Effect of thermo-mechanical parameters on microstructure and mechanical properties of microalloyed steels

454 G.R. Ebrahimi et al. Effect of Thermo-Mechanical Parameters on Microstructure and Mechanical Properties of Microalloyed Steels G.R. Ebrahimi and M. Javdani Metallurgy and Materials Engineering Department, Sabzevar Tarbiat Moallem University, Sabzevar, Iran H. Arabshahi Physics Department, Ferdowsi University of Mashhad, Mashhad, Iran∗ (Received on 29 September, 2010) In this work the effects of controlled rolling parameters and adding of Niobium have been studied. In this order two steel grades with and without Niobium are planed and after steelmaking and continuous casting, rolling process are done. Then, laboratory investigations such as microstructure, mechanical properties and grain size analysis were performed Tensile and Charpy impact tests specimens were machined out of the central part of the rolled billets. The microstructure of the specimens was examined for each experimental condition using optical microscopy. The results indicate that increasing the reheating temperature above the dissolution temperature of Nb (C, N) improved the impact energy values. By increasing the cooling rate from 0.5 to 1.5 ◦ C/s both tensile strength and impact toughness were improved. High elongation percent was also observed on samples reheated at higher temperature and/or cooled with the higher cooling rates. The obtained mechanical properties were related to the characteristics of microstructural components including acicular ferrite, retained austenite, pearlite and ferrite. Keywords: Microalloyed steel, hot rolling, Niobium, microstructure, mechanical properties. 1. INTRODUCTION Nowadays, most of ferritic-perlitic precipitation hardening steel grades are microalloyed steel. These steels after rolling or forging process would be cooled as completely controlled. The properties of these steel grades would be affected by solidification microstructures, thermomechanical process and cooling process after rolling [1]. Niobium has a threefold influence on the mechanical properties of steel which are as grain size refinement during thermomechanical hot forming, precipitation hardening and lowering the γ to α transition temperature. Grain refinement is the only mechanism that simultaneously increases strength, toughness and ductility. Niobium-microalloyed steel has become a standard material in plate and strip for line pipe, automotive and construction use. Until now, the high potential of microalloyed high strength steel has not been used to the same extent in long products. Nb (C, N) precipitates that have formed and grown at high temperature and in austenite phase, prevent from grain coarsening in the subsequent stage of hot deformation. Other diffusion controlled process that occur with solution of niobium in austenite are retarding of γ to α transformation that cause to increase nucleation of ferrite and reduce grain growth rate, forming of quasi-equivalence structures like bainit and finally appearance very fine Nb(C, N) precipitates during transformation that being coherent interface cause to increase strength with precipitation hardening mechanism [2,3]. ∗ Electronic address: 2. EXPERIMENTAL EQUIPMENT, MATERIAL AND PROCEDURE The material used in this investigation was produced in an electric arc furnace equipped with semi-automatic charging system. Secondary steelmaking operations were performed in a ladle furnace with vacuum degassing units to eliminate oxygen and nitrogen as well as inclusion modification capabilities. For improving mechanical properties of structural steels, quantitative amount of Niobium would be added to general composition of these steel. For this purpose after preparation melting of base composition in an electric arc furnace, in steel making process two grades of these steels with and without Niobium are provided. Then in heavy section mill the blooms were hot rolled to a cross section of 125×125 mm2 billets. Finally in light section mill the billets hot rolled to 65 mm diameter bars. Steel bars were inspected by magnetic particles inspection method to identify and remove any possible surface cracks. To reveal prior austenite grain boundaries and to determine the effect of reheating temperature on austenite grain size, specimens with 15 mm diameter and 25 mm height with their axis parallel to the axis of the bar were prepared from the material. They were then heated in an electrical furnace with SiC heating elements between 1000 to 1250 ◦ C for 25 min followed by water quenching. The specimens were then tempered at 450 ◦ C for 4 h to improve grain boundary etching. After usual grinding and polishing operations they were etched in a supersaturated solution of warm picric acid and water with the addition of cupric chloride. Digital pictures were prepared by using optical microscopy and average austenite grain sizes were measured using the linear intercept method. An extensive number of relationships have been proposed for the dissolution temperature of Nb carbonitrides in microalloyed steels. In the present study, the above temperature was estimated using the following relationships proposed by Tamura et al. [4] whose steel compositions are the same as 455 Brazilian Journal of Physics, vol. 40, no. 4, December, 2010 TABLE 1: Chemical composition of steel investigated (wt %). Si Mn S Nb N 1 0.36 0.42 1.08 0.01 - 130 2 3 0.30 0.30 0.39 0.39 0.98 0.97 0.006 0.015 0.036 0.041 220 145 used in this work, Log [Nb] [C] = − (6661/T) + 2.54 Log [Nb] [C] = − (10960/T) + 5.43 Log [Nb] [C] = − (7900/T) + 3.42 The above relationships lead to dissolution temperatures between 1195 ◦ C to 1220 ◦ C for both of compositions [56]. The billets were reheated to 1180 ◦ C or 1240 ◦ C in a continuous walking beam furnace for 60 min. The selection of the reheating temperature was made with the objective to study the effect of smaller austenite grain size and larger carbonitrides (reheating at 1180◦ C) versus large austenite grain sizes and very fine carbonitrides (upon cooling from 1240 ◦ C) on final mechanical properties. Special pyrometer was used to measure the temperature of the billets at the exit of the walking beam furnace. The specimens were then immediately started to deform as shown schematically in figure 1a. After 17 passed rolling and 93% reduction of area, the bars with 65mm diameter were produced. The as rolled bars were then cooled at room temperature with different cooling rates. These were approximately air cooled (0.5 ◦ C/s), and water-spray with forced air cooled (1.5 ◦ C/s). Tensile and impact test specimens were prepared from the center of the deformed bars according to ASTM E8 and E23 standards, respectively. Metallography samples, perpendicular to the rolling direction, were extracted from the extremities of the impact test specimens. They were then cold mounted in bakelite, polished, and etched with 5% nital. An optical microscope instrumented by image analysis software was used for microstructure examination. 3. RE (...truncated)