JP2672889B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention is capable of suitably removing non-metallic inclusions in the molten steel stage, and is a high-grade thin steel sheet used as a material for food cans, beverage cans, etc. It is particularly preferably applied to steel making such as. The present invention relates to a continuous casting method for ordinary steel and a continuous casting method for stainless steel.

<Prior Art> When manufacturing a steel material or a stainless steel material that requires high quality such as a high-grade thin steel sheet applied to food cans, beverage cans, etc. by continuous casting, non-metallic inclusions mixed in this molten steel at the molten steel stage. The removal of the object is extremely important in reducing the defective rate of the product.

Conventionally, removal of such non-metallic inclusions from molten steel (stainless steel molten steel) in continuous casting is achieved by increasing the size of the tundish arranged between the ladle and the mold (mold). In order to extend the residence time of molten steel in the steel, and promote the aggregation of non-metallic inclusions to remove it; by providing a weir, preferably a multi-stage weir in the tundish, and limiting the flow path of the molten steel, A method of extending the residence time of molten steel in the tundish to promote aggregation of non-metallic inclusions; controlling the flow of molten steel flow in the mold by changing the shape of the discharge port of the nozzle such as the immersion nozzle , A method for preventing the entrainment of mold powder from the nozzle discharge holes into the molten steel flow in the mold;

However, these methods cannot sufficiently remove non-metallic inclusions, especially when the molten steel is poured from the tundish into the mold at a high rate or when ladle is replaced in multi-continuous casting. In a steady state, molten steel containing a large amount of non-metallic inclusions is injected into the mold, and the quality of the cast piece cast at this time is significantly deteriorated.

In order to solve such a problem, Japanese Patent Laid-Open Nos. 55-107743 and 58-22317 disclose that the molten steel in the tundish is swirled in the horizontal direction so that non-metallic inclusions in the molten steel intervene. A method of facilitating separation and removal of objects is disclosed.
According to this method, the non-metallic inclusions can be removed relatively well in the steady state, but the non-metal inclusions can still be removed in the non-steady state when the molten steel pouring speed is high or when the ladle is replaced. It is impossible to prevent molten steel containing a large amount of inclusions from being injected into the mold.

<Problems to be Solved by the Invention> An object of the present invention is to solve the problems of the above-described conventional techniques, and to favorably remove nonmetallic inclusions present in molten steel in a continuous casting tundish. Even if the injection speed of molten steel from the tundish into the mold is fast, or even during unsteady conditions such as when the ladle is replaced and before and after the ladle, non-metallic inclusions in the molten steel that are injected into the mold are possible. It is possible to significantly reduce the mixture of slabs, and it is possible to apply slabs cast during non-steady conditions to applications that require high quality, such as high-grade thin steel plates that are the raw material for food cans, beverage cans, etc. The object of the present invention is to provide a continuous casting method of ordinary steel and stainless steel which can be performed.

<Means for Solving the Problem> In order to achieve the above-mentioned object, the present inventors first of all, the high-grade thin steel sheet (hereinafter, referred to as “material for can or beverage can”)
The product defects of high-grade thin steel sheet) were investigated in detail. As a result, inclusions shown in Table 1 below were detected from the defective portion.

As shown in Table 1, many of the inclusions detected from the defect portion of the fine steel sheets _{"CaO-Al 2 O 3 -Na 2} O-MgO-S "
Having the following composition: From the composition, the origin of the non-metallic inclusions is presumed to be ladle slag as shown in Table 1. Therefore, if the ladle slag is prevented from being mixed with the molten steel injected into the mold, product defects can be significantly reduced even when applied to high-grade thin steel sheets.

Since the ladle slag having the above composition is in a molten state in the operating temperature range of continuous casting, it exists in the molten steel flow from the ladle in a suspended state and flows into the tundish together with the molten steel.

Therefore, this ladle slag is injected into the mold together with the molten steel when the injection speed of the molten steel into the mold is fast, when the ladle is replaced, and during unsteady conditions such as before and after,
It cannot be completely separated from the molten steel by the conventional methods such as increasing the size of the tundish, providing a weir on the tundish, and rotating the molten steel of the tundish in the horizontal direction as described above. I can't.

The reason for this is that when the injection speed of molten steel into the mold is high, molten steel injected into the mold with little retention in the tundish, that is, molten steel injected into the mold through a short pass in the tundish occurs. It is considered that this is because the molten steel is poured into the mold while holding the ladle slag in a suspended state.

Immediately after the ladle is replaced, the liquid level of the molten steel in the tundish is lowered, so the inflow velocity of the molten steel from the ladle is high until the liquid level returns to the steady position. Therefore, also at that time, similarly, molten steel is short-passed through the tundish and injected into the mold.

However, with the conventional methods such as increasing the size of the tundish, providing the tundish with a weir, and turning the molten steel into a horizontal swirling flow as described above, it is not possible to prevent this short pass of the molten steel, resulting in high speed. The slab cast into the mold by the above method and the slab cast during a non-steady state cannot be used as a high-grade thin steel sheet.

As a result of intensive studies to solve this problem, the present inventors have come to the conclusion that the following points need to be solved.

First, it prevents the mixing of ladle slag into the molten steel that is poured into the mold during unsteady conditions such as before and after the ladle is replaced, not during so-called steady pouring.

Second, it is necessary to positively separate the ladle slag described above from the molten steel, not by floating separation by aggregation of deoxidized products.

Third, a short path of molten steel from the ladle to the mold,
In particular, it prevents the short path of molten steel that occurs when the ladle is replaced and before and after it.

As a result of further studies to solve these three points, the inventors of the present invention can cancel the flow of the molten steel injected from the ladle into the molten steel injected into the tundish, especially in the non-steady state. The swirling flow makes it possible to completely prevent the molten steel short path even when the molten steel is injected at high speed, or when the ladle is replaced or during unsteady conditions such as before and after the ladle. It has been found that it is possible to separate the ladle slag existing in the molten steel in a suspended state during the swirling of the molten steel from the molten steel by the difference in specific gravity, and thus completed the present invention.

Further, according to the study of the present inventors, there is a similar problem when producing stainless steel by continuous casting,
It was also found that ladle slag can be separated from molten stainless steel by forming a similar swirl flow at least at unsteady times.

That is, the present invention, the molten steel of the ladle flows into the tundish, in continuous casting to inject the molten steel into the mold from the tundish, the molten steel from the tundish into the mold injection rate is 3.0 t / min or more. In order to cancel the flow of molten steel injected into the tundish from the ladle, the molten steel is swirled at a rotation speed of 30 to 60 rpm, and the swirling flow speed is increased at non-steady time than at steady time. A continuous casting method is provided.

Further, in the present invention, the molten steel of the ladle is introduced into the tundish, in continuous casting to inject molten steel into the mold from the tundish, the injection rate of molten steel from the tundish to the mold is 3.0 t / min or more. And, there is provided a continuous casting method characterized in that the molten steel filled in the tundish is made into a swirling flow only when it is unsteady.

Furthermore, in the present invention, in the continuous casting of stainless steel in which the molten stainless steel in the ladle is introduced into the tundish and the molten stainless steel is poured into the mold from the tundish, the molten steel filled in the tundish at least during unsteady state. The present invention provides a continuous casting method for stainless steel, characterized in that

Further, in the continuous casting method and the stainless steel continuous casting method of the present invention, it is preferable to heat the tundish at least during a non-steady state.

In the continuous casting method of the present invention and the continuous casting method of stainless steel, a non-steady state is a state different from a normal state which occupies the majority in the continuous casting step, such as when the ladle is replaced and before and after. It shows all of the above. In addition, before the ladle is replaced, specifically, from the state where the remaining amount of molten steel in the ladle is about 15 wt% with respect to the total amount of molten steel filled in the ladle until the time when the ladle is replaced, After the ladle is replaced, specifically, it means after the replacement of the ladle with a new one, until about 5 wt% of the total amount of molten steel filled in the ladle is discharged.

 Hereinafter, the present invention will be described in more detail.

The continuous casting method of the present invention is applied when the molten steel pouring speed from the tundish to the mold (hereinafter referred to as the molten steel pouring speed) is 3.0 t / min or more. The inventors of the present invention intervened by magnetic particle flaw detection (hereinafter, MT) when commercializing various thin steel plates for food cans produced from slabs continuously cast at molten steel pouring rates of 3.0, 3.5 and 4.0 t / min. Non-destructive inspection of object defects was performed.

The result is shown in Fig. 1. The molten steel pouring rate is 3.0t.
MT at both low speed and steady time at low speed
Although almost no defects are observed, MT defects increase as the molten steel injection speed increases, and it is understood that a large amount of MT defects occur especially in the unsteady part when the molten steel injection speed becomes 4.0 t / min. Therefore, in the first and second aspects of the present invention, the injection rate into the molten steel is set to a relatively high rate of 3.0 t / min or more.

In addition, in the continuous casting method of stainless steel, the injection speed of the molten stainless steel is not particularly limited. This is because in continuous casting of stainless steel, the casting speed is often determined by matching with refining, and for continuous casting, injection at about 1.0 to 2.0 t / min often occurs,
This is because even in this state, the unsteady part may cause deterioration in quality due to a decrease in T / D level.

In the continuous casting method of the present invention, the molten steel (stainless steel molten steel) filled in the exclusive tundish is swirling, specifically, at all times or only in the unsteady state, and in the continuous casting method of stainless steel, at least in the unsteady state. Is a swirling flow of molten steel filled in the tundish so as to cancel the flow of molten steel injected from the ladle.

As described above, most of the causes of inclusion defects in the slab produced by continuous casting are that the ladle slag is mixed in a molten state in the molten steel injected into the mold. Here, the cause of the mixing of the ladle slag includes a short path of molten steel from the ladle to the mold, the inability of the ladle slag mixed in the molten steel in a suspended state to be sufficiently separated, and the like.

On the other hand, in the continuous casting method of the present invention, by swirling the molten steel in the tundish so as to cancel the flow of molten steel injection from the ladle, the short pass of the molten steel is completely prevented, and the molten steel is injected into the mold. It is possible to use only the molten steel for the tundish head, and it is possible to separate inclusions such as ladle slag from the molten steel due to the difference in specific gravity while the molten steel is being swirled in the tundish. Is.

The swirling direction of the molten steel in the tundish is not particularly limited as long as it can cancel the flow of molten steel injected from the ladle, for example, the direction opposite to the flow of molten steel injected from the ladle. Are preferably exemplified.

The method of swirling the molten steel in the tundish is not particularly limited, and various known methods such as a method of applying a magnetic force and a method of blowing an inert gas can be applied.

The speed of the swirling flow is not particularly limited as long as it can sufficiently cancel the flow of molten steel injected from the ladle, but it is usually 30 rpm or more.

If the swirling flow velocity is too high, the rising height of the molten steel near the wall of the tundish will increase,
The required area for refractories will increase accordingly, and the tundish will need to be larger. Here, the difference between the molten steel liquid level height in the state where the molten steel is stationary in the tundish and the liquid level height in the state where the molten steel is swirling is called freeboard, and when the swirling speed of the molten steel exceeds 60 rpm. However, the size of this freeboard becomes too large, and the required amount of refractory material increases accordingly, so the cost of the tundish increases and it is not suitable for practical use. Therefore, in the continuous casting method of the present invention, the swirling speed of the molten steel is preferably 60 rpm or less.

Therefore, the swirling speed of molten steel in the tundish is 30
When it is set to 60 rpm, the occurrence of molten steel defects can be preferably prevented and the economical balance is also good.

In the continuous casting method of the present invention, always or only in the unsteady state, and in the continuous casting method of stainless steel, at least in the unsteady state, the molten steel in the tundish is swirled, and also unsteady. At times, it is preferable to accelerate the swirling flow of molten steel in the tundish more than in the steady state.

FIG. 2 shows, as an example, the results of visual inspection of the defect occurrence index for each slab position when the continuously cast SUS430 slab was commercialized. The top slab is the final casting part of the continuous casting, the bottom slab is the initial casting part of the continuous casting, the pan exchange slab is the part including the before and after the ladle exchange of the continuous casting, and the middle slab is the other parts. It shows the part which was made. Therefore, in the illustrated example, the middle slab is cast during a steady state, and the others are cast during a non-steady state.

From the results shown in FIG. 2, it can be seen that product defects occur at all positions except the top slab. Especially in bottom slabs and pan exchange slabs, product defects occur with a high probability. Here, there is no product defect in the top slab, because when the final pouring state before the pan replacement, the ladle slag does not get caught in the ladle casting is performed at an extremely low speed. This is because the.

As described above, most of the causes of product defects are that ladle slag is mixed in a molten state in the molten steel injected into the mold. Therefore, from the results shown in Fig. 2, the bottom slab, pan exchange slab, and top slab, etc., without adjusting the drawing speed, the ladle slag easily mixes with the molten steel injected into the mold during unsteady conditions. Therefore, it is understood that a countermeasure is necessary.

Therefore, in the continuous casting method of the present invention, the turning speed of the molten steel in the tundish is low at about 30 to 40 rpm in the steady state, and the turning speed is 40 to 60 rp in the non-steady state.
It is preferable to increase the speed to about m.

FIG. 3 shows an example of the adjustment of the swirling speed of the molten steel in the tundish during steady time and unsteady time. In the example shown in Fig. 3, in the steady state, the molten steel is swirled at 10 rpm, the swirling speed starts to be increased about 10 minutes before the ladle is changed, and the molten steel is swirled 10 minutes after the completion of the ladle exchange. The speed has started decelerating. The continuous casting method for stainless steel is not limited to the one in which the molten stainless steel is swirled only in the non-steady state, and the molten stainless steel is swung at a low speed in the steady state and the swirling speed is increased in the non-steady state. Alternatively, the stainless steel may be swung at a constant speed regardless of whether it is stationary or unsteady.

In such a continuous casting method and a stainless steel continuous casting method of the present invention, it is preferable to heat the tundish at least during the non-steady state to keep the temperature of the molten steel to be kept at a certain temperature or higher.

As an example, FIG. 4 shows the relationship between the temperature of the tundish and the slab norokami score when continuously casting a SUS304 slab.

As is clear from FIG. 4, it is necessary to keep the tundish at a temperature above a predetermined level in order to obtain a suitable product with less slag. However, the temperature of the molten steel in the tundish may change at the beginning of pouring when the refractory of the tundish has not reached the prescribed temperature (molten steel temperature) or at the end of pouring when the molten steel temperature in the ladle decreases. Has fallen,
Even with the continuous casting method of the present invention as described above, there are cases where the defective rate at this time cannot be satisfactorily reduced. In the example shown in FIG. 4, as an example, SUS304
However, this point is the same in the continuous casting of steel.

Therefore, in the present invention, it is preferable to heat the tundish at least during the non-steady state, which can further reduce the occurrence of product defects. The heating temperature is not limited, and may be set so that the temperature decrease can be guaranteed depending on the molten steel or stainless molten steel. Further, as a heating method, any of various known methods can be applied.

An example of the tundish heating cycle is also shown in FIG.

Although the continuous casting method of the present invention has been described above in detail, the present invention is not limited to this, and various modifications and improvements may be made without departing from the scope of the present invention. That is.

<Example> Hereinafter, the present invention will be described in more detail with reference to specific examples of the present invention.

[Example 1] A slab for food cans of 260 mm x 1850 mm was manufactured by using the continuous casting method of the present invention. The composition of the slab is C ≦ 0.08
%; Si ≦ 0.01%; Mn ≦ 0.25%; P ≦ 0.025%; S ≦ 0.015%; A1
≤0.1%, other unavoidable ingredients.

The continuous casting machine used was a vertical bending type, and the ladle capacity was 23.
The capacity of 0t and the tundish is 35t, and the tundish has no weir. The molten steel pouring rate from the tundish to the mold was 4.0 t / min.

In addition, the molten steel filled in the tundish can be electromagnetically stirred (a coil is installed outside the tundish wall).
It swung in the direction opposite to the direction of pouring molten steel from the ladle.

Here, when the turning speed is a constant speed of 10 rpm,
Despite the molten steel pouring rate of 4.0 t / min, the product defect index of the steady part could be made equal to that of the steady part of molten steel pouring rate of 3.0 t / min shown in FIG. Furthermore, by increasing the swirling speed to 50 rpm in the unsteady part, the defect index in the unsteady part could be made equal to that in the unsteady part at the molten steel injection speed of 3.5 t / min. In addition, since the temperature drop of the tundish was observed in the unsteady part,
When the tundish was heated at the melting point of the molten steel + 60 ° C for this portion, the occurrence of product defects could be further reduced, and the molten steel injection rate as shown in Fig. 3.
It was possible to make it equivalent to the unsteady time part of 0 t / min. In addition,
The adjusting cycle of the turning speed and the heating was as shown in FIG.

When commercializing the slab for food cans manufactured in this way,
Non-destructive inspection of inclusion defects in non-steady parts was performed by magnetic particle testing (MT).

The results are shown in FIG. It should be noted that FIG. 5 also shows the similar inspection results of the slab for food cans manufactured by the conventional continuous casting method. In the conventional method, production was performed under the same conditions as in the continuous casting method of the present invention example, except that the molten steel was not swirled in the tundish.

Further, FIG. 6 shows the defect-free rate of the manufactured slab for food cans in the steady state and the non-steady state.

From the results shown in FIG. 5 and FIG. 6, by applying the continuous casting method of the present invention, inclusion defects can be significantly reduced as compared with the conventional continuous casting. In addition, since there is no difference in quality between the steady state and the non-steady state, and there are few defects, the entire manufactured slab has high quality requirements such as high-grade thin steel plates for food cans and beverage cans. It can be applied to various uses.

[Example 2] As shown in Fig. 7, the same as Example 1 except that the molten steel was not swirled during the unsteady state and was swirled at a speed of 50 rpm only during the unsteady state (the tundish). Was not heated) to produce a slab for food cans.

The product defects in the unsteady part of the obtained slab are the same as those in the unsteady part of molten steel pouring speed of 3.5 t / min shown in Fig. 1, even though the molten steel pouring speed is 4.0 t / min. I was able to

Example 3 Using the continuous casting method for stainless steel of the present invention, 200 mm
A × 1240 mm SUS304 slab was manufactured.

The continuous casting machine used was a vertical bending type, and the ladle capacity was 16
The capacity of 0t and tundish is 25t, and the tundish has no weir. The molten steel pouring rate from the tundish to the mold was 2.0 t / min.

Further, the molten stainless steel filled in the tundish swirled in the direction opposite to the pouring direction of the molten stainless steel from the ladle by using the same electromagnetic stirring as in Example 1 above only in the non-steady state. The turning speed is 50 rpmd, and the turning speed adjustment cycle is as shown in FIG.

Upon commercialization of the SUS304 slab manufactured in this manner, the inclusion defect on the coil surface after cold rolling was visually inspected.

The results are shown in FIG. It should be noted that FIG. 9 also shows the same inspection results for the SUS304 slab manufactured by the conventional continuous casting method.

In the conventional method, production was performed under the same conditions as in the continuous casting method of the present invention except that the molten steel was not swirled in the tundish.

From the results shown in FIG. 9, by applying the continuous casting method of the present invention, inclusion defects can be significantly reduced compared to conventional continuous casting regardless of whether it is a middle slab or an unsteady slab. In the present embodiment, the molten stainless steel was swirled only during the unsteady state, but the molten stainless steel may be swung at a low speed during the steady state, and the swirling speed may be increased during the unsteady state as described above. Is.

In addition, when the tundish was heated to the melting point of molten steel + 60 ° C in the cycle shown in Fig. 8, the occurrence of product defects could be further reduced, and there was a difference in quality between steady state and non-steady state. Since it has no defects and has few defects, the entire manufactured slab can be applied to applications in which quality is strictly required, such as bright annealed stainless steel cold rolled steel sheet for exterior.

 From the above results, the effect of the present invention is clear.

<Effects of the Invention> As described in detail above, according to the continuous casting method and the stainless steel continuous casting method of the present invention, the ladle slag that is the largest cause of inclusion defects is reliably separated from the molten steel in the tundish. However, since this ladle slag or molten steel is not poured into the mold in a suspended state, the amount of slag contained in the manufactured slab can be greatly reduced, and high quality without inclusion defects can be obtained. A slab can be manufactured. In addition, since there is little difference in the quality between continuous casting steady state and non-steady state, all the produced slabs are food cans, high-grade thin steel plates for beverage cans, and further bright annealed stainless cold-rolled steel plates for exteriors, etc. It can be applied to applications that require high quality.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the injection rate of molten steel from the tundish into the mold and the magnetic particle flaw detection index. FIG. 2 is a graph showing the defect occurrence index for each slab position of the continuously cast SUS430 slab. FIG. 3 and FIG. 7 are graphs showing an example of a cycle of adjusting the rotation speed of the molten steel of the tundish and a heating cycle in the continuous casting method to which the present invention is applied. FIG. 4 is a graph showing the relationship between the temperature of the tundish and the slab norokami score. FIG. 5 is a graph showing the number of defects of magnetic particle flaw detection in a non-steady state of a slab manufactured by the continuous casting method of the present invention and the conventional continuous casting method. FIG. 6 is a graph showing the integrity index of the slab manufactured by the continuous casting method of the present invention and the conventional continuous casting method during steady state and non-steady state. FIG. 8 is a rotation speed adjustment for swirling molten steel in a tundish in a continuous casting method for stainless steel to which the present invention is applied.
It is a graph which shows an example of a heating cycle. FIG. 9 is a graph showing the integrity indexes of the slabs manufactured by the continuous casting method for stainless steel of the present invention and the conventional continuous casting method for stainless steel in the steady state (middle slab) and the non-steady state.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ei Kitaoka 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Hiroyuki Kadowaki 1 Kawasaki-cho, Chiba-shi Chiba Prefecture Kawasaki Steel (56) References JP-A-62-156055 (JP, A) JP-A-55-107743 (JP, A) JP-A-63-137554 (JP, A) JP-A-62-134150 (JP, A) JP 58-192666 (JP, A)

Claims (2)

(57) [Claims] 1. In continuous casting in which molten steel in a ladle is introduced into a tundish and the molten steel is poured from the tundish into a mold, the molten steel is injected from the tundish into the mold at a rate of injection.
The rotation speed of molten steel is 3.0 t / min or more so as to cancel the flow of molten steel injected from the ladle into the tundish.
A continuous casting method, characterized in that a swirling flow of 30 to 60 rpm is used, and the speed of the swirling flow is increased in a non-steady state than in a steady state. 2. The continuous casting method according to claim 1, wherein the tundish is heated at least during a non-steady state.