Continuous casting method

In a continuous casting method for casting an aluminum-deoxidized molten stainless steel 1 by using a continuous casting apparatus 100 in which a long nozzle 3 extending into a tundish 101 is provided at a ladle 2, the molten stainless steel 1 is poured through the long nozzle 3 into the tundish 101, while immersing a spout 3a into the poured molten stainless steel 1, and the molten stainless steel 1 in the tundish 101 is poured into a casting mold 105. A TD powder 5 is sprayed so that the powder covers the surface of the molten stainless steel 1 in the tundish 101, a nitrogen gas is supplied around the molten stainless steel 1, and a calcium-containing material is added to the molten stainless steel 1 in the tundish 101. The surface of the molten stainless steel 1 after casting is ground.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 National Phase Entry Application from PCT/JP2014/075272, filed Sep. 24, 2014, and designating the United States, which claims priority to Japanese Patent Application No. 2013-200838, filed Sep. 27, 2013. The above identified applications are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This invention relates to a continuous casting method.

BACKGROUND ART

In the process for manufacturing stainless steel, which is a kind of metal, molten iron is produced by melting raw materials in an electric furnace, molten steel is obtained by subjecting the produced molten iron to refining including decarburization for instance performed to remove carbon, which degrades properties of the stainless steel, in a converter and a vacuum degassing apparatus, and the molten steel is thereafter continuously cast to solidify to form a plate-shaped slab for instance. In the refining process, the final composition of the molten steel is adjusted.

In the continuous casting process, molten steel is poured from a ladle into a tundish and then poured from the tundish into a casting mold for continuous casting to cast. In this process, a seal gas shielding the molten steel surface from the atmosphere is supplied around the molten steel transferred from the ladle in the tundish to the casting mold in order to prevent the molten steel with the finally adjusted composition from reacting with nitrogen or oxygen contained in the atmosphere, such a reaction increasing the content of nitrogen or causing oxidation.

For example, PTL 1 discloses a method for manufacturing a continuously cast slab by using an argon gas as the seal gas.

CITATION LIST

Patent Literature

Japanese Patent Application Publication No. H4-284945.

SUMMARY OF INVENTION

Technical Problem

However, where the argon gas is used as the seal gas, as in the manufacturing method of PTL 1, the argon gas taken into the molten steel remains on the steel surface and inside thereof in the form of bubbles. The resultant problem is that since the regions including the bubbles degrade the slab quality, surface defect regions from the slab surface to the regions where the bubbles have been formed need to be removed by surface grinding over the entire slab, increasing the cost.

Further, some stainless steel grades include easily oxidizable titanium as a component. When stainless steel of such grades is refined, aluminum deoxidation aimed at removal of oxygen contained in the molten steel is performed by adding aluminum, which reacts with oxygen even more easily, thereby preventing the reaction of titanium with oxygen blown into the steel for decarburization. Aluminum reacts with oxygen and forms alumina, thereby removing the oxygen contained in the molten steel. However, the problem associated with this process is that since alumina has a high melting point of 2020° C., alumina contained in the molten steel precipitates in the casting process in which the temperature of the molten steel decreases, and the precipitated alumina adheres to and deposits on the inner wall of the nozzle extending from the tundish to the casting mold, thereby clogging the nozzle. Yet another problem is that alumina is present as large inclusions on the surface of the solidified slab and inside thereof, thereby creating surface defects.

The present invention has been created to resolve the above-described problems, and it is an objective of the invention to provide a continuous casting method in which surface defects in a slab (solid metal) obtained by casting a molten steel are reduced, while preventing a nozzle extending from a tundish to casting mold from clogging during casting of an aluminum-deoxidized molten steel (molten metal).

Solution to Problem

In order to resolve the above-described problems, the present invention provides a continuous casting method for casting a solid metal by pouring a molten metal, subjected to aluminum deoxidation in a ladle, into a tundish and continuously pouring the molten metal in the tundish into a casting mold, the continuous casting method including: a long nozzle installation step for providing in the ladle a long nozzle extending into the tundish as a pouring nozzle for pouring the molten metal in the ladle into the tundish; a casting step for pouring the molten metal into the tundish through the long nozzle, while immersing a spout of the long nozzle into the molten metal poured into the tundish, and pouring the molten metal in the tundish into the casting mold; a spraying step for spraying a tundish powder so that the powder covers the surface of the molten metal in the tundish; a seal gas supply step for supplying a nitrogen gas as a seal gas around the molten metal sprayed with the tundish powder; a calcium-containing material addition step for adding a calcium-containing material to the molten metal retained in the tundish; and a grinding step for grinding the surface of the cast solid metal.

Advantageous Effects of the Invention

With the continuous casting method in accordance with the present invention, surface defects in a solid metal obtained by casting a molten steel can be reduced, while preventing clogging of a nozzle extending from a tundish to a casting mold during casting of an aluminum-deoxidized molten metal.

DESCRIPTION OF EMBODIMENTS

Embodiment

The continuous casting method according to an embodiment of the invention will be explained hereinbelow in greater detail with reference to the appended drawings. Explained in the below-described embodiment is a method for continuously casting stainless steel including titanium (Ti) as a component, such a stainless steel requiring deoxidation with aluminum in a secondary refining process.

Stainless steel is manufactured by implementing a melting process, a primary refining process, a secondary refining process, and a casting process in the order of description.

In the melting process, scrap or alloys serving as starting materials for stainless steel production are melted in an electric furnace to produce molten iron, and the produced molten iron is transferred into a converter. In the primary refining process, crude decarburization is performed to remove carbon contained in the melt by blowing oxygen into the molten iron in the converter, thereby producing a molten stainless steel and a slag including oxides and impurities. Further, in the primary refining process, the components of the molten stainless steel are analyzed and crude adjustment of the components is implemented by charging alloys for bringing the steel composition close to the target composition. The molten stainless steel produced in the primary refining process is tapped into a ladle and transferred to the secondary refining process.

In the secondary refining process, the molten stainless steel is introduced, together with the ladle, into a vacuum oxygen decarburization apparatus (vacuum degassing apparatus, abbreviated as VOD, referred to hereinbelow as VOD), and finishing decarburization treatment, final desulfurization, removal of gases such as oxygen, nitrogen, and hydrogen, and removal of inclusions are performed. As a result of the above-described treatment, a molten stainless steel having the target properties of a product is obtained. Further, in the secondary refining process, the components of the molten stainless steel are analyzed and final adjustment of the components is implemented by charging alloys for bringing the steel composition close to the target composition.

Referring toFIG. 1, in the casting process, the ladle2is taken out from the VOD and set at a continuous casting apparatus (CC)100. Molten stainless steel1in the ladle2is poured into the continuous casting apparatus100and cast, for example, into a slab-shaped stainless steel1cas a solid metal with a casting mold105provided in the continuous casting apparatus100. The cast stainless steel billet1cis hot rolled or cold rolled in the subsequent rolling process (not illustrated in the figures) to obtain a hot-rolled steel strip or cold-rolled steel strip.

Here, the molten stainless steel1constitutes a molten metal.

The configuration of the continuous casting apparatus (CC)100will be explained hereinbelow in greater detail.

Further, referring toFIG. 1, the continuous casting apparatus100has a tundish101which is a container for temporarily retaining the molten stainless steel1transferred from the ladle2and transferring the molten stainless steel to the casting mold105. The tundish101has a main body101bwhich is open at the top, an upper lid101cthat closes the open top of the main body101band shields the main body from the outside, and an immersion nozzle101dextending from the bottom of the main body101b. In the tundish101, a closed inner space101ais formed inside thereof by the main body101band the upper lid101c. The immersion nozzle101dis opened from the bottom of the main body101bin the inner space101aat the inlet port101e.

Further, the ladle2is set above the tundish101, and a long nozzle3which is a pouring nozzle extending through the upper lid101cinto the inner space101ais connected to the bottom of the ladle2. A spout3aat the lower tip of the long nozzle3is opened in the inner space101a. Sealing is performed and gas tightness is ensured between the long nozzle3and the upper lid101c.

A plurality of gas supply nozzles102are provided in the upper lid101c. The gas supply nozzles102are connected to a gas supply source (not depicted in the figures) and deliver a predetermined gas from the top downward into the inner space101a. The long nozzle3is configured such that the predetermined gas is also supplied into the long nozzle.

A powder nozzle103is provided in the upper lid101c, which is for charging a tundish powder (referred to hereinbelow as “TD powder”)5from the top downward into the inner space101a. The powder nozzle103is connected to a TD powder supply source (not depicted in the figure). The TD powder5is constituted by a synthetic slag agent, or the like, and where the surface of the molten stainless steel1is covered thereby, the following effects are produced on the molten stainless steel1: the surface of the molten stainless steel1is prevented from oxidation, the temperature of the molten stainless steel1is maintained, and inclusions contained in the molten stainless steel1are dissolved and absorbed.

A rod-shaped stopper104movable in the vertical direction is provided above the immersion nozzle101d. The stopper104extends from the inner space101aof the tundish101to the outside through the upper lid101c.

Where the stopper104is configured such that where the stopper is moved downward, the tip thereof can close the inlet port101eof the immersion nozzle101d, and also such that where the stopper is pulled upward from a position in which the inlet port101eis closed, the molten stainless steel1inside the tundish101is caused to flow into the immersion nozzle101dand the flow rate of the molten stainless steel can be controlled by adjusting the opening area of the inlet port101eaccording to the amount of pull-up. Further, sealing is performed and gas tightness is ensured between the stopper104and the upper lid101c.

The tip101fof the immersion nozzle101dprotruding from the bottom portion of the tundish101to the outside extends into a through hole105aof the casting mold105, which is located therebelow, and opens sidewise.

The through hole105ahas a rectangular cross section and passes through the casting mold105in the vertical direction. The through hole105ais configured such that the inner wall surface thereof is water cooled by a primary cooling mechanism (not depicted in the figure). As a result, the molten stainless steel1inside is cooled and solidified and a slab1bof a predetermined cross section is formed.

A plurality of rolls106for pulling downward and transferring the slab1bformed by the casting mold105are provided apart from each other below the through hole105aof the casting mold105. A secondary cooling mechanism (not depicted in the figure) for cooling the slab1bby spraying water is provided between the rolls106.

The operation of the continuous casting apparatus100and the peripheral components thereof when the continuous casting method of the present embodiment is implemented will be explained hereinbelow.

Referring toFIG. 1together withFIG. 2, the ladle2containing inside thereof the molten stainless steel1which includes Ti as a component and has been taken out from the VOD (not depicted in the figure) after the secondary refining process is disposed above the tundish101in the continuous casting apparatus100.

The secondary refining process of the molten stainless steel involves finish decarburization, final desulfurization, removal of gases such as oxygen, nitrogen, and hydrogen, removal of inclusions, and the addition of Ti which is a component.

In the finishing decarburization, oxygen is blown into the molten stainless steel, and carbon contained in the molten stainless steel is removed by a reaction with the blown oxygen and oxidation into carbon monoxide. As a result, the molten stainless steel in the secondary refining process includes oxygen which has not reacted with carbon. In the aforementioned degassing aimed at the removal of oxygen, an alloy including aluminum (Al) which is higher than Ti in reactivity with oxygen is added as a deoxidizer (oxygen scavenging agent) to the molten stainless steel prior to adding Ti which easily reacts with oxygen. The Al contained in the alloy including Al reacts with the oxygen contained in the molten stainless steel and forms alumina (Al2O3). Most of Al2O3aggregates in the molten stainless steel and is separated as slag, but part thereof remains in the molten stainless steel. In other words, Ti which is a component is added to the molten stainless steel after the oxygen contained therein has been removed by adding the alloy including Al. As a result, since Al reacts with oxygen and removes it in the molten stainless steel before the oxygen reacts with Ti, the oxidation of Ti is suppressed.

In the continuous casting apparatus100in which the ladle2containing the aluminum-deoxidized molten stainless steel1is disposed in the tundish101, the long nozzle3is mounted on the bottom of the ladle2, and the tip of the long nozzle3having the spout3aextends into the inner space101aof the tundish101. In this configuration, the stopper104closes the inlet port101eof the immersion nozzle101d.

Then, an argon (Ar) gas4awhich is an inert gas is injected as a seal gas4from the gas supply nozzle102into the inner space101aof the tundish101, and the Ar gas4ais also supplied into the long nozzle3. As a result, the air which is present in the inner space101aand the long nozzle3and includes impurities is pushed out of the tundish101to the outside, and the inner space101aand the long nozzle3are filled with the Ar gas4a. In other words, the region from the ladle2to the inner space101aof the tundish101is filled with the Ar gas4a.

A valve (not depicted in the figure) which is provided at the ladle2is then opened, and the molten stainless steel1in the ladle2flows down under gravity inside the long nozzle3and into the inner space101a. In other words, the interior of the tundish101is in the state illustrated by a process A inFIG. 2.

At this time, the molten stainless steel1which has flowed in is sealed on the periphery thereof with the Ar gas4afilling the inner space101aand is not in contact with the air. As a result, nitrogen (N2) which is contained in air and can be dissolved in the molten stainless steel1is prevented from dissolving in the molten stainless steel1and increasing the concentration of N2component therein. For this reason, the formation of TiN by contact and reaction of the nitrogen component (N) and the Ti contained as a component in the molten stainless steel1is suppressed. TiN forms clusters and is present as large inclusions (for example, with a diameter about 230 μm) in the molten stainless steel1. However, since the formation of large inclusions by TiN is suppressed, the precipitation of TiN as large inclusions is also suppressed in the molten stainless steel1which has been cooled and solidified.

Further, inside the tundish101, the molten stainless steel1which has flowed down from the spout3aof the long nozzle3hits the surface1aof the retained molten stainless steel1. As a result, the Ar gas4ais dragged in and mixed, albeit in a small amount, with the molten stainless steel1. However, the Ar gas4adoes not react with the molten stainless steel1.

Further, inside the tundish101, the surface1aof the molten stainless steel1is raised by the inflowing molten stainless steel1. Where the rising surface1areaches the vicinity of the spout3aof the long nozzle3, the intensity with which the molten stainless steel1flowing down from the spout3ahits the surface1adecreases and the amount of the surrounding gas which is dragged in also decreases. Therefore, the TD powder5is sprayed from the powder nozzle103towards the surface1aof the molten stainless steel1. The TD powder5is sprayed to cover the entire surface1a.

After the TD powder5has been sprayed, a nitrogen (N2) gas4b, which is an inert gas, is injected instead of the Ar gas4afrom the gas supply nozzle102. As a result, inside the inner space101aof the tundish101, the Ar gas4ais pushed out to the outside, and the region between the TD powder5and the upper lid101cof the tundish101is filled with the N2gas4b.

At this time, the TD powder5accumulated in a layer configuration on the surface1aof the molten stainless steel1blocks contact between the surface1aof the molten stainless steel1and the N2gas4band prevents the N2gas4bfrom dissolving in the molten stainless steel1. As a result, contact between the nitrogen component (N) and Ti included as a component in the molten stainless steel1is suppressed and the formation of TiN is suppressed. Therefore, the formation of large inclusions by TiN in the molten stainless steel1is suppressed. Further, the precipitation of TiN as large inclusions is also suppressed in the molten stainless steel1which has been cooled and solidified.

Further, in the secondary refining process, part of Al2O3generated in the deoxidation treatment is not separated as slag and remains in the molten stainless steel1. Since Al2O3has a high melting point of 2020° C., it precipitates and forms clusters in the molten stainless steel1and is also present in the form of large inclusions in the solidified molten stainless steel1. Further, Al2O3precipitated in the molten stainless steel1can adhere and accumulate inside the immersion nozzle101dand in the vicinity thereof, thereby clogging the immersion nozzle101d.

For this reason, a calcium-containing wire (referred to hereinbelow as Ca-containing wire)6, which is a calcium-containing material, is charged into the molten stainless steel1after the TD powder5has been sprayed. The Ca-containing wire6is disposed to extend from the outside of the tundish101through the upper lid101cinto the inner space101aand be immersed through the layer of the TD powder5into the molten stainless steel1. Examples of the Ca-containing wire6include a calcium wire (Ca wire) and a calcium silicon wire (CaSi wire).

Al2O3and Ca contained in the Ca-containing wire6react with each other, thereby changing the Al2O3into calcium aluminate (12CaO.7Al2O3). Since the Ca-containing wire6is decomposed and consumed by reaction with Al2O3, the wire is successively fed into the molten stainless steel1as the reaction proceeds.

The generated 12CaO.7Al2O3has a melting temperature of 1400°, which is substantially lower than the melting point of Al2O3, and dissolves and disperses in the molten stainless steel1. Therefore, 12CaO.7Al2O3does not precipitate as large inclusions, such as formed by Al2O3, in the molten stainless steel1and does not clog the immersion nozzle101dby precipitating and adhering inside and in the vicinity thereof.

However, since the Ca-containing wire6inserted into the molten stainless steel1and dissolved therein reacts with Al2O3, the layer of the TD powder5in the charging region of the Ca-containing wire6is disrupted. In this disrupted region, the N2gas4bcomes into contact and reacts with Ti contained in the molten stainless steel1and TiN is formed, albeit in a very small amount, in the molten stainless steel1. Since the amount of the formed TiN is very small, it precipitates in a very shallow region close to the surface of the cooled and solidified molten stainless steel1.

Therefore, in the molten stainless steel1, the precipitation of Al2O3is suppressed, while the amount of TiN precipitating due to the dissolution of the N2gas4bis reduced. Further, since the Ca-containing wire6is charged into the molten stainless steel1in the tundish101immediately before casting, even when 12CaO.7Al2O3has precipitated, it is dissolved and dispersed.

Further, inside the inner space101aof the tundish101, where the rising surface1acauses the spout3aof the long nozzle3to dip into the molten stainless steel1and the depth of the molten stainless steel1in the inner space101abecomes a predetermined depth D, the stopper104rises. As a result, the molten stainless steel1in the inner space101aflows into the through hole105aof the casting mold105through the interior of the immersion nozzle101d, and casting is started. At the same time, the molten stainless steel1inside the ladle2is continuously poured through the long nozzle3into the inner space101aand new molten stainless steel1is supplied into the inner space101a. The interior of the tundish101at this time is in a state such as illustrated by process B inFIG. 2.

In the course of casting, the outflow rate of the molten stainless steel1from the immersion nozzle101dand the inflow rate of the molten stainless steel1through the long nozzle3are adjusted such that the molten stainless steel1maintains the depth which is close to the predetermined depth D and the surface1aof the molten stainless steel1is at a substantially constant position, while maintaining the spout3aof the long nozzle3in a state of immersion in the molten stainless steel1in the tundish101.

When the molten stainless steel1in the inner space101ahas the predetermined depth D, it is preferred that the long nozzle3penetrate into the molten stainless steel1such that the spout3abe at a depth of about 100 mm to 150 mm from the surface1aof the molten stainless steel1. Where the long nozzle3penetrates to a depth larger than that indicated hereinabove, it is difficult for the molten stainless steel1to flow out from the spout3adue to the resistance produced by the internal pressure of the molten stainless steel1remaining in the inner space101a. Meanwhile, where the long nozzle3penetrates to a depth less than that indicated hereinabove, the surface1aof the molten stainless steel1, which is controlled such as to be maintained in the vicinity of a predetermined position during casting, can change and the spout3acan be exposed. In such cases, the molten stainless steel1which has been poured out hits the surface1aand the N2gas4bcan be dragged in and mixed with the steel.

The molten stainless steel1which has flowed into the through hole105aof the casting mold105is cooled by the primary cooling mechanism (not depicted in the figure) in the process of flowing through the through hole105a, the steel on the inner wall surface side of the through hole105ais solidified, and a solidified shell1bais formed. A mold powder is supplied from a tip101fside of the immersion nozzle101dto the inner wall surface of the through hole105a. The mold powder acts to induce slag melting on the surface of the molten stainless steel1, prevent the oxidation of the surface of the molten stainless steel1inside the through hole105a, ensure lubrication between the casting mold105and the solidified shell1ba, and maintain the temperature of the surface of the molten stainless steel1inside the through hole105a.

The slab1bis formed by the solidified shell1baand the non-solidified molten stainless steel1inside thereof, and the slab1bis grasped from both sides by rolls106and pulled further downward and out. In the process of being transferred between the rolls106, the slab1bwhich has been pulled out is cooled by water spraying with the secondary cooling mechanism (not depicted in the figure), and the molten stainless steel1inside thereof is completely solidified. As a result, by forming a new slab1binside the casting mold105, while pulling out the slab1bfrom the casting mold105with the rolls106, it is possible to form the slab1bwhich is continuous over the entire extension direction of the rolls106from the casting mold105. The slab1bwhich is fed out by the rolls106is cut to form a slab-shaped stainless steel billet1c. Where surface defects such as bubbles and inclusions are present in the stainless steel billet1c, surface grinding is performed to remove uniformly the entire surface layer.

The stopper104is controlled to adjust the opening area of the inlet port101e of the immersion nozzle101dto maintain the surface of the molten stainless steel1inside the through hole105aof the casting mold105at a constant height. As a result, the outflow rate of the molten stainless steel1is controlled. Furthermore, the inflow rate of the molten stainless steel1from the ladle2through the long nozzle3is adjusted such as to be equal to the outflow rate of the molten stainless steel1from the inlet port101e. As a result, the surface1aof the molten stainless steel1in the inner space101aof the tundish101is controlled such as to maintain a substantially constant position in the vertical direction in a state in which the depth of the molten stainless steel1remains close to the predetermined depth D. At this time, the spout3aat the distal end of the long nozzle3is immersed into the molten stainless steel1. Further, the casting state in which the vertical position of the surface1aof the molten stainless steel1is maintained substantially constant, while the spout3ais immersed into the molten stainless steel1in the tundish101, as mentioned hereinabove, is called a stationary state.

Therefore, as long as the casting is performed in the stationary state, in the inner space101a, the molten stainless steel1flowing in from the long nozzle3does not hit the surface1aor the TD powder5and only the layer of the TD powder5is disturbed around the Ca-containing wire6. Therefore, a state is maintained in which the N2gas4bis practically shielded from the molten stainless steel1by the TD powder5. As a result, the dissolution of the N2gas4bin the molten stainless steel1is suppressed. The precipitation of large inclusions formed by TiN and Al2O3in the molten stainless steel1is also suppressed.

When no molten stainless steel1remains inside the ladle2, the long nozzle3is detached from the ladle2and the ladle is replaced with another ladle2containing the molten stainless steel1, while the long nozzle3is left in the tundish101. The long nozzle3is connected again to the replacement ladle2. The casting operation is also continuously performed during the replacement of the ladle2. As a result, the surface1aof the molten stainless steel1in the inner space101aof the tundish101is lowered. The supply of the N2gas4binto the inner space101aand the insertion of the Ca-containing wire6into the molten stainless steel1are also continued during the replacement of the ladle2. The interior of the tundish101at this time is in a state such as illustrated by process C inFIG. 2.

During the replacement of the ladle2, the opening area of the inlet port101eof the immersion nozzle101dis adjusted with the stopper104and the outflow rate of the molten stainless steel1, that is, the casting rate, is controlled such that the surface1aof the molten stainless steel1in the inner space101adoes not fall below the spout3aof the long nozzle3. By continuously casting the molten stainless steel1of the plurality of ladles2in the above-described manner, it is possible to eliminate a seam in the slab1bwhich occurs when the ladle2is replaced. Further, the change in quality of the slab1bin the initial period of casting which occurs each time the ladle2is replaced can be reduced. Further, it is possible to omit a step for retaining the molten stainless steel1in the tundish101until the casting is started, such a step being necessary when the casting is ended for each single ladle2.

Further, when the casting advances so no molten stainless steel1remains in the replacement ladle2, and the casting is ended, the ladle2and the long nozzle3are removed. The interior of the tundish101at this time is in a state such as illustrated by process D inFIG. 2. At this time, there is no new downward flow of the molten stainless steel1, the surface1aand the TD powder5are not disturbed by the falling steel, and only the layer of the TD powder5around the Ca-containing wire6is disturbed. Therefore, the N2gas4bis prevented from dissolving in the molten stainless steel1until the end of the casting. The precipitation of large inclusions in the molten stainless steel1is also suppressed.

Even before the spout3aof the long nozzle3is immersed into the molten stainless steel1in the inner space101a(see process A inFIG. 2), the admixture of the air and Ar gas4acaused by dragging into the molten stainless steel1is reduced because the distance between the spout3aand the bottom of the main body101bof the tundish101is small, the distance between the spout3aand the surface1aof the molten stainless steel1which is being poured is small, and the surface1ais hit by the molten stainless steel1only for a limited short period of time until the spout3ais immersed.

Where the N2gas4bis used instead of the Ar gas as the seal gas when the surface1ais hit by the molten stainless steel1, or where the TD powder5is sprayed on the surface1aand the N2gas4bis used as the seal gas, excessive amount of N2gas4bcan be dissolved in the molten stainless steel1and this component can make the steel unsuitable as a product. In addition, a large amount of inclusions caused by TiN can be formed. Therefore, it may be necessary to dispose of the entire stainless steel billet1cwhich has been cast from the molten stainless steel1remaining in the inner space101ain the initial period of casting until the spout3aof the long nozzle3is immersed. However, by using the Ar gas4ain the initial period of casting, it is possible to fit the components of the molten stainless steel1into the prescribed ranges, without causing significant changes thereof, and to prevent the formation of TiN. Further, in the initial period of casting, the precipitation of large inclusions formed by Al2O3is also small. Therefore, the stainless steel billet1ccast from the molten stainless steel1to which very small amount of air or Ar gas4ahas been admixed in the initial period of casting contains practically no large inclusions and has the required composition. As a result, the billet can be used as a product after shallow surface grinding is performed to remove the large inclusions and bubbles created by the admixed Ar gas4a.

Further, the stainless steel billet1cwhich has been cast over a period of time other than the abovementioned initial period of casting, this period of time taking a major part of the casting interval of time from after the initial period of casting to the end of casting, is not affected by the air or Ar gas4athat has been admixed in the initial period of casting, and it can be also said that the admixture of the N2gas4bis suppressed by the TD powder5. Further, even if the N2gas4bis admixed, it is dissolved in the molten stainless steel1and therefore is unlikely to remain as bubbles. The amount of TiN formed by the reaction thereof with Ti is also very small. The TD powder5also acts to absorb the N component admixed to the molten stainless steel1. Therefore, in the stainless steel1cwhich is cast over a period of time other than the initial period of casting, the nitrogen content does not increase over that after the secondary refining, defects caused by bubbling of the admixed gas are practically absent, and large inclusions formed by TiN are present only within a very shallow surface region.

Further, in a period of time other than the initial period of casting, after the TD powder5has been sprayed over the molten stainless steel1, the Ca-containing wire6is charged and the amount of contained Al2O3is reduced. Therefore, the occurrence of inclusions formed by Al2O3in the stainless steel billet1cis greatly suppressed.

It follows from above, that in the stainless steel billet1ccast over a period of time other than the initial period of casting, surface defects caused by bubbles are prevented and the number of surface defects caused by large inclusions constituted by TiN and Al2O3is greatly reduced. Therefore, even when surface grinding is necessary, a product of desired quality can be obtained by grinding with a very small grinding depth.

EXAMPLES

Explained hereinbelow are the results obtained by examining the effect the Ca-containing wire produced on examples of stainless steel billets cast by using the continuous casting method according to the embodiment.

In the examples, the continuous casting method of the embodiment was applied to a Ti-added ferritic stainless steel. Compared hereinbelow are Examples 1 and 2 in which surface grinding was performed after a slab, which was a stainless steel billet, was cast, Comparative Examples 1 and 2 which were the same as Examples 1 and 2, except that no surface grinding was performed, and Comparative Examples 3 and 4 in which surface grinding was performed after casting a slab by using a continuous casting method different from that of the embodiment.

In Examples 1 and 2, the cast slabs of Comparative Examples 1 and 2 were surface ground to a depth of 2 mm.

In Comparative Examples 3 and 4, a slab was cast without spraying the TD powder by using a short nozzle with a distal end at the level of the lower surface of the upper lid101cas the pouring nozzle and using only the Ar gas as the seal gas in the tundish101depicted inFIG. 1. Further, in Comparative Examples 3 and 4, the Ca-containing wire6was inserted and added to the molten stainless steel1in the tundish101at the time of casting. The cast slab was surface ground to a depth of 2 mm.

Specifications for the chemical compositions of the stainless steels in Examples 1 and 2 and Comparative Examples 1 to 4 are presented in Table 1 below. The specifications for the chemical compositions of the stainless steels in Example 1, Comparative Example 1, and Comparative Example 3 are the same, and the specifications for the chemical compositions of the stainless steels in Example 2, Comparative Example 2, and Comparative Example 4 are the same.

The detection results presented hereinbelow were obtained for the examples by sampling from slabs cast in the stationary state, except for the initial period of casting, and for the comparative examples by sampling from the slabs cast within the period of time equal to the sampling period in the examples from the start of casting.

Casting conditions (type of seal gas, type of pouring nozzle, whether the TD powder was used, and whether the cast slab was surface ground) are presented for the examples and comparative examples in Table 2.

Further, inFIG. 3, the ratio of the number of slabs in which bubble defects were detected from a large number of cast slabs and the ratio of the number of slabs in which defects caused by inclusions were detected from the same slabs are compared for Examples 1 and 2 and Comparative Examples 1 to 4.

As shown inFIG. 3, in Examples 1 and 2, the number of defects caused by inclusions was reduced to zero, with respect to that in Comparative Examples 1 and 2, by surface grinding to a depth of 2 mm. Meanwhile, in Comparative Examples 3 and 4 the number of defects was not zero despite surface grinding to a depth of 2 mm. Therefore, the grinding amount of the slab can be greatly reduced in Examples 1 and 2 with respect to that in Comparative Examples 3 and 4.

The present invention was also applied to steel grades which were obtained by adding an Al-containing alloy as a deoxidizer in the secondary refining process and which included Ti as a component, such as 18Cr-1 Mo-0.5Ti and 22Cr-1.2Mo—Nb—Ti stainless steels, in addition to the above-described steel grades, and the immersion nozzle clogging prevention effect was confirmed.

The continuous casting method according to the embodiment is explained with reference to stainless steels including Ti as a component, but the method can be also effectively applied to stainless steels which require aluminum deoxidation in the secondary refining process and include Nb as a component.

Further, the continuous casting method according to the embodiment is applied to the production of stainless steel, but it may be also applied to the production of other metals.

The control in the tundish101in the continuous casting methods according to the embodiment is applied to continuous casting, but it may be also applied to other casting methods.