JPH0710428B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] 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, sulfur, phosphorus, manganese and the like. , A continuous casting method capable of obtaining a homogeneous metal.

[Conventional technology]

In recent years, offshore structures, storage tanks, steel pipes for oil and gas transportation,
The requirements for material properties such as high-strength wire rods are becoming more and more severe, and it is an important issue to provide homogeneous steel products. Originally, steel should be homogeneous in cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate and partially concentrate during the casting process. Becomes vulnerable. Particularly in recent years, continuous casting has been generally used for the purpose of improving productivity, yield, and energy saving, but a remarkable segregation of components is usually observed in the thickness center of the slab obtained by continuous casting. .

This component segregation significantly impairs the homogeneity of the final product and causes serious defects such as cracks due to stress acting on the steel during the product use process and wire drawing process, etc. . Such component segregation occurs when the residual molten steel flows at the final stage of solidification due to 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, in addition to the flow caused by solidification shrinkage, the flow caused by slab bulging between rolls and roll alignment irregularity, but the most serious of these is solidification shrinkage, In order to prevent segregation, it is necessary to roll down the slab by an amount that compensates for this.

Attempts have been made to reduce segregation by rolling down the slab, and in the continuous casting process, solidification shrinkage of the slab occurs during the period from the liquidus temperature to the solidus temperature of the slab center temperature. A method is known in which the rolling is carried out at a constant rate above the amount to be compensated.

[Problems to be Solved by the Invention]

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

As a result of various investigations by the inventors on the cause of the problem of the conventional method, the segregation improving effect is not recognized in the case of the conventional method, or the segregation rather deteriorates.
Basically, we found that it was due to improper coagulation timing and its range to be reduced.

In Japanese Patent Laid-Open No. 62-275556, the solid fraction at the center of the slab is 0.
The area from the time of reaching the temperature corresponding to 1 to 0.3 to the time of reaching the temperature corresponding to the flow limit solid fraction is per unit time.
It is continuously reduced at a rate of 0.5 mm / min or more and less than 2.5 mm / min, and the region from the time when the temperature of the core of the slab reaches the temperature corresponding to the flow limit solid fraction to the solidus temperature is substantially A continuous casting method in which no reduction is applied is disclosed.

However, the solidified state of the slab is 0.1 at the thickness center solid fraction.
Alternatively, a very long reduction zone is required to set the reduction roll in the temperature range from 0.3 to the fluid limit solid fraction, and a very large reduction force is required to reduce the vicinity of the fluid limit solid fraction. Becomes Thus, the conventional method is not always a simple light reduction method. An object of the present invention is to provide a continuous casting method for solving the problems of the conventional method and obtaining a homogeneous steel material.

[Means for Solving the Problems]

The gist of the present invention is that in the continuous casting method of molten metal in which the slab is drawn out by rolling with at least one pair of rolls in the final stage of solidification, when the upper surface equiaxed crystal ratio is less than 5%, the temperature of the slab center is solid. Amount for compensating the total amount of solidification shrinkage within the solidification time range by installing at least one pair of rolls in the solidification time range from the position corresponding to the phase ratio of 0.25, preferably 0.35 to the position corresponding to the flow limit solid phase ratio. When the upper surface equiaxed crystal ratio is 5% or more, the solidification timing from the position where the temperature at the center of the slab corresponds to the solid fraction 0.1, preferably 0.15 to the position corresponding to the fluidity limit solid fraction The continuous casting method is characterized in that at least one pair of rolls is installed in the range, and the amount that compensates for the total solidification shrinkage amount within the solidification timing range is reduced. When the equiaxed crystal ratio of the upper surface is less than 5%, the temperature at the center of the slab is 0.
25, preferably at a position corresponding to 0.35 to 0.5,
When the upper surface equiaxed crystal ratio is 5% or more, it is preferable to reduce the temperature of the slab center portion in the range corresponding to the solid phase ratio of 0.1, preferably 0.15 to 0.4.

[Work]

As a result of further research on the segregation generation mechanism by an actual machine and a laboratory experiment, the present inventors have found that the influence of molten steel flow on the segregation varies depending on the solidification timing. That is, segregation causes bridging to be formed in the center of the slab to increase the liquid flow resistance, and then the concentrated molten steel between dendrite trees located closer to the skin than the center is attracted to the center by solidification shrinkage suction force and accumulates. V segregation is a passage for the flow of concentrated molten steel at this time, and at the solidification time when concentrated molten steel is most intense, the temperature at which the solidified state of the center of the slab is 0.25 to 0.4 in terms of the central solid fraction The present invention has been accomplished by finding that it is most effective to carry out reduction so as to prevent the flow in this coagulation time range in the region where Further, as a result of research on the accumulation time of the concentrated molten steel, the solidification time at which the concentrated molten steel is most intensely accumulated varies depending on the solidification structure as shown in Fig. 2, and the proportion of equiaxed crystals in the solidification structure on the upper surface of the slab is When the upper surface equiaxed crystal ratio is less than 5%, the solidified state of the central portion of the slab is a region where the temperature is equivalent to 0.25 to 0.4 in the central solid phase ratio, and the upper surface equiaxed crystal ratio is 5% or more. In the case of, the concentration of concentrated molten steel is most intense when the solidified state of the center of the slab is 0.1 to 0.1%.
The equiaxed crystal ratio of the upper surface is 5% in the temperature range equivalent to 0.3
It was found that the low solid fraction region is more likely to be rolled down than below. As described above, the coagulation timing at which the flow should be prevented by the light pressure differs depending on the coagulation structure. The amount of reduction for compensating the total amount of solidification shrinkage between the solid phase ratio A and the critical flow solid phase ratio at the center of the slab was confirmed under the condition that the V segregation observed in the slab was zero. Further, the temperature of the central portion of the slab is calculated in advance by heat transfer calculation based on the casting conditions, or is calculated based on the conditions such as cooling of the slab and the casting speed during casting, and the central solid fraction is Calculate from temperature.

[Example 1] Fig. 1 shows an outline of a continuous casting machine used for the test, and Table 1 shows a typical example of the composition of the molten steel cast. A casting speed change test was carried out for cases where the solidification structure had an upper surface equiaxed crystal ratio of zero and 5% or more, and the accumulation timing of the concentrated molten steel was examined. The size of the cast slab is 310 x 500 mm. Figure 2 shows the timing of concentrated molten steel accumulation. The solidification timing of concentrated molten steel varies depending on the solidification structure.When the top equiaxed crystal ratio is zero, the concentrated molten steel is the most violent in the region where the temperature at the center of the slab corresponds to the solid fraction of 0.25 to 0.4. When they were accumulated and the upper surface equiaxed crystal ratio was 5% or more, the concentrated molten steel accumulated most violently in the region where the temperature at the center of the slab was 0.1 to 0.3 in terms of the solid fraction. Based on the above results, Table 2 shows the results of rolling down so as to prevent the flow during the solidification period when the concentrated molten steel accumulates strongly. It can be seen that the segregation can be improved satisfactorily with a short rolling zone and a small rolling force.

[Example 2] The conditions of the present invention and the conventional method in which V segregation does not occur are compared.
Shown in. When the upper surface equiaxed crystal ratio is 5% or more, a homogeneous cast piece having neither V segregation nor center segregation can be obtained within a narrower rolling range than the conventional method and with a small rolling down force (average roll reaction force of the rolling down roll). When the upper surface equiaxed crystal ratio is less than 5%, the rolling force is about the same as that of the conventional method, but V segregation and center segregation can be made favorable in a narrow rolling range.

[Effects of the Invention] According to the present invention, it is possible to obtain a homogeneous steel material without segregation without making the rolling zone too long, and to secure an equiaxed crystal ratio of 5% or more by low temperature casting or electromagnetic stirring. For example, not only can the length of the rolling zone be shortened, but even if the rolling force is reduced, it is possible to obtain a homogeneous steel material without segregation, which is an excellent light rolling method that is simpler than the conventional method.

[Brief description of drawings]

FIG. 1 is a diagram showing an outline of a continuous casting machine, and FIG. 2 is a diagram showing a relationship between a solidification time and a solidification structure in which concentrated molten steel is intensely accumulated.
A indicates a solidification time at which the concentration of concentrated molten steel is the highest when the equiaxed crystal ratio of the upper surface is 5% or more and B is zero.

Claims (2)

[Claims] 1. In a 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, when the upper surface equiaxed crystal ratio is less than 5%, the temperature at the center of the slab is solid phase. At least one pair of rolls is installed in the coagulation time range from the position corresponding to the rate 0.25 to the position corresponding to the flow limit solid phase ratio, and the amount for compensating the total coagulation shrinkage amount within the coagulation time range is reduced, and If the equiaxed crystal ratio of the upper surface is 5% or more, install at least one pair of rolls in the solidification time range from the position where the temperature of the slab center corresponds to the solid fraction 0.1 to the position where the solid limit of fluidity corresponds Then, the continuous casting method is characterized in that the amount for compensating the total amount of solidification shrinkage within the solidification time range is reduced. 2. When the upper surface equiaxed crystal ratio is less than 5%, the temperature of the slab center portion is reduced in the range corresponding to the solid fraction 0.25 to 0.5, and when the upper surface equiaxed crystal ratio is 5% or more, It is characterized by compensating for the total amount of solidification shrinkage within the solidification time range corresponding to each of the upper surface equiaxed crystal ratios by rolling down the range in which the temperature of the slab center part corresponds to the solid fraction 0.1 to 0.4. The continuous casting method according to claim 1.