JP2532306B2 - Continuous casting method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an impurity element found in the center of thickness of a continuously cast slab, namely sulfur in the case of a steel slab.
The present invention relates to a continuous casting method capable of preventing the segregation of phosphorus, manganese, etc. and obtaining a homogeneous metal.

[0002]

2. Description of the Related Art In recent years, the demands on the material properties of offshore structures, storage tanks, steel pipes for transporting oil and gas, high-strength wire rods, etc. have become more severe, and it is an important issue to provide homogeneous steel rods. There is. 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. In particular, in recent years, continuous casting has been widely used for the purpose of improving productivity, yield improvement, energy saving, etc., but generally significant component segregation is observed in the vicinity of the thickness center of the slab obtained by continuous casting. It

The above-mentioned component segregation 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 of the product or the wire drawing process of the wire rod. Coveted. Such component segregation causes residual molten steel to flow at the end of solidification due to solidification shrinkage force,
It is thought that this occurs because the concentrated molten steel near the solid-liquid interface is washed out and the residual molten steel is progressively concentrated. 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.

As such a cause of flow, in addition to a flow caused by solidification shrinkage, there is a slab bulging between rolls and a flow caused by roll alignment irregularity, and the most serious cause is solidification shrinkage. Therefore, 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 observed during the period from the liquidus temperature to the solidus temperature of the slab center. There is known a method of rolling down at a constant rate equal to or more than the amount compensating for.

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, the segregation rather deteriorates, and it is difficult to sufficiently improve the component segregation. Met. As a result of various investigations by the inventors on the cause of the problems of the conventional method, the segregation improving effect of the conventional method is not recognized, or the segregation rather deteriorates. And found that the range was inappropriate.

Based on such knowledge, the present inventors have
In Japanese Unexamined Patent Publication (Kokai) No. 62-275556, the time from the time when the central portion of the slab reaches the temperature corresponding to the solid fraction of 0.1 to 0.3 to the time corresponding to the temperature corresponding to the fluidity limit solid fraction. The region is continuously reduced at a rate of 0.5 mm / min or more and less than 2.5 mm / min per unit time, and the solidus temperature is reached from the time when the temperature at the center of the slab reaches the temperature corresponding to the fluidity limit solid fraction. A continuous casting method was proposed in which no reduction was applied to the regions up to.

Further, the inventors of the present invention promoted many experiments, and as shown in Japanese Patent Application No. 1-120295, there is a solidification time when the concentrated molten steel is violently accumulated in the center of the slab. However, preventing the flow of this concentrated molten steel during the accumulation period is the most important for improving the segregation,
It was also found that the solidification timing at which the amount of concentrated molten steel is particularly large differs depending on the solidification structure.

Based on this result, as a result of research on a light reduction method for further improving segregation, it was found that at least 1 at the end of coagulation.
In the continuous casting method of molten metal in which the slab is pulled down while rolling down by a pair of rolls, when the upper equiaxed crystal ratio is less than 5%,
The temperature at the center of the slab is 0.25, preferably 0.3
At least one pair of rolls is installed at any position in the solidification time range from the position corresponding to 5 to the position corresponding to the flow limit solid phase ratio, preferably at the upstream side within the solidification time range, The amount that compensates for the total coagulation shrinkage of
Further, when the equiaxed crystal ratio of the upper surface is 5% or more, the solidification timing range from the position where the temperature at the center of the slab corresponds to the solid fraction 0.1, preferably 0.15, to the position corresponding to the flow limit solid fraction Of at least one pair of rolls, preferably at the upstream side within the coagulation timing range, and reducing the amount that compensates for the total coagulation contraction amount within the coagulation timing range. We have come to propose a simple and efficient light reduction method that can be made small.

However, in the continuous casting work, since the pot speed is frequently changed and the casting speed is slowed down or stopped due to sudden top troubles or sudden troubles, the slab may be delayed in reaching the draft zone. It is very difficult to eliminate the occurrence of unsteady slabs in which the casting speed has slowed or stopped in the rolling zone, and segregation of these unsteady slabs is worse than in the steady part.

[0010] In order to prevent the trouble of the wire rod starting from a cast piece having a insufficient segregation improving effect under a light reduction due to such a variation of the casting speed, the trouble occurs even in the portion where the segregation is the worst. To avoid lumpy heating conditions high temperature,
Although it is necessary to select it for a long period of time, this causes an over-action for a segregated slab with good segregation, and during high-temperature heating, yield such as iron loss and decarburized layer generation in the heating furnace occurs. And the workability is deteriorated.

In order to improve such problems, it is necessary to further improve the segregation of the slab of the steady part and simultaneously select the slab of the non-steady part with deterioration of the segregation. The present inventors, as a separation selection method of these segregation deteriorated slab, the segregation improvement effect by light reduction is insufficient due to the rolling speed of the slab and the solidification timing when it reaches the pressing zone inlet roll. We have proposed a patent application for a method for selecting and selecting a stationary part slab (referred to as the proposed method).

[0012]

However, in the above proposed method, as shown in FIG. 1, almost all the length of the cast piece located in the continuous casting machine when the casting speed is reduced or stopped is unsteady in which the segregation is bad. It is determined that the slab is a slab, which is not necessarily a good yield determination method, and it is an important issue to more accurately determine a segregation-deteriorated slab and improve the yield.

In order to solve such a problem, the present inventors have
As a result of research on the segregation determining factors of the slab with varying casting speed, it is possible to select the slab that has a sufficient segregation improvement effect by light reduction and has good segregation, and the slab heating conditions can be kept at low temperature for a short time. Has provided a continuous casting method.

[0014]

The subject matter of the present invention is as follows. (1) In a continuous casting method of molten metal in which a slab is drawn while being pressed by a pair of rolls or more, whether or not the segregation deteriorated slab is generated as the casting speed is reduced and stopped ,
The slab has a solidification time from solidification time A to solidification time B
Average casting speed in the range and rolling start solidification timing C of the cast piece
According to the result of the judgment,
Continuous casting characterized by determining heating conditions in the process
Law.

However, coagulation timing A, coagulation timing B, reduction opening
Start coagulation time C is Ri numerical der previously determined, the center solid phase
The rate, shell thickness, and other indicators are used to indicate the progress of solidification of cast slabs. (2) Segregation data for each cast piece and solidification time range A
Compare the average casting speed between B and B
Solidification time A that represents the transition of the average casting speed that corresponds well,
Coupling time B is adopted, so that
Sequential casting method.

The present invention will be described in more detail below. As a result of research on the segregation determining factors when the casting speed slows down and stops as shown in the example of FIG. 2, the present inventors have found that a slab with poor segregation has a specific value during solidification of the slab. A slab whose average casting speed in the solidification timing range A to B has been slowed and a slab with a reduction start timing C of the slab being delayed, and a slab with deteriorated segregation solidifies from the solidification timing A to the solidification timing B. The present invention has been accomplished by discovering that the separation can be performed with high accuracy by the average casting speed and the rolling start time C during the heating. Coagulation timing and pressure
The lower start time is represented by the central solid fraction here,
It may be expressed as a shell thickness, an unsolidified thickness, or the like. Also, segregation evil
As shown in Fig. 3, the length of the slab that is judged to be a slab is solidified.
0.15 and 0.3 are adopted as the central solid fractions of time A and B
When adopting 0.1, 0.4 is larger than when
It can be seen that the larger the difference between A and B, the longer the length. This inclination
Segregation data and solidification time A for each cast part,
Compare the average casting speed between B,
Can represent changes in the average casting speed that correspond well
A and B are determined in advance. In Figure 3, the result
Is A = 0.15 and B = 0.3.

In order to accurately separate the segregation-deteriorated slab and shorten the slab that is determined to be the segregation-deteriorated slab,
The solidification time range A to B to be controlled may be narrowed so that only the cast product having the worst segregation can be separated. It is considered that which solidification time is adopted as A and B depends on the segregation degree of the obtained cast piece, the variation in segregation, and the process capability such as slab rolling conditions. It is determined by the merits when considering all processes. There is a need to.

According to the method of the present invention described above, FIG.
The values of A and B are for the slabs where segregation is worse as shown in
Can be separated accurately by adjusting
It As a result, as shown in FIG. 4, the segregation deteriorated slab is determined by the conventional method that is selected according to the average casting speed when the slab passes through the reduction zone and the solidification timing when the slab reaches the reduction zone.
The slab length is shortened, and slabs with good segregation are separated with good yield.
It becomes possible to separate.

By thus setting the heating conditions for the slabbing of the slab with favorable segregation selected at a low temperature for a short time, it is possible to greatly save the energy used and the iron yield. The solidification timing of the slab can be quantified by the central solid phase ratio, shell thickness, unsolidified thickness or unsolidified ratio, but here, as shown in Japanese Patent Application No. 2-78940, segregation It was quantified by the central solid fraction, which is estimated to be related to the increase of liquid flow resistance in the center of the slab, which is considered to have the greatest effect on the formation.

The central solid fraction can be calculated as a function of the temperature of the central portion of the slab as in the case of the equation (1), and is the proportion of the solid phase present in the central portion. The temperature of the central portion of the slab is calculated in advance by heat transfer calculation based on the operating conditions, or is calculated based on conditions such as cooling of the slab and casting speed during casting. Central solid phase, which is one indicator of the time of solidification
Rate is determined by casting speed, cooling conditions, slab size, steel type
For example, coagulation time as shown in Japanese Patent Application No. 1-1212487.
The central solid fraction is the time of injection at each slab position.
And the time when each position of the continuous casting machine is reached.
It can be easily calculated from the solidification time. The shell thickness is a function of the same clotting time, unsolidified thickness, it can easily be converted into unsolidified rate. Further, the average casting speed in the solidification timing range in which the solidification timing of the slab solidifies from A to B is determined by the equation 2). Core solid fraction of cast slab = (Tl-T) / (Tl-Ts) 1) Tl: Liquidus temperature of molten steel (° C) Ts: Solidus temperature of molten steel (° C) T: Center of cast slab Temperature (° C) Solidification time Average casting rate during solidification from A to B = L / t (m / min) 2) L: Moving length of the slab while the slab solidifies from A to B (m) t: Solidification time for the cast piece to solidify from A to B (min) Next, the present invention will be described with reference to examples.

[0021]

EXAMPLES Example 1 An outline of a continuous casting machine used for the test is shown in FIG. 5, and a typical example of the composition of the molten steel cast is shown in Table 1. As shown in FIG. 3, the slab with the deteriorated segregation has a central solid fraction of 0.1.
It is a slab in which the average casting speed during solidification from 5 to 0.3 is slower than in the steady part, and a slab in which the rolling start of the slab is delayed. Further, FIG. 6 shows a condition in which the wire segregation becomes good when the whole amount is rolled under the same agglomeration heating conditions (lower temperature and shorter time than conventional). Deterioration of wire rod segregation is observed in the slab where the average casting speed slows down from the steady part while the slab position solidifies from 0.15 to 0.3 in the central solid fraction, and the start of reduction of the slab A slab having a central solid fraction of 0.19 or more. The segregation deterioration of the segregation deterioration unsteady part separated according to the present example is the conventional slab heating condition, and the segregation good slab heating condition is lower than the conventional slab heating condition. 7 shows. It was proved that the segregation of the wire rod was good, and that a homogeneous steel without segregation could be obtained with low energy compared with the conventional method.

Example 2 Table 2 shows a typical example of the composition of molten steel cast in this example. As shown in FIG. 8, the slab where the segregation deteriorates corresponds to the slab having a central solid fraction of 0.25 to 0.5.
A slab with an average casting speed slowing down from the steady part during solidification and a slab with a delayed central solid fraction when the slab reaches the reduction zone. FIG. 9 shows conditions under which the wire segregation becomes good when the slab heating conditions are low temperature and short time. A slab whose average casting speed is reduced while the solidification state of the slab solidifies to 0.24 to 0.5 at the central solid fraction, and a central solid phase when the slab reaches the reduction zone inlet roll Deterioration of segregation of slabs with a rate of 0.24 or more is recognized. As in the case of Example 1, the slab heating conditions of the segregation-deteriorated non-steady part cast separated according to the present example were set to the conventional values, and the segregation good slab rolling conditions of the steady part slab were set to a lower temperature than the conventional values. As a result of shortening the time, it was proved that the segregation of the wire rod was good in its entirety and that a homogenous steel material without segregation could be efficiently obtained with less energy as compared with the conventional method.

Example 3 Table 3 shows, for various steel types and solidification structures, the solidification timing range of the cast slab in which segregation deteriorated due to the deceleration stop of the casting speed and the rolling start solidification timing of the cast slab. Based on the results of Table 3, it becomes possible to select a cast piece site having an insufficient segregation improving effect which occurs with a decrease in casting speed, and improve the slabbing condition of a good segregation site at a low temperature in a short time.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

EFFECTS OF THE INVENTION According to the present invention, by separating a slab with a sufficient segregation improving effect under a light reduction caused by a variation in casting speed and a slab with an insufficient segregation, slab rolling conditions and processes depending on the segregation level are obtained. Is provided, and a continuous casting method is provided that enables obtaining a homogeneous steel material with a low yield and a good yield.

[Brief description of drawings]

FIG. 1 is a view showing a range of a cast piece determined to be a segregation-deteriorated cast piece in a conventional method.

FIG. 2 is a diagram showing an example of fluctuations in casting speed.

FIG. 3 is a diagram showing an average casting speed during solidification of A to B corresponding to a slab in which segregation is deteriorated.

FIG. 4 is a diagram showing a comparison of a slab length determined as a segregation-deteriorated slab with a conventional method.

FIG. 5 is a schematic view of a continuous casting machine.

FIG. 6 is a view showing conditions under which wire segregation becomes good in the case of performing lump heating for a short time at a low temperature in Example 1.

FIG. 7 is a diagram showing the level of segregation of wire rods in which the segregation heating conditions for the segregation-favored portion selected in Example 1 were low temperature and short time, and the segregation-deteriorated cast piece was conventional.

FIG. 8 is a diagram showing an average casting speed (in the case of Example 2) during solidification of a cast piece corresponding to a cast piece having deteriorated segregation into A (A ′) to B.

FIG. 9 is a diagram showing a light reduction condition in which wire segregation becomes good in the case of performing lump heating at low temperature for a short time in Example 2;

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Katsuhiko Kubo 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Co., Ltd. Kimitsu Works (56) Reference JP-A-2-280952 (JP, A)

Claims (2)

(57) [Claims] 1. In a continuous casting method for molten metal in which a slab is drawn while being rolled down by one or more pairs of rolls, whether or not the slab is a segregation-deteriorated slab that is generated as the casting speed is reduced and stopped.
The slab is solidified from solidification time A to solidification time B.
Average casting speed in the solid time range and rolling start solidification of the slab
Judgment is made according to the timing C, and the cast slab according to the judgment result.
Series, characterized in that it determines the heating conditions in the agglomeration process of
Sequential casting method. However, the coagulation timing A, the coagulation timing B, and the rolling start coagulation timing C are
Numerical der that is determined is, the center solid phase ratio, such as shell thickness slab
It is shown by an index showing the progress of coagulation. 2. Segregation data and solidification timing range for each cast piece site
Comparing the average casting speed between the areas A and B, the segregation worst part
During solidification, which represents the transition of the average casting speed that best corresponds to
The period A and the coagulation period B are adopted.
The continuous casting method described.