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Description
High-value speciality steels are often designed for specific large component engineering applications. These steels, which are typically heavily alloyed, offer excellent properties in terms of fatigue, toughness, high-temperature strength and corrosion resistance. However, due to the fact steelmakers are at the start of a multi-partner supply chain, feedback regarding component manufacture and performance in service is often limited. The high cost of production scale material manufacture can make the cost of development trials prohibitive. For this reason, it is desirable to use laboratory-scale trials and computer alloy design to optimise properties and process parameter selection.
In the speciality steel market there is a requirement for large diameter bar. Producing large diameter bar from ingot by hot rolling comes with many challenges including difficulties in achieving through-thickness uniformity. In order to achieve sufficient reduction during hot working larger ingots can be used, but increasing ingot size does not guarantee improved uniformity across the full bar section. During rolling, strain levels, strain uniformity and thermal homogeneity all decrease as bar diameter increases. These factors may manifest themselves in the final product as variation in grain size, second phase distribution and/or segregation levels.
Alloy design coupled with process modelling has been used to predict the extent of segregation during casting and reheating together with strain histories through the thickness of bar stock and microstructural development during hot working with the aim of optimising the process route to consistently achieve specification requirements throughout the product. The modelling predictions have been verified using rapid alloy processing facilities: laboratory ingots of 8kg were cast, homogenised and characterised. Samples were thermo-mechanically processed according to both surface and core time/temperature/strain conditions using a Gleeble HDS-V40. This paper presents case studies demonstrating the alloy design, modelling and RAP route.