| The lightweight design of the car body has always been a core issue in the automotive industry.Energy saving and emission reduction require the automotive industry to develop innovative lightweight vehicles and reduce the quality of all components.In the contemporary automotive industry,the issue of car weight loss is even more important because of the added safety and comfort system.In order to meet the new requirements of the automotive industry and minimize the structural quality,this experiment designed a low density steel with an alloy composition of 0.39C-0.92Mn-2.93Al.The lightweight element A1 is added to achieve the goal of light weight.The alloying element Mn is added to strengthen the matrix and lower the Ms point,and a small amount of silicon is added to avoid surface quality problems during hot rolling and galvanizing.Limited experiments were carried out for verification work to the extent of advanced commercial applications.The phase transform point and the distribution of alloying elements were calculated by the thermodynamic software Jamtpro.Static recrystallization,phase transformation and heat treatment tests were designed in salt bath furnaces and continuous annealing was performed on the simulator.The micro structure was analyzed by optical microscopy and scanning electron microscopy.The hardness is measured by a Vickers hardness tester.Residual austenite is explored by X-ray Diffraction and the mechanical properties test was carried out by a tensile test.The experimental results are as follows:The A1 and A3 point of this experimental steel is 767? and 1227?,respectively.The static recry stallization kinetic equation of experimental steel is(?).The recrystallization activation energy is 278.5 KJ/mol.When the phase transform temperature is sequentially increased from 800?to 830? and 860?,the martensite content obtained at room temperature increases;As the phase transform holding time is extended from 20 s to 40 min,the martensite content increases first,then decreases,and finally increases.The heat treatment was carried out in a salt bath furnace.When the annealing temperature increased from 830? to 860?,XRD showed that the retained austenite in the structure had a(311)y peak.The tested steel annealed at 860 C has the best mechanical properties after 360 C overaging and the strength plastic product is 21.7 GPa·%.The tested steel annealed at 830 C has the best comprehensive mechanical properties after overaging at 400 C and the strength plastic product is 21.9 GPa·%.The extension of the overaging isothermal time optimizes the overall mechanical properties.The length of the L·uers band becomes shorter as the annealing temperature increases.The work hardening process is divided into four stages,of which the second stage shows the influence and magnitude of the TRIP effect.Finally,physical simulation is carried out in the continuous annealing simulator according to the actual equipment and process conditions on site.The continuous annealing micro structure is mainly composed of massive austenite and coarse ?-ferrite matrix.As the overaging temperature increases,the martensite content generally increases and the size continues to decrease.The Al content of the M/A island is higher than the A1 content(molar atomic ratio)in martensite and ferrite.The Mn content in the massive martensite is higher than the Mn content in the ferrite(molar atomic ratio).The best continuous heat treatment process is:830 C annealing holding for 106 s and 440 C over-aging holding for 420 s,the retained austenite content under this process is maximum,the strength plastic product reached 21.8 GPa·%.As the overaging temperature increases,the overall mechanical properties of the material are optimized.In the designed experimental process,the carbon content and yield strength of the retained austenite change in a general trend whlie the tensile strength changes in the opposite direction.The retained austenite volume fraction changes in approximately the same trend of elongation. |
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