| Automobile industry is one of the fastest growing industries in our country and the vehicle population increased year by year. The frequency of bad climates such as the national haze and fog weather has increased sharply simultaneously. How to effectively reduce vehicle exhaust pollution for environment protection caused an extensive concern of the whole society. The lightweight of automobile is an important part of reducing emissions and energy conservation, which mainly consists of material lightweight and structural lightweight. In the material lightweight respect, the iron and steel materials which have more than 70% of the automobile weight are developing in the direction of high strength and ductility, and weight reduction must be on the premise of guarantee the safety of the car body. For now, the AHSS (Advanced High Strength Steel) has been developed to the third generation. In the structural lightweight respect, it is a very effective method using hollow parts instead of solid parts. Therefore, tube hydro forming technology which is an advanced manufacturing technology of hollow variable cross-section light structural parts as characterized by lightweight and integration has gotten more and more attention. Domestic and foreign research institutions and manufacturers have made systematic studies on the tube hydroforming process using the finite element simulation and various experiment methods. However, the researches on the billet of the tube material lagged behind, and also the research on strength-ductility balance steel tube with low cost is rarely reported. In this background, this article combined material lightweight and structural lightweight, and introduced the concept of advanced high strength steel to tube production, and successfully developed the advanced strength-ductility balance DP (Dual Phase) steel and TRIP (transformation induced plasticity) steel seamless tube with the high strength performance close to the third generation of AHSS, and meanwhile studied the microstructure evolution of TRIP steel seamless tube during hydroforming process. Main results are as follows:(1) On the basis of the investigation on the continuous cooling transformation behaviors of the 0.16C-0.31Si-1.6Mn microalloyed steel, the ultra fast annealing process for the DP steel seamless tube was developed by the means of intermediate frequency induction heating. The effects of intercritical annealing temperature and overaging procedure on microstructure and mechanical properties of the DP seamless steel tube were studied. The results show that, at the intercritical annealing temperature of 750?, the grain size and the distribution of martensite phase were homogeneous and the recrystallization of ferrite was sufficiently finished, resulting in a higher product of strength and elongation,16510MPa-%. The transformation mechanism from austenite to martensite/bainite changes from diffusion to diffusionless, the final microstructure changes from bainite to martensite and the volume fraction of martensite increases as the overaging temperature decreases from 400? to 200?. The optimum mechanical properties are achieved at the overaging temperature of 200?.(2) The effects of three heat treatments containing tempering, slowly cooling and pre-heating on the plasticity of the seamless DP steel tube by the means of intermediate frequency induction heating were investigated. The carbides evolution during tempering, the influence of slowly cooling treatment on epiphytic ferrite and quenched microstructure and the effect of initial microstructure on plasticity were discussed. The results show that the size of carbides dimension increased during martensite decomposition and the Vickers-hardness of martensite decreased with increasing tempering temperature. However, the density of movable dislocations within ferrite grains decreased, resulting the yielding platform in stress-strain curves. The optimum tempering temperature was 200?. Although increasing initial temperature before slowly cooling can make new ferrite transform sufficiently, the volume fraction of bainite increased simultaneously, both of the phenomenon have contrary effects to the ductility. At the initial slow cooling temperature of 790?, the volume fraction of bainite decreased and the transformation of new ferrite was just plenary, the performance of the tested tube become better. The initial microstructure of the tube change to disperse cementite clusters instead of pearlite after pre-heating, which make the bainite disappeared after the continuous annealing, as the result of this, the product of the strength and elongation reach 20865MPa·%.(3) The TRIP steel seamless tube was developed using two stage heat treatment. The influence of heat treatment on chemical stability of retained austenite was analyzed. Meanwhile, the TRIP steel seamless tube was applied to a T-shape hydroforming experiment. The microstructure evolution and crack initiation and propagation were investigated with the stress-strain state. The results show that the martensite transformation of retained austenite in the TRIP steel seamless tube took place during deformation, which is called TRIP effect. The TRIP effect was related to the stress-strain state and distribution of retained austenite. The retained austenite transformed easily into martensite under tensile stress condition rather than under compressive stress along the axial direction. The retained austenite located at the boundary of ferrite grain transformed prior to the interior of the grain. The cracks can nucleate at inclusions or inclusions/matrix interfaces, and large cracks can be formed by coalescence. The microcrack growth within ferrite grains was suppressed due to TRIP effect and the retained austenite can effectively arrest crack propagation by absorbing energy and acting as obstacle, retarding failure phenomenon during deformation.(4) The austenite transformation behaviors during intercritical annealing and isothermal holding at bainite region were investigated. We improved the mechanical stability of retained austenite in the TRIP steel seamless tube and the reasonable proportion for each phase in the final microstructure was achieved, finally, the short heat treatment process was developed. At the intercritical annealing temperature of 830?, the reverse austenite preferentially nucleated at perlite enriched region, then at individual perlite region and finally at random carbides region. The film retained austenite with higher mechanical stability was observed at intercritical annealing holding time of 300s. Moreover, these film retained austenite distributed at bainite lath boundaries or within ferrite grains. A certain amount of martensite could be formed in the TRIP steel seamless tube after a very short isothermal treating at bainite zone, and a large density of movable dislocation was induced by accommodate volume expansion around martensite, resulting in disappearance of yielding platform and improvement of strength. The total time for this short process is only one-third of conventional process.(5) The strain hardening behaviors of the TRIP steel seamless tube for above aforementioned two different processes were investigated. The TRIP steel seamless tube produced by the short process possesses a higher strain hardening exponent. Moreover, the decrease in strain hardening exponent is not obvious with increasing true strain due to higher mechanical stability of film retained austenite, and possibility of wrinkling will be significantly lowered when the steel tube is subjected to secondary forming.In the present work, the flexible rapid heat treatment process was developed on the basis of key processes for third generation AHSS and using low-cost chemical composition design. And the high strength and plasticity steel seamless tube with excellent compositions and mechanical properties combinations, satisfy requirement for secondary forming process of steel tube. A novel material was provided for automotive parts with complex shape, large deformation and higher strength. This material seamless tube possesses high practical value. |
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