A Study On Microstructure And Mechanical Properties For Automotive Cold-rolled Low Carbon Steel Under Rapid Thermal Processing

The heating and cooling methods are the key technologies of the contintuous heat treatment lines.The application of rapid cycle annealing (rapid heating, short soaking and rapid cooling)can reduce the length of the continuous annealing furnace to a large extent, refine the steel grain, upgrade the steel strength, and eventually bring a technical revolution to the continuous heat treatment. In order to satisfy the requirements of the weight loss of the car and the development of high strength steel for the new generation of vehicles, the artical try to strengthen the properties of existing steels furtherly through rapid cycle annealing technology. In the present report, the cold rolled steel from BAOSTEEL are chosen to study the effects of rapid cycle annealing on the microstructures and properties of the cold rolled B800NQ, QP980, DC01and DC06steels through Gleeble thermal simulation machine. The relationships among the above processing parameters, the steel microstructures and mechanical properties are established, the related mechanisms are also discussed. The main works and achievements were shown as following:(1) The transformation temperature of B800NQ, QP980, DC01and DC06steels were determinated by thermal expansion experiments. It was found that the Ac1temperature remained basically unchanged but the AC3temperature increased gradually with the increase of the heating rates. The final microstructures for high strength steel B800NQ and QP980were also different at the different cooling rates.(2) Rapid heating can effectively improve the strength and elongation of the B800NQ steel, we can find that rapid heating can significantly refine the grain of B800NQ steel while analysing of microstucture; The effect of soaking temperature on mechanical properties of the B800NQ steel is relatively obvious. The strength become higher when the soaking temperature increased, but the elongation decreased. Above 1000℃, the tensile strength of B800NQ was above1200MPa, increased by about300MPa than the tensile strength(906MPa) that simulating normal annealing technology, while the yield strength is also about900MPa. The soaking time in the rapid cycle annealing has little effect on B800NQ steel, so the soaking time can be identified as5s; Slow cooling segment was more obvious when soaking in the two-phase regions; The yield strength of B800NQ steel increased significantly when the flat amount increased, comprehensive consideration, the elongation can be set at about0.4%.(3)Increasing the heating rate in the low-temperature section, the strength and elongation of the QP980steel can be effectively improved, but at high temperatures there has little change; As the soaking temperature increased, the strength of QP980steel gradually increased, but the elongation decreased. The tensile strength can be achieved1200MPa when soaking at970℃for5s, which increased by about250MPa than the tensile strength(951MPa) that simulating normal annealing technology, at the same time, the total elongation reached18%which decreased not much in comparision with the total elongation(19.7%) of simulating normal annealing; when the partitioning time kept longer, the amount of retained austenite increased and the comprehensive performance was better, but the effect was not obvious when the partitionng time was too long;When heated to870℃rapidly and cooled directly, the tensile strength of QP980was up to1610MPa.(4) Rapid heating can effectively refine the average grain size of the DC01steel, the average grain size gradually decreased from9.46μm to8.12μm; when the soaking temperature was too high, the ferrite grain of DC01will coarsen, so the soaking temperature of DC01should be between800-850℃;Overaging temperature and time has a great impact on the properties of DC01, the high-aging temperature and longer aging time help to improve the comprehensive performance of DC01steel; when overaging at450℃, the tensile strength of DC01reached361MPa, yield strength was245MPa, the total elongation was39.9%, and the uniform elongation was21.2%. Compared to the elongation (37.6%,19%) that simulating normal annealing, the elongation of DC01improved but the strength essentially unchanged through the rapid cycle annealing. (5) Heating rate can significantly refine the grain and improve the elongationof the DC06steel, but the strength changes not much; the mechanical properties of DC06is better at the lower soaking temperature, which is due to that the ferrite grain of DC06will coarsen at high temperature. So the holding temperature should be around810℃in order to keep good properties; it will also lead to grain coarsening when holding time is too long, therefore, the holding time can be around15s; Overaging temperature has little effect on the DC06steel because it has no interstitial atoms, so we can get rid of the aging stage.The results of experiment show that the rapid thermal annealing can not only shorten the heat treatment time but also greatly improve the performance of the tested steel, therefore, rapid thermal processing has a very ambitious future in the industrial applications.