JP6743850B2 - Continuous casting method for round slabs - Google Patents

Description

The present invention relates to a method for continuously casting a round cast slab having a circular cross-sectional shape.

Continuous casting of steel is performed while supplying a mold powder (hereinafter sometimes simply referred to as “powder”) onto the surface of molten steel in a mold. The powder supplied to the molten steel surface is melted to form a molten powder. By allowing the molten powder to flow between the mold (mold) and the solidified shell (slab) to sufficiently lubricate the mold and the slab, it is possible to stably continuously cast steel.

When continuously casting a round slab, compared to a rectangular slab, contact between the inner wall of the mold and the slab is likely to be non-uniform, and cooling is likely to be non-uniform. If the cooling is not uniform, there are problems that vertical cracking and breakout of the slab occur, and if not, the roundness of the cross section of the slab decreases.

As a technique for dealing with such a problem, in Patent Document 1, in a continuous casting method of a round cast piece performed while supplying the mold powder on the molten steel surface in the mold, the mold powder is mixed with CaO/SiO _{2 in} a mass ratio. Is 0.30 to 0.80, the viscosity at 1573K is 1.0 to 3.5 Pa·s, and the powder does not crystallize, and a continuous casting method for round cast pieces is described. This method is to prevent vertical cracking and roundness reduction of round slabs by suppressing the non-uniform inflow of molten powder by amorphizing the mold powder, but in the examples, the viscosity It has been pointed out that when the viscosity of the amorphous mold powder of 4.0 Pa·s or more is increased, the roundness is adversely affected when the viscosity is increased.

Further, in Patent Document 2, when manufacturing a round cast piece with a curved or vertical curved continuous casting machine, [Al%] and [N%] of molten steel components injected into the continuous casting mold are [Al%]. X [N%] ≤ 2 x 10 ^-4 , and the continuous casting method is characterized in that the round slab drawn from the mold is secondarily cooled at a secondary cooling specific water amount of 0.4 L/kg or more. Have been described. This method prevents the roundness from decreasing by setting the specific water amount of the secondary cooling to 0.4 L/kg or more. In Patent Document 2, it is said that the specific water content is preferably less than 1.0 L/kg in order to further prevent warpage of the round cast slab.

Patent Document 3 discloses a continuous casting method for a round cast piece which prevents vertical cracking and bubble defects by stabilizing the crystal formation of melilite from mold powder and slowly cooling it. In Patent Document 3, the specific water amount is preferably 0.2 L/kg or more in order to prevent a decrease in roundness and 1.6 L/kg or less in order to prevent surface cracking during straightening. .. Further, it is said that there was a problem of vertical cracks and bubble defects in a comparative example in which a glassy mold powder having a viscosity of 3.3 to 9.7 poise was continuously cast at a specific water content of 1.2 L/kg.

JP, 2013-151022, A JP 2005-052881A International Publication No. 2011/004507

From the viewpoint of preventing the roundness of the round cast piece from decreasing, from Patent Document 1, a powder having a viscosity of 4.0 Pa·s or more (40 poise or more when converted into a unit) is not preferable, in other words, a powder having a viscosity of less than 40 poise is preferable. In addition, it is described in Patent Document 2 that the secondary cooling specific water amount is 0.4 L/kg or more, and in Patent Document 3 that the secondary cooling specific water amount is 0.2 L/kg or more. .. However, according to the study by the present inventors, even if the powder viscosity and the secondary cooling specific water amount are set in the above ranges to perform continuous casting of round slabs, the roundness may be excellent or may be inferior. Conversely, even if the powder viscosity and the secondary cooling specific water amount are set outside the above ranges, the roundness may be excellent. That is, it has been impossible to obtain a guideline for reliably manufacturing a good billet-shaped round cast piece from the related art.

A decrease in roundness leads to operational problems in pipe making, such as defective biting during pipe making. Therefore, the roundness is generally managed by setting a certain threshold value, and a slab with a roundness value lower than a predetermined threshold value (in other words, a circularity ratio exceeding a predetermined threshold value) cannot be used. It becomes scrap and deteriorates the yield. Therefore, the present inventors have paid attention to the problem of obtaining a guideline for reliably manufacturing a good billet-shaped round cast piece.

That is, the present invention has been made in view of the above problems, and an object thereof is to provide a continuous casting method for a round billet capable of reliably producing a good billet-shaped round cast piece.

As a result of intensive studies by the present inventors in order to solve the above problems, it was found that a decrease in the roundness of the round cast piece tends to occur when the viscosity of the mold powder is high. Regarding the cause, the inventors of the present invention have found that the higher the viscosity of the mold powder is, the harder the molten powder layer is separated from the surface of the slab immediately below the mold, and as a result, the cooling efficiency in the secondary cooling is decreased, and the solidification is reduced. I thought that it might be because the growth of the shell was hindered. Therefore, the higher the viscosity of the powder to be used, the higher the specific water content in the secondary cooling is set to promote the separation of the molten powder layer from the surface of the slab directly under the mold, thereby increasing the cooling efficiency in the secondary cooling. , It is recalled that high roundness can be surely obtained by promoting the growth of the solidified shell. Then, the present inventors have conducted further studies and succeeded in finding a relational expression that should be satisfied by the viscosity of the powder and the specific water amount of the secondary cooling, which is required to obtain a predetermined roundness.

The present invention has been completed based on the above findings, and its gist structure is as follows.
[1] A continuous casting method of a round cast piece, which is performed while supplying a mold powder onto the surface of molten steel in a mold,
As the mold powder, a mass ratio of CaO content to SiO ₂ content (CaO/SiO ₂ ) of 0.30 to 1.0 is used.
Continuous round slab characterized by setting the secondary cooling specific water amount W (L/kg) so as to satisfy the following formula (1) according to the viscosity η (poise) of the mold powder at 1300°C. Casting method.
0.2+0.012 η ≤ W ・・・(1)

[2] According to the viscosity η (poise) of the mold powder at 1300° C., the secondary cooling specific water amount W is set so as to satisfy the following expression (2). Continuous casting method.
W ≤ 0.8 + 0.016 η (2)

[3] The continuous casting method for round slabs according to the above [1], wherein the roundness of the round slabs is 2.5% or less.

[4] The continuous casting method for round slabs according to [2], wherein the warpage of the round slabs is 40 mm or less.

[5] The continuous round cast piece according to any one of [1] to [4] above, wherein the mold powder is an amorphous mold powder having a viscosity η at 1300° C. of 3.5 to 35 poise. Casting method.

According to the continuous casting method for round cast slabs of the present invention, it becomes possible to reliably manufacture good billet-shaped round cast slabs.

3 is a graph showing the relationship between the viscosity of mold powder and the circularity of round slabs obtained in Example 1. 5 is a graph showing the relationship between the specific water content of secondary cooling and the circularity of round slabs obtained in Example 2. It is a figure explaining the definition of the warp of a round cast piece. 5 is a graph showing the influence of the viscosity of the mold powder and the specific water amount of the secondary cooling, which are obtained in Example 3, on the circularity of the round slab. 7 is a graph showing the influence of the viscosity of the mold powder and the specific water amount of the secondary cooling on the warpage of the round slab obtained in Example 3.

The continuous casting method for round cast slabs according to one embodiment of the present invention is performed while supplying mold powder onto the surface of molten steel in the mold.

The mold powder used in the present embodiment has a mass ratio of CaO content to SiO ₂ content, so-called basicity (CaO/SiO ₂ ) of 0.30 to 1.0. As a result, it is possible to prevent the molten powder from flowing nonuniformly between the mold and the solidified shell due to the generation of slugrim, and it is possible to suppress vertical cracking and breakout due to nonuniform cooling. If the basicity (CaO/SiO ₂ ) is less than 0.30, the viscosity of the molten powder will be too high, and it will be difficult for the molten powder to flow between the mold and the solidified shell, resulting in uneven flow in the circumferential direction. , It is easy to cause restraint breakout. On the other hand, if the basicity (CaO/SiO ₂ ) exceeds 1.0, the powder tends to crystallize, and the growth of slugrim reduces the flow path between the mold and the solidified shell, causing the molten powder to flow in the circumferential direction. In this case, uneven flow is likely to occur, and problems such as a decrease in roundness of the cross section of the cast round cast piece are likely to occur. From the above viewpoint, the basicity (CaO/SiO ₂ ) is preferably 0.50 to 0.90.

The viscosity of the molten powder at 1300°C is preferably adjusted to 3.5 to 35 poise. If the viscosity at 1300°C is less than 3.5 poise, the molten powder is easily caught in the molten steel and inclusion defects tend to occur. On the other hand, if the viscosity at 1300°C exceeds 35 poise, the molten powder becomes difficult to flow between the mold and the solidification shell, and the film of the inflowing molten powder becomes locally thin and breaks, causing a break. Out easily occurs. The viscosity of the molten powder at 1300°C is more preferably 10 to 35 poise.

The mold powder used in this embodiment is preferably an amorphous mold powder. As a result, it is possible to prevent the molten powder from flowing nonuniformly between the mold and the solidified shell due to the generation of slugrim, and it is possible to suppress vertical cracking and breakout due to nonuniform cooling. Here, the amorphous mold powder is defined as a mold powder in which an exothermic peak due to crystallization is not detected when the mold powder is heated and melted using a differential thermal analyzer and cooled at 10 K/min. And

In this embodiment, it is important to set the secondary cooling specific water amount W (L/kg) so as to satisfy the following expression (1) according to the viscosity η (poise) of the mold powder at 1300°C.
0.2+0.012 η ≤ W ・・・(1)

That is, the use of a powder having a high viscosity hinders the peeling and dropping of the powder immediately below the mold, and the cooling effect in the secondary cooling zone cannot be sufficiently obtained, which may result in a decrease in roundness. .. In this case, the secondary cooling specific water amount W (L/kg) is set so as to satisfy the above formula (1) according to the powder viscosity η (Poise) at 1300°C (1573K). Specifically, the secondary cooling specific water amount W is adjusted to increase as the viscosity η of the powder used increases. This makes it possible to perform secondary cooling, particularly strong cooling immediately below the mold, promote peeling and falling of the molten powder layer, and effectively cool the slab. As a result, the growth of the solidified shell is promoted and the strength is enhanced, and a round cast piece having a high roundness, that is, a low non-circularity can be obtained.

Here, the non-circularity is a value obtained by measuring the diameter of a cast round cast piece at eight locations in the circumferential direction and dividing the value obtained by subtracting the minimum diameter from the maximum diameter by the maximum diameter. It is defined and expressed by the following equation (3). In order to prevent problems during pipe manufacturing, the non-circularity is required to be below a predetermined reference value such as 2.5%. The roundness is expressed by the following equation (4).
Non-circularity (%) = (maximum diameter-minimum diameter)/maximum diameter x 100 (3)
Roundness (%) = 100-Revenness rate (4)

By setting the secondary cooling specific water amount W (L/kg) so as to satisfy the above expression (1), it is possible to reliably manufacture a round cast slab having an asphericity of 2.5% or less. Further, when it is desired to reliably manufacture a round cast slab having a lower non-circularity of less than 2.0%, the secondary cooling specific water amount W (L/kg) is set so as to satisfy the following formula (1)′. It is preferable.
0.3+0.012η ≤ W ・・・(1)'

Further, in the present embodiment, it is preferable to set the secondary cooling specific water amount W so that the following expression (2) is also satisfied according to the viscosity η (poise) of the mold powder at 1300°C.
W ≤ 0.8 + 0.016 η (2)

That is, if the secondary cooling specific water amount W is too large, the warpage of the round slab may increase. When using an amorphous mold powder, if a high-viscosity amorphous powder is used, the powder film becomes difficult to peel from the slab, and this becomes thermal resistance, so even if the secondary cooling specific water content W is increased to some extent, it will warp. The problem is not noticeable. However, with a low-viscosity amorphous powder, the powder film is likely to peel off from the slab that has been pulled out of the mold, so that the problem of warpage tends to become apparent when the secondary cooling specific water amount W increases. Therefore, the secondary cooling specific water amount W (L/kg) is set according to the powder viscosity η (Poise) at 1300° C. so as to satisfy the above expression (2). Specifically, the secondary cooling specific water amount W is adjusted to decrease as the viscosity η of the powder used decreases. As a result, it is possible to reliably manufacture a round cast piece with less warpage.

Here, the warpage of the round slab is defined as the maximum value of the distance between the line connecting the both ends of the round slab with a length of 11 m and the surface of the slab, as shown in FIG. In order to prevent problems, it is required to be below a predetermined reference value such as 40 mm or less.

By setting the secondary cooling specific water amount W (L/kg) so as to satisfy the above formula (2), it is possible to reliably manufacture a round cast slab having a warp of 40 mm or less. In addition, if you want to reliably manufacture round cast pieces with a warp of less than 20 mm, set the secondary cooling specific water volume W (L/kg) so as to satisfy the following formula (2)'. Is preferred.
W ≤ 0.6 + 0.016 η (2)'

In this way, by setting the secondary cooling specific water amount W (L/kg) so as to satisfy the equations (1) and (2), the required secondary cooling water amount is defined by the viscosity of the mold powder. It has become possible to reliably manufacture a good billet-shaped round slab that achieves both roundness of the round slab and suppression of warpage.

In this embodiment, the viscosity η of the mold powder used is measured in advance. Then, in the operation of continuous casting, the measured powder viscosity η is substituted into the equation (1), preferably also into the equation (2), so that the equation (1) is satisfied, preferably the equation (2). The secondary cooling specific water amount W is determined so as to satisfy the above condition. Then, the secondary cooling is performed under the condition of the determined secondary cooling specific water amount W. In the present specification, “viscosity η at 1300° C. of mold powder” is measured using a rotational viscometer, and specifically, a powder melted at 1400° C. in a graphite crucible using a vertical heating furnace. While adjusting and maintaining the temperature to 1300℃ by controlling the furnace temperature, the graphite cylindrical rotor immersed in the molten powder was rotated by changing the rotation speed to multiple conditions. From the relationship with the torque, the viscosity was determined based on a calibration curve previously obtained using a plurality of standard viscosity liquids.

The component composition of the mold powder used in the present embodiment contains CaO and SiO ₂ as main components so that the basicity (CaO/SiO ₂ ) satisfies 0.30 to 1.0, and the viscosity at 1300° C. is preferably 3.5 to 35 poise. It may be appropriately adjusted so that it becomes amorphous, and is not particularly limited. The viscosity and crystallization characteristics of the molten powder can be adjusted by changing the contents of F, Na ₂ O, Li ₂ O, Al ₂ O ₃ and MgO together with CaO and SiO _{2 in} the powder composition. Further, the physical properties of the molten powder can be adjusted as described above even under the condition that Fe ₂ O ₃ , MnO, TiO ₂ , ZrO ₂ , B ₂ O _{3 and the} like are added in small amounts.

CaO and SiO ₂ are the main components of the mold powder, and their total amount is preferably about 65 to 80 mass %. SiO ₂ and Al ₂ O ₃ increase the viscosity, and CaO, Na ₂ O, Li ₂ O, MgO and F have the effect of decreasing the viscosity, so the viscosity is adjusted by these adjustments. In order to achieve this, it is effective to lower CaO and MgO with respect to SiO ₂ and Al ₂ O ₃ and to adjust the viscosity by actively using Na ₂ O and Li ₂ O. %, SiO ₂ is 30% or more, 50% or less, Al ₂ O ₃ is 5% or more, 18% or less, CaO is 18% or more, 35% or less, MgO is 1.5% or more, 4% or less, Na ₂ O Is preferably 2% or more and 12% or less, Li ₂ O is 1.5% or more and 5% or less, and F is preferably 2% or more and 8% or less.

The components of the steel to which the continuous casting method of the present embodiment is applied are not particularly limited, but in mass %, C: 0.14 to 0.29%, Si: 0.14 to 0.35%, Mn: 0.40 to 1.45%, P: 0.005. To 0.025% and S: 0.001 to 0.015%, optionally further Cu: 0.00 to 0.10%, Ni: 0.00 to 0.20%, Cr: 0.00 to 1.15%, Mo: 0.000 to 0.075%, Co: 0.000 -0.030%, V: 0.000-0.055%, Ti: 0.000-0.040%, B: 0.0000-0.0030%, Nb: 0.000-0.040%, Al: 0.002-0.070%, Ca: 0.0000-0.0035%, and N: 0.002 It can be preferably applied to a low alloy carbon steel containing one or more selected from 0.008% to 0.008% and the balance being Fe and inevitable impurities.

Further, in the present embodiment, the continuous casting speed is not particularly limited, but from the viewpoint of productivity, it is preferable to set the slab drawing speed to 0.8 to 2.4 m/min.

The diameter of the round cast piece manufactured in the present embodiment is not particularly limited, but is preferably 100 to 350 mm, more preferably 170 to 300 mm.

The experiments of Examples 1 to 3 below were performed using the five types of mold powders A to E having the specifications shown in Table 1.

In the following Examples 1 to 3, C: 0.12%, Si: 0.20%, Mn: 0.50%, P: 0.020%, and S: 0.008% by mass%, and the balance being Fe and inevitable impurities. Molten steel having a certain composition was poured into a curved water-cooled mold having a circular cross section and continuously cast at a casting speed of 2.0 m/min to form a round cast piece having a diameter of 210 mm. The radius of curvature of the curved mold and the curved portion of the strand was about 10.5 m.

(Example 1)
Round casting obtained by continuous casting using the five types of mold powders shown in Table 1 (viscosity at 1300°C is 1.9 to 35 Poise) under the condition that the specific water volume for secondary cooling is constant at 0.6 L/kg. The circularity of one piece was measured. This is an investigation of the influence of the viscosity of the mold powder on the circularity, and the results are shown in Table 2 and FIG. As is clear from Table 2 and FIG. 1, in the viscosity range of 1.9 to 35 Poise, it is found that the non-circularity tends to gradually increase as the viscosity of the mold powder increases. , The non-circularity increased more than the standard value of 2.5%.

(Example 2)
It was obtained by continuous casting while changing the specific water amount of the secondary cooling to 0.4 to 1.1 L/kg under the condition that the viscosity of the mold powder at 1300° C. is constant at 20 Poise (that is, the mold powder D is used). The circularity of the round slab was measured. This is an investigation of the influence of the secondary cooling specific water content on the circularity, and the results are shown in Table 3 and FIG. As is clear from Table 3 and FIG. 2, the non-circularity increased more than the standard value of 2.5% under the condition that the formula (1) was not satisfied.

(Example 3)
The viscosity of the mold powder at 1300°C is 3.5 to 35 Poise, and the secondary cooling specific water amount is variously changed in the range of 0.20 to 1.40 L/kg, and continuous casting is performed. The warpage defined by was measured. Table 4 shows the powder viscosity at each level, the secondary cooling specific water content, the value on the left side of the formula (1), the value on the right side of the formula (2), and the evaluation results of the non-circularity and the warp. In Table 4, the circularity rate of less than 2.0% is expressed as "○", the circularity rate of 2.0% to 2.5% is expressed as "△", and the circularity rate of more than 2.5% is expressed as "X". "," above 20 mm and below 40 mm is indicated as "△", and above 40 mm is indicated as "x". Further, the evaluation of the circularity of the round slab under the conditions of the viscosity and the specific water content of each mold powder is shown in FIG. 4, and the evaluation of the warpage of the round slab is shown in FIG.

As is clear from Table 4 and FIGS. 4 and 5, the non-circularity is 2.5% or less when the condition (1) is satisfied, and the warp is 40 mm or less when the condition (2) is satisfied. On the other hand, it can be seen that, under the condition out of the equation (1), the non-circularity deteriorates to over 2.5%, and under the condition out of the equation (2), the warp deteriorates to over 40 mm. Moreover, in order to control the non-circularity to less than 2.0% and the warp to a better level of 20 mm or less, the conditions must satisfy W≧0.3+0.012η and W≦0.6+0.016η, respectively. Is preferred.

As described above, according to the results of Examples 1 to 3, by setting the secondary cooling specific water amount W so as to satisfy the formula (1) according to the viscosity of the powder used in the operation, the non-circularity is 2.5% or less. It is possible to reliably manufacture a round slab with a warp of 40 mm or less by setting the secondary cooling specific water amount W so as to satisfy the formula (2). Can understand.

According to the continuous casting method for round cast slabs of the present invention, it becomes possible to reliably manufacture good billet-shaped round cast slabs.

Claims (5)

A continuous casting method for round slabs performed while supplying mold powder onto the surface of molten steel in a mold,
As the mold powder, a mass ratio of CaO content to SiO ₂ content (CaO/SiO ₂ ) of 0.30 to 1.0 is used.
The viscosity η (poise) of the mold powder at 1300° C. is measured in advance,
At the time of continuous casting operation, the measured viscosity η is substituted into the following formula (1), and the secondary cooling specific water amount W (L/kg) is determined so as to satisfy the formula (1) .
A continuous casting method for round cast slabs, characterized in that secondary cooling is performed under the conditions of the determined secondary cooling specific water amount W.
0.2+0.012 η ≤ W ・・・(1) The secondary cooling specific water amount W is determined so as to substitute the measured viscosity η(poise) of the mold powder at 1300° C. into the following equation (2) and also satisfy the equation (2). Continuous casting method for round slabs.
W ≤ 0.8 + 0.016 η (2) The method for continuously casting a round cast piece according to claim 1, wherein the circularity of the round cast piece is 2.5% or less. The method for continuously casting a round cast piece according to claim 2, wherein the warp of the round cast piece is 40 mm or less. The continuous casting method according to any one of claims 1 to 4, wherein the mold powder is an amorphous mold powder having a viscosity η at 1300°C of 3.5 to 35 poise.

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