JPH04187352A - Continuous casting method and method for continuously casting stainless steel - Google Patents

Abstract

PURPOSE:To sufficiently remove non-metallic inclusion present in a molten steel in a tundish by specifying the pouring speed of the molten steel into a mold from the tundish and making circular flow of the molten steel to be filled up in the tundish. CONSTITUTION:In the case of continuous casting by pouring the molten steel into the mold from the tundish after pouring the molten steel in a ladle into the tundish, the pouring speed of the molten steel into the mold from the tundish is >=3.0t/min and the molten steel to be filled up in the tundish is made to the circular flow. By this method, the non-metallic inclusion present in the molten steel in the tundish can be sufficiently removed.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention is capable of suitably removing non-metallic inclusions during the molten steel stage, and is suitable for producing high-grade thin steel sheets used as materials for food cans, beverage cans, etc. The present invention relates to a continuous casting method for ordinary steel and a continuous casting method for stainless steel, which are particularly suitably applied to steel manufacturing such as steel manufacturing.

<Prior art> When manufacturing steel or stainless steel materials that require high quality, such as high-grade thin steel sheets used for food cans, beverage cans, etc., by continuous casting, non-metallic inclusions mixed in the molten steel at the molten steel stage are used. Removal of objects is extremely important in reducing the rejection rate of products.

Conventionally, the removal of such non-metallic inclusions from molten steel (stainless steel molten steel) in continuous casting has been achieved by increasing the size of the tundish placed between the ladle and the mold. A method of prolonging the residence time of molten steel and promoting agglomeration of nonmetallic inclusions to remove them; by providing a weir, preferably a multi-stage weir, in the tundish and restricting the flow path of the molten steel, A method of prolonging the residence time of molten steel to promote agglomeration of non-metallic inclusions; By changing the shape of the discharge pressure port of a nozzle such as an immersion nozzle, the flow of molten steel in the mold is controlled and A method of preventing mold powder from being drawn into the molten steel flow from a nozzle discharge hole; and the like have been carried out.

However, these methods cannot sufficiently remove non-metallic inclusions, especially when the injection rate of molten steel from the tundish into the mold is high, or when changing the ladle in multi-continuous casting. Sometimes, molten steel containing large amounts of non-metallic inclusions is injected into the mold, significantly reducing the quality of the cast slab.

To solve this problem, Japanese Patent Application Laid-Open No. 55-107743 and Japanese Patent Application Laid-Open No. 58-22317 disclose that non-metallic inclusions in the molten steel are created by creating a horizontal swirl flow in the molten steel in the tundish. A method is disclosed that facilitates the separation and removal of . According to this method, non-metallic inclusions can be removed relatively well under steady state conditions, but non-metallic inclusions can be removed relatively well under steady state conditions. It is impossible to prevent molten steel containing a large amount of inclusions from being poured into the mold.

<Problems to be Solved by the Invention> The purpose of the present invention is to solve the problems of the prior art described above, and to provide a method for effectively removing non-metallic inclusions present in molten steel in a continuous casting tundish. Furthermore, even if the molten steel is poured from the tundish into the mold at a high speed or during unsteady conditions such as when changing the ladle or before and after, non-metallic inclusions may be mixed into the molten steel poured into the mold. It is possible to significantly reduce the amount of steel that is cast during unsteady conditions, and even slabs cast during unsteady conditions can be suitably applied to applications that require high quality, such as high-grade thin steel sheets that are used as materials for food cans, beverage cans, etc. The purpose of the present invention is to provide a continuous casting method for steel and stainless steel.

<Means for Solving the Problems> In order to achieve the above object, the present inventors first solved the problem of product defects in high-grade thin steel sheets (hereinafter referred to as high-grade thin steel sheets) used as materials for food cans and beverage cans. was investigated in detail. As a result, inclusions as shown in Table 1 below were detected from the defective portion.

Table 1 As shown in Table 1, many of the inclusions detected in defective areas of high-grade thin steel sheets are rCaO-AlaOx-NaaO
-MgO-3J.

Furthermore, based on the composition, it is estimated that the origin of these nonmetallic inclusions is ladle slag, as shown in Table 1. Therefore, by preventing ladle slag from being mixed into the molten steel poured 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 a suspended state in the molten steel flow from the ladle 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 molten steel is poured into the mold at a high speed, or when the ladle is replaced, or during unsteady situations such as before and after, and the tundish becomes larger as described above. Conventional methods such as providing a weir in the tundish or horizontally rotating the molten steel in the tundish cannot completely separate the molten steel from the molten steel and cannot be used as high-grade thin steel sheets.

The reason for this is that when the molten steel injection speed into the mold is fast, molten steel is injected into the mold without being retained in the tundish, that is, molten steel is injected into the mold after taking a short bath in the tundish. This is thought to be because the molten steel is poured into the mold with the ladle slag in suspension.

Further, immediately after replacing the ladle, the level of the molten steel in the tundish is low, so the inflow speed of the molten steel from the ladle remains high until the level returns to a normal position. Therefore, in that case as well, molten steel will be injected into the mold through a short bath in the tundish.

However, conventional methods such as increasing the size of the tundish, installing a weir in the tundish, or creating a horizontal swirling flow of molten steel cannot prevent this short bath of molten steel. Slabs produced by pouring into the steel or cast slabs cast during unsteady conditions cannot be used as high-grade thin steel sheets.

The inventors of the present invention have made extensive studies to solve this problem, and have come to the conclusion that the following points need to be solved.

First, it prevents ladle slag from being mixed into the molten steel that is injected into the mold not during so-called steady injection, but during irregular periods such as before and after replacing the ladle.

Secondly, it is necessary to actively separate the ladle slag described above from molten steel, rather than flotation separation by agglomeration of deoxidized products.

Thirdly, it prevents the short bus of molten steel from the ladle to the mold, especially the short bus of molten steel that occurs when changing the ladle or before and after changing the ladle.

As a result of further studies to solve these three points, the inventors of the present invention found that it is possible to counteract the flow of molten steel injected into the tundish from the ladle, especially in unsteady conditions. By creating a swirling flow, it is possible to completely prevent short baths of molten steel even when molten steel is injected at high speed or during unsteady situations such as when changing the ladle or before and after changing the ladle. It was discovered that ladle slag, which exists in a suspended state in molten steel while the molten steel is swirling, can be separated from molten steel by the difference in specific gravity, and the first and second aspects of the present invention were completed. I ended up doing it.

Furthermore, according to the studies of the present inventors, similar problems occur when manufacturing stainless steel by continuous casting, but by forming a similar swirling flow at least during unsteady conditions, ladle slag can be converted into stainless steel molten steel. It was also discovered that the third aspect of the present invention can be separated.

That is, the first aspect of the present invention is continuous casting in which molten steel in a ladle flows into a tundish and the molten steel is injected from the tundish into a mold, and the injection rate of molten steel from the tundish into the mold is 3. Provided is a continuous casting method characterized in that the flow rate is Qt/min or more, and the molten steel filled in a tundish is made into a swirling flow.

Further, it is preferable that the speed of the swirling flow is increased during an unsteady state compared to a steady state.

Furthermore, in a second aspect of the present invention, in continuous casting in which molten steel in a ladle flows into a tundish and the molten steel is injected from the tundish into a mold, the injection rate of molten steel from the tundish into the mold is 3.0 t. /min
As described above, there is provided a continuous casting method characterized in that the molten steel filled into the kundish is made into a swirling flow only during unsteady conditions.

Furthermore, in a third aspect of the present invention, in continuous casting of stainless steel in which molten stainless steel in a ladle flows into a tundish and the molten stainless steel is poured from the tundish into a mold, at least during unsteady operation, the molten stainless steel flows into the tundish. Provided is a method for continuous casting of stainless steel, characterized in that the molten steel to be filled is made into a swirling flow.

Further, in the continuous casting method and continuous casting method for stainless steel ports of the present invention, it is preferable to heat the tundish at least during unsteady operation.

In the continuous casting method of the present invention and the continuous casting method of stainless steel, the unsteady state refers to a state different from the normal state that occupies the majority in the continuous casting process, such as when changing the ladle and before and after that. It shows everything.

In addition, before replacing the ladle, specifically, the remaining amount of molten steel in the ladle is 15 wt% of the total amount of molten steel filled in the ladle.
After the ladle is replaced, specifically, after replacing the ladle with a new ladle, approximately 5wt% of the total amount of molten steel filled in the ladle is discharged. Until then.

The present invention will be explained in more detail below.

The first and second aspects of the continuous casting method of the present invention are applied when the molten steel injection rate from the tundish to the mold (hereinafter referred to as molten steel injection rate) is 3.0 t/min or more. Ru. The inventors obtained 3.0, 3.
When commercializing various thin steel sheets for food cans manufactured from slabs continuously cast at molten steel injection rates of 5 and 4,0 t/min, non-destructive inspection for inclusion defects was conducted using magnetic particle testing (hereinafter referred to as MT). went.

The results are shown in Figure 1, where the molten steel injection rate was 3°0.
When the speed is as low as t/min, at steady state,
Almost no MT large defects are observed in both unsteady conditions, but as the molten steel injection rate increases, the MT large defects increase, and when the molten steel injection rate increases to 4.0 t/min, a large number of MT large defects are observed especially in the unsteady part. I understand that it occurs. Therefore, in the first and second embodiments of the present invention, the injection rate into molten steel is relatively high, such as 3.0 t/min or more.

In addition, in the third aspect, which is a continuous casting method for stainless steel, the injection speed of molten stainless steel is not particularly limited. This is because, in continuous casting of stainless steel, the casting speed is determined by matching with refining. For continuous casting, it is often necessary to inject at a rate of 1.0 to 2.0 t/min, and even in this state, there is a possibility that quality may deteriorate due to a drop in T/D level in unsteady areas. This is because there is.

In the continuous casting method of the present invention, the exclusive tundish is used at all times in the first aspect, only during unsteady conditions in the second aspect, and at least during unsteady times in the third aspect, which is a continuous casting method for stainless steel. Molten steel to be filled (
The molten stainless steel (molten stainless steel) flows into a swirling flow, specifically, the molten steel that is filled into the tundish so as to counteract the flow of molten steel injected from the ladle.

As mentioned above, many of the causes of inclusion defects in slabs manufactured by continuous casting are due to ladle slag being mixed into the molten steel injected into the mold in a suspended state.
The causes of this contamination of ladle slag include a short bath of molten steel from the ladle to the mold, and failure to sufficiently separate ladle slag, which is suspended in the molten steel, in the tundish.

On the other hand, in the continuous casting method of the present invention, the molten steel in the tundish is swirled so as to cancel the flow of molten steel injected from the mold, thereby completely preventing the short path of the molten steel and allowing the molten steel to be injected into the mold. The molten steel can be used only in the tundish head, and while the molten steel is being swirled in the tundish, inclusions for ladle slag can be separated from the molten steel due to the difference in specific gravity.

The direction of swirling of the molten steel in the tundish is not particularly limited as long as it can counteract the flow of molten steel injected from the ladle, but for example, the direction opposite to the flow of molten steel injected from the ladle may be used. A suitable example is given.

The method of swirling the molten steel in the tundish is not particularly limited, and various known methods can be used, such as applying magnetic force or wiping with inert gas.

Further, the speed of the swirling flow is not particularly limited as long as it can sufficiently cancel out the flow of molten steel injected from the ladle, but it may normally be 30 rpm or more.

Note that if the speed of the swirling flow is made too high, the height of the bulge of the molten steel near the wall of the tundish will increase, the required area of the refractory will increase accordingly, and the tundish will also need to be larger. Here, if the difference between the molten steel liquid level height when the molten steel is stationary and the liquid level height when the molten steel is swirling in the tundish is called freeboard, when the molten steel swirling speed exceeds 6 Orpm, This freeboard becomes too large and the required amount of refractories increases accordingly, which increases the cost of the tandish and makes it unsuitable for practical use. Therefore, in the continuous casting method of the present invention, the swirling speed of molten steel is 60rprrl.
It is preferable that it is below.

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

As mentioned above, the second aspect of the continuous casting method of the present invention is only in an unsteady state, and the third aspect is a continuous casting method for stainless steel in which the molten steel in the tundish is turned into a swirling flow at least in an unsteady state. Moreover, in the first aspect of the present invention, it is preferable to increase the speed of the swirling flow of molten steel in the tundish in an unsteady state compared to a steady state.

FIG. 2 shows, as an example, the results of visually inspecting the defect occurrence index for each slab position when commercializing a continuously cast 5Us430 slab. In addition, the top slab refers to the final casting part of continuous casting, the bottom slab refers to the initial casting part of continuous casting, the pot exchange slab refers to the part including before and after changing the amount of continuous casting, and the middle slab refers to the other parts. This shows the part that has been changed. Therefore, in the illustrated example, the middle slab was cast in a steady state, and the others were cast in an unsteady state.

From the results shown in Figure 2, it can be seen that product defects occurred at all locations except the top slab. In particular, there is a high probability of product defects occurring in bottom slabs and pot replacement slabs. Here, the reason why there were no product defects in the top slab is that casting was performed at an extremely low speed that did not cause ladle slag to be drawn in from the ladle when the final pouring state was reached before replacing the ladle. This is because the.

As mentioned above, many of the causes of product defects are due to the presence of suspended ladle slag in the molten steel injected into the mold. Therefore, from the results shown in Figure 2, ladle slag is likely to mix into the molten steel poured into the mold during unsteady conditions, such as the bottom slab, the ladle replacement slab, and even the top slab if the drawing speed is not adjusted. This shows that countermeasures are necessary.

Therefore, in the first aspect of the continuous casting method of the present invention, the swirling speed of the molten steel in the tundish is reduced to 3 during steady state.
It is preferable to perform the rotation at a low speed of about 0 to 40 rpm, and increase the rotation speed to about 40 to 60 rpm during unsteady conditions. Molten steel is injected into the mold at a speed that does not cause MT defects, and the molten steel is turned into a swirling flow during unsteady conditions. Note that the swirling speed of the molten steel during unsteady conditions is about 10 to 60 rpm, preferably about 40 to 60 rpm.

FIG. 3 shows an example of adjusting the swirling speed of molten steel in a tundish in a steady state and an unsteady state. In the example shown in Figure 3, the molten steel is swirled at 10 rpm in steady state, and the swirling speed is increased by one level approximately 10 minutes before the ladle is replaced, and 10 minutes after the ladle is replaced, the molten steel is swirled at 10 rpm. Starting to reduce turning speed. In addition, in the continuous casting method of stainless steel according to the third aspect of the present invention, the molten stainless steel is not limited to the one in which the molten stainless steel is swirled only in an unsteady state, but the molten stainless steel is swirled at a low speed in a steady state, and the molten stainless steel is swirled in an unsteady state. It may be configured to increase the speed, or it may be configured to rotate the stainless steel at a constant speed regardless of whether it is steady or unsteady.

In the continuous casting method and stainless steel continuous casting method of the present invention, it is preferable to heat the tundish at least during unsteady conditions to maintain the temperature of the molten steel filled at a certain level or higher.

FIG. 4 shows the relationship between the temperature of the tundish and the roughness rating of the slab when a 5US304 slab was continuously cast as an example.

As is clear from FIG. 4, it is necessary to maintain the tongue pusher at a predetermined temperature or higher in order to obtain a suitable product with less sagging. However, at the beginning of pouring when the refractories in the tundish have not reached the specified temperature (molten steel temperature), or at the end of pouring when the molten steel temperature in the ladle drops, the temperature of the molten steel in the tundish may drop. Therefore, even with the continuous casting method of the present invention as described above, it may not be possible to satisfactorily reduce the defective rate during this period. In the example shown in Figure 4, 5US is used as an example.
Although 304 is shown as an example, the same applies to continuous casting of steel in this regard.

Therefore, in the present invention, it is preferable to heat the tundish at least during unsteady conditions, thereby further reducing the occurrence of product defects. The heating temperature is not particularly limited, and may be set to a level that can guarantee a temperature reduction depending on the molten steel or stainless steel. Furthermore, any of various known heating methods can be applied.

In addition, an example of the heating cycle of the tundish is also shown in the shaded area in FIG.

Although the continuous casting method of the present invention has been described in detail above, the present invention is not limited thereto, and various changes and improvements may be made without departing from the gist of the present invention. Of course.

<Example> Hereinafter, the present invention will be explained in more detail by giving specific examples of the present invention.

[Example 1] Using the continuous casting method of the present invention, 260mm x 1g5
A slab for a 0 mm food can was manufactured. The composition of the slab is C≦0.08%; Si≦0.01%; MnS2.
25%; P≦0.025%; S≦0.015%; A1≦
0.1%, which is an unavoidable chemical component.

The continuous casting type used was a vertical bending type, and the ladle capacity was 23
0t, the capacity of the tundish is 35t, and the tundish does not have a weir. Further, the injection rate of molten steel from the tundish into the mold was 4.0 t/min.

Further, the molten steel filled in the tundish was swirled in a direction opposite to the injection direction of the molten steel from the ladle using electromagnetic stirring (a coil was installed outside the tundish wall).

Here, when the rotating speed is set to a constant speed of 10 rpm, the product defect index in the steady section is set to the molten steel injection rate shown in Figure 1, even though the molten steel injection rate is 4.0 t/min. It was possible to make it equivalent to the steady state part of 3.0 t/min. Furthermore, in the unsteady part, the turning speed was increased to 5
By increasing the speed to Orpm, it was possible to make the defect index of the unsteady part equivalent to that of the unsteady part at a molten steel injection rate of 3.5 t/min. In addition, a temperature drop in the tundish was observed in the unsteady part, so when the tundish was heated in this part to the melting point of molten steel +60°C, the occurrence of product defects could be further reduced, as shown in Figure 1. However, it was possible to make it equivalent to the unsteady part with a molten steel injection rate of 3.0 t/min. Note that the adjustment cycle for the rotation speed and superheating was as shown in FIG. 3.

When commercializing the food can slab manufactured in this way, a non-destructive inspection for inclusion defects in unsteady parts was performed using magnetic particle testing (MT).

The results are shown in Figure 5. Note that FIG. 5 also shows similar test results for food can slabs manufactured by the conventional continuous casting method. The conventional method was manufactured under the same conditions as the continuous casting method of the present invention except that the molten steel was not swirled in the tundish.

Furthermore, FIG. 6 shows the defect-free rate of the produced slab for food cans in a steady state and an unsteady state.

From the results shown in FIGS. 5 and 6, by applying the continuous casting method of the present invention, inclusion defects can be significantly reduced compared to conventional continuous casting. In addition, there is no difference in quality between steady and unsteady conditions, and there are few defects, so the entire manufactured slab can be used to produce food cans, beverage cans, and other high-grade thin steel sheets that have strict quality requirements. It can be applied to applications where

[Example 2] As shown in Fig. 7, the molten steel was not swirled during the unsteady state, and the molten steel was swirled at a speed of 50 rpm only during the unsteady state (the molten steel was rotated at a speed of 50 rpm). (without overheating) to produce slabs for food cans.

The product defect in the unsteady part of the obtained slab was caused by the molten steel injection rate of 3.5t/min as shown in Fig. 1, although the molten steel injection rate was 4°0t/min.
/min.

[Example 3] Using the continuous casting method of stainless steel of the present invention, 200 m
A 5LIS304 slab of m x 1240 mm was manufactured.

The continuous casting machine used was a vertical bending type, and the ladle capacity was 16
0t, the capacity of the tundish is 25t, and the tundish does not have a weir. Further, the injection rate of molten steel from the tundish into the mold was 2.0 t/min.

In addition, the molten stainless steel filled in the tundish was swirled in a direction opposite to the direction in which the molten stainless steel was poured into the ladle using the same electromagnetic stirring as in Example 1 only during unsteady conditions.

The swing speed is 50 rpm, and the swing speed adjustment cycle is as shown in FIG.

When the 5US304 slab thus produced was commercialized, the surface of the coil after cold rolling was visually inspected for inclusion defects.

The results are shown in Figure 9. Incidentally, FIG. 9 also shows similar test results for a 5US304 slab manufactured by the conventional continuous casting method.

The conventional method was manufactured under the same conditions as 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 the slab is a middle slab or an unsteady slab.

In addition, in this example, the molten stainless steel was rotated only during unsteady conditions, but as mentioned above, the molten stainless steel may be rotated at low speed during steady conditions, and the rotation speed may be increased during unsteady conditions. It is.

In addition, when the tan-dish was heated to 60°C above the melting point of molten steel in the cycle shown in Figure 8, it was possible to further reduce the occurrence of product defects, and there was no difference in quality between steady and unsteady states. Moreover, since it has fewer defects, the entire manufactured slab can be applied to applications with strict quality requirements, such as bright annealed cold-rolled stainless steel sheets for exterior use.

From the above results, the effects of the present invention are clear.

<Effects of the Invention> As described above in detail, according to the continuous casting method and continuous stainless steel casting method of the present invention, ladle slag, which is the biggest cause of inclusion defects, can be reliably removed from molten steel in the tundish. Since this ladle slag is not injected into the mold while suspended in molten steel, the amount of slag contained in the manufactured slab can be extremely reduced, resulting in high quality without inclusion defects. It is possible to produce cast slabs. In addition, since there is little difference in quality between steady and unsteady continuous casting, all of the produced slabs are used to produce high-grade thin steel sheets for food cans and beverage cans, and even bright annealed cold-rolled stainless steel sheets for exterior use. It can be applied to applications that require high quality.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the injection speed of molten steel from the tundish into the mold and the magnetic particle flaw detection defect index. FIG. 2 is a graph showing the defect occurrence index according to the slab position of continuously cast 5US430 slabs. FIG. 3 and FIG. 7 are graphs showing an example of rotational speed adjustment of swirling of molten steel in a tundish and heating cycle in a 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 score of the slab. FIG. 5 is a graph showing the number of defects detected by magnetic particles in unsteady conditions in slabs 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 slabs manufactured by the continuous casting method of the present invention and the conventional continuous casting method in a steady state and an unsteady state. FIG. 8 shows the rotational speed adjustment of the swirling of molten steel in the tundish in the stainless steel continuous casting method to which the present invention is applied;
and a graph showing an example of a heating cycle. FIG. 9 is a graph showing the integrity index of slabs manufactured by the continuous stainless steel casting method of the present invention and the conventional stainless steel continuous casting method in a steady state (middle slab) and in an unsteady state. FIG, 2 Photo Mille L Top Rotating Machine - (9m) FIG, 4 Turnoff 2LM ('C) FIG, 5 FIG, 6 Steady Part - Normal Stem Rotation Speed (rl) , m) Kaikenyun number (r, p, m)

Claims (6)

[Claims] (1) In continuous casting in which molten steel in a ladle flows into a tundish and the molten steel is injected from the tundish into a mold, the injection rate of molten steel from the tundish into the mold is 3.0 t/min or more; , a continuous casting method characterized by creating a swirling flow of molten steel filled in a tundish. (2) The continuous casting method according to claim 1, wherein the speed of the swirling flow is increased during an unsteady state compared to a steady state. (3) In continuous casting in which molten steel in a ladle flows into a tundish and the molten steel is injected from the tundish into a mold, the injection rate of molten steel from the tundish into the mold is 3.0 t/min or more; , a continuous casting method characterized by creating a swirling flow of molten steel that is filled into a tundish only during unsteady conditions. (4) The continuous casting method according to any one of claims 1 to 3, wherein the tundish is heated at least during unsteady operation. (5) In continuous stainless steel casting, in which molten stainless steel in a ladle flows into a tundish and molten stainless steel is injected from said tundish into a mold, the molten steel filled in said tundish is formed into a swirling flow, at least in unsteady conditions. A continuous casting method for stainless steel characterized by: (6) The continuous casting method for stainless steel according to claim 5, wherein the tundish is heated at least during unsteady operation.