| The development of automobile not only brings great convenience to people's life,but also brings serious problems of energy consumption and air pollution.In order to reduce the fuel consumption and reduce the pollution to the environment,the lightweight of automobile is an important direction of the development of automobile industry in recent years and in the future.In the light-weighting process of automobile,besides the structure design,the light-weighting of material plays an extremely important role.It is worth noting that the lightweight of automobiles is based on the premise of ensuring safety.Therefore,it is of great significance to improve the strength of steel for automobiles,and at the same time strive to ensure the strong-plastic product of the materials that characterize the energy-absorbing properties of automobiles.Considering the cost and processing performance,a large number of hot-formed parts are used in the mainstream high-grade domestic automobiles.The typical material is 22 Mn B5,the tensile strength of the automobile parts can reach 1500 MPa,but the plasticity is low,only 5?7%,and the corresponding strength-plastic product is only 7?10GPa%.Based on the previous research work of the project group,the relationship between the physical properties,microstructure and properties of a new low-alloy hot-forming steel based on the design of Cr-Mn alloy is systematically studied in this paper,it provides a basis for the development and industrial popularization of this new type of automobile hot-forming steel.The basic physical parameters of the designed Cr-Mn hot forming steels20Mn2Cr?27Mn2Cr and 20Mn2 Cr Nb are analyzed.The results show that Cr can effectively improve the hardenability of the steels.On the basis of C(0.21?0.27wt%)and Mn(1.66?2.03wt%),adding 1.2?1.5wt%Cr can remarkably improve the hardenability of steel.The critical cooling rate of 20Mn2 Cr and 20Mn2 Cr Nb experimental steels is 10?/s,the critical cooling rate of 27Mn2 Cr is 5?/s,which is much less than 27?/s required by 22 Mn B5.The decrease of critical cooling rate is helpful to the manufacture of hot forming die and the formation of martensite by quenching during hot forming.According to the design of the experimental steel,it is used for hot forming simulation.In this paper,different austenitizing temperature,holding time and cooling rate were used to study the effect of hot forming process parameters on the microstructure and properties of hot forming samples.The austenitizing temperature is 890?,910?,930? and 950?,and the holding time is 5min,8min,10 min,12min and 15 min respectively.After heating and heat preservation,it is cooled to room temperature by water cooling and oil cooling respectively.The results show that when the heating temperature is above 910?,the mechanical properties of the steel decrease due to the growth of austenite grains.However,due to the formation of Nb C in the alloy Nb during heat treatment,the growth of austenite grains can be hindered by Nb C particles,so the mechanical properties do not decrease significantly even if the heating temperature is raised to 950?.With the increase of heating time,the austenite grain did not grow obviously.It is noteworthy that the 27Mn2 Cr automobile hot forming steel studied in this paper shows excellent strength and plasticity after water quenching at 890? and holding time of 10 min.At the strength level of up to 2GPa,the plastic elongation still reaches 10%,and the strength-plastic product achieves 20 GPa%.On the one hand,the preparation of automobile parts with 2GPa is realized,and on the other hand,the strength-plastic product is 200% more than the traditional 22 Mn B5.The realization of the mechanical properties provides an important reference value for the research and development of 2GPa ultra-high strength automobile hot forming steel with good strength-plastic product. |
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