CN107532225B

CN107532225B - Molten cast iron treatment method

Info

Publication number: CN107532225B
Application number: CN201680028691.3A
Authority: CN
Inventors: 藤本亮辅; 本间周平
Original assignee: Toshiba Machine Co Ltd
Current assignee: Shibaura Machine Co Ltd
Priority date: 2015-05-18
Filing date: 2016-05-17
Publication date: 2020-10-23
Anticipated expiration: 2036-05-17
Also published as: WO2016186094A1; JPWO2016186094A1; JP6728150B2; CN107532225A; DE112016002243T5; US20180148805A1; KR20180008612A; KR102107888B1

Abstract

A molten cast iron treatment method comprising the steps of: the inoculant is composed of 15-80 wt% of Si, one of La with the purity of 80-100 wt% as RE or Ce with the purity of 80-100 wt%, Ca and Al, and the balance of Fe and inevitable impurities, and is added into the molten iron so that the addition amount of each component element relative to the molten iron is 0.001-0.009 wt% of La or Ce, 0.001-0.02 wt% of Ca and 0.001-0.02 wt% of Al, thereby inoculating the molten iron.

Description

Molten cast iron treatment method

Technical Field

The present invention relates to a molten metal treatment method for cast iron (including both spheroidal graphite cast iron and flake graphite cast iron). The molten metal treatment method comprises the following steps: particularly effective in improving the mechanical properties of thick-walled cast iron (spheroidal graphite cast iron, flake graphite cast iron).

Background

In the casting of spheroidal graphite cast iron and flake graphite cast iron, it is generally carried out that: the molten metal is inoculated when discharged from a melting furnace into a ladle, when poured from a ladle into a mold, or the like, to improve the mechanical properties (tensile strength, elongation) of the cast iron product.

In a thick cast iron product, the eutectic solidification time for causing graphite crystallization is long, and therefore abnormal graphite or coarse graphite in the metal structure is likely to crystallize. The crystallization of abnormal graphite or coarse graphite causes a decrease in the tensile strength of cast iron. In the ferritic spheroidal graphite cast iron, the elongation of the material is significantly reduced by the crystallization of abnormal graphite or coarse graphite.

The crystallization of abnormal graphite and coarse graphite can be avoided by increasing eutectic clusters by performing appropriate inoculation. With the increase in eutectic cell, the number of graphite grains and the spheroidization rate increase in spheroidal graphite cast iron, and the formation of microscopic A-type graphite is promoted in flake graphite cast iron, and in either case, the mechanical properties can be improved.

In the case of casting relatively thin-walled cast iron articles, it is well known to: in a ladle or a pouring box, inoculation is performed using an inoculant obtained by adding Ca (calcium), Al (aluminum), Ba (barium), Bi (bismuth), etc. to Fe — Si (ferrosilicon).

As described above, the eutectic solidification time becomes long when casting a thick-walled cast iron product. Therefore, if a general inoculant containing Ca, Al, Ba, Bi, etc., which has not only an eutectic cell increasing action but also a graphitization promoting action, is used for casting of a thick-walled cast iron product, abnormal graphite (in the case of spheroidal graphite cast iron, "short rod graphite") or coarse graphite may crystallize. That is, when casting a thick-walled cast iron product, it is necessary to increase the number of eutectic cells and to suppress graphitization more than necessary. Therefore, it is considered that the rare earth element is preferably used as the graphite nucleating agent.

In the casting of a thick-walled flake graphite cast iron product, a molten metal treatment method using an inoculant containing a rare earth element that can sufficiently satisfy the above requirements is not known. Patent document 1 (international publication WO2015/034062a1) describes: a method for spheroidizing a molten metal in the production of a thick spheroidal graphite cast iron product. The method disclosed herein leaves room for further improvement in the efficiency of the graphite in the microminiaturization.

Documents of the prior art

Patent document

Patent document 1: international publication WO2015/034062A 1.

Disclosure of Invention

The present invention aims to provide a molten metal treatment method, particularly an inoculation method, which can suppress the crystallization of abnormal graphite and coarse graphite and suppress the reduction of mechanical properties.

In order to achieve the above object, in the present invention, the amount of RE (rareearth, rare earth element), Ca (calcium), and Al (aluminum) as compounds that become graphite nuclei in the inoculant are optimized to the molten metal, thereby suppressing abnormal graphite and coarse graphite that are crystallized by excessive graphitization.

In the inoculation method according to an embodiment of the present invention, as a graphite inoculant (hereinafter, simply referred to as "inoculant"), an inoculant containing 15 to 80% of any one of Si, La (lanthanum) and Ce (cerium), Ca and Al, and the balance of Fe (iron) and inevitable impurities is used, such that the addition amount of each component element to the molten metal is RE (La or Ce): 0.001-0.009%, Ca: 0.001-0.02%, Al: the inoculant is added to the molten metal in an amount of 0.001-0.02%. In the present specification, percentages indicating contents or amounts added refer to% by weight unless otherwise specified.

In thick cast iron having a eutectic solidification time of 1.0ks or more, RE, Ca and Al exhibit graphitization and promote crystallization of abnormal graphite or coarse graphite. However, by optimizing the amounts of RE, Ca and Al added as described above and using La or Ce alone as RE, the crystallization of abnormal graphite and coarse graphite can be suppressed.

If the amount of Ca or Al added is excessive, not only the crystallization of abnormal graphite or coarse graphite but also the formation of slag or dross is promoted. However, since a clean molten metal can be obtained by optimizing the amounts of Ca and Al added as described above, the occurrence of defects such as slag inclusion and pinholes in the product can be suppressed.

In addition, as described above, by suppressing the amount of RE added, which is expensive and not yet stable with respect to price stability, to a low level, material costs can be reduced and sensitivity to price fluctuations can be reduced.

Drawings

FIG. 1 is a photograph showing the structure of a spheroidal graphite cast iron according to an example of the present invention.

FIG. 2 is a photograph showing a structure of a conventional spheroidal graphite cast iron.

FIG. 3 is a photograph showing the structure of a flake graphite cast iron according to an example of the present invention.

FIG. 4 is a photograph showing the structure of a conventional flake graphite cast iron.

FIG. 5 is a schematic view showing a pouring box method.

FIG. 6 is a schematic view showing a method of inoculating a molten metal into a ladle.

FIG. 7 is a schematic view showing a wire processing method.

FIG. 8 is a schematic view showing a combination of flask inoculation and mold in-mold inoculation.

FIG. 9 is a schematic view showing the stream inoculation and the flask inoculation performed in combination.

Detailed Description

Hereinafter, preferred embodiments of the present invention will be described.

In the cast product of the thick-walled spheroidal graphite cast iron having the eutectic solidification time of 1.0ks or more, by using the molten metal treatment method, particularly the inoculation method according to the embodiment of the present invention, the crystallization of short rod-like graphite as abnormal graphite can be suppressed.

The inoculant used comprises 15-80% of Si, one of La or Ce as RE, Ca and Al, and the balance of Fe and inevitable impurities.

The inoculant can be produced by melting predetermined amounts (described later) of RE, Ca and Al in a molten Fe — Si alloy (ferrosilicon) metal, solidifying the molten metal, and then crushing the solidified molten metal into particles.

As is clear from a known Fe-Si state diagram (for example, refer to ASM HANDBOOK (trademark or registered trademark), volume 3, etc.), the reason why the amount of Si contained in the inoculant is set to 15 to 80% is that the amount of Si melted in the inoculant is increased in this range. When the Si content is 80% or more, other component elements are difficult to be incorporated. Further, it is more preferable to increase the amount of Si to be incorporated by making the Si content in the inoculant to be 15 to 25% or 50 to 60%.

As for RE, only Ce (cerium) or only La (lanthanum) is added, not in the form of an alloy of plural RE (for example, an alloy of Ce: La ═ 2: 1 called "misch metal") or a mixture. By adding only Ce, or only La alone in a proper amount, excellent mechanical properties can be obtained. When only Ce is used as RE, the purity of Ce is preferably 80 to 100 wt%. When only La is used as RE, the purity of La is preferably 80 to 100% by weight. The above ingredient definitions do not exclude the following: for example, when the added RE is Ce, La, which cannot be separated cleanly from Ce, is contained in the added RE as an inevitable impurity.

The amount of RE added to the molten metal is preferably 0.001 to 0.009%. When the amount of RE added is less than 0.001%, the number of eutectic cells decreases in the flake graphite cast iron, and the ability to neutralize the graphite spheroidization inhibitor is insufficient in the spheroidal graphite cast iron, resulting in deterioration of the graphite morphology. When the amount of RE added exceeds 0.009%, the flake graphite cast iron has no significant adverse effect, but the spheroidal graphite cast iron has a problem that a large amount of short rod graphite, which is abnormal graphite, is crystallized. If the graphite shape is deteriorated, the mechanical properties are deteriorated.

The amount of Ca added to the molten metal is preferably 0.001 to 0.020%, and the amount of Al added to the molten metal is also preferably 0.001 to 0.020%. If the amount of Ca or Al added is less than 0.001, the graphite nucleation cannot proceed sufficiently. When the amount of Ca or Al added exceeds 0.020%, abnormal graphite or coarse graphite is likely to be crystallized, and slag or dross is likely to be produced, which may cause defects such as inclusion of slag or pinholes in the product.

The above-mentioned inoculant can be used for inoculation in a furnace from which the original melt (element ) is to be tapped, or can be used for all known inoculation methods such as ladle inoculation, pouring box inoculation, pouring stream inoculation, in-mold inoculation, and wire treatment.

The compositions of inoculants which can suitably be used in the pour-in ladle inoculation, the pouring box inoculation, the pouring stream inoculation and the in-mold inoculation are shown below.

Si：30~80%

RE: 0.1 to 0.6% (La or Ce having a purity of 80 to 100 wt%)

Ca：0.1~1.3%

Al：0.1~2.0%

And the balance: fe and inevitable impurities

The compositions of inoculants that may suitably be used in the wire treatment process are shown below.

Si：30~60%

RE: 0.3 to 1.8% (La or Ce having a purity of 80 to 100 wt%)

Ca：0.1~6.0%

Al：0.1~6.0%

And the balance: fe and inevitable impurities

In the above composition, by controlling the concentrations of Fe and Si to be low and the concentrations of the other component elements to be high, a sufficient inoculation effect can be achieved even with a small amount of wire feeding, and therefore, the inoculation treatment time can be shortened.

In both the case of using any inoculation method and the case of using an inoculant of any composition, the amounts of the respective component elements added to the molten metal are as described above.

The inoculant according to this embodiment does not contain Mg (magnesium). Therefore, in the case of casting spheroidal graphite cast iron products, a different nodulizer from the inoculant used in the inoculation is used for the nodulizing treatment, and the nodulizing treatment is performed separately before the inoculation treatment. The spheroidizing agent used for the spheroidizing treatment can be used by selecting an appropriate substance from known spheroidizing agents. However, from the viewpoint of minimizing the effect on the inoculation treatment, it is preferable to use a nodulizer containing no RE, Ca or Al, for example, a nodulizer of Fe-Si-Mg (e.g., Fe: Si: Mg in a weight ratio of 45: 45: 10, or 30: 30: 20, or 45: 30: 5) system.

It is known that a high effect can be obtained by performing inoculation at a time point as close as possible to the time point of pouring the molten metal into the mold, and in this embodiment, Mg as an element contributing to spheroidization is not contained in the inoculant, spheroidization is performed by using another spheroidizing agent, and inoculation is performed immediately before pouring into the mold after spheroidization, whereby the inoculation effect can be improved.

When the flake graphite cast iron product is cast, the above-mentioned inoculant may be added to the molten metal.

FIG. 6 shows a schematic representation of the pour-in ladle inoculation process. In the case of the ladle inoculation method which is generally used in many cases, an inoculant is filled in a reaction trough (pocket) at the bottom of a ladle, and the original melt at 1400 to 1500 ℃ is discharged into the ladle for inoculation.

In the molten metal treatment of spheroidal graphite cast iron, the inoculant described above is disposed so as to cover the surface of the nodulizing agent filled in the reaction channel, and can also be used as a covering agent for smoothly proceeding the Mg reaction. If the amount of Mg added is large, the reaction becomes vigorous, but the reaction can be smoothly progressed by adding Ca in a large amount within the above-mentioned optimum range (0.001 to 0.02 wt% with respect to the entire molten metal).

Fig. 7 shows a schematic diagram of a thread processing method. The inoculation treatment can be efficiently performed in a short time by the line treatment.

FIG. 8 is a schematic diagram showing the combined practice of flask inoculation and in-mold inoculation. It is also possible to carry out only pouring box inoculation or in-mold inoculation. In general, the mechanical properties of the cast can be further improved by inoculation immediately before casting into a mold. Further, as shown in the schematic view of FIG. 9, stream inoculation and flask inoculation may be performed compositely.

Further, it is also preferable that the molten metal is inoculated a plurality of times by combining 2 or more of inoculation to the molten metal in the ladle, inoculation to the pouring box, in-mold inoculation, and stream inoculation from the time when the molten metal is tapped from the melting furnace into the ladle until the end of pouring into the mold, and by doing so, the mechanical properties of the cast product can be further improved. In the case of performing inoculation a plurality of times, the total of the component elements with respect to the molten metal is set to fall within the above-described range.

Preferably, the molten metal inoculated as described above (in the case of spheroidal graphite cast iron, spheroidization is performed in addition to inoculation) is poured into a mold at 1300 to 1400 ℃, and by doing so, a thick cast product having good mechanical properties can be obtained. The shape of the cast product is not particularly limited, and the inoculation method according to the above embodiment exhibits particularly excellent effects when the inoculation method has a wall thickness such that the eutectic solidification time is 1.0ks or more. When the cast product is larger or thick and the eutectic solidification time is longer, the casting temperature is preferably set to 1280 to 1360 ℃ which is slightly lower, so that good mechanical properties of the cast product are ensured. In the case of spheroidal graphite cast iron, the spheroidizing temperature is preferably 1400 to 1500 ℃.

Examples

Casting experiments were conducted on flake graphite cast iron articles and spheroidal graphite cast iron articles using the inoculants of the examples of the present invention and the inoculants of the comparative examples, respectively. In this experiment, a mold for casting a test piece having a wall thickness of 100mm was used, in which the eutectic solidification time was designed to be 1.2 ks. As described above, if the eutectic solidification time is long, the abnormal graphite and the coarse graphite are easily crystallized, and therefore, the method is suitable for verifying the effect of inoculation.

In the experiment, as previously explained, inoculants made by: predetermined amounts (described later) of RE, Ca, and Al were dissolved in a molten metal of an Fe-50% Si alloy (ferrosilicon), and the molten metal was solidified and then crushed into particles. The inoculant was added to 30kg of the original melt by the pouring box method, and the amounts of the inoculant added to the molten metal (original melt) were as shown in Nos. 1 to 20 of Table 1 below. When casting spheroidal graphite cast iron products, a nodulizer different from an inoculant is arranged at the bottom of a reaction channel at the bottom of a ladle, and nodulizing treatment and inoculation are carried out together. No.1, No.11 are inoculated. In the inoculated test piece, the amounts of Ca added were 0.003%, 0.012%, and 0.03%, the amounts of Al added were 0.003%, 0.012%, and 0.03%, and the amounts of RE added were 0.002%, 0.008%, and 0.020%.

The molten metal of the spheroidal graphite cast iron comprises the following components: 3.5-3.7%, Si: 2.4-2.6%, Mn: 0.5-1.0%, wherein the composition of the flaky graphite cast iron molten metal is C: 3.1-3.2%, Si: 1.5-1.7%, Mn: 0.8 to 0.9 percent.

The obtained test piece was subjected to a tensile test, and the tensile strength and the elongation at break were measured, and the structure was observed.

The test results are shown in Table 1.

[ Table 1]

In Table 1, Nos. 1 to 10 are spheroidal graphite cast iron, and Nos. 11 to 20 are flake graphite cast iron. No.11 to 20 show the mechanical properties and conditions of the flake graphite cast iron.

The following was confirmed for the tensile strength of the spheroidal graphite cast iron. The tensile strength was less than 450MPa in the case of no inoculation (No.1), in the case of RE being Ce + La (mixed rare earth is used as RE) (Nos. 2 to 4), and in the case of RE being 0.02% in addition even if La alone or Ce alone (Nos. 7 and 10). However, in the other cases (Nos. 5, 6, 8, 9), i.e., examples, the tensile strength was 450MPa or more. The following was confirmed for the elongation of the spheroidal graphite cast iron. The examples in which La alone or Ce alone was added as RE and the amounts of RE, Ca and Al were controlled to be low (Nos. 5, 6, 8 and 9) showed significantly higher elongation than the comparative examples (Nos. 1, 2 to 4, 7 and 10).

The following was confirmed with respect to the tensile strength of the flake graphite cast iron. Any of the samples showed a tensile strength of 300MPa or more. By the inoculation, an increase in tensile strength was confirmed. When samples having the same amounts of RE, Ca, and Al added are compared with each other, the samples with La added alone or Ce added alone as RE exhibit higher tensile strength than the samples with RE added in the form of misch metal. When the samples in which La alone was added as RE were compared with each other, the samples (Nos. 15 and 16) in which the amounts of RE, Ca and Al were kept low exhibited higher tensile strength than the sample (No.17) in which RE, Ca and Al were added in a relatively high amount. When the samples in which Ce was added alone as RE were compared with each other, the samples (Nos. 18 and 19) in which the amounts of RE, Ca and Al were kept low exhibited higher tensile strength than the sample (No.20) in which the amounts of RE, Ca and Al were high. The elongation of the flake graphite cast iron was almost not different depending on the presence or absence of inoculation and the amounts of RE, Ca and Al added.

FIG. 1 is an example of a spheroidal graphite cast iron according to the present invention, and FIG. 2 is a photograph showing a structure of a spheroidal graphite cast iron according to a conventional example. FIG. 3 is an example of the flake graphite cast iron of the present invention, and FIG. 4 is a photograph showing the structure of a flake graphite cast iron of a conventional example. By inoculation, the number of graphite particles of the spheroidal graphite cast iron is increased, and the graphite structure of the flake graphite cast iron is microscopic. By using an inoculant containing an optimum amount of RE, Ca and Al, improvement of the structure can be confirmed.

Claims

1. A method for treating molten metal of cast iron, comprising inoculating molten metal of the same cast iron 1 or more times, wherein an inoculant used in each of the 1 or more inoculations contains 15 to 80 wt% of Si, either La having a purity of 80 to 100 wt% as RE or Ce having a purity of 80 to 100 wt%, Ca and Al, and the balance of Fe and unavoidable impurities,

inoculating is performed so that the ratio of the total of the component elements of the inoculant added to the same molten metal by the 1-time or more inoculation to the molten metal is 0.001 to 0.009 wt% of La or Ce, 0.003 to 0.02 wt% of Ca, and 0.001 to 0.02 wt% of Al.

2. The molten cast iron metal treatment method according to claim 1, wherein the inoculant contains 30 to 80 wt% of Si, 80 to 100 wt% of La or 80 to 100 wt% of Ce, 0.1 to 0.6 wt% of Ca, 0.1 to 1.3 wt% of Al, and 0.1 to 2.0 wt%.

3. The molten cast iron metal treatment method according to claim 1, wherein the inoculant contains 30 to 60 wt% of Si, 80 to 100 wt% of La or 80 to 100 wt% of Ce, 0.3 to 1.8 wt% of Ca, 0.1 to 6.0 wt% of Al.

4. The molten cast iron metal treatment method according to any one of claims 1 to 3, wherein the inoculant is in the form of particles having a particle size of 1 to 5 mm.

5. The molten cast iron metal treatment method according to any one of claims 1 to 3, wherein the inoculant is in the form of a block having a length of 5 to 70 mm.

6. A molten cast iron metal treatment method according to any one of claims 1 to 3, wherein the inoculant is composed of grains having a grain size of 0.1 to 1.0mm, and the grains are supplied to the molten iron in a state of being continuously included in a core of the wire.

7. The molten cast iron metal treatment method according to any one of claims 1 to 3, further comprising a step of spheroidization by using a spheroidizing agent other than the inoculant, wherein the treatment temperature of the spheroidization is 1400 to 1500 ℃, and the pouring temperature into the mold is 1270 to 1370 ℃.

8. A molten iron treatment method according to any one of claims 1 to 3, wherein the same molten iron is inoculated 2 or more times, wherein inoculation at the 1 st time is carried out by an in-ladle inoculation method, and spheroidization is carried out by an in-ladle inoculation method simultaneously with inoculation at the 1 st time, and after spheroidization and inoculation by an in-ladle inoculation method, at least 1 of ladle inoculation, in-mold inoculation and pour stream inoculation is carried out as inoculation at the 2 nd time or later.

9. A molten cast iron metal treatment method according to any one of claims 1 to 3, wherein a spheroidizing agent containing magnesium is used for spheroidizing separately from the inoculation treatment based on the inoculant.