| TMCP(Thermo-Mechanical Control Process)is an advanced processing technology which can control and optimize the microstructures of steel materials and improve their performance by combination of controlled rolling and controlled cooling.When TMCP is applied to seamless steel pipes production,high strength and toughness can be obtained,and it is also helpful to realize resource-saving green production.However,there are still many restrictions on the practical application of TMCP in seamless steel pipes production due to the special cross-section shape,wide range of specification,narrow process adjustment window,complex rolling deformation and difficult cooling control.It is necessary to study the microstructure changes of seamless steel pipes during rolling deformation and on-line cooling according to the characteristics of TMCP and the laws of high-temperature deformation and cooling transformation of specific steel products.On this basis,controlled rolling and on-line heat treatment process will be realized by using and exploring the on-line resources.Therefore,taking low alloy steel 30 Mn Cr22 and high alloy steel P91 as the research materials,the high-temperature recrystallization behavior of the two steels was studied,the experimental simulation of TMCP for both kinds of pipes was carried out,the heat transfer and transformation mechanism in controlled cooling of the seamless steel pipes were studied,the microstructure evolution rule and the strengthening and toughening mechanism in TMCP of the seamless steel pipes were discussed,and the microstructures control mechanism of TMCP was proposed.Through the on-line test of typical specifications of both kinds of pipes,the on-line control of microstructures in TMCP of seamless steel pipes was analyzed in detail.The true stress-true strain curves of 30 Mn Cr22 and P91 during single-pass and double-pass hot compression were measured by Gleeble-1500 D thermo-mechanical simulator.The high-temperature flow stress-strain constitutive relation of the two steels were regressed.On this basis,the dynamic and static recrystallization rules of the two steels in the process of piercing,continuous rolling and sizing(reducing)were analyzed,and the controlled rolling strategies of dynamic recrystallization controlled rolling in piercing,static recrystallization controlled rolling in continuous rolling and non-recrystallization controlled rolling in sizing(reducing)for both steels were presented.The whole TMCP for heating,rolling and cooling of 30 Mn Cr22 and P91 steel pipe based on PQF(premium quality finishing)process was simulated by multi-pass hot compression experiment of Gleeble-1500 D thermo-mechanical simulator.Microstructure changes of the two steels in TMCP were studied.The results show that recrystallization refinement,deformation induced phase transformation and dispersed precipitation of the second phase were occurred in TMCP of seamless steel pipes.The microstructures refinement and the fine substructures in deformation and transformation microstructures were hereditary.Dynamic and static recrystallization occured through high-temperature large deformation during piercing and continuous rolling.Grains of pipe billet were refined and the fine grains were kept before sizing(reduceing).The deformed austenite was strengthened by strain accumulation in the non-recrystallization zone at low-temperature during sizing(reducing).Then the transformation structures were refined by deformation induced phase transformation combined with ultra-fast cooling for 30 Mn Cr22 or controlled cooling for P91,and the fine second phases were precipitated dispersedly.So the inheritance of excellent microstructures in TMCP was realized and the synergistic effect of multiple strengthening mechanisms of fine grain,deformation,phase transformation and precipitation was achieved.The heat transfer mechanism,dynamic phase transformation law and rapid cooling transformation strengthening mechanism in controlled cooling of seamless steel pipes were studied in order to realize the final refinement and strengthening in TMCP by controlled cooling.Cooling curves of 30 Mn Cr22 pipe samples were measured under different controlled cooling conditions on the physical simulation platform of seamless steel pipe for heat transfer in controlled cooling.It is found that the key factor affecting the heat transfer of steel pipes in air mist spray controlled cooling is the mixture ratio of air and water,which affects the cooling rate and the transformation structures of steel pipes.The heat fluxes and heat transfer coefficients of the steel pipes in air mist spray controlled cooling were calculated by the reverse heat transfer method.With the increase of cooling water flow rate and compressed air pressure,the cooling effect of the steel pipes was enhanced,the corresponding heat fluxes and heat transfer coefficients were also increased,and a finer lath martensite structure was obtained for the steel pipes.With the decrease of temperature difference(?T),the heat transfer coefficients increase and go through three different stages in turn,namely the film boiling stage at higher temperature,the stable stage at middle temperature and the transition boiling stage at lower temperature.The key to realize the ultra-fast cooling is to improve the interfacial heat transfer coefficient at the film boiling stage of higher temperature.The continuous cooling curves of 30 Mn Cr22 deformed austenite and the dynamic martensitic transformation temperatures of P91 deformed austenite in continuous cooling were measured by Gleeble-1500 D thermo-mechanical simulator,and their dynamic transformation laws were analyzed to guide the determination of TMCP parameters in sizing(reducing)and cooling after rolling.In order to obtain fine and strengthening microstructures,the finishing rolling temperature shoud be 800?,the equivalent true strain shoud be 0.25,and the cooling rate for ultra-fast cooling shoud be more than 35?/s after reducing for 30 Mn Cr22 steel pipes,and the finishing rolling temperature shoud be 990?,the equivalent true strain shoud be 0.2,and the cooling rate for controlled cooling shoud be 1?/s after sizing for P91 steel pipes.It is found by in situ observation of rapid cooling transformation process of 30 Mn Cr22 seamless steel pipe samples using high-temperature laser confocal microscopy that the faster the cooling rate is,the lower the martensitic transformation temperature is,the faster the transformation speed is,the smaller the temperature range of transformation is,and the finer the martensitic block is.When the ultra-fast cooling rate is over 70?/s,the explosive transformation of martensite can be induced and the ultra-fine lath martensite with the feature of sub-crystal refinement can be obtained.The mechanical properties of the steel pipes can be significantly improved by martensitic lath refinement and sub-crystal refinement.Different control strategies were developed to realize on-line control of microstructure in TMCP for typical specifications of 30 Mn Cr22 and P91 seamless pipes based on PQF process.High heating temperature,high-temperature large deformation dynamic recrystallization controlled rolling for piercing,high-temperature static recrystallization controlled rolling for continuous rolling and low-temperature non-recrystallization controlled rolling for sizing(reducing)were adopted for both kinds of pipes.The main difference is that the cooling intensity is different.On-line ultra-fast cooling was used for 30 Mn Cr22 steel pipes,while on-line controlled cooling was used for P91 steel pipes.Through the above control strategies,the significant refinement of microstructures and the great improvement of mechanical properties can be achieved for both kinds of steel pipes.The research and its results are of great value to optimize the process parameters of TMCP for seamless steel pipes based on PQF process,and to explore the microstructure control and strengthening-toughening mechanism in the rolling and cooling process based on TMCP.At the same time,it is hoped that the research can play a role in promoting the implementation of TMCP of seamless steel pipes in China. |
PDF Full Text Request