Analysis of Delamination Causes of J55 Hot Rolled Steel Coil

Using optical metallographic microscope and other analysis methods to analyze the delamination defects found in the process of J 55 hot-rolled steel coil being cut into two during the production of welded pipe. The results show that the reason is that there are serious MnS and CN compound inclusions containing Nb and i in the steel plate, as well as the segregation of C and elements, and the segregation comes from the continuous casting slab.
Key words: 55 hot-rolled steel coil; delamination; segregation

A batch of J 55 hot-rolled steel coils with specifications of 8.94 mm X 1 550 mm, 1035 mm X 1 550 mm was used to produce welded pipes , among which 7 coils were found to exist in the middle of the width direction during the production process of one section and two sections. Irregular 3-7 layers . In order to find out the reason of delamination , the delamination samples of steel coil were taken for metallographic and scanning electron microscope analysis .
1 test
1. Macro inspection
The layers are mostly distributed in the area of about 2/5 of the plate thickness on the longitudinal shear plane ( equivalent to between the inner arc of the slab and the center line ), and the length varies from 13 to 15 mm, which is a macroscopic feature .
1. Chemical composition
Samples were taken from steel coils with delamination defects for chemical composition analysis , and the results are listed in Table 1 . Its chemical composition complies with internal control requirements. 1.3 Metallographic examination
Sampling was taken at the delamination of the steel coil , and the longitudinal shear plane with delamination was directly ground and polished for optical microscope observation .

Table 1 Chemical composition of J55 hot-rolled steel coil ( w b ) %

project	C	Si	mn	P	S	Nb	Ti
sample	0.185	0.21	1.20	0.013	0.0027	0.025	0.013
Internal control value	0. 17〜0.0	0.15〜0.30	1.15〜1.35	<0.02	<0.003	0.025〜0.045	0.008〜0.016

There are gray strip-like sulfides and thick water-red massive inclusions in the gray strips and their extensions . The shape of the massive inclusions is regular, the size is between 11 and 21 "m , and the inclusions are often accompanied by black pores . Judging from the optical characteristics of the inclusions , the inclusions are carbonitrides . See Figures 3 and 4 for the characteristics of the inclusions at the layered extension .

The sample was re-polished and corroded with 3 % nitric acid alcohol solution to observe the metallographic structure . The normal structure was ferrite + pearlite . The black pearlite segregation band shown in Figure 7 .

The delamination defects found during the production of J55 hot-rolled coils during the production of welded pipes are due to the serious composition segregation bands inside the steel plate . The segregation bands are composed of aggregated distribution of MnS and blocky CN compound inclusions containing Nb and i , and segregation of C and other elements . This segregation comes from the continuous casting slab , which is mainly related to factors such as high superheat of molten steel and excessive casting speed . It is suggested to strictly control the tapping temperature and continuous casting process , and adopt casting speed matching the casting temperature to reduce slab segregation .

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