JP5135534B2 - Continuous annealing method and continuous annealing equipment for steel strip with Curie point - Google Patents

Description

  The present invention relates to a continuous annealing method and continuous annealing equipment for a steel strip having a Curie temperature (also referred to as Curie Temperature: Tc), and in particular, a Curie point that can perform uniform annealing in the longitudinal direction of the steel strip. The present invention relates to a continuous annealing method and a continuous annealing facility at an annealing temperature exceeding the Curie point of a steel strip. In addition, as a steel strip having a Curie point to be processed by the continuous annealing method and continuous annealing equipment of the present invention, a grain-oriented electrical steel sheet containing Si ≦ 4.5 mass% and Cr ≦ 18 mass% are contained. Examples thereof include ferritic stainless steel plates and martensitic stainless steel plates.

  In continuous annealing of metal strips such as steel strips, the heating temperature, heating time, etc. are generally strictly controlled. Among them, for example, low iron loss suitable for use as the core of transformers and other electrical equipment. Strict temperature control may be required as in the decarburization annealing process in the manufacturing process of grain-oriented electrical steel sheets. In the production of grain-oriented electrical steel sheets, (a) if the decarburization annealing temperature variation occurs and the arrival at the soaking temperature is delayed, the time required for decarburization is reduced and the decarburization performance deteriorates. When (b) overshoot occurs during heating and the target soaking temperature is exceeded even in a very short time, the oxide layer generated at an early stage inhibits decarburization and degrades decarburization, or This is because it has a problem of causing film defects.

  As an invention relating to the management of such an annealing temperature, Patent Document 1 discloses that a steel plate temperature is heated to 550 to 650 ° C. by a gas heating method using a radiant tube having a large heating capacity, and then reaches the soaking temperature above the above temperature. Uses a tube heater with a large heat generation density and densely arranged heating resistors, so that it can flexibly respond to a wide range of steel plate size changes despite a furnace with a large processing capacity. An invention relating to a method for decarburizing and annealing a grain-oriented electrical steel sheet, in which undershoot is eliminated and stable decarburization annealing is possible, is disclosed.

In addition, regarding the continuous annealing method of the steel sheet, an invention that utilizes an induction heating device to change the plate temperature of the condition change part so that the annealing condition can be smoothly changed from the preceding material to the subsequent material having different annealing conditions. This is disclosed in Patent Document 2.
In Patent Document 3, iron loss is improved by rapidly heating a cold-rolled steel strip to a temperature of 705 ° C. or higher at a heating rate of 230 ° C./second or more during decarburization annealing of grain-oriented electrical steel sheets. In a second or third embodiment, the heating operation uses a special electromagnetic induction heating coil at a fundamental frequency of 450 kHz that provides a heating rate of 1100 to 1200 ° C./sec to a Curie point of 746 ° C. It is disclosed that this is done.

  Further, in Patent Document 4, when accelerated cooling is employed in a thick steel plate manufacturing process, variations in the mechanical properties and shape defects of the steel plate, which are likely to occur due to the high cooling property, and the residual quality are caused. Uniform temperature in the steel sheet by applying heat treatment using an induction heating device that sets the target heating temperature of the steel sheet after accelerated cooling to the magnetic transformation temperature (Curie point) or 700 to 760 ° C. An invention to be solved by hot correction after enhancing the properties is disclosed.

Japanese Patent Laid-Open No. 10-324922 JP 2003-328039 A Japanese Patent Publication No. 06-051887 JP 2006-206927 A

  However, in the invention described in the above-mentioned Patent Document 1, it is clear from the fact that there is no description about the change in the longitudinal direction of the steel strip to be heated, which the steel strip itself has, as is apparent from the hot rolling in the actual steel strip. The problem of inhomogeneity in the longitudinal heatability caused by fluctuations in the longitudinal direction of the temperature history, etc., and the problem of variations in the various characteristics in the longitudinal direction of the resulting steel strip cannot be avoided. there were. For example, the problem of non-uniformity of decarburization in the longitudinal direction of a grain-oriented electrical steel sheet or the problem of film defects, or the problem of non-uniformity of mechanical strength in the longitudinal direction of ferritic stainless steel sheet and martensitic stainless steel sheet However, the invention described in Patent Document 1 could not solve the problem.

Further, the invention described in Patent Document 2 is intended to smoothly change the annealing conditions from the preceding material to the succeeding material having different annealing conditions, and uniformly heating the steel strip in the longitudinal direction. Is not described at all.
Moreover, in invention of the said patent document 3, it is disclosed that the iron loss of an electromagnetic steel plate can be improved by applying the rapid heating by electromagnetic induction heating to the heating to the Curie point of the decarburization annealing of an electromagnetic steel plate. However, there is no disclosure of temperature uniformity in the longitudinal direction of the steel strip.
Moreover, in the invention described in Patent Document 4, the temperature uniformity in the steel sheet is improved by performing a heat treatment in which the heating target temperature of the induction heating device is the magnetic transformation temperature (Curie point) of the steel material or 700 to 760 ° C. Although it is disclosed that it can be applied, it is neither disclosed nor suggested whether it can be applied to continuous annealing of a steel strip having an annealing temperature exceeding the Curie point to enhance temperature uniformity in the steel sheet.

  Then, this invention aims at providing the continuous annealing method and continuous annealing equipment of the steel strip which has a Curie point which can heat the steel strip which has a Curie point very uniformly in the longitudinal direction to the annealing temperature exceeding a Curie point. It is what.

The gist of the present invention for solving the above problems is as follows.
(1) Continuous annealing method at an annealing temperature exceeding the Curie point of a steel strip having a Curie point in a continuous annealing facility comprising a heating zone, a soaking zone, a cooling zone, or a heating zone, a soaking zone, a nitriding zone, and a cooling zone. In, the heating treatment in the heating zone is divided into first to third three regions,
In the first heating zone, the steel strip is heated to 500 ° C. or more and the Curie point Tc (° C.) to less than −50 ° C. by radiation heating means by indirect gas heating or direct gas heating and / or radiation heating means by electric heater,
In the subsequent second heating zone, the heated steel strip is heated to a temperature range of Tc-30 ° C to Tc-5 ° C by a solenoid coil type high frequency induction heating means,
In the final third heating zone, the heated steel strip is heated to a treatment target temperature exceeding the Curie point by radiation heating means by indirect gas heating or direct gas heating and / or radiation heating means by an electric heater. A method for continuously annealing a steel strip having a Curie point.
(2) The steel strip having the Curie point is a cold-rolled grain-oriented electrical steel sheet containing Si ≦ 4.5% by mass, having the Curie point according to (1) above Continuous annealing method for steel strip.
(3) The steel strip having the Curie point is a cold-rolled ferritic stainless steel plate or a martensitic stainless steel plate containing Cr ≦ 18% by mass, as described in (1) above A method for continuous annealing of steel strips having a Curie point.

(4) In a continuous annealing facility comprising a heating zone, a soaking zone, a cooling zone, or a heating zone, a soaking zone, a nitriding zone, and a cooling zone, continuously annealing a steel strip having a Curie point at an annealing temperature exceeding the Curie point, Divide the heating zone into the first to third areas,
In the first heating zone, a radiant heating means by indirect gas heating or direct gas heating and / or a radiant heating means by an electric heater, which heats the steel strip to 500 ° C. or higher and lower than Tc−50 ° C., is provided.
In the second heating zone, a solenoid coil type high frequency induction heating means for heating the steel strip heated in the first heating zone to a temperature range of Tc-30 ° C to Tc-5 ° C is provided,
The third heating zone includes radiant heating means by indirect gas heating or direct gas heating and / or radiant heating means by an electric heater, which heats the steel strip heated in the second heating zone to a processing target temperature exceeding the Curie point. A continuous annealing equipment for a steel strip having a Curie point, characterized by being arranged.

  According to the present invention, since the heating up to the annealing temperature exceeding the Curie point of the steel strip having the Curie point can be performed extremely uniformly in the longitudinal direction, in particular, strict control on the rate of temperature rise of the steel sheet. In continuous annealing of cold-rolled steel strips of directional silicon steel sheets that require uniformity and continuous annealing of ferritic stainless steel strips, martensitic stainless steel strips, etc., quality improvement is achieved by uniformizing the annealing temperature Since the effect is great and the product can be manufactured stably, the industrial effect of the present invention is immeasurable.

BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out the present invention will be described below by taking as an example the production of a directional silicon steel plate and a ferritic stainless steel plate that are particularly effective in the present invention. In addition, it cannot be overemphasized that this invention is not limited to manufacture of a directional silicon steel plate or a ferritic stainless steel plate.
FIG. 1 is a schematic isometric view of a typical continuous heat treatment facility for decarburizing and annealing (including application of an annealing separator) to a finished cold rolled sheet of directional silicon steel.

  The main elements of the manufacturing equipment line are the payoff reel 1 for loading and unwinding the coiled steel strip 60 of directional silicon steel that has been finish cold-rolled, and the leading end of the steel strip. Inlet side shearing machine 2 for cutting parts and preparing for welding, welding machine 3 for continuously joining the ends of steel strips, welding preparation for steel strips and entrance cleaning during welding In order to remove the dirt such as rolling oil and iron, cleaning the surface of the steel strip and the entry side storage looper 4 for storing the steel strip in order to allow the plate to pass through the apparatus 11 and the furnace section 12 without decelerating or stopping. Inlet side cleaning device 11, furnace portion 12 composed of heating, soaking and cooling regions used to decarburize and anneal steel strip, when coil rewinding is completed and outlet side shearing machine 6 is operating In order to allow the steel strip to pass through the entrance side cleaning device 11 and the furnace section 12 without decelerating and stopping. Outgoing storage looper 5 for storing the steel strip, an outgoing cleaning device 13 for cleaning the surface of the annealed steel strip coming out of the furnace, and removing dirt in the furnace, an annealing separator coating device 14, and an annealing separator. It comprises a drying device 15, an exit side shearing machine 6, and a tension reel 7 for rewinding the steel strip in a coil shape.

In such a line, the annealing separator drying apparatus 15 has a highly responsive furnace configuration composed of a furnace material with low thermal inertia and a direct fire burner, and unavoidable annealing during operation of the exit side shearing machine 6. The structure is such that it can quickly respond to stopping and deceleration of the steel strip in the separating material drying device 15.
Further, the tension of the steel strip 60 before and after the furnace section 12 is measured by tension meters 41 and 42, and the tension of the steel strip 60 in the annealing separator drying apparatus 15 is measured by the tension meter 43. The measurement result is fed back to the passing bridle rolls 23 to 26, and the steel strip tension before and after the bridle roll is secured.
Note that the outlet side cleaning device 13 is not necessarily installed when the steel strip in the furnace section 12 is very dirty.

  The finished cold-rolled sheet of directional silicon steel is decarburized and annealed (including application of an annealing separator) in the above-mentioned line, then is subjected to high-temperature annealing, and is further subjected to smooth annealing and becomes a final product.

  FIG. 2 is a diagram schematically showing the configuration of the furnace section 12 according to the prior art in a longitudinal vertical sectional view. The furnace section 12 is generally composed of a heating area 31 by a radiant tube heating method, a soaking area 32 by electric heater heating, a nitriding area 33 by an electric heater heating, and a cooling area 34. Plate thermometers 36, 37, and 38 for monitoring the plate temperature are installed.

  The steel strip 60 whose surface has been cleaned by the inlet side cleaning device 11 is heated in the heating region 31 by the radiant tube method, heated to a decarburization temperature of about 820 ° C., and decarburized and annealed in the soaking zone 32 by the electric heater heating.

  In the heating region 31 by the radiant tube method, the steel strip is heated so as not to cause decarburization, and plate thermometers 36 and 37 installed in the middle of the heating region, and a plate thermometer 38 on the exit side of the heating region. Generally, the temperature of the furnace is controlled while monitoring. Recently, a method of automatically controlling the furnace in the heating region while automatically monitoring the measured values of the plate thermometers 36, 37, and 38 has been adopted.

  3 (a) to 3 (c), one steel strip coil at the position of the plate thermometers 36, 37, 38 in the decarburization annealing of the grain-oriented electrical steel sheet by the equipment of FIGS. An example of the temperature distribution of a longitudinal direction is shown. In this example, the plate thermometer 38 on the heating area exit side is adopted, although the method of automatically controlling the furnace in the heating area while automatically monitoring the measured values of the plate thermometers 36, 37, 38 is adopted. The plate temperature in the longitudinal direction of the steel strip varies. This is because the thermal inertia of the furnace is large and it is difficult to suppress this fluctuation. As a result, such fluctuations greatly affect the reaction of the surface including the decarburization reaction of the steel strip, and quality variations in the longitudinal direction of the steel strip, such as non-uniformity of decarburization and film defects, etc. I was injured.

  The present inventors have investigated and analyzed in detail the plate temperature in the longitudinal heating process of the steel strip, and found that the heating rate fluctuates in the longitudinal direction of the steel strip in one steel plate coil. It was. When the cause of this variation was further analyzed, in the radiant tube furnace used in the continuous heating equipment of the steel strip, the steel plate was heated by radiant heat transfer between the radiant tube and the steel strip, and the temperature of the steel plate was increased. The amount of heat transfer that determines the amount is determined by the emissivity and geometric position of the radiant tube and steel plate, but since the emissivity and geometric position of the radiant tube are unchanged in the short term, the temperature of the steel strip is It was clarified that it changes with the fluctuation of the emissivity of the steel strip. There are many unknown factors that cause the emissivity of the steel sheet to change in the longitudinal direction. However, hot rolling, which is the pre-process for manufacturing cold-rolled steel sheets, is not continuous and is performed in units of slabs (corresponding to steel strip coils). It is presumed that the surface properties change due to longitudinal fluctuations in the plate temperature during hot rolling and uneven cooling processes.

  Moreover, since the emissivity of a steel plate is used for the temperature measurement of a steel plate, if the emissivity changes, the accuracy of the measured value of the plate temperature will deteriorate. Although the plate thermometer using a plurality of wavelengths is slightly improved in accuracy, it cannot escape from this problem.

  As a result of further earnest and repeated research, the inventors of the present invention, in the solenoid coil type high frequency induction heating, the permeability of the steel strip rapidly decreases in the vicinity of the Curie point, and accordingly, the penetration depth increases, Since the heating capacity of the steel strip decreases rapidly, the temperature of the steel strip can be brought close to a constant value without being affected by the emissivity in the longitudinal direction of the steel strip. It came to make this invention which makes the longitudinal direction uniform. Furthermore, when the emissivity of the steel sheet exceeds 700 ° C., the absolute value becomes large, and it has been found that the emissivity of the steel sheet is relatively difficult to be influenced by the state of the plate surface.

  FIG. 4 schematically shows the configuration of the furnace section 12 of a continuous heat treatment facility (FIG. 1) for annealing cold-rolled directional silicon steel, which is one embodiment of the present invention, in a longitudinal vertical sectional view. FIG. Except the furnace part 12, it is the same as the conventional continuous annealing equipment. Compared with the heat treatment line of the prior art of FIG. 2, a solenoid coil type high frequency induction heating device 35 is disposed in the center of the heating zone. In addition, plate thermometers 36 and 37 are installed before and after the solenoid coil type high frequency induction device 35. The steel strip 60 is heated in a heating area (first half) 31A by a radiant tube system, the plate temperature is 500 ° C. or higher, and a predetermined temperature (less than Tc−50 ° C.) lower than 50 ° C. from the Curie point Tc (° C.). After reaching the temperature, the solenoid coil type high frequency induction heating device 35 is heated to a temperature range of Tc-30 ° C to Tc-5 ° C, and then heated to about 825 ° C in the radiant tube type heating region (second half) 31B. Then, decarburization annealing is performed in the soaking zone 32 by electric heater heating.

  If the plate temperature of the steel strip 60 on the inlet side of the solenoid coil type high frequency induction heating device 35 is less than 500 ° C., the required heating allowance by the induction heating device becomes large, and the equipment capacity of the induction heating device for that purpose must be made excessive. In addition to being impractical, when hydrogen is contained in the atmosphere of the heat treatment furnace, it is not possible to secure an atmosphere temperature of 750 ° C. or higher that can avoid the danger of hydrogen explosion. . On the other hand, when the plate temperature is Tc-50 ° C. or higher, it is necessary to make the temperature less than Tc-50 ° C. because the heating variation in the radiant heating cannot be absorbed by the ultimate plate temperature in the induction heating device.

  Further, the sheet temperature of the steel strip 60 on the outgoing side of the solenoid coil type high-frequency induction heating device 35 is too small for the magnetic permeability of the steel strip on the outgoing side if it exceeds Tc−5 ° C. Therefore, it is necessary for the high-frequency induction heating device. The magnetic field becomes large and the required equipment becomes huge, which is not realistic. Also, if the temperature is lower than Tc-30 ° C, the permeability of the steel strip on the exit side is not small, and the heating variation in the radiant heating is high-frequency induction heating. Can not be suppressed. Therefore, the plate temperature of the steel strip 60 on the outlet side of the solenoid coil type high frequency induction heating device 35 needs to be in the temperature range of Tc-30 ° C to Tc-5 ° C.

FIG. 5 shows the temperature in the longitudinal direction of one steel strip coil measured at the positions of the plate thermometers 36, 37, 38 on the exit side of each heating region 31A, 35, 31B in the furnace section 2 according to the present invention. An example of distribution is shown.
As described above, according to the present invention, the solenoid coil type high frequency induction is provided on the exit side of the heating region 31A by the radiant tube method, although the steel strip temperature unevenness exists as in the measurement data of the plate thermometer 36. On the exit side of the heating device 35, the temperature becomes substantially uniform as measured by the plate thermometer 37. Further, on the exit side of the heating region 31B by the radiant tube method, the plate temperature in the longitudinal direction of the steel strip is Like the measurement data of the thermometer 38, it hardly fluctuates and is very stable.
The continuous annealing equipment for steel strip according to the present invention makes it possible to anneal the steel strip of the directional silicon steel plate in the longitudinal direction very uniformly. Therefore, the quality of the obtained directional silicon steel plate is also uniform in decarburization. The film defects were almost eliminated.

  Although FIG. 4 shows an example having the nitriding region 33, the present invention is not limited to a decarburization annealing facility for cold-rolled grain-oriented electrical steel sheets having a nitriding region. It is also effective for decarburization annealing equipment that does not have.

  FIG. 6 is a schematic isometric view of a typical continuous heat treatment facility for bright annealing a ferritic stainless steel cold-rolled sheet. The main elements of the production line are the same as in FIG. 1 except that there is no annealing separator coating device and drying furnace on the furnace exit side.

  FIG. 7 is a diagram schematically showing the configuration of the furnace section 12 according to the prior art in a longitudinal vertical sectional view. The furnace section 12 is generally composed of a heating area 51, a soaking area 52, and a cooling area 54 by a muffle furnace system (indirect heating). The heating area 51 includes a plate for monitoring the plate temperature during heating. Thermometers 56, 57, and 58 are installed.

FIG. 8 is a schematic longitudinal sectional view of the furnace section 12 of a continuous heat treatment facility (FIG. 6) for bright annealing of cold-rolled ferritic stainless steel according to one embodiment of the present invention. FIG. Except the furnace part 12, it is the same as the conventional continuous annealing equipment. Compared with the annealing line of the prior art of FIG. 7, a solenoid coil type high frequency induction heating device 55 is installed in the center of the heating zone.
As in the case of the decarburization annealing equipment for electrical steel sheets, in the conventional furnace configuration, the temperature rising process in the heating zone varied widely, but in the present invention, in the solenoid coil type high frequency induction heating furnace up to the vicinity of the Curie point. It was possible to heat and uniformly heat the steel strip in the longitudinal direction.

In addition, as a steel strip having a Curie point to be treated by the present invention, it is not limited to the cold-rolled steel strip of the directional electromagnetic steel plate exemplified here or the cold-rolled steel strip of a ferritic stainless steel plate, All are valid for steel strips with a Curie point.
Moreover, as the grain-oriented electrical steel sheet containing Si ≦ 4.5 mass% to be processed by the present invention, for example, the directions disclosed in JP 2002-060842 A, JP 2002-173715 A, and the like. Any component system such as a heat-resistant electrical steel sheet may be used, and the component system is not particularly limited in the present invention.

  Further, as a ferritic stainless steel plate containing Cr ≦ 18 mass% to be treated by the present invention, standard steel types such as SUS430 and SUS430J1L of JIS G 4305, Japanese Patent Laid-Open Nos. 05-293595 and 06-002044 are disclosed. In the present invention, the component system is not particularly limited as long as it is a component system such as a ferritic stainless steel sheet disclosed in Japanese Patent Application Laid-Open No. 07-118754.

  Further, as the martensitic stainless steel plate containing Cr ≦ 18 mass% to be treated by the present invention, standard steel types such as SUS410 and SUS420J1 of JIS G 4305, JP-A-07-268561, JP-A-08- Any component system such as the martensitic stainless steel sheet disclosed in Japanese Patent No. 199310 may be used, and the component system is not particularly limited in the present invention.

  The means for heating the steel strip to less than Tc-50 ° C. is not limited to the radiant tube method, and all indirect gas heating or radiant heating means by direct gas heating and / or radiant heating means by electric heaters and This is effective in heating means using an induction heating device. Further, the method of heating from the temperature range of Tc-30 ° C. to Tc-5 ° C. near the Curie point to the processing target temperature is not limited to the electric heater heating method, and radiation by all indirect gas heating or direct gas heating is used. It is effective in a radiant heating means using a heating means and / or an electric heater. In general, Tc-30 ° C exceeds 700 ° C, and in this region, the emissivity of the steel sheet becomes large and the plate surface is relatively less affected by the state of the plate surface. Since the measurement accuracy of the thermometer also increases and the temperature of the steel sheet can be easily controlled, the heating method is not particularly limited at Tc-30 or higher.

  In mass%, C: 0.06%, Si: 3.3%, Mn: 0.1%, P: 0.03%, S: 0.008%, acid-soluble Al: 0.028%, N: A steel slab containing 0.008% and Cr: 0.1% was heated at a temperature of 1150 ° C., and then hot-rolled to a plate thickness of 2.3 mm to form a steel strip coil, and thereafter annealing temperatures 1120 ° C. and 920 C. Two-stage annealing was performed. Furthermore, after cold rolling with a reverse rolling mill to a plate thickness of 0.22 mm, the conventional decarburization annealing equipment (FIGS. 1 and 2) and the decarburization annealing equipment of the present invention (FIGS. 1 and 4) are used. Decarburized and annealed. Then, after performing high temperature annealing, the smoothing annealing was finally performed. In that case, while measuring the steel plate temperature of the heating part exit side of the furnace part 12 with the plate thermometer 38, the film defect rate of the grain-oriented electrical steel plate after smoothing annealing was measured. Table 1 shows the test conditions and test results. The start temperature of induction heating is Tc-A (° C.), the end temperature is Tc-B (° C.), and values A and B are shown in the table. Further, as an evaluation item of the stability of the quality in the coil longitudinal direction, since the decarburization is difficult to continuously measure, the film defect rate (area ratio of the defect portion) capable of continuous measurement was measured.

In Examples 1 and 2 according to the present invention, it is understood that there is almost no variation in the steel plate temperature on the plate thermometer, and as a result, the film defect rate of the steel plate is very small.
On the other hand, in Comparative Example 11 where the induction heating end temperature was too high, the steel sheet did not reach the target temperature, and the test conditions could not be satisfied. Further, in Comparative Example 12 where the induction heating end temperature was too low and Comparative Examples 13 and 14 where the induction heating start temperature was high, the variation in the steel sheet temperature was still not small, and as a result, the film defect rate of the steel sheet was high. In Comparative Example 15 in which induction heating was not used, the steel plate temperature variation was large, and the film defect rate of the steel plate was very large.

  In mass%, C: 0.005%, Si: 0.1%, Mn: 0.1%, Cr: 15%, P: 0.02%, S: 0.01%, N: 0.01% Was heated at a temperature of 1200 ° C., and then hot-rolled to a thickness of 5 mm to form a steel strip coil, and then annealed at 900 ° C. Furthermore, after cold rolling with a reverse rolling mill to a plate thickness of 2 mm, at a temperature of 950 ° C. using the conventional method (FIGS. 6 and 7) and the method according to the present invention (FIGS. 6 and 8) in an annealing facility. Bright annealed. At that time, the steel plate temperature on the exit side of the heating region of the furnace section 12 was measured by a plate thermometer 58, and the hardness of the obtained ferritic stainless steel plate after bright annealing was measured to measure the strength mismatch rate. Table 2 shows test conditions and results (target values A and B are the same as in Table 1).

In Examples 21 and 22 according to the present invention, it can be seen that there was almost no variation in the steel plate temperature, and as a result, the variation in the mechanical strength of the steel plate was very small, and there were no mismatched portions.
On the other hand, in Comparative Example 31, where the induction heating end temperature was too high, the steel sheet did not reach the target temperature, and the test conditions could not be met. Further, in Comparative Example 32 where the induction heating end temperature is too low and Comparative Examples 33 and 34 where the induction heating start temperature is high, the ratio variation of the mechanical strength mismatched portion (insufficient heating) of the steel sheet was still large. In Comparative Example 35 in which induction heating was not used, the ratio of the mechanical strength mismatched portion (insufficient heating) of the steel sheet was very large.

It is a figure which shows typically the typical continuous heat processing equipment for carrying out the decarburization annealing (including application | coating of an annealing separation agent) of the cold-rolled sheet of directional silicon steel by an isometric view. It is a figure which shows typically the typical structure by the longitudinal direction vertical cross-sectional view of the furnace part 12 in FIG. It is a figure which shows the example of the longitudinal direction transition of the plate | board temperature of the steel strip measured by three typical places in the heating area | region 31 of FIG. 2 by a prior art. It is a figure which shows typically the structure by one Embodiment of this invention of the furnace part 12 in FIG. 1 with a longitudinal direction vertical sectional view. It is a figure which shows the example of the longitudinal direction transition of the plate | board temperature of the steel strip measured by each area | region exit side of the heating area | region 31A, 35, 31B of FIG. 4 by this invention. It is a figure which shows typically the typical continuous heat processing equipment for carrying out bright annealing of the cold rolled sheet of a ferritic stainless steel by an isometric view. It is a figure which shows typically the typical structure by the prior art of the furnace part 12 in FIG. 6 with a longitudinal direction vertical sectional view. It is a figure which shows typically the structure by one Embodiment of this invention of the furnace part 12 in FIG. 6 with a longitudinal direction vertical sectional view.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Payoff reel 2 Entry side shearing machine 3 Welding machine 4 Entry side storage looper 5 Entry side storage looper 6 Entry side shearing machine 7 Tension reel 11 Entry side cleaning device 12 Furnace part 13 Entry side cleaning device 14 Annealing separation agent coating device 15 Annealing Separating agent drying furnace 21-26 Bridle roll 31 Heating area by radiant tube system 31A Heating area by radiant tube system (previous stage)
31B Radiant tube heating area (second stage)
32, 52 Soaking area 33 Nitriding area 34, 54 Cooling area 35, 55 Solenoid coil type high frequency induction heating device (heating area)
36, 37, 38 Plate thermometer 41, 42, 43 Tension meter 51 Heating area of muffle type heating furnace 51A Heating area of muffle type heating furnace (previous stage)
51B Heating area of muffle furnace (second stage)
52 Soaking area of the muffle type heating furnace 53 Cooling area of the muffle type heating furnace 56, 57, 58 Plate thermometer 60 Steel strip

Claims (4)

In the continuous annealing method at the annealing temperature exceeding the Curie point of the steel strip having the Curie point in the continuous annealing equipment comprising the heating zone, the soaking zone, the cooling zone, or the heating zone, the soaking zone, the nitriding zone, and the cooling zone, The heat treatment in the heating zone is divided into the first to third regions,
In the first heating zone, the steel strip is heated to 500 ° C. or more and the Curie point Tc (° C.) to less than −50 ° C. by radiation heating means by indirect gas heating or direct gas heating and / or radiation heating means by electric heater,
In the subsequent second heating zone, the heated steel strip is heated to a temperature range of Tc-30 ° C to Tc-5 ° C by a solenoid coil type high frequency induction heating means,
In the final third heating zone, the heated steel strip is heated to a treatment target temperature exceeding the Curie point by radiation heating means by indirect gas heating or direct gas heating and / or radiation heating means by an electric heater. A method for continuously annealing a steel strip having a Curie point.   The continuous steel strip having a Curie point according to claim 1, wherein the steel strip having the Curie point is a cold-rolled grain-oriented electrical steel sheet containing Si ≦ 4.5% by mass. Annealing method.   The steel strip having the Curie point is a cold-rolled ferritic stainless steel plate or a martensitic stainless steel plate containing Cr ≦ 18% by mass, having the Curie point according to claim 1. Continuous annealing method for steel strip. In a continuous annealing facility comprising a heating zone, a soaking zone, a cooling zone, or a heating zone, a soaking zone, a nitriding zone, and a cooling zone, which continuously anneals a steel strip having a Curie point at an annealing temperature exceeding the Curie point, the heating zone Divided into the first to third areas,
The first heating zone is provided with a radiant heating means by indirect gas heating or direct gas heating and / or a radiant heating means by an electric heater, which heats the steel strip to 500 ° C. or more and less than the Curie point Tc (° C.) − 50 ° C. Set up
In the second heating zone, a solenoid coil type high frequency induction heating means for heating the steel strip heated in the first heating zone to a temperature range of Tc-30 ° C to Tc-5 ° C is provided,
The third heating zone includes radiant heating means by indirect gas heating or direct gas heating and / or radiant heating means by an electric heater, which heats the steel strip heated in the second heating zone to a processing target temperature exceeding the Curie point. A continuous annealing equipment for a steel strip having a Curie point, characterized by being arranged.

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