Processing,Microstructures And Mechanical Properties Of A New High Strength Automobile Plate Spring Steel

The automobile plate spring is the most traditional elastic element in the automobile suspension system,since it has the advantages of economical and concise production,reliability,and ease in structural optimization and so on.So it has been widely used in the transportation tools.With the rapid development of the automobile industry,the demand for high strength automobile plate spring steel is increasing year by year.In this thesis a new kind of extra-high strength automobile plate spring steel was used as experimental material and the aim is to provide guidance for industrial trial production on the basis of investigation on its processing,microstructures and mechanical properties.The main contents and results obtained in this thesis are summarized as follows.(1)The continuous cooling transformation(CCT)behavior and hardenability of the experimental steel were studied.The influences of different cooling rates on the decomposition and the products of undercooled austenite were analyzed by thermal expansion method.The CCT diagram and microstructural evolution of the experimental steel were obtained by microstructure observation and hardness measurement.The hardenability curves of the experimental steel at different quenching temperatures were determined by the end quenching test method.The hardenability of the experimental steel was improved in the range of 880℃-940℃ while the heating temperature was increased.With the increase of the distance from the sample end,the microstructure was changed from martensite into martensite plus pearlite and ferrite,and the hardness was gradually decreased.The depth of hardened layer meets the requirement for the industrial production of plate spring with a certain thickness.(2)The reheating schedule having much influence on austenite grain size growth tendency and decarburization performance was studied for the experimental steel.With the increase of the reheating temperature,the austenite grain of the experimental steel increases.In the range of 860℃～1100℃,the grain growth includes slow growth stage and rapid growth stage,while the austenite grain coarsening temperature is about 980℃.The decarburization depth increases with the increase of reheating temperature and holding time,and the complete decarburization layer occurs at 700℃～850℃.The maximum depth of the total decarburization layer is achieved at～1050℃.This temperature is defined as the sensitive decarburization temperature.The decarburization depth at 900℃ shows an exponential law with holding time.(3)Two-stage hot rolling schedule was used to the experimental steel plate in the laboratory hot rolling mill which included controlled rolling both in austenite recrystallization zone(starting rolling temperature:1050℃)and austenite non-recrystallization zone(starting rolling temperature:800℃,air cooling after rolling).The microstructure after hot rolling is ferrite and fine pearlite.(4)The microstructure and mechanical properties of the experimental steel were observed and measured after heat treatment quenched and tempered at different temperatures.The best combination of strength and plasticity was achieved for this steel by appropriate processing and heat treatment.The results showed that the optimal heat treatment schedule for the experimental steel is 940℃×30min(quenching)and 410℃×60min(tempering).In such a case,the tensile and yield strengths are 1825MPa and 1565MPa,respectively,while the elongation is 10.3%and the reduction ratio of shrinkage is 31.4%.(5)Fatigue behavior is an important factor affecting the service life of automobile plate spring.In the frame of simulating the working state of the plate spring,the three-point bending fatigue test was carried out in a high-frequency fatigue testing machine.The fatigue limit of the experimental steel was determined to be 1312MPa,and the S-N curve was depicted.The fracture surface was observed under scanning electron microscope.It was found that the fracture of the specimen originated from the specimen surface with characteristic of cleavage fracture.