Hot work tool steel and material for metal injection molding machine

Abstract

To provide: 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.SOLUTION: 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.SELECTED DRAWING: Figure 2

Description

[Technical field]

[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. [Background technology]

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

[0003] [Patent Document 1] JP 2002-427246 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the hot work tool steel disclosed in Patent Document 1 is required not to be damaged in a relatively short time. Since the required properties for each part are different, the appropriate properties are required for each part. 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, so the cylinder itself needs to be prevented from warping. To meet these requirements, the materials used need to have long-term creep strength.

[0005] The present invention has been made in light of the above circumstances, and has an object to provide a hot work tool steel having excellent long-term creep strength and a material for injection molding machines using said hot work tool steel. [Means for solving the problem]

[0006] That is, the hot work tool steel of one embodiment has a composition containing, 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%, 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. Effect of the Invention

[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. [Brief description of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing an outline of a metal injection molding machine used in one embodiment of the present invention. [Diagram 2] 1 is a graph showing the creep rupture life of each test material obtained from the creep rupture test of the examples. [Diagram 3] 1 is a representative photograph showing the metal structure of some test materials in the examples. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] (Configuration defined in this embodiment) (composition) C: 0.08-0.13% C is an essential element for promoting martensitic transformation and for forming carbides by bonding with Fe, Cr, Mo, V, Nb, W, and other elements in the alloy to increase high-temperature strength. Furthermore, if there is a small amount of carbide, the aggregation and coarsening of the Laves phase, an intermetallic compound of the (Fe,Cr)2(Mo,W) type, is promoted, resulting in a decrease in high-temperature creep strength.

[0011] From this viewpoint, a minimum C content of 0.08% is required. On the other hand, if the C content exceeds 0.13%, the carbides tend to become coarse and the high-temperature creep strength decreases, so the C content is limited to 0.08 to 0.13%.

[0012] Cr: 8.5~9.8% Excessive Cr significantly reduces long-term high-temperature creep strength at around 650°C, so the upper limit for 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 carbides and fine Laves phases, so a minimum of 8.5% Cr is required.

[0013] 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%.

[0014] Mo: 1.0% or less Mo is an element that effectively suppresses the coagulation and coarsening of carbides, dissolves in the alloy to strengthen the matrix, and precipitates finely dispersed Laves phase in the matrix to improve high-temperature strength and high-temperature creep strength, and is added as desired. However, excessive addition of Mo makes it easier to form delta ferrite and promotes the coagulation and coarsening of the Laves phase, so the upper limit is set at 1.0%.

[0015] 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 if Mo is not intentionally contained, it may be contained in an amount of 0.02% or less as an unavoidable impurity.

[0016] V: 0.10~0.25% 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%, it fixes carbon excessively, increasing the amount of carbide precipitates and reducing high-temperature strength, so it is limited to 0.10-0.25%. For the same reason, it is desirable to set the lower limit at 0.18% and the upper limit at 0.22%.

[0017] Nb: 0.03 to 0.08% Nb is an element that forms fine carbides and carbonitrides, improves high-temperature creep strength, promotes grain refinement, and improves low-temperature toughness, and a minimum of 0.03% is required. However, if it is contained in excess of 0.08%, coarse carbides and carbonitrides precipitate, reducing ductility and toughness, so the content is limited to 0.03-0.08%. For the same reason, it is preferable to set the lower limit at 0.05% and the upper limit at 0.07%.

[0018] W: 0.2-3.5% W is an element that effectively suppresses the coagulation and coarsening of carbides, dissolves in the alloy to strengthen the matrix, and precipitates finely dispersed in the matrix as a Laves phase to improve high-temperature strength and high-temperature creep strength, and a minimum of 0.2% is required. On the other hand, if it is contained in excess of 3.5%, delta ferrite is easily formed, and the coagulation and coarsening of the Laves phase is promoted, and excessive addition leads to increased costs, so it is limited to 0.2 to 3.5%. For the same reason, it is preferable to set the upper limit at 3.0%.

[0019] Cobalt: 1.0-3.5% Co inhibits 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, but a large content reduces ductility and high-temperature creep strength and increases costs, so the content is limited to 1.0 to 3.5%. For the same reason, it is preferable to set the lower limit at 1.1% and the upper limit at 3.1%.

[0020] 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 a long period of time, and is also an effective element for improving high-temperature creep strength by adding it in combination with alloy elements such as W and Nb, and a minimum of 0.002% is required. On the other hand, if it is contained in excess of 0.015%, it combines with nitrogen to form a precipitated BN phase, which reduces high-temperature creep ductility and toughness, so its content is limited to 0.002 to 0.015%. For the same reason, it is preferable to set the lower limit at 0.005% and the upper limit at 0.010%.

[0021] N: 0.015~0.025% N combines with Nb, V, etc. to form nitrides, improving high-temperature strength and high-temperature creep strength, but if the N content is less than 0.015%, sufficient high-temperature strength and high-temperature creep strength cannot be obtained.

[0022] On the other hand, if it is contained in excess of 0.025%, it combines with boron to form a precipitated BN phase, The content is limited to 0.015 to 0.025% because the effective action of B is reduced and high temperature creep ductility and toughness are reduced. For the same reason, it is preferable to set the lower limit at 0.010% and the upper limit at 0.020%.

[0023] Ni: 0.2% or less Ni is an impurity element, so it is desirable to reduce it 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.

[0024] Si:0.50% or less Silicon is an impurity that is unavoidably contained. However, excessive silicon content increases segregation within the steel ingot and temper embrittlement susceptibility, so the upper limit is preferably set at 0.50%.

[0025] Mn: 0.1% or less Mn is an unavoidable impurity, but it is acceptable to add it 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 Mn content to 0.1%.

[0026] Mo equivalent (Mo+W / 2): 1.30-2.75 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. In addition, since excessive addition leads to increased costs, the upper limit is set at 2.75%.

[0027] Cr equivalent: 9.0 or less The tendency for δ-ferrite to form increases with an increase in the Cr equivalent, as shown in the following formula, which leads to a decrease in toughness and high-temperature strength. Therefore, it is desirable to limit this 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 more desirable to limit it to 8.0 or less.

[0028] Co+W:5.0 or less Both Co and W are effective in improving high-temperature strength and creep strength, but they are costly. In the present invention, since properties equivalent to or better than those of conventional steels can be obtained even if the Co and W contents are reduced, it is preferable to set the amount of Co+W to 5.0 or less.

[0029] Creep strength: 1100 hours or more to rupture at 650°C and 157MPa For example, when a metal injection molding machine is in operation, the cylinder continues to heat up, especially the tip of the cylinder, which is heated to a maximum of 630°C. Therefore, if the creep strength of the material is low, creep deformation occurs at the joint surface between the cylinder and the nozzle. Also, in order to prevent the entire cylinder from retracting due to the reaction force at the time of 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.

[0030] To prevent such creep deformation, it is desirable for the creep strength of the cylinder material and the nozzle material to be such that the rupture time at 650°C and 157 MPa is 1100 hours or more, and it is even more desirable for the rupture time to be 1200 hours or more.

[0031] (Manufacturing method of the embodiment) The hot working tool steel of this embodiment can be produced by melting in a conventional manner by adjusting each component so as to have a specified composition. In this embodiment, the melting method is not particularly limited. The hot working tool steel obtained as described above has the composition shown above, and has excellent tensile strength, long-term high-temperature creep strength, and melting resistance, as well as good thermal conductivity.

[0032] The hot work tool steel is appropriately processed and provided as a high temperature member. In the present invention, the manufacturing process from the hot work tool steel to the high temperature member is not particularly limited, and mechanical processing such as rolling, forging, bending, grinding, etc. can be appropriately performed. The high temperature member is preferably used in a high temperature state of, for example, 300°C or higher, and is required to have the above-mentioned characteristics. Representative examples of the high temperature member include structural members for casting machines, structural members for injection molding machines, and members for hot forging machines. For injection molding machines, the member can be preferably used as a cylinder material or a nozzle material.

[0033] (Structure of device using embodiment) 1 is a cross-sectional view showing a part of a metal injection molding machine 1 for magnesium used under high temperature conditions, and 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.

[0034] The nozzle 5 of the metal injection molding machine 1 is clamped to a mold 10 . When the metal injection molding machine 1 is operating, the magnesium chips introduced from the hopper 7 are heated, and the cylinder 2, screw 3, cylinder head 4 and nozzle 5 become in a high-temperature state. When a low-melting point metal is injection molded, the high-temperature low-melting point metal moves inside while contacting the inside, and is introduced into the mold 10 from the nozzle 5.

[0035] In the above operation, the cylinder 2, nozzle head 4, and nozzle 5 made of hot work tool steel have excellent high temperature characteristics and resistance to melting damage, and therefore exhibit excellent durability even in the above high temperature conditions. They also have excellent thermal conductivity, and the generation of thermal stress due to heating by the heater is small, making them highly reliable as equipment. Specifically, deformation of the cylinder / nozzle sealing surface and bending of the cylinder itself are effectively prevented. EXAMPLES

[0036] The embodiments of the present invention will be described in detail below. Test materials having the composition 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. Each of the steel ingots thus melted was subjected to a diffusion homogenization treatment and then hot forged into plates with a thickness of 30 mm and a width of 120 mm.

[0037] Test pieces taken from this plate material were subjected to a heat treatment at 1100°C for 5 hours as a quenching treatment, and then air-cooled, and were subjected to a heat treatment at 680°C for 20 hours as a tempering treatment, and then furnace-cooled.

[0038] [Table 1]

[0039] (Creep test) A creep rupture test was carried out under conditions of 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) (Method of creep and creep rupture test for metallic materials).

[0040] In order to evaluate the high temperature creep strength of each test material, a creep rupture test was carried out on the hardened and tempered test pieces at a temperature of 650°C and a stress of 157MPa. Figure 2 shows the relative creep rupture life of each test material, with the inventive material having superior creep rupture strength to the comparative material.

[0041] Furthermore, for test material No. 3, after cutting and polishing, it was etched with 15% hydrochloric acid + 1% picric acid, and then a metal structure photograph was observed. The Cr equivalent was 8.16 or less, and no δ ferrite was observed. [Explanation of symbols]

[0042] 1 injection molding machine 2 Cylinder 3 Screw 4. Cylinder head 5 Nozzles

Claims

1. A hot work tool steel having a composition, in 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, and the balance being Fe and unavoidable impurities.

2. 2. The hot work tool steel according to claim 1, further comprising Mo: 1.0% or less.

3. 2. The hot work tool steel according to claim 1, wherein the content of Ni among the inevitable impurities in said composition is restricted 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 (Mo+W / 2) is 1.30 to 2.

75.

5. 4. The hot work tool steel according to claim 1, wherein in the composition, a Cr equivalent value represented 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: 5.0 or less.

7. 4. A hot work tool steel according to claim 1, which has a rupture time of 1100 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, in 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, and the balance being Fe and unavoidable impurities.

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

10. 9. The material for metal injection molding machines according to claim 8, wherein the inevitable impurities in the composition are regulated to Ni: 0.2% or less.

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

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