Micro-alloyed 690 MPa High-strength Seismic-resistant Refractory Steel Microstructure And Properties

Fire-resistant steel is a kind of low-alloy high-strength steel with good fire resistance.The yield strength at 600 ? in the event of a fire is not less than 2/3 of the room temperature yield strength,and has good high-temperature mechanical properties.The strength of fire-resistant steels currently studied in China is still at 345 MPa,490 MPa and 590 MPa,the high-temperature yield strength is low,the Mo content added to the steel is higher,and the production cost is relatively large.With the increase of building height and frequent occurrence of natural disasters,the yield strength of fire-resistant steel for general construction cannot meet the bearing capacity in complex service environment.Therefore,it is of great significance for the research of fire-resistant steel with high yield strength and excellent seismic performance.In this paper,a low-molybdenum 690 MPa class anti-seismic refractory steel plate was designed through microalloying.The structure and properties of the test steel plates were studied through different process conditions and heat treatment experiments.The test steel plate with the fastest cooling rate after rolling was subjected to two heat treatments of 740 ? sub-temperature quenching and 740 ?sub-temperature quenching + 400 ? tempering.The precipitation mechanism of niobium and titanium in fire-resistant steel is calculated.The rolling process test results show that the structure of the test steel plate after laminar cooling at different cooling rates is ferrite,granular bainite and martensite.The amount of ferrite in the test steel plate showed a decreasing trend with the increase of cooling rate,and the volume fraction of granular bainite showed an increasing trend.When the cooling rate is large,the structure of the test steel plate is mainly composed of lath martensite.As the cooling rate increases,the yield strength and hardness of the test steel plate also gradually increase.The room temperature yield strength of the test plate with the highest cooling rate reached 902 MPa,which met the room temperature strength requirements of the test steel plate,but the high temperature yield strength was less than 460 MPa and failed to reach 2/3 of the room temperature strength,and the plastic toughness was also poor.The test steel plate also needs to be heat-treated to improve its high-temperature yield strength and toughness.The impact test results show that as the cooling rate increases,the impact toughness of the test steel plate gradually decreases.The results of extraction and replication showed that the precipitated phases in the experimental steel were(Nb,Ti)C and Ti C.The test steel plate after rapid water cooling has more martensite structure,and its yield strength at room temperature and high temperature is the highest.However,the toughness is poor,so the test steel plate after rapid water cooling is tempered and heat treated.In this paper,different heat treatment experiments were carried out on the test steel plate with the highest cooling rate after rolling.The structure of the test steel plate after 740 ? sub-temperature quenching is granular bainite,tempered martensite and a small amount of ferrite.After tempering at 400 ?,the volume fraction of granular bainite and ferrite in the structure increased compared with 740 ?subthermal quenching,and the volume fraction of tempered martensite decreased.After different heat treatments,the room temperature tensile strength and yield strength of the test steel plate were reduced compared with before.However,elongation after break,-40 ? impact energy and high temperature yield strength have been greatly improved.The high-temperature yield of the two test steel plates after heat treatment exceeded 460 MPa,which met the high temperature strength requirements of 690 MPa grade fire-resistant steel plates.After 740 ?sub-temperature quenching + 400 ? tempering,the number of precipitated phases in the test steel plate has increased,and the room temperature and high-temperature yield strength are higher than 740 ? sub-temperature quenching.It can be found from the measurement results of retained austenite that the amount of retained austenite is lower after the test steel plate is water cooled.After 740 ?sub-temperature quenching and 740 ? sub-temperature quenching + 400 ?tempering can increase the amount of retained austenite in the test steel plate.The existence of a certain amount of retained austenite is the main reason why the test steel plate maintains good toughness.The first-principles simulation calculation results of the precipitation phase of microalloy carbonitride show that with the increase of Nb content,the binding energy and formation heat of niobium-titanium carbide decrease first and then increase.When the Nb content is 0.5,the binding energy and formation heat energy of(Nb0.5,Ti0.5)C are the lowest,the structure is the most stable,and it is easier to form.The calculation results of the interface energy show that the interface energy of ?-Fe /(Nb0.5,Ti0.5)C interface is the largest and that of ?-Fe / Ti C is the smallest.It shows that the most likely method of niobium-titanium composite precipitation is titanium carbide –based on niobium-titanium carbide composite precipitation.