Papers by Keyword: Ultrafine Grained Steel

Abstract: Development of industries in recent years reveals the essential need to the microalloyed steels with high strength and good ductility. Refinement of Ferrite grains by thermomechanical Treatment is the only lower cost effective method to improve strength and toughness spontaneously in this type of steels. API X70 steel belongs to high strength microalloyed steel group. The manufacturing process of this steel is controlled rolling which is a kind of thermomechanical treatment and it is considered as a grain refining method. In this research, three specimens of API X70 steel were experimentally rolled in order to achieve ultrafine grained microstructure. Rolling operations are designed in such a way that the rolling of these specimens finished at 846, 823 and 800°C. Results of the experiments were analyzed by mechanical tests and microstructures observations. The microstructure observations show that decreasing of finish rolling temperature causes decrease in Ferrite grain size. Results also show that rolling of API X70 steel in the vicinity of A_r3 temperature and high strain rates lead to ultrafine Ferrite grains in microstructure. This is due to the transformation of work hardened austenite to Ferrite. On the other side, Tensile and impact tests show that decreasing of finish rolling temperature causes increasing in yield and tensile strength and also improves the toughness.

Abstract: While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. Ultrafine-grained steels with different carbon contents from ultralow carbon to high carbon were produced through warm caliber rolling. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to martensite steels for all materials. Formability of ultrafine-grained steel is examined by applying to form a M 1.7 micro screw using these ultrafine-grained steels. Screws are formed through the process of cold heading and rolling. Relationship between cold heading, rolling, uniform elongation and reduction in area are investigated to clarify the formability of ultrafine-grained steels. Low-carbon ultrafine-grained steel has excellent cold headability and favorable rolling properties, i.e., excellent formability. Reduction in area is a measure to determine formability on cold heading. Ultrafine grained steel wire with length of several hundred meter were developed with the technology of warm continuous multi-directional rolling. This wire also have a good formability which can form microscrews.

Abstract: In the present research, a combined forward extrusion-equal channel angular pressing (FE-ECAP) was developed and used for production of bulk ultrafine grained steel in the high temperature conditions. In this method, two different deformation steps including forward extrusion and equal channel angular pressing takes place successively in a single die. The deformation process was performed at different deformation start temperatures (800, 930, and 1100 °C). In addition, 3D finite element simulation was used to predict the hot/warm deformation parameters such as strain and temperature variations within the samples during deformation. The results show that the EF-ECAP process is effective in refining the grains from initial size of 32 m to final size of 0.9 m after executing of extrusion and ECAP on as received samples. The main mechanisms of grain refinement were considered to be strain assisted transformation, dynamic strain-induced transformation, and continuous dynamic recrystallization.

Abstract: Titanium microalloyed steel whose grain size is around 3μm has been developed by CSP process. Samples after different plastic deformation were obtained by suddenly stopping the six-stand finishing train during tandem rolling. Experimental results and analysis show that plastic deformation during tandem rolling plays a major role for grain refinement. With the successive severe strain, microstructure of the same stock during tandem rolling is further refined because of repeated recrystallization at higher temperature and strain accumulation at lower temperature. Technological features of CSP process, such as large reduction per pass, rapid cooling after rolling and high solidification and of thin slab, are important reasons for obtaining ultafine-grained structure. Besides these, chemical composition of titanium microalloyed steel has beneficial effects on grain refinement.

Abstract: For engineering components with cracks, it is very important to evaluate the reliability for fracture. The X-ray diffraction method is now widely used to measure non-destructively the loading and residual stresses in crystalline materials. Synchrotron radiation sources provide the X-rays with extremely high intensity as well as a narrow divergence. The high intensity X-rays with a narrow divergence enables stress measurements in a localized region. The strain distribution near the fatigue crack in the steel plate with ultrafine-grained surface layers, called SUF plates, was measured by the polychromatic X-ray from synchrotron radiation at SPring-8. The spatial resolution in the direction parallel to the crack propagation direction was 0.1 mm. The strain distributions at several applied stress levels were determined for six diffraction planes. The measured strain distribution was compared with the result calculated by the FE analysis. The average value of the measured strains for several diffraction planes agreed well with the calculated results.

Abstract: Ultrafine grain refinement to 1 m deteriorates the uniform elongation in the tensile tests of steels. Such loss of ductility has been argued to be an inherent feature of the ultrafine-grained steels. While uniform elongation is a measure of ductility of the material, reduction in area in tensile tests is also an important measure of ductility. Ultrafine grained steels with different carbon contents from ultralow carbon to high carbon were produced through warm caliber rolling and evaluated for their stress-strain behavior along with the reduction in area. It was found that the reduction in area- tensile strength balance is far better than the conventional ferrite+pearlite steels and even superior to bainitic steels for all materials tested in the present study. Formability of ultrafine grained steel is examined by applying to make a micro screw. Good formability was verified by this process.

Abstract: In the present work, low-carbon ultra grained constructional low-alloyed steel were subjected to thermo-mechanical treatment for modification of microstructure. It shows that microstructure after thermo-mechanical treatment is quite dependent on the alloy composition, conditions of hot deformation, grain size of austenite and cooling rate. The research was provide by using the computer program for thermo and thermo – mechanical treatment. The most optimal variant of heat treatment and thermo – mechanical deformation were obtained. The verifications were provided by the dilatometer with possibility of deformation DIL 805A/D.

Abstract: In the present study, ultrafine-grained (UFG: grain size < 1m) HSLA and IF steels were investigated. The tensile test results showed that, because of different strengthening mechanisms, UFG HSLA steel represents finally better ductility than initially more ductile IF steel. The analysis performed in the present study is based on the mechanism of generation the dislocation structures and takes into account effects of precipitation and solid solution strengthening. The results of this analysis, implemented in FEM, enable to capture the moment of plastic instability of ultrafine-grained microalloyed steels. A good convergence of the proposed model was observed for the investigated steels. The fact that ductility, strengthening and deformation mechanisms are strictly connected to each other suggests that the proper use of their synergetic effect may be used to improve the ductility of UFG materials.

Abstract: Ultrafine-grained steels with a grain size of about one micron offer the prospect of high strength coupled with high toughness among conventional steel compositions and are attracting the attention of researchers worldwide. Application of these ultrafine-grained steels to potential engineering structures demands extensive study of their mechanical properties. While there are many studies on the development of ultrafine-grained microstructures through various deformation processing techniques on a spectrum of compositions, fewer studies were reported on the more important aspect of evaluating their mechanical properties. This is to verify the basic assumption that the microstructural refinement at bulk level indeed improves the mechanical properties offering the prospect of a realistic replacement of the existing conventional steels in the near future. As we move towards the ultimate goal of applying these advanced high strength materials, this review article attempts to present a comprehensive picture on the mechanical properties of ultrafine-grained steels with varying carbon contents fabricated by large strain warm deformation. Finally, it is believed that time is ripe for exploring the possible applications of these materials for structural applications.

Abstract: After reheated at different temperatures for 5 minutes, the 400MPa Ultrafine Grained Steel specimens were air-cooled to room temperature, and then carried out the mechanical nanocrystallization surface treatment and structure performance testing. On the basis of comparing the test results on the specimens before and after the mechanical nanocrystallization surface treatment, the process of mechanical nanocrystallization was analyzed briefly. The results show that: as the reheating temperature rising, the trend of grain size growing increases markedly, and the mechanical properties also drop down to different degrees; when the reheating temperature is around 800°C, because of the pearlite spheroidized significantly, its mechanical properties drop the most seriously; after the mechanical nanocrystallization surface treatment, not only its surface form a layer of fine nano-layer (about 100 nm) structure, but also its mechanical properties rise obviously, and the yield strength is over 450MPa.