Research On Microstructure Transformation And Temperature Simulation Of High Strength Steel Of 700MPa Used In Automobile

With the development of economy,automobile plays an increasingly important role in industrial and agricultural production and daily life of people.The universal application of automobile not only facilitates people's production and life,but also brings about problems such as energy consumption and environmental pollution.Reducing vehicle weight can effectively reduce energy consumption and emissions of CO₂ and other waste gases.However,if we blindly reduce the weight of the body in order to reduce energy consumption,it will leave a hidden danger to the safety of the car.It is the development direction of today's automobile manufacturing industry to reduce the vehicle weight without reducing the structural strength of the vehicle body and ensuring the safety of use.In order to avoid being eliminated in the wave of upgrading the steel product structure,steel mills are actively investing manpower and financial resources to develop high strength steel for automobile structure.the article study organization change rule with different cooling conditions by Gleeble-3500 thermal simulator,and study rules of return red of steel belt by numerical simulation software of DEFORM-3D and carry out the industrial trial production to find the best industrial production process parameters in the steel mills.The thermal expansion curves of the experimental steel under different cooling conditions were obtained through the simulation test of Gleeble-3500.The phase transition point was determined by the tangent method.the cooling transition curve of the experimental steel was drawn by combining the microstructure.CCT show that the ferrite/pearlite phase transformation is happen in the steel when the cooling speed is 0.1?/s,and start transformation temperature is 770?.It also shows that ferrite/pearlite and bainite transformation is happen when cooling speed is about 1?/s⁷?/s.the ferrite is polygon and bainite is granular bainite.bainite phase transition occurs and ferrite/pearlite phase disappeared when cooling speed is about10?/s¹16?/s.Ferrite is acicular and bainite changed from granular bainite to up/down bainite.Numerical simulation software was used to simulate laminar flow cooling process.It was found that the"redness"temperature and"redness"time of high strength steel showed a linear relationship with the thickness of steel plate.The fitting functions were:Y=6.4X-26.5 and Y=0.4X-1.6.When intermediate slab thickness is about 35 mm,and finishing temperature is 880?,and coiling temperature is 550?,the trail coils property satisfies EN 10338:2015.Yield strength is 680 MPa.Tensile strength is 850 MPa.Elongation is 13.5%.Trial coil's microstructure is mainly bainite microstructure,and contains a small amount of ferrite and a small amount of pearlite.bainite is granular bainite and ferrite is a porelygon.