Study On Microstructure And Toughness Of High Weathering Hot Rolled H-beam

With the rapid development of railway transportation in China,the weathering steel and sheet used in the railway industry urgently need to be further upgraded in combination with weatherability and strength.The corrosion rate of high weathering steels need to be less than 30% of the general steel corrosion rate.The improvement of weatherability requires adding alloy elements such as Cr,Ni and Cu to the steel,at the same time,it needs to reduce the content of C.It is necessary to adopt TMCP technology and reduce the cooling rate after rolling.These requirements are very difficult for the production of H-beam steel.In view of this,according to the performance requirements of high strength and high weathering H-beam steel for railway vehicles,three kinds of test steel with different C and Cr components are designed in view of the specific characteristics of the H-beam steel production line of Ma steel.The following tests are carried out:The CCT curves of the tested steels were plotted by the metallographic method combined with the thermal expansion method,and the continuous cooling transformation and performance change rules of the tested steels were analyzed.The results show that the ferrite transformation area of the No.1 tested steel with low Cr content is larger,the pearlite transformation area of the three tested steels is smaller,and the No.2 tested steel with lower C content is within the test cooling speed range,no martensitic transformation has occurred.From the perspective of intensity,the cooling rate of No.1 test steel to simply satisfy the tensile strength should be less than or equal to 1 ℃/s.The cooling rate of No.2 steel to meet the pure tensile strength should be greater than or equal to 0.5 ℃/s.The No.3 test steel simply meets the tensile strength cooling rate should be less than or equal to 0.2 ℃/s.The effects of deformation temperature,deformation degree and cooling rate on microstructure transformation and properties were studied by thermal simulation.The results show that,from the point of view of microstructure,No.1 test steel obtained fine ferrite,a few pearlite and small bainitic microstructure at low temperature,large deformation and slow cooling rate and its microstructure and toughness were preferably.The better deformation process scheme: a temperature of 800~850℃ and pressure ratio is greater than 30%,the cooling rate between 0.5~1 ℃/s.No.2 test steel obtained ferrite,bainite and a little pearlite structure at slow cooling rate or large deformation and moderate cooling rate.Its strength and toughness are better,and the better deformation process is as follows: the cooling rate is 0.5~1 ℃/s or the reduction rate is more than 50%,and the cooling rate is about 2℃/s.The ideal microstructure of No.3 test steel is more than 60% deformation of 800~850℃,and the cooling rate is less than 0.5 ℃/s to obtain the structure of fine ferrite and bainite with good toughness.On the basis of ensuring weatherability,in order to obtain the strength and toughness of high weather resistant steel,the composition of No.3 tested steel was optimized,and the effect of tempering process on the microstructure and mechanical properties of high weather resistant hot rolled H steel was emphatically studied.The results show that compared with the tensile properties of the hot-rolled test steel,the elongation of the tempered test steel has been significantly improved,and the yield strength decreases with the increase of the tempering temperature.The tensile strength and micro-hardness with the increase of the tempering temperature is slightly increased,and then decreased.When tempered at 450℃ the impact energy of the test steel is the highest,which meets the design requirement of the impact energy of the test steel at-40℃≥27J.Through comprehensive consideration of mechanical properties,the best tempering process for the test steel is tempering temperature of 450 ℃ for 60 min.