Steel and mold

Abstract

Provided are: a steel material which can be hardened and tempered to an appropriate hardness and which has excellent machinability, impact value, and corrosion resistance; and a mold which uses the steel material. The steel material contains 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 unavoidable impurities. The mold is made of the steel material.

Description

Technical Field

[0001] The present invention relates to steel, and more particularly to steel (specifically pre-hardened steel tempered to a specified hardness by quenching and tempering under specified conditions) for manufacturing molds used in injection molding or blow molding of plastics, molding or processing of rubber or various carbon fiber reinforced plastics, etc., and molds using such steel. Background Art

[0002] Pre-harden steel is steel that has been tempered to a specified hardness and can be machined. Pre-harden steel does not require heat treatment and can be directly used as molds, etc. after machining. Therefore, pre-harden steel is widely used in molds, mold components, etc. used in injection molding or blow molding of plastics, molding or processing of rubber or various carbon fiber reinforced plastics, etc. Various proposals have been made regarding such pre-harden steel and its manufacturing method.

[0003] For example, Patent Document 1 discloses a steel for plastic molding dies with excellent temperature controllability, which, by mass %, satisfies 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 (Cr + Mo) ≤ 10% and 7 ≤ (Cr + 3.3Mo), and the balance is composed of Fe and inevitable impurities.

[0004] The following is described in this document:

[0005] (a) By adjusting the ratio between ferrite-forming elements (Cr, Mo) and austenite-forming elements (Mn, Ni), mirror finish and impact value can be taken into account; and

[0006] (b) When their contents are optimized such that Cr and Mo satisfy a specified relationship, corrosion resistance and thermal conductivity become high.

[0007] Patent Document 2 discloses a steel for molds, which contains, by 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 balance is composed of Fe and inevitable impurities.

[0008] The following content is described in this document: In die steel containing specified elements, when the Al content is set to 0.007 to 0.150%, after being quenched and tempered to a specified hardness, good mirror polishing property, corrosion resistance intermediate between 5% Cr steel and 12% Cr steel, and high impact value can be achieved.

[0009] A die steel is disclosed in Patent Document 3, which contains, by 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 balance is composed of Fe and inevitable impurities.

[0010] The following content is described in this document: In die steel containing specified elements, since the components (especially Ni, Mo, and Al) are optimized, the SA property, tempering hardness, residual stress, machinability, impact value, and corrosion resistance are good.

[0011] When using prehardened steel to manufacture a die, first, die prehardened steel materials need to be manufactured. Die prehardened steel materials are generally manufactured through the processes of melting, refining, casting, homogenization heat treatment, hot working, intermediate heat treatment (normalizing, tempering), spheroidizing annealing (SA), quenching, straightening, and tempering.

[0012] In addition, when SA is not required depending on the steel type, tempering may be performed multiple times, or a tempering process may be added before and after straightening. Regardless of the number of tempering times, the final tempering process also serves to reduce the residual stress.

[0013] Next, a die or die component is manufactured from the prehardened steel material. A die or die component is generally manufactured through the processes of machining, mirror polishing, surface decoration, and surface treatment.

[0014] In addition, surface decoration is a process of imparting a special pattern to the surface through embossing or the like, but it may not be required depending on the use. Also, surface treatment is a process of hardening the surface through nitriding or PVD or the like, but it may not be required depending on the use.

[0015] As particularly important characteristics required for the prehardened steel material manufactured through the above processes, and the die and die component manufactured using the prehardened steel material, the following characteristics are required.

[0016] (1) Tempering hardness (a moderate tempering hardness that can maintain good wear resistance and achieve a high impact value)

[0017] (2) Machining shape soundness (low residual stress to the extent that warping or distortion of the mold during machining can be avoided)

[0018] (3) Machinability (ease of cutting)

[0019] (4) Impact value (a high impact value to the extent that large cracks in the mold can be avoided)

[0020] (5) Corrosion resistance (high corrosion resistance to the extent that it does not rust even when used and stored in a humid environment)

[0021] (6) Mirror finish (high mirror polishability to the extent that it can be polished smoothly)

[0022] (7) Embossing processability (ease of surface decoration)

[0023] Here, in the configuration of Patent Document 1, although the temperature controllability is excellent, further improvement is required for tempering hardness, machinability, and corrosion resistance.

[0024] In addition, in the configuration of Patent Document 2, although the stability of mirror finish and impact value can be ensured by adjusting the Al content, like Patent Document 1, further improvement is required for tempering hardness, machinability, and corrosion resistance.

[0025] In the configuration of Patent Document 3, although tempering hardness and corrosion resistance can be ensured by increasing the N content, further improvement is required for machinability.

[0026] However, in order to increase the N content, a nitrogen addition process (so-called nitrogen addition) under a pressurized state is required, and the introduction of equipment accompanying the nitrogen addition also becomes large-scale.

[0027] In addition, the steel materials disclosed in Patent Documents 1 to 3 are so-called low C-8Cr steels.

[0028] Prior art documents

[0029] Patent documents

[0030] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2010-24510

[0031] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2020-63508

[0032] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2022-83627 Summary of the Invention

[0033] Problems to be solved by the invention

[0034] The problem to be solved by the present invention is to provide a steel material that can be quenched and tempered to a moderate hardness and has excellent machinability, impact value, and corrosion resistance.

[0035] Another problem to be solved by the present invention is to provide a mold that can be quenched and tempered to a moderate hardness and has excellent machinability, impact value, and corrosion resistance.

[0036] Solutions for solving the problems

[0037] To solve the above problems, the steel material according to the present invention comprises:

[0038] 0.090 < C ≤ 0.170 mass%;

[0039] 0.60 < Si ≤ 1.00 mass%;

[0040] 0.10 ≤ Mn ≤ 0.60 mass%;

[0041] 0.003 ≤ P ≤ 0.080 mass%;

[0042] S ≤ 0.012 mass%;

[0043] 0.20 < Cu ≤ 0.58 mass%;

[0044] 0.20 ≤ Ni ≤ 1.10 mass%;

[0045] Cu ≤ Ni;

[0046] 7.70 ≤ Cr ≤ 8.20 mass%;

[0047] 0.70 < Mo ≤ 1.20 mass%;

[0048] 0.10 < V ≤ 0.30 mass%;

[0049] 0.007 ≤ Al ≤ 0.100 mass%;

[0050] 0.001 ≤ N < 0.015 mass%;

[0051] 0.118 ≤ C + N ≤ 0.168 mass%,

[0052] The remaining part is composed of Fe and inevitable impurities.

[0053] The mold according to the present invention is composed of the steel material according to the present invention.

[0054] Advantages of the invention

[0055] The steel involved in the present invention is a kind of low C-8Cr steel. Moreover, as the low C-8Cr steel, due to the optimization of the composition (especially Si, Al, C, V and Mo), the properties of (1) tempering hardness, (2) machining shape soundness, (3) machinability, (4) impact value and (5) corrosion resistance are particularly good.

[0056] Specifically, it has the following characteristics:

[0057] (1) The hardness after tempering is an appropriate value of 35-43 HRC,

[0058] (2) The residual stress after tempering is small,

[0059] (3) The machinability after tempering is equivalent to that of SKD61,

[0060] (4) The impact value after tempering is ≥80 J / cm 2 of an appropriate value,

[0061] (5) The corrosion resistance after tempering is as high as that of martensitic stainless steel.

[0062] Therefore, the steel involved in the present invention has the following advantages:

[0063] (A) The mirror grinding property is also excellent;

[0064] (B) The deformation during machining is small;

[0065] (C) The die machining is easy;

[0066] (D) Cracks are not likely to occur during use;

[0067] (E) Rust is not likely to occur during use and storage. Description of the Drawings

[0068] Figure 1 It is a graph showing the tempering hardness with respect to the value of LMP (Experiment 1).

[0069] Figure 2 It is a graph showing the drill bit machinability (Experiment 2).

[0070] Figure 3 It is a graph showing the end mill machinability (Experiment 3).

[0071] Figure 4 It is a graph showing the impact value (Experiment 4).

[0072] Figure 5 It is a graph showing the corrosion resistance (Experiment 5).

[0073] Figure 6 It is a graph showing the tempering hardness with respect to the value of LMP (Example).

[0074] Figure 7 It is a graph showing the tempering hardness with respect to the value of LMP (Comparative Example).

[0075] Figure 8 It is a graph showing the machinability of the drill bit (Example, Comparative Example).

[0076] Figure 9 It is a graph showing the machinability of the end mill (Example, Comparative Example).

[0077] Figure 10 It is a graph showing the impact value (Example, Comparative Example). Detailed Description of the Invention

[0078] Hereinafter, an embodiment of the present invention will be described in detail.

[0079] [1. Steel]

[0080] [1.1. Composition]

[0081] [1.1.1. Main Constituent Elements]

[0082] The steel involved in the present invention contains the following elements, and the remaining part is composed of Fe and inevitable impurities. The types of additive elements, their composition ranges, and the reasons for their limitations are as follows.

[0083] (1) 0.090 < C ≤ 0.170 mass%:

[0084] If the amount of C is too small, δ-ferrite will be mixed in the pre-hardened steel, deteriorating the mirror polishing property or the embossing processability. In addition, VC or VCN that inhibits the movement of austenite grain boundaries during quenching will become too small, the grains will coarsen, and thus the impact value will decrease. Moreover, when tempering is performed at a temperature of 510°C or higher and 16800 ≤ LMP ≤ 17200 to reduce the residual stress, the tempering hardness will be less than 35 HRC, and a tempering hardness of 35 HRC or higher cannot be achieved.

[0085] Therefore, the amount of C needs to exceed 0.090 mass%. The amount of C is preferably 0.095 mass% or more, and more preferably 0.105 mass% or more.

[0086] On the other hand, if the C content is excessive, VC or VCN will crystallize in a coarse state during the casting of the ingot, reducing the impact value. In addition, pearlite precipitates during quenching, deteriorating the tempering hardness, impact value, mirror finishability, embossing processability, and corrosion resistance. Moreover, since a large amount of Cr is consumed as carbides, the corrosion resistance may sometimes be insufficient. Additionally, cracks are likely to occur during welding repair. Also, the thermal conductivity is reduced.

[0087] Therefore, the C content needs to be 0.170 mass% or less. The C content is preferably 0.165 mass% or less, and more preferably 0.160 mass% or less.

[0088] In the injection molding of resins such as plastics, it is necessary to quickly cure the resin filled in the mold to improve productivity. Therefore, rapid cooling of the mold is required, that is, high thermal conductivity is required. In addition, when selectively flowing a high-temperature fluid or a low-temperature fluid through the flow path in the mold to control the mold temperature, high responsiveness of the mold to heating or cooling is required. Therefore, high thermal conductivity is important.

[0089] (2) 0.60 < Si ≤ 1.00 mass%:

[0090] If the Si content is too low, the machinability will deteriorate. In addition, the corrosion resistance in the atmosphere is also insufficient.

[0091] Therefore, the Si content needs to exceed 0.60 mass%. The Si content is preferably 0.62 mass% or more, and more preferably 0.64 mass% or more.

[0092] On the other hand, if the Si content is excessive, the thermal conductivity will decrease. In addition, δ-ferrite is likely to be mixed in the pre-hardened steel. Moreover, during hot working, a hard and difficult-to-peel scale will form on the surface of the steel, significantly wearing the processing tools.

[0093] Therefore, the Si content needs to be 1.00 mass% or less. The Si content is preferably 0.95 mass% or less, and more preferably 0.90 mass% or less.

[0094] When a corrosion part (rust) is generated due to moisture in the atmosphere during the manufacturing process or storage of the mold, even if there is only one such part, mirror polishing needs to be redone. Redoing the mirror polishing is very time-consuming and costly. In addition, when the rust cannot be removed by mirror polishing, the surface quality of the mold will be damaged. Therefore, corrosion resistance that inhibits the generation of rust is important.

[0095] (3) 0.10 ≤ Mn ≤ 0.60 mass%:

[0096] If the amount of Mn is too small, pearlite will precipitate during quenching. In addition, δ-ferrite will be mixed into the pre-hardened steel. Moreover, the hardenability is insufficient and the impact value inside the steel also becomes low.

[0097] Therefore, the amount of Mn needs to be 0.10 mass% or more. The amount of Mn is preferably 0.15 mass% or more, and more preferably 0.22 mass% or more. Moreover, when the amount of Ni is 0.39 mass% or more, it is more preferably 0.34 mass% or more.

[0098] On the other hand, if the amount of Mn is too large, the SA property will deteriorate. In addition, through tempering at 510 °C or higher, the impact value decreases (temper embrittlement). Moreover, the thermal conductivity also decreases.

[0099] Therefore, the amount of Mn needs to be 0.60 mass% or less. The amount of Mn is preferably 0.56 mass% or less, and more preferably 0.54 mass% or less.

[0100] (4) 0.003 ≤ P ≤ 0.080 mass%:

[0101] P has the effect of finely crushing the cutting chips. Therefore, if the amount of P is too small, the machinability will deteriorate. Moreover, if the amount of P is too small, the high-temperature strength will decrease.

[0102] Therefore, the amount of P needs to be 0.003 mass% or more. The amount of P is preferably 0.005 mass% or more, and more preferably 0.007 mass% or more.

[0103] On the other hand, if the amount of P is too large, the decrease in the impact value will become significant. In addition, P segregates at the grain boundaries, accelerating the corrosion from the grain boundaries, so the corrosion resistance deteriorates.

[0104] Therefore, the amount of P needs to be 0.080 mass% or less. The amount of P is preferably 0.070 mass% or less, and more preferably 0.060 mass% or less.

[0105] (5) S ≤ 0.012 mass%:

[0106] Generally, if the amount of S is too small, the machinability will deteriorate, but the steel involved in the present invention ensures the machinability through the amount of Si. Therefore, the amount of S can be very small. In addition, the lower limit of the amount of S is about 0.0002 mass%.

[0107] On the other hand, if the amount of S is too large, the impact value will decrease. In addition, S segregates at the grain boundaries, accelerating the corrosion from the grain boundaries, so the corrosion resistance deteriorates. Moreover, MnS is formed between S and Mn, and corrosion also occurs from the interface between MnS and the matrix, further deteriorating the corrosion resistance.

[0108] Therefore, the S content needs to be 0.012 mass% or less. The S content is preferably 0.010 mass% or less, and more preferably 0.008 mass% or less.

[0109] (6) 0.20 < Cu ≤ 0.58 mass%:

[0110] If the Cu content is too low, pearlite will precipitate during quenching. In addition, δ-ferrite will be mixed in the pre-hardened steel. Moreover, when tempering is carried out at a temperature of 510 °C or higher and 16,800 ≤ LMP ≤ 17,200 to reduce residual stress, the tempering hardness will be less than 35 HRC, and a tempering hardness of 35 HRC or higher cannot be achieved. Also, the machinability deteriorates and the corrosion resistance is insufficient.

[0111] Therefore, the Cu content needs to exceed 0.20 mass%. The Cu content is preferably 0.22 mass% or higher, and more preferably 0.23 mass% or higher.

[0112] On the other hand, if the Cu content is too high, cracks will occur during hot working. In addition, the SA property deteriorates, and the thermal conductivity and impact value also decrease.

[0113] Therefore, the Cu content needs to be 0.58 mass% or less. The Cu content is preferably 0.54 mass% or less, and more preferably 0.50 mass% or less.

[0114] (7) 0.20 ≤ Ni ≤ 1.10 mass%:

[0115] If the Ni content is too low, δ-ferrite will be mixed in the pre-hardened steel. In addition, the hardenability is insufficient and the impact value inside the steel decreases. Moreover, the effect of suppressing cracks during hot working in the case of a high Cu content also decreases.

[0116] Therefore, the Ni content needs to be 0.20 mass% or higher. The Ni content is preferably 0.30 mass% or higher, and more preferably 0.39 mass% or higher.

[0117] On the other hand, if the Ni content is too high, the SA property will deteriorate and the thermal conductivity will also decrease.

[0118] Therefore, the Ni content needs to be 1.10 mass% or less. The Ni content is preferably 1.00 mass% or less, and more preferably 0.90 mass% or less.

[0119] (8) Cu ≤ Ni

[0120] Ni has the effect of suppressing hot working cracks caused by Cu. However, even if the amount of Ni is appropriate, if the amount of Cu is excessive relative to the amount of Ni, the effect of suppressing hot working cracks may sometimes become smaller.

[0121] Therefore, in order to suppress cracks during hot working, the amount of Cu needs to be equal to or less than the amount of Ni.

[0122] (9) 7.70 ≤ Cr ≤ 8.20 mass%:

[0123] If the amount of Cr is too small, the corrosion resistance will be insufficient. In addition, the impact value also decreases.

[0124] Therefore, the amount of Cr needs to be 7.70 mass% or more. The amount of Cr is preferably 7.75 mass% or more, more preferably 7.80 mass% or more.

[0125] On the other hand, if the amount of Cr is too large, δ-ferrite will be mixed in the pre-hardened steel. In addition, pearlite will precipitate during quenching. In addition, when tempering is carried out at a temperature of 510 °C or higher and 16800 ≤ LMP ≤ 17200 for the purpose of reducing residual stress, the tempering hardness will be less than 35 HRC, and a tempering hardness of 35 HRC or more cannot be achieved. Moreover, the machinability deteriorates and the thermal conductivity also decreases.

[0126] Therefore, the amount of Cr needs to be 8.20 mass% or less. The amount of Cr is preferably 8.15 mass% or less, more preferably 8.10 mass% or less.

[0127] (10) 0.70 < Mo ≤ 1.20 mass%:

[0128] If the amount of Mo is too small, when tempering is carried out at a temperature of 510 °C or higher and 16800 ≤ LMP ≤ 17200 for the purpose of reducing residual stress, the tempering hardness will be less than 35 HRC, and a tempering hardness of 35 HRC or more cannot be achieved. In addition, pearlite will precipitate during quenching. Moreover, the corrosion resistance is also insufficient.

[0129] Therefore, the amount of Mo needs to exceed 0.70 mass%. The amount of Mo is preferably 0.75 mass% or more, more preferably 0.80 mass% or more.

[0130] On the other hand, if the amount of Mo is too large, δ-ferrite will be mixed in the pre-hardened steel. Moreover, the fracture toughness will also deteriorate.

[0131] Therefore, the amount of Mo needs to be 1.20 mass% or less. The amount of Mo is preferably 1.15 mass% or less, more preferably 1.10 mass% or less.

[0132] (11) 0.10 < V ≤ 0.30 mass%:

[0133] If the amount of V is too small, 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 will be less than 35 HRC, and a tempering hardness of 35 HRC or higher cannot be achieved. In addition, VC or VCN that inhibits the movement of austenite grain boundaries during quenching will become too little, so the grains will coarsen. Furthermore, if the grains coarsen during quenching, the impact value will decrease.

[0134] Therefore, the amount of V needs to exceed 0.10 mass%. The amount of V is preferably 0.12 mass% or more, more preferably 0.13 mass% or more.

[0135] On the other hand, if the amount of V is too large, VC or VCN will crystallize in a coarse state during the casting of the ingot, reducing the impact value. Moreover, δ-ferrite will be mixed into the pre-hardened steel.

[0136] Therefore, the amount of V needs to be 0.30 mass% or less. The amount of V is preferably 0.28 mass% or less, more preferably 0.27 mass% or less.

[0137] (12) 0.007 ≤ Al ≤ 0.100 mass%:

[0138] The steel (low C-8Cr steel) related to the present invention has the following specificity: in the case of low Al, even if the grains are fine, the impact value is significantly low.

[0139] Therefore, the amount of Al needs to be 0.007 mass% or more. The amount of Al is preferably 0.010 mass% or more, more preferably 0.020 mass% or more.

[0140] If the amount of Al is too large, alumina becomes too much. As a result, not only does the impact value decrease, but the mirror finishability also deteriorates. The reason is that pinholes are generated due to the peeling off of alumina. Moreover, if the amount of Al is too large, the thermal conductivity will decrease, and the machinability will also deteriorate significantly.

[0141] Therefore, the amount of Al needs to be 0.100 mass% or less. The amount of Al is preferably 0.090 mass% or less, more preferably 0.085 mass% or less.

[0142] In particular, when the amount of O is 0.003 mass% or less, the adverse effects of alumina become less obvious. Therefore, in the range of 0.050 < Al ≤ 0.085 mass% where the lower limit of the amount of Al is increased, the balance of crystal grain size, impact value, mirror finishability, machinability, and thermal conductivity becomes very good.

[0143] (13) 0.001 ≤ N < 0.015 mass%:

[0144] If the amount of N is too small, the amount of AlN that inhibits the movement of austenite grain boundaries during quenching will become too small. Therefore, the grains tend to grow excessively. In addition, δ-ferrite will be mixed into the pre-hardened steel. In addition, 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 will be less than 35 HRC, and a tempering hardness of 35 HRC or higher cannot be achieved. Moreover, the corrosion resistance is insufficient.

[0145] Therefore, the amount of N needs to be 0.001 mass% or more. The amount of N is preferably 0.002 mass% or more, and more preferably 0.003 mass% or more.

[0146] On the other hand, in order to contain a large amount of N, the amount of N in the impurity level of the raw material is insufficient, so N needs to be actively added, and special equipment is required. In addition, during the casting of the ingot, VC or VCN will crystallize in a coarse state, and more coarse AlN will also be formed, reducing the impact value.

[0147] In addition, when the mold is welded and repaired, nitrogen sometimes vaporizes at the welded part, forming voids on the surface. In addition, during welding repair, voids may also be formed inside the mold and exposed to the outside through mirror polishing.

[0148] In order to remove these voids by mirror polishing, it takes a lot of time and cost. In addition, if these voids cannot be removed, the surface quality of the mold will be damaged. Moreover, even if the voids inside the mold do not expose to the outside, they may also cause cracks during use.

[0149] In addition, when B is dissolved, the hardenability can be improved. In order to dissolve B, N that easily combines with B needs to be fixed in advance as a compound. Therefore, by adding Ti or the like that is more likely to form nitrides, N is fixed in the form of TiN or the like. Here, when N is large, the amount of added Ti or the like also needs to increase. As a result, the amount of coarse TiN or the like increases, and the impact value sometimes decreases.

[0150] Therefore, the amount of N needs to be less than 0.015 mass%. The amount of N is preferably 0.013 mass% or less, and more preferably 0.012 mass% or less.

[0151] (14) 0.118 ≤ C + N ≤ 0.168 mass%:

[0152] Even when the amounts of C and N exceed the above lower limit values, if the amount of (C + N) is too small, the above problems may sometimes occur.

[0153] Therefore, the amount of (C + N) needs to be 0.118 mass% or more. The amount of (C + N) is preferably 0.122 mass% or more.

[0154] Even when the amounts of C and N are below the above upper limit values, if the amount of (C + N) is excessive, the above problems may sometimes occur.

[0155] Therefore, the amount of (C + N) needs to be 0.168 mass% or less. The amount of (C + N) is preferably 0.164 mass% or less.

[0156] (15) Inevitable impurities may also be included:

[0157] O ≤ 0.005 mass%,

[0158] B ≤ 0.0002 mass%,

[0159] W ≤ 0.30 mass%,

[0160] Co ≤ 0.30 mass%,

[0161] Nb ≤ 0.004 mass%,

[0162] Ta ≤ 0.004 mass%,

[0163] Ti ≤ 0.004 mass%,

[0164] Zr ≤ 0.004 mass%,

[0165] Ca ≤ 0.0005 mass%,

[0166] Se ≤ 0.03 mass%,

[0167] Te ≤ 0.005 mass%,

[0168] Bi ≤ 0.01 mass%,

[0169] Pb ≤ 0.03 mass%,

[0170] At least one of Mg ≤ 0.02 mass%.

[0171] [1.1.2. Sub-constituent elements]

[0172] The steel material according to the present invention may contain, in addition to the above main constituent elements, one or more elements as follows. The types of additive elements, their composition ranges, and the reasons for their limitations are as follows.

[0173] [A. Group A]

[0174] (16) 0.0002 < B ≤ 0.0080 mass%:

[0175] When the amount of P is large, since P segregated at grain boundaries reduces the grain boundary strength, the impact value sometimes becomes low. To improve the grain boundary strength, addition of B is effective. In addition, when the total amount of alloying elements is small, ferrite or pearlite sometimes precipitates during quenching. To suppress this, addition of B is also effective. To obtain such an effect, the amount of B is preferably more than 0.0002 mass%. The amount of B is preferably 0.0003 mass% or more, more preferably 0.0004 mass% or more.

[0176] If the amount of B is large, compounds with iron (borides, carbon borides) increase, reducing the impact value.

[0177] Therefore, the amount of B is preferably 0.0080 mass% or less. The amount of B is preferably 0.0075 mass% or less, more preferably 0.0070 mass% or less.

[0178] In addition, when adding B for the purpose of improving grain boundary strength, it is meaningless if B forms BN. Therefore, when adding B to steel with a large amount of N, it is necessary to combine N with elements other than B. Specifically, elements such as Ti, Zr, Nb that are more likely to form nitrides are combined with N. These elements are effective even at impurity levels, but if insufficient, the amounts described below are preferably added.

[0179] On the other hand, when it is desired to disperse BN to improve machinability, it is not necessary to adopt a scheme of actively combining N with the above nitride-forming elements.

[0180] [B.B group]

[0181] (17) 0.30 < W ≤ 4.00 mass%:

[0182] Since the steel according to the present invention has a low C content and small amounts of Mo and V, the strength may sometimes be insufficient depending on the use. To strengthen the steel according to the present invention, addition of W is effective. To obtain such an effect, the amount of W is preferably more than 0.30 mass%. The amount of W is preferably 0.50 mass% or more, more preferably 1.00 mass% or more.

[0183] On the other hand, if the amount of W is too large, not only will segregation become significant, but also the raw material cost will increase.

[0184] Therefore, the amount of W is preferably 4.00 mass% or less. The amount of W is preferably 3.90 mass% or less, more preferably 3.80 mass% or less.

[0185] (18) 0.30 < Co ≤ 3.00 mass%:

[0186] To strengthen the steel involved in the present invention, it is effective to replace W or add Co in addition to W. To achieve such an effect, the Co content is preferably more than 0.30 mass%. The Co content is preferably 0.50 mass% or more, more preferably 1.00 mass% or more.

[0187] On the other hand, if the Co content is too high, not only will segregation become significant, but also the raw material cost will increase.

[0188] Therefore, the Co content is preferably 3.00 mass% or less. The Co content is preferably 2.80 mass% or less, more preferably 2.50 mass% or less.

[0189] [C.C group]

[0190] (19) 0.004 < Nb ≤ 0.100 mass%:

[0191] (20) 0.004 < Ta ≤ 0.100 mass%:

[0192] (21) 0.004 < Ti ≤ 0.100 mass%:

[0193] (22) 0.004 < Zr ≤ 0.100 mass%:

[0194] Since the steel involved in the present invention has a low C content and not much V content, VC or VCN that inhibits the movement of austenite grain boundaries during quenching is sometimes insufficient. In this case, depending on the quenching conditions, austenite grains are sometimes prone to excessive growth. To inhibit grain growth, it is effective to add elements that form carbides, nitrides, or carbonitrides (i.e., Nb, Ta, Ti, and / or Zr) to disperse carbides, etc. in the matrix.

[0195] To achieve such an effect, the contents of Nb, Ta, Ti, and Zr are each preferably more than 0.004 mass%. The contents of these elements are each preferably 0.006 mass% or more, more preferably 0.008 mass% or more.

[0196] On the other hand, if the contents of these elements are too high, carbides, nitrides, or carbonitrides will become coarse and the impact value will decrease. In addition, the addition of these elements in excess of the required amount will lead to an increase in raw material costs. Therefore, the contents of Nb, Ta, Ti, and Zr are each preferably 0.100 mass% or less. The contents of these elements are each preferably 0.090 mass% or less, more preferably 0.080 mass% or less.

[0197] In addition, the die steel involved in the present invention may contain any one of Nb, Ta, Ti, or Zr, or may also contain two or more of them.

[0198] [Group D.D]

[0199] (23) 0.0005 < Ca ≤ 0.2000 mass%:

[0200] (24) 0.03 < Se ≤ 0.50 mass%:

[0201] (25) 0.005 < Te ≤ 0.100 mass%:

[0202] (26) 0.01 < Bi ≤ 0.50 mass%:

[0203] (27) 0.003 < Pb ≤ 0.50 mass%:

[0204] The steel involved in the present invention ensures machinability through Si. However, depending on the application, it is sometimes necessary to further improve machinability. To improve machinability, it is effective to add elements known as free-cutting elements (i.e., Ca, Se, Te, Bi, and / or Pb). To obtain such an effect, the contents of Ca, Se, Te, Bi, and Pb are preferably amounts exceeding the above lower limit values, respectively.

[0205] The amount of Ca is preferably 0.0006 mass% or more, more preferably 0.0007 mass% or more.

[0206] The amount of Se is preferably 0.04 mass% or more, more preferably 0.05 mass% or more.

[0207] The amount of Te is preferably 0.006 mass% or more, more preferably 0.007 mass% or more.

[0208] The amount of Bi is preferably 0.02 mass% or more, more preferably 0.03 mass% or more.

[0209] The amount of Pb is preferably 0.04 mass% or more, more preferably 0.05 mass% or more.

[0210] On the other hand, if the contents of these elements are excessive, not only is it easy to crack during hot working, but also the impact value becomes low. Therefore, the contents of Ca, Se, Te, Bi, and Pb are preferably below the above upper limit values, respectively.

[0211] The amount of Ca is preferably 0.1900 mass% or less, more preferably 0.1800 mass% or less.

[0212] The content of Se is preferably 0.48 mass% or less, more preferably 0.46 mass% or less.

[0213] The content of Te is preferably 0.090 mass% or less, more preferably 0.080 mass% or less.

[0214] The content of Bi is preferably 0.450 mass% or less, more preferably 0.400 mass% or less.

[0215] The content of Pb is preferably 0.45 mass% or less, more preferably 0.40 mass% or less.

[0216] In addition, 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.

[0217] [1.2. Characteristics]

[0218] [1.2.1. Hardness]

[0219] The steel material according to the present invention is usually used in a state where it is quenched and tempered to a specified hardness. In the steel material according to the present invention, when the above-described components are applied and the heat treatment conditions are optimized, the hardness after quenching and tempering becomes 35 HRC or more and 43 HRC or less.

[0220] Here, if the hardness after quenching and tempering exceeds 43 HRC, the machinability deteriorates, and the man-hours for manufacturing a mold by machining increase.

[0221] On the other hand, if the hardness after quenching and tempering is less than 35 HRC, the mirror finishability or wear resistance deteriorates.

[0222] In addition, in the steel material according to the present invention, "hardness" means the Rockwell hardness (C scale) measured at 15°C to 35°C using a specimen obtained by the following (a) to (e):

[0223] (a) A square bar having a specimen shape of 11 to 12 mm × 11 to 12 mm × 20 to 25 mm is manufactured;

[0224] (b) The square bar is held at a quenching temperature of 895°C to 975°C for 2 Hr (hours);

[0225] (c) The square bar is cooled from the quenching temperature to 220°C at a cooling rate of 15 to 30°C / min (minutes), and then cooled from 220°C to 100°C or less at an arbitrary cooling rate;

[0226] (d) The square bar is tempered only once under the condition that the tempering temperature T (°C) and the tempering time X (Hr) satisfy the following formulas (1) to (3);

[0227] (e) Remove the decarburized portion on the surface of the square bar after tempering.

[0228] 16800 ≤ LMP ≤ 17200…(1)

[0229] LMP = (T + 273) × (20 + logX) … (2)

[0230] 510°C ≤ T ≤ 590°C … (3)

[0231] If the quenching temperature is within the above range, it is possible to achieve both the desired tempering hardness and the desired impact value.

[0232] If the quenching temperature is less than 895°C, the solution elements will be insufficient, and it will be difficult to stably obtain a hardness of 35 HRC or more. In addition, depending on the composition, the quenching temperature may sometimes be lower than the AC3 point. Therefore, the quenching temperature is preferably 895°C or more.

[0233] On the other hand, if the quenching temperature exceeds 975°C, the coarsening of the grains will become significant, and the impact value will decrease. Therefore, the quenching temperature is preferably 975°C or less.

[0234] Here, LMP (Larson - Miller ParameTer) is one of the temperature - time conversion parameters.

[0235] If LMP is within the above range, it is possible to achieve both low residual stress and good corrosion resistance.

[0236] If LMP is less than 16800, the release of the residual stress will be insufficient. Therefore, LMP is preferably 16800 or more, more preferably 16900 or more.

[0237] On the other hand, if LMP exceeds 17200, due to the large amount of Cr in solid solution, Cr will be consumed in carbide formation, and the amount of Cr in solid solution will decrease. As a result, the corrosion resistance deteriorates. Therefore, LMP is preferably 17200 or less, more preferably 17150 or less.

[0238] The tempering temperature T and the tempering time X are not particularly limited as long as LMP is within the above range. However, if the tempering temperature is too low, a long time may sometimes be required to obtain the specified effect. Therefore, the tempering temperature T is preferably 510°C or more.

[0239] In addition, if the tempering temperature T is less than 510°C, the time to reach LMP = 16800 will exceed 29 Hr, and the productivity will be significantly reduced.

[0240] On the other hand, if the tempering temperature T is too high, the tempering time will be too short, and sometimes the specified effect cannot be obtained for all parts of the pre-hardened steel. Therefore, the tempering temperature T is preferably 590°C or lower.

[0241] For example, if the tempering temperature T exceeds 590°C, the time to reach LMP = 17200 will be less than 0.9 Hr, and it becomes difficult to reach 16800 ≤ LMP ≤ 17200 for all parts (from the center to the surface) within the cross-section of the pre-hardened steel.

[0242] [2. Mold]

[0243] The mold according to the present invention is a mold using the steel according to the present invention.

[0244] In addition, in the present invention, the so-called "mold" includes not only the mold body, but also mold components such as pins assembled to the mold body for use. That is, the "mold" refers to all steel parts in contact with the object to be molded (plastic, vinyl, rubber, etc.).

[0245] [3. Manufacturing method of steel]

[0246] The steel according to the present invention is obtained through the following (a) to (c):

[0247] (a) Melting and casting the raw materials compounded to have a specified composition;

[0248] (b) Performing homogenization heat treatment on the ingot;

[0249] (c) Performing hot working on the ingot after homogenization heat treatment.

[0250] It is also possible to: (d) Perform normalizing on the rough material after hot working as needed;

[0251] (e) Perform tempering on the rough material as needed;

[0252] (f) Perform spheroidizing annealing (SA) on the rough material as needed;

[0253] (g) Perform quenching and tempering on the rough material as needed.

[0254] Since the composition of the steel according to the present invention is optimized, it sometimes already has the required hardness after hot working. Otherwise, it is preferable to perform the required pretreatment after hot working and then perform quenching and tempering.

[0255] The steel tempered to the specified hardness through quenching and tempering is processed into a specified shape and used for various purposes.

[0256] [4. Function]

[0257] [4.1. Characteristics Required for Steel]

[0258] For steel (so-called pre-hardened steel) in a state quenched and tempered to a specified hardness and used for various purposes, as well as for molds and mold components manufactured using such steel, the following characteristics are required as particularly important characteristics.

[0259] (1) Tempering hardness:

[0260] To ensure the strength of the mold, a certain degree of tempering hardness is required. If the tempering hardness is too low, the shape of the mold will change due to wear during use. On the other hand, if the tempering hardness is too high, not only will it be difficult to machine the mold shape, but the impact value will also decrease. Therefore, large cracks are likely to occur in the mold during use. To balance good wear resistance and a high impact value, a tempering hardness of around 30 - 55 HRC is generally required, but this may vary depending on the application.

[0261] (2) Machining shape integrity:

[0262] High "machining shape integrity" means that the expected shape can be obtained after machining. If the residual stress of the pre-hardened steel is high, due to the balanced change of the residual stress after machining, the mold machined from the steel may sometimes undergo strain. This is called machining strain. Steel that is not prone to machining strain is judged to have high machining shape integrity. Steel that can obtain a high hardness even after tempering at a high temperature has high machining shape integrity because the residual stress can be sufficiently released through high-temperature tempering.

[0263] (3) Machinability:

[0264] "Machinability" becomes a problem when machining the pre-hardened steel into the mold shape. In the mold, a circuit for the flow of a cooling or heating fluid is required, and holes for this circuit are formed using tools such as drills. For steel with poor machinability, if the machining speed is not reduced, holes cannot be formed, and the wear of tools such as drills is also large.

[0265] Machinability is greatly affected by hardness. However, even for the same hardness, when the composition of the steel is different, the machinability often varies. For steel, not only the characteristics required for the mold are required, but also high machinability.

[0266] (4) Impact value:

[0267] The "impact value" is preferably as high as possible. If the impact value is low, large cracks are likely to occur in the mold during use. Since it is difficult to repair large cracks, a mold with large cracks must be replaced with a new one. To reduce the mold cost, large cracks in the mold need to be avoided. Therefore, high impact value is required for pre-hardened steel.

[0268] (5) Corrosion resistance:

[0269] The higher the corrosion resistance, the better. The molds involved in the present invention are used for injection molding or blow molding of resins, molding or processing of rubbers or fiber-reinforced plastics, etc. Therefore, due to the corrosive gases generated from the resin during injection molding, the surface of the mold may sometimes be corroded. In addition, during the manufacturing process of the mold or during the storage of the mold, the surface of the mold may sometimes be corroded by the moisture in the atmosphere. Therefore, high corrosion resistance is required for pre-hardened steel.

[0270] (6) Mirror finish:

[0271] "Mirror finish" refers to an index indicating how smooth the surface of the mold can be polished. The smoother it can be polished, the higher the mirror finish is evaluated. Although pre-hardened steel with a high mirror finish is suitable for molds, such steel often has fewer foreign substances (crystals or inclusions, etc.) and a high hardness.

[0272] (7) Embossing processability:

[0273] "Embossing process" refers to a process of imparting specific patterns (concavities and convexities) to the surface of the mold by etching with a chemical solution. Steel that can easily impart patterns is evaluated as having good embossing processability. Since embossing process is generally etching by acid, it often conflicts with corrosion resistance.

[0274] [4.2. Characteristics of the steel involved in the present invention]

[0275] Generally, it is difficult to satisfy the above 7 characteristics simultaneously. In contrast, since the composition of the steel involved in the present invention is optimized, it has appropriate tempering hardness, high machinability, high impact value, and high corrosion resistance.

[0276] In addition, since the required hardness can be obtained even when tempering is performed at a high temperature, the residual stress is released by tempering at a high temperature. Therefore, the machining shape soundness of the steel involved in the present invention is also high.

[0277] The steel involved in the present invention is a kind of low C-8Cr steel. In low C-8Cr steel, if the Si content is relatively increased and the Al content is relatively decreased, the machinability and impact value can be stably high.

[0278] In addition, in low C-8Cr steel, if the Si content is relatively increased, the corrosion resistance in a humid atmosphere can be improved.

[0279] Moreover, in low C-8Cr steel, by optimizing the composition (especially Si, Al, C, V, and Mo), the required hardness can be obtained even when tempering is performed at a high temperature.

[0280] In summary, the properties of the steel involved in the present invention, namely (1) tempering hardness, (2) machining shape soundness, (3) machinability, (4) impact value, and (5) corrosion resistance, are excellent.

[0281] Specifically, it has the following characteristics:

[0282] (1) The hardness after tempering is an appropriate value of 35 - 43 HRC;

[0283] (2) The residual stress after tempering is low;

[0284] (3) The machinability after tempering is comparable to that of SKD61;

[0285] (4) The impact value after tempering is an appropriate value of ≥80 J / cm 2 ;

[0286] (5) The corrosion resistance after tempering is as high as that of martensitic stainless steel.

[0287] Therefore, the steel involved in the present invention has the following advantages:

[0288] (A) The mirror polishing property is also excellent;

[0289] (B) The deformation during machining is small;

[0290] (C) Die machining is easy;

[0291] (D) Cracks are not easily generated during use;

[0292] (E) Rust is not easily generated during use and storage.

[0293] [Examples]

[0294] (Experiment 1: Tempering Hardness)

[0295] [1. Preparation of Specimens]

[0296] In pre-hardened steel for dies, the control of tempering hardness is important. Therefore, the tempering hardness was evaluated among five types of steels, Steel A - Steel E, shown in Table 1.

[0297] In addition, Steel A corresponds to the steel described in Patent Document 1 and has less C, Si, Cu, Mo, and V contents compared to the present invention. Also, Steel E has less C, Si, and Cu contents and more N content compared to the present invention.

[0298] [Table 1]

[0299] C Si Mn P S Cu Ni Cr Mo V Al N C+N Steel A 0.062 0.28 0.29 0.018 0.0007 0.09 0.96 8.01 0.44 0.08 0.038 0.009 0.071 Steel B 0.109 0.81 0.46 0.027 0.0009 0.46 0.87 7.92 1.04 0.23 0.058 0.012 0.121 Steel C 0.131 0.64 0.35 0.019 0.0008 0.24 0.62 7.93 0.95 0.18 0.055 0.008 0.139 Steel D 0.159 0.72 0.23 0.012 0.0007 0.35 0.53 7.96 0.86 0.15 0.052 0.006 0.165 Steel E 0.055 0.27 0.31 0.018 0.0007 0.08 0.97 8.02 0.95 0.18 0.033 0.086 0.141

[0300] [2. Test Method]

[0301] Five test pieces of 11.5 mm × 11.5 mm × 20 - 24 mm were cut out from each steel material. (1) Quenching: After holding the test pieces in a vacuum furnace at 920 °C for 2 Hr, nitrogen was introduced into the furnace, and by balancing heating and cooling, they were slowly cooled to 220 °C at a rate of 30 °C / min. Then, after cooling to below 100 °C, they were taken out of the furnace.

[0302] (2) Tempering: Each test piece was heated in a vacuum furnace to a specified temperature T (540 °C - 560 °C) for 5 Hr, held for a specified time X (2.5 - 7.0 Hr), then nitrogen was introduced into the furnace, and after cooling to below 100 °C, they were taken out of the furnace.

[0303] In addition, the specified temperature T is the temperature of the test piece, not the atmosphere temperature or the furnace wall temperature, and is maintained at T ± 2 °C.

[0304] Then, for each test piece, the Rockwell hardness (C scale) was measured at room temperature (above 15 °C and below 35 °C).

[0305] Here, the combinations of the specified temperature T (tempering temperature) and the specified time X (tempering holding time) are as follows.

[0306] 550 °C × 3.5 Hr: LMP = 16908

[0307] 540 °C × 7.0 Hr: LMP = 16947

[0308] 560 °C × 2.5 Hr: LMP = 16991

[0309] 550 °C × 5.0 Hr: LMP = 17035

[0310] 555 °C × 5.0 Hr: LMP = 17139

[0311] LMP = (tempering temperature T (°C) + 273) × (20 + log(tempering holding time X (Hr))

[0312] [3. Results]

[0313] In Figure 1 The tempering hardness is shown with respect to the value of LMP.

[0314] In the range of 16900 ≤ LMP ≤ 17150 with low residual stress, the desired range of tempering hardness is 35 - 43 HRC (within the single-dot dash line in the figure), and thus, for steel grades other than steel A, all values are within the single-dot dash line.

[0315] In addition, compared with the present invention, steel A has less amounts of C, Si, Cu, Mo, and V.

[0316] (Experiment 2: Machinability 1 (Drill Bit Machinability))

[0317] [1. Preparation of Specimens]

[0318] In prehardened steel for molds, the quality of machinability is important. Therefore, the drill bit machinability was evaluated among five types of steel, namely Steel A, Steel C, Steel F, Steel G, and Steel H shown in Table 2.

[0319] In addition, Steel H is JIS SKD61 (AISI H13). Also, Steel F is a steel obtained by increasing the Si content of Steel C, and Steel G is a steel obtained by increasing the Al content of Steel C.

[0320] [Table 2]

[0321] C Si Mn P S Cu Ni Cr Mo V Al N C+N Steel A 0.062 0.28 0.29 0.018 0.0007 0.09 0.96 8.01 0.44 0.08 0.038 0.009 0.071 Steel C 0.131 0.64 0.35 0.019 0.0008 0.24 0.62 7.93 0.95 0.18 0.055 0.008 0.139 Steel F 0.132 0.82 0.35 0.019 0.0008 0.24 0.89 7.95 0.94 0.18 0.058 0.011 0.143 Steel G 0.131 0.63 0.35 0.017 0.0008 0.26 0.62 7.93 0.95 0.18 0.119 0.007 0.138 Steel H 0.372 0.98 0.44 0.012 0.0009 0.09 0.13 5.21 1.21 0.86 0.026 0.012 0.384

[0322] [2. Test Methods]

[0323] Test pieces of 25 mm × 50 mm × 200 mm were cut out from each type of steel.

[0324] (1-1) Quenching: The test pieces of (low C-8Cr steel other than Steel H) were held at 920 °C for 2 Hr in a vacuum furnace, then nitrogen was introduced into the furnace, and by the balance of heating and cooling, they were slowly cooled to 220 °C at a rate of 15 °C / min. Then, after cooling to below 100 °C, they were taken out of the furnace.

[0325] (1-2) Quenching: The test pieces of (Steel H) were held at 1030 °C for 2 Hr in a vacuum furnace, then nitrogen was introduced into the furnace, and by the balance of heating and cooling, they were slowly cooled to 220 °C at a rate of 15 °C / min. Then, after cooling to below 100 °C, they were taken out of the furnace.

[0326] (2) Tempering: By setting each test piece to an appropriate LMP, the tempering hardness was adjusted to 36.4 - 37.9 HRC.

[0327] Then, for each test piece, the drill bit machinability was evaluated at room temperature.

[0328] In the evaluation of drill bit machinability, a high-speed steel drill bit with a diameter of 5 mm was used. This drill bit was pressed against the test piece at a specified machining speed to drill a hole with a depth of 20 mm. The drill bit was pulled out from the specimen, and a 20-mm hole was drilled again at the specified machining speed at another position. This process was repeated until the tool life at which the 20-mm depth could not be reached due to drill bit wear or breakage. Then, the cutting distance until the tool life was calculated = the number of holes drilled until the tool life × hole depth (20 mm). Next, by changing the machining speed, the cumulative cutting distance was calculated according to the same process.

[0329] [3. Results]

[0330] Figure 2 The correlation between the machining speed and the cutting distance is shown. Among the steel grades located more towards the upper right in the figure, the machinability is better (because the machining speed is high and the cumulative cutting distance is large).

[0331] From Figure 2 it can be seen that as the Si content increases, the machinability improves. Specifically, the machinability is 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: 0.98 mass%).

[0332] In addition, Steel H is SKD61, which is well-known for its good machinability and actually has the best machinability. Also, Steel F has the same machinability as Steel H, and Steel C also shows machinability close to that of Steel H.

[0333] On the other hand, from Figure 2 it can also be seen that as the Al content increases, the machinability deteriorates. Specifically, the machinability is Steel C (Si content: 0.64 mass%, Al content: 0.055 mass%) > Steel G (Si content: 0.64 mass%, Al content: 0.119 mass%). Compared to Steel C, whose machinability is close to that of Steel H, the machinability of Steel G deteriorates to a level close to that of Steel A.

[0334] From the above, it can be known that the machinability of the drill bit is improved by increasing the Si content and deteriorated by increasing the Al content.

[0335] (Experiment 3: Machinability 2 (Machinability of End Mill))

[0336] [1. Preparation of Specimens]

[0337] Similar to Experiment 2, the machinability of the end mill was evaluated for the five types of steel materials, namely Steel A, Steel C, Steel F, Steel G, and Steel H, shown in Table 2.

[0338] [2. Test Method]

[0339] The dimensions of the test pieces, the conditions of quenching and tempering were the same as in Experiment 2, so the description is omitted.

[0340] Then, for each test piece, the machinability of the end mill was evaluated at room temperature.

[0341] In the evaluation of the machinability of an end mill, XDGT1550PDER-G30VP15TF was used as the tool. The feed per tooth of the tool was set to 0.2 mm, the cut was set to 1.0 mm × 4.0 mm, and a test piece was cut by downcut milling at a machining speed of 400 m / min. Then, after cutting a specified distance, the tool wear amount was measured. The tool wear amount was measured again after cutting the specified distance under the same conditions. This process was repeated, and the change in the maximum tool wear amount with respect to the cumulative cutting distance was investigated.

[0342] [3. Results]

[0343] In Figure 3 shows the correlation between the maximum tool wear amount and the cutting distance. Among the steel grades, the one located more towards the lower right in the figure has better machinability (because the tool does not wear even when cutting a long distance).

[0344] From Figure 3 it can be seen that as the Si content increases, the machinability improves. Specifically, the machinability is 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: 0.98 mass%). In addition, Steel H is SKD61, which is well-known for its good machinability and actually has the best machinability. Also, the cutting distance at which the maximum wear increases sharply is G < A < C < F < H, and it can be said that Steel F has the second-highest machinability after Steel H. Regarding Steel C, although the cutting distance at which its maximum wear increases sharply is 60 m, which is smaller than 70 m of Steel F, the difference from Steel F is small, and the maximum wear up to 60 m is the second smallest after Steel H. Therefore, it can be said that Steel C has high machinability not inferior to Steel F.

[0345] On the other hand, from Figure 3 it can be seen that when the Al content increases, the machinability deteriorates. Specifically, the machinability is Steel C (Si content: 0.64 mass%, Al content: 0.055 mass%) > Steel G (Si content: 0.64 mass%, Al content: 0.119 mass%). Compared with Steel C whose machinability is close to that of Steel H, the machinability of Steel G deteriorates compared to Steel A. The deterioration of machinability caused by the increase in Al content is more significant than that of the drill bit's machinability.

[0346] From the above, it can be known that the machinability of the end mill is also improved by the increase in Si content and deteriorated by the increase in Al content.

[0347] (Experiment 4: Impact value)

[0348] [1. Preparation of specimens]

[0349] In pre-hardened steel for molds, the level of the impact value is important. Therefore, the impact values were evaluated among five types of steel, namely Steel A, Steel C, Steel F, Steel G, and Steel H shown in Table 2.

[0350] [2. Test Method]

[0351] Multiple test pieces of 11 mm × 11 mm × 55 mm were cut out from each type of steel.

[0352] (1-1) Quenching: After holding the test pieces (low C-8Cr steel other than Steel H) in a vacuum furnace at 920 °C for 2 Hr, nitrogen was introduced into the furnace, and by the balance of heating and cooling, it was slowly cooled to 220 °C at a rate of 20 °C / min. Then, after cooling to below 100 °C, it was taken out of the furnace.

[0353] (1-2) Quenching: After holding the test pieces (Steel H) in a vacuum furnace at 1030 °C for 2 Hr, nitrogen was introduced into the furnace, and by the balance of heating and cooling, it was slowly cooled to 220 °C at a rate of 20 °C / min. Then, after cooling to below 100 °C, it was taken out of the furnace.

[0354] (2) Tempering: By setting each test piece to an appropriate LMP, the tempering hardness was conditioned to 37.2 - 37.9 HRC.

[0355] Then, each test piece was made into 10 mm × 10 mm × 55 mm and set as a test piece for impact value evaluation. Specifically, the impact value refers to the value obtained by dividing the absorbed energy obtained from the impact test at room temperature (above 15 °C and below 35 °C) using a test piece with a height of 8 mm under the U-notch and a cross-sectional area of the test piece at the lower part of the U-notch of 0.8 cm 2 by the cross-sectional area of the test piece at the lower part of the U-notch.

[0356] Then, in each steel type, the average value of the impact values of 6 test pieces was measured and used as the impact value of each steel type.

[0357] [3. Results]

[0358] In Figure 4 the impact values of each steel type are shown. For low C-8Cr steel, the impact value becomes low when the Al content is low, but since the Al content of Steel A is 0.038 mass%, a high impact value can be ensured. Both Steel C and Steel F obtained by increasing the Si content can ensure a high impact value equivalent to that of Steel A. In addition, Steel G obtained by increasing the Al content of Steel C can also ensure a high impact value. That is, Steel A, Steel C, Steel F, and Steel G of low C-8Cr steel can stably ensure a high impact value exceeding 100 J / cm 2 . In addition, the impact value of Steel H (SKD61) is less than 50 J / cm 2 .

[0359] Based on the above, it can be confirmed that there is no significant deterioration in the impact value due to the increase in the Si content.

[0360] (Experiment 5: Corrosion resistance)

[0361] [1. Preparation of specimens]

[0362] In prehardened steel for molds, the quality of corrosion resistance is important. Therefore, the corrosion resistance was evaluated among five types of steels, namely Steel A, Steel C, Steel F, Steel G, and Steel H shown in Table 2.

[0363] [2. Test method]

[0364] Test pieces of 25.5 mm × 40.5 mm × 12.5 mm were cut out from each steel.

[0365] (1-1) Quenching: After holding the test pieces of (low C-8Cr steel other than Steel H) in a vacuum furnace at 920 °C for 2 Hr, nitrogen was introduced into the furnace, and by the balance of heating and cooling, it was slowly cooled to 220 °C at a rate of 20 °C / min. Then, after cooling to below 100 °C, it was taken out of the furnace.

[0366] (1-2) Quenching: After holding the test pieces of (Steel H) in a vacuum furnace at 1030 °C for 2 Hr, nitrogen was introduced into the furnace, and by the balance of heating and cooling, it was slowly cooled to 220 °C at a rate of 20 °C / min. Then, after cooling to below 100 °C, it was taken out of the furnace.

[0367] (2) Tempering: By setting each test piece to an appropriate LMP, the tempering hardness was adjusted to 35.5 - 37.8 HRC.

[0368] Then, each test piece was made into 25 mm × 40 mm × 12 mm, the surface was mirror-polished and made into a test piece for corrosion resistance evaluation. Then, each test piece was exposed to the atmosphere with a humidity of 95% and a temperature of 50 °C for 24 Hr, and the rust generation condition was evaluated.

[0369] [3. Results]

[0370] In Figure 5The generation status of rust in each steel grade is shown. Rust is prominent in steel H (SKD61). In addition, steel A of low C-8Cr steel has less rust than steel H. It is also known that as the Si content increases, the corrosion resistance improves. Specifically, the amount of rust generated is steel A (Si content: 0.28 mass%) > steel C (Si content: 0.64 mass%) > steel F (Si content: 0.82 mass%). Moreover, even when the Al content increases, the amount of rust generated does not increase. Specifically, the amount of rust generated is the same in steel C (Si content: 0.64 mass%, Al content: 0.055 mass%) and steel G (Si content: 0.64 mass%, Al content: 0.119 mass%).

[0371] From the above, it can be seen that the corrosion resistance is improved by increasing the Si content, and even when the Al content is increased, it will not deteriorate.

[0372] (Examples 1 to 13, Comparative Examples 1 to 6)

[0373] [1. Preparation of specimens]

[0374] Table 3 shows the chemical compositions of 19 steel grades used for evaluation. Although not recorded in the table, other elements in amounts less than the specified amount are sometimes included as impurities.

[0375] In addition, Comparative Examples 1 to 6 are also low C-8Cr steels like the steel involved in the present invention. In addition, the Si content is less in Comparative Examples 1 to 6 than in the present invention.

[0376] [Table 3]

[0377]

[0378] Comparative Example 1 is characterized in that it has the lowest Si content among the 19 steel grades.

[0379] Comparative Example 2 is characterized in that it has the highest Si content among the comparative examples, has more P and S contents than the present invention, and has less Al content than the present invention.

[0380] Comparative Example 3 is characterized in that it has less Cr content than the present invention.

[0381] Comparative Example 5 is characterized in that it has less Al content than the present invention, has more Cu content, and Cu > Ni.

[0382] Comparative Example 6 is characterized in that it is the only one among the comparative examples where the (C + N) content is within the composition range of the present invention and the Mo content is also within the composition range of the present invention.

[0383] 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 deviated from the composition range of the steel involved in the present invention.

[0384] The 19 steel grades shown in Table 3 were each cast into 50 kg ingots. Next, bars with a rectangular cross-section having a height of 40 mm and a width of 65 mm and a length of about 2000 mm were manufactured by hot working, and then cooled to near room temperature.

[0385] Next, the bars were heated to 970 °C and normalized with soaking for 1 Hr.

[0386] In addition, cracks were confirmed in the bars of Comparative Example 5. It is considered that this is because Cu≤Ni was not satisfied, so cracks caused by Cu occurred during hot working.

[0387] [2. Evaluation]

[0388] Various test pieces were made from the above bars, and four properties were evaluated: (1) tempering hardness, (2) machinability, (3) impact value, and (4) corrosion resistance.

[0389] [2.1. Tempering Hardness]

[0390] [2.1.1. Test Method]

[0391] Test pieces of 11.5 mm×11.5 mm×22 mm were cut out from the part of the above bar that avoided the decarburized layer.

[0392] The test method followed the test method of Experiment 1. However, the combinations of the specified temperature T (tempering temperature) and the specified time X (tempering holding time) are as follows.

[0393] 540 °C×4.5 Hr: LMP = 16791

[0394] 540 °C×6.0 Hr: LMP = 16893

[0395] 555 °C×3.0 Hr: LMP = 16955

[0396] 547 °C×7.0 Hr: LMP = 17093

[0397] 547 °C×8.0 Hr: LMP = 17141

[0398] 555 °C×7.0 Hr: LMP = 17260

[0399] LMP = (tempering temperature T (°C) + 273)×(20 + log(tempering holding time X (Hr))

[0400] [2.1.2. Results]

[0401] The results are shown in Table 4. Additionally, in Figure 6 the results of the examples are shown, and in Figure 7 the results of the comparative examples are shown.

[0402] The desired tempering hardness range within 16,800 ≤ LMP ≤ 17,200 with low residual stress is 35 to 43 HRC ( Figure 6 , Figure 7 within the dashed line).

[0403] Among the examples, the tempering hardness of Example 11 is the highest, and that of Example 2 is the lowest. Additionally, the tempering hardness of the other examples is between that of Example 2 and Example 11.

[0404] In Examples 2 and 11, the HRC values with respect to the values of 16,800 ≤ LMP ≤ 17,200 are all within the dashed line (refer to Figure 6 ). That is, in all the examples, the HRC values with respect to the values of 16,800 ≤ LMP ≤ 17,200 are within the dashed line. Additionally, it is assumed that the values change linearly between the respective measurement points.

[0405] [Table 4]

[0406]

[0407] In Example 11, due to the high C content and Mo content, the tempering hardness is the highest.

[0408] Additionally, in Example 2, although the C content is low, by increasing the Mo content and V content, the tempering hardness can be increased, and the HRC value with respect to the value of 16,800 ≤ LMP ≤ 17,200 is within the dashed line (refer to Figure 6 ).

[0409] Figure 7 These are the results of the tempering hardness of the comparative examples, but only in Comparative Example 6, the HRC value with respect to the value of 16,800 ≤ LMP ≤ 17,200 is within the dashed line.

[0410] It is considered that this is because, in Comparative Example 6, the (C + N) content is within the composition range of the present invention, and the Mo content is also within the composition range of the present invention.

[0411] Additionally, Comparative Examples 1 to 4 have a lower (C + N) content compared to the present invention, and Comparative Example 5 has a higher (C + N) content compared to the present invention. Therefore, the tempering hardness of Comparative Examples 1 to 4 is too small, and the tempering hardness of Comparative Example 5 is too large.

[0412] [2.2. Machinability 1 (Drill Machinability)]

[0413] [2.2.1. Test method]

[0414] A test piece of 25 mm × 50 mm × 200 mm was cut from the part of the above-mentioned bar that avoided the decarburized layer.

[0415] The test method followed the test method of Experiment 2 (low C8-Cr steel). In addition, the tempering hardness of the test piece was conditioned to 36.5 - 37.5 HRC.

[0416] The machinability was evaluated using VL 300 Here, "VL 300 " refers to the machining speed (m / min) at which the drill bit reaches the end of its life at the time when the cumulative cutting distance reaches 300 mm. Specifically, the value on the Y-axis corresponding to the X value of 300 mm in the graph with the X-axis being the cutting distance (m) and the Y-axis being the machining speed (m / min) was obtained by interpolation. VL 300 is an index of machining efficiency. The larger this value, the higher the drilling speed, so it can be judged that the steel with good machinability has excellent machining efficiency.

[0417] [2.2.2. Results]

[0418] The results are shown in Figure 8 The VL of Examples 1 - 13 300 was 29.4 m / min or more.

[0419] On the other hand, the VL of Comparative Examples 1 - 6 300 was 28.3 m / min or less.

[0420] In Example 5, since the Si content, P content, and S content were high, the VL 300 became as large as 38.7 m / min. In addition, the VL of Example 2 with relatively high Si content, P content, and S content 300 also became as large as 36.1 m / min.

[0421] On the other hand, in Example 4, since the C content was relatively high and the Si content was relatively low, the VL 300 was the lowest in the examples at 29.4 m / min.

[0422] In addition, among the comparative examples, the VL of Comparative Example 2 300 was the largest among the comparative examples at 28.3 m / min. This is because Comparative Example 2 had the largest Si content, P content, and S content among the comparative examples.

[0423] On the other hand, in Comparative Example 1, since the Si content was the lowest among the 19 steel grades, the VL 300is the lowest value among 19 steel grades, which is 21.6 m / min.

[0424] From the results of the comparative examples, the effects of the Si content, P content, and S content on machinability can also be confirmed.

[0425] [2.3. Machinability 2 (machinability of end mills)]

[0426] [2.3.1. Test method]

[0427] A test piece of 25 mm × 50 mm × 200 mm was cut from the part of the above-mentioned bar that avoided the decarburized layer.

[0428] The test method followed the test method of Experiment 3 (low C-8Cr steel). In addition, the tempering hardness of the test piece was adjusted to 36.5 - 37.5 HRC.

[0429] In the machinability of end mills, the cutting distance is the cutting distance when the tool wear amount reaches 100 μm. Specifically, the value on the X-axis corresponding to the Y value of 100 μm in the graph with the X-axis set as the cutting distance (m) and the Y-axis set as the maximum wear amount (μm) was obtained by interpolation. The larger the cutting distance, the more that can be cut with one tool, and the steel with good machinability can be judged.

[0430] [2.3.2. Results]

[0431] In Figure 9 The results are shown. The overall trend is the same as that in the case of machinability 1 (drill bit machinability). The cutting distances of Example 5 and Example 2 are large, and among the examples, the cutting distance of Example 4 is the lowest.

[0432] In addition, among the comparative examples, the cutting distance of Comparative Example 2 is large, and the cutting distance of Comparative Example 1 is the lowest.

[0433] [2.4. Impact value]

[0434] [2.4.1. Test method]

[0435] A test piece of 11 mm × 11 mm × 55 mm was cut from the part of the above-mentioned bar that avoided the decarburized layer.

[0436] The test method followed the test method of Experiment 4 (low C-8Cr steel). In addition, the tempering hardness of the test piece was adjusted to 36.5 - 37.5 HRC.

[0437] [2.4.2. Results]

[0438] In Figure 10 The results are shown. Among the examples, except for Example 5, the impact value is 145 J / cm 2Above. In addition, the impact value of Example 5 is 82 J / cm 2 . The low impact value of Example 5 is due to the large amounts of P and S.

[0439] Among the comparative examples of low C-8Cr steel, which are the same as the examples, except for Comparative Examples 2 and 5, the impact values are also equivalent to those of the examples.

[0440] The impact values of Comparative Examples 2 and 5 are less than 30 J / cm 2 because the amount of Al is less than that of the present invention.

[0441] In addition, in low C-8Cr steel, if the amount of Al is too small, the impact value will decrease significantly.

[0442] [2.5. Corrosion resistance]

[0443] [2.5.1. Test method]

[0444] Test pieces of 25 mm × 40.5 mm × 12.5 mm were cut from the part of the above-mentioned bar that avoided the decarburized layer.

[0445] The test method followed the test method of Experiment 5 (low C-8Cr steel). In addition, the tempering hardness of the test pieces was adjusted to 36.5 - 37.5 HRC.

[0446] [2.5.2. Results]

[0447] The results are shown in Table 5. The evaluation was set at four levels: S (Superior), A, B, and I (Inferior). The determination of S is a state where almost no rust is observed. The determination of A is a state where although a little rust can be observed, it can be judged that the corrosion resistance is good.

[0448] On the other hand, the determination of B is the following state: rust is conspicuous, and sufficient anti-rust measures need to be taken when used in a humid environment. The determination of I is the following state: rust is very much, and even if sufficient anti-rust measures are taken when used for molds, rust is expected to be significant.

[0449] [Table 5]

[0450]

[0451] Among the examples, Examples 4 and 5 were determined as A, and the others were determined as S.

[0452] In addition, the reason for the A determination of Example 4 is the large amount of C and the small amount of Si.

[0453] In addition, the reason for the A determination of Example 5 is the large amounts of P and S.

[0454] Even in Examples 4 and 5 composed of components that are considered unfavorable for corrosion resistance, good corrosion resistance can be achieved with the Si content within the component range of the present invention.

[0455] Therefore, it can be said that the corrosion resistance of the examples is extremely excellent.

[0456] On the other hand, in the comparative examples, Comparative Example 2 and Comparative Example 3 were judged as I, and the others were judged as B.

[0457] In addition, the reason for the I judgment of Comparative Example 2 is that the Si content is low, and the P content and S content are high.

[0458] Moreover, the reason for the I judgment of Comparative Example 3 is that the Cr content is low.

[0459] [3. Summary]

[0460] Examples 1 to 13 showed good results in tempering hardness, machinability, impact value, and corrosion resistance. On the other hand, although there are steel grades with excellent tempering hardness and impact value in the comparative examples, the machinability and corrosion resistance are insufficient. This is because the Si content for improving machinability and corrosion resistance is low.

[0461] Moreover, in Examples 1 to 13, by setting the LMP within an appropriate numerical range, the residual stress was also reduced.

[0462] From this, it can be known that Examples 1 to 13 have (1) an appropriate tempering hardness of 35 to 43 HRC, (2) low residual stress, (3) good machinability, (4) high impact value, and (5) good corrosion resistance.

[0463] [4. General Applicability]

[0464] In the verification of characteristics, the melted bars were taken as an example, but the steel materials related to the present invention can also be made into powders, blocks, wires, or plates for use.

[0465] For example, if the steel materials related to the present invention are made into powders, the powders can be applied to various layer-by-layer forming such as laminated forming (SLM method or LMD method, etc.) and plasma transferred arc welding (PPW).

[0466] In addition, if the steel materials related to the present invention are made into melted blocks, molds or components can also be manufactured from the blocks.

[0467] Moreover, if the steel materials related to the present invention are made into melted bars or wires, they can be applied to layer-by-layer forming or repair by surfacing the bars or wires using TIG or laser welding, etc.

[0468] Alternatively, a mold or a component can also be manufactured by forming the steel material according to the present invention into sheets and joining a plurality of the sheets together.

[0469] In addition, a mold or a component can also be manufactured by producing segmented molds or components made of the steel material according to the present invention and joining them together.

[0470] As described above, the steel material according to the present invention can be used to manufacture or repair molds or components using raw materials of various shapes and various methods.

[0471] The embodiments of the present invention have been described in detail above, but the present invention is not limited to any of the above embodiments, and various changes can be made without departing from the gist of the present invention.

[0472] Industrial applicability

[0473] The steel material according to the present invention can be used for molds or mold components used in injection molding or blow molding of plastics or resins, molding of rubber, molding of fiber-reinforced plastics, and the like.

[0474] In addition, 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.

[0475] In addition, it is also effective to combine the mold according to the present invention with surface modification (such as shot peening, shot blast, sand blast, nitriding, PVD, PCVD, CVD, plating, DLC coating, etc.).

[0476] In addition, a method of using the mold according to the present invention, in which concavo-convex patterns (referred to as "texturing") are imparted to the surface of the mold by corrosion with a chemical solution, machining, laser processing, etc., and the patterns are transferred to the surface of plastics or resins to impart additional value, is also possible.

[0477] Moreover, the steel material according to the present invention can also be applied to powders or sheets used in layered manufacturing. It can also be made into bars or wires and used for welding repair of molds or mold components.

Claims

1. A kind of steel, characterized in that, Comprising: 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%, The balance consists of Fe and inevitable impurities.

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

3. The steel according to claim 1, wherein The hardness is 35 HRC or more and 43 HRC or less, wherein the "hardness" refers to the Rockwell hardness (C scale) measured at 15°C to 35°C using a specimen obtained through the following (a) to (e): (a) Fabricate a square bar with a specimen shape of 11 - 12 mm × 11 - 12 mm × 20 - 25 mm; (b) Hold the square bar at a quenching temperature of 895°C to 975°C for 2 Hr; (c) Cool the square bar from the quenching temperature to 220°C at a cooling rate of 15 - 30°C / min, and then cool it from 220°C to below 100°C at an arbitrary cooling rate; (d) Temper the square bar only once under the condition that the tempering temperature T (°C) and the tempering time X (Hr) satisfy the following formulas (1) to (3); (e) Remove the decarburized part on the surface of the tempered square bar, 16800 ≤ LMP ≤ 17200 … (1) LMP = (T + 273) × (20 + log X) … (2) 510℃≤T≤590℃ … (3)。 4. A mold, characterized in that It is made of the steel according to claim 1.

Images