Hot work tool steel and material for metal injection molding machine

Abstract

The present invention provides: a hot work tool steel that exhibits excellent creep strength over a long period time; and a material for an injection molding machine using the hot work tool steel. A hot work tool steel according to the present invention has a composition comprising, in mass%, 0.08-0.13% of C, 8.5-9.8% of Cr, 0.10-0.25% of V, 0.03-0.08% of Nb, 1.0-3.5% of Co, 0.2-3.5% of W, 0.002-0.015% of B, and 0.015-0.025% of N, the remaining portion being Fe and unavoidable impurities.

Description

Hot work tool steel and materials for metal injection molding machines

[0001] The present invention relates to a hot work tool steel that can be used as a material for a metal injection molding machine, and to a material for an injection molding machine that uses the hot work tool steel.

[0002] Hot work tool steels with excellent high-temperature properties are used as materials for injection molding machines and other machines used at high temperatures. Examples of hot work tool steels include 5% Cr-based JIS-SKD61 steel and the hot work tool steel disclosed in Patent Document 1. Materials for injection molding machines and other machines are used at high temperatures under stress, so they must have excellent high-temperature creep properties. However, the former SKD61 steel cannot maintain its material properties when used at high temperatures for long periods of time, resulting in poor durability. On the other hand, the hot work tool steel disclosed in Patent Document 1 is used in screws, cylinders, nozzles, and other parts of Mg injection molding machines, which are used at high temperatures.

[0003] Japanese Patent Application Publication No. 2002-427246

[0004] However, the hot work tool steel described in Patent Document 1 is required to not break in a relatively short time. Since the required properties for each part are different, properties appropriate for each part are required. For example, the fastening part at the tip of the cylinder and the joint surfaces of the cylinder and nozzle are constantly maintained at high temperatures, and the cylinder itself needs to be prevented from warping. To meet these requirements, the material used must have long-term creep strength.

[0005] The present invention has been made in light of the above circumstances, and has as its object to provide a hot work tool steel having excellent long-term creep strength and a material for injection molding machines that uses the hot work tool steel.

[0006] That is, the hot work tool steel of one embodiment has a composition containing, by mass%, C: 0.08 to 0.13%, Cr: 8.5 to 9.8%, V: 0.10 to 0.25%, Nb: 0.03 to 0.08%, Co: 1.0 to 3.5%, W: 0.2 to 3.5%, B: 0.002 to 0.015%, N: 0.015 to 0.025%, and the balance being Fe and unavoidable impurities.

[0007] In one embodiment, a material for a metal injection molding machine has the form of the hot work tool steel.

[0008] According to the present invention, excellent creep strength can be obtained using inexpensive elements, and there is an effect that creep rupture over a long period of time can be prevented and excellent durability can be obtained when used at high temperatures.

[0009] 1 is a cross-sectional view showing an outline of a metal injection molding machine used in one embodiment of the present invention. 2 is a graph showing the creep rupture life of each test material obtained from a creep rupture test of the examples. 3 is a representative photograph showing the metal structure of a part of the test material (test material No. 3) of the examples.

[0010] [Hot Work Tool Steel] The hot work tool steel of this embodiment has a composition, in mass%, of C: 0.08 to 0.13%, Cr: 8.5 to 9.8%, V: 0.10 to 0.25%, Nb: 0.03 to 0.08%, Co: 1.0 to 3.5%, W: 0.2 to 3.5%, B: 0.002 to 0.015%, N: 0.015 to 0.025%, and the balance being Fe and unavoidable impurities.

[0011] (Configuration defined in this embodiment) (Composition) C: 0.08 to 0.13% C is an essential element for accelerating martensitic transformation and for forming carbides by bonding with Fe, Cr, Mo, V, Nb, W, etc. in the alloy to increase high-temperature strength. Furthermore, if the amount of carbides is small, (Fe, Cr) ２ The aggregation and coarsening of the Laves phase, which is a (Mo, W) type intermetallic compound, is promoted, resulting in a decrease in high-temperature creep strength.

[0012] From this perspective, a minimum C content of 0.08% is required. On the other hand, if the C content exceeds 0.13%, coarsening of carbides tends to occur and high-temperature creep strength decreases, so the C content is limited to 0.08 to 0.13%.

[0013] Cr: 8.5 to 9.8% Excessive Cr significantly reduces long-term high-temperature creep strength at around 650°C, so the upper limit of the 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 further improves high-temperature creep strength by dissolving in the alloy and precipitating as precipitated carbides and fine Laves phases, so a minimum of 8.5% is required.

[0014] From the above viewpoints, the Cr content is limited to 8.5 to 9.8%. For the same reasons, the lower limit is preferably set to 8.6% and the upper limit to 9.2%, and the lower limit is more preferably set to 8.8%.

[0015] Mo: 1.0% or less Mo is an element that effectively suppresses the aggregation and coarsening of carbides, dissolves in the alloy to solid-solution strengthen the matrix, and precipitates in the matrix as a finely dispersed Laves phase, improving high-temperature strength and high-temperature creep strength. Therefore, Mo may be added as desired. However, excessive Mo content tends to produce delta ferrite and further promotes the aggregation and coarsening of the Laves phase, so the upper limit is preferably 1.0%. In other words, it is preferable for the above composition to contain Mo: 1.0% or less.

[0016] In order to obtain a sufficient effect, the lower limit is preferably set to 0.1%, and the upper limit is preferably set to 0.7% for the same reasons as above. Even when Mo is not intentionally added, it may be contained in an amount of 0.02% or less as an unavoidable impurity.

[0017] V: 0.10 to 0.25% V is effective in improving high-temperature creep strength by forming fine carbides and carbonitrides, and a minimum of 0.10% is required. On the other hand, if it exceeds 0.25%, carbon is excessively fixed, increasing the amount of carbide precipitation and reducing high-temperature strength, so it is limited to 0.10 to 0.25%. For the same reason, it is desirable to set the lower limit at 0.18% and the upper limit at 0.22%.

[0018] Nb: 0.03 to 0.08% Nb is an element that forms fine carbides and carbonitrides, improving high-temperature creep strength and promoting grain refinement to improve low-temperature toughness, and a minimum of 0.03% is required. However, if the content exceeds 0.08%, coarse carbides and carbonitrides precipitate, reducing ductility and toughness, so the content is limited to 0.03 to 0.08%. For the same reason, it is desirable to set the lower limit at 0.05% and the upper limit at 0.07%.

[0019] W: 0.2 to 3.5% W is an element that effectively suppresses the aggregation and coarsening of carbides, dissolves in the alloy to solid-solution strengthen the matrix, and precipitates in the matrix as a finely dispersed Laves phase, improving high-temperature strength and high-temperature creep strength. A minimum of 0.2% is required. On the other hand, if the content exceeds 3.5%, delta ferrite is likely to form, further promoting the aggregation and coarsening of the Laves phase. Excessive addition leads to increased costs, so the content is limited to 0.2 to 3.5%. For the same reason, the upper limit is preferably set at 3.0%.

[0020] Co: 1.0 to 3.5% Co suppresses the formation of delta ferrite and improves high-temperature strength and high-temperature creep strength. To be effective in preventing the formation of delta ferrite, a content of 1.0% or more is necessary. However, if Co is added in large amounts, ductility and high-temperature creep strength decrease and costs increase, so the content is limited to 1.0 to 3.5%. For the same reason, it is desirable to set the lower limit at 1.1% and the upper limit at 3.1%.

[0021] B: 0.002 to 0.015%. B has the effect of suppressing the aggregation and coarsening of precipitated carbides, precipitated carbonitrides, and precipitated Laves phases in prior austenite grain boundaries, martensite packets, martensite blocks, and martensite laths at high temperatures for long periods of time. It is also an effective element for improving high-temperature creep strength when added in combination with alloying elements such as W and Nb. A minimum of 0.002% B is required. However, if the B content exceeds 0.015%, it combines with nitrogen to form a precipitated BN phase, which reduces high-temperature creep ductility and toughness. Therefore, the B content is limited to 0.002 to 0.015%. For the same reasons, the lower limit is preferably 0.005% and the upper limit is preferably 0.010%.

[0022] N: 0.015 to 0.025% N combines with Nb, V, etc. to form nitrides, which improve the high-temperature strength and high-temperature creep strength. However, if the N 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 content exceeds 0.025%, B combines with boron to form a precipitated BN phase, which reduces the effective action of B and reduces high-temperature creep ductility and toughness, so the content is limited to 0.015 to 0.025%. For the same reason, it is desirable to set the lower limit at 0.010% and the upper limit at 0.020%.

[0024] Ni: 0.2% or less Since Ni is an impurity element, it is desirable to reduce its content as much as possible. Therefore, it is set to 0.2% or less. For the same reason, it is desirable to set it to 0.15% or less. In other words, it is preferable to restrict Ni to 0.2% or less in the above composition.

[0025] Si: 0.50% or less Si is an impurity that is unavoidably contained in some cases. However, if Si is contained in excess, it segregates within the steel ingot and increases temper embrittlement susceptibility. Therefore, the upper limit of Si content is preferably set to 0.50%.

[0026] Mn: 0.1% or less Mn is an impurity that may be unavoidably contained, but it may be contained because it is an inexpensive austenite stabilizing element and contributes to improving toughness. However, excessive addition of Mn reduces high-temperature creep strength and increases temper embrittlement susceptibility. Therefore, it is desirable to limit the upper limit of the Mn content to 0.1%.

[0027] Mo equivalent ([Mo%] + [W%] / 2): 1.30 to 2.75 The effects of Mo and W are similar and can complement each other. Therefore, in order to ensure the necessary creep strength, it is desirable to limit the Mo equivalent, defined as [Mo%] + [W%] / 2, to 1.30 or more. Furthermore, because excessive addition leads to increased costs, the upper limit is set to 2.75. That is, in the above composition, it is preferable that the Mo equivalent ([Mo%] + [W%] / 2) be 1.30 to 2.75.

[0028] Cr equivalent: 9.0 or less The tendency for delta ferrite to form increases with an increase in Cr equivalent, as shown in the following formula, resulting in a decrease in toughness and high-temperature strength. Therefore, it is desirable to limit the Cr equivalent to 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%] For the same reason, it is even more desirable to limit it to 8.0 or less.

[0029] Co + W: 5.0% or less Both Co and W are effective in improving high-temperature strength and creep strength, but they increase costs. In the present invention, even if the Co and W contents are reduced, properties equivalent to or better than those of conventional steels can be obtained, so it is desirable to set the amount of Co + W to 5.0% or less.

[0030] Creep strength: Time to rupture at 650°C and 157 MPa is 1,100 hours or more. For example, when a metal injection molding machine is in operation, the cylinder continues to heat up, with the tip of the cylinder in particular reaching a maximum of 630°C. Therefore, if the creep strength of the material is low, creep deformation will occur at the joint surface between the cylinder and the nozzle. In addition, to prevent the entire cylinder from recoiling during injection, the cylinder is pressed against the mold after each injection, so if the creep strength of the material is low, the cylinder will bend.

[0031] To prevent such creep deformation, the creep strength of the cylinder material and the nozzle material is preferably such that the time to rupture at 650°C and 157 MPa is 1100 hours or more. Furthermore, it is even more preferable that the time to rupture is 1200 hours or more. The creep strength can be tested and measured in accordance with JIS Z 2271 (2010) (Methods for creep and creep rupture testing of metallic materials).

[0032] (Manufacturing Method of the Embodiment) The hot work tool steel of the embodiment can be produced by melting in a conventional manner by adjusting each component so as to have a specified composition. The melting method is not particularly limited in this embodiment. The hot work tool steel obtained as described above has the composition shown above, and has excellent tensile strength, long-term high-temperature creep strength, and corrosion resistance, as well as good thermal conductivity.

[0033] The hot work tool steel is appropriately processed and provided as a high-temperature component. The manufacturing process from the hot work tool steel to the high-temperature component is not particularly limited in the present invention, and mechanical processing such as rolling, forging, bending, and grinding can be appropriately performed. Suitable applications for the high-temperature component are those in which it is used at high temperatures, for example, 300°C or higher, and in which the above-mentioned properties are required. Typical applications include structural components for casting machines, structural components for injection molding machines, and components for hot forging machines. For injection molding machines, the steel can be suitably used as a cylinder material or nozzle material.

[0034] [Material for Metal Injection Molding Machine] The material for a metal injection molding machine of this embodiment is made of the above-mentioned hot work tool steel. That is, the material for a metal injection molding machine of this embodiment is made of hot work tool steel having a composition, by mass%, of 0.08 to 0.13% C, 8.5 to 9.8% Cr, 0.10 to 0.25% V, 0.03 to 0.08% Nb, 1.0 to 3.5% Co, 0.2 to 3.5% W, 0.002 to 0.015% B, 0.015 to 0.025% N, and the balance being Fe and unavoidable impurities.

[0035] The details of the hot work tool steel can be directly cited from the description in the "Hot Work Tool Steel" above, and the preferred embodiments are also the same. That is, the hot work tool steel constituting the material for a metal injection molding machine of this embodiment preferably contains 1.0% or less Mo in its composition. Furthermore, it is desirable to restrict Ni to 0.2% or less in the above composition. Furthermore, it is desirable that the Mo equivalent ([Mo%] + [W%] / 2) in the above composition be 1.30 to 2.75.

[0036] 1 is a cross-sectional view showing a part of a metal injection molding machine 1 for magnesium used under high temperature conditions, in which the hot work tool steel of the present invention is used as high-temperature members for a cylinder 2, a screw 3, a cylinder head 4, and a nozzle 5. A heater 6 for heating the cylinder 2, the cylinder head 4, and the nozzle 5 is disposed on the outer periphery of the tip side of the cylinder 2, the cylinder head 4, and the nozzle 5.

[0037] Nozzle 5 of metal injection molding machine 1 is clamped to mold 10. When metal injection molding machine 1 is in operation, magnesium chips introduced from hopper 7 are heated, and cylinder 2, screw 3, cylinder head 4, and nozzle 5 reach high temperatures. When injection molding a low-melting-point metal, the high-temperature low-melting-point metal moves inside while in contact with the inside, and is introduced into mold 10 from nozzle 5.

[0038] In the above operation, the cylinder 2, cylinder head 4, and nozzle 5, which are made of hot work tool steel, have excellent high-temperature properties and resistance to erosion, and therefore exhibit excellent durability even under the above-mentioned high temperatures. They also have excellent thermal conductivity, which minimizes the generation of thermal stress due to heater heating, ensuring high reliability as equipment. Specifically, deformation of the cylinder / nozzle seal surface and deflection of the cylinder itself are effectively prevented.

[0039] Examples of the present invention are described in detail below. Test materials having the compositions shown in Table 1 (the balance being Fe and unavoidable impurities) were melted into 50 kg steel ingots in a vacuum induction melting furnace. The table also shows the Mo equivalent, Cr equivalent, total Co content and W content (Co + W) of the test materials, and creep rupture test results at 650°C and a stress of 157 MPa. The units of values ​​in the columns for each element and Co + W in Table 1 are all "% (mass%)." The units of values ​​in the column for creep rupture test results are "hours." Each of the melted steel ingots was subjected to a diffusion homogenization treatment and then hot forged into a plate material measuring 30 mm thick and 120 mm wide.

[0040] Test pieces taken from this plate material were subjected to a heat treatment at 1100°C for 5 hours as a quenching treatment, followed by air cooling, and then to a heat treatment at 680°C for 20 hours as a tempering treatment, followed by furnace cooling.

[0041]

[0042] (Creep Test) A creep rupture test was carried out at 650°C and a stress of 157 MPa using test materials No. 1 to 12 shown in Table 1. The creep rupture test was carried out in accordance with JIS Z 2271 (2010) (Methods for creep and creep rupture testing of metallic materials).

[0043] To evaluate the high-temperature creep strength of each test material, quenched and tempered test pieces were subjected to creep rupture tests at a temperature of 650°C and a stress of 157 MPa. Figure 2 shows the creep rupture life of each test material, and the inventive material has superior creep rupture strength to the comparative material.

[0044] Furthermore, for test material No. 3, after cutting and polishing, the specimen was etched with 15% hydrochloric acid and 1% picric acid, and then photographs of the metal structure were observed. Figure 3 shows the photographs taken with an optical microscope. The Cr equivalent was 8.16, which is less than 9.0, and no δ-ferrite was observed.

[0045] According to the present invention, it is possible to provide a hot work tool steel having excellent long-term creep strength and a material for injection molding machines using the hot work tool steel.

[0046] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-140827) filed on August 31, 2023, the contents of which are incorporated herein by reference.

[0047] REFERENCE SIGNS LIST 1 injection molding machine 2 cylinder 3 screw 4 cylinder head 5 nozzle 6 heater 7 hopper 10 mold

Claims

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

2. A hot work tool steel according to claim 1, further comprising Mo: 1.0% or less in said composition.

3. A hot work tool steel according to claim 1, wherein the Ni content in said composition is restricted to 0.2% or less.

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

75.

5. A hot work tool steel according to any one of claims 1 to 3, wherein the composition has a Cr equivalent of 9.0 or less, as expressed by the following formula: 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. A hot work tool steel according to any one of claims 1 to 3, wherein in the composition, Co + W: 5.0% or less.

7. A hot work tool steel according to any one of claims 1 to 3, which has a rupture time of 1,100 hours or more at 650°C and 157 MPa.

8. A material for metal injection molding machines made of hot work tool steel having a composition, by mass%, of 0.08-0.13% C, 8.5-9.8% Cr, 0.10-0.25% V, 0.03-0.08% Nb, 1.0-3.5% Co, 0.2-3.5% W, 0.002-0.015% B, 0.015-0.025% N, with the balance being Fe and unavoidable impurities.

9. A material for metal injection molding machines according to claim 8, wherein the composition contains Mo: 1.0% or less.

10. A material for metal injection molding machines as recited in claim 8, wherein the Ni content in said composition is restricted to 0.2% or less.

11. A material for metal injection molding machines according to claim 8 or 9, wherein in said composition, the Mo equivalent ([Mo %] + [W %] / 2) is 1.30 to 2.75.

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