JPH08132105A - Method for preventing surface crack of large width hot-rolling of cast slab - Google Patents

Abstract

PURPOSE: To prevent surface crack at the time of a large width hot-rolling of a cast slab. CONSTITUTION: In this method for preventing surface crack at the time of a large width hot-rolling of the cast slab, as for a steel contg. >=10×10<-3> % Al and >=20ppm N, the surface temp. of the slab is cooled to a temp. not higher than the point Ar3 at the time of continuous rolling, γ→α transformation rate is ensured by the next inequality in accordance with the Al and N contents and, after that, the surface temp. of the cast slab is held at >=1000 deg.C for at least >=30min. X>=0.087×(Al)×(N)×10<7> ... Inequality. Where, X: α transformation rate(%), Al: Al content in steel(wt.%), N: N content in steel(ppm).

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 surface cracks during hot rolling of a slab with a large hot width.

[0002]

2. Description of the Related Art As a method of preventing surface cracks that occur when an aluminum-killed steel slab produced by a continuous casting method is hot-rolled to a width rolling amount of 400 mm or more, conventionally, AlN precipitated in a continuous casting process is used. In order to render the precipitates such as etc. harmless, in the secondary cooling zone in the continuous casting process of the cast slab, the cooling temperature is set to not more than Ar ₃ points and at least 750 ° C.
By heating to 00 ° C. or higher, AlN is not present in the γ grain boundaries, and AlN is included in the grains, so that surface cracking during width rolling has been prevented.

However, since the cracking sensitivities of Al and N, which cause cracking, differ, the cracking cannot be completely prevented even if the above-mentioned cracking prevention measures are carried out under the same conditions. there were. In addition, the cracking susceptibility was not indexed, so the cooling conditions required were unclear.

[0004]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention The present invention is a subject for preventing surface cracks during slab hot rolling, which is necessary according to the levels of Al and N contained in steel. It is to clarify the cooling conditions in the required secondary casting secondary cooling zone from the transformation rate.

[0005]

The present invention has been made to solve the above problems, and the gist thereof is
For steel containing so-called aluminum killed steel containing l ≧ 10 × 10 ⁻³ % and N ≧ 20 ppm, the slab surface temperature during continuous casting is Ar.
Cooling to ₃ points or less, securing at least 20% or more of γ → α transformation rate calculated from the following formula depending on the amounts of Al and N,
After that, the surface temperature of the slab is raised to 1000 ° C or higher for at least 30
It is a method for preventing surface cracks during hot rolling of a slab, which is characterized by holding for at least a minute. X ≧ 0.087 × [Al] × [N] × 10 ⁷ (1) However, X: α transformation rate (%) Al: Al content rate in steel (wt%) N: N content rate in steel (Ppm)

[0006]

[Function] Cracking of an aluminum-killed steel slab produced by continuous casting is caused by the thermal stress applied to the slab during solidification of the steel in a temperature range of 20 from the low temperature γ (austenite) phase region to the α + γ (ferrite + austenite) 2 phase region. Alternatively, it is said that it is generated by an external stress applied by correction, and it is considered that it is a γ intergranular crack caused by nitrides such as AlN continuously precipitating at the γ grain boundary. Therefore, in order to prevent this, the precipitation can be prevented by confining the precipitates such as AlN in the α grains by the γ → α transformation and minimizing the grain boundary precipitates that cause the cracks.

However, as a result of research and development by the present inventors, it was found that the proper amount of the γ → α transformation amount is greatly influenced by the amounts of Al and N contained in the steel, and the present invention is based on this finding. This is a completed product, and the basic idea of the present invention is to secure the minimum required amount of α transformation in accordance with the amounts of Al and N contained in the steel.

FIG. 1 shows the relationship between the product of the amount of Al and N in steel and the γ → α transformation rate, which shows the occurrence of cracking during hot rolling of a slab in Examples described later. The presence or absence is plotted in the same figure. The ∘ mark indicates that the slab surface did not crack, and the X mark indicates that the slab surface cracked.

As can be seen from the figure, as the value of the product of Al and N increases, the amount of γ → α transformation for suppressing the prevention of cracking must be increased. That is, it is understood from this result that the minimum required amount of γ-transformation does not change even if the type of steel changes, as long as the amounts of Al and N contained in the steel are at the same level.

Based on the experimental value in this actual operation, in steel A
The relationship between the product of 1 amount and N amount and the minimum required amount of γ transformation is
The following formula (1) is expressed by a mathematical formula. X ≧ 0.087 × [Al] × [N] × 10 ⁷ (1) However, X: α transformation rate (%) Al: Al content rate in steel (wt%) N: N content rate in steel (Ppm)

In order to obtain the α-transformation amount obtained by the above equation, during continuous casting, the surface temperature of the cast slab is cooled to below the Ar ₃ point, which is the transformation temperature region to α, and the transformation from γ to α is performed. Promote.

FIG. 2 shows an example of the slab temperature history from the continuous casting process to the rolling. Here, the slab is passed through the curved part of the continuous casting machine, and is then strongly cooled at a slab surface temperature of 1 to 2 ° C / s in the horizontal part, cooled to 750 ° C or lower, then reheated again and further heated. After heating to 1000 ° C. or higher in a furnace, wide reduction is performed.

FIG. 3 shows the relationship between the γ → α transformation rate calculated by the equation (1) based on the cooling curve in FIG. 2 and the slab temperature. It is clear that the γ → α transformation rate is increased by cooling the slab and lowering the slab surface temperature to a point below the Ar ₃ point. The slab cooling conditions for securing the amount of α transformation obtained by the above formula (1) are the ferrite (α) shown in FIG.
Cooling may be performed by an appropriate cooling means so that the slab temperature is equal to or lower than the slab temperature obtained from the transformation rate.

[0014]

EXAMPLE Steel having the chemical composition shown in Table 1 was cast with a curved continuous casting machine to obtain a slab size (thickness) of 282 mm ×
A slab with a (width) of 1550 to 1950 mm was manufactured and cooled to 750 ° C. or lower in a secondary cooling zone to obtain slabs having various α transformation rates. This slab was charged into a heating furnace at 850 ° C and
It was heated for 10 minutes, extracted at 1020 ° C., and then subjected to large rolling of the slab width by a sizing mill so that the slab width became 750 to 1430 mm. The results are also shown in Table 2.

[0015]

[Table 1]

As is clear from Table 1, No. 1 having the α transformation rate determined by the equation (1) secured. Regarding Nos. 1, 2, 4 and 6, no surface cracking was observed even when the slab width was rolled, but the α transformation rate was less than the calculated value. Three
For 5 and 7, surface cracking occurred.

[0017]

According to the present invention, for steel containing a predetermined amount or more of Al and N, the slab cooling during continuous casting is properly cooled so as to secure the α transformation amount according to the product of the Al and N amounts. As a result, a good slab can be produced without causing surface cracking even during slab wide rolling.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a product of Al amount and N amount in steel and a γ → α transformation amount.

FIG. 2 is a diagram showing a slab temperature history from a continuous casting process to a rolling process.

FIG. 3 is a diagram showing the relationship between the slab temperature and the γ → α transformation rate.

Claims (1)

[Claims] 1. Al ≧ 10 × 10 ⁻³ %, N ≧ 20 ppm
For steels containing Al, the slab surface temperature is cooled to below the Ar ₃ point during continuous casting, and γ → α is calculated from the following formula depending on the amounts of Al and N
A method for preventing surface cracking during hot rolling of a slab, characterized by ensuring a transformation rate and then maintaining the slab surface temperature at 1000 ° C or higher for at least 30 minutes or more. X ≧ 0.087 × [Al] × [N] × 10 ⁷ (1) However, X: α transformation rate (%) Al: Al content rate in steel (wt%) N: N content rate in steel (Ppm)