Microstructures and properties of ferrite/bainite dual-phase steels with high hole-expanding ratio

The effects of Si and Mn contents on microstructure, mechanical properties and formability of low carbon Si-Mn steels were studied, and the crack propagation of ferrite/bainite dual-phase steel was also investigated. The results showed that the increase in Si content increases the volume fraction of equiaxed ferrite. However, the increase in Mn content increases both strength and ductility, but decreases elongation and hole-expanding ratio. The crack of ferrite/bainite dual-phase steel is formed by the mode of microvoid coalescence. When a microcrack meets the bainite, it mostly propagates along the phase interface between ferrite and bainite and by cutting off ferrite grains. The hot-rolled ferrite/bainite dual-phase steel, which has a hole-expanding ratio of 95% and good property combination, could be produced by designing proper contents of Si and Mn as well as parameters of TMCP.

Dynamic strain-induced ferrite transformation during hot compression of low carbon Si-Mn steels

The dynamic strain-induced transformation (DSIT) of austenite to ferrite was investigated under different undercooling conditions using three low carbon Si-Mn steels. The undercooling of austenite (ΔT) was controlled by varying the cooling rate between austenitization and deformation temperatures. Uniform DSIT ferrite grains (∼2.3 μm) were produced at a relatively high deformation temperature above 840°C using a low carbon high Si steel (0.077C-0.97Mn-1.35Si, mass%) in connection with a larger ΔT. The critical conditions for DSIT were determined based on the flow stress-strain curves measured during hot compression tests. Influence of deformation temperature on DSIT of low carbon Si-added steel was also discussed.

Strain hardening behavior of high performance FBDP, TRIP and TWIP steels

Based on n-value differential equation and microstructural observation, strain hardening behaviors of FBDP, TRIP, and TWIP steels during uniaxial tension were investigated. TRIP steel exhibits both superior strength and ductility than FBDP steel, and TWIP steel displays much higher total and uniform elongations in comparison to FBDP and TRIP steels. The instantaneous n values of FBDP and TRIP steels increase at small strains, reach a maximum value, smoothly decrease at higher strains, and then rapidly drop up to the specimen rupture. The strain hardening of TRIP steel persists at higher strains where that of FBDP steel begins to diminish. TWIP steel exhibits gradually increased instantaneous n values over the whole uniform plastic deformation, implying that TWIP steel shows a much larger strain hardening capability than FBDP and TRIP steels.

Continuous cooling transformation of deformed austenite in highly hole-expandable steels

The effects of Si and Mn contents on transformation temperature _r3, transformed microstructure and mechanical properties of three kinds of low-carbon steels during continuous cooling were investigated. A _r3 rises by 15-25°C when increasing Si content from 0.50% to 1.35%, and it drops by 30-50°C when increasing Mn content from 0.97% to 1.43%. The effect of Mn on A _r3 is more significant than Si. Si stimulates the precipitation of the high-temperature equiaxed ferrite to suppress the bainite transformation, but Mn not only provides the grain refining of transformed microstructure but also stimulates the forming of bainite. The homogeneous and grain refining diphase ferrite/bainite steel (w(Si)=0.56, w(Mn)=1.43) can be obtained after deformed at 850°C and cooled at the rate 30°C/s, of which the tensile strength is up to 654 MPa.

Effects of Si on microstructural evolution and mechanical properties of hot-rolled ferrite and bainite dual-phase steels

Based on the thermo-mechanical controlled process, the effects of Si on microstructural evolution, tensile properties, impact toughness, and stretch-flangeability of ferrite and bainite dual-phase (FBDP) steels were systematically investigated. The addition of Si from 0 to 0.95% promoted the formation of fine and equiaxed ferrite grains, and high Si (0.95%) also resulted in the formation of blocky martensite islands and retained austenite. Yield and tensile strengths, and uniform and total elongations all increased with increasing Si content. Therefore, the tensile strength and ductility balance was improved by Si addition due to the increasing strain-hardening rate. The fractured morphologies after hole-expansion showed that the excellent stretch-flangeability of FBDP steels was associated with the micro-cracks propagating through in ferrite phase as well as the elongated ferrite grains along the direction perpendicular to the crack. 0.95% Si steel had a similar high combination of tensile strength and impact toughness to 0.55% Si steel, and especially 0.95% Si steel exhibited an excellent combination of tensile strength and stretch-flangeability.

Transformation behavior of low carbon steels containing two different Si contents

Continuous cooling transformation behaviors of low carbon steels with two Si contents (0.50% and 1. 35%) were investigated under undeformed and deformed conditions. Effects of Si contents, deformation, and cooling rates on y transformation start temperature (A,_r3), phase microstructures, and hardness were studied. The results show that, in the case of the deformation with the true strain of 0. 4, the length of bainitic ferrite laths is significantly decreased in low Si steel, whereas, the M/A constituent becomes more uniform in high Si steel. An increase in cooling rates lowers the A,_r3 greatly. The steel with higher level of Si exhibits higher A,_r3, and higher hardness both under undeformed and deformed conditions compared with the steel with a lower Si content. Especially, the influence of Si on A_r3 is dependent on deformation. Such effects are more significant under the undeformed condition. The hardness of both steels increases with the increase of cooling rates, whereas, the deformation involved in both steels reduces the hardness.

Comparative study of ni effect on the corrosion behavior of low alloy steels in FGD and acid rain environments

The alloying effect of a small amount of nickel on low alloy steel for application to flue gas desulfurization(FGD) systems was studied. The structural characteristics of the rust layer were investigated by scanning electron microscopy(SEM). The electrochemical properties were examined by means of potentiostatic polarization test, potentiodynamic polarization test, and electrochemical impedance spectroscopy(EIS) in a modified green death solution of 16.9 vol.% H2SO4+0.35 vol.% HC1 at 60°C and an acid rain solution of 6.25 X 10-5 M H2S04+5.5 X 10-3 M NaCl at room temperature. It was found that as the amount of nickel increased, the corrosion rate increased in the modified green death solution, which seemed to result from micro-galvanic corrosion between NiS and alloy matrix. In acid rain solution, the corrosion rate decreased as the amount of nickel increased due to the repulsive force of NiFe204 rust against Cl-ions by electronegativity.

Advanced pattern mining for complex sata analysis

The thesis has researched a set of critical problems in data mining and has proposed four advanced pattern mining algorithm to discover the most interesting and useful data patterns highly relevant to the user’s application targets from the data is represented in complex structures.

Comparison of advanced waste heat recovery systems with a novel oil heating system

Latest trends in waste heat recovery include systems like Thermo Electric Generation (TEG), Rankine cycle, and active warm up systems. The advantages and disadvantages of different approaches are critically discussed and compared with a novel and effective oil heating system that can deliver between 7% and 12% reductions of CO2 emissions and fuel consumption. The comparison includes the expected CO2 and fuel saving potential related to the legal drive cycle as well as real world driving, effects on regulated exhaust emissions, utilisation of resources, maintenance and service, vehicle performance, comfort, noise, and durability.