Precipitation and hot ductility of low C-V and low C-V-Nb microalloyed steels during thin slab casting

Abstract

The as cast hot ductility of low C -V, low C -V -Nb, and niobium microalloyed steels has been investigated using in situ melted tensile specimens, which were subjected to cooling rates and strain rates found typically in thin slab casting. Stress relaxation tests were performed on in situ melted specimens to monitor the kinetics of strain induced precipitation in the above steels. Although the addition of niobium to low C -V and low C -V -N steels increased the temperature at which ductility began to deteriorate, ductility was improved in the low temperature region of the austenite. This was attributed to a delay in NbV(C,N) precipitation in V -Nb steels. Increasing the nitrogen content did not influence the temperature at the onset of ductility loss in low C -V and low C -V -Nb steels, but did increase the temperature marking the onset of the ductility trough. Faster cooling rates led to a deterioration of ductility in low C -V -N and low C -V -Nb -N steels. The as cast ductility of low C -V and low C -V -Nb steels was superior to that as of cast peritectic C -Nb steel as well as reheated peritectic C -V and peritectic C -V -Nb steels. MST/4988Dr Banks

Hot ductility of TWIP steels

“…Banks et al [128] have noted that using cooling rates applicable to thin slab casting; a small addition of Nb (0.015%) can actually be of benefit to the hot ductility of high V (0.085%) low C (0.05%) steels with N contents in the range 0.0065 to 0.012%. Improved ductility in the temperature range 800~900 o C occurred and stress relaxation tests showed that this was due to Nb delaying dynamic and post dynamic precipitation of NbV(C, N).…”

“…V additions have been found to have a much smaller influence than Nb on the hot ductility provided the nitrogen level is low giving rise to a coarser precipitation. There is also some evidence that adding V to an Nb containing steel can improve its ductility by slowing down the rate of precipitation [128].…”

Hot ductility of high Al TWIP steels containing Nb and Nb-V

“…Nb additions have, unfortunately, been found to give rise to poor ductility in peritectic C, HSLA steels (~0.1-0.2%C) making it more difficult to avoid transverse cracking [15,16]. Adding V has on occasions enhanced the ductility in these HSLA steels because it delays precipitation [17,18]. However, when this was tried in the simpler high Al, TWIP steels, free of Ti or B, the Nb-V combination was found to be detrimental to hot ductility; presumably because it increases the amount of precipitation [4].…”

Characterization of Grain-boundary Precipitates after Hot-ductility Tests of Microalloyed Steels

“…Compositions and precipitation temperatures for many of the possible secondary phases in microalloyed steels have been stated in literature for many steel grades. [5][6][7][8][9][10][11] From a literature survey it can be stated that AlN and (Nb, V)CN are generally considered as some of the most detrimental phases for hot ductility. However, when drawing information from published works, it is to consider that a great influence on precipitation sequence and on kinetics is exerted by the actual microstructural condition and the thermal history of the steel.…”

“…[12][13][14][15][16] A relatively large number of papers available in literature refers to as cast steels tested by cooling at various rates from the melting temperature or by reheating the cast structure from room temperature. 1,2,[6][7][8][9]12,13,[17][18][19][20] Under these conditions, the coarse grain structure and the segregation of alloying elements at boundaries strongly affects precipitation. On the contrary, less attention has been paid to hot ductility of wrought steels.…”