K-CARBIDES PRECIPITATION IN TWO HIGH MANGANESE ALLOYS
Two high Mn alloys marked A (24Mn-1.2C-12Al) and B (28Mn-0.7C-8.6Al) were laboratory hot rolled. After rolling in material A coarser k-carbides were detected, sporadically on the austenite grains boundaries and more frequently on the ferrite ones. Reason was un-balanced Mn a C contents in superposition with the rolling deformations and finishing rolling temperature of 950°C. Hardness corresponded to 432HV30 on average. Using light microscope no carbides on grains boundaries were observed in B both after rolling and followed aging at 500°C/6´. After 6 minutes of aging the hardness of B was approx. by 9 % higher unlike the rolled state. The X-ray analysis nucleation of nano-size kcarbides confirmed. After aging of B at 500°C/30´ and/or 60´some coarser k-carbides were detected on ferrite-austenite boundaries and/or austenite-austenite interface. The hardness went up and corresponded to 343 and/or 350HV30. From the X-ray and microstructure analyses follows the 30´ and/or 60´ dwell supported both very fine and coarser k-carbides precipitation. The calculated stacking fault energies (SFE) of both materials complete all solution. The A showed by 9 mJ.m -2 higher SFE than the B. In the A alloy lower Mn content and higher Al one resulted to 2.8 times higher ferrite volume fraction in comparison with the B alloy.
The development of a new Fe-Mn-C austenitic steel for automotive applications
The current strong demand for vehicle lightening from the automobile sector requires flat carbon steel manufacturers to develop new advanced grades capable of fulfilling the increasingly stringent technical requirements of this market. Two basic approaches are possible: - short term strategies for incremental improvements in the mechanical properties of existing products which can then be produced in thinner gauge strips with equivalent functional properties, - longer term solutions involving the development of breakthrough products such as ultra high strength ductile austenitic steels or low-density steels alloyed with light elements (Al, Mg, Si). Arcelor is actively pursuing both these avenues of research. Restricting the discussion to the second point, it is clear that there are many technical hurdles and conflicting requirements to be overcome in order to produce a marketable product. Arcelor Research, in conjunction with TKS, has recently developed an ultra high strength Fe-Mn-C austenitic steel with excellent formability for automotive applications. The X-IP TM 1000 steel composition is optimised to provide the best compromise in ultimate tensile strength (>1000 MPa) and total elongation (>50%) at room temperature. These properties are achieved through the optimisation of the TWIP (TWinning Induced Plasticity) effect by careful control of the stacking fault energy (SFE) and the final microstructure. The austenite matrix is fine grained (grain size <10 µm for hot strips and <3 µm for cold strips), contains little or no cementite and is exempt from martensitic transformations under cold working. The steel can be processed on conventional industrial lines (continuous casting, hot and cold rolling and continuous annealing) in a wide range of formats. In this paper we present the factors that determine the choice of composition (phase stability diagrams and SFE modelling) and we describe the evolution of the microstructure at different stages in the production process. The relationship between the microstructure and the final mechanical properties is discussed. X-IP
Control of reflective cracking
The first is 'fatigue' cracking of the layer. This type of cracking is the result of the application of loads on the pavement and is controlled by appropriate thickness design which 'ill not be discussed in detail here. Thickness design includes not only the design strength and thickness of the stabilised, but other design elements such as delaying reflection cracking, or limiting stresses in the stabilised layer, by adopting a minimum thickness of overlying material. This may include an unbound layer utilising the 'upside down' pavement approach. The actual thickness required of the overlying material varies but for example PIARC (Ref.1) recently reported when the thickness of coated materials exceeds 20 cm the emergence of shrinkage cracks at the surface is generally uncommon; when it is 15 cm thick, the cracks emerge at the surface, but do not shorten the life of the surface course, especially if they are sealed. This experience has been gained 'with legal axle loads of 130 kN, considerably greater than the Australian legal axle load.