JP2003305542A - Continuous casting method for steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for continuously casting a steel slab, which can use high viscosity powder and can obtain a casting having excellent quality in which generation of surface defects and internal defects in the casting has been prevented, and further oscillation of the molding, which was indispensable until now, can be dispensed with in some cases. <P>SOLUTION: The continuous casting method for steel uses a mold for a mold, which has, on the inside surface of the longitudinal side thereof, a first tapered portion having a taper angle within the range from 0.03° to 10°, a second tapered portion, below the first tapered portion, having a tape angle within the range from 0° to the angle smaller than that of the first tapered portion, the transition point of the two tapered portions located below the molten metal surface within the range from 80 mm to 300 mm, and carries out casting at a casting speed not smaller than 1.5 m/min. Further, preferably, lubricating powder having viscosity within the range from 0.2 to 10 Pas is added to the molten steel in the mold. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel, and more particularly to a casting method suitable for continuous casting of steel slabs.

[0002]

2. Description of the Related Art Generally, in continuous casting of steel, by adding powder to the molten metal surface in the mold, lubrication between the mold and the slab, prevention of oxidation of the molten metal surface in the mold, heat retention, capture of floating inclusions, etc. As a result, defects on the surface of the cast slab are prevented and stable operation is aimed at. When this powder is caught in molten steel and remains in the cast slab, it becomes a surface defect (sliver flaw) or an internal defect. Therefore, it is known to increase the viscosity of the powder as much as possible to suppress the inclusion. However, if the powder is made highly viscous, on the contrary, the inflow of the powder may be hindered, resulting in poor lubrication, which may cause cracking or breakout. On the other hand, increasing the casting speed may cause a situation similar to that of increasing the viscosity of the powder.

Conventionally, the viscosity is 3 po in order to obtain a slab with excellent surface properties by reducing the entrainment of powder.
It has been proposed to use a highly viscous powder having an ise (corresponding to 0.3 Pa · s) or more, or to perform continuous casting by using this highly viscous powder and electromagnetic stirring in a mold as needed (Japanese Patent Laid-Open No. 2000- 280051).

On the other hand, in the continuous casting facility for steel slabs, attempts have been made to improve the quality of cast slabs by tapering the inner surface of the mold. For example, in JP-A-6-31418, by adjusting the taper amount of the short side of the mold according to the casting speed,
A continuous casting method for preventing the occurrence of vertical cracks in the vicinity of the slab corners is disclosed. Further, in order to enhance the adhesion between the slab and the mold and promote heat removal, a technique of giving a taper to the long side of the mold is also known (for example, "Kawasaki Steel Technical Report" vol.12 (1980), p66). Furthermore, JP-A-2000-15
In Japanese Patent No. 8106, it is proposed to perform casting by using a highly viscous powder together with a taper on the long side, which improves the quality of the slab and prevents breakout.
And it is trying to extend the mold life.

[0005]

First of all, in the technique disclosed in Japanese Patent Laid-Open No. 2000-280051, a highly viscous powder (3 poise or more, 25 poise in the embodiment) is shown. It is recognized that the use of a) and the combined use with electromagnetic stirring are effective in reducing powder entrainment and achieving an improvement in the surface quality of the slab. However, this conventional technology exemplifies requirements such as electromagnetic stirring, molten steel flow rate, and heating of powder as casting conditions that enable the use of highly viscous powder. There is no mention or suggestion of the taper and its relation to high viscosity powders and casting speeds.

Further, in Japanese Patent Laid-Open No. 6-31418,
Although there is a description that the casting was performed at a maximum casting speed of 5.0 m / min, the characteristic feature of this conventional example is that the taper is only given according to the solidification shrinkage, and the description is related to the viscosity of the powder. In addition, the taper is not large enough to promote the powder inflow, and the taper is limited to the short side. Further, even in the prior art such as the above-mentioned document of giving a taper to the long side of the mold, the purpose of giving the taper is to suppress the wear of the mold, which is different from the present invention, and the taper angle given to the long side is not so great. Not large, and the taper is 1 on the inner surface of the long side.
It is formed in a stepped form.

An object of the present invention is to obtain a slab of excellent quality which enables the use of highly viscous powder in continuous casting of steel slab and prevents the occurrence of surface defects and internal defects of the slab. It is an object of the present invention to provide a continuous casting method of a steel slab that can perform the above-mentioned process and, in some cases, does not require the oscillation of the mold, which has been conventionally required.

[0008]

The gist of the present invention for solving the above problems is as follows. (1) The taper angle is 0.03 ° to 1 on the inner surface of the long side of the mold.
There is a first-stage taper of 0 ° and a second-stage taper below the first-stage taper, the taper angle being from 0 ° to less than the first-stage taper angle. A continuous casting method for steel, characterized by using a slab mold located 80 to 300 mm below the position and casting at a casting speed of 1.5 m / min or more. (2) Further, the viscosity at 1300 ° C. is 0.2 to 10
The continuous casting method according to (1), characterized in that a lubricating powder of Pa · s is used in the mold. (3) The continuous casting method as described in (1) or (2) above, wherein the slab is pulled out without oscillating the mold. (4) The two-step taper provided on the inner surface side of the long side of the mold used for casting is 1500 from the vicinity of the center in the width direction of the long side.
The continuous casting method according to any one of (1) to (3) above, wherein the continuous casting method is performed within a range of mm.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of a mold used for carrying out the continuous casting method for a steel slab according to the present invention, and particularly, schematically shows the side surface of one mold wall constituting the long side of the mold. . In this mold, two tapers 2a and 2b having different angles are formed on the inner surface of the long side 1 of the mold which is in contact with the molten steel 4 and which have different angles along the slab drawing direction (casting direction). The change point 3 of these two-stage tapers 2a and 2b is that
Located within the 0～300Mm, also the angle theta ₁ is 0.03 ° to 10 ° of the upper side of the upper taper 2a, the angle theta ₂ of the lower side of the lower taper 2b from 0 ° theta ₁ It is less than.

The reason why the upper taper 2a of the first step on the inner surface of the long side of the mold in a certain range below the position of the molten metal and the inner surface of the molten metal is strongly tapered with a relatively angle will be described with reference to FIGS. 3 and 4. FIG. 3 is a cross-sectional view schematically showing the behavior of the long side mold wall, the slab and the powder in the vicinity of the molten metal surface of the example of the present invention, and FIG. 4 is the long side of the conventional example in the same region shown for comparison. It is sectional drawing which showed the behavior of a mold wall, a cast piece, and a powder typically.

In continuous casting, the molten steel continuously injected into the mold begins to solidify from its surroundings due to the cooling action from the inner surface of the mold to form a shell and is drawn downward at a constant casting speed. The powder added to the surface of the molten steel melts and covers the surface of the molten steel, and enters between the inner wall of the mold and the solidified shell to perform a lubricating action. In this case, in order to reduce the friction between the mold and the slab to prevent seizure and to perform a stable casting operation, it was usual to add the powder and to add oscillation to the mold for operation.

The solidified shell gradually increases in thickness toward the bottom of the slab, and the slab itself also tends to shrink toward the center, and as shown in FIG. 4, the length of the mold 1'is increased. When the inner surface of the side is almost vertical (including an angle of 0 ° or an angle close to 0 °), a downwardly expanding space 5'is formed between the inner wall of the mold and the slab 6. . The space 5'of the downward spreading type is formed as the slab contracts. The molten powder 8 in the molten powder-filled region 7 is expanded and depressurized by pulling out the slab, so that the slab moves to the mold side and the gap is narrowed, which impedes the fluidity. This tendency becomes more remarkable as the casting speed increases and the powder viscosity increases, which also promotes the depressurization tendency of the downwardly expanding mold space 5 ′. If the inflow of powder is obstructed and its consumption is reduced, the lubrication function of powder is impaired, breakout easily occurs, and cracking of cast slabs occurs. For this reason, conventionally, it has been difficult to increase the casting speed and use a highly viscous powder. For this reason, it was unavoidable that a casting speed below a certain speed was used and a powder having a low viscosity was used.

On the other hand, as shown in FIG. 3, in the case of the present invention in which a mold having a taper 2a having a specific angle is provided on the upper part of the inner surface of the mold 1, the inner wall of the mold tapered surface and the cast piece 6 are used.
A narrower space 5 is formed between and. The lower narrowed mold space 5 tends to have a larger pressure than others because of its shape and the narrowing of the cast piece as it descends. Acts on
The powder 8 in the molten powder-filled region 7 works easily to flow in the slab withdrawing direction, and as a result, the fluidity of the powder is further promoted. Further, when increasing the casting speed and increasing the viscosity of the powder, the amount of powder drawn in along with the slab increases, and the force that pushes the slab 6 in this lower narrowing mold space 5 toward the center inward. Is further increased,
Since the space is made more open, the powder 8 becomes easier to flow in and the powder consumption increases. as a result,
The original function of the powder is fully exerted, the risk of breakout is eliminated, and the occurrence of cracks in the cast piece can be prevented.

The reasons for limitation of the present invention will be described below. First, the two-step taper is provided on the inner surface side of the long side of the mold used in the present invention along the casting direction. The single-step taper makes it difficult to pull out the slab when the taper amount is large. Because it will be. In addition, the reason why the change of the two-step taper is set to be 80 to 300 mm below the molten metal surface position is that this range region (h _{1 in} FIG. ₁ ) matches the range filled with powder under normal continuous casting conditions. And
This is because if the taper with the above-mentioned angle is given in this powder filled region, the object of the present invention can be achieved. The preferred range of h ₁ is 100~200mm.

Further, in the present invention, the upper taper angle θ ₁ above the change point is 0.03 ° to 10 °, and the lower taper angle θ ₂ below the change point is 0 ° to less than the upper taper angle θ _1. I am trying. When the angle θ _{1 of the} upper taper is less than 0.03 °, it is too small to form the above-mentioned narrowed space, and the aim of the present invention is to promote the inflow of powder and increase the powder consumption. Difficult to achieve, prohibiting the use of high casting speeds and the use of highly viscous powders. On the other hand, this angle θ ₁ is 10 °
On the contrary, if it exceeds, the taper is too large, which causes a great resistance to pull out the slab and causes breakout. Therefore, the upper limit is set to 10 °. The angle θ ₁ is preferably in the range of 1 ° to 5 ° in order to achieve the object of the present invention better. The lower taper angle θ ₂ has a lower limit of 0 °, which is as small as possible in consideration of the pulling resistance of the cast slab, and an upper limit thereof does not exceed the upper taper angle θ ₁ . Further, regarding the taper relationship between the upper and lower parts, it is necessary that θ ₁ > θ ₂ is always satisfied. In actual operation, it is preferable that this angle θ ₂ be 0 ° or an angle close thereto.

On the other hand, in the present invention, the casting speed is 1.
The speed of 5 m / min or higher is specified because a casting speed lower than this is not preferable for improving productivity. The higher the casting speed is, the more desirable it is to achieve the object of the present invention, and the more the productivity is improved. Therefore, the upper limit is not specified, but it is about 10 due to practical problems.
The upper limit is a speed of about m / min.

Further, in the present invention, a mold having a two-step taper with the above-mentioned angle is used and the mold is 1.5 m / min.
When performing continuous casting of slabs at the above casting speed, the lubricating powder to be added to the mold molten metal surface is 1300 ° C.
Cast with a powder having a viscosity of 0.2 to 10 Pa · s. In the present invention, by maintaining the upper taper angle of the two-step taper within a specific range, the function of promoting the inflow of the powder is exerted, but this function is further enhanced if the viscosity of the powder is increased. The viscosity was defined in the above high viscosity range. The preferable range of the powder viscosity is 0.5 to 5 Pa · s.

Next, the two-step taper portion provided on the inner surface of the long side of the mold is within a range of 1500 mm from the vicinity of the center (when the long side is viewed in plan) in the width direction of the long side (w ₂ shown in FIG. 2).
It is preferable that the inner wall that is formed in the range 1) and that is closer to the shorter side is vertical or has one step taper. The two-step taper 2a, 2b is 1500 mm in the width direction of the inner surface of the wall of the long side 1.
If it exceeds 5 mm, it may be difficult to move the short side to change the width of the slab. In addition, since the amount of powder consumed tends to decrease near the center of the long side in the width direction, a two-step taper may be formed over a certain range near the center of the long side in the width direction. It is not required to be specified, but preferably about 700
It suffices if there is a two-step taper over ~ 800mm.

In the drawing, the change point 3 of the two-step taper is formed as an angled edge for convenience, but this portion (including other corner portions, of course) is not included in the actual mold manufacturing process. , It does not have an edge shape, but has a slight R. It should be noted that R of this change point can be positively made larger than R which is inevitable in manufacturing, and this point is also included in the scope of the present invention. Two
It is expected that by positively giving a large R to the change point of the step taper, the casting conditions can be continuously and reasonably changed, and the pulling out of the slab and the inflow of powder can be favorably influenced.

As described above, according to the present invention, the specific angle of 2
By using a mold with a step taper and adopting a casting speed above a certain speed, during continuous casting of a slab, the slab is cast without performing the oscillation operation of the mold, which was common in conventional continuous casting. It was possible to pull it out. By selecting the upper taper angle, casting speed and powder viscosity in the appropriate range, the powder will flow smoothly between the mold and the slab, so continuous casting can be performed without any problems even if no oscillation is applied to the mold. be able to. Of course, also in the present invention, the combined use of template oscillation does not hinder anything.

[0021]

EXAMPLES When continuously casting a slab having a cross-sectional size of 250 × 1800 mm, as a mold used for the continuous casting, the size and angle shown in Table 1 (see FIGS. 1 and 2 for each size and angle) are defined as long sides. Casting was performed by appropriately selecting the powder viscosity, the casting speed, and the presence or absence of oscillation. As a result, the amount of powder consumed and the index for judging casting are also shown in Table 1. In each of the examples of the present invention, the powder consumption is at least about 0.4 kg / t,
It was found that a sufficient lubricating action was achieved, and even if the judgment index was bad, only slight cracks were seen. Further, in the present invention, it can be seen that by more concretely defining the casting speed and the range and angle of the two-step taper, a slab with superior quality can be obtained. On the other hand, in the comparative examples that deviate from the conditions of the present invention, cracking is noticeable, and in some cases, breakout occurs.

[0022]

[Table 1]

[0023]

According to the continuous casting method of the present invention described above, it is possible to adopt a high casting speed and use a highly viscous powder, and it is possible to perform a good casting operation without powder entrapment, It prevents surface defects and internal defects from occurring and contributes to the quality improvement of steel slabs. Further, according to the continuous casting method according to the present invention, since it is possible to use a powder having a higher viscosity than before, the powder consumption increases, the function of the powder can be sufficiently exhibited, and stable casting is possible. In addition to the above, it is possible to omit the oscillation of the template, which was conventionally required, depending on the case. Further, increasing the casting speed leads to an improvement in productivity.

[Brief description of drawings]

FIG. 1 is a schematic side view showing an example of a long side inner wall shape of a continuous casting mold for carrying out the method of the present invention.

FIG. 2 is a schematic plan view of a long side inner wall shown in FIG.

FIG. 3 is a cross-sectional view schematically showing the behavior of the long side mold wall, the cast piece, and the powder in the vicinity of the molten metal surface when the present invention is applied.

FIG. 4 is a cross-sectional view schematically showing the behavior of a long side mold wall, a slab and a powder in a conventional example.

[Explanation of symbols]

1 Mold long side 2a Long side upper taper 2b Long side lower taper 3 Upper and lower taper change point 4 Molten steel 5 Lower narrowing type space 5'Upward expanding type space 6 Cast piece 7 Powder filling area 8 Powder 9 Solidifying shell

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B22D 11/20 B22D 11/20 A

Claims (4)

[Claims] 1. A taper angle of 0.0 on the inner surface of the long side of the mold.
A first-stage taper of 3 ° to 10 ° and a second-stage taper below the first-stage taper having a taper angle of 0 ° to less than the first-stage taper angle are provided. Is a continuous casting method for steel, characterized by using a slab mold located 80 to 300 mm below the molten metal surface position and casting at a casting speed of 1.5 m / min or more. 2. The viscosity at 1300 ° C. is 0.2.
The continuous casting method according to claim 1, wherein a lubricating powder of 10 Pa · s is used in the mold. 3. The continuous casting method according to claim 1, wherein the slab is pulled out without oscillating the mold. 4. The two-step taper provided on the inner surface side of the long side of the mold used for casting is formed within a range of 1500 mm from the vicinity of the center in the width direction of the long side. The continuous casting method according to any one of 3 above.