JP2885880B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention prevents the segregation of impurity elements found in the center of the thickness of continuous casting, that is, in the case of steel slab, sulfur, phosphorus, manganese, etc. The present invention relates to a continuous casting method capable of obtaining a suitable metal.

[Conventional technology]

In recent years, requirements for material properties of marine structures, storage tanks, steel pipes for oil and gas transportation, high-strength wires, and the like have become more stringent, and providing a homogeneous steel material has become an important issue. Originally, steel materials should be homogeneous within the cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, which segregate and partially concentrate in the casting process, and steel Vulnerable. In particular, in recent years, the continuous casting method has been widely used for the purpose of improving productivity, yield improvement, energy saving, and the like. However, remarkable component segregation is usually observed in the center of the thickness of a slab obtained by continuous casting. .

The above-mentioned segregation of components significantly impairs the homogeneity of the final product, and causes serious defects such as cracks due to stress acting on the steel during the use process of the product or the wire drawing process of the wire rod. I have. Such component segregation is caused by the residual molten steel flowing due to the solidification shrinkage force or the like at the end of solidification, washing out the concentrated molten steel in the vicinity of the solid-liquid interface, and the residual molten steel progressively becoming concentrated. Therefore, in order to prevent segregation of components, it is important to remove the cause of the flow of the residual molten steel. Causes of such flow include slab bulging between rolls and improper roll alignment, in addition to solidification shrinkage,
The most significant of these is solidification shrinkage, and to prevent segregation, it is necessary to reduce the slab by an amount that compensates for this.

Attempts to improve segregation by rolling down the slab have been made in the past, and in the continuous casting process, the solidification shrinkage of the slab was reduced until the temperature of the slab center reached the liquidus temperature to the solidus temperature. It is known as a method of reducing the pressure at a fixed rate greater than the amount to be compensated.

[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 or the segregation worsens in some cases, and it is difficult to sufficiently improve the component segregation.

The present inventors conducted various investigations on the cause of the problem of the conventional method, and found that the effect of improving segregation was not recognized or the segregation was worsened because the solidification time to be reduced and the range thereof were basically inappropriate. I figured out why.

In Japanese Patent Application Laid-Open No. 62-275556, the area from the point in time when the center of the slab reaches a temperature corresponding to the solid fraction of 0.1 to 0.3 to the point in time when the temperature reaches the temperature corresponding to the flow limit solid fraction is unit time The area from the point when the center of the slab reaches the temperature corresponding to the flow limit solid fraction to the temperature at the solidus line is substantially reduced by continuously lowering at a rate of 0.5 mm / min or more and less than 2.5 mm / min. There is disclosed a continuous casting method in which no reduction is applied to the steel.

Furthermore, as a result of promoting a number of experiments, the inventor quantified the reduction timing and reduction amount of each roll to ensure the effect of improving segregation by light reduction, and controlled the reduction timing and reduction amount to be appropriate. Realized that it was essential.

[Means for solving the problem]

The gist of the present invention is that in a continuous casting method of molten metal, in which a slab is drawn down by at least one pair of rolls at the end of solidification, two or more sets of rolls in which slabs having different solidification times are respectively reduced are taken as one combination. And a and m are determined using the total rolling amount of each set so that A in equation (1) is minimized, and the short side solidification width Bi of each roll calculated from equation (4) The continuous casting method is characterized in that the rolling solidification timing of each roll is quantified by the value of, and the position of the rolling roll is changed so as to obtain an appropriate rolling timing.

A = Σ (K ′ · B′−KB) ² (1) K · B is given by equation (2), and K ′ · B ′ is given by equation (3).

P: Roll reaction force (kg), K: Deformation resistance (kg / mm ² ), B: Short side solidification width (mm) 2 × D, D: Short side solidification thickness (mm), R: Roll radius (mm) , ΔH: Roll reduction amount of each roll or total reduction amount of each group (mm), Subscript i: Roll number, t: Elapsed time (minute) from mold meniscus K ・ B: Average value of multiple rolls [ Action] The present inventor has found that, in the case of a 300 × 500 mm slab, the relationship of the formula (2) is established between the total reduction amount of the multiple rolls and the roll reaction force. Based on this result, if bulging or the like can be ignored, the rolling reduction for each roll is (5)
It becomes an expression. Short-side solidification width at i-roll position in equation (2)
Bi is a function of the time elapsed from the meniscus, and Bi = A ·
It can be approximated as ti ^M.

Δhi = (Pi ² / Ri) · (1 / Ki · Bi) ² (5) ΔH = ΣΔhi P: roll reaction force (kg), K: deformation resistance (kg / mm ² ), B: short side solidification width ( mm) 2 × D, D: Short side solidification thickness (mm), R: Roll radius (mm), Δh: Reduction amount per roll (m
m), subscript i: each roll No., K: average deformation resistance of rolling zone (kg / mm ² ) Therefore, the relationship between Ki / Bi at each roll position and solidification time (ti) is approximated as in equation (4). Assuming that the rolling force (roll reaction force) and the roll radius of each roll of the multiple rolls are constant, the KB in the case of the multiple rolls represented by Expression (1) is ti.
And can be expressed by equation (3). On the other hand, since the actual measurement KB for a large number of rolls can be calculated back from the measured amount of reduction of each segment by the formula (1), (2), (3)
The constants a and m in the equation can be determined by minimizing the sum of the squares of the difference between the actually measured KB and K'B 'expressed by the equation (3). If the values of a and m become clear, Ki and Bi at each roll position can be determined by equation (4) using the time ti required to move from the mold meniscus of the slab to the roll. In the equation (4), if the value of Ki is determined in advance by Bi calculated by the heat transfer calculation and the equation (4) calculated by the present method, Bi can be calculated using a, m, and Ki. The short-side solidification width Bi measured and calculated in this way can be converted into the thickness center solid phase ratio fs of the slab from the relationship between Bi calculated in advance by heat transfer calculation and the thickness center solid phase ratio fs of the slab. According to the present invention, it is possible to quantitatively grasp the rolling timing of each rolling roll under light rolling, and by using the value of the rolling solidification timing of each roll, the casting speed and cooling conditions fluctuate during casting. Even so, it is possible to stably obtain a homogeneous steel material without segregation by changing the rolling roll so that the rolling timing is appropriate.

〔Example〕

Example 1 An outline of a continuous caster on which a test was performed is shown in FIG. 1, and a typical example of a molten steel composition cast is shown in Table 1. The test continuous caster is of a segment reduction type. The amount of reduction of the number of rolls was measured by measuring the displacement of the frame 3 with a dial gauge 5 for three continuous segments as shown in FIG. The obtained K
B is shown in equation (6) as a function of the elapsed time t from the mold. The short side solidification width Bi is calculated using the equation (6) and the relation between the previously measured K and t (7). Bi is (6),
The Bi can be calculated using the equation (7), and Bi can be converted into the thickness center solid phase ratio of the casting side from the relationship between Bi calculated by the heat transfer calculation shown in FIG. 3 and the thickness center solid phase ratio.

K · B = 27.2 t ^1.62 (6) K = 11.3 · t ^0.15 (7) FIG. 4 shows the relationship between the thickness center solid fraction of the slab measured as described above and the position of each reduction roll.

Example 2 The outline of the continuous caster and the molten steel composition in which the test was performed is the same as in Example 1. In this test, the solidification structure was improved by electromagnetic stirring, and the upper surface equiaxed crystal ratio was 5% or more. The slab thickness center solid phase ratio of the reduction zone entrance roll measured by this method is 0.1
The rolling segment was changed by changing the rolling oil pressure of the rolling segment so as not to be smaller. Cut samples of the slab thus cast were taken at a pitch of 7 m in the casting direction, and the segregation level of the slab is shown in FIG. 5 in comparison with the present method and the conventional method. It can be seen that the present method eliminates a slab portion having poor segregation as compared with the conventional method, and a stable slab without segregation can be obtained.

Example 3 The outline of the continuous caster and the molten steel composition in which the test was performed is the same as in Example 1. In this test, there is no equiaxed crystal due to high-temperature casting, and the equiaxed crystal ratio of the upper surface is 0%. The rolling roll was changed by inserting and removing spacers between the rolls of the rolling roll so that the slab thickness center solid phase ratio of the rolling band entrance roll measured by this method did not become smaller than 0.25. FIG. 6 shows the segregation level of the slab obtained by the present method in comparison with the conventional method. It can be seen that the present method has less variation in the segregation in the direction of casting of the slab than the conventional method, eliminates the slab portion with poor segregation, and stably obtains a homogeneous slab without segregation.

〔The invention's effect〕

As described above, the reduction solidification timing of each reduction roll is determined by the present method, and the position of the reduction roll is changed so that the reduction timing is appropriate. Is obtained stably.

[Brief description of the drawings]

FIG. 1 is a view schematically showing a continuous caster in which an experiment was performed, FIG. 2 is a view schematically showing a measurement method, and FIG. 3 is a view showing a relationship between a short-side solidification width and a thickness center solid phase ratio of a slab. Fig. 4 is a diagram showing the measured center solid fraction of the thickness of the slab at each roll position, and Figs. 5 and 6 are diagrams showing a comparison of the segregation level between the conventional method and the present method. 1 ... mold, 2 ... segment, 3 ... frame, 4 ... post, 5 ... dial gauge, 6 ... displacement gauge installation position.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Hideaki Gotota 1 Kimitsu, Kimitsu City, Chiba Prefecture Inside the Kimitsu Works, Nippon Steel Corporation (56) References JP-A-1-271047 (JP, A) (58) Field surveyed (Int.Cl. ⁶ , DB name) B22D 11/00-11/22

Claims (1)

(57) [Claims] In a continuous casting method for a molten metal, in which a slab is drawn down while being rolled down by at least one pair of rolls at the end of solidification, two or more sets of rolls each of which rolls down slabs having different solidification times are set. A set of rolls is prepared, and a and m are determined using the total reduction amount of each set so that A in equation (1) is minimized, and the short-side solidification width Bi of each roll calculated from equation (4) is determined. A continuous casting method characterized in that the rolling solidification timing of each roll is quantified based on the value, and the position of the rolling roll is changed so as to obtain an appropriate rolling timing. A = Σ (K ′ · B′−KB) ² (1) K · B is given by equation (2), and K ′ · B ′ is given by equation (3). P: Roll reaction force (kg), K: Deformation resistance (kg / mm ² ), B: Short side solidification width (mm) 2 × D, D: Short side solidification thickness (mm), R: Roll radius (mm) , ΔH: Roll reduction amount of each roll or total reduction amount of each set (mm), Subscript i: Roll number, t: Elapsed time from mold meniscus (minutes)