JP3341340B2 - 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.

On the other hand, regarding the segregation caused 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, a sufficient solution has not yet been established. 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. Moreover, as described later, the type of slab and the casting conditions,
Furthermore, there are no known low-rate reduction conditions that take into account the relationship with the slab drawing speed and the like.

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. 9: 1 is a medium-sized roll, 2 is a slab, 3 is an unsolidified portion, 4
Indicate the axes, respectively).

[0007]

As described above, in the conventional low-rate rolling technology, even the examination from the angle of how to define the degree of rolling is chaotic, and remains as an unsolved problem. In addition to this, for example, the effect of the C concentration, which tends to cause internal cracks in the slab, has a specific effect, or the degree of superheating of the molten steel in the tundish and the casting speed, which greatly affect the form of the solidified structure. It seems that there is no known idea of how to optimally adjust the rolling reduction by paying attention to the relationship between the two. Therefore, it is desired to establish appropriate rolling conditions including the case where high-carbon steel, which is liable to cause internal cracks due to rolling, is targeted.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and introduces a new concept of a reduction gradient in a low-rate reduction process at the end of solidification.
The above problem was solved by configuring the starting time of the low rate reduction at the end of solidification and the operating conditions of the above-described reduction gradient in consideration of factors such as C concentration, molten steel superheat degree, and slab drawing speed. It has been achieved. That is, the continuous casting method of the present invention means that the reduction is started from the time when the central solid fraction (fs) obtained by the following equation (1) is reached, and the reduction is continued until the central solid fraction reaches 0.8. The gist is that the reduction is applied so that the reduction gradient (S) given as the ratio of the reduction amount to the slab thickness per unit time at this time satisfies the following expression (2). .

[0009]

(Equation 3) Where ΔT: degree of superheat of molten steel in tundish (° C.) Vc: casting speed (m / min) [% C]: carbon concentration of molten steel (%)

[0010]

(Equation 4) In the formula, ΔT, Vc and [% C] have the same meaning as before.

[0011]

The present invention will be described below with reference to the research history leading to the completion of the present invention. It is generally known that high carbon steel slabs are liable to cause internal cracking when excessive reduction is applied at the end of solidification. Therefore, the reduction gradient (in this specification, the ratio of the reduction amount to the slab thickness per unit time is referred to as the reduction gradient,% / m
(given in units of in) must be as small as possible, and there is, of course, the basic proposition of eliminating center segregation and center porosity. Therefore, an appropriate rolling gradient considering these factors comprehensively is required.

[0012] In addition, V which is often found in high carbon steel slabs
It is known that the form of the segregation depends on the width of the equiaxed crystal. Generally, the wider the equiaxed crystal, the wider the width of the V-shaped segregation. This tendency becomes more remarkable as the carbon concentration in the steel increases. Therefore, in order to achieve the intended purpose of low-rate reduction, it is necessary to know the carbon content and the equiaxed crystal content in steel and to determine the optimal rolling start time corresponding to them. On the other hand, since the equiaxed zone becomes wider as the carbon steel becomes higher, the deformation of the solidified shell due to the reduction is easily dispersed among the crystal grains. There is a background that more reduction is required to obtain an improvement effect. From such a viewpoint, when the purpose is to improve center segregation due to low rate reduction at the end of casting, it is indispensable to determine the optimal reduction gradient in accordance with the C content and the amount of equiaxed crystal in the steel. It becomes.

Factors that affect the amount of equiaxed crystals in a slab include casting speed Vc (m / min) and casting temperature (superheat degree of molten steel in a tundish ΔT ° C.). The crystal domain is known to spread. That is, the smaller these values are, the wider the width of V-shaped segregation becomes, and it is necessary to start earlier (low solid phase ratio side) than the reduction required for improving V-shaped segregation.

It is known that the equiaxed crystal region in the slab becomes wider as the carbon content in the steel increases, and as the casting speed Vc and the casting temperature decrease. On the other hand, since the amount of reduction required to improve center segregation increases with the increase of equiaxed crystals, it is necessary to set the carbon content in steel, casting speed, and all of the casting temperature in the end. is there.

[0015] The inventors of the present invention have investigated various test steel types with various C concentrations under various casting and drawing conditions, and as a result, have reached the above-mentioned constituent requirements recommended in the present invention.

First, in the present invention, the starting point of the low rate pressure at the end of solidification is determined based on the solid phase ratio (central solid phase ratio) at the center of the slab. The solid phase ratio at the center of the slab at the end of solidification is determined by comparing the central solid phase ratio obtained by the following formula with a measured value, and comparing this with a reference value.

[0017]

(Equation 5) Where T: slab center temperature% C: slab C concentration

On the other hand, the solid fraction is determined by the carbon concentration in the steel, the degree of superheat of the molten steel in the tundish, and the casting speed, as understood from the above description of the research history. The above equation (1) is the central solid phase ratio obtained by using these as variables, and this central solid phase ratio naturally changes depending on the slab drawing traveling position. Therefore, for each part, the position where the central solid fraction obtained from equation (3) shows a smaller value than the central solid fraction obtained from equation (1) [in other words, the solid fraction at the center of the slab is calculated as above. From the position indicating the value (position on the upstream side (mold side)). 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 necessary, the low rate reduction may be continued up to the center solid phase ratio of 1.0. Particularly in the case of high carbon steel, it is meaningful to prevent the flow of the unsolidified molten metal to the end to prevent the center segregation. That is.

If the low-rate reduction starts after the center solid phase ratio exceeds the above-mentioned value, the low-rate reduction starts too late, and at that time, the coagulation contraction in the late coagulation portion has already started. Induces molten steel flow, resulting in segregation. On the other hand, if the low rate reduction is stopped before the position of the central solid phase ratio of 0.8, which is the fluid phase limit, the low rate reduction is released in a state where molten steel flows due to solidification shrinkage, so the formation of segregation cannot be avoided. . Further, since compensation for coagulation shrinkage is not performed, center porosity is formed.

Next, the degree of low rate reduction, which is one of the most important conditions of the present invention, is controlled according to the concept of the reduction gradient given in the unit of% / min as described above.
This concept numerically indicates the reduction ratio at which the rolling reduction is performed in the slab thickness direction per minute. In the present invention, the reduction ratio is selected from the range satisfying the expression (2). And The upper limit in the expression (2) is a limit at which internal cracks are generated by rolling down, and the lower limit is a limit for showing a segregation improving effect by rolling down. If the upper limit is exceeded, there is a danger that reverse V segregation is likely to occur, so the condition of the above formula (2) must be observed.

The low-rate roll used in the present invention is not particularly limited, and the above-mentioned flat roll and medium-thick roll can be used in the present invention. However, more preferred are the short width rolls described below developed by the present applicant. That is, the flat roll and the middle roll have the following problems.

First, in a flat roll, the shell grown from both sides of the slab toward the center has high rigidity, so that the rolling resistance is large (especially in the case of a bloom slab having a small aspect ratio). The efficiency of the reduction of the cross-sectional area of the solidified portion (rolling efficiency) is poor, so a large amount of rolling is required to prevent segregation, and the load on the roll increases, and the wear of the roll and the bearing becomes severe. is there. In addition, equipment costs and operating costs for responding to the required reduction amount also increase. On the other hand, in the case of a medium-sized roll, since the effective reduction of the slab is applied only at the central portion having a larger diameter 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 with the amount, but if the roll warpage due to the thermal influence from the slab is to be minimized and the rolling accuracy is to be maintained, the diameter of both ends of the roll will be reduced. The roll must be quite large, the diameter of the center of the momentum increases, and therefore the gap (roll pitch) between adjacent short width rolls in the slab drawing direction also increases, and bulging of the slab (by the roll interval) This causes a problem that the effect of preventing segregation and center porosity is lost. Under such circumstances, the present applicant has developed a reduction roll having an effective length of 0.2-0.8 times the slab width, and has already filed a patent application (Japanese Patent Application No. Hei 5 (1993) -195).
-5958).

FIGS. 7 and 8 are explanatory views showing the concept of using a short width roll in the present invention, wherein 1 is a short width roll, 2 is a cast piece, 3 is an unsolidified portion, 4 is a shaft, and 5 is a flat. Indicates a role. FIG. 7 shows a case where short width rolls of the same dimensions are applied from above and below the slab, and FIG. 8 shows a case where short width rolls are applied from the upper side of the slab and the lower side is supported by the flat roll 5. The short width roll 1 has already been described in detail in Japanese Patent Application No. 5-5958, but the point is that the axial length W of the short width roll 1 is substantially shorter than the width dimension W ′ of the slab 2. In particular, those satisfying the following relationship are preferably used. In this specification, W is called an effective length. 0.2W '≦ W ≦ 0.8W ′ (4) More preferably, the relationship of 0.3W ′ ≦ W ≦ 0.7W ′ (5) is satisfied.

Since such a short width roll 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, the roll pitch is 35
It is recommended that it be 0 mm or less.

As is apparent from FIGS. 7 and 8, 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 required reduction amount is also 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 shown in FIG. 7, the center of the slab is reduced from both sides of the slab, and in the arrangement shown in FIG. 8, the center of the slab is reduced 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.

As shown in the arrangement examples of FIGS. 7 and 8 described above, the short-width roll is arranged so as to be lowered from both the upper and lower sides of the slab 2 or only one of the upper and lower sides of the present invention. It is preferable that the rolls are formed to be short width rolls and the opposite side is rolled down as the flat rolls described above. However, it is not necessary that all rolls have the same arrangement over the entire length in the slab drawing direction. For example, FIGS. It is also possible to change the design such that the arrangement configuration is alternately adopted. The present invention can be widely applied to medium to low carbon steels to high carbon steels, and in any case, it has been found that the expected effects can be obtained.

[0027]

EXAMPLE Using various types of steel having a C concentration of 0.6 to 1.20%, the superheat degree of molten steel in a tundish is 10 to 40 ° C., the slab size is 380 × 600 (mm), and the casting speed is 0.4 to 0.7 m.
/ Min, and continuous casting was carried out in combination with electromagnetic stirring in the mold. As the roll for reduction in the reduction roll stand, a short width reduction roll having a width of 300 mm from the upper side (diameter: 3 mm)
00 mm, roll pitch: 320 mm, and the lower side is a flat roll (diameter: 300 mm, roll pitch: 3).
20 mm) to reduce the pressure at a low rate (arrangement configuration in FIG. 8). 20-40% of slab thickness by electromagnetic stirring in the mold
A thick equiaxed crystal region was formed, and a predetermined reduction gradient was applied to a reduction roll stand provided at the final stage of solidification, and reduction was performed until solidification was completed. In addition, V segregation and internal cracking were investigated by sulfur printing of a vertical section of the slab.

FIG. 1 shows a relationship between the solid phase ratio at the time of reduction at 0.8% C and ΔT · Vc. In this case, the rolling gradient was 0.20% / min. The black circles indicate the case where the outside of V segregation remained because the rolling start was too late. On the other hand, open circles indicate the case where V segregation did not remain. As shown in the figure,
As T · Vc increases, the appropriate rolling-start solid phase ratio increases. FIG. 2 shows a similar result at 1.0% C. The rolling gradient was similarly set at 0.20% / min.

The above experiment was conducted on 0.6% C to 1.2% C steel, and from the results obtained, the equation (1) for obtaining an appropriate rolling start solid phase ratio was derived, and the rolling start calculated by calculation was obtained. FIG. 3 shows the relationship between the solid phase ratio and ΔT · Vc.

FIG. 4 shows the reduction gradient and ΔT at 0.8% C.
-Indicates the relationship of Vc. In this case, the rolling start solid phase ratio was determined based on the result obtained by the equation (1) under the condition that V segregation did not occur. In the figure, the black circles indicate the case where the rolling gradient was too large and internal cracks occurred, and the crosses indicate the V segregation V at the center of the slab.
In this case, the tip of the shape remains. ΔT for both upper and lower limits
• Decreases with increasing Vc. FIG. 5 shows a similar result at 1.0% C. Similar experiments were performed at 0.6% C to 1.2%.
% C steel, and from the results obtained, the formula (2) for obtaining an appropriate rolling gradient is derived, and the relationship between the reduction solid phase ratio and ΔT · Vc obtained by calculation is summarized in FIG.

[0031]

The present invention is constructed as described above, and takes into consideration the continuous casting conditions such as the C concentration in molten steel, the superheat degree of molten steel in a tundish, the casting speed, etc., to determine the start timing and the degree of reduction in rolling. As a result, operation under optimal conditions is promised, and center segregation, center porosity,
It has become possible to produce a slab without internal cracks by a continuous casting method. 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.

[Brief description of the drawings]

FIG. 1 is a graph (at 0.8% C) showing the relationship between ΔT · Vc and the rolling-start solid phase ratio.

FIG. 2 is a graph showing the relationship between ΔT · Vc and the rolling start solid phase ratio (at 1.0% C).

FIG. 3 is a graph showing the relationship between ΔT · Vc and the solid phase ratio at the start of rolling (summarization of 0.6 to 1.2% C).

FIG. 4 is a graph showing a relationship between ΔT · Vc and a reduction gradient (at 0.8% C).

FIG. 5 is a graph showing the relationship between ΔT · Vc and the rolling down gradient (at 1.0% C).

FIG. 6 is a graph showing the relationship between ΔT · Vc and the rolling down gradient (summary of 0.6 to 1.2% C).

FIG. 7 is an explanatory view of a concept of using a short width roll in the present invention.

FIG. 8 is an explanatory view of another usage concept of the short width roll in the present invention.

FIG. 9 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

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Masaki Nitta 1 Kanazawa-cho, Kakogawa-shi, Hyogo Prefecture Kobe Steel, Ltd. Kakogawa Works (56) References JP-A-60-121054 (JP, A) JP-A Sho 61-132247 (JP, A) JP 4-22664 (JP, B2) (58) Fields surveyed (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, wherein a central solid fraction (fs) obtained by the following equation (1) is obtained. The reduction is started from the time when the center solid phase ratio is 0.8
And the reduction is applied so that the reduction gradient (S) given as a ratio of the reduction amount to the slab thickness per unit time at this time satisfies the following expression (2). Characteristic continuous casting method. (Equation 1) Where ΔT: degree of superheat of molten steel in the tundish (° C.) Vc: casting speed (m / min) [% C]: carbon concentration of molten steel (%) In the formula, ΔT, Vc and [% C] have the same meaning as before. 2. The reduction according to claim 1, wherein the reduction roll having an effective length of 0.2 to 0.8 times the width of the slab is acted on both or both of the upper and lower sides of the slab. Continuous casting method.