Die steel and die

Abstract

This invention relates to a mold steel and to a mold comprising the mold steel, the mold steel comprising, by mass%, 0.55% ≤ C ≤ 0.70%; 0.30% ≤ Si ≤ 0.60%; 0.55% ≤ Mn ≤ 1.2%; 5.7% ≤ Cr ≤ 6.9%; 1.2% ≤ Mo + W / 2 ≤ 1.6%; 0.55% ≤ V ≤ 0.79%; and 0.005% ≤ N ≤ 0.1%, the balance being Fe and unavoidable impurities, the impurities comprising, by mass%, Al. ≤0.020%, Ni≤0.20%, S≤0.0015% and Cu≤0.10%, and satisfy P1≥24 and 4.9≤P2≤7.3, P1 and P2 are values ​​obtained based on the following formulas (1) and (2), respectively, P1=45-13.6[Si]-7.0([Mo]+[W] / 2)-12.9[Ni](1), P2=7.4[V]+15.8[N]+38.6[Al](2), where [M] represents the content of element M in mass%.

Description

Technical Field

[0001] This invention relates to mold steel and molds, and more particularly, to mold steel for constructing molds such as hot stamping dies and such molds. Background Technology

[0002] In die steels used to construct dies for processing steels through processes such as pressing and forming, high hardness and toughness are desirable from the viewpoint of improving the wear resistance and thermal shock resistance of the dies. Improving the wear resistance and thermal shock resistance of dies used under high-temperature conditions such as warm forming, hot stamping, warm trimming, and punching is crucial. For example, Patent Document 1 discloses a hot-working tool steel containing, by weight, greater than 0.35% and less than 0.45% C, less than 1.00% Si, 0.1% to 1.5% Mn, 0.1% to 1.5% Ni, 4.35% to 5.65% Cr, one or both of W and Mo at 1.5% to 3.5% W / 2 + Mo, 0.5% to 1.5% V, with the amounts of Si and Cr satisfying the relationship Si < (18.7 / Cr) - 3.3, and the balance being Fe and unavoidable impurities. This hot-working tool steel is considered to possess high toughness within a high hardness range. Furthermore, Patent Document 2 discloses a tool steel for warm and hot working, comprising, by weight, 0.45% to less than 0.65% C, 0.60% to less than 1.50% Mn, 3.00% to 5.50% Cr, 2.00% to 3.50% W and Mo (W / 2 + Mo), 0.80% to 1.60% V, 0.30% to 5.00% Co, and 0.005% to less than 0.005% S, with the balance being Fe and unavoidable impurities. This tool steel is considered to possess excellent high-temperature strength and toughness.

[0003] Patent Document 1: JPH04-308059A

[0004] Patent Document 2: JPH02-11736A Summary of the Invention

[0005] In the hot-working tool steel disclosed in Patent Document 1, the highest hardness is 54 HRC. With this hardness, it may be difficult to ensure sufficiently high wear resistance as a mold steel. It is believed that due to the relatively low C and Cr content, it is difficult to achieve high hardness in the hot-working tool steel of Patent Document 1. Increasing the C content can improve the hardness of the mold steel, but with increased hardness, coarse carbides such as crystalline carbides may form, thus potentially reducing toughness even when high hardness is achieved. Furthermore, to improve thermal shock resistance, it is believed that in addition to improving the toughness of the mold steel, increasing the thermal conductivity to prevent large impacts on the mold surface due to localized heating is also effective; however, increasing thermal conductivity was not considered in Patent Documents 1 and 2.

[0006] The purpose of this invention is to provide mold steel and molds with excellent wear resistance and thermal shock resistance.

[0007] To address the aforementioned problems, the steel according to the present invention is a mold steel comprising: by mass % 0.55% ≤ C ≤ 0.70%; 0.30% ≤ Si ≤ 0.60%; 0.55% ≤ Mn ≤ 1.2%; 5.7% ≤ Cr ≤ 6.9%; 1.2% ≤ Mo + W / 2 ≤ 1.6%; 0.55% ≤ V ≤ 0.79%; and 0.005% ≤ N ≤ 0.1%, with the balance being Fe and unavoidable impurities comprising by mass % Al ≤ 0.020% and Ni ≤ 0. 20%, S≤0.0015% and Cu≤0.10%, and satisfy P1≥24 and 4.9≤P2≤7.3, where P1 is the value obtained based on the following formula (1) and P2 is the value obtained based on the following formula (2), P1=45-13.6[Si]-7.0([Mo]+[W] / 2)-12.9[Ni](1), P2=7.4[V]+15.8[N]+38.6[Al](2), where in formulas (1) and (2), [M] represents the content of element M in mass%.

[0008] The steel used for this mold, after quenching and tempering, preferably has a room temperature hardness of 58HRC or higher and 61HRC or lower, and a room temperature thermal conductivity of 20W / (m·K or higher.

[0009] The mold steel may also contain at least one element selected from the group consisting of the following elements, expressed as a percentage by mass: 0.01% ≤ Nb ≤ 0.5%, 0.01% ≤ Zr ≤ 0.5%, and 0.01% ≤ Ta ≤ 0.5%. The mold steel may also contain 0.10% ≤ Co ≤ 1.0% by mass.

[0010] The die steel, in its quenched state, preferably has a grain size of 5 or more as defined in JIS G 0551:2020. The die steel, in its quenched and tempered state, preferably has crystalline carbides with a grain size of less than 25 μm.

[0011] The mold according to the present invention is a mold comprising the steel used for the mold.

[0012] The mold can be a hot stamping mold.

[0013] The mold steel according to the present invention, by comprising the above-described components, possesses both high hardness and high thermal conductivity, and suppresses the formation of coarse carbides and grain coarsening. As a result, this mold steel simultaneously achieves both high wear resistance and high thermal shock resistance. In particular, when P1 ≥ 24, a high improvement in thermal conductivity is obtained. Furthermore, when 4.9 ≤ P2 ≤ 7.3, a high effect on improved toughness is obtained due to grain refinement. As a result, a mold steel with particularly excellent thermal shock resistance is obtained. Limiting the contents of Al, Ni, S, and Cu to predetermined upper limits or lower also contributes to improving thermal shock resistance. In addition, by employing the above-described component composition, molds with excellent wear resistance and thermal shock resistance can be provided, while limiting the content of added alloying elements to a relatively small amount, and avoiding costly manufacturing processes such as powder forming.

[0014] In this paper, when the room temperature hardness of the die steel after quenching and tempering is 58 HRC to 61 HRC and the room temperature thermal conductivity is 20 W / (m·K) or higher, it is possible to obtain high hardness sufficient to improve wear resistance. This prevents the formation of coarse crystalline carbides and grain coarsening caused by using a composition that provides excessive hardness, thereby preventing the associated reduction in toughness and ensuring high thermal shock resistance. Furthermore, since the die steel has sufficiently high thermal conductivity, it can prevent the surface temperature of the die from rising and reduce heat concentration on the surface, thereby enhancing thermal shock resistance.

[0015] When the mold steel also contains at least one element selected from the group consisting of Nb, Zr and Ta in a specific amount, the toughness of the mold steel can be particularly enhanced.

[0016] When the mold steel also contains the aforementioned specific amount of Co, the high-temperature strength of the mold steel is improved.

[0017] When the die steel has a grain size of 5 or more as defined in JIS G 0551:2020 after quenching, or when the die steel has crystalline carbides with a grain size of less than 25 μm after quenching and tempering, the thermal shock resistance of the die steel can be enhanced particularly easily by suppressing the formation of coarse crystalline carbides.

[0018] Because the mold according to the invention comprises the mold steel described above, it possesses excellent wear resistance and thermal shock resistance. Due to these properties, the mold is particularly suitable for use as a hot stamping die. Attached Figure Description

[0019] Figure 1 A schematic cross-sectional view of the cap-shaped bending test used to evaluate abrasion resistance;

[0020] Figure 2 A graph showing the relationship between P1 value and thermal conductivity;

[0021] Figure 3 A graph showing the relationship between P2 value and crystal grain size; and

[0022] Figure 4 A graph showing the results of thermal shock resistance tests relative to S and Cu contents. Detailed Implementation

[0023] The following will describe in detail the mold steel and the mold according to embodiments of the present invention.

[0024] The mold steel according to an embodiment of the present invention comprises the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their composition percentages, and the reasons for limitations are as follows. The composition percentages are expressed in mass percent. In the following text, unless otherwise specified, each property is a value evaluated at room temperature (approximately 25°C). The properties to be evaluated are those of the heat-treated state after quenching at a cooling rate of 9°C / min to 100°C / min from the quenching temperature (e.g., 1,030°C ± 20°C) to 200°C and tempering at 500°C to 600°C.

[0025] [Content of each component element]

[0026] 0.55% ≤ C ≤ 0.70%

[0027] During quenching, carbon dissolves in the matrix phase and forms martensite, thereby increasing the hardness of die steel. Furthermore, carbon also increases the hardness of die steel by forming carbides with chromium, molybdenum, and v.

[0028] By setting the carbon content to 0.55% ≤ C, the amount of carbon dissolved and the amount of carbide formed can be ensured, thereby obtaining high hardness. From the viewpoint of obtaining sufficient wear resistance, die steel preferably has a hardness of 58 HRC or higher through quenching and tempering, but a high hardness of 58 HRC or higher can be easily achieved when the carbon content is 0.55% ≤ C. Preferably, the carbon content can be 0.57% ≤ C.

[0029] On the other hand, excessive carbon content may lead to an increase in coarse carbides, thus potentially reducing the toughness of the die steel. Furthermore, thermal conductivity may also decrease. As a result, it becomes difficult to achieve high thermal shock resistance in die steel. By ensuring a carbon content of ≤0.70%, the formation of coarse carbides is suppressed, and high thermal conductivity is ensured, thereby achieving high thermal shock resistance. In alloy compositions that provide excessive hardness, the formation of coarse carbides and a decrease in thermal conductivity may occur. Therefore, in die steel, it is preferable to limit the hardness to below 61 HRC through quenching and tempering. By ensuring a carbon content of ≤0.70%, the hardness is limited to below 61 HRC, thus easily ensuring high thermal shock resistance. Preferably, the carbon content is ≤0.65%. More preferably, the carbon content is ≤0.64%.

[0030] 0.30% ≤ Si ≤ 0.60%

[0031] Si increases the hardness of mold steel, and the hardness improvement effect can be fully achieved when the Si content is 0.30% ≤ Si. Si also acts as a deoxidizer and improves machinability during mold manufacturing. Preferably, the Si content is 0.40% ≤ Si. More preferably, the Si content is 0.42% ≤ Si.

[0032] On the other hand, excessive Si content reduces the thermal conductivity of the mold steel. Furthermore, coarse-grained carbides may form. Therefore, from the viewpoint of ensuring high thermal conductivity and suppressing the formation of coarse-grained carbides, the Si content is set to Si ≤ 0.60%. Preferably, the Si content is Si ≤ 0.55%.

[0033] 0.55% ≤ Mn ≤ 1.2%

[0034] Mn enhances the hardenability of die steel. Furthermore, Mn effectively enhances the toughness of die steel. From the viewpoint of obtaining high hardenability and toughness, the Mn content is set to satisfy 0.55% ≤ Mn. Preferably, the Mn content is 0.70% ≤ Mn. More preferably, the Mn content is 0.75% ≤ Mn.

[0035] On the other hand, manganese (Mn) is an element that reduces the thermal conductivity of mold steel. Therefore, from the viewpoint of ensuring high thermal conductivity, the Mn content is set to satisfy Mn ≤ 1.2%. Preferably, the Mn content can satisfy Mn ≤ 1.1%.

[0036] 5.7% ≤ Cr ≤ 6.9%

[0037] Cr increases the hardness of die steel. Similar to Mn, Cr improves the hardenability and toughness of die steel. From the viewpoint of obtaining high hardness, hardenability, and toughness, the Cr content is set to be 5.7% ≤ Cr. Preferably, the Cr content is 5.9% ≤ Cr.

[0038] On the other hand, similar to Mn, Cr also reduces the thermal conductivity of mold steel. Therefore, from the viewpoint of ensuring high thermal conductivity, the Cr content is set to satisfy Cr ≤ 6.9%. Preferably, the Cr content can satisfy Cr ≤ 6.7%. More preferably, the Cr content can satisfy Cr ≤ 6.5%.

[0039] 1.2% ≤ Mo + W / 2 ≤ 1.6%

[0040] Mo and W contribute to increasing the hardness of die steel by forming secondary carbides. From the viewpoint of ensuring the desired high hardness of the die steel, the content of Mo and W is set to be 1.2% ≤ Mo + W / 2, where the sum of the Mo and W contents is half (Mo + W / 2). As a result, a high hardness of 58 HRC or higher is easily obtained. Preferably, the content of Mo and W is 1.3% ≤ Mo + W / 2. More preferably, the content of Mo and W is 1.32% ≤ Mo + W / 2.

[0041] On the other hand, Mo and W are elements that reduce the thermal conductivity of mold steel. Furthermore, Mo and W are expensive elements, thus increasing material costs when mold steel contains large amounts of Mo and W. From the viewpoint of ensuring high thermal conductivity and reducing material costs, the content of Mo and W is set to satisfy Mo+W / 2 ≤ 1.6%. Preferably, the content of Mo and W can satisfy Mo+W / 2 ≤ 1.55%.

[0042] 0.55% ≤ V ≤ 0.79%

[0043] V produces pinned particles, which can suppress grain coarsening during quenching. As a result of suppressing grain coarsening, the toughness of the die steel is improved. When V is ≤0.55%, grain coarsening during quenching is effectively suppressed, thereby enhancing toughness. Preferably, the V content is ≤0.57%.

[0044] On the other hand, if the V content is too high, a large amount of coarse carbides will precipitate. Coarse carbides do not contribute to increased hardness. Furthermore, since coarse carbides are the initiation point of cracks, the toughness of the die steel is actually reduced. Therefore, from the viewpoint of suppressing the formation of coarse carbides, the V content is set to satisfy V ≤ 0.79%. Preferably, the V content can satisfy V ≤ 0.75%. More preferably, the V content can satisfy V ≤ 0.72%.

[0045] 0.005% ≤ N ≤ 0.1%

[0046] Nitrogen (N) produces nitrides with a pinning effect, which suppresses grain coarsening during quenching. By suppressing grain coarsening during quenching, the toughness of die steel is improved. Furthermore, nitrides can also act as nuclei for crystalline carbides, refining them through fine dispersion and nucleation. From the viewpoint of fully obtaining these effects, it is preferable to set N to satisfy 0.005% ≤ N. Preferably, the N content is 0.01% ≤ N.

[0047] On the other hand, when the nitrogen (N) content is too high, nitrides aggregate, thus increasing the size of the pinned particles. As a result, the grains become coarser. Furthermore, nitrides, which act as nuclei for crystalline carbides, aggregate, thus increasing the size of the crystalline carbides. From the viewpoint of avoiding grain coarsening and the formation of coarse crystalline carbides, the N content is set to satisfy N ≤ 0.1%. Preferably, the N content can satisfy N ≤ 0.05%. More preferably, the N content can satisfy N ≤ 0.03%.

[0048] The mold steel according to an embodiment of the present invention comprises at least one of C, Si, Mn, Cr, V, N, and Mo and W in predetermined amounts, with the balance comprising Fe and unavoidable impurities. Hereinafter, as unavoidable impurities, Al, Ni, S, and Cu may be included, and their contents are limited to the following ranges.

[0049] Al≤0.020%

[0050] In mold steel, Al tends to form large inclusions, thereby reducing thermal shock resistance. From the viewpoint of suppressing inclusion formation and ensuring high thermal shock resistance, Al is not added to mold steel, but is included only as an unavoidable impurity, and the Al content is limited to 0.020% or less. Preferably, the content is 0.015% or less. More preferably, the content is 0.010% or less.

[0051] Ni≤0.20%

[0052] Ni reduces the thermal conductivity of mold steel. From the viewpoint of ensuring high thermal conductivity, Ni is not added to the mold steel, but is included only as an unavoidable impurity, and the Ni content is limited to 0.20% or less. Preferably, the content may be 0.16% or less. More preferably, the content may be 0.13% or less.

[0053] S≤0.0015%

[0054] Similar to Al, sulfur (S) readily forms large inclusions in mold steel, thereby reducing its thermal shock resistance. From the viewpoint of suppressing inclusion formation and ensuring high thermal shock resistance, sulfur is not added to mold steel, but is included only as an unavoidable impurity, and the S content is limited to 0.0015% or less. Preferably, the content is 0.0012% or less. More preferably, the content is 0.0010% or less.

[0055] Cu≤0.10%

[0056] Similar to Ni, Cu also reduces the thermal conductivity of mold steel. From the viewpoint of ensuring high thermal conductivity, Cu is not added to the mold steel, but is included only as an unavoidable impurity, and the Cu content is limited to 0.10% or less. Preferably, the content may be 0.08% or less. More preferably, the content may be 0.06% or less.

[0057] Examples of unavoidable impurities, other than Al, Ni, S, and Cu, that may be contained in the mold steel according to embodiments of the present invention include P < 0.05%, O < 0.01%, Co < 0.10%, Nb < 0.01%, Ta < 0.01%, Ti < 0.01%, Zr < 0.01%, B < 0.001%, Ca < 0.001%, Se < 0.03%, Te < 0.01%, Bi < 0.01%, Pb < 0.03%, Mg < 0.02%, and rare earth metals (REM) < 0.10%.

[0058] In addition to the essential elements mentioned above, the mold steel according to embodiments of the present invention may optionally contain one or more elements selected from the following elements. The composition ratio of each element, the reasons for limitation, etc., are as follows.

[0059] 0.01%≤Nb≤0.5%, 0.01%≤Zr≤0.5%, 0.01%≤Ta≤0.5%

[0060] Nb, Zr, and Ta produce precipitates that can act as pinning particles to suppress grain coarsening during quenching. Suppressing grain coarsening during quenching and refining the grains improves the toughness of die steels. The lower limit of the element content is set at the level sufficient to produce precipitates that exhibit the pinning effect. The upper limit is set from the viewpoint that suppressing precipitate aggregation prevents them from effectively functioning as pinning particles.

[0061] 0.10% ≤ Co ≤ 1.0%

[0062] Co has the effect of improving the strength of mold steel, especially its high-temperature strength. The lower limit of its content is set at the level required to achieve the effect of improving high-temperature strength. The upper limit is set from the viewpoint of suppressing the reduction of thermal conductivity and reducing material costs.

[0063] [The relationship between the content of constituent elements]

[0064] Next, the relationship between the contents of the constituent elements will be described. In the following text, in the mathematical formulas that define the relationship between the contents of the constituent elements, [M] represents the content of element M in mass % . Furthermore, if the mold steel does not contain elements that are not necessary to be included, their content is set to 0 in the mathematical formulas.

[0065] P1≥24

[0066] P1 is obtained based on the following equation (1).

[0067] P1=45-13.6[Si]-7.0([Mo]+[W] / 2)-12.9[Ni] (1)

[0068] The Si, Mo, W, and Ni contained in equation (1) all reduce thermal conductivity by being dissolved in the mold steel. High thermal conductivity is achieved by limiting the content of these elements to a low level, thereby increasing the P1 value. The following examples also demonstrate that thermal conductivity tends to increase as P1 increases (see...). Figure 2 With P1 ≥ 24, a high thermal conductivity of 20 W / (m·K) or higher is easily obtained. Preferably, P1 can satisfy P1 ≥ 25. More preferably, P1 can satisfy P1 ≥ 26. In mold steels, higher thermal conductivity is preferred, so no upper limit is specifically set for the P1 value, as long as Si, Mo+W / 2, and Ni do not fall below their respective lower limits.

[0069] 4.9 ≤ P2 ≤ 7.3

[0070] P2 is obtained based on the following equation (2).

[0071] P2=7.4[V]+15.8[N]+38.6[Al] (2)

[0072] The V, N, and Al contained in Equation (2) all contribute to the formation of pinning particles, such as carbonitrides and nitrides, which suppress grain coarsening during quenching. As a result of suppressing grain coarsening, the toughness of the die steel is improved. When 4.9% ≤ P2 is satisfied, grain coarsening during quenching is effectively suppressed, thereby enhancing toughness. Preferably, P2 can satisfy 5.0 ≤ P2. More preferably, P2 can satisfy 5.2 ≤ P2.

[0073] On the other hand, when the contents of V, N, and Al are too high, a large number of coarse precipitates are formed. These coarse precipitates are difficult to pin into particles, thus failing to effectively suppress the formation of coarse grains. Furthermore, coarse crystalline carbides and inclusions are easily formed. As a result, the toughness of the die steel is reduced. Therefore, from the viewpoint of preventing these phenomena, P2 is set to satisfy P2≤7.3. Preferably, P2 can satisfy P2≤7.0. More preferably, P2 can satisfy P2≤6.5. As shown in the following embodiments (refer to...) Figure 3 When P2 is too small or too large, it cannot effectively suppress the formation of coarse grains due to the generation of pinning particles. However, when 4.9≤P2≤7.3 is satisfied, it is easy to achieve grain refinement in the quenched state, and a grain size of 5 or more as specified in JIS G0551:2020 (the number of grain size levels also applies below) is obtained.

[0074] Properties of mold steel

[0075] Because the mold steel of the present invention contains the above-mentioned components, both high wear resistance and high thermal shock resistance are achieved. Specifically, the mold steel exhibits high hardness after heat treatment, thus achieving high wear resistance. Simultaneously, the mold steel possesses high toughness and high thermal conductivity. Due to its high thermal conductivity, the mold steel is less susceptible to large impacts on the mold surface caused by localized heating. Therefore, high thermal shock resistance is achieved by possessing both high toughness and high thermal conductivity.

[0076] For example, when die steel achieves a hardness of 58 HRC or higher, and even 59 HRC or higher, through quenching and tempering, it exhibits sufficiently high wear resistance as a die, particularly as a hot stamping die, and can prevent die damage. Especially when the surface of the steel sheet to be processed has a large amount of oxides or has undergone plating, hot stamping dies are prone to wear; however, in these cases, if the die has the high hardness described above, wear can be effectively prevented.

[0077] On the other hand, if the composition of the die steel provides excessively high hardness, for example, if it contains a large amount of carbon, the toughness of the die may decrease due to the formation of coarse crystalline carbides. Furthermore, thermal conductivity may decrease. The decrease in toughness and thermal conductivity leads to a reduction in the die's resistance to thermal shock. Therefore, from the viewpoint of ensuring thermal shock resistance by improving toughness and thermal conductivity, it is preferable to limit the hardness of the die steel in the quenched and tempered state to below 61 HRC. As a result, for example, a high thermal conductivity, such as 20 W / (m·K) or higher, is obtained in the quenched and tempered state, and due to both the improved toughness and improved thermal conductivity, the die steel achieves excellent thermal shock resistance. When forming using a die under conditions accompanied by heating, such as hot stamping, the temperature of the die surface rises instantaneously during forming, thus easily subjecting the die surface to thermal load (thermal shock). However, when the die has high thermal shock resistance, cracks in the die due to thermal shock can be prevented. Therefore, from the perspective of avoiding damage during forming, molds with high mechanical and thermal loads, such as hot stamping dies, should contain materials that have excellent thermal shock resistance in addition to wear resistance.

[0078] Thus, setting the composition of the mold steel to prevent excessively high hardness is a good indicator for improving thermal shock resistance, both in terms of improving toughness and thermal conductivity. Furthermore, by setting the composition so that P1 and P2, determined by equations (1) and (2) above, are within predetermined ranges, the thermal shock resistance of the mold steel can be effectively improved. In other words, a good effect is achieved in improving thermal conductivity when P1 ≥ 24. Moreover, a good effect is achieved in improving toughness when 4.9 ≤ P2 ≤ 7.3, by suppressing the formation of coarse grains. Excellent thermal shock resistance is obtained by combining these effects. Furthermore, limiting the content of Ni and Cu, which are unavoidable impurities, in the mold steel to below predetermined upper limits also helps ensure high thermal conductivity, thereby improving thermal shock resistance. In addition, limiting the content of Al and S, which are unavoidable impurities, to below predetermined upper limits also helps suppress the formation of coarse inclusions, thereby improving thermal shock resistance.

[0079] From the viewpoint of improving toughness, the mold steel according to embodiments of the present invention, in its quenched state, has a grain size of 5 or more, more preferably 7 or more, and even more preferably 9 or more as defined in JIS G 0551:2020. For example, the grain size can be evaluated by grinding and etching a cross-section of the quenched mold steel and measuring the average grain size. Furthermore, in the quenched and tempered state, the grain size of the crystalline carbides in the mold steel can be less than 25 μm. As a result, a high level of improved toughness is achieved by suppressing the formation of coarse crystalline carbides. The grain size of the crystalline carbides is more preferably less than 20 μm. The grain size of the crystalline carbides is evaluated by the maximum value of the diameter of the crystalline carbides generated in the cross-section after appropriate etching of the cross-section of the quenched and tempered mold steel. Further, as described above, the mold steel in its quenched and tempered state preferably has a thermal conductivity of 20 W / (m·K) or more, and more preferably 24 W / (m·K) or more.

[0080] As described above, the mold steel according to embodiments of the present invention comprises a predetermined composition, thereby achieving both high wear resistance and high thermal shock resistance. These properties are achieved while reducing the content of expensive alloying elements such as Mo and W, thereby reducing the material cost of the mold steel. Furthermore, when manufacturing molds, costly manufacturing methods such as powder forming are not required.

[0081] From the viewpoint of achieving the aforementioned high hardness, high thermal conductivity, and suppressing the formation of coarse grains and coarse crystalline carbides, preferred heat treatment conditions for mold steel according to embodiments of the present invention can be exemplified as follows: the molten and cast steel is appropriately forged, and then homogenized at 1,030°C ± 20°C for 45 minutes ± 15 minutes, quenched by cooling at a rate of 9°C / min to 100°C / min, and further tempered at 500°C to 600°C. Furthermore, from the viewpoint of reducing the formation of crystalline carbides, homogenization at 1,150°C or higher is preferred before forging. The contents of Al, Ni, S, and Cu, which are unavoidable impurities, can be adjusted (for example) by the stirring time during refining. The content can be reduced by allowing these impurity elements contained in the molten metal to escape to the upper part of the molten metal.

[0082] Because the die steel according to embodiments of the present invention exhibits high wear resistance and high thermal shock resistance, it is suitable for dies used in applications subject to high mechanical loads under high-temperature conditions, such as warm forming, hot stamping, warm trimming, and punching. In particular, the present invention is preferably applied to dies for hot stamping. However, the present invention is not limited thereto and can also be used for dies for various applications, such as molding resin or rubber materials.

[0083] Example

[0084] The present invention will be described in more detail below through examples.

[0085] [Sample Preparation]

[0086] Mold steels with the compositional compositions shown in Tables 1 and 2 were prepared. Specifically, the steels with the respective compositional ratios were melted in a vacuum induction furnace and then cast into ingots. The resulting ingots were hot-forged and then subjected to homogenization heat treatment at 1,150°C for various tests.

[0087] [Experimental Methods]

[0088] The methods used in each test will be described below. Unless otherwise stated, all evaluations were conducted at room temperature in air.

[0089] <Hardness Measurement>

[0090] Alloy samples were homogenized at 1,030 °C for 60 minutes, followed by quenching at a cooling rate of 9 °C / min. They were then tempered twice, with homogenization at 500 °C to 600 °C for 1 hour followed by air cooling. 10 mm × 12 mm specimens were then collected. After cutting the cross-section of the specimens, the cut surfaces were planar ground, and the hardness was determined at room temperature using the Rockwell C scale (HRC). The highest hardness value displayed within the tempering temperature range of 500 °C to 600 °C was recorded. Hardness values ​​between 58 HRC and 61 HRC were considered to be within an appropriate range.

[0091] <Particle size evaluation of crystalline carbides>

[0092] The grain size of the crystalline carbides was evaluated using samples after hardness testing. In the evaluation, a cross-section of the sample was etched with a etching solution and then observed under a microscope. Ten fields of view were observed at 200x magnification, and a total of 15 mm was measured. 2 The grain size of crystalline carbides in the field of view was determined. In grain size estimation, the white crystalline carbides observed in the image were enhanced by binarization, and the grain size was converted to the equivalent circle diameter. Then, the maximum grain size of the crystalline carbides in the image was recorded. If the obtained maximum grain size was less than 25 μm, the formation of coarse crystalline carbides could be considered sufficiently suppressed.

[0093] <Determination of Thermal Conductivity>

[0094] A 10 mm × 2 mm diameter region was cut from the remaining material used for hardness testing to obtain a sample for thermal conductivity testing. The thermal conductivity of the sample was measured by laser flash method. A thermal conductivity of 20 W / (m·K) or higher can be evaluated as sufficiently high.

[0095] <Evaluation of Crystallization Grain Size>

[0096] Each sample was homogenized at 1,050 °C for 5 hours, then cooled at a rate of 30 °C / min for quenching. The cross-section of the sample was cut, ground, and etched, and a 450 mm² area was observed under a microscope. 2 The average grain size in this region is evaluated according to the grain size level number specified in JIS G 0551:2020 "Test Method for Austenitic Grain Size of Steel", and the presence or absence of grain coarsening due to quenching is also evaluated. If the obtained grain size is 5 or higher according to the grain size level number, it can be considered that the formation of coarse grains has been sufficiently suppressed.

[0097] <Evaluation of Abrasion Resistance>

[0098] To evaluate the wear resistance of die steel, a 30mm × 60mm × 50mm block punch was prepared as a component to simulate the die using the die steel samples. The punch was then quenched and tempered under the condition of obtaining the highest hardness in the hardness test. Figure 1 As shown, a heated steel plate 3 was subjected to cap-shaped bending using a punch 1 and a die 2 obtained through quenching and tempering. The wear resistance of the punch 1 was evaluated by an accelerated test with the gap between the punch 1 and the die 2 set to -15%. The steel plate 3 to be processed was a hot-stamped steel plate with a thickness of 1.2 mm, heated to 980°C. An oxide layer formed on the surface of the steel plate 3. The steel plate 3 was not plated. Multiple processing operations were performed while replacing the steel plate 3. When the punch 1 wore down to the point that it caused problems in stamping within 90 processing operations, the wear resistance was rated as "C", indicating low wear resistance. On the other hand, when the punch 1 wore down but did not cause problems in stamping, the wear resistance was rated as "A", indicating high wear resistance. Furthermore, when the punch 1 showed almost no visually identifiable wear, the wear resistance was rated as "AA", indicating exceptionally high wear resistance.

[0099] <Evaluation of thermal shock resistance>

[0100] Each specimen was cut into 15.5 mm × 15.5 mm diameter pieces and quenched and tempered under the same conditions as those used to evaluate wear resistance. The thermal shock resistance of the obtained specimens was evaluated by repeatedly applying thermal loads using the following process: heating the surface of the specimen by high-frequency heating followed by water cooling as one cycle. This was repeated up to 200 cycles. In cases where large cracks appeared, the thermal shock resistance was rated "C," indicating low thermal shock resistance. Conversely, in cases where only minor cracks appeared, the thermal shock resistance was rated "A," indicating high thermal shock resistance. Furthermore, in cases where no cracks appeared, the thermal shock resistance was rated "AA," indicating exceptionally high thermal shock resistance.

[0101] [Experimental Results]

[0102] Tables 1 and 2 show the composition of each mold steel according to the various embodiments and comparative examples, the values ​​of P1 and P2 calculated based on the composition, and the results of the above-mentioned tests.

[0103] Table 1

[0104]

[0105] Table 1 (continued)

[0106]

[0107] Table 2

[0108]

[0109] Table 2 (continued)

[0110]

[0111] <Content of each component element and properties of mold steel>

[0112] The mold steels according to the embodiments shown in Table 1 contain the compositional composition specified in this disclosure above. The values ​​of P1 and P2 are also within predetermined ranges. Each mold steel according to the various embodiments has a hardness of 58 HRC or higher and 61 HRC or lower, a thermal conductivity of 20 W / (m·K) or higher, and a grain size index of 5 or higher. Furthermore, the maximum grain size of the crystalline carbides is limited to less than 25 μm. Moreover, based on these characteristics, high evaluation results were obtained in abrasion resistance tests and thermal shock resistance tests.

[0113] When comparing the various embodiments, a high correlation was found between hardness and abrasion resistance, with particularly high abrasion resistance (AA) obtained in embodiments with a hardness exceeding 60.5 HRC. Generally, embodiments exhibiting high hardness were confirmed in those with high contents of elements that demonstrate an effect on increasing hardness (such as C, Si, Cr, Mo, and W). On the other hand, particularly high thermal shock resistance (AA) was obtained in samples with a thermal conductivity of approximately 28 W / (m·K). (Refer to...) Figure 2 As described in detail, there is a high correlation between P1 and thermal conductivity, with a tendency to obtain high thermal conductivity in regions where P1 values ​​are large.

[0114] On the other hand, the mold steels of the comparative examples shown in Table 2 do not contain the composition specified in this disclosure. Consequently, high wear resistance and high thermal shock resistance cannot be achieved simultaneously. In each comparative example, specifically Comparative Examples 1 to 16, the content of each element is substantially outside the predetermined range. In these comparative examples, taking the main comparative example as an example, the relationship between the content of each element and its properties will be explained.

[0115] In Comparative Example 1, the carbon content was too low. Consequently, the hardness did not reach 58 HRC, resulting in low abrasion resistance. On the other hand, in Comparative Example 2, the carbon content was too high. Consequently, the hardness exceeded 61 HRC, and crystalline carbides with a particle size of 25 μm or more were formed, resulting in low thermal shock resistance.

[0116] In Comparative Example 3, the Si content was too low. Consequently, the hardness did not reach 58 HRC, resulting in low wear resistance. On the other hand, in Comparative Example 4, the Si content was too high. Consequently, the thermal conductivity did not reach 20 W / (m·K), resulting in low thermal shock resistance.

[0117] In Comparative Examples 5, 6, and 7, the contents of Mn, Cr, and Mo+W / 2 were all too low. Consequently, none of these samples achieved a hardness of 58 HRC, resulting in low wear resistance. On the other hand, in Comparative Example 8, the content of Mo+W / 2 was too high. Consequently, crystalline carbides with a particle size of 25 μm or more were formed, and the thermal conductivity did not reach 20 W / (m·K). As a result, thermal shock resistance was low.

[0118] In Comparative Example 9, the nitrogen content was too low. Consequently, the crystal grain size was less than 5, resulting in low thermal shock resistance. On the other hand, in Comparative Example 10, the nitrogen content was too high. In this case, the crystal grain size was also less than 5. Similarly, crystalline carbides with a grain size of 25 μm or more were formed. As a result, the thermal shock resistance was also low.

[0119] In Comparative Example 12, the V content is too low. Accordingly, the crystal grain size is less than 5, resulting in low thermal shock resistance. On the other hand, in Comparative Example 13, the V content is too high. Accordingly, coarse crystalline carbides with a grain size of 25 μm or more are formed, so the crystal grain size is also less than 5, and the thermal shock resistance is also low.

[0120] <Relationship between P1 and thermal conductivity>

[0121] Hereinafter, the relationship between P1 and thermal conductivity will be discussed. In Figure 2 , in addition to the respective examples, the relationship between P1 and thermal conductivity of some comparative examples is also plotted. Here, as Figure 2 the comparative examples shown, those comparative examples are selected in which the content of at least one of Si, Mo, W and Ni included in the definition of P1 in formula (1) and / or the value of P1 itself is outside the predetermined range. In other words, Figure 2 in addition to the respective examples, Comparative Examples 3, 4, 7 to 13, 27 and 28 are also shown.

[0122] According to Figure 2 , there is a correlation between the P1 value and thermal conductivity. Despite variations, thermal conductivity tends to increase as P1 increases. This is consistent with the fact that the contents of Si, Mo, W and Ni, which cause a decrease in thermal conductivity, have a negative contribution to P1 defined by formula (1). According to Figure 2 shown by the dashed line in , it can be seen that when P1 is 24 or more, the thermal conductivity is 20W / (m·K) or more.

[0123] <Relationship between P2 and crystal grain size>

[0124] Next, the relationship between P2 and crystal grain size will be discussed. In Figure 3 , in addition to the respective examples, the relationship between P2 and crystal grain size of some comparative examples is also plotted. Here, as Figure 3 the comparative examples shown, those comparative examples are selected in which the content of at least one of V, N and Al included in the definition of P2 in formula (2) and / or the value of P2 itself is outside the predetermined range. In other words, Figure 3 in addition to the respective examples, Comparative Examples 4, 7 to 16, 27 and 29 are also shown.

[0125] According to Figure 3, there is a correlation between the P2 value and the crystal grain size, wherein the crystal grain size is small in regions with low P2 and regions with high P2, while the crystal grain size is large in regions where P2 is a median value. This is consistent with the fact that the contents of V, N and Al that contribute to the formation of pinned particles are included in P2 defined by formula (2). When the contents of these elements are too low, pinned particles that contribute to inhibiting grain coarsening cannot be sufficiently formed; conversely, when the contents of these elements are too high, coarse grains are also formed. Therefore, in regions where P2 is neither too small nor too large, the crystal grain size is large, and grain refinement is promoted. As shown by Figure 3 the dotted line in , it can be seen that when P2 is in the range of 4.9 or more and 7.3 or less, the crystal grain size is 5 or more.

[0126] <Relationship Between S Content, Cu Content and Thermal Shock Resistance>

[0127] Finally, the relationship between the contents of S and Cu contained as inevitable impurities and thermal shock resistance will be described. Figure 4 shows the relationship between the contents of S and Cu and the thermal shock resistance evaluation results in each example and each comparative example (Comparative Examples 17 to 26), wherein each comparative example is only a comparative example in which the S content and / or Cu content exceeds a predetermined upper limit. The S content is plotted on the horizontal axis, the Cu content is plotted on the vertical axis, and the thermal shock resistance evaluation results are represented by symbols AA, A and C corresponding to the thermal shock resistance evaluation at the corresponding coordinate positions.

[0128] according to Figure 4 , points with high thermal shock resistance, which represent symbols corresponding to thermal shock resistance evaluations of A and AA, are concentrated in the lower left region where S ≤ 0.0015% and Cu ≤ 0.10%. In regions where the content of at least one of S and Cu exceeds the above range, symbols corresponding to thermal shock resistance evaluation of C are distributed, and thus the thermal shock resistance is low. Therefore, when the contents of S and Cu are increased, the thermal shock resistance of die steel is reduced, but when the contents of S and Cu, which are inevitable impurities, are limited to the range of S ≤ 0.0015% and Cu ≤ 0.10%, high thermal shock resistance is ensured.

[0129] 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.

[0130] This application is based on Japanese Patent Application No. 2022-026456 filed on February 24, 2022, and the content of said Japanese Patent Application is incorporated herein by reference.

[0131] 1: Punch

[0132] 2: Die

[0133] 3: Steel plate.

Claims

1. A type of mold steel, comprising: In terms of mass%, 0.55％≤C≤0.70％； 0.30% ≤ Si ≤ 0.60%; 0.55% ≤ Mn ≤ 1.2%; 5.7％≤Cr≤6.9％； 1.2% ≤ Mo + W / 2 ≤ 1.6%; 0.55% ≤ V ≤ 0.79%; and 0.005％≤N≤0.1％， The balance is Fe and unavoidable impurities, which include, by mass%, Al ≤ 0.020%, Ni ≤ 0.20%, S ≤ 0.0015%, and Cu ≤ 0.10%, and The following conditions must be met: P1 ≥ 24 and 4.9 ≤ P2 ≤ 7.3, where P1 is the value obtained based on equation (1) and P2 is the value obtained based on equation (2). P1 = 45 - 13.6[Si] - 7.0([Mo]+[W] / 2) - 12.9[Ni] (1) P2 = 7.4[V] + 15.8[N] + 38.6[Al] (2) In equations (1) and (2), [M] represents the content of element M in terms of mass%.

2. The mold steel according to claim 1, wherein... In the quenched and tempered state, the steel has a room temperature hardness of 58 HRC or higher and 61 HRC or lower, and a room temperature thermal conductivity of 20 W / (m·K) or higher.

3. The mold steel according to claim 1 or 2, It also includes, by mass percent, at least one element selected from the group consisting of the following elements: 0.01% ≤ Nb ≤ 0.5% 0.01% ≤ Zr ≤ 0.5% and 0.01% ≤ Ta ≤ 0.5%.

4. The mold steel according to claim 1 or 2 further comprises 0.10% ≤ Co ≤ 1.0% by mass.

5. The mold steel according to claim 1 or 2, wherein the steel, in its quenched state, has a grain size of 5 or more grain size levels as defined in JIS G0551:2020.

6. The mold steel according to claim 1 or 2, wherein the steel has crystalline carbides with a particle size of less than 25 μm in the quenched and tempered state.

7. A mold comprising mold steel according to any one of claims 1 to 6.

8. The mold according to claim 7, wherein the mold is a hot stamping mold.

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