Low Carbon Dual Phase Research Articles

Quantitative characterization of local deformation-fracture behavior in ferrite-martensite dual-phase steels with different martensite distributions

Low-carbon dual-phase (DP) steels, composed of a soft ferrite phase and a hard martensite phase, are known as promising advanced high-strength steels (AHSSs) due to their high strength and good ductility at low fabrication cost. However, the deformation behavior of DP steels is still not fully understood because of their complex mixed-phase distribution and mechanical interactions between two phases during deformation. The present study quantitatively investigated the effect of martensite distribution on mechanical properties and local deformation-fracture behavior, using the digital image correlation (DIC) technique. Two types of DP structures were prepared: one with a chained martensite distribution (chained DP) and one with an isolated martensite distribution (isolated DP). The chained DP specimen exhibited a superior tensile property, achieving both high strength and large ductility compared to the isolated DP specimen. DIC strain analysis revealed that the chained DP structure showed relatively homogeneous deformation due to the greater contribution of martensite to plastic deformation. In contrast, the isolated DP specimen experienced significant strain localization in the soft ferrite grains. Despite high global strains, non-plastically deformed zones were observed in the central regions of large, isolated martensite particles. Notable differences in micro-void evolution were also observed between the two specimens. The chained DP specimen had a large number of randomly distributed micro-voids, ranging from 0.5 μm to 2 μm in size. In contrast, the isolated DP specimen contained fewer micro-voids, with some aligned at an angle of ∼65° to the tensile direction, potentially leading to early tensile fracture.

Effect of martensite hardness on mechanical properties and stress/strain-partitioning behavior in ferrite + martensite dual-phase steels

Low-carbon dual-phase (DP) steels composed of ferrite and martensite are widely used in automotive industries due to their good combination of strength and ductility and remarkable strain-hardening ability. Such exceptional mechanical properties of DP steels arise from microstructural deformation heterogeneity due to the different deformation roles of soft ferrite and hard martensite. The present study investigated mechanical properties and local deformation behavior of DP steels having different martensite hardness (strength) using digital image correlation (DIC) technique and in-situ X-ray diffraction (XRD) experiment during tensile deformation. The martensite hardness was reduced by tempering treatment, and its mechanical properties were compared with the as-quenched DP specimen by tensile testing. The tempered DP specimen exhibited reduced strength but enhanced ductility, especially in post-uniform elongation, compared to the as-quenched DP specimen. DIC-strain analysis indicated that the tempering effectively inhibited strain localization, allowing a greater contribution of martensite to plastic deformation. On the other hand, the in-situ XRD analysis revealed that the tempering significantly restricted the stress-increasing potential of martensite. Our results suggest that martensite, acting as a hard domain, plays a significant role in enhancing strain-hardening ability and achieving large uniform elongation due to its high stress-increasing potential. However, the presence of hard martensite also imposes restrictions on post-uniform elongation due to the strong strain localization induced in microstructures.

The effects of heat-treatment parameters on the mechanical properties and microstructures of a low-carbon dual-phase steel

The austenite decomposition phase transformation in a low-carbon dual-phase (DP) steel is studied as a function of inter-critical annealing parameters: annealing time, annealing temperature, and cooling rate. The austenization kinetics are obtained by dilatometric analyses, water-quenched metallography, and thermodynamic calculations, and the austenite formed at different inter-critical annealing temperatures influences the subsequent formation of secondary phases during continuous cooling. Results indicate that the austenite volume fraction increases with both annealing temperature and time; although their effect on the final microstructure and mechanical properties lessens when the cooling rate decreases to the slow cooling regime below air cooling. Interestingly, the yield strength (YS) values of samples cooled at ∼1 °C/s with sand cooling, are 75∼100 MPa greater than the corresponding values at a rate of ∼6 °C/s with air-cooling. The results indicate that the yield strength is positively correlated with yield-point elongation but negatively correlated with the ultimate tensile strength. The effects of the cooling rate on the mechanical properties can be explained by the formation of Cottrell atmospheres of carbon together with the increase of the pearlite volume fraction. Based on the secondary phase components and strength evolutions, there are four possible outcomes for the different cooling rates: (I) martensite (M); (II) ferrite-martensite (FM); (III) coexistence of martensite and pearlite (MP); and (IV) ferrite-pearlite (FP). Our results reveal that the microstructure-property correlation can be illustrated schematically as a function of cooling rates and can thereby direct the heat-treatment parameters required to obtain desirable mechanical properties.

Characterization of local deformation and fracture behavior in ferrite + martensite dual-phase steels having different grain sizes

Low carbon dual-phase (DP) steels composed of soft ferrite and hard martensite have been widely used in the automotive industry due to their good strength-ductility balance and large strain hardening ability. DP steels have a wide variation in mechanical properties depending on several microstructural features such as grain size, phase fraction and distribution. Among them, the grain refinement of DP steels is known to be an effective option for enhancing mechanical performance in strength and ductility (especially post-uniform elongation). However, the exact reason for the significant improvement of post-uniform elongation by grain refinement has not been fully understood. It is considered that the characterization of local deformation behavior and micro-void formation/growth behavior in connection with microstructures is an essential approach for understanding the enhanced post-uniform elongation realized in the fine-grained DP specimen. In the present study, we prepared two kinds of DP specimens with mean ferrite grain sizes of 14.9 μm (coarse-grained DP) and 7.1 μm (fine-grained DP), and carefully investigated local strain distribution of tensile specimen and micro-void formation/growth behavior using digital image correlation (DIC) analysis and SEM observations. The fine-grained DP specimen exhibited a gradual strain localization after necking and had sufficient strain capacity that could endure against fracture. The fine-grained DP structure had a great number of micro-voids in the necked region, but almost all the micro-voids maintained a very small size, which was contrasted with the case of coarse-grained DP specimen containing very large-sized micro-voids. Such a significant difference in micro-void size/number characters between two kinds of DP specimens would be one possible reason for exhibiting greatly different post-uniform elongation behavior.

Micromechanism involved in ultrafine grained ferrite/martensite dual phase steels strengthened by nanoscale Cu-rich precipitates

The strengthening effects of grain refinement and nano-sized coherent precipitation are effective strengthening mechanisms in the development of low-cost advanced high-strength steels. In the present work, low carbon dual phase steels with ultrafine grains and nanoscale copper rich precipitates were produced by a combination of warm deformation and isothermal aging processes. Then, the microstructure and plastic deformation behavior were explored using the EBSD technique and crystal plasticity simulations, and the precipitation mechanism of nanoscale Cu-rich particles was investigated by HRTEM, atom-probe tomography in combination with first-principles calculations. The results indicate that the formation and compositional evolution of the Cu-rich particles are associated with the mixing enthalpy of alloying elements and the attractive interaction between the solute atoms. Besides the precipitation hardening effect, grain refinement improve both strength and ductility of the steels significantly, while more severe deformation will result in the heterogeneous distribution of martensite, then intensify the plastic heterogeneities and early hardening of ferrite grains during tension deformation, thus give rise to a decrease in ductility.

Development of Tailored Structure and Tensile Properties of Thermomechanical Treated Micro Alloyed Low Carbon Dual Phase Steel

Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing parameters. Instead, high technology compact slab production plant offers economic advantages, adequate control and prompt use of the advanced thermomechanical controlled rolling. The current work aims to obtain different structures and tensile properties by physical simulation of direct hot rolled niobium micro alloyed dual phase low carbon steel by varying the metallurgical temperatures of hot strip mill plant. This starts with adaptation of the chemical analysis of a low carbon content to fall far from the undesired peritectic region to avoid slab cracking during casting. Thermodynamic and kinetics calculations by Thermo-Calc 2020 and JMat pro software are used to define the transformation’s temperatures Ae1 and Ae3 as well as processing temperatures; namely of reheating, finishing rolling, step cooling and coiling temperatures. The results show that the increase of finishing rolling temperature from 780°C to 840°C or decreasing either of step cooling duration at ferrite bay from 7 to 4 seconds, enhances yield and tensile strengths, all due to more martensite volume fraction formation. The yield and tensile strengths also increase with decreasing coiling temperature from 330°C to 180°C, which is explained due to the increase of dislocation densities resulted from the sudden shape change during martensite formation at the lower coiling temperature in additional to the self-tempering of martensite formed at higher coiling temperatures which soften the dual phase steel.

Effects of tempering on the mechanical and corrosion properties of dual phase steel

The mechanical and corrosion behaviors of low carbon dual phase (DP) steel were studied based on different tempering conditions. By increasing the time and temperature of tempering, the ultimate tensile strength (UTS), the hardness, and the corrosion current density (icorr) in the polarization curves decrease while the total elongation and the diameter of the semicircular arc in the Nyquist plots increase. Tempering treatment resulted in the appearance of the yield point phenomenon and an initial rise in the yield stress (YS). While the UTS values were related to hardness through a single relationship for both tempered and DP steels, two separate trends were observed for the relation of the YS and hardness. The latter was related to the high work-hardening rate of DP steels that increased the hardness during indentation. The tempering kinetics were also studied and the average value of the Avrami exponent (n) was determined as 0.65, which was interpreted as precipitation on dislocations (theoretical n-value of 0.667). The tempering activation energy was determined as ∼ 40 kJ/mol, which was indicative of the rapid diffusion of carbon atoms along the dislocation core (pipe diffusion), and hence, it was concluded that the kinetics of tempering is essentially controlled by carbon atom diffusion.

Internal Friction Behavior Associated with Martensitic Decomposition in Low-carbon Dual-phase Steel

Internal Friction Behavior Associated with Martensitic Decomposition in Low-carbon Dual-phase Steel

Effect of Silicon Content on the Wear Behavior of Low-Carbon Dual-Phase Steels

The aim of this study is to understand the influence of the silicon content and volume fraction of martensite obtained by intercritical annealing at three temperatures (750, 780, and 800 °C) on the wear behavior of low-carbon dual-phase steels. Dry sliding wear tests were carried out in three sliding intervals (0–200, 200–600, and 600–1000 m) by using a pin-on-disk machine at normal load of 30 N, room temperature, and fixed sliding velocity of 0.5 m/s. The weight loss of the specimens was measured and the wear rate calculated. For all the samples, the wear rate decreased with increasing sliding distance. The results of the present investigation clearly show that the effect of the intercritical annealing temperature is greater than that of the silicon content. Among the three tested temperatures, the wear resistance was more sensitive to the silicon content at the lowest (750 °C). Both the content and hardness of the martensite were factors affecting the wear resistance of the dual-phase steels. The wear response of the samples can be described on the basis of the interaction between the martensite volume fraction and hardness. The role of the silicon content in the oxidation of steels was also examined.

Effect of Intercritical Annealing Time at Pearlite Dissolution Finish Temperature (Ac1f) on Mechanical Properties of Low-Carbon Dual-Phase Steel

The effect of intercritical annealing at Ac1, Ac1f, and Ac3 temperatures followed by water quenching was studied on the microstructure and mechanical properties of a 0.035C-0.27Mn low-carbon steel. During short-term holding at Ac1 and Ac1f, some pearlite remained after quenching and the yield-point phenomenon was still detectable, which was related to the formation of small amount of martensite inside pearlite colonies and on ferrite grain boundaries with marginal effect on the surrounding ferrite phase. By increasing holding time at Ac1f, pearlite disappeared and DP microstructures were achieved with the disappearance of the yield-point phenomenon and the enhancement of the work-hardening capacity. At long holding times, however, the mechanical properties deteriorated mainly due to the coarsening of martensitic islands and disappearance of dynamic strain aging (Portevin–Le Chatelier) effect at room temperature.

Effects of pre-annealing conditions on the microstructure and properties of vanadium-bearing dual-phase steels produced using continuous galvanizing line simulations

It is well known that the fracture behavior and weldability of steel often vary inversely with the carbon content. In the case of thin sheet intended for automotive applications, this means spot weldability and tensile ductility. However, the question arises, what is the reasonable maximum strength attainable in a low-carbon dual-phase (DP) steel under the constraints of: (1) with carbon levels at 0.1 wt%, (2) with the product (UTS × % TE) > 18,000 MPa, and (3) with processing on a simulated continuous hot-dipped galvanizing line. This study has focused on experimental compositions intended to meet DP steel grade properties in excess of 780 MPa UTS. This kind of advanced high-strength steel (AHSS) is an integral part of mass reduction programs for the body-in-white for the automotive industry. The family of steels investigated here is based on a low-carbon, aluminum-killed base steel containing Cr and Mo. In this study, the effects of the presence of V were studied. In addition, there were three processing variables investigated in terms of their respective effects on microstructure and mechanical properties: first, hot band coiling temperature; second, % cold reduction; and third, the thermal path through the CGL process. The thermal paths explored included both a standard galvanizing process and a new supercooling process; these were studied using a CGL simulation. This research revealed that all three variables can have a significant effect on the final microstructure and properties.

Effect of manganese pre-partitioning process on the microstructure and mechanical properties of low carbon dual phase steel

A low carbon (0.11 wt.%) steel with 1.78% Mn was subjected to pre-partitioning quenching and partitioning (PQ&P), quenching and tempering (Q&T), quenching and partitioning (Q&P) to develop ferrite-martensite dual phase (DP) steel. The study suggested that all the treatments resulted into ferrite, martensite (or tempered martensite) and retained austenite microstructure. The amount of retained austenite was ∼ 5%, when the steel was treated by PQ&P process with partitioning time of 100 s, because of Mn pre-partitioning stage increased in the stability of retained austenite. But only ∼ 1% for Q&T and Q&P processes. With the increase of partitioning time from 10 to 100 s in the PQ&P process, the amount of retained austenite was increased, the tensile strength decreased, and yield strength and elongation were both increased. Among the different heat treatment processes, PQ&P sample clearly yields most attractive combination of strength and ductility compared to Q&T and Q&P samples. The experimental steel after pre-partitioning quenching and partitioning (PQ&P) process had higher uniform elongation of 16.3% and strength-ductility balance (UTS × TEL) of 17013 MPa.%. In contrast to conventional Q&T process, 27.3% higher uniform elongation and 7.4% greater strength-ductility balance were obtained.

Enhancement of mechanical properties of low carbon dual phase steel via natural aging

The natural aging behavior of a low carbon dual phase (DP) steel was studied and the effect of martensite content was taken into account. The enhancement of strength and hardness by aging at room temperature was related to quench aging via the formation of fine precipitates in the ferrite grains. It was revealed that increasing the intercritical annealing temperature diminishes this precipitation hardening effect, which was related to the decreased degree of supersaturation of carbon in ferrite. The maximum quench aging effect was observed by annealing the ferritic-pearlitic steel at a subcritical temperature close to the eutectoid temperature, where the solute carbon content of ferrite reaches the maximum value. However, in contrast to aged DP steels, the tensile stress-strain curves showed yield-point phenomenon. The aging effect was rationalized by the Ashby-Orowan relationship and the effect of the silicon in the chemical composition of the studied steel was discussed.

Deformation and annealing behaviour of dual phase TWIP steel from the perspective of residual stress, faults, microstructures and mechanical properties

The present study aims to examine the deformation and annealing behaviour of Fe–0.07C–20Mn–2.6Si–1.6Al TWIP steel in the viewpoint of microstructural characterisation, fault analysis, triaxial residual stress measurement and mechanism of twin formation. 50% cold deformation results in improvement of hardness and tensile strength (428 HV and 1419 MPa, respectively), whereas annealing at 900 °C of the same leads to improvement in ductility (61%) with the decrease in strength (873 MPa). The residual stress is maximum and compressive in nature in deformed sample which has been diminishing after annealing. Hot rolled and hot rolled-solution treated samples develop dual phase microstructure along with annealing twins, whereas fine nano twins and high dislocation density with dual phase microstructure are apparent after 50% cold deformation. The presence of Shockley partial dislocation, overlapping of stacking faults, Lomer-Cottrell lock, intrinsic and extrinsic stacking faults affect the mechanical properties as obstacles of dislocation movements or assisting twin formation. Goss and Brass components of texture are dominating after rolling, whereas annealing results in weakening of rolling texture components. The nucleation mechanism of twin has been consistent with the pole mechanism along with the deviation process. Hence the present work amplifies the present understanding of deformation and annealing behaviour on low carbon dual phase TWIP steel from different characterisation perspectives.

Analysis and design of dual-phase steel microstructure for enhanced ductile fracture resistance

The goal of this paper is to predict how the properties of the constituent phases and microstructure of dual phase steels (consisting of ferrite and martensite) influence their fracture resistance. We focus on two commercial low-carbon dual-phase (DP) steels with different ferrite/martensite phase volume fractions and properties. These steels exhibit similar flow behavior and tensile strength but different ductility. Our experimental observations show that the mechanism of ductile fracture in these two DP steels involves nucleation, growth and coalescence of micron scale voids. We thus employ microstructure-based finite element simulations to analyze the ductile fracture of these dual-phase steels. In the microstructure-based simulations, the individual phases of the DP steels are discretely modeled using elastic-viscoplastic constitutive relations for progressively cavitating solids. The flow behavior of the individual phases in both the steels are determined by homogenizing the microscale calibrated crystal plasticity constitutive relations from a previous study (Chen et al. in Acta Mater 65:133–149, 2014) while the damage parameters are determined by void cell model calculations. We then determine microstructural effects on ductile fracture of these steels by analyzing a series of representative volume elements with varying volume fractions, flow and damage behaviors of the constituent phases. Our simulations predict qualitative features of the ductile fracture process in good agreement with experimental observations for both DP steels. A ‘virtual’ DP microstructure, constructed by varying the microstructural parameters in the commercial steels, is predicted to have strength and ductile fracture resistance that is superior to the two commercial DP steels. Our simulations provide guidelines for improving the ductile fracture resistance of DP steels.

Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase (DP) steels with almost constant martensite contents of 20vol% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature (1043 K, FH1) with fast-heating (300 K/s) show banded ferrite/martensite structure, whereas those soaked at high temperature (1103 K, FH2) with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated (5 K/s) reference material (SH0). The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard–Jaoul technique demonstrate two stages of work hardening for all samples.

Influence of Tempering on Mechanical Properties of Ferrite and Martensite Dual Phase Steel

This study investigated the change in microstructure and mechanical properties by tempering a low carbon dual phase steel composed of ferrite and martensite. The microstructure analysis revealed that tempering process resulted in the carbide precipitation and coarsening of martensite structures. The nanohardness of each phase (martensite and ferrite) decreased with increasing the tempering temperature. The ultimate tensile strength had a linear relationship with nanohardness ratio of martensite and ferrite. On the other hand, the uniform elongation firstly did not change by tempering at the temperature below 400°C, but then decreased by tempering at the temperature above 400°C with decreasing the nanohardness ratio. It was concluded that the nanohardness ratio can be a good parameter for controlling the mechanical properties of dual phase steels.

Effect of carbon on the plastic strain ratio of low carbon dual-phase steels

The effect of carbon on the plastic strain ratio of low-carbon dual-phase steels was investigated in a series of low C-1.6 Mn-0.3Cr-0.2Mo-0.001B steels with carbon contents ranging from 0.021 to 0.048%. The rm value of dual-phase steel could be increased by controlling both the carbon content and the martensite morphology. The highest rm value of 1.23 was obtained in 0.021% C steel annealed at 790 °C according to the typical galvannealing heat cycle. The martensite volume fraction was 5.4%, which was sufficient to eliminate the yieldpoint elongation in as-annealed steel, and this had little deteriorating effect on the rm value. The fine martensite particles between 0.5 μm and 2.0 μm in size were desirable for a high rm value in dual-phase steel.

Microstructures and Mechanical Properties of a New As-Hot-Rolled High-Strength DP Steel Subjected to Different Cooling Schedules

Controlled rolling followed by accelerated cooling was carried out in-house to study the microstructure and mechanical properties of a low carbon dual-phase steel. The objective of the study described here was to explore the effect of cooling schedule, such as air cooling temperature and coiling temperature, on the final microstructure and mechanical properties of dual-phase steels. Furthermore, the precipitation behavior and yield ratio are discussed. The study demonstrates that it is possible to obtain tensile strength and elongation of 780 MPa and 22 pct, respectively, at the two cooling schedules investigated. The microstructure consists of 90 pct ferrite and 10 pct martensite when subjected to moderate air cooling and low temperature coiling, such that the yield ratio is a low 0.69. The microstructure consists of 75 pct ferrite and 25 pct granular bainite with a high yield ratio of 0.84 when the steel is directly cooled to the coiling temperature. Compared to the conventional dual-phase steels, the high yield strength is attributed to precipitation hardening induced by nanoscale TiC particles and solid solution strengthening by high Si content. The interphase precipitates form at a suitable ledge mobility, and the row spacing changes with the rate of ferrite transformation. There are different orientations of the rows in the same grain because of the different growth directions of the ferrite grain boundaries, and the interface of the two colonies is devoid of precipitates because of the competitive mechanisms of the two orientations.

Suppression of Ms temperature by carbon partitioning from carbon-supersaturated ferrite to metastable austenite during intercritical annealing

Three various cooling patterns including gas-jet cooling (∼25°C/s), Fast spraying water cooling (∼60°C/s) and Water quenching (∼1700°C/s) were chosen to study the effects of carbon partitioning on martensitic transformation and mechanical properties in a low carbon dual phase steel. The suppression of Ms temperature by decreasing the cooling rate was investigated by dilatometer experiments. The results show that the Ms suppression was closely associated with the carbon partitioning between carbon-supersaturated ferrite and metastable austenite before martensitic transformation, which leads to the enrichment of alloy elements in austenite. Since no carbon partitioning occurred during water quenching process, more interstitial solid solution of carbon will segregate to the dislocations and thus contribute to the increase in ferritic strength in the quenched steel. The decrease of Ms temperature, the change of martensitic microstructure and the increase of ferritic strength were also used to verify the presence of carbon partition. Besides, the increase in yield and ultimate tensile strengths at higher cooling rate was mainly attributed to the ferrite and interfaces strengthening as well as the suppression of ferritic transformation.