JP2721420B2 - Sour-resistant steel for electric resistance welded steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a steel for electric resistance welded steel pipe having excellent sulfide stress corrosion cracking resistance and having both hydrogen cracking resistance to organic cracking and low temperature toughness. Alternatively, a sour-resistant electric resistance welded steel pipe suitable for oil and gas pipelines used in a humid environment containing carbon dioxide at a high concentration (hereinafter referred to as a sour environment) and in a low-temperature environment such as a cold region is provided. Is what you do.

(Prior Art) In recent years, mining and transportation of petroleum and natural gas in a sour environment containing a high concentration of hydrogen sulfide and the like, and further in a low-temperature environment such as a cold region have been increasing.

In pipelines used for this purpose, hydrogen-induced cracking and sulfide stress corrosion cracking may occur in a sour environment, and low-temperature brittle fracture may occur in a low-temperature environment. With the progress of steel, the development of steel materials with excellent fracture resistance that can suppress the occurrence of these cracks that cause greater damage has become increasingly important.

Many researches and developments have been made on the causes of these cracks and how to prevent them.

Regarding cracking in a sour environment, hydrogen generated by corrosion of steel penetrates and diffuses into the steel in the atomic state and accumulates and molecularizes at the interface between inclusions and ground iron, causing the cracks due to the pressure of hydrogen gas generated. Hydrogen-induced cracking that occurs along the band-like hardened structure that tends to occur in the central segregation part (concentrated segregation part of component elements), which is the final solidification part in steel, or in the presence of load stress It is known that sulfide stress corrosion cracking occurs alone or in combination.

Conventionally, the following means have been used to prevent hydrogen-induced cracking.

(1) A method in which corrosion of steel or intrusion of hydrogen is suppressed by adding Ni, Cu, Cr, etc. (for example, Japanese Patent Application Laid-Open No. 50-97515).

(2) A method for controlling the morphology of MnS, which is the starting point of cracking, by adding Ca, REM, etc. to the sphere (for example, JP-A-53-14606,
JP-A-54-38214).

(3) A method of reducing the content of Mn, P, etc., or reducing the degree of segregation at the center segregated portion by subjecting the cast slab to soaking diffusion (for example, Japanese Patent Application Laid-Open No. 52-111815, Kaisho 50-
No. 97517).

(4) A method of reducing cracking susceptibility by reheating after rolling, performing quenching, tempering or normalizing, and mainly improving the microstructure of the segregated portion.

Etc. have been tried.

On the other hand, as means for preventing sulfide stress corrosion cracking, the above (1) to (4) are effective not only for improving hydrogen-induced cracking resistance but also for improving sulfide stress corrosion cracking resistance. It is known that the addition of the element for suppressing hydrogen intrusion of (2) and the heat treatment of (4) improve the critical stress for the occurrence of sulfide stress corrosion cracking.

Also, a method of appropriately adjusting V and N to form fine VN precipitates to improve sulfide stress corrosion cracking resistance (Japanese Patent Application Laid-Open No.
62-213346) and a method of reducing P, S, O, N and appropriately adjusting the rolling and cooling conditions to obtain the same effect (Japanese Patent Application Laid-Open No. 62-112722). Etc. have also been proposed.

(Problems to be Solved by the Invention) However, the methods that have been conventionally tried have the following problems with respect to the resistance to sulfide stress corrosion cracking.

For example, the method (1) impairs the effect of suppressing the intrusion of hydrogen due to the addition of Cu and Ni in a severe sour environment with a low pH.

In addition, in the method of adding Ni, minute local corrosion progresses from the surface of the steel, and a phenomenon that sulfide stress corrosion cracking is easily promoted also occurs.

Also in the method (4), the quenching and tempering treatments cause an extremely large economic loss in industrial production.

Also, a method of improving the sulfide stress corrosion cracking resistance by adding V and N disclosed in Japanese Patent Application Laid-Open No. 61-213346, not only increases the production cost by adding a large amount of V, but also increases the production cost. It may impair toughness.

Furthermore, JP 62-112722 Patent P by JP, S, O, N quantity limitations and also by a method of combination of rolling and cooling conditions, in particular the tensile strength of 63kgf / mm ² greater than such high-strength steel In A
Other problems such as sulfide stress corrosion cracking caused by oxidized inclusions including l, which are detected as defects in ultrasonic flaw inspection performed after manufacturing ERW steel pipes There is also a high risk of occurrence.

In any case, in the production of steel having excellent sulfide stress corrosion cracking properties, for which the necessity has become increasingly important, and having both hydrogen-induced cracking resistance and low-temperature toughness, the above-mentioned means are required. Is usually combined as appropriate,
A steel material that is excellent in economy and that can completely suppress the occurrence of sulfide stress corrosion cracking and the like has not yet been completed, and there is a great need for further improvement.

The present invention has been established in such a situation, and solves the above-mentioned problems, has excellent sulfide stress corrosion cracking resistance, and has hydrogen-induced cracking resistance and high toughness at low temperatures. It is an object to obtain steel for a sewn steel pipe.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a method for controlling the morphology of MnS, which is 0.004 to 0.024% by weight, and the elimination of oxide-based inclusions. In the following formula, a sour-resistant steel for electric resistance welded steel pipe characterized by containing Ca satisfying a value of 1.5 to 4.5 and substantially not adding Ni and Cu is used.

The reasons for limiting the constituent requirements will be described below.

As described above, steel materials used in a sour environment are required to have both hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance.

Accordingly, the inventors of the present invention have repeated various experimental studies from the viewpoint that the means for suppressing sulfide stress corrosion cracking, which is the main feature of the present invention, must be established under conditions that can prevent hydrogen-induced cracking. The findings shown in FIG. 1 were obtained. Based on this finding, the conditions for suppressing hydrogen-induced cracking are If the value of the conditional expression satisfies 1.5 or more, hydrogen-induced cracking can be prevented even in a severe sour environment of pH ≒ 3 (smaller as the pH value is smaller). If the value of the conditional expression exceeds 4.5, the sulfide stress corrosion cracking resistance is degraded due to an increase in oxide-based inclusions due to excessive addition of Ca, and the occurrence of surface blisters (blisters) due to hydrogen intrusion increases, and economic efficiency is increased. Is found to decrease.

However, meeting these conditions alone cannot stably prevent sulfide stress corrosion cracking.
As a result of the analysis, it was found that oxide inclusions containing Al often originated in cracks, and Al was reduced to 0.024% or less and its formation was suppressed to prevent sulfide stress corrosion cracking. It has been found that the resistance is stable.

For Al, the amount required to obtain the deoxidizing effect
The lower limit was 0.004%.

As described above, Ca and Al corresponding to S and O are added to the minimum necessary amount, and Ni which has a bad influence on sour resistance and Cu which does not exert any effect in a severe sour environment are not substantially added. The above-mentioned conditions are limited because the production of a product is suppressed, and a steel excellent in sulfide stress corrosion cracking resistance and hydrogen-induced cracking resistance can be economically obtained with high productivity.

The sour resistant steel targeted by the present invention, except for the above-mentioned Al and Ca, is described in, for example, the above-mentioned JP-A-62-112722, and as described below, required materials for ordinary sour resistant steel In order to achieve the same effect, the same effect and effect can be obtained by using the type and amount of additive element determined based on the relationship between the effect and effect, which has been confirmed in the field of industry. Can be Therefore, the present invention is intended to include steels including these.

The constituent elements, the reasons for their addition, and their amounts are shown below.

C is to secure the strength of the base metal and welded parts and the internal quality, weldability, HAZ
0.15% or less to prevent toughness deterioration.

Si is added for deoxidation, and is set to 0.6% or less to prevent deterioration of weldability and HAZ toughness.

Mn is set to 0.6% or more and 1.5% or less to secure strength and toughness, and to maintain HIC propagation stopping ability and toughness of base metal and HAZ.

 P is set to 0.007% or less to improve center segregation.

Ti is added to refine the rolled structure and the HAZ structure, and is 0.003 to 0.03% in order to prevent toughness deterioration.

N is set to 0.0035% or less in order to prevent deterioration of HIC resistance due to coarse nitride-based inclusions.

Cr, Mo, Nb, V, and B are each added in one or more types for the purpose of HIC resistance, strength toughness, grain refinement of the structure, promotion of formation of the bainite structure, etc. 1.0,0 each.
5,0.1,1.0,0.005% or less.

(Action) According to the method of the present invention, the formation of oxide-based inclusions (Al ₂ O ₃ clusters and the like, which are the starting points of sulfide stress corrosion cracking, can be suppressed. Cracks can be prevented without adding steel, and it is also possible to prevent defects due to inclusions in ultrasonic flaw inspection after forming ERW steel pipes, and it has excellent economical effects it can.

(Example) Next, an example of the present invention will be described.

API Spec.5L X52 to X80 equivalent steel pipe for sour line pipes
Using a slab of a sour resistant steel having a composition consisting of the chemical components shown in the table, a plate thickness of 7 to 7 was obtained through a heating-continuous hot rolling-cooling process.
14mm steel plates were manufactured.

 Steel numbers 1 to 9 are inventive steels, and 10 to 18 are comparative steels.

Table 2 shows the conditions of the heating-rolling-cooling process, the mechanical properties of the steel sheet, the resistance to hydrogen-induced cracking and the resistance to sulfide stress corrosion cracking.

The hydrogen-induced cracking test uses a test piece 20 mm wide and 100 mm long with a thickness of 1 mm for each of the front and back surfaces of the steel sheet.The sulfide stress corrosion cracking test was performed according to NACE Standard TM0177.
In a Proofring type constant load tensile test method according to Method A specified in -90, a deflection corresponding to 50 to 100% of the yield stress was applied to the test piece.

The immersion liquid is 0.5% CH ₃ COOH-5% N saturated with H ₂ S at 25 ° C.
An aCl aqueous solution (pH ≒ 3) was used, and immersion in the solution was performed for 4 days in the hydrogen-induced cracking test, 30 days in the sulfide stress corrosion cracking test, and the breakage up to that day.

The ratio between the minimum applied stress at which sulfide stress corrosion cracking does not occur and the yield stress (the critical stress ratio at which cracking occurs) was used as an index indicating sulfide stress corrosion cracking resistance.

The hydrogen-induced cracking resistance was determined by using a crack area ratio detected by ultrasonic flaw detection as an index for evaluation.

Steel Nos. 1 to 9 are examples of the present invention having a tensile strength of 〜50 to 75 kgf / mm ² , and have excellent sulfide stress corrosion cracking resistance exceeding all cracking limit stress ratios of 0.80, which is a generally required level. In addition, there is no occurrence of hydrogen-induced cracking, and excellent low-temperature toughness is obtained.

Among the comparative steels, steel Nos. 10 to 12 have a high Al content of 0.031 to 0.055%, so that the sulfide stress corrosion cracking resistance is deteriorated even though the other conditions are within appropriate ranges.

In steel Nos. 13 to 15, hydrogen-induced cracking has occurred because the values of the regulatory conditional expressions for S, O, and Ca are too low.

In steel Nos. 16 to 18, blisters are partially generated because the value of the above conditional expression is too high.

(Effect of the Invention) The present invention described above uses Al and Ca in a limited amount range without substantially adding Ni and Cu, thereby eliminating inclusions in a more economical manner. A steel material with excellent reduced sulfide stress corrosion resistance and excellent hydrogen-induced cracking resistance and toughness at the same time, which is used for oil and gas transportation used in harsh sour environments. It can be used for ERW steel pipe production, and the effect of the invention is extremely large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a conditional expression showing a degree of form control of MnS. Value and sulfide stress corrosion cracking resistance (Crack initiation critical stress ratio:
The larger the better, the better the relationship).

Claims (1)

(57) [Claims] (1) 0.004 to 0.024% by weight of Al and 1.5 to 4.5
A steel for a sour-resistant electric resistance welded steel pipe, characterized by containing Ca satisfying the above, and substantially not adding Ni and Cu.