JPS61235051A - Prevention of internal crack of continuously cast round ingot - Google Patents

Abstract

PURPOSE:To prevent an internal crack by subjecting an ingot in a secondary cooling zone for continuous casting to the cooling satisfying the equation at the point of the time when the recuperation in the cooling thereof is completed. CONSTITUTION:The cooling to satisfy the equation: (TL-TS)/d<=5 deg.C/mm at the point of the time when the recuperation is completed is adopted for the cooling in the secondary cooling zone for continuous casting. In the equation, TS: the surface temp. of the ingot deg.C, TL: the internal temp. of the solidified shell deg.C, d: the thickness of the solidified shell mm.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for preventing internal cracks when producing round slabs by continuous casting.

[Prior art and its problems]

Since the magnitude of the cooling capacity in continuous casting has a large effect on the quality of the slab, the cooling capacity needs to be maintained appropriately depending on the casting speed, the size of the slab cross section, etc.

For example, when high-speed casting is performed, bulging distortion (distortion caused by the shell protruding due to the static pressure of internal molten steel) or correction distortion is likely to occur, so the amount of secondary cooling water is increased as the casting speed increases. In particular, for slabs with large cross-sections such as slabs and blooms, the cooling water amount is generally set in a speed-proportional manner.

The rate of occurrence of internal cracks, which the present invention aims to prevent, also depends on the suitability of the cooling capacity.

In other words, in the secondary cooling zone, the skin of the slab is cooled by cooling water, so it is lower than the temperature inside the shell. Therefore, the temperature of the molten steel inside the shell moves toward the skin, which is called a relapse. A thermal effect occurs, and the epidermis has a history of recuperation.

In the slab cooled in this manner, the shell skin side is thermally contracted by the cooling water, and the inside of the shell is thermally expanded, and thermal stress based on this temperature difference between the inside and outside causes internal cracks. Therefore, the greater the temperature difference between the inner and outer surfaces of the shell, the greater the thermal stress and the greater the rate of occurrence of internal cracks.If the heat capacity of the slab itself, such as a round slab, is small, internal cracks will occur. The rate also increases.

The present invention has been made to solve the above-mentioned problems.

[Technical means]

The present invention cools the slab in the secondary cooling zone of continuous casting by using the following formula Ts: Surface temperature of the slab (
TL: Inside temperature of solidified shell (°C) d: Thickness of solidified shell (°C) It is characterized by cooling that satisfies the following.

[Process of creation of invention]

The present invention was developed in view of the following two points: (1) suppressing thermal stress; and (2) thermal stress can be expressed as the difference in temperature between the inside and outside of the shell per unit thickness, and is based on the following experimental results.

That is, a high carbon material (API-J55), a round steel material with a size of 282φ, was cast at a casting speed of 1.0 m/min and a casting temperature of 1.5 m/min.
During continuous casting at 00'C, the temperature difference between the inside and outside of the shell per unit thickness (hereinafter referred to as "temperature gradient inside the shell") can be reduced by changing the amount of secondary cooling water as shown in Table 1.
Find the temperature gradient inside each shell (indicated by the symbol in Figure 2)
We conducted an evaluation of internal cracks in slabs according to the following. This result is the first
As shown in the figure, (i) The skin temperature of the slab was measured using a thermocouple, but it is also possible to use, for example, a COP (optical pyrometer) or a two-color thermometer.

(i;) Measurement of the internal temperature of the solidified shell is carried out at the liquidus temperature T
I guessed it from LL.

(iii) The shell thickness is measured as shown in Figure 3.
It was determined by driving nail 2 into slab 1 and measuring the remaining length d of the nail. In Figure 3, 3 is a mold, 4 is a mold, and 4 is a mold.
indicates meniscus, and 5 indicates molten steel.

Figure 1, which shows the above experimental results, plots the internal crack evaluation on the vertical axis and the temperature gradient inside the shell on the horizontal axis. Internal cracks were observed in the round slabs, and in cases of ■ to ■, which were within 5°C, almost no internal cracks were observed.

Therefore, in actual operation, the slab temperature TL decreases as it moves away from the meniscus 4, as shown in Fig. 2, but it is necessary to suppress the temperature gradient inside the shell to within 5°C at the time of completion of cooling. This will prevent the occurrence of internal cracks. In FIG. 2, 6 indicates the secondary cooling zone, 7 indicates the non-water cooling zone, and the broken line indicates the wheat skin temperature Ts.

〔effect〕

As explained above, the present invention was achieved by focusing on the fact that the temperature gradient inside the shell is related to internal cracking, and through repeated experiments.When the present invention was applied to continuous casting of round steel materials for one month, the following results were obtained. As shown in Table 2, remarkable effects were obtained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the temperature gradient inside the shell and the evaluation of internal cracks, and FIG. 2 is a graph showing the change in temperature inside and outside the slab, where the solid line shows the skin temperature and the broken line shows the inside temperature. Further, FIG. 3 is a cross-sectional view illustrating a method of measuring shell thickness. D is the temperature gradient inside the shell, 1 is the slab, and 6 is the secondary cooling zone.

Claims (1)

[Claims] When the cooling of the slab in the secondary cooling zone of continuous casting is completed, the following formula (T_L-T_S)/d≦5℃/mm T_S: Skin temperature of the slab (℃) T_L : Inside temperature of solidified shell (°C) d : Solidified shell thickness (mm) A method for preventing internal cracks in a continuously cast round slab, characterized by performing cooling that satisfies the following.