Batch annealing furnace for coils

Abstract

A batch annealing furnace includes a coil support base on which an end face of a coil is mounted and that supports the coil with an axis of the coil being upright, an inner cover that covers an entire body of the coil mounted on the coil support base, and a cooling pipe that extends downward from the upper part of the inner cover to a cavity of the inner peripheral part of the coil mounted on the coil support base and cools the coil from the inner surface side by passing a coolant through the inside of the cooling pipe.

Claims

1. A batch annealing furnace for coils configured to anneal a coil in which a steel sheet is wound, the batch annealing furnace comprising: a coil support base on which an end face of the coil is mounted and that supports the coil with an axis of the coil being upright; an inner cover that covers an entire body of the coil mounted on the coil support base; and a cooling pipe that extends downward from an upper part of the inner cover to a cavity of an inner peripheral part of the coil mounted on the coil support base and cools the coil from an inner surface side by passing a coolant through inside of the cooling pipe, wherein the cooling pipe comprises a double pipe comprising a cylindrical inner pipe and a cylindrical outer pipe that surrounds the inner pipe, the inner pipe serves as an introduction pipeline that introduces the coolant from the upper part of the inner cover toward the coil support base, and an area between the outer pipe and the inner pipe serves as a return pipeline that returns the coolant from the coil support base toward the upper part of the inner cover, and wherein at a location where a direction of flow of the coolant passing through the introduction pipeline and the return pipeline changes, a bottom plate having a semispherical shape convex downward whose diameter is half the radius of the outer pipe or more reverses the direction.

2. The batch annealing furnace for coils according to claim 1, wherein at least one of the introduction pipeline and the return pipeline has a diameter expanded toward downstream.

3. A batch annealing furnace for coils configured to anneal a coil in which a steel sheet is wound, the batch annealing furnace comprising: a coil support base on which an end face of the coil is mounted and that supports the coil with an axis of the coil being upright; an inner cover that covers an entire body of the coil mounted on the coil support base; at least one burner located outside of the inner cover and a heater below the coil support base for heating the coil; and a cooling pipe that extends downward from an upper part of the inner cover to a cavity of an inner peripheral part of the coil mounted on the coil support base and cools the coil from an inner surface side by passing a coolant through inside of the cooling pipe to enable cooling of the coil from the inner surface during heating, wherein: the cooling pipe comprises: an introduction pipeline that introduces the coolant from the upper part of the inner cover toward the coil support base; a curved pipeline that changes a direction of flow of the coolant introduced into the introduction pipeline toward the upper part of the inner cover, the curved pipeline having a U-shape, a cross section of a bottom of the U-shape being semicircular; and a return pipeline that returns the coolant of which direction of flow has changed by the curved pipeline toward the upper part of the inner cover.

4. The batch annealing furnace for coils according to claim 3, wherein the return pipeline comprises two or more return pipelines, the curved pipeline comprises two or more corresponding curved pipelines, and each return pipeline is connected to the introduction pipeline via a corresponding curved pipeline.

5. The batch annealing furnace for coils according to claim 3, wherein at least one of the introduction pipeline and the return pipeline has a diameter expanded toward downstream.

6. The batch annealing furnace for coils according to claim 4, wherein at least one of the introduction pipeline and the return pipeline has a diameter expanded toward downstream.

7. The batch annealing furnace for coils according to claim 3, wherein both the introduction pipeline and the return pipeline have a diameter expanded toward downstream.

8. The batch annealing furnace for coils according to claim 3, wherein an opening is at an end of the introduction pipeline, and the opening has a funnel shape whose diameter expands towards the upper part of the inner cover.

9. The batch annealing furnace for coils according to claim 3, wherein the cooling pipe contains a gas as coolant.

10. The batch annealing furnace for coils according to claim 3, wherein the gas is air, nitrogen, argon, helium, a gas mixture of inert gas and air in which an oxidative gas is reduced, or a reducing gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

(2) FIG. 1 is a schematic diagram (sectional view) illustrating a first embodiment of a batch annealing furnace according to one aspect of the present invention.

(3) FIG. 2 is a schematic diagram (sectional view) illustrating a second embodiment of the batch annealing furnace according to one aspect of the present invention.

(4) FIG. 3 is a schematic diagram (sectional view) illustrating a third embodiment of the batch annealing furnace according to one aspect of the present invention.

(5) FIG. 4 is a drawing illustrating a comparison of flows in the embodiments of the batch annealing furnace according to one aspect of the present invention; the drawing indicates dimensions of models studied.

(6) FIG. 5 illustrates an image of differences in discharge flow (a flow rate of 20 m/s) in the models studied in FIG. 4.

(7) FIG. 6 illustrates an image of differences in discharge flow (a flow rate of 50 m/s) in the models studied in FIG. 4.

(8) FIG. 7 illustrates an image of differences in the displacement of gas passing through a discharge part in the models studied in FIG. 4.

(9) FIG. 8 are graphs illustrating differences in the displacement of gas passing through the discharge part in the models studied in FIG. 4; (a) is an example of discharge flow: a discharge flow rate of 20 m/s, whereas (b) is an example of discharge flow: a discharge flow rate of 50 m/s.

(10) FIG. 9 is a drawing illustrating an example of a heat transfer calculation model.

(11) FIG. 10 are graphs ((a) to (f)) illustrating calculated temperature results and actually measured temperature results in combination for the sake of comparison and a drawing ((j)) illustrating positions on a coil corresponding to the graphs.

(12) FIG. 11 are graphs ((g) to (i)) illustrating calculated temperature results and actually measured temperature results in combination for the sake of comparison and a drawing ((j)) illustrating positions on a coil corresponding to the graphs.

(13) FIG. 12 (a) is a graph illustrating changes over time in stress occurring in a coil, whereas FIG. 12 (b) is a drawing illustrating corresponding directions of the coil in (a).

(14) FIG. 13 is a graph illustrating the maximum stresses (absolute values) occurring in a coil during annealing for comparison, and (b) is a drawing illustrating corresponding directions of the coil in (a).

(15) FIG. 14 is a drawing illustrating a modification to a cooling pipe of the batch annealing furnace for coils according to one aspect of the present invention (a first modification).

(16) FIG. 15 is a drawing illustrating a modification to a cooling pipe of the batch annealing furnace for coils according to one aspect of the present invention (a second modification).

(17) FIG. 16 is a schematic diagram (sectional view) illustrating an example of a conventional batch annealing furnace for coils.

(18) FIG. 17 is a schematic diagram (sectional view) of a first comparative example for illustrating another example of the conventional batch annealing furnace.

(19) FIG. 18 is a schematic diagram (sectional view) of a second comparative example for illustrating the batch annealing furnace for coils according to one aspect of the present invention.

(20) FIG. 19 are drawings for illustrating an example of a structure (a solid structure) of the conventional batch annealing furnace: (a) is a perspective view of the entire furnace; (b) is a sectional view in the axial direction; (c) is an enlarged view of the principal part of (b); and (d) is a drawing illustrating the part of a coil support base in (a) with a part cut away.

(21) FIG. 20 are sectional views of the principal part illustrating the thermal expansion deformation of a coil in the conventional batch annealing furnace; (a) is at heating, whereas (b) is at cooling.

(22) FIG. 21 are sectional views of the principal part illustrating displacement deformation occurring between the inside and the outside along with the thermal expansion deformation of a coil in the conventional batch annealing furnace; (a) is at heating, whereas (b) is at cooling.

DETAILED DESCRIPTION OF THE INVENTION

(23) Described first is how the present invention has been achieved. The inventors of the present invention made investigations on the cause of defects occurring in a coil in detail through the following process to determine a defect occurrence mechanism.

(24) FIG. 16 is a schematic diagram simply illustrating a structure of a conventional batch annealing furnace for coils (hereinafter also referred to as simply a batch annealing furnace). As illustrated in the drawing, this conventional batch annealing furnace 100, in order not to produce temperature unevenness within the furnace, heats an inner cover 7 within a furnace wall 8 from its outside by a plurality of burners 5 and also heats from a furnace bottom 9 side below a coil support base 2 supporting a coil C by a heater 6. This makes the temperature within the furnace nearly uniform. The heating is programmed in advance so as to follow target temperatures.

(25) Temperature within a furnace has been conventionally measured to obtain a temperature distribution within the furnace, and a heating method and the structure of an outer wall of the surface have been changed so as to reduce the distribution. However, only doing so is insufficient, sometimes producing the defects. In this situation, the conventional manufacturing process cannot be omitted completely, resulting in failure in reduction in costs with increased productivity.

(26) Given these circumstances, the inventors of the present invention also measured the temperatures of an inner peripheral part Cn of the coil C, the coil support base 2 supporting the coil C, and the like by thermocouples. At the same time, heat transfer calculation was performed to determine a temperature distribution also in an area for which temperature measurement was unable to be performed by the thermocouple, thereby measuring an influence on the coil C. This has brought about results that were considered unthinkable before.

(27) In other words, it has been conventionally qualitatively considered that the temperature distribution in the inner peripheral part Cn of the coil C would cause elongation strain. As a result of the above heat transfer calculation, however, it has been found that the deformation of the coil C caused by the temperature distribution has larger effect on a plate shape than expected, and that defects such as edge elongation, edge distortion, center elongation, and longitudinal wrinkles, which have been conventionally considered to occur simply by thermal deformation, do not occur due to such a simple manner.

(28) Specifically, when the inside of the furnace is heated from the furnace bottom 9 and outside the inner cover 7, the coil C within the furnace is heated by its thermal radiation to increase the temperature of an outer peripheral part Cs of the coil C first. For this reason, at heating, the outer peripheral part Cs of the coil C has larger thermal expansion than the inner peripheral part Cn, thereby, as represented by the symbol in FIG. 20 (a), a lower end of the outer peripheral part Cs lifts and holds the coil C itself.

(29) In addition, because at heating the temperature of the upper end of the outer peripheral part Cs of the coil C increases, a part corresponding to the coil upper end has a larger amount of thermal expansion, and similarly, the coil lower end elongates by thermal expansion. As a result, the central part of the wound steel sheet is elongated by being dragged by the upper and lower coil elongation, causing center elongation. The outward expansion of the lower end of the outer peripheral part Cs produces not only edge distortion by expansion, but also deformation caused by the fact that the weight of the coil C with an axial direction being upright is supported by this part. This also produces deformation caused by friction with the coil support base 2 (a spacer 4 arranged on an interposed cushion 3) below the coil C when the coil C expands.

(30) Because at cooling the coil C is cooled by radiational cooling, the outer peripheral part Cs of the coil C is cooled first. For this reason, as represented by the symbol in FIG. 20 (b), the coil shape becomes deformed, and the weight of the entire body of the coil C is supported by the lower end of the inner peripheral part Cn of the coil C, leading to coil deformation at the lower end near the inner periphery. In other words, it has been found that attempt to prevent deformation when annealing the coil cannot be achieved simply by relaxation of a temperature increasing rate and a cooling rate or uniform thermal radiation from a furnace wall, which have been conventionally considered.

(31) In addition, as for new defects (a sticking phenomenon of a sheet during annealing) from an unknown cause, the cause has been clarified by a temperature measurement experiment and analysis for these defects. There have been a phenomenon in which a steel sheet as part of a coil sticks after annealing, and its cause has not been known so far. This time, by performing temperature measurement and heat transfer calculation, it has been found that the coil C is deformed by thermal expansion as illustrated in FIG. 21. In other words, as represented by the symbol in FIG. 21 (a) and FIG. 21 (b), it has been found that displacement in a steel sheet may occur in the axial direction of the coil C while annealing the coil C. With respect to this result, when the size of the displacement in the steel sheet at the part where the coil sticks was measured, it has been found that the size is nearly the same as the size of deformation obtained by calculation. Although it cannot be determined in general because various cases can cause this displacement, it is clear from this result that the occurrence of the displacement is caused by the thermal deformation and the thermal stress of the coil.

(32) It has been found that the thermal deformation and the thermal stress also relate to characteristics deterioration in annealing. In other words, the phase transformation for characteristics improvement takes place from heating to soaking of the coil C. In general, in the coil C, the outer peripheral part Cs is first heated by radiation, and at the same time, the inner peripheral part Cn is also heated by radiation. In particular, when attempting to increase the coil temperature up to a target temperature quickly, radiation reaches the inner peripheral part Cn of the coil C, and the temperature within the coil C also increases. When heated also from the furnace bottom 9 in order to increase a temperature increasing rate, radiation is effected from the furnace bottom 9, thereby further heating the inner peripheral part Cn of the coil C and giving a larger temperature increase from the inside. Owing to this, even when heating from the outer peripheral part Cs, a compressive stress is produced within the coil by the expansion of the inner peripheral part Cn, which is considered to cause the coil C to be lifted. When the value is large at the same time, a compressive stress is produced within the coil, which is considered to cause the progress of phase transformation to be hindered.

(33) FIG. 9 is a diagram illustrating a heat transfer calculation model used in the above heat transfer calculation. FIG. 9 (a) illustrates an example of a right half () of a section of a batch annealing furnace (the batch annealing furnace 100 in FIG. 16 or a batch annealing furnace 1 in FIG. 1 described below) and the coil C. Based on this FIG. 9 (a), 15 from the center is modeled as periodic symmetry (illustrated in FIG. 9 (b)). Heating parts are arranged on the wall surface of the furnace wall 8 (illustrated in FIG. 9 (c)) and parts of the furnace bottom 9 (illustrated in FIG. 9 (d)). A thermal flux from the burner 5 of the furnace wall 8 is given to the heating part on the wall surface in FIG. 9(c). The heating parts on the furnace bottom 9 in FIG. 9 (d) set areas in which heating is actually performed with a heating wire and gives a heat flux by the heating wire. Using this heat transfer calculation model, an internal temperature distribution of the coil C is determined by a finite element method, and from the result of this internal temperature distribution, an internal stress of the coil C is determined by numerical calculation. The calculation of the internal stress of the coil C is performed in coupling with the heat transfer calculation; in order to reduce a calculation time, the calculation is performed with weak coupling on the assumption that a local difference of heat expansion is small. As for the internal stress of the coil C, because the influence of high-temperature creep cannot be negligible, the internal stress calculation is performed using data on high-temperature creep in addition to the internal temperature distribution. In addition, as for the coil support base 2, the cushion 3, and the spacer 4 receiving the coil C, heat transfer calculation is also performed concurrently in order to calculate a temperature distribution, and based on this temperature calculation, deformation by heat is calculated. Also considered is the influence of the contact of the coil support base 2, the cushion 3, and the spacer 4 that have been deformed by heat with the coil C. Heat transfer calculation, which will be described below, on the batch annealing furnace 1 (FIG. 1 to FIG. 3) as an embodiment according to the present invention and the batch annealing furnace 100 (FIG. 16 to FIG. 19) as a conventional example and the internal stress calculation of the coil C are performed with the batch annealing furnace as the base of modeling appropriately replaced with the batch annealing furnace 1 or the batch annealing furnace 100 in FIG. 9 (a) by a similar method with a similar model created.

(34) Based on the above knowledge about the defect occurrence mechanism, the inventors of the present invention have achieved the present invention. The following describes an embodiment of a batch annealing furnace according to one aspect of the present invention. This batch annealing furnace performs annealing on a coil in which a steel sheet is cylindrically wound in order to provide the steel sheet with various characteristics.

(35) FIG. 1 illustrates a schematic diagram of a first embodiment of a batch annealing furnace according to one aspect of the present invention. The structure of the batch annealing furnace according to one aspect of the present invention will be described with reference to the schematic diagrams of the conventional batch annealing furnace illustrated in FIG. 16 and FIG. 19 for comparison. Including the above description, similar or corresponding components will be indicated by the same reference symbols.

(36) A big difference between the batch annealing furnace 1 according to the present embodiment illustrated in FIG. 1 and the conventional batch annealing furnace 100 illustrated in FIG. 16 (FIG. 19) is that the batch annealing furnace 1 according to the present embodiment includes a cooling pipe 10, which is not included in the conventional batch annealing furnace 100, in the inner peripheral part Cn of the coil C.

(37) Specifically, as illustrated in FIG. 1, the batch annealing furnace 1 according to the present embodiment and the conventional batch annealing furnace 100 include the coil support base 2 within the furnace wall 8. The coil support base 2 is a base on which an end face of the coil C is mounted and that supports the coil C with an axis of the coil C being upright. The coil C is mounted on the top surface of the coil support base 2 through the cushion 3 and the spacer 4 (the cushion 3 and the spacer 4 are not illustrated in FIG. 1). The inner cover 7 is arranged within the furnace wall 8 so as to collectively cover the coil C and the coil support base 2. In order not to produce temperature unevenness within the furnace, the inner cover 7 within the furnace wall 8 is heated from its outside by the burners 5 and is also heated from the furnace bottom 9 side below the coil support base 2 supporting the coil C by the heater 6. This makes the temperature within the furnace nearly uniform. The heating is programmed in advance so as to follow target temperatures.

(38) The batch annealing furnace 1 according to the present embodiment includes the cooling pipe 10 that extends downward from the upper part of the inner cover 7 to a cavity of the inner peripheral part Cn of the coil C mounted on the coil support base 2 and cools the coil C from the inner surface side by passing a coolant through the inside of the cooling pipe 10. The cooling pipe 10 according to the present embodiment is a double pipe including a cylindrical inner pipe 11 and a cylindrical outer pipe 12 that surrounds the inner pipe 11. The inner pipe 11 is an introduction pipeline that introduces the coolant from the upper part of the inner cover 7 toward the coil support base 2, and an area between the outer pipe 12 and the inner pipe 11 is a return pipeline that returns the coolant from the coil support base 2 toward the upper part of the inner cover 7. The cooling pipe 10 reverses the direction of a flow by a bottom plate 13 having a semispherical shape convex downward whose diameter is half the radius of the outer pipe 12 or more at a location (the lowermost position in the drawing) where the direction of the flow of the coolant passing through the introduction pipeline and the return pipeline changes. An opening (an inlet for the coolant to be passed through the cooling pipe 10) 14 at the upper part of the inner pipe 11 is formed in a funnel shape whose diameter expands toward the upper part.

(39) The coolant to be passed through the cooling pipe 10 is gas, which is preferably air, pure nitrogen gas, an inert gas such as pure argon, or helium, a gas mixture of the inert gas and air in which an oxidative gas such as oxygen or fluorine is reduced, or a gas mixture of a reducing gas such as hydrogen or carbon monoxide and the inert gas.

(40) Descries next are differences in effects between the batch annealing furnace 1 according to the present embodiment illustrated in FIG. 1 and the conventional batch annealing furnace 100 illustrated in FIG. 16 (FIG. 19).

(41) As illustrated in FIG. 16, the coil C has been conventionally annealed with the inner peripheral part Cn of the coil C being a mere cavity. As a result, the coil C is heated plainly with radiation from the inner cover 7 and radiation from the heater 6 on the furnace bottom 9, and when attempting to increase the coil temperature up to a desired temperature, the temperature of the inner peripheral part Cn of the coil C has been inevitably increased. In this situation, as illustrated in FIG. 19 (b), in an attempt to reduce the temperature of the inner peripheral part Cn of the coil C, radiant heat has been conventionally prevented from entering the cavity of the inner peripheral part Cn by arranging a heat insulating material 110 above the coil C. However, because this has been less than perfect to effect radiation even through the heat insulating material 110, and the radiation from the heater 6 on the furnace bottom 9 has also been effected, the temperature inside the coil has been inevitably increased.

(42) In this situation, heating has been conventionally performed with a low temperature increasing rate in order to perform heating so that the inner peripheral part Cn of the coil C is maintained at a lower temperature than the outer peripheral part Cs. However, because the temperature of the inner peripheral part Cn of the coil C is inevitably high during the intra-furnace cooling, it is necessary to perform cooling with a temperature distribution reduced to the extent that coil quality is not affected by reducing a cooling rate. This has been a further cost increase.

(43) In contrast, in order to achieve simultaneously a reduction in annealing time and the maintenance of high quality, the batch annealing furnace 1 according to the present embodiment arranges the cooling pipe 10 within the cavity of the inner peripheral part Cn of the coil C to make a structure that arranges the coils C outside the cooling pipe 10. Thus, the batch annealing furnace 1 extends the cooling pipe 10 downward from the upper part of the inner cover 7 to the cavity of the inner peripheral part Cn of the coil C mounted on the coil support base 2 and passes the coolant through the cooling pipe 10, thereby cooling the coil C from the inner surface side and reducing a temperature increase inside the coil.

(44) Although it is considered that at first glance this batch annealing furnace 1 only includes the cooling pole 10 as compared with the conventional batch annealing furnace 100 illustrated in FIG. 16, there is a great difference therebetween.

(45) Specifically, in the present embodiment, as illustrated by the schematic diagram in FIG. 1, the cooling pipe 10 is arranged within the cavity of the inner peripheral part Cn of the coil C, and the coolant (cooling gas) is passed through the cooling pipe 10 to cool the coil C from its inner peripheral part Cn side. In other words, the cooling pipe 10 of the batch annealing furnace 1 does not directly blow the cooling gas within the furnace, but cools the coil C from inside through radiant heat transfer. The present embodiment, by applying this at heating, enables heating without producing a thermal stress within the coil, and at cooling, enables cooling efficiently at a higher rate than a conventional cooling rate by cooling the coil C from inside.

(46) In contrast, the conventional batch annealing furnace 100 illustrated in FIG. 16 only heats the inner cover 7 from outside by the burners 5 to heat the coil C with the radiant heat of the inner cover 7. As a result, depending on the material of the coil, at the heating, heating and cooling are needed so as to give a stress within a range of not affecting quality inside the coil C, thereby increasing the annealing time. As a result, the conventional batch annealing furnace 100 fails to produce a similar effect to the batch annealing furnace 1 according to the present embodiment.

(47) A first comparative example illustrated in FIG. 17 is an example that extends a mere cylindrical cooling pipe 120 downward to the inside of a coil. This example does not perform active heating and cooling as with the one disclosed in Patent Literature 7. As a result, heated gas enters a gap (recess) between the cooling pipe 120 and the inside of the coil at heating, thereby causing the inside of the coil to be heated, leading to a reduction in a heating time. The same holds true for at cooling. In other words, as Patent Literature 7 illustrates a temperature distribution, this constitution results in a temperature distribution that is convex downward at heating and that is convex upward at cooling in the thickness direction. This still produces a stress, and in order to avoid the stress, it is needed to set heating and cooling rates, which makes this constitution deficient. As a result, the first comparative example still cannot produce a similar effect to that of the batch annealing furnace 1 according to the present embodiment.

(48) Although a second comparative example illustrated in FIG. 18 attempts to achieve a similar effect to the effect produced by the constitution of the batch annealing furnace 1 according to the present embodiment by actively passing a coolant through the mere cylindrical cooling pipe 120, the mere cylindrical cooling pipe 120 does not cause gas as the coolant to enter the pipe smoothly. As a result, the second comparative example still cannot produce a similar effect to that of the batch annealing furnace 1 according to the present embodiment.

(49) Next, in order to verify the effect of the batch annealing furnace 1 according to the present embodiment illustrated in FIG. 1, the shape of the cooling pipe 10 of the batch annealing furnace 1 as a first embodiment and the shapes of cooling pipes of other embodiments according to the present invention were compared with each other by numerical calculation to confirm the effect. Schematic diagrams of comparative shapes (the other embodiments according to the present invention) are illustrated in FIG. 2 and FIG. 3.

(50) A second embodiment illustrated in FIG. 2 is an example that replaces the bottom plate having a semispherical shape convex downward attached to the lower part of the cooling pipe 10 of the first embodiment illustrated in FIG. 1 with a flat plate. A third embodiment illustrated in FIG. 3 adopts the bottom plate of the first embodiment illustrated in FIG. 1 (the semispherical shape convex downward whose diameter is half the radius of the outer pipe or more) and expands the diameter of the outer pipe toward the upper part. Specific model shapes used in the calculation are illustrated in FIG. 4 for comparison, and results related to the calculation are illustrated in FIG. 5 to FIG. 8. FIG. 4 omits the indication of the corresponding same dimensions. The correspondence relations between the embodiments according to the present invention and the respective models are as follows: a model A corresponds to the second embodiment (FIG. 2); a model B corresponds to the first embodiment (FIG. 1); and a model C corresponds to the third embodiment (FIG. 3).

(51) FIG. 5 illustrates flow rate distributions at a discharge rate from a nozzle of 20 m/s, whereas FIG. 6 illustrates flow rate distributions at a discharge rate from the nozzle of 50 m/s for each model. It has been found from the simulation results illustrated in FIG. 5 and FIG. 6 that the bottom of the cooling pipe 10 formed as the semispherical shape convex downward (the models B and C) gives higher flow rates of the gas at the bottom than the bottom of the cooling pipe 10 formed as the flat plate (the model A), and in particular, the model C that expands the diameter of the outer pipe toward its downstream side (upper part) gives the highest flow rate at the bottom of the cooling pipe 10.

(52) In addition, a gas flow in the vicinity of the opening (the volume of the gas passing through the vicinity of the opening) was compared among the models. Flow rate measurement positions P.sub.A, P.sub.B, P.sub.C in the vicinity of the opening of the respective models are illustrated in FIG. 7, and comparison results thereof are illustrated in FIG. 8. It has been confirmed from these results that the bottom of the cooling pipe 10 formed as the semispherical shape convex downward (the models B and C) gives a larger flow than the bottom of the cooling pipe 10 formed as the flat plate (the model A) and that expanding the diameter of the outer pipe toward the downstream side (upper part) (the model C) further increases the flow.

(53) In other words, it is preferable to make the bottom shape of the cooling pipe 10 a smooth semispherical shape convex downward (the first embodiment) for the second embodiment as the constitution cooling the coil C from inside. This enables more effective cooling of the coil C. In addition, expanding the diameter of the outer pipe toward the downstream side (upper part) (the third embodiment) makes it possible to achieve a further cooling effect.

(54) As illustrated in FIG. 1, the embodiments according to one aspect of the present invention installs the cooling pipe 10 at the center of the furnace and passes the coolant through the cooling pipe 10. This can cool the coil C from inside when heating and cooling the coil C, thereby practically eliminating a stress occurring inside the coil C, and as a result, can reduce deformation caused by the temperature unevenness of the coil C, and in particular, can prevent coil defects occurring on the inner periphery and the outer periphery of the coil C (shape defects such as edge elongation (the coil upper part), edge distortion (the coil lower part), center elongation, longitudinal wrinkles, and steel sheet sticking and defects as characteristic degradation such as inability to improve characteristics involving specific phase transformation) and can obtain sheet products having favorable shapes obtained thereby.

EXAMPLE

(55) The following describes an example. An electromagnetic steel sheet is exemplified as a functional material that anneals a coil in which a steel sheet is cylindrically wound. In this case, a stricter condition is added; that is a magnetic property. When there is an excessive internal stress at annealing, recrystallized state deteriorates, and the magnetic property remarkably deteriorates. In view of this, the present example made confirmation with an electromagnetic coil that is sensitive to stress.

(56) The present example employs a small-sized experimental furnace in order to study characteristics deterioration caused by faulty recrystallization during annealing occurring in a conventional coil. In an annealing test by this small-sized experimental furnace, a part of a steel sheet was cut out as a single sheet, and a stress corresponding to a stress occurring inside a coil was applied to the single sheet in advance. When the single sheet was heated in the small-sized experimental furnace, a state of recrystallization by phase transformation of this single sheet (steel sheet) was observed. Characteristics at that time were also measured. Using measurement related to the magnetic property of the electromagnetic steel sheet that is recrystallized by annealing and whose characteristics can be evaluated remarkably, an evaluation of annealing was performed. As a result, it has been found that a higher stress causes characteristics deterioration; the value was about 10 MPa.

(57) Based on the above result, an annealing experiment was performed by a real furnace (coil shape: a sheet width of 1,000 mm; a sheet thickness of 300 m; a coil weight of 8 tons; and an inner diameter of 508 mm). In addition to a conventional temperature pattern, in order to enable a stress in the real furnace to be performed at the above 10 MPa or less, annealing was performed with a heating pattern studied at heat transfer calculation in advance. In performing the real furnace experiment, in order to check whether a temperature distribution obtained by the heat transfer calculation and an experimental value match, a coil was wound with thermocouples put into the coil, and the coil was put into a batch annealing furnace to perform a temperature measurement experiment at the same time. The results are illustrated in FIG. 10 and FIG. 11. The symbol (j) in FIG. 10 and FIG. 11 indicates temperature measurement positions in the coil C. The symbols of graphs in FIG. 10 and FIG. 11 correspond to the symbols of the temperature measurement positions indicated in (j). From the results illustrated in FIG. 10 and FIG. 11, it is found that the temperature measurement results and the results of the temperature distribution of the coil obtained by the heat transfer calculation matched well, which established the validity of the heat transfer calculation method. In view of this, analysis was performed using numerical calculation from there on.

(58) As representative examples of results when performed stress calculation based on the results of the heat transfer calculation described above, stresses in the coil radial direction are illustrated in FIG. 12, and the maximum radial stresses for different inner diameters are illustrated in FIG. 13. The symbol P.sub.0 in FIG. 12 (b) and FIG. 13 (b) indicates the center of a coil section. As is evident from FIG. 12 and FIG. 13, it has been found that the stress occurring inside the coil decreases as the coil inner diameter increases. In addition, it has been found that because an inner diameter of 508 mm gives a stress of nearly 10 MPa, a small fluctuation in annealing conditions may lead to characteristics deterioration. In view of this, a stress causing no characteristics deterioration was set to 6 MPa or less to be on the safe side.

(59) From the results mentioned above, a comparison was performed between a batch annealing time when the batch annealing furnace according to one aspect of the present invention was used and a batch annealing time in the conventional batch annealing furnace illustrated in FIG. 16 (FIG. 19). Other cases were also studied for reference.

(60) As described above, when performing heating and cooling of a coil with thermal radiation in the conventional batch annealing furnace for coils illustrated in FIG. 16 (FIG. 19), the temperature distribution inside the coil deviates to produce an internal stress. In order to resolve it, with respect to FIG. 1 (the cooling pipe 10 whose bottom has the convex semispherical shape) as the first embodiment according to the present invention, FIG. 2 (the cooling pipe 10 whose bottom is the flat plate) as the second embodiment according to the present invention, FIG. 3 (the bottom is the convex semispherical shape and the diameter expands toward the upper part) as the third embodiment according to the present invention, and FIG. 16 as the conventional batch annealing furnace having no cooling pipe for comparison, annealing times were compared and studied by a method shown below.

(61) With respect to (1) annealing using the first embodiment according to the present invention (FIG. 1), (2) annealing using the second embodiment according to the present invention (FIG. 2), (3) annealing using the third embodiment according to the present invention (FIG. 3), and (4) annealing using the conventional batch annealing furnace illustrated in FIG. 16, Table 1 lists a comparison of the annealing times when performing annealing calculation so as to be 6 MPa or less that produces no stress. The annealing time is indicated with a relative ratio with the annealing time of annealing using the conventional batch furnace (FIG. 16) being 1. Accordingly, a smaller value shows a shorter annealing time, thus improving production efficiency.

(62) TABLE-US-00001 TABLE 1 (1) (2) (3) (4) (FIG. 1) (FIG. 2) (FIG. 3) (FIG. 16) First Second Third Conventional embodiment embodiment embodiment example Produced 1 MPa 1 MPa 1 MPa 2 MPa stress or less or less or less or less Annealing 0.6 0.8 0.5 1 time

(63) From the comparison result of the annealing time listed in Table 1, it has been confirmed that the example of the present invention reduces the annealing time as compared with the conventional example by using the cooling pipe and controls the stress to be 6 MPa or less, thereby manufacturing high-quality coils with high productivity.

(64) The shape of the cooling pipe according to the present invention is not limited to the cooling pipe 10 of a double pipe type illustrated in FIG. 1 to FIG. 3. For example, as illustrated in FIG. 14 and FIG. 15, a cooling pipe of an individual pipe type may be configured by combining several pipes. In other words, this cooling pipe 20 includes an introduction pipeline 21 that introduces the coolant from the upper part of the inner cover toward the coil support base, a curved pipeline 22 that changes the direction of the flow of the coolant introduced into the introduction pipeline 21 so as to be directed toward the upper part of the inner cover 7 (not illustrated in the drawing), and a return pipeline 23 that returns the coolant whose direction has been changed by the curved pipeline 22 toward the upper part of the inner cover 7.

(65) When adopting this constitution, it is important to connect the curved pipeline 22 as a turning point to the introduction pipeline 21 and the return pipeline 23 smoothly. As illustrated in FIG. 15, it is preferable that the diameter of at least either one of (both in the drawing) the introduction pipeline 21 and the return pipeline 23 is expanded toward an outlet of the coolant (toward the downstream side).

REFERENCE SIGNS LIST

(66) 1 Batch annealing furnace 2 Coil support base 3 Cushion 4 Spacer 5 Burner 6 Heater 7 Inner cover 8 Furnace wall 9 Furnace bottom 10 Cooling pipe (of a double pipe type) 11 Inner pipe 12 Outer pipe 13 Bottom plate 20 Cooling pipe (of an individual pipe type) 21 Introduction pipeline 22 Curved pipeline 23 Return pipeline 110 Heat insulating material C Coil