JP2006152444A - MELTING AND MANUFACTURING METHOD OF Ti-ADDED ULTRA-LOW CARBON STEEL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for melting and manufacturing a Ti-added ultra-low carbon cold-rolled steel sheet by which casting can be performed without causing nozzle clogging. <P>SOLUTION: When melting and manufacturing the ultra-low carbon Ti-killed steel containing ≤0.020 wt.% C and ≥0.010 wt.% Ti, a Ti-containing alloy is added to a decarbonized molten steel for deoxidization to obtain a killed molten steel with a composition satisfying a relation of Al≤(wt.% Ti)/5. Then, an alloy for adjusting the inclusion composition which contains at least one selected from ≥10 wt.% Ca and ≥5 wt.% REM and at least one selected from Fe, Al, Si and Ti, is added to the killed molten steel to adjust the oxide composition in the molten steel so that it comprises ≤90 wt.% of Ti oxide, ≥10 wt.% but ≤50 wt.% of CaO and/or REM oxide and ≤70 wt.% of Al<SB>2</SB>O<SB>3</SB>. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for melting Ti-containing ultra-low carbon steel, and in particular, nozzle clogging in a tundish nozzle even when continuously casting ultra-low carbon Ti deoxidized steel in which the use of Al, Si, Mn is restricted. In addition, the present invention proposes a melting method that is advantageous for producing a Ti-containing ultra-low carbon cold-rolled steel sheet that does not cause defects, has few non-metallic inclusion defects in the product, and has little rusting.

  As disclosed in Japanese Examined Patent Publication No. 44-18066, ultra-low carbon Ti deoxidized steel for cold rolling, particularly Ti-containing ultra-low carbon cold-rolled steel sheet, was initially degassed with FeTi without using Al as disclosed in This was to produce a Ti-deoxidized steel of an acid type. However, in recent years, Al deoxidized steel to which Al is added in an amount of 0.005 wt% or more has become mainstream in order to stably produce Ti-containing ultra-low carbon steel at low cost.

By the way, in this deoxidation with Al, the oxides produced in the gas agitation and RH degassing apparatus are agglomerated,
Although the method of combining and trying to separate and float is taken, Al oxide (Al ₂ O ₃ ) inevitably remains in the slab. Moreover, since the residual Al ₂ O ₃ has a cluster shape, the apparent specific gravity with respect to the molten steel is small and it is difficult to separate and float, so that cluster-like inclusions of several hundred μm or more are likely to remain in the steel. If the clusters generated in this way are trapped in the surface part of the slab during continuous casting, surface defects such as hege and sliver will occur and the surface cleanliness of the cold-rolled steel sheet will be impaired. In addition, the solid phase Al ₂ O ₃ produced by Al deoxidation adheres and accumulates on the inner wall of an immersion nozzle used for injection from the tundish into the mold in continuous casting, causing the nozzle to be clogged. It was.

  Thus, since there are many problems in the case of Al deoxidized steel, recently, there are many cases where deoxidation is performed with Ti without adding Al. This is because, in the case of Ti deoxidation, the concentration of oxygen reached is higher than that of Al deoxidation and the amount of inclusions is large, but compared to Al deoxidation, cluster-like oxides are less likely to be formed and oxidation of about 5 to 20 μm. This is because the object comes to be dispersed in the steel. Therefore, in this Ti deoxidation, surface defects due to cluster inclusions are reduced. However, in ultra-low carbon steel with Ti concentration of 0.010 wt% or more and Ti / Al ≧ 5, Ti oxide is in a solid phase in molten steel, so the tundish is in the form of taking in metal during continuous casting. As a result of adhesion and growth on the inner surface of the nozzle, the nozzle is blocked.

  For example, as described in Japanese Patent Publication No. 56-29730, in the case of high carbon steel with C ≧ 0.50 wt%, the occurrence of nozzle clogging is small even when Ti ≦ 0.015 wt%. However, in an extremely low carbon steel with C <0.50 wt%, even if the Ti concentration is 0.010 wt%, since the initial oxygen concentration before deoxidation is high, the amount of generated oxide is large and the solidification temperature is also high. Blockage occurs. In particular, in order to ensure excellent deep drawability, in the case where 0.010 wt% or more of Ti is contained, in general, clogging of the tundish nozzle is unavoidable.

As a method for solving such a problem, conventionally, in Japanese Patent Laid-Open No. 8-281391, in Al-less Ti deoxidized steel, the oxygen amount of the molten steel passing through the nozzle is limited as a measure for preventing the tundish nozzle from being blocked. Thus, a method for preventing the growth of Ti ₂ O ₃ growing on the inner surface of the nozzle is proposed. However, in the case of Ti deoxidized steel, the oxygen concentration is about 30 ppm. In this case, it can only be cast up to about 800 tons, and the level control of the hot water surface in the mold becomes unstable as the blockage proceeds, It is not a fundamental solution.

In JP-A-8-281390, as measures for preventing clogging of a tundish nozzle in Al-less Ti deoxidized steel, optimization of molten steel Si concentration and inclusion composition are made to be in the Ti ₃ O ₅ —SiO ₂ form. Thus, a method for preventing the growth of Ti ₂ O ₃ growing on the inner surface of the nozzle has been proposed. However, the increase in Si leads to hardening of the material and deteriorates the plating property. Therefore, it is not a desirable method and is not a fundamental solution for preventing nozzle clogging.

  In Japanese Patent Publication No. 7-47764, deoxidation is performed so that Mn is 0.03 to 1.5 wt% and Ti is 0.02 to 1.5 wt%, and inclusions in the steel are changed to MnO: 17 to A non-aging cold-rolled steel sheet as an inclusion having a low melting point composition composed of 31 wt% MnO—Ti-based oxide is proposed. Certainly, with this technology, since a MnO-Ti-based oxide having a low melting point composition that is in a liquid phase in the molten steel is generated as inclusions, even if the molten steel containing these inclusions is passed through a tundish nozzle, the nozzle It can be injected into the mold without adhering to the surface, and it can be said that it is effective in preventing the tundish nozzle from being blocked.

  However, in carrying out this technique, in order to obtain a MnO—Ti-based oxide containing 17 to 31 wt% of MnO, the difference in affinity between Mn, Ti and oxygen in the relationship between the Mn concentration and the Ti concentration in molten steel Therefore, the concentration ratio of Mn and Ti in the molten steel needs to be wt% Mn / wt% Ti ≧ 100 (Yasuyuki Morioka, Kazuki Morita et al .: Iron and Steel, 81 (1995), p40). Therefore, when the Ti concentration in the steel is 0.010 wt%, in order to obtain a MnO—Ti-based oxide containing 17 to 31 wt% of MnO, the Mn concentration needs to be 1.0 wt% or more. However, when the Mn content exceeds 1.0 wt%, the material is cured, and when the Ti content is less than 0.010 wt%, there is a problem that excellent deep drawability cannot be obtained. Therefore, it is actually difficult to make the inclusion into a MnO—Ti-based oxide containing MnO: 17 to 31 wt%.

Furthermore, in JP-A-8-28139, in Al-less Ti deoxidized steel, as a measure for preventing tundish nozzle clogging, a refractory containing CaO · ZrO ₂ particles is used for the nozzle member, thereby allowing Ti in molten steel. _{When 3} O ₅ is trapped by the nozzle, a method of preventing its growth by forming a low melting point inclusion of TiO ₂ —SiO ₂ —Al ₂ O ₃ —CaO—ZrO ₂ is proposed. However, in this technique, due to the variation in the oxygen concentration in the molten steel, if the oxygen is high, the TiO ₂ concentration in the adhering inclusions will be high and the melting point will not be sufficiently lowered. If the oxygen concentration is low, there is a problem that the nozzle is melted, which is not a sufficient countermeasure.

The object of the present invention is to propose a method for melting Ti-containing ultra-low carbon steel suitable for casting without causing nozzle clogging during continuous casting.
Another object of the present invention is to propose a method of melting that is advantageous for obtaining a Ti-containing ultra-low carbon cold-rolled steel sheet excellent in surface cleaning.
Still another object of the present invention is to propose a melting method that is advantageous for obtaining a thin steel sheet for automobiles with less rusting and less surface defects.

As a result of repeated experiments and researches to solve the above-mentioned problems of the prior art, the inventors have made a method for melting Ti-containing ultra-low carbon steel that can be represented by the gist configuration as described below. Led to the development.
That is, in the present invention, in melting ultra-low carbon Ti deoxidized steel containing 0.010 wt% or more of C with 0.020 wt% or less, the molten steel after the converter steel is first vacuum degassed. And then deoxidizing by adding a Ti-containing alloy to the molten steel after the decarburizing treatment to obtain a deoxidized molten steel having a composition satisfying Al ≦ (wt% Ti) / 5, An inclusion composition adjusting alloy containing one or two of Ca ≧ 10 wt% and REM ≧ 5 wt% and one or more selected from Fe, Al, Si and Ti in the deoxidized molten steel Is added to the oxide composition in the molten steel, Ti oxide: 90 wt% or less, at least one of CaO and REM oxide: 10 wt% or more and 50 wt% or less, Al ₂ O ₃ : 70 wt% or less Features to adjust to This is a method for melting Ti-containing ultra-low carbon steel.

In the present invention, the molten steel after decarburization is predeoxidized with any of Al, Si, and Mn prior to the deoxidation treatment with the Ti-containing alloy, so that the dissolved oxygen concentration in the molten steel is 200 ppm in advance. It is a more preferable embodiment that the Ti oxide having a size of about 5 to 20 μm is dispersed in the molten steel after the Ti deoxidation treatment. The present invention is a _SiO 2, MgO mixed in unavoidable or may be contained within the following 5 wt%.

  As described above, according to the melting method according to the present invention, the clogging of the immersion nozzle does not occur at the time of continuous casting, and when the steel sheet for cold rolled automobile subjected to rolling, annealing, plating treatment is used, A Ti-containing ultra-low carbon steel having excellent surface cleanliness, little rusting, and almost no surface defects due to nonmetallic inclusions can be easily obtained.

  The present invention treats Ti deoxidized ultra-low carbon steel having C of 0.020 wt% or less and containing Ti ≧ 0.010 wt%, and in particular, in the deoxidation treatment, the amount of Ti added depends on the amount of Al. It is characterized in that it is adjusted and the composition and form of inclusions in the molten steel are controlled.

That is, according to the present invention, first, when a Ti—Fe alloy is added and deoxidized in molten steel decarburized by an RH vacuum degassing apparatus, the composition of the deoxidized molten steel is Al ≦ (wt% Ti ) / 5 is characterized by adjustment. In this respect, if the adjustment range is not exceeded, Ti is not deoxidized but Al is deoxidized, and a large amount of Al ₂ O ₃ clusters are generated.
In the present invention, the inclusion is composed of an oxide mainly composed of Ti oxide, and the Ti oxide having a size of about 5 to 20 μm is dispersed in the steel so that the steel sheet for cold rolling is used. Prevents inclusion surface defects.

  However, when the Ti content is Ti <0.010 wt%, in the case of an ultra-low carbon steel with C ≦ 0.020 wt%, it is difficult to ensure deep drawability, and the deoxygenation ability is weak, and the total oxygen concentration Becomes higher. On the other hand, the Ti concentration is preferably 0.15 wt% or less in order to prevent the immersion nozzle due to the generation of a large amount of TiN. Therefore, a preferable Ti content is Ti = 0.0.10 to 0.15 wt%.

In the present invention, the molten steel after the converter steel is decarburized by refining with a vacuum degassing device, and as a deoxidation method after the decarburization treatment, first, the molten steel is made of a Ti-containing alloy such as Fe-Ti. Deoxidation is performed to generate inclusions mainly composed of Ti oxide. As a result, the inclusions do not form a cluster as in the case of deoxidation with Al, and have a size of about 5 to 20 μm and are dispersed in the steel. On the other hand, when deoxidizing with Al until the Al concentration exceeds 0.005 wt%, a huge Al ₂ O ₃ cluster is formed, so that the Ti concentration is increased by adding the Ti-containing alloy thereafter. However, it cannot be sufficiently reduced and remains as cluster inclusions in the steel. For this reason, in the present invention, it is necessary to first deoxidize the molten steel with Ti to produce Ti oxide with Ti ₂ O ₃ ≧ 80 wt%.

Ti oxide of Ti ₂ O ₃ ≧ 80 wt% generated by this Ti deoxidation is dispersed in the steel with a size of about 5 to 20 μm and does not grow into a cluster. Therefore, in the steel sheet for cold rolling obtained according to the method of the present invention, there are almost no surface defects due to cluster inclusions. However, since Ti oxide is in a solid state in molten steel, and ultra-low carbon steel has a high solidification temperature of steel, this Ti oxide takes in the metal on the inner surface of the tundish nozzle during continuous casting. It grows in an oval shape and causes nozzle clogging.

Therefore, in the present invention, after deoxidation with a Ti alloy, Fe, Al, Si and Ti containing at least one of 10 wt% or more of Ca, 5 wt% or more of REM (rare earth element) are selected. The inclusion composition adjusting alloy containing one kind or two kinds or more is added, and the oxide composition in the molten steel is 90 wt% or less of Ti oxide and one or more kinds of CaO and REM oxides are 10 wt% or more and 50 wt%. %, And a low melting point inclusion composition containing a Ti oxide containing 70 wt% or less of Al ₂ O ₃ . As a result, adhesion of Ti oxide to the tundish nozzle can be effectively prevented.

Hereinafter, the reason for limiting the composition of the inclusion composition adjusting alloy added in the present invention will be described. First, FIG. 1 shows the range of the preferable composition of the oxide produced | generated in molten steel under the method of this invention. As can be seen from this figure, in the present invention, the inclusion composition control is performed by adding the inclusion composition adjusting alloy to the molten steel after the deoxidation treatment, thereby controlling the inclusion (oxide) composition in the molten steel. It can be seen that Ti oxide ≦ 90 wt%, CaO, REM oxide: 10 to 50 wt%, Al ₂ O ₃ ≦ 70 wt%. This point will be described in more detail below.

The Ca concentration in the Ca-REM-based inclusion composition adjusting alloy containing at least one of Fe, Al, Si, and Ti added after deoxidation using a Ti alloy is less than 10 wt%, Ce, La When the REM is less than 5 wt%, the Ti ₂ O ₃ concentration in the oxide is 90 wt% or more, and the concentration of CaO, REM oxide (La ₂ O ₃ , Ce ₂ O _3, etc.) is less than 10 wt%. The melting point of does not decrease sufficiently. As a result, inclusions do not form clusters in the steel, but adhere to the inner surface of the nozzle and cause clogging.

The composition of the oxide in molten steel by adding the inclusion composition adjusting alloy is such that Ti ₂ O ₃ is 80 wt% or less, and CaO, REM oxide (La ₂ O ₃ , Ce ₂ O _3, etc.) is 10 wt% or more. It is desirable to do. However, when the concentration of CaO, REM oxide (La ₂ O ₃ , Ce ₂ O _3, etc.) in the inclusions in the molten steel exceeds 50 wt%, the inclusions easily contain sulfur in a liquid phase state. As a result, when the liquid phase inclusions solidify, CaS, REM sulfide (LaS, CeS) is generated around the inclusions, which becomes the starting point of rusting in the steel sheet, and the knowledge that the rusting amount of the steel sheet increases remarkably Has been obtained. Therefore, the concentration of CaO, REM oxide (La ₂ O ₃ , Ce ₂ O ₃ etc.) in the inclusions needs to be 50 wt% or less. Since the specific gravity of REM oxide (La ₂ O ₃ , Ce ₂ O ₃ ) is larger than that of other oxides, if this REM oxide exceeds 50 wt%, the floatability of inclusions in molten steel is poor. Therefore, the total oxygen concentration in the steel is high, and the cleanliness in the cold-rolled steel sheet is deteriorated.

Next, when the Al ₂ O ₃ concentration in the inclusion exceeds 70 wt%, the composition has a high melting point and not only nozzle clogging occurs, but also the inclusions are clustered, resulting in defects in nonmetallic inclusion physical properties in the product plate. Will increase.

  Under the method of the present invention, the yield of the Ti alloy is poor compared to the conventional method of deoxidizing with Al, and the inclusion composition adjusting alloy is expensive because it contains Ca and REM. For this reason, it is economical and preferable to add such an alloy into the molten steel so as to be as small as possible within a range in which the composition of inclusions can be controlled.

  Next, about the addition of a deoxidation material, it preliminarily deoxidizes so that the dissolved oxygen concentration in the molten steel before the addition may become 200 ppm or less. This preliminary deoxidation is performed by stirring the molten steel in a vacuum, deoxidation with a small amount of Al so that Al ≦ 0.005 wt% after deoxidation, and addition of Si, FeSi, Mn, and FeMn.

Next, the reasons for limiting the components other than the additive alloy will be described below.
C: If it exceeds 0.020 wt%, deep drawability in the product cannot be secured, so it is necessary to make it 0.020 wt% or less.
If it exceeds Si: 0.20 wt%, the plating properties deteriorate and the surface cleanliness deteriorates, so it is necessary to make it 0.20 wt% or less.
When Mn exceeds 1.0 wt%, the material is cured, so the content is set to 1.0 wt% or less. On the other hand, when the content exceeds 1.0 wt%, the inclusion becomes an inclusion having a low melting point composition of Ti oxide-MnO, and it is not necessary to add an alloy as in the present invention.
When S exceeds 0.050 wt%, CaS and REM sulfide increase in the molten steel, and not only deep drawability cannot be ensured but also rust is very easily generated in the cold-rolled steel sheet as a product.
Moreover, in this invention, it does not have any problem to further contain 1 type or 2 types of B and Nb according to the necessity of the material of a cold-rolled sheet.

Example 1
After the converter steel is discharged, 300 tons of molten steel is decarburized with an RH vacuum degasser, and the composition of the molten steel is C = 0.0035 wt%, Si = 0.02 wt%, Mn = 0.20 wt%, P = 0.015 wt%, S = 0.010 wt%, and the temperature were adjusted to 1600 ° C. In this molten steel, 0.5 kg / ton of Al was added, and the dissolved oxygen concentration in the molten steel was reduced to 150 ppm. At this time, the Al concentration in the molten steel was 0.003 wt%. Then, the molten steel was deoxidized by adding 1.2 kg / ton of a 70 wt% Ti—Fe alloy. Thereafter, 20 kg% Ca-10 wt% REM-50 wt% Ti-Fe alloy was added to the molten steel at 0.5 kg / ton, and the components were adjusted. The Ti concentration after this treatment was 0.050 wt%, and the Al concentration was 0.003 wt%. Next, a casting experiment was performed using a two-strand slab continuous casting apparatus. As a result of investigating the inclusion in the tundish at this time, it was a spherical inclusion of 65 wt% Ti ₂ O ₃ -15 wt% CaO-10 wt% Ce ₂ O ₃ -10 wt% Al ₂ O ₃ . After casting, there was almost no deposit in the immersion nozzle. This slab was hot-rolled to 3.5 mm, cold-rolled to 0.8 mm, and further annealed at 780 ° C. for 45 seconds. In this annealed plate, only 0.1 / 1000 m or less non-metallic inclusion physical defects were observed. Also, rusting was not a problem as with conventional Al deoxidation.

Example 2
After converter steelmaking, 300 tons of molten steel was decarburized by RH vacuum degassing equipment, C = 0.030 wt%, Si = 0.02 wt%, Mn = 0.25 wt%, P = 0.020 wt% , S = 0.012 wt%, and the temperature was adjusted to 1600 ° C. In this molten steel, 0.5 kg / ton of Al was added, and the dissolved oxygen concentration in the molten steel was reduced to 170 ppm. At this time, the Al concentration in the molten steel was 0.002 wt%. The molten steel was deoxidized by adding 1.4 kg / ton of 70 wt% Ti—Fe alloy. Then, 0.3 kg / ton of 20 wt% Ca-15 wt% REM-40 wt% Al-Fe alloy was added in molten steel, and the component adjustment was performed. The Ti concentration after this treatment was 0.030 wt%, and the Al concentration was 0.004 wt%. Next, casting was performed using a 2-strand slab continuous casting apparatus. As a result of examining the inclusions in the tundish at this time, it was a spherical inclusion of 50 wt% Ti ₂ O ₃ -15 wt% CaO-10 wt% Ce ₂ O ₃ -25 wt% Al ₂ O ₃ . After casting, there was almost no deposit in the immersion nozzle. This slab was hot-rolled to 3.5 mm, cold-rolled to 0.8 mm, and further annealed at 780 ° C. for 45 seconds. In this annealed plate, only 0.02 pieces / 1000 m or less of defects having surface defects and nonmetallic inclusion physical properties were observed. Also, rusting was not a problem as with conventional Al deoxidation.

Comparative Example 1
After converter steelmaking, 300 tons of molten steel was decarburized by RH vacuum degassing equipment, C = 0.030 wt%, Si = 0.02 wt%, Mn = 0.20 wt%, P = 0.015 wt% , S = 0.010 wt%, and the temperature was adjusted to 1600 ° C. In this molten steel, Al was added at 0.7 kg / ton, and the dissolved oxygen concentration in the molten steel was reduced to 170 ppm. At this time, the Al concentration in the molten steel was 0.003 wt%. Then, 1.2 kg / ton of 75 wt% Ti-25 wt% Fe alloy was added to the molten steel to perform deoxidation and component adjustment. The Ti concentration after the treatment was 0.040 wt%, and the Al concentration was 0.002 wt%. Next, this molten steel was cast with a two-strand slab continuous casting apparatus. As a result of investigating the inclusions in the tundish at this time, fine inclusions having a composition of 90 wt% Ti ₂ O ₃ -10 wt% Al ₂ O ₃ were dispersed. After casting, deposits of Ti ₂ O ₃ —Al ₂ O ₃ were observed in the immersion nozzle. This slab was hot-rolled to 3.5 mm, cold-rolled to 0.8 mm, and further annealed at 780 ° C. for 45 seconds. On this annealed plate, defects of surface defects and non-metallic inclusions were found in the 0.05 / 1000 m coil. Also, rusting was not a problem as with conventional Al deoxidation.

Comparative Example 2
After converter steelmaking, 300 tons of molten steel was decarburized by RH vacuum degassing equipment, C = 0.030 wt%, Mn = 0.20 wt%, P = 0.015 wt%, S = 0.010 wt% The temperature was adjusted to 1600 ° C. In this molten steel, Al was added at 1.5 kg / ton, and then 75 wt% Ti-25 wt% Fe alloy was added at 0.5 kg / ton to perform deoxidation and component adjustment. The Ti concentration after the treatment was 0.040 wt%, and the Al concentration was 0.035 wt%. Next, this molten steel was cast with a two-strand slab continuous casting apparatus. As a result of investigating the inclusion in the tundish at this time, it was a 5 wt% Ti ₂ O ₃ -95 wt% Al ₂ O ₃ cluster-like inclusion. After casting, deposits of Al ₂ O ₃ were observed in the immersion nozzle. This slab was hot-rolled to 3.5 mm, cold-rolled to 0.8 mm, and further annealed at 780 ° C. for 45 seconds. In this annealed plate, defects of surface defects and non-metallic inclusions were observed in 0.4 / 1000 m coils.

The inclusion composition controlled by the method of the present invention is shown.

Claims (3)

In melting ultra-low carbon Ti deoxidized steel containing 0.010 wt% or more of C with 0.020 wt% or less, first, the molten steel after the converter steel is first decarburized by a vacuum degassing device, Next, by adding a Ti-containing alloy to the decarburized molten steel and deoxidizing it, a deoxidized molten steel having a composition satisfying Al ≦ (wt% Ti) / 5 is obtained, and then, in the deoxidized molten steel By adding an alloy for adjusting inclusion composition containing one or two of Ca ≧ 10 wt% and REM ≧ 5 wt% and one or more selected from Fe, Al, Si and Ti, The oxide composition in the molten steel is adjusted to Ti oxide: 90 wt% or less, at least one of CaO and REM oxide: 10 wt% or more and 50 wt% or less, and Al ₂ O ₃ : 70 wt% or less. Ti-containing extremely low Carbon steel melting method. Prior to the deoxidation treatment with the Ti-containing alloy, the decarburized molten steel is preliminarily deoxidized with any of Al, Si, and Mn, so that the dissolved oxygen concentration in the molten steel is set to 200 ppm or less in advance. The melting method according to claim 1, wherein 3. The melting method according to claim 1, wherein after the Ti deoxidation treatment, a Ti oxide having a size of about 5 to 20 μm is produced in a state dispersed in the molten steel.

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