Alternatives to pickling : preparation of carbon and low alloyed steel wire rod
Sammanfattning: A well established method for scale removal on hot rolled wire rod is pickling. To avoid pollution problems, the wire rod can be mechanically descaled. By this method, the removed scale can be recirculated to the blast furnace.In batch pickling considerable energy is necessary; e.g. for heating of the pickling baths, ventilation of acid fumes and application of lubricant carrier. Energy consumption of the alternative descaling methods has been assessed and compared to picklingA method was developed to evaluate the remaining scale on mechanically descaled wire rod. Stretching experiments were used to evaluate adhesion of scale to the wire rod surface of different steel grades. Most of the scale was removed on the carbon steel grades and Cr/V alloyed steel grades. More scale remained on silicon alloyed steel grades.Shot blasting with steel shots was studied as an alternative to pickling. This method removed the scale on all studied steel grades, even the silicon alloyed grades. The wire rod surface was much rougher compared to the pickled wire rod surface. The shot blast power consumption was measured and evaluated. The PC strand wire gave more wear of dies otherwise the shot blasted wire rod showed no difference in the wire drawing process compared to pickled wire rod. Downstream problems such as cleaning and adherence of copper were observed.Reverse bending and brushing with steel brushes and brushes with SiC abrasives were studied on wire rod of low carbon and silicon chromium alloyed steel grades. Scale remained in surface valleys after brushing of the low carbon steel grade. The silicon steel grades were not possible to descale by bending and brushing. Surface defects from brushing were observed on wire rods brushed by steel brushes, but not on wire rod brushed by SiC-nylon brushes.A study of shot blasting with recycled glass was done. Due to the lower density, the shot velocity must be higher than for steel shots. Recycled glass gives an irregular surface of the wire, including chip formation, which can improve the lubricant carrying ability of the wire rod surface, but also increase the risk for reduction of the fatigue strength of the wire.The differences of die wear using wire rods descaled by two treatments; pickling and reversed bending has been studied. The study gave a background of wear mechanism and described a method to estimate die wear in wire drawing. The conclusions of the experiments indicate that the method for wire rod preparation seems to be of minor importance for the wear of die. The wear rate of a die used in the sixth draft was one order of magnitude lower than the wear rate of the die in the first draft. Abrasive wear is present presumably from the WC grains, torn out in the adhesive wear, on both dies used on pickled wire rod and reverse bent and steel wool cleaned wire rod.In order to evaluate new methods to descale wire rod, the effect of thermal shock has been studied. An estimation of the stresses, developed in the oxide during cooling after hot rolling and a FEM-simulation during cooling from ambient temperature down to 77 K, has been made. Experiments using liquid nitrogen, flame and steam to descale wire rod have been performed with mild steel and silicon and chromium steel wire rod. The experiments have shown that thermal shock cannot descale the wire rod. Only a descaling effect from the flame has been found.
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