JPS619954A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent internal cracking and to prevent central segregation and the formation of center porosity at the lower rolling reduction by rolling down a billet in the end period of solidification and acting axially compressive force to the billet in a rolling down region. CONSTITUTION:The solid-liquid mixed part of the billet where a solidified part 1 and an unsolidified part 2 are mixed is rolled down by rolling down rolls 3 and the billet is fed by driving pinch rolls 4 on an upper stream side in the light reduction is the end period of solidification of the billet. The movement of the billet is braked by braking pinch rolls 4' on the down stream of the rolling down region by the rolls 3. More specifically, the moving speed lower than at least the moving speed of the billet by the pinch rolls 4 on the upper stream side is provided to the peripheral speed of the rolls 4', by which the compressive force is acted axially to the billet. The tensile stress of the billet generated by the rolling reduction of the rolls 3 is offset or relieved by such compressive force, by which the internal cracking owing to the rolling reduction is prevented.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a light reduction casting method at the final stage of solidification that prevents the center segregation and center porosity of slabs manufactured by a continuous casting machine, or a method for casting unsolidified slabs. The present invention relates to a technology for preventing the generation of internal cracks due to reduction in the in-line reduction method, and also for realizing a more efficient reduction method.

(Conventional technology) Conventionally, as a light reduction technology at the final stage of solidification of cast slabs,
There is a method described in Publication No. 8296 @This late solidification light reduction casting method applies a reduction commensurate with the solidification shrinkage to the slab in which unsolidified portions still remain at the end of solidification, and forms molten steel based on the solidification shrinkage. This technology not only suppresses the flow of steel and prevents center segregation from occurring, but also compresses the center porosity that forms due to bridging at the center of the slab at the final stage of solidification.

However, if a light reduction is applied to the slab with unsolidified parts remaining, and if the amount of reduction per rolled terminal of a roll etc. is large, internal cracks will occur on the solid phase side near the solid-liquid interface due to the reduction. On the contrary, it worsens the internal quality of the slab.

Therefore, the amount of reduction per one terminal is limited, so in order to secure the desired total amount of reduction, measures must be taken such as increasing the number of terminals and extending the area of reduction, which reduces equipment costs. In addition, for steel types that are highly susceptible to cracking, existing rolling equipment and rolling methods cannot avoid the occurrence of internal cracks.
In some cases, it may not be possible to improve the internal quality by applying light pressure.

In addition, in in-line reduction, as with light reduction at the end of solidification, unsolidified slabs are rarely rolled due to problems such as internal cracking due to reduction, and the slabs are reheated and rolled after complete solidification. ing.

In addition, in the late-solidification light reduction casting method, in order to prevent center segregation and center porosity, it is necessary to ensure that the reduction applied to the surface of the unsolidified slab extends to the center of the slab and that the amount of solidification shrinkage in that area is reduced. It is necessary to compensate for this and also to crimp the shrinkage hole.

In a study on crimping by rolling thick plates, it was found that when the thickness of the plate is large compared to the contact area between the rolling terminal such as a roll and the thick plate, the amount of reduction required for crimping increases, that is, the deformation extends to the center. It is known that it can become difficult.

Therefore, in order to prevent center segregation and center porosity and improve internal quality even under light reduction at the final stage of solidification, it is preferable to adopt a reduction method in which the contact area between the slab and the reduction terminal is larger. .

However, due to the environment around the rolling device, it is not possible to secure enough space for the rolling device, and as a result, the contact area between the slab and the rolling terminal is limited, and there are cases in which the rolling down method at the end of solidification is used. For the reasons mentioned above, when the roll diameter is limited, the increase in the contact area due to the increase in the roll diameter promotes bulging, and the increase in bulging has a negative effect on the internal quality of the slab. It becomes difficult to improve the internal quality through light compression at the end of coagulation.

(Problems to be Solved by the Invention) The present inventors have discovered that internal cracks due to reduction can be prevented in the in-line reduction method, which uses light reduction at the final stage of solidification or sizing of unsolidified slabs to achieve the desired cross-sectional dimensions in the continuous casting process. As a result of studying methods to prevent this occurrence and ways to efficiently improve the internal quality even with a small reduction amount such as light reduction at the end of solidification, we found that it is possible to apply compressive force in the axial direction of the slab in the area where the slab is rolled down. By adding a light reduction method at the end of solidification, it is possible to suppress the occurrence of internal cracks, and to produce slabs with significantly improved internal quality compared to non-reduced material, and to achieve more efficient reduction. Furthermore, the present invention provides an in-line reduction method for rolling unsolidified slabs while suppressing the occurrence of internal cracks.

(Second measure to solve the problem) Internal cracks that occur during light reduction in the final stage of solidification of slabs and in-line reduction of unsolidified slabs are caused by tensile stress generated near the solid-liquid interface due to the reduction. .

When performing high-speed casting in a curved continuous casting machine and straightening the slab in an unsolidified state, internal cracks due to tensile stress will occur on the loose side of the slab, similar to light reduction at the end of solidification or in-line reduction. It is known that the degree of this problem increases as the casting speed increases, and a compression casting method (CPC) was developed as a method to solve this problem (Japanese Patent Publication No. 34025/1983).

The present invention prevents internal cracks due to reduction by incorporating the concept of compression casting into light reduction at the final stage of slab solidification and in-line reduction of unsolidified slabs.

When the slab is subjected to light reduction at the final stage of solidification, as shown in FIG. The slab is fed in, and the movement of the slab is braked by a brake pinch roll 4' downstream of the rolling area by the reduction roll 3. That is, the compressive force is exerted in the axial direction of the slab by making the peripheral surface of the brake pinch roll 4' move at least at a lower moving speed than the moving speed of the slab by the driving pinch roll 4 on the upstream side. This compressive force cancels out or relieves the tensile stress in the slab generated by rolling down with the rolling down rolls 3, thereby preventing the occurrence of internal cracks due to rolling down. 1st
If driving pinch rolls or brake pinch rolls other than those shown in the figure are provided in the rolling area, the circumference of the driving pinch roll or brake pinch roll is such that compressive stress is applied to the slab in both the upper and downstream rolling areas. By selecting the speed and removing the tensile stress caused by rolling, it is possible to effectively prevent the formation of internal cracks.

Next, a case will be described in which the present invention is applied to in-line reduction of unsolidified slabs.

As shown in FIG. 2, when rolling is performed using one rolling work roll 6, driving pinch rolls 4 and brake pinch rolls 4' are arranged before and after the rolling work roll 6, and a plurality of rolling work rolls 6 as shown in FIGS. 3 and 4 are used. Rolling work roll 6.6'.

One method for rolling down at 6N is to use an intermediate pinch roll of 4N between each rolling roll 6.6' as shown in Figure 3.
A driving pinch roll 4 provided upstream and a brake pinch roll 4' provided downstream generate compressive stress in the slab. Further, as shown in FIG. 4, a driving pinch roll 4 is arranged further upstream of the most upstream rolling roll 6 among the plurality of rolling rolls, and a brake pinch roll 4' is arranged further downstream of the most downstream rolling roll 6N. During that time, each rolling roll 6.
It is also possible to control the circumferential speed of 6' and 6'' to apply compressive stress to the cross section of the slab to prevent internal cracks due to rolling.

(Example) Using the continuous casting machine shown in FIG. 5 and under the casting conditions shown in Table 1, light reduction at the final stage of solidification of the slab in a state where compressive stress is forcibly generated in the slab according to the present invention, For comparison, light reduction (normal method) at the final stage of solidification was carried out without applying compressive stress to the slab.

In the method of the present invention, a reduction is applied to the slab by the reduction roll 3 shown in FIG.
was arranged. The comparative example is a reduction roll of 3 degrees and a driven pinch roll of 4.
．． The peripheral speeds of both brake pinch rolls 4' were set to be the same. Note that FIG. 5 shows a 5Vi mold.

Table 1 Casting conditions Table 2 Rolling down conditions Further, Table 2 shows the rolling conditions.

Regarding the rolling conditions, in order to fully understand the effect of preventing internal cracks according to the present invention, two levels were set: a condition in which internal cracks occur in the normal method and a condition in which almost no internal cracks occur in the normal method.

FIG. 6 shows the frequency of occurrence of internal cracks under light reduction at the end of solidification according to the method of the present invention and under light reduction at the end of normal solidification. In the light reduction at the final stage of solidification according to the present invention, the internal crack score was significantly reduced compared to the conventional method, confirming the effect of preventing internal cracks by applying compressive force.

FIG. 7 shows the relationship between the amount of reduction and the center segregation degree of S, and FIG. 8 shows the results of measuring the bulk specific gravity at the center of the slab and investigating the internal quality improvement effect of light reduction at the final stage of solidification.

The light reduction at the final stage of solidification using the method of the present invention has the following advantages compared to the conventional method:
It can be seen that even with approximately the same amount of reduction, the S center segregation degree decreases, and the bulk specific gravity increases.

Therefore, it was confirmed that by applying the method of the present invention, even with the same amount of reduction as in the conventional method, a more efficient light reduction at the end of vertical positioning, in which the deformation reaches the center, is realized.

(Effects of the Invention) As explained above, the present invention not only has the effect of preventing or significantly reducing the occurrence of internal cracks due to reduction in late-solidification light reduction casting and in-line reduction of unsolidified slabs, but also According to the method of the present invention, in the light reduction at the end of solidification, a more efficient light reduction at the end of solidification prevents center segregation and center porosity from being generated, and significantly improves the internal quality. can be realized.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the present invention during light pressing of the unsolidified part of the slab, Figure 2
3 and 4 are explanatory diagrams of the present invention during in-line reduction, FIG. 5 is an explanatory diagram of an embodiment of the present invention, FIG. 6 is a diagram of the effect of reducing the occurrence of internal cracks according to the present invention, and FIG. 7 8 is a diagram illustrating the effect of reducing the center segregation degree of S according to the present invention, and FIG. 8 is an explanatory diagram of the effect of increasing the bulk specific gravity of the center according to the present invention. 1: Solidified part, 2: Unsolidified part, 3: Reduction roll, 4:
Drive pinch roll, 4': Brake pinch roll, 4"; Intermediate pinch roll, 5: Mold, 6.6'. 6": Rolling work roll. Fig. 1 Fig. 2 Fig. 4 Buttocks (＞O[>O[>αkon2, Fig. 5 Fig. 6 Fig. 7 Fig. 7 Amount of leg (mm) Fig. 8 Amount of force (ynm)

Claims (1)

[Claims] A continuous casting method characterized in that during slab production in a continuous casting machine, a reduction is applied to the slab at the final stage of solidification, and a compressive force is applied in the axial direction of the slab in the reduction area.