Method for annealing of strip coils

Abstract

A strip coil is uncoiled, heated to a predetermined temperature and coiled again. The heated and coiled strip is maintained at a predetermined temperature for a predetermined time and thereafter gradually cooled.

Description

The present invention relates to a method for annealing cold-rolled strip coils.

The batch type annealing method has been generally employed in the cold rolling mill factories. According to this method the upended three or four strip coils are stacked one upon another and covered with an inner cover and a cylindrical furnace body for annealing. Since the cold roller strip coil is coiled very tight, it must be gradually heated from the outer periphery thereof until the temperature at the center of the coil rises to a predetermined temperature (about 720.degree.C), and be maintained at a predetermined temperature for a predetermined time (about 1.5 to 2 hours). Thereafter the strip coil must be gradually cooled.

Therefore the heat cycle becomes extremely long (for about four or five days) so that the productivity is low. Furthermore when the strip coil is rapidly heated and then cooled, it is subjected to deformations so that the operation in the next stage is adversely affected. Moreover sticking occurs due to the thermal expansion of the strip coil and the quality of the strip is deteriorated. (Sticking refers to the phenomenon that the adjacent strips adhere to each other due to the thermal expansion.)

In order to overcome the above and other problems there has been proposed an improved method in which the open-coiled strip is annealed in a batch type annealing furnace such as before mentioned tight furnace. The open-coiled strip is uniformly and rapidly heated by the convection of the heated atmosphere gases in the annealing furnace. But when the open-coiled strip is heated too rapidly, it is subjected to deformations by ununiformity heating, and the sufficiently quick heating of the strip cannot be attained even by the convection of the high temperature atmosphere gases. Furthermore this method has a defect that the installation cost is extremely high.

There has been also used a continuous annealing line as the method for annealing of especially thin steel sheet such as tin plate which will not require the complete annealing. According to this method the strip is continuously uncoiled and passed through a furnace so that it may be heated and maintained at a predetermined temperature for a predetermined time. Thereafter the strip is gradually cooled. But this method has a distinct defect that the length of the annealing furnace must be considerably increased in order to store the strip in the furnace, which is transported at high speed, at a predetermined temperature for a predetermined time (generally 1.5 to 2 hours) required for the sufficient grain growth in the strip. (Grain growth refers to a phenomenon that a nucleus of crystal is formed and grown in the structure of steel. The larger the grain growth, the better the effect of annealing becomes). In practice the strip is maintained at a predetermined temperature only for 30 to 60 seconds so that the annealed strip is hard and is used only for the manufacture of tin plate. That is, the continuous annealing method of the type described therefore cannot be used for producing complete annealing and drawing quality of cold rolled strips.

The cooling rate is the most important factor in annealing, and it is preferable to cool the strip at as slow cooling rate as possible from the metallurgical viewpoint. However, in practice rapid cooling is generally employed in order to improve the production efficiency. Therefore the strip is cooled quickly from a temperature of about 500.degree.C below a critical range. (At a higher temperataure all of the carbon in steel exists in the form of saturated solid solution, and when steel is cooled rapidly carbon is not precipitated along the equilibrium line in the phase diagram so that the over-saturated solid solution of carbon remains in the ferrite. The tendency of the excess carbon to precipitate as carbide thereby recovering the equilibrium is called age hardening. The slow cooling temperature range in which no age hardening occurs is called a critical range.)

The present invention therefore provides a method for annealing of strip coils which utilizes the advantages of the prior art methods and apparatus and may provide high-quality and drawing-quality strip coils in a highly effective productivity. Briefly stated the present invention is characterized in that a strip is heated, coiled so as to keep heat and then gradually cooled.

The present invention will become more apparent from the following description of one preferred embodiment thereof taken in conjunction with the accompanying drawing in which:

Single FIGURE illustrates a schematic diagram used for the explanation of the annealing method in accordance with the present invention.

In the single FIGURE, reference numeral 1 designates unwinding devices; 2, a welder; 3, an electrolytic cleaning line; 4, a loop car for transporting a strip continuously in a furance area at a predetermined speed in order to prevent from the overheat even when the uncoiling line is stopped while coil preparation and welding; 5, a heating furnace; 6, a nonoxidizing gas purging chamber; 7, rewinders disposed within the purging chamber 6; 8, upenders; 9, a uniform heating furnace; 10, a cooling furnace; 11, strip coils; 12, coil cars; 13, a nonoxidizing gas purging chamber in which are disposed a unwinder 14, a shear 15 and a welder 16; 17, a jet cooling furnace; 18, a rewinder; 19, strips; 20, a temper mill; 21 and 22, shears; and 23, a flying shear disposed within the gas purging chamber 6.

The strips 19 continuously uncoiled from the unwinding devices 1 are fed into the shears 22 so that the off-gage portions at the leading edges of the strips 19 are cut off. Thereafter the strips 19 are formed by the welder 2 into the form of a continuous strip and fed into the electrolytic cleaning line 3, then into the loop car 4 and into the continuous heating furnace 5 where the strip is heated at a temperature about 700.degree.-720.degree.C for 20 - 30 seconds. The heated strip 19 is coiled again by the rewinders 7 in the nonoxidizing purging chamber 6 under the condition that the strip is heated. Since the strip is separated again by the high-speed flying shear 23 when, the strip is coiled up again, so that it is not required to stop the heating process. Heat must be always supplied to the heating furnace 5 and the nonoxidizing gas purging chamber 6 in order to maintain the temperature of the strip 19 at a predetermined constant temperature. The recoiled strip is upended by the upender 8 within the furnace and transported into the uniform heating furnace 9 by a suitable means such as a conveyor or coil car. Within the uniform heating furnce 9 the coil is slowly transported by the coil car 12 or by a walking beam system etc., so that it may remain within the uniform heating furnace 9 for a sufficient grain growth time (1.5 - 2 hours). Thereafter the strip coil 11 is cooled in the cooling furnace 10 for a sufficient time (about 4 hours) while the above mentioned critical range to a temperature from which the coil 11 may be cooled rapidly. The cooled coil is transferred to the rapid cooling process by suitable means such as a conveyor or coil car, downed and mounted on the unwinder 14 within the nonoxidizing gas purging chamber 13 so that the strip coil 11 may be uncoiled and passed through the shear 15 and the welder 16 in the form of a continuous strip. Thereafter the continuous strip is cooled rapidly in the jet cooling furnace, and the annealed strip in coiled again by the rewinding device 18 through the shear 21. Thus the strip annealing process is completed in high productivity.

Unlike the heating furnace, the jet cooling furnace 13 will not be provided with a loop car because the operation of the jet cooling furnace can be stopped while the strip coil is mounted upon the unwinder 14, welded, thread, and removed from the rewinding device 18. The strip coil 11 mounted on the unwinder 14 is relatively loose because the coil has been cooled in the cooling furnace 10 and therefore contracted so that the strip coil must be uncoiled without exerting the tension thereto in order to prevent the strip surface from slippage of each other wrap of coil.

The present invention is not limited to the preferred embodiment described hereinafter with reference to the accompanying drawing. For example in the embodiment described above the strip coils have been described as being upended by the upender 8, but it will be understood that the strip coils may be downended. It is also understood that various modifications and variations can be effected without departing from the true spirit of the present invention.

According to the present invention the coiled strip is heated in the form of a continuously uncoiled strip so that the strip may be rapidly and uniformly heated, maintained at a predetermined temperature for a required time and gradually cooled. Therefore the apparatus required for carrying out the annealing method in accord with the present invention may be made smaller and the strip may be maintained at a predetermined temperature for a long time so that the complete annealing may be effected. Furthermore the strip which has been slowly cooled to a certain temperature is rapidly cooled so that the overall cooling time may be reduced. So not only uniform cooling may be effected, but also the productivity may be increased. In the conventional cold rolling mill factory, the electrolytic cleaning line and the temper mill are installed independently, but according to the present invention the electrolytic cleaning line 3 is installed at the entrance of the heating line whereas the temper mill 20 is installed at the exit of the rapid cooling line thereof so that a continuous cold rolling mill factory may be realized. Therefore the general conception of a cold rolling mill factory is changed, and the labor saving, the high production efficiency, and the reduction in installation cost and in operation cost may be attained.

Claims

What is claimed is:

1. A method for continuously annealing steel strip coils comprising the steps of

a. uncoiling a strip coil,

b. heating the uncoiled strip to an elevated temperature of from 700.degree.C to 720.degree.C for a time period in the range of 20 to 30 seconds,

c. recoiling the heated strip,

d. maintaining the recoiled strip at substantially said elevated temperature for a time period in the range of 1.5 to 2 hours,

e. cooling the heated coil for a time period in the range of 4 hours,

f. uncoiling the cooled strip coil and rapidly cooling the strip, and

g. recoiling the rapidly cooled strip.