JPS60255919A - Method for preventing nitriding in annealing stainless steel - Google Patents

Abstract

PURPOSE:To prevent the generation of nitriding on the surface of a stainless steel material by producing a temper color on the surface of steel with the control of the dew point before the temp. is increased to a temp. at which the nitriding reaction occurs when the stainless steel material is annealed in a nitrogen-contg. atmosphere. CONSTITUTION:A stainless steel material in heated in a nitrogen-contg. atmosphere, and steam is blown in to keep the dew point at an appropriate temp. in the range of about 0-50 deg.C when the surface temp. reaches about 300 deg.C. The temp. of the steel material is kept at an appropriate fixed temp. in the range of about 450-600 deg.C for suitable hours to form an oxide film on the surface and to develop a temper color. When the oxide film is formed, the atmospheric gas is immediately dishcarged to the outside of the furnace, and a dry common atmospheric gas is introduced to return the dew point to a specified value. The annealing is then carried out. Consequently, the nitriding is not generated on the stainless steel material, and the specified annealing can be carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing nitridation during annealing treatment of stainless steel materials.

During annealing of stainless steel materials, AX gas, HNX, etc. are used to prevent carburization, decarburization, oxidation, etc.
This is carried out using gas, DX gas, etc. as an atmospheric gas. The treatment temperature is around 800°C, which is higher than that for general annealing of steel materials.

However, these atmospheric gases all contain several percent or more of nitrogen (Nz) gas (for example, AX gas consists of 75% N2 and 25% N2), so during the annealing process, the material to be treated may be nitrided. is born. The temperature at which nitriding occurs varies depending on the type of steel, but for example, when a 5US304 stainless steel strip coil is annealed in an AX gas atmosphere, nitriding is observed from around 750'C, and after annealing at 850°C for 4 hours. The increase in nitrogen content often reaches 500 ppm or more. Such a nitriding phenomenon causes embrittlement of the treated material, decrease in ductility, decrease in workability, and undesirable changes in material quality, which has been a problem in annealing stainless steel materials.

The present invention solves the above problems and provides a method for preventing nitridation during annealing of stainless steel materials performed in an atmospheric gas containing nitrogen gas.

The method for preventing nitridation of the present invention is to adjust the dew point of the atmospheric gas before the surface temperature of the material to be treated reaches the nitriding reaction temperature during the heating temperature raising process in annealing the stainless steel material. Another feature is that a temper color is produced on the surface of the material to be treated.

That is, the nitridation prevention method of the present invention utilizes the fact that nitridation does not occur on the surface of a stainless steel material exhibiting a temper color. As is well known, temper color is a color that is produced by an extremely thin oxide film that forms on the surface of a treated material by heating it to an appropriate temperature (approximately 300°C or higher) in an atmospheric gas that has a certain oxidizing power. It is.

The annealing treatment of the stainless steel material in the present invention is carried out in the same manner as usual, using AX gas, HNX gas, DX gas, or the like as the furnace atmosphere gas. These atmospheric gases are adjusted to a dry state with a dew point of approximately -20 to -50°C in order to decarburize the surface of the treated material and prevent the formation of scale (thick oxide layer). Even if the treated material is heated to about 300° C. or higher, it is not possible to form an oxide film necessary to prevent subsequent nitridation. In the present invention, in order to form this oxide film, the dew point of the atmosphere in the furnace is adjusted during the heating process of the material to be treated. This dew point adjustment is performed, for example, by attaching a steam supply pipe to the annealing furnace and blowing steam into the furnace through the pipe. The dew point can be adjusted while heating the material to be treated.
The treatment may be carried out at a time when the surface temperature of the material to be treated reaches about 300°C.

Further, the dew point may be adjusted to a predetermined value before starting heating and raising the temperature. It goes without saying that the formation of the oxide film by controlling the dew point must be completed before the surface temperature of the material to be treated reaches the nitriding reaction (d-layer formation temperature of about 750° C.).

Strictly speaking, the six points of atmospheric gas necessary for the formation of an oxide film during heating and temperature rise vary depending on the steel type, chemical composition, and applied temperature of the material to be treated, but the range is approximately 0 to 50°C. Better results can be obtained each time the adjustment is kept within the range. Setting the dew point to 0°C or higher is because if it is lower than that, it is difficult to form an oxide skin II9 that has reliable nitridation prevention ability due to the lack of oxidizing power in the atmosphere. On the other hand, setting the dew point to 50°C or lower is This is because, if it exceeds that range, decarburization of the surface of the material to be treated and generation of thick scale due to excessive oxidation will occur.

Formation of an oxide film on the surface of the material to be treated by adjusting the dew point is carried out as described above when the surface temperature of the material to be treated reaches the nitriding reaction temperature (approximately 7
50℃), but in order to reliably prevent decarburization and scale formation on the surface of the treated material due to temperature rise, the process should be completed before the surface temperature of the treated material reaches approximately 650℃. It is desirable to complete the formation of the oxide film and then immediately replace the atmospheric gas with a normal dry atmospheric gas.

In addition, in order to form the required oxide film while avoiding decarburization and excessive oxidation on the surface of the treated material as described above, it is necessary to heat the material at an appropriate temperature range during heating, for example at a constant temperature of 450 to 600°C. It is also a preferable method to complete the oxide film by holding for an appropriate time, and then replace the atmospheric gas with a normal dry atmospheric gas to return the dew point to the normal state and restart heating and temperature raising.

FIG. 1 shows an example of the heating pattern and dew point adjustment of the material to be treated during annealing. In the figure, curve (a) shows the temperature of the material to be treated, and curve (b) shows the dew point of the atmospheric gas. After the heating of the material to be treated is started, water vapor is blown in when the surface temperature of the material to be treated reaches approximately 300°C to maintain the dew point at an appropriate value of 0 to 50°C. In addition, the temperature of the material to be treated is approximately 450 to 6
By maintaining the temperature at a constant temperature of 00'c for a suitable period of time, an oxide film is formed on the surface. As soon as the formation of an oxide film is observed, the atmospheric gas is immediately discharged to the outside, and normal atmospheric gas for drying is introduced into the furnace to maintain the temperature at six predetermined points (for example, -25°C). At the same time, the heating of the material to be treated is restarted to reach a predetermined annealing temperature (for example, 800 to 850'C), and then maintained at the same temperature for a predetermined time. Since the surface of the material to be treated is coated with an oxide film, the specified annealing treatment can be completed without nitriding even when heated to high temperatures in an atmospheric gas containing nitrogen gas. can.

Next, the nitriding prevention effect according to the present invention will be explained in detail.

An open coil of 13Cr stainless steel strip (equivalent to Sl13403) was heated with AX gas (75
%H2+25%Nz, dew point 25°C) as the atmospheric gas. The nitrogen content (lower value) of the test zone is about 50 ppm. The heating of the material to be treated and the adjustment of the dew point of the atmospheric gas are carried out according to the pattern shown in FIG. First, a steel strip coil (open coil) is placed in a furnace, AX gas is introduced, water vapor is blown into the furnace, and the dew point is set at +20°C. Heating of the copper band coil was started, and when the temperature reached 550°C, the heating was stopped once and the temperature was maintained at the same temperature for 1 hour. As a result, an oxide film is formed on the surface of the copper band coil, producing a blue tempered color.

Next, the atmospheric gas is removed and new AX gas is introduced to maintain the dew point at -25°C, while the copper band coil is heated to raise its temperature and held at a predetermined temperature (T) for a predetermined time to complete annealing. do. The annealing treatment temperature (T) was 820°C, 850°C, and 870'C, and the holding time was set to 4 to 5 hours.

On the other hand, as a comparative example, a similar steel strip coil was annealed according to the conventional general heating temperature increase pattern shown in FIG. 3 without applying a dew point rope for forming an oxide film. The atmospheric gas was the same AX gas (dew point -25°C) as above, and the annealing temperature and holding time were set to the same three conditions as above.

Changes in nitrogen content after annealing of each of the above test steel strips are shown in the fourth table.
As shown in the figure. Copper strip coil of comparative example using conventional method (in the figure, Δ
), an increase in nitrogen content due to nitriding was observed during annealing at 850°C or higher, and at 870°C, the nitrogen content increased by approximately 1.
It has reached 100pp. On the other hand, the steel strip coil treated according to the present invention (indicated by O in the figure) has a temperature of 8
It can be seen that even in the high temperature treatment at 70° C., the increase in nitrogen content is extremely small, a few ppm or less, and nitridation is almost completely prevented.

As described above, according to the method of the present invention, nitridation of the stainless steel strip in an atmospheric gas containing nitrogen gas and the accompanying deterioration of the material can be substantially completely prevented. Furthermore, even in annealing treatment in which an atmospheric gas that does not contain nitrogen gas is used, it is actually difficult to completely seal the actual furnace, and the material to be treated is often nitrided due to atmospheric infiltration. Even in such cases, the problem of nitridation can be solved by applying the method of the present invention to control the dew point and form an oxide film.

[Brief explanation of drawings]

FIGS. 1 and 2 are diagrams showing examples of heating temperature increase and dew point adjustment patterns in the implementation of the method of the present invention, FIG. 3 is a diagram showing heating temperature increase and dew point adjustment patterns in the conventional method, and FIG. It is a graph illustrating the relationship between annealing temperature and nitrogen increase due to nitriding. Agent: Patent attorney Arata Miyazaki, Haruma → Unema → Figure 3: Jun Kado (C)

Claims (1)

[Claims] (1) In annealing a stainless steel material in an atmospheric gas containing nitrogen gas, a temper color is produced on the surface of the steel material by adjusting the dew point of the atmospheric gas before the nitriding reaction temperature is reached during heating. A nitriding prevention method suitable for annealing stainless steel.