WO2023008413A1 - Hot work tool steel having excellent high-temperature strength and toughness - Google Patents

Abstract

The purpose of the present invention is to provide a hot work tool steel having excellent high-temperature strength and toughness The present invention provides a hot work tool steel comprising, in mass%: 0.20% to 0.60% of C; 0.10% or more but less than 0.30% of Si; 0.50% to 2.00% of Mn; 0.50% to 2.50% of Ni; 1.6% to 2.6% of Cr; 0.3% to 2.0% of Mo; 0.05% to 0.80% of V; with the remainder being Fe and unavoidable impurities. The hot work tool steel is in a state of having been quenched and tempered such that a value A of formula A, calculated on the basis of formula A: value A = ([T]+273)(log₁₀[t]+24)/1000 (where [T] represents the quenching temperature (°C) and [t] represents the quenching temperature retention time (h)), is 27.4 to 29.3; and the number of carbides having an equivalent circular diameter of 1 μm or more per 10,000 μm² of the hot work tool steel before use is 150 or less.

Description

Hot work tool steel with excellent high-temperature strength and toughness

The present invention relates to hot work tool steel excellent in high-temperature strength and toughness, which is used as hot tools such as hot forging dies.

Hot tools (for example, molds) for hot press forging, hot extrusion or die casting are generally made of Japanese Industrial Standard (JIS) SKD61 steel, and molds for hot hammer forging are made of JIS SKT4 steel is commonly used. JIS SKD61 steel is a mold steel that has both strength and toughness at a relatively high level. In addition, the toughness of JIS SKD61 steel is insufficient to suppress the extension of thermal fatigue cracks. JIS SKT4 steel emphasizes toughness so that it can withstand a large impact from hammer forging, but it lacks wear resistance due to its low softening resistance. In addition, if the die surface is repeatedly lowered for the purpose of reprocessing, the hardenability is low, causing a decrease in hardness at the center, and cracks and settling occur due to lack of strength. Furthermore, due to its low applicable hardness, it lacks wear resistance and strength, and is not suitable for hot press forging and hot extrusion applications.

In Patent Document 1, in mass %, C: 0.37 to 0.45%, Si: 0.3 to 1.2%, Mn: 0.6 to 1.5%, Ni: 0.3 to 1.5%. 0%, Cr: 1.0 to 2.0%, Mo: 1.1 to 1.4%, V: 0.1 to 0.3%, and the balance consisting of Fe and unavoidable impurities, formula of alloy components Hot work tool steels have been proposed in which the values of L and formula Y are defined within specific ranges. The formula L is −0.4×Si−9.7×Mn+3.7×Ni+54.4×Mo, and the value of the formula L is 54-65. The formula Y is −17.1×C+0.1×Si+0.2×Mn+0.2×Ni+0.5×Cr+Mo+5.0, and the value of the formula Y is 0.0 or more.

However, Patent Document 1 does not consider the carbide precipitation state before the hot work tool steel is used as a hot work tool in hot (high temperature) conditions (hereinafter sometimes referred to as "before high temperature use"). , the high temperature strength was insufficient.

In addition, in Patent Document 2, in mass%, C: 0.10 to 0.70%, Si: 0.10 to 2.00%, Mn ≤ 2.00%, Cr ≤ 7.00%, W and Mo (1/2 W + Mo): 0.20 to 12.00%, V ≤ 3.00%, S: less than 0.005%, O is less than 30 ppm, and the balance is substantially Fe A hot working tool having a composition of

However, in Patent Document 2, neither the range of component fluctuations nor the state of carbide precipitation before high-temperature use was considered, and the toughness was insufficient.

JP 2019-19374 A JP-A-11-106868

Hot work tool steel exhibits softening resistance, that is, high-temperature strength, due to the precipitation of carbides (e.g., MC-based carbides, M ₂ C-based carbides, etc., where M represents a metal element) and carbonitrides when used at high temperatures. can get. However, if there is a large amount of M ₂₃ C ₆ -based carbides at the stage before high-temperature use, the amount of carbides (e.g., MC-based carbides, M ₂ C-based carbides, etc.) and carbonitrides during high-temperature use decreases, and high-temperature No strength. Further, if a large amount of coarse carbides are present before high-temperature use, there is a problem that the toughness is lowered.

Therefore, the problem to be solved by the present invention is to limit the quenching conditions so that the M ₂₃ C ₆ -based carbides, which may remain after pressing and have a small contribution to high-temperature strength, are dissolved in the quenching process. At the same time, it is to obtain excellent toughness by controlling the width of component variation. Then, the solid solution of M ₂₃ C ₆ carbides in the quenching process increases the carbon content in the matrix, and fine carbides (such as MC It is to obtain excellent _high -temperature strength by precipitating fine carbonitrides and fine carbonitrides. That is, the problem to be solved by the present invention is to provide a hot work tool steel having toughness and high temperature strength.

In order to solve the above-mentioned problems, the present inventors have made intensive development, and as a result, by specifying the alloy composition, quenching conditions, carbide state and the range of composition fluctuation, heat It has been found that a tool steel can be obtained.

That is, in order to solve the above-described problems, the present invention provides the following hot work tool steel.
[1] % by mass,
C: 0.20% or more and 0.60% or less,
Si: 0.10% or more and less than 0.30%,
Mn: 0.50% or more and 2.00% or less,
Ni: 0.50% or more and 2.50% or less,
Cr: 1.6% or more and 2.6% or less,
Mo: 0.3% or more and 2.0% or less,
V: 0.05% or more and 0.80% or less, and the balance: a hot work tool steel consisting of Fe and inevitable impurities,
The hot work tool steel has the following formula A:
Value A=([T]+273)( _log10 [t]+24)/1000 (A)
[In the formula, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h). ]
It is in a state of being quenched and tempered so that the value A calculated based on is 27.4 or more and 29.3 or less,
Hot work tool steel, wherein the number of carbides having an equivalent circle diameter of 1 μm or more per 10000 μm ² in the hot work tool steel before use is 150 or less.
[2] The hot work tool steel is represented by the following formulas 1-4:
([C] _max- [C] _min )/[C]≤1.0 (1)
([Cr] _{max-[Cr]min )} /[Cr]≦0.5 (2)
([Mo]max-[Mo] _min )/[Mo] _≤1.5 (3)
([V]max-[V] _min )/[V] _≤1.5 (4)
[In the formula, [C] _max and [C] _min represent the maximum and minimum concentrations of C determined by concentration mapping of hot work tool steel by the electron probe microanalysis method, [Cr] _max and [ Cr] _min represent the maximum and minimum Cr concentrations, respectively, determined in concentration mapping of hot work tool steel by the electron probe microanalysis method, and [Mo] _max and [Mo] _min , respectively, [V] max and [V] min represent the highest and lowest concentrations of Mo determined in the hot work tool steel concentration mapping by the analysis method, and [V] _max and [V] _min are respectively the hot work tool steel concentration mapping by the electron probe microanalysis method. represents the maximum concentration and minimum concentration of V determined in, [C] represents the content of C determined by composition analysis of hot work tool steel by infrared absorption method, [Cr], [Mo] and [ V] respectively represent the contents of Cr, Mo and V determined by the composition analysis of the hot work tool steel by the fluorescent X-ray analysis method. ]
The hot work tool steel according to [1], which satisfies

According to the present invention, there is provided a hot work tool steel having both high-temperature strength and toughness, such as a reduction in hardness from the initial hardness of 14 HRC or less, and an impact value of 70 J/cm ² or more in a Charpy impact test. .

If the range of component variation is specified and well controlled, the internal segregation is small and the impact value in the Charpy impact test is 85 J/cm ² or more, so hot work tool steel with better toughness can be obtained.

In this specification, hot work tool steel before being used in hot (high temperature) as hot work tools such as hot forging dies is referred to as "hot work tool steel before use". The hot tool is, for example, for the purpose of improving workability, or for the purpose of structure control for obtaining desired properties after hot working. The vicinity of the surface is exposed to a considerable temperature (for example, 180 to 1300° C.) due to heat transfer from the surface.

The reason for specifying the chemical composition, the reason for specifying the value A, and the reason for specifying the number of carbides in the hot work tool steel of the present invention will be described below. In addition, % in a chemical component shows the mass %.

C: 0.20% or more and 0.60% or less C is a component for ensuring sufficient hardenability and for obtaining high-temperature strength, hardness and wear resistance by forming carbides and carbonitrides. If C is less than 0.20%, sufficient high-temperature strength cannot be obtained. On the other hand, when C exceeds 0.60%, solidification segregation is promoted, coarse carbides and carbonitrides are formed, and toughness is lowered. In addition, the resulting carbides remain undissolved during quenching, reducing the amount of precipitated carbides and carbonitrides when used as a hot work tool steel at high temperatures, and an improvement in high-temperature strength cannot be expected. Therefore, C should be 0.20% or more and 0.60% or less. Preferably, C is 0.40% or more and 0.60% or less.

Si: 0.10% or more and less than 0.30% Si is a component necessary for securing the deoxidizing effect and hardenability in steelmaking. If the Si content is less than 0.10%, sufficient effects are not exhibited. On the other hand, when Si is 0.30% or more, the toughness is lowered. Therefore, Si should be 0.10% or more and less than 0.30%. Preferably, Si is 0.10% or more and 0.20% or less.

Mn: 0.50% or more and 2.00% or less Mn is a component necessary for securing the deoxidizing effect and hardenability in steelmaking. If Mn is less than 0.50%, sufficient effects are not exhibited. If the Mn content is more than 2.00%, workability is lowered. Therefore, Mn should be 0.50% or more and 2.00% or less. Preferably, Mn is 0.50% or more and 1.40% or less.

Ni: 0.50% or more and 2.50% or less Ni is a component necessary for ensuring hardenability and improving toughness. If Ni is less than 0.50%, sufficient effects are not exhibited. Above 2.50% Ni, the cost becomes too high. Therefore, Ni should be 0.50% or more and 2.50% or less. Preferably, Ni is 1.10% or more and 2.30% or less.

Cr: 1.6% or more and 2.6% or less Cr is a component necessary for ensuring sufficient hardenability. If Cr is less than 1.6%, sufficient hardenability cannot be obtained. On the other hand, when Cr is added in an amount of more than 2.6%, M ₂₃ C ₆ -based carbides mainly composed of Cr and Fe are excessively formed during quenching and tempering, deteriorating high-temperature strength, softening resistance and toughness. Therefore, Cr should be 1.6% or more and 2.6% or less. Preferably, Cr is 1.6% or more and 2.4% or less.

Mo: 0.3% to 2.0% Mo is a useful component for obtaining precipitated carbides that contribute to hardenability, secondary hardening and high-temperature strength. If Mo is less than 0.3%, sufficient effects cannot be obtained. When Mo is more than 2.0%, not only does the effect saturate even if it is excessively added, but also the toughness is reduced due to coarse aggregation of carbides. Moreover, it becomes costly. Therefore, Mo should be 0.3% or more and 2.0% or less. Preferably, Mo is 0.3% or more and 1.7% or less.

V: 0.05% or more and 0.80% or less V precipitates fine, hard carbides and fine, hard carbonitrides during tempering or when used as hot work tool steel at high temperatures, resulting in strength and wear resistance. It is a component that contributes to If V is less than 0.05%, these effects cannot be sufficiently obtained. If V is more than 0.80%, coarse carbides and carbonitrides crystallize during solidification, impairing toughness. Therefore, V is set to 0.05% or more and 0.80% or less. Preferably, V is 0.05% or more and 0.20% or less.

Value A: 27.4 or more and 29.3 or less The hot work tool steel before use has been quenched and tempered so that the value A is 27.4 or more and 29.3 or less.

Value A is calculated based on Equation A below.
Value A = ([T] + 273) (log ₁₀ [t] + 24) / 1000 (A)
In Formula A, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h).
That is, when obtaining the value A, the numerical value of the quenching temperature (°C) is substituted for [T] in Formula A, and the numerical value of the quenching temperature holding time (h) is substituted for [t] in Formula A. do.

The value A is an index for ensuring the solid solubility of carbide by specifying the quenching temperature and holding time. If the value A is less than 27.4, solid solution of carbides due to quenching of the steel of the composition of the present invention becomes insufficient, resulting in insufficient toughness and high-temperature strength when used as a hot tool at high temperatures. On the other hand, if the value A exceeds 29.3, the prior austenite crystal grains become coarse, resulting in a decrease in toughness. Therefore, the value A is set to 27.4 or more and 29.3 or less.

Number of carbides with an equivalent circle diameter of 1 μm or more per 10000 μm ² : 150 or less The amounts of carbides (eg, MC-based carbides, M ₂ C-based carbides, etc.) and carbonitrides precipitated during use as steel at high temperatures are reduced. Carbides (e.g., MC-based carbides, M ₂ C-based carbides, etc.) and carbonitrides contribute to the improvement of high-temperature strength by precipitating during high-temperature use as hot work tool steel, so these are reduced. As a result, sufficient high-temperature strength cannot be obtained. Also, if there are too many carbides with an equivalent circle diameter of 1 μm or more, stress concentrates and acts as a starting point and propagation path for cracks, which impairs toughness. Therefore, in the hot work tool steel before use, the number of carbides having an equivalent circle diameter of 1 μm or more per 10000 μm ² is 150 or less.

The number of carbides having an equivalent circle diameter of 1 μm or more per 10,000 μm ² is measured using a steel material after quenching and tempering, as described in the Examples. The carbides to be measured are, for example, MC-based carbides, M2C-based carbides, M3C _- based carbides _{, M7C3 -} based carbides, _{M23C6 -} based carbides, and the like. Note that M represents a metal element.

The number of carbides having an equivalent circle diameter of 1 μm or more per 10000 μm ² is measured according to the method described in the Examples.

The hot work tool steel prior to use has the following formulas 1-4:
([C] _max- [C] _min )/[C]≤1.0 (1)
([Cr] _{max-[Cr]min )} /[Cr]≦0.5 (2)
([Mo]max-[Mo] _min )/[Mo] _≤1.5 (3)
([V]max-[V] _min )/[V] _≤1.5 (4)
is preferably satisfied.

In formulas 1 to 4, [C] _max and [C] _min are the highest concentration (% by mass) and lowest concentration (% by mass) of C determined by concentration mapping of hot work tool steel by the electron probe microanalysis method, respectively. ), and [Cr] _max and [Cr] _min are respectively the maximum Cr concentration (% by mass) and the minimum concentration (% by mass) determined by concentration mapping of hot work tool steel by the electron probe microanalysis method. where [Mo] _max and [Mo] _min represent the highest concentration (% by mass) and the lowest concentration (% by mass) of Mo determined by concentration mapping of hot work tool steel by the electron probe microanalysis method, respectively, [V] _max and [V] _min respectively represent the highest concentration (% by mass) and lowest concentration (% by mass) of V determined by concentration mapping of hot work tool steel by the electron probe microanalysis method, and [C ] represents the C content (% by mass) determined by the composition analysis of the hot work tool steel by the infrared absorption method, and [Cr], [Mo] and [V] are each determined by the fluorescent X-ray analysis method. The contents of Cr, Mo and V (% by mass) determined by composition analysis of hot work tool steel are shown.

For alloying elements X (X=C, Cr, Mo, V), the value of ([X] _max - [X] _min )/[X] is an index of the variation in composition due to internal segregation of each alloying element. In this specification, the value of ([X] _max - [X] _min )/[X] may be referred to as "the width of the component variation of the alloying element X". If the variation in composition due to internal segregation of each alloying element is large (that is, the value of ([X] _max - [X] _min )/[X] is large), the difference in distribution of carbides and carbonitrides and the difference in deformability Since it becomes large, it will reduce toughness. Therefore, it is preferable to regulate the range of component variation for C, Cr, Mo and V. That is, the hot work tool steel before use preferably satisfies all of formulas 1 to 4.

The value of ([X] _max - [X] _min )/[X] is obtained using a steel material after quenching and tempering, as described in the Examples. After mirror-polishing the L surface of the steel material (surface parallel to the rolling direction and thickness direction of the steel plate, the so-called longitudinal cross section), 0.5 mm × 0 using the electron probe micro analysis method (Electron Prove Micro Analysis: EPMA) Concentration mapping of the alloying element X (X=C, Cr, Mo, V) in the range of .5 mm is performed, and the maximum concentration (% by mass) and the minimum concentration (% by mass) of the metal element X determined by the concentration mapping are calculated. Let them be [X] _max and [X] _min , respectively. The composition analysis of the steel material by the infrared absorption method (analysis of the content of C) and the composition analysis of the steel material by the fluorescent X-ray analysis method (analysis of the content of Cr, Mo and V) are performed, and the C determined by the infrared absorption method [C] is the content (% by mass) of Cr, and [Cr], [Mo] and [V] are the contents (% by mass) of Cr, Mo and V determined by fluorescent X-ray analysis, respectively. Concentration mapping by the electron probe microanalysis method, composition analysis by the infrared absorption method and fluorescent X-ray analysis method are performed according to the methods described in Examples.

The application of the soaking treatment in which the center of the steel ingot is soaked for 10 to 40 hours in the range of 1225°C to 1300°C makes it possible to effectively reduce the value of component fluctuation.

The present invention will be described in further detail below based on examples and comparative examples.
Invention Examples Nos. 1 to 28 and Comparative Examples Nos. 29 to 40 are steels each having the chemical composition shown in Table 1 and the balance being Fe and inevitable impurities. 100 kg of each steel was melted in a vacuum induction melting furnace (VIM) and cast into ingots. Invention Steel No. For Nos. 1 to 20, soaking treatment was performed under the above conditions (that is, soaking treatment in which the center of the steel ingot was soaked at 1225° C. to 1300° C. for 10 to 40 hours). After that, these ingots were heated to 1220° C. and forged into square bars of 15 mm square (15 mm×15 mm). The chemical composition of each bulk steel was confirmed by an infrared absorption method (analysis of the content of C) and a fluorescent X-ray analysis method (analysis of the content of alloying elements other than C). Infrared absorption method and fluorescent X-ray analysis method were performed as described below.

After that, it was heated to 850 to 960° C. and held for various times (30 minutes to 3 hours) to obtain an austenite structure. Then, quenching by oil cooling was performed, and after further heating to 500 to 700° C., tempering by air cooling was performed twice to refining to 39 to 41 HRC. Furthermore, test materials were obtained by machining.
In addition, in Table 1, the balance of each steel is Fe and unavoidable impurities.

(Calculation of value A)
Formula A below:
Value A = ([T] + 273) (log ₁₀ [t] + 24) / 1000 (A)
[In the formula, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h). ]
Based on, the value A of each steel was calculated. Table 2 shows the quenching temperature (°C), holding time (h) and value A of each steel.

(Measurement of carbide content)
After polishing the center of each test material to a mirror surface by buffing, the matrix is corroded with a picric acid alcohol solution, and 30 fields of view where many carbides that look light gray or white are observed are selected, and 10 are observed with an electron microscope. The number of carbides having an equivalent circle diameter of 1 μm or more observed at a magnification of 1,000 was counted by image analysis. When the number of carbides having an equivalent circle diameter of 1 μm or more per 10,000 μm ² was 150 or less, it was rated as “A”, and when the number was greater than this, it was rated as “C”. Table 2 shows the measurement results of the amount of carbide.

(Evaluation of width of component variation)
For each test material, after mirror-polishing the L surface (surface parallel to the rolling direction and thickness direction of the steel plate, so-called longitudinal cross section), using Electron Prove Micro Analysis (EPMA), Concentration mapping of the alloying element X (X=C, Cr, Mo, V) in an area of 0.5 mm×0.5 mm was performed.

EPMA was performed under the following conditions.
Analyzer: EPMA1600 manufactured by Shimadzu Corporation
Accelerating voltage: 15 kV
Beam diameter: 2 μm
Irradiation current: 0.1 μA
Scan mode: Stage scan Step size (measured area once): 2.5 μm × 2.5 μm
Number of steps (number of measurement points): 200 x 200
Measurement time (1 step): 50ms
Analysis crystal: C LS12L
Mo PET
Cr, V LIF

For each test material, composition analysis (analysis of C content) by infrared absorption method and composition analysis (analysis of Cr, Mo and V content) by fluorescent X-ray analysis were performed.

The composition analysis by the infrared absorption method was carried out based on the "infrared absorption method" of JIS Z 2615: 2015 "general rules for carbon determination method of metal materials" using Horiba's carbon sulfur analyzer EMIA-Expert.

Composition analysis by X-ray fluorescence spectroscopy (XRF) was performed based on JIS G 1256:2013 "Iron and steel - X-ray fluorescence analysis method" using MXF-2400 manufactured by Shimadzu Corporation.

The highest concentration (% by mass) and the lowest concentration (% by mass) of the alloying element X determined by concentration mapping are set to [X] _max and [X] _min , respectively, and the content of C (% by mass) determined by the infrared absorption method ) is [C], the contents of Cr, Mo and V (% by mass) determined by fluorescent X-ray analysis are respectively [Cr], [Mo] and [V], and ([X] _max - [ The value of X] _min )/[X] was calculated as the width of component variation. Tables 3A to 3D show the evaluation results of the width of component variation.

In all alloying elements of C, Cr, Mo and V, if the value of ([X] _{max-[X]min )} /[X] satisfies the specified requirements, it is an excellent A sample was evaluated as "A" as a good product, and a sample that had a large fluctuation range even for one component and did not satisfy the prescribed requirements was evaluated as "C" as an inferior product. Table 2 shows the evaluation results of the width of component variation.

The specified requirements for C, Cr, Mo and V are as follows.
([C] _max- [C] _min )/[C]≤1.0
([Cr] _max- [Cr] _min )/[Cr]≤0.5
([Mo] _{max-[Mo]min )} /[Mo]≤1.5
([V] _{max-[V]min )} /[V]≤1.5

(Evaluation of high temperature strength)
After measuring the HRC hardness of each test material, it was further held at 600° C. for 100 hours and then air-cooled. A reduction value of 14 HRC or less was rated as "A", and a reduction value greater than this was rated as "C". Table 2 shows the evaluation results of high-temperature strength.

(Evaluation of toughness)
From each test material, a U-notch test piece of JIS standard (JIS Z2242) No. 3 square 10 mm and length 55 mm is formed, and the test piece is quenched and tempered so that the hardness is 39 to 41 HRC. The toughness was evaluated by performing a Charpy impact test at room temperature. Those with an impact value of 70 J/cm ² or more were rated as "A", those with an impact value of 85 J/cm ² or more were rated as "A ⁺ ", and those with an impact value of less than 70 J/cm ² were rated as "C". Table 2 shows the toughness evaluation results.

Invention Example No. As shown in Tables 1 and 2, all of Nos. 1 to 28 are within the prescribed range of the chemical composition, satisfy the value of Formula A, and have a small number of carbides having an equivalent circle diameter of 1 μm or more. rice field. For this reason, the impact value in the Charpy impact test is 70 J/cm ² or more, the toughness is rated A, the width of decrease from the initial hardness is within 14 HRC, and the high-temperature strength (softening resistance) is also rated A. A hot work tool steel with excellent high temperature strength and toughness was obtained.

Comparative example no. No. 29 had a low high-temperature strength due to a small amount of C.
Comparative example no. In No. 30, the amount of C was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness and high-temperature strength were low.
Comparative example no. In No. 31, the amount of Si was large and the toughness was low.
Comparative example no. In No. 32, the amount of Ni was small and the toughness was low.
Comparative example no. In No. 33, the Cr content was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness and high-temperature strength were low.
Comparative example no. In No. 34, the amount of Mo was small and the high-temperature strength was low.
Comparative example no. In No. 35, the amount of Mo was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness and high-temperature strength were low.
Comparative example no. No. 36 had a small amount of V and a low high-temperature strength.
Comparative example no. In No. 37, the amount of V was large, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the range of component fluctuation was large, so the toughness was low.
Comparative example no. In No. 38, the value of formula A was small and the number of carbides having an equivalent circle diameter of 1 μm or more was large, so the toughness and high-temperature strength were low.
Comparative example no. In No. 39, the value of Formula A was large and the toughness was low.
Comparative example no. In No. 40, the number of carbides having an equivalent circle diameter of 1 μm or more was large, and the toughness and high-temperature strength were low.

Claims (2)

in % by mass,
C: 0.20% or more and 0.60% or less,
Si: 0.10% or more and less than 0.30%,
Mn: 0.50% or more and 2.00% or less,
Ni: 0.50% or more and 2.50% or less,
Cr: 1.6% or more and 2.6% or less,
Mo: 0.3% or more and 2.0% or less,
V: 0.05% or more and 0.80% or less, and the balance: a hot work tool steel consisting of Fe and inevitable impurities,
The hot work tool steel has the following formula A:
Value A=([T]+273)( _log10 [t]+24)/1000 (A)
[In the formula, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h). ]
It is in a state of being quenched and tempered so that the value A calculated based on is 27.4 or more and 29.3 or less,
Hot work tool steel, wherein the number of carbides having an equivalent circle diameter of 1 μm or more per 10000 μm ² in the hot work tool steel before use is 150 or less. The hot work tool steel has the following formulas 1-4:
([C] _max- [C] _min )/[C]≤1.0 (1)
([Cr] _{max-[Cr]min )} /[Cr]≦0.5 (2)
([Mo]max-[Mo] _min )/[Mo] _≤1.5 (3)
([V]max-[V] _min )/[V] _≤1.5 (4)
[In the formula, [C] _max and [C] _min represent the maximum and minimum concentrations of C determined by concentration mapping of hot work tool steel by the electron probe microanalysis method, [Cr] _max and [ Cr] _min represent the maximum and minimum Cr concentrations, respectively, determined in concentration mapping of hot work tool steel by the electron probe microanalysis method, and [Mo] _max and [Mo] _min , respectively, [V] max and [V] min represent the highest and lowest concentrations of Mo determined in the hot work tool steel concentration mapping by the analysis method, and [V] _max and [V] _min are respectively the hot work tool steel concentration mapping by the electron probe microanalysis method. represents the maximum concentration and minimum concentration of V determined in, [C] represents the content of C determined by composition analysis of hot work tool steel by infrared absorption method, [Cr], [Mo] and [ V] respectively represent the contents of Cr, Mo and V determined by compositional analysis of hot work tool steel by X-ray fluorescence spectroscopy. ]
The hot work tool steel according to claim 1, which satisfies