METHOD FOR TREATING MOLTEN CAST IRON

Abstract

A method for treating molten cast iron includes, performing an inoculation treatment to the molten cast iron, with the use of an inoculant containing: 15 to 80 wt % Si; either 80 to 100 wt % purity La or 80 to 100 wt % purity Ce as RE; Ca; Al; and the balance Fe with inevitable impurities, by adding the inoculant to the molten cast iron such that: the addition amount of La or Ce relative to the molten cast iron is 0.001 to 0.009 wt %; the addition amount of Ca relative to the molten cast iron is 0.001 to 0.02 wt %; and the addition amount of Al relative to the molten cast iron is 0.001 to 0.02 wt %.

Description

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 method includes an inoculation treatment that is effective especially for improving the mechanical properties of thick cast iron (spheroidal graphite cast iron and flake graphite cast iron).

BACKGROUND ART

In the field of production of spheroidal graphite cast iron and flake graphite cast iron, molten metal is usually subjected to inoculation treatment when the molten metal is tapped from the melting furnace to the ladle or poured from the ladle to the mold so as to improve the mechanical properties (tensile strength and elongation) of cast iron products.

In the case of thick cast iron products, abnormal or coarse graphite is likely to be crystallized in their metal structure because the eutectic solidification time during which graphite is crystallized is longer. The crystallization of abnormal or coarse graphite reduces the tensile strength of the cast iron. In the case of ferritic spheroidal graphite cast iron, it considerably reduces the elongation of the materials.

The crystallization of abnormal or coarse graphite can be prevented by performing an appropriate inoculation treatment so that the number of eutectic cells can be increased. An increase in the number of eutectic cells increases the number of graphite grains formed and spheroidal graphite rate in the case of spheroidal graphite cast iron and also promotes the formation of fine type A graphite in the case of flake graphite cast iron. In either case, the mechanical properties of the cast iron can be improved.

In the case of casting relatively thin cast iron products, a well-known approach is the inoculation treatment that uses an inoculant that is formed by adding calcium (Ca), aluminum (Al), barium (Ba), bismuth (Bi), and the like to ferrosilicon (Fe—Si) in the ladle or in the runner box.

As stated above, the eutectic solidification time is longer in casting thick cast iron products. Thus, in producing thick cast iron products, the use of a typical inoculant including Ca, Al, Ba, Bi, and the like that have not only eutectic cell increasing effects but also graphitization promoting effects may lead to the crystallization of abnormal graphite (“chunky graphite” in the case of spheroidal graphite cast iron) or coarse graphite. In other words, production of thick cast iron products requires preventing excessive graphitization while increasing the number of eutectic cells. Thus, it is considered to be preferable to use rare earth elements as graphite nucleation materials.

No method for treating molten metal is known in the field of casting of thick flake-graphite cast-iron products that involves the use of an inoculant including rare earth elements which may fully satisfy the above requirements. Patent Document 1 (international patent application WO 2015/034062 A1) discloses a method for performing a spheroidizing treatment on molten metal in producing thick spheroidal-graphite cast-iron products. However, the method disclosed therein still leaves room for further improvement in terms of the efficiency with which to achieve finer graphite.

PRIOR ART DOCUMENT

Patent Document

Patent Document 1: International Patent Application WO 2015/034062 A1

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for treating molten metal, especially an inoculation method, that prevents crystallization of abnormal or coarse graphite and thus prevents degradation of the mechanical properties of the cast iron.

To achieve the above object, the invention optimizes the amounts of rare earth (RE), calcium (Ca), and aluminum (Al), which are included in an inoculant and constitute the compounds forming graphite nuclei, to be added to the molten metal. By optimizing those amounts, it is possible to prevent the crystallization of abnormal or coarse graphite that results from excessive graphitization.

In an inoculation method according to an embodiment of the invention, we use a graphite inoculant (hereinafter referred to simply as “the inoculant”) that includes 15% to 80% Si, either lanthanum (La) or cerium (Ce), Ca, and Al. The materials constituting the rest of the inoculant are iron (Fe) and inevitable impurities. The inoculant is added to the molten metal such that the ratios of RE (La or Ce), Ca, and Al to the molten metal are 0.001% to 0.009%, 0.001% to 0.02%, and 0.001% to 0.02%, respectively. In this specification, the percentages indicative of content or amounts added are all meant as wt % (percent by weight) unless otherwise specified.

In the case of thick cast iron whose eutectic solidification time is 1.0 ks or longer, RE, Ca, and Al contribute to graphitization, promoting the crystallization of abnormal or coarse graphite. However, as stated above, by optimizing the amounts of RE, Ca, and Al added and also using only La or Ce as the RE element, the crystallization of abnormal or coarse graphite can be prevented.

An excessive amount of Ca or Al leads not only to promoting the crystallization of abnormal or coarse graphite but also to promoting the formation of slag and dross. However, the above optimization of the addition amounts of Ca and Al results in clean molten metal. Therefore, the finished product is prevented from having defects such as slag inclusion and pinholes.

Further, as stated above, by reducing the addition amount of the RE, which is expensive and also prone to price fluctuations, material costs can be reduced so that products are less influenced by price fluctuations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph depicting the structure of spheroidal graphite cast iron according to an embodiment of the invention.

FIG. 2 is a photograph depicting the structure of spheroidal graphite cast iron according to a conventional method.

FIG. 3 is a photograph depicting the structure of flake graphite cast iron according to an embodiment of the invention.

FIG. 4 is a photograph depicting the structure of flake graphite cast iron according to a conventional method.

FIG. 5 is a schematic diagram illustrating runner box inoculation.

FIG. 6 is a schematic diagram illustrating sandwich inoculation.

FIG. 7 is a schematic diagram illustrating wire inoculation.

FIG. 8 is a schematic diagram illustrating runner box inoculation and in-mold inoculation performed in a combined manner.

FIG. 9 is a schematic diagram illustrating pouring stream inoculation and runner box inoculation performed in a combined manner.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

A preferred embodiment according to the invention will now be described below.

In casting product that is thick spheroidal graphite cast iron whose eutectic solidification time is 1.0 ks or longer, the use of a molten metal treatment method, especially an inoculation method, according to an embodiment of the invention prevents the crystallization of chunky graphite, which is abnormal graphite.

The inoculant used herein contains 15% to 80% Si; either La or Ce as RE; Ca; Al; and the balance Fe with inevitable impurities.

The inoculant is made by dissolving predetermined amounts of RE, Ca, and Al (described later in detail) in molten metal of ferrosilicon (Fe—Si) alloy, solidifying the molten metal, and then crushing it into granules.

The reason for setting the content percentage of SI in the inoculant in the range of 15% to 80% is that more Si can be dissolved in that range, as is obvious from a known Fe—Si status diagram (see, for example, ASM handbook (trademark or registered trademark) vol. 3). If the content percentage of SI is equal to or greater than 80%, the other constituent elements are less likely to dissolve. For the purpose of further increasing the amount of dissolved Si, it is more preferable to set the content percentage of Si in the inoculant in the range of 15 to 25% or 50% to 60%.

As RE, an RE alloy including several RE substances (e.g., a Ce—La alloy in which Ce:La=2:1, called “mischmetal”) or a mixture of several RE substances is not used. Instead, only cerium (Ce) or lanthanum (La) is added. Adding an appropriate amount of only Ce or only La leads to excellent mechanical properties. If only Ce is to be used as RE, its purity is preferably 80 to 100 wt %. If only La is to be used as RE, its purity is preferably 80 to 100 wt %. The above constituent requirements do not exclude a case where Ce contains La as inevitable impurity which cannot be separated completely from the Ce if the RE to be added is Ce, for example.

It is preferred that the amount of RE added to the molten metal relative to the molten metal be 0.001% to 0.009%. If the amount of RE is less than 0.001%, in the case of flake graphite cast iron, the number of eutectic cells is reduced; while in the case of spheroidal graphite cast iron, graphite shapes may be adversely affected for lack of ability to neutralize the effect of the graphite spheroidization-inhibiting elements. If the amount of RE added exceeds 0.009%, this does not have much influence in the case of flake graphite cast iron, but more chunky graphite, or abnormal graphite, are crystallized in the case of spheroidal graphite cast iron. Bad graphite shape results in degraded mechanical properties.

It is preferred that the amount of Ca added to the molten metal relative to the molten metal be 0.001% to 0.020%. Likewise, the amount of Al added to the molten metal relative to the molten metal is preferably 0.001% to 0.020%. If the amounts of Ca and Al added are less than 0.001%, sufficient graphite nucleation cannot be achieved. Also, if the amount of Ca or Al added exceeds 0.020%, the crystallization of abnormal or coarse graphite is more likely to occur. In that case, slag and dross are also more likely to be formed, resulting in defects in the finished product such as slag inclusion and pinholes.

The above inoculant can be used in the furnace right before the tapping of the melt. Also, it can be used for any known inoculation methods including a sandwich method, pouring box method, a stream inoculation method, an in-mold method, a cored wire method, and so forth.

The following depicts the composition of a preferred inoculant for the sandwich method, the pouring box method, the stream inoculation method, and the in-mold method.

Si: 30% to 80%

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

Ca: 0.1% to 1.3%

Al: 0.1% to 2.0%

Balance: Fe and inevitable impurities

Also, the following depicts the composition of a preferred inoculant for the cored wire method.

Si: 30% to 60%

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

Ca: 0.1% to 6.0%

Al: 0.1% to 6.0%

Balance: Fe and inevitable impurities

In this composition, the concentrations of Fe and Si are lower while the concentrations of the other constituent elements are higher. Thus, a sufficient inoculation effect can be obtained even if the feed amount of the wire is small. As a result, the inoculation treatment time can be shortened.

Whichever inoculation method or inoculant is employed, the amounts of the constituent elements added to the molten metal relative to the molten metal are set as described above.

The inoculants used in the present embodiment do not include magnesium (Mg). Thus, to produce spheroidal graphite cast iron products, a spheroidizing treatment is separately performed prior to the inoculation treatment, using a spheroidizing agent, which is different from the inoculant used in the inoculation treatment. The spheroidizing agent can be selected from any known ones. However, for the purpose of minimizing its influence on the inoculation treatment, it is preferred to use a spheroidizing agent that does not contain RE, Ca, and Al. An example is an Fe—Si—Mg agent (weight ratios of Fe:Si:Mg=45:45:10, 30:30:20, or 45:30:5).

It is known that a higher effect can be obtained if inoculation treatment is performed at a time point as close to pouring of the molten metal into the mold as possible. In light of the above, in the present embodiment, the inoculant used does not include Mg, which element contributes to spheroidization. A spheroidization treatment is performed by using a different spheroidizing agent. After the spheroidization treatment, an inoculation treatment is performed right before pouring. As a result, the inoculation effect can be enhanced.

In producing flake graphite cast iron product, the above inoculant can be added to the molten metal.

FIG. 6 is a schematic diagram illustrating the sandwich method. When the sandwich method which is widely used is employed, the reaction groove (pocket) at the bottom of the ladle is first filled with an inoculant. The base melt at a temperature of 1400° C. to 1500° C. is then poured into the ladle, thereby performing inoculation treatment.

In the molten metal treatment for spheroidal graphite cast iron, the inoculant can be placed over the surface of a spheroidizing agent in the reaction groove so that the inoculant can serve as a covering agent to lessen the reaction of Mg. Although the reaction of Mg may become intense if a large amount of Mg is added, it can be lessened by adding a large amount of Ca within the above-mentioned optimal range (0.001 to 0.02 wt % with respect to the entire molten metal).

FIG. 7 is a schematic diagram illustrating the cored wire method. With the cored wire method, inoculation treatment can be performed efficiently in a short time.

FIG. 8 is a schematic diagram illustrating the pouring box method and the in-mold method performed in a combined manner. It should be noted that only one of the two methods may be solely performed. Generally, by performing inoculation right before the pouring into the mold, the mechanical properties of the casting can be improved much more. Also, as illustrated in FIG. 9, the stream inoculation method and the pouring box method can be performed in a combined manner.

It is also preferred to perform several inoculations for the molten metal during the time period between after tapping from the melting furnace to the ladle and the completion of pouring by selecting two or more inoculation methods from among an inoculation for the molten metal in the ladle, a pouring box inoculation, an in-mold inoculation, and a stream inoculation. This further improves the mechanical properties of the casting. It should be noted, however, that in performing multiple inoculations, the total amount of each constituent element with respect to the molten metal has to fall within the aforementioned range.

It is preferred that the molten metal to which inoculation treatment has been performed (spheroidizing treatment in addition to the inoculation treatment in the case of spheroidal graphite cast iron) be poured into the mold at a temperature of 1300° C. to 1400° C. By doing so, a thick casting with excellent mechanical properties can be obtained. Although the foregoing inoculation methods of the present embodiment are not limited by the shape of castings, they exhibit an excellent effect especially when applied to thick-walled castings in which the eutectic solidification time is 1.0 ks or longer. In the case of larger or thicker castings in which the eutectic solidification time is longer, it is preferred to set the casting temperature to a slightly low temperature, for example, to a temperature between 1280° C. and 1360° C. for the purpose of ensuing favorable mechanical properties of the castings. In the case of spheroidal graphite cast iron, the preferred spheroidization temperature is 1400° C. to 1500° C.

Examples

A casting experiment was conducted in which flake graphite cast iron and spheroidal graphite cast iron were produced using the inoculants according to the above embodiment of the invention (examples) and inoculants as comparative examples. In the experiment, test pieces of 100 nm thick were casted using a mold designed such that the eutectic solidification time was equal to 1.2 ks. Such a long eutectic solidification time is suitable for examining the inoculation effects since it promotes crystallization of abnormal or coarse graphite.

In the experiment, the inoculants were made by, as stated earlier, dissolving predetermined amounts (described below) of RE, Ca, and Al in molten metal of Fe-50% Si (ferrosilicon) alloy, solidifying the molten metal, and then crushing it into granules. These inoculants were added using the pouring box inoculation method such that the addition amounts of the inoculant added to molten metal (melt) of 30 kg were as listed in Table 1, No. 1 through 20. When the spheroidal graphite cast iron product were casted, a spheroidizing agent, different from the inoculants, was placed in the reaction groove at the bottom of the ladle to perform a spheroidizing treatment in addition to the inoculation treatment. In No. 1 and 11, no inoculant was added. In the test pieces to which the inoculants were added, the addition amount of Ca was either 0.003%, 0.012%, or 0.03%, the addition amount of Al being either 0.003%, 0.012%, or 0.03%, the addition amount of RE being either 0.002%, 0.008%, or 0.020%.

The composition of the molten spheroidal graphite cast iron was 3.5% to 3.7% C, 2.4% to 2.6% Si, and 0.5% to 1.0% Mn. The composition of the molten flake graphite cast iron was 3.1% to 3.2% C, 1.5% to 1.7% Si, and 0.8% to 0.9% Mn.

The test pieces thus obtained were subjected to tensile test to measure the tensile strength and break elongation and the structure of them were observed.

The test results are shown in Table 1 below.

TABLE 1
					Tensile
		RE	Ca	Al	Strength	Elongation	Example/
No.	RE	(%)	(%)	(%)	(MPa)	(%)	Comparative Example
1	—	0	0	0	437	9	Comparative Example
2	Ce + La	0.002	0.003	0.003	445	12	Comparative Example
3	Ce + La	0.008	0.012	0.012	439	10	Comparative Example
4	Ce + La	0.2	0.03	0.03	440	9	Comparative Example
5	La	0.002	0.003	0.003	466	19	Example
6	La	0.008	0.012	0.012	455	18	Example
7	La	0.02	0.03	0.03	443	13	Comparative Example
8	Ce	0.002	0.003	0.003	461	20	Example
9	Ce	0.008	0.012	0.012	458	21	Example
10	Ce	0.02	0.03	0.03	449	14	Comparative Example
11	—	0	0	0	309	0.9	Comparative Example
12	Ce + La	0.002	0.003	0.003	317	0.7	Comparative Example
13	Ce + La	0.008	0.012	0.012	322	1	Comparative Example
14	Ce + La	0.02	0.03	0.03	320	0.8	Comparative Example
15	La	0.002	0.003	0.003	346	1.3	Example
16	La	0.008	0.012	0.012	343	1	Example
17	La	0.02	0.03	0.03	327	1.1	Comparative Example
18	Ce	0.002	0.003	0.003	348	1.2	Example
19	Ce	0.008	0.012	0.012	341	1.3	Example
20	Ce	0.02	0.03	0.03	330	1.1	Comparative Example

No. 1 through 10 in Table 1 are spheroidal graphite cast iron while No. 11 through 20 are flake graphite cast iron and depict their mechanical properties and the test conditions.

The following was confirmed as to the tensile strength of the spheroidal graphite cast iron. The tensile strength were below 450 MPa in the following cases: a case where no inoculant was added (No. 1); a case where the RE consisted of Ce and La (mischmetal was used as RE) (No. 2 to 4); and a case where the RE was only La or only Ce but the addition amount of RE was 0.02% (No. 7 and 10). However, in the other cases (No. 5, 6, 8, and 9), namely with the examples, the tensile strength was 450 Mpa or more.

Also, the following was confirmed as to the elongation of the spheroidal graphite cast iron. In the cases where only La or Ce was added as RE and the addition amounts of RE, Ca, and Al were low (No. 5, 6, 8, and 9), namely with the examples, the elongation was much larger than in the comparative examples (No. 1, 2 to 4, 7, and 10).

Further, as to the tensile strength of the flake graphite cast iron, all the test pieces exhibited a tensile strength of 300 MPa or more. It was confirmed that the tensile strength was increased by inoculation. Comparing the test pieces having the same addition amounts of RE, Ca, and Al, the tensile strength is higher in the case where only La or Ce was added as RE than the case where mischmetal was used as RE. Comparing the test pieces added with only La as RE, the tensile strength is higher in the case where the addition amounts of RE, Ca, and Al were small (No. 15 and 16) than the case where the addition amounts of RE, Ca, and Al were large (No. 17). Comparing the test pieces added with only Ce as RE, the tensile strength is higher in the case where the addition amounts of RE, Ca, and Al were small (No. 18 and 19) than the case where the addition amounts of RE, Ca, and Al were large (No. 20). Regarding the elongation of the flake graphite cast iron, there was little difference regardless of inoculation and the amounts of RE, Ca, and Al added.

FIG. 1 is a photograph of showing the structure of a spheroidal graphite cast iron in one example according to the invention, while FIG. 2 is a photograph showing the structure of a spheroidal graphite cast iron in a conventional example. FIG. 3 is a photograph depicting the structure of a flake graphite cast iron in one example according to the invention, while FIG. 4 is a photograph depicting the structure of a flake graphite cast iron in a conventional example. The inoculation increased the number of graphite grains in the spheroidal graphite cast iron and achieved finer graphite structures for the flake graphite cast iron. It was thus confirmed that the structures could be improved with the use of an inoculant having optimal amounts of RE, Ca, and Al.

Claims

1. A method for treating molten cast iron including, performing an inoculation treatment to the molten cast iron, with the use of an inoculant containing: 15 to 80 wt % Si; either 80 to 100 wt % purity La or 80 to 100 wt % purity Ce as RE; Ca; Al; and the balance Fe with inevitable impurities, by adding the inoculant to the molten cast iron such that: the addition amount of La or Ce relative to the molten cast iron is 0.001 to 0.009 wt %; the addition amount of Ca relative to the molten cast iron is 0.001 to 0.02 wt %; and the addition amount of Al relative to the molten cast iron is 0.001 to 0.02 wt %.

2. The method for treating molten cast iron according to claim 1, wherein, in the inoculant, the content of the Si is 30 to 80 wt %, the content of said 80 to 100 wt % purity La or said 80 to 100 wt % purity Ce is 0.1 to 0.6 wt %, the content of the Ca is 0.1 to 1.3 wt %, and the content of the Al is 0.1 to 2.0 wt %.

3. The method for treating molten cast iron according to claim 1, wherein in the inoculant, the content of the Si is 30 to 80 wt/%, the content of said 80 to 100 wt % purity La or said 80 to 100 wt % purity Ce is 0.3 to 1.8 wt %, the content of the Ca is 0.1 to 6.0 wt %, and the content of the Al is 0.1 to 6.0 wt %.

4. The method for treating molten cast iron according to claim 1, wherein the inoculant is in the form of granules each having a diameter of between 1 to 5 mm.

5. The method for treating molten cast iron according to claim 1, wherein the inoculant is in the form of chunks each having a length of between 5 to 70 mm.

6. The method for treating molten cast iron according to claim 1, wherein the inoculant is in the form of granules each having a diameter of between 0.1 to 1.0 mm, and wherein the inoculant are supplied to the molten cast iron with the granules being contained continuously in a core portion of a wire.

7. The method for treating molten cast iron according to claim 1, further including performing a graphite spheroidizing treatment using a spheroidizing agent different from the inoculant, wherein the processing temperature of the graphite spheroidizing treatment is between 1400° C. to 1500° C. and the pouring temperature at which the molten cast iron is poured into a mold is between 1270° C. to 1370° C.

8. A method for treating molten cast iron that performs a spheroidizing treatment and an inoculation by a sandwich method using the inoculant according to claim 1, and thereafter performs an inoculation in at least one of a ladle, a runner box, and a mold, using the inoculant.

9. The method for treating molten cast iron according to claim 8, wherein the method performs combined multiple inoculations including a stream inoculation during pouring into the mold, an in-mold inoculation, and a pouring box inoculation.

10. The method for treating molten cast iron according to claim 1, wherein the method includes performing, separately from the inoculation using the inoculant, a spheroidizing treatment using a spheroidizing agent containing magnesium.

11. The method for treating molten cast iron according to claim 2, wherein the inoculant is in the form of granules each having a diameter of between 1 to 5 mm.

12. The method for treating molten cast iron according to claim 2, wherein the inoculant is in the form of chunks each having a length of between 5 to 70 mm.

13. The method for treating molten cast iron according to claim 2, wherein the inoculant is in the form of granules each having a diameter of between 0.1 to 1.0 mm, and wherein the inoculant are supplied to the molten cast iron with the granules being contained continuously in a core portion of a wire.

14. The method for treating molten cast iron according to claim 2, further including performing a graphite spheroidizing treatment using a spheroidizing agent different from the inoculant, wherein the processing temperature of the graphite spheroidizing treatment is between 1400° C. to 1500° C. and the pouring temperature at which the molten cast iron is poured into a mold is between 1270° C. to 1370° C.

15. The method for treating molten cast iron according to claim 3, further including performing a graphite spheroidizing treatment using a spheroidizing agent different from the inoculant, wherein the processing temperature of the graphite spheroidizing treatment is between 1400° C. to 1500° C. and the pouring temperature at which the molten cast iron is poured into a mold is between 1270° C. to 1370° C.

16. A method for treating molten cast iron that performs a spheroidizing treatment and an inoculation by a sandwich method using the inoculant according to claim 2, and thereafter performs an inoculation in at least one of a ladle, a runner box, and a mold, using the inoculant.

17. A method for treating molten cast iron that performs a spheroidizing treatment and an inoculation by a sandwich method using the inoculant according to claim 3, and thereafter performs an inoculation in at least one of a ladle, a runner box, and a mold, using the inoculant.

18. The method for treating molten cast iron according to claim 2, wherein the method includes performing, separately from the inoculation using the inoculant, a spheroidizing treatment using a spheroidizing agent containing magnesium.

19. The method for treating molten cast iron according to claim 3, wherein the method includes performing, separately from the inoculation using the inoculant, a spheroidizing treatment using a spheroidizing agent containing magnesium.

20. The method for treating molten cast iron according to claim 4, wherein the method includes performing, separately from the inoculation using the inoculant, a spheroidizing treatment using a spheroidizing agent containing magnesium.