Copper Precipitation Hardened, High Strength, Weldable Steel

Steel containing 0.03% carbon, 1.35% copper and 0.84% nickel had yield strength in the 540-625 MPa (78-90 Ksi) range depending on thickness, ultimate tensile strength in the 625-690 MPa (90100 Ksi) range, and 25-30% elongation when air cooled after hot rolling. No brittle heat-affected zone was formed during manual or automatic submerged arc welding without pre-heating or postheating. The fracture toughnesses in the plate and in the heat-affected zone were excellent. Introduction During the past several years an easily weldable, high strength (more than 540 MPa yield), high impact fracture toughness steel (NUCu) has been investigated at Northwestern University with bridge applications in mind. For good weldability without pre-heating and post-heating, the carbon content of the steel was kept low and high strength was achieved by copper precipitation hardening. The steel was designed to be air cooled from hot rolling. Omitting the most expensive alloying elements (Cr and Mo), and eliminating the need for quench and temper heat treatment used in other high strength structural steel alloys reduces the cost of the steel, an important requirement for infrastructure applications. Experimental Methods The initial studies at Northwestern University were conducted on six 220 kg laboratory heats prepared at Inland Steel Company's Research Laboratory by vacuum-induction melting. These steel heats, numbered as NUCul through NUCu6 were hot-rolled to 12.7 mm thick plates and air cooled. A commercial 80 ton steel heat was produced at Oregon Steel Mills, Portland, Oregon. The heat was calcium treated for inclusion shape control. Two slabs were cast using Amsted bottom pressure casting process. The slabs were hot-rolled into plates of several thicknesses 12.7 to 25.4 mm (0.5 to I inch) and air cooled. A hot-rolling temperature of 1150°C or less was specified. The compositions of two laboratory steel heats (NUCu5 and NUCu6) produced by Inland Steel Research Laboratory and the commercial steel produced by Oregon Steel Mills (NUCu-Oregon), are listed in TABLE 1. For good weldability the carbon content in steels was kept low, 0.03 to 0.05%, giving low carbon equivalent (Table 1). Since the steel is not quenched and tempered, Cr and Mo were omitted. Copper concentration was approximately 1.3% to provide targeted better than 485 MPa (70 Ksi) strength through precipitation hardening. Ni was added to prevent hotshortness during hot rolling. Nb and Ti were added to control grain size during hot rolling and welding. TABLE 1. Composition of Experimental NUCu5 and NUCu6, and Commercial NUCu-Oregon Steels (Wt.%) ELEMENT NUCu5-INLAND NuCu6-INLAND NUCu-OREGON