Ridging in stabilized ferritic stainless steels: the effects of casting and hot-rolling parameters

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, auditorium L10. Remote connection: https://oulu.zoom.us/j/62656362296

Topic of the dissertation

Ridging in stabilized ferritic stainless steels: the effects of casting and hot-rolling parameters

Doctoral candidate

Master of Science Suresh Kumar Kodukula

Faculty and unit

University of Oulu Graduate School, Faculty of Technology, Materials and Mechanical Engineering Research Unit

Subject of study

Material Engineering

Opponent

Doctor Bevis Hutchinson, Swerim AB

Second opponent

Professor Pasi Peura, Tampere University

Custos

Professor Emeritus David Porter, University of Oulu

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Ridging in stabilized ferritic stainless steels: the effects of casting and hot-rolling parameters

The appearance of ferritic stainless steels (FSS) is deteriorated when the surface defect known as ridging appears after sheet forming. This research concerns the influence of various sheet production conditions on the severity of ridging in FSS stabilized with titanium and niobium.
A method for quantifying ridging was developed, the combined effect of calcium (Ca) treatment and electromagnetic stirring (EMS) on slab structure and ridging was studied, the influence of hot-rolling finishing temperature on microtexture and ridging was established, and a model relating microtexture to ridging intensity was developed.
The results showed that when casting without Ca treatment, even in the absence of EMS, the slab macrostructure is completely equiaxed and the final rolled, and annealed sheet has a high resistance to ridging. A similar situation prevails in the presence of a small Ca addition provided EMS is applied. However, accurate control of Ca content is difficult, and higher Ca contents lead to slabs having increasingly columnar grain structures and poor ridging resistance even with EMS. Under standard rolling with a finishing rolling temperature of 1000 °C just a two-fold increase in Ca content leads to a notable reduction in ridging resistance despite the use of EMS. Without EMS, the ridging resistance under these conditions is the worst of all the cases tested. However, lowering the hot-rolling finishing temperature to 810 °C increases the amount of recrystallization in the annealed hot band, which randomizes the texture and leads to cold-rolled and annealed sheet that is practically ridging free.
To further understand the mechanism of ridging and its dependence on the direction of strain, the evolution of macrotexture during 15% strain in both the rolling and transverse direction was studied in sheet samples with different susceptibilities to ridging. No correlation was found between macrotexture, e.g., γ-fiber intensity, and the severity of ridging. The microtextures of two samples with similar macrotextures but widely differing resistance to ridging were studied using electron backscattered diffraction. The detrimental influence of clusters of grains with low r-values on ridging was substantiated by predicting surface ridging profiles from the orientations of all grains through the thickness and across a representative width of sheet.
Last updated: 15.11.2021