Simulation of the controlled rolling and accelerated cooling of a bainitic steel using torsion testing A.B. Cota a , R. Barbosa b , D.B. Santos b,* a Department of Physics, Federal University of Ouro Preto, Ouro Preto, Minas Gerais, Brazil b Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Belo Horizonte, Minas Gerais, Brazil Received 31 August 1999 Abstract Controlled rolling, followed by accelerated cooling, was simulated by means of torsion tests. High-strength low-alloy (HSLA) low- carbon (0.08%) bainitic steel containing B, recently developed by the industry as a bainitic steel grade of the API X80 class, was examined. The in¯uence of cooling rate and ®nish-cooling temperature on the microstructure and mechanical properties were studied. The ®nal microstructure was predominantly bainitic. For a ®nish-cooling temperature of 4008C the microstructure consists of ®ne laths of bainitic ferrite with interlath MA constituent, and increase in the cooling rate leads to a continuous increase of the tensile and yield strengths of 158 and 183 MPa, respectively. The analysis of the results enabled the establishment of quantitative relationships between the accelerated cooling variables and the mechanical properties of steel. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Torsion testing; Accelerated cooling; Bainitic steel; Mechanical properties 1. Introduction Recently several investigations have been developed with the objective of studying the in¯uence of the cooling vari- ables (cooling rate and start- and ®nish-cooling tempera- tures) on the microstructure and mechanical properties [1±6] of high-strength low-alloy (HSLA) steels, through simula- tions of rolling and accelerated cooling processes. Such simulations are usually performed by rolling on laboratory or pilots mills and the cooling rates and ®nish-cooling temperatures after rolling are often dif®culties to control. In an attempt to ®nd an alternative to the conventional approach, a new method based on hot torsion testing to simulate controlled rolling and on gas-accelerated cooling was proposed in the present work. Computerized hot torsion testing is the technique that best simulates industrial rolling [7], enabling a excellent control of the thermo-mechanical processing parameters (temperature, deformation per pass, strain rate and inter-pass time). The use of gas-accelerated cooling also permits an easier and more precise control of the cooling rate and ®nish-cooling temperature. This technique was used to study the in¯uence of the cooling rate (Cr) and ®nish accelerated-cooling temperature (T FC ) on the microstructure and mechanical properties of a HSLA low-carbon bainitic steel, recently developed by the metallurgical industry. For the steel investigated the micro- structural evolution and the mechanisms for the re®nement of the microstructure are discussed. Quantitative relation- ships between the cooling variables and the mechanical properties of the steel were obtained by means of multiple regression analysis. 2. Experimental procedure The chemical composition of the steel used in the present investigation is given in Table 1. The steel contains Nb, Ti and V as microalloying elements and B (24 ppm) to enhance hardenability. Tubular torsion specimens (16.5 mm gauge length, 6.5 mm outer and 2 mm inner diameter) were machined from pieces cut longitudinally along the rolling direction. The hot torsion tests were carried out in a servo-hydraulic torsion machine with a computerized MTS TestStar II TM unit and Testware SX TM software, to control the hydraulic servo-valve and the data acquisition system. A thermo- Journal of Materials Processing Technology 100 (2000) 156±162 * Corresponding author. Tel.: 55-031-238-1803; fax: 55-031-238- 1815. E-mail address: dsantos@demet.ufmg.br (D.B. Santos). 0924-0136/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII:S0924-0136(99)00467-7