Die steel and molds

Abstract

To provide a die steel that has both of a high hardness and a high shock value, and shows high resistance to thermal shock, and to provide a die.SOLUTION: A die steel comprises, in mass%, 0.35%≤C≤0.55%, 0.05%≤Si≤0.40%, 1.50%≤Mn≤2.50%, 7.5%≤Cr≤9.0%, 0.90%≤Mo≤2.5%, 0.40%≤V≤0.80%, with the balance being Fe and inevitable impurities. A die is constituted by using such a die steel.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a die steel and a die, and more particularly to a die steel that can be used to construct a die such as a die casting die, and to such a die. [Background technology]

[0002] Many molds are subjected to thermal shock due to repeated use during molding while in contact with heated materials. Repeated thermal shock can lead to damage such as heat checking, shortening the mold's lifespan. In particular, molds used in die-casting processes are prone to heat checking due to thermal fatigue due to repeated heating from the injection of molten metal and cooling from the spraying of mold release agents. Heat checking can grow into large cracks that can transfer to the product and affect product quality. When cracks that could transfer to the product occur in a mold, the mold must be repaired by welding or re-polishing. Alternatively, if the damage to the mold is too great to repair by these methods, the mold must be replaced. Repairing or replacing a mold requires significant effort and cost.

[0003] From the viewpoint of reducing the frequency of repair or replacement of dies due to damage caused by thermal shock and enabling the dies to be used for a long period of time, die steels constituting dies are required to have high impact values ​​(impact resistance, toughness).Patent Documents 1 to 4 each specify the component compositions of alloys in die steels (hot work tool steels) from the viewpoint of increasing the impact values. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-87322 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-224418 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-001572 [Patent Document 4] Japanese Patent Application Laid-Open No. 2002-88443 Summary of the Invention [Problem to be solved by the invention]

[0005] In general, increasing the hardness of a steel material tends to lower its impact value. Therefore, it is difficult to achieve both high hardness and high impact value in die steel, and even in the above-mentioned Patent Documents 1 to 4, the hardness of the die steel remains at 50 HRC or less. However, in order to effectively prevent damage to the die due to thermal shock, it is desirable for the die steel to have high strength in addition to a high impact value. For example, in a die-casting die, it is desirable for the die steel to have a hardness of 55 HRC or more after tempering in order to prevent heat checking.

[0006] The problem to be solved by the present invention is to provide a die steel that has both high hardness and high impact value and exhibits high resistance to thermal shock, and a die. [Means for solving the problem]

[0007] In order to solve the above problems, the mold steel according to the present invention contains, in mass%, 0.35%≦C≦0.55%, 0.05%≦Si≦0.40%, 1.50%≦Mn≦2.50%, 7.5%≦Cr≦9.0%, 0.90%≦Mo≦2.5%, 0.40%≦V≦0.80%, and the balance being Fe and unavoidable impurities.

[0008] Here, it is preferable that the mold steel further contains, by mass %, Ni≦1.1%.

[0009] Furthermore, it is preferable that the value of A obtained by the following formula (1) is A≧22.0. A = Si + Mn + 2Cr + 3Mo + 3.5V (1) In formula (1), each element symbol indicates the content of each element in units of mass %.

[0010] The mold steel has a hardness of 55HRC or more and a Charpy impact value of 20J·cm after being tempered at less than 510°C. -2 It is preferable that the residual γ content after quenching is 15% or more and 20% or less. Furthermore, it is preferable that the decomposition temperature of the residual γ phase is 510°C or more. It is preferable that the mold steel has a martensitic transformation starting point of 190°C or more and 220°C or less. It is preferable that the mold steel has a surface hardness increased by 50HV or more by shot peening after tempering at less than 510°C.

[0011] The mold according to the present invention is made of the above-mentioned mold steel. Here, the mold is preferably a die casting mold. [Effects of the Invention]

[0012] The die steel according to the present invention has the above-described composition, and thus can achieve both high hardness and a high impact value. In die steel, a certain amount of residual γ after quenching can improve the impact value, but if the amount of residual γ is too high, the hardness decreases. However, by having the above-described composition, the amount of residual γ can be controlled within an appropriate range. The die steel achieves both high hardness and a high impact value, which suppresses damage caused by thermal shock, such as heat checking, and contributes to improving the durability of the die.

[0013] Here, when the die steel further contains Ni≦1.1% by mass, the hardenability of the die steel can be improved while ensuring high hardness.

[0014] Furthermore, when the value of A obtained by the above formula (1) is A≧22.0, the decomposition temperature of the residual γ phase is 510°C or higher. As a result, in the mold steel, the amount of residual γ is maintained within an appropriate range even after use of the mold, which involves heating, and a state in which both high hardness and high impact value are achieved is maintained.

[0015] In mold steel, after being tempered at less than 510°C, the hardness is 55HRC or more and the Charpy impact value is 20J·cm-2 In the above cases, if the mold steel has both sufficiently high hardness and a high impact value, the occurrence of damage caused by thermal shock such as heat checking can be effectively suppressed.

[0016] Furthermore, when the amount of retained γ is 15% or more and 20% or less after quenching, it is possible to highly effectively achieve both an improvement in impact value due to the sufficient amount of retained γ phase contained in the mold steel and suppression of a decrease in hardness due to an excessive increase in the amount of retained γ.

[0017] Furthermore, if the decomposition temperature of the residual gamma phase is 510°C or higher, the decomposition of the residual gamma phase is suppressed in the general die-casting mold usage environment, making it easier to maintain a state in which both high hardness and high impact value are achieved even after the mold has been used.

[0018] When the martensitic transformation starting point of the mold steel is 190°C or higher and 220°C or lower, the amount of residual γ after quenching tends to fall within the range of 15% or higher and 20% or lower, and the mold steel tends to have both high hardness and high impact value.

[0019] When the mold steel is one in which the hardness of the surface layer can be increased by 50 HV or more by shot peening after tempering at a temperature below 510°C, the surface hardness of the mold steel can be efficiently increased by stress-induced martensitic transformation due to shot peening. The mold steel of the present invention has a relatively large amount of retained γ due to adjustment of its components, and is therefore likely to undergo an increase in hardness due to stress-induced martensitic transformation.

[0020] The mold according to the present invention is made of the above-mentioned mold steel, and thus has both high strength and high impact value. As a result, even if it is subjected to repeated thermal shocks, the occurrence of damage such as heat checking is suppressed, and the mold can be used for a long period of time. The increased durability of the mold can reduce the labor and cost required for repairing or replacing the mold.

[0021] Here, when the mold is a die-casting mold, it is used in a situation where it is repeatedly subjected to contact with high-temperature molten metal and cooling by a mold release agent, but it is a highly durable mold in which the occurrence of heat checking due to thermal fatigue and the formation of large cracks due to the growth of heat checking are unlikely to occur. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram showing the relationship between the amount of retained γ and tempered hardness. [Figure 2] FIG. 1 is a diagram showing the relationship between the martensitic transformation start point (Ms point) and the amount of retained γ. [Figure 3] 1 is a graph showing the relationship between the tempering temperature and the amount of retained γ for five types of die steel. [Figure 4] 1 is a graph showing the relationship between the A value and the residual γ decomposition temperature. The residual γ decomposition temperature at each data point was obtained from the results in FIG. 3, and the sample numbers assigned to the data points correspond to those in FIG. 3. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, the die steel and die according to the embodiment of the present invention will be described in detail. The die according to the embodiment of the present invention can be constructed using the die steel according to the embodiment of the present invention. The type of die is not particularly limited, but since the die steel has high hardness and a high impact value, it is suitable for use as a die that comes into contact with heated materials, particularly a die for die casting.

[0024] [Component composition] First, the chemical composition of a mold steel according to one embodiment of the present invention will be described. The mold steel according to one embodiment of the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their component ratios, and the reasons for their limitations are as follows. The unit of the component ratios is % by mass.

[0025] 0.35%≦C≦0.55% Carbon has a significant effect on the hardness of mold steel during quenching and tempering. Carbon dissolves in the matrix during quenching and forms a martensite structure, thereby improving the hardness of the mold steel. Carbon also improves the hardness of mold steel by forming carbides together with Cr, Mo, V, etc.

[0026] By setting the C content to 0.35%≦C, the amount of C dissolved and the amount of carbide formed are ensured, and high hardness is obtained. In the mold steel according to this embodiment, from the viewpoint of obtaining sufficient heat check resistance, it is desirable to have a tempered hardness of 55 HRC or more. By setting the C content to 0.35%≦C, it becomes easier to achieve a high tempered hardness of 55 HRC or more. Preferably, 0.37%≦C. More preferably, 0.40%≦C.

[0027] On the other hand, if the C content is excessive, the amount of carbides formed increases, resulting in a decrease in the machinability of the die steel. Furthermore, the martensitic transformation start temperature (Ms) decreases, and the amount of retained γ increases, which in turn reduces the hardness of the die steel. By setting C≦0.55%, it is possible to suppress the decrease in machinability and to easily keep the amount of retained γ within a range of 20% or less. Preferably, C≦0.50%. More preferably, C≦0.45%.

[0028] 0.05%≦Si≦0.40% Si acts as a deoxidizer and improves machinability during mold manufacturing. Adding a small amount of Si also increases the hardness of mold steel. These effects can be fully achieved by setting the Si content at 0.05% or less. The Si content is preferably 0.25% or less. The Si content is more preferably 0.30% or less.

[0029] On the other hand, if the Si content is excessive, the thermal conductivity of the mold steel decreases. Furthermore, the amount of coarse crystallized carbides increases, which may result in a decrease in impact value. Therefore, from the viewpoint of ensuring high thermal conductivity and suppressing the formation of coarse crystallized carbides, the Si content is set to 0.40% or less. Preferably, Si is set to 0.35% or less. More preferably, Si is set to 0.32% or less.

[0030] 1.50%≦Mn≦2.50% Mn has the effect of increasing the hardenability of die steel, that is, the effect of delaying carbide precipitation. Furthermore, Mn lowers the Ms point and increases the amount of residual γ in die steel, thereby increasing the impact value. From the viewpoint of obtaining high hardenability and a sufficiently large amount of residual γ, the Mn content is set to 1.50%≦Mn. Preferably, it is 1.90%≦Mn. More preferably, it is 2.00%≦Mn.

[0031] On the other hand, Mn generates MnS in mold steel. MnS reduces the impact value of mold steel. Therefore, from the viewpoint of ensuring a high impact value, Mn≦2.50% is set. Preferably, Mn≦2.30%. More preferably, Mn≦2.10%. If 1.50%≦Mn≦2.50%, the mold steel will have an excellent balance between the amount of retained γ and the impact value, and will have a retained γ content of 15 to 20% and a strength of 20 J / cm. 2 It becomes easier to achieve both of the above impact values ​​(Charpy impact values).

[0032] 7.5%≦Cr≦9.0% Like Mn, Cr has the effect of improving the hardenability of mold steel. Cr also contributes to secondary hardening when tempering is performed at a temperature of about 400 to 500°C. Cr also lowers the Ms point, increases the amount of residual γ, and stabilizes the residual γ phase. From the viewpoint of obtaining high hardness in mold steel when tempered at less than 510°C and ensuring a sufficient amount of residual γ to increase the impact value, the Cr content is set to 7.5%≦Cr. It is preferably 7.9%≦Cr. It is more preferably 8.0%≦Cr.

[0033] On the other hand, if Cr is contained in excess in the mold steel, the amount of crystallized carbides increases and the impact value decreases. Therefore, from the viewpoint of maintaining a high impact value, Cr≦9.0% is set. Preferably, Cr≦8.5%. More preferably, Cr≦8.1%. If 7.5%≦Cr≦9.0%, the mold steel will have an excellent balance between the amount of retained γ and the impact value, and the amount of retained γ will be 15 to 20% and the impact value will be 20 J / cm. 2 It becomes easier to achieve both of the above impact values.

[0034] 0.90%≦Mo≦2.5% Mo has the effect of improving the hardenability of mold steel. Mo also contributes to secondary hardening when tempering is performed at temperatures of about 500 to 600°C. Furthermore, like Cr, Mo also lowers the Ms point, increases the amount of residual γ, and stabilizes the residual γ phase. From the viewpoint of obtaining high hardness in mold steel when tempered at about 500°C and ensuring a sufficient amount of residual γ to increase the impact value, the Mo content is set to 0.90%≦Mo. Preferably, it is 1.00%≦Mo. More preferably, it is 1.20%≦Mo.

[0035] On the other hand, if Mo is contained in excess in the mold steel, the amount of crystallized carbides increases and the impact value decreases. Therefore, from the viewpoint of maintaining a high impact value, the Mo content is set to Mo≦2.5%. Preferably, Mo≦2.0%. More preferably, Mo≦1.5%. If 0.90%≦Mo≦2.5%, the mold steel will have an excellent balance between the amount of retained γ and the impact value, and the amount of retained γ will be 15 to 20% and the impact value will be 20 J / cm. 2 It becomes easier to achieve both of the above impact values.

[0036] 0.40%≦V≦0.80% V forms pinning particles that suppress the coarsening of crystal grains during quenching. As a result of suppressing the coarsening of crystal grains, a decrease in the impact value of the mold steel is suppressed. By setting the V content to 0.40% or less, the coarsening of crystal grains during quenching is effectively suppressed, and the impact value is increased. Preferably, the V content is 0.45% or less. More preferably, the V content is 0.50% or less.

[0037] On the other hand, if the V content is too high, a large amount of coarse crystallized carbides will be formed. The coarse crystallized carbides will reduce the impact value of the mold steel. Therefore, from the viewpoint of suppressing the formation of coarse crystallized carbides and maintaining a high impact value, V≦0.80% is set. Preferably, V≦0.70%. More preferably, V≦0.60%. If the V content is 0.40%≦V≦0.80%, the impact strength of 20 J / cm 2 This makes it easier to achieve impact values ​​above this.

[0038] The mold steel according to this embodiment may optionally contain Ni in addition to the above-mentioned essential elements. The Ni content and the reasons for limiting it are as follows.

[0039] Ni≦1.1% Ni has the effect of improving hardenability by stabilizing the gamma phase in mold steel and delaying the formation of pearlite. Even if only a small amount of Ni is added, sufficient hardenability can be ensured by other elements, so there is no particular lower limit for the Ni content. However, to obtain a significant effect from the addition of Ni, it is sufficient to keep the Ni content at 0.10% or less.

[0040] On the other hand, if too much Ni is added, the amount of residual γ at the time of quenching becomes excessive, resulting in a decrease in tempered hardness. Furthermore, the stabilization of the residual γ phase results in an excessive amount of residual γ after tempering, resulting in a decrease in tempered hardness. To prevent these phenomena, the Ni content is limited to Ni≦1.1%. Preferably, Ni≦0.50%. More preferably, Ni≦0.20%.

[0041] The mold steel according to this embodiment contains the above-mentioned predetermined amounts of C, Si, Mn, Cr, Mo, and V, and optionally Ni, with the balance being Fe and unavoidable impurities. The unavoidable impurities are assumed to include the following elements: Cu≦0.25%, Al≦0.030%, N≦0.0250%, O≦0.0030%, P≦0.030%, S≦0.020%, etc.

[0042] Of the above unavoidable impurities, N and O are unavoidable because they dissolve in Fe from the air during industrial mold steel production. However, if the amount of N and O mixed in is too high, voids tend to form in the structure. They also form oxides and nitrides in the alloy. In particular, coarse oxides and nitrides can accelerate abrasive wear of molds and reduce impact strength. To prevent these phenomena, it is preferable to keep the N and O contents below the upper limits mentioned above.

[0043] In the mold steel according to this embodiment, it is preferable that the contents of the essential elements C, Si, Mn, Cr, Mo, and V each satisfy the above-mentioned predetermined range, and in addition, the A value defined by the following formula (1) based on the contents of these elements is sufficiently large. A = Si + Mn + 2Cr + 3Mo + 3.5V (1) In formula (1), each element symbol indicates the content of each element in units of mass %.

[0044] Specifically, it is preferable that A≧22.0. As will be shown in the Examples below, according to the findings of the inventors, the A value has a high correlation with the residual γ decomposition temperature (the decomposition temperature of the residual γ phase), and the larger the A value, the higher the residual γ decomposition temperature tends to be (see FIG. 4). When the decomposition temperature of the residual γ phase is high, even if the mold steel is heated by contact with a high-temperature material during use of the mold, the decomposition of the residual γ phase is less likely to occur, making it easier to maintain a predetermined amount of residual γ. When A≧22.0, it is easy to increase the residual γ decomposition temperature to 510°C or higher. It is more preferable that A≧23.0 or A≧25.0.

[0045] [Characteristics of mold steel] The properties of the mold steel according to this embodiment will be described below. In this specification, unless otherwise specified, the various properties are values ​​evaluated at room temperature (approximately 25°C). In addition, in this specification, properties specified for a tempered state refer to hardness and impact value after quenching and tempering at a temperature of less than 510°C. For example, quenching may be performed by soaking at 1030°C followed by gas cooling equivalent to oil quenching. In addition, tempering may be performed by subsequently tempering at 500°C.

[0046] The die steel according to this embodiment has the above-described composition, and thus has both high hardness and a high impact value (high impact resistance and toughness). A high impact value of the die steel increases its resistance to thermal shock, making it less susceptible to damage even when the die is repeatedly heated and cooled. In particular, in die casting dies, repeated heating due to the injection of molten metal and cooling due to the spraying of a mold release agent generates thermal stress, making the design surface prone to heat checking due to thermal fatigue. However, the die steel's high impact value can suppress the occurrence of heat checking. Furthermore, the die steel's high hardness can effectively suppress damage to the die steel due to the application of thermal shock, such as the development of large cracks due to the growth of heat checking. In other words, the die steel's high hardness and high impact value can strongly suppress the occurrence of damage due to thermal shock, such as heat checking.

[0047] In many mold steels, as hardness increases, the impact value tends to decrease, making it difficult to achieve both high hardness and high impact value. However, the mold steel according to this embodiment, having the above-described composition, achieves both high hardness and high impact value, thereby suppressing damage caused by thermal shock, such as heat checking and cracking, and improving the durability of the mold. Since the mold is less likely to be damaged even when repeatedly heated and cooled, the mold can be used for a long period of time without repair or replacement, thereby reducing the labor and cost required for mold repair and replacement.

[0048] The mold steel according to this embodiment has the above-described composition, and therefore has a hardness of 55 HRC or more and a compressive strength of 20 J / cm after being tempered at a temperature of less than 510°C. 2 More preferably, the hardness is 56HRC or more and the impact value is 30J / cm. 2 Although there are no particular upper limits for the hardness and impact value of mold steel, from the viewpoint of the balance between hardness and impact value, the hardness should be approximately 60HRC or less and the impact value should be 40J / cm or less. 2 It is best to leave it as follows:

[0049] The fact that the die steel according to this embodiment achieves both high hardness and high impact value can be explained by the fact that the amount of retained γ is controlled within an appropriate range. If the amount of retained γ is large, the impact value of the die steel improves, while if the amount of retained γ is excessive, the tempered hardness decreases significantly. Therefore, by controlling the amount of retained γ within an appropriate range, it is possible to achieve both high hardness and high impact value. In the die steel according to this embodiment, the amount of retained γ falls within the range of 15% or more and 20% or less, in accordance with the above-mentioned composition. The amount of retained γ being within this range corresponds to the tempered hardness of 55 HRC or more and the impact strength of 20 J / cm 2 The above impact values ​​can be achieved at the same time. The amount of retained γ is more preferably 16% or more and 19% or less. In this specification, the amount of retained γ is evaluated as the volume fraction of the retained γ phase in the mold steel in the state after quenching (as quenched).

[0050] The amount of retained γ correlates with the martensitic transformation start point (Ms point), and the higher the Ms point, the smaller the amount of retained γ (see FIG. 2). The Ms point correlates with the component composition of the mold steel, and tends to decrease as the amount of added elements increases. The mold steel according to this embodiment has an Ms point of 190°C or higher and 220°C, corresponding to the above-mentioned composition. Corresponding to the Ms point being in this range, the amount of retained γ falls within the range of 15% to 20%, which provides high hardness and high impact strength as described above. It is more preferable that the Ms point be 200°C or higher and 210°C or lower. The Ms point can be estimated from the component composition using the following formula (2). Ms[K]=812-423C-7.5Si-30.4Mn-17.7Ni-12.1Cr-7.5Mo (2) In formula (2), each element symbol indicates the content of each element in units of mass %. The above formula (2) is obtained by adding a term for Si to Andrews' formula (KW Andrews, J. Iron Steel Inst., 203 (1965), 721).

[0051] Furthermore, the mold steel according to this embodiment preferably has a residual γ decomposition temperature (the temperature at which the residual γ phase decomposes) of 510°C or higher. If the residual γ decomposition temperature is sufficiently high, the residual γ phase is less likely to decompose even when the mold steel is heated by contact with the high-temperature material to be molded during use of the mold. This makes it easier to maintain the residual γ content within a suitable range that achieves both high hardness and high impact strength. In a typical die casting process, the maximum temperature reached by the mold is approximately 500°C. If the residual γ decomposition temperature is 510°C or higher, the decomposition of the residual γ phase in the die casting mold can be sufficiently suppressed. As described above, the residual γ content exhibits a high correlation with the A value defined by formula (1) based on the component composition. If A≧22.0, the residual γ decomposition temperature can be set to 510°C or higher. A residual γ decomposition temperature of 515°C or higher is even more preferable. While there is no particular upper limit to the residual γ decomposition temperature, the residual γ decomposition temperature in practical mold steels is generally 560°C or lower. The residual γ decomposition temperature may be evaluated as the tempering temperature at which the amount of residual γ becomes 90% or less of the quenched product when tempering is performed after quenching.

[0052] As described above, the mold steel according to this embodiment has an Ms point and a residual γ content within an appropriate range due to the effect of the component composition, and accordingly achieves both high hardness and high impact resistance. For example, the mold steel has a hardness of 55 HRC or more and a strength of 20 J / cm 2 The quenching and tempering conditions may be set so as to obtain the above impact values. A suitable example of the conditions is to soak a steel material that has been melted, cast, appropriately forged, normalized, and annealed at 1000 to 1050°C for about 30 minutes, then quench it by cooling it to 40°C or less at a cooling rate of 20°C / min or more, such as cooling in an oil bath, and then temper it at 490 to 510°C. Furthermore, it is preferable to subject the heat-treated mold steel to shot peening to improve the surface hardness.

[0053] The die steel according to this embodiment has high hardness due to the effect of its chemical composition, and shot peening can further increase the hardness of the surface layer. By imparting strain through shot peening, stress-induced martensitic transformation of the γ phase occurs, thereby increasing the hardness of the die steel. However, the die steel according to this embodiment has a relatively large amount of retained γ due to the effect of its chemical composition, and shot peening effectively improves hardness. For example, in die steel tempered at less than 510°C, the hardening amount of the surface layer due to shot peening is preferably 50 HV or more. In other words, the shot peening treatment should increase the hardness of the surface layer by 50 HV or more. The hardening amount of the surface layer due to shot peening is preferably 70 HV or more, and even more preferably 80 HV or more. For example, after soaking at 1030°C, the die steel may be quenched by gas cooling equivalent to oil quenching, and then tempered at 500°C. The Vickers hardness at a depth of 20 μm from the surface of the mold steel is taken as the hardness of the surface layer, and the increase in hardness after shot peening is taken as the hardening amount. As the conditions for shot peening, the same conditions as those in the examples shown in Table 1 below can be suitably adopted. [Example]

[0054] The present invention will be described in more detail below using examples.

[0055] [Sample preparation] Die steels having various component compositions (unit: mass%) were prepared. Specifically, steels having predetermined composition ratios were melted in a vacuum induction furnace, and then ingots were cast. The obtained ingots were heated at 1250°C for 10 hours, and then hot forged to a diameter of 80 mm. After forging, the ingots were normalized by heating to 1020°C, cooling, and tempering at 680°C for 6 hours. Further, the ingots were annealed by heating to 900°C for 2 hours and cooling to 660°C at a cooling rate of 15°C / h to produce annealed materials.

[0056] [Test method] The properties of each sample were evaluated by the following methods. Unless otherwise specified, the evaluations were carried out at room temperature in the atmosphere.

[0057] <Residual γ amount> Test pieces measuring 12B x 20 mm were cut from the annealed material prepared above and subjected to heat treatment. The heat treatment involved soaking at 1030°C for 1 hour in a vacuum furnace, followed by quenching with gas cooling equivalent to oil quenching. The surface of the quenched specimen to measure the residual γ was polished to a grain size of #1000, and the amount of residual γ was measured using an X-ray stress measurement device.

[0058] <Residual γ decomposition temperature> Test pieces measuring 12B x 20 mm were cut from the annealed material prepared above and subjected to heat treatment. The heat treatment consisted of soaking at 1,030°C in a vacuum furnace for one hour, followed by quenching using gas cooling equivalent to oil quenching. Tempering was then performed at 10°C increments from 460°C to 570°C, producing 12 quenched and tempered specimens for each sample. The surfaces of the quenched and tempered specimens for measuring residual γ were polished to a grain size of #1000, and the amount of residual γ was measured using an X-ray stress analyzer. The tempering temperature at which the amount of residual γ was 90% of the quenched specimen was taken as the residual γ decomposition temperature. To estimate this residual γ decomposition temperature, interpolation was performed for tempering temperatures in 10°C increments.

[0059] <Tempered hardness> Test pieces measuring 12B x 20 mm were cut from the annealed material prepared above and subjected to heat treatment. The heat treatment involved soaking at 1,030°C for one hour in a vacuum furnace, followed by quenching with gas cooling equivalent to oil quenching. The test pieces were then tempered twice for one hour at 500°C, a temperature below the residual γ decomposition temperature. The measurement surface and the contact surface of the tempered pieces were polished to a grain size of #400, and their hardness was measured using the Rockwell C scale.

[0060] <Impact Value> Test pieces measuring 10 mm x 10 mm x 55 mm were cut from the annealed material prepared above and subjected to heat treatment. The heat treatment involved soaking at 1030°C for one hour in a vacuum furnace, followed by quenching with gas cooling equivalent to oil quenching. The test pieces were then tempered twice for one hour at 500°C, a temperature below the residual γ decomposition temperature. JIS No. 3 impact test pieces (2 mm U-notch) were then cut and subjected to Charpy impact testing in accordance with JIS Z 2242. The Charpy impact values ​​were obtained from these tests.

[0061] <Amount of hardening by shot peening> A 30B x 20 mm sample was cut from the annealed material prepared above and subjected to heat treatment. The heat treatment consisted of soaking at 1,030°C for one hour in a vacuum furnace, followed by quenching with gas cooling equivalent to oil quenching. The sample was then tempered twice for one hour at 500°C, a temperature below the residual γ decomposition temperature. The sample was then shot peened (SP). The shot peening conditions are shown in Table 1 below. Shot peening was performed by masking half of the 30B surface of the sample, allowing for comparison of cross-sectional hardness with and without shot peening. After shot peening, the Vickers hardness was measured at a depth of 20 μm from the surface. Shot peening increased hardness, and the difference in hardness between the two treatments was taken as the amount of hardening due to shot peening.

[0062] [Table 1]

[0063] [Test Results] <Test 1> Relationship between tempered hardness, residual γ content, and Ms point First, the results of an investigation into the relationship between tempered hardness, the amount of retained γ, and the Ms point are presented.

[0064] The relationship between the amount of retained γ and tempered hardness evaluated as described above for die steels having multiple component compositions is shown in Figure 1. As can be seen from the figure, the tempered hardness tends to decrease as the amount of retained γ increases.

[0065] Furthermore, Figure 2 shows the relationship between the Ms point and the amount of retained γ for mold steels with multiple chemical compositions. Here, the Ms point is estimated using the above formula (2) based on the chemical composition. As shown in the figure, although there is a large variance, there is a tendency for the amount of retained γ to decrease as the Ms point increases.

[0066] These results indicate a correlation between tempered hardness, residual γ content, and Ms point; the lower the Ms point, the greater the residual γ content, and the greater the residual γ content, the lower the tempered hardness. The Ms point can be estimated from the chemical composition using equation (2). The chemical composition of the mold steel can be adjusted to obtain the Ms point corresponding to the desired amount of residual γ and tempered hardness. Figure 2 also shows an approximate curve approximating the data points. The Ms point corresponding to a residual γ content of 15% to 20% is generally in the range of 190°C to 220°C.

[0067] <Test 2> Component composition and residual γ decomposition temperature Next, the results of an investigation into the relationship between the component composition and the residual γ decomposition temperature are shown.

[0068] Figure 3 shows the relationship between tempering temperature and the amount of residual γ. This shows the results of measuring the amount of residual γ while varying the tempering temperature in the evaluation of the residual γ decomposition temperature described above. #1 to #5 in the figure represent the measurement results for samples with different chemical compositions. As can be seen from the figure, the tempering temperature range in which the data points are distributed varies widely depending on the sample. However, in all samples, the amount of residual γ tends to rapidly decrease from approximately 20% to nearly 0% as the tempering temperature increases. This decrease in the amount of residual γ is due to the decomposition of the residual γ phase. The tempering temperature at which the amount of residual γ decreases to 90% or less of the quenched sample (before tempering) (here, the amount of residual γ is approximately 18% (only #3 is approximately 6%)) was defined as the residual γ decomposition temperature.

[0069] The residual gamma decomposition temperature thus obtained is shown in Figure 4. Here, the residual gamma decomposition temperature is plotted against the horizontal axis, where the A value (Si + Mn + 2Cr + 3Mo + 3.5V) estimated using Equation (1) based on the chemical composition is plotted. The numbers #1 to #5 in the figure correspond to the sample numbers used in the test in Figure 3. As shown in the figure, a good correlation is observed between the A value and the residual gamma decomposition temperature, with the larger the A value, the higher the residual gamma decomposition temperature. This suggests that the A value calculated based on the chemical composition is a good indicator for predicting the residual gamma decomposition temperature. For example, the chemical composition can be set using the A value as an indicator to obtain the desired residual gamma decomposition temperature based on the mold operating conditions, etc. According to the approximation curve in Figure 3, an A value of 22 or higher is required to achieve a residual gamma decomposition temperature of 510°C or higher.

[0070] <Test 3> Component composition and various properties Next, mold steels having various component compositions were prepared and various properties were evaluated based on the findings obtained in the above Tests 1 and 2. Table 2 shows the component compositions and property evaluation results for Examples 1 to 11 and Comparative Examples 1 to 9.

[0071] [Table 2]

[0072] According to Table 2, the die steels of Examples 1 to 11 all have chemical compositions within the ranges specified in the present invention as described above. In Examples 1 to 11, the Ms points are between 190°C and 220°C, and the amount of retained γ is in the range of 15% to 20%. This is the same result as the findings obtained in Test 1 above. Furthermore, Examples 1 to 11 have high tempered hardness of 55 HRC or more and a modulus of 20 J·cm -2 These tempered hardnesses and impact values ​​can be correlated with the amount of retained γ being in the above range.

[0073] Furthermore, in all of Examples 1 to 11, the A value was 22.0 or more, and the residual γ decomposition temperature was 510° C. or more, which is the same result as the findings obtained in Test 2 above.

[0074] In addition, in all of Examples 1 to 11, the amount of hardening by shot peening was 50 HV or more. In other words, the hardness of the surface layer was increased by 50 HV or more through shot peening. This can be associated with the fact that the amount of retained γ was large, at 15% or more.

[0075] On the other hand, Comparative Examples 1 to 9 do not have the component compositions defined in the present invention as described above. All of Comparative Examples 1 to 9 have a tempered hardness of 55 HRC or more and a tensile strength of 20 J cm -2 In Comparative Examples 1 to 6, the Ms point exceeds 220°C and the amount of residual γ is less than 15%, mainly because the content of at least one of Mn, Cr, and Mo is too low. These are considered to be factors that make it impossible to achieve both high hardness and impact value. On the other hand, in Comparative Examples 7 to 9, the content of at least one of Mn, Cr, and Mo is too high, mainly because the content of at least one of Mn, Cr, and Mo is too high, and although the amount of residual γ is sufficiently high or even excessive, the impact value is 20 J cm -2 It does not reach that level.

[0076] The embodiments and examples of the present invention have been described above. The present invention is not particularly limited to these embodiments and examples, and various modifications can be made.

Claims

1. In mass%, 0.35%≦C≦0.55%, 0.05%≦Si≦0.40%, 1.50%≦Mn≦2.50%, 7.5%≦Cr≦9.0%, 0.90%≦Mo≦2.5%, 0.40%≦V≦0.80% and the balance being Fe and unavoidable impurities, A mold steel, wherein the value of A calculated by the following formula (1) is 22.0≦A≦27.

5. A=Si+Mn+2Cr+3Mo+3.5V (1) In formula (1), each element symbol indicates the content of each element in units of mass %.

2. 2. The mold steel according to claim 1, further containing, by mass%, Ni≦1.1%.

3. After tempering at less than 510°C, the hardness is 55HRC or more, and the Charpy impact value is 20J cm －２ The die steel according to claim 1 or 2, wherein

4. 4. The die steel according to claim 1, wherein the amount of retained γ after quenching is 15% or more and 20% or less.

5. 5. A mold steel according to claim 1, wherein the decomposition temperature of the residual γ phase is 510°C or higher.

6. 6. A mold steel according to claim 1, wherein the martensitic transformation start point is 190°C or higher and 220°C or lower.

7. 7. A mold steel according to claim 1, wherein the hardness of the surface layer is increased by 50 HV or more by shot peening after tempering at a temperature of less than 510°C.

8. A mold made of the mold steel according to any one of claims 1 to 7.

9. The mold according to claim 8, which is a die casting mold.

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