In this diploma thesis, the microstructure and the atomic assembly of a micro-alloyed HSLA trial heavy plate has been investigated and compared to the materials mechanical behavior. This has been conducted in thermo-mechanical rolling condition and four additional heat treatment conditions. The aim was to find the influence of austenite conditioning (''pancaking''), annealing (precipitation hardening, etc.) and ''normalizing'' (slow air cooling) on the material properties and microstructure. The micro- and sub-micro structures have been investigated using optical microscopy (LM), electron microscopy (SEM, TEM, HRTEM) and atom probe analysis (3DAP). An etchant that is suitable for distinguishing the individual constituents in the optical microscope has been found. Atom probe analysis revealed the local enrichment and general distribution of the chemical elements in shipping condition. The behaviour of various elements during annealing has been investigated. The evolution of precipitations during annealing has been traced by X-ray diffraction (XRD). The chemical composition of the middle segregation zone has been studied using a laser assisted local electrode atom probe (LEAP), a method for site-specific preparation in steel materials has been developed for this purpose. The measured mechanical properties have been compared to the corresponding values, calculated based on microstructural features.