JP3394730B2 - Continuous casting method of steel slab - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel slabs, and more specifically, to a steel slab in a semi-solidified state which is extracted from a water-cooled mold and secondarily cooled, and further subjected to a light reduction.
This is a technique for forming a solidified body having a healthy internal structure (macro structure).

[0002]

2. Description of the Related Art In order to continuously cast molten steel, a continuous casting machine equipped with a tundish, a water-cooled mold, a cooling water jet nozzle group, a slab drawing device (various roller groups) and the like is used. The casting situation will be described using an example of the curved continuous casting machine shown in FIG. First, the molten steel 2 in the tundish 1 is cast into a water-cooled mold 3, and the portion in contact with the wall surface of the mold 3 is cooled,
It solidifies and is almost formed into the size of the steel slab 4 (hereinafter, simply referred to as a slab) which is a production target. Since the inside of the slab 4 is still unsolidified, after that, it is cooled by passing through a water jet nozzle group called a secondary cooling zone 5 and further cooled on a drawing device (roller group) 6, Let it be a solidified body. Therefore, the obtained cast piece 4 exhibits various internal states (component segregation, crystal structure, pores, etc.) depending on the cooling state. The internal state of the slab 4 is referred to as an internal structure or a macro structure, and as a proper one, it is preferable that the slab 4 is composed of crystal grains as uniform as possible without component segregation or voids. This is because these internal structures have a significant effect on the quality of the cast slab 4 when the slab 4 is later rolled into a product such as a steel plate or steel product. In particular, in recent years, there has been a strong demand from users of wire rod steel to improve the internal structure.

Therefore, in order to improve the internal structure of the slab 4, the slab is solidified while being rolled down on the downstream side of the secondary cooling zone 5. For this reduction,
Many technologies have been developed than before, and light pressure reduction using a pinch roll 7 that supports the moving slab 4 and strong pressure reduction using a separate press type die (for example, continuous forging device) have been put to practical use. There is. If these techniques are used, central segregation will be improved to some extent (in the case of continuous forging pressure, it will be dramatically improved).
Voids also disappear. However, when reduction is performed in a semi-solidified state as shown in FIG. 2, the liquid-solid phase interface 9 (the tip of the interface is the crater end 12 of the solid line 14 and the position where the solidified substance begins to precipitate from the liquid phase is A crack 10 is often generated at the tip of the phase line 13, which is called the crater end 11 of the liquid phase line. When molten steel with concentrated impurities enters into the cracks 10 and solidifies, it also becomes a slab having component segregation. Further, the crack 10 does not re-melt and the solidified defect also occurs. Furthermore, if the above-mentioned strong reduction (for example, continuous forging method) is used, the cost is significantly increased. Therefore, the light reduction is preferable for the slab as a raw material of the wire rod steel.

Therefore, a technique which employs light reduction and is useful for cracking is disclosed in Japanese Patent Publication No. 59-16862. That is, in the drawing process following the secondary cooling zone in continuous casting of molten metal, in the steady drawing process between the liquidus crater end of the slab and the crater end of the solidus line by one or more pairs of reduction rolls. The rolling reduction per pair of rolls is 1.5% or less, and the steady drawing process is continuously rolled down. " In other words, in the vicinity of the crater end of the unsolidified slab, molten steel enriched with impurities enters between the dendrite dendrites at the solidified interface, causing component segregation and voids (called center porosity). It is prevented under a light pressure of about 1.5%.

However, even with this technique, it has been difficult to obtain an internal structure satisfying the strict requirements of wire rod steel users. In particular, when the space between the crater end 11 of the liquid phase line 13 and the crater end 12 of the solid phase line 14 was pressed down only by a pair of pressing rolls, the target internal tissue was significantly deviated.

[0006]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention reduces the center segregation and voids of the slab as compared with the prior art when continuously casting the slab as a raw material for wire rod steel, and rolling An object of the present invention is to provide a continuous casting method of a steel slab that does not cause defects in the formed steel material.

[0007]

In order to achieve the above-mentioned object, the inventor has completed the present invention by reviewing the prior art described in Japanese Patent Publication No. 59-16862 and improving the problems.

That is, according to the present invention, a steel slab of the same steel type that has come out of a water-cooled mold and has an incompletely solidified inside is
After passing through the secondary cooling zone, a slight reduction is applied by a pair of pinch rolls to cool and form a completely solidified body in a continuous casting method of a steel slab.
s, the solid fraction fs is adjusted by changing the secondary cooling rate and the moving speed of the steel slab to determine the solid fraction when the steel slab reaches the pair of pinch rolls. A continuous casting method for a steel slab, characterized by containing in any of the following ranges and performing light reduction at a predetermined reduction rate corresponding to the range.

Fs = 0.55 to 0.60 fs = 0.60 to 0.65 fs = 0.65 to 0.80 In the present invention, the reduction ratio is determined according to the steel type of the steel slab. It is a continuous casting method of a steel slab characterized by selecting from the range of 0.6 to 2.7%.

According to the present invention, the solidification state of the steel slab, specifically, the solid phase ratio fs is lightly reduced within a substantially similar range, or when the solid phase ratios are different, it is suitable and different reduction is performed. Since the steel billet is lightly reduced at a specific rate, a steel slab having a target crystal structure can be obtained.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below together with the background of the invention.

First, the inventor has reviewed the above-mentioned prior art. As a result, as in the prior art, the liquid crystal line 13 is not
Even if a slight reduction is made between the end end 11 and the crater end 12 of the solidus line 14, the cast piece 4 is moving, so if only a pair of the reduction rolls is used, it is reduced only for a short time. It was concluded that in many cases, the slab with the internal structure that does not occur is not formed. Further, in the conventional technique, there is also a problem that it is very difficult to position the portion of the cast slab 4 to be rolled only on the pair of rolling rolls. Furthermore, in the prior art,
Although the restriction for rolling down in the steady drawing process is set, the slab between the crater end 11 of the liquidus line 13 and the crater end 12 of the solidus line 14 is either the liquidus line 13 or the solidus line 14. Since the solidification state varies greatly depending on the temperature (see Fig. 2), the internal structure of each slab remains the same even if the slabs of the same steel type are placed in different positions and lightly pressed for a short time. I was convinced that it was difficult.

On the other hand, the current technology for controlling the solidification state of cast slabs is
Although the cooling speed, the moving speed, and the like in the secondary cooling zone are changed, it is difficult to pinpoint the solidification state when the slab 4 reaches the position of the roll or the like that is lightly rolled down. Therefore, the inventor considered that when continuously casting molten steel of the same steel type, it is necessary to perform the reduction in a substantially similar solidification state or to appropriately adjust the reduction rate if the solidification states are significantly different. Then, the solidification state is the solid phase rate fs (the ratio of the amount of solid to the liquid contained in the solidified slab, 0.5 at the crater end of the liquidus line and 1.0 at the crater end of the solidus line). ), And this idea was embodied in the present invention of the above-described embodiment.

Specifically, when molten steel of the same steel type is cast by the continuous casting machine shown in FIG. 1, the slab 4 has a pair of reduction functions 8.
Before reaching the position of the pinch roll 7 equipped with, the cooling water temperature in the secondary cooling zone 5, the amount of water, and the moving speed of the slab 4 are changed based on the temperature difference of the molten steel 2 in the tundish 1. Then, the solid phase ratio at the time of arrival is within a certain range (for example, fs =
0.55 to 0.60). As long as it stays within this range, the coagulation states are almost similar, so the same predetermined reduction rate (for example, 2.
0%) and continue light reduction. However, control is not successful for some reason (for example, the molten steel temperature in the tundish 1 during casting is lowered), and the solid fraction at the time of reaching the pinch roll 7 having the rolling-down function 8 is fs = 0. Over 60 ~
There is a case where 0.65 or fs exceeds 0.65 and deviates to 0.80. In this case, in the present invention, the reduction rate is changed so that a good internal tissue can be obtained within the range of each solid phase rate, and the reduction is performed lightly. In addition, the rolling reduction that can obtain a good internal structure in each solid phase ratio may be obtained in advance from test operation or conventional operation data. Further, the reduction rate is limited to the range of 0.6 to 2.7% in any steel grade since the present invention is a light reduction technique. If it is less than 0.6%, the effect of reduction is not recognized.
This is because if it exceeds 7%, excessive pressure is applied and a good internal organization cannot be obtained. Further, the solid phase rate when reaching the pinch roll 7 having the rolling-down function 8 is detected by measuring the surface temperature of the slab 4 along its moving line.

[0015]

EXAMPLE As shown in FIG. 1, a curved continuous casting machine in which only one pair of pinch rolls had a rolling-down function 8 was used to test a slab 4 having a size of 300 mm and a width of 400 mm by the method according to the present invention. Manufactured in a regular manner. At that time, a test production was separately performed by a conventional production method (described in Japanese Patent Publication No. 59-16862), and the results of both were compared. The amount of molten steel for one charge is 180 tons. Further, in carrying out the present invention, the drawing speed (moving speed) from the water-cooled mold 3 is 0.
71 to 0.74 m / min, the amount of cooling water in the secondary cooling zone 5 is changed to 0.40 liter / kg, and the solid fraction of the slab 4 when reaching the pinch roll 7 having the rolling-down function 8 is controlled. did. Table 1 shows the changes in the solid phase ratio and the rolling reduction during casting for each steel type, with the values obtained when casting one charge of molten steel for each steel type. From Table 1, the control of the solid fraction is relatively good, and the case of casting in the solid fraction range of 1 region,
It is clear that there is a case where casting is performed in the solid fraction range of a plurality of regions due to poor control.

[0016]

[Table 1]

All of the cast pieces obtained by these test productions were examined for the internal structure of the cross section (existence of component segregation, pores, cracks, etc.), and the results are collectively shown in Table 2.

[0018]

[Table 2]

From Table 2, it is clear that the casting method according to the present invention can produce a steel slab having an excellent internal structure as compared with the conventional method.

[0020]

As described above, according to the present invention, center segregation and voids can be reduced as compared with the conventional one, and continuous casting of a steel slab that does not cause defects in a rolled steel material is possible.
As a result, it has become possible to stably supply the wire rod material.

[Brief description of drawings]

FIG. 1 is a view showing a vertical cross section of a curved continuous casting machine used for carrying out a casting method according to the present invention.

FIG. 2 is a diagram showing cracks generated in a slab by a conventional casting method.

FIG. 3 is a view showing a vertical section of a curved continuous casting machine used in a conventional casting method.

[Explanation of symbols]

1 tundish 2 Molten steel 3 Water-cooled mold (mold) 4 Steel slab (cast slab) 5 Secondary cooling zone 6 Pulling device (roller group) 7 pinch rolls 8 Rolling down function 9 Liquid-solid interface 10 cracks 11 Liquidus tip (crater end) 12 Solidus tip (crater end) 13 liquidus 14 Solidus

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-7-204812 (JP, A) JP-A-7-185763 (JP, A) JP-A-5-220559 (JP, A) JP-A-5- 154633 (JP, A) JP-A-2-151354 (JP, A) JP 59-16862 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/20

Claims (2)

(57) [Claims] 1. A steel slab of the same steel type that has come out of a water-cooled mold and the inside of which is in an incompletely solidified state is passed through a secondary cooling zone and then cooled with a pair of pinch rolls by applying a light reduction.
In a continuous casting method of a steel slab to be a completely solidified body, the degree of the incompletely solidified state is evaluated by a solid phase ratio fs, and the solid phase ratio fs of a secondary cooling rate and a moving speed of the steel slab. Adjusting by changing, the solid phase ratio when the steel slab reaches the pair of pinch rolls is contained in one of the following ranges, and light reduction is performed at a predetermined reduction ratio corresponding to the range. A method for continuously casting a steel slab, characterized by: fs = 0.55 to 0.60 fs = 0.60 exceeded to 0.65 fs = 0.65 exceeded to 0.80 2. The continuous casting method for a steel slab according to claim 1, wherein the reduction ratio is selected from the range of 0.6 to 2.7% depending on the steel type of the steel slab.