JPH11179509A - Continuous casting method of billet cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a billet cast slab from internal crack and to reduce porosity as well as center segregation by holding a reduction face at a temp. lower than that of a non-reduction face when a center solid phase rate of a cast slab is in a specified range and reduction of a specified range quantity of a cast slab diameter is imparted to a cast slab at least a pair of rolls. SOLUTION: When a center solid phase rate of a cast slab is 0.4-0.9, the reduction of 10-40% of a cast slab diameter is conducted. A cast slab diameter is a cast slab diameter for a round billet, a cast slab thickness for a rectangular billet, a side length for a square. A partially solidified reduction device uses at least a pair of horizontal rolls 9, an increase in numbers of pairs of rolls is not always effective but is preferably 3 or less. A cooling method, not limiting, is preferably water spray or mist spray. By cooling upper.lower reduction faces, a non-reduction face is relatively made a high temp., as necessary, a non-reduction face may heated. A temp. difference is preferably 100-300 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting billet slabs for improving center segregation and porosity generated in slabs of carbon steel, stainless steel, high alloy steel and the like.

[0002]

2. Description of the Related Art When a billet slab is manufactured by a continuous casting method, center segregation and porosity are generated. However, electromagnetic stirring and unsolidified light reduction are applied at the time of casting, and a large rolling reduction is performed in a lump and rolling process. It is well-known that the application of the heat treatment results in a level at which center segregation, porosity and the like can be brought about without any problem.

For example, Japanese Patent Application Laid-Open No. 64-4863 discloses a technique for reducing center segregation by using both electromagnetic stirring and unsolidified light pressure.

[0004]

However, even in such prior art, the improvement has not yet been achieved until the sizing process can be omitted, and the center segregation and porosity are further reduced at the slab stage. There is a need to.

[0005] It is considered effective to perform unsolidification reduction with a large rolling reduction at the stage of billet cast slabs. However, performing unsolidification reduction with a large rolling reduction causes internal cracks, and thus this measure is desired. .

[0006]

The present inventor has conducted various experiments, investigations, and studies in order to solve the above-mentioned problems, and has obtained the following findings (A) to (C).

(A) The center segregation and porosity are in a semi-solid state in which a solid phase and a liquid phase coexist when the central solid phase ratio is in the range of 0.4 to 0.9. The slab can be discharged to the upstream side and the porosity can be reduced.

If the center solid fraction of the slab exceeds 0.9, the concentrated residual molten steel in the center is almost solidified with the center segregated, and cannot be discharged to the upstream side of the slab. Also, the porosity is solidified similarly, and it is difficult to crush the porosity.

(B) When the billet slab is unsolidified and reduced,
The slab is deformed in the direction in which the thickness decreases, in the direction in which the width expands, and in the direction in which the slab extends.However, when the surface temperature of the slab is high, the surface layer portion is easy to extend, or in the width expansion direction. Deformation becomes easy, and accordingly, deformation in the thickness direction becomes small. In other words, when the slab is unsolidified,
When the surface temperature of the slab is high, the rate of deformation of the slab in the extending direction is large, the deformation in the thickness direction is small, and the internal permeability in the thickness direction is reduced.

(C) By lowering the pressing surface of the billet slab to a low temperature, deformation in the extending direction and the width direction is suppressed, and by raising the non-pressing surface to a high temperature, deformation in the pressing direction, that is, the thickness direction is suppressed. Accelerated to give good internal permeability. The desired temperature immediately before the reduction of the slab is 800 ° C. to 90 ° C.
0 ° C. and 900 ° C. to 1100 ° C. on the non-pressed lower surface.

The pressing lower surface is a slab surface with which the pressing roll contacts, and the non-pressing lower surface is a slab surface with which the pressing roll does not contact at the time of pressing. For example, when rolling down the billet in the vertical direction, the top surface of the slab is the pressed surface, and the side surface of the slab is the non-pressed surface.

The present invention has been made on the basis of the above findings, and the gist of the present invention is that the center solid phase ratio of a slab is 0.4 to 0.1%.
9. The billet according to claim 9, wherein the pressing surface is kept at a lower temperature than the non-pressing surface side when a reduction of 10% to 40% of the slab diameter is applied to the slab by at least one pair of rolls. This is a continuous casting method for slabs.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a method for producing a slab having a center solid phase ratio of 0.4 to 0.9, wherein a reduction of 10 to 40% of the slab diameter is performed by at least one pair of rolls. This is a method of continuously casting billet slabs in which the pressing surface side of the slab surface is cooled immediately before the reduction and the pressing surface is maintained at a lower temperature than the non-pressing surface side.

The slab diameter is a slab diameter for a round billet, a slab thickness for a square billet, and a side length for a square cross section. These are collectively referred to as a slab diameter in the following description.

The reason why the reduction amount of 10% or more of the slab diameter is required is that the critical reduction amount which compensates for the solidification shrinkage and enables the discharge of the residual molten steel is 10%. The reason for setting it to be 40% or less is that above this, the effect is almost saturated. Preferably it is 15 to 30%.

The reason for setting the center solid fraction of the slab to 0.4 or more is that when the rolling reduction is less than 0.4, internal cracks occur. When the central solid phase ratio is 0.4 or more, internal cracks are once generated in the solidified shell by the reduction, but a compressive force acts as the reduction progresses, and an effect of closing the once generated internal cracks is produced.

The upper limit is set to 0.9, as described above.
If the center solid phase ratio of the slab exceeds 0.9, the concentrated residual molten steel in the center is almost solidified while segregating at the center, and cannot be discharged to the upstream side of the slab. The place where porosity exists is also solidified, and it is difficult to crush porosity.

Although at least one pair of rolls is used in the unsolidification rolling device, it is not always effective to increase the number of roll pairs, and it is preferable to reduce the number of rolls to 3 or less in view of the equipment cost and the effect. A pair of rolls is sufficient if the unsolidification draft can be ensured.

The reason why the upper and lower pressing surfaces of the slab are cooled just before the unsolidification rolling and the temperature is made lower than the temperature of the non-pressing surface is that deformation in the rolling direction, that is, the thickness direction is promoted and good internal permeability is obtained. This is because defects at the center of the slab can be reduced.
The cooling method is not particularly limited, but water spray or mist spray is preferred. By cooling the upper and lower pressing surfaces, the non-pressing surface is relatively heated to a high temperature. If necessary, the non-pressing surface may be heated. The temperature difference is not particularly limited, but is preferably 100 to 300 ° C.

In the present invention, 10% to 40% of the slab diameter is used.
%, Which may greatly impair the original shape of the slab, and immediately after the unsolidification reduction where the temperature is high and the deformation is easy, the forming roll is pressed in the direction perpendicular to the unsolidification reduction direction, For example, if the unsolidification reduction is a horizontal roll, it is preferable to install a forming device having a vertical roll as the forming roll immediately after.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention was carried out by using a curved continuous casting facility shown in FIG. The shape of the billet was a square square billet (0.8% carbon steel, 200 mm × 200 mm cross section), and the casting speed was controlled to 1.8 to 2.2 m / min.

To prevent bulging of the slab, a secondary cooling zone 5 immediately below the mold is provided with a length of 2 m, and no cooling zone is provided up to the secondary cooling zone 8 immediately before the unsolidification pressure. The specific water volume of the secondary cooling immediately below the mold is 0.2 l / kg-steel.

The unsolidification reduction is performed by using a pair of horizontal rolls 9 for rolling down the slab 1 from above and below, and the unsolidification reduction is performed by 2 mm from the mold melt surface.
The horizontal roll 9 was arranged at a position of 2 m. In the secondary cooling zone 8 immediately before the unsolidification pressure reduction, only the upper and lower surfaces of the slab 1 having a length of 2 m are cooled by water spray.
The test was performed at 0.3 l / kg-steel.

The upper surface temperature and the side surface temperature of the slab 1 immediately before entering the horizontal roll 9 were measured with a radiation thermometer. The horizontal roll 9 is a flat roll having a roll diameter of 400 mm, and can be rolled down by hydraulic pressure. The rolling force is 70 ton at maximum.

As a forming roll of the slab 1 after the unsolidification reduction, a vertical forming roll 10 is used.
The slab 1 was placed downstream and formed by pressing down both side surfaces of the slab 1. The roll diameter is a flat roll of 400 mm, and the rolling force is 100 tons at maximum.

In this example, the center solid fraction of the slab was estimated by one-dimensional radial heat transfer calculation of the slab for each casting condition. From this estimation result, the casting speed was adjusted so as to obtain a predetermined center solid phase ratio. In addition, the accuracy of the heat transfer calculation was confirmed by comparing the calculated value of the slab surface temperature with the actual measurement result and the comparison of the thickness of the solidified shell by tacking.

Table 1 shows operating conditions and evaluation results of the examples. Comparative Example 6 is an example in which only casting was performed without performing unsolidification reduction. In Comparative Example 7, the central solid phase ratio was 0.1 to 0.1.
Continuous light rolling is performed between 90 and the rolling reduction rate is 5% (rolling amount 1
0 mm). Only in this case, the horizontal roll 9 shown in FIG. 1 was removed, and a reduction roll segment for light reduction (5 pairs of rolls, a roll diameter of 210 mm, and a segment length of 5 m) was arranged. Secondary cooling before reduction is performed on the lower surface,
Neither was performed on the non-pressed surface.

The evaluation of the examples was conducted by taking 2 m cast slab samples corresponding to the stationary part of the cast after casting, cutting out 11 cross-sectional samples at a pitch of 200 mm, and performing center segregation, porosity and internal cracking. .

The center segregation was performed by sampling a chemical analysis sample having a diameter of 2 mm in a range of 50 mmφ from the center of the slab at a pitch of 5 mm in the radial direction from the center on a diameter straight line passing through the center of the cross section. The carbon concentration was determined by chemical analysis, and the maximum concentration was determined.
The value obtained by dividing the maximum concentration C (% by weight) by the average concentration C ₀ (% by weight), that is, C / C ₀ (−) was defined as the center segregation ratio. A good range of the center segregation ratio was set to 1.03 or less.

The porosity was determined by determining the area of porosity present in the cross section for each sample, and dividing the average value of the sample by the slab cross-sectional area as a porosity area ratio (%). A good range of the porosity is set to 0.02% or less.

Regarding internal cracks, each cross-sectional sample was subjected to sulfur printing, and the presence or absence was visually determined. As shown in Table 1, the internal quality (center segregation, porosity, internal cracks) of the cast pieces in Examples 1 to 6 of the present invention was good.

In Comparative Example 1, the center solid phase ratio at the time of reduction was 0.4%.
Because of the lower, internal cracks occurred. In Comparative Example 2, the porosity and the center segregation could not be improved because the center solid fraction was higher than 0.9. In Comparative Example 3, since the rolling reduction was as low as 5%, porosity and center segregation could not be improved. Comparative Example 4 is a case where the secondary cooling was performed on both the pressure lower surface and the non-pressure lower surface, and Comparative Example 5 was a case where neither was performed. However, both the comparative examples 4 and 5 had the center segregation and the porosity. Did not fall within the good range.
Comparative Example 6 was obtained by simply continuous casting without performing unsolidification reduction, and Comparative Example 7 was obtained by light reduction.
Neither the porosity nor the center segregation of Comparative Examples 6 and 7 were in good ranges as compared with Examples 1 to 6 of the present invention.

[0033]

[Table 1]

[0034]

According to the present invention, porosity and center segregation can be reduced without causing internal cracks in billet slabs.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating a method of the present invention.

[Explanation of symbols]

 1: cast slab, 2: submerged nozzle, 3: molten steel surface, 4: mold, 5: secondary cooling zone immediately below the mold, 6: molten steel, 7: solidified shell, 8: secondary cooling zone immediately before unsolidified pressure, 9: horizontal roll, 10: vertical forming roll, 11: pinch roll

Claims (1)

[Claims] When a reduction of an amount of 10% to 40% of a slab diameter is applied to a slab by at least one pair of rolls when the center solid phase ratio of the slab is 0.4 to 0.9. A continuous casting method for billet slabs, wherein the pressing surface is maintained at a lower temperature than the non-pressing surface side.