Determination of Final Ferrite Grain Size During Multiple-Stage Controlled Cooling of Low-Carbon, Low-Alloy Steels

Ferrite grain size strengthening makes the predominant contribution to the overall strength of ferrite–pearlite structural hollow section steel grades. A fine ferrite grain size is achieved through a two-stage controlled cooling process. First, the material is rapidly cooled with water. This provides a large undercooling, which is the driving force for ferrite to form. The second stage involves slow natural (air) cooling, where the cooling rates and the transition temperature from water to air cooling are carefully controlled. This is crucial to prevent the formation of bainite or martensite. Ferrite grain sizes can be predicted for continuous cooling and isothermal transformation based on the prior austenite grain size, composition and cooling rate/isothermal transformation temperature. However, predictions for multiple-cooling-stage transformations have not been reported. In this work, EN S355-grade steel was used to study ferrite grain size development during continuous cooling, isothermal holding and complex (two-stage or multi-stage) cooling. Dilatometry and microstructure assessment was used to study the relationship between the final ferrite grain size and undercooling at which 40% of the ferrite formed. It was found that any changes in cooling rate/temperature (including a possible ‘bounce back’ in temperature due to latent heat formation) after 40% of the ferrite had formed had a negligible effect on the final ferrite grain size, assuming that re-austenitization or bainite formation was avoided.