JPH01162550A - Method for continuously casting cast slab having excellent cross sectional size accuracy and internal quality - Google Patents

Abstract

PURPOSE:To obtain a round billet having excellent roundness and internal quality by specifying surface temp. of a cast billet in front of pinch rolls and average cooling speed on the surface of the cast billet. CONSTITUTION:In the cast of continuously casting molten steel, the surface of the cast billet is in order forcedly cooled along casting direction from position between 2-15m in front of the most end part of casting direction in the remained molten steel pool to the position of the most end part of the pool, and the solidified shell in the cast billet is shrunk to more than the corresponding rate of the volume shrinkage rate caused by solidified shrinkage, to reduce the cross sectional area of the cast billet and cast. Then, the surface temp. of the cast slab in front of the pinch rolls 3 for drawing the cast billet is made to <=850 deg.C and the average cooling speed for the surface of the cast slab is held to >=30 deg.C/min and cast. By this method, the cast billet having good dimensional accuracy and excellent internal quality with only a little center segregation and center porosity can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a continuous casting method for slabs with excellent dimensional accuracy and internal quality, which are supplied as raw materials for seamless pipes and wire rods.

<Prior Art> A conventional round billet continuous casting method will be explained with reference to FIG.

The molten steel injected into the circular mold 8 is
It is cooled in the secondary cooling zone 1 located below and solidified toward the axis, and then pulled out by a plurality of pinch rolls 3 provided downstream of the secondary cooling zone. This pinch roll is pressed against the round billet by a hydraulic cylinder 5, and the round billet is pulled out by the frictional force generated therein. 11. If the pinch roll is pressed with too much force. The round billet is deformed and the target roundness [(maximum diameter - nominal diameter) / nominal diameter]
This phenomenon occurs when ×100 (%) cannot be obtained, when the casting speed is high and the unsolidified area inside the slab is large at the pinch roll position, or when the degree of secondary cooling is weak and the internal temperature of the slab is high at the pinch roll position. In other words, the smaller the deformation resistance of the slab at the pinch roll position, the greater the deformation. Here, it is desirable to minimize the pressing force with the pinch rolls, but if the pressing force is too small, the slab will fall suddenly in the casting direction, causing problems such as breakout. As a method to prevent this, there is a method of suppressing deformation due to the pressing of the pinch roll by using a pinch roll Wokaliver shape (Tetsu to Hagane 70゜1984.52).
07). If this method is adopted, deformation of the round billet is certainly suppressed, but when casting several types of round billet diameters using the same caliber roll, it becomes difficult to set the shape. On the other hand, in order to improve this problem, if a caliber-shaped steel roll is prepared that matches the diameter of each round billet, it will not only increase the cost but also increase the cost. , an excessive amount of time is spent replacing the rolls.

As mentioned above, the round billet is also required to have sound internal quality.

In other words, center segregation or center porosity in the shaft center of a round billet causes uneven hardness or pores that cause cracks on the inner surface of the pipe (inner surface curvature) during pipe manufacturing, and significant center segregation causes transfer of bearing steel. This causes problems such as deterioration of fatigue life and breakage of hard steel wire during wire drawing.

As a method for keeping the axial center of a slab in good condition, a technique for equiaxed crystallizing the axial center by electromagnetic stirring within a mold is known, for example, as disclosed in Japanese Patent Publication No. 42572/1983.

However, when tM1 stirring is used, although the central segregation as a large segregation band is reduced, island-like so-called semi-macro segregation still remains, and in particular near-net shape, which has a size and shape close to the final product targeted by the present invention. Since the degree of subsequent working of the material is small, there is a problem in that even such semi-macro segregation can result in serious slab defects.

<Problems to be Solved by the Invention> The present invention solves the above-mentioned problems and produces slabs with excellent dimensional accuracy and internal quality, especially round billets with excellent roundness and internal quality. This was developed to provide a continuous casting method.

Means for Solving the Problems〉 As a result of intensive research and repeated experiments on a continuous casting method for slabs with good dimensional accuracy and internal quality, the present inventors have found that the objective can be achieved by forced cooling under certain conditions before the pinch rolls. Based on this knowledge, the present invention was completed.

In continuous casting of molten steel, the surface of the slab is cast from a position 2 to 15 m before the leading edge of the molten metal pool in the pouring direction to the leading edge of the pool as the liquid core core of the slab progresses. A pinch roll part is used to shrink the solidified slab shell by an amount equivalent to the volumetric shrinkage due to the solidification shrinkage by sequential forced cooling along the direction, reduce the cross section of the slab, and then pull out the slab. A series of slabs with excellent cross-sectional dimensional accuracy and internal quality, characterized by casting with the front surface temperature of the slab maintained at 850°C or lower and the average cooling rate of the slab surface at 30°C/m or higher. This is a casting method.

The mechanism of center segregation and center porosity generation is as follows.

■Center segregation: Solute-enriched molten steel discharged during solidification is deposited on the shaft center due to suction and flow due to solidification contraction in the final solidification region.

■Center porosity: Non-uniform solidification interface is generated in the final solidification region, complete solidification occurs above the crater tip, and when the unsolidified steel trapped below that area solidifies, pores are created by the volumetric shrinkage. occurs.

Therefore, center segregation and center porosity can be reduced by compensating for solidification shrinkage.

175mmφ cast at a casting speed of 2.7m/win
Figure 6 shows the segregation distribution at the center of the billet shaft, which has a slab diameter of .

This clearly shows the central segregation and the adjacent negative segregation zone. From this figure, the region where negative segregation begins to occur is in the range of 40-degrees from the axis. That is, it can be seen that there is bulk solute movement in a region with a width of 80 mm centered on the axis. Under the actual casting conditions in which this billet was cast, the area where this solute movement occurs is 8 points from the tip (crater end) of the residual molten steel pool in the billet when viewed from the slab position along the casting direction. This corresponds to the position between ~10m and the leading edge of the molten steel pool. Furthermore, this range will naturally change if billet cross-sectional dimensions, casting speed, cooling conditions, etc. change, but under actual continuous casting conditions, the range is from 2 to 15 m from the leading edge of the molten steel pool to the leading edge of the molten steel pool. It corresponds to the range. Center porosity is also generated within the region where solute movement occurs.

Based on the above results, we decided to suppress the formation of these by compensating for the solidification shrinkage that causes center segregation and center porosity by shrinkage due to forced cooling of the tomato solidified shell. However, the area where forced cooling is performed to compensate for solidification shrinkage is 2-1 points before the tip of the molten steel pool where concentration and molten steel flow occur.
This is from the 5 m position to the tip of the pool, and even if forced cooling is performed in other areas, it will not be effective in reducing center segregation and center porosity.

In order to reduce center segregation using this method, it is necessary to understand the amount of solidification shrinkage at the position where the spray nozzle is placed and the surface temperature that must be secured. Therefore, we first conduct a heat transfer analysis of the billet and The amount of solidification shrinkage was determined from the solidification profile using the following equation (1).

A cross section of the solidified shell is schematically shown in FIG.

∎Assume that the shell profile f is a parabolic body of revolution.

■The shell profile with equal solid fraction f L + f i +1 is congruent between different solid fractions.

Under the above assumptions, the billet changes from Xi to x! The volumetric shrinkage rate l until +1 is, where: η: Volumetric shrinkage rate during solidification β: Solidification shrinkage rate f, ? The solid phase ratio v of the first region is the volume W2 surrounded by planes Xi+Xt, , fl, fsi, mesh width D=mesh length. Next, in order to cool the solidified shell of the billet and compensate for the volumetric contraction during solidification, the following equation (2) must hold true.

ηn1−η”'+1−ηn ・・・・・・・・・・・・
・・・・・・・・・・・・・・・ Song－・・・－・
...(2) Here, ηm: Solidification shrinkage rate ηn+ of the solidified shell, solidification shrinkage rate ηn+ of the solidified shell, : Solidification shrinkage rate of the n+1 layer. The solidification shrinkage rate ηm is expressed by the following equation (3),
The average temperature drop ΔT of the solidified shell is obtained by substituting the difference in the volumetric shrinkage rate of each layer obtained in (1) 2 above.

L: Distance from billet axis to f, = 1 α: Coefficient of surface expansion ΔT: Average temperature drop of the solidified shell. The relationship between the average temperature of the solidified shell and the billet surface temperature determined by the above equation (3) is expressed by the following equation (4) (H,
S CAR5LAW, J, C, LAEGI! R:
”Conduction of he-at-in
5olids'', 0xford tlniversi
ty Press+5econ-d edition
1959 P, 99-100), and the surface temperature is T
Keep it at 1, X/j! Assuming that T is maintained at -1, the temperature profile is approximated as shown in equation (5) below.

Therefore, by controlling the billet surface temperature to T determined by equation (5), volume shrinkage during solidification, which causes center segregation, can be compensated for.

l π 1 n 1 exp (-k x”n”+t/j!”)+X
or '(X)' sin d X' --
-(4)! X2 π (Ta + To) ・sin X ・! e x p (-k x ”・t/l ”),
−−−−−−−−(5) Here, T: Average temperature of solidified shell Tm, : Surface temperature T0 required to maintain T: Initial surface temperature T, : Solidus temperature j! : Distance from fs-1 to the surface.

In order to prevent slab deformation due to force, pinch roll pressing improves the rigidity of the round billet surface layer by forcibly cooling the round billet located in front of the pinch rolls that undergo deformation.

A specific method of the present invention will be explained with reference to FIG.

The secondary cooling zone 1 is located inside the chamber 2, and the billet at the rear of the chamber is normally allowed to cool and is pulled out while being compressed by pinch rolls 3. At this time, the billet surface temperature on the input side of the pinch rolls is as high as about 1000°C, and since there is an unsolidified region, the deformation resistance is low, and the billet is easily deformed under pressure by the pinch rolls. Therefore, we investigated the optimal cooling conditions by varying the billet surface temperature and surface temperature drop rate just before the pinch roll (reduction of 17 kg 10 n'') where the round billet undergoes deformation. The results are shown in Figure 3.
As shown in Figure 4, the surface temperature immediately before the pinch roll is 850°C or less, and the surface temperature drop rate at that time is 30°C.
It was found that the round billet was within the permissible roundness if the temperature was °C/m. Furthermore, center segregation and center porosity can be reduced by controlling the surface temperature to be at or below the surface temperature determined from the calculations of equations (1) to (4) described above.

<Example> (Example 1) As Example 1 of the present invention, C: 0.25% by weight (hereinafter abbreviated as %), St: 0.26%, Mn: 1.30
%, P: 0.010%.

S: o, oos%, l! : Seamless material with a composition of 0.03% is made into a slab diameter of 175mm and 2.3m.
The spray nozzle shown in Fig. 1 was installed to maintain the surface temperature just before the pinch roll at 830°C.
℃, and the average cooling rate before the pinch roll is 32"C/m
The slab was cooled. The cooling zone length of the pinch roll is 3 m. Comparative Example 1 is a case in which cooling is not performed before the pinch roll and cooling to compensate for solidification shrinkage, and Comparative Example 2 is a cooling zone of 20°C/m before the pinch roll.
Cooling was performed at an average cooling rate of .

We measured the roundness of the slabs cast under the three cooling conditions mentioned above, and also investigated the segregation of the billet shaft center. A square piece of wood was cut out, specific gravity was measured, and center porosity was evaluated.

The results are shown in Table 1. From this table, it can be seen that the example of the present invention is effective in improving the roundness of the slab, decreasing the center segregation rate, and reducing the center porosity.The example of the present invention compensates for pinch rolls and solidification shrinkage. Because the cooling areas were close together, it appeared to be continuous forced cooling.

FIG. 2 shows the results of measuring the surface temperature of the slabs of Inventive Example 1 and Comparative Examples 1.2 using a radiation thermometer.

Table 1 (Example 2) C: 0.46%, St: 0.3%, Mn: 0.
93%, P: o, ois%, S: 0.007%
, I/! A material for a seamless pipe having a composition of :o, ooz% was cast at a rate of 3.0 m/min with a slab diameter of 175 mm.

This slab is solidified 3m in front of the vinyl roll! Example 2 of the present invention in which a 6 m area for compensating the amount of il was forcibly cooled, and a case in which only 3 m in front of the pinch roll was cooled in the same manner as Example 2 of the present invention (
A comparison was made in Comparative Example 3). The location of the cooling zone is schematically shown at No. 8.
As shown in the figure, the temperature at the input side of the pinch roll is 842°C1
The average cooling rate was 34°C/m. Table 2 shows the results of comparing the roundness, center segregation, and center porosity of the slabs thus cast. From now on, it has become clear that a slab with excellent center segregation, center porosity, and roundness can be obtained by combining forward cooling with pinch rolls and cooling that compensates for solidification shrinkage at the final stage of solidification.

It should be noted that no cracks were observed on the surface of the forcedly cooled billet, which was presumed to be caused by the cooling.

Table 2 <Effects of the invention> According to the continuous casting method according to the present invention, dimensional accuracy is good;
Moreover, it has become possible to produce slabs with excellent internal quality and less center segregation and center porosity.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram for explaining the method of the present invention, Figure 2 is a graph showing changes in billet surface temperature in the method of the present invention and a comparative example, and Figure 3 is a graph showing the billet surface temperature immediately before pinch rolls. Figure 4 is a graph showing the relationship between the roundness and the rate of surface temperature drop just before the pinch roll, and Figure 5 is a schematic diagram for explaining the conventional method. FIG. 6 is a characteristic diagram showing the segregation distribution in the axial center of a round billet, FIG. 7 is a schematic diagram of a cross section of a solidified slab shell, and FIG. 8 is a schematic diagram for explaining an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Secondary cooling zone, 2... Chamber for secondary cooling zone, 3... Pinch roll, 4... Spray nozzle, 5
...Hydraulic cylinder, 6. Slab, 7. Molten steel pool, 8. Mold. Patent applicant Jll i4 Steel Co., Ltd. Figure 1 Figure 2 Figure 3 Billet surface just before the pinch roll! (℃) Surface temperature drop rate in the cooling zone immediately before the pinch roll (℃/m
) Figure 5 Figure 6 Distance from round billet ring 0 (mm) Figure 7

Claims (1)

[Claims] In a continuous casting method for molten steel, the slab surface is moved in the casting direction from a position 2 to 15 m before the leading edge of the remaining molten metal pool in the casting direction to the pool's leading edge as the liquid core core of the slab solidifies. A pinch roll part for sequentially forced cooling along the solidified slab to shrink the solidified slab shell by an amount equivalent to the volumetric shrinkage due to the solidification shrinkage, reduce the cross section of the slab, cast it, and pull out the slab. Continuous casting of slabs with excellent cross-sectional dimensional accuracy and internal quality, characterized by maintaining the front surface temperature of the slab at 850°C or lower and the average cooling rate of the slab surface at 30°C/m or higher. Method.