Investigation On The Development Of High Grade Pipeline Steels With Ultra Fast Cooling

In order to adapt to the requirement of the economy and safety of pipelineengineering, pipeline steels require high strength, toughness, weldability and highresistance to hydrogen induced cracking (HIC), sulfide stress corrosion cracking(SSCC)and CO2corrosion. The X80pipeline steel has been used as the first choice for the oiland gas pipeline steel grade at home and abroad. X100and X120steels are the focus ofthe future pipeline research and development. Microalloying and thermal-mechanicalcontrol processing (TMCP) are the main technique for the product of pipeline steels.The addition of alloying elements, such as Mn, Mo, Cr, Ni, Nb, Ti, V and B, has theeffects of precipitation strengthening, fine-grained strengthening, solid solutionstrengthening and phase transformation strengthening. Due to the limit of rollingcondition and cooling speed, thermal-mechanical controlled processing can not playfull role in the development of high grade pipeline steels. However, the ultra fastcooling process provides an approach to solve the problems above. In the present work,in order to make full use of rolling and cooling on the control of mechanical propertiesas much as possible, and minimize the consumption of alloy elements, and thus saveenergy and resources, the high grade pipeline steels have been developed applyingultra fast cooling technology.In this paper, according to the requirements of high grade pipeline steels for nationalenergy pipeline construction and the important product development for steel corporation,low cost X80and X100pipeline steels have been investigated by a novel ultra fastcooling technology. The industrial trial production has been carried out on the basis oflaboratory and pilot plant tests. The literature review and current status at home andabroad is presented in the first part of thesis (Chapter1). The second part consists oflaboratory research works (Chapters2–5). The third part is trial production on anindustrial scale (Chapters6–10). The fourth part is the summary and outlook (Chapter11). The recrystallization behavior, high temperature deformation resistance andcontinuous cooling transformation were investigated by means of thermal simulator,thermal dilatometer and optical microscope. The evolution behavior of the oxide scalewas analyzed by high temperature thermogravimetric analyzer, hot simulationexperiment machine and other equipments, The microstructures and mechanicalproperties of high grade pipeline steels were investigated using optical microscope,scanning electron microscope, transmission electron microscope, X-ray diffraction andenergy dispersive system, microhardness tester, universal testing machine, impacttesting machine and drop hammer test machine. The corrosion resistance propertieswere investigated by mans of NaCl, CH3COOH, H2S aqueous solution and autoclave.The dynamic recrystallization mathematical model of X80pipeline steel isexpressed asz exp(356.59/RT)Z=1.61012[sinh （0.012)]4.095andp. Thedynamic recrystallization mathematical model of X100pipeline steel is expressed asZ exp(399.23/RT)Z6.4247.014andp. The static recrystallization activation forX80pipeline steel is QSRX＝393kJ mol-1and the static recrystallization kineticsX SRX1exp[0.693(t/t)0.95equation is0.5]. The static recrystallization activation forX100pipeline steel is QSRX＝365kJ mol-1and the static recrystallization kineticsX SRX1exp[0.693(t/t)0.95equation is0.5]. The high temperature deformationresistance model for X80pipeline steel are0.0σ=3327.758ε0.315109ε exp （-0.002Τ-1.12ε）and for X100pipeline steel is=3155.8080.480.13exp（-0.002T-0.65）. The static and dynamic continuous coolingtransformation curves for X80and X100pipeline steels have been carried out underdifferent cooling rates, which provides theoretical base for the control of ultra fastcooling modes.The low-Mo X80pipeline steel treated by a novel ultra fast cooling process has very similar microstructure and mechanical properties as those of the high-Mo pipelinesteel treated by the conventional accelerated continuous cooling process. The aboveresults have been verified by pilot plant test and industrial trial. It shows that the ultrafast cooling process can obtain superior mechanical properties by reducing alloy cost.The excellent composite mechanical properties are attributed to grain refinement. Thisnovel process provides a new technique to manufacture low cost high strength lowalloy steels. The developed X80and X100pipeline steels have low yield ratio andsuperior low temperature impact toughness. The ultra-fast cooling results in workhardened austenite before phase transformation and also low dynamic transformationtemperature. Also, the precipitation of Nb-Ti carbonitrides is inhibited during ultra fastcooling.The two principles of ultra fast cooling still insist on the conventional TMCP,namely the control of austenitic hardening and the phase transformation of thehardened austenite. Therefore, ultra fast cooling plays an important role in grainrefinement. In the present work, complete thermal deformation occurs at hightemperature, but the steel is still in the state of non-recrystallization in a very shorttime after deformation, and is in a high-energy state which contained a large number of"defects". There are many nucleation sites for the phase transformation. Theimplementation of ultra fast cooling ensures passing through the austenite region in avery short time, resulting in work hardened austenite before phase transformation. Thework hardened state austenite is beneficial to refine the transformed product. The ultrafast cooling pushes transformation to lower temperature, compared with conventionallamellar cooling. The microstructure is transformed at lower temperature and refinedby a larger driving force. Therefore, more fine-grained microstructure and superiortoughness are obtained for ultra fast cooling.According to the NACE standard, the corrosion resistance of the X80pipeline steelproduced by the newly developed ultra fast cooling to sulfide stress corrosion cracking,hydrogen induced cracking and CO2have been investigated. SSCC corrosion results show that the critical stress value to produce cracking is about65﹪s(390MPa).Above the critical value, corrosion sensitivity of the test specimen is high, and thecorrosion resistance is poor. Below the95﹪sloading level, the stress sensitivity isextremely high. HIC corrosion results show that the crack sensitive ratio, the ratio ofcrack length and width are zero. CO2corrosion resistance results show that when thepressure of CO2is0.1MPa, the average corrosion rate is0.6843mm/a. It can be seenthat the X80pipeline steel produced by the ultra fast cooling process has superiorresistance to SSCC, HIC and CO2.The oxide scale of X80steel produced by ultra fast cooling process is differentfrom that of Q235B. The oxide scale structure of the X80has five layers. The ultra fastcooling process and alloying elements are the main reason for the difference of hightemperature oxide scale structure.On the basis of above research results, rational technological route has been made.Fine-grained microstructures and superior mechanical properties of X80and X100pipeline steels have been developed by the implementation of an ultra fast coolingprocess. The excellent composite mechanical properties attribute to grain refinement.This ultra fast cooling process provides a new technique to manufacturesuperior-strength high-toughness low-alloy steels with low cost alloy additions.