JPS6360057A - Reducing method for center segregation in continuously casting bloom - Google Patents

Abstract

PURPOSE:To reduce the center segregation in continuously casting bloom and to improve the breaking rate of wire and the cupping fracture rate for hard steel wire rod by cooling forcedly a short side of cast bloom as preceding to a long side face in the continuously casting bloom having flat section. CONSTITUTION:A volumetric shrinkage quantity in accordance with proceeding solidification of non-solidied part at the center of cast bloom along the casting direction in the range from remaining position of non-solidified molten metal to the completing position of solidification, is compensated by forcedly cooling from the cast bloom surface. In this continuous casting, by forcedly cooling the short side face before forcedly cooling the long side face, the temp. at the short side face becomes to lower than the temp. at the long side face. In this way, the ratio of the recessed quantity at the long side face to the bulging quantity at the short side face is made to large, and so the center segregation is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for reducing center segregation of continuous PJ slabs of flat cross-section material.

[Conventional technology]

In order to reduce the center segregation of continuously cast slabs and blooms, it is necessary to prevent the occurrence of bulging by setting an appropriate roll interval, arranging the rolls, or providing appropriate secondary cooling.

On the other hand, reduction of melt overmaturity, addition of coolant to the mold, application of electromagnetic stirring to the molten steel in the mold, application of ultra-FF waves to the slab in the strand, and furthermore, Techniques to reduce center segregation by applying electromagnetic stirring, light roll reduction, and other methods are widely used. All of these methods aim to make the cast structure equiaxed, to achieve fine dispersion of solutes, and to reduce center segregation.

Reducing the degree of superheating of molten steel is effective in L, which aims to make equiaxed products, but it is necessary to control the casting temperature within a narrow range, which has the disadvantage of hindering operational stability. Although this method is effective in improving product quality, it has the drawback that the melting of the coolant may leave residue and mold slag may be involved, making it easy to cause UT defects.

The application of AFF waves to the slab is effective in principle, but as an implementation technique, there is a problem of fatigue of the application roll, which makes it difficult to put it into practical use.

Considering these points, electric/a stirring within the mold or within the strand has few drawbacks in practical use, and is effective in producing capstone products, and is generally used. However, although it is recognized that center segregation tends to be reduced due to the progress of equiaxed products due to electromagnetic stirring, the fracture of hard steel wire rods made from continuously cast blooms applied with electromagnetic stirring also increases by 1 [magnetic stirring]. No significant improvement effect was observed compared to wire rods made from materials that were not coated.

For example, in a bloom slab for hard steel wire with a slab size of 400 mm x 560 mm, hollow-shaped cavities are formed intermittently along the casting direction at the axial center, and a further characteristic is that there is segregation near the axial center. This is different from the V segregation that occurs at the axial center of the steel ingot, and rather has the form of inverted V segregation in the steel ingot. V segregation occurs in an area with a radius of about 100 mm around the axis, and the central segregation and the adjacent negative segregation zone are clearly recognized. In this example, the region where negative segregation begins to occur is within 40 mm from the axis. That is, it can be seen that there is bulk solute movement in a region with a width of 80 mm centered on the axis.

It is clear from metallurgical observations and numerical calculations that this movement of solute-enriched molten steel is caused by the suction force caused by the solidification contraction of the remaining molten steel in the molten steel pool. Can be done.

Therefore, in order to prevent center segregation, IF should be applied near the axis of the slab (in the case of slabs, the center of thickness). The objective is to prevent the movement of solute-concentrated molten steel generated by the suction force caused by the solidification and shrinkage of the remaining molten steel in the molten steel pool.

As a measure for this, Japanese Patent Application Laid-open No. 52-104420 and Japanese Patent Laid-open No. 52-104420
No. 4-107831 discloses a center segregation reduction method using a roll light downstream method, but with this method, it is extremely difficult to continuously implement rolling reduction commensurate with solidification shrinkage. In other words, if the rolling reduction is small, it is insufficient to prevent the concentrated molten steel from moving downward, and if the rolling reduction is excessive, the C
The residual molten steel moves in the L direction, and the solute concentration increases in the upper part where it moves.

Moreover, a large-scale compression device is required.

In order to solve the problems described in
0-201383, the yield amount as the solidification of the liquid core of the slab progresses along the casting direction from a position 2 to 15 m before the leading edge of the residual molten metal pool in the casting direction to the pool's leading edge position. We proposed a compensation method that compensates for this by shrinking and deforming the solidified shell through forced cooling of the slab surface.

[Problem that the invention seeks to solve]

Although Patent Application L No. 60-201383 has many advantages over conventional methods, subsequent research by the present inventors shows that by further improving the direction of forced cooling, It was found that even better effects can be obtained.

The present invention has been improved based on this knowledge and provides an improved method for eliminating center segregation in continuously cast bloom slabs and producing sound slabs at low cost.

[Means for solving problems]

That is, the present invention deals with the volume gain IMIlj equivalent to 41 as the unsolidified liquid core solidifies along the casting direction from the position where the unsolidified molten metal remains in the slab until solidification is completely completed.
In the continuous APJ manufacturing method of molten metal, which compensates for this by forced cooling from the surface of the slab, the center segregation of the continuously cast slab is reduced, which is characterized in that the short side of the slab is forcedly cooled before the long side. It's a method.

[Effect]

Bloom of high carbon steel (C concentration in molten steel 0.40-1.2%) (slab size 400 mm x 560 mm
) was cast at a pI production speed of 0.45 to 0.58 m/min.

Experiments and studies were conducted to forcefully cool the bloom from a position 24 m above the meniscus to a position where solidification is complete.

At this time, while measuring the shape of the bloom after cooling,
20 samples for analysis were taken at 30 mm intervals in the casting direction using a drill of 5 mm diameter from the axis of the bloom, and the average carbon analysis value (C) was compared with the carbon analysis value (CO) of the surface layer of the slab. The ratio C/Co was determined.

As a result, as shown in FIG.
As shown in the figure, as the short side bulge i; increases, C/C
It turns out that o is large.

When a bloom containing an unsolidified liquid core is straightened using pinch rolls, each slab becomes shaped with a concave long side of about several mm. This is because the subsequent forced cooling of the slab further promotes the deformation of the long side, but if the short side is not properly cooled and its strength is insufficient, the short side will become convex. Due to the overhang, compensation for solidification shrinkage is inadequate.

Therefore, in order to make C/Co close to 1, it is necessary to make the concavity on the long side large and the bulge on the short side small. It is effective to increase the ratio to the short side surface bulge amount.

In the present invention, the short side surface is forcibly cooled before the long side surface is forcibly cooled, so that the temperature of the short side surface becomes lower than the temperature of the long side surface, and as described later, 9! Preferably, there should be a difference of 100°C or more in slab surface temperature.

As a result, it is possible to increase the ratio of the long side concavity 7H to the short side bulge, making it possible to significantly reduce center segregation.

〔Example〕

A hard copper wire molten steel containing 0.8 tons of charcoal and 9% of 1a was cast into a cross-sectional size of 4QQmz x 560mm using a fully curved continuous casting machine at a casting speed of 0.55 m/min.
This slab, which was cast at a water ratio of 0.551/kg in the Z-order cooling zone, was immediately rolled into a billet of 150 mm x 150 mm, and then rolled into a wire rod with a product size of 4.5 mmφ.

During this casting, the surface temperature at the outlet of the pinch roll band at a position 23 m from the meniscus (molten metal surface in the mold) is 950~
The temperature was 1000°C. Furthermore, according to heat transfer calculations and experiments using the riveting method, in the case of the present casting conditions, the final solidification position is considered to be about 34 m from the mecoscus.

During this casting, (1) Pinch roll stripping [[] until the final fi fixed position,
When forced cooling is performed with the same amount of cooling water rIX on each side (
Conventional method) (2) Pinch roll stripping [For about 2 m from 1, the long side is not cooled, only the short side is cooled, and from then on, as in (1), cooling water is applied to each side until the final solidification position. ! (Example I) (3) In the method of (2) above, while actually measuring the temperature of the short side and long side, a sub-tube of cooling water is applied, and Regarding the case where the temperature difference between the surface and the riverside surface is intentionally set to 100℃ or more (Example ■), Naka! We compared the cuppy rupture rates of nine poems.

The results are shown in Fig. 4.The cuppy breakage rate during wire drawing represents the number of breaks (times/m) that occurs when drawing a unit length of wire drawing material. The case of law is 1.
It is shown as a relative value when it is set to 0.

As is clear from Figure 4, by optimizing the cooling method,
Cuppy fractures 41 are significantly reduced compared to the conventional method. This is a result of reduced center segregation due to improved proper cooling.

〔Effect of the invention〕

By improving the forced cooling method according to the present invention, it is possible to further reduce the center segregation of blooms in continuous PJ construction and reduce the wire breakage rate and cuppy breakage rate in hard steel wire rods.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between long-side concave stitching and center segregation.
Figure 2 is a graph showing the relationship between short side bulge and center segregation, Figure 3 is a graph showing center segregation versus the ratio of long side dent and short side bulge rIi, and Figure 4 is a comparison with the example. It is a graph showing a comparison with the frequency of occurrence of cuppy breakage in the example.

Claims (1)

[Claims] 1. From the position where unsolidified molten metal remains in the slab until solidification is completely completed, the amount of volumetric shrinkage due to the solidification of the unsolidified liquid core is calculated from the slab surface along the casting direction. 1. A method for reducing center segregation of continuously cast slabs, characterized in that in continuous casting of flat cross-section materials, the short sides of the slab are forcibly cooled before the long sides.