Influence of Ti microalloying on the formation of nanocrystalline structure in the 201L austenitic stainless steel during martensite thermomechanical treatment

 Abstract

The martensite thermo mechanical treatment was used for the formation of nano/ultrafine grain structure in the 201L austenitic stainless steel containing 0.12 wt% Ti microalloying element. The initial microstructure was provided through homogenizing, hot rolling and solution annealing of the as-cast ingots. The specimens were then cold rolled between 5% and 90% thickness reduction and subsequently annealed at 750–900 1C for various times. The results showed a promoting effect of Ti on the formation of strain-induced martensite (SIM). A nano crystalline austenitic structure with average grain size of 45 nm was achieved by annealing at 900 1C for 60 s through a diffusional transformation mechanism. It was found that precipitation of nanosized TiC particles during the reversion annealing could retard the reversion process and suppressed grain growth in further annealing times. The tensile testing of the thermomechanically processed specimens showed a good combination of high yield strength(
1000 MPa, six times higher than that of the initial coarse-grained steel) and excellent ductility (42% total elongation) for the Ti microalloyed 201L steel due to the SIM formation during the deformation and impressions of the nanosized Ti carbides distributed within the nano/ultrafine grain structure.

Keywords:

201L stainless steel Ti microalloying Nano /ultrafine grained structure Martensite thermomechanical treatment Strain-induced martensite Reversion annealing.

Introduction

Extensive research has been carried out on grain refinement of metals and alloys down to the nano-grain (NG, do100 nm) and ultrafine-grain (UFG, do500 nm) scales in order to improve their mechanical properties. Whereas significant increases in hardness and strength have been reported for nanostructured materials,they often exhibit a very low tensile elongation to failure espe-cially uniform elongation (strain before necking) [1,2]. The reason for the low ductility in NG/UFG metals is plastic instability due to the limited strain hardening capacity that arises from the fact that extremely small grains cannot store dislocations to increase their density by order of magnitude as normally possible in the coarse-grained (CG) metals [3]. It has confirmed that any improvement of the strain hardening ability as a stabilizing mechanism to bring the plastic instability under control will be beneficial to enhancing the homogeneous plastic deformation for NG and UFG materials [4]. Several approaches have been developed in terms of microstruc-tural design for improving ductility of the NG and UFG metalswithout sacrificing their strength. These include dispersion of nanoparticles and nano-precipitates in the nanostructured metal matrix [5], to use transformation induced plasticity (TRIP) [6] and twinning induced plasticity (TWIP) [7] effects, enhancing the fraction of high angle grain boundaries and lowering the density of dislocations [8], and improving strain hardening rate by tailor-ing the stacking fault energy (SFE) [9].A recent thermomechanical route of the formation of NG/UFG structures in metastable austenitic steels called martensite treat-ment is characterized by strain induced martensitic transforma-tion and its reversion to austenite [10–13]. In this approach, severe deformation of austenite at room temperature leads to strain induced transformation of austenite (γ) to martensite (α′) and upon annealing, this heavily deformed strain induced martensite(SIM) transforms back to austenite leading to a noticeable grain refinement. Because of low SFE, austenitic stainless steels (ASSs) are susceptible to deformation induced martensitic transforma-tion. When ASSs are deformed, strain induced ε-martensite withthe hexagonal close packed (hcp) structure and α′-martensite with body centered cubic (bcc) structure are formed from the meta-stable austenite.

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