Study On The Effect Of Intermediate Billet Cooling On Microstructure And Properties Of Hot Continuous Rolling X70 Pipeline Steel

Controlled rolling and controlled cooling is an important process technology for the development and production of high-grade pipeline steel.The controlled rolling is mainly carried out in the austenite complete recrystallization zone and the non-recrystallization zone.The purpose is to refine the austenite grains and introduce enough dislocations and defects to provide more nucleation sites for subsequent phase transitions.In order to achieve the purpose of controlled rolling,it is generally required that the steel is to be waited to the setting temperature after rough rolling of the pipeline steel,which leads to the problem of low production efficiency.In order to improve production efficiency,adding a rapid cooling system after rough rolling for intermediate billet cooling is an effective process solution.However,at present,there is less research on the cooling of the intermediate billet of high-grade pipeline steel based on the hot strip rolling,and the influence of the intermediate billet cooling process on the microstructure and properties of X70 pipeline steel needs to be further revealed.In this paper,X70 pipeline steel was used as the research object,and the influence of intermediate billet cooling on the microstructure of X70 pipeline steel was studied,which provided a theoretical basis for the industrial application of the intermediate billet cooling process.The main research contents of this paper are as follows:(1)The effect of the cooling rate of the intermediate billet on the austenite grain refinement of X70 pipeline steel was studied by thermal simulation experiments.The results show that the cooling rate of the intermediate billet increases from 0.1?/s to 40?/s,the austenite coarsening is inhibited,the austenite grain size before refining is refined from 43 ?m to 28 ?m,and the austenite grain size after the finish rolling is refined from 37 ?m to 27?m.(2)The evolution of the final microstructure of X70 pipeline steel under different intermediate billet cooling rates was studied by thermal simulation experiments.The results show that the microstructure of X70 pipeline steel consists of AF,BF and M/A.As the cooling rate of the intermediate billet increases,the final microstructure of the experimental steel is refined and the percentage of acicular ferrite increases.(3)The effects of different intermediate billet cooling rates on the microstructure and properties of X70 pipeline steel were studied by hot rolling experiments.Compared with the conventional air-cooled process,when the intermediate billet cooling rate is 4?/s,the microstructure of the thickness direction of the experimental steel is refined,and the effective grain size of the central portion of the structure is refined.3.9 ?m to 3.6?m,and the strength is increased by about 20MPa.As the cooling rate of the intermediate billet continued to increase,the micro structure of the experimental steel did not change significantly.(4)Through the hot rolling experiment,the effects of different starting temperatures of the intermediate billet on the microstructure and properties of X70 pipeline steel were studied under the same cooling rate.Compared with the conventional process,the microstructure of the experimental steel under the different starting cooling temperature of the intermediate billet is refined and the performance is improved.When the intermediate billet starting cooling temperature is 1100,1020 and 950?,the obtained experimental steels have good uniformity in the thickness direction,and the effective grain size and mechanical properties are not much different.(5)According to the above experimental rules and the arrangement of field equipment,the corresponding experimental scheme was designed,and the X70 pipeline steel plate was tested by the experimental scheme.The test shows that the microstructure and properties of the experimental steel under the intermediate billet cooling process meet the API standards.The results of industrial trial production show that the X70 pipeline steel is produced by the intermediate billet cooling process,the microstructure and properites meet the standard,and the steel swing time is reduced by about 40s.