JPH07195154A - Method for continuously casting extra-low carbon steel - Google Patents

Abstract

PURPOSE:To provide a method for preventing the longitudinal crack of an extra-low carbon steel developed in a synchronous mold type continuous caster by slow-cooling in the mold. CONSTITUTION:At the time of casting the steel containing <=0.01wt% by using the synchronous mold type continuous caster, the cooling speed on the surface of a cast slab is reduced, and the uniformity of the solidified shell thickness at the initial stage and the reduction of strain in the solidified shell are controlled to prevent the longitudinal crack on the surface of the cast slab. In this case, the cooling speed at the position of 1.5mm depth from the surface of the cast slab is made to be <=-60 deg.C/sec, or the conductive heat from the mold is made to be <=2,500,000Kcal/m<2>.hr. By this method, the extra-low carbon steel cast slab without longitudinal crack can be cast in the synchronous mold type continuous caster.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a slab in continuous casting without causing vertical cracks on the surface of the slab of ultra-low carbon steel.

[0002]

2. Description of the Related Art Vertical cracks generated on the surface of a slab during continuous casting are caused by uneven solidification in the vicinity of a meniscus, and strains are concentrated in a solidification delay portion due to a restraining force that inhibits shrinkage in the width direction. In a fixed casting continuous casting machine, the frequency of vertical cracking is high in the case of medium carbon steel. This is thought to be due to the deformation of the initially solidified shell due to the δ / γ transformation (Tetsu Matsumiya, Takeshi Saeki, Jun Tanaka, Toshihiko Ariyoshi: Iron and Steel, 68).
(1982) 1782). On the other hand, it is said that vertical cracks are unlikely to occur in the case of low carbon steel and ultra low carbon steel.

However, when a synchronous casting continuous casting machine is used, vertical cracks often occur even in extremely low carbon steel. This is considered to be because in the continuous casting machine of the synchronous mold, the cooling condition and the lubricating condition in the mold are different from those in the fixed mold. However, no analysis has been performed on the initial solidification and stress / strain of ultra-low carbon steel, and no measures have been taken to prevent cracking.

[0004]

DISCLOSURE OF THE INVENTION According to the present invention, when performing continuous casting using a synchronous casting type continuous casting machine, ultra low carbon steel is cast without causing vertical cracking, and casting with good surface properties is achieved. The purpose is to provide a piece.

[0005]

Means for Solving the Problems In order to clarify the cause of the occurrence of vertical cracks in ultra-low carbon steel, the present inventors conducted a basic experiment on the non-uniform phenomenon of the initial solidified shell thickness and obtained the following findings. It was

The ultra-low carbon steel has a non-uniform initial solidified shell thickness as compared with the low carbon steel and the medium carbon steel. This is considered to be the cause of vertical cracking. This non-uniform solidification is due to the small frequency of nucleation during initial solidification in the extremely low carbon region. The non-uniform solidification of ultra low carbon steel can be mitigated by reducing the cooling rate during solidification. By reducing the cooling rate, the strain generated in the initial solidified shell can be reduced.

Therefore, if the cooling rate at least on the surface of the cast slab is lowered to make the initial solidified shell thickness uniform and reduce the strain, vertical cracking of the ultra low carbon steel can be prevented.

The continuous casting method for ultra-low carbon steel of the present invention was made on the basis of the above findings.
When casting a steel containing 0.01% by weight or less of C using a synchronous casting continuous casting machine, (1) at least the cooling rate of the surface of the slab to such a rate that vertical cracks do not occur on the surface of the slab. It is characterized by lowering and homogenizing the thickness of the initial solidified shell and reducing the strain accumulated in the solidified shell to prevent the occurrence of vertical cracks on the surface of the slab. Specifically, (2) particularly, at least 1.5 mm depth from the surface of the cast slab surface
Cooling rate at 60 ° C / sec or less, or (3) mold heat removal amount is 2,500,000 kcal / m ²
・ Achieve the purpose by keeping the time below.

[0009]

The operation of the present invention will be described below. Figure 1
Fig. 3 shows the relationship between the cooling rate and the initial solidification nonuniformity in the case of continuously casting a slab of ultra-low carbon steel and low-carbon steel using a synchronous casting continuous casting machine. From the figure, if the cooling rate at a position 1.5 mm deep from the surface of the slab is set to 60 ° C / sec or less, the solidification uniformity of the ultra low carbon steel (SULC) becomes
It is almost equivalent to low carbon steel (LC) with no vertical cracking.

FIG. 2 shows the results of stress-strain analysis of the initially solidified shell for ultra-low carbon steel. 1.5mm depth from the surface of the slab
If the cooling rate at the position is 60 ℃ / sec, 80 ℃ / s
The amount of strain accumulated in the solidified shell is smaller than that of ec. Therefore, the cooling rate at a depth of 1.5 mm from the slab surface is 6
By setting the temperature to 0 ° C./sec or less, the initial solidified shell thickness is made uniform and the strain in the shell is reduced, and as a result, vertical cracking can be prevented.

[0011]

EXAMPLES Next, the effects of the present invention will be described based on examples. (Example 1) When casting steel containing 0.002, 0.01, and 0.05% by weight of C using a synchronous casting continuous casting machine at a casting speed of 6 m / min, the casting coating thickness was set to By changing, the cooling rate was changed. Table 1 shows the relationship between the presence or absence of vertical cracking at that time and the cooling rate at a position 1.5 mm deep from the surface of the slab.

From Table 1, C is 0.002,0.01.
When the cooling rate is 80 ° C./sec in the ultra-low carbon steel of weight%, vertical cracking occurs on the surface of the slab, but the cooling rate is 6%.
When the temperature is 0 ° C / sec or less, vertical cracking can be completely prevented.
Further, in the low carbon steel containing 0.05% by weight of C, vertical cracking did not occur even at the cooling rate of 80 ° C./sec. Therefore, it is not the object of the present invention.

[0013]

[Table 1]

(Embodiment 2) C is 0.002, 0.01,
About the content of 0.05 wt% of steel, casting speed was 6m / min when casting with a continuous casting continuous casting machine, changing the thickness of the casting mold, The results of the investigation are shown in Table 2.

From Table 2, it can be seen that for ultra low carbon steel with C of 0.002,0.01% by weight, the heat removal amount in the mold is 2.5 million kcal / m.
By setting the time to ² hours or less, the occurrence of vertical cracks can be completely prevented. Further, in the case of low carbon steel containing 0.05% by weight of C, vertical cracking does not occur even when the heat removal amount from the mold exceeds 2.5 million kcal / m ² · hour, which is not the subject of the present invention.

[0016]

[Table 2]

[0017]

EFFECTS OF THE INVENTION By carrying out the present invention during continuous casting using a synchronous casting continuous casting machine, it is possible to cast ultra-low carbon steel without causing vertical cracks on the surface of the slab. As a result, it is possible to obtain a slab having a good surface property.

[Brief description of drawings]

FIG. 1 shows a case where a slab is continuously cast on an ultra-low carbon steel and a low-carbon steel using a synchronous casting continuous casting machine.
The figure which shows the relationship between a cooling rate and an initial solidification nonuniformity.

FIG. 2 is a diagram showing the results of stress strain analysis of an initially solidified shell for ultra-low carbon steel.

Continued Front Page (72) Inventor Toshiya Komori 1 Nishinosu, Oita-shi, Oita Prefecture Nippon Steel Co., Ltd. Inside Oita Steel Co., Ltd. Inside the Technology Development Headquarters (72) Inventor Shoichi Aratani 20-1 Shintomi, Futtsu City, Chiba Shin Nippon Steel Co., Ltd.

Claims (3)

[Claims] 1. When casting steel containing 0.01% by weight or less of C using a synchronous casting continuous casting machine, at least the cooling rate of the surface of the slab is such that vertical cracks do not occur on the surface of the slab. A continuous casting method for ultra-low carbon steel, which is characterized by reducing the speed to a uniform speed, homogenizing the initial solidified shell thickness, and reducing the strain accumulated in the solidified shell. 2. The continuous casting method for ultra low carbon steel according to claim 1, wherein the cooling rate at a position 1.5 mm deep from the surface of the slab is set to 60 ° C./sec or less. 3. When casting a steel containing 0.01% by weight or less of C by using a synchronous casting continuous casting machine, the heat removal amount of the casting mold is set to 2,500,000 kcal / m ² · hr or less. A continuous casting method for ultra low carbon steel.