Advanced Materials Research Vols. 1004-1005

Abstract: Retained austenite has an important effect on strength and toughness of 20Mn2SiCrMo bainitic rail steel. In this work, the stability of retained austenite and mechanical properties have been studied. The results show that impact toughness of experimental steel has close relationship with the stability of retained austenite. When tempered at lower than 350°C, retained austenite owns good thermal stability, corresponding to relatively high impact toughness.

Abstract: There is a currently desirable demand for high strength steels with good ductility reduce the weight of steel parts for automobile and train applications. Retained austenite in steels can improve the toughness and plasticity. The austenite reverse transformation + quenching and partitioning (ART + Q&P) process was treated on a 0.2C-Mn-Si-Cr low alloyed steel, a multiphase microstructure composed of intercritical ferrite (IF), martensite, bainite and retained austenite (RA) can be obtained in the low carbon steel. Microstructures of the steel treated by different heat treatments were characterized by SEM and XRD. Results show that the formation of RA in low alloy steel depends on the following: (1) the enrichment of the carbon and manganese in the reversed austenite during the ART step; (2) the secondary enrichment of carbon in retained austenite during the following Q&P step. High fraction of RA (14 vol.%) was obtained through the two-step element enrichment treatment (ART + Q&P). Due to continuous TRIP effect of RA during the deformation, a good combination of strength and plasticity was achieved in our works: the product of strength and elongation is greater than 35 GPa•%, the tensile strength is more than 1230 MPa, the yield strength greater than 890 MPa, the total elongation is about 28.6%.

Abstract: Strain paths during sheet metal forming are always complex and nonlinear. Forming limit diagram (FLD) is a common method to determine failure in the past decades. However, it is only suitable for linear strain path condition. Regarding dual phase steel DP780, a special experiment was designed and carried out on Zwick Cupping equipment to get nonlinear strain paths. And the strain status was analyzed in FLD. It was found that FLD cannot predict failure precisely in this case. A new approach proposed by Stoughton and Yoon which based on polar effective plastic strain was introduced to analyze this nonlinear strain paths condition, the result is in good agreement with experiment, which indicated that Polar Effective Plastic Strain Diagram was an effective and precise tool to determine failure especially for complex nonlinear strain paths forming.

Abstract: In order to clarify the austenite reverse transformation of a 0.2C-3Mn-1.7Si steel during intercritical region with an ultra-slow heating rate of 0.05°C/s, DIL (dilatometer), SEM, XRD and EPMA were carried out. The experimental results indicated that the dilatometric cures exhibited the shape of two “valleys” and the growth of austenite from quenched martensite was composed of three consecutive sub-stages. This phenomenon is associated with Mn-rich and Mn-poor regions in martensite matrix. During the heat treatment,the austenite forms firstly in region enriched in Mn. However, with increasing temperature, the growth of γ is restrained owing to the concentration of Si. Upon the further process of heating, the free energy for the reverse transformation increases, and the residual Mn-depleted regions start to transform at a higher temperature.

Abstract: The surface of hot-dip galvanized high strength low alloy (HSLA) steel easily occurs orange peel in the deformation process. On the other hand, the defects possess a specific directivity and sits at approximately a 45-degree angle to the sheet steel rolling direction. The microstructures and properties of steel specimens with the orange peel defects and the normal were analyzed, which results showed that their microstructures consist of ferrite and few granular pearlite. The yield point elongation of the HSLA steel resulted in the orange peel defects on the surface of sheet stamping and it is associated with skin rolling and stretch rolling process. Further studied on the fine microstructures by means of SEM and electron back scatter diffraction (EBSD) techniques, which was apparent for the defect steel that the orange peel defects were resulted from weak favorable {111} texture might be the key factors aggravating the formation of orange peel defects. It can be concluded that the formation of Cottrell atmospheres caused the yield point elongation by the interaction between dislocation and diffusive solute atoms as basic reason and the directivity of the orange peel defects was related with the LUDERS slip forming. The yield point elongation can be eliminated to avoid the orange peel defects beyond to 1.8% skin-rolling and stretch rolling method with an appropriate annealing technology.

Abstract: 23Co-Ni is an ultra-high strength steel and it needs ultra low nitrogen content in the steel. The removal of nitrogen for 23Co-Ni steel in a 6t/12t Vacuum Induction Melting Furnace (VIM) with different melting processes was studied. The results show that, the longer of the melting time and the higher of the vacuum level, the lower of the final nitrogen contents is. The denitrogenization rate can reach 70% when the melting time is more than 8 hours. The electromagnetic stirring can increase the denitrogenization speed. When the VIM is vacuumed to 1-3 Pa, it can make the nitrogen content reaches 0.0005%-0.0008%.

Abstract: The effect of Mn addition on mechanical property and corrosion behavior of hot rolled 19 % Cr duplex stainless steel was investigated. The ultimate tensile strength of specimen decreased from 950 to 840 MPa with increasing Mn content from 3.6 to 5.5 wt.%, and further decreased to 768MPa with Mn content addition up to 8.1 wt.%. The sample with the addition of 5.5 wt.% Mn exhibited the highest elongation up to 43%. The polarization result revealed that higher value of breakdown potentials E_b was obtained with increasing Mn content from 3.6 to 5.5 wt. %, but which decreased with increasing Mn content to 8.1 wt.%. The electrochemical impedance spectroscopy (EIS) spectra showed higher corrosion prevention capability was obtained due to enhancement of polarization resistance with increasing Mn content from 3.6 to 5.5 wt.%.

Abstract: The model of cellular automaton (CA) for simulating the martensitic transformation of the high strength and elongation steel during quenching was established. The microstructure evolution of martensitic transformation of high strength and elongation steel during continuous cooling was simulated dynamically, in terms of parameters such as the volume fraction and morphology of the martensite and retained austenite were quantitatively, accurately and visually described. The simulation results could provide a theoretical reference for the control of the microstructure and property of high strength and elongation steel.

Abstract: Isatin Schiff base, 3-(4-hydroxyphenylimino) indolin-2-one, was investigated as an corrosion inhibitor in NaCl solution. The structure was analysized by X-ray single diffraction. The inhibition and the mechanism of the title compound on the corrosion of high protective Q235A steel in NaCl solution were screened and discussed. The results indicated that it can inhibit the corrosion with moderate inhibition efficiency in different conditions, and the inhibition mechanism of the corrosion inhibiting may be mainly contributed to the adsorption.

Abstract: Thermal and thermo-oxidative degradation of epoxidized natural rubber containing 25 mol % expoxidation (ENR25) with different four antioxidants were investigated by thermogravimetric analysis (TGA). The four antioxidants, viz., poly-2,2,4-trimethyl-1,2-dihydroqui- noline (RD), N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenedianine (4020), N-isopropyl-N'-phenyl- p-phenylenedi-amine (4010NA), and 2-mercaptobenzimidazole (MB) were used. The results indicate that the ENR25 vulcanizates with antioxidant 4020 or 4010NA is the optimal antioxidant for ENR25.