Effect of cementite characteristics on high-cycle fatigue resistance of cold-rolled tempered martensitic steel

Abstract

Recently, there have been significant efforts to develop high-strength steels in the automotive industry for satisfying safety issues. Tempered martensitic steel, which offers superior mechanical properties and efficiency for mass production, is widely used in automotive industry. Tempered martensitic steels for spring part in automotive industry are made of medium-carbon steels. Medium-carbon tempered martensitic steels have reliability issues such as lower fracture toughness, inferior fatigue properties and poor weldability due to carbide precipitation. Furthermore, the process of steels require a reheating process for convenience of process control. It need too much energy consumption. Recently, efforts are made to produce the springs with low-carbon martensitic steels with cold-rolling process. This steels has similar tensile strength with medium-carbon tempered martensitic steels. Furthermore, this new process could reduce half of process cost compared to the conventional process by skipping reheating process. The cold-rolled tempered martensitic steels have different lath structure and cementite characteristics compared to conventional tempered martensitic steels due to cold-rolling. Microstructure can affect tensile properties and high-cycle fatigue resistance which is necessary for industrial application. However, many studies only have focused recrystallization of martensite by cold-rolling and tempering. There is not study about microstructure and high-cycle fatigue resistance of cold-rolled tempered martensite. Therefore, this study investigated the effect of cold-rolling and tempering on microstructure and high-cycle fatigue of cold-rolled tempered martensitic steel. Firstly, the effect of cold-rolling reduction and tempering temperature on microstructure of cold-rolled tempered martensitic steel was investigated. Cold rolling increases alignment of the lath and decreases thickness of the laths. The amount of cementite precipitation is not affected by the cold-rolling amount, but is mainly influenced by the tempering temperature. High amount of cold-rolling reduction resulted in cementite precipitation at lath boundaries. Low amount of cold-rolling reduction provided random cementite precipitation. During cold-rolling, lath boundaries have high amount of dislocations, which are potential nucleation sites for cementite precipitation. Depending on tempering temperature, cementite morphology significantly changed. Tempering at 200 ℃ caused to sharp lath-type cementite. Tempering at 415 ℃ resulted in spherical and lath-type cementite. Increasing the amount of cold-rolling led to higher yield strength (YS) and ultimate tensile strength (UTS), but sacrificed uniform and total elongations. Cold-rolled specimens showed a higher increase in yield and tensile strength compared to as-received specimen when tempered at 200 ℃. The higher dislocation density in cold-rolled specimens actively induced the cottrell atmosphere, leading to higher strength enhancement. Tempering at 415 ℃ increased the ductility of all specimens, but the degree of increase varied. Specimens with higher cold-rolling reduction exhibited lower increases in ductility due to the specimen thickness effect. Secondly, the effect of cold-rolling reduction and tempering temperature on high-cycle fatigue resistance of cold-rolled tempered martensitic steel was investigated. Most specimens had the similar fatigue limit/tensile strength ratio values ranged from 0.5 to 0.6. The specimens exhibited single crack initiation sites with radial-type cracks. Specimen 70% cold-rolled and 380 ℃ tempered showed the ratio value of 0.4 and multiple spike-type crack initiation sites. High amount of cold-rolling reduction with 380 ℃ tempering resulted in thin lath martensite structure with high amount of fine lath-type cementite at lath boundaries. The fine lath-type cementite concentrated at the lath boundaries caused stress concentration and weakened the lath boundaries during the high-cycle fatigue test, leading to crack initiation along the lath boundaries and the formation of numerous severe spike-type crack initiation sites. Although fine lath-type cementite located in lath boundaries, small amount of cementite does not deteriorate HCF resistance. Spherical cementite concentrated on lath boundaries does not deteriorate high-cycle fatigue resistance due to decreased stress concentration effect. In conclusion, high amount of fine lath-type cementite at lath boundaries was worst case for HCF resistance. A single factor does not deteriorate HCF resistance, instead HCF resistance is influenced by multiple microstructural conditions.