________________ Biography: XU Pingguang (1973), Male, Doctor. Email: xupingguang@tsinghua.org.cn or tomota@mx.ibaraki.ac.jp In Situ Neutron Diffraction of Austenite-to-Ferrite Transformation in Nb-free and Nb-added Low Alloy Steels during Thermo-mechanically Controlled Process XU Ping-guang 1 † , TOMOTA Yo 1 , LUKAS Peter 2 , ADACHI Yoshitaka 3 (1-Graduate School of Science and Engineering, Ibaraki University, Hitachi, 316-8511, Japan; 2-Nuclear Physics Institute, 250 68 Řež, near Pargue, Czech Republic; 3-National Institute for Materials Science, Tsukuba, 305-0047, Japan) Abstract: Microstructure evolution of two Nb-free and Nb-added low alloy steels during step-by-step ferrite transformation and the influences of Nb addition and prior austenite deformation were investigated by in situ neutron diffraction. The ferrite volume fraction estimated from the profile analysis is in a good consistence with that measured in the metallographic microstructure of a specimen quenched from the corresponding temperature. The changes in lattice plane strain during the step-by-step ferrite transformation were analyzed. The evident preferred orientation and residual strain were observed in the prior deformed austenite, especially in the Nb-added steel, which resulted in a higher starting temperature of ferrite transformation. Key words: Neutron diffraction; thermomechanically controlled process; low alloy steels, phase transformation 1 Introduction In order to obtain a bulky material with an ultrafine-grained multiphase microstructure showing a good strength-toughness balance in modern low alloy steels, the advanced thermomechanically controlled process (TMCP) has been developed in the last decades by employing heavy deformation at lower temperature [1-3] . Heavy deformation in non-recrystallization region of austenite is helpful for increasing the ferrite nucleation site and results in the ultrafine ferrite grain by static transformation during cooling process. Meanwhile, the microalloying technique based on precipitating the thermally stable nanometer particle has been investigated to prevent the grain growth in heat affected zone (HAZ) [4] . In order to clarify the formation mechanism of ultrafine grained ferrite, the in situ characterization of microstructure evolution during the austenite-to-ferrite transformation is urgently needed. However, due to the technical difficulty for integrating microstructure evaluation apparatus and TMCP equipments, the relevant investigations have not been carried out yet. Recent development of in situ neutron diffraction technology enables us to find the bulky crystallographic information about the deformation and/or transformation behavior of metallic materials, such as grain rotation, texture, internal stress, phase-volume fraction, grain size and dislocation density [5-7] . Since the neutron beam is too weak to achieve time-division measurement to follow a rapid transformation in a low carbon steel, 2%Mn was added to make austenite to ferrite transformation slower and the austenite-to-ferrite transformation evolution during the step-by-step cooling were investigated in this paper by employing the in situ neutron diffraction. The effects of Nb addition and prior austenite deformation were also studied. 2 Experimental Procedures The chemical compositions of the two experimental steels are 0.2C-2.0Mn (Nb-free steel) and 0.2C-2.0Mn -0.03Nb (Nb-added steel). Two kinds of experiments were performed by using the two relevant machines: one was the in situ neutron diffraction measurement (Test I) and the other was the microstructure observation after quenching a specimen at different temperatures (Test II) during TMCP. A common TMCP schedule was employed as shown in Fig.1. The specimen was heated up to 900°C with a heating rate of 5 °C/s and held there for 0.6ks to obtain single