Hot work tool steel and material for metal injection molding machine
A specially formulated hot work tool steel with specific elemental compositions enhances creep strength and durability, addressing the durability issues of existing steels in injection molding machines by ensuring long-term performance at high temperatures.
Patent Information
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- THE JAPAN STEEL WORKS LTD
- Filing Date
- 2024-08-28
- Publication Date
- 2026-06-03
AI Technical Summary
Existing hot work tool steels used in injection molding machines suffer from poor durability and lack long-term creep strength at high temperatures, leading to rapid damage and performance degradation.
A hot work tool steel composition comprising 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%, and N: 0.015% to 0.025%, with the balance being Fe and unavoidable impurities, to enhance creep strength and durability.
The new steel composition provides excellent long-term creep strength and durability, preventing creep rupture for extended periods at high temperatures, thereby improving the reliability of injection molding machine components.
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Abstract
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 a material for an injection molding machine using the hot work tool steel.BACKGROUND ART
[0002] As a material for an injection molding machine or the like used in a high temperature state, a hot work tool steel having excellent properties at high temperature is used. As the hot work tool steel, a 5% Cr-based JIS-SKD61 steel or a hot work tool steel disclosed in Patent Literature 1 is adopted.
[0003] Since the material for an injection molding machine or the like is used in a state where a stress is applied at a high temperature, the material needs to have excellent creep properties at high temperature. However, the former SKD61 steel cannot maintain material properties when used at a high temperature for a long time, and is poor in durability.
[0004] On the other hand, the hot work tool steel disclosed in Patent Literature 1 is applied to a screw, a cylinder, a nozzle, and the like of a Mg injection molding machine used at a high temperature.CITATION LISTPATENT LITERATURE
[0005] Patent Literature 1: JP2002-427246ASUMMARY OF INVENTIONTECHNICAL PROBLEM
[0006] However, the hot work tool steel disclosed in Patent Literature 1 is required not to be damaged in a relatively short time.
[0007] Since properties required for respective components are different, properties suitable for respective components are required. For example, a fastening portion on a cylinder tip side and a joint surface of the cylinder or the nozzle are always held in a high temperature state, and the cylinder itself needs to be prevented from deflection. In order to meet these requirements, the material to be used needs to have long-term creep strength.
[0008] The present invention has been made in view of the above circumstances, and an object thereof is to provide a hot work tool steel having excellent long-term creep strength and a material for an injection molding machine using the hot work tool steel.SOLUTION TO PROBLEM
[0009] That is, a hot work tool steel according to one embodiment has a composition including, in 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%, and N: 0.015% to 0.025%, with the balance being Fe and unavoidable impurities.
[0010] A material for a metal injection molding machine according to one embodiment includes the hot work tool steel according to one embodiment.ADVANTAGEOUS EFFECTS OF INVENTION
[0011] According to the present invention, it is possible to obtain excellent properties in terms of creep strength with inexpensive elements, and there is an effect that excellent durability is obtained by preventing creep rupture for a long term in use at a high temperature.BRIEF DESCRIPTION OF DRAWINGS
[0012] FIG. 1 is a cross-sectional view schematically illustrating a metal injection molding machine for use in one embodiment of the present invention. FIG. 2 is a graph showing a creep rupture life of each test material obtained from a creep rupture test in Examples. FIG. 3 is a photograph showing a metal structure in a part of a sample material (sample material No. 3) in Examples. DESCRIPTION OF EMBODIMENTS[Hot Work Tool Steel]
[0013] A hot work tool steel according to the present embodiment has a composition including, in 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%, and N: 0.015% to 0.025%, with the balance being Fe and unavoidable impurities.(Configuration Defined in Present Embodiment)(Component Composition)C: 0.08% to 0.13%
[0014] C is an essential element for promoting martensitic transformation and forming a carbide by combining with Fe, Cr, Mo, V, Nb, W, and the like in the alloy to increase strength at high temperature, and when the amount of carbide is small, aggregation and coarsening of a Laves phase, which is a (Fe, Cr) 2 (Mo, W) type intermetallic compound, are promoted, and creep strength at high temperature decreases.
[0015] From such a viewpoint, at least 0.08% of C is required to be included. On the other hand, when more than 0.13% of C is included, coarsening of a carbide is likely to occur and the creep strength at high temperature decreases, so that the content thereof is limited to 0.08% to 0.13%.Cr: 8.5% to 9.8%
[0016] Excessive Cr greatly decreases long-term creep strength at high temperature at around 650°C, so that the upper limit of the content of Cr is set to 9.8%. On the other hand, Cr is an element that enhances oxidation resistance and corrosion resistance at high temperature, and that is precipitated as a precipitated carbide or a fine Laves phase as well as be dissolved in the alloy to improve the creep strength at high temperature, and at least 8.5% or more of Cr is required.
[0017] From the above viewpoint, the content of Cr is limited to 8.5% to 9.8%. Note that, for the same reason, the lower limit is desirably set to 8.6% and the upper limit is desirably set to 9.2%, and the lower limit is further desirably set to 8.8%.Mo: 1.0% or less
[0018] Mo is an element that prevents aggregation and coarsening of the carbide, that is dissolved in the alloy to strengthen the matrix by the solid solution, and that is finely dispersed and precipitated in the matrix as a Laves phase to effectively contribute to improvement in the strength at high temperature and the creep strength at high temperature, and is included as desired. On the other hand, when it is included in excess, delta ferrite is likely to be generated, and the aggregation and coarsening of the Laves phase is promoted, so that the upper limit is desirably set to 1.0%.
[0019] That is, the composition preferably further includes Mo: 1.0% or less.
[0020] Note that, in order to obtain a sufficient effect, the lower limit thereof is desirably set to 0.1%, and the upper limit thereof is desirably set to 0.7% for the same reason as described above.
[0021] When Mo is not actively included, 0.02% or less of Mo may be included as an unavoidable impurity.V: 0.10% to 0.25%
[0022] V is effective for improvement in the creep strength at high temperature by forming a fine carbide or carbonitride, and at least 0.10% of V is required. On the other hand, when it is more than 0.25%, carbon is excessively fixed, and a precipitation amount of the carbide increases to decrease the strength at high temperature, so that it is limited to 0.10% to 0.25%. Note that, for the same reason, the lower limit thereof is desirably set to 0.18% and the upper limit thereof is desirably set to 0.22%.Nb: 0.03% to 0.08%
[0023] Nb is an element that forms a fine carbide and carbonitride to improve the creep strength at high temperature, and that also promotes refinement of crystal grains to improve toughness at low temperature, and thus at least 0.03% of Nb is required. However, when more than 0.08% of Nb is included, a coarse carbide and carbonitride are precipitated to decrease ductility and toughness, so that the content thereof is limited to 0.03% to 0.08%. Note that, for the same reason, the lower limit thereof is desirably set to 0.05% and the upper limit thereof is desirably set to 0.07%.W: 0.2% to 3.5%
[0024] W is an element that prevents the aggregation and coarsening of the carbide, that is dissolved in the alloy to strengthen the matrix by the solid solution, and that is finely dispersed and precipitated in the matrix as a Laves phase to effectively contributes to improvement in the strength at high temperature and the creep strength at high temperature, and thus at least 0.2% of W is required. On the other hand, when more than 3.5% of W is included, delta ferrite is likely to be generated, the aggregation and coarsening of the Laves phase is promoted, and excessive addition thereof leads to an increase in cost, so that the content thereof is limited to 0.2% to 3.5%. Note that, for the same reason, the upper limit thereof is desirably set to 3.0%.Co: 1.0% to 3.5%
[0025] Co prevents generation of delta ferrite and improves the strength at high temperature and the creep strength at high temperature. In order to effectively prevent the generation of delta ferrite, it is necessary to include 1.0% or more of Co, but on the other hand, when a large amount of Co is included, the ductility and the creep strength at high temperature decrease, and the cost further increases, so the content thereof is limited to 1.0% to 3.5%. Note that, for the same reason, the lower limit thereof is desirably set to 1.1% and the upper limit thereof is desirably set to 3.1%.B: 0.002% to 0.015%
[0026] B has an effect of preventing the aggregation and coarsening of a precipitated carbide, a precipitated carbonitride, and a precipitated Laves phase in prior austenite grain boundaries, martensite packets, martensite blocks, and martensite laths over a long time at a high temperature, and is an element effective for improving the creep strength at high temperature by being added in combination with an alloy element such as W or Nb, and thus at least 0.002% of B is required. On the other hand, when more than 0.015% of B is included, it combines with nitrogen to form a precipitated BN phase and creep ductility at high temperature and the toughness decrease, so that the content thereof is limited to 0.002% to 0.015%. Note that, for the same reason, the lower limit thereof is desirably set to 0.005% and the upper limit thereof is desirably set to 0.010%.N: 0.015% to 0.025%
[0027] N combines with Nb, V, or the like to form a nitride, and improves the strength at high temperature and the creep strength at high temperature, but when the content thereof is less than 0.015%, sufficient strength at high temperature and creep strength at high temperature cannot be obtained.
[0028] On the other hand, when more than 0.025% of N is included, it combines with boron to form a precipitated BN phase, the effective action of B is reduced to decrease the creep ductility at high temperature and the toughness, so that the content thereof is limited to 0.015% to 0.025%. Note that, for the same reason, the lower limit thereof is desirably set to 0.010% and the upper limit thereof is desirably set to 0.020%.Ni: 0.2% or less
[0029] Since Ni is an impurity element, it is desirable to reduce Ni as much as possible. Therefore, a content thereof is set to 0.2% or less. Note that, for the same reason, the content thereof is desirably set to 0.15% or less.
[0030] That is, the above composition is preferably restricted such that the content of Ni is 0.2% or less.Si: 0.50% or less
[0031] Si is an impurity that may be unavoidably included, and when Si is excessively included, segregation inside a steel ingot and temper embrittlement susceptibility are increased, so that the upper limit thereof is desirably set to 0.50%.Mn: 0.1% or less
[0032] Mn is an impurity that may be unavoidably included, and may be included because it is an inexpensive austenite stabilizing element and contributes to improvement in the toughness. However, when it is added excessively, the creep strength at high temperature decreases and the temper embrittlement susceptibility is increased. Therefore, the upper limit of the content of Mn is desirably limited to 0.1 %.Mo Equivalent ([Mo%]+[W%] / 2): 1.30 to 2.75
[0033] The effects of Mo and W are the same and can be mutually complemented. Therefore, in order to ensure the required creep strength, it is desirable to limit the Mo equivalent defined by [Mo%]+[W%] / 2 to 1.30 or more. In addition, since excessive addition thereof leads to an increase in cost, the upper limit thereof is limited to 2.75.
[0034] That is, in the above composition, the Mo equivalent ([Mo%]+[W%] / 2) is preferably 1.30 to 2.75.Cr equivalent: 9.0 or less
[0035] Since a generation tendency of delta ferrite increases as a Cr equivalent shown by the following equation increases, which leads to decrease the toughness and the strength at high temperature. Therefore, the value of the Cr equivalent is desirably limited to 9.0 or less.
[0036] Note that, for the same reason, it is desirably limited to 8.0 or less.Co+W: 5.0% or less
[0037] Both Co and W are effective for improving the strength and the creep strength at high temperature, but increase the cost. In the present invention, even when the contents of Co and W are reduced, properties equal to or more than those of the steel in the related art can be obtained, so that the amount of Co+W is desirably set to 5.0% or less.Creep Strength: rupture time at 650°C and 157 MPa is 1100 hours or longer
[0038] For example, during an operation of a metal injection molding machine, a cylinder is continuously heated, and in particular, a cylinder tip is heated at 630°C at the maximum. Therefore, when the creep strength of the material is low, creep deformation occurs on a fastening surface between the cylinder and a nozzle. In addition, since the cylinder prevents the entire cylinder from moving backward due to a reaction during injection, the cylinder is pressed against a mold for each injection, and thus the cylinder is bent when the creep strength of the material is low.
[0039] In order to prevent the creep deformation, it is desirable that a rupture time at 650°C and 157 MPa is 1100 hours or longer as the creep strength of a cylinder material and a nozzle material. Further, the rupture time is more desirably 1200 hours or longer.
[0040] Note that, the creep strength can be tested and measured in accordance with JIS Z 2271 (2010) (Metallic materials-Uniaxial creep testing in tension-Method of test).(Production Method according to an Embodiment)
[0041] The hot work tool steel according to the present embodiment can be melted by a known method by adjusting respective components to have a specified composition. In the present embodiment, the melting method is not particularly limited.
[0042] The hot work tool steel obtained as described above has the composition described above, is excellent in tensile strength, long-term creep strength at high temperature, and erosion resistance, and has better thermal conductivity.
[0043] The hot work tool steel is appropriately processed and provided as a member for high temperature use. Note that, in the present invention, production steps from the hot work tool steel to the member for high temperature use are not particularly limited, and machining such as rolling, forging, bending, and grinding can be appropriately performed. Suitable applications of the member for high temperature use are, for example, applications in which the member for high temperature use is used at a high temperature state of 300°C or higher and the above properties are required, and representative examples of the application include a structural member for casting machines, a structural member for injection molding machines, and a member for hot forging machines. The present invention can be suitably applied to a cylinder material and a nozzle material for injection molding machines.[Material for Metal Injection Molding Machine]
[0044] A material for a metal injection molding machine according to the present embodiment includes the above hot work tool steel. That is, the material for a metal injection molding machine according to the present embodiment includes a hot work tool steel having a composition that includes, in 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%, and N: 0.015% to 0.025%, with the balance being Fe and unavoidable impurities.
[0045] Regarding the details of the hot work tool steel, the description in the above [Hot Work Tool Steel] can be incorporated as it is, and the preferred aspect is also the same.
[0046] That is, the hot work tool steel constituting the material for a metal injection molding machine according to the present embodiment desirably includes Mo: 1.0% or less in the composition.
[0047] In addition, the above composition is desirably restricted such that the content of Ni is 0.2% or less.
[0048] In addition, in the above composition, the Mo equivalent ([Mo%]+[W%] / 2) is desirably 1.30 to 2.75.(Structure of Device Using Embodiment)
[0049] FIG. 1 is a cross-sectional view illustrating a part of a metal injection molding machine 1 for magnesium used in a high temperature state, and a cylinder 2, a screw 3, a cylinder head 4 and a nozzle 5, as the member for high temperature use, are made of the hot work tool steel according to the present invention. In addition, a heater 6 that heats the cylinder 2, the cylinder head 4, and the nozzle 5 is disposed on an outer peripheral portion on a tip side of the cylinder 2, the cylinder head 4, and the nozzle 5.
[0050] The nozzle 5 of the metal injection molding machine 1 is clamped to a mold 10.
[0051] When the metal injection molding machine 1 is operated, a magnesium chip introduced from a hopper 7 is heated, the cylinder 2, the screw 3, the cylinder head 4, and the nozzle 5 are in a high temperature state, and when a low melting point metal is injectionmolded, the low melting point metal at a high temperature moves inside while being in contact therewith, and is introduced into the mold 10 through the nozzle 5.
[0052] In the above operation, the cylinder 2, the cylinder head 4, and the nozzle 5, which are made of the hot work tool steel, have excellent properties at high temperature and erosion resistance, and exhibit excellent durability even in the above high temperature state. In addition, the thermal conductivity is also excellent, the generation of a thermal stress due to heating by the heater is small, and high reliability as a device can be obtained.
[0053] Specifically, deformation of a sealing surface between the cylinder and nozzle and deflection of the cylinder itself are effectively prevented.EXAMPLE 1
[0054] Examples of the present invention will be described in detail below.
[0055] Sample 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. Note that, in the table, the Mo equivalent, the Cr equivalent, the total content of Co and W (Co+W), and the creep rupture test results under the conditions of 650°C, and a stress of 157 MPa of the sample material are also shown. The unit of each element shown in Table 1 and the numerical value shown in the column of Co+W is "% (mass%)". In addition, the unit of numerical values described in the column of creep rupture test result is "hour". Each of the melted steel ingots was subjected to a diffusion and homogenization treatment and then hot forged into a plate material having a thickness of 30 mm and a width of 120 mm.
[0056] A test specimen taken from this plate material was subjected to a heat treatment at 1100°C for 5 hours followed by air-cooling as a quenching treatment, and then subjected to a heat treatment at 680°C for 20 hours followed by furnace cooling as a tempering treatment. [Table 1]Sample material No.CSiMnNiCrMoVNbCoExample10.120.090.090.209.000.400.210.062.0020.120.090.080.199.000.600.210.061.6030.120.090.080.209.000.600.210.061.2040.120.090.080.199.00-0.210.061.2050.110.010.010.019.070.400.210.061.5260.100.010.010.019.070.400.210.062.2770.120.030.010.029.110.410.210.083.0180.100.030.010.208.600.110.210.073.00Comparative material90.200.110.090.079.451.510.20.0057.49100.190.250.340.119.291.540.02-9.83110.130.040.080.199.950.660.180.043.07120.130.030.060.189.960.660.180.045.03 [Table 1] (continued) Sample material No.WBNFeMo equivalentCr equivalentCo+WCreep rupture test result (650°C, 157 MPa)Example11.800.0090.016Bal.1.306.193.80131821.500.0090.016Bal.1.357.403.10134831.500.0090.016Bal.1.358.162.70174042.700.0090.016Bal.1.357.603.90150051.930.0080.019Bal.1.378.183.45178361.920.0080.019Bal.1.367.064.19181271.870.0070.016Bal.1.355.054.88139483.500.0090.02Bal.1.865.716.501964Comparative material92.960.020.011Bal.2.99-0.9610.45962105.68-0.022Bal.4.38-3.3515.51302111.76-0.032Bal.1.544.424.831013121.72-0.035Bal.1.520.386.751405 (Creep Test)
[0057] A creep rupture test was performed under conditions of 650°C and a stress of 157 MPa using sample material Nos. 1 to 12 shown in Table 1. The creep rupture test was performed in accordance with JIS Z 2271 (2010) (Metallic materials-Uniaxial creep testing in tension-Method of test).
[0058] In order to evaluate the creep strength of each test material at a high temperature, the test specimen after quenching and tempering was subjected to a creep rupture test under conditions of a temperature of 650°C and a stress of 157 MPa. FIG. 2 shows a creep rupture life of each test material, and the inventive material is more excellent than the comparative material in terms of creep rupture strength.
[0059] Further, the sample material No. 3 was cut and polished, and then corroded with 15% hydrochloric acid + 1% picric acid, followed by observing a metal structure photograph thereof. FIG. 3 shows a photographic image obtained by photographing the sample material No. 3 using an optical microscope. The Cr equivalent was 8.16, which was 9.0 or less, and no delta ferrite was observed.
[0060] 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 an injection molding machine using the hot work tool steel.
[0061] Although the present invention has been described in detail 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.
[0062] The present application is based on Japanese Patent Application No. 2023-140827 filed on August 31, 2023, and the contents thereof are incorporated herein as reference.REFERENCE SIGNS LIST
[0063] 1injection molding machine 2cylinder 3screw 4cylinder head 5nozzle 6heater 7hopper 10mold
Claims
1. A hot work tool steel having a composition comprising, in 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%; and N: 0.015% to 0.025%, with the balance being Fe and unavoidable impurities.
2. The hot work tool steel according to claim 1, wherein the composition further comprises: Mo: 1.0% or less.
3. The hot work tool steel according to claim 1, wherein the composition satisfies Ni: 0.2% or less.
4. The hot work tool steel according to any one of claims 1 to 3, wherein the composition has a Mo equivalent ([Mo%] + [W%] / 2) of 1.30 to 2.75.
5. The hot work tool steel according to any one of claims 1 to 3, wherein the composition has a value of a Cr equivalent represented by the following equation of 9.0 or less, 6. The hot work tool steel according to any one of claims 1 to 3, wherein the composition comprises: Co+W: 5.0% or less.
7. The hot work tool steel according to any one of claims 1 to 3, having a rupture time at 650°C and 157 MPa of 1100 hours or longer.
8. A material for a metal injection molding machine comprising: a hot work tool steel having a composition that comprises, in 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%; and N: 0.015% to 0.025%, with the balance being Fe and unavoidable impurities.
9. The material for a metal injection molding machine according to claim 8, wherein the composition further comprises: Mo: 1.0% or less.
10. The material for a metal injection molding machine according to claim 8, wherein the composition satisfies Ni: 0.2% or less.
11. The material for a metal injection molding machine according to claim 8 or 9, wherein the composition has a Mo equivalent ([Mo%]+[W%] / 2) of 1.30 to 2.75.