| With the development of pipeline transportation industry, the pipeline steels with high strength, toughness and deformability are paid more and more attention. X100and X120ultra-high strength pipeline steels with good match of strength and toughness, resistant HIC, SSCC, SCC are keys to ensure the security and reduce the operation cost, for which the X100and X120grade pipeline steel will be extensively applied in the future.This paper was based on the problems which include the rigorous requirements of equipments, increasing cost of alloying elements and production efficiency using low temperature large deformation and micro-alloying process by adjusting TMCP. Effect of rolling and cooling processes on microstructures and properties of high grade X100pipeline steel was systematically researched by means of heat simulation technique and mechanical properties test and microscopic analysis method. Meanwhile,"controlled rolling&relaxation&fast cooling" process and "controlled rolling&intermittent fast cooling" process were proposed, and the relationship was studied between strength-toughness combinations and microstructure of high grade X120pipeline steel. The main works involved as follows:(1) The hot deformation behavior and static recrystallization for two tested steels, Mn-Nb-Mo-Ti micro-alloyed X100pipeline steel and Mn-Nb-Mo-B-Ti micro-alloyed X120pipeline steel were studied by using MMS-200thermal simulator. The dynamic recrystallization activation energy and the static recrystallizational activation energy were determined, respectively. Dynamic recrystallization characteristic parameters were also determined. Results show deformation temperature was the main factor that influences the dynamic static recrystallization behavior and the recrystallization behavior, and decreasing deformation temperature inhibits recrystallization. Deformation storage energy increased with increasing strain and static recrystallization rate could be accelerated. Least square method was used to confirm the static recrystallization activation energy and dynamic recrystallization activation energy of X120were higher than that of X100. Peak characteristic parameters and critical characteristic parameters of X120pipeline steels containing boron were also higher than that of X100. For no obvious peak value point strain-stress curves, the existence of dynamic recrystallization could be determined by analyzing the relationship between strain-harden rate and strain.(2) Microstructure and transformation behavior were investigated by the continuous cooling experiments, and the continuous cooling phase transition kinetics for XI00and XI20pipeline steels have been constructed. The results show that the acicular ferrite phase transformation and granular bainite phase transformation were accelerated observably, and the starting temperature of phase transformation were raised notablely by increasing deformation and decreasing deformation temperature. The addition of B could refine lower bainite grains and accelerate lower bainite transformation, thus retarded the formation of ferrite and intermediate transformation microstructures, meanwhile, CCT curves became more flat.(3) The influence of heating temperature, rolling and cooling parameters on microstructure and refinement had been studied by rolling processes simulation test. The results show that the best effect of controlling and refinement for austenite, martensite and retain austenite (M/A) and precipitates could be achieved by all controlling hot working processes, namely, accurate controlling the parameters of reheat temperature, rough rolling, finish rolling and cooling rate.(4) The effects of TMCP and HTP on microstructures and properties of Mn-Nb-Mo-Ti micro-alloyed X100pipeline steel and high Nb pipeline steel were studied, respectively. Compared with the grains obtained by TMCP, the grains by HTP were larger, the strength, elongation and CVN toughness were not improved, while yield ratio dropped sharply. For Mn-Nb-Mo-Ti micro-alloyed XI00pipeline steel, when the started rolling temperature, finished rolling temperature, finished cooling temperature and cooling rate was780℃-800℃,760℃-780℃,440℃-460℃and about30℃/s, respectively, and the yield strength, tensile strength, elongation, CVN toughness at-20℃, ductile-brittle transition temperature (DBTT) was751MPa,894MPa,20.6%,247J, and-62℃, respectively, the comprehensive mechanical properties were better in laboratory. According to the regression formula, the mechanical properties of the tested steel were predicted σs;=769MPa, σb=907MPa, δ5=19.7%, which were well correspondence to actual ones.(5)"Controlled rolling&relaxation&fast cooling" process was proposed by an investigation on refining intermediate transformation microstructures of XI20ultra high strength pipeline steel. The number of acicular ferrite as softer phases of the tested plate rolled by "controlled rolling&relaxation&fast cooling" process was controlled by adjusting start cooling temperature. Dislocation cells and substructure were formed by dislocation interaction during relaxation, then being rapid cooled became harder phases (lower bainite). The resisting large deformation property and yield ratio were optimized by acicular ferrite+lower bainite dual-phase microstructure, and the strength and toughness were improved. The ideal process scheme for producing X120grade pipeline steel was that the tested steel was deformed at final rolling temperature of800℃, cooled to about650℃in air, and cooled at a cooling velocity of above50℃/s with UFC process after about400℃.(6) The effects of rolling and cooling parameters on microstructures and mechanical properties of XI20ultra high strength pipeline steel were studied by means of laboratory hot rolling experiments adopting "controlled rolling&intermittent fast cooling" process. Distortional strain energy accumulate was formed by heavy deformation by lower finished rolling temperature, then was remained at the transition region of γâ†'a by fast cooling, in which acicular ferrite and granular bainite were obtained by means of relaxation. Subsequently, lower bainite and martensite transformation were occurred by cooling to below martensite transformation start (Ms). The most superior process parameter was that finished rolling temperature was about850℃, relaxation was560℃for10seconds, and cooling velocity was over50℃/s. A lot of deformation bands were retained in the harden austenite, which provided nucleation sites and the energy required for the transformation, leading to improvement of mechanical properties of the plates.(7) Effects of on-line heat treatment processes on microstructure and properties of X120pipeline steel were studied. The results show that amount of large angle grain boundaries of the direct quenched-tempered steel plates tempered at450℃for40min was slightly more than that of the two stage quenched-tempered steel plates. When tempering temperature range was450℃to500℃, the tempered steel which had good mechanical properties and mechanical stability could be obtained by two stage quenched-tempered process.In view of pollution reduction and protect the environment, the heat treatment steels with a high strength and toughness and high mechanical stability were obtained by the two stage quenched-tempered process, which were also used to technique reserve of heat treatment. |
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