Ductile cast iron having high strength, high ductility, and high impact value

Abstract

To provide ductile cast iron having high strength, high ductility, and high impact value.SOLUTION: A ductile cast iron of the present invention having high strength, high ductility, and high impact value comprises, in mass%, C: 3.05-3.7%, Si: 2.95-3.4%, Mn: 0.2-0.46%, Cu: 0.02-0.19%, Cr: 0.02-0.11%, and Mg: 0.07% or less, with the balance being Fe and impurities. Optionally, it further comprises P: 0.02% or less and / or S: 0.02% or less. The ductile cast iron has a ferrite area ratio of, preferably, 90% or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to ductile cast iron having high strength, high ductility, and a high impact value, and being suitable as a material for pipes, pipe fittings, water storage tanks, and the like.

Background Art

[0002] Ductile cast iron (nodular graphite cast iron) has higher strength than ordinary cast iron, and in JIS G5502, the tensile strength, elongation, Charpy impact value, etc. are standardized.

[0003] Generally, as the mechanical properties of a material, there is a correlation between tensile strength and elongation. When the tensile strength increases, the elongation decreases, and when the tensile strength decreases, the elongation increases. For this reason, regarding ductile cast iron, various proposals have been made to adjust the balance between tensile strength (in this specification, "tensile strength" and "strength" are synonymous) and elongation and impact value (for example, see Patent Documents 1 to 4).

[0004] The invention according to Patent Document 1 relates to ductile cast iron having high strength and ductility (in this specification, "ductility" and "elongation" are synonymous). By reducing the Si content, a decrease in the impact value is suppressed, and by setting the ratio of the Si content to the total content of Mn and Cu, the pearlite ratio is set to 30 to 55% to ensure strength.

[0005] The invention according to Patent Document 2 relates to a ductile cast iron pipe having high strength and high toughness. By keeping the contents of Sn and Cu within a predetermined range, the area ratio of pearlite in the matrix is set to 50 to 90% to ensure strength, and fine spherical graphite is crystallized in the matrix to obtain high toughness (endurance and elongation).

[0006] The invention according to Patent Document 3 relates to high-rigidity ductile cast iron excellent in strength and toughness. By pearlitizing the matrix structure according to the content of Mn and Cu, the content of P, and the ratio of the total content of Mn and Cu, the strength and impact value (toughness) are improved, and a high Young's modulus is obtained by setting the carbon equivalent value CE calculated by the formula: C(%) + Si(%) / 3 to 3.6 to 4.3%.

[0007] The invention according to Patent Document 4 relates to ductile cast iron having high strength, high ductility, and high toughness (high impact value). By setting the area ratio of ferrite to 20 to 55% and the area ratio of pearlite to 45 to 80%, the mechanical properties of both ferrite and pearlite are made compatible to obtain high tensile strength and elongation.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] In ductile cast iron where tensile strength, ductility, and impact value are emphasized, the above prior art ensures ductility and impact value by setting the Si content in a low range, and improves tensile strength by adjusting the area ratio of pearlite in the matrix structure.

[0010] The ductile cast iron of the present invention obtains ductile cast iron having high strength, high ductility, and high impact value by adjusting the content of Si and the contents of Mn, Cu, and Cr.

[0011] An object of the present invention is to provide a ductile cast iron having high strength, high ductility and a high impact value.

Means for Solving the Problems

[0012] The ductile cast iron having high strength, high ductility and a high impact value according to the present invention is in mass%, C: 3.05 to 3.7%, Si: 2.95 to 3.4%, Mn: 0.2 to 0.46%, Cu: 0.02 to 0.19%, Cr: 0.02 to 0.11%, Mg: 0.07% or less, the balance being Fe and impurities, optionally, P: 0.02% or less, optionally, S: 0.02% or less.

[0013] The area ratio of ferrite is desirably 90% or more.

[0014] The tensile strength is desirably 420 N / mm 2 or more.

[0015] The elongation is desirably 18% or more.

[0016] The Charpy impact value is desirably 100 kJ / m 2 or more.

[0017] The pipe of the present invention is made of the above ductile cast iron having high strength, high ductility and a high impact value.

[0018] Also, the pipe joint of the present invention is made of the above ductile cast iron having high strength, high ductility and a high impact value.

Effects of the Invention

[0019] The ductile cast iron of the present invention has high strength, high ductility and a high impact value.

Modes for Carrying Out the Invention

[0020] Compared with conventional ductile cast iron, the ductile cast iron of the present invention increases the Si content for a predetermined C concentration, solid-solution strengthens the ferrite matrix by heat treatment, and aims to improve the tensile strength. On the other hand, when the Si content is increased, although the tensile strength is improved, there is a problem that the impact value decreases. Therefore, by containing Mn, Cu, and Cr within a predetermined range, a high impact value is obtained.

[0021] The reasons for limiting the components of each alloying element are explained below. Note that all “%” of the content of each element are by mass.

[0022] C: 3.05~3.7% C is contained at least 3.05%, preferably 3.1% in order to ensure the precipitation of graphite necessary for the present invention and to ensure the fluidity of the molten metal. On the other hand, when the content increases, the precipitation of graphite becomes excessive, leading to a decrease in tensile strength and ductility. Therefore, the upper limit of the content is 3.7%, preferably 3.55%, more desirably 3.4%.

[0023] Si: 2.95~3.4% Si has the effect of enhancing the fluidity of the molten metal and promoting the precipitation of graphite. It also suppresses the generation of cementite and pearlite in the matrix structure and promotes ferritization. In addition, since the ferrite matrix is solid-solution strengthened by heat treatment and the tensile strength is improved, it is contained at least 2.95%, preferably 3.0%, more desirably 3.1%. On the other hand, Si is an element that decreases the impact value, and when its content exceeds 3.4%, the decrease in the impact value is significant. Therefore, the upper limit of the content is 3.4%, preferably 3.3%, more desirably 3.2%. Note that it is desirable that the carbon equivalent value CE calculated by the formula: C(%) + Si(%) / 3 is 4.0~4.6.

[0024] Mn: 0.2~0.46% Since Mn improves the impact value, it is contained in an amount of at least 0.2%, preferably 0.25%, and desirably 0.3%. However, when the content increases, the amount of pearlite increases and the pearlite in the matrix structure is stabilized, resulting in a decrease in the impact value. Also, when the amount of Mn is excessive, it causes a decrease in elongation. Therefore, the upper limit is set to 0.45%, preferably 0.43%.

[0025] Cu: 0.02 to 0.19% Since Cu improves the impact value, it is contained in an amount of at least 0.02%. On the other hand, when the content increases, the amount of pearlite increases and the pearlite in the matrix structure is stabilized, resulting in a decrease in the impact value. Therefore, the upper limit is set to 0.19%.

[0026] Cr: 0.02 to 0.11% Since Cr improves the tensile strength by making the structure uniform and fine, it is contained in an amount of at least 0.02%. On the other hand, when the content increases, it promotes the precipitation of pearlite and cementite, resulting in a decrease in the impact value. Therefore, the upper limit is set to 0.11%.

[0027] Mg: 0.07% or less Mg is added as a spheroidizing agent to spheroidize graphite and ensure a predetermined spheroidization rate. If the residual amount of Mg becomes too large, the elongation and impact value decrease. Therefore, the content is 0.07% or less, preferably 0.06% or less, and desirably 0.02% or less.

[0028] The ductile cast iron of the present invention has the balance being Fe and impurities. Impurities are components that are mixed in due to various causes such as raw materials like ore and scrap, and manufacturing processes during the industrial production of cast iron, and are those that are allowed within a range that does not adversely affect the present invention.

[0029] In addition, the ductile cast iron of the present invention can selectively contain the following elements.

[0030] P: 0.02% or less P is an element generally contained as an impurity, but is added to contribute to the improvement of the fluidity of the molten metal during casting. However, if the P content is high, steadite will crystallize and the material will become brittle, so the upper limit of the P content is set at 0.02%.

[0031] S: 0.02% or less S is an element contained as an inevitable impurity in the melting of cast iron. However, if the content is high, it will inhibit the spheroidization of graphite, so its upper limit is specified to be 0.02% or less separately from other impurities.

[0032] The ductile cast iron of the present invention can improve the tensile strength by subjecting it to a first soaking heat treatment in a first temperature range of 970 °C to 1000 °C and a second soaking heat treatment in a second temperature range of 730 °C to 740 °C after casting, thereby solid-solution strengthening the ferrite matrix.

[0033] The ductile cast iron of the present invention has a ferrite area ratio of 90% or more, preferably 92% or more, and desirably 94% or more. By adjusting the ferrite area ratio in this way, ductility can be ensured.

[0034] The ductile cast iron of the present invention has a tensile strength of 420 N / mm 2 or more, preferably 450 N / mm 2 or more, and desirably 470 N / mm 2 or more.

[0035] The ductile cast iron of the present invention has an elongation of 18% or more, preferably 20% or more, and desirably 22% or more.

[0036] The ductile cast iron of the present invention has a Charpy impact value of 100 kJ / m 2 or more, preferably 110 kJ / m 2 or more, and desirably 150 kJ / m 2 or more.

[0037] The ductile cast iron of the present invention has high strength, high ductility, and high impact value, so it is used to supply water for water supply, sewage, agricultural water, wells, fire fighting, power generation, spring water, industrial water, etc. that require earthquake resistance, or It can be applied to pipes and pipe joints for accommodating communication cables, and is particularly suitable for application to buried pipes and buried pipe joints. Of course, it is not limited to buried applications, but can also be applied to pipes for applications where part or all are exposed above the ground. These can be manufactured by centrifugal casting or static casting. Also, for the same reason, the ductile cast iron pipe of the present invention can be applied to water storage tanks installed on the ground or partially or completely buried underground, but the applications are not limited to these examples.

Example

[0038] The molten metal of various alloy components of the invention examples and comparative examples shown in Table 1 was cast into a sand mold to produce a test piece of Y-shaped B No. (thickness 25 mm) specified in JIS G5502:2022. Each test piece was loaded into a heat treatment furnace, subjected to a first soaking heat treatment at a first temperature of 970 °C to 1000 °C, then subjected to a second soaking heat treatment at a second temperature of 730 °C to 740 °C, and then air-cooled heat treatment was performed. In addition, for all test pieces, Sn is 0.025% or less and Al is 0.04% or less.

[0039] For the obtained test pieces, tests for tensile strength, elongation, and impact value were conducted. For tensile strength and elongation, test pieces were prepared in accordance with the metal material tensile test method specified in JIS Z2241:2022 and measurements were taken. Also, for the impact value, test pieces were prepared in accordance with the Charpy impact test method specified in JIS Z2242:2023 and measurements were taken. The test results are shown in Table 1.

[0040]

Table 1

[0041] In Table 1, Nos. 1 to 12 are invention examples of the present invention, and Nos. 101 to 110 are comparative examples.

[0042] In the inventive examples, the area ratio of ferrite is 90% or more in each case, the tensile strength is 420 N / mm 2 or more, the elongation is 18% or more, and the Charpy impact value is 100 kJ / m 2 or more.

[0043] On the other hand, among the comparative examples, Nos. 101, 102, and 103 are examples where the Si content is less than the range of the present invention, and Nos. 104, 107, and 108 are examples where the Si content is more than the range of the present invention. In comparative examples Nos. 101, 102, and 103, since the Si content is less than the range of the present invention, the area ratio of ferrite is small, the tensile strength is high, but the elongation and impact value are low. On the other hand, in comparative examples Nos. 104, 107, and 108, since the Si content is more than the range of the present invention, the ferrite area ratio exceeds 90%, the tensile strength and elongation are excellent, but the impact value is low.

[0044] Also, comparative examples Nos. 105 and 106 are examples where the Mn content is more than the range of the present invention. In comparative example No. 106, the ferrite area ratio exceeds 90%, and the tensile strength and elongation are also excellent, but the impact value is low. In comparative example No. 105, furthermore, since the Cr content is more than the range of the present invention, the precipitation of pearlite and cementite is promoted, the area ratio of ferrite becomes less than 90%, the elongation slightly decreases, and the decrease in the impact value is significant.

[0045] No. 109 is an example where Mn and Cr are more than the range of the present invention, and No. 110 is an example where the Cr content is more than the range of the present invention. In comparative example No. 109, since the Mn content is more than the range of the present invention, the impact value is further decreased. In comparative example No. 110, the ferrite area ratio exceeds 90%, and the tensile strength and elongation are also excellent, but the impact value is low.

[0046] The above description is for explaining the present invention and should not be construed as limiting the invention described in the claims or narrowing the scope. Also, each component configuration of the present invention is not limited to the above embodiment, and it goes without saying that various modifications are possible within the technical scope described in the claims.

Claims

1. By mass percentage, C: 3.05 - 3.7%, Si: 2.95 - 3.4%, Mn: 0.2 - 0.46%, Cu: 0.02 - 0.19%, Cr: 0.02 - 0.11%, Mg: 0.07% or less, the balance being Fe and impurities, Optionally, P: 0.02% or less, Optionally, S: 0.02% or less, and comprising Ductile cast iron having high strength, high ductility and high impact value.

2. The area ratio of ferrite is 90% or more, The ductile cast iron having high strength, high ductility and high impact value according to Claim 1.

3. The tensile strength is 420 N / mm ２ or more. The ductile cast iron having high strength, high ductility and high impact value according to Claim 2.

4. The elongation is 18% or more, The ductile cast iron having high strength, high ductility and high impact value according to Claim 3.

5. The Charpy impact value is 100 kJ / m ２ or more. The ductile cast iron having high strength, high ductility and high impact value according to Claim 4.

6. Comprising ductile cast iron having high strength, high ductility and high impact value according to any one of Claims 1 to 5, Pipe.

7. Comprising ductile cast iron having high strength, high ductility and high impact value according to any one of Claims 1 to 5, Pipe fitting.

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