Hot working tool steel

Abstract

The hot working tool steel of the present invention has a composition comprising 0.38-0.45 mass% of C, 0.50 mass% or less of Si, 0.37-1.00 mass% of Mn, 4.30-5.50 mass% of Cr, 0.11-0.20 mass% of Ni, 1.5-3.2 mass% of Mo+(1 / 2)W, 0.49-0.62 mass% of V, and 0.006-0.025 mass% of N, with the balance being Fe and inevitable impurities. When the content (mass%) of the component X is represented by [X], A determined as A=1050-373.6[C]+28.7[Cr]-150.0[Ni]-127.3[V]+45.9[Mo+(1 / 2)W] exceeds 1000. This makes it possible to obtain a hot working tool steel with excellent 0.2% yield strength at high temperatures. When this hot working tool steel is used to manufacture dies, even if the workpiece is a high-strength steel, it will not plastically deform, allowing high-precision processing.

Description

Technical Field

[0001] The present invention relates to a hot working tool steel having high hardenability and high high temperature strength, which is suitable as a material for dies (including molds) used for warm pressing, die casting, hot extrusion, warm forging, etc. Background Art

[0002] JIS-SKD61 and SKD7 are widely used as raw materials for dies used in warm pressing, die casting, hot extrusion, and warm forging. The two steel grades are used depending on the required properties. SKD61 is generally used for components requiring toughness, while SKD7 is mainly used for components requiring high-temperature strength.

[0003] As a steel material for hot working tools, a hot working tool steel has been proposed that aims to improve both softening resistance (high temperature strength) and toughness by adjusting the contents of Cr and Mo (Patent Document 1). The toughness of the hot working die steel described in Patent Document 1 is 50.3 to 86.6 J / cm in terms of Charpy impact test value. 2 Furthermore, the difference ΔHRC between the initial hardness HRC before quenching and tempering and the HRC after quenching and tempering is 7.3 to 11.1. This ΔHRC is taken as the softening resistance characteristic (high temperature strength).

[0004] In addition, a hot working die steel has been proposed that focuses on the composition and amount of carbides after quenching and tempering, with the goal of improving toughness and high-temperature strength by appropriately controlling them (Patent Document 2). The hot working die steel described in the invention of Patent Document 2 has a Charpy impact value of 30 J / cm 2 Above, the softening amount (ΔHRC) is 13HRC or less.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2013-087322,

[0008] Patent document 2: Japanese Patent Application Laid-Open No. 2017-155306. Summary of the Invention

[0009] Problems to be solved by the invention

[0010] Generally, steel grades such as SKD61, SKD62, and SKD7 are known as materials for molds used in hot working. SKD61 is used for applications requiring heat resistance, heat cracking resistance, and toughness, while SKD7 is used for applications requiring softening resistance and high heat strength.

[0011] On the other hand, in the invention disclosed in Patent Document 1 or Patent Document 2, the high-temperature strength of hot-working tool steel is evaluated by the hardness reduction (softening resistance) of the steel after quenching and tempering to 45HRC and maintaining it at high temperature. This is not a problem as a characteristic evaluation of die steel for casting molten aluminum, such as die casting, but it is not suitable for mold applications in hot extrusion or warm forging, where the strength of the workpiece is high, because the stress applied to the mold material is high and the strength is insufficient. That is, the mold used for hot extrusion will bend during processing due to insufficient strength at a hardness below 40HRC, and the dimensional specifications of the extruded product cannot be met. The inventions of Patent Documents 1 and 2 also include steel with a hardness of 35HRC or less after softening resistance, so that when the strength of the workpiece is high, such as hot extrusion or warm forging, as described above, bending will occur during processing, and products with high dimensional accuracy cannot be obtained.

[0012] The present invention has been made in view of such problems, and its object is to provide a hot working tool steel that can obtain a hot working tool steel with excellent 0.2% yield strength at high temperatures. When using this hot working tool steel to manufacture dies, even if the workpiece is a high-strength material, plastic deformation will not occur, and high-precision processing can be performed.

[0013] Means of solving problems

[0014] The hot working tool steel according to the present invention is characterized by having the following composition, namely, containing:

[0015] C: 0.35-0.50 mass%,

[0016] Si: 0.50 mass% or less,

[0017] Mn: 0.37-1.00 mass%,

[0018] Cr: 4.30-5.50 mass%,

[0019] Ni: 0.20 mass% or less,

[0020] Mo+(1 / 2)W: 1.5-3.2 mass%,

[0021] V: 0.30-0.80 mass%,

[0022] N: 0.006 to 0.025 mass %, and

[0023] The balance is Fe and unavoidable impurities;

[0024] Furthermore, when [X] represents the content (mass %) of component X, A obtained by the following formula exceeds 1000:

[0025] A=1050-373.6[C]+28.7[Cr]-150.0[Ni]-127.3[V]+45.9[Mo+(1 / 2)W].

[0026] In the present invention, for example, it can be set as:

[0027] Ni is 0.11 mass% or more, C is 0.45 mass% or less or 0.38-0.45 mass%, or V is 0.49-0.62 mass%. In addition, Mn can be 0.45 mass% or more, or Mo+(1 / 2)W can be 1.6 mass% or more.

[0028] In addition, in the present invention, for example,

[0029] The critical cooling time for quenching, which is an indicator of hardenability, is preferably 60 minutes or longer.

[0030] and,

[0031] The 0.2% proof stress in a high-temperature tensile test at 500° C. is preferably tempered to 1000 MPa or more by quenching and tempering.

[0032] Effects of the Invention

[0033] According to the present invention, a hot working tool steel having excellent 0.2% yield strength at high temperatures can be obtained. When extrusion or forging is performed using a die made of this steel, the die will not plastically deform even when the counterpart material is high-strength and the process is repeated, thereby enabling the processing of high-precision products.

[0034] Furthermore, because molds using the steel of the present invention prevent plastic deformation, mold life is increased. Furthermore, the steel of the present invention exhibits excellent hardenability, suppressing the reduction in toughness caused by the effects of quenching cooling rates. Furthermore, due to the excellent hardenability of the steel of the present invention, molds using this steel can be manufactured even for large profiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] [ Figure 1 ] shows a graph illustrating tensile properties at 0.2% proof stress.

[0036] [ Figure 2 ] A graph showing the relationship between 0.2% proof stress and elastic strain or plastic strain.

[0037] [ Figure 3 ] A graph showing the relationship between the A value and the 0.2% proof stress (MPa). DETAILED DESCRIPTION

[0038] The present invention will be described in detail below. Hot-working tool steels, particularly steels for hot extrusion dies, require properties that allow them to finish high-strength workpieces into high-precision products, minimizing deflection in the dies used. To achieve these properties, the steels must have a high elastic limit at high temperatures. The present invention was developed based on this insight.

[0039] In practice, it is difficult to experimentally determine the high-temperature elastic limit of mold materials. Therefore, the 0.2% proof stress in a commonly used high-temperature tensile test is used as an indicator of strength up to the plastic deformation region. Figure 1 This is a diagram showing the tensile properties of steel at high temperatures, with elongation (strain) on the horizontal axis and load (stress) on the vertical axis. Figure 2 This is a schematic diagram showing the high temperature tensile properties of the elastic region. Figure 1 In the figure, point P is the proportional limit load, point E is the elastic limit load, point B is the maximum load, and point F is the breaking load. Also, from the point where the elongation is 0.2% at 0 load, a line is drawn parallel to the elastic region of the stress-strain line. The point where the line intersects the stress-strain line is the 0.2% proof stress σ. 0.2 It should be noted that Figure 1 and Figure 2 This is a diagram schematically showing the high temperature tensile properties. The steel type and test temperature are not limited to specific steel types and test temperatures. As mentioned above, since it is difficult to measure the high temperature elastic limit, the 0.2% proof stress σ is used instead. 0.2 As an indicator of elastic limit at high temperature (strength indicator until the plastic deformation zone). It should be noted that the strain when stretched from load 0 to elastic limit load E is elastic strain, and the strain after elastic limit load E is plastic strain. In addition, if the 0.2% proof stress σ is an indicator of elastic limit of steel 0.2 The larger the load, the greater the plastic strain. 0.2 When large steel is used in molds (dies), the deformation in the elastic zone becomes dominant when subjected to processing stress and thermal stress, and the bending generated in the mold becomes smaller. Therefore, the precision of the products processed using the mold is improved, and the mold life is also increased.

[0040] Therefore, the inventors of this application set the 0.2% endurance σ 0.2 As an indicator of the minimum bending of die steel at high temperatures, various experimental studies have been conducted to develop steel that exhibits minimal plastic deformation and allows for high-precision processing. In most cases, high-strength hot extrusion dies are tempered to approximately 49 HRC for use. The inventors of this application measured the 0.2% proof stress (σ) of a high-temperature tensile test on steel tempered to a hardness level that is practical for use. 0.2 , studied the combination of steel components to improve this value.0.2 The target value is set at 1000MPa. Of course, as the number of continuous uses increases, the mold will soften due to the influence of heat, so the mold material is also required to have high resistance to softening. In addition, as mentioned above, since the 0.2% proof stress is high, this suppresses the plastic deformation of the mold used, thereby extending the life of the mold. In addition, the hardenability of the hot working tool steel affects the quenching cooling rate of the center part of the mold. Especially in the case of large molds, if the hardenability of the steel constituting the mold is low, the quenching cooling rate of the center part of the mold becomes slow, so the quenching becomes insufficient, resulting in a decrease in the toughness value, and early breakage of the mold. Therefore, in the present invention, the high hardenability of the hot working tool steel is an important factor.

[0041] Therefore, the present invention proposes a hot working tool steel that can be used as a steel material for a mold, which improves the 0.2% proof stress σ 0.2 , softening resistance and quenching resistance (toughness). As a steel material for molds used under warm working, it has small plastic deformation and can produce high-precision products.

[0042] First, the reasons for adding components and limiting the composition of the hot working tool steel of the present invention will be described.

[0043] C: 0.35-0.50 mass%

[0044] C is dissolved in the matrix of hot working tool steel and is an element that increases the hardness of hot working tool steel and improves wear resistance. In addition, C forms carbides and improves softening resistance at high temperatures. If C is less than 0.35% by mass, these necessary characteristics cannot be met. On the other hand, if C exceeds 0.50% by mass, the toughness of the steel decreases. Therefore, the C content is set to 0.35-0.50% by mass. In addition, C is an element that also affects the A value described later. If it is added in excess, the 0.2% endurance σ at high temperature will decrease. 0.2 Therefore, the upper limit of C is preferably 0.45 mass % or less.

[0045] "Si: 0.50 mass% or less"

[0046] Si can be added because it improves the machinability of steel. On the other hand, if Si is contained in an amount exceeding 0.50 mass%, toughness will be reduced due to the worsening of segregation. Therefore, the Si content is set to 0.50 mass% or less.

[0047] Mn: 0.37-1.00 mass%

[0048] Mn is an element that stabilizes austenite and improves hardenability. If Mn is less than 0.37% by mass, hardenability is significantly reduced. A Mn content of 0.45% by mass or greater is preferred. If Mn exceeds 1.00% by mass, workability and thermal conductivity deteriorate, so the Mn content is set to 0.37-1.00% by mass.

[0049] Cr: 4.30-5.50 mass%

[0050] Cr is an element that improves hardenability and toughness. In addition, Cr forms carbides during tempering after quenching, which has the effect of improving heat resistance. If Cr is less than 4.30 mass%, the hardenability of the steel will be significantly reduced. In addition, Cr is an element that also affects the A value described below. At high temperatures, 0.2% of the steel has a resistance of σ 0.2 The addition of Cr increases the toughness, so it is necessary to add 4.30% by mass or more of Cr. On the other hand, if the Cr content exceeds 5.50% by mass, Cr-based carbides tend to form excessively, which reduces toughness. Therefore, the Cr content is set to 4.30-5.50% by mass.

[0051] "Ni: 0.20 mass% or less"

[0052] Ni is an element that is effective in improving the hardenability of steel, just like Cr. However, if Ni exceeds 0.20% by mass, the machinability of the steel material decreases, so the Ni content is set to 0.20% by mass or less. In addition, Ni is an element that also affects the A value described later. The 0.2% resistance σ at high temperature 0.2 The addition of Ni significantly reduces the Mg content. Therefore, it is not preferable to add a large amount of Ni. For this reason, the upper limit of the amount of Ni added is 0.20 mass %.

[0053] "Mo+(1 / 2)W: 1.5-3.2 mass%"

[0054] Like Cr, Mo and W are elements that are effective in improving hardenability. In addition, Mo and W form carbides during tempering after quenching, which are effective in improving the strength and heat resistance of steel. If the total amount of Mo and 1 / 2 of the W content (Mo+(1 / 2)W) is less than 1.5% by mass, the effect of improving hardenability cannot be achieved. On the other hand, if (Mo+(1 / 2)W) exceeds 3.2% by mass, crystallized carbides are produced, and it is found that hardenability is reduced. Therefore, (Mo+(1 / 2)W) is set to 1.5-3.2% by mass. However, W has an atomic weight approximately twice that of Mo, and when the atomic number is equal, the hardenability is equal, and the degree of their effect is mutually replaceable. Therefore, for the addition of Mo and W, (Mo+(1 / 2)W) is used as an indicator. It should be noted that either Mo or W can be added alone. In addition, Mo and W are elements that also affect the A value described later, and the 0.2% proof stress at high temperatures is increased by their addition. Therefore, it is preferable to set Mo+(1 / 2)W to 1.6 mass % or more.

[0055] V: 0.30-0.80 mass%

[0056] V forms carbides and is an element that is effective in preventing grain coarsening during quenching and improving wear resistance. In order to achieve this effect, it is necessary to contain 0.30% by mass or more of V. However, if V exceeds 0.80% by mass, coarse carbides are formed in the steel, reducing the toughness of the steel, and excessive addition of V will increase the manufacturing cost. Therefore, the V content is set to 0.30-0.80% by mass. In addition, V is an element that also affects the A value described later, and the 0.2% proof stress at high temperature will be reduced by the addition of V. Therefore, the V content is preferably 0.49% by mass or more and 0.62% by mass or less.

[0057] "N: 0.006 to 0.025 mass%"

[0058] Nitrogen forms fine carbides, effectively preventing grain coarsening during quenching and improving machinability. To achieve this effect, a Nitrogen content of 0.006% by mass or greater is required. However, if Nitrogen exceeds 0.025% by mass, coarse carbides are formed, degrading the toughness of the steel. Therefore, the Nitrogen content is set to 0.006-0.025%.

[0059] Thus, in order to achieve the purpose of the present invention, it is necessary to keep the composition of each component within the specified composition range. In particular, it is important to keep the amounts of C, Si, Mn, Cr, Ni, V, and (Mo + (1 / 2) W) within the above ranges. In addition to the above components, for example, B having an effect of improving hardenability, S having an effect of improving machinability, and Ti or Nb having an effect of refining grains may be contained as unavoidable impurities to the extent that they do not deteriorate other properties.

[0060] "A>1000"

[0061] A is obtained by the following mathematical formula 1, where [X] represents the content (mass %) of component X.

[0062] [Mathematical formula 1]

[0063] A=1050-373.6[C]+28.7[Cr]-150.0[Ni]-127.3[V]+45.9[Mo+(1 / 2)W]

[0064] In the present invention, the content of each component is determined so that the A value exceeds 1,000.

[0065] The technical field of the present invention is hot working tool steel used in dies for hot forging, hot extrusion, and die casting. Generally, the steel is tempered to 40-50 HRC by quenching and tempering. The tempered die is then heated to approximately 400-500°C and used as a die material for extruding, forging, or casting the workpiece.

[0066] As mentioned above, the inventors of this application used 0.2% endurance σ 0.2 As an indicator of the necessary properties of the mold material at high temperatures. 0.2% proof stress σ of hot working tool steel 0.2 The influence of alloy composition is not significant between room temperature and 300°C. However, above 500°C, the properties of JIS SKD61 and JIS SKD71, for example, differ. Therefore, the effect of various alloy compositions on the 0.2% yield strength σ at 500°C was studied repeatedly. 0.2 The results show that the high temperature endurance of 0.2% σ 0.2 The relationship with alloying elements is: if [C], [Ni], [V] are added, the A value decreases. On the contrary, if the addition of [Cr] and [Mo+(1 / 2)W] is increased, the A value increases. The higher the A value, the lower the 0.2% endurance σ 0.2 The larger it is, the better the characteristics are.

[0067] The above-mentioned mathematical formula (1) for the A value is obtained as follows. Specifically, the inventors of the present application tempered test pieces manufactured by varying the content of each alloying element to an HRC of 49±1 and conducted a high-temperature tensile test at 500°C. Then, in this high-temperature tensile test, the 0.2% proof stress σ was measured. 0.2 Then, the effect of each element on 0.2% endurance σ was calculated by the least square method (multiple regression analysis). 0.2 The coefficient of the content of each component is obtained from the analysis results of the influence degree, and the formula for calculating the A value is obtained (Mathematical Formula 1).

[0068] Figure 3In the figure, the horizontal axis is A value, and the vertical axis is 0.2% endurance σ 0.2 The relationship between the 0.2% proof stress measured values of the test pieces of each steel material and the A value determined from the composition is plotted in the graph. In this case, the coefficient of the A value is determined by the least square method so that the 0.2% proof stress is most approximated to the A value, which is the above-mentioned mathematical formula 1. In the present invention, if the content of each component is determined so that the approximate formula, that is, the mathematical formula 1, exceeds 1000, then the 0.2% proof stress σ 0.2 The value is approximately greater than 1000.

[0069] like Figure 3 As shown, the higher the A value, the greater the 0.2% endurance σ 0.2 The object of the present invention can be achieved by regulating the contents of the components [C], [Ni], [V], [Cr], and [Mo + (1 / 2)W] so that the A value becomes higher. Specifically, if the threshold value of the A value is A = 1000, then when the contents of the components of the steel material of the present invention satisfy A>1000, the 0.2% proof stress is 1000 MPa or higher. Therefore, in the present invention, the contents of the components are determined so that the A value is greater than 1000.

[0070] Example

[0071] Next, the properties of hot-working tool steels according to the embodiments of the present invention satisfying claim 1 are compared with the properties of hot-working tool steels according to comparative examples that fall outside the scope of the present invention to illustrate the effects of the present invention. The steel materials of Examples 1 to 5 and Comparative Examples 6 to 14, each having the compositions shown in Table 1 below, were melted in a high-frequency induction furnace to produce 20 kg ingots. Comparative Examples 15 and 16 were forged using a forging ratio of 6S or greater from 3 to 6 ton steel ingots melted in a mass-produced electric furnace. These ingots were heated at 1200 to 1280°C for at least 4 hours, then forged. The ingots were then heated at 820 to 870°C for at least 4 hours and then cooled to 400 to 500°C at a cooling rate of 15 to 35°C / hour for annealing. Softening resistance test pieces, hardenability test pieces, tensile test pieces, and Charpy impact test pieces were then collected from the annealed steel materials.

[0072] Then, based on these test pieces, the "hardenability", "toughness", "0.2% proof stress σ" and "hardenability" shown in Table 2 were determined. 0.2 ” and “resistance to softening”.

[0073] In the "hardenability" test, a CCT curve was created using the Formaster test, and the critical cooling time (in minutes) required for bainite formation was determined to determine the quality. Specifically, after holding at 1030°C for 10 minutes and then cooling at a constant rate, the critical cooling time required for bainite formation was rated as 0 if the critical cooling time was 60 minutes or longer, and as x if it was less than 60 minutes. This was used to evaluate the hardenability.

[0074] In the "toughness" test, a JIS No. 3 test piece of 10×10×55 mm was cut and heated at 1030°C for 30 minutes. After cooling to room temperature at a cooling rate of 12.5°C per minute, the test piece was quenched and then tempered at 580-630°C for two or more times until the hardness reached a uniform hardness of 49±1HRC. The impact value was then measured. In the evaluation, the impact value was set to 25J / cm 2 The above is represented as ◎, and 15J / cm 2 Above and below 25J / cm 2 It is expressed as 0, which is less than 15J / cm 2 The case is represented as ×.

[0075] In the "softening resistance" test, after heating at 1030°C for 30 minutes, the steel was cooled to room temperature at a cooling rate of 33.3°C per minute for quenching. After that, it was tempered at 580-630°C for two or more times. Each steel material, which had been tempered to a hardness of 48±1HRC, was held at 600°C for 50 hours. The steel material was then air-cooled and the hardness was measured. The hardness was evaluated based on the difference from the initial tempered hardness, i.e., the hardness reduction ΔHRC. If the difference ΔHRC between the initial tempered hardness and the post-test hardness was 10HRC or less, the softening resistance evaluation was recorded as ◎, if it exceeded 10 and was 13HRC or less, it was recorded as ○, and if it exceeded 13HRC, it was recorded as ×.

[0076] For the "0.2% yield strength" in the high-temperature tensile test, a flanged test piece (JIS G0567) with a parallel portion having a diameter of 6 mm and a length of 30 mm is cut from the steel material. After heating at 1030°C for 30 minutes, it is cooled to room temperature at a cooling rate of 33.3°C per minute for quenching, and then tempered at 580-630°C for more than 2 times to make the hardness uniformly reach 49±1HRC. For the obtained test piece, a tensile test is carried out at a temperature of 500°C in accordance with JIS G0567. The tensile speed is set to 0.3% / min relative to the portion with a parallel portion length of 30 mm. Then, the 0.2% yield strength σ is calculated. 0.2 The results are shown in Table 2 (unit: MPa). 0.2 A value of 1000 MPa or higher was determined to be good.

[0077] [Table 1]

[0078]

[0079] [Table 2]

[0080]

[0081] Table 1 shows the contents of C, Si, Mn, Cr, Ni, Mo, W, V, and N in the steel materials tested, as well as the value of Mo + (1 / 2) W. The remainder of Table 1 is Fe and unavoidable impurities. The contents of the components of Examples 1 to 5 of the present invention satisfy claim 1. In addition, as shown in Table 2, the A value exceeds 1000. Moreover, the hardenability of Examples 1 to 5 is ○, the toughness is ◎ or ○, the softening resistance is ○, and the 0.2% proof stress σ is 0. 0.2 More than 1000MPa.

[0082] On the other hand, although the A value of Comparative Example 6 satisfies the range of more than 1000, the Cr content is too low compared to the range of the present invention, and the Mo+(1 / 2)W content is more than the range of the present invention, resulting in the formation of crystallized carbides, poor hardenability, and the critical cooling time for quenching, which is a hardenability indicator in the present invention, does not meet the requirement of more than 60 minutes. In addition, the Cr content of Comparative Examples 9, 10, 11, and 14 is also too low compared to the range of the present invention, so the hardenability would have been poor like Comparative Example 6. However, in these Comparative Examples 9, 10, 11, and 14, the hardenability was improved by adding other elements. Therefore, as shown in Table 2, the hardenability column is all marked with ○ (critical cooling time for quenching is more than 60 minutes). However, in Comparative Examples 9, 10, 11, and 14, Ni was added beyond the range of the present invention. Therefore, the addition of Ni, which reduces the A value (the coefficient in the formula of the A value is negative), resulted in an A value of less than 1000 and a 0.2% proof stress σ at high temperature. 0.2 In addition, C and V are also high in Comparative Example 9, which is also the main reason for the decrease in A value.

[0083] In Comparative Examples 7, 8, 12, and 13, the A value is low due to the high Ni content. In Comparative Examples 7, 12, and 13, the C content is slightly high, which is also a major factor in the decrease in the A value.

[0084] In Comparative Examples 10, 12, 13, and 14, the A value is low because the content of Mo+(1 / 2)W is low, and the 0.2% yield strength σ is 0.2 Also low.

[0085] In Comparative Example 15, since Si is high, Mn is low, and Mo+(1 / 2)W is low, the softening resistance is low and the A value is also low, so the 0.2% yield strength σ is low. 0.2 In Comparative Example 16, due to the small amounts of Mn and Cr, the hardenability and toughness were low.

[0086] Industrial applicability

[0087] The present invention is effective in manufacturing molds with a high elastic limit and low plastic deformation strain at high temperatures, thereby extending the mold life. Therefore, the present invention is useful as a mold steel material capable of producing high-precision products even when repeatedly extruding or forging a high-strength counterpart.

Claims

1. Hot working tool steel, characterized in that It has the following composition, namely, contains: C: 0.35-0.50 mass%, Si: 0.50 mass% or less, Mn: 0.37-1.00 mass%, Cr: 4.30-4.50 mass%, Ni: 0.11-0.20 mass%, Mo+(1 / 2)W: 1.5-3.2 mass%, V: 0.30-0.80 mass%, N: 0.006 to 0.025 mass %, and The balance is Fe and unavoidable impurities; Furthermore, when [X] represents the mass % content of component X, A obtained by the following formula exceeds 1000: A=1050-373.6[C]+28.7[Cr]-150.0[Ni]-127.3[V]+45.9[Mo+(1 / 2)W].

2. The hot working tool steel according to claim 1, characterized in that C is 0.38 to 0.45 mass %.

3. Hot working tool steel, characterized in that It has the following composition, namely, contains: C: 0.38-0.45 mass%, Si: 0.50 mass% or less, Mn: 0.37-1.00 mass%, Cr: 4.30-4.50 mass%, Ni: 0.20 mass% or less, Mo+(1 / 2)W: 1.5-3.2 mass%, V: 0.30-0.80 mass%, N: 0.006 to 0.025 mass %, and The balance is Fe and unavoidable impurities; Furthermore, when [X] represents the mass % content of component X, A obtained by the following formula exceeds 1000: A=1050-373.6[C]+28.7[Cr]-150.0[Ni]-127.3[V]+45.9[Mo+(1 / 2)W].

4. Hot working tool steel, characterized in that It has the following composition, namely, contains: C: 0.35-0.50 mass%, Si: 0.50 mass% or less, Mn: 0.37-1.00 mass%, Cr: 4.30-4.50 mass%, Ni: 0.20 mass% or less, Mo+(1 / 2)W: 1.5-3.2 mass%, V: 0.49-0.62 mass%, N: 0.006 to 0.025 mass %, and The balance is Fe and unavoidable impurities; Furthermore, when [X] represents the mass % content of component X, A obtained by the following formula exceeds 1000: A=1050-373.6[C]+28.7[Cr]-150.0[Ni]-127.3[V]+45.9[Mo+(1 / 2)W].

5. The hot working tool steel according to claim 1 or 3, characterized in that: V is 0.49 to 0.62 mass %.

6. The hot working tool steel according to claim 1, 3 or 4, characterized in that: Mn is 0.45 mass% to 1.00 mass%.

7. The hot working tool steel according to claim 1, 3 or 4, characterized in that: Mo+(1 / 2)W is 1.6 mass% to 3.2 mass%.

8. The hot working tool steel according to claim 1, 3 or 4, characterized in that: The critical cooling time for quenching, which is an indicator of hardenability, is 60 minutes or longer.

9. The hot working tool steel according to claim 1, 3 or 4, characterized in that: Through quenching and tempering treatment, the 0.2% proof stress of the high temperature tensile test at 500°C is tempered to more than 1000 MPa.

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