【作者】 王琪；

【导师】 姜周华；

【作者基本信息】 东北大学 ， 钢铁冶金， 2020， 硕士

【摘要】 49MnVS3是一种典型的铁素体-珠光体组织的非调质钢,其具有生产周期短、节省能源、成本低廉、切削性能优异、截面硬度均匀等优点,广泛用于制作汽车的曲轴、连杆等。目前,国产49MnVS3仍面临诸多问题:硫化物控制效果差、强度与韧性不匹配性、控锻控冷工艺研究薄弱等问题。本研究创新性地结合稀土铈与金属镁,对非调质钢用钢49MnVS3进行复合处理。本实验利用30 kg真空感应炉,选用纯铈、含Mg质量分数为20.31%的镍镁合金,设计了四组不同配比质量分数的稀土铈、镍镁合金,探究稀土 Ce、金属Mg复合处理对钢中夹杂物、组织、力学性能的影响规律及作用机理。各炉次中 Ce、Mg 质量分数分别为:0%、0.0100%、0.0110%、0.0122%、0.0149%;0%、0.0015%、0.0015%、0.0012%、0.0020%。实验发现 Ce、Mg 收得率波动范围相对较大,加入量越多,收得率会越低,其中Ce表现更为明显。Ce的收得率的范围为12.69%～22.88%,平均收得率为17.97%。Mg的收得率的范围为3.41%～6.14%,平均收得率为5.06%。经Ce、Mg复合处理后,钢中的T.O、S质量分数总体呈下降趋势。钢中的T.O质量分数最低为0.0008%,S质量分数最低为0.05%。较1~#空白对照组相比,钢中的T.O质量分数降低了约55%,S质量分数降低了约23%。通过金相显微镜观察发现,1~#钢中硫化物在晶界析出明显,呈链条状分布。2~#～5~#钢中硫化物在晶界偏聚现象明显减弱,且链条状夹杂减少,球化夹杂显著增多。利用扫描电镜观察发现,1~#钢中的主要夹杂物为Al2O3和MnS两大类。2~#～4~#钢中主要夹杂为Ce-Mn-Mg-O-S复合夹杂,复合夹杂均为球形。Mn-Mg-O-S在夹杂心部位置占比较高,Ce-O-S在夹杂边部位置占比较高。4~#夹杂变性效果最佳,夹杂尺寸小,球化效果明显。5~#钢中很难发现含氧复合夹杂,MnS夹杂也不再单一分布在心部,而呈现分散分布,且复合夹杂中Mn占比明显下降,夹杂物尺寸变大。夹杂物的平均直径在1.71 μm～2.46 μm之间,1~#钢中夹杂物平均直径最大,4~#钢中夹杂的平均直径最小,经Ce、Mg复合处理后,夹杂物的平均直径降低了约44%。2~#和3~#钢中夹杂物平均尺寸相近,分别为2.41 μm和2.43 μm。5~#钢中夹杂物的平均直径增加,夹杂尺寸增加与变性剂质量分数增加,促进夹杂物聚合长大有关。单位面积夹杂物数量呈先减少后增加的趋势,4~#单位面积夹杂物数量最少,为311个/mm2。夹杂物数量的减少与生成含Ce、Mg的氧硫复合夹杂密度低,易于上浮有关。经4%硝酸酒精腐蚀后的锻态试样进行组织观察,发现实验钢的组织为珠光体+网状铁素体+晶内铁素体。1~#钢中的铁素体大体以网状形式在晶界分布,存在极少的晶内铁素体,铁素体占比为11.11%。经Ce、Mg复合处理后,铁素体占比呈先增加后降低的趋势。4~#钢中铁素体占比最高,为19.46%。5~#钢中铁素体占比减少的原因可能与生成的硫化物夹杂与铁素体的错配度高有关,不利于铁素体形核。1~#～5~#钢的珠光体片层间距呈逐渐细化趋势,5~#钢的片层间距为0.26 μm,较1~#钢相比,细化率达53%。珠光体片层间距的细化主要与Ce增加了钢液的过冷度有关。同时发现2~#、3~#、5~#钢局部出现了退化珠光体,这主要由于Mg的加入,使得γ→α的相变温度降低,改变了过冷奥氏体的分解产物。根据常温拉伸和冲击实验结果,发现抗拉强度、屈服强度呈增加的趋势,5~#钢的抗拉强度、屈服强度分别为:907.15 MPa、573.62 MPa。较1~#钢相比,抗拉强度、屈服强度分别提高了 5.10%、7.72%,Ce、Mg复合处理对屈服强度影响较大。5~#钢珠光体占比在经Ce、Mg复合处理的实验钢中最高,珠光体片层间距最细,Ce的固溶强化作用最明显,受多种原因共同作用,5~#钢强度最高。1~#～5~#实验钢的冲击功呈先增加后降低的趋势。4~#钢冲击韧性最佳,为32.89 J,较1~#相比,冲击功增加了 33%。冲击性能的改善主要与含稀土、镁的氧硫化物促进铁素体形核有关。在本研究中,为获得最佳的夹杂物变性效果及最佳的综合力学性能,Ce的最佳质量分数为0.0122%,Mg的最佳质量分数为0.0012%。更多还原

【Abstract】 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%.更多还原