JPS63165053A - Continuous casting method with few center segregation - Google Patents

Abstract

PURPOSE:To reduce the center segregation by detecting the most end position of remained molten steel pool, operating solidification shinkage quantity and the compensating quantity by the surface temp. and next, adjusting water cooling zone length, cooling water quantity and cooling pattern. CONSTITUTION:A moving type rig 2 providing the most end detecting instrument 3 for remained molten metal pool is arranged at downstream side of pinch rolls 4, 9. The detecting 3 is composed of ultrasonic transmitter 11 and receiver 12 facing to both sides of the cast slab 1 and the most end position for remained molten metal is detected, while shifting the longitudinal direction of cast slab 1 through automatic driving carriage 13. The detected signal is inputted to a water quantity control device for cast slab water cooling zone, to obtain solidified profile. Next, by a heat conduction analysis program, the solidification shrinkage quantity and the surface temp. compensating it are calculated to adjust the spray nozzle 5. By optimumly controlling the cooling zone length, cooling water quantity and pattern, the center segregation of cast slab 1 is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for reducing center segregation of continuous slabs.

[Conventional technology]

In order to reduce the center segregation of continuously cast slabs and blooms, it is necessary to prevent the occurrence of bulging by setting an appropriate roll interval, arranging the rolls, or performing appropriate secondary cooling.

On the other hand, reduction of the degree of superheating of molten steel, addition of coolant to the mold, application of electromagnetic stirring to the molten steel in the mold, application of ultrasonic waves to the slab in the strand, and furthermore, application of electromagnetic stirring to the molten steel in the strand. Techniques to reduce center segregation by methods such as application and roll light reduction are widely used. All of these methods aim to make the cast structure equiaxed, to achieve fine dispersion of solutes, and to reduce center segregation.

Although reducing the degree of superheating of molten steel is effective in achieving equiaxed weight, it requires controlling the casting temperature within a narrow range, which has the disadvantage of impeding operational stability.

Addition of a coolant is also effective in achieving equiaxed loading, but the melting of the coolant may leave a residue or cause mold slag to be involved, which has the drawback of easily causing UT defects.

Applying ultrasonic waves to slabs is effective in principle, but as a practical technique, there is a problem of fatigue of the application roll, which makes it difficult to put it into practical use.

Considering these points, electromagnetic stirring within the mold or within the strand has few practical drawbacks, is effective in equiaxed loading, and is generally popular. However, although there is a tendency for center segregation to be reduced due to the progress of equiaxed loading due to electromagnetic stirring, the rupture rate of hard steel wire rods made from continuously cast blooms that are subjected to stirring also decreases when electromagnetic stirring is not applied. No significant improvement effect was observed compared to wire rods obtained from raw materials.

For example, in a bloom slab for hard steel wire with a slab size of 400 mm and a width of 560 mm, hollow-shaped cavities are formed intermittently along the casting direction at the axial center, and what is more characteristic is that the cavity is intermittently formed near the axial center. Accompanied by V segregation, this has a form different from the V segregation that occurs in the axial center of the steel ingot, and rather has the form of inverted V segregation in the steel ingot. The V segregation occurs in a region having a width of about 1QOmm centered on the axis, and the center segregation and the adjacent negative segregation zone are clearly recognized. In this example,
The region where negative segregation begins to occur is within a range of 40 mm from the axis. That is, it can be seen that there is bulk solute movement in an 80 mm wide region centered on the axis.

It can be clarified from metallurgical observations and simple numerical calculations that this movement of solute-enriched molten steel is caused by the suction force associated with the solidification contraction of the remaining molten steel in the molten steel pool. .

Therefore, in order to prevent center segregation, it is necessary to prevent the movement of solute-enriched molten steel generated by the suction force caused by the solidification shrinkage of the remaining molten steel in the molten steel pool near the axis of the slab (thickness center in the case of slabs). That's true.

As a measure for this, Japanese Patent Application Laid-open No. 52-104420 and Japanese Patent Laid-open No. 52-104420
No. 4-107831 discloses a center segregation reduction method using a roll light reduction method, but with this method, it is extremely difficult to continuously implement a reduction commensurate with solidification shrinkage. In other words, if the reduction is small, it is insufficient to prevent the concentrated molten steel from moving downwards, and if the reduction is too large, the concentrated remaining molten steel will move upwards, and the moved upper part will be even worse. This will lead to an increase in solute concentration.

Moreover, a large-scale rolling down device is required.

[Problem that the invention seeks to solve]

The present invention provides a method for eliminating center segregation in continuously cast slabs or bloom slabs and producing sound slabs.

It is thought that the variation in the center segregation rate of the slab material is largely due to the variation in the tip position in the longitudinal direction of the remaining molten metal pool of the slab due to changes in casting conditions.

Therefore, in order to prevent variations in the center segregation rate of the slab material, it is necessary to reliably detect the leading edge position of the remaining molten metal pool during casting.

As a technical means to solve the above problem, the liquid core core of the slab is The amount of shrinkage accompanying the progress of solidification can be compensated for by shrinking and deforming the solidified shell by forced water cooling of the slab surface (#Intercondylar 6O-201383).

However, the leading edge position of the remaining molten metal pool during casting varies depending on the casting conditions and the steel type, which causes the following problems.

(a) If the leading edge of the remaining molten metal pool is behind the end point of water cooling, the expansion of the solidified shell due to recuperation of the slab after water cooling will generate tensile stress at the solidification interface and cause cracks in the center. The suction of concentrated molten steel promotes center segregation.

(b) When the leading edge position of the residual molten metal pool is close to the water cooling zone starting point, the shrinkage effect due to water cooling is small and the amount of shrinkage accompanying the progress of solidification cannot be compensated for.

[Means for solving problems]

In order to solve the above problems, the leading edge position of the residual molten metal pool is detected using a residual molten metal pool position detection device that uses electromagnetic ultrasound, for example, and the position is detected according to the depth of the residual molten metal pool. Calculate the amount of solidification shrinkage of the slab and the surface temperature that should compensate for the solidification shrinkage, adjust the water cooling zone length, cooling water amount, and cooling pattern so that the slab surface reaches this temperature, and solidify the unsolidified volumetric shrinkage. Compensate by shrinking the shell.

[Effect]

FIG. 1 shows a block diagram of the method of the present invention.

The slab is cast at a certain speed and pulled out through pinch rolls 4.9, and the spray nozzle 5 is installed to shrink the solidified slab shell 6 and compensate for the solidification shrinkage of the residual molten steel pool 8. be. The thus cooled slab is conveyed by a roller table 7. At that time, when casting is performed at a certain casting speed or higher, the leading edge position of the remaining molten metal pool 8 is located on the downstream side of the pinch rolls 4 and 9. This has been confirmed by heat transfer calculations and riveting.

In the method of the present invention, for example, as shown in FIG. A detection device 3 for detecting the leading edge position of the residual molten metal pool 8 is installed downstream of the pinch roll 4.9, and this detection device 3 automatically scans to detect the leading edge position of the residual molten metal pool 8. However, the solid phase ratio of residual steel that can be detected with this device is 4
We have confirmed that it is 0%. The detected signal is sent to the water flow control device in the slab water cooling zone, where the solidification profile is determined using a heat transfer analysis program. Considering this solidification profile as a rotating paraboloid as shown in Figure 3,
The amount of solidification shrinkage and the surface temperature at which the solidification shrinkage should be compensated are calculated using the following formula, and the water volume density distribution in the casting direction that achieves that temperature is calculated again and the slab is water-cooled.

■ Assume that the shell profile fst is a parabolic body of revolution.

■ Shell profiles with equal solid fractions f4 and fL+1 are congruent between different solid fractions.

Under the above assumptions, the volumetric contraction rate η of the slab from X[ to X[+1] is We D (xt+t −Xi ) −・−(1
) Here, η: Volumetric contraction rate during solidification β: Solidification contraction rate fsL: Solid phase ratio in the first region ■i: Surface xt + X t+1 + fL +
Volume surrounded by f st W : Slab width D : Slab length 0 Order In order to cool the solidified shell of the slab and compensate for the volume contraction during solidification, the following equation (2) is established. There must be.

1m8ηnet -ηn (2) where, 1m: Contraction rate of solidified shell ηn: Solidification contraction rate of n layer ηn+1:. This is the solidification shrinkage rate at +1 layer. The solidification shrinkage rate ηm is expressed by the following equation (3), and the average temperature drop ΔT of the solidified shell is obtained by substituting the difference in the volume shrinkage rate of each layer determined by the above equation (1).

Here, L: Distance from the axis of the slab to fs = 1 α: Coefficient of surface expansion ΔT: Average temperature drop of the solidified shell, which is the average temperature of the solidified shell determined by equation (3) above, and the slab surface The temperature relationship is expressed by the following equation (4) (H, S
CARSLAW, J.C.JAEGER: ”Con
duction of heat in 5okids''
, Oxford University Press
.

5econd edition 195θP,! 98～
100), and the surface temperature is maintained at Ta, x/u=
1, the temperature profile is approximated as shown in equation (5) below.

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

〔Example〕

Molten hard steel wire with a carbon content of 0.8% by weight was cast into a cross-sectional size of 400 x 560 mm2 by a fully curved continuous casting machine at a casting speed of 0.55 m/min and a water ratio of 0 in the secondary cooling zone.
．． This slab, cast at 55u/kg, immediately
Rolled into a billet of x150mm2, then 4.5m
A wire rod with a product size of mφ was rolled.

During this casting, the surface temperature at the outlet of the pinch roll band at a position 23 m from the meniscus (molten metal surface in the mold) was 950°C.
The temperature was ~1000°C. Furthermore, according to heat transfer calculations and experiments using the riveting method, the final solidification position is considered to be approximately 34 m from the meniscus under the present casting conditions. however,
According to measurements with the residual molten steel pool detection device used in this method, it has been found that the crater end fluctuates by about ±2 m from this position.

During casting, 1) When forced cooling is performed at a constant amount of cooling water on each side from the pinch roll outlet side to the final solidification position (comparative example) 2) The leading edge position of the remaining molten metal pool is detected by the leading edge position detection device of the remaining molten metal pool However, the center segregation of the slab and the cuppy rupture rate during wire drawing were compared in cases where the cooling conditions were changed according to the fluctuations (Example) and 3) where water cooling was not performed (Conventional Example). The results are shown in Table 1, where the center segregation rate of the slab is 600 mm.
It was excavated with a mmφ drill, and the C analysis value of the chips was evaluated by the value assigned to the molten steel C analysis value (Co). In addition, the cuppy breakage rate during wire drawing represents the number of breaks (times/m) that occurs when drawing a unit length of wire drawing material. However, in Table 1, the case where water cooling is not performed is assumed to be 1.0. expressed as a relative value.

From Table 1, it can be seen that in the case of the method of the present invention, the variation in the center segregation rate of slabs becomes smaller and the cuppy fracture rate decreases.

Table 1 [Effects of the Invention] According to the present invention, the center segregation rate of slabs and its variation are reduced, and troubles caused by center segregation in products are eliminated.

[Brief explanation of the drawing]

FIG. 1 is a partial perspective view of a continuously cast slab illustrating an embodiment of the method of the present invention, FIG. 2 is a side view of a detection device of the embodiment, and FIG. 3 is a schematic cross-sectional view of a solidified shell. DESCRIPTION OF SYMBOLS 1... Slab, 2... Mobile rig, 3... Detection device, 4.9... Vinci roll, 5... Spray nozzle. 6... Solidified shell, 7... Roller table, 8... Residual molten metal pool, IT... Ultrasonic transmitter, 12... Receiver, 13... Self-driving trolley

Claims (1)

[Claims] 1. The surface of the slab from a position 2 to 15 m before the leading edge of the remaining molten metal pool in the pouring direction of the continuously cast slab to the pool's leading edge position is examined along the casting direction to prevent solidification of the liquid core core of the slab. As the slab progresses, the solidified slab is sequentially forcedly cooled with a cooling amount equal to or more than the volumetric shrinkage due to the solidification shrinkage, and when the slab is cast with a reduced cross-section, the residual molten steel in the pouring direction is Detects the leading edge of the pool, calculates the amount of solidification shrinkage of the slab and the surface temperature to compensate for the solidification shrinkage based on the detection, and adjusts the length of the water cooling zone, the amount of cooling water, and the cooling pattern to reach the temperature. A continuous casting method with less center segregation.