Large Diameter and Thin Wall X60 Steel Grade Pipelines

process development

1. 1. Process introduction

① The 340 mm continuous rolling tube unit rolling tube billet ① 325 mm x 8.8 mm x 14 000 - 15 000 mm key process is continuous rolling and on-line normalization (equivalent to normalizing), and it is necessary to ensure that the wall thickness deviation of the tube billet is within ± Within 10% , the comprehensive mechanical properties meet the requirements of the standard .
Because the wall thickness of the finished steel pipe is 7.92 mm , the pipe blank is subjected to local induction
When heating thermal expansion to reduce the diameter, the temperature is high and it is easy to "wrinkle" or the port "tears". In order to keep its comprehensive mechanical properties as consistent as possible with the tube blank, the main control process parameters are thermal expansion temperature and speed.

After the tube blank enters the guide area of the mandrel, it enters the induction heating section, and thermal expansion is carried out in the deformation zone of the heating section. The steel pipe and the mandrel reach a stable temperature (about 760 ℃) in the deformation zone . Enter the slow cooling stage, that is, the flattening area and the straightening area. At this time, the temperature of the mandrel is maintained at about 650 °C . Due to the heat conduction of the mandrel, the temperature of the steel pipe drops slowly; when the steel pipe leaves the straightening area, it is air-cooled to room temperature .

Thermal expansion adopts intermediate frequency induction heating, using electromagnetic induction to generate electric eddy current inside the heated material, relying on the energy of eddy current to achieve the purpose of heating. Compared with different phase transformation rules and microstructure characteristics: the critical temperature ( 4c ) of hypoeutectoid steel pearlite to austenite transformation and the complete austenitization transformation temperature ( 4J increase. X60 belongs to hypoeutectoid steel Steel, expanded by induction heating to raise the Aci line.
The Curie point of carbon steel and low alloy steel with rapid temperature rise by induction heating is 760-780 T.  according to the calculation according to the formula and referring to the research results of L360 , it is considered that the thermal expansion temperature should be controlled near Aci (about 760 P is better, both Ensure that the diathermic heating of the steel pipe does not change the basic structure of the steel.
The essence of the push-type thermal expansion steel pipe process is a special heat treatment, and this rapid heat treatment requires penetration heating. The test shows that 86.5% of the heat converted by the induced current is concentrated in the current penetration layer, and the current penetration depth a is calculated by the following formula:

5 =5 030 x aunt f
where p - metal resistivity, Qpm ;
M —the relative magnetic permeability of the metal;
f ———— The current frequency of the power supply, HZ .

1. Determination of resistivity p value. When heated at a temperature above the Curie point, the resistivity of any steel grade is basically similar, which is ( 1.1 ~
   1. x IO -4 Q •cm , so take ll x lO -4 Calculation by Q* cm will not produce obvious deviation ［ 7 ］ .
2. Determination of the relative permeability Bu value. Ferromagnetic steel has a sudden change in magnetic permeability near the Curie point. The relative magnetic permeability of carbon steel and low alloy steel suddenly drops from 200 to 1.0 . When heating at the Curie point and above, all steel grades take Bu = 1.0 .
3. Determination of current frequency f . Generally, when large-diameter steel pipes are heated above the Curie point, the reasonable current frequency is 500-1 000 Hz . At this time, the heating electric efficiency is about 80 % [ 7 ] , and the current frequency f=1 000 Hz for thin-walled pipes .

Substituting p and "eight f values into the formula ( 1) calculates 5 = 16.6 mm , considering the induction current loss caused by the distance between the induction coil and the outer surface of the steel pipe, taking a = 8.3 mm , which is greater than the specified wall thickness of the steel pipe, and belongs to the diathermy type heating.

the four sides of the upper, lower, left, and right sides . The actual measured temperature is 760 ~ 770 °C. Since the heating deformation temperature of the X60 seamless steel pipe is not higher than the phase transition point Ad during the push-type thermal expansion process , it is equivalent to the low-temperature deformation annealing process; and the thermal expansion deformation of the steel pipe is below the austenite recrystallization temperature of the steel, and the metal is carbonized It is difficult to dissolve the material, which increases the strength of the material without changing the matrix structure of the steel; because it is an annealing process at the same time, it can eliminate or reduce the internal stress in the steel, reduce the hardness, and improve the processing performance such as cutting [13 ] .
If the pipe expansion speed is too high, on the one hand, it will cause the steel pipe port to be easily "teared" and "wrinkled" when the diameter is changed by thermal expansion, and on the other hand, the internal stress of the steel pipe will be concentrated ; min .
Appropriate thermal expansion process is adopted in the thermal expansion of the port to ensure that the outer diameter, roundness and wall thickness of the steel pipe port part meet the standard requirements .

• chemical composition

The chemical composition meets the requirements of PSL2 in the API Spec 5L standard ( 44th edition) , and it is based on C-Mn steel and strengthened by adding microalloying elements.

Table 3 $ 406.4 mm x 7.92 mm X60 steel grade seamless steel pipe chemical composition mass fraction%

• Tube blank production process and size control

Tube billet production process: ring furnace heating billet — piercing — ① 340 mm MPM continuous rolling—pipe stripping, one step cooling bed cooling, one step furnace reheating — high pressure water descaling — sizing — cooling — straightening.
Dimensional deviation of tube blank: ± 1% for outer diameter, ± 10% for wall thickness .

• thermal expansion process

Tube blank size inspection — head and tail sawing — thermal expansion — straightening — flaw detection — cutting to length, cutting — hydraulic test — chamfering — final inspection — painting with antirust paint — spraying labels — weighing — storage.

1. Comprehensive Mechanical Properties Analysis of Thermal Expansion Finished Tube

The performance statistics of the thermally expanded finished tube are shown in Table 4 . Test conditions: Tensile properties are taken according to the standard longitudinal strip specimen with shoulder, width 38.1 mm ; transverse impact test, temperature 0 °C, notch V8 , sample size 10 mm x 6.67 mm x 55 mm ; chemical composition is taken from each batch For double samples, the carbon equivalent is calculated according to CEIIW=C+Mn/6+ ( Cr+Mo+V /5+ ( Ni+Cu /15 ) ; the transverse HRC hardness is analyzed, and three values are measured at intervals of 120 ° on the cross-section of the steel pipe .

from the performance statistics table of the thermal expansion finished tube :
①The yield strength is 420 ~ 490 MPa , the tensile strength is 530 ~ 610 MPa , the yield ratio is 0.75 ~ 0.81 , and the elongation after fracture is 33% ~ 40% , indicating that the finished pipe has good plasticity and yield ratio;
② The transverse impact energy at 0 C is greater than 100 J in full-scale conversion , indicating that the steel pipe still maintains good toughness after thermal expansion;
③The maximum value and average value of hardness shall not exceed 12 HRC , which is lower than the standard requirement (not greater than 22 HRC ;
④Carbon equivalent CEiiwW 0.47 , indicating that the steel pipe has good welding performance.
During the final inspection of the steel pipe, the residual magnetism of the groove at the pipe end shall be randomly measured, and the maximum value shall not exceed 20 Gs . Although steel pipe heating is based on the principle of electromagnetic induction, it will not increase the magnetism of the steel pipe because it is non-contact with the induction coil.

Table 4 Performance Statistics of Thermal Expansion Finished Tubes