JPH0318524B2 - - 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, Zr, and Zr alloy. Further, the rolling speed in this specification means the sheet passing speed on the exit side of the rolling mill. 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) is produced on the free surface of the rolled material in contact with the rolling material. 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 significantly reduces the surface precision of the plate, making it necessary to re-grind the rolling rolls. 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 accuracy of rolled products, if the rolling roll shows seizure phenomenon or even if only sticking occurs, the rolling operation should be immediately interrupted and the rolling roll should be replaced with a polished finishing roll. In addition to replacing the rolls and restarting operations, the rolling rolls with stuck metal to be rolled are being 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 oil and setting operating conditions that can suppress oil pits as much as possible and prevent seizure. However, attempts to suppress oil pits cause seizures, and attempts to prevent seizures tend to cause oil pits to occur, which is a problematic tendency. It was thought that it would be extremely difficult to improve this to a certain extent. 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 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 a fixed layer of the rolled metal is formed homogeneously over a certain distance of rolling, 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 performing cold rolling at a rolling speed of less than 100 m/min using a lubricant, a homogeneous adhesion layer is formed on the surface of the roll by a part of the surface metal of the material to be rolled, and then the lubricant is changed. 100m/
The gist of this method is that cold rolling is carried out at a rolling speed of 1 minute or more. 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 are likely to cause sticking are set at the initial stage of rolling, and rolling is continued for a certain distance under these rolling conditions to improve the roll surface. After that, a uniform coating layer is formed on the rolled metal, and then cold rolling is continued.The formation of the initial fixed coating layer causes oil pits to form, an increase in rolling load, etc. It is possible to cold-roll a dense hexagonal metal plate with extremely excellent surface precision without causing any problems, with smooth operation and high productivity. In the present invention, as a means of forming a fixed coating layer on the surface of the roll at the initial stage of rolling, a method is adopted in which the rolling speed is set at a low speed of less than 100 m/min. In other words, in normal cold rolling, as the speed increases, the amount of lubricant brought into the roll bite increases, making it difficult for sticking to occur (on the contrary, oil pits are more likely to occur), and as the speed decreases, the amount of lubricant brought into the roll bite increases, making it difficult for oil pits to occur. It is known that as the amount of lubricant carried in decreases, it becomes more likely that sticking will occur (on the contrary, oil pits will be less likely to occur). The rolling speed at which seizure occurs varies depending on the type of plate to be rolled and the type of lubricant, but various experiments have shown that as long as dense hexagonal metal plates are used, the lubricant provides good lubrication. Even when using a rolling agent, the rolling speed is reduced to 100%.
If the setting is less than 90 m/min (more definitely less than 90 m/min), sticking will occur (see Figure 5 below), and if rolling is carried out for a predetermined time (or a predetermined length) in this state, the rolling rolls will stick. It has been confirmed that it is possible to form a uniformly fixed coating layer of dense hexagonal metal on the surface. After the adhesion coating layer is formed, it can be used for 100m/100m without changing the type of lubricant.
By continuing cold rolling at a speed of more than 1 minute,
Thereafter, a rolled plate with extremely high surface precision can be smoothly produced with almost no oil pits or seizures. FIG. 1 is a graph conceptually showing such changes in rolling speed, in which a so-called dummy tail is supplied immediately after the start of rolling, and then a lubricant is supplied at the same time as the start of rolling. At the start of this rolling, the rolling speed is set to less than 100 m/min (a speed that tends to cause sticking), and rolling is performed at low speed for a predetermined period of time to make the metal to be rolled stick to the surface of the rolling roll. After the fixed coating layer is almost uniformly formed on the roll surface, the rolling speed is set to 100 m/min or higher and cold rolling is continued without changing the type of lubricant. Note that the length of time and rolling speed for forming a homogeneous fixed coating layer vary depending on the type of metal sheet to be rolled, as described above.
The execution time of low-speed rolling may be appropriately determined depending on the rolling speed at that time. That is, as mentioned above, sticking occurs more easily as the rolling speed is lower, so the slower the initial rolling speed to form a homogeneous adhering coating layer, the more the sticking occurs as indicated by broken lines 1 and 2 in Fig. 1. Similarly, the acceleration time T to increase the rolling speed to 100 m/min or more can be accelerated as T1 and T2. The gist of the present invention is to set the rolling speed at the initial stage of rolling and the rolling speed after the period of steady rolling as described above, with the former being low and the latter being high, and during this period the lubricant is changed, i.e. No changes will be made, including changing the base oil from mineral oil to beef tallow. Figure 2 shows a plate thickness of 0.87 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 200 m/min. FIGS. 3 and 4 show microscopic photographs of the surfaces of the cold-rolled sheets obtained. The surface of the plate rolled with coating rolls (Fig. 3) shows no oil pits at all, indicating excellent surface precision, but the surface of the plate rolled with grinding rolls (Fig. 4) shows a depth of 8 to 10 μm.
Many oil pits can be seen. As is clear from FIG. 2, at low cumulative rolling reductions, the coating roll has a higher rolling pressure relative to strain than the ground finish 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 by using coated rolls is not necessarily clear, but the fine metal powder that separates from the surface of the rolled material within the roll bit provides lubrication. This is thought to be due to the formation of a chemical agent and metal soap, which reduces frictional resistance in the boundary lubrication region. In addition, in FIG. 2, 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 in the roll bite area 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 the amount of plastic deformation of the rolled material is small in the early stage of rolling, so a sufficient amount of metal soap is not produced. 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 irregularities that occur when the surface of the rolled material, which is softer than the roll surface, is pressurized by the high-pressure rolling oil present in the roll bit. In the roll, since a relatively soft metal coating layer exists between the hard roll base material and the material to be rolled, this coating layer deforms preferentially and suppresses deformation of the material to be rolled. Table 1 shows the number of roll rotations required to form a fixed coating layer when the most common lubricant for cold rolling Ti plates, tallow-based emulsion (2%), is used and the rolling speed is varied. The results of the investigation are shown, and FIG. 5 shows the change in the friction coefficient of the work roll surface at this time. That is, when a fixed coating layer is formed on the work roll surface, the surface friction coefficient changes due to this coating layer, and as a result, the rolling load changes. In other words, changes in rolling load immediately appear as changes in the friction coefficient, so
The state of formation of the fixed coating layer can be determined by this change.

[Table] As is clear from Table 1 and Figure 5,
When using 2% beef tallow emulsion as a lubricant, sticking occurs if the rolling speed is less than 100 m/min, more certainly 90 m/min or less, and by continuing rolling in this state for a while, A uniform, fixed coating layer can be formed on the roll surface. However, if the rolling speed is set to 100 m/min or higher, no adhesion itself occurs, and no adhesion coating layer is formed, making it impossible to achieve the object of the present invention. The reason why the adhesion situation changes suddenly at a rolling speed of around 100 m/min is considered to be as follows. In other words, when rolling a metal plate, if more than a certain amount of lubricant flows into the roll bit, a complete oil film is formed between the work roll and the material to be rolled, resulting in a so-called fluid lubrication state, and the lubricant When the amount of inflow is below a certain value, an oil film cannot be formed on the entire rolling surface, and metal contact occurs locally, resulting in a so-called boundary lubrication state, and it is thought that sticking occurs at this time. On the other hand, the inflow of lubricant into the roll bite during rolling is significantly affected by the rolling speed.
At low speeds, lubricant entrainment becomes difficult and a boundary lubrication state is likely to occur.On the other hand, as the speed increases, the amount of lubricant inflow increases and a fluid lubrication state is likely to occur, but this lubrication state is similar to that at around 100 m/min. Since the fluid lubrication transitions from fluid lubrication to the occurrence of boundary lubrication at the boundary of the amount of agent inflow, it is thought that there will be intermittent changes at that point, and sticking will occur on the low speed side and no sticking will occur on the high speed side. It will be done. For this reason, in the present invention, the rolling speed "less than 100 m/min" at which the lubrication state shifts to boundary lubrication is required as a condition for forming a fixed coating layer.
It stipulates that The process of forming the fixed coating layer as described above only needs to be performed at the beginning of the first pass after installing the grinding finishing roll, and is not necessary after the second pass and during rolling of the second and subsequent coils. Therefore, the decrease in productivity due to the decrease in rolling speed can be almost ignored. In addition, rolling after forming the adhesive coating layer as described above is carried out under normal rolling conditions, such as selecting a speed of 100 m/min or more from the viewpoint of increasing productivity and preventing further adhesion, or using high-quality lubrication. It is recommended to use an agent. Note that the maximum speed is determined by equipment constraints and rolling technology constraints such as heating 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 setting the speed to less than 100 m/min at the beginning of cold rolling and coating the roll surface with the metal to be rolled, subsequent cold rolling can be carried out smoothly without oil pits or seizure. It is possible to obtain a dense hexagonal metal rolled plate with excellent surface precision. 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 mm.
Cold roll the sheet to a thickness of 0.5 mm using cold rolling rolls according to the rolling schedule shown in the table below.

[Table] In the first pass, the rolling speed was set to 50 m/min only while rolling about 150 m from the tip of the coil, and rolling was performed while supplying rolling oil to uniformly coat the roll surface with Ti, and then the rolling speed was increased. The rolling speed was increased to 300 m/min and rolling was continued while supplying rolling oil. As the rolling oil, a 2% emulsion of beef tallow oil (viscosity 60cst, viscosity 60cst,
at 40°C) was supplied by spray. Further, from the second pass onwards, rolling was carried out while supplying rolling oil from the tip of the coil at a speed of about 300 m/min. 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. 3 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 conceptual explanatory diagram showing changes in rolling speed over time when carrying out the present invention, Fig. 2 is a graph showing the relationship between cumulative strain during cold rolling, outlet plate thickness, and average rolling pressure; Figure 3 is a photomicrograph that substitutes for a drawing showing the surface texture of a rolled plate (Ti plate) obtained by the present invention, and Figure 4 is a photomicrograph that substitutes for a drawing that shows the surface texture of a rolled plate (Ti plate) obtained by the conventional method. , Figure 5 is 2
9 is a graph showing changes in the coefficient of friction on the work roll surface when rolling speeds are varied using % beef tallow emulsion.

Claims (1)

[Claims] 1. In a method of cold rolling a dense hexagonal metal plate using a mill roll that has been finished with grinding, by performing cold rolling at a rolling speed of less than 100 m/min using a lubricant in the initial stage of rolling, Dense rolling, characterized in that a homogeneous adhesion layer is formed on the surface of the roll by a part of the surface metal of the material to be rolled, and then cold rolling is performed at a rolling speed of 100 m/min or more without changing the lubricant. Cold rolling method of hexagonal metal sheet.