Cause analysis of cracking of steel pipe in flattening test

In view of the fracture phenomenon of steel pipe made of strip steel in flattening test, the inclusion, grain size, microstructure, fracture and oxygen content of the fracture sample were inspected by XL30 scanning electron microscope and metallographic microscope. The results show that the main reasons for the flattening fracture are the large amount of inclusions and high oxygen content in the steel.

For the fracture of steel tubes made of strip steel in the flattening test, the fractured specimens were examined for inclusions, grain size, organization, fracture and oxygen content analysis by using XL30 scanning electron microscope and metallographic microscope. The results showed that more inclusions and high oxygen content in the steel were the main causes of the flattening fracture.

1. Overview

Material Q235B steel pipe in the flattening test appears in the non-welding area fracture. For this phenomenon using a variety of methods of analysis and research to find the main reasons for steel fracture.

2. Test methods and results

2.1 Test method

With the help of metallurgical microscopy, scanning electron microscopy, spectroscopy and other methods, the steel pipe made of non-metallic inclusions, grain size, microstructure, fracture, chemical composition and gas content analysis.

2.2 Inclusions, grain size and tissue inspection

Through the steel pipe inclusions, grain size and microstructure inspection found inclusions are mainly divided into three categories: linear composite silicates, chain-like alumina and sulfides, the specific results are shown in Table 1. 3.0 level.
Table.1 Metallographic examination results
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After further magnification of the band organization, many fine diffuse distribution of ferrite matrix on the dot inclusions (see Figure 1), the energy spectrum micro-zone composition analysis for sulfide. The more abundant brittle inclusions in the specimen (see Figure 2) were analyzed by energy spectrum, and their composition mass fractions are shown in Table 2.
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Figure.1 Inclusions in the banded tissue
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Figure.2 Brittle inclusions
Table.2 The results of energy spectral analysis of brittle inclusions %
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2.3 Fracture analysis

Fracture analysis was carried out with the help of scanning electron microscope. The fracture morphology is shown in Figure 5, and the fracture morphology of the whole test surface is wood grain-like. By further observation, the fracture surface is roughly divided into three zones A, B and C, where zone A is the fracture source of fracture, zone B is the fracture extension zone and zone C is the shear lip of final fracture. further magnification of the three zones A, B and C respectively can be observed: zone A, rolling cracks caused by a large number of dense inclusions can be seen, crack deformation is corrugated, fracture morphology cannot be seen, these inclusions are angular and mainly from Package lining and other refractory materials see Figure 4a, can be seen in the A area of the accumulation of a large number of inclusions is caused by rolling cracks and become the source of fracture of the welded pipe fracture; B area is a dense inclusions caused by smooth fracture and fine shear tough nest see Figure 4b; C area is the last fracture site see Figure 4c.
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Figure.3 Fracture morphology
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(a) Enlargement of crack source A

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(b) Enlargement of crack extension area B

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(c) Enlargement of final fracture area C

Figure.4 Further enlargement of fracture morphology

2.4 Chemical composition and gas content

Chemical composition and gas content analysis of the specimen. The chemical composition analysis shows that the chemical composition of the steel pipe is within the range specified in the standard, and the composition is qualified. Gas content analysis shows that the average oxygen content of the steel pipe is 141.26 × 10-6, the oxygen content is high.

2.5 Mechanical properties

Intercepted specimens in the steel pipe to do tensile testing, test data are as follows: yield strength of 365MPa, tensile strength of 455MPa, elongation of 30%, within the requirements of the national standard. Due to limited specimens did not do cold bending test.

3. Analysis of results

Steel pipe flattening fracture specimens of grain size, organization is normal, indicating that the strip rolling process is normal. Table 1 shows that the high level of inclusions in the strip steel is mainly composite silicate inclusions. From the fracture analysis, it can be seen that a large number of inclusions located under the skin of the steel accumulation of rolling cracks formed during the rolling process is the main factor causing flattening cracking. Through analysis, these subcutaneous inclusions are mainly steelmaking pouring system of refractory materials by steel scouring, leaching, etc. into the formation of large particles of inclusions in the steel.

4. Process improvement measures

  • (1) Improve the deoxidation method from Si-Ca-Ba deoxidation to steel core aluminum plus Si-Ca-Ba deoxidation deoxidation method, increase the deoxidation strength to reduce the content of inclusions. Improve the primary carbon pulling hit rate, reduce post-blowing, to avoid secondary oxidation of steel.
  • (2) Improve the argon blowing time, to ensure that each furnace blowing argon not less than 8min, in order to promote the inclusions on the float.
  • (3) The use of high-quality refractory materials to avoid the number of inclusions increase due to steel scouring, erosion.

5. Conclusion

The main causes of fracture in the steel pipe flattening test are.

  • (1) Refractory materials such as steelmaking ladle lining into the steel formation of inclusions in the steel under the skin of a large number of accumulation of rolling cracks generated during the rolling process becomes the source of cracking, the first cracking under the action of external forces.
  • (2) Gas analysis shows that the oxygen content is relatively high, which further indicates that the inclusions in the strip steel causing the fracture are high. Deep deoxidation technology is adopted to optimize the continuous casting process, thus controlling the oxygen content in steel, reducing the content of inclusions in steel and avoiding the appearance of subcutaneous inclusions, so that the plastic toughness of the strip steel is greatly improved.

Author: Wang Hongxia

Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (

(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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  • [3] Wang Y. Analysis of motor flange screw fracture [J]. Physical and Chemical Inspection-Physical Branch, 2004, 40(7): 356-359

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