基于动态相变的热轧Nb-V-Ti微合金化TRIP钢组织及性能

通过单轴热压缩试验,结合扫描电镜以及X射线衍射技术,研究了动态相变前奥氏体晶粒状态对基于动态相变的热轧Nb-V-Ti微合金化TRIP钢复相组织状态及力学性能的影响.与动态相变前奥氏体晶粒为等轴状条件下相比,动态相变前奥氏体晶粒为拉长状条件下,动态相变得到的铁素体转变量较大,最终复相组织中贝氏体含量较少且团径较小,马氏体含量较少,但对残余奥氏体含量及其含碳量影响不明显.与不含微合金化元素的基于动态相变的热轧TRIP钢相比,Nb-V-Ti微合金化TRIP钢的屈服强度和抗拉强度明显提高,而延伸率有所降低.      

相变时间对CSP热轧TRIP钢组织和性能的影响

利用OM、SEM、XRD、EBSD和室温拉伸试验机等研究了CSP热轧TRIP钢中间缓冷时间及贝氏体等温时间对组织和力学性能的影响。结果表明,随着中间缓冷时间的延长,试验钢中的铁素体和残余奥氏体体积分数增加,贝氏体体积分数减少;抗拉强度基本不变,屈服强度逐渐降低,断后伸长率和强塑积变化不明显。中间缓冷时间为6 s时,可满足CSP产线的要求。对贝氏体相变时间的研究表明,当等温时间为15 min时,试验钢中的残余奥氏体主要分布于铁素体/铁素体界面、铁素体/贝氏体界面以及贝氏体中,体积分数约为7.1%,表现出良好的TRIP效应。其抗拉强度、屈服强度、断后伸长率和强塑积分别达到744.0 MPa、522.5 MPa、29.3%和21.8 GPa·%,力学性能最优。当等温时间延长至50 min时,试验钢中的贝氏体含量增加,残余奥氏体体积分数减少至2.7%,强塑积明显下降。     

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 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.     

Microstructure Characterization and Mechanical Properties of TRIP-aided Steel under Rapid Heating for Different Holding Time

In order to develop a comprehensive understanding about the effect of different holding time under rapid heating on the microstructural evolution and mechanical properties of transformation-induced plasticity (TRIP)steel, continuous annealing process simulations were performed using a thermal system with resistance heating method. The morphology and distribution of all phases present in the microstructure and the mechanical properties of TRIP steel were revealed.It appeared that the final tensile strength of the TRIP steel increased and retained austenite car-bon content decreased with increasing holding time.An overlap between ferrite recrystallization and austenitization was observed during intercritical holding.In addition,the work hardening of the samples was evaluated by calculat-ing the instantaneous n value as a function of the true strain.The difference in work hardening behavior corresponds to the rate of the retained austenite transformation during straining,which can be attributed to the carbon content and the morphology of the retained austenite.     

退火时间对冷轧中锰TRIP钢组织和力学性能的影响

采用CCT-AY-Ⅱ型钢板连续退火机模拟分析了退火时间对中锰TRIP钢0.1C-7Mn组织性能的影响规律.利用扫描电镜、透射电镜、电子背散射衍射和X射线能量色散谱等研究了不同工艺下制备的0.1C-7Mn钢的微观组织和成分,利用X射线衍射法测量了残留奥氏体量,利用拉伸试验测试了其力学性能.0.1C-7Mn钢在650℃保温3 min退火后获得最佳的综合力学性能,其强度为1329 MPa,总延伸率为21.3%,强塑积为28 GPa·%.分析认为,0.1C-7Mn钢的高塑性是由亚稳奥氏体的TRIP效应和超细晶铁素体共同提供的,而高强度是由退火冷却过程中奥氏体转变的马氏体和拉伸变形过程中TRIP效应转变的马氏体的强化作用造成的.      

Mechanical Property and Microstructural Characterization of C-Mn-Al-Si Hot Dip Galvanizing TRIP Steel

 Mechanical properties and microstructure in high strength hot dip galvanizing TRIP steel were investigated by optical microscope (OM), transmission electron microscope (TEM), X-ray diffraction (XRD), dilatometry and mechanical testing. On the heat treatment process of different intercritical annealing (IA) temperatures, isothermal bainitic transformation (IBT) temperatures and IBT time, this steel shows excellent mechanical properties with tensile strength over 780 MPa and elongation more than 22%. IBT time is a crucial factor in determining the mechanical properties as it confirms the bainite transformation process, as well as the microstructure of the steel. The microstructure of the hot dip galvanizing TRIP steel consisted of ferrite, bainite, retained austenite and martensite during the short IBT time. The contents of ferrite, bainite, retained austenite and martensite with different IBT time were calculated. The results showed that when IBT time increased from 20 to 60 s, the volume of bainite increased from 14.31% to 16.95% and the volume of retained austenite increased from 13.64% to 16.28%; meanwhile, the volume of martensite decreased from 7.18% to 1.89%. Both the transformation induced plasticity of retained austenite and the hardening of martensite are effective, especially, the latter plays a dominant role in the steel containing 7.18% martensite which shows similar strength characteristics as dual-phase steel, but a better elongation. When martensite volume decreases to 1.89%, the steel shows typical mechanical properties of TRIP, as so small amount of martensite has no obvious effect on the mechanical properties.     

Effect of Microstructure in TRIP Steel on Its Tensile Behavior at High Strain Rate

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不同工艺下低碳Mn-Si钢的组织与力学性能

通过Gleeble-1500热模拟压缩试验,借助光学显微镜、扫描电镜、X射线衍射及拉伸试验等,研究一种低碳Mn-Si钢在基于热轧动态相变的热轧TRIP钢工艺和基于贝氏体等温处理工艺下的组织与力学性能,比较了通过两种工艺获得的不同复相组织状态对材料的加工硬化能力的影响.结果表明:实验钢在基于动态相变的热轧TRIP钢工艺下获得了以细晶铁素体为基体和贝氏体、残余奥氏体组成的复相组织,而在基于贝氏体等温处理工艺下得到了以板条贝氏体为基体和残余奥氏体组成的复相组织,前者中残余奥氏体含量较高且其碳含量也较高.实验钢具有以板条贝氏体为基体的复相组织时屈服强度和抗拉强度较高;但由于残余奥氏体稳定性较差,实验钢的加工硬化能力较弱,导致其均匀延伸率和总延伸率较小.      

Effects of Holding Temperature for Austempering on Mechanical Properties of Si-Mn TRIP Steel

A new type of high strength steel containing a significant amount of stable retained austenite was obtained by austempering immediately after intercritical annealing. This sort of low carbon steel only contains alloying elements of silicon and manganese rather than nickel and chromium. Its mechanical properties were enhanced considerably due to strain-induced martensite transformation and transformation-induced plasticity (TRIP) of retained austenite when it was strained at temperatures between Ms and Md, because retained austenite was moderately stabilized due to carbon enrichment by austempering. Austempering was carried out at different temperatures and 400℃ was found to be optimal. Tensile strength, total elongation and strength-ductility balance reached the maximum values and the product of tensile strength and total elongation exceeded 30 135 MPa % when the TRIP steel was held at 400℃ and strained at 350℃.     

连续退火的无Si TRIP钢的组织和力学性能

在实验室用Gleeble3500热模拟试验机制备了一种无Si TRIP钢.利用拉伸试验机、扫描电镜、透射电镜、X射线衍射以及热膨胀仪对其力学性能、微观组织和相变规律进行研究,在此基础上分析了贝氏体相变温度和时间对力学性能和残余奥氏体的影响.无Si TRIP钢呈现出良好的整体力学性能,抗拉强度分布在740~810 MPa,延伸率均在25%以上,最高可达32%以上;贝氏体等温温度为420℃时能获得最佳的综合力学性能,抗拉强度随贝氏体相变时间增加而下降,延伸率随之上升,而屈服强度没有显著变化.无Si TRIP制的铁素体晶粒大小约为3~4μm,比含Si TRIP钢铁素体晶粒细小;残余奥氏体的体积分数在8%~10%,比含Si TRIP钢低约3%;420℃保温300 s后贝氏体相变基本结束,而碳的扩散仍然在进行;无Si TRIP钢贝氏体相变速率比含Si TRIP钢快,贝氏体相变总量也更多.      

Effect of deformation schedule on the microstructure and mechanical properties of a thermomechanically processed C-Mn-Si transformation-induced plasticity steel

Thermomechanical processing simulations were performed using a hot-torsion machine, in order to develop a comprehensive understanding of the effect of severe deformation in the recrystallized and nonrecrystallized austenite regions on the microstructural evolution and mechanical properties of the 0.2 wt pct C-1.55 wt pct Mn-1.5 wt pct Si transformation-induced plasticity (TRIP) steel. The deformation schedule affected all constituents (polygonal ferrite, bainite in different morphologies, retained austenite, and martensite) of the multiphased TRIP steel microstructure. The complex relationships between the volume fraction of the retained austenite, the morphology and distribution of all phases present in the microstructure, and the mechanical properties of TRIP steel were revealed. The bainite morphology had a more pronounced effect on the mechanical behavior than the refinement of the microstructure. The improvement of the mechanical properties of TRIP steel was achieved by variation of the volume fraction of the retained austenite rather than the overall refinement of the microstructure.     

Nb含量对TRIP钢组织和性能的影响

利用Gleeble-1500热模拟机对不同Nb含量的TRIP钢的热轧过程进行了热模拟实验.研究了Nb含量对TRIP钢组织和性能的影响.实验结果表明：Nb的加入使得实验钢的组织得到细化,贝氏体量增多;增加Nb含量,残余奥氏体量和残余奥氏体含碳量有所降低,宏观维氏硬度值增加;Nb的质量分数为0.014%时,残余奥氏体量为19.2%、残余奥氏体含碳量为1.422%、宏观维氏硬度为258HV和抗拉强度为851 MPa.     

热轧相变诱发塑性钢的热模拟实验

 利用Gleeble 1500热应力 应变模拟机研究了铌含量、热变形参数(终轧温度和卷取温度)对相变诱发塑性(TRIP)钢组织和性能的影响。实验结果表明：不含铌实验钢的残余奥氏体量、残余奥氏体相中的碳含量、宏观维氏硬度和抗拉强度与常规低碳硅锰系TRIP钢的水平相当；增加铌含量，残余奥氏体量和残余奥氏体相中的碳含量有所下降，而宏观维氏硬度和抗拉强度提高；铌含量为0014%、终轧温度为780 ℃、卷取温度为400 ℃时，残余奥氏体量、残余奥氏体相中的碳含量与宏观维氏硬度和抗拉强度具有最佳组合。     

退火温度对超细晶中锰TRIP钢组织性能的影响

为研究连续退火工艺生产中锰TRIP钢汽车板的可行性,在钢板连续退火模拟机CCT-AY-域上研究了590~710℃不同退火温度下保温3 min对低碳中锰钢组织性能的影响.利用扫描电镜、透射电镜、电子背散射衍射和X射线能谱分析等微观分析方法对实验钢进行了组织结构和成分表征,利用X射线衍射法测量了残余奥氏体量,通过拉伸试验机测试了钢的单轴拉伸性能.结果表明:保温3 min时,随着保温温度的升高,残奥含量先增加后减少.在650℃退火时断后伸长率（21.3%）和强塑积（28 GPa·%）获得最大值,抗拉强度达到1330 MPa.马氏体基体通过回复,而残余奥氏体通过孪晶,获得超细晶组织.亚稳奥氏体的TRIP效应和超细晶铁素体（马氏体）共同提供了实验钢高的塑性.实验钢真实应力-应变曲线上呈现锯齿状现象,且稳定阶段加工硬化指数远高于传统TRIP钢.      

Sheet metal forming behaviour and mechanical properties of TRIP steels

Recently various kinds of high-strength sheet steels have been developed to meet the requirements of the automotive industry such as passive safety, weight reduction and saving energy. Usually the main problem of high-strength steels is their inferior ductility. Multiphase steels however show a very good combination of strength and formability so that the applicable region of high-strength steels has been widely enlarged. Multiphase steels have been developed for various purposes because of their ability to tailor properties by adjusting the type, the amount, and the distribution of different phases. Especially new developed triple-phase steels which make use of the TRIP effect (transformation induced plasticity) can further improve formability as well as strength due to the transformation of retained austenite to martensite during the deformation. In this work the transformation behaviour and the mechanical properties of low alloyed TRIP steels were investigated. The influence of the annealing parameters on transformation behaviour and on the amount of retained austenite were determined. In addition the temperature dependence of the mechanical properties and the effect of testing speed on the formability were studied. The investigation was carried out on seven different TRIP steels with different chemical compositions, especially the influence of the microalloying element niobium was considered. For reasons of comparison various mild and high-strength steels were tested parallel to the TRIP steels. It was found that the investigated TRIP steels offer very attractive combinations of elongation and strength values. An interesting temperature dependence of the mechanical properties can be observed, in such a way that the elongation values of the TRIP steels possess a maximum between +50 and +100°C. Due to its effect on grain size and on precipitation behaviour the addition of niobium leads to higher strength values without a strong decrease in ductility. In general, the mechanical properties are strongly affected by the type and the distribution of the different phases. The most important parameters, however, to influence the mechanical behaviour are the amount and the stability of the retained austenite, which are mainly controlled by the heat treatment and the chemical composition.     

热处理对TRIP 590钢微观组织及力学性能的影响

采用Gleeble3800热模拟机对TRIP钢拉伸试样进行不同工艺条件的快速热处理模拟实验,并采用金相分析、显微硬度测试等方法对试样进行组织观察和性能测试,目的是通过适宜的热处理工艺促使材料微观组织中出现适量的残余奥氏体组织,增强该材料在变形过程的相变诱导塑性(TRIP)效应,强化材料结果表明:在两相区内,TRIP钢中的残余奥氏体含量随着退火温度和退火时间的增加而增大,以25℃/s缓慢加热到700℃,再以150℃/s的速率快速加热到820℃保温120s后淬火处理,处理后的试样,残余奥氏体体积分数达到13％,显微硬度最高,达到262HV.     

Effects of heat treatment and alloying elements on the microstructures and mechanical properties of 0.15 wt pct C transformation-induced plasticity-aided cold-rolled steel sheets

The main emphasis of this study has been placed on understanding the effects of manganese and silicon additions and of heat-treatment (intercritical annealing and isothermal treatment) conditions on the microstructures and mechanical properties of 0.15 wt pct C transformation-induced plasticity (TRIP)-aided cold-rolled steel sheets. The steel sheets were intercritically annealed and isothermally treated at the bainitic region. Microstructural observation and tensile tests were conducted, and volume fractions of retained austenite were measured. Steels having a high manganese content had higher retained austenite fractions than the steels having a low manganese content, but showed characteristics of a dual-phase steel such as continuous yielding behavior, high tensile strength over 1000 MPa, and a low elongation of about 20 pct. The retained austenite fractions and mechanical properties varied with the heat-treatment conditions. In particular, the retained austenite fractions increased with decreasing intercritical annealing and isothermal treatment temperatures, thereby resulting in the improvement of the elongation and strength-ductility balance without a serious decrease in the yield or tensile strength. These findings suggested that the intercritical annealing and isothermal treatment conditions should be established in consideration of the stability of austenite and the solubility of alloying elements in the austenite formed during the intercritical annealing.     

Microstructure and Mechanical Properties of a Low-Carbon Mn-Si Multiphase Steel Based on Dynamic Transformation of Undercooled Austenite

The microstructure evolution of 0.20C-2.00Mn-2.00Si steel treated by the thermomechanical process to manufacture hot-rolled, transformation-induced plasticity (TRIP) steel based on dynamic transformation of undercooled austenite was investigated using a Gleeble 1500 (Dynamic Systems, Inc., Poestenkill, NY) hot simulation test machine in combination with light microscope (LM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The mechanical properties of this steel with different multiphase microstructures were also analyzed using room-temperature tensile tests. The results indicated that the multi-phase microstructures consisting of fine-grained ferrite with a size of 1–3 μm, bainite packets, and retained austenite and martensite were formed for the used steel by a thermo-mechanical process involving dynamic transformation of undercooled austenite, controlled cooling, isothermal bainite treatment and water-quenching. With the increase in the strain of hot deformation of undercooled austenite, the fraction of ferrite increased, that of bainite decreased, and that of martensite increased. At the same time, the fraction of retained austenite (RA), as well as the carbon content of RA, first increased and then decreased. For the used steel treated by such process, the tensile strength is about 1200 MPa with a total elongation of about 20 pct, and the product of tensile strength and total elongation can be up to 25,000 MPa × pct.     

Heredity Characteristic From Hot Rolled Microstructure to Annealed Microstructure in High Strength TRIP Steels

 The influence of hot-rolled process on microstructure in TRIP steel and the heredity characteristic from the hot-rolled microstructure to annealed microstructure are investigated. The results show there are two kind of hot-rolled microstructures under different coiling temperatures. One is composed of coarse grains of ferrite, pearlite and bainite, and the other is composed of small grains of ferrite, bainite and austenite. After annealing, the first kind of hot-rolled microstructure is greatly refined, and volume fraction, carbon content of austenite increase significantly. However, it have little changes in grain size, volume fraction and carbon content of austenite after the second kind of hot-rolled sheet is annealed. There are also differences in distribution of retained austenite between the two annealed microstructures observed by EBSD and TEM technology. Retained austenite in the first annealed microstructure distributes mainly inside of the polygonal ferrite in the form of spot, only little retained austenite on bainte ferrite boundary, however retained austenite in the second annealing microstructure allocates several places, such as inside of polygonal ferrite, on polygonal ferrite boundary, on bainte ferrite boundary.