Analysis of Surface Cracks of 12CrlMoVG Seamless Steel Tube

Aiming at a large number of steel pipe surface crack defects in the production process of 12CrlMoVG seamless steel pipe in a domestic enterprise, the types of defects and the reasons for the formation of cracks were analyzed by using metallographic analysis combined with scanning electron microscope observation and energy spectrum analysis. The results show that the essence of steel pipe surface cracks is folding, and the reason for the formation of steel pipe surface folding lies in the surface cracks of the tube blank. Therefore, the key to eliminating crack defects on the steel pipe surface is to control the surface quality of the billet and bar during continuous casting and hot rolling.
Key words: seamless steel pipe; crack; tube blank; surface quality

12CrlMoVG is a pearlitic low-alloy heat-strength steel with strong heat strength, durable plasticity, oxidation resistance and good welding performance. It is widely used in components such as boiler superheaters and reheaters at £580 °C . In the process of drawing seamless steel pipes, the tube blanks are subjected to complex strains such as extrusion, stretching and torsion, and any small defects on the surface and inside may further expand and form serious defects, which requires the tube blanks to have good Surface and internal quality, especially surface quality. Due to the complex formation factors of surface defects of tube billet steel, it is difficult to carry out stable control. Although scholars at home and abroad have done a lot of research on steel cracks, the causes of cracks on the surface of tube billet steel are different for different steel types under different production conditions, which needs to be analyzed in combination with the actual production process. In order to improve the competitiveness of the enterprise and increase production efficiency, a medium-sized factory of a large domestic steel company developed and produced steel for high value-added tube blanks on the basis of the original product series. surface cracks, resulting in a large number of waste products. Combined with the quality objections fed back by users, using metallographic analysis, scanning electron microscopy and energy spectrum analysis, the cause of cracks is systematically analyzed, and a preliminary solution is proposed.

1 experiment
1.1 Experimental materials
The test materials were taken from 090 mmxlO mml2CrlMoVG steel pipe produced by a domestic iron and steel company. The basic production process of the steel pipe was EAFtLF/VDt bloom continuous casting - hot charging and hot delivery - tube billet rolling- > piercing- > rolling - finishing. After finishing in the production process, it was found that there were serious cracks on the surface of the steel pipe. In order to analyze the state and cause of the crack, a section of the sample with a crack length of 80 mm was cut near the surface crack of the steel pipe . The chemical composition of steel pipe material is (mass fraction): C 0.08 ~ 0.15 ; Si 0.17 ~ 0.37 ; Mn 0.40 ~ 0.70; S < 0.035 ; P<0.035 ; Cr 0.90 - 1.20 ; Mo 0.25 ~ 0.35 ; .
1.2 Test method
30 mm x 15 mm was cut from the pre-cut steel pipe. After grinding and polishing, it was corroded with 4% nitric acid alcohol solution for microstructure and crack observation. For metallographic analysis , structure observation was performed under OLYMPUS BX51 optical microscope, and crack morphology observation and energy spectrum analysis were performed under LEO-1450 scanning electron microscope.
2 Experimental results and discussion
Figure 1 shows the macroscopic morphology of steel pipe surface cracks. It can be seen from Figure 1 that the surface of the steel pipe does not show the standard crack shape extending inward along the surface, but the folds formed by the surface extrusion. It can be seen from Figure 2 that the crack extends from the surface to the center at a certain angle, with an angle of 15 to 20 from the surface . angle, which is a typical surface folded crack morphology.
When the metal forms surface folding cracks during the pipe threading process, the tube blank or capillary tube is subjected to complex strains such as extrusion, stretching, and torsion in the three seamless steel pipe production processes of pipe threading, pipe rolling, and diameter reduction. There are cracks in the billet, even if it is a small crack, under the action of alternating stress fields such as tension and compression, these cracks will not only fail to weld but will also expand, resulting in large areas of spiral inner and outer bending defects on the surface of the steel pipe to varying degrees, and The degree of exposure and expansion varies with the genetic degree of tube blank defects and the amount of rolling deformation. There will be a large amount of oxides in the folding cracks, which are produced due to the cracks in the tube blank being exposed to the air and oxidized during subsequent heating and thermal processing. In the process of piercing, rolling and reducing from Figure 3 that the inside of the crack is filled with a large number of gray inclusions, and the center presents a rheological phenomenon under force, indicating that in the process of steel pipe production, the surface defects of the tube blank are deformed and expanded under the influence of axial and tangential complex stresses at the same time . Energy spectrum analysis shows that the main components of the gray inclusions in the crack are Fe and O. It can be seen from Figure 2(a) , (b) that the matrix structure of 12CrlMoVG steel is mainly composed of ferrite and pearlite. However, the content of pearlite is significantly reduced near the crack, and the structure near the crack is mainly ferrite, indicating that the carbon content near the crack is low and there is a decarburization layer. During the heating and hot rolling process of the tube billet steel, the surface will be oxidized and decarburized due to high temperature. When the folding defect is formed during the pipe passing process, the surface folding will cause the surface oxide and decarburization layer to become the edge of the crack. It can be concluded that the surface cracks of the 12CrlMoVG steel pipe are folded.
The main reason for the formation of surface folds in the production process of the pipe is that there are quality defects in the raw material tube blank, such as folds on the surface of the tube blank, serious subcutaneous air holes and subcutaneous inclusions of the tube blank, poor cleaning of the surface of the tube blank or cracks, ears, etc. , Wrong side, etc. Therefore, further analysis of the raw material pipe blanks that form the folded steel pipes is called

The surface defects of the billet, especially the longitudinal cracks on the surface, are related to the quality of the continuous casting billet. After the continuous casting billet exits the mold, the weak part of the surface quenching layer cannot bear the static pressure of the molten steel and the transverse shrinkage stress generated during the condensation process, and the subsequent The mechanical stress accompanying the straightening process increases the probability of longitudinal cracks in the continuous casting slab. Due to the existence of thermal stress and structural stress, the quality defects of continuous casting slabs will gradually expand due to the existence of thermal stress and structural stress in the process of heating and thermal processing.

In the process of continuous casting slab rolling, the upper and lower sides of the rolled piece are continuously rolled for multiple passes, and the defects such as air bubbles and cracks in the continuous casting slab are easily welded on the upper and lower sides, and the processed rolled The noodle quality is good. However, on the side, especially at the radial seam, the rolled piece has not been processed for several consecutive passes, and the defects are easy to expand and are exposed on the surface. From the perspective of the rolling production process of the plant, there is only one turnover in 4 rough rolling passes , and the number of turnovers is seriously insufficient, which is an important reason for the expansion of cracks in the tube billet during the rolling process. On the other hand, due to the uneven deformation and the oxidized scale pits formed on the surface of the billet, intermediate cracks and wrinkles will be formed in the depressions, which is another cause of cracks during the rolling process of the tube billet.

Conclusion Through the metallographic and scanning electron microscope analysis of the surface crack defects of the billet steel during the production process, it is shown that the surface defects of the steel pipe are mainly the folds caused by the pipe passing process, and this type of fold comes from the surface defects of the pipe billet. When there are serious surface cracks in the billet steel bar, large areas of spiral inner and outer bending defects on the surface of the steel pipe will be caused in different degrees under the action of alternating stress fields such as tension and compression during the threading process. The formation factors of cracks on the surface of billet steel are complex, mainly due to the many subcutaneous bubbles on the surface caused by the smelting and continuous casting process control process. Therefore, strictly controlling the surface quality of the tube blank is the key to solving the surface folding of the steel tube. Strictly controlling the continuous casting process, reasonable heating system, and increasing the number of turning times in the rolling process are important measures to ensure the surface quality of 12CrlMoVG seamless steel pipes. It is suggested to adopt VD process and appropriate slow cooling process to improve the quality of continuous casting slab on site, and the specific process improvement measures need further on-site analysis. references:

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