冷拔与时效对10.9级紧固件非调质钢的影响

Influence of cold drawing and aging treatment on microalloyed steels for grade 10.9 fasteners

非调质钢具有性能优良、节能、制造成本低、生产周期短,并有利于环境保护等突出优点,因此非调质钢被誉为“绿色钢材”。铁素体-珠光体型非调质钢是使用量最大、应用范围最广的非调质钢,但随着紧固件用钢对强塑性和安全系数要求的日益提高,目前缺乏对10.9级紧固件用非调质钢强塑性的系统研究。采用SEM、EBSD、XRD、TEM、硬度和拉伸测试等手段,研究了变形量和时效处理对一种低碳Nb-V系铁素体-珠光体型冷作强化非调质钢(MFT9)组织结构和力学性能的影响。结果表明,相较于热轧态,随着减面率增大,经拉拔后的钢屈服强度升高了245~400 MPa,抗拉强度升高了125~280 MPa,硬度升高了24HV~67HV,屈强比先增大后下降,由0.71增加到0.94后下降到0.91,塑性有所降低。经50%减面率冷变形后,位错密度增加,由5.104×10^(14)cm^(-2)升高到1.140×10^(16)cm^(-2),而经时效处理后位错密度相较于冷拔态无明显变化。热轧态下晶粒取向主要为〈101〉和〈111〉,冷变形后晶粒取向主要转变为〈001〉和〈111〉,晶粒择优取向。减面率为50%的试验钢经时效处理后,随着时效温度升高,强度和硬度先增大后减小,屈强比变化不大,塑性得到改善。时效处理能够使碳氮化物和铁素体基体中的碳氮原子发生扩散至位错处,形成柯氏气团,起到钉扎位错的作用,因此能够提高材料的强度和硬度。试验钢满足10.9级紧固件要求的工艺为减面率50%、时效温度300℃保温时间2 h。

Microalloyed steel has outstanding advantages,such as excellent performance,energy saving,low manufacturing cost,short production cycle,and is conducive to environmental protection,consequently the non-quenched and tempered steel is known as"green steel".Ferritic-pearlitic non-quenched and tempered steel is the highest consumption and the most widely used non-quenched and tempered steel.However,there is a lack of systematic research on the strength and plasticity of non-quenched and tempered steel for grade 10.9 fasteners,with the increasing demand for the strength,plasticity and safety factor of the steel for fasteners.The effects of cold drawing deformation and aging treatment on the mechanical properties of a low-carbon Nb-V ferrite-pearlitic non-quenched and tempered steel(MFT9)was investigated by means of SEM,EBSD,TEM,hardness and tensile test.The results show that,the yield strength after drawing is increased by 245-400 MPa,the tensile strength is increased by 125-280 MPa,hardness is increased by 24HV-67HV,compared with the hot rolled state,with the increase of the area reduction ratio.Furthermore,the yield ratio increases first and then decreases,it increases from 0.71 to 0.94 and then decreases to 0.91.Dislocation density increases from 5.104×10^(14) cm^(-2) to 1.140×10^(16) cm^(-2) after cold deformation with drawing area reduction ratio of 50%,but the dislocation density does not show significant change after aging treatment compared to as cold drawing.The grain orientation is mainly〈101〉and〈111〉in the hot rolled state,nevertheless,the grain orientation is mainly transformed to〈001〉and〈111〉while the grains are optimally oriented after cold deformation.The tested steel with drawing area reduction ratio of 50%after aging treatment,the strength and hardness increases and then decreases with the increase of aging temperature,the plasticity has been improved but the yield ratio do not change clearly.Carbon and nitrogen atoms in the carbonitrides and ferrite matrix diffuse into dislocations through aging treatment,which form Cottrell atmosphere and play a role in pegging dislocation,therefore enhance the strength and hardness of materials.The optimal process of tested steel for grade 10.9 fasteners is area reduction ratio of 50%,aging at 300℃and holding time for 2 h.