JPH01210157A - Method for preventing surface longitudinal crack on continuous cast slab - Google Patents

Abstract

PURPOSE:To surely prevent longitudinal crack on slab surface by adding exothermic powder developing exothermic reaction by melting with molten steel in the initial stage of casting on the molten steel surface in a mold at the time of continuously casting the medium carbon steel slab having the specific C content range. CONSTITUTION:In the initial stage of the casting at the time of continuously casting the medium carbon steel containing 0.09-0.15wt.% C, the exothermic powder as replacing with the ordinary mold powder is added on the molten steel surface in the mold. The exothermic powder is desirable to be one having >=5kcal/g calorific value and at the time of melting on the molten steel, the exothermic reaction is generated with thermit reaction and the powder becomes high temp. state and is rapidly melted. Then, molten powder having high fluidity is flowed on the whole range to width direction of the mold. Therefore, temp. at each part is raised and also temp. distribution to the width direction is uniformized because of the good fluidity thereof. By this method, the longitudinal crack caused by relative lowering the molten steel temp. at center part in the width direction, is surely prevented. Further, the above-mentioned exothermic powder is used to the one having, for example, the composition shown in the table.

Description

Detailed Description of the Invention [Industrial Field of Application] The present invention provides a method for reducing the amount of carbon generated on the long sides of a slab during continuous casting of a medium carbon steel slab with a carbon content of 0.09 to 0.15% by weight. This invention relates to a method for preventing vertical cracking.

[Conventional technology]

When continuously casting a medium carbon steel slab with a carbon content of 0.09 to 0.15% by weight, the higher the casting speed, the more likely vertical cracks will occur on the long side surfaces of the slab.

In particular, as shown in Figure 5, 1 to 2 charges (c
h) It tends to occur frequently in the eyes. When this vertical crack occurs,
A care process for the slab is required, and as a result, hot direct heating (hot charging) and hot direct rolling (direct rolling) are not possible, which is a major hindrance to energy saving.

The reason why medium carbon steel is easy to crack is that the C content is between 0.09 and 0.09.
If it is 15% by weight, peritectic solidification occurs, and the amount of shrinkage during solidification is large, resulting in local gaps between the mold and the solidified shell, making it easy to generate a non-uniform solidified shell, resulting in cracking due to thermal stress. This is thought to be due to the fact that

In addition, the reason why cracks are more likely to occur due to high-speed casting is that, especially when high-speed casting is performed at 1.1 m/min or higher, the amount of powder injected between the mold and the solidified shell becomes partially excessive or insufficient. It is thought that the solidified shell becomes non-uniform in the width direction of the slab, causing thermal stress and cracking.

Therefore, in order to prevent such vertical cracks, the following measures have been conventionally taken. That is, (11) a method for optimizing the viscosity of powder and ensuring uniform inflow of powder, (
2) Joining a metal with low thermal conductivity to the inner surface of the mold copper plate,
Alternatively, a method of reducing the amount of heat removed from molten steel by forming grooves, and (3) a method of fluidizing the molten metal between the immersion nozzle and the long side of the mold (Japanese Patent Laid-Open No. 172663/1983) have been proposed. There is.

[Problem to be solved by the invention]

Methods (1) and (2) above have a considerable improvement effect when the casting speed is relatively slow, especially below 1.0 m/min, but when casting at high speeds of 1.1 m/min or more, vertical It was not possible to completely prevent cracking.

On the other hand, method (3) above aims to prevent vertical cracking and increase the casting speed, but even with this method, it is difficult to sufficiently fluidize the molten metal, and the flow remains only locally and vertically. It is considered difficult to reliably prevent cracking.

Therefore, an object of the present invention is to provide a method that can reliably prevent vertical cracks on the surface of a slab.

[Means to solve the problem]

In order to solve the problem of vertical cracking on the surface of continuous casting slabs of medium carbon steel containing 0.09 to 0.15% by weight of carbon, the present inventors investigated the temperature of molten steel in the mold, the state of slag bears, the occurrence of vertical cracks, etc. We conducted many experiments from various fields. As a result, the following was found.

That is, (1) as shown in FIG. 2, there is a high probability that vertical cracks will occur near the immersion nozzle. (2) As shown in FIG. 3, the temperature of the molten steel is low near the immersion nozzle, which corresponds to the site where vertical cracks occur.

(3) The molten steel temperature is low at the early stage of pouring (Fig. 4).
This corresponds to the vertical crack occurrence situation shown in the figure.

(4) The shape of the slag rim 2a shown in FIG. 6 is non-uniform in the width direction, and the slag rim 2a is particularly large at the center in the width direction, making it difficult for powder to flow into that area and easily causing vertical cracks.

Therefore, based on the above-mentioned findings (1) to (4), the following conclusion was reached. In other words, the cause of vertical cracks on the slab surface is poor powder inflow conditions at the initial stage of casting (the temperature of the molten steel at the center in the width direction is lower than the temperature at other parts in the width direction).

The present invention was completed based on this knowledge. The gist is:
When manufacturing a medium carbon steel slab containing 0.09 to 0.15% by weight of carbon by continuous casting, an exothermic powder that is melted by the molten steel and exhibits an exothermic reaction is added to the surface of the molten steel in the mold at the initial stage of casting. It is characterized by:

[For production]

In the present invention, heat-generating powder is added to the mold in place of normal mold powder at the early stage of casting.

This exothermic powder, more preferably has a calorific value of 5 Kca
When the exothermic powder of 1/g or more is melted on the molten steel in the mold, it generates heat due to a thermite reaction, reaches a high temperature, and melts quickly. The powder, which has become molten and highly fluid, flows throughout the mold in the width direction, increasing the temperature of each part, and also makes the temperature distribution in the width direction uniform due to its good fluidity. Thereby, it is possible to reliably prevent vertical cracking caused by a relative decrease in fJ'W4 temperature at the central portion in the width direction.

〔Example〕

Next, the effects of the present invention will be clarified through examples.

First, FIG. 1 shows the casting part of the continuous casting machine used in this example. A two-hole immersion nozzle 1 attached to the lower part of the tundish that receives the molten steel from the ladle is immersed in the molten steel M of the mold 2, and the molten steel M flows from the discharge port 1a of the nozzle 1 in the direction of the short side of the mold 2. It is discharged. 3
A is a heat generating powder according to the present invention, 3B is a molten powder,
4 is a solidified shell.

Molten steel having the composition shown in Table 1 was placed in the mold (Fig. 1) of such a continuous casting machine (curving radius 10 m) at a casting speed of 1.5 m.
Casting at m/min, 250mmt x 1600m
mW continuous casting slabs were produced.

However, in order to clarify the effects of the present invention, 11hl
The strands were cast using a conventional method, and for the second strand, as shown in FIG. 1, exothermic powder having the composition shown in Table 2 was used at the initial stage of casting. By using this exothermic powder, the temperature of the molten steel at the initial stage of casting increased by about 8 to 10°C.

Table 2 As a result, when the frequency of occurrence of vertical cracks in the obtained slabs was investigated, the results were as shown in Fig. 7. As can be seen from the results in FIG. 7, in the method of the present invention, the incidence of vertical cracks was reduced to about 115 in the conventional method.

〔Effect of the invention〕

As described above, according to the present invention, the occurrence of vertical cracks on the slab surface can be significantly reduced even when medium carbon steel is cast at high speed.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the mold part of a continuous casting machine according to the method of the present invention, Figures 2 to 5 are graphs showing the occurrence of vertical cracks based on various factors in the conventional method, and Figure 6 is a diagram showing the occurrence of vertical cracks based on various factors in the conventional method. FIG. 7 is a plan view showing the state of slag bear formation, and is a graph showing the incidence of vertical cracking when the method of the present invention is applied, together with the conventional method. DESCRIPTION OF SYMBOLS 1... Immersion nozzle, 1a... Discharge port, 2... Mold, 2a... Slag rim, 3A... Exothermic powder, 3
B... Molten powder, 4... Solidified shell, M...
Molten steel. Patent applicant: Sumitomo Metal Industries, Ltd.

Claims (1)

[Claims] (1) When manufacturing medium carbon steel slabs containing 0.09 to 0.15% by weight of carbon by continuous casting, exothermic powder that is melted by the molten steel and exhibits an exothermic reaction is placed in the mold on the surface of the molten steel at the initial stage of casting. A method for preventing longitudinal cracking on the surface of a continuously cast slab, characterized by adding additives.