Grain refinement and strengthening of austenitic stainless steels during large strain cold rolling

Abstract

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.

Microstructural Changes and Strengthening of Austenitic Stainless Steels during Rolling at 473 K

“…It should be noted that the dependencies in Figure 7 differ only in the coefficient B, which is greater for 304L steel. Almost the same value of m and much larger values of B and A have been reported for the same steels subjected to rolling at room temperature [15] that reflects slowing down the transformation kinetics with an increase in deformation temperature. Note here that an increase in rolling temperature to 573 K fully suppresses martensitic transformation in these steels [42].…”

“…Thus, a decrease in the mean grain size results from increasing the number of ultrafine grains with a size of D UFG . Therefore, the mean austenite/martensite grain size (D ε ) in the present steels during rolling at 473 K can be expressed as follows [15,28]:…”

“…Among various methods proposed to achieve the large strains, rolling is the most efficient method for production of the sizable semi-products of austenitic stainless steels. A number of researches were devoted to the deformation microstructure and mechanical properties of austenitic stainless steels (including 304 and 316 series) after cold rolling at room temperature [9][10][11][12][13][14][15][16]. The frequent operation of twinning and martensitic transformation during cold rolling was reported to result in the rapid refinement of the microstructure.…”

“…The frequent operation of twinning and martensitic transformation during cold rolling was reported to result in the rapid refinement of the microstructure. As a result of cold rolling to large strains, austenitic stainless steels experienced substantial strengthening; their yield strength could exceed 1500 MPa [10][11][12]15]. On the other hand, plasticity of these cold worked steels degraded remarkably down to a few percent of tensile elongation.…”

“…The strengthening of metals and alloys owing to large strain cold or warm deformation is generally discussed in terms of either grain boundary strengthening through the Hall-Petch relationship [21,22], or dislocation strengthening [23,24], or modified Hall-Petch-type relationship including dislocation strengthening [25][26][27]. It should be noted that both structural parameters, namely, the grain size and the dislocation density, that evolved during cold to warm rolling can be expressed by functions of true strain [15,28,29]. Such approach that could allow expressing the yield strength as a function of true strain seems to be quite beneficial.…”