Advanced High Strength Steels Research Articles

Effect of magnetic field on mechanical properties of an advanced high strength steel sheet

The present study deals with the experimental investigations during plastic deformation of a dual phase steel sheet in uniaxial tension under a magnetic field, applied in the longitudinal direction of the tensile specimen. To ascertain the changes in tensile properties, the experiments are performed with and without magnetic field. The microstructural and texture investigations are done using field emission scanning electron microscopy in the samples taken from the middle of the tensile tested with 20% of gauge length (20% engineering strain) with and without magnetic field. The results show an increase in compressive residual stress, microhardness and anisotropy of the sheet when tested with magnetic field which may affect the formability of tested sheet. It is also observed that the magnetic field affects the texture during plastic deformation and renders a strong texture as compared to that of specimen tested without magnetic field.

Effect of hydrogen on the microstructure and mechanical properties of FeCCrNiBxSi advanced high strength steels

This research provided a thorough knowledge of the hydrogen embrittlement in AHSS models based on FeCCrNiB0Si (AHSS1) and FeCCrNiB2Si (AHSS2) alloy systems. The results obtained by electron backscatter diffraction (EBSD) showed the local deformation in the cracked regions. Thermal desorption spectroscopy (TDS) analysis showed despite the higher ratio of irreversible to reversible traps in AHSS2 compared to AHSS1, the susceptibility to HE in AHSS2 was greater than AHSS1 due to the addition of boron and the γ/Cr2B interfaces’ decohesion in the presence of hydrogen. Higher Taylor factor grains in H-charged samples were more likely to suffer plastic instability.

Microstructural Design and Mechanical Enhancement of δ-TRIP Steel Through Mo Micro-alloying and Isothermal Bainitic Transformation

This research was done to investigate third-generation Advanced High-Strength Steel (AHSS) to improve mechanical properties and reduce production costs in the automotive industry. The potential of TRIP-assisted steel has been studied to address this issue, with a special focus on δ-TRIP steels. This research studied the δ-TRIP samples, which were micro-alloyed with 0.05 and 0.30 wt.% Mo, with isothermal bainitic transformation (IBT) time parameters. Microstructural analyses were conducted using optical microscopy (OM), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectrometry (EDX). The volume percentage of retained austenite (RA) phase, critical to achieving the requisite mechanical characteristics, was determined using X-ray diffraction (XRD). The analysis of the samples revealed a complex, multi-phased structure comprising δ-ferrite matrix, α-ferrite, bainite, RA, and martensite. The quantity and morphology of RA significantly influenced the resulting mechanical characteristics. The experimental results demonstrated yield stresses (YS) of 400–440 MPa, ultimate tensile strength (UTS) of 790–870 MPa, and total elongations to failure (TEL) of 24–34% in δ-TRIP steels containing 0.05 and 0.30 wt.% Mo alloy. The optimal combination of UTS and TEL (UTS × TEL) is achieved at an impressive 850 MPa × 32% (27200 MPa.%). This remarkable achievement was made possible by adding only 0.30 wt.% Mo and IBT, which were carried out at 450°C for 10 min. Focusing on the effects of Mo-alloy content and heat treatment duration on microstructure and mechanical characteristics, this study provided insights into enhancing the quality of δ-TRIP steel while decreasing manufacturing costs.

Investigations on hydrogen-assisted cold cracking of laser welded AHSS

AbstractThis study aims to investigate the impact of various surface conditions prior to welding on the susceptibility of materials to cold cracking, including an analysis of fracture surfaces. Additionally, a novel method is introduced for quantifying the presence of diffusible hydrogen using thermal desorption analysis (TDA). This method allows for the determination of diffusible hydrogen concentration in thin sheet welded joints without use of welding consumables. Three different cold-rolled Advanced high-strength steel (AHSS) samples with different surface conditions, such as coatings, lubrication, or water, are examined to assess their susceptibility to cold cracking. In addition to measuring the diffusible hydrogen content in both the base material and the coating, the overall hydrogen content of the base material is also measured using the melt extraction (ME) method. The new method for quantifying diffusible hydrogen in weld metal is applied to investigate different welding variations, intentionally introducing hydrogen through coatings and hydrogenous fluids on the sheet surface. By combining the assessment of cracking susceptibility and hydrogen content, a better understanding of critical hydrogen levels leading to hydrogen-assisted cracking (HAC) is achieved. The results of this study demonstrate that the occurrence of cold cracking in specific AHSS samples increases when either lubrication or both coating and water or lubricant are added. Additionally, the presence of diffusible hydrogen in the welds of all materials is found to increase with the introduction of hydrogenous layers to the material. Although a critical hydrogen content is identified, no clear correlation between the amount of hydrogen and cracking susceptibility can be determined. These findings have significant implications for the welding of cold-formed AHSS, particularly in the automotive industry where safety and lightweight design are of paramount importance.

A finite element model with multi-flow fields for the quenching process of mill liner component made of bainite and martensite: simulation and experimental validation

Controlling the type and location of microstructure transformation through heat treatment processes is the key to improving the property of workpieces, especially they are large-sized and highly thick irregular. Such as mill liner, the worst wear in mining machinery, of which microstructure inhomogeneity is key to developing cost-efficient mining process. In order for advanced high strength steel (AHSS) to be used in mining machinery, its microstructure uniformity must be optimized, usually by adjusting the process parameters of the heat treatment process to optimize the microstructure distribution uniformity. However, the research on the simulation of large-sized and highly thick irregular heat treatment process is limited. Previous studies usually controlled the heat transfer process by using the heat transfer coefficient as the boundary condition, but only applied to small size and simple structure of the workpiece. As the heat transfer coefficient is a process quantity, its size depends on the thermal property parameters of the fluid on both sides of the wall, the flow rate, the shape of the solid surface, the thermal property number of the material and other factors, which is not suitable for large-sized and highly thick irregular. Therefore, for the large-sized and highly thick irregular heat treatment simulation process, the equivalent heat transfer coefficient of flow field was introduced as the boundary condition, and the temperature field, microstructure field and stress field were coupled to improve the accuracy of the numerical model. The accuracy of the finite element model is verified by comparing the temperature simulation error of less than 10% through temperature measurement experiments of the quenched workpiece. The evolution process of heat transfer in large-sized and highly thick irregular water bath quenching is obtained through the finite element model data. The difference in the flow rate of the wall fluid is the fundamental reason for the difference in cooling effect, and the wetting process of the wall is the secondary reason. At the same time, the corresponding microstructure and surface hardness distribution of different cooling regions are obtained, which can provide more accurate guidance for the optimization of heat treatment process of large size and thick special-shaped parts.

Effect of Change in Current Density on Hydrogen Embrittlement of Advanced High-Strength Steel S960MC during Hydrogenation

Effect of Change in Current Density on Hydrogen Embrittlement of Advanced High-Strength Steel S960MC during Hydrogenation

Numerical and experimental study of underwater friction stir welding of 1Cr11Ni2W2MoV heat-resistant stainless steel

Due to the benefits of solid-state joining, friction stir welding (FSW) has seen an increase in applications in the aircraft and automotive industries. However, short tool life and heat-affected zone (HAZ) softening are the two main issues with the FSW of advanced high-strength steel. In this study, the underwater friction stir welding (UFSW) of 1Cr11Ni2W2MoV steel was investigated and compared to the Normal friction stir welding (NFSW). Besides, a 3-D thermo-mechanical finite element model was developed to understand the effect of the different cooling mediums and tool rotation rates on the thermal cycles, plastic deformation, tool wear, and microstructure evolution. The simulation results revealed that the stir zone (SZ) of the UFSW exhibits lower peak temperatures and smaller plastic strain compared to the NFSW. The experimental results showed that a high tool rotation rate can be used during UFSW without causing overheating of the workpiece and tool materials resulting in high joint quality and low tool wear. The M23C6 and Fe3C carbide fractions in the HAZ at UFSW are lower than that at NFSW. The UFSW reduces the soft HAZ's width and increases its hardness. The ultimate tensile strength was increased by 17 %, and the HAZ width was decreased by 35 % at UFSW compared to that at NFSW.

Evaluation of microstructures and mechanical properties of deltha trip steel with different vanadium contents

This research studies the effect of adding 0.12 and 0.25 wt% vanadium on the microstructure and mechanical properties of a delta-TRIP steel. Optical microscopy and scanning electron microscope (SEM) were employed to investigate the microstructure of the steel. In addition, hardness measurements and tensile testing were used for the evaluation of the mechanical properties. The X-ray diffraction (XRD) analysis was also applied to analyze the present phases. The structure of the three studied steels before applying the heat treatment cycle consisted of delta-ferrite, allotriomorphic ferrite, martensite, and pearlite. The heat treatment cycle led to the stability of the austenite phase. The addition of 0.12 wt% V resulted in the enhancement of the mechanical properties so that the best combination of strength (866 MPa) and elongation percentage (41 %) was achieved in this steel. Nonetheless, the addition of 0.25 wt% V deteriorated the mechanical properties since the increase in the vanadium content promoted the formation of martensite, decreased the percentage of the retained austenite, and weakened the mechanical properties. However, adding 0.12 wt% V improved the mechanical properties since it increased the strength through solid-solution strengthening and precipitation hardening; no brittle martensite was formed, and lamellar δ-ferrite was achieved. The steel containing 0.12 wt% V, exhibiting a formability index of 35.7 GPa%, is in the range of the third-generation advanced high-strength steels.

New insights into the effect of Al on the selective oxidation of advanced high-strength steels prior to hot-dip galvanizing

Advanced high-strength steels (AHSSs) enable the manufacture of lighter, safer and more energy-saving body-in-white. However, the poor galvanizability induced by the surface selective oxidation during annealing makes the production of high-grade galvanized AHSS quite challenging. It is generally believed that replacing Si with Al enhances the galvanizability of AHSS, but a recent industrial practice shows the opposite for a high Al steel grade, which indicates complex competing mechanisms introduced by Al. Here, we study the selective oxidation of a model system Fe-Mn-Si-Al systematically by varying the Al content and dew point of annealing atmosphere. We suggest that Al has a dual-side effect on the galvanizability of AHSS via: i) promoting the outward diffusion and subsequent external oxidation of Mn and Si, ii) deteriorating the surface chemistry of galvanizing by enhancing the Si/Mn ratio of surface oxides, iii) facilitating the formation of amorphous film-like external oxides, and iv) a comparatively low trend for Al to segregate to the surface, where the first three function negatively while the last positively. Our work would change the perspective away from a focus solely on the deceleration effect of Al to also consider its acceleration on the surface selective oxidation, insights critical for developing strategies against galvanizing problems of AHSSs.

High temperature oxidation behaviors of steels at initial stages in air

The initial stage oxidation behaviors of commercial low carbon steel (LCS) and advanced high strength steel (AHSS) were comparatively studied at 1150 °C in air. Two reaction fronts were in situ observed during the oxidation of LCS, which was not found for AHSS. It was confirmed that the early detachment of LCS occurred after 30-s oxidation, however, the scale of AHSS maintained adherence due to the formation of a liquid phase. Besides, the scale thickness increased parabolically with oxidation time for both LCS and AHSS, with the estimated parabolic rate constants of 4.7 × 10−6 µm2/s and 4.9 × 10−6 µm2/s, respectively.

Microsegregation Influence on Austenite Formation from Ferrite and Cementite in Fe–C–Mn–Si and Fe–C–Si Steels

The production reality of sheet steels from casting to the end product is such that in the cases of ultra- and advanced high-strength steels, we have to deal with the segregation of elements on macro- and microlevels. Both can have a significant impact on the microstructure formation and resulting properties. There are several production stages where it can influence the transformations, i.e., casting, hot rolling process and annealing after cold rolling. In the present work, we focus on the latter, and more specifically, the transformation from ferrite–cementite to austenite, especially the nucleation process, in cold-rolled material. We vary the levels of two substitutional elements, Mn and Si, and then look in detail at the microsegregation and nucleation processes. The classical nucleation theory is used, and both the chemical driving force and strain energy are calculated for various scenarios. In the case of a high Mn and high Si concentration, the nucleation can thus be explained. In the cases of high Mn and low Si concentrations as well as low Mn alloys, more research is needed on the nuclei shapes and strain energy.

Recurrent Neural Networks and Three-Point Bending Test on the Identification of Material Hardening Parameters

The continuous evolution of metallic alloys in the automotive industry has led to the development of more advanced and flexible constitutive models that attempt to accurately describe the various fundamental properties and behavior of these materials. These models have become increasingly complex, incorporating a larger number of parameters that require an accurate calibration procedure to fit the constitutive parameters with experimental data. In this context, machine learning (ML) methodologies have the potential to advance material constitutive modeling, enhancing the efficiency of the material parameter calibration procedure. Recurrent neural networks (RNNs) stand out among various learning algorithms due to their ability to process sequential data and overcome limitations imposed by nonlinearities and multiple parameters involved in phenomenological models. This study explores the modeling capabilities of long short-term memory (LSTM) structures, a type of RNN, in predicting the hardening behavior of a sheet metal material using the results of a standardized experimental three-point bending test, with the aim of extending this methodology to other experimental tests and constitutive models. Additionally, a variable analysis is performed to select the most important variables for this experimental test and assess the influence of friction, material thickness, and elastic and plastic properties on the accuracy of predictions made by neural networks. The required data for designing and training the network solutions are collected from numerical simulations using finite element methodology (FEM), which are subsequently validated by experiments. The results demonstrate that the proposed LSTM-based approach outperforms traditional identification techniques in predicting the material hardening parameters. This suggests that the developed procedure can be effectively applied to efficiently characterize different materials, especially those extensively used in industrial applications, ranging from mild steels to advanced high-strength steels.

Testing, numerical modeling and design of perforated advanced high-strength steel channel section columns

This paper examines the influence of perforations on the buckling instability and load-bearing capacity of advanced high-strength steel channel section (C-section) columns. Experimental tests were first conducted on 19 column specimens made of complex phrase steel HC700CP980 under axial compression, followed by finite element (FE) analysis. Two section types, the flat web C-section and the web-stiffened C-section, were considered, with perforations categorised as web-only or both web and flanges. Material tests and initial imperfection measurements were conducted and reported. Using test results, finite element models were established and parameter analysis was carried out. Analysis of test results and finite element data revealed that perforations had a noticeable impact on the buckling deformation of flat web C-sections but had a minor effect on load-bearing capacity. In contrast, web-stiffened C-sections exhibited intensified deformation post-perforation, significantly reducing their axial stiffness and load-bearing capacity. The Direct Strength Method (DSM) was assessed based on combined test and FE data. It revealed that the DSM resulted in a high level of inaccuracy and scatter in the load-bearing capacity predictions of high-strength steel C-sections after perforation. To address this, a modified method incorporating a perforation strength reduction factor was proposed, offering straightforward and intuitive calculations with improved design accuracy and consistency.

Characterizing the mechanical deformation response of AHSS steels: A comparative study of cyclic plasticity models under monotonic and reversal loading

This study evaluates the performance of a user-defined combined hardening modeling method for advanced high-strength steels (AHSS) under monotonic and reversal loading conditions. The plastic behavior of TWIP980 and TRIP980 AHSS sheet metals is investigated using a cyclic plasticity modeling approach. The model incorporates an isotropic von Mises yield criterion and a single-term Chaboche nonlinear kinematic hardening rule. Monotonic and reversal loading stress-strain curves are predicted and compared with experimental results. The model accurately captured the Bauschinger effect for both materials, but it needs help to effectively model the permanent softening behavior observed in TWIP980 steel. Overall, the proposed modeling method agrees well with experimental results for monotonic loading and accurately represents the Bauschinger effect and transient behavior during reversal loading. However, better improvements are needed to capture the permanent softening behavior of TWIP980 steel.

Numerical modeling of liquid metal embrittlement initiation during resistance spot welding of third generation advanced high strength steel – Effect of welding schedule and electrode alignment

Third generation (GEN3) advanced high-strength steels are increasingly used to reduce vehicle body structure weight for improved fuel economy. Zinc-induced liquid metal embrittlement (LME) has become a prevalent challenge when these steels are welded by resistance spot welding (RSW). In the literature, various numerical models have been developed to study LME during RSW of GEN3 steels. Those models tend to have a limited utility in predicting how various factors such as RSW parameters affect LME cracking. In this study, a novel LME calculation method has been developed by integrating an electro-thermo-mechanical finite element model of RSW process with an LME ratio for crack initiation established from hot tensile testing data. The onset of different types of LME cracking is tracked throughout the weld region over the entire course of RSW including the final cooling stage after electrode retraction. The model is validated using an existing set of literature data that involves different welding schedules, electrode geometries and electrode misalignments. The local temperature and stress conditions during RSW are calculated and used to understand how different welding parameters affect the severity of LME cracking. It is found that electrode misalignment exacerbates crack initiation on outer surfaces of the spot weld due to the tensile stress arisen from the off-centered squeeze force as well as the reduced surface cooling to electrodes. Initiation of cracks between the steel sheets, taking place mainly after electrode retraction, is not significantly affected by the welding schedule and the electrode tip diameter.

Research on the Causes and Treatment of Weld Zone Fracture in Cold Rolled High Strength Steel Rolling Process

In response to the poor quality of laser welding of cold-rolled advanced high-strength steel and the problem of easy fracture at the weld seam during the rolling process, the reasons for the fracture at the weld seam were first analysed using scanning electron microscopy and other equipment, to clarify the fracture characteristics and organizational distribution. Combined with the characterization results of mechanical properties, the reasons for the fracture at the weld seam during the rolling process were elucidated; Then, the influence of welding process on weld quality is studied from four aspects: laser power, welding speed, pre heat power, and post heat power; Based on the above analysis, a typical cold rolled advanced high-strength steel laser welding technology has been developed. By optimizing welding parameters such as welding power, speed, and preheating before and after, the quality of the weld seam and the cup protrusion effect have been ensured. At the same time, a reasonable rolling process optimization plan has been proposed; After applying the research results to the site, the quality of the weld seam has been significantly improved, and the occurrence rate of band breakage at the high-strength steel weld seam has decreased from an average of 3.6% per month to 0.13%. The band breakage defect at the weld seam has been effectively treated.

Mitigating hydrogen embrittlement of advanced high-strength steel by controlling carbides (cementite and alloy carbides) and microstructural modification

This work aims to demonstrate that the resistance to hydrogen embrittlement of conventional advanced high-strength steel (AHSS) with a bainitic ferrite structure can be significantly enhanced through two metallurgical strategies. Firstly, a smaller fraction of cementite in a mixture of bainitic ferrite and lath-martensite structures is realized by an intercritical annealing process with rapid cooling. Secondly, a higher fraction of alloy carbides (MC) with a mean size of less than 30 nm is formed through the combined addition of Ti, Mo, and V as micro-alloying elements. The reduction of cementite precipitation in the matrix leads to an increased resistance to hydrogen evolution/adsorption on the surface under cathodic polarization. Moreover, a large number of V-bearing nanoprecipitates ((Ti,Mo,V)-carbides and V-carbides) provides intermediate trapping for hydrogen atoms, effectively delaying the diffusion kinetics of hydrogen towards potential crack-forming areas in the matrix. Consequently, the lower strain reduction during slow strain rate test and the higher resistance to cracking during the four-point bending test in acidic aqueous solutions are manifested in the newly designed AHSS.

Strength of Bolted Lap Shear Connections with Advanced High-Strength Steel Sheets

Strength of Bolted Lap Shear Connections with Advanced High-Strength Steel Sheets

Behavior of TRIP-aided medium Mn steels investigated by in situ synchrotron X-ray diffraction experiments and microstructure-based micromechanical modelling

Medium Mn steels belong to a new generation of advanced high-strength steels whose superior mechanical properties are explained by their ultrafine-grained ferrite/austenite/martensite microstructures and a possible transformation induced plasticity (TRIP) related to the stability of retained austenite. The mechanical behaviour of a set of model medium Mn steels is investigated during tensile testing using a combination of high-energy X-ray diffraction (HEXRD) and digital image correlation (DIC) measurements. HEXRD allows for the time-resolved determination of transformation kinetics and in situ stress partitioning among the different constituting phases. DIC provides precise spatiotemporal information on the strain evolution along the gauge length, particularly at the position of the diffracting volume. These experiments served to calibrate an innovative mean field micromechanical model which accounts for the local behaviour of each phase, as well as the strain-induced martensitic transformation (SIMT) of retained austenite. The work-hardening of both austenite and ferrite is modeled using a dislocation-based size-sensitive approach which includes kinematic hardening contributions. The behaviours of fresh and strain-induced martensite are predicted using a genuine model derived from the continuous composite approach. The model for SIMT is based on a thermodynamic assessment of the stability of retained austenite. The overall model is thus sensitive to the size of the microstructure components, their local chemistry, and their respective stability.

Hydrogen Diffusion and Trapping Behaviour in DP Steels: An Electrochemical Hydrogen Permeation Study

Advanced high-strength steels (AHSSs) are widely used in the automotive industry as they possess high strength-to-weight ratio. This allows designing light vehicles with better passenger safety, lower fuel consumption, and CO2emissions. Dual-phase (DP) steels are very attractive among the AHSSs because of their optimal combination of high tensile strength (up to 1.2 GPa), high ductility (10 % or more), and good formability due to their two-phase microstructure (ferrite and martensite). However, the use of DP steel is associated with an increased risk of mechanical degradation in the presence of hydrogen environment, i.e., hydrogen embrittlement (HE). The HE susceptibility of DP steel is mainly determined by hydrogen entry, diffusion, and trapping events in the microstructure (i.e., hydrogen interaction with metal). There are various methods to study the above mechanism in metals like hydrogen charging-tensile test, thermal desorption spectroscopy, silver decoration, scanning kelvin probe microscopy, electrochemical hydrogen permeation, etc. Among these, electrochemical hydrogen permeation using Devanathan - Stachurski (DS) cell is a simple and most widely used technique.In this work, electrochemical hydrogen permeation was used to understand the role of microstructural features on hydrogen diffusion and trapping behaviour in DP-980 steel. It was identified that the phase fraction and its morphological features influence the hydrogen diffusion behaviour. The challenges in layout, design and integration of conventional DS cell into a tensometer and the effect of pre-strain (ex-situ) and tensile stress (in-situ) loading on hydrogen diffusion and trapping in DP-980 will be discussed. The applied elastic stress (i.e., 50 % of YS) during the permeation test resulted in a slight increase in Dapp from 5.4 ± 0.1 × 10- 7 to 5.9 ± 0.69×10-7 cm2s-1 and a slight reduction in NT from 1.08 ± 0.01 × 1021 to 1.0 ± 0.13× 1021 sites cm-3 compared to the no-stress condition. At 110 % of YS, the highest value of iss is 169 ±10.6 (µAcm-2) and a significant increase in the value of Capp to 3.6 ± 0.3 × 10-4 (molHcm-3 ), whereas no significant change in values of Dapp and NT were observed at 110 % YS compared to no stress and 50 % of YS. This could be due to the elastic deformation of martensite at 110 % of YS. The lattice expansion of martensite under elastic tensile stress can accommodate more interstitial hydrogen, thereby higher steady-state permeation current is observed. At 125 % of YS, the value of Dapp (2.86×10-7 cm2s-1) is approximately 50 % less compared to the value observed for the un-stressed specimen (5.4 ± 0.1×10-7 cm2s- 1). The value of NT (2.11×1021 cm-3) is approximately two times higher than that of the value (1.08×1021 cm-3 ). Plastic deformation of both ferrite and martensite results in increased dislocation density (hydrogen trap sites) at 125 % of YS. The sample failed during the hydrogen permeation test and when it is subjected to 125 % YS. It exhibited distinct fractographic features along the thickness of the sample. Quasi-cleavage and intergranular cracks were observed near the cathodic surface. Whereas, dimples and microvoids which are complete ductile features, were observed near the anodic surface. At 125 % of YS a significant decrease in the value of DL 0.6×10-6 cm2s-1 was observed. Whereas the value of NTrev increased to 4.0 ± 0.2×1019 cm-3 from 3.0 ± 0.15×1019 cm-3.An attempt was made to study the hydrogen transport behaviour at the local spot by appropriate modification of conventional DS cell. The fabrication challenges and the potential application of such an instrument will be elucidated.