JPH0584555A - Continuous casting method - Google Patents

Abstract

PURPOSE:To provide a continuous casting method which can make blooming heating condition for blooming a cast bloom having good segregation condition, by separately selecting the deteriorated segregation cast bloom which is insufficient to segregation improving effect, in the light rolling reduction continuous casting method. CONSTITUTION:The deteriorated segregation cast bloom in the light rolling reduction is decided by the cast bloom thickness at the fixed position in the rolling reduction zone and separated to select the heating condition for blooming. The segregation which can improve by the heating condition for blooming, is beforehand clear and from the limited casting bloom thickness corresponding to this, the deteriorated segregation cast bloom and the good cast bloom are separated. The heating condition for blooming in the good segregation cast bloom is made to low temp. in short time. The consumed energy is a little and the iron yield is good and the wire rod having stable quality can be supplied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents segregation of impurity elements found in the center of thickness of continuously cast slabs, that is, in the case of steel slabs, segregation of sulfur, phosphorus, manganese and the like to obtain a homogeneous metal. The present invention relates to a continuous casting method that can be performed.

[0002]

2. Description of the Related Art In recent years, the demands on the material characteristics of offshore structures, storage tanks, steel pipes for carrying oil and gas, and tensile strength wire rods have become stricter, and it has become an important issue to provide homogeneous steel rods. There is. Originally, steel should be homogeneous in the cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, which segregate during the casting process and become partially concentrated, resulting in brittleness. Become. Especially in recent years, the continuous casting method has been widely used for the purpose of improving productivity, yield improvement, and energy saving, but usually noticeable segregation of components is observed in the vicinity of the thickness center of the slab obtained by continuous casting. To be done. This component segregation seriously 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 and the wire drawing process of the wire rod. .

It is believed that such component segregation occurs because the residual molten steel flows at the end of solidification due to solidification shrinkage force and the like, the concentrated molten steel near the solid-liquid interface is washed out, and the residual molten steel progressively thickens. .. 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. Among these, 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 improve segregation by rolling down the slab, and in the continuous casting process, the slab is cooled from the liquidus temperature to the solidus temperature until the slab center temperature rises from the liquidus temperature to the solidus temperature. There is known a method of rolling at a constant rate above the amount that compensates for the solidification shrinkage of. However, the conventional continuous casting method has a problem that the segregation is hardly improved depending on the conditions, and the segregation is rather deteriorated in some cases, and it is difficult to sufficiently improve the segregation of the components.

Further, by studying the reason for the deterioration of segregation, the time when the temperature of the central portion of the slab reaches the temperature corresponding to the solid fraction of 0.1 to 0.3 is reached. Area up to 0.5 mm / min or more per unit time of 2.5 mm
It is continuously reduced at a rate of less than 1 minute, and the region from the time when the temperature of the core of the slab reaches the solidification rate limit to the solidus temperature is not substantially reduced. The method according to Japanese Patent Application No. 62-27556 is well known.

Furthermore, the present inventors previously filed Japanese Patent Application No. 1-12.
As presented in No. 0295, there is a solidification time when the concentrated molten steel accumulates violently in the center of the slab, and preventing the flow of the concentrated molten steel during the accumulation time is the most effective for improving the segregation. The solidification timing at which the concentrated molten steel is particularly large depends on the solidification structure. Based on this result, as a result of research on a light reduction method for further improving segregation, in a continuous casting method of molten steel metal in which a slab is drawn while being pressed by at least a pair of rolls at the final stage of solidification, a top surface equiaxed crystal ratio of 0 to 5% is obtained. In this case, the temperature at the center of the slab is 0.2
5, preferably any position in the solidification time range from the position corresponding to 0.35 to the position corresponding to the flow limit solid fraction,
Preferably, at least a pair of rolls is installed on the upstream side in the solidification timing range to reduce the amount that compensates for the total solidification shrinkage amount in the solidification timing range, and 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 central part of the slab corresponds to the solid phase ratio of 0.1, preferably 0.15 to the position corresponding to the flow limit solid phase ratio, preferably the solidification time range At least a pair of rolls is installed on the upstream side of the inside, and a simple and efficient light reduction is proposed in which the reduction range for reducing the amount for compensating the total amount of solidification shrinkage within the solidification timing range can be reduced.

[0008]

However, in the continuous casting operation, sufficient reduction cannot be performed at an appropriate solidification time due to troubles of the cooling device such as deceleration of the casting speed and damage of the cooling tip, troubles of the reduction device, and the like. Many experiments have revealed that the segregation is aggravated. As described above, in order to prevent the trouble of the wire rod whose starting material is the ingot with insufficient segregation improving effect due to the light reduction, in order to avoid the occurrence of trouble even in the worst segregation area, the slab heating condition is set to a high temperature, Need to be long. For this reason, over-action occurs on the slab of steady part with good segregation,
Further, in high temperature heating, there are problems such as reduction of yield such as iron loss and generation of decarburized layer in the heating furnace and deterioration of workability, and solving these problems is an important issue.

[0009]

The gist of the present invention is as follows.

In a continuous casting method for molten metal in which a slab is drawn out while being rolled down by one or more pairs of rolls, a slab that has deteriorated in segregation due to troubles in a cooling system and a rolling system and fluctuations in casting speed is determined, and the segregation level is determined. A continuous casting method characterized by selecting appropriate slab rolling conditions and processes.

A continuous casting method as described above, characterized in that the segregation deteriorated slab is judged by the slab thickness.

The above continuous casting method is characterized in that the segregation-deteriorated cast slab is determined as the sum of the maximum segregated slab thickness with good segregation and 2 mm.

A continuous casting method as described above, characterized in that the segregation deterioration limit slab thickness is calculated by this method to determine the segregation deterioration slab.

[0014]

As a result of studying the case where segregation deteriorates under light pressure, segregation deteriorates when the casting speed is slowed down as shown in Fig. 1 or when a trouble occurs in the cooling device or the rolling down device. is there. As shown in FIGS. 2 and 3, the cast pieces in which the segregation is worse are thicker than the cast pieces in which the segregation is good. Based on this result, it is possible to selectively separate the cast pieces in which the segregation is deteriorated due to the cast piece thickness. Is. Furthermore, as a result of research on the method of separating the segregation that deteriorates segregation due to the thickness of the slab, the critical slab thickness that corresponds to the critical segregation of the slab that does not cause trouble in the wire even if the slab heating condition is low temperature for a short time is quantified. It has been found that the slabs with good segregation can be separated and selected by the following steps from 1 to 3 by changing the ratio.

As shown in FIG. 3, the critical maximum segregated grain size corresponding to the slab heating conditions to be controlled is determined.

The maximum segregated particle size measured and analyzed in advance,
(1) The marginal reduction amount is determined from the relationship with the reduction amount defined by equation (1).

[0017]

[Equation 1] Insufficiency of reduction = Measured slab thickness-Maximum slab thickness with good segregation (1)

Next, the control limit slab thickness is calculated by the equation (2), and the segregation deteriorated slab and the segregation good slab are separated by the control limit slab thickness.

[0019]

[Equation 2] Control limit slab thickness = maximum slab thickness for good segregation + insufficient rolling reduction (2)

By selecting the conditions for heating the slab of the selected slab with good segregation at a low temperature for a short time, it is possible to significantly reduce the energy used and the iron yield, and to stably produce a good quality product. it can.

The cast piece thickness was measured as shown in FIG.
It is also possible to use a laser thickness meter or measure a roll interval with a displacement meter set in the roll.

[0022]

EXAMPLES Next, the present invention will be explained by examples.

Example 1 An outline of a continuous casting machine which has been tested is shown in FIG. Table 1 shows a typical example of the composition of the molten steel cast. The slab with the segregation deteriorated is a slab with a large slab thickness as shown in FIG. As shown in FIG. 3, the limit maximum segregation grain size → the limit amount of rolling reduction was calculated and the limit cast piece thickness was calculated by the equation (2). As a result, the limit cast piece thickness was 309 mm. FIG. 6 shows a case where the entire amount of the slab is rolled under a slab heating condition of low temperature and short time. In the case where the whole amount was set to the low temperature and short time slab heating condition, segregation was observed in the wire in some cases. On the other hand, a slab having a slab thickness of 309 mm or more is conventionally subjected to high-temperature and long-time slab heating, and a slab having a slab thickness of less than 309 mm is set to a lower slab heating condition than before.
FIG. 7 shows the result of the slab heating condition for a short time. The segregation of all the wire rods was good, saving energy consumption, improving yield, and stabilizing quality.

[0024]

[Table 1]

Example 2 Table 2 shows the marginal reduction amount in various steel types and solidification structures. Segregation is good due to the control limit cast thickness calculated by the formula (2) by using the critical insufficiency of critical reduction and the maximum cast thickness at which segregation is good. Energy saving and improvement of iron yield by making slab slab heating conditions lower and shorter than before.
And the quality was stabilized.

[0026]

[Table 2]

[0027]

Industrial Applicability According to the present invention, it becomes possible to separate a slab which has an insufficient effect of improving the segregation due to the reduction of the casting speed and the trouble of the cooling system and the reduction system of the continuous casting machine due to the light reduction. By selecting the slabbing condition and process according to the level, it is possible to obtain a steel material having a good iron yield and a homogeneous steel material with less energy than in the past.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of fluctuations in casting speed.

FIG. 2 is a diagram showing the relationship between the segregation-deteriorated slab and the slab thickness, which are caused by the reduction of the casting speed.

FIG. 3 is a diagram showing a relationship between a cast piece thickness, a rolling reduction shortage amount and segregation in a cast piece in a steady portion.

FIG. 4 is a diagram showing a method for measuring the thickness of a cast slab.

FIG. 5 is a schematic view of a continuous casting machine that has been tested.

FIG. 6 is a diagram showing segregation of wire rods when the entire amount of cast slab is rolled under a slab heating condition at a lower temperature for a shorter time than conventional.

FIG. 7: A segregation-deteriorated slab is identified by this method, and the segregation-deteriorated slab is subjected to conventional high-temperature long-time slab heating conditions, and segregation-satisfactory segregation slab is heated at a low temperature for a short time. FIG.

[Explanation of symbols]

 1 Dial gauge 2 Water-cooled rod 3 Rolling roll 4 Slab

Front Page Continuation (72) Inventor Hidenori Tsuchida 1 Kimitsu, Kimitsu City, Chiba Nippon Steel Corporation Kimitsu Works

Claims (4)

[Claims] 1. In a continuous casting method of molten metal in which a slab is drawn out while being rolled down by one or more pairs of rolls, segregation-deteriorated slabs are determined by segregation-deteriorated slabs caused by troubles in a cooling system and a rolling system and fluctuations in casting speed. A continuous casting method characterized in that slabbing conditions and processes are selected according to the level. 2. The continuous casting method according to claim 1, wherein the segregation-deteriorated slab is judged by the slab thickness. 3. The continuous casting method according to claim 2, wherein the segregation-deteriorated cast slab is determined as a value obtained by adding 2 mm to the maximum segregated slab thickness at which segregation is aggravated. 4. The continuous casting method according to claim 2, wherein the segregation deterioration deterioration cast piece thickness is calculated by this method to determine the segregation deterioration deterioration cast piece.