High-Strength Low-Carbon Ferritic Steel Containing Cu-Fe-Ni-Al-Mn Precipitates

Abstract

An investigation of a low-carbon, Fe-Cu-based steel, for Naval ship hull applications, with a yield strength of 965 MPa, Charpy V-notch absorbed impact-energy values as high as 74 J at -40°C, and an elongation-to-failure greater than 15 pct, is presented. The increase in strength is derived from a large number density (approximately 10 23 to 10 24 m -3 ) of copper-iron-nickelaluminum-manganese precipitates. The effect on the mechanical properties of varying the thermal treatment was studied. The nanostructure of the precipitates found within the steel was characterized by atom-probe tomography. Additionally, initial welding studies show that a brittle heat-affected zone is not formed adjacent to the welds.

Atom-probe study of Cu and NiAl nanoscale precipitation and interfacial segregation in a nanoparticle-strengthened steel

“…The Cu particles in martensite are enriched in Cu ( ∼ 44.6 ± 5.8 at.%) but contain a significant amount of Ni ( ∼ 12.2 ± 3.8 at.%), Al ( ∼ 20.3 ± 4.7 at.%), Mn ( ∼ 9.5 ± 3.4 at.%), and Fe ( ∼ 13.5 ± 4.0 at.%), indicating that the composition of the Cu particles in martensite is far from equilibrium. In addition, a pronounced segregation of Ni, Al, and Mn at the Cu particle/martensite interface can be observed, consistent with previous studies on Cu and NiAl nanoparticle-strengthened martensitic steels [15][16][17][18][19][20][21][22]. For the NiAl particles in martensite (Figure 4(f)), the enrichment of not only Ni and Al, but also Mn and Cu change monotonically toward the center of the NiAl nanoparticles, and the concentration of Ni, Al, Mn, Cu, and Fe of the particles are estimated to be 39.1 ± 2.3, 26.8 ± 2.1, 9.7 ± 1.4, 15.4 ± 1.7, and 8.6 ± 1.3 at %, respectively.…”

“…Therefore, the Cu nanoparticles in the fcc austenite phase are expected to directly form with a fcc equilibrium structure of Cu, the formation of which is different from the initial structure of bcc Cu nanoparticles in the bcc ferrite/martensite phase [26,27]. In addition, the segregation of Ni has been observed at the interface between the Cu nanoparticles and the austenite matrix, which is similar to but less pronounced than that as observed for the Cu precipitation in ferrite/martensite [15][16][17][18][19][20][21][22]. It is known that the heat of mixing between Fe and Cu is large and positive (+13 kJ mol −1 ), whereas that for both Fe-Ni and Cu-Ni pairs is very small (−4 and +2 kJ mol −1 , respectively) [28].…”

“…Practically, for many high-strength steels, the matrix is tailored to have a mixture of two or more phases, rather than simply a single phase, to obtain an optimum combination of high strength and high ductility [11][12][13][14]. Among them, Cu and NiAl nanoparticle-strengthened steels represent an important class of advanced highstrength steels, which have attracted significant attention in recent years due to their good combination of high strength, good ductility, and good weldability [15][16][17][18][19][20][21][22]. It was found that there are two types of pathways for the formation of the NiAl/Cu co-precipitates in the martensite phase (i.e.…”

“…It should be pointed out that the matrix of most Cu and NiAl nanoparticle-strengthened steels is primarily martensite, but contains a small amount of austenite, the amount of which depends on the alloy composition and heat-treatment histories. While most of the previous studies were focused mainly on the understanding of precipitation behavior of Cu and NiAl nanoparticles in the martensite and ferrite phases [15][16][17][18][19][20][21][22], limited detailed information is available on the precipitation behavior and related mechanism in the austenite phase.…”

Shear and tensile plastic behavior of austenitic steel TRIP-120 compared with martensitic steel HSLA-100

“…HSLA-100 has been extensively investigated in the past two decades. Studies have been completed to optimize heat treatments (Dhua et al 2003) and also to investigate relationships between strength and microstructure (Vaynman et al 2008). The mechanical performance of HSLA-100 has been characterized through studies of its fracture behavior (Das et al 2006;Densley and Hirth 1998) and its ballistic resistance (Martineau et al 2004).…”