Tempering microstructure and mechanical properties of pipeline steel X80

The tempering microstructure and mechanical properties of X80 steel used for heating-bent pipe were analyzed. The results show that the microstructure of X80 steel tempered at 550 ℃ and 600 ℃ is bainitic ferrite (BF)+granular bainite (GB), and partial ferrite laths in BF merge and broaden. The interior sub-lath boundary of some GB begins to disappear due to merging, the M/A constituent (a mixture of martensite plus retained austenite) in GB is orbicular. At the two tempering temperatures the tested X80 steel shows a certain degree of tempering stability. After being tempered at 650 ℃, the microstructure of X80 steel is GB+quasi-polygonal ferrite(QF), and the original BF laths have merged to form smaller GB crystal grains. The reason is that the steel shows better match of strength and toughness. After being tempered at 700 ℃ , the microstructure of X80 steel is composed mainly of QF, which can improve the plasticity but decline severely the yield strength of X80, and the M/A constituent assembles and grows up at the grain boundary of QF, resulting in excellent lower low-temperature toughness of X80.

Microstructure Characteristics and Mechanical Properties of X80 Pipeline Steels

The relation between microstructure characteristics and mechanical properties of X80 pipeline steels was investigated using optical microscopy, scanning electron microscopy, etc. It is shown that the structure consists of polygonal ferrite （PF）, quasi-polygonal ferrite （QPF）, acicular ferrite （AF）, and granular bainitic ferrite （GF）. With increasing volume fraction of M-A islands （below 3%）, the yield strength increases. With increasing content of higher angle grain boundaries（HAGBs）, the yield strength, elongation, and DWTT properties at -15 ℃ increase, and the volume fraction of M-A islands reaches its highest point in the steel containing the most volume fraction of GF.

Effects of Cooling Paths on Through-Thickness Microstructure and Mechanical Properties of Heavy Gauge X80 Pipeline Steel

The effects of various cooling paths on uniformity of through-thickness microstructure and mechanical properties of X80 pipeline steel of 22.0 mm in thickness were studied. The finite difference method was employed to calculate the temperature field during cooling. It was confirmed by the experimental result and temperature field calculation that the optimizing process was achieved by the ultra-fast cooling with medium cooling capacity（cooling rate of ＊23 K/s）followed by ultimate cooling capacity（cooling rate of ＊50 K/s）. After optimization, the experimental steel displayed much uniform microstructure and the deviation of through-thickness hardness was controlled within 20 HV. In addition,the yield strength, tensile strength and elongation of the experimental steel were 621, 728 MPa and 21.5%, respectively,meeting the requirements of the API standard for X80 pipeline steels.

Effect of Nb on Mechanical Properties of HAZ for High-Nb X80 Pipeline Steels

The mechanical properties of heat affected zone （HAZ） of two commercial high-Nb X80 grade pipeline steels with different alloy elements were investigated using thermal simulation performed on a Gleeble-3500 thermal simulator. The results showed that the high-Nb steels have excellent weldability. Ernbrittlement regions appear in coarse grain heat affected zone （CGHAZ） and intercritically heat affected zone （ICHAZ） ~ Softening region appears in fine-grain heat affected zone （FGHAZ）, and the strength here was even lower than 555 MPa as required in the standard. Meanwhile, with the increase of heat input, the strength and the toughness of HAZ of steel with high Nb, C and lower alloy decrease notably. Therefore, take into account the welding procedure during manufacture of weld pipe, suitable amount of alloy elements, such as Cr, Ni, Cu, Mo and so on, is necessary for high Nb X80 heavy- thick steel plate.

X80级管线钢的组织和力学性能

对成分(%)为:0.04C,1.87Mn,0.27Mo,0.06Nb,0.006V,0.017Ti的X80级管线钢进行了组织和冲击韧性、强度试验,并与X70级管线钢进行对比。试验结果表明,当X80钢的组织为针状铁素体和细小弥散的贝氏体时,该钢有较好的强韧性,抗拉强度达750 MPa,屈强比接近0.85,高于X70级管线钢。

X系列管线钢的力学性能和显微组织比较

文中主要研究X系列管线钢力学性能和显微组织,通过全谱直读式光谱仪进行化学成分分析;通过金相显微镜对管线钢进行显微组织分析和晶粒度评级;通过拉伸试验、低温冲击试验及维氏硬度试验进行力学性能测试。经过分析得出结论:L415/X60的强度和硬度较高,塑性与韧性较好。

钛、锆元素对ODS钢中氧化物析出相的分布特征及材料力学性能的影响

以氧化物弥散强化(Oxides dispersion strengthened,ODS)钢合金粉为研究对象,利用合金粉的真空等温热处理来模拟ODS钢的烧结成型过程。增大ODS钢中合金元素的含量,以此研究合金元素对氧化物析出相分布特征、种类及合金粉力学性能的影响。利用XRD、XAFS、SAXS等技术手段检测球磨及真空等温热处理不同时间后合金粉中氧化物析出相的分布特征、种类变化情况,同时测量各合金粉的硬度。XRD实验结果表明,当合金中添加了钛元素和锆元素后,除铁素体衍射峰外,热处理过程中还形成了Cr_(2)O_(3)相、Y_(2)Ti_(2)O_(7)相及Y_(4)Zr_(3)O_(12)相。而在不含钛元素和锆元素的合金粉中只检测到Cr_(2)O_(3)相和铁素体的存在。XAFS实验结果表明,添加了钛元素和锆元素后,合金粉的XAFS曲线发生改变,与纯钇元素及纯氧化钇样品的曲线不同,含钛元素和锆元素的合金粉经真空等温热处理0.5 h后,其中氧化物析出相的分布密度最高为2.73×10^(22)/m^(3);氧化物析出相达到分布峰值的时间早于不含钛元素和锆元素的样品,说明钛元素和锆元素会促进氧化物相的形成。同时,含钛元素和锆元素的合金粉的硬度也高于不含钛元素和锆元素的合金粉样品。

一种扁钢管柱-X型支撑钢框架结构的抗震性能研究

钢结构具有自重轻、强度高、抗震性能好、易于装配等特点,是最理想的住宅之一,近年来在国家和地方政策的倡导下取得了较大的发展。传统钢结构住宅主要采用钢框架结构、钢框架支撑结构、钢框架剪力墙结构等,然而这些结构体系普遍存在“凸梁凸柱”问题,影响建筑使用空间和居住体验。为此,在传统钢框架结构的基础上提出了一种扁钢管柱-X型支撑钢框架结构,该结构体系采用截面宽度在200 mm以内的扁钢管柱和窄翼缘钢梁,并通过设置X型支撑来增大结构抗侧刚度,可以较好地解决“凸梁凸柱”问题。针对该新型结构体系,开展了以下工作:1)利用PKPM结构设计软件对分别采用扁钢管柱-X型支撑钢框架结构和传统钢框架结构的某学校学生宿舍楼进行了多遇地震作用下的结构受力性能及整体指标分析,包括结构周期、地震作用下结构最大层间位移角和最小刚度比,并对比了两种结构体系的用钢量;2)利用SAUSAGE软件选择了2条天然地震波和1条人工地震波,对该结构体系进行了罕遇地震下的结构弹塑性分析,统计了结构在各地震工况下的弹塑性层间位移角以及构件性能水平;3)利用MIDAS FEA有限元软件对该体系基本结构单元进行了静力推覆分析,得到了其破坏模式和延性性能。研究结果表明:1)通过合理布置扁钢管柱-X型支撑单元,结构在多遇地震作用下的各项指标均满足GB 50011—2010《建筑抗震设计规范》要求,同时具有一定的经济性,与传统钢框架结构相比节约用钢量3%左右。2)结构在罕遇地震作用下表现出良好的抗震性能,大部分构件均未出现中度以上损坏,各工况下结构最大弹塑性层间位移角均小于1/50,满足GB 50011—2010要求。3)通过静力有限元推覆分析计算得出结构位移延性系数为8.5,表明结构具备良好的延性和抗推覆性能,同时可以看出柱在与支撑相连部位有较明显的损坏,因此后续可对该部位进行加强构造措施。