Effect of retained austenite and nonmetallic inclusions on the thermal/electrical properties and resistance spot welding nuggets of Si-containing TRIP steels

Five advanced high-strength transformation-induced plasticity(TRIP) steels with different chemical compositions were studied to correlate the retained austenite and nonmetallic inclusion content with their physical properties and the characteristics of the resistance spot welding nuggets. Electrical and thermal properties and equilibrium phases of TRIP steels were predicted using the JMatPro? software. Retained austenite and nonmetallic inclusions were quantified by X-ray diffraction and saturation magnetization techniques. The nonmetallic inclusions were characterized by scanning electron microscopy. The results show that the contents of Si, C, Al, and Mn in TRIP steels increase both the retained austenite and the nonmetallic inclusion contents. We found that nonmetallic inclusions affect the thermal and electrical properties of the TRIP steels and that the differences between these properties tend to result in different cooling rates during the welding process. The results are discussed in terms of the electrical and thermal properties determined from the chemical composition and their impact on the resistance spot welding nuggets.

Thermal stability of retained austenite and mechanical properties of medium-Mn steel during tempering treatment

The thermal stability of retained austenite（RA）and the mechanical properties of the quenched and intercritical annealed 0.1C-5Mn steel with the starting ultrafine lamellar duplex structure of ferrite and retained austenite during tempering within the range from 200 to 500°C were studied by X-ray diffraction（XRD）,transmission electron microscopy（TEM）and tensile testing.The results showed that there was a slight decrease in the RA volume fraction with increasing tempering temperature up to 400°C.This caused a slight increase in the ultimate tensile strength（UTS）and a slight decrease in the total elongation（TE）;thus,the product of UTS to TE（UTS×TE）as high as 31GPa·% was obtained and remained nearly unchanged.However,aportion of the RA began to decompose when tempered at 500°C and thus caused a^35% decrease of the RA fraction and a^16%decrease of the value of UTS×TE.It is concluded that the ultrafine lamellar duplex structure is rather stable and the excellent combination of strength and ductility could be retained with tempering temperature up to 400°C.Thus,thermal processes such as galvanization are feasible for the tested steel provided that their temperatures are not higher than 400°C.

Stability of Retained Austenite Through a Combined Intercritical Annealing and Quenching and Partitioning(IAQP) Treatment

Intercritical annealing（IA） at various temperatures followed by quenching and partitioning（IAQP） treatments was conducted on a cold-rolled Fe-0.2C-1.42Si-l.87Mn（wt%） sheet steel.Optimized microstructure and enhanced mechanical properties were achieved through appropriate adjustment of IA temperatures.The steel which was annealed at1,033 K for 600 s,then quenched to 573 K and partitioned at 693 K for 20 min,designated as 1033 QP steel,exhibits maximum 16.3 vol% retained austenite（RA） with good mechanical properties（ultimate tensile strength 886 MPa and total elongation 27%）.It was found that the thermal and mechanical stabilities of RA are mainly influenced by the combined effect of its average carbon content and amount of adjacent martensite.Furthermore,the transformation-induced plasticity effect increased the peak n-values observed at the second stage of the work hardening of IAQP steels.

Effect of Microstructure on Tensile Behavior and Mechanical Stability of Retained Austenite in a Cold-Rolled Al-Containing TRIP Steel

In the present study, a quenching treatment prior to two-stage heat treatment was conducted on a Fe-0.28C-1.55Mn-2.06Al transformation-induced plasticity steel to tailor the final microstructure. Compared with the microstructure of the ferrite, bainite and blocky retained austenite obtained by conventional two-stage heat treatment, the microstructure subjected to quenching plus two-stage heat treatment was composed of the ferrite, lath bainite and film-like retained austenite. The corresponding tensile behavior and mechanical stability of retained austenite were investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The results show that the mechanical stability of blocky retained austenite grains is lower and most of them transform to martensite during the tensile deformation, which leads to higher ultimate tensile strength and instantaneous work hardening exponent. Film-like retained austenite has relatively higher stability, which could cause sustained work hardening and high ductility as well as product of strength and elongation.

低碳Si-Mn系TRIP钢的热处理工艺对组织的影响

低碳Si Mn系TRIP钢有着复杂的显微组织 ,主要由多边形铁素体 (F) +无碳贝氏体 (B) +残留奥氏体(AR)组成。本试验采用了彩色金相法 ,并结合X ray衍射、SEM和TEM等手段研究了低碳Si Mn系TRIP钢显微组织与工艺的关系 ,发现随着两相区退火温度的升高 ,最终显微组织中铁素体基体体积分数变小 ,并且贝氏体量增多 ,残留奥氏体的稳定性呈起伏式变化 ;在贝氏体转变区的等温温度过高或过低 ,均使最终显微组织中残留奥氏体体积分数减少 ;在贝氏体转变区等温时 ,所形成贝氏体表现出粒状的特征。

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 Msand 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 ℃.

Si－Mn系TRIP钢残余奥氏体与应变的关系

The equation which reflects the relationship between the retained austenite and strain has been proposed and clear TRIP can be obtained while the S value (An index of retained austenite stability) is less than 6.5 for Silicon-Manganese TRIP steel

相变诱发塑性的影响因素研究进展

对国外相变诱发塑性影响因素的研究进展做了概述 ,介绍了TRIP效应及其影响因素。重点概括了化学成分对残留奥氏体量和稳定性的影响 ,残留奥氏体量和相变转化率及残留奥氏体中碳含量对相变诱发塑性的影响。介绍了影响TRIP效应的其它因素 ,如变形条件、轧制工艺和热处理制度等。这些论述有助于相变诱发塑性的进一步研究和应用。

超级贝氏体组织中残余奥氏体的TRIP效应研究

针对超级贝氏体组织中残余奥氏体的应力诱发相变及对钢的力学性能影响,设计试验钢60Mn2Si Cr,经900℃完全奥氏体化保温30 min,在260℃盐浴炉中等温处理12 h后,将试样在疲劳试验机上进行加载试验;利用扫描电镜(Scanning electron microscope,SEM)、透射电镜(Transmission electron microscope,TEM)、X射线衍射仪(X-ray diffracmeter,XRD)和拉伸试验机等仪器设备对加载前后的样品,分别进行显微组织形貌观察和相组成的定性定量检测分析以及力学性能测试;结果显示,等温处理后样品的显微组织为超级贝氏体(贝氏体铁素体BF+残余奥氏体AR),残余奥氏体体积分数为9.4%,其含碳量为1.296%;经加载后试样显微组织中残余奥氏体体积分数下降至6.1%,含碳量达1.439%;钢的强塑积提高近20%。这说明等温处理获得超级贝氏体组织的钢,经施加载荷给予应力作用,显微组织中残余奥氏体发生转变,即超级贝氏体组织中的残余奥氏体能够通过相变诱发塑性(Transformation induced plasticity,TRIP)效应的产生,提高钢的力学性能。

低碳硅-锰系Q＆P钢的热处理工艺及实验室研究

在GLEEBLE3800热模拟机上模拟了低碳Si．Mn系Q＆P（Quenching and Partitioning）钢的控冷热处理工艺。结果表明，Q＆P钢具有高的抗拉强度（σb=1050MPa）和良好的塑性（δ=25％）。采用扫描和透射电镜进行了组织观察与分析，发现其组织主要由板条马氏体和8％～15％的残留奥氏体组成，残留奥氏体呈膜状。利用XRD技术对残留奥氏体进行了定量分析并用扫描电镜进行了断口观察。结果表明，拉伸试样纤维区为比较典型的韧窝状形貌，体现出良好的韧性断裂特征，残留奥氏体在组织中起到了相变诱发塑性的作用。

高强中锰TRIP钢的残余奥氏体含量及其稳定性

设计了一种中锰相变诱导塑性(TRIP)钢,利用全新的热处理工艺对其进行处理,研究了其残余奥氏体含量及其稳定性,并对该钢的显微组织和力学性能进行了分析。结果表明:中锰TRIP钢退火后的残余奥氏体体积分数均在39%以上,且奥氏体在变形过程中绝大部分转变为马氏体,提高了钢的塑性和强度;在630℃退火可使该钢的抗拉强度大于1 000 MPa,伸长率大于30%,强塑积大于30GPa.%,残余奥氏体体积分数为51.4%。

应力状态对TRIP钢残余奥氏体稳定性的影响

通过单向拉伸、平面应变和双向等拉实验研究了宏观应力状态对相变诱发塑性(TRIP)钢中残余奥氏体稳定性的影响.实验中残余奥氏体在不同应变量的体积分数通过X-射线衍射法测量,并引入应力三维度水平来表征不同应力状态.结果表明,不管何种变形模式,TRIP钢中残余奥氏体的体积含量都随塑性应变的增大而减少,而且应力三维度水平越高,TRIP钢的相变速率越快,残余奥氏体的力学稳定性越差.基于此,给出了能够表征不同应力状态的应变诱发马氏体相变动力学方程.

冷轧TRIP钢中残余奥氏体的热稳定性

为了研究冷轧TRIP钢中残余奥氏体的热稳定性,利用差示扫描量热法(Differential scanning calorimetry,DSC)分析0.25C-1.78Mn-1.23Al-0.54Cu-0.33Ni冷轧相变诱导塑性(Transformation induced plasticity,TRIP)钢中残余奥氏体的热分解,采用Kissinger方法测量残余奥氏体热的分解温度和激活能。发现TRIP钢中残余奥氏体热分解温度要高于普通淬火钢。利用Johnson-Mehl-Avrami(JMA)方程获得动力学参数n与A,预测TRIP钢贝氏体区等温过程中残余奥氏体转变动力学。结果表明,残余奥氏体在420℃等温时,显示出较高的稳定性,即使等温600 s也仍有90%的奥氏体不发生分解。X射线衍射结果还表明,420℃等温时,由JMA方程预测的残余奥氏体量与实际值吻合较好。

TRIP钢板在单向拉伸变形下的相变行为研究

TRIP钢的塑性取决于残余奥氏体的含量及其相变速度.研究TRIP钢中残余奥氏体在单向拉伸变形下的相变行为,首先回顾了TRIP钢板的研究进展,然后对一种低碳硅锰系TRIP700钢板在单向拉伸变形下的力学性能和相变性能进行了试验研究,并用扫描电镜方法分析了该TRIP钢在变形过程中微观组织的变化.结果表明,该低碳硅锰系TRIP钢板兼有高强度和高韧性的特点,在单向拉伸变形中残余奥氏体的含量与等效应变的关系可用来logfγ0-logfγ=kε.表示.

不同应变方式下TRIP钢中残余奥氏体的体积分数随应变量的变化

对TRIP钢板在单向拉伸、双向拉伸和平面应变3种应变方式下残余奥氏体的体积分数随应变量变化的规律进行了实验研究。结果表明:在变形过程中,残余奥氏体的体积分数随应变量的增加而减小;该变化量不仅与应变量有关,还与应变方式有关;平面应变时变化最大,双向拉伸次之,单向拉伸下变化最小。还对同一零件不同区域残余奥氏体的含量对零件形状精度的影响进行了讨论。

相变诱发塑性钢的研究进展

介绍国外以高温形变热处理、连续冷却形变热处理以及临界区退火加中温等温淬火等方法在低合金硅锰钢中获得相变塑性的原理和途径 ,并叙述了冷却过程中的计算机相变模拟原理。

镍含量对2200 MPa级超高强度钢力学性能的影响

通过比较含N i量(质量分数)分别为1.92%和4.92%的中碳Cr-N i-Mo系超高强度钢不同回火温度下的力学性能,得到了残留奥氏体分解及其对试验钢力学性能影响的规律。结果表明,200℃回火后两种试验钢的强度、伸长率和-40℃冲击吸收功分别>2200 MPa、>10%和>10 J。4.92%N i钢的屈服强度低,在拉伸过程中残留奥氏体应变诱导马氏体相变和相变诱发塑性(TR IP),伸长率有明显提高。随着回火温度的升高,残留奥氏体分解,4.92%N i钢的屈强比升高,应变硬化指数和伸长率均下降。

两相区退火温度对TRIP780钢组织和性能的影响

利用金相显微镜、X-射线衍射等方法,研究了0.16C-1.47Si-1.64Mn-0.02Nb-0.02Ti冷轧TRIP钢不同两相区退火温度对组织和力学性能的影响。结果表明,实验钢在840℃退火180s和420℃等温240s处理后,可以获得53.0%的铁素体、38.6%的贝氏体、8.4%的残余奥氏体,此时可以获得较佳的相变诱发塑性和好的强韧性配合,其强塑积可达22.4×103MPa.%,较低或较高的退火温度都将减少残余奥氏体含量。

TRIP1000钢的连续退火工艺及组织性能

对低碳硅锰钢进行了一系列的两相区退火和贝氏体区等温处理,通过拉伸试验测试热处理后的力学性能,采用X-Ray衍射分析方法检测了热处理后残余奥氏体含量的变化,并利用扫描电镜对热处理后的显微组织进行观察。结果表明,两相区退火温度为820℃,贝氏体区等温温度为420℃,等温240 s时,TRIP1000钢呈铁素体、贝氏体和残余奥氏体三相组织,晶粒大小为2～4μm,残余奥氏体及其碳含量较高,TRIP1000钢具有优异的力学性能,强塑积达到23088 MPa.%。

Nb含量对TRIP钢组织和性能的影响

利用G leeb le-1500热模拟机对不同Nb含量的TR IP钢的热轧过程进行了热模拟实验.研究了Nb含量对TR IP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.