Ferrite-bainite Steel Research Articles

Achieving an excellent balance of strength, plasticity, and stretch flangeability in a 1000 MPa grade TRIP-assisted bainitic ferrite steel

To address the requirements for safety enhancement, weight reduction, and emission reduction, a novel transformation-induced plasticity-assisted bainitic ferrite (TBF) steel was processed by involving a pre-quenching (PQ) process. This steel was designed with varying PQ temperatures to achieve different fractions of intercritical ferrite and bainite, and its influence on mechanical properties of TBF steels was thoroughly investigated through tensile tests and hole expansion tests. The results demonstrated that the introduction of high-temperature PQ treatment before heat treatment can dramatically accelerate the formation of austenite during the secondary annealing process, thereby facilitating a more sufficient bainitic transformation in the subsequent isothermal process. With the increase of PQ temperature from 750 to 900 °C, abundant intercritical ferrite was replaced by lathy bainite. In comparison with conventional specimens, the high-temperature PQ treatment could reduce the fraction of intercritical ferrite and martensite-austenite (M-A) islands, in favor of delaying the initiation of microvoid and microcrack and improving local formability. In summary, the P-900 specimen exhibited optimal comprehensive mechanical properties, with the highest yield strength of 657 MPa, ultimate tensile strength of 1081 MPa, elongation of 28.7%, and hole expansion ratio of 35.1%.

Stretch Flangeability of Low Carbon Micro-alloyed Ferrite-Pearlite and Ferrite-Bainite Steel

Stretch Flangeability of Low Carbon Micro-alloyed Ferrite-Pearlite and Ferrite-Bainite Steel

Enhancing ductility of the TRIP aided bainitic ferrite steel by Mn heterogeneity introduced via reversion: Towards the 3rd generation

A novel approach to enhance the ductility of the transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel was proposed in this work. Lamellar lath Mn-heterogeneity was introduced into the single austenite grain through austenite-reversion from martensite by slow-heating or intercritical annealing. Acicular austenite is formed accompanied with apparent Mn enrichment occurring during reversion, and such a lamellar lath Mn-inhomogeneity is remained even after full-austenitization. Highly stable retained austenite (RA) is formed at the original Mn-enriched region after austempering with higher Mn and carbon contents than those of traditional austempering from Mn-homogenized austenite. In addition, higher total fraction and larger ratio of film-like RA is obtained in the heterogeneous-austenite than homogeneous one. These contribute to the ductility improvement of the TBF steel.

Simultaneous improvement of strength and uniform elongation of ferrite-bainite steel with heterogeneous networked microstructure

A novel heterogeneous networked dual-phase structure, composed of bainite grains surrounding the ferrite grains and forming a networked structure, was successfully introduced into the low-carbon ferrite-bainite steel through a two-step heat treatment process. The ferrite-bainite steel with the networked structure exhibited a high ultimate tensile strength of 872 ± 39 MPa, a good uniform elongation of 15.59 %, and a high total elongation of 22.79 %. Meanwhile, it possessed excellent work hardening behaviors compared to the ferrite-bainite steel with an island feature. The improvement of the comprehensive mechanical properties was attributed to the influence of hetero deformation induced (HDI) hardening and the synergistic deformation of the dual-phase structure.

Microstructural evolution and carbides precipitation behavior and their effects on mechanical property of Nb–Ti microalloyed FB590 steel

Ferrite-bainite dual phase steels have been widely used in automobile manufacturing industry due to their high strength and excellent hole expansion performance. In this study, the effects of coiling temperature on mechanical properties and hole expansion performance of low carbon Nb–Ti micro-alloyed FB590 ferrite-bainite steel have been studied. Additionally, the microstructure evolution and carbide precipitation behavior during hot rolling and coiling processes have been investigated. The influence mechanism of microstructures and carbides on both mechanical properties and hole expansion performance has also been discussed. The results suggest that the fraction of the retained austenite and the martensite decreases gradually with increasing coiling temperatures from 450 °C to 500 °C. The main reason for the formation of martensite and retained austenite in steel CT450 coiled at 450 °C is that the coiling temperature is near the martensitic transformation temperature Ms. Increasing the coiling temperature to 500 °C which is above the Ms leads to ferritic and bainitic structure. The carbides precipitation behavior has been studied by TEM, thermodynamic and kinetics calculation. The results show that the size of the carbides increases with increasing coiling temperatures from 450 °C to 500 °C. Most of the large size carbides above 100 nm with high percent of Ti are the undissolved particles in austenite during slab heating. The medium size particles between 50 nm and 100 nm and enriched in Ti precipitate during rough rolling at higher temperature, while the particles smaller than 50 nm enriched in Nb precipitate during finish rolling and coiling processes. The strength of the steel is effectively increased by the fine carbides and martensite formed at low coiling temperature of 450 °C. With increasing coiling temperature from 450 °C to 500 °C, the steel's strength decreases from 630 MPa to 591 MPa, while the HER are improved from 76.8 % to 119.9 % due to the reduction of martensite and retained austenite. Although the strength is a little reduced due to the coarsening of carbides and reducing of the martensite, the plasticity and hole expansion performance is improved significantly with the coiling temperature of 500 °C.

Unstable stress-triaxiality development and contrasting weakening in two types of high-strength transformation-induced plasticity(TRIP) steels: Insights from a new compact tensile testing method

The impact of high stress triaxiality on work hardening in transformation-induced plasticity (TRIP) steel has been widely acknowledged, particularly through measurements of the austenite fraction. Understanding this TRIP behavior is crucial for predicting material fracture in press-forming processes. However, the actual flow stresses under high-stress-triaxiality conditions remain largely undetermined. To address this gap, we developed a new tensile testing method using tiny notched round bars to investigate stress-triaxiality-induced work hardening in TRIP steels. The specimens were analyzed using two-dimensional micrometry to allow finite element analyses to identify the flow stress. Additionally, we conducted in situ tensile tests in which their crystal lattice stresses were monitored using synchrotron X-ray diffraction (XRD) to realize mechanism analyses of the unexpected work-hardening behavior identified by the developed tensile testing method. Our combined approach revealed a mutual, unstable increase in the flow stress and stress triaxiality in the TRIP-aided bainitic ferrite steel, which reduced the hardening exponent coefficients and thus induced a higher stress triaxiality. In contrast, the TRIP-aided martensitic steel exhibited a weakening behavior, characterized by a significant decrease in the hardening exponent coefficients in the case of the sharpest notch. XRD analyses showed that microstructural heterogeneity led to an extraordinarily high hydrostatic stress in the austenite phase, accounting for these contrasting behaviors. This finding challenges the established consensus on TRIP steels and suggests the need for a revised framework for their application in press-forming, taking into account stress-triaxiality conditions.

Thermal characterization by dilatometry tests of a ferritic-bainitic (FB) steel for automotive application

The main objective of this article is to deep the knowledge of the thermal behavior of ferrite-bainite (FB) steel in correlation with the structural aspects that justify the mechanical properties. Samples of a DP-590 steel were tested by dilatometry considering a heating rate of 0.17 °C/s, up to a temperature of 1150 °C and different cooling rates between 0.03 °C/s to 100 °C/s, in air atmosphere. The results of the thermal behaviour were correlated with a microstructural study using optical microscopy in order to establish the final proportion of ferrite and bainite that is achieved at each cooling rate considered. As the cooling rate increases, the diffusion processes are limited so that hard second phases are obtained. In this steel, the presence of bainite was corroborated. The mechanical strength evolution associated with the cooling rate was determined through microhardness measurements. On this base, the steel's continuous CCT curves was built.

Neural network modelling of the Nb effect on mechanical properties of the ferritic bainitic dual-phase steel

ABSTRACT The production of low-alloy advanced high-strength steels such as the dual-phase (DP) steel with ferritic-bainitic microstructure using a hot strip mill is challenging in terms of consistent mechanical properties-based DP steel without violating the hot strip mill rolling capacity. In the present study, the Artificial Neural Network (ANN) technique was employed to develop tensile strength, yield strength and %Elongation models which can cater to complex relationships between the DP steel mechanical properties as a function of steel composition and rolling parameters. Furthermore, ANN models were used to predict the Nb effect on the mechanical properties of the DP steel. Model prediction and validation confirmed that niobium has a notable influence on mechanical properties. Finally. Nb’s role during hot rolling was reviewed and justified.

Temperature dependence of hydrogen-assisted damage evolution and fracture behavior in TRIP-aided bainitic ferrite steel

We systematically investigated the effects of hydrogen and deformation temperature on the micro-damage evolution and fracture behavior of the TRIP-aided bainitic ferrite steel using slow strain rate tensile tests and microstructure examinations. For hydrogen-uncharged condition, the micro-damage evolution behavior was not dependent on temperature, except for −100 °C. The exceptional behavior at −100 °C was attributed to the occurrence of brittle fracture. Hydrogen uptake significantly increased the number density of micro-damage in all deformation temperatures. The fracture mode of hydrogen-charged specimens changed from quasi-cleavage and ductile fractures to cleavage, quasi-cleavage, and intergranular fractures with decreasing deformation temperature. Quasi-cleavage fracture features were attributed to the crack growth process consisting of repetition of small crack initiation/blunting (or void initiation) and crack coalescence, whereas the cleavage and intergranular surfaces with smooth facets were due to the occurrence of hydrogen-reduction cohesive strength of cleavage planes and prior austenite grain boundaries.

Effect of microstructure on impact toughness, texture, and nano-indentation on heat-affected zones (HAZ) in a ferrite-bainite steel

Effect of microstructure on impact toughness, texture, and nano-indentation on heat-affected zones (HAZ) in a ferrite-bainite steel

StressLife: A Short-Time Approach for the Determination of a Trend S-N Curve in and beyond the HCF Regime for the Steels 20MnMoNi5-5 and SAE 1045.

Within the scope of this research, a new short-time procedure designated as StressLifeHCF was developed. Through a combination of classic fatigue testing and non-destructive monitoring of the material response due to cyclic loading, a process-oriented fatigue life determination can be carried out. A total of two load increases and two constant amplitude tests are required for this procedure. By using data from non-destructive measurements, the parameters of the elastic approach according to Basquin and the plastic approach according to Manson-Coffin were determined and combined within the StressLifeHCF calculation. Furthermore, two additional variations of the StressLifeHCF method were developed in order to be able to accurately describe the S-N curve over a wider range. The main focus of this research was 20MnMoNi5-5 steel, which is a ferritic-bainitic steel (1.6310). This steel is widely used for spraylines in German nuclear power plants. In order to validate the findings, tests were also performed on an SAE 1045 steel (1.1191).

Synergistic effects of hydrogen and deformation temperature on mechanical properties of TRIP-aided bainitic ferrite steel

Mechanical properties of TRIP-aided steel with bainitic ferrite matrix in the presence of hydrogen were investigated at the deformation temperature range of −100 to 100 °C. In hydrogen-uncharged condition, the yield strength (YS) first increased and then remained almost unchanged, while the highest uniform elongation (UEl) was observed at a deformation temperature of 21 °C. Hydrogen charging slightly increased the YS and deteriorated the UEl at all the temperature ranges. In the hydrogen-uncharged and hydrogen-charged specimens, the transformation ratio of retained austenite increased with a decrease in the deformation temperature; however, the hydrogen charging decreased the transformation ratio. The fracture behavior drastically changed when hydrogen was introduced, owing to which deformation-induced martensitic transformation occurred and hydrogen played dual roles in accelerating brittle-like cracking.

Effects of thermo-mechanical treatments on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel

The effects of deformation at 900 °C and 1050 °C along with various cooling rates on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel are investigated. Without prior high temperature deformation, the microstructures can be polygonal ferrite, pearlite, granular ferrite, acicular ferrite or bainite ferrite, depending on the cooling rate. High temperature deformation moves the transformation curves toward the left side in the continuous cooling transformation (CCT) diagram and promotes the formation of polygonal ferrite, granular ferrite, and acicular ferrite, which leads to a lower hardness. Electron backscattering diffraction (EBSD) results indicated that with a cooling rate of 40 °C/s, high proportion of low angle grain boundary (LAGB), large orientation gradient, and high dislocation density and residual stress can be obtained by deformation at high temperature before cooling. Low cooling rate reduces the proportion of LAGB, orientation gradient, dislocation density, and residual stress due to the release of residual stress and the consumption of dislocations. Deformation before cooling slightly lowers the transformation start temperature (Ar3) while accelerating the transformation speed of the supercooled austenite. The yield and tensile strength increase with the increase in cooling rate. Furthermore, the impact energy value increases with the increase of cooling rate, and it slightly decreases when the microstructure fully changes to bainite ferrite owing to the significant increase in strength. Impact toughness is more sensitive to strength at low temperatures.

Feasibility Study on Additive Manufacturing of Ferritic Steels to Meet Mechanical Properties of Safety Relevant Forged Parts.

Additive manufacturing processes such as selective laser melting are rapidly gaining a foothold in safety-relevant areas of application such as powerplants or nuclear facilities. Special requirements apply to these applications. A certain material behavior must be guaranteed and the material must be approved for these applications. One of the biggest challenges here is the transfer of these already approved materials from conventional manufacturing processes to additive manufacturing. Ferritic steels that have been processed conventionally by forging, welding, casting, and bending are widely used in safety-relevant applications such as reactor pressure vessels, steam generators, valves, and piping. However, the use of ferritic steels for AM has been relatively little explored. In search of new materials for the SLM process, it is assumed that materials with good weldability are also additively processible. Therefore, the processability with SLM, the process behavior, and the achievable material properties of the weldable ferritic material 22NiMoCr3-7, which is currently used in nuclear facilities, are investigated. The material properties achieved in the SLM are compared with the conventionally forged material as it is used in state-of-the-art pressure water reactors. This study shows that the ferritic-bainitic steel 22NiMoCr3-7 is suitable for processing with SLM. Suitable process parameters were found with which density values > 99% were achieved. For the comparison of the two materials in this study, the microstructure, hardness values, and tensile strength were compared. By means of a specially adapted heat treatment method, the material properties of the printed material could be approximated to those of the original block material. In particular, the cooling medium/cooling method was adapted and the cooling rate reduced. The targeted ferritic-bainitic microstructure was achieved by this heat treatment. The main difference found between the two materials relates to the grain sizes present. For the forged material, the grain size distribution varies between very fine and slightly coarse grains. The grain size distribution in the printed material is more uniform and the grains are smaller overall. In general, it was difficult and only minimal possible to induce grain growth. As a result, the hardness values of the printed material are also slightly higher. The tensile strength could be approximated to that of the reference material up to 60 MPa. The approximation of the mechanical-technological properties is therefore deemed to be adequate.

A dual-scale FE simulation of hole expansion test considering pre-damage from punching process

The effect of the hole-edge condition on the hole-expansion ratio (HER) of ferrite-bainite (FB) dual-phase steel is analyzed by developing a dual-scale finite element (FE) approach. The proposed dual-scale approach consists of macroscale simulation of hole expansion test (HET) and microscale simulation for capturing localized inhomogeneous deformation at the microstructure level. The macroscale HET simulation considers the experimentally measured hole-edge geometry and hardness profile under different clearance conditions. The deformation history in the region of interest calculated from the macroscale simulation was transferred to the microscale simulation as boundary conditions. In the microscale simulation, the deformation inhomogeneity within the two-phase microstructure was calculated by considering the dislocation pile-up at the grain boundaries. In the experimental aspect, the geometry and surface roughness of the hole edge, which are factors for quantifying the damage from the punching process, were measured using a confocal microscope. The work hardening of the material was quantitatively analyzed by measuring the hardness profile in the radial direction from the hole edge. For the work hardening due to the punching process, the measured hardness is converted into an equivalent plastic strain, which is transferred to both macro-and microscale models in the form of pre-strain. The predicted HERs are compared with the experimentally measured HER values, and it is confirmed that the damage caused by punching is critical to the hole expansion formability of the dual-phase FB steel.

Methodological features of microstructural heterogeneity estimation by the thickness of steel plates

The relationship of anisotropy coefficients estimated both by textural analysis and direct measurements has been studied using the example of low carbon low-alloy plate steel for Arctic application. The parallel between the anisotropy coefficients values and visual assessment of the structural inhomogeneity has been established, and also use cases of anisotropy coefficients to search for the “structure-property” relationships have been discussed. All the investigated coefficients with different accuracy describe the basic trends of heterogeneity changes by the thickness of the plate, but only the coefficient found by textural analysis makes it possible to evaluate the heterogeneity at any thickness point of the plate adequately, and this is always uniquely coincided with the visual assessment. The volume fraction of the coarse structural constituents with the highest anisotropy of ferritic-bainitic steel, estimated by direct measurements, can also be useful when searching for the “structure-properties” relationships.

Effect of austempering treatment on the microstructure and mechanical properties of 0.4C–1.5Si-1.5Mn TRIP-aided bainitic ferrite steel

In this study, the effects of austempering treatment on the microstructure, mechanical properties, and fracture mechanisms of Fe-0.4C-1.5Si-1.5Mn transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel were investigated. When the austempering time increased from 0 to 3600 s, the volume fraction of retained austenite and its carbon concentration initially increased and then decreased. The increase in carbon concentration during austempering may be attributed to the formation of fine bainitic ferrite, representing the matrix and in turn the formation of retained austenite with a film-type morphology. When the austempering time was increased from 0 to 1000 s, the TBF steel showed excellent mechanical properties such as high yield and tensile strengths, and uniform and total elongations. Meanwhile, void density increased significantly in the initial stages and then slightly decreased; in contrast, the trend observed for mean crack length was opposite to that of void density leading to the change in the fracture mode of the TBF steels from intergranular to quasi-cleavage and dimple fractures. It could be inferred that the observed improvement in mechanical properties, especially the good ductility of TBF steels, at the optimal austempering time is due to the suppression of crack propagation and crack blunting to form voids owing to the effective deformation-induced transformation of metastable retained austenite.

Development of Direct Hot-Rolled Ultralow-Carbon Pre-peritectic Ferrite-Bainite Dual-Phase Steel for a Compact Slab Production Plant

The current work developed a direct hot-rolled ultralow-carbon ferritic-bainitic dual-phase “FBDP” steel with high ductility values in accordance with a compact slab production CSP plant. Four ultralow-carbon alloys with chemical compositions of 0.045% C, 0.28 Si%, 0.91% Mn and different Cr weight contents of 0.59, 1, 1.56 and 2% were determined within the pre-peritectic region from phase diagrams plotted by Thermo-Calc 2020. These alloys were melted and cast into blocks, and then, a certain thermomechanical schedule was applied using the actual data obtained from the log files of a CSP plant. The cooling schedule was designed with the aid of CCT and TTT curves plotted by JMat Pro. The results show that hot-rolled FBDP steel contains a substantial amount of bainite from 10 to 28% and negligible martensite. The increase in Cr led to an increase in the bainite volume fraction and an increase in the yield strength and tensile strength, while the elongation decreased slightly. The strain hardening exponent (n-value) reached values ranging from 0.28 to 0.36, while a value of 0.18 was found in the literature for martensitic DP steel. Stroke-force curves plotted from the Erichsen test achieved high Erichsen index values from 14 to 18 mm.

Roles of Hydrogen Content and Pre-strain on Damage Evolution of TRIP-aided Bainitic Ferrite Steel

Roles of Hydrogen Content and Pre-strain on Damage Evolution of TRIP-aided Bainitic Ferrite Steel

Effects of Retained Austenite on Hydrogen Embrittlement in TRIP-aided Bainitic Ferrite Steel Sheet

Effects of Retained Austenite on Hydrogen Embrittlement in TRIP-aided Bainitic Ferrite Steel Sheet