3.4 粗轧机工作辊的使用

up and work rolls
worn work rolls and new back-up rolls
立辊轧制形成的板坯狗骨头形状 会增加板带边部轧制时的应力。 结合层区域因此处于高疲劳，在 轧辊寿命的最后阶段（工作层消 worn back-up and work rolls 耗了50-70%后），每个换辊周期 之后,都要用超声波检测结合层， 特别是检查位于轧制区域边部的 结合层。 © 2009 Innse Cilindri – All Rights Reserved
MAIM AIMS主要目标: increment of mill productivity through : 通过以下方法增加产量 - increment of trip duration - 增加轧制周期 - reduction of roll changes - 减少换辊次数 MAIN PROBLEMS : 主要问题： - wear resistance - 耐磨性 - bar surface quality –辊面质量 - slippage tendency - 打滑趋势 - firecrack resistance - 抗热裂纹性 EVOLUTION OF ROLL QUALITY :轧辊质量的评估 Carbon Steel rolling :碳钢轧制： Hi Cr Steel 高铬钢 semi HSS (HSS) 半高速钢 （高速钢） Stainless Steel rolling :不锈钢轧制 Hi Cr Steel 高铬钢 HSS 高速钢 REMARKS : 备注 The change of roll grade into well developed grade permits to get better results as far wear resistance, but it is necessary to pay attention in using, to avoid risks of spallings connected with higher values of residual compressive stresses of the shell using more alloyed grades 轧辊材质的改进提高了轧辊的耐磨性，但我们在使用中必须注意避免剥落的风险, 这 是因为工作层中添加更多的合金材料使得残余压应力变大。
SHELL THICKNESS The increment of shell thickness, mainly for new high alloyed grades (semi HSS and HSS), should be anyway limited in order to avoid the creation of excessive values of residual stresses in the core. For instance, HSS rolls with diameter 1200 mm should have a maximum total shell thickness of 145 mm in the part with lower thickness. In fact for these rolls (due to the vertical centrifugation) it is possible to have a difference in thickness through the barrel length of 15 ÷ 20 mm, and consequently a shell thickness of 165 mm in the bottom part of the casting : this value is considered as the limit.
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WEAR PROFILE In the wear profiles it is notable bigger wear ( + 20 ÷ 50 %) of bottom roll compared with top roll; therefore the wear control should be made on the bottom roll : usually for this roll wear of 1,5 mm on diameter is not exceeded. It is recommended anyway to do not exceed 1 ÷ 1,2 mm. Further dressing must remove totally any crack present on the barrel (cracks could originate spallings). Tests through Eddy Currents (and eventually through UT too) permit to verify the presence of cracks. In case of surface free from defects, after elimination of wear, normally a further dressing of 0,5 ÷ 1 mm on diameter permit to obtain a suitable surface of the roll for next use. The aim of increment of trip duration must be pursued anyway for each specific mill, for each specific kind of roll, and for each kind of rolled steel. Remarks : on the basis of our experience, it is possible to avoid the complete removal of firecracking network if it is uniform distributed on the barrel surface and if it is very small in size.
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INCREMENT OF TRIP DURATION is the main aim for work rolls of roughing stands. In the last years there was a continuous increment rolling campaigns. Presently the target is to reach and to exceed 50.000 tons in Carbon Steel rolling with 5/7 passes in reversing stands. Of course, the aims of increment of rolling campaigns must be pursued paying strong attention to avoid rolling accidents (in particular risks of spalling of barrel edges). In fact, excessive values of work roll wear, together with the back-up roll wear, could cause too high values of stresses in the zones closed to the barrel edges, as shown hereunder. The formation of dog bone profile of the slab by the Edger can increase these values of stress in correspondence of the strip edges Being the bonding area subjected to high fatigue, it is opportune to make, mainly in the last part of the roll life (after 50 ÷ 70 % of shell consumption), after each trip, UT tests of the bonding area, taking care in particular of the area of rolling edge.
Fig. 5
new back-up and work rolls
Fig. 6
worn work rolls and new back-up rolls
worn back-up and work rolls Fig. 5 Fig. 5
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F ig. 5 F ig. 5
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These tests can permit to take under control these areas as far the possible formation of misbondings due to fatigue for the heavy rolling conditions. To limit the problem of misbonding during the use (normally more frequent in second part of roll life) rolls are normally cast with increased shell thickness at scrap diameter up to 25 mm.