Manufacturing method of nodular graphite cast iron product

Abstract

To produce a nodular graphite cast iron product with even and high surface hardness.SOLUTION: A manufacturing method of a nodular graphite cast iron product comprises: a casting step of casting a nodular graphite cast iron article; a soaking and holding step of soaking and holding the nodular graphite cast iron article to diffuse carbon derived from graphite into the base metal texture and austenitizing the base metal texture; a hardening step of performing hardening to the soaked and held nodular graphite cast iron article; a tempering step of performing tempering to the nodular graphite cast iron article after the hardening step; and a high frequency induction hardening step of performing high frequency induction hardening to the nodular graphite cast iron article after the tempering step. The tempering step is conducted by soaking and holding the nodular graphite cast iron article at a tempering temperature between 550 and 600°C, or soaking and holding the nodular graphite cast iron article in the condition that the surface hardness after the tempering step of a high frequency induction hardening object portion of the nodular graphite cast iron article becomes 33HS to 56HS (Shore hardness).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a spheroidal graphite cast iron product, and more particularly to a technique for manufacturing a spheroidal graphite cast iron product having a uniform and high surface hardness by heat treatment including induction hardening. [Background technology]

[0002] Spheroidal graphite cast iron can be hardened by quenching, so it is often used for parts that require high hardness and high wear resistance. However, when spheroidal graphite cast iron is induction hardened, problems arise such as uneven surface hardness and cracks occurring during induction hardening. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Nakata et al. "Continuous Cooling Transformation Behavior of Spheroidal Graphite Cast Iron During Heat Treatment" Casting 64 (1992) 8 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention provides a technique for producing spheroidal graphite cast iron products having uniform and high surface hardness. [Means for solving the problem]

[0005] According to one embodiment of the present invention, A method for manufacturing a spheroidal graphite cast iron product, comprising: a casting process for casting spheroidal graphite iron products; a soaking step of soaking the spheroidal graphite cast iron product to diffuse carbon derived from graphite into a matrix structure and convert the matrix structure to austenite; a quenching step of quenching the spheroidal graphite cast iron product that has been soaked; a tempering step of tempering the spheroidal graphite cast iron product after the quenching step; an induction hardening step of performing induction hardening on the spheroidal graphite cast iron product after the tempering step; Equipped with The tempering process is carried out by soaking the spheroidal graphite cast iron product at a tempering temperature between 550 and 600°C, or by soaking the spheroidal graphite cast iron product under conditions such that the surface hardness of the induction hardened portion of the spheroidal graphite cast iron product after the tempering process is 33HS to 56HS (Shore hardness).

[0006] The spheroidal graphite cast iron products manufactured by the above manufacturing method can be suitably used for, for example, large machine tool parts. The spheroidal graphite cast iron products can also be suitably used for components that require wear resistance, such as industrial machinery, agricultural machinery, and excavators. [Effects of the Invention]

[0007] According to the above embodiment, a spheroidal graphite cast iron product having a uniform and high surface hardness can be manufactured. [Brief explanation of the drawings]

[0008] [Figure 1] This is a copy of an optical microscope photograph of an as-cast spheroidal graphite cast iron product (etched with nital). [Figure 2] This is a copy of an optical microscope photograph of a spheroidal graphite cast iron product after hardening (with nital etching). [Figure 3] This is a copy of an optical microscope photograph of a spheroidal graphite cast iron product after tempering at 500°C (with nital etching). [Figure 4] This is a copy of an optical microscope photograph of a spheroidal graphite cast iron product after tempering at 580°C (with nital etching). [Figure 5] This is a copy of an optical microscope photograph of a spheroidal graphite cast iron product after tempering at 640°C (with nital etching). [Figure 6]This is a copy of an optical microscope photograph of a spheroidal graphite cast iron product after tempering at 640°C, at a higher magnification than Figure 5 (with nital etching). [Figure 7] This is a copy of an optical microscope photograph (no etching) of a spheroidal graphite cast iron product after tempering at 640°C, at a higher magnification than in Figure 5. [Figure 8] 10 is a graph showing the results of an experiment investigating the relationship between hardness after a tempering process and hardness after an induction hardening process. DETAILED DESCRIPTION OF THE INVENTION

[0009] A preferred embodiment of the present invention (hereinafter also referred to as "this embodiment") will be described below. A spheroidal graphite cast iron product (this term means a final product) according to this embodiment can be manufactured by subjecting a spheroidal graphite cast iron product (this term means an intermediate product) to a heat treatment (heat treatment process) consisting of a series of steps.

[0010] The manufacturing process of the spheroidal graphite cast iron product according to this embodiment is as follows: - a casting process for casting spheroidal graphite iron products; - A pre-heat treatment process in which part of a series of heat treatment processes (quenching process and tempering process described below) is performed on the spheroidal graphite cast iron product; - a rough processing step of performing rough processing such as grinding (for example, lathe processing) on ​​at least a portion of the spheroidal graphite cast iron product (for example, a portion to be subjected to induction hardening); - A post-heat treatment process in which the other part of the heat treatment process (induction hardening) is performed on the spheroidal graphite cast iron product; - a finishing process in which at least a portion of the spheroidal graphite cast iron product (for example, a portion to be subjected to induction hardening) is subjected to finishing processing such as polishing as necessary (and in some cases painting or plating) to obtain the final spheroidal graphite cast iron product; It can also be configured to include

[0011] The manufacturing process of the spheroidal graphite cast iron product according to the modified embodiment is as follows: - a casting process for casting spheroidal graphite iron products; - a rough processing step of performing rough processing such as grinding on at least a portion of the spheroidal graphite cast iron product (for example, a portion to be subjected to induction hardening); - a heat treatment process in which a series of heat treatments are performed on the spheroidal graphite iron castings; - a finishing process in which at least a portion of the spheroidal graphite cast iron product (for example, a portion to be subjected to induction hardening) is subjected to finishing processing such as polishing as necessary (and in some cases painting or plating) to obtain the final spheroidal graphite cast iron product; Includes.

[0012] Each step will be explained below. Note that the roughing and finishing steps are the same as those generally performed, so explanations will be omitted.

[0013] [Casting process] First, the casting process will be described. Preferably, casting is performed using molten spheroidal graphite cast iron adjusted so that the spheroidal graphite cast iron product has the following chemical composition. The casting itself can be performed using a general casting method (molten metal treatment, pouring) using a general mold (sand mold) for spheroidal graphite cast iron. Preferably, secondary inoculation is performed to refine the structure and stabilize the mechanical properties.

[0014] The spheroidal graphite cast iron product preferably has In mass%, C: 3.4-3.8% Si: 2.0 to 2.5% Mn: 0.3 to 0.6% Mg: 0.035 to 0.060% Cr: 0.02 to 0.10% Sn: 0.005 to 0.020% S: 0.03% or less Cu: 0.05 to 0.10% The composition contains Fe and unavoidable impurities as the balance. In this specification, unless otherwise specified, all percentages indicating contents are by mass%.

[0015] The above-mentioned composition is characterized by the fact that the contents of Si, Cr, Mo, Sn, and Cu are kept low compared to general spheroidal graphite cast iron in order to maintain strength and reduce variation.

[0016] Specifically, the content of each element is determined based on the following concept.

[0017] C (carbon): 3.4%~3.8% The C content is the same as that of current spheroidal graphite cast iron and is within the proven chemical range. If the C content is less than 3.4%, shrinkage cavities are likely to occur, and if the C content is 3.8% or more, strength decreases, so the C content was set within the above range.

[0018] Si (silicon): 2.0% to 2.5% The Si content is set within the above range because if the Si content is 2.5% or more, hardenability decreases, and if the Si content is 2.0% or less, the degree of solid solution strengthening of ferrite by Si decreases, resulting in a decrease in strength. Note that in this embodiment, the hardenability of the material is important, so the importance of the upper limit of the Si content (2.5%), which has a significant effect on hardenability, is greater than the importance of the specified values ​​of the other components.

[0019] Mn (manganese): 0.3 to 0.6% The Mn content is equivalent to that of current spheroidal graphite cast iron and is within the proven composition range.

[0020] Mg (Magnesium): 0.035-0.060% (residual amount) The amount of Mg is the same as that of current spheroidal graphite cast iron and is within the proven composition range.

[0021] Cr (chromium): 0.02 to 0.10% Sn (tin): 0.005 to 0.020% Cr and Sn are added in the minimum amounts necessary to stabilize the pearlite structure. If the Cr content exceeds 0.1% or the Sn content exceeds 0.02%, pro-eutectoid cementite may form, significantly reducing workability.

[0022] S (sulfur): 0.03% or less Within the current control range for spheroidal graphite cast iron, if S is 0.03% or more, spheroidization is hindered and mechanical properties (tensile strength) decrease, so the increase in S from the raw material is minimized.

[0023] Cu (copper): 0.05 to 0.10% Cu segregates around the spheroidal graphite and has the effect of stabilizing pearlite. Because Cu is relatively expensive, the amount added was kept low, and the minimum amount added was set to achieve the above effect.

[0024] The above composition is suitable in the sense that it is easy to obtain good results when the heat treatment according to this embodiment is performed, but it is not necessary to completely satisfy all of the above composition. As mentioned above, the amount of Si is important, but it is also acceptable for the other components to deviate slightly from the above ranges.

[0025] The above composition (chemical components) is measured in accordance with the method specified in JIS G5502, 12.4 Analytical Tests defined for spheroidal graphite cast iron products. Here, measurements are made using JIS G1253 Spark Discharge Optical Emission Spectroscopy.

[0026] In the casting of the spheroidal graphite cast iron product according to this embodiment, the general inoculation and spheroidization treatments that are usually performed on molten spheroidal graphite cast iron can be performed. It is preferable to add 0.05 wt% to 0.10 wt% of an Fe-Si-based inoculant to the molten iron as secondary inoculation. This secondary inoculation refines the crystal grains and homogenizes the structure, resulting in a uniform hardness after heat treatment.

[0027] The above-mentioned spheroidal graphite cast iron products, in the as-cast state, satisfy the mechanical properties of, for example, FCD450-10 as defined in JIS G5502. The matrix structure is mostly ferrite (with a small proportion of pearlite), with spheroidal graphite dispersed throughout (see Figure 1). It is also possible to cast products with the above composition that, in the as-cast state, satisfy the mechanical properties of FCD400-18 or FCD600-3.

[0028] [Heat treatment process] Next, the heat treatment process will be described. The heat treatment process is A soaking process in which spheroidal graphite cast iron products are soaked at a high temperature (soaking temperature) a quenching step of quenching the spheroidal graphite cast iron product held at the soaking temperature; a tempering process for tempering the spheroidal graphite cast iron product after the quenching process; an induction hardening step of performing induction hardening on a desired portion of the spheroidal graphite cast iron product after the tempering step; It is equipped with: Each of the sub-steps constituting the heat treatment step will be described below.

[0029] <Soaking heating process> The soaking process is performed to diffuse graphite-derived carbon (C) into the matrix structure of the spheroidal graphite cast iron (originally composed mainly of ferrite) and to austenitize the matrix structure (to make it quenchable). The soaking temperature is at least 850°C. An appropriate temperature can be selected from 850°C to 980°C. The lower limit of 850°C is the temperature required for the carbon (C) of the spheroidal graphite to diffuse into the matrix structure at a practically acceptable rate, while the upper limit of 980°C is determined by manufacturing equipment considerations (higher temperatures require atmospherically controlled furnaces such as vacuum furnaces, but these are not practical for large castings due to cost considerations). A particularly suitable soaking temperature is, for example, 920°C. The soaking time can be selected from, for example, 2 to 4 hours depending on the shape and dimensions of the spheroidal graphite cast iron.

[0030] <Quenching process> The quenching process is carried out to maintain the graphite-derived carbon (C) diffused into the matrix structure in a substantially diffused state. For this purpose, quenching is preferably carried out under conditions where the cooling rate is 2.3°C / sec. Quenching is preferably carried out by water cooling or oil cooling, but forced air cooling is also possible depending on the size of the workpiece (spheroidal graphite cast iron product). It is preferable that the matrix structure after quenching becomes completely martensite. However, an incompletely quenched structure in which pearlite or bainite is mixed with martensite is also acceptable.

[0031] <Tempering process> The tempering process is carried out for the purposes of imparting appropriate toughness to areas of the spheroidal graphite cast iron product other than those to be induction hardened, releasing residual stress generated during the hardening process, and preventing cracks during the subsequent induction hardening process.If machining is performed between the tempering process and the induction hardening process, the tempering process is also carried out for the purpose of reducing the hardness of the spheroidal graphite cast iron product to a level that allows rough machining.The purpose described in this paragraph is also referred to as "purpose 1."

[0032] The tempering process is carried out under conditions that can achieve the above-mentioned Objective 1. When tempering is performed normally, the matrix structure (structure other than spheroidal graphite) becomes almost entirely a sorbite structure (a mixed structure of fine cementite and fine ferrite). However, if tempering is performed excessively, the matrix structure becomes a ferrite and fine graphite structure (or a mixed structure of large ferrite grains and a mixed structure of ferrite and pearlite). In other words, since carbon is not uniformly dispersed in the matrix structure in a state where it is combined with iron (Fe), the matrix structure is not uniformly austenitized during the subsequent induction hardening process (at which the region to be hardened is rapidly heated in a short time), and therefore, the matrix structure is not hardened. However, in the matrix structure of a sorbite structure, carbon is uniformly dispersed in a state where it is combined with iron (Fe), so this problem does not occur.

[0033] The tempering temperature in the tempering process is preferably 550°C to 600°C, and particularly preferably around 580°C. The tempering time depends on the size of the workpiece, but can be, for example, around 4 to 6 hours. If the tempering temperature exceeds 600°C, large ferrite precipitates become noticeable, making it impossible to obtain sufficient hardness through induction hardening in the subsequent process. If the tempering temperature is increased to 640°C, the matrix structure becomes mostly ferrite (and fine graphite).

[0034] On the other hand, if the tempering temperature is less than 550°C, the above-mentioned Object 1 cannot be sufficiently achieved. For example, if the tempering temperature is lowered to a temperature below 550°C, such as 500°C, the amount of cementite in the matrix structure after tempering increases (i.e., the amount of C in the matrix structure increases). This results in greater expansion when martensitic transformation occurs during induction hardening, making quench cracking more likely to occur. In addition, a large amount of cementite in the matrix structure means that the matrix structure has low toughness. This results in the area near the area where martensitic transformation occurs during induction hardening being unable to keep up with the expansion of the area where martensitic transformation occurs, making cracking more likely to occur.

[0035] For production control purposes, it is preferable to measure the surface hardness of the workpiece after the tempering process is completed. The standard surface hardness value of the workpiece is preferably 33HS to 56HS (Shore hardness). If the hardness is less than 33HS, large ferrite precipitation becomes noticeable, and if induction hardening is performed in this state, the hardness will decrease and the variation will increase. If the hardness exceeds 56HS, quench cracks will easily occur during induction treatment. The above phenomenon corresponds to the change in the matrix structure mentioned above. Workpieces with a hardness exceeding 56HS have a hypereutectoid structure (a structure with excessive cementite) and have low toughness.

[0036] In order to more reliably avoid quench cracks and also to avoid insufficient hardness, it is more preferable to set a larger safety margin and set the reference surface hardness value of the workpiece at 37HS to 49HS (Shore hardness).

[0037] Taking the example of spindle-related parts of machine tools, when the tempering temperature in the tempering process is 580°C, the average surface hardness of the workpiece will be about 42HS.

[0038] Since the hardness of a casting is expected to vary to some extent, it is also possible to measure the hardness at multiple points (for example, 10 to 20 points) and compare the average value with the above-mentioned reference value.

[0039] If the surface hardness of the workpiece is measured after the tempering process and is found to exceed the upper limit of the set standard surface hardness value (56 HS (49 HS if a larger safety margin is taken)), the workpiece may be re-tempered at the same tempering temperature, for example. By re-tempering in this way, the workpiece can be salvaged.

[0040] <Induction hardening process> After the tempering process (and possibly after further rough surface polishing), an induction hardening process is performed. For production control purposes, it is preferable to measure the surface hardness of the workpiece after the induction hardening process. The target hardness is, for example, 40 HRC (HRC: Rockwell C scale hardness) or higher, but the target hardness varies depending on the required performance of the part (wear resistance, etc.), so it is not limited to this value. The upper limit of the target hardness is, for example, 55 HRC, but this upper limit also varies depending on the required performance of the part (wear resistance, etc.), so it is not limited to this value. When particularly high wear resistance is required, a hardness of approximately 60 to 65 HRC may be used. The upper limit of the hardness of eutectoid steel is also approximately 65 HRC, and the upper limit of the hardness of spheroidal graphite cast iron does not exceed 65 HRC.

[0041] After the induction hardening step, a low-temperature tempering step may be carried out at a temperature of about 150°C to 200°C, if necessary.

[0042] The results of a heat treatment test conducted on an actual product (here, machine tool spindle-related parts) are shown below. The samples were soaked at 920°C for two hours, then quenched by water cooling, and then tempered at temperatures of 500°C, 580°C, and 640°C for four hours, and the structure of each sample was observed. For comparison, the structure of a sample that was not tempered after quenching was also observed. The structure was observed after mirror polishing, without corrosion, and by nital etching.

[0043] The as-cast texture is shown in Figure 1 for reference. The texture of the untempered sample was martensitic (see Figure 2). The texture of the samples tempered at 500°C and 580°C was sorbite (see Figures 3 and 4). The matrix structure of the 640°C tempered sample was ferrite + microspheroidal phase (see Figures 5 to 7). Microspheroidal phase was also observed in the 500°C and 580°C tempered samples, but the number of microspheroidal phases confirmed in the 640°C tempered sample was significantly higher than in the other samples. Furthermore, the microspheroidal phase was hardly detected in the untempered sample. The microspheroidal phase could be confirmed even without etching (see Figure 7).

[0044] From the above, it is thought that the fine spheroidal phase is the result of carbon (C) that existed in martensite or cementite being precipitated by tempering. It is also thought that the precipitation of carbon causes the matrix to become ferrite. If the matrix becomes ferrite, it will not be able to be hardened by induction hardening in the subsequent process.

[0045] Here is some data on the amount of Si. When actual products (spindle-related parts) with 2.82% Si and all other element contents within the above specifications were subjected to the tempering process, large variations in hardness were observed. The hardness range of the normal area was within the above-mentioned specified value of 33HS to 56HS, but the hardness of the abnormal area was 32HS. This is thought to be due to a decrease in hardenability caused by an excessive amount of Si. Note that for parts with an Si content of 2.0 to 2.5%, the overall hardness was within the above-mentioned specified value.

[0046] Furthermore, the relationship between hardness after the tempering process and hardness after the induction hardening process will be explained with reference to the graph in Fig. 8. In the graph in Fig. 8, the horizontal axis represents the surface hardness (Shore hardness HS) of the workpiece (spindle-related part) after the tempering process, and the vertical axis represents the surface hardness (Rockwell C scale hardness HRC) of the workpiece (spindle-related part) after the induction hardening process.

[0047] The plots within the dashed-line region C in the graph of Figure 8 represent data for workpieces that did not achieve sufficient hardness after induction hardening. If the hardness after tempering was 32 HS or less, it was impossible to achieve the target hardness of 40 HRC or more after induction hardening. In this case, the base structure after tempering was a ferrite-rich structure, as shown in Figure 5. When induction hardening is performed on a ferrite-based structure, the sorbite structure converts to martensite, resulting in a slight increase in hardness. However, the hardness of the original ferrite structure remains almost unchanged, resulting in insufficient overall hardness. Furthermore, when workpieces with such an inhomogeneous structure are hardened, cracks may occur. It was confirmed that when the hardness after tempering was 33 HS or more, the base structure was largely a sorbite structure (ferrite may be present in parts at the lower hardness limit). For these reasons, it was concluded that a hardness of 33 HS or more after tempering is preferable. If the hardness after the tempering process is 37 HS or more, it is guaranteed that almost all of the texture will be a sorbite texture, as shown in Figures 3 and 4.

[0048] The plots in area D surrounded by a dashed line in the graph of Figure 8 are data for workpieces in which cracks occurred after the induction hardening process. According to the inventor's analysis, when the Shore hardness after the tempering process is high at 57 HS or more, the matrix structure is a hyper-eutectoid structure with a large amount of cementite and low matrix toughness, which is thought to make cracks more likely to occur when quenched.

[0049] In the graph of FIG. 8, the plots within region A surrounded by a thin solid line (Shore hardness after tempering of 33 HS to 56 HS) did not cause cracking after induction hardening, nor did they cause insufficient hardness. If a sufficient safety margin is required, it is more preferable to refer to the plots within region B surrounded by a solid line (Shore hardness after tempering of 37 HS to 49 HS).

[0050] According to the above embodiment, a spheroidal graphite cast iron product having a uniform and high surface hardness can be manufactured.

Claims

1. A method for manufacturing a spheroidal graphite cast iron product, comprising: a casting process for casting spheroidal graphite iron products; a soaking step of soaking the spheroidal graphite cast iron product to diffuse carbon derived from graphite into a matrix structure and convert the matrix structure to austenite; a quenching step of quenching the spheroidal graphite cast iron product that has been soaked; a tempering step of tempering the spheroidal graphite cast iron product after the quenching step; an induction hardening step of performing induction hardening on the spheroidal graphite cast iron product after the tempering step; Equipped with The tempering step is carried out by soaking the spheroidal graphite cast iron product at a tempering temperature between 550 and 600°C, or by soaking the spheroidal graphite cast iron product under conditions such that the surface hardness of the induction hardened portion of the spheroidal graphite cast iron product after the tempering step is 33 HS to 56 HS (Shore hardness).

2. a hardness testing step of performing a hardness test on the induction hardening target portion after the tempering step and before the induction hardening step; If the surface hardness of the induction hardening target portion measured in the hardness testing step is within a predetermined reference range, the induction hardening step is continued as is, If the surface hardness of the induction hardened portion measured in the hardness testing step exceeds the upper limit of the standard range, the spheroidal graphite cast iron product is re-tempered by soaking it again at the tempering temperature used in the tempering step. The method of claim 1.

3. 2. The manufacturing method according to claim 1, wherein in the quenching step, the spheroidal graphite cast iron product held at the soaking temperature is cooled under conditions such that the cooling rate is 2.3°C / second or more.

4. The spheroidal graphite cast iron product cast in the casting step is In mass%, C: 3.4-3.8% Si: 2.0-2.5% Mn: 0.3-0.6% Mg: 0.035-0.060% Cr:0.02~0.10% Sn: 0.005-0.020% S: 0.03% or less Cu: 0.05-0.10% and the remainder being Fe and unavoidable impurities. The method of claim 1.

5. 2. The manufacturing method according to claim 1, wherein the spheroidal graphite cast iron product cast in the casting step is made of a material equivalent to FCD400-18, FCD450-10, or FCD600-3 specified in JIS G5502.

6. The manufacturing method according to claim 5, wherein the Si content of the spheroidal graphite cast iron product is 2.0 to 2.5 mass%.

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