Resistance to Temper Softening of Low Carbon Martensitic Steels by Microalloying of V, Nb and Ti

Abstract

The resistance to temper softening in low carbon martensite with its underlying origin, by microalloying of strong carbide-forming alloying elements (V, Nb and Ti) to an Fe-0.1C-1.5Mn-0.05Si (mass%) alloy, was investigated in this study. With similar hardness in as-quenched condition in all the alloys used, the hardness of tempered martensite is increased by V, Nb and Ti additions, particularly after treatment at higher temperature with longer time. The increment in hardness becomes larger by more amount of V addition, while with almost the same amount of microalloying additions, Nb and Ti provide larger strengthening than that of V. Atom probe measurements have revealed that a high density of nano-sized alloy carbides are formed in those alloys with V, Nb and Ti additions at 923 K, where large secondary hardening was observed. At 723 K, where there is some resistance to temper softening, however, almost no precipitation of V, Nb and Ti can be detected. The X-ray line profile analysis of the tempered alloys implies that the reduction in dislocation density during tempering is strongly retarded by V, Nb and Ti additions. This should be the major reason for their resistance to temper softening at relatively lower temperature, even without nano-precipitation of alloy carbides.

Strengthening of Low Carbon Steel by Nano-sized Vanadium Carbide in Ferrite and Tempered Martensite

“…1(c)). Although a small amount of cementite precipitated in tempered martensite was observed in our previous study, 9) no obvious change can be observed in the optical micrograph even after prolonged tempering for 43.2 ks (Fig. 1(d)).…”

“…Among them, the magnitude of precipitation strengthening is always overestimated by the prediction using conventional Orowan model, whereas the Ashby-Orowan model with the consideration of interaction between dislocations bypassing the particles gives more accurate prediction. 4,9,13,14) In addition, Pythagorean summation rather than simple linear summation of precipitation strengthening and dislocation strengthening shows better agreement with the experimental results. 12) Based on the above findings, isothermal ferrite transformation and tempering process of martensite under the same conditions in a V-microalloyed low carbon steel were performed in this study, to make the direct comparison of interphase precipitation and age precipitation behaviors via quantification of carbide dispersion using three-dimensional atom probe (3DAP).…”

“…Electron backscatter diffraction (EBSD) measurements with an accelerating voltage of 25 kV, a current of 10 nA and a step size of 0.5 μm were performed in a scanning electron microscope (SEM: JSM-7001F) to measure the crystallographic grain size, which was defined as the average intercept length of the boundaries with misorientation larger than 2° following our previous works. 9,12,13) The dispersion of nano-sized VC precipitates was analyzed using three-dimensional atom probe (3DAP: CAMECA LEAP-4000HR), for which the specimens were prepared by focused ion beam (FIB: FEI QUANTA 3D) and measured in a voltage pulse mode with a pulse fraction of 20%, a pulse rate of 200 kHz, a specimen temperature of 50 K. VC precipitates were detected from the 3DAP datasets using cluster analysis based on the maximum separation method, 15) whose details can be found in our previous report. 16) To ensure statistical reliability, more than 3 specimens for each condition of heat treatment with a volume more than 10 6 nm 3 (10 −21 m 3 ) were measured.…”

“…and tempering process, which corresponds to the secondary hardening of bainitic and martensitic structures. [7][8][9] The other one is the newly developed interphase precipitation, which occurs as a result of nucleation at the migrating interphase boundary during ferrite transformation. 10,11) Recently, the contributions to strength of these two types of precipitation were comparatively studied in a vanadium (V)-microalloyed low carbon steel (Fe-0.1C-0.8Mn-0.2Si-0.3V (mass%)) by some of the present authors.…”

Carbide Precipitation in a Low Alloyed Steel during Aging Studied by Atom Probe Tomography and Thermodynamic Modeling

“…It is of interest to follow the precipitation sequence in the martensite of HSLA steels, and precipitation at dislocations has been reported to occur in several studies. Zhang et al studied the softening of steels alloyed with V, Nb, and Ti aged at 350-750 • C [13]. An increase in hardness was achieved through alloying when aging at around 600 • C. Characterization using APT revealed a network of precipitates located at dislocations after aging for a short time, 5 min at 650 • C. Shrestha et al used APT to confirm that Nb-rich clusters evolve to very fine nanosized precipitates in a Nb-alloyed HSLA steel during aging at 800 • C for 10, 20, and 30 s [14].…”