Hot rolled seamless steel pipe

Hot rolled seamless steel pipes is relative to the cold-rolled, cold rolling is below the recrystallization temperature of the rolling and hot rolled is carried out at above the recrystallization temperature of the rolling.

Hot-rolled seamless steel pipe is a large category of seamless steel pipe, which is divided according to the production method. Hot rolling is relative to cold rolling, cold rolling is carried out rolling at room temperature, and hot rolling is carried out rolling at the crystallization temperature. Seamless steel pipe is named relative to the welded steel pipe, seamless steel pipe is usually made of round steel perforation and reprocessing, welded steel pipe is usually welded by steel plate in different ways. The diameter of hot-rolled seamless steel pipe is usually above 38mm. Seamless steel pipe with smaller diameter can be obtained by cold rolling or cold drawing.

Hot-rolled seamless steel pipe: hot rolled is relative to the cold-rolled, cold rolling is below the recrystallization temperature of the rolling, and hot rolled is carried out at above the recrystallization temperature of the rolling.

Hot-rolled seamless steel pipe production base deformation process can be summarized as three stages: perforation, extension and finishing.

Billet is fed into the heating furnace, the temperature is about 1200 degrees Celsius. Fuel for a hydrogen or acetylene. The furnace temperature control is critical to wear air tube round, after the release of pressure puncher. which are generally more common puncher tapered roller punch this punch, high production efficiency, product quality, perforated expanding volume, wearing a variety of steels. After perforation, round tube has been three-roll cross rolling, rolling or extrusion.

Generally process of hot-rolled seamless steel pipe: round bars → heating → piercing → three-roll cross rolling, rolling or extrusion → detached → sizing (or reducing) → cooling → parison tube → straightening → hydrostatic test (or flaw detection) → mark → storage.

After extrusion off pipe sizing. Sizing mill by the high-speed rotation of the cone drill into the Billet punch to form a steel pipe. The steel pipe inner diameter is determined by the length of the outer diameter of the sizing mill drill. After sizing, the steel pipe to enter the cooling tower by the water spray cooling, the steel pipe after cooling, it is necessary to be straightening. Sent by conveyor pipe by straightening the metal detection machine (or hydrostatic test) to conduct an internal flaw. Steel pipe internal issues such as cracks, bubbles will be detected.

Advantages

hot rollded seamless steel pipe can damage the cast microstructure of the steel ingot, refinement of the crystal grains of the steel, and eliminate the defects of the microstructure, so that the the steel organization compacting, improve the mechanical properties. This improvement is reflected in the rolling direction, so that the steel is no longer to a certain extent isotropic; pouring the formation of bubbles, cracks, and osteoporosis, under high temperature and pressure can also be welded together.

Disadvantages:

1. After the hot rolling, the nonmetallic inclusions in the interior of a steel (mainly sulfides and oxides, as well as the silicate) was pressed into a sheet, stratified (laminated) phenomenon appears. The layering of the steel in the thickness direction by the pull performance deteriorated significantly, and may appear in the weld shrinkage interlayer tear. Weld shrinkage induced local strain often reach several times the yield point strain, the strain is much greater than the load caused;
2. Residual stress caused by uneven cooling. The residual stress is the stress of internal self-phase equilibrium in the absence of external force, the hot-rolled steel of various cross-sectional has a residual stress such Usually steel sectional dimension is the greater, the greater the residual stresses. Residual stress is self-phase equilibrium, but the steel members in the performance external force or have a certain influence. Such as deformation, stability, anti-fatigue may adversely.
3. Hot-rolled steel products, poor control of thickness and edge width. We are familiar with the thermal expansion and contraction, even if the beginning of the hot-rolled out are standard length, thickness, or there will be some negative final after cooling, this negative differential edge width wider the increasing thickness of the performance of the more obvious.

So for large steel for steel edge width, thickness, length, angle, and sideline no law requires too precise.

Separation

Located on the inner surface of the steel pipe was vertical distribution, was raised helical, massive metal separation or break-like dissection.

Straight inner flap

Located on the inner surface of the steel pipe was vertical distribution, showing a symmetrical or a single line-shaped folded with through long, there are local.

Irregular hole

Located in the surface of the pipe was vertical distribution, ① has one or two walls 180 °out of thickening, or in the surface of the pipe and the wall thickness of the mandrel separation point has thickened shape, also known as the inner drum kit. ② steel inner garden was a hexagonal shaped uneven thickness, also called hexagon.

Wall contraction

Located on the inner surface of the steel pipe, steel transverse section thinnest steel pipe inner surface depressions, localized thinning of the wall thickness of the severe contraction almost tear.

Rolled scar

Longitudinal direction of the inner surface of steel nails like scarring, bumps or lumps folded inner pipe surface indentation.

Inside folding

Located at the inner surface of the end portion of the steel pipe, local or flaky longitudinal spiral or semi-spiral folding irregular distribution.

Rolling off

Located within the longitudinal surface of the pipe wall was a partial or full length in the outer concave or convex corrugations on the outer surface of the pipe through the long longitudinal grooves prescribe obvious symmetry, generally rectilinear, individual to slash shape.

Tear

Located on the tube longitudinal steel surface showed varying degrees of transverse rupture, diamond-shaped and oval-shaped hole through the tubular body.

Fold the double-slit

Located on the longitudinal surface of the pipe wall obliquely extending into a pair of longitudinal cracks, the cracks sometimes scattered distribution on the pipe circumference sometimes appear symmetrical.

Double centerfold

Located on the longitudinal surface of the pipe wall obliquely extending into a pair of longitudinal cracks, the cracks sometimes scattered distribution on the pipe circumference sometimes appear symmetrical.

Outer folding

① on the outer pipe presents regular folding triangle shaped, double-slit straight single slit straight or irregular sheet folded like.

② Presenting a generally continuous or intermittent stitch sewing machine-like or staggered 60 °, 120 °, 180 ° sewing stitch-shaped folded pipe in the longitudinal direction of the outer。

③ outer longitudinal folded on a spiral steel pipe.

④ a longitudinal direction of the pipe surface through long or short slash dot folded severe staggered two or three of 120 °.

Requirements on pipe billet for hot-rolled seamless tube is related to and pipe varieties, uses and piercing methods.

For general-purpose steel pipe tube quality requirements can be relaxed more, but the quality of important applications of tube steel and high-alloy steel pipe requirements must be strictly. Stress conditions better or lesser amount of deformation perforation method, without affecting pipe performance conditions, the tube surface and internal quality requirements may be slightly lower. Stress disadvantaged two-roll rotary piercing, if a large amount of deformation perforation, then the tube surface and internal quality have strict requirements. In short, steel tube technical condition is to ensure the quality and smooth production process as the basis to determine, and with different methods vary perforation, with the technical conditions of steel to improve and increase. Technical conditions can be found on a variety of tube standards and technical protocols.

Such as general automatic rolling machine for tube requirements are as follows:

(1) Deviation in Table 1 tube diameter Dp. Tube bending of any part of the ≤6mm / m, tube end face of the cut slope ≤6 ~ 8mm, flattened tube end face of ≤8% Dp;

(2) Tube surface without cracks, hairline, scarring, scaly layer, folding, non-metallic inclusions and residual shrinkage. Not allowed height or depth of more than 0.5mm small grooves, pitting, ears and height of more than two-sided ears of 1mm. Cleanup tube defects deeper than 0.05Dp;

(3) Low magnification tube: not allowed ≥1 level of residual shrinkage, bubbles, anti-leather, white spots and cracks. For the average carbon structural steel billet, which generally loose ≤3 level, center porosity ≤3 level, segregation ≤3 level, subcutaneous bubbles ≤2.