| As the level of offshore engineering equipment and technology continues to increase,offshore oil and gas exploration and development will surely move towards deep sea areas and polar regions.Offshore engineering steels with higher strength,better low temperature toughness and resistance to lamellar tearing are the development direction of offshore platform steels in the future.In this paper,the domestically produced 690 MPa grade rack steel is used as a starting point to study its phase transition and microstructure uniformity.Based on this,the alloy composition is optimized.Its continuous cooling transformation behavior;with the goal of improving the low temperature toughness of-60℃,the heat treatment process and strengthening and toughening mechanism of three 785 MPa grade rack steels were studied and analyzed.The main works are as follows::(1)Continuous austenite transformation curves(CCT curves)of two 690 MPa grade rack steels were measured.When the cooling rate of 160 mm 1.5Ni rack steel is less than 1℃/s,the structure is bainite;when the cooling rate is 1～2℃/s,the structure is bainite and martensite;when the cooling rate is greater than 2℃/s,the structure is martensite.When the cooling rate of 210 mm thick 2.8Ni rack steel is less than 0.5℃/s,the structure is bainite;when the cooling rate is 0.5-1℃/s,the structure is bainite and martensite;when the cooling rate is greater than 1℃/s,the structure is martensite.As the austenitizing temperature increases,the grain size of the 2.8Ni rack steel increases,and it coarsens significantly at 1200℃.With the increase of cooling rate,the starting point of martensite transformation of 2.8Ni rack steel decreases.2.8Ni rack steel has the best hardenability,and it is easier to obtain martensite at low cooling rates.(2)The microstructure and properties of two 690 MPa grade rack steel plates in the thickness direction were studied.The strength of the 1.5Ni rack steel plate gradually decreases from the surface to 1/2.The impact energy of the two types of rack steel plates decreased with the decrease of experimental temperature.The impact energy of 1/4,1/2 and 1.5Ni rack steel plates decreased in order,and the maximum impact energy at-60℃ of 1/4 was 39 J.The impact energy of the 2.8Ni rack steel plate is relatively balanced at various locations,and the maximum of the impact energy at-60℃ 1/2 is 210 J.On the surface of the 1.5Ni rack steel plate,1/4 is lath martensite,and 1/2 is a mixed structure of lath martensite and lath bainite.2.8Ni rack steel plate is lath martensite at every position.The microstructure of the 1.5Ni rack steel plate gradually thickened from the front and back;the microstructure of the 2.8Ni rack steel plate was uniform at all positions.Mn,Ni,and Mo segregation occurred at 1/2 of the 1.5Ni rack steel plate;Mn and Ni segregated at 1/2 of the 2.8Ni rack steel plate.The electroslag remelting can improve the uniformity of the overall performance of the rack steel extra thick plate and reduce the thickness effect.(3)The three 785 MPa grade test steels designed under the alloy system of 2.8Ni rack steel have good hardenability.When the cooling rate is less than 0.5℃/s,the structure of 3NiNbTi steel and 3Ni steel is bainite,while 3.7Ni steel has lath martensite in the structure at 0.2℃/s;when the cooling rate is greater than 1℃/s,the three test steel structures are basically all martensite.Nb and Ti can increase the temperature of steel Ac1 and Ac3.3NiNbTi steels Ac1(700℃)and Ac3(800℃)are nearly 20℃ higher than 3Ni steels Ac1(680℃)and Ac3(780℃).Ni can improve the stability of supercooled austenite.The M point of 3.7Ni steel is 360℃,which is 20℃ lower than the Ms point of 3Ni steel at 380℃.(4)As the tempering temperature increases,the strength of the three test steels gradually decreases.The tempering strength at 650℃ is the lowest:3NiNbTi steel has Rm of 893 MPa,Rp0.2 of 805 MPa,and hardness of 292 HV;3Ni steel Rm is 852 MPa,Rp0.2 is 781 MPa,hardness is 271 HV;3.7Ni steel has Rm of 864 MPa,Rp0.2 is 796 MPa,and hardness is 278 HV.3NiNbTi steel and 3.7Ni steel are tempered and brittle at 300～500℃,3Ni steel is tempered and brittle at 200～550℃,When tempered at 650℃ and impacted at60℃,the impact energy of the three steels is 200 J,162 J,and 203 J,all of which have good impact toughness.With the increase of the tempering temperature,the size of the 3NiNbTi steel plate and the proportion of the large-angle grain boundaries change little,and the density of the large-angle grain boundaries gradually increases.The addition of Nb and Ti can significantly refine the austenite grains and improve the strength and impact toughness.The addition of Nb and Ti and the increase of Ni content can effectively increase the proportion and density of large-angle grain boundaries,at the same time refine the size of the slat bundles and slat blocks,hinder crack propagation,and improve impact toughness.The three designed test steels can meet the requirements of 785 MPa grade rack steel.3NiNbTi steel is tempered at 650℃,and 3Ni steel and 3.7Ni steel are tempered at 600℃ to obtain the best strength and toughness matching. |
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