JPS6086262A - Life extending heat treatment of core metal for preparing seamless steel pipe - Google Patents

Abstract

PURPOSE:To enhance the life of a core metal by stably forming dense scale having good close adhesiveness, by holding a core metal for preparing a seamless steel pipe comprising Cr-Ni type low alloyed steel at a specific temp. under heating in an atmosphere specified in the ratio of steam and O2. CONSTITUTION:A core metal for preparing a seamless steel pipe made of a 3Cr- 1Ni type low alloyed steel is placed in an atmosphere of a gaseous mixture consisting of 50vol% or more of steam, 0.5-5.0vol% of O2 and the remainder of inert gas and having a volumetric ratio of O2 to steam of 1/3 or less. Said core metal is held at 1,000 deg.C or less, pref., about 950 deg.C under heating in this atmosphere and scale sufficiently penetrated into the metal is rapidly developed. By this method, dense scale with good close adhesiveness having resistance against abrasion, molten loss or baking can be certainly formed on the surface of the core metal and the life of the core metal can be improved to a large extent.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for extending the life of a core metal for a seamless steel pipe.

Seam i is generally used when manufacturing seamless steel pipes with a steel pipe hot perforating machine or rolling machine such as a Mannesmann type pipe making machine.
Core metal for steel pipe manufacturing is heated to high temperatures and subjected to high pressure during pipe manufacturing, so localized wear, melting, and seizure on the surface, which adversely affect product quality, are significant, and its lifespan is therefore shortened. It is no exaggeration to say that this is mainly affected by local wear, melting damage, or seizure.

For this reason, measures to suppress wear, erosion, seizure, etc. of seamless steel pipe cores have been studied from various perspectives, and influential reports and proposals have been made. It's here.

Among these, applying heat treatment to the core metal for seamless steel pipes to form scale on the surface is effective in suppressing the occurrence of wear, melting damage, seizure, etc. of the core metal base, and extending the life of the core metal. The report that it is possible to achieve
No. 3) or Japanese Patent Laid-Open No. 56-62922, etc., proposes measures to extend the life of seamless steel pipe cores based on the same purpose.

The former is known to extend the life of a core metal for hot drilling or rolling by forming an oxide layer on its surface during heat treatment. ``Core metal for seamless steel pipes (
3Cr-INi) at 900 to 940°C, when the temperature reaches around 500°C, the introduction of steam into the furnace is started, and after being maintained at 900 to 940°C for a certain period of time, it is cooled to 500°C. By adopting a method in which the injection of steam is stopped when the temperature reaches below ℃, and the amount of steam during injection always accounts for 10% or more of the internal volume of the furnace, as shown in Figure 1 ( (As shown in the schematic diagram, "The thickness of the direct oxidation layer 3 in contact with the core metal base 2 under the surface oxidation layer l is 30μ or more on the core metal surface, and this direct oxidation layer 3
This proposal proposes that the depth of penetration into the core metal base 2 (in Fig. 1, the part indicated by reference numeral 4 is the penetration part) forms a scale of 10 μm or more, and the latter is shown in Fig. 1. In order to form the scale shown, it was proposed that ``the inside of the furnace be made into a C○--CO2 atmosphere.''

Indeed, it is known that the method of forming scale on the surface of the core metal for seamless steel pipes is a relatively inexpensive and easy-to-work method for improving the life of the core metal. Forming scales as described in JP-A No. 19363 or JP-A-56-62922 1 (scales as shown in FIG. 1) improves the peeling resistance of the scales and However, in the course of subsequent detailed studies by the present inventors, it was found that even if the scale forming means shown in the above-mentioned publications were adopted as they were, the first It is difficult to reliably manufacture a core metal with a scale with good peeling resistance as shown in the figure with good reproducibility, and the characteristics of the formed scale vary widely, resulting in a stable extension of the life of the core metal. It became clear that this was not possible.

From the above-mentioned viewpoint, the present inventors have discovered that 3Cr-I, which has been considered suitable as a core metal material in terms of durability, etc.
Stably and reliably apply a dense scale with good adhesion that is highly resistant to abrasion, melting, and seizure on the surface of the core metal for seamless steel pipes made of Ni-based low alloy steel. As a result of repeated research in order to find a method to form scales with good resistance and thereby further extend the life of the core metal, we found that (a) In order to form scale on the surface of the core metal and extend its life, it was found that the scale to be formed is As mentioned above, it is important that the oxide layer 3 is dense and has good adhesion, and for this purpose, it is necessary to generate scale that directly oxidizes the oxide layer 3 and bites into the core metal base 2, as shown in FIG. As a means of achieving this, the above-mentioned Special Publication Publication No.
Although it is effective to blow steam into the heat treatment furnace atmosphere as described in Publication No. 63 (C), it is not sufficient by itself, and in addition to this, it is necessary to appropriately control the 02 content in the heating atmosphere. (b) is essential for stably forming a scale with a desired shape; (b)
The 02 in the atmosphere in the heat treatment furnace, together with the water vapor, has a large effect on the properties of the scale on the surface of the core metal, and it greatly influences the porosity of the scale and the degree of penetration into the core metal, so it is dense and has good adhesion. In order to form a good scale, it is necessary to adjust the ratio of 02 in the heating atmosphere. Person in charge 0
2, and the volume ratio of [02: water vapor] is [l, 3
], and heat treatment at a temperature not exceeding 000°C is the necessary condition for stably forming a dense and well-adherent scale in a short time; We have come to the knowledge shown in (C).

The present invention has been made based on the above knowledge, and includes the following steps: A seamless steel pipe core made of 3Cr-INi low alloy steel is heated to 50% water vapor capacity.

02: 0.5-50 capacity.

By heating and holding at a temperature of 1000°C or less in a mixed gas atmosphere in which the remainder is substantially inert gas and the volume ratio of 02 to water vapor is less than U/3, it is possible to form a close contact. It is characterized by having excellent properties and forming a dense scale on the surface of the metal core to extend its life.

In addition, in the method of this invention, the core material is 3Cr-1N.
The i-series low alloy steel is used because, as mentioned above, this steel is said to be suitable as a core metal material in terms of durability, etc., and usually C: 0.25 to Q, 35 weight. %.

Sj: OIi~08 Weight Section.

Mn: 0.4-08 weight section.

Cr: 2.50 to 3.50 weight ratio.

Ni: 1.00 to 2.50% by weight.

It has a chemical composition that includes.

The inert gases still available are N2 gas and Ar gas.

In addition to He gas, a mixture of these gases can also be used.

Next, in the heat treatment method of the present invention, the reason why each ratio of the amount of water vapor and the amount of 02 in the heating atmosphere, the volume ratio of 02 to 1 water vapor, and the heating holding temperature are each limited to specific values will be explained.

a) Amount of 02 If water vapor is the only oxidizing gas in the heating atmosphere, the oxidation rate is low and the desired scale cannot be formed, so mixing a predetermined amount of 02 is an essential condition in the method of the present invention.

If the 02 ratio in the heating atmosphere is less than the 05 volumetric ratio, scale that bites into the base metal will be formed, but its growth will be extremely slow and it will take a long time to form the scale to a predetermined thickness. In short, it is not practical, and on the other hand, 0
If the ratio exceeds 5.0 volume 1i%, the scale becomes porous and the mechanical strength deteriorates even if the amount of water vapor is increased, and the scale that has penetrated into the base metal does not develop (i.e., Because grain boundary oxidation becomes insufficient), some products are extremely easy to peel off, so the amount of 02 is reduced to 05 to 50.
He was designated as the person in charge of Yōkuma.

In other words, if there is water vapor in the heated atmosphere,
By coexisting with the capacity factor 02, a scale that penetrates sufficiently into the base metal material quickly develops.

Figure 2 (a) shows 20 volume% H2Q -0'B-volume%
02-In a N2 mixed gas atmosphere, a plug used for Mannesmann type pipe making made of 3Cr-INi low alloy steel was
It is a microscopic photograph showing the state of the scale when heated to 0°C, held for 3 hours, and then cooled down in the furnace. This is a micrograph showing the state of a school formed under the same conditions as above except that N2 mixed gas was used, and the relative softness of both shows that the oxidation rate is very low in an atmosphere where the oxidizing gas is N20 only. is clear. Figure 2 shows that when 02 is added and mixed with steam, even if the amount of 02 is small, the oxidation rate increases, grain boundary oxidation is significantly promoted, and the penetration into the base metal is sufficient. It can be seen that many dense scales are formed as they develop.

In addition, Fig. 2(C) and Fig. 2(d) show that 0 in the atmosphere.
2 is a micrograph showing the state of the scale formed under the same conditions as above except that the ratio was set to exceed 50 i%,
From these figures, when the 02 ratio exceeds the 50 capacity ratio,
The oxide layer in contact with the base metal base becomes porous [Figure 2 (C)], or the oxide layer no longer bites into the base metal base [Figure 2 (C)].
Figure 2(d)]. And, such scales tend to peel off.

b) Capacity ratio of 02 to water vapor The formation of scales that are likely to peel off is also affected by the ratio of the amount of water vapor to the amount of 02 in the atmosphere, that is, the value k expressed by As a result of the experiment, the 02 ratio was 50 volumes and 11 tons.
% and when k is smaller than 1/3, a dense scale as shown in Fig. 2(b) and which has sufficiently penetrated into the gold base is produced. It was confirmed that when the ratio is 1/3 or more, it is not possible to stably form a good scale. Therefore, 02 against water vapor
The capacity ratio (k) was set to be less than 1/3.

C) Amount of water vapor The main oxidizing raw gas in the heating atmosphere used in the method of the present invention is water vapor, but in order to grow scale uniformly, the amount of water vapor must be 50 volume fft% or more. It is.

That is, if the water vapor is reduced while maintaining the above-mentioned value below 1/3, it is recognized that the thickness of the oxide scale becomes uneven from the point where the water vapor is reduced. This is because, due to the influence of gas flow within the furnace, oxidation progresses more rapidly on the surfaces that are often exposed to the gas than on other parts. This phenomenon becomes noticeable when the amount of water vapor is less than 5.0 volumes - 1 inch, and becomes less noticeable when the amount of water vapor is less than 50 volumes.

For this reason, the amount of water vapor was determined to be 50 volumes or more.

Note that the upper limit of the amount of water vapor is determined only by the relationship with the amount of 02, and is not limited by other factors.

d) Heating and holding temperature The heating and holding temperature in the heating atmosphere needs to be 1000°C or less. This is because when the temperature exceeds 1000° C., grain boundary oxidation becomes difficult to proceed, and scale intrusion into the base metal does not occur. and,
This phenomenon is also well known as a high-temperature oxidation phenomenon of metals.

The penetration of scale into the base metal due to grain boundary oxidation is greatest when heated and held at approximately 850°C, but in order to ensure the thickness of the Fe-Cr-0 oxide layer in contact with the base metal, A high temperature is preferable, and therefore about 950° C. is suitable as the heating holding temperature.

However, the lower limit of the heating holding temperature is not particularly limited and may be determined according to the normal heat treatment conditions required for the core metal, but if the temperature is lower than 600°C, the scale growth rate will decrease. It is desirable to heat it to 600° C. or higher because it becomes smaller and is less affected by the atmosphere.

The heating holding time may be determined depending on the desired scale thickness.

Generally, when the heating temperature (oxidation temperature) is constant, if the scale thickness is X and the heating holding time (oxidation time) is t, then constant) relationship.

Of the scales, the direct oxidation layer shown by numeral 3 in Figure 1 is useful for extending the life of the tool (core metal), but this layer is composed of water vapor and 5
When heated and held at 950°C in an atmosphere of 0 volume-meter or higher, it grows according to the formula % formula % (). From this equation, it is possible to easily derive the heating holding time that will give the desired thickness scale.

As mentioned above, this invention provides water vapor: 50 volume or more,
02: contains 0.5 to 5.0 volume, the remainder is substantially inert gas, and the volume ratio of 02 to water vapor is 1
In a mixed gas atmosphere with a value smaller than /3, 3Cr-IN
The core metal for seamless steel pipes made of i-series low alloy steel is heated and maintained at a temperature of 1000℃ or less. Gas replacement is performed so that the atmosphere becomes the above-mentioned mixed gas atmosphere. In particular, it is desirable that the atmosphere in the furnace be the above-mentioned atmosphere when the temperature of the core metal is about 600° C. or higher.

Next, the present invention will be explained by examples.

Example l 0, 3C-3Cr- used in Mannesmann type pipe making machine
20 capacity water vapor to the INi steel first piercer plug.
Heat treatment by holding at 950°C for 3 hours and then cooling in the furnace in an electric furnace in which the atmosphere was adjusted by mixing 02 in various proportions in N2 mixed gas (however, the atmosphere adjustment was performed at 600°C or higher) The service life of these plugs was investigated.

The results obtained are shown in FIG. In addition, the "rolling number" in FIG. 3 (() is shown as the average value of 6 plugs in actual operation.

From the results shown in Figure 3, it is clear that a plug with a long life can be obtained when the 02 ratio in the heating atmosphere is between 0.5 and 50 by volume, and the 02 ratio in the atmosphere can be adjusted appropriately. The importance of this can be confirmed. , Example 2 3.0% by volume 02-N2 mixed gas was mixed with water vapor in various proportions to adjust the heating atmosphere, but under the same conditions as in Example 1, 0, 3 C-3
The first piercer plug made of Cr-'I Ni@ was processed and its service life was investigated and is shown in FIG.

The results shown in FIG. 4 also show that in order to provide a long life to the plug, the proportion of water vapor in the heated atmosphere needs to be 10.0% by volume or more.

In addition, the reason why the number of rolled pieces at the steam amount and the 50 volume ratio is small is because the O2/H2O capacity ratio is as large as 315.
In the atmosphere where 02 ratio is 1.0 capacity factor, ■
A good lifespan was shown as indicated by the mark. in this way,
02/H20 even if the amount of water vapor in the atmosphere is around 50% by volume
It can be seen that by reducing the value of , the plug can have a longer life.

Example 3 0,3C-3Cr-IN used in Mannesmann type pipe making machine
i steel i second piercer plug, 20 capacity steam N2
Heat treatment was carried out in an electric furnace in which the atmosphere was adjusted by mixing 02 in various proportions in the mixed gas, and the mixture was held at 950°C for 3 hours and then cooled in the furnace (however, the atmosphere was adjusted at 600°C or higher). The service life of these plugs was investigated.

The results obtained are shown in FIG.

It is clear from the results shown in FIG. 5 that the life of the plug is significantly extended by the treatment of the present invention.

Example 4 The same conditions as in Example 3 except that various amounts of water vapor were used as the atmospheric gas in a water vapor-02-N2 mixed gas with a 02 ratio of 1.0 volume ratio and 3.0 volume ratio. 0.3 C-3Cr for Mannesmann type pipe making machine
-The second viaser plug made of INi steel was treated and its service life was investigated and is shown in FIG.

From the results shown in FIG. 6, the same matters as in Example 2 can be confirmed.

As described above, according to the present invention, the surface of the seamless steel pipe core metal has high resistance to wear, melting damage, seizure, etc. It is possible to form a dense scale with good adhesion, stably, reliably, and with good workability, and it brings about industrially useful effects such as making it possible to efficiently and significantly improve the life of the core metal. It is.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional diagram of scales formed on the surface of a core metal for seamless steel pipes, and FIG. Figure (a), Figure 2 (b), Figure 2 (C) and Figure 2 (
d) are examples of different heating atmosphere conditions;
Figure 4 is a diagram showing the relationship between the amount of 02 in the heated atmosphere and the life of the first piercer plug. Figure 4 is a diagram showing the relationship between the amount of water vapor in the heating atmosphere and the life of the first piercer plug. The figure is a diagram showing the relationship between the amount of 02 in the heated atmosphere and the life of the second piercer plug.
FIG. 3 is a diagram showing the relationship with the piercer plug life. In the drawings, 1... Surface oxidized layer, 2... Core metal base, 3... Direct oxidized layer, 4... Biting part. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person

Claims (1)

[Claims] 3Cr-INi low alloy steel seamless steel pipe core metal, water vapor: 5. O capacity section or above. 02: 0.5-5.0 capacity section. In a mixed gas atmosphere in which the remainder is substantially inert gas and the volume ratio of 02 to water vapor is less than 1/3, the adhesion is achieved by heating and holding at a temperature of 1000°C or less. 1. A heat treatment method for extending the life of a core metal for seamless steel pipes, which is characterized by forming a fine scale with excellent properties on the surface of the core metal.