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The chemical composition of the C–Mn–Si steel with Nb and Ti microadditions was established. The rolling and cooling conditions of the thermomechanical processing of DP-type and TRIP-type sheets were developed. The conditions of the thermomechanical processing were selected on a basis of the determined DTTT c -diagram of plastically deformed undercooled austenite and dissolution kinetics of TiN and NbC in the austenite. It was found that thermomechanically processed sheets have more fine-grained structure and better mechanical properties than after the classical underhardening or isothermal quenching from the temperature slightly higher than A c1 of the investigated steel.

Strain hardening behavior and the corresponding microstructure evolution of the high-manganese steel with additions of Si and Al were investigated in this study. Thermomechanically processed and solution-heat-treated sheet steels were compared under conditions of interrupted tensile tests. Relationships between microstructure and strain hardening were assessed for different strain levels using light microscopy and scanning electron microscopy techniques. It was found that the deformation of both steels at low strain levels was dominated by dislocation glide before the occurrence of mechanical twinning. The amount of twins, slip lines, and bands was increasing gradually up to the point of necking. As the strain level increased, dislocation density within twinning areas becomes higher, which enhances the strength, the work hardening exponent, and the work hardening rate of the investigated high-manganese sheet steels.

Praca dotyczy problematyki spawania laserowego stali typu CPW 800 z mikrododatkami Ti i Nb. Próby spawania laserowego blach walcowanych termomechanicznie prowadzono techniką głębokiego przetopienia z wykorzystaniem lasera na ciele stałym. Przeprowadzono próby przy różnej energii liniowej procesu, a następnie wykonano badania metalograficzne makroskopowe i mikroskopowe oraz oceniono twardość materiału rodzimego, strefy wpływu ciepła i spoiny. Określono wpływ energii liniowej spawania na szerokość spoiny. Twardość spoiny wynosi około 400 HV1 niezależnie od energii liniowej spawania, a dominującym składnikiem strukturalnym złącza jest martenzyt niskowęglowy. Przedstawiono wyniki badania wytrzymałości na rozcią- ganie połączeń spawanych.

W artykule dokonano przeglądu wysokowytrzymałych stali AHSS stosowanych w motoryzacji na odpowiedzialne elementy struktury nośnej samochodów. Przeanalizowano zasady projektowania składu chemicznego i mikrostruktury stali wielofazowych typu DP, TRIP i CP, a także właściwości mechaniczne i technologię wytwarzania blach. Scharakteryzowano metody łączenia blach cienkich dla motoryzacji. Szczególną uwagę poświęcono spawalności stali AHSS, wskazując na główne trudności technologiczne występujące podczas zgrzewania oporowego i spawania laserowego. Artykuł uzupełniono wynikami badań własnych spawalności stali CP i TRIP. Weldability of high-strength multiphase AHSS steels Abstract The review of advanced high-strength AHSS steels used in automotive industry for important elements of the body-in-white has been presented in the paper. The principles of the design of chemical composition and microstructure of multiphase DP, TRIP and CP-type steels as well as mechanical properties and manufacturing tech- nology of sheets have been analysed. Joining methods used for automotive steel sheets have been characterized. The special attention has been paid to weldability of AHSS steels with indicating major technological problems occurring during their resistance spot welding and laser welding. The paper contains some authors’ results on weldability of CP and TRIP steels.

The paper addresses the macro- and microsegregation of alloying elements in the new-developed Mn-Al TRIP steels, which belong to the third generation of advanced high-strength steels (AHSS) used in the automotive industry. The segregation behaviour both in the as-cast state and after hot forging was assessed in the macro scale by OES and by EDS measurements in different structural constituents. The structural investigations were carried out using light and scanning electron microscopy. A special attention was paid to the effect of Nb microaddition on the structure and the segregation of alloying elements. The tendency of Mn and Al to macrosegregation was found. It is difficult to remove in Nb-free steels. Microsegregation of Mn and Al between austenite and ferritic structural constituents can be removed.

The heat treatment conditions for the medium-carbon steel strengthened by plastic deformation-induced martensitic transformation of the retained austenite (Transformation Induced Plasticity or TRIP) were developed. After heat treatment according to an optimum variant, the structure of the investigated steel is ferritic bainite, with a 23% fraction of retained austenite showing thermal stability. It was found that the bainite contains a very small fraction of carbides. Their precipitation is hampered by a Si concentration of 1.35%. The retained austenite occurs as regular grains of 3 m in diameter or as films between bainite laths. On the basis of the tensile test, it was found that the steel is characterised by high strength properties (UTS = 1120 MPa, YS = 790MPa) and excellent ductile properties (TEL = UEL = 34%). The high strength and excellent ductile properties as well as the instantaneous work-hardening exponent n ~ 0.4 show that optimum utilisation of the TRIP effect improves the mechanical and technological properties of the steel.

The work deals with dilatometric studies of a new-developed advanced high-strength bainitic 3Mn–1.5Al steel. Ferritic, bainitic and martensitic phase transformations are investigated in detail in respect of their temperature range forming and microstructures produced under various conditions of both continuous and isothermal cooling. The equilibrium temperatures of A e1 and A e3 and phase composition of the investigated steel were initially calculated whereas critical temperatures of A c1 and A c3 as well as the decomposition of retained austenite were determined upon heating. The major tests consisted of controlled cooling of undeformed or plastically deformed austenite using the dilatometer within the cooling rate range of 2–0.5 °C s−1. The effects of the cooling rate and deformation at temperatures of 900 and 1,050 °C on the phase transformation behaviour and microstructure were explained. The final experiment was carried out using a thermo-mechanical simulator under conditions of multi-step deformation and isothermal holding of the steel at 400 °C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques.