汤忖江
, 关海龙
TANG Cunjiang, GUAN Hailong
中图分类号: TG142.1
文章编号: 1005-3093(2016)06-0409-09
通讯作者:
收稿日期: 2015-12-2
网络出版日期: 2016-06-25
版权声明: 2016 《材料研究学报》编辑部 《材料研究学报》编辑部
基金资助:
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摘要
采用TMCP工艺获得了5种不同贝氏体体积分数的铁素体/贝氏体(F/B)多相钢。通过纵向力学性能分析, 并结合修正C-J分析方法研究了以F/B多相组织为特征的大变形管线钢的应变硬化行为, 获得了F/B多相钢中贝氏体体积分数与应力比、屈强比的关系, 并通过修正C-J分析对此现象的机理进行了合理的阐释。结果表明, 大变形管线钢的弹性形变阶段主要对应修正C-J分析中的第I阶段, 塑性形变阶段包括修正C-J分析第II和第III阶段, 屈服点(应变0.5%)附近阶段可跨越第I和第II阶段。F/B多相钢中各阶段的应变硬化能力存在显著差异, 并且其应变硬化行为呈现出与贝氏体体积分数相关的特性。通过适宜的组织调控可以实现管线钢强度和塑性的最佳匹配。应力比Rt1.5/Rt0.5适宜用于表征管材屈服点附近的应变硬化能力, 应力比Rt2/Rt1、Rt5/Rt1均适宜用于表征X70级管线钢塑性阶段应变硬化能力, Rt2/Rt1较适宜用于表征X80级管线钢塑性阶段应变硬化能力。
关键词：
Abstract
Five ferrite/bainite (F/B) multi-phase steels with different volume fractions of bainite were obtained by TMCP process. The strain hardening behavior of high deformability pipeline steel with F/B multi-phase was studied by the analysis of longitudinal mechanical properties and modified C-J analysis. The relationships between volume fraction of bainite and stress ratio as well as yield ratio were analyzed, and relevant mechanisms were illustrated by modified C-J analysis. The results show that the stage of elastic deformation of high deformability pipeline steel mainly corresponds to stage I in modified C-J analysis, and the stage of plastic deformation consists of stage II and stage III; and the stage near yield point (0.5% strain) can go across stage I and stage II. However, the strain hardening capability of each stage is obviously different from each other, and the strain hardening behavior is closely related to the volume fraction of bainite in F/B multi-phase steel. The optimal matching between strength and plasticity of pipeline steel can be achieved by controlling the microstructure suitably. The stress ratio of Rt1.5/Rt0.5 is appropriate to describe the strain hardening capability near the yield point, and the stress ratios of Rt2/Rt1 and Rt5/Rt1 are appropriate to represent the strain hardening capability of plastic deformation stage in X70 grade pipeline steel. The stress ratio of Rt2/Rt1 is suitable to characterize the strain hardening capability of plastic deformation stage in X80 grade pipeline steel.
Keywords：
管道输送是石油天然气最经济、高效、安全和环保的运输方式[1]。随着石油和天然气需求上升, 油气铺设管线已经延伸到极地、海洋和地质不稳定等环境恶劣的地区[2]。然而, 管道通过冻土层、海底、地震地带以及塌陷和滑坡等地区时常常要承受一定的塑性变形, 从而发生扭曲、屈折、断裂等破坏, 引发失效事故, 这就要求管道钢管应具有足够高的抗变形能力[3, 4]。因此, 开发能承受大的变形而不发生失效的大变形管线钢(high deformability pipeline steel)成为高性能管线钢的一个重要发展方向。
管材的变形能力或形变容量通常采用应力比、屈强比、均匀伸长率和应变硬化指数(strain/work hardening exponent)等描述。在工程中通常用应力比控制管材的应变硬化能力, 常用的表达形式有Rt1.5/Rt0.5, Rt2/Rt1和Rt5/Rt1等, 其中Rt1-Rt5分别表示应变1%-5%时的应力。屈强比, 即Rt0.5/Rm, 可以用于表征管材开始塑性变形到最后断裂前的形变容量[5]。管材塑性形变阶段对进一步形变的抵抗能力可以用应变硬化指数表征[6], 提高管材的应变硬化指数是提高其变形能力的有效途径[7]。对于大变形管线钢而言, 在高强韧性基础上, 较低的屈强比、较高的应力比、均匀伸长率和应变硬化指数是体现其优异形变性能的重要指标[8]。
大变形管线钢组织通常为双相或多相[5, 9], 其中, 铁素体/贝氏体(Ferrite/Bainite, F/B)复相、贝氏体/MA(martensite-austenite)等组织已成为大变形管线钢的主要组织调控手段[7, 10]。大变形管线钢的形变能力与其应变硬化行为相关, 但究其根本则取决于多相组织的微观力学行为, 而组织类型的多样化使得多相钢的形变机理变得异常复杂。目前对大变形管线钢组织的形变机制[11-13]、应变硬化行为及其硬化能力已有研究[7, 13, 14], 但对大变形管线钢的应变硬化行为、硬化能力与多相组织之间的关系研究较少。此外, 在表征管材应变硬化能力的各参数的研究中, 例如应变硬化指数[12, 14, 15]和屈强比[4, 16, 17]等有研究, 然而对应力比的相关研究却鲜见报道[18]。
本文通过热机械处理工艺(thermo mechanical controlled processing, TMCP)获得不同贝氏体体积分数的F/B多相钢, 对以F/B多相组织为特征的大变形管线钢应变硬化行为进行研究, 着重分析和阐释贝氏体分数与应力比、屈强比和形变机制之间的关系。
为获得不同贝氏体体积分数的F/B多相钢, 采用如表1所示化学成分的实验钢。通过热膨胀法测得Ar3为678℃, Ar1为575℃。采用如图1所示的TMCP工艺获得5种不同贝氏体体积分数的实验钢。实验钢在实验室轧机上进行轧制, No.1-No.5号实验钢坯厚度为42 mm。钢坯加热至1200℃保温60 min, 按照二阶段轧制工艺, 分别在再结晶区(2道次)和非再结晶区(3道次)轧制成厚度6 mm的钢板。再结晶区、非再结晶区总压下率分别为62-64%、60-63%。No.5号实验钢轧后空冷至Ar3以上温度水淬; No.4、No.3、No.2号实验钢轧后空冷至Ar3-Ar1之间不同温度水淬, 开冷温度依次由高到低; No.1号实验钢轧后空冷至室温。沿轧板纵向(轧向)切取金相和拉伸试样, 分别用于观察实验钢微观组织和测量其力学性能及应力-应变曲线, 拉伸应变速率低于5×10-3 s-1。金相试样表面经机械研磨、抛光, 用4%硝酸酒精溶液侵蚀约5-8 s, 采用BX51M型光学显微镜观察轧板纵向截面组织形貌。采用定量金相法测定实验钢中贝氏体体积分数。
表1 实验钢化学成分(质量分数,%)
Table 1 Chemical composition of experimental steels (mass fraction,%)
| C | Si | Mn | Ti | Nb | Ni+Cr+Cu |
|---|---|---|---|---|---|
| 0.04 | 0.22 | 1.75 | 0.015 | 0.095 | 0.5 |
目前, 对双相钢或多相钢应变硬化行为分析常见的分析方法有Hollomon分析(Hollomon analysis)、C-J分析(differential Crussard-Jaoul analysis)、修正C-J分析(modified Crussard-Jaoul analysis)和瞬时应变硬化指数n'(n'=d(lnσ)/d(lnε))等[19, 20]。其中, 修正C-J分析对组织的弹塑性变化敏感[21], 适用于对F/B多相钢组织的形变与应变硬化能力的分析。修正C-J分析基于式(1), 变换后得式(2), 对lnσ-ln(dσ/dεp)曲线进行分析[22]。
式中, εp为塑性真应变, ε0为初始真应变, σ为真应力, c为材料常数,
图2为通过TMCP工艺获得实验钢的微观组织。可以看出, No.1号实验钢组织类型为单一针状铁素体组织, 其中弥散分布着粒状MA; No.2-No.4号实验钢为针状铁素体和板条贝氏体多相组织, 含有极少量先共析铁素体; No.5号实验钢为单一板条贝氏体组织。
图2 TMCP工艺获得实验钢的微观组织
Fig.2 Microstructures of experimental steels obtained by TMCP process (a) No.1, (b) No.2, (c) No.3, (d) No.4, (e) No.5. Note: AF-acicular ferrite, PF-proeutectoid ferrite, B-bainite
实验钢中贝氏体体积分数和纵向力学性能如表2所示。可以看出, 随着实验钢中贝氏体体积分数增加, 屈服强度和抗拉强度升高, 均匀伸长率和总伸长率降低。图3为实验钢纵向拉伸工程应力-应变曲线, 各曲线均呈圆屋顶型, 具有该特征应力-应变曲线的管线钢其形变能力优于具有屈服平台应力-应变曲线的管线钢[7]。
表2 实验钢中贝氏体体积分数和纵向力学性能
Table 2 Volume fraction of bainite and longitudinal mechanical properties of experimental steels
| Experimental steel | Volume fraction of Bainite/% | Engineering value | ||||
|---|---|---|---|---|---|---|
| Rt0.5/MPa | Rm/MPa | UEL/% | Agt/% | TEL/% | ||
| No.1 | 0 | 513 | 668 | 11.8 | 12.2 | - |
| No.2 | 27.9 | 520 | 774 | 10.8 | 11.5 | 31.7 |
| No.3 | 47.0 | 560 | 848 | 9.0 | 9.6 | 28.9 |
| No.4 | 65.9 | 638 | 914 | 6.1 | 6.7 | 25.7 |
| No.5 | 100 | 914 | 1031 | 1.4 | 1.6 | - |
图4为实验钢中贝氏体体积分数对应力比和屈强比的影响, 图5为实验钢中贝氏体体积分数对应力的影响。由图4可见, 不同贝氏体体积分数的实验钢的应力比曲线存在明显差异: 随着贝氏体体积分数增大, 应力比Rt1.5/Rt0.5呈现先增大后减小的趋势。当贝氏体体积分数约为60%时达到最大值; 应力比Rt2/Rt1、Rt5/Rt1在贝氏体体积分数低于30%时变化不明显, 当贝氏体体积分数超过30%后, 这两个比值均呈现明显下降的趋势, 其中Rt5/Rt1下降幅度高于Rt2/Rt1; 屈强比随着贝氏体体积分数增大明显呈先降低后升高的趋势, 当贝氏体体积分数约为50%时出现最小值。随着实验钢中贝氏体体积分数提高, Rt1.5/Rt0.5和屈强比曲线分别出现最大值和最小值, 由于这两个比值中均含有Rt0.5项, 此现象极有可能与实验钢的屈服点(应变0.5%)附近形变行为有关。
图4 实验钢中贝氏体体积分数对应力比和屈强比的影响
Fig.4 Effect of volume fraction of bainite on stress ratio and yield ratio of experimental steels
图5 实验钢中贝氏体体积分数对工程应力的影响
Fig.5 Effect of volume fraction of bainite on engineering stress of experimental steels
图5为实验钢中贝氏体体积分数对工程应力的影响。可以看出, 随着实验钢中贝氏体体积分数增加, 应力Rt0.5、Rt1、Rt1.5、Rt2、Rt5和Rm均增大, 而Rt0.5增大幅度在贝氏体体积分数为30%-80%之间减缓。Rt5在贝氏体体积分数超过70%后呈现明显下降趋势, 一方面是由于管材的塑性恶化至一定程度, 如最大力伸长率(elongation at maximum force, Agt)低于5%, 较早出现颈缩现象导致管材流变应力降低, 因此, 应力比Rt5/Rt1可能低于Rt2/Rt1。
图6为实验钢的修正C-J分析曲线, 各阶段分段如图中虚线所示。由图6可以看出, 修正C-J分析曲线中, 第I阶段转变至第II阶段, 第II阶段转变至第III阶段通常呈现平缓过渡, 为渐变的过程。No.1、No.2和No.3号实验钢均呈现三阶段硬化行为, 但No.3号实验钢的三阶段硬化特点不明显, 呈现出由三阶段向二阶段过渡的特点, No.4和No.5号实验钢呈现出二阶段硬化行为。由此可知, F/B多相钢的应变硬化行为呈现与贝氏体体积分数相关的特性, 即随着贝氏体体积分数增加, 其应变硬化行为呈现由三阶段硬化行为向二阶段硬化行为转变。在修正C-J分析中, 双相钢/多相钢硬化行为通常不会超过三阶段[19, 24]。对各阶段的机理阐述存在一些差异, 但总的来说均是从组织形变和位错演化角度进行阐述。从组织形变演化角度而言, 第I阶段主要与软相(铁素体)形变相关, 第II阶段主要与软相和硬相(贝氏体或马氏体)的一致性形变(协调或者非协调形变)相关[19, 25], 第III阶段应与动态回复相关[15, 24, 26]。从位错演化方式而言, 由于软相强度低从而易形变(实验中观测到软相先形变[27]), 第I阶段主要是位错在软相中的滑移、排列和缠结[13, 15, 26]; 随着载荷提高, 软相与硬相发生一致性形变[19, 25], 硬相中位错和滑移系开动[11], 此为第II阶段; 在第III阶段中, 由于已形成的位错结构难以显著提高材料流变应力, 导致材料的应变硬化能力将下降[24]。
通过对实验钢修正C-J分析曲线中的各阶段进行线性拟合可得其斜率1-
表3 实验钢的修正C-J分析中各阶段应变硬化能力和转折应变(εt)
Table 3 The strain hardening capability of each stages and transition strain (εt) of experimental steel in modified C-J analysis
| Experimental Steel | Stage I | Stage II | Stage III | Engineering strain/% | |||||
|---|---|---|---|---|---|---|---|---|---|
| 1- | 1/ | 1- | 1/ | 1- | 1/ | Transition strain | Elongation at maximum force (Agt) | ||
| εt1 (stage I-II) | εt2 (stage II-III) | ||||||||
| No.1 | -14.8 | 0.06 | -4.6 | 0.18 | -13.4 | 0.07 | 0.6 | 3.3 | 12.2 |
| No.2 | -6.5 | 0.13 | -6.9 | 0.13 | -9.3 | 0.10 | 0.8 | 4.6 | 11.5 |
| No.3 | -4.9 | 0.17 | -9.6 | 0.09 | -11.3 | 0.08 | 0.8 | 3.1 | 9.6 |
| No.4 | -4.0 | 0.20 | -12.5 | 0.07 | 0.7 | 6.7 | |||
| No.5 | -3.3 | 0.23 | -24.3 | 0.04 | 0.4 | 1.6 | |||
从表3可以看出, εt1值变化范围0.4%-0.8%; εt2值变化范围3.1%-4.6%。由此可知, 大变形管线钢的弹性形变阶段主要对应修正C-J分析第I阶段, 塑性形变阶段包括修正C-J分析第II和第III阶段, 屈服点(应变0.5%)附近阶段可跨越第I和第II阶段。通常情况下, 转折应变随着F/B多相钢中硬相体积分数增加有降低趋势[28], 即第I阶段转变至第II阶段, 第II阶段转变至第III阶段均会提前。从表3可以看出, No.2-No.5号实验钢的εt1、εt2均呈现降低趋势, 而No.1与No.2-No.5号实验钢的εt1、εt2变化趋势不一致。由此表明, 实验钢中贝氏体体积分数超过30%后, 转折应变εt1、εt2均随贝氏体体积分数增大而减小, εt1减小说明贝氏体较早产生弹塑性形变。然而No.1号实验钢的εt1、εt2变化规律呈现异常现象, 应主要是由于单一组织铁素体钢在形变、位错和亚结构演化机制等方面与F/B多相钢存在一定差异所致[26, 29, 30]。
图7显示了转折应变εt1、εt2在实验钢应力-应变曲线上的位置(图中实线和虚线分别为转折应变的实测值和拟合直线), 0.5%、1%、1.5%、2%、5%应变分别用竖直线表示。应力比Rt1.5/Rt0.5和屈强比两个比值在实验钢中贝氏体体积分数约为0-90%时跨越了修正C-J分析的第I和第II阶段, 只有No.5号实验钢基本位于第II阶段。由于第I和第II阶段应变硬化能力随实验钢中贝氏体体积分数增大呈现相反的变化趋势, 变化幅度也较大, 因此导致随贝氏体体积分数增大, 应力比Rt1.5/Rt0.5和屈强比分别出现最大值和最小值。与应力比Rt1.5/Rt0.5不同的是: 实验钢中贝氏体体积分数为0-50%时, 屈强比还跨越了修正C-J分析的第III阶段。但第III阶段的应变硬化能力随着实验钢中贝氏体体积分数增加变化不明显, 因此, 第III阶段应变硬化行为不会对屈强比和应力比规律产生显著影响。应力比Rt2/Rt1完全位于第II阶段。当修正C-J分析呈现二阶段硬化特点时, 应力比Rt5/Rt1位于第II阶段; 当修正C-J分析呈现三阶段硬化特点时, Rt5/Rt1大部分位于第II阶段, 小部分位于第III阶段, 如图7所示。随着实验钢中贝氏体体积分数提高第II阶段应变硬化能力降低, 但第III阶段不会对应力比规律产生显著影响, 因此, 导致Rt2/Rt1和Rt5/Rt1随着实验钢中贝氏体体积分数增加而减小。由此可知, 通过C-J分析能够合理地阐释了贝氏体体积分数变化对应力比和屈强比的影响。
图7 修正C-J分析中转折应变(εt1、εt2)在实验钢工程应力-应变曲线的位置
Fig.7 Transition strain (εt1, εt2) of modified C-J analysis in engineering stress-strain curves of experimental steels
从图7中可以看出, 应力比Rt1.5/Rt0.5适宜用于表征管材屈服点附近的应变硬化能力, Rt2/Rt1和Rt5/Rt1可用于表征管材塑性阶段的应变硬化能力。Rt1.5/Rt0.5与Rt2/Rt1、Rt5/Rt1所表述的应变硬化含义有较大差异: 管材屈服点附近和塑性形变阶段应变硬化行为机制存在较大差异, 并且硬化能力差异明显, 因此, Rt1.5/Rt0.5与Rt2/Rt1、Rt5/Rt1不宜混淆使用。同时, 若管材最大力伸长率(Agt)分别低于1.5%、2%、5%, 则Rt1.5/Rt1、Rt2/Rt1、Rt5/Rt1值将包含过颈缩点后出现损伤的一部分应变硬化能力, 这是一个值得注意的方面。
图8为应力比Rt1.5/Rt0.5、Rt2/Rt1、Rt5/Rt1与屈服强度的关系, 竖直虚线区间为管线钢强度级别所规定的屈服强度范围(API SPEC 5L (2012))[31]。直线、虚线分别为Rt1.5/Rt0.5、Rt2/Rt1、Rt5/Rt1在X70、X80强度级别范围内拟合直线, 曲线为拟合曲线。可以看出, 随着实验钢屈服强度提高, Rt1.5/Rt0.5呈现先增大后减小的趋势, Rt1.5/Rt0.5最大值出现在X80强度级别范围内; 在X70-X80强度级别范围内, Rt5/Rt1值高于Rt2/Rt1, Rt5/Rt1随着管材屈服强度提高有降低趋势, 而Rt2/Rt1变化不大。通常, 实验钢屈服强度随着贝氏体体积分数增大而升高, 因此, Rt1.5/Rt0.5和Rt5/Rt1有如图4所示类似的变化规律, 但在X70-X80强度级别范围内Rt2/Rt1下降趋势不明显。
图8 在X70-X80强度级别范围内实验钢的屈服强度(Rt0.5)与应力比的关系
Fig.8 The relationship between yield stress (Rt0.5) and stress ratio of the experimental steels in the region of X70-X80 grade
表4为应力比Rt1.5/Rt0.5、Rt2/Rt1和Rt5/Rt1在X70、X80强度级别范围内通过线性拟合所得的斜率绝对值以及基于应力比样本估计的标准偏差, 简称应力比标准差。在各强度区间上应力比拟合直线斜率可以表示其值的稳定性, 应力比标准差可以观察其值的离散程度[32]。从表4中可以看出, 在X70-X80强度级别范围内, Rt1.5/Rt0.5的斜率和标准差较大, 分别为0.0006-0.0025、0.053-0.107; Rt2/Rt1和Rt5/Rt1的斜率和标准差较小, Rt2/Rt1的斜率和标准差分别为0.0002-0.0003、0.011-0.014; Rt5/Rt1的斜率和标准差分别为0.0003-0.0006、0.016-0.026。通过对比Rt2/Rt1和Rt5/Rt1的斜率及标准差可以发现, Rt2/Rt1斜率和标准差均比Rt5/Rt1小, 表明Rt2/Rt1值较稳定。由于实验钢屈服点附近阶段可跨越修正C-J分析中的第I和第II阶段。通常情况下, 随着管材中组织体积分数变化, 第I和第II阶段应变硬化能力变化较大, 导致管材屈服点附近的应变硬化能力波动较大。Rt1.5/Rt0.5表征了管材屈服点附近的应变硬化行为, 因此导致Rt1.5/Rt0.5波动较大。Rt2/Rt1和Rt5/Rt1表征管材塑性阶段的应变硬化行为。修正C-J分析呈现三阶段硬化特点时, Rt5/Rt1大部分位于第II阶段, 小部分位于第III阶段, 第III阶段是动态回复阶段, 这是导致应力比Rt5/Rt1降低幅度稍大于应力比Rt2/Rt1的原因之一。另一方面, Rt5/Rt1大于Rt2/Rt1是由于管材持续应变硬化所致。
表4 X70-X80强度级别范围内实验钢的应力比线性拟合斜率绝对值和标准差
Table 4 The absolute value of the linear fitted slope of stress ratio and standard deviation of experimental steels in the region of X70-X80 grade
| The grade of experimental steels (interval of yield stress/MPa) | Slope of linear fitting (absolute value) | Standard deviation of stress ratio (sample estimation) | ||||
|---|---|---|---|---|---|---|
| Rt1.5/Rt0.5 | Rt2/Rt1 | Rt5/Rt1 | Rt1.5/Rt0.5 | Rt2/Rt1 | Rt5/Rt1 | |
| X70 (485-635) | 0.0025 | 0.0002 | 0.0003 | 0.107 | 0.014 | 0.016 |
| X80 (555-705) | 0.0006 | 0.0003 | 0.0006 | 0.053 | 0.011 | 0.026 |
对于X70级实验钢而言, Rt2/Rt1和Rt5/Rt1的斜率分别为0.0002、0.0003, 其标准差分别为0.014、0.016。由于Rt2/Rt1和Rt5/Rt1值变化均不大, 并且比较稳定, 较适宜用于表征X70级管线钢塑性阶段应变硬化能力。对于X80级实验钢而言, Rt2/Rt1和Rt5/Rt1的斜率分别为0.0003、0.0006, 标准差分别为0.011、0.026, Rt5/Rt1值变化稍大于Rt2/Rt1, Rt2/Rt1值变化不大, 并且比较稳定, Rt2/Rt1较适宜用于表征X80级管线钢塑性阶段应变硬化能力, 如表4所示。
标准中对X70-X80级大变形管线的应力比作出规定: 应力比Rt1.5/Rt0.5、Rt2/Rt1、Rt5/Rt1分别不应低于1.07-1.10、1.01-1.04、1.08[33-35]。目前已有研究结果表明[36], 为了使钢管的屈曲应变满足至少为1.5%的水平, 管体材料纵向拉伸试验的Rt2/Rt1和Rt5/Rt1分别需要超过1.04 和1.08。图9为X70-X80强度级别范围内实验钢的应力比与标准规定应力比的关系。图9中的水平直线为Rt1.5/Rt0.5、Rt2/Rt1和Rt5/Rt1值, 分别为1.10、1.04和1.08。可以看出, X70-X80级实验钢容易满足Rt1.5/Rt0.5、Rt2/Rt1和Rt5/Rt1的需求, 此强度级别大变形管线钢可以采用F/B多相组织设计。并且, 通过适宜的组织调控可以实现管线钢强度和塑性的最佳匹配。
图9 X70-X80强度级别范围内实验钢的应力比与标准规定应力比的关系
Fig.9 The relationship between the stress ratio of the experimental steels in the region of X70-X80 grade and the specified values of stress ratio in the standard
1. 大变形管线钢的弹性形变阶段主要对应修正C-J分析中的第I阶段, 塑性形变阶段包括修正C-J分析第II和第III阶段, 屈服点(应变0.5%)附近阶段可跨越第I和第II阶段。F/B多相钢的各阶段的应变硬化能力存在差异, 随着F/B多相钢中贝氏体体积分数增加, 第I阶段应变硬化能力升高, 第II阶段应变硬化能力降低, 第III阶段应变硬化能力变化不大。其应变硬化行为呈现出与贝氏体体积分数相关的特性, 通过适宜的组织调控可以实现管材强度和塑性的最佳匹配。
2. 应力比Rt1.5/Rt0.5和屈强比(Rt0.5/Rm)在形变过程中跨越了修正C-J分析第I、II阶段, 应力比Rt2/Rt1全部位于第II阶段, 应力比Rt5/Rt1中大部分或全部位于第II阶段。第I和第II阶段应变硬化能力随F/B多相钢中贝氏体体积分数增加呈现相反的变化趋势, 导致Rt1.5/Rt0.5在贝氏体体积分数约60%时出现最大值, 屈强比在贝氏体体积分数约50%时出现最小值, Rt2/Rt1和Rt5/Rt1随着贝氏体体积分数增大而减小。屈强比、Rt5/Rt1可能有一部分位于第III阶段, 但第III阶段应变硬化能力随F/B多相钢中贝氏体体积分数变化不明显, 此阶段应变硬化行为不会对屈强比和应力比的规律产生显著影响。
3. 应力比Rt1.5/Rt0.5适宜用于表征管材屈服点附近的应变硬化能力, 应力比Rt2/Rt1和Rt5/Rt1可用于表征管材塑性阶段的应变硬化能力。其中, Rt2/Rt1和Rt5/Rt1较适宜用作X70级管线钢塑性阶段应变硬化能力表征参数; Rt2/Rt1较适宜用作X80级管线钢塑性阶段应变硬化能力表征参数。X70-X80级大变形管线钢可以采用F/B多相组织设计。
The authors have declared that no competing interests exist.
| [1] |
Development of high strength material and pipe production technology for grade X120 line pipe , in:
The increasing demand for natural gas will further influence the type of its transportation in the future, both from the strategic and economic point of view. Long-distance pipelines are a safe and economic means to transport the gas from production sites to end users. High-strength steels in grade X80 are nowadays state of the art. Grade X100 was recently developed but not yet utilised. The present-day technical limitations on the production of X120 line pipe namely the steel composition, the pipe forming and the welding are addressed in this paper. Production test results on X120 pipes are presented to describe the materials properties. A low carbon and low PCM steel with VNbTiB microalloying concept is used. In the plate rolling the main attention is turned to the heavy accelerated cooling. The large spring back that occurs during the U-forming step of the UOE process is one of the most complex aspects in forming X120. To handle this aspect FEM calculations were used to modify the forming parameters and to optimise the shape of the U-press tool. For optimising the existing welding procedure with respect to an avoidance of HAZ softening, a low heat input welding technology and new welding consumables were developed.
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| [2] |
The challenges for pipeline projects and development trend of pipeline steel ,管道工程面临的挑战与管线钢的发展趋势 ,
现代油气管线正面临着高压输送以及低温、大位移、深海、酸性介质等恶劣环境的挑战。为适应管 线工程的经济性和安全性的要求,管线钢的发展趋势是高强度、高韧性、大变形性、厚壁、高抗腐蚀性和在恶劣环境下的焊接性。通过合金设计中的微合金化和多元 合金化、控制轧制和控制冷却工艺提高管线钢的强度与韧性,满足长距离管线输送能力的经济性要求和应对恶劣环境的安全性要求。
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| [3] |
Strain-based design for Pipeline and development of pipe steels with high deformation resistance ,油气管道基于应变的设计及抗大变形管线钢的开发与应用 ,
地震和地质灾害是引发油气输送 管道失效的主要原因之一。基于应变的设计是预防这类失效的有效手段。在运用设计措施解决管道承受大位移、大应变问题的同时,选择合适的管线钢及钢管,也可 使管道具有抗大应变的能力。抗大变形管线钢应具有的主要特性是:应力应变曲线为Round House型、屈强比较低、形变强化指数高、均匀塑性变形延伸率高,其组织包含有硬相和软相的双相或多相。管道环焊缝接头应为高匹配(焊缝及热影响区的强 度高于母材)。
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| [4] |
Research and development of large deformability pipeline steels ,大变形管线钢的研究和开发 ,
为了适应管道的大位移环境和高 强度管线钢发展的需要,研制和开发了大变形管线钢。研究结果表明,大变形管线钢的主要性能特征是在保证高强韧性的同时,要求具有低的屈强比、较高的均匀伸 长率和形变强化指数;大变形管线钢的组织特征为B+F和B+M/A双相组织。通过适度加速冷却方法、双相区加速冷却方法和延迟加速冷却方法,可以获取 B+F大变形管线钢;通过在线配分技术,可以获取B+M/A大变形管线钢。
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| [5] |
Significant technical progress in the West-East Gas Pipeline projects-Line One and Line Two ,西气东输一, 二线管道工程的几项重大技术进步 ,
<p>提高管线钢强度级别和管道输送压力是天然气输送管道的发展趋势，是输气管道技术进步的重要标志。近年来，我国管道企业和相关科研院所联合攻关，取得了一批关键技术成果，使西气东输（为便于区别，以下称为西气东输一线）和西气东输二线等国家重点管道工程的设计压力和钢管强度级别达到或领先于同时期的国际水平。这些成果包括：①研制了X70、X80钢级高性能管线钢及焊管、管件；②突破国际上螺旋缝埋弧焊管的使用禁区，确立了具有中国特色的“大口径高压输送主干线螺旋缝埋弧焊管与直缝埋弧焊管联合使用”的技术路线；③在国内首次研究了高压输气管道动态断裂与止裂问题，分别采用Battelle简化公式和Battelle双曲线法预测了西气东输一线和西气东输二线等管道延性断裂的止裂韧性；④在国内首次研究了油气管道基于应变的设计方法，解决了该设计方法及抗大变形管线钢管在强震区和活动断层管段应用的技术难题；⑤研究解决了高强度焊管的腐蚀控制和应变时效控制技术等。上述成果对降低输气管道建设成本、保障管道运行安全具有重要意义。</p>
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| [6] |
Key issues in the specification of high strain line pipe used in strain-based designed districts of the 2nd west to east pipeline , in: |
| [7] |
Design concept and production of high deformability linepipe , in:
Extensive studies to develop high deformability linepipe have been conducted. In the case of linepipes laid in seismic region or permafrost field, higher resistance to buckling against large strain induced by ground movement is required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, based on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine MA constituents was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure was evaluated by axial compression and bending tests, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Plate manufacturing technologies for producing recent high strength linepipe steel and the concept for microstructure control for improving deformability were also introduced in this paper.
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| [8] |
Deformation and fracture behavior of X80 pipeline steel with excellent deformability ,X80大变形管线钢的变形与断裂行为 ,
采用光学和扫描电子显微镜过对X80大变形管线钢在拉伸和冲击载荷下的变形和断裂过程的实验观察，研究了X80大变形管线钢的变形和断裂行为。结果表明：对于由贝氏体＋铁素体组成的双相大变形管线钢，在变形过程中，塑性变形优先在铁索体中进行，随着塑性变形量的增加，双相组织的形态逐渐沿作用力方向呈现明显的位向分布。双相组织中裂纹的产生有3种典型方式，即夹杂物形核、相界面形核以及铁素体或贝氏体基体形核。裂纹的扩展与所受的应力状态有关，在较低应力状态下时，裂纹主要通过铁素体扩展；只有在较高应力状态下，裂纹才穿越贝氏体。在实验观察和分析基础上建立了贝氏体＋铁素体大变形管线钢的变形和断裂模型。
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| [9] |
The Development of Third-generation Pipeline Steel in China , in: |
| [10] |
Control of microstructures of ferrite/bainite (F/B) dual-phase steels and analysis of their resistance to deformation behavior ,铁素体/贝氏体 (F/B) 双相钢组织调控及其抗变形行为分析 ,
本文针对低C, 高Mn和高Nb化学成分, 采用轧后弛豫控制相变的组织调控技术得到5种不同铁素体/贝氏体(F/B)体积含量的双相组织</br>钢. 用改进的C-J分析方法分析了软相(铁素体)含量, 研究了晶粒尺寸对加工硬化性能的影响, 以及以铁素体和贝氏体为主的软硬相</br>混合组织的塑性变形协调关系. 并用电子背散射(EBSD)技术验证了双相组织在均匀塑性变形阶段的协调变形行为. 结果表明, 软硬相</br>的合理比例有利于提高加工硬化程度(高<em>R</em><sub>t1.5</sub>/<em>R</em><sub>t0.5</sub>), 降低屈强比, 同时能保证较高的均匀变形能力. 铁素体与贝氏体</br>之间的协调变形是提高双相组织钢应力比和均匀伸长率的主要机制.
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| [11] |
Effects of cooling process after rolling on microstructure and yield ratio of high-strain pipeline steel X80 ,轧后冷却制度对 X80 级抗大变形管线钢组织和屈强比的影响 ,
<p>利用SEM和TEM原位拉伸方法研究了轧后冷却制度对X80级抗大变形管线钢组织的影响及低屈强比的微观机理. 结果表明: 采用轧后弛豫+控制冷却的工艺可以获得铁素体 +贝氏体双相组织, 弛豫终止温度是影响铁素体体积含量和晶粒大小的决定因素. 当弛豫终止温度区间为690-705 ℃时, 试样的强度和塑性达到了较好的匹配, 满足X80级抗大变形管线钢的性能要求. 弛豫终止温度越低, 铁素体体积含量越高, 晶粒尺寸越大, 屈强比越低. 对拉伸过程进行动态原位观察的结果表明, 铁素体(软相)和贝氏体(硬相)的协调变形机制是屈强比降低的原因.</p>
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| [12] |
Effect of volume fraction of bainite on microstructure and mechanical properties of X80 pipeline steel with excellent deformability , |
| [13] |
Influence of dual-phase microstructures on the properties of high strength grade line pipes ,
The influence of dual-phase microstructures on mechanical properties of X70, X80, and X90 line pipes is investigated. It is found that the line pipes with dual-phase microstructures possess both larger uniform elongation and higher hardening exponent, especially for high grade steel X90. The tensile deformation of dual-phase line pipe does not follow the trend predicted by the Hollomon formula, and a stable strain-hardening exponent is not found. This stress-strain behavior is different from the normal line pipe. In the initial stage of plastic deformation, the strain-hardening capacity of dual-phase line pipe increases rapidly. However, it reaches a stable stage after 2.0% total strain. The dual-phase pipeline steel is composed of soft phase (polygonal ferrite) and hard phase (bainite), and thus the relatively soft ferrite is good for its deformability. Besides, the fraction of large angle grain boundaries in the dual-phase microstructures is greater than that of the normal line pipe, which is proven to be critical for improving the resistance to plastic deformation and crack propagation.
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| [14] |
Influence of bainite/martensite-content on the tensile properties of low carbon dual-phase steels ,
This investigation aims to examine structure–property relations of ferrite–bainite dual-phase (FBDP) steels and to compare these against that of ferrite–martensite dual-phase (FMDP) steels. For this purpose, a series of FBDP and FMDP steels containing wide variation (20–90%) of the harder constituents have been prepared from a low carbon Nb-micro-alloyed base material by suitable heat treatments. Hardness and tensile properties of the developed steels have been examined against the volume fraction of bainite or martensite. The nature of variation of the estimated mechanical properties such as hardness, yield and tensile strength, percentage elongation and strain-hardening exponent with the amount of the harder constituents of the FBDP and FMDP steels exhibits subtle to significant differences. These differences have been explained using the influence of the nature of the microstructural constituents and their mutual interactions. Low carbon FBDP steel with 60–70% bainite appears to possess excellent potential for structural applications.
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| [15] |
Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels ,
A series of dual-phase (DP) steels containing finely dispersed martensite with different volume fractions of martensite (V m) were produced by intermediate quenching of a boron- and va
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| [16] |
Analysis and commentary on yield ratio of pipeline steel ,管线钢屈强比分析与评述 ,
论述了管线钢屈强比的作用和要 求,分析了屈强比对钢管形变容量、缺陷容量及承载能力的影响。综合部分标准对管线钢屈强比的要求,指出屈强比超标或对屈强比的大小有疑虑时,应结合材料的 应力-应变曲线及其均匀伸长率、形变硬化指数和静力韧度等综合进行评价。并从强化机制、化学成分、控轧控冷工艺和显微组织等方面,分析了管线钢屈强比的控 制因素。
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| [17] |
Effect of microstructure on the yield ratio and low temperature toughness of linepipe steels ,
The present study aims at elucidating the effects of microstructural features on the yield ratio and toughness of high strength linepipe steels. The main emphasis has been placed on understanding the effects of constituents on the properties. Several alloy systems with different constituents, i.e. ferrite-pearlite steels, ferrite steels with acicular ferrite as second phase, acicular ferrite steels with ferrite as second phase, and bainite steels, have been investigated. Experimental results show that while the refinement of ferrite grain size improves both yield strength and low temperature toughness of ferrite-base steels, it increases the yield ratio. Modification of matrix from ferrite to acicular ferrite or bainite results in improvements in both yield strength and yield ratio. However, bainite steels have worse low temperature toughness (i.e., higher DBTT) than the other types of steels. It has been shown that the low temperature toughness of acicular ferrite steels can be improved by the introduction of polygonal ferrite as a second phase. This is mainly due to the refinement of effective grain size by the introduction of second phases. The relationship between the yield ratio and work hardening exponent has also been established using the Swift equation. Based on the results, the optimum microstructure for a better combination of strength, toughness and yield ratio is suggested to be the one having second phase of polygonal ferrite in an acicular ferrite or bainite matrix.
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| [18] |
Effects of thermo-mechanical control process on microstructure and mechanical properties of X80 grade high-strain line pipe steel ,控轧控冷工艺对 X80 级抗大变形管线钢组织与性能的影响 , |
| [19] |
Influence of martensite morphology on the work-hardening behavior of high strength ferrite-martensite dual-phase steel ,
<a name="Abs1"></a>This study concerns influence of martensite morphology on the work-hardening behavior of high-strength ferrite–martensite dual-phase (DP) steel. A low-carbon microalloyed steel was subjected to intermediate quenching (IQ), step quenching (SQ), and intercritical annealing (IA) to develop different martensite morphologies, i.e., fine and fibrous, blocky and banded, and island types, respectively. Analyses of work-hardening behavior of the DP microstructures by differential Crussard–Jaoul technique have demonstrated three stages of work-hardening for IQ and IA samples, whereas the SQ sample revealed only two stages. Similar analyses by modified Crussard–Jaoul technique showed only two stages of work-hardening for all the samples. Among different treatments, IQ route has yielded the best combination of strength and ductility due to its superior work-hardening behavior. The influence of martensite morphology on nucleation and growth of microvoids/microcracks has been correlated with the observed tensile ductility.
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| [20] |
The strain hardening behaviour of dual-phase steel ,
The strain hardening behaviour of a dual-phase steel was investigated using uniaxial tension load-elongation data, recorded by a computer in very small intervals. The double-logarithmic true stress-strain curve derived from the data is interpreted in terms of three separate stages of deformation behaviour. Within each stage the curve is approximated by a parabola and the differential (instantaneous) n values are derived analytically without making any prior assumptions about an arbitrary strain hardening law. It is shown that for each stage the n value is not constant and that n values changing linearly with log ϵ provide a better approximation to the experimental results.
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| [21] |
Theoretical model for the tensile work hardening behaviour of dual-phase steel ,
The modified Crussard-Jaoul analysis was employed to describe the work hardening behaviour (the ln(dσ/d03) vs. ln σ curves) of a 1020 dual-phase steel with quenching and quenching plus tempering treatments and with various volume fractions of martensite ( V m ), which demonsrated that this dual-phase steel exhibits two stages of work hardening in the range of plastic deformation. The modified law of mixture was used to simulate the tensile stress-strain and the ln(dσ/d03) vs. ln σ curves for the steel. The simulations were divided in different ways in terms of the deformation state of martensite. For the steel with quenching treatment and with V m < 50%, the work hardening behaviour in the first stage of deformation can be well simulated with a model in which martensite deforms elastically and ferrite deforms plastically. For the steel with quenching treatments and with V m > 50% and the steel with quenching plus tempering treatments and with V m over the whole test range (33–85%), the work hardening behaviour in the first stage of deformation was well simulated with the assumption that martensite deforms elastically and then deforms partly elastically and partly plastically while ferrite deforms plastically. For the steel in all the above-mentioned cases, the work hardening behaviour in the second stage of plastic deformation was simulated with a model in which both phases deform plastically.
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| [22] |
Concerning the analysis of tensile stress-strain data using log dσ/dεp versus log σ diagrams , |
| [23] |
Mechanical properties of 0.40 pct C-Ni-Cr-Mo high strength steel having a mixed structure of martensite and bainite , |
| [24] |
Tensile stress-strain analysis of dual-phase structures in an Mn-Cr-Si steel , |
| [25] |
Tensile stress-strain analysis of cold worked metals and steels and dual-phase steels ,
A study has been made of the applicability of the differential Crussard-Jaoul (C-J) analysis that assumes the Ludwik power relation, the modified C-J analysis based on the Swift formula, and the Hollomon analysis to uniaxially prestrained metals and steels and high strength, formable, dual-phase steels. The pure aluminum and copper metals and a series of plain carbon steels with carbon ranging from 0.10 to 1.05 pct were uniaxially prestrained by a given amount of strain under ambient temperature. A plain carbon steel with carbon of 0.10 pct was utilized in manufacturing the dual-phase steels. An empirical analysis exhibited the limited applicability of the C-J analysis for the interpretation of the stress-strain relationship of uniaxially prestrained metals and steels. The C-J analysis was also less sensitive to changes in the deformation behavior of the dual-phase steels in which the ferrite matrix and the shape and distribution of the second phase martensite were altered by three heat treatments. The modified C-J analysis was most suitable for describing work-hardening of uniaxially prestrained metals and steels. This analysis revealed that the dual-phase steels deformed in two stages. The first stage was associated with deformation of the ferrite matrix, and the second stage was associated with uniform straining of ferrite and martensite. The more generally used Hollomon curves deviated from linearity over all the uniform strain range regardless of the uniaxially prestrained metals and steels and dual-phase steels. Thus, the Hollomon parameters could not be assigned to an entire curve.
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| [26] |
Dislocation substructure as a function of strain in a dual-phase steel ,
Dislocation structures in the ferrite of a C-Mn-Si dual-phase steel intercritically annealed at 810°C were characterized at various tensile strains by transmission electron microscopy At strains which corresponded to the second stage on a Jaoul-Crussard plot of strain hardening behavior, the dislocation density in the ferrite is inhomogeneous, with a higher density near the martensite. The third stage on a Jaoul-Crussard plot corresponds to the presence of a well-developed dislocation cell structure in the ferrite. The average cell size during this stage is smaller than the minimum size reported for deformed iron, and the cell size was inhomogeneous, with a smaller cell size near the martensite.
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| [27] |
Microscopic deformation behaviour of martensitic-ferritic dual-phase steels ,
The deformation behaviour of the two phases of three plain carbon dual–phase steels after various treatments has been studied using a scanning electron microscope equipped with a tensile straining stage. The distribution of strains between the ferrite and martensite phases, as well as among the different grains of each phase, was observed to be inhomogeneous. The martensite/ferrite strain ratio, which defines the degree of uniformity of straining between the phases, depends on the microstructural parameters of the steels: it increases with increasing volume fraction of martensite, but decreases as the carbon content of the martensite increases. Tempering at various temperatures causes a decrease in the martensite/ferrite microhardness ratio and hence causes an increase in the strain ratio. The macroscopic strain of the specimen at which the martensite begins to deform was also found to be dependent on the microstructural parameters. Regions of applicability of the existing theories of the strength of dual–phase steels can be estimated according to the deformation condition of the martensite. MST/235
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| [28] |
Dependence of strain hardening exponent on the volume fraction and carbon content of martensite in dual phase steels during multistage work hardening , |
| [29] |
Effect of microstructure on strain-hardening behaviour of a Ti-IF steel grade ,
Thermomechanical processing parameters were adjusted during the processing of Ti-IF steel sheet to obtain microstructures with different grain sizes and precipitation states. The grain size and precipitation state were fully characterized for each specimen in order to investigate the effect of each on mechanical properties. Uniaxial tensile tests were performed at a strain rate of 2 · 10-3s-1 at room temperature. Relationships between strain-hardening coefficient, n, and mechanical properties were analysed. Differences in measured n-values between the different specimens are associated to a change in yield strength resulting from hardening effects of precipitates and grain size at the beginning of plastic deformation. The role of grain size and precipitation state on strain-hardening behaviour is discussed in terms of their effect on dislocation structure evolution. A strain transition exists where dislocation tangles evolve towards well-defined dislocation cells. It is shown in the present study that the entangled dislocation density is very sensitive to the microstructure for the Ti-IF steel studied while the dislocation cell size appears to be insensitive to the microstructure.
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| [30] |
Dynamic stress-strain behavior for acicular ferrite steel ,针状铁素体钢的动态应力-应变行为 ,
利用Hopkinson压杆技术对X70管线钢进行了冲击压缩实验,研究了在高应变率变形过程中钢的组织演变和动态应力--应变行为. 结果表明: 经过适当热处理后X70管线钢具有以针状铁素体为主的显微组织. 在103s-1应变率条件下, 该钢发生了明显的应变强化与应变率强化, 且最大应变也随应变率提高而增加; 在铁素体板条内形成的大量位错胞亚晶结构和铁素体组织的显著细化, 是该钢高应变率增强增塑的主要机制.
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| [31] |
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| [32] |
Standard deviation and standard error of arithmetic mean ,标准差与标准误 ,
对容易引起混淆的统计量"标准差"和"标准误"从意义、特征、计算公式、符号表示等方面作了准确描述与区分,并对统计学结果表示中"平均数±标准差""平均数±标准误"的符号表示进行了统计分析,指出了存在问题.通过原因分析,提出了避免二者混淆和不规范的符号表示的一些应对措施.
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| [33] |
Progress in research and application of pipeline steels with high deformation resistance in China ,国内抗大变形管线钢研究及应用进展 ,
随着国内长输油气管道的建设和发展,适用于地震断裂带等大应变地区的高强度抗大变形管线钢得到有效开发和应用。基于目前国内对抗大变形管线钢的研究成果,分析和总结了抗大变形管线钢的设计基础（拉伸应变、压缩应变）和典型特点（双相组织、低的屈强比、高的均匀延伸率和形变强化指数、典型的拱顶型应力-应变曲线）,从轧制工艺、化学成分、组织性能、焊接工艺、腐蚀性能等方面分别探讨了抗大变形管线钢的研究进展及其影响因素,概述了目前国内相关标准要求、钢厂和钢管厂加工制造水平以及抗大变形管线钢的研究应用现状,研究成果有利于抗大变形管线钢的应用和推广。
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| [34] |
Development of X70 SAWL pipe with large deformation resistance ,
X70 抗大变形直缝埋弧焊管的开发 ,
针对基于应变设计地区管道建设,尤其是中缅管线项目所需的X70 抗大变形焊管进行研究开发.采用铁素体+贝氏体双相组织的高应变能力的钢板,结合新开发的制管工艺,使批量研制的X70抗大变形焊管具有良好的强度、均匀 延伸率、应力比等优点.通过对制管前后的拉伸性能进行对比,发现大变形钢板制管后屈服强度、抗拉强度大幅提高,而均匀延伸率、应力比等指标有所下降；热时 效试验表明,时效处理后钢管纵向屈服强度、抗拉强度均升高,而均匀延伸率、应力比等指标进一步下降.
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| [35] |
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| [36] |
Analysis of local buckling strain of line pipe ,管线钢管局部屈曲应变分析与计算 ,
针对基于应变的管线设计所使用的管线钢管进行变形能力评价，为基 于应变设计地区用的钢管产品标准的指标制订提供技术基础。本文以精确建模的数值仿真计算作为钢管全尺寸弯曲变形试验的有效补充，对钢管进行了屈曲变形行为 的研究；针对钢管产品试制中不同的材料性能进行了仿真计算，最终提出：两个与钢管临界屈曲应变相关的应力比指标尺Rt5．0/Rt1．0和 Rt2．0／Rt1．0分别为1．04和1．08。该研究结果为基于应变设计地区使用的钢管标准的制订提供了技术支持。
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