This study is concerned with issues related to laser welding of Si-Al type TRIP steels with Nb and Ti microadditions. The tests of laser welding of thermomechanically rolled sheet sections were carried out using keyhole welding and a solid-state laser. The tests carried out for various values of heat input were followed by macro- and microscopic metallographic investigations as well as by microhardness measurements of welded areas. A detailed microstructural analysis was carried out in the penetration area and in various areas of the heat affected zone (HAZ). Special attention was paid to the influence of cooling conditions on the stabilisation of retained austenite, the most characteristic structural component of TRIP steels. The tests made it possible to determine the maximum value of heat input preventing the excessive grain growth in HAZ and to identify the areas of the greatest hardness reaching 520 HV0.1.

This work presents the results of a microstructural characterization of welds in Nb-microalloyed TRIP steel with silicon partially replaced by aluminum. Tests of laser welding of thermomechanically processed sheet samples were carried out using keyhole welding and a solid-state laser. Welding penetration tests were conducted for heat input values between 0.037 and 0.048 kJ/mm. Identification of different microstructural constituents was carried out using light microscopy and scanning electron microscopy in the fusion zone (FZ), heat-affected zone (HAZ), and base metal. Special focus was put on the effect of cooling conditions on the stabilization of retained austenite in different zones. The intercritical, fine-grained, and coarse-grained regions of the HAZ were identified. It was determined that enriching austenite with carbon in the intercritical HAZ stabilizes this phase at a level close to the base metal, i.e., a 15% volume fraction. Despite a high cooling rate in the FZ and HAZ, interlath retained austenite is also present in these zones. The research involved microhardness measurements and characterizing non-metallic inclusions formed in the fusion zone. A good correlation between microstructures formed in different weld regions and microhardness results was obtained.

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The work concerns the studies on non-metallic inclusions occuring in laser-welded Si-Al TRIP steel containing Nb and Ti microadditions. Laser welding tests of 2 mm thick thermomechanically rolled sheets were carried out using keyhole welding and a solid-state laser. The results of laser welding in the air atmosphere for the heat input value of 0.048 kJ/mm are included. The distribution, type and chemical composition of non-metallic inclusions formed in the base metal, heat-affected zone, and fusion zone are analysed in detail. It was found that the base metal contains rare, fine oxysulphides. Their chemical composition was modified by rare earth elements. Numerous oxide inclusions of a various size and a chemical composition occur in the fusion zone. The dependence between a size of particles and their chemical composition was observed. A microstructure of steel was assessed using light microscopy and scanning electron microscopy techniques.Keywords: TRIP steel, non-metallic inclusions, laser welding, multiphase microstructure, retained austenite, fusion zone, heat-affected zone Praca dotyczy badań wtrąceń niemetalicznych występujących w przetapianej laserowo stali TRIP typu Si-Al zawierającej mikrododatki Nb i Ti. Próby spawania laserowego odwalcowanych termomechanicznie taśm o grubości 2 mm przeprowadzono z wykorzystaniem lasera na ciele stałym i techniki głębokiego przetopienia. Przedstawiono wyniki przetapiania laserowego pró-bek z energią liniową 0.048 kJ/mm w atmosferze powietrza. Szczegółowej analizie poddano rozkład, rodzaj i skład chemiczny wtrąceń niemetalicznych tworzących się w materiale rodzimym, strefie wpływu ciepła i strefie przetopionej. Stwierdzono, że w materiale rodzimym występują nieliczne, drobne wtrącenia siarczkowo-tlenkowe, których skład chemiczny został zmodyfikowany przez pierwiastki ziem rzadkich. W strefie przetopionej występują liczne wtrącenia tlenkowe o zróżnicowanej wielkości i składzie chemicznym. Zaobserwowano zależność między wielkością wtrąceń a ich składem chemicznym. Mikrostrukturę stali określono z wykorzystaniem mikroskopii świetlnej i skaningowej mikroskopii elektronowej.

The present study aims to characterize the weldability of a multiphase, automotive steel containing Al and Si additions from the point of view of its tendency to form non-metallic inclusions. Laser welding tests of 2-mm-thick sheets were performed using the keyhole-welding mode and a solid-state laser. The tests were carried out in air and with the use of an argon atmosphere. The distribution, type and chemical composition of the non-metallic inclusions formed in the base metal and fusion zones were analysed. The effect of applying the protective gas on the type and amount of non-metallic inclusions was determined using light and scanning electron microscopy. The chemical composition of the identified particles was assessed using the EDS method. It was found that a protective gas has a beneficial effect on reducing the non-metallic inclusions, but only to a limited extent. The boundary between the complex oxides and the pure aluminium oxides was determined to be 2-3 μm. Keywords: TRIP steel, laser welding, non-metallic inclusions, protective gas, Ar atmosphere, oxidation Namen te {tudije je opredeliti varivost s stali{~a tvorbe nekovinskih vklju~kov v ve~faznem jeklu, ki vsebuje Al in Si, za avtomobilsko industrijo. Izvedeni so bili varilni preizkusi z laserjem na 2 mm debelih plo~evinah z V zarezo. Preizkusi so bili izvedeni na zraku in v za{~itni atmosferi argona. Analizirana je bila razporeditev, vrsta in kemijska sestava nekovinskih vklju~kov v osnovnem materialu in v coni zlivanja. Vpliv uporabe za{~itnega plina na vrsto in koli~ino nekovinskih vklju~kov je bil dolo~en s pomo~jo svetlobne in vrsti~ne elektronske mikroskopije. Kemijska sestava najdenih delcev je bila ugotovljena z metodo EDS. Ugotovljeno je bilo, da ima za{~itni plin ugoden vpliv na zmanj{anje nekovinskih vklju~kov le v omejenem obsegu. Meja med kompleksnimi oksidi in~istimi oksidi aluminija je bila dolo~ena kot 2-3 μm.