JPH1043848A - Cast slab internal crack prevention method in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent internal crack in high speed casting by adding Ca from an immersion nozzle in a mold and stabilizing Ca concentration near the center of cast slab thickness to high degree and fixing/reducing molten S with Ca. SOLUTION: In an immersion nozzle 3 to pour a molten steel 2 in a mold 6, supply holes 3c, which are pierced in a nozzle tip face from an introducing hole 3a arranged at the upper side wall of immersion nozzle through a supply route 3b extending up to the inner side of side wall, are formed in series, Ca or Ca alloy 12 is supplied from the supply holes 3c in the molten steel 2 in mold 6, Ca concentration near the center of cast slab thickness is locally increased. Here, a quantity to add Ca or Ca alloy 12 in the molten steel 2 in mold 6 is supplied so that a sampling analysis value of molten steel in mold satisfies the formula. Thus, generation of internal crack is suppressed without increasing a secondary cooling capacity, in particular the high speed casting of >=2.0m/min is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing slab internal cracks in continuous casting.

[0002]

2. Description of the Related Art As shown in FIG. 1, molten steel 2 in a ladle 1 is passed through a long nozzle 3 through a tundish 4 to continuously cast steel.
And then continuously inject the molten steel 2 from the immersion nozzle 5 into the mold 6. And mold 6
The molten steel 2 injected into the inside is quenched here to form a solidified shell 7 on the inner wall of the mold 6. It is cooled and completely solidified, and is continuously drawn out as a slab 9 by a pinch roll 10. Reference numeral 11 denotes a mold powder.

[0003] In recent years, for the purpose of improving productivity,
In order to achieve the production of high-temperature slabs and the stabilization of the direct-feed rolling process, continuous casting has been accelerated. In establishing this high-speed casting technique, one of quality problems is cracking inside the slab (hereinafter abbreviated as "internal cracking"). The occurrence of this internal crack is caused by an increase in strain at the solidification interface. The distortion acting on the solidification interface includes bulging distortion, bending distortion, correction distortion, roll misalignment distortion, distortion due to thermal stress, and the like. Among them, bulging distortion is considered to be the main. Therefore, to reduce the strain acting on the solidification interface,
It is effective to reduce bulging by strengthening the cooling. For example, Japanese Patent Application Laid-Open No. 57-187150 proposes strengthening the cooling with secondary cooling water for the purpose of preventing the occurrence of internal cracks.

However, when the secondary cooling is strengthened, the temperature of the slab is lowered, so that the surface slab is generated. In addition, it is difficult to maintain the slab at a high temperature, which is required for direct feeding in the continuous casting-rolling process. Therefore, when strengthening the secondary cooling, it is necessary to limit the casting speed to about 2.0 m / min or less.

On the other hand, it is known that the susceptibility of internal cracks is greatly affected by the S concentration, and it has been reported that the reduction of S is effective for preventing internal cracks. In connection with this S reduction, Japanese Patent Application Laid-Open Nos. 56-144051 and 56-51859 show that the addition of a Ca alloy and a REM alloy in molten steel can suppress the occurrence of internal cracks. Although reported, the problem remains where the effectiveness is particularly poor in high speed casting.

[0006]

SUMMARY OF THE INVENTION Accordingly, the present invention has been made by providing a method of preventing internal cracks in continuous casting without excessively cooling the slab temperature.
An object of the present invention is to realize a continuous casting-rolling direct feed process in which internal cracks are suppressed during high-speed casting at 2.0 m / min or more and a slab kept at a high temperature is required.

[0007]

Means for Solving the Problems The present invention relates to a continuous casting of steel, in which molten steel is poured into a mold by adding Ca or a Ca-containing alloy into the molten steel in the mold from a dipping nozzle. This is a method for preventing internal slab cracking in continuous casting, characterized by stabilizing the Ca concentration near the center of the slab thickness to a high level, and fixing and reducing dissolved S as sulfide by Ca.

Here, Ca or a Ca-containing alloy is preferably added so that the analysis value of the molten steel sample in the mold satisfies the following formula: 0.008 ≧ [% S] − [% Ca] /1.25. As the Ca-containing alloy, Ca-Si alloy, Ca-Al alloy, Ca-Si-RE
Suitable for M alloy.

[0009]

As described above, the occurrence of internal cracks in continuous casting can be prevented by strengthening the secondary cooling water and reducing the S value in the components. However, there is a limit in terms of equipment to increase the amount of cooling water, and a high temperature slab is required to promote the direct feed of the continuous casting-rolling process. It is difficult to avoid. Further, to reduce S,
In general, a desulfurization process is required, which not only leads to an increase in steelmaking cost, but also prolongs the time required for the production process, which makes it difficult to realize the process from the viewpoint of productivity.

Since S has a low interfacial tension, it has been reported that S is not only easily concentrated at the starting point of internal cracking, but also increases the embrittlement zone where hot ductility is zero.
It is also necessary to keep the S value within the limit of internal crack generation corresponding to the casting speed and the amount of secondary cooling water.

In addition, the occurrence of internal cracks in the vertical bending type continuous casting machine, which is the current mainstream system, becomes remarkable at the end of solidification when the bulging strain and the width of the embrittlement zone become large. Therefore, it is particularly important to prevent internal cracks at a depth of 1/4 to 1/2 of the thickness of the slab (corresponding to a depth on one side and a depth of 1/4 to 3/4 on both sides).

In contrast to the above technical background, in the conventional method of reducing internal cracking by adding a Ca alloy or a REM alloy, the concentration of Ca or REM becomes almost uniform in the thickness direction of the slab, so that the internal It was difficult to fix S with Ca until the low level of S required for crack prevention was reached.

In the present invention, however, Ca or a Ca-containing alloy, preferably in the form of fine particles, is blown into molten steel in a mold from a dipping nozzle to locally reduce the Ca concentration near the center in the slab thickness direction. The high concentration can be achieved, and the S amount required for the occurrence of internal cracks can be achieved even in high-speed casting.

That is, as shown in FIGS. 2 and 3,
A supply port 3c is formed in a series on the immersion nozzle 3 for injecting the molten steel 2 into the mold 6 through a supply passage 3b extending from the introduction hole 3a provided in the upper side wall to the nozzle tip surface through a supply path 3b extending inside the side wall. Ca or Ca alloy 12 from supply port 3c
Is supplied into the molten steel 2 in the mold 6, the Ca concentration near the center of the slab thickness can be locally increased.

Here, Ca or a Ca alloy is molded
It is preferable to supply the amount to be added to the molten steel 2 so that the analysis value of the molten steel sample in the mold satisfies the following formula: 0.008 ≧ [% S] − [% Ca] /1.25. Because
By causing the above-mentioned Ca amount to exist in the molten steel, the dissolved S amount Sr required to prevent the occurrence of internal cracks can be reduced to 1/4 to 1 /
This is because it can be achieved at two depth positions.

The immersion depth of the immersion nozzle is 250 mm
It is preferable to make the above. Further, as an addition condition of Ca or a Ca alloy, for example, in order to achieve a high yield, a Ca-Si alloy composed of 70% Ca-30% Si in a granular shape of 1 mm or less is used as a carrier gas at 5.0 liter / liter. mi
The conditions are as follows: n is used, and the feed rate is 0.8 kg / min.

[0017]

EXAMPLE In a continuous casting facility for actual production of a 220 mm thick slab, continuous casting of molten steel having a composition of C: 0.16 wt%, P: 0.02 wt% and S: 0.010 wt% was performed.
Casting speed: 2.0 m / min and specific water volume: 2.0 liter /
kg, 70% Ca-30% Si
An a-Si alloy (granules having a particle diameter of 1 mm or less) was added at a rate of 0.8 kg / min at a rate of 0.8 kg / min with an Ar gas flow rate of 5.0 liter / min in the manner shown in FIG.
For comparison, continuous casting was also performed in a case where the Ca alloy was not added (Comparative Example A) and a case where the Ca alloy was added in an amount equivalent to 25 ppm of Ca from the ladle (Comparative Example B).

For the slab continuously cast by the above method,
S in the thickness direction was analyzed to determine the amount of dissolved S, and the degree of internal cracking was investigated. First, FIG. 4 shows the distribution of the dissolved S amount in the slab thickness direction. Note that the dissolved S amount Sr
Is defined by the following equation. Where MS: S content MCa: Ca atomic weight

FIG. 4 shows that in the method of the present invention, the slab thickness is 1/4 to 1/1.
2 It can be seen that the Ca concentration at the (center) position has a high value and the Sr value has a low value. In the figure, the Sr value of the internal crack generation limit estimated from the strain analysis is indicated by a two-dot chain line as a reference, but it can be seen that the distribution almost close can be achieved by employing the method of the present invention. .

Table 1 shows a comparison of the amount of internal cracks generated. Here, the amount of occurrence of internal cracks was evaluated using the S-section of the slab, and the evaluation method followed the method shown in FIG. That is, in a sample taken in the casting direction at the center of the slab width, the internal crack index ICR when the internal crack length is a _i (mm) is as follows. ICR = {a _i ² / (L × T)} × 10000, where L: sample length (mm) T: sample thickness (mm)

[0021]

[Table 1]

As shown in Table 1, the adoption of the method of the present invention resulted in almost no occurrence of internal cracks, and made it possible to produce defect-free cast slabs. In the slab according to the method of the present invention, there is no occurrence of surface cracking, and in the method of the present invention, the Ca alloy is added through the immersion nozzle. There were no generation defects and no blowhole defects.

[0023]

According to the present invention, the occurrence of internal cracks in high-speed casting can be suppressed without increasing the secondary cooling capacity. Therefore, high-speed casting at 2.0 m / min or more, which was conventionally difficult to cast, can be stably performed, so that productivity is dramatically improved. Also,
It is possible to obtain a slab kept at a high temperature, which greatly contributes to the realization of a continuous casting-rolling direct feed process.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a continuous casting process.

FIG. 2 is a schematic view showing a Ca addition procedure of the present invention.

FIG. 3 is a schematic view showing a cross section of an immersion nozzle.

FIG. 4 is a view showing a distribution of a dissolved S concentration in a slab thickness direction.

FIG. 5 is a diagram illustrating the definition of an internal crack index.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Molten steel 3 Long nozzle 4 Tundish 5 Immersion nozzle 6 Mold 7 Solidification shell 8 Guide roll 9 Cast piece 10 Pinch roll 11 Mold powder 12 Ca

Continuing from the front page (72) Inventor Koichi Tozawa 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. None) Kawasaki Steel Corporation, Mizushima Works

Claims (2)

[Claims] In the continuous casting of steel, when pouring molten steel into a mold, Ca or a Ca-containing alloy is added from the immersion nozzle into the molten steel in the mold to reduce the Ca concentration near the center of the slab thickness. A method for preventing internal slab cracking in continuous casting, characterized by stabilizing to a high level and fixing and reducing dissolved S as sulfide by Ca. 2. The method according to claim 1, wherein Ca or a Ca-containing alloy is added so that the analysis value of the molten steel sample in the mold satisfies the following formula: 0.008 ≧ [% S] − [% Ca] /1.25. .