| With the rapid development of the automobile industry,demand for high quality automotive spring steel is also needed.In the future,automotive spring steel will develop towards high strength and functionality(corrosion resistance,heat resistance,wear resistance).In recent years,although China spring steel industry has made great progress,some high premium spring steels are still required to be imported.Spring steel with extra-high strength and high fatigue resistance is the research and development focus.This thesis deals with a new type of extra-high strength plate spring steel.On the basis of the previous studies,the emphasis is placed on the relationship between the heat treatment process,micrsotructure and mechanical properties for the industrially trial produced product.This study will provide a theoretical basis for plate spring steel with good comprehensive mechanical property and fatigue behavior.The main contents and results are summarized as follows.(1)An industrially trial produced plate spring steel was used as the experimental material,the continuous cooling transformation behavior was studied.The effect of cooling rates on the phase transformation behavior and the CCT curves of the experimental steel were explored by means of thermal expansion method,hardness measurement and metallographic analysis.The results show that the critical austenitic transformation temperatures Ac1 and Ac3 of the experimental steel are 675? and 775?,respectively.When the cooling rate is lower than 0.5?/s,the microstructure consists of ferrite and pearlite at room temperature;When the cooling rate is 1?/s,the microstructure consists of bainite and martensite at room temperature;When the cooling rate ranges 2?40?/s,the microstructure is the full martensite at room temperature.The HRC hardness of the experimental steel is increased with the increase of cooling rate.(2)The decarburization behavior of the experimental steel was studied.At the temperature range of 700??900?,the depth of decarburization layer increases with increasing heating temperature.The depth of decarburization decreases in the temperature range of 900??1050?.The complete decarburization layer occurs in the temperature range of 750??850?,i.e.the sensitive decarbururization temperature region.The depth of decarburization layer increases with extending of holding time.(3)The heat treatment process of the experimental steel was studied.The microstructure and mechanical properties of the experimental steel were analyzed and compared for two different rolling temperatures and different heat treatment processes,then the appropriate heat treatment process was determined.Finally,quenching process is determined to be 940?×30 min,tempering process is 430?×60 min.In this case the optimal mechanical properties were obtained.Under these conditions,when the start rolling temperature is 1050? and the finish rolling temperature is 820?,the tensile strength of the experimental steel is 1818 MPa,the yield strength is 1617 MPa,the elongation to failure is 9.5%and the area reduction ratio is 31.2%.When the start rolling temperature is 950? and the finish rolling temperature is 820?,the tensile strength of the experimental steel is 1898 MPa,the yield strength is 1633 MPa,the elongation to failure is 8.1%and the area reduction ratio is 34.3%.(4)The bench fatigue test was conducted for the first batch of industrially trial produced leaf spring steel.The experimental results were analyzed and key points of production processes were determined for the second batch of leaf spring steel.The experimental results showed that fatigue lives of some plate spring steel met the design requirement.Further analysis showed that the Al and Ca oxide inclusions caused the reduced fatigue life.The second batch of leaf spring steel was trial produced after strictly controlling the amount of deoxidizer Al in the smelting process.In this case,the experimental steels met the technical requirements and the production process of new automobile plate spring steel was stabilized. |
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