Effect of Deformation on Continuous Cooling Phase Transformation Behaviors of 780 MPa Nb‐Ti Ultra‐High Strength Steel

For the purpose of achieving the reasonable rolling technology of 780 MPa hot‐rolled Nb‐Ti combined ultra‐high strength steel, the effect of deformation and microalloy elements Nb and Ti on phase transformation behaviors was investigated by thermal simulation experiment. The results indicated: the deformation promoted ferritic transformation; due to the carbon content of the experimental steel was lower (<0.12% wt), the deformation indirectly impacted perlitic transformation through promoting ferritic transformation; the effect of the deformation on bainitic transformation was subject to condition whether proeutectoid ferrite precipitated before bainitic transformation. At low cooling rate of 0.5 °C/s, Nb and Ti promote transformation process γ → α, but that not good for refining the ferrite grain; at high cooling rate of 25 °C/s, Nb and Ti to a certain extent promote bainitic transformation. The recrystallization stop temperature of experimental steel was greater than 1000 °C, and phase transformation point A_r3 was 764 °C. In order to obtain the fully bainite microstructure in the practical rolling process, the cooling rate should be controlled above 15 °C/s, the start finish rolling temperature between 950–980 °C, the finishing temperature between 830–850 °C, the coiling temperature between 450–550 °C.     

Study on Isothermal Precipitation Behavior of Nano‐Scale (Nb,Ti)C in Ferrite/Bainite in 780 MPa Grade Ultra‐High Strength Steel

In order to precisely control the nano‐scale (Nb,Ti)C precipitate in hot‐rolled 780 MPa Nb–Ti microalloying C–Mn steel, isothermal precipitation behavior of nano‐scale (Nb,Ti)C precipitate in the ultra‐high strength steel was investigated by the thermal simulation experiments. The results indicated that defects of deformed supercooled austenite became the preferential nucleation sites of nano‐scale (Nb,Ti)C precipitate and ferrite, so there was a competition mechanism for austenitic defects between ferritic transformation and precipitate nucleation. Bainitic transformation could effectively freeze austenitic defects, and additional defects are formed because of volume expansion in bainitic transformation process, so bainitic transformation could promote precipitate nucleation. However, precipitate was impacted by both nucleation driving force and atom diffusibility, so the peak temperature of nano‐scale (Nb,Ti)C precipitate was 550°C. On the basis of the above theoretical results, hot rolling experiments results showed that when the coiling temperature was 550°C, the yield strength and tensile strength were 710 and 790 MPa, respectively, and the microstructure of hot‐rolled steels was mainly bainitic ferrite, and a large number of <10 nm nano‐scale (Nb,Ti)C precipitates were obtained. Precipitation strengthening contribution to reached 325 MPa.     

Microstructure and properties of hot‐rolled high strength multiphase steels for automotive application

Effects of alloying with combinations of the elements Mo, Cr and B on the bainite transformation behaviour and microstructure of hot‐rolled high strength sheet steels microalloyed with mass contents of Ti and Nb, 0.05 or 0.15 % C and 1.5 % Mn have been studied. The relationships between microstructures formed in the steels coiled at various temperatures and their mechanical properties have been investigated. The 0.15 % C microalloyed steel alloyed with Mo,Cr and B with a complex bainitic microstructure was found to have distinctive high performance behaviour combining continuous yielding, high tensile strength and plasticity after coiling in a wide temperature region. The strain hardening of the micro‐constituents typical for the investigated steels has been analysed to have a better understanding of the mechanical properties of complex phase microstructures in low alloy ferrous alloys. It was found that bainitic ferrite with austenitemartensite islands as a second phase leads to high strength and adequate elongation. The features of the bainite formation in the Mo, Cr and B alloyed CMn steel microalloyed with Ti and Nb during slow cooling from temperatures between 650 and 550 °C was studied by dilatometry.     

780MPa级热镀锌用TRIP钢退火工艺及组织演变

研究了热镀锌用高强TRIP钢的退火工艺对性能的影响和组织演变规律.结果表明:实验用钢可获得780.00MPa以上的抗拉强度和24.00%以上的断后延伸率;两相区加热温度和贝氏体保温时间对钢的力学性能具有显著影响,两相区加热温度为850℃,贝氏体保温时间为30s时,实验用钢能获得最佳的综合力学性能;在贝氏体中温相变后,仍有部分亚稳奥氏体(碳含量较低)在后续冷却过程中发生马氏体相变,从而导致钢退火后的微观组织由铁素体、贝氏体、残余奥氏体和马氏体组成.     

A Study of the Influence of Thermomechanical Controlled Processing on the Microstructure of Bainite in High Strength Plate Steel

Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.     

The effect of an intercritical heat treatment on temper embrittlement of a Ni-Cr-Mo-V rotor steel

The effect of an intercritical heat treatment on tempor embrittlement has been investigated for a rotor steel containing 0.25 pct C, 3.5 pct Ni, 1.7 pct Cr, 0.5 pct Mo, 0.1 pct V, and deliberate additions of phosphorus, tin, or antimony. Both martensitic and bainitic steels were held at the intercritical temperature of 1380°F (750°C) for times up to 40 h and were then quenched or cooled to obtain martensitic or bainitic transformation. The steels were then tempered, followed by water quenching or step cooling from the tempering temperature. The residual ferrite maintained a fine plate-like shape even after 40 h at the intercritical temperature. Embrittlement induced by step cooling from the final tempering was mark edly reduced by the intercritical treatment as compared to the embrittlement observed after conventional heat treatment; for example, AFATT, the increase in the Charpy V-notch 50 pct shear fracture transition temperature caused by step cooling, was reduced by at least 80°F (45°C) as a result of the intercritical treatment of steels containing 0.02 pct P. Molybdenum effectively reduced AFATT in intercritlcally heat-treated steels as well as in conventionally treated steels. Possible mechanisms for reducing temper embrittlement with the intercritical treatment are discussed.     

Micro‐scale Strain Distribution in Hot‐worked Duplex Stainless Steel

A modified microgrid technique has been applied to a laboratory‐made duplex stainless steel, to experimentally simulate the local state of deformation of the austenite‐ferrite microstructure of low‐alloy steels subject to intercritical deformation. A sample containing such a microgrid was deformed by plane strain compression at high temperature under conditions representative of hot rolling processes. The distortion of the microgrid after hot deformation revealed, in a quantifiable manner, the plastic flow of both phases and different deformation features. The micro‐strain distributions measured can be used to validate the models predicting the hot deformation of low alloyed C‐Mn steels during intercritical rolling.     

Detailed study of the transformation mechanisms in ferrous TRIP aided steels

Development of TRIP aided ferrous alloys is one answer to the demand for weight decrease in the automotive industry. The microstructure of hot rolled and cold rolled TRIP steels is quite complex and the optimisation of such steel products requires a detailed understanding of the mechanisms of phase transformation, during thermomechanical treatment as well as during mechanical testing or metal forming. We present in this paper the results obtained at Irsid concerning the study of austenite stabilisation through bainitic transformation during thermal treatment and its transformation into martensite during mechanical testing. First of all, the characterisation methods are presented. An effort has to be put on this point due to the refinement of the microstructure of TRIP steels, especially the size of austenite and martensite islands. Carbon replicas for the observation by means of transmission electron microscopy (TEM) are used to analyse the morphological features of the microstructure ‐ nature of the constituents, size and shape ‐ and the composition of cementite particles present in the steels. The mean value for this carbon content in retained austenite is deduced from X‐ray diffraction measurements. Then the kinetics of bainitic transformation are discussed as well as cementite precipitation. The typical composition of the steel studied is 0.5 % C, 1.5 % Mn. The use of 0.5 % C steels facilitates the study of bainitic transformation by avoiding the ferrite formation usually occurring in TRIP steels. Cementite nucleation appears at the ferrite/austenite interface without any partitionning of substitutional elements. To satisfy thermodynamic equilibrium at the interface, the silicon content on the cementite side is very low and high on the austenite side. Then, carbon diffusion towards austenite is delayed and, as a consequence, cementite growth is also delayed. As the diffusion kinetics are low at 400 °C, cementite keeps this “non partitioned” composition, even after 3 hours holding. At 500 °C, diffusion kinetics are higher and cementite composition approaches that predicted by equilibrium. Finally, the stability of retained austenite during mechanical testing is studied. Before and after mechanical testing the morphological characteristics of the microstructure (austenite island size and elongation) are analysed by TEM replicas and image analysis. There is a high density of very small austenite islands but they represent only a small fraction of the total retained austenite. These results confirm and quantify the size effect on austenite stabilisation during deformation.     

Correlation of rolling condition,microstructure, and low-temperature toughness of X70 pipeline steels

Correlation of rolling conditions, microstructure, and low-temperature toughness of high-toughness X70 pipeline steels was investigated in this study. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot rolling, and their microstructures were varied by rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below −100 °C when the finish cooling temperature range was 350 °C to 500 °C, showing excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85 pct, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture occurred at some finish cooling temperatures.     

Hot Mechanical Properties of 24Cr‐14Ni High Alloy Billets

24Cr‐14Ni alloys have gained importance in high temperature applications. Because of δ‐ferrite and α phase formation, 24Cr‐14Ni austenitic stainless steel billets are difficult to hot work. The mechanical properties at high temperature of such stainless steels are investigated on a hot tensile test machine according to hot‐rolling conditions, under different time and temperature regimes. These 24Cr‐14Ni stainless steels were also hot rolled under various reduction ratios. The influences of the reduction ratio on the hot mechanical properties and phase transformation from δ‐ferrite into σ phase in 24Cr‐14Ni stainless steels are discussed in detail. The results obtained can be a contribution to improve the hot rolling of this high alloy stainless steel.     

Microstructure Evolution During Hot Deformation of a Micro-Alloyed Steel

In the present investigation, hot deformation by uniaxial compression of a microalloyed steel has been carried out, using a deformation dilatometer, after homogenization at 1200 °C for 20 min up to strains of 0.4, 0.8 and 1.2 at different temperatures of 900, 1000 and 1100 °C, at a constant strain rate of 2 s^?1 followed by water quenching. In all the deformation conditions, initiation of dynamic recrystallization (DRX) is observed, however, stress peaks are not observed in the specimens deformed at 900 and 1000 °C. The specimens deformed at 900 °C showed a combination of acicular ferrite (AF) and bainite (B) microstructure. There is an increase in the acicular ferrite fraction with increase in strain at all these deformation temperatures. At high deformation temperature of 1100 °C, coarsening of DRXed grains is observed. This is attributed to the common limitations involved in fast quenching of the DRXed microstructure, which leads to increase in grain size by metadynamic recrystallization (MDRX). The strain free prior austenite grains promote the formation of large fraction of both bainite and martensite in the transformed microstructures during cooling. The length and width of bainitic ferrite laths also increases with increase in deformation temperature from 900 to 1100 °C and decrease in deformation strain.     

Grain Size Dependence of Austenite Decomposition in Air-Cooled 16MnCr5 Steel

A hot-rolled and controlled rolled 16MnCr5 steel was analyzed after similar industrial cooling conditions. The hot rolled steel had a ferrite–bainite microstructure whereas the controlled rolled steel had a ferrite–pearlite microstructure. The prior austenite grain size was found to be the controlling factor based on a cooling analysis. The effect of prior austenite grain size on the bainite start temperature had to be considered in the transformation model.     

Microstructural Changes after Control Rolling and Interrupted Accelerated Cooling Simulations in Pipeline Steel

The γ‐α transformation and final microstructure in pipeline steel was studied by carrying out a number of physical simulations of industrial hot rolling schedules. Particularly, the effect of the reheating temperature, deformation and cooling parameters on the transformation temperatures and final grain size were considered with a goal to obtain an appropriate thermo‐mechanical processing route which will generate appropriate microstructures for pipeline applications. The CCT diagram of the steel was derived experimentally by means of dilatometric tests. Hot torsion experiments were applied in a multi‐deformation cycle at various temperatures in the austenite region to simulate industrial rolling schedules. By variation of the reheating temperature, equivalent strain, and accelerated cooling, different types of microstructures were obtained. It was found that the deformation increases the transformation temperatures whereas the higher cooling rates after deformation decrease them. Post‐deformation microstructure consists of fine bainitic‐ferrite grains with dispersed carbides and small amount of dispersed martensite/austenite islands which can be controlled by varying the reheating temperature, deformation and post‐deformation cooling. The detailed microstructure characteristics obtained from the present work could be used to optimize the mechanical properties, strength and toughness of pipeline steel grades by an appropriate control of the thermo‐mechanical processing.     

Effect of thermomechanical processing on the retained austenite content in a Si-Mn transformation-induced-plasticity steel

The influence of hot deformation on the microstructure of a hot-rolled Si-Mn transformation-induced-plasticity (TRIP) steel was evaluated in an effort to better control retained austenite content. In this study, axial compressive strains varying in amounts from 0 to 60 pct were imposed in the austenite phase field, and effects on the formation of polygonal ferrite, bainite, and retained austenite were determined. In addition, modifications in simulated coiling temperature from 420 °C to 480 °C and cooling rates from the rolling temperature, between 10 °C/s and 35 °C/s, were assessed. Fast cooling rates, low coiling temperatures, and low degrees of hot deformation were generally found to decrease the amount of polygonal ferrite and increase retained austenite fraction. Unexpectedly, a sharp increase in polygonal ferrite content and decrease in retained austenite content occurred when the fastest cooling rate, 35 °C/s, was coupled with extensive hot deformation and high coiling temperatures. This effect is believed to be due to insufficient time for full recovery and recrystallization of the deformed austenite, even in the absence of intentional microalloying additions to control recrystallization kinetics. The resultant decrease in hardenability allowed the ferrite transformation to continue into the holding time at high (simulated) coiling temperatures.     

Effect of Thermo‐Mechanical Process on Phase Transformation Behaviors of V–Ti–N Microalloyed Steel

Thermo‐mechanical simulation tests were performed on V–Ti–N microalloyed steel under three hot working conditions by using Gleeble‐3800 thermo‐mechanical simulator to study the effects of hot deformation and post‐deformation holding process on the continuous cooling transformation behaviors of overcooled austenite. The continuous cooling transformation diagrams (CCT diagrams) were determined by thermal dilation method and metallographic method. The effects of the hot deformation, post‐deformation holding, and cooling rate on the microstructure evolution were analyzed. The results show that deformation promotes ferrite and pearlite transformation. In addition, deformation leads to an increase in bainite start temperature, which becomes more markedly with the increase in cooling rate. The post‐deformation holding process is much favorable to promote carbonitride precipitation of the microalloying elements, which contributes to ferrite nucleation and smaller austenite grains. As a result, an increase in ferrite quantity and a decrease in ferrite grain size can be observed. And further more, the post‐deformation holding process reduces the effect of hot deformation on the bainite start temperature.     

Effect of the Charging Temperature on the Hot Ductility of Nb‐Containing Steel in the Simulated Hot Charge Process

In order to develop a comprehensive understanding of the effect of hot charging temperature on the hot ductility of a Nb‐containing steel, direct hot charging process was simulated by using a Gleeble thermo stress/strain machine. Three kinds of thermal histories were introduced to assess the hot ductility of the steel during continuously cast, hot charging, and cold charging process by means of hot tensile test in relation to surface cracking of hot charging processed steel slabs. The ductility of the specimens charged at the temperature within the range of ferrite/austenite two‐phase region and charged at the temperature just below the A_r1 of the steel is largely reduced. These results can be ascribed to the retained ferrite films at the boundaries of austenite encouraging voiding at the boundaries and these voids gradually link up to give failure around 750°C, and the combination of inhogeneous austenite grain size and precipitations aggravating the ductility trough by encouraging grain boundary sliding at 950°C. The steel via the conventional cold charge process experienced a complete phase transformation from austenite to ferrite and pearlite structure during the cooling to the ambient temperature. This steel can be charged into a reheating furnace and rolled without experiencing hot embrittlement due to the recrystallization and the precipitates are trapped inside a newly formed grain of austenite. In comparison with the hot ductility results, the hot tensile strength is only slight influenced by the charging temperature.     

Effect of hot‐rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

TRIP‐assisted multiphase steels have been thoroughly studied in the cold‐rolled and annealed state. The effects of hot‐rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP‐assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low‐carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium‐carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non‐recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low‐carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work‐hardening capabilities. In the case of the medium‐carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low‐carbon steel, the improvement of the work‐hardening behaviour was attributed to an Interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.     

New Duplex Microstructure of Grain Boundary Allotriomorphic Ferrite/Granular Bainite

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TRIP钢在分段冷却过程中的相变行为

 建立了热轧低碳Si Mn系TRIP钢相变动力学模型。将该模型用于模拟热轧TRIP钢分段冷却过程中显微组织的演变,并定量分析了热变形和冷却速率对热变形奥氏体相变行为的影响。结果表明,预测值与实测值符合较好;降低终轧温度、提高终轧变形量,延长两相区缓冷时间都能促进铁素体相变,从而有利于提高残余奥氏体中的碳含量。     

Low-Temperature Toughening Mechanism in Thermomechanically Processed High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels were fabricated by varying thermomechanical processing conditions such as rolling and cooling conditions in the intercritical region, and the low-temperature toughening mechanism was investigated in terms of microstructure and the associated grain boundary characteristics. The steels acceleratedly cooled to relatively higher temperature had lower tensile strength than those acceleratedly cooled to room temperature due to the increased volume fraction of granular bainite or polygonal ferrite (PF) irrespective of rolling in the intercritical region, while the yield strength was dependent on intercritical rolling, and start and finish cooling temperatures, which affected the formation of PF and low-temperature transformation phases. The steel rolled in the intercritical region and cooled to 673 K (400 °C) provided the best combination of high yield strength and excellent low-temperature toughness because of the presence of fine PF and appropriate mixture of various low-temperature transformation phases such as granular bainite, degenerate upper bainite (DUB), lower bainite (LB), and lath martensite (LM). Despite the high yield strength, the improvement of low-temperature toughness could be explained by the reduction of overall effective grain size based on the electron backscattered diffraction (EBSD) analysis data, leading to the decrease in ductile-to-brittle transition temperature (DBTT).