JPH0649523A - Smelting method for aluminum killed steel for cold rolling - Google Patents

Abstract

PURPOSE:To continuously cast the above steel without generating blister defects by determining the amt. of the Ca to be added by the total oxygen concn. in a molten steel, blowing an inert gas to the molten steel to absorb CaO Al2O3 inclusions in slag and not blowing the inert gas to the inside wall of a nozzle at the time of casting. CONSTITUTION:T[Ca] is determined to attain 1.4>=R>=0.7 when total Ca concn. T[Ca] total oxygen concn. T[O]=R is designated from the total oxygen concn. T[O] in the molten extra-low carbon aluminum killed steel with which secondary refining ends. The required amt. of the Ca is then added to the molten steel. The inert gas is blown to the molten steel added with the Ca to form and float the CaO-Al2O3 inclusions which are absorbed in slag. The blowing is stopped when the Ca concn. attains <=10ppm. Further, the inert gas is not blown at all to the inside wall of the upper nozzle, sliding nozzle and immersion nozzle of a tundish at the time of continuously casting this molten steel. As a result, the molten aluminum killed steel for cold rolling with which the generation of the blister defects is prevented is smelted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting aluminum-killed steel for cold rolling, and in particular, it does not involve blowing an inert gas into the inner wall of a tundish upper nozzle, a sliding nozzle or a dipping nozzle as in the prior art. Even if you do not do not come to the blockage,
In addition, the present invention relates to a method for melting aluminum-killed steel for cold rolling in which Ca does not generate rust due to its effect even if Ca is added to remove Al ₂ O ₃ .

[0002]

2. Description of the Related Art Recent advances in secondary refining techniques such as vacuum degassing techniques have enabled continuous casting of ultra-low carbon aluminum killed steel. Degassing technology made this possible, and as a result, it became possible to manufacture high-cleanliness steel with few non-metallic inclusions. However, when an ultra-low carbon cold-rolled steel sheet is annealed, the surface of the annealed steel sheet has a width of 1 to 4 mm and a length of 1 to 4 mm.
It was found that a 6 cm raised portion was generated. This raised portion is a so-called "blister defect", which often occurs especially in the case of an ultra-low carbon steel having a carbon content of 0.015% by weight or less. It was a cause of a large drop in the stay. It has been found that the cause of the blistering defect is greatly influenced by non-metallic inclusions in steel, particularly Al ₂ O ₃ .

In the case of ultra-low carbon steel, since there is a marked difference in the deformation strength between the matrix of base metal and inclusions, when cold rolling is performed, the portion of the matrix that is soft with respect to hard Al ₂ O _{3 is used} . Has a larger elongation, and as a result, a void is formed at the boundary between the two. The void is proportional to the size of the Al ₂ O _3, the more the voids increases the larger the Al ₂ O ₃ inclusions. However when annealed cold-rolled steel sheet having such voids, the but of H ₂ atmosphere gas AX gas or the like into the void from entering, because the solubility of H ₂ in particular during quenching cooling is reduced, of H ₂ in the void The partial pressure rises and the surface of the steel sheet in the vicinity of Al ₂ O ₃ swells, resulting in what is called a “blister defect”. Such a "blister defect" does not occur when the void is small, that is, when the size of the Al ₂ O ₃ inclusion is small, because the increase in the internal pressure is small. Furthermore, according to the study of the trap to the steel in Al ₂ O ₃ to produce the blister defects, most of the part of Al ₂ O ₃ aggregated adhered to the immersion nozzle are mainly composed of Al ₂ O ₃ It has been found to have been carried away from the surface of the immersion nozzle. Therefore, in order to prevent this blistering defect, it is most important to prevent Al ₂ O ₃ from adhering to the immersion nozzle.

From this point of view, conventionally, an inert gas was blown into the immersion nozzle, and Ca or Ca-Si was added into the molten steel.
It was added Ca alloy etc., also suppress aggregation deposition amount of the Al ₂ O ₃ inclusions and a low melting point of CaO-Al ₂ O ₃ inclusions, Al ₂ O ₃ to the immersion nozzle by its melting point lowering amount Techniques for doing so are disclosed. For example, in Japanese Patent Laid-Open No. 61-276756, "2 to 40 wt% of steel is added by adding Ca or a Ca alloy to a molten aluminum killed steel containing C≤0.015 wt% at the melting stage or continuous casting. A method for preventing blistering defects in an ultra-low carbon cold-rolled steel sheet is characterized by treating ppm ultra-low carbon so as to form CaO-Al ₂ O ₃ -based inclusions. "

Further, in Japanese Patent Laid-Open No. 1-99761, "When continuously casting aluminum killed steel, a refractory cylinder whose lower end is immersed in molten steel of a tundish within a distance of 1 m from the mounting center position of a tundish nozzle is arranged. However, 5 to 20 ppm of Ca is added to the amount of molten steel passing through the tundish nozzle in the refractory cylinder, and a continuous casting method of aluminum killed steel is disclosed. However, in the method of blowing an inert gas such as argon into the immersion nozzle, the blown inert gas is trapped by the solidified shell in the mold, there is a problem that becomes the origin of blowholes or swelling defects of the slab, and Ca or Ca
In the method of adding the alloy, the amount of Ca added to the steel is
When the residual amount of steel exceeds 10 ppm, there is a problem that rust tends to occur, and on the other hand, Al ₂ O of the immersion nozzle
_{3 In} order to prevent blockage due to cohesive adhesion, there is a trade-off problem that the Ca concentration in the steel needs to be 20 to 50 ppm in most cases.

[Problems to be Solved by the Invention] As described above, the technique of adding Ca to molten aluminum killed steel is an extremely effective means from the viewpoint of preventing clogging of the nozzle portion. If there is a residual amount that exceeds, there is a problem that rust is likely to occur, and it is extremely difficult to determine the appropriate amount of Ca added. However,
The present inventors obtained the following knowledge during the Ca addition experiment of aluminum killed steel. And Ca amount to be added to aluminum-killed molten steel, resulting in CaO-Al ₂ O ₃ inclusions refractory compositions in the balance of the total oxygen concentration T [O] was produced and crumble in the steel. In particular, when [O] in the steel is higher than the Ca concentration in the molten steel, inclusions having a variation in the ratio of xCaO · yAl ₂ O ₃ are generated, and CaO · 2Al ₂ O ₃ , CaO · 6Al
_{It has been found that} high melting point inclusions such as ₂ O ₃ are generated, and these high melting point inclusions selectively adhere to the upper nozzle of the tundish, the sliding nozzle, the dipping nozzle, etc., making continuous casting impossible. From the above findings of the present inventors, an object of the present invention is to provide T [Ca] / in continuous casting of aluminum killed steel.
By providing an appropriate amount of T [O] and maintaining the appropriate amount, it is possible to eliminate the gas injection by the inert gas in the nozzle part, and to provide an effective melting method for cold-rolled aluminum-killed steel. There is.

[0007]

The subject matter of the present invention is as follows. That is, (1) in the method for melting ultra-low carbon aluminum-killed steel for cold rolling, the total Ca concentration T in the ultra-low-carbon aluminum-killed steel is T
[Ca] / total oxygen concentration T [O] = R1.4 ≦ R ≦ 0.7
And determining the T [Ca] from T [O] so, the steps to be absorbed into the slag to float the CaO-Al ₂ O ₃ based inclusions generated by blowing inert gas into the molten steel, A method for smelting aluminum-killed steel for cold rolling, characterized in that the upper wall of the tundish for continuous casting, the sliding nozzle and the inner wall of the immersion nozzle are not blown with an inert gas at all. (2) At the stage where the CaO-Al ₂ O ₃ -based inclusions generated by blowing an inert gas into the molten steel are floated and absorbed by the slag, they are inert when the Ca concentration in the molten steel reaches 10 ppm or less. The method for smelting aluminum-killed steel for cold rolling according to (1) above, wherein the blowing of gas is stopped. (3) refractory as means for removing the molten steel in place of the step of absorbing the slag to float the CaO-Al ₂ O ₃ based inclusions generated by blowing inert gas, CaO-Al ₂ O ₃ inclusions The method for producing an aluminum-killed steel for cold rolling according to (1) or (2) above, wherein the molten steel is treated through a material filter.

The present inventors conducted experiments on the steps (A), (B) and (C) shown in FIGS. (A) Secondary refining by RH The molten steel 4 roughly decarburized in the converter and tapped in the ladle 2 is transferred to the RH vacuum degassing device 6 shown in FIG. After carrying out secondary refining such as degassing, it was completely deoxidized with Al or Al slag to produce aluminum-killed steel. The molten steel 4 is covered with a converter slag 8. (B) Addition of Ca Addition of Ca to the secondary refined molten steel 4. Fig. 2 shows a Ca alloy wire wound around an uncoiler fed by an injector, and a molten steel 4 in a ladle 2 through a guide pipe.
Although the method of forcibly adding to the above is schematically shown, any other method may be used, such as Ca alloy powder such as Ca-Si is pneumatically fed by Ar or the like and blown through a lance. (C) Floating separation of CaO / Al ₂ O ₃ inclusions by blowing inert gas (A) T in molten steel 4 after RH degassing treatment by (A)
[O] was about 35 ppm as indicated by RH in FIG. T immediately after adding Ca to the molten steel 4 by (B)
[Ca] = 30 ppm and T [O] = 30 pp
It was m. That is, in this case, T [Ca] / T [O]
= 30/30 = 1, for example, a porous plug provided at the bottom of the ladle 2 or a lance 10 as shown in FIG.
Bubbling is performed by blowing an inert gas such as Ar through Ar to generate Ar bubbles 9, and CaO—Al ₂ O ₃ -based inclusions are precipitated and floated to be absorbed by the slag. T [Ca] = 1
It became 0 ppm and T [O] = 12 ppm, and reached point F shown in FIG.

Since the T [O] in the ultra-low carbon aluminum killed steel used as the test material is almost constant at 35 ppm at the end of the RH secondary refining as described above,
The same test was repeated by varying the amount of Ca added by the operation as shown in FIG. 2 according to the item (B), and the C remaining in the molten steel 4 was changed.
Many tests were conducted within the limit that the amount of a was 10 ppm or less, but if T [Ca] / T [O] = R, 1.4 ≧ R ≧ 0.7 .................... (1) Other than, CaO.2Al ₂ O ₃ or CaO.
Since high melting point inclusions such as 6Al ₂ O ₃ are formed (1)
Limited to the scope of the formula. Further, in the step (C) shown in FIG. 3, bubbling with Ar is carried out so that CaO.Al ₂ O ₃ -based inclusions are levitated, and this is absorbed by the slag.
a] Ar gas blowing was stopped when ≦ 10 ppm. Therefore, 12CaO · 7Al ₂ O ₃ which is apparent from the CaO-Al ₂ O ₃ binary phase diagram is generated.
CaO.Al ₂ O ₃ inclusions with a low melting point of about 450 ° C are
It can be generated only by limiting the range within the shaded area in FIG. 4, that is, the range of the above formula (1).

Next, an experiment conducted by the present inventors on the bubbling process using an inert gas such as Ar in the item (C) shown in FIG. 3 will be described. The same test material as above
[O] = 35 ppm for molten steel after RH secondary refining,
T [Ca] and T [O] immediately after Ca was added according to (B) were as follows. T [Ca] = 30 ppm T [O] = 30 ppm T [Ca] / T [O] = R = 1 [Ca] by bubbling Ar into this molten steel by bubbling,
The result of observing the change of [O] is as shown in FIG. That is, [Ca] decreases with the lapse of time of Ar bubbling, but [O] becomes 6 at 13 minutes after the start of Ar bubbling.
Reoxidation was carried out with the level of ppm reaching the minimum, and the level reached 13 ppm 20 minutes after the start of bubbling. Therefore, the bubbling by Ar should be stopped when [Ca] ≦ 10 ppm, and the bubbling time by Ar should be 1
It turns out that 0 minutes is the optimum, and 15 minutes should not be exceeded. It should be noted that in the Ca residual steel material, CaS, C
The problem of rust generation starting from aO is known, and the number of rust generations of Ca-containing steel materials (number / c
m ² ) is as shown in FIG. 6, and also in the present invention, the CaO-Al ₂ O ₃ -based inclusion floating separation process by the inert gas injection is stopped when the Ca concentration in the molten steel becomes 10 ppm or less. It was decided to.

In the above-mentioned present invention, the step of causing the CaO-Al ₂ O ₃ type inclusions generated after the addition of Ca to float and to be absorbed by the slag has been described. This step shown in FIG. 3 is an inert gas bubbling method. Not limited to this, it is also possible to use centrifugal separation with a refractory filter or filtration means. Thus, (A) shown in FIGS.
[Ca] ≦ 10 pp after finishing the steps of (B) and (C)
The result of continuous casting of aluminum killed ultra low carbon steel of m, [O] ≦ 10 ppm is shown in FIG. This continuous casting is carried out by so-called "gas-free casting" in which an inert gas such as Ar or N ₂ is not blown into any of the nozzle part, that is, the upper nozzle of the dunnishu, the sliding nozzle and the dipping nozzle. FIG. 7 shows the height of the molten metal surface, the change in the casting speed, and the opening of the sliding nozzle as an index from the start to the end of casting, depending on the elapsed time of casting of the charge. As is apparent from FIG. 7, even if the "gas-free casting" is carried out according to the present invention, there is no clogging of the nozzle portion during the casting, the molten metal surface having a substantially constant height, the substantially constant casting speed, and With the sliding nozzle kept at a substantially constant opening, it was possible to carry out continuous casting, which was much more stable than when the conventional inert gas was blown.

[0012]

INDUSTRIAL APPLICABILITY The present invention relates to the melting of ultra-low carbon aluminum killed steel for cold rolling, T [Ca] / T [O] depending on T [O] in the molten steel that has been subjected to secondary refining such as RH. = R = 1.4 ≧ R ≧ 0.7, the amount of added Ca T [Ca] is determined according to T [O] so as to form a CaO.Al ₂ O ₃ type inclusion having a relatively low melting point. Moreover, after that, a method of continuously casting molten steel in which CaO / Al ₂ O ₃ inclusions generated by blowing an inert gas such as Ar are floated and absorbed in the slag is used. As in the continuous casting of aluminum killed steel, the method of performing so-called "gas-free casting", in which no inert gas is blown into the upper nozzle of the tundish, the sliding nozzle, or the immersion nozzle, is adopted. The following effects were achieved. (A) In the present invention, after Ca is added to the molten steel, CaO · A
Although an inert gas such as Ar is blown in order to float the l ₂ O ₃ -based inclusions, the blowing of the inert gas is stopped at the time when T [Ca] ≦ 10 ppm is reached, and the CaO · Al ₂ floated.
Regarding molten steel after absorbing O ₃ -based inclusions in slag,
This is a method of continuous casting without blowing the inert gas at all into the nozzle part, eliminating the conventional method of continuously casting while injecting the inert gas into the nozzle part (the steel material by this method is called process material). In the case of the method of the invention (hereinafter, the steel material by this method is referred to as a gas-free material), the number of blowholes (pieces / m) in the cold rolled material is as shown in FIG.
The gas-free material according to the present invention is far superior, and in comparing the number of inclusions Al ₂ O ₃ contained therein, the gas-free material according to the present invention is one of the conventional process materials as shown in FIG.
/ 4 or less, which is remarkably small, and in the comparison of the defective rate (%) of product quality due to internal defects (O marks) and surface defects (● marks) in other product steels, as shown in FIG. It was found that the gas-free material according to the present invention was significantly superior to the process material according to 1/5 to 1/6. (B) Since the melting method according to the present invention is extremely easy to carry out and is extremely stable, it is considered to be established as a gas-free method as a melting method for ultra-low carbon aluminum killed molten steel.

[Brief description of drawings]

[Figure 1]

[Fig. 2]

FIGS. 1 to 3 show steps in a method for smelting aluminum-killed steel for cold rolling according to the present invention, FIG. 1 shows a secondary refining step of RH for molten steel roughly decarburized in a converter, and FIG. Al ₂ O ₃ in molten steel that has undergone secondary refining has a low melting point of CaO ・ Al
_In order to precipitate as a ₂ O ₃ -based inclusion, Ca is added to contain a limited amount of T [Ca] according to T [O]. FIG. 3 is a schematic cross-sectional view showing any step of blowing CaO / Al ₂ O ₃ -based inclusions into the slag to absorb the slag.

FIG. 4 T in molten steel in a melting process according to the present invention
Total Ca concentration added according to [O] (ppm) T [C
a] and changes in T [O] and T [Ca] in the bubbling process caused by blowing an inert gas such as Ar from T [Ca]
FIG. 6 is a relationship diagram of T [Ca] and T [O] showing a limit range according to the present invention where / T [O] = R 1.4 ≧ R ≧ 0.7.

5 shows T [Ca] and T in the course of bubbling time showing the Ar bubbling effect according to the present invention shown in FIG.
It is a diagram which shows the change of [O].

FIG. 6 Residual Ca in cold rolled aluminum-killed steel for cold rolling
It is a diagram which shows the change of the number of rust generations which originates in CaS and CaO by content (ppm).

FIG. 7 shows changes in molten metal level of molten steel in a mold, changes in casting speed, and sliding nozzles from the start point to the end point of continuous casting (gas-free casting) without blowing an inert gas into the nozzle portion according to the present invention. It is a diagram which shows the change of an opening degree.

FIG. 8 is a comparative diagram comparing the number of blowholes (pieces / m) between an aluminum-killed steel material for cold rolling (gas-free material) manufactured according to the present invention and a steel material (process material) obtained by blowing Ar gas into a conventional nozzle portion. Is.

FIG. 9 is a comparison diagram for comparing the Al ₂ O ₃ inclusion number index in the steel material of the gas-free material manufactured according to the present invention and the process material by the conventional method.

FIG. 10 is a comparison diagram comparing the defect rate (%) due to internal defects (circles) and surface defects (circles) between the gas-free material manufactured according to the present invention and the process material manufactured by the conventional method.

[Explanation of symbols]

 2 Ladle 4 Molten Steel 6 Vacuum Degasser 8 Ca Wire 9 Ar Bubble 10 Lance

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[Procedure amendment]

[Submission date] March 31, 1993

[Procedure 3]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a secondary refining process by RH during a melting process of an aluminum killed steel for cold rolling according to the present invention.

FIG. 2 is a schematic diagram showing a step of adding Ca for precipitating Al ₂ O ₃ in molten steel that has undergone the secondary refining in the melting step of the present invention as CaO and Al ₂ O ₃ type inclusions having a low melting point. FIG.

FIG. 3 is a schematic diagram of molten steel added with Ca during the melting process of the present invention.
by blowing an inert gas r such is a schematic sectional view showing a step of absorption in the slag to float the CaO · Al ₂ O ₃ inclusions.

FIG. 4 T in molten steel in a melting process according to the present invention
Total Ca concentration added according to [O] (ppm) T [C
a] and changes in T [O] and T [Ca] in the bubbling process caused by blowing an inert gas such as Ar from T [Ca]
FIG. 6 is a relationship diagram of T [Ca] and T [O] showing a limit range according to the present invention where / T [O] = R 1.4 ≧ R ≧ 0.7.

5 shows T [Ca] and T in the course of bubbling time showing the Ar bubbling effect according to the present invention shown in FIG.
It is a diagram which shows the change of [O].

FIG. 6 Residual Ca in cold rolled aluminum-killed steel for cold rolling
It is a diagram which shows the change of the number of rust generations which originates in CaS and CaO by content (ppm).

FIG. 7 shows changes in the molten metal surface of the mold content steel, changes in the casting speed, and sliding nozzles from the start to the end of continuous casting (gas-free casting) without blowing inert gas into the nozzle portion according to the present invention. It is a diagram which shows the change of an opening degree.

FIG. 8 is a comparative diagram comparing the number of blowholes (pieces / m) between an aluminum-killed steel material for cold rolling (gas-free material) manufactured according to the present invention and a steel material (process material) obtained by blowing Ar gas into a conventional nozzle portion. Is.

FIG. 9 is a comparison diagram comparing the Al ₂ O ₃ inclusion number index in the steel material between the gas-free material manufactured according to the present invention and the process material by the conventional method.

FIG. 10 is a comparison diagram comparing the defect rate (%) due to internal defects (circles) and surface defects (circles) between the gas-free material manufactured according to the present invention and the process material manufactured by the conventional method.

[Explanation of Codes] 2 Ladle 4 Molten Steel 6 Vacuum Degassing Device 8 Ca Wire 9 Ar Bubbles 10 Lance

Claims (3)

[Claims] 1. A melting method of an ultra low carbon aluminum killed steel for cold rolling, wherein all C is contained in the ultra low carbon aluminum killed molten steel.
a concentration T [Ca] / total oxygen concentration T [O] = R1.4 ≦ R
The step of determining T [Ca] from T [O] so that ≦ 0.7, and CaO-Al ₂ O ₃ -based inclusions generated by blowing an inert gas into the molten steel are levitated into the slag. A step of absorbing the molten steel, and in the continuous casting of the molten steel, the inner wall of the upper nozzle of the tundish, the sliding nozzle and the dipping nozzle are characterized in that no inert gas is blown at all. Melting method for rolled aluminum killed steel. 2. The Ca concentration of the molten steel is 10 p when the CaO—Al ₂ O ₃ type inclusions generated by blowing an inert gas into the molten steel are floated and absorbed by the slag.
The method for smelting aluminum-killed steel for cold rolling according to claim 1, wherein the blowing of the inert gas is stopped when the temperature reaches pm or less. 3. Removal of CaO—Al ₂ O ₃ inclusions instead of the step of levitating the CaO—Al ₂ O ₃ inclusions generated by blowing an inert gas into the molten steel and absorbing the inclusions in the slag. The method for smelting aluminum-killed steel for cold rolling according to claim 1 or 2, wherein the molten steel is treated through a refractory filter as a means.