KR20120102081A - Steel with high temper resistance - Google Patents

Abstract

As a steel with high tampering resistance, the composition of the alloying element is essentially, in mass%, 0.20 to 0.50 C, 1.0 or less Si, 0.030 or less P, 3.0 to 4.0 Cr, Mo of 1.5 to 4.0, 0.1 V of ˜2.0, Co of 1.5 or less, balance consists of Fe and unavoidable harmful substances. The steel is produced by a process comprising ingot casting and hot / cold forging or used with a cast structure; Or by atomization or diffusion of molten metal, such as powder metallurgy, powder injection or spray formation.

Description

Steel with high tamper resistance {STEEL WITH HIGH TEMPER RESISTANCE}

The present invention relates to steel constructed for hot metal forming tools, even if the steel to be formed typically withstands temperatures above 600 ° C., even if the steel can be used in processes at even lower or even room temperature. The steel has a composition classified as a high temperature processing tool steel, the main characteristic of which is that the resistance to hardness loss at high temperature-so-called tempering resistance is large, while maintaining high toughness and proper thermal conductivity and hardenability. will be. This effect is possible by carefully constructing the alloy and setting the optimum range of the elements P, Si, Mo and Cr.

The term hot working tool is applied to a large number of hot forming operations, is used in the industrial field, and also focuses on the production of parts for machine applications, in particular automotive parts. The most popular high temperature forming process is forging, extrusion or casting of nonferrous alloys. Other applications carried out at high temperatures, typically at temperatures above 500/600 ° C., may also be classified as hot processing. In this application, molds, dies, punches, inserts and other forming apparatus are classified under the general term: high temperature machining tools. Such tools are typically made of steel and require special properties to withstand high temperatures and mechanical characteristics of the process in which they are used.

Among the important properties of hot work steels, the following are noteworthy: high temperature resistance, more specifically tamper resistance, toughness, hardenability, and physical properties such as thermal conductivity and specific heat, which correlate with thermal diffusivity, and coefficient of thermal expansion.

In forging applications, high temperature forging of steel, in particular mechanical construction steels applied to automotive parts, is noteworthy. In this operation, forged billets withstand temperatures of 1100 ° C. and above. In the forming process, the longer the contact period, the higher the temperature, the surface of the tool is heated. As a result, the generated heat requires high high temperature resistance from the steel used. The steel hardening mechanism for hot die forming is generally induced by precipitation of fine carbides. Notable are Mo or W carbide, M ₂ C-type, or V carbide, MC-type. For high Cr steels, Cr-rich M ₇ C ₃ carbides are noteworthy but have Mo and V in solid solution. Despite their high stability, such carbides tend to integrate after prolonged periods at high temperatures, typically 550 ° C. or higher, which are easily reached within the tool operating range. As a result, the hardness of this region is reduced, causing wear and high temperature plastic deformation, and causing breakage of the tool.

Thus, improving the resistance of the material to the hardness loss or the tempering resistance is important for improving the performance of tools operating at high temperature conditions. Examples of such applications are tools used for hot forging of steel parts or other metal alloys, extrusion of nonferrous alloys and dies for casting of nonferrous alloys (the latter two applications being more important for Al alloys). The same applies to other applications, such as extrusion or casting of nonferrous alloys. For example, in forging applications of steel, the temperature of the preform to be forged is approximately 1200 ° C. Even considering the short connection time (seconds) with the tool, the surface of the tool is heated considerably, causing a hardness loss due to tampering of this surface. Considering the extrusion of aluminum alloys or other nonferrous alloys, the temperature of the billets is low in the range of 400 to 600 ° C. However, in this application, the contact time is considerably longer (ten minutes to tens of hours of operation). In addition, local friction generated by the tool / aluminum contact increases heating, increases the hardness loss of the tool steel, and eventually causes wear of the steel. In a pressurized casting die, molten metal is injected at high pressure and high temperature (approximately 700 ° C.) and also heats the surface of the die. In this case, fracture occurs mainly due to thermal fatigue cracking caused by continuous heating and cooling of the die processing surface. However, high heat exchange between the molten aluminum and the die surface favors heating of the surface area, leading to loss of hardness as in other applications, and consequently to fatigue cracking processes.

Thus, the mechanism of post-heat hardness loss is critically important for high temperature actuated tool steels; Therefore, it is desirable to increase the strength of the material against this phenomenon. In view of the steel used, improved high temperature resistance is usually obtained by using higher grade elements which form secondary carbides such as Mo, W and V or by solid solution liquid hardening. Although effective for increasing high temperature resistance, excessively increasing the element grade suggests reduced toughness, poor thermal diffusivity and a significant increase in conductivity or production cost. Given that expensive raw materials are used as sources of the alloying elements Mo, W and V, the economic factors are of practical importance today.

For a better understanding of the present application, some of the conventional steels currently used are described below (the chemical compositions are summarized in Table 1). Notable are H11 and H13 steels as tool steels mainly used for high temperature machining. Such materials include 5% Cr to provide adequate hardenability and also support high temperature resistance, 0.9% V and 1.2% Mo to improve high temperature resistance, and generally low grade P and S to improve proper toughness. . However, in order to improve the tampering resistance, higher grades of Mo are needed. DIN 1.2365 and DIN 1.2367 are steels used for this purpose. These steels have a high grade of Mo to improve high temperature resistance. However, when the content of the element is increased in the structure of DIN 1.2367 steel, the toughness, thermal conductivity, and thermal diffusivity tend to be lowered. In 1.2365 steel, the thermal conductivity degradation is offset by increasing Mo content and also decreasing Cr content. However, lower Cr content lowers hardenability and limits the application in large tools. It is important to pay attention to the thermal conductivity and toughness properties. In processing, an increase in thermal conductivity is important so that the temperature difference between the tool steel formed material and the tool core can be quickly homogenized, thereby reducing stress and thermal cracking. And, in the event of cracking, the toughness of the material is also important because it delays propagation and consequently thermal fatigue damage. Thus, as illustrated by the steels DIN 1.2367 and DIN 1.2365, it is evident that increasing the Mo content alone is not sufficient to improve the overall properties of the hot work steel.

More recently, new steel types have been developed, which are disclosed in PI 9909160-7. This material, like DIN 1.2367, has a high Mo content, but a low Si and P content to improve toughness. In this case, the increase in cost is avoided by not using a high Mo content, but the advantage for high temperature resistance compared to steel H13 is not significant.

In this given scenario, there is a clear need for tool steels having a high temperature resistance higher than that of conventional steel H13, without using excessive alloying elements that can affect thermal conductivity and material cost. In addition, the materials used should be characterized by high hardenability which can be applied to large tools.

Thus, the steel of the present invention will meet this need.

The first object of the present application was to investigate the influence of Cr and Mo content in hot working tool steels to distinguish some synergy and high temperature resistance between two elements. More specifically, a more significant effect on the high temperature strength could be observed when the Cr content decrease followed by the Mo content increase. In addition, Cr content reduction improves thermal conductivity, thereby reversing the adverse effects of high Mo content. On the other hand, the Cr content must be carefully balanced because, as mentioned above, very low values impair the hardenability and also limit the application to tools with an average partial dimension of at least 100 mm. Thus, in the materials of the present invention, a chemical composition having an ideal arrangement of Cr and Mo contents, which can overcome the tamper resistance properties of conventional steels, without having to significantly increase the cost and having adequate thermal conductivity, toughness and hardenability, can be obtained. Indicates.

In order to satisfy the above conditions, the steel of the present invention has a composition of alloying elements composed of the following in mass%:

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. If chemically similar to W, Mo can be substituted for W, with 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 may be partially or wholly substituted by Nb, followed by a 1.0% Nb: 0.5% V ratio.

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

0.030 P max, preferably 0.015 P max, typically 0.010 P max.

The balance is iron and metal or nonmetallic hazardous substances that are unavoidable in the steel forming process, and the nonmetallic hazardous substances include, but are not limited to:

0.10 S maximum, preferably 0.020 S maximum, typically 0.008 S maximum.

Up to 1.5 Al, Mn or Co, preferably up to 1.0 Al, Mn or Co, typically up to 0.5 Mn, Al and Co.

Then, the ratio in the specification with respect to the composition of a new material is described. The stated percentages are regarded as mass%.

C: Carbon is primarily responsible for martensite curing under low temperature conditions. Together with alloying elements, carbon acts upon secondary curing, which is important for curing at high temperatures. For this effect, a carbon content of at least 0.20% is recommended, preferably at least 0.30%. On the other hand, very high C contents cause excessive precipitation of carbides in particulate form during quenching (particularly when the Mo and V contents are high), resulting in increased hardness and volume of secondary carbides. Thus, toughness is generally impaired. The C content should be limited to a maximum of 0.50%, preferably 0.40% or less. This limitation also serves to reduce the amount of retained austenite and prevents problems associated with dimensional instability and embrittlement.

Cr: Chromium content should be at least 3.0%, preferably at least 3.5%, since this element is advantageous for the hardenability which is important in the application of large tools. However, the Cr content should be limited. The present invention includes the concept of reducing Cr content to improve tamper resistance. This has a significant effect because the final tampering resistance is higher than that of conventional steel. The mechanism causing this effect is thought to be related to the shape of the secondary Cr carbide, M ₇ C ₃ -type, which dissolves the primary carbides Mo and V to be formed. Therefore, if the Cr content is low, the volume of the M ₇ C ₃ carbide is also lowered, thereby increasing the amount of Mo and V available for secondary curing. As a result, if the alloy Cr content is lower than that of conventional alloys, the tampering resistance is significantly higher. As with the PI 9909160-7 alloy, there is also significant tamper resistance gain. This is important because the alloy has an Mo equivalent grade (Mo is an expensive alloying element), and the present invention has been shown to be able to reach high high temperature resistance values without excessively increasing the Mo content. For all these effects, the Cr content should be 5.0% or less of the conventional steel content, preferably Cr content or less. Finally, the ideal Cr content required to maximize the tamper resistance distinguished in the present invention should be set between 3.0% and 4.0%. In addition to the heat resistance properties, the low Cr content improves thermal conductivity, and this property is maintained even if the Mo content is raised. Therefore, this short Cr range aims to carefully control the maximum tampering resistance and proper thermal conductivity.

Mo and W: High Mo contents are used in the alloy of the present invention to improve the tamper resistance properties. This makes it possible to form chemically stable Mo-rich tampering carbides, in particular M ₂ C carbide. Therefore, the alloy of the present invention should contain at least 1.5%, preferably at least 2.0%. On the other hand, excessively high Mo grades can adversely affect toughness due to precipitation of pro- eutectic carbides during quenching, and can also significantly increase the cost of the alloy, and such applications can be applied to various tools. Do not. Therefore, the Mo content should be limited to 4.0%, preferably 3.0% or less. Tungsten and molybdenum produce similar effects in the tool steels of the present invention and form M ₂ C or M ₆ C secondary carbides. Thus, they can be specified together through the tungsten equivalent relation (Weq) given by the sum of W + 2Mo, which normalizes the difference in atomic weight between two elements.

V: Vanadium is of primary importance in the formation of MC secondary carbides. Since these carbides are very thin, they block the movement of the dislocation lines, increasing the mechanical strength. V also improves particulate growth and enables high austenitization temperatures (1000 ° C. or higher). For this effect, V should be at least 0.1%, preferably at least 0.3%. However, excessively high V grades can result in difficult-to-solubilize carbides, thereby reducing toughness. Therefore, the V content should be 2.0% or less, preferably 1.0% or less.

Si: Silicon has a great influence on the secondary curing and toughness. Si, at high levels, increases the secondary hardness to a quench temperature of 600 ° C. However, studies of the present invention have shown that low Si content is important in reducing hardness loss under high temperature conditions, thereby increasing the tamper resistance. Low Si content also significantly increases toughness and has the effect of applying the present invention. Therefore, the Si content in the material of the present invention should be 1.0% or less, typically 0.5% or less.

P: Since this element separates on the particulate surface, the reduction in P content also causes a significant increase in toughness, thereby reducing aggregates on the surface. Therefore, the P content should be 0.030% or less, usually 0.05% or less.

Residual Elements: Other elements such as Mn and Al should be regarded as hazardous substances associated with or inherent in the manufacturing process. Therefore, the Mn and Al content should be limited to 1.5%, preferably 1.0% or less. The Co content should also be limited to the same value, due to the beneficial effect on high temperature resistance and the significant effect on alloy cost. In terms of inclusions, the sulfur content must be controlled because such inclusions can induce cracks in operation; Thus the S content should remain below 0.050%, preferably below 0.020%.

The alloy can be produced as a rolled or forged product through spray molding or continuous casting of conventional or special processes such as powder metallurgy, wire rods, bars, wires, sheets, and strips, as described above.

The accompanying drawings refer to the description of experiments performed, the contents of which are as follows.

1 shows the effect of P and Si on alloys 1-8 in post-tampering toughness and hardness,
2 is a view comparing alloys 1 to 8 showing P influence on toughness according to the quench temperature;
FIG. 3 shows the carbide distribution in high and low Si content alloys, better distributed in low Si content alloys, illustrating good toughness;
FIG. 4 is a diagram comparing the hardness reduction with time at 600 ° C., showing the tampering resistance, showing that the alloy tampering resistance becomes higher as it moves further to the right;
FIG. 5 is a diagram comparing thermal conductivity values for some alloys studied; FIG.
6 is a diagram comparing the toughness of alloys 9-12 and alloys IP1 and IP2; A drawing showing that the data is produced by a non-notch collision test (7 × 10 mm test specimen) and Charpy V, and
FIG. 7 shows a hot forging punch applied industrially produced IP 2 steel, compared to conventional H13 steel, a) wear fracture and cracking; b) a diagram showing the drop in the working area (distance from the surface = 0) as a hardness profile.

Example 1 Influence of Silicon and Phosphorus

Eight experimental ingots were initially created to evaluate the effect of Si and P on conventional H11 steels. The composition is shown in Table 2. Hardness and impact results are shown in FIG. 1. It should be noted that strong Si affects hardness for quench temperatures of 500 ° C. or less, even if the same effect is not observed for quench temperatures of 600 ° C. or more; The hardness of both high and low Si alloys is about the same. P influences were compared in FIG. 2 for different tampering temperatures. In this case, the reduction of the P content significantly improves the toughness of the Si rich alloy but this effect can be observed to be less noticeable for low Si content alloys.

Thus, the results indicate that the best combination in terms of toughness is an alloy of low P and Si content. Alloys of high Si content are practical only when the hardness value of 52 HRC or higher is used, for example when a tampering temperature of 600 ° C. or lower is also adopted. In this case, the reduction of the P content is even more important.

The reason for this important effect of Si and P, although not fully defined, appears to be related to the formation of secondary carbides in the initial scientific results carried out by the inventors of the patent. In alloys of high Si content, secondary carbides tend to concentrate in the high diffusion region (las or particulate surface) due to the difficulty imposed on the cementite forming process by Si. On the other hand, for alloys of low Si content, cementite rapidly forms, leading to a better distribution of secondary carbides formed at high temperatures. 3 shows an image of transmission electron microscopy illustrating this observation.

Example 2 Influence of Cr and Mo

To help define the effects of Cr and Mo, seven additional experimental ingots were produced, including four conventional steels: the steels disclosed in H11, H13 and PI 9909160-7 and the two alloys proposed in the present invention ( See Table 3). The two compositions reduce the Si and P content described in Example 1 but also lead to separate balances of Cr and Mo.

As mentioned above, the purpose of the IP 1 and IP 2 alloys is to obtain greater resistance to hardness loss, i.e., tamper resistance. Thus, the hardness reduction is evaluated after different exposure periods at 600 ° C., and the results are shown in Table 4. The increase in time follows the logarithmic scale, as shown in the chart of FIG. 4B. In these results, comparing the alloys IP 1 and H11, the high temperature resistance is simply increased by changing the Si and P content (but since P does not function in the carbide forming process, this effect is considered to be only related to Si ).

However, the gain in high temperature resistance by reducing the Si content is not sufficient to produce results that are significantly larger than H13. Thus, high Mo content with low Cr content was used in IP 2 alloy. In this case, a significant change in high temperature resistance could be demonstrated, giving increased hardness after the same exposure period. And, as shown in FIG. 4A, the same hardness degradation obtained for H13 steels appears after a much longer period than in the case of alloy IP 2. For example, the hardness is reduced from 45 HRC to 35 HRC at a temperature of 600 ° C. after a 25 hour exposure period, while the same phenomenon occurs in the IP 2 alloy only after 60 hours.

This marked improvement in tampering resistance is associated with a reduction in Cr content as well as an increase in Mo content. This effect is evident after comparing the difference between the PI 2 alloy and Alloy 12 (Patent PI 9909160-7). It also demonstrates the high tampering resistance obtained for alloy 11.

Despite the interesting impact on the tamper resistance, the Cr content should not be reduced to an excessively low level to prevent hardenability damage, thus limiting this application to large tools. This is considered as a major disadvantage of the conventional DIN 1.2365 steel (alloy 11), ie it has good tampering resistance but low hardenability. Table 5 describes the Cr × hardenability problem based on the test results of the dilatometer. An IP 2 composition having a lower Cr content than the H13 steel (alloy 10), in this embodiment, may be considered ideal to provide increased tampering resistance, but is not as low as DIN 1.2365 (alloy 11). The high Mo content of alloy IP 2 helps to achieve a suitable level of hardenability, which includes the effect of reducing Cr content and also ensures application in large tools.

Another advantage of using a lower Cr content than Alloy 12 and other conventional alloys is that they can maintain adequate thermal conductivity. As shown in FIG. 5, this property tends to decrease as the Mo content increases (compare Alloy 12 and Alloy 10) and also increases as Cr content decreases (alloys 11 and IP 2). ). Thus, in addition to being considered ideal for high temperature resistance, the combination of Cr and Mo contents of the IP 2 alloy allows to maintain thermal conductivity at much higher levels than in conventional H13 steels (alloy 10).

An excellent advantage is that there are other benefits of IP 1 and IP 2 alloys over conventional H11, H13 and DIN 1.2365 alloys (alloys 9 to alloy 11). These results can be compared in FIG. 6. It should be noted that the gain of the IP 2 alloy in related alloy 11 has a similarly high tampering resistance. In other words, in addition to good hardenability, the balance of IP 2 chemical composition makes this alloy more pronounced than alloy 11. In this case, the effect is primarily related to low Si and P content as described in Example 1.

Example 3: Field Test

Field studies are described below and IP 2 alloys are compared to steel H13 in forging tools. The results are analyzed based on the failure mode and the properties of the material.

The process is treated with high speed warm forging (see FIG. 7A). Although the forged billet is exposed to temperatures lower than the usual hot forging temperature, the high treatment rate allows the contact between the heated billet and the matrix to extend, heating the surface. This process is formulated at high cooling conditions, improving thermal shock to the surface area.

Process data

Product: Shaft End

Tooling: warm forging precision punch

Forged materials: modified SAE 1045 and 1050 steels

Billet Temperature: Approx 900 ℃

Cooling: Precipitation

Blow application speed: high

Previously used tool steel: AISI H13 (hardness: 53 HRC)

Tested steel: IP 2 at same hardness

7A shows the punch analyzed after the end of the punch life. Because this type of forging produces parts with high dimensional precision, deviations of several tens of millimeters ensure the quality of the manufactured parts. End of life of such parts is caused by the occurrence of wear and thermal cracks on the protruding and rounded surfaces (see FIG. 7A). After the end of the life of these parts, the matrix is broken and analyzed. 7b shows hardness related data for distance from contact surface; It should be noted that the hardness decreases close to the subsurface area. Abrasion, in fact, is related to the loss of hardness during machining, regardless of core hardness. Since low hardness surfaces are more susceptible to thermal cracking, heat generation is also associated with hardness loss. Therefore, increased tampering resistance is important for further increasing tool life time.

The steel, IP 2 of the present invention is then tested and approved for application, increasing the tool life time to 56%. In figures, 5000 parts made of H13 steel can be forged until the end of the life of the tool, in this figure increased to 7500 parts made of IP 2 steel (FIGS. 4 and 6); A comparative analysis of the tempering curves and hardness versus time developed for H13 steels (alloy 10) and IP 2 steels provides a better understanding of this phenomenon. In both cases, the hardness of the steel decreases when subjected to high temperatures, and the larger this decrease, the longer the time and the higher the temperature used. However, the stability of alloy IP 2 is better at high temperatures. Thus, in the forging process, breakage will occur after a greater number of strokes, indicating an observed yield gain.

Claims (10)

For steels with high tamper resistance,
The composition of the alloying element is essentially, in mass%,
0.20 to 0.50 C, 1.0 or less Si, 0.030 or less P, 3.0 to 4.0 Cr, 1.5 to 4.0 Mo, 0.1 to 2.0 V, 1.5 or less Co, balance consisting of Fe and unavoidable harmful substances Steel with high furring resistance. For steels with high tamper resistance,
The composition of the alloying element is essentially, in mass%,
0.30 to 0.50 C, 0.8 or less Si, 0.020 or less P, 3.0 to 4.0 Cr, 2.0 to 3.0 Mo, 0.1 to 1.0 V, 1.0 or less Co, balance consisting of Fe and unavoidable harmful substances Steel with high furring resistance. For steels with high tamper resistance,
The composition of the alloying element is essentially, in mass%,
0.30 to 0.45 C, 0.5 or less Si, 0.015 or less P, 3.2 to 3.9 Cr, 2.0 to 3.0 Mo, 0.3 to 1.0 V, 1.0 or less Co, balance consisting of Fe and inevitable harmful substances Steel with high furring resistance. For steels with high tamper resistance,
The composition of the alloying element is essentially, in mass%,
0.30 to 0.40 C, 0.4 or less Si, 0.010 or less P, 3.5 to 3.9 Cr, 2.2 to 2.8 Mo, 0.3 to 0.8 V, 0.5 or less Co, balance consisting of Fe and inevitable harmful substances Steel with high furring resistance. The method according to any one of claims 1 to 4,
Mo: W substitution ratio is a steel with high tamper resistance, characterized in that 1 Mo: 2 W. 6. The method according to any one of claims 1 to 5,
Vanadium: Niobium / Titanium substitution ratio is 1 V: 2 Nb / 1 Ti, characterized in that the high tempering resistance steel. 7. The method according to any one of claims 1 to 6,
High tempering resistance steel, characterized in that it is applied to molds, matrices and utility tools to form solid or liquid materials at room temperature or up to 1300 ° C. 7. The method according to any one of claims 1 to 6,
Metal forming tools subjected to temperatures between 300 and 1300 ° C. High tempering resistance steels, which are also used for forging, extrusion or casting of ferrous or nonferrous alloys. Highly tamper resistant steels produced by processes including ingot casting and hot / cold forming or even used with casting structures. High tempering resistance steel produced by processes involving atomization or diffusion of molten metal, such as powder metallurgy, powder injection or spray forming processes.

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