METHOD AND APPARATUS FOR HEATING A STRIP OF METALLIC MATERIAL IN A CONTINUOUS ANNEALING FURNACE

Abstract: The present invention relates to method and apparatus for heating a strip of metallic material in a continuous annealing furnace and more particularly improvement of or relating to method and apparatus for heating a strip of metallic material in a continuous annealing furnace in which annealing of the strip is continuously carried out in such a manner that gas serving to adjust temperature of the strip is blown toward the strip through a plurality of gas jet nozzles which are arranged on the one side or both the sides of the strip, wherein temperature and flow rate of the strip are properly determined to a required level in response to changing of the operating conditions such as heat cycle, line speed, thickness of strip, width of strip or the like. Further, the present invention relates to method and apparatus for heating a strip of metallic material in a continuous annealing furnace, wherein temperature of the strip is controlled to reach a target temperature by heating or coolling the strip by means of gas jet or the like having excellent respondency at a part of the heating zone in the continuous annealing furnace whereby irregular annealing of the strip is inhibited effectively.

Description: Background of the Invention (i) Field of the Invention The present invention relates to method and apparatus of heating a strip of metallic material in a continuous annealing furnace. (ii) Related Art Statement The FR 240 66 67 discloses a method for controlling the strip temperature within a continuous annealing furnace comprising a preheating zone as well as a speed heating zone. A further method is known from US 43 16 717. Whereby the processing temperature within the chamber is changed in response to a change in the gange of a well consisting of two joint metal strips. For a better understanding of the invention another prior art should, however, been explained now. As shown in Fig. 6, a typical conventional continuous annealing furnace for continuously annealing a strip of metallic material such as cold rolled steel sheet, tin plated steel sheet or the like is so constructed that the strip 1 is unreeled from a payoff reel and it is then introduced into the furnace via cleaning tank, looper or the like. The furnace is provided with a plurality of rolls (that are called helper rolls) R in bath the upper and lower areas thereof and the strip 1 is subjected to heating or cooling at a temperature in the range of 650 °C to 900 °C in dependence on mechanical properties required for a product of strip while it moves up and down in the vertical direction in the area as defined between the upper and lower rolls R. After completion of annealing the strip has required metallic properties such as high tensile strength, capability of deep drawing or the like at the state of room temperature. In the recent years requirements have been raised from users for improved method and apparatus for continuously annealing a strip of metallic material having different thickness and width in accordance with different heat cycle independence on required mechanical properties of the product of strip, because there is a tendency of carrying out production in the form of many kinds and small quantity. In the conventional furnace the strip 1 in the heating zone is heated up to an elevated temperature by radiation of thermal energy in accordance with the radiant tube system. However, it is pointed out that the conventional furnace has a problem that temperature of the strip to be heated can not be controlled quickly in response to variation of the heat cycle required for the strip, because temperature of each of the radiant tubes has large time constant. For instance, when thickness of the strip 1 increase, that is, a strip having thickness more than that of the preceding strip is continuously treated and therefore the thick strip having large heat capacity moves through the heating zone, there is a necessity for raising temperature of the radiant tubes to a higher level. However, due to the fact that the radiant tubes themselves have large time constant in the range of 10 to 20 minutes, the strip 1 can not reach a predetermined temperature within a very short period of time after an intensity of combustion of the burners relative to the radiant tubes is changed. In the meanwhile it is acceptable to change line speed of the strip 1. When line speed of the strip 1 is left unchanged until the preceding thin strip 1 moves past the heating zone of the furnace, it results that the fore end part of the following thick strip is heated insufficiently. In practice, it was reported that a part of strip having very long length of 2000 to 5000 m was annealed insufficiently. When line speed of the following thick strip is reduced by a necessary extent in order to assure that it reaches a required temperature, it results that temperature of the strip is raised up excessively and thereby its is annealed excessively. This leads to production of a strip which has mechanical property softer than generally required one. Alternatively, when line speed of the strip is changed to an intermediate level, it is found that the preceding strip becomes softened while a part of the following strip is annealed insufficiently. On the contray, in the case where thickness of a strip to be annealed decreases in the cource of its moving through the heating zone in the furnace, it is obvious that reverse phenomenon will be recognized to the foregoing case. In the past time, users were generally willing to receive a product of strip which was softened to a level above required mechanical properties from the viewpoint of excellent workability. In the recent years, however, automation has been increasingly employed for elastic working process of metallic plate or the like material and this leads to a tendency that metallic material softened in the above-described manner is not always willingly received by users. Thus, products which are uniformly treated as required become important for them. However, this causes the jointed area where two strips having different thickness are jointed to one another to be subjected to irregular treating by a considerably long distance. Therefore, the conventional annealing method can not be employed. To obviate the above-mentioned problem concerning the jointed area where thickness of strips varies there was made a proposal that a dummy strip was interposed between two strips to be annealed and operating conditions of the furnace were changed properly during movement of the dummy strip through the heating zone. As a result, however, it is found that the furnace has a reduced treating capability. In the meanwhile, it is necessary that a possibly large quantity of strips having the same size or material are continuously annealed from the viewpoint of operation of the furnace at a high efficiency. This leads to a necessity that a large quantity of strips are kept in storage as inventry in the area located in proximity of the continuous annealing furnace in order to facilitate operation of the furnace as planned. As a result, inventory cost increases and moreover there occurs such an inconvenience that production can not be carried out in the acceptable timing relation as required. Further, in the case where thick strip is shifted to thin strip in the cource of annealing operation or in the case where thin strip is shifted to thick strip in the reverse manner, there occurs the following problem, particularly when difference of thickness between adjacent strips is remarkably large. For instance, in the case there thin strip is shifted to thick strip, gas having higher temperature is blown toward the moving strip through gas jet nozzles which are exposed to radiant tubes having lower temperature immediately after shifting of thickness is effected in that way. As a result, a high intensity of thermal stress is generated in the gas jet nozzles and this leads to a fear of causing deformation, damage or the like with the gas jet nozzles. Generally, the conventional continuous annealing furnace employed for continuously annealing a strip of metallic material is so constructed that preheating zone, heating zone, soaking zone and cooling zone (inclusive excessive aging zone in the case where excessive aging treatment is required for the strip) are arranged one after another as seen from the inlet side of the furnace. Heating in the preheating zone is achieved by direct heating with the use of exhaust gas which is delivered from the heating zone and the soaking zone or by blowing hot air toward the strip of which temperature is raised up to an elevated level by heat exchanging with exhaust gas. Further, heating in the heating zone as well as in the soaking zone is achieved by means of a plurality of radiant tubes. On the other hand, cooling in the cooling zone is achieved in accordance with roll cooling system, gas jet cooling system or cooling tube system. In the meanwhile, temperature of strip at the outlet of the heating zone is controlled to reach a target temperature by controlling line speed in such a manner that a value of (thickness of strip) x ( line speed) is kept constant while temperature of the heating zone is left unchanged, when thickness of a strip is changed to another one with the same heat cycle used during the whole operation. In the case where the existing heat cycle is changed to another one, temperature of the strip at the outlet of the heating zone is controlled by changing the preset temperature in the heating zone. However, it is found that the conventional continuous annealing furnace has a drawback that the heating zone has slow heat responsibility relative to temperature thereof and it takes 20 to 30 minutes when the preset temperature of the heating zone is changed to another one and thereby there appears difference of temperature, for instance, 100°. Accordingly, material rejection equivalent to the length of about one coil takes place due to insifficient heating, for instance, when line speed is held at a level of 300 mpm. This means that there is a necessity for preparing a dummy coil having a length as mentioned above. However, a period of time for which the dummy coil moves past the heating zone in the furnace does not make any contribution to production and moreover using of the dummy coil is not preferable from the viewpoint of thermal energy saving. Further, when such a dummy coil is used for the furnace, extra operations such as welding of the dummy coil before it enters the heating zone, cutting of the same after it leaves there and handling of the same in the area extending from the inlet to the outlet of the heating zone. Another drawback of the conventional continuous annealing furnace is that when thickness of a strip is changed to another one with the same heat cycle employed therefor, material rejection takes place by a certain distance in the area located before and behind the welded point of the strip, because another line speed can not be quickly determined in response to changing of thickness of the strip. To obviate the above-mentioned drawback, temperature of the strip at the outlet of the heating zone is kept within the extent of allowable temperature by limiting an amount of changing of thickness of strip, for instance, within ± 15% of thickness of the preceding strip whereby rejection due to material failure is inhibited. However, such a countermeasure as mentioned above makes it complicated to design operation schedule relative to a strip to be annealed and control a number of coils in a coil storage house. 4. Summary of the Invention Hence, the present invention has been made in the foregoing background in mind. (I) It is an object of the present invention to provide a method of heating a strip of metallic material in a continuous annealing furnace with the aid of radiation of thermal energy from a plurality of radiant tubes which assures that heating temperature can be quickly changed for the strip when operating conditions such as heat cycle, line speed or the like are changed. (II) It is other object of the present invention to provide a method of heating a strip of metallic material in a continuous annealing furnace with the aid of radiation of thermal energy from a plurality of radiant heat tubes which assures that temperature response time in the heating zone is shortened when operating conditions such as heat cycle, thicknes of strip or the like are changed and a plurality of gas jet nozzles are inhibited from being subjected to a high intensity of thermal stress at that time. (III) It is another object of the present invention to provide an apparatus for heating a strip of metallic material in a continuous annealing furnace which assures that temperature of the strip is quickly raised or lowered to a level of target temperature to effectively heat or cool the strip without any necessity for complicated operations and utilization of dummy coil as seen with the conventional furnace. To accomplish the above objects there are proposed according to the present invention the method and apparatus for heating a strip in a continuous annealing furnace, as explained in detail in the claims. The present invention will be described in more details below as to continuous heating means required in the case where thin strip is shifted to thick strip. According to the invention an intensity of combustion in the radiant tube burners is quickly raised up to a level corresponding to thus shifted thick strip before shifting is effected. It should be noted that quick temperature increase does not occur due to the fact that the radiant tubes themselves have large heat capacity but an amount of thermal energy required for thin strip becomes excessive gradually. For the reason it is necessary that an amount of thermal energy which becomes excessive gradually is removed at the same time when an intensity of combustion in the radiant tube burners is raised up. To this end an amount of cooling gas is gradually increased so that it is blown toward the strip. Blowing of cooling gas is interrupted when thickness of the strip to be annealed is changed. Since the present invention consists in that gas to be blown through the gas jet nozzles is supplied gradually and occurrence of thermal stress due to gas blown through the gas jet nozzles is inhibited effectively. Thus, a period of response time in the heating zone can be shortened when thickness of strip is changed. (IV) Further, there is proposed according to another aspect to the present invention a method of heating a strip of metallic material in a continuous annealing furnace which is characterized in that the strip is heated or cooled by means of gas jet having excellent thermal respondency at a part of the heating zone in the furnace in inresponse to changing of operating conditions such as heat cycle, line speed, thickness of strip or the like whereby heating temperature of the strip is controlled to reach a target temperature. (V) Further, there is proposed according to another aspect of the present invention an apparatus for heating a strip of metallic material in a continuous annealing furnace which is characterized in that it includes a strip temperature controlling zone in a part of the heating zone and the strip temperature controlling zone is provided with means for heating or cooling the strip by using gas jet having excellent thermal respondency. According to the invention as defined in the preceding paragraphs (IV) and (V) the continuous annealing furnace is provided with a strip temperature controlling zone located in a part of the heating zone where heating is effected in accordance with radiant tube system and thereby temperature of a strip to be annealed can be controlled to reach to a target level by blowing heating or cooling gas jet directly toward the strip to quickly raise or lower the existing temperature. Thus, operation of the furnace is carried out properly without any complicated handling as well as utilization of dummy coil. By the way, an amount of thermal energy Q s received on or radiated from a strip to be annealed can be obtained in accordance with the following formulas for the case where heating or cooling is effected with the aid of radiant tubes, gas jet or rolls. (1) In the case where heating or cooling is effected with the use of a plurality of radiant tubes where  cq : total thermal conductive coefficient T f : furnace temperature (particularly, furnace wall temperature which is affected by temperature of radiant tubes) T s : temperature of strip to be annealed (2) In the case where heating or cooling is effected by means of gas jet Q s = KVn ( T g - T s ) ---- (2) where K : constant V : flow speed of gas n : constant T g : temperature of gas (3) In the case where heating or cooling is effected with the use of a plurality of rolls Q s = αt ( T R - T S ) ---- (3) where α : constant t : period of time for which strip to be annealed comes in contact with rolls under the influence of winding angle and the number of rolls T R : temperature on the surface of rolls When an amount of thermal energy Q s received on strip to be annealed is changed, that is, when heat cycle and thickness of the strip LS are changed, there is a necessary for changing furnace temperature T f in the case where heating is effected with the use of radiant tubes. However, due to the fact that furnace wall and radiant tubes have large thermal capacity it can not be expected that furnace temperature T f is changed quickly. However, in the case where heating or cooling is effected by means of gas jet, an amount of thermal energy received on strip to be annealed can be easily and quickly changed by changing flow speed of gas. Further, in the case where heating or cooling is effected by means of rolls, an amount of thermal energy received on strip to be annealed can be easily and quickly changed by changing winding angle of rolls relative to the strip, and the number of rolls about which the strip is wound, that is, period of time for which the strip comes in contact with the rolls. As means for changing flow speed of gas jet it is recommendable to employ a damper of which function is to adjust a flow rate of gas jet. Further, in the case where a plurality of rolls are employed for the purpose of heating or cooling it is recommendable to use driving rolls which are able to carry out thrusting relative to the strip. (VI) The present invention consists in that a plurality of gas jet means for blowing toward a strip to be annealed gas of which temperature is determined to a required level to adjust temperature of the strip are arranged at the position located adjacent to radiant tubes in the area extending from the rear part of the heating zone to the rearmost end of the same. Namely, there is proposed according to further another aspect of the present invention an apparatus for heating a strip of metallic material in a continuous annealing furnace which is characterized in that annealing of the strip is continuously carried out in such a manner that the fore end part of gas jet means through which gas serving to adjust temperature of the strip is located at the fore end of the rear part of the heating zone in response to an amount of variation of thermal load in the range of 20 to 30 %, temperature and flow rate of the gas being properly adjusted to a required level in response to changing of the operating conditions such as heat cycle, line speed, thickness of strip or the like, and the rear end part of the gas jet means is extended to the rearmost end of the heating zone or over the whole soaking zone. When a strip having different thickness over the whole length thereof is introduced into the continuous annealing furnace of the invention, an intensity of combustion in radiant tube burners is adjusted properly and gas of which temperature is determined to a required level to adjust temperature of the strip is blown toward the strip through a plurality of gas jet means for a short period of time. Owing to the arrangement made in that way it is assured that quick temperature controlling is achieved properly while compensating for low temperature respondency of the radiant tubes. Further, since the gas jet means are arranged in the area extending from the rear part of the heating zone to the rearmost end of the same, proper temperature controlling can be achieved from the leading end of the strip while the preceding heat cycle is shifted to another one. Finally, advantageous features of the present invention will be described below. (I) As described above, the present invention consists in that gas of which temperature and flow rate can be adjusted as required is blown toward a strip of metallic material on the one side or both the sides of the latter and that gas of the above-mentioned type is blown toward the strip from the area as defined between adjacent radiant tubes. Thus, proper heating can be carried out within a very short period of time in response to changing of thickness of the strip or the like factor in the cource of operation of the furnace. As a result, reduction of yielding rate and increased loss of products caused by changing thickness of the strip can be inhibited effectively. (II) The present invention consists in that an intensity of combustion in radiant tubes is changed before operating conditions such as heat cycle, thickness of strip or the like are changed and at the same time a flow rate of gas blown through gas jet nozzles is changed gradually. Thus, for instance, temperature response time in the heating zone can be shortened when thickness of strip to be annealed is changed. This leads to an advantageous feature that reduction of yielding rate and increased loss of products caused by changing thickness of the strip can be inhibited effectively. Another advantageous feature of the invention is that there does not take place deformation or damage due to thermal stress gener ated by the gas jet nozzles. (III) Further, the present invention consists in that the heating zone is provided with a strip temperature controlling zone whereby temperature of the strip at the outlet of the heating zone can be easily controlled to reach a target level in response to changing of heat curve, line speed or thickness of strip. This leads to advantageous features that there is no necessity for complicated operations as are seen with the conventional furnace, it becomes possible to widen the extent of deviation from a predetermined thickness of strip, for instance, to ± 50 % and moreover utilization of dummay coil is not required any more. 6. Brief Description of the Drawings The accompanying drawings will be briefly described below. Fig. 1 is a frahmental schematic vertical sectional view of a continuous annealing furnace in which an embodiment of the invention is carried out, particularly illustrating how the heating zone is constructed. Fig. 2 is a cross-sectional view of the heating zone in the continuous annealing furnace taken in line IV - IV in Fig. 1. Fig. 3(A) is a schematic side view of a pebble heater used for the heating zone, particularly illustrating how temperature varies during heat storing as time elapses. Fig. 3(B) is a schematic side view of the pebble heater used for the heating zone similar to Fig. 3(A), particularly illustrating how temperature varies during heat radiating as time elapses. Figs. 4 (A) to (C) are a diagram respectively which shows a relation of thickness of strip to be annealed vs. time when thin strip is shifted to thick strip. Figs. 5(A) to (C) are a diagram similar to Figs. (A) to (C) respectively which shows a relation of thickness of strip to be annealed vs. time when thick strip is shifted to thin strip. Fig. 6 is a schematic sectional side view of a conventional continuous annealing furnace. Fig. 7 is a fragmental schematic vertical side view of the continuous annealing furnace in accordance with an embodiment of the invention, particularly showing an essential part in the furnace. Figs. 8(A) and (B) are a graph respectively which shows a relation of temperature of strip vs. distance from furnace inlet in the continuous annealing furnace including heating zone, soaking zone and quenching zone. Figs. 9(A) and (B) are a graph similar to Figs. 8(A) and (B) respectively which shows a relation of temperature of strip vs. distance from furnace inlet in the continuous annealing furnace of the type including no soaking zone. Fig. 10 is a schematic vertical sectional view of the continuous annealing furnace of the invention. Fig. 11 is a schematic vertical sectional view of a conventional continuous annealing furnace similar to Fig. 10. Fig. 12 is a graph including heat curves for a strip of metallic material in the area extending from inlet of preheating zone to outlet of heating zone in a conventional continuous annealing furnace, particularly showing a relation of temperature of strip vs. distance from furnace inlet. Fig. 13 is a graph showing a relation of temperature of strip vs. time in the area extending to outlet of heating zone in a conventional continuous annealing furnace. Fig. 14 is a graph including heat curves for a strip of metallic material in the area extending from inlet of preheating zone to outlet of heating zone in the continuous annealing furnace of the invention similar to Fig. 12, particularly showing a relation of temperature of strip vs. distance from furnace inlet, and Fig. 15 is a graph showing a relation of temperature of strip vs. time in the continuous annealing furnace of the invention similar to Fig. 13. Next, description will be made below as to the first embodiment of the invention with reference to Figs. 1 and 2. Fig. 3 is a fragmental schematic vertical sectional view of a heating furnace which is employed for carrying out the invention. The drawing shows the case where heating is achieved by means of a plurality of radiant tubes from both the sides of the strip. In the drawings reference numeral 1 designates a strip of metallic material, reference numeral 2 does a plenum chamber, reference numeral 3 does a gas jet nozzle, reference numeral 4 does a radiant tube, reference numeral 5 does a furnace wall which is lined with thermal insulating material having small heat capacity such as ceramic fiber or the like and reference numeral 6 does a gas feeding duct through which gas is introduced into the plenum chamber 2. Further, reference numeral 10 designates pebble-shaped heat storing medium (hereinafter referred to simply as pebble) made of material having a high melting temperature such as ceralic or the like, reference numeral 11 does a filled structure which is filled with the pebble 10 (hereinafter referred to as pebble heater), reference numeral 12 does a gas feeding duct through which hot gas having a temperature in the range of 1200 to 1300 °C is introduced into the pebble heater 11, reference numeral 13 does a HN gas feeding duct through which HN gas (mixture gas of hydrogen and nitrogen) having a comparatively low temperature is introduced into the pebble heater and reference numeral 14 does a bypass duct for HN gas. Hot gas is fed into the pebble heater 11 through the gas feeding duct 12 from the top side of the pebble heater 11 and it is then discharged from the bottom of the same. On the other hand, HN gas is fed into the pebble heater 11 through the feeding duct 13 from the bottom side of the pebble heater 11 and it is then delivered to the plenum chamber 2 from the top of the same. Fig. 2 is a cross-sectional view of the heating furnace taken in line IV - IV in Fig. 1. In the drawing reference numeral 7 designates a combustion burner which is used exclusively for the radiant tube 4 and reference numeral 8 does a discharging duct through which HN gas flowing out of the plenum chamber 2 is discharged to the outside. It should be noted that thus discharged HN gas may be reused by flowing back to the HN gas feeding duct 13. Refering to Fig. 1 again, for instance, in the case where steady operation is performed by heating the strip 1 having the same thickness, heating is achieved merely by means of a plurality of radiant tubes. When operating conditions such as heat cycle, thickness of strip, width of strip, line speed or the like are caused to vary, for instance, when the following strip has an increased thickness compaired with the thickness of the preceding strip and thereby an intensity of heating is required to increase, hot gas which is previously heated up to an elevated temperature in the range of 1200 to 1300 °C with the aid of a heater which is not shown in the drawings is first introduced into the pebble heater 11 through the duct 12 during steady operation of the furnace as mentioned above. At this moment distribution of temperature of the pebble 10 in the pebble heater 11 is as shown in Fig. 3(A). As is apparent from the drawing, temperature of the pebble 10 varies in such a manner that it comes closer to temperature of gas during heat storing, as time elapses. Thus, temperature in the pebble heater 11 can be maintained at a level of that of hot gas in that way. Next, an intensity of combustion of the radiant tube burners is caused to increase immediately after the strip 1 having an increased thickness enters the furnace. At the same time HN gas is supplied into the pebble heater 11 from the bottom side thereof through the duct 13. This causes distribution of temperature in the pebble heater 11 to vary as shown in Fig. 3(B) which illustrates how temperature in the pebble heater 11 varies during heat radiating. Since HN gas having lower temperature is brought in contact with the hot pebble 10 having large heat capacity, it results that temperature of HN gas increases rapidly. As a result, gas of which temperature at the outlet of the pebble heater 11 is raised up to a level of the maximum temperature (1200 to 1300 °C) of the pebble heater 11 within a period of several seconds can be fed into the plenum chamber 2 for 10 to 20 minutes until temperature of the radiant tubes reaches a steady state whereby temperature of the strip can be raised up to a predetermined temperature. Accordingly, gas jet having high temperature can be blown toward the strip 1 having an increased thickness for a very short period of time compaired with the number of radiant tubes immediately after the strip 1 has had an increased thickness. This means that temperature of the strip 1 can be instantaneously raised up to a predetermined level of temperature, resulting in the length of a part of the strip 1 where annealing is carried out insufficiently being reduced remarkably. On the other hand, for instance, in the case where thickness decreases, a part of HN gas having lower teme-temperature near to room temperature is caused to bypass so that it is mixed with the other part of HN Gas which has been heated up to an elevated temperature. Thus, by properly adjusting a ratio of mixing, gas having a properly determined lower level of temperature can be supplied to the furnace within a period of several seconds in response to variation of thickness of the strip. The present invention has been described above with respect to the case where a vertically extending strip of metallic material is subjected to heating on both the side thereof. It should of cource be understood that it should not be limited only to this but it may be applied to the case where the furnace has a horizontally extending heating zone as well as the case where heating is generally carried out for a strip of metallic material in accordance with radiant tube system. Further, the present invention should not be limited to the case where the pebble heater (heat storing type heater with heat storing medium filled therein) is employed for the furnace but other kind of means for adjusting temperature of gas and flow rate of the same may be employed for the same purpose. (Second Embodiment) Further, the heating method as illustrated in Fig. 1 will be described in more details with reference to Figs. 4(A) to (C) as well as Figs. 5(A) to (C). First, Fig. 4 shows the case where thickness of the strip varies in such a manner that a thin strip is shifted to a thick strip, wherein Fig. 4(A) illustrates how thickness of the strip varies as time elapses, Fig. 4(B) does how temperature of the radiant tubes varies as time elapses and Fig. 4(C) does how a flow rate of cooling gas jet varies as time elapses. As is apparent from Fig. 4(B), when thin strip is to be shifted to thick one, operation for raising temperature of the radiant tubes is initiated at time of two hours before shifting is effected in that way. It should be noted that temperature is gradually raised because the radiant tubes themselves have large time constant. This causes the thin strip to be gradually subjected to excessive heating until thickness shifting is completed. Thus, to assure that the thin strip maintains proper temperature during heating, a flow rate of cooling gas jet is caused to gradually increase for the purpose of cooling it until thickness shifting takes place. Next, Fig. 5 shows the case where thickness of the strip varies in such a manner that a thick strip is shifted to a thin strip, wherein Fig. 5(A) illustrates how thickness of the strip varies as time elapses, Fig. 5(B) does how temperature of the radiant tubes varies as time elapses and Fig. 5(C) does how a flow rate of cooling gas jet varies as time elapses. As is apparent from Fig. 5(B), when thick strip is to be shifted to thin strip, operation for lowering temperature of the radiant tubes is initiated at time of two hours before shifting is effected in that way. It should be noted that temperature is gradually lowered because the radiant tubes themselves have large time constant. This causes the thick strip to be gradually subjected to heating with a reduced amount of thermal energy until thickness shifting is completed. To compensate for shortage of thermal energy, a flow rate of gas of which temperature is determined higher than that of the strip is caused to gradually increase and heating is effected for the strip with an increased flow rate of gas until thickness shifting takes place. The present invention has been described above with respect to the case where a strip of metallic material is subjected to heating on both the sides thereof with the aid of a number of radiant tubes which are arranged one above another in the vertically aligned relation. It should of cource be understood that it should not be limited only to this but it may be applied to the case where the furnace has a heating zone having the trapezoidal configuration as seen from the side as well as the case where heating is generally carried out for a strip of metallic material in accordance with the conventional radiant tube system. Further, the present invention should not be limited to the case where the pebble heater (heat storing type heater with heat storing medium filled therein) is employed for the furnace but other kind of means for adjusting temperature of gas and flow rate of the same may be employed for the same purpose. (Third Embodiment) Fig. 7 is a schematic vertical sectional side view of an essential part in the continuous annealing furnace in accordance with the fourth embodiment of the invention. As shown in Fig. 7, the furnace includes a plurality of heating zones comprising a heating zone 114 and a soaking zone 115. As is apparent from the drawing, a number of plenum chambers 121 serving as gas jet means are arranged in the spaced relation with a number of radiant tubes 119 located in the proximity of the the plenum chambers 121 in the area extending from the rear part of the heating zone 114 to the rearmost end of the soaking zone 115, that is, over the area including the rear part of the heating zone 114 and the whole soaking zone 115. In this embodiment, for instance, when a strip 111 which has an increased thickness for the purpose of increasing a production rate is to be supplied to the continuous annealing furnace 112, an intensity of combustion of the burners for the radiant tubes 119 in both the heating zone 114 and the soaking zone 115 is raised up and HN gas which is heated to a required elevated temperature with the aid of gas jet means is blown toward the moving strip 111 until temperature of the radiant tubes 119 reaches a required high level. As a result, the strip 111 can be heated up to a required level of temperature without any time delay. It should be noted that since gas jet means are arranged over the area including the rear part of the heating zone 114 and the whole soaking zone 115, the strip 111 of which thickness is changed in response to change of production rate can be controlled to maintain a proper temperature, starting with the foremost end part of the strip 111. If gas jet means are arranged only in the intermediate part of the heating zone, variation of temperature of the radiant tubes 119 located behind gas jet means as seen in the direction of movement of the strip 111 is caused to delay whereby the foremost end part of the strip 111 leaves the heating zone before it reaches the predetermined level of temperature. In view of the above-mentioned fact the scope of area at the fore end part of the heating zone where gas jet means are arranged should be determined in dependence on an extent of fluctuation of thermal load (normally about 20 %) corresponding to fluctuation of an amount of thermal load which is obtainable by composite multiplication of heat cycle or line speed of the strip 111 to be annealed and thickness of the strip and temperature difference equivalent to an extent of increasing of temperature of the strip. It is preferable that gas jet means are arranged in the area extending from the position where an amount of thermal load on the strip 111 is reduced by 20 to 30 % in the heating zone 114 to the rearmost end position of the latter. If the area where gas jet means are arranged is determined small, there is a fear of causing such a malfunction that the strip 111 to be annealed is heated higher than the predetermined annealing temperature before it reaches the area where they are arranged, that is, so-called superheating, for instance, when the strip has a reduced thickness. Fig.8(A) illustrates how temperature of the strip to be annealed varies in the furnace as constructed in accordance with this embodiment. As is apparent from the drawing, temperature of the strip is raised up at a higher rate than in the case of the normal operating state as represented by a dotted line, for instance, when thickness of the strip is reduced and thereby an amount of thermal load decreases. However, when it reaches the area Z where gas jet means are arranged, it is restrained within the predetermined level of temperature. Next, Fig. 8(B) illustrates how temperature of the strip to be annealed varies in the furnace as constructed in accordance with a modified embodiment of the invention where the area Z there gas jet means are arranged is divided into two sections. In this embodiment gas jet means are additionally arranged in the intermediate area of the heating zone 114. Next, Figs. 11(A) and (B) are a graph similar to Figs. 8(A) and (B) respectively which show the case where the present invention is applied to a continuous annealing furnace which is not provided with the soaking zone 115 in Fig. 7. Obviously, in the continuous annealing furnace which is not provided with the soaking zone 115 a heating area is constituted merely by the heating zone 114. Accordingly, gas jet means are arranged in the area located at the rear part of the heating zone 114. The present invention has been described above with respect to the case where thickness of the strip 111 is reduced and an amount of thermal load decreases. When thickness of the strip, width of the same and line speed increase and thereby an amount of thermal load is caused to increase, HN gas comprising a mixture gas having a required high temperature is introduced into the plenum chambers 121 whereby the strip 111 can maintain a required high annealing temperature for a period of time until temperature generated by means of the radiant tubes 119 is raised up to a required high level of temperature. (Fifth Embodiment) Fig. 10 schematically illustrates how a continuous anealing furnace f is constructed in accordance with the fifth embodiment of the invention. In this embodiment the furnace includes a preheating zone a , heating zones b -1 and b -2, a soaking zone d and cooling zones e -1, e -2 and e -3. A strip temperature controlling zone c is constituted by a part of the heating zone b and includes a cooling zone which is operated in accordance with gas jet system. It is preferable that heating and cooling means for the strip temperature control zone c is constructed in such a system that it has quick respondency and temperature of the strip can be easily controlled. A method of carrying out heating and cooling with the aid of gas jet or rolls may be employed as system as mentioned above. In the illustrated embodiment the method of carrying out heating and cooling with the aid of gas jet is employed. Specifically, function of the strip temperature controlling zone is to lower the existing temperature of the strip which has been excessively heated or raise the existing temperature of the strip which has been insufficiently heated when heat cycle, line speed, thickness of strip or the like factor are charged. Thus, temperature of the strip at the outlet of the heating zone can be maintained at an intended level of temperature. Fig. 11 schematically illustrates how the conventional continuous annealing furnace is constructed for steel strips which are subjected to rolling at a lower temperature and Fig. 12 shows heat curves which extend from the preheating zone to the outlet of the heating zone in the conventional continuous annealing furnace. In Fig. 12 preference letter A designates a heat curve which was obtained when a strip of cold rolled steel having a thickness of 1.0 mm and a width of 1200 mm was annealed at a line speed of 300 mpm, whereas reference letter B does a heat curve which was obtained when a strip of cold rolled steel having a thickness of 0.75 mm and a width of 1200 mm was annealed at a line speed of 300 mpm. As is readily apparent from comparision between the curves A and B for cold rolled steel strip which were obtained by operating the conventional continuous annealing furnace, there occurs temperature difference of about 70 °C at the outlet of the heating zone when both the cold rolled steel strips A and B are annealed at the same line speed and the cold rolled steel strip B is excessively heated by 50 °C relative to a target temperature of strip of 780 °C + / - 20 / 0. Further, Fig. 13 illustrates how strip temperature T s at the outlet of the heating zone varies when preset temperature T g in the heating zone of the conventional annealing furnace is changed from 950 °C to 850 °C. The drawing shows that about 20 minutes is required until the temperatute T g reaches 850°C and similarly about 20 minutes is required until the temperature T s is lowered from 780°C to the target temperature of 740°C + / - 20 / 0. Next, Fig. 14 shows heat curves which are obtainable when the method of the invention is employed. In the drawing reference letter C designates a heat curve which was obtained in the same manner as in the case of the heat curve A when a strip of cold rolled steel having a thickness of 1.0 mm and a width of 1200 mm was annealed at a line speed of 300 mpm, whereas reference letter D does a heat curve in the same manner as in the case of the heat curve B when a strip of cold rolled steel having a thickness of 0.75 mm and a width of 1200 mm was annealed at a line speed of 300 mpm. A target temperature of 780 °C can be reached at the outlet of the heating zone by lowering a temperature of cold rolled steel D to 610 °C in the strip temperature controlling zone c . Further, when line speed x is changed to 1.0t x 300 mpm - 0.75 x mpm after the welded point of the strip moves past the heating zone, the heat curve which is scribed thereafter becomes same to that in the case of the cold rolled steel strip. Next, Fig. 15 is a graph which illustrates how the preset temperature T g at the heating zone varies when it is changed from 950°C to 850°C. In the drawing reference letters T s designates temperature of the strip at the outlet of the heating zone which is controlled in accordance with the method of the invention, whereas reference letters T c does temperature of the strip at the outlet of the strip temperature controlling zone. Similarly to the conventional method it takes about 20 minutes until temperature of the strip at the heating zone is lowered from 950°C to 850°C but temperature of the strip T s at the outlet of the heating zone can be controlled to a level of target temperature by controlling temperature of the strip T c at the outlet of the strip temperature controlling zone. Incidentally, feedback controlling for which a strip temperature measuring meter is used at the outlet of the heating zone is employed as a method of controlling temperature of the strip. Function of the controlling zone has been described above with respect to the case where preset temperature of a strip at the heating zone is changed to the lower side but controlling can be effected at the quitely same manner as in the foregoing case also in the case where it is changed to the higher temperature side.

Claims: A method of heat-treating a strip of metallic material in a continuous annealing furnace containing a radiant heating tube system and a plurality of gas jet nozzles arranged on one or both sides of the annealing furnace which comprises continuously introducing the strip of metallic material to be treated into the annealing furnace and heat-treating said metallic material with the heat from the radiant tube system and a heating or cooling gas introduced through said gas jet nozzles to the metallic strip, whereby the introduction of the heating or cooling gas cooperates with the radiant heat from the radiant tube systems for effectively controlling the temperature of the metallic strip being treated to the annealing temperature irrespective of changes in operating conditions such as the heat cycle, line speed, thickness of the metallic strip and the width of the metallic strip, said gas being introduced to said strip for the period of time until the temperature of the radiant tube system reaches a predetermined temperature so that the strip is always treated at its proper annealing temperature. The method of heat-treating a strip of metallic material of claim 1 wherein the temperature of the heating or cooling gas is controlled by heating the gas in a heater, by passing the gas around the heater and by selectively blending desired portions of said heated and unheated by-pass gas and the flow rate thereof to achieve the desired temperature of the gas introduced to the metallic strip. The method of heat-treating a strip of metallic material of claim 1 wherein the gas is introduced for short periods of time in anticipation of changes in operation conditions. The method of heat-treating a strip of metallic material of claim 1 wherein the gas jet nozzles are disposed between adjacent radiant tubes. The method of claim 4 wherein the intensity of radiant tube burners is changed before the operating conditions are changed and the temperature and flow rate of the gas are gradually changed in response to the change in temperature of the radiant tubes until the operating conditions are changed in order to maintain the temperature of the metal strip at a constant level. An apparatus for heat-treating a strip of metallic material in a continuous annealing furnace which comprises a heating zone having an inlet and an outlet therefor, means for introducing a strip of metallic material into said heating zone and removing said strip from said heating zone, a radiant heating tube system disposed on one or both sides of said metallic strip within said heating zone, gas distribution means disposed on one or both sides of said metallic strip, means for introducing a gas to said strip of metallic material through said gas distribution means, and means for controlling the temperature and flow rate of the gas introduced to said gas distribution means based on the temperature emanating from the radiant heating tube system. The apparatus for heat-treating a strip of metallic material of claim 6 wherein the gas distribution means include a plurality of gas jet nozzles which are interposed between the radiant tubes of the radiant tube system. The apparatus for heat-treating a strip of metallic material of claim 7 wherein the gas distribution means further includes a plenum chamber which communicates with said plurality of gas jet nozzles. The apparatus for heat-treating a strip of metallic material of claim 7 wherein the temperature control means is a heat exchanger means for heating said gas and a by-pass line for bypassing said heat excahnge means and communicating with an outlet side of said heat exchange whereby the temperature of the gas introduced to the gas distribution means can be effectively controlled.

Inventors: FUKUSHIMA, TAKEO HIROSHIMA SHIPYARD   ENGINE WORKS, FURUKAWA, KUSUO KAWASAKI STEEL CORP., HARADA, MASAHIRO HIROSHIMA TECHNICAL INST., NAKAJIMA, YASUHISA CHIBA WORKS, KAWASAKI STEEL COR, OHTA, NORIO CHIBA WORKS, KAWASAKI STEEL CORP., SATO, KUNIAKI CHIBA WORKS, KAWASAKI STEEL CORP., SOEDA, NAOHIKO CHIBA WORKS, KAWASAKI STEEL CORP., YANAGI, KENICHI HIROSHIMA TECHNICAL INST., Fukushima, Takeo Hiroshima Shipyard Engine Works
Applicants: KAWASAKI STEEL CORPORATION, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1985
Main classification: C21D9
IPC codes: C21D9