Blackening treating method of stainless steel strip surface and blackening treating furnace

Abstract: A blackening treating furnace (1), comprising a soot generation burner (4) for blowing an incomplete combustion flame toward the both surfaces of a stainless steel strip (3) passed in continuously, a pair of secondary air nozzles (5) installed at a position for enclosing the incomplete combustion flame (9) for blowing a secondary air to the surface of the stainless steel strip (3) in the vertical direction thereto or at a slight inclination toward the front end middle direction of the incomplete combustion flame (9), and a flame guide air nozzle (6) installed between the pair of secondary air nozzles (5) and an exhaust duct (7) for sucking and discharging after fluidizing the combustion reaction flame (9') of the incomplete combustion flame (9) and secondary air along the running direction (10) of the stainless steel strip (3) and the secondary air nozzles (5), thereby injecting the flame guide air toward the combustion reaction flame (9') in the vertical direction of running direction of the stainless steel strip (3) or at an inclined angle, is installed at the upstream side of the continuous annealing furnace (2) to deposit the soot on the surface of the stainless steel strip (3) efficiently, uniformly and stably, so that the cold-rolled stainless steel strip may be annealed continuously.

Description: Blackening treating method of stainless steel strip surface and blackening treating furnace The present invention relates to a method of blackening treatment of a stainless steel strip surface for depositing soot uniformly and stably on the surface of a stainless steel strip moved through a treating furnace positioned at the upstream side of a continuous annealing furnace for continuous annealing of cold-rolled stainless steel strip, and a blackening treating furnace preferable for executing the same method. Generally, a cold-rolled stainless steel strip is annealed by being passed along a series of continuous annealing line comprising, for example, an annealing step and a pickling step in order to eliminate processing strain or the like caused by rolling. Such annealing has been provided widely by an open-air continuous annealing furnace such as of the horizontal type (catenary type). Such continuous annealing furnace is designed to heat the stainless steel strip mainly by the direct ignition burner, and therefore the stainless steel strip is heated chiefly by radiation heat. However, since the surface of the stainless steel strip product is required to have a gloss and uniform finish after annealing, the strip is cold-rolled in the cold rolling step before annealing process in order to provide a substantial or advanced surface gloss. As a result of such high surface gloss of the stainless steel strip, the heat absorption rate is extremely low, and it is difficult to obtain high production efficiency of the annealing process. Accordingly, in order to raise the production efficiency of the annealing process, there have been attempted a method of preheating by combustion waste gas in forced convection or preheating of combustion air, a method of increasing radiation heat by raising the annealing furnace temperature over the heating temperature of stainless steel strip to increase the temperature difference between the material temperature and the furnace wall temperature, a method of improving heat transfer efficiency by direct contact of a high temperature burner flame with the stainless steel strip, and combinations of such methods. It also has been proposed to raise production efficiency by extending the length of the heating run or zone of the annealing furnace. These methods, however, involve the following problems. First of all, in the method of heating by increasing the radiation heat by raising the temperature difference between the material temperature and the furnace wall temperature, the passing speed of the stainless teel strip to be heated continuously varies, or the quantity of heat transferred to the stainless steel strip becomes uneven due to contamination of the surface of stainless steel strip or the like, or material abnormality may be caused by extremely exceeding the desired material temperature, or, in a worst case, the stainless steel strip is melted down in the annealing furnace, among other troubles. In the heating method by direct contact of burner flame with stainless steel strip, it is effective if the temperature of the stainless steel strip is low. However, when the temperature of the stainless steel strip exceeds a certain range, the surface of the stainless steel strip may be extremely and locally oxidized, or if the passing speed of the stainless steel strip varies, the temperature of the stainless steel strip can be extremely raised to produce material abnormality. In the method of extending the heating zone of the continuous annealing furnace, in a new furnace, an increased equipment cost is required for the portion of extension, or in an existing furnace it takes much time and cost for modification. Further, after extension of the heating zone of the annealing furnace, the basic unit of the fuel is increased, and the heating efficiency is lowered. Accordingly, as a method of enhancing the heat absorption rate of the stainless steel strip surface without sacrificing the desired properties and quality such as gloss of the stainless steel strip surface and without causing other difficulties, a method of blackening treatment of the surface of stainless steel cold-rolled strip with soot at the upstream side of the radiation heating zone of the annealing furnace was proposed by the present applicant in the Japanese Unexamined Patent Publication (KOKAI) No. JP-A 1-119628 (1989). In such conventional method of blackening treatment of the stainless steel strip surface, plural soot generation burners are needed to blacken the stainless steel strip surface uniformly with soot. If the soot is deposited on the surface of stainless steel strip by burning the fuel with plural soot generation burners, the soot is not deposited uniformly. Thus, in the subsequent continuous annealing furnace, the heat absorption rate is not always raised, and the fuel consumption is increased, as confirmed experimentally by the present inventor. The reasons are as follows: since the heat absorption rate of soot has a specific value, if the soot is deposited over a long period of time, the heat absorption rate is not raised above a certain value. Since the soot is generated in the following steps from the hydrocarbon gas of the fuel, the soot progressed up to the oxidation step no longer contributes to the heat absorption rate because its adhesion to the stainless steel strip is reduced extremely, and the deposited soot is easily peeled off or is vaporized to be in non-blackening state due to convection in the forced convection preheating zone or heating zone at the upstream side of the continuous annealing furnace in which the strip is passed after blackening treatment: (Pyrolysis) --- (Generation of nucleus) --- (Surface growth, combining) --- (Grouping) --- (Oxidation) Therefore, unless controlled so as not to progress up to the oxidation step by suppressing the soot generation step within the soot grouping step at the exit side of the blackening treating furnace, depending on the passing speed of the stainless steel strip, it is impossible to deposit uniformly and stably on the surface of the stainless steel strip soot which adheres smoothly to the stainless steel strip and is not easily peeled off or vaporized and is capable of obtaining sufficient heat absorption. Since in the soot generation burner hydrocarbon gas of the fuel, oxygen, air and oxygen-enriched air are burned incompletely at a low air ratio of 0.3 or less, it is necessary to feed combustion air properly in order to progress, while controlling, the soot generation step. However, it is necessary to control the furnace temperature to be at a relatively low temperature. If the incomplete combustion flame of the hydrocarbon of the fuel injected from the soot generation burner toward the stainless steel strip surface burns at low air ratio as mentioned above, since the majority of the inside of such flame is composed of incomplete combustion flame of relatively low temperature while the outside is a high temperature complete combustion flame, it is necessary to lower the combustion temperature by injecting secondary air of relatively low surface temperature from a secondary air nozzle toward the stainless steel strip surface which is to be passed around the flame injected from the soot generation burner in order to lower the temperature of the outside complete combustion flame. That is, the combustion reaction rate of the hydrocarbon gas in the fuel, or the soot generation rate, varies with the low temperature secondary air volume injected from the secondary air nozzle, and the temperature of the furnace atmosphere changes accordingly. However, if the low temperature secondary air volume injected from the secondary air nozzle is increased, the combustion reaction is promoted, and the flame temperature of the incomplete combustion flame goes up. As the flame temperature elevates, the temperature of the furnace atmosphere rises. Hence the combustion reaction speed increases, soot generation decreases and the soot deposite becomes unsatisfactory. Thus, it is difficult to control the combustion reaction rate of the hydrocarbon gas of fuel, that is, the soot generation reaction rate, only by the secondary air volume injected from the secondary air nozzle or its temperature. EP 0 120 373 A1 discloses a method and a burner system for coating chills with soot before casting. For improving the adherance of the soot layer a driving gas is added to the fuel and/or the combustion flame is enclosed by an envelopping gas jet. It is hence a primary object of the invention to provide a method of blackening treatment of stainless steel strip surface capable of generating soot efficiently in a blackening treating furnace installed separately from a continuous annealing furnace at the upstream side of the continuous annealing furnace, in order to anneal at high production efficiency by maintaining a desired target without sacrificing surface properties, by performing continuous annealing of stainless steel strip in, for example, an open-air horizontal or vertical continuous annealing furnace, depositing the soot uniformly, stably and efficiently on the surface of the stainless steel strip being passed through, and easily controlling the combustion reaction rate of hydrocarbon gas of the fuel, that is, the soot generation reaction rate and its deposit state by solving the problems of the prior art, and a blackening treating furnace in a structure suited for executing this method. The present inventor intensively studied the above problems, and found the solution as defined in claims 1 and 5, optional features of the invention being set out in the dependent claims. The incomplete combustion flame formed by the incomplete burning of the fuel by the soot generation burner and the secondary air separately supplied and injected so as to enclose this incomplete combustion flame are blown toward the surface of the stainless steel strip passed continuously into the blackening treating furnace. The relationship of the two components is controlled to be within a range so as not to raise the flame temperature of the incomplete combustion flame by the secondary air. A flame guide air is supplied separately and is injected toward the stainless steel strip surface side in the vertical direction to the running direction of the stainless steel strip or at an inclined angle thereto until the combustion reaction flame of the incomplete combustion flame and secondary air blown to the stainless steel strip surface is sucked and discharged into the exhaust duct, and the combustion reaction flame is fluidized along the stainless steel strip surface in the same direction as the running direction of the stainless steel strip, as far as possible. The flame guide air is properly injected so as not to raise the temperature of the furnace atmosphere in this fluidized flame. Heat withdrawal from the furnace is increased so as to keep low the temperature of the furnace atmosphere. The soot can be deposited on the stainless steel strip surface uniformly, stably and efficiently by more easily controlling the combustion reaction rate or the soot generation reaction rate only by the supply of air into the furnace, the injection method, and the proper air volume. Thus, as for the soot generated in the process of burning the incomplete combustion flame along the surface of the stainless steel strip, the secondary air volume blown onto the surface of the stainless steel strip is decreased within a necessary limit together with the incomplete combustion flame in order to prevent progress of the soot generation step up to the oxidation step preferably by cooling the secondary air. The progressed combustion reaction flame of the incomplete combustion flame is fluidized toward the running direction of the stainless steel strip while pressing along the surface of the stainless steel strip without diffusing into the blackening treating furnace by the flame guide air, thereby cooling the furnace atmosphere so that the temperature may not rise too much. In order to further lower internal atmospheric temperature of the furnace, the entire wall of the blackening treating furnace is enclosed with a water-cooled box, such that a greater cooling effect may be obtained. By also lowering the temperature of the internal atmosphere of the furnace, the soot generation reaction rate may be made moderate, while the soot generation step easily may be controlled to stay within the grouping step, not advancing to the oxidation step, depending on the temperature of the flame guide air and injection volume. Therefore, it is possible to control operation so that the combustion step in the blackening treating furnace may always take place at a constant position not advancing to the oxidation step, depending on the passing speed of the stainless steel strip, by increasing the flow of flame guide air when the passing speed of the stainless steel strip is fast, or by decreasing the flow of flame guide air when the passing speed is slow. Hence, in the blackening treating furnace, since the soot generation step is controlled within the grouping state, not progressing further, it is possible to generate soot to be smoothly formed on the stainless steel strip. The combustion reaction flame containing such soot can be pressed against the surface of the stainless steel strip, so that the soot in a highly density state may be maintained in contact with the strip for a long period of time. Therefore, waste of fuel is decreased, and the soot may be uniformly and stably deposited so as not to be easily peeled off or vaporized. As described herein, the method of blackening treating of the stainless steel strip surface and the blackening treating furnace of the stainless steel strip surface of the invention are simple and may be executed at a relatively low cost. Industrial values are great, including, among others, the following effects. (1) If the passing speed of the stainless steel strip varies, it is possible to control the soot generation step of the incomplete combustion flame formed by incomplete combustion of the fuel by the soot generation burner, and the distribution thereof within the furnace depending on its passing speed. Soot of excellent depositing property may be stably generated, and such soot at high density may be kept in contact with the stainless steel strip surface for a long period of time. As a result, the soot may be efficiently, uniformly and stably deposited on the surface of the stainless steel strip. (2) Consequently, peeling or vaporization of soot does not easily occur in the continuous annealing furnace, and the blackened state of the strip may be maintained longer than before. Therefore, the heat absorption rate of stainless steel strip may be stably enhanced, and the annealing may be achieved without abnormality, and the annealing process may be realized at high speed and at a stabilized high quality level. (3) Fuel consumption for blackening treatment may be reduced, and the basic unit is lowered. Thus, energy may be saved in the entire annealing process including the continuous annealing furnace. Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description, taken with reference to the drawings, wherein: Fig. 1 is a schematic view showing a furnace for blackening treating of the surface of stainless steel strip according to the invention and installed separately from a continuous annealing furnace upstream thereof ; Fig. 2 is a sectional view showing the structure of the blackening treating furnace; Fig. 3 is an enlarged view of essential parts of a soot generation burner shown in Fig. 2. Fig. 4 is a schematic view showing the soot depositing process in the blackening treating furnace shown in Figs. 2, 3; Fig. 5 is a front view showing the relationship between the soot generation burner 4 and a secondary air nozzle in the blackening treating furnace shown in Figs. 2, 3; and Fig. 6 is a sectional view taken along line A-A in Fig. 5. Now referring to the drawing, preferred embodiments of the invention are described below. In the drawings, numeral 1 denotes a furnace for blackening treating of the surface of a stainless steel strip 3 according to the invention. A continuous annealing furnace 2 of, for example, open-air horizontal (catenary) type, is installed at the downstream side of the blackening treating furnace 1. Stainless steel strip 3 is coated with a soot deposited on the surface with in the blackening treating furnace 1 as shown in Fig. 1, and is immediately inserted into the continuous annealing furnace 2 to be heated and annealed. At the upstream side of the continuous annealing furnace 2, a forced convection preheating zone 2' may be also installed in order to blow high temperature gas. Within the blackening treating furnace 1 , at both upper and lower sides of the stainless steel strip 3, soot generation burners 4 are arranged for blowing incomplete combustion flames formed by incomplete combustion of the fuel toward the upper and lower surfaces of the stainless steel strip 3. Thereby, the soot is deposited on both the upper and lower sides of the stainless steel strip 3, thus achieving the blackening treatment. The soot generation burner 4 is a flat burner extended in the widthwise direction of the stainless steel strip 3, as shown in Fig. 2 and Fig. 4. The burner 4 comprises a burner nozzle 11 having burner nozzle holes 12 (Fig. 6) spaced substantially at equal intervals along the widthwise direction of the stainless steel strip 3. At the side of the burner nozzle 11 directed toward strip 3, a pair of secondary air nozzles 5 that are elongated in the direction parallel to the burner nozzle 11 are installed, at the position enclosing anincomplete combustion flame 9 injected from the burner nozzle holes 12 of the burner nozzle 11. Secondary air nozzle holes 13, (Fig.6) of the secondary air nozzles 5 are installed in directions slightly inclined toward the incomplete combustion flame 9 or in the vertical direction to the surface of the stainless steel strip 3. The secondary air nozzle holes 13 are injecting secondary air cooled to 20°C or less, preferably which is supplied from outside the blackening treating furnace 1, aside from the oxygen, air, and oxygen-enriched air supplied into the soot generation burner 4. The incomplete combustion flame 9 blown from the burner nozzle holes 12 of the burner nozzle 11 of the soot generation burner 4 contains much unburnt gas including soot resulting from incomplete combustion of hydrocarbon gas of fuel, oxygen, air and oxygen-enriched air at a low air ratio of 0.3 or less. Therefore, when the secondary air is blown onto the surface of the stainless steel strip 3, the combustion reaction is further progressed somewhat by such low temperature secondary air. The incomplete combustion flame 9 is enclosed or enveloped by the secondary air injected from the secondary air nozzle holes 13 of the pair of secondary air nozzles 5. A combustion reaction flame 9' of the incomplete combustion flame 9 blown toward the surface of the stainless steel strip 3 and the secondary air are sucked and discharged into an exhaust duct 7 installed at the top of the blackening treating furnace 1, as schematically shown in Fig. 4. The gas sucked and discharged from the exhaust duct 7 may be effectively used in the blackening treating furnace 1 and in the continuous annealing furnace 2 at the downstream side thereof. Between the pair of secondary air nozzles 5 and the exhaust duct 7, an arbitrary number of flame guide air nozzles 6 are disposed. The flame guide air nozzles 6 are also injecting flame guide air cooled at low temperature, preferably below 20°C, toward the surface of the stainless steel strip 3, in the vertical direction to the running direction 10 of the stainless steel strip 3 or at an inclined angle thereto. The combustion reaction flame 9' of the incomplete combustion flame 9 and secondary air still contains much unburnt gas including soot that still is not burnt completely. Therefore, the combustion reaction is further promoted by the flame guide air injected from the flame guide air nozzle 6, and therefore the incomplete combustion flame 9 is not directly discharged from the exhaust duct 7. Thus, the incomplete combustion flame 9 formed by incomplete combustion of the fuel by the soot generation burner 4 progresses in its combustion reaction while varying the furnace internal atmospheric temperature, depending on the air flow and air temperature of the secondary air and flame guide air. Therefore, the combustion reaction may be retarded by lowering the air temperature and decreasing the flow, and accelerated by raising the air temperature and increasing the flow. Generally, however, when the air flow is increased, the flame temperature rises, and the combustion reaction is accelerated depending on the temperature elevation, and although it is difficult to control the combustion reaction rate only by the secondary air flow and its temperature, it is possible to suppress the elevation of the flame temperature by lowering the furnace atmospheric temperature and arrest an increase of combustion reaction rate that would be caused by elevation of flame temperature. This can be done by controlling within a proper range the combustion reaction rate of the incomplete combustion flame 9 formed by the soot generation burner 4 without increasing the secondary air flow injected from the secondary air nozzle holes 13 of the pair of secondary air nozzles 5, by burning properly by further feeding flame guide air injected from the flame guide air nozzle 6 to the combustion reaction flame 9' of the secondary air and incomplete combustion flame 9 flowing toward the running direction 10 of the stainless steel strip 3 after being blown toward the surface of the stainless steel strip 3 from the soot generation burner 4, by cooling the secondary air and flame guide air at low temperature as far as possible, preferably 20°C or less, and more preferably by enclosing the furnace body in a water-cooled box 8. By the use of such means, the soot generation process can be controlled only by the adjustment of the air flow rate.

Claims: A method of blackening treatment of a surface of a stainless steel strip (3) moved through a treating furnace (1) positioned upstream of a continuous annealing furnace (2), said method comprising: incompletely combusting a fuel in a soot generation burner (4) to thereby generate an incomplete combustion flame (9) including soot and directing said incomplete combustion flame (9) toward the surface of the stainless steel strip (3), characterized by injecting secondary combustion air into said treating furnace (1), separately from said soot generation burner (4) at positions to enclose said incomplete combustion flame (9) and thereby combusting further said fuel from said incomplete combustion flame (9) to form a combustion reaction flame (9'), injecting flame guide air, separately from said soot generation burner (4) and said secondary combustion air, into said treating furnace (1) toward the surface of the stainless steel strip (3) in a vertical direction to its running direction or in a direction inclined thereto, and causing said combustion reaction flame (9') to flow along the surface of the stainless steel strip (3) in its running direction. A method according to claim 1, wherein air cooled to 20 °C or less is used as the secondary combustion air and flame guide air. A method according to claim 1 or 2, wherein the temperature of the furnace atmosphere is lowered by enclosing the furnace body with a water-cooled box (8). A method according to claim 1 to 3, wherein the soot generation burner (4) is supplied with hydrocarbon gas, oxygen, oxygen-enriched air and air for an incomplete burning at an air ratio of 0.3 or less. A furnace for blackening treatment of a surface of a stainless steel strip (3), said furnace (1) being suitable to be separately positioned at the upstream side of a continuous annealing furnace (2), said furnace (1) comprising a soot generation burner (4) for generating an incomplete combustion flame (9) by incompletely combusting a fuel and directing said incomplete combustion flame (9) toward the surface of the stainless steel strip (3) passed continuously into said furnace (1), characterized by a pair of secondary air nozzles (5) disposed at a position enclosing the incomplete combustion flame (9) for injecting a secondary air in the vertical direction to the surface of the stainless steel strip (3) or a slight inclination toward the front end middle direction of the incomplete combustion flame (9) and thereby combusting further said fuel from said incomplete combustion flame (9) to form a combustion reaction flame (9'), and a flame guide air nozzle (6) for injecting a flame guide air toward the combustion reaction flame (9') in the vertical direction to the running direction of the stainless steel strip (3) or at an inclined angle thereto, said flame guide air nozzle (6) being disposed between the pair of secondary air nozzles (5) and an exhaust duct (7) for sucking and discharging the combustion reaction flame (9') of the incomplete combustion flame. A furnace according to claim 5, wherein the soot generation burner (4), the pair of secondary air nozzles (5) attached to the soot generation burner (4), and flame guide air nozzle (6) are disposed in both directions of the upper and lower side, or right or left sides of the stainless steel strip (3) passed continuously in the blackening treating furnace (1). A furnace according to claim 5 or 6, wherein the furnace body is enclosed with a water-cooled box (8). A furnace according to claim 5 to 7, wherein the fuel used in the soot generation burner (4) is hydrocarbon gas, and oxygen, oxygen-enriched air and air for burning it incompletely at an air ratio of 0.3 or less.

Inventors: ADACHI TAKAKATSU, ADACHI, TAKAKATSU
Applicants: CHUGAI RO KOGYO KAISHA LTD, NISSHIN STEEL CO LTD, CHUGAI RO CO., LTD., NISSHIN STEEL CO., LTD.
Application year: 1992
Main classification: C21D1
IPC codes: C21D9, C23C8, C21D1