JPS61288018A - Manufacture of steel pipe having high strength and low yield ratio - Google Patents

Abstract

PURPOSE:To manufacture the titled steel pipe, by cooling rapidly a high temp. steel pipe after hot rolling or a steel pipe reheated to high temp., then applying local temper treatment or local temper rapid cooling to only a surface hardened layer of steel pipe by high frequency heating method. CONSTITUTION:At manufacturing the steel pipe for line for transporting liquid or gas such as oil and natural gas contg. sulfide, the steel pipe is cooled rapidly from highly heated state after hot rolling or reheated to quenching temp. then cooled rapidly and quenched. The steel pipe is temper treated locally only at the surface hardened layer by a high frequency induction coil, or heated by the high frequency induction coil and rapid cooling treated while the surface hardened layer is temper treated locally. The high strength steel pipe for line pipe having low surface hardness and yield point is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a high-strength steel pipe for line pipes with maximum hardness and low yield point.

(Conventional technology) With the rapid progress in energy development in recent years, the transportation of oil or natural gas over long distances has come to be carried out on a large scale and over long distances. There is a strong demand for manufacturing steel pipes.

Newly drilled oil and natural gas wells due to depletion of energy resources often contain sulfides, and the reaction between sulfides and iron is often used in the pipes that transport oil and natural gas and the tanks that store them. The hydrogen generated by this process diffuses into the steel, causing hydrogen-induced cracking and sulfide stress corrosion cracking, which sometimes causes accidents in which the contents leak. Furthermore, the environment in which excavation is carried out is becoming more severe year by year, and construction methods are changing in terms of safety, construction efficiency, etc. For the above reasons, the materials used are also tending to have higher strength, and there is a shift toward demands for materials with maximum hardness and low yield ratio. In general, as a method for improving the strength of steel materials, for example, quenching and tempering treatments (Q
T treatment) or controlled rolling characterized by rolling at a low temperature as disclosed in, for example, JP-A-54-11017 and JP-A-54-102223. However, the yield ratio characteristics mentioned above can be expected to be improved by controlled rolling, but the normal Q
T treatment results in high strength grade, hardness and yield ratio. Steel for line pipes may not necessarily be manufactured by controlled rolling due to the balance with not only tensile properties and hardness but also toughness, weldability, and other mechanical properties, or due to rolling constraints such as seamless pipes. There are some cases where this is not possible, and it is extremely important industrially to have the possibility of manufacturing by QT treatment.

(Problems to be Solved by the Invention) In view of the current situation, the present invention aims to improve strength, weldability, etc. in place of the conventional QT treatment, which is difficult to obtain high strength, maximum hardness, and low yield ratio. The purpose of this invention is to provide a method for producing a material that has a low maximum hardness, a high average hardness, and a low yield ratio without impairing any other properties.

(Means for Solving the Problems) In other words, the gist of the present invention is to rapidly cool a steel pipe that has been hot-rolled or reheated to a high temperature from a high temperature, and then apply a high-frequency heating method to the surface hardening layer of the steel pipe. A method for manufacturing a high-strength, low-yield-ratio steel pipe characterized by subjecting only the steel pipe to a local tempering treatment or a local tempering quenching treatment. Next, details of the manufacturing method of the present invention and reasons for limitations will be explained. First, there are no particular limitations on the manufacturing history of the material before entering the process of the present invention. What is important is that the steel pipe has been rapidly cooled from a high temperature in order to obtain high strength, and whether it is a plate-wound steel pipe or one that has been subjected to controlled rolling, the subsequent effects will not be lost. Normally, the hardness distribution within the wall thickness section of the above-mentioned material varies depending on the component system, the cooling method, the size of the tube, etc., but it is generally higher at the strongly cooled end and decreases as the distance from the cooled end increases. The hardness of this outer surface hardening layer is preferably as low as possible because it does not affect sour resistance properties such as hydrogen-induced cracking resistance. This material is subjected to tempering treatment (Q
When T) is applied, the hardness gradient within the wall thickness section becomes smaller than that of the plain pipe (AsQ), but the overall level decreases, as shown in FIG. 1. In other words, it is shown that the strength of the material is significantly reduced by performing a tempering treatment that reduces the hardness of the outer surface hardened layer, which affects the sour resistance characteristics. Next, duplex steel, low-temperature tempering, etc. are well-known methods for obtaining a low yield ratio.The yield ratio is the ratio of yield strength to tensile strength, and from a microstructural perspective, plastic It is thought that this is determined by the mixing ratio of the soft tissue that is easily deformed and the hardened tissue that has tensile strength.

However, normal tempering treatment tends to reduce this mixing ratio obtained during hardening, resulting in an increase in yield ratio.

As mentioned above, reducing the outer surface hardening layer and lowering the yield ratio, which affect sour resistance, are contradictory, and the current QT
The process could not solve this problem. In order to solve this problem, the present inventors have discovered that only the outer surface hardened layer can be tempered by utilizing the skin effect of high frequency current. Fortunately, the metallographic structure of the steel material after quenching is different depending on the composition system, but in the case of line pipe steel with API equivalent In addition, it has a mixed structure of i4-lite, dispersed bainite, and island-shaped martensite, and has excellent material properties even without tempering.

In tempering treatment using the skin effect of high-frequency current, the frequency, output, etc. vary depending on the hardness gradient of the material, size, coil used, etc., and are selected depending on the purpose of the user. The important thing is to locally temper the surface hardened layer of the material on the living body, and the larger the tempering temperature gradient is made to match the hardness gradient, the greater the effect will be. For this reason, when performing local tempering treatment on the hardened layer on either the outer surface of the pipe or the inner surface of the pipe,
By performing local tempering treatment on the opposite surface while cooling with water, the effect becomes even greater. Furthermore, by rapidly cooling the same surface while performing local tempering, it is possible to prevent softening in areas other than the target areas caused by the heat transfer effect, so it goes without saying that the local tempering can be performed more effectively.

By the treatment method described above, an excellent steel material with high strength, low maximum hardness, and low yield ratio, suitable for line pipe steel, can finally be obtained. Let's discuss an example.

Table 1 shows the chemical composition of the steel samples.

FIG. 2 shows a schematic diagram of the example and part of the specifications of the implementation apparatus, in which a steel pipe 1 was heated by a high-frequency coil 2 and subjected to a tempering treatment in which the steel pipe was rapidly cooled from the inner and outer surfaces. In the figure, numeral 3 indicates a cooling water jet nozzle, and a indicates a water cooling mode. Table 2 shows some of the material properties of steel pipes obtained by applying local tempering treatment using the high-frequency heating method according to the present invention (method of the present invention) to test steel pipes with the components shown in Table 1 that were quenched immediately after rolling. , compared with the characteristics of a steel pipe obtained by quenching and tempering (conventional method) using a normal gas heating furnace.

Although the toughness properties are slightly lower on average when using the method of the present invention, as far as the balance of strength and toughness is concerned, there is no problem with the decrease in quality. Also, if the level etc. becomes a problem, it can be adequately addressed by methods such as changing the component system. On the other hand, although the strength is high on average, the yield ratio is at a low level. Moreover, according to the method of the present invention, the maximum hardness can be controlled to a large extent, and the hardness gradient in the thickness direction can also be sufficiently controlled. As described above, the method of manufacturing a steel pipe according to the present invention can provide a low yield ratio characteristic that could not be achieved using conventional methods, and is of great industrial benefit.

[Brief explanation of drawings]

Figure 1 shows the hardness changes in the wall thickness direction (distance from the outer surface) of the raw tube (AfiQ) and the tempered tube (QT), and Figure 2 shows the tempering treatment method used in the example of the present invention. FIG. 1: Steel pipe 2: High frequency coil 3: Cooling water injection nozzle Figure 1 Figure 2

Claims (2)

[Claims] (1) After hot-rolling or reheating a steel pipe to a high temperature, the steel pipe is rapidly cooled from a high temperature, and then only the surface hardened layer of the steel pipe is locally tempered using a high-frequency heating method. Manufacturing method for yield ratio steel pipes. (2) High strength characterized by rapidly cooling a steel pipe that has been hot-rolled or reheated to a high temperature from a high temperature, and then rapidly cooling while locally tempering only the surface hardened layer of the steel pipe using a high-frequency heating method. A manufacturing method for large and low yield ratio steel pipes.