Abstract: The microstructure and mechanical properties of ferritic rolling low carbon steel are investigated by metallurgical microscope, thermal simulation testing machine, electron backscattered diffraction (EBSD) and universal tensile test machine. The finishing temperature of the transition from austenite to ferrite changed from 680°C to740 °C with different cooling rates, which was obvious lower than that of the interstitial free steel. The deformation stress of low carbon steel was larger than that of interstitial free steel. In addition, the deformation stress of the low carbon steel was more sensitive to the deformation rate than that of the interstitial free steel. The microstructure at the surface layer of the hot rolling plate was composed of fully recrystallized grains while the microstructure in the center was composed of fibrous deformed grains. The ferritic rolling low carbon steel has lower yield ratio and higher elongation than that of normal rolling low carbon steel.
Abstract: In this paper, a new type of automotive 1500 MPa grade hot-formed steel without boron but containing niobium was subjected to thermoforming experiments. The phase transition point and Continuous Cooling Transformation (CCT) curve of the hot-formed steel were measured by thermal dilatometer, and then the best austenitizing parameters was determined. The microstructure of the cold-rolled sheet and the hot-formed steel sheet were observed by electron microscopy. The microstructure of the steel sheet after hot forming was studied by X-ray diffraction (XRD) method to determine whether the microstructure after hot forming had residual austenite. The influence of residence conditions on its mechanical properties was studied. The experimental results has shown that the microstructure of the original cold-rolled sheet is mainly composed of ferrite and pearlite. After thermoforming, the basic microstructure are martensite and a small amount of ferrite; When the hot forming parameters is that 900 °C of the heating temperature, 3 min of the holding time, 8 s of the residence time, quenching temperature is the room temperature, the new 1500 MPa grade hot formed steel has the best mechanical properties that the tensile strength is 1519 MPa, the yield strength is 1060 MPa, the yield ratio is 0.73, and the elongation reaches 10.52%. The result shows that the new 1500 MPa grade hot formed steel could obtain excellent mechanical properties through a reasonable process under the premise of ensuring hardenability.
Abstract: The cause of drawing fracture of SWRH82B wire rods was analyzed by using optical microscopy, scanning electron microscope - energy dispersive spectrometer and electron probe micro-analyzer - wavelength dispersive spectrometer. A multivariate diffusion model was established in Thermo-Cale, and the effects of temperature and time on diffusion behavior of alloys were studied. Results show that cementite network and martensite in the center area of rod is main cause of tensile fracture. There is serious segregation of chromium and manganese in the central area. The CCT curve moves to right, and critical cooling rate of martensite decreases. With high cooling rate, time for eutectoid transition is insufficient, and martensite transformation occurs in segregation band. The segregation of phosphorus further worsen the brittleness of steel. With increase of heating temperature and duration of heating time, segregation in final product is reduced, and content of cementite network and martensite decreases. When the temperature is maintained at 1050 °C for 600 s, there is no segregation of phosphorus and carbon. The diffusion of chromium is even when temperature is maintained at 1150 °C for 5400 s, and an even diffusion of manganese is obtained when temperature is maintained at 1200 °C for 3000 s. In stelmor air cooling process, the key point is keeping cooling rate low to extend holding time, and to optimize microstructure and properties.
Abstract: Gear steel is a ferritic steel. In the rolling process, the ideal structure is ferrite + pearlite, and bainite or martensite is not expected. However, due to the high alloy content, the hardenability is good, and the bainite or martensite structure is very likely to be generated upon cooling after rolling. In this paper, phase transformation rules during continuous cooling of 20CrMnTi with and without deformation were studied to guide the avoidance of the appearance of bainite or martensite in steel. A combined method of dilatometry and metallography was adopted in the experiments, and the dilatometer DIL805A and thermo-simulation Gleeble3500 were used. Both dynamic and static continuous cooling transformation (CCT) diagrams were drawn by using the software Origin. The causes of those changes in starting temperature, finishing temperature, starting time and transformation duration in ferrite-pearlite phase transformation were analyzed, and the change in Vickers hardness of samples with different cooling rate was discussed. The results indicate that with different cooling rate, there are three phase transformation zones: ferrite-pearlite, bainite and martensite. Deformation of austenite accelerates the occurrence of transformation obviously and moves CCT curve to left and up direction. When the cooling rate is lower than 1 °C/s, the phases in samples are mainly ferrite and pearlite, which is the ideal microstructure of experimental steel. As the cooling rate increases, starting temperature of ferrite transformation in steel decreases, starting time reduces, transformation duration gradually decreases, and the Vickers hardness of samples increases. Under the cooling rate of 0.5 °C/s, ferrite transformation in deformed sample starts at 751.67 °C, ferrite-pearlite phase transformation lasts 167.9 s, and Vickers hardness of sample is 183.4 HV.
Abstract: In this study, based on the existing 20LH5 austenitic stainless steel for sieve, the influence of different solution treatment parameters on the microstructure and properties were studied. Through the simulation before the experiment, the suitable theoretical solution treatment temperature range for the steel plate is 800 °C~1100 °C according to the empirical formula, and the optimal holding time range is 5 s-15 s. In the optimal temperature range and the best solution treatment time, the gradient was set in the interval and then cooled by water and air after heating. The results show that the grain of the sample steel before pre-treatment (cold rolled then pre-annealed before leaving the factory) is coarser, and the microstructure of the steel plate after solution treatment has obvious refinement tendency and a large number of annealing twins are formed. The softening effect is remarkable. At 800 °C + 15 s + water cooling and 950 °C + 10 s + water cooling, the tensile strength of the two processes are greater than 1000 MPa, the yield strength of the two processes are greater than 440 MPa, and the elongation of the two processes are greater than 46%.The steel sheets obtained by solution treatment under both experimental parameters can achieve good mechanical properties and meet the expected specifications of the products.
Abstract: Titanium and niobium were applied to stable the carbon or nitrogen which dissolved ferritic stainless steel for improving the anti-corrosion performance. The titanium nitride and niobium carbide had been formed during solidification processing. For understanding those precipitates how to influence the casting macrostructure, three steels that had different content of niobium and the fixed content of titanium had been designed. The result showed the casting macrostructure of ingot and the grain size of the centre-equiaxed crystal zones had different tendency. And the titanium nitride and niobium carbide had interacted.
Abstract: In this paper, the Steel Plate Heat Commercial (SPHC) that produced by RiZhao Steel’s Endless Strip Production (ESP) line was taken as the research object. The phase transition points under different cooling rates were measured by DIL805A thermal expansion instrument and then the static Continuous Cooling Transformation (CCT) curve was plotted. The rolling process of ferritic zone was simulated by Gleeble-3800 hot compression tester. The microstructure evolution of SPHC under different temperatures and different strain rates were analyzed, and the hot compression deformation behavior was studied. The experimental result has shown that when the cooling rate of low carbon steel is lower than 15 °C·s-1, the microstructure is mainly composed of ferrite and a small amount of pearlite and tertiary cementite. The experimental material showed a mixed crystal phenomenon when the deformation reached 50% at 780 °C. The fitting calculation has shown that the deformation activation energy of the ferrite zone rolling is 112 kJ·mol-1, and the relationship between the deformation energy storage and the temperature compensation strain rate factor was established. Subsequently, the above experimental results were verified in the RiZhao Steel’s ESP line, which laid the experimental foundation for the use of ferrite rolling technology for endless strip production.
Abstract: Two annealing processes have been designed to process a medium Mn steel, namely, the traditional direct two-phase annealing process and the pre-quenching treatment before two-phase annealing process, called two stage annealing process. The experimental results indicated that different annealing processes resulted in completely different microstructures, and a significant difference in mechanical properties. The microstructures of the steel after direct two-phase annealing process were coarse tempered martensite matrix and retained austenite. In this case, the optimum mechanical properties with total elongation of 29.87 %, tensile strength of 932 MPa and UTS*TE of 27.84 GPa•% were achieved after annealing at 625 °C for 18 h. The annealing process with pre-quenching treatment could realize two kinds of retained austenite, including acicular austenite and blocky austenite. The process with pre-quenching treatment could improve mechanical properties as well as shorten the optimal annealing time. The steel with two stage annealing process achieved optimal mechanical properties after pre-quenching and annealing at 625 °C for 4 h with tensile strength of 1177 MPa, total elongation of 30.92 % and UTS*TE of 36.39 GPa•%.
Abstract: The equipment and technology characteristics of endless strip production line in Rizhao steel were analyzed and the achievements of endless strip production in Rizhao steel were illustrated with detailed productive data in 2017. The achievements of endless continuous casting process was introduced from the aspects of the mold level deviation, the casting speed and the tundish temperature. The achievements of endless continuous rolling process was described from the rolling length, the mechanical properties and the microstructure at different position along the width. The achievements of endless continuous casting and rolling process was discussed from the product qualification rate, the yield rate, the thin strip ratio, the accident rate and the energy consumption. The productive practice of endless strip production (ESP) shows that the longest number of continuous casting furnace reached to 15 heats (4500 tons), the longest rolling length in single casting period was 182 km, the qualified rate of products was up to 99.73%, the yield rate was up to 97.51% and the thin strip ratio had a maximum value of 40.57%. At the same time, its accident rate was below 2.02% and its energy consumption was less than 27.26 kgce·ton-1. These achievements indicated that the ESP line in Rizhao steel had the advantages of low cost, good performance and green manufacturing, so the endless rolling technology will be rapidly promoted and the hot rolled thin products by ESP will further expand the market.
Abstract: The microstructure and mechanics of 15CrNiMo steel with different quenching temperatures (800°C, 825°C, 850°C, 875°C, 900°C) and different quenching holding time (15min, 25min, 35min) were studied for steel 15CrNiMo for roller bit. The relationship between the properties of different quenching parameters and the mechanical properties such as microstructure, impact toughness and hardness was analyzed. The results show that the microstructure of the annealed precursor is mainly ferrite and pearlite. With the increase of quenching temperature, the ferrite is gradually reduced, the lath martensite is gradually increased, and the martensite grains tend to grow. The strength increases first and then decreases, and the plasticity increases slightly. The quenching and holding time has no obvious effect on the test steel, and the austenitization of the core is the best time. The best quenching temperature is 850°C, the best holding time is 25 min, (tempering temperature is 220°C, tempering holding time is 50 min) and the mechanical properties are optimal.