Cheng Jia Shang

Dislocation-Precipitate Interaction and Its Effect on Thermostability of Bainite in a Nb-Bearing Steel

After bainitic transformation, the dislocations formed in deformed austenite remained to be pinned by the precipitates so that thermostability of the bainitic ferrite was improved. Coarsening of the precipitates accompanied by their distribution density change occurred during reheating. After long reheating, further precipitates nucleated in bainite. Dislocations inside laths getting rid of pinning of precipitates and their polygonization play the precursor to the evolution of microstructures, in which lath boundaries disappeared gradually.

Formation and Control of the Acicular Ferrite in Low Carbon Microalloying Steel

The influence of processing parameters on the acicular ferrite formation for the low carbon microalloying steel was studied. The results showed that the fraction of acicular ferrite could be controlled by the cooling process. The acicular ferrite/ bainitic ferrite dual phase structure can be formed. The multi-phase microstructure is ultra fine. The hardness is sensitively affected by the acicular ferrite fraction.

The Influence of Oxide Inclusion on Austenite Grain Size and Heat Affected Zone Toughness for Low Carbon Steels

The low carbon steels were smelted with special oxide introduction technique and the HAZ properties has been studied with thermal simulation. The optical microscope, SEM and TEM were used to analyze the composition, size and distribution of the inclusions, and the mechanical properties after thermal simulation were also investigated. The influence of oxide inclusions on the austenite grain size was also studied. The results show that after the smelting the inclusion is complex, in the core is Ti oxides about 1-3 micron and around it is MnS. When the reheat temperature is below 1000, the size of austenite grain is the same for experimental steel and base steel. However, when the reheat temperature is over than 1100, the size of austenite grains in experimental steel is one third of that in base steels. After thermal simulation, with the t8/5 increasing the toughness of HAZ decreased. The austnite grain size also increased. The microstructure is composed of intergranular ferrite and intragranular acicular ferrite. Therefore by introducing the fine oxide inclusion to the steel the austenite grain was refined and during the phase transformation the acicular ferrite formed at inclusions at first. These two factors are the main causes to improve the toughness of heat affected zone for steels produced by oxide metallurgy technique.

Precipitation Behavior of Steels with Various Copper during Continuous Cooling

The precipitation behavior of several Cu-bearing steels with various copper contents during continuous cooling has been studied. The optical microscope and HRTEM were employed to study the influence of cooling rate on the precipitation process. Also, the hardness of samples with different processes is tested. The results show that when the steels was cooled at a cooling rate between 0.1-1°C/s with the cooling rate increasing the second phase precipitates become finer but the precipitates become denser. When the cooling rate is 1°C /s the density of the second phase precipitates are the largest. When the cooling rate is quicker than 1°C /s as the cooling rate increase the precipitates become finer and fewer. The hardness tests also show that the sample will get the highest hardness. When the samples are cooled at a rate larger than 5°C /s， there is few precipitates in samples. The copper-rich second phase form by Inter-phase precipitation, and the copper-rich phase i.e. G.P zone is the main cause to strengthen the alloy. As the copper content varies from 1.5wt% to 2.5wt% the highest hardness could be obtain when the samples is cooled at a rate of 1°C /s and the density of the precipitates is the largest

Physical Simulation and Metallurgical Evaluation of Heat-Affected Zone during Laser Welding of Ultrafine Grain Steel

Two major challenges in relation to laser welding are abrupt change in metallurgical aspects and actual assessment of the mechanical properties due partly to very narrow heat affected zone (HAZ) and partly to high mechanical properties gradient. The rapid thermal cycle of laser welding imposed on the HAZ was physically simulated using a Gleeble™ dynamic simulator equipped with a special isothermal quenching device (ISO-Q™), and a relatively large volume of HAZ with a homogeneous microstructure was obtained. The thermal cycles were determined from actual laser welding followed by laser tempering. Estimations of microstructure and mechanical properties of the simulated HAZs of an ultrafine grain steel imposed by laser welding with or without post-weld laser tempering were performed. The results indicate that the simulated HAZs, depending on the thermal history, are composed of lathy martensite with different pocket size and dislocation density. The impact toughness of as-welded HAZ is improved in contrast to the base material, but is further degraded by a following laser tempering, which, however, alleviates the abrupt change in hardness of as-welded HAZ.

Evolution of Microstructures in a Low Carbon Bainitic Steel during Reheating

Cooled in water after isothermal relaxation of deformed austenite for different time, a Nb-bearing microalloyed steel always exhibited synthetic microstructures of bainitic ferrite, granular bainite and acicular ferrite. When these samples were reheated to and held at 650°C or 700 °C, the non-equilibrious microstructures tended to evolve into equilibrious ones, accompanied by obvious change of hardness. The rate of microstructures evolution was closely related to relaxation time of deformed austenite. The sample relaxed for 60s displayed the highest thermal stability, while microstructure evolution was quickest in the sample relaxed for 1000s even though it was softest before reheating. By hardness measurement, it was found that softening was not only process occurring during reheating, in which hardness fluctuated with time. There were two peaks in hardness-time curve of each sample having undergone relaxation, while single peak occurred in the curve of the sample not being relaxed. These results indicate that thermal stability of microstructures is determined by their history of formation.

Recrystallization and Grain Coarsening Control in Processing High Niobium Microalloyed Line Pipe Steels

The effect of solute niobium in retarding coarsening kinetics of austenite in upstream thermo mechanical processing of high niobium (0.1wt%Nb) low interstitial steel is analyzed. Solute drag effect of niobium in retarding boundary mobility in static recrystallization is examined in thermo-mechanical rolling of high Nb microalloyed steel. The importance of austenite grain refinement prior to pancaking in compact strip rolling of high Nb microallyed steel as a means to increase surface area to volume ratio of pancaked austenite grain is emphasized. This is to promote adequate nucleation sites for phase transformation even under conditions low total rolling reduction below temperature of no recrystallization.

The Influence of Subgrain Size on the Bainite Refinement for Steels

The relaxation-precipitation-controlling phase transformation (RPC) technique after deformation at non-recrystallization zone to refine the intermediate transformation microstructure has been simulated on a Gleeble-1500 thermo-simulator. The optical microscope, SEMTEMPTA(particle tracking autoradiography) technique to reveal the boron distribution were employed to study the variation of austenite grain size and subgrain size, the features of microstructure after RPC, precipitation and the evolution of dislocation configuration during the relaxation and the boron distribution. The results show that after relaxation at non-recrystallization zone, the subgrain formed inside an original austenite grain. With the relaxation time increasing, the size of the subgrains increased and the misorientation also increased. During the cooling after the relaxation the boron can also segregate at the boundaries of subgrains and the boron segregation can reveal the subgrains forming in deformed austenite before phase transformation. It has been found that during the relaxation strain induced precipitates occurs and these precipitates can pin the subgrain boundary and make it more stable. Comparing the subgrain size demonstrated by PTA with the optical microstructure a conclusion can be drawn that the packet of bainite generally cannot break through the boundaries of subgrains, so the subgrain appearing at the relaxation stage can confine the growth of the microstructure during the transformation in succeeding and the final bainite is refined.

A Study on Cu and Nb Precipitation during High Temperature Tempering in Low Carbon Steels

The microstructure evolution and precipitation behavior of two low carbon steels are studied, with 0.05C-0.77%Nb added in one steel and (0.03C-)1.63Cu-0.74%Nb added in the other as a comparison. In the Cu-Nb steel tempered at 600°C for 18 hrs, there are two peaks in the particle size distribution figure, one between 2-3nm formed by NbCN precipitates, and the other, 10-12nm for Cu precipitates. The TEM observation on carbon replica shows that the average particle diameter of NbCN precipitate is 2.81±0.78nm in C-Nb steel, while 4.23±0.95 nm in Cu-Nb steel with lower carbon. The analysis shows that this size increase of NbCN not only decreases the precipitation strengthening, but also weakens significantly the pinning effect on the dislocations, which results in a more serious microstructure softening in Cu-Nb steel.

Effect of Nb Content on Hot Flow Stress, Dynamic Recrystallization and Strain Accumulation Behaviors in Low Carbon Bainitic Steel

Compressive deformation behaviors of low carbon steels with different Nb contents were investigated in the temperature range 900oC to 1100oC and strain rates from 0.05s-1 to 2s-1 by single pass deformation. Multi-pass compressive deformation processes were also carried out to examine strain accumulation under different Nb contents. In single pass deformations, dynamic recrystallization (DRX) can be observed in the case of low strain rate and high temperature, and the higher Nb steel exhibits higher deformation activation energy (Qdef) and critical strain value (εc) for the onset of DRX. However during multi-pass compression process (interval time of 3-4s), the higher Nb steel has larger strain accumulation between passes, so it is easier for high Nb steel that DRX happens during hot strip rolling process, which starts at relative high rolling temperature.