Process for continuously annealing steel strip

Abstract: To provide equipment for continuously annealing a steel strip which has been cold-rolled, intended to stably operate an annealing furnace by preventing the slip of bridle rolls and hearth rolls. The diameter D of bridle rolls (2) is set so that a surface pressure p expressed by the equation p = h(σ1 + σ2)/D becomes not lower than 10 kPa (kilopascal); wherein σ1 is unit tension of the portion of a steel strip (1) which is on the upstream side of bridle rolls (2) and hearth rolls (3) in a furnace; σ2 unit tension of the portion of the steel strip (1) which is on the downstream side of the rolls (2, 3); h the thickness of the steel strip; and D the diameter of the bridle rolls (2).

Description: Technical Field The present invention relates to a process for continuously annealing a steel strip after cold rolling, wherein the term "steel strip" refers to steel products ultimately applicable to production of cans, steel furniture, automobiles, etc. after continuous annealing, and, if necessary, tin plating, zinc plating, etc. Background Art (Prior Art) Generally, the higher the tension of a steel strip in the furnace in an apparatus for continuous annealing, the better from the viewpoint of preventing walk or fluttering on one hand, and the lower the tension, the better from the viewpoint of preventing heat buckle. To satisfy such contradicting requirements, it is an ordinary operating practice to decrease the tension of steel strip in a heating furnace or a soaking furnace where the steel strip is at a high temperature and increase the tension in a cooling furnace or an overaging furnace where the steel strip is at a relative low temperature. Such tension distribution can be obtained by finely adjusting and controlling a hearth roll speed, and when it is desired to change the tension radically at a position in the furnace, it is an ordinary practice to provide a bridle roll in the furnace. Fig. 1 shows one example of that practice, where sets of hearth rolls 3 are so arranged in the individual furnaces as to guide a steel strip 1 into the individual furnaces from the inlet side to the outlet side and a set of bridle rolls 2 is arranged just before a rapid cooling furnace 6 in which the steel strip 1 that has passed through a heating furnace 4 and a soaking pit 5 is to be exposed to a gas jet stream of a high speed, and the tension of the steel strip 1 becomes largest after passing through the set of bridle rolls 2. Then, the steel strip 1 passes through the successive quenching furnace 6, an overaging furnace 7 and a final cooling furnace 8 under the largest tension. (Problems to be solved by the Invention) A set of bridle rolls 2 and sets of hearth rolls 3 provided for these purposes must surely restrict the steel strip 1 so as to prevent any slip. Actually, the bridle rolls 2 and hearth rolls 3 have often slipped, particularly when the thickness of steel strip becomes smaller, and have failed to perform their proper functions. An object of the present invention is to solve these problems and to provide a process for continuously annealing a steel strip, capable of performing functions of bridle rolls and heath rolls so satisfactorily as to attain the stable operation of the apparatus for annealing. Disclosure of the Invention To solve the above-mentioned problems, the present invention provides a process as given in the following items (1) to (3): (1) Process for continuously annealing a steel strip in furnaces comprising sets of rolls so arranged in the furnaces as to guide a steel strip fed into the furnaces from the inlet side to the outlet side, at least one roll of the sets of rolls having such a diameter D with the exception of diameters ≥ 800mm, as to make a surface pressure p defined by the following equation (I) 10 kPa (kilopascal) or more: P = h (σ1 + σ2) / D(I) wherein the unit tension of the steel strip at the inlet side to the roll is σ1, the unit tension at the outlet side to the roll is σ2, the thickness of the steel strip is h and the diameter of the roll is D. US-A-4 296 919 discloses annealing steel strip in a continuous heat treating furnace comprising a cooling zone; the diameter of rolls guiding the strip in the cooling zone amounts to a minimum disclosed in the US-A in order to avoid strain induced phase transformation resulting from bending the strip. Exemplified are roll diameters of 800 and 1000 mm which, together with the disclosed parameters for tension and thickness of the strip, would meet the requirements of the present invention and are, therefore, disclaimed. Disclaimed are all roll diameters ≥ 800mm such that the invention only relates to a diameter D of the at least one roll smaller than 800mm. (2) Process as described in the above-mentioned item (1), wherein at least one roll of the sets of rolls is a bridle roll. (3) Process as described in the above-mentioned item (1), wherein at least one roll of the sets of rolls is a hearth roll. (Function) A maximum tension T2 that cannot generate any slip technologically can be given by the following equation (II): T2 = T1 · exp (µ)(II) wherein the tension at the inlet side to bridle rolls 2 is T1, the tension at the outlet side is T2, winding angles of a steel strip 1 around bridle rolls 2a, 2b and 2c are 1, 2 and 3, respectively, and sum total ( 1 + 2 + 3 ) is , and the coefficient of friction is µ, as shown in Fig. 2. In other words, the tension ratio T2/T1 depends solely upon parameter µ and is independent on the absolute values of the tensions. However, the actual bridle rolls in the apparatus are liable to undergo slip when the absolute values of the tensions become smaller even in the same tension ratios. That is, it seems that the coefficient of friction µ is susceptible to tensions. Number of the bridle rolls is not limited, and only one bridle roll will do. Number of the bridle rolls can be determined from  derived by inverse operation of the equation (II). As a result of measuring the coefficient of friction at slip generation limits of hearth rolls and bridle rolls having different diameters in an actual apparatus for continuous annealing, it has been found that the coefficient of friction is a function of surface pressure p given by the following equation (III). p = (T1 + T2) / (DW)(III) wherein the diameter of a bridle roll is D and the width of a steel strip is W, as shown in Fig. 3. An important fact is, as is apparent from Fig. 3, that the critical point of the coefficient of friction is at a surface pressure p of about 10 kPa (the surface pressure at this critical point will be hereinafter called a critical surface pressure pc). In a region whose surface pressure is at or above pc, the bridle rolls and the hearth rolls are in a state of normally restricting a steel strip, whereas in a region whose surface pressure is below pc they are in a slip state, that is, the friction is in a state of kinetic friction. In order to make the bridle rolls and hearth rolls normally function, their diameters must be so set that the surface pressure can be at or above pc. Generally, the object of hearth rolls is to convey a steel strip without changing tensions on the steel strip. The present inventor has found that the friction is in a state of kinetic friction below a surface pressure of 10 kPa, as is apparent from Fig. 3, and a slip occurs even due a slight difference in the tension between the inlet side and the outlet side to a hearth roll. The slip phenomenon, which appears even if there is no substantial difference in the tension, seems surprising, but since the control unit to motors driving the hearth rolls generally has no means for detecting a true travelling speed of steel strip, and the roll peripheral speed is thus controlled to an instructed value (same as the peripheral speed of a bridle roll), and thus such a phenomenon seems to appear. Furthermore, the roll peripheral speed is not controlled as instructed, because the hearth roll diameter thermally expanded in furnaces at an elevated temperature cannot be detected exactly, and thus this fact also promotes appearance of such a phenomenon. The slip presence leads to generation of defects on the steel strip, or buildups (fixation of iron component in the form of small spherical projections that also lead to defects of steel strip) on the surface of hearth roll due to continued slip. These problems can be solved by selecting appropriate roll diameters so as to give a critical surface pressure or more according to the present invention. Normal operation is so carried out that the unit tension σ of a steel strip can be kept constant, and thus the equation (III) can be given as the following equation (IV), when a correlation of tension T = σhW is taken into the equation (III): p = h (σ1 + σ2) / D(IV) Brief Description of the Drawings Fig. 1 is a schematic side view showing an arrangement of bridle rolls and hearth rolls in the furnaces according to the present invention. Fig. 2 is a diagram illustrating the general relation between the winding angles and tensions of a steel strip around bridle rolls. Fig. 3 is a diagram showing actual measurements of relations between the surface pressure and coefficient of friction of bridle rolls and hearth rolls as basic data for the present invention. Modes for carrying out the Invention Examples are given below. Example 1 One design example of bridle rolls based on the above-mentioned findings will be given below. Under the following conditions, i.e. unit tension of a steel strip at the inlet side to the bridle roll σ1 = 0.7 kg/mm 2 , unit tension at the outlet side σ2 = 1.2 kg/mm 2 , thickness of steel strip h = 0.25 mm, number of rolls = 3, winding angle of steel strip around the respective rolls = 180° and tension ratio at the respective rolls is constant ( = σ2/σ1 = 1.2 ), a critical surface pressure pc = 10 kPa is inserted into the equation (IV) to obtain D1, D2 and D3. The results are D1 = 390 mm, D2 = 460 mm and D3 = 550 mm. Actual differences in the roll diameter give a complicatedness to their manufacturing and maintenance, and all roll diameters are set to constant 390 mm or less in view of safety. Data shown in Fig. 3 relate to smoothly finished bridle rolls and hearth rolls, each made of steel, and are those obtained at a line speed of 350 to 600 mpm. Example 2 One design example of hearth rolls based on the above-mentioned findings will be given below. Tensions of a steel strip at the inlet side and the outlet side to the hearth rolls are generally constant ( σ1 = σ2 ), and the equation (IV) can be applied to the hearth rolls as conditions for preventing slip occurrence in the same manner as in the case of bridle rolls. Under conditions of σ1 = σ2 = 0.7 kg/mm 2 and thickness of steel strip h = 0.25 mm, roll diameters D will be 350 mm or less. When the surface finishing roughness of bridle rolls or hearth rolls or the line speed considerably differs, there would be a possibility for a change in the critical surface pressure pc defined by Fig. 3, but there is no change in the basic concept of setting the bridle roll diameter or the hearth roll diameter to give the critical surface pressure or more. The concept of selecting roll diameters so that the surface pressure p can be at least such a critical value as not to cause any slip can be applied not only to the bridle rolls, but also to all the hearth rolls, as mentioned above. When the hearth roll diameter cannot be reduced due to the dimensional limit of the apparatus, as in a heating furnace provided with radiant tubes, care must be paid to the design and adjustment of the hearth roll control system so as not to give a driving force in excess of the necessary torque to the conveying of a steel strip by the hearth rolls. For example, speed instructions common to the furnaces must be limited so as not to cause an error in the speed over a predetermined value (a few mpm). It is also another technical knowledge suggested by the present invention to give an appropriate suspending characteristic, etc. Industrial Utility of the Invention As described above, bridle rolls and hearth rolls arranged in an apparatus for continuous annealing can give a sure tension-amplifying action to a steel strip as their proper function according to the present invention, thereby attaining a high level, stable operation in the process for continuously annealing the steel strip, and also preventing serious defects such as occurrence of defects (flaws) due to a slip between the steel strip and the rolls.

Claims: A process for continuously annealing a steel strip in furnaces comprising sets of rolls so arranged in the furnaces as to guide a steel strip fed into the furnaces from the inlet side to the outlet side, at least one roll of the sets of rolls having such a diameter D with the exception of diameter ≥ 800mm as to make a surface pressure p defined by the following equation (I) 10 kPa (kilopascal) or more : p = h (σ1 + σ2) / D (I) wherein the unit tension of the steel strip at the inlet side to the roll is σ1, the unit tension at the outlet side to the roll is σ2, the thickness of the steel strip is h and the diameter of the roll is D. A process according to Claim 1, wherein at least one roll of the sets of rolls is a bridle roll. A process according to Claim 1, wherein at least one roll of the sets of rolls is a hearth roll.

Inventors: IKEGAMI HIROSHI, IKEGAMI, HIROSHI, IKEGAMI, HIROSHI, TECHNICAL DEVELOPMENT BUREAU
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1993
Main classification: C21D9
IPC codes: C21D9