Effect of alloying elements on the microstructure‐property relationship in thermomechanically processed C‐Mn‐Si TRIP steels

The effect of additions of Nb, Al and Mo to Fe‐C‐Mn‐Si TRIP steel on the final microstructure and mechanical properties after simulated thermomechanical processing (TMP) has been studied. The laboratory simulations of discontinuous cooling during TMP were performed using a hot rolling mill. All samples were characterised using optical microscopy and image analysis. The volume fraction of retained austenite was ascertained using a heat tinting technique and X‐ray diffraction measurements. Room temperature mechanical properties were determined by a tensile test. From this a comprehensive understanding of the structural aspect of the bainite transformation in these types of TRIP steels has been developed. The results have shown that the final microstructures of thermomechanically processed TRIP steels comprise ~ 50 % of polygonal ferrite, 7 ‐12 % of retained austenite, non‐carbide bainitic structure and martensite. All steels exhibited a good combination of ultimate tensile strength and total elongation. The microstructure‐property examination revealed the relationship between the composition of TRIP steels and their mechanical properties. It has been shown that the addition of Mo to the C‐Si‐Mn‐Nb TRIP steel increases the ultimate tensile strength up to 1020 MPa. The stability of the retained austenite of the Nb‐Mo steel was degraded, which led to a decrease in the elongation (24 %). The results have demonstrated that the addition of Al to C‐Si‐Mn‐Nb steel leads to a good combination of strength (~ 940 MPa) and elongation (~ 30 %) due to the formation of refined acicular ferrite and granular bainite structure with ~7 ‐ 8 % of stable retained austenite. Furthermore, it has been found that the addition of Al increases the volume fraction of bainitic ferrite laths. The investigations have shown an interesting result that, in the Nb‐Mo‐Al steel, Al has a more pronounced effect on the microstructure in comparison with Mo. It has been found that the bainitic structure of the Nb‐Mo‐Al steel appears to be more granular than in the Nb‐Mo steel. Moreover, the volume fraction of the retained austenite increased (12 %) with decreasing bainitic ferrite content. The results have demonstrated that this steel has the best mechanical properties (1100 MPa and 28 % elongation). It has been concluded that the combined effect of Nb, Mo, and Al addition on the dispersion of the bainite, martensite and retained austenite in the ferrite matrix and the morphology of these phases is different than effect of Nb, Mo and Al, separately.     

Effects of Niobium Addition on Microstructures and Formability in Si‐Al‐Mn TRIP Cold‐rolled Steels

The effects of Nb addition on microstructures and formability in Si‐Al‐Mn TRIP cold‐rolled steels were investigated. These steels were intercritical annealed at 770 °C for 5 min, and isothermally treated at 400 °C for 3 min. Microstructural observation, tensile tests and forming limit diagram (FLD) tests were conducted, and the changes of retained austenite volume fraction as a function of tensile strain were measured by using an X‐ray diffractometer. The results showed that Nb addition makes grain size refined, the volume fraction of ferrite increase and that of bainite decrease, however, obviously it does not affect the volume fraction and carbon content of retained austenite. The Nb addition increased the stability of retained austenite owing to grain refinement. With Nb addition, increase in strength, ductility, strain hardening exponent and formability could be achieved simultaneously. These findings indicate that Nb addition can be a new direction of microalloying design for the low carbon TRIP steels with excellent formability and high stability of retained austenite.     

等温时间对低硅含铝热轧TRIP钢组织性能的影响

 为了实现低硅含铝热轧TRIP钢的工业应用，以低硅含铝热轧TRIP钢为研究对象，采用扫描电子显微镜、透射电子显微镜、拉伸试验和X射线衍射等试验方法，研究了不同等温时间对试验钢显微组织和力学性能的影响。结果表明，试验钢的显微组织主要由多边形铁素体、贝氏体铁素体和残余奥氏体组成，随着等温时间的增加，板条贝氏体的体积分数升高，粒状贝氏体的体积分数降低；当等温时间为20 min时，试验钢的综合力学性能最佳，抗拉强度为732.25 MPa，断后伸长率为36%，强塑积为26.36 GPa·%；残余奥氏体的体积分数和碳含量先升高后降低，等温时间为20 min时试验钢表现出较强的加工硬化行为。     

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si Mn TRIP steel was investigated by means of microstructural ob servation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.     

Role of carbon and alloying elements in the formation of bainitic ferrite

One approach to the prediction of the carbon content of austenite, remaining after the precipitation of bainitic ferrite, is based on the assumption that bainitic ferrite during growth inherits the carbon content of the parent austenite. An alternative approach is based on the assumption that bainitic ferrite grows with a low carbon content and there is no major difference between Widmanstätten ferrite and bainitic ferrite. The two approaches are now compared using information from alloyed steels with considerable amounts of Si, where the formation of cementite is retarded. The former approach does not account for the effect of Mn and fails severely at low alloy contents. The latter approach seems more promising but is not without difficulties. In particular, in order to explain the effects of Cr and Mo, it seems necessary to introduce a kinetic effect, presumably caused by solute drag.     

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.     

Forming Limit Curve (FLC) and Fracture Mechanism of Newly Developed Low‐Carbon Low‐Silicon TRIP Steel

The Forming‐Limited Diagram (FLD) of intercritically annealed 0.11C‐1.65Mn‐0.62Si TRIP‐assisted steel was investigated. The high FLD₀ value of this new low carbon TRIP steel was indicative of a superior formability. The micro‐structural changes during deformation and fracture were studied in detail. The polygonal ferrite phase was found to plastically deform first and deformed most at larger strains. Fracture was initiated by micro‐voids nucleated at ferrite grain boundaries, within ferrite grains or at the interface between ferrite and the harder phases. Cracks were formed after micro‐voids grew, coalesced, and expanded in one direction. When crack tips reached the bainite phase or the martensite/austenite constituent, the cracks propagated along the boundary of these phases. Cracks reaching retained austenite islands caused stress‐induced martensite transformation at the crack tip. The direction of motion of the cracks also changed in this case.     

2种含铝TRIP钢连续冷却转变动力学分析

用热膨胀法,通过对含1.5%w（Al）和含w（Al）1.0%＋w（Si）0.5%的2种TRIP钢的显微组织观察,绘制了2种不同w（Al）、w（Si）的TRIP钢的静态CCT曲线,研究了含Al TRIP钢在冷却过程中的相转变,分析了冷却速率及不同w（Al）对微观组织和贝氏体转变动力学的影响。结果表明,Al元素加速TRIP钢冷却时铁素体转变和贝氏体转变,但是推迟珠光体转变。     

1000MPa级高延伸率Q&P钢的实验室研究

以C-Si-Mn系TRIP钢成分为基础,设计了四种不同Si和Mn含量的合金成分,并采用不同两相区奥氏体化温度的淬火—配分(QP)工艺进行处理,得到了兼具高强度和高塑性的QP钢。其中,当奥氏体化温度为820℃时,0.18C-1.8Si-2.2Mn(质量分数,%)钢和0.18C-1.8Si-2.5Mn钢在抗拉强度达到1 000 MPa以上的同时断后延伸率仍不低于20%,显示了极佳的强塑性结合。利用SEM和XRD等对热处理材料的显微组织进行了表征,结果显示,其显微组织为铁素体、板条马氏体和一定量的残余奥氏体,残余奥氏体多呈块状且被铁素体所包围,且奥氏体化温度为820℃时,材料中的残余奥氏体含量和平均碳浓度均较高。更多且稳定的残余奥氏体在变形过程中发生TRIP效应,可以在不显著降低材料强度的情况下更有效地改善材料的塑性,这也是四种试验用钢经820℃的QP工艺处理后显示出更佳强塑性结合的主要原因。     

Martensite Transformation and Its Control in DP,TRIP and TWIP Steels

Designing of alloy concept and process for DP,TRIP and TWIP steels stressing at martensite transformation are analyzed.For DP steel,austenite volume percent and its carbon content at different intercritical temperatures are calculated as well as the tensile strength of the steel,which meet well with the experimental result.The condition for dissolution of carbide is discussed by experiments and predicted by kinetic estimation.Several sample TRIP steels are prepared and their concentration profiles are calculated showing different diffusion characteristics of elements.Calculation also shows carbon enrichment is successful in this stage through the quick diffusion of carbon from ferrite to austenie.In order to maintain the austenite stability or to prevent precipitation of cementite,minimum cooling rate from the intercritical zone to over aging stage is obtained through kinetic simulation.Bainite transformation is estimated,which indicates the carbon rerichment from ferrite of bainite structure to austenite in this stage is also successful.Thermal HCP martensite transformation and the strain induced martensite transformation in TWIP steel is introduced.Relationship between transformation and mechanical properties in the steel is also mentioned.     

硅含量对低碳钢连续冷却中转变组织的影响

通过热模拟压缩实验,对C、Mn含量基本相同,w（Si）对低碳钢在850℃变形后、以30℃/s冷却到室温时的组织演变的影响。结果表明：w（Si）由O．50％增加到0．99％时,硅的增加促进了先共析铁索体的形成,而w（Si）由0．99％增加到1．41％时,硅的增加促进了粒状贝氏体的形成。同时Si对组织中先共析铁素体的晶粒尺寸以及体积分数的变化也有一定的影响。     

Effect of Cooling Method on Microstructure and Mechanical Properties of Hot-Rolled C-Si-Mn TRIP Steel

 The controlled cooling technology following hot rolling process is a vital factor that affects the final microstructure and mechanical properties of the hot-rolled transformation induced plasticity (TRIP) steels. In the present study, low alloy C-Si-Mn TRIP steel was successfully fabricated by hot rolling process with a 450 hot rolling mill. To maximize the volume fraction and stability of retained austenite of the steel, two different cooling methods (air-cooling and ultra-fast cooling “AC-UFC” and ultra-fast cooling, air-cooling and ultra-fast cooling “UFC-AC-UFC”) were conducted. The effects of the cooling method on the microstructure of hot-rolled TRIP steel were investigated via optical microscope, transmission electron microscope and conversion electron Mssbauer spectroscope. The mechanical properties of the steel were also evaluated by conventional tensile test. The results indicated that ferrite and bainite in the microstructure were refined with the cooling method of UFC-AC-UFC. The morphology of retained austenite was also changed from small islands distributing in bainite district (obtained with AC-UFC) to granular shape locating at the triple junction of the ferrite grain boundaries (obtained with UFC-AC-UFC). As a result, the TRIP steel with a content of retained austenite of 1152%, total elongation of 32% and product of tensile strength and total elongation of 27552 MPa·% was obtained.     

Effect of Si,C and Mn on the formation of bainite in ferrite-bainite steel

High-strength steels have been widely applied to automotive chassis parts.In order to form complex shapes,high hole expansion rates and high formability are required.Dual phase (DP) steel has a good formability,but a poor hole expansion rate.In this circumstance,another kind of steel which has a microstructure of ferrite-bainite,rather than ferrite-martensite,has been found to be an alternative solution.It is called FB steel.This steel with Si,C and Mn additions are applied in this study.A two-step cooling process is used to get the desired F+ B microstructures.Continuous cooling transformation (CCT) diagrams are made with deformation and without deformation,and starting times and temperatures of the phase transformations of interest are obtained.It is shown that Si,C and Mn contents in the steel strongly affect the shapes and positions of the CCT diagrams,as well as the final microstructures of FB steel.An increase of the Si content can promote the formation of ferrite and move the CCT diagram toward the left.However,when Si content is too high,when comparing to carbon and manganese contents,the formation of bainite will be retarded because of the formation of more ferrite.It increases the amount of C in a solid solution in the untransformed austenite and promotes the formation of pearlite.C and Mn can inhibit the formation of ferrite and retard the accumulation of C in austenite.Therefore,the appropriate balance of C,Si and Mn contents in steels will be able to help in obtaining the desired microstructure.     

Effect of Bake‐Hardening on the Structure‐Property Relationship of Multiphase Steels for the Automotive Industry

The effect of a bake‐hardening (BH) treatment on the microstructure and mechanical properties has been studied in C‐Mn‐Si TRansformation Induced Plasticity (TRIP) and Dual Phase (DP) steels after: (i) thermomechanical processing (TMP) and (ii) intercritical annealing (IA). The steels were characterized using X‐ray diffraction, transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). All steels showed high BH response. However, the DP and TRIP steels after IA/BH showed the appearance of upper and lower yield points, while the stress‐strain behavior of the TRIP steel after TMP/BH was still continuous. This was due to the higher volume fraction of bainite and more stable retained austenite in the TMP/BH steel, the formation of plastic deformation zones with high dislocation density around the “as‐quenched” martensite and “TRIP” martensite in the IA/BH DP steel and IA/BH TRIP steel, respectively.     

TRIP钢 TMP和热处理过程中的组织演变

介绍了热轧TRIP钢TMP(形变热处理)的精轧阶段(奥氏体未再结晶区变形)、输送台冷却(γ-α两相区的相变)和卷取阶段(贝氏体转变)热轧TRIP钢组织演变的影响因素;冷轧TRIP钢临界区(γ-α区)退火和贝氏体转变区等温处理时的组织演变及其演变过程中TRIP钢残余奥氏体中碳的富集的研究进展。     

Effect of microstructure on the stability of retained austenite in transformation-induced-plasticity steels

Two Fe-0.2C-1.55Mn-1.5Si (in wt pct) steels, with and without the addition of 0.039Nb (in wt pct), were studied using laboratory rolling-mill simulations of controlled thermomechanical processing. The microstructures of all samples were characterized by optical metallography, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The microstructural behavior of phases under applied strain was studied using a heat-tinting technique. Despite the similarity in the microstructures of the two steels (equal amounts of polygonal ferrite, carbide-free bainite, and retained austenite), the mechanical properties were different. The mechanical properties of these transformation-induced-plasticity (TRIP) steels depended not only on the individual behavior of all these phases, but also on the interaction between the phases during deformation. The polygonal ferrite and bainite of the C-Mn-Si steel contributed to the elongation more than these phases in the C-Mn-Si-Nb-steel. The stability of retained austenite depends on its location within the microstructure, the morphology of the bainite, and its interaction with other phases during straining. Granular bainite was the bainite morphology that provided the optimum stability of the retained austenite.     

Forming response of retained austenite in a C‐Si‐Mn high strength TRIP sheet steel

TRIP sheet steels typically consist of ferrite, bainite, retained austenite, and martensite. The retained austenite is of particular importance because its deformation‐induced transformation to martensite contributes to excellent combinations of strength and ductility. While information is available regarding austenite response in uniaxial tension, less information is available for TRIP steels with respect to the forming response of retained austenite in complex strain states. Therefore, the purpose of this work was to study the austenite transformation behaviour in different strain paths by determining the amount of retained austenite before and after forming. Forming experiments were performed on a high strength 0.19C‐1.63Si‐1.59Mn TRIP sheet steel 1.2 mm in thickness in two different strain conditions, uniaxial tension (ε₁ = ‐2ε₂) and balanced biaxial stretching (ε₁ = ε₂). Specimens were formed to strains ranging from zero to approximately 0.2 effective (von Mises) strain. Specimens were tested both longitudinally and transverse to the rolling direction in uniaxial tension, and subtle mechanical property differences were found. The volume fraction of austenite, determined with X‐ray diffraction subsequent to forming, was found to decrease with increasing strain for both forming modes. Some modification in the crystallographic texture of the ferrite was observed with increasing strain, in specimens tested in the balanced biaxial stretch condition. This trend was not evident in the uniaxial tensile test results. Slight differences were found in the transformation behaviour of the austenite when formed in different strain conditions. More austenite transformed in specimens tested parallel to the rolling direction than transverse to the rolling direction in uniaxial tension. The amount of austenite transformed during biaxial stretching was determined to be greater than the amount transformed in uniaxial tension for specimens tested transverse to the rolling direction at an equivalent von Mises strain. The amount of austenite that transformed in biaxial tension, however, was comparable to the amount of austenite that transformed in specimens tested longitudinal to the rolling direction in uniaxial tension.     

Microstructure,Mechanical Properties,and Abrasive Wear Behaviour of Si‐Mn‐Cr‐B Cast Steel as a Function of Carbon Concentration

The microstructures, mechanical properties and abrasive wear behaviour of five kinds of Si‐Mn‐Cr‐B cast steels were studied. The steels investigated contained X wt.% C with X= 0.15, 0.25, 0.35, 0.45, 0.55, 2.5 wt.% Si, 2.5 wt.% Mn, 0.5 wt.% Cr, 0.004 wt.%B . The results showed that the A_c1temperatures increased and A_c3 and M_s temperatures decreased with increasing carbon concentration. From the continuous cooling transformation (CCT) curves, it was discovered that the incubation period of pearlitic transformation was prolonged and the transformation curves of pearlite and bainite were separated significantly with rising carbon concentration. At lower carbon concentration, the normalized structure of Si‐Mn‐Cr‐B cast steel consisted mainly of granular bainite and M‐A islands. The normalized microstructures of the cast steel changed from granular bainite gradually to needle‐like bainite, upper bainite, and lower bainite with rising carbon concentration. The tensile strength and hardness of Si‐Mn‐Cr‐B cast steel increased and impact and fracture toughness decreased with increasing carbon content. The wear testing results showed that the wear resistance of Si‐Mn‐Cr‐B cast steel improved with higher carbon content but was obviously unchanged beyond the carbon concentration of 0.45%. The best balance of properties of Si‐Mn‐Cr‐B cast steel is obtained at the carbon concentration range of 0.35 ‐ 0.45%C.     

C-Mn-Si系超级贝氏体钢连续冷却转变（CCT）曲线

采用热膨胀法和金相法,通过Gleeble-1500热模拟试验机测定C-Mn-Si系低碳(/%:0.11C、1.15Si、1.85Mn、0.032Al、0.003 Ti、0.002 4N)和中碳(/%:0.35C、1.11Si、1.82Mn、0.041Al、0.002 Ti、0.004 2N)贝氏体钢在0.5～30℃/s的冷却速度下连续冷却时的膨胀曲线,确定相变点,并结合显微组织,借助Origin软件分别绘制出两种钢的连续冷却转变(CCT)曲线。结果表明,0.11%C钢当冷却速度≤1℃/s时获得铁素体+贝氏体+马氏体组织,冷却速度≥2℃/s时为贝氏体+马氏体组织,0.35%C钢冷却速度≥0.5℃/s即可获得贝氏体+马氏体组织;随碳含量增加,贝氏体和马氏体转变温度均降低。