Microstructure characterization and strengthening behavior of dual precipitation particles in Cu-Ti microalloyed dual-phase steels

Copper particles that directly nucleated on the interphase precipitated TiC within the ferrite matrix were found in the Cu-Ti bearing dual-Phase (DP) steel after a multistep heat treatment process. The formation of dual precipitates within ferrite grains should be considered a separated phase transformation process because only titanium carbides form initially when austenite undergoes decomposition reaction; subsequently, copper particles heterogeneously nucleate on these carbides. The above separated precipitation behavior leads to a gentle tempering behavior of Cu-Ti microalloyed DP steel. For example, in Cu microalloyed DP steel, the changes in the average Vickers hardness of ferrite and martensite after 9 h of tempering treatment were 165 to 210 and 407 to 278, respectively. However, in the Cu-Ti microalloyed DP steel, the average Vickers hardness of ferrite and martensite increased from 235 to 249 and 339 to 353, respectively, after 9 h of tempering. Fractography analysis revealed the mixed mode fracture characteristic due to the DP steel consisting of soft ferrite and hard martensite. However, fewer brittle-feature cleavage facets were found in the Cu-Ti microalloyed DP steel after 5 h of tempering treatment, indicating the suppression of damage due to ferrite and martensite interface decohesion. (C) 2019 The Authors. Published by Elsevier Ltd.