Complicated Interaction of Dynamic Recrystallization and Precipitation During Hot Deformation of Ultrahigh-Strength Stai

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INTRODUCTION

LOW-ALLOYED, ultrahigh-strength steel, such as the well-known 300M steel grade, has been widely used due to its excellent combination of ultrahigh ultimate tensile strength (UTS) of around 1700 MPa and good toughness. Its poor corrosion resistance, however, leads to a remarkably reduced service life in the case of attacking environments. Thus, it has to be coated with cadmium, which is harmful to the environment and shall be banned in the future. Therefore, ultrahigh-strength stainless steel (UHSSS) is then considered to be an alternative for this application[1,2] because it has a good combination of corrosion resistance, ultrahigh strength, and high toughness. Recently, we have developed a new type of UHSSS containing high contents of Cr, Ni, Co, XIAOHUI WANG, Ph.D. Student, is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xue Yuan Lu, Beijing 100083, P.R. China, and also Researcher with the Division of Special Steels, Central Iron and Steel Research Institute, 76 Xue yuan Nan Lu, Beijing 100081, P.R. China. ZHENBAO LIU, Senior Engineer, is with the Division of Special Steels, Central Iron and Steel Research Institute. HAIWEN LUO, Professor, is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing. Contact e-mail: [email protected]. Manuscript submitted April 18, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

and Mo. It exhibits a UTS higher than 1900 MPa, excellent corrosion resistance, and good toughness.[3,4] Nevertheless, the heavy alloying leads to high resisting force and poor hot plasticity during hot working and, unfortunately, to frequent cracking. Therefore, we have to optimize the hot working process to suppress cracking via recrystallization;[5–7] the latter can restore the hot plasticity of work hardened materials and lead to a significant grain refinement. Dynamic recrystallization (DRX) often occurs during the hot deformation of stainless steels because they have low stacking fault energy (SFE).[8–10] DRX is usually characterized by the nucleation and growth of new grains at pre-existing grain boundaries. Figure 1(a) schematically shows typical flow stress–strain curves, resulting from different restoration mechanisms during hot deformation. When dynamic recovery (DRV) operates during deformation, stress first increases due to the work hardening and then approaches to a constant value, rS1.[11] When DRX occurs, the flow curve usually exhibits a peak, followed by a lower and steady flow with the proceeding deformation. DRX is initiated at the critical stress, rc, which is usually smaller than the peak stress, rp. Mirzadeh and Najafizadeh have managed to determine the critical stress for DRX from the curve of strain hardening rate, h, vs strain.[12] At the critical stress, the local bulges of grain boundaries grow

and occupy the prior boundaries and finally form the so-called ‘‘necklace structure’’ until the peak stress.[13,14] In particular, Poliak and Jonas[15] have demonst