JP2014097518A - High area-reduction ratio rolling method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simplify the whole rolling equipment by realizing the low aspect ratio/high area-reduction ratio rolling, in hot rolling of a bar-wire.SOLUTION: When imparting back tension to a pre-rolling steel material, the steel material 2 is heated so as to become the highest temperature just before biting-in, and the back tension corresponding to yielding force of a biting-in part is imparted to the steel material by specifying two conditions of the roll 1' diameter ratio and the roll peripheral velocity ratio, and drawing is induced before biting in the steel material. Widening disappears by pre-reduction in the draft-width both directions, and an increase in the aspect ratio by draft is restrained, and the high area-reduction ratio of the draft ratio or more is provided. Stable over-tension rolling becomes possible without rupture and a reduction cut by the high temperature setting of the biting-in part and front tension after rolling, and the low widening/high area-reduction ratio is provided.

Description

  The present invention relates to a method for rolling a long product such as a steel wire rod, steel bar, or bar steel manufactured by hot rolling.

  The target steel material is finished to a predetermined dimension by repeated hot rolling using a steel piece as a material. Since the area reduction rate is usually 10 to 30% per pass, a large number of rolling mills are required, resulting in high equipment costs, and high operating costs such as energy loss, roll wear, and maintenance of incidental equipment.

The first reason that the area reduction ratio is in the above range is that the cross-sectional aspect ratio (= width / thickness) becomes excessive when the pressure is reduced to obtain a high area reduction. In the next pass that is squeezed in the width direction, taole, buckling, twisting, etc. are likely to occur, and the desired shape is not easily obtained.
The second reason is that as the rolling reduction increases, the widening increases at an accelerated rate, and the increase in the area reduction rate with respect to the rolling down decreases rapidly.
The third reason is that once the aspect ratio becomes excessive, the excess circumference becomes inevitable due to the excessive perimeter, so that uneven skin accumulation is unavoidable in subsequent rolling, and wrinkle damage is likely to occur. This is because local accumulation of the decarburized layer is amplified.
As described above, mere large pressure not only does not provide a sufficient area reduction ratio, but also causes an excessive aspect ratio to induce a reduction in surface quality.

In the following, the preceding examples of the high area reduction rolling method will be examined.
Non-Patent Document 1 describes a PCRM method (Planetary Cross Rolling Mill, planetary inclined rolling mill). According to this method, the cylindrical material can be stretched by about 6 times (reduction rate of about 84%) by drawing and rolling by rotating and revolving a conical inclined three-way roll. However, due to the speed limitation due to the structure of the rolling mill, it can be applied effectively to rough rolling (approximately 0.1 m / s) after intermediate rolling (speed of 1 to 100 m / s) where the running speed increases at an accelerated rate. ) Is not applicable.

  According to the method disclosed in Patent Document 1, the area reduction rate can be improved to 55% by applying a forward / backward tension of about 85% of the yield stress of the material to the material in reversible cold rolling of a thin plate. . In the conventional method, it is at most about 40% due to the problem of shape control. Although there is a significance that the conventional action limit is expanded by the reduction under the tensile stress, the effect is several tens of percent on the conventional extension.

  Non-Patent Document 2 proposes a theory and specific measures for reducing the reduction in area due to widening and improving rolling efficiency. According to this, when a material under tension of Y / √3 (Y: yield stress of material) is rolled down, no widening occurs and the rolling reduction ratio is reduced. As a result, it can be understood that the increase in the aspect ratio is suppressed. Although the effect is expected to increase further when the tension exceeds the above value, there is no experiment or mention about the specific deformation behavior. It is also questionable whether such rolling can be done easily.

  Patent Document 2 discloses a rolling method that develops the above theory and dramatically increases the area reduction rate in hot rolling. According to this, by increasing the rear tension to the strength of the yielding force of the roll biting part with respect to the workpiece material supplied to the rolling mill at a constant speed, the workpiece is stretched immediately before biting, and the surface is reduced. It is possible to significantly increase the rate and change the widening accompanying the reduction to erasure or contraction to suppress an increase in the cross-sectional aspect ratio. A new predicate called over-tensile rolling is used to strengthen the backward tension beyond the conventional elastic limit to the plastic range.

  By an appropriate combination of three conditions: rolling ratio (= thickness after rolling / thickness before rolling), roll diameter ratio (= roll diameter / thickness before rolling), and speed ratio (speed after rolling / speed before rolling), 1 It has been theoretically elucidated that it is possible to reduce the number of passes equivalent to several passes of conventional rolling, and it has also been demonstrated as a fragmentary experiment.

  When a rolling mill according to the theory was manufactured and subjected to a verification test, it was proved that there was no major mistake in the theory because the rolling was performed with a low widening and a high area reduction rate. However, drawing out often occurred, which proved to be a critical obstacle to the practical application of the high area reduction rolling.

Edited by Japan Iron and Steel Institute, Steel Handbook 4th Edition, Volume 3-2, Volume 3, 10 ・ 4 ・ 1-4 Special rolling mill Saito, Utsunomiya et al .: Plasticity and processing 40-465 (1999-10), p.966-p.970: “Development of stretch-controlled rolling mill and application to flat wire rolling”

JP2000-197907 Patent No. 4284396

  It is a significant problem for those skilled in the art to greatly reduce the number of rolling passes and to simplify the entire rolling equipment by processing rods and linear materials with a high reduction in area. In that case, 1) Rolling efficiency decreases and the surface reduction rate reaches its peak at a mere high pressure, and the aspect ratio becomes excessive, and the subsequent forming, that is, generation of dimensional / shape defects, wrinkles, and local decarburization defects occurs. 2) The above-described inclined rolling method is effective for rough rolling, but cannot be used in a region where the rolling speed is high after intermediate rolling. 3) The result of suppressing widening by applying tension. In particular, there is a problem that the effect is limited in the method of improving the area reduction rate.

  In order to solve the above-mentioned problems, an over-tensile rolling method (patented as a rolling method that can achieve a remarkably high reduction in area even when the rolling speed is high and that does not cause difficulty in subsequent rolling because the aspect ratio is suppressed. Document 2) has been proposed, but in the demonstration experiment, it was found that there was a frequent occurrence of throttling due to excessive tension. This invention makes it the subject which should solve generation | occurrence | production prevention of drawing out in this over tension rolling.

The cause of the full-throttle in the demonstration experiment is gradually revealed that various conditions are involved, and there is no need to modify or reinforce the theory, and it is solved through optimization of work factors through trial and error The following inventions were made.
In the first aspect of the invention, when rolling a rod, wire, or strip steel material hot while applying tension in the rolling direction, the rolling mill itself is drawn into the steel material supplied to the rolling mill while maintaining a constant speed. The force generates a rear tension equal to the yield force of the steel material at the roll bite entrance, induces the steel material to stretch at the entrance to increase the area reduction rate, and eliminates the widening due to rolling to increase the cross-sectional aspect ratio. In the low aspect ratio and high area reduction rolling method characterized by suppressing, heating is added so that the temperature distribution of the steel material reaches the maximum temperature at the roll bite inlet on the upstream side of the rolling mill, and the metal structure Is austenite, and a forward tension is applied to the downstream side.

The second invention is the rolling method according to the first invention, wherein the roll diameter ratio γ is set according to the equation (1).
μ ² γ> 2 / (1−h) +2+ (1-h) / 2 −−−− (1)
However,
h: Total reduction ratio (= Thickness G after rolling of material / Thickness Ho before rolling)
μ: Coefficient of friction between roll and steel γ: Roll diameter ratio (= roll diameter 2R / thickness Ho before rolling)

3rd invention adjusts roll peripheral speed Vi in the range according to (2) Formula with respect to the steel material of a rectangular cross section, according to (3) Formula, and the surface reduction ratio r according to (4) Formula. The rolling method according to the second aspect of the invention is characterized in that the range is adjusted.
1 / h <Vi / Vo ≦ 1 / (he × h) ----- (2)
1 / h> α ≧ he / h ----- (3)
he × h ≦ r <h −−−−− (4)
However,
he = [− B−√ (B ² −4AC)] / 2A −−−−− (5)
A = 9, B = -6h-2F / k, C = h (h + 2F / k)
he; Drawing reduction ratio (= thickness He just before biting / thickness Ho before rolling)
F: Friction index (= μ ² γ)
Vo: Speed of the supplied steel before rolling
Vi; speed after rolling α; specific aspect ratio (= section aspect ratio after rolling / aspect ratio before rolling)
r; Area reduction ratio (= cross-sectional area after rolling / cross-sectional area before rolling = 1-area reduction ratio)

  A fourth invention is a rolling method according to the first invention, the second invention or the third invention, wherein the heating method is a method of directly energizing a steel material, and one of the electrodes is a roll of a rolling mill. .

  In the above invention, by specifying the heating and drawing conditions, the first effect is: “For a bar, wire, or strip-shaped steel material supplied at a constant speed, the tension acting on the material by the pulling force by the roll is reduced. Since it is equal to the force, stretching is induced in the material by the roll bite. As a result, even if the rolling reduction ratio is set higher than the conventional one, the widening due to rolling disappears or turns into a narrowing width, the excessive increase in the aspect ratio is suppressed, and the subsequent rolling problem is solved, and the surface area is reduced more than the rolling reduction ratio. The rate can be easily obtained. Further, as a first condition for quantification, a roll diameter ratio that generates a reduction force of a size necessary for stretching is defined by the equation (1), and as a second condition, the reduction force is converted into a drawing force required for stretching. Since the ratio of the roll peripheral speed / material speed is defined by the formula (2) or (3), not only a low aspect ratio and a high area reduction ratio can be obtained, but they can be adjusted extensively. Become. The processing corresponding to the conventional six passes can be performed in one pass, and the number of rolling mills required in the rolling mill can be greatly reduced. ] Is reinforced.

  Second effect, “The structure of the rolling mill is mainly a general double rolling mill with a relatively large roll diameter, and no special mechanism or design is required. The entire rolling equipment is combined with the first effect. Is cheap and simple. ] Is reinforced.

The third effect is that “the ratio of simple stretching is large when the cross-section is reduced and the aspect ratio is kept small, so that abnormal accumulation and local accumulation of the side skin due to the reduction seen in conventional rolling are less likely to occur. That is, the problem of local decarburization defects and wrinkle scratches is eliminated. A significant reduction in the number of passes is also effective in reducing various surface flaws associated with the passes. ] Is reinforced.
The fourth effect, “processing energy efficiency is improved due to the fact that no widening occurs, power loss due to roll contact, cooling of steel materials, etc. is small. ] Is reinforced.

It is explanatory drawing of the low aspect-ratio and high area reduction rolling method of the steel materials of this invention. Dimensional symbols in the rolling situation are shown. The example 1 which adds the heating apparatus of this invention is shown. The example 2 which adds the heating apparatus of this invention is shown. The theory and actual relationship between speed ratio and area reduction ratio are shown.

Hereinafter, embodiments will be described with reference to the drawings.
FIG. 1 is a schematic explanatory diagram of a low aspect ratio / high area reduction rolling apparatus for carrying out the present invention. The apparatus mainly includes a rolling mill 1 having a flat roll 1 ′, a steel material supplying means 3 (eg, a pinch roll, a rolling mill) for supplying a steel material 2 to the rolling mill 1 at a constant speed, and the steel material 2 to the rolling mill 1. A direct current heating apparatus power source 4 for heating on the upstream side of the steel, a circuit 5 having the steel material 1 as a part of the circuit, and a steel material extracting means 6 (for example, which is disposed in front of the rolling mill 1 and applies a forward tension to the steel material after rolling. : Pinch roll).

First, a steel material 2 having a square cross section is supplied to a rolling mill 1 having a flat roll 1 'at a constant speed, and the steel material 2 between the steel material supply means 3 and the flat roll 1' is energized at the same time as it is engaged with the roll 1 '. As a result, the temperature is gradually increased toward the downstream and reaches the maximum temperature at the point of contact with the flat roll 1 ′. At this point, the temperature is about 900 ° C. or higher.
Next, the peripheral speed Vi of the roll 1 ′ is gradually increased toward a predetermined value that is twice or more the steel material supply speed, and at the same time, the roll gap G is reduced to a predetermined reduction ratio. Although the steel material 2 is caught in the flat roll 1 ′, the circumferential speed greatly exceeds the supply speed, so that a rear tension is generated in the steel material 2.

In the present invention, since the roll diameter ratio γ is set large, the rolling force is large, and therefore the pulling force P (= friction coefficient × rolling force) is large, and the tension is increased without slipping. On the other hand, the steel has a small cross-sectional area at the bite, but is smaller than the main body, and since it is softer at high temperatures, this part is in the stage where the tension reaches the bite yield force (generation of over tension). Stretching occurs.
FIG. 2 shows a situation in which a steel material having a square cross section is being squeezed by a roll and dimension symbols. The film is stretched at substantially the same ratio in the rolling direction and the width direction. As a result of stretching, the roll bite is successively reduced, the pulling force is reduced, the amount of stretching is reduced, and it is stabilized at a certain equilibrium point. Accordingly, the steel material (thickness He) having a reduced square cross section is thereafter reduced by the flat roll 1 ′.
As a mechanical condition for stabilizing over-tensile rolling, an appropriate combination of a roll diameter ratio, a reduction ratio, and a speed ratio is necessary, and the conditions have been elucidated in the preceding examples.

On the other hand, the first condition on the stable operation of over-tensile rolling is that the biting portion is at a higher temperature than the upstream side and the yield force is minimized in the tensile section.
The second condition is that the rolled steel material does not hinder the roll pull-in force even slightly. Therefore, the steel material 2 extruded from the rolling mill must be always pulled forward by a pinch roll as the steel material drawing means 6.

As the heating method, the above-described direct energization is appropriate. In some cases, a high-frequency induction heating coil 7 may be provided as shown in FIG.
As an energization heating method, as shown in FIG. 4, a method of providing an energizing roll 8 between the steel material supply means 3 and the flat roll 1 ′, energizing both sides, and grounding both ends is more desirable.

In over tension rolling, the total reduction is distributed to the stretching of the biting portion and the net rolling after biting. The former area reduction ratio r is the square of the drawing reduction ratio he, and the latter area is reduced in width by the rear tension, so that rolling efficiency is improved, and when combined, the area reduction ratio is remarkably improved. The reduction ratio (= cross-sectional area after rolling / cross-sectional area before rolling) is the reciprocal of the speed ratio (= rolling speed / feeding speed).
On the other hand, as for the cross-sectional shape, the total width becomes equal to or less than that before rolling when the reduced width due to stretching and the small widening corresponding to the net rolling are added together, the increase in aspect ratio due to rolling is remarkably suppressed.
From the above, for example, the roll diameter ratio is 50, the reduction ratio (= thickness after rolling / thickness before rolling) is 0.5, the speed ratio (rolling speed / feeding speed) is 2.5, and the area reduction ratio is 0.6 (normal four passes). Ratio) is obtained, and the aspect ratio falls within 1.8.

The embodiment of the present invention has been qualitatively described above. Hereinafter, the theoretical basis of the present invention and the quantitative relationship between various factors will be clarified.
Assuming that the ratio of the material supply speed to the roll peripheral speed and other factors are in an appropriate state for a material with a square cross section, the material is stretched, and the sum of the tension generated in the material and the retraction force by the roll is the maximum. Equilibrium with the pulling force μ × P, and the following equation is established.
Yield force + roll retraction force = roll pulling force
Y × He × He + ΔH × P = μ × P = μ × Y × L × He (6)
L = √ (ΔH × R) ----- (7)
ΔH = He-G ----- (8)
Y: Yield stress, He: Steel thickness after drawing, P: Rolling force,
L: Roll / steel contact length, ΔH: Net reduction amount,
The explanation of the symbols is also shown in paragraph [0015], paragraph [0016] and FIG.

Rearranging the above equation gives a quadratic equation.
A′He ² + B′He + C ′ = 0 −−−−− (9)
Here, γ = 2R / Ho, μ ² γ = F, and both sides are divided by Ho ² to make it dimensionless to obtain a solution.
Ahe ² + Bhe + C = 0 −−−− (10)
he = [− B−√ (B ² −4AC)] / 2A −−−−− (5)
A = 9, B = -6h-2F, C = h (h + 2F)
That is, the drawing reduction ratio he is calculated from the total reduction ratio h and the friction index F.

The occurrence of stretching means that the following formula is established, and when substituting into the formula (5), the formula (1) is derived.
he <1.0
F = μ ² γ> 2 / (1−h) +2+ (1-h) / 2 −−−− (1)
That is, the first necessary condition for over-tensile rolling is expressed by equation (1).
The value of γ is usually about 3 to 20 from rough rolling to finish rolling, but the vicinity of the upper limit is so for structural reasons and there is no special reason. From the equation, it can be understood that at least 40 or more is required for over-tensile rolling, and a roll diameter that greatly exceeds the conventional level is required. The above is the specific basis for the roll diameter ratio γ described in the second invention.

The specification of the speed ratio that is the second condition for over-tensile rolling will be described.
In the state where normal rolling is maintained, the following formula is established from the constant quantity rule.
Vi x Si = Vo x So ---- (11)
Vo: Steel material entry side speed, Vi: Steel material delivery side speed, Si: Steel material delivery side cross-sectional area, So: Steel material entry side cross-sectional area The cross-sectional area ratio Si / So is expressed by the following formula in the case of a rectangular cross section.
Si / So = He ² × Hr / He / Ho ² = he ² × hr
On the other hand, from the definition
he × hr = He / Ho × G / He = G / Ho = h
Therefore, Si / So = he × h ---- (12)
From the above, the speed ratio Vi / Vo is as follows. If the speed ratio exceeds this, the pulling force is lost and the balance is lost, resulting in slip rolling.
Vi / Vo = 1 / (he × h) ---- (13)

The above explanation is a case where maximum stretching is obtained. On the contrary, when the speed ratio is decreased by a small amount, the increase in the pulling force is relatively superior to the yield force, and the stretching is maintained, and the specific aspect ratio α is slightly increased and the surface reduction ratio r is slightly increased. This action continues until the draw reduction ratio he is 1.0. Therefore, it is understood that the phenomenon of over-tension rolling in which stretching occurs up to the roll bite can occur within a certain range with respect to the speed ratio, and the amount of deformation changes depending on the speed ratio. The lower limit speed at which over-tensile rolling appears is as follows if he = 1.0 on the right side of the equation (17).
Vi / Vo> 1 / h ---- (14)
Summarizing the above, the speed ratio range in which over-tensile rolling appears is as follows.
1 / h <Vi / Vo ≦ 1 / (he × h) ----- (2)
The above formula is the second condition for over-tensile rolling, and is the specific basis for the speed ratio Vi / Vo in the third invention.

The specific aspect ratio α at the above upper limit and lower limit (= cross-sectional aspect ratio after rolling / aspect ratio before rolling) will be described. Ignoring widening approximately,
α ≒ He / G = he / h
Corresponding to the speed ratio in the equation (2), α changes within the range represented by the following equation.
1 / h> α ≧ he / h ----- (3)

Find the area reduction ratio r at the upper and lower limits of the speed ratio. In the equation (12), Si / So actually represents the reduction ratio r. Therefore, the following equation can be easily obtained.
h> r ≧ he × h −−−−− (4)
From the above, when the total reduction ratio h is given, the drawing reduction ratio he, the rolling reduction ratio hr, the area reduction ratio r, the specific aspect ratio α, and the speed ratio Vi / Vo are calculated.

  Example 1 An experiment was conducted using a demonstration overtensile rolling apparatus as shown in FIG. The specimen is a carbon dioxide arc welding wire having a wire diameter of 5.5 mm. The roll diameter is 250 mm. The wire supply speed was 25 mm / s, the rolling speed was set to twice that, the low gap of the rolling mill was lightly reduced, and heated to about 800 ° C. with a high-frequency induction heating device. As a result, mild tensile rolling was maintained in a sliding state. When the gap was gradually reduced, the slip suddenly disappeared, and the rolling speed increased rapidly. The cross-sectional area before rupture was halved, and temporary but over-tensile rolling was confirmed. It was understood that when both the wire speeds were reduced by half and the heating temperature was raised to about 950 ° C., the rolling continuation time increased, and high temperature heating was necessary. It is interpreted that the ductility of the ferrite region is insufficient. However, rolling did not continue for a long time, and drawing out occurred. In simple hot rolling, it was found that the difference in which the biting portion preferentially extends was unstable.

  Example 2: A direct energizer was added to compensate for the lack of capacity of the induction heating device. One of the electrodes was a rolling mill roll. As a result, the temperature became higher in the downstream, and the biting portion reached a maximum temperature of about 1000 ° C. It became clear that stretching of the biting portion was stable, and overtensile rolling of several tens of meters was possible at a predetermined speed, and softening of the biting portion was effective. However, there was often a throttling. It was found from the record of the feeding speed and rolling speed that it occurred after a slight fluctuation in speed, and it was necessary to supply at a constant speed.

Example 5: When an interlocking mechanism was added to keep the ratio between the supply speed and the rolling speed constant to eliminate the speed ratio fluctuation, the over-tensile rolling became stable.
When a little resistance acted on the running of the wire on the exit side of the rolling mill, drawing was generated after a momentary slip and broke. When slipping was observed, kinks (extrusion) occurred and rolling was impossible. When the guide was always pulled out, the diaphragm was not cut. It was found that it was necessary to apply tension constantly.
As shown in FIG. 5, when the speed ratio was 2.5 times with an appropriate reduction amount, a surface reduction rate of 60% was achieved as theoretically. In the case of 3.0 times, a new slip occurred, which was 55% compared with 67% of theory. However, sliding rolling was stable. The set reduction ratio was close to the limit of conditional expression (2).

DESCRIPTION OF SYMBOLS 1: Rolling machine 1 ': Flat roll 2: Steel material 3: Steel material supply means 4: Electrical heating power supply 5: Electrical heating circuit 6: Steel material extraction means 7: High frequency induction heating coil 8: Current roll

Claims (4)

  When rolling rods, wires, and strip-shaped steel materials hot while applying tension in the rolling direction, the roll bite entrance is brought about by the pulling force of the stretcher itself against the steel materials supplied to the rolling mill while maintaining a constant speed. A backward tension equal to the yield force of the steel material is generated at the inlet, the steel material is stretched at the entrance to increase the area reduction rate, and the expansion due to rolling is eliminated to suppress an increase in cross-sectional aspect ratio. In the rolling method with a low aspect ratio and high area reduction ratio, in the upstream side of the rolling mill, heating is added so that the temperature distribution of the steel material becomes the highest temperature at the roll bite inlet, and the metal structure is austenite, and the downstream A rolling method characterized by applying a forward tension to the side. The rolling method according to claim 1, wherein the roll diameter ratio γ is set according to the equation (1).
μ ² γ> 2 / (1−h) +2+ (1-h) / 2 −−−− (1)
However,
h: Total reduction ratio (= Thickness G after rolling of material / Thickness Ho before rolling)
μ: Coefficient of friction between roll and steel γ: Roll diameter ratio (= roll diameter 2R / thickness Ho before rolling) By adjusting the roll peripheral speed Vi within the range according to the formula (2) for the steel having the rectangular cross section, the specific aspect ratio α is adjusted according to the formula (3), and the surface reduction ratio r is adjusted within the range according to the formula (4). The rolling method according to claim 2, wherein:
1 / h <Vi / Vo ≦ 1 / (he × h) ----- (2)
1 / h> α ≧ he / h ----- (3)
he × h ≦ r <h −−−−− (4)
However,
he = [− B−√ (B ² −4AC)] / 2A −−−−− (5)
A = 9, B = -6h-2F / k, C = h (h + 2F / k)
he; Drawing reduction ratio (= thickness He just before biting / thickness Ho before rolling)
F: Friction index (= μ ² γ)
Vo: Speed of the supplied steel before rolling
Vi; speed after rolling α; specific aspect ratio (= section aspect ratio after rolling / aspect ratio before rolling)
r; Area reduction ratio (= cross-sectional area after rolling / cross-sectional area before rolling = 1-area reduction ratio)   The rolling method according to claim 1, wherein the heating method is a method of directly energizing a steel material, and one of the electrodes is a roll of a rolling mill.

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