WO2011078500A2 - Cgi cast iron and a production method for the same - Google Patents

Abstract

The present invention relates to cast iron, and more specifically relates to CGI (Compacted Graphite Iron) cast iron in which the amounts of carbon (C), silicon (Si), manganese (Mn), copper (Cu), tin (Sn) and magnesium (Mg) are controlled during production, thereby improving the castability and also giving a stable tensile strength and yield strength and giving an appropriate range of hardness, and also to a production method for the same.

Description

CVI cast iron and its manufacturing method

The present invention relates to cast iron and a method for manufacturing the same, and more particularly, by controlling the content of the components added to the iron, castability is improved, having a stable tensile strength and yield strength and a manufacturing method thereof will be. The cast iron according to the present invention corresponds in particular to hypereutectic compacted graphite iron (CGI) cast iron which is applicable to the cylinder block of a high power diesel engine.

Due to the recent tightening of environmental regulations, it is inevitable to reduce the amount of environmental pollutants such as Cox and NOx emitted from engines.

In the case of diesel engines, it is necessary to increase the explosion pressure of the engine in order to reduce the emission of environmental pollutants such as Cox and NOx. As such, in order to increase the explosion pressure of the engine, the strength of the cylinder block of the engine must be increased.

The material commonly used in conventional cylinder blocks is cast iron, common grade cast iron is usually gray cast iron. Gray cast iron is called gray cast iron because it is gray when the carbon is separated and formed into graphite when cast.

In general, cast iron has a lot of difference depending on the shape, size, and distribution of graphite contained in the matrix. Generally, the cast iron, called cast iron, has a tensile strength of about 15 to 20 kg / mm ² . Gray cast iron as described above has excellent castability, vibration damping ability, and thermal conductivity, but has a limitation in being used as a cylinder block material of a high explosion pressure engine because of its low strength.

Spherical graphite cast iron is a cast iron that has improved the properties of such gray cast iron. Spherical graphite cast iron is cast iron that improves toughness by converting graphite present in ordinary cast iron (gray cast iron) tissues into spherical tissues from the original lobe-like tissues. Such nodular cast iron is also called nodular cast iron or ductile cast iron. The spheroidal graphite cast iron has excellent abrasion resistance, heat resistance, corrosion resistance, and the like, and has a higher elastic modulus than general gray cast iron, has a Brinell hardness of about 200, and is also better than normal cast iron having the same hardness.

By the way, spherical graphite cast iron has the high strength required by the cylinder block, but lacks castability to make a complicated shape, and has a low thermal conductivity, and thus is limited to be applied to a cylinder block having a complicated shape.

Accordingly, CGI (compacted graphite iron) cast iron, which has excellent castability, vibration damping ability and thermal conductivity of gray iron, and has both high strength and constant elongation of spheroidal graphite iron, has recently been applied as a next-generation cylinder block material.

The CGI cast iron precisely increases the content of magnesium (Mg) when tapping a nodular graphite iron melt obtained by melting a nodular cast iron material in a furnace with a ladle, which is a mechanism for transferring a molten metal melted in a furnace to another place. Can be manufactured by controlling.

In order to secure stable mechanical properties (tensile strength) in CGI cast iron, it is necessary to precisely control the melting / melting temperature and the amount of magnesium. For this purpose, precise control device, the skill of the operator, and the use of high grade pig iron with low impurity content Things are required. In spite of such precise control, there are problems such as frequent defects in material and casting of CGI cast iron due to errors generated in various requirements such as magnesium content, tapping location, tapping temperature, tapping speed, and the like.

In the present invention, by controlling the content of carbon (C), silicon (Si), manganese (Mn), copper (Cu), tin (Sn) and magnesium (Mg) in a range to prevent stable physical properties and casting failure To provide cast iron with tensile strength and yield strength in the range of 500-600MPa and 350-450MPa, respectively.

The present invention has an object to provide a cast iron having a stable physical properties and structure by precisely controlling the amount of magnesium (Mg). In particular, it is an object of the present invention to provide cast iron that can be applied to a cylinder block of a high power high horsepower diesel engine.

The present invention also aims to establish a chemical composition and manufacturing method capable of producing cast iron for cylinder blocks applicable to high power high horsepower diesel engines having stable tensile strength and yield strength and having appropriate hardness.

The present invention is 3.65 to 3.75 wt% of carbon (C), 2.0 to 2.25 wt% of silicon (Si), 0.3 to 0.6 wt% of manganese (Mn), 1.2 to 1.4 wt% of copper (Cu), and tin (Sn). ) 0.07 to 0.10% by weight, magnesium (Mg) 0.008 to 0.018% by weight, phosphorus (P) 0.04% by weight or less, sulfur (S) 0.02% by weight and the remaining amount of iron (Fe) is provided.

According to an example of the present invention, the tensile strength of the cast iron is in the range of 500 to 600 MPa. In addition, according to another embodiment of the present invention, the yield strength of the cast iron (Yield Strength) is in the range of 350 to 450Mpa. Meanwhile, the Brinell hardness value (BHW) of the cast iron is in the range of 255 to 280.

According to an example of the present invention, the carbon equivalent (CE: Carbon Equivalent) of the cast iron is in the range of 4.35 to 4.5.

According to an example of the present invention, the nodularity of graphite formed by carbon in the cast iron may be about 5 to 20%.

The present invention also relates to 3.65 to 3.75 wt% of carbon (C), 2.0 to 2.25 wt% of silicon (Si), 0.3 to 0.6 wt% of manganese (Mn), 0.04 wt% or less of phosphorus (P), and sulfur (S). ) Melting the cast iron material containing 0.02% by weight or less and the remaining amount of iron (Fe) in the furnace to produce a cast iron wontang; 1.2 to 1.4 wt% of copper (Cu), 0.07 to 0.10 wt% of tin (Sn), and 0.008 to 0.018 wt% of magnesium (Mg) are placed in a ladle, which is a container for tapping the molten cast iron molten metal in the furnace. Making; The prepared cast iron wontang is melted with the ladle disposed by 1.2 to 1.4 wt% of the copper (Cu), 0.07 to 0.10 wt% of tin (Sn), and 0.008 to 0.018 wt% of magnesium (Mg) to prepare a cast iron melt. step; Determining the amount of magnesium to be added by grasping the magnesium content contained in the cast iron melt contained in the ladle; Adding magnesium to the cast iron melt contained in the ladle in an amount of magnesium to be added determined above; And injecting the molten cast iron added with magnesium into a mold.

In the present invention, after a predetermined amount of magnesium (Mg) and a suitable amount of copper (Cu) and tin (Sn) is added to a ladle, which is a container for tapping the molten cast iron molten metal in a furnace, the cast iron molten metal is poured into the ladle to become stable CGI. The graphite is crystallized to obtain CGI cast iron having stable mechanical properties.

According to an example of the present invention, the carbon equivalent (CE: Carbon Equivalent) in the cast iron wontang is adjusted to be about 4.35 ~ 4.5.

In addition, according to an example of the present invention, the tapping temperature of the raw water is adjusted to 1520 ± 10 ℃.

According to an example of the present invention, the magnesium addition step may be inoculated magnesium using a magnesium inoculant in the form of a wire (wire).

In the case of the cast iron according to the present invention, it is possible to predict the spheroidization rate of the graphite contained in the cast iron through the content of magnesium contained in the molten cast iron, it is possible to estimate the range of the strength according to the spheroidization rate.

In the present invention, the content of magnesium may vary depending on the strength required, so that the content of magnesium may be 0.008 to 0.018% by weight in order to be applied to a cylinder block of a high power diesel engine.

According to the present invention, the amount of magnesium (Mg) is precisely controlled, and the amount of copper (Cu) and tin (Sn) is controlled so that tensile strength in the range of 500 to 600 MPa, yield strength in the range of 350 to 450 MPa, and 255 Cast iron having Brinell hardness in the range of from 280 to 280 may be provided.

Cast iron according to the present invention has a stable tensile strength and yield strength and has an appropriate hardness, it can be applied to the manufacture of cylinder block applicable to high power high horsepower diesel engine.

According to the present invention, it is also possible to produce a CGI cast iron having a homogeneous structure with a high strength enough to be used for cylinder blocks for high horsepower diesel engines by finely controlling the amount of magnesium (Mg). In addition, by controlling the amount of copper (Cu) and tin (Sn) of the alloying elements, it is possible to produce a CGI cast iron that can have a variety of hardness and tensile strength.

1 is a graph illustrating the relationship between magnesium (Mg) content and the spheroidization rate of graphite.

FIG. 2 is a graph illustrating the relationship between the spheroidization rate of graphite, tensile strength, and yield strength. FIG.

Figure 3 is a diagram showing the relationship between magnesium (Mg) content and tensile strength and the appearance of the representative structure of cast iron. For reference, 1 MPa corresponds to 1 N / mm ² .

Figure 4 briefly shows an example of the manufacturing process of cast iron according to the present invention.

Hereinafter, the present invention will be described in more detail with reference to specific examples.

Cast iron according to the present invention is 3.65 to 3.75% by weight of carbon (C), 2.0 to 2.25% by weight of silicon (Si), 0.3 to 0.6% by weight of manganese (Mn), 1.2 to 1.4% by weight of copper (Cu), 0.07-0.10 weight percent tin (Sn) and 0.008-0.18 weight percent magnesium (Mg); Phosphorus (P) comprises up to 0.04% by weight and sulfur (S) comprises up to 0.02% by weight; It contains the balance of iron (Fe). Cast iron of the present invention is based on iron (Fe),

Next, the components and the content of the cast iron in the present invention will be described.

1) Carbon (C) 3.65 ~ 3.75wt%

Carbon is added for crystallization of compacted graphite. In the cast iron according to the present invention, when the carbon content is less than 3.65 wt%, the chilling behavior is observed in the thin-walled part, and 3.75 wt% Exceeding% causes graphite spheroidization shrinkage and poor flow. Therefore, in order to prevent such a defect in a high-strength cylinder block having a variety of thickness in the present invention, the content of carbon is limited to 3.65 ~ 3.75% by weight.

2) Silicon (Si) 2.0 ~ 2.25 wt%

Silicon, when added in the optimum ratio with carbon, maximizes the amount of compacted graphite and increases the strength of cast iron. In the cast iron according to the present invention, if the silicon content is less than 2.0% by weight, the problem of lowering the amount of reinforced graphite crystals occurs, and if the content exceeds 2.25% by weight, the ductility is lowered, so that the content is 2.0 to 2.25% by weight. Set it.

3) Manganese (Mn) 0.3 ~ 0.6% by weight

Manganese is added for the refinement of graphite and stabilization of pearlite. In the cast iron according to the present invention, when the content of manganese is less than 0.3% by weight, the hardness decreases, and when it exceeds 0.6% by weight, brittleness increases, so that the content of 0.3 ~ Set to 0.6% by weight.

4) Copper (Cu) 1.2 ~ 1.4 wt%

Copper is an element for strengthening graphite, and is an element necessary for securing strength because it serves to promote and refine pearlite production. In the cast iron according to the present invention, if the copper content is less than 1.2% by weight, the lack of strength is caused, but even if the content exceeds 1.4% by weight, there is no addition effect corresponding to the excess. Therefore, in the present invention, the content of copper is set to 1.2 to 1.4% by weight.

5) Tin (Sn) 0.07 ~ 0.10 wt%

Tin is a very powerful pearlite production promoting element and, like copper, is added for the purpose of improving strength. In the cast iron according to the present invention, when the content of tin is less than 0.07% by weight, a decrease in strength is caused, and when it is added in excess of 0.10% by weight, brittleness is rapidly increased, so the content is set to 0.07 to 0.10% by weight. do.

6) Magnesium (Mg) 0.008 ~ 0.018 wt%

Magnesium plays a role in the nodularity of graphite and at the same time promotes the nucleation and growth of compacted graphite. In the cast iron according to the present invention, when the magnesium content is less than 0.008% by weight, the graphite is flattened, and when the content of magnesium is more than 0.018% by weight, the spheroidization rate of the graphite is increased to cause shrinkage, so that the content is 0.008 to 0.018% by weight. It is limited to a range.

7) Phosphorus (P) 0.04 wt% or less

Phosphorus is also a kind of impurities that are naturally added in the manufacturing process of cast iron in the air. This phosphorus also serves to stabilize the pearlite, but when the content exceeds 0.04% by weight, brittleness increases rapidly, which is associated with shrinkage defects due to segregation. Therefore, in the cast iron according to the present invention, it is preferable to manage the phosphorus content to be 0.04% by weight or less.

It is practically difficult to make the phosphorus content to be zero in the cast iron raw material components, even if the phosphorus content is 0 in the cast iron raw material components will be contained in the manufacturing process of cast iron. Therefore, in the present invention, it is important that the content of phosphorus does not exceed 0.04% by weight.

8) Sulfur (S) 0.02 wt% or less

Sulfur acts as a source of compacted graphite, but when the content exceeds 0.02% by weight, the addition of more magnesium is required for the production of reinforced graphite. That is, if the content of sulfur in the magnesium content is limited to more than a certain range, the problem that the graphite is compacted (compacted graphite) occurs. Therefore, in the cast iron according to the present invention, the content of sulfur should be controlled to 0.02% by weight or less.

It is practically difficult to make the sulfur content in the cast iron raw material also equal to zero, and even if the sulfur content in the cast iron raw material is zero, sulfur will be contained in the manufacturing process of the cast iron. Therefore, in the present invention, it is important that the content of sulfur does not exceed 0.02% by weight.

9) Iron (Fe)

Iron is the main body of cast iron according to the invention. The remaining component other than the above components is iron.

According to an example of the present invention, the carbon equivalent is set to 4.35 to 4.5. In the cast iron according to the present invention, when the carbon equivalent is less than 4.35, a tendency of the thinning of the thin-walled part occurs, and when it exceeds 4.5, shrinkage is caused by excessive generation of primary graphite. And since the flow failure occurs, the range is limited to 4.35 ~ 4.5. At this time, the carbon equivalent (CE) is defined as carbon + (silicon +) X 1/3, the value can be adjusted to control the properties and quality of the product.

According to one embodiment of the present invention, the tensile strength of the cast iron (Tensile Strength) is 500 ~ 600MPa, yield strength (Yield Strength) is 350 ~ 450MPa.

According to an example of the present invention, in the cast iron, the nodularity rate of the graphite formed by the carbon is 5 to 20%.

The manufacturing process of the cast iron according to the present invention will be described with reference to FIG.

According to the manufacturing method of cast iron according to the present invention, first, the carbon (C) 3.65 ~ 3.75% by weight, silicon (Si) 2.0 ~ 2.25% by weight, manganese (Mn) 0.3 ~ 0.6% by weight, phosphorus (P) The cast iron material containing more than 0 and less than 0.04% by weight, sulfur (S) is more than 0 and less than 0.02% by weight and the remaining amount of iron (Fe) in the furnace 100 to prepare a cast iron melt 110 molten metal.

1.2 to 1.4 wt% of copper (Cu), 0.07 to 0.10 wt% of tin (Sn), and magnesium (ladle), which is a container for tapping the molten cast iron molten metal melted in the furnace, Mg) to prepare a ladle 200 by placing by 0.008 ~ 0.018% by weight.

The produced cast iron melt bath was melted with the ladle 200 disposed by 1.2 to 1.4 wt% of copper (Cu), 0.07 to 0.10 wt% of tin (Sn), and 0.008 to 0.018 wt% of magnesium (Mg). The melt 110 is prepared.

According to an example of the present invention, the tapping temperature may be adjusted to be 1520 ± 10 ° C.

On the other hand, according to an example of the present invention, the carbon equivalent (CE: Carbon Equivalent) in the cast iron wontang can be adjusted to be 4.35 ~ 4.5.

The amount of magnesium to be added is determined by grasping the magnesium content contained in the molten cast iron contained in the ladle 200.

Here, since the cast iron wontang was transferred to the ladle which originally contained magnesium, the molten cast iron contained in the ladle contains a predetermined amount of magnesium. Nevertheless, in order to take into account the loss of magnesium during ladle transfer and to control the magnesium content more precisely, once again the magnesium content contained in the molten cast iron contained in the ladle is again determined, and when it is determined that addition of magnesium is necessary, Add magnesium.

According to an example of the present invention, the thermal analyzer 300 may be used to determine the magnesium content.

The amount of magnesium to be added determined above is added to the cast iron melt contained in the ladle. According to an example of the present invention, magnesium may be added using magnesium 500 in the form of wire.

In addition to magnesium, other inoculants commonly used in the manufacture of cast iron may be added. For example, silicone based inoculants may be further added. The silicone-based inoculant may be purchased commercially available. The type and content of the inoculant can be easily selected and determined by those skilled in the art as needed. The other inoculants may also be wire 500.

Subsequently, the molten solution to which magnesium is added is injected into the mold 400 to complete cast iron.

<Example 1-10 and Comparative Example 1-10>

To cast iron according to Example 1-10 and Comparative Example 1-10 according to the composition of Table 1 below.

Table 1 Unit: weight% division C Si Mn Cu Sn Mg P S Fe Example 1 3.650 2.240 0.440 1.360 0.090 0.017 0.031 a very small amount Remaining amount Example 2 3.680 2.190 0.400 1.340 0.090 0.010 0.031 a very small amount Remaining amount Example 3 3.700 2.110 0.390 1.410 0.080 0.011 0.031 a very small amount Remaining amount Example 4 3.710 2.080 0.400 1.330 0.100 0.009 0.034 a very small amount Remaining amount Example 5 3.680 2.140 0.380 1.260 0.080 0.011 0.031 a very small amount Remaining amount Example 6 3.680 2.180 0.430 1.340 0.090 0.018 0.026 a very small amount Remaining amount Example 7 3.710 2.130 0.400 1.290 0.070 0.013 0.031 a very small amount Remaining amount Example 8 3.690 2.050 0.400 1.350 0.100 0.014 0.033 a very small amount Remaining amount Example 9 3.710 2.130 0.400 1.290 0.070 0.013 0.031 a very small amount Remaining amount Example 10 3.700 2.140 0.430 1.320 0.090 0.017 0.030 a very small amount Remaining amount Comparative Example 1 3.810 2.220 0.110 1.150 0.090 0.012 0.034 a very small amount Remaining amount Comparative Example 2 3.710 2.090 0.290 0.820 0.070 0.008 0.024 a very small amount Remaining amount Comparative Example 3 3.780 1.970 0.110 0.870 0.070 0.000 0.019 a very small amount Remaining amount Comparative Example 4 3.690 2.220 0.080 1.100 0.120 0.011 0.036 a very small amount Remaining amount Comparative Example 5 3.780 2.010 0.110 0.890 0.070 0.000 0.018 a very small amount Remaining amount Comparative Example 6 3.620 2.230 0.480 1.650 0.090 0.020 0.028 a very small amount Remaining amount Comparative Example 7 3.720 2.090 0.460 1.390 0.090 0.022 0.029 a very small amount Remaining amount Comparative Example 8 3.710 2.090 0.430 1.430 0.100 0.024 0.025 a very small amount Remaining amount Comparative Example 9 3.650 2.190 0.370 1.230 0.090 0.020 0.031 a very small amount Remaining amount Comparative Example 10 3.690 2.370 0.290 0.810 0.070 0.017 0.023 a very small amount Remaining amount

First, according to the composition of Table 1, a raw water containing carbon (C), silicon (Si), manganese (Mn), phosphorus (P) was prepared. Sulfur (S) is an element that is inevitably included in the manufacturing raw material and manufacturing process of cast iron, but not added separately, but the content is less than 0.02% by weight.

Before tapping, a carbon equivalent (CE) was measured using a CE meter to adjust the content of carbon, and the raw water was prepared by adjusting it to 1,146 ± 1 ° C based on TL (liquid line temperature).

Ladle was prepared by adding magnesium (Mg), copper (Cu) and tin (Sn), and the raw water was tapped into the ladle while maintaining the tapping temperature uniformly around about 1,520 ° C.

The amount of magnesium to be added was determined by thermal analysis of the tapping hot water in consideration of the amount of magnesium to be finally included, and magnesium in the form of a wire was adjusted in the mold at 1,410 ± 10 ° C.

The carbon equivalent (C.E.), tensile strength (TS), yield strength (YS), hardness (Hardness) and nodularity of the cast iron prepared according to the composition of Table 1 were measured and shown in Table 2. Hardness here is Brinell hardness, and HBW Brinell hardness value.

TABLE 2 division CE Tensile Strength (N / mm ² ) Yield strength (N / mm ² ) Hardness (HBW) Nodularity (%) Example 1 4.410 514.0 422.0 267.3 17.8 Example 2 4.420 521.0 342.5 259.5 8.5 Example 3 4.410 537.0 418.5 260.5 7.5 Example 4 4.410 550.8 418.8 273.3 11.6 Example 5 4.400 562.5 424.5 265.0 18.0 Example 6 4.420 570.0 434.0 271.3 19.9 Example 7 4.430 576.3 427.3 261.8 18.2 Example 8 4.380 579.5 441.8 279.0 14.4 Example 9 4.430 584.0 430.8 272.5 18.3 Example 10 4.420 584.8 438.3 269.0 21.1 Comparative Example 1 4.560 356.3 371.0 261.5 9.2 Comparative Example 2 4.410 412.5 345.8 244.3 17.5 Comparative Example 3 4.440 480.3 379.5 248.0 Not measurable Comparative Example 4 4.440 480.7 372.0 256.7 5.1 Comparative Example 5 4.460 481.0 390.0 248.0 Not measurable Comparative Example 6 4.370 602.8 443.8 275.3 22.0 Comparative Example 7 4.430 609.0 433.3 261.8 36.4 Comparative Example 8 4.420 629.0 442.5 273.8 29.7 Comparative Example 9 4.390 636.3 462.3 281.8 42.0 Comparative Example 10 4.490 654.3 448.0 266.0 59.3

As seen above, cast iron according to an embodiment of the present invention has a tensile strength in the range of 500 to 600 MPa (N / mm ² ), and yield strength of 350 to 450 MPa (N / mm ^2). It can be seen that the HBW Brinell hardness value is in the range of 255 to 280.

As described above, the cast iron according to the present invention has stable tensile strength and yield strength and has an appropriate hardness, and thus can be easily applied to the manufacture of a cylinder block applicable to a high power high horsepower diesel engine.

For reference, the results of observing the relationship between the spheroidization rate and magnesium content of the cast iron is shown in FIG. As shown in FIG. 1, in the case of the CGI cast iron according to the present invention, the sphericity ratio was found to be in the range of 5 to 20%.

The relationship between the tensile strength and yield strength according to the spheroidization rate of the CGI cast iron prepared above may be referred to FIGS. 2 and 3. As shown in Figures 2 and 3, the tensile strength and the yield strength can be cast iron in the range of 500 ~ 600MPa and 350 ~ 450Mpa, respectively, to have a good quality.

Claims (9)

Carbon (C) 3.65-3.75 wt%, Silicon (Si) 2.0-2.25 wt%, Manganese (Mn) 0.3-0.6 wt%, Copper (Cu) 1.2-1.4 wt%, Tin (Sn) 0.07- Cast iron comprising 0.10% by weight, magnesium (Mg) 0.008-0.018% by weight, phosphorus (P) 0.04% by weight or less, sulfur (S) 0.02% by weight and the balance of iron (Fe). The cast iron according to claim 1, wherein the tensile strength is 500 to 600 MPa. The cast iron according to claim 1, wherein yield strength is 350 to 450 MPa. The cast iron according to claim 1, wherein the carbon equivalent (CE) is 4.35 to 4.5. The cast iron according to claim 1, wherein the nodularity rate of the graphite formed by the carbon is 5 to 20%. 3.65-3.75 wt% of carbon (C), 2.0-2.25 wt% of silicon (Si), 0.3-0.6 wt% of manganese (Mn), 0.04 wt% or less of phosphorus (P) 0, sulfur (S) 0 Melting a cast iron material containing more than 0.02% by weight and the balance of iron (Fe) in a furnace to produce a cast iron wontang; 1.2 to 1.4 wt% of copper (Cu), 0.07 to 0.10 wt% of tin (Sn), and 0.008 to 0.018 wt% of magnesium (Mg) are placed in a ladle, which is a container for tapping the molten cast iron molten metal in the furnace. Making; The prepared cast iron wontang is melted with the ladle disposed by 1.2 to 1.4 wt% of the copper (Cu), 0.07 to 0.10 wt% of tin (Sn), and 0.008 to 0.018 wt% of magnesium (Mg) to prepare a cast iron melt. step; Determining the amount of magnesium to be added by grasping the magnesium content contained in the cast iron melt contained in the ladle; Adding magnesium to the cast iron melt contained in the ladle in an amount of magnesium to be added determined above; And Injecting the molten magnesium cast iron into a mold; Cast iron manufacturing method comprising a. The method of claim 6, wherein the carbon equivalent (CE: Carbon Equivalent) in the cast iron wontang manufacturing method characterized in that the 4.35 ~ 4.5. The method of claim 6, wherein the tapping temperature is adjusted to 1520 ± 10 ℃. The method of claim 6, wherein in the magnesium addition step, the magnesium is added by using magnesium in a wire form.

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