Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)
29

Development of a Bar-joining Apparatus
for Endless Hot Rolling

KENJI HORIITOSHIHIRO USUGIHIDEAKI FURUMOTO

Endless hot rolling is a production process for finish rolling, that is carried out in abath-less manner, while contributing to higher productivity and yield. Bar joining prior tofinishing rolling is one of the key technologies for endless rolling. In contrast to the conventional
method, in which the areas to be joined are heated and then joined together, Mitsubishi-HitachiMetals Machinery (MH), Inc. and POSCO have recently developed a unique solid-state joining
process capable of joining bars very quickly (in less than 0.5 seconds) and reliably without aspecial heating process. An apparatus using this new joining process has been incorporated into
the No. 2 hot rolling mill at the POSCO Pohang Works, where full-scale endless hot rolling isalready in operation.

|1. Introduction
Completely continuous hot rolling, which can be regarded as the ultimate process for hot
strip rolling, was first employed in commercial production in 1996. It is a technology for joining
intermittently supplied rough bars and carrying out finish rolling in an endless manner. Endlessrolling can prevent bar bending in the width direction at the head end and pinching at the tail end,
which have been operational problems in intermittent rolling (batch rolling). This permits stable
continuous rolling, resulting in uniform quality over the entire coil length. Endless rolling can also
lead to increased productivity since it eliminates mill idle time. Moreover, the process makes it
possible to produce extremely thin strips (1.0 mm thickness) via flying-gauge changing technology.
Figure 1 comparatively illustrates the equipment used for two types of hot strip rolling: batchrolling and endless rolling.

Figure 1 Comparison between two types of hot strip rolling equipment

Joining is one of the key technologies for endless rolling. A combination of induction heating
and upset joining1, and laser welding2 are two of the joining technologies thus far adopted. Since
these methods both require a relatively long joining time, the joining machine must travel a long
distance with the rough bars, thus necessitating larger equipment. For wider application to endless
hot rolling, MH has endeavored to provide a joining process capable of instantly joining bars via asimple joining machine while ensuring a joint strength comparable with that of the base metal. As aresult of these endeavors, the company developed super-deformation joining (hereafter referred toas SDJ) 3,4. SDJ is a solid-state joining process that requires only a short amount of time, and isaccomplished by overlapping two rough bars, then bringing together a pair of knives to shear thejoint position. SDJ is able to provide a joint strength that is high enough to withstand hot rollingwithout the formation of fractures. A joining machine based on SDJ technology was incorporated
into the No. 2 hot rolling mill at the POSCO Pohang Works (hereafter referred to as P2H), where
full-scale endless rolling has been in operation since January 20075. Since that time, increased
productivity, expanded production, quality improvements, and other benefits have been confirmed.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)30

P2H now produces 100,000 tons of coils per month by endless rolling, and has set a record forcontinuously carrying out endless rolling for one hour, producing 45 bars (more than 1,000 tons).
This paper presents a description of the SDJ technology developed by MH and POSCO, the resultsof a joint test and the results of applying SDJ to endless hot rolling into the existing line.

|2. Outline and characteristics of the SDJ technology
2.1

SDJ process

Figure 2 depicts the SDJ process. In step 1, highly pressurized water is ejected onto theoverlapping surfaces that are to become a joint area, and scale is removed. In step 2, the following
bar is accelerated/decelerated so that the tail of the preceding bar and the head of the following bar
overlap by the amount required for joining. In step 3, the bars are joined together by the shearing
action of the knives. In step 4, crops generated during the joining process are cut off at theirconnections and removed.

2.2

SDJ mechanism

The SDJ apparatus joins bars by moving the top and bottom knives in the shearing direction.
Figure 3 schematically illustrates the joining process from start to finish, and shows the changes instate during the process. In the pressing step (a), air is eliminated at the overlapping surface tofacilitate a vacuum condition. As indicated in the detailed drawing of Figure 3(a), the portion of theoverlapping surfaces that does not come into contact with the knives is formed into a joint during
the subsequent shearing step. In the shearing step (b), a portion of the overlapping surfaces iselongated by press and support forces, and scale is broken into segments to form scale-freesurfaces. The scale-free surfaces are maintained since adhesion at the joint interface issimultaneously enhanced. At the end of the shearing step, a large upsetting force, which is theresultant of the support and press forces, is applied to complete the joining process. In addition,joining is presumably accelerated by the heat that is generated during the shearing step.

Figure 2 SDJ process

Figure 3 Schematic illustration of the joining process from start to finish, showing changes
in state

As the above discussion indicates, the SDJ mechanism can be regarded as solid-state
joining6, in which bars are joined using super-deformation to create scale-free surfaces, and moving
the atoms on both surfaces closer to each other so that metal bonding can be accomplished via theupsetting force.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)31|3. Results of the SDJ joint tests
3.1

Test method

We constructed a basic SDJ test apparatus capable of joining bars up to a maximum width of
100 mm, as well as a wide-width SDJ test apparatus capable of joining bars up to 450 mm in width,and conducted an SDJ test using this equipment4. Figure 4 shows the joint test procedure carriedout using the test apparatus. To simulate the temperature of the joined material and the overlapping
surface conditions in actual hot rolling, the bars were piled on top of one another in an electric
furnace, and heated to 1,200–1,300°C. After being taken out of the furnace, the bars were exposedto the atmosphere to form scale on the overlapping surfaces. Partial descaling was performed on thejoining area of the two bars using highly pressurized water. The bars were again overlapped before
being placed on the test apparatus to initiate the joining process. After the two bars were joined,
crops were removed by manually bending the crop connections several times. Tensile test
specimens were collected from the joined material and subjected to analysis. The material was
carbon steel (hereafter referred to as CS) with a tensile strength of 394-436 MPa.

Figure 4 Test apparatus joining procedure

3.2 Test results and discussion(1) Joint strength: Figure 5 shows the relationship between location relative to the bar width
direction and the strength ratio βj of the joint to the base metal. The ratio βj is obtained by
dividing the tensile strength of the joint (hereafter referred to as joint strength) by the tensile
strength of the base metal (394 MPa). The joint strength was fairly uniform in the width
direction, except near the bar edges, and the ratio βj varied approximately from 0.70 to 0.85.
An observation of tensile fracture surfaces revealed that there were poorly joined areas
within several millimeters of the bar edges. This indicates that the reason why bar edges havea lower joint strength is that they are affected by these poorly joined areas. The joint strength
had a tendency to be higher for increased descaling pressure pd, and this is assumed to be dueto the fact that higher pressure descaling results in less scale.
(2) Metallographic joint analysis: Figures 6(a) and (b) show a macrograph and a microstructureof a CS joint cross-section, respectively, while Figure 6(c) shows a microstructure of the basemetal cross-section. Figure 6(b) is an enlarged view of the joint center in the thickness
direction that is circled in Figure 6(a). The structure was composed of ferrite and pearlite in
the vicinity of the joining line. Compared with the unjoined area of the base metal, finer
crystal grains were present along the joint interface, and were merged with the base metal
grains. As a result, the joining line could not be clearly identified. Additionally, minute scalesegments (which are presumed to have been broken up and formed during the joiningprocess) were observed in the crystal grains. The presence of these fine crystal grains
indicates that they were formed by super-deformation and recrystallization that took placewhen the bars were joined. Hence it can be concluded that super-deformation during thejoining process is instrumental to the rapid joining and integrating of bars.
(3) Joint state changes after rolling: Figure 7 shows a cross-sectional view of the joint afterrolling. After one rolling pass, the steps at the joint were flattened, and the joint angle θj wasalso reduced. Top and bottom crop removal edges still existed on the extension of the joining
line, but they were not serious enough to cause strip breakage.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)32

Figure 5 Relationship along bar
width direction and strength
ratio βj of the joint to basemetal
Material joined: CS. Size: 28 mm
thickness  200 mm width. Tensile
strength of base metal: 394 MPa.

Figure 6 Cross-sectional photos of the CS joint. Tensile strength of base metal: 436 MPa.

Figure 7 Joint cross-sectional shape
before and after rolling

Material joined: CS. Size: 28 mm
thickness  450 mm width. Tensile
strength of base metal: 436 MPa.

|4. Results of applying SDJ to endless hot rolling into the existing line
4.1

Layout of the endless rolling line

Figure 8 illustrates the layout of the endless rolling line. The primary equipment between theroughing and finishing mills (from right to left) are the coil box, crop shear, leveler, partial
descaling device, overlapping device, joining machine and crop-removing device. The coil box
dispenses the rough bars after rough rolling, serving as heat-retention and to adjust the join timing.
The crop shear cuts off abruptly deformed portions created at the head and tail ends of the bars byrough rolling. The joining apparatus is made up of the remaining components, from the partial
descaling device to the crop-removing device. The partial descaling device ejects highlypressurized water onto the rough bar surfaces to remove scale immediately prior to joining. The
overlapping device overlaps the preceding and following bars by utilizing the deflection at the tailend of the preceding bar. The crop-removing device is installed close to the joining machine, andremoves crops directly after joining for immediate disposal from the rolling line.

Figure 8 Layout of the endless rolling line

4.2

Structure of the joining machine

Figure 9 illustrates the action of the joining machine applied to the existing line. It has amechanism similar to that of a pendulum flying shear7. The crankshaft has two eccentricities, witheccentricity 1 connected to the top knife and top clamp, and eccentricity 2 connected to the bottom
knife and bottom clamp. Rotation of the crankshaft moves the top and bottom knives closer
together or farther apart. The crankshaft is also connected to the pendulum shaft via a lever so thatrotation of the crankshaft is converted to a back-and-forth motion of the pendulum housing, parallelto the direction of motion of the rough bars. When overlapping rough bars enter the joining
machine, the crankshaft is rotated counterclockwise. The top and bottom knives then come into

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)33

contact with the rough bars at a speed approximately equal to the speed of the bars. As the knives
approach each other, the bars are sheared to complete the joining process. The knives move apartwhen the crankshaft undergoes further counterclockwise rotation from the position shown in Figure9(b). Since they are subject to a combination of up/down movement and pendulum motion, theknives follow a quasi-elliptic trajectory.

Figure 9 Joining action of a pendulum type joining machine

In the conventional method, which is a combination of induction heating and the upset
joining process, the faces of the joining edges are heated for 3–5 seconds to reach the desired
joining temperature before being upset. Thus the joining machine must travel a long distance withthe rough bars1. By contrast, the SDJ technology completes the joining process in less than 0.5
seconds. It is therefore possible to implement speed synchronization with the rough bars, even witha pendulum machine, which is fixed to a foundation. Accordingly, the length of the joining
machine, from the descaling device to the crop-removing device, can be reduced to approximately
15 m so that the machine can be compactly installed in comparison to a method that requires thejoining machine to move along the line.

4.3

Joining and rolling at P2HFigure 10 shows the strength ratio βj of the joint to the base metal, measured relative to thebar width direction, for material joined at P2H. The ratio βj was about 0.70 when no partial
descaling was performed prior to joining, and reached 0.85–1.00 with partial descaling, resulting inan average ratio of 0.90 or higher. When βj equaled 1.00, fractures developed in the base metal
rather than at the joint interface5. Partial descaling was performed with a descaling pressure pd of
25 MPa and a water ejection distance of 100 mm. The material joined was low-carbon steel, which
has a lower tensile strength than the CS used with the test apparatus. Nevertheless, the measured βjvalues were comparable to those obtained with the test apparatus.

4.4

Benefits of endless hot rolling

The following are typical benefits obtained from endless hot rolling using the equipment
described above.
(1) Increase in productivity: Figure 11 presents an example of the amount of time saved byendless hot rolling compared to batch rolling. In batch rolling, idle time accumulates fromthe instant the tail end of a bar goes beyond the final stand to the instant the line starts rollingthe next bar. In addition, the batch rolling speed is lower at the head end of a bar, andincreases after the head end has passed through the final stand. Moreover, in thin-gaugebatch rolling, the rolling speed decreases before the tail end passes through the final stand.
On the other hand, endless rolling involves no idle time, and bars can be rolled at maximum
speed from the second bar to the second-from-last bar, even in thin-gauge rolling. Thus,endless rolling can significantly reduce processing time.
Figure 12 shows the rate of productivity of endless rolling compared to batch rolling.
Under the conditions indicated in Figures 11 and 12, the productivity of endless rolling is
25–30% higher than that of batch rolling, allowing the finishing mill to be utilized to its
maximum capability.
(2) Quality stabilization: Figure 13 shows an example of the product quality obtained fromendless rolling at P2H. The coiling temperature, which affects the mechanical strength andmetallurgical structure of the coil, is uniform while the strip thickness and crown arecontrolled to fit the desired specifications. In other words, the quality is well stabilized.

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)34

Figure 10 Strength ratio βj of the joint to basemetal relative to the bar width direction
for material joined by production
versionMaterial joined: low-carbon steel. Size: 31 mmthickness  1,200 mm width. Tensile strength of basemetal: 308–319 MPa.

Figure 11 Example of time saved by endless
hot rolling compared to batch rolling
Strip size: 1.40 mm thickness  1,219 mm width.
Number of bars: 5. Weight per bar unit width: 18.0kg/mm. Bar weight: 22.0 tons.

Figure 12 Rate of productivity by endlessrolling compared to batch rolling

Figure 13 Example of the product quality
obtained from endless rollingNumber of bars: 5. Weight per bar unit width: 18.0kg/mm. Bar weight: 22.0 tons. Idle time between two
consecutive bars during batch rolling: 20 s.

Material rolled: SAE1022. Number of endlessly rolledcoils: 15

4.5

Equipment-related advantages of SDJ over the conventional method

Figure 14 depicts the layouts of endless rolling equipment for different bar-joining
processes. The layouts were compared with each other by setting the maximum bar speed during
joining at 70 m/min, and setting the distance La, between the joining machine and position F1, andthe distance Lb, between the coil box and the joining machine, to be the same. For the method thatcombines induction heating and upset joining, the equipment length was established as the bar
moves a distance of 20 m when the maximum bar speed is 60 m/min8. Considering the bar length
in the two rolling passes prior to the final rough rolling, the distance between the roughing mill andthe coil box must be at least 45 m. Hence the distance shown in Figure 14(a) between the roughing
mill and position F1 is 119 m, since 45 + 74 = 119. On the other hand, the required distance for theSDJ process, shown in Figure 14(b), can be reduced to 100 m, since 45 + 55 = 100. Thus the SDJ
technology allows for more compact equipment than the conventional method. As a result, endlessrolling can be incorporated into a wider range of existing lines where line space is limited.

Figure 14 Layouts of endless rolling equipment for different bar-joining processes
Maximum bar moving speed during joining: 70 m/min

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 3 (Sep. 2009)35|5. Conclusions(1) SDJ, a highly reliable, fast joining technology, eminently suited to the bar-joining process forendless hot rolling, has been developed, tested, and incorporated into an existing line.
(2) With the adoption of SDJ, it is possible to install a joining machine to the foundation while
reducing the overall length of the endless rolling equipment, including the coil box. As aresult, the feasibility of endless rolling is enhanced in many cases, including at a large
number of existing facilities.
Acknowledgments
The present research was initiated by Hitachi, Ltd., prior to the establishment ofMitsubishi-Hitachi Metals Machinery, Inc. It was therefore promoted in collaboration withPOSCO. The authors would like to thank Dr. Takao Funamoto of Hitachi Kyowa Engineering Co.,
Ltd., and Dr. Jongsub Lee of POSCO, as well as many others, for their contributions over the years.

References
1. Nikaido, H. et al., Endless Hot Strip Rolling at No. 3 Hot Strip Mill, Kawasaki Steel Technical Report, Vol.
28 No. 4 (1996), pp. 224-230.
2. Matsuo, S. et al., Outline of Continuous Finish-rolling at Oita Works, Current Advance in Materials andProcess, Vol. 13 (2000), p. 1056.
3. Horii, K. et al., In-line Bar Joining – A Key Technology for Endless Hot Rolling, AISE Steel Technology,
Vol. 79 No. 6 (2002), pp. 55-60.
4. Horii, K. et al., Development of Super Deformation Joining Technology for Endless Hot Rolling,
Proceeding of the 58th Japanese Joint Conference for the Technology of Plasticity (2007), pp. 443-444.
5. Lee, J.S. et al., Introduction of Endless Hot Rolling at Pohang Works of POSCO, Current Advance inMaterials and Process, Vol. 20 (2007), p. 987.
6. Hashimoto, T. et al., Kosou-yosetsu & Rou-zuke，Sanpo Publications (1979), pp. 19-29.
7. Fukui, K. et al., Heavy Duty Pendulum Type Flying Shear for Hot Strip Mill, Hitachi Review, Vol. 61 No. 9(1979), pp. 7-12.
8. Yamazaki, T. et al., Control of Endless Hot Strip Rolling Process for No. 3 Hot Strip Mill at Chiba Works,Current Advance in Materials and Process, Vol. 10 (1997), p. 1092.

Authors
Kenji Horii
Manager,
Technical Development
Department,
Mitsubishi-Hitachi MetalsMachinery (MH), Inc.
Hideaki Furumoto
Manager,
Hiroshima Research &Development Center,
Headquarters

Toshihiro Usugi
Rolling Mill Design Department,
Hiroshima Works,
Mitsubishi-Hitachi Metals
Machinery (MH), Inc.