Study On Microstructure And Properties Control For 1000 MPa Grade Cr-mo High Strength Structure Steel

Since entering the 21st century,offshore oil has become an important direction of petroleum exploitation in the future,but the traditional oil steel is difficult to meet the requirements of the exploitation in the new century.To ensure the stability and safety of drill pipe and casing,more higher strength and toughness of petroleum steel must be required in the exploitation process,and how to improve strength and toughness simultaneously has become an urgent problem to be solved.To solve the problem of adjust and control on strength and toughness,the controlled rolling and deep cryogenic treatment（DCT）is one of useful ways on the improvement of strength and toughness,but there are still relatively few studies on the microstructure evolution and strength-toughness mechanism of high strength-toughness steel in petroleum.Therefore,the study of microstructure evolution and strength-toughness mechanism of ultra-high strength steel（UHSS）by controlled rolling and DCT has an important theoretical significance and engineering application value.The bainite and martensite（B/M）high strength steel was investigated in the paper,carried out the continuous cooling hot simulation,DCT treatment in phase transformation zone,controlled rolling and DCT treatment experiments,studied the phase transformation behavior of undercooled austenite,analyzed the effects of DCT on microstructure evolution and strength-toughness,revealed the influence mechanism of controlled rolling and DCT on strength-toughness,and obtained a new method for adjusting the strength-toughness.The main research and conclusions are as follows:（1）The phase transformation behavior of undercooled austenite was studied via the continuous cooling and cyclic cooling processes,analyzed the influence of cooling parameters on the B/M phase transformation.The result shows that hardness was increased with the cooling rates in 0～50℃/s.The hardness and stability of microstructure was decreased while the cooling rates over 80℃/s.The microstructure is mainly composed of lath bainite（LB）,lath martensite（LM）,acicular ferrite（AF）,grain boundary ferrite（GBF）and retained austenite（RA）.And it undergone the transformation stage of LB+LM+AF+GBF+（RA）（0～2℃/s）→LB+LM+AF+RA（2～30℃/s）→LM+RA（50～80℃/s）during the change of cooling rate.Combined with microstructure characterization and hardness analysis,the appropriate cooling rate can be selected for strength and toughness control within 2～50℃/s.In the 900～1100℃,the phase transformation temperature was decreased with the cyclic cooling temperature and times;the phase transformation rate of B/M was increased with temperature and decreased with the cyclic times.（2）The relationship of strength-toughness and controlled cooling process in different phase transformation zone was investigated,analyzed the effect of cooling path on the microstructure transformation and the precipitation of carbides,and clarified the influence of DCT on strength and toughness.The result shows that the heat preservation in austenite zone and cooling process is more benefited to the increase of strength than the dual phase zone process.The tensile strength R_m and yield strength R_p0.2 were up to 1886 MPa and 1372 MPa under water cooling process.R_m and R_p0.2were increased with DCT time in austenite zone and decreased in dual phase zone.The microstructure is mainly LM+LB+RA+carbides in the DCT treatment of austenite zone,and carbides changes from grain boundary to intragranular precipitation with the increase of DCT time.The microstructure is LM+LB+F+RA+carbide in DCT treatment of dual phase zone,which is benefited to reduce the local stress and improve the strength and toughness.（3）The effects of controlled-rolling and controlled-cooling processes in austenite zone and austenite+dual phase on the strength-toughness were studied,analyzed the relationship of rolling methods and microstructure transformation such as B/M and RA.The result shows that the controlled-rolling and controlled-cooling process in austenite+dual phase zone is more benefited to improve the strength and toughness,R_m can reach more than 2000 MPa under controlled rolling and water cooling but will reduce plasticity.R_m and R_p0.2 increase with the DCT time,R_m and elongation A%can reach 1938 MPa and 4.4%.The main microstructure is refined LM+LB+RA in the austenite zone rolling process,and DCT for a short time after rolling is not conducive to the precipitation of carbides.Besides,the austenite+dual phase rolling process is more benefited to improve the strength and toughness via increasing the dislocation,precipitates and{332}<113>texture.（4）The effects of controlled rolling and DCT parameters in austenite zone+dual phase zone on the strength and toughness were studied,expounded the influence mechanism of controlled rolling cryogenic process on the strength and toughness.The refined LM,LB and dislocations was increased in controlled rolling of austenite+dual phase zone.beside,the DCT treatment promoted the precipitation of carbides and transformation of RA,both of which improve the strength and toughness together.The increase of refined LM,LB and precipitation is contributed to impede dislocation movement and hinder the plastic deformation furthermore.Based on the deformation induced phase transition mechanism,the strength and toughness can be further improved via RA transformed to the refined LM.（5）The combination of controlled rolling and DCT treatment is benefited to improve the strength-toughness properties concurrently.The controlled rolling in austenite+dual phase zone and DCT treatment 4 h has the most obvious effect on improving the strength and toughness properties.Combined with the stress relief process,it is expected to produce more than 2000MPa grade UHSS for petroleum,which has important theoretical guidance and engineering significance to produce ultra-high strength structural steel in the future.