Steel material and mold

Abstract

To provide a steel material which is hardly largely cracked when used for a die casting mold, a low pressure casting mold or the like, and to provide a mold using the steel material.SOLUTION: The steel contains 0.32 ≤ C ≤ 0. 42mass%, 0.25 ≤ Si ≤ 0. 40mass%, 0.10 ≤ Mn ≤ 0. 60mass%, 6.60 <Cr ≤ 7. 50mass%, 1.10 ≤ Mo ≤ 2. 90mass%, 0.18 ≤ V ≤ 0. 30mass%, 0.001 ≤ Al ≤ 0. 050mass%, 0.001 ≤ N ≤ 0. 050mass%, and 0.001 ≤ P ≤ 0. 040mass%, with the balance being Fe and an unavoidable impurity. The mold is made of such a steel material.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a steel material and a mold, and more particularly to a mold used for various casting processes such as die casting and low-pressure casting, and for molding plastics, and a steel material for such a mold. [Background technology]

[0002] "Die casting" is a method of producing a casting by injecting molten metal into a mold at high speed and solidifying it within the mold. Die casting has the advantage of being able to produce castings with high dimensional accuracy in a short period of time. "Low-pressure casting" is a method of producing a casting by injecting molten metal into a mold at low pressure and speed and solidifying the molten metal within the mold. Low-pressure casting takes longer than die casting, but has the advantage of producing castings with less gas entrapment. Hereinafter, molds used in environments where temperature fluctuations occur repeatedly (for example, molds used for various types of casting such as die casting and low-pressure casting, and molds used for molding and processing plastics) will also be simply referred to as "molds."

[0003] The manufacturing process of steel materials used for dies generally includes a melting step, a refining step, a homogenizing heat treatment step, a hot working step, a normalizing step, a tempering step, and a spheroidizing annealing step. Note that one or both of the normalizing step and the tempering step may be omitted. Furthermore, a mold is manufactured from the steel material manufactured as described above. The manufacturing process of a mold generally includes a rough machining step, a quenching step, a tempering step, a precision machining step, and a surface treatment step. However, the surface treatment step may be omitted.

[0004] Because dies are manufactured through the many processes described above, the steel used for dies is required to have spheroidizing annealing (SA) properties, machinability, etc. Furthermore, dies manufactured from such steel are required to have impact resistance, heat check resistance, softening resistance, etc. However, it is generally not easy to manufacture steel that simultaneously satisfies these multiple properties.

[0005] Therefore, various proposals have been made in the past to solve this problem. For example, Patent Document 1 discloses a steel material containing predetermined amounts of C, V, Mn, Cr, Cu, Ni, Si, Mo, Al, and N, with the balance being Fe and unavoidable impurities.

[0006] During use, die-casting dies are repeatedly filled with molten metal and sprayed with a mold release agent on the design surface. As a result, the dies are repeatedly subjected to thermal stress during use. Meanwhile, the interior of the dies is usually provided with water-cooling holes to cool the dies. Therefore, when the mold is used repeatedly, cracks may occur in the stress concentration areas due to thermal stress. Furthermore, if the mold continues to be used with a crack, the crack may grow and eventually connect the design surface of the mold to the water-cooling holes inside the mold. Hereinafter, the phenomenon in which the design surface and the water-cooling holes are connected by a crack is also referred to as "large crack."

[0007] When large cracks occur in a mold, the cooling medium (water, hot water) or its steam leaks from the water-cooling holes onto the design surface. This results in a deterioration in the quality of the casting. Therefore, when a large crack occurs in a mold, the mold must be replaced, which increases the cost of the casting and reduces productivity. However, there have been no examples of steel materials proposed that are resistant to the initiation and propagation of cracks even when subjected to repeated thermal stresses, or steel materials that are resistant to large cracks when used as molds. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2024-046069 Summary of the Invention [Problem to be solved by the invention]

[0009] The problem to be solved by the present invention is to provide a steel material that is less likely to cause large cracks when used in die-casting dies, low-pressure casting dies, and the like. Another object of the present invention is to provide a mold using such a steel material. [Means for solving the problem]

[0010] In order to solve the above problems, the steel material according to the present invention comprises: 0.32≦C≦0.42mass%, 0.25≦Si≦0.40mass%, 0.10≦Mn<0.60mass%, 6.60 <Cr≦7.50mass%、 1.10≦Mo≦2.90mass%, 0.18≦V≦0.30mass%, 0.001≦Al≦0.050mass%, 0.001≦N≦0.050mass%, and 0.001≦P≦0.040mass%, and the balance is Fe and unavoidable impurities.

[0011] The mold according to the present invention is made of the steel material according to the present invention. [Effects of the Invention]

[0012] There are three patterns of large mold cracks: (a) Pattern 1: A crack occurs near the surface of the water-cooled hole, progresses toward the design surface, and penetrates all the way to the design surface. (b) Pattern 2: A crack occurs near the design surface, progresses toward the water-cooling hole, and penetrates all the way to the cooling hole. (c) Pattern 3 is a combination of Pattern 1 and Pattern 2.

[0013] Pattern 1 cracks occur in rusted areas (corroded areas) on the surface of the mold's water cooling holes. Areas where the rust has penetrated into the steel become stress concentration areas, making cracks more likely to occur. To prevent the occurrence of Pattern 1 cracks, it is necessary to improve the corrosion resistance of the steel. Pattern 2 cracks, also known as heat checks, occur as a result of repeated thermal stress on the mold. To prevent the occurrence of pattern 2 cracks, it is necessary to increase the strength of the steel and reduce the thermal stress.

[0014] Furthermore, the cracks that occur propagate due to mechanical and thermal stresses acting on the mold during use. Furthermore, if the mold has low toughness, the cracks are more likely to propagate. To suppress crack propagation, it is necessary to improve the high-temperature strength of the steel, improve the thermal conductivity of the steel, and improve the toughness of the steel. In the present invention, the components are optimized, so that the occurrence and growth of large cracks caused by Pattern 1 and Pattern 2 can be suppressed. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram of the process of crack initiation, propagation, and penetration. [Figure 2] FIG. 1 is a diagram showing the relationship between temperature, 0.2% proof stress, and hardness. [Figure 3] The SA structure before quenching (left), the fully quenched and tempered structure (middle), and the incompletely quenched and tempered structure (right).

[0016] [Figure 4] The hardened structure of steel with a high content of dispersed particles (top image) and the hardened structure of steel with a low content of dispersed particles (bottom image) are shown. [Figure 5] FIG. 1 is a diagram showing the relationship between the transformation point and the structure. [Figure 6] FIG. 1 is a diagram showing the relationship between hardenability and structure.

[0017] An embodiment of the present invention will be described in detail below. [1. Steel materials] [1.1. Composition] [1.1.1. Main constituent elements] The steel material according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0018] (1) 0.32≦C≦0.42mass%: The greatest effect of C is that it can provide a hardness (≧38HRC) that can suppress the initiation and propagation of cracks during high-temperature tempering (i.e., tempering under conditions that satisfy the following formulas (1) to (3)). By optimizing the C content, a hardness of 38HRC or more can be obtained when tempering is performed under the following conditions. As a result, high-temperature strength is improved.

[0019] 16900≦LMP≦17800 …(1) LMP = (T + 273) × (20 + log X) … (2) 530℃≦T≦630℃ …(3) however, LMP is the Larson-Miller parameter, T is the tempering temperature [℃]. X is the holding time at the tempering temperature [H]. The temperature and holding time are not the temperature and holding time of the furnace but the temperature and holding time of the steel material.

[0020] The effect of C is not only to ensure hardness, but also to (a) The effect of increasing the softening resistance of steel (the effect of suppressing the decrease in hardness in high-temperature environments), (b) The effect of ensuring fine dispersed particles (carbides and carbonitrides) during quenching heating and suppressing the growth of austenite crystal grains, thereby refining the structure. (c) The effect of reducing the transformation temperature to refine the structure and increase toughness There is.

[0021] If the C content is too low, it may be difficult to ensure the strength and / or softening resistance of the steel material. Furthermore, if the C content is too low, the Ms point of the steel material increases, and even if the martensite is formed, a fine structure cannot be obtained, making it difficult to ensure toughness. Therefore, the C content must be 0.32 mass% or more. The C content is preferably 0.33 mass% or more, or 0.34 mass% or more.

[0022] On the other hand, excessive C content increases the amount of coarse inclusions (carbides, carbonitrides, carboborides, etc.) that are generated. Furthermore, because Cr is consumed in the formation of carbides and carbonitrides, the amount of Cr dissolved in the matrix decreases, which can reduce the corrosion resistance of the steel. Furthermore, the amount of C dissolved in the matrix also increases, which can reduce the thermal conductivity of the steel. Furthermore, steel with a high C content may be at higher risk of quench cracking. This is because the transformation point is excessively lowered, and the transformation will not be completed unless the quenching end temperature is lowered. The lower the transformation temperature, the higher the quenching stress, so the lower the quenching temperature, the higher the risk of quench cracking. Therefore, the C content must be 0.42 mass% or less. The C content is preferably 0.41 mass% or less, or 0.40 mass% or less.

[0023] (2) 0.25≦Si≦0.40mass%: Optimizing the amount of Si makes it possible to achieve a balance between machinability and thermal conductivity. Generally, the higher the Si content, the better the machinability of the steel. On the other hand, the lower the Si content, the higher the thermal conductivity of the steel. Therefore, it is preferable to select an optimal Si content, taking into consideration the balance between machinability and thermal conductivity.

[0024] If the Si content is too low, the machinability and / or toughness of the steel may decrease, so the Si content must be 0.25 mass% or more. On the other hand, excessive Si content may increase the amount of coarse inclusions (carbides, carbonitrides, carboborides, etc.) produced. Furthermore, the amount of Si dissolved in the matrix increases, which may reduce the thermal conductivity of the steel. Furthermore, excessive Si content may reduce the toughness of the steel during high-temperature tempering (high-temperature tempering embrittlement). Therefore, the Si content must be 0.40 mass% or less.

[0025] In particular, when high machinability is important, the Si content is preferably 0.35 to 0.40 mass%, whereas when high thermal conductivity is important, the Si content is preferably 0.25 to 0.30 mass%.

[0026] (3) 0.10≦Mn<0.60mass%: Optimizing the Mn content allows for a balance between hardenability and spheroidizing annealing properties. Generally, the greater the Mn content, the better the hardenability of the steel material. On the other hand, the smaller the Mn content, the better the spheroidizing annealing properties of the steel material. Therefore, it is preferable to select an optimal Mn content in consideration of the balance between hardenability and spheroidizing annealing properties.

[0027] If the Mn content is too low, the toughness of the steel material may decrease. Therefore, the Mn content must be 0.10 mass% or more. The Mn content is preferably 0.20 mass% or more, or 0.30 mass% or more.

[0028] On the other hand, an excessive amount of Mn may reduce the spheroidizing annealing property and / or thermal conductivity of the steel material, and may also reduce the transformation point of the steel material excessively, increasing the amount of retained austenite and reducing the toughness. Furthermore, if the Mn content is excessive, the toughness of the steel may decrease during high-temperature tempering (high-temperature temper embrittlement). Such high-temperature temper embrittlement is particularly likely to become apparent when the Si and P contents are relatively high. Therefore, the Mn content must be less than 0.60 mass%, and is preferably 0.57 mass% or less, or 0.55 mass% or less.

[0029] (4) 6.60 <Cr≦7.50mass%: Optimizing the Cr content makes it possible to achieve a balance between corrosion resistance, high-temperature strength, and softening resistance. Generally, the higher the Cr content, the better the corrosion resistance and / or high-temperature strength of the steel. Furthermore, the higher the Cr content, the better the suppression of seizure during die casting of Al. This is because the higher the Cr content, the more likely it is that an oxide film with low reactivity with Al will be formed. On the other hand, the lower the Cr content, the better the softening resistance of the steel. Therefore, it is preferable to select an optimal Cr content in consideration of the balance between corrosion resistance, high-temperature strength, and softening resistance.

[0030] If the Cr content is too low, the spheroidizing annealing property, corrosion resistance, and / or toughness of the steel may be reduced. Therefore, the Cr content must be more than 6.60 mass%. The Cr content is preferably 6.63 mass% or more, or 6.66 mass% or more.

[0031] On the other hand, an excessive Cr content may result in a decrease in the machinability of the steel. In particular, when the Si content is relatively low, an excessive Cr content may significantly decrease the machinability. Furthermore, an excessive Cr content may result in a decrease in the thermal conductivity and / or softening resistance of the steel. Furthermore, an excessive Cr content may excessively decrease the transformation point of the steel, increasing the amount of retained austenite and decreasing the toughness. Therefore, the Cr content must be 7.50 mass% or less. The Cr content is preferably 7.30 mass% or less, or 7.00 mass% or less.

[0032] (5) 1.10≦Mo≦2.90mass%: By optimizing the Mo content, it is possible to achieve a balance between toughness, high-temperature strength, and softening resistance. Generally, the higher the Mo content, the better the high-temperature strength, softening resistance, and / or corrosion resistance of the steel material. On the other hand, the lower the Mo content, the better the toughness of the steel material. Therefore, it is preferable to select an optimal Mo content in consideration of the balance between toughness, high-temperature strength, and softening resistance.

[0033] If the Mo content is too low, the high-temperature strength, softening resistance, corrosion resistance, and / or toughness of the steel may decrease. Therefore, the Mo content must be 1.10 mass% or more. The Mo content is preferably 1.20 mass% or more, or 1.30 mass% or more.

[0034] On the other hand, if the Mo content is excessive, the machinability and / or toughness of the steel may be reduced. Therefore, the Mo content must be 2.90 mass% or less. The Mo content is preferably 2.80 mass% or less, or 2.70 mass% or less.

[0035] (6) 0.18≦V≦0.30mass%: Optimizing the V content allows for a balance between toughness, high-temperature strength, and softening resistance. Generally, the higher the V content, the better the high-temperature strength and / or softening resistance of the steel. On the other hand, the lower the V content, the fewer coarse inclusions (carbides, nitrides, carbonitrides) there are, and the better the toughness of the steel. Therefore, it is preferable to select an optimal V content in consideration of the balance between toughness, high-temperature strength, and softening resistance.

[0036] If the V content is too low, it may be difficult to ensure the high-temperature strength and / or softening resistance of the steel. Furthermore, if the V content is too low, the amount of fine dispersed particles (carbides, nitrides, and carbonitrides) during quenching may be insufficient, resulting in coarsening of grains and a decrease in toughness. Therefore, the V content must be 0.18 mass% or more. On the other hand, if the V content is excessive, the amount of coarse inclusions (carbides, nitrides, carbonitrides) increases, which may reduce the toughness of the steel material. Therefore, the V content must be 0.30 mass% or less.

[0037] In particular, when emphasis is placed on reducing foreign matter, the V content is preferably 0.18 to 0.24 mass%, whereas when emphasis is placed on making the crystal grains finer, the V content is preferably 0.24 to 0.30 mass%.

[0038] (7) 0.001≦Al≦0.050mass% Optimizing the Al content can stabilize the toughness of the steel material. Since the steel material according to the present invention has a low V content, Al is utilized for grain refinement. Generally, the higher the Al content, the more the grain coarsening during quenching is prevented, resulting in a fine-grained structure and improved toughness. A fine-grained structure is obtained by increasing the amount of fine Al-based dispersed particles (nitrides). On the other hand, an excessive Al content increases the amount of coarse foreign matter (alumina), which in turn reduces toughness. Therefore, it is preferable to select an optimal Al content in consideration of stabilizing toughness.

[0039] If the Al content is too low, the number of fine Al-based dispersed particles (nitrides) may decrease. If the number of dispersed particles decreases, the crystal grains become coarse during quenching, and toughness decreases. Therefore, the Al content must be 0.001 mass% or more. The Al content is preferably 0.003 mass% or more, or 0.005 mass% or more.

[0040] On the other hand, if the Al content is excessive, the amount of coarse foreign matter (alumina) increases, which may actually reduce toughness. Therefore, the Al content must be 0.050 mass% or less. The Al content is preferably 0.040 mass% or less, or 0.035 mass% or less.

[0041] (8) 0.001≦N≦0.050mass%: Optimizing the N content can stabilize the toughness of the steel material. Since the steel material according to the present invention has a low V content, N is utilized for grain refinement. Generally, the greater the N content, the greater the number of fine dispersed particles (nitrides and carbonitrides), improving high-temperature strength and softening resistance. Furthermore, the greater the N content, the better the corrosion resistance of the steel material. On the other hand, if the N content is excessive, the amount of coarse inclusions (nitrides and carbonitrides) increases, which actually reduces toughness. Therefore, it is preferable to select an optimal N content in consideration of stabilizing toughness.

[0042] If the N content is too low, the number of fine dispersed particles (nitrides, carbonitrides) may decrease during quenching. If the number of dispersed particles decreases, the crystal grains become coarse and the toughness decreases. Therefore, the N content must be 0.001 mass% or more. The N content is preferably 0.003 mass% or more, or 0.005 mass% or more.

[0043] On the other hand, if the N content is excessive, the amount of coarse inclusions (nitrides, carbonitrides) increases, which may actually reduce toughness. Therefore, the N content must be 0.050 mass% or less. The N content is preferably 0.040 mass% or less, or 0.030 mass% or less.

[0044] (9) 0.001≦P≦0.040mass% By optimizing the P content, it is possible to achieve a balance between machinability, toughness, and corrosion resistance. Generally, the greater the P content, the better the machinability and / or corrosion resistance of the steel. On the other hand, the smaller the P content, the better the toughness. Therefore, it is preferable to select an optimal P content in consideration of the balance between machinability, toughness, and corrosion resistance.

[0045] Here, the reason why the addition of P improves toughness and corrosion resistance is thought to be because a P-enriched layer is formed on the surface of the water-cooled hole, and this concentrated layer suppresses corrosion and crack propagation. The P-enriched layer may or may not be what is known as "rust (a substance containing Fe oxides)." Regardless of whether the P-enriched layer is "rust," it is thought that this layer functions as a barrier layer, preventing direct contact between water and the steel, the formation of new rust inside the steel, and the penetration of hydrogen into the steel.

[0046] If the P-enriched layer is "rust," the main timing for its formation is as follows: (a) When quenching and tempering a rough-machined mold with water cooling holes, (b) At the time of casting, or (c) The period during which the mold was removed from the machine for maintenance. And so on. If the P-enriched layer is not "rust," it is mainly formed during quenching and tempering of a roughly machined mold with water-cooling holes.

[0047] If the P content is too low, the machinability and / or corrosion resistance of the steel may be reduced. Therefore, the P content must be 0.001 mass% or more. The P content is preferably 0.003 mass% or more, or 0.005 mass% or more.

[0048] On the other hand, if the P content is excessive, P may segregate at grain boundaries, reducing the toughness of the steel material. Furthermore, if the P content is excessive, the amount of P dissolved in the matrix increases, reducing the toughness of the matrix itself. Therefore, the P content must be 0.040 mass% or less. The P content is preferably 0.035 mass% or less, or 0.030 mass% or less.

[0049] 1.1.2. Inevitable impurities The steel material according to the present invention may contain unavoidable impurities. The elements that may be contained as impurities in the steel material according to the present invention and the contents thereof are as follows. O≦0.006mass%, Cu≦0.20mass%, Ni≦0.20mass%, S≦0.006mass%, W≦0.30mass%, Co≦0.30mass%, Nb≦0.004mass%, Ta≦0.004mass%, Ti≦0.004mass%, Zr≦0.004mass%, B≦0.0002mass%, Ca≦0.0005mass%, Se≦0.03mass%, Te≦0.005mass%, Bi≦0.01mass%, Pb≦0.03mass%, Mg≦0.02mass%.

[0050] In the present invention, the term "content" refers to the "average element amount in the steel material" including areas with high segregation, areas with low segregation, and areas with average segregation. (a) A method of dissolving a predetermined mass of steel (preferably 1 gram or more per element) in acid for analysis; (b) A method of irradiating the surface of steel with X-rays etc.

[0051] [1.1.3. Sub-constituent elements (1)] The steel material according to the present invention may further contain Cu and Ni in addition to the above-mentioned main constituent elements and unavoidable impurities. The ranges of the Cu and Ni contents and the reasons for their limitations are as follows.

[0052] (1) 0.20 <Cu≦0.80mass%: If the Cu content is too low, the toughness of the steel material may decrease, so the Cu content is preferably more than 0.20 mass%. On the other hand, if the Cu content is excessive, the spheroidizing annealing property and / or toughness of the steel material may be reduced. Furthermore, if the Cu content is excessive, cracks may be more likely to occur during hot working. Therefore, the Cu content is preferably 0.80 mass% or less.

[0053] (2) 0.20 <Ni≦1.60mass%: If the Ni content is too low, the toughness of the steel material may decrease, so the Ni content is preferably 0.20 mass% or more. On the other hand, if the Ni content is excessive, the spheroidizing annealing property and / or toughness of the steel material may be reduced. Furthermore, if the Ni content is excessive, grain boundary oxidation may become apparent during hot working. If grain boundary oxidation becomes apparent, descaling property deteriorates and surface defects become more likely to occur. Therefore, the Ni content is preferably 1.60 mass% or less.

[0054] (3) 0.45≦Cu+Ni≦2.00mass%: By optimizing the Cu and Ni contents, it is possible to achieve a balance between spheroidizing annealing properties and toughness, and also to achieve a balance between thermal conductivity and corrosion resistance. Generally, the greater the Cu and / or Ni contents, the more improved the toughness and / or corrosion resistance of the steel material. On the other hand, the smaller the Cu and / or Ni contents, the more improved the spheroidizing annealing properties and / or thermal conductivity of the steel material. Therefore, it is preferable to select the optimal Cu and Ni contents in consideration of the balance between spheroidizing annealing properties, toughness, thermal conductivity, and corrosion resistance.

[0055] The reason why the addition of Cu and / or Ni improves toughness and corrosion resistance is thought to be that a concentrated layer of Cu and / or Ni is formed on the surface of the water-cooled hole, which inhibits corrosion and crack propagation. This concentrated layer is thought to function as a barrier layer, inhibiting direct contact between water and the steel, the formation of new rust inside the steel, and the penetration of hydrogen into the steel. The main timing for the formation of a Cu and / or Ni concentrated layer is during quenching and tempering of a rough-machined die with water cooling holes drilled therein.

[0056] If the total content of Cu and Ni is too low, the toughness of the steel may decrease. Therefore, the total content is preferably 0.45 mass% or more. The total content is more preferably more than 0.50 mass%, or more preferably more than 0.84 mass%. On the other hand, if the total content of Cu and Ni is excessive, the spheroidizing annealing property and / or toughness of the steel may be reduced. Therefore, the total content is preferably 2.00 mass% or less. The total content is more preferably 1.75 mass% or less, or 1.50 mass% or less.

[0057] (4) 0.5Cu≦Ni: When the steel material according to the present invention contains Cu and Ni and the total amount thereof is within the above-mentioned range, if the ratio of Ni to Cu is inappropriate, the steel material may crack during hot working. In order to suppress cracking of the steel material during hot working, it is preferable that 0.5Cu≦Ni.

[0058] [1.1.4. Sub-constituent elements (2)] The steel material according to the present invention may further contain, in addition to the above-mentioned main constituent elements, Cu, Ni, and unavoidable impurities, one or more elements selected from the group consisting of the following groups A to D. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0059] [A. Group A] (1) 0.30 <W≦2.00mass%: (2) 0.30 <Co≦1.00mass%:

[0060] The steel material according to the present invention may contain either W or Co, or may contain both, in order to increase the strength.

[0061] When W and / or Co are added, in order to obtain such effects, the W content is preferably more than 0.30 mass%, and the Co content is preferably more than 0.30 mass%. On the other hand, excessive W and / or Co content may result in a decrease in thermal conductivity, toughness, fatigue strength, and / or heat check resistance. Therefore, the W content is preferably 2.00 mass% or less. The Co content is preferably 1.00 mass% or less.

[0062] [B. Group B] (3) 0.004 <Nb≦0.100mass%: (4) 0.004 <Ta≦0.100mass%: (5) 0.004 <Ti≦0.100mass%: (6) 0.004 <Zr≦0.100mass%:

[0063] To prevent grain coarsening, it is effective to add carbide-forming elements (Nb, Ta, Ti, Zr) to the steel material according to the present invention. The steel material according to the present invention may contain any one of these carbonitride-forming elements, or may contain two or more of them.

[0064] In order to suppress excessive grain growth of austenite grains, the content of each carbonitride-forming element is preferably greater than the above-mentioned lower limit. On the other hand, if the content of the carbonitride-forming elements is excessive, the amount of coarse carbides, carbonitrides, and / or nitrides increases, which may cause a decrease in toughness. Therefore, the content of each carbonitride-forming element is preferably equal to or less than the above-mentioned upper limit.

[0065] [C. Group C] (7) 0.0002 <B≦0.0080mass%: When the P content is high, the P segregating at the grain boundaries reduces the grain boundary strength, resulting in a low impact value. Adding B is effective in improving the grain boundary strength. To obtain this effect, the B content is preferably more than 0.0002 mass%. On the other hand, if the B content is excessive, the impact value and fatigue strength may decrease due to borides, so the B content is preferably 0.0080 mass% or less.

[0066] When adding B to improve grain boundary strength, B must exist alone in the steel (without forming a compound). It is meaningless for B to form BN. Therefore, when adding B to steel containing N, it is necessary to combine N with elements other than B. Specifically, it is preferable to add nitride-forming elements (elements of group B) such as Ti, Zr, and Nb, which easily form nitrides, and combine these with N. The "elements of group B" are effective even at the impurity level, but if there is a deficiency, it is preferable to add the specified amount described above. On the other hand, when it is desired to disperse B in order to improve machinability, it is not necessary to take measures to actively combine N with nitride-forming elements.

[0067] [D. Group D] (8) 0.006 <S≦0.180mass%: (9) 0.0005 <Ca≦0.0500mass%: (10)0.03 <Se≦0.50mass%、: (11)0.005 <Te≦0.100mass%: (12)0.01 <Bi≦0.50mass%: (13)0.03 <Pb≦0.50mass%:

[0068] The steel material according to the present invention may contain any one of the free-cutting elements (S, Ca, Se, Te, Bi, Pb), or may contain two or more of them.

[0069] In order to obtain sufficient free-cutting properties, the content of each free-cutting element is preferably greater than the respective lower limit values. On the other hand, excessive free-cutting elements may cause cracking during hot working. Furthermore, excessive free-cutting elements may reduce toughness, fatigue strength, and / or heat-checking resistance. Therefore, the content of each free-cutting element is preferably equal to or less than the upper limit indicated above.

[0070] [1.2. Uses of steel] The steel material according to the present invention can be used as a material for dies used in various processes. Details of the dies will be described later. Furthermore, the steel material according to the present invention can be processed into a rod or wire, and the rod or wire can be used as a weld repair material.

[0071] [2. Mold] The mold according to the present invention is made of the steel material according to the present invention.

[0072] 2.1. Steel material, shape, mass, size Details of the steel material are as described above, and therefore will not be described here. In the present invention, the term "mold" refers to a member having a portion that comes into contact with a workpiece (metal, plastic, vinyl, rubber, etc.). In the present invention, the mass and size of the mold are not particularly limited, and optimal values ​​can be selected depending on the purpose.

[0073] [3. Steel Manufacturing Methods] The steel material according to the present invention is (a) Melting and casting raw materials blended to form a predetermined composition; (b) subjecting the ingot to a homogenizing heat treatment; (c) Hot working the ingot after homogenization heat treatment This is obtained by:

[0074] In addition, after hot processing, (d) If necessary, normalize the raw material after hot working; (e) Temper the raw material as necessary; (f) If necessary, perform spheroidizing annealing (SA) on the raw material. (g) If necessary, the raw material may be quenched and tempered.

[0075] In the present invention, the conditions for each step are not particularly limited, and it is preferable to select the optimum conditions depending on the purpose.

[0076] [4. Effect] [4.1. Causes of large cracks] Figure 1 shows a schematic diagram of the process of crack initiation, propagation, and penetration. There are three major crack patterns: (a) Pattern 1: A crack occurs near the surface of the water-cooled hole, progresses toward the design surface, and penetrates all the way to the design surface. (b) Pattern 2: A crack occurs near the design surface, progresses toward the water-cooling hole, and penetrates all the way to the cooling hole. (c) Pattern 3 is a combination of Pattern 1 and Pattern 2.

[0077] Table 1 shows the factors that cause crack initiation and propagation. Pattern 1 cracks occur in rusted areas (corroded areas) on the surface of the mold's water cooling holes. Areas where the rust has penetrated into the steel become stress concentration areas, making cracks more likely to occur. To prevent the occurrence of Pattern 1 cracks, it is necessary to improve the corrosion resistance of the steel. Pattern 2 cracks, also known as heat checks, occur as a result of repeated thermal stress on the mold. To prevent the occurrence of pattern 2 cracks, it is necessary to increase the strength of the steel and reduce the thermal stress.

[0078] Furthermore, the cracks that occur propagate due to mechanical and thermal stresses acting on the mold during use. Furthermore, if the mold has low toughness, the cracks are more likely to propagate. To suppress crack propagation, it is necessary to improve the high-temperature strength of the steel, improve the thermal conductivity of the steel, and improve the toughness of the steel.

[0079] [Table 1]

[0080] [4.2. Directions for suppressing crack initiation] Table 2 shows the direction to suppress crack occurrence. To prevent mold corrosion, it is necessary to improve the corrosion resistance of the steel itself. The elements C, Cr, Mo, and Si in steel all affect the corrosion resistance of steel. Therefore, optimizing their content can improve the corrosion resistance of steel. This also helps prevent cracks from occurring due to corrosion on the surface of the water-cooled hole.

[0081] To prevent heat checking in a mold, it is necessary to increase the strength of the mold and reduce the thermal stress acting on the mold. C, Cr, Mo, and V in steel all affect the strength of the steel at high temperatures. In addition, C, Cr, Mo, V, and Si in steel all affect the softening resistance of the steel. Therefore, optimizing the content of these elements can improve the strength of the mold. Furthermore, C, Si, Mn, Cr, Cu, and Ni in steel all affect the thermal conductivity of the steel, so optimizing their content can reduce the thermal stress acting on the mold.

[0082] [Table 2]

[0083] 4.3. Directionality for suppressing crack propagation Table 3 shows the direction to suppress crack propagation. To suppress crack propagation, it is important to reduce the mechanical and thermal stress acting on the mold and to improve the mold's toughness. Elements that affect mold heat checks (elements (b), (c), and (d) in Table 2) also affect crack propagation. Therefore, optimizing the content of these elements can suppress the propagation of cracks caused by mechanical and thermal stress.

[0084] In order to improve the toughness of a mold, it is necessary to increase the toughness of the steel material. In order to increase the toughness of the steel material, it is also necessary to increase the toughness of the matrix itself, to make the structure finer, and to reduce the amount of retained austenite in the quenched state. Of these, C, Si, Mo, and P in the steel material all affect the toughness of the matrix itself, so optimizing their contents can improve the toughness of the matrix itself.

[0085] Furthermore, C, V, Si, N, and Al in steel all affect the refinement of the steel's structure. Furthermore, Mn, Cr, Cu, and Ni in steel all affect the steel's hardenability. Therefore, optimizing the content of these elements can refine the structure. Furthermore, C, Mn, Cr, Mo, Cu, and Ni in steel all affect the amount of retained austenite in the quenched state, so optimizing the content of these elements can reduce the amount of retained austenite.

[0086] [Table 3]

[0087] [4.4. Specific methods for suppressing crack generation] [4.4.1. Improving the corrosion resistance of steel: Table 2(a)] To prevent cracks from forming in corroded areas, it is necessary to prevent rust from forming. This reduces the number of locations where cracks can start due to stress concentration, such as depressions on the steel surface at the interface with the rust and sharp areas of rust that have penetrated into the steel. These depressions and sharp areas of rust that have penetrated into the steel may or may not be prior austenite grain boundaries.

[0088] [4.4.2. Strength Improvement of Steels at High Temperatures: Table 2(b)] In this application, "high temperature range" refers to a temperature range of 50°C or higher. Thermal stress acts on the design surface, which is heated by contact with the molten metal. The mold may also be subjected to mechanical stresses due to casting (mold deformation due to clamping, mold deformation due to injection and pressure load). If the strength is sufficiently high in the temperature range where such stresses act (approximately 300 to 650°C), fatigue during contact with the molten metal or cast product can be reduced, and heat checking can be suppressed. On the other hand, thermal stress also acts on the design surface that is cooled by the application of the release agent. If the strength is sufficiently high in the temperature range where such stress acts (approximately 150 to 300°C), fatigue during cooling by the application of the release agent will be reduced, and heat checking can be suppressed.

[0089] [4.4.3. Improving the softening resistance of steel: Table 2(c)] In order to prevent cracks from occurring on the design surface, it is necessary to prevent the design surface from softening (reducing hardness) when heated by contact with the molten metal. Figure 2 shows the relationship between temperature, 0.2% yield strength, and hardness. As shown in Figure 2, high-temperature strength has a positive correlation with hardness. Therefore, if the decrease in hardness is suppressed, high-temperature strength can also be maintained.

[0090] [4.4.4. Improving the thermal conductivity of steel: Table 2(d)] The smaller the temperature difference within the cross section of the mold (the temperature difference between the design surface and the interior), the weaker the thermal stress acting on the design surface. Thermal stress becomes particularly problematic when heating due to contact with molten metal and when cooling due to the application of a mold release agent. Reducing the temperature difference within the cross section is effective in reducing thermal stress at these times. To do this, it is necessary to speed up the transfer of heat, in other words, to increase thermal conductivity.

[0091] [4.5. Specific methods for suppressing crack propagation due to mechanical stress] 4.5.1. Improving the strength of steel To prevent cracks from growing, it is necessary to prevent thermal deformation of the mold. To do this, the strength of the steel material must be increased. If mechanical deformation caused by casting (deformation due to mold clamping, injection, pressure load, etc.) is reduced, stress will also be reduced. Increasing the strength of the mold can prevent plastic deformation caused by stress and the growth of fatigue cracks.

[0092] [4.5.2. Reduction of temperature distribution] The design surface of a die-casting mold is heated during casting, creating a temperature difference between the design surface and the back side. As a result, the mold warps, with the hot design surface rising. This warping is corrected by clamping and injection, and mechanical stress acts on the mold. Reducing the temperature difference between the design surface and the back side is effective in reducing warping. To do this, the thermal conductivity of the mold needs to be high.

[0093] [4.6. Specific methods for suppressing crack propagation due to thermal stress] In die casting, the design surface is repeatedly exposed to molten metal and then coated with a mold release agent. As a result, large temperature fluctuations occur in the mold during use. These temperature fluctuations also cause cracks to grow. To suppress the growth of cracks caused by temperature fluctuations, the thermal conductivity of the mold must be increased.

[0094] [4.7. Specific methods for suppressing crack propagation by improving toughness] [4.7.1. Improving the toughness of the matrix itself: Table 3(e)] When elements dissolve in iron or steel, toughness often decreases. Therefore, toughness can be ensured by reducing the amount of solute elements. On the other hand, solute elements also have the effect of increasing toughness by refining the quenched structure. Therefore, it is preferable to optimize the content of solute elements, taking into account the balance between improving the steel matrix itself and refining the structure, thereby improving toughness.

[0095] [4.7.2. Refinement of Structure: Table 3(f)(g)] An effective way to increase toughness is to transform the fine-grained austenite phase into martensite. In other words, it is important to refine the structure and to quench it. To refine the austenite grains during quenching, two conditions must be met: the SA structure before quenching must be fine-grained, and there must be many dispersed particles during quenching.

[0096] Figure 3 shows the SA structure before quenching (left), the fully quenched and tempered structure (middle), and the incompletely quenched and tempered structure (right). In Figure 3, the upper part shows the SA structure and quenched and tempered structure of a steel material with a fine grain structure. The lower part shows the SA structure and quenched and tempered structure of a steel material with a coarse grain structure.

[0097] As shown in Figure 3, when the SA structure is fine and the quenching rate is high, a fine structure is obtained and toughness is increased. Even if incomplete quenching occurs due to a slow quenching rate, if the SA structure is fine, the bainite will also be fine. On the other hand, when the SA structure is coarse, martensite becomes slightly coarse even when the quenching rate is high. Furthermore, when the SA structure is coarse and incomplete quenching occurs at a low quenching rate, bainite becomes significantly coarse.

[0098] Figure 4 shows the quenched structure of steel with many dispersed particles (top image) and the quenched structure of steel with few dispersed particles (bottom image). In Figure 4, the right image is a low-magnification photograph, and the left image is a high-magnification photograph. Figure 4 shows that steel with many dispersed particles has a fine quenched structure, while steel with few dispersed particles has a coarse quenched structure. From the above, in order to improve toughness, it is necessary to refine the SA structure by combining the components and heat treatment conditions, and to disperse an appropriate amount of dispersed particles during quenching.

[0099] [4.7.2. Reduction of retained austenite in the as-quenched state: Table 3(h)] If austenite remains after quenching, it will transform into bainite during tempering. Bainite reduces toughness. Therefore, to improve toughness, the less retained austenite there is in the quenched state, the better. C, Mn, Cr, Mo, Cu, and Ni all affect the amount of retained austenite, so optimizing the content of these elements can reduce the amount of retained austenite in the quenched state.

[0100] 4.8. Effect of heat treatment conditions 4.8.1. Effect of transformation temperature When the critical cooling rate for transformation into a single martensite phase (i.e., hardenability) is the same, the steel that transforms at a lower temperature has a finer structure.

[0101] Figure 5 shows the relationship between the transformation point and the microstructure. In Figure 5, Steel A and Steel B have the same critical cooling rate X1 for achieving a single martensite phase. However, Steel A has a high transformation temperature Ms, while Steel B has a low transformation temperature Ms. When such Steel A and Steel B are cooled at the critical cooling rate X1, Steel A develops a coarse microstructure, while Steel B develops a fine microstructure. The reason Steel A develops a coarse microstructure is thought to be that the bainite formed during the cooling process is easily coarsened due to the high temperature at which it transforms. Therefore, in order to improve the toughness of steel, it is preferable to lower the transformation temperature Ms. In particular, the amount of C has a large effect on the transformation temperature Ms. Increasing the amount of C is effective in lowering the transformation temperature Ms.

[0102] 4.8.2. Effect of Critical Cooling Rate Similarly, when the critical cooling rate for transformation into a martensite single phase (i.e., hardenability) is different, the steel that transforms at a lower temperature will have a finer structure.

[0103] The relationship between hardenability and structure is shown in Figure 6. In Figure 6, Steel C is a steel material with low hardenability (high critical cooling rate X1) and low transformation temperature Ms. On the other hand, Steel D is a steel material with high hardenability (low critical cooling rate X3) and high transformation temperature Ms. When steel D is cooled at a cooling rate of X2, the entire surface of steel D becomes martensite, but the structure becomes coarse. On the other hand, when steel C is cooled at a cooling rate of X2, steel C forms bainite, but the structure does not become coarse because the transformation temperature is low. Therefore, in order to improve the toughness of the steel material, it is preferable to lower the transformation temperature Ms. [Example]

[0104] (Examples 1 to 13, Comparative Examples 1 to 6) 1. Sample Preparation 1.1. Preparation of square bars A small-section steel material was manufactured using a small-sized (150 kg) ingot. Test pieces made from this steel were subjected to heat treatment simulating industrial manufacturing methods (large mold material and manufacturing methods for large molds). This allows for an accurate evaluation of the properties of the steel material when it is made into a mold manufactured using industrial methods.

[0105] Nineteen types of steel with chemical compositions shown in Table 4 were evaluated. Comparative Example 1 corresponds to SKD61. Comparative Examples 2 and 3 are commercially available hot work die steels. Comparative Examples 4 to 6 are steels similar to those of Examples 1 to 13, but with an increased Mn content and / or a decreased Cr content.

[0106] The 19 steel types shown in Table 4 were each cast into a 150 kg ingot. The ingots were then subjected to homogenization heat treatment, hot working, intermediate heat treatment, and spheroidizing annealing to obtain two square bars. The square bars measured 58 mm thick, 86 mm wide, and 1500 mm long. In order to make the solidification rate closer to that of large industrial ingots, the small 150 kg ingot was kept warm or heated. The detailed conditions for each process are as follows:

[0107] (a) Homogenization heat treatment: 1200℃×12H (b) Hot working: The ingots taken out of the homogenizing heat treatment furnace were worked while still hot, and finished into square bars measuring 58 mm thick and 86 mm wide. (c) Intermediate heat treatment: Normalizing at 1020°C and tempering at 680°C. Either or both of the normalizing and tempering may be omitted. (d) Spheroidizing annealing: In an atmospheric furnace, the sample was cooled from 900°C to 650°C at a rate of 15°C / H. After that, the sample was taken out of the furnace and allowed to cool naturally.

[0108] [Table 4]

[0109] 1.2. Preparation of rough-machined test pieces The following roughly machined test pieces were prepared from the square bars. (a) Corrosion resistance: 12.5mm x 40.5mm x 25.5mm (b) High temperature strength: φ18mm×120mm (c) Softening resistance: 12mm×12mm×20mm (d) Thermal conductivity: φ18mm×30mm (e) Toughness: 11mm x 11mm x 55mm (f) Water cooling hole crack and heat check (die cast): 200.5mm x 45.5mm x 40.1mm. A through hole with a diameter of 15mm x 200mm was installed as the water cooling hole at a depth of 5.1mm from the design surface (200.5mm x 45.5mm surface).

[0110] [1.3. Quenching and Tempering] The roughly machined test pieces were quenched and tempered in a vacuum to a hardness of 41.5 to 42.5 HRC. The quenching temperature was 970°C (recommended: 960 to 1000°C) for Examples 1 to 13 and Comparative Examples 4 to 6, and 1030°C (recommended: 1010 to 1050°C) for Comparative Examples 1 to 3. After holding at the quenching temperature for 2 hours, quench cooling was performed. There were two types of quench cooling processes: "Process A" and "Process B."

[0111] "Process A" is a quenching and cooling process that simulates the inside of a large mold. Cooling was performed from the quenching temperature to 500°C at 8°C / min, and from 500°C to 200°C at 1.5°C / min. Cooling was performed arbitrarily from 200°C to 100°C, and the material was removed from the vacuum furnace at 100°C. Cooling from 200°C to 100°C was not controlled because the cooling history in this temperature range does not have much effect on the steel properties.

[0112] "Process B" is a quenching and cooling process designed for the surface of a large mold. The material was cooled from the quenching temperature to 500°C at a rate of 30°C / min, and then from 500°C to 200°C at a rate of 5°C / min. The cooling from 200°C to 100°C was the same as "Process A."

[0113] The conditions for tempering to a hardness of 41.5 to 42.5HRC were selected based on the appropriate heating temperature, holding time, and number of treatments depending on the steel type. The heating temperature was 560 to 625°C, the holding time was 1 to 3 hours, and the number of treatments was 2 to 4 times. The roughly processed test pieces used to evaluate (b) high-temperature strength, (d) thermal conductivity, and (e) toughness were subjected to the process A. The roughly machined test pieces used to evaluate (a) corrosion resistance, (c) softening resistance, and (f) water-cooled cracking and heat check were subjected to the process B.

[0114] 1.4. Preparation of test specimens for evaluation The following evaluation test pieces were prepared from the above-mentioned roughly processed test pieces. (a) Corrosion resistance: 12mm x 40mm x 25mm, mirror polished (b) High temperature strength: M14mm×100mm, parallel part φ8 (c) Softening resistance: 12mm x 12mm x 20mm (roughly processed test piece shape) (d) Thermal conductivity: φ10mm×2mm (e) Toughness: 10mm x 10mm x 55mm, Charpy impact test specimen, V-notch (f) Water cooling hole cracking and heat check (die cast): 200mm x 45mm x 40mm. The water cooling holes were placed 5mm deep from the design surface (200mm x 45mm surface). The surface of the water cooling holes was left as rough. Depending on the steel type, elements were concentrated on the surface of the water cooling holes after quenching and tempering.

[0115] 2. Test Method 2.1. Corrosion Resistance Corrosion resistance was evaluated by a humidity test. The test pieces were exposed to an atmosphere of 50°C and 95% humidity for 48 hours. Corrosion resistance was evaluated by counting the number of rust spots with a diameter of 1 mm or more that appeared on a 40 mm x 25 mm surface. The details of the evaluation are as follows: S rating: Number of rusted areas ≦ 2 Grade A: 2 pieces < number of rusted parts ≦ 5 pieces B: 5 < number of rusted areas ≦ 8 C rating: 8 or fewer rust spots

[0116] [2.2. High temperature strength] Tensile tests were carried out at 500°C using test pieces. High-temperature strength was evaluated as the absolute value of tensile strength. Details of the evaluation are as follows. S rating: 975MPa<tensile strength Grade A: 970MPa<tensile strength≦975MPa ·B rating: 965MPa<tensile strength≦970MPa C rating: Tensile strength ≦ 965 MPa

[0117] [2.3. Softening resistance] The test pieces were held at 560°C for 79 hours, and then their hardness was measured at room temperature. Softening resistance was evaluated as the decrease in hardness from the initial hardness, i.e., the difference (ΔH=H1-H2) between the initial hardness (H1) and the hardness (H2) after holding at 560°C for 79 hours. The details of the evaluation are as follows: ·S judgment: ΔH<4.0HRC ·A judgment: 4.0HRC≦ΔH<4.5HRC ·B judgment: 4.5HRC≦ΔH<5.0HRC ·C judgment: 5.0HRC≦ΔH

[0118] 2.4. Thermal Conductivity Using the test pieces, the thermal conductivity was measured at room temperature by the xenon flash method. The thermal conductivity of each sample was evaluated as a ratio to that of Comparative Example 1 (SKD61). The details of the evaluation are as follows. ·S judgment: 1.20<ratio ·A judgment: 1.15<ratio≦1.20 ·B judgment: 1.05<ratio≦1.15 ·C judgment: ratio ≦1.05

[0119] 2.5. Toughness An impact test was carried out at room temperature using test pieces (with a V notch). The toughness was evaluated as the average value of the absorbed energy of 10 test pieces. The details of the evaluation are as follows. S rating: 14J or less average absorbed energy A rating: 12J≦average absorbed energy<14J B rating: 10J≦average absorbed energy<12J C rating: Average absorbed energy <10J

[0120] 2.6. Water Cooling Hole Cracks and Heat Checks A recess measuring 200 mm x 45 mm x 40 mm was made in part of a die-casting mold roughly measuring 200 mm x 205 mm x 55 mm. The above-mentioned test piece (200 mm x 45 mm x 40 mm in size, with a φ15 mm x 200 mm through-hole formed as a water-cooling hole at a depth of 5 mm from the design surface) was placed in the recess. In this state, 1000 shots were cast. After casting, the test piece was removed from the mold and cut at the center. The cut surface was evaluated for water cooling hole cracking and heat check.

[0121] Water cooling hole cracks were evaluated based on the distance (L1) between the tip of the crack that had developed from the water cooling hole and the design surface. The greater the distance (L1), the lower the risk of water leakage due to the crack penetrating. The details of the evaluation are as follows: ·S rating: 4mm <L1 ·A rating: 3mm <L1≦4mm ·B rating: 2mm <L1≦3mm ·C judgment: L1≦2mm

[0122] The heat check was evaluated by the depth (L2) of the crack that progressed from the design surface toward the water cooling hole. The shallower the depth (L2), the lower the risk of water leakage due to the progression of the crack. The details of the evaluation are as follows. ·S judgment: L2<0.3mm ·A rating: 0.3mm <L2≦0.8mm ·B judgment: 0.8mm <L2≦1.2mm ·C judgment: 1.2mm <L2

[0123] [3. Results] The results are shown in Table 5. From Table 5, the following can be seen.

[0124] 3.1. Corrosion Resistance In all of Comparative Examples 1 to 6, the corrosion resistance was rated C. The main reason for this is thought to be the low Cr content (Cr≦6.28 mass%). In contrast, none of Examples 1 to 13 were rated C for corrosion resistance, and all had corrosion resistance sufficient for use in die-casting dies without any problems.

[0125] The corrosion resistance of Examples 1 and 2 was rated B, probably because the Cr content was slightly low (Cr<7.0 mass%). In Example 7, although the Cr content was the lowest, the corrosion resistance was judged as A. This is presumably because the C content was slightly low and the Mo content was slightly high. Furthermore, in some examples, the corrosion resistance was judged as S. This is presumably because the Cr content and the Mo content were relatively high.

[0126] [3.2. High-temperature strength] Both Comparative Example 1 and Comparative Example 2 correspond to general-purpose steels with high Si and low Mo. In both Comparative Example 1 and 2, the high-temperature strength was judged as C. Both Comparative Example 3 and Comparative Example 6 correspond to high-performance steels with low Si and high Mo. In both Comparative Example 3 and Comparative Example 6, the high-temperature strength was judged as A. Comparative Example 4 is a steel with medium Cr and Mo contents, and the high-temperature strength was judged as B. Furthermore, Comparative Example 5 is a steel with low Cr as well as low Mo contents, and the high-temperature strength was judged as C. The decrease in high-temperature strength is presumably because as the Cr content decreases, the softening resistance improves, but the Cr dissolved in the matrix decreases.

[0127] In contrast, none of Examples 1 to 13 had a high-temperature strength judged as C. The high-temperature strength of some examples was judged as S. This is presumably because the Mo content was high (2.0 mass% < Mo). From the comparison between Example 1 and Example 2, it was found that when the Mo contents were equivalent, the lower Si resulted in higher strength.

[0128] [3.3. Softening resistance] In both Comparative Example 1 and 2, the softening resistance was judged as C. One of the reasons is presumably because the Mo content was low (Mo < 1.30 mass%). Another reason is presumably because the hardenability was low (Mn + Cr < 6.60 mass%). In Comparative Example 3, the softening resistance was judged as B. This is presumably because although the Mo content was high, the hardenability was not very high (Mn + Cr content = 6.59 mass%). In Comparative Example 5, although the Mo content was the lowest, the softening resistance was judged as A. This is presumably because the hardenability was high (Mn + Cr = 7.23 mass%) and the Cr content was low.

[0129] In both Comparative Example 4 and Comparative Example 6, the softening resistance was judged as S. This is presumably because in addition to the low Cr content and Mo content, the hardenability was high (7.0 mass% < Mn + Cr). In contrast, in all of Examples 1 to 13, the softening resistance was judged as S or A. Although some of the examples have a low Mo content, it is considered that the softening resistance is ensured due to the high hardenability (7.0 mass% < Mn + Cr).

[0130] [3.4. Thermal Conductivity] In Comparative Example 1, Comparative Example 4, and Comparative Example 5, the thermal conductivity was judged as C. This is presumably because the Si content and / or the Cr content was high. In Comparative Example 2, the thermal conductivity was judged as A. This is presumably because the Si content and / or the Cr content was lower than in Comparative Examples 1, 4, and 5. In Comparative Example 3 and Comparative Example 6, the thermal conductivity was judged as S. This is presumably because the Si content was 0.26 mass% and the Cr content was less than 6.2 mass%.

[0131] In contrast, none of Examples 1 to 13 had a thermal conductivity judged as C. Some of the examples had a thermal conductivity judged as S. This is presumably because the Si content and / or the Cr content was low. Also, some of the examples had a thermal conductivity judged as B. This is presumably because the Si content exceeded 0.30 mass%.

[0132] [3.5. Toughness] Comparative Examples 1 to 3 were rated C for toughness. This is thought to be due to poor hardenability (Mn+Cr<6.60 mass%). Looking at the breakdown of the 10 test pieces, Comparative Examples 1 and 2 included many specimens with absorbed energy of 8 J or less. Foreign matter was noticeable on the fracture surfaces of these low-toughness materials. The foreign matter is thought to be V-based carbides, V-based carbonitrides, or V-based nitrides that precipitated during solidification. The particularly low toughness of Comparative Examples 1 to 3 is thought to be due to the coarse structure that facilitates crack propagation, as well as the fact that these foreign matter served as the starting point for crack initiation. In Comparative Examples 4 to 6, the toughness was rated A or S. This is thought to be because the V content was low (V<0.30 mass%), resulting in little inclusions and high hardenability.

[0133] In contrast, none of Examples 1 to 13 was rated C for toughness. The toughness of Examples 5 and 7 was rated B. This is thought to be because the hardenability was not high among the Examples (Mn+Cr≦7.02 mass%). In some examples, the toughness was rated A. This is thought to be because the Mn+Cr content was moderate (7.03 to 7.17 mass%). Furthermore, the toughness of some examples was judged to be S. This is thought to be due to the large amount of Mn+Cr (Mn+Cr<7.17 mass%).

[0134] [3.6. Water cooling hole cracks] In all of Comparative Examples 1 to 6, the water cooling hole cracking was rated C. This is thought to be due to low corrosion resistance. It is also thought to be influenced by the low content of P, Cu, and Ni that form the barrier layer. In contrast, Examples 1 to 13 did not include any examples in which the water cooling hole cracking was rated C. In Examples 1 and 2, the water cooling hole cracking was rated B. This is thought to be due to the low contents of Cr, P, Cu, Ni, etc. On the other hand, in Examples 3 to 13, the water cooling hole cracking was rated as S or A. This is thought to be due to the large content of the elements that affect the corrosion resistance described above.

[0135] [3.7. Heat Check] In Comparative Examples 1 and 5, the heat check was rated C. This corresponds to the fact that both the thermal conductivity and the high-temperature strength were rated C. The heat check for Comparative Examples 2 and 4 was rated B. This is thought to be because either the high-temperature strength or the thermal conductivity was rated A or B, while the other was rated C. The heat check for Comparative Examples 3 and 6 was rated A. This is thought to be because the thermal conductivity was rated S and the high-temperature strength was rated A.

[0136] In contrast, none of Examples 1 to 13 received a heat check rating of C. Example 10 received a heat check rating of B. This is thought to be because both the thermal conductivity and high-temperature strength were rated B. All other examples received a heat check rating of A or S. This is thought to be because the thermal conductivity, high-temperature strength, and toughness were all high, and these properties were effectively combined.

[0137] [Table 5]

[0138] 4. Versatility Although the above verification was based on the assumption of a die-casting mold, the steel material according to the present invention can be applied to molds and parts used in various castings, not limited to die-casting. (a) Forging, in which the material is heated and processed; (b) Hot stamping (a method in which steel sheets are heated, formed, and tempered), (c) extrusion processing; (d) plastic and vinyl injection and blow molding; (e) Molding and processing of rubber and fiber-reinforced plastics It can be applied to molds and parts used in

[0139] In addition, during the verification, the characteristics were evaluated at 42HRC, but the hardness can be adjusted to a wide range of 32HRC to 52HRC depending on the application and applied to molds and parts. The steel material according to the present invention can be formed into a rod shape and used to manufacture molds and parts, or the steel material according to the present invention can be formed into a rod or wire shape and used for incremental shaping or repair by build-up welding (TIG or laser welding, etc.).

[0140] If the steel material according to the present invention is formed into a plate shape, it is possible to manufacture dies and parts by joining them. Of course, it is also possible to manufacture dies and parts by joining members made of the steel material according to the present invention. As described above, the steel material according to the present invention can be applied to members having various shapes. Furthermore, dies and parts can be manufactured or repaired by various methods using materials of various shapes made of the steel material according to the present invention.

[0141] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0142] The steel material according to the present invention can be used as a mold or a part thereof for various manufacturing methods such as casting, forging, hot stamping, extrusion, injection molding, and blow molding.

Claims

1. 0.32≦C≦0.42mass%, 0.25≦Si≦0.40 mass%, 0.10≦Mn<0.60 mass%, 6.60<Cr≦7.50mass%, 1.10≦Mo≦2.90mass%, 0.18≦V≦0.30mass%, 0.001≦Al≦0.050 mass%, 0.001≦N≦0.050 mass%, and 0.001≦P≦0.040 mass%, and the balance being Fe and unavoidable impurities.

2. 0.20<Cu≦0.80 mass%, and 0.20<Ni≦1.60mass% further comprising 0.45≦Cu+Ni≦2.00mass% The steel material according to claim 1, which satisfies the above.

3. 0.5Cu≦Ni The steel material according to claim 2, further satisfying the following:

4. The steel material according to claim 1, further comprising one or more groups selected from the group consisting of the following groups A to D: Group A: 0.30<W≦2.00 mass%, and / or 0.30<Co≦1.00 mass%. Group B: 0.004<Nb≦0.100mass%, 0.004<Ta≦0.100mass%, 0.004<Ti≦0.100mass%, and 0.004<Zr≦0.100mass% At least one element selected from the group consisting of: Group C: 0.0002<B≦0.0080 mass%. Group D: 0.006<S≦0.180mass%, 0.0005<Ca≦0.0500mass%, 0.03<Se≦0.50mass%, 0.005<Te≦0.100mass%, 0.01<Bi≦0.50 mass%, and 0.03<Pb≦0.50mass% At least one element selected from the group consisting of:

5. A mold made of the steel material according to claim 1.

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