JPH09234551A - Continuous casting method for preventing segregation and internal crack - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simply produce a cast slab having the similar shape as a product shape without internal crack and segregation defect by deciding rolling reduction rate in each roll pair so as to become in a specific range, in a continuous casting of molten metal drawing while executing the rolling reduction to the cast slab. SOLUTION: The rolling reduction rate Δh of each rolling roll pair 3, 3 decided so as to be in the range of the formulas I, II, III, IV and V. εc>ε=[β(R +D)-Δh/2)/cosβ-1d]/1d×100... (I). Wherein, εc is crack limit average rolling reduction strain in development of the internal crack (%), R is radius of the rolling reduction roll (mm), D is shell thickness of solid phase ratio 1 (mm), 1d is contacting length (mm), Δh is rolling reduction rate (mm), ΔT is super heat in the mold ( deg.C), (f) is the solid phase ratio at the molten metal temp. in the mold and β is angle (radian) forming a line connecting the position of shell thickness D at the position of the roll contacting with the cast slab and the roll center and the perpendicular line in the roll center.

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 in which a pressure is reduced from directly below a mold under an in-machine pressure to prevent the occurrence of internal cracks and center segregation and to produce a slab having a shape as close as possible to a product. Regarding

[0002]

2. Description of the Related Art In recent years, demands for material characteristics of offshore structures, storage tanks, steel pipes for transporting oil and gas, high-strength wire rods, etc. have become more severe, and at the same time, establishment of a method for manufacturing these at lower cost is an important issue. It has become.

When the quality of the slab for realizing these is examined first, it is ideal that the slab is homogeneous in the cross section, however, inclusions, segregation, porosity, etc. occur in the slab. Affect the properties of steel materials. Among these defects, segregation occurs when impurity elements such as sulfur, phosphorus and manganese contained in the molten steel are partially concentrated in the casting process.

One of the causes of such segregation is cracking that occurs during casting, and the cracking is caused by roll misalignment, bulging, straightening bending, vertical bending back, or solidification caused by the recent light pressure reduction. It is caused by the rolling strain generated at the liquid interface, and at that time, impurity elements such as sulfur, phosphorus, and manganese concentrated in the spaces between dendrite trees are sucked into the cracks to form segregation, which causes internal cracks.

As a measure for preventing these internal cracks, it is effective to minimize the strain generated at the solid-liquid interface, and it is necessary to prevent roll misalignment, limit the casting speed, or limit the amount of reduction. There is.

Further, another important segregation that impairs the homogeneity of the slab is center segregation. At the center of the slab obtained by continuous casting, center segregation and V segregation in which impurity elements are concentrated are observed. To be done.

[0007] This has been described by the present inventors first in [Materials and Processes] vol. 2 (1989), 1269], bridging that occurs at the center of the slab at the end of solidification and unevenness of the solid-liquid interface, etc., partially increases the liquid passage resistance at the center and Thick molten steel between dendrite trees,
By coagulation contraction suction force, it is sucked and accumulated in the central part.

The reason why the liquid passing resistance at the center of the cast slab increases is that the solid fraction of the central portion locally increases due to bridging and unevenness of the solid-liquid interface, and the dendrite of the central portion having a large solid fraction is increased. Molten molten steel with a low melting point such as wood is sucked by downward solidification shrinkage, and the solute such as dendrite in the upper part where the solute such as dendrite, from which this low melting point concentrated molten steel has escaped, is not concentrated This is because molten steel with a high temperature enters and locally solidifies.

As a measure for preventing center segregation and V segregation, there is a method of controlling the solidification structure into equiaxed crystals or the like in which tree flow is unlikely to occur due to electromagnetic stirring, and solidification shrinkage which causes flow of concentrated molten steel or the like. In recent years, a method of compensating the surface of a slab by lightly reducing the surface has been developed.

On the other hand, in consideration of a further process saving method, it is conceivable to increase the casting speed or omit the rolling process.
As well known, in order to omit the rolling step, a method such as a twin roll method for realizing a shape close to a product shape at the casting stage and an in-machine reduction method for rolling in the solidification step have been proposed.

In all of the above processes, it is indispensable to establish measures for preventing internal cracks and segregation such as V segregation and center segregation. Especially under internal pressure of a continuous casting machine, internal cracks and segregation are prevented from occurring. The establishment of technology to maximize the amount of reduction is an important issue.

[0012]

By the way, in order to prevent segregation such as V segregation and center segregation, it is necessary to reduce the concentrated molten steel so that it does not accumulate. It is necessary to control the strain at the solid-liquid interface caused by the stress below the critical strain possessed by the material. Conventionally, Sugiya et al. [Iron and Steel, 68
(1982), A149], the type of formula (4) is well known.

[0013]

## EQU4 ## ε = 1.15 × 3d × δ × 100/1 ² (4) where ε: rolling strain, d: shell thickness, δ: rolling amount, 1: roll pitch

According to the equation (4), the rolling strain under roll rolling increases as the roll pitch decreases and the rolling amount increases. In order to reduce the rolling strain, the shell thickness and the roll pitch during rolling are increased to reduce the rolling strain. The conclusion is that the amount should be small, and the term of roll pitch does not match the actual situation.

In the above equation (3), internal cracking does not occur,
It is not possible to study how to secure as large a reduction amount as possible by using a large number of rolls with a limited reduction zone length. Therefore,
It is necessary to establish a reduction method that reduces the segregation and internal cracks to a shape as close as possible to the product.

The present invention has been made to solve the above problems, and provides a continuous casting method for preventing segregation and internal cracking.

[0017]

The gist of the present invention is as follows.

The present invention is characterized in that, in a continuous casting method for molten metal in which a cast slab is drawn out while being pressed down, the rolling down amount Δh of each roll is determined so as to fall within the range of formulas (5), (6) and (7). This is a continuous casting method that prevents segregation and internal cracking.

[0019]

(Equation 5) However, β = tan ⁻¹ [ld / (R + D−Δh / 2)] ld = (R · Δh) ^0.5

[0020]

[Equation 6] εc = 5.09 + 23.8 · f ΔT ≦ 0 ° C. (6)

[0021]

[Equation 7] εc = 4.55−0.76 · ΔT ΔT> 0 ° C. (7)

In the above equation, εc: Cracking limit average rolling reduction (%) of internal cracking, R: Rolling roll radius (mm), D: Shell thickness with solid phase ratio of 1 (mm), ld: Contact length ( mm), Δh: Reduction amount (mm), ΔT: Superheat in mold (° C), f: Display of melt temperature solid phase ratio in mold, β: Position of shell thickness D at position where roll and slab contact and roll It is the angle (in radians) formed by the line connecting the centers and the perpendicular to the center of the roll.

In the continuous casting method for preventing the above segregation and internal cracking, the diameter of each rolling roll is 50 mm to 350 mm.
A group of reduction rolls composed of two or more rolls was prepared, and the reduction amount of each roll was determined so as to fall within the range of formulas (5), (6) and (7), and the central solid fraction of the slab was determined. 0.3-0.
It is a continuous casting method characterized by producing internal slabs having a shape close to the product shape by preventing internal cracking and segregation due to rolling by rolling down to 6.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail with reference to the following embodiments.

The inventors of the present invention have studied the rolling behavior and internal cracking behavior of the slab under rolling, and the following formula (8) is established between the rolling force and the rolling amount of the slab. Have already been described [(Iron and Steel (1987), p. 20.
7)].

[0026]

[Equation 8] P = k (R · Δh) ^0.5 (8) However, P = reaction force, k: deformation resistance, R: reduction roll radius, Δh: reduction amount

The relationship of the above equation (8) is established by the fact that the rolling relationship in the drawing showing the deformation behavior of the slab based on the unsolidified rolling behavior of FIG. 1 is between the rolling behavior of the slab and the roll. It means that it is generally established. In FIG. 1, 3 is a rolling roll, 12 is a slab, R is a roll radius, D is a shell thickness, Δh is a reduction amount, and ld is a contact length.

Based on these results, the present inventor further researched the conditions for preventing the occurrence of internal cracks in a cast piece under a light pressure. Figure 2 shows the results of the maximum strain reduction experiment by the reduction of each thickness position using plastisine. Strain that occurs at the position where the distance of the plastisine from the surface layer is D and the position where the roll starts to contact with plastisine. It shows the relationship with the distance.

The generated strain at each position in the thickness direction has a maximum value,
The position where the maximum value is generated is closer to the contact start position between the roll and the plasticine (cast slab) in the surface layer, and is closer to directly below the axial center of the roll in the thickness center of the slab. The strain that actually leads to internal cracking is considered to be the maximum strain thus generated.

The present inventor has studied a simple index which can easily express such strain in relation to equipment conditions such as roll diameter. As a result, the maximum strain generated in the shell thickness D of the cast slab is shown in FIG. According to the present invention, it was found that there is a good correlation with the average strain ε of the shell thickness D shown in the following equation (9), as shown in the relationship between the maximum strain measured at each thickness position and the average strain based on the geometrical shape. Accomplished

[0031]

[Equation 9] Where β = tan ⁻¹ [ld / (R + D−Δh / 2)] ld = (R · Δh) ^0.5

In the above equation (9), ε: average rolling reduction (%), R: radius of the rolling roll (mm), D: solid fraction 1
Shell thickness (mm), ld: contact length (mm), Δh: reduction amount (mm), β: position of shell thickness D at the position where the roll and the slab come into contact with the line connecting the roll center and the roll center It is the angle (radian) formed by the perpendicular.

FIG. 3 also plots the relationship between the critical strain at which cracking occurs in single-point straightening bending of carbon steel and the average rolling strain ε calculated by equation (9) when rolling reduction occurs at light pressure. doing. A good correlation is observed between the actual strain generated at the distance D from the surface layer including the result of plasticin and the average rolling strain ε at the shell thickness D calculated by the equation (9).

Therefore, the amount of reduction of each roll capable of preventing internal cracking can be calculated by the equation (9) in relation to the equipment conditions such as the diameter of the reduction roll. FIG. 4 is a drawing showing the relationship between the number of rolls that can be inserted per unit length of cast slab and the roll diameter. The smaller the roll diameter, the larger the number of rolls that can be inserted into the unit length.

FIG. 5 shows the mold 1 of the experimental apparatus shown in FIG.
After cooling the slab with spray 2 directly under the water,
It is a figure which shows the relationship between the measured cracking limit reduction amount and the superheat of the molten metal 4 in the mold 1 which arrange | positioned the reduction roll 3 of 00 mm.

FIG. 7 is a drawing showing the relationship between the crack limit average rolling reduction εc calculated by the above equation (5) using the rolling reduction amount and roll diameter shown in FIG. 5 and the superheat in the mold. is there. As the superheat in the mold is smaller, cracks are less likely to occur, the amount of reduction per roll can be increased, and the cracking limit average rolling reduction εc increases.

In particular, in the case of semi-solid casting in which the superheat in which the temperature of the molten metal in the mold is lower than the liquidus temperature is 0 ° C. or less, the cracking limit reduction amount per roll that does not cause cracking is significantly increased. The shape of the slab can be controlled with a small number of rolls.

In FIG. 8, as shown in FIG. 9, a plurality of reduction rolls 3 and 11 consisting of two or more are installed directly below the mold 1, and a cooling temperature adjusting water spray 7 is arranged between the reduction rolls. For the amount of reduction and roll diameter of each roll, refer to (5)
(6) 0.1% C when determined by the formula (7) and reduced
It is drawing which shows the influence of the central solid fraction of the position of 9 immediately after the completion | finish roll 11 of the reduction | decrease of the un-solidification reduction on the segregation of steel.

If the central solid fraction immediately after the final roll under the unsolidified pressure by a large number of rolls is less than 0.3, the slab 1
Segregation is observed in No. 2, and segregation can be prevented by setting the end point of the reduction of the final roll of the multiple rolls to 0.3 or more in the central solid fraction.

The reason why the center segregation can be improved is as follows.
As schematically shown in FIG. 10, the shell thickness of solid phase ratios 0.1 to 0.3 shown by the curves 13 and 14 instantly moves to the central portion due to the reduction, and the concentrated solidified steel is intensely accumulated in the central solid phase. Rate 0.
This is because the elapsed time of 1 to 0.4 can be shortened.

The segregation is observed by macro etching, and the shell thickness and solid fraction of the slab are calculated by calculating the temperature at each position by heat transfer calculation using the cooling water amount, the slab thickness and each physical property. The rate was calculated by the equation (10) using the calculated temperature at each position.

[0042]

[Equation 10] Solid fraction = (T-Tsl) / (T11-Tsl) (10) where T: Calculated temperature at each position calculated by heat transfer calculation Tsl: Solidus temperature of molten metal T11: Liquidus temperature of molten metal

[0043]

EXAMPLES Table 1 shows the quality and shape of the slab thus produced. According to the method of the present invention, it is possible to produce a slab having no segregation, no internal cracking, and a shape close to the product shape. The shape of the slab can be further adjusted by cutting the slab after adjusting the temperature of the slab with the water spray 10 shown in FIG. 9 installed after the above-mentioned internal pressure reduction, or continuously supplying it to the next step and rolling it as it is. Alternatively, it is heated in a heating furnace and then rolled to prepare.

[0044]

[Table 1]

[0045]

As described above, according to the continuous casting method of the present invention, in the continuous casting method of molten metal in which a slab is drawn while being pressed, the rolling reduction amount of each roll is a crack limit average rolling reduction determined by a predetermined formula. By rolling in the range of strain or less, there is no internal crack or segregation defect, and a slab with a shape close to the product shape can be manufactured by a simple method, and the rolling step can be omitted, and a high quality slab can be obtained. Can be manufactured.

[Brief description of drawings]

FIG. 1 is a drawing showing an outline of deformation behavior of a slab based on unsolidified rolling behavior.

FIG. 2 is a drawing showing the maximum strain due to rolling at each thickness position.

FIG. 3 is a diagram showing the relationship between the measured maximum strain at each thickness position due to rolling and the average strain based on the geometrical shape.

FIG. 4 is a diagram showing the number of rolls that can be inserted into a unit length.

FIG. 5 is a drawing showing the effect of superheat in a mold on the amount of reduction per roll capable of preventing internal cracking.

FIG. 6 is a drawing showing an experimental apparatus that clarifies the influence of superheat in a mold on the critical reduction amount at which internal cracking occurs.

FIG. 7 is a drawing showing the influence of superheat in a mold on the crack limit average rolling reduction εc calculated by equation (5) capable of preventing internal cracking.

FIG. 8 is a view showing the influence of the end solidification timing under the internal pressure on the segregation.

FIG. 9 is a drawing showing an experimental apparatus for conducting an experiment on the influence of internal pressure reduction of a multi-roll machine on segregation.

FIG. 10 is a drawing showing the reason why analysis can be improved by the method of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Water spray nozzle 3 Rolling roll 4 Molten metal 5 Shell thickness of solid phase ratio 1 6 Immersion nozzle 7 Water spray nozzle for slab temperature adjustment 8 Shell thickness of solid phase ratio 9 Central solid phase ratio immediately after final roll under internal pressure 10 Water spray nozzle for adjusting slab temperature 11 Final roll in machine 12 Slab 13 Shell thickness with solid phase ratio 0.1 14 Shell thickness with solid phase ratio 0.3

Claims (2)

[Claims] 1. In a continuous casting method for molten metal, in which a slab is drawn out while being rolled down, the rolling down amount Δh of each roll is (1)
(2) A continuous casting method for preventing segregation and internal cracking, which is determined so as to fall within the range of equations (3). [Equation 1] However, β = tan ⁻¹ [ld / (R + D−Δh / 2)] ld = (R · Δh) ^0.5 [Equation 2] εc = 5.09 + 23.8 · f ΔT ≦ 0 ° C. ………… (2) [ Mathematical Expression 3 εc = 4.55−0.76 · ΔT ΔT> 0 ° C. (3) In the above equation, εc: Cracking limit average rolling reduction of internal cracking (%) R: Rolling roll radius (mm ) D: Shell thickness with solid phase ratio of 1 (mm) ld: Contact length (mm) Δh: Reduction amount (mm) ΔT: Superheat in mold (° C) f: Melt temperature solid phase ratio display in mold β: With roll Angle formed by the line connecting the shell thickness D at the position where the slab comes into contact with the roll center and the perpendicular to the roll center (radian) 2. The diameter of each rolling roll is 50 mm to 350.
mm, a set of reduction rolls consisting of two or more multi-rolls was made, and the reduction amount of each roll was determined to fall within the range of formulas (1), (2), and (3), and the central solid fraction of the slab was determined. Is 0.3-
The continuous process for preventing segregation and internal cracking according to claim 1, wherein the production of a cast product having a shape close to that of the product is prevented by preventing internal cracking and segregation due to the reduction by pressing down to 0.6. Casting method.