Study On Rolling Process And Performance Of Q500NHE Weathering H-beam For Building

In recent years,under the strong advocacy and support of the Chinese government,prefabricated buildings and steel residential buildings will become the mainstream of the construction market.At the same time,the Chinese national development plan proposes that urbanization and housing construction should reach more than 60%by 2020,so structural steel for construction is already facing huge demand.H-beam is named by its efficient H-shaped cross-section.It is widely used in critical structural parts such as beams and columns in buildings due to its excellent mechanical properties,bending resistance,and seismic performance.As the main structural component of high-rise and large-scale buildings,the development of new high-strength H-shaped steel has become the fundamental guarantee for the rapid development of national construction.In addition,the corrosion problem of structural steel for construction cannot be ignored due to numerous accidents and losses caused by corrosion every year.As a significant steel material country,China bears enormous expenses in the maintenance and repair of corrosion.Based on the knowledge of physical metallurgy,material science,and corrosion electrochemistry,a series of researches has been carried out on the rolling process and corrosion behavior of hot-rolled weathering H beams with a yield strength of 500 MPa.(1)Based on the investigation of related literature,two kinds of experimental steel with yield strength up to 500 MPa were designed by using low C,low P,low S and optimizing the proportion of alloying elements Cu,Cr and Ni,and based on the strengthening effects of microalloying elements Nb and Ti.Subsequently,Thermo-Calc thermodynamic software was used to plot and analyze the change curve of the content of each phase of the experimental steel with temperature under the equilibrium state.The phase transformation point and continuous cooling phase transformation behavior of the experimental steel were measured by the thermal expansion method.The results show that the optimized composition and microstructure of the experimental steel can meet the strength requirement while ensuring the weather resistance and welding performance.(2)A single-pass compression test was carried out on Q500NHE experimental steel,and the influence of deformation temperature,deformation rate,and degree of deformation on deformation resistance were analyzed.The results show that the deformation temperature has the most significant effect on the deformation resistance of the experimental steel.The rise of the deformation temperature will accelerate the recovery and softening of the metal,which will offset the work hardening of the deformed metal and reduce the deformation resistance.When the deformation temperature is higher than 1100?,the stress-strain curves of the experimental steel in the strain rate range of 1?10 s-1 are all recrystallized.Then the constitutive equation and the deformation resistance model were constructed,which provided a reference basis for the calculation of the rolling force and the verification of the rolls in production.(3)Simulating rolling tests were carried out on the flanges of Q500NHE weathering H-beam with different thickness specifications.Two-stage rolling were adopted,and different precision rolling temperatures are controlled by controlling the precision rolling start temperature.The results indicate that at the same target thickness,as the final rolling temperature decreases,the content of Bainite in steel and the yield ratio decreases,while the impact energy increases.Analyses and quantitative calculations were carried out about the increments of Hall-Petch strengthening,precipitation strengthening,solid solution strengthening,and dislocation strengthening of the experimental steel with a thickness of 12 mm under different rolling processes.The results show that when the final rolling temperature is 860?,the yield strength is 455 MPa,the average impact energy is 199 J,the Hall-Petch strengthening increment is 210 MPa,and the precipitation strengthening increment is 157.1 MPa.The total increase of the solid solution strengthening and dislocation strengthening is 47 MPa;when the final rolling temperature is 927?,the yield strength is 490 MPa,the average impact energy is 83 J,the Hall-Petch strengthening increment is 167 MPa,the precipitation strengthening increment is 157.1 MPa,and the total increase of the solid solution strengthening and dislocation strengthening is 171.9 MP a.Analysis of the experimental results shows that the best strength and toughness can be obtained under the final rolling temperature of 890?.The yield strength is 510 MPa,and the average impact energy is 111 J,which meets the standard requirements.(4)The atmospheric corrosion behavior of Q500NHE experimental steel was studied through a periodic infiltration test.The experimental results show that the average corrosion rate of the experimental steel is much lower than that of the comparative steel during the entire experimental period,and it decreases with the increase of the corrosion time and gradually flattens.XRD method was used to analyze the composition of the corrosion products.The corrosion products are mainly composed of ?-FeOOH and ?-FeOOH,and the?-FeOOH content gradually increases with the increase of corrosion time.Using EPMA to scan the surface elements of the rust layer by line scanning and surface scanning,it was found that different levels of enrichment of components such as Cu,Cr,and Ni appeared in the rust layer.The enrichment of Cu and Cr in the rust layer can help to generate a dense and robust protective rust layer(inner rust layer),which can effectively prevent the penetration of foreign ions,and plays a particular protective role.(5)The experimental steels were obtained with different microstructure(Ferrite+Bainite,Ferrite+Pearlite,and single-phase Bainite)by changing the cooling rate.Subsequently,a periodic infiltration experiment was carried out to study the influence of different microstructures on the atmospheric corrosion behavior of weathering steel.The results show that,due to the potential difference of the multiphase structure,corrosion will be accelerated in the initial stage,so the average corrosion rate of the experimental steel with single-phase Bainite structure is the lowest.Line-scanning analyzed the rust layer elements after 72 hours of corrosion,and it was found that the enrichment of Cr and Cu elements in the Bainite-structure experimental steel was almost twice of Ferrite+Bainite and Ferrite +Pearlite experimental steels.With the increase of corrosion time,due to the effect of anti-corrosion elements in later stages of corrosion and galvanic corrosion between structures been gradually weakened,the average corrosion rate of experimental steels with three different structures tended to be consistent.(6)CO2 gas shielded welding experiments were carried out on Q500NHE experimental steel.The tensile test of the welded joint after welding found that its strength and elongation were slightly lower than those of the base material,but met the requirements of the standard.The impact test at-40? was performed on steel plates in different parts of the welded joint.It is found that the impact energy at the weld and fusion line is lower than that in the heat-affected zone.The average impact energy is about 50 J,while the impact energy in the heat-affected zone is about 150 J.The decrease in impact energy is mainly due to a large number of inclusions/second phase particles in the welded joint,which deteriorates its impact performance.The structure of the welded joint is mainly composed of Bainite,and the hardness distribution is relatively uniform.The hardness ranges from 215 to 250 HV,and the heat-affected zone appears as a high hardness zone.