JPH0573507B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention is aimed at preventing 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, and obtaining a homogeneous metal. This relates to a continuous casting method that can be used. [Prior art and problems to be solved by the invention] In recent years, requirements for material properties for offshore structures, storage tanks, steel pipes for oil and gas transportation, high-tensile wire rods, etc. have become more severe, and it is becoming increasingly difficult to obtain homogeneous steel materials. Providing this information 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 in the center of the thickness of slabs obtained by continuous casting. . The above-mentioned component segregation significantly impairs the homogeneity of the final product and causes serious defects such as cracking due to stress acting on the steel during the product usage process and wire drawing process, so there is an urgent need to reduce it. There is. Such component segregation occurs when the remaining 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 remaining molten steel progressively becomes concentrated. Therefore, in order to prevent component segregation, it is important to eliminate the cause of the flow of residual molten steel. 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 the most important cause of these is solidification shrinkage. 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 pressure is reduced at a constant rate greater than the amount to be compensated. [Problems to be Solved by the Invention] However, the conventional continuous casting method has problems such as hardly any segregation improvement effect being observed depending on the conditions, and in some cases, segregation may even worsen. It has been 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 causes segregation to worsen is basically due to pressure. It was determined that this was caused by an inappropriate coagulation timing and range. The present inventor previously disclosed in Japanese Patent Application Laid-Open No. 62-275556 that from the point in time when the center of the slab reaches a temperature corresponding to a solid fraction of 0.1 to 0.3, to the point in time when the temperature corresponds to the flow limit solid fraction. area per unit time
Continuous rolling is performed at a rate of 0.5 mm/min or more and less than 2.5 mm/min, and 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 apply reduction. Furthermore, based on numerous experimental results, the present inventor has found that
By assuming several formulas and comparing them with the experimental results, we have come to propose a more advanced continuous casting method. 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 slab. [Means for Solving the Problems] According to the present invention, the region from the time when the central solid fraction of the slab reaches a temperature corresponding to 0.1 to 0.3 to the time when the temperature corresponds to the flow limit solid fraction is , a continuous casting method for molten metal in which a plurality of pairs of rolls having a hydraulic reduction mechanism are used to continuously reduce and draw the molten metal, characterized in that the reduction force value of the hydraulic reduction is increased in two or more stages as the slab solidifies. A continuous casting method is provided. [Operation] According to the continuous casting method of the present invention, the hydraulic reduction mechanism is operated from the time when the central solid fraction of the slab reaches a temperature corresponding to 0.1 to 0.3 to the time when the temperature corresponds to the flow limit solid fraction. It is continuously rolled down by a plurality of pairs of rolls. The reduction force value of the hydraulic pressure is increased in two or more stages as the slab solidifies. Here, it is preferable that the rolling force value of the hydraulic pressure is increased in three stages as the solidification of the slab progresses, and the rolling force value at each stage is set to the following (1), (2), and (3). Each rolling force value is defined as follows, and the following inequalities (4) and (5)
It is preferable to increase it so as to satisfy (6) or (6). P ₀ : Hydraulic pressure reduction force value when the central solid fraction is 0.1 to 0.3 (1) P ₁ : Hydraulic pressure reduction force value when the central solid fraction is 0.3 to 0.5
(2) P ₂ : Hydraulic pressure reduction force value when the central solid fraction is 0.5 to 0.7
(3) P ₀ <P ₁ <P ₂ (4) 20<P ₀ <50 50≦P ₁ <90 90≦P ₂ <200 [Ton/Roll] (5) or 2.0<P ₀ ² /R<10 10.0≦P ₁ ² /R＜33 33≦P ₂ ² /R＜168 (6) P: Roll reaction force (Ton) R: Roll radius (mm) [Example] First, refer to Figure 1 and read the book. An example of a continuous casting machine to which the continuous casting method according to the invention is applied will be schematically explained. In order to reduce segregation, the key is to prevent solidification shrinkage flow at the final stage of solidification, and the solid phase ratio in the center of the slab is
In order to prevent solidification shrinkage flow in the range of 0.1 or 0.3 to 0.7, it is necessary to increase the amount of reduction (reduction speed) toward the downstream of the reduction zone. The present invention is provided as a simple method for achieving this. FIG. 1 shows a continuous casting machine to which the continuous casting method according to the present invention is applied, and specifically shows an example of 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 the tundish 2 through a sliding nozzle 11 at the bottom. It is designed so that it can be removed. 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 (molten metal level position in the tundish) M is constant. being done. The molten steel in the tundish 2 is injected into the mold 3 at a constant rate by controlling the vertical movement of a stopper 21 that closes the bottom of the tundish. 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 (primary cooling). The molten steel (slab) temporarily cooled by the mold 3 is continuously drawn out by rollers. The slab pulled out from the mold 3 is, for example,
It is spray cooled in a spray zone, bent by a plurality of group rolls and pinch rolls, and then fed to a light reduction zone. The light reduction zone consists of multiple reduction rolls R ₄₅ , R ₄₆ ,
It consists of R ₄₇ ,... Further, in FIG. 1, rolls R ₄₂ , R ₄₃ , R ₄₄ have their rolling force values (hydraulic rolling force value) limited (controlled) by the same hydraulic system, and rolls R ₄₅ , R ₄₆ , R ₄₇ , R ₄₈ , For R ₄₉ and R ₅₀ , the rolling force value is limited by the same hydraulic system, and the roll
R ₅₁ , R ₅₂ , and R ₅₃ are designed so that their reduction force values are limited by the same hydraulic system. And roll
The rolling force values of R ₄₅ to _{R 50} are made larger than those of rolls R ₄₂ , R ₄₃ , and R ₄₄ , and rolls R ₅₁ ,
The rolling force values of R ₅₂ and R ₅₃ are set larger than the rolling force values of rolls R ₄₅ to _{R 50} . Here, the light reduction zone refers to the reduction zone (roll (R ₄₂ ), R ₄₃ , R ₄₄ ,
R ₄₅ , R ₄₆ , . . ), the area includes reduction rolls R ₄₅ , R ₄₆ , R ₄₇ , . . . that perform light reduction on the slab. That is, it is configured such that the value of the rolling force under hydraulic pressure increases in three stages as the solidification of the slab progresses. Here, the rolling force value of each of the above three stages is
It is best to define (1), (2), and (3) and limit each rolling force value (tons/roll) to satisfy the following inequality (4), (5), or (6). preferable. P ₀ : Hydraulic pressure reduction force value when the central solid fraction is 0.1 to 0.3 (1) P ₁ : Hydraulic pressure reduction force value when the central solid fraction is 0.3 to 0.5
(2) P ₂ : Hydraulic pressure reduction force value when the central solid fraction is 0.5 to 0.7
(3) P ₀ <P ₁ <P ₂ (4) 20<P ₀ <50 50≦P ₁ <90 90≦P ₂ <200 [Ton/Roll] (5) or 2.0<P ₀ ² /R<10 10.0≦P ₁ ² /R＜33 33≦P ₂ ² /R＜168 (6) P: Roll reaction force (Ton) R: Roll radius (mm) Here, the roll at the unbending point
The roll upstream from R ₄₂ , i.e. roll R ₄₁ ,
R ₄₀ , R ₃₉ , . . . are not subjected to hydraulic pressure, but the reduction amount is limited by a spacer. Tests conducted using the continuous casting method of the present invention using the continuous casting machine shown in FIG. 1 will be described below. here,
The feature of this light reduction is that the hydraulic system of the reduction roll is divided into three (or two) parts, and the roll hydraulic system of each divided block is made the same. In addition, overpressure is prevented by inserting a spacer between the bearings of the rolls upstream of the rolling zone. Table 1 shows the composition of the molten steel tested, and compared with the conventional method in which the molten steel composition shown in Table 1 is cast using the continuous casting machine shown in Fig. 1, and the rolling force of all rolls in the rolling zone is constant. did. The size of the slab was 300 x 500 mm, and details of this example are shown in Table 2. Here, in Table 2, for example, roll No. 45 corresponds to roll R ₄₅ . In addition, in Table 2, roll Nos. 40', 41', and 42' are roll R ₃₉ (roll No. 39) and roll R ₄₀ (roll No. 40).
It shows the role inserted between.

【table】

[Table] The 1/2 width cross section of the slab cast in this manner was corroded to investigate the occurrence of segregation, and the results are shown in Table 3 in comparison with the conventional method. The grain size of the maximum segregated grains observed in half the width of the slab was smaller in this method than in the conventional method, proving that the present invention is a superior light reduction method to the conventional method.

〔Effect of the invention〕

As described above in detail, the continuous casting method according to the present invention increases the thickness center of the continuously cast slab by increasing the rolling force value of hydraulic rolling in two or more stages as the slab solidifies. It is possible to obtain a homogeneous metal by preventing the segregation of impurity elements found in the parts.

[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. (Explanation of symbols) 1...Ladle, 2...Tundish, 3...Mold, 11...Sliding nozzle, 21...Stopper.

Claims (1)

[Claims] 1. A region from the time when the center solid fraction of the slab reaches a temperature corresponding to 0.1 to 0.3 to the time when the temperature corresponds to the flow limit solid fraction A method for continuous casting of molten metal in which a pair of rolls is used to continuously reduce and pull out the molten metal, characterized in that the reduction force value of the hydraulic reduction is increased in two or more stages as the slab solidifies. . 2. The continuous casting method according to claim 1, wherein the reduction force value of the hydraulic pressure reduction is increased in three stages as solidification of the slab progresses. 3 Define the rolling force values at each stage as shown in (1), (2), and (3) below, and each rolling force value can be calculated using the following inequality (4).
The continuous casting method according to claim 2, which satisfies (5) or (6). P ₀ : Hydraulic pressure reduction force value when the central solid fraction is 0.1 to 0.3 (1) P ₁ : Hydraulic pressure reduction force value when the central solid fraction is 0.3 to 0.5
(2) P ₂ : Hydraulic pressure reduction force value when the central solid fraction is 0.5 to 0.7
(3) P ₀ <P ₁ <P ₂ (4) 20<P ₀ <50 50≦P ₁ <90 90≦P ₂ <200 [Ton/Roll] (5) or 2.0<P ₀ ² /R<10 10.0≦P ₁ ² /R＜33 33≦P ₂ ² /R＜168 (6) P: Roll reaction force (Ton) R: Roll radius (mm)