| With the rapid development of automobile industry,the pollution of vehicle emission is increasingly serious and the lightweight of automobile spare part has a great significance on energy-saving and emission-reduction.As the key component of heavy truck,which plays an important role in carrying the weight of the car and transmitting the load,the mechanical strength,stiffness,and elongation of axle housing have a great influence on vehicle performance.At present,the axle housing made of ductile iron cannot gradually meet the practical use requirements of heavy truck.Therefore,there is an urgent need for high-strength cast steel with a simple process and low cost to replace the ductile iron,which will significantly improve the use of safety and meet the market requirements.Given the above background,this article aims to design a new low-alloy cast steel with high strength under the condition of low cost and simple heat treatment.By studying on the effect of different alloying elements on microstructure and properties of steel,carbon,silicon,manganese,chromium,and other alloys with little amount(RE,Ti,B)are put into the steel.In this paper,the effect of Mn content on microstructure and properties of the steel is studied systematically and the heat treatment process of the test steel has been optimized.The main results of the paper are drawn as follows:1)with the increase of Mn content,the hardenability of the experimental steel is increased significantly.The microstructure gradually changes from diffusive transformation to non-diffusive transformation.For the steel with Mn content of 2%and 2.5%,the mixed microstructure with bainite,martensite and ferrite can be obtained after the intercritical heat treatment under the condition of air cooling due to the great hardenability.Therefore,the Mn content of 2%and 2.5%is the optimized compositions of experimental steel with the excellent mechanical properties.2)Two groups of experimental steels with optimized composition are respectively normalized and tempered at different temperatures.The optimized heat treatment process for experimental steels with different compositions is slightly different.For the steel with 2%Mn,the sample was heated to 900℃and cooled by air-cooling,then reheated to 815℃and tempered at 380℃,the steel exhibits the excellent combination of strength and ductile(σ_b is 1120 MPa andδis17.6%).However,for the steel with 2.5%Mn,the best comprehensive properties was obtained when it was heated to 900℃and cooled by air-cooling,then reheated to 815℃and tempered at 250℃(σ_b is 1380 MPa andδis 13%).The mechanical properties of the two kinds of experimental steels have all reached and exceeded the objective of design.3)In order to meet the requirements of different positions with different properties in axle housing,a new gradient heat treatment process was designed to prepare the novel structural gradient material.By controlling the cooling rate in different regions of steel,the gradient microstructures from granular bainite+upper bainite to martensite and ferrite+pearlite to martensite were successfully obtained.Both of them achieve the objective of high plasticity at one end and high strength at the other end,which meets the requirements of axle housing.Moreover,it is noticeable that a transition region with a superior combination of tensile strength and ductility(1700 MPa and 11.1%)was successfully produced,which can be attributed to the formation of acicular carbide-free bainite and the refinement of martensite substructures.It may have a more potential application in fabrication of multi-property automobile parts.4)The high-strength cast steel with tensile strength 1100 MPa has been developed and the microstructure is M/B dual phase,the process is simple and cost is low,and successfully industrialized.The heat treatment process was successfully optimized to two-step heat treatment,the properties are:σ_b≥1100 MPa,δ≥15%after normalizing at 900℃and tempering at 380℃. |
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