Deformation Induced Ferrite Research Articles

A novel dual-strengthening technology combining external second phase nanoparticle method and hot compression deformation process in micro-alloyed medium carbon steel

Herein, a novel dual-strengthening technology that combines the external second-phase nanoparticle addition method with the hot compression deformation tests was carried out to investigate the effects of surface-modified nanoparticles (NPs) and deformation conditions on the characteristics of inclusions and the evolution of microstructure in the micro-alloyed (MA) medium carbon steels. The results revealed that the inclusions in the steels were divided into single phase inclusions and composite phase inclusions. With the addition of NPs, Al2O3 inclusions were replaced by MgAl2O4 spinel, and the composite inclusions mainly consisted of TiN–VN, MgAl2O4–TiN and MgAl2O4–TiN–MnS inclusions, and the same types of inclusions became significantly smaller in size. According to the EDS results, some of the inclusions acted to induce ferrite nucleation, and the MgAl2O4–TiN–MnS composite inclusion was more favorable for the nucleation of acicular ferrite (AF). The deformation induced ferrite (DIF) started to form at the deformation temperature of 750 °C and the maximum stress peak in the NPs-bearing steel was 516 MPa, which was 28.4% higher than that in the original steel. The proportion of ferrite increased as the deformation temperature decreased. The dispersed NPs contributed to the ferrite nucleation and the grain refinement as the inclusion-modifying agent, which improved the material properties, and the relevant strengthening mechanisms were explained.

Achieving ultra-high elongation of 1401% in a warm-rolled medium Mn lightweight steel with dual-phase lamellar structure

The present work reports a dual-phase lamellar structure to combine ultra-high superplastic elongation and relatively low deformation temperature in a novel Al-added medium Mn lightweight steel. The specimens were controlled rolling in a two-phase zone; Hence the microstructure contained a high-volume fraction of residual austenite in the ferrite matrix. The ultra-high elongation of 1401% was obtained at the deformation temperature of 775 °C, which is more than 50 °C lower than others reported of the cold-rolled sheets. Interestingly, near 800 °C, due to the enhanced DIFT effect, the ductility was reduced by temperature rise, which is opposite to the condition of the cold-rolled sheets. The ultrahigh elongation at 775 °C was ascribed to the continued recovery and recrystallization of the deformation-induced ferrite, which not only achieves dynamic grain refinement but also maintains the balance of stress distribution between the two phases.

Formation mechanism and control of transverse corner cracks in fine blanking steel

Owing to the extreme difficulty in their prevention, transverse corner cracks have been frequently observed in fine blanking steel produced on straight arc caster. In this study, the transient thermal module of Ansys was used to calculate the corner temperature distribution of a continuous casting slab. According to the actual cooling rate, a Gleeble 3500 thermal simulator was used to simulate the bending process of continuous casting, and the thermoplastic curves of test steel grades were drawn. Through the observation of fracture morphology and metallographic examination, typical precipitation detection and phase transformation temperature detection, the mechanism of corner cracks in experimental steel grades was determined and the corresponding control measures were formulated. The results showed that under the original process conditions, the corner temperature of the continuous casting slab was in the brittle temperature range of the steel grade when it entered the bending section. During the cooling process of the continuous casting slab, all Ti elements dissolved into the matrix, and the main precipitates were (Cr, Fe)XCY, while the grain boundary pinning was weak. In the high temperature brittle region, deformation-induced ferrite was produced at grain boundaries, which formed a stress concentration during the bending process which led to plastic reduction. The mixed control mode of strong cooling in the cooling stage and weak cooling in the reheating stage could not only ensure the strength of the slab shell in the vertical section, but also promoted the growth of the ferrite film induced by intergranular deformation in the strong cooling stage. It increased the maximum allowable stress of ferrite at the grain boundary. By controlling the temperature and cooling rate in the reheating stage, the growth of ferrite at the grain boundaries and the nucleation of ferrite in grains were accelerated, the stress distribution at grain boundaries and grains was uniform, and the corner cracks were eliminated.

Effect of Annealing on the Microstructure, Texture and Mechanical Properties of a Dual-Phase Ultrahigh-strength TWIP Steel

The present study investigates the effect of annealing time and temperature on the microstructural restoration (by recovery and recrystallization), texture evolution and tensile properties of a dual-phase TWIP steel. The samples, which were initially hot rolled-air cooled followed by being in solution treated condition, subsequent 50 pct cold rolled condition and different annealing conditions (temperatures ranging from 500 °C to 1000 °C for 5 minutes to 2 hours), were subjected to microstructural characterization using optical and transmission electron microscopy, electron backscatter diffraction analysis, tensile testing and fractographic study. The deformation-induced ferrite (DIF) transformation due to cold deformation, the evolution of the grain structure and texture in both austenite and ferrite, and the change in the DIF fraction (accompanying the recrystallization annealing treatment) are critically analyzed. The optimum combination of strength and ductility is achieved in the partially recrystallized samples annealed at 700 °C for 30 minutes (UTS: 859 MPa, total elongation: 37 pct) and 900 °C for 30 minutes (UTS: 708 MPa, total elongation: 63 pct).The effects of DIF on the evolution of microstructure and texture during annealing and the final properties are discussed.

Extension of the Conventional Press Hardening Process by Local Material Influence to Improve Joining Ability

Press hardened structural components are a key factor in lightweight car design and thus in reducing vehicle mass while increasing crash safety. The use of quenched 22MnB5 (Usibor 1500) has been established in hot sheet forming for the production of safety-relevant car body components. In order to expand the field of application for press-hardened components, a process-reliable joining technique is essential. Ultra-high-strength components can be joined with other parts in car bodies using the resistance spot welding process. Here, challenges like uneven welding lens formations with an incorrect connection in multi-sheet joints arise. Mechanical joining processes, for example self-pierce riveting, can only be used to a limited extent due to the high hardness of the hardened parts. For this purpose, an annealing treatment is often carried out in order to reduce the strength of the material after press hardening. Another possibility to create softened areas is the introduction of local deformation in the austenitic material. The phase areas in the continuous cooling transformation diagram are shifted to shorter cooling times, which enables the development of deformation-induced ferrite. The local thinning and softening improves joinability by means of mechanical and thermal joining processes but increases the forming force. Therefore, in this study the process of hot forming and local deformation is first pre-estimated using numerical simulation. The required force for the deformation and an optimal positioning, as well as the possible number of deformation punches, are investigated. Furthermore, the first experimental results of the feasibility of locally thinned and softened sheets are presented. In addition, joining tests by resistance spot welding and self-pierce riveting are carried out on the generated specimen to illustrate the practical effectiveness of the local thinning and the use of deformation-induced ferrite for critical joints.

Effects of plastic deformation on austenite transformation in Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo ultra-high strength steel during continuous cooling

Dynamic continuous cooling-transformation (D-CCT) diagram is an essential research method to predict mechanical properties of ultra-high strength steel (UHSS) before material processing. In this study, D-CCT diagrams of Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo at different cooling rates of 0.5–15 °C s−1 were plotted by a combined method of dilatometry and metallography. Influences of deformation on microstructure, transformation behavior, and carbon diffusion were elucidated. Proeutectoid ferrite transformation behavior was discussed in detail based on strain induced nucleation kinetics. When the cooling rate increased above 1 °C s−1, the initial austenite grains were divided into blocks by newly forming complex microstructures, which were mainly consisted of martensite (M), bainite (B), deformation induced ferrite (DIF) and retained austenite (RA). Moreover, the decrease of proeutectoid ferrite and the increase of granular bainite (GB) resulted in the strength increased linearly at the cooling rate of 0.25–2 °C s−1. Increasing cooling rate has a remarkable effect on the tensile strength of the UHSS up to 1 °C s−1. Further increase in the cooling rate from 1 to 15 °C s−1, the results showed a relatively small effect on the intensity value. Besides, carbon diffusion from α-phase (face-centered-cubic microstructure) in DIF can enhance the stability of γ-phase (body-centered cubic microstructure) and delay the transformation of γ-phase to GB. Furthermore, attributed to the diffusion path of carbon decreases, the proeutectoid ferrite transformation was restrained, and γ-phases were transformed into GB as raising the cooling rate.

Deformation and annealing behaviour of a low carbon high Mn TWIP steel microalloyed with Ti

ABSTRACTA low carbon high Mn, Ti microalloyed dual phase TWIP steel has been processed through cold rolling and annealing. X-ray diffraction reveals the maximum austenite (≈92%) in HRACST sample whereas, the 50CD sample shows 29% ferrite. The microstructure of HRAC and HRACST samples reveal austenite grains with annealing twins and deformation induced ferrite (DIF). The higher amount of DIF along with deformation twins form during cold deformation. Annealing at 500°C shows recovery, whereas at 700°C shows partial recrystallisation and at 900°C reveals almost full recrystallisation. TEM microstructures of the 900°C for 30 min samples reveal annealing twins with TiC particle. Strong Brass {110}<112> and Goss{110}<001> texture components are observed in HRAC, HRACST and 50CD samples. Goss Twin (GT) {113}<332> and Copper Twin (Cu-T) {552}<115> components are observed in 50CD sample. Addition of Ti results in an average grain size of 20 μm. Maximum YS (1176 MPa) and UTS (1283 MPa) values with the lowest ductility of 11% have been obtained for the 50CD sample which is related to the formation of extensive deformation twin and a higher fraction of DIF. 700°C-30 min and 700°C-60 min samples show an increase in ductility (23% and 34%, respectively) with a marginal decrease in tensile strength (1054 MPa). Annealing at 900°C shows ductility restoration up to 60% with higher tensile strength compared to HRACST sample. Ductile fracture of HRAC and HRACST samples transform to brittle fracture in the 50CD sample. Annealing at 900°C for 30 min shows ductile fracture with some (Fe, Mn)S and TiC particles.

The effect of thermomechanical controlled processing on recrystallisation and subsequent deformation-induced ferrite transformation textures in microalloyed steels

The evolution of texture components for two experimental 0.06 wt% C steels, one containing 0.03 wt% Nb (Nb steel) and the second containing both 0.03 wt% Nb and 0.02 wt% Ti (Nb–Ti steel), was investigated following a new thermomechanical controlled process route, comprising first deformation, rapid reheat to 1200 °C and final deformation to various strains. Typical deformation textures were observed after first deformation for both steels. Following subsequent reheating to 1200 °C for various times, the recrystallisation textures consisted primarily of the α- leftlangle {011} rightrangle //RD texture fibre with a weak γ-{111}//ND texture fibre, similar to deformation textures, indicative of the dominance of a strain-induced boundary migration mechanism. The texture components after finish deformation were different from the rough deformation textures, with a strong α- leftlangle {011} rightrangle //RD texture fibre at the beginning, and then the strong peaks move to (111) leftlangle {1bar{2}1} rightrangle and (111) leftlangle {bar{1}bar{1}2} rightrangle textures due to the deformation-induced ferrite (DIF) transformation. The effect of Ti on the recrystallisation textures and deformation textures has also been analysed in this study. The results illustrate that Ti significantly influences the γ-{111}//ND texture fibre. Finally, the textures after deformation and recrystallisation in the austenite were calculated based on the K–S orientation relationship between the austenite and ferrite. This allowed the understanding of the mechanism of recrystallisation between first and final deformation and the DIF textures during phase transformation.

Effect of Deformation Parameters on Microstructure and Properties During DIFT of X70HD Pipeline Steel

Grain refinement is a critical approach to improve the strength of materials without damaging the toughness. The grains of deformation-induced ferrite are considerably smaller than those of proeutectoid ferrite. Grain refinement is crucial to the application of deformation-induced ferrite. The composition of ferrite and bainite or martensite is important in controlling the performance of X70HD pipeline steel, and cooling significantly influences the control of their ratio and grain size. By analyzing the static and dynamic phase-transition points using Gleeble-3800 thermal simulator, thermal simulations were performed through two-stage deformations in the austenite zone. Ferrite transformation rules were studied with thermal simulation tests under different deformation and cooling parameters based on the actual production of cumulative deformation. The influence of deformation parameters on the microstructure transformation was analyzed. Numerous fine-grain deformation-induced ferrites were obtained by regulating various parameters, including deformation temperature, strain rate, cooling rate, final cooling temperature and other parameters. Results of metallographic observation and microtensile testing revealed that the selection of appropriate parameters can refine the grains and improve the performance of the X70HD pipeline steel.

An ultrahigh strength steel with ultrafine-grained microstructure produced through intercritical deformation and partitioning process

Abstract In this work, a novel design scheme in which deformation-induced ferrite transformation (DIFT) was applied to produce fine-grained steel and the quenching is controlled by quenching and partitioning (Q&P) process has led to the development of a new kind of steel. This steel possesses excellent mechanical properties and the ductility can be further improved without compromising strength because the refined microstructure contains martensite, retained austenite and deformation-induced ferrite. The highest elongation of 15% allied with strength of 1700 MPa is obtained through hot deformation followed by Q&P treatment at 300 °C. The microstructure evolutions are discussed in terms of the current knowledge of the Q&P process and the experimental observations. The results show that the designed multiphase steels are a promising candidate for the development of the third generation of advanced high strength steels.

Effects of Manganese Content on Solidification Structures, Thermal Properties, and Phase Transformation Characteristics in Fe-Mn-Al-C Steels

The solidification structures and the thermal properties of Fe-Mn-C steel ingots containing different manganese contents have been investigated and the phase transformation characteristics have been revealed by Thermo-Calc to assist development of the continuous casting technology of Fe-Mn-C steels. The results show that the thermal conductivity of the 0Mn steel is higher than that of the 3Mn steel. The thermal conductivity of the 6Mn steel is the lowest in the three kinds of steels below 1023 K (750 °C) and the highest above 1173 K (900 °C). The 0Mn steel has the highest value of the proportion of equiaxed grain zone area in the three kinds of steels, whereas the 3Mn steel has the lowest value of it in the steels. Manganese has the effect of promoting the coarsening of grains. The microstructure is martensite and a little retained austenite (3.8 mass pct) in the 6Mn steel, whereas the microstructure is bainite in the 3Mn steel. The 0Mn steel is characterized by ferrite and pearlite. The mean thermal expansion coefficients of the steels are in the range from 1.0 × 10−5 to 1.6 × 10−5 K−1, and the determinations of mold tapers of the 6Mn and 3Mn steels can refer to low-carbon steel. Using RA <60 pct as the criterion, the third brittle temperature region of the 6Mn steel is 873 K to 1073 K (600 °C to 800 °C), whereas those of the 3Mn steel and the 0Mn steel are 873 K to 1123 K (600 °C to 850 °C) and 873 K to 1173 K (600 °C to 900 °C), respectively. In the 6Mn and 3Mn steels, the deformation-induced ferrite (DIF) forms in sufficient quantities cause the recovery of the ductility at the low temperature end. However, since low strains are present when straightening, sufficient quantities of DIF cannot be formed. Thus, the ductility of the 6Mn and 3Mn steels cannot be improved during the continuous casting process. Manganese has the effect of enlarging the austenite phase region and reducing the δ-ferrite phase region and α-ferrite phase region.

Hot deformation behavior of HSLA steel Q690 and phase transformation during compression

A hot compression experiment was conducted on the High Strength Low Alloy (HSLA) steel Q690E, using Gleeble-3500 thermal/mechanical simulator, with a temperature of 950°C and maximum compression amount of 80%. The deformation induced ferrite (DIF) was obtained under different compression amounts and cooling conditions. The stress–strain diagram during compression was mapped and the micro-hardness of the thermal simulated specimens measured. Results show that a 40% compression effectively accelerates the formation of DIF and refines the grain size. The grain sizes after 80% deformation are in a concentrating distribution ranges from 1μm to 3μm, and the average grain size is 2.34μm. The formation of DIF leads to an obvious drop on the stress–strain curve and makes it shows a hump-like distribution characteristic, and also leads to a decrease of microhardness. The mechanism of DIFT is analyzed. A mathematical model is established between the effective grain boundary area and the compression amount. And its calculations indicate that the increasing of the effective grain boundaries area caused by the compressive deformation is one of the essential reasons for DIFT. The ferrite phase transformation driving force can be increased by the dislocation energy, and the critical dislocation energy to drive DIFT in this experiment is calculated to be 17.56J/mol.

Ferrite Evolution During Isothermal Process in a High Deformability Pipeline Steel

In order to ensure the safety of long-distance oil and natural gas transmission pipeline installed in seismic and/or permafrost region, high strength pipeline steel with excellent deformability has been developed. The ferrite and bainite dual phase pipeline steel is a very important kind of high deformability pipeline steel. Polygonal ferrite is a key microstructure in ferrite and bainite dual phase deformability pipeline steel. Ferrite evolution during isothermal process at 700 °C after 50% deformation at 800 °C was conducted by using a Gleeble-3800 thermal simulator, and microstructure was characterized by using an optical microscope, a scanning electron microscope and a transmission electron microscope. There are two types of ferrite, ferrite with high density dislocation and ferrite with a little dislocation. There is about 7% (volume percent) deformation induced ferrite (DIF) for compression of 50% at 800 °C and strain rate of 1 s−1. During the isothermal process at 700 °C, with the holding time increasing, ferrite volume percent, ferrite grain number and average ferrite grain size increase. As the holding time is prolonged, dislocation recovery occurs in DIF. There are secondary phases in ferrite when the holding time is too long, and secondary phases and dislocation formation in dislocation pinning.

Martensitic Phase Transformation from Non-isothermally Deformed Austenite in High Strength Steel 22SiMn2TiB

Non-isothermal compressive deformation was performed on high strength steel 22SiMn2TiB for the study of martensitic phase transformation from deformed austenite. The transformation start temperature M s decreased with the increase of deformation from 0 to 50 pct, and the variation of deformation rate (0.1 and 10 s−1) and the appearance of deformation-induced ferrite and bainite showed no influence on the change of M s temperature. The deformation at both the rates increased the volume fraction of retained austenite; however, the carbon content of retained austenite decreased at 10 s−1 and remained basically unchanged at 0.1 s−1. The yield strength of austenite at M s temperature and the stored energy in deformed austenite were experimentally obtained, with which the relationships between the change of M s temperature and the thermodynamic driving force for martensitic phase transformation from deformed austenite were established by the use of the Fisher-ADP–Hsu model. And finally, the transformation kinetics was analyzed by the Magee–Koistinen–Marhurger equation.

Effect of Different Strain on Microstructure Evolution of Medium Carbon Steel

Uniaxial hot compression simulation experiment at 700°C with different true strain was carried out to study the microstructure evolution of medium carbon steel, the predominant mechanism on the cementite softening has been explored, the experimental results show that the volume fraction of deformation induced ferrite (DIF) increased with increasing true strain and even exceeds the equilibrium content. With the increase of DIF, more and more carbon atoms congregated in the boundaries such as the interface of DIF and the interphase of DIF/deformation austenite. Carbon congregation provides the right carbon content and the optimized microstructure for divorced decomposition during the process of controlled cooling. Therefore spherical or rod-like cementite and degenerated pearlite can be obtained.

Effect of thermo-mechanical process on the microstructure and secondary-deformation behavior of 22MnB5 steels

22MnB5 steel specimens were deformed at 923K and 693K to three strain levels to study the effect of applied strain level on the microstructure and secondary-deformation behavior. As the steel was deformed at 923K, deformation induced ferrite transformation (DIFT) occurred even when a small strain of 0.044 was applied, and the volume fraction of deformation induced ferrite (DIF) increases with increasing applied strain level. When deformed at 693K, deformation induced bainite transformation (DIBT) was observed when the applied strain was larger than 0.109. The incubation period for DIFT is shorter than that for DIBT, but the DIBT proceeds much faster than DIFT. Sub-size tensile specimens were cut from the hot deformed 22MnB5 steel specimens, and digital image correlation technique was employed to investigate the secondary-deformation behavior of the sub-size tensile specimens at room temperature. It is found that the appearance of DIF or DIB (deformation induced bainite) decreases the yield strength and ultimate tensile strength (UTS) but increases the elongation and strength–ductility product of the hot deformed 22MnB5 steel specimens compared with the as-quenched 22MnB5 steel specimen with full martensite.

Quantification of Phase Transformation Kinetics under Thermomechanical Conditions Using Dilatometry Data

Using dilatometric technique, it is the purpose of present research to quantify the kinetics of austenite decomposition during cooling after isothermal hot compression. To reach this aim, based on lattice parameters and thermal expansion coefficients of different phases, a new model was proposed. In this model, the contributions of both isotropic and non-isotropic dilatations were considered. The model was applied to 22MnB5 steel to predict formation of bainite and martensite. Furthermore, the model was developed in order to determine the final fraction of deformation induced ferrite (DIF).

Mechanical Properties and Temper Resistance of Deformation Induced Ferrite in a Low Carbon Steel

The microstructures and mechanical properties of deformation induced ferrite (DIF) in the low carbon steel Q235 under different deformation temperatures have been investigated systematically. Through deformation induced ferrite transformation (DIFT), ferrite grain can be refined to 3 mu m and accounts for above 85% of the overall fraction. Yield strength of DIF (>500 MPa) is increased by up to 100% compared with the conventional low carbon steel. Comparison of microstructure and mechanical properties in the Q235 steel with DIF and tempered DIF microstructure illustrates that the strengthening mechanism of DIF microstructure is the combination of grain boundary strengthening and carbon supersaturated strengthening. Electron back-scattered diffraction (EBSD) analysis and high magnification scanning electron microscopy (SEM) observation denote that high-angle grain boundary among ultrafine ferrite grain and the transformation product of retain austenite membrane along ferrite boundaries are responsible for the stability of ferrite grain size during tempering process. Transmission electron microscopy (TEM) analysis demonstrates that the transformation product of retained austenite membrane between ferrite grain boundaries is cementite.

Effect of microstructural features on the hot ductility of 2.25Cr–1Mo steel

Some factors contributing to the hot ductility losses of a 2.25Cr–1Mo steel were identified over the temperature range 750–950 °C, after the specimens were austenitized at 1000 °C, furnace cooled to different temperatures, and held there for sufficient periods of time, followed by tensile testing. There were two types of ferrite present in the microstructure, namely, pro-eutectoid ferrite and deformation-induced ferrite. The pro-eutectoid ferrite was only formed below Ar 3 (∼825 °C), which was nucleated on the inclusions and distributed uniformly. Nevertheless, the deformation-induced ferrite was formed in a much wider temperature range. It was distributed mainly along austenite grain boundaries above Ar 3, and around the pro-eutectoid ferrite below Ar 3. The deformation-induced ferrite had a primary effect on the hot ductility, which was mainly responsible for a hot ductility trough. There was a peak in the quantity of deformation-induced ferrite between 800 and 900 °C, which was just corresponding to the hot ductility trough. The morphology of ferrite was also essential. The net-like structure of ferrite formed along austenite grain boundaries was the most deleterious to the hot ductility.

The Ferrite Transformation in Hot Deformed 0.036% Nb Austenite at Temperatures Above the Ae3

A 0.036% Nb microalloyed steel was deformed in torsion over the temperature range 816–896°C in a 2%H2–Ar gas atmosphere. Strains of 0.5–5.0 were applied at strain rates of 0.04 and 0.4 s−1. The experimental parameters were varied in order to study the effects of strain, strain rate and temperature on the formation of ferrite by dynamic transformation (DT) at temperatures above the Ae3. The critical strain for ferrite formation by DT was 0.5 and the volume fraction formed increased with strain and slightly with strain rate. It was also observed that the applied strain has a far greater influence on the transformation than the time. Average ferrite grain sizes of 2 to 3.5 μm were produced, the size increasing with the transformation temperature and decreasing strain rate. By comparison with the behavior of plain C steels, it is evident that the addition of niobium slows the reverse transformation to a considerable degree. Two stages were detected in the reverse transformation: i) in stage I, observed during the initial 200 s of isothermal holding, the deformation-induced ferrite was fairly stable; ii) in stage II, observed after 200 s of holding, the reverse transformation began to take place, going to completion in about 400 s. The results of these experiments support the view that it is the stored energy of the ‘inhomogeneously’ distributed dislocation (i.e. those in shear bands and sub-boundaries) that provides the driving force for such “non-equilibrium” transformation.