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

Abstract

The microstructural changes in 304L and 316L austenitic stainless steels during plate rolling with 95% rolling reduction at a temperature of 473 K and their effect on strengthening were studied. The microstructure evolution was associated with deformation twinning and microshear banding. The latter ones involved ultrafine crystallites, which rapidly evolved in strain-induced ultrafine austenite grains as a result of fast increase in misorientations between them. Besides the ultrafine austenite crystallite evolution, the microshear bands assisted local appearance of deformation martensite, which attained about 25 vol.% and 3 vol.% at total strain of 3 in 304L and 316L steels, respectively. Both the microshear banding and the martensitic transformation promoted the formation of ultrafine grains with a size of less than 1 µm. The strain dependence of the ultrafine grain fraction obeyed a modified Johnson-Mehl-Avrami-Kolmogorov function. The deformation grain size and dislocation density that develop during rolling could also be expressed by exponential functions of true strain. Incorporating the revealed relationships between the strain and the microstructural parameters into modified Hall–Petch-type equation, unique expression for the yield strength of processed steels was obtained. The dislocation strengthening was the largest contributor to the strength, especially at small to medium strains, although grain size strengthening increased during rolling approaching that from dislocations at large strains.

Microstructures and Mechanical Properties of Steels and Alloys Subjected to Large-Strain Cold-to-Warm Deformation

“…In spite of apparent simplicity, Equation ( 5) provides fairly good agreement with experimental data for various materials (Figure 8) [74,75]. Note here that Equation ( 5) well predicts the mean grain size even in metastable austenitic steels experiencing partial martensitic transformation during cold deformation (304L in Figure 8a), because the grain refinement due to the transformation can be discussed with the same speculation leading to Equation (4).…”

“…The strain dependencies for the grain size and the dislocation density (Equations ( 5) and ( 7)) predict a power-law function between the microstructural parameters, i.e., the deformation grain size and dislocation density. Figure 11 shows the relationship between the grain size (the transverse grain size in the case of unidirectional straining) that developed at various deformation stages and the corresponding dislocation density evolved in these grains/subgrains [75,88,[95][96][97][98][99]. It is clearly seen in Figure 11 that a linear function with a slope of −2 generally holds between the grain size and the dislocation density plotted in double log scale.…”

“…Taking ρ 0 << ρ and r = 1 for simplicity, the strain effect on the evolution of the dislocation density in various metallic materials is displayed in Figure 10. The symbols in Figure 10 indicate the experimental values of dislocation density obtained by either direct counting the individual dislocations in grain/subgrain interiors, using transmission electron microscopy (TEM) [55,75,88] or evaluated by orientation imaging microscopy (OIM) [74,88] with automatic analysis of electron backscattered diffraction (EBSD) patterns. The lines in Figure 10 correspond to approximation by Equation (7).…”

“…Figure 11. Relationship between the grain size and dislocation density in various steels, i AISI stainless steels, high-manganese steel (12Mn), highly alloyed austenitic stain less stee and copper alloys processed by cold-to-hot working[75,88,[95][96][97][98][99].…”

High-Temperature Precipitation Design-of-Experiments Simulation in Low-Alloy Cr–Mo–Ni Hot Forging Steel

“…If the lamellar spacing indicated in nm in Figure 4 is analyzed, it is difficult to connect strain or strain rate with the spacing of martensite or bainite as a function of the position in the deformed bar. On the other hand, this spacing, clearly related to hardness (Figure 2), with higher hardness when spacing is smaller [18], is a function of the cooling rate: zones near the surface will cool at a higher rate than those at the interior of the part. Moreover, differences in hardness were more evident at the upper zone (Figure 3), which suggests that PAGS is very important when defining the mechanical properties and that lamellar spacing is only part of the phenomenon.…”