Mechanical properties and microstructure of low carbon ultra-high strength steels (UHSS) microalloyed with boron
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Mechanical properties and microstructure of low carbon ultra-high strength steels (UHSS) microalloyed with boron I. Mejía1, A. García de la Rosa1, A. Bedolla-Jacuinde1 and J.M. Cabrera2,3 1
Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, Edificio “U-5”, Ciudad Universitaria. 58060-Morelia, Michoacán. MÉXICO. [email protected] 2 Departament de Ciència del Materials i Enginyeria Metal·lúrgica, ETSEIB, Universitat Politècnica de Catalunya, Av. Diagonal 647. 08028-Barcelona, SPAIN. 3 Fundació CTM Centre Tecnològic, Av. de les Bases de Manresa 1. 08242-Manresa (Barcelona), SPAIN. [email protected] ABSTRACT The aim of this research work is to study the effect of boron addition on mechanical properties and microstructure of a new family of low carbon NiCrVCu advanced high strength steels (AHSS). Experimental steels are thermo-mechanically processed (TMP) (hotrolled+quenched). Results show that the microstructure of these steels contains bainite and martensite, predominantly, which nucleate along prior austenite grain boundaries (GB). On the other hand, tensile tests reveal that the TMP steels have YS (0.2% offset) of 978 MPa, UTS of 1140 MPa and EL of 18%. On the basis of exhibited microstructure and mechanical properties, these experimental steels are classified as bainitic-martensitic complex phase (CP) advanced ultra-high strength steels (UHSS). INTRODUCTION Recent years have seen many developments in steel technology and manufacturing processes to build vehicles of reduced weight and increased safety. For this purpose Advanced High Strength Steels (AHSS) have been developed. The AHSS include newer types of steels such as dual phase (DP), transformation-induced plasticity (TRIP), complex phase (CP), B steels (BS), and martensitic steels (MART), which are primarily multi-phase steels, and contain ferrite, martensite, bainite, and/or retained austenite in quantities enough to produce outstanding mechanical properties [1-2]. Researchers have studied the B effect in steels for a long time [3-7] particularly due to its potential to increase steel hardenability. Nowadays, with the development of modern steelmaking technologies, the technique for controlling B additions is mature and B steels are extensively applied in the construction of heavy machinery, building structures, marine platforms and pipelines [8-11]. It has been well-established that B atoms segregate towards austenite GB and increase hardenability of steel by suppressing the nucleation of ferrite. There are four mainly explanations about the B effect mechanism >12@: (i) B segregation to austenite GB reduces the grain boundary energy, and so the number of preferential nucleation sites for ferrite, (ii) B reduces the self-diffusional coefficient of iron at GB and decreases the nucleation rate of ferrite, (iii) GB are a preferential nucleation sites for ferrite and when B segregates to GB, these sites will vanish, and (iv) Fine borides form along the boundaries and are coherent with the matrix; in this case, it i
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