Materials Science Forum Vols. 500-501

Abstract: High performance commercial micro alloyed steels contain elements such as vanadium, which leads to a fine dispersion of vanadium carbide precipitates. The precipitation state, in terms of volume fraction and size distribution, plays a significant role in final mechanical properties of the material. Different austenitisation heat treatments were performed on a model ternary alloy FeCV. Precipitation states were characterised combining different experimental techniques. TEM was used to identify the chemical composition of observed precipitates. ICP mass spectroscopy was performed to measure the volume fraction of precipitates. The size distribution was studied by SEM. Results are characteristic of a coarsening regime.

Abstract: Austenite grain growth in microalloyed steels is governed by the coarsening of fine precipitates present at grain boundaries below the grain coarsening temperature. Zener model is widely used in metals to describe the pinning effect of second phase particles precipitated in the matrix. In this work it has been discussed whether grain boundary or volume diffusion is the rate controlling process for the coarsening of the niobium carbonitrides. Calculations on austenite grain growth kinetics, obtained coupling Zener theory and both rate controlling processes of precipitate coarsening, have been compared against experimental austenite grain size results under nonisothermal heating conditions. In this sense, it has been concluded that the coarsening of niobium carbonitrides is mainly controlled by volume diffusion of Nb in austenite.

Abstract: A database for thermodynamic information of sulfides and carbo-sulfide in steels has been developed, where the thermodynamic properties of the relevant constituting sub-system of the complex system have been assessed by the CALPHAD (Calculation of Phase Diagrams) approach. The utility of this database is demonstrated in examples of phase diagram calculations of low carbon steels and stainless steels. The calculated results are in good agreement with experimental data. The developed thermodynamic database would be useful in predicting the formation of sulfides and carbo-sulfides in the commercial steels.

Abstract: From a “macroscopic” point of view, steel composition is assumed to vary smoothly along its microstructure. A closer look reveals that, on the atomic level the material composition does not change so smoothly. Single atoms jump randomly along the crystal lattice due to their thermal energy. These random jumps create sporadic zones of the crystal with higher concentration of certain species, and they are responsible for many phenomena, such as precipitation, Ostwald ripening, some phase transformations… This paper proposes a model to simulate the evolution of C-N-V precipitates in microalloyed steels heat treated in the range of warm temperatures (800-900 °C); when the matrix is austenite (fcc), thus taking into account for the local composition fluctuations. The model works by dividing the space into very small cells, containing a single atomic cell each. If during the random movement of atoms a cell that touches a precipitate reaches some critical composition, it is very easy to stick it to the precipitate by changing its “phase”. But it is also possible that some atoms escape from the precipitate by jumping to the austenitic matrix. Both processes happening simultaneously, and which one is leading depends on the atoms energy, i.e. system temperature.

Abstract: The effects of matrix microstructure and features of non-metallic inclusion (morphology, type, volume fraction and size) on cleavage initiation in medium-carbon Ti-V-N and V-N microalloyed resulphurized forging steels have been determined by examining fracture surfaces produced in room temperature Charpy impact tests. The steels were generally Al-deoxidised but one V-N steel was Si-deoxidised. It has been found that, in the Ti-treated steel, having a ferrite-pearlite microstructure, brittle fracture initiation occurred at cracked coarse (Ti,V)(C,N) single phase or [(Ti,V)(C,N)/Al2O3/MnS] multi-phase inclusions. In the Ti-free steels, cleavage initiation was dependent on matrix microstructure and non-metallic inclusions. In the low strength Ti-free steels, with a ferrite-pearlite microstructure, the absence of a continuous grain boundary ferrite layer led to initiation from interfacing pearlite colonies. For the bainitic microstructure, cleavage initiated close to the notch, but the microstructural feature responsible could not be identified. For the ferritepearlite microstructure in the Si-deoxidised V-N steel, cleavage initiated at cracked Mn-Al-(Ca) silicate inclusions. The higher matrix strength and more continuous nature of grain boundary allotriomorphic ferrite in the V-N steel deoxidised with Al was associated with cleavage initiation from V-rich (V,Ti)(C,N)-containing inclusions. These were generally of smaller size than those in the Ti-treated steels.

Abstract: The relationship between mechanical properties and pearlite microstructure was investigated using various heat treatments on a hypereutectoid steels containing 1% carbon with different levels of vanadium and silicon. Specimens were heat treated at various temperatures ranging from 900 to 1200°C and transferred to salt bath conditions at 550, 580 and 620°C to examine the structural evolution of pearlite. The results show that the thickness of the cementite network increases with increasing reheat temperature. This is likely due to the larger austenite grain size reducing the grain boundary area available for proeutectoid cementite nucleation. It was found that the vanadium and silicon additions increased the strength of hypereutectoid steels through refinement of the microstructure and precipitation strengthening.

Abstract: The high level of residual nitrogen in Electric Arc Furnace (EAF) steels is one of the major factors influencing the performance of the finished product. For high tensile strength galvanised wire applications, nitrogen in interstitial solid solution can severely limit drawability and formability. This problem can be controlled simply and effectively by adding nitride-forming elements to the molten steel so that the nitrogen is removed from solution. Vanadium additions are especially beneficial in high strength steels because the removal of nitrogen as vanadium-nitride can cause extensive precipitation strengthening. This investigation concerns commercial grade steels microalloyed with vanadium and rolled to 5.5mm rod, under controlled Stelmor cooling conditions. This rod is used to produce 2.5mm high tensile strength galvanised wire. The aim of the research was to determine the optimum vanadium/nitrogen (V/N) ratio and Stelmor cooling profile for the vanadium steel rod. This was achieved by extensive production and laboratory trials followed by mechanical and microstructural analyses of the product.

Abstract: The tensile strength levels required for spring and gear applications are far higher than those commonly associated with other uses of microalloyed steels. In addition, springs and gears are manufactured using highly specific processes. This is why we present first of all a brief overview of the context and objectives of these developments, in order to assess their scope. The work presented here confirms that the effects of Niobium are not simply those usually associated with the precipitation of Nb(CN) carbonitrides: inhibition of recrystallisation, control of austenitic grain coarsening and precipitation hardening . Account must also be taken of the interactions between the Nb and other elements, as well as the slowing down of g => a + carbides transformation observed in certain cases. The number of mechanisms (or interactions) identified clearly shows why there is still a broad range of choices for optimising steel products and processes according to the target application, the production characteristics and the desired value for money ratios.

Abstract: This paper analyses the application of vanadium microaddition for the production of high strength 16mm diameter wire-rods. Laboratory trials, simulating industrial cooling conditions after hot rolling, were made in a range between 3 and 8°C/s. The results show that introducing vanadium means that it is possible to optimise chemical composition by reducing elements susceptible to segregation. Besides, high strength values are maintained by means of precipitation hardening. The influence of vanadium microalloying on the crystallographic ferrite unit size was also evaluated.

Abstract: A novel thermal treatment, rapid transformation annealing (RTA), has been applied to six different cold rolled low-carbon (LC) steel sheets with the aim of refining their microstructure. The process involves rapid heating to just above the austenite (g) to ferrite (a) transformation temperature and subsequent rapid cooling to room temperature. Grain sizes around 2 µm in two different Nb-Ti HSLA steels, 5 µm in a Ti-LC steel and 6 µm in a plain LC (0.037%C) steel have been produced using fast cooling rates (200°C/s). Non-equiaxed structures are obtained in a Nb-Ti HSIF steel and in a plain LC (0.135%C) (CM) steel due to their higher Mn content. However, very fine equiaxed grains (2 µm) are obtained by rapid intercritical annealing (RIA) in the CM steel. Irrespective of the microalloying concept, the grain growth of recrystallized a grains before their transformation was inhibited in CM and in both HSLA steels. This inhibition is connected with the overlapping of a recrystallization and a-g transformation processes which is essential in order to achieve extreme grain refinement either by RTA or RIA.