Strength-ductility and corrosion resistance match mechanism of bainite/martensite dual phase 30Cr15MoY alloy steel

Abstract

Due to the requirements of strength-ductility and corrosion resistance combination for remanufacturing roller box of short stress path rolling mill, bainite/martensite dual phase 30Cr15MoY alloy steel was fabricated by direct laser metal deposition (DLMD), and the microstructure evolution, mechanical and corrosion properties were studied by varying the laser energy volume density (EVD) of DLMD. The research results revealed that the microstructures of the specimens were bainite, martensite and grain boundary precipitates. Affected by phase content and grain boundary, the tensile strength of specimens increased while the total elongation increased first and then decreased with the increment of EVD. The specimen with EVD of 171 J⋅mm−3 exhibited the best strength and ductility combination with the tensile strength of 1137 MPa and the total elongation of 11.2%. Micro cracks formed and propagated due to the aggregation and coalescence of voids, and the fracture exhibited typical feature of dimple fracture. Influenced by residual stress, phase content and grain boundary, the corrosion resistance increased first and then decreased with increasing EVD, the specimen with EVD of 171 J⋅mm−3 had the highest corrosion resistance (Icorr: 2.37✕10−6 A⋅cm−2 and Ecorr: 357.03 mV). The corrosion mechanisms were pitting corrosion and selective corrosion, bainite and martensite formed the micro-battery in electrolyte, and the bainite as a micro anode was corroded firstly. The relationship model associated with strength-ductility properties and corrosion resistance of DLMD dual phase 30Cr15MoY alloy steel was established. The specimen with EVD of 170.5 J⋅mm−3 corresponding to bainite content of 58.97% gets the best combination of strength-ductility and corrosion resistance. The model is expected to be utilized in the optimization of mechanical properties and corrosion resistance of DLMD dual phase steel.