All of Shenhao seamless steel tubes and pipes are fabricated from round steel bars through a series of processes including billets inspection, billets cutting, heating, piercing, rolling, sizing and reducing, straightening, flaw diction, visual and dimensional inspection, etc. As a leading manufacturer of seamless round steel tubing, we boast the most up to date process in the industry.
A. Billets Acceptance Inspection
All billets for making seamless steel tubes come direct from large sized iron and steel plants in China such as Tianjin Iron and Steel (Group) Co., Ltd. After receiving the round steel bars, we will take some samples and send them to the lab for inspection of chemical compositions and macro test using metallographic microscope, so as to ensure all raw materials can meet the requirements.
In addition, billets appearance will be inspected to make sure there are no flaws such as cracks, rolling skins, and others on their surface. Only qualified billets can be transported to the next process.
B. Billets Cutting
After inspection, qualified billets will be cut into a number of sections according to steel grade, furnace number, specification, quantity and length. We use band sawing machine with high automation. It features low energy consumption, low metal loss and high cutting quality.
End slope of the billets is 3mm or less and length tolerance is controlled within 0-20mm. After cutting, each billet will be marked with steel grade, furnace number and specification. Billet number in each lifting is at least 3. For special steel, color should be marked on its end to avoid confusion.
C. Billets Heating
Billets will be heated in an annular furnace in a diameter of 13.5m. Such furnace is characterized by excellent seaming performance, uniform heating and high automation due to use of mechanical arm for billets feeding and discharging. Heating temperature ranges from 1150℃ to 1280℃. Rated heating capacity of the furnace is 25t/h and the maximum heating capacity is up to 50t/h.
In recent years, to reduce air pollution, Shenhao has changed the combustion system from coal gas system to natural gas system, thus realizing dual use of these two fuels and reducing air pollution.
Discharged from the furnace, the billets will receive piercing process that is the most importance shaping process in production of hot rolled seamless pipes. It will make solid billets into hollow billets. We use Mannesmann two-roll rotary piercers which are controlled and operated by experienced workers, ensuring that the hollow billets come with uniform wall thickness and high dimensional accuracy, smooth inner and outer surface without cracks, scratches, scabs, wrinkles and other flaws.
Moreover, the whole piercing process is automatically controlled by Siemens PLC control system, considerably reducing labor strength and improving production efficiency.
After piercing, the hollow billets will be conveyed into the rolling mill for secondary rolling. We employ the AR rolling mill that is provided with tapered rolls. Such rolls maintain increasing surface linear speed at elongating direction, thus reducing irregular deformation, improving product quality and wall thickness uniformity, eliminating internal thread and decreasing flaws.
AR rolling mill uses hydraulic cylinder to control the mandrel for rolling so as to increase the length of working surface and reduce mandrel consumption. One mandrel can be used for rolling of about 3000 steel pipes. The whole rolling mill is controlled through a Siemens PLC control system with touch screen. This can improve rolling precision, and at the same time reducing labor intensity and increasing production efficiency.
F. Sizing and Reducing
The sizing and reducing is of great importance in the whole seamless steel tube production processes. It is a process used to size the rolled tubes to specified dimensions. Shenhao uses 12 three-roll stretch reducing mills which come with individually driven roller stands.
The most remarkable feature of this reducing mill is its individual drive, enabling the speed of stands to be set and adjusted independently. This will improve the wall thickness accuracy of steel tubes. Especially, it improves the non-uniformity of longitudinal wall thickness during stretch reducing and reduces irregular wall thickness increase and other flaws caused by extrusion.
We employ six-roller hyperbolic straightening machine with adjustable speed. This tube straightening machine comes with highly rugged base. Straightening angle of the rollers can be adjusted, ensuring up to over 3/4 length of tubes in direct contact with the roller surface. Based on reasonable rolling reduction, it can achieve the optimal straightening effect, combined with small out of roundness, no straightening mark, and straightness within 1mm/m.
H. Eddy Current Flaw Inspection
To enhance the inner quality of seamless steel tubes, we install an eddy current flaw inspection equipment on the production line to conduct online inspection to each tube. According to the standards from customers, unqualified products in this process will be not permitted to enter the next process. We have two nondestructive eddy current flaw inspectors, namely NEM180 and NEM219. They feature high automation and can inspect round steel pipe in 108-219mm diameters. Inspection speed is 10-60m/min. Circumferential sensitivity is less than or equal to 3db.
I. Visual and Dimensional Inspection
Each batch of steel tubes that we produce must receive sampling and physical and chemical analysis. Qualified tubes after analysis will be subject to visual and dimensional inspection. Only fully qualified products have right to enter the processes of marking, bundling, weighting and warehousing.