Advanced Materials Research Vols. 887-888

Abstract: In this paper, the influence of heat treatment temperature on the microstructure and mechanical properties of cast steel (ZG25MnNi) was investigated. The results showed that normalizing treatment can effectively refine cast microstructure of ZG25MnNi and maximum hardness achieved at 900°C. After normalized at 900°C, the sample tempered at 500°C,530°C,560°C, 590°C, 620°C, 650°C respectively. With the tempering temperature increasing, the samples tensile strength increased gradually and elongation decreased gradually. When tempered at 530-590°C, the sample had a relatively high strength plastic product. The results showed that the best normalizing temperature is 900°C and the optimum tempering temperature is 530-590°C.

Abstract: The flowing stress of a highstrength steel before unloading is different with the yield stress of the same sample extended in air after unloading and charging with hydrogen. The difference is the hydrogeninduced additive stress, which can help the external stress to enhance the plastic deformation. The hydrogeninduced additive stress, σ_ad, induced through being immered in a 3.5% NaCl solution with pH=4 increases linearly with the yield strength of the sample, e.g., σ_ad=106.6 + 0.14σ_ys. On the other hand, hydrogeninduced additive stress increases linearly with the logarithm of hydrogen concentration in the samples with the yield strengths of 900 and 1050 MPa, i.e, σ_ad=55.5 + 63.6 lnC₀ (σ_ys=900 MPa) and σ_ad=23.5 + 64.2 lnC₀ (σ_ys=1050 MPa). To sum up, the hydrogeninduced additive stress is σ_ad=260 + 0.226σ_ys + 63.9 lnC_0.

Abstract: In order to clarify the influence of milling time on the atomized precursor powders for 9Cr-ODS (Oxide Dispersion Strengthened) steel, the morphology and microstructure of atomized powders and mechanical alloying (MA) powders with different milling time were investigated by scanning electron microscopy (SEM), laser diffraction particle size analyzer, X-ray diffraction (XRD) and Vickers-hardness tester. The experimental results showed that the atomized powders are spherical in shape with the particle size of ~ 47 μm and the grain size of ~ 2 μm. With the increase of MA time, the morphology of MA powders becomes from spherical to lamellar structure, then to equiaxed shape. The mean powder size increases firstly then decreases. The grain size decreases quickly at the initial stage of milling process (0-2 h) then trends to reach a saturation value. The Vickers-hardness increases rapidly at the initial stage of milling, then reaches a saturation value.

Abstract: 6CrW2MoVSi steel has a refined and even microstructure after heat treatment, the average size of annealing carbide is 0.6 μm; quenching martensite is mainly lath-shaped martensite and only a small amount of acicular martensite, and the size of quenching acicular at 950 °C is smaller than 2.5 μm. The curve of quenching hardness increasing with quenching temperature rising is divided into three sections. In the middle section of quenching between 910 °C ~ 980 °C, quenching hardness presents slow rising trend. After higher temperature quenching, there are low and high temperature tempering precipitation hardening zones. At 220 °C ~ 240 °C tempering temperature, precipitation hardness is HRC54 ~ 58. At 540 °C ~ 570 °C tempering temperature, precipitation hardness is HRC52 ~ 56.

Abstract: The article studies on sections microstructure of 430ferritic stainless steel after tension, the tensile temperatures are the 1073K, 1173K, 1223K, 1273K, 1323K and 1423K. The transverse sections (vertical tensile direction) of fractured specimens microstructure of 430ferritic stainless steel were observed and compared with those of longitudinal sections (parallel tensile direction). Moreover, we compare microstructure of transverse section specimens with the salt water-cooled condition and air-cooled condition. The optical micrograph of fractured tensile specimens of 430stainless steel after cooling to room temperature indicated that the volume fraction of the martensite is gradually increased and then declined from 1073K to 1423K. At 1223K, the martensite content is highest. At 1423K, martensite is sharply reduced and disappeared, the microstructure of 430ferritic stainless steel is almost all of ferrite and grain boundary obviously observed. Due to tensile deformation, the morphology of martensite is massive in the transverse section specimens. Whereas, the strip-type morphology of martensite was observed in the longitudinal section specimens. The cooling rate impact on the microstructure was also discussed.

Abstract: Focusing on improving the galvanizability of the third generation automotive steel, the effect of surface oxides morphologies on the galvanizability was studied. The results show that the surface oxide types of sample steels by XPS analysis after annealing in different conditions are the same, only MnO and Cr₂O₃ were detected and no complex oxides exist on the surface. Morphologies of surface oxides can greatly influence the galvanizability of the third generation automotive steel, nodule-like oxides surface can contribute to better wettability and inhibition layer than vitreous film-like oxides surface. Galvanizing panels of nodule-like oxides surface steels only show pinhole-sized bare spots, while panels galvanized from vitreous film-like oxide surface steels reveal larger areas of bare spots and uncoated areas. Inhibition layer observed in galvanizing panels of nodule-like oxides surface steels are compact but not homogeneous, some inhibition layer grains are fine, and others are coarse. While the inhibition layer grains of panels galvanized from vitreous film-like oxide surface steels have a non-compact morphology with some particularly fine equiaxed crystals which developed deficiently.

Abstract: In this research, influence of annealing process on microstructure and mechanical performance of Nb-Ti-stabilized 430 ferritic stainless steel were investigated. In order to obtain the optimal annealing process, metallographic observation, SEM detection and tensile test were carried out. It is found that the microscopic structure is composed of fine and uniform isometric recrystallization grain after annealing. Optimum microstructure and mechanical properties can be achieved while annealed at 950 °Cfor 90 seconds. The annealed sample can obtain the optimum microstructure and mechanical properties under such annealing process. The yield platform is eliminated and the average plastic strain ratio is further improved to 1.269, which reflected a well deep drawability of the Nb-Ti-stabilized 430 ferritic stainless steel compared to SUS 430 stainless steel.

Abstract: In the data processing of iron ore metallurgical properties, the polynomial least squares method or the assumed functions are usually employed to fit the performance curve. This is stuck at a high degree of the polynomial, where the equation group is prone to morbid problems. To solve this problem, the use of orthogonal functions as the least squares method may help draw out the reductioln metallurgical performance curves.

Abstract: A high grade non-oriented electrical steel final annealing product was processed by stress relief annealing experiments under pure hydrogen atmosphere using different process parameters. The samples were compared in the aspects of magnetic properties and anisotropy, then analyzed the phenomena concerned with grain size, texture and precipitates aspects. The experiments showed that the samples magnetic properties were most improved in the 850 degrees stress relief annealing experiment, thus providing a reference method for non-oriented silicon steel stress relief annealing experiments and to obtain low core loss non-oriented silicon steel.

Abstract: The Q&P process (Quenching and Partitioning) is a heat treating method for high-strength low-alloyed steels. It delivers the desired combinations of high strength and adequate ductility. These properties are achieved thanks to the unique martensitic microstructure with a certain volume fraction of stable retained austenite. Retained austenite imparts plasticity to the otherwise brittle martensitic structure. Optimum mechanical properties are achieved by using correct processing parameters and chemistry of the material. The experimental material was a steel with 0.47 % carbon alloyed with silicon, manganese and chromium. The purpose of the effort was to optimise the heat treating parameters in order to obtain a strength level above 2000 MPa and an elongation of no less than 10%. In the first step, the appropriate austenitizing temperature was identified. In the second, effects of various quenching temperatures and cooling rates on the microstructure evolution were explored. In the third, the impact of raising the partitioning temperature on stabilization of retained austenite was examined. Adjustment of the parameters led to a strength of more than 2300 MPa and an elongation of 8 %.