Materials Science Forum Vols. 500-501

Abstract: Transformation induced plasticity (TRIP) steel combines high strength and high ductility that makes it particularly suitable for forming. Martensite within a ferrite matrix is usually obtained either by continuous casting of slabs followed by hot rolling (which is the fastest method, hence the most economical one, producing, however, relatively thick products) or by the continuous casting of slabs followed by hot rolling, cold rolling and annealing (the method used for thin products). High cooling rates, low coiling temperatures and low reduction during hot deformation were generally found to suppress the formation of polygonal ferrite and promote the presence of retained austenite. This paper focuses on development and modifications of two CMnSi-based TRIP steels with 0,23 % C;1,4 % Mn; 1,9 % Si; ( 0,08 % Nb) by means of laboratory thermomechanical processing. Description of experimental devices for the analysis of transformation plasticity under tensioncompression loading is given. Experiments were carried out on the simulator for thermaldeformation cycles SMITWELD and TANDEM was used for thermomechanical processing on the laboratory rolling mill. The maximum volume fraction of retained austenite and the resulting optimum combination of tensile strength and ductility were achieved in testing heats. Special attention was paid to volume fraction changes of single phases and to changes in morphology of phases. The results suggest that rather short isothermal bainite transformation times are sufficient to obtain TRIP microstructure. The influence of parameters of thermomechanical processing such as the amount of strain, forming temperature and austenitization time and temperature on microstructures of TRIP steels were evaluated.

Abstract: The recent worldwide surge of steel consumption, mainly of low-strength carbon grades, has created raw-materials shortages and price increases. These supply-demand strains could be relaxed by satisfying engineering needs with less steel. However, materials used for such a substitution must combine high weight reducing potential with low cost. Microalloyed (MA) steels are cost- effective substitutes, since their high strength is the result of grain refinement and precipitation hardening. The optimum alloy design of MA steels combines superior properties with lowest processing cost. The growing use of EAF and thin slab casting technology improve the economics of MA steels, especially when alloyed with vanadium. The monetary value of weight reduction is sufficient to increase the profitability of steel makers and to lower the material cost to steel users. This “win-win” situation is financed by the elimination of efforts spent in producing inefficient steel, yielding an increase in wealth formation. To gain acceptance of substitution by the consumer, a long-term strategic plan is needed to be implemented by the beneficiaries – steel producers and steel users. The successful substitution is of importance to the national economy, resources and energy conservation, and the environment. Since microalloyed steels, used as a replacement for carbon steels, offer low-cost weight savings, they deserve to be classified as advanced structural materials.

Abstract: Among modern weldable high strength steels, low carbon microalloyed steels have been widely used for linepipe, construction, and automobile industries. One of the major technical components to successfully produce these steels is to effectively use precipitation strengthening. In the present paper, the effect of an aging treatment on the microstructure and mechanical properties of a low carbon Nb-microalloyed steel is analyzed.

Abstract: The possibility of mechanical properties improvement in cast low carbon manganese steels for thin-walled castings via appropriate microalloying and heat treatment was studied. The steels (0.15 C and 1.2 Mn) microalloyed by vanadium, titanium and niobium were undergone the solution heat treatment. Mechanical testing and detailed microstructural analyses were performed using light, transmission and scanning electron microscopy. Precipitation processes in individual steels were discussed and the steel with the most promising composition was selected for following experiments.

Abstract: The mechanical properties of a bainitic microstructure with slender ferrite plates (20-65 nm in thickness) in a matrix of carbon-enriched retained austenite were characterized. The microstructure is generated by isothermal transformation at temperatures in the range 200-300°C. A yield strength as high as 1.5 GPa and an ultimate tensile strength between 1.77 to 2.2 GPa was achieved, depending on the transformation temperature. Furthermore, the high strength is frequently accompanied by ductility (£ 30%) and respectable levels of fracture toughness (< 45 MPa m0.5). This unusual combination of properties is attributed to the exceptionally fine scale of the carbidefree bainitic microstructure and the associated retained austenite.

Abstract: To improve the strength properties of vanadium bearing low carbon steel, nitrogen is often added to the liquid steel. The source of the nitrogen addition can be in many different forms. The recovery of nitrogen from the addition is variable due to the low solubility of nitrogen in steel. In this work, nitrogen-enriched alloy (Nitrovan) was added under open atmosphere. To deduce the nitrogen role, two alloys were chosen that having the same vanadium content. One of them was Ferro-Vanadium as a source of vanadium, whereas Nitro-Vanadium used as a source of vanadium and nitrogen. Ferro-vanadium as well as Nitro-vanadium was added separately in the ladle after completely melting of carbon steel and proper superheat using 100 Kg induction furnace. The effect of adding nitrogen-enriched alloy on mechanical properties of the steel was investigated. For this purpose, four heats were produced and cast into sand moulds. The general trend of results shows higher mechanical properties through increasing nitrogen content. The experimental work indicates that enhanced nitrogen content promotes the precipitation of V(C,N) and decreases the particles size of V(C,N) precipitates. Also, under the same level of vanadium content, the tensile strength and yield strength of the nitrogen-enhanced steels increases consistently compared to the steels added 80% Ferro-Vanadium. An empirical formula, correlating the mechanical properties of the steel and its composition, was obtained.

Abstract: The chemical composition of a new vanadium-nitrogen containing steel grade (17Mn2NV) and process technology for production of At500S-class reinforcing bar, have been designed. The steel alloyed with vanadium and nitrogen possesses a uniquely high level of strength, fatigue, high ductility and notch toughness. At the same time it possesses good high temperature properties, resistance to stress-corrosion and mechanical ageing and its lower carbon level should ensure good weldability. Production and application of this reinforcing steel will enable considerable improvement in the quality, reliability and longevity of reinforced concrete structures, used in extreme Arctic and seismic regions and will ensure considerable lowering of cost.

Abstract: This article offers an overview on the ways and means of the application of Nb in quenched and tempered high-strength steel plates. Thereby, the outstanding role of Nb to control the austenite microstructure during rolling or heat treatment and to contribute to effective precipitation hardening is discussed. It is shown, that Nb is very effective to retard the transformation processes during quenching. For high-strength constructional steels with up to 1100 MPa yield strength and for wear resistant steels, the improvement in strength and toughness properties in Nb microalloyed steels as compared to Nb-free steels is pointed out. A remarkable effect is the improvement in toughness and brittle fracture resistance due to a very fine microstructure and finely dispersed Nb carbonitrides in a martensitic microstructure. Fields of application for the new Nb microalloyed high-strength structural steels are presented too.

Abstract: Hot- and cold-rolled steel sheets are the most important substrate materials for enamelling applications. Currently, industry is actively seeking ways to widen the area of application of hotrolled steel sheets for double-face enamelling. Another important task is search for new steel compositions that might substitute for currently used open-coil annealed, cold-rolled steel sheet for direct white enamelling applications. The purpose of this research project was the investigation of influence of chemical composition and processing conditions of several microalloyed steels on mechanical properties and fish-scale resistance of hot and cold rolled sheets. The results show that a high fish-scale resistance of the hot-rolled steel is achieved by creating a high volume fraction of fine precipitates (nitrides, carbides, carbonitrides, sulphides, etc.). By partial replacement of titanium with other carbide and nitride forming elements, such as niobium and/or boron, the suitability of cold-rolled IF steels for white direct-on enamelling can be remarkably improved without open-coil annealing. However, the processing conditions have to be tuned to ensure high fish-scale resistance without impairing formability.