GB2107623A - Low-head bow type continuous casting machine for making high-temperature, high-quality strand - Google Patents

Abstract

The casting machine is characterized by positioning a strand straightening section in an area of from 6% to 35% of the total length of the machine. Preferably the machine is not higher than 5m. and preferably the strand is withdrawn at at a speed of at least 1.1 m/min. The strand can be fed directly to a rolling mill without need for reheating. <IMAGE>

Description

SPECIFICATION Low-head bow type continuous casting machine for making high-temperature high-quality strand This invention relates to a bow type continuous casting machine that can be installed and operated at low cost and which produces steel strands without internal cracks, transverse surface cracks and edge cracks.

Conventionally, refined molten steel is poured into an ingot mould, the resulting ingot is soaked in a re-heating furnace or "soaking pit" and rough-rolled into a slab or bloom which is cooled to ordinary temperatures, the cooled slab or bloom is subjected to scarfing for removal of any surface defect, and the scarfed slab is reheated and rolled into hot strips or rails. Rapid progress is now being made in the use of methods for casting molten steel in a long, continuous bar, or strand, which is cooled to ordinary temperatures, subjected to scarfing, and reheated in preparation for rolling into hot strips or rails. This method, called casting (formerly continuous casting), is much simpler than the conventional ingot casting process and has two advantages over the latter: a higher yield of useful product and elimination of heating in the soaking pit.

To further increase the usefulness of the strand casting process, a continuous casting machine meeting the following two requirements must be developed: (1) it can be installed and operated at very low cost; and (2) it makes effective use of the sensible heat of the molten steel because it reduces the thermal energy necessary for reheating a long, continuous bar, or strand which has been cooled to ordinary temperature, to the temperature suitable for hot rolling prior to conducting the hot rolling, or it eliminates a step of the reheating by producing a hot bar or strand directly suitable for rolling. To reduce the installation and operating cost of the costing machine, (a) the weight of the machine must be reduced, (b) the casting speed must be increased with a view to reducing the operating cost per unit weight of the molten steel, and (c) the machine must permit easy maintenance.To meet requirement (a), the machine must have low rigidity without sacrificing its performance and the strand must be supported with fewer support and guide rolls of smaller diameter. If the casting speed is increased to satisfy requirement (b), a strand with a small thickness of solidified shell passes through the casting machine, and this causes the strand to bulge with attendant internal cracks or centre segregation in the strand. The curved section of the casting machine defines the path along which the strand travels, and the distance between each of the rolls to support, guide and drive the strand must be precisely set along this path.

Therefore, maintenance of a casting machine which involves replacement of the rolls is inherently very difficult, and to meet requirement (c), the length of the curved section must be decreased.

To comply with requirement (2), i.e. making effective use of the sensible heat of the molten steel to reduce or eliminate the need of heating the strand in preparation for its rolling, the casting machine must produce a strand of good quality that is free from surface cracks and other flaws that must be removed before the rolling step. The term "a strand of good quality" means a strand free from centre segregation, internal cracks, surface flaws and nonmetallic inclusions, and in particular, a strand that is free from surface defects such as surface cracks or edge cracks that must be detected and removed after being cooled to ordinary temperatures.

Bow type continuous casting machines are currently in wide use, and, in most cases, the strand emerging from a curved mould is straightened before the centre of its cross section is completely solidified. Typically, this is achieved by using a large radius of curvature (10-13 m) for the mould and first arc and a casting speed of 0.7 to 2.0 m, together with closely arranged rolls and spray cooling.

During the strand casting, the interior of the strand is placed under the high static pressure of continuous molten steel leaving the mould, so the casting machine must have great rigidity to support and guide the strand reliably and provide and accurate path for its travel. On the other hand, the high static pressure of the molten steel has a tendency to produce a bulging strain in the strand, so in most continuous casting machines used today, the strand emerging from the mould is supported and guided by closely arranged sets of rolls to reduce bulging, and hence undesired bulging strain.

As another feature, the strand is rapidly cooled with water (flow rate : 1.0 litre/kg or more) in the secondary cooling zone to form a stronger solidified shell, and at the same time, the strand is withdrawn at low speed to produce a thicker and hence stronger solidified shell. However, if the temperature of the solidified shell is decreased and its thickness increased, the sensible heat of the molten steel cannot be effectively used to reduce or eliminate the thermal energy necessary for rolling the strand.

If a curved strand that has been cooled rapidly to provide a colder solidified shell and which has been withdrawn at low speed to form a thicker shell is straightened, internal cracks, transversal surface cracks or edge cracks will develop in the strand. To minimize the strain that develops upon bulging and straightening, and hence to prevent the development of the above defects in the strand, the following three techniques are currently used: use of closely arranged rolls to support and guide the strand in the curved section of the casting machine, distribution of the possible strain by "multi-point straightening" using many different radii of curvature, and rapid cooling of the strand to produce a sufficiently strong solidified shell to prevent bulging.

Therefore, in the conventional casting machine, the straightening section starts at a point 1 5.7 to 20.4 m away from the meniscus in the mould as measured along the length of the strand. At this point, the strand has a surface temperature of between 700 and 900"C and a solidified sheel with an estimated thickness of about 80 to 1 20 mm. If the strand has a cross section measuring 250 mm thick and 1 800 mm wide, the solidified shell accounts for 64% to 96% of the thickness of the strand.If such a strand is straightened, an edge crack rate of 10 to 30% and an evaluated internal crack point (C'1.5%) of 4 to 5% are unavoidable even with the stateof-the-art casting machine, and the strand cannot be passed on to the rolling step until after it has been cooled to ordinary temperatures and these defects have been removed.

Stahl and Eisen, vol. 95 (1975), No. 16, pp. 733-741 describes a process wherein a curved strand (thickness: 1 50 mm, width: 600 mm) having a radius of curvature of 3.9 m is cast from a curved mould at a rate of 0.9 m/min and 0.4 m/min, subjected to secondary cooling with water spray, and straightened at many points. This process is capable of reducing the static pressure of the molten steel and suggests a way to achieve partially one of the objects of the present invention, i.e. great reduction in the installation and operating cost of a continuous casting machine. As Stahl and Eisen teaches, a low-head casting machine has small rigidity but enables satisfactory strand casting.However, this reference does not propose a method to attain another object of the present invention, i.e. casting molten steel into a strand at a high speed, say 1.7 m/min, to reduce the operating cost per unit weight of the strand. The reference also does not teach a technique to produce a strand of good quality that is necessary for making effective use of the sensible heat of the molten steel to reduce or eliminate the thermal energy needed for rolling the strand.

The present invention provides a low-head bow type continuous casting machine which comprises a curved mould, a section for supporting and guiding a curved strand, cooling zones, a section for straightening the curved strand, and a horizontal roll zone, the straightening section being positioned in an area of from 6% to 35% of the total length of the machine.

The present invention enables a continuous casting machine to be installed and operated at low cost, and to produce a strand of good quality that enables effective use to be made of the sensible heat of the molten steel to reduce the thermal energy necessary for rolling the strand or to eliminate its need entirely. The low-head bow type continuous casting machine preferably has a height of 3 to 5 m and has a straightening section positioned in an area ranging from 6% to 35% of the total length of the casting machine as measured from the meniscus of molten steel.

The total length of the casting machine means the distance between the meniscus in the mould and the point at which the solidification of the strand is completed (the point of 0% being the meniscus and that of 100% being the point of completion of solidification), plus a certain variation in the point of completion of solidification in the transverse direction of the strand and possible variation in the point of completion of solidification in the longitudinal direction of the strand due to varying operating conditions. However, for the purpose of the following discussion, the total length of the continuous casting machine is simply defined as the distance between the meniscus in the mould and the point of completion of solidification.

Reference is now made to the accompanying drawings, in which: Figure 1 is a graph showing the relation between the percent solidification of a strand and the straightening strain; Figure 2 is a graph showing the relation between the temperatures of the corners of a strand and its percent solidification; Figure 3 is a graph showing the effect of the head of molten steel on the bulging height of a strand; the term "head of molten steel" in Figs. 3 and 4 means a distance between the meniscus level of molten steel and the underside level of the straightended part of the strand; Figure 4 is a graph showing the effect of the head of molten steel on the bulging strain of a strand according to the pitch of rolls that support and guide the strand and to the average temperature in cross section of the strand at the leaving end of the machine, the shaded portion a of Fig. 4 showing the area of bulging strain developed with a conventional continuous casting machine; ; Figure 5 is a graph showing the effect of the radius of curvature on the straightening strain in a strand according to the number of straightening points, the curves b, c and d showing the straightening at one point, at two points and at three points, respectively, the curve e showing the critical straightening strain for development of internal fissure, and the curve showing the straightening at five points; Figure 6 (a) is a graph showing the effects of spraying with water and a gas/water mixture on the surface temperature of a strand as measured in its longitudinal direction, and Fig. 6 (b) is the same as Fig. 6 (a) except that the surface temperature of the strand is measured in its tranverse direction;; Figure 7 is a graph showing the relation between the average temperature in cross section of a strand 250 mm thick and the casting speed, the line g showing the critical temperature for direct strand feed to a rolling mill; and Figure 8 is a schematic representation of one embodiment of the low-head bow type continuous casting machine of the present invention.

To reduce the installation and running cost of a strand casting process, the present invention uses a low head bow type continuous casting machine that includes a curved mould with a large radius of curvature, has a machine height of 5.0 m or less, preferably 4.5 m or less, and which is capable of high-speed casting at a rate of at least 1.1 m/min, preferably more than 1.5 m/min. As will be described hereunder, the low head of the machine and its ability to effect high-speed casting, coupled with the provision of straightening points in an area ranging from 6% to 35% of the total length of the machine, are three important factors not only for reducing the initial cost and running cost of the strand casting process but also for making effective use of the sensible heat of molten steel.The latter effect reduces the thermal energy necessary for reheating a long, continuous bar, or strand, which has been cooled to ordinary temperature, to the temperature suitable for hot rolling prior to conducting the hot rolling, or it eliminates the step of reheating by producing a hot bar or strand directly suitable for rolling.

By using a continuous casting machine that is capable of high-speed casting at a rate of at least 1.1 m/m preferably more than 1.5 m/min, and which has a height of 5 m or less, the initial cost of the overall strand casting process can be reduced (due partly to the smaller weight of the machine and partly to the decreased height of the housing in which the machine is installed) and, at the same time, the operating cost of the machine per unit weight of the strand to be cast is decreased.If the speed of casting is at least 1.1 m/min, particularly more than 1.5 m/min, the strand has to pass through the casting machine before a solidified shell having a satisfactory thickness forms, and this increases the chance of bulging, but using a low-head machine with a height of not more than 5 m, the static pressure of molten metal is redued by at least half and the amount of possible bulging can be decreased.

Another object of the present invention is to make effective use of the sensible heat of molten steel because it reduces the thermal energy necessary for reheating a long, continous bar, or strand, which has been cooled to ordinary temperature, to the temperature suitable for hot rolling prior to conducting the hot rolling, or it eliminates the step of reheating by producing a hot bar or strand directly suitable for rolling. In order to achieve this, a) the strand must be fed to a rolling mill at a temperature of at least 1100 C, preferably at least 1200"C, and b) the strand must be free from such defects as transverse surface cracks or edge cracks.To meet these requirements, the continuous casting machine of the present invention is to designed that it achieves a casting speed of at least 1.1 m/min, preferably at least 1.5 m/min, to feed the rolling line with a strand having a temperature of at least 1100 C, preferably at least 1200"C.

Since the machine is of a low head type with a height of not more than 5.0 m, the straightening points are arranged in an area from 6% to 35% of the total length of the machine so as to produce a strand of good quality at a casting speed of at least 1.1 m/min, preferably at least 1.5 m/min. The lower limit of 5% is determined by the following factors. The curved mould used in a bow type continuous casting machine or first arc of the curved strand cannot have a radius of curvature smaller than 1.5 m because a jet of molten steel supplied through an injection nozzle causes breakout in the solidified shell forming within the mould.With a curved mould having a radius of curvature of 1.5 m, the straightening of the curved strand is completed at a point of 2.4 m apart from the meniscus, and if the total length of the casting machine is 40 m which is a conceivably maximum value for currently used machines, the value of 2.4 m is 6.0% of the machine length, hence the lower limit of 6%. By arranging the straightening point within an area of from 6% to 35% of the total length df the casting machine, a thin solidified shell is formed in the curved strand at the individual straightening points, and as a result, the critical strain for crack development is increased to reduce the chance of surface cracks occuring.According to experiments conducted by the present inventors, cracks develop easily in the strand if it is straightened at a temperature between 700"C and 900"C (brittleness range), but if the straightening points are confined in an area of 6% to 35% of the total length of the casting machine, straightening can be completed in a temperature range higher than 900"C and outside the brittleness range.

In the continuous casting machine of the present invention, not only is the straightening section provided in an area of from 6% to 35% of the total length of the machine but also it comprises 5 or more straightening points to distribute possible straightening strain among these points. In addition, the machine supports and guides the strand with rolls of smaller diameter which, coupled with the small height of the machine ( < 5 m), helps reduce the weight of the machine. What is more, the rolls are arranged at a pitch of 200-400 mm (which is conventionally 500-600 mm) and at least one roll is divided into two or more segments to reduce the distance between the two points supporting points of the roll axis.For example, a roll divided into two segments has one axis, two roller members and four bearings, and two bearings of the four bearings are located between the two roller members. Because of this arrangement, the need of backup rolls can be eliminated without sacrificing the ability to absorb the reaction force from the strand being straightened.

A theoretical aspect of the present invention described hereunder should not be construed as limiting the scope of the invention. The present inventors have made experimental studies on a method of strand-casting molten steel with a bow type machine to determine the conditions for straighening the strand without causing transverse surface cracks, internal cracks or edge cracks, and found the conditions under which the sum of the bulging strain (web) and the straightening strain (Eu) is smaller than the critical strain for crack development (Ec).

When a strand is subjected to straightening and any other deformation, a crack will develop in a certain temperature range (brittleness range) where the critical strain for crack development (Cc) is low. For ordinary steels, this brittleness range is from 700 to 900"C. Therefore, it is important that the strand be subjected to any deformation at a temperature outside this range, and it is desirably straightened in a temperature range higher than 900"C. However, the strain developing in the strand is the sum of the bulging strain and straightening strain, so if the strand is straightened in a high temperature range, the strength of the solidified shell forming in the strand is reduced and the bulging strain due to the static pressure of molten steel is increased.This causes a tensile strain at the solid-liquid interface to develop internal cracks and other internal defects in the rolled product.

To meet these two conflicting requirements and reduce the bulging strain, the present invention uses a low-head bow type continuous casting machine preferably having a height of 3 to 5 m (this is effective in reducing the static pressure of molten steel) and provides a straightening section in an area not exceeding 35% of the total length of the machine so as to effect straightening of the strand in an area wherein the percent solidification of the strand is not more than 55%. By so doing, the strand can be straightened in an area where the critical strain for crack development (Cc) is about twice as large as in the conventional technique. This is apparent from Fig. 1.The relation between the height of bulging and the height of the casting machine is shown in Fig. 3, from which one can see that if the height of the machine expressed by the head of molten steel is 5 m or less, the bulging strain is from about a half to a third of the strain that develops in the conventional casting machine having a height between 10 and 14 m.Another advantage of straightening the strand in an area wherein the percent solidification is not more than 55% is that in this area the temperature at the corners of the strand is more than 900"C. Therefore, by straightening the strand with its solidification controlled to be not more than 55%, the two conflicting requirements of increasing the critical strain for crack development and maintaining the temperature of the corners of the strand (which are liable to cool rapidly) outside the brittleness range ( > 900 C) can be satisfied at the same time.

Still another advantage of straightening the strand to form a thin solidified shell (not more than 55% of solidification) is that because of the high temperature of the strand, any stress applied to the strand relaxes 10 to 100 times faster than in the conventional technique, and this again helps minimize the formation of cracks.

When a strand with a concave surface is straightened, a tensile force works in its longitudinal direction, and when a strand with a convex surface is straightened, a compressive force works in the same direction. Therefore, assuming a neutral plane extends in the longitudinal direction of a curved strand to divide it into a concave side and a convex side, a tensile force works on the former and a compressive force works on the latter, the magnitude of each force being proportional to the distance from the neutral plane in the direction of the width of the strand.

The tensile force that works in the longitudinal direction of the strand being straightened is one cause for the formation of surface cracks and internal cracks but if the strand is straightened with its solidification controlled to be not more than 55%, the constricting action of the solidified shell is greatly reduced and the neutral plane shifts from the centre toward the concave side, and as a result, the tensile force acting on the concave side the magnitude of which is proportional to the distance from the neutral plane is decreased, and this is another factor that contributes to minimizing the chance of crack development.

As described in the foregoing, by straightening the strand in a region where its percent solidification is more than 55%, the critical strain for crack development can be increased to a level much higher than achieved in the conventional technique, and the corners of the strand and its surface have a temperature of 1000"C or more which is outside the brittleness range, and furthermore, because of this high temperature, any stress applied to the strand can be relieved 10 to 100 times faster than in the conventional machine and in consequence, a straight strand of good quality can be produced.

To obtain a hot, flawless product, the strand must be straightened in a temperature region higher than 900on, and to this end, a low-head bow type casting machine with a height of 3 to 5 mm is necessary and, at the same time, the straightening section must be positioned in an area not exceeding 35% of the total length of the machine, or an area where the percent solidification of the strand is not more than 55%. To meet this second requirement, use of a low head machine is not enough, and a casting speed of at least 1.1 m/mim, preferably 1.5 m/min, is necessary. The use of a low head machine means casting with a great curvature or a small radius of curvature, and this necessitates distributing a great straightening strain by straightening the strand at many points, preferably at least 5 points.Fig. 5 shows the effect of machine height and the number of straightening points on the strain within the strand.

Obviously, with a high-speed machine having a height of not more than 5 m, the strand must be strained at 5 points or more.

In high-speed casting, the strand leaving the mould is supported and guided by rolls while forming a thin solidified shell. Since the thin solid shell has a high tendency to bulge out, some means is necessary for preventing it in addition to the use of a low-head casting machine reducing the static pressure of molten steel. For this purpose, the present invention uses rolls of smaller diameter spaced apart by a smaller pitch than is conventionally used (200-400 mm in comparision with the conventional 500-600 mm pitch). When rolls of a small diameter are arranged in a small pitch, they are liable to deflect and are unable to withstand the reaction force from the non-stationary part of the strand. To avoid this problem, at least one roll is divided into two or more segments and the distance between the two supporting points of the roll axis is reduced.The relation between the roll pitch and bulging strain is shown in Fig. 4 from which one can see that by reducing the roll pitch by 10%, the amount of bulging strain developing in a strand cast under high temperatures through a low head machine ( < 5 m) is equal to that of bulging strain that occurs in a strand cooled rapidly in the conventional high head machine. In Fig. 4, the solid straight lines indicate a roll pitch of 350 mm, and the dashed straight lines indicate a roll pitch of 315 mm (10% reduction from the 350 mm pitch).

In the low-head bow type casting machine of the present invention, the strand emerging from the mould is guided by a roller apron consisting of segmented rolls and is straightened in a zone composed of sets of pinch rolls.

The requirement that the strand be straightened in an area not exceeding 35% of the total length of the low head bow type casting machine can be related to a solidification not exceeding 55% by the following equation: Percent solidification at straightening point =

I,: distance between meniscus and straightening point lo: total length of the machine Therefore, a solidification of 55% is equivalent to 11/lo = 0.3025.

For example, the case of Ii -- x 100 = 17.9 (%) 1o (wherein 1, and 1o are equal to 9 m and 50.4 m, respectively), is exemplified as one of the preferred embodiments according to the present invention.

This preferable embodiment is briefly explained as follows: The low-head bow type continuous casting machine of a height of 3.2 m according to one embodiment of the present invention is illustrated in Fig. 8, wherein the numeral 1 indicates a curved mould; 2 is a roller apron comprising 8 pairs of driven or free rolls spaced at a small distance to support and guide a curved strand 3 that has been withdrawn from the curved mould 1 as an arc having a radius of curvature of 3 m. The roller apron 2 is followed by 1 7 sections, the first of which is a first straightening section 4 composed of nine rollers. The first of the nine rollers starts to straighten the strand having a radius of curvature of R1, and this is started in the part of the strand having a solidification ratio of 55% or less.In the first straightening section 4, the strand is straightened at nine points C, to C9 having increasing radii of curvature of R2 to R,,, respectively. The partially straightened strand is then passed through a second straightening section 5 where the strand is subjected to multi-point straightening at points C,O to C,8, having increasing radii of curvature of R to Rag and at point C,8, the strand, now straightened completely has an infinite radius of curvature. The straight strand is then directed into a horizontal roll zone composed of levelling units 6 to 20. In Fig. 8, levelling units 9 to 1 2, 1 7 and 1 8 are abbreviated.The strand leaving the horizontal roll zone is cut into lengths in a cutting station and the individual cuts are delivered to a run-out table 25 as hot and high-quality products. In Fig. 8, the numeral 26 indicates a ladle. The hot curved strand 3 leaving the mould contains molten steel which gradually solidifies as the strand is passed through the cooling zones. In Fig. 8, the meniscus of molten steel is indicated at 27, and the point at which the solidification of the steel is completed is indicated at 28. In Fig. 8, 1, is equal to a distance between the meniscus point 27 and the last straightening point C18, and 1o is equal to a distance between the meniscus point 27 and the solidification point 28.

To minimize the formation of surface defects on a strand and to hold it at high temperatures, the strand travelling through the casting machine must be cooled mildly and uniformly. The cooling effect of spraying pressurized water is compared with spraying with a gas-liquid mixture (e.g. air-water) in respect of the longitudinal direction of the strand [Fig. 6 (a)] and in respect of the transverse direction of the strand [Fig. 6 (b)]. Whether in the longitudinal or transverse direction, cooling with a gas-liquid mixture (such as air-water) is effective in obtaining a uniform temperature distribution on the surface of the strand. According to the present invention, spraying with a gas-liquid mixture is used in at least one cooling zone.

Figure 7 shows the relation between the speed of casting (withdrawing) a strand 250 mm thick and the average temperature in cross section of the strand. As is clear from the Fig, the strand must be withdrawn at a speed of at least 1.1 m/min, preferably at least 1.5 m/min, to enable a cast strand to be directly fed to a hot roll mill while retaining the desired average temperature of 1100 C in its cross section.

The present invention is now described in greater detail by reference to the following examples and comparative example which are given here for illustrative purposes only and are by no means intended to limit its scope.

Example I The apparatus was as shown in Fig. 8. A slab of medium-carbon Al-Si killed steel measuring 250 mm thick and 1000 mm wide was continuously cast with a bow type machine having a height of 3.2 m and a radius of curvature for the first arc (defined by the curved mould) of 3 m.

The other parameters for the strand casting were as follows.

(1) Number of straightening points; 1 8 (2) Roll pitch: 196-302 mm (3) Roll diameter: 140-230 mm (4) Roll type: segmented rolls (5) Cooling system: Slab travelling in a region of the first 7 m below the mould was sprayed with an air-water mixture.

(6) Casting speed: V=2 m/min max.

When the average casting (withdrawing) speed was 1.7 m/min, a slab of good quality having an average temperature in cross section of 1200"C was obtained at the exit end of the casting machine. The slab was fed directly to a hot rolling mill without heating and rolled into a hot strip having a thickness of 2.3 mm which left the finishing mill at 890"C. A hot strip coil of good quality was obtained.

The distance between the meniscus and the last straightening point 1, was equal to 9 m and the total length of the machine 1o was equal to 50.4 m. The ratio of 1, to 1o was as follows: 1, X 100= 17.9% 1o Example 2 The apparatus was a shown in Fig. 8. A slab measuring 250 mm thick and 1000 mm wide was continuously cast with a bow type machine having a height of 3.2 m and a radius of curvature for the first arc of 3 m.The other parameters of the strand casting were as follows: (1) Casting speed: V = 1.7 m/min (2) Water spraying rate: 0.81/keg (3) Thickness of solidified shell d < 60 mm (at straightening points): (4) Straightening roll diameter: 300-320 mm (5) Roll pitch: 200-400 mm (6) Number of straightening points: 1 8 The strand was checked for any defect and the results were as follows: surface defect 0.5%, and internal cracks 0%.

A typical bow type continuous casting machine has a foot roll zone in the curved mould and sets of pinch rolls in the straightening zone. By arranging the rolls in segments as described hereinabove, roll replacement can be made very easily and the machine can be adapted to cast various sizes of strand.

The distance between the meniscus and the last straightening point 1, was equal to 9 m and the total length of the machine 1o was equal to 50.4 m. The ratio of 1, to 1o was as follows: Ii x 100= 17.9% o Comparative Example 1 The procedure of Example 2 was repeated using the following conditions: (1) Casting speed: V = 0.7-0.5 m/min (2) Water spraying rate: 1.81/keg (3) Thickness of solidified shell (at straightening points): d = 70-90 mm The result of checking for any defect was as follows: surface defect 20% , and internal cracks 30%.

The continuous casting machine of the present invention has the following advantages: (1) it is lightweight and can be installed in a low housing, so the intitial cost of the machine can be reduced by about 20-30%, (2) it is capable of high-speed casting (1.7-2.0 m/min), hence high efficiency in strand production; (3) because of its ability to complete strand casting at high temperatures the machine can feed the strand into a hot rolling mill directly without a re-heating step, and saves thermal energy equal to about 100,000 to 200,000 Kcal/ton; (4) because of its low height, the machine has a short curved section and permits all rolls to be replaced at one time, which leads to easy maintenance and high reliability of the machine; and (5) the machine feeds a strand of good quality to the rolling line.

Claims (7)

1. A low-head bow type continous casting machine which comprises a curved mould, a section for supporting and guiding curved strand, cooling zones, a section for straightening the curved strand, and a horizontal roll zone, the straightening section being positioned in an area of from 6% to 35% of the total length of the machine.

2. A continuous casting machine according to Claim 1, which is not higher than 5.0 m.

3. A continuous casting machine according to Claim 1 or 2, wherein said straightening section has a multi-point straightening mechanism for straightening the curved strand at 5 or more points.

4. A continuous casting machine according to any of the preceding claims, wherein said support/guide section and/or the straightening section comprises closely arranged rolls at least one of which consists of two or more segments.

5. A continuous casting machine according to any of the preceding claims, wherein at least one of said cooling zones has means for cooling the curved strand with a sprayed gas-liquid mixture.

6. A continuous casting machine according to any of the preceding claims, wherein said support/guide section and/or said straightening section has means for withdrawing the curved strand at a speed of at least 1.1 m/min.

7. A continuous casting machine substantially as hereinbefore described with reference to any of the Examples and/or the accompanying drawings.