Microalloyed steel

About: Microalloyed steel is a research topic. Over the lifetime, 2183 publications have been published within this topic receiving 33586 citations.

The effect of the initial microstructure of the X70 low-carbon microalloyed steel on the heat affected zone formation and the mechanical properties of laser welded joints

Abstract: In this paper, the heat affected zone (HAZ) of laser welded joints of the X70 steel were studied by the transmission electron microscopy method. The effect of the initial microstructure (coarse-grained hot-rolled and fine-grained after cross-helical rolling) on the HAZ formation and the mechanical characteristics of the welded joints were shown. It was found that the microstructure in the inter-critical HAZ of the steel after cross-helical rolling was more dispersed, homogeneous, and uniform compared to that of the coarse-grained hot-rolled one due to the initial fine-grained ferrite-bainitic-pearlite microstructure and the absence of pronounced ferrite-pearlite banding in the base metal. The character of the microhardness value distribution in the HAZ of the steel after cross-helical rolling was smooth with the gradual decrease from 370 down to 185 HV as shifted towards the base metal. In the HAZ of the coarse-grained hot-rolled steel, the heterogeneous microhardness value (up to 640–670 НV) distribution was revealed. The reason was the upper degenerate bainite microstructure with high residual stresses, characterized by laths up to 2.0–2.5 μm long and a high martensitic-austenitic constituent fraction (10–16%) of a slender shape along the boundaries of bainite laths. The conclusion was drawn that one of the ways to reduce the brittleness of the laser welded joints could be using the initially fine-grained steels possessing the homogeneous (mainly bainitic) microstructure.

Role of carbon and nitrogen content on microstructural homogeneity in thin slab direct rolled microalloyed steels

Abstract: This paper analyses the effect of carbon and nitrogen content on the austenite microstructural homogeneity before transformation in the thin slab direct rolling of Nb and Nb–V microalloyed steels. The study was made with the help of a microstructural hot working model adapted to the metallurgical peculiarities associated with thin slab direct rolling. The results show that an increase in carbon content from 0·04 to 0·09% in 0·05%Nb microalloyed steels requires a significant increase in the initial rolling temperature in order to avoid the presence of isolated as cast austenite grains prior to transformation. Similarly, an increase in nitrogen content from 30 to 120 ppm does not imply changes as drastic as in the case of carbon. In both situations the changes required in the rolling temperature can be explained by the interaction between post-dynamic softening mechanisms and strain induced precipitation kinetics. In this context, the incidence of different final gauge thicknesses on microstructural...

Effect of Cooling Rate on Oxidation Behaviour of Microalloyed Steel

Abstract: Oxidation characteristics of a microalloyed low carbon steel were investigated by a hot rolling mill combined with acceleration cooling system over the cooling rate range from 20 to 70°C/s. The effects of cooling rate after hot rolling on microstructure and phase composition of oxide scale were examined. The results showed that the increase of the cooling rate has a significant influence on the decrease of the grain size and surface roughness of oxide scale. A higher cooling rate promotes the formation of retain wustite and primary magnetite precipitation while suppression of eutectoid α-iron precipitates. This provides the possibility to enhance potential contribution of magnetite precipitates with preferable ductility, and hence fabricates a desired oxide-scale structure under continuous post cooling conditions considering a suitable cooling rate.

ANN model to predict the effects of composition and heat treatment parameters on transformation start temperature of microalloyed steels

Abstract: The paper presents some results of the research connected with the development of new approach based on the artificial neural network (ANN) of predicting the transformation start temperature of the phase constituents occurring in five steels after continuous cooling The independent variables in the model are chemical compositions (C, Mn, Nb, Mo, Ti, N, Cu, P, S, Si, Al, V), austenitizing temperature, initial austenite grain size and cooling rate over the temperature range of the occurrence of phase transformations For purpose of constructing these models, 138 different experimental data were gathered from the literature The data used in the ANN model are arranged in a format of fourteen input parameters that cover the chemical compositions, initial austenite grain size and cooling rate, and output parameter which is transformation start temperature In this model, the training and testing results in the ANN have shown strong potential for prediction of effects of chemical compositions and heat treatments on phase transformation of microalloyed steels

Deformation Induced Ferrite Transformation

Abstract: DIFT Processing is a new promising technology for ferrite grain refinement. In terms of thermodynamics, a prominent feature of DIFT distinguished from static transformation without deformation is the addition of deformation stored energy into the transformation driving force, which leads to an increase in the Ae3 temperature or/and greatly accelerates the transformation at a temperature between Ae3and Ar3. In terms of kinetics, DIFT is a nucleation dominant process; on the contrary, the continuous cooling transformation or isothermal transformation without deformation is a grain growth dominant process. The mechanism of DIFT is still ambiguous. Our recent study shows that DIFT possibly has a unique mechanism which could involve limited diffusion of carbon that is different from both massive transformation and pro-eutectoid ferrite transformation. Based on DIFT theory, new TMCP procedures have been developed to produce ultrafine grained plain low carbon steel and microalloyed steel.