The deformation microstructures and mechanical properties of a 316L austenitic stainless steel subjected to large strain cold or warm plate rolling are studied. The cold or warm rolling was carried out at room temperature or 573 K, respectively, to different total true strains up to 3. The structural changes are characterized by the development of nanocrystalline structures with the transverse grain size of about 80 or 160 nm at room temperature or 573 K, respectively. The evolution of nanocrystalline structure under cold working conditions (room temperature) is assisted by the development of mechanical twinning and partial martensitic transformation that result in rapid grain refinement. The development of nanocrystalline structures during cold/warm working is accompanied by significant strengthening. The yield strength approaches 1 680 or 1 070 MPa after cold or warm rolling to a total strain of 3, respectively.
Abstract:The microstructural changes leading to nanocrystalline structure development and the respective tensile properties were studied in a 304L stainless steel subjected to large strain cold rolling at ambient temperature. The cold rolling was accompanied by the development of deformation twinning and martensitic transformation. The latter readily occurred at deformation microshear bands, leading the martensite fraction to approach 0.75 at a total strain of 3. The deformation twinning followed by microshear banding and martensitic transformation promoted the development of nanocrystalline structure consisting of a uniform mixture of austenite and martensite grains with their transverse sizes of 120-150 nm. The developed nanocrystallites were characterized by high dislocation density in their interiors of about 3 × 10 15 m −2 and 2 × 10 15 m −2 in austenite and martensite, respectively. The development of nanocrystalline structures with high internal stresses led to significant strengthening. The yield strength increased from 220 MPa in the original hot forged state to 1600 MPa after cold rolling to a strain of 3.
The microstructure/texture evolution and strengthening of 316 L-type and 304 L-type austenitic stainless steels during cold rolling were studied. The cold rolling was accompanied by the deformation twinning and micro-shear banding followed by the strain-induced martensitic transformation, leading to nanocrystalline microstructures consisting of flattened austenite and martensite grains. The fraction of ultrafine grains can be expressed by a modified Johnson-Mehl-Avrami-Kolmogorov equation, while inverse exponential function holds as a first approximation between the mean grain size (austenite or martensite) and the total strain. The deformation austenite was characterised by the texture components of Brass, {011}<211>, Goss, {011}<100>, and S, {123}<634>, whereas the deformation martensite exhibited a strong {223}<110> texture component along with remarkable γ-fibre, <111>∥ND, with a maximum at {111}<211>. The grain refinement during cold rolling led to substantial strengthening, which could be expressed by a summation of the austenite and martensite strengthening contributions.
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