Toughness Control Of Al Added Low Density High Strength Steel Plate

In order to cope with energy shortage and environmental pollution,the lightweight of construction equipment has become the development direction in the field of construction equipment manufacturing.Taking dump truck as an example,using high strength steel plate to reduce the thickness of structural parts is the most effective way to achieve lightweight.However,there are some limitations to achieve lightweight simply by this way alone.The stiffness of structural parts decreases which leads to structural deformation and even safety accidents when the thickness of the structure decreases.Therefore,developing hot-rolled plate with the advantages of low density,high strength and high hardness to reduce the density of steel plate effectively by Al added is the reasonable way to reduce the weight of equipment and ensure the stiffness of structural parts at the same time.The stability of ferrite is improved by Al added in the steel,which makes the steel in the ferrite/austenite dual-phase zone of at the rolling temperature.Based on the Al added composition,the ferrite/martensite dual-phase steel with lamination structure was prepared by rolling and heat treatment in dual-phase zone,and the high toughness of the steel was obtained by delaminated.Based on this idea,two kinds of low density steels with Alcontent of 3 wt%and 6 wt%were designed.Three different processes were used to prepare the experimental steels.Finally,a kind of low density steel with high strength,high toughness,low density and a certain extent plastic deformation ability was obtained.On this basis,the effects of process parameters on the properties of Al-containing steel were compared and analyzed,and the influence of delamination fracture on high toughness of steel plate and the fracture behavior under unidirectional tensile load were further analyzed.In this paper,the design of Al added low density and high strength plate,the relationship between preparation process and structure and properties,the mechanism of high toughness and tensile fracture behavior has been researched and discussed.The conclusions are as follows:(1)The alloy phase diagram was calculated by Thermo-calc thermodynamic calculation software.The result shows that the low density steel with 0 wt%,3wt%and 6wt%Al element was in the ferrite and austenite dual-phase region at the temperature range of 1200?A1 point.The austenite eutectoid C content increased with the addition of Al element,which resulted in the increase of lattice parameters and volume expansion when ferrite and cementite transform to austenite.The simulation of the structure after deformation shows that the structure after deformation and quenching in the two-phase zone is greatly affected by quenching temperature.The volume fraction of austenite in 3Al steel decreases with the decrease of quenching temperature.The volume fraction of austenite in 6Al steel reaches its maximum at 950?.(2)Based on the design of 3Al steel and 6Al steel,samples were prepared by on-line quenching by high temperature rolling,on-line quenching by hot rolling and off-line quenching by hot rolling.The structure of sample has strong anisotropy.The structure of 3 Al steel is mainly martensite with strip ?-ferrite,while that of 6Al steel is mainly massive delta ferrite with strip martensite.The mechanical properties of 3Al-1R steel prepared by on-line quenching,high temperature rolling are the best,yield strength is 1114 MPa,tensile strength is 1698 MPa,higher than that of 0A1-900Q,elongation after fracture is 8.2%,impact energy is 391.0 J at-40%.However,brittle fracture occurs in samples of 6Al steel during deformation for poor toughness and plasticity of delta ferrite at room temperature.(3)The impact fracture and crack growth of low density steel were analyzed by using SEM.The results show that the ?-ferrite in low density steel caused by the Al added is brittle fracture when deformed at room temperature.The martensite in 3Al steel restrain the crack propagation,while the high carbon content of martensite in 6Al steel results in brittle fracture when deformed.The ?-ferrite in martensite caused by low quenching temperature,which would be the nucleation site of cracks,resulting in local stress concentration and decrease of toughness.(4)The delamination occurs in the impact sample of 3Al-1R and 3Al-2R prepared by on-line qunching and a ladder type decrease and load platforms appear during the crack propagation stage of load-deflection curve.By observing the structure of the delamination position,it is found that the crack propagates transversely along the strip ?-ferrite,resulting in a larger secondary crack leading to the delamination of the fracture.When the specimen continues to propagate along the depth of the notch,the renucleation of the crack is required.In the load-displacement curve,the loading platform is the crack renucleation process.In the 3Al-1R and 3Al-2R impact samples,the crack growth process is as follows:crack nucleates and prapagates-crack turns into a secondary crack-crack renucleation and propagation until fracture.(5)Based on the analysis of the structure anisotropy and the micro-crack morphology near the fracture surface of low density steel,two types of micro-cracks were formed in the deformation process of 3Al steel due to the discontinuity of strain at the martensite-ferrite interface.The first type of micro-cracks nucleated at the martensite-? ferrite interface,resulting in local stress concentration;the second type of micro-cracks nucleated at the martensite-? ferrite interface.Two kinds of microcracks propagation and connect,the sample eventually breaks.The cleavage crack of the T-direction specimen of 3Al steel propagates along the length direction of strip ferrite,which results in serious stress concentration and brittle fracture of the sample.Both martensite and ferrite in 6Al steel are brittle fracture,which results in low elongation.