JPH0479738B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a low machine height multi-point correction curved continuous casting machine having a radius of curvature of 3 to 5 m.

(Prior art) Conventional Japanese Patent Application Laid-Open No. 14062/1983 discloses that in a multi-point straightening curved continuous casting machine with a low machine height, the straightened inside of the slab at each straightening point is cast within a range of 0.35% or less. There is a proposal for producing slabs without internal cracks.

However, according to experiments conducted by the inventors, when casting slabs with a ratio of wide face to narrow face of 4 or more in a low machine height multi-point straightening curved continuous casting machine with a curvature radius of 3 to 5 m, the narrow face cooling device From a conventional point of view, if it is about 2m below the mold, there will be a part where the narrow side is extremely reheated,
Misalignment deformation of the short pieces occurred, as well as concentration of correction strain.

(Objective of the Invention) As a result of intensive investigation into the causes of the occurrence of internal cracks, the present invention has developed a low-pressure casting machine with a radius of curvature of 3 to 5 m that can efficiently cast slabs with a wide-to-narrow ratio of 4 or more without internal cracks. A high multi-point correction curved continuous casting machine is provided.

(Means for Solving the Problems) The gist of the present invention is to use a low machine height multi-point straightening curved continuous casting machine with a radius of curvature of 3 to 5 m for casting slabs with a wide-to-narrow ratio of 4 or more. 1st from directly below
This is because the narrow-sided cooling device is placed beyond the correction point position.

The present invention will be explained below.

When manufacturing steel slabs using a small arc continuous casting machine with a small radius of curvature, it is necessary to straighten the slabs and draw them out horizontally, but at this time the internal strain acting on the solidification interface is large and may cause internal cracks. tends to occur. As a countermeasure, multi-point correction is performed to disperse correction distortion. However, since the straightening is performed using a row of rolls, the number of straightening points cannot be increased indefinitely due to the roll diameter and geometric constraints for horizontal drawing.

Furthermore, in order to reduce distortion, the more the number of correction points is increased, the earlier it is necessary to start correction.
On the other hand, in such a low machine height multi-point straightening curved continuous casting machine, the static pressure of molten steel is small in the horizontal section after straightening is completed and until final solidification, so bulging is less likely to occur and it is difficult to cast at high speed. , internal cracks are less likely to occur and high productivity can be achieved.

However, in a low machine height multi-point straightening curved continuous casting machine in which the distance from the meniscus to the first straightening point position is short, manufacturing at high speed means straightening with a thin shell. In conventional single-point straightening curved continuous casting machines with a large radius of curvature of around 10.5 m, the solidified shell thickness during straightening is a thick shell of 80 to 100 mm. There was no known decrease in unique benefits, which will be described later.

The narrow side cooling of the conventional curved continuous casting equipment with a high machine height of 10.5 m and a high machine height of 1 point, which casts steel slabs with a ratio of wide side width to narrow side width of 4 or more, was carried out as part 8.
This will be explained based on the diagram.

1 is the mold, 2 is the same constant radius R= as the mold
3 is a straightening band for mechanically bending back the slab and straightening it into a straight line;
is the horizontal part that supports the slab until it completely solidifies. Wide-surface cooling sprays are arranged above and below between the rolls of the circular arc section 2, correction band 3, and horizontal section 4. On the other hand, the narrow surface is supported by a narrow surface support roll 5 at the arcuate portion directly below the mold, and a narrow surface cooling spray 6 is disposed thereon.

This narrow surface arrangement spray ends at most 3 m from the meniscus 7. The reason for this is that the need for narrow-sided spraying is mainly to prevent bulging and breakout from occurring in the thin shell directly below the mold 1. This is because the film becomes sufficiently thick and there is no need to worry about breakouts.

The required cooling length of the narrow surface from the viewpoint of preventing breakout is considered to be approximately constant regardless of the type of continuous casting machine. That is, even in the low-height 15-point straightening continuous casting machine with a curvature radius of 3 m shown in Fig. 1, the narrow surface cooling length was determined from the viewpoint of breakout prevention, and was set to 0.2.
% [C] I started casting steel, but the casting size was 250.
mm x 1050 mm, casting speed is 1.7 m/min, the cooling length of the narrow surface is up to 2 m from the meniscus, and the straightening zone is from 3 m (first straightening point position) to 7 m (first straightening point position) from the meniscus. 15 orthodontic point positions).

When the corrected internal strain is calculated using the following formula under these casting conditions, it is found to be sufficiently small at 0.23% at the maximum position.

Si=K√ εui=(D/2−Si)×(1/Ri−1/Ri−1)×100 where Si is the solidified shell thickness at roll i, li
is the distance of roll i from the meniscus, and K is the coagulation fixation number, which is 25 mm·min _-1/2 . Further, V is the casting speed, D is the slab thickness, and Ri and Ri-1 are the curvature radii of the front and rear rolls i.

In this way, even though the correction strain was distributed over 15 points, internal cracks occurred on the top surface. This internal crack was not accompanied by a negative segregation zone indicating molten steel flow, and it was clear that it was not caused by a misset of the rolls.
As shown in Figure 2, since the surface temperature 8 is low and the cracks are limited to the upper surface, it is highly unlikely that the cracks are due to bulging.

By the way, by radioisotope addition method etc.
When the starting point of the crack was accurately determined, it was confirmed that the crack started from 2 m below the meniscus. FIG. 3 shows the results of converting the crack starting point observed by S-print to the position l _c from the meniscus using the following equation.

l _c = V・(a+λ) ² /K ² where a is the shell thickness λ from the surface to the start of cracking
is the crack penetration depth from the solidification interface, and 3 mm is used based on the radioisotope addition test.

As shown in Figure 3, 1m from the first correction point position.
The occurrence of straightening cracks from above is an extremely puzzling phenomenon since it is not a bulging crack. However, since the crack is limited to the upper surface, the correction strain reverses by about 1 m due to some reason and is concentrated.

Although such a phenomenon was completely unknown in the past, as a result of the in-machine coagulation test, the deviation _δs of the short piece oscillation mark 10 shown in Fig. 4 was found to be 2 m below the meniscus as shown in Fig. 5. 3m at the border
It became clear that this had happened suddenly.

This misalignment occurs because the slab is a thin shell.
The energy required to deform a cross section is smaller than that of a flat surface, and deformation occurs concentratedly in areas with low rigidity. This is considered to be because such deformation occurs because less total energy is required than when correction is performed in a highly rigid area while keeping the cross section flat. That is, since the parts with weak rigidity are preferentially deformed, deformation of elements 11 to 12 shown in FIG. 6 and resulting displacement are observed.

If we look closely at the position where this deviation occurs, it corresponds to the area where the cooling of the narrow surface ends exactly at the narrow surface temperature 9 in FIG. 2a, and reheating occurs rapidly. That is, it is a discontinuous and weak portion within the narrow surface in the casting direction. Deformation concentrates at such narrow surface recuperation positions, causing the aforementioned internal cracks. Once a crack opens, even a small strain will cause the crack to grow and become a harmful defect.

Based on the above investigation, tests were conducted with various changes in steel type and narrow surface cooling length, and the results are shown in FIG. 7.
Moreover, FIG. 2b shows the wide surface temperature 8 and the narrow surface temperature 9 when the narrow surface is cooled to 8 m from the meniscus.

The casting conditions shown in Figure 7 are based on the casting speed of electromagnetic steel.
Other than the low speed of 0.8 m/min, the conditions are the same as in Figure 1. The reason why the casting speed of electromagnetic steel is slow is that it is extremely prone to internal cracking, but also because it is prone to bulging and has a high risk of breakout.

From Figure 7, although there are differences depending on the steel type under these conditions, the narrow-face cooling device is placed from directly below the mold beyond the first straightening point position, and narrow-face cooling is performed from directly below the mold beyond the first straightening point position. It is clear that internal cracks can be effectively prevented by this method, and that this method is extremely effective.

(Effects of the Invention) As described above, the present invention prevents the concentration of straightening strain by preventing heat recovery on the narrow surface, and can be used in a low machine height multi-point straightening curved continuous casting machine with a bending radius of 3 to 5 m. , it is possible to cast highly crack-sensitive steel grades at high speed without causing internal cracks,
It is extremely useful industrially.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of a 3mR low machine height 15-point straightening curved continuous casting machine, and Figure 2 is a chart of the surface temperature in the spray cooling zone, where a shows narrow-plane cooling up to 2 m from the meniscus and b shows narrow-plane cooling up to 8 m. Fig. 3 is a diagram explaining the position of crack occurrence, Fig. 4 is an illustration of shear deformation of the narrow surface, Fig. 5 is a diagram explaining the position of misalignment, and Fig. 6 is a diagram explaining the occurrence of misalignment and distortion. A schematic diagram of the relationship, Fig. 7 is a chart of internal crack reduction test by narrow surface cooling, and Fig. 8 is an explanatory diagram of a conventional 10.5 mR high machine height single point correction curved continuous casting machine. 1... Mold, 2... Arc part, 3... Orthodontic band, 4
...Horizontal part, 5...Narrow surface support roll, 6...Narrow surface cooling device, 11...Element before deviation occurs, 12...
Elements after misalignment occurs.

Claims (1)

[Claims] 1. In a low machine height multi-point straightening curved continuous casting machine with a curvature radius of 3 to 5 m that casts slabs with a ratio of wide face to narrow face of 4 or more, the narrow face cooling device is installed from directly below the mold beyond the first straightening point position. A curved continuous casting machine with a low machine height and multi-point correction.