JP7750044B2 - Die steel and molds - Google Patents

Description

The present invention relates to mold steel and molds, and more specifically to mold steel that can be used to construct molds such as die-casting molds, and to such molds.

Many molds are subjected to thermal shock due to repeated use during molding while in contact with materials heated to high temperatures. 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 subject to repeated heating from the injection of molten metal and cooling from the spraying of mold release agents, which generates thermal stress and makes the design surface prone to heat checking due to thermal fatigue. As the heat check grows, it can become a large crack that can transfer to the product and affect its quality. When a crack occurs in a mold that could transfer to the product, the mold must be repaired by welding or re-polishing. Alternatively, if the damage to the mold is too severe to repair by these methods, the mold must be replaced with a new one. Repairing or replacing a mold requires significant effort and cost.

To reduce the frequency of mold repairs and replacements due to damage caused by thermal shock and enable molds to be used for a long period of time, mold steels that make up molds are required to have high impact values (impact resistance, toughness). Patent documents 1 to 4 each specify the alloy composition of mold steels (hot work tool steels) with the aim of increasing the impact value.

JP 2013-87322 A Japanese Patent Application Laid-Open No. 2007-224418 JP 2011-001572 A Japanese Patent Application Laid-Open No. 2002-88443

In general, increasing the hardness of steel 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 the case of die-casting dies, it is desirable for the hardness to be 55 HRC or more after tempering in order to prevent heat checking.

The problem that this invention aims to solve is to provide a mold steel and mold that combine high hardness and high impact value and exhibits high resistance to thermal shock.

In order to solve the above problems, the mold steel of the present invention contains, by 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 consisting of Fe and unavoidable impurities.

Here, the mold steel may further contain, by mass%, Ni≦1.1%.

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 %.

The die steel preferably has a hardness of 55 HRC or more and a Charpy impact value of 20 J cm ^-2 or more after tempering at less than 510°C. Furthermore, the amount of residual γ after quenching is preferably 15% or more and 20% or less. Furthermore, the decomposition temperature of the residual γ phase is preferably 510°C or more. The die steel preferably has a martensitic transformation start point of 190°C or more and 220°C or less. The die steel preferably has a surface hardness of 50 HV or more after tempering at less than 510°C by shot peening.

The mold according to the present invention is made from the above-mentioned mold steel. Here, the mold is preferably a die-casting mold.

The mold steel of the present invention, having the above-mentioned chemical composition, achieves both high hardness and a high impact value. In mold steel, a certain amount of residual γ after quenching has the effect of improving the impact value, but if the amount of residual γ is too high, the hardness decreases. However, by having the above-mentioned chemical composition, the amount of residual γ can be controlled within an appropriate range. When mold steel achieves both high hardness and a high impact value, damage caused by thermal shock, such as heat checking, is suppressed, contributing to improved mold durability.

Here, if the mold steel further contains Ni≦1.1% by mass, the hardenability of the mold steel can be improved while maintaining high hardness.

Furthermore, when the value of A calculated by the above formula (1) is A≧22.0, the decomposition temperature of the residual γ phase is 510°C or higher. This means that the amount of residual γ in the mold steel 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.

In the case where a die steel has a hardness of 55 HRC or more and a Charpy impact value of 20 J cm ^{or more} after being tempered at a temperature of less than 510°C, the die steel has both a sufficiently high hardness and a high impact value, and therefore can effectively suppress the occurrence of damage caused by thermal shock, such as heat checking.

Furthermore, if the amount of residual γ is 15% or more and 20% or less after quenching, it is possible to achieve a high degree of both an improvement in impact value due to the sufficient amount of residual γ phase contained in the mold steel and suppression of a decrease in hardness due to an excessive increase in the amount of residual γ.

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.

When the martensitic transformation start point of the mold steel is between 190°C and 220°C, the amount of residual γ after quenching tends to fall within the range of 15% to 20%, making it easier for the mold steel to achieve both high hardness and high impact resistance.

When mold steel is tempered at less than 510°C and the surface hardness is increased by 50 HV or more by shot peening, the surface hardness of the mold steel can be efficiently improved by stress-induced martensitic transformation caused by shot peening. The mold steel of the present invention has a relatively high amount of retained γ due to the adjustment of its composition, making it more likely to undergo hardness improvement through stress-induced martensitic transformation.

The mold of the present invention is made from the above-mentioned mold steel, and therefore has both high strength and a high impact value. As a result, even when subjected to repeated thermal shocks, the occurrence of damage such as heat checking is suppressed, making it possible to use the mold for a long period of time. The increased durability of the mold also reduces the labor and costs required for mold repair and replacement.

Here, if the mold is a die-casting mold, it will be used in conditions where it will be repeatedly exposed to contact with high-temperature molten metal and cooling by mold release agents, but it will be a highly durable mold that is less likely to experience heat checks due to thermal fatigue or large cracks due to the growth of heat checks.

FIG. 1 is a diagram showing the relationship between the amount of retained γ and tempered hardness. FIG. 1 is a diagram showing the relationship between the martensitic transformation start point (Ms point) and the amount of retained γ. 1 is a graph showing the relationship between the tempering temperature and the amount of retained γ for five types of die steels. 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.

The die steel and die according to an embodiment of the present invention are described in detail below. The die according to an embodiment of the present invention can be constructed using the die steel according to an embodiment of the present invention. The type of die is not particularly limited, but because 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 casting die.

[Component composition]
First, the chemical composition of a die steel according to one embodiment of the present invention will be described. The die 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 mass %.

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

By setting the C content to 0.35% or less, the amount of dissolved C and the amount of carbide formed are ensured, resulting in high hardness. In the mold steel according to this embodiment, it is desirable for the tempered hardness to be 55 HRC or higher in order to obtain sufficient heat check resistance. However, by setting the C content to 0.35% or less, it becomes easier to achieve a high tempered hardness of 55 HRC or higher. Preferably, the C content is 0.37% or less. Even more preferably, the C content is 0.40% or less.

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 mold steel. Furthermore, the martensitic transformation start temperature (Ms) decreases, and the amount of residual γ increases, which actually reduces the hardness of the mold steel. By limiting C to 0.55%, it is possible to suppress the decrease in machinability and to easily keep the amount of residual γ to 20% or less. Preferably, C is 0.50% or less. Even more preferably, C is 0.45% or less.

0.05%≦Si≦0.40%
Silicon has the effect of acting as a deoxidizer and of improving machinability during mold manufacturing. Furthermore, even a small amount of silicon added increases the hardness of mold steel. By setting the Si content at 0.05% or less, these effects can be fully achieved. Preferably, the Si content is 0.25% or less. More preferably, the Si content is 0.30% or less.

On the other hand, if the Si content is excessive, the thermal conductivity of the mold steel will decrease. Furthermore, the amount of coarse crystallized carbides will increase, which may result in a decrease in impact value. Therefore, from the perspective 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 0.35% or less. Even more preferably, Si 0.32% or less.

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.

On the other hand, Mn generates MnS in mold steel. MnS reduces the impact value of the 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 it will be easier to achieve both a residual γ amount of 15 to 20% and an impact value (Charpy impact value) of ²⁰ J/cm2 or more.

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 temperatures of approximately 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 by tempering at temperatures below 510°C and ensuring a sufficient amount of residual γ to increase the impact value, the Cr content is set to 7.5%≦Cr. Preferably, it is 7.9%≦Cr. More preferably, it is 8.0%≦Cr.

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 is set to 9.0% or less. Preferably, Cr is set to 8.5% or less. More preferably, Cr is set to 8.1% or less. If 7.5% or less Cr is set to 9.0%, the mold steel will have an excellent balance between the amount of retained γ and the impact value, and it will be easier to achieve both a retained γ amount of 15 to 20% and an impact value of 20 J/cm2 or ^more .

0.90%≦Mo≦2.5%
Mo has the effect of improving the hardenability of mold steel. Furthermore, Mo contributes to secondary hardening when tempering is performed at temperatures of about 500 to 600°C. Furthermore, Mo, like Cr, 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 by tempering 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.

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 the Mo content is 0.90%≦Mo≦2.5%, the mold steel will have an excellent balance between the amount of retained γ and the impact value, and it will be easier to achieve both a retained γ amount of 15 to 20% and an impact value of 20 J/cm2 or ^more .

0.40%≦V≦0.80%
V generates 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.

On the other hand, if the V content is too high, a large amount of coarse crystallized carbides will be generated. The coarse crystallized carbides will reduce the impact value of the mold steel. Therefore, from the viewpoint of suppressing the generation 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%, it becomes easier to achieve an impact value of 20 J/cm2 or ^more .

The mold steel according to this embodiment may optionally contain Ni in addition to the essential elements described above. The Ni content and the reasons for limiting it are as follows:

Ni≦1.1%
Ni has the effect of improving hardenability by stabilizing the γ 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, in order to obtain a significant effect of Ni addition, it is sufficient to keep the Ni content at 0.10% or less.

On the other hand, adding too much Ni results in an excessive amount of residual γ at the time of quenching, resulting in a decrease in tempered hardness. Furthermore, 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 should be limited to Ni≦1.1%. Preferably, Ni≦0.50%. Even more preferably, Ni≦0.20%.

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

Of the above unavoidable impurities, N and O are unavoidable when industrially producing mold steel, as they dissolve in Fe from the air. However, if the amount of these elements mixed in is too high, voids are more likely to form in the structure. They also form oxides and nitrides in the alloy. Coarse oxides and nitrides in particular can accelerate abrasive wear of molds and reduce impact values. To prevent these phenomena, it is preferable to keep the N and O content below the upper limits mentioned above.

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 %.

Specifically, it is preferable that A≧22.0. As will be shown in the Examples below, according to the inventor's findings, the A value has a high correlation with the residual gamma decomposition temperature (the decomposition temperature of the residual gamma phase), and the larger the A value, the higher the residual gamma decomposition temperature tends to be (see Figure 4). When the decomposition temperature of the residual gamma phase is higher, even if the mold steel is heated by contact with high-temperature materials during mold use, decomposition of the residual gamma phase is less likely to occur, making it easier to maintain a specified amount of residual gamma. If A≧22.0, it is easy to raise the residual gamma decomposition temperature to 510°C or higher. It is even more preferable that A≧23.0, or A≧25.0.

[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 when tempered at less than 510°C after quenching. 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.

The mold steel according to this embodiment, having the above-described chemical composition, has both high hardness and a high impact value (high impact resistance and toughness). A high impact value of the mold steel increases its resistance to thermal shock, making it less susceptible to damage even when the mold is repeatedly heated and cooled. In particular, in die-casting molds, 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 mold steel's high impact value can suppress the occurrence of heat checking. Furthermore, the mold steel's high hardness can effectively suppress damage to the mold 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 mold steel's high hardness and high impact value can strongly suppress the occurrence of damage to the mold steel due to thermal shock, such as heat checking.

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 of this embodiment, having the above-mentioned composition, achieves both high hardness and high impact value, suppressing damage to the mold caused by thermal shock, such as heat checking and cracking, and increasing the durability of the mold. Because 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 repairing or replacing it, reducing the labor and cost required for mold repair and replacement.

The die steel according to this embodiment has the above-described chemical composition, and thus after tempering at a temperature of less than 510°C, achieves both a hardness of 55 HRC or more and an impact value (Charpy impact value) of 20 J/ ^cm2 or more. More preferably, the hardness is 56 HRC or more and the impact value is 30 J/ ^cm2 or more. Although there are no particular upper limits set for the hardness and impact value of the die steel, it is generally preferable to set the hardness to 60 HRC or less and the impact value to 40 J/ ^cm2 or less from the viewpoint of the balance between hardness and impact value, etc.

The fact that the die steel according to this embodiment achieves both high hardness and high impact strength can be explained by the fact that the amount of residual γ is controlled within an appropriate range. While a large amount of residual γ improves the impact strength of the die steel, an excessive amount of residual γ significantly reduces the tempered hardness. Therefore, by controlling the amount of residual γ within an appropriate range, both high hardness and high impact strength can be achieved. In the die steel according to this embodiment, the amount of residual γ falls within the range of 15% to 20% in accordance with the above-described composition. The amount of residual γ falling within this range enables both a tempered hardness of 55 HRC or more and an impact strength of 20 J/ ^cm2 or more to be achieved. It is more preferable that the amount of residual γ be 16% or more and 19% or less. In this specification, the amount of residual γ is evaluated as the volume fraction of the residual γ phase in the die steel after quenching (as quenched).

The amount of retained γ correlates with the martensitic transformation start point (Ms point), and the higher the Ms point, the lower the amount of retained γ (see FIG. 2). The Ms point also 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 to 220°C, corresponding to the above-described composition. Corresponding to the Ms point being within this range, the amount of retained γ falls within the range of 15% to 20%, which provides high hardness and high impact value, 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 the Andrews formula (K.W. Andrews, J. Iron Steel Inst., 203 (1965), 721).

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

As described above, the die steel according to this embodiment has an appropriate range of Ms point and residual γ content due to the effect of its chemical composition, and accordingly, achieves both high hardness and high impact strength. For example, the quenching and tempering conditions can be set so as to obtain a hardness of 55 HRC or higher and an impact strength of 20 J/ ^cm² or higher. A suitable example of the conditions is to melt, cast, appropriately forge, normalize, and anneal the steel, soak it at 1000 to 1050°C for approximately 30 minutes, then quench it by cooling it to 40°C or lower at a cooling rate of 20°C/min or higher, such as cooling in an oil bath, and then temper it at 490 to 510°C. Furthermore, it is preferable to perform shot peening on the heat-treated die steel to improve its surface hardness.

The mold steel according to this embodiment has high hardness due to its chemical composition, but shot peening can further increase the hardness of the surface layer. By imparting strain through shot peening, stress-induced martensitic transformation of the gamma phase occurs, increasing the hardness of the mold steel. However, the mold steel according to this embodiment has a relatively high amount of retained gamma due to the chemical composition, which effectively improves hardness through shot peening. For example, in mold steel tempered at temperatures below 510°C, the hardening of the surface layer due to shot peening should be 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 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 steel may be quenched by gas cooling equivalent to oil quenching, followed by tempering 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 amount of hardening. The shot peening conditions can be suitably the same as those in the examples shown in Table 1 below.

The present invention will be explained in more detail below using examples.

[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 at 900°C for 2 hours and cooling to 660°C at a cooling rate of 15°C/h to produce annealed materials.

[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.

<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 consisted of soaking at 1,030°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 #1,000, and the amount of residual γ was measured using an X-ray stress measurement device.

<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 entailed soaking at 1,030°C for 1 hour in a vacuum furnace, followed by quenching with gas cooling equivalent to oil quenching. Tempering was then performed at 460°C to 570°C in 10°C increments, 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 measurement device. 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.

<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 test surface and the ground surface of the tempered pieces were polished to a grain size of #400, and their hardness was measured using the Rockwell C scale.

<Impact Value>
Test pieces measuring 10 mm x 10 mm x 55 mm were cut out 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. Then, tempering was performed twice for 1 hour at 500°C, a temperature below the residual γ decomposition temperature. Subsequently, JIS No. 3 impact test pieces (2 mm U-notch) were taken and subjected to a Charpy impact test in accordance with JIS Z 2242. The Charpy impact values were obtained from these.

<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 entailed 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 subjected to shot peening (SP). The shot peening conditions were as 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 conditions was taken as the amount of hardening due to shot peening.

[Test Results]
<Test 1> Relationship between tempered hardness, amount of retained γ, and Ms point First, the results of investigating the relationship between tempered hardness, amount of retained γ, and Ms point are shown.

Figure 1 shows the relationship between the amount of retained γ and tempered hardness evaluated as described above for mold steels with multiple component compositions. The figure shows a tendency for tempered hardness to decrease as the amount of retained γ increases.

Furthermore, Figure 2 shows the relationship between the Ms point and the amount of residual γ for mold steels with multiple chemical compositions. Here, the Ms point is estimated using the above formula (2) based on the chemical composition. The figure shows that, although there is a large variance, the amount of residual γ tends to decrease as the Ms point increases.

These results show that there is a correlation between tempered hardness, residual gamma content, and Ms point; the lower the Ms point, the greater the residual gamma content, and the greater the residual gamma content, the lower the tempered hardness tends to be. The Ms point is a quantity that can be estimated from the chemical composition as shown in formula (2), and the chemical composition of the mold steel can be adjusted to obtain an Ms point corresponding to the desired residual gamma content and tempered hardness. Figure 2 also shows an approximate curve that approximates the data points, and the Ms point corresponding to a residual gamma content of 15% to 20% is generally in the range of 190°C to 220°C.

<Test 2> Component Composition and Residual γ Decomposition Temperature Next, the results of an investigation into the relationship between component composition and residual γ decomposition temperature are shown.

Figure 3 shows the relationship between tempering temperature and residual gamma content. This shows the results of measuring the residual gamma content while varying the tempering temperature in the evaluation of the residual gamma 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 residual gamma content tends to rapidly decrease from approximately 20% to nearly 0% as the tempering temperature increases. This decrease in the residual gamma content is due to the decomposition of the residual gamma phase. The tempering temperature at which the residual gamma content drops to 90% or less of the quenched sample (before tempering) (here, the residual gamma content is approximately 18% (only #3 is approximately 6%)) was defined as the residual gamma decomposition temperature.

The residual gamma decomposition temperature obtained in this manner is shown in Figure 4. Here, the horizontal axis represents the A value (Si + Mn + 2Cr + 3Mo + 3.5V) estimated using formula (1) based on the component composition, and the residual gamma decomposition temperature is plotted against the A value. The numbers #1 to #5 in the figure correspond to the sample numbers used in the test in Figure 3. The figure shows a good correlation 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 component composition is a good indicator for predicting the residual gamma decomposition temperature. For example, the component composition can be set using the A value as an indicator to obtain the desired residual gamma decomposition temperature based on the mold usage 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.

<Test 3> Elemental Composition and Various Properties Next, die steels having various elemental compositions were prepared and various properties were evaluated based on the findings obtained in Tests 1 and 2. Table 2 shows the elemental compositions and the results of property evaluation for Examples 1 to 11 and Comparative Examples 1 to 9.

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 190°C or higher and 220°C or lower, and correspondingly, the amount of retained γ is in the range of 15% or higher and 20% or lower. This is similar to the findings obtained in Test 1 above. Furthermore, Examples 1 to 11 achieve both a high tempered hardness of 55 HRC or higher and a high impact value of 20 J cm ^-2 or higher. These tempered hardnesses and impact values can be correlated with the amount of retained γ being within the above ranges.

Furthermore, in all of Examples 1 to 11, the A value was 22.0 or higher. The residual gamma decomposition temperature was 510°C or higher. These results are similar to those obtained in Test 2 above.

In addition, in all of Examples 1 to 11, the amount of hardening due to shot peening was 50 HV or more. In other words, shot peening increased the hardness of the surface layer by 50 HV or more. This corresponds to the high amount of residual gamma, at 15% or more.

On the other hand, Comparative Examples 1 to 9 do not have the component compositions defined in the present invention as described above. None of Comparative Examples 1 to 9 achieves both a tempered hardness of 55 HRC or more and an impact value of 20 J·cm ⁻² or more. In Comparative Examples 1 to 6, the Ms point exceeds 220°C and the amount of residual γ is less than 15%, mainly due to the content of at least one of Mn, Cr, and Mo being too low. These factors are thought to be one of the reasons why high hardness and impact value cannot be achieved simultaneously. 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 due to the content being too high, and although the amount of residual γ is sufficiently high or excessive, the impact value does not reach 20 J·cm ⁻² .

The above describes embodiments and examples of the present invention. The present invention is not particularly limited to these embodiments and examples, and various modifications are possible.

Claims (9)

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 %. The mold steel according to claim 1, further containing, by mass%, Ni≦1.1%. 3. A mold steel according to claim 1 , which has a hardness of 55 HRC or more and a Charpy impact value of 20 J·cm ⁻² or more after being tempered at a temperature of less than 510° C. 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. A mold steel according to claim 1 , wherein the decomposition temperature of the residual γ phase is 510°C or higher. 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. 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. A mold made of the mold steel according to any one of claims 1 to 7 . The mold according to claim 8 , which is a die casting mold.