Study On The Microstructural Control And Mechanical Properties In A Low Carbon Low Alloyed Steel With High Strength And Good Ductility

Low alloyed steels are widely used in steels structures such like building facilitis, oil pipelines, bridges and shipbuilding, etc due to its excellent mechanical properties and good weldability. Nowadays, with the high-speed development of industrialization and increasing consciousness of energy-saving and environment-protecting, the requirement for the engineering structures and lightweight equipment becomes more and more higher. For this reason, it is urgent to develop high performance low alloyed steels. By the transformation-induced-plasticity (TRIP) effect of retained austenite, the strength and ductility can be enhanced simultaneously. Hence, how to obtain retained austenite in low carbon low alloyed steel becomes the key point to develop high performance low alloyed steel.In this paper, we introduce an innovative multi-step intercritical heat treatment to create stable retained austenite in low carbon low alloyed steel. The micro structural evolution rule and its relationship with the mechanical properties were systematically investigated through colour metallography, scanning electron microscopy (SEM), transmission electronic microscopy (TEM) and high resolution TEM technology. On the basis of the present investigation, we build up a thorough route for retained austenite control, and obtain the prototype steel with high strength and good ductility.After the first step intercritical annealing, the microstructure primarily comprised of intercritical ferrtite and bainite/martensite. Both ferrite and bainite/martensite were lath-like in alternate arrangement. Bainite/martensite was transformed from reversed austenite during the cooling process, and inherited high alloy concentration from the reversed austenite. Because of the enrichment of alloying elements in reversed austenite, the Ac/temperature was decrease 20℃, which means the next-step intercritical heat treatment can be conducted at lower temperature. After the second-step intercritical tempering, a multi-phase microstructure consisting of ferrite, retained austenite and tempered bainite/martensite was obtained. Ferrite was still in shape of lath, and retained austenite had the morphology of granular and lath, distributing dispersly at the phase boundaries and lath interface. When the experimental steel experienced the third-step tempering, the microstructure remained the same without too much change.Retained austenite was formed through the enrichment of alloying elements such as C, Mn, Ni and Cu during the intercritical tempering process. The reversed austenite was enriched with Mn and Ni after annealing, however, the enrichment was not enough to stabilize austenite to room temperature, while after intercritical tempering, the reversed austenite became much more stable by the repeatedly enrichment of Mn, Ni and Cu to be remained to room temperature. During the third-step tempering process, the retained austenite remained stable, with volume fraction at high level as the tempering temperature and time changed. The increase in C content maintained retained austenite stable at higher intercritical tempering temperature, and the decrease in Cu and Ni concenteration decreased the volume fraction of retained austenite. The volume fraction of retained austenite was not influenced by the decrease of Nb content.During the multi-step intercritical heat treatment, the niobium-containing precipitates were formed in each step. The size of the precipitates decreased with the temperature decreased. The niobium precipitates were spherical, oblong and irregular, and mainly in size range of 2-10 nm, dispersing in the ferrite matrix. The precipitation of copper occurred in the ferrite and retained austenite during the seond and third step, respectively. Copper precipitates had large size of 10-30 nm and in shape of spherical. The copper precipitation was not uniform in ferrite matrix, because some ferrite that was transformed from alloy-rich bainite/martensite contained high copper content and relatively had high density of copper precipitates after tempering. Copper precipitates exsit in form of Fe-Cu B2 crystal structure in ferrite matrix with nanometer scale, and complex phase that consit of copper core and other alloying elements. The exsistence of nano-scale B2 structural precipitates significantly increased the precipitation densitiy which is beneficial to the precipitation strengthening effect. The copper precipitation in retained austenite increased the strength of retained austenite, significantly improved the stability of retained austenite.The annealing temperature has little impact on the volume fraction of retained austenite, with the increase in annealing temperature, the retained austenite content remained above 20%. While the intercritical tempering temperature determined the final volume fraction of retained austenite. The retained austenite increased first, arriving the maximum value, and then decreased again when the intercritical temperature increased. The highest content of retained austenite was obtained at temperature slightly above Acr. The yield strength of low alloyed steel can be further enhanced through putting the third-step tempering process ahead of intercritical tempering process. This is because of the yiled strength was determined by the strength of matrix that was obtained in the former step. The multi-step intercritical heat treatment effectively combined the multi-phase microstructure, the TRIP effect of retained austenite and the precipitation hardening effect together, contributing to the excellent combination of strength and ductility in low carbon and low alloyed steel. The prototype low alloyed steel have outstanding mechanical properties:yield strength> 700 MPa, uniform elongation> 20%, and total elongation> 30%.