JPH0318525B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of cold rolling a dense hexagonal metal plate, and more particularly to a cold rolling method that can produce the same metal plate with good surface precision with high productivity. The close-packed hexagonal metal sheet to be subjected to cold rolling in the present invention is a general term for metal sheets having a close-packed hexagonal crystal structure, such as Ti, Ti alloy, and Zr alloy. In the cold rolling of metal plates, it is essential to use rolling oil to prevent seizure, but when rolling oil with good lubricity is used, fluid lubrication occurs between the rolling rolls and the rolled material, and high-pressure rolling oil can be used. It is known that unevenness (so-called oil pits) occurs on the free surface of the rolled material in contact with it. In particular, in metals such as the dense hexagonal metals, which have significantly different deformation resistance depending on crystal orientation, oil pits are likely to occur because crystals in orientations with lower deformation resistance are easily deformed. It is not uncommon for pits to reach a depth of more than ten microns. Since these oil pits significantly impede the surface precision of the final product, how to reduce the size of the oil pits generated during the rolling process is an important issue in cold rolling of this type of difficult-to-work metal sheet. . The allowable limit of this oil pit varies depending on the required surface accuracy determined depending on the product's use, but it is often required that the depth be 1 to 2 μm or less. On the other hand, if rolling oil with poor lubricity is used to prevent oil pits from occurring, boundary lubrication occurs between the rolling rolls and the rolled material, which increases the friction coefficient in roll bite, increases the rolling load, and increases the rolling load. The rolled material sticks to the rolls. If rolling is continued in such a state, the rolled material will become locally stuck to the rolls, resulting in so-called seizure. When such seizure occurs during the rolling process, the coefficient of friction at the roll bite portion increases rapidly, making it impossible to continue rolling. Furthermore, the occurrence of seizure deteriorates the surface precision of the roll, and the surface precision of the Ti cold-rolled plate also deteriorates significantly, making it necessary to re-grind the rolling roll. Conventionally, it has been thought that sticking in the early stage of rolling causes seizure, and rolling operations have been carried out to prevent sticking as much as possible from the early stage of rolling. In addition, in the final finishing pass, which greatly affects the surface of the rolled product, if the rolling roll shows a seizure phenomenon, or even if only sticking occurs, the rolling operation should be immediately stopped and the rolling roll replaced with a polished finishing roll. At the same time, the mill rolls with stuck rolled metal were sent to the grinding and repair process. Under these circumstances, in the cold rolling of metal sheets as mentioned above, improvement research is being conducted with a focus on developing lubricating oils that can suppress oil pits as much as possible and prevent seizures, and setting operating conditions. However, attempts to suppress oil pits tend to cause seizures, and attempts to prevent seizures tend to cause oil pits to occur, which is a problem. It was thought that it would be extremely difficult to improve this to a certain degree. In view of these circumstances, the inventors of the present invention have attempted to establish a technology that can solve the oil pit problem and the seizure problem at the same time, particularly for dense hexagonal metal plates, and can obtain the same metal plates with good surface precision. We have been conducting various research. As a result, if the rolled metal is rather actively fixed to the surface of the rolling roll at the initial stage of cold rolling, and the fixed layer of the rolled metal is uniformly fixed by rolling a certain distance, this homogeneous The fixed metal fine powder becomes a kind of coating layer, making it possible to continue rolling without oil pits or seizure, and producing the same metal plate with excellent surface precision.This is a new phenomenon that cannot be predicted from conventional wisdom. I found out the facts. The present invention is provided as a result of research based on such knowledge, and has a structure in which, in a method of cold rolling a dense hexagonal metal plate using a mill roll that has been ground, the initial stage of rolling is By not supplying rolling oil or by supplying rolling oil with poor lubricity, a homogeneous adhesion layer is formed on the surface of the rolling roll by part of the surface metal of the material to be rolled, and then a lubrication layer with excellent lubricity is formed. The gist is that cold rolling is carried out while supplying the agent. Conventionally, it was believed that when seizure occurs in the cold rolling process as mentioned above, the rolling load increases and operability decreases, and the surface precision of the rolled product also deteriorates significantly.However, in the present invention, As described above or as detailed below, rolling conditions that tend to cause sticking are set at the initial stage of rolling, and rolling is continued for a certain distance to make the rolled metal uniform on the roll surface. After that, normal cold rolling is continued, and due to the initial formation of a fixed coating layer, there are no problems such as the formation of oil pits or an increase in rolling load, and the surface is smoothed. A dense hexagonal metal plate with extremely high precision can be cold rolled with smooth operation and high productivity. In the present invention, as a means of forming a fixed coating layer on the surface of the rolling roll at the initial stage of rolling, a method of cold rolling is employed in which no lubricant is supplied or rolling oil with poor lubricity is supplied at the early stage of rolling. We are hiring. Whether or not sticking occurs on the surface of the rolling roll depends on the type of lubricant. In other words, when rolling a metal plate, seizing does not occur when a complete oil film is formed between the work roll and the material to be rolled, creating a so-called fluid lubricant state, but an oil film forms over the entire rolling surface. When this occurs, metal-to-metal contact occurs partially, resulting in a so-called boundary lubrication state, and sticking occurs. A lubricant with good lubricity has a high ability to form an oil film and is easy to obtain a fluid lubrication state, whereas a lubricant with poor lubricity has a low ability to form an oil film and tends to form a boundary lubrication state locally. If a lubricant with good lubricity is used, sticking will not occur, but if a lubricant with poor lubricity is used, sticking will occur. The present invention makes good use of this phenomenon and intentionally uses a lubricant with poor lubricity in order to form a fixed coating layer at an early stage. In the same way, when lubricating oil is not used, adhesion progresses rapidly, and a homogeneous adhering coating layer is formed at a roll rotation speed of less than 20 revolutions. After forming a fixed coating layer in this way, if cold rolling is continued while supplying a normal lubricant (the most common is tallow-based emulsion type lubricant), oil pits and seizures will almost always occur thereafter. It is possible to smoothly manufacture rolled plates with extremely high surface precision without any problems. Figure 1 conceptually shows the process of cold rolling. Immediately after the start of rolling, a so-called dummy tail is fed,
Next, cold rolling begins. At this time, by initially supplying no lubricant or by supplying a lubricant with poor lubrication performance, adhesion is actively caused in the rolling portion to form a homogeneous coating layer. After a fixed coating layer is uniformly formed on the roll surface, cold rolling is continued while supplying a normal lubricant with excellent lubrication performance, but if a fixed coating layer is formed at the beginning of rolling, Thereafter, cold rolling can proceed smoothly with almost no oil pits or seizure. Figure 1 shows a plate with a thickness of 0.9 using a small two-high rolling mill.
This figure shows the relationship between the average rolling pressure and cumulative strain for each pass when a Ti plate with a width of 60 mm is cold rolled. The diameter of the rolling roll used is
Two types of 160mm rolls were used: a roll with a grinding finish and a similar roll with a fixed coating layer of Ti (hereinafter referred to as coating roll).
% concentration beef tallow emulsion lubricant (viscosity:
60cst/40℃) was used. The rolling speed is 200mpm. FIGS. 2 and 3 show microscopic photographs of the surface of the cold-rolled sheet obtained. The surface of the plate rolled with coating rolls (Fig. 2) shows no oil pits at all, showing excellent surface precision, but the surface of the plate rolled with grinding rolls (Fig. 3) has no oil pits. Numerous oil pits of 8 to 10 microns can be seen. As is clear from FIG. 1, at low cumulative rolling reductions, the coating roll has a higher rolling pressure relative to strain than the ground finishing roll, and the rolling efficiency is lower. However, with a grinding finish roll, the average rolling pressure gradually increases due to work hardening of the rolled material as the rolling reduction increases, whereas when a coating roll is used, the average rolling pressure increases gradually during the first pass and the second pass.
In the pass, the average rolling pressure is higher than in the case of the ground finish roll, but after that it shows a decreasing tendency, and despite the increase in work hardening (cumulative strain) of the rolled material, the average rolling pressure is 130 kg/ It is stable around mm ² . That is, in the normal cold rolling method, the rolling pressure increases monotonically as the cumulative reduction rate increases, so the rolling pressure at the end of rolling becomes extremely large, but with the method of the present invention, the cumulative reduction rate increases. However, since the rolling pressure is stable at a relatively low value, cold rolling can be carried out smoothly to the end without extremely reducing the rolling efficiency at the final stage of rolling. The reason why the rolling pressure decreases in the middle to late stages of rolling when using coated rolls is not necessarily clear, but the fine metal powder that separates from the surface of the rolled material in the tool bit acts as a lubricant. This is thought to be due to the formation of metallic soap, which reduces frictional resistance in the boundary lubrication region. In addition, in FIG. 1, it is not clear why the rolling pressure is large in the low cumulative reduction region when a coating roll is used. However, the generation of metal soap at the bit part is related to the amount of plastic deformation of the rolled material, and there is a theory that a certain amount of plastic deformation is necessary in order to generate a large amount of metal soap. This is presumed to be because a sufficient amount of metal soap is not generated because the amount of plastic deformation of the rolled material is small in the early stage of rolling. The reason why oil pits are significantly suppressed by using a coating roll can be considered as follows. As mentioned above, oil pits are thought to be caused by the high-pressure rolling oil present in the roll bite pressurizing the surface of the rolled material, which is softer than the roll surface, resulting in unevenness.
Since a relatively soft metal coating layer exists between the hard roll base material and the rolled material, this coating layer deforms preferentially and suppresses the deformation of the rolled material. The fixing coating layer forming step as described above only needs to be carried out at the beginning of the first pass after installing the grinding finishing roll, and is not necessary in the second pass and thereafter and in the rolling of the second and subsequent coils. The length of the rolled material to be rolled in the coating layer forming step may be shorter if rolling oil with poor lubricity is used, and therefore, the length of the rolled material is shortest when no rolling oil is supplied. 1st
The tip part of the rolled material used to form the coating layer of the roll during rolling is slightly thicker than other parts due to the large friction coefficient between the roll and the rolled material during rolling. This plate thickness deviation is within a range that can be completely eliminated in the second and third passes. Furthermore, the surface of the rolled material becomes exactly the same as other parts in the second and third passes, so there is no product loss. After forming the fixed coating layer as described above, it is preferable to use a lubricant with excellent smoothness in subsequent rolling operations and to perform the rolling operation at high speed from the viewpoint of improving productivity. Note that the maximum speed is determined by equipment constraints and rolling technology constraints such as heat of the rolls, and is not limited by the reduction in surface accuracy due to oil pits. The present invention is constructed as described above, and its effects can be summarized as follows. By coating the roll surface with the metal to be rolled at the beginning of cold rolling, subsequent cold rolling can be carried out smoothly without oil pits or seizure, resulting in excellent surface accuracy. A dense hexagonal metal rolled plate can be obtained. In conventional cold rolling of Ti plates, etc.,
For example, as disclosed in JP-A No. 56-165502, in order to prevent oil pits, etc., it is necessary to take measures such as making the diameter of the rolling roll as small as possible and shortening the arc length of contact between the roll and the rolled material. Therefore, it was thought that it would be practically difficult to use a roll with a large diameter, but with the present invention, even if a roll with a large diameter is used, it can be cooled smoothly without causing oil pits or seizures. Inter-rolling can be performed, contributing to improved productivity. Example: A Ti plate with a thickness of 3.0 mm and a width of 900 mm is used, and the diameter is 200 mmφ.
Cold roll the sheet to a thickness of 0.5 mm using cold rolls according to the rolling schedule shown in the table below.

[Table] When rolling, start rolling by setting the rolling speed to 300 m/min, and in the first pass, the tip of the coil
Rolling was carried out without supplying rolling oil only while rolling a portion equivalent to 50 m, and after forming a homogeneous Ti coating layer on the work roll surface,
Thereafter, rolling was carried out while spraying rolling oil (2% emulsion of beef tallow oil, viscosity 60cst at 40°C). Further, from the second pass onwards, rolling was performed while supplying rolling oil from the tip of the coil at the above-mentioned predetermined rolling speed. The rolling reduction ratio in the first and second passes is a little less than 10%, but from the third pass onwards, rolling is performed at a reduction ratio of 15% or more, and the final rolling load is in the range of 600 to 700 tons. Cold rolling could be performed stably up to pass. FIG. 2 is a photomicrograph substituted for a drawing showing the surface properties of a cold-rolled Ti plate obtained by this method, and shows excellent surface precision with almost no oil pits.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the relationship between cumulative strain during cold rolling, exit side plate thickness, and average rolling pressure, and Fig. 2 is a drawing-substituting microscope showing the surface texture of the rolled plate (Ti plate) obtained by the present invention. The photograph and FIG. 3 are micrographs used as drawings showing the surface properties of a rolled plate (Ti plate) obtained by the conventional method.

Claims (1)

[Claims] 1. In a method of cold rolling a dense hexagonal metal plate using a mill roll that has been ground, by rolling without supplying rolling oil or supplying rolling oil with poor lubricity at the initial stage of rolling, A dense hexagonal metal plate characterized in that a homogeneous adhesion layer is formed on the surface of the rolling roll by a part of the surface metal of the material to be rolled, and then cold rolling is performed while supplying a lubricant with excellent lubricity. cold rolling method.