JPH02299754A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent segregation of impurity elements at center part of thickness in a cast slab by executing rolling reduction treatment when solid phase ratio at the center part in the cast slab becomes the specific value in the continuous casting for molten metal continuously drawing the cast slab. CONSTITUTION:In a twin casting type continuous casting machine, the rolling reduction zone is matched with the position where the solid phase ratio of the center part in the cast slab becomes about 0.35. That is, the position where the solid phase ratio of the slab becomes 0.35 is positioned near the starting position of rolling reduction zone of the roll R43 or R44. By this method, with only a few rolls of the roll R43, R44, etc., and with little rolling reduction force (hydraulic pressure), the segregation of impurities at the center part in the continuously cast slab can be prevented.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention prevents the segregation of impurity elements found in the center of the thickness of continuously cast slabs, such as sulfur, phosphorus, and manganese in the case of steel slabs. This invention relates to a continuous casting method that can produce homogeneous metal.

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

In recent years, steel pipes for offshore structures, storage tanks, oil and gas transportation,
Requirements for material properties such as high-tensile wire rods are becoming increasingly strict, and providing homogeneous steel materials has become an important issue. Originally, steel should be homogeneous in its cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate and become partially concentrated during the casting process. becomes vulnerable. Particularly in recent years, continuous casting methods have become popular for purposes such as improving productivity and yield and saving energy, but noticeable component segregation is usually observed at the center of the thickness of slabs obtained by continuous casting. Ru.

The above-mentioned component segregation significantly impairs the homogeneity of the final product.
Stress acting on steel during the product use process and the wire drawing process can cause serious defects such as cracks, so there is a strong desire to reduce this. Such component segregation occurs when the residual molten steel flows at the final stage of solidification due to solidification contraction force, washes out the concentrated molten steel near the solid-liquid interface, and the residual molten steel progressively becomes concentrated. Therefore, in order to prevent component segregation, it is important to eliminate the cause of residual molten steel flowing.

Causes of such molten steel flow include flow caused by solidification shrinkage, as well as flow caused by slab bulging between rolls and roll misalignment, but solidification shrinkage is also an important cause of the amount of steel. In order to prevent segregation, it is necessary to reduce the slab by an amount that compensates for this.

Attempts have been made to improve segregation by compressing the slab, which involves solidifying and shrinking the slab during the period when the temperature at the center of the slab reaches from the liquidus temperature to the solidus temperature during the continuous casting process. A method is known in which the compensation is reduced by a certain percentage on the volume sales.

[Problem to be solved by the invention]

However, with conventional continuous casting methods, there are problems such as hardly any segregation improvement effect being observed depending on the conditions, and in some cases, segregation may even worsen, making it difficult to sufficiently improve component segregation. .

The present inventors have conducted various investigations into the causes of such problems in the conventional method, and have found that the reason why the conventional method does not have an effect on improving segregation or even worsens segregation is that the pressure It was discovered that this was caused by inappropriate coagulation timing and range.

The present inventor previously disclosed in Japanese Unexamined Patent Publication No. 62-275556 that the solid phase ratio in the center of the slab is 0. The area from the time when the temperature corresponds to 0.3 to 0.5 mm per unit time to the time when the temperature corresponds to the flow limit solid fraction is 0.5 mm.
The area from the time when the center of the slab reaches a temperature corresponding to the flow limit solid fraction to the solidus temperature is substantially We proposed a continuous casting method that does not require excessive rolling reduction.

Furthermore, the present inventor has proposed a more advanced continuous casting method by making several assumptions based on the results of numerous experiments and comparing them with the results of the experiments.

An object of the present invention is to obtain a homogeneous metal by preventing the segregation of impurity elements found in the center of the thickness of a continuously cast piece.

[Means to solve the problem]

According to the present invention, there is provided a method for continuous casting of molten metal in which a slab is continuously drawn, wherein the central solid fraction of the slab is 0.25.
Provided is a continuous casting method characterized in that the slab is subjected to a reduction treatment when the value becomes 0.50 from 0.50. 1. It is preferable that the reduction treatment for the slab be performed when the central solid fraction of the slab is about 0.35. Further, it is preferable that the rolling treatment for the slab is performed by increasing the rolling force value toward the downstream of the rolling zone where the center solid fraction of the slab increases, and rolling the slab at a constant rolling rate. .

[For production]

According to the continuous casting method of the present invention, the central solid fraction of the slab is 0.
．． 25 to 0.50, the slab is subjected to rolling treatment. The reduction treatment for this slab is particularly
It is preferable to carry out this process when the central solid fraction of the slab is about 0.35. Further, the rolling force value of the slab increases as it goes downstream of the rolling zone where the central solid fraction of the slab increases,
The slab is processed to be rolled down at a constant rolling reduction rate.

Thereby, it is possible to prevent the segregation of impurity elements found in the center of the thickness of the continuously cast slab, and to 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 explained with reference to FIG.

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, specifically a twin cast circular arc type continuous casting machine. As shown in the figure, in this continuous casting machine, a ladle 1 filled with molten steel is placed above a tundish 2, and the molten steel in the ladle 1 is poured into a sliding nozzle 1 at the bottom.
1 and then poured into tundish 2. Here, the opening degree of the sliding nozzle 11 is controlled according to the entire weight of the tundish 2 containing the molten steel poured from the ladle 1, so that the meniscus (the scene position in the tundish) M is kept constant. There is.

The molten steel in the tundish 2 is controlled by moving the stopper 21 that closes the bottom of the tundish upward and downward.
It is injected into the mold 3 at a constant rate. The bottom of the mold 3 is also open, and the molten steel injected into the mold 3 is cooled on the side wall of the mold 3 to which cooling water is supplied, and is solidified from the outside (secondary cooling). The molten steel (slab) that has been primarily cooled by the mold 3 is continuously drawn out by rollers.

The slab pulled out from the mold 3 is spray cooled in spray zones (spray rolls) S, R, and -
Furthermore, a plurality of (Nα1 to Nα5) group roles G,
R, and pinch rolls P and R1 to bend it and supply it to the rolling band. Here, N
o, 2 group courses include E M S (Ele
A magnetic smoother (ctro Magnetic Smoother) is provided to perform electromagnetic stirring of the slab at this position.

In the continuous casting machine to which the continuous casting method of the present invention is applied, the position where the central solid fraction of the slab is 0.25 to 0.50 is the rolling zone of the continuous casting machine (position from roll R43 to roll R93). Rolling down treatment (light rolling down) is performed in such a way that the In particular, the position where the central solid phase ratio of the slab is approximately 0.35 is aligned with the rolling zone, that is, the position where the central solid phase ratio of the slab is approximately 0.35 is the starting point of the rolling zone of roll Rss or R44. If it is placed close to the position, only a small number of rolls such as roll R43* R44 are required, and a small rolling force (hydraulic pressure) is required.
This can prevent the segregation of impurities in the center of continuously cast slabs.

The continuous casting method of the present invention will be explained in detail below.

Light reduction is very effective in improving bloom segregation.

It has become clear that in order to reduce segregation to the maximum extent possible through light reduction, it is necessary to optimize the amount of reduction within an appropriate reduction range. However, in order to further reduce segregation to the utmost limit and obtain a slab without segregation, it is necessary to quantitatively understand the segregation improvement effect in relation to the rolling timing and rolling amount of each roll. For the purpose of quantifying the above, we further analyzed the casting speed change test data reported in the previous report and created a model formula for center segregation formulation.

Casting conditions for the analyzed tests Table 1 shows the casting conditions for the tests analyzed using a twin-cast arc type continuous caster (see Figure 1), and the maximum segregated grain size observed in the etch print of the obtained slab ( Figure 2 shows the relationship between the thickness (center part) and casting speed.

Analysis methodBased on the appropriate rolling timing of the light rolling test and the Cu addition laboratory experiment using a modified mold, center segregation occurred between the dendrite trees at the final stage of solidification, and concentrated molten steel between equiaxed grains occurred between the dendrite trees, etc. It is thought that the liquid flowed through areas with low flow resistance and accumulated in the center of the slab. In the present invention, the following method was used to quantify the accumulation timing and amount of concentrated molten steel.

■] Blocking the solidification period based on the elapsed time from the upper mold and material balance method Divide the solidification period into blocks based on the elapsed time from the mold, and connect the material balance formula for each block to achieve the thickness center of the slab. We examined the connected substance balance equation (hereinafter referred to as the substance balance equation for central solute accumulation) that indicates the amount of solute accumulated in the central part, used the substance balance equation for central solute accumulation as the fitting equation, and used multiple regression to formulate the central segregation formulation. A model formula was created. The appropriateness of the block division method was determined by multiple regression using the following method, and the coefficients of each term adopted the values in the optimal division method.

2) Optimization of the block division method and method for determining the coefficient It is thought that the magnitude of the residual molten steel flow during the solidification period (block) where the accumulation of concentrated molten steel is most intense has a large correlation coefficient with center segregation, and the division method If it is adapted to the actual situation of concentrated molten steel accumulation, the correlation coefficient between the actual measured value and the predicted value obtained from multiple regression is considered to be the maximum, so the block division method is used to maximize the correlation coefficient between the predicted value and the actual measured value. Therefore, the coefficient was determined and calculated so as to be consistent with the formula of Ateihome.

Examination of center segregation formulation model equation 1) Center solute mass determining substance balance equation i) Thickness Divide the solute mass that accumulates at the center of the light pressure zone using the time elapsed from molding as in the example in Figure 3, Examine the material balance of each block. tj minutes from the mold (distance from the mold is Vctj
(m) residual molten steel cross section) If the elapsed slab cross section is j cross section, then j
The amount of solute components flowing into the cross section is expressed by equation (1). In addition, the solute flowing out from the j+l cross section after t j+1 minutes has passed from the mold is contained in the solidified phase a and the residual molten steel phase between the j cross section and j±1 cross section (hereinafter referred to as j block), and the outflow components of each phase are The amount can be expressed by equations (2) and (3). Therefore, the material balance of each block is expressed by equation (4). The amount of components in the residual molten steel phase flowing out from the previous block becomes the amount of components flowing into the next block, so if these are connected, the amount of components accumulated at the center of thickness per unit time can be expressed by equation (5).

Block inflow component it (residual molten steel) = ρff1 (Vc + Uj) ijl:j (g/m1n
) (1) Block outflow component amount (residual molten steel) = pA ・(Vc+Uj+1) −5j+1 ・C
j+1 (g/m1n) (2) Block outflow component amount (
Solidification phase) 1ρ, l/cSsj-Yoi-Ssj (g/m1n)
(3) Material balance of each block: pH・(Vc+1Ij)・5j−Cj=CJI・
(Vc+LIj+1) ・Sj+1・Cj+1+ρ,・
Vc-Ssj -Ce 汀 (4) Amount of component accumulated per unit time at the center of thickness: Vc-5e-Ce=(Vc+IJ
,) ・S+ ・C, −M ic・ΣSsj・β5j−
Csj (g/m1n) (5) vc: Casting speed (Cm/m1n) Uj: Residual molten steel flow rate (Cm/m1n) ρ,: Solidification phase density (g/c) Sj: J cross-sectional residual molten steel area (C- 111) ρl: Molten steel density (g/c1B) Ssj: Solidification area in j block (d) M = (ρ, /ρ
1) Cj = j average component concentration of cross-sectional residual molten steel phase Csj: j average component concentration of block solidified phase βsj: j average solid phase volume fraction of block solidified phase Ce: Segregative average component concentration Se: Segregation C cross-sectional area ( cTM) To consider equation (5) using the convention of light pressure, Csj,
Quantification of Ssj is necessary. First, let's consider Ishi J.

1f) Average concentration of solidified phase in j block (6 pieces) ■Concentration of solidified phase in j cross section (CsD Based on the appropriate rolling down period of the light rolling test and the results of the Cu addition laboratory experiment using dumbbell molds, the accumulation period of concentrated molten steel is determined by the slab Center segregation, which is estimated to occur at the end of solidification when a solid phase occurs at the center of the thickness, occurs when concentrated molten steel flows and accumulates in areas with low liquid flow resistance, such as between dendrite trees and branched dendrite trees. On the other hand, negative segregation bands are observed on the upper and lower surfaces of the central segregation, as shown in Figure 4.

The cause of this negative segregation is thought to be the result of cleaning between the dendrite trees by the flow of residual molten steel at the final stage of solidification, and the dendrite cleaning model was used to calculate the final stage of solidification (solidification contraction flow rate "io",
049cm/sec, and solidification rate #0.0039cm/
5ea)', the effective partition coefficient becomes approximately 1, and the negative segregation in the vicinity of the central segregation cannot be explained. The cause of this negative segregation zone near the center is estimated to be traces of the concentrated molten steel between the trees being carried away to the thickness center by the suction force of solidification shrinkage at the final stage of solidification, based on the results of the Cu addition laboratory experiment by the authors mentioned above. It is thought that this concentrated molten steel flowing out from between the trees corresponds to the amount accumulated in the center, and the amount of concentrated molten steel accumulated in the center increases as the amount of solidification shrinkage increases. From these facts, it is estimated that Csj tends to become smaller when there is a large amount of concentrated molten steel flowing between dendrite trees, etc., and the solidified phase concentration C at cross section j
sj was assumed as shown in equation (6).

Csj =Aj-Vj +C5jo
(6) Vj = Sj - Uj (satisfied when the fluid flow resistance between dendrite trees is small) (7) Vj: Solidification contraction rate downstream of the j cross section (cn! / m
i n ) Sj: Residual molten steel area of j cross section (, ff1) U
j: average flow velocity of residual molten steel at j cross section (C1117m1n) C
Let us examine whether the knowledge conventionally obtained in the j-section solidified phase concentration method of sj, : Uj = O can be explained by equation (6). According to the appropriate reduction timing through casting speed change tests and the results of Cu addition laboratory experiments using dumbbell molds, the solid phase ratio of concentrated molten steel at the center of the thickness of the slab is approximately 0.25 to 0.
Accumulate in the range of 50. Based on this result, when the solid fraction at the center of thickness is less than 0.2, even if the residual molten steel flows, the concentrated molten steel does not accumulate and Csj becomes C5jo (
6) It is estimated that Aj in the equation is close to zero. On the other hand, when the solid fraction at the center of thickness is 0.25 to 0.50, concentrated molten steel such as between dendrites tends to flow and accumulate due to the suction force of solidification contraction.
Aj is considered to be large. Furthermore, even if the solidification progresses and the resistance to liquid flow between the dendrite trees increases, the concentrated molten steel does not accumulate and it is estimated that Aj in equation (6) is close to zero. In this way, the difference in concentrated molten steel accumulation depending on the solidification period is given by the difference in the magnitude of Aj in equation (6). Also (6)
Csj in Eq.

is Csj when the flow velocity of the residual molten steel in the j cross section is zero. Since the effective distribution coefficient is considered to be 1 when the residual molten steel flow velocity in the j cross section is zero, Csj is the same as the average concentration of residual molten steel in the j cross section Cj. This Cj is estimated to be equal to the bulk concentration when the solidification stage is very early, and is estimated to be the average concentration of the molten steel between dendrites, which has a relatively high solid phase ratio, when the solidification stage is at the final stage when concentrated molten steel accumulates. The analysis was performed on the assumption that Csj at each solidification stage is approximately constant in the case of the same component system. Note that Vj in equation (6)
is the sum of solidification shrinkage speeds downstream of the j cross section, which will be described later.

■J block average concentration of solidified phase (drunk) Based on the above study, the average solidified phase concentration of J block was assumed to be the average of the solidified phase concentrations at the block entrance and exit cross sections, as shown in equation (8).

C5j=(Csj+Csj+1)/2=((Aj-Vj
+Aj+iVj+1)+(Csjo +Csj+1o)
) / 2 (8) ij ) Quantification of Ssj The solidified area Ssj within 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 expressed as a function of the elapsed time from the mold by using a regression equation.

tv) Reduction of solidification shrinkage rate (Vj) due to light reduction When there is no light reduction, the amount of solidification shrinkage per unit time downstream of section j is the average flow velocity of residual molten steel at section j, Uj, and the cross-sectional area of j, Sj.
Since it is considered to be the product of

Vj = Sj −Vc ・tx (cIIY/!l1
n) (9) Vc・α=Uj α
=(ρs pi!-)/pitρ, =7.3,
ρf=7.0 (g/2d) On the other hand, when there is a substantial reduction, the solidification shrinkage rate, which causes residual molten steel flow, is partially reduced due to the movement of the solid-liquid interface due to the light reduction (10
) can be shown by the formula.

Vj= (Sj-Vc ・ct-vj) (d/In
1n) (10) ■j: Total solid-liquid interface movement volume per unit time downstream from the j cross section under light pressure (cn/m
1n) Here, vj is the sum of the solid-liquid interface movement volume per unit time, which is generated by rolling down the slab from the j cross-section with downstream rolls, and can be expressed by equation (11) (hereinafter abbreviated as effective rolling volume). ).

vj=(Σ-1・ii・Δhi)・Vc (cffl
/m1n) (11) i: roll # Δhi: i roll reduction amount (+nm/roll) ηi: i
Reduction efficiency with rolls wi: unsolidified width with i rolls (mm) Therefore, when there is a light reduction, the solidification shrinkage rate that causes residual molten steel flow in the j cross section can be expressed by equation (12).

Vj=Sj−Vc・ct−(Σ−1・ii・Δhi
)l/c(ci!/m1n) (12) Based on the results of the above i) ii) 1ii) iv), and since Uj in equation (5) is smaller than Vc, if Uj is omitted, then per meter of slab. The amount of components accumulated at the center of the thickness can be expressed by equation (13), and if equation (8) is adopted as the value of Csj, equation (14) is obtained.

5e-Ce-S1・C1-M・ΣSSj・βsj-Drunkenness.

+M -ΣSsj・β5j-Tr7vT (57m)
(13) Se-Ce-S1・C1-M・ΣSSj・Ss
j・(C3jo+Csj+1)/2 +M・ΣSS
j・Ssj・((Aj−Vj +Aj +1 ・Vj
+1) ) / 2 (57m) (14) In equations (13) and (14), the first term on the right side is a value determined by determining the population cross section, and the second term on the right side is the value obtained when there is no residual molten steel flow as described above. Since it is the sum of the values corresponding to the product of the concentration of the solidified block residual molten steel and the amount of solidified steel, it is considered to be a constant.
It becomes a constant including the first and second terms on the right side. If the coefficients of each term in equations (12) and (14), including this constant, can be determined, center segregation can be calculated by giving light reduction conditions such as the amount of reduction of the slab by each roll and the arrangement of the rolls. The segregation model formula is completed. (12), (
In formula 14), Ssj, Sj and unsolidified width (wi)
can be calculated by heat transfer calculation, and Ssj is a value determined once the solidification progress period and solidification structure are determined, and is estimated to be about 1. Further, the rolling efficiency (li) can be given by equation (15), where the value of Δhi was calculated from the rolling behavior of the cold piece in this test. According to the above results, in equation (14), Aj and Csj are unknown parameters, and the other values can be obtained by calculation once the casting conditions and light reduction conditions are determined.

Δhi=P2/R・(Ki-Bi) 2Ki-Bi=
9.06(j!/Vc) '・7q4(15) P: Roll reaction force (kg) R: Roll radius (1wl) Ki:
Slab deformation resistance at roll position (kg/dish 2) Bi:i
Solidified thickness of slab short side at roll position (111111) li:
The distance from the meniscus to the i-roll (m), where Aj
The coefficients and constants of each term including Δhi and the unsolidified width (
wi) Using Vj calculated from rolling efficiency (li) and Sj (calculated with solid fraction = 037), values other than Vj were calculated by performing multiple regression analysis on the results of casting speed change tests in which the rolling timing was variously changed. The evaluation has been decided. The segregation used in the analysis was not the component amount per meter of slab shown in equation (14), but the maximum segregated grain size.

Note that in equations (12) and (14), when there is no light reduction (vj = O), the amount of component accumulation at the center is expressed as equation (16).

Se −Ce=S, −C, −M−ΣSsj・Ssj・
(Csjo+Csj+1o)/2+M・ΣSs゛・s”
A'S' 18° + to needle top u-vc - color L7 imaginary (16) Here M-Ssj of each block
・Ssj・ ((Aj−Sj+Aj+1・Sj+1)・
α)/2 (underlined in formula 16) is the component accumulation amount of the block when there is no light reduction. The value obtained by dividing this value by the block time (hereinafter abbreviated as the segregation agglomeration index) is the unit casting speed in the absence of light reduction, the amount of component accumulation per block unit time, and the unknown parameter (Aj and Csj
If o) can be determined, the degree of segregation and aggregation at each solidification stage can also be evaluated based on the magnitude of the segregation and aggregation index.

2) Optimizing the division method and determining the coefficients By performing multiple regression analysis on the casting speed change test results using equation (14) as the fitting formula, we determined the division method of the rolling zone and (1)
4) The coefficients of each term in the formula were calculated as follows.

i) Appropriate method of dividing the reduction range If the dividing method is suitable for the actual situation of accumulation of concentrated molten steel, it is thought that the correlation coefficient between the measured maximum segregated grain size and the predicted maximum segregated grain size obtained by multiple regression will be maximum. Therefore, the dividing method was selected to maximize the correlation coefficient between the measured value and the predicted value of the maximum segregated grains, 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). (17
) The maximum segregated grain size calculated using the formula (Figure 5) is in very good agreement with the measured maximum segregated grain size, as described later (Figure 5).
0.07078V30. S+0.15485Vff4
(17) Vj=Sj 4c・α−(Σgai・li・Δ
hi) l/cVc (am/m1n) Here, the subscript of ■: elapsed time from the mold (win
) E: Time required to reach thickness center solid fraction of 0.7 (minutes) 3) Relationship between reduction blade, roll arrangement, casting speed, etc. and segregation In equation (17), the reduction by the downstream roll of the reduction zone is the It also affects the flow of residual molten steel, and in order to express segregation as a relational expression with the reduction amount of each roll, roll arrangement, etc., it is convenient to separate the VcO term from Vj and transform it as shown in Equation 18). Furthermore, it can be generalized by showing the solid phase percentage at the center of thickness of the slab, which indicates the progress of solidification of the time elapsed from the molding. (18), (15), (
By combining Equations 19), it was possible to formulate the relationship between center segregation and light reduction equipment conditions such as casting speed, reduction blade, roll arrangement, etc., and it became possible to calculate segregation under various light reduction conditions.

Maximum segregated grain size = -29,311+0.87617 ・
Vc −(1,767・10−” ・vj = (Σ−
1・wi・Δhi)Vc Vc (cm/m1n) Here, the subscript ■: Central solid fraction of slab (fcj) vj:
Effective reduction volume between the upper and lower fcj of v (ctl/m i
n) wi: Unsolidified width (C11), ηi: Rolling efficiency,
Δhint Roll reduction amount (C111), i: Roll N.

Here, Δhi is the equation (15) described above, and j2i/Vc is the elapsed time from the molding.

Δh i = P i ” / Ri・(Ki-Bi)
"K1-B1 = 9.06 (l i/Vc)"
”' (15) Pi: Reaction force of each roll (k
g) R1: Radius of each roll (-) Ki: Slab deformation resistance at i roll position (kg/mm")
Bi: Solidified thickness of slab short side at i roll position (llffl
)21: Distance from meniscus to i-roll (m)l
i = Lr + (N-Nr) ・Rp
(19) Lr: Distance from the meniscus to the light reduction start roll (m) Nr: Light reduction start roll NO N: N of the light reduction roll.

Rp: Roll pitch (m) The distance 21 from the meniscus to each roll is expressed by equation (19), where Rp is the roll pitch. The relationship between the maximum segregated grain size and the casting speed in the casting speed change test was calculated using the center segregation formulation model equation (18), and the results are shown in FIG. 5 in comparison with the measured data. It can be seen that the calculated values and the measured values match very well.

4) Aj of each solidification period and accumulation period of concentrated molten steel (F)
Aj and the segregation aggregation index described above were calculated using the coefficients of each term in the equation. Here, as mentioned above, the larger Aj is, the easier it is to segregate (the effective distribution coefficient is smaller), and the larger the segregation aggregation index is, the larger the amount of agglomeration of concentrated molten steel per unit time is. The relationship between Aj and the solid fraction at the center of slab thickness (hereinafter abbreviated as fsc) is shown in FIG. 6, and the relationship between the segregation agglomeration index and fsc is shown in FIG. It is recognized that Aj tends to gradually increase as solidification progresses, but the reason for the change in behavior of Aj needs to be investigated in the future. On the other hand, the segregation aggregation index is very large, with a thickness center solid fraction (fsc) of around 0.35. When fsc is smaller or larger than 0.35, the segregation agglomeration index is small and is about 1/20 of that when fsc is 0.35, and it is estimated that the solidification period when concentrated molten steel is intensely accumulated is relatively narrow. Thickness center solid fraction (fsc) is 0.
The reason why the segregation agglomeration index is small when it is 35 or less is thought to be because the residual molten steel flows through areas with a relatively low solid fraction, so there is little flow accumulation of concentrated molten steel between the high solid fraction dendrite trees where the degree of concentration is severe. It will be done.

Moreover, the reason why the segregation aggregation index is small when fsc is larger than 0.35 is thought to be because the amount of solidification within the block decreases as solidification progresses, and also because the liquid flow resistance between dendrite trees increases.

In the above, the central solid fraction of the slab is 0.25 to 0.5
0, the rolling reduction treatment to be carried out is to increase the rolling force value toward the downstream of the rolling zone where the center solid fraction of the slab increases, and to roll the slab at a constant rolling rate. . For example, the rolling force value of each rolling roll may be increased depending on the time required for the slab to reach each rolling roll from the meniscus of the mold, or the rolling force value of each rolling roll may be increased depending on the position of the slab from the meniscus of the mold to each rolling roll. It can be configured to increase the pressure value of each reduction roll.

〔Effect of the invention〕

As detailed above, the continuous casting method according to the present invention includes:
By performing the reduction treatment on the slab when the central solid fraction of the slab is from 0.2 to 0.5,
A homogeneous metal can be obtained by preventing the segregation of impurity elements found in the center of the thickness of continuously cast slabs.

[Brief explanation of the drawing]

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

Claims (5)

[Claims] 1. A method for continuous casting of molten metal in which a slab is continuously drawn out, wherein the central solid fraction of the slab is from 0.25 to 0.50.
A continuous casting method characterized in that, when the slab is rolled, the slab is subjected to a reduction treatment. 2. 2. The continuous casting method according to claim 1, wherein the reduction treatment of the slab is performed when the central solid fraction of the slab is about 0.35. 3. The rolling process for the slab is performed by increasing the rolling force value toward the downstream of the rolling zone where the central solid fraction of the slab increases, and rolling the slab at a constant rolling rate. The continuous casting method according to claim 1. 4. 4. The continuous rolling method according to claim 3, wherein the rolling process for the slab is performed such that the rolling force value of each rolling roll is increased in accordance with the time required for the slab from the meniscus of the mold to each rolling roll. Casting method. 5. The continuous casting method according to claim 3, wherein the rolling process 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. .