An in-depth microstructural characterisation is usually required to establish the process-microstructure-properties relationship and optimize the material. However, it remains a challenge to differentiate and accurately quantify bcc phases characteristic of high strength steels (HSS): ferrite, bainite, martensite. To address this problem, we propose a correlative approach by using a combination of dilatometry, CCT diagrams, SEM, EBSD and in-depth chemical composition analysis. A multi-phase thick-section S690QT HSS, containing a mixture of ferrite, tempered bainite and tempered martensite was investigated. The phase fractions were found to vary significantly through the thickness due to cooling rate gradients and elemental segregation in the plate’s mid-thickness. The slower cooling in the mid-thickness led to a higher ferritic content compared to the outer plate areas. Furthermore, in mid-thickness, segregation of elements such as Mo, Mn and Cr strongly influence the phase transformations, resulting in higher martensitic and lower ferritic content in enriched than in depleted zones.