Hot work tool steel with excellent thermal conductivity

Abstract

To provide a hot work tool steel that has all of high thermal conductivity, high hardness, high softening resistance, and high toughness, and is a die steel applicable to die casting, hot stamping or the like.SOLUTION: A hot work tool steel has, in mass%, C: more than 0.35 to 0.70%, Si: 0.01 to 1.20%, Mn: 0.01 to 1.50%, Cr: 0.40 to 4.00%, Cu: 0.05 to 3.50%, one or two of Mo and W (Mo: 3.00% or less and W: 6.00% or less, and Mo+5 / 6 W: 1.50% or more and Mo+1 / 2 W: 3.00% or less), V: more than 0.10 and to 0.55%, with the balance being: Fe and inevitable impurities.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to hot work tool steel having excellent thermal conductivity. In particular, among hot work tool steels, it relates to die steel for use in dies used in high-temperature environments such as die casting and hot stamping.

Background Art

[0002] In recent years, in the die casting field, due to the strengthening of aluminum parts for the purpose of weight reduction of automobiles and the shortening of the processing pitch of part forming for the purpose of productivity improvement, the mechanical and thermal loads on die casting dies have been increasing. As a result, problems such as wear, large cracks, and heat checks are likely to occur in the dies. In order to address these problems, die materials are required to have excellent hardness and toughness. In hot stamping, wear of the die due to scale generated on the surface of the steel sheet, which is the material to be processed, is a problem, and die materials are required to have high hardness and softening resistance.

[0003] Furthermore, dies for die casting and hot stamping have cooling circuits formed inside, and the cooling efficiency by the cooling water flowing through here greatly affects the production cycle speed. As a method of enhancing the cooling efficiency, there is an increase in the thermal conductivity of the die. Therefore, in order to meet the requirements for improving the production cycle speed for the purpose of improving productivity described above, high thermal conductivity is required as a characteristic of the material.

[0004] Prior arts attempting to improve the above requirements have been proposed in Patent Documents 1 to 8. First, Patent Document 1 proposes die casting die steel that does not contain Cu. However, since the unmentioned Cu remains within the range of inevitable impurities, bainite, which is an incomplete quenched structure, is likely to form, and there is a problem of insufficient toughness.

[0005] In addition, die steels containing more than 1.50% Mn have been proposed to ensure hardenability (see, for example, Patent Document 2). However, when Mn is added in excess in this way, there is a problem that the thermal conductivity decreases.

[0006] In addition, die steels containing 0.1% or less of V have been proposed to ensure machinability (see, for example, Patent Document 3). However, when the amount of V is this small, there is a problem that the hardening and tempering hardness is insufficient.

[0007] In addition, a molding tool using a die steel containing more than 0.55% of V has been proposed to suppress the coarsening of γ crystal grains during quenching (see, for example, Patent Document 4). However, when V is added in excess in this way, there is a problem that the thermal conductivity decreases.

[0008] In addition, die steels containing 4.00% or more of Cr have been proposed to ensure corrosion resistance (see, for example, Patent Document 5). However, excessive addition of Cr causes a problem that the thermal conductivity decreases.

[0009] A pre-hardened steel for die-casting dies containing 0.35% or less of C has been proposed to suppress welding cracks (see, for example, Patent Document 6). However, when the amount of C is this small, there is a problem that the hardening and tempering hardness is insufficient.

[0010] In addition, hot-work tool steels with Mo + 1 / 2·W greater than 3.0% have been proposed (see, for example, Patent Document 7). However, excessive addition of Mo or W causes a problem that the thermal conductivity decreases.

[0011] In addition, hot-work tool steels containing 3% or more of Ni have been proposed (see, for example, Patent Document 8). The steels described in the claims and examples of this document contain more than 3% of Ni. However, when the content is 3% or more, there is a problem that the thermal conductivity decreases due to excessive addition of Ni.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0013] The problem to be solved by the present invention is to provide a hot - working tool steel that has high thermal conductivity, high hardness, excellent softening resistance, and high toughness and is applicable to die - casting, hot stamping, etc. Hot - working tool steel is tool steel used as a tool for processing workpieces in the hot - working temperature range.

Means for Solving the Problems

[0014] As a result of intensive development by the inventor, by specifying the alloy components within a specific range and further specifying the steel structure and carbide state, it has been found that a hot - working tool steel having high thermal conductivity, high hardness, excellent softening resistance, and high toughness can be obtained.

[0015] The first means for solving the problem is, by mass%, C: more than 0.35 to 0.70%, Si: 0.01 to 1.20%, Mn: 0.01 to 1.50%, Cr: 0.40 to 4.00%, Cu: 0.05 to 3.50%, One or two of Mo and W, where Mo: 3.00% or less, W: 6.00% or less, and Mo + 5 / 6·W: 1.50% or more, Mo + 1 / 2·W: 3.00% or less, V: more than 0.10 to 0.55%, and the balance consists of Fe and inevitable impurities, and is a hot work tool steel characterized by this.

[0016] Its second means is a hot work tool steel characterized in that, in addition to the chemical components of the first means, it has Ni: 0.01 to 2.99% by mass, and the balance consists of Fe and inevitable impurities.

[0017] Its third means is a hot work tool steel characterized in that, in addition to the chemical components of either the first means or the second means, it has N: 0.001 to 0.040% by mass, and the balance consists of Fe and inevitable impurities.

[0018] Its fourth means is a hot work tool steel characterized in that, in addition to the chemical components of any one of the first to third means, it has Al: 0.001 to 0.300% by mass, and the balance consists of Fe and inevitable impurities.

[0019] Its fifth means is a hot work tool steel characterized in that, in addition to the chemical components of any one of the first to fourth means, the balance consists of Fe and inevitable impurities, and it has a martensite single-phase structure.

[0020] Its sixth means is a hot work tool steel characterized in that, in addition to the chemical components of any one of the first to fourth means, the balance consists of Fe and inevitable impurities, and the types of carbides are M 3 C, M 7 C 3 、M 2 C, MC, and it consists of any 1 to 4 of these.

[0021] The seventh means is the hot work tool steel described in the fifth means, and further the type of carbide is M 3 C, M 7 C 3 、M 2 C, or any one to four of MC, and is a hot work tool steel characterized by this.

Advantages of the Invention

[0022] Now, the hot work tool steel of each means of the present invention described above, in the quenched and tempered state, has a high thermal conductivity of 25.0 W / m·K or more at room temperature, a high hardness of 48.0 HRC or more at room temperature, and after being held at 600 °C for 100 hours after quenching and tempering, shows a high softening resistance with a hardness of 32.0 HRC or more at room temperature, and a Charpy impact value of 20 J / cm at room temperature 2 or more, resulting in high toughness. Therefore, when using the hot work tool steel of each means of the present invention, it can be made to have high thermal conductivity, high hardness, high softening resistance, and high toughness, which are suitable for die steel.

[0023] In the present invention, High thermal conductivity means that the thermal conductivity at room temperature after quenching and tempering is 25.0 W / m·K or more, High hardness means that the hardness at room temperature after quenching and tempering is 48.0 HRC or more, High toughness means that the Charpy impact value at room temperature after quenching and tempering is 20 J / cm 2 or more, High softening resistance means that the hardness at room temperature after being held at 600 °C for 100 h after quenching and tempering is 32.0 HRC or more.

Embodiments for Carrying Out the Invention

[0024] First, the reasons for defining the chemical components in the hot work tool steel of the means for solving the problems of the present invention, and the reasons for defining the structure and the type of carbide of this hot work tool steel will be explained below.

[0025] C: More than 0.35 to 0.70% or less C is an element that strengthens the matrix by solid solution and promotes precipitation strengthening by forming carbides. By the way, when C is as low as 0.35% or less, sufficient hardening and tempering hardness cannot be obtained. On the other hand, when C exceeds 0.70% and becomes excessive, it promotes segregation and reduces toughness. Therefore, C is set to more than 0.35% and less than or equal to 0.70%, and preferably, C is more than 0.55% and less than or equal to 0.70%.

[0026] Si: 0.01 - 1.20% Si is an element necessary as a deoxidizer during steelmaking. By the way, when Si is less than 0.01%, it cannot be sufficiently deoxidized during steelmaking. On the other hand, when Si is more than 1.20%, it dissolves in the matrix without forming carbides and reduces the thermal conductivity. Therefore, Si is set to 0.01% or more and 1.20% or less. In addition, when Si is 0.05% or more, there is an effect of improving the hardness by dissolving in the matrix. Therefore, preferably, Si is 0.05% or more and 1.00% or less.

[0027] Mn: 0.01 - 1.50% Mn is an element necessary as a deoxidizer during steelmaking. By the way, when Mn is less than 0.01%, it cannot be sufficiently deoxidized. On the other hand, when Mn is more than 1.50%, it dissolves in the matrix and reduces the thermal conductivity. Therefore, Mn is set to 0.01% or more and 1.50% or less, preferably Mn is 0.05% or more and less than 0.92%, and more preferably Mn is 0.10% or more and less than 0.50%.

[0028] Cr: 0.40 - 4.00% Cr is an element necessary to improve hardenability and suppress the decrease in toughness due to bainite formation. By the way, when Cr is less than 0.40%, sufficient toughness cannot be obtained. On the other hand, when Cr is excessive over 4.00%, it dissolves in the matrix and reduces the thermal conductivity. Also, when Cr is too much, M that is likely to coarsen at high temperature during tempering 6 C carbide and M 23 C 6Carbides precipitate and the softening resistance decreases. Therefore, Cr is set to 0.40% or more and 4.00% or less, preferably Cr is 0.45% or more and less than 2.60%, and more preferably Cr is 0.50% or more and less than 2.10%.

[0029] Cu: 0.05 - 3.50% Cu is an element necessary to improve hardenability and suppress the decrease in toughness due to bainite formation. By the way, if Cu is less than 0.05%, sufficient toughness cannot be obtained. On the other hand, if Cu is more than 3.50%, it dissolves in the matrix and reduces the thermal conductivity. Therefore, Cu is set to 0.05% or more and 3.50% or less, preferably Cu is 0.10% or more and 3.00% or less.

[0030] Contains either one or both of Mo and W Mo: 3.00% or less W: 6.00% or less Mo + 5 / 6·W: 1.50% or more Mo + 1 / 2·W: It is 3.00% or less Mo and W are elements necessary to promote secondary hardening during tempering and increase the hardening and tempering hardness. If there is too much Mo or W, the Mo or W remaining in the matrix increases and the thermal conductivity decreases. Therefore, Mo + 1 / 2·W is 3.00% or less, Mo: 3.00% or less, and W: 6.00% or less. On the other hand, if there is too little Mo or W, sufficient hardening and tempering hardness cannot be obtained. Therefore, Mo and W contain one or both, and Mo + 5 / 6·W: 1.50% or more.

[0031] V: More than 0.10 to 0.55% V is an element that promotes secondary hardening during tempering and increases the hardening and tempering hardness. However, if V is 0.10% or less, sufficient hardening and tempering hardness cannot be obtained. On the other hand, if V is more than 0.55%, the V remaining in the matrix increases and the thermal conductivity decreases. Therefore, V is more than 0.10 and 0.55% or less, preferably V is 0.25% or more and less than 0.45%.

[0032] Ni: 0.01 to 2.99% Ni is an element that does not necessarily need to be added. However, like Cr, it is an element that improves hardenability and suppresses the decrease in toughness due to bainite formation. By the way, when Ni is contained in an amount exceeding 2.99%, it dissolves in the matrix and decreases the thermal conductivity. Therefore, Ni is set to 2.99% or less, and preferably, Ni is 0.01% or more and 2.00% or less.

[0033] N: 0.001 to 0.040% N does not necessarily need to be added, but it is an element effective in significantly increasing the hardening and tempering hardness, similar to C. By the way, when N is added in an excessive amount exceeding 0.040%, it takes time during smelting, leading to an increase in the cost during smelting. Therefore, N is set to 0.001% or more and 0.040% or less, and preferably, N is 0.001% or more and 0.030% or less.

[0034] Al: 0.001 to 0.300% Al does not necessarily need to be added, but it is an element that dissolves in the matrix and improves the hardness. By the way, when Al is less than 0.001%, the hardness is not improved as it dissolves in the matrix. On the other hand, when Al is more than 0.300%, it dissolves in the matrix and decreases the thermal conductivity. Therefore, Al is set to 0.001 or more and 0.300% or less, and preferably, Al is 0.005% or more and 0.150% or less.

[0035] Next, the structure of hot work tool steel will be described. Structure in the hardened and tempered state: Martensite single phase If there is bainite, which is an incomplete hardened phase, in the structure after hardening and tempering, the toughness will decrease significantly. Therefore, when used as a hot stamping or die casting mold, a sufficient mold life cannot be obtained. Thus, in the present application, it is desirable to be a martensite single phase.

[0036] Types of carbides in hot work tool steel in the hardened and tempered state: M 3 C, M 7 C 3 , M2 Consisting of any one to four types of C and MC M exists in hot work tool steel in the quenched and tempered state 6 C carbides and M 23 C 6 When C carbides or M 6 C carbides and M 23 C 6 Since the carbides are carbides that tend to coarsen at high temperatures, the softening resistance of the steel material decreases. Therefore, the types of carbides in hot work tool steel in the quenched and tempered state are M 3 C carbides, M 7 C 3 carbides, M 2 It is desirable that the C carbides, MC carbides be any one type, any two types, any three types, or four types.

[0037] Next, the embodiments of the present invention will be sequentially described with reference to the table as appropriate.

[0038] As examples of the present invention, Table 1 and Table 2 below show the chemical compositions of Invention Steels No. 1 to No. 66. Table 3 also shows the chemical compositions of Comparative Steels No. 67 to 83. The balance is Fe and unavoidable impurities. These Invention Steels and Comparative Steels No. 1 to No. 83 of these chemical compositions were each melted in a vacuum induction melting furnace, and 100 kg steel ingots of each No. obtained were hot forged and drawn into drawn materials in blocks having a width of 65 mm and a height of 30 mm. Next, after annealing these drawn materials of each No. at 870 °C, round bars having a diameter of 16 mm and a length of 160 mm were each collected from an intermediate position between their surfaces and centers. Furthermore, after holding these round bars at 1030 °C for soaking, quenching was performed by air cooling, and then tempering was performed twice at 570 to 670 °C. Thereafter, the structure and precipitated carbide species of each steel material in the quenched and tempered state were observed, and investigations were carried out on various characteristics shown in Tables 4 to 6.

[0039] The above various characteristics are thermal conductivity (W / m·K), quenching and tempering hardness (HRC), and Charpy impact value (J / cm showing toughness2 ) and the hardness (HRC) after high-temperature holding, i.e., the softening resistance.

[0040]

Table 1

[0041]

Table 2

[0042]

Table 3

[0043] 1) For the microstructure observation of each sample in the quenched and tempered state, the surface parallel to the forging direction was polished until it became a mirror surface, then etched with nital, and this was carried out using a scanning electron microscope (SEM).

[0044] 2) The identification of carbides present in the quenched and tempered state was carried out by electrolytic extraction of precipitates from the sample after quenching and tempering, and then X-ray diffraction.

[0045] 3) For the measurement of thermal conductivity, the laser flash method was used. Each sample in the quenched and tempered state was finished into a cylindrical shape with a diameter of 10 mm × 1 mm and subjected to the test. The thermal conductivity was measured at room temperature.

[0046] 4) The quenched and tempered hardness was measured at room temperature using a Rockwell hardness tester. For each sample in the quenched and tempered state, the hardness of the surface perpendicular to the forging direction was measured.

[0047] 5) Toughness was evaluated by the Charpy impact test at room temperature. The test specimens were made from samples of the steel material after quenching and tempering. The test specimen shape was a 2 mm U-notch Charpy test specimen, and the notch direction was perpendicular to the forging direction.

[0048] 6) The softening resistance was evaluated by measuring the hardness at room temperature with a Rockwell hardness tester after holding a sample of the steel material after quenching and tempering at 600 °C for 100 hours and then air-cooling it.

[0049] Regarding the microstructure in 1) and the types of carbides in 2) above, they are shown in Tables 1 to 3. The symbol M for the microstructure in the table means a single-phase martensite. M + B is a mixed microstructure of martensite and bainite. Also, for the carbide types, ○ means that the identified carbides are M 3 C carbides, M 7 C 3 carbides, M 2 C carbides, MC carbides only, meaning that no other carbide types were observed. × means that M 3 C carbides, M 7 C 3 carbides, M 2 C carbides, MC carbides, in addition to M 6 C carbides, M 23 C 6 carbides were contained.

[0050] Regarding the thermal conductivity measured in 3) above, the quenching and tempering hardness measured in 4), the Charpy impact value (toughness) measured in 5), and the hardness after high-temperature holding (softening resistance) measured in 6), they are shown for the inventive steel in Tables 4 and 5 below, and further shown for the comparative steel in Table 6 below.

[0051]

Table 4

[0052]

Table 5

[0053]

Table 6

[0054] The steel of the invention of the present application has the chemical compositions of each No. 1 to 66 in Tables 1 and 2, has a single martensite structure, and the carbide species is M 3 C carbide, M 7 C 3 carbide, M 2 C carbide, and only MC carbide. And as shown in each No. 1 to 66 in Tables 4 and 5, they have a thermal conductivity of 25.0 W / m·K or more, a quenching and tempering hardness of 48.0 HRC or more, a Charpy impact value of 20 J / cm 2 or more, and a hardness after high-temperature holding of 32.0 HRC or more. Thus, the hot-work tool steel of the present application, by focusing on the component composition, structure, and carbide, can be suitably applied to die steels and the like as a hot-work tool steel having high thermal conductivity, high hardness, high softening resistance, and high toughness when in a quenched and tempered state.

[0055] On the other hand, in the comparative steel, in the chemical compositions of each No. 67 to 83 in Table 3, some deviate from the component ranges specified in the present application. Also, the structure is not a single martensite but a mixed structure of martensite and bainite, and among the carbide species, there are those containing M 3 C carbide, M 7 C 3 carbide, M 2 C carbide, and those containing other than MC carbide. Regarding the properties of these comparative steels, as shown in each No. 67 to 83 in Table 6, any one of the properties of thermal conductivity, hardness, softening resistance, and toughness is inferior to that of the hot-work tool steel of the present application.

[0056] The comparative examples will be further detailed below. In No. 67, as shown in Table 3, C is 0.32%, less than the specification, and as shown in Table 6, the quenching and tempering hardness is 44.3 HRC, lower than the specification. In No. 68, as shown in Table 3, C is 0.73%, more than the specification, and as shown in Table 6, the Charpy impact value is 14.1 J / cm 2 lower than the specification. In No. 69, as shown in Table 3, Si is 1.28%, more than the specification, and as shown in Table 6, the thermal conductivity is 23.6 W / m·K, lower than the specification. In No.70, as shown in Table 3, Mn is 1.57%, which is more than the specified value. As shown in Table 6, the thermal conductivity is 24.3 W / m·K, which is lower than the specified value. In No.71, as shown in Table 3, Cr is 0.31%, which is less than the specified value, and the structure is a mixed structure of martensite and bainite. As shown in Table 6, the Charpy impact value is 14.6 J / cm 2 which is lower than the specified value. In No.72, as shown in Table 3, Cr is 4.28%, which is more than the specified value. As shown in Table 6, the thermal conductivity is 24.1 W / m·K, which is lower than the specified value. In No.73, as shown in Table 3, Cr is 4.85%, which is more than the specified value, and in the carbide species, there are M 3 C carbides, M 7 C 3 carbides, M 2 C carbides, and in addition to MC carbides, other carbides are also included. As shown in Table 6, the thermal conductivity is 23.4 W / m·K, which is lower than the specified value, and the hardness after high-temperature holding is also as low as 31.1 HRC. In No.74, as shown in Table 3, Cu is 0.03%, which is less than the specified value, and the structure is a mixed structure of martensite and bainite. As shown in Table 6, the Charpy impact value is 14.0 J / cm 2 which is lower than the specified value. In No.75, Cu is 3.62%, which is slightly more than the specified value. As shown in Table 6, the thermal conductivity is 23.4 W / m·K, which is lower than the specified value. In No.76, as shown in Table 3, Mo is 3.12%, which is more than the specified value, and Mo + 1 / 2·W is also 3.12%, which is more than the specified value. As shown in Table 6, the thermal conductivity is 23.1 W / m·K, which is lower than the specified value. In No.77, as shown in Table 3, W is 6.21%, which is more than the specified value, and furthermore, Mo + 1 / 2·W is 3.11%, which is more than the specified value. As shown in Table 6, the thermal conductivity is 24.9 W / m·K, which is lower than the specified value. In No.78, as shown in Table 3, Mo + 1 / 2·W is 3.24%, which is more than the specified value. As shown in Table 6, the thermal conductivity is 23.1 W / m·K, which is lower than the specified value. In No.79, as shown in Table 3, Mo + 5 / 6·W is 1.45%, less than the specified value, and as shown in Table 6, the quenching and tempering hardness is 43.4 HRC, lower than the specified value. In No.80, as shown in Table 3, V is 0.09%, less than the specified value, and as shown in Table 6, the quenching and tempering hardness is 46.7 HRC, lower than the specified value. In No.81, as shown in Table 3, V is 0.59%, more than the specified value, and as shown in Table 6, the thermal conductivity is 24.3 W / m·K, lower than the specified value. In No.82, as shown in Table 3, Ni is 3.14%, more than the specified value, and as shown in Table 6, the thermal conductivity is 23.5 W / m·K, lower than the specified value. In No.83, as shown in Table 3, Al is 0.325%, more than the specified value, and as shown in Table 6, the thermal conductivity is 23.7 W / m·K, lower than the specified value.

Claims

1. By mass percentage, C: more than 0.35% to 0.70%, Si: 0.01% to 1.20%, Mn: 0.01% to 1.50%, Cr: 0.40% to 3.30% Cu: 0.29% to 3.50% One or two of Mo and W, where Mo: 3.00% or less, W: 6.00% or less, and Mo + 5 / 6·W: 1.50% or more, Mo + 1 / 2·W: 3.00% or less, V: more than 0.10% to 0.55%, and the balance consists of Fe and inevitable impurities, characterized as a hot work tool steel.

2. In addition to the chemical composition described in Claim 1, having Ni: 0.01% to 2.99% by mass percentage, and the balance consists of Fe and inevitable impurities, characterized as a hot work tool steel.

3. In addition to the chemical composition described in any one of Claim 1 or Claim 2, having N: 0.001% to 0.040% by mass percentage, and the balance consists of Fe and inevitable impurities, characterized as a hot work tool steel.

4. In addition to the chemical composition described in any one of Claims 1 to 3, having Al: 0.001% to 0.300% by mass percentage, and the balance consists of Fe and inevitable impurities, characterized as a hot work tool steel.

5. In addition to the chemical composition described in any one of Claims 1 to 4, the balance consists of Fe and inevitable impurities, and it is characterized as a hot work tool steel with a single-phase martensite structure.

6. A hot work tool steel comprising, in addition to the chemical components according to any one of claims 1 to 4, the balance consisting of Fe and inevitable impurities, wherein the types of carbides are M 3 C, M 7 C 3 、M 2 C, and characterized in that it consists of any 1 to 4 types of MC.

7. The hot work tool steel according to claim 5, further characterized in that the types of carbides are any one to four of M 3 C, M 7 C 3 、M 2 C, MC, and the hot work tool steel is characterized by being composed of any one to four of them.

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