Precipitation behaviors of X80 acicular ferrite pipeline steel

The precipitation behaviors of X80 acicular ferrite pipeline steel were investigated by using transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The results show that dendritic precipitates in the as-cast steel slabs precipitate mainly in grain boundaries, and these dendritic precipitates dissolve and re-precipitate to two kinds of carbonitrides: Ti- and Nb-rich (Ti, Nb)(C, N) carbonitrides during reheating. Four types of precipitates mainly exist in the hot rolled plate: Ti-ri...     

Influences of microstructure and texture on crack propagation path of X70 acicular ferrite pipeline steel

The aspects of two pipeline steels with different technologies were investigated by using transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD).The microstructure presents a typical acicular ferrite characteristic with free particles of martensite/austenite (M/A) constituent,which distributes in grains and at grain boundaries.The bulk textures of the pipeline steel plate are {112}<110> and <111> fibers,respectively,and the {112} <110> component is the favorable texture benefiting for drop weight tear test.Moreover,low angle boundaries and low coincidence site lattice boundaries are inactive and more resistant to fracture than high energy random boundaries.     

热加工对管线钢针状铁素体组织形成的影响

以X70管线钢为实验材料,研究不同变形量和冷却速率对管线钢显微组织的影响。结果表明,在奥氏体未再结晶区进行适量的变形,从而形成位错、形变带和胞状组织等缺陷,可以增加铁素体在奥氏体晶内的形核位置和形核率,增大相变驱动力,有利于在随后的冷却过程中得到晶粒细小的针状铁素体组织;其中变形量2ε=0.4、冷却速率为30~60℃/s（油冷）下冷却的试样,能够得到最佳的针状铁素体组织,可以满足工程上要求组织中针状铁素体占80%以上的要求。     

Toughening mechanisms of a high-strength acicular ferrite steel heavy plate

An ultra-low carbon acicular ferrite steel heavy plate was obtained with an advanced thermo-mechanical control process-relaxed precipitation controlled transformation (TMCP-RPC) at Xiangtan Steel, Valin Group. The heavy plate has a tensile strength of approximately 600 MPa with a lower yield ratio. The impact toughness of the heavy plate achieves 280 J at ?40°C. The fine-grained mixed microstructures of the heavy plate mainly consist of acicular ferrite, granular bainite, and polygonal ferrite. The high str...     

Effect of acicular ferrite on banded structures in low-carbon microalloyed steel

The effect of acicular ferrite （AF） on banded structures in low-carbon microalloyed steel with Mn segregation during both iso- thermal transformation and continuous cooling processes was studied by dilatometry and microscopic observation. With respect to the iso- thermal transformation process, the specimen isothermed at 550℃ consisted of AF in Mn-poor bands and martensite in Mn-rich bands, whereas the specimen isothermed at 450℃ exhibited two different morphologies of AF that appeared as bands. At a continuous cooling rate in the range of 4 to 50℃/s, a mixture of AF and martensite formed in both segregated bands, and the volume fraction of martensite in Mn-rich bands was always higher than that in Mn-poor bands. An increased cooling rate resulted in a decrease in the difference of martensite volume fraction between Mn-rich and Mn-poor bands and thereby leaded to less distinct microstrucmral banding. The results show that Mn segregation and cooling rate strongly affect the formation of AF-containing banded structures. The formation mechanism of microstructural banding was also discussed.     

X80管线钢中氮化物和碳化物析出热力学分析

基于规则熔体理论和平衡反应基础,对连铸工序X80管线钢中碳化物、氮化物析出进行热力学分析,研究Ti、Nb及Al的碳化物、氮化物析出规律。结果表明,固液两相区,TiC0.005N0.995和TiN分别在1785K和1784K时开始析出;奥氏体相区,碳化物、氮化物析出先后顺序为NbC0.64N0.36、NbC、NbN、AlN、TiC,相应析出温度为1457、1404、1354、1267、1226K;γ→α相变过程中,TiC发生相间析出,Nb主要以NbC形式析出,AlN析出量较少。     

Effect of inclusions on the formation of acicular ferrite in Ti-bearing non quenched-and-tempered steel

Nucleation of acicular ferrite and its influence factors in non quenched-and-tempered steel was studied by using TEM and thermodynamic calculation. The results show that the complex particles with a center made of Ti oxide, Al2O3, and silicate and an outside made of a small quantity of mixture of TiN and MnS are able to act as ferrite nucleation nuclei. The acicular ferrite percent- age changes little with Ti. When the oxygen content was 80 ppm, the volume percentage of acicular ferrite decreased due to an in- crease in allotriomorphic ferrite. The larger the cooling rate and the shorter the incubation time, the finer the titanium oxide and the higher the nucleation ratio of acicular ferrite.     

炉卷轧机X70级管线钢组织与性能研究

采用低碳、低硫和微合金化成分设计,在1200-1220℃加热温度、790-830℃终轧温度、500-550℃终冷温度控制工艺下进行热轧处理,对3500 mm炉卷轧机生产的X70管线钢进行组织性能检验。结果表明,所生产的X70管线钢均满足西气东输二线X70级管线钢的技术要求。     

热送热装工艺对X80管线钢析出行为的影响

为了研究热送热装X80管线钢铸坯析出行为,使用了析出物定量分析、TEM和EDS测试方法,分析研究了析出物的成分、形貌、分布、质量分数以及频度分布情况.结果表明：两种工艺制度下,铸坯中析出物主要在奥氏体晶界析出,尺寸较大.室温装炉铸坯析出物在冷却和加热保温过程中发生析出、回溶和再析出行为,枝晶状的析出物大部分回溶并重新析出为两类碳氮化物,即富Ti和富Nb的（Ti,Nb）（C,N）碳氮化物.1100℃热装态的铸坯在加热和保温过程中将主要发生析出行为,析出颗粒的总质量分数要低于室温装炉条件,并且铸坯中固溶Nb的质量分数要远高于室温装炉条件.     

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deforma- tion band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.     

Microstructure homogenization control of Nb-bearing X65 pipeline steel by the CSP process

In order to get the controlled methods of microstructure homogenization and high strengthening-toughening combination by compact strip production (CSP) rolling,the dynamic recrystallization characteristics of each pass were obtained during CSP rolling using a Gleeble-1500 thermal mechanical simulator.Then,the CSP process was simulated by laboratory rolling experiment.Through thermal mechanical simulation experiment and laboratory rolling experiment,a design idea of Nb-bearing pipeline steel by CSP can be ob...     

Effects of solution environments under disbonded coatings on the corrosion behaviors of X70 pipeline steel in acidic soils

A rectangle crevice assembly was used to study the effects of cathodic protection(CP) potential,crevice thickness,holiday size,bubbling CO2,and surface condition on the chemical and electrochemical environment of the local solution under disbonded coatings.It is found that the cathodic protection removes dissolved oxygen from the crevice and thus shifts the solution to a more alkaline state.Furthermore,the potential of the steel reaches the protected potential range.The available protection distance increas...     

Inclusion variations and calcium treatment optimization in pipeline steel production

SiCa line and SiCaBaFe alloy were injected into liquid pipeline steel at the end of LF refining as calcium treatment,and samples were taken from the ladles,mould,and slabs.Analysis of Ca content and inclusions shows that Ca content in steel decreases obviously in the following process after calcium treatment; the compositions,morphology,and sizes of inclusions also vary much in the production; primary inclusions in the ladles prior to calcium treatment are mainly Al2O3 inclusions,but they turn to fine irregular CaS-CaO-Al2O3 compound inclusions after the treatment,then become fine globular CaO-Al2O3 inclusions in the mould,and finally change to a few larger irregular CaS-CaO-Al2O3 complex inclusions in the slabs.Thermodynamic study reveals that inclusion variations are related with the preferential reactions among Ca,Al2O3,and S and the precipitation of S in CaO-Al2O3 inclusions with high sulfur capacity.New evaluation standards for calcium treatment in high-grade pipeline steel were put forward according to the inclusion variations and requirements of pipeline steel on inclusion controlling,and the calcium process was studied and optimized.     

X60管线钢连铸过程碳、氮化物的析出热力学研究

连铸工艺中不同温度区间内析出的碳化物、氮化物对钢的高温力学性能有重要影响。基于规则溶液理论对X60管线钢在液相、凝固过程及奥氏体相3个阶段析出的微合金元素碳化物、氮化物进行热力学分析与计算,研究析出物的析出相阶段、析出温度及析出顺序,探讨固—液两相阶段由于溶质元素富集而导致析出物溶度积增大、增大析出可能性的因素。计算结果表明：TiN在高温固—液两相区中开始析出,析出温度为1 781 K,奥氏体相区第二相析出物为NbC,NbN,AlN,TiC。     

Study on M／A islands in pipeline steel X70

The free structure of M/A islands in pipeline steel X70 has been studied by transmission electron microscope (TEM). It is shown that the M/A islands are about 1-2 μm in size and distribute at the grain boundary of irregular massive ferrite or acicular ferrite in the microstructure of the steel undergoing TMCP (thermo-mechanical controlled processing). The analysis of diffraction contrast shows that the M/A islands consist of retained austenite and some martensite lamellae different in size and orientation. The microtwinning and midrib exist in the lamella of lenticular martensite, which exhibit the typical character of high-carbon martensite. The influences of TMCP parameters on M/A islands have been studied carefully. With the increase of the cooling speed, the amount of M/A islands decrease slightly and the morphology of M/A islands changes to thin dispersive short bars from thick irregular long strips.     

X80级管线钢的开发研究

根据X80级管线钢技术要求,对湘潭钢铁（集团）有限公司试制X80级管线钢进行成分设计、相变规律和再结晶规律研究,开发出了φ1016×22．0mmX80级热轧钢板。检验结果表明,各项性能指标均满足技术要求。     

Effects of welding wire composition and welding process on the weld metal toughness of submerged arc welded pipeline steel

The effects of alloying elements in welding wires and submerged arc welding process on the microstructures and low-temperature impact toughness of weld metals have been investigated.The results indicate that the optimal contents of alloying elements in welding wires can improve the low-temperature impact toughness of weld metals because the proentectoid ferrite and bainite formations can be suppressed,and the fraction of acicular ferrite increases.However,the contents of alloying elements need to vary along with the welding heat input.With the increase in welding heat input,the contents of alloying elements in welding wires need to be increased accordingly.The microstructures mainly consisting of acicular ferrite can be obtained in weld metals after four-wire submerged arc welding using the wires with a low carbon content and appropriate contents of Mn,Mo,Ti-B,Cu,Ni,and RE,resulting in the high low-temperature impact toughness of weld metals.     

X80管线钢变形能力的微观结构设计

在原有X80管线钢微观结构的基础上,通过不同的热处理工艺引入等轴状铁素体作为塑性相来提高管线钢的塑性变形能力,分析等轴状铁素体含量对管线钢强度、塑性和冲击韧性的影响。结果表明,随着等轴状铁素体含量的增加,管线钢的抗拉强度、屈服强度和冲击韧性逐渐下降,但均匀伸长率增加。     

Hydrogen induced cracking of X80 pipeline steel

The hydrogen-induced cracking (HIC) behavior of X80 pipeline steel was studied by means of electrochemical charging, hydrogen permeation tests, tension test, and scanning electron microscopy (SEM). The experimental results indicate that the increase of charging time and charging current density or the decrease of the solution pH value leads to an increase of the hydrogen content in X80 steel, which plays a key role in the initiation and propagation of HIC. It is found that the majority of macro-inclusions w...