JP7147477B2 - Continuous casting method for billet slab - Google Patents

Description

The present invention is especially useful when continuously casting billet slabs having a cross-sectional area (hereinafter simply referred to as "cross-sectional area") of 500 cm ² or less while stably and reliably suppressing the occurrence of center porosity. The present invention relates to a continuous casting method capable of suppressing bending occurring in parts.

When a seamless steel pipe is produced by rolling or forging a continuously cast billet slab (hereinafter also simply referred to as “slab”), the quality of not only the outer surface of the slab but also the inner part is important. Since the soundness of slabs is strongly required, quality control of the cast slab core is important.

For this reason, in the continuous casting of slabs, various methods have been proposed for the purpose of suppressing the occurrence of center porosity. The present applicant also proposed a continuous casting method that stably and reliably suppresses the occurrence of center porosity in the slab by performing strong cooling at the final stage of solidification in continuous casting, and exhibits the effect of improving the internal quality of the slab. This is proposed in Patent Document 1.

By carrying out the continuous casting method proposed in Patent Document 1, desired effects can be obtained in continuous casting of various types of steel such as carbon steel, low-alloy steel, high-alloy steel, and stainless steel. can.

However, in the continuous casting method proposed in Patent Document 1, intensive cooling is performed at the final stage of solidification, so particularly in the case of small cross-section slabs with a cross ^- sectional area of 500 cm was likely to occur. As shown in FIG. 4, when the tip portion 1a of the slab 1 is bent, the tip portion 1a collides with the carry-out table 2 when the slab 1 is carried out from the continuous casting facility, and the carry-out of the slab 1 is hindered. There is

JP-A-2009-6367

The problem to be solved by the present invention is that when a small cross-section slab having a cross-sectional area of 500 cm ² or less is continuously cast by the method proposed in Patent Document 1, bending is likely to occur at the tip of the slab. The problem is that there are cases where it is difficult to carry out.

In order to suppress the bending of the tip of the slab, which tends to occur when continuously casting a slab with a cross-sectional area of 500 cm ² or less by the method proposed in Patent Document 1, the inventors performed a hardening at the final stage of solidification. The effects of cooling and bending occurring at the tip of the slab were investigated.

As a result, in the method proposed in Patent Document 1, since the slab is strongly cooled at the final stage of solidification, cooling unevenness in the circumferential direction of the slab can occur due to clogging of the spray tip for injecting the cooling water, adhesion of scale, etc. Investigations by the inventors have revealed that this is easy to do.

In particular, when the tip of the slab, which is the part where casting is started, is strongly cooled at the final stage of solidification, not much time has passed since the start of casting, and the continuous casting machine and cooling water are cold, so cooling unevenness is noticeable. bending is likely to occur.

Based on the above investigation results, the inventors have come up with the idea of suppressing uneven cooling by weakening the cooling in the final stage of solidification at the tip of the cast slab. Specifically, it was investigated to gradually increase the specific water quantity (cooling water quantity) for cooling in the final stage of solidification for the tip of the cast slab.

As a result, in relation to the surface temperature of the slab when passing through the inlet of the cooling zone in the final stage of solidification, the specific water amount of the cooling water at the tip of the slab is set to a predetermined specific water amount or less, so that the tip of the slab can be It was found that the bending of the

The present invention has been made based on the knowledge obtained by the studies of the inventors.
That is, the present invention
In a cooling zone in the final stage of solidification installed at a predetermined distance in the casting direction from the meniscus of molten steel in the mold, the surface temperature when the tip of the billet slab passes through the entrance of the cooling zone in the final stage of solidification falls within a predetermined temperature range. This is a continuous casting method for billet slabs, in which the billet slabs adjusted as described above are water-cooled, and solidification is completed in the cooling zone in the final stage of solidification.

Then, when continuously casting the billet slab by the above method,
The specific water content of the cooling water in the cooling zone at the final stage of solidification is gradually increased from a specific water content smaller than a predetermined specific water content at the tip of the billet slab as the billet slab progresses. The most important feature is that the specific water content is set to the predetermined specific water content after the tip of the cooling zone passes through the inlet of the cooling zone in the final stage of solidification for a predetermined length.

The present invention gradually increases the specific water content of cooling water when the tip of the slab passes through the cooling zone in the final stage of solidification from a specific water content smaller than a predetermined specific water content as the slab progresses. Therefore, it is possible to suppress uneven cooling of the front end portion of the cast slab in the cooling zone at the final stage of solidification.

In the present invention, the position where the cooling zone in the final stage of solidification is installed is a position where productivity in continuous casting can be secured and stable operation can be achieved, for example, a range of 15 to 45 m in the casting direction from the meniscus of molten steel in the mold. is within.

Further, in the present invention, the surface temperature of the cast slab when passing through the inlet of the cooling zone at the end of solidification is 900 to 1200°C. If the temperature is less than 900° C., a phase transformation from the γ phase to the α phase occurs, the surface of the slab expands, and the effect of suppressing the generation of center porosity is likely to be impaired. On the other hand, if the temperature exceeds 1200° C., the cooling becomes non-uniform and the effect of suppressing the center porosity becomes unstable.

Further, in the present invention, the predetermined specific water content of the cooling zone in the final stage of solidification is determined by the outer diameter and material of the slab to be cast.

According to the results of continuous casting by the inventors of low-carbon steel, three types of alloy steel (Cr content of 1% by mass and 2% by mass, and low-carbon Ni steel), casting When the outer diameter of the slab was 191 mm or less, the occurrence of center porosity could be suppressed by setting the specific water content to 0.07 to 0.13 (L/kg-steel) regardless of the material.

On the other hand, it is necessary to increase the specific water content as the outer diameter of the slab to be cast becomes larger than 191 mm. Also, as the outer diameter of the cast slab to be cast increases, it becomes more susceptible to the influence of the material. For example, when the outer diameter of the cast slab to be cast exceeds 200 mm, it is 0.21 (L / kg-steel) for general low carbon steel, and the specific water content (0 It was confirmed that the occurrence of center porosity can be suppressed by setting the ratio to 0.10 (L/kg-steel) or 0.01 (L/kg-steel).

However, even if the specific amount of cooling water is more than 0.21 (L/kg-steel) or less than 0.01 (L/kg-steel), the occurrence of center porosity cannot be effectively suppressed. Since the properties inside the slab deteriorate, it is desirable to determine it within the range of 0.01 to 0.21 (L/kg-steel) according to the outer diameter and material of the slab to be cast.

Further, in the present invention, a specific water content smaller than a predetermined specific water content means that even if cooling unevenness occurs in the circumferential direction at the tip of the slab due to clogging of the spray tip, adhesion of scales, etc., bending will not occur. It is the amount of specific water that does not occur and does not necessarily have to be 0 (L/kg-steel).

Further, in the present invention, the length of the tip portion of the slab for which the specific water content of the cooling water in the cooling zone at the final stage of solidification is the predetermined specific water content may be 15 m from the tip of the slab. This is because, at a distance of 15 m or more from the tip of the slab, even if the specific water content is set in advance, there is no bending that hinders carrying out of the slab from the continuous casting facility.

The present invention exhibits great effects when continuously casting small cross-section slabs having a cross-sectional area of 500 cm ² or less. This is because when the cross-sectional area exceeds 500 cm ² , even if cooling at the end of the slab in the final stage of solidification is not weakened, the tip of the slab will not be bent enough to interfere with carrying out.

In the present invention, the manner in which the specific water content is gradually increased as the slab advances from a state of less than a predetermined specific water content may be linear or stepwise.

In the present invention, the specific water content of the cooling water when the tip of the slab passes through the final solidification cooling zone is gradually increased from a specific water content smaller than a predetermined specific water content as the slab progresses. . Therefore, it is possible to suppress uneven cooling of the front end portion of the cast slab in the cooling zone in the final stage of solidification, and it is possible to suppress bending that hinders carrying out from the continuous casting facility.

When the slab is continuously cast, the surface temperature of the slab when the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification, and when the part 5m from the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification. Fig. 3 is a diagram showing the relationship between the specific water content of the cooling water and the bending that occurs at the tip of the cast slab. FIG. 4 is a diagram showing the relationship between the casting length of a slab and the specific water content, in an example of the case where the specific water content is increased stepwise in the cooling zone at the final stage of solidification. FIG. 4 is a diagram showing the relationship between the casting length of a slab and the specific water content, in an example of linearly increasing the specific water content in the cooling zone at the final stage of solidification. FIG. 4 is a plan view showing a state in which a cast slab having a curved front end portion is being transported by a carry-out table of a continuous casting facility;

An object of the present invention is to suppress bending occurring at the tip of a slab having a cross-sectional area of 500 cm ² or less, particularly in the case of continuous casting while stably and reliably suppressing the occurrence of center porosity. .

In order to achieve the above purpose, the specific water content of the cooling water when the tip of the slab passes through the cooling zone in the final stage of solidification is gradually increased from a specific water content smaller than a predetermined specific water content to gradually increase as the slab advances. This was achieved by increasing

First, an example of the study results that led to the discovery of the continuous casting method for billet slabs of the present invention will be described.

When continuously casting low-carbon steel slabs having outer diameters of 191 mm, 225 mm, and 310 mm and having the chemical compositions shown in Table 1 below, the inventors set a cooling zone at the end of solidification at a position 30 to 35 m from the meniscus position. , the specific amount of cooling water when the tip of the slab passes was increased stepwise from 0 as the slab progressed.

Outer diameter of continuously cast slab, surface temperature of slab when tip of slab (casting length is 0m) passes through the inlet of cooling zone in final stage of solidification, part from tip to 5m of slab, 5~ Table 2 below shows the specific water volume V of the cooling water when the portion up to 10 m, the portion from 10 to 15 m, and the portion after 15 m pass through the inlet of the cooling zone in the final stage of solidification. Table 2 below also shows the presence or absence of bending at the tip of the cast slab after the completion of cooling in the final stage of solidification.

No. In 1 to 4, the specific water volume V of the cooling water was set to 0.00 (L/kg-steel) when the portion from the tip of the slab to 5 m passed through the inlet of the cooling zone in the final stage of solidification. Then, the specific water volume V of the cooling water when the portion 15 m or more from the tip of the slab passes through the inlet of the cooling zone in the final stage of solidification was measured using No. 1 with an outer diameter of 191 mm. For slabs 1 and 2, 0.07 and 0.13 (L/kg-steel) were used in consideration of the slab surface temperature when the tip of the slab passes through the inlet of the cooling zone in the final stage of solidification. No. 225 mm outside diameter. 0.18 and 0.17 (L/kg-steel) for 3 and 4 slabs.

On the other hand, the specific water volume V of the cooling water when the portion 5 to 10m from the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification is The amount of water shown in Table 2 was set to be less than the amount of specific water V when passing through.

In addition, the specific water amount V of the cooling water when the portion 10 m to 15 m from the tip of the slab passes through the inlet of the cooling zone in the final stage of solidification is the specific water amount V of the portion 5 to 10 m from the tip of the slab. , the intermediate specific water content of the above-mentioned specific water content V of the portion 15 m or more from the tip of the slab.

Then, when the tip of the cast slab was cooled in the cooling zone in the final stage of solidification with the above specific water content, the occurrence of bending at the tip of the cast slab was investigated.

For comparison, the conventional method in which the specific amount of cooling water in the cooling zone at the final stage of solidification is constant was also examined for bending at the leading edge of the cast slab (Nos. 5 to 7).

Regarding the presence or absence of bending in Table 2, when the bending of the tip of the cast slab makes it difficult to carry out from the continuous casting equipment, it is evaluated that bending has occurred. evaluated. It can be evaluated that no bending occurred when the amount of displacement in the direction perpendicular to the advancing direction at the tip of the slab up to 15 m from the tip is 50 cm or less.

No. 1 to No. The surface temperature of the slab when the tip of the slab in 7 passes through the entrance of the cooling zone in the final stage of solidification, and the specific amount of cooling water when the part 5 m from the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification , and the presence or absence of bending at the tip of the cast slab is shown in FIG.

As shown in Fig. 1, when the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification, the surface temperature of the tip of the slab changes the temperature of the casting when the part 5m from the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification. It was found that the specific water content at which bending occurs at one tip changes.

From Fig. 1, when the surface temperature of the slab tip passing through the entrance of the cooling zone in the final stage of solidification is T (°C), the part 5 to 10m from the tip of the slab is the entrance to the cooling zone in the final stage of solidification. It has been found that if the specific water volume V of the cooling water passing through is set to (5.7×10 ⁵ )×exp ^{−0.016 T} (L/kg-steel) or less, bending does not occur at the tip of the slab.

In other words, the specific water volume V of the cooling water when the part 5 to 10m from the tip of the slab passes through the entrance of the cooling zone in the final stage of solidification is It was found that the amount of specific water should be greater than the amount of water passing through, and not more than (5.7×10 ⁵ )×exp ^{−0.016 T} (L/kg-steel). However, if the specific water content is too small, the specific water content at 10 m or more from the tip of the cast slab increases rapidly, thereby promoting the occurrence of bending. Therefore, a more preferable lower limit of the specific water content is -0.02 (L/kg-steel).

The present invention has been made based on the results of the inventors' studies described above.
That is, the continuous casting method for billet slabs of the present invention is
In a cooling zone in the final stage of solidification installed at a predetermined distance in the casting direction from the meniscus of molten steel in the mold, the surface temperature when the tip of the billet slab passes through the entrance of the cooling zone in the final stage of solidification falls within a predetermined temperature range. When water-cooling the billet slab adjusted as above and completing the solidification of the billet slab in the final solidification cooling zone,
The specific water amount of cooling water in the cooling zone at the final stage of solidification is gradually increased from a cooling water amount smaller than a predetermined specific water amount at the tip of the billet slab as the billet slab progresses, The specific water content is set to the predetermined specific water content after the tip of the has passed through the inlet of the cooling zone in the final stage of solidification for a predetermined length.

In the present invention, when the specific water content of cooling water in the cooling zone at the final stage of solidification is increased stepwise as shown in FIG. It is desirable to determine the specific water content V (L/kg-steel) when a portion separated by a distance of .

L = 0 to 5 m: V = 0 (L/kg-steel) (1)
L=5-10m: V≦(5.7×10 ⁵ )×exp ^−0.016T (L/kg-steel) (2)
L = 10 to 15 m: intermediate specific water content between formula (2) and formula (4) (3)
After L=15m: 0.01 to 0.21 (L/kg-steel) (4)

T in the above formula (2) is the surface temperature (° C.) when the tip of the cast slab passes through the inlet of the cooling zone in the final stage of solidification. Further, the specific water content indicated by the above formula (4) is appropriately set within the range of 0.01 to 0.21 (L/kg-steel) according to the outer diameter and material of the slab to be continuously cast. means the specific water content.

The starting position of the specific water content determined by the above formula (2) is set to 5 m from the tip of the slab because if it is set to the tip side from the position, the effect of weakening the cooling at the end of solidification at the tip of the slab is obtained. because it makes you weaker. On the other hand, if it is 5 m behind the tip of the slab, and the specific water content of the portion 15 m or later from the tip of the slab is as large as, for example, 0.21 (L / kg-steel), cast from that position. This is because the increase in specific water content between the tip of the strip and 15 m becomes too rapid.

The mode of gradually increasing the specific water content in the cooling zone at the end of solidification as the slab progresses is not limited to the stepwise increase as shown in FIG. 2, but the linear increase as shown in FIG. may

When increasing the cooling water flow rate linearly, increase linearly from the tip end of the slab to 15 m until the specific water content of the portion 15 m from the tip end of the slab reaches the specific water content of the above formula (4). Just do it. At this time, it is desirable that the specific water volume V of the cooling water when the portion 5 m away from the tip of the cast slab passes through the inlet of the cooling zone in the final stage of solidification satisfies the above formula (2).

For example, in the example shown in FIG. The specific water content V of the cooling water is set to 0 (L/kg-steel) at this time, and the specific water content is increased linearly connecting the two points of the specific water content at the tip of the slab and the specific water content at the part separated by 15 m. is.

Of course, the present invention is not limited to the above examples, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in each claim.

For example, in the example shown in FIG. 2, the specific water volume V of the cooling water is set to 0 (L/kg-steel) when the portion from the tip of the slab to 5 m passes through the inlet of the cooling zone in the final stage of solidification. However, it may not necessarily be 0 depending on the surface temperature when the tip of the cast slab passes through the inlet of the cooling zone in the final stage of solidification.

In the example shown in FIG. 2, the specific amount of cooling water in the cooling zone at the final stage of solidification at the tip of the slab is changed in three stages. However, if the specific water content of the portion 15 m or more from the tip of the slab is small, such as 0.01 (L / kg-steel), the portion up to 5 m from the tip of the slab and the tip of the slab The length may be changed in two stages: a portion from 5m to 15m from the tip of the slab, and a portion after 15m from the tip of the slab.

In the example shown in FIG. 3, the specific water amount V of the cooling water when the tip of the cast slab passes through the inlet of the cooling zone in the final stage of solidification is set to 0 (L/kg-steel). (L/kg-steel) or more. Conversely, the specific water volume V of the cooling water may be set to 0 (L/kg-steel) until the portion less than 5 m from the tip of the cast slab passes through the inlet of the cooling zone in the final stage of solidification.

Further, the examples shown in Table 2 above are obtained by continuously casting low-carbon steel slabs having outer diameters of 191 mm, 225 mm and 310 mm and having chemical compositions shown in Table 1. However, even if the steel other than the low-carbon steel shown in Table 1 is used, the method of the present invention can be applied to continuous casting of slabs of various steel grades targeted by Patent Document 1, which is the premise of the present invention. , it has been confirmed that bending of the tip of the slab can be suppressed.

1 slab 1a tip

Claims (6)

A cooling zone in the final stage of solidification set in the mold at a position within the range of 15 to 45 mm in the casting direction from the meniscus of molten steel, and the surface temperature when the tip of the billet strand passes through the entrance of the cooling zone in the final stage of solidification is 900 ° C. When water-cooling the billet slab adjusted to a temperature range of 1200 ° C. and completing the solidification of the billet slab in the final solidification cooling zone,
The specific water content of the cooling water in the cooling zone at the final stage of solidification is gradually increased from a specific water content smaller than a predetermined specific water content at the tip of the billet slab as the billet slab progresses. a continuous casting method for billet slabs having a cross- sectional area of 500 cm ² or less , wherein the specific water content is set to the predetermined specific water content after the tip of the billet has passed 15 m or more from the entrance of the cooling zone in the final stage of solidification. 2. The method according to claim 1, wherein the predetermined specific water content is determined in a range of 0.01 to 0.21 (L/kg-steel) according to the outer diameter and material of the slab to be cast. continuous casting method for billet slabs. 3. The continuous billet slab according to claim 1 or 2, wherein the mode of gradually increasing the specific amount of cooling water in the cooling zone at the final stage of solidification as the billet slab advances is stepwise. casting method. When the specific water content is increased stepwise, the specific water content V (L/kg-steel) of the cooling water when the portion separated by the distance L from the tip of the billet slab passes through the inlet of the cooling zone in the final stage of solidification is 4. The continuous casting method for billet slab according to claim 3, wherein the determination is made according to the following equation.
L = 0 to 5 m: V = 0 (L/kg-steel) (1)
L=5-10m: V≦(5.7×10 ⁵ )×exp ^−0.016T (L/kg-steel) (2)
L = 10 to 15 m: intermediate specific water content between formula (2) and formula (4) (3)
After L=15m: 0.01 to 0.21 (L/kg-steel) (4)
T: Surface temperature (°C) when the tip of the cast slab passes through the entrance of the cooling zone in the final stage of solidification The mode of gradually increasing the specific water content of the cooling water in the cooling zone at the final stage of solidification as the billet slab advances is to increase the specific water content of any part less than 5 m from the tip of the slab and the specific water content of any part less than 5 m from the tip of the slab 3. The continuous casting method for a billet slab according to claim 1, wherein the line is a line connecting two points of the specific water content of the separated portions. The specific water content V (L/kg-steel) when the portion 5 m away from the tip of the billet slab passes through the inlet of the cooling zone in the final stage of solidification is (5.7×10 ⁵ )×exp ^−0.016T (L /kg-steel) or less.
T: Surface temperature (°C) when the tip of the cast slab passes through the entrance of the cooling zone in the final stage of solidification

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