High-Strength Low-Carbon Ferritic Steel Containing Cu-Fe-Ni-Al-Mn Precipitates
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TRODUCTION
APPROXIMATELY 20 years ago, the United States Navy developed a high-strength low-alloy (HSLA) steel, denoted HSLA-100, with a yield strength of 100 ksi (689 MPa), as a replacement for HY-100 steel to reduce fabrication costs in ship construction.[1–4] HSLA-100 possesses similar strength and toughness values as HY100, but because of the reduced nominal carbon content, it is weldable without preheat, thereby reducing fabrication costs.[3–5] To compensate for the decrease in strength on reducing the C concentration, Cu was added to HSLA-100 for precipitation strengthening, whereas Cr, Ni, and Mo were added to increase hardenability.[2–7] The thermal processing of HSLA-100, a solutionizing (austenitizing) and quenching step followed by tempering at 620 C to 690 C, produced a tempered martensitic steel containing Cu precipitates.[2–4] Studies by Foley et al.[8,9] have demonstrated that tempering of HSLA100 overages the Cu precipitates, thereby reducing their strengthening contribution. Earlier research on steels alloyed with Cu, however, showed that significant strengthening could be attained without tempering.[10–13] To develop a precipitationstrengthened ferritic variant of HSLA-100 steel, Mo and Cr were removed, reducing hardenability, and the thermal processing was simplified to hot rolling followed by air cooling. These changes produced a ferritic HSLA SEMYON VAYNMAN, Research Professor and MORRIS E. FINE, Professor, Member of Graduate Faculty, Department of Materials Science and Engineering, and DIETER ISHEIM, Research Assistant Professor, and DAVID N. SEIDMAN, W.P. Murphy Professor, Department of Materials Science and Engineering and Northwestern University Center for Atom-Probe Tomography (NUCAPT), are with Northwestern University Evanston, IL 602083108. Contact email: [email protected] R. PRAKASH KOLLI, Senior Metallurgist, is with Global Customer Analytical, Nalco Company, Naperville, IL 60563 and SHRIKANT P. BHAT, Manager, is with ArcelorMittal Steel Global Research & Development, East Chicago, IN 46312. Manuscript submitted May 11, 2007. Article published online January 9, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
plate steel with a yield strength in excess of 482 MPa (70 ksi), a Charpy V-notch (CVN) absorbed impactenergy better than 136 J (100 ft-lbs) at -40 C, improved weldability, and enhanced atmospheric corrosion resistance.[8,14–18] This steel, ASTM A710 grade B, was used in construction of a demonstration bridge in northern Illinois. The body-centered-cubic (bcc) nanoscale Cu precipitates within the steel are coherent with the matrix providing significant precipitation strengthening. Goodman et al. first documented this in a binary Fe-Cu steel many years ago by using field-ion and atom-probe microscopies.[19,20] Additional increases in yield strength were achieved by changing the thermal processing to hot rolling followed by quenching into water and subsequently isothermal aging. The result was a high-strength lowcarbon (HSLC) steel with yield strengths of approximately 700 MPa
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