Piyada Suwanpinij

Investigation of the Effect of Deformation on γ‐α Phase Transformation Kinetics in Hot‐Rolled Dual Phase Steel by Phase Field Approach

Dual phase steels, consisting of hard martensite particles in a ductile ferritic matrix, offer high strength and deformability at the same time. Additionally, they are cost effective by a dilute alloying concept. In industrial production, two manufacturing concepts have been implemented: intercritical annealing of cold rolled sheet, or hot rolling. The current work has investigated the effect of deformation on the γ-α phase transformation kinetics in the dual phase steel production using the hot rolling scheme. The pancaked austenite grains containing denser nucleation sites have a strong influence on the ferrite transformation kinetics. In addition, the multiplication of dislocations which results in the increase in elastic strain energy and dislocation core energy contributes to some acceleration in ferrite growth kinetics. A modelling approach for the γ-α phase transformation kinetics in dual phase steels has been developed employing the phase field theory. The nucleation behaviour, i.e. the number and size of nuclei developed after an elapsed time as well as their nucleation sites which were evaluated from microstructure analysis, and the increase in the driving force of grain growth were integrated into this model.

Analysis of Crack Development, Both Growth and Closure, in Steel Oxide Scale Under Hot Compression

Numerical analysis based on the finite element method allowed evaluation of conditions for crack development in oxide scale during compression in hot metal forming, particularly in the hot rolling of steel. In addition to cracks formed ahead of contact with the roll, through‐thickness cracks can also occur under roll pressure within the roll gap. The modeling has revealed that sometimes cracking that started at the uppermost oxide scale layer was stopped at the thin oxide scale layer nearest to the stock surface. Alternatively, crack closure within the roll gap was possible at high temperature in places where both viscous sliding and ductile behavior of the scale were more pronounced. Many aspects of the modeling are supported by experimental observations.

The Synchrotron Radiation for Steel Research

The synchrotron X-ray radiation is a great tool in materials characterization with several advantageous features. The high intensity allows clear interaction signals and high energy of X-ray yields higher sampling volume. The samples do not need extra preparation and the microstructure is therefore not affected. With the tunability of the X-ray energy, a large range of elements and features in the samples can be investigated by different techniques, which is a significant difference between a stand-alone X-ray tube and synchrotron X-ray. Moreover, any experimental equipment can be installed through which the synchrotron beam travels. This facilitates the so-called in situ characterization such as during heat treatment, hot deformation, chemical reaction or welding. Although steel which possesses rather high density requires very high energy X-ray for large interaction volume, lower energy is still effective for the investigation of local structure of nanoconstituents. This work picks up a couple examples employing synchrotron X-ray for the characterization of high strength steels. The first case is the quantification of precipitates in high strength low alloyed (HSLA) steel by X-ray absorption spectroscopy. The other case is the in situ X-ray diffraction for phase fraction and carbon partitioning in multiphase steels such as transformation induced plasticity (TRIP) steel.

Quantification of Vanadium Precipitates after Reheating Slab Steel by Synchrotron X-Ray Absorption Spectroscopy (XAS)

High Strength Low Alloy (HSLA) steels or microalloyed steels are developed in order toimprove the strength and toughness compared with conventional carbon steels. During the reheatingprocess at 1250-1300 °C for a few hours, the furnace consumes large amount of energy, and the slabsuffers from thick oxide scale. This results in significant mass loss. The long reheating time ensuresmaximum dissolution of the microalloying elements, which must be kept to precipitate duringcooling at the end of the hot rolling process. To minimise the reheating time and save the energyconsumption, this research studied the dissolution kinetics of vanadium in HSLA steel. Vanadium isa main microalloying element added to provide higher strength mainly by precipitation hardening. Itis supposed to be dissolved readily according to the solubility limit. The samples were reheated to1200 °C and 1250 °C for 0, 10, 30, and 60 s. After that the fraction of vanadium dissolved in the solidsolution and the remaining undissolved phases of VC, CN, and V(C,N) were measured bysynchrotron XAS. As soon as the sample reaches as low temperature as 1200 °C, a large atomicfraction of 0.878 of vanadium can be dissolved in the solid solution.

SIMULATION OF PRECIPITATION IN V-CONTAINING HSLA STEEL FOR THE STRENGTHENING ENHANCEMENT

The precipitation of the microalloying elements in high strength low alloyed (HSLA) steel controls the strength of the steel greatly through grain refinement and particle hardening mechanisms. The current work simulates the precipitation of vanadium a hot rolling process for the optimized strengthening effect in a microalloyed steel. Taking into account the effect of deformation, cause of the drastic increase in the dislocation density, namely higher nucleation site density, it can be clearly seen that the precipitation of all species at dislocations, dominate the precipitation kinetics. The diffuse interface effect on the interfacial energy as well as a volumetric misfit of AlN at dislocations is also taken into account. The latter is because of its significant difference in the lattice parameter from the matrix. The presence of AlN at dislocations does not override that of V(C,N) as found in other cases with low density of dislocation. Slow cooling rate in the process ensures the consumption of the microalloying elements which in turn strengthen the product and minimise the production cost. The experimental verifications for the precipitates are performed by scanning transmission electron microscopy (STEM) as well as X-ray absorption spectroscopy (XAS) from synchrotron radiation.