Producing A615 / A615M High Strength Construction Re-Bars Without Use of Microalloys: Part 2

Abstract

The metallurgy of ASTM A615/A615M Gr. 60 steels made from three different chemistries was studied to suggest an economically advantageous route to produce a steel grade that saves the extra cost of alloying elements. Metallographic examinations, along with microhardness and XRD studies, were performed to rate the steel chemistries based on their superheats. This study of the steel grades revealed that producing steel for requisite standards like ASTM 615/A615M Grade 60 may not be dependent on starting superheat but on the chemistry and rolling process. Study of the three chemistries A, B and C indicated that the standards were met in all 3 chemistries; however, sample A had the lowest cost chemistry and therefore is a suggested route for this product. 1. Introduction. ASTM A615/A615M Grade 60 standard calls for a minimum 60 Ksi (420MPa) yield strength with no upper limit even on tensile strength. The standard only specifies that phosphorus (P) be no more than 0.06%. The resulting plain carbon steel is expected to have basic ferrite - pearlite microstructure with minimum grain size ASTM 5. Because every steel mill is different and processes vary, some steel mills use some microalloying to achieve fine grain size in pursuit of the aim microstructure. This process of compositional and process variations can produce yield strengths typically in the range of 350 to 700 MPa (50 - 100 Ksi). Microalloying, which enhances physical properties through ferritic grain refinement, is often supplemented by precipitation and or dislocation strengthening. Hall-Petch type of strengthening is determined to suggest that a decrease of ferritic grain size from ASTM 6 - 8 to ASTM 12 - 13 is accompanied by an increase of 30 Ksi (210 MPa) in yield strength. Admittedly the other good effect of fine grains besides strengthening is good ductility or toughness. Two common microalloying elements V and Nb are used in industry to achieve this goal and several authors (1-6) have studied their use. It is known that the behavior of individual micro alloying elements classifies them as mildly carbide forming or strongly carbide forming. The two micro alloying elements (V and Nb) used in this study do qualify as strong carbide formers. They therefore stabilize the α phase. This essentially means that they reduce the γ phase field. Any of the elements in solid solution in α strengthen the ferrite matrix in steel. These elements differ in their contribution to hardening and the extent to which they reduce plasticity as they add a certain increment to strength. Furthermore, compared to other microalloying elements like Nb and Ti, vanadium exhibits essential differences. The Swedish Institute of Metals Research (6) found that the solubility of vanadium carbonitrides, in particular, is much larger and the solubility of vanadium's nitride is about two orders of magnitude smaller than its carbide, contrary to Nb but similar to Ti. Vanadium has higher solubility in austenite than niobium, and its carbonitrides (V(C,N)) dissolve more easily prior to hot rolling than NbC. Consequently, vanadium is an excellent choice for strong and easily controllable precipitation strengthening, but it is expensive.