Effect Of Hot Forming Process Parameters On The Mechanical Properties And Microstructure Of B1500HS Boron Steel

With the continually improvement of the people's material life condition and increasingly serious problems on energy crisis and environmental pollution, automobile industry is much more challenged, the general trend in the development of the automobile industry, therefore, was aimed to safety, environmental protection, energy saving and comfort. One of the most efficient solution to enable the energy conservation and emission reduction is the lightweighting of the car body. The research of the lightweighting design has become an important subject for the design and manufacture of modern automible. Taking the advantages, namely benign formability of materials at high temperature and phase transformation strengthening of materials when cooling down which enable the car parts with integral morphology, precise size and high strength, of the stamping deformation and quenching, hot stamping forming technique has been extensively concerned. The parameters of the hot stamping process have essential influence on the formability of the plate material.The U-type stamping parts of B1500HS boron steel were systematically investigated, the contents of which showed that:1. The mold temperature control equipments have been used to study the effect of heating and mold temperature on the mechanical properties and microstructure of the different parts of B1500HS boron steel, the results showed that: For the different local position of the U-type parts, of which the mechanical performance and stability were: bottom > flange >straight well; the U-type parts were incomplete quenching after heat preservation at 850? for 5 min, the properties of the hot stamping parts were not suitable for design; when the austenization temperature of B1500HS exceeded 900? and increased, the austenite grain size increased,which resulted in the decreasing of the mechanical performance of the hot stamping parts; with the increasing of the mold temperature in the case of constant austenization temperature, the reducing of the mechanical performance occurred; large amount of bainite showed up when the mold temperature was higher than 300?.2. The effect of punching perssure and holding time on the mechanical properties and microstructure of B1500HS boron steel has been studied by controlling the stamping pressure, the results showed that: In the initial stage, the small protrusions at the interface produced elastic, plastic and creep deformation under thermal coupling, the effective contact area increased linearly with the increase of the load, indicating the similar linear increase of the heat transfer coefficient. When the pressure reached 40MPa,the small protrusions produced plastic flow at the interface, indicating the strain release, which led the interface contact coefficient stable. The grain was coarse and the mechanical performance was poor when short time pressure keeping. The strength and hardness of the parts increase with the increase of the time of pressure keeping. Uniformly dispersed martensite was obtained when the time of pressure keeping was 7.5s, of which the strength and hardness was relatively high.3. The thermal expansion of B1500HS curve was drawn by high temperature phase-change instrument DIL805A, the effect of heating and cooling rate on the mechanical properties and microstructure of the different parts of B1500HS boron steel has been studied combined with metallographic observation and hardness test. The results showed that: The austenization of sample did not initiate until 800?. It was more difficult for pearlite to nucleated on the pre-eutectoid phase (ferrite) located at grain boundary of austenite when the cooling rate was higher due to the incubation period of the pearlite, which resulted in the reduction amount of the pearlite transformed. The rapid quenching temperature and the increase of heat preservation would increase the Ms point. Rapid cooling rate during continous cooling would increase the subcoold temperature, which increase the volume fraction of the martensite and lower the Ms point. The sample was not austenited when heated to 900? at a heating rate of 50?/s. The ununiformity of the diffusion of the C atoms aggravated at a high heating rate due to the diffusion-type phase transformation of the austenite, which aggravated the inhomogeneity of the microstructure and retained the banding-like microstructure. And the prolonging of the heat preservation could alleviate the inhomogeneity.