Flake graphite cast iron product and its manufacturing method

Abstract

To provide a flake graphite cast iron product having uniformity in hardness / structure while maintaining required hardness.SOLUTION: A flaky graphite cast iron product according to the present disclosure has a composition comprising, by mass, 2.9 to 3.2% C, 1.1 to 1.5% Si, 0.7 to 1.1% Mn, 0.1 to 0.2% Cr, 0.06 to 0.10% Sn, 0.01 to 0.03% S, 0.4 to 0.8% Cu, and the balance Fe with inevitable impurities, and further satisfying CE: 3.3 to 3.6 and CE≤-0.33×Si+4.26. CE is a carbon equivalent obtained by CE=C content (mass%)+1 / 3×Si content (mass%).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a flake graphite cast iron product and a manufacturing method thereof. Generally, the term "cast iron" refers to both a casting and a material for the casting. The same applies in this disclosure. Furthermore, in this disclosure, the term "cast iron product" may be used to refer to a casting. [Background technology]

[0002] Typically, machine tool beds and tables are manufactured by machining flake graphite cast iron. When the table is guided relative to the bed by a sliding guide mechanism, particularly high machining precision is required for the sliding surfaces. It is usually difficult to make the hardness and / or structure of flake graphite cast iron uniform throughout the product. When flake graphite cast iron with varying hardness / structure is machined (cutting, grinding, etc.), the amount of processing varies from part to part. This increases the number of work steps required to ensure the desired precision. To suppress the variation in hardness / structure from part to part, chills are installed in the mold to adjust the cooling rate. However, it is difficult to achieve sufficient uniformity in hardness / structure. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-158010 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides a technology for optimizing the chemical composition of flake graphite cast iron to increase the uniformity of hardness / structure while maintaining the required hardness. [Means for solving the problem]

[0005] A flake graphite cast iron product according to one embodiment of the present disclosure includes: In mass%, C: 2.9-3.2% Si: 1.1 to 1.5% Mn: 0.75 ~1.1% Cr: 0.1 to 0.2% Sn: 0.06 to 0.10% S: 0.01 to 0.03% Cu: 0.4 to 0.8% with the remainder being Fe and unavoidable impurities, moreover CE: 3.3~3.6 and CE≦-0.33×Si+4.26 (Here, CE is the carbon equivalent calculated by the formula: CE = C content (mass%) + 1 / 3 × Si content (mass%)) Satisfy.

[0006] The present disclosure further provides a method for producing a flake graphite cast iron product having the above composition, the method comprising pouring molten cast iron from a ladle into a mold and allowing it to solidify in the mold. Inoculation can be performed as needed. [Effects of the Invention]

[0007] According to the above embodiment, a flake graphite cast iron product having high uniformity in hardness / structure while maintaining the required hardness can be obtained. [Brief explanation of the drawings]

[0008] [Figure 1] This shows a front view (right) and a ZZ cross-sectional view (left) of a test piece for hardness testing. [Figure 2] 1 is a graph showing the C amount and Si amount of the samples used in the test. [Figure 3] 1 is a graph showing known data for explaining the conditions necessary to obtain a hypoeutectic structure in flake graphite cast iron. [Figure 4] 1 is a graph showing known data illustrating the relationship between the contents of various elements and graphitization time during the eutectic reaction and the eutectoid reaction in flake graphite cast iron. [Figure 5] 1 is a graph showing publicly known data showing the relationship between the Mn content and the tensile strength and Brinell hardness for each S content in flake graphite cast iron. DETAILED DESCRIPTION OF THE INVENTION

[0009] Below, a flake graphite cast iron product according to one embodiment of the present disclosure will be described.

[0010] First, the concept behind the alloy design of the flake graphite cast iron product according to this embodiment will be described below. Typically, machine tool beds and tables are made of a material equivalent to FC350, as specified in "JIS G5501-1995 Gray Iron Castings." The flake graphite cast iron according to this embodiment is designed to generally satisfy the strength required by FC350 (tensile strength of 350 MPa in a separately cast test piece specified in G5501-1995), as described above, and to have a standard deviation of Brinell hardness (HB) of approximately less than 1.0 (as determined by the test method described below). Regarding strength, the FC350 standard does not need to be strictly met; it is sufficient to generally satisfy the FC350 standard. Rather, emphasis is placed on hardness and structural stability to achieve stable workability. In addition, the tensile strength can be increased by keeping the C content low and increasing the Mn content within the composition range described below, so if a tensile strength of 350 MPa or more is required, such adjustments can be made.

[0011] The flake graphite cast iron product according to this embodiment is In mass%, C: 2.9-3.2% Si: 1.1 to 1.5% Mn: 0.75 ~1.1% Cr: 0.1 to 0.2% Sn: 0.06 to 0.10% S: 0.01 to 0.03% Cu: 0.4 to 0.8% The remainder consists of Fe and unavoidable impurities.

[0012] The ranges of composition (including CE value ranges) of various alloying elements defined in this specification include both ends of the range, and the numerical values ​​at both ends of the range are rounded up or down to the nearest digit. Specifically, for example, with respect to Si, 1.05% and 1.54% should be understood to be within the range of 1.1 to 1.5%.

[0013] In addition to the component ranges of the individual alloy elements described above, the flake graphite cast iron according to this embodiment further comprises: The carbon equivalent (CE) represented by the formula "CE = C content (mass%) + 1 / 3 × Si content (mass%)" is 3.3 to 3.6%, and Inequality “CE≦-0.33×Si content (mass%)+4.26” The present invention is characterized in that:

[0014] Hereinafter, the concept of alloy design for flake graphite cast iron according to this embodiment will be described. Regarding C and Si, C: 2.9 to 3.2% (1) Si: 1.1 to 1.5% (2) CE: 3.3 to 3.6 (3) CE≦-0.33×Si+4.26 (4) By doing so, the amount of graphite crystallization is minimized, maintaining hardness. Also, Variation is minimized by optimizing the amounts of Mn, Cr, Cu, Sn, and S added. The range of each component element is determined based on experimental results and / or publicly known knowledge.

[0015] First, an experiment conducted to determine the range of component elements will be described.

[0016] As shown in Figure 1, a test sample was cast by sand casting into a flaky graphite iron casting that was a rectangular parallelepiped with overall dimensions of 1050 mm x 550 mm x 300 mm and had a groove with dimensions of 1050 mm x 35 mm x 20 mm formed in the center of the top surface. The casting method was quite common.

[0017] Various components of cast iron were melted in a melting furnace to produce a molten metal having the above-described composition according to this embodiment. The molten metal was transferred from the melting furnace to a ladle, poured from the ladle into a mold, and solidified to obtain a flake graphite cast iron product. General manufacturing equipment for flake graphite cast iron products was used for casting. All samples except for Samples No. 21 and 22 were inoculated by pouring the mixture into a ladle.

[0018] After casting the samples, the castings were cut at the grooves, and both sides of the grooves were measured with a digital hardness tester, and the results were converted to Brinell hardness (HB). On each side of the grooves, measurements were taken at 10 points along the longitudinal direction, 3 mm deep and 10 mm apart, at 100 mm intervals. In other words, hardness measurements were taken at a total of 40 points for each casting of each composition, and the average value and standard deviation were calculated.

[0019] In addition, when each sample was cast, a separate cast specimen was prepared as specified in "JIS G5501-1995 Gray Iron Castings." Tensile tests were conducted on the test pieces of each composition, and the average value of the tensile strength (N=2) was calculated.

[0020] The experimental results are shown in Table 1. In Table 1, the letters before the numbers indicate the component elements of interest, but in the following explanation, the letters will not be referred to and the sample numbers will be listed only by the numbers. The components shown in Table 1 are the analytical values ​​of analysis samples taken from the ladle before pouring.

[0021] [Table 1]

[0022] The determination of the contents of various constituent elements will be explained below with reference to Table 1. The graph in Figure 2 plots the Si and C contents of each sample in Table 1. The graph in Figure 2 plots straight lines representing C = 2.9, C = 3.2 (corresponding to the upper and lower limits of the above-mentioned condition 1), Si = 1.1, Si = 1.5% (corresponding to the upper and lower limits of the above-mentioned condition 2), CE = 3.3, CE = 3.6 (corresponding to the upper and lower limits of the above-mentioned condition 3), and CE = -0.33 × Si + 4.26 (corresponding to the upper limit of CE in the above-mentioned condition (4)). The inequality CE ≦ -0.33 × Si + 4.26 representing condition (4) can be rewritten as "C + 1 / 3Si ≦ -0.33 × Si + 4.26," which can be further rewritten as "C ≦ 4.26 - 0.66Si."

[0023] The setting of the C and Si contents will be described in detail below. First, regarding condition (4), CE≦-0.33×Si+4.26 (4) By setting the temperature to 4.276, a hypoeutectic structure with minimal graphite crystallization can be obtained. A hypoeutectic structure is advantageous for achieving stable hardness. The above condition (4) is based on known knowledge and can be derived from the Fe-C-Si stable ternary phase diagram (enlarged view near the eutectic temperature) shown in Figure 3. The graph in Figure 3 reveals that the relationship between the CE value and the Si content, which determines whether or not a hypoeutectic structure can be obtained, is CE = 0.33Si + 4.276. In this embodiment, 4.276 was changed to 4.26 to ensure a sufficient margin for obtaining a hypoeutectic structure. The above condition (4) was derived based on this. Figure 3 is taken from "Hideo Nakae: High-Temperature Strength, Shrinkage, and Dimensional Accuracy of Molds (2014)." The reduction of approximately 0.02% was achieved to prevent hypereutectic formation, based on the actual results of a ±0.02% variation due to instrument accuracy.

[0024] Incidentally, it is clear from the graph in Figure 2 that satisfying the above conditions (1) to (3) automatically satisfies condition (4), and condition (4) has no other meaning than to reliably guarantee the formation of a hypoeutectic structure.

[0025] Conditions (1) to (3) were determined primarily based on the experimental results described below. In particular, the test results for Samples 3, 7, and 15 indicate that by setting the C content within the range of 2.9 to 3.2%, the desired strength and stable hardness can be achieved, at least within this range. Furthermore, the test results for Samples 8, 10, and 14, in which the Si content was significantly varied, indicate that when the Si content was too high, such as 1.82% in Sample 14, the standard deviation of hardness increased. While not directly related to the experimental evaluation items, it has been found that when the Si content is lower than the lower limit of 1.1%, solidification begins too early during casting, resulting in poor molten metal flow. Therefore, the lower limit of the Si content was set at 1.1%, and the upper limit was set at 1.1 to 1.5%, taking into account the fact that Sample 10, which showed good results, had a Si content of 1.49%. It can be seen that by setting the Si content within this range, the desired strength and stable hardness can be achieved (hereinafter, this is also referred to as "good properties being obtained").

[0026] The CE value was set to 3.3 to 3.6 (condition (3)) in order to reliably obtain a hypoeutectic structure even if the Si content changed slightly.

[0027] In the graph of Figure 2, satisfying all of conditions (1) to (3) is equivalent to the data plot being above line C=2.9 and below line C=3.2, to the right of line Si=1.1 and to the left of line Si=1.5%, above line CE=3.3 and below line CE=3.6, and below line C=4.26-0.66Si. In the graph of Figure 2, samples 7, 11, 14, 17, 21, and 23 are outside the area enclosed by the lines. However, as mentioned above, the C content, S content, and CE value have two significant digits, and the values ​​are rounded to the nearest significant digit. In other words, it should be noted that samples 7, 11, 17, and 23 can be considered to be within the area enclosed by the lines.

[0028] [Mn and Sn] By setting the Mn and Sn contents within appropriate ranges, the graphitization inhibition effect during the eutectic reaction is reduced and pearlitization during the eutectoid reaction is promoted. For more information, please refer to Figure 4. Figure 4 is quoted from "Inoyama Naoya et al.: Casting 62 (1990) 7." Figure 4 shows the relationship between the content (mass%) of each element and the graphitization time (min). The upper part of Figure 4 shows the graphitization time during the eutectic reaction, indicating that the longer the graphitization time, the greater the degree of graphitization inhibition. The lower part of Figure 4 shows the graphitization time during the eutectoid reaction, indicating that the longer the graphitization time, the more pearlitization is promoted, and the shorter the graphitization time, the more ferritization is promoted. By making the matrix structure entirely pearlite, tensile strength can be improved. In this embodiment, since a hypoeutectic structure is being aimed for, it is preferable to inhibit graphitization, and since a high strength of approximately FC350 is being aimed for, it is preferable to promote pearlitization. From this viewpoint, a high content of Mn and Sn is preferable, but adding more Mn and Sn than necessary is not preferable because it increases the sensitivity of hardness to the cooling rate during solidification, making it more likely that uneven hardness will occur. In this embodiment, the amounts of Mn and Sn are determined as follows based on experimental results and taking the above findings into consideration.

[0029] Sample No. in Table 1 3,13 From the test results of 24, the amount of Mn 0.75 It can be seen that good results are obtained within the range of 1.13% or less. The test results for sample No. 23 show that when only Mn is high and the Sn content is low, the standard deviation of hardness becomes large. The test results for samples No. 17 and 19 show that when the Sn content is low, down to around 0.03%, sufficient tensile strength cannot be obtained even if the Mn content is relatively high (0.85%). On the other hand, if the Mn content is relatively high, good properties are obtained even with an Sn content of 0.06%. Sample No. 1, 16 The Mn content is low (0.68%, 0.72% ) In this case, the Sn content is 0.06% or 0.07%Even if the amount is high, sufficient tensile strength is not obtained. From the test results of samples No. 8 to No. 11, good properties are obtained when the Mn content is 0.92 to 1.04% and the Sn content is 0.10%. Even if the Mn content is 0.94% as in sample No. 20, when the Sn content is increased to 0.15%, the hardness increases and the standard deviation deteriorates significantly. Considering the above, the Mn content 0.75 The Mn content was set to 1.1%, and the Sn content was set to 0.06 to 0.10%. At least within these ranges of Mn content and Sn content, good properties were obtained.

[0030] [About S] The above-mentioned Mn content 0.75 In the range of ~1.1%, samples No. 3, 5, 8-11, 13,19, 24, it can be seen that when the S content is 0.01% or 0.03%, the hardness and tensile strength are stable. Increasing only the S content, as in sample No. 6 (when the Sn content is low), does not result in sufficient hardness, and the standard deviation also increases.

[0031] The relationship between the S content and the Mn content has already been reported (see Figure 4). Figure 4 is quoted from "Kanno et al., Foundry Engineering 85 (2013) 7." The upper part of Figure 4 shows the relationship between the Mn content and tensile strength for each S content, and the lower part shows the relationship between the Mn content and Brinell hardness for each S content. The data in Figure 4 is not for a material equivalent to FC350, but for a flake graphite cast iron with lower strength, but the trend is the same. It is known that Mn and S form compounds that serve as nuclei for graphite nuclei and eutectic cell formation, so it is important that they exist in a certain ratio. In this embodiment, the Mn and Sn contents are first determined based on the above-mentioned concept, and the optimum S content for the determined Mn content is determined based on experimental results. It is determined to be 0.01 to 0.03%. The Mn content 0.75 It is clear from the experimental results in Table 1 that when the S content is 0.01 to 0.03%, good properties are obtained when the S content is 0.01 to 0.03%, and when the S content is 0.06 to 0.10%.

[0032] [About Cr] The test results for samples No. 2 and No. 4 show that when the Cr content is 0.3% or more, hardness is obtained, but the standard deviation becomes larger. This is because adding more Cr than necessary increases the sensitivity of hardness to the cooling rate during solidification. In Table 1, samples No. 10 and No. 13 show that good properties are obtained when the Cr content is 0.09% and 0.19%. For this reason, the Cr content was set to 0.1 to 0.2%.

[0033] [About Cu] Cu has little effect on the formation of cementite during eutectic solidification, and its effect on the standard deviation of hardness is also small (see Figure 4 for this point). However, if the Mn content is above a certain level, the addition of Cu has the effect of stabilizing pearlite. According to the test results in Table 1, good properties are obtained with a Cu content of 0.4 to 0.8%. Even if the Cu content is within this range, when the Cu content is relatively low, around 0.45%, as in sample No. 25, the Mn content of 0.61% (which is 0.75 If the Mn content is lower than 1.1%, the tensile strength will be significantly lowered to 327 MPa. 0.75 %. Although it is believed that there will be no problem with the performance of the alloy even if the Cu content is 0.8% or more, adding more Cu than necessary increases the cost of the alloy, and is therefore not rational. Taking the above into consideration, the Cu content was set at 0.4 to 0.8%.

[0034] As described above, the alloy according to the above embodiment has an alloy component content adjusted to an appropriate range, thereby enabling a hypoeutectic structure to be obtained, and the hardness and the sensitivity of the structure to the cooling rate during solidification to be kept low, thereby improving hardness and uniformity.

[0035] [About vaccinations] In casting the flake graphite cast iron product according to the above embodiment, it is also preferable to inoculate the molten metal using an inoculant, for example, an Fe-Si-based inoculant. A known treatment method can be used as appropriate for the inoculant treatment. Specifically, for example, primary inoculation can be performed by pouring the material into a ladle or by adding the material to the surface of the ladle.

[0036] As time passes after inoculation in the ladle, the inoculation effect decreases due to the fading phenomenon, so in such cases, secondary inoculation can be performed by pouring the molten metal into the sluice gate or by using an inoculant installed in the sluice gate. When performing secondary inoculation, an Fe-Si-based inoculant can be added in an amount equivalent to, for example, 0.05 to 0.10% of the molten metal weight.

[0037] Furthermore, tertiary inoculation such as in-mold inoculation (in-mold inoculation) can be performed. When in-mold inoculation is performed, an Fe-Si-Ca-based inoculant can be additionally inoculated in an amount equivalent to, for example, 0.05 to 0.10% of the molten metal weight.

[0038] Such secondary or tertiary inoculations are expected to restore the inoculation effect that has been reduced due to fading.

[0039] Inoculation can refine grain size, minimize shrinkage cavity size, and increase tensile strength. Samples No. 21 and 22 were not inoculated, while the other samples were. For example, comparing Sample No. 3 with Sample No. 21, the latter exhibited a worse standard deviation in Brinell hardness. In other words, proper inoculation can further uniformize the hardness throughout the entire cast product. While differences in hardness tend to occur between the edge and center of a product due to differences in cooling rate, proper inoculation can minimize these differences. This can reduce variations in resistance to cutting or grinding during machining, especially in the manufacture of machine tool beds and tables.

Claims

1. A flake graphite cast iron product, In mass%, C: 2.9-3.2% Si: 1.1-1.5% Mn: 0.75-1.1% Cr: 0.1-0.2% Sn: 0.06-0.10% S: 0.01-0.03% Cu: 0.4-0.8% with the remainder being Fe and unavoidable impurities, moreover CE: 3.3-3.6 and CE≦−0.33×Si+4.26 (where CE is the carbon equivalent calculated by the formula: CE = C content (mass%) + 1 / 3 × Si content (mass%)) Flake graphite cast iron products that satisfy the above requirements.

2. A step of pouring molten cast iron from a ladle into a mold and solidifying it in the mold; a step of performing primary inoculation before pouring into the ladle; a step of performing a secondary inoculation in the sluice gate, or a secondary inoculation in the sluice gate and a tertiary inoculation in the mold, when a predetermined time has elapsed after the inoculation in the ladle; A method for manufacturing a flake graphite cast iron product, comprising: The flake graphite cast iron product In mass%, C: 2.9-3.2% Si: 1.1-1.5% Mn: 0.75-1.1% Cr: 0.1-0.2% Sn: 0.06-0.10% S: 0.01-0.03% Cu: 0.4-0.8% with the remainder being Fe and unavoidable impurities, moreover CE: 3.3-3.6 and CE≦−0.33×Si+4.26 (where CE is the carbon equivalent calculated by the formula: CE = C content (mass%) + 1 / 3 × Si content (mass%)) A method for manufacturing flake graphite cast iron products that satisfy the above.

3. A manufacturing method as described in claim 2, wherein after inoculation in the ladle, secondary inoculation is carried out in the hanging weir and tertiary inoculation is carried out in the mold, and the tertiary inoculation is carried out using an Fe-Si-Ca-based inoculant.

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