EP1870182B1

EP1870182B1 - Process for the casting of molten alloy

Info

Publication number: EP1870182B1
Application number: EP06731431.0A
Authority: EP
Inventors: Setsuo Mishima; Yasushi Ishimoto; Takanori Aikawa
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2005-04-11
Filing date: 2006-04-07
Publication date: 2016-10-19
Anticipated expiration: 2026-04-07
Also published as: WO2006109739A1; CN101155653A; EP1870182A4; CN101155653B; EP1870182A1; JPWO2006109739A1; JP4548483B2

Description

Technical Field

The present invention relates to a process for casting a molten alloy, which can prevent segregation such as centerline segregation and/or inverse V segregation, and form a fine structure.

Background Art

Conventionally, the vacuum arc remelting process (VAR) and the electroslag remelting process (ESR) are frequently used as a process for the casting of a molten alloy, which form less segregation and a fine structure is obtained. Since a molten alloy is solidified in a solidification space enclosed by the wall of a water-cooled mold in these processes while a molten alloy pool is formed, the solidification space is small so that solidification is made in a stacking manner, which is commonly referred to as stacking solidification.
The stacking solidification can decrease generation of segregation, such as centerline segregation or inverse V segregation, caused in casting into an ingot by virtue of a small solidification space. There is also an advantage that fine and homogenous structure is obtained, since the cooling rate can be increased by the use of a water-cooled mold.
Although the remelting processes have advantageous features in this manner, both VAR and ESR need to manufacture a remelting electrode, and many processes and energy for remelting are required.
Patent Document 1 discloses, as a process for solution of such problem, a technology based on the ESR process in which the effect of refinement can be expected by slag reaction. The process includes producing fine droplets of molten alloy without the use of an electrode, adding a heated and melted slag layer to the molten alloy, and drawing an ingot from a bottom of a refined layer.
Patent Document 1: JP-A-62-4840
JP 2004 098092 A discloses a casting process with the features of the preamble of claim 1. A similar casting process is also suggested in JP 10 328 792 A . In EP-A-0 448 773 , the inner walls of the mold are lined with heat-conductive ceramic tiles. The article "Continuous casting of steel: a close connection between solidification studies and industrial process development", M. Bobadilla et a., Materials Science and Engineering, Vol. A173, , 1993, pp. 275-285, suggests the placement of metallic or refractory inserts against the inner wall of the mold. JP 11 300 448 A shows a heat insulator provided inside or at the outer surface of the wall of the mold.

Disclosure of the Invention

Problems to be solved by the Invention

The specific process described in the Patent Document 1 is a continuous casting process of holding a molten steel in a refining tank and drawing the molten steel from a water-cooled mold provided separately. Although this is referred to as stacking solidification in the Patent Document 1, stacking solidification in the technical field of the ESR and VAR processes is one solidified in a stacking manner as described above, and is different from that in the process disclosed in the Patent Document 1.
Since Patent Document 1 employs the continuous casting process, which is effective in producing the effect of refinement by slag which is one of the effects in application of ESR, but in which the ingot is forced to be drawn only with its surface solidified, the Patent Document 1 involves a problem that there is a possibility of generation of such deficiency as centerline segregation or center porosity, especially in high alloy. It also involves a problem that it is not possible to produce a small solidification space to provide a fine and homogeneous structure, which is an important advantage of ESR.
It is an object of the invention to provide a novel process for the casting of molten alloy, which is free from the adverse effects brought about by the solidified slag in drawing an ingot, so that segregation is suppressed in a solidified steel ingot and fine structure are attained, as well as surface texture of the steel ingot is improved.

Means for Solving the Problems

The inventors of the invention have found that a small molten alloy pool, which is approximate to that in ESR, can be formed in a solidification space, when the molten alloy is supplied to the solidification space, in which slag is held, under the condition of a very low casting rate of not more than 0.3 m/min. It has been also found that an ingot having a fine and homogeneous structure can be obtained by slag having the effects of thermal insulation and shielding of a molten alloy pool surface from outside air. However, this process has confronted problems, which are not involved in ESR, that refinement of the structure is obstructed by a slag layer present on an outer periphery of an ingot, and that crack is generated on the slag layer and a solidified shell as to cause effusion and breakout of the molten steel in a worse case. The cause therefor has been searched. It has been ascertained that, a part or a whole of a solidified slag portion formed on the meniscus on the inside wall of a water-cooled mold descends inadvertently together with the ingot when an ingot is drawn, or that an initially solidified shell is broken by resistance when it leaves the solidified slag portion.
Therefore, in order to realize a stable work, it is necessary to eliminate the adverse effect brought about by the solidification of slag in drawing an ingot. This is thought to be a problem in behaviors of a mold wall, a solidified shell, and slag, which contact dynamically at a very low speed, and a peculiar problem in the case where the presence of slag and the very low feeding rate of a molten alloy are combined together.
The inventors have reached the invention ascertaining that the direct cause for roughening the surface texture of the steel ingot, when stacking solidification like ESR is performed by supplying a molten alloy toward a water-cooled mold made of metal such as copper, iron, etc. through slag at a very low rate, resides in that the extraction of heat in the vicinity of a boundary between a solidified shell and slag, that is, in the vicinity of a meniscus position of the molten alloy pool is excessively large to lead to solidification of slag in a wide range, and drawing is made while a heat insulating layer is formed, or the cause resides in breakage of an initially solidified shell. They find that a steel ingot is considerably improved in surface texture, without inhibiting cooling, by arranging a heat insulator in the vicinity of the boundary.
The invention provides a process for casting a molten alloy as defined in claim 1, the process comprising pouring the molten alloy into a solidification space enclosed by an inside wall of a water-cooled mold and holding slag therein from a container holding the molten alloy therein to solidify the molten alloy while forming a molten alloy pool, and drawing an ingot vertically from a lower section of the water-cooled mold depending on the pouring rate of the molten alloy, wherein a heat insulator for suppressing an extraction of heat from slag is arranged in an upper section of the mold, the insulator having an inside wall shaped to be contiguous to the inside wall of the water-cooled mold.
In the invention, preferably, an upper surface position of a meniscus in the molten alloy pool is controlled to come within a range where the heat insulator is arranged, in a steady state of casting.
The inside wall of the heat insulator in the invention is contiguous to the inside wall of the water-cooled mold on a downstream side.
The heat insulator is made of a graphite material.
The casting rate of the molten alloy in the invention is 0.01 to 0.1 m/min, more preferably at most 0.08 m/min, and still more preferably at most 0.05 m/min.
In the invention, casting is carried out while slag is heated by heating means. Slag applied in the invention is one having a low melting point of 500 to 1400°C and a thickness of 20 to 100 mm.
As a molten alloy applied to the invention, such an alloy can be applied as are especially difficult in obtaining fine grains and decreasing component segregation, such as tool steel such as cold die steel, hot die steel, or high-speed tool steel, other high alloy steel, or alloy steel applied to ESR.
Specifically, the invention is desirably applied to the casting of a molten alloy which contains, by mass%, Fe as a main component, not more than 3.0% of C, and not less than 5% of other metallic elements than Fe, and more desirably the molten alloy contains 0.1 to 3.0 % of C, by mass%.

Advantages of the Invention

According to the invention, since a stackingly solidified ingot can be obtained directly from a molten alloy, it is possible to ensure low cost and high productivity. The invention is especially made effective because of a considerable decrease in processes when it is applied to the manufacture of a high alloy which is liable to generate segregation and deterioration in surface texture.

Best Mode for Carrying Out the Invention

As described above, the invention has an important feature in a casting process for feeding a molten alloy into a solidification space, in which slag is held, at a very low rate while the extraction of heat from slag is suppressed by a heat insulator arranged on an upper portion of a water-cooled mold.
Specifically, in the invention, a molten alloy is poured into the solidification space which is enclosed by the inside wall of a water-cooled mold and in which slag is held, from a container, such as tundish, which holds the molten alloy therein.
In order to achieve stacking solidification without the use of an electrode employed in the remelting method, a molten alloy is poured at a very low rate of not higher than 0.3 m/min and the molten alloy as fed is rapidly solidified while forming a molten alloy pool. Therefore, a solidification space is enclosed by the inside wall of a water-cooled mold.
According to the invention, an ingot is vertically drawn from the lower portion of the water-cooled mold depending on the pouring rate of the molten alloy (so that upper surface position of a meniscus be maintained substantially in a predetermined position). Thereby, the meniscus having a predetermined shape can be formed to obtain a stacked solidified ingot having a fine and homogeneous structure similar to that by ESR.
However, the roughened surface of an ingot cannot be improved only by the above method, and thus the invention employs an approach, in which casting is made while the extraction of heat from slag is suppressed by a heat insulator arranged on the upper portion of a water-cooled mold and having an inside wall contiguous to the inside wall of the water-cooled mold.
The heat insulator enables suppressing slag from being excessively solidified at a boundary between the slag and a solidified shell, and drawing of an ingot is inhibited from causing simultaneous descent of a solid slag and breakage of a solidified slag. Thus, it is possible to prevent effusion of a molten steel caused by inadvertent crack of slag or crack of an underdeveloped solidified shell, enabling not only an improvement in ingot surface but also an improvement in ingot cooling.
In addition, the reason why an inside wall shaped to be continuous to the inside wall of a water-cooled mold is provided in the invention is that in case where the inside wall forms a large, discontinuous step or a clearance, a surplus stress acts on a solidified shell as formed when it slides on a mold wall surface, and thus the solidified shell is broken to lead to degradation of an ingot surface.
In order to guide an unstable, solidified shell formed on the inside wall of the heat insulator to the inside wall of the water-cooled mold on a downstream side without the action of a surplus stress, it is effective that the heat insulator be shaped to have an inside wall having a substantially the same shape as that of the inside wall of the water-cooled mold on the downstream side in a cross section perpendicular to a direction in which an ingot is drawn.
The heat insulator according to the invention is positioned within that range, which is effective in restricting the extraction of heat from slag. The solidification of slag has a most adverse effect on the boundary between slag in the vicinity of the mold and the meniscus of a molten alloy pool, that is, in a position where a solidified shell is formed.
The invention relates to dynamic casting and thus is effective to demonstrate the effect of suppressing the extraction of heat from slag by the heat insulator especially in a position where a solidified shell is formed, in the steady state of casting.
The position where the solidified shell is formed is close to an upper surface position of a meniscus of the molten alloy pool. Since the upper surface position of the meniscus is easily detected as an object to be controlled, it is desired to control the upper surface position of the meniscus in the molten alloy pool to be within the range where the heat insulator is arranged, in the steady state of casting.
In the invention, a lower end position of the heat insulator arranged in the vicinity of a boundary between a solidified shell and slag preferably comes within the range of 100 mm downward from a controlled position of the molten alloy surface (upper surface position of the meniscus), since the original cooling function is degraded when the heat insulator excessively covers the water-cooled mold. Taking into consideration fluctuation of the controlled position of the molten alloy surface during casting, the lower end position of the heat insulator preferably comes out of the range of 10 mm downward from the upper surface position of the meniscus.
On the other hand, an upper end position of the heat insulator is preferably arranged above an upper surface of the slag. This is for the sake of handling in mounting of the heat insulator and thermal insulation over a slag region.
Furthermore, in order to suppress growth of the solidification of slag on an inside surface of the heat insulator, it is applicable, according to need, to decrease heat loss by thickening the heat insulator in a region corresponding to a slag layered portion or providing with a further heat insulating layer on an outer periphery of the heat insulator.
As a heat insulator in the invention, preferable is a material which has excellent resistance to slag errosion and slidability in addition to heat resistance. While ceramics, etc. can be used, ceramics containing graphite is effective with respect to slidability. Furthermore, a graphite material having excellent resistance to slag errosion, moldability, and slidability is arranged.
In addition, it does not matter whether a container for holding a molten alloy in the invention comprises any one as well as one called tundish. For example, it may be a container having heating means according to need.
While a casting mold formed by a water-cooled mold preferably has a circular shape from the viewpoint of uniformity in a configuration of solidification, it may have elliptical or rectangular shape taking into consideration a shape of an ingot, manufacturability, etc. It is desired, in order to make a small solidification space, that a water-cooled mold be made of a metal having excellent heat conductivity, such as iron or copper.
In the invention, slag is important to insulation of heat and shielding of a surface of the molten alloy pool from outside air, as well as to the refining action such as trapping of inclusion, or desulfurizing of a molten alloy.
A method of feeding a molten alloy is not limited in the invention. It is expected that the refining effect is improved owing to slag reaction when the molten alloy is poured so as to pass through the slag. In this case, however, there is a possibility that flow of a molten alloy at the time of pouring agitates slag to trap the slag, which becomes an inclusion in the ingot. When a molten alloy is fed with the use of a dipping nozzle which reaches a molten alloy pool, on the other hand, the refining effect owing to slag reaction cannot be expected much, but generation of an inclusion due to agitation of slag can be prevented. Accordingly, it is desirable to appropriately select the method of feeding a molten alloy, according to the invention, depending on a required quality and taking into consideration the relationship with slag.
According to tests made by the inventors of the invention, the following novel effects, which are not found in ESR, can be obtained by regulating the properties of slag.
First, while slag has a refining effect and serves the function of thermal insulation and shielding of the surface of a molten alloy pool from an outside air, slag having a melting point over 1400°C cannot be adequately melt in some cases only by transferred heat from the molten alloy and thus a solidified slag shell surrounding the molten alloy pool grows excessively to form an abnormal structure deeply in the outer layer of an ingot in some cases. In contrast, when slag has a melting point not higher than 1400°C, slag receives heat from the molten alloy to be suppressed in solidification, so that formation of an abnormal structure in the outer layer of the ingot is considerably prevented. An effect is also obtained that an unnecessarily thick slag skin is not formed on the surface of an ingot, since such slag having a low melting point is also low in viscosity.
It is possible to separately provide heating means for the purpose of temperature control on slag. It is simple and effective as heating means to use Joule heat obtained by carrying current through slag. In order to suppress growth of slag solidification on the inside surface of a heat insulator without temperature rise of a whole slag, an electrode for current-carrying is arranged so that current flows concentratedly on the outer periphery of slag. Heating of slag enables suppressing generation of an abnormal structure on a surface layer and mitigating the detrimental quality of slag having a high melting point. When slag is excessively heated up to high temperatures, heat transferred from slag to a molten alloy causes a decrease in cooling rate. Slag temperature is preferably below the casting temperature.
Slag having such a low melting point forms, together with the effect of a heat insulator, an appropriate slag solidified layer on the side of the heat insulator, and permits little slag to enter between an ingot and a water-cooled mold, so that generation of crack is suppressed on a solidified shell. Thereby, such slag is preferable since direct contact between a molten alloy and the water-cooled mold is avoided, and an ingot having a favorable casting surface can be drawn along the water-cooled mold.
Slag having a melting point below 500°C is not practical. Rather, slag having a low melting point of 500 to 1400°C is used.
In order to produce such effect of slag, a slag layer preferably has a thickness of 20 mm to 100 mm.
The reason why a casting rate of the molten alloy is not higher than 0.3 m/min in the invention is that, when the casting rate is too large, it is hard to obtain a structure which is homogeneous and less in segregation which stacking solidification aims, and there is a possibility that slag may be trapped in the molten alloy. In particular, the casting rate is not higher than 0.1 m/min, and preferably, it is not higher than 0.05 m/min. Taking into consideration productivity, the casting rate is not lower than 0.01 m/min.

Example 1

Fig. 1 shows an example of a casting process, according to the invention, in which an apparatus embodying the invention is used. Fig. 1 shows a cross section of the apparatus comprising a tundish 10 which holds a molten alloy 11, a water-cooled mold 2 made of iron, and an elevator 20 which draws an ingot. A graphite sleeve 3 serving as a heat insulator is arranged on an upper portion of the water-cooled mold. The water-cooled mold is shaped to have a length of 400 mm, and the upper portion having a length of 200 mm on which the graphite sleeve is arranged has an inside diameter of 471 mm, while a lower portion has an inside diameter of 450 mm. The graphite sleeve having a length of 200 mm, an inside diameter of 450 mm, and an outside diameter of 470 mm is mounted inside the upper portion of the water-cooled mold. A secondary cooling zone 30 is arranged below the water-cooled mold.
A shield 14 serving to shut off the molten alloy from an outside air and an electrode 15 for carrying current to slag is arranged as additional means.
In the apparatus shown in Fig. 1, flow 13 of a molten alloy is poured from a nozzle 12 on a bottom of the tundish 10, which holds the molten alloy 11, into the water-cooled mold 2, which defines a solidification space including slag 1 therein. Slag are melted beforehand and introduced into the mold in the early stage of casting.
A molten alloy pool 4 is controlled to be formed having a meniscus upper surface position "A" distant by 50 mm from a lower end "B" of the graphite sleeve 3 mounted inside the water-cooled mold 2 and serving as a heat insulator. Thereby, it is possible to form a solidified shell on an inside wall of the heat insulator.
Specifically, an elevator 20 is lowered to draw an ingot 5 according to a poured quantity of the molten alloy 11, and thus it is possible to advance stacking solidification while maintaining a meniscus position constant. The ingot drawn out of the water-cooled mold is mist-cooled in the secondary cooling zone 30.
An experiment of casting was made using the apparatus shown in Fig. 1. The electrode 15 for carrying current to slag was not used.
A molten alloy was held in the tundish and cast into the water-cooled mold in which slag having the composition and the melting point shown in Fig. 1 was held so as to have a thickness of 50 mm. Two types of steel corresponding to JIS SKD11 and SKH51 by mass% were used as the molten alloy. TABLE 2 indicates compositions of the molten alloy.
Temperature of the molten alloy was 1500°C and the casting rate was approximately 0.02m/ min (20 mm/min), and a length of up to 3m was cast.
As a Comparative example, casting was carried out without installing a graphite sleeve (heat insulator) in the apparatus shown in Fig. 1.
TABLE 3 indicates thicknesses of skin slags of thus obtained ingots. TABLE 4 indicates results of measurements of a secondary dendrite arm spacing DAS II in positions of D/8, D/4 and D/2 (where D indicates a diameter of an ingot) from a surface of a sectioned specimen at a longitudinal position of 1/2 of an ingot length.
It is found from TABLE 3 that an ingot having a smooth surface was obtained owing to the effect of the heat insulator in the invention and an ingot substantially was free of skin slag. In the Comparative example, it is found that a slag skin having a large thickness as large as several mm was formed since a slag solidified layer was drawn together with an ingot.
As indicated in TABLE 2, it is found that a DAS II value being an index of the cooling rate was small and a fine structure was obtained, since skin slag is not substantially formed in the invention.
Subsequently, the ingot as obtained was subjected to hot forging at 1100°C until it was made 115 mm square. After surfaces of the 115 mm square ingot were ground by 2 mm, the ingot was subjected to die marking and the presence of any cracks was examined. TABLE 5 indicates the results.

It is found that no cracks were generated at the time of hot forging in the invention since the ingot was free of slag skin and smooth on surfaces. On the other hand, it was confirmed in the Comparative example that a thick slag skin was formed, and surfaces of an ingot have irregularly bleeds of a molten alloy, and that crack was generated at the time of hot forging, which could not be removed by grinding by 2 mm, and the crack remained.

[TABLE 1]

SLAG COMPOSITION (mass%)
MELTING POINT (°C)	CaO	Al₂O₃	CaF₂	SiO₂
1320	36	27	27	10

[TABLE 2]

COMPOSITION OF MOLTEN ALLOY (mass%)
STEEL TYPE	C	Si	Mn	Cr	Mo	W	V	Fe
SKD11	1.50	0.30	0.40	12.0	1.0	-	0.3	Bal.
SKH51	0.85	0.25	0.35	4.1	5.0	6.5	2.0	Bal.

[TABLE 3]

THICKNESS OF SKIN SLAG
	HEAT INSULATOR	STEEL TYPE	THICKNESS OF SKIN SLAG (mm)
INVENTION	PRESENT	SKD11	0
INVENTION	PRESENT	SKH51	0
COMPARATIVE EXAMPLE	ABSENT	SKD11	3
COMPARATIVE EXAMPLE	ABSENT	SKH51	4

[TABLE 4]

RESULTS OF DAS II MEASUREMENT
	HEAT INSULATOR	STEEL TYPE	DAS II (µm)
	HEAT INSULATOR	STEEL TYPE	D/8	D/4	D/2
INVENTION	PRESENT	SKD11	89	163	154
INVENTION	PRESENT	SKH51	75	153	145
COMPARATIVE EXAMPLE	ABSENT	SKD11	102	191	186
COMPARATIVE EXAMPLE	ABSENT	SKH51	95	181	177

[TABLE 5]

SITUATION OF RESIDUAL CRACK AFTER GRINDING
	HEAT INSULATOR	STEEL TYPE	RESIDUAL CRACK
INVENTION	PRESENT	SKD11	ABSENT
INVENTION	PRESENT	SKH51	ABSENT
COMPARATIVE EXAMPLE	ABSENT	SKD11	PRESENT
COMPARATIVE EXAMPLE	ABSENT	SKH51	PRESENT

Example 2

An experiment of casting was made with the apparatus shown in Example 1 added with an electrode 15 for carrying current to slag. A cylindrical-shaped graphite electrode was used as the electrode 15. The electrode 15 for carrying current to slag was immersed into slag having the composition in TABLE 1. Electric current was carried to the slag to heat the slag. Temperature was simultaneously measured in an intermediate position between the graphite electrode and a molten alloy surface, and slag temperature was controlled at 1400°C by a current value. The slag had a thickness of 50 mm and the molten alloy have a composition of SKD11 indicated in TABLE 2.
Other conditions are same as those in Example 1. Temperature of the molten alloy was 1500°C and the casting rate is approximately 0.02 m/min (20 mm/min). A length of up to 3m was cast.
Consequently, it was confirmed that the ingot was smooth and substantially free of skin slag even when electric current was carried to the slag for heating.
A structure in the vicinity of a surface of a sectioned specimen in a position corresponding to 1/2 of an ingot length was observed. TABLE 6 indicates depths of bad structures in a surface layer, and TABLE 7 indicates DAS II measured values in positions of D/8, D/4 and D/2 from a surface thereof (where D indicates a diameter of the ingot).
It is found from TABLE 6 that growth of a solidified slag layer can be suppressed owing to temperature control by heating of slag, thus enabling promoting homogeneity of an ingot surface structure. It is also found from TABLE 7 that slag heating at 1400°C had little influence on a DAS II value and a fine structure was maintained. The reason for this is thought to be that the extraction of heat by a water-cooled mold and in a secondary cooling zone is effective since skin slag was not generated even when slag heating was performed.
Since a smooth ingot could be obtained even when slag is heated, no residual crack was confirmed as in the case of the ingot of Example 1, for which the heat insulator was used, while observation of a die mark was carried out to samples grinded by 2 mm of a hot forged ingot of 115 mm square. [TABLE 6]

DEPTH OF BAD STRUCTURE OF SURFACE LAYE (SKD11)

HEAT INSULATOR SLAG HEATING DEPTH OF BAD STRUCTURE OF SURFACE LAYE (mm)

INVENTION PRESENT PRESENT 3

INVENTION PRESENT ABSENT 11

[TABLE 7]

RESULTS OF DAYS II MEASUREMENT

HEAT INSULATOR SLAG HEATING DAS II (µm)

D/8 D/4 D/2

INVENTION PRESENT PRESENT 87 165 153

PRESENT ABSENT 89 163 154

Industrial Applicability

According to the invention, a metal structure can be made fine and a steel, which is low in cost and excellent in steel product performance, can be supplied by performing such rapid solidification, so that a wide demand is expected from the viewpoint of source saving and energy saving.

Brief Description of the Drawing

[Fig. 1] Fig. 1 is a conceptional view showing an example of an apparatus suitable to carry out the process of the invention.

Claims

A process for casting a molten alloy, comprising:
pouring the molten alloy from a container holding the molten alloy therein into a solidification space enclosed by an inside wall of a water-cooled mold with holding slag therein to solidify the molten alloy while forming a molten alloy pool; and

drawing an ingot vertically from a lower section of the water-cooled mold;
characterized in that
the molten alloy is poured at a casting rate of 0.01 to 0.1 m/min;
a heat insulator made of a graphite material for suppressing an extraction of heat from slag is arranged in an upper section of the mold, the insulator having an inside wall shaped to be continuous to the inside wall of the water-cooled mold; and
the slag is heated at its periphery with an electrode, the slag having a melting point of 500 to 1400°C and a thickness of 20 to 100 mm from the molten alloy.
The process according to claim 1, wherein an upper surface position of a meniscus of the molten alloy pool is controlled, in a steady state of casting, to be within a range in which the heat insulator is arranged.
The process according to claim 1 or 2, wherein the heat insulator has the inside wall having the same cross-sectional shape in perpendicular to a direction in which the ingot is drawn, as that of the inside wall of the water-cooled mold on a downstream side.
The process according to any one of claims 1 to 3, wherein the casting rate of the molten alloy is 0.01 to 0.05 m/min.
The process according to any one of claims 1 to 4, wherein the molten alloy contains Fe as a main component, not more than 3.0% of C, and not less than 5% of any other metallic element than Fe, by mass %.