Influence of cooling rate and MnS inclusions on hot ductility of steels

Abstract

The influence of cooling rate on the hot ductility of plain C-Mn steels containing 0'1-0'7%C has been examinedfor tensile specimens tested in the temperature range 1000-550°C. Increasing the cooling rate from 10 to 60 K min -1 in all cases caused the ductility to deteriorate. This deterioration was found to be associated with the presence of a finer sulphide distribution in the ferrite surrounding the austenite grains, as well as a reduction in the thickness of the ferrite covering. Decreasing the strain rate from 3 x 10-3 to 5·3 X 10-4 S-l led to a smaller influence of cooling rate. However, ductility was always poorer at the lower strain rate as grain boundary sliding was enhanced. Evidence is also submitted to indicate that MnS inclusions can encourage the production of deformation induced ferrite, leading to an earlier recovery in ductility at the low temperature end of the trough in the hot ductility curve. MSTj3758The authors are in the

Hot ductility of TWIP steels

“…This has become particularly important with the advent of thin slab casting, where cooling rates are much higher than in conventional continuously cast steel. Generally, the average cooling rate from the mould to the straightening temperature are 60~100 o C/min for conventional casting (220 to 250mm thick slab) and It is found that increasing the cooling rate to the test temperature after solution treatment results in deeper troughs Fig.2.20 [75]. This has been shown to be related to a finer sulphide re-precipitation at the γ grain boundaries produced by the higher cooling rate so that again the spacing between the particles is reduced making it easier for ductile cracks to link up and give failure [75].…”

“…This has been shown to be related to a finer sulphide re-precipitation at the γ grain boundaries produced by the higher cooling rate so that again the spacing between the particles is reduced making it easier for ductile cracks to link up and give failure [75]. It should be appreciated that for a given inclusion volume fraction, although finer inclusions will generally increase the energy required for ductile fracture at room temperature because they are less deformable, this does not apply to the present situation where the failures are intergranular and the linking up of the cavities is more important than their ease of formation [75].…”

“…Considerable work [75,77,78] has been carried out into the influence of cooling rate in the range 25 to 200 o C/min on hot ductility for a wide variety of steels including Ti containing steels. Invariably increasing the cooling rate results in worse ductility, Fig.2.21, and this is even so for plain C-Mn steels.…”

“…Sulphur can impair hot ductility by weakening the grain boundary regions for a variety of reasons (a)S segregation to the boundary [114], (b) the formation of low melting point Fe-S compounds [115] and (c) the effect of Mn sulphides on the formation of cavities [15], which then link up to produce low ductility intergranular failure. Although the segregation of atomic S may play a part in influencing the hot ductility of HSLA steels, there is much evidence to indicate that it is the sulphides that have the dominating influence on fracture [46,75,100,101].…”

Study on MnS Precipitation Thermodynamics of DL05 Cable Steel

“…Increasing the Mn content in the steel will increase the probability of the combination of sulfur and manganese, and the formula (1) in the equilibrium forward movement, which can hinder the formation of FeS and other low melting point compounds in the austenite grain boundaries. theoretically to achieve this goal, manganese sulfur ratio to reach 7 [3][4][5].…”