| This paper mainly takes the 80 mm thick SA537CL1 steel currently produced by Nanjing Iron and Steel Co.,Ltd.For pressure vessels has insufficient strength after normalizing and post-weld heat treatment(PWHT),especially the serious deterioration of the impact toughness at the core at-50℃.Compared with the similar Q370 R and high-Ti steel plates with different additions of Nb,V and Ti,the microstructure and mechanical properties are compared,and the preliminary optimization plan for SA537CL1 steel is proposed,the optimization effect is evaluated,and further improvement of the composition and process of this steel is proposed.The main conclusions of the study are as following:(1)Under different processes(hot rolling → normalizing → PWHT)of SA537CL1 and Q370 R steel,the microstructures along the thickness direction are mainly ferrite and pearlite,but the core structure is uniform,and a large number of them are formed due to Mn segregation.The abnormally high hardness structure(granular bainite)of the ferrite seriously deteriorates the impact toughness of the core;,V carbonitride)the number of precipitates is much more than SA537CL1,making it slightly higher than SA537CL1 in both normalizing and PWHT processes.The coarsening of second-phase particles of PWHT is the main factor that causes the strength of the steel to decrease compared to the normalized state.(2)The high Ti steel plate with higher Ti content than Q370 steel was used for the test,and its microstructure under different processes was similar to that of Q370 R.As the Ti content of this steel is further increased,it is found that the number of second-phase precipitated particles in the ferrite is significantly more than that of Q370 steel under normalizing or PWHT process,and the size is smaller,making its strength even under PWHT process.It can still maintain a higher level,and the elongation after breakage does not decrease significantly.(3)The SA537CL1 steel is optimized by appropriately reducing the C and Mn contents,appropriately increasing the V and Ti microalloying contents,and further decreasing the normalizing temperature.It was found that a slight decrease in Mn could not alleviate the abnormal structure formed due to Mn segregation;on the other hand,a decrease in C content drastically reduced the pearlite content.Owing to the increase of V and Ti content,the number of second-phase particles in normalizing and PWHT processes is significantly increased compared with that before optimization,and the size is refined.In terms of mechanical properties,when the pearlite content of the steel is significantly reduced after optimization,the strength of the steel under normalizing and PWHT processes is slightly lower than that before optimization,which means that the secondary carbide has a significant strengthening effect;at the same time,it is found that the low-temperature impact of the core is The toughness is obviously improved,and it has basically met the stricter technical requirements of users.The mechanism is the reduction of pearlite,the dispersion strengthening of secondary carbides and the refinement of ferrite.(4)The fundamental solution to improve the low-temperature impact toughness of the core must start from the source processes such as continuous casting and hot rolling,and effectively suppress the Mn segregation in the core.In addition,it is necessary to ensure sufficient C and Mn content,and further increase the composite addition of Nb,V,and Ti to enhance the precipitation strengthening effect,and ensure that the strength of the steel reaches relevant standards under normalizing and PWHT processes. |
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