中碳微合金非调质钢36MnVS4连杆胀断缺陷分析

Analysis on bulging fracture defects for medium carbon microalloy non-quenched and tempered steel 36MnVS4 connecting rod

通过合金成分分析、显微组织定量表征、断口形貌观察及力学性能测定,对中碳微合金非调质钢36MnVS4连杆断口掉渣和断口不齐缺陷进行分析。在VW 50030标准和TCCMI 13.1—2021标准下进行比较,结果表明:4支连杆组织为珠光体+铁素体,均无异常组织,晶粒度均为5.5~6.0级,不符合标准要求;缺陷连杆的抗拉强度、屈服强度、断后伸长率符合标准,但断面收缩率为26.5%~27.4%,明显低于合格连杆的32.8%~33.5%的断面收缩率,未达到要求。断口不齐的主要原因为断口附近的Ca、Si氧化物夹杂物,在断口侧面发现激光切槽的位置较深,不仅改变胀断轨迹,还降低断面收缩率;掉渣的主要原因为断口附近出现微孔型断裂韧窝,改变了断口表面状态。最后提出采用控锻控冷技术精确控制组织性能是解决问题的关键。

Through analysis of alloy composition,quantitative characterization of microstructure,observation of fracture morphology and measurement of mechanical properties,the slag dropping at fracture and irregular fracture defects of medium carbon microalloy nonquenched and tempered steel 36MnVS4 connecting rod were analyzed.Under VW 50030 and TCCMI 13.1—2021 standards,the results show that the structures of four connecting rods are pearlite+ferrite without abnormal structure,and the grain size is all Grade 5.5-6.0,which does not meet the standard requirements.The tensile strength,yield strength and elongation after fracture of the defective connecting rods meet the standards,but the section shrinkage is 26.5%-27.4%,which is significantly lower than that(32.8%-33.5%)of the qualified connecting rods and does not meet the requirements.The main reason of irregular fracture is the Ca and Si oxide inclusions near the fracture.The position of laser cut groove is found to be deep on the side of fracture,which not only changes the bulging fracture trajectory,but also reduces the section shrinkage.The main reason of slag dropping is that microporous fracture dimples appears near the fracture,which changes the surface state of fracture.Finally,it is proposed that the use of thermo mechanical control process to accurately control the microstructure and properties is the key to solve the problem.