Numerical Investigation On Springback And Thinning Of22MnB₅in The Hot Forming Process

In recent years, with the development of cars, the main themes are energy saving andemission reduction, safety and environmental protection, with lightweight and mechanicalperformance of high strength steels, they are widely applied in the automotive industry. Butthe performance characteristics such as high strength, high hardness of high strength steels,there are many problems in the stamping process, especially springback and thinning.Temperature is one of the key factors which have great effects on thermal stamping forming.So how to balance the relationship between the thinning rates and springback of high strengthsteel by controlling the temperature is the key to forming process of high strength steel. Inthis paper, we took the stamping samples of22MnB5material as the research object in thecondition of neglecting the effect of heat conduction, heat convection and other factors on thetemperature and researched the influence of the process parameters on springback andthinning rate at different temperature under isothermal condition through the AUTOFORMsimulation. Using simple samples forming to guide the forming of complex samples canprovide the basis reference data for the practical engineering application. The mainconclusions as following:1. The influence of process parameters on springback:(1) With blank holder stress increases, the springback of U stamping samples decreases.Under the blank holder stress increases from35KN to80KN, at500℃temperature, thespringback angle reduces from95.191°to92.362°, and at700℃temperature, the springbackangle reduces from91.699°to90.624°. Larger blank holder stress can reduce springback, butit will increase the risk of cracks in samples. When the temperature exceed650℃and theblankholder stress is above60KN, the cracks of samples will emerge.(2) The friction coefficient affect springback by affecting the friction stress between themold and the sample, When the forming temperature exceed500℃, the friction coefficienthas influence on springback as like blankholder stress, the springback angle decreases withfriction coefficient increase: under the friction coefficient increases from0.05to0.15, at500℃temperature, the springback angle reduces from95.92°to94.39°, and at650℃ temperature, the springback angle reduces from93.84°to91.56°, at700℃temperature, thespringback angle reduces from93.33°to90.24°.(3) Springback increases with the increase of strain rate. Under strain rate increases from0.01s-1to1.00s-1, at500℃temperature, the springback angle increases from94.64°to97.64°, and the springback angle increases from91.32°to94.74°at650℃temperature.(4) The springback angle decreases with the increase of the samples’ thickness, but theeffects are significant just at lower temperature. When the thickness increases from1.2mm to1.8mm at300℃temperature, the springback angle reduces from122.7°to110.7°.2. The influence of different process parameters on thinning:(1) The thinning rates increase with the blank holder stress increases. Blank holder stresshas less effects on thinning rate when the temperature below500℃. At300℃temperature,when the blank holder stress increases from35KN to160KN, the thinning rate increases justby6.06%. At500℃temperature, when the blank holder stress increases to80KN, thethinning rate increases by8.93%. At650℃temperature, when the blank holder stress isabove50KN, the cracks of samples will emerge.(2) The thinning rates increase with the increases of the friction coefficient, and at thehigher temperature, the effects are more obvious: under the friction coefficient increases from0.05to0.25, at300℃temperature, the thinning rate increases by4.99%, and at500℃temperature, the thinning rate increases by38.3%. In addition, the necking phenomenon willappear for samples when the coefficient of friction in the stage of0.20~0.25.(3) The thinning rates decrease with the increase of strain rates and the temperature playan important role among them, with the increase of temperature, the influence effects ofstrain rates on thinning rates are more obvious. Under strain rate increases from0.01s-1to1.00s-1, at300℃and650℃temperature, the thinning rate decreases by0.04%and5.17%.(4) The thickness of blank has few effects on the thinning rate. Under the thickness ofsample increases from1.2mm to1.8mm, at300℃and700℃temperature, the thinning rateincreases by2.9%and3.5%.3. FLC curve is an important basis for the forecast of cracks on plate forming process. Inthis paper, we acquire some FLC curves of22MnB5at different temperatures. From the FLCcurves, we can know that, at300℃temperature, the major principal strain is0.725, minimum strain is-0.705and the straight slope is about-0.76, and at700℃temperature, themajor principal strain is1.09, minimum strain is-0.521and the straight slope is about-1.42.4. The influence of hot forming process on forming properties of B-pillars for cars:During the isothermal forming process, with the increases of the deformationtemperature, the thinning of the B-pillar is more serious, and the springback of the B-pillar issmaller. At500℃temperature, thinning rate is33.6%and the maximal normal springback is1.48mm, and at650℃temperature, the thinning rate is41.9%and the maximal normalspringback is1.16mm.5. Through the analysis of U stamping samples with the thickness of1.95mm andB-pillars for cars, it is concluded that the thickness thinning rates in the same strain place areidentical for stamping samples with different shapes. In the process of isothermalforming, the forming properties are similar for different shapes of stamping samples, so viathe analysis of the forming process of simple stamping samples can provide basic referencedata for the forming of the complex samples.