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X70 JCOE big straight seam welded pipe mechanical widening process
Jan 30, 2019

X70 JCOE big straight seam welded pipe mechanical widening process


According to domestic TMCP microalloy pipeline steel yield strength greater Rt0.5 changes the status quo, targeted for X70 pipeline steel in one power plant adopted the JCO + mechanical expanding manufacturing craft, obtained the corresponding yield strength, tensile strength, showed, and the change of impact toughness curves, points plate steel tube under different hole enlargement rate of mechanical properties change rule of microstructure on mechanical properties and function mechanism, this paper expounds the selection of hole enlargement technology practice, half given JCOE straight seam submerged arc welding pipe production line of X70 pipeline steel plate suggest acceptance limit.



The acceptance conditions of raw steel sheet for welded pipe are generally in accordance with the steel pipe standards, so there is a certain fluctuation range for the mechanical property acceptance of steel sheet for steel pipe manufacturers.For each raw material supplier, there are not only different carbon equivalent, alloy ratio, but also different rolling cooling process, and pipe production and the use of different molding process and different diameter, processing hardening and bauschinger effect are also different.Therefore, it is necessary for the pipe factory to establish a personalized steel pipe production process on the basis of the composition and properties of the sub-plate steel plate, so as to reduce the production cost on the premise of ensuring the quality of steel pipe.


  1. Material property test before and after tube making


1.1 test materials


The materials used in the test are X70 steel grade steel plates with yield strength exceeding 500Mpa and between 485-500mpa produced by a steel plant.Its main chemical components are:



C 0.05
Si 0.09
Mn 1.54
P 0.009
S 0.002
Mo 0.179
Nb 0.05
V 0.00
Ti 0.007


1.2 pipe making technology


The JCOE forming process is basically pure bending deformation, the inner surface of the steel tube undergoes transverse compression deformation, the outer surface undergoes transverse tensile deformation, and the entire tube wall undergoes transverse tensile deformation during overexpansion and deformation.The flattening process of the composite wall thickness specimen adopted is pure anti-bending deformation, the inner surface of the steel tube is tensile deformation, and the outer surface is compressive deformation.Therefore, after the completion of pipe making, the yield strength of steel pipe may be reduced due to the bauschinger effect.The diameter of 1016x21mm steel tube is produced with the same grade X70 steel plate of different strength


Yield strength Mpa 485-500 (>500)Width of milling edge back plate mm 3083 (3087)Molding step length/step frequency 15 (15)Diameter gain % 1.2 (1.0)Circumference after diameter enlargement


1.3 test results


The test results of the main mechanical properties before and after pipe making showed that the yield strength of the steel plate exceeded 500Mpa, and there were unqualified items after pipe making.


2 analysis of test results


2.1 impact of JCOE tube manufacturing process on the performance of steel tubes


The average yield strength, tensile strength and tensile strength of the manual inspection were 490.1mpa, 587.5mpa and 616.25mpa, respectively. The average yield strength and tensile strength increased by 8.9% (43.9mpa), 28.75mpa, 0.82-0.90 and 0.031 respectively.Although the yield strength and tensile strength of the steel sheet have been significantly improved after the adoption of 1.2% diameter increase, the yield strength still has a risk point (482 Mpa, 590 Mpa), and the yield strength ratio of the safety index has also increased significantly and is close to the acceptance limit of 0.90 (5% is allowed to be less than or equal to 0.92).Therefore, in order to control the risk of raw materials to the lowest level, it is suggested that the yield strength of X70 pipeline steel plate should be controlled above 500 Mpa in the factory inspection.


The hardening effect of using 1.2% diameter enlargement rate is much greater than that of pre-bending forming, diameter enlargement and sample flattening process. The yield strength of steel tube decreases due to the bauschinger effect. The strength of steel tube is higher than that of steel plate, and the ratio of buckling strength generally increases by 0.02-0.07.


For steel plates with an average yield strength of 550 Mpa and an average tensile strength of 625.6 Mpa for in-plant inspection, the average yield strength and tensile strength of steel tubes were 535.6 Mpa and 625.6 Mpa, respectively, after pipe sampling with 1.0% diameter enlargement rate. The average yield strength decreased slightly by about 15 Mpa on average and the tensile strength hardly changed.The mean ratio decreased from 0.88 to 0.86.There is a risk point (505 Mpa, 625 Mpa).However, it is also far higher than the acceptance limit of steel pipe (485 Mpa, 570 Mpa).


At the same temperature (-20 c), the mean impact work of steel plates with different yield strength is different. The mean impact work of steel plates with yield strength less than 500 Mpa is 324J.For steel plates with yield strength greater than 500 Mpa, the impact force is 344J.After pipe making (-10 c), steel plates with yield strength greater than 500 Mpa are adopted with a 1.0% diameter increase rate, and the mean impact work of the base metal is 437J.For steel plates with yield strength less than 500 Mpa, a 1.2% expansion rate is adopted, and the average impact energy of the parent metal is 404J, and the diameter increase from 1.0% to 1.2%, and the average impact energy of the parent metal is about 33J.The toughness indexes before and after pipe making were compared and the changes were obvious.


2.2 influence of microstructure on the properties of steel tubes


The X70 pipeline steel produced by TMCP rolling process is micro-alloy steel, with the content of C only 0.05%.The ferrite grains were refined obviously by controlled rolling. The grain refinement can improve the strength and toughness.The loss of strength due to the decrease of pearlite content can be compensated by plate exit strengthening and dislocation strengthening.


The effective bainite grain size cannot be measured under an optical microscope because the large and small Angle grain boundaries cannot be identified under an optical microscope.Therefore, it is necessary to use dark field image to detect electron diffraction spots systematically.Transmission-electron microscopy (TEM) images of the grain structure of multilateral ferrite and bainite were obtained.It can be seen that the average statistical grain size of bainite is several times that of bainite.


Another important difference is that there is a high dislocation density in bainite.The measurement of dislocation density involves considerable cost of electron microscopy experiments because, in addition to counting the number of dislocations, a focused diffraction beam is used to measure the thickness of many points on the thin film.Because of the high dislocation density in bainite, it is easy to realize multiple slip.


Steel plate after JCO forming, under the 1.2% rate of hole enlargement, the steel tube flattening test results of tensile test 柆 yield strength is higher than plate yield strength of 28 Mpa, suggesting that steel tube forming and expanding process of work hardening effect is greater than the pressing bauschinger effect of tensile samples.


After the steel plate was formed by JCO, the yield strength test results of the flattening tensile sample of steel tube were decreased by 15 Mpa compared with the yield strength of the plate at the rate of 1.0% diameter enlargement, indicating that the bauschinger effect of the flattening tensile sample was greater than the hardening effect during the steel tube forming and diameter enlargement process.


3 conclusion


(1) the diameter enlargement rate shall be flexibly selected according to the performance and chemical composition of raw materials.(2) in order to improve the production efficiency, the pipe mill should develop the acceptance conditions for the mechanical properties of steel plates according to their own equipment and process conditions.(3) for the combination of JCO+ mechanical expansion, the minimum yield strength for acceptance of X70 steel grade steel plate is 500mpa.(4) measure the residual stress under different diameter expansion rates, and optimize the diameter expansion.