Ai Min Zhao

Effect of Ausforming Temperature on Bainite Transformation of High Carbon Low Alloy Steel

The effect of ausforming temperature on bainite transformation of high carbon low alloy steel was studied by in situ experiments using a Gleeble 3500 thermal and mechanical testing system. Morphology and crystallography of ausforming bainite were examined by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). It has been found that deformation at all temperatures range from 230°C to 600°C can accelerate low temperature bainite transformation, and transformation rate increased with deformation temperature reduced. Quantitative X-ray analysis shows that the volume fraction of retained austenite was about 35.84% after deformation and isothermal transformation for 20 hours, it was approximately the same amount with austempering bainite transformation process (no strain) which austenite volume fraction was about 32.01%. Low temperature bainite formation can be accelerated with a smaller increase amount of retained austenite by deformation at a low temperature range of 230~600 oC.

Effect of Annealing Temperature on Microstructure and Properties of Ultra Purified Ferritic Stainless Steel Containing Copper

The effects of annealing temperature on microstructure, mechanical properties, formability, and texture evolution were analyzed in the article. The microstructure of the steel obtained through different annealing processes were investigated by means of transmission electron microscopy; The micro texture of steel was measured by using electron back scattering diffraction analysis; The relationship between Cu precipitates and matrix was analyzed by using the selected area diffraction technology. The results show that when annealing temperature was 700 ~ 850 °C, the yield strength and tensile strength first decreased slightly and then increased, while the elongation accordingly first increased then decreased slightly. The best mechanical property and formability were obtained at 800 °C. Cu precipitates reduced with the increase of annealing temperature and it accorded with K - S relationship with matrix. The grains near the {111} < 112 > orientation grew up selectively. The higher the temperature, the more the γ fiber texture content. But at higher temperature (850 °C), γ texture was damaged and the content was reduced.

Bare Spot Defect on a Hot Dip Galvanized DP Sheet Strip

Bare spots defect of galvanealed (GA) in High Strength Dual Phase (DP) steel strip with 1.5 % Mn contain was studied in detail. The surface morphologies of spot defects before and after partial and complete removal of the Zn layer, as well as the interface between the outermost coating layer and the sheet substrate were analyzed by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray Photoelectron spectroscopy (XPS), glow discharge optical emission spectrometry (GDOES) and laser scanning confocal microscope(LSCM). It was found that the bare spots defect was composed of a large number of pits with different sizes and depths. There were a lot of Fe - Zn alloy particles distributed in the pits, and many MnO formed on the surface, no effective Fe2Al5 inhibition layer formed. The results showed that the main reasons for the bare spots defect of the GA in High Strength DP steel strip are as follows: a silicon oxide film forms on the substrate during annealing prior to hot dipping because of Mn gathered on the surface of steel strip, thus strip surface wettability with liquid zinc is deteriorated and preventing the formation of the Fe2Al5 inhibition layer during hot dipping. In this paper, the dew point control process is introduced creatively, by increasing the dew point and the hydrogen content in the furnace area, and the bare spot defects on the surface of the high strength duplex steel galvanized sheet are solved.

Low Temperature Mechanical Properties of Power Transmission Tower Steel

The tensile and impact tests were used to study the mechanical properties under different temperatures of 300 mm large-scale angle steel at different positions, especially the tensile strength, yield strength, total elongation and impact toughness in the range of-40 oC to 20 oC. The results showed that different regions had great differences in the microstructures and impact toughness, in which the size of edge region was the smallest and the impact toughness was the best. However, the coarsened grain of heat affected zone at weld region had deteriorated to the low temperature impact toughness. When the impact energy was 34 J, the ductile-brittle transition temperature of weld, center, vertex and edge were-7.2 oC, -33.0 oC, -31.5 oC and far less than-40 oC, respectively. Meanwhile, because the banded structure was detrimental to the ductility, the elongation of rolling direction was lower than vertical direction. The strength of weld region was higher than other locations, but the elongation was obviously decreased.

Wear Resistance Research of Advanced High Strength Steels

The wear performance and wear mechanism under two-body abrasion of five advanced high strength steels, i.e. Nanobainite (NB) steel, Tempered Martensitic (TM) steel, Dual Phase (DP) steel, Transformation Induced Plasticity (TRIP) Steel and Twining Induced Plasticity (TWIP) steel were studied. By using the scanning electron microscopy (SEM), we investigated the wearing surface. Phase transformation strengthening behavior was also be discussed by analyzing the surface and sub-surface after abrasion. The results showed that micro-cutting was the major role of wear mode in the condition of two-body abrasion. In the circumstance of two-body abrasion, hardness was an important factor, the property of wear resistance enhanced while the hardness increased except for TM steel. NB steel possessed the best wear resistance which was 1.71 times higher than that of TWIP steel. The retained austenite transformed into martensite which can improve the hardness so that it enhanced the wear resistance of NB steel.

Study of the Microstructure and Mechanical Properties of a Medium Manganese Quenching and Partitioning (Q&P) Steel

The microstructure and mechanical properties of a medium manganese quenching and partitioning (Q&P) steel for automobile were investigated by optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and mechanical property test. The grain size and recovery degree were greatly affected by annealing temperature normally. The result shows that the medium manganese steel after quenching and partitioning (Q&P) heat treatment exhibited good mechanical properties. The maximum tensile strength and yield strength was 1280MPa and 1421MPa at 600°C, respectively. Additionally, the product of strength and plasticity could reached to 40472MPa×% at 640°C. Annealing temperature also had a great influence on the volume of retained austenite which increases linearly with the rise of annealing temperature as well.

Effect of Boron and Chromium on Hardenability in 0.33% C Cold Heading Steel

The effect of B and Cr on hardenability in 0.33% C cold heading steel was studied. The results show that the hardenability enhanced from adding 0.0020% B or 0.28% Cr. The length of high hardness stability region of the Jominy Curve obviously increased with adding B,and the hardness of low hardness stability region of the Jominy Curve increased significantly with adding Cr. The ideal critical diameter DI of the experimental steels was calculated, the DI is 20.0 mm of the base steel with 0.33% C and no other alloying element addition, but the DI becomes 29.5 mm with adding 0.28% Cr and 46.4mm with adding 0.0020% B. The hardenability of the test steels was calculated by the nonlinear equation and compared with the experimental results. The calculated Jominy curve did not reflect characteristics of 2# steel with B and needs to be further optimized.

Effect of Annealing Time on the Formability of a Ultra Purified Ferritic Stainless Steel Containing Copper

For developing of a ultra-purified ferritic anti-bacterial stainless steel, 1.4 Cu % was added in 21% Cr ferritic stainless steel. The effect of annealing time on the formability was analyzed. The micro texture of steel was measured by using electron back scattering diffraction analysis. The Cu precipitates through different annealing periods were investigated by means of transmission electron microscopy. Average plastic strain ratio was calculated by the r values at 0, 45°and 90° directions measured by tensile testing. The results shows that with the extension of annealing time, the morphology of the Cu precipitates became into rod from spherical and the size increased. Average plastic strain ratio increased first and then decreased, while the change trend of anisotropy index was opposite. The higher formability is due to the {111} texture components increase significantly, while more {001}<110> component at 60 min was adverse for the formability. And anisotropy index was determined by γ components and {110}<110> components.

Microstructure and Mechanical Properties of TRIP-Aided Steels with Different Heat Treatments

The microstructures, mechanical properties and retained austenite characteristics of TRIP-aided steels with three different heat treatments were studied in this paper. The results indicated that the designed annealing treatments resulted in completely different matrices and the morphologies of second phase, and a significant difference in mechanical properties. The TAM steel was found to have fine annealed martensite lath matrix and inter lath acicular retained austenite, and possessed an excellent combination of strength and elongation which attributed to the highest retained austenite volume fraction and carbon concentration. For TPF steel, the higher instability and lower carbon content of retained austenite and the soft matrix resulted in the lowest ultimate tensile strength and total elongation. While in TBF steel, the stability of retained austenite was lower than that in TAM steel but higher than that in TPF steel. The ultimate tensile strength of TBF was significantly higher than the TAM and TPF steels, but the ductility of TBF steel was lower than TAM steel.

A Temperature Field Analysis for Contraction and Collapse of 72AU2 Hot-Rolled Steel

Hot rolled coil after winding cooled to room temperature in the storage room often takes 4 to 5 days. The process of coil cooling sometimes produces collapse defects in industrial applications. In this research, the cooling process after coiling of Shougang Group 72AU2 hot-rolled strip steel was taken as the research material. We used finite element method (FEM) software ABAQUS to analyze the temperature field of coil after coiling. At first, a temperature field model of hot-rolled coil during cooling was established by the finite element method. The simulation results show that, the maximum temperature difference is 206°C, when the cooling-time is about 50th min. The highest temperature node was about at 44% apart from inside diameter distance position in the thickness direction, and finally moved to the node which is at 26% apart from inside diameter distance position. Temperature field calculation considered the anisotropy of the heat transfer. The temperature of each node can be obtained by this simulation, which cannot be obtained from field measurements and experiments. Thereby this research has a significant impact on further research on the causes of the contraction and collapse.