Regulation Of Retained Austenite In Ultra-high Strength Dual-phase Steel And Its Effect On Mechanical Properties

Ultra-high-strength dual-phase steel is a kind of high-performance steel that satisfies the development demand of the third-generation advanced high-strength steel(3GAHSS)and has high strength and high plasticity.At present,low temperature bainite steels and quenching-carbon partitioning(Q&P)martensitic steels are representative of such ultra-high strength dual phase steels.The high strength of ultrahigh-strength dual-phase steels mainly comes from the bainitic ferrite or martensite laths in the structure,while retained austenite induces the TRIP effect when subjected to external stress.It will further increase the strength and hardness while maintaining good plasticity and toughness.Therefore,how to obtain the excellent mechanical structure by regulating the retained austenite in ultra-high strength dual phase steel has become an important scientific problem for the development and application of such advanced high-strength steels.In this paper,the effects of alloying elements(Ni)on the microstructure(especially the retained austenite volume fraction,size and carbon distribution)were analyzed by advanced research methods such as scanning electron microscopy(SEM),X-ray diffraction(XRD),and electron backscattered diffraction(EBSD).Two different heat treatment processes for Q&P martensite and low temperature bainite were designed for the experimental steels with the same chemical composition,and the mechanics and three-body abrasive wear properties of the two treatments were compared.The microstructure was observed by optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The carbon content in retained austenite was detected by electron probe microanalysis(EPMA)to investigate the relationship among retained austenite,TRIP effect and abrasive wear performance in ultra-high strength dual phase steel.The main results are as follows:1.The addition of Ni decreased the driving force of bainite transformation,significantly delayed the nucleation of bainite,and greatly prolonged the time required for the transformation of low temperature bainite.2.After addition of Ni element,the volume fraction of retained austenite in the microstructure after the low-temperature bainite transformation was significantly increased(from 19.3 vol% to 39.60 vol%),and more retained austenite was retained atroom temperature.The average size of retained austenite was also increased(from 0.654± 0.232 ?m to 0.822 ± 0.336 ?m,by 25.7%)in the low temperature bainitic steel.3.Through the addition of Ni alloying elements,the volume fraction and average size of retained austenite are regulated and controlled,which can promote the occurrence of TRIP effect,so that the total elongation and the product of strength and elongation of the steel sample were greatly increased by 15.5%(by 63.2%)and 25.90GPa·%(by 53.0%),resulting in better mechanical properties.4.The carbon content in the film-like retained austenite was higher(by approximately 34 ~ 37%)than that in the blocky retained austenite;this resulted in the film-like retained austenite to have a higher stability and being conducive to the TRIP effect.5.During the three-body abrasive wear tests,for both the treated samples,the surface hardness increased as a result of the transformation of carbon-enriched retained austenite to martensite under strain(TRIP effect),which led to obviously less abrasion weight losses in the later cycles.6.Compared with the low temperature transformed bainitic sample,the Q&P martensitic sample has higher austenite content and a greater depth of deformed layer which provides a better TRIP effect.As a result,the Q&P sample exhibits better three-body abrasive wear performance.