hot work tool steel having high temperature strength and toughness

Abstract

The purpose of this invention is to provide a hot work tool steel with excellent high-temperature strength and toughness, comprising, by mass%, C: 0.20% to 0.60%, Si: 0.10% to 0.30%, Mn: 0.50% to 2.00%, Ni: 0.50% to 2.50%, Cr: 1.6% to 2.6%, Mo: 0.3% to 2.0%, V: 0.05% to 0.80%, and the balance being Fe and unavoidable impurities. This hot work tool steel is designed to achieve the following value A: A = ([T] + 273)(log 10 [t]+24) / 1000…(A) [where [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h).] The calculated value A is the state of quenching and tempering in the hot work tool steel with a temperature between 27.4 and 29.3 μm before use. 2 The number of carbides with an equivalent circle diameter of 1 μm or more is less than 150.

Description

Technical Field

[0001] This invention relates to hot work tool steel with excellent high-temperature strength and toughness, which is used as hot forging dies and other hot work tools. Background Technology

[0002] Generally, hot-working tools (e.g., dies) used in hot stamping, forging, hot extrusion, or die casting commonly use Japanese Industrial Standard (JIS) SKD61 steel, while dies used in hot hammer forging commonly use JIS SKT4 steel. JIS SKD61 steel is a relatively high-level die steel that combines both strength and toughness; however, its toughness may not be sufficient due to the high incidence of premature damage caused by cracks during use. Furthermore, the toughness of JIS SKD61 steel is insufficient to suppress the development of hot fatigue cracks. JIS SKT4 steel prioritizes toughness to withstand the significant impact of hammer forging; however, its low resistance to softening results in insufficient wear resistance. Additionally, if the die surface is repeatedly cut for refurbishment, its low hardenability leads to a decrease in hardness at the center, resulting in insufficient strength and subsequent cracking and breakage. Moreover, its low applicable hardness results in insufficient wear resistance and strength, making it unsuitable for hot stamping, forging, and hot extrusion applications.

[0003] Patent Document 1 discloses a hot-work tool steel, comprising, by mass percent, 0.37–0.45% C, 0.3–1.2% Si, 0.6–1.5% Mn, 0.3–1.0% Ni, 1.0–2.0% Cr, 1.1–1.4% Mo, 0.1–0.3% V, and the balance Fe and unavoidable impurities. The values ​​of the alloy composition formulas L and Y are specified within specific ranges. Furthermore, formula L is -0.4×Si - 9.7×Mn + 3.7×Ni + 54.4×Mo, and the value of formula L is 54–65. Additionally, 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 formula Y is 0.0 or higher.

[0004] However, Patent Document 1 does not consider the carbide precipitation state of hot work tool steel before it is used as a hot work tool in a hot state (high temperature) (hereinafter sometimes referred to as "before high temperature use"), resulting in insufficient high temperature strength.

[0005] In addition, Patent Document 2 discloses a tool for hot working, the composition of which, by mass%, is: C: 0.10-0.70%, Si: 0.10-2.00%, Mn≤2.00%, Cr≤7.00%, W and Mo, individually or in combination (1 / 2W+Mo): 0.20-12.00%, V≤3.00%, and S: less than 0.005%, O less than 30ppm, with the balance consisting substantially of Fe.

[0006] However, Patent Document 2 did not take into account the range of changes in composition or the state of carbide precipitation before high-temperature use, resulting in insufficient toughness.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2019-19374

[0010] Patent Document 2: Japanese Patent Application Publication No. 11-106868 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] Hot work tool steels achieve resistance to softening, i.e., high-temperature strength, by precipitating carbides (e.g., MC-based carbides, M2C-based carbides, where M represents a metallic element) and carbonitrides during high-temperature use. However, in the stage before high-temperature use, if M... 23 If there are many C6-based carbides, the amount of carbides (such as MC-based carbides, M2C-based carbides, etc.) and carbonitrides precipitated during high-temperature use will be reduced, resulting in lower high-temperature strength. In addition, if a large number of coarse carbides are present before high-temperature use, there will be a problem of low toughness.

[0013] Therefore, the problem that this invention seeks to solve is to limit the quenching conditions to reduce the amount of M that may remain after forging and contribute little to high-temperature strength. 23 C6-series carbides undergo solid solution treatment during the quenching process, and excellent toughness is achieved by controlling the range of compositional changes. Furthermore, due to the M... 23 The solid solution of C6-based carbides increases the carbon content in the matrix. When used as a hot work tool steel at high temperatures, this leads to the precipitation of fine carbides (e.g., MC-based carbides, M2C-based carbides, etc.) and fine carbonitrides, which contribute significantly to high-temperature strength, resulting in excellent high-temperature strength. Therefore, the problem this invention aims to solve is to provide a hot work tool steel possessing both toughness and high-temperature strength.

[0014] Problem-solving methods

[0015] In order to solve the above-mentioned problems, the inventors have carried out intensive research and development, and found that by specifying the alloy composition, quenching conditions, carbide state and composition variation range, hot work tool steel with both excellent high-temperature strength and toughness can be obtained.

[0016] In order to solve the above-mentioned problems, the present invention provides the following hot work tool steel.

[0017] [1] A hot work tool steel, comprising, by mass%,

[0018] C: Above 0.20% and below 0.60%

[0019] Si: ≥0.10% and <0.30%

[0020] Mn: ≥0.50% and ≤2.00%

[0021] Ni: 0.50% or more and 2.50% or less

[0022] Cr: 1.6% or more but less than 2.6%

[0023] Mo: 0.3% or more and 2.0% or less

[0024] V: Above 0.05% and below 0.80%, and

[0025] Balance: Fe and unavoidable impurities.

[0026] Hot work tool steel is defined as steel that has been quenched and tempered in a manner where the value A, calculated according to the following formula, is 27.4 or higher and 29.3 or lower.

[0027] Value A = ([T] + 273)(log 10 [t]+24) / 1000…(A)

[0028] [In the formula, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h).]

[0029] In hot work tool steel before use, per 10000μm 2 The number of carbides with an equivalent circle diameter of 1 μm or more is less than 150.

[0030] [2] The hot work tool steel described in [1] satisfies the following formulas 1 to 4:

[0031] ([C] max - [C] min ) / [C]≤1.0…(1)

[0032] ([Cr)) max -[Cr] min ) / [Cr]≤0.5…(2)

[0033] ([Mo)) max -[Mo] min ) / [Mo]≤1.5…(3)

[0034] ([V] max - [V] min ) / [V]≤1.5…(4)

[0035] In the formula, [C] max and [C] min The values ​​represent the highest and lowest concentrations of C determined by concentration mapping of hot-work tool steels using electron probe microscopy, respectively. [Cr] max and [Cr] min represents the highest and lowest Cr concentrations determined by concentration mapping of hot-work tool steels using electron probe microscopy, respectively, and [Mo]. max and [Mo] min [V] represents the highest and lowest concentrations of Mo determined by the concentration mapping of hot-work tool steel using electron probe microscopy. max and [V] min [[C]] represents the highest and lowest concentrations of V determined by concentration mapping of hot work tool steel using electron probe microscopy, [Cr] represents the C content determined by compositional analysis of hot work tool steel using infrared absorption spectrometry, and [Cr], [Mo], and [V] represent the Cr, Mo, and V contents determined by compositional analysis of hot work tool steel using X-ray fluorescence spectrometry, respectively.

[0036] The effects of the invention

[0037] According to the present invention, a hot work tool steel with both high-temperature strength and toughness is provided, with a reduction in initial hardness within 14 HRC and an impact value of 70 J / cm² in a pendulum impact test. 2 The above, etc.

[0038] If the variation range of the composition is specified and well controlled, internal segregation will be small, and the impact value of the pendulum impact test will be 85 J / cm. 2 Therefore, hot work tool steel with better toughness can be obtained. Detailed Implementation

[0039] In this specification, hot work tool steel used as hot work tools such as hot forging dies before being used in a hot state (high temperature) is referred to as "hot work tool steel before use". Hot work tools, for example, are used for the purpose of improving machinability or for controlling the microstructure to obtain desired properties after hot working, because they come into contact with the workpiece heated to a high temperature, and therefore are exposed to the corresponding temperature (e.g., 180 to 1300°C) near the heat transfer surface of the workpiece.

[0040] The reasons for specifying the chemical composition, the specified value A, and the specified number of carbides in the hot work tool steel of the present invention will be explained below. Also, the % in the chemical composition represents mass%.

[0041] C: Above 0.20% and below 0.60%

[0042] Carbon (C) is a component used to ensure sufficient hardenability and to facilitate the formation of carbides and carbonitrides, thereby obtaining high-temperature strength, hardness, and wear resistance. If C is below 0.20%, sufficient high-temperature strength cannot be obtained. On the other hand, if C is above 0.60%, it promotes solidification segregation, resulting in the formation of coarse carbides and carbonitrides, which reduces toughness. Furthermore, the carbides formed do not dissolve during quenching and remain, thus reducing the amount of carbides and carbonitrides precipitated when used as hot-work tool steel at high temperatures, and improving high-temperature strength cannot be expected. Therefore, C is 0.20% or more and 0.60% or less. Preferably, C is 0.40% or more and 0.60% or less.

[0043] Si: 0.10% or more but less than 0.30%

[0044] Si is a necessary component to ensure effective deoxidation and hardenability in steelmaking. If Si is below 0.10%, it cannot achieve its full effect. On the other hand, if Si is above 0.30%, it leads to a decrease in toughness. Therefore, Si is between 0.10% and 0.30%. Preferably, Si is between 0.10% and 0.20%.

[0045] Mn: ≥0.50% and ≤2.00%

[0046] Mn is a necessary component to ensure deoxidation and hardenability in steelmaking. If Mn is below 0.50%, it cannot achieve its full effect. If Mn is above 2.00%, it leads to a decrease in workability. Therefore, Mn should be between 0.50% and 2.00%. Preferably, Mn should be between 0.50% and 1.40%.

[0047] Ni: 0.50% or more and 2.50% or less

[0048] Ni is a necessary component to ensure hardenability and improve toughness. If Ni is below 0.50%, it will not be effective enough. If Ni is above 2.50%, the cost becomes too high. Therefore, Ni is between 0.50% and 2.50%. Preferably, Ni is between 1.10% and 2.30%.

[0049] Cr: 1.6% or more and 2.6% or less

[0050] Cr is a necessary component to ensure adequate hardenability. Insufficient hardenability is not achieved when Cr content is below 1.6%. On the other hand, if more than 2.6% Cr is added, during quenching and tempering, the chromium (M), primarily composed of Cr and Fe, will... 23 Excessive formation of C6-based carbides reduces high-temperature strength, softening resistance, and toughness. Therefore, the Cr content should be 1.6% or more and 2.6% or less. Preferably, the Cr content should be 1.6% or more and 2.4% or less.

[0051] Mo: 0.3% or more and 2.0% or less

[0052] Mo is a useful component for obtaining precipitated carbides that contribute to hardenability, secondary hardening, and high-temperature strength. If Mo is below 0.3%, sufficient effect cannot be achieved. If Mo is above 2.0%, even excessive addition not only saturates the effect but also results in coarse agglomeration of carbides, reducing toughness. Furthermore, it leads to high costs. Therefore, Mo is between 0.3% and 2.0%. Preferably, Mo is between 0.3% and 1.7%.

[0053] V: Above 0.05% and below 0.80%

[0054] During tempering or high-temperature use as hot-work tool steel, v precipitates fine, hard carbides and carbonitrides, which contribute to strength and wear resistance. If v is less than 0.05%, these effects cannot be fully achieved. If v is more than 0.80%, coarse carbides and carbonitrides crystallize during solidification, hindering toughness. Therefore, v is between 0.05% and 0.80%, preferably between 0.05% and 0.20%.

[0055] Value A: 27.4 or higher, 29.3 or lower

[0056] Before use, the hot work tool steel is quenched and tempered with an A value of 27.4 or higher and 29.3 or lower.

[0057] Value A is calculated according to the following formula A.

[0058] Value A = ([T] + 273)(log 10 [t]+24) / 1000…(A)

[0059] In formula A, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h).

[0060] That is, when calculating A, substitute the value of the quenching temperature (°C) into [T] in formula A, and substitute the value of the quenching temperature holding time (h) into [t] in formula A.

[0061] Value A is an indicator used to ensure the solid solubility of carbides by specifying the quenching temperature and holding time. If value A is lower than 27.4, the solid solubility of carbides in the composition of this invention through steel quenching is insufficient, resulting in insufficient toughness and high-temperature strength when used as a hot-working tool at high temperatures. On the other hand, if value A exceeds 29.3, the coarsening of the original austenite grains leads to a decrease in toughness. Therefore, the value A is set to be 27.4 or higher and 29.3 or lower.

[0062] per 10000μm 2 Number of carbides with an equivalent circle diameter of 1 μm or more: less than 150

[0063] In hot work tool steel before use, if there is an excessive amount of carbides with an equivalent circle diameter of 1 μm or more, the carbon content in the matrix will be insufficient, resulting in a reduction in the amount of carbides (e.g., MC-based carbides, M2C-based carbides, etc.) and carbonitrides that precipitate during high-temperature use of hot work tool steel. The precipitation of carbides (e.g., MC-based carbides, M2C-based carbides, etc.) and carbonitrides during high-temperature use of hot work tool steel contributes to increased high-temperature strength; therefore, a reduction in their amount will result in insufficient high-temperature strength. Furthermore, an excessive amount of carbides with an equivalent circle diameter of 1 μm or more leads to stress concentration and acts as a crack initiation and propagation path, thus hindering toughness. Therefore, in hot work tool steel before use, the amount of carbides should be less than 1 μm per 10,000 μm of carbon. 2 The number of carbides with an equivalent circle diameter of 1 μm or more is less than 150.

[0064] per 10,000 μm 2 The number of carbides with an equivalent circle diameter of 1 μm or larger was measured using quenched and tempered steel, as described in the examples. The carbides being measured, for example, are MC-based carbides, M2C-based carbides, M3C-based carbides, M7C3-based carbides, and M... 23 C6 series carbides, etc. Also, M indicates a metallic element.

[0065] per 0000μm 2 The number of carbides with an equivalent circle diameter of 1 μm or more was measured in accordance with the method described in the examples.

[0066] Hot work tool steel, before use, preferably satisfies the following formulas 1 to 4:

[0067] ([C] max - [C] min ) / [C]≤1.0…(1)

[0068] ([Cr)) max -[Cr] min ) / [Cr]≤0.5…(2)

[0069] ([Mo)) max -[Mo] min ) / [Mo]≤1.5…(3)

[0070] ([V] max - [V] min ) / [V]≤1.5…(4)

[0071] In equations 1 through 4, [C] max and [C] minThe values ​​represent the highest and lowest concentrations (mass%) of C determined by concentration mapping of hot-work tool steel using electron probe microscopy, respectively. [Cr] max and [Cr] min The values ​​represent the highest and lowest concentrations (mass%) of Cr determined by concentration mapping of hot-work tool steel using electron probe microscopy, respectively. [Mo] max and [Mo] min V represents the highest and lowest concentrations (mass%) of Mo determined by concentration mapping of hot-work tool steel using electron probe microscopy. max and [V] min [V] represents the highest and lowest concentrations (mass%) of V determined by concentration mapping of hot work tool steel by electron probe microanalysis, respectively; [C] represents the content of C (mass%) determined by compositional analysis of hot work tool steel by infrared absorption spectrometry; and [Cr], [Mo], and [V] represent the contents (mass%) of Cr, Mo, and V determined by compositional analysis of hot work tool steel by X-ray fluorescence analysis, respectively.

[0072] Regarding alloying element X (X = C, Cr, Mo, V), ([X]) max -[X] min The value of () / [X] is an indicator of the compositional deviation caused by the internal segregation of each alloying element. In this specification, ([X]) / [X] is sometimes used as an indicator. max -[X] min The value of () / [X] is called the "composition variation range of alloying element X". If the composition deviation caused by the internal segregation of each alloying element is large (i.e., ([X] / [X]), then the variation range of the alloying element X is large. max -[X] min A larger value for () / [X] results in a greater difference in the distribution and deformation capacity of carbides and carbonitrides, thus reducing toughness. Therefore, it is preferable to control the variation range of C, Cr, Mo, and V in the composition. That is, the hot work tool steel before use preferably satisfies all formulas 1 to 4.

[0073] ([X] max -[X] minThe value of ) / [X], as described in the examples, was obtained using quenched and tempered steel. After mirror polishing the L-plane of the steel (the plane parallel to the rolling direction and thickness direction of the steel plate, the so-called longitudinal cross-section), electron probe microanalysis (EPMA) was used to map the concentration of alloying element X (X = C, Cr, Mo, V) within a range of 0.5 mm × 0.5 mm. The highest and lowest concentrations (mass%) of metallic element X determined by this concentration mapping were set as [X]. max and [X] min The compositional analysis of the steel (analysis of C content) was performed by infrared absorption spectrometry and by X-ray fluorescence spectrometry (analysis of Cr, Mo, and V content). The C content (mass %) determined by infrared absorption spectrometry was denoted as [C], and the Cr, Mo, and V contents (mass %) determined by X-ray fluorescence spectrometry were denoted as [Cr], [Mo], and [V], respectively. Concentration mapping by electron probe microscopy and compositional analysis by infrared absorption spectrometry and X-ray fluorescence spectrometry were performed according to the methods described in the examples.

[0074] Applicable to the temperature range of 1225℃~1300℃, the homogenization heat treatment of the center of the steel ingot for 10~40 hours can effectively reduce the value of composition variation.

[0075] Example

[0076] The present invention will now be described in more detail based on embodiments and comparative examples.

[0077] Invention Examples No. 1-28 and Comparative Examples No. 29-40 are steels containing the chemical composition and balance Fe as described in Table 1, as well as unavoidable impurities. 100 kg of each steel was melted in a vacuum induction melting furnace (VIM) and cast into ingots. For Invention Steels No. 1-20, a homogenization treatment was performed under the conditions described above (i.e., homogenization treatment at 1225°C to 1300°C, maintaining the center of the ingot for 10 to 40 hours). Subsequently, these ingots were heated to 1220°C and forged into square sections of 15 mm (15 mm × 15 mm). The composition of each batch of steel was confirmed by infrared absorption spectroscopy (analysis of the content of C) and X-ray fluorescence analysis (analysis of the content of alloying elements other than C). Infrared absorption spectroscopy and X-ray fluorescence analysis were performed as described later.

[0078] Subsequently, the material was heated to 850–960°C and held for various times (30 minutes to 3 hours) to obtain an austenitic structure. It was then oil-quenched, reheated to 500–700°C, and subjected to two air-cooling tempering processes to a tempering temperature of 39–41 HRC. Finally, the material was machined to obtain the test material.

[0079] Furthermore, in Table 1, the balance for each steel is Fe and unavoidable impurities.

[0080] (Calculation of value A)

[0081] Calculate the value A for each steel according to the following formula A. The quenching temperature (°C), holding time (h), and value A for each steel are shown in Table 2.

[0082] Value A = ([T] + 273)(log 10 [t]+24) / 1000…(A)

[0083] [In the formula, [T] represents the quenching temperature (°C), and [t] represents the quenching temperature holding time (h).]

[0084] (Determination of carbide content)

[0085] After mirror polishing the center of each test material, the matrix was etched with a picric acid alcohol solution. Areas with abundant light gray or white carbides were selected from 30 fields of view. Image analysis was used to count the number of carbides with an equivalent circle diameter of 1 μm or larger, observed at 10,000x magnification using an electron microscope. The number of carbides per 10,000 μm was determined. 2 Carbides with an equivalent circle diameter of 1 μm or more and fewer than 150 are designated as "A", while those with more than 150 are designated as "C". The results of the carbide quantity determination are shown in Table 2.

[0086] (Evaluation of the magnitude of component variation)

[0087] In each test material, after mirror polishing of its L-surface (the surface parallel to the rolling direction and thickness direction of the steel plate, the so-called longitudinal cross-section), the concentration mapping of alloying element X (X = C, Cr, Mo, V) within a range of 0.5 mm × 0.5 mm was performed using Electron Probe Micro Analysis (EPMA).

[0088] EPMA is performed under the following conditions.

[0089] Analytical device: EPMA1600 manufactured by Shimadzu Corporation

[0090] Accelerating voltage: 15kV

[0091] Beam diameter: 2μm

[0092] Irradiation current: 0.1μA

[0093] Scanning mode: Stage scanning

[0094] Step size (area measured in one measurement): 2.5μm × 2.5μm

[0095] Number of steps (number of measurement positions): 200×200

[0096] Measurement time (1 step): 50ms

[0097] Spectroscopic crystal: CLS12L

[0098] Mo PET

[0099] Cr, V, LIF

[0100] For each test material, compositional analysis (analysis of C content) was performed by infrared absorption method and compositional analysis (analysis of Cr, Mo and V content) was performed by X-ray fluorescence analysis.

[0101] Compositional analysis was performed using infrared absorption spectrometry with the EMIA-Expert carbon and sulfur analyzer manufactured by Horiba Corporation, according to the "infrared absorption method" of JIS Z 2615:2015 "Conventional method for carbon quantification of metallic materials".

[0102] Compositional analysis was performed by X-ray fluorescence analysis (XRF) using a Shimadzu MXF-2400, in accordance with JIS G 1256:2013 "Iron and steel - X-ray fluorescence analysis method".

[0103] The highest and lowest concentrations (mass%) of alloying element X, determined by the concentration mapping, are denoted as [X]. max and [X] min Let the content of C (mass%) determined by infrared absorption spectroscopy be [C], and the content of Cr, Mo, and V (mass%) determined by X-ray fluorescence spectroscopy be [Cr], [Mo], and [V], respectively. Calculate ([X]... max -[X] min The value of ) / [X] is used as the component variation range. The evaluation results of the component variation range are shown in Tables 3A to 3D.

[0104] Among all alloying elements of C, Cr, Mo, and V, ([X]) max -[X] minWhen the value of ) / [X] meets the specified requirements, it is considered an excellent level of meeting the specified requirements for the range of component variation, and is rated as "A". Even if the range of variation of one component is large, it is considered to fail to meet the specified requirements, and is rated as "C". The evaluation results of the range of component variation are shown in Table 2.

[0105] The requirements for C, Cr, Mo and V are as follows.

[0106] ([C] max - [C] min ) / [C]≤1.0

[0107] ([Cr)) max -[Cr] min ) / [Cr]≤0.5

[0108] ([Mo)) max -[Mo] min ) / [Mo]≤1.5

[0109] ([V] max - [V] min ) / [V]≤1.5

[0110] (Evaluation of high-temperature strength)

[0111] After measuring the HRC hardness of each test material, it was kept at 600℃ for 100 hours, then air-cooled, and the HRC hardness at room temperature was measured. The high-temperature strength was evaluated based on the reduction in hardness from the initial value. A reduction of less than 14 HRC was designated as "A", and a reduction greater than this was designated as "C". The evaluation results of the high-temperature strength are shown in Table 2.

[0112] (Evaluation of resilience)

[0113] U-shaped test pieces, each 10 mm square and 55 mm long, conforming to JIS standard (JIS Z 2242), were formed from the tested materials. The test pieces were then quenched and tempered to achieve a hardness of 39–41 HRC. Pendulum impact tests were conducted at room temperature to evaluate toughness. The impact value was 70 J / cm². 2 The above are "A", especially 85J / cm 2 The above is "A" + "Below 70J / cm" 2 The value is "C". The evaluation results for toughness are shown in Table 2.

[0114] Table 1

[0115] Table 1: Invention Examples (No. 1-28) and Comparative Examples (No. 29-40)

[0116]

[0117] Table 2

[0118] Table 2: Invention Examples (No. 1-28) and Comparative Examples (No. 29-40)

[0119]

[0120] Table 3A

[0121] Table 3A: Invention Examples (No. 1-28) and Comparative Examples (No. 29-40)

[0122] No. [C]max [C]min ([C]max-[C]min) / [C] 1 0.26 0.16 0.5 2 0.78 0.49 0.5 3 0.41 0.25 0.5 4 0.80 0.40 0.7 5 0.71 0.43 0.6 6 0.41 0.22 0.7 7 0.51 0.27 0.7 8 0.54 0.31 0.6 9 0.44 0.23 0.7 10 0.49 0.29 0.6 11 0.72 0.39 0.6 12 0.74 0.43 0.6 13 0.63 0.42 0.4 14 0.35 0.19 0.6 15 0.44 0.26 0.5 16 0.69 0.45 0.5 17 0.51 0.26 0.7 18 0.55 0.27 0.7 19 0.56 0.32 0.6 20 0.62 0.34 0.6 21 0.70 0.32 <![CDATA[ 1.1 ]]> 22 0.78 0.42 0.7 23 0.57 0.34 0.6 24 0.74 0.45 0.6 25 0.98 0.39 <![CDATA[ 1.3 ]]> 26 0.51 0.32 0.5 27 1.00 0.44 <![CDATA[ 1.2 ]]> 28 0.87 0.37 <![CDATA[ 1.2 ]]> 29 0.15 0.08 0.7 30 1.60 0.58 <![CDATA[ 1.3 ]]> 31 0.29 0.18 0.5 32 0.63 0.34 0.6 33 0.30 0.17 0.7 34 0.42 0.27 0.5 35 0.87 0.49 0.6 36 0.32 0.19 0.6 37 0.40 0.23 0.6 38 0.46 0.25 0.6 39 0.47 0.23 0.7 40 0.65 0.35 0.6

[0123] Table 3B

[0124] Table 3B: Invention Examples (No. 1-28) and Comparative Examples (No. 29-40)

[0125] No. [Cr]max [Cr]min ([Cr]max-[Cr]min) / [Cr] 1 2.98 2.04 0.4 2 2.66 1.87 0.4 3 2.71 2.03 0.3 4 2.74 1.91 0.3 5 3.12 2.19 0.4 6 2.15 1.59 0.3 7 1.99 1.39 0.4 8 2.02 1.44 0.3 9 2.20 1.55 0.4 10 3.09 2.11 0.4 11 2.12 1.50 0.3 12 2.26 1.63 0.3 13 2.66 1.88 0.4 14 3.00 2.13 0.3 15 2.54 1.81 0.3 16 2.16 1.47 0.4 17 2.24 1.64 0.3 18 2.22 1.58 0.3 19 2.14 1.44 0.4 20 3.09 2.17 0.4 21 3.17 2.16 0.4 22 3.38 2.12 <![CDATA[ 0. 6]]> 23 2.09 1.42 0.4 24 2.83 2.05 0.3 25 3.23 2.00 0.6 26 3.50 2.00 <![CDATA[ 0.7 ]]> 27 3.36 1.96 <![CDATA[ 0.6 ]]> 28 3.90 2.20 <![CDATA[ 0.7 ]]> 29 2.66 1.83 0.4 30 2.60 1.82 0.3 31 2.02 1.46 0.3 32 2.56 1.67 0.4 33 4.41 2.70 <![CDATA[ 0.6 ]]> 34 1.95 1.32 0.4 35 2.42 1.59 0.4 36 2.89 2.07 0.3 37 2.15 1.53 0.3 38 1.86 1.27 0.4 39 2.13 1.55 0.3 40 2.64 1.80 0.4

[0126]

Table 3C

[0127] Table 3C: Invention Examples (No. 1-28) and Comparative Examples (No. 29-40)

[0128] No. [Mo]max [Mo]min ([Mo]max-[Mo]min) / [Mo] 1 1.24 0.50 0.9 2 2.77 1.07 1.0 3 0.87 0.31 1.1 4 2.02 0.79 0.9 5 2.31 0.85 1.1 6 1.97 0.67 1.1 7 1.78 0.64 1.0 8 1.45 0.51 1.0 9 0.72 0.25 1.2 10 1.91 0.68 1.1 11 0.47 0.18 0.9 12 3.20 1.32 0.9 13 3.26 1.09 1.2 14 2.69 0.86 1.2 15 2.92 1.19 1.0 16 0.94 0.40 0.9 17 1.49 0.56 1.0 18 2.45 0.99 0.9 19 2.69 1.00 1.1 20 0.81 0.29 1.0 21 2.21 0.83 1.0 22 2.70 0.93 1.1 23 2.93 0.76 <![CDATA[ 2.0 ]]> 24 0.91 0.34 1.1 25 1.38 0.46 1.2 26 5.28 1.48 <![CDATA[ 1.9 ]]> 27 1.81 0.57 1.2 28 4.90 1.40 <![CDATA[ 1.8 ]]> 29 2.51 1.00 1.0 30 2.81 0.95 1.1 31 2.63 0.99 1.1 32 2.07 0.82 1.0 33 0.81 0.28 1.1 34 0.15 0.07 0.9 35 4.90 0.95 <![CDATA[ 1.6 ]]> 36 2.62 1.01 1.0 37 2.25 0.86 1.1 38 1.78 0.64 1.1 39 1.99 0.70 1.2 40 1.58 0.54 1.2

[0129]

Table 3D

[0130] Table 3D: Invention Examples (No. 1-28) and Comparative Examples (No. 29-40)

[0131] No. [V]max [V]min ([V]max-[V]min) / [V] 1 0.96 0.37 1.0 2 1.30 0.42 1.2 3 0.34 0.13 1.0 4 1.21 0.42 1.1 5 1.34 0.44 1.1 6 0.86 0.32 1.1 7 0.12 0.04 1.2 8 1.32 0.45 1.1 9 1.30 0.38 1.2 10 0.52 0.17 1.1 11 0.72 0.24 1.1 12 0.19 0.07 1.1 13 0.09 0.03 1.1 14 1.39 0.46 1.2 15 0.47 0.16 1.1 16 0.17 0.06 1.2 17 0.89 0.32 1.1 18 1.02 0.31 1.2 19 1.14 0.35 1.2 20 0.95 0.34 1.1 21 1.33 0.40 1.2 22 0.19 0.07 1.1 23 1.09 0.41 1.0 24 0.17 0.05 <![CDATA[ 1.7 ]]> 25 0.55 0.20 1.1 26 0.14 0.05 1.1 27 0.20 0.05 <![CDATA[ 1.9 ]]> 28 0.80 0.18 <![CDATA[ 1. 8]]> 29 0.79 0.29 1.1 30 0.53 0.16 1.2 31 0.18 0.05 1.2 32 0.81 0.28 1.1 33 1.05 0.41 1.0 34 0.90 0.30 1.2 35 0.88 0.30 1.1 36 0.04 0.01 1.1 37 2.61 0.72 <![CDATA[ 1.6 ]]> 38 1.23 0.37 1.2 39 1.04 0.32 1.2 40 0.22 0.07 1.2

[0132] Invention Examples No. 1 to 28, as shown in Tables 1 and 2, all have chemical compositions within the specified range, satisfying the value of Formula A, and exhibiting a low number of carbides with an equivalent circle diameter greater than 1 μm. Therefore, the impact value in the pendulum impact test shows 70 J / cm. 2 The above results show that the toughness is rated A, the reduction in initial hardness is within 14 HRC, and the high-temperature strength (resistance to softening) is also rated A, thus obtaining a hot work tool steel that combines excellent high-temperature strength and toughness.

[0133] Comparative example No. 29 has low high-temperature strength because of its low C content.

[0134] Comparative Example No. 30 has a high C content, resulting in a large number of carbides with an equivalent circle diameter of 1 μm or more and a large variation in composition, thus exhibiting low toughness and high-temperature strength.

[0135] In Comparative Example No. 31, the Si content is high and the toughness is low.

[0136] In Comparative Example No. 32, the amount of Ni is low, resulting in low toughness.

[0137] In Comparative Example No. 33, the amount of Cr is high, the number of carbides with an equivalent circle diameter of more than 1 μm is high, and the composition varies greatly, resulting in low toughness and high-temperature strength.

[0138] In Comparative Example No. 34, the amount of Mo is low, resulting in low high-temperature strength.

[0139] Comparative Example No. 35, due to its high Mo content, has a large number of carbides with an equivalent circle diameter of 1 μm or more, resulting in a large variation in composition, thus exhibiting low toughness and high-temperature strength.

[0140] In Comparative Example No. 36, the amount of V is low, resulting in low high-temperature strength.

[0141] Comparative Example No. 37, due to its high V content, has a large number of carbides with an equivalent circle diameter of 1 μm or more, resulting in a large variation in composition and thus low toughness.

[0142] Comparative Example No. 38, because the value of Formula A is small, has a large number of carbides with an equivalent circle diameter of 1 μm or more, resulting in low toughness and high-temperature strength.

[0143] In Comparative Example No. 39, the value of Equation A is large, indicating low toughness.

[0144] In Comparative Example No. 40, there were more carbides with an equivalent circle diameter of 1 μm or more, but lower toughness and high-temperature strength.

Claims

1. A hot work tool steel, comprising, by weight % C: Above 0.20% and below 0.60% Si: ≥0.10% and <0.30% Mn: ≥0.50% and ≤2.00% Ni: 0.50% or more and 2.50% or less Cr: 1.6% or more but less than 2.6% Mo: 0.3% or more and 2.0% or less V: Above 0.05% and below 0.80%, and Balance: Fe and unavoidable impurities. Hot work tool steel is quenched and tempered in a manner such that the value A calculated according to the following formula is 27.4 or higher and 29.3 or lower. Value A = ( [T] + 273) (log 10 [T] + 24) / 1000... (A) In the formula, [T] represents the quenching temperature in °C, and [t] represents the holding time at the quenching temperature in hours. The number of carbides of 1 μm or more in diameter of equivalent circle in the hot work tool steel before use is 150 or less per 10,000 μm 2 of equivalent circle The hot work tool steel satisfies the following formulas 1 to 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 [Cr] represents the highest and lowest concentrations of C determined by concentration mapping of hot-work tool steels using electron probe microscopy, respectively. max and [Cr] min [Cr] represents the highest and lowest concentrations of Cr determined by concentration mapping of hot-work tool steels using electron probe microscopy, and [Mo] represents the lowest concentrations of Cr. max and [Mo] min [V] represents the highest and lowest concentrations of Mo determined by the concentration mapping of hot-work tool steel using electron probe microscopy. max and [V] min [ ] represents the highest and lowest concentrations of V determined by concentration mapping of hot work tool steel by electron probe microanalysis, [C] represents the C content determined by compositional analysis of hot work tool steel by infrared absorption spectrometry, and [Cr], [Mo] and [V] represent the Cr, Mo and V contents determined by compositional analysis of hot work tool steel by X-ray fluorescence analysis, respectively.

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