.440 Mpa, Class Of High-strength Deep Drawing Steel (if) Developed

The steel with high-C high-Nb type composition system was used in this study. The C content in the steel is about 0.006% in wt%, Nb/C atomic ratio is higher than 1.5 and appropriate amount of solid solution strengthening elements Mn and P is added in order to guarantee the strength level of the steel. The recrystallization behavior during annealing process of steels, both high C-Nb L604 and traditional L485, was studied by using a Gleeble-1500 simulator. The results show that the recrystallization finishing temperatures are about 750℃for high C-Nb L604 and 730℃for traditional L485, respectively. The recrystallization finishing temperature of high C-Nb IF steel was 20℃higher than that of traditional high strength IF steel. Based on this result, effects of steel compositions and processing parameters on microstructures and mechanical properties of high-C-Nb high strength IF steel were investigated by using salt bath annealing trial.In various parameters of hot-rolling and annealing process, the annealing temperature is a major factor influencing microstructure and grain size of high C-Nb steel. When annealing temperature changes from 790℃to 850℃, the high C-Nb steel shows the same ferrite-structure, but the ferrite grain size increases from 5.5μm to 6.3μm.When annealing temperature is 870℃, the steel shows ferrite-bainite structure. The ferrite grains of high C-Nb steel have a small growing tendency and are much finer than that of traditional IF steel. The analysis of precipitates for high C-Nb steel annealed at 810℃in salt bath shows that the main precipitation phases of stabilized elements are Nb (C, N), FeNbP and Ti (N, C) phase.90.5% of the total Nb and 81.0% of the total Ti in the steel are in precipitation phases, respectively. P is a solid solution strengthening element, but 55% of the total P is also in precipitates. In the steel, the total precipitation amount of stabilized elements is 0.237%. With respect to particle size, most particles are smaller than 36nm, accounting for 87.5% of total amount of precipitates. The particles, which contribute to the steel strength and are finer the 18nm, account for about 50% of the total precipitates and the average size of them is about 6.0nm.The main factor affecting mechanical property of high C-Nb steel is annealing temperature. The optimum annealing temperature of high C-Nb IF steel should be controlled in the range of 790℃-830℃. The experimental steel-high C-Nb L604, when coiled at 640℃and annealed in the range of 790℃-830℃, shows the following properties:tensile strength is 480～510MPa, elongation is about 30% and rm is 2.0. The study of effects of hot-rolling parameters on mechanical properties of high C-Nb annealed sheet shows that higher slab reheating temperature and higher coiling temperature are beneficial to improving plasticity of the sheets. This study indicates that, when coiled at 710℃, annealed in the range of 790℃-830℃, high C-Nb IF steel can completely meet the requirements of strength, plasticity and formability of IF steel with a strength level of 440MPa. Appling higher coiling temperature is beneficial to strength stability of high C-high-Nb steel, which reduces changes in yield strength arising from the dissolution of NbC during the annealing process.According to the contribution to steel strength, the strengthening sequence for high C-highNb IF steel is determined as follows:grain refinement strengthening, precipitation strengthening and then solid solution strengthening. While for traditional IF steel, the sequence changes to grain refinement strengthening, solid solution strengthening and then precipitation strengthening. High Nb and C contents and Nb/C ratio in High C-highNb IF steel lead to the formation of much more small-sized precipitates that play key role of grain refinement and precipitation strengthening.To better meet the requirements of mechanical properties of IF steel with a strength level of 440MPa, the optimized compositions and processing parameters for high C-Nb IF steel are presented based on the results of this study. In order to obtain an appropriate strength level for high C-Nb IF steel, Mn and P contents can be reduced properly. Among various processing parameters, the following parameters are recommended:Reheating temperature 1200℃, finish rolling temperature 910℃and coiling temperature 680℃are used in hot-rolling process. Cold rolling reduction is around 80%. In annealing process, the annealing temperature is in the range of 790℃to 830℃, then using the corresponding cooling process (depending on the different products).