GB2513253A

GB2513253A - Spheroidal graphite cast iron and method of manufacturing the same

Info

Publication number: GB2513253A
Application number: GB1405632.9A
Authority: GB
Inventors: Yoshiaki Wada; Toru Kayahara; Takanobu Sugimoto; Tomohiro Tanaka
Original assignee: Hitachi Zosen Corp
Current assignee: Hitachi Zosen Corp
Priority date: 2013-03-29
Filing date: 2014-03-28
Publication date: 2014-10-22
Anticipated expiration: 2034-03-28
Also published as: JP5506973B1; GB2513253B; RU2551724C1; GB201405632D0; JP2014194052A

Abstract

Spheroidal graphite cast iron contains (by weight): 3.5-4.0 % carbon, 1.7-2.3 % silicon, less than 0.2 % manganese, less than 0.1 % chromium, 0.04-0.06 % magnesium, 0.10-0.20 % copper, 0.01-0.02 % sulphur and the balance being iron and impurities. The cast iron is made by adding a first sulphur containing inoculant to molten metal before pouring and a second sulphur containing inoculant to the molten metal during pouring. No ferrite annealing is performed on the cast iron.

Description

SPHEROIDAL GRAPHITE CAST IRON AND METHOD OF MANUFACTURING

THE SAME

Field of the Invention

The present invention relates to spheroidal graphite cast iron having high low-temperature toughness and a method of manufacturing the same.

Background of the Invention

In recent years, power supply equipment has shifted from consuminç fuel expected to be exhausted, e.g., oil or nuclear power to environment-friendly and renewable natural energy (including wind power and solar power) Wind power generator sets particularly achieve low power generation cost with short construction times and thus have become widespread over the world. Furthermore, wind power generation is a key national technology that is expected to be developed to a higher level.

Such a wind turbine generator system has a wind-power propeller mounted several tens meters above the ground or * *. the sea. Thus, the propeller or a device (a speed up gear or the like) connected to the propeller in a wind turbine * ** generator system provided in a cold climate area may be disposed under a low-temperature (e.g., -20°C) environment.

A gear box and a bearing component for the speed up gear have complicated shapes and thus can be manufactured only by casting under present circumstances.

Thus, cast irons with high low-temperature toughness have been demanded. In response to the demand, a proposed spheroidal graphite cast iron contains a large number of fine spheroidal graphite particles distributed on a ferrite base (For example, see Japanese Patent No. 2716063) . According to experimental data in Japanese Patent No. 2l6063, the spheroidal graphite cast iron has high toughness at -40°C. Furthermore, according to a description of Japanese Patent No. 2716063, the toughness and extension of the spheroidal graphite cast iron are improved by ferrite annealing.

The spheroidal graphite cast iron described in Japanese Patent No. 2716063 contains relatively a large quantity of Ni, which is an expensive element, naturally leading to high cost. Moreover, Bi added to the spheroidal graphite cast iron can prevent graphitization and form finer spheroidal graphite particles but leads to difficulty in quality governing because of its low yields and fluctuating. Furthermore, ferrite annealing in the manufacturing of the spheroidal graphite cast iron increases the number of steps. This disadvantageously increases the cost of the spheroidal graphite cast iron described in Japanese Patent No. 2716063.

An object of the present invention is to provide spheroidal graphite cast iron and a method of manufacturing the same which can considerably reduce the cost with excellent mechanical properties and high low-temperature toughness. * **

Disclosure of the Invention

* In order to solve the problem, spheroidal graphite cast iron according to a first aspect of the present invention is spheroidal graphite cast iron used at a sub-zero temperature, the spheroidal graphite cast iron containing 3.5 to 4.0 wt% of C, 1.7 to 2.3 wt% of Si, less than 0.2 wt% of Mn, less than 0.1 wt% of Cr, 0.04 to 0.06 wt% of Mg, Cu, 5, and other incidental impurities, the spheroidal graphite cast iron having a remainder containing Fe, wherein the spheroidal graphite cast iron contains 0.10 to 0.20 wt% of Cu and 0.01 to 0.02 wt% of S. A method of manufacturing spheroidal graphite cast iron according to a second aspect of the present invention is a method of manufacturing the spheroidal graphite cast iron according to the first aspect by pouring molten metal, the method including: a first inoculating step of adding an inoculant containing S to the molten metal before the pouring; and a second inoculating step of adding an inoculant containing S to the molten metal during the pouring.

A method of manufacturing spheroidal graphite cast iron according to a third aspect of the present invention, wherein ferrite annealing is not performed in the method cf rnanufacturLng the spheroidal graphite cast iron according to the second aspect.

The spheroidal graphite cast iron and the method of manufacturing the same can considerably reduce the cost while obtaining excellent mechanical properties and high low-temperature toughness. 0**

Brief Description of the Drawings

FIGS. 1A to 1D show the mechanical properties, low- * *, temperature toughness, and Si contents of spheroidal ::. graphite cast iron and a comparative material according to a first embodiment of the present invention, FIG. 1A showing the relationship between 0.2% proof stress and a Si content, FIG. TB showing the relationship between tensile strength and a Si content, FIG. TC showing the relationship between an impact value and a Si content at -20°C, FIG. lD showing the relationship between an impact value and a Si content at -40°C; FIGS. 2A to 2D show the mechanical properties, low-temperature toughness, and Si contents of spheroidal graphite cast iron, a comparative material and a conventional material according to a second embodiment of the present invention, FIG. 2A showing the relationship between 0.2% proof stress and a Si content, FIG. 2B showing the relationship between tensile strength and a Si content, FIG. 2C showing the relationship between an impact value and a Si content at -20°C, FIG. 20 showing the relationship between an impact value and a Si content at -40°C.

Description of the Embodiments

Spheroidal graphite cast iron and a method of manufacturing the same will be described below according to a first embodiment of the present invention. In the present embodiment, a low temperature means a sub-zero temperature (below 0°C) The spheroidal graphite cast iron will be first described below.

The spheroidal graphite cast iron contains 3.5 to 4.0 wt% of C(carbon), 1.7 to 2.3 wt% of Si(silicon), less than 0.2 wt% of Mn(manganese), less than 0.1 wt% of Cr(chromium), 0.04 to 0.06 wt% of Mg(magnesium), and 0.025 wt% of P(phosphorus) The spheroidal graphite cast iron particularly * ** features 0.10 to 0.20 wt% of Cu(copper) and 0.01 to 0.02 wt% of S(sulfur) . The spheroidal graphite cast iron * contains other incidental impurities and a remainder containing Fe (iron) The reason for setting the composition range will be discussed below.

The ranges of the contents of C and Si were set such that a CE value (C + Si/3) is about 4.3. This is because a CE value and mechanical strength are correlated with each other so as to obtain predetermined mechanical strength of spheroidal graphite cast iron. The Si content was set at 1.7% to 2.3% in compliance with DIN (Deutsche Industrie Norm) shown in FIGS. 1 and 2 because a large Si content increases 0.2% procf stress and tensile strength but reduces an impact value at -20°C. As shown in FIGS. 1A to 10 and 2A to 20, an EN-GJS-400U-L equivalent has a 0.2% proof stress of at least 220 MPa, a tensile strength of at least 30 MPa, and an impact value of at least lOJ at _2000.

A Mn content was set smaller than 0.2 wt% because a large Mn content generates pearlite.

A Cr content was set smaller than 0.1 wt% because a large Cr content generates carbide.

A Mg content was set at 0.04 to 0.06 wt% for spheroidization of graphite.

The content of P, which is an incidental impurity, was set at 0.325 wt% or less in this example but is not particularly limited.

The composition ranges Df Cu and S will be specifically described below within the scope of the t...

present invention.

Generally, cast iron containing Cu has excellent mechanical properties but reduced low-temperature toughness. In other words, 0.2% proof stress and tensile strength improve, but an impact value decreases at a low- * ** temperature. However, the present inventors found that excellent mechanical properties and high low-temperature * toughness are achieved if the Cu content is 0.10 to 0.20 wt%. Specifically, a-Fe(ferrite) containing at least 0.10 wt% of dissolved Cu achieves excellent mechanical properties and high low-temperature toughness. However, the solubility of Cu(room temperature) in a-Fe(ferrite) is 0.20 wt%. Solubility exceeding 0.20 wt% leads to reduced low-temperature toughness. Hence, 0.10 to 0.20 wt% is a preferably Cu content. Unlike in Japanese Patent No. 2716063, this can achieve excellent mechanical properties and high low-temperature toughness without adding expensive Ni with the graphite cast iron according to the present invention. A large number of fine spheroidal graphite particles are desirably distributed evenly in cast iron to obtain higher low-temperature toughness. In this case, the use of an ordinary spheroidizing agent forms spheroidal graphite particles larger than a desired size. However, the present inventors confirmed that the inoculation of S finely distributes spheroidal graphite particles into a desired size. Theoretically, S generates MgS (magnesium sulfide) or the like, which is expected to form the nuclei of spheroidal graphite particles. This finely disperses spheroidal graphite particles by inoculating S without adding Bi that leads to difficulty in quality governing as described in Japanese Patent No. 2716063.

A specific example of the method of manufacturing the spheroidal graphite cast iron will be described below.

jExampie 1] : A 5-ton low-frequency melting furnace was used. A tapping temperature was set at 1480°C after quality governing.

Subsequently, a Fe-Si-Mg alloy was added as a spheroidizing agent into a ladle. In this case, 1.1% of * *. the alloy was added by a sandwich method. Moreover, a Fe- ::::; Si-Ca-S alloy (first inoculant) was added into the ladle * as primary inoculation (first inoculating step) . In this case, 0.3% of the alloy was added by the sandwich method.

After that, a Fe-Si-Ca-S alloy (second inoculant) was directly added into a pouring flow (molten metal flow during pouring) as secondary inoculation (second inoculating step) . In this case, the temperature of addition was 1350°C while the quantity of addition was 0.1%. The secondary inoculation is performed to reliably obtain the effect of inoculation, finely generating a large number of spheroidal graphite particles. In other words, the secondary inoculation is performed to compensate for the effect of the primary inoculation deteriorating with the passage of time from the primary inoculation to pouring. Moreover, a sample with a main body was made collectable by JIS (Japan Industrial Standards) G5502.

The sample was not heat-treated, that is, the sample was left in an as-cast condition.

A tensile test piece conformed to JIS14A while an impact test piece conformed to JIS4 (V notch) . Tables 1 and 2 show the components of the collected sample and the mechanical properties and low-temperature toughness of the test piece with comparative materials (1) to (3) . FIG. 1 shows the contents of Table 2.

[Table 1]

C Si Mn P S Cu Mg Comparative 3.76 2.18 0.14 0.016 0.012 -0.046 material (1) Comparative 3.79 2.02 0.13 0.015 0.010 -0.049 material (2) Comparative 3.82 1.74 0.13 0.014 0.012 -0.058 * * material (3) *. .* Present **.

invention 3.83 2.10 0.13 0.016 0.012 0.19 0.059 material

___________________ _________ _________ _________ I ___________________ * .* * * *

* ** [Table 2J * *

* 0.2% * ** Tensile Impact Impact proof Extension strength value value stress (%) (MPa -20°C (J) -40°C (J) ______________ (MPa) _________ __________ __________ __________ comparative 249 397 26.4 14.5 7.1 material (1) Comparative 237 381 26.3 16.8 -material (2) ________ __________ __________ __________ ___________ Comparative 221 372 26.1 17.9 15.1 material (3) ________ __________ __________ ___________ Present inventLon 256 412 24.6 16.0 9.1 material I __________ __________ __________ As shown in FIG. 1, the comparative material showed positive correlation between a mechanical property and a Si content and negative correlation between an impact value at a low temperature and a Si content, whereas the present invention material had more excellent mechanical properties and higher low-temperature toughness than the comparative material.

[Example 2]

In a method of manufacturing spheroidal graphite cast iron according to Example 2, heat treatment for stress relief annealing was performed after the pouring of the manufacturing method of Example 1. Other points of Example 2 are identical to those of Example 1.

Specifically, in Example 2, materials were kept in heat treatment for stress relief annealing at 590°C for five hours and then were slowly cooled (furnace cooling) in a fUrnace. Tables 3 and 4 show the components of a collected sample and the mechanical properties and low-temperature toughness of a test piece with comparative materials (1) to (3) and a conventional material. FIG. 2 shows the contents of Table 4. The conventional material is a material equivalent to a comparative material (3) * * .. The Ni content of the conventional material is equal to * 4 the Cu content of the present invention material.

* [Table 3]

c Si Mn P S Ni Cu Mg comparative 3.76 2.18 0.14 0.016 0.012 --0.046 material (1) comparative 3.79 2.02 0.13 0.015 0.010 --0.049 material (2) ______ ______ comparative 3.82 1.74 0.13 0.014 0.012 --0.058 material (3) Conventional 3.88 1.73 0.10 0.016 0.013 0.19 0.047 material Present invention 3.33 2.10 0.13 0.016 0.012 -0.19 0.059 material

[Table 4]

0.2% Tensile Extension Impact Impact proof strength (%) value value stress (MPa) -20°C (J) -40°C (J) _______________ (MPa) __________ ___________ __________ __________ comparative 248 396 25.3 12.1 9.4 material (1) Comparative 234 381 25.5 17.0 10.0 material (2) comparative 220 372 26.1 18.3 16.7 material (3) Conventional 223 368 27.0 17.6 15.9 material Present invention 254 408 23.9 15.9 13.2 material As shown in FIG. 2, the comparative material showed positive correlation between a mechanical property and a Si content and negative correlation between an impact value at a low temperature and a Si content, whereas the present invention material had more excellent mechanical properties and higher low-temperature toughness than the comparative material. Toughness remarkably improved particularly at -40°C in the present invention.

The spheroidal graphite cast iron and the method of manufacturing the same according to the present invention has been able to improve the mechanical properties and low-temperature toughness of the spheroidal graphite cast iron. Furthermore, the absence of expensive Ni and 61 leading to difficulty in quality governing allowed considerable cost reduction. Moreover, according to the manufacturing method, heat treatment is not performed or heat treatment is performed only for stress relief annealing without ferrite annealing. This reduced the number of steps and the cosc.

In Examples 1 and 2, the first inoculant and the second inoculant were Fe-Si-Ca-S alloys but are not particularly limited to this. The inoculants are simply required to contain S.