Microalloyed Medium Carbon Steel Research Articles

Coupled Precipitation Behavior of MnS–Nb(C, N) Composite Inclusion in Medium Carbon Micro-alloyed Steel

Coupled Precipitation Behavior of MnS–Nb(C, N) Composite Inclusion in Medium Carbon Micro-alloyed Steel

Continuous cooling transformation in micro-alloyed medium-carbon steel and optimization of the forging process for half-shaft production

Abstract Micro-alloyed medium-carbon steel is increasingly used as a cost-effective alternative to quenched and tempered steel in the production of half-shaft components. Current research emphasizes controlling the microstructure during thermomechanical processing to achieve the desired structural properties after cooling. This study investigated the effect of continuous cooling transformation on the microstructure and mechanical properties of micro-alloyed medium-carbon steel. At a cooling rate of 0.5 °C s−1, only pearlite transformation occurred. At rates between 1 °C s−1 and 8 °C s−1, both bainite and martensite transformations were observed, while only martensite formed at rates exceeding 12 °C s−1. Subsequently, a four-factor, three-level orthogonal experiment was designed based on the actual production process for half shafts. The optimal forging parameters were identified as a heating temperature of 1000 °C, a deformation temperature of 920 °C, a deformation extent of 15%, and a cooling rate of 0.5 °C s−1. The study offered solutions to enhance microstructural uniformity and effectively manage abnormal bainite and martensite formations, thereby establishing a foundation for the high-quality application of micro-alloyed medium-carbon steel in half-shaft components.

Fracture failure analysis and prevention of the rod wheel-side half shaft manufactured from micro-alloyed medium-carbon steel

The newly developed rod wheel-side half shaft manufactured from micro-alloyed medium-carbon steel broke during the test. The fracture occurred in the thermal deformation affected transition zone between the rod and the spline. It is necessary to study its causes and prevention mechanisms. First of all in this paper, the uniformity of the whole rod micro-structure and its mechanical properties were investigated. No obvious abnormalities were found. The finite element model was used to analyse the stress distribution of the transition zone under an actual load. The transition zone had a significant stress gradient of 300 MPa. The tensile test proved that it was prone to quasi-dissociation fracture. Second, the micro-structure of the transition zone was analysed. The abnormal growth of the parent austenite grain (35 % higher than the rod) led to the growth of the martensite lath. It increased the high energy grain boundary by 6 % and caused hardness to fluctuate. Finally, it was determined that the growth of the parent grain size mainly resulted from the energy input and incomplete re-crystallisation introduced in the upsetting process. Numerical simulations and engineering tests showed that the method of increasing the forging ratio can effectively improve the uneven distribution of grains. After optimization, the recrystallization area in the transition zone was more than 85 %, and the grain uniformity and mechanical properties remained stable.

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.

The consequences of hot deformation and control cooling on the microstructure of medium carbon microalloyed steel − 38MnSiVS5 grade

Medium carbon micro-alloyed steels are mostly being utilized in the application where quenched and tempered steels are needed for cost-productive processing. However, the impact toughness of micro-alloyed steels is inferior to that of quenching & tempered steels, which resist their use in critical applications. Conventional techniques are used for hardness enhancement of micro-alloyed steels due to reduction in carbon percentage, the addition of relevant chemical constituent such as titanium to achieve the reduction in the amount of pearlite and controlling the microstructure during thermomechanical processing, subsequently after cooling and forging. In this research work, the effect of hot deformation and control cooling temperature on the microstructure of medium carbon micro-alloyed steel 38MnSiVS5 was studied. Different approaches were performed to improve the impact toughness and hardness and found out that forced air cooling of the micro-alloyed steel 38MnSiVS5 forgings resulted in the finest microstructure and exhibiting a higher hardness of 45.93 RC.

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.

Simulation, microstructure and austenite reconstruction of a medium carbon micro-alloyed steel subjected to an austenitising bending process

The austenitising bending process was conducted on a medium-carbon micro-alloyed steel by Gleeble 3500 thermomechanical simulator to comparatively investigate the microstructural and mechanical evolution through thickness of the bent plate. A combination of bainite ferrite, lath martensite and acicular ferrite with a presence of partial dynamic recrystallisation was the main microstructural features. The increased work hardening in the compressive and tension zones after austenitising bending operation can be ascribed to synergistic effects of dislocation pile-up/cells, and finer prior austenite grains. Also, the reliability of this study was verified from the consistency between the results of simulation and experiment, which would provide a valuable reference for the practical manufacturing technology of railway spring clips.

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.

Influence of hot compressive parameters on flow behaviour and microstructure evolution in a commercial medium carbon micro-alloyed spring steel

Abstract The spring fastener that connects sleepers and rail in high-speed railway is with a high responsibility since its quality directly affects the safety of the railway system. This paper investigates the isothermal hot compression behaviour of a commercial medium carbon micro-alloyed spring steel (raw material) by a Gleeble 3500 thermal-mechanical simulator, which was used to produce spring fasteners. The dependency of the flow behaviour and microstructure evolution under different deformation conditions were systematically studied. The results revealed that the peak stress points are not necessarily recognised as the dynamic recrystallisation (DRX) behaviour because the partial occurrence of DRX may appear in flow stress curves with no stress peak. The metallurgical events that happen during the process of hot compression are close related to the phase transformation products. In the prior austenite grains, high dislocation density can promote the nucleation acicular ferrite. Nevertheless, the DRX behaviour consumed the stored energy, which is conducive to the transformation of bainite and martensite. Besides, the microstructure evoluted from prior austenite grains occupies a larger fraction of middle angle grain boundaries and higher mean local strain compared to the counterparts of DRX grains. The findings would provide useful references for the industrial optimisation in the hot forming process of a spring fastener using Si-Cr medium carbon spring steels.

Anomalous Refinement of the Austenite Grain upon High-Strain-Rate Hot Deformation of Microalloyed Medium-Carbon Steel

Dynamic recrystallization (DR) of austenite of a medium-carbon steel microalloyed with titanium, niobium, vanadium, and boron has been studied in the course of deformation treatment in the temperature range of 850–1180°C using strain rates of 1–100 s–1. It has been established that the threshold deformation corresponding to the onset of the DR increases in a regular way with increasing strain rate from 1 to 10 s–1 and decreases sharply at 100 s–1. In this case, fine grains (D ≈ 8 μm) are formed, which evidences the occurrence of a specific mechanism of the formation of the austenite structure.

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.

Modeling of mass flow behavior of hot rolled low alloy steel based on combined Johnson-Cook and Zerilli-Armstrong model

Accuracy and reliability of numerical simulation of hot rolling processes are dependent on a suitable material model, which describes metal flow behavior. In the present study, Gleeble hot compression tests were carried out at high temperatures up to 1300 °C and varying strain rates for a medium carbon micro-alloyed steel. Based on experimental results, a Johnson-Cook model (JC) and a Zerilli-Armstrong (ZA) model were developed and exhibited limitation in characterizing complex viscoplastic behavior. A combined JC and ZA model was introduced and calibrated through investigation of strain hardening, and the coupled effect of temperature and strain rate. Results showed that the combined JC and ZA model demonstrated better agreement with experimental data. An explicit subroutine of the proposed material model was coded and implemented into a finite element model simulating the industrial hot rolling. The simulated rolling torque was in good agreement with experimental data. Plastic strain and stress distributions were recorded to investigate nonlinear mass flow behavior of the steel bar. Results showed that the maximum equivalent plastic strain occurred at 45° and 135° areas of the cross section. Stress increased with decreasing temperature, and the corresponding rolling torque was also increased. Due to the extent of plastic deformation, rolling speed had limited influence on the internal stress of the bar, but the relative rolling torque was increased due to strain rate hardening.

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.