JPH0550201A - Light rolling reduction method in continuous casting - Google Patents

Abstract

PURPOSE:To provide light rolling reduction method in the continuous casting to reduce the center segregation being generated at a center part of thickness in a continuously cast slab. CONSTITUTION:In the case casting speed reduces, by controlling a roll interval, the light rolling reduction range is shifted to the upstream side, and in the case the casting speed increases, the light rolling reduction range is shifted to the downstream side, and the rolling reduction speed in the light rolling reduction range is kept to the same of 0.5-1.2mm/min as before reducing or increasing the casting speed. The passing time from meniscus at the position of starting the rolling reduction is made to the time of solid phase ratio is 0.1-0.3, and at the position of ending the rolling reduction is made to the time becoming 0.5-0.7 solid phase ratio.

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, such as sulfur, phosphorus and manganese in the case of steel slabs, to obtain a homogeneous metal. The present invention relates to a light reduction method in continuous casting which can be performed.

[0002]

2. Description of the Related Art In recent years, the demands on the material properties of offshore structures, storage tanks, steel pipes for transporting oil and gas, high-strength wire rods, etc. have become more severe, and it is an important issue to provide homogeneous steel rods. There is. Originally, steel should be homogeneous in the cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate in the casting process and partially concentrate, so steel Becomes vulnerable. In particular, in recent years, continuous casting has been generally used for the purpose of improving productivity and yield, and energy saving, but generally significant component segregation is observed in the thickness center of the slab obtained by continuous casting. It

The above-mentioned component segregation significantly impairs the homogeneity of the final product and causes serious defects such as cracks due to stress acting on the steel in the use process of the product and the wire drawing process of the wire rod. Coveted. Such component segregation occurs when the residual molten steel flows at the end of solidification due to the solidification shrinkage force and the like, the concentrated molten steel near the solid-liquid interface is washed out, and the residual molten steel progressively concentrates. Therefore, in order to prevent the segregation of the components, it is important to remove the cause of the flow of the residual molten steel.

Such molten steel flow causes include flow due to solidification shrinkage, flow due to slab bulging between rolls and roll alignment irregularity, and the most serious cause is solidification shrinkage. Therefore, in order to prevent segregation, it is necessary to roll down the slab by an amount that compensates for this.

[0005] Attempts have been made to improve segregation by rolling down cast slabs, and in the continuous casting process,
A method is known in which, while the temperature of the slab center is from the liquidus temperature to the solidus temperature, the slab is rolled down at a constant rate equal to or more than the amount that compensates for solidification shrinkage.

[0006]

However, the conventional continuous casting method has a problem that the segregation-improving effect is hardly recognized depending on the conditions or the segregation rather deteriorates depending on the conditions, so that the segregation of the components is sufficient. It was difficult to improve. Furthermore, when performing the light reduction, if the roll interval setting in the light reduction range is fixed, the position where solidification is higher than usual becomes the upstream side when the casting speed decreases, and it deviates from the appropriate reduction range and becomes the center of the light reduction. The effect of suppressing segregation cannot be obtained. Further, when the casting speed increases, the solidification position moves to the downstream side and deviates from the proper reduction range.

[0007]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a light reduction method when the casting speed fluctuates and the progress of solidification in the continuous casting machine changes. Moved to the upstream side, when the casting speed is increased, the light reduction range is moved to the downstream side, and the roll interval is controlled so that the reduction speed in the light reduction range is kept the same as before or after the reduction or increase of the casting speed, The elapsed time from the meniscus at the reduction start position is the time when the solid fraction is 0.1 to 0.3, and the elapsed time from the meniscus at the reduction end position is the solid fraction is 0.5 to 0.7. It is a light reduction method in continuous casting in which the reduction rate within the reduction range is controlled to 0.5 to 1.2 mm / min.

[0008]

Operation and Embodiments The operation of the present invention will be described below with reference to the illustrated embodiments.

The present invention is a method for controlling the roll interval in the light reduction region when the casting speed fluctuates and the progress of solidification in the continuous casting machine changes.

FIG. 1 is a diagram showing an embodiment of the present invention.
The vertical axis represents the roll interval (mm), and the horizontal axis represents the distance (m) from the meniscus. The roll interval of the continuous casting machine is set to 0 to 0.5 mm per 1 m in a region where solidification does not proceed in the central portion (solid fraction <0.1) to compensate for solidification shrinkage. In this embodiment, 320 mm to 314 mm
I have narrowed down to. Further, a reduction of 0.5 to 2 mm per 1 m is performed from the region of solid fraction 0.1 to 0.3 to the region of solid fraction 0.5 to 0.7, and molten steel flow accompanying solidification shrinkage is performed. Perform a light reduction to suppress and prevent center segregation.
The pressure is reduced from 4 mm to 300 mm.

FIG. 2 is a diagram showing an example of roll interval control when the casting speed changes. In the figure, the solid line indicates the roll interval before the casting speed is reduced, the alternate long and short dash line indicates the roll interval during the casting speed fluctuation, and the dotted line indicates the roll interval after the casting speed is reduced. If the casting speed decreases, move the light reduction range to the upstream side,
By controlling the roll interval so as to keep the rolling speed within the light rolling range the same as before casting speed reduction, proper rolling can be performed during and after casting speed fluctuation, and molten steel flow can be prevented,
It is possible to prevent center segregation. Similarly, when increasing the speed, it is possible to prevent center segregation by moving the light reduction range to the downstream side and keeping the reduction speed constant.

FIG. 3 is a view showing a method of controlling the roll interval when the casting speed varies. The vertical axis represents the total amount of reduction (mm) from the start of the light reduction, the horizontal axis represents the elapsed time from the meniscus, and the slope of the straight line (dR / dt) is the reduction rate.

The elapsed time from the meniscus at each roll position can be calculated by the formula 1.

[0014]

[Equation 1]

T: elapsed time from meniscus L _i : casting length up to the roll position at which casting speed is Vi

The elapsed time from the meniscus at the reduction start position is the time at which the solid phase ratio becomes 0.1 to 0.3, and the elapsed time from the meniscus at the reduction end position is at the solid phase ratio of 0.
The time is 5 to 0.7.

The solid phase ratio is a function of the elapsed time from the meniscus, and is represented by, for example, equation 2.

[0018]

[Equation 2]

F _s : solid fraction a _n : constant t: elapsed time from meniscus

Thus, when the casting speed fluctuates,
The elapsed time from the meniscus at the reduction start position and the reduction end position is within the appropriate range, and the reduction speed (mm
/ Min) is controlled within a suitable range obtained from FIG.

FIG. 4 is a diagram showing an appropriate rolling speed.
The vertical axis represents the segregated particle size (m
m), the horizontal axis represents the average rolling speed (mm / min) when the slab passes through the solid fraction of 0.15 to 0.7. When the average rolling speed is between 0.5 and 1.2, molten steel flow is prevented and segregation is at a good level, but when the rolling speed is less than 0.5, molten steel flow is not sufficiently suppressed, and center segregation occurs. Getting worse. On the other hand, if the rolling speed is higher than 1.2, molten steel flow occurs in the opposite direction to the normal one, and the center segregation deteriorates.

[0022]

EFFECTS OF THE INVENTION According to the present invention, even when the casting speed fluctuates, an appropriate light pressure reduction condition is established, molten steel flow is suppressed, and center segregation can be prevented.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram showing an example of roll interval control when a casting speed changes.

FIG. 3 is a diagram showing a way of thinking of roll interval control when the casting speed is changed.

FIG. 4 is a diagram showing an appropriate rolling speed.

Claims (1)

[Claims] 1. A light reduction method when the casting speed fluctuates and the solidification progress in the continuous casting machine changes. When the casting speed decreases, the light reduction range is moved to the upstream side to increase the casting speed. In this case, the light rolling range is moved to the downstream side, the roll interval is controlled so that the rolling speed in the light rolling range is kept the same as before the casting speed was decreased or increased, and the elapsed time from the meniscus at the rolling start position. The solid fraction is 0.1
0.3 time, the elapsed time from the meniscus at the reduction end position is the time when the solid fraction becomes 0.5 to 0.7, and the reduction speed within the reduction range is 0.5 to 1.2 mm / mi
A method of light reduction in continuous casting controlled to n.