JPH0452226A - Production of high strength seamless steel tube - Google Patents

Abstract

PURPOSE:To produce a high strength seamless steel tube by forming a round billet into a tube stock by means of hot piercing rolling, exerting heat treatment under specific conditions, performing outside diameter drawing, and then carrying out hardening and tempering. CONSTITUTION:In the case where cooling is exerted from a temp. not lower than the Ac, point down to ordinary temp. after hot piercing rolling, cooling is performed at a cooling velocity where the steel structure is formed into bainite or martensite structure. Since the neucleus of recrystallization is formed of a small lath structure of bainite or martensite when reheating is exerted up to <=(Ac3 +100 deg.C) after the steel structure is formed into bainite or martensite structure, fine austenite grains can be obtained after reheating. Hardening temp. at the time of quench-and-temper treatment is regulated to a temp. between Ac3, and the above reheating temp., and tempering temp. is <=Ac1. By carrying out quench-and-temper treatment under the above temp. conditions, the high strength seamless steel tube having fine austenite grain size can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to a method for manufacturing seamless steel pipes that require high strength, such as steel pipes for oil wells, and includes tensile strength, external pressure crushing strength, toughness, delayed fracture resistance, Production of high-strength seamless steel pipes with excellent performance such as hydrogen sulfide stress cracking resistance Conventional technology Seamless steel pipes used for oil well pipes, etc. have properties such as tensile strength, external pressure crushing strength, toughness, Since delayed fracture resistance, hydrogen sulfide stress cracking resistance, etc. are required, a common method is to perform heat treatment after hot pipe manufacturing to increase the strength.

More specifically, a round steel billet heated in a heating furnace is hot-pierced and rolled using a Mannesmann drilling machine, and then the raw tube is elongated and rolled using a rolling mill such as a plug mill, mandrel mill, or pilger mill to reduce the diameter and thin the wall. After that, a high-strength seamless steel pipe is obtained by quenching and tempering heat treatment.

Figure 2 shows an example of a conventional process heat treatment pattern for seamless steel pipes, in which the material is heated to a predetermined temperature in a heating furnace, hot-pierced and rolled, then reheated and subjected to outside diameter drawing to form a pipe. After that, the seamless steel pipe is quenched and tempered to form a martensitic structure.

Problems to be Solved by the Invention In terms of performance, a seamless steel pipe manufactured using a working heat treatment pattern as shown in FIG. 2 satisfies the quality required for steel pipes for oil wells and the like. However, in harsh operating environments with high concentrations of hydrogen sulfide, such as in modern sour crude oil and sour gas transportation pipelines, hydrogen-induced cracking, stress corrosion cracking, etc. have become major problems, and there are There is now a need for seamless steel pipes that exhibit increased resistance.

In view of these circumstances, the present invention exhibits excellent resistance to hydrogen-induced cracking, sulfide stress corrosion cracking, etc. even under severe usage conditions with high hydrogen sulfide concentrations, and has high tensile strength and crushing strength. This paper attempts to propose a method for manufacturing high-strength seamless steel pipes with improved toughness.

Means for Solving the Problems This invention provides a method of forming a martensite or bainitic structure of the steel by cooling it from a temperature above A c s to room temperature after hot piercing rolling, and then cooling the steel structure to a temperature below A c s +100°C. After reheating and drawing the outer diameter, after air cooling, Ac
The main purpose of this method is to heat the pipe to a temperature above s and below the reheating temperature, and then perform quenching and tempering treatments.After the pipe is made, the pipe is made into a paynite or martensitic structure, and then reheated to form a fine pipe. The main feature is that the grains have an austenite grain size.

Function In this invention, the austenite grain size is refined to increase the yield ratio, which represents the ratio of tensile yield stress/tensile strength, and to improve external pressure crushing strength, toughness, delayed fracture resistance, and hydrogen sulfide stress cracking resistance. This is because it improves sex, etc.

When cooling from the Acs temperature or higher to room temperature after hot piercing rolling, cooling is performed at a cooling rate at which the steel structure becomes a bainite or martensitic structure.

When the steel structure is made into a bainite or martensite structure and then reheated to a temperature below A c s + 100°C, recrystallization nuclei become bainite or martensite /J)
Since it has a small lath structure, fine austenite grains can be obtained after reheating. Here, the reheating temperature is A (s +
The reason why the temperature is set not to exceed 100° C. is that the effect of crystal grain growth during reheating becomes large and the effect of austenite grain refinement cannot be obtained.

The quenching temperature in the quenching/tempering treatment is Ac
s or more and less than or equal to the reheating temperature, and the tempering temperature is A.
C+ or less. In other words, heating for quenching is necessary to convert the bainite or martensite structure into austenite, so it is necessary to exceed Acs, but if the heating temperature is too high, the grains will grow larger and the effect of the austenite refining treatment in the previous step will be reduced. is lost, so the upper limit needs to be set below the reheating temperature.

By performing the quenching and tempering treatment under the above temperature conditions, a high-strength steel pipe with fine austenite grain size can be obtained.

Embodiment FIG. 1 illustrates a working heat treatment pattern of the present invention, in which a material is heated to a predetermined temperature in a heating furnace, and after hot piercing rolling, it is cooled from a temperature of Acs or higher to room temperature by, for example, shower cooling. The steel structure is martensite or bainite. Next, it is reheated to a temperature of A c x + 100°C or less, subjected to outer diameter drawing, and after air cooling
This is a method in which a fine-grained tempered martensitic structure is obtained by heating to a temperature above the reheating temperature and below the reheating temperature, and performing quenching and tempering treatments.

Example 1 After making the steel having the composition shown in Table 2 into a bloom and further into a billet, according to the processing heat treatment pattern shown in Fig. 1,
A seamless steel pipe (size: 177φX-10, 36t) was manufactured by hot piercing rolling and drawing.

That is, in this example, as shown in Table 1, 125
After heating to 0°C and holding for half an hour, it was hot open-pierced and rolled to 1000
It was rapidly cooled from a temperature of 0.degree. C. to room temperature (20.degree. C.) by shower cooling. Subsequently, it was reheated to a temperature of 920°C and heated to 174°C.
After holding for a time, the outer diameter is drawn, air cooled, heated to 880℃ and quenched (water cooling at a cooling rate of -20℃/sec), then heated to 700℃ and 600℃ and air cooled. Then, tempering was performed.

For comparison, Table 1 also shows the processing heat treatment conditions and quality of pipes of the same size manufactured from steel of the same composition according to the conventional processing heat treatment pattern shown in FIG. In this conventional example, as shown in Table 1, the billet is 1250
After heating to ℃ and holding for half an hour, it was hot open-pierced and rolled to 1000℃.
It was air-cooled from a temperature of 100°C to 800°C, reheated to a temperature of 950°C, and subjected to outer diameter drawing. Thereafter, quenching was performed at a quenching temperature of 920° C., and the same tempering as above was performed.

From the results in Table 1, it can be seen that according to the method of the present invention, the yield ratio is higher and the external pressure crushing strength, Charpy impact test value, and hydrogen sulfide stress cracking test value are all higher than those of the conventional method. Recognize.

As described in detail below, the method of the present invention allows for appropriate processing in the hot seamless pipe manufacturing method that includes heating and reheating, and the method for manufacturing high-strength seamless steel pipes by quenching and tempering heat treatment. By selecting the heat treatment conditions, it is possible to obtain a fine-grained tempered martensitic structure, which has the great effect of producing a high-strength seamless steel pipe with excellent steel pipe performance.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a working heat treatment pattern according to the present invention. FIG. 2 is a diagram showing an example of a conventional processing and heat treatment pattern.

Claims (1)

[Claims] In a method for manufacturing high-strength seamless steel pipes in which after hot piercing and rolling, the pipe is reheated and subjected to outer diameter drawing, and then quenched and tempered, after hot piercing and rolling, the steel pipe is cooled from a temperature of Ac_3 or higher to room temperature. The steel structure is made into a martensite or bainite structure, and then reheated to a temperature of Ac_3 + 100°C or less, subjected to an outer diameter drawing process, and after air cooling, heated to a temperature of Ac_3 or more and the reheating temperature or less, and subjected to quenching and tempering treatment. A method for producing high-strength seamless steel pipes.