TPT July 2009

 Figure 1-b : Schematic illustration of the simple and compact medium size mill and inline heat treatment facilities

line. The mandrel bar is drawn back to the entry side of the mill after rolling (the full retract method). Using a toe-angle and high expansion piercer, which produces a thin hollow shell, the succeeding compact mandrel mill requires only 5-stands. Precise roll gap set up is carried out by a hydraulic screw-down mechanism, considering the roll separating force and mill rigidity. By using the hydraulic screw-down mechanism, roll gaps are changed to control the length of each tube and to obtain uniform longitudinal wall thickness distribution of the tube. 6. Sizer After the mandrel mill, a 12-stand extracting sizer is located. This works as an extracting mill, which extracts the tube from the mandrel bar, and as a sizer, which rolls the outside diameter of the tube to the finishing size. The rolling line is compact and simple, omitting the reheating furnace that is usually installed between the mandrel mill and the sizer. In the conventional 3-roll sizer roll gaps are fixed, but in this sizer roll gaps can be changed by a screw-down mechanism. It is believed to be the first time this mechanism was adopted for the 3-roll sizer. 7. In-line heat treatment The in-line heat treatment facility is installed just after the pipe- making line to guarantee the quenching temperature above Ar3 transformation temperature, and to improve the uniformity of mechanical properties by homogeneous heating. In-line heat treatment equipment is introduced for shortening the delivery time, cutting the energy cost and producing high performance tubes. Tubes are quenched from the outside and inside simultaneously. After quenching, tubes are tempered by the tempering furnace. In summary, the significant features of this production line are: • Connective line from round billet casting facility to pipe mill line • Walking beam furnace • Development of the high toe-angle and high-expansion piercing technology • 5-stand compact mandrel mill • Extracting sizer, with a screw-down mechanism for changing the roll gap • In-line heat treatment apparatus High-toe-angle and high-expansion piercing technology Figure 2-a shows a schematic illustration of a conventional piercer; the newly developed piercer is illustrated in figure 2-b. With a conventional piercer, where the diameter of the billet and the diameter of hollow shell are equal, it is difficult to pierce to the thin hollow shell. With the newly developed piercer, the diameter of the

 Figure 2-a : Schematic illustration of conventional piercer and a photograph of the billet and hollow shell

 Figure 2-b : Schematic illustration of high-toe-angle and high-expansion piercer and a photograph of the billet and hollow shell

billet is smaller than the diameter of the hollow shell, so that the area reduction is decreased and the thinner hollow shell can be pierced; because the area reduction is small the motor power of the piercer can be reduced. The high-toe-angle and high-expansion piercer has a special feature that prevents making inside defects on the hollow shell in comparison with conventional piercer. Figure 3 shows two regions of the piercing process. In the first region, the billet bites between the main rolls and touches the edge of the plug. In the second region the billet is rolled between the main rolls and plug. In the first region the billet is subjected to compressive stress and tensile stress. The stresses make it easy for the billet to be pierced, however increased stress makes more inside defects on the hollow shell. With the newly developed piercer the billet is pierced using less stress than a conventional piercer, so the hollow shell has fewer inside defects.

 Figure 3 : Schematic illustration of the piercing process

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J uly 2009

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