Effects of Nano Precipitates in Austenite on Ferrite Transformation Start Temperature during Continuous Cooling in Nb–Ti Micro-alloyed Steels

Abstract

In the present paper, transformation behavior during continuous cooling in non-deformed and hot deformed Nb-Ti micro-alloyed steels was investigated by using the thermal dilation method. Ti content in Nb-Ti bearing steels varied from 0 to 0.031 mass% with Nb content being kept to be constant. Thermal dilation curves were measured at different cooling rates, from which continuous cooling transformation (CCT) curves were built up. For non-deformed Nb-Ti steels, it was observed that ferrite transformation start temperatures (A r3 ) decreased with increasing Ti content up to 0.015 mass%, leveled off in the range of 0.015 to 0.027 mass% Ti, and drastically decreased thereafter. For hot deformed Nb-Ti steels, A r3 temperature did not exhibit significant difference with Ti addition lower than 0.027 mass%, and decreased drastically by further increasing Ti content. Austenite grain size (D g ), Nb-Ti precipitates and residual strain were taken into account to explain the variation of A r3 temperatures. Based on the experimental results, mathematical models for the calculation of A r3 for non-deformed and hot deformed Nb-Ti micro-alloyed steels were developed.KEY WORDS: Nb-Ti micro-alloyed; CCT curve; A r3 temperature; transformation; nano precipitate.across the specimen center, where the thermocouple was welded. Fernández et al. 20) calculated the amount of Nb and Ti precipitated at different temperatures for either Nb or Nb-Ti micro-alloyed steel, which showed that a reheating temperature of 1 220°C was proper for the steels used in this work. Figure 1 shows the schematic drawing for the thermo-mechanical treatment. The reheating temperature for these steels was chosen to be 1 220°C and held for 5 min to be fully austenitized. After reheating, the specimens were cooled down to 900°C at a cooling rate of 5°C/s, being held for 20 s and cooled down to 200°C at different cooling rates from 0.5 to 30°C/s. To study the effect of hot deformation on phase transformation behavior in the steels, a compression with a true strain of 0.693 was applied to the specimen at a strain rate of 5 s Ϫ1 after being held for 10 s at 900°C, with the laser beam being able to follow the welding spot on the specimen center surface during the deformation.After thermo-mechanical treatments, all specimens were cut in the middle along longitudinal direction across the thermocouple welding points and prepared for optical metallographic examination in the conventional way to observe the transformed microstructure by using an image analysis system, CCT curves were drawn by the temperatures determined on the dilatometric curves with the help of optical metallographic observations.To measure the austenite grain sizes prior to transformation, some of the samples were quenched from temperatures just above A r3 determined by the dilatometric tests. Carbon extraction replicas from the quenched samples, which were continuously cooled to A r3 at a rate of 1°C/s before quenching, were prepared to observe the precipitates prior to transformation by TEM (transmis...

Influence of Nb/Mo Alloying on Phase Transformations and Microstructures in 0.05C-1.5Mn-Nb-Mo Microalloyed Steels during Thermomechanical Simulation

“…grains that act as nucleation sites for ferrite transformation, 7) which allows for refinement of the final microstructure. 8) The accelerated cooling step serves to avoid the polygonal ferrite transformation region and retard phase transformation to lower temperatures to produce fine microstructures.…”

“…Niobium also forms fine strain-induced precipitates which can pin grain boundaries, slow defect recovery, and cause incomplete recrystallization, which assists in strain accumulation. 1,7,[21][22][23][24][25][26] Solute effects also influence the hardenability of the steel. Along with the solute drag effect, solute Nb and Mo lowers the activity of carbon.…”