Analysis of surface cracking causes of Q355B angle steel
For the quality problem of surface cracks in the production of Q355B angle steel in a plant, the chemical composition, microstructure, and inclusions of cracked angle steel were analyzed using spectral analysis, metallurgical microscopy, scanning electron microscopy, and energy spectrometer. The results show that the cracks originate from the casting billets, whose surface bubbles, cracks, and other defects are the main reasons for the cracks in the finished angle steel material. According to the actual production situation on site, the surface cracking of Q355B angle steel was eliminated by improving the smelting quality and optimizing the continuous casting process.
Q355B is a kind of low-alloy high-strength structural steel mainly used in bridges, vehicles, ships, construction, and pressure vessels. Its comprehensive performance is excellent, with good low-temperature resistance, weldability, and cutting performance. However, its surface cracks and other quality defects have potential safety risks and bring certain economic losses to production enterprises and users. In the inspection process of Q355B angle steel of an enterprise, surface cracks were found in some finished products. To identify the specific causes of crack formation, organizational analysis of the cracked angles was carried out, and measures to reduce or eliminate cold-forming fractures were proposed to effectively control the surface cracking of the angle steel.
1. Experimental scheme
Table of Contents
1.1 Research object
The random defective material in the rolling process, i.e., the intermediate rolled material and the finished defective material, were cut, and their macroscopic morphology was observed, as shown in Figure 1. The composition of the hot-rolled angle was examined by an ICP emission spectrometer and oxygen and nitrogen analyzer, and the results are shown in Table 1.
The main chemical composition was determined to be by the specified requirements. Checking the relevant literature, it is known that the w(O) in the normal furnace of the angle steel produced in China’s northern high-quality plants is between 10×10-6-20×10-6, and w(N) is around 90×10-6. And the w(O) in the specimen is 22×10-6, w(N) is 96×10-6, and the w(O) in the specimen is slightly higher than the normal value.
Figure.1 Macroscopic shape of cracks in intermediate rolled and finished materials
Table.1 Chemical composition of Q355B angle steel %
1.2 Research method
The selected test samples were first ultrasonically cleaned to prevent impurities that could affect the subsequent experimental observations. Usually, when studying the problem of surface defects, only metallographic and scanning electron microscopic observations are made at the defects. In this experiment, to further study the causes of surface crack defects, the metallographic characteristics of the specimens after polishing and corrosion were first observed by optical microscopy. Then, the crack defects and the matrix were observed and examined with scanning electron microscopy and energy spectrometry. Based on the above experimental results, the causes of Q355B angle steel surface cracks are analyzed.
2. Experimental results and analysis
2.1 Surface crack microscopic morphology and metallographic organization
Two samples were cut longitudinally, ground, and polished and then etched with 4% nitric acid alcohol by mass fraction, and the microscopic morphology and metallographic organization of the surface cracks were observed under the optical microscope. Figure 2 on the next page shows the microscopic morphology and normal metallographic organization of the cracks on the surface of the intermediate rolled and finished material. It is found that both are ferrite and pearlite near the cracks, and there is an obvious decarburization layer; the depth of the decarburization layer of intermediate rolled material can reach 455μm, and the depth of decarburization layer of finished material can reach 553μm. Meanwhile, from the comparison of normal metallographic organization in Figures 2-2 and 2-4, both are ferrite and pearlite, and the grain organization is more refined with the increase of rolling passes.
2.2 Crack electron microscopic analysis
Scanning electron microscopy of the intermediate rolled material in the direction of the extended crack is shown in Figure 3; the presence of the crack surface is observed to be different from the matrix of the material, combined with the energy spectrometer results show that the granular material is iron oxide, the presence of the reason may be the billet rolling heating process, the crack surface of the steel matrix is oxidized. The electronic image of the intermediate rolled material in the vertical crack direction is shown in Figure 4. The EDS line scan of the depth of the crack found that region 1 is the normal matrix composition, Si, Mn, and O elements to region 2, the crack location, the content suddenly increased, Si, Mn elements to region 3 content decreased. In contrast, the O element content is still high. Fe element content change process is exactly the opposite of the O element content change process; C element The elemental content of C decreases slightly in region 3. According to the EDS line scan results, Si, Mn, and O elements are enriched with high content at the crack location. There may be MnO/SiO2 oxide inclusions, while MnO and SiO2 belong to oxide and brittle inclusions that are not easily deformable, and their presence aggravates crack generation and expansion.
Figure.2 Microscopic morphology of cracks and normal metallographic organization of intermediate rolled material and finished material
Figure.3 Particle-like material at the crack of the intermediate rolled material
The electron image of the finished material along the crack direction under SEM is shown in Fig. 5, and it is observed that there are substances different from the matrix on the crack surface. The electronic image of the crack direction of the finished material is shown in Fig. 6, and the EDS line scan of the crack depth shows that area 1 is the composition of the crack area, and the O element content is high; area 2, the crack location, the Si content suddenly increases, and the content of each element returns to normal in area 3.
Figure.4 EDS line scan of intermediate rolled material cracks
Figure.5 Granular material at the crack of the finished material
Figure.6 EDS line scan of cracked finished material
2.3 Analysis and Discussion
The decarburization layer is present in the cracks of both the intermediate rolled steel and the finished steel, and decarburization generally requires a high temperature (above 800°C) and enough time for the carbon atoms to diffuse from the inside out to form CO or CO2 with O in the air, which leads to decarburization around the cracks. The rolling process, on the other hand, does not meet the decarburization conditions, and basically, no decarburization occurs. Therefore, cracks may arise from the oxidation of the cast billet surface cracks or pores during the heating process in the furnace.
The surface morphology of the cast billet is shown in Figure 7 on the following page. By observation, it is found that there is a certain length of surface cracks and a certain number of pores on the billet’s surface, which further indicates that the cracks of the finished product originate from the cast billet. The surface defects of the cast billet are further oxidized internally during the heating process. The presence of calcium, magnesium, and silicon oxides around the cracks and the disruption of the matrix continuity cause the cracks to extend and expand during the rolling process of the steel. The inability to weld this defect together during the rolling process is also evidenced by the consistency of the change in the content of each element, as seen by the line scan results of the intermediate rolled material and the finished material.
Figure.7 Cast billet surface morphology
3. Conclusion
The surface cracking defect of Q355B steel finished material produced in this batch mainly originates from the cast billet’s surface cracking and air bubbles. After the cast billet is heated in the furnace, the defective area produces certain decarburization, iron oxides, and brittle inclusions of oxides such as calcium, magnesium, and silicon, which are not easily deformed and cannot be welded together in the subsequent rolling process, thus producing the macroscopic longitudinal surface cracking of this finished material. Therefore, in response to this defect, corrective measures were analyzed and formulated from smelting and continuous casting, respectively, focusing on controlling deoxidation and degassing, reducing the gas content of steel, improving the accuracy of the casting machine to arc, improving the second cooling strength, etc., which reduced the mechanical stress and thermal stress, improved the surface quality of cast billet, and eliminated the phenomenon of Q355B angle steel surface cracks.
Author: Liu Hongchun
Source: China Angle Steel Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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