Transformation of cross cracks of continuously casted billet in rolled product

Abstract

Increased requirements to the surface quality of hot-rolled products used in automobiles manufacturing and endeavor to decrease t rejections of the metal with surface defects, stipulate the necessity to increase the quality of the initial continuously casted billet (CCB). Cracks, formed in the process of crystallization, which are transformed into surface defects of rolled products during hot deformation are attributed to the surface defects of CCB. Classification of defects presented, reasons of cross cracks on CCB and their locations considered. In the course of the experiment the samples, cut from two CCBs in the place, where cross cracks were visually revealed after shot blasting and hot etching in the 50% solution of chloric acid. The cross cracks on the surface of a sample were revealed in the zone of the big side, and the other – on a rib. The longitudinal polished sections were cut from the place of cross cracks location. At the study of the polished sections multiple cracks were discovered. The width of cracks opening was different – from 0.01 up to 0.3 mm, depth in the section of the study – up to 10 mm. The locations of the cross cracks on the billets were marked by notches from two sides (method of “marked” defects). After that the billets were rolled at a one-stand reversible mill 850 for the diameter of 100 mm. At the visual inspection of the rolled product surface, obtained from the defect CCB, surface defects in the form of discontinuity flaws were discovered. The discontinuity flaws looked as metal ruptures of longitudinal and cross orientation and sometimes having a form of tongue. The degree of defects development depends on the depth and the crack location on the initial CCB. The defects which location corresponds to the CCB side have less opening, and defects which location corresponds to the initial billet rib looked as a rough fissure. Cross samples were cut in the places of defects locations on the surface of rolled product for metal science study. At the cross polished sections the defect cavity have several branches, the cavity walls are twisting, around the cavity branches and next to them, aggregation of small globular oxides and massive decarburization were observed. The defects of rolled products surface were classified as rolled-out cracks (cross orientation). The cracks depth in the section under study was 0.6–3.5 mm. Based on the results of the study the surface defects of rolled products were properly classified and reasons of their arising revealed, that will allow avoiding their formation in the future. Based on the proper classification and avoiding of the surface defects reasons, the rolled products rejects by surface defects will be considerably decreased.