Showing papers on "Microalloyed steel published in 2016"

Effect of nitrogen on the microstructures and mechanical properties in simulated CGHAZ of vanadium microalloyed steel varied with different heat inputs

Abstract: The microstructures and mechanical properties of simulated weld coarse heat affected zone (CGHAZ) of two kinds of vanadium microalloyed steel, i.e. V–Ti and V–N–Ti steel, had been studied using the welding thermal simulation method on a Gleeble-3800 thermal simulator, under different welding heat input. The yield strength of simulated CGHAZ decreases with heat input increasing. The 50% fraction appearance transition temperature (50% FATT) of simulated CGHAZ in V–N–Ti steel decreased at first, and then increased. While for the V–Ti steel, 50% FATT increased with t 8/5 increasing. The V–N–Ti steel had a better combination of strength and toughness of simulated CGHAZ compared with V–Ti steel. Increasing nitrogen cannot only refine the austenite grain size, but also simulate V-rich cap part forming on the TiN core part which can be as intragranular ferrite nucleation positions, resulting in refining the effective grain size and raising cleavage fracture stress of simulated CGHAZ. The detrimental effect of the free N on toughness can be remedied by austenite grain and effective grain size refinement.

Thermo-mechanically controlled processed ultrahigh strength steel: Microstructure, texture and mechanical properties

Abstract: A low-carbon microalloyed steel containing high Ni and Cu content has been developed and subjected to thermo-mechanical processing by varying the finish rolling temperature (FRT∼850–750 °C) and cooling rates (air cooling and water quenching). Microstructures of air cooled samples consist of granular bainite and lath or plate-like bainite, whereas, water quenched samples exhibit a mixture of lower bainite and lath martensite. A refinement in microstructure has been noticed with the decrease in FRT and increase in cooling rate. Transmission electron microscopy demonstrates the presence of coarse (Ti, Nb)C precipitates (~90–160 nm) and fine Cu precipitates (<20 nm). Macro-texture and micro-texture results reveal the dominance of Goss and rotated Goss texture components, which strengthened with the decrease in FRT and increase in cooling rate. The proposed steel composition and TMCP schedule have offered YS ∼ 1000 MPa, UTS ∼ 1400 MPa, total elongation greater than 10% maintaining a low YS: UTS ratio (0.68–0.80). Such a satisfactory combination of tensile properties achieved in as-cooled or as-quenched conditions (without the need of any tempering treatment) makes the steel suitable for automotive application.

Constitutive Analysis of Dynamic Recrystallization and Flow Behavior of a Medium Carbon Nb-V Microalloyed Steel

Abstract: The dynamic recrystallization (DRX) and flow behavior of a medium carbon Nb-V microalloyed steel was investigated using hot isothermal compression experiments in a wide range of temperatures (1123-1473 K) and strain rates (0.01-10 s−1). The flow stress curves were analyzed comprehensively, and it was found that the flow stress of this steel is higher than C-Mn steel and V microalloyed steel. All the curves obtained can be ranged into three principal types: work hardening, dynamic recovery, and DRX. The DRX behavior of this steel was investigated, including critical strain, kinetics of DRX, and microstructure. The constitutive equation to predict the flow stress of the tested steel was also developed, and the analysis result indicates that the developed model has a high accuracy in predicting the flow stress during hot deformation.

The Determining Role of Finish Cooling Temperature on the Microstructural Evolution and Precipitation Behavior in an Nb-V-Ti Microalloyed Steel in the Context of Newly Developed Ultrafast Cooling

Abstract: We have studied here the impact of finish cooling temperature on the microstructural evolution and precipitation behavior in Nb-V-Ti microalloyed steel through thermo-mechanical simulation in the context of newly developed ultrafast cooling system. The microstructural evolution was studied in terms of morphology and crystallography of precipitates using high-resolution transmission electron microscopy. At finish cooling temperature of 933 K and 893 K (660 °C and 620 °C), the microstructure primarily consisted of polygonal ferrite, together with a small amount of wedge-shaped acicular ferrite and lamellar pearlite, while, at 853 K and 813 K (580 °C and 540 °C), the microstructure consisted of lath bainite with fine interlath cementite and granular bainite with martensite/austenite (M/A) constituent. In all the finish cooling temperatures studied, the near-spherical precipitates of size range ~2 to 15 nm were randomly dispersed in ferrite and bainite matrix. The carbide precipitates were identified as (Nb,V)C with NaCl-type crystal structure. With a decrease in the finish cooling temperature, the size of the precipitates was decreased, while the number density first increased with a peak at 893 K (620 °C) and then decreased. Using Ashby–Orowan model, the contribution of the precipitation strengthening to yield strength was ~149 MPa at the finish cooling temperature of 893 K (620 °C).

The determining impact of coiling temperature on the microstructure and mechanical properties of a titanium-niobium ultrahigh strength microalloyed steel: Competing effects of precipitation and bainite

Abstract: We elucidate here the influence of coiling temperature on the microstructure and mechanical properties, in an ultrahigh strength titanium-niobium microalloyed steel. The objective was to underscore the impact of coiling temperature on the nature and distribution of microstructural constituents (including different phases, precipitates, and dislocation structure) that significantly contributed to differences in the yield and tensile strength of these steels. Depending on the coiling temperature, the microstructure consisted of either a combination of fine lath-type bainite and polygonal ferrite or polygonal ferrite together with the precipitation of microalloyed carbides of size ~2–10 nm in the matrix and at dislocations. The microstructure of steel coiled at lower temperature predominantly consisted of bainitic ferrite with lower yield strength compared to the steel coiled at higher temperature, and the yield to tensile strength ratio was 0.76. The steel coiled at higher temperature consisted of polygonal ferrite and extensive precipitation of carbides and was characterized by higher yield strength and with yield strength/tensile strength ratio of 0.936. The difference in the tensile strength was insignificant for the two coiling temperatures. The observed microstructure was consistent with the continuous cooling transformation diagram.

Processing-structure-mechanical property relationship in Ti-Nb microalloyed steel: Continuous cooling versus interrupted cooling

Abstract: The process parameters associated with thermo-mechanical controlled processing (TMCP) of steels play an important role in influencing the ultimate mechanical properties. The study of TMCP parameters have not received the required attention. In this regard, we elucidate here the impact of finish cooling temperature on interrupted cooling and compare with continuous cooling on microstructural evolution and precipitation behavior and associated mechanical properties in Ti-Nb microalloyed steels. The microstructural evolution was studied via transmission electron microscopy and electron back scattered diffraction (EBSD). The microstructure of continuously cooled and interrupted cooled steels with different finish exit temperatures consisted of polygonal ferrite, bainite and martensite/austenite constituent. However, the fraction of different microstructural constituents was different in each of the experimental steels. Similarly, there were differences in the distribution and average size of (Nb, Ti)C precipitates. The aforementioned differences in the microstructure and precipitation introduced differences in tensile properties. Furthermore, electron back scattered diffraction studies indicated distinct variation in average grain area and high angle boundaries between continuously cooled and interrupted cooled steels.

Phase reverted transformation-induced nanograined microalloyed steel: Low temperature superplasticity and fracture

Abstract: The novel concept of phase reverted transformation involving severe cold deformation of martensite at room temperature followed by controlled annealing was adopted to obtain nanograined (NG) microalloyed steel The NG microstructure was characterized NG (

Precipitation in Microalloyed Steel by Model Alloy Experiments and Thermodynamic Calculations

Abstract: Precipitation in microalloyed steel has been studied by applying thermodynamic calculations based on a description of the Gibbs energies of the individual phases over the full multicomponent composition range. To validate and improve the thermodynamic description, new experimental investigations of the phase separation in the cubic carbides/nitrides/carbonitrides in alloys containing Nb, V, Mo, and Cr, have been performed. Model alloys were designed to obtain equilibrium carbides/carbonitrides that are sufficiently large for measurements of compositions, making it possible to study the partitioning of the elements into different precipitates, showing distinctly different composition sets. The reliability of the calculations, when applied to multicomponent alloys, was tested by comparing with published experimental studies of precipitation in microalloyed steel. It is shown that thermodynamic calculations accurately describe the observed precipitation sequences. Further, they can reproduce several important features of precipitation processes in microalloyed steel such as the partitioning of Mo between matrix and precipitates and the variation of precipitate compositions depending on precipitation temperature.

Hot ductility trough elimination through single cycle of intense cooling and reheating for microalloyed steel casting

Abstract: Surface transverse cracking, especially corner cracking, is prone to generate in continuously cast slabs of microalloyed steels The method of surface structure control (SSC) was supposed to the best way to avoid the detrimental defects However, the mechanism of improving hot ductility by SSC and the specific parameters to control the process are still unclear for the reasonable adoption in production In the present work, the impact of cooling rate, holding temperature and holding time on austenite decomposition, and the austenite grain size before and after intense cooling were investigated by thermal simulation method With the increase of cooling rate, it is observed that the phase is transformed from austenite → grain boundary film-like alltromorph ferrite → Widmanstatten ferrite plates (or intragranular ferrite plates) → bainite+martensite Mostly important, the film-like ferrite can be eliminated through intense cooling and the following reheating, but the austenite grain size is not observed to b

Effect of silicon carbide particle size on microstructure and properties of a coating layer on steel produced by TIG technique

Abstract: Surface engineering is essential to prolong life of engineering components subjected to deterioration in service from wear, erosion or corrosion, separately or in combination. This is possible by, for example, incorporating ceramic particles into a molten substrate surface, which may result in the partial or complete dissolution of the particles, and precipitation of a new phase, giving enhanced surface protection. Here, a single-track surface zone of a microalloyed steel was preplaced with a layer of SiC particles 1 and 75 μm in size, melted at a constant heat input with a tungsten inert gas torch using argon as shielding gas. The aim was to compare the melted track without SiC particles and when using SiC particles on the generated temperature, microstructure, melt dimension and hardness of the re-solidified surfaces. Previous work has shown that microstructural changes occur along the track length due to preheating of the un-melted surface ahead of the molten zone. This effect was explored by i...

Low temperature superplasticity and thermal stability of a nanostructured low-carbon microalloyed steel.

Abstract: We describe here for the first time the low temperature superplasticity of nanostructured low carbon steel (microalloyed with V, N, Mn, Al, Si, and Ni). Low carbon nanograined/ultrafine-grained (NG/UFG) bulk steel was processed using a combination of cold-rolling and annealing of martensite. The complex microstructure of NG/UFG ferrite and 50-80 nm cementite exhibited high thermal stability at 500 °C with low temperature elongation exceeding 100% (at less than 0.5 of the absolute melting point) as compared to the conventional fine-grained (FG) counterpart. The low temperature superplasticity is adequate to form complex components. Moreover, the low strength during hot processing is favorable for decreasing the spring back and minimize die loss.

Evaluation of AH36 microalloyed steel welded joint by submerged arc welding process with one and two wires

Abstract: Shipbuilding is going through a period of revitalization, growth and technological advancement. One component of these innovations is to improve welding techniques and materials for optimizing processing time, reducing costs and to improve properties and ship performance. The aim of the present work is to evaluate the welding of microalloyed shipbuilding steel, AH36, using the submerged arc welding (SAW) process with one and two wires. The mechanical properties of the welded joint will be presented using microhardness and Charpy V-notch testing. For metallographic characterization, the base metal and welded joint were etched with 2% Nital and Klemm's I reagent. Scanning electron microscopy, X-ray diffraction and colored etching were used to quantify and verify the presence of martensite and retained austenite, the MA microconstituent. The results obtained from Charpy impact tests and another mechanical test can be correlated with the associated microstructure.

Surface-crack formation in the manufacture of microalloyed steel pipe

Abstract: Deform-3D software is used to model surface-crack formation in the rolling of thick sheet on a 5000 mill. The calculations show that the temperature field and stress state are relatively uniform over the width of the sheet, except at the edges. The minimum temperature of the surface layers of metal hardly depends on the sheet thickness and the relative reduction per pass. The Cockroft–Latham criterion is used to analyze the probability of steel failure in rolling. Analysis shows that metal failure may occur at points where ferrite appears, especially at the instant that the metal leaves the deformation region. Considerable tensile stress acts at those points.

The effect of manganese content on mechanical properties of high titanium microalloyed steels

Abstract: In this work, in order to achieve an optimum combination of high strength, ductility and toughness of high Ti microalloyed steel, extensive research efforts were exerted to study the effect of soaking temperature, manganese and sulfur content on properties of titanium steels. Precipitation hardening of Ti-bearing steels has been found to vary with different soaking temperature. Higher strength was achieved in these steels at higher soaking temperature due to dissolution of more TiC, Ti 4 S 2 C 2 and little TiN, which lead to re-precipitation of fine carbides with greater volume fraction. The results of transmission electron microscope (TEM)analysis indicates that there were more and finer TiC precipitates coherent or semi-coherent with the ferrite matrix in the high manganese content steel than in low manganese content steel. The marked improvement in strength is also associated with low sulfur content. TiC particles smaller than 20 nm in 8Ti-8Mn steel help enhance strength to higher than 302 MPa compared with 8Mn steel.

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

Abstract: The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.

Optimization of resistance spot welding parameters for microalloyed steel sheets

Abstract: Abstract The paper presents the results of resistance spot welding of hot-dip galvanized microalloyed steel sheets used in car body production. The spot welds were made with various welding currents and welding time values, but with a constant pressing force of welding electrodes. The welding current and welding time are the dominant characteristics in spot welding that affect the quality of spot welds, as well as their dimensions and load-bearing capacity. The load-bearing capacity of welded joints was evaluated by tensile test according to STN 05 1122 standard and dimensions and inner defects were evaluated by metallographic analysis by light optical microscope. Thewelding parameters of investigated microalloyed steel sheets were optimized for resistance spot welding on the pneumatic welding machine BPK 20.

Extending an empirical and a fundamental bainite start model to a continuously cooled microalloyed steel

Abstract: The empirical model proposed by Lee and a fundamental model for the isothermal bainite start temperature ( B s ) are applied in this work to a microalloyed steel undergoing continuous cooling transformation. The fundamental model for B s is based on displacive transformation of bainite. The method proposed in this paper is termed the Bainite Intercept Temperature method (BIT). Using this method it is possible to determine the volume fraction of the phase that forms before bainite, i.e., quasi-polygonal ferrite. BIT analysis is applied to continuous cooling where a two phase microstructure is formed of quasi-polygonal ferrite and mixed bainitic ferrite and bainite/martensite. The estimated intrinsic hardness of the components based on BIT is compared with isothermal transformation results and the hardness is calculated based on the contribution of strengthening mechanisms.

Prediction of microstructure and mechanical properties of line pipe welded joints based on hardness map

Abstract: In this article, the variation of hardness in cross-section of X52 microalloyed steel joints before and after post-weld heat treatment and also the relationship of hardness with ultimate tensile strength, yield strength and microstructure have been investigated. For this purpose, the specimens were prepared in two conditions of as welded and post-weld heat treated, and the hardness maps of their cross-section were provided. Then, some equations were suggested to correlate the obtained hardness with the grain size and tensile and yield strengths. The equations made it possible to predict tensile and yield strengths with high accuracy. According to the results, the maximum hardness and strength belong to the weld junction. Normalisation of the joints at 950°C reduced significantly the hardness difference (and also the strength difference) in various joint areas and homogenised the microstructure considerably. The grain size distribution was also modelled acceptably for the post-weld heat treated joint, and ...

Effect of solidification rate on microstructure evolution in dual phase microalloyed steel

Abstract: In steels the dependence of ambient temperature microstructure and mechanical properties on solidification rate is not well reported. In this work we investigate the microstructure and hardness evolution for a low C low Mn NbTi-microalloyed steel solidified in the cooling rate range of 1–50 Cs−1. The maximum strength was obtained at the intermediate solidification rate of 30 Cs−1. This result has been correlated to the microstructure variation with solidification rate.

The effect of warm deforming and reversal austenization on the microstructure and mechanical properties of a microalloyed steel

Abstract: The influence of warm deforming and reversal austenization on the microstructure and mechanical properties of a microalloyed steel was elucidated. Grain refinement induced by warm deforming and reversal austenization was studied by thermal simulation experiments, and its effect on properties was assessed by thin-plate hot rolling experiments. The study suggested that austenite was refined to ~10 µm after reversal austenization, which was further refined to ~5 µm after reversal austenization and application of warm deforming. Warm deforming recrystallized the microstructure during the reheating process, when the reheating rate was less than 2 °C/s. While recrystallization was inhibited at reheating rates greater than 5 °C/s. The ultimate microstructure obtained from the refined austenite via combination of warm deforming and reversal austenization, comprised of fine-grained (4.7±3.2 µm) ferrite and pearlite with small colony size (1.3±0.6 µm). On the other hand, the fine-grained microstructure was characterized by reduced extent of coarse precipitation (9.3±3.4 nm), compared to the coarse-grained thermo-mechanical controlled processing (TMCP) plate with dispersed and fine precipitation (4.0±1.3 nm), which was responsible for lower strength (~70 MPa) of the fine-grained plate than the coarse-grained TMCP plate. However, the fine-grained plate had an excellent low temperature toughness resulting from fine grain size, coarse precipitation, and small pearlite colony size.