JPH1058106A - Continuous casting method reducing center porosity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To drastically reduce the center porosity without deteriorating the crack and the center segregation of a cast slab by combinedly executing light rolling reduction at the end stage of solidifying the cast slab and rolling reduction of the center porosity. SOLUTION: The rolling reduction is executed at the position where unsolidified part 2 exists in the cast slab 1, with rolls 3 for light rolling reduction, and at the rear part thereof, the rolling reduction is executed with rolls for center porosity rolling roll. The condition of the light rolling reduction is that the rolling reduction is executed at the position of the cast slab whose temp. at the center part corresponds to 0.05-0.7 solid phase ratio, at 4-20mm draft. The rolling reduction for reducing the center porosity is executed at the position of the cast slab whose temp. at the center part corresponds to >=0.08 solid phase ratio, at 5-20% in the range of draft. This method can be applied to the continuously cast slab having about 50-300mm thickness. Particularly, in the case of producing a thick plate material from the thin slab. It is effective to reduce the product defect that the rolling reduction for reducing the center porosity is executed at the step of the cast slab.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a continuous casting method for reducing the center porosity of a continuous cast slab.

[0002]

2. Description of the Related Art In a central portion of a continuously cast slab, small holes, so-called center porosity, which are formed due to solidification shrinkage of molten steel are formed. The center porosity often causes UST failure of an extremely thick steel plate or an inner surface defect of a seamless steel pipe.

In particular, when trying to reduce the load of the rolling step by reducing the thickness of the cast slab and reducing its cross section, the center porosity is not crimped and tends to cause product defects. In addition, center porosity often poses a problem in relation to hydrogen defects, and a technique for reducing the center porosity at the slab stage is required.

In recent years, many techniques have been developed to reduce the segregation at the center of a continuous cast slab by reducing the unsolidified slab lightly. However, such light reduction requires a large center porosity. It is known that it is effective in reducing the amount.

For example, the inventors of the present invention disclosed in Japanese Patent Laid-Open No.
As disclosed in JP-A-79265, the solid fraction at the center of the slab is 0.05 to the flow limit solid fraction (solid fraction of 0.1%).
It has been confirmed that by performing the two-step light reduction in the range of about 7), the maximum segregation particle size is significantly reduced, and the center porosity having a large size is significantly reduced accordingly.

However, even with the above-described light rolling method, the solid fraction at the center of the slab at the final rolling position is 0.7 or less,
Subsequent solidification shrinkage forms a center porosity of 1 to 2% or more by volume. That is, the conventional light reduction method is effective in reducing the size of the center porosity, but the reduction in the volume ratio is not necessarily sufficient, and further improvement is required.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method capable of further reducing the center porosity as compared with the conventional continuous casting slab light reduction method. Therefore, in the present invention, the slab is reduced in the early stage of solidification, without the accompanying deterioration of the slab cracks and center segregation, with the object of presenting a reduction method capable of significantly reducing the center porosity. I do.

[0008]

The gist of the present invention to solve the above-mentioned problems is to provide a continuous casting method of molten metal in which a slab is drawn down while being solidified before solidification. At least a pair of rolls are installed at the slab position corresponding to the phase ratio of 0.05 to 0.7 to reduce the slab by 4 to 20 mm, and the temperature at the center of the slab is 0.8% or more. A continuous casting method for reducing center porosity, characterized in that at least one pair of rolls is installed at one position and a cast piece is reduced at a reduction rate of 5 to 20%.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic view showing an example of the arrangement of a reduction roll according to the present invention. A slab 1 is reduced by a light reduction roll 3 at a position where an unsolidified portion 2 is present, and further rearward. , The center porosity reduction roll 4 is used to reduce the pressure.

In the present invention, the reason for combining the light reduction with the reduction of the center porosity in this way is to reduce the segregation of the center of the slab by the light reduction and to reduce the segregation particles in the center of the slab by the light reduction. This is because the diameter is reduced or the equiaxed crystal ratio is increased to reduce the size and dispersion of the center porosity, thereby facilitating the pressing of the center porosity.

In addition, the reduction of the center porosity of the continuous cast slab before or immediately after the solidification is performed is that the deformation resistance outside the slab is large and the deformation resistance near the center is small. This is because this is the most advantageous in performing center porosity crimping.

In the present invention, the conditions under light pressure are to reduce the center segregation to some extent and to reduce the size of the center porosity.
What is necessary is just to be able to disperse | distribute, for that purpose, it is good to reduce the slab by 4 to 20 mm at the slab position where the temperature of the slab center part corresponds to the solid phase ratio of 0.05 to 0.7.

The reason why the position of the light reduction is set to a position where the temperature at the center of the slab corresponds to the solid phase ratio of 0.05 to 0.7 is that the effect of the light reduction is significantly reduced outside this range. In addition, the reduction amount of 4 to 20 mm is insufficient when the reduction amount is 4 mm or less to compensate for the solidification shrinkage, which is the original purpose of the light reduction, and when the reduction amount is 20 mm or more, the internal cracks are reduced. This is because there is a risk of occurrence.

From the viewpoint of ideally reducing center segregation as a method under light pressure, for example, the temperature at the center of the slab is 0.05 to 0.
At the position corresponding to 25, a first step of light reduction with a reduction gradient of 0.2 to 3.0 mm / m, and a solid phase ratio of 0.25 to 0.6
It is preferable to perform a second-stage reduction with a total reduction amount of 4 to 20 mm at a position corresponding to the following.

The solid fraction in the present invention is defined by the following equation as a function of the temperature T at the center of the slab. Solid phase ratio = (T ₁ −T) / (T ₁ −T _s ) where T _l : liquidus temperature of molten steel (° C.) T _s : solidus temperature of molten steel (° C.) T: cast slab It is the center temperature (° C).

The center temperature T of the slab can be calculated in advance by operating conditions such as cooling conditions and casting speed. Therefore, the solid phase ratio is a function of the solidification time as shown in JP-A-2-303661, if the casting speed, cooling conditions, slab size, and steel type are determined. It can be easily converted to thickness, unsolidified rate, etc.

In the present invention, the reduction for reducing the center porosity is performed at a slab position where the temperature of the slab center is 0.8% or more of the solid fraction and a reduction ratio of 5 to 20%. And That is, the center porosity reduction roll 4 in FIG. 1 is installed at a position where the solid fraction is 0.8 or more under standard operating conditions (particularly, standard casting speed).

When a large reduction is applied to the slab at a position where the solid fraction is less than 0.8, the molten steel at the center of the slab flows to deteriorate the center segregation. This is because, at the position, the center segregation does not deteriorate, and only the pressing of the center porosity is effectively performed.

The reason for setting the rolling reduction at this time in the range of 5 to 20% is that if the rolling reduction is less than 5%, the pressing effect of the center porosity is insufficient, and even if the rolling reduction exceeds 20%, the center porosity cannot be reduced. This is because the crimping effect reaches a plateau and only increases the load on the screw-down device.

The appropriate reduction rate of the center porosity reduction roll depends on the size and dispersion state of the center porosity before the reduction, and therefore, the condition of the light reduction.

When an appropriate light reduction is performed at the position of the slab where the effect of the light reduction is the greatest, for example, at a position where the temperature at the center corresponds to the solid phase ratio of 0.5 to 0.7, the center porosity is reduced. Are small and uniformly dispersed, so that the center porosity can be sufficiently pressed by reducing the center porosity with a reduction ratio of about 5 to 10%.

On the other hand, when the condition of the light reduction is other than the above, the size of the center porosity is large and the local porosity may be locally uneven, so that a reduction ratio close to 20% is required.

The center porosity reduction roll is preferably installed at a position where the center solid phase ratio is 1.0 or less. If the center solid phase ratio of the slab becomes 1.0 or more, the deformation resistance sharply increases due to a decrease in the temperature of the slab, and the rolling efficiency becomes poor, and the center portion may not be effectively lowered.

In the method of the present invention, the slab thickness is 50 to 300.
It can be applied to continuous cast slabs of about mm, especially when manufacturing thick plates from thin slabs, reducing the center porosity at the slab stage is an effective way to reduce product defects. Become.

[0025]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows an outline of a continuous casting machine which has been subjected to a test of the continuous casting method of the present invention. A test was performed by installing a light reduction roll and a center porosity reduction roll as shown in FIG. 1 in the reduction zone of FIG. Table 1 shows typical examples of the composition of the molten steel that was cast.

[0026]

[Table 1]

The section size of the slab is 150 to 300 m in thickness.
m, a test was conducted within the range of 900 to 2500 mm in width, and the casting speed was set to 0.65 to 0.7 m / min as a standard,
The test was performed in the range of 0.75 m / min.

In both the examples and comparative examples, two-stage light reduction considered to be effective in reducing center segregation was performed. That is, first, the first stage of the first step is performed at a position where the temperature at the center of the slab corresponds to the solid phase ratio of 0.05 to 0.25, with a reduction gradient of 1 mm / m. At the position corresponding to
The second stage of light reduction was performed at 6 mm (a reduction gradient of 1.8 mm / min).

In the embodiment, the center porosity is reduced in addition to the above-described light reduction, and the comparative example is the case where the center porosity is not reduced and the center porosity is reduced.

As the conditions for the reduction of the center porosity, when the casting speed is 0.65 to 0.7 m / min, the position of the center solid fraction of 0.8 is 5% and the position of the center solid fraction is 1.0. 5%, for a total of 10% reduction. Also, when the casting speed fluctuates in the range of 0.55 to 0.75 m / min,
A total reduction of 20% or less was applied at the position of the central solid phase ratio of 0.8 to 1.0.

FIG. 3 shows a comparison of the center porosity volume ratios of the slabs in the example and the comparative example. As can be seen from the figure, in the example, when the casting speed is 0.65 to 0.7 m / min, the center porosity volume ratio is almost 0, and even when the casting speed varies, the volume ratio is approximately 1% or less. Met. On the other hand, in the comparative example, the center porosity volume ratio was in the range of 0.5 to 2.0%, and the effect of reducing the center porosity by the method of the present invention was confirmed.

[0032]

According to the method of the present invention, the center porosity can be reduced without causing cracks in the slab and deterioration of center segregation by performing a combination of light reduction and center porosity reduction at the end of solidification of the continuous cast slab. It became possible to greatly reduce.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of the arrangement of a pressing roll in the present invention.

FIG. 2 is a diagram showing an outline of a continuous caster on which a test of the present invention has been performed.

FIG. 3 is a diagram showing a comparison of a center porosity volume ratio of a slab in an example and a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Cast piece 2 Unsolidified part 3 Roll for light reduction 4 Roll for center porosity reduction

Claims (1)

[Claims] In a continuous casting method of a molten metal in which a slab is drawn down while being pressed before solidification, at least a pair of slab positions are located at a slab position where a temperature of a slab center portion corresponds to a solid phase ratio of 0.05 to 0.7. Rolls are set to reduce the slab by 4 to 20 mm, and at least a pair of rolls are set at a slab position where the temperature of the slab center is 0.8% or more of the solid phase ratio, and the reduction rate is 5 to 20%. A continuous casting method that reduces center porosity by rolling down the slab.