Straightening Fracture Analysis of N80 Hot Rolled Seamless Steel Tube

The N80 hot-rolled seamless steel pipe fractured during the straightening process after rolling . The fracture morphology, microstructure and microdomain composition of the tube were analyzed by optical microscope , EM and EPMA . The results show that the main reason for the straightening fracture of the steel pipe is the presence of hard and brittle martensite distributed in bands in the microstructure of the steel pipe . The generation of martensite is due to the uneven cooling of the steel pipe in the finishing section and the excessive local cooling intensity . When rolling N80 hot-rolled seamless steel pipe products, try to keep the final rolling at about 600 t , which can effectively reduce the band structure of the steel pipe; strictly control the cooling process of the finishing rolling to avoid the pipe fittings staying in the strong cold zone, which can avoid the formation of hard and brittle of martensite .
Key words: N80 steel; fracture morphology; microstructure; straightening; crack Analysis on straightening fracture of hot-rolled N80 seamless steel tube
The N80 hot rolled seamless steel tubes were fractured during straightening． The fracture morphology ， microstructure and microarea chemical composition of the tube were analyzed by optical microscope， SEM and energy spectrum etc． The results show that the factures are mainly caused by the hard-brittle martensite in banded distribution in the steel． During finish rolling process ， the inhomogeneous cooling of the tube results in local intensive cooling which leads to the formation of the martensite． Therefore ， the finish-rolling temperature should be controlled about 600 t to reduce the banded structure and the cooling process should be strictly controlled to avoid the tube stay in the intensive cooling region and restrain the formation of the hard-brittle martensite．
Key words: N80 steel ; fracture morphology ; microstructure; straightening; crack

N80 grade hot-rolled seamless steel pipe is mainly used for oil production well pipes in domestic oil fields, and belongs to one of the oil-specific series . For oil pipes, a high-risk product, once a quality problem occurs during use, it will cause huge and irreparable losses . Usually, in the production process of N80 oil pipe, straightening and cracking are easy to occur, and the cracking types include longitudinal cracking or fragmentation along the pipe . The reason is mostly related to the rolling cooling process of steel pipe production . In this paper, optical microscope , electron microscope and other means are used to inspect the fracture accident occurred in the straightening process of a certain N80 hot-rolled seamless steel pipe, analyze the specific reason of the fracture and put forward the corresponding process improvement measures .
In addition, strict flaw detection procedures should be carried out for the blind area of the pipe end, and its flaw detection level should not be lower than that of the pipe body . For related testing such as the determination of the content of non-metallic inclusions, it is also necessary to increase the testing requirements and strictly control the level of non-metallic inclusions .

1. Test process and results

1. 1 Macroscopic appearance of the fractured sample
The production process is tube billet heating ( 1200 ° C ) , piercing , continuous rolling, finishing rolling and sizing , cooling (cooling bed) —straightening (rolling 10 times) —acceptance — storage . The steel pipe starts to crack in the radial direction of the pipe, and then expands and fractures in an arc along the longitudinal direction of the pipe. Its macroscopic appearance is shown in Figure 1 ( a ) and ( b ) .

1. 2 Chemical Composition Analysis
from the source of steel pipe cracking ( position No. 2 marked on the steel pipe in Figure 1 ), and its chemical composition was tested by ICP-AES inductively coupled plasma emission spectrometer spectrometry. The results are shown in Table 1 . The chemical composition of the broken pipe complies with API5CT " American Petroleum Institute Casing and Tubing Specifications" .
Table 1 Chemical composition of the ruptured seamless steel tube ( mass fraction, % ) Table 1 Chemical composition of the ruptured seamless steel tube ( wt% )

project	C	Si	mn	P	S
broken tube	0 . 31	0. 36	1.45	0. 018	0.0056
standard value	0.23-0.33	0.35-0.65 1.44-1.76		W 0.024	W 0.015

1. 3 metallographic inspection
from the steel pipe crack source ( position No. 2 ), ground and polished , corroded by 4% nitric acid ethanol solution, and observed under a ZEISS Axiovert 200 MAT optical microscope . According to GB/T 10561-2005 " Determination Standard Grading Chart Microscopic Examination Method of Non-metallic Inclusion Content in Steel " , the inclusion level is evaluated
Don't be: A0 . 5. B0 . _ 5. D0 . _ 5 .

Observing the microstructure of the steel pipe, the microstructure near the fracture, the inner wall of the pipe is ferrite + pearlite + a small amount of bainite distributed in a band shape. Continue to observe the inside and find that martensite appears in the structure . That is, in the middle of 1/3 of the tube wall thickness , there is a wide band of ferrite + pearlite + bainite + martensite between the ferrite + pearlite bands, and the bainite + martensite mixed The organization is also banded along the longitudinal distribution; the banding of the microstructure of the outer wall of the tube is still obvious . It is observed that the microstructure of the intact steel pipe far away from the fracture is bainite + ferrite + pearlite, of which bainite is the main one, and the distribution is relatively uniform, and only a small amount of banded structure exists near the inner wall of the tube .
1. 4 Fracture microscopic morphology analysis
The fracture sample was taken from the crack source of the steel pipe, cleaned by ultrasonic vibration and then observed under the QUANTA400 scanning electron microscope . Radial river patterns are formed in the place where the fracture facets fluctuate up and down, showing that the crack propagation path is discontinuous, that is, the crack is formed locally and expands locally . The entire fracture shows obvious quasi-cleavage brittle fracture characteristics .
1. 5 Composition analysis of the micro-area at the fracture
In order to further determine whether the cracking of the steel pipe is related to the composition segregation of the steel, EPMA1610 electronic probe was used to test the composition of the micro-area at the fracture of the steel pipe, and the distribution curves of C , Si , Mn , P , S through the wall thickness elements were obtained, as shown in Fig. 6 show . The results show that the content of detected elements is evenly distributed throughout the thickness of the pipe wall, and no component segregation is seen .

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