Advanced Materials Research Vols. 399-401

Abstract: The morphology of microstructure and PAG was observed by means of LEICA DMIRM metallurgical microscope, the phase was measured by D8 ADVANCE X-ray diffractometer (XRD), and the martensitic transformation point is tested on DIL805A dilatometer after ST. The results show that the PAG size of C250 steel has not a clear growth. The recrystallisation has the priority of nucleation in the PAG boundary. The morphology of martensite presents irregular bans and stringers before recrystallisation, whereas presents blocky thereafter. The intermetallic compounds re-dissolve by ST. With the prolonged of ST time, the Ms point increases before recrystallisation, decreases thereafter; the Mf point does not have a clear change.

Abstract: A Fe-0.05C-2.94Mn-1.87Si steel is heat treated using a two-stage isothermal holding process to obtain allotriomorphic ferrite and bainite. Two kinds of allotriomorphic ferrite are obtained, one with only carbon partitioning and the other, alloying element partitioning. It is observed that the allotriomorphic ferrite stimulates the adjacent bainite to select the similar variant on the side where near K-S relationship is maintained between ferrite and prior austenite. The longer the border length of the allotriomorphic ferrite, the larger the stimulated bainite area. The statistical measurement shows that the alltriomorhpic ferrite with alloying element partitioning stimulates such bainite variant selection as well as that with only carbon partitioning.

Abstract: The behavior of carbon and sulfur segregations of 82B hard wire steel produced under different continuous casting conditions has been systematically studied by the combination of field trial and laboratory analysis. The results showed three factors such as superheat degree, casting speed and F-EMS which influenced the segregation of billet worked differently with the variations of inner positions of the billet and segregation elements. For the central segregations of carbon and sulfur, the dominant factor was casting speed. Under high casting speed, proper increase of secondary cooling intensity could effectively decrease carbon and sulfur segregations of billet, especially for the central position of billet. The hydraulic simulation of the tundish showed the optimal casting speed should be 1.8m/min.

Abstract: There are lath martensite and a little austenite in the microstructure of samples quenched. The original austenite grain sizes ranges from 7.9μm to 74.1μm, which grows up gradually with the increasing of temperature quenching. So do the martensite acicular bundle. During the process of tempering at different temperatures after quenching at 1050°C, austenite grain size becomes bigger with the temperature increasing, and martensite acicular bundle becomes thinner. The content of austenite ascends to the peak at 650°C then it decreases. The mechanical properties (σb =958.87 MPa, δ=20.44%, HRC=30.9) of the samples are the best, when quenching temperature is 1050°C and tempering temperature is 600°C.

Abstract: In the present work, physical model experiments were carried out to clarify the inclusion removal at the free surface of steel bath. Polyethylene particles were used to simulate the non-wetting inclusions like alumina and silica. The influence of gas flow rate and bubble size on the inclusion removal at the free surface was evaluated. It is demonstrated that not all particles are removed when they arrive at the free surface of liquid bath, and those which are not removed will get back to the bath. It is found an annular particle layer is formed by the removed particles at free surface, which can capture other particles arriving at the free surface. However, the attachment of particles to the annular particle layer is not stable, and re-entrainment of particles occurs at high gas flow rate. It is shown the overall particle removal is determined by a balance of removal and re-entrainment. The particle removal constant increases with the increase in the gas flow rate, but decreases with the increase in bubble size. The equilibrium concentration of particles increases with the increase in gas flow rate and bubble size.

Abstract: The V、Ti、Nb micro-alloyed Q345B structural steel was made the hot tensile test by Gleeble-1500D thermal simulation testing machine. It analyses the change of fracture strength and reduction of area with temperature at the deformation condition of strain rate 1.5*10^-3/S and deformation temperature 700-1300°C(△T=100°C) were analyzed. The Q345B structural steel exists two obvious brittle zones, the first temperature area of brittle(Ⅰ) is solidification temperature~1300°C,the second temperature area of brittle(Ⅲ) is 850~740°C. Through observation of fracture morphology and microstructure by using of scanning electron microscopy and optical microscopy, and the fracture reasons were discussed.

Abstract: Microstructure and mechanical properties of 60Si2Mn steel by 870 °C prequenched and second sub-temperature quenching was studied. The results showed that the hardness of the 60Si2Mn steel first increased and then decreased with increasing second sub-temperature quenching temperature, and the highest hardness was obtained when sub-temperature quenching temperature was 800 °C.The sample after second sub-temperature quenching was composed of martensite and a little ferrite phase. The abundance of ferrite phase decreased with the increase in second sub-temperature quenching temperature. Fine martensite was obtained by second sub-temperature quenching, which may be relate to the grain refinement of austenitic phase and austenitic nucleation sites. The mechanical properties of the 60Si2Mn steel after 870 °C prequenching and second sub-temperature quenching are better than that of conventional samples after 870 °C quenching.

Abstract: The effects of high carbon content on the microstructure and tensile properties of Fe-20Mn-3Cu-xC TWIP steels were studied by OM, XRD, SEM analysis. The experimental results indicate that the amount of carbide and micro-porosity increased with the increasing of carbon content. The microstructures in the undeformed and deformed region of Fe-20Mn-3Cu-xC steels exhibit fully austenitic phase. The fractography of tensile samples reveals typical tough fracture microstructure with isometric dimples. The number of carbide in the as-cast structure, the yield strength and the tensile strength increase with the increasing of carbon addition, but the elongation rate increases at first and then decreases. TWIP steel has the optimum property when the carbon addition is 1.3 mass pct, of which the tensile strength is 1040.59 MPa, the elongation percentage is 98.08% and the strength-elongation product achieves 102061.07MPa•%. Fe-20Mn-3Cu-xC steels have high strain hardening capacity, and strain hardening rate (θ) and strain hardening index (n) both increase with the increasing of C addition.

Abstract: The high temperature deformation law of nitriding steel 25Cr5MoA over the strain rate range 0.001S^-1~20S^-1 and temperature range 850°C to 1150°C was studied in the thermal simulation testing machine Gleeble-1500. Under a certain strain rate and a certain deformation degree, the flow stress decreased with the increase of deformation temperature. Work hardening of nitriding steel 25Cr5MoA was strong when the true strain was less than 0.2, otherwise the flow stress increased slowly, even dropped. High temperature deformation flow stress of nitriding steel 25Cr5MoA was influenced by the deformation temperature and strain rate. When the strain rate was 0.1S^-1, true stress-true strain curve exhibited a dynamic recrystallization model, and with the increase of deformation temperature, peak flow stress shift left. When deformation degree was 0.69, the strain rate was 1S^-1, and when deformation temperature was in the region of 850°C~1050°C, true stress-true strain curve exhibited a dynamic recovery model. And when the deformation temperature was in the region of 1100°C~1150°C, it showed a dynamic recrystallization model. Dynamic recrystallization diagrams of nitriding steel 25Cr5MoA were also established.

Abstract: Two X70 pipeline steel plates with different finishing cooling temperature showed different DWTT indexes. In order to investigate the influences of microstructure and texture on toughness EBSD and X-ray were employed. Plates with lower toughness had higher low angle boundary proportion than the other, which is different from the previous research. Then the macro textures of the two plates were obtained by the X-ray diffractometer. It was shown that the ‹110›//ND were dominant in both of the two plates. The effects of several components on the toughness were analyzed. This work shows that the {112}‹110› component is the favorable texture benefiting the toughness. ‹111›//ND has direct influence on the yield stress and toughness.