Effect of rolling conditions on ferrite refinement of low-carbon steel
Refinement of the ferrite grains provides a promising approach to simultaneously improving both the strength and the toughness of steels. Among recent techniques to obtain ultrafine ferrite, dynamic strain induced transformation (DSIT) is used. This type of treatment is sensitive to steel composition, deformation temperature, the prior austenite grain size, strain and cooling rate. This work is aiming to improve the mechanical properties of steels using ferrite grain refining through trials on industrial scale using a compact strip production plant (CSP) with hot strip mill (HSM) containing six stands F1-F6 in EZDK Steel Company. The chemical composition of the trials was C (~0.046), Si (0.073) Mn (0.50) and Ti (0.008). The effect of final deformation temperature of 750, 775, 800 and 815°C on the microstructure and mechanical properties was studied. In addition the effect of strain rate variation at the final two rolling stands was correlated with the ferrite refining mechanisms. The ferrite grain size attained after refining varies from 10 μm to be 6 μm. Yield and tensile strength increased slightly through changing deformation temperature, with the advantage of keeping almost the original high ductility. Toughness of the processed steel increased with ferrite grain refining.
Processing and Evaluation of Ferritic-Bainitic Multi-phase Steel
Production of bainite-ferrite multi-phase steels on industrial CSP scale provides approach to improve mechanical properties along with large cost reduction opportunity. In terms of hot-rolling conditions, chemical composition, and microstructure; characterizing ferritic-bainitic steels and their difference from ferritic-pearlitic steels regarding properties and cost becomes of a great importance to understand the optimum conditions for production. This work is aiming to improve mechanical properties of steels by producing bainite-ferrite steel through industrial scale runs on the  6 stands  hot strip mill (HSM) with at EZDK steel using low alloy Al-killed steel. The effect of hot rolling coiling temperature and alloy content on final properties was studied. In addition to characterizing differences between ferrite-bainite steels and conventional ferrite-pearlite steels. Ferrite-bainite steel has been successfully produced in different conditions, showing up to 27% (~100 MPa) improvement in yield strength, 8% (42 MPa) in tensile strength, 13% in total elongation, and 11% (27 J) in impact toughness. Vanadium microalloying has proven to have almost no effect on mechanical properties of ferrite-bainite steels upon its rising from 0.045% to 0.063% which is not the case in ferrite-pearlite steels. Ferrite-bainite steel offers a great margin for alloy cost saving compared to ultra-low carbon microalloyed steels (up to 56%) through replacing chemical strengthening with phase transformation strengthening. We expect this new approach to significantly reduce the cost of production without sacrificing the quality.
Experimental investigation of the geotechnical properties and microstructure of lime-stabilized saline soils under freeze-thaw cycling
Abstract The Song-Nen Plain in northeastern China holds one of the three largest expanses of soda saline-alkali soil in the world. Due to its disadvantageous geotechnical properties, the saline soil in this area is unable to meet the needs of construction projects. This study evaluates the stabilizing effect of adding lime to such soils. Specifically, it investigates how the physicochemical and mechanical properties and microstructure of soda saline-alkali soil are altered by lime and how these changes are affected by both freeze-thaw cycling and the duration of curing. The results reveal that adding lime to saline soil transforms clay particles into sand and silt particles and causes the pore size distribution (PSD) to become bimodal, with small-pore and macropore populations. An increase in lime content leads to an increase in the unconfined compressive strength (UCS) but a gradual decrease in both the cation exchange capacity (CEC) and plastic index. Freeze-thaw cycles break up coarse particles and aggregate finer particles, increasing the diameter of macropores and decreasing the USC; however, the effect on specimens with >9% lime is negligible. It is further shown that the geotechnical properties of the soil change immediately after the addition of lime, indicating that the cation exchange reaction takes place very quickly. Furthermore, the pozzolanic reaction leads to an increase in the percentage of sand-sized particles and a decrease in porosity after 28 days of curing. Overall, these results indicate that it is feasible to use lime to improve the engineering properties of soda saline-alkali soil in this area.