Endless rolling involves the joining of the tail end of a preceding transfer bar with the head end of the following transfer bar after the roughing stands but prior to the finishing stands. Primetals Technologies was the first company in the world to develop the related equipment for induction-heating joining (IHJ) and super deformation joining (SDJ) to enable endless rolling. Furthermore, the dynamic Pair Cross mill can also be installed to deal with flying strip-thickness changes and increasing thermal crown resulting from continuous rolling.
The concept of endless hot rolling was introduced to meet demands for diverse products of hot rolled steel sheets using a cost-effective process compared to the conventional batch-type process. Endless rolling involves joining of the tail end of a preceding bar and the head end of a following bar after rolling in the roughing stand(s). The repetitively joined hot bars are subsequently finish-rolled, which results in the endless rolling. This process has the following advantages over the conventional batch-type process:
- Expansion of product line-up: Production of thinner, wider and harder steel sheets; Stable rolling of thin and hard-to-roll materials
- Increase in productivity: Great reduction in mill idle time, and constant rolling speed with reduction of mill acceleration and deceleration time
- Improvement of product quality: Improvement in accuracy and uniformity of strip thickness, crown, and width
- Energy savings: Reduction of the mill acceleration and deceleration frequencies
- Improvement of yield: Minimizing of miss-rolling by applying tension at head and tail of strip
Fig.1- Comparison of IHJ and SDJ
Induction Heating Joining (IHJ)
JFE Steel Chiba No.3 HSM is the first HSM in the world to realize endless hot strip rolling. The Key to Endless Rolling is the Induction Heating Joining Machine that was developed by Primetals Technologies jointly with JFE Steel.
Fig.2- IHJ sequence | Fig.3- IHJ components
Super Deformation Joining (SDJ)
The Super Deformation Joining Machine that was developed by Primetals Technologies jointly with POSCO was installed in 2006 and is now in operation at No.2 Hot Strip Mill in Pohang Works, POSCO. The procedure of SDJ consists of four steps: partial descaling, overlapping, joining, and crop disposal.
- Partial descaling by high-pressure water is first performed on the area to be joined before overlapping
- The head of the following bar is partially overlapped on the tail of the preceding bar by increasing the speed of the following bar
- Both bars are joined by the shearing action of the stationary pendulum type-joining machine
- Crops on both sides are pushed off from the joint and disposed of
SDJ does not require any heating and the joining process is very rapid. Also, the equipment size is smaller than the IHJ.
Fig.4- Layout of SDJ | Fig.5- Design image of SDJ
Fig.6- SDJ in operation | Fig.7- SDJ process
The following items are key technologies to implement the successful endless rolling.
- Automatic Profile & Flatness Control (Dynamic APFC)
- Shear to split long strip before coiling
- Dynamic switching over of coilers during high- speed rolling and coiling
Primetals Technologies is the only supplier in the world that can supply a complete endless rolling system, and five hot strip mills are under operation with our endless rolling technology, with remarkable accomplishments. Primetals Technologies offers, in connection with the above, the following proven technologies for the customer.
- Dynamic Pair Cross Mill
- High-speed shear
- Advanced down coilers with switch-over function (Option: Carrousel coiler)
Fig.1- Dynamic APFC | Fig.2- High-speed shear | Fig.3- Coilers with switch-over function
Dynamic Pair Cross Mill
The Dynamic Pair Cross Mill consists of mill stands in which the angle at which the roll axes cross can be adjusted during the rolling process. The feature allows the control of strip crown and strip shape during rolling (Dynamic APFC) in order to deal with flying strip thickness changes and growth of roll thermal crowns which occur during continuous rolling.
The application and number of Dynamic Pair Cross Mill stands are customized in accordance with product range and other requirements.
The Dynamic Pair Cross Mill can also be effective for normal batch rolling for compensation of thermal expansion of the work rolls and changes in other rolling conditions in the transfer bar.
High Speed Shear
The High Speed Shear splits the continuous strip produced by endless rolling prior to coiling. It is a rotary-type shear equipped with an eccentric mechanism. Before shearing, the drums are rotating with a gap in order to let the strip pass through. At shearing the eccentric mechanism narrows the gap and knives mounted on the drum approach the shearing position. The speed of the drum is synchronized with the speed of strip at shearing.
High-Speed Down Coiler with Threading Direction Change
Endless rolling requires the coiling facility to have special features. Threading in the active coiler starts immediately after the preceding strip had passed. The threading speed is higher than in normal batch rolling. In order to continue coiling without a time gap, the coiler is equipped with a switch-over function which can change the direction of threading at high speed. Pinch roll with an offset adjustment system is the core technology for this. To ensure stable threading at the carry-over table between No. 1 and No. 2 coilers, a pneumatic strip feeding system is introduced. Also, the down coiler its self has a special configuration of 4 four wrapper rolls which enables smooth and quality coiling under demanding conditions.
Primetals Technologies were the first in the world to develop a hot rolling carrousel coiler. This is an alternative option to the down coiler.
Fig.4- Carrousel Coiler | Fig.5- Carrousel Coiler in operation
POSCO Pohang Works
Type of plant
Full-scale endless hot rolling
Joining bars very quickly (in less than 0.5 seconds) and reliably without a need for heating. Equipment using this new joining process has been incorporated into the No. 2 hot rolling mill at POSCO’s Pohang Works, enabling full-scale endless hot rolling. Here the other key technologies for endless rolling, such as High speed shear and Advanced down coilers with switch-over function are also applied. This project was a revamping of the existing, conventional batch hot rolling mill.