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. 1. Introduction Constantly rising prices of energy sources and environmental aspects force car manufacturers to reduce fuel consumption. The basic way to achieve this goal is to reduce a complete vehicle weight. To this end, manufacturers seek new grades of materials, mainly high-strength steels, enabling the significant reduction of cross sections of both structural and panelling materials. In addition, such materials should demonstrate technological processability, that is, primarily forming and welding. Recently, resistance welding is often replaced by laser welding. Materials which meet the aforesaid requirements include Advanced High-Strength Steels (AHSS) dedicated for automotive industry. They are one of the most important achievements of today’s metallurgy. Depending on their microstructure, steels can be divided into dual phase steels (DP), complex phase steels (CP), martensitic steels (MS), and steels with the transformation induced plasticity effect (TRIP) [1–3]. The greatest prospects of development due to the effective combination of high strength and plasticity characterise multiphase TRIP-aided steels. The microstructure of these steels is composed of a ferritic matrix containing bainitic-austenitic islands with a dozen or so fraction of thermally stable retained austenite which, however, is mechanically unstable. The retained austenite undergoes a martensitic transformation [2–4], which partly takes place during the cold technological forming of sheets, in a car manufacture process. The remaining retained austenite undergoes a martensitic transformation only in the case of greater deformations, for example, caused during a car
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