JP2000288706A - Method for continuously casting steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the occurrence of center segregation to a level having practically no problem, even in the case of generating the change of casting speed. SOLUTION: In a continuous casting method for executing rolling reduction to a cast slab in the continuous casting process, in 0.3-0.7 mm/m of rolling reduction gradient, the rolling reduction is executed so that the rolling reduction gradient X (mm/m) and the solidified shell thickness Y (mm) satisfy the equation Y=50X+45, within range from a position where the solidified shell thickness becomes >=70 mm before the center solid phase ratio at the thickness center part of the cast slab at the casting time in the permissible min. casting speed reaches 0.3, to a position where the center part of the cast slab at the casting time in the permissible max. casting speed reaches a fluid limit solid phase ratio.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a practical problem in which the center segregation of a slab slab manufactured by continuous casting is reduced by a continuous casting process over a wide range as possible from the initial stage to the final stage. The present invention relates to a method for continuously casting steel to reduce the level to a level free from defects.

[0002]

2. Description of the Related Art Segregation (central segregation) occurring at the center of the slab slab slab thickness in a continuous steel casting method is one of the important issues to be solved. As a technique for improving such center segregation, means such as application of electromagnetic stirring technology, low-temperature casting, and reduction of roll pitch to prevent bulging in a continuous casting machine have been adopted. The results are up.

On the other hand, segregation caused by molten steel flow accompanying solidification shrinkage at the end of solidification is described in, for example, JP-B-59-16682, JP-B-3-6855, and 3-8.
No. 863, No. 3-8864, No. 4-20696, No. 4-22664, No. 5-30548, etc., the roll interval at the end of solidification is controlled to reduce unsolidified slabs. Techniques for rolling down are known. According to these techniques, at an appropriate time in the process of continuous casting (for example, an appropriate time between the time when the central solid fraction is 0.1 to 0.3 and the flow limit solid fraction), Reduction speed (for example, 0.5
It is known that it is effective to reduce the pressure by 2.0 mm / min). And, for the parts cast at the target casting speed by these light reduction technologies,
It is said that the center segregation can be reduced to a level at which there is no practical problem.

[0004]

SUMMARY OF THE INVENTION However, Patent 2
No.593933, No.2593377, No.259338
As disclosed in JP-A Nos. 4-6, 2532306, etc., even in the above-described technique, center segregation occurs at a portion where the casting speed is changed from the start to the completion of solidification. There was a problem that it was easy.

In each of the above patents, in order to cope with such a problem, the segregation deteriorated slab caused by the change of the casting speed is determined, and the heating condition (rolling heating condition) of the segregation deteriorated slab before rolling is determined. ) At a high temperature for a long time to eliminate the center segregation generated.

However, setting the heating condition before rolling the slab to a high temperature for a long time causes an increase in cost and confusion in slab distribution. Also, in the continuous casting process, it is a fact that the casting speed is inevitably changed due to the existence of the following regions (1) to (3). For this reason, even when the casting speed is changed, it is a reality that it is desired to guarantee the center segregation quality that has no practical problem over the entire region from the start of casting to the end of casting. . (1) Acceleration range in which the casting speed in the early stage of casting is increased to a certain speed (2) Deceleration region in the last stage of casting to reduce shrinkage cavities in the final casting slab The present invention has been made in view of the above circumstances, and its purpose is to reduce the generation of center segregation to a practically acceptable level even when the casting speed is changed. An object of the present invention is to provide a continuous casting method of steel that can be improved.

[0007]

The continuous casting method of the present invention that solves the above-mentioned problems is a continuous casting method in which a slab slab is reduced in a continuous casting process, wherein a solidified shell thickness at an allowable maximum casting speed is reduced. The section from the position of 70 mm or more to the position where the center of the slab when casting at the allowable maximum casting speed reaches the flow limit solid fraction is 0.3 to 0.7.
The gist is that the reduction is performed so that the reduction gradient X (mm / m) and the thickness of the solidified shell Y (mm) satisfy the following formula (1).

[0008] Y = 50X + 45 (1)

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have set a time period for applying a reduction especially for the purpose of maintaining the center segregation to a level at which there is no practical problem even at a portion where the casting speed is lower than a target speed. The study was carried out from the viewpoint of extending to the upstream side of the strand. As a result, the reduction is started from a position where the slab thickness when casting at the allowable maximum casting speed becomes 70 mm or more, and the target maximum casting speed (this is called “allowable maximum casting speed” in the present invention). 0.3-0.7m until the center of slab reaches the flow limit solid fraction
If the rolling gradient is reduced to m / m and the rolling gradient and the slab thickness are further reduced so as to satisfy a predetermined relationship, the center segregation is improved to a level that does not pose a practical problem in a wide range over the entire casting length, It has been found that other defects such as internal cracks can also be prevented.

In order to reduce center segregation by rolling down the slab at the end of solidification, the center solid phase ratio of the solidified slab should be reduced to 0.1%.
It is known that it is effective to reduce the molten steel from the position of 1 to 0.3 to the extent that the molten steel flow due to solidification shrinkage is prevented at an appropriate time in the region until the center reaches the flow limit solid fraction. I have.

In view of the above, in the conventional continuous casting process, a target casting speed is determined, and at that casting speed, the roll interval of a portion corresponding to the region where the center solid fraction is in the above range is adjusted. When cast at a casting speed, a predetermined amount of reduction is applied to the above-mentioned region. Such a situation will be described with reference to the drawings.

FIG. 1 is a diagram for explaining the timing of light pressure reduction in the conventional continuous casting process. That is, in FIG. 1, the target casting speed (allowable maximum casting speed) is 1.
At this casting speed, the center solid phase ratio was 0.1 m / min.
~ 0.3 (indicated by fs = 0.3 in the figure), the roll corresponding to a region ranging from the flow limit at which the central solid phase ratio becomes 0.8 (fs = 0.8) By adjusting the interval, a light reduction with a reduction gradient of 0.6 mm / m is applied to this portion.

However, if the operating conditions are set so as to apply such a light reduction, the casting speed is, for example, from 1.2 m / min to 0.5 m / min while the part is solidified.
When the speed is reduced to 0.1 to 0.3 (f
From the time when s = 0.3, the central solid phase ratio is 0.8 (fs =
As shown in FIG. 1, the region up to the flow limit solid phase ratio of 0.8) moves to the upstream side of the casting, and the appropriate rolling region sequentially deviates from the side where the central solid phase ratio is low. The level will not be improved.

In order to solve the above-mentioned problem, the present inventors set the slab reduction start timing to a more upstream side as shown in FIG. It is considered effective to start rolling from a position corresponding to 0.1 to 0.3 and to reduce the pressure in the section up to the position where the center reaches the flow limit solid fraction when the casting speed is maximum. Was. However, when the operating conditions are set in this way, some problems as described below have occurred.

(1) Target casting speed (allowable maximum casting speed)
Since the pressure is reduced from a thinner region of the solidified shell than when casting is performed, strain acts on the solidified interface due to the reduction, and internal cracks are easily generated. For this reason, when the reduction is started at a time when the thickness of the solidified shell is small, or when the reduction gradient (reduction amount) is increased, internal cracks occur, and a high quality cast piece cannot be obtained.

(2) When the casting speed is reduced, the slab is reduced after the center of the slab exceeds the flow limit solid fraction, and the slab in this region is strongly reduced with a large reduction gradient. Then, due to the small non-uniformity of solidification, confinement in which the concentrated molten steel remains in the center of the slab thickness occurs, and local center segregation worsens rather.

The present inventors have studied from various angles the optimum rolling conditions that do not cause such inconveniences. As a result, if the above structure is used and the rolling is lightly performed, the above-mentioned object is successfully achieved. That is, the present invention has been completed.

Hereinafter, the operation and effect of the present invention will be described more specifically with reference to examples. However, the following examples are not intended to limit the present invention. All of them are included in the technical scope of the present invention.

[0019]

[Example] Using a normal steel having a C concentration of 0.5 to 0.6%,
Slab continuous casting with a slab cross section size of 280 mm x 2100 mm was performed. At this time, rolling at the end of solidification was performed using a roll having a diameter of 280 mm and an initial roll interval of 350 mm.

The roll interval (rolling gradient:
It was investigated how the center segregation level changes according to the change in the average casting speed when casting at 0.6 mm / m). At this time, the center segregation site of the slab was drilled at a pitch of 30 mm in the width direction with a drill having a diameter of 5 mm.
A chip was collected and analyzed for C concentration, and the maximum C concentration in the width direction was defined as Cmax, and the center segregation was evaluated by dividing this value by the C concentration C0 of the bulk (Cmax / C0). The average casting speed means an average casting speed from the meniscus to the solidification position.

FIG. 3 shows the results, and it can be seen that the center segregation level deteriorates as the casting speed decreases from the target value of 1.2 mm / m.

Next, the casting speed dependence of center segregation when the roll was reduced at the roll interval (reduction gradient: 0.6 mm / m) shown in FIG. 2 was examined. The results are shown in FIG. 4 together with the results of FIG. 3 (indicated by a circle in FIG. 3). At a casting speed of 1.2 m / min, compared to the case where the roll spacing pattern shown in FIG. 1 was used. Although the center segregation level has a large variation and deteriorates, the change of the center segregation is small with respect to the change of the casting speed, and almost falls within the practical center segregation level (Cmax / C0: 1.3 or less). You can see that there is.

FIG. 5 is a graph showing the effect of the reduction gradient at the casting speed of 1.2 m / min and the change in the thickness of the solidified shell at the start of the reduction on the degree of internal cracks in the slab. FIG. 6 is a graph showing the relationship between the draft and the center segregation level. From these results, the reduction gradient required to achieve the practical center segregation level: the minimum reduction start time at the reduction of 0.3 mm / m or more (FIG. 6) is 7 mm in the solidified shell thickness.
It can be seen that it is 0 mm (FIG. 5).

On the other hand, FIG. 7 shows the change in the center segregation level when the average casting speed (the average casting speed until the relevant portion reaches the solidification position from the meniscus) is reduced using the rolling gradient as a parameter. It is a graph. From this result, it is apparent that when the rolling reduction gradient exceeds 0.7 mm / m, the balance of the center segregation of the deceleration time is deteriorated, and the cast slab quality is deteriorated.

From the above results, in the present invention, the rolling start timing is set when the solidified shell thickness is 70 mm and the rolling gradient is specified to be 0.3 to 0.7 mm / m. It is necessary to appropriately define the relationship between the thickness of the solidified shell and the reduction gradient. That is, as is apparent from FIG. 5, if the thickness of the solidified shell is too thin compared to the rolling gradient, there is a risk of occurrence of internal cracks. Therefore, in the present invention, based on the results of FIG. Is defined so as to satisfy the above equation (1).

FIG. 8 shows a comparison of the ratio of the portion where the center segregation level which is practically problematic is achieved (the center segregation good portion) before the application of the present invention (prior art) and after the application. These results were obtained under the condition that the (charge / casting number) was 3. However, before the application, the center segregation favorable region was 65%.
%, But after application, it has been improved to 90%.

[0027]

The present invention is configured as described above, and is capable of continuously changing casting speed during casting and minimizing deterioration of center segregation of the slab due to unsteady portions at the beginning and end of casting. The method has been realized.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the timing of light pressure reduction in a conventional continuous casting process.

FIG. 2 is a diagram for explaining the timing of light pressure reduction in the continuous casting process of the present invention.

FIG. 3 shows a roll interval (rolling gradient: 0.6 m) shown in FIG.
10 is a graph showing how the center segregation level changes according to the change in the average casting speed when casting at m / m).

FIG. 4 shows the roll interval (rolling gradient: 0.6 m) shown in FIG.
3 is a graph showing the casting speed dependence of center segregation when the rolling is reduced at m / m).

FIG. 5 is a graph showing the effect of a reduction gradient at a casting speed of 1.2 m / min and a change in thickness of a solidified shell at the start of reduction on the degree of internal cracking of a slab.

FIG. 6 is a graph showing a relationship between a rolling gradient and a center segregation level.

FIG. 7 is a graph showing a change in the center segregation level when the average casting speed is reduced using the rolling gradient as a parameter.

FIG. 8 is a graph showing a comparison of the ratio of a portion (center segregation good portion) where a center segregation level, which is a practical problem, is achieved before application of the present invention (prior art) and after application.

Claims (1)

[Claims] In a continuous casting method in which a slab cast is reduced by a continuous casting process, a cast slab is cast at a maximum allowable casting speed from a position where a thickness of a solidified shell at an allowable maximum casting speed is 70 mm or more. In the section up to the position where the center reaches the flow limit solid fraction, the rolling gradient is 0.3 to 0.7 mm / m, and the rolling gradient X (mm / m) and the solidified shell thickness Y (mm) are as follows. A continuous casting method for steel, characterized by reducing the pressure so as to satisfy the formula (1). Y = 50X + 45 (1)