Polygonal Ferrite Matrix Research Articles

Effect of Cr on the phase transformation and interphase nanoprecipitation behaviours of high-strength microalloyed steels

In this work, the microstructures and mechanical properties of interphase precipitation strengthening microalloyed steels were investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy, atom probe tomography and tensile tests. Microstructural analysis shows that the studied steels coiled at 620 °C possess a microstructure consisting of a single-phase polygonal ferrite matrix and disc-liked interphase-precipitated carbides. Tensile test results show that with the addition of Cr, the yield strength increased from 644 MPa to 728 MPa and 748 MPa for 0Cr, 0.1Cr and 0.2Cr steels, respectively, without a substantial decrease of ductility. Thermodynamics analysis indicates that the addition of Cr increases the driving force of γ → α transformation, which was responsible for grain refinement and decrease of sheet spacing of nanoprecipitates. The strengthening contributions from solid solution, dislocations, grain boundaries and interphase precipitation were calculated, and the results indicate that the decrease of interphase-precipitated carbides sheet spacing and the refinement of grain size were responsible for the strength enhancement of Cr microalloyed steel.

Effects of Silicon and Manganese Contents on V-Bending in High-Strength TRIP-Aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

Effects of Silicon and Manganese Contents on V-Bending in High-Strength TRIP-Aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

Effect of Carbon Content on V-Bending in High-Strength TRIP-Aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

Effect of Carbon Content on V-Bending in High-Strength TRIP-Aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

Effects of Si and Mn Contents on V-bending in High Strength TRIP-aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

Effects of Si and Mn Contents on V-bending in High Strength TRIP-aided Dual-Phase Steel Sheets with Polygonal Ferrite Matrix

Development of high strength steel sheet with improved strainhardenability for automotive application

Advance high strength steel with minimum UTS of 780 MPa is industrially developed utilizing continuous annealing line (CAL) and Gleeble thermo mechanical simulation. An outline of superior elongation, improved strain hardenability, enhanced strength of developed Fe – C – Mn – Si TRIP assisted steel is described. Correlation amid Simulated result and industrially annealed steel are stabilized for microstructure and mechanical property. Annealing condition is optimized for best combination of strength and formability accordingly. CCT diagram for the selected composition from JMatPro is utilized to optimize rapid cooling rate and over aging section temperature. Final microstructure of developed steel comprises tempered martensite, granular bainite with retained austenite distributed in polygonal ferrite matrix. An evaluation of developed TRIP steel is carried out with the help of microstructure and XRD analysis. It was concluded that strain hardening coefficient of new steel is comparable to that of drawing grades attributable to about 13 % retained austenite in microstructure.

Effect of substitution of Si by Al on the microstructure and mechanical properties of bainitic transformation-induced plasticity steels

The effect of partial or full substitution of Si by Al on the microstructure and mechanical properties has been extensively studied in multi-phase transformation-induced plasticity (TRIP) steels with polygonal ferrite matrix, but rarely studied in bainitic TRIP steels. The aim of the present study is to properly investigate the effect of Al and Si on bainite transformation, microstructure and mechanical properties in bainitic steels in order to provide guidelines for the alloying design as a function of process parameters for the 3rd generation advanced high strength steels (AHSS). It is shown from the dilatometry study, microstructural investigations and tensile properties measurements that the Al addition results in an acceleration whereas Si addition leads to a retardation in bainite transformation kinetics. The addition of Al retards the decomposition of austenite into pearlite and carbides at holding temperatures higher than 450°C whereas Si retards the decomposition of austenite into carbides at temperatures lower than 450°C. Consequently, the Al-added bainitic steel has a better strength-elongation combination at bainitic holding temperatures higher than 450°C while Si-added steel has a better strength-elongation combination at temperatures lower than 450°C. The higher yield strength of Al-added steel is mainly attributed to its finer bainitic lath. The higher tensile strength of Si-added steel is not only related to the stronger contribution of Si on work hardening during deformation, but also due to the higher volume fraction of martensite or martensite/austenite (MA) blocks in all heat treatment conditions, as well as the lower mechanical stability of retained austenite in this steel.

Microstructure and Mechanical Properties of V-Nb Microalloyed Ultrafine-Grained Dual-Phase Steels Processed Through Severe Cold Rolling and Intercritical Annealing

Ultrafine-grained (UFG) dual-phase (DP) steel was produced by severe cold rolling (true strain of 2.4) and intercritical annealing of a low carbon V-Nb microalloyed steel in a temperature range of 1003 K to 1033 K (730 °C to 760 °C) for 2 minutes, and water quenching. The microstructure of UFG DP steels consisted of polygonal ferrite matrix with homogeneously distributed martensite islands (both of size <1 µm) and a small fraction of the inter lath films of retained austenite. The UFG DP steel produced through intercritical annealing at 1013 K (740 °C) has good combination of strength (1295 MPa) and ductility (uniform elongation, 13 pct). The nanoscale V- and Nb-based carbides/carbonitrides and spheroidized cementite particles have played a crucial role in achieving UFG DP microstructure and in improving the strength and work hardening. Analysis of work hardening behavior of the UFG DP steels through modified Crussard–Jaoul analysis showed a continuously varying work hardening rate response which could be approximated by 2 or 3 linear regimes. The transmission electron microscopy analysis on post tensile-tested samples indicated that these regimes are possibly related to the work hardening of ferrite, lath, and twin martensite, respectively.

Examination of phase transformation kinetics during step quenching of dual phase steels

Employing step quenching method for producing dual phase (DP) steels, kinetics of austenite to ferrite phase transformation was studied. In this regard, a series of samples were austenitized at 960 °C for 600 s. These specimens were then annealed at different intercritical temperatures, for different times between 0 and 7200 s, followed by water quenching. Detailed microstructural analysis was employed to study the variation of volume fraction as well as morphology of ferrite phase with intercritical annealing time. Moreover, mechanism of ferrite formation, by intercritical annealing treatment, was studied. It was seen that at initial annealing times a thin layer of allotriomorphic ferrite was formed at the primary austenite grain boundaries. Increasing the annealing time led to growth of this layer and formation of DP steel incorporating martensite islands distributed within polygonal ferrite matrix, after water quenching. Employing Johnson-Mehl-Avrami-Kolmogorov approach, a model was then successfully developed to predict formation of ferrite during intercritical annealing treatment. The Avrami exponent was found to be 0.56, indicating that the austenite to ferrite phase transformation, within the intercritical temperature region, was diffusion controlled, governed by one-dimensional growth.

Effect of Nb-Mo additions on precipitation behaviour in V microalloyed TRIP-assisted steels

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation-induced plasticity-assisted steels with an acicular/bainitic ferrite matrix were investigated by a heat treatment up to the austenite formation range. It was found that during the heating stage the acicular/bainitic ferrite matrix resisted recrystallisation, while cementite and martensite were decomposed and austenite was formed in the acicular/bainitic ferrite. Both Nb-V-Mo and V containing steels after the heat treatment showed a microstructure consisting of a polygonal ferrite matrix with small islands of pearlite. During these transformations, the microscopy observations showed that 0.04 wt% Nb and 0.08 wt% Mo additions to the 0.16 wt% V microalloyed steel considerably reduced the growth-coarsening of microalloy precipitates.

Influence of ferrite matrix and precipitation status on the mechanical properties of low carbon low alloy steel during high temperature tension

Utilizing the inhomogeneity of temperature and stress status during heavy plate rolling, with controlled cooling, polygonal ferrite and quasi-polygonal ferrite with two grain sizes for each were obtained, and their precipitation status varied with different microstructures. GLEEBLE 3500 was used to investigate the influence of microstructure and precipitation status during high temperature tensile deformation. The results revealed that ferrite matrix microstructure and nucleation of precipitates were determined by the overall cooling process, while the final precipitation status was mainly determined by the cooling rate within the temperature range of (Ti,Mo)C precipitation during the subsequent isothermal process. Also, dislocations and subboundaries acting as containers facilitated the growth of precipitates along some particular directions. Pre-existing substructures, such as low-angle boundaries and statistically-stored dislocations, along with small precipitates, influenced the formation of subgrains and dislocation cells during high temperature deformation. These newly-generated substructures involved rotation or rearrangement of dislocations, and finally translated into recrystallized grains. Additionally, the grain size was closely related to the probability of interactional of subboundaries. Although the polygonal ferrite matrix showed more stable stress status, the quasi-polygonal ferrite matrix with fine precipitates and small grains exhibited better mechanical properties.

Impact properties of low-alloy transformation-induced plasticity-steels with different matrix

The effects of matrix types on Charpy impact properties were investigated in Fe–0.2%C–1.5%Si–1.5%Mn (mass%) transformation-induced plasticity steels. The steels with annealed martensite and bainitic ferrite matrix possessed higher upper shelf Charpy impact absorbed energy than the steel with polygonal ferrite and martensitic matrix. In addition, the low ductile–brittle fracture appearance transition temperatures were achieved in annealed martensite and martensite types in comparison with those of other steels.

The impact of thermo-mechanical controlled processing on structure-property relationship and strain hardening behavior in dual-phase steels

We elucidate here the impact of thermo-mechanical controlled processing (TMCP) in governing nucleation of second phase in Nb-Ti microalloyed dual-phase steels. A wide range of mechanical properties were obtained through change in the microstructure, depending on the TMCP cooling schedule. Polygonal ferrite matrix with martensite as the second phase exhibited highest initial strain hardening rate, lowest yield strength, and highest tensile strength as compared to the situation when bainite or bainite-martensite was the second phase. The Crussard–Jaoul (C-J) strain hardening behavior of granular bainite and granular bainite-martensite as the second phase exhibited one-stage characterized by linear and parabolic behavior, respectively. On the other hand, two stages were observed in steel containing martensite as the second phase, viz., linear stage I and parabolic stage II.

Microstructure and mechanical properties of economical dual phase wheel steels based on CSP line with later ultra fast cooling

To attain the economical wheel steels with high strength, the microstructure and properties of steel with microstructure consisting of ferrite and martensite were studied. The transformation behaviour of the economical wheel steels under continuous cooling condition and stepped cooling condition were researched and the 11·0-mm thickness wheel steel strips with simple chemical composition were developed on compact strip production (CSP) line with later ultra fast cooling (UFC) system. The results indicate that the dynamic continuous cooling transformation (CCT) curves of low carbon Mn–Cr steel is characterised by a large ferrite region, and the stepped cooling pattern is more beneficial to achieve ferrite–martensite microstructure than continuous cooling pattern. In order to obtain adequate polygonal ferrite matrix during stepped cooling process, the early cooling temperature should be controlled in the range of 700–660°C and the isothermal holding time should be ensured. The 11·0-mm thickness dual phase (DP) wheel steel strips are achieved on CSP line via control cooling technology based on later UFC system. Tensile strength increment of C–Mn–Cr steels can reach 130–170 MPa when the F-P microstucture is replaced by F-M microstucture. The wheel steel strips with tensile strength above 590 MPa, yield ratio under 0·62 and elongation above 28% have been used for wheel discs, whose weight achieve reduction of 16–22%.

Microstructure and Mechanical Properties of TRIP Steel with Annealed Martensite

The microstructure and mechanical properties of cold rolled TRIP steel containing C 0.2, Si 0.5, Mn 1.5, Al 1. 3, and Nb+V 0. 13 (mass%) with annealed martensite (TAM steel) were investigated using optical microscopy, field emission gun scanning electronic microscope (FEG SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and mechanical testing. The microstructure of the TAM steel mainly consisted of polygonal ferrite, bainite, annealed martensite and retained austenite. The martensite after annealing did not spheroidize, which consisted of annealed lath martensite structure and interlath second phase. Compared with the traditional TRIP steel with polygonal ferrite matrix (TPF steel), the TAM steel has more excellent elongation rate over 32%. The TAM steel also has better strain hardening behavior than the TPF steel. The excellent elongation and strain hardening behavior of TAM steel result from high retained austenite stability of the TAM steel, which is attributed to its fine distribution and medium strength ratio of second phase to matrix.

A Study on the Surface Characteristics of Dual Phase Steel by Electron Backscatter Diffraction (EBSD) Technique

Dual phase steels have a microstructure comprising of a polygonal ferrite matrix together with dispersed islands of martensite. There are clear differences between the image quality (IQ) map of the dual phase and the corresponding ferritic/pearlitic structures, both in the as-heat treated and cold rolled conditions. Electron backscatter diffraction (EBSD) techniques were used to study the evolution substructure of steel due to plastic deformation. The martensite-ferrite and ferrite-pearlite interfaces were observed. The interface can be a source of mobile dislocations which the bands seem to originate from the martensite islands. In particular, the use of image quality is highlighted.

ポリゴナルフェライトを母相に有する0.2%C-TRIP鋼板のバーリングに及ぼすMn添加量の影響

ポリゴナルフェライトを母相に有する0.2%C-TRIP鋼板のバーリングに及ぼすMn添加量の影響

On the Practice of the Microstructure Adjustment to Improve the Strength-Plastic Comprehensive Performance of a C-Si-Mn Alloyed High-Strength Steel

Four continuous annealing cycles were adopted to get four kinds of multi-phase steels using 0.2C-1.5Si-1.8Mn [wt%] alloyed raw steel and the microstructures and mechanical properties were investigated. The results show that traditional ferrite-martensite dual phase steel has a tensile strength of more than 1000MPa, but has the lowest strength-plastic product of only 18GPa%. The traditional ferrite, bainite and residual austenite TRIP steel has the highest elongation of 31% and hardening exponent of 0.24, but its tensile strength is just over 800MPa. Introducing hard matrix such as acicular bainitc ferrite or martensite to replace the polygonal ferrite matrix, meanwhile introducing metastable austenite, is conducive to obtain high strength and good ductility. Those kinds of modified TRIP steel with hard matrix can reach above 20GPa% of the strength-plastic product on the 1000MPa grade.

Mechanical Properties and Retained Austenite Transformation Mechanism of TRIP-Aided Polygonal Ferrite Matrix Seamless Steel Tube

Through the comparison of microstructure for polygonal ferrite (PF) matrix transformation induced plasticity (TRIP) seamless steel tube at different positions before and after tensile rupture, the transformation behavior of retained austenite (RA) was studied. The results showed that there were no yield points in tensile process and the splendid elongation and tensile strength were contributed by the uniform ferrite/bainite grains and the transformation of RA. The stability of RA was to some extent in inverse proportion with the ability of transformation induced plasticity. The coarse retained austenite located in ferrite and ferrite/bainite laths were all transformed into martensite during the tensile process.

Characterization of inclusions of X80 pipeline steel and its correlation with hydrogen-induced cracking

In this work, the microstructures of an X80 pipeline steel were characterized, and their susceptibilities to hydrogen-induced cracking (HIC) were investigated by hydrogen-charging, electrochemical hydrogen permeation and surface characterization. It is found that the microstructure of X80 pipeline steel consists of a polygonal ferrite and bainitic ferrite matrix, with martensite/austenite (M/A) constituents distributing along grain boundaries. The inclusions existing in the steel include those enriched with Si, Al oxide, Si–ferric carbide and Al–Mg–Ca–O mixture, respectively. The majority of inclusions are Si-enriched. Upon hydrogen-charging, cracks could be initiated in the steel in the absence of external stress. The cracks are primarily associated with the Si- and Al oxide-enriched inclusions. The diffusivity of hydrogen in X80 steel at room temperature is 2.0 × 10 −11 m 2/s, and the estimated hydrogen trapping density in the steel is as high as 3.33 × 10 27 m −3.

Transformation Induced Plasticity Behaviors of TRIP Steels with Different Heat Treatment Processes

Two-stage heat treatment process which has guiding significance for continuous annealing TRIP steel producing was applied in this research. Different matrixes such as polygonal ferrite matrix, bainite ferrite matrix and annealed martenite matrix were obtained through different heat treatment processes. Compared the transformation-induced plasticity (TRIP) behaviors of three different tested samples，and the corresponding process for required product properties can be chosen. It was found that the needle-like retained austenite obtained in AMT steel was isolated from other microstructures while the retained austenite in the other two samples appeared to be equiaxed or network structure. The elongation, yield ratio and stability of retained austenite in AMT steel were all higher than that in PFT or BFT steels. BFT steel possesses highest tensile strength and lowest elongation while the yield ratio, RA content and carbon concentration in RA were all lowest for PFT steel.