Precipitation Strengthening of Micro-Alloyed Steels Thermo-Mechanically Processed by Ultra Fast Cooling (original) (raw)

Materials Science Forum, 2012

Abstract

ABSTRACT Thermo-mechanical Controlled Processing (TMCP) is one of the greatest achievements in steel industry in the 20th century, which, however, depends too much upon low temperature rolling for the refinement of austenite grains, causing great loss in terms of productivity. To overcome this disadvantage, a new processing route with ultra fast cooling as the core has been proposed, and pilot rolling and industrial trials were carried out. As compared to conventional accelerated cooling, the ultra fast cooling can achieve cooling rate up to 300°C/s for 3 mm thick strip and highly homogeneous cooling by the pressurized water spraying. In the present work, the metallurgical backgrounds for ultra fast cooling in thermo-mechanical processing were studied and elucidated. By the pilot hot rolling experiments with a lean composition of a typical 600 MPa grade high strength steel, it has been found that the application of ultra fast cooling (UFC) at the exit of hot rolling mills can improve the strength by as much as 100 MPa as compared to the conventional TMCP. The strengthening mechanism lies in that the ultra fast cooling immediately after hot rolling may further improve the strengthening effects by precipitation, grain refinement, and dislocation hardening. The theoretical calculations and experiments indicate that the grain refinement, dislocation hardening, and precipitation in the TMCP with in-front UFC have caused the strength increments of 36, 34 and 54 MPa over the conventional TMCP with ACC, respectively. The microstructure characterization showed that the density of high angle grain boundaries had been increased, and the average size of precipitates had been reduced from about 34 nm to 10 nm with the cooling pattern changing from ACC to the application of UFC. The theoretical estimation indicates that when the cooling profile is changed from the conventional ACC to UFC+ACC, and to UFC, precipitation strengthening accounts for more and more strength increment in the improved strength of hot rolled micro-alloyed steels.

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