JP2561180B2 - Continuous casting method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents segregation of impurity elements found in the center of thickness of continuously cast slabs, that is, in the case of steel slabs, segregation of sulfur, phosphorus, manganese and the like to obtain a homogeneous metal. The present invention relates to a continuous casting method.

[0002]

2. Description of the Related Art In recent years, demands for material characteristics of offshore structures, storage tanks, steel pipes for transporting oil and gas, high-strength wire rods, etc. have become more and more strict, and it is an important issue to provide homogeneous steel rods. .

Originally, steel should be homogeneous in cross section, but steel generally contains impurity elements such as sulfur, phosphorus and manganese, which are segregated in the casting process and partially concentrated. Steel becomes fragile due to

Particularly, in recent years, the continuous casting method has been generally popular for the purpose of improving productivity, yield, and energy saving. However, a significant segregation of components is usually present in the vicinity of the center of the thickness of a cast product obtained by continuous casting. Is observed.

[0005] This segregation of components significantly impairs the homogeneity of the final product, and causes serious defects such as cracking due to stress acting on the steel during the use of the product or the wire drawing process of the wire. Coveted.

It is considered that such component segregation occurs because the solidified liquid shrinkage force of the residual molten steel at the end of solidification flows to wash out the concentrated molten steel near the solid-liquid interface, and the residual molten steel progressively thickens. ing. Therefore, in order to prevent the segregation of the components, it is important to eliminate the cause of the flow of the residual molten steel.

The causes of such a flow include a flow caused by solidification shrinkage, a flow caused by slab bulging between rolls, and an irregular roll alignment. Among these, the most serious cause is solidification shrinkage. In order to prevent segregation, it is necessary to reduce the slab by an amount that guarantees this.

[0008] Attempts have been made to reduce segregation by rolling down the slab, and in the continuous casting process, the slab is squeezed between the liquidus temperature and the solidus temperature of the slab center. There is known a method of rolling at a constant rate above the amount that guarantees the solidification shrinkage of.

However, the conventional continuous casting method has a problem that the segregation improving effect is hardly recognized depending on the conditions, and in some cases, the segregation rather deteriorates.
It was difficult to sufficiently improve the component segregation.

As a result of various investigations on the causes of the problems of the conventional method, the present inventors have found that the effect of improving the segregation of the conventional method is not recognized or that the segregation is rather deteriorated. The coagulation time to be lowered and the range were found to be inappropriate.

Based on these findings, the present inventor previously found that in Japanese Patent Application No. 62-275556, the flow limit solidification was started from the time when the central portion of the slab reached a temperature corresponding to a solid fraction of 0.1 to 0.3. The region up to the temperature corresponding to the phase ratio is continuously reduced at a rate of 0.5 mm / min or more and less than 2.5 mm / min per unit time, and the slab center part corresponds to the flow limit solid fraction region from the time the temperature until the solidus temperature was proposed continuous casting method not exert substantial pressure. The flow limit solid fraction is the flow rate of molten steel.
It is the upper limit of possible solid fraction, and the solid fraction is about 0.7
It is.

[0012] Further, the present inventor has promoted many experiments, and as shown in Japanese Patent Application No. 1-120295, the concentrated molten steel is violently accumulated in the central portion of the slab, and the center of solidification shrinkage flow. There is a solidification time that has a large effect on segregation, and preventing the flow during the accumulation time of this concentrated molten steel has the greatest effect of improving the segregation, and the solidification time that greatly affects this segregation varies depending on the solidification structure. I found out.

As a result of research on a method for further improving segregation based on these findings, in the continuous casting method of molten metal in which the slab is drawn while being pressed by at least one pair of rolls at the final stage of solidification, the upper half part of the slab cross section And the equiaxed crystal
Top equiaxed crystal ratio defined by the length ratio in the thickness direction of the slab
If the upper surface equiaxed crystal ratio is less than 5%, the temperature at the center of the slab is from the position corresponding to the solid fraction 0.25, preferably 0.35 to the position corresponding to the fluidity limit solid fraction. Up to at least one pair of rolls are installed at any position in the coagulation timing range, preferably upstream in the coagulation timing range, to reduce the amount of total coagulation contraction within the coagulation timing range, and When the equiaxed crystal ratio is 5% or more, the solidification timing range from the position where the temperature of the slab center portion corresponds to the solid fraction 0.1, preferably 0.15 to the position corresponding to the flow limit solid fraction is arbitrary. Position, preferably at least one pair of rolls is installed on the upstream side within the coagulation timing range, and the amount that compensates for the total coagulation shrinkage amount within the coagulation timing range is reduced, simple and efficient. I proposed a simple light reduction method first.

[0014]

However, when an attempt is made to increase the casting speed to secure the production amount by this method, the segregation deteriorates as the casting speed increases, and even if the casting speed is increased, the segregation can be prevented. The development of a light reduction method that can be stabilized at a low level is an issue.

Further, when the casting speed is slowed down or stopped due to a joint between different steels, etc., a slab that segregation is deteriorated is recognized.
The problem is the development of a light reduction method that can shorten the length of the slab where segregation deteriorates even if the casting speed slows or stops.

In order to solve such a problem, the present inventor has variously changed the rolling pattern and studied the optimum light rolling method. As a result, in the case of columnar solidification, segregation is stabilized at a low level even if the casting speed is increased. It is possible to achieve a wide range of casting speeds where segregation is good, and segregation deteriorates when the casting speed is slowed or stopped due to dissimilar steel type seams etc. I proposed earlier.

In view of the above problems, the present invention has further advanced the research on the case of equiaxed crystal solidification, and as a result, the casting speed range in which segregation is good with a smaller rolling force and the continuous casting speed is increased as compared with the case of columnar crystal solidification. It is possible to stabilize the segregation at a low level even if the speed is increased, and it is possible to shorten the length of the slab where segregation deteriorates even when the continuous casting speed slows down or stops due to dissimilar steel type seams, etc. We have come to provide a casting method.

[0018]

The present invention has the following gist.

(1) In the continuous casting method of molten metal in which a slab is drawn while being pressed by at least one pair of rolls at the final stage of solidification, the upper surface equiaxed crystal ratio is controlled to be 5% or more,
At least one pair of rolls is installed on the upstream side in the solidification time range from the position where the temperature of the central portion of the slab corresponds to the solid fraction of 0.15 to the position where the solid fraction of the flow limit corresponds to 4 to 20 mm, and the central solid fraction is 0.02 to 0.15, and the reduction amount per unit length of the cast piece is 0.2 to 3.0 mm / m. Is a continuous casting method.

(2) In the continuous casting method described in (1) above, the temperature at the center of the slab is reduced so that the total reduction amount of the solid fraction 0.15 to 0.6 is 4 to 20 mm, and The central solid fraction is 0.02 to 0.15, and the amount of reduction per unit length of the cast piece is 0.2 to 3.0 mm / m.

[0021] (3) above (1) or (2) in a continuous casting process according to control so that the upper surface equiaxed Akiraritsu becomes 5% or more, when the casting speed is the fastest in the continuous casting operation
Relationship between center solid fraction of slab and distance from mold meniscus
The continuous casting is performed by setting the arrangement of the reduction rolls, the reduction gradient, and the reduction amount based on the above.

[0022]

The operation of the present invention will be described in detail below.

In order to solve the above-mentioned problems, the present inventor conducted a test in which the time of rolling and solidification and the amount of rolling down were variously changed when the upper surface equiaxed crystal ratio of the slab was controlled to 5% or more by electromagnetic stirring of the mold. As a result of studying a reduction method with a good segregation improving effect, as shown in FIG. 1, in addition to the reduction area A, in a continuous casting machine, a slab in the area B located upstream of A and having a central solid fraction smaller than A is produced. The present invention has been accomplished by finding that segregation becomes good even if the casting speed is increased as shown in FIG.

Table 1 below is a table comparing the conditions of the central solid fraction, the total reduction amount, and the reduction gradient for the inventive example and the conventional example in this case.

[0025]

[Table 1] Region A: central solid fraction 0.15 to 0.6 Region B: central solid fraction 0.02 to 0.15

[0026] That is to control the top equiaxed Akiraritsu of the slab 5% or more, the region of 0.15 to 0.6 the slab rolling clotting time of the setting the casting speed at the center solid phase ratio and the area A
The flow of solidification time when concentrated molten steel is most accumulated
By reducing the total reduction amount to 10 to 16 mm
To prevent the total amount of reduction in the area A from becoming 10 to 16 mm, and the solidification time of the slab is 0.0 at the central solid fraction.
By performing the reduction so that the reduction gradient in the region B of 2 to 0.15 becomes 0.6 mm / m, it is possible to reduce the reason for the mismatch of the coagulated tissue.
Therefore, it is possible to prevent the occurrence of local areas where the liquid flow resistance decreases.
The concentrated molten steel gathers in the center.
Wide casting by reducing the passage of concentrated molten steel flow when stacking
The present invention has been accomplished by finding that segregation does not deteriorate in the velocity range and stabilizes at a low level.

Region A and region B where segregation is further improved
As a result of the research on the rolling reduction conditions of No. 2, the segregation is favorable as shown in FIG. 3, in which the rolling gradient of the region B is 0.2 to 3 mm / m and the total rolling amount of the region A is 4 to 20 m.
It was found to be in the range of m. That is, the reduction of area B
By making the gradient 0.2 to 3 mm / m, concentrated molten steel
Of the coagulation structure that becomes the bud of V segregation, which is the flow path of water
It is possible to prevent
By setting the reduction amount to 4 to 20 mm,
It is possible to prevent the flow during the solidification period when accumulation is intense.
By realizing the two conditions of
The improvement effect is stable. The flow due to the solidification shrinkage of the slab
To prevent this, reduction of the solid-liquid interface that compensates for solidification shrinkage is necessary.
It is important. When the normal slab is rolled, the amount of reduction is the surface reduction.
Means quantity. Transfer of surface reduction of cast slab to solid-liquid interface
If the ratio of the reduction ratio is defined as the rolling reduction efficiency, the rolling reduction efficiency is
There is a difference in size, and the smaller the slab cross section is, the smaller the rolling reduction efficiency is.
Therefore, even if the size of the slab becomes smaller, it actually flows.
The total amount of surface reduction required to prevent
Less affected by the size of the piece. In addition, the draft is the slab
The amount of reduction per unit length of the
Set by the amount of narrowing down per unit. The reduction amount is
Method using a reduction gradient that gradually narrows
Although there is a method to control the amount of reduction by force,
The downward gradient is the reduction in the low solid fraction region and the amount of reduction in this region.
Has a large effect on the variation of segregation fluctuation,
It is preferable to set the above-mentioned reduction gradient, and
Any method of controlling the reduction amount may be used.

On the other hand, in the light reduction technique, it is indispensable to reduce the internal crack so as not to generate it. Internal cracks occur when the amount of reduction of one roll is excessive, and therefore area B
The reduction gradient of No. 1 and the amount of reduction of each roll in the area A are determined within a range in which internal cracking does not occur, and the reduction gradient of the area B and the total reduction amount of the area A are reduced by satisfying the conditions of FIG. It is possible to obtain a slab with good segregation and no internal crack.

The rolling time and pressure of the examples of the present invention shown in Table 2
Decrease in slope and amount of reduction at the highest casting speed required for operation
Fig. 4 shows the effect of casting speed on segregation.
You. That is, the rolling time is the fastest casting speed required for operation.
Core solid fraction of the slab calculated by heat transfer
Determined by the relationship of the distance from Lumeniscus, Table 2
As described above, the temperature of the central portion of the slab is reduced so that the total reduction amount downstream of 0.15 in the solid fraction is 12 mm, and the temperature of the central portion of the slab is 0.02 in the solid fraction. from slab position of the slab center solid phase ratio of casting speed on the narrowing of the roll gap of the slab position of 0.15 in the polarization <br/> analysis in the case of setting so as to be 0.6 mm / m effect is shown in Figure 4.

[0030]

[Table 2]

The influence of the casting speed on the segregation is U-shaped as shown in the figure, and the segregation is deteriorated if the casting speed is too slow or too fast. The reason for this is the variation in casting speed.
The casting rate is slow because the solid fraction of the cast slab changes.
When the temperature drops, the solid fraction in the center of the cast slab is high.
If the casting speed becomes faster due to shifting to the index side, roll down
The solid fraction in the center of the cast slab shifts to the lower solid fraction side
by. The reduction pattern of the present invention is the most necessary for production capacity.
If set on the high speed side , the segregation becomes good even when the casting speed has to be reduced, and the range of the casting speed at which the segregation becomes good becomes wider.

That is, if the reduction pattern of the example of the present invention is set in accordance with the highest speed required for operation, even when casting is performed at a speed slower than usual for matching with a converter, secondary refining, etc. A slab with stable segregation at a low level can be obtained.

When it is necessary to slow down and stop the casting speed like a joint of different steel types, it becomes possible to shorten the length of the slab where segregation deteriorates. The solidification timing of the slab can be quantified by the central solid fraction, shell thickness, unsolidified thickness, or unsolidified rate, but here, the solidification rate of the central part, which is considered to have the most influence on the occurrence of segregation. The quantification was performed by the central solid fraction, which is considered to be related to the increase in liquid flow resistance.

The central solid fraction is a ratio of the solid phase existing in the center calculated as a function of the temperature of the center of the slab, as shown in the following mathematical formula 1. The temperature of the central portion of the slab is calculated in advance by heat transfer calculation based on cooling conditions, operating conditions such as casting speed, or during cooling during casting, based on conditions such as cooling and casting speed.

This central solid fraction depends on the casting speed, cooling conditions,
Once the slab size and steel type are determined, it is a function of solidification time, and can be easily converted to shell thickness, unsolidified thickness, and unsolidified rate, which are also functions of solidification time.

[0036]

## EQU1 ## T1: liquidus temperature of molten steel (° C.) Ts: solidus temperature of molten steel (° C.) T: slab Center temperature (℃)

Further, in the case of the equiaxed crystal solidification as in the present invention, since the solidification time required for improving the segregation is the reduction of the slab having a low central solid fraction and the short solidification time, the rolling force is smaller than that in the case of the columnar crystal solidification. Can improve the segregation.

According to the present invention, even if the casting speed is increased to secure the production amount, it is possible to prevent the segregation from deteriorating, and due to the trouble of the converter, the secondary refining furnace, etc. in the steelmaking plant, Even when the casting speed has to be reduced, it is possible to obtain a stable slab with low segregation.

Further, even when the casting speed is slowed or stopped due to a joint work between different steel types, it is possible to shorten the length of the cast piece in which segregation is deteriorated.

As described above, according to the present invention, there is provided a continuous casting method which is excellent in productivity and operability and is capable of obtaining a homogeneous steel material without segregation.

[0041]

EXAMPLES The present invention will be described below with reference to examples.

Example 1 FIG. 5 shows a schematic structure of a continuous casting machine which has been tested, and Table 3 below shows a typical example of the composition of the molten steel cast. In FIG. 5, 1 and 2 are electromagnetic stirrers, 3 is a reduction band, 4 is a segment of a continuous casting machine, and 5 is a cast piece.

[0043]

[Table 3]

At the casting speed of 1.0 m / min, the central solid fraction of the slab was 0.02 to 0.15 , and the rolling gradient was 0.
FIG. 4 shows the relationship between the segregation and the casting speed of the obtained slab, which is set to 6 mm / m and the central solid fraction is 0.15 to 0.6 so that the total reduction amount is 12 mm. . According to the method of the present invention example, segregation becomes better than the conventional example over a wide range of casting speed.

Example 2 The continuous casting machine used for the test, the composition of the molten steel, the set draft and the casting speed in the steady part were the same as in Example 1 above. FIG. 6 shows segregation of unsteady cast slabs whose casting speed has been slowed down and stopped due to a joint between different steels, comparing the present invention with the conventional method.

It has been proved by the present invention that the length of the cast piece in which the light reduction condition becomes inappropriate and the segregation deteriorates can be shortened by reducing or stopping the casting speed.

Example 3 The following Table 4 shows the speed-up effect when the present invention is applied to various steel grades and the effect of improving segregation-deteriorated cast slab accompanying deceleration. In the case of the example of the present invention, the upstream rolling reduction range B (0.02 in the central solid fraction)
.About.0.15) and downstream reduction range A (0.15 in the central solid fraction)
.About.0.7) , and in the conventional example, only the downstream rolling range B was rolled down, and in each case, the upper surface equiaxed crystal ratio was 5% or more.

[0048]

[Table 4]

In any of the steel types, the solidification timing is reduced at an early stage, and the reduction force can be small. According to the present invention, it is possible to realize the acceleration with the good segregation, and the generation is caused by the reduction or the stop of the casting speed. It has been proved that it is possible to shorten the length of the slab that has deteriorated segregation.

[0050]

As described above, according to the continuous casting method of the present invention, sulfur found in the thickness center portion of the continuously cast slab,
Prevents segregation of impurity elements such as phosphorus and manganese, and has a wide casting speed range where segregation is good even with a smaller rolling force than in the case of columnar solidification, and segregation is lowered even if casting speed is increased. It is possible to stabilize, and even when the continuous casting speed slows down or stops due to seams of different steel types, etc.
It is possible to shorten the length of the cast piece in which segregation deteriorates, and it is possible to obtain a continuous cast piece that is homogeneous and has a good yield.

[Brief description of drawings]

FIG. 1 is a drawing showing the concept of applying a reduction gradient upstream of a reduction zone according to the present invention.

FIG. 2 is a drawing comparing segregation improving effects of the present invention and a conventional method.

FIG. 3 is a drawing showing the relationship between the reduction gradient in the region B and the total reduction amount in the region A, in which the segregation effect is favorable.

FIG. 4 is a drawing showing the effect of casting speed on segregation when the present invention is applied (Example 1).

FIG. 5 is a drawing showing a schematic structure of a continuous casting machine used in a test.

FIG. 6 is a drawing showing an example (separate example 2) of the effect of improving segregation of unsteady cast pieces according to the present invention.

[Explanation of symbols]

 1, 2 electromagnetic stirrer 3 reduction zone 4 segment 5 slab

Claims (3)

(57) [Claims] 1. In a continuous casting method for molten metal, in which a slab is drawn while being rolled down by at least one pair of rolls at the final stage of solidification, in the slab thickness direction occupied by equiaxed crystals in the upper half of the slab cross section.
Is controlled so that the upper surface equiaxed crystal ratio defined by the length ratio becomes 5% or more, and the upper limit of the flow limit solidity at which molten steel can flow from the position where the temperature of the slab center corresponds to the solid fraction 0.15 At least one pair of rolls is installed on the upstream side in the solidification time range up to the position corresponding to the phase ratio, and the rolls are rolled down so that the total rolling amount is 4 to 20 mm, and the central solid fraction is 0.02 to 0. .15, the amount of reduction per unit length of cast slab is 0.2 to 3.0 mm / m.
The continuous casting method is characterized by rolling down so that 2. The temperature of the slab center is reduced so that the total reduction amount of solid fraction 0.15 to 0.6 is 4 to 20 mm,
2. The continuous casting method according to claim 1, wherein the reduction amount is 0.2 to 3.0 mm / m per unit length of the cast slab having a central solid fraction of 0.02 to 0.15. 3. A controlled to top isometric Akiraritsu becomes 5% or more, cast when the casting speed is the fastest in the continuous casting operation
Relationship between the central solid fraction of a piece and the distance from the mold meniscus
The continuous casting method according to claim 1 or 2 , wherein the arrangement of the reduction rolls, the reduction gradient, and the reduction amount are set based on the above.