JPH0835009A - Production of martensitic stainless steel excellent in corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To produce a martensitic stainless steel excellent in CO2 corrosion cracking resistance by subjecting a low C-Cr-Ni-Cu-Mo steel of specific composition to two-stage tempering treatment under specific temp. conditions. CONSTITUTION:A steel, having a composition consisting of, by weight, 0.005-0.1% C, 0.05-1% Si, 0.1-2% Mn, <=0.025% P, <=0.015% S, 12-16% Cr, 0.1-4% Ni, 0.1-3% Cu, 0.1-3% Mo, 0.005-0.2% Al, 0.005-0.1% N, and the balance Fe with inevitable impurities, is used. This steel is subjected to first-stage tempering treatment at 640-770 deg.C and then to second-stage tempering treatment at a temp. in the range between 600 deg.C and the Ac1 point. By this tempering treatment in two stages, the whole structure is formed into tempered martensitic structure and softened. By this method, the sawing of a tube stock after rolling can be facilitated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing martensitic stainless steel having excellent corrosion resistance, particularly CO ₂ corrosion resistance for oil country tubular goods and line pipes.

[0002]

BACKGROUND OF THE INVENTION Wells for collecting oil or natural gas.
The environment of the door has become more and more severe in recent years, and mining
In addition to increasing the depth, wet carbon dioxide (CO₂) Or sulfide water
Elementary (H ₂S), chlorine ion (Cl^-) Etc.
The number of wells that contain water is increasing. Because of this
It is known that carbon steel and low alloy steel corrode significantly
ing.

In an oil well environment containing a large amount of carbon dioxide, a martensitic stainless steel containing 0.2% of C and 12 to 13% of Cr such as AISI 420 steel is widely used as a relatively inexpensive alloy steel. Has been done. However, at temperatures above 120 ° C, AISI 420
Steel also corrodes. Therefore, in a CO ₂ environment of 120 ° C. or higher, duplex stainless steel containing 22 to 25% of Cr is used. However, duplex stainless steel is an expensive material for use in a CO ₂ environment. So AISI42
There has been a long-awaited need for the development of a grade having an intermediate performance between 0 steel and duplex stainless steel (withstanding high temperature CO ₂ environment of 180 ° C. or lower) and price. This steel is referred to below as low C- (1
2-16)% Cr-Ni-Cu-Mo system Martensitic stainless steel.

This low C- (12-16)% Cr-Ni-
Since Cu-Mo-based martensitic stainless steel has extremely high hardness of the tube material after slabbing, it is impossible or extremely difficult to saw the tube material. was there.

[0005]

The present invention is a mass production AISI.
Trouble in manufacturing with the same process as 420 steel (that is, low C- (12-16)% Cr-Ni-Cu-M having the property of withstanding in a high temperature CO ₂ environment of 180 ° C. or less).
It is an object of the present invention to solve the problem that it is difficult to saw a pipe material after slabbing of o-type martensitic stainless steel) and to establish a manufacturing process capable of industrial mass production.

In order to solve the above manufacturing trouble, the Vickers hardness of the low C- (12-16)% Cr-Ni-Cu-Mo type martensitic stainless steel is set to 27.
Must be 0 or less.

[0007]

As a result of various experiments to solve the above problems, low C- (12 to 1
The present invention was completed by finding a manufacturing method for softening a Cr-Ni-Cu-Mo system martensitic stainless steel (6%). In the present invention, C: 0.005-0.1% by weight%, S
i: 0.05 to 1%, Mn: 0.1 to 2%, P: 0.0
25% or less, S: 0.015% or less, Cr: 12 to 16
%, Ni: 0.1 to 4%, Cu: 0.1 to 3%, Mo:
0.1-3%, Al: 0.005-0.2%, N: 0.
A steel containing 005 to 0.1% and the balance of Fe and unavoidable impurities was subjected to a first-stage tempering treatment in a temperature range of 640 ° C. or higher and 770 ° C. or lower, and then 600 ° C. or higher Ac.
_The gist is a method for producing martensitic stainless steel with excellent corrosion resistance, which comprises performing a second-stage tempering treatment in a temperature range of ₁ point or less.

[0008]

After performing the first-stage tempering treatment at a temperature of 640 ° C. or higher and 770 ° C. or lower as described above, 600 ° C. or higher and Ac
_The reason why the low C- (12-16)% Cr-Ni-Cu-Mo system martensitic stainless steel is softened by performing the second stage tempering treatment at a temperature of ₁ point or less is as follows. I'm guessing.

Low C- (12-16)% Cr-Ni-Cu
-Mo type martensitic stainless steel 640 ℃ or more 7
The structures produced by performing the first-stage tempering treatment in the temperature range of 70 ° C. or lower are tempered martensite and fresh martensite. Further, Cu is precipitated from the matrix. If it exceeds 770 ° C, the amount of fresh martensite is larger than the amount of tempered martensite, and the hardness is too high. Therefore, the upper limit of the tempering temperature of the first stage was set to 770 ° C. On the other hand, when the temperature is lower than 640 ° C., Cu is finely precipitated to be hardened, so that the lower limit is set to 640 ° C.

[0010] Fresh martensite is produced by a single-stage tempering treatment. Since the structure has a high hardness, it is considered that when the second-stage tempering treatment is performed once again at the Ac ₁ point or less, the whole structure becomes a tempered martensite structure and is softened. Therefore, the upper limit of the tempering temperature of the second stage was set to Ac ₁ point. Here, the lower limit of the second-stage tempering treatment is set to 600 ° C. Even if the tempering treatment is performed at a temperature lower than 600 ° C., since it is a low temperature tempering, sufficient tempering treatment cannot be performed, and therefore the hardness does not change. That is, the Vickers hardness exceeds 270, which makes it difficult to saw the pipe material in an actual production line.

Next, the reasons for limiting the composition range of the target steel will be described below. C: C is an element necessary for producing martensitic stainless steel. If it is less than 0.005%, it becomes difficult to make the structure into a martensitic single phase, and if it exceeds 0.1%, Cr carbide is generated. Since a large amount is present and the CO ₂ corrosion resistance is deteriorated, the content range is set to 0.005 to 0.1%.

Si: Si is an element necessary for deoxidation, but if it is less than 0.05%, its effect is not sufficient, and if it is added in excess of 1%, the impact toughness decreases, so the content range is set. It was set to 0.05 to 1%. Mn: an element effective for deoxidizing and securing strength,
If it is less than 0.1%, the effect is not sufficient, and if it is added in excess of 2%, the effect is saturated.
It was set to 1-2%.

Cr: Cr is the most basic and essential element that constitutes martensitic stainless steel, and is resistant to CO
₂ It is an element necessary for imparting corrosiveness, but if the content is less than 12%, the corrosion resistance is not sufficient, while if it is added in excess of 16%, it becomes difficult to form a martensite single phase. The content range was 12 to 16%. In order to obtain a martensite single phase, it is desirable that the content be 12% or more and 15% or less.

Al: Al is an element necessary for deoxidation, and if the content is less than 0.005%, its effect is not sufficient, and if it exceeds 0.2%, it is a coarse oxide inclusion. Remains in the steel and reduces toughness, so the content range was made 0.005 to 0.2%. N: N is essential because it is an austenite forming element, but if it is less than 0.005%, it is difficult to form a single phase of martensite at room temperature, and if it exceeds 0.1%, the impact toughness of the base material decreases, so The content range is 0.005 to 0.
It was set to 1%.

P: P is an element that reduces toughness, so the upper limit content is made 0.025%. S: S, like P, is an element that reduces toughness, so the upper limit content was made 0.015%. Ni: Ni is an austenite forming element that stabilizes martensite and improves CO ₂ corrosion resistance.
If less than 4%, the effect is not sufficient, and if added over 4%, the effect is saturated and the cost increases, so the content range was made 0.1 to 4%. Desirably 2 to 4
%.

Cu: Cu, like Ni, is an austenite-forming element and improves CO ₂ corrosion resistance, but if it is less than 0.1%, its effect is not sufficient, and if it exceeds 3%, the cost rises, so its inclusion. The amount range was 0.1 to 3%. It is preferably 1.0 to 3.0%. Mo: Mo is an element effective for improving CO ₂ corrosion resistance or pitting corrosion resistance, but if it is less than 0.1%, its effect is not sufficient, and if it exceeds 3%, ferrite tends to be generated. In addition, the content range was set to 0.1 to 3%. It is preferably 0.1 to 2.0%.

[0017]

EXAMPLES Table 1 shows the chemical composition of the test steel and the heat treatment conditions for the production. Hardness of each sample steel and 180 ° C, CO ₂ 4
Table 1 also shows the corrosion rate at 0 atm. Table 2 shows tempering conditions as the heat treatment conditions shown in Table 1. No.
In any of the steel grades 1 to 8, if the first stage tempering temperature is 640 ° C. or higher and 770 ° C. or lower and then the second stage tempering process is performed at 600 ° C. or higher and Ac ₁ or lower, the Vickers hardness becomes 270 or lower. It can be seen that it is easy to saw the pipe material after the slabbing in the actual production line. Also N
o. It can be seen that in any of the steel types 1 to 8, the corrosion rate in the 180 ° C. CO ₂ environment is 0.1 mm / y or less and the CO ₂ corrosion resistance is good. Furthermore, FIG.
The influence of the tempering temperature on the hardness when tempered at 00 ° C to 750 ° C is shown. Ac of steel type 4 at this time
_One point is 645 ° C. The mark Δ indicates that the tempering treatment was performed only once at various temperatures from 600 ° C. to 750 ° C. A circle indicates that the first-stage tempering treatment was performed at 750 ° C. and then the second-stage tempering treatment was performed at 640 ° C., ×
The mark indicates 6 after the first tempering treatment at 670 ° C.
It was subjected to the second stage tempering treatment at 40 ° C. 60
The hardness is extremely high and the cutting of the pipe material becomes extremely difficult by only one tempering treatment in the temperature range from 0 ° C to 750 ° C. On the other hand, when the first-stage tempering process is performed in the temperature range of 670 ° C. or 750 ° C. and then the second-stage tempering process is performed at the temperature of 640 ° C., Vickers hardness of 270 is obtained.
It was found to soften below. As a result, it becomes extremely easy to saw the pipe material on the actual production line.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

According to the present invention, it is possible to mass-produce martensitic stainless steel excellent in CO ₂ corrosion resistance.

[Brief description of drawings]

FIG. 1 is a tempering temperature-hardness curve diagram when tempering is performed in a temperature range of 600 ° C. or higher and 750 ° C. or lower.

Claims (1)

[Claims] 1. C: 0.005 to 0.1% by weight, Si: 0.05 to 1%, Mn: 0.1 to 2%, P: 0.025% or less, S: 0.015 % Or less, Cr: 12 to 16%, Ni: 0.1 to 4%, Cu: 0.1 to 3%, Mo: 0.1 to 3%, Al: 0.005 to 0.2%, N: Steel containing 0.005 to 0.1% and the balance of Fe and unavoidable impurities is subjected to the first-stage tempering treatment in a temperature range of 640 ° C. or higher and 770 ° C. or lower, and then 600 ° C. or higher with an Ac ₁ point. A method for producing martensitic stainless steel having excellent corrosion resistance, which comprises performing a second-stage tempering treatment in the following temperature range.