左越
, 潘涛
ZUO Yue, PAN Tao
中图分类号: TG142
文章编号: 1005-3093(2016)06-0401-08
通讯作者:
收稿日期: 2015-10-18
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
利用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)以及拉伸和冲击试验等方法研究了V(0.03%-0.12%)(质量分数, 下同)、Si含量(0.32%-0.89%)对中碳(0.54%)珠光体车轮钢显微组织及力学性能的影响。结果表明: 提高V含量细化了实验钢的奥氏体晶粒尺寸、珠光体团尺寸及其片层间距, 并且提高了铁素体体积分数。随着V含量的提高, 由于VC沉淀强化和细化晶粒的作用, 室温屈服强度和-20℃冲击韧性得到改善; 但软相(先共析铁素体)增多, 室温抗拉强度降低。提高Si含量显著降低了铁素体体积分数和细化了珠光体片层间距, 略细化奥氏体晶粒和珠光体团尺寸; Si也促进VC的析出但作用很小。Si主要以固溶强化和细化片层间距的方式提高屈服强度和抗拉强度。结合适中含量的V(0.07%-0.08%)微合金化和较高含量的Si(0.8%-0.9%)合金化, 可以使中碳珠光体钢获得较好的强韧性匹配。
关键词：
Abstract
The effect of V and Si on the microstructure and mechanical properties of medium-carbon pearlitic steels for wheel was studied by means of OM、SEM and TEM, as well as tensile and impact tests. The results showed that the austenite grain size, the pearlite colony size and interlamellar spacing were significantly refined by increasing V content, which also led to an increase in the volume fraction of proeutectoid ferrite of the steels. With the increasing of V content, the yield strength at room temperature and the impact toughness at -20℃were enhanced due to precipitation strengthening and grain refinement effects of VC. However, the tensile strength at room temperature was decreased due to the increasing of the soft phase, i.e., proeutectoid ferrite. The increase of Si content resulted in the great decrease of proeutectoid ferrite and the significant refinement of pearlite interlamellar spacing but the slight refinement of austenite grain size. Si addition also promoted the VC precipitation but had only a little influence. The yield- and tensile-strength were enhanced mainly by the effect of solid solution strengthening and the refinement of pearlite interlamellar spacing due to Si addition. The balance of strength and toughness in medium-carbon pearlite steels could be effectively optimized by microalloying with the combination of medium 0.07%-0.08%V(mass fraction) and relatively high 0.8%-0.9%Si (mass fraction).
Keywords：
车轮是高速列车的关键行走部件, 必须具有高安全性和耐磨性, 要求材料具有良好的强韧性匹配。目前, 世界上采用的高铁车轮钢为中高碳珠光体钢, 采用特殊的踏面淬火热处理方式获得以片层珠光体为主的显微组织, 其强韧性主要取决于先共析铁素体和珠光体组织的比例与亚结构尺寸。细化奥氏体晶粒是改善珠光体钢强韧性匹配的有效方法, 在生产工艺不大幅度改变的前提下, 进行微合金化是细化高铁车轮钢晶粒的重要手段和发展趋势[1, 2]。
V微合金化在中高碳钢热处理过程中以固溶或析出形式存在, 细化晶粒、沉淀强化、调控淬透性等作用多样, 因而应用较多[3-8]。有研究[7]表明, 在0.78%(质量分数, 下同)C含量的钢中添加0.1%V能够推迟珠光体转变, 细化珠光体团尺寸和片层间距。文献[8]认为, 对于0.52%C含量的珠光体车轮钢, 将V含量从0.05%提高到0.06%可以细化晶粒尺寸, 提高塑韧性和屈服强度。而V微合金化对中碳高铁车轮钢显微组织与强韧性的影响尚缺乏系统研究。如何让固溶与析出的V含量、以及V与C、合金元素的含量达到合理匹配水平从而获得理想的显微组织组成和强韧性匹配, 成为中碳高铁车轮钢研究的焦点问题。Si是固溶强化效果非常显著且不显著影响淬透性的合金元素, 因此, 提高Si和V的含量是提高高铁车轮强韧性的有效措施之一。文献[9]对中碳非调质钢的研究表明, Si和V含量的增加细化了奥氏体晶粒, 提高了先共析铁素体的硬度。但Si和V影响高铁车轮钢显微组织与强韧性的协同作用还需要进一步阐明。
本文主要研究V和Si对中碳珠光体高铁车轮钢显微组织和强韧性的影响规律, 并揭示Si与V的协同作用机理, 为车轮钢的合金化设计和综合性能的优化提供相应的实验与理论依据。
实验钢的制备工艺流程为: 真空感应炉冶炼→锻造→轧制→热处理。铸坯的化学成分如表1所示, 1、2、3#为高Si(~0.85%)变V(0.03%-0.12%)试样, 2#和4#为中V(~0.075%)变Si(0.32%-0.89%)试样。轧制试样尺寸为250 mm(长度, 轧向)×125 mm(宽度)×60 mm(厚度), 轧制工艺为: 加热至1280℃保温2 h, 出炉经多道次连续轧制后空冷至室温, 终轧温度高于900℃。热处理试样尺寸为65 mm×62 mm×15 mm(厚度), 热处理工艺为: 860℃保温1 h, 然后采用风冷+雾冷的方式进行连续冷却至500℃以下, 随后进行风冷至室温, 然后在520℃回火2 h。冷却过程中700~500℃温度区间内的平均冷却速度约为2℃/s。奥氏体晶粒的检测方法为: 试样加热至860℃保温1 h后水淬至室温, 经机械研磨、抛光后在室温用添加少许洗涤剂的过饱和苦味酸水溶液腐蚀原始奥氏体晶界。其他试样的显微组织采用3%的硝酸酒精溶液进行浸蚀。
表1 实验钢化学成分
Table 1 Composition of studied steels (%, mass fraction)
| No. | C | Si | Mn | P | S | Cr | V | Als | N |
|---|---|---|---|---|---|---|---|---|---|
| 1#(0.88Si-0.03V) | 0.54 | 0.88 | 0.78 | 0.0079 | 0.0077 | 0.17 | 0.030 | 0.020 | 0.0014 |
| 2#（0.89Si-0.074V） | 0.55 | 0.89 | 0.78 | 0.0068 | 0.0067 | 0.16 | 0.074 | 0.020 | 0.0018 |
| 3#（0.87Si-0.12V） | 0.54 | 0.87 | 0.78 | 0.0072 | 0.0078 | 0.18 | 0.120 | 0.021 | 0.0016 |
| 4#（0.32Si-0.075V） | 0.53 | 0.32 | 0.77 | 0.0068 | 0.0083 | 0.16 | 0.075 | 0.016 | 0.0010 |
对实验钢进行力学性能测试, 室温拉伸实验按照GB/T228-2010在WE-300拉伸试验机上进行, 试样为直径5 mm、标距25 mm的标准拉伸试样。冲击实验按照GB/T229-2007在JBN-300N冲击试验机上进行, 试样为尺寸10 mm×10 mm×55 mm的标准V型缺口冲击试样, 实验温度为-20℃。利用VH-5维氏硬度计进行显微硬度测量, 其中载荷为10 g, 加载时间为10 s。
利用光学显微镜(OM)、扫描电子显微镜(SEM)和透射电子显微镜(TEM)对实验钢的显微组织进行观察和分析。在光学显微镜下观察奥氏体晶粒和显微组织形貌, 在S-4300型场发射扫描电子显微镜下观察珠光体片层和珠光体团以及冲击断口形貌。利用点计数法在OM照片上统计先共析铁素体的体积分数, 利用截线法测量奥氏体晶粒尺寸、珠光体团尺寸和平均珠光体片层间距。利用JEM-2100F高分辨透射电镜对实验钢中的析出相进行观察和分析。
实验钢的奥氏体晶粒形貌的金相照片如图1所示, 可见随着V含量的提高, 奥氏体晶粒尺寸不断减小, 统计结果如表2所示, 0.12%V对奥氏体晶粒的细化最为显著, 平均奥氏体晶粒尺寸约15 μm。增加Si含量后奥氏体晶粒尺寸略有减小。
图1 实验钢奥氏体晶粒形貌的金相照片
Fig.1 OM images showing austenite grain morphology of the steels (a) 1#; (b) 2#; (c) 3#; (d) 4#
表2 实验钢的奥氏体晶粒尺寸、先共析铁素体体积分数、珠光体团尺寸与片层间距
Table 2 Austenite grain size, proeutectoid ferrite volume fraction, pearlite colony size and interlamellar spacing of studied steels
| No. | Austenite grain size/μm | Ferrite volume fraction / % | Pearlite colony size/μm | Pearlite interlamellar spacing /μm |
|---|---|---|---|---|
| 1# | 23.1±1.2 | 4.3 | 9.8±1.3 | 0.172±0.02 |
| 2# | 20.1±0.5 | 12.2 | 7.4±0.9 | 0.147±0.02 |
| 3# | 14.9±0.7 | 21.4 | 5.8±0.4 | 0.140±0.02 |
| 4# | 21.4±0.6 | 19.5 | 7.7±0.8 | 0.159±0.02 |
实验钢的显微组织的金相和SEM照片如图2、3所示, 四种钢的组织均为先共析铁素体和片层珠光体, 对先共析铁素体的含量、珠光体团尺寸和片层间距的统计测量结果如表2所示。可以看出, V含量从0.03%增加到0.12%, 奥氏体晶粒、珠光体团尺寸以及片层间距逐渐细化, 铁素体体积分数显著增加。但V含量从0.074%增加到0.12%, 珠光体片层间距的细化效果很小。比较2#与4#钢, Si含量从0.32%增加到0.89%, 先共析铁素体的含量显著减少, 珠光体片层间距得到显著细化, 但珠光体团尺寸的细化效果很小。
图2 实验钢先共析铁素体/珠光体组织的金相照片
Fig.2 OM images of proeutectoid ferrite and pearlite of the steels (a) 1#; (b) 2#; (c) 3#; (d) 4#
图3 实验钢先共析铁素体/珠光体组织的SEM照片
Fig.3 SEM images of proeutectoid ferrite and pearlite of the steels (a) 1#; (b) 2#) (c) 3#; (d) 4#
实验钢的力学性能如表3所示, V含量从0.03%增加到0.12%, 屈服强度逐渐增加而抗拉强度逐渐下降, 屈服强度提高的幅度(50 MPa)高于抗拉强度的下降幅度(22 MPa)。从断后延伸率和-20℃KV2冲击功可以看出, V含量的增加总体改善了实验钢的塑韧性。比较2#与4#钢, Si含量从0.32%增加到0.89%, 屈服强度和抗拉强度分别提高了46 MPa和96 MPa, 而塑韧性变化不大。
表3 实验钢的室温拉伸性能和-20℃冲击性能
Table 3 Tensile and impact properties of experimental steels, respectively at room temperature and -20℃
| No. | Yield strength / MPa | Tensile strength / MPa | Total elongation / % | KV2 /J (-20℃) |
|---|---|---|---|---|
| 1# | 511 | 914 | 19.0 | 10 |
| 2# | 531 | 907 | 21.0 | 14 |
| 3# | 561 | 892 | 21.0 | 16 |
| 4# | 485 | 813 | 22.0 | 14 |
3.1.1 V的存在形式与作用 V是强碳氮化物形成元素, 但由于V(CN)在奥氏体中的固溶度较大, 在实验钢的奥氏体基体中以固溶或固溶与析出的形式存在, 并且固溶析出程度与V的含量有关。利用Thermo-Calc计算了三种V钢(分别相应于1-3#钢)奥氏体固溶的C和V含量随奥氏体化温度的变化, 如图4所示, 从图中可以看出, 在860℃奥氏体化时, 2#(0.074%V)和3#(0.12%V)钢中的V并未完全固溶, 以VC形式析出的V含量分别约为0.025%和0.07%。图5为3#钢中析出相的TEM照片, 从能谱分析可知析出相为VC, 在先共析铁素体和珠光体中分别析出。而2#与3#钢860℃奥氏体化时固溶的V含量基本相同, 约0.05%。相应地, 固溶C含量由0.54%降低到约0.53%和0.52%。析出的VC数量越多, 对奥氏体晶粒的细化作用也越强, 同时固溶C含量的降低也适当减小了奥氏体化时的过热度, 对奥氏体晶粒的细化也有一定的作用, 所以3#(0.12%V)钢具有更小的奥氏体晶粒尺寸。固溶C含量的降低, 奥氏体晶粒的细化, 使得连续冷却过程中形成的先共析铁素体含量增多。此外, 奥氏体晶粒尺寸的减小也有利于珠光体团尺寸的减小[10]。而1#(0.03%V)钢在860℃奥氏体化时, C和V全部固溶在奥氏体中, 固溶V起到的溶质拖曳作用也较小, 因而得到的奥氏体晶粒尺寸较大。
图4 名义V含量对中碳钢奥氏体固溶C/V含量的影响
Fig.4 The effect of nominal V content on dissolved C/V in austenite in medium carbon steels
图5 0.87Si-0.12V钢中VC析出相的TEM照片 (a) 先共析铁素体中; (b) 珠光体中; (c) 析出相的EDS能谱
Fig.5 TEM images of VC precipitates in 3# steel (a) VC with proeutectoid ferrite; (b) VC with pearlite; (c) EDS of VC
珠光体片层间距主要取决于珠光体转变的温度(过冷度), 转变温度越低, 珠光体片层间距越细, 合金元素通过影响珠光体相变点以及珠光体形核长大过程来影响珠光体的片层间距和团尺寸[11, 12], 固溶V含量增加将增大C原子的扩散激活能, 提高奥氏体的稳定性, 增加珠光体转变时的孕育期, 从而降低珠光体相变温度, 这得到了文献[7]的研究结果证实。本实验中1#、2#、3#钢的V含量逐渐增加, 2#、3#钢860℃奥氏体化时固溶的V含量基本相同但高于1#钢(图4), 这导致了2#、3#钢的珠光体片层间距差别不大, 但较1#钢明显细小, 而2#、3#钢的珠光体团尺寸比1#钢的明显细化, 这是前两者奥氏体晶粒尺寸较细小和相变温度较低导致的。3#钢的珠光体团尺寸更细小, 则主要是由于其奥氏体晶粒尺寸更细小。
3.1.2 Si的作用 Si是非渗碳体形成元素, 提高Ae1和Ae3温度。对比2#钢与4#钢, Si含量从0.32%增加到0.89%, 使得Ae1、Ae3温度分别提高了10℃、14℃, 从而减小了奥氏体化时的过热度, 有利于细化奥氏体晶粒和珠光体团, 但本实验结果显示这种影响不大。
利用Formastor热膨胀仪测量2#(0.89Si-0.074V)与4#(0.32Si-0.075V)钢在1℃/s~5℃/s冷却速率下的铁素体/珠光体相变温度。这种冷速范围相应于高铁车轮关键部位在端淬热处理时的冷速范围。如图6所示, Si含量提高后, 铁素体/珠光体相变的开始温度与终了温度呈降低趋势, 冷却速度越快, 相变温度的降低程度越大。这与固溶V的作用[7]类似。由于Si不溶于渗碳体中, 珠光体相变过程中Si必然发生长程扩散, Si含量越高, 扩散时间越长, 将连续冷却珠光体相变推向低温方向的趋势就越大。本实验中, 提高Si含量推迟了珠光体相变以及提高了平衡相变温度, 增大了珠光体转变时的过冷度, 从而显著细化珠光体片层间距。
图6 2#和4#钢不同冷速下的相变温度(860℃奥氏体化)
Fig.6 Phase transformation temperatures of 2# and 4# steels under different cooling rates (austenitizing at 860℃)
3.1.3 Si-V协同作用 如图7所示, 利用Thermo-Calc计算了两种Si钢(分别相应于2#与4#钢)奥氏体中固溶C和V含量随温度的变化曲线。可见, 在860℃时0.89Si钢中未溶的VC要多于0.32Si钢, Si含量的增加提高了V的析出温度, 即促进了VC在奥氏体中的析出。这是由于Si提高了C的活度, 减小了VC在奥氏体的固溶度[13, 14], 未溶VC的增多也有利于奥氏体晶粒的细化。此外, 有报道称Si也能促进VC在铁素体中的析出[14]。从本实验条件与研究结果来看, Si含量从0.32%增加到0.89%, 对VC在奥氏体和铁素体中析出的影响均很小。
图7 Si含量对中碳钢奥氏体固溶C/V含量的影响
Fig.7 The effect of Si content on dissolved C/V in austenite in medium carbon steels
3.2.1 强度 对实验钢的显微组织中铁素体和珠光体分别进行了显微硬度测量, 如图8所示, 随着V含量和Si含量的增加, 铁素体和珠光体的硬度都增加, 铁素体硬度的增加主要是由于VC的沉淀强化和Si的固溶强化作用, 珠光体硬度的增加除了上述原因以外, 还与片层间距得到细化有关。
图8 实验钢的先共析铁素体和珠光体的显微硬度
Fig.8 The microhardness of proeutectoid ferrite and pearlite of the steels
V通过以下三种主要方式对强度产生影响[3]: (a)奥氏体中析出或未溶的VC颗粒细化奥氏体晶粒; (b)奥氏体中固溶的V影响淬透性; (c)在先共析铁素体和珠光体铁素体中的沉淀析出。前两者通过细化组织和组织类型的相对比例来影响强度, 后者通过沉淀强化作用提高强度。本文中实验钢的组织均为先共析铁素体和珠光体, 复相组织的屈服强度主要取决于软相的屈服强度, 抗拉强度符合混合物定律, 取决于两种组织的抗拉强度以及体积分数。V含量对先共析铁素体尺寸的影响不大(1~3#钢的先共析铁素体尺寸差别不大), 但以沉淀强化方式提高了先共析铁素体的强度即提高了实验钢的屈服强度。虽然V细化了珠光体片层间距, VC析出强化了珠光体铁素体, 但更显著地增加了先共析铁素体的体积分数、减少了珠光体的体积分数, 综合结果使得抗拉强度有所降低。
Si对实验钢强度的影响主要通过3个方面: (1)在铁素体中的固溶强化; (2)减少先共析铁素体含量; (3)细化珠光体片层间距。屈服强度主要受首个因素影响, 抗拉强度则由三个因素共同决定。Si固溶在铁素体中, 每1%Si对屈服强度的贡献约为83 MPa[1]。2#钢与4#钢相比, 0.57%Si的增量所产生的强度增量约为47 MPa, 与实际差异相吻合, 这说明Si的固溶强化作用是提高屈服强度的主要因素。实际情况中, 增加Si使先共析铁素体尺寸略微减小, 也有利于提高屈服强度, 但提高幅度很小。增加Si增加了珠光体的体积分数和细化了珠光体片层间距, 这些因素的综合作用使得抗拉强度的提高幅度比屈服强度的更大。
3.2.2 韧性 对于铁素体-珠光体钢的韧性, 研究表明[15-17], 铁素体体积分数和晶粒尺寸、珠光体团尺寸和珠光体片层间距以及奥氏体晶粒尺寸等组织结构参数都会对其产生一定的影响。增加铁素体体积分数以及减小铁素体晶粒尺寸有利于提高韧性, C含量较高时(组织中珠光体占主要比例), 珠光体块(珠光体中铁素体取向基本一致的区域)和珠光体团的作用更为重要, 细化奥氏体晶粒可以细化珠光体块和团尺寸[10, 18-20], 增加大角度铁素体界面密度。大角度界面对裂纹起阻碍作用, 因而改善韧性。
增加V含量明显增加了铁素体体积分数, 减小了奥氏体晶粒、珠光体团尺寸, 且组织分布更为均匀, 显著改善韧性; 而珠光体片层间距的细化和沉淀强化对韧性的损害作用相对较小。本实验证实了V含量从0.03%增加到0.12%时, -20℃V型缺口冲击功随之提高, 说明V含量增加有利于改善中碳钢的韧性。
增加Si含量使固溶强化作用增强, 同时使铁素体体积分数和珠光体片层间距减小; 其对奥氏体晶粒尺寸有细化作用, 但作用很小; 此外, 对于中高碳钢, 增加1%Si使韧脆转变温度提高约48.7℃[15], 所以增加Si含量总体上对韧性应是不利的。而由于V和Si的协同作用, 在V微合金化钢中增加Si含量能够促进VC的析出, 有利于进一步提高VC的细化晶粒和沉淀强化作用, 对韧性也会有一定的改善。从实验结果来看, Si含量从0.32%增加到0.89%时, -20℃ V型缺口冲击功未有变化。
从冲击断口的形貌(如图9)可以看出, 4种钢的断裂形貌均以准解理断裂为主。随着V含量的增加, 解理面尺寸随之减小, 特别是V含量增加到0.12%时, 韧窝数量显著增多。根据韧窝带的形貌, 判断其可能来源于先共析铁素体组织。2#钢-20℃冲击时完全处于准解理断裂状态, 冲击功处于下平台。以2#钢为基准, 4#钢在-20℃冲击时也处于下平台且下平台冲击功与2#钢很接近。3#钢在-20℃冲击断裂时包含了一定的韧性断裂, 因此, 冲击功更高。而1#钢的-20℃冲击功最低, 则可能是由于其下平台冲击功最低。
图9 实验钢冲击断口形貌
Fig.9 Impact fracture morphology of the studied steels (a) 1#; (b) 2#; (c) 3#; (d) 4#
通过对V和Si在中碳珠光体车轮钢中作用规律的研究可以看出, 添加V提高了实验钢的屈服强度和韧性, 但降低了抗拉强度, 添加Si同时提高了实验钢的屈服强度与抗拉强度, 但提高抗拉强度的幅度更大。因此, 适中V含量(0.07-0.08%)和较高Si含量(0.8-0.9%)的匹配添加可以使珠光体车轮钢获得较好的强韧性匹配。
1. V含量从0.03%增加到0.074%, 在860℃淬火温度下, V由全固溶转变为部分固溶, 继续增加V含量则会增加奥氏体中的VC析出量, 降低奥氏体的固溶C含量, 基本不改变奥氏体中固溶V含量。
2. V含量从0.03%增加到0.074%, 显著细化奥氏体晶粒、珠光体团尺寸以及片层间距, 并显著增加铁素体体积分数。继续增加V含量至0.12%, 除珠光体片层间距的细化效果很小以外, 其他变化趋势仍与上述相同。
3. Si含量从0.32%增加到0.89%, 对奥氏体晶粒、珠光体团尺寸起细化作用, 但作用很小; 而细化珠光体片层间距和减少铁素体体积分数的作用明显。
4. V含量从0.03%增加到0.12%, 通过沉淀析出提高了先共析铁素体的强度(硬度), 通过沉淀析出和细化片层间距提高了珠光体的强度(硬度), 但显著增加了软相(先共析铁素体)的体积分数, 因此提高了屈服强度, 改善了低温韧性(-20℃ V型缺口冲击功), 但降低了抗拉强度。Si含量从0.32%增加到0.89%, 通过固溶强化与细化片层间距、降低铁素体体积分数同时提高了屈服强度和抗拉强度, 且提高抗拉强度的作用更显著, -20℃ V型缺口冲击功不变。
5. 适中V含量(0.07-0.08%)和较高Si含量(0.8-0.9%)的匹配添加可以使珠光体车轮钢获得较好的强韧性匹配。
The authors have declared that no competing interests exist.
| [1] |
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| [2] |
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| [3] |
Microstructure and mechanical properties of medium-carbon ferrite-pearlite steel microalloyed with vanadium , |
| [4] |
Intragranular ferrite nucleation in medium-carbon vanadium steels ,
In this study, the mechanism of intragranular ferrite nucleation is investigated. It is found that intragranular ferrite idiomorphs'' nucleate at vanadium nitrides which precipitate at manganese sulfide particles during cooling in the austenite region. It is observed that intragranular ferrite has the Baker-Nutting orientation relationship with vanadium nitride which precipitated at manganese sulfide. According to classical nucleation theory, the proeutectoid ferrite nucleation rate depends on the following factors: (1) the driving free energy for ferrite nucleation, (2) the diffusivity of carbon atoms in austenite, and (3) the increase in the interfacial energy associated with ferrite nucleation. In the Baker-Nutting orientation relationship, the lattice mismatch across the habit planes is likely to be very small. Depleted zones of solute atoms such as vanadium are assumed to be formed in the austenite matrix around precipitates. The effect of the depleted zones on factors (1) and (2) is estimated thermodynamically and it is proved that those effects are negligibly small. Thus, the authors conclude that the most important factor in nucleation kinetics of intragranular ferrite is the formation of precipitates which can develop coherent, low energy interfaces with ferrite.
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| [5] |
Effect of pearlite morphology on impact toughness of eutectoid steel containing vanadium ,
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| [6] |
Precipitation of VC in ferrite and pearlite during direct transformation of a medium carbon microalloyed steel ,
Transmission electron microscopy of an air-cooled medium carbon (0.5 wt pct) steel containing 0.1 wt pct vanadium has shown that VC precipitates by the interphase mecha-nism during transformation to both proeutectoid and pearlitic ferrite. Depending upon the rate of transformation, a considerable proportion of the available vanadium may remain in supersaturated solid solution and can be precipitated as VC upon subsequent aging at 700掳C. It was found that the proportion of proeutectoid ferrite, the interlamellae pearlite spac-ings and the VC precipitate dispersion parameters all decreased with increasing cooling rate in as-transformed material.
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| [7] |
The effects of V on phase transformation of high carbon steel during continuous cooling ,V对高碳钢连续冷却时组织转变的影响 ,
通过在Gleeble1500热模拟试验机上的模拟加热和冷却实验, 研究了连续冷却条件下不同含量的V对高碳钢组织转变的影响, 并测定了相关力学性能. 实验结果表明:V的加入降低了高碳钢珠光体转变温度, 推迟了珠光体转变的CCT曲线, 细化了珠光体片层, 增加了高碳钢珠光体组织的强度和硬度,从V降低珠光体转变开始温度和降低珠光体团长大速率两个方面分析了V降低珠光体平均转变温度的原因. 实验还观察到在1 ℃/s冷速下高碳钢原奥氏体晶界上存在少量铁素体, 加入0.1%V对晶界铁素体的析出有抑制作用,而加入0.2%V到0.3%V则促进铁素体在晶界明显析出, 相关力学性能的测定表明晶界铁素体的出现改善了高碳钢塑性.
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| [8] |
Anisotropy and rolling contact fatigue of railway wheels ,
Modes of rolling contact fatigue failure are described and categorized for railway wheel treads and rims. The overall strength and endurance of railway wheels is investigated with special focus on material anisotropy and fatigue characteristics. An improved metallurgical design of the wheel material is proposed and tested. A procedure for defect-tolerant design of the wheel, addressing anisotropic fatigue properties, is outlined.
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| [9] |
The effect of silicon and vanadium alloying on the microstructure of air cooled forged HSLA steels ,
The paper presents comparison of results of microstructure investigations of HSLA steels: 38MnSi6 and SiV-steel. The main objective of this investigation has been to find the influence of the concentration of silicon increased to 1.26% and vanadium to 0.16% on the microstructure of SiV-steel in comparison to 38MnSi6. It has been found that SiV-steel represents higher volume fraction of proeutectoid ferrite, which is more evenly distributed in the structure, with finer prior austenite grain size. Higher concentration of vanadium and nitrogen characteristic of SiV-steel results in non-random dispersion of V(CN) precipitations. It has also been observed that microhardness of the proeutectoid ferrite, characteristic for SiV-steel, is about 70HV0.2 higher than in 38MnSi6 steel.
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| [10] |
Phase transformation from fine-grained austenite ,
Microstructure formed by diffusional or martensitic transformation from fine-grained austenite of which grain size is smaller than 5 μm was studied. Grain refinement of austenite was established through two kinds of reversion processes; (1) cyclic transformation between martensite and austenite and (2) reverse transformation from tempered and cold-rolled lath martensite (or pearlite). In the process of (1), the fine austenite structures whose grain sizes of 5–10 μm are obtained. Refinement of austenite grain size results in the increase of hardness. In the process of (2), austenite grain size can be refined down to about 2 μm in low-carbon Mn steels by microalloying through pinning of austenite grain growth by alloy carbides. The ferrite grain size after continuous cooling transformation becomes finer as austenite grain size is refined. However, the grain size ratio of austenite and ferrite, dα/dγ, increases by refining austenite grain size. For the austenite of grain size smaller than 5 μm, the ferrite grain size becomes coarser than that of austenite for slow cooling. A similar trend in the change of ferrite grain size by refinement of austenite was recognized for isothermal pearlite transformation in eutectoid alloys. Thus, it is suggested that extensive accelerated cooling is important to obtain fine-grained ferrite by diffusional transformations from the fine-grained austenite. Packet and block sizes of lath martensite in low carbon steels are also refined by decreasing the austenite grain size. Several packets and blocks are formed even from the austenite matrix of 2 μm in grain size.
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| [11] |
An investigation of manganese partitioning during the austenite-pearlite transformation using analltical electron microscopl ,
The analltical electron microscope, EMMA-4, has been used to studl the partitioning of manganese betleen cementite and ferrite during the austenite → pearlite transformation in a eutectoid steel. At high reaction temperatures the manganese partitioned preferentialll to the cementite at the pearliteaustenite interface, lhile at lol temperatures there las no partitioning of manganese in the freshll formed pearlite. The no-partition temperatures measured for eutectoid steels containing 1.08 and 1.80 lt.% Mn lere 683 and 649°C, respectivell, and lere in good agreement lith values of 683 and 645°C calculated from thermodlnamic data. Manganese segregation to cementite occurred at all temperatures after pearlite had formed and continued until eluilibrium values lere reached. The grolth rate and interlamellar spacing of pearlite have been measured as a function of transformation temperature and used to examine the rate controlling process for pearlite grolth at temperatures belol the no-partition temperature.
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| [12] |
A review of the data on the interlamellar spacing of pearlite ,
After defining interlamellar spacing the various optical and electron optical methods for measuring spacing are outlined. It is clear for both isothermal and forced velocity transformation conditions that pearlite can grow at a constant velocity with a range of true spacings. The minimum true spacing and mean true spacing are not related by a constant factor, but this may vary from system to system and with temperature in a given system. The relationship between interlamellar spacing and temperature for isothermal growth conditions and between translation velocity and spacing for forced-velocity growth conditions is reviewed for a range of steels and nonferrous alloys. It is seen that the velocity-spacing relationship for the two modes of transformation is the same. For isothermal transformation a linear relationship between reciprocal spacing and temperature is generally observed, but for steels containing alloy additions there is little evidence of the predicted inflexion corresponding to a temperature at which alloy partitioning at the transformation front ceases. The lack of precise interfacial energy data makes it difficult to determine reliably the relationship between measured and critical spacings, although it seems likely to be in accord with the maximum growth rate or maximum rate of entropy production optimization criteria.
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| [13] |
Alloy additions on solubility of alloy carbides in steels ,
As a supplement to a previous extensive study of ordering phenomena in a series of Ni-Mo alloys, a resistometric investigation of Ni-15at.%Mo alloy representing the (alpha + beta) region was carried out. The results illustrate a continuous change of ordering mechanisms, which takes place when the Mo content increases. The estimated vacancy migration energy is close to the values determined for other Ni-Mo alloys in the alpha and (alpha + beta) phase domains.
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| [14] |
Pearlite phase transformation in Si and V steel ,
Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the phase transformation and microstructure of pearlitic steels. Both alloy additions were found to result in the formation of nonlamellar products in the vicinity of austenite grain boundaries in hypereutectoid compositions (0.77 to 0.95 wt pct C). The products comprise discrete initial cementite particles and grain boundary ferrite, which is embedded with interphase precipitates of vanadium carbide. As the carbon content is increased further (up to 1.05 wt pct), the amount of grain boundary ferrite gradually decreases without any dramatic change in the morphology of the initial cementite particles. No continuous embrittling grain boundary cementite network was formed. The aspect ratios of the grain boundary cementite particles were decreased from 60:1 to 25:1 by the addition of the alloy elements. A compre-hensive model has been suggested to explain these effects. Other effects of these alloy elements on the microstructure of pearlitic steels have also been examined. For given austenitization conditions, an increase in carbon and vanadium content produced a decrease in austenite grain size. Silicon was found to increase the rate of interphase precipitation of vanadium carbides.
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| [15] |
Some aspects of the structure-property relationships in high-C ferrite-pearlite steels , |
| [16] |
Fracture toughness of medium-high carbon steel for railroad wheel ,
Railroad wheel brittle fracture is catastrophic, and can cause a derailment. Such usage conditions as load and speed of railroad vehicles have become severe in recent years. Therefore, more precise evaluation of wheel strength and safety has been desired. From recent development of fracture mechanics, it is clear that in order to prevent the wheel fracture, fracture toughness of medium-high carbon steel for railroad wheels should be raised. This paper describes the effect of carbon, manganese, aluminum solution on the fracture toughness. Ferrite volume fraction and grain refinement were analysed to obtain the relationship between fracture toughness and material characteristics in medium-high carbon steel.
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| [17] |
Effect of interlamellar spacing on the mechanical properties of 0.65% C steel , |
| [18] |
The role of microstructure on the strength and toughness of fully pearlitic steels ,
An experimental program was carried out to clarify the structure-property relationships in fully-pearlitic steels of moderately high strength levels, and to identify the critical microstructural features that control the deformation and fracture processes. Specifically, the yield strength was shown to be controlled primarily by the interlamellar pearlite spacing, which itself was a function of the isothermal transformation temperature and to a limited degree the prior-austenite grain size. Charpy tests on standard and fatigue precracked samples revealed that variations in the impact energy and dynamic fracture toughness were dependent primarily on the prior-austenite grain size, increasing with decreasing grain size, and to a lesser extent with decreasing pearlite colony size. These trends were substantiated by a statistical analysis of the data, that identified the relative contribution of each of the dependent variables on the value of the independent variable of interest. The results were examined in terms of the deformation behavior being controlled by the interaction of slip dislocations with the ferrite- cementite interface, and the fracture behavior being controlled by a structural subunit of constant ferrite orientation. Preliminary data suggests that the size of such units are controlled by, but are not identical to, the prior-austenite grain size. Possible origins of this fracture unit are considered.
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| [19] |
Microstructures dominating the ductility of eutectoid pearlitic steels .URL 摘要
ABSTRACT Microstructures which control the ductility of eutectoid pearlite steel were experimentally examined. A concept of pearlite block was proposed as an area in which crystallographic orientation of ferrite is constant. It was revealed that the pearlite block acts as a unit of slip and fracture of pearlite steels. The refinement of austenite grains, which is effective when improving the ductility of pearlite steels, was found to be due to the resulting refinement of pearlite blocks. 17 refs.
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| [20] |
The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel ,
The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite; i.e. , localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic structures.
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