JPH03107423A - Stress corrosion cracking resisting steel tube and its production - Google Patents

Abstract

PURPOSE:To produce a stress corrosion cracking resisting steel tube while obviating the necessity of complicated stages by introducing inert gas, etc., into a high strength steel tube in an austenite region and forming a decarburized soft steel layer on the internal surface of the tube at the time of applying heat treatment, such as normalizing, to the high strength steel tube. CONSTITUTION:At the time of subjecting a high strength steel tube to heat treatment consisting of normalizing, normalizing and tempering, or hardening and tempering, an inert gas, such as Ar and N2, a nonoxidizing gas free from O2, or a low-O2 gas containing trace amounts (<= about 5%) of O2 is introduced into the tube in an austenite region. By this method, the amount of O2 in the tube can be controlled and a decarburized soft steel layer can be formed on the internal surface of the pipe, by which stress corrosion cracking resistance can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a stress corrosion cracking resistant steel pipe having a mild steel layer on its inner surface and a method for manufacturing the same.

[Conventional technology]

Tensile strength 60 kg obtained by performing specified heat treatment
High-strength steel pipes with f/an2 or higher are known to suffer from stress corrosion cracking in sour, chemical, hydrogen, and other environments, leading to destruction.

This is a phenomenon unique to high-strength steel pipes, and does not normally occur with mild steel materials. It is also known that stress corrosion cracking is less likely to occur in double pipes made of mild steel materials.

[Problem that the invention seeks to solve]

Therefore, it is desirable to use such a double pipe in locations where high strength is required in environments such as sour, chemical, and hydrogen environments. However, there are various restrictions in manufacturing double pipes, requiring a complicated manufacturing process and equipment capable of handling the process, and furthermore, the manufacturing cost is extremely high.

The present invention was created in view of the above-mentioned problems in the prior art, and aims to provide a stress corrosion cracking resistant steel pipe and its manufacturing method that does not require a complicated manufacturing process, is inexpensive, and can be applied to mass production. It is something to do.

[Means for solving problems]

Therefore, the stress corrosion cracking resistant steel pipe of the present invention has Ar in the pipe in the austenite region during heat treatment of high-strength steel pipe that performs normalizing, normalizing and tempering, or quenching and tempering.
A decarburized mild steel layer is formed on the surface layer of the pipe by introducing an inert gas such as N2 or N2, a non-oxidizing gas that does not contain O2, or a low O2 gas that contains a very small amount (5% or less) of O2. It is something.

In other words, if the above gas is injected in this temperature range, the amount of O2 inside the pipe can be suppressed, the inner surface layer is decarburized, a mild steel layer is formed, and the stress corrosion cracking resistance of that part is improved. It will improve.

If an oxidizing gas is blown in instead of such a gas, oxidized scale will be generated on the surface layer and a decarburized layer will not be obtained.

Therefore, the gas used was limited to inert gas, non-oxidizing gas, or low O2 gas as described above.

Regarding the blowing of the gas, when the heat treatment is carried out in a continuous processing line, it is carried out by blowing gas into the heating zone, and when the heat treatment is carried out in a batch manner, it is carried out before the heat treatment. This is done by blowing gas into the pipe and sealing both ends.

Incidentally, examples of the base material composition of high-strength steel pipes include the following.

〔Example〕

One embodiment of the present invention will be described in detail below.

The present inventors quenched and tempered steel pipes A and B having the compositions shown in Table 2 below, respectively, in a batch process.

- For some of them, Ar gas was blown into the tube and both ends were sealed before quenching, and for the rest, quenching was performed as is. After tempering, a test piece was cut out and soaked in NACE solution (0.5% acetic acid saturated with hydrogen sulfide) under a tensile stress of 8°% YS. SCC on the surface was observed. In addition, in order to limit the test surface, the other surfaces except the inner surface were coated with resin. The results are shown below.

4-Table As shown in the above table, when the tube was filled with Ar gas during heating, stress corrosion on the surface did not progress, and the tube was beautiful with no cracks or the like.

〔Effect of the invention〕

According to the present invention, a mild steel layer can be formed on the inner surface of the pipe by a simple operation such as gas introduction during heat treatment, and a steel pipe with excellent stress corrosion cracking resistance can be obtained, which is an excellent effect. have.

Claims (2)

[Claims] (1) During heat treatment of high-strength steel pipes for normalizing, normalizing and tempering, or quenching and tempering, inert gas, non-oxidizing gas, or low O_2 gas is introduced into the pipe in the austenite region, and the surface layer inside the pipe is heated. Stress corrosion cracking resistant steel pipe with a decarburized mild steel layer. (2) During heat treatment of high-strength steel pipes for normalizing, normalizing and tempering, or quenching and tempering, inert gas, non-oxidizing gas, or low O_2 gas is introduced into the pipe in the austenite region, and the surface layer inside the pipe is heated. A method for producing a stress corrosion cracking resistant steel pipe characterized by producing a decarburized mild steel layer.