Research On Microstructure Evolution Of High Product Of Strength And Elongation Of Steels Treated By DQ&P Process

The advanced high strength steel(AHSS)can not only guarantee the automobile lightweight,but also promote and guarantee the safety of modern car plate materials.The development of AHSS based on the Q&P(Quenching and partitioning)attains more attention.On the premise of the ultra fast cooling system and based on the Q&P(Quenching and partitioning)technology put forward by J.G.Speer,the DQ&P(direct quenching and partitioning)technology is proposed.For the hot rolled strip of Si-Mn series,the behavior of microstructure evolution of high product of strength and plastic hot rolled DQ&P steel.In order to improve the plasticity of the materials,the ferrite is added to the lath martensite and retained austenite.In this paper,the main research contents and results are as follows:(1)The effects of the quenching temperature,the partitioning temperature,the partitioning time and the degree of deformation of DQ&P steels on the microstructures and macro hardness were explored by thermal simulation testing machine under the condition of isothermal partitioning.The amount of residual austenite is largest when the deformation is 880 C.With the increase of time,there are carbide particles between the lath martensite,which presents the trend from point to slender rod then to small block form.The amount of residual austenite and macro hardness are largest when the partitioning time is 150s.(2)The effects of the quenching temperature,the process of relaxation of DQ&P steels on the microstructures and macro hardness were explored by thermal simulation testing machine under the condition of coiling partitioning.From the experiment results,we can know that in the condition of non-isothermal partitioning,the content of residual austenite and macro hardness reach maximum respectively at the 240 ? and 280 ? of quenching temperature.Though the amount of retained austenite is lower,the carbon of retained austenite is higher after joining the process of relaxation.(3)From the analysis of element distribution,macro hardness,the content anddistribution of residual austenite we can know that the range of variation of the microstructure and property of two kinds of partitioning process are similar.The process of non-isothermal partitioning can obtain similar or even better mechanical property with isothermal partitioning under the condition of selecting optimal parameters.(4)By hot rolling experiments,the effects of quenching temperature and relaxation on the microstructure and properties of DQ&P steel were studied under the condition of coiling partitioning deformed in austenite.Because of lath martensite in the microstructure,the strength of steels quenched at 280? is up to 1247MPa.Joining the relaxation process to increase the content of ferrite,the strength dropped to 1038MPa at the same quenching temperature.The product of strength and elongation remains about 17000MPa%,when the quenching temperature changes from 220? to 360?.The product of strength and elongation of steels with relaxation process increases obviously contrast to 123 00MPa%obtained without the relaxation process.(5)By hot rolling experiments,the effects of quenching temperature on the microstructure and properties of DQ&P steel were studied under the condition of coiling partitioning deformed in two-phase.The strength and the elongation of steels deformed in two phase zone increases because of the refinement of the grain.So the product has been further improved.The value of product is up to 26610MPa%.At this point,the strength is 983MPa,and the elongation is 27.13%.In addition,the content of retained austenite reduces sharply by measuring the samples after fracture,which can illustrate that the steels take place TRIP effect during tensile deformation,and increases the plasticity of the steels.