Cold Treatment Temperature On The High Strength Steel Organization And Mechanical Properties Of Low Alloy Forging State

Low alloy high strength structural steels are widely used in pipe rackflanges. As in the Northeast of China, the working temperature of the flangeswill be-30~-50℃, it is meaningful to investigate the effects of cold-treatment onthe strength, toughness of the steels.In the paper, and shock test, the influences of cold-treatment temperatureon the mechanical properties of Q345C、Q420C、Q460C、CF62were studied bytension tests and shock tests. Moreover, the microstructure and fracture of thesteels were studied,too. The main results are follows.(1)At various temperature besiding25℃、0℃、-10℃、-20℃、-40℃and-50℃,.determined the indicators by tensile tests.The tensile strength and yieldstrength of the steels with decreasing temperature is gradually increased.(2)By testing the impact aborption energy at various temperature besiding25℃、0℃、-10℃、-20℃、-40℃and-50℃, determined the impact aborptionenergy under different temperature. Through the precentage of fibrous area infracture surface at various temperature, the dutile-brittle transiton temperaturesof four kinds of steels are-30℃、-35℃、-40℃and-50℃, respectively.(3)From the metallography of Q345C and CF62, obvious band structure,serious coarse grains are oberserved, which is the main reason of low impactabsorption energy for this two steels. CF62is constituted by temperedmartensite and pearlite in small size, which is helpful for toughness and strengthat low temperature.(4)The electron microprobe is adopted to identify the inclusion in Q345Cand CF62. More many surphide, silicate andAl2O3, etc exist in Q345C. Theinclusion is harmful to the low-temperature toughness. However, a few surphideand silicate can be found in CF62, which have slight impact on toughness of thesteels.(5)SEM and EDS is employed to observe and to analyze the second phrasein CF62. The result shows that the precipitates are NbN, TiN, etc. Thoseprecipitates greatly promote the refined crystalline strengthening and precipitation strengthening, improve the strength and low-temperature toughnessof CF62.