连续屈服、高强屈比中锰钢的工艺设计与组织调控
Process Design and Microstructure Control of Medium Manganese Steel with Continuous Yield and High Strength Yield Ratio
通讯作者: 定 巍,adingwei@126.com,主要从事先进高强钢的开发与研究
责任编辑: 毕淑娟
收稿日期: 2022-10-19 修回日期: 2022-11-28
基金资助: |
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Corresponding authors: DING Wei, associate professor, Tel:
Received: 2022-10-19 Revised: 2022-11-28
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作者简介 About authors
张光莹,男,1996年生,硕士生
为了探究残余奥氏体稳定性对于中锰钢塑性失稳的影响和作用规律,本工作针对中锰钢(0.2C-5Mn-0.5Si-1.5Al,质量分数,%),设计了预处理加临界退火的热处理工艺,得到2种残余奥氏体,并对其成分、形貌及稳定性进行了分析,进而讨论了残余奥氏体对力学性能的影响。结果表明,珠光体+马氏体+铁素体的多相初始组织退火后能够获得2种不同稳定性的残余奥氏体:源自珠光体具有较高Mn含量的薄膜状残余奥氏体;主要源自于马氏体具有适当Mn含量的块状残余奥氏体。块状残余奥氏体稳定性较差,在塑性变形初始阶段发生相变,有助于消除Lüders应变;薄膜状残余奥氏体稳定性较高,在变形中后期发生相变,有助于获得高强塑性。含双稳定性残余奥氏体的试样不仅保持了高抗拉强度(> 1000 MPa)和高断裂延伸率(> 30%),而且还具有连续屈服和高强屈比的特点。
关键词:
Researches have focused on the development of lightweight and high-performance steel materials to ensure automobile safety. Medium manganese steel is a potential candidate owing to its excellent mechanical properties and low production cost. However, the problem of plastic instability (Lüders strain, Portevin-Le Chatelier effect) is one of the main factors restricting the development of medium manganese steel. Therefore, resolving the plastic instability of medium manganese steel is a prerequisite for its development and hence to ensure the benefits of its mechanical qualities. Many studies have found that the stability of retained austenite is directly related to the plastic instability of medium manganese steel. In this work, cold rolled low-carbon medium manganese steel is selected, and multi-stable retained austenite is obtained by designing pretreatment and critical annealing process. The phase transformation of the retained austenite with different stability in each stage of the tensile process and its influence on the mechanical properties are studied. The results show that the microstructure containing pearlite + ferrite + martensite is obtained after pretreatment of medium manganese steel. After annealing, pearlite transformed into filmy retained austenite and ferrite phase; while martensite transformed into blocky retained austenite. Mn content in the filmy retained austenite is higher than that in the blocky retained austenite, making the filmy retained austenite more stable than the blocky one. The blocky retained austenite has poor stability, and phase transformation occurs at the initial stage of plastic deformation, eliminating Lüders strain. In contrast, the filmy retained austenite has high stability, and phase transformation occurs in the middle and late deformation, contributing toward its high strength and plasticity. The specimens containing double-stable retained austenite not only maintain the tensile strength (> 1000 MPa) and high fracture elongations (> 30%), but also have the characteristics of continuous yield and high strength yield ratio.
Keywords:
本文引用格式
张光莹, 李岩, 黄丽颖, 定巍.
ZHANG Guangying, LI Yan, HUANG Liying, DING Wei.
冷轧热处理中锰钢整体力学性能优异,但是塑性失稳严重,塑性失稳主要包括Lüders应变和Portevin-Le Chatelier (PLC)效应,塑性失稳有损成形性能。研究[11]表明,冷轧热处理中锰钢的塑性失稳与残余奥氏体的稳定性密切相关:冷轧热处理中锰钢的残余奥氏体稳定性高且差异小,马氏体相变集中发生在变形量较大的区域,因此不能在形变前期提供加工硬化来弱化Lüders应变,而且容易导致严重的PLC效应。因此,合理调控残余奥氏体稳定性,使其在更大应变范围内发生马氏体相变,是优化和改善中锰钢力学性能的关键。影响残余奥氏体稳定性的因素有很多,包括残余奥氏体的化学成分、晶粒尺寸、形貌、晶体取向以及相邻相等[12],其中化学成分、晶粒尺寸以及形貌对稳定性影响更为显著。研究[13]表明,当退火前组织中存在珠光体时,在退火过程中珠光体中的渗碳体和附近的铁素体转变为奥氏体,此时还有少量铁素体未转变,因此冷却后得到间隔分布的铁素体和残余奥氏体,并且这种残余奥氏体的Mn含量很高。Zhang等[14]使用热轧Fe-0.42C-3.71Mn-1.45Si (质量分数,%)钢,通过调控热处理工艺,得到含珠光体的原始组织,进行淬火-配分工艺后得到了薄膜状和块状的残余奥氏体,这2种残余奥氏体在拉伸变形过程中表现出了不同的稳定性,在保证整体力学性能的前提下,优化了材料局部力学性能。
冷轧中锰钢存在塑性失稳现象可以通过设计不同稳定性的残余奥氏体来改善。鉴于此,本工作通过预处理获得珠光体+马氏体+铁素体的三相组织,在临界退火后的最终组织中获得了具有不同化学成分和形貌的2种残余奥氏体,研究了2种残余奥氏体的特点,分析了残余奥氏体稳定性对力学性能的影响,探讨不同稳定性残余奥氏体在不同变形阶段的作用机理。
1 实验方法
实验所用中锰钢的化学成分(质量分数,%)为:C 0.18,Mn 5.45,Al 1.62,Si 0.50,Fe余量。在50 kg中频感应熔炼炉冶炼,浇铸冷却后开坯锻造成60 mm厚的钢坯。钢坯在箱式加热炉中随炉加热至1200℃保温1.5 h,经5道次热轧后冷却至600℃保温1.5 h,然后空冷至室温。热轧板为3 mm厚的钢板,热轧后酸洗,最终经多道冷轧成1.4 mm厚的钢板。
冷轧钢板按照图1所示的工艺进行热处理,所得试样命名为PA730-5。具体热处理过程如下:首先,将钢板在800℃保温5 min,得到奥氏体(austenite,A)和铁素体(ferrite,F)的双相组织,随即在550℃下保温2 h后水淬,得到珠光体(pearlite,P) +马氏体(martensite,M) +铁素体的多相组织,此过程称为预处理(pretreatment);经过预处理的试样在730℃退火5 min,其微观组织为铁素体+残余奥氏体(retained austenite,RA),其中残余奥氏体有2种,分别为由珠光体转变而成的富Mn薄膜状残余奥氏体(RA+)和马氏体转变而成的贫Mn块状残余奥氏体(RA-)。为了对比,引入不经预处理的钢板直接在730℃退火3 min,对应的试样命名为A730-3。
图 1
图 1
热处理工艺示意图
Fig.1
Schematic of heat treatment process (A1—start temperature of pearlite transformed to austenite, A3—temperature of all ferrite transformed to austenite, A—austenite, F—ferrite, P—pearlite, M—martensite, RA—retained austenite, RA+—Mn-rich RA, RA-—Mn-poor RA)
PA730-5试样进行机械研磨抛光,使用体积分数为4%的硝酸酒精溶液腐蚀后,分别在Axio Vret A1型金相显微镜(OM)和Sigma300型扫描电镜(SEM)下进行微观组织观察,并依据GB/T15749—2008定量金相测量方法,使用OM自带软件对珠光体含量(体积分数)进行统计。A730-3和PA730-5试样进行研磨抛光,用体积分数为15%的高氯酸酒精溶液电解腐蚀,在SEM下对试样进行组织观察。PA730-5试样经机械研磨至厚度50 μm后双喷减薄,随后利用带有能谱仪(EDS)的JEM-F200型透射电镜(TEM)测量残余奥氏体中的元素含量。
A730-3和PA730-5试样研磨抛光后,用体积分数为20%的高氯酸酒精溶液电解抛光去除应力,利用D8 Advance型X射线衍射仪(XRD)对试样中的残余奥氏体进行定量分析,用
式中,Iγ 为奥氏体(200)、(220)、(311)衍射峰积分强度,Iα 为铁素体(200)、(211)衍射峰积分强度。
利用
式中,XC、XMn和XAl分别为残余奥氏体中C、Mn和Al的质量分数,a为残余奥氏体的晶格常数,由
式中,λ为X射线波长,θ为衍射角,h、k、l为晶面指数。使用XRD峰半高宽数据,采用
式中,β为铁素体XRD峰的半高宽(FWHM);b为Burgers矢量模,取值为 2.48 × 10-10 m。
根据GB/T228—2002,将A730-3和PA730-5试样制成标距为25 mm的非比例拉伸试样,在CMT-30型电子试验机下进行拉伸实验,加载速率2 mm/min。根据
式中,
2 实验结果
2.1 微观组织构成与残余奥氏体特性
图2为A730-3和PA730-5试样退火前后微观组织的SEM像及PA730-5试样奥氏体转变示意图。如图2a所示,原始冷轧板的微观组织为马氏体和铁素体。经过预处理后微观组织由珠光体、铁素体和马氏体组成(图2c),其中珠光体体积分数约占22%。A730-3和PA730-5试样退火后最终组织如图2b和d所示,均由铁素体和残余奥氏体组成,A730-3试样的铁素体晶粒更为细小。A730-3试样最终组织中残余奥氏体主要为块状残余奥氏体。而PA730-5试样中的残余奥氏体呈块状和薄膜状,块状残余奥氏体源自于马氏体,薄膜状残余奥氏体源自于珠光体[12],与其相邻的薄膜状铁素体交替呈层状分布,很大程度遗传了珠光体的形貌[14,18]。PA730-5试样2种形貌残余奥氏体获得的原理如图2e所示。
图2
图2
A730-3和PA730-5试样退火前后的SEM像和奥氏体转变示意图
Fig.2
SEM images of A730-3 (a, b) and PA730-5 (c, d) samples before (a, c) and after (b, d) annealing, and schematics of austenite transformation of PA730-5 sample (CEME—cementite) (e)
图3为A730-3和PA730-5试样残余奥氏体及铁素体晶粒尺寸分布。如图3a所示,A730-3中残余奥氏体晶粒尺寸多分布于0.2~0.9 μm之间,约占晶粒总数的65.06%,这表明A730-3试样残余奥氏体晶粒尺寸相对集中,均匀且细小。从图3b可以看出,因为PA730-5中存在2种不同形貌的残余奥氏体,其残余奥氏体晶粒尺寸分布出现2个峰值:薄膜状残余奥氏体平均晶粒尺寸在0.15 μm左右,在0.1~0.3 μm形成一个峰值;块状残余奥氏体平均晶粒尺寸在1 μm左右,在0.8~1.2 μm又形成一个峰值。从图3c中可见,PA730-5中块状残余奥氏体和铁素体平均晶粒尺寸要大于A730-3块状残余奥氏体和铁素体平均晶粒尺寸。
图3
图3
A730-3和PA730-5试样的残余奥氏体晶粒尺寸分布图和平均晶粒尺寸
Fig.3
RA grain size distributions of A730-3 (a) and PA730-5 (b) samples, and average grain sizes of each phase in the microstructure (c)
图4为PA730-5试样退火后块状和薄膜状残余奥氏体的TEM像、选区电子衍射(SAED)花样及EDS分析。根据图4a和d中插图的SAED花样可确定所选区域为残余奥氏体。由图可见,块状残余奥氏体晶粒尺寸约为1 μm (图4a)。薄膜状残余奥氏体与铁素体呈片层状分布,奥氏体薄膜厚度约100 nm;薄膜状残余奥氏体周围铁素体中并无明显的位错,这是因为PA730-5经过多次热处理,晶粒发生回复再结晶导致位错密度有所降低(图4d)。从Mn元素的EDS面扫描结果可知,薄膜状残余奥氏体中Mn元素富集程度明显高于块状残余奥氏体(图4b和e),这是因为Mn元素在珠光体中的富集程度远高于马氏体,在短时间的临界退火过程中,Mn元素没有得到充分扩散,因此在薄膜状残余奥氏体中得到富集。为了直观地体现2种残余奥氏体中Mn含量的差异,采用EDS点扫描的方式测量图4a和d中点1~20的Mn元素含量,所得结果如图4c和f所示。可见,块状残余奥氏体中Mn含量较低,且分布不均,其平均Mn元素含量约为6.18% (质量分数,下同),而薄膜状残余奥氏体中平均Mn元素含量相对较高,约为9.94%。Mn元素对于残余奥氏体的稳定性尤为重要,其稳定性可以通过马氏体相变的驱动力(ΔGch)来描述[17]:
图4
图4
PA730-5试样退火后块状和薄膜状残余奥氏体的TEM像、SAED花样及EDS分析
Fig.4
TEM images and SAED patterns (insets) (a, d), and corresponding EDS element mappings (b, e) and EDS point concentrations (c, f) of Mn element of blocky (a-c) and film (d-f) RA in PA730-5 sample after annealing (Red dots in Figs.4a and d show EDS scanning points)
式中,MC、MMn为残余奥氏体中C和Mn的摩尔分数,经过换算得块状和膜状残余奥氏体中C和Mn的摩尔分数分别为3.16%、5.99%和3.17%、9.69%;T为热力学温度(298 K)。通过
图5
图5
PA730-5试样拉伸断裂后距离断口不同位置的取样位置示意图及SEM像
Fig.5
Schematic of sample location (a) and corresponding SEM images (b-d) away from fracture successively in PA730-5 sample
图6为A730-3和PA730-5试样拉伸前和拉伸断裂后断口处的XRD谱。由图可见,A730-3和PA730-5试样在拉伸前存在很高的残余奥氏体衍射峰(图6a),这表明试样中存在大量的残余奥氏体,而拉伸断裂后残余奥氏体的衍射峰不明显(图6b)。经计算,拉伸前A730-3和PA730-5试样残余奥氏体体积分数分别为29.18%和28.11%;拉伸断裂后,A730-3和PA730-5试样在断口处的残余奥氏体体积分数分别为7.91%、3.74%。研究[17]表明,细小颗粒状残余奥氏体的稳定性高于薄膜状残余奥氏体,而薄膜状残余奥氏体的稳定性又高于块状残余奥氏体。如图2b所示,A730-3试样中存在细小颗粒状的残余奥氏体,因其稳定性高,在拉伸的过程中难以发生马氏体相变,从而导致断后A730-3中的残余奥氏体含量相对较高。
图6
图6
A730-3和PA730-5试样拉伸前和拉伸断裂后断口处的XRD谱
Fig.6
XRD spectra of A730-3 and PA730-5 samples before (a) and after (b) tensile fracture
利用
式中,VRA0为初始残余奥氏体含量;VRA为应变为δ时的残余奥氏体含量;k反映残余奥氏体的机械稳定性,其值越大,残余奥氏体的机械稳定性越低。经过计算,A730-3和PA730-5试样断口处的k分别为3.04和5.93,显然A730-3试样残余奥氏体的机械稳定性比PA730-5试样高,这就意味着在变形量相同的条件下,PA730-5中残余奥氏体先发生相变。
2.2 力学性能
图7为A730-3和PA730-5试样的工程应力-应变曲线。由图可知,A730-3试样的工程应力-应变曲线表现为较长的屈服平台(Ⅰ)和剧烈的曲线抖动(Ⅱ),表明该试样出现了严重塑性失稳,体现为明显的Lüders应变和PLC效应;PA730-5试样为连续屈服,没有Lüders应变,虽然工程应力-应变曲线在第Ⅱ阶段也出现了很微弱的抖动,但其PLC效应相较A730-3已经显著弱化。
图7
图7
A730-3和PA730-5试样的工程应力-应变曲线
Fig.7
Engineering stress-strain curves of A730-3 and PA730-5 samples (PLC—Portevin-Le Chatelier)
A730-3和PA730-5试样退火后的力学性能如表1所示。可见,2个试样均拥有良好的力学性能,其抗拉强度均超过1000 MPa,断后延伸率在30%以上,强塑积在30 GPa·%以上。值得注意的是,PA730-5试样的力学性能呈现出更合理的强屈比。
表1 A730-3和PA730-5试样退火后的力学性能
Table 1
Sample | YS MPa | UTS MPa | TE % | UTS / YS | UTS × TE GPa·% |
---|---|---|---|---|---|
A730-3 | 922 | 1041 | 43 | 1.12 | 44.76 |
PA730-5 | 652 | 1025 | 34 | 1.57 | 34.85 |
图8为A730-3和PA730-5试样的加工硬化指数曲线。根据变形机制和曲线特征,加工硬化指数曲线分为3个阶段[20,21]:第1阶段,曲线迅速下降,这与铁素体形变有关;第2阶段,曲线开始上升,这是因为发生了TRIP效应的强化作用;进入到第3阶段后,残余奥氏体含量逐渐减少,TRIP效应逐渐减弱,加工硬化速率逐渐降低,直至结束。由图8可知,加工硬化指数曲线进入第2阶段的初始阶段,PA730-5试样的加工硬化指数要高于A730-3试样,这表明PA730-5试样的TRIP效应更积极。第2阶段的中后段,2个试样的硬化指数剧烈波动,此时PA730-5试样中相对稳定的薄膜状残余奥氏体开始转变,表现出了良好的硬化能力,A730-3试样中残余奥氏体也开始大量发生相变,TRIP效应的强化效果明显。随着曲线进入第3阶段,2个试样中仅剩余少量过于稳定的残余奥氏体,此时加工硬化指数急剧下降。
图8
图8
A730-3和PA730-5试样的加工硬化指数曲线
Fig.8
Work hardening index curves of A730-3 and PA730-5 samples
3 分析讨论
3.1 残余奥氏体稳定性对Lüders应变与PLC效应的影响
PA730-5试样中不同稳定性的残余奥氏体组织在保证了中锰钢力学性能优势的前提下,实现了连续屈服,消除了Lüders应变,弱化了PLC效应。由图7可知,A730-3试样拉伸曲线呈现明显的Lüders应变(很长的屈服平台);而PA730-5试样的拉伸曲线则没有屈服平台,为连续屈服。屈服平台会导致拉伸试样表面出现Lüders带,从而影响试样表面质量,应尽量避免。研究[30~32]表明,冷轧中锰钢出现Lüders应变是一种普遍现象,而导致Lüders应变出现的主要原因是变形开始阶段组织中可动位错少[33]。对于中锰钢,如果有一定量残余奥氏体在变形的开始阶段发生马氏体相变,则有助于弱化甚至完全消除Lüders应变[34]。根据图3c可知,PA730-5试样组织中存在晶粒尺寸较大、稳定性较差的块状残余奥氏体。这部分残余奥氏体能在塑性变形初期发生马氏体相变,引起周围铁素体中出现可移动位错,进而增加组织中的可移动位错,从而弱化甚至消除Lüders应变。如图9a所示,在PA730-5试样断后的组织中,马氏体周围的铁素体晶界处出现高密度位错(dislocation),这表明在拉伸前期,块状残余奥氏体相变提供了可动位错。
图9
图9
PA730-5试样断后的TEM像和Lüders带运动示意图
Fig.9
TEM image of PA730-5 sample after fracture (a) and schematic of motion of Lüders band (b) (VL—Lüders band moving rate, VC—tensile rate)
根据
式中,VC为拉伸仪器的夹头速率,VL为Lüders带移动速率。其中,VC为定值(2 mm/min),所以l与VL成反比,即Lüders带运动越快,Lüders应变越小。如图9b的示意图所示,块状的马氏体提供了位错,使得VL增大,因此钢在拉伸前期没有Lüders应变。
3.2 高强屈比
式中,ky为比例常数,对于低碳钢取值为17.4 MPa·mm1/2;d0为铁素体晶粒尺寸。经计算,可得到A730-3和PA730-5试样的σg分别为644和488 MPa。
式中,
PA730-5试样中存在2种稳定性不同的残余奥氏体,稳定性较差的块状残余奥氏体在第1阶段(图7)发生相变,避免了屈服点延伸,强度得到提升;在第2、3阶段,稳定性高的薄膜状残余奥氏体发生相变,使得强度和塑性得到提升;同时,PA730-5试样中发生相变的残余奥氏体含量更高,TRIP效应表现优异,因此在位错和晶粒细化对强度贡献更小的情况下,使得PA730-5试样仍然获得了与A730-3试样相同水平的抗拉强度,这也是PA730-5试样具有高强屈比的原因。
4 结论
(1) 以珠光体+马氏体+铁素体的多相组织为初始组织进行临界退火后,能够获得2种不同稳定性的残余奥氏体:Mn元素富集(9.94%)的薄膜状残余奥氏体,源自于珠光体;适当Mn含量(6.18%)的块状残余奥氏体,主要源自于马氏体。
(2) PA730-5试样中的2种残余奥氏体的稳定性差异较大,在整个变形过程中都有TRIP效应:稳定性较差的块状残余奥氏体,在塑性变形初始阶段发生相变,有助于消除Lüders应变;稳定性较高的薄膜状残余奥氏体,在变形中后期发生相变,有助于获得高强塑性。
(3) PA730-5试样不仅保持了高抗拉强度(> 1000 MPa)和高断裂延伸率(> 30%),而且还具有连续屈服和高强屈比的特点。
参考文献
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[J].研究了两相区退火温度对一种新型冷轧中锰钢(0.2C-5Mn-0.6Si-3Al,质量分数,%)显微组织及拉伸性能的影响。结果表明,在退火温度为730 ℃时,冷轧中锰钢可获得优异的强度与塑性配合,即抗拉强度为1062 MPa,总伸长率为58.2%,强塑积为61.8 GPa%。随着退火温度升高,逆转变奥氏体逐渐粗化,且由片层状组织形态逐渐向等轴状组织形态转变,在一定退火温度下可获得奥氏体晶粒尺寸分布较为宽泛的多尺度的组织形态。这种多尺度组织形态的残余奥氏体具有适当的机械稳定性,能够产生连续不断的相变诱发塑性(TRIP)效应。连续不断的TRIP效应与铁素体在变形过程中的良好配合,是冷轧中锰钢获得高强度、高塑性的主要原因。冷轧中锰钢拉伸断裂的裂纹主要萌生于软相的铁素体(δ-铁素体)及超细晶铁素体与形变诱导马氏体(残余奥氏体)的界面处。
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A novel cold-rolled medium Mn steel with an ultra-high product of tensile strength and elongation
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[J].The mechanical properties of a novel heterogeneous cold-rolled medium Mn steel were investigated by means of mechanical testers, in situ EBSD (electron back-scattered diffraction) and SDTEM (spherical differential transmission electron microscope). The results show that the sample annealed at 680°C consists of multiple microstructure of austenites (granular shape, blocky shape, and lath-like shape) and fine ferrite grains. The heterogeneous steel has ultimate tensile strength of 1.27 GPa, total elongation of 54.5% and product of strength and elongation of 69.3 GPa·%. During tensile deformation the granular-shape austenite with a low C/Mn content preferentially transforms into martensite ahead of the blocky-shape and lath-like austenite with high C/Mn content, and the multi-type microstructure of austenite with various stability lead to a continuous TRIP effect in a large strain region, which is responsible for the excellent properties of the heterogeneous medium Mn steel. In addition, austenite grain boundaries or austenite/ferrite interfaces are the preferred nucleation zone of martensite during deformation. The effect of Mn/C content on austenite stability readily overrides those of grain size.
一种新型高强塑积异质冷轧中锰钢的力学性能
[J].使用原位电子背散射衍射(EBSD)和球差透射电镜(ACTEM)等手段,研究了新型异质结构中锰TRIP钢在拉伸过程中微观组织的演变机制和力学性能。结果表明,在680℃退火后的实验钢中生成了多形貌、多尺度的异质奥氏体结构(颗粒状、块状、片层状奥氏体)和铁素体组织,其抗拉强度为1272 MPa,总延伸率为54.5%,强塑积高达69.3 GPa·%。在拉伸过程中C/Mn含量较低的颗粒状奥氏体先发生相变,而C/Mn含量较高的块状和片层状奥氏体在较大的应变范围内逐渐发生相变,从而导致高强度与高塑性的良好匹配。结果还表明,马氏体相变优先在奥氏体晶界/相界附近的区域形核。与晶粒尺寸相比,C/Mn元素对奥氏体稳定性的作用更重要。
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[J].
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