Cause Analysis and Countermeasures for Cracking of 27 SiMn Seamless Steel Tubes

In the process of rolling 27SiMn seamless steel tubes， there are cracks on the tubes．The chemical compositions of the tubes are tested as well as the micro analysis is made with metallurgical microscope and scanning electron micro- scope．The results show that the non - metallic inclusions are the main causes for cracks． In the paper， the qualitative analysis of the inclusions is made and the sources are found out as well as the appropriate measures are proposed．
Key words: seamless steel tubes; stress; cracks; non - metallic inclusions

27SiMn thick-walled seamless steel pipe produced by the seamless factory has a pipe body specification of ① 299.00 mm X 40.00 mm , and longitudinal partial cracks appear in the pipe body during the rolling process . To this end, take back the problematic pipe sample for inspection and analysis of the cause of cracking . Steel pipe production process: Top-bottom combined blowing converter smelting— LF furnace refining t Round billet continuous casting t Tube billet heating t Primary piercing T Secondary piercing T Automatic pipe rolling T Equalizing T Reheating T Sizing

1. Physical and chemical performance testing

The macroscopic feature of the fracture is that the pipe body is partially cracked along the longitudinal direction, and there is a certain precession angle (about 16° ), and there is no obvious scratch on the surface of the pipe body . Sampling was taken separately to test the chemical composition of the pipe body. At the same time, two typical metallographic samples were taken from the part with serious cracks for high-magnification inspection. The test results are as follows .
27 SiMn Seamless Steel Tubes Chemical Composition Analysis

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	Table 1 Detection results of chemical composition of 27SiMn			%
	C Si	mn	P	S
detection value	0. 28 1. 17	1. 22	0. 014	0.010
standard requirement	0.24-0.32 1.10-1.40	1.10-1.40	W0.030	W0.030

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Table 2 Detection results of residues and content of five harmful elements							%
	Cr	NiCu _	Pb	sn	As	Sb	Bi
detection value	0. 017	0.006 0.007	0.005	0.007	0.003	0.0003	0.0001
standard requirement	W 0.30	W 0.30 W 0.30

From the test results in Table 1 and Table 2 , the chemical composition of the pipe body is controlled within the scope of the standard requirements, and the content of residual elements and five harmful elements is also low, which have little effect on the occurrence of cracking. 2 . 2 high magnification metallographic analysis
From the test results in Table 3 , the structure of the matrix is ferrite and pearlite, and the outer surface of the fracture of sample 1# has been oxidized , decarburized, and has a tendency to grow grains (see Figure 1 ); The level of inclusions in the matrix is relatively high, and there are large grain inclusions (see Figure 2 ); the grain size is controlled within the 8.0-8.5 level, and there is no mixed crystal phenomenon .

		Table 3	High magnification metallographic inspection results
Sample number	organize	Grain Size Decarburization Layer Inclusion / Grade
		/ level	/ mm ABCD Ds
0#	F+P	8.0	0 0. 5 1.5 1. 0 1.0 2
1#	F+P	8.5	0.2 1. 0 2 1. 5 1.0 1. 5

Analysis of crack growth
After the formation of micro-cracks, the stress at the tip of the cracks increases as the cracks expand, while the effective cross-section of the steel part decreases. The crack size is generally much smaller than the critical crack length . In order to improve productivity, the method of rapid reheating was adopted on site. Due to the small specific volume of austenite, the temperature rises too fast, so that the surface layer first undergoes austenite phase transformation and shrinks rapidly, resulting in a large tangential tensile stress, which is the expansion of cracks. Provide plenty of energy . Since the inclusions are distributed close to the outer surface of the tube body, according to the law of least resistance, cracks will expand to the outer surface to form longitudinal cracks, and the grains tend to grow, indicating that the tube blank is slightly overheated, reducing the bonding force between grains. Oxidation and decarburization at the cracking site fully illustrate this point .
During the crack growth process, if the tensile stress decreases or encounters a compressive stress region, the crack growth will stop; if the crack growth encounters the ductile phase, the ductile phase will produce plastic deformation, absorb the deformation work to relax the stress, and make the crack tip curvature The increase of the radius and passivation increases the resistance to crack growth, making it difficult for the crack to continue to expand or slow down the expansion speed, so the crack will also change direction, but generally it is rarely cracked. 4 Improvement measures and effects

1. 1 Improvement measures

In order to prevent the molten steel from being involved in refining slag, the working pressure and flow rate of argon , and enough soft blowing time should be well controlled, so that the final deoxidized product and deep desulfurized product can float into the slag under sufficient conditions, and finally ensure that the cast slab has Good pristine cleanliness .
In order to prevent the steel pipe from cracking due to excessive temperature rise during the reheating process, low-temperature preheating or slowing down the heating speed should be adopted, and the heating temperature and holding time should be strictly controlled to ensure uniform heating without overheating .
4. 2 Implementation effect
After tracking the production of the subsequent 27SiMn thick-walled seamless steel pipe, the material of the pipe body has been significantly improved, the content of non-metallic inclusions has been greatly reduced, and there has been no cracking of the pipe body, which proves that the improvement measures are practical and effective .
27 SiMn Seamless Steel Tubes Conclusion
( 1 ) Argon blowing is insufficient in the later stage of refining, and the inclusions are not fully floated and separated and remain in molten steel .

1. In the continuous casting stage, the inclusions in the liquid phase cavity are captured by the inner arc side and cannot float to the liquid surface of the crystallizer, and gather on the inner arc side of the round tube billet .
2. There are many non-metallic inclusions in the tube blank, especially the brittle inclusions and non-deformable inclusions with large particles, which will produce spiral cracks in the cross rolling process, and the rapid temperature rise in the reheating process will make the outer surface of the tube body due to phase change. Shrinkage produces tangential tensile stress, and the crack expands further to cause local cracking .

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