Qualified billet is a precondition for pipe production. Due to the deformation characteristics of skew rolling perforation, strict technical requirements for the tube billet quality(especially surface quality) are put forward.

1,Technical requirements

(1) the pipe diameter and its allowable deviation:

The pipe diameter of 75 mm, allowable deviation is + 1.0-0.5 mm;

The diameter of 80-90 mm , allowable deviation is + 0.8-1.3 mm;

The diameter of 95-120 mm, allowable deviation is + 1.0-1.7 mm.

(2)Tube billet length: generally 2.3-6 meters (according to mutual agreement).

(3)Bending: per meter local bending is less than 10 mm without straightening device, the total curvature is less than 1.0% of the total length.

(4)Tube billet head chamfer Angle: when the pipe diameter is less than 96 mm,the cutting angle is less than 6 mm;Tube billet is 95 ~ 120 mm in diameter, cut slope is less than 8 mm.After peeling or scalping,tube billet surface roughness Ra is 25 microns.If exceed the allowable deviation,it will not only make perforation nip condition turn bad, and directly affect the quality of perforation and the mill adjustment.

2, the tube billet surface quality directly affects the surface quality of the finished steel pipe.

In the production of hot rolled steel pipe, first of all,to inspect strictly

the state of tube billet surface.The defects such as scar,crack, grooving, folding, dent, handle, porosity, pitting, sand holes and non-metal inclusion is not allowed in the outer surface of the tube billet.

When the defect depth is more than 0.7 ~ 0.7 mm, defects can't be burn with tube billet heating, but any defect remained in the perforated tube billet surface will extend in the process of deformation, the defect in any defects on the surface of the steel tube surface,longer and deeper ,finally causing waste.The above local defects should be cleared in time, but the depth of defects treatment does not allow more than 5% of the pipe diameter.

3, the macrostructure of the tube billet.

The internal quality of tube billet is generally based on the macrostructure.

There shouldn’t be visible defects to the naked eye of grade 1 in the macrostructure, such as residual shrinkage cavity, skull patch, delamination, bubbling, sand holes, non-metallic inclusions, white spots and crack, etc.

For those used in high alloy steel pipe with special requirements for the production, in addition to the examine of the macrostructure,the inspection for the microstructure is also necessary (i.e., metallographic microstructure organization) , to determine the content and distribution of nonmetallic inclusion (e.g., sulphide, oxide and carbide) and identify the banded structure and decarburization layer, etc.

The general loose, loose, center segregation, subcutaneous bubble defects such as the standard sample after rating, shall not exceed the following requirements: production of general-purpose ordinary carbon structural steel pipe generally loose grade 4 or less center loose category 3 or less partial analysis level 3 or less depth of subsurface blisters, 2 mm or less in the production of high pressure boiler pipe, the structure of the alloy tube, tube billet generally loose grade 2 center loose or less or less partial analysis level 2 or less subcutaneous bubble is not permitted in depth

4, the microstructure of the tube billet .

For the alloy and high-alloy steel pipe used in the production of the special requirements of the tube, in addition to checking the low times outside organization, but also for the microstructure (ie metallurgical high-powered organization) test to determine the non-metallic inclusions (such as sulfide, oxide matter and carbide) forms of content and distribution, identifying band structure and decarbonization layer.

The influence of the tube billet’s merits and demerits on the quality of the structural carbon steel pipe:

The merits of the billet quality is the primary factor in determining the quality of seamless tube.

When pipe diameter is too big or ovality too big,it can make the bite condition when perforation turns bad,and will also promote the occurrence of folds as large amount of compression of the pipe billet diameter.

There are two bites in the process of rotary piercing,the first bite is the in the instant moment when the pipe billet and roll start to contact, the roll drives the tube base move and the tube billet dragged into the deformation zone, called primary biting. When the metal gets into the deformation zone and meets the piercing plug,it continues to get into the deformation zone overcoming the immediate axial resistance,called for secondary bite.

In order to guarantee the reasonable perforation process and to obtain high quality seamless steel pipe,we must put forward strict requirements for the geometry size,macrostructure and surface state of the tube billet.