JP2008137013A - Continuous casting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the risk of the development in operating technology on a real production scale in a continuous casting method by which a hollow slab is pressure-welded so as to produce a solid slab and which has not been performed heretofore. <P>SOLUTION: Equipment designing is performed in such a manner that a hollow slab press-welding system is made possible in a drawing locus composed of a 3/4 circumference and rectilinear propagation by curved type continuous casting. In the equipment, the dimensions/shape of a mold and a drawing rate are suitably set, and a slab is perfectly solidified to the center part thereof. The cross-sectional aspect ratio or the like in the slab are optimized for securing efficiency of the casting. Rolling straightening is acted for solving bending strain to the circular arc radius which is too small. In this way, the conventional ordinary production and production in the new method are suitably changed over. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a curved continuous casting method of steel.

When manufacturing a steel slab from molten steel, the total cost is greatly reduced by adopting the continuous casting method, but there are many problems. For example, in the case of a slab, the equipment cost is extremely high. Regardless of the cross-sectional shape, defects inherent to continuous casting such as center segregation, porosity and internal cracks have not been sufficiently solved.

Patent Documents 1 and 2 propose a continuous casting method that 1) dramatically improves casting efficiency and 2) eliminates internal defects. The principle is that slabs in which molten steel is cast into the mold and the outer skin is formed are continuously drawn downward along the arc from the mold, and beyond the semicircle until the center part of the slab is solidified A hollow cast slab is formed by pulling upward from the surface over a static iron equivalent height of about atmospheric pressure (about 1.4 m), and then the slab is pressed by a pressure rolling mill to press the inner surfaces together. Is a continuous casting method using solid slabs.
In terms of quality, since there is no solidification end point, the core defect is eliminated, and it consists of substantially columnar crystals and becomes more homogeneous.
When the solidification section length is the same as that of the conventional method, the casting efficiency is improved several times.

Patent Document 3 discloses that the structure of the continuous casting method is low in machine height, and a series of physical distribution from the melting furnace to the slab unloading is renewed. The slab drawing locus consists of 3/4 circumference and subsequent horizontal drawing. According to this method, the ladle on the carriage that has received the molten steel immediately below the melting furnace is directly transferred and suspended from the carriage traveling on the GL (ground level) to the ladle rack in the continuous casting machine. The slab is curved, passes through the lowest point on the first basement floor, rises, stretches at the 3/4 circle point on the second floor above the ground, and travels horizontally.
In the case of this method, 1) a ladle crane which is indispensable in a normal continuous casting method is not required. 2) Therefore, the steel factory building is extremely light and has low installation costs. 3) Automation of ladle handling becomes easy. 4) The layout between the melting furnace and the casting machine where the logistics are mixed becomes simple and safe. 5) The unloading of the slab and the rolling floor are at the same level, and effects such as easy maintenance are produced.

Patent Document 4 discloses a method for efficiently producing low-cost and high-efficiency blooms and billets of arbitrary dimensions from a slab by a novel method obtained by improving the continuous casting method.
By this method, the slab cross-sectional dimension can be adjusted to the optimum shape for the steelmaking process, and can be adjusted to the optimum shape for rolling by a sizing mill.

Patent Document 5 discloses a method of efficiently manufacturing a beam blank by simultaneously reducing and pressing both the long and short pieces of a slab-shaped hollow cast piece in the method.
Patent Document 6 discloses a method of improving the method and modifying it to a concentric structure to obtain a precision hardened steel.

As described above, the continuous casting method is not yet put into practical use even though it is an innovative method that is expected to have extremely excellent characteristics and great effects. The reason for this is that although the research and development stage has been completed, development of mass production technology requires the development of actual production scale equipment (50 to 100 t / h) and the accumulation of operational experience, and the initial equipment costs for that. This is to overestimate both the burden of experimental operation costs and the risk. Therefore, it would be extremely advantageous if the risk can be reduced and the burden can be directly contributed to production without burden.

Patent Publication No.2989737 Patent Publication No. 3218361 Japanese Patent Publication No.3684731 Japanese Patent Publication No.3677572 Patent Publication 2001-205397 Patent Publication 2002-346710

As described above, the innovative continuous casting method “from the time when the molten steel is cast into the mold and the outer slab is formed is continuously drawn downward along the arc from the mold until the center of the slab is solidified. A hollow slab is formed by pulling upward beyond the semi-circle and further exceeding the atmospheric pressure equivalent static iron pressure height (about 1.4 m) from the casting surface, and then the slab is reduced by a pressure rolling mill. Practical application of `` continuous casting method in which the inner surfaces are pressed against each other to form solid slabs '' has been stagnant due to the large burden and risk of initial equipment costs and experimental operation costs in developing actual operation technology, and the present invention The task is to significantly reduce the burden and risk.

The following basic measures are taken to solve the above problems.
1) Using a continuous casting machine having the same structure as the innovative continuous casting method,
2) A general fully solidified continuous cast slab is subjected to daily production under appropriate casting conditions, and 3) The production test on an actual production scale by the innovative method is appropriately performed by changing the casting conditions.

The present invention provides a drawing locus in which a slab continuously drawn out along a circular arc downward from a mold turns upward beyond the lowest point M, is stretched at a 3/4 circumferential point T, and is drawn horizontally. In the continuous casting method of steel, 1) the arc radius R is set to 3 to 5 m, and 2) the position at which the center portion of the slab solidifies is 3/8 circle point P and 1/2 circle point +1.4 m position Q. 3) It is a continuous casting method characterized in that stretching is performed while applying a reduction of 2% or more by a pair of rolls at a point T.
In the present invention, the casting efficiency is enhanced by increasing the aspect ratio of the slab cross section.

According to the above invention, by setting appropriate casting conditions in the same continuous casting machine, both general continuous solidification type continuous casting and hollow continuous slab pressure welding type innovative continuous casting can be achieved at a production scale production efficiency. Can operate. In the former, products are used for daily production, and in the latter operation, it is easy to identify problems in mass production and search for improvement measures. In other words, it can be used for new technology development as appropriate while being produced, and it is easy to shift to mass production as a new product as it is with the developed technology.

As a second effect, the effect on the vertical and horizontal layout disclosed in Patent Document 3 can be obtained despite the production of the conventional method. That is, 1) A ladle crane which is indispensable in a normal continuous casting method is not required. 2) Therefore, the steel factory building is extremely light and has low installation costs. 3) Automation of ladle handling becomes easy. 4) The layout between the melting furnace and the casting machine where the logistics are mixed becomes simple and safe. 5) The unloading of the slab and the rolling floor are at the same level, and effects such as easy maintenance are produced.

Hereinafter, embodiments will be described with reference to the drawings.
FIG. 1 is a schematic side view illustrating a continuous casting method embodying the present invention.
Molten steel 2 is cast from the tundish 1 into a curved mold 3 having a rectangular cross section. The slab 4 with the outer shell formed by the mold 3 is drawn down along the arc by the pinch roll 6 and further cooled through the secondary cooling zone 5, exceeding the 3/8 round point P, and forming a semicircle. Furthermore, it is drawn upward from the casting surface beyond the point Q corresponding to the atmospheric pressure equivalent to the static iron pressure (about 1.4 m), straightened by the straightening machine 7 arranged from the 3/4 round point T, and horizontal. And is cut by a cutting machine 8 into a steel piece.

In the continuous casting method, the arc radius R is set to 3 to 5 m. The drawing speed V and the cooling strength of the secondary cooling zone are adjusted according to the mold size (diameter or short side size) so that the solidification of the slab center ends between the points P and Q. The machine length (= the length from the casting surface to the solidification end point) L is π × R + 1.4 m. Since it solidifies to the center of the slab, the solid slab 10 is pulled out and guided to the straightening machine 7.

The straightening machine 7 is composed of two or more roll pairs in the front and rear, and the leading roll pair is arranged at a point T and functions as a rolling mill 9. After the second roll, it interacts with the rolling mill 9 to have a function of correcting and guiding the bending. When the solid slab 10 is straightened, it is corrected while being reduced by a rolling mill 9 at a reduction rate of 2% or more.

In the present invention, the casting efficiency is premised on ensuring a normal production level. The machine length directly linked to the casting efficiency is smaller than that of the conventional high-efficiency continuous casting machine due to the size of the bending radius. As a result, efficiency is disadvantageous. As a countermeasure, the mold cross-sectional dimensions are set appropriately. A rectangular shape is desirable for improving efficiency, and the cross-sectional aspect ratio is a decisive condition.
With the above equipment and working method, the same continuous solidification type continuous casting as in the past is performed with the same casting efficiency as in the past.

As shown in FIG. 2, solidification does not end at the Q point and the solidification does not end at the Q point when the three dimensions of the mold size (short side size or diameter), the drawing speed, and the secondary cooling zone cooling strength are appropriately combined in the above equipment. The molten core 11 is detached to form a hollow slab 12. The hollow slab is reduced by a rolling mill 9 so that the solidified shell inner surfaces are pressed against each other to form a solid slab 13. That is, the continuous casting method of the present invention can be easily converted to a known continuous casting method of a hollow slab pressure welding system only by changing the casting conditions.

The gist, elements, background of the specified conditions, reasons, grounds, etc. will be described below.
Operation that can easily switch between two continuous casting methods with one continuous casting machine, that is, on the one hand, a hollow pressure welding system that can be expected to dramatically improve quality and efficiency, and on the other hand, complete with conventional quality and efficiency The reason for operating the solidification method is as follows.
Although the development basis for the former method has been completed, it has not yet been put into practical use. It is necessary to develop, improve, and accumulate operation technology depending on the actual production scale, but it is too expensive as a facility for development testing only, and it feels very risky. If the test machine can be shared with daily production without any problems, it is extremely effective as a risk mitigation measure. This is the main point / object of the present invention.

Regardless of which method is used for the above purpose, the casting efficiency is premised on the actual production scale (eg, 50 to 100 t / h, and in some special steels, 20 to 30 t / h). As disclosed in Patent Documents 1 and 2, the casting efficiency in the case of the hollow method is disclosed that if the arc radius R is 4.5 m, the above-described efficiency can be easily obtained for the slab cross-sectional shape of a circle, square, or rectangle. Has been.
Therefore, as a first problem, it is necessary to prove that the above efficiency can be obtained under practical conditions even in the case of the complete solidification system.

The casting efficiency Pc in the case of the complete solidification method is set based on the following calculation.
Casting efficiency Pc = Cross section area S × density ρ × drawing speed V
Slab cross-sectional area S = (Cast slab short side dimension B) ² x Slab cross-sectional aspect ratio β
Drawing speed V = machine length L / coagulation time t
That is, if the solidification time is known for the slab dimensions, the efficiency can be easily calculated.

First, consider the solidified shell thickness formula. In the case of infinite one-dimensional solidification on one side, as is well known, the shell thickness d is in the relationship of time t and equation (1). The proportional coefficient k is a solidification constant depending on the cooling strength.
d = k√t ---------- (1)
In actual continuous casting, the above equation is not accurate because it is almost two-dimensionally solidified. The solidification rate varies depending on the mold cross-sectional shape. In the case of a square, the above equation can be approximated up to a shell thickness ratio (= solidified thickness / final solidified thickness) of about 0.5.

An approximate expression for the two-dimensional solidification of the circular cross-section slab is derived from the steady heat transfer expression (2) of the hollow cylinder.
Q = 2πλ (θ ₁ -θ ₂ ) / (lnr ₁ -lnr ₂ ) --- (2)
Here, Q: radial heat flux, λ: thermal conductivity, θ ₁ : inner surface temperature, θ ₂ : outer surface temperature, r _1,2 : inner, outer radius. Solidification with a minute shell thickness Δr proceeds in a minute time Δt.
When the cooling surface temperature Ts is constant and λ (Tm−Ts) / H / ρ = K
r × ln (Ro / r) × Δr = K × Δt (3)
Here, Tm: solidified shell inner surface temperature, Ts: slab surface temperature, H: solidification latent heat, ρ: density, r: solidified shell inner surface radius, Ro: slab section radius.
Expression (3) is an approximate expression, and the progress of coagulation can be calculated by a sequential calculation method.

FIG. 3 shows the progress of solidification by the two-dimensional solidification formula (3) for various diameters of the circular cross-section slab in comparison with the one-dimensional solidification formula (1). In the first half of solidification, both curves approximate, but in the second half, they deviate rapidly, and the solidification end time is about 0.5 times for all dimensions.

FIG. 4 shows the progress of solidification of a 160 mm diameter circular slab and a 160 mm thick slab in comparison with one-dimensional solidification.
The progress of solidification of the square cross section is easily assumed to be located between the case of the circular cross section and the one-dimensional case, and further between the square and the one-dimensional solidification in the rectangle. When the internal shape of the solidified shell in the case of a square is measured from an actual breakout slab, it is almost circular with a shell thickness ratio of 0.7. From the calculation and actual condition, the solidification end time is estimated to be about 0.7 times of one-dimensional solidification. From here, the rectangle is considered to be about 0.8.
If only the solidification end time is a problem and the progress is ignored, a simple approximate expression for calculating the solidification coefficient k corresponding to the cross-sectional shape and the solidification time t based on the k value is substituted.

That is, the solidification constant k for one-sided infinite one-dimensional solidification may be corrected to k _1, k _2, k ₃ corresponding to the cross-sectional shape. FIG. 4 and various actual measurement data are corrected to show only the result as follows.
Circular cross section _{k 1 = 1 / √0.5 × k} = 1.4k -------- (4)
Square _{k 2 = 1 / √0.7 × k} = 1.2k -------- (5)
Rectangular _{k 3 = 1 / √0.8 × k} = 1.1k -------- (6)
The solidification time t for each shape is derived from the above equation.
Circular cross section t ₁ = D ² / 4k ₁ ² -------- (7)
Square t ₂ = B ² / 4k ₂ ² -------- (8)
Rectangular t ₃ = B ² / 4k ₃ ² -------- (9)
Similarly, the casting efficiency P of each shape is derived from the above formula.
Circular section P ₁ = πρk ₁ ² L -------- (10)
Square P ₂ = 4ρk ₂ ² L -------- (11)
Rectangular P ₃ = 4ρk ₃ ² Lβ -------- (12)
Here, D is the diameter, B is the short side thickness, ρ is the steel density, L is the machine length, and β is the rectangular aspect ratio.
From the structure of the formula, it is understood that the casting efficiency is independent of the cross-sectional dimension of the slab.

FIG. 5 shows the influence of the arc radius and the slab section aspect ratio on the casting efficiency obtained from the above equation. From the figure, in the case of R = 4 m, only about 25 t / h can be obtained for both a circle and a square. On the other hand, if the aspect ratio is a rectangle of 2 or more, the solidification time slightly increases and the drawing speed has to be reduced. However, since the cross-sectional area increases in proportion to the aspect ratio, the required efficiency can be easily obtained.

From the above description, in the continuous casting method having a drawing trajectory composed of a 3/4 circumference and a straight line, if the arc radius is about 4 m, in the case of the hollow pressure welding method, casting of 50 t / h or more regardless of the cross-sectional shape of the slab. On the other hand, in the case of the complete solidification system, an equivalent efficiency can be obtained by making the cross section into an appropriate rectangle.
If the cross-sectional shape of the slab is different from the desired cross-sectional shape of the steel slab, it can be corrected to an arbitrary cross-sectional dimension by following the sizing process disclosed in Patent Document 4.

As a second problem, the problems related to the arc radius will be examined. In the curved continuous casting method, the curved inner surface is pulled when the bending is straightened. Cracking occurs when the slab skin is fragile. In order to prevent this, the size of the bending radius is restricted. The value depends on the production type and the required level, but is usually designed to be 1% or less. In some cases, the bending distortion may be further reduced by correcting in multiple stages. The same applies when casting vertically and bending.

The bending strain is calculated by the following formula.
Bending strain = slab thickness / arc diameter The arc radius of a continuous casting machine currently in practical use is designed to be 6 m or more according to the above conditions.
The curvature radius R of the new continuous casting method described in the series of patent documents described above is characterized by being relatively small compared to the conventional bending method. In practice, 6 m or less is appropriate. In the related patent documents described above, the problem of bending strain has not been mentioned or disclosed.

In the present invention, the bending problem is solved by rolling. That is, rolling is applied at the T point without performing simple bending during straightening. In rolling in the case of a rectangular section, since the widening is not large, most of the reduction is stretched, and the tensile stress on the inner surface of the arc due to bending is eliminated or reduced.
Strictly speaking, it is not the tensile stress itself but the stress gradient that opens the fragile portion. Since the entire skin is stretched by rolling, the gradient is reduced to suppress the opening.

The reason why the arc radius R is specified as 3 to 5 m in the present invention corresponds to appropriate conditions for carrying out the hollow pressure welding type continuous casting method, and the conventional complete solidification method is also suitable for actual production by various measures. This is because it is a range. If the length exceeds 5 m, there is room for the latter method, but facilities and work are redundant for the former method. If it is less than 3 m, the latter method is not practical because the production efficiency is lowered.

The reason for specifying the position of the solidification end point as between PQ is that the machine length is insufficient upstream from the point P and the casting efficiency is insufficient. This is because the molten core is detached at the Q point downstream from the Q point and a cavity is generated, which corresponds to the preceding hollow pressure welding method.

The reason why the rolling reduction is 2% or more is as follows. The slab bending strain is 1 to 4% from the slab thickness assumed to be the aforementioned arc radius. Rolling is effective for reducing the strain to 1% or less as in the prior art. When the rolling reduction is 10% or less, most of the rolling strain becomes stretching strain. In the rectangular cross section, the widening is slight and almost stretched. The rolling stress is compression in the rolling direction at the skin portion. Accordingly, the film is stretched while being compressed, and the bending tensile strain is actively relieved. Even if the rolling reduction is less than 2%, it is effective depending on the conditions, but the lower limit is set to 2% in consideration of fluctuations in manufacturing conditions. Although there is no particular upper limit, 10 to 30% is implemented from the case of continuous casting inline rolling.

The above measures are also effective for steel types to which Al, Ti, B, etc., which are prone to cracking during bending, or to which surface cracks are likely to occur during rolling after reheating, are processed on the surface of the slab skin by roughening the roll surface. Increasing the compressive strain microscopically or further recrystallizing makes it more effective.

To reverse the actual situation of the present invention, while using a continuous casting machine of the hollow slab pressure welding method, the work of the conventional complete solidification method, which has a lower casting efficiency and lower quality compared with the method, is improved. It is to be implemented appropriately. The reason for adopting a method that retreats from progress is to develop an innovative operation technique of the former method while maintaining daily production as before, as described above.

The hollow pressure welding method is designed as a single strand in principle. Conventional methods are usually designed with multiple strands to ensure the required casting efficiency. The present invention is a single strand for the purpose of the invention.

Although it is not an experimental example as an example, Table 1 shows the result of arranging the design conditions and expected performance in comparison with the conventional case. It can be seen from the table that both types of continuous casting methods can be achieved on an actual production scale by appropriately setting the slab short side thickness, the slab cross-sectional aspect ratio, the drawing speed and the cooling strength.

In the above table, in the case of hollow pressure welding, the cooling strength of the secondary cooling zone is set weak, and the drawing speed is slightly reduced. In the case of the present invention, the drawing speed is close to the upper limit possible.
The drawing speed of the conventional method is set slightly smaller than the equipment performance. The reason is related to slab quality.

According to the continuous casting method of the present invention, a conventional product can be produced with a conventional casting efficiency. If the casting conditions are appropriately combined, an innovative hollow slab pressure welding continuous casting method can be implemented on an actual production scale. As a result, the latter operation technology will be developed, and its practical application and spread will be accelerated. In parallel with development and improvement, products and manufacturing will gradually shift from the old method to the new method, and quality improvement and new product development will progress. There is virtually no management risk and the new method can be adopted.

It is a schematic side view which illustrates the continuous casting method of this invention. It is a schematic side view which performs the continuous casting of a hollow pressure welding system using the installation of this invention. It is a figure showing progress of solidification of a circular section slab. It is a figure showing progress of coagulation of various cross-sectional shapes. It is a figure which shows the relationship between a casting efficiency, a circular arc radius, and a cross-sectional aspect ratio.

Explanation of symbols

1: Tundish 2: Molten steel 3: Mold 4: Cast slab 5: Secondary cooling zone 6: Pinch roll 7: Straightening machine 8: Cutting machine 9: Rolling mill 10: Solid slab 11: Molten core 12: Hollow casting Piece 13: Solid slab

Claims (2)

A continuous steel with a drawing locus in which a slab continuously drawn out along a circular arc downward from a mold turns upward beyond the lowest point M and is stretched at a 3/4 circumferential point T and drawn horizontally. In the casting method, 1) the arc radius R is 3 to 5 m, and 2) the position where the center of the slab solidifies is between the 3/8 circle point P and the 1/2 circle point +1.4 m position Q point. 3) A continuous casting method characterized by stretching at a T point while applying a reduction of 2% or more with a pair of rolls. The continuous casting method according to claim 1, wherein the casting efficiency is increased by increasing the cross-sectional aspect ratio of the slab.

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