Design Of Non-quenched And Tempered Cast Steel With Low Alloy, High Strength And Toughness And Control Of Its Microstructure And Properties

Along with the automobile industry rapid development as well as the urgent demand forreducing weight of the automobile spare part, the development of high strength steel withlow price and simple craft is the inevitable trend and the basic way to reduce the oilconsumption and emissions, enhance the safety. At present, the axle housing made of ductileiron breaks from time to time since the heavy commercial vehicle overloads, which causesgreat economic loss. Compared with the ductile cast iron, the low-alloy cast steel exhibitshigher strength and toughness. If the axle housing made of ductile iron were replaced by theone made of low-alloy steel, its stability would increase remarkably, which reduces the breakof axle housing greatly and has significant economic and social benefit. Given the abovebackground, this paper is trying to design a low-alloy non-quenching and tempering caststeel with high strength under the condition of low cost and simple heat treatment.This paper has considered the functions of various alloying elements and established thealloy steel design route by adding Si, Mn, Cr as well as some other alloy elements with fewamount. Three kinds of steel are designed through controlling cost of raw material strictly aswell as considering the influence of alloy content on the organization performance. And thebrands of the steels are ZG15Si2Mn3CrB, ZG25Si2Mn2CrB, ZG35Si2Mn2CrB.ZG15Si2Mn3CrB, ZG25Si2Mn2CrB, ZG35Si2Mn2CrB are tempered at300℃、340℃、360℃、380℃respectively after pretreatment normalizing at880℃and normalizing at900℃. And then the effect of temper temperature on microstructure and mechanicalproperties of steel are studied. The research results show that ZG15Si2Mn3CrB exhibitsmechanical properties that σb≥1300MPa,≥12%with microstructure of the bainite andmartensite, retained austenite after tempering at300℃; ZG25Si2Mn2CrB exhibitsmechanical properties σb≥1400MPa,≥17%with microstructure of the bainite and lathmartensite, retained austenite after tempering at360℃; ZG35Si2Mn2CrB exhibitsmechanical properties σb≥1400MPa,≥12%with microstructure of the needle-like bainite，grain bainite, lath martensite, retained austenite after tempering at380℃; tempering temperature at which steel has the highest tensile strength raises with the increase of carboncontent of steel; the steel exhibits better mechanical performance since the stability ofretained austenite increases after tempering at appropriate temperature.ZG15Si2Mn3CrB, ZG25Si2Mn2CrB, ZG35Si2Mn2CrB are conducted subcriticalnormalizing experiments and the test results show that ZG25Si2Mn2CrB exhibitsmechanical properties thatσb≥1200MPa,≥18%with the microstructure consisted ofcarbide free bainite, granular bainite, martensite, ferrite. When ZG25Si2Mn2CrB hold in twoin the two-phase region, austenite was divided by undissolved ferrite which would limit thecoarsening of austenite. After wind cooling, finer prior austenite grains transform intomartensite, bainite and parts of austenite and part of austenite were preserved, which wascalled retained austenite. And strength and toughness of steel were improved significantly bythe finer martensite, bainite and retained austenite.In this paper ZG25Si2Mn2CrB was quenched in NJ quenching medium with theproperties that cooling fast in high temperature area and cooling show in mediumtemperature region and mechanical properties, microstructure of ZG25Si2Mn2CrB wastested after tempering at a low temperature. The test results show that ZG25Si2Mn2CrBexhibits mechanical properties that σb≥1700MPa,≥14%with microstructure containinglarge amount of lower bainite. Large amount of lower bainite in microstructure demonstratesthat NJ medium has the properties that cooling fast in high temperature area and coolingshow in medium temperature region.