Irregularity of microhardness and microstructure of low-carbon steel rolled in a two-stand rolling-leveling mill

Abstract

The relevance of the study is proved by two factors. One of them is the necessity to study the nature of the change in the microstructure of the cross-section of extra-thin tinplate made of TS 435 steel (analogue of 08ps steel) skin-passed in a new DSR-1250 mill of the Miory Metal Rolling Plant. The second factor is the need to develop an effective method for obtaining accurate geometry characteristics of an oblique cut of tinplate 0.19 mm or less thick. The purpose of the work is to determine the changes in microhardness and microstructure over the thickness of tinplate samples and identify the existence of a layer microstructure necessary for this type of flat-rolled products. The study was carried out on the selected samples of tinplate of TS 435 steel 0.19 mm thick. This tinplate was rolled from an annealed strip, 0.224 mm thick. The total magnitude of reduction in the mill was 15 %, and the reduction in the skin-pass stand was at least 3 %. The authors carried out measurements of microhardness at different points over the thickness of the selected tinplate samples. The microhardness values over the strip thickness were averaged using 6th degree polynomial interpolation. To study the grain dimension, a number of microstructure images were taken in various areas over the sheet thickness with ×500 magnification. The microstructure studies showed a pronounced strain microstructure with grains elongated in the direction of rolling. At the very boundary of metal contacting the rolls, the grains received the greatest deformation. The highest values of microhardness were identified in two zones adjacent to both strip surfaces and in the central layers along the strip thickness. The change in the microhardness values along the sheet thickness has a wave-like character with three pronounced zones of increase in hardness and two zones of a decrease in its values. The zones with the lowest microhardness values are located between the zones with the maximum values.