setting the optimum ranges of the elements P, Si, Mo and Cr.
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, 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 characteristics of the processes in which they are employed.
Among their key properties of hot work steels, the following stand out: hot resistance, more specifically tempering resistance, toughness, hardenability and physical properties such as thermal conductivity, specific heat, both correlated to thermal diffusivity, and thermal expansion coefficient.
For forging applications, hot forging of steels stands out, especially steels for mechanical construction applied to auto parts. In such operations, the forged billet withstands temperatures above 1100° C. During the forming process, it heats up the surface of the tool, given that the higher the contact period, the higher the temperature. Consequently, the heat generated requires high hot resistance from the steel used. The steel hardening mechanism for hot die forming is chiefly induced by the precipitation of fine carbides. Noteworthy are the Mo or W carbides, M2C-type, or V carbides, MC-type. For high-Cr steels, Cr-rich M7C3 carbides also stand out, but also with 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 within the tool operating range. As a consequence, the hardness of the area decreases, causing wear and hot plastic deformation, resulting in tool failure.
Improving the resistance of the material against loss of hardness or tempering resistance, is thus critical to improving the performance of the tools that operate under high temperature conditions. Examples of such applications are tools used in hot forging of steel parts or other metal alloys, extrusion of non-ferrous alloys and dies for casting non-ferrous alloys (being the latter two applications more important to Al alloys). The same goes for other applications, such as extrusion or casting of non-ferrous alloys. E.g., in forging steel applications, the temperature of the pre-forms to be forged is about 1200° C. Even considering the short contact time with the tools (seconds), their surface heat up significantly, causing loss of hardness due to tempering of these surfaces. Considering the extrusion of aluminum alloys or other non-ferrous alloys, the billet temperature is lower, ranging from 400 to 600° C. However, in these applications, the contact time is significantly longer (tens of minutes to hours of operation). Moreover, local friction generated by the tool/aluminum contact intensifies heating, increasing the loss of hardness of the tool steel and, consequently, leading to steel wear. In pressurized casting dies, the molten metal is injected at high pressure and temperature (around 700° C.), also heating the surface of the die. In this case, failure is due mainly to thermal fatigue cracking caused by the successive heating and cooling of the working surface of the die. But, the high heat exchange between molten aluminum and the die surface favors the heating of the surface areas, generating, as in other applications, loss of hardness and, consequently, triggering the fatigue cracking process.
This mechanism of post-heating loss of hardness is, therefore, critically important to hot work tool steels; thus, increasing the strength of the material against this phenomenon is something desirable: Concerning the steel employed, improved hot resistance is usually obtained through the use of higher grades of those elements that form secondary carbides such as Mo, W and V, or by solid solution hardening. Although effective in increasing hot resistance, the excessive increase of the grade of these elements implies reduced toughness, poor thermal diffusivity and conductivity or significant increase of production costs. This latter economic factor is really important nowadays, given the high cost of raw materials used as a source of alloy elements Mo, W and V.
To provide a better understanding of this invention, we describe below some of the state-of-the-art steels currently used (chemical composition is summarized in Table 1). The H11 and H13 steels stand out, as these are the tool steels mostly used for hot works. These materials contain 5% Cr providing adequate hardenability and to assist with hot resistance, 0.9% V and 1.2% Mo to improve hot resistance, and generally low grades of P and S to promote adequate toughness. However, for improving tempering resistance, higher Mo grades would be needed. The DIN 1.2365 and DIN 1.2367 are steels used for such purpose. They have a high Mo grade to improve hot resistance. However, if the content of this element is increased within the structure of the DIN 1.2367 steel, toughness and thermal conductivity and diffusivity tend to drop. In the 1.2365 steel, this thermal conductivity reduction is counterbalanced by increasing the Mo content and decreasing the Cr content. However, the lower the Cr content, the lower the hardenability, limiting the applications in large tools. It is important emphasize that attention should be paid to the thermal conductivity and toughness properties. During the work, the increase in thermal conductivity is important such that the tool steel is able to homogenize quickly the difference of temperature between the material formed and the tool core, thus reducing stresses and thermal cracking. And, in case cracking occurs, the toughness of the material is also critical because it delays propagation and, consequently, thermal fatigue damage. Thus, it is clear that only increasing the Mo content, as exemplified by steel DIN 1.2367 and DIN 1.2365, is not sufficient for overall improvement of the properties of hot work steels.
A new steel type has been developed more recently and is described in PI 9909160-7. This material has, similarly to DIN 1.2367, higher Mo content, but lower Si and P content to improve toughness. In this case, cost increase is avoided by not using a high Mo content, but the hot resistance gain is not significant in comparison to steel H13.
Given this scenario, it is evident the need for a tool steel with hot resistance higher than that of state-of-the-art steel H13, but without using excessive alloying elements which might affect thermal conductivity and material cost. Also, the material used should feature high hardenability, which allows it to be applied to large tools.
Therefore, the steel of the present invention will fulfill all these needs.
The initial purpose of the invention was to investigate the influence of the Cr and Mo content on hot work tool steels that allowed identifying some synergy between the two elements and hot resistance. More specifically, when an increase of the Mo content is followed by a reduction of the Cr content, a more significant effect on the hot strength can be observed. In addition, a reduced Cr content improves thermal conductivity, thereby reversing the negative effect of a higher Mo content. On the other hand, the Cr content must be carefully balanced, because very low values, as previously mentioned, impair hardenability and limit the application to tools with average section dimension above 100 mm. Therefore, the material of the present invention shows a chemical composition with ideal arrangement of the Cr and Mo contents, capable of overcoming the tempering resistance properties of state-of-the-art steels, with no significant cost increase and proper thermal conductivity, toughness and hardenability.
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:
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- 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. Given its chemical similarity to W, Mo can be replaced with W, a 2 W: 1 Mo ratio by mass.
- 0.1 to 2.0 V, preferably 0.3 to 1.0 V, typically 0.5 V; V can be partially or fully replaced with Nb, following a 1.0% Nb: 0.5% V ratio.
- up to 1.0 Si, preferably up to 0.5 Si, typically 0.3 Si
- Max 0.030 P, preferably max 0.015 P, typically max 0.010 P.
Balance by 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:
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- Max 0.10 S, preferably max 0.020 S, typically max 0.008 S.
- Max 1.5 Al, Mn or Co, preferably up to 1.0 Al, Mn or Co, typically below 0.5 Mn, Al 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. Together with the alloying elements, carbon acts in the secondary hardening, important for the hardening at high temperature. For such effects, carbon contents of at least 0.20% are recommended, preferably above 0.30%. 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), and result in increased hardness and volume of secondary carbides. Thus, toughness is generally impaired. the C content should be limited to a maximum value of 0.50%, preferably below 0.40%. 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 3.0%, preferably greater than 3.5%, 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. This is an important effect, because the final tempering resistance is higher than that of state-of-the-art steels. The mechanisms that cause this effect are thought to 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 volume of M7C3 carbides and, thus, the greater the amount of Mo and V available for secondary hardening. The end result is a significantly higher tempering resistance when the alloy Cr content is lower than that of state-of-the-art alloys. Even in relation to the PI 9909160-7 alloy, there is a significant tempering resistance gain. This is significant because the alloys have Mo equivalent grades (Mo is a pricey alloying element), showing that the present invention is able to reach high hot resistance values without excessively increasing the Mo content. For all such effects, the Cr content should be below the 5.0% content of conventional steels, being the preferred Cr content lower than 4.0%. Finally, the ideal Cr content required to maximize tempering resistance identified in the present invention should be set between 3.0% and 4.0%. In addition to the heat resistance property, a lower Cr content improves thermal conductivity, also preserving this property as the Mo content rises. Therefore, this shorter Cr range aims at a careful adjustment for maximum tempering resistance and adequate thermal conductivity.
Mo and W: high Mo contents are used in the alloy of the present invention to improve tempering resistance properties. This is possible by the formation of chemically-stable, Mo-rich tempering carbides, especially the M2C carbide. Thus, the alloy of the present invention must contain at least 1.5%, preferably above 2.0%. On the other hand, excessively high Mo grades can harm toughness due to precipitation of pro-eutectic carbides at the time of quenching, and may significantly increase the cost of the alloy, making its application in many tools unfeasible. Hence, the Mo content should be limited to 4.0%, preferably below 3.0%. Tungsten and molybdenum produce similar effects in the tool steel of the present invention, forming M2C or M6C secondary carbides. Thus, they can be jointly specified through the tungsten equivalent relationship (Weq) given by the sum W+2Mo, which normalizes the differences in atomic weight between the two elements.
V: Vanadium is primarily important for the formation of MC secondary carbides. Because they are very thin, these carbides block the movement of dislocation lines, increasing mechanical strength. V also improves grain growth, allowing high austenitizing temperatures (above 1000° C.). For such effects, V must be above 0.1%, preferably above 0.3%. However, excessively high V grades may generate primary, difficult-to-solubilize carbides, thus reducing toughness. Hence, the V content should be lower than 2.0%, preferably below 1.0%.
Si: silicon produces a strong effect on secondary hardening and toughness. At high levels, Si increases the secondary hardness up to quenching temperatures of 600° C. However, the study of the present invention showed that a lower Si content was important to reduce the loss of hardness under high temperature conditions, thereby increasing tempering resistance. A lower Si content also results in significant increase of toughness, having this effect been applied to the present invention. Therefore, the Si content of the material of the present invention must be lower than 1.0%, typically below 0.5%.
P: reduction of the P content also results in significant increase of toughness, because this element can be segregated on the grain surface and, thereby, diminishes cohesion in these surfaces. Therefore, P content should be lower than 0.030%, typically below 0.015%.
Residual Elements: Other elements such as Mn and Al should be considered as deleterious substances associated with the steelmaking deoxidation processes or inherent to the manufacturing processes. Hence, the Mn and Al content should be limited to 1.5%, preferably below 1.0%. The Co content should also be limited to the same values, due to its beneficial effect on hot resistance and strong impact on the alloy cost. 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%.
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 figures attached herein have been referenced to in the description of the experiments carried out, and their contents are listed below:
Eight experimental ingots were initially produced to evaluate the effect of Si and P on the state-of-the-art H11 steel. The compositions are shown in Table 2. The hardness and impact results are shown in
Therefore, the results show that the best combination, in terms of toughness, would be alloys with low P and Si content. Alloys of high Si content are only viable for situations when hardness values higher than 52 HRC are employed and, for such, tempering temperatures below 600° C. are also adopted. In these cases, the decrease of the P content is even more critical.
The reasons for these significant Si and P effects have not been fully defined, but early scientific results conducted by the inventors of this patent show a relationship with the formation of secondary carbides. In alloys with high Si content, the secondary carbides tend to concentrate in areas of high diffusion (lath or grain surfaces), because of the difficulties imposed by Si onto the cementite formation process. On the other hand, in alloys with low Si content, cementite is rapidly formed, leading to a better distribution of secondary carbides formed at higher temperatures.
To assist in the definition of the effect of Cr and Mo, seven additional experimental ingots have been produced, comprehending four state-of-the-art steels: H11, H13 and the steel described in PI 9909160-7 and two alloys proposed for the present invention (see Table 3). These two compositions lead to a reduction of the Si and P contents described in example 1 but also to distinct balances of Cr and Mo.
As mentioned, the purpose of the IP 1 and IP 2 alloys is to obtain greater resistance to loss of hardness, i.e., tempering resistance. Therefore, hardness reduction after different exposure periods at 600° C. was evaluated and the results are shown in Table 4. The time increments followed a logarithmic scale, as depicted in the chart of
However, this gain in hot resistance by reducing the Si content is not sufficient to produce results significantly higher than those of H13. Therefore, a higher Mo content together with a lower Cr content in the IP 2 alloy was used. For that case, a significant variation of the hot resistance could be verified, which provided increased hardness after the same exposure period. And, as shown in
This significant tempering resistance improvement is related not only to the increase of the Mo content but also to the reduction of the Cr content. This effect is clear after comparing the differences between alloy PI2 and alloy 12 (patent PI 9909160-7). It also explains the high tempering resistance results obtained for alloy 11.
Despite the interesting effect on tempering resistance, Cr contents should not drop to excessively low levels to prevent damaging the hardenability and thus, limiting its application in large tools. This can be considered the major setback of the state-of-the-art DIN 1.2365 steel (alloy 11), i.e., excellent tempering resistance but low hardenability. Table 5 illustrates these Cr×hardenability issues based on dilatometer test results. The IP 2 composition can be considered ideal under this aspect, with Cr content lower than that of steel H13 (alloy 10), to provide increased tempering resistance, but not as low as that of steel DIN 1.2365 (alloy 11). The higher Mo content of alloy IP 2 also helps achieving proper hardenability levels, which compensates the effect resulting from the Cr content reduction and ensures its application in large tools.
A further advantage of using a lower Cr content than that of alloy 12 and other state-of-the art alloys is the ability to maintain adequate thermal conductivity. As shown in
Superior tenacity is another gain of alloys IP 1 and IP 2 in relation to the state-of-the-art H11, H13 and DIN 1.2365 alloys (alloys 9 to 11). These results can be compared in
A field study is detailed next, with allow IP 2 being compared to steel H13 in forging tools. The results were analyzed based on the failure modes and on the properties of materials.
The process in question deals with high-speed warm forging (see
Process data
Product: shaft end.
Tool: Warm forging precision punch
Forged Material: Modified SAE 1045 and 1050 steels
Billet temperature: approximately 900° C.
Cooling: Intense, water-cooled.
Blow application speed: high.
Tool steel previously used: AISI H13 (hardness: 53 HRC).
Steel tested: IP 2, at same hardness.
The steel of the present invention, IP 2, was then tested and approved for the application, increasing the tools life time by 56%. In numerical values, 5000 parts made of H13 steel could be forged up to the end of the tool's life and this figure increased to 7500 parts made of steel IP 2; the comparative analysis of the tempering curves and hardness vs. time developed for steels H13 (alloy 10) and IP 2,
Claims
1-10. (canceled)
11. STEEL WITH HIGH TEMPERING RESISTANCE comprising a composition of alloying elements consisting essentially of, in percentage by mass, C between 0.20 and 0.50, Si at maximum 0.30, P lower than 0.030, Cr between 3.0 and 4.0, Mo between 1.5 and 4.0, V between 0.1 and 2.0, Co lower than 1.5, being the remaining composed of Fe and inevitable deleterious substances.
12. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim 11, comprising a composition of alloying elements consisting essentially of, in percentage by mass, C between 0.30 and 0.50, Si at maximum 0.30, P lower than 0.020, Cr between 3.0 and 4.0, Mo between 2.0 and 3.0, V between 0.1 and 1.0, Co lower than 1.0, being the remaining composed of Fe and inevitable deleterious substances.
13. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim 12, comprising a composition of alloying elements consisting essentially of, in percentage by mass, C between 0.30 and 0.45, Si at maximum 0.30, P lower than 0.015, Cr between 3.2 and 3.9, Mo between 2.0 and 3.0, V between 0.3 and 1.0, Co lower than 1.0, being the remaining composed of Fe and inevitable deleterious substances.
14. STEEL WITH HIGH TEMPERING RESISTANCE, according to claim 13, comprising a composition of alloying elements consisting essentially of, in percentage by mass, C between 0.30 and 0.40, Si at maximum 0.30, P lower than 0.010, Cr between 3.5 and 3.9, Mo between 2.2 and 2.8, V between 0.3 and 0.8, Co lower than 0.5, being the remaining composed of Fe and inevitable deleterious substances.
15. STEEL WITH HIGH TEMPERING RESISTANCE according to claim 11, wherein a Mo/W replacement ratio is 1 Mo:2 W.
16. STEEL WITH HIGH TEMPERING RESISTANCE according to claim 11, wherein a Vanadium: Niobium/Titanium replacement ratio is 1 V:2 Nb/1 Ti.
17. USE OF STEEL WITH HIGH TEMPERING RESISTANCE according to claim 11, for molds, dies and multiple-use tools, for formation of solid or liquid materials, at room temperature or at temperatures up to 1300° C.
18. USE OF STEEL HIGH RESISTANCE TEMPERING according to claim 11, for metal-forming tools subject to temperatures between 300 and 1300° C., and also to other applications such as forging, extrusion or casting ferrous or nonferrous alloys.
Type: Application
Filed: Nov 10, 2010
Publication Date: Dec 20, 2012
Inventors: Celso Antonio Barbosa (Parque Nove Campinas-Campinas), Rafael Agnelli Mesquita (Sao Manoel - American - SP)
Application Number: 13/510,205
International Classification: C22C 38/52 (20060101); C22C 38/46 (20060101); C22C 38/44 (20060101);