Study On Heat Treatment Process,Microstructure And Properties Of 590 MPa Grade Copper-Bearing High Strength Hull Steel

In this thesis,effect of the secondary quenching temperature（T_L）,quenching cooling rate（quenching method）and tempering temperature on properties,microstructure and second phase precipitation behavior of ultra-low carbon copper-bearnging hull steel were studied by means of solid solution+aging（QT）,solid solution+intercritical quenching+aging（QLT）heat treatment.The results show that:（1）Intercritical heat treatment process has a significant effect on the comprehensive mechanical properties of the steel.Compared with QT process,the tested steel treated by QLT process can significantly improve the level of low temperature toughness（in the range of 80 to 100 J）on the premise of ensuring the matching of excellent strength and toughness of steel.The yield strength ratio of steel can be reduced from 0.97 to less than0.9.The effect of secondary quenching temperature（T_L）on the strength and toughness of steel can be divided into two stages:when T_L is between 720⁷80℃,the strength and toughness of steel is relatively stable,in which the yield strength is in 656⁶75 MPa,the tensile strength is in 726⁷51 MPa,and the low temperature toughness at-84℃is in308³23 J;When T_L temperature is in the range of 780 to 840℃,the strength increases and the toughness decreases with the increase of T_L temperature.The yield strength increased from 657.5 MPa to 817 MPa,the tensile strength increased from 732.5 MPa to845.5 MPa,and the low temperature impact energy at-84℃decreased from 323 J to 266 J,with a decrease of more than 50 J.The effect of tempering temperature on the strength and toughness of steel is as follows:No matter under the condition of QT or QLT,the strength of steel decreases and the low temperature toughness increases with the increase of aging temperature.（2）Intercritical heat treatment process has a significant effect on the microstructure of steel.Under the Q process,the microstructure of the steel is single lath bainite.And under the QL process,the microstructure shows different characteristics at different T_L temperatures.When T_L is in the range of 720℃and 780℃,the microstructure is a mixture lamellar of ferrite and secondary martensite,and with the increase of T_L,the size of secondary martensite increases,the volume fraction increases,and the volume fraction of ferrite decreases.When T_L increases to 810℃,the fine lath bainite structure will be formed after water cooling,in addition to the existence of ferrite and secondary martensite,due to the complete austenite in the local area of the matrix.When T_L continue increases to840℃,because the temperature is higher than the A_c3 temperature of steel,the matrix is completely austenitized,and the lath bainite structure is formed after water cooling.Under QLT process,the matrix structure of the tested steel only recovers after tempering at560℃,and the microstructure is still a mixed structure with lamellar distribution of ferrite and secondary tempered martensite,and the effective grain size has no obvious change with its corresponding QL state.（3）Intercritical heat treatment process has a certain effect on the precipitation behavior of Cu in steel.Under the Q process,Cu is completely dissolved in the matrix.And under the QL process,when T_L is at 720℃,there are dispersed Cu precipitates,the size of which is about 30 to 50 nm;When T_L further increased,Cu is completely dissolved in the matrix.Under QLT process,there are large number of copper-rich precipitates in the samples after tempering at 560℃.When T_L is in the range of 720⁷80℃,the precipitation behavior of rich Cu in ferrite and secondary martensite is different.The size of precipitated phase in secondary martensite is relatively larger.Average size is 28.0 nm,but the size of precipitated phase in ferrite is smaller,and average size is 17.1 nm.That’s due to the difference in alloying element content between the two tissues.The secondary martensite rich in Cu is formed after intercritical secondary quenching,and the concentration of solid solution Cu is higher than that of ferrite.When T_L temperature continues to increase and the matrix is completely austenitized to form fine lath bainite,the microstructure is uniformly fine and dispersed copper-rich precipitated phase.（4）Under QLT process,the change of strength and toughness of steel is determined by the evolution of microstructure and the change of precipitated phase.When T_L is between 720⁷80℃,with the increase of T_L temperature,the strengthening and softening increment caused by the above two factors basically reach equilibrium,in which the strengthening factor is the increase of small angles grain boundary density and the refinement of effective grain size.The softening factor is that the density of precipitated phase decreases and the soft phase ferrite decreases gradually.Therefore,strength and toughness of the tested steel is relatively stable.When T_L temperature continues to increase,the dislocation strengthening,precipitation strengthening and fine grain strengthening increment increase,at the same time,the soft phase ferrite in the steel disappears,which leads to the continuous increase of strength and the decrease of toughness.（5）The quenching method has a significant effect on the mechanical properties of the steel.Under the QLT process,accelerated cooling is beneficial to obtain higher strength and low temperature toughness at the same time.Under the two cooling conditions of oil cooling and water cooling,the tensile strength of steel is basically the same,and the yield strength is obviously different,which is 611 MPa and 665 MPa,respectively.There is also a significant difference in toughness.The impact energy at-84℃is 280 J and 325 J,and the FATT₅₀ is-120℃and-155℃,respectively.The main mechanisms of rapid cooling to improve the mechanical properties of steel are as follows:rapid cooling is beneficial to obtain lath substructure and small effective grain size in the process of quenching,and plays a great contribution to the improvement of steel strength.At the same time,more large angle grain boundaries can be obtained,and the possibility of forming M/A hard brittle phase in steel can be reduced,thus the ability of steel to hinder crack propagation in low temperature environment can be improved,and the low temperature toughness level of steel can be greatly improved.Therefore,improving the quenching cooling rate is an effective way to improve the comprehensive mechanical properties of the steel for the ultra-low carbon copper bearing high strength hull steel.