JPH0669606B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention prevents segregation of impurity elements found in the center of thickness of continuously cast slabs, that is, in the case of steel slabs, sulfur, phosphorus, manganese and the like. The present invention relates to a continuous casting method capable of obtaining a homogeneous metal.

[Prior art and problems to be solved by the invention]

In recent years, offshore structures, storage tanks, steel pipes for oil and gas transportation,
The requirements for material properties such as high-strength wire rods are becoming more and more severe, and it is an important issue to provide homogeneous steel products. Originally, steel should be homogeneous in cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate and partially concentrate during the casting process. Becomes vulnerable. Particularly in recent years, continuous casting has been generally used for the purpose of improving productivity, yield, and energy saving, but a remarkable segregation of components is usually observed in the thickness center of the slab obtained by continuous casting. .

The above-mentioned component segregation significantly impairs the homogeneity of the final product,
Reduction in stress is desired because it causes serious defects such as cracks caused by stress acting on steel in the use process of products and the drawing process of wire rods. Such component segregation occurs when the residual molten steel flows at the final stage of solidification due to solidification shrinkage force and the like, the concentrated molten steel near the solid-liquid interface is washed out, and the residual molten steel progressively concentrates. Therefore, in order to prevent the segregation of the components, it is important to eliminate the cause of the flow of the residual molten steel.

Such molten steel flow causes include, in addition to the flow caused by solidification shrinkage, the flow caused by slab bulging between rolls and roll alignment irregularity, but the most serious of these is solidification shrinkage, In order to prevent segregation, it is necessary to roll down the slab by an amount that compensates for this.

Attempts have been made to reduce segregation by rolling down the slab, and in the continuous casting process, the solidification shrinkage of the slab is compensated during the period from the liquidus temperature to the solidus temperature of the slab center temperature. A method is known in which the rolling is carried out at a constant rate equal to or more than the amount to be worked.

[Problems to be solved by the invention]

However, the conventional continuous casting method has a problem that the segregation improving effect is hardly recognized depending on the conditions, and in some cases, segregation rather deteriorates, and it is difficult to sufficiently improve the component segregation. .

As a result of various investigations on the cause of the problem of the conventional method, the present inventors find that the segregation improving effect is not recognized in the case of the conventional method, or the segregation rather deteriorates. It was found that it was due to improper coagulation timing and its range.

The present inventor previously mentioned in JP-A-62-275556,
The area from the time when the central part of the slab reaches the temperature corresponding to the solid fraction of 0.1 to 0.3 to the temperature corresponding to the flow limit solid fraction of 0.5 mm / min or more and less than 2.5 mm / min per unit time The continuous casting method in which the rolling is continuously reduced in a ratio, and the region from the time when the temperature of the slab center reaches the solid phase limit to the solidus temperature is not substantially reduced Proposed.

Furthermore, the present inventor has proposed a further advanced continuous casting method by assuming some formulas from a large number of experimental results and comparing them with the experimental results.

An object of the present invention is to prevent the segregation of the impurity element found in the thickness center portion of the continuously cast slab and obtain a homogeneous metal.

[Means for Solving the Problems]

According to the present invention, there is provided a continuous casting method for molten metal in which a cast piece is continuously drawn out, and a thickness center solid fraction of the cast piece is 0.35.
To 0.40, one or more rolls are installed in a temperature range corresponding to 0.40 to 0.40, and the reduction treatment is performed. The reduction treatment increases the reduction force value as it goes downstream of the reduction zone where the central solid fraction of the slab increases. However, there is provided a continuous casting method, characterized in that the slab is treated.

[Action]

According to the continuous casting method of the present invention, one or more rolls are installed in a temperature range corresponding to the thickness center solid phase ratio of 0.35 to 0.40, and the rolling treatment is performed on the slab. In this rolling reduction, the rolling force value is increased toward the downstream of the rolling zone where the central solid fraction of the slab increases, and the slab is treated.

As a result, it is possible to prevent the segregation of the impure element found in the center of the thickness of the continuously cast slab and obtain a homogeneous metal.

〔Example〕

First, an example of a continuous casting machine to which the continuous casting method according to the present invention is applied will be schematically described with reference to FIG.

FIG. 1 is a diagram showing a continuous casting machine to which the continuous casting method according to the present invention is applied, specifically, an example of a twin cast arc type continuous casting machine. As shown in the figure, in this continuous casting machine, the ladle 1 filled with molten metal is placed above the tundish 2, and the molten steel in the ladle 1 passes through the sliding nozzle 11 at the bottom into the dundish 2. It is designed to be poured. Here, the opening of the sliding nozzle 11 is controlled according to the weight of the entire tundish 2 containing the molten steel poured from the ladle 1 so that the meniscus (the position of the molten metal in the tundish) M becomes constant. ing.

The molten steel in the tundish 2 is poured into the mold 3 at a constant rate by vertically moving the stopper 21 that closes the bottom of the tundish. The bottom of the mold 3 is also opened, and the molten steel injected into the mold 3 is cooled by the side wall of the mold 3 to which cooling water is supplied and solidified (primary cooling) from the outside. The molten steel (cast slab) primarily cooled by the mold 3 is continuously drawn out by the roller.

The slab pulled out from the mold 3 is spray-cooled in a spray band (spray roll) SR, and is further bent by a plurality of (No. 1 to No. 5) group rolls GR and pinch rolls PR to a rolling band. It is designed to be supplied. Here, for the No. 2 group role,
EMS (Electro Magnetic Stirrer) is installed,
At this position, the slab is electromagnetically stirred.

In the continuous casting machine to which the continuous casting method of the present invention is applied, the thickness center solid fraction of the slab is at a position where the temperature becomes a temperature corresponding to 0.25 to 0.50 in the rolling zone of the continuous casting machine (from roll R ₄₃ to roll R ₅₃ . Position), the reduction process (light reduction) is performed. In particular, the position where the central solid fraction of the slab is about 0.35 to 0.4 is matched with the draft zone, that is, the position where the central solid fraction of the slab is about 0.35 is the draft zone of roll R ₄₃ or roll R ₄₄ . By using a small number of rolls, such as rolls R ₄₃ and R _{44, with} a small rolling down force (hydraulic pressure), it is possible to prevent the segregation of impurities in the center of the continuous cast slab by setting it near the start end position of be able to.

Hereinafter, the continuous casting method of the present invention will be described in detail.

Light reduction is very effective in improving bloom segregation. It has been clarified that it is necessary to optimize the amount of reduction within an appropriate reduction range in order to reduce segregation to the maximum by light reduction. However, in order to further reduce segregation to the limit and obtain a slab without segregation, it is necessary to quantitatively grasp the effect of improving segregation in relation to the time and amount of reduction of each roll. A casting speed change test was carried out for the purpose of the above quantification, and a central segregation formulation model formula was created by analyzing the data.

Casting conditions of the analyzed test The continuous casting machine that carried out the test is a twin cast arc type (first
Table 1 shows the casting conditions of the test analyzed in Fig. 2), and Fig. 2 shows the relationship between the maximum segregated grain size (thickness center portion) observed in the etch print of the obtained slab and the casting speed.

Analysis method The present inventor is based on a laboratory experiment in which Cu is added as a tracer at the final stage of solidification using a deformed mold that can reveal segregation in a lab, and the concentration of concentrated molten steel that is the cause of segregation is severe.
The present inventors have come to the knowledge that it is a relatively limited solidification timing range after the liquid passage resistance at the center of the thickness of the slab has increased. In the present invention, in order to quantify the accumulation timing and amount of concentrated molten steel, analysis was performed by the following method.

1) Blocking of solidification timing and material balance formula according to elapsed time from mold For the above-mentioned reasons, the solidification timing is divided into blocks based on the elapsed time from mold as a scale, and the material balance formula of each block is connected to the slab. A macroscopic linked substance balance equation (hereinafter abbreviated as the substance balance equation for central solute accumulation) showing the solute mass accumulated in the central part of the thickness of was investigated. Using this material balance equation for central solute accumulation as a fitting equation, a central segregation formulation model was created by multiple regression, and based on this result, the effect of light reduction on the concentrated molten steel accumulation and central segregation at each solidification time Was evaluated. The optimization of the block division method was determined by the following method by multiple regression, and the coefficient of each term adopted the value in the optimum division method.

2) Optimization of block division method and determination method of coefficient If the division method is suitable for the actual situation of concentrated molten steel accumulation, the correlation coefficient between the measured value and the predicted value obtained from multiple regression is considered to be the maximum. , The block division method was determined so that the correlation coefficient between the predicted value and the actual measurement value was the maximum and did not contradict the fitting equation, and the coefficient was calculated.

Center segregation formulation Study of model formula 1) Central mass balance determining substance balance formula i) Melt mass accumulating in the center of thickness Light pressure zone is divided as in the example of Fig. 3 with the elapsed time from the mold as a scale, Consider the material balance of each block. Tj minutes from the mold (The distance from the mold is Vc · tj
(M) Residual molten steel cross section) If the cross section of the slab that has passed is taken as j section, the amount of solute components flowing into j section is given by equation (1).
Further, the solute flowing out from the j + 1 cross section after tj + 1 minutes from the mold is contained in the solidified phase solidified between the j cross section and the j + 1 cross section (hereinafter referred to as j block) and the residual molten steel phase, and the outflow component amount of each phase is (2 ), (3) can be shown. Therefore, the material balance of each block is given by equation (4). Since the amount of the component in the residual molten steel phase flowing out from the preceding block becomes the amount of the inflowing component of the next block, if these are connected, the amount of the component accumulated in the thickness center portion per unit time can be expressed by the equation (5).

Block inflow component amount (remaining molten steel) = ρ _l · (Vc + Uj) · Sj · Cj (g / min) (1) Block outflow component amount (remaining molten steel) = ρ _l · (Vc + Uj + 1) · Sj + 1 · Cj + 1 (g / min) )
(2) Block outflow component amount (solidification phase) = ρ _s · Vc · Ssj · ▲ · βsj (g / min) (3) Material balance of each block: ρ _l · (Vc + Uj) · Sj · Cj = ρ _l・ (Vc + Uj + 1) ・ Sj +
1 ・ Cj + 1 ＋ ρ _s・ Vc ・ Ssj ・ ▲ ▼ (4) Amount of component accumulated in the thickness center per unit time: Vc ・ Se ・ Ce = (Vc + U ₁ ) ・ S ₁・ C ₁ -M ・ Vc ・ ΣSsj ・ βs
j ・ ▲ ▼ (g / min) (5) Vc: Casting speed (cm / min) Uj: Residual molten steel flow rate (cm / min) ρ _s : Solidified phase density (g / cm ³ ) Sj: j Section residual molten steel area (Cm ² ) ρ _l : Molten steel density (g / cm ³ ) Solidification area in Ssj: j block (cm ² ) M = ρ _s / ρ _l Cj = j area Average component concentration of residual molten steel phase ▲ ▼: J block Average component concentration of solidification phase βsj: Average solid phase volume fraction of J block solidification phase Ce: Average component concentration in segregation Se: C cross-sectional area of segregation (cm ² ) To consider equation (5) in relation to light reduction Is ▲
▼, It is necessary to quantify Ssj. Therefore, first consider ▲ ▼.

ii) j Block solidification phase average concentration (▲ ▼) j Section solidification phase concentration (CSj) Based on the appropriate reduction time of the light reduction test and the results of the laboratory experiment of Cu addition by the deformed mold, the concentration time of the concentrated molten steel is slab It is presumed that the solid phase is generated in the center of the thickness of the steel, and the center segregation is due to the fact that the concentrated molten steel such as between dendrite trees and between the dendrite trees and between the branch dendrite trees are It is considered that it was generated by fluidized accumulation in a portion having a small liquid passage resistance such as V segregation. On the other hand, negative segregation zones are observed on the upper and lower surfaces of the center segregation as shown in FIG. The cause of this negative segregation is considered to be the result of washing between dendrite trees due to residual molten steel flow at the end of solidification, and the end-of-solidification stage (solidification contraction flow velocity ≈ 0.049 cm / sec and solidification rate ≈ 0.0039 cm / sec) was determined by the dendrite intertree washing model The effective partition coefficient is about 1, and the negative segregation near the center cannot be explained.
The cause of the negative segregation in the vicinity of the center is caused by the inventor described above.
Based on the results of the Cu addition laboratory experiment, it is estimated that the concentrated molten steel between the trees at the end of solidification was carried away to the thickness center by the solidification shrinkage suction force, and the concentrated molten steel flowing out from this tree was used as the central accumulation amount. This is considered to be the case, and the larger the solidification shrinkage amount, the larger the central portion accumulation amount of this concentrated molten steel. From these things Cs
It is estimated that j tends to decrease as the amount of concentrated molten steel flowing between dendrite trees increases, and the solidification phase concentration Csj in the j cross section is assumed as in equation (6).

Csj ＝ －Aj ・ Vj ＋ Csj ₀ (6) Solidification shrinkage velocity downstream of Vj: j section (cm ³ / min) Sj: j section residual molten steel area (cm ² ) Uj: j average residual molten steel flow velocity (cm) / Min) Csj ₀ : Uj = 0 j-section solidification concentration Next, let us examine whether the conventionally obtained knowledge can be explained by equation (6). According to the results of a Cu addition laboratory experiment using a deformed mold, the concentrated molten steel has a solid fraction of 0 at the thickness center of the slab.
Accumulates at temperatures lower than 15 equivalents. Based on this result, when the thickness center solid fraction is less than 0.15,
Even if the residual molten steel flows, the concentrated molten steel does not accumulate and Csj is Csj.
_It becomes ₀ , and it is estimated that Aj in equation (6) is close to zero. On the other hand, it is considered that Aj is large because the concentrated molten steel such as dendrite tree having a solid fraction in the center of thickness greater than 0.15 is likely to flow and accumulate due to solidification shrinkage suction force. In addition, it is estimated that Aj in Eq. (6) is close to zero even when the solidification progresses and the liquid resistance between the dendrite trees increases, because the concentrated molten steel does not accumulate. Thus, the difference in concentrated molten steel accumulation depending on the solidification time is given by the difference in the size of Aj in Eq. (6). Csj ₀ in Eq. (6) is Csj when the flow velocity of the residual molten steel in the j-section is zero, and the effective distribution coefficient is considered to be 1 when the flow velocity of the residual molten steel is zero, so Csj ₀ is the residual molten steel in the j-section. Average concentration
It is the same as Cj. This Cj has a very early coagulation time,
In the final stage of solidification, which is equal to the bulk concentration and where concentrated molten steel accumulates, it is estimated to be the average concentration of dendrite interstitial molten steel with a relatively high solid fraction. The analysis was performed assuming that Csj ₀ at each solidification time was almost constant in the case of the same component system. It should be noted that Vj in the equation (6) is the sum of solidification contraction speeds downstream of the j cross section and will be described later.

Average concentration of solidification phase of j block (▲ ▼) Based on the above examination, the average concentration of solidification phase of the j block was assumed to be equal to the solidification phase concentration of the cross section of the inlet and the outlet of the block, and was assumed as in the equation (8).

▲ ▼ = (Csj + Csj + 1) / 2 = {-(Aj · Vj + Aj
+ 1 · Vj + 1) + (Csj ₀ + Csj + 1 ₀ } / 2 (8) iii) Quantification of Ssj The solidified area Ssj in the j block can be calculated as the difference between the j cross section and the unsolidified area of j + 1. The unsolidified area Sj of the j cross section can be calculated by heat transfer calculation, and can be shown as a function of elapsed time from the mold by making a regression equation.

iv) Reduction of solidification shrinkage rate (Vj) by light reduction When there is no light reduction, the solidification shrinkage amount per unit time downstream from the j-section is the average residual steel flow velocity Uj of the j-section and the j-section area Sj.
It is considered to be the product of
The solidification contraction rate downstream of the j-section can be expressed by equation (9).

Vj = Sj ・ Vj ・ α (cm ³ / min) (9) Vc ・ α = Uj α = ρ _s −ρ _l ) / ρ _l ρ _s = 7.3, ρ _l = 7.0 (g / cm ³ ) Meanwhile, light pressure reduction If there is, the solidification shrinkage rate that causes the residual molten steel flow can be expressed by Eq. (10) because a part of the solidification shrinkage rate is reduced due to the movement of the solid-liquid interface due to light pressure reduction.

Vj = (Sj · Vc · α-vj) (cm ³ / min) (10) vj: Total volume of solid-liquid interface movement per unit time (cm ³ / min) downstream from the j-section under light pressure, where vj is j The total volume of solid-liquid interface movement per unit time generated by pressing down the cast piece with a roll downstream from the cross section can be expressed by equation (11) (hereinafter referred to as effective rolling down volume).

vj = (Σwi · ηi · Δhi) · Vc (cm ³ / min) (11) i: Roll # Δhi: i Roll reduction amount (mm / roll) ηi: Roll reduction efficiency wi: i Roll Unsolidified width (mm) Therefore, when there is a slight reduction, the solidification shrinkage rate that causes the residual molten steel flow in the j cross section can be expressed by equation (12).

Vj = Sj ・ Vc ・ α- (Σwi ・ ηi ・ Δhi) ・ Vc (cm ³ / mi
n) (12) Based on the results of i) ii) iii) iv), and since Uj in Eq. (5) is smaller than Vc, if Uj is omitted, the slab accumulates at the thickness center per unit length. The amount of ingredients is (13)
It can be shown by a formula, and if formula (8) is adopted as the value of Csj, formula (14) is obtained.

In Eqs. (13) and (14), the first term on the right-hand side is a value determined by determining the inlet cross section, and the second term on the right-hand side is the product of the concentration and the amount of solidification of each solidified block residual molten steel when there is no residual molten steel flow as described above. Is considered to be a constant because it is the sum of the values corresponding to, and is a constant including the first term and the second term on the right side. Including this constant, (1
If the coefficient of each term in Eqs. 2) and (14) can be determined, the center segregation can be calculated by giving the conditions of light reduction such as the amount of reduction of the slab by each roll and the roll arrangement. Complete. In Eqs. (12) and (14), Ssj and Sj and the unsolidified width (wi) can be calculated by heat transfer calculation, and βsj is a value that is determined if the time of solidification progress and solidified tissue are determined, and is estimated to be about 1. To be done. Further, the rolling reduction efficiency (ηi) can be calculated by elasto-plastic analysis using the high temperature physical properties of steel, and becomes smaller as the solidification progresses. The value of Δhi can be given by the equation (15) calculated from the rolling behavior of the cold pieces in this test. According to the above results, in equation (14)
Aj and Csj ₀ are unknown parameters, and other values can be found by calculation if casting conditions and light reduction conditions are determined.

Δhi = P ² / R ・ (Ki ・ Bi) ² Ki ・ Bi ＝ 9.06 (l / Vc) ^1.794 (15) P: Roll reaction force (kg) R: Roll radius (mm) Ki: Casting at i roll position One-sided deformation resistance (kg / mm ² ) Bi: Solid thickness of short side of cast piece at roll position (mm) li: i Distance from meniscus to i-roll (m) where coefficient of each term including Aj and For the constant, using equation (14) as a fitting equation, and using Vhi calculated from Δhi and uncoagulated width (wi) reduction efficiency (ηi) and Sj (calculated at solid fraction = 0.7) shown in equation (15), By performing multiple regression analysis on the results of casting speed change tests with various reduction times
Values other than Vj were evaluated and determined. The segregation used in the analysis was the maximum segregated grain size, not the component amount per 1 cm of the slab shown in equation (14).

If there is no light reduction in equations (12) and (14) (vj =
The amount of components accumulated in the center of 0) is given by equation (16).

Se ・ Ce = S ₁・ C ₁ -M ・ ΣSsj ・ βsj ・ (CSj ₀ + Csj + 1
₀ ) / 2 + M ・ ΣSsj ・ βsj ・ {Aj ・ Sj + Aj + 1 ・ Sj + 1)
・ Vc ・ α} / 2 (16) where M ・ Ssj ・ βsj ・ {Aj ・ Sj + Aj + 1 of each block
・ Sj + 1) · α} / 2 (underlined in Equation 16) is the component accumulation amount of the block when the light reduction is inside. The value obtained by dividing this value by the block time (hereinafter abbreviated as segregation aggregation index) is the unit casting speed when there is no light reduction, the amount of component accumulation per j block unit time, and the unknown parameter (Aj and Cs
If j ₀ ) can be determined, the degree of segregation and aggregation at each solidification time can also be evaluated by the size of the segregation and aggregation index.

2) Optimization of division method and determination of coefficient By applying multiple regression analysis of casting speed change test results using equation (14) as the fitting method, and
The coefficient of each term in the formula was calculated below.

i) Appropriate partitioning method of reduction range If the partitioning method is suitable for the actual condition of concentrated molten steel accumulation, it is considered that the correlation function between the actually measured maximum segregated grain size and the predicted maximum segregated grain size obtained by multiple regression is maximized. Therefore, the dividing method was selected so that the correlation between the actual measured value and the predicted value of the maximum segregated grains was maximized, and the results are shown in Table 2. Based on the results in Table 2,
The maximum segregated grain size can be expressed by equation (17). The maximum segregated grain size calculated by Eq. (17) agrees very well with the actually measured maximum segregated grain size, as described later (Fig. 5).

Here, subscript of V: elapsed time from mold (min) E: time required for solid fraction at the thickness center of 0.7 (minutes) 3) Relationship between rolling force, roll arrangement, casting speed, etc., and segregation In Eq. (17), the rolling by the rolling roll downstream of the rolling belt has an effect on the residual molten steel flow upstream of that, and segregation can be reduced by the amount of rolling by roll or roll. In order to show it as a relational expression with the roll arrangement etc., it is more convenient to separate the term of Vj from Vc and transform it as shown in Eq. (18), and the elapsed time from the mold is It can be generalized by showing the solid fraction at the center of thickness. By combining Eqs. (18), (15), and (19), it is possible to formulate the relationship between center segregation and light reduction equipment conditions such as casting speed, reduction force, roll arrangement, etc. Segregation calculation is now possible.

Here, subscript of v: central solid fraction of cast slab (fcj) vj: effective rolling volume between upper and lower fcj of v (cm ³ / min) wi: unsolidified width (cm), ηi: rolling efficiency, Δhi : i Roll reduction amount (cm), i: Roll No where Δhi is the above-mentioned formula (15), and li / Vc is the elapsed time from the mold.

Δhi ＝ Pi ² / Ri ・ (Ki ・ Bi) ² Ki ・ Bi ＝ 9.06 (li / Vc) ^1.794 (15) Pi: Reaction force of each roll (kg) Ri: Radius of each roll (mm) Ki: i Mold slab deformation resistance at roll position (kg / mm ² ) Bi: Slab solidified thickness at roll position (mm) li: Distance from meniscus to i roll (m) li = Lr + (N-Nr) ・Rp (19) Lr: Distance from meniscus to light reduction start roll (m) Nr: Light reduction start roll No N: Light reduction roll No Rp: Roll pitch (m) Distance li from meniscus to each roll is roll pitch To
If Rp, it becomes the formula (19). Fig. 5 shows the result of calculation of the relationship between the maximum segregation grain size and the casting speed in the casting speed change test using the model formula of the central segregation formula (18) in comparison with the measured data. It can be seen that the calculated value and the measured value agree very well.

4) Aj at each solidification time and accumulation time of concentrated molten steel The above-mentioned Aj and segregation aggregation index were calculated using the coefficients of each term in Eq. (17). As described above, the larger Aj is, the more segregated (the effective distribution coefficient is small), and the larger the segregation aggregation index is, the larger the aggregation amount of the concentrated molten steel per unit time is. The relationship between Aj and the solid fraction in the thickness center of the slab (hereinafter abbreviated as fsc) is shown in FIG. 6, and the relationship between the segregation aggregation index and fsc is shown in FIG. Aj tends to increase as coagulation progresses, but the reason for the behavioral change of Aj needs further study. On the other hand, the segregation agglomeration index is large when the solid center solid fraction (fsc) is 0.35 to 0.4, and the segregation agglomeration index is the largest when fsc is 0.37. When fsc is smaller or larger than this, the segregation aggregation index is small and 1 / of fsc is 0.37no
It is about 5 to 1/10, and it can be seen that the solidification time at which the concentrated molten steel is intensely accumulated is relatively narrow. Thickness center solid fraction (fsc)
The reason why the segregation aggregation index is smaller when it is smaller than 0.35 to 0.4 is that the residual molten steel flows in a relatively low solid fraction region and the flow accumulation of the concentrated molten steel between the high solid fraction high dendrite trees is small. it is conceivable that. The reason why the segregation aggregation index is small when fsc is larger than about 0.35 to 0.4 is that the solidification amount in the block decreases as the solidification progresses, and the liquid flow resistance between the dendrite trees increases.

In the above, the thickness center solid fraction of the slab is set to a rolling roll in a temperature range corresponding to 0.25 to 0.50, as the rolling treatment to be performed, especially the solidification shrinkage flow at the thickness center solid fraction of 0.35 to 0.4 can be prevented. Need to be reduced. In order to efficiently achieve this, it is preferable to reduce the concentration immediately after the concentrated molten steel is accumulated, that is, the thickness center solid fraction of the slab is around 0.35 to 0.4. Further, in the case of rolling down the rolling zone downstream of the rolling zone where the central solid fraction of the slab increases, the rolling-down efficiency decreases, so the rolling force value may be increased and rolled down the downstream side of the rolling zone. For example, the slab increases the reduction force value of each reduction roll according to the time required from the meniscus of the mold to each reduction roll, or the slab has a position from the meniscus of the mold to each reduction roll. It can be configured to increase the pressure value of each reduction roll.

〔The invention's effect〕

As described above in detail, the continuous casting method according to the present invention,
The thickness center solid fraction of the slab is set to one or more rolls in a temperature range corresponding to 0.35 to 0.40, and the rolling treatment for the slab is performed downstream of the rolling zone where the central solid fraction of the slab increases. By increasing the rolling force value in accordance with the above, it is possible to prevent the segregation of the impurity element found in the central portion of the thickness of the continuously cast slab and obtain a homogeneous metal.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a continuous casting machine to which the continuous casting method according to the present invention is applied, FIG. 2 is a diagram showing a relationship between segregation in a cast piece and a casting speed, and FIG. 3 is a diagram showing the continuous casting machine. Fig. 4 is a diagram for explaining a method of dividing the light reduction zone, Fig. 4 is a diagram showing negative segregation around the thickness center of the cast piece, and Fig. 5 is a diagram showing the relationship between the calculated maximum segregated grain size and the casting speed in the cast piece. Fig. 6 is a diagram showing the relationship between the parameter Aj and the solidification timing of the slab, and Fig. 7 is a diagram showing the accumulation timing of the concentrated molten steel in the slab. (Explanation of symbols) 1 ... Ladle, 2 ... Tundish, 3 ... Mold, 11 ... Sliding nozzle, 21 ... Stopper.

Claims (3)

[Claims] 1. A continuous casting method for molten metal, wherein a slab is continuously drawn out, wherein the slab has a thickness center solid fraction of 0.35 to
Install one or more rolls in the temperature range equivalent to 0.40,
The rolling treatment is carried out, and the rolling treatment is such that the rolling force value is increased as it goes downstream of the rolling zone where the central solid fraction of the slab increases, and the slab is treated. Continuous casting method. 2. The reduction treatment for the cast slab is such that the reduction force value of each of the reduction rolls is increased according to the time required for the slab to reach from each meniscus of the mold to each of the reduction rolls. The continuous casting method according to item. 3. The rolling reduction treatment for the slab is performed by increasing the rolling force value of each rolling roll according to the position of the slab from the meniscus of the mold to each rolling roll. The continuous casting method described.