CONTINUOUS STRIP STEEL PROCESSING LINE HAVING DIRECT FIRING FURNACE

Abstract: This furnace is provided with a plurality of reducing flame burners, which are capable of forming a non-­ equilibrium region, i.e. a reducing region in a flame, so as to heat the strip steel in a non-oxidizing or reducing condition. These burners are arranged in predetermined positions with respect to the strip steel in the direction of the processing line with pitches which are determined relative to the inner dia­ meter of the burners. In a continuous processing line having a direct firing furnace and a successive atmospheric control furnace, the direct firing furnace can be constructed as men­ tioned above, and a seal chamber having various means for preventing the intrusion of the outside air can be provided on the outlet side of the atmospheric control furnace. In a con­ tinuous processing line having a preheating furnace, a direct firing furnace and an indirect heating furnace in succession, the direct firing furnace can be constructed as mentioned above, intermediate chambers for preventing the travelling of the furnace gas between the adjacent furnaces, and an after-burning chamber for completely burning a combustion exhaust gas which is supplied to the preheating furnace.

Description: This invention relates to a continuous treating line for steel strips having a heating furnace by directly flaming where the heating is available without causing oxidation or with reduction. Known heating manners in a continuous treating line for a steel strip are a direct flame heating system and an indirect heating system using radiant tubes. The former is more excellent in heating efficiency in comparison with the latter, and since it may burn out a cold rolling oil a cleaning facility may be omited, it has been used widely in a hot-dip zinc plating line, a continuous annealing line for electromagnetic steel plates, and others. However, in the conventional direct flame heating system, oxidation of the steel band is remarkable, and there arises a big problem causing roll pickup thereby . Such conventional ones, so-called direct flame heating system without causing oxidation have been proposed for continuous heat treating facilities in Japanese Patent Publication No. 44,133/83 and Japanese Patent Publication No. 29,651/84. These systems lower air ratios (less than 1.4 → 0.6) in combustion control zones in response to increasing strip temperatures (Max. 900°), and carry out the heating while controlling oxidation of the steel strip. These systems are called non-oxidation systems, but actually they are slight oxidation systems. Since oxidizing gases such as C₂ or H₂O are much contained in gases generated in combustion, thickness of oxidized film after heating comes up to from less than 50Å to 500-1000Å. Therefore, when such a system is applied to the continuous annealing facility, it is inevitable that hydrogen in the atmosphere is made high in concentration (about 20%) in a soaking zone following a heating zone, so as to reduce the oxidized film, otherwise the direct flame heating furnace is, at its exit, furnished with a treating zone serving as a forcibly reducing zone, so as to reduce the oxidized film with hydrogen of high concentration (more than about 50%). Continuous annealing facilities recently developed have become large scaled aiming at decreasing production costs. In these facilities, if the above mentioned direct flame heating zone were built in 1 PASS (a steel strip runs once within the heating zone), a furnace would be very tall, and there would be the problem that the steel strip flutters as running or controlling of the furnace is difficult. Therefore, the heating zone could not be but built in 2 PASS or more. However, in such direct flame heating zone of plural PASSes, the roll within the heating zone is caused with roll pickup due to the oxidized film and the surface appearance of the steel strip is remarkably spoiled. For avoiding the roll pickup, complicated instruments should be required, for example, an isolation chamber filled with a pro­ tecting atmosphere for housing the roll within the furnace, as disclosed in Japanese Patent Application Laid Open No. 53-54100 (1978). Also in the roll protecting system, it is very difficult to exactly provide a sealing between the roll isolation chamber and the heating furnace at high temperature. The protecting atmospheric gas must be much supplied to fully charge the protecting atmosphere in the isolation chamber. As a heating treatment in the continuous hot-dip zinc plating line, so-called Senzimir system is known. In the continuously annealing cycle, this system heats the steel by directly flaming it up to about 400 to 450°C and subsequently indirectly heats it up to about 750°C in a forcibly reducing atmosphere. But this system has difficulties as follows. (1) indirect heating time is long, and heating efficiency is bad, (2) forcibly reducing atmosphere, i.e. (H₂ + N₂) having high H₂ concentration is required, and a problem arises about safety, and (3) when the soaking is performed, the length of the furnace is made large. In view of Senzimir system, another system has been deve­ loped and is now used widely, which carries out a slight oxidation heating in so-called NOF furnace an subsequently carries out reduction by indirect heating in a following weak reduction furnace. But this system also has the following difficulties. (1) indirect heating time is long, (2) in the weak oxidation (weak reduction process) a surface activity of the steel strip is not fully obtained which is required for plating adhesion, and (3) the length of the furnace is made large. In addition, the conventional continuous annealing line has further problems as follows. 1st problem: In the continuous heat treating furnace installed with the direct flame heating furnace, the steel strip is heated up to 500 to 800°C without causing oxidation or with reduction, and subjected to the treatments of heating, soaking and rapid cooling, and it is cooled down to less than about 150°, where oxidation in the atmosphere is no problem, and left in the air. The furnace is maintained therein in positive pressure of about + 5 to 15 mmWC so as to prevent the invasion of the atmosphere and is charged with the atmospheric gas (3 to 20% H₂ and the rest being N₂) for reduction and prevention of oxidation. For a sealing mechanism which leads out the heat-treated steel strip into the air, a seal roll system and a water envelop system are taken into consideration. But these systems have difficulties as follows. (1 ) Seal roll system This System is provided with a pair of seal rolls at an exit of the steel band, but spaces are inevitably formed between upper and lower rolls at both sides of the steel strip and between the furnace wall and the rolls. Therefore, a perfect seal could not be promised. Ordinarily, a bit of atmospheric gas is blown out from said spaces to prevent the invasion of the air. However, if a negative pressure were caused in the furnace due to changes in combustion within the heating furnace, the air would enter thereinto. (2) Water envelop system This system seals the exit of the atmospheric furnace by means of a water enveloping tank, and passes the steel strip therethrough. But steam will enter the furnace as the air invades. 2nd problem: In the continuous treating line installed with the indirect heating furnace following the direct flame heating furnace, the atmospheric gas of the indirect heating furnace flows into the direct flame heating furnace, and further a mixture of the combustion exhaust gas of said heating furnace and the atmospheric gas flows into the preheating furnace, and-­ finally streams outside. In this case, the pressure within the furnace is extremely fluctuated by the mutual interferences in the respective zones due to changings in flows therein and this phenominon could not be dealt with. Further, if the combustion exhaust gas containing non-burnt contents is introduced into the preheating furnace, the temperature of the exhaust gas becomes low at the exit thereof, and perfect combustion of the non-burnt content is difficult. The present invention is to remove the above mentioned defects involved with the conventional continuous treating line, and provide this kind of an improved continuously treating line. It is an object of the invention to provide a continuous treating line which may heat the steel strip without causing oxidation or with causing reduction in a direct flame heating furnace installed therein. It is another object of the invention to provide a continu­ ous treating line which may carry out the heating and the soaking on the steel strip at high temperatures without causing oxidation or with causing reduction. It is another object of the invention to provide a continu­ ous treating line which makes a non-oxidizing condition at least when the steel strip gets out of the direct flame heating furnace or when the steel strip contacts the roll within the furnace so that the roll pickup may be prevented. It is another object of the invention to provide a continu­ ous treating line which shortens the time of the indirectly heating and the length of the whole furnace and by which a surfce activity of the steel strip required for plating adhesion may be sufficient. It is another object of the invention to provide a continu­ ous treating line comprising the direct flame heating furnace and a subsequent atmosphere furnace, wherein the air is exactly prevented from invasion thereinto. It is another object of the invention to provide a continu­ ous treating line comprising in order a preheating furnace, the direct flame heating furnace and the indirect heating furnace, wherein the gas is checked to move to another furnace, so that the mutual interference due to fluctuation of the furnace gas is prevented. It is another object of the invention to provide a continu­ ous treating line comprising in order the preheating furnace, the direct flame heating furnace and the indirect heating furnace, wherein the combustion exhaust gas is utilized for efficiently performing the preheating of the steel strip. The essential features of the invention are recited in claim 1. The direct flame heating furnace installed in the continu­ ous treating line has a plurality of heating burners of reduction type. These burners may form a range having products in the intermediate combustion and not having free oxygen in the flame, that is, non-equilibrium range of the air and the fuel. In the invention, the heating burners of the reduction type are so arranged that the flame collides against the steel strip on its surface almost perpendicularly within said non-equilibium range, and said plural burners are positioned with spaces where (inner diameter of burner)/(pitch of burner) is more than 0.3. The present invention is applicable to the continuously treating line of every embodiment having the direct flame heating furnace. In the direct flame heating furnace having the heating burner of the reduction type, temperatures are about 900°C where the steel strip can be heated without causing oxidation and with causing reduction. If the steel strip is heated at higher temper­ tures, an indirect heating furnace is installed following the direct flame heating furnace. If the treating line is a continuous annealing line, there are embodiments as follows : (1) a line, which is provided, in order, with the direct flame heating furnace, the indirect heating furnace and a cooling furnace where cooling is mainly carried out with rolls, and is provided with a temper rolling machine at an exit of a finally treated steel strip; (2) a line, which is provided with a preheating furnace at an upper side of the direct flame heating furnace of the above line; (3) a line which is provided with a cleaning facility at an upper side of the preheating furnace of the above line (2); (4) a line, which is provided, in order, with the direct flame heating furnace, the indirect heating furnace and a cooling furnace where cooling is mainly carried out with liquid and with a temper rolling at an exit of a finally treated steel strip; (5) a line, which is provided with a preheating furnace at an upper side of the direct flame heating furnace of the above line (4); and (6) a line which is provided with a cleaning facility at an upper side of the preheating furnace of the above line (5). Passing through these lines, the steel strip may be continuously annealed efficiently without problems arising about oxidation. If the treating line is a continuous hot-dip zinc plating line, there are a line which has, in order, the direct flaming oxidizing furnace _ the direct flame reducing furnace (direct flame heating furnace having a heating burner of the reduction type) _ a soaking furnace of indirect heating system _ a hot-dip zinc plating device, or a line which has the direct flame reducing furnace and a subsequent hot-dip zinc plating device, not having said soaking furnace. If the treating line has the direct flame heating furnace and a following atmosphere furnace, it may be provided with a sealing chamber at an upper side of an exit sealing roll of the atmosphere furnace and further it may be provided with measures for detecting the pressure of the atmosphere furnace and measures for blowing the sealing gas into the sealing chamber when the pressure is lowered, thereby to prevent the invasion of the air into the atmosphere. If the treating line has in order the preheating furnace, the direct flame heating furnace and the indirect heating furnace, it may be provided with intermediate chambers between said furnaces, and further it may be, if required, provided with an after-burning chamber between the direct flame heating furnace and the preheating furnace, said after burning chamber effecting perfect combustion on the combustion exhaust gas within the direct flame furnace for supplying it to the preheating furnace, thereby to prevent the furnace gas from moving into the other furnace, and check the mutual interference between the furnaces due to the fluctuation of the gas pressure within the furnace, and further utilize the combustion exhaust gas for carrying out the preheating of the steel strip efficiently. In the continuous treating lines of the above mentioned various embodiments, the heating burners of the reduction type may be provided all over the available heating range of the direct flame heating furnace. However, said burner has a smaller capacity than generally used burners of the non-reducing type (diffusion type). If these burners are installed over the full available length of the furnace, a lot of burners should be closely disposed, otherwise required heat capacity is not kept. It is reasonable to dispose said burners only where necessary and in enough ranges, and following structures may be adopted. A first adaptable embodiment is that the reducing burners are placed such that the steel strip is in the non-oxidizing condition when the steel strip gets out of at least the direct flame heating furnace. Its content is as follows: In an available length of the furnace, the burner of the reduc­ ing type is disposed at said pitch (inner diameter of burner /pitch of burner ≧ 0.3) in the length of the furnace exit of more than γ% which is obtained by the following equation, while the burner of the non-reducing type is disposed in a remaining available length of the burner. Herein, T IN : Temperature of the steel strip at an entrance of the direct flame heating furnace (°K) T OUT : Temperature of the steel strip at an exit of the direct flame heating furnace (°K) A (T) : Reducing rate of the steel strip (A/sec) [= 127000 e -(6433/T) ] T * : Temperature (°K) of the steel strip at boundary between position of the reducing burner and position of the non-reducing burner B (T) : Oxidizing rate of the steel strip (A/sec) and position of the reducing burner 1 [= 69200 e -(6126/T) ] F (T) : Heating rate (°K/sec) [= dT/dt] A second adaptable embodiment is that the reducing burners are placed such that the steel strip is under the non-oxidizing con­ dition at least when it contacts the roll in the furnace, thereby avoiding the roll pickup and sending the steel strip under the non­ oxidizing condition from the direct flame heating furnace. Its contents is that the reducing burners are disposed at the region of the PASS-exit including the heating region just before at least the passing rolls at the exit of the direct flame heating furnace. In the invention, the reducing burners are employed in a determined condition which may form non-equilibrium range of the air and the fuel in the flame, where the products in the intermediate combustion stay and free oxygen does not stay. A region which has finished combustion reaction and contains CO₂, H₂O, N₂, H₂, CO and others, that is a semi-equilibrium range, is oxidizing, and on the other hand said non-equilibrium range containing intermediate ion, radical and others shows reduction, and such a burner renders the flame to collide against the steel strip almost perpendicularly in the non-­ equilibrium range, so that the steel strip can be heated with­ out oxidation. Figs. 3 and 4 show one example of said heating burner of the reduction type, where a plurality of combustion air jetting outlets 2 are formed with spaces in an inner wall 6 of a cylindrical burner tile 1 in circumferential direction there­ of, and a plurality of fuel gas jetting outlets 3 are formed centrally of the burner, and in addition the combustion air outlet 2 and the fuel gas outlet 3 are composed as follows _ (a) the combustion air outlet 2 is formed such that an air jetting direction has an angle ϑ1 of not more than 60° with respect to a tangent of an inner circumference of the burner tile; (b) a distance N in an axial direction of the burner between the combustion air outlet 2 and the fuel gas outlet 3 is determined from _ 0.1 to + 0.4D (D: inner diameter of burn­ er), wherein (-) is valid when the fuel gas outlet is positionned at the side of the exit of the burner tile or behind the combustion air outlet and (+) is valid in the contrary case ; and (c) a distance L from the combustion air outlet 2 to the exit of the burner tile is determined from 0.6 to 3D (D: same). The thus composed burner forms non-equilibrium range of the air and the fuel in a determined scope in the flame by controlling the air ratio to not more than 1.0. That is, the heating burner may rapidly provide combustion by swirling flow of the air from the air outlet 2 and the fuel gas from the center of the burner, and form a range not containing non-­ reacting free oxygen, i.e., non-eqilibrium range of the air and the fuel stably and widely since the flame contains many products in the intermediate combustion over a determined scope outside of the burner exit. Fig. 5 shows one example of the non-equilibrium range of the air and the fuel in the flame to be formed by the burner, measured with an ion detecting probe, where a high value of electric current implies that an ion strength is large and said range contains many products in the intermediate combus­ tion. According to this fact, the non-equilibrium range is formed over the determined range outside of the burner exit, and outside of this outlet a semi-equilibrium range is formed containing CO₂, H₂O, N₂ and others. Fig. 6 shows reduction heating characteristics of the burner, that is, limit temperatures up to which a steel strip may be heated without causing oxidation (limit temperature for thin plate or ordinary steel). The steel strip may be heated up to about 900°C in a range between 0.85 and 0.95 of the air ratio without causing oxidation. Besides the heating burner the present invention may use a so-called radiant cup burn­ er as a reducing burner. The radiant cup burner rapidly burns a mixture of the air and the fuel gas having been mixed in advance in a hemi-spherical cup of the burner tile for providing rapid combustion reaction so as to increase temperature of the inner surface of the burner tile, and heats the strip by the radiant heat conduction from said inner surface. This burner has a characteristic to bring about the flow flux of high temperatures in a range of the high temperature of the heat material. If the fuel gas is burnt at the air ratio of not more than 1.0, it is possible to form the non-equilibrium range. This radiant burner depends upon a pre-mixture of the combustion air and the fuel gas, and so the combustion air can not be preheated and the heating without oxidation is limited to the temperature of 750°C, and if heating is required at higher temperatures, this burner is not applicable. In this point, the burner as shown in Fig. 3 may utilize the preheated air, and it is possible to heat without oxidation up to about 900°C. Further the temperature of the flame is increased by this utilization of the preheated air, and the reducing action by the products in the intermediate reaction can be effective­ ly increased in comparison with the radiant burner. In the invention, the above-mentioned heating burners of the reduction type are arranged in a line direction with spaces of (inner diameter of burner)/(pitch of burner) of more than 3. According to the inventors' studies, it was found that if the burners were positioned with large spaces in the line direc­ tion, the steel strip was oxidized by the combustion gas (hemi-­ equilibrium gas) staying between the burners. The inventors investigated conditions for maintaining the steel strip as a whole reduced, which is reduced by the flame and oxidized by the combustion gas in relation to the inner diameter of the heat­ ing burner and the pitch thereof in the line direction. It was found that the reduction rate A (T) of the steel strip facing the non-equilibrium range and the oxidation rate B (T) thereof facing the oxidizing combustion as between the heating burners were obtained by the following equations. Each of the rates is applicable to the combustion gas of calorific value of 2000 Kcal/Nm³. A (T) = 127000 e -(6433/T) (Å/sec) B (T) = 69200 e -(6126/T) (Å/sec) Herein, T: Temperature of steel band (°K) Assuming that the inner diameter is d, the pitch of the heating burner is P and γ = d/P. For the steel strip to be heated by the burners to maintain the reduced condition, a below mentioned formula must be established. Herein, A (T) and B (T) are functions of the temperature. According to said formula, B (T)/(A (T) + B(T)) is almost 0.3 within the temperature range of 0 to 1000°C. Figs. 1 and 2 show the disposing arrangements of the reducing burners (a), and Fig. 1 is a zigzag arrangement and Fig. 2 is a parallel arrangement. In each of the two arrangements, the burners (a) are placed so that the ratio d/P of the inner diameter d and the pitch p in the line direction is more than 0.3. In the conventional burners used in, e.g., NOF, the non-­ equilibrium range is not formed distinctly from other ranges. Therefore, if an observable flame directly contacts the steel strip, it is extremely oxidized, and generally the burners are positioned to make the flame parallel to the width of the steel strip so that the flame does not directly contact it. On the other hand, the heating burners of reduction type to be employed in the invention are installed for heating the steel strip by the non-equilibrium range to be formed in the length of the flame. For this purpose, the burners are positioned so that the flame collides with the steel strip almost perpendicula­ ly in the non-equilibrium range. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are explanatory views showing arrangements of the heating burners in this invention; Figs. 3 and 4 are one example of the heating burner to be used in the direct flame heating furnace in the continuous treating line of the invention, and Fig. 3 is a vertical cross-­ sectional view thereof and Fig. 4 is a cross-sectional view seen from IV-IV line; Fig. 5 is a graph showing one example of measuring non-­ equilibrium range forming scope within the heating burner of Figs. 3 and 4; Fig. 6 is a graph showing reduction heating characteristics of the heating burner; Fig. 7 is an explanatory view showing one embodiment of the invention; Fig. 8 is an explanatory view showing another embodiment of the invention; Fig. 9 is a graph showing transitions of actions of causing oxidizing film and temperatures of the steel strip; Fig. 10 is a cross sectional view showing another embodiment of the continuously treating line of the invention; Fig. 11 is a graph showing transitions of actions of causing oxidizing film and temperatures of the steel strip; Figs. 12 to 14 are respective explanatory views showing embodiments of the continuous treating lines of the invention; Fig. 15 is an explanatory view showing in detail a cleaning facility within the line of Fig.14; Fig. 16 is a graph showing a relation between an air ratio of combustion exhaust gas and limit temperature of preheating without causing oxidation within the preheating furnace of Figs. 13 and 14; Figs. 17 to 19 are explanatory views showing embodiments of the continuous treating lines of the invention; Figs. 20 (A) and (B) are explanatory views of continuous hot-­ dip zinc plating lines of the invention; Fig. 21 is a graph comparing the plating line of Fig. 20(A) and foregoing plating lines with respect to curves of increasing temperatures of the respective annealing cycles; Fig. 22 is an explanatory view showing another embodiment of the present inventive line; Fig. 23 is an explanatory view showing a further embodiment of the invention; Fig. 24 is a partially enlarged view of an intermediate chamb­ er within the line of fig. 23; Figs. 25 to 29 are graphs showing characteristics of the heat­ ing burner shown in Figs. 3 and 4, wherein Fig. 25 is a relation between distance N from the burner exit, gas temperature, and O₂ concentration and ion strength, when a distance N in the burner axial direction between a fuel gas jetting outlet and an air jetting out­ let is _ 0.25, and Fig. 26 is a relation between the distance N in the burner axial direction from the fuel gas jetting outlet to the air jetting outlet and free O₂ remaining distance L0 in the burner axial direction, and Fig. 27 is a relation between the distance L from the burner exit, gas temperature, O₂ concentration and ion strength, when the distance N is + 0.1D, and Fig. 28 is a relation between the fuel gas jetting outlet, the distance N of the air jetting outlet and temperature Tb of a rear wall of a burner tile, and Fig. 29 is a relation between the distance L from the air outlet to the burner exit and the distance L R till termination of the non-equilibrium range. MOST PREFERRED EMBODIMENTS FOR PRACTISING THE INVENTION Since the heating burners of the reduction type are placed in said arrangements all over the available heating scope of the heating furnace by directly flaming, the steel strip is always heated without causing oxidation but with causing reduction, whereby non-oxidation can be provided exactly. Fig. 7 shows one example thereof, where said burners (a) are placed in the whole available heating scope, i.e., the whole scope requiring the heating in a continuously heating furnace 8 of 1 PASS type, and it is seen that the heating burners (a) are positioned with spaces at both side in the length of the fur­ nace. Such heating burners of the reduction type can heat the steel strip all along in the non-oxidation condition in the whole heating scope of the direct flame heating furnace. However, this kind of burner has a smaller heat capacity than the ordinarily used burner of the non-reduction type (diffusion burner), and if these burners are placed over the whole heating scope, they should be close to the next ones for keeping the required heat capacity. Therefore, the present invention places said heating burners of the reduction type in required and enough ranges (exit side) only, and places the burners of non-reduction type in the remaining ranges (entrance side). The invention determines "non-oxidizing condition of the steel strip" at least when the steel strip gets out from the direct flame heating furnace. Then, the invention adopts the basic disposing embodiment where the reduction burners are disposed only at the deter­ mined scope on the exit side of the direct flame heating fur­ nace, and the conventional non-reduction burners are disposed at the remaining parts in the length thereof, i.e., the entrance side, whereby the non-reduction burners at the entrance compensate the lack of the heat capacity of the reduction burners at the exit, and the oxidized film deposited on the steel surface by the non-reduction burner is reduced by the reduction burner at the exit side, so that the steel strip leaving the direct flame heating furnace is transferred in the non-oxidation condition. That is to say, a plurality of heating burners is placed all over the available length of the furnace, and with respect to said available length, the burners positioned in the furnace length at the exit side of more than γ% which is obtained by a below-mentioned equation, are said reduction burners, and the burners positioned at the entrance side in the remaining length are said non-reduction burners. Herein, T IN : Temperature of the steel strip at an entrance of the direct flame heating furnace (°K) T OUT : Temperature of the steel strip at an exit of the direct flame heating furnace (°K) A (T) : Reducing rate of the steel strip (A/sec) [= 127000 e -(6433/T) ] T * : Temperature (°K) of the steel strip boundary between position of the reducing burner and position of the non-reducing burner B (T) : Oxidizing rate of the steel strip (A/sec) and position of the reducing burner 1 [= 69200 e -(6126/T) ] F (T) : Heating rate (°K/sec) [= dT/dt] The above-mentioned composition determines the positioning scope of the reduction burners in order to make the thickness of the oxidized layer 0 by utilizing the reduction rate A (T) and the oxidation rate B (T) in the non-equilibrium range (reduction burner) of the reducing range in the flame and the semi-equili­ brium range (non-reduction burner) of the oxidizing range. The oxidation amount of the steel strip within the furnace is determined by the contacting time with said two ranges, and according to the inventors' studies, it was found that said reduction rate A (T) and oxidation rate B (T) were obtained by the below-mentioned equations. These rates are applicable to the combus­ tion gas of calorie 2000 Kcal/Nm³ and more. A (T) = 127000 e -(6433/T) (Å/sec) B (T) = 69200 e -(6126/T) (Å/sec) Herein, T: Temperature of the steel strip (°K) Therefore, assuming that T * is the temperature of the steel strip at the boundary between the positioning ranges of the reduction burners and the non-reduction burners in the length of the furnace, the thickness of the oxidized film at the exit of the furnace can be obtained by the below-mentioned formula. Herein, T IN : Temperature of the steel strip at an entrance of the direct flame heating furnace (°K) T OUT : Temperature of the steel strip at an exit of the direct flame heating furnace (°K) F (T) : heating rate (°K/sec) [= dT/dt] It is possible to obtain the temperatures T * of the steel strip at the boundary where the thickness of the oxidized film is 0 and since the steel strip is considered to increase the temper­ ature at a constant rate in the whole available heating scope in length of the furnace, necessary part γ% of the reducing range in the full available length of the furnace for making the oxidized film zero, that is, of the range of the furnace length where the reduction burners should be placed, can be obtained by a below-mentioned equation (2) from the temperature of the steel strip at said boundary. In accordance with γ calculated as above, the reduction burners are positioned in the length part at the furnace exit of the full available length, and the non-reduction burners are positioned in the remaining length. Fig. 8 shows one example specified as mentioned above, where 7 designates the preheating furnace, 8 is the direct flame heating furnace, and (S) is the steel strip. The heating burners 8 to be disposed in the full avail­ able length of the furnace are divided into #1 to #6. For example, when the reduction heating range requires more than 24% of the full furnace length, the reduction burners are placed at #5 and #6 (range of about 30% of the full length), and the non-reduction burners are placed in the remaining #1 to #4. Fig. 9 shows one example of generation of the oxidized film and the transition of the temperature of the steel strip, and it is seen that the oxidized film generated in the range (sligh oxidation heating range) where non-reduction burners are positioned, is reduced to the oxidized thickness of the base sheet in the range (reduction heating range) where the reduction burners are positioned, and the steel strip is put to almost non-oxidation condition. Roll pickup due to the oxidation can be prevented if the steel strip is non-oxidized just before the passing rolls at the exit of PASS composing the direct flame heating furnace. The invention sets said conditions, aiming at "non-oxi­ dizing condition of the steel strip". In this case, the reduc­ tion burners are with said pitch ((inner diameter of burner)/­ (pitch of burner) ≧ 0.3) at the exit range of PASS including the heating range just before the passing rolls at the exit sides of PASSes of the direct flame heating furnace. In said composition, with respect to each of PASSes composing the direct flame heating furnace, the non-reduction burners of large heat capacity are positioned at the extrance side, whereby the calorie is fully kept, and the reduction burners which may reduce the oxidized film formed on the steel surface at the entrance heating range, are positioned, and the steel strip is sent to a next PASS under the non-oxidizing condition or a soaking zone. Fig. 10 shows one example thereof which is applied to the heating furnace of a 2 PASS type, where (I) is a first PASS, (II) is a second PASS, and 10a to 10d are passing rolls. In this composition, heating groups 11 having a plurality of said reduction burners (a) in the line direction are disposed at the heating ranges just before the passing rolls 10b and 10d at the exits of PASSes, while heating groups 12 by the non-reduction burners are disposed in the remaining range. Heating burner groups shown in Fig. 10 use burners of nozzle mix type and are placed so that a semi-equilibrium range thereof collides with the steel strip. In the present embodiment, shielding plates 13 are project­ ed for protecting the passing rolls 10b and 10c from direct radiation from the furnace at the exit of the first PASS (I) and at the entrance of the second PASS (II). In such a heating furnace, the steel strip (S) is oxidized to a certain extent by the heating burner groups 12 at the ent­ rances and the intermediate ranges of PASSes, but the oxidized film is reduced by the heating burners 11 just before the pass rolls 10b and 10d, and passes the rolls 10b, 10c, 10d under the non-oxidizing condition and to the following indirectly heating furnace. Fig. 11 shows one example of generation of the oxidized film and the transition of the temperature of the steel strip in the first PASS, and it is seen that the oxidized film generated in the range (slight oxidation heating range ) where non-reduct­ ion burners are positioned, is reduced to the oxidized thickness of the base sheet in the range (reduction heating range) where the reduction burnes are positioned, and the steel strip is sent to the second PASS under almost non-oxidation condition. Temperatures of heating the steel strip without causing oxi­ dation but with causing reduction are about 900°C, and if it is required to heat the steel band to higher temperatures, the in­ direct heating furnace 9 may be installed next to the furnace 8. The continuous treating lines invention according to the invention will be two basic embodiments (though not excluding others) of a con­ tinuous annealing line and a continuous hot-dip zinc plating line. An explanation will be made of the continuous annealing line. There are embodiments of the continuous annealing line as follows; (1) a line, which is provided, in order, with the direct flame heating furnace, the indirect heating furnace and a cooling furnace where cooling is mainly carried out by rolls, and which is provided with a temper rolling machine at an exit of a finally treated steel band; (2) a line, which is provided with a preheating furnace at an upper side of the direct flame heating furnace of the above line; (3) a line which is provided with a cleaning facility at an upper side of the preheating furnace of the above line (2); The steel strip is exposed to the reduction heating by the reduction burners in the furnace and sent to the indirect heating furnace under the non-oxidizing condition. The steel strip is heated and removed from a rolling oil in the heating furnace. In the subsequent indirect heating furnace, the steel strip is soaked in the reducing atmosphere. Since the steel strip is sent into this furnace under almost non-oxidation condition, the atmo­ sphere has sufficient weak reducibility to an extent keeping the non-oxidation condition (H₂: 3 to 10%). A following cooling furnace mainly cools the rolls, and at its rear side an overaging furnace is generally installed, and the steel strip gets out from the cooling furnace and to a temper rolling machine. In the indirect heating furnace (soaking zone), it is possible to control H₂ concentration low, and since the steel strip can be rapidly cooled to the overaging temperatures by cool­ ing the rolls, the re-heating is no longer necessary for the overaging treatment, so that the energy is largely saved in com­ parison with the conventional annealing furnace. Further, with the heating without oxidation and the cooling rolls, the oxida­ tion is avoided, and a pickling facility is not required at all. Since the invention adopts the direct flame heating system excellent in responsibility of thermal load and the water cooling roll system easy in controlling the cooling cycle, the heating cycle can be exactly changed in accordance with a material or a desired property, and the steel strip can be treated, irrespect­ ively of the furnace temperatures, sheet thickness or width. In the above line (2), the preheating furnace is installed in front of the heating furnace, and the steel strip is preheated by an exhaust gas introduced from said furnace and guided to the heating furnace. The heating time is short in the continuous anneal­ ing, and the operation is carried out at higher temperatures than the batch annealing. Especially, in the facility having the direct flame reducing furnace as this invention, the operation is carried out, aiming at the high speed annealing, and the heating temperatures are set higher. By the preheating, the load for heating in the heating furnace is decreased, thereby to make possible desired high temperature and high speed annealing. The oxidation on the steel surface by the preheating is reduced by said heating furnace, so that the preheating is carried out at the high temp­ eratures of 250 to 500°C and the rolling oil is burnt thereby. In the direct flame heating system, the heating rate is high so that the heating temperature (final temperature) is high and extra energy is required as much accordingly, but by preheating the gradient of rising temperature is lowered and the heating temperature is not increased more than required. In the above line (3), the cleaning facility is installed in front of the preheating furnace, which mainly removes iron powders attached to the steel surface. The surface of the steel strip having been cold-rolled is covered with rolling oils, iron powders (rolling dusts) and others. The rolling oil is burnt away as said above, but the iron powders are accumulated or circulated to ether with the atmospheric air in the furnace and kept between the roll and the steel strip and makes flaws on the steel surface. In this facility, the iron powders are removed by said cleaning instrument. In case the continuous annealing is performed on the steel strip of high Si, P, Mn, Ti, Cr, and since these elements cause oxidized films which are difficult to be reduced, the air ratio of the combustion gas is often lowered in order to lighten oxidation by the preheating and the direct flame heat­ ing (heating prior to reduction heating). Then, the burning off characteristic of the rolling oil on the steel surface is more or less lowered but said cleaning instrument compensates said lowering. Fig. 12 shows an embodiment corresponding to the above line (1), and there are installed the direct flame heating furnace 8, indirectly heating furnace 9, cooling furnace 14, overaging furnace 15 and a finally cooling furnace 16 in order from the entrance side, and at the exit of the furnace 16 the temper mill 18 is disposed. The indirect heating furnace 9 is provided with radiant tubes as conventionally. In this annealing line, the direct flame heating furnace 8 has the reducing function and the steel strip is transferred to the indirect heating furnace 9 under the non-oxidizing condition, and therefore this furnace 9 has enough of such an atmosphere to an extent that the steel is not oxidized, that is, the atmosphere of H₂: 3 to 10% preferably 4 to 6%. In the cooling furnace 14, a plurality of cooling rolls 19 (ordinarily: water cooling roll) are provided and a contact length between the cooling rolls 19 and the steel strip is changeable, so that the temperature may be controlled at cooling end. It is preferable to use chromium rolls of hard property for the rolls of the temper mill 18. Especially preferable rolls are seen in Japanese Patent Applications No. 41009/85 and No. 41011/85. The rolls therein hardly get pressing flaws by the steel edges, and the steel surface is exactly prevented from scratches by flaws of the rolls, and coarse printing to the steel strip may be kept, so that the continu­ ous annealing of cycle free in the width of the steel strip is possible. In the prior art, in order to prevent influences of the edge marks or roll flaws, steel strips to be treated are combined in that the steel widthes are narrower successively. By using the hard chromium rolls, restrictions as mentioned are released, and the steel strips can be combined, irrespect­ ively of the wide sizes. Fig. 13 shows an embodiment corresponding to the above line (2), and the preheating furnace 20 (2 PASS) is placed before the direct flame heating furnace 8, into which the combustion waste gas is introduced from the furnaces 8 or 9 so as to accelerate the preheating of the steel band (S). According to the inventors' investigations, it was found that the oxidation of the steel strip was regulated by the preheating temperature and the air ratio when the combustion exhaust as was generated, and the steel strip could be preheated with scarece­ ly causing oxidation by using, in response to the preheating temperature, the combustion waste gas which was different in the air ratio when the combustion took place, and concretely, if the combustion waste gas generated at the air ratio of more than 1.0 was used, when the steel strip was preheated as shown in Fig. 16 in the range of less than 280°C, and if the combus­ tion waste gas generated at the air ratio of less than 1.0 was used, when preheating more than 280°C, the steel strip could be preheated effectively with almost non-oxidation, irrespectively of the preheating temperatures. As seen above the preheating without causing oxidation is possible in the preheating furnace 20 by regulating the air ratio of the combustion gas. In the following furnace 8, since the oxidized film is reduced, oxidation to a certain extent is permitted in the preheating furnace 7, and as a result, the preheating allowance temperature can be increased by about 50°C as shown with a chain line A of Fig. 16, whereby it is possible to perform the preheating of about 400°C even at the air ratio of about 1.0, and burn off the rolling oil from the steel surface. Fig. 14 shows an embodiment corresponding to the above line (3), and the cleaning facility 22 is provided before the preheating furnace 20 via an entrance looper 21 in order to remove the iron powders. This facility 22 can have a simple structure, since it aims at removing iron powders or dusts. Fig. 15 shows one example of the cleaning facility, where 23 is an alkaline chamber, 24 are brush rolls, 25 are backup rolls, 26 are hot water spray nozzles, 27 is a hot water rinse chamber, and 28 is a dryer. By this facility, the iron dusts or powders are cleansed. Other examples of the continuously annealing line are, (4) a line, which is provided, in order, with the direct flame heating furnace, the indirect heating furnace and a cooling furnace where cooling is mainly carried out with liquid and wich is provided with a temper rolling machine at an exit of a finally treated steel strip; (5) a line, which is provided with a preheating furnace at an upper side of the direct flame heating furnace of the above line (4); and (6) a line which is provided with a cleaning facility at an upper side of the preheating furnace of the above line (5). Passing through these lines, the steel strip may be continuously annealed efficiently without problems arising about oxidation. In these compositions, the steel strip is subjected to the non-oxidation but reduction heating as in the above lines (1) to (3) and to the soaking with the reducing atmosphere in the indirectly heating furnace. Since the steel strip is sent to the indirectly heating furnace under almost non-oxidized condition, and freed from a new oxidized film caused by the liquid cooling at the intermediate pickling, the atmosphere therein has sufficient weak reducibility to an extent of maintaining the non-oxidation (H₂ : 2 to 5%). In the cooling furnace, rapid cooling is carried out with liquid, and the steel strip is cooled to the room temperature, or overaging or tempering temperatures by hot water. Thus the steel strip is freed from said oxidized film and subjected to the overaging or tempering treatment in the overaging furnace, and is treated by the temper rolling machine after the final cooling zone. Although said continuously annealing facility carries out the rapid cooling with the hot water or cold water after the heating and soaking, the steel strip of high surface qua­ lity may be produced by using the direct flame heating furnace which enables the non-oxidation heating. In other words, if the cooling after the heating and soaking is performed with the water, the oxidized film is inevitably formed on the steel surface. In the conventional combination of the direct flame heating furnace and the liquid cooling system, even if the reducing furnace is provided after the direct flame heating furnace, the oxidized film remains and a further oxidized film is formed by the liquid cooling. Such phenomena are remarkable in the materials of high Si, Mn, P, Cr or Ti which generate strong oxidized films. In this point, in the present continuous annealing line, the steel strip is sent, under non-oxidizing condition, from the direct flame heating furnace which enables reduction heat to the indirect heating furnace _ the cooling furnace, the intermediate pickling facility is sufficent only to remove the oxidized film created by the rapid cooling, and it is perfectly removed by the pickling. Further, this line is provided with the intermediate pickling facility before the overaging furnace in addition to the final pickling facility, thereby to increasing the removing effect of oxidized film. In the pickling, if strong acid is used, there arises a problem that Fe(OH)₂ harmful to the sur­ face treatment is generated. If the pickling is done at the final side, i.e., after the overaging furnace, Fe(OH)₂ stays on the steel surface as it is, and this causes various troubles to the modifying treatment of the steel band. In this regard, the instant line is provided with the pickling facility before the overaging furnace, and Fe(OH)₂ generated does not remain, since it is reduced by the reducing atmospheric gas within the subsequent overaging furnace, and therefore the pickling with a strong acid is substantially possible. Further, if the annealing is carried out at the high temperature by directly flaming, the steel strip is often attached with a bit of carbon on its surface, but this carbon could be removed by the inter­ mediate pickling. In line (5), the preheating is done in the preheating furnace as in line (2), and in line (6) the treatment is done in the cleaning facility as in line (3). Although the steel surface is oxidized by the preheating, and as the direct flame heating furnace is provided which can reduce this oxidation and the intermediate pickling chamber is provided, the preheating can be performed at the higher temperatures as 250 to 600°C in the preheating furnace. In the direct flame heating furnace, the zone of the reduction burners must always keep the combustion for perform­ ing the reduction heating on the steel surface, the thermal load of the heating furnace due to differences in thickness of the steel strip is controlled by extinguishing the burners of other zones, but if the preheating is carried out with an auxiliary combustion furnace, a delicate control of the thermal load may be effected in the heating of thin materials. Fig. 17 shows an embodiment corresponding to line (4), where seen from the entrance side there are disposed the direct flame heat furnace 8, indirect heating furnace 9, cool furnace 14, intermedi­ ate pickling 29, overage treatment 15 and final cool furnace 16. At the exit side of the final cooling furnace 16, a temper mill 18 is placed. The indirect heating furnace 9 has a radiant tube system as mentioned above, and the intermediate pickling is provided and therefore the soaking zone has sufficient the atmosphere to an extent not oxidizing the steel strip, H₂: 2 to 5%, preferably 3 to 4%. The steel strip (S) is rapidly cooled by immersion into the water in the cooling furnace 14. In the water a nozzle sprays to the steel strip to remove the steam film. The intermediate pickling facility 29 comprises a pickl­ ing chamber 30, rinse chamber 31 and dryer 32. For example, the pickling treatment is HC1 5% and 40 to 60°C × 1.5 sec, and the water rinse is 80°C water. In the overaging furnace 15, the steel strip (S) is subjected to the overaging treatment or the tempering treatment in the weak reducing atmosphere. It is preferable to use the same temper mill 18 as the one in line (1). Fig. 18 shows an embodiment corresponding to line (5) where the preheating furnace 20 is provided before the direct flame heating furnace 8, and this preheating furnace is as the one above in line (2). Fig. 19 shows an embodiment corresponding to line (6), where the cleaning facility 22 is provided before the preheat­ ing furnace 20 via the entrance looper 21, and this cleaning facility is as the one above in line (3). As the temper rolling machine in lines (1) to (6), a tension leveller may be used in addition to the temper mill. After the cooling facility, a plating apparatus may be employed as a zinc plating apparatus. If the present invention is applied to the continuous annealing line, a following heat cycle will be adopted. After heating in the direct heating furnace, the steel strip maintained at a determined temperature range for more than 5 seconds in the heating and soaking furnace of the indirect heating. Nuclei of crystal grains appear and grow after re­ crystalizing temperature is over in a last half of the heating zone, and said soaking time is the shortest time for crystal grains to grow up to determined grain diameter. The steel strip heated and soaked as mentioned if required, maintained till the determined temperature, and rapidly cooled at more than 40°C/sec. For improving the aging property of the product, solute C effected by the heating and soaking should be precipitated for a period of time as short as possible in the overaging furnace, following the rapid cooling. The above cooling rate is necessary for producing solute C in an oversaturated condition so as to realize said precipitation of short time. The faster the cooling rate, the higher the degree of oversaturation, and the overaging time is shorter. Thus, the minimum cooling speed is regulated. The steel strip having passed through a continuous heating treatment becomes a product, after the overaging treatment _ final cooling, if required. The below mentioned (1) to (7) show actual heating cycle of the continuous annealing line comprising the preheating fur­ nace _ direct flame heating furnace _ indirect heating fur­ nace _ gas jet cooling furnace _ roll cooling fur­ nace _ overaging furnace _ finally cooling furnace. (1) Preheating furnace: using the combustion exhaust gas of 1200 to 1400°C from the direct flame heating furnace for preheating the cooled steel strip to 250 to 330°C; (2) Direct flame heating furnace: reducing the preheated steel strip to 430 to 800°C by means of the direct flame heating burners; (3) Indirect heating furnace : since the heating of the direct flame heating furnace is limited to 900°C, and if heating of more than 900°C is required, such heating is made in this furnace. When the steel strip comes up to the upper limit, it is soaked for 5 to 120 seconds in the weak reducing atmosphere; (4) Gas jet cooling furnace: moderately cooling the steel strip in the soaking zone to temperature of starting rapid cooling (550 to 750°C); (5) Roll cooling furnace: contacting the steel strip to the water cooled rolls so as to rapidly cool it to 250 to 400°C at more than 40°C/sec for temp­ ing; (6) Overaging furnace: performing the overaging treatment by maintaining the steel strip at 400 to 150°C for more than 30 seconds; (7) Finally cooling furnace: cooling the overaged steel strip less than 150°C and leaving it in the air A further explanation will be made of another basic embodi­ ment of a continuous hot-dip zinc plating line. Fig. 20 (A) shows one example of the continous hot-dip zinc plating line, where 33 is a direct flame oxidizing fur­ nace, 34 is a direct reduction furnace, 35 is a soaking fur­ of indirect heating system, and a hot-dip zinc plating furnace is installed after the soaking furnace 35. The fur­ nace 34 is arranged with the above-mentioned reduction heat­ ing burners with the determined spaces. Fig. 20 (B) shows another example of the same, where the soaking furnace is not located, and the plating apparatus is placed after the direct flame reduction furnace 34. In this line, the steel strip (S) is perfectly heated and soaked in said furnaces 33 and 34. Therein, the steel strip (S) is at first subjected to the direct flame heating in the direct flame oxidizing furnace 33 and the oil is removed and is oxidized. Subsequently, the steel strip is forcibly subjected to the direct flame reduc­ tion heating in the reduction furnace 34 and is freed from the oxidized film generated on the steel surface. Thus, the forcible oxidation _ forcible reduction are effected with the oxidation in the direct flame oxidizing furnace. After said heating, in the line as shown in Fig. 20 (B), the steel strip is instantly immersed into a zinc pot. In the line of Fig. 20 (A), the steel strip is heated in the oxidation furnace 33 and the reduction furnace 34 up to a determined temperature, and sent to the soaking furnace 35 under the non-oxidizing condition. This soaking furnace 35 is the indirect heating system, but since the steel strip from the reduction furnace 34 is non-oxidized it does not require in principle the reducing gas and the atmosphere gas is sufficient with an inert gas. However, actually, due to leakage in the furnace, it is preferable that H₂ stays there and compensates it, to the extent of not more than 5%. Fig. 21 compares the line of Fig. 20 (A) and the convent­ ional lines (Senzimir and NOF) with respect to the curves of the rising temperatures of the annealing cycles. In Fig. 20 (A), the heating is made by directly flaming, and the heating efficiency is higher than the conventional ones, and heating comes up to the determined temperature with in a short period of time. As a result, the furnace length is shortened considerably. In addition to the basic structure of the above-mentioned direct flame heating furnace, the below-mentioned composi­ tions may be employed. (1) In a line comprising the direct flame heating furnace and an atmosphere furnace, the atmosphere furnace is provided with a seal chamber at an upper part of sealing rolls at an exit for preventing invasion of the air into the atmosphere furnace, and provided with a means for detecting pressure in said atmosphere furnace and a means for blowing the sealing gas into the sealing chamber when the furnace pressure is decreased. (2) In a line comprising the preheating furnace, the direct flame heating furnace and the indirect heating furnace, an intermediate chamber is provided between the furnaces for preventing the furnace gas from moving into the other furnace, and for preventing mutual interference between the furnaces due to changes in the furnace pressure. (3) In a line comprising preheating furnace, the direct flame heating furnace and the indirect heating furnace, an after-burning chamber is provided between the direct flame heating furnace and the preheating furnace, which perfectly burns the combustion exhaust gas within the heating furnace and supplies it into the preheating furnace, in addition to the object mentioned above (2), for effectively preheating the steel strip by utilizing the combustion exhaust gas. Fig. 22 shows one example of the above line (1). The line comprises, in order from the entrance, the preheating furnace 7, the direct flame heating furnace 8 and the atmosphere furnace 36. The atmosphere furnace 36 comprises the soaking and the cooling zones, and if required, an overaging zone. The atmosphere furnace 36 is provided, at its exit, with the seal rolls 37 and a seal chamber 38 is furnished above the rolls 37. The seal chamber 38 is provided with a sealing gas blowing device 39, and a pressure gauge 40 is provided for measuring the furnace pressure. The steel strip (S) passes through the preheating furnace 7, is heated up to 500 to 800°C in the non-oxidation or the reduction, heated to higher temperature in the atmosphere furnace 36, and subjected to the soaking, rapid cooling and overaging, and rapidly cooled less than about 150°C, and taken out from the furnace after the sealing rolls 37. In the direct flame heating furnace 8, the combustion gas at high temperature is supplied to the burners by the fuel gas 41 and the combustion air 42. The combustion gas directly collides with the steel strip to heat it to the deter­ mined temperature, and there after it becomes an exhaust gas 43 and preheats the steel strip to 200 to 450°C in the pre­ heating furnace 7 and is exhausted from a chimney via a furnace pressure control damper 4 and an exhaust fan 45. Between the heating furnace 8 and the atmosphere furnace 36, a passage is provided for preventing invasion of the com­ bustion exhaust as into the atmosphere furnace, but this passage should have a size large enough for passing the steel strip (100 mm × 2000 mm). The passage does not serve the gas sealing, and accordingly variances in pressure of the heating furnace 8 will be variances in pressure of the atmosphere furnace 36. Therefore, a furnace pressure gauge 48 is positioned at a position where the atmosphere gas 46 flows to the heating furnace 8 from the atmosphere furnace 36, and a furnace pressure control damper 44 controls the pressure therein to be + 5 to 15 mmWC. The furnace pressure can be maintained constant, but when the combustion in the direct flame heating furnace 8 is changed, e.g., when one of the various zones is extinguished, it is often too late to close the furnace pressure control damper 44, so that the pressure in the atmosphere furnace 36 is nega­ tive for a short period of time (around 5 to 10 seconds). In this case, the seal rolls 37 are prepared for a part of the steel strip (S) which gets out from the atmosphere furnace 36, but a large space is formed at said part and the air easily invades thereinto. In the invention, a sealing chamber 38 is provided at an upper part of the sealing rolls 37 in order to avoid the inva­ sion of the air. A distance between the sealing chamber 38 and the atmosphere furnace 36 is throttled, so that the gauge 40 detects the low pressure of the atmosphere 36 (e.g., lower 5 mmWC than a determined pressure), and in this interval the sealing gas is blown into the sealing chamber 38 from a blow­ ing device 39. Said sealing gas may be sole N₂ or an atmosph­ ere gas containing H₂ 3 to 20%. The blowing amount of the seal gas is sufficient with 300 to 600 Nm³/1 for 10 to 20 seconds, though depending upon the throttling size. Since it is necessary to continuously blow a bit of N or the atmosphere gas even in the normal time, an interruption valve 47 should be equipped with a bypath pipe of small dia­ meter, or another pipe should be connected to the sealing chamber. Fig. 23 shows an embodiment of the above lines (2) and (3) . This continuous line comprises, in order from the entrance side, the preheating furnace 7, direct flame heating furnace 8, indirect heating furnace 9 and gas jet cooling furnace 58. After the cooling furnace 58, there are installed a cooling furnace by roll cooling, an overaging furnace and exit facility. A first intermediate chamber 49a is positioned between the pre­ heating furnace 7 and the heating furnace 8, a second intermediate chamber 49b is positioned at an upper turning point of the heat furnace 8, and a third intermediate chamber 49c is positioned between the heat furnace 8 and and indirect heat furnace 9. A detailed explanation will be made of the structures of the intermediate chambers 49a, 49b, 49c, referring to Fig. 24. 49 designates the intermediate chamber, 50 shows rolls holding the steel strip (S), 51 are sealing plates with a space therebetween via the steel strip (S), 52 are sealing rolls, 53 are labyrinth seals via the steel strip (S). A distance between the seal rolls 52 may be approached till a few millimeters. The sealing rolls may depend upon an inner cooling or the water cooling. If the water cooling is not carried out, the rolls of heat resisting steel or ceramics will be used. The labyrinth seal 53 is made of heat resisting material, and protects the sealing roll 52 from the heat radiation from higher temperature in the furnace. The sealing plate 51 is used as a final seal and is not always required, but it is positioned just after the sealing roll 52 and it can be brought quite near to the steel strip (S). This is a large sealing effect. A distance between the sealing plate 51 and the labyrinth seal 53 is about 50 to 100 mm. In these sealings, the labyrinth seals 53 make rough sealing, and the seal rolls 52 make normal sealing, and the seal plates 51 make a further sealing. In Fig. 23, the temperature of the first intermediate chamber 49a is not so high but about 300°C, and a measure for protect­ ing the rolls is not especially necessary. The amosphere in the first intermediate chamber 49a may be a reducing gas (H₂ + N₂) or the combustion exhaust gas. Sealings can be provided to separate independent furnaces. In Fig. 23, the heating furnace is composed of two PASSes between which the intermediate chamber 49b is positioned. The reducing atmosphere (H₂ + N₂) is preferable in the second and third chambers 49b and 49c for protecting the rolls, and especially the third chamber 49c must contain the reducing atmosphere for pre­ venting the exhaust gas of the heating furnace from entering into the indirect heat furnace 9. Between the direct flame heating furnace 8 and the pre­ heating furnace 9, there is an after-burning chamber 54 which perfectly burns the combustion exhaust gas of the heating furnace 8 and supplies it to the preheating furnace 7. The temperature of the exhaust gas at the exit of the heating furnace 8 is 800 to 1200°C, which is higher than that of the spontaneous flaming of non-combustion, and the part of non-combustion can be easily burnt by only supplying the air in the after-burning chamber 54, whereby said part of non-­ combustion in the exhaust gas is not released into the air, and the temperature of the exhaust gas is increased to accelerate the preheating of the steel strip. The burning chamber 54 is divided at its exit into two routes to the side of the preheating furnace 7 and the exhaust side, and a suit­ able amount of the exhaust gas is led to the preheating furnace. When the atmosphere as of the third chamber 49c and the atmosphere gas of the indirect heating furnace 9 flow into the reduction heating range of the heating furnace 8, the reduc­ ing ability is lowered. The temperatures of these atmosphere gases are near the soaking temperatures of the steel strip (700 to 900°C) but lower than the temperature of the combustion gas (1400 to 1600°C), and if the atmosphere gas enters the reduc­ tion furnace, the gas temperature of the reduction heating range is lowered. Therefore, a second after-burning chamber 57 is provided, and the atmosphere gases of the third chamber 49c and the indirect heat furnace 9 are led to the second after-burning chamber 57 to solve this problem. Referring to Figs. 3 and 4, the structure of the heat­ ing burner shown therein will be explained. The reference numeral 59 designates the fuel gas nozzle projected from the inner wall 4 of the burner tile, and in the present embodiment, the fuel gas jetting outlets 3 are formed with spaces circumferential of the fuel gas nozzle 59. In this burner, the air outlet 2 is defined with an air supply angle O, because the combustion air is effected with a swirling flow within the burner tile, and by this swirling flow a negative pressure is formed at the inner side of the burner, and by this negative pressure the gas is recirculated and the combus­ tion is accelerated, whereby a stable non-equilibrium range of the air and the fuel is formed. The air supply angle O is 60° at the maximum, preferably 20 to 40°, for providing stable swirling of the air flow . When a distance N in the burner axial direction between the air outlet 2 and the fuel gas outlet 3 is (-) side, the gas temp­ erature is high and the product in the intermediate combustion is spread widely, but on the other hand, free O₂ (non-reacted O₂) is spread in the axial length. For exactly forming non-equilibrium range, it is necessary to minimize a free O₂ existing distance in the axial direction of the burner, and a limit therefor is _ 0.1D. Fig. 25 investigates relationship, when said distance N is _ 0.25D, between the distance in the burner axial direction from the burner exit, the gas temperature in the burner tile, O₂ concentration and ion strength. It is shown that when N is at (-) side, a remaining distance L₀ of the free O₂ existing in the axial direction is large. Fig. 26 shows relationship between said distance N and said free O₂ existing distance. It is shown that when N is larger in (-) side than - 0.1D, L₀ becomes rapidly large. Therefore, a limit in (-) side is - 0.1D. Fig. 27 shows relationship, when N is + 0.1D, between the dis­ tance in the burner axial direction from the burner exit, O₂ con­ centration, ion strength and gas temperature. In Figs. 26 and 27, when N is at (+) side, no problem is involved with O₂ concentration, and a suitable non-equilibrium range is formed at a spot where the distance from the burner exit is more than 0.5D. When N is at (+) side, the suitable non-equilibrium range is formed, but if exceeding + 0.4D, the air and the fuel are not fully mixed. In this regard, in the invention, the mixture of the air and the fuel gas is accelerated by jetting the fuel gas from the center of the nozzle during rapid rotation of the air, but if N is made exceedingly large, said acceleration is not fully obtained, and a stable non-equilibrium range could not be expected. So, N is + 0.4D at the maximum. Thus, the distance N is - 0.1D to 0.4D. When N becomes larger, the temperature of the inner end wall of the burner tile is increased. Fig. 28 shows the relationship between the distance N and the temperature Tb of the inner end wall of the burner tile. When N is + 0.25D, Tb is 1400°C, and in general, up to this temperature, the ordinary heat resisting material may be used. When N is + 0.4D, the inner end wall of the burner tile is increased to more than 1800°C, and then a high heat resistant material should be used for the burner tile. The distance L between the air outlet 2 and the exit 5 of the burner tile has a close relation to a scope of forming the non-­ equliribrium range. If L exceeds 3D, the non-equilibrium range is formed only at a part just after the exit of the burner tile. If L is less than 0.6D, the flame is shaped like a flower petal, and non-equilibrium range is not exactly formed centrally of the burner. Therefore, it is preferable to determine L in the scope of 0.6 to 3.0D. When a thin steel strip is subjected to a continuous heating, a distance between the burner tile exit 5 and the steel strip should be more than 100 mm, otherwise the steel strip contacts the burner. Therefore it is preferable to form non-equilibrium range in the flame in a scope as wide as possible, including the steel strip passing place at a determined distance from the burner exit. Fig. 29 shows the relation betwen the distance L and a distance L R from the burner exit to a termination of non-equilibrium range (end opposite to burner, e.g., "A" point of Fig. 27). If L exceeds 3D, the non-equilibrium range is formed only just after the exit of the burner tile. As L becomes smaller, non-equilibrium range is enlarged, but if L is at a range (X) of less than 0.6D, the flame is formed like a flower petal just after the burner tile, and the required range is not formed centrally of the burner. Thus, the distance L from the air outlet 2 to the exit 5 of the burner tile is 0.6D to 3.0D. In the above-mentioned structure of the burners, if the swirl­ ing flow of the air from the combustion air outlet 2 is too strong, the dispersion of the temperature is not uniform in a diameter direction of the burner, and as a result, it is often difficult to form the non-equilibrium range. In such a case, in order to moderate the air swirling and to uniformly disperse the temperature, the invention may adopt solely or in combination the following stru­ tures, that is, a structure, where a jetting direction of the fuel gas outlet 3 is not perpendicular with respect to a tangent line of the outer circumference of the fuel nozzle, and the fuel gas flow is made a swirl flow contrary to the air flow from the air outlet 2, that is, the fuel gas flow collides with the air swirling flow in a contrary direction, or a structure, where the fuel gas outlet 3 has a jetting direction made oblique with respect to the burner axial line, or a structure, where an air outlet 2 is given an oblique angle (twist angle) toward the burner opening with respect to the diameter direction of the burner tile. The following structures can be also adopted, a structure, where in order to enlarge the heating area by the burner, the diameter of the burner is expanded at an inner wall of the opening end, or a structure, where in order to make easy forming of the air outlet 2, a swirling flow passage of the combustion air is formed, following the circumferential direction of the burner in the tubular burner tile, and a plurality of the combustion air outlets are formed to make said swirling flow passage communicate with the inner side of the burner. The present invention may be applied to the continuous annealing line, the hot dip-Zn plating line and others.

Claims: 1. A continuous treating line for a steel strip having a heating furnace by directly flaming, wherein said heating furnace is provided with a plurality of heating burners of reduction type which may, in a flame, form non-equilibrium range of the air and a fuel with products in an intermediate combustion and without free oxygen, and said furnace is arranged with said burners such that the flame collides against the steel strip on its surface perpendicularly within said non-equilibrium range, and said arrangement of the burn­ ers is made with spaces where (inner diameter of burner)/­ (pitch of burner) in a line direction is more than 0.3. 2. A continuous treating line as claimed in claim 1 having said heating furnace by directly flaming and an indirect heating furnace subsequent thereto. 3. A continuous treating line as claimed in claim 1 having, in order, an oxidizing furnace by directly flaming, a reducing furnace by directly flaming, an indirect heating furnace and a hot-dip zinc plating apparatus. 4. A continuous treating line as claimed in claimed 1 having an oxidizing furnace by directly flaming, a reducing furnace by directly flaming and a hot-dip zinc plating apparatus. 5. A continuous treating line as claimed in claim 1 having said heat­ ing furnace by directly flaming and an atmosphere furnace, wherein said atmosphere furnace is provided with a seal chamber at an upper part of sealing rolls at an exit, and provided with a means for detecting pressure in said atmosphere furnace and a means for blowing the sealing gas into the sealing chamber when the furnace pressure is decreased. 6. A continuous treating line as claimed in claims 1 and 2 having, in order, a pre-heating furnace, said heating furnace by directly flaming and said indirect heating furnace, wherein an intermediate chamber is provided between said furnaces for preventing the furnace gas from moving into an other furnace. 7. A continuous treating line as claimed in claim 6, wherein an after-burnning chamber is provided between said heating furnace and the preheating furnace for perfectly burning the combustion exhaust gas within said heating furnace and supplying it into the pre-heating furnace. 8. A continuous treating line as claimed in claims 2 and 6, wherein a cooling furnace is provided behind said indirect heating furnace where cooling is mainly carried out with rolls, and a temper rolling machine is provided at an exit of a finally treated steel strip. 9. A continuous treating line as claimed in claims 6 and 8, wherein a cleaning facility for the surface of the steel strip is provided before said pre-heating furnace. 10. A continuous treating line as claimed in claims 2 and 6, wherein a cooling furnace is provided behind said indirect heating furnace where cooling is mainly carried out with liquid, further an intermediate pickling facility and an overaging furnace, are provided as well a temper rolling machine at an exit of a finally treated steel strip. 11. A continuous treating line as claimed in the preceding claims, wherein said burners are arranged with said spaces all over available heating scope of said heating furnace. 12. A continuous treating line as claimed in the preceding claims, wherein with respect to the available length of the furnace, said burners are arranged with said spaces in the length of an exit of more than γ% which is obtained by following equation, and burners of non-reduction type are arranged in a remaining available length, Herein, T IN = Temperature of the steel strip at an entrance of the direct flame heating furnace (°K) T OUT = Temperature of the steel strip at an exit of the direct flame heating furnace (°K) A (T)= Reducing rate of the steel strip (A/sec) [= 127000 e -(6433/T) ] T * : Temperarure (°K) of the steel strip at boundary between position of the reducing burner and position of the non-reducing burner B (T) : Oxidizing rate of the steel strip (A/sec) and position of the reducing burner 1 [= 69200 e -(6126/T) ] F (T) : Heating rate (°K/sec) [= dT/dt] 13. A continuous treating line as claimed in the preceding claims, wherein said burner is arranged with said space at each exit range including the heating range just before the passing roll at the exit of each of the PASSes.

Inventors: ABE, MASAHIRO, ARIMA, KOICHIRO, FUKUDA, SHUZO, FUKUNAKA, SHIRO, KANETOU, SHUJI, NAKAYAMA, MICHIO
Applicants: NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1986
Main classification: C21D9
IPC codes: C21D9