KR20240041334A - Hot-worked tool steel with excellent high-temperature strength and toughness - Google Patents

Abstract

The purpose of the present invention is to provide hot tool steel with excellent high-temperature strength and toughness, and in mass%, C: 0.20% to 0.60%, Si: 0.10% to 0.30% less, 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 remainder: A hot tool steel consisting of Fe and inevitable impurities, where the hot tool steel has the following formula (A):
Value A= ([T]+273)(log ₁₀ [t]+24)/1000... (A)
[Wherein, [T] represents the quenching temperature (℃), and [t] represents the quenching temperature maintenance time (h)]
Provided is a hot-worked tool steel that has been quenched and tempered so that the value A calculated based on is 27.4 or more and 29.3 or less, and the number of carbides with an equivalent circle diameter of 1 μm or more per 10,000 μm ² in the hot-worked tool steel before use is 150 or less. .

Description

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

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

Hot tools (e.g., molds) for hot press forging, hot extrusion, or diecast generally use Japanese Industrial Standards (JIS) SKD61 steel, and molds for hot hammer forging use JIS SKT4 steel. It is used universally. JIS SKD61 steel is a mold steel that combines both strength and toughness at a relatively high level, but early failure due to cracking during use often occurs, so it is not necessarily sufficient in terms of toughness. Additionally, the toughness of JIS SKD61 steel is insufficient to suppress the extension of thermal fatigue cracks. JIS SKT4 steel focuses on toughness so that it can withstand large impacts caused by hammer forging, but has low softening resistance and therefore lacks wear resistance. Additionally, if the engraving surface is repeatedly lowered for the purpose of recycling, hardenability is low, so hardness decreases in the center, and cracks, deformation, etc. occur due to insufficient strength. Additionally, since the applicable hardness is low, 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 1.0%, Cr: 1.0 to 2.0%, Mo: 1.1 to 1.4%. , V: 0.1 to 0.3%, and the balance Fe and inevitable impurities, and the values of the formula L and formula Y of the alloy components are defined in a specific range. And 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 to 65. In addition, the formula Y is -17.1

However, in Patent Document 1, the state of carbide precipitation before the hot tool steel is used hot (high temperature) as a hot tool (hereinafter sometimes referred to as “before high temperature use”) was not considered, and the high temperature strength was insufficient. .

Additionally, 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 alone or in combination (1/2W+Mo): 0.20 A tool for hot working with a composition of ∼12.00%, V≤3.00%, S: less than 0.005%, O of less than 30 ppm, and the balance substantially consisting of Fe has been proposed.

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

Japanese Patent Publication No. 2019-19374 Japanese Patent Publication No. 11-106868

Hot-worked tool steel can obtain softening resistance, that is, high-temperature strength, by precipitating carbides (for example, MC-based carbides, M ₂ C-based carbides, etc., where M represents a metal element) and carbonitrides when used at high temperatures. However, if there is a lot of M ₂₃ C ₆ type carbide in the stage before high temperature use, the amount of precipitation of carbide in use at high temperature (e.g. MC type carbide, M ₂ C type carbide, etc.) and carbonitride decreases, and high high temperature strength is obtained. I don't lose. Additionally, if a large amount of coarse carbide exists before use at high temperatures, there is a problem that toughness is lowered.

Accordingly, the problem to be solved by the present invention is to limit the quenching conditions by limiting the quenching conditions so that M ₂₃ C ₆ -based carbides, which can remain after cutting and have a small contribution to high-temperature strength, are dissolved in solid solution in the quenching process. and, at the same time, controlling the range of component variation to obtain excellent toughness. In addition, the amount of carbon in the matrix increases due to the solid solution of the M ₂₃ C ₆ type carbide in the quenching process, and fine carbides (for example, MC type carbide) that greatly contribute to high temperature strength during high temperature use as hot-worked tool steel are formed. , M ₂ C-based carbides, etc.) and fine carbonitrides, excellent high-temperature strength is obtained. That is, the problem to be solved by the present invention is to provide hot-worked tool steel with toughness and high-temperature strength.

In order to solve the above-mentioned problem, the present inventors conducted careful development and, as a result, obtained hot-worked tool steel with excellent high-temperature strength and toughness by specifying the alloy composition, quenching conditions, carbide state, and range of composition variation. I found something I could do.

That is, in order to solve the above-described problems, the present invention provides the following hot-worked tool steel.

[1] In 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

Residue: Fe and inevitable impurities

As a hot tool steel consisting of,

Hot tool steel, formula (A):

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

[Wherein, [T] represents the quenching temperature (℃), and [t] represents the quenching temperature maintenance time (h)]

It is quenched and tempered so that the value A calculated based on is 27.4 or more and 29.3 or less,

Hot-worked tool steel in which the number of carbides with a circle equivalent diameter of 1 μm or more per 10,000 μm ² in the hot-worked tool steel before use is 150 or less.

[2] Hot tool steel, following formulas (1) to (4):

([C] _max -[C] _min )/[C]≤1.0 … (One)

([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 highest and lowest concentrations of C determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively, and [Cr] _max and [Cr] _min represents the highest and lowest concentrations of Cr determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively, and [Mo] _max and [Mo] _min , respectively, represent the highest and lowest concentrations of Cr determined by concentration mapping of the hot tool steel by the electronic probe microanalysis method. represents the highest and lowest concentrations of Mo determined by concentration mapping, and [V] _max and [V] _min represent the highest and lowest concentrations of V determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively. , [C] represents the content of C determined by composition analysis of the hot tool steel by infrared absorption method, and [Cr], [Mo], and [V] represent the content of C determined by composition analysis of the hot tool steel by the infrared absorption method, and [Cr], [Mo], and [V] represent the content of C determined by composition analysis of the hot tool steel by the infrared absorption method, respectively Indicates the content of Cr, Mo and V determined by composition analysis]

The hot tool steel described in [1] that satisfies.

According to the present invention, a hot-worked tool steel is provided that has both high-temperature strength and toughness, such that the decrease from the initial hardness is within 14 HRC and the impact value in the Charpy impact test is 70 J/cm ² or more.

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

In this specification, hot tool steel before being used hot (high temperature) as a hot tool such as a hot forging mold is referred to as “hot tool steel before use.” Hot tools come into contact with a workpiece heated to a high temperature, for example, for the purpose of improving machinability or controlling the structure to obtain desired characteristics after hot working, so the surface is affected by heat transfer from the workpiece. It is used by exposing the surrounding area to significant temperatures (for example, 180-1300°C).

Hereinafter, in the hot tool steel of the present invention, the reason for specifying the chemical composition, the reason for specifying the value A, and the reason for specifying the number of carbides will be explained. And, % in chemical components represents mass%.

C: 0.20% or more and 0.60% or less

C is an ingredient to ensure sufficient hardenability and to obtain high-temperature strength, hardness, and wear resistance by forming carbides and carbonitrides. If C is less than 0.20%, sufficient high temperature strength is not obtained. On the other hand, when C exceeds 0.60%, solidification segregation is promoted, coarse carbides and carbonitrides are formed, and toughness is reduced. In addition, since the formed carbides remain unsolidified during quenching, the amount of precipitated carbides and carbonitrides decreases when used at high temperatures as hot-worked tool steel, and improvement in high-temperature strength cannot be expected. Accordingly, C is set to 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 but less than 0.30%

Si is a component necessary to ensure the deoxidation effect and hardenability in steelmaking. If Si is less than 0.10%, sufficient effect is not achieved. On the other hand, if Si is 0.30% or more, toughness decreases. Accordingly, Si is set to 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 an ingredient necessary to ensure the deoxidation effect and hardenability in steelmaking. If Mn is less than 0.50%, the effect is not sufficient. If Mn is more than 2.00%, processability is reduced. Accordingly, Mn is set to 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 necessary ingredient to ensure hardenability and improve toughness. If Ni is less than 0.50%, sufficient effect is not achieved. If Ni is more than 2.50%, the cost becomes excessive. Accordingly, Ni is set to 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 necessary component to ensure sufficient hardenability. If Cr is less than 1.6%, sufficient hardenability cannot be obtained. On the other hand, if more than 2.6% of Cr is added, excessive M ₂₃ C ₆ carbides mainly composed of Cr and Fe are formed during quenching and tempering, and the high temperature strength, softening resistance, and toughness are reduced. Accordingly, Cr is set to 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% or more and 2.0% or less

Mo is a useful ingredient for obtaining precipitated carbides that contribute to hardenability, secondary hardening, and high temperature strength. If Mo is less than 0.3%, sufficient effect is not obtained. If Mo is more than 2.0%, not only is the effect saturated even if added in excess, but also coarse agglomeration of carbides causes a decrease in toughness. Additionally, costs increase. Accordingly, Mo is set to 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 is a component that precipitates fine, hard carbides and fine, hard carbonitrides during tempering or during high-temperature use as hot-worked tool steel, contributing to strength and wear resistance. If V is less than 0.05%, these effects are not sufficiently obtained. If V is more than 0.80%, coarse carbides and carbonitrides crystallize during solidification, impairing toughness. Accordingly, V is set to be 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 higher and 29.3 or lower

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

The value A is calculated based on the following formula (A).

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 calculating the value A, the value of the quenching temperature (°C) is substituted for [T] in equation (A), and the value of the quenching temperature holding time (h) is substituted for [t] in equation (A).

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, the solid solution of carbide by quenching of the steel in the component of the present invention becomes insufficient, and therefore the toughness and high temperature strength when used at high temperature as a hot tool are insufficient. On the other hand, when the value A exceeds 29.3, the toughness decreases due to coarsening of the prior austenite crystal grains. Therefore, the value A is set to 27.4 or more and 29.3 or less.

Number of carbides with a diameter of 1㎛ or more equivalent to a 10000㎛ ² sugar circle: 150 or less

In hot-worked tool steel before use, if there are too many carbides with an equivalent circle diameter of 1 μm or more, the amount of carbon in the matrix is insufficient, and carbides that precipitate during high-temperature use as hot-worked tool steel (e.g., MC-based carbides, M ₂ C-based carbides, etc.) and the amount of carbonitride decreases. Carbides (e.g., MC-based carbides, M ₂ C-based carbides, etc.) and carbonitrides contribute to improving high-temperature strength by precipitating during high-temperature use as hot-worked tool steel. Therefore, if they decrease, sufficient high-temperature strength may not be obtained. do. Additionally, if there is too much carbide with an equivalent circle diameter of 1 μm or more, stress is concentrated and acts as the origin and propagation path of cracks, thereby impairing toughness. Accordingly, in hot-worked tool steel before use, the number of carbides with an equivalent circle diameter of 1 µm or more per 10000 µm ² is set to 150 or less.

The number of carbides with a size of 1 μm or more in circle equivalent diameter per 10,000 μm ² is measured using steel materials after quenching and tempering, as described in the examples. The carbides to be measured include, for example, MC-based carbides, M ₂ C-based carbides, M ₃ C-based carbides, M ₇ C ₃ -based carbides, and M ₂₃ C ₆ -based carbides. And, M represents a metal element.

The number of carbides with an equivalent circle diameter of 1 µm or more per 10,000 µm ² was measured according to the method described in the Examples.

The hot tool steel before use has the following formulas (1) to (4):

([C] _max -[C] _min )/[C]≤1.0 … (One)

([Cr] _max -[Cr] _min )/[Cr]≤0.5 … (2)

([Mo] _max -[Mo] _min )/[Mo]≤1.5 … (3)

([V] _max -[V] _min )/[V]≤1.5 … (4)

It is desirable to satisfy.

In equations (1) to (4), [C] _max and [C] _min are the highest concentration (mass%) and lowest concentration (% by mass) of C determined by concentration mapping of hot-worked tool steel by the electronic probe microanalysis method, respectively. mass %), and [Cr] _max and [Cr] _min represent the highest concentration (mass %) and lowest concentration (mass %) of Cr determined by concentration mapping of hot-worked tool steel by the electronic probe microanalysis method, respectively. , [Mo] _max and [Mo] _min represent the highest concentration (% by mass) and lowest concentration (% by mass) of Mo determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively, and [V] _max and [V] _min , respectively, represent the highest concentration (mass %) and lowest concentration (mass %) of V determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, and [C] is the infrared absorption method. represents the content of C (mass %) determined by composition analysis of hot tool steel using X-ray fluorescence analysis, and [Cr], [Mo] and [V] are Cr, Mo and Indicates the content of V (mass%).

For alloy element X (X=C, Cr, _Mo , _V ), the value of ([ . In this specification _, the value of ( _[ When the non-uniformity of components due to internal segregation of each alloy element is large (i.e., when the value of ([X] _max -[X] _min )/[ It reduces toughness. Accordingly, it is desirable to regulate the width of component variation for C, Cr, Mo, and V. In other words, it is desirable that the hot tool steel before use satisfies all of equations (1) to (4).

The value of ([X] _max -[X] _min )/[X] is obtained using steel after quenching and tempering, as described in the examples. After mirror polishing the L-side of the steel (a surface parallel to the rolling direction and thickness direction of the steel sheet, the so-called longitudinal cross-section), using Electron Prove Micro Analysis (EPMA), 0.5 Concentration mapping of alloy element Let be [X] _max and [X] _min , respectively. Composition analysis of the steel material (analysis of the content of C) by the infrared absorption method and composition analysis of the steel material (analysis of the content of Cr, Mo, and V) by the fluorescent X-ray analysis method were performed, and the content of C determined by the infrared absorption method ( mass%) is set to [C], and the content rates (mass%) of Cr, Mo, and V determined by fluorescence X-ray analysis are set to [Cr], [Mo], and [V], respectively. Concentration mapping by electronic probe microanalysis, composition analysis by infrared absorption and X-ray fluorescence analysis were performed according to the methods described in the examples.

In the range of 1225°C to 1300°C, application of a soaking treatment that keeps the center of the steel ingot cracked for 10 to 40 hours makes it possible to effectively reduce the value of component variation.

[Example]

Hereinafter, the present invention will be described in more detail based on examples and comparative examples.

Invention Example No. 1 to 28 and Comparative Example No. 29 to 40 are steels composed of the chemical components shown in Table 1, the balance Fe, and inevitable impurities. 100 kg of each steel was melted in a vacuum induction melting furnace (VIM) and formed into ingots. Invention River No. For items 1 to 20, soaking treatment was performed under the above conditions (i.e., soaking treatment in which the center of the steel ingot was soaked for 10 to 40 hours in the range of 1225°C to 1300°C). After that, this ingot was heated to 1220°C and shortened into a 15 mm x 15 mm square lumber. The component composition of each bulk steel was confirmed by an infrared absorption method (analysis of the content of C) and a fluorescence X-ray analysis method (analysis of the content of alloy elements other than C). Infrared absorption and X-ray fluorescence analysis were performed as described later.

After that, the austenite structure was obtained by heating to 850 to 960°C and maintaining it for various times (30 minutes to 3 hours). Next, quenching with oil cooling was performed, and after further heating to 500 to 700°C, air cooling tempering was performed twice and tempering was performed to 39 to 41 HRC. Additionally, test materials were obtained through mechanical processing.

In Table 1, the remainder of each steel is Fe and inevitable impurities.

(Calculation of value A)

Equation (A):

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

[Wherein, [T] represents the quenching temperature (℃), and [t] represents the quenching temperature maintenance time (h)]

Based on this, the value A for each steel was calculated. The quenching temperature (℃), holding time (h), and value A of each steel are shown in Table 2.

(Measurement of the amount of carbide)

After mirror-polishing the center of each specimen by buffing, the matrix was corroded with a picric acid alcohol solution, and an area where a lot of light gray or white carbide was observed was selected at 30 views, and the circle equivalent was observed at 10,000 times with an electron microscope. The number of carbides with a diameter of 1 μm or more was measured by image analysis. The number of carbides with a circle equivalent diameter of 1 μm or more per 10,000 μm ² was set to 150 or less as “A,” and the number larger than this was set to “C.” The measurement results of the amount of carbide are shown in Table 2.

(Evaluation of the width of component variation)

For each specimen, the L-surface (a plane parallel to the rolling direction and thickness direction of the steel sheet, the so-called longitudinal cross-section) was mirror-polished and then used Electron Prove Micro Analysis (EPMA). , concentration mapping of alloy element X (X=C, Cr, Mo, V) in the range of 0.5 mm

EPMA was performed under the following conditions.

Analysis device: EPMA1600, manufactured by Shimadzu Manufacturing Co., Ltd.

Acceleration Voltage: 15kV

Beam diameter: 2㎛

Irradiation current: 0.1μA

Scan Mode: Stage Scan

Step size (area of one measurement): 2.5㎛×2.5㎛

Number of steps (number of measurement points): 200×200

Measurement time (1 step): 50 ms

Spectroscopic determination: 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 fluorescence X-ray analysis were performed.

Composition analysis by infrared absorption method was based on the "infrared absorption method" of JIS Z 2615:2015 "General rules for carbon determination of metallic materials" using a carbon sulfur analysis device EMIA-Expert manufactured by Horiba Works. And he did it.

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

The highest concentration (mass%) and _lowest concentration (mass _% ) of alloy element ], and the content rates (mass %) of Cr, Mo, and V determined by fluorescence X-ray analysis are [Cr], [Mo], and [V], respectively, ([X] _max -[X] _min )/[ The value of [X] was calculated as the width of component variation. The evaluation results of the width of component variation are shown in Tables 3A to 3D.

For all alloying elements of C, Cr, Mo and V, if the value of ([X] _max -[X] _min )/[X] satisfies the regulatory requirements, Those that were excellent were evaluated as “A”, and those that had a large fluctuation range even for one component and did not satisfy the regulatory requirements were evaluated as “C” as inferior. The evaluation results of the width of component variation are shown in Table 2.

The regulatory 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 specimen, it was further maintained at 600°C for 100 hours, cooled in air, and the HRC hardness at room temperature was measured, and the high-temperature strength was evaluated based on the decrease from the initial hardness. Those with a reduction value of 14 HRC or less were designated as “A”, and those with a reduction value greater than this were designated as “C”. The evaluation results of high temperature strength are shown in Table 2.

(Evaluation of personality)

From each test material, a U-notched test piece made of No. 3 10 mm Toughness was evaluated by performing a Charpy impact test in a laboratory. Those with an impact value of 70 J/cm ² or more were designated as “A”, those with an impact value of 85 J/cm ² or more were designated as “A ⁺ ”, and those with an impact value of less than 70 J/cm ² were designated as “C”. The results of the toughness evaluation are shown in Table 2.

[Table 1] Invention examples (No. 1 to 28) and comparative examples (No. 29 to 40)

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

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

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

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

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

Invention Example No. As shown in Tables 1 and 2, 1 to 28 are all within the specified range of chemical components and satisfy the value of formula (A), and the number of carbides with an equivalent circle diameter of 1 μm or more is also small. Accordingly, the impact value in the Charpy impact test is more than 70J/cm ² , the toughness is rated A, the reduction from initial hardness is within 14HRC, and the high temperature strength (softening resistance) is also rated A, and has excellent high temperature strength and A hot-worked tool steel with toughness was obtained.

Comparative Example No. 29 had a low high-temperature strength because the amount of C was small.

Comparative Example No. In 30, the amount of C is large, the number of carbides with an equivalent circle diameter of 1 μm or more is large, and the range of compositional variation is large, so the toughness and high temperature strength are low.

Comparative Example No. 31 had a large amount of Si and low toughness.

Comparative Example No. 32 had a small amount of Ni and low toughness.

Comparative Example No. In No. 33, the amount of Cr was large, the number of carbides with an equivalent circle diameter of 1 μm or more was large, and the range of compositional variation was large, so the toughness and high-temperature strength were low.

Comparative Example No. 34 had a small amount of Mo and low high temperature strength.

Comparative Example No. In 35, the amount of Mo was large, the number of carbides with an equivalent circle diameter of 1 μm or more was large, and the range of compositional variation was large, so the toughness and high temperature strength were low.

Comparative Example No. 36 had a small V amount and low high-temperature strength.

Comparative Example No. In No. 37, the V amount was large, the number of carbides with an equivalent circle diameter of 1 μm or more was large, and the range of component variation was large, so the toughness was low.

Comparative Example No. In 38, the value of formula (A) is small and the number of carbides with an equivalent circle diameter of 1 μm or more is large, so the toughness and high temperature strength are low.

Comparative Example No. 39, the value of formula (A) was large and the toughness was low.

Comparative Example No. 40 had a large number of carbides with an equivalent circle diameter of 1 μm or more, and the toughness and high-temperature strength were low.

Claims (2)

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
Residue: Fe and inevitable impurities
As a hot tool steel made of,
The hot tool steel, formula (A) below:
Value A=([T]+273)(log ₁₀ [t]+24)/1000... (A)
[Wherein, [T] represents the quenching temperature (℃), and [t] represents the quenching temperature maintenance time (h)]
It is quenched and tempered so that the value A calculated based on is 27.4 or more and 29.3 or less,
Hot-worked tool steel in which the number of carbides with a circle equivalent diameter of 1 μm or more per 10,000 μm ² in the hot-worked tool steel before use is 150 or less. According to paragraph 1,
Hot tool steel, following formulas (1) to (4):
([C] _max -[C] _min )/[C]≤1.0 … (One)
([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 above formula, [C] _max and [C] _min represent the highest and lowest concentrations of C determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively, and [Cr] _max and [Cr] _min represents the highest and lowest concentrations of Cr determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively, and [Mo] _max and [Mo] _min represent the highest and lowest concentrations of Cr determined by concentration mapping of hot tool steel by the electronic probe microanalysis method, respectively. Represents the highest and lowest concentrations of Mo determined by concentration mapping of tool steel, and [V] _max and [V] _min are, respectively, the highest and lowest concentrations of V determined by concentration mapping of hot tool steel by the electronic probe microanalysis method. , [C] represents the content of C determined by composition analysis of the hot tool steel by infrared absorption method, and [Cr], [Mo], and [V] represent the content of C determined by composition analysis of the hot tool steel by the infrared absorption method, respectively, Indicates the content of Cr, Mo and V determined by composition analysis]
Hot-worked tool steel that satisfies the .