X70管线钢的应变时效行为

采用扫描电子显微镜(SEM)、透射电子显微镜(TEM)等手段研究了时效温度和预应变量对X70管线钢的显微组织和力学性能的影响。结果表明,由于时效温度比较低(最高300℃),时效前后X70钢的组织都由粒状贝氏体+针状铁素体+M/A组成,没有明显的变化。时效前后抗拉强度的变化也不明显。随着时效温度的提高活化的间隙原子增多,钉扎的位错数量增加,屈服强度提高;当时效温度高于200℃时可供钉扎的位错开始大量回复,屈服强度逐渐降低。随着时效温度的提高柯氏气团及析出物的数量增加,使材料硬化,冲击韧性逐渐下降。随着预应变量的增大可被间隙原子钉扎的位错数量增加,屈服强度逐渐升高。随着预应变量的增大硬质相M/A边界处产生的应力集中不断增大,产生微裂纹的概率不断提高,使冲击韧性逐渐下降。     

高钢级管线钢应变时效行为分析

应变时效现象会造成管线钢屈服强度和屈强比升高,应力一应变曲线出现明显屈服平台。通过对管线钢及钢管制造和使用过程的分析,指出应变时效行为对钢管生产过程质量控制、现场环焊缝对接以及管线服役性能的稳定性都有不良影响。通过减小钢管管体试样制备温度影响、严格控制防腐层制备工艺加热条件、降低管线钢中的C、N含量以及采用双相显微组织并在终轧时加速冷却后立即进行热处理等方法,可以有效降低应变时效效应。最后,对管线钢的应变时效行为的定量表征提出了建议。     

基于临界区加速冷却的(B+F)X80大变形管线钢的组织和性能研究

采用热模拟、力学性能测试和材料显微分析等试验技术,对X80管线钢在临界区加速冷却工艺下的组织性能变化规律进行了研究。结果表明,通过临界区加速冷却,X80管线钢可获得贝氏体+铁素体(B+F)双相组织。随着始冷温度的上升,试验钢的贝氏体含量增加,铁素体含量降低,导致屈服强度增高,塑性降低。当始冷温度为840℃时,显微组织以细小、多位向分布的贝氏体为主,辅以高密度位错的多边形铁素体。这种(B+F)双相组织使得试验钢的屈强比为0.80、均匀伸长率为10.0%、形变强化指数为0.12,满足了大变形管线钢的技术要求。     

一种抗大变形管线钢的微观组织与拉伸性能的相关性研究

对比研究了3组X80抗大变形管线钢的拉伸性能和显微组织,讨论微观组织尤其是第二相组态对力学性能的影响.结果表明,3组X80抗大变形管线钢的拉伸应力-应变曲线均呈圆屋顶型的连续屈服态,且纵向屈强比均小于0.85;硬相M/A组元对X80钢产生明显的第二相强化作用,随着M/A相含量的增加,材料屈服强度增大,屈强比主要受控于软硬结合的AF+M/A组元双相组织,硬相M/A组元的体积含量与屈强比表现出非线性关系.     

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.     

板弹簧真空时效处理工艺研究

针对空间用板弹簧的使用要求,开展Ti-15-3钛合金材料的真空时效处理工艺研究。通过调整时效温度和时效时间,分别对板弹簧进行时效处理。第一批次板弹簧时效温度450℃,第二批次时效温度520℃,第三批次采用双级时效(第一级温度450℃,第二级温度520℃)。对所有三个批次的板弹簧试验件进行拉伸试验,试验结果显示第一种时效处理工艺处理的ti-15-3材料获得了很高的拉伸强度,但屈服强度较差。采用第二种时效处理工艺和第三种时效处理工艺的ti-15-3材料抗拉强度和屈服强度都很高,但是第二种时效处理工艺获得的延伸率较为离散,选择第三种时效处理工艺作为板弹簧产品时效处理工艺。     

Mg-3.04Li-0.77Sc合金的高应变率变形局部化行为

利用Hopkinson压杆对固溶及固溶+时效处理状态的Mg-3.04Li-0.77Sc合金进行高应变率冲击压缩实验,研究了固溶和时效处理后该合金的高应变率变形行为。结果表明,随着应变率提高,固溶及固溶+时效态Mg 3.04Li-0.77Sc合金的动态变形表现为应变率弱化效应。固溶处理可增大合金的最大应变,而时效处理可显著提高该合金的动态屈服强度。组织分析表明,在高应变率冲击载荷下,固溶及固溶+时效态Mg-3.04Li-0.77Sc合金产生了明显的剪切变形局部化现象。热软化及其促使的变形局部化和微裂纹沿变形带产生并扩展是该合金动态变形表现为应变率弱化效应的主要原因。     

应力波载荷作用下X70管线钢的应力-应变行为

利用Hopkinson压杆加载装置对国产轧制态X70管线钢进行了冲击压缩实验,研究了其在高速冲击过程中的组织演变和动态应力-应变行为.结果表明:轧制态X70管线钢是以针状铁素体为主的混合组织;在ε≤2.5×103s-1条件下,铁素体板条内形成了大量的位错胞亚晶结构,随着应变率增加,针状铁素体组织显著细化,致使X70管线钢产生了明显的增强增塑现象.     

预变形对X90管线钢显微组织和力学性能的影响

采用拉伸试验、冲击试验、光学显微镜(OM)、扫描电镜(SEM)、显微硬度测试仪等研究了0.5%~6%预拉伸变形对X90管线钢显微组织及力学性能的影响。结果表明:随着预拉伸变形量的增加,X90管线钢晶粒增大,位错塞积导致强度增加,均匀延伸率下降,呈现典型的加工硬化特点,抗拉强度的增幅要小于屈服强度,屈强比增大;随着变形量的增加冲击吸收功逐渐由291J减小至235J,冲击试样断口的韧窝减小,伴随第二相粒子析出;显微硬度中间层较边缘区增加少,预拉伸在6%时边缘显微硬度为325HV。X90管线钢的预拉伸在4%以内能保证管线钢的正常服役。     

单向预应变对X70大变形管线钢拉伸性能的影响

对X70大变形管线钢进行应变量小于2.5％的单向预拉伸和预压缩应变处理,而后卸载,再次拉伸至断裂,研究单向预应变对管线钢拉伸性能的影响.结果表明:同种单向预应变时,不同预应变量下管线钢的弹性变形、屈服变形和形变强化特征基本相同,弹性应变总体上随预应变量的增大而增大;预拉伸后再次拉伸时,管线钢在应变ε小于0.5％即发生屈...     

Interpretation of ductile fracture toughness temperature dependence of a low strength steel in terms of a local approach

Fracture toughness and notch ductility tests were performed on two heats of A508 steel tested over the temperature range between 100°C and 450°C. Both types of experiments showed that the materials exhibited a ductility trough at temperatures close to 300°C. At this temperature tensile tests showed the existence of strain aging phenomenon. Tests on axisymmetric notched tensile specimens were used to derive the critical value for void growth, R_c/R₀, used in a model for ductile fracture. A good correlation between J_Ic and R_c/R₀ was observed. This was used to predict the variations of J_Ic with temperature. A reasonable agreement between the predicted values and the experimental results is observed.     

Effects of microstructural variables on the deformation behaviour of dual-phase steel

An expression for the stress of martensite in dual-phase steel was developed, which shows the interdependence of the stress of martensite and strain hardening in the ferrite matrix and the contribution of microstructural variables (the volume fraction of martensite f_m, ferrite grain size d_f, and martensite particle size d_m). The onset of plastic deformation of martensite in dual-phase steel was predicted to depend on its yield strength and the microstructural variables, and this was verified by the modified Crussard-Jaoul analysis. It was found that for this dual-phase steel, refining the grain size and increasing f_m increase the flow stress and raise the strain hardening rate at low strains, but little affect the strain hardening rate at high strains. The effect of the ferrite grain size on the flow stress of this dual-phase steel was found to obey the Hall-Petch relation, i.e. σ = σ₀^e + K^ed_f^−1/2, where the Hall-Petch intersection σ₀^e and slope K^e are functions of strain, f_m and d_m. The effects of the plastic deformation of martensite and the microstructural variables on the strain hardening rate and the Hall-Petch behaviour were analysed in terms of the densities of statistically stored dislocations and geometrically necessary dislocations using the previously developed theoretical model.     

X100级管线钢的动态塑性变形行为研究

采用原位拉伸扫描电镜试验对X100级管线钢的动态塑性形变行为进行观察,并运用EBSD微观取向分析技术对形变前后管体组织的取向变化进行分析。结果表明,X100级管线钢的微观组织由针状铁素体、粒状贝氏体和M/A岛组成。在拉伸应力的作用下针状铁素体首先发生形变,随着应变的增加,针状铁素体形变累积到一定程度后,导致粒状贝氏体发生形变。针状铁素体边界和贝氏体基体上的M/A组织钉扎位错使变形不易发生。由EBSD取向可知,晶体在发生形变后轧面的{110}晶面方向沿拉伸形变方向转动。由扫描电镜照片观察到,在外加应力作用下,夹杂物成为微裂纹形核核心并随着外加应力的增加而扩展,最后连接,导致裂纹贯穿基体直至失效。     

Influence of Mo content on microstructure and mechanical properties of high strength pipeline steel

The effect the Mo content on the microstructure and mechanical properties of high strength pipeline steel X80 was investigated in the present study. Optical microscopy, scanning electron microscopy and transmission electron microscopy were used to investigate the microstructure of the steel. It was concluded that, the proportion of acicular ferrite increased as the amount of Mo content increased, which resulted in the enhancement of yield and tensile strength; on the other hand, the yield ratio decreased and the toughness deteriorated as the quantity of M–A increased.     

X90钢级管线钢预精焊接头的性能研究

为综合研究X90管线钢的焊接性,选用国内某钢厂轧制的X90管线钢卷板,利用预精焊工艺制备试验钢管4根,采用金相分析、扫描电镜(SEM)断口分析、夏比V型缺口冲击试验、拉伸、弯曲、硬度等试验,研究了焊接接头各个区域的组织和性能.试验结果表明:内外焊缝区组织均为针状铁素体,热影响区(HAZ)粗晶区晶粒粗化严重,主要组织为粒状贝氏体和贝氏体铁素体,在原奥氏体晶界和贝氏体板条内部存在块状或条状的(马氏体-奥氏体)M-A组元;HAZ冲击功离散性较大,出现了单值较低(45 J)的试样,SEM断口分析呈现典型的解理断裂特征;焊接接头抗拉强度805~815 MPa,断裂位置均在HAZ;焊接接头反弯试样易在HAZ出现裂纹和脆断现象;HAZ硬度在220~250 HV之间,较母材下降30 HV左右.HAZ是X90预精焊钢管焊接接头的薄弱环节,为提高X90管线钢的焊接稳定性,应重点研究精焊内外热循环双热影响亚区的组织转变和脆化机理.     

Strain aging on the yield strength to tensile strength ratio of UOE pipe

Strain aging of microalloyed steel pipe can occur at the relatively low temperatures associated with the pipe coating process and/or during long-term storage. A Box–Behnken statistical design was used to determine the significant strain aging variables that affect the longitudinal yield strength to tensile strength (Y/TS) ratio for three uncoated X70 UOE pipes. The strain aging variables examined include time, temperature, steel composition/microstructure (vis-a-vis the C/Nb ratio) and position through the pipe wall thickness. Metallographic and electron backscattered diffraction examinations were undertaken to determine the grain size and phase percentages of the as-received pipe steel. Both position in the pipe and the C/Nb ratio were found to have a statistically significant effect on the yield strength to tensile strength ratio.     

Prediction of constant-amplitude fatigue life to failure under pulsating tension by use of the local-strain approach

The aim of this investigation was to compare local-strain approximation (LSA) life predictions with pulsating-tension tests to failure. It was shown that, in the case of pulsating tension, the ratio N_i/N_f does not change significantly and therefore an LSA calculation can be of use for predicting A-M-N lines for R0. The accuracy of the prediction is associated with the choice of the k-value in Neuber's relation K_σK_ε=k². The -values that give the best life predictions were obtained close to those of the corresponding notch factors for pulsating tension (R=S_min/Smax=0). Because of the size and surface effects (which are not taken into account in LSA), and in view of the difficulty in knowing whether the specimen with a given K_F-value has exactly the same cyclic parameters as those used in the LSA calculations, it is then preferable to use a k-value proved to give prediction results in agreement with some pulsating-tension tests. This k-value may be used for all combinations of S_a and S_m (S_aS_m and S_max<S_y). The results are valid for a specific specimen material, geometry, size and surface finish.     

Effect of controlled rolling/controlled cooling parameters on microstructure and mechanical properties of the novel pipeline steel

The study of controlled rolling/controlled cooling process parameters which affect the microstructure and mechanical properties of a novel pipeline steel has been optimized by the orthogonal experiment with four factors and three levels in this paper.However,the parameters of thermo-mechanical control process(TMCP)optimized by the Gleeble-3500 hot simulator could not satisfy performance requirements of the X100 pipeline steel.In order to improve the performance of this steel,the influence of finish cooling temperature(FCT) on the microstructure and property is studied in detail.It is found that,as this steel is thermo-mechanically treated by this set of parameters(the start heating temperature,finish rolling temperature(FRT),FCT and cooling rate of 1,180℃,810℃,350℃ and 35℃/s,respectively),the microstructures are mainly composed of granular bainite(GB)and acicular ferrite(AF).The effective grain sizes are below 20μm;the steel reaches the optimal balance between the strength and the toughness;the yield strength is 695 MPa;the tensile strength is 768 MPa;the elongation is 16.6%;the impact energy is 262 J at room temperature.All indexes could meet the requirements of X100 pipeline steel.     

An evaluation of the resistance of pipeline steels to initiation and early growth of stress corrosion cracks

Safety and reliability are basic requirements on the operation of systems of high-pressure pipelines. One of the dangerous mechanisms endangering the safe operation is stress corrosion cracking (SCC). In this work, SCC initiation in two types of pipeline steels––a carbon pipeline steel and an X60 high-strength thermo-mechanically treated steel––in carbonate solution was studied. An influence of surface microstructure, different stress/strain conditions and temperature was evaluated. The X60 steel was more sensitive to the SCC process in comparison with the carbon steel. Surface local microscopical inhomogeneities were the predominant initiation sites of microcracks.     

显微带细化组织和两相组织对低Cr合金钢高温断裂行为的影响

针对低Cr合金钢进行轧制工艺设计,分别制备出显微带细化的1#组织和以马氏体/铁素体为特征的2#组织,在550℃、650℃、750℃对其进行高温拉伸实验。拉伸曲线和断后组织的分析结果表明,温度的升高和应变速率的减小能够促进再结晶过程,最终不但导致抗拉强度的降低,同时也减弱了组织内部滑移系统对亚结构界面运动的影响;然而具有2#两相组织的钢板其650℃的抗拉强度却比550℃的高。对组织和析出状态分析后认为,650℃变形时铁素体晶粒中大量形成的M₇C₃析出物显著提高了铁素体基体的强度,进而通过平衡其与回火马氏体之间的性能关系,使拉伸性能得以改善。