Development Of High Strength And Plasticity TRIP Seamless Steel Tube And Investigation Of Its Hydroformability

The light weight of transport machinery has been receiving an increasing attention as the concept of low-carbon, energy-saving and environment-friendly was advocated in the recent years. At present, using the hollow structures take the place the solid ones is being paid more attentions, which is one effective way to reduce the weight of transport machinery. For the purpose of obtaining the hollow structures with higher strength and lower the produce cost, the tube with high strength as well as good formability are required. In this thesis, the manufacturing technique of TRIP steel was firstly used in the steel tube production field. The TRIP seamless steel tube with a microstructure of ferrite, bainite, retained austenite and a bit of martensite was successfully produced. The mechanical properties and hydro formability of the TRIP seamless steel tube were investigated via conventional tensile test, flaring test, ring tensile test and free hydraulic bulge test. The effect of the heat treatment process on the microstructure, mechanical properties and hydroformability was also analyzed. The main new findings are summarized as follows.(1)The TRIP seamless thin-walled steel tube was successfully produced with a production flow of melt, forging, piercing, cold drawing and heat treatment proces.The microstructure of the TRIP seamless steel tubes consist of ferrite, bainite, retained austenite and a bit of martensite. The TRIP seamless steel tubes exhibit good mechanical properties in the axial direction. The total elongation of most of the TRIP seamless steel tubes are near or more than30%. The product of strength and total elongation of tube D-II reached up to the high value of28269MPa·%. The total elongation of tube F-V was as high as39.8%and its retained austenite volume fraction was also very high, about10.26%.(2) The effect of the heat treatment process on the microstructure and mechanical properties of the TRIP seamless steel tube was investigated.The effects of the on the microstructure, mechanical properties in the axial direction, the retained austenite volume fraction and its morphology of the TRIP seamless steel tube were mainly studied. The results showed that for the predetermined intercritical annealing temperature, the volume fraction of retained austenite increased first then decreased with the increase of intercritical annealing holding time. With the same isothermal bainite treatment temperature, the volume fraction of retained austenite increased first then decreased with the increase of isothermal bainite treatment time and it exhibited the same influence rules on the total elongation as well as product of strength and total elongation.(3) The hydroformability of the TRIP seamless steel tube was evaluated with flaring test. The influences of the conical tool vertex angle and heat treatment process on the flaring limit were studied.The study mainly focused on the flaring limits of TRIP seamless steel tube B, C and D. The effect of the conical tool vertex angle on the flaring limits of TRIP seamless steel tube B, C and D was analyzed. The influences of the heat treatment condition on the TRIP seamless steel tube C and D were investigated. It was found that the flaring limits of the tubes decreased with the decrease of the conical tool vertex angle. The flaring limits of TRIP seamless steel tube C and D increased first then decreased with the increase of isothermal bainite treatment time from3min to9min. The largest flaring limit was obtained with the isothermal bainite treatment time of6min.(4) The determination method of the material constants of Oyane ductility fracture criterion was developed. In order to study the general applicability of this method, the TRIP seamless steel tube and seamless carbon steel tube were used.Firstly, the flaring tests with two of the three conical tools with the vertex angles of20°,30°and60°were carried out experimentally and numerically to obtain the material constants of a and b values. Secondly, the flaring limits of the tubes with the third conical tool was obtained with material constants a and b values by FEM simulation. Compare the flaring limits obtained by FEM simulation and experiment to confirm the accuracy of the determination method of material constants. Finally, flaring tests of TRIP seamless steel tube and seamless carbon steel tube were carried out to confirm the general applicability of the method developed in this study. The results showed that this method was reliable.(5) The ring tensile test equipment was fabricated and the calculation method for stress, strain and the anisotropy coefficient (r-value) in the circumferential direction of the tube was proposed. The FEM simulation was used to confirm the accuracy of this method. 〢ccording to the geometrical and mechanics relationships of the ring tensile test, the instantaneous stress and strain in the circumferential direction can be obtained.②The anisotropy coefficient was calculated with the strain grids.③The FEM simulation of the ring tensile test was executed. Compare the true stress-strain curve of the material input to the simulation process and the one obtained by the calculation method developed in this study with the simulation results to verify the availability of this method. It was found that these two curves agreed to each other very well and the method for determination of stress and strain developed in this study was available.(6) The circumferential mechanical properties of the TRIP seamless steel tube A, B, E and F were studied with ring tensile test. The effects of the heat treatment condition on the circumferential mechanical properties of the TRIP seamless steel tube were also investigated.The ring tensile test was carried out at room temperature with a fixed crosshead speed of2.5mm/min. The results showed that the tensile strength and yield strength in the circumferential direction of all the samples were around600MPa and350MPa, respectively. The total elongations in the circumferential direction of all the samples were high, near or over30%. The tube E-VI exhibited a highest product of strength and ductility value of19502MPa·%. Its total elongation was near30%and the tensile strength reached up to655MPa. It has a good combination of strength and ductility.(7) The free hydraulic bulge test apparatus were developed. The method for determining the effective stress-strain curve with free hydraulic bulge test was proposed. The effects of deformation rate and heat treatment process on the maximum internal pressure and bulge height were investigated.The rate of pressure rise of0.2MPa/s and0.5MPa/s was conducted in the free hydraulic bulge test of TRIP seamless steel tube F. It demonstrated that:①The over low deformation rate was unfavorable to obtain the good hydroformability of the TRIP seamless steel tube;②For the predetermined intercritical annealing condition, the maximum internal pressure decreased with the increase of isothermal bainite treatment time from4min to6min. For the given isothermal bainite treatment condition, the maximum increased and the maximum bulge height decreased with the intercritical annealing time form5min to1Omin.The TRIP seamless steel tube developed in this study has a good composite mechanical properties and hydroformability which can meet to the requirement to the material of the hydroforming process. The part with complex shape, heavy deformation and high strength can be produced with TRIP seamless steel tube. It has a high utility value.