JPH05220556A - Continuous casting method - Google Patents

Abstract

PURPOSE:To improve the consumed energy and the yield in blooming heating of a cast slab cast with light rolling reduction casting and the realize the stability of quality. CONSTITUTION:In a continuous casting method improving the deteriorated segregation caused by delay of starting of the rolling reduction by suitabilizing the rolling reduction condition, the deteriorated segregation cast slab caused by reduction of the casting speed under the light rolling reduction is discriminated and separated by center solid phase ratio in the cast slab at the time of reducing the cast speed, and the blooming heating condition of the good segregation cast slab is executed at low temp. and in short time. Therefore, the consumed energy is little and the iron yield is good and the stable quality product 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 oil and gas transportation, 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, or 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 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, the reduction of the casting speed is unavoidable in the continuous casting operation,
As a result of this reduction in casting speed, it has become clear from many experiments that there are slabs in which segregation deteriorates. 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 of molten metal in which a slab is drawn out while being rolled down by one or more pairs of rolls, segregation-deteriorated slab due to slowing of the casting speed is judged by the solidification timing of the slab when the casting speed is slowed down. Then, the continuous casting method is characterized in that slab heating conditions and processes are selected according to the segregation level.

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 due to slowing of the casting speed The solid phase fraction of the slab when the casting speed is reduced The continuous casting method is characterized by selecting the slab heating condition and process according to the segregation level.

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, a part or all of the solidification time when the casting speed is decelerated is 0.15 to 0.40 as a central solid fraction. A continuous casting method, characterized in that the slab heating conditions for segregation-deteriorated slabs within the range are high temperature and long time, and the slab heating conditions for other segregated good slabs are low temperature and short time.

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, a casting speed deceleration test is performed in advance to quantify the central solid fraction at the time of deceleration of the segregation-deteriorated slab. A continuous casting method characterized in that a slab with deteriorated segregation is determined by using and the slab heating condition and process are selected according to the segregation level.

A part or all of the solidification time when the casting speed is decelerated is measured by the central solid fraction in advance and determined to be within the range of the central solid fraction of the segregation deteriorated slab. The continuous casting method described above in which the slab heating conditions for other slabs with good segregation are set to low temperature and short time for a long time.

[0015]

The present invention will be described in detail below.

The light reduction is very effective as a measure for improving the segregation. However, this effect of improving segregation becomes smaller due to the deceleration of the casting speed, and some of the slabs affected by the deceleration have the segregation worse than in the steady part. The first measure for preventing the segregation from deteriorating due to the reduction of the casting speed is to realize an operation at a constant casting speed. However, it is very difficult to reliably realize a constant casting speed operation in continuous casting operation. A second countermeasure based on such an actual situation is to accurately select and separate segregation-deteriorating cast pieces when the casting speed is reduced, and to set the segregation good slab heating conditions to a low temperature for a short time, The segregation-deteriorating cast slab is heated at a high temperature for a long time as in the conventional case. In order to realize this technique, it is important to establish a method for selecting a slab with segregation that is deteriorated as the casting speed is reduced.

In order to solve such a problem, the inventors of the present invention have conducted a test study on the mechanism of deterioration of segregation when casting is performed with a deceleration pattern of the casting speed shown in FIG. 1, and as a result, as shown in FIG.
The segregation deteriorated slab due to the slowing of the casting speed, the slab that was in a specific solidification time among the slabs located in the continuous casting machine when the casting speed was decelerated (segregation deterioration part) It was found that it was a cast piece (segregation-deteriorated portion) located between them. As a result of further research, the segregation deterioration of the slab (segregation deterioration part) located between the mold and the draft zone inlet among these segregation deterioration slabs was found in Japanese Patent Application No. 3-103939.
It was found that this can be improved by optimizing the rolling conditions such as rolling pattern as shown in No. When the reduction condition is optimized, the segregation deteriorated due to the deceleration of the casting speed is only the segregation (segregation deteriorated part) at a specific solidification time during deceleration. As a result of examining the solidification history of this segregation-deteriorated slab, the segregation-deteriorated slab is a slab whose solidification time is in the range of 0.15 to 0.40 in terms of the central solid fraction as shown in FIG. The present invention has been accomplished by finding that a slab that has deteriorated in segregation due to deceleration can be separated as shown in FIG.

As a result of further research by the present inventors, the solidification timing at the time of deceleration of a slab in which segregation deteriorates due to the reduction of the casting speed is different depending on the solidification structure, steel type and slab shape as shown in Table 1. I found that. Therefore, in order to accurately separate the segregation-deteriorated slab, it is preferable to quantify the solidification timing of the segregation-deteriorated slab by a casting speed deceleration test for the target steel type, solidification structure, and slab shape. . By separating the segregated slabs with deteriorated segregation selected in this way and reducing the slab heating conditions for the slabs with good segregation to a low temperature for a short time, it is possible to greatly reduce the energy used and the iron yield, and to obtain high quality products. Can be stably produced.

[0019]

[Table 1]

The solidification time of the slab is indicated by the central solid fraction. The solidification time at which the thickening of the thickened molten steel is considered to start is the solidification time at which the liquid flow resistance at the center of the slab increases. It is considered that the central solid fraction, which indicates the proportion of the solid phase in the central portion, has the greatest effect on this increase in liquid flow resistance, and is therefore the most appropriate index for the solidification timing of segregation. The central solid fraction is calculated as a function of the temperature at the center of the slab, as shown in equation (1). The temperature of the center of the slab is calculated in advance by heat transfer calculation based on cooling conditions, operating conditions such as casting speed, or based on conditions such as cooling during casting and casting speed. This central solid fraction is a function of solidification time if the casting speed, cooling conditions, slab size, and steel type are determined, and can be easily converted to the same solidification time function of shell thickness, unsolidified thickness, and unsolidified ratio. You can

[0021]

[Number 1] center of the slab solid fraction _{= (T 1 -T) / (} T 1 -T s) (1) T 1: molten steel liquidus temperature (℃) T _s: solidus temperature of the molten steel (℃) T: Core temperature of the slab (℃)

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

[0023]

Example 1 FIG. 5 shows an outline of a continuous casting machine that has been tested. The rolling reduction pattern was set by the method described in Japanese Patent Application No. 3-103939. Typical examples of cast molten steel composition
Shown in. With the casting speed pattern shown in FIG. 1, the casting speed was decelerated, and the effect of the casting speed deceleration on segregation was investigated.
The solidified structure had an upper surface equiaxed crystal ratio of 30%. As for the slab in which the segregation was deteriorated, the solidification time during deceleration of the casting speed was a slab with a central solid fraction of 0.18 to 0.32 as shown in FIG. FIG. 6 shows the case where the slab is not selected and separated by this method, and the entire amount of the slab is rolled under the slab heating conditions at a low temperature for a short time. When the whole amount is rolled under a slab heating condition at a low temperature for a short time, segregation may be observed in the wire. On the other hand, FIG. 7 shows that the slab heating conditions for the slabs whose solidification time during deceleration is 0.18 to 0.32 at the central solid phase rate are high temperature and long time, and slab heating for other slabs with good segregation. The segregation of the wire when the conditions are low temperature and short time is shown. The segregation of all the wire rods was good, saving energy consumption, improving yield, and stabilizing quality.

[0024]

[Table 2]

[0025]

[Example 2] A casting speed deceleration test was conducted on various solidified structures, steel types, and slab shapes by the same method as in Example 1.
The slab in which segregation deteriorates as the casting speed decreases is shown in Example 1.
Similarly, as shown in Table 1, in the case of a cast piece having a specific solidification time when the casting speed was decelerated, the segregation deterioration deteriorated, and the solidification time during deceleration of the cast piece differed depending on the steel type, solidification structure, and shape of the cast piece. It was In this way, the slab heating conditions for segregation-deteriorated slabs selected in this way are kept at high temperature for a long time as before, and the slab heating conditions for slabs with good segregation are set at a low temperature for a short time, thus saving energy consumption and iron. We were able to improve yield and stabilize quality.

[0026]

EFFECTS OF THE INVENTION According to the present invention, it becomes possible to separate a slab with insufficient segregation improving effect due to light reduction due to the reduction of casting speed, and the slab heating conditions and steps are selected according to the segregation level. As a result, it is possible to obtain a steel material having a good iron yield and a uniform steel material with less energy than conventional ones.

[Brief description of drawings]

FIG. 1 is a diagram showing a deceleration pattern of a casting speed in which a test is performed.

FIG. 2 is a diagram showing the relationship between casting length and segregation at the start of deceleration.

FIG. 3 is a diagram showing a central solid fraction at the time of deceleration of a cast piece in which segregation is deteriorated as the casting speed is decelerated.

FIG. 4 is a diagram showing a method of separating a slab that is deteriorated in segregation due to a reduction in casting speed.

FIG. 5 is a diagram showing an outline 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.

Claims (5)

[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, the segregation time of the slab when the casting speed is reduced when the casting speed is reduced The continuous casting method is characterized by selecting the slab heating condition and process according to the segregation level. 2. In a continuous casting method for molten metal, wherein a slab is drawn out while being rolled down by one or more pairs of rolls, segregation-deteriorated slab due to slowing of the casting speed causes the solid core of the slab to be cast when the casting speed is slowed down. A continuous casting method characterized by selecting slab heating conditions and steps according to the segregation level as judged by the phase ratio. 3. 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, a part or all of the solidification time when the casting speed is reduced is 0.
A continuous casting method characterized in that the slab heating conditions for segregation-deteriorated slabs in the range of 15 to 0.40 are high temperature and long time, and the slab heating conditions for other segregation-satisfactory slabs are low temperature and short time. 4. A continuous casting method for molten metal in which a slab is drawn while being pressed by a pair of rolls or more, a casting speed deceleration test is performed in advance to quantify the central solid fraction during deceleration of a segregation-deteriorated slab, A continuous casting method characterized by determining a segregation-deteriorated slab using this result and selecting a slab heating condition and process according to the segregation level. 5. A slab heating condition for a segregation-deteriorated slab within a range of a center-solid fraction of a segregation-deteriorated slab which is determined by measuring a part or all of the solidification time when the casting speed is decelerated. 5. The continuous casting method according to claim 4, wherein the temperature is set to a high temperature for a long time, and the slab heating conditions for other slabs with good segregation are set to a low temperature for a short time.