City Research Online

Abstract

The current work investigated the influence of alloying composition and thermal treatment on the hot ductility of TWIP (twinning induced plasticity) steel and mechanical properties of HSLA (high-strength low-alloy) steel at room temperature. For TWIP steel, the present work simulated the continuous casting process in the laboratory using hot tensile tests at the temperature range of 700°C 1100°C to assess likelihood of transverse cracking. The tensile specimens were soaked at 1250°C then cooled at 12 or 60°C min-1 to the test temperature and then strained to failure at 3 x 10-3 s-1. Reduction of area was measured after failure to indicate whether the steel can be cast without transverse cracking occurring. The processing parameters involved the Ti/N content ratio and the cooling rate. Ductility was always good (reduction of area > 40% in the straightening temperature range 800–1000°C, the value required to avoid transverse cracking), independent of Ti/N ratio or cooling rate. A high product of [Ti][N] was found to give improved ductility and a higher N level gave some further benefit. The good ductility is due to B segregation strengthening the grain boundaries and the low S level (0.005%S) limiting the volume fraction of MnS inclusions and restricting AlN precipitation to the matrix. The addition of Nb is beneficial as the precipitation of Nb(CN), on the Ti particles at higher temperatures, will coarsen the precipitates so not only is the fine deformation-induced precipitation avoided but a coarser precipitation enhances ductility. Increasing the cooling rate resulted in a small improvement in ductility due to the refinement of the particle size in the matrix. For HSLA steel, three steels with 0.02%Al, 0.16%Al and 0.16%Al, 0.018%Nb have been examined and their strength and impact behaviour obtained. For the Nb free steels, 0.16%Al steel had a similar strength to the 0.02%Al containing steel~300MPa, but better impact behaviour (40°C lower ITT) with an impact transition temperature (ITT) of -90°C which is attributed to grain size refinement and refinement of the grain boundary carbides. Nb encouraged Widmanstatten ferrite and the formation of lower transformation products like martensite and cannot be recommended as a suitable addition to achieve the benefits of adding higher Al additions. The present work shows that the addition of Nb to this high Al containing steel, although beneficial to strength, giving a lower yield strength (LYS) of 385 MPa, close to that given by some of the control rolled steels, gives very poor impact behaviour with an ITT of only -30°C. The improvement of strength is mainly a result of precipitation hardening by NbCN with some benefit from grain refinement while the deterioration of impact behaviour is due mainly to the formation of lower transformation products, Widmanstatten ferrite and coarse grain boundary carbides. Increasing the cooling rate from 17 or 33°C min-1 resulted in a small deterioration in ITT due to the increase of the volume fraction of Widmanstätten ferrite. The finer grain size produced on control rolling prevented the formation of these deleterious phases

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	Doctoral Theses School of Science & Technology > School of Science & Technology Doctoral Theses School of Science & Technology > Engineering

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