SS400 Hot-rolled Coiled Tubes

Excerpt: The hot-rolled coiled tube made of SS400 cracked after welding, and the main reason for the cracked coiled tube was found through research and analysis of the cracked sample: surface cracks in the continuous casting slab
The grains cause the rolling and folding of the surface of the steel plate and finally cause the cracking of the coiled tube.
Key Words: Coiled Tube Cracking Continuous Casting Slab Surface Crack Rolling Folding

made of SS400 is first rolled into a round pipe, and then welded into a pipe fitting with a high-frequency welding machine. After half of the welding, a longitudinal crack is found at a distance of 300 to 350mm from the weld . The crack is parallel to the weld. After research and analysis, it is found The reason for the cracking of the coiled tube.
2 test method
For the sent hot-rolled plate sample, conduct macroscopic inspection, intercept the composition sample to detect the chemical composition of the steel plate, select the cracked part to intercept the metallographic sample, and use the Olympus GX 51 metallographic microscope for metallographic analysis . The macro picture of the sample is shown in Figure 1 0
3.1 Macro inspection
Through macroscopic inspection, it is found that the fracture has a layering phenomenon visible to the naked eye. One side of the fracture is very smooth, and the other side is a normal fracture shape. By measurement, the thickness of the smooth part of the fracture is 2mm .

Figure 2 Fracture site) R piece
3.2 Chemical composition analysis
The chemical composition analysis results are shown in Table 1, and the chemical composition of the steel plate meets the standard requirements after testing.
Decay 1 chemical composition test results

name	C S1		mn	P	S
	0.17	0.19	0.73	0.017	0.028
standard	<0.21	<0.30	< 1.40	< 0.035	Gain 0.035

3.3 Metallographic analysis
Through metallographic analysis, it was found that the grades of inclusions inside the steel plate matrix were all 0.5 . After corrosion, it was found that the structure of the steel plate was ferrite plus pearlite, which was a normal structure with a grain size of 11 .
The extended cracks of delamination and matrix were observed under a metallographic microscope, and it was found that there was a serious decarburization layer at the crack site, and there were relatively serious internal oxidation particles near the crack, and the defect site had high temperature oxidation characteristics, and the deformation streamline was distributed along the crack.

It can be seen from the metallographic picture that the main crack is a cleavage crack, the crack part has high temperature oxidation characteristics, there are serious internal oxidation particles on both sides of the crack, the bifurcated branch cracks also have high temperature oxidation characteristics, and the end of the junction crack under the layer In the round ton, the nearby inclusions have traces of rolling deformation, the banded structure develops along the crack, and the branch crack has nothing to do with the existing structure.
4 discussions
4.1 Defect nature
From the test results, it can be seen that there is a layering phenomenon visible to the naked eye at the defect site. The fracture side is very smooth, and there are cracks extending to the matrix at the layering site. There is a serious decarburization layer near the crack, and there are serious internal oxidation particles nearby. It has obvious high-temperature oxidation characteristics. Such defects must be formed after high temperature for a long time. It is also found that the cracks have nothing to do with the existing structure. Therefore, it can be determined that the defects are caused by cracks on the surface of the continuous casting slab and folded by rolling.
4.2 Reasons for formation
The customer used the steel plate to coil the pipe, and the steel plate was plastically deformed, and the stress on the outside had exceeded the yield strength of the steel plate < 78, causing the steel plate to bend, but there was stress concentration at the crack tip, making the stress exceed the tensile strength of the steel plate < rb , cracking occurs.
in conclusion
The reason for the cracking of the steel coil pipe is that there are rolling folds on the surface of the steel plate, and the rolling folds are caused by cracks on the surface of the continuous casting slab.

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