JPH0995729A - Vacuum heat treatment of steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vacuum heat treatment method of a steel material which can restrain or prevent the development of discoloration even in the case of raising the temp. in a short time. SOLUTION: The steel material 4 to be treated is charged into a vacuum furnace 1 by enclosing into a steel-made vessel 5 and on the other hand, an oxygen getter 6 occluding hydrogen is charged into the vacuum furnace 1 and/or the vessel 5 to execute the heat treatment. The hydrogen occluded in the oxygen getter 6 and the outgas (oxygen) discharged from the furnace wall in the temp. raising process are reacted to restrain the lowering of the vacuum degree in the furnace and then, the development of discoloration can be restrained or prevented even in the case of raising the temp. in the short time, and the heat treatment efficiency can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum heat treatment method for steel products, and in particular, it is possible to suppress or prevent discoloration that occurs when high alloy steel products are heat-treated at low temperature without lowering the heat treatment efficiency. The present invention relates to a vacuum heat treatment method for steel materials.

[0002]

2. Description of the Related Art When a steel material is heated in an atmosphere gas, various methods are known as a method for preventing an oxide scale or discoloration generated on the surface of the steel material by reacting with oxygen present in the atmosphere gas.

For example, in Japanese Patent Publication No. 56-35728, in heating pre-plastic working of a hard-to-oxidize steel material whose scale is difficult to remove, the top surface of a steel material having a rectangular plate section is removed.
While the surface is coated with an antioxidant, only the top surface of the steel material is covered with a thin steel sheet whose dimension in the direction perpendicular to the direction of conveyance is slightly larger than the steel material dimension, and it is placed in an atmospheric gas heating furnace. A method of preventing the occurrence of is shown.

Further, Japanese Patent Laid-Open No. 63-11616 discloses a method of preventing discoloration by wrapping a steel material to be treated with aluminum foil and heat-treating it in an atmospheric gas furnace in the final heat treatment of the steel material. Has been done.

[0005]

[Problems to be Solved by the Invention]
The method disclosed in 6-35728 is effective when used for heating prior to hot plastic working, but since the antioxidant is applied, the antioxidant locally adheres to the surface of the steel material after heating. However, it has a drawback that it cannot be applied to the final heat treatment depending on the application of the product steel material due to poor surface quality.

For example, it is made of a high alloy such as a Ni-based alloy or a high Cr-high Ni alloy that is used by being incorporated in a heat generator such as a steam generator of a nuclear power generation facility or a water heater of various equipment. For a small diameter long tube having a length of 20 mm or less and a length of more than 20 m, it is prohibited to perform blasting or polishing for surface cleaning after the final heat treatment in order to prevent residual surface stress. Therefore, the above-mentioned Japanese Patent Publication No. 56-
The technology disclosed in Japanese Patent No. 35728 cannot be applied.

On the other hand, the method disclosed in the above-mentioned Japanese Patent Laid-Open No. 63-11616 can be applied to the product steel material which cannot be subjected to the surface cleaning treatment after the final heat treatment, but it can be applied to the small diameter long pipe. In order to apply heat treatment to a large number of steel products such as 800 pieces at a time in product steel products, it takes a lot of man-hours for packaging using the aluminum foil and has a drawback that it is not industrial. There is. Further, the method disclosed in Japanese Patent Laid-Open No. 63-11616 is heat treatment in an atmospheric gas furnace, the effect of preventing discoloration is unstable, and the nuclear power generation requires extremely strict quality control for its safety. It also has a drawback that it is insufficient to be applied to a small diameter long tube made of a high alloy used by being incorporated in a steam generator in a facility.

Therefore, the final heat treatment of a product steel material such as a high-alloy small-diameter long tube used by being incorporated in the steam generator of the nuclear power generation facility is performed in a vacuum furnace, more specifically, in an inorganic refractory furnace wall. It is carried out using a vacuum truck-type vacuum furnace with a structure.

The vacuum furnace has an excellent effect of preventing discoloration because the oxygen partial pressure in the furnace can be lowered by increasing the degree of vacuum and the amount of oxygen present in the atmosphere is smaller than that in the atmosphere gas furnace. However, it is extremely difficult in operation to raise the degree of vacuum of the industrial batch carriage type vacuum furnace to a high degree of vacuum without discoloration.

Therefore, conventionally, several tens to several hundreds of small-diameter long pipes are stacked on a trolley at upper and lower intervals and placed in a furnace, while the small-diameter long pipes to be treated are made of steel. Metals that are more reactive with oxygen than the higher alloys of the material (eg Ti,
An oxygen getter material made of Cu) was placed in the furnace to heat-treat a large amount of steel materials at once.

However, even when the above batch trolley type vacuum furnace is used and heat treatment is carried out in a state where a metal oxygen getter material having a higher reactivity with oxygen than the steel to be treated is placed in the vacuum furnace. , To increase the efficiency of heat treatment,
Discoloration occurs when heat-treated by raising the temperature for a short time, and it is necessary to raise the temperature for a long time in order to prevent the occurrence of discoloration, and there is a problem that the heat treatment efficiency cannot be increased.

That is, in the heat treatment using the above-mentioned vacuum furnace, usually, heating / heating is started from the time when the degree of vacuum in the furnace reaches a predetermined degree of vacuum, and the temperature is maintained at a predetermined temperature for a predetermined time. Done. In this case, outgas (for example, oxygen, carbon monoxide, etc.) is released from the furnace wall while the temperature in the furnace reaches 150 to 400 ° C during the temperature rising process, and the released amount depends on the specifications of the used furnace. It is a fixed amount. However, when the temperature is raised for a short time, a large amount of this is released at once, and the degree of vacuum in the furnace is extremely lowered despite the presence of the oxygen getter material, and discoloration occurs at this point. On the other hand, when the temperature is raised for a long time, the release of outgas is slowed down, and in combination with the presence of the oxygen getter material, the decrease in the degree of vacuum in the furnace is greatly suppressed, and the occurrence of discoloration is suppressed or occurs. However, the heat treatment requires a long time.

FIG. 3 is a diagram showing an example of the relationship between the temperature inside the furnace and the degree of vacuum. The figure (a) shows the case where the temperature is raised for a short time, and the figure (b) shows the case where the temperature is raised for a long time. Shows.

The present invention has been made in view of the above circumstances, and its object is to provide a vacuum heat treatment method for a steel material capable of suppressing or preventing discoloration even if heat treatment is performed by raising the temperature for a short time in order to increase the heat treatment efficiency. To provide.

[0015]

Means for Solving the Problems As a result of various experiments and researches for achieving the above-mentioned object, the present inventor has found that an oxygen getter material in which hydrogen is occluded in a vacuum furnace, specifically, a hydrogen occluding titanium material. It has been found that when the oxygen getter material is charged and arranged and heat treated, the occurrence of discoloration is suppressed or prevented even if the temperature is raised for a short time. In this case, it was also found that the effect of preventing discoloration from occurring is further improved by housing and encapsulating the steel material to be treated in a steel container, charging it in a vacuum furnace, and performing heat treatment.

The present invention was made based on the above findings, and its gist resides in the following vacuum heat treatment methods for steel materials (1) and (2).

(1) A vacuum heat treatment method for a steel material, characterized in that, when heat-treating a steel material in a vacuum furnace, an oxygen getter material having hydrogen stored therein is placed in the vacuum furnace and heat treated.

(2) When heat-treating a steel material in a vacuum furnace, an oxygen getter material containing hydrogen is charged and arranged in the vacuum furnace while the steel material is contained and enclosed in a steel container. A method for vacuum heat treatment of a steel material, which comprises heat treatment.

In the methods (1) and (2) of the present invention, it is preferable to use, as the oxygen getter material, a metal material having a higher reactivity with oxygen than the steel to be treated.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The heat treatment method of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a schematic view showing an example of an embodiment of the present invention using the batch carriage type vacuum furnace. In the figure, reference numeral 1 is a vacuum furnace that is long in a direction perpendicular to the plane of the drawing, and has a plurality of multi-tiered racks 3 each having one sidewall standing upright.
The opening / closing wall 1a allows entry and exit of the carriage 2 having A steel container 5 accommodating and enclosing a large number of steel materials 4 to be treated is placed on the multi-stage rack 3, and a hydrogen storage oxygen target material 6 is arranged on the side of the steel container 5.

In this state, the vacuum pump 7 is started to increase the degree of vacuum in the furnace, and the degree of vacuum is a predetermined degree of vacuum, for example, 5 × 10 5.
^{When the} temperature reaches ^-5 Torr, the heating means (not shown) raises the temperature to the heat treatment temperature at a predetermined heating rate, and the temperature is maintained at that temperature for a predetermined time.

On the other hand, during this period, the vacuum pump 7 is continuously moved so that the degree of vacuum in the furnace becomes a predetermined degree of vacuum, for example, 1 × 10 ^-5 Torr or more.

In this case, if the temperature rising rate is increased, a large amount of outgas is discharged from the furnace wall at once, as described above. However, in the present invention, the hydrogen storage oxygen target material 6 is arranged in the furnace, and the hydrogen stored in the hydrogen storage oxygen target material 6 and the oxygen in the outgas react rapidly to form water vapor. Then, this steam is discharged to the outside of the furnace by activating the atmospheric gas discharge pump 8. As a result, the degree of vacuum in the furnace, in other words, the oxygen partial pressure in the furnace does not drop extremely. Further, the atmosphere in the steel container 5 is hardly affected by the outgas release. Therefore, discoloration does not occur in the steel 4 to be treated, or even if it occurs, it becomes extremely slight. As a result, in the present invention, the heating rate can be increased and heating can be performed, and the heat treatment efficiency can be increased by the amount by which the heating rate can be increased.

In the example shown in FIG. 1, the steel 4 to be treated is contained and enclosed in the steel container 5 and heat-treated, but the steel container 5 can be omitted. In this case, although the degree of discoloration is slightly deteriorated as compared with the case where the steel container 5 is used, there is no problem.

The steel container 5 comprises a bottom box 5a and a lid box 5b. The inner dimension of the lid box 5b is slightly larger than the outer dimension of the bottom box 5a, as shown in FIG. Thus, the lid box 5b is fitted on the open end side of the bottom box 5a to cover the open surface thereof. For this reason, there is a minute gap between the fitting surfaces of the two, and the air in the container is also degassed when the vacuum pump 7 is degassed, so that the degree of vacuum becomes the same or almost the same as the degree of vacuum in the furnace. Become. On the other hand, when a large amount of outgas is released from the furnace wall during the temperature rising process and the degree of vacuum in the furnace sharply decreases, the gap between the fitting surfaces is very small.
The decrease in the degree of vacuum in the container is suppressed.

As the steel container 5, it is preferable to use a material having a lower oxidation resistance than the steel 4 to be treated. For example, when the steel 4 to be treated is a Ni-based alloy, it is preferable to use a container made of stainless steel. This is because when a container made of low-alloy steel or carbon steel, which is inferior in oxidation resistance, is used, a large amount of scale is generated in the container itself, which is removed and adheres to the surface of the steel material 4 to be treated, which causes deterioration of the surface quality. This is because there is almost no fear of this in the case of stainless steel.

The arrangement position of the hydrogen storage target material 6 in the furnace does not have to be specified in particular, but it is preferable to arrange it as close to the furnace wall as possible. This is to quickly capture oxygen in the outgas discharged from the furnace wall and avoid a decrease in the degree of vacuum in the furnace as much as possible.

As the hydrogen storage oxygen target material, any metal can be used as long as it can store a large amount of hydrogen, but a metal more reactive with oxygen than the steel 4 to be treated is used. preferable. This is because, in addition to the stored hydrogen, the metal itself preferentially reacts with the oxygen in the furnace in preference to the steel 4 to be treated and reduces the oxygen that reacts with the steel 4 to be treated, causing more discoloration of the steel 4 to be treated. It is difficult to do. For example, when the steel 4 to be treated is a Ni-based alloy, it is preferable to use Ti, Cu or the like, and particularly the heat treatment temperature of the product (80
It is most preferable to use Ti, which has a melting point higher than that of Cu (1083 ° C.), which is 1660 ° C., and is stable at high temperatures.

The amount of hydrogen to be occluded does not need to be specified, but the larger the amount, the more preferable. For example, JIS
-H4650, H4657, H4630, H46
Rods made of pure Ti or Ti alloy specified in 31, etc.,
When using a tube or plate, the hydrogen supply flow rate is 30 to 1
It is possible to use the one that has been subjected to the hydrogen storage treatment of heating and holding for 0.5 to 2 hours in a hydrogen furnace of 50 Nm ³ / hr and a temperature of 300 to 900 ° C. Of course, various commercially available hydrogen storage alloys can be used, but they are expensive, so it is economical to use the above-mentioned appropriate metals after hydrogen storage treatment by heating and holding them in a hydrogen furnace.

[0031]

[Example] For a small diameter long tube made of NCF690TB having an outer diameter of 17.4 mm, a wall thickness of 1.02 mm and a length of 24000 mm, a batch truck type vacuum furnace whose schematic diagram is shown in FIG. While heating and holding 800 tubes at 725 ° C for 10 hours, various discoloration prevention measures are taken, while the temperature rising rate is variously changed to perform the final heat treatment.
The minimum value of decrease in the degree of vacuum in the furnace during the temperature rising process was measured, and the occurrence of discoloration after heat treatment was investigated.

When heating and raising the temperature, the degree of vacuum in the furnace is 5 × 10.
^The degree of vacuum was maintained at 1 × 10 ⁻⁵ Torr or more at the time of heating and holding at 725 ° C., starting at ⁻⁵ Torr.

A steel container made of SUS304 was used.

Further, as the oxygen target material, JIS is used as the oxygen target material which does not absorb hydrogen in the conventional method.
A pure Ti tube made of TTH28 specified in -H4631 is used as the hydrogen storage oxygen target material in the method of the present invention.
The hydrogen supply flow rate was 110 Nm ³ / in a pure Ti pipe made of H28.
In a hr hydrogen furnace, a hydrogen storage treatment of heating and holding at 800 ° C. for 1 hour was used, and three of each were placed at a distance of 50 mm at a position 100 mm apart from the inner surface of the furnace wall.

The discoloration after the heat treatment is as follows:
^{Those 2} per 100 mm ² or more discoloration occurs and there discoloration occurred was evaluated Searching for the occurrence number ratio.

The results are shown in Table 1 together with the measures for preventing discoloration, the rate of temperature increase, and the minimum value of decrease in the degree of vacuum in the furnace during the temperature increasing process.

[0037]

[Table 1]

As is clear from the results shown in Table 1, in the case of the method of the present invention (Nos. 5 and 6), the heating rate was 8
When a heat treatment of 5.3 ° C./hr (required time from room temperature to 725 ° C. 8.5 hr) is applied, the degree of vacuum in the furnace during the temperature rising process is reduced to 5 × 10 ⁻⁴ Torr, Due to the action of hydrogen stored in the target material, and further the action of the steel container, the discoloration occurrence rate is 0.1 to 0.2.
% (Occurred in 1 to 2 out of 800), which was extremely small.

On the other hand, in Comparative Example No. 2 in which the target material which does not absorb hydrogen was used and the heat treatment was performed at the same temperature rising rate as the example of the present invention without using the steel container, the discoloration occurrence rate was 1%. (8 out of 800). In addition, in Comparative Example No. 4 in which a steel container was used but a target material that did not occlude hydrogen was used and heat treatment was performed at the same temperature rising rate as that of the present invention example, the discoloration occurrence rate was 0.6% (8%).
It occurred in 5 out of 00). Further, similarly to No. 2 of Comparative Example, a temperature rising rate of 12 was used without using a target material that does not occlude hydrogen and without using a steel container.
In No. 1 of the comparative example, which was accelerated to 0.8 ° C / hr (required time from normal temperature to 725 ° C for 6 hr), the discoloration occurrence rate was 3.5% (occurred in 28 out of 800) and was extremely high. did.

On the other hand, the temperature rising rate was set to 6 without using a target material that does not absorb hydrogen and without using a steel container.
In Comparative Example No. 3 which was heat-treated with a delay of 0.4 ° C./hr (time required to reach from room temperature to 725 ° C. 12 hr),
The rate of occurrence of discoloration was 0.2%, which was the same result as that of the example of the present invention.

As is clear from the comparison between No. 3 of the comparative example and Nos. 5 and 6 of the present invention described above, according to the method of the present invention, the time required for the heat treatment can be significantly shortened. It can be seen that the heat treatment efficiency can be improved.

[0042]

EFFECTS OF THE INVENTION According to the heat treatment method of the present invention, even if the vacuum degree in the furnace is lowered by heating for a short time, the oxygen in the outgas released from the furnace wall by the hydrogen stored in the oxygen target material is removed. Since it is rendered harmless, the occurrence of discoloration can be effectively suppressed, and the time required for the heat treatment can be shortened, so that the heat treatment efficiency can be improved.

[Brief description of drawings]

FIG. 1 is a schematic view showing an embodiment of a vacuum heat treatment method of the present invention.

FIG. 2 is a diagram showing an example of a steel container used for housing and enclosing a material to be treated.

FIG. 3 is a diagram showing an example of the relationship between the temperature in the vacuum furnace and the degree of vacuum; FIG. 3 (a) is a diagram showing heating during short-time heating, and FIG. 3 (b) is a diagram showing heating during long-time heating. is there.

[Explanation of symbols]

 1: Vacuum furnace, 1a: Opening / closing furnace wall, 2: Truck, 3: Multi-stage rack, 4: Steel to be treated, 5: Steel container, 6: Hydrogen storage oxygen target material, 7: Vacuum pump, 8: Atmospheric gas discharge pump.

Claims (3)

[Claims] 1. When heat treating a steel material using a vacuum furnace,
A vacuum heat treatment method for a steel material, comprising placing an oxygen getter material containing hydrogen stored in a vacuum furnace for heat treatment. 2. When heat treating a steel material using a vacuum furnace,
A method for vacuum heat treatment of a steel material, comprising: placing an oxygen getter material in which hydrogen is occluded in a vacuum furnace, and heat-treating the steel material in a state where the steel material is housed and enclosed in a steel container. 3. The vacuum heat treatment method for a steel material according to claim 1, wherein the oxygen getter material is made of a metal having a higher reactivity with oxygen than the steel material to be treated.