A Study Of Strength And Toughness For An Ultrafine-grain Size Ferrite/Cementite Steel

Grain refinement is an active research area currently. Ultrafine grained steel has its potential in combining high strength with low ductile-to-brittle transition temperature simultaneously. Carbon manganese steels are environmentally friendly materials because they involve no costly alloying and are easy to recycle comparing to the plain alloy steels. Ultrafine grained carbon manganese steels are obtained from plain carbon manganese steels by grain refinement technology and are believed to have a good prospects in industrial applications.Microstructures、tensile properties、impact properties and impact fracture morphology of the rolled and annealed ultrafine grained carbon manganese steels were investigated in this work. Charpy impact test was determined from373K to4.2K and tensile test was measured between323K and77K. The fracture surface and microstructure characteristics were analyzed by scanning electron microscopy and electron back scattering diffraction. The experimental results are listed as follows:1. The as-rolled microstructures were characterized by a ferrite matrix with an ultrafine grain size containing nano-sized globular cementite particles.The average ferrite grain size ranged from less than a micrometer to a few micrometers. The cementite particles were less than100nm in diameter and showing heterogeneous dense distributions at some specific sites.2. The ferrite grain size of the annealed microstructures presented a bimodal distribution, consisted of the large size regions and the small size regions. The ferrite grain size in the large size regions increased from1μm to5.5μm with increasing annealing time, whereas the grain size in the small size regions didn’t change much (only from0.5μm to0.7μm). The area fraction of the the small size regions decreased with increasing annealing time and almost disappeared after annealing for600min at873K.3. The as-rolled and annealed samples had similar stress-strain curves. The as-rolled samples showed higher yield stress value but lower uniform elongation and total elongation, compared to the annealed samples. The yield stress decreased with increasing annealing time for the annealed microstructures.4. The as-rolled and annealed samples had similar Charpy transition curves. The as-rolled samples showed lower upper shelf energy(about220J) but higher lower shelf energy(about25J), compared to the annealed samples which had a lower upper shelf energy above250J and a lower shelf energy almost nil respectively. The Charpy transition temperature of the as-rolled microstructure was about150K, lower than210K of the annealed microstructure.5. The ductile-to-brittle transition temperature of the annealed samples were lower than the Charpy transition temperature, both of them increased, and the former approached the latter with increasing annealing time at873K.34figures,3tables,97references.