Metal working – Method of mechanical manufacture – Combined manufacture including applying or shaping of fluent...
Reexamination Certificate
1999-07-26
2002-10-15
Echols, P. W. (Department: 3726)
Metal working
Method of mechanical manufacture
Combined manufacture including applying or shaping of fluent...
C029S03300H, C072S204000
Reexamination Certificate
active
06463652
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to an apparatus and methods for manufacturing hot rolled steel sheets, using continuous casting equipment and a plate reduction press apparatus, with a high production efficiency, high quality and low cost.
2. Prior Art
1. According to the prior art of manufacturing hot rolled steel sheets, steel sheets (strips) are manufactured by hot rolling a continuously cast slab; the slab is reheated in a heating furnace, rough and finish rolled to a predetermined plate thickness, cooled on a runout table to a predetermined temperature, and then reeled into a coil using a coiler.
Such a conventional rolling system known in the prior art and described above (called “batch rolling” for short) leaves the worked material in an untensioned state during the period from the time that the leading end of a hot rolled steel sheet leaves a group of finish rolling mills to the time it is coiled by a coiler, and during the period from the time that the trailing end of the hot rolled sheet leaves the group of finish rolling mills to the time that it has been completely coiled in the coiler, and as a consequence, particularly with a thin steel sheet, the leading and trailing ends of the sheet become extremely distorted with a wave shape on the runout table. As a result, the leading and trailing ends of the steel sheet are not cooled satisfactorily and the quality of the material often become defective, which may lead to a reduction in product yield.
In batch rolling, the maximum length of a hot rolled steel sheet depends on the maximum dimensions of a slab that can be rolled, that is, the thickness and length of a slab that,can be inserted into a heating furnace. In addition, because the trailing end of a steel sheet moves unstably on the runout table during batch rolling as described above, the speed of rolling the leading end is reduced to about 600 mpm, and after the leading end of the steel sheet has been reeled onto the coiler, the speed is increased to the normal rolling speed of more than 1,000 mpm, then immediately before the trailing end of the steel sheet leaves the group of finish rolling mills, the speed is decreased, according to a predetermined sequence of controlling the speed. As a result, the time taken to roll the entire steel sheet is longer than the time it would have taken to roll the steel sheet from the leading end to the trailing end at the normal, constant speed, so consequently the production efficiency is low. Furthermore, there is an idle time between rolling one steel sheet and rolling the next steel sheet, which further aggravates the production efficiency.
In contrast to such a batch rolling process as described above, a rolling method has been proposed in which a slab with a plate thickness of less than 100 mm is cast continuously, rolled through all the stages up to finish rolling without cutting the slab at all, and after the slab has been made into a hot rolled steel sheet with a redetermined plate thickness, the sheet is cut. However, because the production capacity of a continuous casting machine is lower than that of a rolling mill, this method cannot yield a satisfactory throughput.
Under these circumstances, various methods have been proposed in the prior art, aimed at avoiding the problem of the low yield in batch rolling and assuring high productivity, regarding the methods of manufacturing hot rolled steel sheets using a slab with a plate thickness of more than 100 mm.
First, to solve the problem of the low yield caused by defective material in the leading and trailing ends of a hot rolled steel sheet, a method is proposed in which the trailing end of a preceding sheet bar (after the material has been rough rolled) and the leading end of the present sheet bar are joined together, and a plurality of sheet bars are continuously finish rolled to produce a hot rolled steel sheet (called the “continuous hot rolling method” for short).
With this method of continuous hot rolling, when n sheet bars are joined into one steel sheet, for example, the steel sheet is subjected to a constant tension between the finish rolling mill and the coiler, therefore when the steel sheet formed from n coils of steel sheets is rolled, material defects due to wave distortions on the runout table are produced only in the portion corresponding to the leading end of the first coil, and the other portion corresponding to the trailing end of the n-th steel coil, so that compared to batch rolling, the yield is higher. In addition, the low-speed rolling operation to keep the leading and trailing ends of the steel sheet moving stably on the runout table is required only for the portions corresponding to the leading end of the first coil and the trailing end of the n-th coil, and the other portions of the steel sheet can be rolled at a normal, constant speed, therefore compared to batch rolling, the rolling time is shorter and the efficiency of production is correspondingly higher. Moreover, there is no idle time during rolling of the entire steel sheet comprised of individual sheet bars joined together, which also contributes to a higher efficiency of production.
However, the roughing-down rolling used in this continuous hot rolling method is the same as that of batch rolling, so that planar, defective shapes known as tongues or fish tails are produced at the leading and trailing ends of each sheet bar. Consequently, before joining sheet bars, such planar defects at the leading and trailing ends of each sheet bar must be removed before finish rolling. Therefore, assuming n slabs are rough rolled, when the n sheet bars are joined, 2n portions (crops) are cut off (the number of such crops is the same as for batch rolling), so a reduction in the yield concerned cannot be avoided. In addition,. when joining sheet bars, portions to be joined must be heated, so defective material caused by the effects of heating, occur, although the effect is slight. Also the strength of the joints in the sheet bars is adversely affected in the continuous hot rolling method and may be so low that the production line might be stopped accidentally because a joint breaks during finish rolling.
When a slab is cast continuously, cut losses are produced during slab cutting and finishing, but the continuous hot rolling method gives rise to the same amount of cut losses as the batch rolling method because the length of the slab is identical to that used in batch rolling. In addition, when only slabs taken from one heating furnace are used in the continuous hot rolling method, a 100% efficiency of the rolling mill cannot be achieved, since the heating efficiency of the heating furnace is lower than the rolling efficiency of the rolling mill.
The unexamined Japanese patent publication No. 106403, 1982 proposes a line of continuous hot rolling facilities in which the ends of a preceding slab and the present slab are joined together, and the joined slabs are continuously rolled by a group of planetary mills and another group of finish rolling mills.
In this system, the slabs are connected together and rolled continuously, so the reduction of the yield caused by crop cutting can be avoided, but because the strength of the joints is low as in the case of the unexamined Japanese patent publication No. 89190, 1992, the joint may possibly break during rolling.
The unexamined Japanese patent publication No. 106409, 1982 proposes continuous hot rolling facilities in which a slab produced by a rotary caster is rolled continuously by a group of planetary mills and another group of finish rolling mills, and the unexamined Japanese patent publication No. 85305, 1984 offers a continuous hot rolling line in which a slab is produced by a rotary caster, the slab is rolled by a cast rolling mill, and after the rolled slab has been reeled up once inside a coil box, it is rolled to a predetermined plate thickness by a group of finish rolling mills.
The aforementioned unexamined Japanese patent publication No. 85305, 1984 describes that a slab with a thi
Dodo Yasushi
Fujii Yasuhiro
Higuchi Kinichi
Honjou Hisashi
Ide Kenichi
Echols P. W.
Griffin & Szipl, P.C.
Ishikawajima-Harima Heavy Industries Co. Ltd.
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