Hot work tool steel
A hot work tool steel with a controlled composition of C, Si, Mn, Ni, Cr, Mo, and V, using A and B value formulas, addresses quench cracking issues, ensuring high toughness and resistance in complex-shaped tools.
Patent Information
- Application Number
- JP2021017386
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- Priority Date
- 2020-03-06
- Filing Date
- 2021-02-05
- Publication Date
- 2026-01-21
- Estimated Expiration
- 2041-02-05
AI Technical Summary
Conventional hot work tool steels face issues with quench cracking during quenching and cooling, especially when they have complex shapes, leading to defective tools due to the difficulty in removing significant cracks.
A hot work tool steel with a specific composition range of C, Si, Mn, Ni, Cr, Mo, W, and V, controlled by the A and B value formulas, to enhance toughness and suppress quench cracking, ensuring high toughness and resistance to quench cracking.
The solution effectively suppresses quench cracking and maintains excellent toughness in hot work tools, even with complex shapes, by optimizing the chemical composition and transformation behavior during quenching and cooling.
Smart Images
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Abstract
Description
[Technical Field]
[0001] The present invention relates to a hot work tool steel that is optimal for various hot work tools such as press dies, forging dies, die casting dies, and extrusion tools, and to the hot work tools. [Background technology]
[0002] Hot work tools are used while coming into contact with high-temperature or hard workpieces, and therefore must have toughness that can withstand impacts. Conventionally, hot work tool steels have been made of, for example, JIS SKD61 alloy tool steels. In response to recent demands for further improvements in toughness, alloy tool steels with improved chemical compositions of the SKD61 alloy tool steels have been proposed (Patent Documents 1 to 6).
[0003] Hot work tool steels are typically manufactured by starting with a steel ingot or a billet formed by blooming the ingot, which is then subjected to various hot working processes and heat treatments to produce a desired steel material, which is then annealed. The resulting hot work tool steel is typically supplied to hot work tool manufacturers in an annealed state with a low hardness, where it is machined into the shape of the hot work tool and then quenched and tempered to achieve the desired working hardness. After achieving this working hardness, the tool is typically subjected to finish machining. The toughness of hot work tool steels is evaluated in this quenched and tempered state (i.e., the state corresponding to the hot work tool). [Prior art documents] [Patent documents]
[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-104519 [Patent Document 2] European Patent Application Publication No. 2194155 [Patent Document 3] Japanese Patent Application Publication No. 6-322483 [Patent Document 4] Japanese Patent Application Publication No. 63-203744 [Patent Document 5] Japanese Patent Application Publication No. 11-90611 [Patent Document 6] Japanese Patent Application Publication No. 2018-131654 Summary of the Invention [Problem to be solved by the invention]
[0005] However, when quenching and tempering hot work tool steel, if the machined hot work tool steel has a complex tool shape, "quench cracks" can occur during quenching and cooling, originating from recesses or the like. If the quench cracks are significant, they are difficult to remove even in subsequent finish processing, resulting in defective hot work tools. In this regard, Patent Documents 1 and 2 leave room for further consideration in achieving excellent toughness and quench crack resistance. An object of the present invention is to provide a hot work tool steel and a hot work tool that are excellent in toughness and resistance to quench cracking. [Means for solving the problem]
[0006] In view of the above problems, the present inventors have conducted extensive research and have found, through detailed analysis of the transformation behavior during quenching and cooling, that there is a suitable range of components in hot work tool steel that can achieve high toughness while suppressing the occurrence of quench cracking.
[0007] That is, the present invention provides a hot work tool steel consisting of, in mass%, C: 0.35-0.39%, Si: 0.25-0.35%, Mn: 0.55-0.65%, Ni: 0-0.15%, Cr: 5.20-5.50%, Mo and W alone or in combination (Mo+½W): 1.25-1.50%, V: 0.70-0.85%, with the balance being Fe and impurities, wherein the relationship of the contents of each element calculated by the following formulas 1 and 2 satisfies the following relationship: A value: 6.40-7.00 and B value: 0.90-1.00. The values in brackets [ ] in formulas 1 and 2 indicate the content (mass%) of each element. Formula 1: A value = -0.7[%Si]+1.5[%Mn]+1.3[%Ni]+0.9[%Cr]+0.6[%(Mo+1 / 2W)]+0.3[%V] Formula 2: B value = 1.9[%C]+0.043[%Si]+0.12[%Mn]+0.09[%Ni]+0.042[%Cr]+0.03[%(Mo+1 / 2W)]-0.12[%V]
[0008] The present invention provides a hot working tool comprising, in mass%, C: 0.35-0.39%, Si: 0.25-0.35%, Mn: 0.55-0.65%, Ni: 0-0.15%, Cr: 5.20-5.50%, Mo and W alone or in combination (Mo+½W): 1.25-1.50%, V: 0.70-0.85%, with the balance being Fe and impurities, wherein the relationship of the contents of each element calculated by the following formulas 1 and 2 satisfies the following formula: A value: 6.40-7.00 and B value: 0.90-1.00. The values in brackets [ ] in formulas 1 and 2 indicate the content (mass%) of each element. Formula 1: A value = -0.7[%Si]+1.5[%Mn]+1.3[%Ni]+0.9[%Cr]+0.6[%(Mo+1 / 2W)]+0.3[%V] Formula 2: B value = 1.9[%C]+0.043[%Si]+0.12[%Mn]+0.09[%Ni]+0.042[%Cr]+0.03[%(Mo+1 / 2W)]-0.12[%V]
[0009] Of the above impurities, the hot work tool steel or hot work tool of the present invention preferably contains, in mass %, P: 0.05% or less and S: 0.01% or less. Furthermore, the hot work tool steel or hot work tool of the present invention preferably contains, among the above impurities, O: 0.003% or less and N: 0.03% or less, in mass %. [Effects of the Invention]
[0010] According to the present invention, it is possible to provide a hot work tool steel that can suppress quench cracking during quenching and has excellent toughness after quenching and tempering, and a hot work tool made of the hot work tool steel. [Brief explanation of the drawings]
[0011] [Figure 1] FIG. 2 is a diagram showing the shape of a test piece used in a quench cracking test in the examples. [Figure 2]1 is a photograph, substituted for a drawing, showing a corner of the groove bottom of a test piece of an example of the present invention after a quench cracking test of the example was carried out. [Figure 3] 10 is a photograph, substituted for a drawing, showing the corner of the groove bottom of a test piece of a comparative example after the quench cracking test of the example was carried out. DETAILED DESCRIPTION OF THE INVENTION
[0012] The present invention is characterized in that a hot work tool steel (or hot work tool) with excellent toughness and quench crack resistance has been achieved by adjusting the content of each element constituting the component composition to an optimum and limited range. In other words, by providing a hot work tool steel with the above component composition, it is possible to suppress quench cracking during quench cooling and to impart high toughness after quenching and tempering, even if the manufacturing method and quenching and tempering conditions remain unchanged.
[0013] Hardening is a process in which hot work tool steel is heated to the austenite temperature range and then cooled (rapidly cooled) to transform the structure into martensite or bainite. When hot work tool steel is hardened, the timing of transformation occurs later inside the steel than on the surface, which causes differences in expansion at different points in the steel. When hot work tool steel has a complex shape, such as the shaped surfaces of various molds, stress concentrates in the recesses (corners), making it prone to quench cracking.
[0014] In hot work tool steel, elements such as Cr, Mn, Mo, W, and Ni that improve hardenability can be added to impart excellent toughness after quenching and tempering. However, this increases the amount of expansion that occurs during transformation during quenching and cooling, which can cause quench cracking to become more pronounced. Therefore, in the present invention, by carefully analyzing the transformation behavior during the quenching and cooling, it has been found that there is a suitable range of components in a hot work tool steel that can obtain high toughness while suppressing the occurrence of quench cracking. The details of the component composition of the hot work tool steel (or hot work tool) of the present invention will be described below.
[0015] C: 0.35 to 0.39 mass% (hereinafter simply referred to as "%") Carbon is a basic element of hot work tool steel; some of it dissolves in the matrix to provide strength, while the other part forms carbides to improve wear resistance and seizure resistance. However, excessive addition of carbon reduces hot strength and promotes quench cracking during quenching and cooling. Therefore, the carbon content is set to 0.35 to 0.39%, preferably 0.36% or more, and more preferably 0.38% or less.
[0016] Si: 0.25 to 0.35% Silicon acts as a deoxidizer during steelmaking and also improves machinability. However, excessive silicon content leads to the formation of acicular bainite in the quenched and tempered structure, reducing tool toughness. Furthermore, by suppressing the precipitation of cementite-based carbides in the bainite structure during quenching and cooling, silicon indirectly promotes the precipitation, aggregation, and coarsening of alloy carbides during tempering, reducing high-temperature strength. This also promotes quench cracking during quenching and cooling. Therefore, the silicon content is set to 0.25 to 0.35%, preferably 0.27% or more, more preferably 0.29% or more, and more preferably 0.33% or less, and even more preferably 0.31% or less.
[0017] Mn: 0.55 to 0.65% Mn is an element that improves hardenability, suppresses the formation of ferrite, and contributes to improving toughness after quenching and tempering. It is also an effective element for obtaining appropriate quenching and tempering hardness. Furthermore, when present in the structure as non-metallic inclusions such as MnS, it is an element that is highly effective in improving machinability. However, too much Mn increases the viscosity of the matrix, reducing machinability. It also promotes quench cracking during quenching and cooling. Therefore, the Mn content is set to 0.55 to 0.65%, preferably 0.57% or more, more preferably 0.59% or more, and even more preferably 0.60% or more. Furthermore, it is preferably 0.63% or less.
[0018] Ni: 0 to 0.15% Ni is an element that suppresses the formation of ferrite. It also, along with Cr, Mn, Mo, and W, gives hot work tool steel excellent hardenability, and is an effective element in preventing a decrease in toughness by forming a martensite-based structure even in the case of a slow quench cooling rate. It is also an element that has the effect of essentially improving the toughness of the matrix. However, too much Ni reduces the high-temperature strength of the hot work tool. It also increases the viscosity of the matrix, reducing machinability. It also promotes quench cracking during quenching and cooling. Therefore, in the present invention, it is important to strictly control the upper limit of Ni content in order to ensure the quench cracking resistance of the hot work tool steel. By satisfying the A and B values defined by the formulas 1 and 2 described below, it is possible to impart excellent toughness to the hot work tool without Ni content. Therefore, Ni content is limited to 0.15% or less, preferably 0.14% or less, more preferably 0.12% or less, and even more preferably less than 0.10%. When Ni is an impurity, the lower limit can be set to 0%.
[0019] Cr: 5.20~5.50% Cr is an element that is effective in improving hardenability and toughness. It also forms carbides in the structure, strengthening the matrix and improving wear resistance, and is a basic element of hot work tool steel, contributing to improved temper softening resistance and high-temperature strength. However, excessive addition of Cr can cause a decrease in high-temperature strength. It also promotes quench cracking during quench cooling. Therefore, the Cr content is set to 5.20 to 5.50%, preferably 5.25% or more, more preferably 5.30% or more, and preferably 5.45% or less, and more preferably less than 5.40%.
[0020] Mo and W alone or in combination (Mo+1 / 2W): 1.25~1.50% Mo and W are elements that can be added alone or in combination to improve hardenability and toughness, as well as to impart strength by precipitating fine carbides during tempering, thereby improving softening resistance. Because W has approximately twice the atomic weight of Mo, it can be defined as (Mo + 1 / 2W). (Naturally, either one or both may be added.) However, excessive Mo or W content reduces machinability and promotes quench cracking during quench cooling. Therefore, the Mo and W content is set to 1.25 to 1.50% based on the Mo equivalent relationship of (Mo + 1 / 2W). It is preferably 1.30% or more, more preferably 1.35% or more, and also preferably 1.45% or less, more preferably 1.43% or less, and even more preferably 1.41% or less. In the present invention, since W is an expensive element, all of the W can be replaced with Mo. In this case, the Mo content is 1.25 to 1.50% (the same applies to the preferred range). However, W can be contained as an impurity.
[0021] Within the above-mentioned range of Cr or Mo equivalent, it is preferable to adjust the Cr or Mo equivalent to a lower value, particularly when further improvement in quench cracking resistance is important. Adjusting the Cr or Mo equivalent to a lower value, and preferably adjusting both the Cr and Mo equivalents to a lower value, acts to lower the B value calculated by Equation 2 described below.
[0022] V: 0.70~0.85% V forms carbides and has the effect of strengthening the matrix and improving wear resistance. It also increases temper softening resistance and inhibits grain coarsening, contributing to improved toughness. It is also an effective element for inhibiting quench cracking during quench cooling. However, too much V leads to a decrease in machinability. Therefore, V content is set to 0.70 to 0.85%, preferably 0.72% or more, and more preferably 0.80% or less. More preferably, it is 0.78% or less.
[0023] The hot work tool steel (or hot work tool) of the present invention may contain P, S, O, and N as impurities. ·P: 0.05% or less P is an element that can be unavoidably contained. It segregates at prior austenite grain boundaries during heat treatment such as tempering, embrittling the grain boundaries and degrading the toughness of hot work tools. Therefore, in order to maintain the toughness of hot work tools, it is preferable to restrict its content to 0.05% or less, more preferably 0.03% or less, even more preferably 0.015% or less, and even more preferably 0.01% or less. On the other hand, in the case of the present invention, the hot work tool steel has a composition that satisfies the above, particularly the relationships of formulas 1 and 2 described below, thereby ensuring excellent toughness. Therefore, a certain amount of P content is acceptable. For example, even if the P content is 0.006% or more (preferably 0.007% or more, more preferably 0.008% or more), excellent properties can be maintained.
[0024] ·S: 0.01% or less S is an element that can be unavoidably contained. It deteriorates the hot workability of the material before hot working and causes cracks in the material during hot working. Therefore, in order to improve the hot workability, it is preferable to restrict S content to 0.01% or less. It is more preferable to restrict S content to 0.005% or less. It is even more preferable to restrict S content to 0.002% or less. It is even more preferable to restrict S content to 0.001% or less.
[0025] ·O: 0.003% or less O (oxygen) is an element that can remain in steel as an impurity. Since O deteriorates the toughness of hot work tools by forming coarse inclusions, it is generally considered preferable to have a low O content. Therefore, in order to maintain the toughness of hot work tools, it is preferable to restrict the O content to 0.003% or less. It is more preferably 0.0025% or less, even more preferably 0.002% or less, and even more preferably 0.0015% or less. It is particularly preferably 0.001% or less. On the other hand, in the case of the present invention, the excellent properties are ensured by the fact that the composition of the hot work tool steel satisfies the above, particularly the relationships of the formulas 1 and 2 described below. Therefore, a certain amount of O content is tolerable. For example, even if the O content is 0.0003% or more (preferably 0.0004% or more, more preferably 0.0005% or more, and even more preferably 0.0007% or more), the excellent properties can be maintained.
[0026] ·N: 0.03% or less Nitrogen (N) is also an element that can remain in steel as an impurity and deteriorates the properties of hot work tools, so a lower N content is generally preferred. Therefore, in order to maintain the properties of hot work tools, it is preferable to limit the N content to 0.03% or less. More preferably, it is 0.025% or less, even more preferably 0.02% or less, and even more preferably 0.015% or less. On the other hand, in the case of the present invention, the hot work tool steel has a composition that satisfies the above, particularly the relationships of formulas 1 and 2 described below, thereby ensuring excellent properties. Therefore, a certain amount of N content is acceptable. For example, even if the N content is 0.005% or more (preferably 0.007% or more, more preferably 0.008% or more, even more preferably 0.01% or more, and even more preferably 0.012% or more), excellent properties can be maintained.
[0027] The fact that the above O and N contents are tolerable means that vacuum melting can be omitted in the material melting process, and that atmospheric melting can be used, which is effective in improving manufacturing efficiency. As a result, steel ingots can be made larger, making it possible to use large hot working tools (steel).
[0028] Co, Cu, and Ti are also elements that may remain in steel as impurities. In the present invention, the lower the contents of these elements, the better. In this case, Co: 1% or less, Cu: 0.25% or less (preferably 0.15% or less), and Ti: 0.05% or less (preferably 0.01% or less) are fully acceptable and are the preferred upper limits of the present invention (including the case of 0%).
[0029] A value calculated by formula 1: 6.40 to 7.00 Equation 1: A value = -0.7 [% Si] + 1.5 [% Mn] + 1.3 [% Ni] + 0.9 [% Cr] + 0.6 [% (Mo + 1 / 2 W)] + 0.3 [% V] (The values in brackets indicate the content (mass%) of each element.)
[0030] In the present invention, it is important to control the A value calculated by the above formula 1 to "6.40 or more" in the chemical composition of the above-mentioned hot work tool steel (or hot work tool). In other words, formula 1 quantifies the influence of each element on the "toughness" of the hot work tool steel. The "A value" calculated by formula 1 is an index value that indicates the degree of "toughness" possessed by a hot work tool steel with a certain chemical composition. In the case of the hot work tool steel of the present invention, elements that affect the toughness after quenching and tempering include "Si, Mn, Ni, Cr, Mo, W, and V." The present inventors have found that, of these elements, Si acts to reduce toughness, while Mn, Ni, Cr, Mo, W, and V act to improve toughness. The present inventors assigned "plus" coefficients to Mn, Ni, Cr, Mo, W, and V, which act to improve toughness, and "negative" coefficients to Si, which acts to reduce toughness. They also determined the value (absolute value) of each coefficient according to the degree to which it affects the improvement or reduction of toughness, thereby completing the above formula, which allows the balance between the content of each element, which changes mutually, and toughness to be evaluated from the chemical composition of the hot work tool steel.
[0031] In accordance with the above-mentioned coefficients, "increasing" the A value calculated by the above formula 1 means improving the toughness of the hot work tool steel while minimizing the effects on other properties required of the hot work tool steel, including the quench cracking resistance described below. In the present invention, the A value is set to "6.40 or more." This improves the hardenability during quenching and cooling, making it possible to maintain a high level of toughness after quenching and tempering. The A value is preferably "6.45 or more." More preferably, it is "6.50 or more." Even more preferably, it is "6.55 or more." In addition, in order to improve the toughness of the hot work tool steel, there is no particular need for an upper limit to the A value, as long as the elements Si, Mn, Ni, Cr, Mo, W, and V constituting Formula 1 satisfy their respective component ranges. However, if the A value is large, the quench cracking resistance of the hot work tool steel will deteriorate, particularly due to the influence of the high contents of Cr, Mo, and W. Therefore, the A value is set to "7.00 or less" in accordance with its relationship with the B value described below. Preferably, it is "6.90 or less." More preferably, it is "6.80 or less." Even more preferably, it is "6.70 or less."
[0032] B value calculated by formula 2: 0.90 to 1.00 Equation 2: B value = 1.9 [%C] + 0.043 [%Si] + 0.12 [%Mn] + 0.09 [%Ni] + 0.042 [%Cr] + 0.03 [%(Mo + 1 / 2W)] - 0.12 [%V] (The values in brackets indicate the content (mass%) of each element.)
[0033] In the present invention, it is important to control the B-value calculated by the above formula 2 in the chemical composition of the above-mentioned hot work tool steel (or hot work tool) to "1.00 or less." In other words, formula 2 quantifies the degree of influence of each element on the "quench cracking resistance" of the hot work tool steel. The "B-value" calculated by formula 2 is an index value that indicates the degree of "quench cracking resistance" possessed by a hot work tool steel with a certain chemical composition. In the case of the hot work tool steel of the present invention, elements that affect quench cracking during quenching and cooling include "C, Si, Mn, Ni, Cr, Mo, W, and V." The inventors have found that, of these elements, C, Si, Mn, Ni, Cr, Mo, and W act to reduce quench cracking resistance, while V acts to improve quench cracking resistance. The inventors assigned a negative coefficient to V, which acts to improve quench cracking resistance, and a positive coefficient to C, Si, Mn, Ni, Cr, Mo, and W, which act to reduce quench cracking resistance. They then determined the value (absolute value) of each coefficient according to the degree to which it improves or reduces quench cracking resistance, thereby completing the above formula, which allows the balance between the content of each element, which changes relative to one another, and quench cracking resistance to be evaluated from the chemical composition of the hot work tool steel.
[0034] By defining the above coefficients, "reducing" the B-value calculated by the above formula 2 means improving the quench crack resistance of the hot work tool steel while minimizing the effects on other properties required of the hot work tool steel, including the above toughness. In the present invention, the B-value is set to "1.00 or less." In particular, this B-value must be strictly controlled. This makes it possible to accommodate the difference in expansion that occurs in the hot work tool steel during quench cooling, and to suppress quench cracking during quench cooling. In order to improve the quench cracking resistance of the hot work tool steel, the lower limit of the B value is not particularly required, as long as the elements C, Si, Mn, Ni, Cr, Mo, W, and V constituting Formula 2 satisfy their respective component ranges. However, if the B value is small, the toughness of the hot work tool steel will deteriorate, particularly due to the influence of the reduced contents of Cr, Mo, and W. Therefore, the B value is set to "0.90 or more" in accordance with the relationship with the above-mentioned A value, etc. Preferably, it is "0.92 or more." More preferably, it is "0.95 or more."
[0035] The quenching and tempering temperatures described above, which are related to the effects of "suppressing quench cracking during quenching and cooling" and "improving toughness after quenching and tempering" of the present invention, vary depending on the chemical composition and target hardness of the material, but it is preferable that the quenching temperature be approximately 1000 to 1100°C and the tempering temperature be approximately 500 to 650°C. The quenched and tempered hardness is preferably 50 HRC or less. Preferably, it is 40 to 50 HRC. More preferably, it is 41 HRC or more. Even more preferably, it is 42 HRC or more. Furthermore, it is more preferably 48 HRC or less. Even more preferably, it is 46 HRC or less. [Example]
[0036] A 10-ton arc melting furnace was used to produce steel ingots (approximately 1.3 m in size) with the chemical composition shown in Table 1. 3) was melted. This steel ingot was subjected to a soaking treatment at a temperature of 1200°C or higher, and then hot forged at a temperature between 1000 and 1250°C to produce a steel material measuring approximately 300 mm thick and over 400 mm wide. This steel material was then annealed at a temperature between 850 and 900°C to produce hot work tool steels of Samples 1 to 5 (inventive examples) and 11, 12, and 13 (comparative examples). Table 1 also shows the A and B values calculated using Equations 1 and 2 according to the present invention. The A and B values are rounded to two decimal places. The effects of the A and B values according to the present invention can be evaluated using these rounded values.
[0037] [Table 1]
[0038] <Quench cracking test> A 300mm x 300mm x 300mm block was cut from the sample, and a 50mm wide, 100mm deep groove was machined into one side of it to create a concave test specimen (Figure 1). The corners of the recess (groove bottom) were finished with a 2.0R radius of curvature. For samples 1 to 5, a 1.5R radius was used to maximize the likelihood of cracking. These test specimens were quenched at temperatures between 1020 and 1030°C. The quenching was performed using oil quenching, and the specimens were removed from the oil after the temperature at their centers reached 200 to 250°C. They were then heated to a tempering temperature (500 to 650°C) to achieve a target hardness of 43HRC. The surface of the test specimens corresponding to the hot-working tool was then subjected to penetrant testing (color check) to confirm the presence or absence of cracking at the groove bottom corners.
[0039] <Charpy impact test> Charpy impact test specimens (ST orientation, 2 mm U-notch) were taken from the samples and quenched and tempered. The quenching temperature was 1030°C, and the quenching cooling was performed using pressurized gas. Simulating the center of a large-sized actual hot work tool steel, the cooling time (referred to as the semi-cooling time) from the quenching temperature (1030°C) to a temperature of [quenching temperature + room temperature (20°C)] / 2 (525°C) was approximately 90 minutes. After quenching, the specimens were tempered at various temperatures between 500 and 650°C to achieve a target hardness of 43HRC, equivalent to that of a hot work tool. After finish machining, the Charpy impact test was performed.
[0040] <Evaluation of quench cracking resistance and toughness> The results of the quench cracking test and the Charpy impact test are shown in Table 2. For samples 1 to 5 of the present invention, the impact strength was 30 J / cm 2 The above Charpy impact values were obtained. Furthermore, in the inventive samples 1 to 5, no quench cracks were observed at the corners of the groove bottom (Fig. 2). Furthermore, no quench cracks were observed even in the test piece with a concave curvature radius of 1.5R. In contrast, the comparative sample 11 has a small A value of 30 J / cm 2 The above Charpy impact values were not achieved. Furthermore, comparative sample 13 had a large B value, and quench cracks occurred at the corners of the groove bottom. This was also the case with comparative sample 12. Although sample 12 had reduced Si and Mo contents and increased V content compared to sample 13, and the curvature radius of the recess was 2.0R, quench cracks occurred at the corners of the groove bottom (Figure 3; the streaks are the penetrant).
[0041] [Table 2] [Example]
[0042] A 10-ton arc melting furnace was used to produce steel ingots (approximately 1.3 m in size) with the chemical composition shown in Table 3. 3) was melted. This steel ingot was subjected to a soaking treatment at a temperature of 1200°C or higher, and then hot forged at a temperature between 1000 and 1250°C to produce a steel material measuring approximately 300 mm thick and over 400 mm wide. This steel material was then annealed at a temperature between 850 and 900°C to produce hot work tool steels of Samples 6 and 7 (invention examples). Table 3 also shows the A and B values calculated using Equations 1 and 2 according to the present invention. These A and B values are rounded to two decimal places. The effects of the A and B values according to the present invention can be evaluated using these rounded values.
[0043] [Table 3]
[0044] <Charpy impact test> Charpy impact test specimens (ST orientation, 2 mm U-notch) were taken from the samples and quenched and tempered. The quenching temperature was 1030°C, and the quenching cooling was performed using pressurized gas. Simulating the center of a large-sized actual hot work tool steel, the cooling time (referred to as the semi-cooling time) from the quenching temperature (1030°C) to a temperature of [quenching temperature + room temperature (20°C)] / 2 (525°C) was approximately 90 minutes. After quenching, the specimens were tempered at various temperatures between 500 and 650°C to achieve a target hardness of 43HRC, equivalent to that of a hot work tool. After finish machining, the Charpy impact test was performed.
[0045] <Evaluation of toughness> The results of the Charpy impact test are shown in Table 4. Inventive samples 6 and 7, the impact strength was 30 J / cm 2 The above Charpy impact values were obtained.
[0046] [Table 4]
[0047] Furthermore, in samples 6 and 7 of the present invention, the B value is controlled to 1.00 or less, and therefore excellent quench crack resistance similar to that of samples 1 to 5 of the present invention can be expected.
Claims
1. In mass%, C: 0.35 to 0.39%, Si: 0.25 to 0.35%, Mn: 0.57 to 0.65%, Ni: 0 to 0.15%, Cr: 5.20 to 5.50%, Mo and W alone or in combination (Mo+1 / 2W): 1.25 to 1.50%, V: 0.70 to 0.85%, the balance being Fe and impurities; A hot work tool steel characterized in that the relationship between the contents of the elements calculated by the following formulas 1 and 2 satisfies the following: A value: 6.40 to 7.00; and B value: 0.90 to 1.
00. Formula 1: A value = -0.7 [%Si] + 1.5 [%Mn] + 1.3 [%Ni] + 0.9 [%Cr] + 0.6 [% (Mo + 1 / 2W)] + 0.3 [%V] Formula 2: B value = 1.9 [%C] + 0.043 [%Si] + 0.12 [%Mn] + 0.09 [%Ni] + 0.042 [%Cr] + 0.03 [% (Mo + 1 / 2W)] - 0.12 [%V] The contents (mass%) of each element are shown in brackets [ ].
2. The hot work tool steel according to claim 1, characterized in that it consists, in mass%, of C: 0.35-0.39%, Si: 0.25-0.35%, Mn: 0.59-0.65%, Ni: 0-0.15%, Cr: 5.25-5.50%, Mo and W alone or in combination (Mo+1 / 2W): 1.25-1.50%, V: 0.70-0.80%, and the balance being Fe and impurities.