| In order to improve the competitiveness of 12 MnNiVR steel,Nb was added to steel and TMCP process was used to replace quenched-tempered heat treatment for production,so as to reduce cost and improve efficiency.In this paper,the Gleeble-3500 thermal simulation machine was used to simulate the TMCP process of 12 MnNiVR test steel with high Nb and the quenched-tempered heat treatment of 12 MnNiVR steel with low Nb.The structure and mechanical properties of the two test steel in different processes were compared,and the characteristics of strengthening methods were studied.The simulation of large heat input welding was carried out for two kinds of test steels.The effect of Nb on the microstructure and low temperature toughness of CGHAZ in large heat input welding was studied.The simulation of quenched-tempered heat treatment of 15 Nb test steel and TMCP process at different cooling rates of 30 Nb test steel were carried out.The microstructure and mechanical properties of two kinds of test steels under different process were studied.The yield strength was described according to the linear superposition of strengthening factors,the results show that precipitation strengthening of 15 Nb steel was the highest.Increase the content of Nb and improve the cooling speed of TMCP could make the yield strength to achieve the same level with quenched-tempered heat treatment,part of the precipitation strengthening transformation to grain boundary strengthening and dislocation strengthening.Three kinds of large heat input welding simulation were carried out for two kinds of test steels.The microstructure and mechanical properties of CGHAZ under different heat inputs were studied.The results showed that after increasing Nb content,the Ar3 in 30 Nb test steel under different heat input was lower than that in 15 Nb test steel,and the granular bainite in CGHAZ was smaller.Only when the heat input reached 150kJ/cm did polygonal ferrite(PF)structure appear in 30 Nb test steel.The prior austenite grain size of 30 Nb test steel under different heat input was smaller than 15 Nb test steel,and the relatively small prior austenite grain size improved the low temperature toughness of large heat input welding CGHAZ.By comparison,there was a relevant threshold value of prior austenite grain size.When the prior austenite grain size of CGHAZ is less than this threshold value,it can maintain good toughness at low temperature.According to the above results,the industrial trial production of Nb microalloyed 12 MnNiVR was carried out.The mechanical properties of the industrial trial production steel meet the national standard requirements,and the low temperature toughness of CGHAZ under large heat input welding was excellent. |
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