JP7479787B2 - Hot work tool steel with excellent thermal conductivity - Google Patents

Description

This invention relates to die steel, and in particular to die steel used in high-temperature environments such as die casting and hot stamping.

In recent years, in the field of die casting, the mechanical and thermal load on die casting dies has increased due to the strengthening of aluminum parts to reduce the weight of automobiles and the shortening of part forming processing pitches to improve productivity. As a result, dies are prone to problems such as wear, large cracks, and heat checking. To address these issues, die materials with excellent hardness and toughness are required. In addition, in hot stamping, die wear due to scale that occurs on the surface of the steel plate, the material being processed, has become an issue, so die materials are required to have high hardness.

Furthermore, die casting dies and hot stamping dies have cooling circuits inside, and the cooling efficiency of the cooling water flowing through the cooling circuit has a significant impact on the production cycle speed. One way to increase cooling efficiency is to increase the thermal conductivity of the die. Therefore, in order to meet the demand for faster production cycle speeds with the aim of improving productivity as mentioned above, a material with high thermal conductivity is required as a property.

In light of this background, a high-toughness, high-strength hot die steel has been proposed as a conventional technique, which contains, by mass%, C: 0.30 to 0.50%, Si: 0.10 to 0.50%, Mn: 0.10 to 1.00%, Cr: 4.00 to 6.00%, Mo: 1.40 to 2.60%, V: 0.20 to 0.80%, Ti: 0.0030% or less, N: 0.0120% or less, the balance being Fe and unavoidable impurities, and further, Mo and Cr satisfy the relational expression 0.33×[%Cr]−0.37<[Mo%]<4.45−0.44×[%Cr] (see, for example, Patent Document 1).
However, although this hot work die steel takes into consideration softening resistance at high temperatures from a state in which it is tempered to 44 to 46 HRC, there is no mention of the size of the carbides present after quenching and tempering, and this is insufficient for obtaining steel having high thermal conductivity of 25.0 W/m K or more at room temperature.
In addition, the proposed hot work die steel does not have a high hardness exceeding 46.0 HRC, and when used as a material for hot stamping dies, wear occurs in the dies, and sufficient die life cannot be obtained.

As another prior art, a die steel has been proposed that aims to increase the thermal conductivity of the die, enable rapid cooling of the die, enable high cycle production of products, improve product quality and reduce the defect rate even under high cycle production, and further reduce the thermal stress of the die, extend the die life, and obtain the strength, toughness, and high temperature properties required for a die while keeping the amount of rare additives low. The die steel has a composition of 0.35<C≦0.50, 0.01≦Si<0.19, 1.50<Mn<1.78, 2.00<Cr<3.05, 0.51<Mo<1.25, 0.30<V<0.80, 0.004≦N≦0.040, balance Fe and unavoidable impurities (see, for example, Patent Document 2). However, since this die steel has a high Mn content, the thermal conductivity is easily reduced by the excessive addition of Mn. In addition, the Cr content is low, making it difficult to obtain toughness.

In addition, as a mold steel aiming for good properties in terms of machinability, thermal conductivity, and hardness, it contains, by mass, C: over 0.45 to 0.60%, Si: 0.05 to 0.20%, Mn: 0.3 to 0.7%, Ni: 0.2 to 0.5%, Cr: 0.2 to 0.5%, V: 0.03 to 0.1%, Al: 0.01 to 0.03%, S: less than 0.010%, O: 0.0030% or less, N: 0.02% or less, and the balance being Fe and unavoidable impurities. A steel for plastic molding dies with excellent thermal conductivity has been proposed, in which die steel has a composition regulated to P: 0.015% or less, Cu: 0.30% or less, and Mo: 0.20% or less among unavoidable impurities, and is heated at 900 to 1050°C, normalized with air cooling to form a two-phase structure of ferrite and pearlite, and then heated at 500 to 650°C and tempered with furnace cooling to a hardness of 180 to 220 HV (see, for example, Patent Document 3). However, this die steel has a low V content, so the quenching and tempering hardness is insufficient.

In terms of mass%, 0.35<C<0.55 mass%, 0.003≦Si<0.300 mass%, 0.30<Mn<1.50 mass%, 2.00≦Cr<3.50 mass%, 0.003≦Cu<1.200 mass%, 0.003≦Ni<1.380 mass%, 0.50<Mo<3.29 mass%, 0.55<V<1.13 mass%, 0.0002≦N<0.1200 mass% A die steel has been proposed that contains 0.55 < Cu + Ni + Mo < 3.29 mass%, with the remainder being Fe and unavoidable impurities, has a hardness of more than 33 HRC to 57 HRC, has a prior austenite grain size number of 5 or more at the time of quenching, and has a thermal conductivity λ at 25°C measured using a laser flash method of more than 27.0 [W/m·K] (see, for example, Patent Document 4). However, this die steel lacks toughness because the Cr content is insufficient.

In addition, a die steel has been proposed that has the following composition in mass %: 0.15<C<0.43, 0.20<Si<0.52, 5.32<Cr<5.72, -0.05814 x [Cr] + 0.4326 < Mn < -0.2907 x [Cr] + 2.4628... (1) (where [Cr] in formula (1) represents the % Cr content), 0.72<Mo<1.60, 0.20<V<0.61, with the remainder being Fe and unavoidable impurities (see, for example, Patent Document 5). However, this die steel contains an excessive amount of Cr, which tends to reduce the thermal conductivity. Also, the examples in this document often contain small amounts of C and Mo, resulting in insufficient quench-tempered hardness.

Furthermore, a mold steel has been proposed that has the following composition, in mass %, 0.15<C<0.43, 0.20<Si<0.52, 4.00<Cr<5.72, -0.05814 x [Cr] + 0.4326 < Mn < -0.2907 x [Cr] + 2.4628... formula (1) (where [Cr] in formula (1) represents the Cr content in percentage), 0.72<Mo<1.60, 0.20<V<0.61, with the balance being Fe and unavoidable impurities (see, for example, Patent Document 6). However, the example of this mold steel has a low C content and is insufficient in quench-and-temper hardness.

The steel also contains, by mass%, C: 0.30-0.50%, Si: 0.10-0.50%, Mn: 0.10-1.0%, Cr: 4.5-5.4%, Mo: 1.4-2.4%, W: 1.0% or less, Mo+W/2: 1.7-2.4%, V: 0.30-0.70%, with the balance being Fe and unavoidable impurities. The types and ratios of residual carbides observed after quenching and tempering are such that the ratio of carbides consisting of _{M6C, M2C, and MC to the total carbides consisting of M26C6 , M6C , M2C} , and MC is 5.0 or more, the total carbide area ratio is 2% or less per 0.01 ^mm2 cross-sectional area, and the Charpy impact value is 30 _J /cm A hot work die steel having excellent high-temperature strength and toughness, in which the hardness (H) is ^0.2 or more and the softening amount (ΔHRC) is 13HRC or less, has been proposed (see, for example, Patent Document 7).
However, although this hot work die steel has been examined for its softening resistance after tempering to 44-45 HRC, there is no mention of the size of the carbides present after quenching and tempering, and it is insufficient for obtaining a steel having a high thermal conductivity of 25.0 W/m·K or more at room temperature and a high hardness of more than 46.0 HRC after quenching and tempering. When used as a material for hot stamping dies, wear occurs in the dies, and a sufficient die life cannot be obtained.

JP 2013-87322 A JP 2011-94168 A JP 2010-13716 A JP 2017-53023 A JP 2015-221933 A JP 2015-224363 A JP 2017-155306 A

In die steel, carbides such as _{M23C6 , M6C} , _M2C , and MC are precipitated by quenching and tempering. The inventors have conducted a detailed study on the effect of the state of these carbides on thermal conductivity, and have found that increasing the size of the precipitated carbides is effective in increasing the thermal conductivity after quenching and tempering.
On the other hand, carbides are also a factor that greatly affects hardness, and it has been discovered that an increase in the size of carbides actually leads to a decrease in hardness.
Based on these conflicting findings, the inventors have discovered that by specifying the range of the chemical components of the hot work tool steel and the range of the sizes of the carbides after quenching and tempering of this hot work tool steel, it is possible to obtain a hot work tool steel that combines high thermal conductivity, high hardness and high toughness.

The problem that the present invention aims to solve is to provide a hot work tool steel, which is a die steel that combines high thermal conductivity, high hardness and high toughness and can be used in die casting, hot stamping, etc.

The first means of the present invention to solve the above problems is a hot work tool steel that contains, by mass%, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and is characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm, and has high thermal conductivity, high hardness, and high toughness.

The second means is a hot work tool steel that contains, in mass %, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, Ni: 0.01-2.00%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and that has high thermal conductivity, high hardness, and high toughness, characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm.

The third means is a hot work tool steel that contains, in mass %, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, N: 0.001-0.040%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and that has high thermal conductivity, high hardness, and high toughness, characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm.

The fourth means is a hot work tool steel that contains, in mass %, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, Ni: 0.01-2.00%, N: 0.001-0.040%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and that has high thermal conductivity, high hardness, and high toughness, characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm.

The fifth means is a hot work tool steel that contains, in mass%, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, Al: 0.001-0.080%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and that has high thermal conductivity, high hardness, and high toughness, characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm.

The sixth means is a hot work tool steel that contains, in mass%, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, Ni: 0.01-2.00%, Al: 0.001-0.080%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and that has high thermal conductivity, high hardness, and high toughness, characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm.

The seventh means is a hot work tool steel that contains, in mass%, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, N: 0.001-0.040%, Al: 0.001-0.080%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and that has high thermal conductivity, high hardness, and high toughness, characterized in that the average equivalent circle diameter of the carbides present in the steel after quenching and tempering is 150-400 nm.

The eighth means is a hot work tool steel that contains, in mass%, C: 0.34-0.55%, Si: 0.01-0.50%, Mn: 0.01-1.50%, Cr: 3.50-5.32%, V: over 0.10-1.00%, Ni: 0.01-2.00%, N: 0.001-0.040%, Al: 0.001-0.080%, and one or two of Mo: 4.00% or less or W: 8.00% or less, and satisfies (Mo+W): 1.80% or more and (Mo+W/2): 4.00% or less, with the balance being Fe and unavoidable impurities, and is characterized in that the average equivalent circle diameter of the carbides in the steel after quenching and tempering is 150-400 nm. The hot work tool steel has high thermal conductivity, high hardness, and high toughness.

The hot work tool steel according to the above-mentioned means of the present invention is a hot work tool steel with high thermal conductivity, high hardness and high toughness, such that the size of the carbides after quenching and tempering is an average equivalent circle diameter of 150 to 400 mm, the thermal conductivity at room temperature after quenching and tempering is high at 25.0 W/m·K or more, the hardness after quenching and tempering is high at over 46.0 HRC, and further the Charpy impact value after quenching and tempering is high at 30 J/ ^cm2 or more. Since it has high hardness, high toughness and high thermal conductivity, it exhibits excellent properties as a die steel used in high temperature environments such as die casting and hot stamping.

Before describing the embodiments of the present invention, we will explain the reasons for limiting the chemical components, etc., in the present invention. Note that % in the description is % by mass.

C: 0.34 to 0.55%
C is an element that strengthens the matrix by dissolving in steel and promotes precipitation strengthening by forming carbides. If C is less than 0.34%, sufficient quench-temper hardness cannot be obtained. On the other hand, if C is more than 0.55%, it promotes segregation and reduces toughness. Therefore, C is set to 0.34-0.55%, and preferably 0.35-0.50%.

Si: 0.01 to 0.50%
Silicon is an element necessary as a deoxidizer during steelmaking. Silicon is an element that improves hardness by dissolving in the matrix, but if the amount of silicon is less than 0.01%, the deoxidization is not sufficient. On the other hand, if the amount of silicon is more than 0.50%, silicon does not form silicon carbides but dissolves in the matrix, greatly reducing the thermal conductivity. Therefore, silicon is set to 0.01 to 0.50%, and preferably 0.05 to 0.40%.

Mn: 0.01 to 1.50%
Mn is an element necessary as a deoxidizer during steelmaking. If the Mn content is less than 0.01%, the steel will not be sufficiently deoxidized. On the other hand, if the Mn content is more than 1.50%, the Mn content will dissolve in the matrix and reduce the thermal conductivity of the steel. Therefore, the Mn content is set to 0.01 to 1.50%, and preferably 0.10 to 1.20%.

Cr: 3.50 to 5.32%
Cr is an element necessary for improving the hardenability of steel and suppressing the decrease in toughness due to the formation of bainite. If the Cr content is less than 3.50%, sufficient toughness cannot be obtained. On the other hand, if the Cr content is more than 5.32%, it dissolves in the matrix and decreases the thermal conductivity of the steel. Therefore, the Cr content is set to 3.50 to 5.32%, and preferably 4.00 to 5.25%.

V: more than 0.10 to 1.00%
V is an element that promotes secondary hardening during tempering of steel and increases the quenching and tempering hardness. If V is 0.10 or less, sufficient quenching and tempering hardness cannot be obtained. On the other hand, if V is more than 1.00%, the amount of V remaining in the matrix increases, lowering the thermal conductivity. Therefore, V is set to more than 0.10 to 1.00%, and preferably 0.25 to 0.85%.

(Selective Essential Ingredients)
Mo and W: one or two are contained, Mo: 4.00% or less, W: 8.00% or less, (Mo+W): 1.80% or more, (Mo+W/2): 4.00% or less Mo and W are elements that promote secondary hardening during tempering of steel and increase quenching and tempering hardness. Therefore, in the present invention, Mo and W are considered as optional essential components, and either one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and further, the total content of both elements is (Mo+W): 1.80% or more, and further, (Mo+W/2) is 4.00% or less.
If (Mo+W) is less than 1.80%, sufficient quenching and tempering hardness cannot be obtained. Therefore, (Mo+W) is set to 1.80% or more, preferably 1.90% or more. On the other hand, Mo: 4.00% or less, W: 8.00% or less, if there is more Mo or W than this, the amount of Mo or W remaining in the matrix increases, lowering the thermal conductivity. Also, if either element is added in excess, the amount of Mo or W remaining in the matrix increases, lowering the thermal conductivity, so (Mo+W/2) for both elements as a whole is set to 4.00% or less, preferably 3.50% or less.

(Additional Ingredients)
Ni: 0.01 to 2.00%
Although it is not essential to add Ni, like C, it is an effective element for increasing the quenching and tempering hardness of steel, and is added as necessary. On the other hand, excessive addition of Ni more than 2.00% leads to an increase in refining time and cost. Therefore, Ni is set to 0.01 to 2.00%, and preferably 0.01 to 1.50%.

N: 0.001 to 0.040%
Although it is not essential to add N, like C, it is an effective element for increasing the quenching and tempering hardness of steel, and is added as necessary. On the other hand, excessive addition of N more than 0.040% leads to an increase in refining time and cost. Therefore, N is set to 0.001-0.040%, and preferably 0.001-0.030%.

Al: 0.001 to 0.080%
Although it is not essential to add Al, it is an element that forms Al nitrides to suppress the coarsening of crystal grains during quenching, and can be added as necessary. On the other hand, the formation of excess Al nitrides reduces toughness. Therefore, the Al content is set to 0.001 to 0.080%, and preferably 0.005 to 0.060%.

(Regarding the size of carbides)
Average circle equivalent diameter of carbides present after quenching and tempering of steel: 150 to 400 nm
In die steel, carbides are precipitated by quenching and tempering, and the larger the size of the precipitated carbides, the higher the thermal conductivity. Therefore, in order to obtain sufficient thermal conductivity, the average equivalent circle diameter of the carbides needs to be 150 nm or more.
Carbide is also a factor that contributes to the amount of precipitation strengthening, and the more carbide precipitates, the more difficult it becomes for dislocations to move because the precipitate particles become obstacles, but if the carbides become coarse, it leads to a decrease in hardness. Therefore, the average equivalent circle diameter of carbides present after quenching and tempering is set to 150 to 400 nm, preferably 160 to 390 mm.

Next, embodiments of the invention will be described below.
Steels consisting of the chemical components of the invention steels No. 1 to 31 in Table 1 and the comparative steels No. 32 to 47 in Table 2, with the balance being Fe and unavoidable impurities, were melted in a vacuum melting furnace, and 100 kg of steel ingots were obtained. From each of these steel ingots, a block with a width of 65 mm and a height of 30 mm was hot forged and stretched. Next, the forged and stretched material was held at 870°C for 2 hours, slowly cooled and annealed, and then samples were taken from the intermediate position between the surface and the center. Each of these samples was quenched at 1030°C, oil-cooled, and tempered twice at 550 to 680°C, and then air-cooled. The average circle equivalent diameter of carbides in the forged and stretched steel of each of these samples was measured, and the results are shown in the invention steels No. 1 to 31 in Table 1 and the comparative steels No. 32 to 47 in Table 2.

Thermal conductivity, quenched and tempered hardness, and Charpy impact value, which are the steel properties of the forged and stretched materials of these samples, were also measured, and the results are shown in Table 3 for the inventive steels No. 1 to 31, and in Table 4 for the comparative steels No. 32 to 47.

The test pieces for observing the carbides in the steel in the above measurement of the average circle equivalent diameter of the carbides were prepared by polishing the surface parallel to the forging direction of the quenched and tempered sample and using the extraction replica method. A region of ¹⁰⁰ μm2 in total area of the test pieces was observed using bright field images of a transmission electron microscope, and the diameter of a perfect circle equivalent to the area of each carbide was calculated by image analysis. The average value was recorded as the circle equivalent diameter of the carbides in Tables 1 and 2.

The laser flash method was used to measure the thermal conductivity, a characteristic of the steel of the above forged and stretched material. After quenching and tempering, the sample was finished into a cylindrical shape of 10 mm diameter x 1 mm for testing, and a pulsed laser was irradiated onto one side of the disk to obtain the thermal diffusivity in the thickness direction. This was then multiplied by the specific heat at constant pressure and the density to obtain the thermal conductivity at room temperature after quenching and tempering, which was then used as the thermal conductivity in Tables 3 and 4. A thermal conductivity of 25.0 W/m·K or more at room temperature after quenching and tempering is called high thermal conductivity.

The quenched and tempered hardness, which is a characteristic of the steel of the above forged and stretched material, was measured with a Rockwell hardness tester. In this case, the Rockwell hardness was obtained by measuring the surface perpendicular to the forging direction of the sample in the quenched and tempered state, and was taken as the quenched and tempered hardness in Tables 3 and 4. A hardness of more than 46.0 HRC after quenching and tempering is evaluated as high hardness.

The toughness, which is a characteristic of the steel of the above forged and stretched material, was evaluated by a Charpy impact test. In this case, the specimen was made from a sample after quenching and tempering. The test piece shape was a 2 mm U-notch Charpy test piece, and the notch direction was measured in a direction perpendicular to the forging direction, and the Charpy impact values in Tables 3 and 4 were obtained. A Charpy impact value of 30 J/ ^cm2 or more after quenching and tempering is evaluated as having high toughness.

Comparative steel No. 32 in Table 2 has a C content of 0.33%, which is lower than the range of the invention, and therefore has a quenched and tempered hardness of 42.2 HRC in Table 4, which is lower than the hardness of over 46.0 HRC obtained in the present invention.

Comparative steel No. 33 in Table 2 has a C content of 0.56%, which is higher than the range of the invention, and therefore has a Charpy impact value in Table 4 of 20.1 J/ ^cm2 , which is lower in toughness than the 30 J/ ^cm2 or more obtained in the present invention.

Comparative steel No. 34 in Table 2 has a Si content of 0.51%, which is lower than the range of the invention, and therefore has a thermal conductivity of 23.8 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 35 in Table 2 has a Mn content of 1.51%, which is higher than the range of the invention, and therefore has a thermal conductivity of 22.3 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 36 in Table 2 has a Cr content of 3.49%, which is lower than the range of the invention, and therefore has a Charpy impact value in Table 4 of 28.9 J/ ^cm2 , which is lower in toughness than the 30 J/ ^cm2 or more obtained in the present invention.

Comparative steel No. 37 in Table 2 has a Cr content of 5.33%, which is higher than the range of the invention, and therefore has a thermal conductivity of 24.8 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 38 in Table 2 has a Mo content of 4.01%, which is higher than the range of the invention, and (Mo+W/2) is also high at 4.01%. Therefore, the thermal conductivity in Table 4 is 24.4 W/m·K, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 39 in Table 2 has a W content of 8.02%, which is higher than the range of the invention, and (Mo+W/2) is also high at 4.01%. Therefore, the thermal conductivity in Table 4 is 23.7 W/m·K, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 40 in Table 2 has a Mo+W value of 1.79%, which is lower than the range of the invention, and therefore has a quenched and tempered hardness of 43.3 HRC in Table 4, which is lower than the 46.0 HRC obtained in the present invention.

Comparative steel No. 41 in Table 2 has a Mo+W/2 value of 4.02%, which is higher than the range of the invention, and therefore has a thermal conductivity of 22.9 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 42 in Table 2 has a V content of 0.10%, which is lower than the range of the invention, and therefore has a quenched and tempered hardness of 43.1 HRC in Table 4, which is lower than the hardness of over 46.0 HRC obtained in the present invention.

Comparative steel No. 43 in Table 2 has a V content of 1.02%, which is higher than the range of the invention, and therefore has a thermal conductivity of 23.2 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 44 in Table 2 has a Ni content of 2.02%, which is higher than the range of the invention, and therefore has a thermal conductivity of 24.3 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 45 in Table 2 has an Al content of 0.094%, which is higher than the range of the invention, and therefore has a Charpy impact value in Table 4 of 16.2 J/ ^cm2 , which is lower in toughness than the 30 J/ ^cm2 obtained in the present invention.

Comparative steel No. 46 in Table 2 has a carbide equivalent circle diameter of 123 nm, which is lower than the range of the invention, and therefore has a thermal conductivity of 24.6 W/m·K in Table 4, which is lower than the 25.0 W/m·K obtained in the present invention.

Comparative steel No. 47 in Table 2 has a carbide circle equivalent diameter of 430 nm, which is higher than the range of the invention, and therefore has a quenched and tempered hardness of 45.4 HRC in Table 4, which is lower than the hardness of over 46.0 HRC obtained in the present invention.

Claims (8)

In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, Ni: 0.01 to 2.00%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, N: 0.001 to 0.040%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, Ni: 0.01 to 2.00%, N: 0.001 to 0.040%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, Al: 0.001 to 0.080%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, Ni: 0.01 to 2.00%, Al: 0.001 to 0.080%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, N: 0.001 to 0.040%, Al: 0.001 to 0.080%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm. In mass%, C: 0.34 to 0.55%, Si: 0.01 to 0.50%, Mn: 0.01 to 1.50%, Cr: 3.50 to 5.32%, V: over 0.10 to 1.00%, Ni: 0.01 to 2.00%, N: 0.001 to 0.040%, Al: 0.001 to 0.080%,
Further, one or both of Mo: 4.00% or less and W: 8.00% or less are contained, and (Mo + W): 1.80% or more and (Mo + W / 2): 4.00% or less are satisfied;
A hot work tool steel consisting of the remainder Fe and unavoidable impurities is in a quenched and tempered state,
A hot work tool steel having high thermal conductivity, high hardness and high toughness, characterized in that the average equivalent circle diameter of carbides present in the steel after quenching and tempering is 150 to 400 nm.