Research On The Formability Of BR1500HS High Strength Steel Laser-welded Tube At Elevated Temperatures

The hot forming technology of high strength steels has been mainly used in automotive lightweight Engineering, hot forming parts of high strength steels can not only reduce the weight, but also improve the safety. Compared with hot stamping of high strength steel plate, hot forming of high strength steel tube can not only reduce weight by useing lightweight materials, but also get more weight reduction through structural design. But its research and application is not enough thorough and extensive.In this paper, the oxidation behavior, formability, mechanical propertiesand microstructure evolution in the hot forming of BR1500 HS laser-welded tube were studied through experiments and theoretical analysis.Heating experiments of BR1500 HS laser-welded tube were carried out. The effects of heating temperature, heating rate, holding time and heating atmosphere on the oxidation behavior were studied. Found: below 600℃, its oxidation was slow, above600℃was its rapid oxidation temperature range; heating in nitrogen and rapid heating could significantly inhibit its oxidation; by rapid heating and short holding time, mother material could get small initial austenite grain and avoid its excessive growth, thus to get better plasticity in forming and higher strength after quenching.The influences of heating rate, holding time and temperature on the bulge properties of BR1500 HS laser-welded tube were studied by bulge tests. It was found that the tubes through rapid heating to austenitizing temperature, holding 0-1min and bulging within 850-950℃got large limit expansion rate. The effect of the welded seam on the deformation coordination was analyzed by numerical simulation, it was found that geometrical defects and the matching coefficient of weld seam had big influence:the tube bulging with relatively high matching coefficient got relatively big limit expansion rate, the region near the welded seam and the offside were the risk area of rupture; the tube bulging with relatively low matching coefficient got relatively small limit expansion rate, the welded seam was the risk area of rupture. Rapid heating and short holding time could obtain relatively large matching coefficient of welded seam,thus obtaining large limit expansion rate.The influences of forming temperature, welded seam position, forming speed,internal pressure on the formability of profiled cross section parts of BR1500 HS laser-welded tube were studied. It was found that its hot forming performance improved greatly compared with cold forming, but under 650℃, the influnce of thermal effects on the rebound was serious, reasonable hot forming temperature range was 850-950. The location of welded seam had little influence on its hot forming performance, but it had a great influence on its pre cold forming. When its extent of deformation was large, the welded seam was easy to crack. Forming speed had great influence on its hot formability, forming too slow easily led to cracking. Reasonable internal pressure could promote its material to fill die and improve the effect of quench, so as to ensure the dimensional accuracy and mechanical properties of forming parts.