JPH0530548B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention is aimed at preventing the segregation of impurity elements found in the center of the thickness of continuously cast slabs, such as sulfur, phosphorus, and manganese in the case of steel slabs, and obtaining a homogeneous metal. This is about a continuous casting method that can be used. (Prior Art) In recent years, requirements for material properties of offshore structures, storage tanks, steel pipes for oil and gas transportation, etc. have become more severe, and providing homogeneous steel materials has become an important issue. Originally, steel materials should be homogeneous in the thickness direction, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, which segregate and become partially concentrated during the casting process. Therefore, the steel becomes brittle. Particularly in recent years, continuous casting methods have become popular for purposes such as improving productivity and yield and reducing energy use, but noticeable component segregation is usually observed in the center of the thickness of slabs obtained by continuous casting. Ru. Such component segregation significantly impairs the homogeneity of the final product and causes serious defects such as the occurrence of cracks due to stress acting on the steel during the use of the product, so there is a strong desire to reduce it. Such component segregation occurs when residual molten steel at the final stage of solidification flows due to solidification contraction force, washes out concentrated molten steel near the solid-liquid interface, and progressively thickens the remaining molten steel. Therefore, in order to prevent component segregation, it is important to eliminate the cause of residual molten steel flowing. To do this, the bulging of the slab between the rolls should be minimized and the slab should be rolled down by an amount equivalent to the amount of solidification shrinkage. It is known to be effective. Attempts to improve segregation by rolling down slabs have been made for a long time; for example,
As described in Publication No. 16862, there is a known method in which the slab is rolled down at a constant rate during the continuous casting process until the temperature at the center of the slab reaches from the liquidus temperature to the solidus temperature. . However, these conventional methods have the following serious drawbacks, which make it difficult to sufficiently improve component segregation. That is, in the case of the conventional method, as the reduction amount increases, the segregation form of the final solidified portion changes from spot-like segregation to linear segregation. Normally, component segregation at the center of the thickness takes the form of spot-like high concentration areas with a diameter of about 1 mm dispersed in the casting direction and width direction. Hereinafter, this type of segregation will be referred to as spot segregation. On the other hand, the linear segregation caused by increasing the rolling reduction is a type of segregation in which the high concentration part is continuous in the casting direction and width direction, and its characteristics are, for example, in Tetsu to Hagane A201 (1983). ) is also detailed. This linear segregation has a much narrower width than spot segregation, and is usually 0.1 to 0.5 at the slab stage.
mm or less, and at first glance it appears that segregation has been significantly improved. However, when this slab is rolled,
For example, hydrogen-induced cracking (hereinafter referred to as
When investigating the area ratio (referred to as HIC), it was found that the crack area ratio increased compared to the case of spot segregation, and therefore, in order to suppress the flow of molten steel at the final stage of solidification, a reduction appropriate to the amount of solidification shrinkage was applied. It turns out that this actually has the opposite effect. FIG. 3 schematically shows this relationship. A similar tendency is also observed in cracks in thick plate welds (HAZ cracks). As a result of investigating the cause of this, the inventors found that in the case of linear segregation, when observed in a plane parallel to the wide surface of the slab (hereinafter referred to as the Z section), the segregated areas are connected in a network. It was found that this clearly remained in the product after rolling, and that continuous high-concentration areas served as preferential propagation paths for cracks, making the product extremely brittle. Therefore, in the conventional method, as the reduction amount increases, the material quality of the product tends to improve to a certain extent, but if the reduction amount is further increased, the material quality deteriorates again and rapidly, resulting in molten steel flow due to solidification shrinkage. There was a serious problem in that, in order to avoid the occurrence of linear segregation, it was necessary to keep the amount of reduction much smaller than the amount of solidification shrinkage, even at the expense of preventing the occurrence of linear segregation. For this example iron and steel A201
(1983), although reduction commensurate with the amount of solidification shrinkage is originally the most essential measure to improve segregation, there are many cases in which the application of this method has been abandoned and electromagnetic stirring has been adopted as another measure. . The electromagnetic stirring method is an effective measure to reduce the negative effects of flow, but it alone cannot completely prevent the negative effects of flow due to solidification shrinkage, and especially in the case of slab casting, there is not always sufficient segregation at this stage. I can't say it's a solution. (Problems to be Solved by the Invention) An object of the present invention is to solve the problems of the conventional method and provide a continuous casting method for obtaining a homogeneous steel material. (Means for Solving the Problems) The gist of the present invention is that in continuous casting of molten metal in which slabs are continuously drawn, unsolidified slabs are continuously reduced during casting, and the amount of reduction is is 0.5 mm/min to 2.0 mm/min from the time when the center of the slab reaches the liquidus temperature to the time when the solid phase ratio reaches the flow limit, and after that, the center of the slab reaches the solidus temperature. The continuous casting method is characterized by a casting rate of 0.3 mm/min or less in the region up to . The present invention will be explained in further detail below. The present inventors conducted systematic research on slab reduction in order to find a means to solve the problems of the conventional method described above. First, we investigated the amount of reduction necessary to compensate for the amount of solidification shrinkage. Usually, in a continuously cast slab, in addition to segregation in the center, V-shaped segregation (V-segregation) can be seen as shown in FIG. Since this V segregation is caused by solidification shrinkage, by observing the number of occurrences, it is possible to know whether the reduction amount is sufficient for the solidification shrinkage amount. The present inventors discovered the following two facts by observing such phenomena. One of them is related to the concept of rolling reduction.In order to compensate for the amount of solidification shrinkage, the weight is not the rolling reduction (unit: mm) per roll, but the range of several meters near the crater end (solidification tip). I learned that the average rolling speed (mm/min) is The rolling speed here refers to the amount by which a given point on the slab is rolled down per unit time during the process of passing between a plurality of rolls. When setting the roll spacing in actual operation, the amount of reduction per unit length in the casting direction (that is, the amount of narrowing of the roll spacing) is determined by the value obtained by dividing the above reduction speed by the drawing speed, that is, the reduction gradient (unit: mm/m). You can know. Another fact concerns the amount of reduction (hereinafter referred to as the appropriate amount of reduction) to compensate for solidification shrinkage in just the right amount. If the reduction amount is less than 0.5 mm/min, V segregation will occur in the casting direction, but if the reduction amount is too large, exceeding 2.0 mm/min, V segregation will occur in the opposite direction to the casting direction (i.e., in the direction of the meniscus). Direct V segregation (hereinafter referred to as reverse V segregation) occurs. Therefore, the appropriate rolling reduction amount at which neither V segregation nor reverse V segregation occurs is 0.5 to 2.0 mm/min.
Therefore, in the present invention, the reduction amount in the region from the time when the center of the slab reaches the liquidus temperature to the time when the flow limit solid phase ratio is reached is defined as 0.5 to 2.0 mm/min. The appropriate amount of reduction essentially changes depending on the casting speed when considered in terms of the amount of reduction per unit length (mm/m), but when expressed in terms of the reduction speed (mm/m). Since it is expressed as a substantially constant value regardless of the casting speed, it is specified in this unit in the present invention. However, the appropriate reduction amount varies depending on the thickness, width, and cooling conditions of the slab, so it is preferably 0.5 to 1.5 mm/min for normal slabs, and 1.0 to 2.0 mm/min for blooms or billets. do. Next, we investigated the segregation form. As mentioned above, from the point when the temperature at the center of the slab reaches the liquidus temperature,
If reduction is carried out in the entire region up to the solidus temperature, the number of V segregations will decrease as the reduction amount increases in a range smaller than the appropriate reduction amount, and the segregation will be improved accordingly. Even in the case of small reductions where the occurrence cannot be prevented, the segregation form of the final solidification zone has already changed from spot-like to linear, and the resistance has deteriorated.
Deteriorates the properties of the final product such as HIC properties. By the way, as a result of numerous casting tests, the present inventors have found that the segregation form of the final solidified zone is determined at the very end of solidification, and that after the time when the central part of the thickness of the slab reaches the flow limit solid fraction. The amount of reduction in the area is 0.3
It has been found that by setting the flow rate to less than mm/min, the occurrence of linear segregation can be prevented and the final solidified portion segregation form can always be made into a fine spot shape. Here, the flow limit solid fraction is the upper limit solid fraction at which molten steel can flow,
The solid phase ratio is between 0.6 and 0.8. In the upstream region (hereinafter referred to as stage I) where the solid fraction at the center of the thickness of the slab is smaller than the flow limit solid fraction, the molten steel continues in the casting direction, and the molten steel flows in the casting direction due to solidification contraction. Since this will cause the residual molten steel to thicken, it is necessary to prevent flow by reducing the amount by enough to compensate for solidification shrinkage. By setting the reduction amount in this region to an appropriate value, it is possible to prevent the occurrence of V segregation and reverse V segregation, and to obtain a good slab with extremely little segregation. The appropriate reduction amount is 0.5 as mentioned above.
~2.0mm/min. The important discovery that forms the basis of this invention is that stage I
This means that the magnitude of the reduction amount at step does not affect the segregation form of the final solidified part. The segregation form of the final solidification zone is the region from the time when the solid fraction at the center of the thickness of the slab becomes larger than the flow limit solid fraction until the temperature at the center reaches the solidus temperature (hereinafter referred to as stage).
Determined by the amount of reduction at. If the amount of reduction on the stage is too large, linear segregation will occur, but it was confirmed that this can be prevented by reducing the amount of reduction to 0.3 mm/min or less, and that a fine spot-like form of segregation can be ensured. A conceptual diagram of a stage and a stage according to the present invention is shown in FIG. The present inventors further investigated the influence of rolling conditions on center porosity and found that center porosity can be significantly reduced by performing appropriate rolling on the stage. If an excessive reduction is applied on the stage, the porosity will further decrease, but in this case, only a very small porosity will be reduced, and the appropriate reduction on the stage will be sufficient to improve the material quality. The present invention will be explained below based on examples. Example 1 Molten steel made in a converter and adjusted by adding Ca to the target composition of Table 1-1 was heated to 210 mm thick.
It was continuously cast with a slab cross-sectional size of 1580 mm width and then rolled into thick plates.

【table】

[Table] Samples were taken from slabs immediately after continuous casting, and the center segregation index, final solidified zone segregation form, and number of V segregations were investigated. In addition, samples were taken from the thick plates after rolling, HIC tests were conducted, and the HIC cracking incidence was investigated. The results are shown in Table 2. The center segregation index is an index that indicates the thickness of a high concentration area (segregation spot) that is 1.3 times or more of the ladle value of Mn in steel, and the higher the value, the higher the concentration of the component. This shows that segregation is large. During continuous casting, for steels A and B to which the present invention is applied, the reduction amount at the stage is 0.85 mm/min for a given casting speed.
The amount of reduction at the stage is 0.1mm/min to 0.3mm/min.
Before casting, the roll spacing was adjusted in advance so that The casting speed is set so that the point at which the solid fraction in the center reaches 0.7 is at the boundary of the roll segment.
The speed was set at 1.2m/min. Steels C, D, E, F, and G are comparison steels. Steels C and D are examples of inverted V segregation occurring due to excessive reduction in stage, and steel E is an example in which reverse V segregation occurred due to excessive reduction in stage. Steels F and G are examples where linear segregation occurred, and V segregation occurred because the reduction amount at the stage was too small. In the case of comparative steel, the HIC cracking incidence was 50 to 90%, and in particular, significant V segregation occurred because the reduction amount in stage I was 0.
Steel G, which had linear segregation in the final solidified part due to excessive reduction at the stage, had the highest crack occurrence rate. On the other hand, the steel to which the present invention was applied had an HIC cracking incidence of 9% or less for the same composition system, and the center segregation was slight, showing a significant difference from the comparison steel, demonstrating the superiority of the present invention. Ta.

[Table] Example 2 Using the composition shown in Table 1-2 as the target component, molten steel produced in a converter was continuously cast into a bloom with a cross-sectional size of 300 mm x 500 mm, and then rolled into a wire rod. As in Example 1, samples were taken from slabs immediately after continuous casting, and the center segregation index, final solidified part segregation form, and V
The number of segregated pieces was investigated. The results are summarized in Table 3. Steels A and B to which the present invention is applied reduce the amount of reduction at the stage.
1.7-1.8mm/min, reduction amount on stage 0.1mm/min
The test was conducted by varying the speed within a range of 0.3 mm/min.
The casting speed was 0.6 m/min. Steels C, D, E, H, and G are comparison steels. Steels C and D are examples where the amount of reduction at the stage was too large and inverted V segregation occurred, and steel E was an example where the amount of reduction at the stage was too large and caused a line. This is an example where V segregation occurs due to the reduction amount in the stage being too small.

[Table] As shown in Table 3, in the steel to which the present invention was applied, V segregation and inverted V segregation did not occur, the segregation form was in the form of fine spots, and the center segregation index was low, making it a remarkable difference from the comparison steel. was recognized, and the superiority of the present invention was also demonstrated in continuous bloom casting.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between each solidification stage, the amount and range to be reduced according to the present invention, and Fig. 2 is a schematic diagram of center segregation and V segregation seen in continuously cast slabs.
FIG. 3 is a diagram showing the relationship between the reduction amount and the hydrogen-induced crack area ratio according to the conventional method.

Claims (1)

[Claims] 1. In continuous casting of molten metal by continuously drawing slabs, the unsolidified slab is continuously reduced during casting, and the amount of reduction is controlled from the point at which the center of the slab reaches the liquidus temperature to the flow limit. 0.5 mm/min to 2.0 mm/min in the area up to the point where the solid phase rate is reached, and 0.3 mm/min in the area after that until the center of the slab reaches the solidus temperature.
A continuous casting method characterized by a casting rate of less than mm/min.