JP2006225727A - Method for producing extra-low-carbon steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an extra-low-carbon steel by which the surface flaw can surely be prevented by preventing an agglomerated body of inclusions in molten steel and finely dispersing inclusions in a steel plate. <P>SOLUTION: This method for producing the extra-low-carbon steel is characterpzed in that in the decarburize-treatment of the extra-low-carbon steel by using a vacuum-degassing apparatus, when the carbon concentration reaches to 0.005-0.01 mass%, a pre-deoxidation is performed by adding Al into the molten steel, and further decarburization is performed to ≤0.004 mass% carbon concentration while controlling dissolved oxygen concentration to 0.025-0.045 mass%, then the pre-deoxidation-strengthening is performed by further adding Al into the molten steel to control the dissolved oxygen concentration in the molten steel within 0.005 mass% to <0.025 mass%, thereafter the molten steel in which the Ti is added, further at least La and Ce are added, is cast. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing an ultra-low carbon steel material that is excellent in workability and formability and hardly generates surface defects.

The molten steel refined in a converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this excess oxygen is generally deoxidized by Al, a strong deoxidizing element with a strong affinity for oxygen. Is. However, Al generates Al ₂ O ₃ inclusions by deoxidation, and these aggregate and coalesce into coarse alumina clusters of several hundred μm or more.

This alumina cluster causes surface flaws during the production of the steel sheet and greatly deteriorates the quality of the thin steel sheet. In particular, ultra-low carbon molten steel, which is a material for thin steel sheets with a low carbon concentration and a high dissolved oxygen concentration after refining, has a very high amount of alumina clusters, a very high rate of surface defects, and Al ₂ O ₃ intervening. Measures to reduce things are a major issue.

On the other hand, conventionally, the inclusion adsorption flux of Patent Document 1 is added to the surface of the molten steel to remove Al ₂ O ₃ inclusions, or the CaO flux is injected into the molten steel using the injection flow of Patent Document 2. A method of adsorbing and removing Al ₂ O ₃ inclusions has been proposed and implemented.

On the other hand, as a method that does not remove the Al ₂ O ₃ inclusions but does not generate them, there is also a method for melting molten steel for thin steel sheets that is deoxidized with Mg and hardly deoxidized with Al as disclosed in Patent Document 3. It is disclosed.

JP-A-5-104219 JP 63-149057 A Japanese Patent Laid-Open No. 5-302112

However, in the method of removing Al ₂ O ₃ inclusions disclosed in Patent Document 1 and Patent Document 2, surface flaws are not generated in Al ₂ O ₃ inclusions generated in a large amount in ultra-low carbon molten steel. It is very difficult to reduce to the extent.

In addition, Mg deoxidation that does not generate Al ₂ O ₃ inclusions as disclosed in Patent Document 3 has a high vapor pressure of Mg and a very low yield to molten steel. In order to deoxidize molten steel having a high dissolved oxygen concentration with only Mg, a large amount of Mg is required.

  In view of these problems, the present invention presents a method for producing an ultra-low carbon steel material that can reliably prevent surface flaws by preventing aggregation and coalescence of inclusions in molten steel and finely dispersing inclusions in the steel sheet. Objective.

  Here, steel materials such as a slab obtained by casting molten steel, a hot-rolled steel plate obtained by hot-rolling the slab, and a cold-rolled steel plate obtained by cold-rolling the hot-rolled steel plate are combined, In the invention, it is defined as a steel material.

In order to solve the above-described problems, the present invention has the following configuration.
(1) In decarburization processing of ultra-low carbon molten steel using a vacuum degassing apparatus, when the carbon concentration reaches 0.005 to 0.01 mass%, Al is added to the molten steel and preliminary deoxidation is performed. The carbon concentration is further decarburized to 0.004% by mass or less while controlling the dissolved oxygen concentration to 0.025 to 0.045% by mass, and further Al is added to the molten steel for preliminary deoxidation strengthening. And the dissolved oxygen concentration in the molten steel is 0.005 mass% or more and less than 0.025 mass%, then Ti is added, and further molten steel to which at least La and Ce are added is cast. A method for producing carbon steel.

  (2) In the decarburization process of ultra-low carbon molten steel using a vacuum degassing apparatus, when the carbon concentration reaches 0.005 to 0.01% by mass, Al is added to the molten steel and preliminary deoxidation is performed. And after decarburizing to a carbon concentration of 0.004% by mass or less while controlling the dissolved oxygen concentration to 0.025 to 0.045% by mass, further adding Al to the molten steel for preliminary deoxidation strengthening And the dissolved oxygen concentration in the molten steel is 0.005 mass% or more and less than 0.025 mass%, then Ti is added to make the Ti concentration 0.003-0.4 mass%, and at least La, Ce Is added to cast a molten steel having a total concentration of Ce and La of 0.0005 to 0.03% by mass.

  (3) In decarburization processing of ultra-low carbon molten steel using a vacuum degassing device, when the carbon concentration reaches 0.005 to 0.01 mass%, Al is added to the molten steel and preliminary deoxidation is performed. And after decarburizing to a carbon concentration of 0.004% by mass or less while controlling the dissolved oxygen concentration to 0.025 to 0.045% by mass, further adding Al to the molten steel for preliminary deoxidation strengthening And stirring for 3 minutes or more to make the dissolved oxygen concentration in the molten steel 0.005 mass% or more and less than 0.025 mass%, and then adding Ti to make the Ti concentration 0.003 to 0.4 mass Furthermore, a method for producing an ultra-low carbon steel material characterized by casting molten steel in which at least La and Ce are added and the total concentration of Ce and La is 0.0005 to 0.03% by mass.

  (4) When the molten steel is cast, the ultra low carbon steel material according to any one of (1) to (3), wherein the molten steel is cast using a mold flux having a viscosity at 1300 ° C. of 4 poise or more. Method.

  (5) The method for producing an ultra-low carbon steel material according to any one of (1) to (4), wherein the molten steel is continuously cast when the molten steel is cast.

  According to the present invention, since inclusions in molten steel can be finely dispersed, it is possible to produce ultra-low carbon cast slabs and ultra-low carbon steel sheets with excellent workability and formability that can reliably prevent surface flaws. It becomes.

  The present invention is described in detail below.

The molten steel decarburized in the converter or vacuum processing vessel contains a large amount of dissolved oxygen, and this dissolved oxygen is usually almost deoxidized by the addition of Al (reaction of [Formula 1]). A large amount of Al ₂ O ₃ inclusions are produced.
2Al + 3O = Al ₂ O ₃ [Formula 1]

  These inclusions aggregate and coalesce with each other immediately after deoxidation to form coarse alumina clusters of several hundred μm or more, which cause surface defects during the production of the steel sheet.

  On the other hand, in order not to generate alumina clusters, it may be possible to deoxidize the dissolved oxygen after decarburization treatment with a deoxidation material other than Al, such as Mg, but in this case, a large amount of expensive deoxidation element is used. Therefore, it is not a practical process in terms of manufacturing cost.

Therefore, the present inventors preliminarily deoxidize part of the dissolved oxygen during the decarburization with Al as long as the decarburization reaction is not inhibited, and further deoxidize all the dissolved oxygen with Al after the decarburization is completed. Instead, by adding additional Al so as to leave dissolved oxygen and strengthening the preliminary deoxidation, the amount of inclusions of Al ₂ O ₃ is levitated and removed in a short time to such an extent that it does not cause harm, and then Ti and at least Ce We devised deoxidation by adding La, and achieved both quality improvement and manufacturing cost reduction.

  Here, the addition of at least Ce and La means that either Ce is added, La is added, or both Ce and La are added.

  Normally, when decarburization is performed using a vacuum degassing apparatus, for example, from 0.04 mass% to 0.004 mass% or less in the extremely low carbon region, the dissolved oxygen concentration is reduced to 0 from the viewpoint of securing the decarburization speed. Usually, it is processed as 0.05 mass% or more.

  However, when the C concentration is 0.01% by mass or less, since 0.05% by mass or more of dissolved oxygen becomes excessive with respect to the subsequent decarburization reaction, Al is added at the end of decarburization to perform preliminary deoxidation. Thus, even if the dissolved oxygen concentration is 0.025 to 0.045% by mass, an extremely low carbon molten steel having a carbon concentration of 0.004% by mass or less can be melted.

  On the other hand, preliminary deoxidized Al added at the end of decarburization reacts with excess dissolved oxygen to produce alumina inclusions. However, since decarburization is in progress, the vacuum degree of the vacuum degasser is high and the stirring power of the molten steel is high. Further, since the CO gas bubbles generated by the decarburization reaction capture the alumina inclusions and float in the molten steel, the alumina inclusions can be separated and removed very efficiently.

  As described above, the present inventors usually have a range of dissolved oxygen concentration that does not inhibit the decarburization reaction at the end of the decarburization, which is considered to always inhibit the decarburization reaction when Al is added. In the present invention, it was newly found that alumina inclusions can be removed very efficiently by carrying out Al predeoxidation as much as possible. Based on these findings, the following Al predeoxidation method was invented.

  The addition time of preliminary deoxidized Al during decarburization is the end of decarburization when the carbon concentration is in the range of 0.005 to 0.01 mass%. If the carbon concentration is less than 0.005% by mass, the time until completion of decarburization is too short, so that the alumina inclusions cannot be sufficiently floated and separated. If the carbon concentration exceeds 0.01% by mass, the pre-deoxidized Al is added. Is too early, and the decarburization reaction is hindered.

  The preliminary deoxidized Al at the end of decarburization needs to be added so that the dissolved oxygen concentration is 0.025 to 0.045% by mass.

  If the dissolved oxygen concentration is less than 0.025% by mass, the decarburization reaction is inhibited. Therefore, if the dissolved oxygen concentration exceeds 0.045% by mass, the amount of Al added for preliminary deoxidation strengthening after decarburization is increased, and alumina is added. This is because the floating efficiency of inclusions decreases.

  In the molten steel subjected to Al preliminary deoxidation treatment at the end of decarburization, alumina inclusions are almost floated and separated at the end of decarburization, but dissolved oxygen remains at 0.025% by mass or more. For this reason, Al is further added after the decarburization to strengthen the preliminary deoxidation, but when the Al preliminary deoxidation is strengthened and the dissolved oxygen concentration is reduced to less than 0.005% by mass, alumina inclusions are contained in the molten steel. It is produced in large quantities and remains without being able to float and separate.

  For this reason, the Al preliminary deoxidation strengthening after the end of decarburization needs to have a dissolved oxygen concentration of 0.005 mass% or more and less than 0.025 mass%.

  In the molten steel subjected to the Al pre-deoxidation and Al pre-deoxidation strengthening treatments, dissolved oxygen still remains in an amount of 0.005% by mass or more, so that a sufficient material as an ultra-low carbon steel sheet cannot be secured. Therefore, the present inventors deoxidize the molten steel after the Al predeoxidation strengthening with Ti so as not to generate alumina inclusions.

  However, in the molten steel deoxidized with Ti, titania inclusions are formed even if the dissolved oxygen concentration is lowered by Al preliminary deoxidation and preliminary deoxidation strengthening. This titania inclusion causes a problem that the ladle nozzle is blocked when molten steel is poured from the ladle into the tundish.

  As a result of various studies on elements that modify titania inclusions so that the ladle nozzle and tundish nozzle are not blocked, Ce and La are effective, and the titania inclusions are converted to Ce oxide-titania system. It was found that adhesion to ladle nozzles and tundish nozzles was suppressed by modifying to La oxide-titania and Ce oxide-La oxide-titania composite inclusions.

  Not only is it difficult to adhere to the dish nozzle, but also the inclusions are less likely to agglomerate and coalesce in the molten steel, thereby preventing the formation of cluster-like inclusions that cause surface defects.

  From these findings, in order to finely disperse inclusions in the molten steel while preventing clogging of the ladle nozzle and tundish nozzle, Ti is added to the molten steel after Al predeoxidation strengthening, and at least La It is very effective to add Ce to modify inclusions in molten steel to Ce oxide-titania, La oxide-titania, and Ce oxide-La oxide-titania composite inclusions. .

  By casting such molten steel, a steel material in which inclusions are finely dispersed can be produced.

  In the above deoxidation treatment, the addition amount of Ti and at least the addition amounts of La and Ce affect the cohesiveness and material of inclusions, and therefore there is a preferable addition range. If the Ti concentration is less than 0.003% by mass, the material of the ultra-low carbon steel sheet is likely to deteriorate. If the Ti concentration exceeds 0.4% by mass, inclusion modification is likely to be insufficient even if Ce or La is added. The Ti concentration is preferably 0.003 to 0.4 mass%.

  Further, if the total concentration of Ce and La is less than 0.0005% by mass, the modification of inclusions is likely to be insufficient. Therefore, if the total concentration of Ce and La exceeds 0.03% by mass, over-reforming causes Ce oxides— Since it is difficult to become a titania-based, La oxide-titania-based, Ce oxide-La oxide-titania-based composite inclusion, the total concentration of Ce and La is preferably 0.0005 to 0.03% by mass.

  In addition, after Al is added by Al preliminary deoxidation strengthening after completion of decarburization, Ti may be immediately added to deoxidize, but when it is necessary to further reduce alumina inclusions remaining in the molten steel Can promote the floating separation of alumina inclusions by providing a stirring time of 3 minutes or more from the addition of Al for preliminary deoxidation strengthening. This is because if the stirring time is less than 3 minutes, it is difficult to see the effect of reducing alumina inclusions.

  When continuously casting the molten steel of the present invention, Ce oxide-titania, La oxide-titania, Ce oxide-La oxide-titania composite inclusions are absorbed in the mold flux with the lapse of casting time, At the same time, the viscosity of the mold flux may decrease. The decrease in the viscosity of the mold flux promotes flux entrainment and causes defects due to the mold flux.

  For this reason, when continuously casting the molten steel of the present invention, it is effective to design the mold flux viscosity high in advance in consideration of viscosity reduction due to inclusion absorption. From experimental knowledge, it is preferable to set the viscosity of the mold flux at 1300 ° C. to 4 poise or more because defects due to the mold flux do not occur.

  Further, the mold flux has a lubrication function between the mold and the slab, and does not particularly define the upper limit of viscosity as long as the function is not impaired.

  The present invention is also applicable to ingot casting and continuous casting. If continuous casting is used, the present invention is not only applied to a slab continuous casting having a thickness of about 250 mm, but the mold thickness of the continuous casting machine is thinner, for example, 150 mm or less A sufficient effect is exhibited even for continuous casting of a thin slab, and it is possible to obtain a slab with extremely little surface flaws.

  Moreover, a steel plate can be manufactured from the slab obtained by the said method by normal methods, such as hot rolling and cold rolling.

  Hereinafter, the present invention will be described with reference to examples and comparative examples.

  Example 1: Using a vacuum degassing apparatus, 300 t of molten steel having a dissolved oxygen concentration of 0.06% by mass and a carbon concentration of 0.04% by mass was decarburized. When the carbon concentration reaches 0.007% by mass, 70 kg of predeoxidized Al is added to the molten steel to maintain the dissolved oxygen concentration at 0.04% by mass, and finally the carbon concentration is 0.003% by mass. Until decarburized.

  Thereafter, 120 kg of preliminary deoxidized strengthened Al was added to reduce dissolved oxygen to 0.006% by mass. Next, 40 kg of Ti was added to obtain a Ti concentration of 0.01% by mass, and 15 kg of La—Ce alloy was further added to melt molten steel having a total concentration of La and Ce of 0.003% by mass.

  This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The viscosity at 1300 ° C. of the mold flux used for casting was 6 poise. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm.

  Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred.

  In addition, we investigated changes in the opening of the ladle nozzle and the tundish nozzle during casting, but there was no phenomenon that the nozzle opening opened when the nozzle was closed, and stable operation was possible with a constant nozzle opening. .

  Example 2: 300t of molten steel having a dissolved oxygen concentration of 0.07 mass% and a carbon concentration of 0.045 mass% was decarburized using a vacuum degassing apparatus. When the carbon concentration reaches 0.005 mass%, 150 kg of predeoxidized Al is added to the molten steel to maintain the dissolved oxygen concentration at 0.027 mass%, and finally the carbon concentration is 0.0025 mass%. Until decarburized.

  Thereafter, 27 kg of preliminary deoxidized strengthened Al was added to reduce dissolved oxygen to 0.02 mass%, and the molten steel was refluxed for 3 minutes and stirred. Next, 150 kg of Ti was added to a Ti concentration of 0.04% by mass, and 35 kg of Ce alloy was further added to melt a molten steel having a Ce concentration of 0.01% by mass. This molten steel was cast into a thin slab having a thickness of 70 mm and a width of 1800 mm by a continuous casting method. The viscosity of the mold flux used at the time of casting at 1300 ° C. was 5 poise.

  The cast slab was cut into a length of 10,000 mm to form one coil unit. The thin slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm.

  Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, no surface defects occurred.

  Moreover, the opening changes of the ladle nozzle and the tundish nozzle during casting were almost constant, the nozzles were not blocked, and the operation was stable.

  Comparative Example 1: Using a vacuum degassing apparatus, 300 t of molten steel having a dissolved oxygen concentration of 0.06% by mass and a carbon concentration of 0.04% by mass was decarburized all at once to a carbon concentration of 0.003% by mass. Thereafter, 330 kg of Al was added to produce molten steel having an Al concentration of 0.04% by mass.

  This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by a continuous casting method. The viscosity of the mold flux used at the time of casting was 2 poise at 1300 ° C. The cast slab was cut to a length of 8500 mm to make one coil unit. The slab thus obtained was hot-rolled and cold-rolled by a conventional method, and finally formed into a cold-rolled steel sheet having a thickness of 0.7 mm and a coil width of 1800 mm.

  Regarding the slab quality, visual observation was performed on the inspection line after cold rolling, and the number of surface defects generated per coil was evaluated. As a result, surface defects of 5 pieces / coil were generated on average on the slab.

  During the casting, the changes in the opening of the ladle nozzle and the tundish nozzle were investigated. As the casting time passed, the opening of the tundish nozzle opened and the nozzle was clogged. For this reason, the molten metal surface fluctuation | variation in a casting_mold | template has become large, and operativity deteriorated.

Claims (5)

  In the decarburization process of ultra-low carbon molten steel using a vacuum degassing device, when the carbon concentration reaches 0.005 to 0.01% by mass, Al is added to the molten steel and preliminary deoxidation is performed. After decarburizing to a carbon concentration of 0.004% by mass or less while controlling the oxygen concentration to 0.025 to 0.045% by mass, Al is further added to the molten steel for preliminary deoxidation strengthening. An ultra-low carbon steel material characterized in that the dissolved oxygen concentration in the steel is 0.005 mass% or more and less than 0.025 mass%, then Ti is added, and at least La and Ce are cast. Production method.   In the decarburization process of ultra-low carbon molten steel using a vacuum degassing device, when the carbon concentration reaches 0.005 to 0.01% by mass, Al is added to the molten steel and preliminary deoxidation is performed. After decarburizing to a carbon concentration of 0.004% by mass or less while controlling the oxygen concentration to 0.025 to 0.045% by mass, Al is further added to the molten steel for preliminary deoxidation strengthening. The dissolved oxygen concentration is 0.005 mass% or more and less than 0.025 mass%, then Ti is added to make the Ti concentration 0.003-0.4 mass%, and at least La and Ce are added. A method for producing an ultra-low carbon steel material, comprising casting molten steel having a total concentration of Ce and La of 0.0005 to 0.03 mass%.   In the decarburization process of ultra-low carbon molten steel using a vacuum degassing device, when the carbon concentration reaches 0.005 to 0.01% by mass, Al is added to the molten steel and preliminary deoxidation is performed. After decarburizing the carbon concentration to 0.004% by mass or less while controlling the oxygen concentration to 0.025 to 0.045% by mass, further adding Al to the molten steel to perform preliminary deoxidation strengthening, Stir for 3 minutes or more, and make dissolved oxygen concentration in molten steel 0.005 mass% or more and less than 0.025 mass%, then add Ti to make Ti concentration 0.003-0.4 mass, A method for producing an ultra-low carbon steel material, characterized in that at least La and Ce are added to cast molten steel having a total concentration of Ce and La of 0.0005 to 0.03% by mass.   The method for producing an extremely low carbon steel material according to any one of claims 1 to 3, wherein the molten steel is cast using a mold flux having a viscosity at 1300 ° C of 4 poise or more.   The method for producing an ultra-low carbon steel material according to any one of claims 1 to 4, wherein the molten steel is continuously cast when the molten steel is cast.