Influence of Ti(C,N) precipitates on austenite growth of micro-alloyed steel during continuous casting

Abstract

A ccording to thermal mechanical control process and relaxation-precipitation controlling phase transformation technique during hot rolling process, the microstructure of steel can be refined by the addition of Ti-Nb microalloys [1,2] . A significant improvement in strength, toughness and weldability of steel can be obtained by Ti(C,N) precipitation. However, the hot ductility of micro-alloyed steels remains very poor during continuous casting, which leads to the formation of transverse cracks during the straightening process [3,4] . The influence factors of hot ductility in the third brittle zone include the size of austenite grain, carbonitride precipitation and pro-eutectoid ferrite film [5,6] . Research concerning the impact of Ti(C,N) precipitation on the growth of austenite mainly focuses on the hot Abstract: Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 ℃·s rolling process at current stage [7][8][9][10][11] . However, there has been few investigations of austenite growth during continuous casting, which affects the ductility of steel directly. The formation of coarse austenite grain during soft cooling of strand surface below oscillation marks in slabs usually deteriorates ductility and causes transverse cracks in the straightening process. Therefore, the prediction of austenite grain size in micro-alloyed steels is very important.The main challenge in the size prediction of austenite grains is the computational complexity of f(t)/r(t) in the Andersen-Grong equation, where f is the volume fraction and r the radius of the precipitates, which is related to the nucleation and growth of precipitates at each state including temperature and time. By implementing some simulation models, Miettinen et al [12] fitted the results of Yasumoto's experimental data [13] using Equation (1). For the steel grade with carbon equivalent of 0.17%, the maximum grain size was found at the highest temperature for totally austenitic structure. Bernhard et al. [14] analyzed the influence of pinning

Effect of Different Thermal Schedules on Ductility of Microalloyed Steel Slabs during Continuous Casting

“…Combined with the second phase precipitation model [28,29], for the Nb, C, and N components of the test steel, the precipitation-time-temperature curves of Nb(C,N) in austenite are calculated and presented in Figure 16, In the typical "C shape" curves, t 0.05 and t 0.95 are the start and finish times of precipitation, respectively. It is shown that the nose temperature for boundary nucleation is 980 • C, and for homogeneous nucleation it is 850 • C. Comparing the influence of three kinds of thermal schedules on ductility of HG785 steel slab in III brittleness area, presented in Figure 17 [30], thin ferrite film will be formed in the brittle trough area 800 °C under the pattern 1 and pattern 2 thermal schedules. Intergranular failure can occur during the austenite to ferrite transformation when a thin ferrite film has been formed around the austenite grains [31].…”

“…Those precipitates reduce the bonding force of the grain boundary and hinder the slipping of the grain boundary, and the temperature fluctuation will promote the precipitation of (Ti,Nb)(C,N), which can seriously reduce the ductility of steel [14,32]. Comparing the influence of three kinds of thermal schedules on ductility of HG785 steel slab in III brittleness area, presented in Figure 17 [30], thin ferrite film will be formed in the brittle trough area 800 • C under the pattern 1 and pattern 2 thermal schedules. Intergranular failure can occur during the austenite to ferrite transformation when a thin ferrite film has been formed around the austenite grains [31].…”

The effect of TiN precipitated particles on the austenite grain and hardenability of 20CrMnTi gear steel

“…where w C represents the mass fraction of C, A C represents the relative atomic mass of C and [C] represents the solid solution amount of C. The nucleation of solid precipitates occurs first at the grain boundary and then at dislocation, and the uniform nucleation has not happened [35,36]. When the new phase nucleates at grain boundaries, the critical nucleation size, and the critical nucleation work can be calculated by Equations ( 13)-( 15).…”

Removal Mechanism of Microscale Non-Metallic Inclusions in a Tundish with Multi-Hole-Double-Baffles

“…Owing to the increasingly stringent quality requirements, many studies have investigated the equipment, process, and theoretical principles of continuous casting, particularly for the tundish, which is the final metallurgical element reactor [1][2][3][4][5][6]. To improve the removal of inclusions by the tundish, many flow control devices, such as the turbulence inhibitor, weir-dam, baffle with holes, and filter and gas-curtain device, have been developed to optimize the flow field, increase the residence time, and promote inclusions' removal [7][8][9][10][11][12][13][14][15][16][17][18].…”