Zuqing Sun

Deformation enhanced transformation and dynamic recrystallization of ferrite in a low carbon steel during multipass hot deformation

Abstract Multipass hot deformation with an industry-acceptable pass reduction has been investigated by hot compression. A fine ferrite grain size can be achieved by carefully controlling the interaction of the deformation-enhanced transformation and dynamic recrystallization of ferrite. By the development of a ferrite grain size of 3–5 μm, the yield strength of Q235 grade plain carbon steel can be doubled, without significant loss of ductility, compared with material produced by conventional processing.

Effect of the alloying element chromium on the room temperature ductility of Fe3Al intermetallics

Abstract The effect of the alloying element chromium on the room temperature ductility of Fe 3 Al intermetallics has been investigated by considering its effect on (1) intrinsic ductility related to the microstructure, etc.; (2) extrinsic ductility related to the surface state. Based on the dissociation energy, the effect of atom interactions on the room temperature ductility is discussed after considering the site occupation of the substitutional element Cr at sublattice sites in DO 3 type stoichiometric Fe 3 Al intermetallics. The deduced results are in good agreement with those obtained from tensile tests of the samples with different structure and surface states in air and vacuum, and the observation of slip traces and APB trails left by imperfect dislocation slipping.

Improvement of room temperature tensile properties for Fe3Al-based alloys by thermomechanical and annealing processes

Abstract The effects of thermomechanical and annealing processes on room temperature tensile properties of Fe 3 Al-based alloys have been investigated. It is found that the thermomechanical process affects the room temperature tensile properties of Fe 3 Al-based alloys greatly. It is very important to control the hot deformation process so that a fine grain microstructure can be obtained before the final warm-rolling process. A higher room temperature ductility combined with a higher yield stress can be obtained by refining grains of Fe 3 Al-based alloys and obtaining the B2 ordered structure with a suitable surface state, such as oil film, Al 2 O 3 film and an aluminium-poor region near the surface. The intrinsic factors, such as microstructure and crystal structure, and extrinsic factors, such as surface condition, must be considered when the sensitivity to environmental embrittlement is investigated. An advanced thermomechanical process, named controlled thermomechanical process, to further improve room temperature tensile properties has been developed. It consists of hot forging an ingot to 20 mm at 1000–1200°C, hot rolling it to 10 mm at 850–1000°C, warm rolling to 5 mm thickness at 500–680°C, then annealing at 800°C for 1 h, then warm rolling to 2 mm thickness at 500–680°C.

On the effect of the B2 thermomechanical treatment in improving the room temperature ductility of Fe3Al-based alloys

The effect of the B2 thermomechanical treatment on the room temperature ductility of Fe3Al-based alloys has been systematically investigated by studying the order structure, the microstructure, the surface state and the texture evolution during rolling and annealing. A comparison between a single crystal Fe3Al-alloy and the polycrystalline Fe3Al alloy is presented in order to discuss the effect of the pancake shaped grains and the transverse grain boundaries on the room temperature ductility. The experimental results demonstrate that the order structure, microstructure and texture are mainly responsible for the improved intrinsic ductility, while an appropriate surface state can suppress the extrinsic environmental embrittlement by delaying the formation of microcracks. The pancake shaped grains hardly influence the hydrogen embrittlement, but could be responsible for the improved ductility at room temperature by increasing the resistance to crack propagation.

Formation of ultrafine grained ferrite in low carbon steel by heavy deformation in ferrite or dual phase region

Abstract The microstructural refinement supplies an alternative approach to improve the strength of low carbon steel without loss of room temperature ductility. This paper reports the methods to obtain the ultrafine grained ferritic microstructure by the heavy deformation of low carbon steel in ferrite or dual phase region. Various initial microstructures before deformation were selected. The deformation microstructure was analyzed by optical microscopy and electron back-scattered diffraction (EBSD) technique. It has been demonstrated that the equiaxed ferritic microstructure with a grain size of around 3xa0μm was obtained. The coarse pre-eutectoid ferrite dynamically recrystallized to form the ultrafine grained ferrite and the undercooled austenite was transformed to the ultrafine ferrite by the strain-induced austenite to ferrite transformation.

Modeling of microstructural evolution during dynamic recrystallization in coarse Nb microalloyed austenite

Abstract The aim of the current study was to investigate the microstructural evolution during dynamic recrystallization in coarse Nb microalloyed austenite in thin slab direct rolling (TSDR) processing. A model was developed to predict the change of the austenite grain size during the dynamic recrystallization, by using the law of mixtures. The equations initially developed for partial static recrystallization were used for partial dynamic recrystallization, by adjusting the value of the constant. The results show that the change of the austenite grain size can be reasonably described by using the equations developed according to the law of mixtures.

An analytical simulation of solidification behavior within deposited preform during spray forming process

Abstract Solidification within spray deposited preform is one of the key processing stages of spray forming, which has a substantial influence on the formation of the final microstructure of the preform. For the sake of optimizing the process and getting desired microstructure and properties of the preform, the solidification behavior should be fully understood. In this investigation, a concept of ‘virtual’ solidification layer was applied to establish an analytical model to simulate the solidification behavior within the preform during spray forming process, which will provide clearer physical pictures of the solidification behavior. According to the model developed, a simulation of the solidification behavior was performed for a typical Al–Cu alloy. The results show that both the processing parameters (such as deposition rate and temperature of spray cone at the point of impingement) and the thermophysical properties of the material are the important factors controlling the solidification behavior. Even for the same thermodynamic state of spray cone at the point of impingement, the top surface temperature of the preform will change conspicuously as variation of the processing parameters or thermophysical properties of the material takes place, which will cause the variation of microstructure and properties of the preform. Therefore, the principle of taking the thermodynamic state of spray cone at the point of impingement as the key factor controlling the spray deposition process should be modified adequately. A new parameter (the surface temperature of the preform) for the processing control of spray forming was proposed.

Mechanical properties of fine-grained dual phase low-carbon steels based on dynamic transformation

Abstract The fine grained dual phase (FG-DP) steel with ferrite grains of 2-4.5 μm and martensite islands smaller than 3 μm was obtained through the mechanism of deformation-enhanced ferrite transformation (DEFT). Mechanical properties of the steel were tested at room temperature. The results indicated that with a similar volume fraction of martensite (about 20vol%), FG-DP steel exhibited a superior combination of higher strength and more rapid strain hardening at low strains compared with the coarse-grained dual phase (CG-DP) steel obtained by critical annealing. The combination of higher strength, large elongation, and more rapid strain hardening of FG-DP steel can be attributed to the fine ferrite grain and finely dispersed martensite islands. In addition, the uniformly distributed martensite islands in FG-DP steel have smaller interspacing compared with that of CG-DP steel. So, at the initial plastic deformation stage, the plastic deformation of ferrite was restrained and more pronounced load was transferred from ferrite to martensite. The plastic deformation of martensite in FG-DP steel started earlier.

Characteristics of microstructural evolution during deformation-enhanced ferrite transformation in Nb-microalloyed HSLA steel

Abstract Microstructure evolution during deformation of undercooled austenite at 760°C was investigated in Nb-microalloyed steel by using SEM (scanning electron microscope), TEM (transmission electron microscope), and EBSD (electron backscattered diffraction). It is indicated that during deformation-enhanced ferrite transformation (DEFT) in Nb-microalloyed steel, the incubation period is prolonged, and the higher strain is needed to accomplish ferrite transformation. Therefore, the transformation kinetics curves move to high strain parallelly; and the transformation kinetics curves of Nb-microalloyed steel can be divided into three stages. At the first stage, the solute drag effect of Nb and the consumption of strain energy for the dynamic precipitation of Nb (CN) led to a long incubation period, and at the second stage, ferrite transformation was accelerated significantly and fine Nb (CN) precipitates restrict the grain growth of ferrite effectively. The results also showed that DEFT in Nb-microalloyed steel is still a nucleation dominated process, and during the microstructure evolution the interchange of and texture was obtained.

Room temperature mechanical behavior of B2-ordered Fe3Al single crystals

Abstract A detailed study of room temperature mechanical behavior of B2-ordered Fe 3 Al single crystals had been conducted. It was found that the yield strength, plastic elongation and work hardening rate were changed with orientation. The orientation near [110] has 42% plastic elongation. At near [001] orientation, the plastic elongation is only 8.2%. The high work-hardening rate of the orientation near [110] was confirmed to be the result of superdislocation interaction. The dissociation of 2-fold superdislocations into single dislocation led to cross-slip, which corresponded to the parabolic stage of stress–strain curve. The low work-hardening rate of near [112] was due to the fact that only one single primary slip system is activated and the superdislocations remained 2-fold until the specimen was fractured.