JPH0777642B2 - A method for preventing edge cracking in cold rolling of difficult-to-process materials - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention prevents edge cracking and increases the total rolling reduction when cold-rolling a difficult-to-work material such as titanium / titanium alloy. The present invention relates to a cold rolling method which enables the obtained rolling and omits the steps of annealing and side trimming.

(Prior Art) Generally, when a difficult-to-process material such as titanium / titanium alloy is cold-rolled, ear cracking often occurs. Once ear cracking occurs in the material, the strip breaks in the worst case, and if not, the trim margin increases and the yield decreases.

It is empirically known that such ear cracking usually occurs when the total rolling reduction becomes higher than a certain level. As a measure against this, when the approach taken is approaching the total rolling reduction that is expected to cause edge cracking, the rolling reduction and rolling speed are reduced, and the condition of the side edge portion of the material is tensionless and Carry out rolling while watching. Then, the process of cutting off the rolling early, annealing, performing side trimming as necessary, and then performing cold rolling again is repeated to manufacture a product having a target plate thickness.

Thus, cold rolling of difficult-to-work materials is difficult,
A rolling method was desired for efficiency.

As a technique for preventing edge cracks in cold rolling of a difficult-to-work material, a method of joining a dissimilar material having large deformability to the side edge of the material and cold rolling, as disclosed in JP-A-52-41149. , As disclosed in JP-A-59-92104,
There is known a method of heat-treating a material side edge portion, and a method of preliminarily width-reducing a raw material (starting material) as disclosed in JP-A-57-14402.

The inventors have confirmed the effectiveness of these prior arts by experiments.

The hard-to-work material used in the experiment is Ti-5Al-2.5Sn, and the plate thickness is 5 mm and the plate width is 50 mm. This material, roll diameter: 160m
m, backup roll diameter: 480 mm, four-stage rolling mill, using mineral oil as a lubricant, cold rolling at a rolling reduction of 5 to 10% per pass. Ear cracking occurs at a total rolling reduction of around 40%. It has been found.

When the longitudinal cross-sectional shape of the material side edge is rectangular, the depth of ear cracking is smaller and the trimming margin is smaller. However, it was impossible to prevent ear cracking itself.

Therefore, first, when titanium was welded to the side edge portion of the material and cold-rolled, ear cracks did not occur even at a total reduction of 70%.

However, considering the actual operation, it is very difficult and impractical to weld the titanium material to both side edges of the strip over the entire length of the strip.

Next, when both edges of the strip were heat-treated and rolled, no effect was observed. The heat treatment of both edges of the strip has resulted in a work-hardening process with a total reduction ratio of 30.
Although it may be effective if it is carried out in the order of%, in actual operation, it is necessary to devise a method to prevent quenching of the rolled material from the point where the rolling lubricant is used, and this method is also practical. is not.

After applying a width reduction of 2 mm to the material, and then rolling,
Ear cracks did not occur up to a total rolling reduction of 45%. However, the plate thickness of both side edge parts of the strip, which is obtained by applying a width reduction of 2 mm to the material, is 1 mm larger than the center part in the width direction, and in actual rolling, it is not only impossible to wind the strip, There is a problem that the shape (flatness) of the strip is deteriorated.

As mentioned above, there is still a problem in the conventional technology for preventing the ear crack in the cold rolling of the difficult-to-work material.

(Problems to be Solved by the Invention) The present invention can prevent the occurrence of edge cracks during cold rolling of a difficult-to-work material such as titanium / titanium alloy and can achieve a high total rolling reduction. In order to provide a process capable of omitting the annealing process and the side trimming process.

(Means for Solving Problems) The gist of the present invention is to provide a width reduction device on the inlet side of a rolling mill when cold-rolling a side-trimmed difficult-to-process material, so that the material has ΔW / H A method for preventing edge cracking in cold rolling of a difficult-to-work material is characterized by having a pass for applying a width reduction in the range of 0.05 to 0.50.

However, ΔW: width reduction amount [mm] H: plate thickness [mm] The present invention will be described in detail below.

The inventors conducted a cold rolling experiment of a difficult-to-work material using the rolling mill shown in FIG.

In FIG. 1, 1 is a difficult-to-process material, which is a Ti-5Al-2.5Sn material in this embodiment. 2 is a width reduction device, which is provided on the rolling mill entrance side. Reference numeral 3 is a work roll, and 4 is a backup roll. Roll length is 400m each
m. The above-mentioned difficult-to-process material having a plate thickness of 5 mm and a plate width of 50 mm was cold-rolled with a four-high rolling mill having a work roll diameter of 160 mm and a back-up roll diameter of 480 mm. Strip tension, 10 kgf / mm ² in the rolling mill inlet side is 15 kgf / mm ² at the delivery side of the rolling mill.

The rolling reduction per pass is 5 to 10%. Rolling speed is 10
m / min, rolling lubricant is a 5% emulsion of mineral oil (60
C) was used.

From this experiment, it was found that ear cracking occurs at a total reduction rate (cold rolling rate) of about 30%. The occurrence of edge cracking at a total rolling reduction of 10% lower than that in the cold rolling experiment of the cut sheet (sheet) is considered to be due to the effect of the tension applied to the strip.

Therefore, the inventors decided to apply width reduction to the strip at the stage when the total reduction rate exceeds 25% during multi-pass cold rolling.

The results are shown in FIGS. 2 (a) and 2 (b). That is, by performing the width reduction, the plate thickness at the plate end portions such as B and C becomes smaller than the plate thickness at the center of the plate from the state of A in which the entrance side plate profile as shown in FIG. 2 (a) is not subjected to the width reduction. Is also the reason to change to a thicker state. Compared with A, the thickness of the plate edge is 0.1 mm for B and C for C
0.2mm thicker. The exit side stress distribution when such an entrance side plate profile was rolled was obtained by calculation and is shown in FIG. 2 (b).

The calculation is performed with the slit model considering the tension feedback mechanism shown in Chapter 4 of "Theory and practice of strip rolling" (published by the Iron and Steel Institute of Japan). The mechanism by which the stress at the plate end is reduced by increasing the plate thickness at the plate end can be explained by the formula (1) as shown on page 100 of the above-mentioned document.

σ _F (j) = Ε _P (Δ _L −Δε _L (j)) + σ _FM (1) In the equation (1), Ε _P is the Young's modulus of the rolled material, σ _FM is the exit side average stress, and Δ _L is This is the elongation difference ratio in the divided section where the outgoing stress matches σ _FM . σ _F (j) and Δε _L (j) are the j-th outgoing stress and elongation difference rate of the slit model.

By increasing the plate thickness at the plate end, the difference in elongation (strain) at the plate end increases. Therefore, Δε in equation (1)
_{Since L} (j) becomes large, σ _F (j), that is, the outgoing stress at the plate end portion decreases.

Under the rolling conditions of this time, the stress distribution on the outgoing side of A, which is not subjected to the width reduction, is very large at the plate edge, whereas the stress on the outgoing side of B and C subjected to the width reduction is small. Entry side plate thickness 3.65 mm
At the time of (total rolling reduction of 27%), a plate profile as shown in FIG. 2 (a) was width-rolled and rolled at a rolling reduction of 6%. However, C did not cause ear cracking. By repeating such an experiment, the optimum entrance side plate profile which does not cause ear cracking and has no problem in strip shape was examined.

Based on this result, the inventors set ΔW / H to be the width reduction amount and H to be the plate thickness on the rolling mill entrance side, and ΔW / H was 0.05 to 0.5.
We found that the range of 0 is good. If ΔW / H is less than 0.05, there is no effect of preventing ear cracking, while if it exceeds 0.5, the strip shape (flatness) is impaired and the winding shape is deteriorated.

As mentioned above, during the rolling schedule in which ear cracking is predicted to occur, by applying width reduction to the strip with ΔW / H in the range of 0.05 to 0.50, ear cracking does not occur even at a total reduction of 50%. It was Surface cracking occurred when the total rolling reduction reached 60%. Therefore, the range of the effect of the present invention is a total rolling reduction of up to about 60%.

In this way, when using the conventional technology,
According to the present invention, it was possible to perform cold rolling up to about 60% in total rolling reduction, which could only be cold rolled up to about 30%.

The inventors consider this mechanism as follows.

That is, as shown in FIG. 2, by increasing the plate pressure at the strip-side edge portion, the stress state applied to the strip-side edge portion during rolling changes from tensile stress to compressive stress. This compressive stress suppresses the growth of fine cracks generated during rolling.

In FIG. 1, the width reduction device is provided only on the entrance side of the rolling mill, but in the case of a reverse mill,
It goes without saying that it is desirable to provide a width reduction device on both the inlet and outlet sides of the rolling mill.

(Effects of the Invention) Since the present invention is configured and operated as described above, a difficult-to-process material such as titanium / titanium alloy is intervened with a step such as annealing or side trimming in between. Since it can be cold-rolled without any need, the production process can be simplified, and an extremely great effect can be achieved in terms of productivity, cost, and the like.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of the present invention, and FIG. 2 is a stress distribution change diagram showing the cause of the present invention. In the figure, 1: material, 2: width reduction device, 3: work roll,
4: Backup roll.

Claims (1)

[Claims] 1. When cold-rolling a side-trimmed difficult-to-process material, a width reduction device is provided on the inlet side of the rolling mill, and a path is provided to apply width reduction to the material in the range of ΔW / H = 0.05 to 0.50. A method for preventing edge cracking in cold rolling of a difficult-to-work material characterized by being hardened. ΔW: Width reduction (mm) H: Plate thickness (mm)