Regulation Mechanism Of Retained Austenite Microstructure In The Nanostructured Dual-phase Steels And Its Effect On The Mechanical Properties

Nano-structured super bainite (also known as low-temperature bainite) and Q&P(quenching-partitioning) martensitic dual-phase steels are high-performance structuralsteels which have a unique combination of ultra-high strength, high ductility and goodtoughness. The strength of nano-structured dual-phase steels is attributed tonano-size/sub-micro bainitic-ferrite or martensite laths, while the ductility andtoughness are associated with retained austenite. When improving the ultra-highstrength is usually accompanied the plasticity and toughness significantly decrease insteel, therefore, it has become an important scientific problem in the field of materials toachieve fine microstructure and excellent mechanical properties by means of controllingthe retained austenite structure. The microstructure, phase fraction and carbon contentof steels are analyzed by OM (optical microscopy), SEM (scanning electronmicroscopy), TEM (transmission electron microscopy), XRD (X-ray diffraction) andEDS (energy dispersive spectrometer). The mechanical properties and wear resistanceare texted by micro-hardness, nano-indentation, universal testing machine and abrasiontester. Utilizing reasonable alloys design, austenite grain size and newly multi-step heattreatment processes to control the microstructure of retained austenite. Simultaneously,the thermal stability is studied by researching the structure and content of retainedaustenite, and the mechanical stability is studied by researching the TRIP effect ofretained austenite. Through the studies, the regulation mechanism of retained austenitemicrostructure in the nano-structured dual-phase steels and its effect on the mechanicalproperties are explored. The main contents are as follows:High-carbon (0.95wt%) and medium-carbon (0.30wt%) content of alloy steelshave been designed to perform bainite transformation. The microstructure consists ofcoarser sub-micro (330–360nm) bainitic-ferrite laths, retained austenite and martensitein medium-carbon steel, with lower volume fraction (10–18vol%) and carbon content(0.62–0.86wt%) of retained austenite. Free-containing and containing Co (3.87wt%)and Al (1.37wt%) of high-carbon alloy steels have been designed to perform bainitetransformation. The microstructure consists of finer nano-size (~33nm) bainitic-ferritelaths and retained austenite in high-carbon steels containing alloys of Co and Al, withlower volume fraction (16–32vol%) and higher carbon content (1.48–1.80wt%) ofretained austenite, simultaneously, the degree of reduction for volume fraction and carbon content of retained austenite are lower than free-containing Co and Al bainiticsteel in the tempering process. Different austenitizing temperature of high-carbon steelshave been designed to perform bainite transformation. The length of bainitic sheavesand the volume fraction of bainite improve with increasing austenite grain size, due toincrease the austenitizing temperature. It is significantly that reduction the volumefraction and carbon content of retained austenite in bainitic steel with lower carboncontent, but some block of M/A (mixed martensite and austenite) is formed in thismicrostructure. It is seem that the volume fraction of retained austenite is reduced andcarbon content of retained austenite is increased in super bainitc steel with containingCo and Al, simultaneously, adding alloys of Co and Al can concurrently suppress theprecipitation of carbides and delay the decomposition of the retained austenite. It isindicated that the coarsening austenite grain size can accelerate bainitic transformationand reduce the volume fraction of retained austenite, in addition, there is a criticalaustenite grain size (~41μm).The newly multi-step heat treatment process of low temperature bainitetransformation&deep cryogenic treatment and low temperature bainite transformation&partitioning process in high-carbon steel have been designed. Multiphasemicrostructure consists of nano-structured bainitic-ferrite laths, retained austenite,newly formed nano-size martensite (or ferrite) plates and fine carbides, can make thesize of retained austenite is significantly reduced and the volume fraction of retainedaustenite is obviously decreased. The studies have shown that the retained austenite isapparently refinement due to the untransformed austenite is further transformation intonano-size martensite plates and carbides in super bainitic steel in cryogenic treatment,and the retained austenite is significantly refinement due to the untransformed austeniteis further decomposed into nano-size ferrite plates and carbides in super bainitic steel inpartitioning process. The two-stage low temperature bainite transformation process needa long transformation time (~4days), in contrast, deep cryogenic treatment orpartitioning process only require much shorter time (~2hrs) to significantly reduce thesize and decrease the volume fraction of retained austenite.Thermal stability of retained austenite in phase transformation and tempering areresearched by super bainite and Q&P martensite processes, the microstructure of thisfamily of steels with a significant amount of retained austenite, consists of nano-size bainitic-ferrite (or martensite) laths and retained austenite (or containing some finecarbides). Meanwhile, the volume fraction and carbon content of retained austenite inQ&P martensitic steel are equal or even higher than the super bainitic steel. Duringtempering process, the volume fraction and carbon content of retained austenite arereduced, due the decomposition of retained austenite and precipitation of carbides inboth super bainitic and Q&P martensitic steels. However, the reduction of retainedaustenite content and tempering hardness in Q&P martensitic are lower than superbainitic steels. So the thermal stability of retained austenite for phase transformation andtempering process in Q&P martensitic is significantly higher than (or equal) the superbainitic steels.Mechanical stability of retained austenite under stress are researched by tensile andwear experiments, the super bainitic and Q&P martensitic steels have excellent strengthand ductility. But the elongation of the super bainitic steel is relatively low (the totalelongation is5.8%) in low-temperature (200℃) bainite transformation. Corresponding,the Q&P martensitic steel has superior ductility (the total elongation is14–20%) inmedium-temperature (450℃) partitioning process. The Q&P martensitic steelsignificantly enhance the hardness of the worn surface (from674to762HV1) andincrease the thickness of the deformed layer (from3.3to2.1μm) in wear process,compared to the super bainitic steel. The results show that the formation of Fe-Cclusters, carbides and twinning of retained austenite flakes of the microstructure insuper bainitic steel, resulting the low stability of retained austenite and this retainedaustenite would not lead to the optimum TRIP effect under stress (in tensile and wearprocess). But Q&P martensitic steel has a higher stability of retained austenite and canoccure strain-induced martensitic transformation under stress, thereby providing asuperior TRIP effect.