X70 Steel Grade HFW Submarine Pipeline Steel Pipe

Abstract : successfully developed the X70 steel grade HFW submarine pipeline steel pipe in 2009 after first developing the X65 steel grade HFW steel pipe for submarine transportation in 2006 . The research and development process of X70 steel grade HFW submarine pipeline steel pipe is introduced . The research results show that the developed X70 steel grade submarine pipeline steel pipe meets the requirements of DNV-OS-F101-2007 submarine pipeline technical specification and GB/T 9711.3-1999 standard ( C grade steel pipe), and has achieved the expected effect.

Key words : HFW submarine pipeline steel pipe; research and development process; steel pipe performance

High frequency resistance welding ( HFW ) is an efficient method of producing pipeline steel pipes . With the improvement of HFW steel pipe manufacturing technology and the improvement of inspection methods , domestic HFW steel pipes have been successfully applied to submarine oil and gas pipelines and replaced imports . At the same time, as the depth of wells for oil and gas exploitation on the seabed becomes deeper and the working conditions become more and more complex , the specific requirements for high strength , high impact toughness , high pressure resistance , corrosion resistance , low temperature resistance and collapse resistance of steel pipes are further increased. . CNOOC Jinzhou Pipeline Co., Ltd. successfully developed HFW steel pipes for submarine transportation for the first time in 2006. The specifications are ① 355.6 mm X 11.1 mm , steel grade X65 , and the products were used in the Haikou - Wenchang project . Production of steel pipes for multiple subsea pipeline projects such as the Zhouzhou Project , Luda Project , Bozhong Project , Jidong Nanbao Oilfield Project , Jinzhou 25-1 Project , and the steel grades are X60 or X65 respectively .

2009 , the company successfully developed X70 steel grade HFW steel pipe for the Boxi - Bonan joint gas transmission project , and implemented the DNV-OS-F101 submarine pipeline standard . The physical and chemical properties , metallographic structure and material requirements are more specific and more accurate than API 5L strict . This paper mainly introduces ① the production process and product test of 323.9 mm x 14.3 mm X70 grade HFW pipeline steel pipe for subsea use .

1. Material requirements for X70 steel grade HFW steel pipes
   1. chemical composition

X70 steel grade is smelted by oxygen-blown converter process and processed by vacuum degassing and calcium treatment . The steel coil is produced by thermomechanical controlled rolling process ( TMCP ) . The trace alloy composition control and controlled cooling and controlled rolling process of X70 steel grade ensure that the material obtains reliable toughness and strength . The chemical composition of the materials is shown in Table 1 .

1. 1. mechanical properties

Submarine pipelines have strict intervals for yield strength and tensile strength

Control , the requirements for steel pipes are that both the maximum yield strength and the maximum tensile strength shall not exceed the minimum specified value of 120 MPa . Since the steel plate has work hardening phenomenon and Bauschinger effect in the process of pipe making and testing , the yield strength of the material The interval requirements for tensile strength and tensile strength are very strict . Due to the full application of strain theory in the design of submarine pipelines , transverse and longitudinal mechanical performance tests are required for products , and the transverse and longitudinal yield ratios are stipulated to be less than 0.90 . In terms of material control, the impact on the quality and performance of the product should be fully considered . Therefore, the parameters that meet the product manufacturing process should be proposed in the raw material technical specification , and the key points of material control should be discussed with the steel coil manufacturer , and the results after the material is accepted in the factory It shows that the performance index meets the technical requirements . X70 steel grade steel coil yield strength range control interval situation .

 

 

Microstructure
It is stipulated that the metallographic structure of the X70 steel grade steel coil is acicular ferrite , and the acicular ferrite structure is conducive to reducing the Bauschinger effect . The grain size is not less than grade 10 .

1. Trial production process

HFW630 unit of CNOOC Jinzhou Pipeline Co., Ltd. has a production diameter range of ① 219 ~ 610 mm , a thickness range of 5.0 ~ 19.1 mm , and a steel grade of X70 . The unit is equipped with TBS forming equipment from ABBEY Company of the United States , and a solid-state high-frequency welding machine from Thermatool Company of the United States with a power of 1200 kW is used . This welding machine has dual functions of induction welding and contact welding , and has automatic temperature control and accurate temperature display. advantage . The online heat treatment of the weld seam adopts 4 sets of 500 kW intermediate frequency heat treatment equipment manufactured by Thermatool Company of the United States to ensure that the heat treatment in the entire thickness direction of the weld seam is sufficient and there is no residual untempered martensite structure in the weld seam .

1. 1. Production process route

Raw material inspection T uncoiling butt welding T loop storage T milling T plate UT ( automatic ) T steel pipe forming T steel pipe welding T weld UT inspection T weld online heat treatment T air cooling T water cooling T sizing straightening T flying saw cutting Steel pipe repair—hydraulic test—offline weld UT ( automatic )— >■ pipe end UT ( hand probe ) —finished product inspection—spray marking—final inspection into the warehouse .

1. 1. Trial production and production status

In September 2008 , the company organized the trial production of ① 323.9 mm X 14.3 mm X70 steel grade submarine pipeline steel pipes , and organized the welding process evaluation , and revised and re-determined important parameters such as welding process parameters and heat treatment processes . Successful trial production of X70 steel grade submarine pipeline steel pipe . Before April 2009 , a total of 12,000 tons of X70 steel grade HFW submarine pipeline steel pipes were produced , and the product performance pass rate was 100% . At the same time , the research and development results were applied to market development and mass production .

1. 1. Core technology and control points

Submarine pipeline steel pipes ① 323.9 mm x 14.3 mm and ① 323.9 mm x 15.9 mm not only have high steel grades , but also have large wall thicknesses , with D / t values of 22.7 and 20.4 respectively . The forming strength of the steel pipes has reached the design limit of the production line ; Under pressure conditions, the amount of burrs in the weld is large and difficult to remove ; high-frequency welding power and intermediate frequency heat treatment temperature need to be appropriately increased to ensure that the welding process meets the design requirements ; under hydrostatic pressure test conditions, the standard pressure requirement reaches 40 MPa or more . All these conditions bring difficulties to product development and trial production .
( 1 ) In view of the large internal burr in the weld and it is difficult to remove , the internal burr removal equipment is improved . The original internal support type is changed to the cantilever type internal burr removal equipment , and the inner burr holder is increased , and the inner burr is removed with a ring-shaped inner burr with a diameter of ① 65 mm , which solves the problem of internal burr removal .
( 2 ) Considering the three aspects of welding speed , opening angle and high-frequency welding machine power comprehensively , under the power of 1200 kW high-frequency welding machine , the utilization efficiency of the equipment is improved by reducing the inner diameter of the induction coil and adopting Japanese TDK magnetic rods to meet the requirements of process requirements .
( 3 ) In order to obtain high-strength and high-toughness HFW steel pipes , sufficient heat treatment should be performed on the weld to improve the microstructure of the weld and its heat-affected zone ( HAZ ) . Since the wall thickness of the steel pipe is 14.3 mm , it is very important to ensure sufficient heat treatment in the entire wall thickness direction , so the selection of heat treatment temperature should be appropriate . During the research and development process, comparative analysis was carried out around different heat treatment temperature ranges . Fine ferrite grains were formed in the 9507 heat treatment samples , while coarse-grained bainite was formed in the 10507 heat treatment samples . The technical conditions of the submarine pipeline steel pipe require that the grain size of the weld seam and the heat-affected zone after heat treatment be above grade 9 of the ASTM E112 standard . In order to avoid coarse grains on the outer wall of the steel pipe and ensure sufficient heat treatment on the inner wall , the heat treatment of 1 000 ± 307 is determined temperature . According to the analysis data of the trial production , the addition of Mo element can make the heat-treated ferrite grains thinner . Since the content of Mo element is considered in the production of raw materials , the ferrite grains in the post-weld heat treatment HAZ can be refined. expected effect .
( 4 ) ① The hydraulic test pressure of a 323.9 mm X 14.3 mm steel pipe is 40.6 MPa , and the seal ring of a conventional hydraulic machine will fail if less than 10 steel pipes are tested under a pressure of 40.6 MPa . After analyzing the reasons for the damage of the sealing ring , it is considered that the lip of the sealing ring is mainly torn under high pressure , and the service life of the sealing ring and the quality of the hydraulic process are improved by improving and thickening the two side lips of the sealing ring . The improved sealing ring can continuously test 260 steel pipes with the same steel grade and wall thickness .

1. Research results
   1. Steel pipe mechanical properties
      1. tensile properties

X70 steel grade submarine pipeline steel pipe is 485 MPa , the transverse yield strength of steel coils in the range of 540-580 MPa accounts for about 33% ( Figure 1 ), and the transverse yield strength of steel coils in the range of 500-540 MPa accounts for about 58% . % , after the steel pipe is made , the transverse yield strength of the steel pipe is reduced to about 10% in the range of 540-580 MPa , and the transverse yield strength is increased to about 82% in the range of 500-540 MPa , which shows that the transverse yield strength of the steel pipe is due to Bauschinger The effect has decreased to a certain extent . Considering the influence of the Bauschinger effect , the lower limit value of the transverse yield strength of raw materials should be about 30 MPa higher than the specified lower limit value , so as to avoid the decrease of the transverse tensile properties of steel pipes due to the Bauschinger effect beyond the standard lower limit value regulations .
The transverse yield ratio of raw materials in the range of 0.76 - 0.84 accounts for about 80% , and the transverse yield ratio of steel pipes in the range of 0.78 - 0.84 accounts for about 76%. The degree of reduction , so the transverse yield ratio after the tube does not change much .
materials in the range of 490-500 MPa accounts for about 60 % , and that in the range of 500-520 MPa accounts for about 30 % . The average increase in longitudinal yield strength is about 40 MPa . The longitudinal yield ratio has been greatly increased , from the lowest longitudinal yield ratio of raw materials 0.72 to the lowest longitudinal yield ratio of steel pipes 0.78 , and from the highest longitudinal yield ratio of raw materials 0.85 to the highest longitudinal yield ratio of steel pipes 0.90 , reaching the limit value specified in the standard . The main reason for the increase in longitudinal yield strength is that the yield strength increases due to work hardening during the forming process of the steel plate , while the change in tensile strength is small , so the yield ratio increases greatly . Considering the effect of work hardening , the upper limit of the longitudinal yield strength of raw materials is appropriately reduced to avoid the yield ratio or yield strength exceeding the standard requirements due to the increase of longitudinal yield strength .

1. Impact toughness

1 ) Impact toughness of steel pipe body
the requirements of the submarine pipeline technical specification DNV-OS-F101 , the Charpy impact test is carried out on the steel pipe body , and the ductile-brittle transition curve is drawn ( Figure 4 ). The Charpy impact test is carried out according to ASTM A370 . The Boxi - Bonan Submarine Linepipe Project requires Charpy impact tests at -15 7 , the average impact energy of raw materials is above 270 J , and the shear area is 100% . The impact test is carried out on the steel pipe body , and the average impact energy of the pipe body is equivalent to or slightly lower than that of the raw material .

2 ) Impact toughness of weld
In order to further analyze the quality of the steel pipe weld ( see Figure 5 for the metallographic structure of the weld ), energy spectrum analysis was performed on the ductile fracture and brittle fracture of the weld fusion line impact specimen, and it was found that the oxygen content in the ductile fracture was extremely low , and in the The oxygen content in the brittle fracture is relatively high . A high oxygen content indicates the presence of oxides in the weld , which originate from raw materials on the one hand and are generated during welding on the other hand . From the perspective of HFW welding process, the source of inclusions is analyzed . The small extrusion force does not squeeze out all the oxides , the opening angle is too small or the input heat is too large to cause the inclusions to reflow . These factors may lead to the presence of oxides in the weld . Therefore , appropriate extrusion force and a larger opening angle should be used during welding , and the quality of the weld should be controlled according to the second situation [ 1 ] , so as to avoid the occurrence of weld inclusions affecting the impact toughness of the weld .

The impact toughness of the weld includes three positions : one is that the V -notch is opened at the weld fusion line ( F ); the other is that the V -notch is opened at the weld fusion line +2 mm ( F+2 ); the third is that the V -notch is opened +5 mm at the weld fusion line ( F+5 ). The impact toughness of the weld fusion line is lower than that of the heat-affected zone and the base metal ( R ) ( Fig. 6 ).
related to the treatment process . In order to improve the low temperature impact toughness of the weld , also

1. Hardness of the weld

After the heat treatment of the weld seam and the heat affected zone , it is measured after being corroded by nitric acid alcohol . The heat treatment width of the outer surface is 27 mm , and the heat treatment width of the inner surface is 20 mm , indicating that the heat treatment of the inner and outer surfaces of the weld is sufficient , and there is no untempered martensite body exists . The hardness of the weld fusion line , the hardness of the heat-affected zone and the hardness of the base metal are basically the same . The measurement data are shown in Table 2 .
Table 2 Weld hardness measurement data

1. Flattening test of weld seam

The steel pipe weld was subjected to a flattening test according to the API 5L standard , and the weld was free of cracks until the distance between the two plates was 215 mm ( that is, 2/3 of the original outer diameter ) ; 1/3 of the weld seam and the base metal have no cracks ; flatten until the pipe wall fits , the base metal of the pipe body does not appear to be delaminated , and the weld seam has no cracks .

1. Steel pipe NDT inspection
2. In the trial production and mass production process of X70 steel grade submarine line pipe , 4 non-destructive testing stations were set up to effectively test the quality of base metal and weld seam . They are online plate UT , online weld UT , offline weld UT , pipe end UT and groove magnetic particle inspection .
3. X70 steel grade submarine line pipe raw material plate is 100% layered detection , using 80- channel in-line scanning with 100% coverage of the steel plate , and the sensitivity comparison test block is ① 6 mm flat-bottomed hole . 100% inspection of steel pipe welds , using 24- channel automatic flaw detection equipment for inspection , N5 grooves , ① 1.6 mm through holes and ① 1.6 mm horizontal holes to calibrate the detection sensitivity of artificial defects .
   1. Geometric dimensions of the steel pipe

The diameter tolerance of the steel pipe end is ① 323.9 mm ± 1.4 mm , and the ellipticity requirement is 2.9 mm . After the steel pipe is sized and straightened , the actual diameter is 323.7 - 325.0 mm , and the ellipticity is controlled to W 1.3 mm . The geometric dimensions are shown in Table 3 .

Table 3 Geometric dimensions of X70 steel grade HFW steel pipes

serial number	Weld HV10	Heat affected zone H V10	Base material H V10
1	195	186	184
2	186	189	181
3	191	190	188
average	191	188	184

Tube body /mm			Pipe end /mm			Curvature / ( mm • m -1 )
Weld Direction	Vertical weld direction	Ellipticity	Weld Direction	Vertical weld direction	Ellipticity
324.7	323.8	1.1	324.6	323.9	1.2	0.49

1. epilogue

Through the research and development of X70 steel grade HFW subsea pipeline steel pipe , the following conclusions are drawn :

1. Since the steel pipe has work hardening and is affected by the Bauschinger effect in the process of manufacturing and testing , the minimum transverse yield strength of the raw material should be increased by 30 MPa than the specified minimum value , and the longitudinal yield strength of the raw material should be reduced by 10 MPa than the specified minimum value , so that the performance of the steel pipe after pipe making can meet the specified requirements .
2. The key process technologies such as forming , welding and heat treatment of X70 steel grade submarine line pipe are reasonably controlled , and the appropriate method of large opening angle and high extrusion force after forming can maintain stable weld quality .
3. The developed HFW submarine pipeline steel pipe body and weld not only have high strength and impact toughness , but also have a relatively stable yield ratio .
4. After heat treatment, the hardness of the weld seam is equivalent to that of the base metal , and the grain size of the weld seam and the base metal are both above grade 10 of the ASTM E112 standard , and the weld seam and the base metal achieve the same toughness effect .

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