Hot work tool steel and raw material for metal injection molding machine

Abstract

Provided are: a hot work tool steel having excellent creep strength over a long period of time; and a starting material for an injection molding machine using the hot work tool steel. This hot work tool steel has a composition comprising, in mass%, 0.08 to 0.13% of C, 8.5 to 9.8% of Cr, 0.10 to 0.25% of V, 0.03 to 0.08% of Nb, 1.0 to 3.5% of Co, 0.2 to 3.5% of W, 0.002 to 0.015% of B, and 0.015 to 0.025% of N, with the balance being Fe and unavoidable impurities.

Description

Technical Field

[0001] This invention relates to hot work tool steel that can be used as raw material for metal injection molding machines, and raw material for injection molding machines using the hot work tool steel. Background Technology

[0002] In the raw materials used for injection molding machines and the like, which operate at high temperatures, hot work tool steel with excellent high-temperature properties is used. As the hot work tool steel, 5% Cr-based JIS-SKD61 steel or the hot work tool steel shown in Patent Document 1 is adopted. In the raw materials used in injection molding machines, which are subjected to stress at high temperatures, excellent high-temperature creep characteristics are required. However, in SKD61 steel, the material properties cannot be maintained and its durability is poor if used at high temperatures for extended periods. On the other hand, the hot work tool steel shown in Patent Document 1 is suitable for screws, cylinders, nozzles, etc. of Mg injection molding machines used at high temperatures. Existing technical documents Patent documents

[0003] Patent Document 1: Japanese Patent Application Publication No. 2002-427246 Summary of the Invention

[0004] However, in the hot work tool steel shown in Patent Document 1, it is imperative to avoid breakage in a relatively short period of time. Because the required characteristics of each component differ, it is necessary to seek characteristics suitable for each component. For example, the fastening part at the front end of the cylinder block and the mating surface between the cylinder block and the nozzle must always be kept at a high temperature, and it is necessary to prevent the cylinder block itself from deflecting. To meet these requirements, the materials used need to have long-term creep strength.

[0005] This invention was made against the background of the above-mentioned reasons, and its purpose is to provide a hot work tool steel with excellent long-term creep strength and a raw material for injection molding machines using the hot work tool steel.

[0006] That is, the hot work tool steel of one embodiment has the following composition: by mass%, it contains C: 0.08~0.13%, Cr: 8.5~9.8%, V: 0.10~0.25%, Nb: 0.03~0.08%, Co: 1.0~3.5%, W: 0.2~3.5%, B: 0.002~0.015%, N: 0.015~0.025%, with the remainder consisting of Fe and unavoidable impurities.

[0007] One embodiment of the raw material for a metal injection molding machine includes the form of the aforementioned hot work tool steel. Invention Effects

[0008] According to the present invention, the following effects are achieved: excellent creep strength can be obtained with inexpensive elements, and excellent durability is obtained by preventing creep fracture over long periods of time during use at high temperatures. Attached Figure Description

[0009] Figure 1 This is a cross-sectional view showing a general outline of a metal injection molding machine used in one embodiment of the present invention. Figure 2 It is a graph showing the creep rupture life of each test material obtained from the creep rupture test of the embodiments. Figure 3 This is a representative photograph showing the metal structure in a sample (sample No. 3) of the embodiment. Detailed Implementation

[0010] [Hot work tool steel] The hot work tool steel of this embodiment has the following composition by mass%, C: 0.08~0.13%, Cr: 8.5~9.8%, V: 0.10~0.25%, Nb: 0.03~0.08%, Co: 1.0~3.5%, W: 0.2~3.5%, B: 0.002~0.015%, N: 0.015~0.025%, with the remainder consisting of Fe and unavoidable impurities.

[0011] (The components specified in this embodiment) (Composition) C: 0.08~0.13% C is an indispensable element for promoting martensitic phase transformation and combining with Fe, Cr, Mo, V, Nb, W and other elements in the alloy to form carbides, thereby improving high-temperature strength. Moreover, if there are few carbides, it will promote the aggregation and coarsening of the Laves phase, which is an intermetallic compound of type (Fe,Cr)2(Mo,W), leading to a decrease in high-temperature creep strength.

[0012] Based on this viewpoint, it is necessary to contain at least 0.08% C. On the other hand, if the C content exceeds 0.13%, it is easy to cause the carbides to coarsen and reduce the high-temperature creep strength. Therefore, its content is limited to 0.08~0.13%.

[0013] Cr: 8.5~9.8% Since excessive Cr significantly reduces the long-term high-temperature creep strength near 650°C, the upper limit of Cr content is set at 9.8%. On the other hand, Cr is an element that improves oxidation resistance and high-temperature corrosion resistance, and also improves high-temperature creep strength by precipitating carbides and fine Laves phases while dissolved in the alloy, so a minimum content of 8.5% is required.

[0014] Based on the above points, the Cr content is limited to 8.5% to 9.8%. Furthermore, for the same reasons, it is desirable to set the lower limit at 8.6% and the upper limit at 9.2%, with a further desire to set the lower limit at 8.8%.

[0015] Mo: 1.0% or less Mo is an element that effectively inhibits the aggregation and coarsening of carbides, strengthens the matrix through solid solution in the alloy, and improves high-temperature strength and high-temperature creep strength by finely dispersing the Laves phase in the matrix. Therefore, Mo is included as desired. On the other hand, if Mo is present in excess, it is easy to form δ-ferrite, which further promotes the aggregation and coarsening of the Laves phase. Therefore, it is desirable to set the upper limit to 1.0%. That is, in the above composition, it is preferable to contain 1.0% or less of Mo.

[0016] Furthermore, in order to achieve full effect, it is desirable to set the lower limit to 0.1%, and for the same reasons mentioned above, it is desirable to set the upper limit to 0.7%. Even when not actively containing Mo, it may contain less than 0.02% Mo as an unavoidable impurity.

[0017] V: 0.10~0.25% V is effective in improving high-temperature creep strength by forming fine carbides and carbonitrides, and should be set to a minimum of 0.10%. On the other hand, if it exceeds 0.25%, it will over-fix carbon, increase the amount of carbide precipitation and reduce high-temperature strength, so it is limited to 0.10~0.25%. In addition, for the same reason, it is desirable to set the lower limit to 0.18% and the upper limit to 0.22%.

[0018] Nb: 0.03~0.08% Nitrogen (Nb) is an element that forms fine carbides and carbonitrides to improve high-temperature creep strength and promotes grain refinement to improve low-temperature toughness; therefore, its content needs to be set to a minimum of 0.03%. However, if the Nb content exceeds 0.08%, coarse carbides and carbonitrides will precipitate, reducing ductility and toughness; therefore, it is limited to 0.03~0.08%. Furthermore, for the same reason, it is desirable to set the lower limit to 0.05% and the upper limit to 0.07%.

[0019] W: 0.2~3.5% W is an element that effectively inhibits the aggregation and coarsening of carbides, strengthens the matrix through solid solution in the alloy, and improves high-temperature strength and high-temperature creep strength by finely dispersing the Laves phase in the matrix. Therefore, a minimum content of 0.2% is required. On the other hand, if the W content exceeds 3.5%, δ-ferrite is easily formed, further promoting the aggregation and coarsening of the Laves phase. Excessive addition leads to increased costs; therefore, the content is limited to 0.2% to 3.5%. Furthermore, for the same reason, it is desirable to set an upper limit of 3.0%.

[0020] Co: 1.0~3.5% Co inhibits the formation of δ-ferrite and improves high-temperature strength and high-temperature creep strength. To effectively prevent the formation of δ-ferrite, a content of more than 1.0% Co is required. On the other hand, if the content of Co is too high, the ductility and high-temperature creep strength will decrease, and the cost will increase. Therefore, the content is limited to 1.0% to 3.5%. Furthermore, for the same reason, it is desirable to set the lower limit to 1.1% and the upper limit to 3.1%.

[0021] B: 0.002~0.015% Bo (B) is an element that effectively suppresses the aggregation and coarsening of precipitated carbides, carbonitrides, and Laves phases within the original austenite grain boundaries, martensite packets, martensite blocks, and martensite laths at high temperatures and for extended periods. Furthermore, it is effective in improving high-temperature creep strength through composite addition with alloying elements such as W and Nb, and therefore requires a minimum content of 0.002%. On the other hand, if the B content exceeds 0.015%, it will combine with nitrogen to form precipitated BN phases, reducing high-temperature creep ductility and toughness. Therefore, its content is limited to 0.002~0.015%. Moreover, for the same reason, it is desirable to set the lower limit at 0.005% and the upper limit at 0.010%.

[0022] N: 0.015~0.025% N combines with Nb, V, etc. to form nitrides, which improve high-temperature strength and high-temperature creep strength. However, when its content is less than 0.015%, sufficient high-temperature strength and high-temperature creep strength cannot be obtained.

[0023] On the other hand, if the nitrogen content exceeds 0.025%, it will combine with boron to form a precipitated BN phase, which reduces the effective effect of boron and decreases high-temperature creep ductility and toughness. Therefore, its content is limited to 0.015~0.025%. Furthermore, for the same reason, it is desirable to set the lower limit to 0.010% and the upper limit to 0.020%.

[0024] Ni: below 0.2% Since Ni is an impurity element, it is desirable to minimize it as much as possible. Therefore, it is set to be below 0.2%. Furthermore, for the same reason, it is desirable to set it to be below 0.15%. That is, in the above composition, it is preferred to limit Ni to 0.2% or less.

[0025] Si: below 0.50% Si is an impurity that is sometimes unavoidable, but if there is an excessive amount of Si, it will increase the segregation and temper embrittlement sensitivity inside the steel block. Therefore, it is desirable to set its upper limit at 0.50%.

[0026] Mn: below 0.1% Mn is sometimes unavoidable as an impurity, but it is an inexpensive austenite stabilizing element and contributes to improved toughness, so its presence is permissible. However, excessive addition can reduce high-temperature creep strength and increase susceptibility to temper embrittlement. Therefore, it is desirable to limit the upper limit of Mn content to 0.1%.

[0027] Mo equivalent ([Mo%]+[W%] / 2): 1.30~2.75 Mo and W have the same effect and can complement each other. Therefore, to ensure the required creep strength, it is desirable to limit the Mo equivalent, specified as [Mo%] + [W%] / 2, to 1.30 or higher. Furthermore, excessive addition would increase costs; therefore, the upper limit is limited to 2.75. That is, in the above composition, the preferred Mo equivalent ([Mo%] + [W%] / 2) is 1.30 to 2.75.

[0028] Cr equivalent: below 9.0 The tendency for δ-ferrite formation increases with increasing Cr equivalent, as shown in the following formula, thus leading to a decrease in toughness and high-temperature strength. Therefore, it is desirable to limit the value of this Cr equivalent to 9.0 or below. Cr equivalent=[Cr%]+6[Si%]+4[Mo%]+1.5[W%]+11[V%]+5[Nb%]-40[C%]-2[Mn%]-4[Ni%]-30[N%]-2[Co%] Furthermore, for the same reason, a further expectation is set below 8.0.

[0029] Co+W: Below 5.0% Both Co and W are effective in improving high-temperature strength and creep strength, but they increase costs. In this invention, even by suppressing the content of Co and W, properties equivalent to or better than conventional steels are obtained; therefore, it is desirable to set the amount of Co+W to 5.0% or less.

[0030] Creep strength: The fracture time at 650℃ and 157MPa is over 1100 hours. For example, during the operation of a metal injection molding machine, the cylinder is continuously heated, especially the front end, which can reach a maximum temperature of 630°C. Therefore, if the creep strength of the raw material is low, creep deformation will occur at the mating surfaces of the cylinder and the nozzle. Furthermore, to prevent the cylinder from retracting due to the reaction force during injection, the cylinder is pressed against the mold with each injection; therefore, if the creep strength of the raw material is low, it can cause the cylinder to deflect.

[0031] To prevent these creep deformations, the creep strength of the cylinder block and nozzle materials is expected to have a fracture time of 1100 hours or more at 650°C and 157 MPa. Furthermore, it is even more desirable for the fracture time to be 1200 hours or more. In addition, creep strength can be tested and determined in accordance with JIS Z 2271 (2010) (Test Methods for Creep and Creep Fracture of Metallic Materials).

[0032] (Manufacturing method of the embodiment) The hot work tool steel of this embodiment can have its components adjusted to achieve the specified composition and can be smelted using common methods. As for this embodiment, the smelting method is not particularly limited. The hot work tool steel obtained above has the composition shown above, excellent tensile strength, long-term high-temperature creep strength, and resistance to melting loss, and also has good thermal conductivity.

[0033] Hot work tool steel is appropriately processed and provided as a high-temperature component. Furthermore, as with this invention, the manufacturing process from hot work tool steel to the high-temperature component is not particularly limited, and machining processes such as rolling, forging, bending, and grinding can be appropriately performed. Suitable applications for this high-temperature component include, for example, applications requiring the aforementioned properties at high temperatures above 300°C; representative applications include structural components for casting machines, structural components for injection molding machines, and components for hot forging machines. For injection molding machines, it can be appropriately used for cylinder block materials and nozzle materials.

[0034] [Raw materials for metal injection molding machines] The raw material for the metal injection molding machine in this embodiment includes the aforementioned hot work tool steel. Specifically, the raw material for the metal injection molding machine in this embodiment includes hot work tool steel with the following composition (by mass%): C: 0.08~0.13%, Cr: 8.5~9.8%, V: 0.10~0.25%, Nb: 0.03~0.08%, Co: 1.0~3.5%, W: 0.2~3.5%, B: 0.002~0.015%, N: 0.015~0.025%, with the remainder consisting of Fe and unavoidable impurities.

[0035] For details regarding hot work tool steels, the descriptions in the above-mentioned section on "Hot Work Tool Steels" can be directly cited, and the preferred method is the same. That is, the hot work tool steel that constitutes the raw material for the metal injection molding machine in this embodiment is expected to contain 1.0% or less of Mo in its composition. Furthermore, in the above composition, it is desirable to limit Ni to 0.2% or less. In addition, in the above composition, the desired Mo equivalent ([Mo%] + [W%] / 2) is 1.30 to 2.75.

[0036] (Structure of the device used in the embodiment) Figure 1 This is a cross-sectional view of a portion of a magnesium injection molding machine 1 used at high temperatures. The cylinder 2, screw 3, cylinder head 4, and nozzle 5 are made of the hot work tool steel of the present invention as high-temperature components. In addition, a heater 6 is provided on the outer periphery of the front end side of the cylinder 2, cylinder head 4, and nozzle 5 to heat the cylinder 2, cylinder head 4, and nozzle 5.

[0037] The nozzle 5 of the metal injection molding machine 1 is closed to the mold 10. When the metal injection molding machine 1 is working, the magnesium particles introduced from the hopper 7 are heated, and the cylinder 2, screw 3, cylinder head 4 and nozzle 5 become high temperature. In the case of injection molding of low melting point metal, the high temperature low melting point metal is in contact with and moves inside, and is introduced into the mold 10 from the nozzle 5.

[0038] In the aforementioned operation, the cylinder body 2, cylinder head 4, and nozzle 5, made of hot work tool steel, exhibit excellent high-temperature characteristics and resistance to melting, demonstrating excellent durability even at the aforementioned high temperatures. Furthermore, their excellent thermal conductivity reduces the generation of thermal stress caused by the heater, resulting in high reliability for the machine. Specifically, it can effectively prevent deformation of the sealing surface of the cylinder / nozzle and deflection of the cylinder itself. Example 1

[0039] The embodiments of the present invention are described in detail below. Samples with the composition shown in Table 1 (the remainder being Fe and unavoidable impurities) were melted into 50 kg steel blocks using a vacuum induction melting furnace. The table also shows the Mo equivalent, Cr equivalent, total Co and W content (Co+W), and creep rupture test results under conditions of 650°C and 157 MPa stress for the above samples. The values ​​for each element and the Co+W column in Table 1 are in "% (mass%)". The creep rupture test results are in "hours". After undergoing diffusion homogenization treatment, the melted steel blocks were hot-forged into plates with a thickness of 30 mm and a width of 120 mm.

[0040] For the test pieces collected from the plate, as a quenching treatment, they were subjected to heat treatment at 1100℃ for 5 hours and then air-cooled; as a tempering treatment, they were subjected to heat treatment at 680℃ for 20 hours and then furnace-cooled.

[0041] Table 1

[0042] (Creep test) Creep fracture tests were conducted using specimens No. 1 to 12 shown in Table 1 at 650°C and a stress of 157 MPa. The creep fracture tests were performed in accordance with JIS Z 2271 (2010) (Metallic materials, creep and creep fracture test methods).

[0043] To evaluate the creep strength of the test materials at high temperatures, a creep fracture test was conducted on the quenched and tempered test pieces under the conditions of 650℃ and 157MPa stress. Figure 2 The figure shows the creep fracture life of each test material, and the invented material has superior creep fracture strength compared with the comparative material.

[0044] Furthermore, regarding sample No. 3, after cutting and grinding, the metal structure was observed after etching with 15% hydrochloric acid + 1% picric acid. Figure 3 The image shown is a photograph taken using an optical microscope. The Cr equivalent is 8.16, and below 9.0, no δ-ferrite was observed.

[0045] According to the present invention, a hot work tool steel with excellent long-term creep strength and a raw material for injection molding machines using the hot work tool steel can be provided.

[0046] The present invention has been described in detail with reference to specific embodiments, but various changes and modifications can be made without departing from the spirit and scope of the invention, which will be apparent to those skilled in the art. This application is based on Japanese Patent Application No. 2023-140827, filed on August 31, 2023, the contents of which are incorporated herein by reference. Explanation of reference numerals in the attached figures 1 Injection Molding Machine 2 cylinders 3 screws 4. Cylinder head 5 nozzles 6 heaters 7. Hopper 10. Molds.

Claims

1. A hot work tool steel having the following composition by mass%, C: 0.08~0.13%, Cr: 8.5~9.8%, V: 0.10~0.25%, Nb: 0.03~0.08%, Co: 1.0~3.5%, W: 0.2~3.5%, B: 0.002~0.015%, N: 0.015~0.025%, with the remainder consisting of Fe and unavoidable impurities.

2. The hot work tool steel according to claim 1, wherein, The composition contains less than 1.0% Mo.

3. The hot work tool steel according to claim 1, wherein, In the composition, Ni is limited to 0.2% or less.

4. The hot work tool steel according to any one of claims 1 to 3, wherein, In the composition, the Mo equivalent is 1.30 to 2.75, wherein the Mo equivalent is [Mo%] + [W%] / 2.

5. The hot work tool steel according to any one of claims 1 to 3, wherein, In the composition, the Cr equivalent value shown by the following formula is 9.0 or less. Cr equivalent=[Cr%]+6[Si%]+4[Mo%]+1.5[W%]+11[V%]+5[Nb%]-40[C%]-2[Mn%]-4[Ni%]-30[N%]-2[Co%].

6. The hot work tool steel according to any one of claims 1 to 3, wherein, In the composition, Co+W is less than 5.0%.

7. The hot work tool steel according to any one of claims 1 to 3, wherein, The fracture time at 650℃ and 157MPa is more than 1100 hours.

8. A raw material for a metal injection molding machine, comprising hot work tool steel having the following composition by mass%: C: 0.08~0.13%, Cr: 8.5~9.8%, V: 0.10~0.25%, Nb: 0.03~0.08%, Co: 1.0~3.5%, W: 0.2~3.5%, B: 0.002~0.015%, N: 0.015~0.025%, with the remainder consisting of Fe and unavoidable impurities.

9. The raw material for a metal injection molding machine according to claim 8, wherein, The composition contains less than 1.0% Mo.

10. The raw material for a metal injection molding machine according to claim 8, wherein, In the composition, Ni is limited to 0.2% or less.

11. The raw material for a metal injection molding machine according to claim 8 or 9, wherein, In the composition, the Mo equivalent is 1.30 to 2.75, wherein the Mo equivalent is [Mo%] + [W%] / 2.

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