| At present,our country’s steel production is the largest in the world,but from the perspective of the structure of steel products,there is still a certain gap compared with developed countries.In the context of continuous pursuit of high strength and high plasticity,the use of quenching and partitioning technology to improve the strength and plasticity of steel has been rapidly developed.So for most scholars focus on adding alloying elements to the steel to change the performance.However,there is less concern about the combination low-cost steel with existing industrialized production.Therefore,from an industrial persperctive,this thesis combines the hot rolling process of steel enterprises with the Q&P process in order to obtain Q&P steel with excellent comprehensive performance with low price and low energy consumption.In the experiment,metallographic microscopy(OM)and scanning electron microscopy(SEM)were used to observe the morphology of the steel.The electronic universal testing machine and the microhardness tester are used to test the variations of strength,elongation and hardness,revealing the influence of the structure changes on the performance.The main research contents of this thesis are as follows:(1)After holding at 910℃ in the γ phase,the Q&P treatment showed that the crystal grains grow gradually with the increase of the partitioning temperature.Yield strength and elongation have opposite trends.The overall performance reaches the maximum after 10 s of partitioning at 350℃,the tensile strength is 580 MPa,the yield strength is 412 MPa,the elongation is 31.5%,and the plastic product is 18270MPa·%.As the partitioning temperature increases the partitioning time for a significant decrease in tensile strength and yield strength will be shortened accordingly.(2)The Q&P treatment was carried out at 800℃ in the dual phase α+γ zone.The results show that the introduction of soft phase ferrite can significantly increase the elongation of the experimental steel.The elongation is 42.2% after being partitioned at 400℃ for 10 s.The strength has also reached its maximum.Short-time partition at low temperature and long-time partition at high temperature have a significant effect on the increase of the hardness of the experimental steel.The dimple morphology of the fracture can reflect the size and quantity of martensite.The dimples produced by martensite fracture are about 10μm,and the extremely small dimples are produced after ferrite tensile fracture.(3)After the single-phase zone which is kept at 910℃,the hot rolling quenching partitioning process is carried out.The results show that the grains are obviously refined after the hot rolling process.Under the partition of 350℃,the yield strength and tensile strength reached their maximum values,481 MP and 596 MPa,respectively,and the elongation exceeded 30%.After 10 s partition at 400℃,the tensile fracture has obvious tearing edges,accompanied by a large number of small dimples.This fracture morphology will significantly reduce the strength and plasticity of the material.Martensite and bainite are at 60° or parallel distributed,which generate a secondary crack,and the secondary crack expands gradually.(4)The hot rolling quenching partitioning process was carried out in the dual-phase zone after holding at 800℃.The results showed that after introducing a proper amount of ferrite,the elastic deformation range of the stress-strain curve was expanded and the elongation was significantly increased.The yield strength is almost doubled after 10 s partition at 350℃,and the overall performance is the best.The minimum tensile strength of all the process conditions is 548 MPa,the minimum yield strength is 450 MPa,and the elongation has shown a trend of first increasing and then decreasing. |
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