| N80-1non-quenched and no-tempered seamless OCTG have many advantages such as lowcost, saving production resources, reducing production cycle and environmental pollution, butthe properties were always quite inconsistent, especially the impact toughness often tended to beon the low side, which affected the yield rate of the product and the safety of the applicationconsiderably. All in all, it was due to lack of effective process control technology and relatedtheoretical research. However, neither the conventional process control technology nor theresearch method was applicable since the process control of N80-1non-quenched andno-tempered seamless OCTG is very unique. In this paper, a new process control technology andrelated theories for N80-1seamless OCTG were studied based on tests using industrial steels36Mn2V and40Mn2V in combination with the process control features and actual parametersfor N80-1seamless OCTG, and verified in industrial practice. The main research and resultswere as follows:The controllable range for continuous rolling and sizing temperature for N80-1seamlessOCTG, the non-controllable parameters, such as the deformation degree, deformation speed ofcontinuous rolling and sizing and the air cooling rate of pipes corresponding to80regular sizeswere determined by collecting, measuring and calculating on the spot. The mathematicalrelationship between the air cooling rate and pipe sizes has been established with averageaccuracy rate at about96.70%, which provided the research of process control technology withreliable parameters.Influence of deformation parameters on deformation resistances, dynamicalre-crystallization behaviors, grain refinement, and related theoretical basis were researchedsystematically by hot simulation tests of continual rolling and sizing processes, observation ofaustenitic grain and analysis of microalloyed carbonitride precipitation. By setting up thedeformation resistance mathematical relationship (with fitting degree of about96.96%and97.33%) for the tested36Mn2V and40Mn2V steels respectively, the deformation resistancerange for continuous rolling and sizing has been worked out. By finding out dynamicre-crystallization activation energies for the two steels at about361.96KJ/mol and381.66KJ/mol which are52.35KJ/mol and72.05KJ/mol higher than plain C-Mn steels, comprehensivedynamic re-crystallization charts have been made for the two steel grades. A comprehensiveconclusion was drawn that the controlled temperature of continuous rolling for dynamicre-crystallization of fine grains shall be higher than or equal to1050℃, with the optimumtemperature at1050℃(under which the average grain size can reach about tens of micrometers).The precipitation of the carbonitride of Ti and V especially the V (N, C)，evidently delayed thedynamic re-crystallization behaviors and refined the austenitic grain size.Influence of steel grades and process parameters on properties, microstructures andprecipitations were analyzed systematically, and the law (behaviour) of property change andmicrostructure transition, the precipitating characters of microalloyed carbonitride of the tested steels and related theoretical basis were explored, through comprehensive hot simulation tests ofcontinuous rolling+reheating+sizing+cooling process, property test, microscope and TEMobservation. The quantitative relationship between the properties of36Mn2V and40Mn2V steelsand process parameters has been extrapolated, and the mathematical relationship between theyield strength and the tensile strength of the two steels has been established with deviation ofabout±10MPa between the calculated and test values. The influence of steel grades, processparameters on the microstructure and the carbonitride precipation of V coincided with that on theproperties. In which, the sizing temperature and cooling rate were the key parameters influencingthe microstructure and the properties, the strength value increased with the increasing oftemperature and cooling rate, and impact toughness value decreased with the increasing oftemperature, it first raised slowly with the increasing of cooling rate, but quickly decreased afterreaching a peak value (the corresponding critical cooling rate decreased as the temperaturedecreased). For the microstructure, bainite transformation was the bright feature, and it was themain factor that led to the remarkable drop of toughness value. The higher the temperature, themore easily bainitic structure occurred, when the cooling rate exceeded some criticalvalue(consistent with the critical cooling rate of impact toughness peak), the transformation ofbainite occurred. Main precipitations of V (N, C) and VC precipitated during the sizing processand the cooling process respectively, the temperature was the key impact factor, the higher thetemperature, the less V(N,C), the more VC. In which, VC was the main precipitation matter(accounting for more than75%) with the smallest size (5~20nm) and most uniformly distributed.Compared with steel36Mn2V, steel40Mn2V contained more pearlite and carbonitride of V butless ferrite, and the transformation of bainite occurred more easily, while the yield strength,tensile strength values were91.1MPa and157MPa higher on average, and the impact toughnessvalues were21.5J lower on an average.Basic principle and mechanism of strengthening and toughening for N80-1non-quenchedand no-tempered seamless OCTG were discussed and researched, combining with processparameters, microstructures, precipitations and properties. Strengthening and toughening effectswere caused by the superimposition of microstructure and precipitation of V carbonitride andtheir interaction. In which, VC and V (N, C) precipitation strengthening effects were the keystrengthening mechanism with VC strengthening effect the most remarkable. Microstructure wasthe main factor for toughening effect, but contradictory between strengthening and tougheningeffect. Ferrite and intragranular-ferrite were the key toughening mechanism; the strengtheningeffect of pearlite and bainite was notable, but disadvantageous to toughening especially thebainite, which embrittles remarkably. To acquire good comprehensive properties, fine F+Pmicrostructure and a mass of VC precipitations should be gained for strengthening andtoughening effects.The process control technology were explored and researched, based on the results aboveand in combination with the process control features and actual parameters. In the main propertyinfluencing factors, steel grades could be chosen rationally, temperature could be controlledeffectively, and cooling rate could be adapted to pipe sizes properly in a manner to, perfectly suitthe process control features of N80-1non-quenched and no-tempered seamless OCTG. Among them, toughness is the weakest link for steels of the type, which should be considered as toppriority, using the critical cooling rate as the important basis. As all the80researched pipe sizeswere completely covered by the applicable size ranges of steels36Mn2V and40Mn2V, theappropriate steel grades for selection, the rational control temperature range and optimal value ofcontinuous rolling and sizing were determined for each pipe size, and a suit of new processcontrol technology was established. The yield strength test values for all the sizes remainedstable at the range of600~700MPa and the impact toughness test values were far more than37J,completely satisfying the target control requirements.Based on the research results of process control technology, a database software has beendesigned using software VC++6.0, that can be utilized to access the basic parameters of differentsizes, the selected steel grades and process control parameters, to forecast deformation resistance,properties and other information. By industrial application, the new process controllingtechnology has been proved to be feasible and accuracy, the actual yield strength values werestable at the range of615~675MPa and actual impact values were more than45J with averagedeviation about±20MPa and-8~+6J respectively compared with research values, whichcompletely satisfied the acceptance requirements of API5CT and greatly improved the one-timeproduction yield rate. |
PDF Full Text Request