JP7766565B2 - Flake graphite cast iron product and its manufacturing method - Google Patents

Description

This disclosure relates to flake graphite cast iron products and methods for manufacturing the same. Generally, cast iron can refer to both castings and the material used to make them. This also applies in this disclosure. Furthermore, in this disclosure, the term "cast iron product" may be used to refer to castings.

Machine tool beds and tables are typically 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 entire product. When flake graphite cast iron, which has variations in hardness/structure, is machined (cutting, polishing, etc.), variations in the amount of processing occur from part to part. This increases the amount of work required to ensure the desired precision. To suppress variations 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.

JP 2015-158010 A

This disclosure provides a technology that optimizes the chemical composition of flake graphite cast iron to maintain the required hardness while improving hardness/structural uniformity.

A flake graphite cast iron product according to one embodiment of the present disclosure includes:
In mass%,
C: 2.9-3.2%
Si: 1.1-1.5%
Mn: 0.75 to 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,
Furthermore, CE: 3.3 to 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%))
Satisfy.

The present disclosure further provides a method for producing a flake graphite cast iron product having the above composition. This production method includes pouring molten cast iron from a ladle into a mold and allowing it to solidify in the mold. Inoculation can be performed as needed.

According to the above embodiment, it is possible to obtain flake graphite cast iron products with high hardness/structure uniformity while maintaining the required hardness.

1 shows a front view (right) and a ZZ cross-sectional view (left) of a test piece for hardness testing. 1 is a graph showing the C content and Si content of samples used in the test. 1 is a graph showing known data for explaining the conditions necessary to obtain a hypoeutectic structure in flake graphite cast iron. 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. 1 is a graph showing 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.

Below, a flake graphite cast iron product according to one embodiment of the present disclosure is described.

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." As mentioned above, 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 separate cast test piece specified in G5501-1995) 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 in order to obtain stable workability.
In addition, the tensile strength can be increased by keeping the C content low and increasing the Mn content within the component ranges described below, so if a tensile strength of 350 MPa or more is required, such adjustments can be made.

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

The composition ranges (including CE value ranges) for 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 to the nearest digit. Specifically, for Si, 1.05% and 1.54% should be understood to be within the range of 1.1 to 1.5%.

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:

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

First, we will explain the experiments conducted to determine the range of component elements.

As shown in Figure 1, a test sample was cast using sand casting to create a flaky graphite iron casting with a rectangular parallelepiped shape measuring 1050 mm x 550 mm x 300 mm overall and a groove measuring 1050 mm x 35 mm x 20 mm formed in the center of the top surface. The casting method was quite conventional.

Various cast iron components were melted in a melting furnace to produce a molten iron having the above-described composition according to this embodiment. The molten iron was transferred from the melting furnace to a ladle, from which it was poured into a mold. The poured molten iron solidified to produce 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 using a ladle.

After the samples were cast, the castings were cut at the grooves, and both sides of the grooves were measured with a digital hardness tester, with the results converted to Brinell hardness (HB). Measurements were taken at 10 points along the length of each side of the groove, 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.

In addition, when each sample was cast, separate cast specimens were prepared in accordance with JIS G5501-1995 Gray Iron Castings. Tensile tests were conducted on test pieces of each composition, and the average tensile strength (N=2) was calculated.

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 referenced and the sample numbers will be indicated only by the numbers. The components shown in Table 1 are the analytical values of analysis samples taken from the ladle before pouring.

The determination of the content of each component element will be explained below, with reference to Table 1. The graph in Figure 2 plots the Si and C amounts for 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 x Si + 4.26 (corresponding to the upper limit of CE in the above-mentioned condition (4)). The inequality CE ≦ -0.33 x Si + 4.26 representing condition (4) can be rewritten as "C + 1/3Si ≦ -0.33 x Si + 4.26," which can be further rewritten as "C ≦ 4.26 - 0.66Si."

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 of Molds, Shrinkage, and Dimensional Accuracy (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.

By the way, from the graph in Figure 2, it is clear that satisfying the above conditions (1) to (3) automatically satisfies condition (4), and condition (4) has no other significance than ensuring that a hypoeutectic structure is obtained.

Conditions (1) through (3) were determined primarily based on the experimental results described below. In particular, the test results for Samples 3, 7, and 15 demonstrate 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 that range. Furthermore, the test results for Samples 8, 10, and 14, in which the Si content was significantly varied, demonstrate that an excessively high Si content of 1.82%, as in Sample 14, increases the standard deviation of hardness. While not directly related to the experimental evaluation items, it has been found that if the Si content is lower than the above-mentioned lower limit of 1.1%, solidification begins too early during casting, resulting in poor molten metal flow. Therefore, the lower limit for 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 produced favorable results, had a Si content of 1.49%. It can be seen that by keeping the Si content within this range, the desired strength and stable hardness are obtained (hereinafter, this is also referred to as "good properties are obtained").

The CE value was set to 3.3 to 3.6 (condition (3)) to ensure that a hypoeutectic structure was obtained even if the Si content changed slightly.

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 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 No. 7, 11, 14, 17, 21, and 23 are outside the area enclosed by the above lines. However, as mentioned above, the C content, S content, and CE value have two significant digits, and the values are rounded to one digit below the least significant digit. In other words, please note that samples No. 7, 11, 17, and 23 can be considered to be within the area enclosed by the above lines.

[Regarding 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.: Castings 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, but 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.

The test results for Samples No. 3 , 13, and 24 in Table 1 show that good results were obtained when the Mn content was in the range of 0.75 to 1.13%. The test results for Sample No. 23 show that when the Mn content was high and the Sn content was low, the standard deviation of hardness increased. The test results for Samples No. 17 and 19 show that when the Sn content was as low as 0.03%, sufficient tensile strength was not obtained even with a relatively high Mn content (0.85%). On the other hand, when the Mn content was relatively high, good properties were obtained even with a Sn content of 0.06%. When the Mn content was low (0.68%, 0.72% ), as in Samples No. 1 and 16 , sufficient tensile strength was not obtained even with a Sn content of 0.06% or 0.07% . The test results for Samples No. 8 to 11 show that good properties were obtained with a Mn content of 0.92 to 1.04% and a Sn content of 0.10%. Even when 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 significantly worsens. Taking the above into consideration, the Mn content is set to 0.75 to 1.1% and the Sn content to 0.06 to 0.10%. At least within these ranges of Mn and Sn content, good properties are obtained.

[About S]
Within the above-mentioned Mn content range of 0.75 to 1.1%, it can be seen from Samples Nos. 3, 5, 8 to 11, 13, 19, and 24 that when the S content is 0.01% or 0.03%, the hardness and tensile strength are stable. It can also be seen from Sample No. 6 that increasing only the S content (when the Sn content is low) does not provide sufficient hardness and the standard deviation also increases.

The relationship between S content and Mn content has already been reported (see Figure 4). Figure 4 is taken from "Kanno et al., Foundry Engineering 85 (2013) 7." The upper part of Figure 4 shows the relationship between Mn content and tensile strength for each S content, while the lower part shows the relationship between Mn content and Brinell hardness for each S content. The data in Figure 4 is not for FC350-equivalent material, but for flake graphite cast iron with lower strength, but the trends are similar. 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 then the optimal S content for the determined Mn content is determined based on experimental results. It is determined to be 0.01 to 0.03%. It is clear from the experimental results in Table 1 that when the Mn content is 0.75 to 1.1% and the Sn content is 0.06 to 0.10%, good properties are obtained by setting the S content to 0.01 to 0.03%.

[Regarding Cr]
The test results for Samples No. 2 and 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 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%.

[Regarding Cu]
Cu has little effect on cementite formation during eutectic solidification and also has little effect on the standard deviation of hardness (see Figure 4 for this point). However, when the Mn content is above a certain level, Cu addition 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 within this range, when the Cu content is relatively low at approximately 0.45%, as in Sample No. 25, the tensile strength drops significantly to 327 MPa when the Mn content drops to approximately 0.61% (which is outside the 0.75 to 1.1% range). For this reason, the lower limit of the Mn content is set at 0.75 %. Although it is believed that a Cu content of 0.8% or more will not cause any problems with the performance of the alloy, adding more Cu than necessary increases the cost of the alloy and is therefore not reasonable. Taking the above into consideration, the Cu content is set at 0.4 to 0.8%.

As described above, the alloy according to the above embodiment has an appropriate range of alloy component contents, which allows for the formation of a hypoeutectic structure, while also reducing the hardness and the sensitivity of the structure to the cooling rate during solidification, thereby improving hardness and uniformity.

[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.

As time passes after inoculation in the ladle, the inoculation effect decreases due to fading, so in such cases, secondary inoculation can be performed, such as inoculating the molten metal flow at the inlet weir or inoculating using an inoculant installed within the inlet weir. When performing secondary inoculation, an additional Fe-Si-based inoculant can be inoculated in an amount equivalent to, for example, 0.05 to 0.10% of the molten metal weight.

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

Such secondary or tertiary vaccinations are expected to restore the vaccination effectiveness that has been reduced due to fading.

Inoculation can refine the crystal grains, minimize the size of shrinkage cavities, 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, it can be seen that the latter had a worse standard deviation of Brinell hardness. In other words, proper inoculation can make the hardness of the entire cast product more uniform. While differences in cooling rate tend to occur between the edges and center of a product, proper inoculation can reduce these differences. This can reduce variations in resistance to cutting or polishing during machining, such as in the manufacture of machine tool beds and tables.

Claims (3)

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,
Furthermore, CE: 3.3 to 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. 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 is composed of:
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,
Furthermore, CE: 3.3 to 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. The manufacturing method according to claim 2, wherein after the inoculation in the ladle, a secondary inoculation in the sluice gate and a tertiary inoculation in the mold are performed, and the tertiary inoculation is performed using an Fe—Si—Ca-based inoculant.