JP3341338B2 - 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 capable of minimizing segregation and center porosity at the center of a slab.

[0002]

2. Description of the Related Art In a continuous casting method, one of the important issues is how to reduce segregation and center porosity generated in the center of a slab. Regarding segregation prevention, segregation dispersion technology represented by application of electromagnetic stirring technology, implementation of low-temperature casting, or addition of a heterogeneous nucleation promoting substance has been put to practical use, and in order to reduce the impurity concentration in molten steel. In addition, the introduction of advanced cleaning technology and the introduction of a bulging prevention technology during the slab drawing process have been implemented, and considerable results have been obtained.

[0003] However, a sufficient solution has not yet been established for the segregation induced by the flow of molten steel accompanying the solidification shrinkage at the end of solidification or the formation of center porosity which is a direct result of the solidification shrinkage. It is.

[0004] Therefore, in recent continuous casting technology, a large number of rolling rolls are provided in the final stage of the slab drawing process, and a late solidified slab having an unsolidified portion left in the center is reduced at a low pressure reduction rate. Has been proposed. When such a low rate reduction is applied, the flow of the molten steel is suppressed to contribute to the prevention of segregation, and at the same time, the compensation for solidification shrinkage is performed to prevent the generation of center porosity, thereby providing a continuous casting product without casting defects. It is possible to do.

[0005] Such low-rate reduction application techniques are disclosed in Japanese Patent Publication Nos. 59-16682, 3-8663, and 3-8663.
No. 8864, No. 3-6855, No. 4-20696,
The ones described in JP-A-4-22664 are known. These known techniques are defined as a position where low-rate reduction is performed (the meaning of a section from the start to the end of low-rate reduction in the final stage of the drawing process in consideration of the unsolidified state of the slab center, hereinafter the same). ), A tentatively unified concept (a concept based on the solid phase ratio at the center) is presented. For the degree of reduction, for example, the reduction ratio (1.5% or less) and the ratio (0.5 to 2) .5 mm / min), 0.6 mm reduction per unit time
Various concepts such as ξ1.1ξ (ξ is ４ of the flattening ratio) have been presented, and it seems that the concept has not yet reached a definitive concept.

On the other hand, as a specific apparatus technology for performing the above-described low-rate reduction, for example, as described in JP-A-50-55529 and JP-B-54-38978, cast slabs are disclosed. A method of applying a reduction by using a roll having an effective length equal to or longer than the width (generally called a flat roll), and a method of applying a roll length as disclosed in Japanese Patent Publication No. 2-56982. A roll whose diameter at the center in the length direction is larger (larger than the diameter at both ends of the roll) in a range shorter than the width of the slab (referred to as a medium roll in this specification).
(See FIG. 5: 1 is a medium-sized roll, 2 is a slab, 3 is an unsolidified portion, 4
Indicate the axes, respectively). As for the evaluation of these conventional techniques, first, in the former technique in which the entire width of the slab is reduced by a flat roll, the shell grown from both sides of the slab toward the center shows high rigidity, so that the reduction resistance is large (particularly, This is notable for bloom cast slabs with a small aspect ratio), and the efficiency (reduction efficiency) of reducing the cross-sectional area of the unsolidified portion at the center is poor.
It has been pointed out that a large amount of reduction is required to prevent segregation, the load applied to the roll is increased, and the wear of the roll and the bearing becomes severe. In addition, equipment costs and operating costs for responding to the required reduction amount also increase. On the other hand, in the latter method, since the effective reduction of the slab is applied only at the central portion having a diameter larger than the both ends of the roll, the reduction resistance by the shell is small, and therefore, the reduction efficiency is improved, and the reduction is relatively small. It is evaluated that the effect of preventing segregation and center porosity is high even in the amount. The applicant of the present application has separately filed a patent application for a short-width roll obtained by further improving such a medium-thick roll (Japanese Patent Application No. 5-5958: see FIGS. 3 and 4 described later). The short width roll will be described later.

[0007]

As described above, in the conventional low-rate rolling reduction technique, for example, it is chaotic to study how to define the degree of reduction, and it remains as an unsolved problem. . For example, it is desired to establish appropriate rolling conditions including a case where high-carbon steel, which is liable to cause internal cracks by rolling, is targeted.

The use of the above-mentioned medium-sized roll has the advantage that the amount of reduction required for exhibiting an effective segregation preventing effect and a center porosity eliminating effect can be reduced, but the roll due to the thermal influence from the slab. In order to maintain the rolling accuracy by minimizing the warpage of the roll as much as possible, the roll must have a fairly large diameter at both ends of the roll. There is also a problem that the interval (roll pitch) between the roll and the short width roll becomes large, the bulging of the slab (expansion of the slab caused by the roll interval) becomes large, and the effect of preventing segregation and center porosity is lost. Therefore, it is desired to establish a rolling condition that does not cause such a problem even when a short width roll is used.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and uses a short width roll in place of the above-mentioned middle roll and introduces a new concept of a draft gradient. By doing so, the above object has been achieved. That is, the continuous casting method of the present invention means that the slab width is 0.2 to 0.
A rolling roll having an effective length of eight times is acted on from above or below or from either of the slabs, and starts rolling from the point where the center solid fraction of the slab is at most 0.05, and The gist of the invention is to reduce the range up to a phase ratio of 0.8 so that the reduction gradient with respect to the slab thickness per unit time is 0.06 to 0.25% / min.

[0010]

3 and 4 are explanatory views showing the concept of using a short width roll in the present invention. In the figure, 11 is a short width roll, 2 is a slab, 3 is an unsolidified portion, 4 is a shaft, and 5 is a flat. Indicates a role. FIG. 3 shows a case where short width rolls of the same size are applied from above and below the slab, and FIG. 4 shows a case where short width rolls are applied from the upper side of the slab and the lower side is supported by flat rolls 5. The short width roll 11 has already been described in detail in Japanese Patent Application No. 5-5958.
Is substantially shorter than the width dimension W ′ of the slab 2, and in the present invention, those satisfying the following relationship are used. In the present specification, W is defined as an effective length. Is called. 0.2 W ′ ≦ W ≦ 0.8 W ′ (1) More preferably, the relationship of 0.3 W ′ ≦ W ≦ 0.7 W ′ (2) is satisfied.

In the present invention, one of the points is to use a roll stand that satisfies the above-described effective length condition. That is, since the rolling roll of the present invention has a short axial length, it exhibits sufficient rigidity without having a particularly large diameter. Therefore, the roll diameter can be reduced and the roll pitch can be shortened, so that it is possible to suppress bulging which is a drawback of the conventional technique using a medium-thick roll. From the viewpoint of preventing bulging, it is recommended that the roll pitch be 350 mm or less.

As is clear from FIGS. 3 and 4, the short width roll of the present invention can efficiently and intensively reduce the central portion of the slab where the unsolidified portion 3 exists, thereby preventing segregation and center porosity. The amount of reduction required for the operation is small, and the operating cost can be reduced. In addition, since the friction between the roll surface and the roll shaft is reduced, the maintenance cost of the equipment can be reduced. In the arrangement of FIG. 3, the center of the slab is lowered from both sides of the slab, and in the arrangement of FIG. 4, the center of the slab is lowered from the upper side of the slab. As long as the low pressure reduction that satisfies the gradient condition is performed, there is no substantial difference between the segregation prevention and the center porosity prevention effects.

If the above expression (1) is not satisfied, for example, W
Is smaller than 0.2 W ', the unsolidified portion 3 cannot be rolled down over its entire width, and the desired effects such as segregation prevention cannot be achieved. On the other hand, if W exceeds 0.8 W ', the rolling resistance is reduced by the solidified shell, so that segregation prevention or the like cannot be achieved at a low rate of pressure. The short-width roll according to the present invention is, as shown in the arrangement example of FIGS. 3 and 4 described above, arranged so as to be pressed down from both the upper and lower sides of the slab 2, or only one of the upper and lower parts of the present invention. Although it is configured to be a short width roll and the other side is reduced as the flat roll described above, it is not necessary that the entire arrangement be the same arrangement over the entire length of the slab drawing direction.
All design changes such as alternately adopting the four arrangements are included in the technical scope of the present invention. Next, the position where the low rate reduction is performed in the roll configuration and the roll arrangement as described above is determined based on the central solid phase ratio obtained by the following formula.

[0014]

(Equation 1) Where T: slab center temperature% C: slab C concentration

That is, the position where the center solid phase ratio obtained by the above formula is at most 0.05 [in other words, the position upstream of the position where the solid phase ratio shows a value of 0.05 in the center of the slab (on the mold side). ), The low rate reduction is started. Then, while the center solid fraction gradually increases downstream, the low rate reduction is continued, and the solidification of the unsolidified material remaining in the center of the slab proceeds to reach the limit showing fluidity. The low rate reduction is continued without fail until the central solid phase ratio reaches 0.8.

If the low rate reduction starts after the center solid phase ratio exceeds 0.05, the low rate reduction starts too late, and at that time, the coagulation contraction in the last stage of solidification has already started. In this way, molten steel flows and segregation occurs. On the other hand, if the low-rate reduction is stopped before reaching the fluid phase limit solid phase ratio of 0.8, the low-rate reduction is canceled in a state where molten steel flows due to solidification shrinkage, so that segregation cannot be avoided. Further, since compensation for coagulation shrinkage is not performed, center porosity is formed.

Finally, the degree of low rate reduction, which is the most important condition of the present invention, is controlled according to the concept of reduction gradient given in the unit of% / min. This concept numerically indicates at what reduction ratio the rolling reduction is performed per minute in the slab thickness direction.
6 to 0.25% / min, preferably 0.06 to 0.13
% / Min. 0.06% /
With a reduction gradient smaller than min, it cannot be said that the reduction is sufficient, and the effect of improving segregation is hardly obtained. On the other hand, when a large reduction exceeding 0.25% / min is applied, reverse V segregation may occur on the contrary, leading to deterioration of center segregation. When the rolling at a low rate was performed while maintaining such a rolling gradient condition, defects such as segregation and center porosity were improved to a level that could be almost ignored. In particular, when a low rate reduction was applied in the range of 0.06 to 0.13% / min, there was no internal cracking, and a nearly perfect cast product was obtained. In addition, the present invention can be widely applied from a medium / low carbon steel to a high carbon steel, and in any case, it was found that the expected effect was obtained.

[0018]

EXAMPLE Using various steel types having a C concentration of 0.7 to 1.20% (see Table 1), a slab size of 380 × 600 (mm), a casting speed of 0.6 m / min, and continuous use of electromagnetic stirring in a mold. Casting was performed.

[0019]

[Table 1]

From the stage where an equiaxed crystal region having a thickness of 20 to 40% of the slab thickness is formed at the center of the slab, 300 mm from the upper side
Roll of short width reduction (diameter: 300mm, roll pitch:
320 mm), and the lower side was subjected to low rate reduction using a flat roll (diameter: 300 mm, roll pitch: 320 mm) (arrangement configuration in FIG. 4).

FIG. 1 shows an unsolidified region (solid fraction of 0.1% at the center of the slab).
10 is a graph showing the relationship between the reduction gradient and the degree of internal cracking or center segregation at a flow rate of 0.05 to a flow limit solid phase ratio of 0.8). The degree of center segregation was represented by the maximum value of C analysis values in 30 samples continuously collected at a pitch of 10 mm from the center of the slab toward the casting direction using a 5 mm φ drill. As is clear from the figure, the maximum center segregation degree is remarkably deteriorated in the region where the reduction gradient is too low, while the maximum center segregation degree based on the reverse V segregation is increased when the reduction gradient is too high, and the internal cracking is also increased. The number increased significantly.

FIG. 2 shows the degree of center segregation when the solid phase ratio at the start of rolling is variously changed when the rolling gradient is fixed to 0.10% / min and the center solid phase ratio is reduced to 0.8. Indicates a change. As is clear from the figure, the central solid phase ratio is 0.05.
It can be seen that the segregation level is low and stable when the low rate reduction is started from the following points.

[0023]

Since the present invention is constructed as described above, it is possible to continuously cast a slab having no center segregation, center porosity, internal cracks, etc. under a low rate of pressure that does not cause roll wear or shaft wear. It became possible to manufacture. In particular, it has been confirmed that an excellent effect can be exerted even on a casting which is liable to produce a casting defect such as bloom continuous casting. In addition, since no bulging occurs, the center segregation is stably eliminated.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the present invention from the viewpoint of a rolling gradient.

FIG. 2 is a graph showing the effect of the present invention from the viewpoint of the center solid fraction at the start of rolling.

FIG. 3 is an explanatory view of the concept of using a short width roll in the present invention.

FIG. 4 is an explanatory view of another usage concept of the short width roll in the present invention.

FIG. 5 is an explanatory view of a conventional medium-sized roll.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Medium roll 2 Cast piece 3 Unsolidified part 5 Flat roll 11 Short width roll

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kenzo Ayata 2222-1, Ikeda, Ikeda-cho, Onoe-cho, Kakogawa-shi, Hyogo Prefecture Kobe Steel, Ltd. Kakogawa Research Area (56) References JP-A-60-121054 (JP) (A) JP-A-61-132247 (JP, A) JP-B-4-22664 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/128 350 B22D 11/20

Claims (2)

(57) [Claims] 1. A method of applying a rolling force to a slab being drawn in a final stage in a slab drawing step of a continuous casting method, comprising a reduction having an effective length of 0.2 to 0.8 times a slab width. The roll is acted on from above and below or from either one of the slabs, rolling starts from the point where the center solids fraction of the slab is at most 0.05, and the range up to the center solids ratio of 0.8 is defined as a unit. The rolling gradient with respect to the slab thickness per hour is 0.06-0.25% /
A continuous casting method characterized by reducing the pressure to a minimum value. 2. The continuous casting method according to claim 1, wherein the reduction is performed at a reduction gradient of 0.06 to 0.13% / min.