摘要
先进第三代中锰汽车钢具有强塑积高、成本低廉等优点,应用前景广阔,但其焊缝区的淬硬性大,影响后续加工和使用。采用光纤激光器对6mm厚中锰汽车钢进行焊接,研究了焊接热输入对焊缝成形、组织、冲击韧性和断裂行为的影响。结果表明,当热输入为150~500J/mm时,激光焊接属于深熔焊,焊缝成形较好;焊缝区的硬度随着热输入的增大呈减小趋势。当热输入较小时,焊缝组织为马氏体,随着热输入的增加,焊缝中有少量的贝氏体组织生成;焊缝在-40℃温度下的冲击韧性随着热输入的增加先增加后降低,当热输入为300J/mm时,冲击韧性最高,断口呈韧性断裂;中锰钢焊缝组织中原奥氏体晶界、马氏体板条束界和板条块界均对裂纹的扩展起阻碍作用,裂纹扩展功与平均有效晶粒尺寸呈反比关系,与大角度晶界密度呈正比关系。
Objective Medium manganese automobile steel is the third generation of advanced automobile steels developed to meet the requirements of lightweight,safety,and low cost.Its structure at room temperature is submicron lath ferrite and austenite two-phase structure,which can improve the strong plastic volume through the transformation induced plastic effect and make up for the deficiencies of low plastic properties of the first generation steel and high cost of the second generation steel.Thus,medium manganese steel has a broad prospect in industrial applications.Laser welding is apromising welding method for medium manganese steel,due to its advantages of high efficiency,narrow heat affected zone and small deformation.During the laser welding of medium manganese steel,however,the high cooling rate promotes easily the generation of martensite structure in the weld metal,resulting in reducing the weld toughness of medium thick plate and influencing subsequent processing.At present,the researches on weld microstructure,toughness,and crack propagation behaviors of the medium manganese steel are insufficient.Therefore,this paper studies the influences of different laser welding heat inputs on weld microstructure,hardness,and toughness of a 6mm thick hot rolled medium manganese steel plate,and determines the microstructural unit controlling weld crack propagation,providing a theoretical basis for its industrial applications.Methods First,the 6kW IPG fiber laser is used for laser welding of the medium manganese steel plate,and X-ray is used for the nondestructive test of welds.Second,the cross-sectional morphologies of welded joints are observed by the microscope.The Charpy pendulum oscillographic impact test is carried out on the weld seam at-40℃.The weld microstructure and impact fracture morphology are observed by scanning electron microscope(SEM)or transmission electron microscope(TEM).The weld hardness of each process is measured by the microhardness tester.Finally,the electron back scatter diffraction(EBSD)is used to analyze the grain orientation and crack propagation path of welds.Results and Discussions The 6 mm thick medium manganese steel is laser-welded.When the heat input is150--500J/mm,the laser welding technique used in this paper belongs to deep fusion welding,and the fusion widths of the upper and lower surfaces of the weld increase with the increase of heat input(Fig.3).When the welding heat input is small,the weld microstructure of the medium manganese steel is lath martensite and some twin martensite(Fig.4).With the increase of heat input,the prior austenite grains gradually grow up,bainite is generated in the weld microstructure,and the number of bainite increases slightly with the increase of heat input(Fig.5).The weld hardness of the medium manganese steel is obviously higher than that of the base metal,and it decreases with the increase of heat input(Fig.6).The impact toughness of the medium manganese steel weld increases first and then decreases with the increase of heat input at-40℃(Fig.7).When the laser welding heat input is 300J/mm,the mixed microstructure of the weld has the smallest effective grain size(Table 5)and the highest high-angle grain boundary density(Table 6),which improves the crack initiation and propagation energies(Table 4),and presents the ductile fracture(Fig.9).The prior austenite grain boundary,martensitic lath bundle boundary,and lath block boundary in the weld microstructure of the medium manganese steel all hinder the crack propagation(Fig.11).The crack propagation energy in the weld zone is inversely proportional to the average effective grain size(>15°angle grain boundary)(Table 5),and positively proportional to the high angle grain boundary density.Conclusions When the heat input of the manganese steel in laser welding is 150--500J/mm,the weld forming is better.When the heat input is small,the weld microstructure is martensite,and with the increase of heat input,a small amount of bainite is generated in the weld.The prior-austenite grain boundary,martensitic packet boundary,and block boundary in the weld microstructure of the medium manganese steel can hinder crack propagation.The crack propagation energy in the weld zone is inversely proportional to the average effective grain size(>15°high-angle grain boundary)and positively proportional to the high-angle grain boundary density.The impact toughness of the weld at-40℃increases first and then decreases with the increase of heat input.When the welding heat input is 150J/mm,the dislocation group and the twin martensite structure reduce the crack initiation energy and make it easy to crack,and the fracture is a cleavage fracture.When the heat input is 300J/mm,the effective grain size decreases,the high-angle grain boundary density is the highest,the crack initiation and propagation energies increase,and the fracture is a ductile fracture.When the heat input is greater than 300J/mm,the effective grain size increases,the high-angle grain boundary density decreases,and the crack propagation energy decreases.When the heat input is 500J/mm,the highangle grain boundary density is the lowest,the crack propagation energy decreases sharply,and the fracture is a cleavage fracture.
作者
王艳杰
赵琳
彭云
曹洋
Wang Yanjie;Zhao Lin;Peng Yun;Cao Yang(Institute of Welding,Central Iron and Steel Research Institute,Beijing 100081,China;College of Materials Engineering,North China Institute of Aerospace Engineering,Langfang,Hebei 065000,China)
出处
《中国激光》
EI
CAS
CSCD
北大核心
2022年第8期221-231,共11页
Chinese Journal of Lasers
基金
国家国际科技合作专项资助(2015DFA51460)
北华航天工业学院重点项目(SZZX-2020-07)。
关键词
激光技术
中锰钢
显微组织
硬度
韧性
大角度晶界
laser technique
medium manganese steel
microstructure
hardness
toughness
high-angle grain boundary
