JP3375862B2 - Method for producing ultra-low carbon steel without blowholes - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an ultra-low carbon steel having no blowholes or blister defects. 2. Description of the Related Art In recent years, there has been an increasing demand for improved workability of cold rolled steel sheets and cold rolled plated steel sheets. Increasingly. Cold rolled steel sheets are generally manufactured by hot rolling, then cold rolling, and then an annealing process. However, the annealed steel sheets may show raised blowholes (bulging defects). In many cases, these defects are found at the final stage of the cold-rolled steel sheet manufacturing process, so that cost or process damage is serious. As a cause of the blowhole defect, it is considered that argon bubbles are blown in to prevent continuous casting powder from being caught or clogging of the immersion nozzle. As a conventional technique for preventing blowhole defects, a method in which a casting speed is specified for each casting width (Japanese Patent Laid-Open No. 2-70354).
JP) and a method of argon gas Komu blown by mixing N ₂ gas (JP 62-38747 JP), a method of heating in advance blowing argon (Japanese Patent Laid-Open No. 4-17967) is disclosed ing. [0003] However, these methods not only cannot completely prevent blowhole defects, but also have their own downsides in terms of operation and quality. Specifically, the regulation of the casting speed by casting width significantly impairs the productivity, and argon gas and N ₂
In crowded blown mixed gas of the gas portion of blowing N ₂ is dissolved and absorbed in the molten steel, the workability of the finished product, there is a significantly impair fear moldability. In addition, in the method of preheating the blown argon, the blowpipe attached to the immersion nozzle glows red, causing a sudden change in temperature between the immersion nozzle and the blowpipe to deform the fitting part and seal. There was a risk of inducing defects. In view of these problems, an object of the present invention is to solve the impediments to operation and product quality and to provide a method for casting ultra-low carbon steel that does not completely generate blowhole defects at the product stage. . [0004] In order to solve the above-mentioned problems, the present inventor casts ultra-low carbon steel under various casting conditions using a curved continuous caster, and the distribution of bubbles in the slab at that time. A comparison was made between the situation and the occurrence of blowholes in products. As a result, it was found that only argon bubbles existing within 30 mm from the surface layer in the slab of ultra-low carbon steel form blowholes in the product after rolling. In other words, 30 minutes from the slab surface
It has been found that argon bubbles trapped at a depth of more than mm do not form blowholes. The present invention has been made based on such knowledge, and in order to manufacture a slab that does not completely generate blowhole defects, it is necessary not to trap argon bubbles at a solidification interface within a portion within 30 mm from the slab surface layer. It is. It is not necessary to specifically limit whether or not argon bubbles exist on the center side of the slab more than 30 mm from the slab surface layer. As a specific method, using a curved continuous caster, when casting ultra-low carbon steel, install an electromagnetic stirrer at least inside the curve immediately below the mold, the following conditions by the electromagnetic stirrer This is a method for producing an ultra-low carbon steel in which blowholes do not occur by adding molten steel flow to the solidification interface at least on the inner side of the curve at least in a satisfactory region. Zm ≦ Z ≦ Vc · 900 / (k ² ) where Z: distance from meniscus to electromagnetic stirring position (m) Vc: casting speed (m / min) Zm: effective length of mold: meniscus Distance from the mold to the lower end of the mold (m) k: solidification constant (mm / min ^0.5 ) Next, examples of the present invention will be described with reference to comparative examples. FIG. 1 is an explanatory diagram showing an example of the present invention, and FIG. 2 is an explanatory diagram showing a comparative example. In general,
To cast extremely low carbon steel slabs with a curved continuous caster,
As shown in FIGS. 1 and 2, the molten steel produced in the converter and the secondary refining process is transported to a continuous casting machine in a molten steel pot,
The molten steel is poured into a continuous casting mold 3 through an intermediate container 1 called a tundish using a dipping nozzle 2. The poured molten steel is cooled by the mold 3 and the subsequent secondary cooling, and forms a solidified shell 5 from the surface. The solidified shell is pulled out while being held by the rolls 4 and completely solidified in a continuous casting machine, then cut to a predetermined length by a cutting machine, and turned into a slab to be subjected to the subsequent hot rolling process. Sent. In this process, argon gas is generally blown into the immersion nozzle 2 as a measure against nozzle clogging, but in the conventional method shown in FIG. The molten steel flow 7 flows deep down and is trapped in the upper surface side solidified shell 5 during the floating process, and the argon bubbles 9 cause a swelling phenomenon in a lower process, resulting in “bulging defects”. In addition, "blowing defect" observed in the product inspection of ultra-low carbon steel of titanium killed or niobium titanium killed,
It is a bulge having a width of 1 to 4 mm and a length of several to several hundreds of mm, and is remarkably generated when it is cast by a curved continuous casting machine or when it is cast at a high casting speed. Next, FIG. 3 schematically shows a cross-sectional observation of the blister defect portion observed in the cold-rolled steel sheet, and the position of the defect in the slab is determined from the position from the steel sheet surface to the rolling ratio. Is shown in FIG. 4, and FIG. 5 shows the effect of preventing the occurrence of blistering defects by applying electromagnetic stirring immediately below the mold according to the present invention. From these figures, the present inventors considered the mechanism of the occurrence of blistering defects in titanium-killed and niobium-titanium-killed ultra-low carbon steel as follows. 1) The argon bubbles blown from the immersion nozzle etc.
The flow of the molten steel injected from the immersion nozzle penetrates to a depth from the meniscus where the rising velocity of the argon bubbles and the descending flow velocity of the molten steel balance. 2) Argon bubbles may be trapped at the position of the thickness of the solidified shell on the upper surface of the slab corresponding to the depth of penetration of argon bubbles from the meniscus. 3) When a steel sheet is manufactured from this slab, H atoms in the steel and in the atmosphere diffuse during rolling and annealing and collect in the argon bubbles. 4) The H atoms collected in the argon bubbles become H2 molecules as the temperature of the steel sheet decreases, and the hydrogen partial pressure in the argon bubbles becomes extremely high. 5) When the capturing position of the argon bubbles in the slab is close to the surface, the partial pressure of hydrogen in the argon bubbles exceeds the strength of the steel sheet on the surface side of the argon bubbles, and the thin portion of the surface layer is “bulging”. It is detected as a raised defect in the surface observation of the steel sheet. Investigation of the floating behavior of inclusions and air bubbles inside the continuous caster revealed that it was trapped on the upper surface side of the curved portion of the curved continuous caster or the vertical bending continuous caster. . In other words, it is common that almost no argon bubbles that cause blister defects are captured on the outside of the curve (the lower surface side of the slab). In addition, even if the casting speed is limited with a curved continuous caster, it is impossible to completely float and remove the argon bubbles, and a continuous caster having a vertical portion corresponding to the depth to which the argon bubbles reach. New construction or remodeling had disadvantages such as requiring huge construction costs and construction periods. The inventor of the present invention has made many observations on the position of capturing argon and the occurrence of blistering defects, and as shown in FIG.
It is apparent from the findings of the present inventor that although present as trapped up to mm, those detected as blistering defects are limited to argon bubbles trapped in the surface layer from the slab surface to 30 mm. became. Based on this finding, in order to cast ultra-low carbon steel having no blistering defect with a curved continuous caster, as shown in FIG. 1, an electromagnetic stirrer 6 was installed at least inside the curve immediately below the mold, The electromagnetic stirring device 6
As a result, it was confirmed that it is effective to add the molten steel flow 8 to the solidification interface at least on the inside of the curve in a portion satisfying the following conditions. Zm ≦ Z ≦ Vc · 900 / (k ² ) where Z: distance from meniscus to electromagnetic stirring position (m) Vc: casting speed (m / min) Zm: effective length of mold: meniscus Distance from the mold to the lower end of the mold (m) k: solidification constant (mm / min ^0.5 ) Note that this solidification constant k varies strictly in the range of 20 to 27 depending on the type of steel component and secondary cooling conditions, Under the cooling condition described above, it may be assumed that k ≒ 25. According to the application of the present invention, the argon bubbles 9 blown from the immersion nozzle 2 are sunk deeply downward by the molten steel flow 7 from the immersion nozzle 2, but are caused by the electromagnetic stirring action of the electromagnetic stirring device 6 shown in FIG. In addition, it is possible to prevent the argon bubbles 9 from being trapped in the upper surface side solidified shell 5, and to avoid occurrence of a blister defect in a lower process. EXAMPLE A molten steel having the composition shown in Table 1 as a representative chemical component was prepared by using a mold having an effective length of 0.8 m and a radius of curvature of 10.5.
As shown in FIG. 1, casting is performed at various casting speeds using a curved continuous casting machine having a length of m, and a stirring force acts directly below the mold and on the inside of the curve within a range of 1 to 1.5 m from the meniscus. An electromagnetic stirrer was installed, and the occurrence of blister defects due to its application was investigated. [Table 1] The casting size was 245 mm thick and 1500 mm wide. The slab thus cast was subjected to hot rolling and cold rolling to finally form a 0.7 mm × 1500 mm cold-rolled steel strip (coil). Thereafter, the occurrence of blistering defects in the cold-rolled steel strip was arranged in FIG. Here, the blister generation index is an index indicating the number of blister defects observed per coil. As shown in FIG. 5, the occurrence of the blistering defect is reduced in the conventional curved continuous casting machine due to a decrease in the casting speed.
Even at a very low casting speed of 0.5 m / min, it was impossible to completely prevent blistering defects. On the other hand, when electromagnetic stirring is applied to the inside of the curve immediately below the mold, the molten steel flow 8 that hinders trapping at the solidification interface of the solidification shell inside the curve, where argon bubbles are likely to be trapped.
Can be added. Therefore, by the application of the present invention, the effect of suppressing blistering defects was remarkable, and blistering defects could be completely prevented even at a casting speed of 1.5 m / min without lowering the casting speed. As described above, by applying the method of the present invention, when producing a cast piece of ultra-low carbon steel by curved continuous casting, an electromagnetic stirring device is installed immediately below the mold.
At least by adding molten steel flow to the solidification interface inside the curve,
By preventing the capture of argon bubbles, blistering defects could be completely prevented without lowering the casting speed, and the product yield could be significantly improved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram showing an embodiment of the present invention. FIG. 2 is an explanatory diagram showing a comparative example. FIG. 3 is an explanatory view showing a cross section of a blister defect generating portion. FIG. 4 is an explanatory view showing a position where an argon bubble causing a blistering defect is captured in a slab. FIG. 5 is an explanatory diagram showing an effect of the present invention. [Description of Signs] 2 Immersion nozzle 3 Mold for continuous casting 5 Solidified shell 6 Electromagnetic stirring device 8 Molten steel flow

Claims (1)

(57) [Claims 1] When casting ultra-low carbon steel using a curved continuous caster, an electromagnetic stirrer is installed at least inside the curve immediately below the mold, and the electromagnetic stirrer is provided. A flow of molten steel is added to at least a solidification interface on the inside of a curve of a portion satisfying the following conditions: Zm ≦ Z ≦ Vc · 900 / (k ² ) where Z: distance from meniscus to electromagnetic stirring position (m) Vc: casting speed (m / min) Zm: effective length of mold: meniscus Distance from the mold to the lower end of the mold (m) k: solidification constant (mm / min ^0.5 )