Studium vlivu válcování za studena a žíhání na mikrostrukturu a mechanické vlastností pásů z vybraných HSLA ocelí

Abstract

The PhD thesis is focused on the study of microstructural changes and their effects on the resulting mechanical properties of selected strips from microalloyed steels after cold rolling and annealing with purpose for the design, optimization and recommendations of processing conditions in cold strip rolling mills. Samples of micro-alloyed steels in the form of strips with dimensions of approximately 4 x 25 x 400 mm were in multiple passes rolled in the range of relative height reduction 5 to 75 %. Particular strains were realized at room temperature in the hydraulically prestressed laboratory mill. Afterwards the laboratory mill products were annealed in the vacuum furnace with the protective gas atmosphere consisting of N2+H2. The annealed samples, as well as the work-hardened samples (with the cold rolled structure) were tensile tested at room temperature to gain mechanical properties. The obtained results - yield strength, ultimate tensile strength and their ratio expressed as well as elongation in percentage and in selected cases hardness were summarized were summarized in dependence on cold deformation before annealing. Using conventional optical microscopy and in selected cases electron microscopy the cause of mechanical properties development were discussed. The experiment clearly proved that the appropriate option of parameters of cold deformation and subsequent recrystallization annealing can effectively influence the complex of mechanical properties of investigated steels. Strength properties may be influenced in an average interval of about 330 MPa and plastic properties in the range of approximately 16 %. Maximum strength and minimum of plastic values was achieved at mode with the lowest applied annealing temperature after deformations which in the material are unable to accumulate enough energy for the next course of recrystallization. On the contrary, relatively high values of plastic properties are achieved by mode with the highest applied annealing temperature. Based on structural analysis the causes of mechanical properties development could be discussed. It was confirmed that the cold rolling in combination with the applied annealing modes significantly affects not only the resulting ferrite grains character, but also distribution, size and shape of precipitates. Those in most cases, effect of cold deformation during subsequent annealing grew, which reflected a decrease in their surface density and, in selected cases, changing the morphology. Especially in the case of steel S500MC was also showed the occurrence of new precipitates, which in the matrix in the state after hot rolling were not present. Given that the steel S460MC with very similar chemical composition was not the occurrence of emerging precipitates in the scale observed, it can be stated that the availability of new particles is associated not only with the applied degree of cold deformation in combination with a suitable annealing schedule, but also with history of the hot rolling process. On the basis of conducted experiments it may be said that a microstructural mechanism which affects in a dominant way the resulting mechanical properties is the mechanism of strain hardening of the metal, in combination with softening caused by recrystallization. Process of dispersion strengthening can partly compensate the softening effect of recrystallization (i.e. subdue the intensity of fall in strength properties) For all three studied steels the mathematical description of hardening curves for maximum height relative deformation applied to 75 %, influenced by the initial state of structure after hot or cold deformation in combination with recrystallization annealing at 650 ° C was performed. Based on the statistical methods was found that the studied steels S420MC and S460MC can be in operating conditions through the appropriate choice of used parameters treated to the three grades of strips steels designated for subsequent cold forming (according to EN 10268). The obtained results may find applications in real conditions during optimization of heat treatment investigated microalloyed steels in cold strip rolling mills. From a theoretical point of view are interesting findings of the effect of cold deformation in combination with annealing to the character of precipitates, which is area currently mapped only sporadically.