Motion characteristics of floating mandrel continuous rolling pipe
Occlusal stage
Hidden continuous rolling stage
Steel throwing stage
Set rolling speed
Motion characteristics of limited mandrel continuous rolling pipe
Deformation characteristics of floating mandrel continuous rolling pipe
Pass system
Hole side wall
Extension coefficient
Wall shrinkage
Selection of pass and deformation parameters of limited mandrel continuous rolling pipe
Determination of rolling force and rolling torque
Rolling force
Rolling torque
Slub phenomenon
The basic theory of kinematics, deformation, rolling force and braking torque of continuous pipe mill and the formation of "slub"
Motion characteristics of floating mandrel continuous rolling pipe When floating mandrel continuous rolling pipe, the perforated capillary inserted into mandrel is generally processed into hollow pipe after 8 stands continuous rolling. The whole tube rolling process includes three rolling stages: biting, steady continuous rolling and steel throwing, and its kinematic characteristics are the characteristics of time-displacement relationship in the tube rolling process (see figure 1).
Figure 1 Characteristic diagram of time-displacement relationship in continuous pipe rolling process
Broken line abcd- spindle head speed change; Dashed line ABCD- speed change at the end of spindle
Solid line Aa'b'c'd'- capillary head speed change; Solid line A'B'C'D'- capillary tail speed change
The biting stage starts from the first 1 mill and ends at the last one. The biting process is an unstable rolling process. When the tube head Va'b' is deformed when entering each frame, the moving speed increases with the increase of elongation coefficient (that is, the stepping acceleration changes). The step increment of pipe head speed is △ v (n-1) → n = (μ n-1) v n-1. Where μn is the spreading coefficient of the nth frame; V n- 1 is the rolling exit speed of the first n- 1 stand. The Va'b' at the end of the pipeline is determined by the biting speed of 1 frame, which can be assumed to remain unchanged.
Because the free-floating long mandrel is a rigid tool, the moving speed of the head Vab and the tail VAB of the mandrel is the same, and it also changes with the step change of pipeline speed. However, the step increment of mandrel speed is always less than the increment of tube head speed. If the outlet speed of the eighth rack is V8( 1→8), the mandrel speed is the average of 1 ~ 8 rack speeds. If the mandrel speed is accelerated from VD [1→ (n-1→ n]] to Vd[ 1→n], the mandrel speed step increment is △ vdn = {VD [1-n]-VD [1→ (n 0。 The step change of tube head speed causes the step change of mandrel speed, and the alternating mandrel speed in turn causes the change of actual tube discharge speed on each rolling mill, which depends on the step increment of mandrel speed and friction conditions. The change of the actual outlet velocity of the pipeline can be expressed by the following formula:
△V ' n( 1→n)= F2△Vdn/(f 1+F2)
Where △V'n( 1→n) is the incremental change of the actual outlet speed of the pipe on the nth rolling mill due to the step change of the mandrel speed when the pipe is continuously rolled at 1 ~ n; F 1 is the friction coefficient between the drum and the outer surface of the pipeline; F2 is the friction coefficient between the mandrel and the inner wall of the pipe.
There are one bite (at the moment when the pipe head contacts the roller, the pipe is dragged into the deformation zone by the friction between the rotating roller and the metal, and the diameter reduction begins) and two bites (at the moment when the inner surface of the pipe contacts the mandrel, the axial resistance of the mandrel is overcome by the friction between the rotating roller and the metal, and the pipe is dragged into the wall reduction zone). For the 1 stand of continuous pipe mill, because the roller table is generally used to feed steel, it can be considered that the first bite and the second bite without external thrust are realized. However, the engagement between the second frame and the subsequent frame has the reverse thrust given by the previous frame, and the first and second engagement conditions can be improved.
The first biting conditions of the continuous pipe mill 1 stand are as follows:
tanα≤f
The secondary biting conditions of the continuous pipe mill 1 stand are as follows:
tanα2≤(2f-tanα)/ 1+2ftanα
Where alpha is a first occlusion angle; α2 is the second occlusion angle; F is the coefficient of friction.
In the steady-state continuous rolling stage, after the pipe head enters the N-th rolling mill, the pipe is between the L ~ N-th rolling mills at the same time, until the capillary tail is thrown out by the 1 th rolling mill, and the continuous rolling of the pipe begins. In the process of steady continuous pipe rolling, the pipe head speed Vb'c', pipe tail speed VB'C', mandrel head speed Vbc and mandrel tail speed VBC all keep constant motion. The pipe outlet speed of each rolling mill is constantly improving. The pipe head speed is much higher than the pipe tail speed, that is, VB' c' >: VB'C', Vb'c'=μ εVB'C' (where μ ε is the total elongation of 1 ~ n frames). The mandrel has a constant average speed, and its head and tail speeds are consistent, which is lower than the outlet speed of the nth tube, that is, Vbc=VBC= constant, while VB' c' >:Vbc & gt；; VB 'C language.
In the stable continuous rolling stage, there are three stands in different rolling states: lagging stand, synchronous stand and leading stand. In the working system of N-stand continuous rolling pipe, there is a speed synchronization surface (or mandrel neutral surface K) on the whole contact length between mandrel and the inner surface of the pipe, that is, the metal flow rate in a certain K section of one stand deformation zone is equal to mandrel speed. This kind of intermediate rack is called synchronous rack (or K rack). The stands in front of synchronous stands are called lagging stands, that is, the speed of metal in these stands lags behind that of mandrel; Each frame after the synchronization frame is called the leading frame, that is, the speed of the metal in these frames is ahead of the speed of the mandrel. When biting steel, the synchronization frame gradually changes from 1 frame to k frame; When throwing steel, the synchronization frame changes from the k-th frame to the n-th frame.
The steel throwing stage starts from the capillary end of 1 rolling mill and ends at the scrap pipe end thrown by the last rolling mill.
In the process of steel throwing, tube head speed Vc'd', tube tail speed VC'D', mandrel head speed Vcd and mandrel tail speed VCD all have the characteristics of step acceleration at the same time. The step change of mandrel speed is greater than that of pipe outlet speed, that is, VCD >；; Vc' d. When the pipe end is thrown out of the 1 rolling mill, a resistance to the mandrel disappears and the mandrel is accelerated. Step increment of mandrel speed △Vd=V d(2→8) -V d( 1→8). When casting steel, the step increment of the outlet speed of the pipe tail is greater than that of the outlet speed of the pipe head when biting.
In a rolling cycle, the floating continuous rolling tube with long mandrel will have (2n- 1) changes in motion state, which will cause 2n changes in tube outlet speed and (2n- 1) changes in mandrel speed. This complex alternating relationship of motion speed will inevitably directly affect the stress-strain state and plastic flow law of metal in rolling deformation zone through the transmission of various forces.
In the process of steady continuous pipe rolling, the rolling speed Vi and ni of any stand can be calculated and preset according to the principle that the second flow rate of metal passing through each stand on any cross section in the deformation zone is equal.
f 1v 1 = F2 v2 =…FiVi = const
And Vi=πDKini/60.
Then f (I-1) dk (I-1) n (I-1) = fidkini.
When the tension (or thrust) between racks is considered,
f(I- 1)DK(I- 1)n(I- 1)= FiDKiniS(I- 1)→I
n(I- 1)= NID ki/DK(I- 1)Fi/F(I- 1)S(I- 1)→I
Because μ1= f 0/f1; μ2 = f 1/F2； ……μI = Fi/Fi
therefore
Where DK(i- 1) is the working roll diameter of the previous roll, mm; DKi is the working diameter of the roller of the latter frame, mm; Fi- 1 is the cross-sectional area of the outlet of the deformation zone in the previous frame, mm2；; Fi is the cross-sectional area of the outlet of the deformation zone of the latter frame, mm2；; μi is the expansion coefficient of the ith frame; S(i- 1)→i is the tension (or thrust) coefficient between (i- 1) frame and I frame.
In modern continuous pipe mill, micro-tension (or thrust) rolling is generally adopted. In order to ensure the stability of rolling and avoid serious mandrel sticking, the tension coefficient of 1% is adopted between stands 1 ~ 2 and 2 ~ 3, and the tension coefficient of 0.5% ~ 0.8% is adopted between intermediate stands to ensure the stability of rolling process and the dimensional accuracy of waste pipes. The thrust coefficient between the last two engines is ≤ 1%, which is convenient for rod loosening. See table 1 for the distribution of tensile coefficient of each frame.
Table 1 distribution of tension coefficient of each stand of continuous pipe mill
unit
transmit
The tension coefficient of each frame is 5 (,. ) one,
type
1~2
2~3
3~4
4~5
5~6
6~7
7~8
8~9
independent
transmit
1.0 1
1.0 1
1.008
1.008
1.005
l
1. object-oriented (=ObjectOriented)
O.99
O.99
common
transmit
1. 12~
1. 15
1.08~
1. 10
1.06
1.05
1.04
1.00~
1.02
1.00
1. object-oriented (=ObjectOriented)
Setting of rolling speed When the roll speed and main motor speed of each stand are preset on the floating mandrel mill, the rolling speed from the last mill to the 1 th mill is usually calculated by the reverse method.
The calculation program for the set roll speed series of modern continuous pipe mill (8 stands) is as follows:
According to the roller speed of each stand and the reducer speed ratio I of each stand, the main motor speed of the standby stand can be converted and set.
The work roll diameter DKi is determined by the following formula: dki = da+△-λ1b.
Where Da is the diameter of the roller body, mm; △ refers to the roll gap (8 ~ 10 mm for the first frame and 4 ~ 6mm)； for other frames); B is the passing height, mm; λ 1 is the pass coefficient, which is determined by Figure 2.
The motion characteristics of the limited mandrel continuous rolling pipe are as follows: the speed of the mandrel is constant during the rolling process, and there is basically no "slub" defect caused by the intermittent rolling state of metal flow when floating mandrel is rolled.
The principle of determining mandrel speed is that mandrel speed must be lower than the rolling speed of any stand, so that all stands are in the same direction of differential rolling. Generally, the speed of the mandrel is lower than the average moving speed of the rolled piece in the first stand.
The influence of mandrel speed on the rolling process is that the lower the mandrel speed, the greater the speed difference of the same rolled piece and the greater the post tension, which can reduce the rolling pressure, reduce the spread, promote the extension and improve the dimensional accuracy of the rolled steel pipe. The speed of mandrel should not be too low, because the speed difference is too large and the friction heat is too large, which will lead to serious wear of mandrel and reduce its service life. In general, the limit speed of mandrel is 0.7 ~ 1.5 mm/s, and the length of mandrel working section is about15 mm. ..
Pass side wall angle αB/ (. )
a
Pass side wall angle αB/ (. )
b
0 0.04 0.08 0. 12 0. 16 0.20
o . 02 0.06 0. 10 0. 14 o . 18
Eccentric torque e/mm
C
Fig. 2 is a schematic diagram for determining the value of λ 1
A- circular hole with straight inverted wall; B- circular arc sidewall circular hole type
C- elliptical pass
1-μ=2.0; 2-μ= 1.5; 3-μ= 1. 1
Fig. 3 curve of stopping speed Vd of mandrel
A- quickly feed the mandrel and position it; B- limited speed rolling
C- mandrel quick return
The limit speed curve of mandrel is shown in Figure 3. The position of the mandrel during rolling is shown in Figure 4.
Deformation characteristics of floating mandrel continuous rolling pipe The deformation characteristics of floating mandrel continuous rolling pipe include pass system, pass side wall, elongation coefficient and wall thickness reduction.
Fig. 4 Working position diagram of mandrel
1, 2- fast feed positioning of mandrel; 3,4-The tube head fills the deformation area of each frame; The 5- mandrel is rolled at a constant speed, and the tails of the 6- and 7- tubes are gradually separated from the deformation zone of each frame.
Pass system in modern floating mandrel continuous pipe mill, generally adopts elliptical round hole type. 1 The rolling mill (or the first two types) adopts elliptical pass with arc side wall slope, which can ensure the necessary extension when the diameter reduction is large and is easy to adjust after wear. The middle frame (such as frame 2 ~ 6) is mainly used to reduce the deformation of the wall, which can be a circular hole with the slope of the arc side wall or an elliptical hole with decreasing eccentricity. In the latter two types, in order to ensure the dimensional accuracy of the rolled hollow tube and facilitate the rod breaking, the small side wall (or no side wall) round hole type is adopted. Figure 5 shows the pass system and metal filling of eight floating mandrel continuous pipe mills.
When the pass width is b and the pass height is dk, the ratio of width to height of the hole ξ=b/dk (or pass ellipticity system) indicates the pass ellipticity. When ξ= 1, the pass is circular, and the greater ξ, the greater the ellipticity of the pass. When ξ = 1.25 ~ 1.35, the lateral flow of metal in the pass is relatively free, which easily leads to uneven lateral wall thickness. ξ& lt； When the temperature is 1.24, the deformation of the metal along the circumference of the pass is relatively uniform, and the transverse wall thickness unevenness is small when rolling the pipe, but the rod is not easy to break. Table 2 lists the ξ values of the pass system on the continuous rolling pipe.
Fig. 5 Pass system and metal filling diagram of floating mandrel mill.
The function of the side wall of the hole is to ensure the normal biting of the pipe, at the same time, to compress and clamp the outer diameter of the pipe, and to obtain longitudinal extension to avoid the generation of ears. In the first few stands of the continuous pipe mill, it is generally chosen that the slope of the side wall of the pass is large, which is beneficial to the lateral flow of metal and relatively free to spread, which can reduce the friction resistance of the pipe to the mandrel and make it possible for the metal to obtain a larger longitudinal extension. However, excessive sidewall slope will increase the non-contact area at the sidewall of the pass, which may lead to uneven wall thickness, over-full pass, and even longitudinal cracks and ears. However, in the latter two machines, smaller side wall inclination should be selected to ensure uniform deformation and dimensional accuracy of waste pipes. The slope of the pass sidewall can be expressed by the pass sidewall angle α b = arccos dk/b. Table 3 lists the distribution of the sidewall angle α b of each stand of the continuous pipe mill.
Table 2 Distribution of pass F value of each stand of continuous pipe mill
Rack serial number
1
2
three
four
five
six
seven
eight
nine
Hole width-height ratio} value
1.20~ 1.25
1.20~ 1.25
1.Z5~ 1.30
1.25~ 1.3C
1.25~ 1.30
1.24~ 1.25
1.24~ 1.25
1.06~ 1.20
1.OO~ 1.02
Extension coefficient The total extension coefficient of the floating mandrel continuous pipe mill is 4 ~ 6. The pass extension coefficient in each stand can be determined by semi-parabolic distribution. Because of the high temperature in the first three passes, large reduction can be used to quickly reduce the diameter and wall thickness, and the wall thickness reduction rate can reach 70%. However, the deformation in intermediate frames (such as 4 ~ 6 frames) gradually decreases. The deformation of the last two units should be very small, so as to ensure the dimensional accuracy of the waste pipe and easy to break the rod. See Table 4 for an example of the distribution of each stand extension system on the continuous pipe mill.
Table 3 Distribution of sidewall angle c|B of each stand hole of continuous pipe mill
Rack serial number
1
2
three
four
five
six
seven
eight
nine
Corner fork of hole side wall
45。 ~50。
40。 ~45。
40。 ~45。
40。 ~45。
40。 ~45。
40。 ~45。
40。 ~45。
30。 ~32。
28~~30。
Table 4 Examples of distribution of extension coefficient of each stand of continuous pipe mill
Rolling mill type
Expansion coefficient anus per frame
l
2
three
four
five
six
seven
eight
nine
7 rack
1.35~ 1.45
1.45~ 1.50
1.45~ 1.50
1.27~ 1.5C
1. 16~ 1.20
1. 10
1.05
9 rack
1.20~ 1.45
1.20~ 1.55
1.20~ 1.40
1. 15~ 1.35
1. 15~ 1.30
1. 10~ 1.25
1.02~ 1. 10
1.02~ 1.03
1.003~
1.005
Table 5 Example of wall thickness reduction distribution of each stand of continuous pipe mill
Rack serial number
1
2
three
four
five
six
seven
eight
nine
The wall reduction is,/mm.
4.2
6.3
4.4
3.4
2.O
1.3
O.4
O
O
The wall reduction rate is equal to/%
30
45
44.9
44. 1
37
30
1 1.7
O
The distribution of wall thickness reduction of each rack can be determined by parabolic empirical formula:
δSi =[0.04 17+(7-I)2/40]δS∈
Where, Δ Si is the wall thickness reduction of the top of the hole in the I-frame, mm; I is the serial number of the rack; Δ s ∑ is the total wall thickness reduction in continuous rolling pipe, mm. See Table 5 for an example of the wall thickness reduction distribution of each stand of the continuous pipe mill.
Selection of pass and deformation parameters for continuous rolling of limited mandrel. Because the stripper is cancelled, the mandrel pulls out the steel pipe from the front end of the mandrel when the tube is removed. Because differential rolling is beneficial to the longitudinal extension of metal, the pass with small ellipticity can be adopted when rolling the limited mandrel. The width-height ratio of the hole is 1.0 ~ 1.03, and the wall thickness reduction and total extension coefficient can be larger, and the maximum total extension coefficient can be larger.
Determination of rolling force and rolling torque
When the tube is rolled on the mandrel, there are two zones of reduced diameter and wall along the length of the deformation zone, and the rolling force is as follows:
P=pc 1F 1+pc2F2
Where pc 1 is the average rolling unit pressure in the reducing area, MPa；; Pc2 is the average rolling unit pressure in the wall reduction zone, MPa；; F 1 is the horizontal projection of the contact surface with reduced area, mm2；; F2 is the horizontal projection of the contact surface of the reduced area of the wall, mm2.
The average unit pressure in the decompression zone is:
pc 1 =ηKf2S0/Dcp
Where S0 is the capillary wall thickness, mm; Dcp is the average diameter of the pipeline in the variable diameter area, mm; Kf is deformation resistance, MPa；; η is the influence coefficient of the outer zone on the average unit pressure;
Where l 1 is the length of the reduced area.
The average unit pressure in the wall reduction zone is:
Pc2=K( 1+m)
Where k =1.15kf; M is the influence coefficient of external friction on the average unit pressure, m = 2f1L2/s0+sk; F 1 is the friction coefficient between the metal and the drum; L2 is the length of the wall reduction zone, mm; S0 is the pipe wall thickness before rolling, mm; SK is the wall thickness of the rolled pipe, mm.
When rolling a pipe with a pass with a side wall, the horizontal projection of the total contact area of the deformation zone is:
Where f is the horizontal projection of the total contact area, mm2；; Dmin is the roll diameter at the top of the pass, and Dmin=D 1 -dk, mm; D 1 is the diameter of the roller ring, mm; Dk is the passing height, mm; B is the pass width, mm.
The horizontal projection of the contact area of the wall reduction area is:
F2=(δ0+2So)l2
Where δ0 is the diameter of mandrel, mm; S0 is the wall thickness of the pipe rolled by the previous frame, mm; L2 is the length of the wall reduction zone, mm. ..
The horizontal projection of the contact area of the reduced area is:
F 1=F-F2
The rolling force can be obtained by calculating the sound pc 1, pc2, F 1 and F2 respectively.
Rolling torque The rolling torque on the continuous pipe mill should include the rolling torque in the reducing area and the wall reducing area, the torque of front and rear tension (or thrust) and the axial torque acting on the contact surface between the steel pipe and the mandrel, namely
Where, Mr is the total rolling torque acting on the rollers of any stand of the continuous pipe mill; P 1, P2 is the length of the reduced diameter region and the reduced wall region; QH and qh are the front and rear tension (or thrust) between adjacent frames (when the torque generated by them is in the same direction as that generated by P 1 and P2, a+sign is used in the formula, and a "one" sign is used in the opposite direction); R 1 is the distance between the center line of the roller and the center line of the mandrel; Q is the axial force on the contact surface between the steel pipe and the mandrel, and Q=pc2πδ0L2f2 (where δ0 is the mandrel diameter; F2 is the friction coefficient between metal and mandrel, F2 = 0.08 ~ 0. 1).
Because of the post tension, the rolling pressure is about 30% lower than that of floating mandrel continuous rolling pipe, and the energy consumption is 20% ~ 30% lower.
Due to the step change of mandrel speed, a prominent problem reflected in the quality of hollow tube is the irregular change of outer diameter and wall thickness in the longitudinal direction. The longitudinal difference (periodic bulging) between the outer diameter and the wall thickness of this hollow tube is called the slub phenomenon. According to the longitudinal dimension difference between the outer diameter and the wall thickness of the hollow tube, the front and rear sections along the rolling direction are divided into front slubs and rear slubs. As shown in Figure 6, segment B is the anterior bamboo joint and segment D is the posterior bamboo joint.
The formation mechanism of slub is an important research topic of modern continuous rolling management theory. Generally speaking, the cause of slub is that there are 2n times of alternating intermittent rolling in the process of floating mandrel continuous pipe rolling, especially the step change of mandrel speed, which causes the plastic deformation of metal during unstable rolling and the discontinuity of its flow in the deformation zone.
The technological measures for controlling slubs are as follows:
(1) In terms of process operation, reasonable layout and extension; Improve the friction conditions of mandrel (such as selecting mandrel lubricant and spraying method, improving mandrel wear resistance and reducing surface roughness, etc.). ); By improving the pass design, the pass of the rear frame adopts a larger side opening, which reduces the clamping force of the pipe on the mandrel, is beneficial to the longitudinal flow of metal and weakens the front slub phenomenon;
(2) In the aspect of equipment improvement, the structure of variable stiffness rolling mill is adopted to eliminate the non-uniformity of longitudinal dimension of waste pipe;
(3) In the electrical control, the step increment of mandrel acceleration or sudden tension thinning is offset by using the speed forced landing control link of the back slub, the sudden tension control link of the tube head and the tube tail, and the dynamic rapid drop compensation link of steel biting, so as to improve the longitudinal dimension accuracy of the hollow tube.