CCT Curves of Low-Carbon Mn-Si Steels and Development of Water-Cooled Bainitic Steels

CCT curves of Mn-Si steels with different manganese contents or carbon contents were determined. The results show that the transformation range of bainite can be separated from that of ferrite when the manganese content approaches a certain content, and the incubation period of ferrite increases more significantly than that of bainite transformation with the increase of carbon content in Mn-Si steels. Furthermore, water-cooled bainitic steels without adding expensive alloying element were developed. Granular bainite was obtained when a bar with diameter of 300 mm was cooled by water, and a mixed microstructure of granular bainite and martensite was obtained in watercooled plate with thickness of 40 mm. The developed water-cooled bainitic Steels containing no expensive alloying element showed a good combination of strength and toughness. The tensile strength, yield strength, and toughness （AKU at --20℃ ） of bar with diameter of 300 mm after water cooling were higher than 850 MPa, 620 MPa, and 65 J, respectively, and those of plate with thickness of 40mm after water cooling were higher than 1000 MPa, 800 MPa, and 50J, respectively.

Phase transformation of TRIP-aided multiphase steel during continuous cooling

The phase transformation characteristics of a high-strength TRIP-aided multiphase cold-rolled steel during continuous heating at different cooling rates were studied by means of dilatometry,and the critical temperatures were also determined.The samples were fully austenitized at 1 050 ℃ and then cooled at different cooling rates ranging from 0.5 ℃/s to 100 ℃/s.The continuous cooling transformation （CCT） curves were obtained for the experimental steel.The experimental results showed that a high cooling rate depressed the formation of ferrite and pearlite and promoted the formation of balnite and martensite,leading to a higher hardness.A large amount of martensite in high-strength TRIP-aided multiphase cold-rolled steel can be obtained at cooling rates in excess of 50 ℃/s.The experimental results provide guidelines for cooling control and heat treatment in real steel production.

Microstructures and Mechanical Properties of X100 Pipeline Steel Strip

Dynamic CCT （continuous cooling transformation） curves and effects of microstructures and M/A islands of X100 hot rolled strip on mechanical properties were studied by means of a thermal simulator, a scanning electron mi- croscope （SEM）, a transmission electron microscope （TEM）, a digital micro-hardness tester and Image-Pro Plus analysis software. The results show that high contents of C and Mo can make transformation lines of acicular ferrite and bain- ite shift rightward. High Mo tested strip has higher strength and micro-hardness and lower toughness than the other one because lath bainite appears instead of quasi-polygonal ferrite. Tested strip with granular bainite, lath bainite and M/A islands has better mechanical properties and in the microstructures lath bainite content is around 36.50% and M/A islands are fine and disperse in the matrix.

Microstructure Transformation Behaviour of 610 MPa HSLA Steel Plate for 150000 m^3 Oil Storage Tank Construction

Static and dynamic continuous cooling transformations （CCT） of 610 MPa HSLA steel plate for 150000 m^3 oil storage tank construction were measured by using an MMS200 thermomechanical simulation system. It was found that with the increase in cooling rate, the proeutectoid ferrite, pearlite, granular bainite, and lath bainite appeared in the transformed microstructure. Compared with those of static CCT, the transformation points of acicular ferrite, granular bainite, and lath bainite in the dynamic CCT curve were shifted to the left, resulting in an increased acicular ferrite and bainite transformation area.

Analysis of Crystallization Behavior of Mold Fluxes Containing TiO_2 Using Single Hot Thermocouple Technique

The crystallization behavior of mold fluxes containing 0-8 mass% TiO2 was investigated using the single hot therrnocouple technique （SHTT） and X ray diffraction （XRD） to study the possible effects on the coordination of heat transfer control and strand lubrication for casting crack sensitive peritectic steels. Time-temperature-transforma tion （TTT） and continuous-cooling transformation （CCT） curves were plotted using the data obtained from SHTT to characterize the crystallization of the mold fluxes. The results showed that crystallization of the mold fluxes during isothermal and non-isothermal processes was suppressed with TiO2 addition. From the TTT curves, it could be seen that the incubation and growth time of crystallization increased significantly with TiO2 addition. The CCT curves showed that the crystallization temperature initially decreased, and then suddenly increased with increasing the TiO2 content. XRD analysis suggested the presence of cuspidine in the mold fluxes with lower TiO2 content （〈4 mass%） , while both perovskite and cuspidine were detected in the mold fluxes when the TiO2 content was increased to 8 mass%. In addition, the growth mechanisms of the crystals changed during the isothermal crystallization process from interface controlled growth to diffusion-controlled growth with increasing the TiO2 content.