Mechanical properties of X70 spiral submerged arc welded steel tube

Mechanical properties of X70 spiral submerged arc welded steel tube：

Grade & material: X70

Type & model: Φ1016mm×14.6mm spiral submerged arc welded steel pipe

Production requirements: API SPEC 5L X70 acicular iron body, oxygen converter or electric furnace smelting, vacuum degassing, calcium and micro titanium treatment, low carbon microalloyed killed steel; TMCP production steel sheet (roll), grain size 10 or finer, Non-metallic inclusions ≤ 2.

Tensile properties: 

Steel pipe: σs (σt0.5)=485~620 MPa      σb≥570 MPa      yield ratio ≤0.90
Steel plate (roll): σs (σt0.5)=530~620 MPa      σb≥570 MPa       yield ratio ≤0.92
Elongation requirements are calculated as ≥22% according to API SPEC 5L
Weld seam σb≥570 MPa

Toughness:

DWTT: steel plate (roll) (-15 °C)     single SA% ≥ 70    average SA% ≥ 85
Steel pipe : (-5 ° C)                  single SA% ≥ 70    average SA% ≥ 85
Charpy impact: transverse direction of the pipe (-20 °C)  single SA% ≥ 80     average SA% ≥ 90    Single Cv ≥ 140J      Average Cv ≥ 190J

Weld and heat affected zone (-20 °C): Single SA% ≥ 30   Average SA% ≥ 40    Single Cv≥60 J    Average Cv≥90

In addition to Nb, V, Ti, X70 pipeline steel also contains a small amount of Ni, Cr,Cu and Mo, delay the formation of ferrite to a lower temperature, which is conducive to the formation of needles ferritic and lower bainite. The microstructure of the X70 steel plate is mainly acicular ferrite. Add a small amount of granular bainite, the band structure is less than 2.0, the grain is fine, basic it is from 11th to 12th. Therefore, X70 pipeline steel is essentially a acicular ferrite type of high strength, high toughness pipeline steel.

X70 grade coil is deformed and hardened due to its special structure. The degree of influence is high, greatly reducing the impact of the Bauschinger effect, yield strength loss the loss is only about 10 MPa.

Since the tensile strength is mainly determined by the saturation value of the work hardening of the material,after the pipe is made, the tensile strength value shows a slight increase, which makes the post-tube yield ratio the average value is reduced, which means that the plastic deformation ability of the steel pipe is enhanced.

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