JP3577357B2 - Method for producing ultra-low carbon steel with excellent surface properties - Google Patents

Description

【００１６】
比較例であるＮｏ．５とＮｏ．８は介在物中のチタン濃度が５４ｍａｓｓ％超、Ｎｏ．６とＮｏ．７は介在物中のアルミ濃度が１３ｍａｓｓ％超であり、冷間圧延製品の表面欠陥指数は０．４５以上であるのに対して、本発明の方法では、鋳片中の介在物の組成がチタン濃度が４２ｍａｓｓ％以上５４ｍａｓｓ％以下かつアルミ濃度が３ｍａｓｓ％以上１３ｍａｓｓ％以下で残部が酸素および不可避的不純物元素よりなる組成であり、冷間圧延製品の表面欠陥指数が０．１５以下と比較例の１／３以下になっている。
【００１７】
【発明の効果】
本発明の方法により、製造コストを上げることなく、鋳片中の介在物の組成を冷間圧延製品での表面欠陥が出現しにくい組成に制御することができ、冷間圧延製品の表面欠陥を低減することができた。
【図面の簡単な説明】
【図１】鋳片中の介在物組成に及ぼすアルミ脱酸後の酸可溶アルミ濃度とチタン添加後のチタン濃度の影響を示す図である。
【図２】冷間圧延製品の表面欠陥指数に及ぼすアルミ脱酸後の酸可溶アルミ濃度とチタン添加後のチタン濃度の影響を示す図である。[0001]
[Industrial applications]
The present invention relates to a method for producing ultra-low carbon steel having excellent surface properties, which is used for automobiles, household appliances, furniture, containers, and the like.
[0002]
[Prior art]
Conventional ultra-low carbon steel uses a vacuum decarburizer to melt undeoxidized molten steel having a carbon (hereinafter referred to as C) concentration of 0.01 mass% or more and 0.06 mass% or less in a steel smelting furnace. It has been manufactured by decarburizing to a C concentration of 0.005 mass% or less and then adding aluminum to deoxidize. When aluminum is added to molten steel, alumina is generated as a deoxidation product. Further, the acid-soluble aluminum in the molten steel is oxidized by contact with the atmosphere or slag, and alumina is generated. Alumina generated in the molten steel floats and is absorbed by the slag or adheres to a refractory of a ladle or a tundish and is removed from the molten steel, but many of the alumina remains in the steel.
[0003]
The alumina is exposed on the surface of the steel sheet during rolling, and becomes a surface defect. Therefore, in order to reduce the surface defects of the steel sheet, it is necessary to reduce the alumina remaining in the steel. To reduce the amount of alumina in steel, reduce the amount of alumina generated during deoxidation, promote removal of generated alumina, and prevent oxidation of acid-soluble aluminum in molten steel by air or slag. There is a way.
[0004]
In ultra-low carbon steel, the amount of alumina produced during deoxidation is substantially determined by the dissolved oxygen concentration before aluminum deoxidation in order to keep the dissolved oxygen concentration after aluminum deoxidation at 0.001 mass% or less. Keeping in mind that dissolved oxygen must be at least 0.03 mass% for decarburization, the dissolved oxygen concentration before aluminum deoxidation cannot be reduced unnecessarily. Is difficult to reduce.
[0005]
As a method of accelerating the removal of the generated alumina, a method of increasing the reflux time of molten steel after deoxidizing aluminum or a method of blowing an inert gas from a gas inlet installed at the bottom of a ladle is generally performed. . Prolonging the reflux time of the molten steel results in prolongation of the processing time, and lowers productivity. The method of blowing an inert gas from a gas inlet installed at the bottom of a ladle increases the production cost due to maintenance of the gas inlet and an increase in the unit of inert gas.
[0006]
As a method of preventing oxidation of acid-soluble aluminum in molten steel, there is a known technique of strengthening an Ar seal of a tundish. As a method of preventing oxidation by slag, deoxidation is performed on slag in a ladle. Agent was added, and T.G. A method for reducing the Fe concentration to 5 mass% or less is disclosed in Japanese Patent Application Laid-Open No. Hei 2-30711. In the case where the Ar seal of the tundish is strengthened, the unit consumption of Ar gas increases and the cost increases. T. in slag The method of lowering Fe requires a deoxidizing agent for slag and increases the cost.
[0007]
[Problems to be solved by the invention]
As described above, it is difficult to reduce the residual amount of alumina in steel without increasing the manufacturing cost with the conventional technology. Furthermore, it is difficult to eliminate alumina in steel even if the above measures are taken. Therefore, it is an object to control the composition of inclusions within a range in which surface defects are hardly generated without increasing the manufacturing cost.
[0008]
[Means for Solving the Problems]
The present invention was invented in order to advantageously solve the above-mentioned problems, and the gist of the present invention is to produce a steel having a carbon concentration of 0.005 mass% or less by using a vacuum degassing apparatus. After decarburizing treatment, aluminum is deoxidized to make the acid-soluble aluminum concentration 0.005 to 0.015 mass%, and then titanium is added to make the titanium concentration 0.02 to 0.10 mass%. This is a method for producing an ultra-low carbon steel having excellent surface properties.
[0009]
[Action]
Normally, in ultra-low carbon steel, titanium is added to improve processing characteristics. The essence of the present invention is to control the composition of inclusions in the steel to a composition that hardly causes surface defects by using titanium and aluminum added for deoxidation. The present inventor has conducted systematic experiments to find that the composition of inclusions that are unlikely to cause surface defects is such that the titanium concentration is 42 mass% or more and 54 mass% or less, the aluminum concentration is 3 mass% or more and 13 mass% or less, and the balance is oxygen and inevitable impurity elements. It was clarified that the composition consisted of:
[0010]
Therefore, a deoxidation method for controlling the inclusion composition within this range was studied, and the acid-soluble aluminum concentration after aluminum deoxidation and the titanium concentration after titanium addition were important for controlling the inclusion composition. Revealed.
[0011]
FIG. 1 shows the effect of the acid-soluble aluminum concentration after aluminum deoxidation and the titanium concentration after titanium addition on the composition of inclusions in the slab. The composition of inclusions is adjusted by setting the acid-soluble aluminum concentration after aluminum deoxidization to 0.005 mass% or more and 0.015 mass% or less, and the titanium concentration after titanium addition to 0.02 mass% or more and 0.10 mass% or less. The composition can be controlled so that the above-mentioned surface defects are hardly generated.
[0012]
FIG. 2 shows the effect of the acid-soluble aluminum concentration after aluminum deoxidization and the titanium concentration after titanium addition on the surface defect index of the cold-rolled product. Here, the surface defect index is an index of the number of surface defects per 100 m of the coil. By making the acid-soluble aluminum concentration after deoxidizing aluminum 0.005 mass% or more and 0.015 mass% or less and the titanium concentration after adding titanium to 0.02 mass% or more and 0.10 mass% or less, the cold-rolled product The surface defect index can be reduced.
[0013]
In the method of the present invention, aluminum added to molten steel is pure aluminum, one or a mixture of two or more aluminum-containing alloys, and titanium may be any of pure titanium and one or a mixture of two or more titanium-containing alloys. , The effect is equivalent.
Further, any of RH, DH, VOD, and tank degasser may be used as the vacuum degassing device, and the effects are the same.
[0014]
【Example】
C: 0.03 to 0.05 mass%, Si: 0.001 to 0.03 mass%, Mn: 0.05 to 0.2 mass%, P: 0.01 to 0.02 mass% produced in the converter. , S: 0.005 to 0.02 mass%, O: 0.03 to 0.1 mass%, molten steel containing an unavoidable impurity element is put on a ladle, and the C concentration is measured using an RH vacuum degassing apparatus. Was decarburized to 0.005 mass% or less. The dissolved oxygen concentration was measured, aluminum was added according to the value, and titanium was further added. The acid-soluble aluminum concentration after aluminum deoxidation and the titanium concentration after titanium addition were changed as shown in Table 1. No. Nos. 1 to 4 are the methods of the present invention. 5 to 8 are comparative examples. These molten steels were continuously cast, hot-rolled and cold-rolled under the same conditions, and the surface defect indexes of the products were compared. The results are shown in Table 1.
[0015]
[Table 1]

[0016]
No. of Comparative Example. 5 and No. 5 No. 8 has a titanium concentration in the inclusions of more than 54 mass%, 6 and no. In No. 7, the aluminum concentration in the inclusions was more than 13 mass%, and the surface defect index of the cold-rolled product was 0.45 or more, whereas in the method of the present invention, the composition of the inclusions in the slab was Comparative example in which the titanium concentration is from 42 mass% to 54 mass% and the aluminum concentration is from 3 mass% to 13 mass% and the balance is composed of oxygen and unavoidable impurity elements, and the surface defect index of the cold-rolled product is 0.15 or less. Is 1/3 or less.
[0017]
【The invention's effect】
By the method of the present invention, it is possible to control the composition of inclusions in a slab to a composition in which surface defects in a cold-rolled product are unlikely to appear without increasing the production cost, and to reduce the surface defects of the cold-rolled product. Could be reduced.
[Brief description of the drawings]
FIG. 1 is a graph showing the effects of the concentration of acid-soluble aluminum after deoxidation of aluminum and the concentration of titanium after addition of titanium on the composition of inclusions in a slab.
FIG. 2 is a graph showing the influence of the acid-soluble aluminum concentration after aluminum deoxidization and the titanium concentration after titanium addition on the surface defect index of a cold-rolled product.

Claims (1)

In melting steel having a carbon concentration of 0.005 mass% or less, after decarburizing in a vacuum degassing apparatus, aluminum is deoxidized, and the acid-soluble aluminum concentration is reduced to 0.005 to 0.015 mass%. And then adding titanium to adjust the titanium concentration to 0.02 to 0.10 mass%, thereby producing a very low carbon steel having excellent surface properties.

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