JPH0711030B2 - Manufacturing method of high strength steel pipe with low hardness and yield ratio - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high strength steel pipe having a low hardness and a low yield ratio.

(Prior art and its problems) Due to the rapid progress of energy development in recent years, pipeline transportation of oil or natural gas has become large-scale and long-distance, and high strength is ensured from the safety of destruction. Manufacturing of steel pipe is required. In addition, oil wells and natural gas wells that are newly drilled due to depletion of energy resources often contain sulfides, and pipes that transport oil and natural gas and tanks that store them contain sulfides and iron. Hydrogen generated by the reaction diffuses into the steel, causing hydrogen-induced cracking and sulfide stress corrosion cracking, sometimes causing accidents in which the contents are leaked.

As for cracks caused by sulfides contained in petroleum, etc., the hardness generally affects the production of low hardness steel pipes. As a method of manufacturing a steel pipe in response to the demand for such characteristics, Japanese Patent Laid-Open Nos. 54-111731,
The quenching and tempering treatment method (QT treatment) introduced in JP-A-55-73849 and the like, and the low temperature as introduced in JP-A-53-52228 and JP-A-54-118325. There is a controlled rolling method (CR method) for rolling in.

In recent years, there has been a demand for shortening the pipeline construction work period.From the conventional laying method for welding short steel pipes on site, long steel pipes have been welded in advance at a pipe manufacturing factory to a long shape to form a coiled product. There is a need to manufacture a low yield point steel pipe with good workability because it is being transferred to a laying method in which only the required parts are welded while being linearly stretched locally. However, high strength and good workability (low hardness, low yield point) are contradictory properties, and a steel pipe satisfying both properties cannot be obtained by the above-described pipe manufacturing method. Fig. 1 shows that the degree of decrease in hardness is extremely large as shown in the change in hardness in the direction of depth (plate thickness) from the outer surface of the steel pipe after hot rolling (AsQ) and after Q-T treatment.
Also, as shown in Table 1, high strength steel pipes generally have a high yield point,
It has a high yield ratio. That is, the conventional high-strength steel pipe does not have safety, crackability, and workability at the same time.

(Means for Solving Problems) In view of such a current situation, the present invention replaces the conventional QT treatment in which it is difficult to obtain a material having a high strength and a maximum hardness and a low yield ratio, strength, weldability, etc. The present invention aims to provide a method for producing a steel pipe having a low maximum hardness and a low yield ratio without impairing other properties. That is, the present invention is a controlled cooling heat treatment method utilizing the deviation of the cooling rate in the cross section of the steel pipe, by finding that the steel pipe intended by the present invention is manufactured by forcibly cooling the outer surface layer of the steel pipe. is there. The gist is that the steel pipe after hot rolling or reheated to a high temperature of Ac ₃ transformation point or higher is heated from Ar ₃ transformation point or higher temperature to the outer surface layer of the steel pipe at a rate faster than natural cooling at 550 ° C. Cool to a temperature of 400 ° C., then cool to a lower temperature at a faster rate. Alternatively, it is a method for producing a high-strength steel pipe having a low hardness and a low yield ratio, which is tempered at a temperature not higher than the Ac ₁ transformation point.

Hereinafter, the present invention will be described in detail.

Steel pipes that retain high temperature heat after normal hot rolling or hot control rolling or steel pipes heated to a temperature higher than the Ac ₃ transformation point for the purpose of heat treatment are cooled from a temperature higher than the Ac ₃ transformation point. In the process, the outer surface layer of the steel pipe is cooled to an arbitrary stop temperature of 550 ° C to 400 ° C at a faster rate than natural cooling (air cooling). This temperature range is to obtain a steel pipe having a low maximum hardness, a high strength and a low yield ratio (YR) by causing α transformation in a part of the outer surface layer of the steel pipe to prevent an increase in hardness due to strong cooling. At a high temperature of 550 ° C. or higher, which is lower than this temperature range, the amount of α transformation is small, and the remaining γ phase is transformed into a low temperature transformation-forming structure by cooling at a high speed which is subsequently performed, which is not suitable for the invention. Further, at a low temperature of over 400 ° C, the amount of α transformation of the entire steel pipe increases and high strength cannot be obtained.

In addition, this temperature range is slower than natural cooling. It has been found that the α-transformation amount of the entire steel pipe increases at the cooling speed, and the effect of increasing the strength by subsequent cooling at a high speed decreases, and the effect aimed at by the present invention cannot be sufficiently exerted.

The steel pipe cooled under the temperature conditions and cooling conditions as described above is subsequently cooled to a low temperature at a higher speed in order to obtain high strength. In this way, the steel pipe with only the outer surface layer of the steel pipe accelerated cooled has a hardness (average in the plate thickness direction) that combines strength and workability with the hard layer at the steel pipe surface layer and the soft layer at the center of the steel pipe. A high-strength steel pipe having a low hardness) and a low yield ratio can be manufactured and provided as a product. Further, in the present invention, in order to remove the internal strain generated in the surface layer portion of the steel pipe by the rapid cooling, the tempering treatment is performed at a low temperature below the Ac ₁ transformation point. Hardness is tempered,
It is manufactured into a steel pipe with a balanced yield ratio and strength.

The steel pipe manufactured by the above-described manufacturing method has a suitable strength as a steel for line pipes and has a low hardness and a low yield ratio.

Next, examples of the present invention will be described.

Using the steel materials with the composition shown in Table 2 as the test materials, the surface layer of the steel pipe heated to a high temperature (930 ° C) higher than the Ac ₃ transformation point after hot rolling was allowed to cool naturally with a water-water mixed refrigerant. It was cooled to an arbitrary temperature at a high speed (average cooling rate: 30 ° C./second), and then cooled to room temperature by rapid cooling (average cooling rate: 150 ° C./second) injecting a larger amount of gas-water mixed refrigerant. Further, the characteristics of the steel pipe when tempered at a temperature of 450 ° C. are shown in Table 3 together with the comparative method. Note that the comparative method is used when a large amount of gas-water mixed refrigerant is injected from a temperature of 930 ° C to room temperature and cooled at once (As
Q), or the surface layer of the steel pipe is cooled to an arbitrary temperature of 550 ° C or higher at a rate faster than natural cooling (average cooling rate: 30 ° C / sec) with a mixture of steam and water, and then a large amount of steam is added. Rapid cooling with mixed refrigerant injection (average cooling rate: 150 ° C / sec)
And cooled to room temperature. Further, the characteristics of the steel pipe when tempered at an arbitrary temperature of Ac ₁ or less are shown.

The toughness characteristics of the present invention are slightly lower on average than those of ordinary QT materials, but no significant deterioration is observed in terms of strength-toughness balance. In terms of strength, the material of the present invention has a high average hardness, but has a low maximum hardness and a low yield ratio.

As described above, the method of manufacturing a steel pipe according to the present invention can obtain a low yield ratio characteristic that cannot be achieved by the conventional idea, and is extremely advantageous in industrial terms.

[Brief description of drawings]

Fig. 1 is a diagram showing the hardness change in the thickness direction (distance from the outer surface) of the plain tube (AsQ) and tempering tube (QT).

Continuation of front page (72) Inventor Anpyo Murata, 1-1-1 Emitsu, Hachimanto-ku, Kitakyushu, Kitakyushu, Fukuoka Inside Nippon Steel Co., Ltd. 3rd Technical Research Laboratory (56) References Japanese Patent Publication No. Sho 56-18046 (JP, B2)

Claims (2)

[Claims] 1. A steel pipe after hot rolling or reheated to a temperature higher than the Ac ₃ transformation point is heated to a temperature higher than the Ar ₃ transformation point and the outer surface layer of the steel pipe is heated to 550 ° C. at a speed faster than natural cooling. ~ 400 ℃
A method for producing a high-strength steel pipe having a low hardness and a low yield ratio, which is characterized by cooling to a low temperature and then to a low temperature at a higher speed. 2. A steel pipe after hot rolling or reheated to a high temperature of Ac ₃ transformation point or higher, from the high temperature of Ar ₃ transformation point or higher, the outer surface layer of the steel pipe is 550 ° C. at a speed faster than natural cooling. ~ 400 ℃
A method for producing a high-strength steel pipe having a low hardness and a low yield ratio, which is characterized by cooling to a temperature of ₁ , then to a low temperature at a higher speed, and then tempering at a temperature not higher than the Ac ₁ transformation point.