Chemistry And Process Design For Achieving Nano-Cementite Precipitate In Hypoeutectoid Steel

Nowadays, the steel industry is facing the increasingly serious shortage of raw material, energy supply and environmental pollution. From the perspective of sustainable development, the development of low cost, high performance, environmentally friendly products becomes the important direction of the steel industry. As the most economic strengthening method, cementite has drawn extensive attention of steel researchers. In plain carbon steel, the phase fraction of cementite can be as large as10%without any increase in production cost. If cementite can be effectively refined to the nanometer scale, it can also produce an equivalent or even larger strengthening effect than microalloy carbonitride. In this paper, through the reasonable design of chemistry and NG-TMCP process, the mechanism of divorced pearlite transformation is investigated. This is targeted to the full use of strengthening effect of cementite. The chief original work of this paper is as follows:(1) Using "ultra fast cooling (UFC)+thermo-mechanical treatment (TMT)" process, hot rolling of Fe-0.17%C-0.7%Mn was carried out at laboratory scale. The obtained microstructure is observed by optical microscope (OM) and scanning electron microscope (SEM). And the nano-scale cementite precipitate in both pro-eutectoid ferrite and bainite matrix was analyzed by transmission electron microscope (TEM). The tensile properties and impact toughness was evaluated. Results showed that good impact toughness was achieved, and the strength increased with the decreasing of UFC stop temperature.(2) Based on transformation thermodynamics and kinetics theory, the effect of Cr on austenite-to-ferrite and austenite-to-cementite transformation has been analyzed. Through reasonable design of chemistry, the nucleation of cementite can be promoted. Further, due to the kinetic difference of these two transformation process, the cooperative pearlite transformation is destroyed and therefore, the dispersed nano-scale cementite were achieved. The idea in the design of chemistry was then validated by kinetic calculation with Dictra.(3) Static isothermal transformation experiments were conducted for Fe-C-Mn-Cr steel. Large number of dispersed nano-scale cementite precipitates with the average size of50nm had formed at600℃and560℃. This has proved the feasibility of the chemistry design for divorced pearlite transformation. The orientation relationship and the partition of Cr and Mn between cementite particle and the surrounding ferrite matrix were also analyzed by TEM. (4) Laborotary-scle hot rolling was done for Fe-C-Mn-Cr steel. Using UFC technology, the "ferrite/bainite+dispersed nano-scale/submicron cementite" microstructure was obtained by controlling the UFC stop temperature in the range of600～445℃, proving the effectiveness of the design of chemistry and process in Fe-C-Mn-Cr steel. Compared with the lamellar cementite obtained by traditional ACC cooling, the dispersed cementite obtained by divorced pearlite transformation did not make extra contribution to the strength of hypoeutectoid steel, but obviously improved its formability. The hole-expansion ratio can be as large as87.3%.