JPH03114643A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent segregation of impurities at center part of a continuous cast slab and to produce the continuous cast slab having uniform material by continuously executing rolling reduction to the continuous cast slab drawn from a mold with the different rolling reduction rate according to temp. at the center unsolidified part at the time of continuously casting molten steel. CONSTITUTION:The molten steel is poured into the mold for continuous casting and continuously drawn from the mold as the continuous cast slab under condition of existence of the unsolidified part in the inner part. In the unsolidified part at center part of the continuous cast slab, the impurities of S, P, Mn, etc., are accumulated and the segregation is developed and the quality of continuous cast slab is deteriorated. In order to prevent this, the rolling reduction is continuously executed to the continuous cast slab. In this case, the rolling reduction is executed to the cast slab during the range from the time point when the temp. at center part of the continuous cast slab becomes liquids temp. for this steel to the time point when the above temp. becomes fluidity limit solidus ratio at 0.5-2.0mm/min the rolling reduction rate and during the range till the temp. in the center part of continuous cast slab becomes solidus temp. after that at 0.3mm/min the rolling reduction rate to solidify the unsolidified part at the center part.

Description

[Detailed Description of the Invention] (Field of Industrial Application) The present invention prevents 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. It relates to a continuous casting method that can produce homogeneous metal.

(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 saving energy, but noticeable component segregation is usually observed in the center of the thickness of slabs obtained by continuous casting. . 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 or the like, washes out the concentrated molten steel near the solid-liquid interface, and the residual molten steel progressively becomes concentrated. Therefore, in order to prevent component segregation, it is important to eliminate the cause of the flow of residual molten steel. 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 that this is effective.

Attempts to improve segregation by compressing slabs have been made for a long time, and for example, as described in Japanese Patent Publication No. 16862/1986, the temperature at the center of a slab is lowered from the liquidus temperature in a continuous casting process. A method is known in which a slab is rolled down at a constant rate until it reaches its 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 form will be referred to as spot segregation. On the other hand, 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 described, 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 at the slab stage.
It is ~0.5 mm or less, and it appears that segregation has been significantly improved. However, when this slab is rolled,
For example, hydrogen-induced cracking (hereinafter referred to as HIC), which has sour resistance properties,
When examining the area ratio (referred to as ``solidification shrinkage''), it was found that the crack area ratio actually increased compared to the case of spot segregation.Therefore, in order to suppress the flow of molten steel at the final stage of solidification, it is effective to apply a reduction commensurate with the amount of solidification shrinkage. It turned out that it had the opposite effect. FIG. 3 schematically shows this relationship. A similar trend is
It has also been observed in cracks in thick plate welds (HAZ cracks).

As a result of investigating the cause of this, the present 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 cross section), the segregated areas are connected in a network pattern. It was found that it clearly remained in the product after rolling, and that the continuous high concentration areas served as preferential propagation routes 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 will deteriorate again and rapidly. Therefore, in order to avoid the occurrence of linear segregation, even at the expense of preventing the occurrence of molten steel flow due to solidification shrinkage, the reduction amount is much smaller than the amount of solidification shrinkage. There was a serious problem in that the amount of reduction had to be limited. For this reason, for example, iron and steel A201
(1,983), although reduction commensurate with the amount of solidification shrinkage is an essential measure to improve segregation, we abandoned its application and adopted electromagnetic stirring as another measure. There are many examples. 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 0.5 mm/min to 2.0 mm/min in the region from the time when the center of the slab reaches the liquidus temperature to the time when the solid fraction reaches the flow limit, and after that, the center of the slab becomes solid. 0 in the region up to the phase line temperature.
The continuous casting method is characterized by a casting rate of 3 mm/min or less.

The present invention will be explained in more 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. In addition to segregation in the center, continuous cast slabs usually exhibit V-shaped segregation (V-segregation) as shown in FIG. 2. Since this (1) 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 these concerns the idea of the amount of reduction.What is important in compensating for the amount of solidification shrinkage is not the amount of reduction per roll (unit: mm).
It was learned that the rolling down speed (mm/min) is the average rolling speed in a range of several meters near the crater end (solidification tip). 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 (i.e., 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). I can do it. 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 7 minutes, segregation will occur in the casting direction, but on the other hand, if the reduction amount is less than 2.0 mm, segregation will occur in the casting direction.
If it becomes too large, exceeding mm 7 minutes, ■ segregation (hereinafter referred to as reverse ■ segregation) occurs in the direction opposite to the casting direction (that is, in the direction of the meniscus). Therefore, the appropriate rolling reduction amount that does not cause either (1) segregation or reverse (3) segregation is 0.5 to 2.0 mm and 7 minutes.

Therefore, in the present invention, the reduction amount in the region from the time when the central part 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 and 7 minutes.

The appropriate amount of reduction 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/min). Since it is expressed as a substantially constant value regardless of the casting speed, this 11th place is specified in the present invention. However, the appropriate reduction amount varies depending on the thickness and width of the slab and the cooling conditions, so it is preferably 0.5 to 1.5 mm/min for normal slabs, and 1.0 to 2 mm/min for blooms or billets. .0
anus/min.

Next, we examined the segregation morphology. As mentioned above, if the reduction is carried out in the entire range from the time when the temperature at the center of the slab reaches the liquidus temperature until it reaches the solidus temperature, V segregation will occur as the reduction amount increases in a range smaller than the appropriate reduction amount. The number of pieces decreases, and segregation is improved accordingly, but even in the case of small reductions where the occurrence of V segregation cannot be prevented, the segregation form in the final solidified part already changes from spot-like to linear, resulting in poor HIC resistance. deteriorate the properties of the final product. 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. It has been found that by setting the reduction amount in the area to 0.3 mm/minute, the occurrence of linear segregation can be prevented and the final solidified portion segregation form can always be made into a fine splotch shape. Here, the flow limit solid fraction is the upper limit solid fraction at which molten steel can flow, and is a value of 0.6 to 0.8. In the upstream region (hereinafter referred to as stage ■) where the solid fraction at the center of the thickness of the slab is smaller than the flow limit solid fraction, molten steel continues in the casting direction, and solidification shrinkage causes the molten steel to flow in the casting direction.
Since this causes the residual molten steel to thicken, it is necessary to reduce the amount of pressure to compensate for solidification shrinkage to prevent flow. By setting the reduction amount in this region to an appropriate value, it is possible to prevent the occurrence of V segregation and reverse ■ segregation, and obtain a good slab with extremely little segregation. The appropriate reduction amount is 0.5 to 2.0 mm and 7 minutes as described above.

An important discovery that forms the basis of the present invention is that the magnitude of the reduction in stage I does not affect the segregation form of the final solidified portion. The segregation form of the final solidification zone occurs in 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. If the amount of reduction in stage ■ is too large, linear segregation will occur, but this can be prevented by reducing the amount of reduction to 0.3 mm and 7 minutes or less.
It was confirmed that it was possible to secure a fine spot-like segregation morphology.

A conceptual diagram of Stage I and Stage H 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 was significantly reduced by carrying out appropriate rolling at Stage I. If an excessive reduction is applied in stage (2), the porosity 0- is further reduced, but in this case, only a very small porosity is reduced, and the proper reduction in stage (2) is sufficient to improve the material quality.

Next, the present invention will be explained based on examples.

Example 1 Using the composition shown in Table 1-1 as the target composition, molten steel was melted in a converter and the composition was adjusted by adding Ca, to a thickness of 210 mm x 1580 m.
The slab cross-section size was continuously cast with a width of m, and then rolled into a thick plate.

Table 1-1 Composition of test steel (%) Table 1-2 Composition of test steel (%) A sample was taken from the slab immediately after continuous casting, and the central segregation index, final solidified part segregation form, and number of segregation pieces were determined. investigated. In addition, samples were taken from the thick plates after rolling and subjected to an HIC test to investigate the incidence of HIC cracking, and the results are shown in Table 2. The center segregation index is an index of the thickness of a high concentration area (segregation spot) that is 1.3 times or more of the Mn concentration value in steel, based on the dollar value of Mn in steel. The larger the value, the greater the component segregation. In continuous casting, for steels A and B to which the present invention is applied, the reduction amount in stage I is 0.85 mm/min and the reduction amount in stage II is 0.1 mm/min to 0.3 m+n for a predetermined casting speed. /
Before casting, the roll spacing was adjusted in advance so that The casting speed was set to 1.2 m/min so that the point at which the solid fraction in the center reached 0.7 was at the boundary of the roll segments. Steels C, D, E, F, and G■ are comparison steels. Steels C and D are examples where reverse ■ segregation occurred due to excessive reduction at stage ■, and ME is an example where the amount of reduction at stage ■ was too large. In mF and G, the amount of reduction in stage (2) was too small, resulting in linear segregation (2). In the case of the comparative steel, the HIC cracking incidence was 50 to 90%, and in particular, significant segregation occurred because the reduction amount at stage (■) was 0, and the reduction amount at stage (■) was excessive. Therefore, steel G, which has linear segregation in the final solidified part, has 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.

3 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. In the same manner as in Example 1, samples were taken from slabs immediately after continuous casting, and the center segregation index, the final solidified part segregation form, and (1) the number of segregated pieces were investigated. The results are summarized in Table 3.

For 42 steels to which the present invention is applied, the reduction amount at stage I is 1.7~
1.8 mm/min, the amount of reduction at stage ■ is 0.1 mm/min.
The test was conducted by changing the time within the range of 0.3 mm to 7 minutes. The casting speed was 0.6 m/min.

Steel Ha, two, ho, he, toba is comparative steel, steel Ha.

2 is an example where the amount of reduction at stage I was too large and reverse ■ segregation occurred; steel E is an example where the amount of reduction at stage 2 was too large and linear segregation occurred; to the steel, the amount of reduction at stage I This is an example where ■ segregation occurred due to an insufficient amount of .

■ As shown in Table 3, in the steel to which the present invention is applied, V segregation and reverse ■ segregation do not occur, the segregation form exhibits a fine splotch I ~ shape, and the central segregation index is low, which is significantly different from the comparative steel. Differences were observed and the superiority of the present invention was also demonstrated in continuous bloom casting.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between each solidification stage, reduction amount and range according to the present invention, Figure 2 is a schematic diagram of center segregation and ■ segregation observed in continuously cast slabs, and Figure 3 is a conventional method. FIG. 3 is a diagram showing the relationship between the amount of rolling reduction and the hydrogen-induced crack area ratio. 7

Claims (1)

[Claims] In continuous casting of molten metal, in which slabs are continuously drawn,
During casting, the unsolidified slab is continuously rolled down, and the reduction amount is
In the region from the time when the center of the slab reaches the liquidus temperature to the time when the solid fraction reaches the flow limit, the rate is 0.5 mm/min to 2.0 mm/min.
A continuous casting method characterized in that the casting speed is 0 mm/min and thereafter 0.3 mm/min or less in the region until the center of the slab reaches the solidus temperature.