Control Cooling And Tempering Process Research Of Marine Engineering Structure Steel Grade NVF550after Controlled Rolling

At present, marine engineering equipment are proposed for high strength, lowtemperature impact toughness, excellent welding performance for the steel with yieldstrength of550MPa which used in ocean engineering structure. Such demand has itsimportant meanings: first it can reduce the weight of equipment, second it can improvethe quality of equipment manufacturing. So, the yield strength of550MPa steel usedin marine engineering structure possess the above requirements, the key is to use themethod of low carbon bainite micro alloying composition with low carbon equivalentto solve the matching relationship between strong toughness and welding performance.Due to low carbon bainite steel micro-alloy high strength structure supplied in thecondition of control rolling and cooling has the properties of low carbon equivalent,strong toughness and weldability, etc., marine engineering structure steel grade NVF550are taken as the research object in this article. To optimizing its organizationalperformance, the thermal simulation technique, microstructure and performancemeasurement method are used to study the marine used steel grade NVF550. The mainwork includes: measured the CCT curve of NVF550steel using Gleeble-1500thermalsimulation testing machine; research the static continuous change rule of steel, and theeffect of cooling rate on the steel group through analysis of the phase transition curveand microstructure. The cooling process of required structure is determined through theanalysis microstructure. And simulate rolling for NVF550is carried out in thelaboratory conditions. Microstructure and performance is studied on rolling pattern andcombined with metallographic microstructure analysis, and compares the differentproperties at different tempering temperature, and optimum tempering temperature is determined. Optimal process program under the laboratory condition is verified byindustrial production.the main subjects are focus on the follows:1、The research results show that when cooling speed for NVF550grade steel is8℃~15℃/s/s, microstructure is the transformation accicular ferrite+granularbainite, it keeps becoming more refine with the increase of cooling rate. Phase transitiontemperature range is500℃~607℃.2、 Experimental steel has great rolling plasticity, good strength, and Lowtemperature brittle ductile transition occurs under60℃, which satisfy the requirementsof GJ712and Det Norske Veritas Rules.3、The comprehensive properties of test steel are greatly improved throughtempering process. Considering the demands of ship rules, and adopt appropriatetempering temperature at about650℃, the comprehensive mechanical properties quitemeet requirements of GB712and Det Norske Veritas Rules.4、Using start temperature of roughing rolling of1000℃or higher, rough rollingfinishing temperature1000℃or higher, accumulative deformation rate more than60%;starting temperature finishing rolling of880℃or less, finishing temperature820℃orless and the accumulative deformation rate more than60%, cooling speed>10℃/s,and self tempering temperature480~540℃, tempering temperature650℃afterrolling. Industrialization trial production of the steel plates can satisfy the requirementsof the ship rule with good welding performance.