JPS5837384B2 - Continuous annealing method for titanium and titanium alloy strips - Google Patents

Description

Detailed Description of the Invention The present invention provides titanium and titanium alloy strips (hereinafter abbreviated as titanium strips) hardened by cold rolling.
The present invention relates to a method of annealing a titanium strip without coloring the surface and to obtain a surface quality that is advantageous for use, and particularly relates to a continuous annealing method in which a hardened layer is simultaneously formed on the surface of a titanium strip.

Conventionally, titanium tubes are manufactured as welded tubes or seamless tubes, and welded tubes are manufactured by cutting a titanium strip into strips, then roll forming, and then performing, for example, TIG welding.

In many cases, the mechanical properties of the titanium materials used for these welded pipes are controlled primarily by controlling only the amount of oxygen, and these materials are generally referred to as industrially pure titanium.

However, if special corrosion resistance such as crevice corrosion is required, a titanium alloy containing a small amount of palladium may be used.

These titanium and titanium alloys are cold rolled into thin sheets, wound into coils, and often batch annealed in high vacuum, but may also be annealed in high purity inert gas.

All of these methods are aimed at preventing the reaction between titanium material and oxygen or nitrogen.

By this conventional annealing method, a titanium strip of uniform material quality can be obtained without surface coloring. However, when this titanium strip is used as a raw material for manufacturing a welded titanium tube, for example, the annealed titanium l-IJ tube Because titanium is soft, surface scratches are likely to form during the forming process using forming rolls, correction rolls, etc. As a result, these scratches remain on the surface of manufactured welded titanium tubes, reducing the commercial value, and in general, titanium Because of their excellent corrosion resistance, pipes tend to become thinner, and the occurrence of such surface flaws is undesirable because it reduces the effective wall thickness.

In addition, titanium tubes are often used as heat transfer tubes for heat exchangers, but when assembling the heat exchanger, surface flaws may occur when the titanium tubes are stretched and inserted into tube sheets or buttress plates. It was something that could be granted.

From the above-mentioned viewpoints, the present invention provides a continuous annealing method that solves the problem of titanium strips treated by conventional annealing methods, that is, the problem of easy surface scratches. When continuously annealing the subsequent titanium strip in a gas atmosphere, the heating temperature was set to 75%.
The temperature is 0 to 850℃, and the atmospheric gas is 30 to 200p.
I) argon containing III oxygen, or 30-2
By using argon containing 00pIIIl of oxygen and 0.5 to 10% by volume of nitrogen, oxygen or both oxygen and nitrogen are dissolved in solid solution on the surface of the titanium strip after annealing without surface coloring. The present invention is characterized by a continuous annealing method for titanium strips in which a hardened layer is formed, and this hardened layer makes it extremely difficult for surface defects to occur.

Note that in batch annealing, when gas is introduced, only the edges of the coil are hardened, but in continuous annealing, where the entire surface of the material is exposed to the atmospheric gas, the reaction with the gas occurs uniformly over the entire surface, which is even more effective. It has the following.

Further, when carrying out the continuous annealing method of the present invention, it is desirable that the hardness of the surface hardened layer formed on the titanium strip after annealing is 240 to 350 in Witzkers hardness. This is because if the hardness is less than 240, the formation of surface flaws cannot be controlled, whereas if the hardness exceeds 350, the titanium strip itself tends to become brittle.

In addition, in the continuous annealing method of this invention, the heating temperature is set to 75%.
The reason for limiting the temperature to 0 to 850°C is to provide sufficient hardness to the surface of the natan strip to prevent surface flaws from forming in the 750°C heat and temperature history. On the other hand, if the heating temperature exceeds 850℃, abnormal length of crystal grains will occur, the yield strength will decrease significantly, and it will be difficult to control the tension applied to the strip during continuous annealing. This is due to reasons such as:

In addition, the oxygen content in the argon atmosphere gas was also increased to 30
The reason for limiting the nitrogen content to ~200ppIn and 0.5 to 10% by volume is that oxygen content of less than 30Wm and nitrogen content of less than 0.5% by volume are sufficient to prevent surface flaw formation. Surface hardening layer, i.e. Witzkaas hardness 2
A surface hardening layer of 40% or higher cannot be formed on the surface of the titanium l-IJ, while a surface hardening layer of 200% or higher cannot be formed for oxygen.
When pIM[l and nitrogen exceed 10% by volume, the formation of titanium oxide or titanium nitride on the surface of the titanium strip becomes significant.
This is because, in addition to the surface becoming colored, the titanium strip also becomes brittle, making it impossible to form the desired hardened surface layer.

Next, the continuous annealing method of the present invention will be explained using examples and comparing with comparative examples.

Example ASTM-B-265- with a plate thickness of 0.6 mm after cold rolling
Grade 2 pure titanium coils were prepared, and these coils were unwound and passed continuously through a Matsufuru continuous annealing furnace for a heating holding time of 30 seconds, under the operating conditions shown in Table 1. Annealing treatment was performed.

In Table 1, Comparative Methods 1 to 6 were conducted under operating conditions in which the conditions marked with * were outside the range of conditions defined in the present invention.

Next, for each of the pure titanium strips annealed under various conditions, the Witzkars hardness of the surface and center of the thickness was measured.
The state of coloration on the strip surface was observed, and the results are shown in Table 1.

As is clear from the results shown in Table 1, Comparative Methods 1 and 2 have lower oxygen content and nitrogen content in the argon atmosphere gas than the range of the present invention, and the heating temperature is lower than the range of the present invention. Comparison method 5 that falls below the range
In this case, it is not possible to form the desired hardened surface layer, so the hardness of the surface area is insufficiently improved compared to the center of the thickness of the strip, and it is difficult to prevent the formation of surface defects. On the other hand, in Comparative Methods 3 and 4, the oxygen content and nitrogen content in the argon atmosphere gas were higher than the range of this invention, and the heating temperature was also higher than the range of this invention. In the case of Comparative Method 6, the surface hardening layer becomes too thick and the difference in hardness between the surface and center of the strip becomes large, making the strip itself brittle and causing oxides and nitrides to form. The surface of the strip became colored depending on the shape of the object, and Comparative Method 6 caused grain coarsening of the strip.

On the other hand, in Methods 1 to 9 of the present invention, a hardened layer with an appropriate hardness that is extremely hard to cause surface flaws is formed on the surface of the strip, and moreover, a hardened layer is formed on the surface of the strip and at the center of the thickness. The difference in hardness is also appropriate, and a strip in a good annealed state without coloring can be obtained.

These results can naturally be expected for, for example, a titanium alloy made by adding a small amount of palladium to pure titanium.

As mentioned above, according to the continuous annealing method of the present invention, an annealed state in which a colorless hardened surface layer is formed that is extremely resistant to surface flaws during the manufacture of welded titanium tubes or when assembling products thereof, etc. Industrially useful effects such as the ability to continuously produce titanium strips and titanium alloy strips at low cost are brought about.

Claims (1)

[Claims] 1. When continuously annealing titanium and titanium alloy strips after cold rolling in a gas atmosphere, the heating temperature was set to 75%.
The temperature is 0 to 850℃, and the atmospheric gas is 30 to 200p.
Argon containing lllll of oxygen, or 30-2
By using argon containing 00ppII1 of oxygen and 0.5 to 10% by volume of nitrogen, the above steps I-IJ
A continuous annealing method for titanium and titanium alloy strips, characterized in that an arbitrary hardened surface layer is formed on the surface of the tip and annealing is performed without coloring the surface.