The microstructure and properties of a quenched and tempered low-carbon-manganese-niobium steel
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The effects of rolling p a r a m e t e r s (deformation schedule and finishing temperature), cooling rate (direct quench and air cool), and heat treatment have been investigated for a low carbon-manganese-niobium steel. Microstructure was characterized by optical and transmission electron microscopy, and correlated with tensile and impact properties. The best combination of strength and toughness was obtained by controlled rolling low in the austenite temperature range, finishing close to the austenite-ferrite transformation temperature, and direct quenching. Such a treatment produces a fine acicular ferrite containing a substructure of subgrains and a high density of mobile dislocations. Subsequent heat treatment effects a very small strength increase due to the limited amount of niobium available for precipitation and due to partial r e c o v e r y of the random dislocation substructure during aging.
THE
current interest in the metallurgy of highstrength low-alloy steels is due to their potential use as linepipe, automotive and structural materials. The materials requirements for each application, and the alloy development trends have been discussed at a recent symposium. 1 F o r the particular case of linepipe steel, the important requirements are yield strength, r e s i s t a n c e to unstable crack propagation, weldability, and economy of production. Current steels for large-dia m e t e r gas lines are usually fully killed, low-carbon, microalloyed steels. 2'3 The pipe is formed directly from controlled-rolled, air-cooled or spray-cooled skelp. The principal strengthening mechanism is grain refinement which is achieved by the combined effects of controlled rolling and microalloy additions of elements such as Nb, V, A1, or Ti. Precipitation hardening, substructural strengthening, and solidsolution strengthening also contribute to the yield strength. The required ductile-brittle transition temperature is achieved p r i m a r i l y by grain refinement and low-pearlite content. The required impact shelf energy is achieved by low-sulfur content and additions of r a r e earth elements for sulfide shape control. Striking improvements in the yield strength and impact toughness have been made in recent y e a r s by manipulating these metallurgical variables .1-3 However, it may be difficult to surpass the current 65 to 70 ksi (448 to 483 MPa) strength level by further small modifications to alloying and processing. To produce 0.5 in. to 0.75 in. (12.7 to 19.1 mm) thick controlled-rolled plate with 80 to 100 ksi (552 to 690 MPa) yield strength would require additional alloying, higher mill loads, and even more precise control of the rolling schedule, all of which would contribute enormously to the cost of the product. Even then it is not clear if the toughness and weldability requirement could be met. Therefore, it appears that a distinctly different processing sequence will have to be developed to produce such high-strength linepipe steel.
J. D. BOYD is Head, Ferrous Metals Section, Canada Centre for Minerals and En
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