Influence of thermal history on hot ductility of steel and its relationship to the problem of cracking in continuous casting

Abstract

A variety of heating and cooling programmes have been examined for plain C-Mn and high strength low alloy steels to examine their suitability in a hot tensile test for assessing the likelihood of transverse cracking occurring in the straightening operation. A tensile test temperature of 800uC was chosen for comparison, this being the temperature that generally results in poor ductility. For steels with 1?4-1?75%Mn, the simple procedure of heating to y1300uC to take all the microalloying additions into solution, followed by cooling to the test temperature, was found to be the easiest and most suitable. For high strength low alloy steels containing Ti and low Mn, high S steels, melting is required. For these steels, it is also advisable to have both primary rapid cooling followed by a slower secondary cooling stage, simulating more accurately the actual industrial operation. The addition of thermal oscillations to simulate slab roll contact makes the cycle even closer to the commercial process and generally led to a small decrease in reduction in area values. Because of its complexity, this latter method would not be generally recommended for steels showing wide trough behaviour (high Mn, peritectic C steels), and melting followed by primary and secondary cooling is sufficient. For narrow troughs (low Mn, low C steels), which require melting and where the minimum ductility will be at a temperature of .800uC, the more complex procedure will be required. It will be necessary to obtain the full hot ductility curve using a cycle that, as well as melting and having primary and secondary cooling, also incorporates commercial thermal oscillations or at least some limited thermal oscillations.

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon, Low Mn Steels

“…[4][5][6][7][8]13,14] Consequently, in situ melting, ''as-cast conditions'' are necessary to incorporate the total S content and thus avoid misleading results about the influence of S on hot ductility. [4,5,7,8,13,14] Banks [14] found that low Mn and high S steels, in particular, required in situ solidification test conditions, and hot ductility tests were improved further by incorporating thermal oscillations to better simulate the continuous casting process. It has been demonstrated that simulating the complex cooling patterns experienced during continuous slab casting affects hot ductility results.…”

Hot ductility of TWIP steels

“…This can be combined with the complex cooling patterns that are experienced in the secondary cooling zone before straightening, although unfortunately this has rarely been used. Some of the more recent work [41] has included both primary and secondary cooling to better simulate the commercial process rather than have one average cooling rate.…”

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

“…Ideally, to simulate the continuous casting and straightening operation more closely "in situ melting" should be used and two cooling rates, a fast cooling rate for the primary cooling, followed by a slower cooling rate for secondary cooling should be incorporated [14]. However, in-situ melting to obtain a satisfactory tensile specimen, free of porosity and choosing silica tubing which doesn't react with the Ti present in the steel encounters serious practical difficulties, so before embarking on this melting route, the advantages of melting over "reheating," need to be weighed up carefully in advance.…”

“…In this exercise two cooling rates, 12 o C/min min and 60 o C/min have been examined. Since 12 o C/min is often the cooling rate for the secondary cooling stage in the continuous casting process, an average cooling rate of 12 o C/min, may indeed, as has been shown from recent work [14] be the more suitable for simulating the industrial process.…”