Showing papers on "Microalloyed steel published in 1992"

Modelling Microstructure and Its Effects during Multipass Hot Rolling

Abstract: By collaborative work the Sheffield Leicester Integrated Model for Microstructural Evolution in Rolling (SLIMMER) has been developed for hot rolling of flat products. The background physical metallurgy is presented together with the expressions used to describe microstructure evolution for a range of ferrous and non-ferrous metals. The finite difference thermal model at the heart of SLIMMER computes heat loss to air, descalers, rolls and water cooling while allowing for oxidation and deformation heating. The use of temperature compensated time enables isothermally determined equations for microstructure evolution to be applied to practical non-isothermal conditions. Rolling loads and torques are calculated using Sims theory with an accurate prediction of mean flow stress. Examples of rolling niobium microalloyed steel plate and the effect of initial grain size illustrate the capabilities of SLIMMER and show some of the validation of the predictions.

Study of correlation between fractal dimension and impact energy in a high strength low alloy steel

Abstract: The search for a possible correlation between fractal dimension and toughness is a topic of current interest. A series of tests have been carried out to determined fractal dimensiona ( D F ) of impact fracture surfaces of microalloyed steel specimens, broken at subambient temperatures. The magnitudes of D F ) have been determined uisng slit island and profilometry techniques and these are referred as D s and D p , respectively. The values of D s are found, unlike some previous reports, to bear a positive linear correlation with toughness (CVN) indexed by impact energy. This result is explained on the basis of the dependence of D s on fracture micromechanism. The method of estimating D p is found to be associated with some inherent difficulties.

Recrystallization kinetics of microalloyed steels deformed in the intercritical region

Abstract: A plain carbon and two microalloyed steels were tested under interrupted loading conditions. The base steel contained 0.06 pct C and 1.31 pct Mn, and the other alloys contained single additions of 0.29 pct Mo and 0.04 pct Nb. Double-hit compression tests were performed on cylindrical specimens of the three steels at 820 °C, 780 °C, and 740 °C within the α + γ field. A’softening curve was determined at each temperature by the offset method. In parallel, the progress of ferrite recrystallization was followed on quenched specimens of the three steels by means of quantitative metallography. It was observed that, in the base steel, a recrystallizes more slowly thany. The addition of Mo retards recrystallization and has a greater influence on γ than on α recrystallization. This effect is in agreement with calculations based on the Cahn theory of solute drag. Niobium addition has an even greater effect on the recrystallization of the two phases. In this steel, the recrystallization of ferrite was incomplete at the three intercritical temperatures. Furthermore, the austenite remained completely unrecrystallized up to the maximum time involved in the experiments (1 hour). The metallographic results indicate that the nucleation of recrystallization occurs heterogeneously in the microstructure, the interface between ferrite and austenite being the preferred site for nucleation.

Effect of controlled rolling on texture development in a plain carbon and a nb microalloyed steel

Abstract: The effect of finish rolling temperature was investigated on texture formation in a plain C and a 0.034% Nb microalloyed steel. When finish rolled at 1020°C (i.e. within the γ recrystallization range), the textures in both steels contain the {001} and {110} components. The sharpness of the {001} component generally increases with decreasing finish rolling temperature down to 630°C, while the {110} component gradually weakens and finally disappears after ferrite rolling. The microalloyed steel displays much sharper texture than the plain C steel when finish rolled at 870°C (i.e. within the γ pancaking range for the Nb steel) and at 730°C (in the γ+α intercritical range). After finish rolling at 870°C, the major texture components in the microalloyed steel are {113} and {332} , in addition to the above two, while the plain C steel texture only contains some low intensity maxima. When finish rolled at 730°C, weak peaks appear at {223} and {554} in the plain C steel and stronger ones at {4411} and {554} in the microalloyed steel. After warm rolling at 630°C, the major texture components in both steels are {223} , {554} and {001} .The {001} and {110} components are obtained, by transformation, from the {100} (cube) and {122} (twinned cube) components of the recrystallized γ. By contrast, the {113} and {332} components originate, respectively, from the {112} (copper) and {110} (brass) components of the unrecrystallized γ. During continued rolling in the γ+α or α range, these transformation texture components are further modified by deformation and ultimately give rise to the stable end orientations which constitute the well-known warm rolling texture in steels.

Thermodynamic calculation and experimental verification of the carbonitride-austenite equilibrium in Ti-Nb microalloyed steels

Abstract: The sublattice-regular solution model has been adapted to describe the thermodynamics of complex carbonitrides. This model has been applied to titanium- and niobium-bearing microalloyed steels for calculation of the mole fraction and composition of the carbonitride precipitates and the residual solute levels in the austenite. Both experimental results and calculations show that titanium nitride predominantly forms at very high temperatures and titanium-niobium carbides go to completion at low temperatures. Quantitative agreement between the experimental measurements and the predictions for carbonitride compositions as a function of temperature is demonstrated.

Development of {111} texture during cold rolling and recrystallization of a CMnV dual-phase steel

Abstract: Four different dual-phase structures were produced in a vanadium microalloyed steel by air cooling and water quenching from the austenitizing temperature (910°C) and then intercritically annealing at 750 and 810°C, followed by a final water quenching. Steels with different distributions of ferrite and martensite were deformed 60% by cold rolling and subsequently recrystallized at 650 and 800°C. Texture measurements, using the conventional pole figure technique and also the orientation distribution function methods, were carried out on the cold-rolled as well as the recrystallized materials. Reasonably strong {111} fibre texture are observed for the cold-rolled materials. Recrystallization textures which are similar to the deformation textures also show a strong {111} fibre, particularly for the material recrystallized at 800°C. Differences between the intensities of the {111} texture produced by the 800 and 650°C anneals are reflected in the corresponding r values which are generally poor in this alloy. The sharper intensity of the {111} texture in the material recrystallized in the intercritical temperature range has been explained as due to selective consumption of grains other than {111} during the α → γ transformation.

Structure-thermomechanical parameters relationship and Zener-Hollomon equation for two vanadium microalloyed steels

Abstract: Hot torsion tests were conducted to simulate forging processes in two vanadium microalloyed steels, types 40MVS6 and 30MSiV62, varying both the conditions of test temperature and time as well as the strain and strain-rate conditions, in order to obtain final results comparable to the classic Q+T steels, direct from the forge. The Zener-Hollomon equations were also calculated, indicating the conditions of application which theoretically permit the prediction of forging parameters, in accordance with the needs or possibilities of an industrial plant.

Dissolution and coarsening of large niobium carbonitrides in a microalloyed steel

Abstract: Observations and measurements of the kinetics of coarsening and dissolution of large cuboidal niobium carbonitrides during solution treatments of a high nitrogen niobium microalloyed steel are reported. At temperatures between 1473 and 1573 K a competitive coarsening and dissolution process was established where the larger niobium carbonitrides grew at the expense of the smaller, or employing niobium and nitrogen which remained in solid solution. In this temperature range growth or dissolution rates and critical sizes could be determined from the analysis of the evolution of particle size distribution. At higher temperatures (1623–1723 K), only a dissolution process existed, where the dissolution rates as a function of particle size was found to increase with increasing temperature.

Multiphase microalloyed steel

Abstract: A steel of particular utility in forging applications has a composition, in weight percent, of from about 0.05 to about 0.35 percent carbon, from about 0.5 to about 2.0 percent manganese, from about 0.5 to about 1.75 percent molybdenum, from about 0.3 to about 1.0 percent chromium, from about 0.01 to about 0.1 percent niobium, from about 0.003 to about 0.06 percent sulfur, from about 0.003 to about 0.015 percent nitrogen, from about 0.2 to about 1.0 percent silicon, balance iron plus conventional impurities. The steel may be worked in the austenite region to produce a well-conditioned austenite structure, cooled to transform the microstructure to a mixture of ferrite and bainite, and then cold forged to a final form. The steel may also be hot forged without first producing the well conditioned austenite. Heat treating of the final product is not required.

Micro-analysis of welds using the field-ion microscope/atom probe

Abstract: Small addition of titanium and boron to welding consumables improves drastically the toughness of welded structures. To investigate the behaviour of titanium and boron on an atomic scale, an atom probe field-ion microscopic study was undertaken on two grades of welds; one with only titanium and one with titanium and boron. It was observed that boron forms pairs with carbon and small clusters with oxygen and carbon. The “bulk” composition in weight per cent was calculated for about 20 000 ions and showed considerable difference from the results obtained from other analytical techniques.

Effects on Mn and Cr Contents on the Toughness of Medium Carbon Microalloyed Steel.

Abstract: The influence of carbon, manganese and chromium contents on the toughness of medium carbon microalloyed steels was investigated by an instrumented Charpy impact testing machine and by the microstructural observation with an electron microscope.The pearlite area fraction increases when carbon, manganese and chromium contents increase. On the other hand, increasing manganese and chromium contents simultaneously makes the interlamellar spacing of pearlite smaller. Chromium has larger effect than manganese in this respect.The hardness becomes higher and the toughness becomes lower when increasing the carbon content. However, in the case of manganese and chromium, both hardness and toughness are improved as far as the bainite structure does not appear. This phenomenon is related to the fact that manganese and chromium change the pearlite morphology and increase the crack initiation energy in the impact tests.Among the steel grades tested in this research, 1.1% manganese and 0.5% chromium is the best combination to get the highest toughness.

Effect of Al content on critical CTOD properties in heat affected zones of C‐Mn microalloyed steel

Abstract: Crack Tip Opening Displacement (CTOD) properties in HAZ of 490 N/mm2 class steels were investigated from viewpoint of Al content in steel. The C-Mn microalloyed steels with Al content of 0.015 and less than 0.001 mass% were selected in this study.The critical CTOD property in HAZ of low Al content steel was more excellent than that in HAZ of conventional Al content steel. The microstructure in HAZ of low Al content steel mainly consisted of fine ferrite. On the other hand, the microstructure in HAZ of conventional Al content steel consisted of upper bainite with M-A constituent.Ferrite formation in low Al content steel was easier than that in conventional Al content steel at usual cooling rate during welding, because the region of ferrite formation was shifted to a higher cooling rate at low Al content.In HAZ of conventional Al content steel, M-A constituent formation is easy due to the suppression of ferrite formation because of the decrease in interfacial energy between γ and α. This is base on condencing the carbon atoms in γ side of γ/α interface due to delaying the diffusion rate of carbon atoms in γ region.From these results, the effects of low Al content on the improvement of the critical CTOD property in HAZ are affecting the transformation behaviour through promoting the diffusion of carbon atoms, resulting in fine ferrite microstructures without M-A constituent.

Effect of heat input and postweld heat treatment on microstructure and mechanical properties in niobium-containing microalloyed steel weldments

Abstract: The paper describes the results of work on the effect of heat input, number of passes and postweld heat treatment on microstructure and mechanical properties of weld metal and heat affected zone of a microalloyed steel containing niobium. 6 mm thick plates with a single V-groove were joined by submerged-arc welding and metal-arc active gas welding with flux-cored wire electrode. Weldments were postweld heat-treated (1 h, 600 degrees C) and furnace cooled. The tests included microstructural examination, bend, Charpy-V, tensile tests and hardness survey. An increase in niobium content accompanied by relatively slower cooling rates (due to high heat input) decreases the amount of acicular ferrite and increases proeutectoid ferrite, upper bainite and polygonal ferrite in the weld metal resulting in the decrease of toughness. In contrast, increase in heat input did not affect the toughness of the heat-affected zone significantly. There was a gradual decrease of tensile strength with increase in heat input. The structure with predominantly acicular ferrite was relatively harder than the one containing predominantly proeutectoid ferrite. The number of passes improved the toughness of the weld metal. Postweld heat treatment did not affect the toughness of either weld metal or heat-affected zone significantly.