JP2561180C - - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an impurity element found in the center of the thickness of a continuously cast slab, that is, in the case of steel slab, sulfur, phosphorus, manganese or the like. The present invention relates to a continuous casting method capable of preventing segregation and obtaining a homogeneous metal. 2. Description of the Related Art In recent years, demands for material properties of marine structures, storage tanks, steel pipes for oil and gas transportation, high-strength wires, and the like have been increasing strictly. Has become. [0003] Originally, steel materials should be homogeneous in cross section, but steel is generally sulfur,
It contains impurity elements such as phosphorus and manganese, which are segregated during the casting process and partially concentrated to make the steel brittle. In recent years, in particular, continuous casting has been widely used for the purpose of improving productivity, improving yield, saving energy, and the like. However, in the vicinity of the center of the thickness of a slab obtained by continuous casting, usually, remarkable component segregation occurs. Is observed. [0005] The 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 process of the product or the wire drawing process of the wire rod. Coveted. [0006] It is considered that such component segregation occurs because the residual molten steel flows due to the solidification shrinkage force of the residual molten steel at the end of solidification, and the concentrated molten steel near the solid-liquid interface is washed out, and the residual molten steel is progressively concentrated. ing. Therefore, in order to prevent component segregation, it is important to remove the cause of the flow of the residual molten steel. As a cause of such a flow, besides a flow caused by solidification shrinkage, a flow caused by slab bulging between rolls or a misalignment of rolls, and the like, the most serious of which is solidification shrinkage. In order to prevent segregation, it is necessary to reduce the slab by an amount that guarantees this. [0008] Attempts to improve segregation by rolling down a slab have been made in the past, and in a continuous casting process, the temperature between the slab center temperature and the solidus temperature during the time from the liquidus temperature to the solidus temperature.
There is known a method of reducing the slab at a fixed rate equal to or more than an amount that guarantees the solidification shrinkage of the slab. [0009] However, the conventional continuous casting method has a problem that the segregation improving effect is hardly recognized depending on the conditions or the segregation is rather deteriorated in some cases, and it is difficult to sufficiently improve the component segregation. Was. 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 basically by the pressure. The coagulation time to be lowered and the range were found to be inappropriate. Based on these findings, the inventor of the present invention disclosed in Japanese Patent Application No. 62-275556 that the flow limit solidification was started from the time when the temperature at the center of the slab reached a temperature corresponding to a solid fraction of 0.1 to 0.3. The region until 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 center of the slab corresponds to the flow limit solid phase ratio. A continuous casting method has been proposed in which the region from the time when the temperature reaches the temperature to the solidus temperature to the time when the temperature reaches the solidus temperature is substantially not reduced. The flow limit solid phase ratio is the upper limit solid phase ratio at which molten steel can flow, and the solid phase ratio is a value of about 0.7. Further, the present inventor has promoted a number of experiments, and has previously obtained Japanese Patent Application No. 1-120.
As presented in No. 295, the concentrated molten steel violently accumulates in the center of the slab, and there is a solidification period that has a large effect on the center segregation of the solidification shrinkage flow, which prevents the flow of the concentrated molten steel during the accumulation period. It has been found that the segregation improvement effect is the largest, and the solidification time, which has the greatest effect on this segregation, differs depending on the solidification structure. As a result of studying a method for further improving segregation based on these findings, it has been found that, in a continuous casting method of molten metal in which the slab is drawn down by at least one pair of rolls at the end of solidification, the upper half of the slab cross section is obtained. When the upper surface equiaxed crystal ratio (hereinafter abbreviated as upper surface equiaxed crystal ratio) defined by the length ratio of the slab thickness direction occupied by the equiaxed crystal is less than 5%, the temperature of the slab central portion is changed to the solid phase. At least one pair of rolls at any position in the solidification time range from a position corresponding to the rate of 0.25, preferably 0.35 to a position corresponding to the flow limit solid phase rate, preferably upstream in the solidification time range. In order to compensate for the total solidification shrinkage within the solidification time range, when the equiaxed crystal ratio of the upper surface is 5% or more, the temperature at the center of the slab is 0.1%, preferably 0%. From the position corresponding to .15 to the flow limit solid fraction At least one pair of rolls at any position in the coagulation time range up to a certain position, preferably at the upstream side in the coagulation time range, and reducing the amount to compensate for the total coagulation shrinkage in the coagulation time range. A simple and efficient light reduction method was first proposed. [0014] However, when the casting speed is increased by this method to secure the production amount,
Deterioration of segregation is recognized with an increase in casting speed, and development of a light reduction method capable of stabilizing segregation at a low level even when the speed is increased is an issue. Further, when the casting speed is reduced or stopped at a joint of different steel grades or the like, slabs in which segregation worsens are recognized. Therefore, even when the casting speed is reduced or stopped, the length of the slabs in which segregation worsens is reduced as much as possible. The challenge is to develop a light reduction method that can be shortened. The inventor of the present invention has studied various optimum rolling reduction methods by changing the rolling pattern in order to solve such a problem. As a result, in the case of columnar solidification, even if the casting speed was increased, segregation was stabilized at a low level. A continuous casting method that enables a wide casting speed range in which segregation is favorable, and a reduction in casting length when casting speed is reduced or stopped at seams of different steel grades, etc. Proposed earlier. The present invention has been made in view of the above problems, and further studied on the case of equiaxed solidification.
The casting speed range where segregation is good with a smaller rolling force than in the case of columnar crystal solidification is wide, the segregation can be stabilized at a low level even if the continuous casting speed is increased, and the continuous casting speed with different steel grade joints etc. The present invention has provided a continuous casting method capable of shortening the length of a slab in which segregation is deteriorated even when the speed is reduced or stopped. Means for Solving the Problems The present invention has the following gist. (1) In a continuous casting method of molten metal in which the slab is drawn down by at least one pair of rolls at the end of solidification, the upper surface equiaxed crystal ratio is controlled to be 5% or more, and the center of the slab is controlled. At least one pair of rolls is installed on the upstream side within the solidification time range from the position where the temperature corresponds to the solid fraction of 0.15 to the position corresponding to the flow limit solid fraction, and the total reduction amount is 4 to 20 mm. Characterized in that the amount of reduction per unit length of the slab having a center solid phase ratio of 0.02 to 0.15 is 0.2 to 3.0 mm / m. It is a casting method. (2) In the continuous casting method described in the above (1), the temperature at the center of the slab is 0.
It is reduced so that the total reduction amount of 15 to 0.6 is 4 to 20 mm, and the reduction amount per unit length of the slab having a central solid phase ratio of 0.02 to 0.15 is 0.2 to 3.0. 0mm / m
The pressure is reduced so that Hereinafter, the present invention will be described in detail together with the operation. In order to solve the above-mentioned problems, the present inventor conducted a test in which the reduction solidification timing and the reduction amount were variously changed when the upper surface equiaxed crystal ratio was controlled to 5% or more by mold electromagnetic stirring. As a result of studying a rolling method with a good segregation improvement effect, as shown in FIG. As shown in FIG. 2, it has been found that the segregation is improved even when the casting speed is increased, and the present invention has been accomplished. Table 1 below is a table in which the conditions of the center solid phase ratio, the total reduction amount, and the reduction gradient are compared for the present invention example and the conventional example in this case. [Table 1] Region A: Central solid fraction 0.15 to 0.6 Region B: Central solid fraction 0.02 to 0.15 That is, the upper surface equiaxed crystal ratio of the slab is controlled and set to 5% or more. The region of 0.15 to 0.6 in terms of the center solid phase ratio is defined as the region A in which the rolling solidification time of the slab at the casting speed is 0.15 to 0.6. A region B in which the total reduction amount in the region A is reduced to 10 to 16 mm and the solidification time of the slab is 0.02 to 0.15 in the center solid phase ratio. By reducing the rolling gradient to 0.6 mm / m, it is possible to prevent the occurrence of a portion where the fluid resistance is locally reduced due to inconsistency of the coagulated structure or the like (function of region B). The flow path of the concentrated molten steel when the concentrated Segregation without degrading at a forming speed range, and finding that stable low underwent no present invention. As a result of further study on the rolling conditions of the regions A and B where the segregation is good, the rolling conditions under which the segregation is good are as shown in FIG.
At 3 mm / m, it was found that the total reduction amount in the region A was in the range of 4 to 20 mm.
In other words, by setting the rolling gradient in the region B to 0.2 to 3 mm / m, it is possible to prevent the occurrence of an inconsistent portion of the solidification structure that becomes the bud of V segregation, which is the flow path of the concentrated molten steel. Yes, by setting the total reduction amount of the region A to 4 to 20 mm, it is possible to prevent the flow during the solidification time when the concentration of the concentrated molten steel is intense. By realizing these two conditions, the segregation due to the light reduction is improved. The effect stabilizes. In order to prevent flow due to solidification shrinkage of the slab, it is necessary to reduce the solid-liquid interface to compensate for solidification shrinkage. Usually, the amount of reduction when the slab is reduced means the amount of surface reduction. If the ratio of the surface reduction of the slab to the solid-liquid interface is defined as the reduction efficiency, the reduction efficiency differs depending on the size of the slab, and the smaller the slab section, the lower the reduction efficiency. Even if the size of the slab is reduced, the total surface reduction required to actually prevent flow is not reduced, and is not significantly affected by the size of the slab. The rolling gradient is the amount of reduction per unit length of the slab, and is usually set by the narrowing amount per unit length of the roll interval. The amount of reduction can be determined by a method using a reduction gradient in which the roll interval is gradually narrowed or a method in which the reduction amount is controlled by a reduction force. The reduction gradient in the region B is the reduction in the low solid phase ratio region and the reduction in this region. Since the influence of the amount on the variation of the segregation fluctuation is large, it is preferable to set the above-described reduction gradient, and any method of controlling the reduction amount in the region A may be used. On the other hand, in the light reduction technology, it is essential to reduce the pressure so as not to cause internal cracks. The internal crack is generated when the amount of reduction of one roll is excessive. Therefore, the reduction gradient of the area B and the reduction amount of each roll in the area A are determined within a range in which the internal crack does not occur. By reducing the total rolling amount of A so as to satisfy the condition of FIG. 3, it becomes possible to obtain a cast piece having good segregation and no internal cracks. FIG. 4 shows the influence of the casting speed on segregation by setting the rolling timing, the rolling gradient, and the reduction amount in the example of the present invention shown in Table 2 at the highest casting speed required for operation. That is, the rolling time is determined by the relationship between the center solid phase ratio of the slab calculated by heat transfer calculation and the distance from the mold meniscus at the highest casting speed required for operation, and as shown in Table 2, the center of the slab is Temperature is 12 mm, the total reduction amount downstream of 0.15 in solid phase ratio.
And reducing the roll interval from the slab position where the temperature at the center of the slab is 0.02 in solid phase ratio to the slab position where the solid phase ratio in the center of the slab is 0.15. The effect of casting speed on segregation when set to 0.6 mm / m is shown in FIG. [Table 2] The influence of the casting speed on the segregation has a U-shape as shown in the figure, and the segregation worsens if the casting speed is too slow or too fast. The reason for this is that the solid phase ratio of the slab to be reduced changes due to fluctuations in the casting speed.If the casting speed decreases, the solid phase ratio at the center of the slab to be reduced shifts to the higher solid fraction. When the casting speed is increased, the solid phase ratio at the center of the slab to be reduced shifts to the low solid phase ratio side. If the rolling pattern of the present invention is set at the highest speed required for production capacity, segregation will be good even when the casting speed has to be reduced, and the casting speed range in which segregation will be good will be widened. That is, if the rolling pattern of the example of the present invention is set in accordance with the highest speed required in the operation, the casting is performed at a speed lower than usual for matching with the converter, secondary refining, etc. A slab with a low level of segregation is obtained. If the casting speed has to be reduced and stopped as in the case of different steel grade joints, it is possible to shorten the length of the slab where segregation worsens. The solidification time of the slab can be quantified by the center solid phase ratio, shell thickness, unsolidified thickness or unsolidified ratio. It was quantified by the central solid fraction, which is considered to be related to the increase in the flow resistance. The center solid fraction is calculated as a function of the temperature at the center of the slab as shown in the following equation 1,
It is the ratio of the solid phase present at the center. The temperature at the center of the slab is calculated in advance by heat transfer calculation based on operating conditions such as cooling conditions and casting speed, or is calculated based on conditions such as cooling and casting speed during casting. The center solid phase ratio is a function of solidification time when the casting speed, cooling conditions, slab size, and steel type are determined. Similarly, the shell thickness, unsolidified thickness, and unsolidified ratio, which are functions of solidification time, are easily determined. Can be converted to ## EQU1 ## Further, in the case of equiaxed solidification as in the present invention, the solidification time required for improving segregation is the reduction of the slab with a low center solid fraction and a short solidification time. Can improve segregation. According to the present invention, even when the casting speed is increased in order to secure the production amount, it is possible to prevent the segregation from deteriorating. Also, due to troubles in the converter, secondary refining furnace, etc. in the steelmaking plant, Even when the casting speed has to be reduced, it is possible to obtain a slab in which segregation is low and stable. In addition, even when the casting speed is reduced or stopped due to a different steel type joint operation or the like, it is possible to shorten the length of the slab where the segregation worsens. As described above, according to the present invention, there is provided a continuous casting method capable of obtaining a homogenous steel material having good productivity and operability as compared with the related art and having no segregation. The present invention will be described below with reference to examples. Example 1 FIG. 5 shows a schematic structure of a continuous casting machine on which a test was performed, and Table 3 below shows a typical example of a composition of molten steel cast. In FIG. 5, reference numerals 1 and 2 denote electromagnetic stirrers, 3 denotes a reduction zone,
Is a segment of a continuous casting machine, and 5 is a cast slab. [Table 3] When the casting speed is 1.0 m / min, the center solid phase ratio of the slab is 0.02 to 0.15.
And the central solid phase ratio was set to be 0.15 to 0.6 so that the total reduction amount was 12 mm, and the relationship between the segregation of the obtained slab and the casting speed was compared with the conventional example. A comparison is shown in FIG. According to the method of the present invention, segregation is better than the conventional example over a wide range of casting speed. Example 2 The continuous casting machine, the molten steel composition, the set pressure gradient, and the casting speed in the steady portion where the test was performed are the same as those in Example 1 described above. FIG. 6 shows the segregation of the unsteady slab in which the casting speed was reduced and stopped due to the joint of different steel types, comparing the present invention and the conventional method. According to the present invention, it has been proved that by reducing and stopping the casting speed, the condition of light reduction becomes inappropriate, and the length of the slab where segregation is deteriorated can be shortened. Example 3 Table 4 below shows the effect of increasing the speed and the effect of improving the segregation slab due to deceleration when the present invention is applied to various steel types. In the case of the example of the present invention, the upstream reduction range B (0.02 to 0.15 in center solid phase ratio) and the downstream reduction range A (0.15 to 0.7 in center solid phase ratio) are reduced. Only the rolling range B was reduced, and the equiaxed crystal ratio of the upper surface was 5% or more in all cases. [Table 4] In any of the steel types, the steel can be reduced at an early stage of solidification, and the rolling force can be reduced. According to the present invention, it is possible to increase the speed with good segregation.
It has been proved that it is possible to shorten the length of the cast slab which deteriorates due to segregation. As described above, according to the continuous casting method of the present invention, segregation of impurity elements such as sulfur, phosphorus, and manganese, which are observed at the center of the thickness of the continuous cast slab, is prevented. Even with a lower rolling force than columnar solidification, the casting speed range where segregation is good is wide, and even if the casting speed is increased, segregation can be stabilized at a low level, and different steel joints etc. Thus, even when the continuous casting speed is reduced or stopped, it is possible to shorten the length of the slab where segregation worsens, and it is possible to obtain a continuous cast slab with a good yield and uniformity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a drawing showing a concept of applying a draft gradient upstream of a draft zone in the present invention. FIG. 2 is a drawing comparing the segregation improvement effects of the present invention and a conventional method. FIG. 3 is a graph showing a relationship between a reduction gradient in a region B where a segregation effect is good and a total reduction amount in a region A. 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 for a test. FIG. 6 is a view showing an example (Example 2) of an effect of improving segregation of slabs in an unsteady part of the present invention. [Explanation of symbols] 1, 2 Electromagnetic stirrer 3 Reduction zone 4 Segment 5 Slab

Claims (1)

Claims: 1. In a continuous casting method of molten metal in which a slab is drawn down by at least one pair of rolls at the end of solidification, a slab occupied by equiaxed crystals in an upper half portion of a slab cross section. The upper limit flow rate at which the molten steel can flow from a position where the temperature of the center of the slab corresponds to the solid phase ratio of 0.15 by controlling the upper surface equiaxed crystal ratio defined by the length ratio in the thickness direction to be 5% or more. At least one pair of rolls is installed on the upstream side within the solidification time range up to the position corresponding to the limit solid phase ratio, the roll is reduced so that the total reduction amount is 4 to 20 mm, and the central solid phase ratio is 0.02. A continuous casting method characterized in that the reduction is performed so that the reduction amount per unit length of the slab is from 0.2 to 3.0 mm / m. 2. The method according to claim 1, wherein the temperature at the center of the slab is from 0.15 to 0.6 and the total reduction amount is 4%.
２０20 mm, and reduced so that the amount of reduction per unit length of the slab having a center solid phase ratio of 0.02 to 0.15 is 0.2 to 3.0 mm / m. 1
The continuous casting method described.