Cause Analysis of Low Tensile Plasticity in Normal Direction for Hot-Rolled Thick Plate of HSLA Steel

To investigate the cause for the low tensile plasticity in normal direction of high-strength low-alloy steel thick plate, tensile fracture observation, inclusion statistic, and microstructure observation of thick plate and solidification structure observation of continuous casting slab are carried out. The experimental results reveal that many MnS inclusions are only distributed on the platform of the tensile fracture and they only appear in the center of thick plate together with martensite and severe macrosegregation of elements C and Mn. It is concluded that the aggregated large-sized MnS inclusions are the direct cause for reducing the tensile plasticity. Stress concentration on a single MnS inclusion during tensile testing can occur and cause a limited brittle fracture area. Aggregation of MnS inclusions creates a large brittle fracture area, which significantly reduces the tensile plasticity. The root cause of the aggregation of MnS inclusions is the well-developed columnar crystals of continuous casting slab. The well-developed columnar crystals push high concentration of solutes into the residual liquid-phase zone in the center of continuous casting slab and create severe central macrosegregation which eventually lead to the appearance of aggregated large-sized MnS inclusions with martensite in the center of thick plate.