Creating Weldable High-Strength Structural Steel with Nanostructuring

Abstract

Exploitation of the Arctic calls for the creation of economical high-strength steel capable of operating at low temperatures. Research shows that, to that end, means of controlling the steel structure must be identified, so as to create quasi-isotropic fragmented nanostructure within the metal. The formation of finegrain structure is possible by a combination of intense plastic deformation with recrystallization and phase transformations. To confirm the theoretical and experimental preconditions for this process, experiments are conducted on the Gleeble-3800 instrument and a Kvarto-800 four-high rolling mill. Experimental steel melts with a carbon equivalent C_equ = 0.44–0.87% are investigated. In simulation on the Gleeble-3800 instrument, compressive deformation is applied in two stages: roughing at 1080–1100°C; and finishing at 950 and 820°C. That simulates the deformation cycle in industrial Kvarto-5000 four-high mills. The grain size in the steel is decreased from 6.5 to 2.2 μm after deformation at 950°C and 1.1 μm after deformation at 810°C. Fragments smaller than 500 nm constitute 20–37% of the steel structure. In the steel with C_equ = 0.44–0.65%, the yield point is 500–700 MPa, which is 40% greater than the standard values. In the steel with C_equ = 0.65–0.87%, the yield point is 700–1150 MPa. These values are obtained with increase in nickel content in the steel to 3%. At higher Ni concentrations, no improvement in yield point is seen. After rolling on the Kvarto-800 mill, with C_equ = 0.60–0.87% in one pass (with 70% reduction) at 1100°C and direct quenching with subsequent tempering at 600°C, the yield point is 1060 MPa. In this case, variation in the Ni content and C_equ has little influence on the yield point. The steel consists of bainite (mean grain size 6.9–8.3 μm), with a large dislocation density (1–2) × 1015 m^–2 and considerable fragmentation within the grain. On the basis of the new technology, a group of low-temperature steels with yield points of 270–690 MPa and C_equ = 0.32–0.65% is created. The thickness of the steel sheet is up to 130 mm; it may operate at temperatures as low as –60°C. Such steel may be used for atomic icebreakers, other Arctic vessels, and fixed and floating drilling platforms for oil and gas extraction from Russia’s Arctic shelf. This research demonstrates the possibility of creating structural steels with relatively little alloying (up to 20–30%) and with standardized chemical composition.