Microalloyed Medium Carbon Research Articles

Development of Neural Networks to Study Flow Behavior of Medium Carbon Microalloyed Steel during Hot Forming

In the present article, the application of an artificial neural network (ANN) model whose function is the development of plastic instability maps of a medium carbon microalloyed steel during the hot forming process is studied. Secondly, we proceed to create another ANN capable of providing the recrystallized grain size in the steady state resulting from forming deformation. We start from the experimental data of a medium carbon microalloyed steel obtained by hot compression tests with strain rates that vary between 10−4 s−1 and 3 s−1 and in a range of temperatures between 900 °C and 1150 °C. These experimental data are used to train the proposed ANN and obtain flow curves. Finally, the processing maps are developed by applying the dynamic materials model (DMM), according to which the safe hot forming domains and the plastic instability domains of the studied material are delineated. The comparison between the ANN and the experimental maps is carried out. It is ascertained that the optimal regions of forging in the ANN maps coincide with those obtained in the experimental maps. In addition, a study of the influence of the microstructure on the behavior of the studied steel during hot forming is carried out.

Determining the Role of Acicular Ferrite Carbides in Cleavage Fracture Crack Initiation for Two Medium Carbon Microalloyed Steels.

Two medium-carbon microalloyed steels with a predominant acicular ferrite microstructure were investigated in this study in order to determine the initial micro-crack formation mechanism and the role of acicular ferrite structure in cleavage fracture. In order to ensure cleavage fracture, samples were investigated at -196 °C for uniaxial tension and four point bending fracture. Previous investigations have shown that cleavage fracture for steels with a predominant acicular ferrite microstructure has not been initiated by the fracture of coarse TiN particles as in ferrite-pearlite, bainite, or martensitic microalloyed steels. The average maximal thickness of cementite plates measured in this work is 0.798 µm and 0.966 µm, for V and TiV steel, respectively. The corresponding stress values required for their fracture according to Griffith's equation are 1970 MPa and 1791 MPa, respectively. Estimated values of the effective surface energy for the V steel with an average cementite volume fraction of 3.8% range from 40 Jm-2 to 86 Jm-2, and for the TiV steel with an average cementite volume fraction of 18.3% range from 55 Jm-2 to 82 Jm-2. The fracture of coarse cementite plates was found to not to be responsible for the cleavage fracture initiation in case of both steels.

A novel dual-strengthening technology combining external second phase nanoparticle method and hot compression deformation process in micro-alloyed medium carbon steel

Herein, a novel dual-strengthening technology that combines the external second-phase nanoparticle addition method with the hot compression deformation tests was carried out to investigate the effects of surface-modified nanoparticles (NPs) and deformation conditions on the characteristics of inclusions and the evolution of microstructure in the micro-alloyed (MA) medium carbon steels. The results revealed that the inclusions in the steels were divided into single phase inclusions and composite phase inclusions. With the addition of NPs, Al2O3 inclusions were replaced by MgAl2O4 spinel, and the composite inclusions mainly consisted of TiN–VN, MgAl2O4–TiN and MgAl2O4–TiN–MnS inclusions, and the same types of inclusions became significantly smaller in size. According to the EDS results, some of the inclusions acted to induce ferrite nucleation, and the MgAl2O4–TiN–MnS composite inclusion was more favorable for the nucleation of acicular ferrite (AF). The deformation induced ferrite (DIF) started to form at the deformation temperature of 750 °C and the maximum stress peak in the NPs-bearing steel was 516 MPa, which was 28.4% higher than that in the original steel. The proportion of ferrite increased as the deformation temperature decreased. The dispersed NPs contributed to the ferrite nucleation and the grain refinement as the inclusion-modifying agent, which improved the material properties, and the relevant strengthening mechanisms were explained.

Shallow and deep learning of an artificial neural network model describing a hot flow stress Evolution: A comparative study

In recent years, the utilization of artificial neural networks (ANNs) as regression models to solve the issue of hot flow stress forecasting has become a standard approach. In a connection with this kind of regression issue, employed ANNs are usually learned via a shallow learning technique while only limited attention has been paid to a deep learning method. In the frame of the submitted research, the shallow learning approach is thoroughly compared to the deep learning techniques which are based on the use of a Restricted Boltzmann Machine (RBM) and an Auto-Encoder (AE). To do so, these learning techniques are applied on a feed-forward multi-layer ANN describing the experimental hot flow curve dataset of micro-alloyed medium carbon steel. In comparison with the shallow learning method, both deep learning approaches provided higher accuracy in the network response – especially in the case of a higher number of hidden layers. The results have also shown that neither the RBM-based deep learning method nor the AE-based method had a significant effect on the duration of the necessary calculations. However, it turned out that the RBM-based method can, under certain conditions, lead to a more reliable network performance.

Cleavage Fracture of the Air Cooled Medium Carbon Microalloyed Forging Steels with Heterogeneous Microstructures.

Cleavage fracture of the V and Ti-V microalloyed forging steels was investigated by the four-point bending testing of the notched specimens of Griffith-Owen’s type at −196 °C, in conjunction with the finite element analysis and the fractographic examination by scanning electron microscopy. To assess the mixed microstructure consisting mostly of the acicular ferrite, alongside proeutectoid ferrite grains and pearlite, the samples were held at 1250 °C for 30 min and subsequently cooled instill air. Cleavage fracture was initiated in the matrix under the high plastic strains near the notch root of the four-point bending specimens without the participation of the second phase particles in the process. Estimated values of the effective surface energy for the V and the Ti-V microalloyed steel of 37 Jm−2 and 74 Jm−2, respectively, and the related increase of local critical fracture stress were attributed to the increased content of the acicular ferrite. It was concluded that the observed increase of the local stress for cleavage crack propagation through the matrix was due to the increased number of the high angle boundaries, but also that the acicular ferrite affects the cleavage fracture mechanism by its characteristic stress–strain response with relatively low yield strength and considerable ductility at −196 °C.

A Control‐Cooled Hot‐Forged Medium Carbon V‐Microalloyed Steel: Strengthening Breakdown in Conjunction with the Synchrotron X‐Ray Absorption Spectroscopy

This article represents a few practical objectives for industrial research on a microalloyed medium carbon ferritic‐pearlitic 38MnSiVS6 steel, fabricated by hot forging followed by controlled cooling. The first is to determine the achievable yield strength (YS) in different process windows using an industrial‐scale sample. Second, it quantifies the contribution of different strengthening mechanisms, where less attention has been paid in the literature for the studied steel group. The process–microstructure–mechanical properties relationship is characterized by means of image analysis, synchrotron X‐Ray absorption spectroscopy (XAS), hardness, impact, and tensile tests. XAS is the key in this study to evaluate the fraction precipitated of the microalloying element to correlate its effectiveness in strengthening. It can be concluded that vanadium precipitates between 60 and 73 at‐%, which contributes to the additional YS of 221–349 MPa within the studied process window. The smaller precipitated fraction of 7–12% at a higher cooling rate shows a strengthening fraction above 100 MPa of improvement. The refined precipitates enable a lower overall precipitation amount down to 60%, which can be a good hint in case alloying optimization is required.

On a New Two‐Step Air Cooling Method Following Thermomechanically Controlled Rolling: Microstructural Evolution and Mechanical Properties of a High‐Strength Multiphase Medium Carbon Microalloyed Steel

A new two‐step air cooling (TSAC) process following finish rolling is developed for producing a multiphase (ferrite–bainite–martensite [FBM]) microstructure in a medium carbon V‐microalloyed steel without any post‐forming heat treatment. In TSAC, step 1, designated as natural air cooling (NAC − 4.2 and 5.1 °C s−1); step 2, representing two different cooling methods, namely, forced air cooling (FAC − 7 and 7.71 °C s−1) and controlled air cooling (CAC − 1.63 and 1.75 °C s−1), are adopted from two critical transformation temperatures. Both cooling methods result in multiphase (FBM) microstructures which consist of polygonal ferrite (PF), bainite (B), martensite (M), retained austenite (RA), and vanadium carbonitride (V (C, N)) precipitates. In both steels, the fraction of RA lies in the range of 8–11%. The FAC steel exhibits a tensile strength (TS) in the range of 1652–1671 MPa and ≈4–7% of total elongation, whereas the CAC steel displays a maximum TS of 1100 MPa and 17% elongation at fracture. The FAC steel exhibits a higher work‐hardening rate compared to a CAC steel. A transmission electron microscopy (TEM) analysis of tested samples reveals that stable RA plays a crucial role in improving the work‐hardening capability of these multiphase steels.

Effects of cooling after rolling and heat treatment on microstructures and mechanical properties of Mo–Ti microalloyed medium carbon steel

The effects of four different processes, direct quenching + tempering (DQ + T), direct quenching + reheating and quenching + tempering (DQ + RQ + T), forced air cooling + reheating and quenching + tempering (AC + RQ + T), and furnace cooling + reheating and quenching + tempering (FC + RQ + T), on microstructures and precipitates of Mo–Ti microalloyed medium carbon steel were investigated by OM, SEM, TEM, EBSD and mechanical tests. The width of martensite laths (0.22 ± 0.01 μm) produced in DQ + RQ + T was similar to that generated in DQ + T and was narrower than those formed during AC + RQ + T and FC + RQ + T. The effective grain size of the DQ + RQ + T sample was found to be the smallest (0.98 ± 0.38 μm), thus it yielded the finest (Ti, Mo) C precipitates, manifested high strength and excellent plastic toughness, and generated uniform transverse and longitudinal properties. The strength of the DQ + RQ + T sample (longitudinal Rm = 1773 MPa) was slightly lower than that of the DQ + T samples and its elongation (A = 11.0%) and impact energy at −40 °C (Akv = 50 J/cm2) were greater than those of the other three samples. Before the RQ process, the DQ cooling process was employed in order to generate large angle grain boundaries to provide more nucleation sites for austenitization. Moreover, the DQ process inhibited the precipitation of Ti during cooling and then formed fine (Ti, Mo) C precipitates during reheating of RQ to prevent austenite grain growth. Therefore, the DQ + RQ + T process generated fully refined austenite grains, made the martensite structure and (Ti, Mo) C precipitates more uniform, and enhanced the effects of fine grain strengthening and precipitation strengthening. In addition, the plasticity and the toughness of the fine and uniform equiaxed grains were also significantly improved, and the formation of acicular cementites was impeded.

Austenite grain growth and its effect on splitting fracture property of a V–N microalloyed medium carbon steel connecting rod

The growth behaviour of austenite grain in a V–N microalloyed medium carbon steel and its effect on splitting fracture property were investigated by mechanical tests, fracture morphology and microstructure analysis. When the heating temperature is 800 °C, the austenite grain size is 7.6 μm, and the fracture surface is uneven with the impact energy of 235.9 J. When the heating temperature increases to 1100 °C, the austenite grain size grows up to 65.5 μm and the impact fracture surface is much more even with the impact energy of 13.6 J. It is believed that the coarse austenite grain size of 65.5 μm is beneficial for the splitting fracture property of V–N microalloyed medium carbon steel connecting rod. In addition, austenite grain sizes of V–N microalloyed medium carbon steel increase with the increasing in heating temperature and holding time. Then, a grain growth model is established to predict austenite grain growth behaviour.

Effect of Small Amount of Rare Earth Element Ce on the Mechanical Properties of Medium Carbon Microalloyed Steel

With the development of rare earth streated steel, the application and theory research of rare earth element in microalloyed steels are becoming more deeply and widely. So far, many investigations have been made on the effect of rare earth element on low carbon microalloyed steel. However, the effect of rare earth element on medium carbon microalloyed steel have not been clarified yet. In this work，the effect of small amount of rare earth element Ce on mechanical properties of medium carbon microalloyed steel were investigated by SEM, EDS and EBSD. The results show that the addition of rare earth elements can improve the comprehensive mechanical properties of medium carbon microalloyed steel, the strength-ductility balance increases from 23.57GPa% to 27.49 GPa%. In particular, the plasticity and toughness are improved obviously. The elongation rate and the impact toughness are increased by 14.8% and 19.5%, respectively. The rare earth elements did not markedly refine grain size and change matrix structure. However, the addition of rare earths increased the proportion of high-angle grain boundary from 33.2% to 40.2%. In addition, the fracture analysis with scanning electron microscopethe show that rare earth can make the inclusions denatured and hence spherical grain with small size can be obtained. The EDS result shows that rare earth and harmful elements P and O can cause inclusion, which purifies the molten steel and hinders the formation of microalloy element inclusions.

Tensile and impact behaviour of thermo mechanically treated and micro-alloyed medium carbon steel bar

In the present study, a comparison between thermo mechanically treated (TMT) and direct air cooled micro-alloyed hot rolled (MAHR) steel bar has been made in terms of microstructure, texture and mechanical properties. Tensile and Charpy impact tests were performed as a part of the mechanical investigation. Tensile strength and impact toughness values of TMT bar were higher than those of MAHR bar. It was also noticed that TMT bar exhibited better resistance to both ductile and brittle fracture at high and low temperatures compared to MAHR steel. This can be corroborated owing to the presence of fine tempered martensite at the periphery, a mixed microstructure of granular bainite, ferrite, and degenerated pearlite as an intermediate layer and a mixture of ferrite and pearlite at the core of the TMT bar. On the other hand, a coarse ferrite-pearlite phase mixture was present throughout the cross section for MAHR steel and sounded responsible for lower strength and impact toughness properties. Additionally, fractions of higher angle grain boundaries (HAGBs) in the core and smaller martensite packet around the rim section were also contributed beneficially to resist the cleavage crack propagation in TMT rebar. The higher intensity of gamma fiber of MAHR rebar has contributed lowest ductile-brittle transition temperature. Numerous cleavage cracks which were initiated from inclusions or carbide particles were also found responsible for crack propagation during both ductile and brittle types of fracture.

On the modified grain-size-distribution method to evaluate the dynamic recrystallisation fraction in AISI 304 stainless steel

The purpose of the present work was to verify the grain size distribution (GSD) method, which was recently proposed by one of the present authors as a method for evaluating the fraction of dynamic recrystallisation (DRX) in a microalloyed medium carbon steel. To verify the GSD-method, we have selected a 304 stainless steel as a model system and have measured the evolution of the overall grain size distribution (including both the recrystallised and unrecrystallised grains) during hot compression at 1,000 °C in a Gleeble machine; the DRX fraction estimated using the GSD method is compared with the experimentally measured value via EBSD. The results show that the previous GSD method tends to overestimate the DRX fraction due to the utilisation of a plain lognormal distribution function (LDF). To overcome this shortcoming, we propose a modified GSD-method wherein an area-weighted LDF, in place of a plain LDF, is employed to model the evolution of GSD during hot deformation. Direct measurement of the DRX fraction using EBSD confirms that the modified GSD-method provides a reliable method for evaluating the DRX fraction from the experimentally measured GSDs. Reasonable agreement between the DRX fraction and softening fraction suggests that the Kocks-Mecking method utilising the Voce equation can be satisfactorily used to model the work hardening and dynamic recovery behaviour of steels during hot deformation.

Effect of aging-deformation-treatment on the formation of intragranular ferrite in V-microalloyed steel

ABSTRACTThe influence of the aging-deformation-treatment on the formation of intragranular ferrite (IGF) in a vanadium microalloyed medium carbon steel (Fe–0.34C–1.53Mn) was studied. Effects of aging time on the volume fraction of IGF and austenite grain size were also investigated. The effect of the aging-deformation recrystallisation process on the amount of ferrite and the mechanism of IGF formation was discussed. The results show that aging-deformation-treatment makes the precipitation position of the carbonitride transfer from austenite grain boundary to intragranular, and the precipitated intragranular carbonitrides become the nucleation cores of the IGF during the recrystallisation. The content of precipitated carbonitrides, and the volume fraction of ferrites and the grain size of austenites, increase with the increasing aging time.

Process Integrated Heat Treatment of a Microalloyed Medium Carbon Steel: Microstructure and Mechanical Properties

Air-water spray cooling was employed during a heat treatment to enhance the mechanical properties of microalloyed medium carbon steel test cylinders (38MnVS6, 88 mm diameter). Using appropriate cooling times and intensities, the test cylinders’ surfaces could be quenched and subsequently self-tempered by the residual heat of the core. Simultaneously, it was possible to keep the core regions of the cylinders in the bainitic regime and carry out a quasi-isothermal holding. The resulting microstructures consisted of tempered martensite (near-surface) and bainite with pearlite and ferrite (core). Compared to the standard heat treatment (controlled air cooling), the tensile properties (proof stress and ultimate tensile strength) could be improved for both near-surface and core regions with the adapted spray cooling. A hardness profile with 450 HV10 surface hardness and a hardening depth of more than 11 mm could be realized. In addition, an increase of the impact toughness for the core was achieved, resulting in approximately 25 J charpy impact energy. This is a substantial improvement compared to standard heat treatment procedure and values reported in the literature and can be attributed to the reduced pearlite volume fraction and the increased amount of fine bainite.

Dry sliding wear of an induction-hardened, high-silicon medium-carbon microalloyed steel

Microalloyed (MA) medium carbon steels are promising materials as a replacement of quenched and tempered (Q–T) grades. The dry sliding behaviour of an induction hardened medium carbon MA steel, compared to Q–T steel, in the same surface hardening condition, was investigated. The MA steel showed a higher frictional stability and wear resistance than the Q–T steel. The transition from the mild oxidative wear regime to the severe adhesive wear regime occurred under more severe sliding condition for the MA steel. This can be ascribed to the superior hardness of the bulk material and also to its higher tempering stability, induced by the strengthening precipitates, that enhance the load bearing capacity of the MA steel and hence the stability of the protective oxide layers.

Microstructural effects on high-cycle fatigue properties of microalloyed medium carbon steel 38MnVS

High-cycle fatigue properties of medium carbon microalloyed (MA) steel 38MnVS with different microstructural characteristics were studied by rotating bending fatigue test. Low alloy steel 40Cr with quenched and tempered (QT) microstructure was also used for comparison. The results show MA steel in the as-forged condition has the best fatigue property mainly owing to the significant precipitation strengthening of fine V(C,N) precipitates and much fine ferrite–pearlite microstructure, which is even better than that of the QT steel 40Cr. The formation of film-like ferrite along coarse prior austenite grain boundary and coarse bainitic type microstructure as well as the coarsening of the initial fine precipitates significantly deteriorated the MA steel's fatigue properties. SEM examination of fatigue fracture surfaces revealed that the fatigue fracture mechanism of MA steel is different from that of QT steel. It is concluded that microstructure of ferrite–pearlite type MA steels should be carefully controlled after rolling and/or forging to obtain improved fatigue properties.

Effect of vanadium on the high-cycle fatigue fracture properties of medium-carbon microalloyed steel for fracture splitting connecting rod

Abstract The present investigation effort was made to study the effect of V up to 0.45% on the high-cycle fatigue properties of medium-carbon microalloyed (MA) steel 37MnSiVS, for the development of new crackable MA forging steel with excellent fatigue properties. The results show that the amount of V(C,N) precipitates increases with increasing V content and most of the precipitates are less than 5 nm. Owing to the significant precipitation strengthening effect of these nanosized particles, the hardness increase of ferrite with increasing V content is higher than that of pearlite and accordingly a decrease of pearlite/ferrite hardness ratio. Therefore, both fatigue strength and fatigue strength ratio increase with increasing V content and excellent fatigue properties could be obtained when V content is higher than about 0.28%. The fatigue crack growth (FCG) behavior is similar for all the three 37MnSiVS samples with an exponent m ≈ 3.5. It is concluded that V can improve the fatigue properties of ferrite–pearlite steel mainly through precipitation strengthening and therefore it is anticipated that MA steel’s fatigue property could be further improved as well as more fine V(C,N) particles be obtained.

Mechanical Properties and Precipitation Behavior of Ti-Mo Microalloyed Medium-Carbon Steel during Ultrafast Cooling Process

We describe here the mechanical property evolution and precipitation hardening behavior under different cooling conditions including ultrafast cooling (UFC) + air cooling process and accelerated cooling (ACC) + air cooling process in a Ti-Mo microalloyed medium carbon steel is described here. The results demonstrate that the cooling procedure after hot rolling has a significant influence on the mechanical properties of the microalloyed steel. The yield strength and tensile strength that were obtained by ultrafast cooling (UFC) + air cooling process were higher than those from accelerated cooling (ACC) + air cooling process, while the elongation was slightly reduced. Microstructural characterization indicated that grain refinement and precipitation hardening were the primary reasons for the increase in strength of the experimental steel. Ultrafast cooling increased the density of dislocations and refined the grain size. Average size of precipitates containing Ti and Mo was 3~6 nm in ultrafast cooling (UFC) + air cooling process, while average precipitate size obtained by accelerated cooling (ACC) + air cooling process was 6~9 nm. Keywords: Ti-Mo medium-carbon steel; Precipitation; Ultra fast cooling process; accelerated cooling process

Phase Transformation under Continuous Cooling Conditions in Medium Carbon Microalloyed Steels

Several 35CrMo4 and 38MnV7 steels with different additions of Ti and V were manufactured by electroslag remelting. The influence of the alloying and microalloying elements on phase transformation at different cooling rates was studied and the continuous cooling transformation diagrams were plotted. In order to optimize the heat treatment and improve the mechanical properties, the range of cooling rates leading to a fully bainitic microstructure (without ferrite, pearlite and especially without martensite) was determined. Bainite and martensite transformation start temperatures (Bs, Ms) were also established and compared with the values predicted by empirical equations. The important role of precipitates (especially V carbonitride particles) on final microstructure and mechanical properties was assessed.

Modeling the High Temperature Flow Behavior and Dynamic Recrystallization Kinetics of a Medium Carbon Microalloyed Steel

In this study, the hot deformation behavior of a medium carbon microalloyed steel was investigated. The hot compression test was conducted in the temperature range of 1000-1200 °C under strain rates of 0.01, 0.1 and 1 s−1. It has been observed that the flow stress increases with a decrease in temperature and/or an increase in strain rate. Furthermore, dynamic recrystallization (DRX) is found to be the main flow softening mechanism in almost all deformation conditions. Material parameters of the constitutive equations are found to be strain dependent. Their relationship with strain is identified by a fourth order polynomial fit. Then, a constitutive model is developed to predict the flow stress of the material incorporating the strain softening effect. The accuracy of the proposed model for the flow stress is evaluated by applying the absolute average error method. The result of 6.08% indicates a good agreement between predicted and experimental data. Moreover, the critical characteristics of DRX are extracted from the stress-strain curves at different deformation conditions. It is found that by increasing the strain rate at a constant temperature or decreasing deformation temperature under a constant strain rate, the recrystallization curve shifts to the higher strains. The kinetics of DRX increases with increasing deformation temperature or strain rate.