Papers by Author: Cheng Jia Shang

Abstract: 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.

Abstract: Stress relaxation was studied in a series of low carbon, high Mn microalloyed steels containing 0.012, 0.06 and 0.1 wt% Nb. The stress-relaxation curves were modeled using a physically-based model that takes into account the time evolution of precipitation, recovery and recrystallization as well as their interactions. The results confirm that high Mn-high Nb design can offer distinct advantage over the low-Mn design for the application of near net shape processing.

Abstract: The properties and precipitation behavior of Cu-bearing steels have been investigated. The optical microscope and transition electron microscope were employed to study the influence of interrupted cooling and quenching temperature on the precipitation behavior. Also, the properties of samples with different quench processes were tested. The results show that when the steel is interruptedly cooled and quenched from 650-700°C, with the quenching temperature increasing the volume fraction of martensite becomes larger and the hardness becomes higher. When the microstructure is ferrite the second-phase precipitates occurs and they are proved copper-rich particles. However there are no obvious precipitates in martensite. The copper-rich second phase forms by the way of inter-phase precipitation.

Abstract: 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.

Abstract: 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

Abstract: Two kinds of ULCB steel were designed with different Nb content. The yield strength of the steel which was made by adding 0.09%Nb, TMCP (Thermo Mechanical Controlling Processing) and tempering can achieve more than 900MPa. As compared with 0.05%Nb steel, adding 0.09%Nb can refine the austenite grain significantly. Through the analysis by TEM and EDX, a large amount of Nb precipitation particles can be observed in the steel with 0.09% Nb tempered at 650°C for 1h and these particles are generally small than 20nm.

Abstract: The inside corrosion of failed automotive mufflers collected in China was investigated and the composition of the automotive exhaust gas condensate collected from HONGQI automobile was analyzed. According to the analyzed result of collected condensate’s composition, the corrosion resistance of a new designed high Cr stainless steel (B439M) bearing Nb and Ti and a low Cr T409L stainless steel were studied with a condensate corrosion test method which simulates the inside corrosion of automotive mufflers. The life of the two materials was estimated by extreme value analysis of the maximum corrosion depth obtained by the dip dry test (DDT). The life of type B439M steel was 1.6 times as long as that of type T409L steel. To clarify the corrosive process of the simulated condensate test, the electron work function (EWF) on the two stainless steels’ surface was evaluated. It was demonstrated that the surface of new designed stainless steel exhibited markedly improved resistances to corrosion during a simulated condensate test cycle and the corrosive process of simulated condensate test was evaluated and discussed.

Abstract: Dynamic continuous stiffness measurement (CSM) nanoindentation method was applied to measure nanohardness in low carbon microalloying steel. The matrix hardness for quenched phase and acicular ferrite calculated through analysis of curves is less fluctuated than the raw results, which shows that the analysis of the curves is helpful for determining matrix hardness.

Abstract: In this paper, the very rapid thermal cycles imposed on an ultrafine grain steel plate were successfully simulated using a Gleeble™ simulator. The simulated heat affected zone was obtained suitable for Vickers hardness and impact toughness examination of laser weld that originally bear a narrow zone with high hardness gradient. Effect of preheat treatment on the mechanical behavior of laser HAZ was estimated. It was indicated that the simulated HAZ was composed of fine martensitic ferrite laths with average width in 65nm and 175nm in the case of preheat at 200°C and not, respectively. The laths were cramped by retained austenite film within the prior austenite grain with average size of 9.5 and 6.5μm, much less than that of base metal. The impact toughness of HAZ was improved by laser thermal cycle comparing to that of base metal, and there is no obvious brittle-ductile transition under temperatures from -80°C to 20°C. Preheat treatment of steel plate prior to laser welding resulted in an effective reduction of the maximum hardness and alleviation of the hardness gradient in the weld.

Abstract: By employing the new developed relaxation-precipitation-controlling phase transformation (RPC) technique in large scale production the bainitic steels with ultra fine bainite has been obtained. These bainitic steels have good synergistic properties. With the aid of thermal simulation the refinement mechanism of RPC technique has also been investigated. The optical microscope, scanning electron microscope, transmission electron microscope and Electron back scattering diffraction technique were employed to study the features of microstructure produced by RPC technique, precipitation and the evolution of dislocation configuration during the relaxation. The results show that when produced by RPC technique the microstructure of the steel is mainly ultra-fine lath bainite packets, and these bainite packets block each other. It is also found that during the relaxation the dislocation cells form and strain induced precipitation occurs, the dislocation cell pinned by the precipitates can confine the bainite transformation. After the relaxation during the cooling the acicular ferrite forms at first and in succeeding the bainite transformation is blocked by the acicular ferrite and the bainite is refined effectively.