Steel material and mold

Abstract

To provide a steel material that can be heat-treated to have an appropriate hardness and has excellent machinability, impact value, and corrosion resistance, and a mold that includes the steel material.SOLUTION: The steel material comprises 0.090<C≤0.170 mass%, 0.60<Si≤1.00 mass%, 0.10≤Mn≤0.60 mass%, 0.003≤P≤0.080 mass%, S≤0.012 mass%, 0.20<Cu≤0.58 mass%, 0.20≤Ni≤1.10 mass%, Cu≤Ni, 7.70≤Cr≤8.20 mass%, 0.70<Mo≤1.20 mass%, 0.10<V≤0.30 mass%, 0.007≤Al≤0.100 mass%, 0.001≤N<0.015 mass%, and 0.118≤C+N≤0.168 mass%, with the balance being Fe and inevitable impurities. The mold includes the steel material.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a steel material, and more specifically to a steel material (particularly a pre-hardened steel that has been quenched and tempered under specified conditions to achieve a specified hardness) for manufacturing molds used in injection molding and blow molding of plastics, and molding and processing of rubber and various carbon fiber reinforced plastics, and a mold using this steel material. [Background technology]

[0002] Pre-hardened steel refers to steel that has been tempered to a predetermined hardness and is machineable. Pre-hardened steel does not require heat treatment and can be used as is for dies and the like after cutting. For this reason, pre-hardened steel is widely used for dies and die parts used in injection molding and blow molding of plastics, and molding and processing of rubber and various carbon fiber reinforced plastics. Various proposals have been made regarding such pre-hardened steel and methods for manufacturing the same.

[0003] For example, Patent Document 1 discloses a steel for plastic molding dies with excellent temperature controllability, which contains, in mass %, C: 0.03% to 0.25%, Si: 0.01% to 0.40%, Mn: 0.10% to 1.50%, P: ≦0.30%, S: ≦0.050%, Cu: 0.05% to 0.20%, Ni: 0.05% to 1.50%, Cr: 5.0% to 10.0%, Mo: 0.10% to 2.00%, V: 0.01% to 0.10%, N: ≦0.10%, O: ≦0.01%, Al: ≦0.05%, and satisfies the conditions (Cr + Mo) ≦ 10% and 7 ≦ (Cr + 3.3Mo), with the remainder being Fe and unavoidable impurities.

[0004] The same document states: (a) By adjusting the ratio of ferrite forming elements (Cr, Mo) and austenite forming elements (Mn, Ni), it is possible to achieve both specularity and impact value, and (b) When the contents of Cr and Mo are optimized so that they satisfy a predetermined relationship, corrosion resistance and thermal conductivity are improved. is stated.

[0005] Patent Document 2 discloses a mold steel containing, in mass%, 0.045≦C≦0.090, 0.01≦Si≦0.50, 0.10≦Mn≦0.60, 0.80≦Ni≦1.10, 6.60≦Cr≦8.60, 0.01≦Mo≦0.70, 0.001≦V≦0.200, 0.007≦Al≦0.150, 0.0002≦N≦0.0500, and the remainder being Fe and unavoidable impurities.

[0006] The document describes that when the Al content is 0.007 to 0.150% in mold steel containing predetermined elements, good mirror polishability, corrosion resistance intermediate between 5% Cr steel and 12% Cr steel, and high impact value can be achieved after the mold steel is tempered to a predetermined hardness.

[0007] Patent Document 3 discloses a mold steel containing, in mass%, 0.070≦C≦0.130, 0.01≦Si≦0.60, 0.02≦Mn≦0.60, 0.003≦P≦0.150, 0.005≦Cu≦1.50, 0.005≦Ni≦0.80, 7.50≦Cr≦8.40, 0.70≦Mo≦1.20, 0.01≦V≦0.30, 0.010≦Al≦0.120, 0.015≦N≦0.095, and the remainder being Fe and unavoidable impurities.

[0008] The document describes that mold steel containing specified elements has excellent SA properties, tempered hardness, residual stress, machinability, impact value, and corrosion resistance because the components (particularly Ni, Mo, and Al) are optimized.

[0009] When manufacturing a mold using pre-hardened steel, it is first necessary to manufacture a pre-hardened steel material for a mold, which is generally manufactured through the steps of melting, refining, casting, homogenizing heat treatment, hot working, intermediate heat treatment (normalizing and tempering), spheroidizing annealing (SA), quenching, straightening, and tempering. Depending on the steel type, SA may not be necessary, tempering may be performed multiple times, or a tempering process may be performed before or after straightening. Regardless of the number of tempering steps, the final tempering process also serves to reduce residual stress.

[0010] Next, a mold or mold part is manufactured from the pre-hardened steel material. The mold or mold part is generally manufactured through the steps of machining, mirror polishing, surface decoration, and surface treatment. Surface decoration is a process of giving a special pattern to the surface by embossing, etc., but this may not be necessary depending on the application. Surface treatment is a process of hardening the surface by nitriding or PVD, etc., but this may not be necessary depending on the application.

[0011] The following are particularly important properties that are required of the pre-hardened steel material produced through the above-mentioned process, and of the molds and mold parts produced using the same. (1) Tempered hardness (moderate tempered hardness that can achieve high impact value while maintaining good wear resistance) (2) Integrity of the processed shape (low residual stress that can prevent warping or twisting of the mold during machining) (3) Machinability (ease of cutting) (4) Impact value (high enough to prevent major cracking of the mold) (5) Corrosion resistance (high corrosion resistance to the extent that rust does not occur even when used or stored in a humid environment) (6) Mirror finish (high level of mirror polishability that allows smooth polishing) (7) Grain processing ability (ease of surface decoration)

[0012] Here, although the configuration of Patent Document 1 is excellent in temperature controllability, further improvements are required in tempered hardness, machinability, and corrosion resistance. Furthermore, in the configuration of Patent Document 2, the mirror finish and stability of the impact value could be ensured by adjusting the amount of Al. However, as in Patent Document 1, further improvements were required in the tempered hardness, machinability, and corrosion resistance.

[0013] In the structure of Patent Document 3, temper hardness and corrosion resistance could be ensured by increasing the amount of N, but further improvement was required in terms of machinability. However, in order to increase the amount of N, a nitrogen addition process under pressurized conditions (so-called nitrification) is required, and the installation of equipment for nitrification is also large-scale. The steel materials disclosed in Patent Documents 1 to 3 are so-called low C-8Cr steels. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-24510 [Patent Document 2] Japanese Patent Application Publication No. 2020-63508 [Patent Document 3] Japanese Patent Publication No. 2022-83627 Summary of the Invention [Problem to be solved by the invention]

[0015] The problem to be solved by the present invention is to provide a steel material that can be tempered to an appropriate hardness and has excellent machinability, impact value, and corrosion resistance. Another problem to be solved by the present invention is to provide a mold that can be tempered to an appropriate hardness and has excellent machinability, impact value, and corrosion resistance. [Means for solving the problem]

[0016] In order to solve the above problems, the steel material according to the present invention comprises: 0.090 <C≦0.170mass%、 0.60 <Si≦1.00mass%、 0.10≦Mn≦0.60mass%, 0.003≦P≦0.080mass%, S≦0.012 mass%, 0.20 <Cu≦0.58mass%、 0.20≦Ni≦1.10mass%, Cu≦Ni 7.70≦Cr≦8.20mass%, 0.70 <Mo≦1.20mass%、 0.10 <V≦0.30mass%、 0.007≦Al≦0.100mass%, 0.001≦N<0.015mass%, 0.118≦C+N≦0.168mass%, and the balance is Fe and unavoidable impurities.

[0017] The mold according to the present invention is made of the steel material according to the present invention. [Effects of the Invention]

[0018] The steel material according to the present invention is a type of low C-8Cr steel, and because the elements (particularly Si, Al, C, V, and Mo) of the low C-8Cr steel are optimized, the steel exhibits excellent properties, particularly (1) tempered hardness, (2) processed shape integrity, (3) machinability, (4) impact value, and (5) corrosion resistance. in particular, (1) The hardness after tempering is an appropriate value of 35 to 43 HRC. (2) Low residual stress after tempering (3) Machinability after tempering is comparable to SKD61. (4) Impact value after tempering is ≥ 80 J / cm 2 is the appropriate value of (5) After tempering, the corrosion resistance is as high as that of martensitic stainless steel. It has the following characteristics.

[0019] For this reason, the steel material according to the present invention has the following properties: (A) Excellent mirror polishability, (B) There is little deformation during processing. (C) Mold processing is easy, (D) Less likely to crack during use (E) Rust is unlikely to occur during use or storage. This has the advantage that: [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a diagram showing tempered hardness versus LMP value (Experiment 1). [Figure 2] FIG. 10 is a diagram showing drill machinability (Experiment 2). [Figure 3] FIG. 10 is a diagram showing end mill machinability (Experiment 3). [Figure 4] FIG. 10 is a diagram showing impact values (Experiment 4). [Figure 5] FIG. 10 is a graph showing corrosion resistance (Experiment 5). [Figure 6] FIG. 1 is a diagram showing tempered hardness relative to LMP values (Examples). [Figure 7] FIG. 10 is a diagram showing tempered hardness relative to LMP values (Comparative Example). [Figure 8] FIG. 1 is a diagram showing drill machinability (Examples and Comparative Examples). [Figure 9] FIG. 1 is a diagram showing end mill machinability (Examples and Comparative Examples). [Figure 10] FIG. 1 is a diagram showing impact values (Examples and Comparative Examples). DETAILED DESCRIPTION OF THE INVENTION

[0021] An embodiment of the present invention will be described in detail below. [1. Steel materials] [1.1. Composition] [1.1.1. Main constituent elements] The steel material according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0022] (1) 0.090 <C≦0.170mass%: If the carbon content is too low, delta ferrite is mixed into the pre-hardened steel, degrading its mirror polishability and embossability. Furthermore, the amount of VC or VCN, which inhibits the movement of austenite grain boundaries during quenching, becomes too low, resulting in coarsening of the grains and a decrease in impact strength. Furthermore, when tempered at a temperature of 510°C or higher and with a hardness of 16,800≦LMP≦17,200 to reduce residual stress, the tempered hardness is less than 35HRC, making it impossible to achieve a hardness of 35HRC or higher. Therefore, the C content must be more than 0.090 mass%, and is preferably 0.095 mass% or more, and more preferably 0.105 mass% or more.

[0023] On the other hand, if the C content is too high, coarse VC or VCN crystallizes during ingot casting, reducing impact strength. Furthermore, pearlite precipitates during quenching, reducing tempered hardness, impact strength, mirror polishability, grainability, and corrosion resistance. Furthermore, a large amount of Cr is consumed as carbides, which can result in insufficient corrosion resistance. Furthermore, cracks are more likely to occur during welding repairs. Furthermore, thermal conductivity also decreases. Therefore, the C content must be 0.170 mass% or less, preferably 0.165 mass% or less, and more preferably 0.160 mass% or less.

[0024] In the injection molding of plastics and other resins, it is necessary to quickly solidify the resin filled in the mold to increase productivity. Therefore, the mold must be able to cool quickly, i.e., it must have high thermal conductivity. Furthermore, when controlling the mold temperature by selectively flowing high-temperature or low-temperature fluid through the flow channels within the mold, the mold must be highly responsive to heating and cooling. For this reason, high thermal conductivity is important.

[0025] (2) 0.60 <Si≦1.00mass%: If the Si content is too low, the machinability deteriorates and the corrosion resistance in the atmosphere also becomes insufficient. Therefore, the Si content must be more than 0.60 mass%, preferably 0.62 mass% or more, and more preferably 0.64 mass% or more.

[0026] On the other hand, too much Si reduces thermal conductivity, increases the likelihood of delta ferrite being mixed into pre-hardened steel, and forms a hard, indestructible oxide scale on the steel surface during hot working, significantly abrading the processing tools. Therefore, the Si content must be 1.00 mass% or less, preferably 0.95 mass% or less, and more preferably 0.90 mass% or less.

[0027] If corrosion (rust) occurs due to moisture in the air during the mold manufacturing process or storage, even in just one spot, the mold must be mirror-polished again. Redoing mirror polishing is extremely time-consuming and costly. Furthermore, if the rust cannot be removed by mirror polishing, the surface quality of the mold will be compromised. For this reason, corrosion resistance, which prevents rust from occurring, is important.

[0028] (3) 0.10≦Mn≦0.60mass% If the Mn content is too low, pearlite precipitates during hardening. Also, delta ferrite is mixed into the pre-hardened steel. Furthermore, hardenability is insufficient, and the impact value inside the steel is also low. Therefore, the Mn content must be 0.10 mass% or more. The Mn content is preferably 0.15 mass% or more, and more preferably 0.22 mass% or more. Furthermore, when the Ni content is 0.39 mass% or more, the Mn content is more preferably 0.34 mass% or more.

[0029] On the other hand, if the Mn content is too high, the SA property deteriorates. Also, the impact value decreases when tempered at 510°C or higher (high-temperature temper embrittlement). Furthermore, the thermal conductivity also decreases. Therefore, the Mn content must be 0.60 mass% or less, preferably 0.56 mass% or less, and more preferably 0.54 mass% or less.

[0030] (4) 0.003≦P≦0.080mass% Since P has the effect of breaking down cutting chips into smaller pieces, if the P content is too low, machinability will deteriorate and, furthermore, if the P content is too low, high-temperature strength will decrease. Therefore, the amount of P must be 0.003 mass% or more, preferably 0.005 mass% or more, and more preferably 0.007 mass% or more.

[0031] On the other hand, if the amount of P is too high, the impact value drops significantly. Also, P segregates at grain boundaries, accelerating corrosion from the grain boundaries, which deteriorates corrosion resistance. Therefore, the amount of P must be 0.080 mass% or less, preferably 0.070 mass% or less, and more preferably 0.060 mass% or less.

[0032] (5) S≦0.012 mass%: Generally, if the S content is too low, machinability deteriorates, but the steel material according to the present invention ensures machinability through the Si content. Therefore, the S content can be quite low. The lower limit of the S content is about 0.0002 mass%. On the other hand, if the S content is too high, the impact value will decrease. Furthermore, S segregates at grain boundaries, accelerating corrosion from the grain boundaries, thereby worsening corrosion resistance. Furthermore, S combines with Mn to form MnS, which causes corrosion from the interface between MnS and the matrix, further worsening corrosion resistance. Therefore, the amount of S must be 0.012 mass% or less, preferably 0.010 mass% or less, and more preferably 0.008 mass% or less.

[0033] (6) 0.20 <Cu≦0.58mass%: If the Cu content is too low, pearlite precipitates during hardening. Also, delta ferrite is mixed into the pre-hardened steel. Furthermore, when tempered at a temperature of 510°C or higher and with a hardness of 16,800≦LMP≦17,200 to reduce residual stress, the tempered hardness is less than 35HRC, making it impossible to achieve a tempered hardness of 35HRC or higher. Furthermore, machinability deteriorates and corrosion resistance is insufficient. Therefore, the Cu content must be more than 0.20 mass%, preferably 0.22 mass% or more, and more preferably 0.23 mass% or more.

[0034] On the other hand, if the Cu content is too high, cracks will occur during hot working, SA properties will deteriorate, and the thermal conductivity and impact value will also decrease. Therefore, the Cu content must be 0.58 mass% or less, preferably 0.54 mass% or less, and more preferably 0.50 mass% or less.

[0035] (7) 0.20≦Ni≦1.10mass% If the Ni content is too low, delta ferrite will be mixed into the pre-hardened steel. It will also result in insufficient hardenability and a low impact value inside the steel. Furthermore, if the Cu content is too high, the effect of suppressing cracking during hot working will also be reduced. Therefore, the Ni content must be 0.20 mass% or more, preferably 0.30 mass% or more, and more preferably 0.39 mass% or more.

[0036] On the other hand, if the amount of Ni is too large, the SA property deteriorates and the thermal conductivity also decreases. Therefore, the Ni content must be 1.10 mass% or less, preferably 1.00 mass% or less, and more preferably 0.90 mass% or less.

[0037] (8) Cu≦Ni Ni has the effect of suppressing hot working cracks caused by Cu. However, even if the Ni content is appropriate, if the Cu content is excessive relative to the Ni content, the effect of suppressing hot working cracks may be reduced. Therefore, in order to suppress cracking during hot working, the amount of Cu must be equal to or less than the amount of Ni.

[0038] (9) 7.70≦Cr≦8.20mass%: If the Cr content is too low, the corrosion resistance will be insufficient and the impact value will also decrease. Therefore, the Cr content must be 7.70 mass% or more, preferably 7.75 mass% or more, and more preferably 7.80 mass% or more.

[0039] On the other hand, if the Cr content is too high, delta ferrite will be mixed into the pre-hardened steel. Furthermore, pearlite will precipitate during quenching. Furthermore, when tempered at a temperature of 510°C or higher and with a hardness of 16,800≦LMP≦17,200 in order to reduce residual stress, the tempered hardness will be less than 35HRC, making it impossible to achieve a tempered hardness of 35HRC or higher. Furthermore, machinability will deteriorate, and thermal conductivity will also decrease. Therefore, the Cr content must be 8.20 mass% or less, preferably 8.15 mass% or less, and more preferably 8.10 mass% or less.

[0040] (10)0.70 <Mo≦1.20mass%: If the Mo content is too low, the tempered hardness will be less than 35HRC when tempered at a temperature of 510°C or higher and under the conditions of 16800≦LMP≦17200 in order to reduce residual stress, and a tempered hardness of 35HRC or more cannot be achieved. In addition, pearlite will precipitate during quenching. Furthermore, corrosion resistance will be insufficient. Therefore, the Mo content must be more than 0.70 mass%, preferably 0.75 mass% or more, and more preferably 0.80 mass% or more.

[0041] On the other hand, if the Mo content is too high, delta ferrite is mixed into the pre-hardened steel material, and the fracture toughness is also deteriorated. Therefore, the Mo content must be 1.20 mass% or less, preferably 1.15 mass% or less, and more preferably 1.10 mass% or less.

[0042] (11)0.10 <V≦0.30mass%: If the V content is too low, when tempered at a temperature of 510°C or higher and with a temperature range of 16800≦LMP≦17200 in order to reduce residual stress, the tempered hardness will be less than 35HRC, making it impossible to achieve a tempered hardness of 35HRC or higher. Furthermore, the amount of VC or VCN, which inhibits the movement of austenite grain boundaries during quenching, will be too low, causing the grains to coarsen. Furthermore, coarsening of the grains during quenching will result in a decrease in impact value. Therefore, the amount of V must be more than 0.10 mass%, preferably 0.12 mass% or more, and more preferably 0.13 mass% or more.

[0043] On the other hand, if the V content is too high, coarse VC or VCN crystallizes during ingot casting, lowering the impact value and also causing δ-ferrite to be mixed into the pre-hardened steel. Therefore, the amount of V must be 0.30 mass% or less, preferably 0.28 mass% or less, and more preferably 0.27 mass% or less.

[0044] (12) 0.007≦Al≦0.100mass%: The steel material (low C-8Cr steel) according to the present invention has the unique feature that when the Al content is low, the impact value is significantly reduced even if the crystal grains are fine. Therefore, the Al content must be 0.007 mass% or more, preferably 0.010 mass% or more, and more preferably 0.020 mass% or more.

[0045] If the Al content becomes too high, excessive alumina will result. As a consequence, not only does the impact value decrease, but the mirror surface polishing property also deteriorates. This is because pinholes are generated due to the detachment of alumina. Furthermore, when the Al content becomes too high, the thermal conductivity decreases and the machinability also deteriorates significantly. Therefore, the Al content needs to be 0.100 mass% or less. Preferably, the Al content is 0.090 mass% or less, and more preferably 0.085 mass% or less.

[0046] In particular, when the O content is 0.003 mass% or less, the adverse effects of alumina are not manifested. Therefore, in the range of 0.050 < Al ≤ 0.085 mass% where the lower limit of the Al content is increased, the balance of crystal grain size, impact value, mirror surface polishing property, machinability, and thermal conductivity becomes very good.

[0047] (13) 0.001 ≤ N < 0.015 mass%: If the N content becomes too low, the amount of AlN that suppresses the movement of austenite grain boundaries during quenching becomes too small. For this reason, the crystal grains tend to grow excessively. Also, δ-ferrite is mixed into the pre-hardened steel material. Moreover, when tempering is carried out at a temperature of 510°C or higher and 16,800 ≤ LMP ≤ 17,200 for the purpose of reducing residual stress, the tempering hardness becomes less than 35 HRC, and a tempering hardness of 35 HRC or higher cannot be achieved. Furthermore, the corrosion resistance is insufficient. Therefore, the N content needs to be 0.001 mass% or more. Preferably, the N content is 0.002 mass% or more, and more preferably 0.003 mass% or more.

[0048] On the other hand, to incorporate a large amount of N, the amount of N present as an impurity in the raw materials is insufficient, so active addition of N is required, which requires specialized equipment. Furthermore, when the ingot is cast, VC or VCN crystallizes in a coarse state, resulting in an increase in coarse AlN, which reduces the impact value. Furthermore, when a mold is repaired by welding, nitrogen can vaporize at the weld, forming voids on the surface. Furthermore, voids can form inside the mold during welding repair, and these voids can be exposed to the outside by mirror polishing. Removing these voids by mirror polishing takes a lot of time and costs a lot of money. If these voids cannot be removed, the surface quality of the mold will be impaired. Furthermore, voids inside the mold can cause cracks during use, even if they are not exposed to the outside. Furthermore, when B is dissolved, hardenability can be improved. To dissolve B, N, which easily bonds with B, must be fixed as a compound. For this reason, N is fixed in the form of TiN by adding elements such as Ti, which are more likely to form nitrides. Here, if there is a lot of N, it is necessary to add more Ti. As a result, coarse TiN and other elements increase, which can reduce the impact value. Therefore, the amount of N must be less than 0.015 mass%, preferably 0.013 mass% or less, and more preferably 0.012 mass% or less.

[0049] (14) 0.118≦C+N≦0.168mass%: Even if the C content and N content exceed the above lower limit, if the (C+N) content becomes too small, the above problems may occur. Therefore, the (C+N) content must be 0.118 mass% or more, and is preferably 0.122 mass% or more. Even when the C content and N content are equal to or less than the upper limit values, if the (C+N) content is too high, the above problems may occur. Therefore, the (C+N) content must be 0.168 mass% or less, and is preferably 0.164 mass% or more.

[0050] (15) Inevitable impurities O≦0.005 mass%, B≦0.0002 mass%, W≦0.30mass%, Co≦0.30mass%, Nb≦0.004mass%, Ta≦0.004 mass%, Ti≦0.004mass%, Zr≦0.004mass%, Ca≦0.0005 mass%, Se≦0.03mass%, Te≦0.005 mass%, Bi≦0.01 mass%, Pb≦0.03 mass%, or Mg≦0.02mass%, may be included.

[0051] [1.1.2. Sub-constituent elements] The steel material according to the present invention may further contain one or more of the following elements in addition to the above-mentioned main constituent elements. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0052] [A. Group A] (16)0.0002 <B≦0.0080mass%: When the P content is high, P segregating at the grain boundaries reduces the grain boundary strength, which may result in a low impact value. Adding B is effective in improving the grain boundary strength. Furthermore, when the total amount of alloying elements is low, ferrite and pearlite may precipitate during quenching. Adding B is also effective in suppressing this. To achieve this effect, the B content is preferably more than 0.0002 mass%. The B content is preferably 0.0003 mass% or more, and more preferably 0.0004 mass% or more.

[0053] If the amount of B is large, the amount of compounds with iron (borides, boron carbohydrates) increases, lowering the impact value. Therefore, the amount of B is preferably 0.0080 mass% or less, more preferably 0.00075 mass% or less, and even more preferably 0.00070 mass% or less.

[0054] When adding B to improve grain boundary strength, it is meaningless if B forms BN. Therefore, when adding B to steel with a high N content, it is necessary to combine N with elements other than B. Specifically, N is combined with elements such as Ti, Zr, and Nb, which are more likely to form nitrides. These elements are effective even at the impurity level, but if they are insufficient, it is preferable to add them in the amounts described below. On the other hand, if it is desired to disperse BN in order to improve machinability, it is not necessary to take measures to actively combine N with the nitride-forming elements described above.

[0055] [B. Group B] (17)0.30 <W≦4.00mass%: The steel material according to the present invention has a low C content and small amounts of Mo and V, and therefore may have insufficient strength depending on the application. To increase the strength of the steel material according to the present invention, it is effective to add W. To achieve this effect, the W content is preferably more than 0.30 mass%. The W content is preferably 0.50 mass% or more, and more preferably 1.00 mass% or more.

[0056] On the other hand, if the amount of W is too large, not only will segregation become significant, but the material cost will also increase. Therefore, the W content is preferably 4.00 mass% or less, more preferably 3.90 mass% or less, and even more preferably 3.80 mass% or less.

[0057] (18)0.30 <Co≦3.00mass%: In order to increase the strength of the steel material according to the present invention, it is also effective to add Co instead of or in addition to W. To obtain such an effect, the Co content is preferably more than 0.30 mass%. The Co content is preferably 0.50 mass% or more, and more preferably 1.00 mass% or more.

[0058] On the other hand, if the amount of Co is too large, not only will segregation become significant, but the material cost will also increase. Therefore, the Co content is preferably 3.00 mass% or less, more preferably 2.80 mass% or less, and even more preferably 2.50 mass% or less.

[0059] [C. Group C] (19)0.004 <Nb≦0.100mass%: (20)0.004 <Ta≦0.100mass%: (21)0.004 <Ti≦0.100mass%: (22)0.004 <Zr≦0.100mass%: The steel material according to the present invention has a low carbon content and a low vanadium content, which may result in a shortage of VC or VCN, which inhibits the migration of austenite grain boundaries during quenching. In this case, depending on the quenching conditions, excessive austenite grain growth may occur. To inhibit grain growth, it is effective to add elements that form carbides, nitrides, or carbonitrides (i.e., Nb, Ta, Ti, and / or Zr) and disperse carbides and the like in the matrix. To achieve this effect, the contents of Nb, Ta, Ti, and Zr are each preferably greater than 0.004 mass%. The contents of these elements are each preferably 0.006 mass% or more, and more preferably 0.008 mass% or more.

[0060] On the other hand, if the content of these elements is too high, the carbides, nitrides, or carbonitrides become coarse, and the impact value decreases. Furthermore, adding more than necessary of these elements increases the material cost. Therefore, the content of Nb, Ta, Ti, and Zr is preferably 0.100 mass% or less. The content of each of these elements is preferably 0.090 mass% or less, and more preferably 0.080 mass% or less. The mold steel according to the present invention may contain any one of Nb, Ta, Ti, and Zr, or may contain two or more of them.

[0061] [D. Group D] (23)0.0005 <Ca≦0.2000mass%: (24)0.03 <Se≦0.50mass%: (25)0.005 <Te≦0.100mass%: (26)0.01 <Bi≦0.50mass%: (27)0.003 <Pb≦0.50mass%: Although the machinability of the steel material according to the present invention is ensured by Si, further improvement of machinability may be required depending on the application. To improve machinability, it is effective to add elements known as free-cutting components (i.e., Ca, Se, Te, Bi, and / or Pb). To obtain such effects, it is preferable that the contents of Ca, Se, Te, Bi, and Pb each exceed the lower limit value mentioned above.

[0062] The Ca content is preferably 0.0006 mass% or more, and more preferably 0.0007 mass% or more. The amount of Se is preferably 0.04 mass% or more, and more preferably 0.05 mass% or more. The amount of Te is preferably 0.006 mass% or more, and more preferably 0.007 mass% or more. The amount of Bi is preferably 0.02 mass% or more, and more preferably 0.03 mass% or more. The amount of Pb is preferably 0.04 mass% or more, and more preferably 0.05 mass% or more.

[0063] On the other hand, if the content of these elements is too high, not only will the steel be prone to cracking during hot working, but the impact value will also be low. Therefore, the contents of Ca, Se, Te, Bi, and Pb are each preferably equal to or less than the upper limit values mentioned above.

[0064] The Ca content is preferably 0.1900 mass% or less, and more preferably 0.1800 mass% or less. The amount of Se is preferably 0.48 mass% or less, and more preferably 0.46 mass% or less. The amount of Te is preferably 0.090 mass% or less, and more preferably 0.080 mass% or less. The Bi content is preferably 0.450 mass% or less, and more preferably 0.400 mass% or less. The amount of Pb is preferably 0.45 mass% or less, and more preferably 0.40 mass% or less. The steel material according to the present invention may contain any one of Ca, Se, Te, Bi, and Pb, or may contain two or more of them.

[0065] [1.2. Characteristics] 1.2.1. Hardness The steel material according to the present invention is usually used in a state where it has been tempered to a predetermined hardness. When the components and heat treatment conditions of the steel material according to the present invention are optimized, the hardness after tempering becomes 35 HRC or more and 43 HRC or less.

[0066] If the hardness after tempering exceeds 43HRC, the machinability deteriorates, increasing the number of steps required to manufacture dies by machining. On the other hand, if the hardness after tempering is less than 35 HRC, mirror polishability and wear resistance will be impaired.

[0067] In addition, in the steel material according to the present invention, "hardness" means (a) Sample shape: Prepare a square bar of 11-12 mm x 11-12 mm x 20-25 mm. (b) The square bar is quenched at a temperature of 895°C to 975°C for 2 hours. (c) cooling the square bar from the quenching temperature to 220°C at a cooling rate of 15 to 30°C / min, and further cooling from 220°C to 100°C or less at an arbitrary cooling rate; (d) tempering the square bar only once under conditions where the tempering temperature T (°C) and the tempering time X (Hr) satisfy the following formulas (1) to (3); (e) Removing decarburized portions from the surface of the tempered square bar. The Rockwell hardness (C scale) is measured at 15 to 35°C using a sample obtained by the method described above. 16800≦LMP≦17200 …(1) LMP = (T + 273) × (20 + log X) … (2) 510℃≦T≦590℃ …(3)

[0068] When the quenching temperature is within the above range, it is possible to achieve both the desired tempered hardness and the desired impact value. If the quenching temperature is less than 895°C, the amount of solute elements will be insufficient, making it difficult to consistently achieve a hardness of 35HRC or higher. Furthermore, depending on the composition, the quenching temperature may be lower than the Ac3 point. Therefore, a quenching temperature of 895°C or higher is preferred. On the other hand, if the quenching temperature exceeds 975°C, the crystal grains become significantly coarse and the impact value decreases. Therefore, the quenching temperature is preferably 975°C or less.

[0069] Here, LMP (Larson-Miller Parameter) is one of the temperature-time conversion parameters. When the LMP is within the above range, low residual stress and good corrosion resistance can be achieved at the same time. Residual stress is not sufficiently released when the LMP is less than 16,800. Therefore, the LMP is preferably 16,800 or more, and more preferably 16,900 or more. On the other hand, if the LMP exceeds 17200, the amount of dissolved Cr is so large that the Cr is consumed in the formation of carbides, resulting in a decrease in the amount of dissolved Cr. This results in a deterioration in corrosion resistance. Therefore, the LMP is preferably 17200 or less, and more preferably 17150 or less.

[0070] The tempering temperature T and tempering time X are not particularly limited as long as the LMP is within the above range. However, if the tempering temperature is too low, it may take a long time to achieve the desired effect. Therefore, the tempering temperature T is preferably 510°C or higher. If the tempering temperature T is less than 510°C, the time required to reach LMP=16800 exceeds 29 hours, resulting in a significant decrease in productivity. On the other hand, if the tempering temperature T is too high, the tempering time becomes too short, and the desired effect may not be obtained for all parts of the pre-hardened steel material. Therefore, the tempering temperature T is preferably 590°C or less. For example, if the tempering temperature T exceeds 590°C, the time required to achieve LMP=17200 will be less than 0.9 hours, making it difficult to achieve 16800≦LMP≦17200 for all parts (from the center to the surface) within the cross section of the pre-hardened steel material.

[0071] [2. Mold] The mold according to the present invention uses the steel material according to the present invention. In this invention, the term "mold" includes not only the mold itself but also mold parts such as pins that are attached to it when used. In other words, the term "mold" refers to all steel parts that come into contact with the molded object (plastic, vinyl, rubber, etc.).

[0072] [3. Steel Manufacturing Methods] The steel material according to the present invention is (a) Melting and casting raw materials blended to form a predetermined composition; (b) subjecting the ingot to a homogenizing heat treatment; (c) The ingot after the homogenization heat treatment is hot worked. This is obtained by:

[0073] (d) If necessary, normalize the raw material after hot working; (e) Temper the raw material as necessary; (f) If necessary, perform spheroidizing annealing (SA) on the raw material. (g) If necessary, the raw material may be quenched and tempered.

[0074] Since the steel material according to the present invention has an optimized composition, it may have the required hardness after hot working. If this is not the case, it is preferable to carry out the required pretreatment after hot working, followed by quenching and tempering. Steel materials that have been quenched and tempered to a predetermined hardness are processed into predetermined shapes and used for various purposes.

[0075] [4. Effect] [4.1. Required properties of steel] The following characteristics are particularly important for steel materials that are tempered to a predetermined hardness and used for various purposes (so-called pre-hardened steel materials), as well as for molds and mold components manufactured using such steel materials:

[0076] (1) Tempered hardness: A certain level of tempered hardness is necessary to ensure the strength of the mold. If the tempered hardness is too low, the shape will change due to wear during use as a mold. On the other hand, if the tempered hardness is too high, not only will it be difficult to machine into the mold shape, but the impact value will also decrease. This makes it more likely that large cracks will occur during use as a mold. To achieve both good wear resistance and high impact value, a tempered hardness of around 30 to 55 HRC is usually required, although this varies depending on the application.

[0077] (2) Machining shape soundness: High "machined shape integrity" means that the intended shape can be obtained after machining. If pre-hardened steel has high residual stress, the mold machined from the steel may become distorted due to a change in the balance of residual stress after machining. This is called machining distortion. Steel that is less likely to suffer from machining distortion is considered to have high processed shape integrity. Steel that can maintain high hardness even when tempered at high temperatures has high processed shape integrity because high-temperature tempering can fully release residual stress.

[0078] (3) Machinability: "Machinability" becomes an issue when pre-hardened steel is machined into the shape of a mold. Molds require circuits for the flow of cooling and heating fluids, and the holes that form these circuits are formed using tools such as drills. Steel with poor machinability cannot be formed with holes unless the machining speed is reduced, and tools such as drills are subject to significant wear. Machinability is greatly influenced by hardness. However, even if the hardness is the same, the machinability often differs depending on the steel's composition. Steel is required to have not only the properties necessary for molds, but also high machinability.

[0079] (4) Impact Value: A high "impact value" is preferable. If the impact value is low, the mold is more likely to crack severely during use. Large cracks are difficult to repair, so molds with large cracks must be replaced with new ones. To reduce mold costs, it is necessary to prevent large cracks from occurring in the mold. For this reason, pre-hardened steel is required to have a high impact value.

[0080] (5) Corrosion resistance: High corrosion resistance is preferable. The mold according to the present invention is used for injection molding or blow molding of resin, molding or processing of rubber or fiber-reinforced plastics, etc. Therefore, the mold surface may be corroded by corrosive gases generated from the resin during injection molding. Furthermore, the mold surface may be corroded by moisture in the air during the mold manufacturing process or during storage of the mold. For this reason, high corrosion resistance is required for pre-hardened steel materials.

[0081] (6) Specularity: "Specularity" is an index of how smoothly the surface of a mold can be polished. The smoother the surface can be polished, the higher the specularity is evaluated. Pre-hardened steel with a high specularity is suitable for molds, and such steel often has few foreign matter (crystallized particles, inclusions, etc.) and is also very hard.

[0082] (7) Graining processability: "Texturing" refers to the process of creating a specific pattern (unevenness) on the surface of a mold by corroding it with a chemical solution. Steel that can easily be patterned is evaluated as having good texturing properties. Since texturing generally involves corrosion by acid, it is often at odds with corrosion resistance.

[0083] [4.2. Characteristics of the steel material according to the present invention] Generally, it is difficult to simultaneously satisfy the seven characteristics described above. In contrast, the steel material according to the present invention has an optimized composition, and therefore has appropriate tempered hardness, high machinability, high impact value, and high corrosion resistance. Furthermore, since the required hardness can be obtained even when tempered at a high temperature, residual stress is also released by tempering at a high temperature, and therefore the steel material according to the present invention has high shape integrity after processing.

[0084] The steel material according to the present invention is a type of low C-8Cr steel. When the Si content in the low C-8Cr steel is relatively increased and the Al content is relatively decreased, both the machinability and the impact value can be stabilized at high levels. Furthermore, in low C-8Cr steel, increasing the Si content relatively can improve corrosion resistance in a humid atmosphere. Furthermore, by optimizing the components (particularly Si, Al, C, V, and Mo) in low-C-8Cr steel, the required hardness can be obtained even when tempered at high temperatures.

[0085] In summary, the steel material according to the present invention has good properties such as (1) tempered hardness, (2) processed shape integrity, (3) machinability, (4) impact value, and (5) corrosion resistance. in particular, (1) The hardness after tempering is an appropriate value of 35 to 43 HRC. (2) Low residual stress after tempering (3) Machinability after tempering is comparable to SKD61. (4) Impact value after tempering is ≥ 80 J / cm 2 is the appropriate value of (5) After tempering, the corrosion resistance is as high as that of martensitic stainless steel. It has the following characteristics.

[0086] For this reason, the steel material according to the present invention has the following properties: (A) Excellent mirror polishability, (B) There is little deformation during processing. (C) Mold processing is easy, (D) Less likely to crack during use (E) Rust is unlikely to occur during use or storage. This has the advantage that: [Example]

[0087] (Experiment 1: Tempered hardness) 1. Sample Preparation Controlling tempered hardness is important for pre-hardened steel materials for molds. Therefore, tempered hardness was evaluated for five steel materials, Steel A to Steel E, shown in Table 1. Steel A corresponds to the steel material described in Patent Document 1, and has smaller amounts of C, Si, Cu, Mo, and V than the steel material of the present invention. Steel E has smaller amounts of C, Si, and Cu, and a larger amount of N than the steel material of the present invention.

[0088] [Table 1]

[0089] 2. Test Method Five test pieces measuring 11.5 mm x 11.5 mm x 20-24 mm were cut from each steel material. (1) Quenching: The test pieces were held at 920°C for 2 hours in a vacuum furnace, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 30°C / min. After cooling to below 100°C, they were removed from the furnace. (2) Tempering: Each test piece was heated in a vacuum furnace to a predetermined temperature T (540°C to 560°C) for 5 hours, and then held for a predetermined time X (2.5 to 7.0 hours). Nitrogen gas was then introduced into the furnace to cool the specimen to below 100°C, and the specimen was then removed from the furnace. The predetermined temperature T was not the ambient temperature or the furnace wall temperature but the temperature of the test piece, and was maintained at T±2°C. Then, the Rockwell hardness (C scale) of each test piece was measured at room temperature (15° C. or higher and 35° C. or lower).

[0090] Here, the combinations of the predetermined temperature T (tempering temperature) and the predetermined time X (tempering holding time) are as follows: 550℃×3.5Hr:LMP=16908 540℃×7.0Hr:LMP=16947 560℃×2.5Hr:LMP=16991 550℃×5.0Hr: LMP=17035 555℃×5.0Hr:LMP=17139 LMP = (tempering temperature T (℃) + 273) x (20 + log (tempering holding time X (Hr)))

[0091] [3. Results] Figure 1 shows the tempered hardness versus LMP value. The desired tempered hardness range in the range of 16900≦LMP≦17150 where residual stress is low is 35 to 43 HRC (within the dashed line in the figure), so all values for steel types other than Steel A are within the dashed line. Steel A has smaller amounts of C, Si, Cu, Mo, and V than the steel of the present invention.

[0092] (Experiment 2: Machinability 1 (Drill Machinability)) 1. Sample Preparation Machinability is an important factor in pre-hardened steel for dies. Therefore, the drill machinability of five steels, Steel A, Steel C, Steel F, Steel G, and Steel H, shown in Table 2, was evaluated. Steel H is JIS SKD61 (AISI H13). Steel F is Steel C with an increased Si content, and Steel G is Steel C with an increased Al content.

[0093] [Table 2]

[0094] 2. Test Method Test pieces measuring 25 mm x 50 mm x 200 mm were cut out from each steel material. (1-1) Quenching: (Low C-8Cr steels other than Steel H) Test pieces were held at 920°C for 2 hours in a vacuum furnace, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 15°C / min. After cooling to below 100°C, they were removed from the furnace. (1-2) Quenching: (Steel H) test pieces were held at 1030°C for 2 hours in a vacuum furnace, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 15°C / min. After cooling to below 100°C, they were removed from the furnace. (2) Tempering: Each test piece was subjected to an appropriate LMP, and the tempered hardness was set to 36.4 to 37.9 HRC. Then, the drill machinability of each test piece was evaluated at room temperature.

[0095] To evaluate drill machinability, a 5 mm diameter high-speed steel drill was used. This drill was pressed against the test piece at a specified cutting speed, drilling a hole 20 mm deep. The drill was then removed from the test piece and another 20 mm hole was drilled at a different location at the specified cutting speed. This procedure was repeated until the end of the tool life when the drill could no longer reach a depth of 20 mm due to wear or breakage. The cutting distance until tool life = number of holes drilled until tool life x hole depth (20 mm) was then calculated. Next, the cutting speed was changed and the cumulative cutting distance was calculated using the same procedure.

[0096] [3. Results] The correlation between cutting speed and cutting distance is shown in Figure 2. The steel types located in the upper right corner of the figure have better machinability (due to the high cutting speed and the large cumulative cutting distance). Figure 2 shows that machinability improves as the Si content increases. Specifically, the machinability was Steel A (Si content: 0.28 mass%) < Steel C (Si content: 0.64 mass%) < Steel F (Si content: 0.82 mass%) < Steel H (Si content: 0.82 mass%). Steel H is SKD61, which is known for its good machinability, and in fact had the best machinability. Steel F also had machinability equivalent to Steel H, and Steel C also showed machinability similar to Steel H.

[0097] On the other hand, Figure 2 also shows that machinability deteriorates as the Al content increases. Specifically, the machinability order was Steel C (Si content: 0.64 mass%, Al content: 0.055 mass%) > Steel G (Si content: 0.64 mass%, Al content: 0.119 mass%). While Steel C had machinability close to that of Steel H, Steel G's machinability deteriorated to a level close to that of Steel A. From the above, it was found that drill machinability improved with an increase in the Si content, and deteriorated with an increase in the Al content.

[0098] (Experiment 3: Machinability 2 (End mill machinability)) 1. Sample Preparation As in Experiment 2, the end mill machinability of five steel materials, namely, Steel A, Steel C, Steel F, Steel G, and Steel H shown in Table 2, was evaluated.

[0099] 2. Test Method The size of the test specimen and the conditions for quenching and tempering were the same as those in Experiment 2, and therefore the explanation will be omitted. Then, the end mill machinability of each test piece was evaluated at room temperature.

[0100] To evaluate end mill machinability, an XDGT1550PDER-G30VP15TF tool was used. The test pieces were cut using down cutting with a tool feed rate of 0.2 mm per tooth, a depth of cut of 1.0 mm x 4.0 mm, and a machining speed of 400 m / min. After cutting a specified distance, the tool wear was measured. A specified distance was cut again under the same conditions, and the tool wear was measured. This procedure was repeated to investigate the change in maximum tool wear over the cumulative cutting distance.

[0101] [3. Results] Figure 3 shows the correlation between the maximum wear amount of the tool and the cutting distance. The steel grades located in the lower right in the figure have better machinability (because they do not wear the tool even when cutting a long distance). From Figure 3, it was found that the machinability improves as the Si content increases. Specifically, the machinability was Steel A (Si content: 0.28 mass%) < Steel C (Si content: 0.64 mass%) < Steel F (Si content: 0.82 mass%) < Steel H (Si content: 0.82 mass%). Note that Steel H is SKD61, which is known for its good machinability, and in fact, it had the best machinability. Also, Steel F had machinability equivalent to that of Steel H, and Steel C also showed machinability close to that of Steel H.

[0102] On the other hand, from Figure 3, it was also found that the machinability deteriorates as the Al content increases. Specifically, the machinability was Steel C (Si content: 0.64 mass%, Al content: 0.055 mass%) > Steel G (Si content: 0.64 mass%, Al content: 0.119 mass%). For Steel C, whose machinability was close to that of Steel H, the machinability of Steel G deteriorated compared to that of Steel A. The deterioration of machinability due to an increase in the Al content was more significant than in the case of drill machinability. From the above, it was found that the end mill machinability is also improved by an increase in the Si content and deteriorated by an increase in the Al content.

[0103] (Experiment 4: Impact value) [1. Preparation of samples] In pre-hardened steel materials for molds, the level of the impact value is important. Therefore, the impact values of five steel materials, namely Steel A, Steel C, Steel F, Steel G, and Steel H shown in Table 2, were evaluated.

[0104] [2. Test method] A plurality of test pieces of 11 mm × 11 mm × 55 mm were cut out from each steel material. (1-1) Quenching: (Low C-8Cr steels other than Steel H) Test pieces were held at 920°C for 2 hours in a vacuum furnace, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 20°C / min. After cooling to below 100°C, they were removed from the furnace. (1-2) Quenching: (Steel H) test pieces were held in a vacuum furnace at 1,030°C for 2 hours, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 20°C / min. After cooling to below 100°C, they were removed from the furnace. (2) Tempering: Each test piece was subjected to an appropriate LMP and tempered to a tempered hardness of 37.2 to 37.9 HRC.

[0105] Each test piece was 10 mm x 10 mm x 55 mm and used as a test piece for impact value evaluation. Specifically, the impact value was evaluated when the height below the U-notch was 8 mm and the cross-sectional area of the test piece below the U-notch was 0.8 cm. 2 This is the value obtained by dividing the absorbed energy obtained by conducting an impact test at room temperature (15°C to 35°C) using a test piece of this material by the cross-sectional area of the test piece below the U-notch. Then, for each steel type, the average impact value of six test pieces was measured and defined as the impact value of that steel type.

[0106] [3. Results] Figure 4 shows the impact values for each steel type. Low C-8Cr steels have low impact values when the Al content is low, but Steel A has an Al content of 0.038 mass%, so it was able to ensure a high impact value. Steels C and F, which have increased Si content, both ensured high impact values equivalent to Steel A. Steel G, which has an increased Al content than Steel C, also ensured a high impact value. In other words, the low C-8Cr steels Steel A, Steel C, Steel F, and Steel G all achieved a high impact value of 100 J / cm 2 The impact value of steel H (SKD61) was 50 J / cm. 2 It was less than. From the above, it was confirmed that increasing the Si content did not significantly worsen the impact value.

[0107] (Experiment 5: Corrosion resistance) 1. Sample Preparation Corrosion resistance is important for pre-hardened steel for molds. Therefore, the corrosion resistance of five steels, Steel A, Steel C, Steel F, Steel G, and Steel H shown in Table 2, was evaluated.

[0108] 2. Test Method Test pieces measuring 25.5 mm x 40.5 mm x 12.5 mm were cut out from each steel material. (1-1) Quenching: (Low C-8Cr steels other than Steel H) Test pieces were held at 920°C for 2 hours in a vacuum furnace, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 20°C / min. After cooling to below 100°C, they were removed from the furnace. (1-2) Quenching: (Steel H) test pieces were held in a vacuum furnace at 1,030°C for 2 hours, and then nitrogen gas was introduced into the furnace. By balancing heating and cooling, the test pieces were slowly cooled to 220°C at a rate of 20°C / min. After cooling to below 100°C, they were removed from the furnace. (2) Tempering: Each test piece was subjected to an appropriate LMP, and the tempered hardness was set to 35.5 to 37.8 HRC. Each test piece was then cut to 25mm x 40mm x 12mm, and its surface was mirror-polished to create a test piece for evaluating corrosion resistance. Each test piece was then exposed to an atmosphere with a humidity of 95% and a temperature of 50°C for 24 hours, and the occurrence of rust was evaluated.

[0109] [3. Results] Figure 5 shows the rust formation status for each steel type. Rust was noticeable on Steel H (SKD61). Steel A, a low-C-8Cr steel, had less rust than Steel H. It was also found that corrosion resistance improved as the Si content increased. Specifically, the amount of rust formed was Steel A (Si content: 0.28 mass%) > Steel C (Si content: 0.64 mass%) > Steel F (Si content: 0.82 mass%). Furthermore, the amount of rust did not increase even with an increase in the Al content. Specifically, the amount of rust formed was similar for Steel C (Si content: 0.64 mass%), Steel G (Si content: 0.64 mass%), and Steel G (Si content: 0.119 mass%). From the above, it was found that the corrosion resistance improved with an increase in the Si content, and did not deteriorate with an increase in the Al content.

[0110] (Examples 1 to 13, Comparative Examples 1 to 6) 1. Sample Preparation The chemical compositions of the 19 steel types used in the evaluation are shown in Table 3. Although not listed in the table, other elements may be contained in amounts less than those specified as impurities. It should be noted that Comparative Examples 1 to 6 are low C-8Cr steels, similar to the steel material according to the present invention, and also have a lower Si content than the steel material according to the present invention.

[0111] [Table 3]

[0112] Comparative Example 1 is characterized by having the smallest Si content among the 19 steel types. Comparative Example 2 is characterized by having the largest Si content among the comparative examples, a larger P content than the present invention, a relatively larger S content, and a smaller Al content than the present invention. Comparative Example 3 is characterized by a smaller Cr content than the present invention. Comparative Example 5 is characterized by a smaller amount of Al and a larger amount of Cu than the present invention, with Cu>Ni. Comparative Example 6 is the only comparative example in which the (C+N) content is within the range of the composition of the present invention, and the Mo content is also within the range of the composition of the present invention. In Comparative Examples 1 to 6, at least three of the 12 main elements (C, Si, Mn, P, S, Cu, Ni, Cr, Mo, V, Al, N) of the present invention are outside the composition range of the steel material of the present invention.

[0113] The 19 steel types shown in Table 3 were each cast into a 50 kg ingot. Then, they were hot worked into rods approximately 2000 mm long with a rectangular cross section of 40 mm high and 65 mm wide, and cooled to near room temperature. The bar was then heated to 970°C and subjected to normalizing by soaking for 1 hour. Cracks were observed in the bar material of Comparative Example 5. It is believed that the cracks were caused by Cu during hot working because Cu≦Ni was not satisfied.

[0114] 2. Evaluation Various test pieces were prepared from the above bar material and evaluated for four properties: (1) tempered hardness, (2) machinability, (3) impact value, and (4) corrosion resistance.

[0115] 2.1. Tempered hardness 2.1.1 Test Method A test piece measuring 11.5 mm x 11.5 mm x 22 mm was cut out from the part of the bar above avoiding the decarburized layer. The test method was the same as that of Experiment 1. However, the combinations of the predetermined temperature T (tempering temperature) and the predetermined time X (tempering holding time) were as follows: 540℃×4.5Hr:LMP=16791 540℃×6.0Hr:LMP=16893 555℃×3.0Hr:LMP=16955 547℃×7.0Hr:LMP=17093 547℃×8.0Hr:LMP=17141 555℃×7.0Hr:LMP=17260 LMP = (tempering temperature T (℃) + 273) x (20 + log (tempering holding time X (Hr)))

[0116] [2.1.2. Results] The results are shown in Table 4. In addition, the results of the example are shown in FIG. 6, and the results of the comparative example are shown in FIG. The desired tempered hardness range in the range of 16800≦LMP≦17200, where the residual stress is low, is 35 to 43 HRC (within the dotted lines in Figs. 6 and 7). Among the examples, the tempered hardness was the largest in Example 11 and the smallest in Example 2. The tempered hardness of the other examples was between Example 2 and Example 11. In both Examples 2 and 11, the HRC values for the range 16800≦LMP≦17200 were within the dotted line (see FIG. 6). That is, in all Examples, the HRC values for the range 16800≦LMP≦17200 were within the dotted line. It is assumed that the values change linearly between each measurement point.

[0117] [Table 4]

[0118] In Example 11, the amount of C and the amount of Mo were large, and therefore the tempered hardness was the highest. In Example 2, although the C content was small, the tempered hardness could be increased by increasing the Mo and V contents, and the HRC value for the value of 16,800≦LMP≦17,200 was within the dotted line (see FIG. 6).

[0119] FIG. 7 shows the results of tempered hardness for the comparative examples, and only in Comparative Example 6 did the HRC value for the value of 16800≦LMP≦17200 fall within the dotted line. This is thought to be because in Comparative Example 6, the (C+N) amount is within the range of the composition of the present invention, and the Mo amount is also within the range of the composition of the present invention. The amounts of (C+N) in Comparative Examples 1 to 4 were smaller than those of the present invention, while the amount of (C+N) in Comparative Example 5 was larger than those of the present invention. As a result, the tempered hardness in Comparative Examples 1 to 4 was too small, while the tempered hardness in Comparative Example 5 was too large.

[0120] [2.2. Machinability 1 (Drill Machinability)] 2.2.1. Test Method A test piece measuring 25 mm x 50 mm x 200 mm was cut out from the part of the bar material avoiding the decarburized layer. The test method was the same as that of Experiment 2 (low C8-Cr steel). The tempered hardness of the test specimens was 36.5 to 37.5 HRC.

[0121] Machinability is VL 300 Here, "VL 300 " refers to the cutting speed (m / min) at which the drill reaches the end of its life when the cumulative cutting distance reaches 300 mm. Specifically, in a diagram where the X axis is the cutting distance (m) and the Y axis is the cutting speed (m / min), the value of the Y axis corresponding to an X value of 300 mm is calculated by interpolation. VL 300 is an index of machining efficiency, and the larger this value, the faster holes can be drilled, so the steel can be judged to have excellent machining efficiency and good machinability.

[0122] [2.2.2. Results] The results are shown in Figure 8. VL of Examples 1 to 13 300 On the other hand, the VL of Comparative Examples 1 to 6 was 29 m / min or more. 300 was less than 29 m / min.

[0123] In Example 5, the Si content, P content, and S content were high, so VL 300 The VL of Example 2, which has a relatively large amount of Si, P, and S, was 38.7 m / min. 300 The speed was also large at 36.1 m / min. On the other hand, in Example 4, the C content is relatively high and the Si content is relatively low, so VL 300 was 29.4 m / min, the lowest among the examples.

[0124] In addition, in the comparative example, VL of Comparative Example 2 300 The speed was 28.3 m / min, the largest among the comparative examples. This is because Comparative Example 2 had the largest amounts of Si, P, and S among the comparative examples. On the other hand, in Comparative Example 1, the Si content is the lowest among the 19 steel types, so VL300 The cutting speed was 21.6 m / min, the lowest among the 19 steel types. The results of the comparative examples also confirm the effects of the Si content, P content, and S content on machinability.

[0125] [2.3. Machinability 2 (End mill machinability)] 2.3.1 Test Method A test piece measuring 25 mm x 50 mm x 200 mm was cut out from the part of the bar material avoiding the decarburized layer. The test method was the same as that of Experiment 3 (low C-8Cr steel). The tempered hardness of the test specimens was 36.5 to 37.5 HRC.

[0126] In end mill machinability, the cutting distance is the cutting distance at which the tool wear reaches 100 μm. Specifically, it is the value of the X-axis corresponding to a Y value of 100 μm in a diagram where the X-axis is the cutting distance (m) and the Y-axis is the maximum wear (μm) and is determined by interpolation. The longer the cutting distance, the more can be removed with one tool, and the steel can be judged to have good machinability.

[0127] [2.3.2. Results] The results are shown in Figure 9. The overall tendency was similar to that of machinability 1 (drill machinability), with Examples 5 and 2 having large cutting distances, and Example 4 having the shortest cutting distance among the Examples. Among the comparative examples, the cutting distance of Comparative Example 2 was large, and the cutting distance of Comparative Example 1 was the smallest.

[0128] 2.4. Impact Value 2.4.1 Test Method A test piece measuring 11 mm x 11 mm x 55 mm was cut out from the part of the bar above, avoiding the decarburized layer. The test method was the same as that of Experiment 4 (low C-8Cr steel). The tempered hardness of the test specimens was 36.5 to 37.5 HRC.

[0129] [2.4.2. Results] The results are shown in Figure 10. In all examples, except for Example 5, the impact value was 145 J / cm 2 The impact value of Example 5 was 82 J / cm 2 The reason why the impact value of Example 5 is low is because the P amount and S amount are large.

[0130] The comparative examples, which were made of low C-8Cr steel like the examples, except for comparative examples 2 and 5, also had impact values equivalent to those of the examples. The impact value of Comparative Example 2 and Comparative Example 5 was 30 J / cm 2 The reason why the amount of Al is less than that of the present invention is that the amount of Al is smaller than that of the present invention. In addition, in low C-8Cr steel, if the amount of Al is too small, the impact value becomes significantly low.

[0131] 2.5. Corrosion Resistance 2.5.1 Test Method A test piece measuring 25 mm x 40.5 mm x 12.5 mm was cut out from the part of the bar material avoiding the decarburized layer. The test method was the same as that of Experiment 5 (low C-8Cr steel). The tempered hardness of the test specimens was 36.5 to 37.5 HRC.

[0132] [2.5.2. Results] The results are shown in Table 5. The evaluation was divided into four levels: S (Superior), A, B, and I (Inferior). Grade S indicates that almost no rust was observed. Grade A indicates that some rust was observed, but the corrosion resistance was judged to be good. On the other hand, grade B indicates that the rust is noticeable and that sufficient rust prevention measures must be taken when used in a humid environment. Grade I indicates that there is a lot of rust and that if used in a mold, the rust is expected to be noticeable even if sufficient rust prevention measures are taken.

[0133] [Table 5]

[0134] Among the examples, Examples 4 and 5 were rated A, and the others were rated S. The reason that Example 4 was rated A is that the C content was large and the Si content was small. The reason that Example 5 was rated A is that the P and S amounts were large. Even in Examples 4 and 5, which have component constitutions thought to be unfavorable for corrosion resistance, good corrosion resistance was achieved by using an Si content within the component range of the present invention. Therefore, it can be said that the corrosion resistance of the example is very excellent.

[0135] On the other hand, among the comparative examples, Comparative Example 2 and Comparative Example 3 were rated I, and the others were rated B. The reason why Comparative Example 2 was rated as I is that the amount of Si was small, and the amounts of P and S were large. The reason why Comparative Example 3 was rated as I is because the amount of Cr was small.

[0136] 3. Summary Examples 1 to 13 showed good results in tempered hardness, machinability, impact value, and corrosion resistance. On the other hand, although some of the comparative examples had excellent tempered hardness and impact value, their machinability and corrosion resistance were insufficient. This is because the amount of Si, which improves machinability and corrosion resistance, was low. Furthermore, in Examples 1 to 13, the residual stress was also reduced by setting the LMP within an appropriate numerical range. Therefore, it was found that Examples 1 to 13 have (1) appropriate temper hardness of 35 to 43 HRC, (2) low residual stress, (3) good machinability, (4) high impact value, and (5) good corrosion resistance.

[0137] 4. Versatility In the verification of the properties, a molten rod material was used as an example, but the steel material according to the present invention can also be used in the form of powder, block, wire, or plate material. For example, if the steel material according to the present invention is made into powder, the powder can be applied to various types of sequential manufacturing such as additive manufacturing (SLM method, LMD method, etc.) and plasma build-up welding (PPW).

[0138] Furthermore, if the steel material according to the present invention is made into a melted block, it is possible to manufacture dies and parts from the block. Furthermore, if the steel material according to the present invention is made into a melted rod or wire, the rod or wire can be applied to sequential shaping or repair in which build-up is performed using TIG or laser welding or the like.

[0139] Alternatively, it is also possible to manufacture dies and parts by using the steel material according to the present invention as plate material and joining a plurality of plate materials together. It is also possible to manufacture molds and parts by preparing separate molds and parts made of the steel material according to the present invention and joining them together. As described above, the steel material according to the present invention can be used in various shapes and by various methods to manufacture and repair molds and parts.

[0140] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0141] The steel material according to the present invention can be used for molds and mold parts used in injection molding and blow molding of plastics and resins, molding of rubber, molding of fiber-reinforced plastics, and the like. Furthermore, the mold according to the present invention can be used for injection molding or blow molding of plastics or resins, molding of rubber, molding of fiber-reinforced plastics, and the like.

[0142] It is also effective to combine the mold according to the present invention with surface modification (shot peening, shot blasting, sand blasting, nitriding, PVD, PCVD, CVD, plating, DLC coating, etc.). It is also possible to give the surface of the mold according to the present invention an uneven pattern by chemical corrosion, mechanical processing, laser processing, etc. (this is called "embossing"), and then transfer this pattern to the surface of plastic or resin to add value. Furthermore, the steel material according to the present invention can be applied to powders and plates used in additive manufacturing, and can also be made into rods or wires for use in welding repair of molds and mold parts.

Claims

1. 0.090<C≦0.170mass%, 0.60<Si≦1.00mass%, 0.10≦Mn≦0.60 mass%, 0.003≦P≦0.080 mass%, S≦0.012mass%, 0.20<Cu≦0.58 mass%, 0.20≦Ni≦1.10 mass%, Cu≦Ni, 7.70≦Cr≦8.20mass%, 0.70<Mo≦1.20mass%, 0.10<V≦0.30mass%, 0.007≦Al≦0.100mass%, 0.001≦N<0.015mass%, and 0.118≦C+N≦0.168mass%, and the balance being Fe and unavoidable impurities.

2. The steel material according to claim 1, further comprising one or more members selected from the group consisting of the following groups A to D: Group A: 0.0002<B≦0.0080mass%, Group B: 0.30<W≦4.00 mass%, and / or 0.30<Co≦3.00mass%. Group C: 0.004<Nb≦0.100mass%, 0.004<Ta≦0.100mass%, 0.004<Ti≦0.100mass%, and 0.004<Zr≦0.100mass% At least one selected from the group consisting of: Group D: 0.0005<Ca≦0.2000mass%, 0.03<Se≦0.50mass%, 0.005<Te≦0.100mass%, 0.01<Bi≦0.50 mass%, and 0.03<Pb≦0.50mass% At least one selected from the group consisting of:

3. 2. The steel material according to claim 1, having a hardness of 35 HRC or more and 43 HRC or less. However, the "hardness" means (a) Sample shape: A square bar of 11-12 mm x 11-12 mm x 20-25 mm was prepared. (b) The square bar is held at a quenching temperature of 895°C to 975°C for 2 hours; (c) cooling the square bar from the quenching temperature to 220°C at a cooling rate of 15 to 30°C / min, and further cooling from 220°C to 100°C or less at an arbitrary cooling rate; (d) tempering the square bar only once under conditions where the tempering temperature T (°C) and the tempering time X (Hr) satisfy the following formulas (1) to (3); (e) Removing decarburized portions from the surface of the tempered square bar. The Rockwell hardness (C scale) is measured at 15 to 35°C using a sample obtained by the method described above. 16800≦LMP≦17200 ... (1) LMP=(T+273)×(20+logX)…(2) 510 ° C ≦ T ≦ 590 ° C ... (3)

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

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