Linxiu Du

Austenite Grain Ultrarefinement and the Formation of Nanocrystallized Structure through Transformation of Low Carbon Steels

The effects of chemical composition, original microstructure, and heating rate on austenite grain refinement were investigated, and the results indicated that ultrafine austenite with grain size 1 µm can be obtained from the original microstructure of ferrite/pearlite after being warm rolled and cold deformed, and the ultrafine austenite could also be obtained through combined addition of Nb–V–Ti and by properly increasing heating rate. The transformation of the ultrafine austenite was also investigated, and the results indicated that when the heavy deformation was applied below Ar3 during cooling, the uniform and equiaxed ferrites with grain size of 0.1 ∼ 0.3 µm, close to nanosize, can be obtained. The main mechanism of deformation-induced transformation of ultrafine austenite to ferrite is that the nucleation of ferrite along austenite boundary is enhanced by the boundary slipping of austenite grains.

Influence of soaking temperature on transformation behaviour and precipitate coarsening of new cold rolled weathering steel containing niobium and titanium

Abstract The continuous cooling transformation (CCT) behaviour and precipitate coarsening of a new cold rolled weathering steel containing Nb and Ti were investigated. Three different soaking temperatures (780, 820 and 860°C) were applied to the steel. CCT diagrams were obtained by Formastor-F II automatic phase transformation analysis. The microstructures and second precipitated particles were examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The initial sample microstructures were identified as cold rolled ferrite and pearlite structures. Results showed that increasing soaking temperatures would decrease ferrite start temperature (Fs) and increase martensite start temperature (Ms). This was mainly attributed to the changes in austenite grain size as well as the average carbon content. At the lowest cooling rate of 0·5°C s−1, a small amount of martensite structure could be observed. Ms increased smoothly with lower cooling rates and showed a dramatic increase at elevated cooling rates, at which point a coarse structure was readily obtained from transformation of coarse grained austenite and composed of bainite and martensite structures in the inner and peripheral regions respectively. Niobium/titanium precipitate coarsening took place during the soaking process: coarsening became serious as soaking temperature increased.

Strain driven grain boundary sliding and/or rotation during tensile deformation of ultrafine grained C–Mn steel

Abstract The microstructure evolution of ultrafine grained C–Mn steel during tensile deformation was investigated using scanning electron microscopy. The surface morphologies and orientation imaging micrographs at different locations near the fractures were discussed. No obvious evident work hardening was identified and partially attributed to the strain driven grain boundary motion of grain rotation and/or grain boundary sliding, especially at the initial stage, while the dislocation activities gradually participate in as deformation proceeds.

Influencing factors for obtaining ultrafine austenite grains with initial microstructure of warm-rolled ferrite/pearlite

Three warm-rolled ferrite/pearlite microstructures were prepared by rolling at 500°C, and the austenitizing characteristics were discussed in conjunction with deformation during the heating stage. The results indicated that the final austenite grain size was sensitive to the deformation direction of the initial warm-rolled microstructure. The transient microstructure at a given temperature was the most important influencing factor on the austenitizing characteristic combined with deformation. Moreover, the hot-rolled microstructure also had to be prepared in an optimal state because of its direct effect on the warm-rolled microstructure.

Effect of austenite stability on toughness, ductility, and work-hardening of medium-Mn steel

ABSTRACT We elucidate here the stability of reversed austenite (RA) and its effect on mechanical properties in 0.05C–5.0Mn steel. With increased annealing temperature from 903 to 943u2009K, the volume fraction of RA was increased from ∼19 to ∼42%, while its stability decreased linearly because of reduced C and Mn enrichment. The tensile strength increased from 845 to 970u2009MPa, impact toughness at 233u2009K decreased from 135 to 98u2009J. The large volume fraction of RA with poor stability can significantly improve the work-hardening ability, but little contribution to impact toughness. The discontinuous transformation-induced plasticity effect is resulted from RA with different degree of stability because of difference in grain size and morphology. This paper is part of a Thematic Issue on Medium Manganese Steels.

Effect of deformation on deformation-induced carbides and spheroidization in bearing steel

Three different rolling processes were performed on bearing steel to study the effect of deformation-induced carbides (DIC) on spheroidization. The results indicated that the effect of DIC on spheroidizing process was twofold. On the one hand, a small number of DIC precipitated at the grain boundaries and deformation bands were beneficial to the spheroidizing process. On the other hand, substantial precipitation of network of deformation-induced carbides hindered the spheroidizing process. Based on the state of DIC, the rolling temperature above Arcm was divided into three parts: traditional rolling region (TRR), critical temperature region (CTR) and induced dual-phase region (IDR). There was near absence of DIC precipitated under the conditions of TRR, and the number of carbides increased with the decrease of rolling temperature. The degree of spheroidization increased when the specimens were rolled in the CTR with the same annealing time, whereas decreased in IDR. And the range of CTR decreased with the increase of pass reduction and decrease of rolling passes. The result of selected-area electron diffraction showed that the nature of DIC was M7C3, which was different from the carbide particles formed during the annealing treatment. The formation of DIC led to a decrease in the C and Cr content of the austenite, which was beneficial for spheroidization. In addition, the existence of DIC also provided nucleation sites for newly formed carbide particles and resulted in a carbon poor area which transformed to ferrite. Finally, ferrite matrix and spheroidized carbides would form directly instead of forming the lamellar pearlite by traditional cooperative mechanism.