Effect Of Pulse Current On The Microstructure And Mechanical Properties Of 45 Steel

Steels have become the most important basic production in modern society and the material basis for the development of country,due to its plasticity,toughness,strength and economy.So far,no material has been available to replace the existing status of steel in the future.More than 90%of steel is used as structural material.Strength and toughness are the most important requirements for steel.The improvement of steel toughness has always been the main direction of this field.From the strengthening mechanism of steel,the refinement of microstructure and the reasonable regulation between hard and soft phases,are important ways to develop a new generation of steel.Pulse current provides the transient high energy non-equilibrium condition,and rapid heating rate?10⁴-10⁵K/s?is beneficial to the refinement or even ultra-fineness of miscrto-structure.By optimizing the technological process,the pulse current processing method can realize the microstructure and macrostructure between the soft and the hard phases of materials.Most of China's steel production is ferrite or pearlite microstructure.If the pulse current can enhance the toughness of 45 steel,it will be a new energy-efficient process for the development of a new generation steels.This will greatly reduce the energy consumption and simplifying technological process,and has the extremelyl significance.In this paper,the pulse current processing method is used to explore the feasibility of its method for strengthening and toughening of 45 steel.By optimizing the process of pulse current,the strengthening effect of the pulse current on 45 steel is further explored.Compared with the traditional heat treatment,the refinement effect of the microstructures such as martensite,ferrite and prior austenite and the corresponding mechanical properties are studied.The mechanism of the pulse current on the toughess enhancement of 45 steel is analyzed by combining theory.The main results obtained in the thesis as following:Compared with the conventional quenching,the plastomer or pearlite microstructure of45 steel by the austenite quenching treatment,the prior austenite grains is refined from 37?m to 15?m.The martensite was refined,the width of the slab was refined to 150 nm,and the tensile strength was significantly increased from 637 MPa in the initial state to 1987 MPa.It is due to the high strength of the nano-scale carbon supersaturated solid solution martensite.Through the cyclic?N=3?austenite quenching treatment of 45 steel,the average austenite grain size reaches 3?m,and the grain appears ultra-fine phenomenon occurs.After rapid cooling,a martensite structure of less than 100 nm is obtained.The ultra high strength45 steel was obtained,the tensile strength reached 2600 MPa,the breaking elongation rate was 9.8%,and the strong plastic product reached 25 GPa%.It is due to the pulse current can effectively reduce the nucleation barrier,and the rapid heating increases the degree of supercooling,resulting in the increase of nucleation rate.Therefore,the relatively large austenite grains have many places nucleation at the same time,and some small austenites are formed,which are rapidly cooled and then transformed into martensite,so that a relatively large crystal is decomposed into some smaller cystals.This process is repeated again,and the smaller crystalss obtained in the previous time are re-refined,and the ultra-fine crystals are obtained by repeating.This refinement will not continue forever.The grain boundary area becomes larger as the crystal grains become smaller.The interface energy or the free energy of system will be higher.When the energy supplied is not enough to increase the free energy of system,the grain refinement will stop.Moreover,grain growth is a spontaneous process,and when the heating temperature is too high or the holding time is too long,the crystal grains become larger.The introduction of pressure causes 45 steel to undergo high temperature deformation when the temperature of pulse current reaches near AC₃.The deformation amount is 37.5%,the average austenite grain size is 5?m,the martensite lath width is only 7 nm,and there is a residual austenite film between the laths,which achieves a good combination of high strength and high plasticity.The tensile strength reached 1893 MPa,the breaking elongation rate was 22%,and the strong plastic product reached 42 GPa%.It is due to the pulse current acts to make the grains ultra-fine,and the austenite undergoes dynamic recrystallization under strain,and the TRIP effect occurs,so that the plasticity is improved.Ultrafine martensite and ferrite structure play a decisive role in strength,and retained austenite is responsible for plasticity and toughness.The soft and hard phase structure of the mother-of-pearl is used as a biological prototype.The bionic coupling design of 45 steel is carried out,and the bionic coupling unit is prepared on the surface of 45 steel by pulse current treatment.The tensile strength and elongation of the pulsed current bionic coupling treatment samples were 1193 MPa and18.1%,respectively,which were 51.8%and 13.2%higher than the conventional quenched and tempered sample?786 MPa and 16%?.This is due to the synergy of the dam effect,the local bearer effect and the relay effect.In conclusion,this study proves that the pulse current method can realize the refinement of 45 steel microstructure and the reasonable regulation of hard and soft phases through different process.It is found that the toughness of 45 steel is improved,and the good comprehensive performance is obtained.Pulse current combined with high temperature deformation can be a fast and efficient method for preparing ultra high strength steels that meet the requirements of a new generation steels.