Influence of titanium and nitrogen on hot ductility of C–Mn–Nb–Al steels

Abstract

T he influence of small additions of titanium on the hot ductility of C-Mn-Nb-Al steels has been examined. T itanium and nitrogen levels varied in the ranges 0•014-0•045 and 0•004-0•011 wt-%, respectively, so that a wide range of T i/N ratios could be studied. T he tensile specimens were cast and cooled at average cooling rates of 25, 100, and 200 K min−1 to test temperatures in the range 1100-800°C and strained to failure at a strain rate of 2×10−3 s−1. It was found that ductility in the titanium containing niobium steels improved with a decrease in the cooling rate, an increase in the size of the titanium containing precipitates, and a decrease in the volume fraction of precipitates. Coarser particles could be obtained by increasing the T i/N ratio above the stoichiometric ratio for T iN and by testing at higher temperatures. However, ductility was generally poor for these titanium containing steels and it was equally poor when niobium was either present or absent. For steels with ~0•005 wt-%N ductility was very poor at the stoichiometric T i/N ratio of 3•4 : 1. Ductility was better at the higher T i/N ratios but only two of the titanium containing niobium steels gave better ductility than the titanium free niobium containing steels and then only at temperatures below about 950-900°C. One of these steels had the lowest titanium addition (0•014 wt-%), thus limiting the volume fraction of fine T i containing particles and the other had the highest T i/N ratio of 8 : 1. However, even for these two steels ductility was worse than for the titanium free steels in the higher temperature range. T he commercial implications of these results are discussed.MST /4232

The Influence of Ti on the Hot Ductility of Nb-bearing Steels in Simulated Continuous Casting Process.

“…In an attempt to reduce the frequency of these surface defects, extensive studies on the high temperature embrittlement of microalloyed steels have been undertaken since early 1980's by Suzuki et al, 1,2) Ouchi and Matsumoto 3) and by Mintz and his coworkers, [4][5][6][7][8] among many others (see a recent review in Ref. 8)).…”

“…It is well known that the hot ductility trough in microalloyed steels is controlled by deformation-induced ferrite film in the two phase austenite/alpha-ferrite region and grain boundary sliding in the low temperature region of austenite phase. [1][2][3][4][5][6][7][8] The latter is usually encouraged by fine precipitation. Moreover, this low ductility can be almost fully recovered at the high temperature region of austenite phase due to the occurrence of dynamic recrystallisation.…”

“…Moreover, this low ductility can be almost fully recovered at the high temperature region of austenite phase due to the occurrence of dynamic recrystallisation. 3,[5][6][7][8] Some practical experiences indicate that the addition of Ti reduces the incidence of transverse cracking and improves the surface quality. 8,9) Turkdogan 10) suggested that in the Nbmicroalloyed steels most of Nb can precipitate on the coarse TiN particles before the straightening stage of the slab, leaving less Nb in solution to precipitate out in a more detrimental form during straining.…”

Effect of High Temperature Deformation on the Hot Ductility of Nb Microalloyed Steel

“…[3][4][5][6][7] Even though extensive work has been done to resolve this problem, material losses because of hot cracking still persist. The pro cess of crack removal reduces productivity, and cracks can even lead to scrapping of a coiI.…”

“…A considerable amount ofresearch has been conducted to improve the hot ductility of Nbcontaining steels, mostly by the addition of alloying elements such as Ti, [6,79,81,113] Ca, [143] Zr,[I13] and y [113]. Boron has been considered as another possible element likely to improve the hot ductility in the Nb-containing steel.…”