Experimental Research On Rare Earth And Magnesium Composite Treatment Of 49MnVS3 Non-quenched And Tempered Steel

49MnVS3 is a typical type of non-quenched and tempered steel with ferrite-pearlite structure.It has the advantages of short production cycle,energy saving,low cost,excellent cutting performance,cross-section hardness and uniform strength.It is widely used in automobile crankshaft,connecting rod.At present,the production of 49MnVS3 in China still faces many problems:the problem of sulfide accurately control;the problem of the mismatch of strength and toughness;the weak research on the process of controlled forging and cooling.In this study,a new method of combing rare earth cerium and magnesium to treat non-quenehed and tempered 49MnVS3 steel was proposed.In this experiment,a 30 kg vacuum induction furnace was chosen.Pure cerium and nickel-magnesium alloy with Mg content of 20.31%were selected.Four groups of rare earths and nickel-magnesium alloys with different proportions were designed to explore the effect mechanism of Mg-Ce composite treatment on inclusions,structure and mechanical properties of 49MnVS3 steel.The contents of Ce in each furnace were:0%,0.0100%,0.0110%,0.0122%,and 0.0149%,respectively.In addition the contents of Mg were:0%,0.0015%,0.0015%,0.0012%and 0.0020%,respectively.The range of Ce and Mg yields fluctuated widely.The yield decreased with the increase of adding amount,which was more obvious for Ce.The yield of Ce and Mg were located at the range of 12.69%～22.88%and 3.41%～6.14%respectively.Furthermore,the average yield of Ce and Mg were 17.97%and 5.06%respectively.The content of T.O and S in steel showed a general downward trend after steel were treated by Ce and Mg multiple.In details,the minimum T.O and S in steel were 0.0008%and 0.05%respectively.Compared with the 1~#blank group,the total oxygen content in the steel reduced by about 55%,and the sulfur content reduced by about 23%.It was obvious that the sulfide inclusions in 1~#steel sample precipitated along the grain boundary and distributed in a chain shape under the observation by metallographic microscope.On the contrary,the segregation of sulfide inclusions of 2~#～5~#steel samples was obviously weakened,and the shape of sulfide inclusions transformed from chain like to spheroidal.Scanning electron microscope observation revealed that the main inclusions in 1~#steel sample were Al2O3 and MnS.The main inclusions in 2~#～4~#steel samples were Ce-Mn-Mg-O-S composite inclusions with spherical shape.A typical elements distribution of composite inclusion could be described:an inclusion with Mn-Mg-O-S in the core,and Ce-O-S was wrapped around the edge of it.The best treatment result was found in 4~#steel samples.The inclusions had the minimum average diameter and count.In addition,most of morphology of inclusions was spherical.The effect of inclusion modification became worse when the content of Ce and Mg was too much.In details,the MnS inclusions presented a distributed distribution but not concentrated in the core.In addition,the content of Mn in composite inclusions decreased significantly.Finally,the size of inclusions increased sharply.The average diameter of inclusions experienced a fluctuation.It decreased from 2.46 μm in 1~#steel sample to the bottom 1.71 μm in 4~#steel sample before rebound to 1.97μm in the 5~#sample.And the reason for the increase in size in 5~#steel sample was that the content of denaturant increases,which would increase the probability of the inclusions growing up increases.The number of inclusions per unit area showed a trend of first decreasing and then increasing.The number of inclusions per unit area of 4~#was the smallest,311/mm2.The formation of oxygen-sulfur composite inclusions containing Mg and Ce were low in density and easy to float,which could explain the decrease of the number of inclusions per unit area.The forged samples after 4%nitric acid alcohol corrosion were observed for structure.It was found that the microstructure of experimental steel was pearlite+mesh ferrite+intragranular ferrite.The ferrite in 1~#is generally distributed in the form of a network along the grain boundary,there was very little intragranular ferrite,the ferrite content was 11.11%.After combined treatment with Mg and Ce,the ferrite content increased first and then decreased.4~#ferrite had the highest content,19.46%.The reason for the decrease in the proportion of 5~#ferrite might be related to the high mismatch between the generated sulfide inclusions and ferrite,which was not conducive to ferrite nucleation.The pearlite lamellar spacing in 1~#～5~#steel samples was gradually refined.The lamellar spacing of 5~#was 0.26μm,which was 53%thinner than that of 1~#.The refinement of the spacing between the pearlite sheets might be related to Ce increasing the supercooling degree of the molten steel.At the same time,degenerated pearlite was found in 2~#,3~#)and 5~#.This might be due to the addition of Mg,which lowered the phase transition temperature of γ→α and changed the decomposition products of supercooled austenite.According to the experimental results of tensile and impact at room temperature,it was found that the tensile strength and yield strength increased.The tensile strength and yield strength of 5~#were 907.15 MPa and 573.62 MPa,respectively.Compared with 1~#steel sample,the tensile strength and yield strength were increased by 5.10%and 7.72%,respectively.The composite treatment of Mg and Ce had a greater influence on the yield strength.The content of 5~#pearlite was the highest in the experimental steel treated with Mg and Ce composite,the spacing of the pearlite lamella was the finest,the solid solution strengthening effect of Ce was the most obvious.So the 5~#strength was the highest due to a variety of reasons.The impact energy of 1~#～5~#experimental steel showed a trend of increasing first and then decreasing.4~#had the best impact toughness of 32.89 J.Compared with 1~#steel sample,the impact energy increased by 33%.The excellent impact performance was mainly related to the promotion of ferrite nucleation by rare earth and magnesium oxysulfide.In this study,in order to obtain the best inclusion treatment effect and the best comprehensive mechanical properties,the optimal content of Mg was 0.0012%and the optimal content of Ce was 0.0122%.