JP3257224B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method.

[0002]

2. Description of the Related Art The continuous casting method has the great advantages that the yield is good, the lumping step is unnecessary, and the productivity is high, and the final slabs such as slabs, blooms, billets, etc. are continuously produced directly from molten steel. It is widely used as a casting method that can be manufactured.

When molten steel is cast by this continuous casting method, there is a problem that C, P, S, etc. are segregated (concentrated) in the center of the slab, and therefore components that are easily segregated, especially bearings In the case of steels containing a large amount of C, such as steel and spring steel, there is a problem that it is difficult to adopt a continuous casting method. As a means for suppressing this segregation, a method of subjecting a slab coming out of a mold to a light reduction treatment is known.

It is considered that the phenomenon that components such as C are concentrated and segregated at the center of the slab is caused by solidification of molten steel progressing from the outer periphery toward the center.
In particular, it is considered that a concentrated molten steel flow is caused by solidification shrinkage at the time of final solidification of the liquid phase remaining in the central portion, and this leads to concentrated segregation of C and the like.

[0005] The light rolling treatment described above prevents the flow of concentrated molten steel due to solidification shrinkage by compressively deforming the unsolidified portion of the slab at least by the volume reduction during solidification shrinkage, thereby preventing segregation during solidification. It is to suppress.

[0006] However, since the above-mentioned light rolling process is based on the premise that the center portion is rolled in an unsolidified state, depending on the rolling conditions, the risk of cracking at the solidification front surface may occur due to the magnitude of tensile stress accompanying deformation of the unsolidified interface. Is accompanied.

[0007] The occurrence of cracks in the solidification front surface causes the infiltration of concentrated molten steel such as C, S, and P in the solidification front surface into the cracked portion, which can be a harmful defect equivalent to the center segregation at the product stage.

[0008] Therefore, in the light rolling process, in order to prevent the above-mentioned cracks from occurring, large tensile stress is not generated at the unsolidified interface, and the central unsolidified portion is efficiently changed in volume (compression deformation). It is necessary to subject the slab to pressure treatment, and it has been desired to establish a technique capable of realizing the pressure treatment.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and the gist of the present invention is to inject molten steel into an opening at the upper end of a water-cooled mold and solidify the molten steel. In the continuous casting method of continuously drawing a slab from the lower end open portion, the slab is cast into a circular cross section, and the central solid phase ratio before the molten steel in the center is completely solidified is 0.2 to 0. .8, the slab is lightly reduced in the flat part of the roll having the flat part.

[0010]

Operation and Effect of the Invention During continuous casting, molten steel
As shown in (2), the phase shifts to a complete solid phase region C via a complete liquid phase region A and a solid / liquid mixed region B where a liquid phase and a solid phase are mixed. In the present invention, the center solid phase ratio is the weight ratio of the solid phase in the center of the slab (the perfect solid state is 1.0). The center solid fraction is, for example, the specific heat of the slab,
It can be estimated from the slab cross-sectional temperature distribution obtained by heat transfer calculation based on data such as thermal conductivity and molten metal temperature. Thus, the present invention provides a method for reducing the weight of a slab cast into a circular cross section at the flat portion of a roll at a position where the solid phase ratio at the center is 0.2 to 0.8 in the solid-liquid mixing region as described above. It is characterized by rolling down. Here, the roll having a flat portion is a roll whose pressing surface against a slab having a circular cross section can all form a flat surface, and may have any shape as long as a contact surface with the slab is flat. .

[0011] As described above, in the light rolling process, a large tensile stress is not generated at the unsolidified interface in order to prevent the occurrence of internal cracks in the slab, and the central unsolidified portion is sufficiently changed in volume (compression deformation). It is necessary to reduce the slab efficiently as described above.

In order to find appropriate conditions therefor, the present inventor has proposed a slab E (3) having a square cross section as shown in FIG.
50 mm square) and slab D (350 mm round) with circular cross section
The analysis of stress and deformation under light pressure using the flat portion of the roll was performed by computer calculation using the finite element method, and the results shown in FIGS.

In FIG. 4A, the abscissa indicates the rolling reduction relative to the cast slab, and the ordinate indicates the volume reduction rate of the central unsolidified portion as an index. Here, the rolling reduction refers to the area reduction rate of the slab cross section before and after the rolling. 4 (b) and 4 (c)
Indicates the magnitude of the tensile stress in the X-axis direction (perpendicular to the axis) and the Z-axis direction (axial direction) at the unsolidified interface generated when the rolling reduction is changed.

These results indicate that the reduction required to obtain the same rate of volume change in the center unsolidified portion is significantly different between a slab D having a circular cross section and a slab E having a rectangular cross section. This shows that the slab D having a circular cross section requires significantly less than the slab E having a square cross section.

Regarding the tensile stress generated at the unsolidified interface, when the volume change rate at the center is the same, FIG.
From the results (b) and (c), it can be seen that the slab D having a circular cross section is significantly smaller than the slab E having a square cross section.

That is, from these results, when the slab is cast into a circular cross-section and light reduction is performed using the flat portion of the roll, the tensile stress generated at the unsolidified interface can be reduced with a small reduction ratio. It can be seen that the central unsolidified portion can be efficiently reduced in volume.

The present invention has been made under such knowledge, and according to the present invention, the occurrence of tensile stress in a cast slab is suppressed to a small value, and segregation of C and the like is prevented while preventing the occurrence of cracks. Can be prevented and the quality of the slab can be improved.

The present invention is particularly effective when applied to steel types having a C% of 0.5% or more.

Of course, the present invention can be applied to a steel type having a smaller C% than this. In this case as well, the effect of being able to efficiently deform the central portion while suppressing the tensile stress in the slab and to prevent segregation is obtained. I can play. An important feature of the present invention is that the above-mentioned light pressure reduction is performed at the position of the central solid fraction of 0.2 to 0.8.

In the present invention, the center solid phase ratio is 0.2 at light pressure.
The reason for performing at the position of ~ 0.8 is that even if the reduction treatment is performed at the position where the center solid phase ratio is higher than 0.8, the liquid phase at the center almost loses fluidity at this stage. This is due to the fact that a sufficient effect by light pressure reduction cannot be obtained, and at a position smaller than 0.2, on the contrary, the ratio of the liquid phase is too high, and the fluidity is too high, so that the same effect by light pressure reduction is also obtained. This is because the effect cannot be obtained.

In the present invention, the light rolling is performed for the purpose of preventing the segregation of components such as C, P, S and the like in the central portion while preventing cracking. In this sense, the rolling reduction is 1.0 to 1.0.
The range is preferably 3.0%, and more preferably 1.5 to
It is in the range of 2.5%.

[0022]

Next, embodiments of the present invention will be described in detail. <Example 1> A SUJ2 material having a C content of 1.0% was cast with a circular cross section, a thickness of 350 mmφ, and a drawing speed of 0.4.
Continuous casting was performed under the conditions of m / min.

At this time, a two-stage roll in which flat rolls are arranged in two upper and lower stages at a position of 0.4 to 0.5 of the center solid phase ratio is used, and the rolling reduction of the slab is 2%. Was lightly reduced. The roll may have a flat portion, and a roll having a V-hole shape may have a flat portion. Analysis of the C content in the cross section of the obtained slab showed that FIG.
The result shown in (a) was obtained, and there was no internal crack.

On the other hand, for comparison, C was similarly analyzed on a slab (a slab having a circular cross section) in which the above light reduction treatment was not performed, and the result shown in FIG. 1 (b) was obtained. Was done.

From these results, it can be seen that by casting the slab into a circular cross section and applying a slight reduction using a flat roll, segregation at the center can be favorably suppressed without causing internal cracks.

<Example 2> SUP7 having a C content of 0.6%
The material was subjected to continuous casting under the same conditions as in Example 1, and the C content in the cross section of the obtained slab was examined.
The result of (a) was obtained, and there was no internal crack.
FIG. 2B shows the result when the light reduction process was not performed.

From these results, it is found that the SUP7 material also has a circular cross-sectional shape of the slab, and that the slab is subjected to a light rolling treatment using a flat roll at the position of the central solid phase ratio of 0.2 to 0.8. Thus, it can be seen that the segregation of C at the center can be favorably suppressed.

Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be applied to continuous casting of other various materials, and various changes can be made without departing from the gist of the present invention. It can be implemented in an added mode.

[Brief description of the drawings]

FIG. 1 is a diagram showing an analysis result of a C amount of a cross section of a cast slab obtained in one example of the present invention, in comparison with a case where light reduction was not performed.

FIG. 2 is a view showing an analysis result of a C amount of a cross section of a cast slab obtained in another example of the present invention, in comparison with a case where light reduction was not performed.

FIG. 3 is an explanatory diagram showing a test method for confirming the effect of light reduction under the present invention.

FIG. 4 is a diagram showing the results obtained in the same test.

FIG. 5 is an explanatory diagram for explaining a position where light reduction is performed in the present invention.

[Explanation of symbols]

 A Complete liquid phase region B Solid-liquid mixed region C Complete solid phase region D Circular cast slab E Square slab slab

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-7-108358 (JP, A) JP-A-5-337510 (JP, A) JP-A-3-155441 (JP, A) JP-A-4-108 17948 (JP, A) JP-A-50-85523 (JP, A) JP-A-3-124352 (JP, A) JP-A-62-275556 (JP, A) JP-A-62-158555 (JP, A) JP-A-3-198964 (JP, A) JP-A-63-215353 (JP, A) JP-A-3-138056 (JP, A) JP-A-1-162551 (JP, A) JP-A-6-1422863 (JP, A) JP-A-3-110001 (JP, A) JP-B-59-16862 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11 / 00

Claims (1)

(57) [Claims] 1. Inject molten steel into an upper end opening of a water-cooled mold,
In a continuous casting method in which the slab is continuously drawn from the lower end open portion of the mold while solidifying the slab, the slab is cast into a circular cross-section, and the central solid phase ratio before the molten steel in the center is completely solidified. Wherein the cast slab is lightly reduced at the flat portion of a roll having a flat portion at a position of 0.2 to 0.8.