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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.

Two 5Mn-1.5Al TRIP steels with and without Nb microaddition were developed in the present study. The steels contain bainite, martensite, interlath retained austenite and martensite- austenite islands. The paper presents the results of the compression tests carried out at various temperatures using the Gleeble simulator. To analyze the kinetics of static recrystallization in these steels, a softening kinetics were determined in a double-hit compression test. It was found that the dynamic recovery is a main thermally activated process occurring during hot deformation. The Nb microalloyed steel has higher flow stresses and peak strains than the Nb-free steel. A solute drag effect of Nb results in a slower softening kinetics of Nb containing steel. The effects of Mn on the retardation of Nb(C,N) precipitation and hot deformation characteristics are also discussed.

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 multistep deformation and isothermal holding of the steel at 400°C. Microstructural features were revealed using light microscopy and scanning electron microscopy techniques.

The electrochemical corrosion properties of 26Mn-3Si-3Al and 27Mn-4Si-2Al austenitic steels in two different states were studied in 0.1 M H 2 SO 4 and 3.5% NaCl using potentiodynamic polarization tests. The effect of cold deformation on the microstructure and corrosion behavior of steels was analyzed. In acid solution, both steels exhibited lower corrosion resistance than in chloride solution independently on the steel state (hot-rolled, cold-worked). Cold deformation decreases the corrosion resistance, though this effect is smaller than the effect of chemical composition related to the combined Al + Si addition. All steels showed the evidence of pitting corrosion. The intensive dissolution of Fe and Mn takes place in the acid medium.

WALCOWANE TERMOMECHANICZNIE STALE ŚREDNIOMANGANOWE ZAWIERAJĄCE AUSTENIT SZCZĄTKOWYChemical composition of four medium-Mn steels containing a various Mn content (3 and 5%) have been proposed in the present work. The two steels are base steels whereas the other two contain Nb microaddition. Thermomechanical rolling tests of 3.3 mm sheets have been carried out using a semi-industrial hot strip rolling line. Detailed investigations of the identification of structural constituents using light microscopy and scanning electron microscopy techniques have been performed. X-ray method has been applied to determine an amount of retained austenite and its C content. Significant microstructural parameters were revealed using an EBSD technique. It was found that the Mn addition affects strongly a microstructure type, stability of retained austenite and mechanical properties determined with a static tensile test. The steels containing 3% Mn are characterized by a good combination of strength and ductility whereas the tensile strength up to 1300 MPa is possible to obtain for the higher Mn content steels.Keywords: medium-Mn steel, bainitic steel, thermomechanical rolling, retained austenite stability, controlled cooling W pracy zaprojektowano cztery składy chemiczne stali średniomanganowych zawierających 3 i 5% Mn. Dwie stale to stale bazowe, a pozostałe dwie zawierają mikrododatek Nb. Przeprowadzono próby walcowania termomechanicznego taśm o grubości 3.3 mm, stosując półprzemysłową linię walcowania na gorąco. Przeprowadzono szczegółowe badania identyfikacji składników strukturalnych z zastosowaniem mikroskopii świetlnej i skaningowej mikroskopii elektronowej. Udział austenitu szczątkowego i stężenie C w tej fazie wyznaczono metodą rentgenowską. Metoda EBSD została użyta do ilościowego wyznaczenia istotnych parametrów mikrostrukturalnych. Stwierdzono, że dodatek Mn ma silny wpływ na rodzaj otrzymanej mikrostruktury, stabilizację austenitu szczątkowego oraz własności mechaniczne wyznaczone w statycznej próbie rozciągania. Stale zawierające 3% Mn charakteryzują się dobrym połączeniem wytrzymałości i plastyczności, a stale o wyższym stężeniu Mn pozwalają uzyskać wytrzymałość na rozciąganie do 1300 MPa.

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.

Detailed studies on microstructure-property relationships of thermomechanically processed medium-Mn steels with various manganese contents were carried out. Microscopic techniques of different resolution (LM, SEM, TEM) and X-Ray diffraction methods were applied. Static tensile tests were performed to characterize mechanical properties of the investigated steels and to determine the tendency of retained austenite to strain-induced martensitic transformation. Obtained results allowed to characterize the microstructural aspects of strain-induced martensitic transformation and its effect on the mechanical properties. It was found that the mechanical stability of retained austenite depends significantly on the manganese content. An increase in manganese content from 3.3% to 4.7% has a significant impact on the microstructure, stability of γ phase and mechanical properties of the investigated steels. The initial amount of retained austenite was higher for the 3Mn-1.5Al steel in comparison to 5Mn-1.5%Al steel-17% and 11%, respectively. The mechanical stability of retained austenite is significantly affected by the morphology of this phase. case, the phase transformation kinetics depends significantly on the austenite deformation degree [5,8,9]. In case of the first generation AHSSs, the main advantage of the TMP is the ability to refine the ferrite grain size by controlling the austenite pancaking [10,11]. Moreover, the thermomechanical treatment can increase the amount of retained austenite to obtain the optimal TRIP (TRansformation Induced Plasticity) effect [12,13]. In case of medium manganese steels (third generation AHSS), the TMP is also a good alternative for the cold-rolling process [14][15][16]. However, most publications on medium-Mn steels concern the cold-rolling process and subsequent inter-critical heat treatment.Cold-rolled medium-Mn TRIP steels are susceptible to plastic instabilities associated with dynamic strain aging (DSA) and serrated flow (PLC-Portevin-Le Chatelier) effects due to the heat treatment required after cold-rolling. From a technological point of view, the PLC and DSA effects must be avoided. The DSA phenomenon gives rise to non-homogeneous plastic flow during the sheet-forming processes and may lead to surface defects on formed parts [17][18][19]. Our previous reports on thermomechanically processed medium-Mn sheet steels indicate that the problem can be avoided [7].Newly developed fine-grained ferrite-austenite or bainitie-austenite steels contain manganese in a range of 3-12%, while carbon content is ca. 0.1-0.2%. These steels contain also aluminum and silicon (1-3%) additions which delay the carbides formation during the bainitic transformation. The increased Mn amount leads to obtain the high fraction of retained austenite (~10-30%). The Mn addition also increases the carbon solubility and lowers the cementite precipitation temperature [20][21][22]. Al is added to partially replace silicon due to the problems related to galvanizing, hot-rolling and welding [23][24][25]. However, Gir...