Abstract: Ultra-high strength seismic steel bar breaks during bending test. Macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, metallographic inspection, central segregation test and other methods were used to analyze the reasons for the fracture of curved samples. The results show that the heating temperature before hot rolling is high, and the austenite grains grow after deformation, which increases the size of the pearlite group and the pearlite content. Slow cooling causes reticular ferrite to precipitate along austenite grain boundaries, reducing the plasticity and toughness of the material, and eventually causing the specimen to fracture during the bending test.
Keywords: ultra-high strength seismic steel bar; bending fracture; pearlite mass; Reticulated ferrite
Catalog number: TB31 Bibliographic identification code: B Article number: 1001-4012 (2023) 05-0030-04
With the development of the domestic construction industry, complex structures such as large-scale public buildings, high-rise residential buildings and major scientific and technological infrastructure engineering buildings have higher and higher requirements for the bearing capacity of hot-rolled ribbed steel bars. At the same time, in order to reduce the loss of people's lives and property caused by natural disasters such as earthquakes, typhoons, and debris flows, and to reduce the total amount of construction steel projects, so as to achieve the purpose of energy conservation and emission reduction, earthquake-resistant steel bars with high strength, high ductility, high toughness and high yield ratio have been widely used in construction, bridges, electric power, petroleum and other fields.
Recently, the ultra-high strength seismic steel bar produced by a company was broken during the bending test. Macroscopic observation, chemical composition analysis, scanning electron microscopy (SEM) analysis, metallographic inspection, central segregation test and other methods were used to study the fracture samples of this batch, and compared with the same batch of normal bending samples to determine the cause of bending fracture, so as to provide direction for the optimization of the production process.
1 Physical and chemical tests
1.1 Macroscopic observation
The macroscopic morphology of the fracture of the bending fracture specimen is shown in Figure 1, which can be seen from Figure 1: the fracture of specimen 1~4 is flat, without obvious necking and deformation, gray fibrous, with obvious radiation patterns, belonging to the brittle fracture of the positive fracture type; The fracture originates on the surface, and cracks are present in the fracture.
1.2 Chemical composition analysis
The SPECTRO Lab11 direct-reading spectrometer was used to analyze the chemical composition of specimen 1 and the same batch of bent normal specimens, and the results are shown in Table 1, which shows that the chemical composition of specimen 1 meets the technical requirements.
1.3 SEM analysis
The fracture of specimen 1 was observed using the ZEISSEVO18 tungsten filament SEM and the results were shown in Figure 2. It can be seen from Figure 2 that no defects such as cracks and inclusions were found in the crack source area, the crack source area and the crack propagation area were both quasi-cleavage, and there were short and discontinuous river pattern morphology at the fracture, and more tear ridge characteristics were visible, and there were secondary cracks along the crystal.
1.4 Metallographic Inspection
Metallographic specimens were intercepted on specimen 1, specimen 2 and bent normal specimens of the same batch and observed using a ZEISSAxioObserver.7m optical microscope. A large number of gray sulfides were found in short strips on the longitudinal cross-sections of both specimen 1 and 2 (see Figure 3). A large number of short strips of gray sulfide are distributed over the entire longitudinal section of the same batch of bent normal specimens (see Figure 4). According to the A method in GB/T10561-2005 "Measurement of Non-metallic Inclusions Content in Steel, Rating Chart Microscopic Examination Method", non-metallic inclusions were evaluated for specimen 1, sample 2 and the same batch of bending normal samples, and the results are shown in Table 2.
Ethanol nitrate solution with a volume fraction of 4% was used to corrode sample 1, sample 2 and the same batch of bent normal sample, and the microstructure morphology was observed, and the results were shown in Figure 5~7. It can be seen from Figure 5~7 that the microstructure of sample 1 and sample 2 is massive ferrite + reticular ferrite + sheet pearlite + a small amount of heinite; The entire longitudinal section of specimen 1 was distributed with pearlite clusters surrounded by reticular ferrite, and the pearlite cluster was relatively large in size, with an average intercept of 67.3 μm, and the average intercept of ferrite in the entire longitudinal section was 14.1 μm, and the pearlite content from the edge to the center was 70%, 73%, and 70% in three positions. The longitudinal section area of specimen 2 contains a large number of pearlite clusters surrounded by reticular ferrite from 1/2 radius to the center position, the average intercept of the pearlite group is 47.6μm, the edge structure is relatively uniform, the average intercept of the ferrite of the entire longitudinal section is 16.8μm, and the pearlite content from the edge to the center of the three positions is 58%, 66%, 68% respectively; The microstructure of the same batch of bent normal specimens is lumpy ferrite + reticular ferrite + sheet pearlite + a small amount of weinchite, the edge tissue is relatively uniform, 1/2 radius to the center position distribution There are bands along the processing direction, the content of reticular ferrite is less, basically semi-closed, the average intercept of the pearlite cluster is 40μm, the average intercept of the entire longitudinal section ferrite is 14.1μm, and the pearlite content from the edge to the center of the three positions is 46%, 52%, 55%.
1.5 Central segregation test
Sampling was taken on sample 2, referring to YB/T4413—2014 "Metallographic evaluation method for center segregation of high carbon steel wire rods", and observed it after corrosion by using a nitro acid ethanol solution with a volume fraction of 4%, and it can be seen that the central segregation of sample 2 is grade 0, that is, no obvious segregation is found in the central area (see Figure 8).
2 Comprehensive analysis
From the macroscopic and microscopic morphology and necking deformation of the fracture of the bending fracture specimen, it can be judged that the fracture nature of the specimen is quasi-cleavage fracture in brittle fracture, and the initial crack source exists on the surface of the specimen, and no obvious defects are found at the fracture. The chemical composition of the fractured specimen meets the design requirements.
The content of Class A sulfides and Class C silicates in specimen 1 and 2 is large, and Class A sulfides are distributed in the entire longitudinal section of the specimen, indicating that non-metallic inclusions are one of the factors leading to bending and fracture of the specimen. The entire longitudinal section of sample 1 is distributed with reticular ferrite, the average intercept of the pearlite mass is 67.3 μm, the average intercept of ferrite is 14.1 μm, the size of the pearlite group is large, and the average content of pearlite is 71%; The edge structure of the longitudinal section of sample 2 is relatively uniform, the size of ferrite from the edge to the center of the three positions is basically the same, the content of reticular ferrite gradually increases, the size of pearlite mass increases, the pearlite content gradually increases, the average content of pearlite is 64%, of which the average content of pearlite from 1/2 half diameter to the center position is 67%.
More reticular ferrite content, more pearlite content, and larger size of pearlite groups will reduce the plasticity and toughness of the material. In subconcentric steels, as the carbon content increases, the pearlite content increases, and the slow cooling after thermal deformation will promote the formation of reticular ferrite. Under the condition of ensuring sufficient deformation of the billet, with the increase of heating temperature, the size of austenite grains increases, which will lead to an increase in the size of pearlite clusters and an increase in pearlite content [1-3]. Combined with the microstructure characteristics of sample 2, the central segregation test was carried out, and no central segregation phenomenon was found. The billet is heated to 1200 °C before rolling, the heating temperature is too high, the cooling rate after heat deformation is slow, so that the austenite grains grow rapidly, and at the same time provide thermodynamic conditions for the diffusion of carbon atoms, the precipitation of ferrite makes the carbon atoms enter the untransformed austenite, which promotes the co-analysis transformation of the untransformed austenite, the size of the pearlite group increases, the pearlite content increases, and a network of ferrite is formed along the grain boundary, which reduces the plasticity and toughness of the material.
The chemical composition, non-metallic inclusion grade, sulfide distribution pattern and bending fracture specimen of the same batch of bending normal specimens are basically the same, indicating that non-metallic inclusions are not the main factors causing bending fracture of the specimen. The microstructure of the normal sample bent in the same batch is finer, the content of reticulated ferrite is less, the average content of pearlite is 51%, and there is a band structure obviously along the processing direction, indicating that the heating temperature of the material is lower when deformed. Therefore, the reticular ferrite formed slowly after heating temperature and slow cooling after thermal deformation is the main cause of specimen bending fracture.
3 Conclusion
The reasons for the bending and fracture of ultra-high strength seismic steel bars are: the heating temperature before hot rolling is too high, the size of the pearlite mass increases, the pearlite content increases, and the plasticity and toughness of the material decrease; The reticular ferrite precipitated along the grain boundary during slow cooling seriously cuts the connection between pearlite, which seriously reduces the plasticity and toughness of the material, and finally causes the steel bar to break during the bending test.
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