Ferrite-bainite Dual Phase Steel Research Articles

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.

Effect of Coiling Temperature on Microstructure and Properties of Ferritic-Bainitic Dual-Phase Steels

In this study, a 780 MPa grade ferritic-bainitic dual-phase steel with excellent matching of strength-plasticity and formability was developed using thermomechanical control processing. Optical microscopy, Scanning electron microscopy, and Electron Backscatter Diffraction techniques were used to characterize the microstructure comprehensively, and the effects of coiling temperature on the microstructure, the strength-plasticity, and hole-expansion ratio of the test steels were thoroughly investigated. The results showed that the test steel had an excellent combination of ferrite and bainite at the coiling temperature of 520 °C, 23.7 and 76.3%, respectively, with a hole expansion ratio of 58.5 ± 2.8%. The uniformity of the microstructure was the key to obtaining a high expansion ratio in ferrite-bainite dual-phase steels. The test steels formed granular bainite at low-temperature coiling, while polygonal ferrite was promoted at high-temperature coiling. The effect of coiling temperature on grain size is small. Dislocations were redistributed during high-temperature coiling, resulting in a decrease in dislocation density. The higher elongation and hole expansion rate at higher coiling temperatures were attributed to increased polygonal ferrite content, reduced grain size, and enhanced TRIP effect. When coiling at low temperatures, the agglomeration of polygonal ferrite or granular bainite tends to result in a non-uniform distribution of the soft and hard phases of the matrix. At the same time, the strong texture parallel to the rolling direction has a significant difference in plasticity in different directions, leading to non-uniform deformation, which is liable to stress concentration, causing crack nucleation and extension in the hole expanding process, thus reducing the hole expansion performance.

Corrosion mechanism of advanced high strength dual-phase steels by electrochemical noise analysis in chloride solutions

The corrosion behavior of ferrite-martensite and ferrite-bainite dual-phase steels in NaCl, CaCl2, and MgCl2 solutions was studied by cyclic potentiodynamic polarization (CPP) and electrochemical noise (EN) techniques. The results showed a trend for mixed corrosion due to no passivation by a protective oxide layer. For the ferrite-martensite dual-phase steels, the ferrite phase was preferentially corroded compared with the martensite phase in test solutions. Bainite-microstructure in ferrite-bainite dual-phase steels was preferentially corroded compared with ferrite-phase in CaCl2 and MgCl2 solutions. The corrosion behavior in test solutions in dual-phase steels could be divided into three steps: dissolution in anodic-phase/microstructure, non-uniform corrosion, and localized corrosion. The results indicated that factors such as galvanic coupling between ferrite/martensite, ferrite/bainite, metallic-matrix/inclusion-phases, metallic-matrix/secondary phases, and self-corrosion of each phase and microstructure contributed to the non-uniform and localized corrosion in dual-phase steels.

The Effect of Bainite Volume Fraction on Wear Behavior of AISI 4340 Ferrite–Bainite Dual-Phase Steel

The Effect of Bainite Volume Fraction on Wear Behavior of AISI 4340 Ferrite–Bainite Dual-Phase Steel

Machinability study and optimization of tool life and surface roughness of ferrite: Bainite dual phase steel

Aim of this present research work is to obtain the machining parameters to optimize the tool life and surface roughness for ferrite-bainite dual phase steel. Machinability tests are carried out using orthogonal array of 27, the Taguchi method, in which the machining parameters are considered as control factors. The effect of speed, feed and depth of cut on tool life and surface roughness of dual phase structure steel is analysed using ANOVA. Regression analysis is used to obtain the equations for predicting the tool life and surface roughness. Experiment is conducted using uncoated carbide insert tool by varying the process parameters. Optimum tool life and surface is analysed using Response Surface Methodology. Hardness and microstructure revealed the dual phase condition in different intercritical zones. It is found that hardness improves as the intercritical temperature is increased from 750 to 770°C. Experimental results prove that dual phase structure has better machining characteristics at an intercritical temperature of 750°C.

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.

Study on Fracture Mechanism and Microstructural Evolution in 780 MPa Grade FB Steel with a High Hole Expanding Ratio

FB780 steel with a high hole-expanding ratio, as ferrite–bainite dual-phase steel, is an indispensable material to manufacture different automobile parts. In the present study, uniaxial tension tests and metallographic analyses were conducted to reveal the fracture mechanism in FB780 steel. TiN particles ruptured and formed micron-sized crack sources due to stress concentrations, and voids were generated by dislocation pile-ups in Fe3C particles. Intragranular dislocations mainly accumulated around second-phase particles and grain boundaries. And these dislocations started to manifest intertwining characteristics with the prolonged deformation and formed serious stress concentrations, thus finally, turned into the crack sources. Hence, the aggregation and growth of crack sources and voids resulted in ductile fracture. In addition, the fraction of low-angle grain boundaries in ferrite grains increased with the prolonged deformation. Moreover, the γ fiber texture and the {001} texture in the tensile deformation area facilitated the propagation of cracks. Small-sized newborn grains appearing in the tensile fracture mainly accumulated at low-angle grain boundaries and facilitated center separation during fracture.

Electrochemical Noise Measurements of Advanced High-Strength Steels in Different Solutions

Advanced high-strength steels (AHSS), are commonly used in the manufacture of car bodies, as well as in front and rear rails, and safety posts. These components can be exposed to corrosive environments for instance, in countries where de-icing salts are used. In this work, the corrosion behavior of four AHSS steels with dual-phase [ferrite-martensite (DP) and ferrite-bainite (FB)] steels were studied by means of electrochemical noise (EN) measurements according to the ASTM G199-09 standard in NaCl, CaCl2 and MgCl2 aqueous solutions at room temperature. The direct current (DC) trend data from EN were removed by a polynomial method of statistical and spectral analysis. According to the noise resistance (Rn) values obtained for the DP and FB dual-phase steels, both the martensite/bainite content and morphology of the phase constituents have an important effect on the corrosion behavior of these steels. The L.I. (localization index) (0.00054 to 0.15431), skewness (−6.18 to 7.35) and kurtosis (high values 37.15, 74.84 and 106.52) were calculated. In general, the results indicated that the main corrosion process is related to uniform corrosion. Corrosion behavior of AHSS steels exposed in NaCl solution could be related to the morphology of the phase constituents exposed in NaCl, CaCl2 and MgCl2 solutions.

Comparison of the fatigue performance of ferrite–pearlite and ferrite–bainite dual-phase steels

Recent demands for large vessels and deep water exploration have increased the requirements for thicker plates to ensure the sufficient strength of such structures. The use of thicker plates, however, has an adverse influence on the fatigue performance. To enhance the fatigue life, post-weld treatments, such as weld-toe grinding, TIG (tungsten inert gas) dressing, hammer peening, and high-frequency mechanical impact (HFMI) treatments are commonly employed. On the other hand, these techniques require additional fabrication time and production cost. Therefore, there is keen interest with respect to thick steel plates with improved fatigue strength. This study examined the fatigue characteristics of conventional steel (Ferrite–Pearlite, F–P steel) and Ferrite–Bainite dual-phase steel (F–B steel). The fatigue behavior of both steels was investigated by a series of fatigue tests with three different types of welded joints. In case of gusset and cruciform-welded joints, fatigue strength of F–B steel is higher than that of F–P steel. F–B steel is known to exhibit improved fatigue performance associated with the existence of bainite microstructure. In addition, the fatigue data in this study are validated against the existing literatures. Based on the fatigue test results, a modified fatigue design curve is suggested from this study. In order to discuss the effects of fatigue crack path, the fatigue crack growth rate (FCGR) of both steels was investigated in terms of Paris’ law. Finally, microstructural analysis was conducted to discuss the fatigue performance.

Study of Microstructure and Mechanical Properties of Hot-Rolled Ultra-High Strength Ferrite-Bainite Dual Phase Steel

Effects of thermo-mechanical control processing (TMCP) on microstructure and mechanical properties of hot-rolled ultra-high strength ferrite-bainite dual phase steel were investigated on a laboratory hot rolling mill. The results have shown that the microstructure containing ferrite and bainite can be obtained by TMCP. Ultimate tensile strength of all the specimens exceeded 1000MPa. Finish rolling temperatures affect the mechanical properties of ultra-high strength ferrite-bainite dual phase steel. Ultimate tensile strength reached 1078MPa at relatively low finish rolling temperature because of the ferrite grains refined. Fast cooling after low temperature rolling results in the ferrite grains refined and the formation of martensite islands. As a result, the product of ultimate tensile strength and total elongation (Rm×A50) of specimen 4 with fast cooling after low temperature rolling reaches the maximum value (18096MPa%).

The role of fibrous morphology on the Charpy impact properties of low carbon ferrite-bainite dual phase steel

Primarily ferrite-pearlite microstructure having coarse ferrite grain size (24µm) and high pearlite fraction (42%) offered YS ∼ 575MPa with poor impact properties such as, upper shelf energy (USE) of only 30J and ductile brittle transition temperature (DBTT) as high as 27°C in an industrially hot-rolled plate of 0.25wt% C steel. In order to improve the strength along with the impact properties by developing ferrite-bainite microstructures, two different types of heat-treatments, namely step-cooling (SC) and intermediate cooling (IC) treatments, were carried out on the as-received material. Significant improvement in strength along with the impact toughness (with YS of 740MPa, USE of 222J and DBTT of − 57°C) has been achieved by developing fibrous microstructure, with alternate thin-films (2–4µm thick) of ferrite and bainite through intermediate cooling (IC) treatment. Fine film-like structure with large orientation difference across the ferrite-bainite interface boundaries not only increased the strength but also resulted in frequent deflection in cleavage crack propagation path which improved the low-temperature impact toughness and reduced the DBTT.

Effects of non-recrystallization zone reduction on microstructure and precipitation behavior of a ferrite-bainite dual phase steel

A dual-phase pipeline steel with high deformability was processed by a two-stage controlled rolling and followed by relaxation process before accelerated cooling. The mechanical properties of the samples were studied, and the effects of different reductions in the second rolling stage on the microstructure and precipitation behavior of the steel were analyzed using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), positron annihilation technique (PAT) and physicochemical phase analysis. The samples mainly consisted of ferrite, bainite, and a small amount of martensite/austenite (M/A) islands. When the reduction increased to 80% in the second rolling stage, lower yield ratio and better impact toughness were obtained, and the microstructure with 49.6% (volume fraction) ferrite and higher dislocation density became more homogenous. The nominal chemical formulas of M(C, N) for sample A and B were (Nb0.735Ti0.260Cr0.005)(C0.420N0.580) and (Nb0.757Ti0.237Cr0.006)(C0.464N0.536), respectively and the phase structure of precipitates of both samples were almost the same. The strain induced more precipitation of M(C, N) in the non-recrystallization zone of austenite than in the intercritical zone. Also the fraction of M(C, N) and the precipitation strengthening effectiveness were promoted by the increase of reduction.

Microscopic deformation and strain hardening analysis of ferrite–bainite dual-phase steels using micro-grid method

The local strain measurement method using nanometer-scaled micro grids printed on the surface of a specimen by an electron lithography technique (the micro-grid method) has been established. Microscopic deformation behavior of the ferrite–bainite steels with different bainite volume fraction, 16% and 40% of bainite, was evaluated. Strain localization in the ferrite phase adjacent to the ferrite/bainite boundary was clearly observed and visualized. Highly strained regions expanded toward the inner region of the ferrite phase and connected each other with an increase of macroscopic strain. The existence of hard bainite phase plays an important role for inducing strain localization in the ferrite phase by plastic constraint in the boundary parallel to the tensile direction. In order to obtain further understanding of microscopic deformation behavior, finite element analysis using the representative volume element, which is expressed by the axisymmetric unit cell containing a hard phase surrounded by a soft phase matrix, was conducted. It was found that the macroscopic stress–strain behavior of ferrite–bainite steels was well simulated by the unit cell models. Strain concentration in the ferrite phase was highly enhanced for the ferrite-40% bainite steel, and this imposed higher internal stress in the bainite phase, resulting in higher strain hardening rate in the early stage of the deformation. However, smaller ferrite volume fraction of ferrite-40% bainite steel induced bainite plastic deformation in order to fulfill the macroscopic strain of the steel. Accordingly, strain hardening capacity of the ferrite-40% bainite steel was reduced to a significant degree, resulting in a smaller uniform elongation than the ferrite-16% bainite steel.

Effect of Strengthening Phase on Deformation Behaviour during Uniaxial Tension of Hot-rolled Dual Phase Steel

Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite (F) + martensite (M) or ferrite (F) + bainite (B) with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength (YS/TS) and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible deformation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.

Fatigue crack growth behaviour of ferrite–bainite dual phase steels

The effect of bainite content on the fatigue threshold characteristics of ferrite–bainite dual phase steels is reported. Five different ferrite–bainite dual phase microstructures are prepared through suitable heat treatment and are subjected to hardness, tensile and fatigue tests apart from microstructural characterisations. Fatigue threshold ΔKth values of the steels are estimated using an earlier reported non-conventional method and the data are analysed to study the role of bainite content on ΔKth. The ΔKth values first decrease and then increase with increasing bainite content in the investigated steels. The nature of variation of ΔKth with bainite content has been explained using the characteristics of the constituent phases.

Short Crack Growth Behavior in Ferrite-Bainite Dual Phase Steels

The effect of bainite content on the short crack growth behavior of three ferrite bainite dual phase steels has been examined using a rotating bending machine together with examinations of the microstructural constituents associated with the fatigue crack path. The magnitude of ΔKthsc for ferrite bainite dual phase steels, in general, is found to increase with increasing amount of bainite. Critical examinations of the crack paths indicate that it is inter-granular for steels with < 70% bainite while it is intra-granular for steels containing > 70% bainite.

Corrosion behavior of each phase in low carbon microalloyed ferrite–bainite dual-phase steel: Experiments and modeling

In situ observation of the initial corrosion behavior of a low carbon microalloyed ferrite–bainite dual-phase steel showed that the corrosion originated from the inside of ferrite and ferrite boundary. In addition, a model for describing the corrosion behavior of each phase in multiphase steel was established. Based on this model, a method to quantitatively assess the corrosion rate of each phase was presented by white light interference, and the relationship between the surface roughness and corrosion morphology was also established. Meanwhile, the galvanic corrosion at phase-scale and the influence of phase distribution on service safety of multiphase steel were discussed.

In-situ investigation on the fatigue crack propagation behavior in ferrite-pearlite and dual-phase ferrite-bainite low carbon steels

The purpose of this study was to describe the roles of microstructure types and grain boundary characteristics in fatigue crack propagation behavior in ferrite-pearlite steel and ferrite-bainite steel. The ferrite-bainite dual-phase steel was obtained by intermediate heat treatment conducted on ferrite-pearlite low carbon steel. This paper presents the results from investigation using constant stress-controlled fatigue tests with in-situ scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and fatigue fractography analysis. Microscopic images arrested by in-situ SEM showed that the second hard bainite phase distributed in the soft ferrite matrix had a significant effect on preventing the cracks opening compared with pearlite, and that the cracks in ferrite-bainite steel were “locked” in the second hard bainite phase while the crack propagation path in ferrite-pearlite steel was more tortuous. Moreover, the fatigue fracture surface analysis and the coincidence site lattice (CSL) obtained by EBSD indicated that low-Σ CSL grain boundaries in ferrite-bainite steel distributed more uniformly, which has a more significant effect on the resistance of crack propagation. It was revealed that ferrite-bainite dual-phase microstructures could inhibit the fatigue crack propagation more effectively than ferrite-pearlite microstructures.

Development of Laminar Flow Cooling of Ultra-High Strength Ferrite-Bainite Dual Phase Steel

In the present paper, controlled cooling in different ways was performed using a laboratory hot rolling mill in ultra-high strength hot rolled ferrite-bainite dual phase (DP) steel. The results have shown that the final microstructures of DP steel comprise ferrite, bainite and a small amount of retained austenite and martensite. DP steel has a tensile strength ranging from 1010 to 1130MPa and yet retains considerable total elongation in the range of 14–17%. The addition of Mn and Nb to DP steel leads to the maximum ultimate tensile strength, yield strength and the product of ultimate tensile strength and total elongation due to the formation of retained austenite and granular bainite structure. Laminar flow cooling after hot rolling results in a significant increase in the quantity of ferrite and bainite due to the suppression of pearlite transformation, and as a result, the present steel possesses high strengths and good toughness.

Influence of temperature field on the microstructure of low carbon microalloyed ferrite–bainite dual-phase steel during heat treatment

Abstract To investigate the quantitative relation between the temperature and the microstructure of low carbon steel, the low carbon microalloyed ferrite–bainite dual-phase (FBDP) steel was prepared by heat treatment. On a series of positions chosen in the sample which was cut from the treated FBDP steel, the volume fractions of bainite in the microstructures were measured and the corresponding temperatures were also obtained through the finite element method. The results show that not only the volume fraction of bainite increases linearly, but also the size of bainite increases with the increase of the outset water-cooling temperature. In addition, by comprehensively evaluating the microstructure and the temperature, it can be estimated that the lowest bainite formation temperature for this steel is about 550 °C, which had been proved by the experiment.