JP2003062648A - Continuous casting method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method for steel wherein the surface cracking of a cast steel piece is prevented securely even when the casting speed changes. SOLUTION: A continuous casting machine having a re-bending correction section inside itself produces a cast steel piece containing one or more elements of Al of 0.02% mass or more, Nb of 0.004% mass or more, Ti of 0.004% mass or more and V of 0.001% mass or more wherein part of the cast steel piece of which casting speed inside a mold is below the preset value is perceived and that part of the cast steel piece is cast without water pouring at least in the re-bending correction section.

Description

Description: BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a continuous casting method of steel for preventing a surface slab of a steel slab. 2. Description of the Related Art In recent years, Nb, V, T
The production of low alloy steels containing various alloying elements such as i. However, with the addition of these alloying elements, surface cracks called lateral cracks or lateral cracks may occur on the slab surface during continuous casting, which is a problem in manufacturing. These surface cracks are caused by AlN and N near the surface.
Occurs when a straightening stress is applied to correct slabs along austenite (γ) grain boundaries weakened by precipitation of bCN, etc., or along proeutectoid ferrite (α) precipitated at γ grain boundaries. It is known to Therefore, γ
Severe surface cracking occurs when is corrected in the temperature range where α is transformed into α. Therefore, usually, a method of avoiding a temperature range in which the hot ductility of the surface is reduced (hereinafter, referred to as an embrittlement temperature range) to a high temperature side and performing correction to prevent surface cracking is adopted.
For example, JP-A-11-33688 discloses the technique. [0003] According to the prior art, in the constant speed portion where the casting speed is constant, the slab surface temperature can be maintained at a high level in the straightening portion by slow cooling the secondary cooling, and surface cracks can be prevented. . However, it is difficult to control the slab surface temperature when the casting speed fluctuates in the initial stage of casting or when changing the pot, and even if the technique disclosed in Japanese Patent Application Laid-Open No. 11-33688 is used, surface cracks may occur in the portion where the surface temperature has decreased. There was an easy task. SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method for steel capable of reliably preventing a slab surface crack even when the casting speed varies. The gist of the present invention resides in the following continuous casting method. That is, by a continuous casting machine having a slab correction section, Al: 0.02%, Nb:
0.004%, Ti: 0.004%, V: 0.001%
In a method for producing a slab containing at least one element of the following, a slab part whose casting speed in a mold is lower than a preset value is recognized, and at least a secondary part in a straightening section is returned to the part. A continuous casting method for steel, wherein the cooling zone is cast without water injection. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the relationship between casting speed fluctuation and surface cracking was examined, and the effect of casting speed fluctuation was considered. FIG.
Fig. 6 shows the relationship between the casting speed fluctuation of the slab and the rate of occurrence of surface cracks.
The casting speed shown in FIG. 1 refers to a reference casting speed (in this case, 1....) For each slab after cutting the slab with a cutter in the width direction at a predetermined interval in the longitudinal direction at the end of the continuous casting machine. 2m
pm) refers to the minimum value at which the casting speed has decreased most from the mold to the straightening portion. From this result, it can be seen that the slab that fluctuated at a lower speed with respect to the reference casting speed of 1.2 mpm has a higher surface cracking occurrence rate. Next, the relationship between the position at which the casting speed fluctuates and the slab surface temperature was investigated in a continuous casting machine. Captain 4
For a vertical bending type continuous casting machine having a length of 5 m (reference casting speed of 1.2 mpm), when casting at a constant reference casting speed from the continuous casting machine mold to the end of the continuous casting machine, the casting speed in the casting mold is equal to the reference casting speed. However, when there was a low speed fluctuation under the mold, there was a low speed fluctuation below the reference casting speed in the mold, but under the mold, casting was performed at a constant reference casting speed. FIG. 2 shows the result of determining the surface temperature. In both cases, the time during which the speed was lower than the reference speed and the casting speed were performed under the same conditions. From FIG. 2, the case where a low speed fluctuation in the mold (in FIG. 2) was observed. In addition, it shows that the surface temperature has decreased to the brittle temperature region in the straightening portion to which the bending return stress is applied. Therefore, when the casting speed fluctuates at a low speed lower than the reference speed in the mold as described above, it can be said that cracks are likely to occur in the straightening portion to which the bending stress is applied. In view of the above, for the slab portion subjected to a low-speed fluctuation in the mold, the surface cracks are prevented by improving the cooling method of the secondary cooling zone of the continuous casting machine (the cooling zone in the continuous casting machine below the mold). I considered how to do it. As described above, the cause of the occurrence of surface cracks is a decrease in the surface temperature of the slab at the straightening portion. Therefore, it was considered that the surface cracking could be prevented by preventing the temperature reduction in the straightening portion and avoiding the brittle temperature range at a high temperature even in a portion where the casting speed was reduced in the mold. One means is to reduce the amount of cooling water from the upper part of the secondary cooling zone in the continuous casting machine for the part where the casting speed has decreased in the mold, and to prevent the slab surface temperature from decreasing. In addition, it is necessary to control the slab temperature in consideration of the growth of the solidified shell inside the slab, and it is industrially difficult to control the slab temperature. As a result of repeated studies by the present inventors, the following has been found. The location where the surface cracks occur is the straightening portion of the continuous casting machine, and the location of occurrence is between 5 mm from the outermost surface or at most the surface layer in the width direction of the slab. Therefore, in order to prevent surface cracks, it was considered that it is sufficient to avoid the embrittlement temperature range of at most 5 mm from the surface layer at least in the bent back straightening portion where the temperature decreases. On the other hand, at the position of the straightening section, which is referred to as bending back in the continuous casting machine, the inside of the slab that passes is not completely solidified, and is in a molten state at a higher temperature (more than 1500 ° C) than the slab's outermost surface. I came up with the idea of using the inside of the slab, which has a higher temperature than the slab outermost surface, as a heat source. That is, the present inventors have conceived to prevent the surface crack of the slab by using the heat source inside the slab and making the temperature of at least the vicinity of the surface of the slab higher than the embrittlement temperature in the bent-back straightening section. is there. Specifically, for a part that has undergone low-speed fluctuations in the mold, its position is tracked in a continuous casting machine, and water is injected into the slab of the secondary cooling zone at least within the section of the slab bending and straightening section. Is stopped, whereby the slab surface is reheated by the heat source inside the slab and the temperature rises, so that the brittle temperature range can be avoided at a high temperature to prevent surface cracking. There is an effect if no water injection is performed only in at least the bending correction section, but in order to further increase the temperature of the slab,
If bulging is not hindered by the growth of the solidified shell, it is possible to make no water injection even before the position of the straightening portion of the continuous casting machine. Next, the concept of the casting speed condition in the mold without water injection will be described. FIG. 3 shows an example of the relationship between the lowest casting speed in the mold and the temperature of the coldest point of the slab in the bend correction section for the slab portion that has undergone speed fluctuations in the mold. For example, the brittle temperature obtained by the components of the slab is 9
With respect to steel types having a temperature of 30 ° C. or lower, waterless casting is performed on a portion where the casting speed in the mold is 0.95 mpm or lower. FIG. 4 shows a case where the casting speed is 0.95 mpm in the mold when the length of the continuous casting machine is 45 m. The calculation result of the slab surface temperature is shown about the case cast without water injection in the straightening part.
In the case of casting without water injection in the bent back straightening section, as shown in FIG. 4, the slab surface regains heat in the bent back straightening section, and becomes more than 930 ° C. in the brittle temperature range in the bent back straightening section. High temperatures can be avoided. Next, the chemical composition of the steel that is the subject of the present invention will be described. In the following description, “%” representing the content of alloying elements in steel means “% by mass”. The method of the present invention is intended for steel having high surface cracking susceptibility, that is, Al and Nb,
Ti and V are respectively Al: 0.02% or more, Nb:
0.004% or more, Ti: 0.004% or more, V: 0.
It is intended for steel having high surface cracking susceptibility containing at least one element of 001% or more. Al and N
When each of b, Ti, and V is less than the above values, there is no influence of embrittlement, and no surface flaw is generated even if the present invention is not used. The five elements of C, Mn, Si, P, and S are not particularly limited, and the range may be limited according to the properties of the obtained steel. Next, the effects of the method of the present invention will be described with reference to examples. As the continuous casting machine, a vertical curved continuous casting machine having a length of 45 m was used. This continuous casting machine has a mold length of 0.8 m, has two strands, performs 7-point bending at a portion of 2.5 to 4.1 m from the meniscus in the mold, and has a portion of 14.0 to 17.0 m from the meniscus. Is a continuous casting machine that performs 5-point bending correction. [Table 1] Table 1 shows the composition and the brittle temperature of the steels to be tested.
Shown in The embrittlement temperature of the test steel is around 930 ° C. In addition, the coldest temperature of the slab at the point where the bending is corrected is 930.
C is the site of 0.95 mpm in the mold (see FIG. 3). Based on this study, tracking of the position in the continuous casting machine was performed for a portion where the casting speed in the mold was 0.95 mpm or less, and the slab was cast without water injection in the straightening portion. In order to verify the effect of preventing surface cracking due to no water injection in the bending and straightening section, one strand is used as a test strand and no water injection is performed, and the other strand is compared to perform normal casting. The refined results of the cast slabs were evaluated. Since it is difficult to determine the occurrence of surface cracks as it is with black scale, a 3 mm-thick scarf was applied to the slab surface and evaluated by visual observation. Table 2 shows the state of occurrence of surface cracks at the slab stage and the surface temperature (calculation) in the straightening portion. An N = 16 test was conducted, and N = 2 surface cracks occurred on the comparative strand side (occurrence rate: 12.5%), while the test strand side was suppressed without occurrence of surface cracks (occurrence rate). : 0
%). [Table 2] According to the present invention, the surface cracks of the continuous slab can be effectively suppressed. As a result, the yield was improved as well as the straightness rate due to the untouched surface, which was able to greatly contribute to the reduction in manufacturing cost.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a correlation between a variation in casting speed and cracking. FIG. 2 is a graph showing a transition of a slab surface temperature from a meniscus to a machine length of 20 m. FIG. 4 shows an example of the relationship between the cold spot temperature of the bent back straightening part. FIG.

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Tomoaki Yoshiyama             Oita City, Oita City             Inside Oita Works of Iron Corporation

Claims (1)

Claims: 1. A continuous casting machine having a portion for correcting a cast slab in a continuous casting machine, wherein Al: 0.0
2% or more, Nb: 0.004% or more, Ti: 0.004
% Or more, V: in a method for producing a slab containing at least one element of 0.001% or more, a slab part whose casting speed in a mold is lower than a preset value is recognized, and A continuous casting method for steel, characterized in that at least a secondary cooling zone in a section of a straightening section is cast without water injection.