Mold for continuous casting of steel

Abstract

To improve practicality in a shape-stabilized mold.SOLUTION: In a normal copper mold to be used for continuous casting of steel, thermal deformation due to thermal expansion occurs with an increase in the number of uses, thereby hindering stability of quality and operation. A novel graphite mold solves the problem of expansion and deformation, but is practically difficult to use in operation due to its fragility. In the present invention, an upper part of the mold where deformation occurs is made of graphite to which prestress is introduced, and a lower part of the mold where scratches, wear, and impacts become problems is formed of a heat-resistant and wear-resistant metal instead of copper. For inevitable damage generated in the graphite part during discharge of a top portion of a slab remaining in the mold due to a casting accident, a mechanism is incorporated to immediately retreat the mold upward at the time of accident occurrence, and separate the mold from the slab, thereby improving durability of the mold.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a double-open continuous casting mold for use in the continuous casting of steel. [Background technology]

[0002] The molds used in continuous casting of steel are made of copper, which has high thermal conductivity, and are tubular with top and bottom openings, with the exterior water-cooled. Molten steel poured from above cools and solidifies on contact with the inner wall of the mold, forming the outer skin of the cast slab. The cast slab is continuously drawn downward, passes through a subsequent spray device, and is cooled until solidification is complete. During this time, the quality of the slab is greatly affected by the cooling conditions. Vertical cracks, horizontal cracks, cracks, wrinkles, dents on the surface of the slab, vertical cracks under the skin, internal cracks, and diamond-shaped deformation in the cross section are mainly caused by inadequate cooling in the mold. The worst case scenario caused by the mold is breakout, which is leakage of molten steel due to damage to the outer skin.

[0003] To prevent the above defects, various improvements have been made, such as improving the mold shape (e.g., vertical inclination, corner shape and curvature), improving the wall properties (e.g., plating, forming fine irregularities), improving lubrication (e.g., molten powder lubrication, mold vibration conditions), and uniform cooling by stirring the molten steel inside the mold. Even if a mold of the appropriate shape is used under the appropriate casting conditions and the required quality is maintained, as the number of times the mold is used increases, the conditions will deviate from the appropriate conditions and defects will occur frequently. Depending on the situation, the mold will be replaced. Normally, the number of times it can be used is several tens to several hundred times. The removed mold will be discarded or repaired and reused. Durability of molds is an issue that cannot be ignored not only in terms of mold cost but also in terms of both product quality and production efficiency.

[0004] The damage to molds caused by increased use mainly consists of deformation, wear, and rough surface. Deformation can cause internal cracks in the slab through uneven cooling, and many other defects. Internal cracks that occur cannot be repaired, causing significant damage. The deformation is mainly caused by the uneven thermal expansion of the mold. The mold wall temperature is highest just below the pouring surface where it comes into contact with the molten steel, at about 300-400°C, while the outer surface of the mold is cooled by water to about 150-200°C. Directly above this area, the molten steel and the mold surface repeatedly come into contact and separate due to the up and down vibration of the mold, causing the thermal load to fluctuate periodically, resulting in a thermal effect different from that of the above area. Below the contact point, the formation of a skin and its shrinkage causes separation from the mold (void formation) at uneven locations, and the thermal load drops sharply, which is also significantly different from the contact point. This uneven cooling causes strange distortions in the thermal expansion of the mold wall.

[0005] During use, the mold is maintained in a distorted state due to the temperature difference between the inside and outside surfaces, plus the difference in thermal load between the top and bottom three stages. Furthermore, metal creep gradually accumulates with the number of uses, causing a unique deformation to progress. After use, the shape of the mold, which was previously flat on all sides, has been deformed to an inward convex shape centered on the area directly below the casting surface. It is presumed that the internal expansion during use causes further deformation into an inward convex shape. This deformation is typical of tubular molds used mainly for billets, but is also the case for plate molds used for blooms and slabs. In cylindrical molds, even slight distortion of the mold cross section can easily cause vertical cracks.

[0006] We will consider previous examples of suppressing deformation of molds that occurs with repeated use. First, block molds (thick walls) cut from copper ingots were less prone to deformation and were advantageous for quality control, but they were expensive and had problems such as the cross section gradually expanding due to modification processing, so today they are not used for anything other than beam blank molds.

[0007] Patent Document 1 discloses a shape-stable mold that solves the problem of thermal deformation of the mold. According to this, the problems of thermal expansion and deformation are solved by using graphite as the mold material, which has an extremely small coefficient of thermal expansion compared to metals. The dissolution of graphite into molten steel and the problem of melting damage are prevented by a metal coating applied to the mold surface, and this is solved. The problem with this method is that compared to copper molds, it is more vulnerable to impacts and shocks, which causes many difficulties in the process. As a countermeasure, a metal coating is applied to the outside of the mold, but it is presumed that this is not sufficient given the allowable coating thickness.

[0008] Patent Document 2 discloses a method for overcoming the weakness of the graphite mold. According to this method, hoops are shrink-fitted onto the outer periphery of the mold to introduce compressive stress in the circumferential and radial directions, and bolts are also used to tighten the mold axially to apply compressive stress in the axial direction. This method has the effect of replacing reinforced concrete with prestressed concrete. Although this method improves the brittleness problem, it is still not sufficient compared to copper molds.

[0009] In addition to the fragility of the graphite mold, two other problems were identified. The first was the tendency for wear to occur in the lower half of the mold. A detailed investigation of mold deterioration reveals that, as mentioned above, deformation of the mold wall directly below the pouring surface is responsible for surface and internal cracks in the slab, while double skin defects are caused by scratches on the mold surface, and diamond-shaped deformation is caused by the above-mentioned deformation of the mold wall as well as wear and uneven cooling of the lower half of the mold. Wear of the lower half, uneven cooling, and diamond-shaped deformation are mutually related causes and effects, and the mold is replaced or discarded as appropriate. With graphite molds, even slight wear will cause the metal coating to disappear, and the mold will then wear out rapidly, making it unsustainable.

[0010] The second problem is that in the event of an emergency stoppage of pouring or withdrawal due to a breakout (a break in the slab's outer shell causing molten steel to leak), the stopped slab is melted down in various places and pulled out of the machine along with the roller apron that supports it. The top of the slab remains after solidification in the mold. This remaining part cannot be easily pulled out due to deformation caused by cooling. In the case of an assembled mold, it is removed by blooming, and in the case of a tubular mold, it is melted down with an oxygen or gas torch. In this case, the metal-coated graphite mold will immediately become unusable due to melting and burning. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 5635717 [Patent Document 2] Patent No. 6557808 Summary of the Invention [Problem to be solved by the invention]

[0012] The molds used in continuous casting of steel are usually made of water-cooled copper, which does not pose any particular problems in terms of work, but in terms of quality, defects such as cracks can occur due to the gradual accumulation of thermal deformation, and the molds are not durable enough for high-quality products. A shape-stable mold made of graphite solves this problem, and while it does not undergo thermal deformation, it is vulnerable to breakage and wear due to the brittleness of graphite. Wear progresses at an accelerated rate in the lower half of the mold, where scratches are more likely to occur. Furthermore, when casting or withdrawal is halted due to a breakout or other problem, the top of the slab remaining in the mold cannot be removed due to deformation and tension of the slab. Gas cutoff is unavoidable, but cutoff immediately damages the graphite mold. The present invention aims to solve the following problems in graphite molds: 1) to address wear of the lower half of the mold, and 2) to prevent damage to the mold caused by remaining cast pieces during various post-accident treatments. [Means for solving the problem]

[0013] The first invention of the present application is a water-cooled mold for continuous casting of steel, which is composed of two cooling zones, upper and lower, The upper stage is a cylindrical mold with an internal water channel made of graphite, the mold surface is metal-coated, and the mold periphery is shrink-fitted over the entire length. The mold also has a clamp with a water supply hole on the top surface and a clamp with a drainage hole on the bottom surface. The two clamps are connected by a tie rod, applying tension to them, thereby introducing compressive stress in the tangential, radial, and axial directions of the mold. The lower section is a mold for continuous casting of steel, which is made of metal and has a thickness greater than or equal to the normal shell thickness of the slab at the mold outlet, providing the strength to restrain and correct abnormal deformation of the slab shell, and the mold surface is made of a heat-resistant and wear-resistant metal.

[0014] The second is a mold for continuous casting of steel according to the first invention, characterized in that it is provided with a mechanism for retracting the upper stage, or the entire upper and lower stages, or the vibration table on which the mold is loaded, so that in an emergency, at least the upper stage can be separated from the cast piece remaining in the mold by retracting it.

[0015] Here, the normal shell thickness is the thickness that can be derived from the following coagulation formula. d=k√tt=L / V d: shell thickness (mm), k solidification constant (usually 25 ± 2 mm / √ min), t: time (min) L: effective mold length (m), V: withdrawal speed (m / min) [Effects of the Invention]

[0016] The continuous casting mold of the present invention is composed of two levels, upper and lower, and does not use conventional copper as the mold material. The upper level is made of water-cooled graphite, making it a shape-stable mold, which improves the horizontal uniformity of cooling during initial solidification and results in excellent cast quality. The lower section is made of water-cooled, strong, wear-resistant metal, which not only eliminates the wear and tear problems of graphite, but also has far superior strength and wear resistance compared to conventional copper, solving the brittleness and wear problems of conventional all-graphite molds and extending the mold life. Since a wide area of ​​the lower part of the mold is wear-resistant, diamond-shaped deformation within the mold is suppressed, and the correction roller guide that is usually installed directly below the mold outlet is no longer necessary.

[0017] The operation of removing the top of a slab remaining in a mold due to a casting accident is complicated, and graphite molds are susceptible to fatal damage during gas cutting. However, with the present invention, in the event of an accident, the mold can be quickly withdrawn to an upper position and separated from the top of the slab, making it easy to dispose of the remaining slab without damaging the mold. This solves the practical problems of graphite molds. Breakout processing also takes less time because there is no need to change molds. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a mold used in the continuous steel casting apparatus of the present invention. [Figure 2] 1 shows an example of the structure of the upper part of the mold of the present invention. [Figure 3] 1 shows an example of the structure of the lower part of the mold of the present invention. [Figure 4] This shows a method for retracting a mold used in a curved continuous casting machine. [Figure 5] This shows how to deal with the remaining slab in the mold when a breakout occurs. DETAILED DESCRIPTION OF THE INVENTION

[0019] The mold portion of the continuous casting apparatus of the present invention will be described with reference to FIG. The mold section consists of a mold 1 (area indicated by dashed line A), a vibration table 2 that carries the mold 1 and vibrates it up and down, and an evacuation mechanism 4 that, together with the vibration table 2, evacuates the mold 1 upward in an emergency, separating the mold 1 from the top of the cast piece 3. Mold 1 consists of two water-cooled molds, upper and lower, with the upper mold 5 (wavy line B) made of graphite to eliminate thermal expansion and eliminate mold deformation problems, and the lower mold 6 (wavy line C) made of heat-resistant and wear-resistant metal with sufficient thickness to provide greater strength and durability than conventional copper molds. The vibration table 2 is no different from the conventional one. A drive unit 7 applies up and down vibrations at an amplitude of several mm to several tens of mm at a rate of approximately 100 times per minute to prevent the molten steel from sticking to the mold wall. In the case of a vertical mold, the retraction mechanism 4 pushes the vibration table 2 vertically upward, preferably using two to four synchronized hydraulic cylinders or synchronized screw jacks.

[0020] FIG. 2 shows the upper mold 5, the upper drawing being a view from above, and the lower drawing being a vertical cross-sectional view including the central axis. The upper mold 5 is composed of a mold body 21 and two types of compressive stress introducing members. The mold body 21 has a cylindrical outer shape, is made of graphite, has water cooling holes 22, has a mold cavity 23 bored therein, and has a metal coating 24 on the bored mold surface. The compressive stress introducing member consists of a shrink-fit steel pipe 25 that fastens the mold body 21, upper and lower clamps 26, 27, and a tie rod 27 that fastens the clamps 26, 27 and tightens the mold body in the axial direction. The holding metal fitting 26 has a water supply hole 28, and the pressing metal fitting 27 has a drainage hole 29. The mold surface of the holding metal fitting 27 comes into contact with the outer shell of the cast slab, and is therefore padded with a heat-resistant and wear-resistant metal. The shrink-fit steel pipe 25 introduces compressive stresses in the tangential and radial directions into the cylindrical mold, and the tie rod 27 introduces compressive stresses in the axial direction.

[0021] 3 shows the lower mold 6. The lower mold 6 is a tubular steel mold 31 having strong upper and lower flanges 32, 33, and is equipped with a water cooling device 34. The mold surface (inner surface) is made of a heat-resistant and wear-resistant metal wall 35. The lower mold 6 needs to have sufficient strength to cope with abnormal deformation of the slab shell and abnormal tension of the damaged slab. The wall thickness must be greater than the shell thickness. A conventional tubular copper mold is insufficient. The upper flange 32 functions to firmly connect to the upper mold 5, and the lower flange 33, together with the tubular portion, restrains the mold from being pushed apart by the diamond-shaped deformation of the slab in the event of an accident.

[0022] FIG. 4 shows a method for retracting the mold in a curved continuous casting machine. In this type of continuous casting, the hole of the mold 41 is curved with the same curvature as the drawing radius (the distance between the center of curvature 43 and the pinch roll 47). Therefore, the vibration locus must have the same curvature, and the same curvature must also be maintained when the mold is evacuated upward in the event of an accident. The mold 41 is fixed to a vibration table 42 mounted on the other end of a lever 44, which has its fulcrum at the center of curvature 43, and is vibrated up and down by a vibration drive unit 45. The mold is evacuated by an evacuation mechanism 46 that pushes up the lever 44. The evacuation mechanism 46 can be operated with a single hydraulic cylinder.

[0023] Figure 5 shows the state of breakout. In many cases, molten steel in a slab 52 leaks out directly below a mold 51. Some of the leaked molten steel explodes and scatters, while some adheres to support rollers 53. The procedure for dealing with the accident is to first gas-cut the strand 52 between the mold 51 and the support roller 53 (arrow 54). After cutting the strand at the bottom of the roller apron, the roller apron is pulled out of the continuous casting machine and replaced with a new roller apron. Any cast 52 remaining in the mold in a stretched state is removed by gas cutting immediately afterwards or after the mold is removed. If the mold can be replaced quickly, the mold is removed from the vibration table and replaced with a new mold.

[0024] In the mold of the present invention, the remaining slab 52 in the mold cannot be gas-cut. The oxygen gas flame penetrates beyond the metal coating and damages the graphite. As a countermeasure, when a breakout occurs, the mold is quickly evacuated upward to separate it from the top of the slab. While the upper mold or the entire mold can be evacuated, it is safer to evacuate the entire vibration table. The retraction mechanism is an essential requirement when using graphite molds.

[0025] The dimensional relationship between the two-stage mold is explained below. Normally, the mold length is around 1m. A close look at the durability of a conventional copper mold reveals that the range of thermal deformation that protrudes into the mold is approximately 200-300mm directly above and directly below the casting surface. This is an area that is subject to extremely high thermal load. Therefore, there is no need to extend the graphite portion any lower. It is desirable for the length of the upper stage to be less than half of the total length. Wear and scratches are the main causes of deterioration in the lower part of the mold. Graphite has less mechanical strength than copper, so using graphite in the lower part is actually harmful. Since the thermal load is reduced in the lower part, copper is not necessarily essential. Therefore, it is more desirable to construct the lower part from a stronger metal than copper. In the present invention, the lower mold surface is made of heat-resistant and wear-resistant metal. High-speed steel or heat-resistant Ni alloy, which are used for rolls and guides in hot rolling, are suitable. The length of the lower stage is preferably at least half the length of the mold. To withstand wear, not only is the material necessary, but a certain degree of thickness and length is also required to ensure resistance to expansion. It is desirable for the wall thickness to be greater than the shell thickness. The method for calculating the shell thickness has already been described. [Example]

[0026] For the graphite upper mold material, 400mm long x 400mm diameter was cut out from a 16-inch graphite electrode rod for an arc furnace due to its quality and price advantages, and a 160mm x 250mm mold hole was bored in the center. The mold surface was plated with 0.5mm thick Ni. The effective length of the graphite section was 300mm. The thickness of the upper and lower clamps shall be approximately 30 mm. Non-magnetic stainless steel is used for shrink-fit steel pipes and tie rods for prestressing, so that they do not impede the penetration of electromagnetic forces, if necessary. The thickness of the lower mold, including the flange, will be 20 mm, which is larger than that of conventional copper molds (thickness is approximately 10 mm). This will provide significant reinforcement compared to conventional molds in terms of both thickness and yield strength. The length will be 420 mm, making the effective mold length 750 mm. The stroke of the retraction mechanism of the newly adopted vibration table must be greater than or equal to the effective length, and designing it will not be particularly difficult for those skilled in the art. [Industrial Applicability]

[0027] The present invention can be applied to improving the quality of current continuous cast materials. [Explanation of symbols]

[0028] 1;Mold 2;Vibration table 3;Slab top 4;Retraction mechanism 5;Upper mold 6;Lower mold 7;Driver 21;Mold body 22;Water cooling hole 23;Mold hole 24;Metal coating 25;Shrink-fit steel pipe 26, 27;Pressure bracket 28;Water supply hole 29;Drain hole 31;Tubular mold 32;Upper flange 33;Lower flange 34;Cooling device 35;Heat-resistant and wear-resistant metal wall 41;Mold 42;Vibration table 43; Bending center 44; Lever 45; Vibration device 46; Retraction mechanism 47; Pinch roll 51; Mold 52; Casting shell 53; Support roll 54;Fusing part 55;Leaking molten steel

Claims

1. A water-cooled mold for continuous casting of steel, comprising two cooling zones, an upper and lower zone, the upper zone having a cylindrical outer shape and an internal water channel, made of graphite, the mold surface being metal-coated and the mold periphery being shrink-fitted over its entire length, the upper surface of the mold being provided with a clamp having a water supply hole, and the lower surface being provided with a clamp having a drainage hole, and the two clamps are connected by tie rods under tension, thereby introducing compressive stress in the tangential, radial and axial directions of the mold, respectively. A mold for continuous casting of steel, characterized in that the lower section is made of metal, has a thickness greater than or equal to the normal shell thickness of the slab at the mold outlet, and has the strength to restrain and correct abnormal deformation of the slab shell, and the mold surface is made of a heat-resistant and wear-resistant metal.

2. 2. A mold for continuous casting of steel according to claim 1, characterized in that a mechanism is provided for retracting the upper stage, or the entire upper and lower stages, or the entire vibration table on which the mold is loaded, upward, so that in an emergency, at least the upper stage can be separated from the cast piece remaining in the mold by retracting it.

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