Method and apparatus for continuously rolling steel billets

Abstract: A plurality of materials are rolled in an upstream stand (10). While transfer speed of a succeeding material and the front end position thereof are detected, the transfer speed and the rear end position of a preceding material are detected by a measuring roll (28). The transfer speed of the succeeding material is controlled by adjusting at least the drive speed of the pinch rolls (27) so as the front end of the succeeding material to contact with the rear end of the preceding material at a specified position in a travelling welder (24). The rear end of the preceding material is joined with the front end of the succeeding material by flash-butt welding using the travelling welder (24) to form a continuous material while letting the travelling welder (24) move at a speed synchronous with the transfer speed of the continuous material. The welded portion on the continuous material is ground and/or cut to remove burr therefrom. After removing the burr, the continuous material is continuously rolled by a down-stream stand (11), and apparatus for carrying out the above-described method.

Description: Background of the Invention Field of the Invention The present invention relates to a method for continuously rolling by continuously welding the rolled steel billets at an intermediate stage of a rolling line after applying several passes of rolling of billets, and relates to an apparatus therefor. The expression "material" as used in the following description especially refers to steel billets. Description of the Related Art A known continuous rolling energy-saving method to produce wire, rod, or shape steel with high efficiency comprises the steps of discharging billets from a heating furnace one at a time, welding the rear end of a preceding billet with the front end of a succeeding billet by a single unit of travelling flash-butt welder, removing the burr from the welded portions using a scarfer or the like, heating thus formed continuous billets, otherwise called strands, to a temperature necessary for rolling in an induction heating furnace, then continuously rolling the continuous billet in a rolling mill group (disclosed in, for example, unexamined Japanese patent publication No. 52-43754(1977)). Alternatively, there is a continuous rolling method in which billets discharged from a heating furnace are joined together by welding to form a continuous billet, and the continuous billet is heated again in the heating furnace, wherein also a single unit of travelling flash-butt welder is used (disclosed in, for example, examined Japanese patent publication No. 57-11722(1982)). Shortening the cycle time for treating a single billet is an important issue in the continuous rolling of billets. In this respect, the weight of an ordinary billet being in a range of from 0.5 to 2 ton, the necessary cycle time to treat a single billet is within 1 min. to assure the production capacity of 70 to 80 ton/hr or more. Since conventional continuous rolling processes use only one on-line travelling flash-butt welder, as described above, the welding time inherent to the travelling flash-butt welder is difficult to shorten. Consequently, it is impossible to realise a cycle time of less than 1 min. in the prior art. An applicable measure is to locate two travelling welder units to realize a short cycle time operation. However, a system of two travelling welders increases investment cost and requires a large capacity power source. Furthermore, an applicable measure is to reduce the cross sectional area of billets, through rolling in a specific number of rolling mill groups, to leave the intermediate rolled materials in a free state, to adjust the position of the intermediate rolled materials, and to perform the weld-joining to reduce the welding time. The measures, however, have a disadvantage that, when the intermediate rolled materials become long, the measures cannot be adapted owing to the limitation of interstand distance. JP-A-60-184408 discloses a method for producing sheet bars-by roughly rolling steel slabs using a first roughing mill. These steel bars of constant cross-section are joined by rolling and thereby pressure-welding the overlapping end parts of the two sheet bars. After pressure welding of the steel bars, the joined steel bars are treated in a finishing mill. In DE-A-28 36 338 a method and an apparatus for continuously rolling continuous billets are disclosed which form the basis for the preambles of claims 1 and 2 below. Summary of the Invention It is an object of the present invention to provide a method of and an apparatus for continuous rolling which shortens the cycle time of billet treatment without requesting a large-scale modification of existing facilities. This object is solved by a method having the features of claim 1, and by an apparatus comprising the features of claim 2. A further embodiment of the apparatus is claimed in claim 3. Brief Description of the Drawing Fig. 1 illustrates a construction of the continous rolling apparatus in the first mode according to the present invention. Fig. 2 is a time chart indicating an example of motion of continous rolling apparatus in the first mode according to the present invention. Fig. 3 illustrates a construction of the continous rolling apparatus in the second mode according to the present invention. Description of the Preferred Embodiment 1. The first mode: Fig. 1 illustrates the arrangement of apearatus for continuous rolling of a mode according to the present invention. The figure shows a fourth stand 10 and a fifth stand 11 in the rolling mill group. The first stand (not shown) through the fourth stand 10 are assumed to structure the roughing mill group, and the fifth stand 11 and succeeding stands are assumed to structure the intermediate rolling stands. Between the fourth stand 10 and the fifth stand 11, there arranged a travelling hot saw 21, a descaler 22, a table 23, a travelling flash-butt welder 24 (hereinafter referred to simply as the "travelling welder"), a travelling burr-removing machine 25, and an induction heating unit 26, in series. The following is the description of the functioning state of the apparatus of Fig. 1. A material 50 left from the fourth stand 10 enters the travelling hot saw 21 where the front end thereof is crop-cut. The crop-cutting is done to obtain a suitable shape (or existence of no significant collapse at edge) for succeeding flash-butt welding (FBW). Even when a travelling cutting machine is applied instead of the hot saw, the cutting machine shall minimize the collapse of material. The material 50 after passed the fourth stand 10 becomes free. Then the material 50 is sent by the table 23 so as the material 50 to catches up with the rear end of the preceding material when the rear end arrives at the center of home position of the travelling welder machine 24. The descaler 22 located at inlet of the travelling welder 24 removes scale at the clamped portions on front end and rear end of the transferred materials. The travelling burr-removing machine 25 is located after the moving end of the travelling welder 24. The travelling burr-removing machine 25 conducts burr-removal at the welded portion of the materials welded by the travelling welder 24 while moving thereof. The materials welded by the travelling welder 24 are cooled at their clamped portions during welding. The induction heating unit 26 located at inlet of the fifth stand 11 heats the cooled portions, then the unit 26 sends the materials to the fifth stand 11. The cut of the material at its rear end is conducted at a point that the rear end passes through the home position of the travelling hot saw 21. At that moment, if the material is in completely free state (or in a state that the material passed through the fourth stand and that the material is not clamped by the travelling welder 24), then the cutting in a stopped state is also- possible. Fig. 2 is a time chart illustrating an example of motion of continuous rolling apparatus of Fig. 1. For example, the material 2 ○ discharged from the fourth stand 10 is crop-cut at its front end by the travelling hot saw 21. At that moment, the material 2 ○ is still caught by the fourth stand 10, so the travelling hot saw 21 conducts the cutting action while moving at a speed synchronous with the transfer speed of the rolled material. When the material 2 ○ passes through the fourth stand 10, the table 23 feeds the material 2 ○ so that the front end thereof catches up with the rear end of the preceding material 1 ○ at a point that the rear end of the preceding material 1 ○ arrives at the center of the waiting position in the travelling welder 24. During the feeding period, the rear end of the material 2 ○ passes through the travelling hot saw 21 so that the rear end of the material 2 ○ is cut. The cutting of rear end of the material 2 ○ is generally conducted while moving thereof. Since, however, during the moving period, the material 2 ○ is in free state, the cutting may be done by stopping the movement of the material and may be done in a stopped state, if only the cycle time allows. The travelling welder 24 begins to move at a point that the front end of the succeeding material 2 ○ catches up with the rear end of the preceding material 1 ○ , and conducts flash-butt welding (FBW) after clamping the materials 1 ○ and 2 ○ . According to the chart of Fig. 2, the rolling cycle of billet is selected to 40 sec. including 20 sec. for welding and 18 sec. for returning. Since the material treated in this step is an intermediate material, the cross sectional area of the material is narrower than that of billet (about one fourth after the fourth stand), and welding in a short time is available. After completing the welding step, the travelling burr-removing machine 25 conducts burr-removal in a moving state. Fig. 2 shows a burr-removal cycle of 12 sec. including 6 sec. for burr-removal and 6 sec. for returning. The induction heating unit 26 located at the inlet of the fifth stand 11 conducts heating and soaking of the materials. Since, however, the time between the material leaving the fourth stand 11 and entering the fifth stand is 120 sec. or less, and since there occurs not significant temperature reduction, rather heating and soaking of the locally cooled portion at clamped portions are important. In this respect, it is effective that the induction heating unit 26 is also a travelling type, if space allows. 2. The second mode: Fig. 3 illustrates the arrangement of an apparatus for continuous rolling in a mode according to the present invention. The figure shows an upstream stand 10 and a down-stream stand 11 in the rolling mill group. The first stand (not shown) through the upstream stand 10 are assumed to structure the rough roll group. A travelling cutting machine 21 is located upstream of the exit of the upstream stand 10. A set of pinch rolls 27 the driving speed of which is controllable is located downstream of the travelling cutting machine 21. A measuring roll 28, a descaler 22, a travelling flash-butt welder 24 (hereinafter referred to simply as "travelling welder"), and a travelling burr-removing machine 25 are arranged in series downstream the pinch rolls 27. A travelling induction heating unit 26 is located at the inlet of the downstream stand 11. Since the mode has a short distance between stands, and the occurring temperature reduction of the material is very little, no stationary induction heating unit for heating the total material is applied. At the inlet of each of the travelling cutting machine 21, the pinch rolls 27, and the measuring roll 28, a respective material detector 30, 31, and 32, is positioned. The detector 30 detects the front end and the rear end of the material, and the detection signal generated from the detector 30 is used for controlling the movement of the travelling cutting machine 21. The detector 31 detects the front end of the material, and the detector 32 detects the rear end of the material. The drive control unit 40 receives the detection signals generated from the detector 32, the rotary encoder 42 attached to the drive motor 41 which drives the pinch rolls 27, and the rotary encoder 43 attached to the measuring roll 28, and controls the drive motor 44 of the upstream stand 10 and the drive motor of the pinch rolls 27 based on the received detection signals. The drive control unit 45 receives the detection signal generated from the detector 30 to control the travelling cutting machine 21. The following is the description of the functioning state of the apparatus of Fig. 3. A material rolled in the upper stand 10 arrives at the detector 30 where the arrival of the front end of the material is detected. The drive control unit 45 drives the travelling cutting machine 21 based on the detection signal to conduct crop-cutting of the front end of the material. When the detector 30 detects the rear end of the material, the movement of the travelling cutting machine 21 is controlled in a similar manner to conduct crop-cutting of the rear end of the material. The cutting is done to obtain a suitable shape (or existence of no significant collapse at edge) for succeeding flash-butt welding (FBW). Accordingly, the travelling cutting machine 21 is requested to be one that generates very little collapse at cutting edge. A travelling saw may be applied as the travelling cutting machine 21. The material which was crop-cut at its end in the travelling cutting machine 21 then arrives at the detector 31 where the arrival of its front end is detected. On arrival of the front end of the material at the detector 31. the material is pinched by the pinch rolls 27 slightly rearwards of its front end. The speed of the material at that moment at the exit of the upstream stand 10 is set to a slightly higher value than the inlet speed of the continuous material to the downstream stand 11 (determined by the rotary encoder 43 in the measuring roll 28), and the pinch rolls 27 are also driven synchronously with the speed of the material. After the succeeding material is pinched by the pinch rolls 27, the transfer speed and the front position of the succeeding material are detected by the pinch rolls 27. That is, the transfer speed of the material is detected by the rotary encoder 42, and the front end of the material is detected by the detector 31, thus tracking the movement of the material using a drive control und 40 to detect the transfer speed and the front end position of the succeeding material. As for the preceding material, the transfer speed is detected by the rotary encoder 43 in the measuring roll 28, and the rear end position is detected by the detector 32, thus tracking the movement of the preceding material using the drive control unit 40 to detect the transfer speed and the front end position of the succeeding material. The drive control unit 40 computes the transfer speed of the upstream material so that the front end of the succeeding material and the rear end of the preceding material contact each other at a target position in front of the travelling welder 24, based on the relative relation between the transfer speed and front end position of the succeeding material and the transfer speed and rear end position of the preceding material, then controls the rotational speed of the drive motor 44 of the upstream stand 10 and the drive motor 41 of the pinch rolls 27. Through the control described above, the rear end of the preceding material and the front end of the succeeding material contact each other at a target position. The drive control unit 40 detects the contact of materials from the increased torque (increased drive current) on the pinch rolls 27 or from the thrust force applied to the pinch rolls 27 towards the upstream side, and controls the transfer speed (rolling speed) at the upstream side to be synchronized with the rolling speed at the downstream side. When the succeeding material and the preceding material arrive at the home position of the travelling welder 24 in an adjoined and synchronously running state the travelling welder 24 begins the welding of these materials while moving by itself synchronously with the transfer speed of the materials. The descaler 22 which is located at the inlet of the travelling welder 24 performs descaling at the clamped portion at both front end and rear end of the transferred material. After the travelling welder 24, the travelling burr-removing machine 25 is positioned which conducts burr-removal at the portion welded by the travelling welder 24 in a moving mode. The material welded by the travelling welder 24 is cooled at its clamped portion during welding. The induction heating unit 26 located at the inlet of the downstream stand 11, however, heats the locally cooled portion before transferring the material to the downstream stand 11. According to the above-described welding process, the transfer speed of the succeeding material is controlled against the transfer speed of the preceding material, in response to respective welding conditions for flash-butt welding and for upset-welding. The control is performed through the control of an upset cylinder which is built into the travelling welder 24. The speed control of the upstream (succeeding) material corresponding to the control of the upset-cylinder differs with variables such as billet size, intermediate material size, and distance between the welder and the upstream stand. 1) In the case that the rear end of the succeeding material passes through the upstream stand 10 and that the succeeding material becomes free before the welding begins, (or that the succeeding material passes through the upstream stand 10 and the succeeding material is not clamped by the travelling welder 24), the welder clamps the succeeding material, then the pinch rolls 27 release the material. That is, the case occurs for a short intermediate material passed through the upstream stand 10. The case is controlled in a similar procedure as in the prior art. 2) In the case that the cross sectional area of the intermediate material is relatively narrow and the length thereof is long, (50 sq. mm to 100 sq. mm), the speed variation is absorbed by a material loop in the height direction, between the upstream stand 10 and the pinch rolls 27. Since the speed variation in that case is slight, the necessary height of the material loop is very small (100 to 200 mm), and the material loop height is easily formed if only the distance between the upstream stand 10 and the pinch rolls 27 is secured. 3) In the case that the cross sectional area of the intermediate material is wide and the length thereof is large, the formation of a material loop in the height direction is difficdt so that the speed control is applied by an integral motion of the upstream stand 10 and of the pinch rolls 27. Since the cycle of the change of transfer speed of the material is at a level of several Hz, conventional tension control is applicable. As described above, according to the present invention, materials that were reduced in their cross sectional area by rolling in the first rolling mill group are joined together by flash-butt welding in a travelling welder to form a continuous material, and the burr on the welded portions of the continuous material is removed, followed by continuous rolling of the continuous material in a second rolling mill group. Accordingly, a shortening of the billet treatment cycle time is realizedwithout necessiating significant modification of existing facilities, and a succeeding material rolled in and discharged from the first rolling mill group is pinched by a set of pinch rolls, and the transfer speed and the front end position of the succeeding material are detected, also the transfer speed and the rear end position of the preceding material are detected. Then the transfer speed of the succeeding material is controlled so that the front end of the succeeding material contacts the rear end of the preceding material at a specified position in a travelling welder through the adjustment of the drive speed of the pinch rolls or both the drive speed of the pinch rolls and the first rolling mill group. Thus the materials which were reduced in the cross sectional area thereof by rolling are successively welded together by the travelling welder to form a continuous material. As a result, the welding time is shortened, the continuous welding of billets is performed independent of the billet size and the intermediate material length, and the cycle time for treating billets is shortened. Since the method according to the present invention is not limited by billet size, the method is applicable to a rolling mill group having relatively short distance between both stands.

Claims: A method for continuously rolling steel billets comprising the steps of: clamping a rear end of a preceding steel billet and a front end of a succeeding steel billet and joining the clamped ends by flash-butt welding to form a continuous steel strand using a travelling welding machine (24) while moving thereof at a speed synchronous with the transfer speed of the steel billets removing burr from welded portions on the continuous steel strand; and continuously rolling the de-burred continuous strand in a rolling mill group (11) located downstream of the burr-removing step (25), characterized by the steps of: i. reducing the cross-section of the steel billets, by an additional rolling mill group (10) located upstream of the travelling welding machine (24), to about one quarter of the cross-section of the steel billets; ii. crop-cutting the front end and rear end of each succeeding reduced cross-section billet, in a travelling crop-cutter (21); iii. detecting the transfer speed of the succeeding steel billet and the position of the cropped front end thereof, and also the transfer speed of the preceding steel billet and the position of the cropped rear end thereof between the travelling crop-cutter (21) and the travelling welding machine (24), iv. controlling the transfer speed of the succeeding steel billet so as to bring the front end of the succeeding steel billet into contact with the rear end of the preceding steel billet at a specified position in the travelling welding machine (24); and v. induction heating the continuous steel strand at those locations which were clamped in the travelling welding machine. An apparatus for continuously rolling steel billets comprising: a travelling welding machine (24) for clamping a rear end of a preceding steel billet and a front end of a succeeding steel billet and joining the clamped ends by flash-butt welding to form a continuous steel strand while moving by itself at a speed synchronous with the transfer speed of the. steel billets, a travelling burr-removing machine (25) for removing burr from welded portions on the continuous steel strand, and a rolling mill group (11) located downstream of the burr-removing machine (25) for continuously rolling the continuous steel strand after removing the burr, characterized in further comprising: i. an additional rolling mill group (10) located upstream of the travelling welding machine (24) for reducing the cross-section of the steel billets to about one quarter of the cross-section of the steel billets. ii. a travelling crop-cutter for crop-cutting the front end and rear end of each succeeding reduced cross-section billet; iii. means (31, 32) for detecting the transfer speed of the succeeding steel billets, comprising a front end detector (31) for detecting the front end of the succeeding steel billet,a measuring roll (28) located upstream of the travelling welding machine (24) for detecting the transfer speed of the preceding steel billet which is transferred before the succeeding steel billet, a rear end detector (32) located at the inlet of the measuring roll (28) for detecting the rear end of the preceding steel billet, iv. a drive control unit to which signals from the front end detector (31), the measuring roll (28) and the rear end detector (32) are entered, and in which a desired speed of a succeeding billet is determined so as to bring said billet into contact with the rear end of the preceding steel billet at a specified position in the travelling welding machine (24), and these determined variables are controlled, wherein the measuring roll (28), the travelling welding machine (24), the travelling burr-removing machine (25) and the rolling mill group (11) are in linear arrangement; and v. induction heating means to heat those locations in the continuous steel strand which were clamped in the travelling welding machine. An apparatus for continuously rolling steel billets according to claim 2, characterized in that the additional rolling mill group (10) is a rough-stand mill group.

Inventors: MATSUO GIICHI, OKAWA SUSUMU, OKUSHIMA KOJI, MATSUO, GIICHI, OKAWA, SUSUMU, OKUSHIMA, KOJI
Applicants: NIPPON KOKAN KK, NKK CORPORATION
Application year: 1997
Main classification: B21B15
IPC codes: B21B1, B21B15, B23K11, B21B37, B21B39