Steel materials and molds

Abstract

To provide a steel material that is good in all three properties of machinability, impact value, and softening resistance.SOLUTION: The steel material contains 0.19≤C≤0.31 mass%, 0.010≤V≤0.180 mass%, Mn / Cr>0.150, Mn≤1.50 mass%, 5.60≤Cr≤6.60 mass%, Cu+Ni≤0.84 mass%, 0.60≤Si≤1.40 mass%, 0.60≤Mo≤2.00 mass%, 0.001≤Al≤0.080 mass%, and 0.003≤N≤0.040 mass%, with a balance being Fe and unavoidable impurities. It is preferable that the steel material has a mass of a 3000 kg or more, has a minimum dimension (Lmin) of the dimensions (L1, L2, L3) in longitudinal, lateral, and height directions of 300 mm or more, and / or has a hardness of the center part of 35 HRC or more and 45H RC or less. The mold is made of such a steel and has a mass of 2000 kg or more.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] This invention relates to steel materials and molds, and more particularly to steel materials suitable for manufacturing molds that are large in both mass and size, and to molds using the same. [Background technology]

[0002] Because molds are repeatedly subjected to stress and heat during use, the steel used for molds is required to have excellent properties in multiple areas, such as hardness, impact resistance, heat check resistance, and wear resistance. Therefore, various proposals have been made regarding steel materials possessing these properties.

[0003] For example, Patent Document 1 discloses a hot tool steel containing predetermined amounts of C, Si, Mn, Cr, Mo, and V, with the remainder being Fe and unavoidable impurities. The document states: (A) When the Si content is set to 0.01 mass% or more and less than 0.25 mass%, a hot work tool steel can be obtained that has machinability sufficient to industrially enable processing into mold shapes, and a higher thermal conductivity than general-purpose mold steel (e.g., JIS SKD61), and, (B) By optimizing the Mn, Cr, Mo, and V content, a hot work tool steel with high hardenability and high impact resistance can be obtained. It is stated.

[0004] The mold manufacturing process generally involves, (a) A first step in manufacturing steel materials suitable for mold manufacturing, (b) A second step in which a mold is manufactured from the obtained steel material. It is equipped with.

[0005] The first process (manufacturing process for steel materials for molds) includes various steps. The main steps are melting, refining, casting, homogenization heat treatment, hot working, normalizing, tempering, and spheroidizing annealing. Of these, one or more of the normalizing, tempering, and spheroidizing annealing steps may be omitted.

[0006] The second process (the process of manufacturing a mold from steel) has the following two processes. The first process is the HT process. The HT process generally (a) A process of machining (rough machining) spheroidized annealed steel into a rough mold shape, (b) A process of quenching (H) and tempering (T) the rough machined mold, (c) A process of performing finish machining on the mold that has been quenched and tempered, (d) A process of surface modification on the finished mold as necessary and includes.

[0007] The second process is the PH process. The PH process generally (a) A process of quenching and tempering steel (not necessarily spheroidized annealed steel) to temper (prehardening (PH)) the steel to an appropriate hardness, (b) A process of machining (finish machining) the tempered steel, (c) A process of surface modification on the machined mold as necessary and includes. For example, in the case of a die-cast mold, most of them are manufactured by the HT process, but when the required hardness of the mold is low, the PH process may be adopted.

[0008] The steel used in the PH process and the properties required for the mold manufactured by the PH process are (1) Good machinability, (2) High impact value when the quenching rate is low, and (3) Small hardness difference between the surface and the center (hardness homogeneity) are.

[0009] Also, in order to obtain a high impact value even when the quenching rate is low, (a) There are few coarse foreign substances, (b) There are few carbides and carbonitrides distributed in a dot pattern, and (c) High hardenability It is necessary to satisfy the three factors of However, it is not easy to manufacture a steel material that satisfies all of the above three characteristics.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] The problem to be solved by the present invention is to provide a steel material suitable for the PH process, that is, a steel material having good cutting performance, impact value, and hardness homogeneity. Another problem to be solved by the present invention is to provide a steel material having good cutting performance, impact value, and hardness homogeneity even when both the mass and size of the steel material are large. Furthermore, another problem to be solved by the present invention is to provide a mold manufactured from such a steel material.

Means for Solving the Problems

[0012] In order to solve the above problems, the steel material according to the present invention 0.19 ≦ C ≦ 0.31 mass%, 0.010 ≦ V ≦ 0.180 mass%, Mn / Cr > 0.150, Mn ≦ 1.50 mass%, 5.60 ≦ Cr ≦ 6.60 mass%, Cu + Ni ≦ 0.84 mass%, <C 0.40 ≦ Si ≦ 1.40 mass%, 0.60 ≦ Mo ≦ 2.00 mass%, 0.001 ≦ Al ≦ 0.080 mass%, and 0.003 ≤ N ≤ 0.040 mass% It contains [a certain component], with the remainder consisting of Fe and unavoidable impurities.

[0013] The steel material according to the present invention is Its mass is 3000 kg or more. Of the vertical dimension (L1), horizontal dimension (L2), and height dimension (L3), the smallest dimension (L min It is preferable that the width is 300 mm or more. Furthermore, the steel material according to the present invention is preferably one in which the hardness of the core is 35 HRC or more and 45 HRC or less. Furthermore, the mold according to the present invention is manufactured from the steel material according to the present invention and has a mass of 2000 kg or more. [Effects of the Invention]

[0014] The steel material according to the present invention has two main features. The first feature is that the carbon (C) and nitrile (V) content is relatively low. This makes it possible to suppress the decrease in impact value caused by coarse foreign matter and carbides and carbonitrides distributed in a dotted pattern. On the other hand, when the carbon and nitrile content is low, the austenite grains tend to coarseen during quenching. However, even with low carbon and nitrile content, by adding appropriate amounts of Al and N and adjusting the quenching conditions, it is possible to suppress the decrease in impact value caused by the coarsening of austenite grains during quenching.

[0015] The second major feature is that, while individually defining the Cr and Mn content, a parameter called "Mn / Cr" was introduced to find the optimal range for Mn and Cr content. Optimizing the Mn and Cr content suppresses the decrease in impact value caused by reduced hardenability and improves hardness homogeneity. In particular, "spheroidizing annealing (SA) properties" and "hardenability," and "hardenability" and "hardness homogeneity," are properties whose influence from elements is inversely related. Of these, steel materials used in the PH process do not necessarily require SA properties. Since the steel material according to the present invention has optimized Cr and Mn content, it is possible to achieve both hardenability and hardness homogeneity.

[0016] Furthermore, "machinability" and "heat check resistance" are generally properties whose influence from elements is inverse. Of these, for steel materials used in the PH process, machinability is more important than heat check resistance. Since the Si content of the steel material according to the present invention is optimized, it is possible to improve machinability without significantly reducing heat check resistance. [Brief explanation of the drawing]

[0017] [Figure 1] This figure shows the relationship between the amount of carbon (C) and the impact value. [Figure 2] This figure shows the relationship between the V amount and the impact value. [Figure 3] This figure shows the ranges of C and V. [Figure 4] This figure shows the effect of Mn and Cr content on impact value when the quenching rate is low.

[0018] [Figure 5] This figure shows the effect of Cr on softening resistance. [Figure 6] This figure shows the range of Mn and Cr content. [Figure 7] This figure shows the effect of Si content on machinability. [Figure 8] This figure shows the effect of Mo on softening resistance. [Figure 9] This is a schematic diagram illustrating the cutting position of the test specimen. [Modes for carrying out the invention]

[0019] One embodiment of the present invention will be described in detail below. [1. Steel materials] [1.1. Composition] [1.1.1. Main constituent elements] The steel material according to the present invention contains the following elements, with the remainder being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows.

[0020] (1) 0.19 ≤ C ≤ 0.31 mass%: Fine particles (carbides and carbonitrides) with a diameter of less than 0.5 μm function as "pinning particles" that suppress the growth of austenite crystal grains during quenching. If the carbon content is too low, there will be insufficient pinning particles during quenching. As a result, the crystal grains may coarseen, and the steel's properties such as impact strength, fracture toughness, and ductility may deteriorate. Furthermore, if the carbon content becomes too low, the martensitic transformation onset temperature (Ms point) becomes excessively high. As a result, hardenability increases, but impact strength may decrease.

[0021] Furthermore, if the carbon content is too low, it becomes difficult to obtain a hardness of 35 HRC or higher (preferably 36 HRC or higher) by tempering at 560-600°C. Therefore, the amount of C must be 0.19 mass% or more. Preferably, the amount of C is 0.20 mass% or more, and more preferably 0.21 mass% or more.

[0022] On the other hand, if the carbon content is excessive, coarse carbides and carbonitrides may crystallize during casting. These become "foreign matter" that reduces impact strength. It is difficult to completely dissolve and eliminate coarse foreign matter through heat treatment (homogenization heat treatment, normalizing). Coarse foreign matter may remain even after quenching and tempering, and may be observed with a diameter exceeding 3 μm. Foreign matter that remains without being completely dissolved can become the starting point for fracture, causing a decrease in impact strength and fatigue strength.

[0023] Furthermore, when ingots are formed into block or rod-shaped steel materials by hot working, if the cooling rate after hot working is slow, carbides and carbonitrides may precipitate in a dotted pattern, reducing the impact value. This phenomenon becomes more pronounced when the carbon content is excessive. Therefore, the amount of C must be 0.31 mass% or less. Preferably, the amount of C is 0.30 mass% or less, and more preferably, 0.29 mass% or less.

[0024] (2) 0.010 ≤ V ≤ 0.180 mass%: V combines with C and / or N in the steel to form carbides, carbonitrides, and / or nitrides. All of these function as pinning particles. Therefore, if the amount of V is too low, there will be insufficient pinning particles during quenching. Furthermore, if the amount of V becomes too low, the degree of secondary hardening during tempering will be reduced. As a result, it becomes difficult to obtain a hardness of 35 HRC or higher (preferably 36 HRC or higher) when tempered at 560-600°C. Therefore, the amount of V must be 0.010 mass% or more. Preferably, the amount of V is 0.013 mass% or more, and more preferably 0.016 mass% or more.

[0025] On the other hand, excessive V content increases the amount of coarse foreign matter and / or carbides and carbonitrides distributed in a point-like pattern. As a result, a decrease in impact value may become more pronounced. Furthermore, excessive V content increases the cost of steel, which is counterproductive to resource conservation. Therefore, the amount of V must be 0.180 mass% or less. Preferably, the amount of V is 0.170 mass% or less, and more preferably 0.160 mass% or less.

[0026] (3) Mn / Cr > 0.150: The ratio of the mass of manganese (Mn / Cr) to the mass of chromium (Cr) in steel affects its spheroidizing annealing (SA) properties. Generally, the smaller the Mn / Cr ratio, the better the SA properties. Therefore, for steel materials used in the hardening process, it is preferable to reduce the Mn / Cr ratio in order to ensure high SA properties that allow for sufficient softening with simple SA. However, maintaining a low Mn / Cr ratio limits the amount of manganese that can be added, thus limiting the improvement in hardenability.

[0027] In contrast, for steel materials used in the PH process, SA (saltability) is of low importance. On the other hand, when dealing with large steel materials or molds in the PH process, high hardenability is important. To obtain high hardenability, the Mn / Cr ratio must be greater than 0.150. Preferably, the Mn / Cr ratio is 0.155 or higher, and more preferably 0.160 or higher.

[0028] (4) Mn ≤ 1.50 mass%: Mn affects hardenability. Generally, the higher the Mn content, the better the hardenability. However, if the Mn content is excessive, the thermal conductivity decreases and the heat check resistance deteriorates. Also, if the Mn content is excessive, the amount of retained austenite increases. Retained austenite may decompose during tempering, but whether it decomposes or not, retained austenite can adversely affect the properties of the steel. Therefore, the Mn content should be 1.50 mass% or less. Preferably, the Mn content is 1.45 mass% or less, and more preferably 1.40 mass% or less.

[0029] (5) 5.60 ≤ Cr ≤ 6.60 mass%: Cr affects hardenability, hardness homogeneity, and high-temperature strength. If the Cr content is too low, hardenability and hardness homogeneity decrease. High-temperature strength also decreases. Therefore, the Cr content needs to be 5.60 mass% or more. Preferably, the Cr content is 5.70 mass% or more, and more preferably 5.80 mass% or more.

[0030] On the other hand, excessive chromium content reduces softening resistance. That is, while the mold surface in contact with the molten metal during die-casting becomes hot, the mold surface heated to high temperatures becomes more susceptible to softening. When the high-temperature strength decreases due to softening, the heat check resistance also deteriorates. Furthermore, softening becomes more pronounced in the region exceeding the maximum hardness, making it difficult to adjust the temper hardness. This is because hardness is sensitive to fluctuations in furnace temperature.

[0031] Furthermore, excessive chromium content reduces thermal conductivity. As a result, thermal stress increases, and heat check resistance deteriorates. Also, excessive chromium content causes chromium-based carbides to precipitate at grain boundaries after hot working, which reduces impact strength. Moreover, when the Si content is 0.50 mass% or less, increasing the chromium content significantly reduces machinability. Therefore, the amount of Cr must be 6.60 mass% or less. Preferably, the amount of Cr is 6.50 mass% or less, and more preferably, 6.40 mass% or less.

[0032] (6) Cu + Ni ≤ 0.84 mass%: In this invention, as described above, hardenability and softening resistance are ensured by the balance between Cr and Mn (Cr amount, Mn amount, Mn / Cr ratio). In contrast, Cu and Ni both have the effect of improving hardenability and softening resistance, but the effect is not very significant. Rather, if the amount of Cu + Ni becomes excessive, the adverse effects described later become noticeable. Therefore, the total amount of Cu and Ni is specified, and the upper limit of that total amount is set to a range in which the effect on hardenability is small.

[0033] The "hardenability property value" is an indicator of the effect of alloying elements on improving the hardenability of steel. A higher hardenability property value indicates a greater effect on hardenability. The hardenability property value is determined for each type of alloying element and the amount added. The hardenability of steels with different compositions is evaluated by adding the hardenability property values ​​according to the type and amount of alloying elements.

[0034] Here, the hardenability property value when 0.10 mass% of Mn is added is 0.125. On the other hand, the hardenability property value when 0.42 mass% of Ni is added is 0.062, and the hardenability property value when 0.42 mass% of Cu is added is also 0.062. In other words, the hardenability property value (additional value) when 0.42 mass% of Cu and Ni are added (total 0.84 mass% added) is 0.124. This value is almost the same as the hardenability property value when 0.10 mass% of Mn is added (=0.125). This means that when the amount of Cu + Ni is 0.84 mass% or less, the effect on improving hardenability is small. When the amount of Cu + Ni is around 0.84 mass%, the effect on improving high-temperature strength is also small.

[0035] On the other hand, when the Cu+Ni content reaches about 0.84 mass%, various problems become apparent. Specifically, these include increased cracking during hot working, increased retained austenite, decreased thermal conductivity, and increased costs. Therefore, the Cu+Ni content needs to be 0.84 mass% or less. In this invention, since the condition Mn / Cr > 0.150 must be met, the total amount of Mn and Cr that contribute to improved hardenability exceeds 6.44 mass% (see Figure 6). From this, it is clear that if the Cu+Ni content is 0.84 mass% or less, it will not significantly affect hardenability. The Cu+Ni content is preferably 0.78 mass% or less, and more preferably 0.72 mass% or less.

[0036] (7) 0.40 ≤ Si ≤ 1.40 mass %: If the Si content is too low, machinability decreases, making it difficult to industrially and stably machine large molds. In particular, since the steel material of the present invention is intended for the manufacture of large molds, a large amount of material needs to be removed, and good machinability is required. Therefore, the Si content needs to be 0.40 mass% or more. Preferably, the Si content is 0.45 mass% or more, and more preferably 0.50 mass% or more.

[0037] On the other hand, when the amounts of C, V, and N are high, and the amount of Si is excessive, there may be an increase in coarse precipitates and / or carbides and carbonitrides distributed in a point-series pattern. Furthermore, for steel materials used in the PH process, good machinability is prioritized over high heat check resistance. However, if the Si content is excessive, the thermal conductivity decreases. As a result, when used as a die-casting mold, thermal stress increases, and heat check resistance may deteriorate. If the heat check resistance decreases excessively, it becomes difficult to use it as a die-casting mold. Therefore, the Si content must be 1.40 mass% or less. Preferably, the Si content is 1.30 mass% or less, and more preferably 1.20 mass% or less.

[0038] (8) 0.60 ≤ Mo ≤ 2.00 mass%: If the amount of Mo is too low, the degree of secondary hardening during tempering will be reduced. Therefore, if the amount of Mo is too low, it will be difficult to obtain a hardness of 35 HRC or higher (preferably 36 HRC or higher) when tempered at 560-600°C. In addition, softening resistance and high-temperature strength may be insufficient, and heat check resistance may deteriorate. For this reason, the amount of Mo needs to be 0.60 mass% or higher. Preferably, the amount of Mo is 0.70 mass% or higher, and more preferably, 0.80 mass% or higher.

[0039] On the other hand, excessive Mo content reduces machinability. In particular, when Si content is low, excessive Mo content significantly reduces machinability. Furthermore, excessive Mo content can reduce fracture toughness. This tendency becomes more apparent when Si content is high. Moreover, excessive Mo content increases the cost of steel materials, which is counterproductive to resource conservation. Therefore, the Mo content needs to be 2.00 mass% or less. Preferably, the Mo content is 1.95 mass% or less, and more preferably 1.90 mass% or less.

[0040] (9) 0.001 ≤ Al ≤ 0.080 mass%: The steel material according to the present invention has significantly lower carbon and vitrification content than existing hot die steel (SKD61). Therefore, the amount of vitrification-based carbides, carbonitrides, and nitrides that act as pinning particles during quenching is less than in SKD61. Accordingly, in the present invention, AlN particles are also used in combination to suppress the growth of austenite crystal grains.

[0041] If the Al content is too low, it becomes difficult to reduce oxygen during refining, which can lead to an increase in oxides and a decrease in impact strength. Also, if the Al content is too low, there will be insufficient AlN, which acts as pinning particles. As a result, the austenite grains may coarse during quenching, which can lead to a decrease in impact strength, fracture toughness, and / or ductility. Therefore, the Al content must be 0.001 mass% or more. Preferably, the Al content is 0.002 mass% or more, and more preferably 0.003 mass% or more.

[0042] On the other hand, if the amount of Al is excessive, the amount of coarse alumina particles increases, which may reduce the impact strength and fatigue strength. Also, the thermal conductivity may decrease, and the heat check resistance may worsen. Therefore, the amount of Al needs to be 0.080 mass% or less. Preferably, the amount of Al is 0.070 mass% or less, and more preferably 0.060 mass% or less. Furthermore, when adding calcium (Ca) to improve machinability, the amount of aluminum (Al) is extremely important in optimizing the compound's morphology.

[0043] (10) 0.003 ≤ N ≤ 0.040 mass%: In this invention, the amount of N is specified along with the amount of Al in order to disperse AlN particles in the austenite phase during quenching. If the amount of N is too low, there will be insufficient AlN to act as pinning particles. As a result, the austenite grains may coarseen during quenching, and the impact value, fracture toughness, and / or ductility may decrease. Also, if the amount of N is too low, there may be insufficient amounts of V-type carbonitrides and nitrides, which also act as pinning particles. Therefore, the amount of N needs to be 0.003 mass% or more. Preferably, the amount of N is 0.004 mass% or more, and more preferably 0.005 mass% or more.

[0044] On the other hand, adding an amount of N exceeding the amount that can be adjusted by normal refining requires the active addition of N using specialized equipment, which increases material costs. Furthermore, if the amount of N is excessive, the amount of coarse precipitates and / or carbides and carbonitrides distributed in a point-series pattern may increase. This tendency becomes apparent when the amounts of C, Si, and V are high. Also, if the amount of N is excessive, there may be an excessive amount of coarse AlN, which may reduce the impact value. Moreover, if the amount of N is excessive, when the mold is welded and repaired, the N in the steel may vaporize, causing the formation of defects inside or on the surface of the weld. Therefore, the amount of N needs to be 0.040 mass% or less. Preferably, the amount of N is 0.038 mass% or less, and more preferably 0.036 mass% or less.

[0045] (11) Inevitable impurities: The steel material according to the present invention may contain unavoidable impurities. The elements that may be contained as impurities in the steel material according to the present invention and their content are as follows. P≦0.03mass%, S≦0.006mass%, O≦0.006mass%, W≦0.30mass%, Co≦0.30mass%, B≦0.0002mass%, Nb≦0.004mass%, Ta≦0.004mass%, Ti≦0.004mass%, Zr≦0.004mass%, Ca≦0.0005mass%, Se≦0.03mass%, Te≦0.005mass%, Bi≦0.01mass%, Pb≦0.03mass%, Mg ≤ 0.02 mass%.

[0046] In the present invention, "content" refers to the "average elemental amount of the steel" obtained by dissolving a predetermined mass of steel (preferably 1 gram or more per elemental analysis) in an acid, including parts with high segregation, parts with low segregation, and parts with average segregation, and then using a chemical analysis method.

[0047] [1.1.2. Sub-constituent elements] In addition to the main constituent elements and unavoidable impurities described above, the steel material according to the present invention may further contain one or more elements as follows. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0048] [A. Group A] (12) 0.30 <W≦2.00mass%: The steel material according to the present invention has lower carbon and vitrification content than conventional hot-die steel, and therefore may lack sufficient strength depending on the application. In such cases, adding ethanol (W) is effective for increasing strength. To obtain this effect, the amount of W is preferably more than 0.30 mass%. More preferably, the amount of W is 0.80 mass% or more.

[0049] On the other hand, if the amount of W is excessive, the material cost will increase. Furthermore, it may lead to deterioration of mechanical properties and increased anisotropy due to the manifestation of segregation. Therefore, the amount of W is preferably 2.00 mass% or less. More preferably, the amount of W is 1.50 mass% or less.

[0050] (13) 0.30 <Co≦1.00mass%: Co, like W, has the effect of improving strength. Therefore, if the strength is insufficient, adding Co is effective in increasing the strength. To obtain this effect, the amount of Co is preferably more than 0.30 mass%. More preferably, the amount of Co is 0.50 mass% or more.

[0051] On the other hand, if the amount of Co is excessive, the material cost will increase. Furthermore, it may lead to deterioration of mechanical properties and increased anisotropy due to the manifestation of segregation. Therefore, the amount of Co is preferably 1.00 mass% or less. More preferably, the amount of Co is 0.90 mass% or less. Furthermore, the steel material according to the present invention may contain either Co or W, or it may contain both.

[0052] [B. Group B] (14) 0.0002 <B≦0.0080mass%: When the amount of phosphorus (P) in steel is relatively high, the P segregates at the grain boundaries, reducing the grain boundary strength and decreasing the impact strength. Adding boron (B) is effective in improving grain boundary strength. For grain boundary strength to improve, B must exist independently (without forming compounds) in the steel. If B forms BN, the effect of adding B is lost. Therefore, when adding B to steel containing nitrogen (N) for the purpose of improving grain boundary strength, it is necessary to bond the N with elements other than B.

[0053] Specifically, it is preferable to bond nitrogen with nitride-forming elements such as Ti, Zr, and Nb, which readily form nitrides. While these elements are effective even at impurity levels, if they are deficient, it is preferable to add an amount exceeding the impurity level. Furthermore, BN has the effect of improving the machinability of steel. Therefore, when adding B for the purpose of improving machinability, it is not necessary to actively add nitride-forming elements to the steel.

[0054] To obtain the effects described above, the amount of B is preferably 0.0002 mass% or more. More preferably, the amount of B is 0.0003 mass% or more, and even more preferably, 0.0004 mass% or more. On the other hand, adding more B than necessary does not make a difference in effect and offers no practical benefit. Furthermore, an excessive amount of B increases the cost of the steel. Therefore, the amount of B is preferably 0.0080 mass% or less. More preferably, the amount of B is 0.0075 mass% or less, and even more preferably, 0.0070 mass% or less.

[0055] [C. Group C] (15) 0.006 <S≦0.180mass%: (16) 0.0005 <Ca≦0.0500mass%: (17) 0.03 <Se≦0.50mass%: (18) 0.005 <Te≦0.100mass%: (19) 0.01 <Bi≦0.50mass%: (20) 0.03 <Pb≦0.50mass%:

[0056] In the steel material according to the present invention, the addition of free-cutting elements is effective in improving machinability. Specifically, examples of free-cutting elements include S, Ca, Se, Te, Bi, and Pb. The steel material according to the present invention may contain one of these free-cutting elements, or it may contain two or more of them.

[0057] To obtain sufficient machinability, it is preferable that the content of each free-machining element is greater than the lower limit value mentioned above. On the other hand, if the content of free-cutting elements is excessive, the material becomes prone to cracking during hot working. Furthermore, excessive free-cutting elements may reduce impact strength, fatigue strength, and heat check resistance. Therefore, it is preferable that the content of each free-cutting element be below the above-mentioned upper limit.

[0058] [D. Group D] (21) 0.004 <Nb≦0.100mass%: (22) 0.004 <Ta≦0.100mass%: (23) 0.004 <Ti≦0.100mass%: (24) 0.004 <Zr≦0.100mass%:

[0059] The steel material according to the present invention may have carbonitride-forming elements other than V and Al added to increase the amount of carbides, carbonitrides, and / or nitrides. Specific examples of carbonitride-forming elements include Nb, Ta, Ti, and Zr. The steel material according to the present invention may contain one of these carbonitride-forming elements, or two or more.

[0060] To suppress excessive grain growth of austenite crystal grains, it is preferable that the content of each carbonitride-forming element is greater than the lower limit mentioned above. On the other hand, if the content of carbonitride-forming elements is excessive, carbides, carbonitrides, and / or nitrides will crystallize in a coarse state during casting. These coarse crystallized particles remain as foreign matter without disappearing during homogeneous heat treatment, SA (saturation), and quenching, causing a decrease in impact strength and fatigue strength. Therefore, it is preferable that the content of each carbonitride-forming element be below the above-mentioned upper limit.

[0061] [1.2. Characteristics of Steel Materials] [1.2.1. Mass and Size] As mentioned above, the required properties for steel materials used in the PH process and molds manufactured in the PH process are machinability, impact resistance, and hardness homogeneity. Among these three properties, a problem with large steel materials is the low impact resistance inside large molds manufactured from large steel materials.

[0062] One reason why impact resistance decreases in large molds is that large foreign matter tends to crystallize inside large steel materials. This is because the solidification rate during ingot manufacturing is low inside large steel materials. The second reason why impact strength decreases in large molds is that the slow cooling rate after hot working makes it easy for carbides and carbonitrides to precipitate in a series of dots. The third reason why impact resistance decreases in large molds is that the quenching rate decreases within the large steel material.

[0063] The steel material according to the present invention has low carbon and vinyl content, and optimized manganese and chromium content, so it is less affected by large foreign matter, low cooling rates after hot working, and low quenching rates. In other words, even when both mass and size are large, the steel material according to the present invention can achieve a high level of balance between the three characteristics of machinability, impact resistance, and hardness homogeneity.

[0064] For example, by optimizing the composition and manufacturing conditions of the steel, it is possible to obtain steel with a mass of 3000 kg or more, in addition to all three of the above characteristics reaching a practical level. Further optimization of the composition and manufacturing conditions of the steel makes it possible to manufacture steel with a mass of 4000 kg or more, or even 5000 kg or more.

[0065] Furthermore, by minimizing the composition and manufacturing conditions of the steel material, in addition to possessing the above characteristics, the smallest size (L) among the vertical dimension (L1), horizontal dimension (L2), and height dimension (L3) is achieved. min Steel material with a diameter of 300 mm or more can be obtained. Further optimization of the composition and manufacturing conditions of the steel material results in L min It is possible to manufacture steel materials that are 350 mm or longer, or even 400 mm or longer. Here, "vertical dimension (L1)", "horizontal dimension (L2)", and "height dimension (L3)" refer to the lengths of the three sides of the smallest rectangular prism that circumscribes the steel material.

[0066] [1.2.2. Hardness] [A. Definition] In this invention, "hardness of steel" means, (a) Cut a test piece from near the center (region with slow quenching rate) or near the outer edge (region with fast quenching rate) of the cross-section of the steel material that has been tempered to a predetermined hardness. (b) The Rockwell C scale hardness measured at room temperature using the specimen. It refers to.

[0067] Figure 9 shows a schematic diagram illustrating the cutting position of the test specimen. For example, if the steel material is a block material 10 with dimensions a[mm]×b[mm]×c[mm] (a≦b≦c, a≧300mm), a first material 12 of dimensions a[mm]×b[mm]×d[mm] is cut from near the center in the c-axis direction. d is not particularly limited, but 30 to 70 mm is preferred. "Near the center" refers to the central region when the block material 10 is divided into three equal parts along the c-axis direction (the direction of the maximum length of the block material 10). The first material 12 is cut from any position in this "near the center".

[0068] Next, a second material 14 with dimensions e[mm] × f[mm] × d[mm] is cut from the center of the ab surface of the first material 12. The values ​​of e and f should preferably be selected to be optimal depending on the size of the ab surface, the evaluation purpose, etc. For example, if a = 600 to 800 mm and b = 900 to 1100 mm, then e = 90 to 120 mm and f = 130 to 160 mm are preferable. Furthermore, a third material 16 measuring g[mm] × g[mm] × d[mm] is cut from the outer circumference of the first material 12. The cutting position of the third material 16 is not particularly limited, but the corner of the first material 12 is preferred. The value of g is not particularly limited, but 40 to 60 mm is preferred. In addition, test pieces for hardness measurement are cut from the second material 14 and the third material 16, respectively, and the hardness is measured using these.

[0069] More specifically, "hardness of the core" refers to the Rockwell C-scale hardness measured using a sample cut from the core of the first material 12. "Hardness of the outer periphery" refers to the Rockwell C scale hardness measured using a sample that has not been decarburized and was cut from the outer periphery of the first material 12. The "center" is defined as the distance from the centroid of the cross-section of the first material 12 to the outer edge of the cross-section, drawing arbitrary straight lines radially from the centroid to the outer edge of the cross-section, with A being the intersection point of each line with the outer edge of the cross-section, and R being the distance from the centroid to intersection A. A When this is done, R is calculated from the centroid of the cross-section of the first material 12. A This refers to the region up to / 2. The "outer perimeter" refers to the area outside the central part.

[0070] [B. Heat treatment conditions] The heat treatment conditions (quenching conditions and tempering conditions) for tempering steel materials are not particularly limited, as long as the desired hardness can be achieved. The optimal quenching temperature varies depending on the composition of the steel. Typically, the quenching temperature is between 880°C and 980°C. The cooling rate during quenching varies depending on the size of the steel and the cooling method. For large steel pieces, the cooling rate in the core is usually less than 10°C / min during the cooling interval from 800°C to 200°C. Similarly, the optimal tempering temperature varies depending on the composition of the steel. The tempering temperature is typically 560-600°C. If the desired hardness cannot be achieved in a single tempering cycle, it is preferable to repeat the tempering process multiple times.

[0071] [C. Hardness homogeneity] The steel material according to the present invention can achieve high hardness even when its mass and size are relatively large. Furthermore, the difference in hardness between the outer periphery, where the cooling rate is fast, and the central area, where the cooling rate is slow, is small, meaning that it exhibits high hardness homogeneity.

[0072] By optimizing the composition and manufacturing conditions of the steel, the hardness (H1) of the outer surface at room temperature becomes 36-44 HRC. Further optimization of the composition and manufacturing conditions of the steel results in H1 being 37-43 HRC or 38-42 HRC. By optimizing the composition and manufacturing conditions of the steel, the core hardness (H2) at room temperature becomes 35-45 HRC. Further optimization of the composition and manufacturing conditions of the steel results in an H2 of 36-44 HRC or 37-43 HRC. Furthermore, when the composition and manufacturing conditions of the steel material are optimized, the absolute value of the difference between H1 and H2 (ΔH = |H1 - H2|) becomes 3.5 HRC or less.

[0073] [1.2.3. Impact value] In the present invention, the "impact value of the steel material" means (a) An impact test piece is cut out from near the center of the cross-section of the steel material quenched and tempered to a predetermined hardness (a region where the solidification rate and quenching rate are slow), (b) The impact value obtained by conducting an impact test at 15 to 35°C .

[0074] The cutting position of the impact test piece is near the center of the cross-section of the steel material. That is, the impact test piece is cut out from the second material 14 shown in FIG. 9. Also, regarding the heat treatment conditions (quenching conditions and tempering conditions) for quenching and tempering the steel material, since they are as described above, the description is omitted.

[0075] The "impact test piece" means a test piece conforming to JIS Z2242 (10 mm × 10 mm × 50 mm, arc radius at the notch tip: 1 mm, notch depth: 2 mm, cross-sectional area of the test piece at the bottom of the notch: 0.8 cm 2 ). The "impact value (J / cm 2 )" means the value obtained by dividing the absorbed energy [J] by the cross-sectional area (0.8 [cm 2 ) at the bottom of the notch of the test piece. [[ID=3​​​​​​​​​​​Specifically, by optimizing the composition and manufacturing conditions, the average impact value becomes 25 [J / cm²]. 2 Steel material is obtained that is ] or higher and has a low impact value ratio of 30% or less. Further optimization of the composition and manufacturing conditions results in an average impact value of 26 [J / cm²]. 2 ] or more, or 27 [J / cm] 2 This concludes the explanation. Furthermore, by further optimizing the composition and manufacturing conditions, the low impact value rate can be reduced to 20% or less, or even 10% or less.

[0077] [2. Molds] The mold according to the present invention is made of the steel material according to the present invention and has the following characteristics.

[0078] [2.1. Mass and Size] The steel material according to the present invention is characterized by excellent machinability, impact resistance, and hardness homogeneity, even when its mass and size are relatively large. Therefore, by using such steel material, a mold with excellent impact resistance and hardness homogeneity can be obtained, even when its mass and size are relatively large.

[0079] By optimizing the mold composition and manufacturing conditions, a mold can be obtained that has impact resistance and hardness homogeneity at a practical level, as well as a mass of 2000 kg or more. Further optimization of the mold composition and manufacturing conditions makes it possible to manufacture molds with a mass of 3000 kg or more, or even 4000 kg or more.

[0080] Furthermore, by minimizing the mold composition and manufacturing conditions, in addition to possessing the above characteristics, the smallest dimension (L') among the vertical dimension (L'1), horizontal dimension (L'2), and height dimension (L'3) is min A mold with a diameter of 250 mm or more can be obtained. Further optimization of the mold composition and manufacturing conditions will result in L' min Even molds with dimensions of 300mm or more, or 350mm or more, can be manufactured. Here, "vertical dimension (L'1)", "horizontal dimension (L'2)", and "height dimension (L'3)" refer to the lengths of the three sides of the smallest cuboid that circumscribes the mold.

[0081] [2.2. Hardness, Impact Value, Hardness Homogeneity] The steel material according to the present invention is particularly suitable as a steel material used in the PH process. Molds manufactured in the PH process have hardness, impact value, and hardness homogeneity equivalent to that of tempered steel material. Details of hardness, impact value, and hardness homogeneity are as described above and will not be explained further.

[0082] [3. Methods for manufacturing steel materials] The method for manufacturing steel materials according to the present invention is (a) A first step of dissolving raw materials that have been blended to a predetermined composition, refining the molten metal, and casting the molten metal into a mold, (b) A second step of homogenizing the ingot, (c) A third step in which the ingot after homogeneous heat treatment is hot-worked, (d) A fourth step in which the rough shape of the material after hot working is normalized, if necessary, (e) A fifth step in which the rough material is tempered as needed, (f) A sixth step in which the raw material is spheroidized and annealed as needed, (g) The seventh step involves quenching and tempering the raw material. It is equipped with.

[0083] [3.1. 1st step] First, raw materials formulated to achieve a predetermined composition are melted, the molten metal is refined, and the molten metal is cast into a mold (first step). The melting conditions, refining conditions, and casting conditions are not particularly limited, and the optimal conditions can be selected according to the purpose.

[0084] [3.2. 2nd step] Next, the ingot is subjected to homogenization heat treatment (second step). Homogenization heat treatment is performed to homogenize the components by diluting the segregation of components that occurred during solidification and by dissolving as much of the foreign matter that crystallized during solidification as possible. The conditions for homogenization heat treatment are not particularly limited, and the optimal conditions can be selected according to the purpose.

[0085] [3.3. Third step] Next, the ingot, after homogeneous heat treatment, is subjected to hot working (third step). Hot working is performed to transform the ingot into a rough shape with the desired form. The conditions for hot working are not particularly limited, and the optimal conditions can be selected according to the purpose.

[0086] [3.4. 4th step] Next, if necessary, the rough-shaped material after hot working is normalized (step 4). Normalization is performed when it is necessary to homogenize and refine the structure of the rough-shaped material. The normalization conditions are not particularly limited, and the optimal conditions can be selected according to the purpose. Note that the normalization step can be omitted.

[0087] [3.5. 5th step] Next, if necessary, the rough material is tempered (Step 5). Tempering is performed when it is necessary to temper the martensite or bainite that has formed during the cooling process after normalizing, or when it is necessary to precipitate carbides in preparation for spheroidizing annealing. The tempering conditions are not particularly limited, and the optimal conditions can be selected according to the purpose. Note that the tempering step can be omitted.

[0088] [3.6. 6th step] Next, if necessary, the raw material is subjected to spheroidizing annealing (step 6). Spheroidizing annealing is performed during the quenching heating process in step 7 to obtain fine austenite crystal grains. Note that the spheroidizing annealing step can be omitted.

[0089] [3.7. 7th step] Next, the rough-shaped material is quenched and tempered (step 7). Quenching and tempering, which are performed after quenching, are carried out to temper the steel to a predetermined hardness. The quenching and tempering conditions are not particularly limited, as long as the predetermined hardness is achieved.

[0090] [4. Method for manufacturing molds] The mold according to the present invention is (a) The first step involves machining (finishing) the steel material which has been tempered to a suitable hardness (pre-hardened (PH)), (b) A second step in which the machined mold is surface modified, if necessary. It is equipped with. The methods and conditions for each process are not particularly limited, and the most suitable ones can be selected according to the purpose.

[0091] [5. Effect] The required characteristics of the steel materials used in the PH process and the molds manufactured in the PH process are: (1) Machinability, (2) Impact value when the hardening speed is low, and (3) Hardness homogeneity Therefore, the reasons why these three characteristics are necessary will be explained below, mainly using die casting as an example.

[0092] [5.1. Machinability] Steel materials used for machining are required to minimize tool wear even at high processing speeds. Excessive tool wear increases the frequency of tool replacement, thereby raising processing costs. On the other hand, reducing the processing speed to avoid tool wear reduces processing efficiency. For these reasons, steel materials used for molds are required to be machineable efficiently and at low cost, in other words, to have "good machinability."

[0093] When manufacturing molds using the HT process, rough machining is performed on steel softened to 98 HRB (approximately 238 HV) or less through spheroidizing annealing, so even if the amount of material removed is large, it is not as problematic as in the PH process. In contrast, the PH process involves machining large quantities of steel that has been hardened to 35 HRC (approximately 345 HV) or higher through quenching and tempering. Therefore, if the machinability is poor, the process will not be industrially viable.

[0094] On the other hand, steels with good machinability generally contain a large amount of Si, P, and / or S. Molds made from such steels generally have poor heat check resistance. "Poor heat check resistance" means that heat checks occur easily and progress quickly. The reasons for this are explained below.

[0095] High-silicon steel has low thermal conductivity. When molds made from steel with low thermal conductivity are used in die casting, the temperature amplitude of the mold surface becomes large, resulting in high thermal stress. Molds made from high-P steel have low toughness. Therefore, using such molds in die casting makes crack formation and propagation easier. Furthermore, high-sodium (S) steel contains a relatively large amount of sulfides. In molds manufactured from such steel, the sulfides act as crack initiation points and propagation paths, making crack formation and propagation easier.

[0096] In other words, steel materials containing relatively large amounts of Si, P, and / or S have good machinability. However, when such steel materials are used as molds, high thermal stress acts on the matrix. As a result, heat checks, which are thermal fatigue cracks, occur and tend to propagate. In other words, "good machinability" and "good resistance to heat checks" are contradictory.

[0097] In contrast, the steel material according to the present invention is substantially free of P and S, and has an optimized Si content. Furthermore, by optimizing the composition of the steel material and the quenching and tempering conditions, the steel material can be tempered to a hardness suitable for the PH process. Therefore, the steel material according to the present invention exhibits good machinability without significantly impairing heat check resistance.

[0098] [5.2. Impact value at low hardening speed] In the PH process, molds are manufactured from steel that has been tempered to a predetermined hardness. Therefore, the steel used to manufacture the molds needs not only hardness but also a high impact resistance. This is because molds with a high impact resistance are less prone to cracking.

[0099] To obtain a high impact value, the following three conditions must be met. That is, (a) There are few large foreign objects, (b) There are few carbides or carbonitrides distributed in a dotted pattern. (c) High hardenability That is the case.

[0100] [5.2.1. Few large foreign objects] "Foreign matter" refers to substances with a different composition from the matrix, such as carbides, nitrides, carbonitrides, sulfides, and oxides. In this invention, "coarse foreign matter" refers to foreign matter with a size (equivalent to a circle diameter) of 3 μm or more.

[0101] When stress is applied to a mold, coarse foreign matter is likely to become the starting point for cracks and the propagation path of those cracks. Therefore, to obtain a high impact value, it is better to have as little coarse foreign matter as possible. Foreign matter can consist of those containing one type of metallic element and those containing two or more types of metallic elements. Conventional die-casting mold steel has high carbon and vitamin (V) content, so coarse foreign matter usually consists of carbides and carbonitrides containing vitamin V. The size and amount of vitamin V carbides and carbonitrides are influenced not only by the chemical composition of the steel but also by the solidification rate during casting, the temperature and time of the homogenization heat treatment, and other factors.

[0102] [5.2.2. There are few carbides or carbonitrides distributed in a dotted pattern.] If the cooling rate after hot working is low, carbides or carbonitrides may precipitate at the austenite grain boundaries, depending on the composition. The shape of these carbides and carbonitrides can be rod-shaped, V-shaped, W-shaped, or wavy, and the size in the direction where the length is greatest is 0.5 to 3 μm. Although these carbides and carbonitrides are smaller in size than the "coarse foreign matter" mentioned above, they are distributed in a series of points, intermittently connected at the grain boundaries. Therefore, the presence of such carbides and carbonitrides makes the material more susceptible to fracture at the grain boundaries, and the impact value is greatly reduced.

[0103] [5.2.3. High hardenability] As the size of the steel material increases, the cooling rate during quenching decreases. This trend is particularly pronounced within the steel material. Therefore, when manufacturing molds using the PH process, as mold sizes have increased in recent years, the cooling rate inside the steel material during quenching decreases. As a result, a decrease in the impact strength of molds machined from large steel materials using the PH process has become a problem.

[0104] [5.2.4. High Impact Value] For the reasons described above, there is a strong demand for steel materials that can achieve high impact values ​​even at low quenching rates, in other words, "steel materials with good hardenability." "Good hardenability" means, in other words, that the steel does not produce coarse bainite even at low quenching rates.

[0105] On the other hand, steels with good hardenability tend to have poor SA properties. This is because carbides do not easily precipitate during the slow cooling of SA in such steels, and ferrite transformation does not proceed easily, making it difficult to obtain an SA structure (a structure in which carbides are dispersed within the ferrite matrix). In contrast, steel materials subjected to the PH process do not necessarily need to be considered in terms of SA properties. However, spheroidizing annealing may be performed to obtain fine austenite crystal grains during the subsequent quenching heating process. The steel material according to the present invention exhibits good hardenability because the Cr and Mn content is optimized to be suitable for such a PH process.

[0106] [5.3. Hardness homogeneity] To temper steel to a specified hardness (35-45 HRC), quenching and tempering are performed. When quenching large pieces of steel, the quenching rate differs between the surface and the interior. Consequently, the ratio of phases such as martensite, bainite, and retained austenite may differ between the surface and the interior. If tempering is performed in this state, the hardness will not be the same on the surface and the interior. Furthermore, when a mold is made from such steel (by machining it), the interior of the steel is exposed as the surface of the mold. Therefore, the hardness of the machined mold surface (the interior side of the PH steel) differs from the hardness of the steel surface.

[0107] The difference in hardness is a difference in steel material properties, and as mentioned above, the difference in hardness between the surface and interior of PH steel is an important factor in ensuring mold performance. A smaller difference in hardness between the surface and interior is preferable, and this state is evaluated as having "good hardness homogeneity." In the present invention, the Cr and Mn content are optimized, resulting in good hardness homogeneity.

[0108] [5.4. Softening resistance] The surface of a die-casting mold experiences a temperature increase due to contact with molten metal. As the number of casting shots increases, the cumulative exposure time to high temperatures also increases, which can lead to a decrease in the hardness of the mold surface. This softening results in a decrease in high-temperature strength, and consequently, a deterioration in heat check resistance.

[0109] For the reasons stated above, die-casting molds require not only the three characteristics mentioned above, but also resistance to softening, i.e., "high softening resistance." However, it should be noted that steel materials with reduced chromium content and increased softening resistance have low high-temperature strength. This is because low-chromium steel has poor solid solution strengthening at high temperatures. A decrease in high-temperature strength degrades heat check resistance. In other words, "good softening resistance" and "good heat check resistance" are contradictory. In contrast, the steel material according to the present invention has optimized Cr and Mn content. Therefore, the steel material according to the present invention exhibits good softening resistance without significantly impairing hardenability or heat check resistance. [Examples]

[0110] [1. Verification tests for suitable elemental amounts] [1.1. Overview] The objectives to be achieved in this invention are reiterated below. (1) Machinability (2) Impact value when the hardening speed is low (a) There are few large foreign objects. (b) There are few carbides or carbonitrides distributed in a point-like pattern (hereinafter, these are collectively referred to as "grain boundary carbides"). (c) High hardenability (3) Hardness homogeneity

[0111] The following verification tests focused on all categories except (2)(a). There are three reasons for this. The first reason is that (2)(a) can only be accurately verified with steel manufactured from industrial-sized ingots (mass of 8 tons or more) with a low solidification rate. The second reason is that actually manufacturing large steel materials using ingots weighing more than 8 tons would result in excessive costs and research time. The third reason is that the effect of (2)(a) is very large, so in order to accurately verify the effect of (2)(b) or (2)(c) on the impact value, the effect of (2)(a) must be eliminated.

[0112] Therefore, a steel material with a small cross-section (diameter: 82 mm x length: approximately 3000 mm) was manufactured from an ingot with a high solidification rate (a small ingot with a mass of 150 kg). Next, test pieces made from this steel material were subjected to heat treatment that simulated an industrial manufacturing method (i.e., a method for manufacturing large mold steel materials and large molds). By doing so, it is thought that the properties of the steel material "other than (2)(a)" when molds are manufactured from industrial-sized ingots can be properly evaluated. On the other hand, (2)(a) was evaluated using steel materials actually manufactured from industrial-sized ingots with a low solidification rate (ingots with a mass of 8 tons or more).

[0113] [1.2. Verification Test of the Upper Limit of C Quantity] [1.2.1. Sample Preparation] [A. Making a round bar] Below, we examined the decrease in impact value when the amount of carbon (C) exceeded 0.31 mass%. The steel composition (mass%) was 1.20Si-0.06Cu-0.11Ni-1.31Mn-5.89Cr-1.68Mo-0.019Al-0.027N-0.18V, with the carbon content systematically varied. These steel types were cast into 150kg ingots. After manufacturing the ingots, homogenization heat treatment, hot working, normalizing, and tempering were performed. In this verification, normalizing and tempering were performed, but these processes may be omitted. Also, although not performed in this verification, spheroidizing annealing (SA) may be performed after hot working or tempering. Using the above process, a tempered steel material (round bar) with a diameter of approximately 82 mm and a length of approximately 3000 mm was manufactured.

[0114] [B. Heat treatment of a square bar simulating hot working] Ten square bars measuring 12mm x 12mm x 55mm were made from a tempered round bar. The resulting square bars were heated to reproduce the austenite grain size during industrial hot working. Specifically, they were held at 1240°C for 2H in a vacuum. Although heat treatment is not always performed in a vacuum in industrial processes, the heat treatment was performed in a vacuum for the purpose of this verification, as it simulates the temperature history.

[0115] Next, the square bar underwent a heat treatment simulating the cooling process after hot working of an ingot into a larger steel material. Specifically, following the 2H holding period at 1240°C mentioned above, it was cooled to 1000°C at a rate of 1°C / min, and then cooled from 1000°C to 600°C at a rate of 0.5°C / min. In addition, depending on the composition, carbides or carbonitrides may precipitate in a point-like pattern at the austenite grain boundaries within this temperature range.

[0116] In the temperature range below 600°C, an inert gas was introduced into the vacuum furnace and pressurized to 3-4 Torr (0.40-0.53 kPa). The square bars were then rapidly cooled by forcing convection of the inert gas. Cooling rates below 600°C do not simulate the large-sized steel materials manufactured industrially. However, since the purpose of this evaluation is to investigate the "effect of grain boundary carbides precipitated at grain boundaries in the high-temperature range after hot working," the objective can be achieved even if the cooling history below 600°C is as described above. Grain boundary carbides do not disappear even after "normalizing-tempering-quenching-tempering" after hot working, and ultimately remain in the mold, significantly reducing the impact value.

[0117] [C. Normalizing, tempering, and quenching / tempering of square bars] Next, the square bars, which had undergone heat treatment simulating hot working, were subjected to normalizing and tempering in a vacuum, in accordance with industrial manufacturing methods. Furthermore, the tempered square bars were vacuum quenched. Specifically, the square bars were held at 920°C for 1 hour in a vacuum. Next, an inert gas was introduced into the vacuum furnace and pressurized to 3-4 Torr (0.40-0.53 kPa), and the square bars were rapidly cooled by forcing convection of the inert gas, cooling them to below 200°C.

[0118] The cooling time from 920°C to 200°C during quenching was less than 60 minutes. In other words, the cooling of the square bar differs from the cooling of large steel materials manufactured industrially. However, since the purpose of this evaluation is to investigate the "effect of grain boundary carbides precipitated at grain boundaries in the high-temperature range after hot working," the objective can be achieved even if the quenching is rapid.

[0119] Next, the hardened square bars were subjected to further tempering. Tempering was carried out by holding the bars at 560-600°C for 2 hours, followed by cooling to below 200°C. Furthermore, tempering was added. Specifically, the square bar described above was held at 560-600°C for a predetermined time, and then cooled to below 200°C. This process was repeated at least once to temper the square bar to 39.5-40.5 HRC. The holding temperature, time, and number of treatments were varied depending on the type of steel (carbon content). This is because different carbon content results in different resistance to softening.

[0120] [1.2.2. Test Method] Impact test specimens were prepared from square bars tempered to 39.5-40.5 HRC. The shape of the impact test specimens conformed to JIS Z2242 (10mm x 10mm x 50mm, notch tip arc radius: 1mm, notch depth: 2mm, specimen cross-sectional area at the bottom of the notch: 0.8cm²). 2 The impact test specimens obtained were used to conduct impact tests at 15-35°C. The impact value used for evaluation was [J / cm²]. 2 ] refers to the absorbed energy [J] at the cross-sectional area of ​​the bottom of the notch in the test specimen, 0.8 cm². 2 This is the value obtained by dividing by , and represents the average value of 10 test specimens.

[0121] [1.2.3. Results] The required impact value for die-casting molds is 20 J / cm for molds with low loads. 2 In summary, for molds with high loads, the load is 25 J / cm². 2 That concludes the explanation. The impact value is 30 J / cm². 2 With the die-casting molds described above, the risk of breakage is significantly reduced. In this evaluation, the quenching is performed using rapid cooling, but in actual large molds where quenching is performed using slow cooling, the impact value is 5 J / cm². 2 The degree of impact decreases. Therefore, the threshold for impact value used to determine whether something is good or bad is set to 25 J / cm². 2 That's what I decided.

[0122] Figure 1 shows the relationship between carbon content and impact value. Unlike large molds, the impact test specimens used in the test underwent rapid quenching. Nevertheless, when the carbon content was excessive, the impact value reached 20 J / cm². 2Some test specimens were found to have a value less than 25 J / cm². This indicates that the influence of grain boundary carbides precipitated in the high-temperature range after hot working is very significant. Figure 1 shows that the impact value was 25 J / cm². 2 The above indicates that the steel grade has a carbon content of 0.31 mass% or less.

[0123] [1.3. Verification test of the upper limit of the V quantity] [1.3.1. Fabrication of square bars] Below, we examined the decrease in impact value when the V amount exceeded 0.180 mass%. The steel composition (mass%) was set to 0.31C-1.36Si-0.04Cu-0.13Ni-1.22Mn-5.96Cr-1.38Mo-0.018Al-0.032N, with the V content systematically varied. These steel types were cast into 150kg ingots. Ten square bars were then prepared in the same manner as the verification test for the C content.

[0124] [1.3.2. Test Method] The impact value was measured in the same manner as in the verification test of the amount of carbon (C).

[0125] [1.3.3. Results] Figure 2 shows the relationship between the V quantity and the impact value. Unlike large molds, the impact test specimens used in the test underwent rapid quenching. Nevertheless, when the V quantity was excessive, the impact value reached 20 J / cm². 2 Some test specimens were found to have a value less than 25 J / cm². This indicates that the influence of grain boundary carbides precipitated in the high-temperature range after hot working is very significant. Figure 2 shows that the impact value was 25 J / cm². 2 The above indicates that the steel grade has a V content of 0.180 mass% or less.

[0126] [1.3.4. Suitable ranges for C and V amounts] Figure 3 shows the ranges for carbon (C) and velocity (V). In this invention, the carbon (C) and velocity (V) are defined considering "hardness," "amount of coarse foreign matter," "amount of intergranular carbides precipitated at the grain boundaries after hot working," and "amount of pinned particles." Conventional hot work die steel has a carbon content of 0.32 mass% or more and a V content of 0.30 mass% or more. In contrast, the steel material according to the present invention has a carbon content of 0.31 mass% or less and a V content of 0.180 mass% or less, so the ranges for carbon (C) and velocity (V) differ from those of conventional steel.

[0127] Furthermore, the present invention differs from conventional steel in terms of quenching temperature. Conventional steel is quenched at a high temperature of 1010 to 1040°C in order to sufficiently solidify C and V. In contrast, the present invention, which has low levels of C and V, can be quenched at a low temperature of 880 to 980°C. Therefore, the steel material according to the present invention is (a) CO2 emissions can be reduced, (b) The deformation of the hardened steel is small, and cracks are less likely to occur in the steel. This has the advantage.

[0128] [1.4. Verification tests of Mn and Cr content] [1.4.1. Sample Preparation] The following section examines the effects of Mn and Cr content on hardenability and impact resistance. The steel composition (mass%) was set to 0.25C-0.81Si-0.08Cu-0.09Ni-1.78Mo-0.05V-0.028Al-0.011N, with systematically varied Mn and Cr content. These steel types were cast into 150kg ingots. Subsequently, in the same manner as the C content verification test, ten 12mm x 12mm x 55mm square bars and one 12mm x 12mm x 20mm small piece were prepared.

[0129] The square bars and small pieces described above were held in a vacuum at 920°C for 1 hour, and then quenched. The cooling rate during quenching was 8°C / min from 920°C to 750°C, 5°C / min from 750°C to 500°C, and 0.5°C / min from 500°C to 200°C. The cooling rate from 200°C to below 100°C was not specifically controlled. The above quenching process is one example that assumes the slowest internal cooling rate when quenching a large steel material weighing over 3000 kg. Since the phase transformation is almost complete when the temperature reaches 200°C, the cooling rate from there to below 100°C was not specifically controlled.

[0130] Next, the square bars and small pieces were tempered. Tempering was performed by holding them at 560°C for 2 hours, followed by cooling to below 200°C. Furthermore, tempering was added. Specifically, the square bars and small pieces described above were held at 560-600°C for a predetermined time, and then cooled to below 200°C. This process was repeated at least once to temper the square bars and small pieces to 39.5-40.5 HRC. The holding temperature and time, and the number of treatments were varied depending on the type of steel (Mn content and Cr content). This is because different Mn and / or Cr content result in different softening resistances.

[0131] [1.4.2. Test Method] [A. Impact Test] Impact test specimens were prepared from square bars tempered to 39.5-40.5 HRC. The shape of the impact test specimens conformed to JIS Z2242 (10mm x 10mm x 50mm, notch tip arc radius: 1mm, notch depth: 2mm, specimen cross-sectional area at the bottom of the notch: 0.8cm²). 2 The impact test specimens obtained were used to conduct impact tests at 15-35°C. The impact value used for evaluation was [J / cm²]. 2 ] refers to the absorbed energy [J] at the cross-sectional area of ​​the bottom of the notch in the test specimen, 0.8 cm². 2 This is the value obtained by dividing by , and represents the average value of 10 test specimens.

[0132] [B. Softening resistance] The softening resistance was evaluated using small pieces tempered to 39.5-40.5 HRC. The tempered pieces were held at 560°C for 24 hours and then cooled to room temperature. Subsequently, the HRC hardness was measured at room temperature.

[0133] [1.4.3. Results] [A. Impact Test] Figure 4 shows the effect of Mn and Cr content on impact value when the quenching rate is low. In Figure 4, "×" indicates an impact value of 25 J / cm 2 This indicates that the value is less than 25 J / cm², and "○" means the impact value is 25 J / cm². 2 More than 30J / cm 2 This indicates that the impact value is less than 30 J / cm², and "●" means the impact value is 30 J / cm². 2 This indicates the above values. The solid line represents the correlation line for Mn / Cr = 0.150, and the dashed line represents the correlation line for Cr = 5.60 mass%. Additionally, the dotted line represents the correlation line for Mn+Cr = 6.60 mass%, and the double dotted line represents the correlation line for Mn+Cr = 6.80 mass%.

[0134] From Figure 4, (a) If Mn / Cr > 0.15 and Cr ≥ 5.60 mass%, the impact value is 25 J / cm 2 This is the case. (b) If Mn+Cr≧6.60mass%, the impact value will be high, and (c) If Mn+Cr≧6.80mass%, the impact value will become even more stable at a higher level. I understood that. Based on the above, it was confirmed that the steel material according to the present invention exhibits high hardenability under conditions where the influence of coarse foreign matter and grain boundary carbides is minimal.

[0135] [B. Softening resistance] Figure 5 shows the effect of Cr on softening resistance. The smaller the decrease in hardness after heat treatment compared to the initial hardness of 40 HRC, the higher the softening resistance. From Figure 5, it can be seen that when the Cr content exceeds 6.60 mass%, the deterioration of softening resistance is significant.

[0136] [1.4.4. Preferred ranges for Mn and Cr content] Based on the above, a suitable range for the Mn and Cr content was determined. Figure 6 shows the range for the Mn and Cr content. The area enclosed by the four lines in Figure 6 is the range of the steel material according to the present invention. The range of Mn and Cr content for conventional hot work die steel is Mn < 0.80 mass% and Cr < 5.80 mass%. This range falls outside the lower left region of the range shown in Figure 6. From Figures 6 and 3, it is clear that the composition of the steel material according to the present invention is completely different from that of conventional hot work die steel.

[0137] [1.5. Verification Test of the Lower Limit of Si Content] [1.5.1. Preparation of Test Specimens] The following section examines the effect of Si content on machinability. The steel composition (mass%) was set to 0.22C-1.09Mn-0.07Cu-0.18Ni-6.12Cr-1.01Mo-0.04V-0.023Al-0.016N, with the Si content systematically varied. These steel types were cast into 150kg ingots. Subsequently, tempered steel bars (round bars) with a diameter of approximately 82mm and a length of approximately 3000mm were prepared in the same manner as the carbon content verification test.

[0138] Next, a 50mm x 25mm x 200mm block was cut from the tempered round bar. Then, the block was quenched and tempered under the same conditions as the verification tests for Mn and Cr content, and the block was tempered to 39.5-40.5 HRC.

[0139] [1.5.2. Test Method] The block was cut with a cutting tool, and the wear of the cutting tool was measured. The cutting distance at which the wear of the cutting tool reached 300 μm was defined as the tool life. A longer cutting distance indicates better machinability.

[0140] [1.5.3. Results] Figure 7 shows the effect of Si content on machinability. When the Si content is low, machinability is poor, and the number of cutting steps becomes enormous. In particular, for large molds, the amount of material removed is large, so poor machinability makes industrial viability difficult. From Figure 7, it can be seen that when the Si content is 0.40 mass% or higher, machinability close to that of SKD61, which is evaluated as having "very good machinability," can be obtained when the Si content is 0.60 mass% or higher. Furthermore, it can be seen that even if the Si content is further increased, the machinability approaches saturation.

[0141] [1.6. Verification tests of the lower and upper limits of Mo content] [1.6.1. Preparation of Test Specimens] The following section examines the effect of Mo content on softening resistance. The steel composition (mass%) was set to 0.29C-0.70Si-1.39Mn-0.07Cu-0.12Ni-5.79Cr-0.03V-0.026Al-0.010N, with the amount of Mo systematically varied. These steel types were cast into 150kg ingots. Subsequently, tempered steel bars (round bars) with a diameter of approximately 82mm and a length of approximately 3000mm were prepared in the same manner as the verification test for the amount of carbon.

[0142] Next, small pieces measuring 12mm x 12mm x 20mm were cut from the tempered round bar. Subsequently, the small pieces were quenched and tempered under the same conditions as the verification tests for Mn and Cr content, and the pieces were tempered to 39.5-40.5 HRC.

[0143] [1.6.2. Test Method] The softening resistance was evaluated using small pieces tempered to 39.5-40.5 HRC. The tempered pieces were held at 560°C for 24 hours and then cooled to room temperature. Subsequently, the HRC hardness was measured at room temperature.

[0144] [1.6.3. Results] Figure 8 shows the effect of Mo on softening resistance. Softening resistance decreased in both cases where the amount of Mo was too low and too high. The hardness of molds manufactured in the PH process is often 35 HRC or higher. From Figure 8, it can be seen that in order to ensure a hardness of 35 HRC or higher even after heat treatment at 560°C × 24H, the amount of Mo should be between 0.60 mass% and 2.00 mass%.

[0145] [2. Verification tests using large ingots] [2.1. Overview] In the verification test of suitable elemental amounts, small-sized (150 kg) ingots were used to manufacture steel with a small cross-section, and test pieces made from this steel were subjected to heat treatment simulating industrial manufacturing methods (manufacturing methods for large mold steel and large molds). This allowed for a proper evaluation of the properties "other than 2(a)" that would occur if the steel were manufactured using industrial methods and used as a mold.

[0146] On the other hand, in the following examples, the effects of the present invention were confirmed using ingots with a mass of 8 tons or more. In this case, the steel material was hardened and tempered, and the internal impact value was verified. That is, item "2(a)" above was verified. Other properties have already been verified in verification tests for suitable elemental amounts.

[0147] [2.2. Sample Preparation] [2.2.1. Preparation of block materials] Table 1 shows the compositions of the steels whose properties were verified (Examples 1-13, Comparative Examples 1-3). Comparative Example 1 corresponds to JIS SKD6 (AISI H11). Comparative Example 2 corresponds to a commercially available steel with adjusted Si-Mn-Cr ratio from SKD6, and is a steel with superior hardenability and heat check resistance compared to SKD6. Comparative Example 3 is a steel in which the C and V content exceeds the upper limits of the present invention. Although not shown in Table 1, all steels contain impurity elements such as P within the range that does not exceed the upper limits mentioned above.

[0148] [Table 1]

[0149] These steels were cast into ingots with a mass of approximately 21 tons. Because the solidification rate of a 21-ton ingot is even slower than that of a 10-ton ingot, coarse impurities are more likely to affect the impact value. Under these adverse conditions, the appropriateness of the carbon (C) and vinyl (V) content was verified.

[0150] A 21-ton ingot was subjected to high-temperature, long-duration homogenization heat treatment, followed by hot working to obtain a block material measuring 740mm x 1060mm x 2440mm (approximately 15 tons). The difference in mass between the ingot and the block material (approximately 6 tons) is due to the mass of the portion removed due to quality or shape issues.

[0151] The heat treatment conditions for the above block materials were set according to the type of steel. For Examples 1 to 13, quenching was performed from 920°C and tempering was performed at 560°C to 600°C. For Comparative Examples 1 to 3, after normalizing, tempering, and spheroidizing annealing, quenching was performed from 1030°C and tempering was performed at 580°C to 630°C. In this way, the surface hardness of all 16 types of steel was set to a pH state of approximately 40 HRC.

[0152] [2.2.2. Preparation of the second and third materials] Areas with a high concentration of coarse foreign matter are near the center where the solidification rate is low. Therefore, as shown in Figure 9, the first material 12 was cut from near the center in the c-axis direction of the block material 10 (a=740mm, b=1060mm, c=2440mm, w≒15000kg). Next, the second material 14 was cut from approximately the center of the ab-face of the first material 12, and the third material 16 was cut from the end of the first material 12. In this experiment, d=35mm, e=95mm, f=135mm, and g=50mm were used.

[0153] [2.3. Test Method] [2.3.1. Hardness] A 15mm x 15mm x 35mm piece was cut from the corner of the second material 14 (95mm x 135mm x 35mm). A 15mm x 15mm x 35mm piece was also cut from near the center of the third material 16 (50mm x 50mm x 35mm). These pieces were ground and polished to achieve the required parallelism and surface roughness for hardness measurement. The Rockwell C scale hardness (H1 for the outer periphery, H2 for the center) was measured using these pieces at room temperature.

[0154] [2.3.2. Impact Test] Twenty impact test specimens measuring 10mm x 10mm x 55mm were prepared from the second material 14 (95mm x 135mm x 35mm). Impact tests were conducted using the obtained impact test specimens at temperatures ranging from 15 to 35°C.

[0155] [2.4. Results] [2.4.1. Hardness] Table 2 shows ΔH, H1, and H2. In Examples 1-13 and Comparative Example 3, the hardness H1 of the outer periphery of the block material was 36.7-42.8 HRC, and the hardness H2 of the center was 38.8-43.7 HRC. Furthermore, the absolute value ΔH (hardness homogeneity) of the difference between H1 and H2 was 0.5-3.4 HRC. In other words, it was reconfirmed that high hardenability and high hardness homogeneity are observed when Mn / Cr > 0.150 and Cr ≥ 5.60 mass%. Because ΔH is small, the difference in properties between the outer periphery and the center of the steel material is also small, and it is expected that molds made from any part of the steel material will have high and stable performance.

[0156] In Comparative Examples 1 and 2, the hardness H1 of the outer periphery of the block material was 41.2 and 41.8 HRC, respectively. The hardness H2 of the central part was 45.3 and 45.5 HRC, respectively. The ΔH was approximately 4 HRC. In other words, in Comparative Examples 1 and 2, Mn / Cr ≤ 0.150 and Cr ≤ 5.60 mass%, resulting in poor hardenability and hardness homogeneity. Due to the large ΔH, there is a significant difference in properties between the outer periphery and the center of the steel material, raising concerns that the performance of the mold will vary greatly depending on which part of the steel material it is made from.

[0157] [2.5.2. Impact Test] Table 2 shows the average impact value, 20 [J / cm²]. 2 This indicates the number of items less than 30 [J / cm²] and the low impact value rate. Examples 1 to 13 all have an average impact value of 30 [J / cm²]. 2 The impact value was 20 [J / cm²] or higher, and the low impact value rate was 30% or less. Examples 1 to 13 had high hardenability, and because the C and V content was low, there were few coarse foreign matter and carbides and carbonitrides distributed in a dotted pattern. As a result, even when material cut from near the center of a large cross-section material with a low solidification rate was slow-hardened, the impact value remained stable at a high level. However, there were not absolutely no coarse foreign matter originating from C and V, and there were also foreign matter that did not contain C or V, so the impact value was 20 [J / cm²]. 2 Test specimens with a value less than ] also occurred at a low probability.

[0158] On the other hand, Comparative Examples 1-3 had an average impact value of 30 [J / cm²]. 2 The impact value was less than 30%, and the low impact value rate exceeded 30%. Comparative Examples 1-3, in addition to having low hardenability, had high C and V content, resulting in a large amount of coarse foreign matter and carbides and carbonitrides distributed in a dotted pattern. As a result, when material cut from near the center of a large cross-section material with a low solidification rate was slow-hardened, the impact value decreased. Comparative Example 3 is a steel with increased carbon (C) and vinyl content compared to Examples 1-13. As a result, while Comparative Example 3 has high hardenability, it contains more coarse foreign matter and carbides and carbonitrides distributed in a dotted pattern. Consequently, the impact value of Comparative Example 3 was lower than that of Examples 1-13. In other words, it is clear that for large cross-section materials, impact value cannot be evaluated solely by hardenability, and the importance of reducing carbon and vinyl content was confirmed.

[0159] Based on the results in Table 2, it can be concluded that the large molds (mass of 2000 kg or more) manufactured from the steel materials of Examples 1 to 13 using the PH process also exhibit higher impact values ​​than molds manufactured from existing steel with high carbon and vinyl content.

[0160] [Table 2]

[0161] [3. Versatility] The above verification was conducted assuming die-casting molds, but the present invention is not limited to die-casting and can be applied to molds and parts used in various casting processes. In addition to casting, it can also be applied to molds and parts used in forging, hot stamping, extrusion, resin injection molding, resin blow molding, molding or processing of rubber and fiber-reinforced plastics, and other processes. Furthermore, in the above verification, the steel material was hardened from 920°C, tempered at 560-600°C, and tempered to approximately 40 HRC to evaluate its properties. However, a wide range of hardness levels can be achieved by adjusting the hardening temperature and tempering temperature according to the application, and this allows for the application of steel materials with a wide range of hardness levels to molds and parts.

[0162] While a molten block was used for verifying its properties, the steel material according to the present invention can also be used in the form of powder, rods, wires, or plates. For example, if the steel material according to the present invention is made into a powder, it can be applied to various sequential molding processes such as additive manufacturing (SLM method, LMD method, etc.) and PPW. Furthermore, if the steel material according to the present invention is used as a molten rod, molds and parts can be manufactured from it. Alternatively, if the steel material according to the present invention is used as a molten rod or wire, it can be applied to additive manufacturing and repair by building up material using TIG welding or laser welding. Furthermore, if the steel material according to the present invention is used as a plate material, it is possible to manufacture molds and parts by joining them together. Of course, it is also possible to manufacture molds and parts by joining members made from the steel material according to the present invention.

[0163] As described above, the steel material according to the present invention can be applied to members having various shapes. Furthermore, molds and parts can be manufactured or repaired using various methods from materials of various shapes made from the steel material according to the present invention.

[0164] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0165] The steel material according to the present invention can be used as a mold or its components in various processes such as casting, forging, hot stamping, extrusion, injection molding, and blow molding.

Claims

1. 0.19≦C≦0.31mass%, 0.010≦V≦0.180mass%, Mn / Cr>0.150, Mn≦1.50mass%, 5.60≦Cr≦6.60mass%, Cu+Ni≦0.84mass%, 0.40≦Si≦1.40mass%, 0.60≦Mo≦2.00mass%, 0.001 ≤ Al ≤ 0.080 mass%, and, 0.003≦N≦0.040mass% It contains, with the remainder consisting of Fe and unavoidable impurities. Steel material.

2. Furthermore, the steel material according to claim 1 contains one or more groups selected from groups A to D below. Group A: 0.30 < W ≤ 2.00 mass%, and, 0.30<Co≦1.00mass% One or two selected from among Group B: 0.0002<B≦0.0080mass% Group C: 0.006<S≦0.180mass%, 0.0005<Ca≦0.0500mass%, 0.03<Se≦0.50mass%, 0.005<Te≦0.100mass%, 0.01 < Bi ≤ 0.50 mass%, and, 0.03<Pb≦0.50mass% One or more selected from among them Group D: 0.004<Nb≦0.100mass%, 0.004<Ta≦0.100mass%, 0.004 < Ti ≤ 0.100 mass%, and, 0.004<Zr≦0.100mass% One or more selected from among them

3. Its mass is 3000 kg or more. Vertical dimension (L 1 ), horizontal dimension (L 2 ), and the height dimension (L 3 ) of which the smallest dimension (L min ) is 300 mm or more The steel material according to claim 1.

4. The steel material according to claim 1, wherein the hardness of the central part is 35 HRC or more and 45 HRC or less.

5. A mold manufactured from the steel material described in claim 4, having a mass of 2000 kg or more.

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