JP2593377B2 - Continuous casting method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents the segregation of impurities such as sulfur, phosphorus, manganese and the like found at the center of the thickness of a continuous cast slab, that is, in the case of steel slab, to obtain a homogeneous metal. The present invention relates to a continuous casting method that can be used.

[0002]

2. Description of the Related Art In recent years, requirements for material properties such as marine structures, storage tanks, oil and gas carrying steel pipes, and tensile strength wire rods have become increasingly severe, and providing a homogeneous steel material has become an important issue. I have. Originally, steel materials should be homogeneous in cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, which segregate during the casting process and are partially concentrated to be brittle. Become. In particular, in recent years, continuous casting 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 vicinity of the center of the thickness of a slab obtained by continuous casting. Is done. This segregation of components significantly impairs the homogeneity of the final product, and causes serious defects such as cracks caused by stress acting on the steel during the use process of the product and the wire drawing process. .

It is considered that such component segregation occurs because the residual molten steel flows due to the solidification shrinkage force and the like in the last stage of solidification, and the concentrated molten steel near the solid-liquid interface is washed out, and the residual molten steel is progressively concentrated. . Therefore, it is considered important to remove the cause of the flow of residual molten steel in order to prevent component segregation.
Have .

[0004] Such flow causes include flow caused by solidification shrinkage, slab bulging between rolls and flow caused by improper roll alignment, among others. Among these, the most serious cause is solidification shrinkage. In order to prevent segregation, it is necessary to reduce the slab by an amount that compensates for this.

[0005] Attempts to improve segregation by rolling down a slab have conventionally been made. In a continuous casting process, the slab is heated until the temperature at the center of the slab falls from the liquidus temperature to the solidus temperature. A method is known in which the pressure is reduced at a constant rate equal to or more than the amount that compensates for the coagulation shrinkage of the material. However, in the conventional continuous casting method, there is a problem that segregation is hardly improved depending on conditions, or segregation is rather deteriorated in some cases, and it is difficult to sufficiently improve component segregation.

The reason for the deterioration of segregation was studied, and the time from the time when the center of the slab reached a temperature corresponding to the solid phase ratio of 0.1 to 0.3 to the temperature corresponding to the flow limit solid phase ratio was obtained. 0.5mm / min to 2.5mm per unit time
/ Min. In a continuous manner at a rate of less than / min, and substantially no reduction is applied to the region from the time when the center of the slab reaches the temperature corresponding to the flow limit solid fraction to the temperature at the solidus temperature. The method according to Japanese Patent Application No. 62-27556 is well known.

Further, the inventors of the present invention have previously reported in Japanese Patent Application No.
As presented in No. 0295, there is a solidification period in which the concentrated molten steel violently accumulates in the center of the slab, and preventing the flow of the concentrated molten steel during the accumulation period is most effective for improving segregation, and The solidification time when the amount of concentrated molten steel accumulation is particularly large depends on the solidification structure. As a result of studying a light reduction method for further improving segregation based on this result, in a continuous casting method of molten steel metal in which the slab is drawn down by at least a pair of rolls at the end of solidification, the upper surface equiaxed crystal ratio is 0 to 5%. If the temperature of the slab center is 0.2
5, any position in the solidification time range from a position corresponding to preferably 0.35 to a position corresponding to the flow limit solid fraction,
Preferably, at least one pair of rolls is installed on the upstream side in the solidification time range to reduce the amount for compensating the total solidification shrinkage in the solidification time range, and when the upper surface equiaxed crystal ratio is 5% or more. , Any position in the solidification time range from the position where the temperature of the slab central portion corresponds to the solid fraction of 0.1, preferably 0.15 to the position corresponding to the flow limit solid fraction, preferably the solidification timing range A simple and efficient light reduction in which at least a pair of rolls is installed on the upstream side of the inside and the reduction range in which the amount of compensating for the total coagulation shrinkage within the coagulation time range is reduced can be reduced.

[0008] In the above research results, the most important point for improving segregation by light reduction is to secure an appropriate reduction timing and reduction amount. However, in the continuous casting operation, sufficient reduction cannot be performed at an appropriate solidification time, and segregation may worsen, such as fluctuation of the crater end due to a reduction in casting speed that cannot be avoided during the casting operation. In order to prevent the trouble of the wire starting from the slab including the slab having the insufficient segregation improvement effect due to the improper rolling condition, the occurrence of the trouble should be avoided even in the site where the segregation is the worst. It is necessary to select a high temperature and a long time as the lumping heating conditions, but if the lumping heating conditions are set to a high temperature and a long time, overheating occurs in the slab of the stationary part where segregation is good, and iron loss in the heating furnace also occurs. Further, problems such as a decrease in yield due to generation of a decarburized layer and deterioration of workability occur.

[0009] The present inventors have solved the above problem.
First, the average casting speed in the specific solidification time range during the solidification of the slab and the rolling start solidification time of the slab are separated and selected from the unsteady portion slab which has insufficient segregation improvement effect by light reduction, segregation. We proposed a method to produce a segregation-free wire with less energy by setting good slab slab heating conditions to low temperature and short time, and setting segregation deterioration slab slab heating conditions to high temperature and long time as before. .

[0010]

However, as a result of further research, not only the casting speed was reduced but also
Segregation may worsen due to a trouble in the cooling device such as breakage of the cooling chip or a trouble in the rolling device. When segregation worsens due to such a cause, it is not possible to separate the segregation worse cast slab by the conventional method. It is an important issue to judge the segregation deterioration slab including these and stabilize the quality.

[0011]

The gist of the present invention is as follows.

In a continuous casting method for molten metal in which a slab is drawn while being pressed down by one or more pairs of rolls, troubles in a cooling device and a pressing device and fluctuations in a casting speed cause improper reduction timing and reduction amount, resulting in deterioration of segregation. The slab thickness of the segregation-exacerbated slab measured with a thickness gauge at the specified position of the reduction zone, and the specific quantification by the central solid phase ratio, shell thickness, unsolidified thickness or unsolidified ratio during solidification of the slab Judgment by the average casting speed of the solidification time range and the start solidification time of the reduction of the slab, and the slab heating conditions according to the segregation level
A continuous casting method characterized by selecting .

[0013]

A specific solidification time range is determined by terminating the casting.
Slab thickness and position in continuous caster when casting speed is reduced
Between slab segregation data and solidification time range A to B
By comparing the average casting speeds of
From solidification time A to solidification time representing the corresponding change in average casting speed
B. The range of solidification time between B
Continuous casting method.

[0015] The thickness of the cast slab and the casting speed
Solidification corresponding to the slab where the segregation worsens with deceleration
Average casting speed in the time range and the slab reached the reduction zone
Of slabs with worse segregation based on
Said above Continuous casting method.

[0016]

The present inventors have further studied the items to be managed in the event of troubles in the cooling device or the screw-down device in order to solve the above-mentioned problems. We have invented a continuous casting method that can select good cast slab sites and make the ingot heating conditions low temperature and short time.

[0017] As a result of the casting speed as shown in FIG. 1 has been studied segregation determinants in the case of deceleration and stop, slab segregation is deteriorated, as shown in FIG. 2, from the slab solidification time A Coagulation time range to coagulate until coagulation time B
A slab average casting speed is decelerated, and elucidated that the reduction start clotting time of the slab C is delayed bloom. However, slabs in which segregation deteriorates are also found in the steady parts other than the casting speed fluctuation parts, and the cause is the slab whose slab thickness is larger than that of the steady part where segregation is good as shown in FIG. Based on these findings, the segregation-exacerbated slab under light pressure shows that the slab solidifies from solidification time A to solidification time B.
A slab average casting speed is decelerated during the, the cast slab rolling start coagulation time of the slab C is delayed, a slab further slab in the steady portion becomes thicker. Furthermore, as a result of studying the control method, the critical slab thickness to be controlled can be determined by the following procedure.

Determination of Critical Rolling Deficiency As shown in FIG. 3, (1) the relationship between the rolling reduction and the segregation defined by equation (1) is quantified, and the wire rod segregation is reduced even if the heating condition of the lumps is low and short. The critical pressure deficiency corresponding to the maximum segregated particle size that is favorable is determined.

[0019]

## EQU1 ## Insufficiency of reduction = measured slab thickness-maximum slab thickness at which segregation is good (1)

Determination of Control Limit Slab Thickness The control limit slab thickness is determined by Equation (2) using the critical rolling shortage.

[0021]

[Equation 2] Control limit slab thickness = Maximum slab thickness at which segregation is good + Critical rolling deficiency (2)

A slab thicker than the control limit slab thickness whose slab thickness is determined as described above is similar to a slab having a slow casting speed in a specific solidification time range or a slab having a delayed rolling start time. Bulking heating conditions must be high temperature and long time. The thick slab is caused by troubles in the cooling device and the drafting device of the continuous casting machine.
Can be According to the method of the present invention, it is possible to determine the segregation-deteriorated cast slab due to the reduction of the casting speed and the trouble of the continuous caster, and the conditions for the segregation-good cast slab slab heating selected at a low temperature, Shorter time saves a great deal of energy and iron yield, and stabilizes quality.

The solidification time of the slab can be quantified by the center solid phase ratio, shell thickness, unsolidified thickness or unsolidified ratio .
Beginning to start integration of the concentrated molten steel, it is thought to be the most influence on the generation of segregation, to increase the resistance to fluid passage of the slab center
It was quantified by the percentage of the central solid phase, which was presumed to be related to the timing of the recovery.

The central solid phase ratio is given by the following equation (3).
It can be calculated as a function of the temperature of the slab center,
It is the ratio of the solid phase present at the center. The temperature of the slab central portion is calculated in advance by heat transfer calculation based on operating conditions, or is calculated based on conditions such as cooling of the slab during casting and casting speed. This central solid phase ratio is a function of solidification time if casting speed, cooling conditions, slab size, steel type are determined, and also a function of solidification time, shell thickness, unsolidified thickness,
It can be easily converted to the uncoagulated rate. Whether the slab is solidification time A
The average casting speed in the solidification time range from solidification to solidification time B is determined by equation (4).

[0025]

Equation 3] billet center solid phase ratio of _{= (T 1 -T) / (} T 1 -T s) (3) T 1: molten steel liquidus temperature (℃) T _s: solidus temperature of the molten steel (° C) T: Central temperature of slab (° C)

[0026]

## EQU4 ## Average casting speed during solidification from solidification time A to solidification time B = L / t (m / min) (4) L: Movement of the slab while the slab solidifies into AB Length (m) t: Solidification time for the slab to solidify into AB (min)

The thickness of the slab was measured by a thickness gauge shown in FIG.

[0028]

Next, the present invention will be described with reference to examples.

Example 1 FIG. 5 shows an outline of a continuous caster on which a test was performed. Table 1 shows typical examples of the composition of the molten steel that was cast.

[0030]

[Table 1]

As shown in FIGS. 2 and 3, there are the following three cases where the segregation is deteriorated.

As shown in FIG. 2, the slab has an average casting speed lower than that of the steady part while the slab solidifies to a center solid phase ratio of 0.15 to 0.3.

As shown in FIG. 2, a slab whose rolling start of the slab is delayed.

A slab having a thick slab thickness as shown in FIG.

As a result of rolling the slab with good segregation and the slab with poor segregation of under low-temperature and short-time slab heating conditions, FIG.
As shown in the figure, a part where the segregation deteriorated was recognized in the wire. On the other hand, the segregation deteriorated cast slab was subjected to the high temperature and long time lumping heating conditions as before, and the other segregation good slabs were rolled under the low temperature and short time sizing heating condition as conventional, as shown in FIG. The segregation of the wire was good in all cases, which proved that it was possible to save energy used, improve iron yield, and stabilize quality.

Example 2 Table 2 shows conditions for deteriorating segregation due to a reduction in casting speed, and conditions for deteriorating segregation due to troubles in the cooling device and the screw-down device, for various steel types and solidification structures. The slab slab heating conditions that became these conditions were set to the high temperature and long time as before, and the slab heating conditions for other segregation good slabs were set to the low temperature and short time, so that a slab with good total segregation was obtained. It has been proved that it is possible to save energy used, improve iron yield, and stabilize quality.

[0037]

[Table 2]

According to the present invention, it is possible to separate a slab having an insufficient segregation improvement effect due to a reduction in casting speed and a light reduction caused by a trouble in a cooling system and a reduction system of a continuous casting machine. Blob heating conditions can be selected according to
As a result, it is possible to obtain a homogeneous steel material with good yield with less energy than before.

[Brief description of the drawings]

FIG. 1 is a diagram showing a variation example of a casting speed.

FIG. 2 is a view showing a segregation-exacerbated slab due to a change in casting speed and a method of determining separation thereof.

FIG. 3 is a diagram showing an example of deterioration of segregation when the thickness of a slab is thicker than a slab of a steady portion.

FIG. 4 is a view showing a method of measuring a slab thickness.

FIG. 5 is a diagram schematically illustrating a continuous caster on which a test is performed.

FIG. 6 is a diagram showing segregation of a wire rod when the heating conditions for lumping are set to a lower temperature and a shorter time than before.

FIG. 7 is a diagram showing wire rod segregation when segregation deteriorated slabs are discriminated by the present method, the segregation deteriorated slabs are used as conventional heating conditions, and the slab heating conditions for good segregation slabs are set to low temperature and short time.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Dial gauge 2 Water-cooled bar 3 Roll-down roll 4 Slab 5 Thickness gauge

Claims (3)

(57) [Claims] 1. In a continuous casting method of molten metal in which a slab is drawn while being rolled down by one or more pairs of rolls, troubles in a cooling device and a rolling device and fluctuations in a casting speed cause improper rolling timing and amount, resulting in segregation. Quantification of the deteriorated segregated slab with the thickness of the slab measured with a thickness gauge at the specified position of the reduction zone, and the center solid phase ratio, shell thickness, unsolidified thickness or unsolidified ratio during solidification of the slab Judgment was made based on the average casting speed in the specified solidification time range and the solidification time at the start of rolling of the slab, and the slab heating conditions according to the segregation level were selected.
Continuous casting method characterized by the choice of: 2. The casting is completed for a specific solidification time range.
The slab thickness and the casting speed
Between the segregation data and the solidification time range A to B
Comparing the average casting speeds to best deal with areas where segregation has deteriorated
From solidification time A to solidification time B representing the average casting speed change
2. The continuous casting method according to claim 1 , wherein a solidification time range is set between the two . 3. The thickness of the cast slab and the casting speed.
Solidification corresponding to the slab where the segregation worsens with deceleration
Average casting speed in the time range and the slab reached the reduction zone
Of slabs with worse segregation based on
The continuous casting method according to claim 1, wherein: