METHOD OF HEATING STEEL SHEET IN CONTINUOUS ANNEALING AND CONTINUOUS ANNEALING FACILITY

Abstract

A method of heating a steel sheet and a continuous annealing facility therefor wherein the temperature of the steel sheet in the longitudinal direction and sheet width direction is uniformized and overheating of the steel sheet exceeding the soaking temperature as the target heating temperature is prevented. T is a value of not less than a variation range of the steel sheet temperature when the sheet temperature is controlled by feedback control in the heating furnace but not more than of a heating capacity of the steel sheet in the semi-soaking furnace.

Claims

1. A method of heating a steel sheet in a continuous annealing facility comprising a direct heating furnace, a soaking furnace and a cooling furnace, wherein a direct semi-soaking furnace is disposed between the heating furnace and the soaking furnace; the steel sheet is heated in the heating furnace so that a steel sheet temperature at an exit side of the heating furnace reaches (a target soaking temperature-T); and a furnace temperature in the semi-soaking furnace is set to the target soaking temperature of the steel sheet and the steel sheet is heated so that the steel sheet temperature reaches the target soaking temperature at a position in the semi-soaking furnace, wherein T is a value of not less than the variation range of the steel sheet temperature when the sheet temperature is controlled by feedback control in the heating furnace, and is a value of not more than of the heating capacity for the steel sheet in the semi-soaking furnace.

2. The method of heating a steel sheet according to claim 1, wherein the value of T is made large when a flow rate of a fuel supplied to a direct burner in the semi-soaking furnace reaches the lower limit of a fuel supply capacity in the semi-soaking furnace, while the value of T is made small when it reaches the upper limit of the fuel supply capacity in the semi-soaking furnace.

3. The method of heating a steel sheet according to claim 1, wherein the flow rate of the fuel supplied to the direct burner in the semi-soaking furnace falls within the range from the lower limit of the fuel supply capacity1.2 to the upper limit of the fuel supply capacity0.8 in the semi-soaking furnace.

4. A continuous annealing facility for a steel sheet comprising a direct heating furnace, a soaking furnace and a cooling furnace, wherein a direct semi-soaking furnace is disposed between the heating furnace and the soaking furnace; the steel sheet is heated in the heating furnace so that a steel sheet temperature at an exit side of the heating furnace reaches (a target soaking temperature-T); and the furnace temperature in the semi-soaking furnace is set to the target soaking temperature of the steel sheet and the steel sheet is heated so that the temperature thereof reaches the target soaking temperature at a position of the semi-soaking furnace, wherein T is a value of not less than the variation range of the steel sheet temperature when the sheet temperature is controlled by feedback control in the heating furnace, and is a value of not more than of the heating capacity for the steel sheet in the semi-soaking furnace.

5. The method of heating a steel sheet according to claim 2, wherein the flow rate of the fuel supplied to the direct burner in the semi-soaking furnace falls within the range from the lower limit of the fuel supply capacity1.2 to the upper limit of the fuel supply capacity>0.8 in the semi-soaking furnace.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a diagram illustrating a method of controlling a steel sheet temperature in a continuous annealing facility.

[0017] FIG. 2 is a graph showing an example of a change of a general heat transfer coefficient .sub.CG with a lapse of time in a continuous annealing.

[0018] FIG. 3 is a diagram illustrating a method of controlling a steel sheet temperature provided by adding feedback control to the method shown in FIG. 1.

[0019] FIG. 4 is a diagram illustrating a method of controlling a steel sheet temperature in a continuous annealing facility provided with a semi-soaking furnace according to the invention.

[0020] FIG. 5 is a graph comparatively showing a change of a sheet temperature with a lapse of time measured at an exit side of a semi-soaking furnace, comparing the case with and without an operation of the semi-soaking furnace according to the invention.

[0021] FIG. 6 is a graph comparatively showing a temperature variation in the longitudinal direction of a steel sheet (3) and a temperature difference in the sheet width direction thereof, comparing the case with or without an operation of the semi-soaking furnace according to the invention.

DESCRIPTION OF EMBODIMENTS

[0022] An embodiment of the invention will be described below with the drawings.

[0023] FIG. 1 shows a method of controlling a steel sheet temperature (sheet temperature) in a heating furnace and a soaking furnace at a first-half part of a continuous annealing facility for a steel sheet comprising a direct heating furnace, a soaking furnace and a cooling furnace. In FIG. 1, a steel sheet 1 is introduced from a left side of the figure to a heating furnace 2, heated to a soaking temperature as a target heating temperature (target soaking temperature) before it reaches an exit side of the heating furnace (point A in FIG. 1), introduced into a soaking furnace 3, kept at the soaking temperature for a given time, and then cooled. In the heating furnace 2, a furnace temperature setting value of the heating furnace 2 is calculated based on conditions of a material (steel sheet) to be treated (sheet thickness, sheet width, specific heat and so on) and annealing conditions (sheet threading speed, atmosphere gas, general heat transfer coefficient .sub.CG and so on) input in a host computer, whereby flow rates of fuel and air supplied to the heating furnace 2 are automatically controlled to achieve the furnace temperature setting value. In the soaking furnace 3, the furnace temperature is set to the soaking temperature as the target heating temperature of the steel sheet, and flow rates of fuel and air supplied to the soaking furnace 3 are automatically controlled to attain the furnace temperature setting value.

[0024] Moreover, there are various methods for determining the furnace temperature setting value of the heating furnace by the host computer. For example, the setting value can be determined by a convergent calculation using a heat transfer model equation as shown by the following equation (1):

T.sub.sX=2.Math..sub.CG.Math.(T.sub.f.sup.4T.sub.s.sup.4)/C.sub.p.Math..Math.D.Math.L.sub.s (1)

, wherein T.sub.s is a sheet temperature at exit side (K), X is a heating length (m), .sub.CG is a general heat transfer coefficient (general heat absorption ratio), is the Stefan-Boltzmann constant (J/s.Math.m.sup.2K.sup.4). T.sub.f is a furnace temperature (K), C.sub.p is a specific heat (J/kg.Math.K), is a specific gravity (kg/m.sup.3), D is a sheet thickness (mm), and L.sub.s is a sheet threading speed (m/s).

[0025] In the exit side of the heating furnace 2 (point A in FIG. 1), as described above, it is necessary that the temperature of the steel sheet (sheet temperature) is precisely heated to the soaking temperature as the target heating temperature. However, the conditions input to the host computer are not always constant and vary from hour to hour. Especially, in a heating furnace where the burner used for heating is not a radiant tube type but a direct type, a change of the general heat transfer coefficient .sub.CG with a lapse of time is large. FIG. 2 shows an example of actual measurement results of the general heat transfer coefficient .sub.CG during the period from the start-up of the furnace to the elapse of 24 hours when a hot-rolled steel sheet having a sheet width of 1052 to 1062 mm is subjected to a hot-band annealing at 1000 C. in a continuous annealing facility provided with a heating furnace using a direct-type burner. In such a continuous annealing facility where the general heat transfer coefficient .sub.CG largely varies, it is difficult to set the furnace temperature of the heating furnace with high precision, and hence it is impossible to control the sheet temperature at the exit side of the heating furnace (point A) to the given target soaking temperature.

[0026] In order to solve the above problem, as shown in FIG. 3, the furnace temperature is adjusted by arranging a sheet temperature measuring gauge 4 at the point A of the exit side of the heating furnace to measure the sheet temperature at the exit side of the heating furnace, feedbacking the measurement result to a furnace temperature control system to control flow rates of fuel and air supplied to the heating furnace so as to render the sheet temperature at the point A of the exit side of the heating furnace into a soaking temperature as a target heating temperature. In FIG. 3, an actual measurement value PV of the sheet temperature at the point A of the exit side of the heating furnace measured by the sheet temperature measuring gauge 4 and the soaking temperature SV previously input as a target heating temperature are compared, and the setting temperature of the heating furnace is corrected in accordance with the difference between both values.

[0027] By adopting the control method of the steel sheet temperature shown in FIG. 3, the sheet temperature at the exit side of the heating furnace can be controlled to the soaking temperature as the target heating temperature with a variation range of C. However, there are problems as follows. [0028] (1) It is difficult to control the furnace temperature with high precision, because the heat capacity of the heating furnace is very large and the change of the furnace temperature is slow by the feedback control mentioned above even when the gain is increased.

[0029] (2) The soaking temperature is desirable to be higher in order to improve the product properties, but the excessively high sheet temperature adversely affects the product properties. In the latter case, it is necessary to avoid such overheating that exceeds the target soaking temperature toward a plus side. Also, the heating exceeding the target soaking temperature is unfavorable from a viewpoint of heat energy.

[0030] In order to deal with the above problems, as shown in FIG. 4, the invention proposes a method of heating a steel sheet where a semi-soaking furnace 5 is disposed between a heating furnace 2 and a soaking furnace 3; the steel sheet is heated in the heating furnace 2 so that the sheet temperature at the exit side of the heating furnace reaches (a soaking temperature-T); the furnace temperature is set to the soaking temperature as a target heating temperature in the semi-soaking furnace 5; and the steel sheet is heated so as to reach the soaking temperature at a position before the exit side of the semi-soaking furnace 5, i.e. at a position in the semi-soaking furnace 5 (point B shown in FIG. 4).

[0031] Here, T is necessary to be a value of not less than , wherein ( C.) is the variation range with respect to the average value of the steel sheet temperature at the exit side of the heating furnace when the furnace temperature is controlled by feedback control based on the sheet temperature measured at the exit side of the heating furnace (point A in FIG. 4). Here, a is defined as a value of 3 times of a standard deviation a of the sheet temperature at the exit side of the heating furnace. When T is less than C., there is a possibility that the steel sheet temperature may partially exceed the soaking temperature as a target heating temperature at the exit side of the heating furnace, when the steel sheet temperature unexpectedly increases in the feedback control of the furnace temperature in the heating furnace.

[0032] When the furnace temperature of the semi-soaking furnace is set to the soaking temperature as the target heating temperature, it is necessary that 2T is not more than , i.e., T is not more than of , wherein a temperature increasing quantity of the steel sheet capable of being heated in the semi-soaking furnace, i.e. a heating capacity of the steel sheet in the semi-soaking furnace is ( C.). When T is more than /2, there is a possibility that steel sheet cannot be partially heated to the soaking temperature as the target in the semi-soaking furnace, when the steel sheet temperature unexpectedly decreases in the feedback control of the furnace temperature in the heating furnace. Moreover, T is preferably not more than 0.4 of , more preferably not more than 0.3 of . The steel sheet heating capacity in the semi-soaking furnace can be determined by the above heat transfer model used when the furnace temperature is set for the heating furnace.

[0033] In the heating method according to the invention, the steel sheet can be heated to the target soaking temperature without being overheated, at a position before it arrives at the exit side of the semi-soaking furnace, and also heated uniformly in the sheet width direction. When T is too small, even though it satisfies the above conditions, the sheet temperature reaches the target soaking temperature in the first half portion of the semi-soaking furnace to substantially bring about the prolongation of the soaking time. Therefore, when the acceptable range to the soaking time is severe, T is preferably set so that the steel temperature can reach the soaking temperature at a position closest to the exit side of the semi-soaking furnace as much as possible. Concretely, although it depends on the length of the semi-soaking furnace, T is preferably set so that the steel sheet temperature can reach the soaking temperature in the latter half range of the semi-soaking furnace, more preferably in the latter range.

[0034] Also, the steel sheet heating capacity of the semi-soaking furnace according to the invention is largely dependent on supply capacities of fuel and air supplied to the direct burner in the semi-soaking furnace, particularly a supply capacity (supply flow rate) of the fuel, and also has an influence on the setting value of T. In the heating method of the steel sheet according to the invention, therefore, it is preferable that T is set to a large value when the actual value of the flow rate of the fuel supplied to the direct burner in the semi-soaking furnace reaches the lower limit of the supply capacity (the fuel supply capacity is sufficient), while T is set to a small value when it reaches the upper limit of the supply capacity (the fuel supply capacity is insufficient).

[0035] From the viewpoint that the steel sheet is stably heated to the target soaking temperature in the semi-soaking furnace, the upper limit of T is preferably set according to the steel sheet heating capacity within the range from the lower limit of supply capacity1.2 to the upper limit of supply capacity 0.8 of the flow rate of the fuel supplied to the direct burner in the semi-soaking furnace. More preferably, it is the range from the lower limit of supply capacity 1.3 to the upper limit of supply capacity0.7.

[0036] A sheet temperature measuring gauge 6 is disposed at the exit side of the semi-soaking furnace shown in FIG. 4 (point C shown in FIG. 4). The sheet temperature measuring gauge 6 measures the sheet temperature at the exit side of the semi-soaking furnace, and is not used in the feedback control of the furnace temperature in the semi-soaking furnace. It may be used in the feedback control as a matter of course. Also, it is preferable that the sheet temperature measuring gauge 6 at the point C can measure sheet temperatures in at least three points of widthwise central portion and both widthwise end portions of the steel sheet to calculate the temperature difference in the sheet width direction of the steel sheet.

EXAMPLES

[0037] A hot-rolled steel sheet having a sheet thickness of 2.0 mm and a sheet width of 1100 mm is subjected to a heat treatment at a soaking temperature of 1000 C. in a continuous annealing facility, shown in FIG. 4, that is comprised of a direct heating furnace, a soaking furnace, and a cooling furnace and disposed with a direct semi-soaking furnace having a function according to the invention between the heating furnace and the soaking furnace. Moreover, the semi-soaking furnace is constructed by applying the function as the semi-soaking furnace according to the invention to the last half portion of the conventional heating furnace separated from the first half portion thereof, and is possible to be used as the conventional heating furnace when the semi-soaking function is necessary.

[0038] In this case, the heat treatment is conducted under two conditions when the function of the invention is developed by operating the semi-soaking furnace, that is, when the furnace temperature is set to the soaking temperature and the steel sheet temperature at the exit side of the heating furnace is set to (soaking temperature-T) to control T to an adequate range according to the invention (Invention Example), and when the semi-soaking furnace is stopped to operate and used in a part of the conventional heating furnace (Comparative Example), where sheet temperatures at three point of the widthwise central portion and both widthwise end portions of the steel sheet are continuously measured with a sheet temperature measuring gauge disposed in the exit side of the semi-soaking furnace (sheet temperature measuring gauge 6 shown in FIG. 4).

[0039] FIG. 5 shows a change of a temperature in the widthwise central portion of the hot-rolled steel sheet actually measured at the exit side of the semi-soaking furnace with a lapse of time, comparing the case with and without the operation of the semi-soaking furnace. Moreover, the temperature in the vertical axis of FIG. 5 is a temperature when an average value in Invention Examples is 0 C. As seen from this figure, the changing quantity of the temperature in the longitudinal direction of the steel sheet is reduced by not more than , from 3: 10.3 C. to 4.3 C., by disposing the semi-soaking furnace (wherein is a standard deviation). In the conventional technique, the value of T at the exit side of the heating furnace is set to a larger value, with concern over overheating of the steel sheet. In Invention Example, there is found to be no concern in this regard as a result of the above, so that the value of T can be made small, which enables the steel sheet to be heated to the soaking temperature promptly.

[0040] FIG. 6 shows a temperature difference in the sheet width direction of the steel sheet (difference between the highest temperature and the lowest temperature in the sheet width direction) in comparison between Invention Example and Comparative Example, in addition to the changing quantity of the temperature in the longitudinal direction of the steel sheet shown in FIG. 5. As seen from this figure, the temperature difference in the sheet width direction can be reduced by not more than , from 9.2 C. to 4.0 C., by adopting the semi-soaking furnace according to the invention.

INDUSTRIAL APPLICABILITY

[0041] Moreover, the above description of the invention is explained on the premise that the semi-soaking furnace is a direct type. The semi-soaking furnace according to the invention is not limited to the direct type, and may be a radiant tube type from a viewpoint of increasing an accuracy in the control of the sheet temperature.

REFERENCE SIGNS LIST

[0042] 1: steel sheet (steel strip)

[0043] 2: heating furnace

[0044] 3: soaking furnace

[0045] 4: sheet temperature measuring gauge

[0046] 5: semi-soaking furnace

[0047] 6: sheet temperature measuring gauge