JP2006055889A - Continuous casting method repeatedly using tundish under hot-state - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a casting method by which a high cleanliness steel can continuously be cast by preventing the contamination of molten metal caused by oxidation of remained steel in a tundish while repeatedly using the tundish under a hot-state. <P>SOLUTION: In a continuous casting method for repeatedly using the tundish just after the casting, to the successive casting under hot-state, this method is performed as the followings, in which after charging flux containing 5-30 mass% carbonaceous grains having 0.5-5 mm grain diameter and 1,200-1,500°C fusing temperature, into the molten steel poured into the tundish from a ladle at the last in a pre-casting, by 20-200 kg, the pre-casting is completed, and thereafter, the remained steel and remained slag in the tundish are discharged, and after cleaning an upper nozzle in the tundish, a sliding gate and the inner part of an immersion nozzle with gas containing oxygen, the successive casting is started within 45 min from the completion of the pre-casting without preheating in the tundish. In the above method, basicity of the flux is desirable to be 5-20. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a continuous casting method of steel in which a tundish is repeatedly used hot, and more particularly to a continuous casting method of steel that can prevent contamination of molten steel due to oxidation of the remaining steel in the tundish. .

  In continuous casting of steel, when the tundish is repeatedly used hot, it is known that the molten steel is contaminated due to oxidation of the remaining steel in the tundish. The prevention of contamination is extremely important for casting a clean slab, and many techniques have been disclosed as countermeasures therefor as described below.

  In Patent Documents 1 to 3, there is a method in which the content of lower oxides such as FeO or MnO is reduced by modifying or deoxidizing slag in a tundish with a deoxidizing material such as Al. Nos. 6 to 6 disclose a flux and a processing method in which the composition of the tundish flux that facilitates the discharge of residues in the tundish and the melting point thereof are adjusted. Further, Patent Documents 7 and 8 disclose a method of increasing the heat retaining property by containing carbon in the tundish flux or relaxing the oxidation degree of the atmosphere by the generated CO gas.

  Patent Document 9 discloses a method for appropriately controlling the melting point of the slag in the tundish and the time from the end of casting to evacuation to increase the slag discharge rate. Patent Document 10 discloses that the tundish is tilted. A method of intensively heating using an oxygen-enriched gas burner to promote the discharge of residual steel and residue. Patent Document 11 discloses that the time for preheating with the gas burner of the tundish is shortened and the remaining in the tundish is removed. Methods for suppressing oxidation of steel are disclosed.

  Patent Documents 12 to 14 disclose a method for suppressing oxidation of the remaining steel by reducing the amount of oxygen supplied to the burner for tundish heating from the amount of combustion equivalent oxygen, and Patent Document 15 utilizes a plasma torch. A non-oxidizing heating method for tundish is disclosed in Patent Documents 16 to 18, in which a tantalum using a high-temperature inert gas or reducing gas or electric heating means is used to prevent oxidation of the remaining steel in the tundish. Each method for preheating a dish is disclosed.

  Patent Document 19 discloses a method in which carbon is attached to the inner wall of the tundish, preheated with a gas burner, and the oxidation of the remaining steel attached to the inner wall of the tundish is suppressed by the generated CO gas. Discloses a method in which molten steel is held in a tundish for a long time at the start of casting, and slag deoxidation or a modifier is added to promote the floating of nonmetallic inclusions.

  However, as disclosed in Patent Documents 1 to 3, a method for reducing the content of a lower oxide such as FeO or MnO by modifying or deoxidizing slag in a tundish with a deoxidizing material such as Al is disclosed in Since the lower oxide attached to the inner wall of the dish has no effect, the effect of reforming or deoxidation was insufficient. In addition, as disclosed in Patent Documents 4 to 6, the component composition and melting point of the tundish flux are adjusted, and as disclosed in Patent Document 9, the melting point of the slag in the tundish and the end of casting are discharged. Although the method of promoting the discharge of the residue in the tundish by properly managing the time until dredging has a certain effect, it uses a gas burner during the preheating of the tundish after discharging, so its combustion gas Therefore, there is a problem that the remaining steel is oxidized and the generation of FeO cannot be prevented.

Although the method of adding carbon to the tundish flux has a certain effect for reducing the oxidation degree of the atmosphere in the tundish by the generated CO gas, the tundish addition flux described in Patent Document 7 includes Since a large amount of Na ₂ O is contained, a large amount of gas is generated when added to the tundish, which is not preferable. Moreover, since the flux described in Patent Document 8 has a CaF ₂ content that is too high, damage to the tundish inner wall refractory increases when the input amount is large. Therefore, even if a large amount of carbon is added to the flux, the amount of flux input must be suppressed, and therefore the amount of carbon input is limited.

  As disclosed in Patent Document 10, the method of strongly heating using an oxygen-enriched gas burner in a state where the tundish is tilted to promote the discharge of the remaining steel and residue, the refractory is damaged by the strong heating. There is a problem that it is easy. Further, as disclosed in Patent Document 11, the preheating time of the tundish by the gas burner is shortened, or as disclosed in Patent Documents 12 to 14, the amount of oxygen supplied to the gas burner is set higher than the combustion equivalent oxygen amount. The method of lowering could not exhibit a sufficient effect for suppressing oxidation of the remaining steel.

  Further, as disclosed in Patent Documents 15 to 18, the method of preheating the tundish using a plasma torch or using a high-temperature inert gas or reducing gas or electric heating means is used to oxidize the remaining steel. Although prevention is effective, in order to secure a sufficient amount of heat for preheating, large and expensive equipment is required, and it has been difficult to increase the effect commensurate with equipment costs.

  Further, as disclosed in Patent Document 19, a method of preheating with a gas burner after carbon is attached to the tundish inner wall and suppressing oxidation of the remaining steel attached to the tundish inner wall with the generated CO gas is constant. Although the effect of is recognized, there are the following problems. That is, a large amount of carbon is required to completely suppress the oxidation of the remaining steel, and if the carbon is not completely burned by the start of the next casting, carburization occurs in the molten steel in the next casting. . Conversely, if the carbon burns out during preheating, the remaining steel will be oxidized thereafter. Due to such operational complexity, it was difficult to obtain a stable effect only by this method.

  And, as disclosed in Patent Document 20, a method of holding the molten steel in the tundish for a long time at the start of casting and introducing slag deoxidation or a modifier to promote the floating of nonmetallic inclusions is as follows. When oxidation of the remaining steel in the dish is significant, there is a problem that the generated nonmetallic inclusions cannot be lifted up.

  As described above, when the tundish is repeatedly used hot, many problems remain to be solved to prevent contamination of the molten steel due to oxidation of the remaining steel in the tundish. .

Japanese Patent No. 3257263

JP-A-10-314899 JP 2003-236648 A Japanese Patent No. 2894190 Japanese Patent No. 2946155 Japanese Patent No. 3265189 Japanese Patent Laid-Open No. 11-77260 JP 2001-96345 A Japanese Patent Laid-Open No. 10-291055 JP 2001-300703 A JP-A-9-94641 JP-A-4-22567 JP-A-4-238656 JP-A-7-299548 JP-A-9-47850 JP-A-8-159664 JP-A-8-257708 Japanese Patent Laid-Open No. 9-295110 JP 2002-35901 A JP-A-8-1288

As described above, the conventional tundish hot repeated use technology has the following problems. That is, (1) In the method of modifying the slag in the tundish with a deoxidizing material, the modification effect does not reach even the lower oxide adhering to the inner wall, and the melting point of the flux and the time to discharge are managed. In this method, the remaining steel is oxidized during preheating of the tundish. (2) In the method of adding carbon to the flux, damage to the refractory due to coexisting components such as Na ₂ O increases. (3) The method of strongly heating the tundish with a burner and discharging the remaining steel, etc. easily damages the refractory, and even the method of heating the burner with an oxygen amount less than the combustion equivalent cannot suppress oxidation of the remaining steel . (4) The method of preheating a tundish using a plasma torch, an inert gas, a reducing gas or an electric heating means requires expensive equipment. (5) The method of preheating with a burner after attaching carbon to the inner wall of the tundish and suppressing the oxidation of the remaining steel requires a large amount of carbon, and it is difficult to control the combustion of the carbon to suppress the oxidation of the remaining steel . (6) In the method in which the molten steel is held for a long time and the slag deoxidizer is added to float the nonmetallic inclusions, the inclusions may not be able to float.

  The present invention has been made in view of the above problems, and its problem is to completely prevent contamination of molten steel caused by oxidation of the remaining steel in the tundish in continuous casting in which the tundish is repeatedly used hot. Another object of the present invention is to provide a continuous casting method capable of stably casting clean steel such as bearing steel and case-hardened steel whose inclusion concentration standards are strict.

  In order to solve the above-described problems, the present inventor has studied a method of continuously casting clean steel while repeatedly using hot tundishes based on the conventional problems. The following (a) and (b) The present invention was completed with the knowledge of

  (A) Highly clean steel is continuously cast stably by maintaining the low oxide content in the refractory working surface of the tundish inner wall and the slag adhering to the inner wall surface that is repeatedly used in the hot. can do.

  (B) In repeated use of the tundish immediately before casting of the high cleanliness steel, the pre-heating of the tundish is not performed after discharging the remaining steel and residue, and the oxidation of the remaining steel in the tundish is suppressed using carbon. It is effective to reduce the lower oxide in the residue and start casting the high cleanliness steel within a short time after the end of the pre-casting.

  This invention is completed based on said knowledge, The summary exists in the continuous casting method shown to following (1)-(5).

  (1) A continuous casting method in which the tundish immediately after being used for continuous casting is repeatedly used for the next continuous casting while still in a hot state, and 5 to 30 masses of carbonaceous particles having a particle size of 0.5 to 5 mm. %, And a casting temperature of 1200-1500 ° C. is introduced into the surface of the molten steel poured into the tundish from the final ladle in the pre-casting, and then the pre-casting is finished, and then in the tundish After draining the remaining steel and residue, the upper nozzle, sliding gate and immersion nozzle in the tundish are cleaned with oxygen-containing gas, and within 45 minutes from the end of pre-casting without preheating the tundish. A continuous casting method characterized by starting the next casting (hereinafter also referred to as “first invention”).

(2) In the continuous casting method according to (1), in addition to the carbonaceous particles, the flux is less than 15% Al ₂ O ₃ , less than 10% MgO and 5% F in mass%. Continuous casting method (hereinafter referred to as “Continuous casting method”) containing ˜25%, further containing Na ₂ O, Li ₂ O and K ₂ O in a total of less than 5% and satisfying the relationship represented by the following formula (1): Also referred to as “second invention”).

5 ≦ (CaO + MgO) / SiO ₂ ≦ 20 (1)
Here, CaO, MgO, and SiO ₂ represent mass contents (%) of the respective components.

  (3) In the continuous casting method according to (1) or (2), when discharging the remaining steel and residue in the tundish after the completion of the pre-casting, the remaining steel is tilted when the tundish is tilted. And a continuous casting method for discharging residue (hereinafter also referred to as “third invention”).

  (4) In the continuous casting method according to any one of (1) to (3), after discharging the remaining steel and residue in the tundish, carbonaceous particles or combustion is caused on the metal adhesion site on the inner wall of the tundish. After spraying or spraying the organic material that carbonizes by alone or with a metal containing Al or Ca, after 5 minutes or more have elapsed, pouring the molten steel to be cast next from the ladle into the tundish is started. Continuous casting method (hereinafter also referred to as “fourth invention”).

  (5) In the continuous casting method according to the above (1) to (4), the tundish immediately after continuous casting is repeatedly used in a hot state a plurality of times, and at least one of the plurality of repeated uses is used. When the inside of the tundish is preheated using a burner and the burner is preheated prior to the pre-casting or a plurality of past castings including the pre-casting, the carbonaceous matter is attached to the metal adhesion site on the inner wall of the tundish. The continuous casting according to any one of claims 1 to 4, wherein the next casting is performed using a tundish preheated with a burner after spraying or spraying particles or an organic substance carbonized by combustion. Method (hereinafter also referred to as “fifth invention”).

  In the present invention, “in the hot state” means that the surface temperature of the refractory is kept at 300 ° C. or higher without cooling the inside of the tundish using a cooling means. That means.

  “Pre-casting” is a series of castings performed immediately before the next casting, and means a casting operation in which molten steel in one or more ladles is continuously cast.

  “Next casting” refers to a casting operation performed immediately after the pre-casting, in which molten steel in one or more ladles is continuously cast. Hereinafter, the casting of molten steel for each ladle is also referred to as “charge”.

  “End casting” means to finish pouring from the tundish to the mold, and “Start next casting” means to start drawing the slab from the mold in the next casting. It means to do.

  The “pre-burner preheating time” refers to the most recent burner preheating time in the past.

  “Carbonaceous particles” mean particles having a carbon content of 75 to 90% by mass, and examples thereof include coke particles, coal particles, and charcoal particles.

“Organic substance carbonized by combustion” means an organic substance carbonized by combustion, for example,
This includes wood chips and compressed sawdust.

  The “metal containing Al or Ca” means a metal Al simple substance, a metal Ca simple substance, a Ca—Al alloy, a Ca—Si alloy, or the like.

  According to the continuous casting method of the present invention, the contamination of the molten steel caused by oxidation of the remaining steel in the tundish is completely eliminated while using the tundish that has been disadvantageous for the casting of clean steel. In particular, it is possible to stably cast clean steel such as bearing steel and case-hardened steel with strict inclusion concentration standards.

  The contents of the continuous casting method of the present invention, and the reason and the preferable range for defining the scope of the present invention as described above will be described in more detail below.

(A) 1st invention 1st invention is the method of materializing the above-mentioned knowledge (b) mainly. That is, as described above, it is a continuous casting method in which the tundish immediately after being used for continuous casting is repeatedly used for the next continuous casting in a hot state, and carbonaceous particles having a particle diameter of 0.5 to 5 mm are 5 Pre-casting is terminated after 20 to 200 kg of flux having a melting temperature of 1200 to 1500 ° C. is added to the surface of the molten steel injected into the tundish from the final ladle in the pre-cast, The remaining steel and residue in the tundish are discharged, and the upper nozzle, sliding gate and immersion nozzle in the tundish are cleaned with a gas containing oxygen, and then the pre-casting is not preheated in the tundish. This is a continuous casting method in which the next casting is started within a minute.

1) Flux feeding The flux is charged in the final ladle molten steel (charge) in order to promote the discharge of residue.

  The reason why 5-30% by mass of carbonaceous particles is contained in the flux is as follows. That is, when the content of the carbonaceous particles is less than 5% by mass, the oxidation of the remaining steel in the tundish by carbon and the reduction action of the lower oxide in the residue are insufficient, while the content is 30% by mass. This is because the carbon content in the flux shifts to the molten steel and the risk that the component composition of the molten steel fluctuates increases when the content exceeds 5%. A preferred range for the content of carbonaceous particles is 10 to 20% by mass.

  In addition, as above-mentioned, a carbonaceous particle means a particle | grain with a carbon content rate of 75-90 mass%, for example, coke particle | grains, coal particle | grains, charcoal particle | grains, etc. correspond.

  Moreover, the particle size of the carbonaceous particles is set to 0.5 to 5 mm. If the particle size is less than 0.5 mm, the carbonaceous particles are burned down in a short time after being put into the molten steel, and the effect of suppressing oxidation of the remaining steel or lower in the residue This is because the reduction effect of the oxide does not last. On the other hand, if the particle size exceeds 5 mm, the combustion time becomes longer and carbonaceous particles remain until the start of the next charge, changing the composition of the molten steel. This is because the risk of making it increase.

  The melting temperature of the flux was set to 1200 to 1500 ° C. The melting temperature of the tundish refractory was large when the melting temperature was less than 1200 ° C., and the flux was completely melted when the melting temperature was higher than 1500 ° C. This is because it becomes difficult.

  The amount of flux input to the molten steel in the tundish was set to 20 to 200 kg. If the input amount was less than 20 kg, the flux did not spread sufficiently in the tundish, and the effect of flux input could not be obtained. This is because if the input amount exceeds 200 kg and is too large, flux melting defects are likely to occur. In addition, the preferable range of the flux input amount is 40 to 160 kg.

2) Time to wash tundish and next casting After discharging the remaining steel and residue in the tundish, it is necessary to clean the upper nozzle, sliding gate and immersion nozzle with a gas containing oxygen. . This is to wash away non-metallic inclusions such as Al ₂ O ₃ adhering to the inner surfaces of these refractories during pre-casting.

  The gas containing oxygen used for cleaning may be pure oxygen or air. After washing the inner surface of the refractory as described above, the immersion nozzle may be reused as it is or may be replaced with a preheated new one. Since it is difficult to replace the upper nozzle and the sliding gate within a short time until the start of the next casting, the upper nozzle and the sliding gate are usually reused as they are.

Thereafter, the reason why the inside of the tundish is not preheated until the start of the next casting is as follows. That is, when the inside of the tundish is preheated using a gas burner, the remaining steel (metal) that is contained in the combustion gas and adheres to the inner surface of the tundish due to CO ₂ or H ₂ O that has oxidizing power at high temperatures. This is because it is oxidized to produce FeO, which contaminates the molten steel in the next casting. Note that the oxidation by CO ₂ or H ₂ O described above is excessive even if the air-fuel ratio of the burner (the ratio of the supply air amount to the stoichiometrically required amount of combustion air) is adjusted so that oxygen becomes insufficient. It is difficult to prevent it within the range of realistic operating conditions that do not cause wrinkles.

  Starting the next casting within 45 minutes from the end of the previous casting minimizes the temperature drop of the tundish refractory, prevents the temperature of the molten steel from dropping at the start of the next casting, This is to promote the floating of the metal inclusion. When the time from the end of the pre-casting to the start of the next casting exceeds 45 minutes, the inclusion content in the molten steel at the initial stage of the next casting tends to increase. If the time from the end of the pre-casting to the start of the next casting is within 30 minutes, better results can be obtained.

(B) 2nd invention 2nd invention prescribes | regulates the composition of the flux for tundish injection for materializing said knowledge (b). That is, the flux in the first invention contains, in addition to the carbonaceous particles, less than 15% Al ₂ O ₃ , less than 10% MgO and 5 to 25% F, and further Na ₂ This is a continuous casting method that contains less than 5% in total of O, Li ₂ O, and K ₂ O and that satisfies the relationship represented by the following formula (1) in terms of chemical composition.

5 ≦ (CaO + MgO) / SiO ₂ ≦ 20 (1)
A preferable range of the basicity ((CaO + MgO) / SiO ₂ ) is 5 to 20. When the basicity is less than 5, it is not preferable because the SiO ₂ content is high and contaminates the molten steel. When the basicity is higher than 20, the melting temperature (melting point) of the flux becomes excessively high. It is because it is not preferable. In addition, the more preferable range of the said basicity is 5-10.

Al ₂ O ₃ may or may not be contained, but it is preferably contained when the melting point of the flux is lowered. A preferable range of the content is less than 15% by mass. An Al ₂ O ₃ content of 15% by mass or more is not preferable because the viscosity of the flux becomes excessively high. If the Al ₂ O ₃ content is 5% by mass or more, the melting point is appropriately lowered, so that it is more preferable.

  MgO may or may not be contained. However, when MgO is contained, it is preferable that the content is within a range of less than 10% by mass because an effect of suppressing melting damage of the MgO refractory can be obtained. However, if the MgO content is 10% by mass or more, the melting point of the flux becomes excessively high, which is not preferable. In addition, it is more preferable to make MgO content rate contain in less than 5 mass%.

  A preferable range of the F content is 5 to 25% by mass. This is because if the F content is less than 5% by mass, the melting point and viscosity of the flux become excessively high, and if the content exceeds 25% by mass, the refractory melts and is not preferable. is there. In addition, the more preferable range of F content rate is 5-15 mass%.

Na ₂ O, Li ₂ O and K ₂ O may not be contained, but when they are contained, the effect of lowering the melting point of the flux is obtained when the total content is contained in a range of less than 5% by mass. Is preferable. However, if these alkali metal oxides are contained in a total amount of 5% by mass or more, a large amount of gas is generated when the flux is melted, and the operability is deteriorated. In addition, when these alkali metal oxides are contained, if they are contained in the range of 3% by mass or more, an appropriate melting point lowering effect can be obtained, which is more preferable.

  Table 1 shows an example of the chemical composition of the flux used in the examples described later.

  In the table, Flux No. 1 has a melting temperature in the range of 1200 to 1500 ° C., satisfies all the flux conditions specified in the first invention and the second invention, This is the flux used in B. Flux number 2 is a flux that has a melting temperature of less than 1200 ° C., does not satisfy the flux conditions defined in the first and second inventions, and is used in test number E of the comparative example.

(C) 3rd invention The 3rd invention prescribes | regulates the discharge method of the remaining steel and residue for materializing said knowledge (b). That is, when discharging the remaining steel and residue in the tundish after the completion of the pre-casting, the continuous casting method discharges the remaining steel and residue by tilting the tundish.

  Since the remaining steel and residue in the tundish can be efficiently discharged by tilting the tundish, it is preferable to tilt the tundish. In particular, when tundish charging flux of 40 kg or more is added, it is preferable to tilt the tundish because the effect of discharging the remaining steel and residue is further enhanced.

(D) Fourth Invention The fourth invention is a method for preventing oxidation of the residual steel in the tundish and reducing the residue for embodying the finding (b). That is, after discharging the remaining steel and residue in the tundish, the carbonaceous particles or the organic substance that is carbonized by combustion, alone or together with the metal containing Al or Ca, on the metal adhesion site of the inner wall of the tundish, This is a continuous casting method in which pouring of molten steel to be cast next is started from the ladle into the tundish after 5 minutes or more has passed after spraying or spraying.

  According to the above method of the fourth invention, compared with the method of the first invention in which a flux containing carbonaceous particles is introduced into the molten steel in the tundish, the oxidation of the remaining steel in the tundish is prevented, and The effect of reducing the residue can be further enhanced. This is because a part of carbonaceous particles contained in the flux can be supplemented to flow out and be lost when the remaining steel and residue are discharged.

  After the spraying or spraying of carbonaceous particles or organic substances that are carbonized by combustion, the pouring of the next casting ladle from the ladle to the tundish starts with carbonaceous particles or substances that have been carbonized by combustion. This is to ensure a reduction reaction time of the slag.

As a method for reducing the residue in the tundish, there is a method using metal Al, metal Ca, and alloys thereof, but these methods generate non-metallic inclusions such as Al ₂ O ₃ and CaO, These are inferior to the method of adding carbonaceous particles in which the product after reaction is CO gas in that they become new sources of contamination during the next casting. In addition, the method of adding carbonaceous particles has the effect of preventing oxidation of the remaining steel by the generated CO gas, whereas the method of adding the above metals and alloys does not have the effect of adjusting the atmosphere in the tundish. In this case, the antioxidant action is inferior.

  In the fourth invention, carbonaceous particles or the like may be sprayed or sprayed together with Al, Ca or an alloy thereof as well as a single substance. Use appropriate amounts of Al, Ca, and their alloys, which have a stronger reducing action and faster reaction speed than carbonaceous particles, in combination with carbonaceous particles and organic substances that are carbonized by combustion, and take advantage of each advantage. As a result, a better effect can be obtained.

(E) Fifth Invention The fifth invention is a tundish preheating method for embodying the above knowledge (a).
That is, the tundish immediately after continuous casting is repeatedly used in the hot state a plurality of times, and the tundish is preheated using a burner at least once among the plurality of repeated uses, and the precast or the front At the time of preheating the gas burner, which was performed prior to multiple previous castings, including casting, the burner was sprayed or sprayed on the ingots on the inner wall of the tundish after spraying or spraying carbonaceous particles or organic substances carbonized by combustion. This is a continuous casting method in which the next casting is performed using the tundish preheated.

As mentioned above, only the method of preheating after spraying or spraying carbon or an organic substance that carbonizes by combustion on the ingot wall of the tundish inner wall prevents oxidation of the remaining steel and stabilizes the reduction of lower oxides in the residue. It is difficult to obtain the effect. However, the effect of maintaining the inside of the tundish repeatedly used repeatedly hot and maintaining the content of lower oxides such as FeO, MnO, and SiO ₂ at a low level can be sufficiently obtained. Therefore, by combining the method of preheating after spraying or spraying carbon or an organic substance that is carbonized by combustion on the bare metal adhesion site on the inner wall of the tundish with the methods of the first to fourth inventions, further effects are exhibited. Can be made.

Conversely, when the methods of the first to fourth inventions are carried out, a large amount of lower oxidation such as FeO, MnO, SiO ₂ is present in the refractory working surface of the inner wall of the tundish and in the deposits on the inner wall. When a material is contained, these oxides cause an oxidation or reduction reaction with Al in the steel during casting to produce Al ₂ O ₃ and contaminate the molten steel. Therefore, in order to ensure the cleaning effect of the molten steel in the continuous casting method of the present invention, a method of preheating after spraying or spraying carbon or an organic substance that is carbonized by combustion on a bare metal adhesion site in the tundish inner wall It is preferable to reduce the contamination source of molten steel by the method of the fifth aspect of the invention in combination.

  In the fifth invention, it is preferable not to spray or spray carbonaceous particles or the like on the inner wall of the tundish together with a metal containing Al or Ca. The reason is as follows. That is, since the time until the next casting is relatively long, there is no need to increase the reduction rate of the slag in the residue, and the mass transfer rate of the elemental component to be reduced is slow in the solidified slag, and therefore Al Or it is because the reduction reaction by Ca does not reach a wide range. Further, the action of preventing oxidation of the remaining steel (metal) using the inside of the tundish as a reducing atmosphere is superior to carbonaceous particles, and Al or Ca has almost no action.

  In order to confirm the effect of the continuous casting method of the present invention, a casting test by repeated use of the following tundish was conducted and the result was evaluated.

  The cast piece to be cast was a round billet and a slab. A curved continuous casting machine was used for casting the round billet, and a vertical bending type continuous casting machine having a 3 m vertical portion was used for casting the slab. Moreover, the component composition of the clean steel in the test of the present invention example and the comparative example is mass%, C: 0.2 to 1.0%, Si: 0.2%, Mn: 0.3 to 0.8% , Sol. Low alloy steels in the range of Al: 0.03-0.04%, Cr: 0.5-1.5% and Mo: 0-0.5% were targeted.

  In Tables 2 and 3, the details of each test condition and the cleanliness of the slab as a test result are compared and shown.

  In Tables 2 and 3, “flux” means flux for charging into molten steel in the tundish, and “in-nozzle cleaning” cleans the inside of the upper nozzle, sliding gate and immersion nozzle in the tundish. Means that.

  “T. [O]” (total oxygen content) described in the column of cleanliness of steel refers to the total content (% by mass) of dissolved oxygen in the steel and oxygen contained in the oxide. T. T. In the analysis of [O], in the case of a round billet, the position of (1/2) R on the top side of the billet (R is the radius of the billet), and in the case of a slab, on the top side of the slab (1/2). The sample was cut out from the position of width and (1/4) thickness.

  Test numbers A to D are tests on examples of the present invention that use a flux having a melting temperature in the range of 1200 to 1500 ° C. and satisfy the conditions specified in the first invention. On the other hand, the test numbers E and F are comparative examples that do not satisfy at least one of the carbon particles in the flux, the melting temperature, and the time from the end of the previous casting to the start of the next casting, which are the conditions specified in the first invention. This is a test.

(Test of test number A)
Test No. A was used when casting clean steel at the 102nd charge after using the tundish of a round billet continuous caster with a mold having an inner diameter of 360 mmφ for hot 101 charge casting. This is a test to which the invention method is applied. In test number A, in the casting of 101 charge before the relevant charge, the tundish that was preheated by the gas burner was reused 18 times, and before each preheating by the gas burner, the carbon-based spray was applied according to the preheating time. That is, it sprayed on the tundish inner wall in the range of 40-100 kg by the method of spraying so much that preheating time is long.

  For spraying, using a spraying device modified from a sprayer for refractory, water corresponding to 20% of the mass of the carbon-based spraying agent was mixed and sprayed. Further, in the stationary part of the final charge (third charge) of the pre-casting that is a triple casting, the flux containing the coke particles and having the chemical component composition indicated by the flux number 1 in Table 1 is contained in the tundish. 80 kg of molten steel was charged and coke particles were dispersed by utilizing the fact that the flux melted and spread in the tundish.

  Immediately after completion of the pre-casting, the tundish was moved to the maintenance position and tilted to discharge the remaining steel and residue. After tilting for about 30 seconds, the tundish was erected, 60 kg of coke particles were sprayed into the tundish, and the upper nozzle, sliding gate and immersion nozzle inside the tundish were cleaned with oxygen.

Thereafter, the immersion nozzle was replaced and immediately moved to the casting position, and the next casting was started 25 minutes after the completion of the previous casting. Between the time when the tundish was upright and the start of casting, Ar gas of 60 Nm ³ / h was continuously blown into the tundish to prevent oxidation of the steel by the atmosphere.

  Thus, as a result of casting 1% carbon-containing steel having a target standard of total oxygen content (T. [O]) of 9 ppm or less, after cutting 13 tons (t) of the specified crop amount at the beginning of casting. The main body slabs were slabs of quality that passed the target standard.

(Test of test number B)
Test No. B shows the present invention when casting a clean steel at the 86th charge after using a tundish of a round billet continuous casting machine having a mold with an inner diameter of 310 mmφ for hot 85 charge casting. This is a test using the method. In Test No. B, the 85-charge casting before the relevant charge was performed 13 times for reusing the tundish preheated by the gas burner, but no carbon-based spray was used before preheating by the gas burner.

  In addition, 120 kg of flux No. 1 containing coke particles was introduced into the molten steel in the tundish at the stationary part of the final charge (5th charge) of the precast, which is a five-cast casting, and the flux melted and the tundish The coke grains were dispersed using the spreading inside.

  Immediately after completion of the pre-casting, the tundish was moved to the maintenance position and tilted to discharge the remaining steel and residue. After tilting for about 1 minute, erect the tundish, sprinkle 40 kg of coke particles and 15 kg of Ca-Al alloy in the tundish, and oxygen inside the upper nozzle, sliding gate and immersion nozzle of the tundish. Washed.

Thereafter, the immersion nozzle was not changed, but immediately moved to the casting position, and the next casting was started 26 minutes after the completion of the previous casting. During the period from when the tundish was erected until the start of casting, 60 Nm ³ / h Ar gas was continuously blown into the tundish to prevent atmospheric oxidation of the steel.

  In this way, T.W. As a result of casting 0.2% carbon-containing steel with a target standard of [O] of 20 ppm or less, the main slab after cutting the specified crop amount of 10t at the initial casting was of a quality that passed the target standard. .

In test number B, since the condition of the fifth invention is not satisfied, the FeO content in the residue on the surface of the tundish inner wall is high, and this is reduced by Al in the molten steel to produce Al ₂ O ₃ . Since the molten steel being cast was contaminated, the inclusion content in the slab increased after the secondary refining, and T.I. A phenomenon that was not observed in Test No. A, in which the value of [O] was slightly high, was confirmed. However, the conditions specified in the first to fourth inventions were satisfied, and the molten steel contamination preventing effect satisfying the target standard was sufficiently obtained.

(Test of test number C)
In test number C, a slab continuous caster tundish equipped with a mold having an inner dimension of 230 mm × 1250 mm in cross section is used for casting 11 charges hot, and then clean steel is cast at the 12th charge. In this case, it is a test to which the method of the present invention is applied. In test number C, the tundish was reused three times without preheating with a gas burner in the casting of 11 charges before the corresponding charge. Also, in the steady part of the pre-cast charge, which is a single casting (single charge continuous casting), 60 kg of flux containing coke particles is introduced into the molten steel in the tundish, and the flux melts and spreads into the tundish. Then, coke particles were dispersed.

Immediately after the completion of the pre-casting, the tundish was immediately moved to the maintenance position and the remaining steel and residue were discharged while the inside of the upper nozzle, sliding gate and immersion nozzle was cleaned with oxygen while being upright. Thereafter, the immersion nozzle was not changed, and the nozzle was immediately moved to the casting position, and the next casting was started 21 minutes after the end of the previous casting. Between the end of the pre-casting and the start of the next casting, 100 Nm ³ / h Ar gas was continuously blown into the tundish to prevent atmospheric oxidation of the steel.

  According to the above method, T.W. As a result of casting a 0.2% carbon-containing steel having a target standard of [O] of 15 ppm or less, all the main body slabs after cutting 11t of the specified crop amount at the initial casting passed the target standard.

  In the test number C, the number of times of tundish hot repeated use carried out until the relevant charge is relatively small. Therefore, since the amount of slag in the tundish was so small that it could be discharged together with the remaining steel from the immersion nozzle without tilting the tundish, the remaining steel and residue were discharged with the tundish upright. did. Further, since the tundish was not tilted, a large amount of coke particles remained in the tundish using the flux in the pre-casting charge. Therefore, the effect of coke particles persisted without spraying new carbon particles after discharging the remaining steel and residue.

  As described above, depending on the situation of repeated use of the tundish, even if the conditions defined in the third invention or the fourth invention are not satisfied, a necessary effect can be obtained.

(Test of test number D)
Test No. D was used when casting clean steel at the 69th charge after using the tundish of a round billet continuous casting machine with a mold having an inner diameter of 360 mmφ for hot 68 charge casting. It is a test to which the method of the invention is applied. In test number D, the tundish that was preheated by the gas burner in the casting of 68 charges before the relevant charge was reused 11 times. Among them, before the gas burner preheating performed most recently prior to the pre-casting, 80 kg of carbon-based spray was sprayed on the inner wall of the tundish, and then preheated for 3 hours to reduce FeO on the inner wall, Lower oxide was reduced.

For spraying, a spraying device modified from a refractory spraying machine was used, and water corresponding to 20% of the mass of the carbon-based spraying agent was mixed and sprayed. In addition, in the steady portion of the final charge (second charge) of the precast, which is a double casting, 60 kg of flux containing coke particles is introduced into the molten steel in the tundish, and the flux melts and spreads into the tundish. Was used to disperse the coke particles. However, the basicity of the flux ((CaO + MgO) / SiO ₂ ) was as low as 2 and did not satisfy the conditions specified in the second invention, which was somewhat disadvantageous for cleaning the molten steel.

  Immediately after completion of the pre-casting, the tundish was moved to the maintenance position and tilted to discharge the remaining steel and residue. After tilting for about 30 seconds, the tundish was erected, 50 kg of coke particles were sprayed into the tundish, and the upper nozzle, sliding gate and immersion nozzle were cleaned with oxygen.

Thereafter, the immersion nozzle was not changed, and immediately moved to the casting position, and the next casting was started 25 minutes after the completion of the previous casting. During the period from when the tundish was upright to when casting was started, 60 Nm ³ / h Ar gas was continuously blown into the tundish to prevent atmospheric oxidation of the steel.

  In this way, T.W. As a result of casting 0.6% carbon-containing steel having a target standard of [O] of 20 ppm or less, all of the main body slabs after cutting 13t of the specified crop amount at the initial casting passed the target standard. As described above, the test number D does not satisfy the condition claim 2 defined in the second invention. A sufficient effect was obtained for clean steel casting with a comparatively loose target standard where the target standard of [O] was 20 ppm or less.

(Exam number E)
Test No. E was obtained when the tundish of a round billet continuous casting machine having a mold having an inner diameter of 360 mmφ was used for casting 112 charges hot and then casting clean steel at the 113th charge. It is the test about the comparative example which did not apply this method. In test number E, in the casting of 112 charges before the corresponding charge, tundish reuse that was preheated by a gas burner was performed 18 times, but no carbon-based spray was used. Therefore, many lower oxides were accumulated on the inner wall of the tundish.

In addition, 60 kg of flux having the chemical composition indicated by the flux number 2 in Table 1 was charged into the molten steel in the tundish at the stationary part of the final charge (third charge) of the pre-cast which is a triple casting. However, since the carbonaceous particles are not contained in the flux, the effect of reducing the lower oxide in the slag by carbon and reducing the oxygen potential of the atmosphere in the tundish cannot be obtained. Moreover, since the basicity ((CaO + MgO) / SiO ₂ ) of the flux was as low as 1.5, the cleaning effect of the molten steel was lowered. Furthermore, since the melting temperature of the flux was as low as less than 1200 ° C., the tundish refractory melted greatly.

  Immediately after completion of the pre-casting, the tundish was moved to the maintenance position and tilted to discharge the remaining steel and residue. After tilting for about 30 seconds, the tundish was erected and the inside of the upper nozzle, sliding gate and immersion nozzle was cleaned with oxygen.

  After the discharge of residual steel and residue due to the tundish tilt, no reducing agents such as carbonaceous particles were added to the tundish, so the reduction of lower oxides in the slag by carbon and oxygen in the atmosphere in the tundish The potential reduction effect was not obtained.

Thereafter, the immersion nozzle was not changed, but immediately moved to the casting position, and the next casting was started 21 minutes after the completion of the previous casting. Between the time when the tundish was upright and the start of casting, 80 Nm ³ / h Ar gas was continuously blown into the tundish to prevent atmospheric oxidation of the steel.

  According to the above method, T.W. Although the 0.6% carbon-containing steel with a target standard of [O] of 20 ppm or less was cast, even if the specified crop amount 13t at the initial casting was cut, the initial oxygen content of the main slab was 20 ppm of the target standard. The value was higher than that and failed to meet the target standards. Thus, test number E did not satisfy any of the conditions defined in the present invention. The target standard for [O] was 20 ppm or less.

(Test of test number F)
In test number F, a slab continuous caster tundish equipped with a mold having an inner dimension of 230 mm × 1250 mm in cross section is used for casting 9 charges hot, and then clean steel is cast at the 10th charge. In this case, it is a test for a comparative example in which the method of the present invention was not applied. In test number F, in the casting of 9 charges before the corresponding charge, the tundish was reused twice without preheating with a gas burner. Also, in the steady part of the pre-cast charge, which is a single cast (single charge continuous casting), 60 kg of flux containing coke particles is introduced into the molten steel in the tundish, and the flux melts and spreads in the tundish. Then, coke particles were dispersed.

Immediately after the completion of the pre-casting, the tundish was immediately moved to the maintenance position, and the remaining steel and residues were discharged while the upper nozzle, sliding gate and immersion nozzle were cleaned with oxygen while being upright. Thereafter, the immersion nozzle was replaced and moved to the casting position, and the next casting was started 65 minutes after the completion of the previous casting. Between the end of the pre-casting and the start of the next casting, 60 Nm ³ / h Ar gas was continuously blown into the tundish to prevent atmospheric oxidation of the steel.

  In this way, T.W. [O] 0.2% carbon-containing steel with a target standard of 15 ppm or less was cast, but even if 11t of the specified crop amount at the initial stage of casting was cut, the oxygen content in the initial slab of the main body was 15 ppm of the standard It exceeded the target standard and failed. Thus, in test number F, despite the operation under the same conditions as in test number C, the time from the end of the previous casting to the start of the next casting was as long as 65 minutes. Since the specified conditions were not satisfied, the steel cleaning action was impaired in the early stage of casting, resulting in failure to meet the target standards as described above.

According to the continuous casting method of the present invention, the contamination of the molten steel caused by oxidation of the remaining steel in the tundish is completely eliminated while using the tundish that has been disadvantageous for the casting of clean steel. In particular, it is possible to stably cast clean steel such as bearing steel and case-hardened steel with strict inclusion concentration standards. Therefore, the method of the present invention is a casting method that can be widely applied in the field of continuous casting that requires the production of slabs with extremely high cleanliness while saving energy and simplifying the process.

Claims (5)

  A continuous casting method in which the tundish immediately after being used for continuous casting is repeatedly used for the next continuous casting while remaining in a hot state, comprising 5 to 30% by mass of carbonaceous particles having a particle size of 0.5 to 5 mm, Pre-casting is finished after 20 to 200 kg of flux having a melting temperature of 1200 to 1500 ° C. is poured into the molten steel poured into the tundish from the final ladle in the pre-casting, and then the remaining steel and residue in the tundish After the upper nozzle, sliding gate and immersion nozzle in the tundish are cleaned with oxygen-containing gas, the next casting is started within 45 minutes from the end of the previous casting without preheating the tundish. A continuous casting method characterized by: The flux contains, in addition to the carbonaceous particles, less than 15% Al ₂ O ₃ , less than 10% MgO and 5 to 25% F, and further Na ₂ O, Li ₂ O. The continuous casting method according to claim 1, further comprising: less than 5% in total and K ₂ O, and a chemical composition satisfying a relationship represented by the following formula (1):
5 ≦ (CaO + MgO) / SiO ₂ ≦ 20 (1)
Here, CaO, MgO, and SiO ₂ represent mass contents (%) of the respective components.   The continuous steel according to claim 1 or 2, wherein when the remaining steel and residue in the tundish after the completion of the pre-casting is discharged, the remaining steel and residue are discharged by tilting the tundish. Casting method.   After discharging the residual steel and residue in the tundish, the carbonaceous particles or the organic substance carbonized by combustion is sprayed on the ingot site of the inner wall of the tundish alone or with a metal containing Al or Ca or The continuous casting method according to any one of claims 1 to 3, wherein pouring of molten steel to be cast next is started from the ladle into the tundish after elapse of 5 minutes or more after spraying. The tundish immediately after continuous casting is repeatedly used in a hot state a plurality of times, and the tundish is preheated by using a burner at least once among the plurality of repeated uses, and the precast or the precast At the time of pre-heating of the burner that was performed prior to multiple past castings, including carbonaceous particles or organic substances that carbonize due to combustion, were sprayed or sprayed on the ingot site of the inner wall of the tundish, using a burner The continuous casting method according to any one of claims 1 to 4, wherein the next casting is performed using a preheated tundish.