Rapid heating apparatus of continuous annealing line

Abstract

In a continuous annealing line for steel sheets comprising a heating zone, a soaking zone and a cooling zone, two or more induction heating devices are arranged in series in a front half part of the heating zone, and a heating stop region of 130 m in length or a slow heating region having a heating rate of more than 0 C./s but not more than 10 C./s is provided in a temperature zone that the temperature of the steel sheet between two or more induction heating devices is 250 C. to 600 C. Even if the steel sheet is rapidly heated at a heating rate of not less than 50 C./s with such a rapid heating apparatus of the heating zone, the temperature distribution in the steel sheet is uniformized to realize the quality improvement of steel sheet shape or magnetic properties and so on.

Claims

1. A method for rapid heating in a continuous annealing line provided with a rapid heating apparatus, the method comprising forming an electrical steel sheet that includes a final cold-rolled material having Si: 8.0 mass % or less by: heating the steel sheet at a heating rate of not less than 50 C./s using two or more induction heating devices arranged in series in a front half portion of a heating zone; and adjusting a temperature of the heating zone in a heating rate adjusting region provided between two of the two or more induction heating devices by holding temperature constant or slow heating the steel sheet at a heating rate in a range of more than 0 C./s and 10 C./s or less, wherein the heating rate adjusting region has a length in a range of 1 m to 30 m.

2. The method for rapid heating according to claim 1, wherein, in the adjusting step, the temperature is held constant or the steel sheet is slow heated for a duration in a range of 1 to 10 s.

3. The method for rapid heating according to claim 1, wherein the length of the heating rate adjusting region is in a range of 3 m to 30 m.

4. The method for rapid heating according to claim 1, wherein the heating rate adjusting region holds the temperature of the steel sheet in a range of 250 to 600 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view illustrating a construction of a continuous annealing line in the conventional technique.

(2) FIG. 2 is a schematic view illustrating a rapid heating apparatus with an induction heating device.

(3) FIG. 3 is a schematic view illustrating a rapid heating apparatus according to the invention.

(4) FIG. 4 is a schematic view illustrating another rapid heating apparatus according to the invention.

(5) FIG. 5 is a diagram explaining a heating pattern of Example 1 with the rapid heating apparatus of FIG. 3.

(6) FIG. 6 is a diagram explaining a heating pattern of Example 2 with the rapid heating apparatus of FIG. 4.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(7) The rapid heating apparatus according to the invention will be described concretely by taking a case of applying to a continuous annealing line for primary recrystallization annealing of a grain-oriented electrical steel sheet as an example below.

(8) FIG. 1 schematically shows a general constitution example of a furnace part of a continuous annealing line used for subjecting a grain-oriented electrical steel sheet cold rolled to a final thickness (product thickness) to a primary recrystallization annealing or a primary recrystallization annealing combined with decarburization annealing. The furnace part of the continuous annealing line is commonly comprised of a heating zone 2 of a radiant tube heating system, a soaking zone 3 of an electric heater heating system and a cooling zone 4. A steel sheet 1 transported from an entrance side of the heating zone 2 (left side of FIG. 1) to the inside of the furnace is heated to a given soaking temperature in the heating zone 2, kept at the soaking temperature in the soaking zone 3 for a given time, cooled to about room temperature in the cooling zone 4, and then carried out to the outside of the furnace (right side of FIG. 1). In FIG. 1 is shown the behavior of transporting the steel sheet through the inside of the horizontal type furnace in the horizontal direction, but the steel sheet may be reciprocally moved in the furnace through a plurality of transporting rolls disposed up and down or right and left inside the furnace to perform heat treatment. Moreover, the temperature of the steel sheet is controlled, for example, by a sheet temperature measuring device 5153 or the like arranged in an exit side of the each zone.

(9) FIG. 2 shows the heating zone of the conventional technique wherein an induction heating device 21 capable of performing rapid heating is arranged before a heating zone 20 of a radiant tube heating system to perform rapid heating from room temperature to a given temperature at once and thereafter the heating is performed to a soaking temperature with a radiant tube.

(10) The reason why the magnetic properties are improved by rapidly heating with the induction heating device as mentioned above is due to the fact that the generation of Goss orientation ({110}<001> orientation) in <110>//ND as nuclei for secondary recrystallization can be promoted by the rapid heating as previously mentioned to thereby refine secondary recrystallized texture.

(11) However, when the steel sheet is heated by using the induction heating device as shown in FIG. 2 in the heating zone as previously mentioned, the temperature distribution in the steel sheet, particularly in the widthwise direction thereof becomes easily non-uniform because the heating rate of the steel sheet is high. For example, the temperature difference in the widthwise direction (highest temperature-lowest temperature) as measured by a sheet temperature measuring device 61 disposed at an exit side of an induction heating device 21 of FIG. 2 may come up to 150 C. Consequently, vertical wrinkling, warping or the like is caused in the steel sheet to deteriorate the shape of the product, or the steel sheet is wound around a transporting roll in the furnace to cause narrowing and serious operation troubles such as breakage of the steel sheet in the furnace and so on are caused in the worst case.

(12) In order to solve the above problems, therefore, according to the invention, the induction heating device 21 of FIG. 2 is divided into two apparatuses 21a and 21b, and these apparatuses are separately arranged through a duct portion 21c, and a heating stop region is provided on the way of a region performing rapid heating as shown in FIG. 3. By providing the heating stop region is kept the steel sheet at a given temperature on the way of the heating for a given time, so that the non-uniform temperature in the steel sheet, particularly in the widthwise direction thereof is mitigated by thermal diffusion and hence it is possible not only to improve the poor shape but also to reduce variation in the magnetic properties.

(13) Alternatively, the induction heating device 21 of FIG. 2 is divided into two apparatuses 21a and 21b, and these apparatuses are separately arranged through a duct portion 21c, and a slow heating region is provided on the way of a region performing rapid heating as shown in FIG. 4. It is preferable that the duct portion 21c as the slow heating region is provided in its inside with a pair of heating devices 21d capable of slowly heating the steel sheet at a heating rate of more than 0 C./s but not more than 10 C./s, which are, for example, arranged so as to face to front and rear faces of the steel sheet. By providing these heating devices can be mitigated the temperature variation in the steel sheet, particularly in the widthwise direction thereof more effectively as compared to the heating device of FIG. 3 having no pair of heating devices. As a result, it is possible not only to mitigate the non-uniform temperature in the steel sheet, particularly in the widthwise direction thereof to improve the poor shape but also to further decrease the variation in the magnetic properties.

(14) The inventors consider the mechanism of improving the temperature variation with such a heating device as follows.

(15) At first, when the slow heating region with a certain length is provided between the induction heating devices as one of the constructional features according to the invention, the temperature variation in the steel sheet formed by the induction heating zone is reduced by thermal diffusion when the steel sheet passes through the slow heating region. Further, when heating means such as heater or the like as another constructional feature according to the invention is disposed in the slow heating region, the effect of uniformizing the temperature in the widthwise direction by the heating means is joined in addition to the above thermal diffusion in the steel sheet, so that a temperature rising margin in the high temperature portion of the steel sheet is small, while a temperature rising margin in the low temperature portion becomes larger and hence the heating may be performed while positively mitigating the temperature variation. As a result, it is possible to more reduce the temperature variation as compared to the case of providing no heating means.

(16) The heating means of the heating device 21d is not particularly limited, and any means such as electric heater, radiant tube, high-temperature gas blowing and the like may be used as long as the temperature variation in the steel sheet can be mitigated.

(17) The reason why the magnetic properties are further improved by providing the duct portion 21c, i.e. the heating stop region or slow heating region (heating rate adjusting region) as compared to the conventional rapid heating is due to the fact that an adequate amount of the stored strain energy of <111>//ND orientation can be released by providing the heating rate adjusting region on the way of the heating process and the existing ratio of <110>//ND orientation to <111>//ND orientation in the primary recrystallized texture can be further relatively enhanced as compared to the conventional technique.

(18) It is preferable that the duct portion 21c is provided in a temperature zone of 250 C. to 600 C. for obtaining the above improving effect. Because, the invention is based on a technical idea that recrystallization superiority of <111>//ND is decreased by keeping at a temperature zone of causing decrease of dislocation density and no recrystallization for a short time or slowly heating such a zone. Therefore, the above effect is not obtained in a zone of lower than 250 C. anticipating substantially no movement of dislocation, while when the zone exceeds 600 C., recrystallization of <111>//ND starts and hence the generation of {110}<001> orientation cannot be promoted even if the temperature of higher than 600 C. is kept or the slow heating is performed at such a temperature. The lower limit of the temperature in the region providing the duct portion is more preferably not lower than 350 C., further preferably not lower than 400 C., while the upper limit thereof is more preferably not higher than 550 C., further preferably not higher than 520 C.

(19) Also, the length of the duct portion 21c or the length of the heating rate adjusting region is preferable to be in a range of 130 m. When the length is less than 1 m, the time of stopping the heating (keeping at a constant temperature) or the slow heating time is too short, and the temperature uniformization in the steel sheet is not sufficient and the effect of improving the shape or magnetic properties is not obtained. While, when it exceeds 30 m, the heating stop time or the slow heating time becomes too long and the recovery of the rolled structure is promoted and hence there is a risk of causing poor secondary recrystallization. Incidentally, the time of the heating stop or the slow heating (time required for passing the steel sheet through the duct portion 21c) is preferable to be a range of 110 seconds. The lower limit of the length of the heating rate adjusting region is more preferably not less than 3 m, further preferably not less than 5 m, while the upper limit thereof is more preferably not more than 20 m, further preferably not more than 10 m.

(20) In order to obtain the effect of improving the magnetic properties, the region of performing the rapid heating with the induction heating device is preferable to be a temperature region from room temperature to at least 700 C. The reason why the upper limit is 700 C. is due to the fact that the temperature of preferentially recrystallizing <111>//ND is not higher than 700 C. and if the rapid heating is performed up to a temperature exceeding 700 C., not only the effect of improving the magnetic properties is saturated, but also the energy required for heating is undesirably increased.

(21) Also, in order to obtain the effect of improving the magnetic properties, the heating rate in the region arranging the induction heating device is preferable to be not less than 50 C./s. When it is less than 50 C./s, the effect of improving the magnetic properties is not sufficient. However, when the rapid heating is performed at a rate of not less than 350 C./s, the effect of improving the magnetic properties is saturated, so that the upper limit is preferable to be about 350 C./s. The lower limit of the heating rate is more preferably not less than 60 C./s, further preferably not less than 70 C./s. while the upper limit thereof is more preferably not more than 300 C./s, further preferably not more than 250 C./s. Moreover, the heating rate means an average heating rate of a heating time obtained by subtracting a time of the heating stop region or the slow heating region from a time of heating from room temperature to at least 700 C.

(22) Although the above is described on a case that the induction heating device 21 is divided into two parts 21a and 21b, the induction heating device in each of 21a and 21b may be one or may be comprised of plural parts. When it is comprised of plural parts, the induction heating device in each of 21a and 21b on the side of the duct portion 21c is movable in the carrying direction of the steel sheet. For example, the induction heating device constituting 21a is moved toward the side of 21b or the induction heating device constituting 21b is moved toward the side of 21a, whereby the position of the duct portion 21c, i.e. the temperature zone of the heating stop region or the slow heating region may be varied.

(23) By applying the rapid heating apparatus according to the invention to the continuous annealing of the steel sheet subjected to the rapid heating as mentioned above, not only the shape of the steel sheet can be improved, but also the texture after the recrystallization can be controlled, so that it is effective to apply to primary recrystallization annealing of a final cold rolled sheet for a grain-oriented electrical steel sheet containing not more than 8.0 mass % of Si or primary recrystallization annealing combined with decarburization annealing thereof, or to annealing of a final cold rolled sheet for a non-oriented electrical steel sheet. Moreover, the Si content is preferably not less than 1.5 mass %, more preferably not less than 2.5 mass %, while the upper limit thereof is more preferably not more than 6.0 mass %, further preferably not more than 4.0 mass %.

EXAMPLE 1

(24) A cold rolled steel sheet with a final thickness of 0.3 mm produced from a slab comprising C: 0.070 mass %, Si: 3.35 mass %, Mn: 0.10 mass %, Al: 0.025 mass %, N: 0.012 mass %, S: 0.01 mass %, Se: 0.01 mass %, Sb: 0.01 mass %, Sn: 0.05 mass % and the remainder being Fe and inevitable impurities is subjected to primary recrystallization annealing combined with decarburization annealing with a continuous annealing furnace constructed with a heating zone, a soaking zone and a cooling zone. The heating zone of the continuous annealing furnace is designed so as to arrange an induction heating device 21 before a heating zone 20 of radiant tube heating system and further divide the induction heating device 21 into two parts 21a and 21b relatively movable into a carrying direction of the steel sheet and provide a duct portion 21c as a heating stop region between 21a and 21b as shown in FIG. 3.

(25) The heating with the above heating equipment is performed by two heating patterns as described in FIG. 5 and the followings.

(26) Pattern A: A heating pattern of the invention wherein a duct portion 21c of about 2.5 m (heating stop region) is provided between induction heating devices 21a and 21b and the sheet is heated from room temperature (20 C.) to 500 C. at 75 C./s in the first 21a, kept at a temperature of 500 C. for about 2 seconds in the duct portion 21c, heated to 700 C. at 75 C./s in the subsequent 21b and thereafter heated up to a soaking temperature of 850 C. at an average heating rate of 15 C./s with a radiant tube.

(27) Pattern B: A heating pattern of the conventional technique wherein the sheet is heated from room temperature (20 C.) to 700 C. at 75 C./s at once without providing a duct portion (heating stop region) between induction heating devices 21a and 21b and thereafter heated up to a soaking temperature of 850 C. at an average heating rate of 15 C./s with a radiant tube.

(28) In this case, a temperature distribution of the steel sheet in its widthwise direction is measured by sheet temperature measuring devices 61a, 61c and 61b disposed in an exit side of the induction heating device 21a, an exit side of the duct portion 21c and an exit side of the induction heating device 21b to determine a difference between highest temperature and lowest temperature, and at the same time the shape of the steel sheet at the exit side of the induction heating device 21b is measured with a laser displacement gauge to determine a steepness.

(29) Thereafter, the steel sheet subjected to primary recrystallization annealing combined with decarburization annealing according to the above heating pattern is coated on its steel sheet surface with an annealing separator, dried, subjected to finish annealing and further to flattening annealing combined with baking and shape correction of an insulating film according to usual manner to obtain a product sheet of a grain-oriented electrical steel sheet.

(30) Test specimens are taken out from the product sheet thus obtained at 5 places in total of a leading end and a tailing end of a coil and positions of , 2/4 and from the leading end over full length of the coil, and magnetic flux density B.sub.8 and iron loss W.sub.17/50 thereof are measured according to an Epstein method defined in JIS C2550. Among the measured values, poorest magnetic flux density B.sub.8 and iron loss W.sub.17/50 are made a guarantee value in the coil.

(31) The results are shown in Table 1. As seen from these results, the temperature difference in the widthwise direction of the steel sheet during the heating is largely mitigated, and hence not only the shape of the steel sheet is made stable to improve the sheet threading performance through the annealing equipment, but also the scattering of the magnetic properties in the steel sheet is decreased to improve the guarantee value in the coil.

(32) TABLE-US-00001 TABLE 1 Temperature difference in widthwise direction ( C.) Temperature Temperature difference Temperature difference at exit side difference at exit side Evaluation Guarantee value in coil of induction at exit side of induction of steel Magnetic Iron loss Heating heating device of duct portion heating device sheet shape flux density W.sub.17/50 pattern 21a ( C.) 21c ( C.) 21b ( C.) (steepness) (%) B.sub.8 (T) (W/kg) Remarks A 60 29 22 0.6 1.930 0.947 Invention Example B 60 38 1.2 1.927 0.958 Comparative Example

EXAMPLE 2

(33) A cold rolled steel sheet with a final thickness of 0.3 mm produced from a slab comprising C: 0.070 mass %, Si: 3.35 mass %, Mn: 0.10 mass %, Al: 0.025 mass %, N: 0.012 mass %, S: 0.01 mass %, Se: 0.02 mass %, Sb: 0.02 mass %, Sn: 0.02 mass % and the remainder being Fe and inevitable impurities is subjected to primary recrystallization annealing combined with decarburization annealing with a continuous annealing furnace constructed with a heating zone, a soaking zone and a cooling zone. The heating zone of the continuous annealing furnace is designed so as to arrange an induction heating device 21 before a heating zone 20 of radiant tube heating system and further constitute the induction heating device 21 with two parts 21a and 21b relatively movable into a carrying direction of the steel sheet and provide a duct portion 21c with a heating device 21d capable of slowly heating the steel sheet between 21a and 21b as shown in FIG. 4.

(34) The heating with the above heating equipment is performed by two heating patterns as described in FIG. 6 and the followings.

(35) Pattern C: A heating pattern of the invention wherein a duct portion 21c of about 2.5 m (slow heating region) is provided between induction heating devices 21a and 21b and the sheet is heated from room temperature (20 C.) to 500 C. at 75 C./s in the first 21a, slowly heated at a heating rate of 2.0 C./s for 2.0 seconds in the duct portion 21c, heated to 700 C. at 75 C./s in the subsequent 21b and thereafter heated up to a soaking temperature of 850 C. at an average heating rate of 15 C./s with a radiant tube.

(36) Pattern D: A heating pattern of the conventional technique wherein the sheet is heated from room temperature (20 C.) to 700 C. at 75 C./s at once without providing a duct portion 21c (slow heating region) between induction heating devices 21a and 21b and thereafter heated up to a soaking temperature of 850 C. at an average heating rate of 15 C./s with a radiant tube.

(37) In this case, a temperature distribution of the steel sheet in its widthwise direction is measured by sheet temperature measuring devices 61a, 61c and 61b disposed in an exit side of the induction heating device 21a, an exit side of the duct portion 21c and an exit side of the induction heating device 21b to determine a difference between highest temperature and lowest temperature, and at the same time the shape of the steel sheet at the exit side of the induction heating device 21b is measured with a laser displacement gauge to determine a steepness.

(38) Thereafter, the steel sheet subjected to primary recrystallization annealing combined with decarburization annealing according to the above heating pattern is coated on its steel sheet surface with an annealing separator, dried, subjected to finish annealing and further to flattening annealing combined with baking and shape correction of an insulating film according to usual manner to obtain a product sheet of a grain-oriented electrical steel sheet.

(39) Test specimens are taken out from the product sheet thus obtained at 5 places in total of a leading end and a tailing end of a coil and positions of , 2/4 and from the leading end over full length of the coil, and magnetic flux density B.sub.8 and iron loss W.sub.17/50 thereof are measured according to an Epstein method defined in JIS C2550. Among the measured values, poorest magnetic flux density B.sub.8 and iron loss W.sub.17/50 are made a guarantee value in the coil.

(40) The results are shown in Table 2. As seen from these results, the temperature difference in the widthwise direction of the steel sheet during the heating is largely mitigated, and hence not only the shape of the steel sheet is made stable to improve the sheet threading performance through the annealing equipment, but also the scattering of the magnetic properties in the steel sheet is decreased to improve the guarantee value in the coil.

(41) TABLE-US-00002 TABLE 2 Temperature difference in widthwise direction ( C.) Temperature Temperature difference Temperature difference at exit side difference at exit side Evaluation Guarantee value in coil of induction at exit side of induction of steel Magnetic Iron loss Heating heating device of duct portion heating device sheet shape flux density W.sub.17/50 pattern 21a ( C.) 21c ( C.) 21b ( C.) (steepness) (%) B.sub.8(T) (W/kg) Remarks C 60 20 15 0.4 1.933 0.938 Invention Example D 60 38 1.2 1.927 0.958 Comparative Example

INDUSTRIAL APPLICABILITY

(42) The rapid heating apparatus according to the invention can change the texture of the steel sheet into preferable properties through the control of the recrystallization behavior. Therefore, the invention is not limited to the field of the grain-oriented electrical steel sheets and can be applied to fields of non-oriented electrical steel sheets requiring the control of the texture, thin steel sheets for automobiles requiring deep drawability, thin steel sheets requiring control of anisotropy and so on.

DESCRIPTION OF REFERENCE SYMBOLS

(43) 1: steel sheet (steel band) 2: heating zone 20: radiant tube heating portion of heating zone 21, 21a, 21b: induction heating device 21c: duct portion (heating stop region or slow heating region) 21d: slow heating device 3: soaking zone 4: cooling zone 51, 52, 53: sheet temperature measuring device 61a, 61b, 61c: sheet temperature measuring device in induction heating region