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{"words":"$head_words:前驱体+$head_words:碳热还原+$head_words:\\(Ti+$head_words:W\\)\\(C+$head_words:N+$head_words:B\\)固溶体粉末+$head_words:碳源+$head_words:氮源","themeword":"$head_words","params":"$title:碳\\/氮源对碳热还原\\(Ti,W\\)\\(C,N,B\\)固溶体粉末合成的影响"}
&&&碳/氮源对碳热还原(Ti,W)(C,N,B)固溶体粉末合成的影响
碳/氮源对碳热还原(Ti,W)(C,N,B)固溶体粉末合成的影响
Effect of different carbon and nitrogen sources on synthesis of(Ti,W)(C,N,B) solid solution powders
采用"前驱体+碳热还原"的方法,利用XRD、SEM研究了不同碳源和氮源对合成(Ti,W)(C,N,B)固溶体粉末的物相组成和微观形貌的影响.结果表明:以木糖和氮气分别作为碳源和氮源,能在1200℃下合成相成分单一的纳米晶(Ti,W)(C,N,B)固溶体粉末,且操作工艺简单、易控;选用炭黑和大分子量的酚醛树脂作为碳源,尿素作为氮源时,碳热还原反应和固溶反应难以充分进行,反应产物存在大量钛氧化物的中间相(如Ti3O5、Ti2O3等)和单质W等杂质相.
摘要： 采用"前驱体+碳热还原"的方法,利用XRD、SEM研究了不同碳源和氮源对合成(Ti,W)(C,N,B)固溶体粉末的物相组成和微观形貌的影响.结果表明:以木糖和氮气分别作为碳源和氮源,能在1200℃下合成相成分单一的纳米晶(Ti,W)(C,N,B)固溶体粉末,且操作工艺简单、易控;选用炭黑和大分子量的酚醛树脂作为碳源,尿素作为氮源时,碳热还原反应和固溶反应难以...&&
Abstract：
(Ti,W)(C,N,B) powders were prepared from precursors by the carbothermal reduction method. Meanwhile,the effects of different carbon and nitrogen sources on phase composition and morphology of reaction products were investigated by XRD and SEM. The results show that nanocrystalline(Ti,W)(C,N,B)solid solu-tion powders with single-phase structure are prepared at 1200℃ when xylose and nitrogenare are used as carbon and nitrogen sources,respectively,and the operation process is simple and easy to control. When carbon black and phenolic resin with high molecular weight are used as carbon source,urea is used as nitrogen source,the car-bothermal reduction and solid solution process are easily hindered,which result in a large number of intermediate phases of titanium oxide(such as Ti3O5,Ti2O3,etc.)and impurity phase(such as W) in reaction products.
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3秒自动关闭窗口金属学报(中文版), ):
退火温度对冷轧7Mn钢拉伸行为的影响及模拟研究
Effects of Intercritical Annealing Temperature on the Tensile Behavior of Cold Rolled 7Mn Steel and the Constitutive Modeling
阳锋1, 罗海文2, 董瀚3,
1 钢铁研究总院 北京 1000812 北京科技大学冶金与生态工程学院 北京 1000833 上海大学材料科学与工程学院 上海 200072
YANG Feng1, LUO Haiwen2, DONG Han3,
1 Central Iron and Steel Research Institute, Beijing 100081, China2 School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China3 School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China
&Cite this article:
YANG Feng, LUO Haiwen, DONG Han. Effects of Intercritical Annealing Temperature on the Tensile Behavior of Cold Rolled 7Mn Steel and the Constitutive Modeling. Acta Metallurgica Sinica[J], ):
859-867 doi:10..315
国家自然科学基金项目No.U1460203;
中图分类号:
TG335.5&&&&
18)06-0859-09
Supported by National Natural Science Foundation of China (No.U1460203);
Received:&&&
利用EBSD、TEM和XRD等手段研究了退火温度对冷轧中锰钢7%Mn-0.3%C-2%Al (质量分数)组织和力学性能的影响,并借助具物理冶金意义的本构模型探讨了冷轧中锰钢退火后的拉伸和加工硬化行为。实验结果表明,随着退火温度的上升,逆转变奥氏体的机械稳定性逐渐降低,使得应变诱导马氏体的转变速率快速上升。在700 ℃退火时,逆转变奥氏体的稳定性适中,此时材料的综合力学性能最优。模拟结果表明,奥氏体稳定性对材料的拉伸行为有决定性的影响。退火温度偏低则奥氏体稳定性过高,材料的加工硬化率和均匀延伸率都较低;若退火温度适中则奥氏体稳定性也适中,变形时能持续地产生TRIP效应硬化基体,使材料的加工硬化率和均匀延伸率均较高;退火温度偏高会导致奥氏体稳定性过低,应变诱导马氏体会在短期内大量形成,致使材料的抗拉强度较高但均匀延伸率降低。
奥氏体稳定性
相变诱导塑性效应
Medium Mn steel is composed of sub-micron grained ferrite and austenite, the unstable austenite may transform to martensite during plastic straining. Although the mechanical properties of medium Mn steel could be easily tested by tensile test, it is quite difficult to directly measure the influences of different constituent phases on the tensile and work hardening behavior. Thus, at the present work, EBSD, TEM, XRD and a constitutive model based on dislocation density have been used to study the effects of intercritical annealing (IA) temperature on the tensile properties and work hardening behavior of a newly designed medium Mn steel, Fe-7%Mn-0.3%C-2%Al (mass fraction). Experimental results showed that with the increase of IA temperature, the mechanic stability of reverted austenite decreased gradually and the kinetics of strain induced martensite rose rapidly. The stability of the reverted austenite was moderate when intercritically annealed at 700 ℃, this led to the best plasticity and the optimal mechanical properties. Simulated results exhibited that the mechanic stability of austenite has a decisive influence on the tensile behavior of the material. The austenite stability will be too high if the IA temperature is lower, and this will lead to the lower work hardening rate an when the IA temperature is moderate, the stability of austenite will be optimum, consequently strain-induced martensite would be progressively produced during straining and result in the higher work hardening rate and prolonge the stability of austenite will be too lower if the IA temperature is higher, thus larger volume fraction of strain-induced martensite would be formed in a short period, and this would result in the higher tensile strength but the inferior uniform elongation.
Key words:
medium Mn steel
austenite stability
TRIP effect
随着汽车工业对节能、环保和安全性要求的不断提高,高强塑积的汽车结构用钢越来越成为人们关注的焦点。近二三十年来,汽车用钢得到了迅速发展,美国学者根据汽车用钢的强塑积(即抗拉强度与断后延伸率的乘积)把汽车用钢划分为三代[]。第一代汽车钢以无间隙原子钢、双相钢和低合金相变诱导塑性钢(phase transformation induced plasticity,TRIP)为代表,强塑积在10~20 GPa%,目前已难以满足汽车工业对轻量化和高安全性的双重要求。第二代汽车钢以具孪生诱导塑性(twinning induced plasticity,TWIP)的奥氏体钢和TWIP钢为代表,强塑积达50~70 GPa%,但其添加了大量的Cr、Ni、Mn等元素,其成本较高且冶炼生产存在一定困难。近年发展起来的第三代汽车用钢中Mn含量约为3.5%~12.0% (质量分数,下同),利用热轧或冷轧钢板在退火过程中发生的逆转变奥氏体来形成亚微米级的奥氏体和铁素体双相组织,奥氏体在应变过程中发生TRIP效应来提高钢的塑性和强度,其优异的综合力学性能不仅可以满足汽车轻量化和碰撞安全性的要求,还可以保证汽车零部件的成型性。
目前,中锰钢的研究已从初期的Fe-Mn-C系[,,,]发展到Fe-Mn-C-Al(-Si)系[,,,,]。合理的合金成分设计可以极大地提高中锰钢的综合力学性能。在文献[]中,作者讨论了退火温度对新型冷轧中锰钢7%Mn-0.3%C-2%Al (以下简称冷轧7Mn钢)组织和力学性能的影响。实验结果表明,随着退火温度的升高,逆转变奥氏体的晶粒尺寸逐渐增大且其中固溶的Mn和C含量逐渐降低,使得奥氏体的机械稳定性显著下降。拉伸实验表明,冷轧7Mn钢在700 ℃退火1 h后可获得机械稳定性适中的奥氏体,在变形时持续转变为马氏体从而使材料的力学性能达到最优。虽然实验结果显示冷轧7Mn钢的机械性能主要受奥氏体稳定性的影响,但目前仍不清楚铁素体、奥氏体和应变诱导马氏体在变形过程中各自的拉伸和加工硬化行为以及这3相是如何影响材料的力学性能的,因为这些数据难以通过实验手段来直接测量。孙朝阳等[]利用基于位错密度的本构模型成功模拟了Fe-22Mn-0.6C型TWIP钢在不同应变速率下的真应力-真应变曲线,并且分析了孪晶与滑移机制的相互作用及对宏观变形的影响。模拟数据表明,孪生速率与滑移速率间呈现负相关,即变形前期,孪生速率较大而滑移速率较小,孪生趋于饱和时,孪生速率降低而滑移速率快速上升。Lee等[,]也利用本构模型分析了Fe-10Mn-0.3C-3Al-2Si型中锰钢在变形过程中各组成相的流变应力和加工硬化行为,模拟计算的真应力-真应变曲线和加工硬化率曲线与实测结果吻合良好。其模拟结果还表明,Fe-10Mn-0.3C-3Al-2Si经冷轧并在800 ℃退火后,亚稳奥氏体晶粒中的TRIP效应的强化效果比TWIP效应的要更显著。孙朝阳等[]和Lee等[,]的工作表明,可以利用本构模型来深入分析TWIP钢或中锰钢在变形过程中的力学行为。为此,本工作将借助基于位错密度的本构模型来研究冷轧7Mn钢的拉伸和加工硬化行为,并结合实验结果来阐明不同温度退火后中锰钢力学行为差异的内在机制。
1 实验方法
7Mn钢由真空感应炉冶炼,其化学成分(质量分数,%)为：Mn 7,C 0.3,Al 2,Fe余量。铸锭在1200 ℃保温2 h后在1200 ℃至850 ℃间锻造,随后炉冷至室温。锻坯加热至1200 ℃后保温1 h,在1200 ℃至850 ℃间进行热轧,轧后水冷至室温,热轧板的最终厚度约为4 mm。热轧板在700 ℃保温30 min后进行冷轧,冷轧压下量为70%。随后将冷轧轧板在680、700和720 ℃分别保温1 h后空冷至室温。为方便描述,将不同温度退火的试样分别称为S680、S700和S720。
用于Quanta 650扫描电镜电子背散射衍射(EBSD)观测的试样经机械研磨后利用10%的高氯酸酒精溶液电解抛光,EDSD扫描步长为0.05 μm,利用HKL Channel5软件处理扫描数据。用于H-800透射电镜(TEM)观测的试样研磨至35 μm后用6%的高氯酸酒精溶液在-20 ℃双喷电解抛光,以备TEM观测精细组织。用平行段长度和宽度分别为50和10 mm的标准拉伸试样,在WDW-300E型拉伸机上进行室温拉伸实验,夹头移动速率为2 mm/min。X射线衍射(XRD)实验在SmartLab衍射仪上进行,样品的扫描区域为40°~100°,扫描速率为1°/min。将试样加载至预定变形量后卸载,以测量不同变形量下的奥氏体含量及位错密度;对于拉断的试样,在远离断口的平行段取样。利用下式计算材料中的奥氏体含量(Vγ)[]：
式中,Iγ为奥氏体的{200}、{220}、{311}衍射峰积分强度的平均值, Iα为铁素体{200}、{211}衍射峰积分强度的平均值。利用修正的Williamson-Hall方程[,]测量奥氏体的位错密度,测量方法见文献[]。利用Jade6.0软件对XRD数据进行寻峰处理并计算衍射峰的半高宽和积分强度。
2 实验结果
所示为不同温度退火试样的EBSD像,图中白色区域为铁素体,灰色区域为奥氏体,黑色实线表示大角晶界,灰色实线为小角晶界。可以看到冷轧退火后铁素体和奥氏体晶粒基本都呈等轴状,与热轧退火后的板条状形貌有很大不同[,]。此外,铁素体的晶界中大部分为小角晶界,即退火后铁素体仍然没有发生再结晶。Cao等[]认为这是由于铁素体晶粒中较高的Mn含量拖曳晶界、抑制了晶界的迁移。统计了退火后的逆转变奥氏体和铁素体的晶粒尺寸,如所示。可见,随退火温度的升高,逆转变奥氏体晶粒的平均尺寸增加,而铁素体的平均晶粒尺寸略有减小。文献[]中测量了冷轧7Mn钢退火后奥氏体的化学成分,数据表明奥氏体晶粒内固溶的Mn和C含量随退火温度的升高而降低。由于奥氏体的晶粒尺寸和成分决定了其机械稳定性,故随退火温度的升高奥氏体的稳定性是逐渐降低的。为S680~S720的TEM像。可以看到虽然没有发生再结晶,铁素体晶粒内部的位错密度仍然很低,Han等[]称这种现象为大回复。
图1-6-859/img_1.png图1
S680、S700和S720的EBSD像 Fig.1
EBSD phase maps of S680 (a), S700 (b) and S720 (c) (The white phase is ferrite and the gray phase is austenite, the black lines are high-angle grain boundaries with misorientation angles of over 15°, the gray lines are low-angle grain boundaries with misorientation angles of 2°~15°)
Fig.1-6-859/img_1.png图1
S680、S700和S720的EBSD像 Fig.1
EBSD phase maps of S680 (a), S700 (b) and S720 (c) (The white phase is ferrite and the gray phase is austenite, the black lines are high-angle grain boundaries with misorientation angles of over 15°, the gray lines are low-angle grain boundaries with misorientation angles of 2°~15°)
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S680、S700和S720的EBSD像
EBSD phase maps of S680 (a), S700 (b) and S720 (c) (The white phase is ferrite and the gray phase is austenite, the black lines are high-angle grain boundaries with misorientation angles of over 15°, the gray lines are low-angle grain boundaries with misorientation angles of 2°~15°)
图2-6-859/img_2.png图2
S680、S700和 S720的TEM像 Fig.2
TEM images of S680 (a), S700 (b) and S720 (c) (γ denotes austenite grains and the rest are ferrite grains)
Fig.2-6-859/img_2.png图2
S680、S700和 S720的TEM像 Fig.2
TEM images of S680 (a), S700 (b) and S720 (c) (γ denotes austenite grains and the rest are ferrite grains)
在新窗口打开
S680、S700和 S720的TEM像
TEM images of S680 (a), S700 (b) and S720 (c) (γ denotes austenite grains and the rest are ferrite grains)
铁素体和奥氏体晶粒尺寸及应变诱导马氏体动力学的相关参数
The average grain sizes of ferrite and austenite and parameters for calculating martensite volume fraction (VM)
所示为S680~S720在不同变形量下的马氏体含量[]。可以看到马氏体的转变动力学与奥氏体的稳定性有关,奥氏体稳定性越低则马氏体的形成速率越快。此外,马氏体含量随应变量呈S形上升,故利用Olson-Cohen模型[]对马氏体的转变动力学进行模拟:
[ 1 - exp ( - β
式中,VM和Vγ0分别为马氏体体积分数和奥氏体初始含量;ε为真应变;α、β和n为与马氏体转变动力学相关的参数[],取值列于。所示为S680~S720在室温下的真应力-真应变曲线。可以看到,随着退火温度的升高,冷轧7Mn钢的屈服强度逐渐降低,而抗拉强度则单调升高。材料的延伸率随退火温度先升高后降低,在700 ℃退火时达到最高,对应的工程应变约为68%;此时材料的强塑积也到达最优,约为65 GPa%[],远高于Fe-Mn-C系的中Mn钢,甚至不输于一般的TWIP钢。文献[]对Fe-Mn-C系和Fe-Mn-C-Al系中锰钢进行了比较,认为Al元素的加入增加了逆转变奥氏体的稳定性,使其在变形过程中缓慢而持续地转变成马氏体,从而提高了材料的延伸率。从中还可以看到,S680的拉伸曲线在均匀变形区很光滑,没有出现应力锯齿;而S700和S720在塑性变形时出现了不同类型的应力锯齿。这表明冷轧7Mn钢经700和720 ℃退火1 h后在拉伸过程中发生了动态应变时效。关于退火温度对冷轧中锰钢动态应变时效的影响在文献[]中有详细讨论,其认为奥氏体晶粒尺寸和层错能决定了拉伸时应力锯齿是否产生,而奥氏体稳定性决定了应力锯齿的类型。
图3-6-859/img_3.png图3
不同温度退火后马氏体体积分数随应变量的变化[16]及拟合曲线 Fig.3
VM of S680~S720 after deformed to various strains[16] and the corresponding fitted curves
Fig.3-6-859/img_3.png图3
不同温度退火后马氏体体积分数随应变量的变化[16]及拟合曲线 Fig.3
VM of S680~S720 after deformed to various strains[16] and the corresponding fitted curves
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不同温度退火后马氏体体积分数随应变量的变化[]及拟合曲线
VM of S680~S720 after deformed to various strains[] and the corresponding fitted curves
图4-6-859/img_4.png图4
冷轧7Mn钢经不同温度退火后的真应力-真应变曲线 Fig.4
True stress-true strain curves of cold-rolled 7Mn steel after annealed at different temperatures
Fig.4-6-859/img_4.png图4
冷轧7Mn钢经不同温度退火后的真应力-真应变曲线 Fig.4
True stress-true strain curves of cold-rolled 7Mn steel after annealed at different temperatures
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冷轧7Mn钢经不同温度退火后的真应力-真应变曲线
True stress-true strain curves of cold-rolled 7Mn steel after annealed at different temperatures
所示为S700在不同变形量下的TEM像。当真应变量为0.095时,奥氏体晶粒内部有大量的层错,如a所示,选区电子衍射(SAED)表明,此时没有马氏体产生。当应变量增加至0.35后,奥氏体晶粒内部可以观察到大量的应变诱导马氏体,如b所示,奥氏体的暗场像如c所示。可以看到应变诱导马氏体侧向扩展逐渐蚕食母相奥氏体,SAED谱表明此时有孪晶马氏体的产生,这与文献[]的观测结果是一致的。
图5-6-859/img_5.png图5
S700在真应变为0.095和0.35时的TEM像 Fig.5
TEM images of deformed microstructures in S700 after the strain of 0.095 (a) and 0.35 (b) and the dark-field image of austenite in Fig.5b (c) (Insets show the SAED patterns of the circles in Figs.5a and b. SF—stacking fault, γ—austenite, M—strain induced martensite)
Fig.5-6-859/img_5.png图5
S700在真应变为0.095和0.35时的TEM像 Fig.5
TEM images of deformed microstructures in S700 after the strain of 0.095 (a) and 0.35 (b) and the dark-field image of austenite in Fig.5b (c) (Insets show the SAED patterns of the circles in Figs.5a and b. SF—stacking fault, γ—austenite, M—strain induced martensite)
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S700在真应变为0.095和0.35时的TEM像
TEM images of deformed microstructures in S700 after the strain of 0.095 (a) and 0.35 (b) and the dark-field image of austenite in Fig.5b (c) (Insets show the SAED patterns of the circles in Figs.5a and b. SF—stacking fault, γ—austenite, M—strain induced martensite)
3 本构模型
上一节中讨论了退火温度对冷轧7Mn钢组织和性能的影响,本节将介绍一种基于位错密度的本构模型,用以模拟计算冷轧7Mn钢的拉伸和加工硬化行为。由于冷轧7Mn钢退火后由铁素体和亚稳奥氏体2相组成,且亚稳奥氏体在变形过程中会转变成马氏体,故冷轧7Mn钢的流变应力与铁素体、奥氏体和应变诱导马氏体这3相的流变应力和体积分数有关。在变形过程中铁素体的体积分数保持不变,马氏体的体积分数可由式(2)描述,而奥氏体含量Vγ=Vγ0-VM。故冷轧7Mn钢的流变应力可由混合定律表示[,]：
式中,σt、σf、σγ和σM分别为总的应力、铁素体的应力、奥氏体的应力和马氏体的应力,Vf、Vγ和VM分别为铁素体、奥氏体和马氏体的体积分数。
材料的微应变dεt也可用混合定律表示,即：
式中,dεf、dεγ和dεM分别为铁素体、奥氏体和马氏体的微应变。由于铁素体、奥氏体和马氏体的应力不同,故在变形过程中3相的应变分配也会不同。Bouaziz等[]认为多相材料在变形时应力和应变的分配符合等功原则,即变形时外加载荷所做的功平均分配于各相中,可由下式表示：
联立式(4)和(5)便可求得各相微应变的表达式。
各相的流变应力由下式计算：
式中,下标i代表铁素体、奥氏体或马氏体,
分别为固溶强化项、晶界强化项和位错强化项。值得注意的是,Jian等[]在研究低层错能的镁合金时,发现该材料在变形过程中会发生位错与层错的交互作用,从而导致材料强度的提高。严格意义上讲,计算亚稳奥氏体流变应力的时候还应该考虑层错的影响。但由于层错对基体强度的影响还没有得到广泛研究且缺乏成熟的物理模型,所以本工作中暂不考虑层错对流变应力的贡献。由于在拟合马氏体的强度时通常只考虑固溶元素和位错密度的影响,而不考虑马氏体尺寸的作用,故本工作中也不考虑马氏体的晶界强化。铁素体、奥氏体和马氏体的固溶强化可由下列3式分别表示[,]：
分别为铁素体中C、Mn和Al的质量分数,
为奥氏体中C和Mn的质量分数,
为马氏体中固溶的C含量。文献[]的研究表明,冷轧7Mn钢经680~720 ℃退火1 h后铁素体和奥氏体晶粒内的Mn和Al含量与ThermalCalc软件计算的平衡含量相近,故模拟计算时使用平衡的C、Mn和Al含量计算铁素体、奥氏体和马氏体的固溶强化。注意马氏体的C含量与奥氏体是相等的。
铁素体的晶界强化可由下式表示：
式中,Kα为铁素体的Hall-Petch系数,dα为铁素体的晶粒尺寸。由于奥氏体在变形时有板条状的应变诱导马氏体产生(如c所示),它们会分割奥氏体晶粒,致使奥氏体在变形时逐渐细化,从而促进了所谓的动态Hall-Petch效应。故奥氏体的晶界强化可由式(11)表达：
式中,Kγ为奥氏体的Hall-Petch系数,L为奥氏体的有效界面间距,可由下式表示[]：
1 / L = 1 /
式中,dγ为奥氏体的晶粒尺寸,λM为马氏体板条的平均间距,iM为调节系数。λM可由下式计算[]：
式中,cM为马氏体板条的平均厚度,取为0.2 μm。
铁素体、奥氏体和马氏体的位错强化如下：
式中,αi为常数,Mi、Gi和bi分别为各相的Taylor因子、切变模量和Burgers矢量模,取值如所示;ρi为各相的位错密度,是应变量的函数。铁素体和奥氏体的初始位错密度ρ0与热处理温度有关,具体数值见。应变诱导马氏体的初始位错密度ρ0定为1×1015 m-2。ρi随应变量的变化规律可由Kocks-Mecking模型表示[,]：
式(15)中右边括号里的第1项和第2项分别表示由晶界和林位错引起的位错密度的增殖,第3项表示由动态回复引起的位错密度的降低。Λi为各相的有效界面间距,Λα和ΛM分别等于dα和cM,Λγ可由式(12)表示(Λγ=L),此时iM=1。
分别表示各相位错的增殖系数和动态回复系数。值得注意的是铁素体和奥氏体中
的取值,由于发生马氏体相变时会产生体积膨胀,形成的形变诱导马氏体会同时挤压周围的奥氏体和铁素体,故
的取值应该与马氏体的体积分数有关,具体取值见。Pi为与晶粒尺寸有关的系数,Bouaziz等[]定义其为位错避免被晶界吸收的几率,其表达式如下：
为临界晶粒尺寸。位错密度的增殖与晶粒尺寸有很大关系[],当实际晶粒尺寸小于临界尺寸时位错湮灭的速率会增加,这样晶粒尺寸越小其位错越不容易增殖,从而加工硬化率也就越低。
室温下冷轧7Mn钢中各相的材料参数及拟合系数
Parameters used in the constitutive model calculations for cold rolled 7Mn steel at room temperature
4 模拟结果与讨论
所示为模拟计算与实测的奥氏体位错密度。由于变形量增大后拉伸试样的部分奥氏体衍射峰消失,故S700和S720只测量真应变小于0.22时的位错密度。从中可以看到,模拟计算的奥氏体位错密度与实测值在数值和变化趋势上基本一致。a和b分别为模拟计算与实测的真应力曲线和加工硬化率曲线。从a中可以看到,对于S680和S700而言,模拟与实测的真应力-真应变曲线吻合很好,但模拟的S720的真应力在真应变大于0.2后与实测值有一定偏差,这可能与两方面的因素有关：一是模型本身的精度,二是实验过程中难以精确地测量马氏体的转变动力学。由于马氏体的体积分数不是原位测量的,对于不同的拉伸试样,化学成分的宏观偏差以及热处理时微小的温度差异都可能导致马氏体的转变动力学有一定差异,从而对模拟与实测结果造成影响。从b中还可以看到,由于模型中没有考虑静态应变时效(Lüders应变)和动态应变时效(应力锯齿)的影响,所以模拟值与实测的加工硬化率也有一定的偏差。但总体而言模拟的真应力和加工硬化率的数值及随真应变的变化趋势与实测数据吻合较好。综合和7来看,可以认为本工作所采用的本构模型能较好地描述冷轧7Mn钢在拉伸过程中的组织与力学性能的演变。
图6-6-859/img_6.png图6
奥氏体位错密度的实测值[16]与模拟值 Fig.6
Measured[16] and calculated dislocation densities (ρ) of austenite
Fig.6-6-859/img_6.png图6
奥氏体位错密度的实测值[16]与模拟值 Fig.6
Measured[16] and calculated dislocation densities (ρ) of austenite
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奥氏体位错密度的实测值[]与模拟值
Measured[] and calculated dislocation densities (ρ) of austenite
图7-6-859/img_7.png图7
实测与计算的真应力-真应变曲线及加工硬化率(WHR)曲线 Fig.7
Measured and calculated true stress-true strain curves (a) and corresponding curves of work hardening rate (WHR) (b)
Fig.7-6-859/img_7.png图7
实测与计算的真应力-真应变曲线及加工硬化率(WHR)曲线 Fig.7
Measured and calculated true stress-true strain curves (a) and corresponding curves of work hardening rate (WHR) (b)
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实测与计算的真应力-真应变曲线及加工硬化率(WHR)曲线
Measured and calculated true stress-true strain curves (a) and corresponding curves of work hardening rate (WHR) (b)
由式(3)可知,冷轧7Mn钢的流变应力由铁素体、奥氏体和马氏体共同提供。对式(3)的左右两侧求微分,可得
/ d ε = d (
) / d ε + d (
) / d ε + d (
,即冷轧7Mn钢的总加工硬化率也由铁素体、奥氏体和马氏体共同提供。下面将利用本文介绍的本构模型来分析不同温度退火后的冷轧7Mn钢在拉伸过程中各组成相的流变应力和加工硬化率的变化。模拟计算的总流变应力和各组成相的流变应力如a、c和e所示,对应的总加工硬化率及各相的加工硬化率如b、d和f所示。
图8-6-859/img_8.png图8
S680~S720总的及各组成相的真应力-真应变曲线和加工硬化率曲线的计算值 Fig.8
The calculated true stress-true strain curves (a, c, e) and curves of work hardening rate of the composite and each constituent phase (b, d, f) in S680 (a, b), S700 (c, d) and S720 (e, f)
Fig.8-6-859/img_8.png图8
S680~S720总的及各组成相的真应力-真应变曲线和加工硬化率曲线的计算值 Fig.8
The calculated true stress-true strain curves (a, c, e) and curves of work hardening rate of the composite and each constituent phase (b, d, f) in S680 (a, b), S700 (c, d) and S720 (e, f)
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S680~S720总的及各组成相的真应力-真应变曲线和加工硬化率曲线的计算值
The calculated true stress-true strain curves (a, c, e) and curves of work hardening rate of the composite and each constituent phase (b, d, f) in S680 (a, b), S700 (c, d) and S720 (e, f)
由a可知,经680 ℃退火后,冷轧7Mn钢的屈服强度由铁素体和奥氏体共同提供。铁素体提供的屈服强度约为510 MPa,奥氏体提供的约为370 MPa。考虑到变形初期奥氏体的体积分数只有约30%,可以认为奥氏体的屈服强度比铁素体的要大,这与文献[]的描述是一致的。随着变形量的增加,铁素体的流变应力略有上升,而奥氏体的流变应力增加得稍明显。研究[,]表明,在单相铁素体材料中,晶粒越细则材料的加工硬化能力越低,故S680中铁素体的加工硬化率偏低主要是其晶粒尺寸偏小的缘故。S680中奥氏体的晶粒尺寸也细小(如所示),但其加工硬化率较铁素体而言偏高,这说明铁素体和亚稳奥氏体对应变的响应是不一样的。在变形的中后期,S680中有少量的马氏体形成,也能提供一部分流变应力。S680拉伸时总的及各组成相的加工硬化率如b所示。在应变量小于0.1时,材料的加工硬化率大部分由奥氏体提供,小部分由铁素体提供;应变量大于0.1后,形变诱导马氏体所提供的加工硬化逐渐上升,在真应变约0.17时达到峰值,此后缓慢下降。在铁素体、奥氏体和马氏体的共同作用下,S680总加工硬化率由变形初期的约1700 MPa缓慢下降,当真应变介于0.10~0.17时基本保持不变,随后持续降低,直到其与真应力相等时S680开始颈缩。
S700总的及各组成相的流变应力如c所示。对比c和a,可以看到虽然S700的屈服强度比S680低,但S700真应力曲线的斜率明显高于后者,即S700的加工硬化率比S680的要高。S700的屈服强度偏低是因为其铁素体和奥氏体晶粒尺寸都比S680的要大,如所示。S700加工硬化率较高的原因主要有两个,一是因为在变形前期奥氏体的加工硬化率较高;二是在变形中后期奥氏体产生明显的TRIP效应,导致总的加工硬化率保持在较高水平,如d所示。由于S700中的奥氏体晶粒尺寸较大,故在变形初期位错的增殖速率较高,导致其加工硬化率较S680中的奥氏体偏高。此外,由于S700中奥氏体的稳定性降低[],在变形时会不断转变为马氏体从而持续产生TRIP效应。由于体积分数不断减少,奥氏体提供的流变应力在真应变大于0.2后逐渐下降,其加工硬化率在真应变约为0.2时降为负值;同时,新形成的马氏体的流变应力和加工硬化率则显著增加,弥补了奥氏体体积分数减少造成的影响。在3相的共同作用下,S700的加工硬化率由变形初期的约2000 MPa下降至真应变为0.13时的1350 MPa,此后由于马氏体的快速形成,加工硬化率重新增加至真应变为0.28时的1600 MPa,随后加工硬化率再次下降,直至与真应力相交。可以看到由于奥氏体产生了明显的TRIP效应,使得S700的加工硬化率与真应力相交时的应变量大大提高,即显著增加了材料的均匀延伸率。
当退火温度增加至720 ℃后,逆转变奥氏体的尺寸变得更大,且其中固溶的Mn和C含量比S680和S700中的都低,故其稳定性进一步降低。若用
Δ V / Δ ε
表示马氏体的转变速率(
分别代表马氏体含量和真应变的变化量),根据中的数据可知S700中马氏体的平均转变率为0.57,而S720则高达0.91,这表明在S720中由于奥氏体稳定性的降低使得马氏体的体积分数快速上升,导致材料的流变应力也快速增加,最终使得S720的流变应力在真应变0.1~0.2之间呈现明显的S形特征,如e所示。S720的加工硬化率随之呈现明显的3段式特征[],即随变形量的增加加工硬化率首先下降,然后上升最后又下降,如f中的I、II、III所示。值得指出的是,S700的加工硬化率曲线也呈现3段式特征,但由于马氏体的转变速率偏慢,使得第II段的最高值和最低值差别不大,导致真应力曲线的S型特征并不明显。
(1) 随着退火温度的上升,冷轧7Mn钢中逆转变奥氏体的机械稳定性逐渐降低,使得应变诱导马氏体的转变速率快速上升。在700 ℃退火后,逆转变奥氏体的稳定性适中,此时材料的综合力学性能最优。
(2) 利用基于位错密度的本构模型研究了冷轧7Mn钢的拉伸和加工硬化行为,模拟计算值与实测结果总体上吻合良好,因此可以利用该本构模型来分析冷轧7Mn钢经不同温度退火后力学行为差异的原因。
(3) 奥氏体稳定性对冷轧7Mn钢的真应力和加工硬化率曲线有决定性的影响。680 ℃退火后,逆转变奥氏体的稳定性偏高,变形时不能产生明显的TRIP效应,材料的加工硬化率和均匀延伸率都较低;700 ℃退火后,奥氏体的稳定性适中,在变形过程中持续产生TRIP效应,使材料的加工硬化率在较大的应变范围内维持在较高的水平,最终材料的综合力学性能达到最优;退火温度增加至720 ℃后,逆转变奥氏体的稳定性过低,在较小的应变范围内几乎全部转变为马氏体,导致材料的抗拉强度较高但均匀延伸率偏低。
The authors have declared that no competing interests exist.
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Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 102mass02pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 302mass02pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 302mass02pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 102GPa with a total elongation over 2502pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.
[本文引用: 1]
Lee S, De Cooman
B C. Tensile behavior of intercritically annealed 10 pct Mn multi-phase steel[J]. Metall. Mater. Trans., 2014, 45A: 709
The exceptional elongation obtained during tensile testing of intercritically annealed 1002pct Mn steel, with a two phase ferrite–austenite microstructure at room temperature, was investigated. The...
[本文引用: 5]
Lee S, De Cooman
B C.Effect of the intercritical annealing temperature on the mechanical properties of 10 Pct Mn multi-phase steel[J]. Metall. Mater. Trans., 2014, 45A: 5009
Intercritically annealed 10 pct Mn steel has been shown to exhibit an excellent combination of strength and ductility due to the plasticity-enhancing mechanisms of mechanical twinning and strain-induced martensite transformation occurring in sequence. This mechanical behavior is only achieved for a multi-phase microstructure obtained after annealing within a specific intercritical temperature range. A model for the selection of the optimal intercritical annealing temperature was developed to achieve a high strength-ductility balance for 10 pct Mn multi-phase steel. The model considers the room temperature stacking fault energy and the thermodynamic stability of the retained austenite.
[本文引用: 4]
Cai Z H, Ding H, Misra R
D K, et al. Austenite stability and deformation behavior in a cold-rolled transformation-induced plasticity steel with medium manganese content[J]. Acta Mater., 2015, 84: 229
We elucidate here the impact of grain size and manganese concentration on the austenite stability and the deformation behavior of a cold-rolled transformation-induced plasticity (TRIP) steel with a nominal chemical composition of Fe–11Mn–4Al–0.2C (wt.%). Intercritical hardening at 770°C led to a ferrite–austenite mixed microstructure, which was characterized by an excellent combination of ultimate tensile strength of 1007MPa and total elongation of 65% and a three-stage work-hardening behavior. The grain size was a critical factor in governing the stability of austenite and the optimal grain size for maximum stability was observed to be 650.6μm. The superior mechanical properties are attributed to the discontinuous TRIP effect and the cooperative deformation of ferrite, where the discontinuous effect is a consequence of the non-uniform distribution of manganese, which is responsible for introducing varying degrees of stability in the austenite phase.
[本文引用: 1]
Park S J, Hwang B, Lee K H, et al.Microstructure and tensile behavior of duplex low density steel containing 5 mass% aluminum[J]. Scr. Mater., 2013, 68: 365
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Yang F, Luo H W, Hu C D, et al.Effects of intercritical annealing process on microstructures and tensile properties of cold-rolled 7Mn steel[J]. Mater. Sci. Eng., 2017, A685: 115
Influences of intercritical annealing temperature and duration on the microstructures and tensile properties of a newly designed medium Mn steel, Fe-7 wt.% Mn-0.3 wt.% C-2 wt.% Al, have been studied and discussed in this paper. Two types of cold-rolled ferritic microstructures, i.e. cell-like and lath-like, could transform to granular and lamellar austenite grains respectively during the subsequent intercritical annealing (IA). Both the IA temperature and duration strongly affect the fraction of retained austenite that have transformed during tensile deformation and the resultant tensile properties. In comparison with a Al-free 7Mn steel, it is found that the addition of Al has led to the reduced fraction but enhanced stability of RA thus, they may transform gradually during deformation, which can make the maximum contribution to the sustainable work hardening. The developed steel exhibits the product of strength and plasticity up to 66 GPa路%, which is much better than the Al-free 7Mn steel and almost one of the best tensile properties among the existing medium-Mn steels but with relatively lean alloying.
[本文引用: 6]
(孙朝阳, 黄杰, 郭宁等. 基于位错密度的Fe-22Mn-0.6C型TWIP钢物理本构模型研究[J]. 金属学报, 2014, 50: 1115)
Sun C Y, Huang J, Guo N, et al.A physical constitutive model for Fe-22Mn-0.6C TWIP steel based on dislocation density[J]. Acta Metall. Sin., 2014, 50: 1115
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Li Z, Wu D.Effects of hot deformation and subsequent austempering on the mechanical properties of Si-Mn TRIP steels[J]. ISIJ Int., 2006, 46: 121
Abstract In the present paper, effects of hot deformation and subsequent austempering on the mechanical properties of hot rolled Si-Mn TRIP steels were investigated. Thermomechanical controlled processing (TMCP) was conducted by using a laboratory hot rolling mill, in which three different kinds of finish rolling reduction, temperatures and austemperings with various isothermal holding duration were applied. The results have shown that polygonal ferrite, granular bainite and larger amount of stabilized retained austenite can be obtained by controlled rolling processes. Ultimate tensile strength, yield strength and total elongation increase with increasing the amount of deformation and decreasing finish rolling temperature due to the stabilization of retained austenite. Tensile strength and total elongation can reach the maximum values (791 MPa and 36%, respectively), and isothermal holding 20 min at 400 C after hot deformation has been proved to be the optimum treatment.
[本文引用: 1]
Ungár T, Borbély A.The effect of dislocation contrast on X-ray line broadening: A new approach to line profile analysis[J]. Appl. Phys. Lett., 1996, 69: 3173
The x‐
ay line profiles of an ultrafine grained copper crystal, produced by equal‐channel angular pressing, were measured by a special high resolution diffractometer with negligible instrumental line broadening. The analysis of the line breadths and the Fourier coefficients have shown that taking into account the contrast caused by dislocations on line profiles gives new scaling factors in the Williamson–Hall plot and in the Warren–Averbach analysis, respectively. When strain is caused by dislocations the new procedure proposed here enables a straightforward determination of particle size and strain, the latter in terms of the dislocation density. 08 1996 American Institute of Physics.
[本文引用: 1]
Ungár T, Dragomir I, Révész ?, et al.The contrast factors of dislocations in cubic crystals: The dislocation model of strain anisotropy in practice[J]. J. Appl. Cryst., 1999, 32: 992
It has been shown recently that in many cases strain anisotropy in powder diffraction can be well accounted for by the dislocation model of the mean square strain. The practical application assumes knowledge of the individual contrast factors C of dislocations related to particular Burgers, line and diffraction vectors or to the average contrast factors C04. A simple procedure for the experimental determination of C04 has been worked out, enabling the determination of the character of the dislocations in terms of a simple parameter q. The values of the individual C factors were determined numerically for a wide range of elastic constants for cubic crystals. The C04 factors and q parameters were parametrized by simple analytical functions, which can be used in a straightforward manner in numerical analyses, as e.g. in Rietveld structure refinement procedures.
[本文引用: 1]
Yang F, Luo H W, Zhang S L, et al.On the characteristics of Portevin-Le Ch?telier bands in cold-rolled 7Mn steel showing transformation-induced plasticity[J]. Int. J. Plast., 2018, 103: 188
Strain localization during tensile deformation of cold-rolled and annealed 7Mn steel were investigated under various strain rates and deformation temperatures. The retained austenite grain size, strain rate and deformation temperature all have remarkable influences on the appearance of PLC bands. Higher strain rate clearly suppressed the formation of PLC bands whilst deformation temperature had a more complicated influence. During the room-temperature deformation at the quasistatic strain rate of 6. 614 /s, Lüders bands appeared in all specimens whilst PLC bands only in the specimens that were annealed above 700°C. Digital image correlation (DIC) analysis showed the specimen annealed at 700°C exhibited the type A PLC bands during the entire p whilst the one annealed at 720°C showed the more complicated type A+B bands before 600s and the type A bands afterward. All of these phenomena have been discussed in relation to the interaction of C atoms/C-Mn pairs and dislocations for sound interpretation.
[本文引用: 7]
Han J, Lee S J, Jung J G, et al.The effects of the initial martensite microstructure on the microstructure and tensile properties of intercritically annealed Fe-9Mn-0.05C steel[J]. Acta Mater., 2014, 78: 369
The effects of the initial microstructure of α′ martensite on the microstructural evolution during intercritical annealing and the tensile properties of annealed specimens were investigated for Fe–9Mn–0.05C (wt.%) steel. The hot-rolled specimen with fully α′ martensitic microstructure showed a mixed microstructure of lath-shaped ferrite (αL) and austenite (γL) after intercritical annealing. The αL grains had a high density of dislocations due to inactive recovery, and also had a low Mn concentration. The γL grains had a low density of dislocations and high Mn and C concentrations. The αL and γL grains were deformed simultaneously during the tensile test because the αL grains were as hard as the γL grains due to their high dislocation density, resulting in continuous yielding. The cold-rolled specimen with a deformed α′ martensite microstructure exhibited a mixed microstructure of globular-shaped ferrite (αG) and austenite (γG) after intercritical annealing. The αG grains had a low dislocation density due to active recovery, and also had a low Mn concentration. The γG grains had a low dislocation density and high Mn and C concentrations. The soft αG grains with a low dislocation density were easily deformed at the early stage of the tensile test, resulting in discontinuous yielding and a large yield point elongation.
[本文引用: 2]
Olson G, Cohen M.Kinetics of strain-induced martensitic nucleation[J]. Metall. Trans., 1975, 6A: 791
Intersections of shear bands in metastable austenites have been shown to be effective sites for strain-induced martensitic nucleation. The shear bands may be in the form of (hcp) martensite, mechanical twins, or dense bundles of stacking faults. Assuming that shear-band intersection is the dominant mechanism of strain-induced nucleation, an expression for the volume fraction of martensite vs plastic strain is derived by considering the course of shear-band formation, the probability of shear-band intersections, and the probability of an intersection generating a martensitic embryo. The resulting transformation curve has a sigmoidal shape and, in general, approaches saturation below 100 pct. The saturation value and rate of approach to saturation are determined by two temperature-dependent parameters related to the fee-bee chemical driving force and austenite stacking-fault energy. Fitting the expression to available data on 304 stainless steels gives good agreement for the shape of individual transformation curves as well as the temperature dependence of the derived parameters. It is concluded that the temperature dependence of the transformation kinetics (an important problem in the development of TRIP steels) may be minimized by decreasing the fee, bec, and hep entropy differences through proper compositional control.
[本文引用: 2]
Yen H W, Ooi S W, Eizadjou M, et al.Role of stress-assisted martensite in the design of strong ultrafine-grained duplex steels[J]. Acta Mater., 2015, 82: 100
This work explains the occurrence of transformation-induced plasticity via stress-assisted martensite, when designing ultrafine-grained duplex steels. It is found that, when the austenite is reduced to a fine scale of about 300nm, the initial deformation-induced microstructure can be dominated by parallel lamellae of ε martensite or mechanical twinning, which cannot efficiently provide nucleation sites for strain-induced martensite. Hence, α′ martensite nucleation occurs independently by a stress-assisted process that enhances transformation-induced plasticity in ultrafine-grained austenite. This metallurgical principle was validated experimentally by using a combination of transmission Kikuchi diffraction mapping, transmission electron microscopy and atom probe microscopy, and demonstrated theoretically by the thermodynamics model of stress-assisted martensite.
[本文引用: 1]
Bouaziz O, Buessler P.Iso-work increment assumption for heterogeneous material behaviour modelling[J]. Adv. Eng. Mater., 2004, 6: 79
Publication & Iso‐work Increment Assumption for Heterogeneous Material Behaviour Modelling.
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Jian W W, Cheng G M, Xu W Z, et al.Physics and model of strengthening by parallel stacking faults[J]. Appl. Phys. Lett., 2013, 103: 133108
We have recently reported that parallel stacking faults (SFs) can tremendously increase the strength of a magnesium alloy. The strengthening is found to increase linearly with the reciprocal of the mean SF spacing, d. In this study we analyze dislocation interactions with SFs, and then propose a physics-based model to explain the observed relationship between yield strength and SFs spacing. Similar to the empirical Hall-Petch relationship for grain size, it is expected that this strengthening mechanism will hold true for a variety of materials engineered with parallel spaced stacking faults over a wide range of fault spacing.
[本文引用: 1]
Seo E J, Cho L, Estrin Y, et al.Microstructure-mechanical properties relationships for quenching and partitioning (Q&P) processed steel[J]. Acta Mater., 2016, 113: 124
The quenching and partitioning (Q&P) processing of advanced high strength steels has been shown to result in a distinct improvement of their strength and ductility. The selection of the quench temperature during Q&P processing makes it possible to obtain a wide range of mechanical properties for the final martensite-austenite microstructures. In the present study, a physically-based model describing the relationship between the microstructure and the mechanical properties of Q&P processed steels is developed. The model is applied to the mechanical properties of a Q&P processed medium Mn steel.
[本文引用: 1]
Liang Z Y, Wang X, Huang W, et al.Strain rate sensitivity and evolution of dislocations and twins in a twinning-induced plasticity steel[J]. Acta Mater., 2015, 88: 170
The present work investigated the effect of strain rates (103 to 103s1) on the deformation behaviour of a twinning-induced plasticity (TWIP) steel. The strain rate sensitivity was studied in terms of instantaneous strain rate sensitivity (ISRS) and strain rate sensitivity of work-hardening (SRSW). While ISRS concerns the instantaneous flow stress change upon strain rate jump, SRSW deals with the subsequent modification in microstructure evolution, i.e. change of work-hardening rate. The present TWIP steel demonstrates a positive ISRS which remains stable during deformation and a negative SRSW, i.e. lower work-hardening rate at higher strain rate. Synchrotron X-ray diffraction experiments indicate that the negative SRSW should be attributed to the suppression of dislocations and deformation twins at high strain rate. This unexpected finding is different to conventional face-centred cubic (fcc) metals which generally show enhanced work-hardening rate at higher strain rate. A constitutive model which is strain rate- and temperature-dependent is developed to explain the stable ISRS and the negative SRSW. The modelling results reveal that the stable ISRS should be attributed to the thermally-activated dislocation motion dominated by interstitial carbon atoms and the negative SRSW should be due to the suppression of the dislocations and deformation twins caused by the adiabatic heating associated with high strain rate deformation.
[本文引用: 1]
Bouaziz O, Allian S, Scott C.Effect of grain and twin boundaries on the hardening mechanisms of twinning-induced plasticity steels[J]. Scr. Mater., 2008, 58: 484
New experimental data related to the grain size and the Bauschinger effects have been obtained from a high-manganese austenitic twinning-induced plasticity steel. As the data show a strong contribution of kinematic hardening to the mechanical behaviour, a new physical-based model describing the isotropic and kinematic hardening is presented and validated in relation to the grain size and the twin spacing during straining.
[本文引用: 1]
Mecking H, Kocks U F.Kinetics of flow and strain-hardening[J]. Acta Metall., 1981, 29: 1865
The kinetics of glide at constant structure and the kinetics of structure evolution are correlated on the basis of various experimental observations in pure f.c.c. mono- and polycrystals. Two regimes of behavior are identified. In the initial regime, the Cottrell-Stokes law is satisfied, hardening is athermal, and a single structure parameter is adequate. With increasing importance of dynamic recovery, be it at large strains or at high temperatures, all of these simple assumptions break down. However, the proportionality between the flow stress and the square-root of the dislocation density holds, to a good approximation, ov mild deviations arc primarily ascribed to differences between the various experimental techniques used. A phenomenological model is proposed, which incorporates the rate of dynamic recovery into the flow kinetics. It has been successful in matching many experimental data quantitatively.
[本文引用: 1]
Estrin Y, Mecking H.A unified phenomenological description of work hardening and creep based on one-parameter models[J]. Acta Metall., 1984, 32: 57
A phenomenological treatment of plastic deformation is proposed which makes it possible to describe in a unified way the plastic behavior of a material both under dynamic loading and in creep. The treatment is based on the notion of a unique structure parameter that determines the mechanical state of the material. By combining an evolution equation for this structure parameter with a kinetic equation, which relates the strain rate to the stress at a fixed value of the structure parameter, a complete description of plastic deformation is achieved. The evolution is viewed as that towards a steady state defined by a dynamic equilibrium of athennal work hardening (associated with the storage of dislocations) and strain-rate and temperature dependent work softening (associated with the annihilation of dislocations). Analytical solutions of the resulting set of equations are given for two different models based on physically most plausible assumptions concerning the hardening and competing softening processes. The corresponding expressions for the two deformation modes provide a basis for converting stress-strain data into creep data without any adjustable parameters. This is exemplified by a good agreement between a measured creep curve and the one predicted from stress-strain data in the case of room temperature deformation of polycrystalline copper. Commonly used techniques for evaluating creep and work hardening data are critically discussed.
[本文引用: 1]
Bouaziz O, Estrin Y, Bréchet Y, et al.Critical grain size for dislocation storage and consequences for strain hardening of nanocrystalline materials[J]. Scr. Mater., 2010, 63: 477
We consider strain hardening of nanostructured materials and propose a physically based interpretation of their low strain hardening capability in terms of a reduced storage rate of dislocations. The model suggested provides a modification of the Kocks–Mecking–Estrin evolution law for dislocation storage for nanostructured materials and predicts a critical grain size below which the strain hardening rate drops off.
[本文引用: 1]
Cheng S, Spencer J A, Milligan W W.Strength and tension/compression asymmetry in nanostructured and ultrafine-grain metals[J]. Acta Mater., 2003, 51: 4505
The recent literature is reviewed with respect to the strength-limiting deformation mechanisms in nanocrystalline and ultrafine-grain metals. Based on these results, a deformation mechanism map is proposed for FCC metals with ultrafine-grain sizes. In the absence of flaw-controlled brittle fracture, it is concluded that the strength-limiting mechanism in metals with grain sizes between approximately 10 and 500 1000 nm is dislocation emission from grain boundary sources. A simple model for the strength in this regime of grain sizes is developed from classical dislocation theory, based on the bow-out of a dislocation from a grain boundary dislocation source. The model predicts not only the strength as a function of grain size, but also the observed tension/compression asymmetry of the yield strength. The tension/compression asymmetry arises from the pressure dependence of the dislocation self-energy during bow-out. The pressure dependence is a function of material and grain size, consistent with experimental observations. Finally, the model provides a physical basis for a pressure-dependent yield criterion.
[本文引用: 1]
Hazra S S, Pereloma E V, Gazder A A.Microstructure and mechanical properties after annealing of equal-channel angular pressed interstitial-free steel[J]. Acta Mater., 2011, 59: 4015
The evolution of microstructure, microtexture and mechanical properties during isothermal annealing of an ultrafine-grained interstitial-free steel after eight passes of route B room temperature equal-channel angular pressing (ECAP) was studied. The microstructure and microtexture were characterized by electron back-scattering diffraction, and mechanical properties were assessed by shear punch and uniaxial tensile testing. Homogeneous coarsening via continuous recrystallization of the ECAP microstructure is accompanied by minor changes in the 63% high-angle boundary population and a sharpening of the original ECAP texture. This is followed by abnormal growth during the final stages of softening due to local growth advantages. Linear correlations between shear and tensile data were established for yield, ultimate strength and total elongation. After yield, the changes in uniaxial tensile behaviour from geometrical softening after ECAP to load drop, L眉ders banding and continuous yielding after annealing is attributable to a coarsening of the microstructure.
[本文引用: 1]
Liu J, Zhu G, Mao W, et al.Modeling of critical grain size for shifting plasticity enhancement to decrease by refining grain size[J]. Mater. Sci. Eng., 2014, A607: 302
The effect of grain size on plasticity was investigated based on the theories of dislocation pile-up and Frank–Read (FR) source activation in ultrafine grained materials. The possible reason of the experimental phenomena of decrease in plasticity as grain size refined in ultrafine grained materials was theoretically analyzed and the critical grain size for plasticity decrease was proposed. The results predicted that there was a critical grain size of about 4–502μm with the present parameters of X80 microalloyed steel where the plasticity would shift from increase to decrease as grain refined, which well agreed with the experimental data described in the references in ultrafine grained materials. The mechanism of decreasing in plasticity as grain size refined was dominantly due to the decrease of probability of activation of FR source. The modeling for predicting the critical grain size has been built up which would be very helpful to optimize microstructure for obtaining excellent combination of strength and plasticity.
[本文引用: 1]
... 随着汽车工业对节能、环保和安全性要求的不断提高,高强塑积的汽车结构用钢越来越成为人们关注的焦点.近二三十年来,汽车用钢得到了迅速发展,美国学者根据汽车用钢的强塑积(即抗拉强度与断后延伸率的乘积)把汽车用钢划分为三代[1].第一代汽车钢以无间隙原子钢、双相钢和低合金相变诱导塑性钢(phase transformation induced plasticity,TRIP)为代表,强塑积在10~20 GPa%,目前已难以满足汽车工业对轻量化和高安全性的双重要求.第二代汽车钢以具孪生诱导塑性(twinning induced plasticity,TWIP)的奥氏体钢和TWIP钢为代表,强塑积达50~70 GPa%,但其添加了大量的Cr、Ni、Mn等元素,其成本较高且冶炼生产存在一定困难.近年发展起来的第三代汽车用钢中Mn含量约为3.5%~12.0% (质量分数,下同),利用热轧或冷轧钢板在退火过程中发生的逆转变奥氏体来形成亚微米级的奥氏体和铁素体双相组织,奥氏体在应变过程中发生TRIP效应来提高钢的塑性和强度,其优异的综合力学性能不仅可以满足汽车轻量化和碰撞安全性的要求,还可以保证汽车零部件的成型性. ...
... 目前,中锰钢的研究已从初期的Fe-Mn-C系[2,3,4,5]发展到Fe-Mn-C-Al(-Si)系[6,7,8,9,10].合理的合金成分设计可以极大地提高中锰钢的综合力学性能.在文献[11]中,作者讨论了退火温度对新型冷轧中锰钢7%Mn-0.3%C-2%Al (以下简称冷轧7Mn钢)组织和力学性能的影响.实验结果表明,随着退火温度的升高,逆转变奥氏体的晶粒尺寸逐渐增大且其中固溶的Mn和C含量逐渐降低,使得奥氏体的机械稳定性显著下降.拉伸实验表明,冷轧7Mn钢在700 ℃退火1 h后可获得机械稳定性适中的奥氏体,在变形时持续转变为马氏体从而使材料的力学性能达到最优.虽然实验结果显示冷轧7Mn钢的机械性能主要受奥氏体稳定性的影响,但目前仍不清楚铁素体、奥氏体和应变诱导马氏体在变形过程中各自的拉伸和加工硬化行为以及这3相是如何影响材料的力学性能的,因为这些数据难以通过实验手段来直接测量.孙朝阳等[12]利用基于位错密度的本构模型成功模拟了Fe-22Mn-0.6C型TWIP钢在不同应变速率下的真应力-真应变曲线,并且分析了孪晶与滑移机制的相互作用及对宏观变形的影响.模拟数据表明,孪生速率与滑移速率间呈现负相关,即变形前期,孪生速率较大而滑移速率较小,孪生趋于饱和时,孪生速率降低而滑移速率快速上升.Lee等[7,8]也利用本构模型分析了Fe-10Mn-0.3C-3Al-2Si型中锰钢在变形过程中各组成相的流变应力和加工硬化行为,模拟计算的真应力-真应变曲线和加工硬化率曲线与实测结果吻合良好.其模拟结果还表明,Fe-10Mn-0.3C-3Al-2Si经冷轧并在800 ℃退火后,亚稳奥氏体晶粒中的TRIP效应的强化效果比TWIP效应的要更显著.孙朝阳等[12]和Lee等[7,8]的工作表明,可以利用本构模型来深入分析TWIP钢或中锰钢在变形过程中的力学行为.为此,本工作将借助基于位错密度的本构模型来研究冷轧7Mn钢的拉伸和加工硬化行为,并结合实验结果来阐明不同温度退火后中锰钢力学行为差异的内在机制. ...
... 图1所示为不同温度退火试样的EBSD像,图中白色区域为铁素体,灰色区域为奥氏体,黑色实线表示大角晶界,灰色实线为小角晶界.可以看到冷轧退火后铁素体和奥氏体晶粒基本都呈等轴状,与热轧退火后的板条状形貌有很大不同[2,3].此外,铁素体的晶界中大部分为小角晶界,即退火后铁素体仍然没有发生再结晶.Cao等[2]认为这是由于铁素体晶粒中较高的Mn含量拖曳晶界、抑制了晶界的迁移.统计了退火后的逆转变奥氏体和铁素体的晶粒尺寸,如表1所示.可见,随退火温度的升高,逆转变奥氏体晶粒的平均尺寸增加,而铁素体的平均晶粒尺寸略有减小.文献[11]中测量了冷轧7Mn钢退火后奥氏体的化学成分,数据表明奥氏体晶粒内固溶的Mn和C含量随退火温度的升高而降低.由于奥氏体的晶粒尺寸和成分决定了其机械稳定性,故随退火温度的升高奥氏体的稳定性是逐渐降低的.图2为S680~S720的TEM像.可以看到虽然没有发生再结晶,铁素体晶粒内部的位错密度仍然很低,Han等[17]称这种现象为大回复. ...
... [2]认为这是由于铁素体晶粒中较高的Mn含量拖曳晶界、抑制了晶界的迁移.统计了退火后的逆转变奥氏体和铁素体的晶粒尺寸,如表1所示.可见,随退火温度的升高,逆转变奥氏体晶粒的平均尺寸增加,而铁素体的平均晶粒尺寸略有减小.文献[11]中测量了冷轧7Mn钢退火后奥氏体的化学成分,数据表明奥氏体晶粒内固溶的Mn和C含量随退火温度的升高而降低.由于奥氏体的晶粒尺寸和成分决定了其机械稳定性,故随退火温度的升高奥氏体的稳定性是逐渐降低的.图2为S680~S720的TEM像.可以看到虽然没有发生再结晶,铁素体晶粒内部的位错密度仍然很低,Han等[17]称这种现象为大回复. ...
... 目前,中锰钢的研究已从初期的Fe-Mn-C系[2,3,4,5]发展到Fe-Mn-C-Al(-Si)系[6,7,8,9,10].合理的合金成分设计可以极大地提高中锰钢的综合力学性能.在文献[11]中,作者讨论了退火温度对新型冷轧中锰钢7%Mn-0.3%C-2%Al (以下简称冷轧7Mn钢)组织和力学性能的影响.实验结果表明,随着退火温度的升高,逆转变奥氏体的晶粒尺寸逐渐增大且其中固溶的Mn和C含量逐渐降低,使得奥氏体的机械稳定性显著下降.拉伸实验表明,冷轧7Mn钢在700 ℃退火1 h后可获得机械稳定性适中的奥氏体,在变形时持续转变为马氏体从而使材料的力学性能达到最优.虽然实验结果显示冷轧7Mn钢的机械性能主要受奥氏体稳定性的影响,但目前仍不清楚铁素体、奥氏体和应变诱导马氏体在变形过程中各自的拉伸和加工硬化行为以及这3相是如何影响材料的力学性能的,因为这些数据难以通过实验手段来直接测量.孙朝阳等[12]利用基于位错密度的本构模型成功模拟了Fe-22Mn-0.6C型TWIP钢在不同应变速率下的真应力-真应变曲线,并且分析了孪晶与滑移机制的相互作用及对宏观变形的影响.模拟数据表明,孪生速率与滑移速率间呈现负相关,即变形前期,孪生速率较大而滑移速率较小,孪生趋于饱和时,孪生速率降低而滑移速率快速上升.Lee等[7,8]也利用本构模型分析了Fe-10Mn-0.3C-3Al-2Si型中锰钢在变形过程中各组成相的流变应力和加工硬化行为,模拟计算的真应力-真应变曲线和加工硬化率曲线与实测结果吻合良好.其模拟结果还表明,Fe-10Mn-0.3C-3Al-2Si经冷轧并在800 ℃退火后,亚稳奥氏体晶粒中的TRIP效应的强化效果比TWIP效应的要更显著.孙朝阳等[12]和Lee等[7,8]的工作表明,可以利用本构模型来深入分析TWIP钢或中锰钢在变形过程中的力学行为.为此,本工作将借助基于位错密度的本构模型来研究冷轧7Mn钢的拉伸和加工硬化行为,并结合实验结果来阐明不同温度退火后中锰钢力学行为差异的内在机制. ...
... 图1所示为不同温度退火试样的EBSD像,图中白色区域为铁素体,灰色区域为奥氏体,黑色实线表示大角晶界,灰色实线为小角晶界.可以看到冷轧退火后铁素体和奥氏体晶粒基本都呈等轴状,与热轧退火后的板条状形貌有很大不同[2,3].此外,铁素体的晶界中大部分为小角晶界,即退火后铁素体仍然没有发生再结晶.Cao等[2]认为这是由于铁素体晶粒中较高的Mn含量拖曳晶界、抑制了晶界的迁移.统计了退火后的逆转变奥氏体和铁素体的晶粒尺寸,如表1所示.可见,随退火温度的升高,逆转变奥氏体晶粒的平均尺寸增加,而铁素体的平均晶粒尺寸略有减小.文献[11]中测量了冷轧7Mn钢退火后奥氏体的化学成分,数据表明奥氏体晶粒内固溶的Mn和C含量随退火温度的升高而降低.由于奥氏体的晶粒尺寸和成分决定了其机械稳定性,故随退火温度的升高奥氏体的稳定性是逐渐降低的.图2为S680~S720的TEM像.可以看到虽然没有发生再结晶,铁素体晶粒内部的位错密度仍然很低,Han等[17]称这种现象为大回复. ...
... 当退火温度增加至720 ℃后,逆转变奥氏体的尺寸变得更大,且其中固溶的Mn和C含量比S680和S700中的都低,故其稳定性进一步降低.若用
Δ V / Δ ε
表示马氏体的转变速率(
分别代表马氏体含量和真应变的变化量),根据图3中的数据可知S700中马氏体的平均转变率为0.57,而S720则高达0.91,这表明在S720中由于奥氏体稳定性的降低使得马氏体的体积分数快速上升,导致材料的流变应力也快速增加,最终使得S720的流变应力在真应变0.1~0.2之间呈现明显的S形特征,如图8e所示.S720的加工硬化率随之呈现明显的3段式特征[3],即随变形量的增加加工硬化率首先下降,然后上升最后又下降,如图8f中的I、II、III所示.值得指出的是,S700的加工硬化率曲线也呈现3段式特征,但由于马氏体的转变速率偏慢,使得第II段的最高值和最低值差别不大,导致真应力曲线的S型特征并不明显.
... 目前,中锰钢的研究已从初期的Fe-Mn-C系[2,3,4,5]发展到Fe-Mn-C-Al(-Si)系[6,7,8,9,10].合理的合金成分设计可以极大地提高中锰钢的综合力学性能.在文献[11]中,作者讨论了退火温度对新型冷轧中锰钢7%Mn-0.3%C-2%Al (以下简称冷轧7Mn钢)组织和力学性能的影响.实验结果表明,随着退火温度的升高,逆转变奥氏体的晶粒尺寸逐渐增大且其中固溶的Mn和C含量逐渐降低,使得奥氏体的机械稳定性显著下降.拉伸实验表明,冷轧7Mn钢在700 ℃退火1 h后可获得机械稳定性适中的奥氏体,在变形时持续转变为马氏体从而使材料的力学性能达到最优.虽然实验结果显示冷轧7Mn钢的机械性能主要受奥氏体稳定性的影响,但目前仍不清楚铁素体、奥氏体和应变诱导马氏体在变形过程中各自的拉伸和加工硬化行为以及这3相是如何影响材料的力学性能的,因为这些数据难以通过实验手段来直接测量.孙朝阳等[12]利用基于位错密度的本构模型成功模拟了Fe-22Mn-0.6C型TWIP钢在不同应变速率下的真应力-真应变曲线,并且分析了孪晶与滑移机制的相互作用及对宏观变形的影响.模拟数据表明,孪生速率与滑移速率间呈现负相关,即变形前期,孪生速率较大而滑移速率较小,孪生趋于饱和时,孪生速率降低而滑移速率快速上升.Lee等[7,8]也利用本构模型分析了Fe-10Mn-0.3C-3Al-2Si型中锰钢在变形过程中各组成相的流变应力和加工硬化行为,模拟计算的真应力-真应变曲线和加工硬化率曲线与实测结果吻合良好.其模拟结果还表明,Fe-10Mn-0.3C-3Al-2Si经冷轧并在800 ℃退火后,亚稳奥氏体晶粒中的TRIP效应的强化效果比TWIP效应的要更显著.孙朝阳等[12]和Lee等[7,8]的工作表明,可以利用本构模型来深入分析TWIP钢或中锰钢在变形过程中的力学行为.为此,本工作将借助基于位错密度的本构模型来研究冷轧7Mn钢的拉伸和加工硬化行为,并结合实验结果来阐明不同温度退火后中锰钢力学行为差异的内在机制. ...
... 目前,中锰钢的研究已从初期的Fe-Mn-C系[2,3,4,5]发展到Fe-Mn-C-Al(-Si)系[6,7,8,9,10].合理的合金成分设计可以极大地提高中锰钢的综合力学性能.在文献[11]中,作者讨论了退火温度对新型冷轧中锰钢7%Mn-0.3%C-2%Al (以下简称冷轧7Mn钢)组织和力学性能的影响.实验结果表明,随着退火温度的升高,逆转变奥氏体的晶粒尺寸逐渐增大且其中固溶的Mn和C含量逐渐降低,使得奥氏体的机械稳定性显著下降.拉伸实验表明,冷轧7Mn钢在700 ℃退火1 h后可获得机械稳定性适中的奥氏体,在变形时持续转变为马氏体从而使材料的力学性能达到最优.虽然实验结果显示冷轧7Mn钢的机械性能主要受奥氏体稳定性的影响,但目前仍不清楚铁素体、奥氏体和应变诱导马氏体在变形过程中各自的拉伸和加工硬化行为以及这3相是如何影响材料的力学性能的,因为这些数据难以通过实验手段来直接测量.孙朝阳等[12]利用基于位错密度的本构模型成功模拟了Fe-22Mn-0.6C型TWIP钢在不同应变速率下的真应力-真应变曲线,并且分析了孪晶与滑移机制的相互作用及对宏观变形的影响.模拟数据表明,孪生速率与滑移速率间呈现负相关,即变形前期,孪生速率较大而滑移速率较小,孪生趋于饱和时,孪生速率降低而滑移速率快速上升.Lee等[7,8]也利用本构模型分析了Fe-10Mn-0.3C-3Al-2Si型中锰钢在变形过程中各组成相的流变应力和加工硬化行为,模拟计算的真应力-真应变曲线和加工硬化率曲线与实测结果吻合良好.其模拟结果还表明,Fe-10Mn-0.3C-3Al-2Si经冷轧并在800 ℃退火后,亚稳奥氏体晶粒中的TRIP效应的强化效果比TWIP效应的要更显著.孙朝阳等[12]和Lee等[7,8]的工作表明,可以利用本构模型来深入分析TWIP钢或中锰钢在变形过程中的力学行为.为此,本工作将借助基于位错密度的本构模型来研究冷轧7Mn钢的拉伸和加工硬化行为,并结合实验结果来阐明不同温度退火后中锰钢力学行为差异的内在机制. ...
... 目前,中锰钢的研究已从初期的Fe-Mn-C系[2,3,4,5]发展到Fe-Mn-C-Al(-Si)系[6,7,8,9,10].合理的合金成分设计可以极大地提高中锰钢的综合力学性能.在文献[11]中,作者讨论了退火温度对新型冷轧中锰钢7%Mn-0.3%C-2%Al (以下简称冷轧7Mn钢)组织和力学性能的影响.实验结果表明,随着