The Microstructure and Mechanical Properties of Ultrafine Grained Plain C‐Mn Steels

An ultrafine microstructure was produced in plain C‐Mn steels with different carbon contents (0.15 ‐ 0.3 mass% C) by heavy warm deformation. The rolling was simulated by the plane strain compression test with a simulated post rolling coiling. The final microstructure consists of an ultrafine grained ferrite matrix with the average grain size of 1.1 ‐ 1.4 μm and spheroidized cementite particles of two different size groups. The fraction of high‐angle grain boundaries maintained in the range of 60% to 65%. With the increase of C content from 0.15 mass% to 0.3 mass% the strength increases by about 100 MPa, while the total elongation of 23% hardly changes. The (specific) upper shelf energy decreases from 320 J/cm² to 236 J/cm² but a rather low ductile‐to‐brittle transition temperature (DBTT) of about 206 K does not rise with increasing C content. The ultrafine steel with higher C content (0.3 mass%) exhibits a superior strength‐toughness combination.