Strengthening Mechanisms in Thermomechanically Processed NbTi-Microalloyed Steel

Abstract

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly. The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075°C to 825°C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly.

Superior mechanical properties of microalloyed steels processed via a new technology based on austenite conditioning followed by warm deformation

“…For the same number of deformation passes, the AC 2 WD-technology resulted in 150-200 MPa higher YS and UTS with similar El for a 5 times lower Ti content (compare this work with [30] in Table 1). For the NbTi-steel, accelerated cooling followed by warm deformation resulted in about 2 times higher YS and UTS for the same total strain (compare this work with our previous work on the NbTi-steel [10] in Table 1). For other steel grades containing similar Nb and Ti contents and various Mo content, similar or slightly higher strength and ductility were obtained here for up to 5.5 times lower Mo content after a more simple processing schedule (compare this work with [32 -40] in Table 1).…”

“…Two processing schedules were applied to each of two steels ( Figure 1): without holding after warm deformation or with holding. The processing involved:  austenitising at 1250 °C for 300 s, followed by cooling to 1100 °C at a cooling rate of 1 °Cs -1 ; this reheating procedure was used in accordance with the previous study on NbTi-steel [10] in order to compare the previous and current results;  first deformation at 1100 °C to 0.35 strain at 5 s -1 strain rate; this temperature was chosen to be well above Tnr for the studied steels, which is 900 °C and 975 °C for the Ti-and NbTi-steels, respectively [11];  second deformation at 975 °C to 0.50 strain at 5 s -1 strain rate, these parameters had to allow complete DRX in the Ti-steel and partial recrystallization in the NbTi-steel [12];  cooling to 675 °C at a cooling rate of 40 °Cs -1 ; this temperature was chosen to be below A1  750 °C for both steels, and the cooling rate had to assure retention of deformed microstructure after the second deformation and prevention of pearlite formation;  third deformation at 675 °C to 0.25 strain at 5 s -1 strain rate gave sufficient work hardening;  holding at 600 °C for 300 s was supposed to promote Nb precipitation in the NbTi-steel [13][14][15][16][17] and, possibly, facilitate the TiC precipitation in the Ti-steel [18][19][20]; to verify the effect of holding on microstructure and mechanical properties, a schedule without holding was also tested. General microstructure characterisation for the four studied conditions was carried out using optical and scanning electron microscopy.…”

Influence of Heat Treatments on the Microstructural Evolution and Resultant Mechanical Properties in a Low Carbon Medium Mn Heavy Steel Plate

“…[40] Meanwhile, in order to consider the effect of retained austenite grain size on the martensite transformation, the Eq. [4] derived by Yang and Bhadeshia [38] was used, as follows:…”

“…Therefore, the M s ¢ temperatures considering the chemical composition and grain size were estimated to be 293 K, 301 K, 295 K, 309 K, and 346 K (20°C, 28°C, 22°C, 36°C, and 73°C) for the heat treatments of 1, 2, 5, and 10 hours @ 873 K (600°C) and 2 hours @ 893 K (620°C) by Eqs. [3] and [4], respectively, indicating that increasing the isothermal holding temperature can significantly lower the stability of retained austenite, while prolonging the isothermal holding time can only slightly lower the stability of retained austenite.…”

“…The strong steels attract attention. [1][2][3][4] The achievement of enhanced strength and toughness can be obtained by the additions of Ni, Cr, Cu, Mo, Nb, V, and Ti and the optimum designs of thermomechanical control process and heat treatments, which can enhance the strength and toughness by grain refinement, solid-solution strengthening, precipitation hardening, and dislocation strengthening. [5][6][7] However, there are some limitations in these strong steels: (1) The microstructure and mechanical properties are always inhomogeneous along the thickness direction for heavy steel plate.…”