Continuous casting method for steel

Abstract

Steel is continuously cast in a mold at a casting speed of 2.5 to 10 m/min while the mold is oscillated so as to satisfy the relationship 450≦(S×f/Vc)≦1000, wherein S is the stroke of mold oscillation in mm, f is the frequency of mold oscillation in cycles per minute, and Vc is the casting speed in m/min.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a continuous casting method for steel. In particular, it relates to a high speed continuous casting method which is capable of forming thin cast slabs having a good surface quality without the occurrence of operational problems such as sticking breakout.

[0003] This invention also relates to a method of manufacturing thin steel sheet from a thin or medium thickness cast slab formed by continuous casting.

[0004] 2. Description of the Related Art

[0005] In a typical method for manufacturing thin steel sheets, a cast slab formed by continuous casting is cooled to room temperature at the completion of casting and is then subjected to hot rolling to obtain a desired sheet thickness. Due to the need to reheat the cast slab from room temperature before hot rolling can take place, this method is inefficient from the standpoint of energy consumption.

[0006] Recently, a method has been developed in which cast slabs produced by continuous casting are subjected to hot rolling without first being cooled to room temperature. In this method, if the cast slab which emerges from a casting machine is sufficiently thin, the load of rough rolling of the slab can be reduced, further increasing the efficiency of the method.

[0007] When performing continuous casting of thin or medium thickness slabs, the thickness of the mold cavity (the internal dimensions of the mold cavity in the thickness direction of a casting formed by the mold) of a mold using for casting is a small value of 50-120 mm, compared to a thickness of 200-300 mm for a mold cavity for casting of ordinary slabs, Due to the small thickness of the mold cavity, a continuous casting line for thin cast slabs must operate at a higher casting speed than a continuous casting line for thicker slabs in order to produce the same output per unit length of time.

[0008] When operating at a high casting speed, it is necessary to prevent sticking breakout, which refers to a phenomenon in which the solidified shell of a slab sticks to the interior surface wall of the mold due to a rupture of the film of the lubricant (which is typically a powder prior to melting) present between the interior surface of the mold and the solidified shell. The sticking solidified shell breaks, and molten steel inside it leaks out from the partially solid cast slab. This phenomenon can cause serious problems. Methods of preventing sticking breakout include varying the material properties of the lubricant powder (such as lowering its solidification temperature or viscosity) and varying the oscillation conditions of the mold.

[0009] However, if the solidification temperature of a lubricant powder is lowered in order to prevent sticking breakout, uneven solidification of the cast slab occurs, because the amount of heat removal, through the walls of the mold increases. As a result, it becomes easy for surface defects referred to as longitudinal cracks to occur in the cast slab. In addition, if the viscosity of the lubricant powder is decreased, the lubricant powder tends to flow into the mold unevenly in the widthwise direction, and it again becomes easy for longitudinal flaws to occur. Therefore, there is a limit of the extent to which the material properties of lubricant powder can be varied in order to prevent sticking breakout.

[0010] There have been many proposals of ways of varying mold oscillating conditions during continuous casting, such as in Japanese Published Unexamined Patent Applications Hei 2-197359/1990, Hei 4-231 159/1992, Hei 6-15425/1994, Hei 7-16718/1995, and Hei 9-234549/1997, However, in the casting methods described in those publications in which oscillation is applied to a mold, the casting speed is at most 2.2 m/min, so those methods are not applicable to high speed casting or casting of thin or medium thickness slabs.

[0011] Japanese Published Unexamined Patent Application Hei 8-187562/1996 discloses a method in which a mold oscillating frequency f can be set to an optimal value in the range of 96-204 cycles per minute up to a casting speed (Vc) of 5.0 m/min, and the mold oscillating stroke a (mm) is varied so as to satisfy the relationship a=(1.9−2.5)×Vc.

[0012] However, at a low frequency of 96-204 cycles per minute at which oscillation is performed in that method, there is a tendency for a cast slab to be easily stuck, the pitch of oscillation marks (the distance in the lengthwise direction of a cast slab between oscillation marks) formed on the cast slab becomes very long, and there is a tendency for longitudinal cracks which are weak against contraction in the widthwise direction due to solidification to be easily generated.

[0013] Furthermore, in the above-mentioned publications, the stroke of mold oscillation is varied as the casting speed increases, so a conventional cam-type oscillating mechanism can not be employed, making it difficult to apply those methods to existing casting equipment. Thus, the methods in those publications are not very practical.

[0014] Moreover, in the above-mentioned publications, there is no mention of mold dimensions, and in particular, there is no mention of the manufacture of thin cast slabs. There is also no disclosure concerning a method for casting of thin or medium thickness cast slabs immediately followed by hot rolling,

SUMMARY OF THE INVENTION

[0015] The present invention provides a high speed continuous casting method in which sticking breakout does not occur, in which surface defects such as longitudinal flaws do not occur, and which is capable of manufacturing thin or medium thickness slabs suitable for forming thin steel sheets.

[0016] The present invention also provides a method for manufacturing thin steel sheet in which thin or medium thickness cast slabs can be continuously cast at a high speed and immediately thereafter subjected to hot rolling.

[0017] The present inventors realized that in order to perform stable casting at a high casting speed of at least 2.5 m/min, the oscillating conditions of a mold are important. As a result of various experiments involving varying of mold oscillating conditions, it was found that if mold oscillation is performed so as to satisfy the relationship 450≦S×f/Vc≦1000, wherein S is the stroke of mold oscillation in mm (4 mm≦S≦15 mm), f is the frequency of oscillation in cycles per minute, and Vc is the casting speed in m/min, continuous casting can be performed in a stable manner without the occurrence of sticking breakout and without the formation of surface defects such as longitudinal cracks.

[0018] The present inventors also found that at a casting speed of 2.5-10 m/min, manufacture of a thin or medium thickness cast slab can be effectively performed with a mold cavity thickness of 50-120 mm, and that thin or medium thickness cast slabs which could not be readily obtained by conventional technology can be economically manufactured.

[0019] Thus, in one form of the present invention, a continuous casting method comprises continuously casting steel at a casting speed of at least 2.5 m/min and at most 10 m/min while oscillating a mold so as to satisfy the relationship

450≦S×f/Vc≦1000

[0020] wherein S is the stroke of mold oscillation in mm (S=4 to 15 mm), f is the frequency of mold oscillation in cycles per minute, and Vc is the casting speed in m/min.

[0021] A slab formed by a casting method according to the present invention may be subjected to hot rolling after casting to form steel sheet or other product having a desired thickness. Preferably hot rolling is performed without letting the cast slab cool to room temperature between casting and rolling, and more preferably without letting the cast slab cool to below its Ac3 point. When the cast slab is to be hot rolled into sheet, the mold preferably has a mold cavity with a thickness of 50-120 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a graph showing the relationship between the frictional force between a mold and a cast slab and casting speed.

[0023] FIG. 2 is a graph showing the relationship between the pitch of oscillation marks on a cast slab and (S×f/Vc).

[0024] FIG. 3 schematically illustrates a continuous casting machine which can be used in the present invention.

[0025] FIG. 4 illustrates the relationship between casting speed and (S×f/Vc).

DESCRIPTION OF PREFERRED EMBODIMENTS

[0026] A continuous casting method according to the present invention can be carried out on any continuous casting machine suitable for continuous casting at a speed of at least 2.5 m/min. The casting machine is not restricted to any particular shape. For example, it may have a curved shape, a vertical shape, or other shape.

[0027] The dimensions of the cavity of the casting mold of the continuous casting machine can be selected in accordance with the size of the cast slab which is to be formed. For example, when used to form usual slabs, the mold cavity may have a thickness of 120 mm or above. When used to form thin or medium thickness cast slabs, the mold cavity will typically have a thickness of 50-120 mm and preferably 70-110 mm.

[0028] The mold can be equipped with any type of oscillating mechanism capable of oscillating the mold in the lengthwise direction of a cast slab with the desired stroke and frequency. Examples of suitable mechanisms include conventional oscillating mechanisms with hydraulic actuators, conventional cam-operated oscillating mechanisms, and conventional oscillating mechanisms employing levers and cranks. The velocity profile of the mold during each cycle of oscillation is not restricted. A sinusoidal velocity profile (in which the displacement of the mold from a reference position varies sinusoidally with respect to time) is generally easy to implement, but other profiles can be employed.

[0029] FIG. 1 shows the relationship between the frictional force acting between the interior surface of a mold and a cast slab and the casting speed when only the stroke of oscillation of a mold was varied.

[0030] From this figure, it can be seen that the frictional force between a mold and a cast slab increases as the stroke of oscillation of the mold decreases, and that the frictional force abruptly increases when the stroke of oscillation is less than 4 mm. This increase in frictional force is caused by the inflow of lubricant powder between the mold surface and the cast slab being suppressed when the stroke of is oscillation is small. On the other hand, when the stroke exceeds 15 mm, the frictional force between the mold and the cast slab abruptly decreases, leading to an increase in the number of longitudinal cracks, which are thought to be caused by excess inflow of lubricant powder between the mold and the cast slab. Therefore, during high speed casting according to the present invention at a casting speed of at least 2.5 m/min, the optimal stroke of mold oscillation is 4 mm.

[0031] The reasons why the stroke S (mm) of mold oscillation, the oscillation frequency f (cycles per minute), and the casting speed Vc (m/min) are expressed by 450≦S×f/Vc≦1000 in the present invention are as follows.

[0032] FIG. 2 shows the relationship between the value of (S×f/Vc) and the pitch (mm) of oscillation marks observed in a series of continuous casting experiments in which the stroke of mold oscillation S (mm), the oscillation frequency f (cycles per minute), and the casting speed Vc (m/min) were varied. S was varied from 4 to 10 mm.

[0033] As can be seen from FIG. 2, the pitch of oscillation marks decreased as the value of (S×f/Vc) increased. This tendency became more marked as the stroke of oscillation increased. When the stroke of mold oscillation was 15 mm and (S×f/Vc) was less than 450, the pitch of oscillation marks was an extremely large value of at least 33.3 mm. As a result, due to shrinkage of the slab within the mold in the widthwise direction of the mold resulting from solidification of the slab, a large number of longitudinal flaws formed between oscillation marks. When (S×f/Vc) exceeded 1000, the pitch of oscillation marks became a small value of less than 15 mm, the number of oscillation marks per unit length of a cast slab increased, resulting in an increase in lubricant powder-related defects in the surface of a cast slab, and the quality of a coil formed from the slab decreased. This is partly due to an increase in the range of fluctuation of the meniscus in the mold cased by an increase in the stroke and frequency of mold oscillation, and partly due to an increase in the amount of inclusions of lubricant powder caused by collapse of the tip of the shell formed near the meniscus during casting. The inclusion of lubricant powder increases in proportion to an increase in the number of oscillation marks.

[0034] When the stroke of oscillation is small, the pitch between oscillation marks becomes extremely small, so longitudinal cracks are no longer generated. However, the frictional force between the mold and the cast slab increases due to a decrease in the inflow of lubricant powder between the mold and the slab, In addition, if S×f/Vc is less than 450, the number of oscillation marks contacting the surface of molten lubricant powder in the mold decreases, so the amount of molten lubricant powder which is carried into the mold decreases, and the danger of sticking breakout increases. For this reason, when the stroke of oscillation is 4 mm, it is difficult to perform stable operation unless S×f/Vc is at least 450.

[0035] When the stroke has a small value of 4 mm, if S×f/Vc exceeds 1000, the increase of the mold oscillation frequency proceeds more than when the stroke is 15 mm. For this reason, large fluctuations of the meniscus in the mold take place, and there is a tendency for lubricant powder-related defects to increase.

[0036] Accordingly, when performing high speed continuous casting at a casting speed of at least 2.5 m/min, in order to carry out optimal operation in which there is no danger of sticking breakout and the quality of the cast slab and coils formed from the slab is maintained, the mold is oscillated with a stroke in the range of 4-15 mm while satisfying the relationship 450≦S×f/Vc≦1000. Preferably, (S×f/Vc) is 500-900.

[0037] The frequency of oscillation f is usually 80-2500 cycles per minute. This range of frequencies can also be employed in the method of the present invention. More preferably, the frequency f is 100-500 cycles per minute.

[0038] When the method of the present invention is used to perform continuous casting of thin or medium thickness cast slabs, casting is preferably performed at a casting speed of 2.5 to 10 m/min and preferably 2.5 to 8 m/min using a mold with a mold cavity having a thickness of 50-120 mm. When manufacturing a thin or medium cast slab, a casting speed of at least 2.5 m/min and preferably less than 8 m/min is desirable in order to achieve economical operation. There is no strict upper limit on the casting speed, but it is difficult to perform stable operation if the casting speed exceeds 10 m/min, so preferably the casting speed is at most 10 m/min.

[0039] If the thickness of the mold cavity exceeds 120 mm, it is not possible to omit rough rolling of the resulting slab, while if the thickness is less than 50 mm, it is difficult to perform continuous casting. Therefore, in cases in which it is desired to omit rough rolling of the cast slab, the thickness of the mold cavity is 50-120 mm, and preferably 70-110 mm.

[0040] After a thin or medium thickness cast slab is formed by a continuous casting method according to the present invention, the slab will typically be subjected to hot rolling to form steel sheet or other rolled product having a desired thickness. Preferably, the hot rolling is performed without the slab being allowed to cool to room temperature, and more preferably without it being allowed to cool below its Ac3 point between the completion of casting and the start of hot rolling. In this case, the casting speed during continuous casting is preferably less than 8 m/min so that hot rolling can be efficiently performed immediately after continuous casting. An upper limit on the casting speed can be determined based on the capacity of the rolling equipment to which the cast slab is supplied and is preferably sufficiently high that the cast slab is not cooled to room temperature and preferably is not cooled to below the AC3 point between the completion of casting and the completion of rolling. If desired, a heating furnace may be disposed in the manufacturing line to maintain the cast slab at a suitable temperature for hot rolling.

[0041] Next, the effects of the present invention will be described in more detail by the following examples.

EXAMPLES

[0042] Continuous casting of steel slabs was performed using a continuous casting machine like the one schematically illustrated in FIG. 3. The casting machine had an overall length of 15 meters and included a vertical portion with a length of 1 meter. Molten steel 1 in the form of low carbon aluminum-killed steel with a carbon content of 0.05 percent was poured into a mold 2, and continuous casting was performed at a casting speed in the range of 2.5-10 m/min. During casting, the mold 2 was oscillated by an unillustrated oscillating mechanism in the direction of casting. A cast slab which was pulled from the mold 2 was supported by guide rolls 3 in the vertical and curved portions of the casting machine. The slab was gradually cooled from the surface thereof, and solidification of the molten steel was completed at solidification point 5. After solidification, the slab was pulled out of the continuous casting machine by pinch rolls 4 and was cut by an unillustrated cutting apparatus. The slab was then transported to an unillustrated hot rolling line without being cooled to below room temperature and preferably without being cooled to below its Ac3 point, using a heating furnace if necessary.

[0043] The mold 2 had a cavity with a width of 1500 mm and a thickness of 120 mm. During continuous casting, the stroke of oscillation of the mold 2 and the value of (S×f/Vc) were varied.

[0044] The mold 2 was equipped with a plurality of thermocouples for sensing the temperature of the mold wall. The outputs of the thermocouples were monitored, and variations among the outputs were analyzed using a known algorithm to determine the occurrence of sticking breakout within the mold 2. An alarm was generated when breakout was detected. The stability of operation during high speed casting was evaluated based on the number of breakout alarms generated per charge, with zero alarms per charge being an acceptable value. The surface condition of each cast slab was evaluated based on the number of sticking marks per 1 meter length of the slab (number per charge). Zero marks was considered an acceptable value.

[0045] The quality of each cast slab was evaluated by a longitudinal crack length index equal to the length of longitudinal cracks per meter of a cast slab (m/m). In addition, the surface of each cast slab was machined to a depth of 1 mm, i.e., the top 1 mm of the surface was removed, and the machined surface was observed with a microscope to count the number of subsurface inclusions having a size of at least 50 micrometers per 10 cm2 of area. Furthermore, a coil defect index was determined by the following formula:

[0046] Coil defect index=(weight in tons of coils having defects in one charge)/(total weight of coils in one charge).

[0047] The results are shown in Table 1. Examples 1-6 are examples of the present invention, while Examples 7-28 are comparative examples outside the range of the present invention. In the columns for Operational Stability and Quality, 0 indicates good, x indicates poor, and xx indicates very poor.

[0048] In Examples 1-6 according to the present invention, there were no breakout alarms and no sticking marks. The longitudinal crack length index was 0-0.01 m/m, the number of subsurface inclusions in the cast slabs was at most 3 inclusions per 10 cm2, and the coil defect index was at most 0.02, so these examples had good values with respect to stability of operation and quality.

[0049] In Examples 7-13, which were comparative examples, when the stroke of mold oscillation was 3 mm, breakout alarms and sticking marks occurred, and these examples were evaluated as having poor or very poor operational stability.

[0050] Similarly, in Examples 22-28, when the stroke of oscillation was 16 mm, although there were no breakout alarms or sticking marks, there were many longitudinal cracks, the longitudinal crack length index was at least 0.1 m/m, and the coil defect index was at least 0.2, so these examples were evaluated as having poor or very poor quality.

[0051] In Examples 14 and 15, when the stroke of oscillation was 4 mm and (S×f/Vc) was less than 450, there were no longitudinal cracks, the number of subsurface inclusions in the cast slab was small, and the coil defect index and the evaluation with respect to quality were good. However, both breakout alarms and sticking marks occurred, so these examples were evaluated as poor with respect to operational stability.

[0052] In Examples 16 and 17, when the stroke of oscillation was 4 mm and (S×f/Vc) was greater than 1000, there were no breakout alarms or sticking marks, but the number of subsurface inclusions in the cast slab increased, and the coil defect index was 0.1-0.2, so these examples were evaluated as being poor with respect to quality.

[0053] In Examples 18 and 19, when the stroke of oscillation was 15 mm and (S×f/Vc) was less than 450, there were no breakout alarms or sticking marks, but longitudinal cracks occurred, the longitudinal crack length index was at least 0.1, and the coil quality index was 0.1-0.2, so these examples were evaluated as being poor with respect to quality.

[0054] In Examples 20 and 21, when the stroke of oscillation was 15 mm and (S×f/Vc) was greater than 1000, there were no breakout alarms or sticking marks, but the number of subsurface inclusions in the cast slab increased to 20-30 per cm2, and the coil defect index was at least 0.1, so these examples were evaluated as having poor quality.

[0055] FIG. 4 illustrates the significance of(S×f/Vc) based on the above results. In the figure, the cross-hatched region indicates the region in which the desired effects of the present invention are exhibited.

[0056] Next, thin cast slabs obtained by Examples 1-6 were subjected to hot rolling after high speed continuous casting without being cooled to below the Ac3 point. Not rolling was performed without rough rolling to obtain hot rolled steel sheet with a thickness of 4 mm.

[0057] The resulting steel sheet did not have surface defects such as ground burs, and hot rolled steel sheet having an excellent surface condition was efficiently manufactured. 1 TABLE 1 Oscillation Breakout Sticking Longitudinal Subsurface Coil Example Stroke mark pitch alarms marks per crack length inclusions defect Operational No. (mm) Sf/Vc (mm) per charge charge index (mm) per 10 sq cm index stabilty Quality THIS 1 4 450 2.9 0 0 0 1 0.01 0 0 INVENTION 2 4 600 6.7 0 0 0 2 0.01 0 0 3 4 1000 4.0 0 0 0 1 0.01 0 0 4 15 450 33.3 0 0 0.01 3 0.02 0 0 5 15 600 25.0 0 0 0.01 3 0.02 0 0 6 15 1000 15.0 0 0 0.01 3 0.02 0 0 COMPARATIVE 7 3 450 6.7 8 4 0 2 0.02 xx 0 EXAMPLES 8 3 500 6.0 7 3 0 2 0.01 xx 0 9 3 600 5.0 6 3 0 1 0.01 x 0 10 3 700 4.3 6 2 0 2 0.01 x 0 11 3 800 3.8 5 2 0 3 0.01 x 0 12 3 1000 3.0 5 2 0 20 0.10 x x 13 3 1050 2.9 4 2 0 30 0.15 x x 14 4 350 11.4 5 2 0 2 0.01 x 0 15 4 400 10.0 3 1 0 1 0.01 x 0 16 4 1050 3.8 0 0 0 7 0.08 0 x 17 4 1100 3.6 0 0 0 20 0.12 0 x 18 15 350 42.9 0 0 0.2 2 0.2 0 x 19 15 400 37.5 0 0 0.1 2 0.1 0 x 20 15 1050 14.3 0 0 0.01 20 0.1 0 x 21 15 1100 13.6 0 0 0.01 30 0.13 0 x 22 16 450 35.6 0 0 0.5 0 0.3 0 xx 23 16 500 32.0 0 0 0.4 0 0.2 0 x 24 16 600 26.7 0 0 0.35 0 0.2 0 x 25 16 700 22.9 0 0 0.2 0 0.22 0 x 26 16 800 20.0 0 0 0.2 1 0.25 0 x 27 16 1000 16.0 0 0 0.15 25 0.3 0 xx 28 16 1050 15.2 0 0 0.12 35 0.3 0 xx

Claims

1. A continuous casting method comprising continuously casting steel in a mold at a casting speed of at least 2.5 m/min and at most 10 m/min to form a slab while oscillating the mold so as to satisfy the relationship

450≦(S×f/Vc)≦1000

wherein S is the stroke of mold oscillation in mm (4 mm≦S≦15 mm), f is the frequency of mold oscillation in cycles per minute, and Vc is the casting speed in m/min.

2. A method as claimed in

claim 1 wherein the mold has a cavity with a thickness of 50-120 mm.

3. A method as claimed in

claim 1 wherein the mold has a cavity with a thickness of 70-110 mm.

4. A method as claimed in

claim 1 including hot rolling the slab after casting to form thin steel sheet without allowing the slab to cool to below its Ac3 point between casting and hot rolling.

5. A method as claimed in

claim 4 wherein the casting speed Vc is less than 8.0 m/min.

6. A method as claimed in

claim 1 wherein the following relationship is satisfied.

500≦(S×Vc)≦900