JPH0422551A - Continuous casting method - Google Patents

Abstract

PURPOSE:To stably obtain a cast slab having uniform quality without any segregation by setting two or more sets of roll pairs differ in the solidified times, using the total rolling reduction quantity of each set to quantify the rolling reduction solidified time for each roll and changing the rolling reduction roll. CONSTITUTION:Two or more sets of the roll pairs differing in the solidified times are set. The constants (a) and (m) in the equations 2, 3 are decided so that the actual value K B, the total of square of difference of K B shown in the equation 3 and A in the equation 1 become the min. By deciding (a) and (m), the rolling reduction solidified time to each roll is quantified with value of solidified width Bi in short side to each roll calculated from the equation 4 and the rolling reduction roll is charged so as to become the suitable rolling reduction time. By this method, the rolling reduction solidified time to each rolling reduction roll is decided, and by changing position of the rolling reduction roll so as to make the rolling reduction time suitable, the cast slab having uniform quality without any segregation can be stably obtd.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention aims to eliminate impurity elements found in the center of thickness of continuous casting,
In other words, in the case of steel slabs, the present invention relates to a continuous casting method that can prevent the segregation of sulfur, phosphorus, manganese, etc., and can obtain homogeneous metal.

[Prior art] In recent years, steel pipes for offshore structures, storage tanks, oil and gas transportation,
The requirements for material properties such as high-tensile wire rods are becoming more and more stringent, and it is important to provide homogeneous steel materials.Originally, steel materials should be homogeneous in cross section, but steel is generally It contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate during the casting process and become partially concentrated, making the steel brittle. 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,
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. This component segregation is caused by the residual molten steel flowing due to solidification contraction force at the final stage of solidification, washing out the concentrated molten steel near the solid-liquid interface, and the remaining molten steel becoming progressively concentrated (
caused by Therefore, in order to prevent component segregation, it is important to eliminate the cause of the flow of residual molten steel.

Causes of such flow include solidification shrinkage, slab bulging between rolls, roll alignment irregularities, etc. Of these, the most important cause is solidification shrinkage, and to prevent segregation, it is necessary to compensate for this. It is necessary to reduce the slab by the same amount.

Attempts have been made in the past to improve segregation by reducing the slab, and in the continuous casting process, the solidification shrinkage of the slab is suppressed during the period when the temperature at the center of the slab reaches from the liquidus temperature to the solidus temperature. It is known as a method 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, conventional continuous casting methods have problems such as hardly any segregation improvement effect being observed depending on the conditions, and in some cases, segregation may even worsen. It was difficult to improve.

As a result of various investigations into the causes of problems in the conventional method, the present inventors found that the reason why no segregation improvement effect was observed or the segregation worsened was basically due to inappropriate solidification timing and range of reduction. I discovered that it was for a reason.

JP-A No. 62-275556 has already described the process from the time when the center of the slab reaches a temperature corresponding to a solid fraction of 0.1 to 0.3 to the time when the temperature corresponds to the flow limit solid fraction. Area per unit time 0.5mm/min or more2.
Continuous reduction at a rate of less than smm/min, and substantially no reduction in the area from the time when the center of the slab reaches a temperature corresponding to the flow limit solid fraction until it reaches the solidus temperature. A continuous casting method is disclosed.

Furthermore, as a result of numerous experiments, the inventor of the present invention quantified the rolling timing and rolling amount for each roll to ensure the segregation improvement effect of light rolling, and controlled the rolling timing and rolling amount to be appropriate. I realized that it is essential to do so.

[Means for Solving the Problem] The gist of the present invention is to provide a continuous casting method for molten metal in which a slab is pulled out while being compressed by at least one pair of rolls at the final stage of solidification. The feature is that 2 sets, preferably 3 or more sets of rolls are made, each set having 1 or more rolls, and the rolling solidification time of each roll is quantified using the total rolling reduction amount of each set, and the rolling rolls are changed. This is a continuous casting method. In this case, a and m are determined so that A in equation (1) is minimized, and (4)
It is preferable to quantify the roll-down solidification time for each roll based on the value of the short side solidification width B1 for each roll calculated from the formula, and to change the roll-down roll so that the roll-down time is appropriate.

A= (K'・B'-K -B12 (-B in 11 is (
According to the equation (2), K'·B' is given by the equation (3).

P=K・Br shoulder-H (2) K′・B′=5 stone sword] 7-possible (3) K i
・B i = a ・t i ff1 (4) P:
Roll reaction force (kgL K: Deformation resistance (kg/+nn+
21B, Short side solidification width (T+2XD, D: Short side solidification thickness (
mml, roll radius (mm), rolling amount of △H110-le or total rolling amount of each set fmm), subscript 1, each roll number,,
t: Elapsed time from mold meniscus (minutes), K-B: Average value of multiple rolls [Function] The present inventor calculated the total reduction amount of multiple rolls and the roll reaction force in the case of a 300 x 500 mm slab. Between (2
) was found to hold true. Based on this result, when bulging and the like can be ignored, the rolling reduction amount for each roll is expressed by equation (5). In equation (2), the short side solidification width Bit at the 10th position is a function of the elapsed time from the meniscus, and can be approximated as Bi=A-ti.

△h i = (Pi”/Ri)・(1/KiBi)
2(5)△H=Σ△hi P・Roll reaction force fkgl, K, deformation resistance fkg/m
m") B: Short side solidified width (mm) 2XD, D: Short side solidified thickness f+nml, R: Roll radius fmm), △h: Reduction amount per 1 roll (mm), Subscript 1: Each roll number.

K: Average deformation resistance of the rolling band (kg/mm") Therefore, the relationship between K1-B1 and solidification time (til) at each roll position is expressed as (
4) Approximate as shown in the formula, and if the rolling edge (roll reaction force) and roll radius of each roll of multiple rolls are constant, (1
) in the case of a large number of rolls can be expressed by equation (3) using ti. On the other hand, in the actual measurement of a large number of rolls, -B can be calculated backwards from the measured drop amount of each segment using equation (1), so f21, f31
The constants a and m in the formula can be determined by actual measurement so that the sum of the squares of the differences between -B and (3) is minimized. Once the values of a and m are clear, K1-B1 at each roll position can be calculated using the time ti required for the slab to move from the mold meniscus to the relevant roll (4
) can be determined by the formula. In equation (4), the value of i is calculated using Bi calculated by heat transfer calculation and this method (4)
If determined in advance by the formula, B using a, m, and Ki
i can be calculated. The short side solidification width B measured and calculated in this way
i is the thickness center solid fraction f of the slab from the relationship between Bi calculated by preliminary heat transfer calculation and the thickness center solid fraction fs of the slab
According to the present invention, it is possible to quantitatively grasp the rolling timing of each rolling roll during light rolling, and by using the value of the rolling solidification stage of each roll, the casting speed and Even if the cooling conditions change during casting, it is possible to stably obtain a homogeneous steel material without segregation by changing the reduction roll so that the timing of reduction is appropriate.

[Example] Example 1 The outline of the continuous casting machine in which the test was carried out is shown in FIG. 1, and a typical example of the composition of the cast molten steel is shown in Table 1. The test continuous casting machine is a segment rolling type. The rolling reduction amount of multiple rolls is continuous 3
Regarding the segments, the displacement of the frame 3 was measured using a dial gauge 5 as shown in FIG. Obtained-E
is shown in equation (6) as a function of elapsed time t from the mold. The short side solidification width B1 is calculated using equation (6) and equation (7), which is the relationship between K and t measured in advance. B1 is (6),
It can be calculated using equation (7), and Bi can be converted to the center-of-thickness solid fraction of the slab from the relationship between Bi and the center-of-thickness solid fraction calculated by the heat transfer calculation shown in FIG.

K −B =27.2t' 62 (to 61 = 11
．． 3. to, +s (71 or more) (Figure 4 shows the relationship between the measured solid fraction at the center of thickness of the slab and the position of each rolling roll.

Table 1 (%) Example 2 The continuous casting machine used in the test and the outline of the molten steel composition are shown in Example 1.
is the same as In this test, the solid phase ratio at the center of the thickness of the cast slab of Roll Roll rolled by rolling was measured using the 0-piece method, which improves the solidification structure by electromagnetic stirring and ensures a top surface equiaxed crystal ratio of 5% or more.
The rolling segment was changed by changing the rolling hydraulic pressure of the rolling segment so as not to become smaller. Cut samples were taken at a pitch of 7 m in the casting direction of the cast slabs cast in this way, and the segregation level of the slabs was compared with that of this magazine and the conventional method.
As shown in the figure. This paper shows that compared to the conventional method, there are no slab parts with bad segregation, and homogeneous slabs without segregation can be stably obtained.

Example 3 Continuous casting machine and ? @The outline of the steel composition is the same as in Example 1. In this test, there were no equiaxed products due to high temperature casting, and the top surface equiaxed crystal ratio was 0%. The rolling roll was changed by inserting and removing a spacer between the rolls of the rolling roll so that the solid fraction at the center of slab thickness of the rolling roll that was measured in this magazine did not become smaller than 0.25. Comparing the segregation level of slabs obtained by this paper with the conventional method, the six methods shown in Figure 6 have smaller variations in segregation in the slab casting direction than the conventional method, eliminate areas of slabs with poor segregation, and reduce segregation. It can be seen that homogeneous slabs with no grains can be stably obtained.

[Effects of the invention] As described above, by determining the rolling solidification timing for each rolling roll using this publication and changing the rolling roll position so that the rolling timing is appropriate, areas with poor segregation are eliminated and slabs are made homogeneous without segregation. A stable slab can be obtained.

Figure 2 is a diagram showing the outline of the measurement method, Figure 3 is a diagram showing the relationship between short side solidification width and slab thickness center solid fraction, Figure 4 is the slab thickness at each roll position measured. Figures 5 and 6 are diagrams showing a comparison of the segregation levels of the conventional method and this publication.

l...mold, 2...segment, 3...frame, 4...post, 5...dial gauge, 6...
・Displacement meter installation position.

Figure 1 Figure 7'1'2 7F3 figure Figure 4 Distance between meniscus forces (m)

Claims (2)

[Claims] (1) In the continuous casting method of molten metal, in which the slab is pulled out while being compressed by at least one pair of rolls at the final stage of solidification, two or more sets of rolls are made with rolls at different solidification stages as one set, and each set is completely A continuous casting method characterized by quantifying the rolling solidification time for each roll using the rolling reduction amount and changing the pressure roll. (2) Create two or more sets of rolls with rolls with different solidification times as one set, and use the total pressure of each set to determine a and m so that A in equation (1) is minimized, and ( 4) The continuous casting method according to claim 1, wherein the rolling solidification time for each roll is quantified based on the value of the short side solidification width Bi for each roll calculated from the formula, and the rolling rolls are changed so that the rolling time is appropriate. A=(K'·B'-K·B)^2 (1) K·B is given by equation (2), and K'·B' is given by equation (3). P=K・B√(R・ΔH)(2) K′・B′=√(1/Σ(1/a・t^m)^2)(3
) Ki・Bi=a・ti^m (4) P: Roll reaction force (kg), K: Deformation resistance (kg/mm^
2), B: Short side solidified width (mm) 2 x D, D: Short side solidified thickness (mm), R: Roll radius (mm), △H: Reduction amount of one roll or total reduction amount of each set ( mm), subscript i: each roll number.
, t: Elapsed time from mold meniscus (minutes)