FUNDAMENTAL APPROACHES IN THE DEVELOPMENT OF HIGH-STRENGTH STRUCTURAL EASY-TO-WELD STEELS WITH NANOSTRUCTURING

The Russian Arctic region development considered as one of the most important national tasks to be realized makes it necessary the creation of economic high-strength cold-resistant steels. Investigations have shown that for this purpose to be achieved it is necessary to develop the mechanism of structure control aimed at forming a quasiisotropic fragmented structure in the metal volume to be refined till the nanolevel. The fine-grained structure formation is most affected by an intensive plastic strain combined with recrystallization and phase transformations. A number of investigations were conducted using a plastometer Gleeble-3800 and a rolling mill Quarto-800 for supporting theoretical and experimental assumptions. The experiments were carried out on the experimental heats of steels with the carbon equivalent CE  =  0.44  –  0.87  %. In modeling of the technological processes on the plastometer Gleeble-3800, compression deformation was conducted in 2  stages – roughing at 1100  –  1080  °С and finishing at 950  °С and 820  °С thatsimulated a strain cycle on the industrial mills Quarto-5000. It has been found that the steel grain was refined from 6.5 to 2.2  µm after deformation at 950  °С and to 1.1  µm – at 810  °С. 20  –  37  % of fragments of less than 500  nm were recorded in the steel structure. For steel with CE  =  0.44  –  0.65  % the yield strength changed from 500  MPa to 700  MPa that was 40  % higher than the level of standard values. For steel with CE  =  0.65  –  0.87  %, the yield strength increased from 700  MPa to 1150  MPa. This strength value was achieved with increasing the nickel content of steel up to 3  %, with further increase in its concentration the yield strength remained the same. After rolling the steel with CE  =  0.60  –  0.87  % on the mill Quarto-800 in one pass with a 70  % deformation at temperature of 1100  °С and direct quenching followed by tempering at 600  °С, the yield strength of 1060  MPa, thus, the effect of the nickel content and the change of CE within the stated limits on the steel hardening were insignificant. The steel structure is bainite with the average grain size of 8.3  –  6.9  µm with a high dislocation density of (1  –  2)·1015  m–2 and a great extent of intragranular fragmentation. Based on the new technology, a group of cold-resistant steels with yield strength of 270  –  690  MPa and CE  =  0.32  –  0.65  % up to 130  mm in thickness and operating temperatures down to –60  °С has been developed. These steels were used in the building of nuclearpowered ice-breakers, ice-going ships, stationary and floating drilling platforms for the exploration and extraction of hydrocarbons in the Arctic Russian offshore zones. The completed work shows the possibility of developing structural steels with a significantly lower level of alloying (by 20  –  30  %) as compared to the steel analogues as well as that of unifying the chemical composition of steels.

structure, thermomechanical controlled processing, plastic strain, plastometer, yield strength, fragmentation, carbon equivalent, cold-resistant steels

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