Abstract
The warm stamping technology is a promising technology to meet the needs of car weight reduction and energy conservation. In order to compare with the mechanical properties of the traditional hot-stamped boron-alloyed steel 22MnB5, a new warm-stamped niobium-alloyed steel 22Mn3SiNb was designed and tested. The optimal heating parameters for warm forming process were explored through mechanical tests, and the process of their microstructure evolution was investigated by scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD), etc. The experimental results indicate that the optimal heating parameters for the niobium-alloyed steel 22Mn3SiNb are a heating temperature of 800 °C and a soaking time of 5 min. Compared to the hot-stamped boron-alloyed steel 22MnB5 under their respective optimal heating parameters, the properties and microstructure characteristics of 22Mn3SiNb are greatly improved, and nearly no decarburized layer is found on the surface of the niobium-alloyed steel 22Mn3SiNb. In addition, the addition of Nb produces the effects of grain refinement and precipitation strengthening due to the introduction of plenty of nano-precipitated particles and dislocations. In the end, it can be predicted that the new warm-stamped niobium-alloyed steel will replace the conventional hot-stamped boron-alloyed steel.
Highlights
The rapid development of the automobile industry aggravates the increasing consumption of energy and serious pollution of the environment, lightening of the automobile is one of the most paramount measures to reduce fuel consumption and exhaust emissions [1,2]
The martensite lath and precipitated particles were examined by the transmission electron microscope (TEM) (Hitachi H-8100, Hitachi, Tokyo, Japan) at 200 kV, samples were obtained by carbon extraction replication on Cu-grids, and the 400 μm foils were mechanically thinned to about 50 μm, and were fabricated by twin-jet electrolytic polishing in a mixed solution (95 mL alcohol + 5 mL perchloric acid) at about −30 ◦ C by the twin-jet electropolisher (DJ2000, Dedong Technology Co., Beijing, China)
The microstructures of the 22Mn3SiNb steel after continuous conversion at the different cooling rates are shown in Figure 4, and all the microstructures are martensite through metallographic observation
Summary
The rapid development of the automobile industry aggravates the increasing consumption of energy and serious pollution of the environment, lightening of the automobile is one of the most paramount measures to reduce fuel consumption and exhaust emissions [1,2]. Lower heating temperature decreases energy usage, improves service life of high-temperature structural components, and reduces the investment cost of production line. (2) The excellent hardenability of the medium manganese steel increases the uniformity of mechanical properties of different parts of the components, so that the warm stamping technology can produce large-sized automobile components. The addition of Nb, Ti, and Nb + Ti to 22MnB5 has achieved good microstructure and performance at a lower cooling rate after hot-stamped, but the austenitizing temperature is still at a high temperature, around 950 ◦ C [18,19,20]. Through comparison with the traditional hot-stamped boron-alloyed steel 22MnB5, the optimum parameters of warm forming process, microstructure evolution, and properties analysis were investigated
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