JPS62158822A - Manufacture of high strength steel tube having low hardness and yield ratio - Google Patents

Abstract

PURPOSE:To manufacture a high strength steel tube having low hardness and yield ratio by cooling the outer surface part of a steel tube from a high temp. to a specified temp. at a cooling rate higher than the air cooling rate and further cooling it to a low temp. at a much higher cooling rate. CONSTITUTION:The outer surface part of a steel tube hot rolled, or subjected to reheating to a high temp. is cooled from a high temp. to 550-400 deg.C at a cooling rate higher than the air cooling rate. It is further cooled to a low temp. at a much higher cooling rate. Tempering is then carried out at the Ac1 transformation point or below are required.

Description

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

(Conventional technologies and their problems) In recent years, with the rapid progress in energy development, oil or natural gas has come to be transported by pipelines on a large scale and over long distances. There is a request for manufacturing steel pipes. Additionally, as energy resources become depleted, newly drilled oil valves and natural gas wells often contain sulfides, and the pipes that transport oil and natural gas and the tanks that store them often contain sulfides and iron. Hydrogen generated by the reaction diffuses into the steel, causing hydrogen-induced cracking and sulfide stress corrosion cracking, sometimes resulting in leakage of contents.

Since cracks caused by sulfides contained in petroleum and the like are generally affected by hardness, there is a demand for manufacturing steel pipes with low hardness. As a manufacturing method for steel pipes that meet the demands for such characteristics, the quenching and tempering treatment method (Q-T treatment) introduced in JP-A-54-117311 and JP-A-55-7:1849, etc. Furthermore, as introduced in JP-A-53-52228 and JP-A-54-118325, there is a controlled rolling method (CR method) that rolls at low temperatures, and is generally widely used. There is.

In addition, recently there has been a demand for shortening the construction period for vibrating lines, so instead of the conventional installation method of welding short steel pipes on-site, short steel pipes are welded into long lengths at a pipe manufacturing factory in advance to form coils. There is a trend towards a new construction method in which pipes are transported in bulk, stretched into a line on site, and welded only at the necessary points, and there is a growing demand for the production of low-yield point steel pipes with good workability. However, high strength and good workability (low hardness, low yield point) are contradictory properties, and a steel pipe that satisfies both properties has not been obtained by the above-mentioned pipe manufacturing method. Figure 1 shows the hardness change in the direction from the outer surface of the steel pipe (from the outside surface to the thickness) for the as-hot-rolled (AsQ) and Q-T treated steel pipes.
In addition, as shown in Table 1, high-strength steel pipes generally have a high yield point,
High yield ratio. In other words, conventional high-strength steel pipes do not have safety, crackability, and workability at the same time.

Table 1 (Means for Solving the Problems) In view of the current situation, the present invention provides a material with high strength, low maximum hardness, and low yield ratio, in place of the conventional non-QT treatment, which is difficult to obtain. The purpose of this invention is to provide a method for producing steel pipes that have a low maximum hardness and a low yield ratio without impairing other properties such as weldability. In other words, the present invention is a controlled cooling heat treatment method that utilizes the deviation in cooling rate within the cross section of the steel pipe, and it has been discovered that the objective steel pipe of the present invention can be manufactured by forcibly cooling the outer surface layer of the steel pipe. be. The gist of this method is to cool a steel pipe that has been hot-rolled or reheated to a high temperature from a high temperature to a temperature of 550°C to 400°C at a faster rate than natural cooling, and then Cool to low temperature at high speed. Alternatively, it is a method for producing high-strength steel pipes with low hardness and yield ratio, which is tempered at a temperature roughly below the Ac+ transformation point.

The present invention will be explained in detail below.

In the process of cooling a steel pipe that has undergone high temperature heat after normal hot rolling or hot controlled rolling, or a steel pipe that has been heated to a high temperature for the purpose of heat treatment, the outer surface layer of the steel pipe is allowed to cool naturally. Faster speed than (air cooling) 550℃ ~ 4
Cool to an arbitrary stop temperature of 00°C. This temperature range is
This method produces α transformation in a part of the outer surface layer of the steel pipe to prevent an increase in hardness due to strong cooling, thereby obtaining a steel pipe with low maximum hardness, high strength, and low YR. 550 below this temperature range
At temperatures as high as .degree. C. or higher, the amount of alpha transformation is small and subsequent rapid cooling causes the remaining gamma phase to transform into a low-temperature transformation-generated structure and increase hardness, which is not suitable for the present invention. Also 4
At temperatures as low as 00°C, the amount of alpha transformation in the entire steel pipe increases, making it impossible to obtain high strength.

Furthermore, in this temperature range, if the cooling rate is slower than natural cooling, the amount of alpha transformation in the entire steel pipe increases, and the effect of increasing the strength due to subsequent cooling at a high rate decreases, so that the desired effect of the present invention is not sufficient. I discovered that I was unable to demonstrate my abilities.

The steel pipe that has been cooled under the temperature and cooling conditions described above is then cooled at a faster rate to a lower temperature in order to obtain high strength. In this way, a steel pipe in which only the outer surface layer of the steel pipe is acceleratedly cooled has a hard layer on the surface layer and a soft layer in the center of the steel pipe, which is hard and has an average hardness in the thickness direction that balances strength and workability. It is possible to manufacture high-strength steel pipes with low yield ratios and use them in products. Furthermore, in the present invention, tempering treatment is performed at a low temperature below the Ac+ transformation point in order to remove internal strain generated in the surface layer of the steel pipe due to rapid cooling. The tempering process produces steel pipes with balanced hardness, yield ratio, and strength.

Steel pipes manufactured by the above-mentioned manufacturing method have appropriate strength as steel for line pipes, and have low hardness and yield ratio.

Next, examples of the present invention will be described.

Using a steel material with the composition shown in Table 2 as a test material, the surface layer of the steel pipe heated to a high temperature (930°C) after hot rolling was heated at a rate faster than natural cooling (average cooling rate ,3
0℃/sec) to a desired temperature, followed by rapid cooling (average cooling rate, 1
50° C./sec) to room temperature and then cooled. Furthermore, the steel pipe is heated to a temperature of 4
The properties of the steel pipe when tempered at 50°C are listed in Table 3 along with comparative methods. The comparative method involves injecting a large amount of air-water mixed refrigerant from a temperature of 930°C to room temperature to cool it all at once.
), or the surface layer of the steel pipe is heated to a temperature of 550°C using a steam/water mixed refrigerant at a faster rate than natural cooling (average cooling rate = 30°C/sec).
Rapid cooling (cooling to any temperature above ℃) and then injecting a large amount of air/water temperature mixed refrigerant (
The mixture was cooled to room temperature at an average cooling rate of 150° C./sec. In addition, the properties of the steel pipes tempered at the desired temperatures listed below are listed.

Regarding the toughness properties, the toughness properties of the present invention are generally slightly lower than those of ordinary QT materials, and as far as the strength-toughness ratio is concerned, there is no significant decrease in toughness. Although the strength of the materials of the present invention is uniformly high, both the maximum hardness and yield ratio are at low levels.

As described above, the method for producing steel pipes according to the present invention provides 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 wall thickness direction (ASQ) and tempered tube (QT).
A diagram showing changes in hardness (distance from the outer surface).

Claims (2)

[Claims] (1) After hot rolling or reheating to a high temperature, the outer surface layer of the steel pipe is cooled from the high temperature to a temperature of 550°C to 400°C at a faster rate than natural cooling, and then at an even faster rate. A method for manufacturing high-strength steel pipes with low hardness and yield ratio, characterized by cooling to low temperatures. (2) After hot rolling or reheating to a high temperature, the outer surface layer of the steel pipe is cooled from the high temperature to a temperature of 550°C to 400°C at a faster rate than natural cooling, and then at an even faster rate. A method for producing a high-strength steel pipe with low hardness and yield ratio, characterized by cooling to a low temperature at a temperature of 100 mL, followed by tempering at a temperature below the Ac_1 transformation point.