Finish rolling method for production of round cross-sectional shape materials

In the rolling of metal material to round cross-sectional shape through a succession of three roll stands with the horizontal-horizontal-vertical, or vertical-vertical-horizontal rolls arranged in this order in the rolling down direction for achievement of increased accuracy of dimensional control at finish, an initial metal stock of specific oval cross-section is rolled to an intermediate and small reduction through the first and second stands forming specific oval groove, and then to a finish and small reduction through the third stand forming a round groove.

BACKGROUND OF THE INVENTION 
This invention relates to a method of rolling metal materials to round 
cross-sectional shape, and more particularly to improvements of the 
rolling method with respect to the accuracy of dimensional control for 
finish gauge. 
In the rolling of metals such as steel to a round cross-sectional shape, 
attempts have been made to increase the accuracy of dimensional control 
for finish gauge by removing tension from the work being rolled in such a 
manner, as, for example, to permit formation of a loop between successive 
two roll stands, or, upon detection of the tension to automatically 
control the peripheral speed of the work roll. These methods are, however, 
because they fundamentally suffer from the limitation of the response 
speed and reliability of automatic control and are not suitable for use in 
achieving additional improvement of the accuracy of dimensional control 
over the heretofore attained level of about +0.2 to about +0.4 mm, 
depending upon the sizes of rolled products. Also, there are disadvantages 
of increasing the cost of equipment necessary to perform them, and of 
requiring additional spare wide floor space on which the equipment is to 
be installed. Further, significant numbers of rolling situations are 
encountered where secondary working operations will be found necessary as 
the heretofore attained level of dimensional control is not sufficiently 
high. For example, a considerable percentage of metal bars and wires after 
having been finish rolled must be further subjected to drawing or likewise 
forming applications. 
Where a small reduction is desired, it is impossible to use the 
conventional rolling method adapted to produce a round cross-sectional 
shape, because this method must be operated with a large reduction which 
in turn calls for a large increase in the diameter of the work roll with 
the corresponding increase in the distance between the successive grooves. 
This leads to a high probability of the occurrence of twist in the work 
being rolled so that the percentage of rolled products which will be found 
acceptable with respect to the finish dimensions is decreased. 
On the other hand, Morgan block mills are known which are representative of 
the various types of block mills used in the intermediate and/or finish 
rolling of steel material, but their primary aim is to achieve the 
speed-up of production run and the compactness of the mill equipment 
itself with the help of employing a relatively axial ratio of the oval 
groove, so that the reduction can not be limited to small orders. Thus, 
the deviation of the actual values of the dimensions of the finished 
products from the specific and ideal ones is of the order of .+-.0.2 mm. 
In addition thereto, it is recognized that it is necessary to provide an 
entrance roller guide positioned in front of a finishing groove for the 
purpose of preventing impartment of twist to the work being rolled. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention has for the general object to provide a 
novel finish rolling method for production of round cross-sectional shapes 
with high accuracy of dimensional control at the finish. The features 
which may be considered to be characteristic of the present invention will 
be explained below. 
The first feature effective particularly when applied to the finish rolling 
of steel material is to employ a train of three roll stands with the first 
stand being of the horizontal roll type, the second stand being of the 
horizontal roll type and the third stand being of the vertical roll type, 
or an alternative train of the vertical-vertical-horizontal roll types, 
arranged adjacent to each other in this order in the rolling direction. 
The second feature is that the first and second stands are provided with 
respective passes of oval rolling configuration (groove), or a groove 
having similar functions thereto, specified, in terms of the axial ratio, 
i.e. ratio of the major diameter to the minor diameter, of the 
cross-sectional area of an intermediate so produced, as ranging from 1.09 
to 1.31 for subsequent rolling to a finish gauge which falls within a 
range of not less than 5 mm to less than 40 mm in diameter (hereinafter 
abbreviated as "finish gauge of low level"), and as ranging from 1.05 to 
1.21 for subsequent rolling to a finish gauge which falls within a range 
of 40 mm to 200 mm in diameter (hereinafter abbreviated as "finish gauge 
of high level"), while the third stand is provided with a round or 
likewise groove. 
In order to maintain the above axial ratio, the dimensions of grooves of 
the first and the second stands are defined below: 
1. when a final product of 5 to not larger than 40 mm diameter is to be 
obtained 
Major diameter: 
EQU (1.1412D + 2.20)mm .about. (1.1256D + 1.95) mm 
Minor diameter: 
EQU (0.985D - 0.1)mm .about. 0.998Dmm 
2. when a final product of 40 to not larger than 200 mm diameter is to be 
obtained 
Major diameter: 
EQU (1.0912 D + 4.02)mm.about.(1.153D + 6.15)mm 
Minor diameter: 
EQU 0.985D .about. 0.998D 
in which D represents a distance between two points at which a straight 
line perpendicular to the roll gap center line and passing through a 
central point of the groove intersects the upper and lower roll profile 
curves in the final stand. 
The third feature is that the dimensions of the cross-sectional area of the 
starting rolling stock are limited to specific values for the major and 
minor axes. For the rolling to the finish gauge of low level, there are 
given a range of 1.07D + 0.5 mm to 1.18D + 0.4 mm for the major axis, and 
a range of 1.01D + 0.2 mm to 1.05D + 0.4 mm for the minor axis. For the 
rolling to the finish gauge of high level, there are given a range of 
1.04D + 1.5 mm to 1.10D + 3.5 mm for the major axis, and a range of 1.01D 
+ 0.2 mm to 1.02D + 1.5 mm for the minor axis. D represents a distance 
between two points at which a line perpendicular to the roll gap center 
line and passing through a central point of the groove intersects the 
upper and lower roll profile curves in the final stand. 
As the fourth feature, the first and second stands are provided with 
respective passes of oval likewise grooves with the values of the major 
diameter thereof being in a range of 1.1412D + 2.02 mm to 1.2565D + 1.95 
mm and the minor diameter thereof being in a range of (0.985D-0.1) mm to 
0.998D mm for the final gauge of low level and in a range of 1.0912D + 
4.02 mm to 1.1535D + 6.15 mm and in a range of 0.985D mm to 0.998D mm for 
the final gauge of high level. 
The fifth feature is that all the roll stands are positioned as spaced 
apart from each other along a common pass axis with a very short distance. 
In numerical terms, the roll axis separation between the first and second 
stands is made shorter than (5.6D + 400) mm; while the roll axis 
separation between the second and third stands is made shorter than (4.6D 
+ 320) mm.

DETAILED DESCRIPTION OF THE INVENTION 
It is well known that the width expansion of the work being rolled and the 
work guide adjusting capabilities of the rolling mill depend upon the size 
of the work. In the present invention, therefore, a variety of operable 
work sizes are classified into two groups in terms of the size of finish 
gauge, one group of which is asigned to a range of not less than 5 mm to 
less than 40 mm (herein referred to as "finish gauge of low level"), and 
another group which is assigned to a range of 40 mm up to 200 mm inclusive 
(herein referred to as "finish gauge of high level"). The term "size of 
finish gauge" herein used refers to the specific dimension defined in 
connection with FIG. 1, wherein there is shown a pair of horizontal rolls 
of the No. 3 stand forming a finishing round groove, or likewise groove 
having an equivalent function thereto. From FIG. 1 on, the distance D 
between the two points at which a line perpendicular to the roll gap 
center line and passing through a central point of the pass opening area 
intersects the upper and lower roll profile curves is identified as the 
diameter of finish gauge. 
The present invention makes use of starting stocks of oval or likewise 
cross-section which may be manufactured in a conventional manner, as, for 
example, by the loop control method or by use of any one of the various 
block mills, or, in some cases, by the control-free rolling method. The 
term "oval or likewise cross-section" includes oval, cross section, 
cross-section formed by a pair of central arcs having the same curvature, 
cross-section formed by a pair of arcs having the same curvature and each 
having a pair of side arcs at both sides having a different curvature from 
that of the central arc, and octagonal and hexagon cross-sections. Now 
hereinafter the above cross-sectional shapes are called simply "oval 
cross-sectional shape". 
The starting stock usuable in the present invention is specified with 
respect to its cross-section before introduction to the initial pass of 
the rolling mill in terms of three parameters, namely, the major axis, 
minor axis and major axis-to-minor axis ratio or axial ratio. The 
definition of these three terms will be understood from FIG. 2, wherein 
the starting stock is shown as being about to enter the No. 1 stand, while 
the cross-section of the starting stock is adjusted in angular position 
relative to the orientation of the No. 1 pass, and wherein the length of 
the cross-section measured in coincidence with the roll gap center line is 
designated by a.sub.0, and the length of the cross-section measured in 
coincidence with a line perpendicular to the roll gap center line and 
passing a central point of the pass opening area is designated by b.sub.0. 
Hence, we call the lengths, a.sub.0, and, b.sub.0, "major axis" and "minor 
axis" respectively, and the ratio a.sub.0 /b.sub.0 "axial ratio". It is 
required that the axial ratio must be limited to a numerical value ranging 
from 1.03 to 1.21 for the finish gauge of low level, and otherwise ranging 
from 1.02 to 1.17 for the finish gauge of high level. It is further 
required that the major and minor axis for the finish gauge of low level 
must be limited to respective values ranging from 1.07D + 0.5 mm to 1.18D 
+ 0.4 mm and from 1.01D + 0.2 mm to 1.05D + 0.4 mm respectively, and those 
for the finish gauge of high level to respective values ranging from 1.04D 
+ 1.5 mm to 1.10D + 3.5 mm from 1.01D + 0.2 mm to 1.02D + 1.5 mm 
respectively. 
The fulfillment of the all above requirements will lead to the possibility 
of performing that portion of the rolling process which operates with the 
first and second roll stands without the necessity of using the otherwise 
necessary entrance roller guide or likewise guide means for sustaining the 
normal position of the work being rolled while nevertheless preventing 
impartment of torsion into the work, and further to the possibility of 
limiting the total reduction from the initial pass to the finishing pass 
to smaller orders than was previously possible for the purpose of 
achieving an improvement of the accuracy of dimensional control for the 
finishes so produced. Another advantage is that whilst the conventional 
guide method requires no reduction, the first and second stands of the 
present invention may be operated with a slight reduction of the work so 
that the intimity of contact between the work and roll can be improved to 
a large extent sufficient to prevent impartment of torsion into the work 
during the rolling operation at the final stand. 
When the upper limits of the various ranges specified above are violated, 
therefore, the total reduction is correspondingly increased, thus being 
responsible for occurrence of increasingly ununiform plastic flow in the 
work being rolled, as the plastic flow is very delicate, so that the work 
tends to distort and that the width expansion is increased, thereby it 
being made difficult to control the dimensions of the finished product 
with high accuracy. When the lower limits are violated, because of the 
unduly small axial ratio of the initial stock, the first and second stands 
can not serve to prevent occurrence of torsional moment of the work by the 
third stand which will result in a decrease of the accuracy of dimensional 
control. 
According to the present invention, there are further specific requirements 
for the shaping pass of the second stand. At first, this pass must be 
formed to an oval opening configuration or likewise configuration having 
an equivalent function thereto, because the final or third stand is 
provided with a round opening configuration or likewise configuration 
having an equivalent function thereto, as will be readily understood by 
those skilled in the art. The cross-sectional shape of the work to be 
formed by this second pass must be specified to account for the facts 
that, in order for the successing or third stand to provide finished 
products with improved dimensions, the reduction from the second stand to 
third stand is required to be as small as possible, and that the second 
stand must function as an entrance guide for supporting the work against 
the third stand. On this account, according to the invention, this 
cross-sectional shape is specified in terms of the axial ratio defined in 
connection with FIG. 3, wherein that portion of the work which is bited 
between the rolls of the second stand is shown as assuming a possible 
cross-section area of oval configuration in the shaping pass, and wherein 
the length of the cross-section of the work measured in coincidence with 
the roll gap center line is designated by a.sub.2, and the length of the 
cross-section measured in coincidence with a line perpendicular to the 
roll gap center line and passing through the center of the length a.sub.2 
is designated by b.sub.2. From FIG. 3 on, we call a.sub.2.b.sub.2 the 
axial ratio referred to. The specific range of values for the axial ratio 
is from 1.09 to 1.31 when the finish gauge is of low level, and from 1.05 
to 1.21 when of high level. 
The fourth feature of that the intermediate gauge to which the above 
sepcified starting stock is to be rolled through the first and second 
stands is specified as follows: The intermediate rolled product be oval in 
cross-section or analogous thereto, having for the final gauge of low 
level the major axis ranging from 1.0712D + 0.52 mm to 1.1865D + 0.45 mm 
and the minor axis ranging from 0.985D - 0.1 mm to 0.998D, and having for 
the final gauge of high level the major axis ranging from 1.0412D + 1.52 
mm to 1.1035D + 3.65 mm and the minor axis ranging from 0.985D to 0.998D 
(mm). 
In order to insure that the rolling of the work can be carried out without 
causing the impartment of torsion to the work, it is of importance to 
require that the length of contact between the peripheral surface of the 
workpiece and the roll profile in the second stand as defined by l in 
connection with FIG. 4 is maximized. As the size and configuration of that 
portion of the workpiece which is introduced in the second stand and the 
groove height of the second stand can not be subject to large change, it 
is proven that the maximization of the contact length, l, can be realized 
only be increasing the radius of curvature of the groove of the second 
stand. Increasing this radius of curvature will, however, cause approach 
of the pass to more round configurations which results in a decrease in 
the ability of the second stand as the entrance guide against the third 
stand. It follows that there must be set forth a compromise between the 
requirements of maximizing the contact length, l, and of fulfilling the 
foregoing specific requirement. This compromise is related to the groove 
width defined in connection with FIG. 5, wherein the distance between the 
two points at which the oval groove profile curve intersects the left and 
right flat roll surfaces constituting roll gaps together with the 
respective surfaces of the opposed roll is designated by A.sub.2 which we 
call the groove width referred to. This definition of the groove width 
applies to the initial or first stand to be described later, in which also 
the groove width is referred to as A.sub.1 and the minor diameter as 
B.sub.1. A specific range of values for this groove width (A.sub.1 or 
A.sub.2) is from 1.1412D + 2.02 mm to 1.2565D + 1.95 mm when the finish 
gauge is of low level, and from 1.0912D + 4.02 mm to 1.1535D + 6.15 mm 
when of high level, and specific range of values for the minor diameter 
B.sub.1 or B.sub.2 is from (0.985D - 0.1) mm to 0.998D for the finish 
gauge of low level and 0.985D to 0.998D for the finish gauge of high 
level. The minor diameter B.sub.1 or B.sub.2 represents a distance between 
two points at which a line perpendicular to the roll gap center line and 
passing through a central point of the groove interdects the upper and 
lower roller profile curves in the respective stand. 
Consideration will now be given to the groove configuration of the first 
stand. The first stand is positioned on the work entrance side of the 
second stand, and is operated in such a way that a slight reduction is 
imposed on the work in the second stand. For this purpose the minor 
diameter of the groove of the first stand is made sligthly larger than 
that of the second stand. From the point of preventing the twisting or 
torsion of the rolling work, it is desirable that the groove width of the 
first stand is equal or almost equal to that of the second stand so as to 
give uniform reduction all around the rolling work in the second stand. 
The first stand has its own function. Thusly, the first stand maintains a 
required attitude of the work piece so as to stabilize its attitude at the 
time of biting in the second and third stands. 
According to the present invention, the first and the second stands are 
spacedly arranged with a specific distance therebetween as mentioned 
later, and the work piece is held at least at points, namely by the first 
and second stands so that the twisting or torsion of the work piece being 
rolled is completely prevented in the third stand. 
As understood from the above description, the first stand as well as the 
second stand is essential, and without the first stand, the desired 
results of the present invention can not be obtained. On the other hand, 
however, an additional stand or stands similar to the first stand may be 
provided before the first stand without deviating from the scope of the 
present invention. 
Further according to the present invention, the three stands of the 
character described above are arranged in unison with the limitation of 
the distance between the adjacent stands for the purpose of preventing 
occurrence of torsion of the work being rolled. On this account, according 
to the present invention, there are set forth two specific requirements, 
one of which is that the distance between the roll centers of the first 
and second stands be made not larger than (5.6D + 400) mm, and another 
requirement which is that the distance between the roll centers of the 
second and third stands be made not larger than (4.6D + 320)mm. It is 
desirable that the above distances are as short as possible from the 
structural requirements of the stands. If the distances are longer than 
the above upper limits, torsion of the work piece being rolled appears in 
the third stand. Unlike the prior art, the present invention operates with 
a slight reduction for each of the first, second and third stands, so that 
the size of each of the work rolls can be decreased very much to readily 
fulfill the above specific requirements. 
The present invention is advantageous because the starting stock as defined 
herein can be produced by the conventional art including the conventional 
continuous rolling method. 
According to the present invention, the slight reduction rolling prevents 
occurrence of torsion of the work being rolled and provides finished 
rolled products with dimensional errors falling within a range of .+-.0.1 
to 35 0.20 mm depending upon the sizes of the products. 
We have now discovered that the finish rolling to round cross-sectional 
shape can be performed with the limitation of the total reduction to very 
small orders to effect an increase in the accuracy of dimensional control 
for the finishes so produced over the heretofore attained level. In this 
connection, it should be explained that, as the reduction is minimized, 
use can be made of work rolls of smaller diameters with decrease in the 
size of each of the roll stands so that the distance between the roll axis 
of the successive two stands is decreased. Further, when the total 
reduction is made on smaller orders, the absolute value of width expansion 
of the part of the work-piece being rolled which is bited by a pair of 
rolls forming a pass becomes smaller with decrease in absolute error in 
estimating the width expansion. Therefore, it is possible to evaluate the 
cross-sectional area of that part of the work-piece which assumes the 
rolling pass previously with higher precision, and further to minimize the 
absolute value of variation of the width expansion which will be caused by 
the introduction of tension and compression. In addition thereto, the 
elongation of that part of the work-piece which extends forwardly beyond 
the first roll stand can be also minimized to make it easier to regulate 
the volume speeds in such a manner as to maintain the tension and 
compression at minimum during the entire rolling process. 
The advantages of the present invention will be more specifically 
illustrated below: 
1. The dimensional accuracy of the final product is very high, and 
secondary workings such as drawing step can be omitted, so that the 
production cost can be lowered remarkably. 
2. Complicated speed control as conventionally required in the rough and 
intermediate steps are no more necessary, thus lowering the capital cost 
remarkably. 
3. No space is required for forming a loop etc., so that the mill line can 
be shortened and power consumption required by the mill operation and 
control can be saved. 
4. A final product of dimensional accuracy as obtained by the conventional 
art can be made very easily without dimensional control meter equipments. 
Further, the dimensional condition of the starting and finishing ends of 
the rolled product can be improved remarkably, so that rejects decreases 
improving the production yield. 
5. As the rolling mill embodying the present invention is so small that it 
can be easily added to the existing equipment or freshly equipped in a new 
mill line. 
Referring now to FIG. 6, there is shown one embodiment of the finish 
rolling method according to the present invention. A starting stock 1 
(FIG. 6a) is rolled to a thickness, b.sub.1, by a pair of horizontal rolls 
2 and 3 of No. 1 stand forming an oval groove 4 or likewise groove with a 
groove width A.sub.1 and a minor diameter B.sub.1 (FIG. 6b), then to a 
thickness, b.sub.2, by a pair of horizontal rolls 5 and 6 of No. 2 stand 
forming an oval groove 7 or likewise groove with a groove width A.sub.2 
and a minor diameter B.sub.2 (FIG. 6c) and then to a round cross-sectional 
shape of finish gauge by a pair of vertical rolls 8 and 9,No. 3 stand 
forming a round groove 10 or likewise groove (FIG. 6d) whereby a rolled 
product 11 having highly improve finish dimensions (shift error from the 
true round within a range of .+-.0.1 to .+-.0.20 mm depending upon the 
size of the finished product) can be obtained (FIG. 6e). 
FIG. 7 shows a schematic plan view of a rolling mill comprising the No. 1, 
No. 2 and No. 3 stands of FIG. 6 unified in a single frame 12 and 
associated with a mill motor 13 and a reduction gear train 14. 15 is a 
pass line. 16 is No. 1 stand of the horizontal roll type, 17 is No. 2 
stand of the horizontal roll type, and 18 is No. 3 stand of the vertical 
roll type. 
FIG. 8 shows the details of the rolling mill of FIG. 7. The rotational 
speed of the motor 13 is reduced by the gear train 14 to a predetermined 
speed at which the rolls incorporated in the stands 16, 17 and 18 are 
driven for rotation. 
The above mentioned rolling mill train is composed of a succession of the 
first horizontal-type stand, the second horizontal-type stand and the 
third vertical-type stand, but may composed of an alternative succession 
of the first vertical, the second vertical and the third horizontal stand 
to effect an equivalent result to the above, provided that all the 
specific requirements of the invention are fulfilled. 
EXAMPLE 1 
In rolling a carbon steel JIS-S45C for mechanical construction to a round 
bar of 11.0 mm in diameter at a temperature of 900.degree. C and a rolling 
speed of 20m/sec., use was made of the succession of roll stands of FIG. 
6. The numerical values of the various design parameters were as follows. 
The unit is in millimeter. 
Starting stock: 
EQU a.sub.0 = 12.65 .+-. 0.3; 
EQU b.sub.0 = 11.6 .+-. 0.3 
No. 1 stand: 
EQU A.sub.1 = 15.45; 
EQU b.sub.1 = 10.98 
no. 2 stand: 
EQU A.sub.2 = 15.4; 
EQU b.sub.2 = 10.8 
no. 3 stand; 
EQU D = 11.13 
the thus produced finish of round bar was found to have a diameter of 11.0 
.+-. 0.1 mm. 
EXAMPLE 2 
In rolling a carbon steel JIS-S15C for mechanical construction to a round 
bar of 70 mm in diameter at a temperature of 800.degree. C and a rolling 
speed of 1.7m/sec., use was made of the train of roll stands of FIG. 6. 
The numerical values of the various design parameters were as follows. The 
unit is in millimeter. 
Starting stock: 
EQU a.sub.0 = 74.60 .+-. 0.3; 
EQU b.sub.0 = 71.50 .+-. 0.3 
No. 1 stand: 
EQU A.sub.1 = 75.15; 
EQU b.sub.1 = 69.86 
no. 2 stand: 
EQU A.sub.2 = 75.64; 
EQU b.sub.2 = 69.54 
no. 3 stand: 
EQU D = 71.05 
the thus obtained finish of round bar was found to have a diameter of 70 
.+-. 0.13 mm. 
The present invention has been described in connection with the rolling of 
steel material for the finish gauge of 5 up to 200 mm in cross-sectional 
size. However, it is evident that the present invention is applicable to 
the rolling of other metals for finish gauges outside that range without 
diminishing the above mentioned effectiveness.