JPH07136749A - Continuous casting method of ni containing steel - Google Patents

Abstract

PURPOSE:To prevent the surface crack of cast slab in the continuous casting of Ni contained steel. CONSTITUTION:A reducing flame burner 2 is arranged between rollers 1, 1 placed in the roller apron zone of continuous casting machine. The surface of a cast slab 3 passed through the roller apron zone is directly heated by reducing flame. By reducing/removing the oxidized scale generated on the surface of cast slab 3, the cast slab 3 is made uniformly, cooled also heating the surface of cast slab 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for Ni-containing steel capable of reducing surface cracking of a slab.

[0002]

2. Description of the Related Art In continuous casting of steel, cracking may occur on the surface of a slab due to bending and straightening in a roller apron band. Particularly, in Ni-containing steel whose ductility decreases in the temperature range when passing through the roller apron zone, this surface cracking is remarkable. Therefore, in continuous casting of Ni-containing steel,
Various measures have been proposed to prevent surface cracks.

For example, from the viewpoint of enhancing the high temperature ductility of the material itself, measures for optimizing the concentrations of Al and N in steel are as follows.
It is proposed by JP-A-57-26141.
In addition, measures to control the cooling pattern so that the temperature of the slab when passing through the bending / straightening zone does not enter the embrittlement temperature zone,
JP-A-57-32862, JP-A-1-22864
It is proposed by the publication No. 4 and the like.

[0004]

Although these measures are certainly effective in preventing surface cracks, they have not been sufficiently effective in actual continuous casting. The reason is considered as follows.

In adjusting the components in the steel, it is considered that the amount and adjustment range of the components for preventing cracking are limited because it is necessary to impart predetermined mechanical properties to the slab. In the control of the cooling pattern, there is a limit to the control range, and since the temperatures of the corners of the slab and other parts greatly differ, it is difficult to manage the entire surface of the slab to an appropriate temperature. Conceivable.

Due to such circumstances, surface cracking cannot be completely prevented in continuous casting of Ni-containing steel. Therefore, the surface of the cast slab after casting has to be refined and maintained, which results in an increase in manufacturing cost.

An object of the present invention is to provide a continuous casting method for Ni-containing steel, which does not have the restrictions found in the component adjustment and cooling control, and can significantly reduce the crack defects on the surface of the slab as compared with the prior art. It is in.

[0008]

As described above, in the continuous casting of Ni-containing steel, it is well known that the cause of the surface crack of the slab is related to the surface temperature of the slab. However, regarding the surface temperature of the cast slab, it has been considered that even if there is a temperature difference between the corner portion and the other portion, there is no large temperature difference in the flat portion other than the corner portion.

However, according to the investigation by the present inventors, in continuous casting of Ni-containing steel, an oxide scale, which is peculiar to Ni-containing steel and is hard to peel off, is generated on the surface of the cast slab, and the oxide scale does not peel on the surface. It was found that the heat transfer coefficient becomes large due to the change in the boiling state of, so that the remaining scale part becomes partly low in temperature, and the surface crack is remarkably generated in this part.

On the other hand, in the low NO _x burner disclosed in Japanese Utility Model Publication No. 58-18007, stable combustion is possible at a low air ratio, and a flame in a reducing atmosphere can be stably obtained due to the low air ratio. It is known that it is possible. Then, various non-oxidizing heatings are performed by direct flame heating using this reducing flame burner.

From these facts, the inventors of the present invention installed a reducing flame burner in the roller apron zone of a continuous casting machine and directly heat the surface of the Ni-containing steel slab with the reducing flame burner.
The present invention has been completed by thinking that it is possible to remove the oxide scale that causes surface cracking, and by supporting the idea by various experiments.

The present invention relates to the continuous casting of Ni-containing steel,
A gist of a continuous casting method for Ni-containing steel is characterized in that the surface of a slab passing through a roller apron band disposed below a mold of a continuous casting machine is heated by an open flame with a reducing flame burner.

[0013]

[Function] By directly heating the surface of the slab with a reducing flame burner, the oxide scale that is generated on the surface of the slab and is hard to peel off, which is peculiar to Ni-containing steel, is reduced and removed, and the surface of the slab in the roller apron zone is removed. While the cooling is made uniform, the surface temperature of the slab rises due to its heating, and these prevent surface cracks in the slab.

The direct flame heating by the reducing flame burner is roughly classified into heating for the long side surface of the slab and heating for the corner portion.

As shown in FIG. 1, the long side surface is heated by installing a reducing flame burner 2 between the adjacent rollers 1 and 1 of the roller apron band so that the long side surface of the cast slab 3, usually both sides. This is done by directly applying a reducing flame. By heating the long side surface with an open flame, oxide scale is reduced and removed from the long side surface, the cooling of the long side surface is made uniform, and the temperature decrease is suppressed, and the occurrence of surface cracks is suppressed.

In the case of direct flame heating of the long side surface, it is preferable to arrange a plurality of reducing flame burners 2 in the width direction in order to apply the reducing flame to the entire width direction of the slab 3. The heating position is preferably as high as possible under the mold, specifically, in the range from immediately below the mold to the start of spray cooling. This is because the uniform cooling starts from the time when the oxide scale is removed, so that even if the oxide scale is removed near the bending / straightening area, the surface temperature of the slab cannot be made uniform during bending / straightening. However, there are many cases where it is actually better to shift downward due to problems such as breakout.

When the long side surface is heated by direct flame, the surface temperature of the slab rises as a whole, but it is unavoidable that the corner portion is cooled more than other portions. Heating the corner portion suppresses partial cooling of the corner portion.

It is desirable that the direct flame heating of the corner portion is performed together with the heating of the long side surface. The heating position is mainly aimed at raising the temperature, so that it is desirable that the heating position is immediately before a force such as bending or straightening is applied, specifically, immediately before the straightening roll. In this heating, the four corners may be directly heated by the four burners, or the short side surface may be directly heated depending on the length of the short side of the slab.

The reducing flame burner may be of any type as long as it is a burner capable of performing stable combustion at a low air ratio. From the viewpoint of reducing flame formation, the air ratio is 1.1 to 0.8.
Is desirable. As the fuel, for example, C gas can be used. It goes without saying that the distance to the surface of the slab is selected so that the reducing flame hits the surface sufficiently.

With regard to Ni-containing steel, if even a small amount of Ni is contained in the steel, the oxide scale will be less likely to peel off and surface cracks will occur due to the oxide scale. 5-10% N with remarkable cracking
The present invention is also effective for i steel.

Needless to say, the above-described component adjustment and cooling control can be combined with the present invention.

[0022]

EXAMPLES Examples of the present invention will be described below, and the effects of the present invention will be clarified by comparison with conventional examples.

During continuous casting of 0.7% Ni steel, the long side surfaces (both sides) of the slab were heated directly by a reducing flame burner in the roller apron zone immediately below the mold (1 m from the meniscus). Further, in the roller apron band in front of the straightening zone (16 m from the meniscus), four corners of the cast slab were directly heated by a reducing flame burner. Table 1 shows the steel types. The slab size is 1800 mm (long side) × 300 mm (short side).

[0024]

[Table 1]

[0025] The reducing flame burner is based on Jitsuko Sho-58-1800.
The low NO _x burner shown in Japanese Patent No. 7 was used. Five units were installed in the width direction of the slab for heating the long side surface, and one unit was opposed to each corner for heating the corners. The fuel per burner is C gas 65 Nm ³ /
h, the air was 246 Nm ³ / h, and the air ratio was 0.80. Exhaust gas components are CO: 3.7%, CO ₂ : 8%, O ₂ :
It was 0%.

With respect to four patterns of "no heating", "heating only long side surface", "heating only corner portion", and "heating long side surface and corner portion", temperature transition of the long side surface and corner surface The results of investigation of the temperature transition are shown in FIG. Moreover, the result of having investigated the occurrence state of the surface crack of the steel slab for each pattern is shown in FIG. “No heating” is the conventional example, and the others are the present invention examples.

2 and 3, the case 1 has "no heating", the case 2 has "only the long side surface", the case 3 has "only the corner portion", and the case 4 has "the long side surface and the corner portion". The case of "heating" is shown.

In the case 1 without heating, the temperature of the long side surface at the entrance of the straightening region dropped to near 800 ° C, and the surface temperature of the corner portion dropped to about 700 ° C, which is lower than that.
Further, since the display temperature of the long side surface is the average temperature, the temperature was lowered to below 800 ° C. in the portion where the oxide scale was not peeled off. As a result, on the surface of the cast piece after casting, 50 surface cracks were generated per 1 m of one side.

In case 2 in which only the long side surface was heated, the temperature of the long side surface after heating increased as compared with case 1, and was maintained at 850 ° C. or higher in the straightening region. Although the surface temperature of the corner portion also increased by about 100 ° C, it is considerably lower than the temperature of the long side surface, and is about 800 ° C in the straightening region. Since the oxide scale is removed, the display temperature of the long side surface is applied to the entire long side surface. As a result, the number of surface cracks was drastically reduced to 0.02 per meter on one side.

In case 3 in which only the corners are heated,
Up to the heating position before the correction area, the same tendency as in case 1 is shown, but in the heating area, the corner temperature rises by about 100 ° C,
The temperature on the long side also rose slightly. As a result, the number of surface cracks was 0.1 per 1 m on one side. This is more than in case 2, but significantly less than in case 1 without heating.

In case 4 in which the long side surface and the corner portion are heated, the temperature of the long side surface and the temperature of the corner portion rise after the heating of the long side surface and further the temperature of the corner portion after the heating of the corner portion. Increased by about 100 ° C. As a result, the number of surface cracks was the smallest, 0.012 per 1 m on one side.

[0032]

As is apparent from the above description, the continuous casting method for Ni-containing steel according to the present invention has a problem in continuous casting of Ni-containing steel by directly heating the surface of the slab with a reducing flame burner. It is possible to significantly reduce the surface cracks. Therefore, the number of steps in the adjusting and maintenance process or the process itself is reduced, which contributes to a reduction in product cost. Further, for steel types that were difficult to cast due to surface cracking, casting is facilitated, and the effect is great.

[Brief description of drawings]

FIG. 1 is a side view of a roller apron belt showing an embodiment of the present invention.

FIG. 2 is a graph showing the transition of the surface temperature of a slab for the present invention example and the conventional example.

FIG. 3 is a graph comparing the occurrence rate of surface cracks between the present invention example and the conventional example.

[Explanation of symbols]

 1 apron roller 2 reducing flame burner 3 slab

Claims (1)

[Claims] 1. In continuous casting of Ni-containing steel, the surface of the slab that passes through a roller apron zone disposed below the mold of a continuous casting machine is heated by a direct flame with a reducing flame burner. Continuous casting method for steel.