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 × 1015 m−2 and 2 × 1015 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.
Highlights
The large strain deformations are considered as promising methods for development of advanced structural steels and alloys with enhanced mechanical properties [1,2]
The grain refinement in these steels is promoted by an intensive grain subdivision, which is associated with deformation twinning followed by strain-induced martensitic transformation [9,10,11,12,13]
An early deformation is accompanied by the frequent development of deformation twinning, which is typical feature of austenitic steels with low stacking fault energy [9,10,13,20], followed by the martensitic transformation
Summary
The large strain deformations are considered as promising methods for development of advanced structural steels and alloys with enhanced mechanical properties [1,2]. The significant improvement of mechanical properties of metallic materials subjected to severe plastic deformations is commonly attributed to the strain-induced ultrafine-grained or, even, nanocrystalline structures [3,4,5]. The efficiency of cold working for processing the high-strength ultrafine-grained/nanocrystalline products depends remarkably on the kinetics of grain refinement during plastic deformation. The grain refinement in these steels is promoted by an intensive grain subdivision, which is associated with deformation twinning followed by strain-induced martensitic transformation [9,10,11,12,13]. In spite of a number of research works dealing with nanocrystalline stainless steels processed by large strain cold working, the mechanisms of microstructure evolution, i.e., a role of deformation twinning and strain-induced martensite, and their contribution to strengthening are still unclear
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