Development of process for thermomechanical treatment of ultrahigh strength steel prepared by electroslag refining

Abstract

Thermomechanical treatment (TMT) is the simultaneous use of work hardening, and grain refinement along with solid solution and precipitation strengthening. In this investigation, four alloys, with a base composition of 0 . 28%C, 1 . 0%Mn, 4 . 2%Cr, 1 . 0%Mo, 0 . 34%V, were prepared by electroslag refining (ESR) and by addition of small amounts of Ti and Nb and by increasing Cr and V to 4 . 8 and 0 . 48% respectively. In two of the alloys a yield strength in excess of 1550 MPa was obtained in the as cast quenched and tempered condition. Attempts were made to further increase the yield strength by thermomechanical treatment. The process parameters for thermomechanical treatment were optimised by adopting procedures such as calculation of stability of precipitates, hot compression test, determination of cooling rates in different coolants, and modelling of TTT and CCT diagrams. The process involved prerolling of the ESR ingot to a bar at 1200uC, followed by hot rolling in two passes starting from 950uC and finishing at 850uC with equal deformation of 25% in each pass to convert the bar into plates. These were immediately cooled in one of the cooling media: air, polymer-water solution (1 : 1 . 5) and oil. Yield strength in excess of 1750 MPa was obtained in oil cooled specimens of the alloy with titanium addition and that where Cr and V were increased. The niobium added specimen gave strengths, similar to that obtained for the base alloy, in spite of the fact that the as cast alloy had shown very high strengths, presumably because of the high soaking temperatures and grain growth. Air cooling gave the lowest strengths and oil cooling the highest.

Development of Ultra High Strength Steel by Electroslag Refining and Thermomechanical Treatment

“…The final rolling was carried out at 1123 K followed by oil quenching. The temperature for final rolling was selected above the AC1 (austenite start) temperature close to 1073 K as was predicted for similar kind of steel containing 0.28 wt% C and 4.2 wt% Cr [13].…”

Development of Ultrahigh Strength Low Alloy Steel through Electroslag Refining Process

“…It is expected that at 930°C most of the precipitates goes to the solution except vanadium carbonitride which is expected to be present about 70-80 % as calculated by CHEMSAGE software. 13,14) Further increase of the austenite temperature leads to gradual decrease of the volume fraction of V (C, N).…”

“…Only vanadium carbonitrides formed during cooling after solidification remains partly undissolved. 13,14) Chromium and molybdenum would have precipitated into these preexisting vanadium carbonitride precipitates during cooling and subsequent tempering and formed complex carbides. As the size of the precipitates are smaller and distributed uniformly in the ferrite matrix, the precipitation hardening seem to play an important role for obtaining high strength.…”

“…air, oil and polymer-water mixture and it is found that during oil cooling its value is about 29 K/s. 14) Therefore, it is expected that the oil-cooled as-quenched specimens consist of fully martensitic microstructure. It is also predicted from Fig.…”

“…13) Therefore, in some alloys about 0.07 % titanium is added with the basic composition and improvement in mechanical properties was reported. 14,15) In this study for further improvement of mechanical properties especially to increase the toughness values, the basic steel is alloyed with 1-3 % of nickel. In some alloys about 0.07 % titanium was added.…”