JPH07179987A - High strength steel excellent in hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To produce a high strength steel excellent in hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance by preparing a steel of specific composition, in which metallic structure is composed of acicular ferrite phase and C inclusions are specified. CONSTITUTION:A high strength steel consisting of, by weight, 0.02-0.06% C, 0.03-0.50% Si, 0.5-2.0% Mn, <=0.02% P, <=0.005% S, 0.001-0.005% Ca, 0.01-0.05% sol.Al, <=0.0015% O, further one or >=2 kinds among <=0.5% Cu, <=0.5% Ni, <=0.5% Cr, and <=0.5% Mo, and the balance essentially Fe and in which the metallic microstructure of the steel is composed of acicular ferrite and the number of C type inclusions of >=5mum diameter in the steel is regulated to <10pieces/mm<2>, is prepared. By this method, the high strength steel, excellent in hydrogen induced cracking resistance and sulfide stress corrosion cracking resistance, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to hydrogen-induced crack resistance (HIC resistance) not only in the base metal part but also in the welded joint part of a line pipe used in a humid hydrogen sulfide environment.
And high-strength steel for line pipes having excellent sulfide stress corrosion cracking resistance (SSC resistance).

[0002]

2. Description of the Related Art In recent years, oil fields containing a large amount of hydrogen sulfide,
With the rapid development of gas fields, high resistance to hydrogen-induced cracking has come to be required for the line pipes used for these transportations. That is, in such a wet hydrogen sulfide environment, hydrogen atoms generated on the steel surface by the corrosion reaction penetrate into the steel, the hydrogen atoms are trapped at trap sites such as nonmetallic inclusions, and by gasification, So-called hydrogen-induced cracking (HIC)
And sulfide stress corrosion cracking (SSC: Sulfide StressCorr
This is because osion cracking) may occur and the pipe may be destroyed.

The former is a crack that occurs parallel to the rolling direction in the unloaded state, and the latter is a crack that occurs in the sheet thickness direction under stress. These cracks tend to occur as the strength of the pipe increases. With the increase in strength of line pipes, line pipes having more excellent HIC resistance and SSC resistance have been required. HIC, SSC
As a countermeasure, the method of reducing S, Ca, RE
Method for controlling morphology of inclusions such as MnS by adding M
(For example, JP-A-54-38214 and JP-A-63-140033) have been adopted.

Since HIC and SSC originate from inclusions that are stretched by rolling like MnS, the above method reduces MnS by reducing the amount of S in steel, and further Ca and REM. Is a method of spheroidizing the form of these sulfide inclusions. Also,
As a method of reducing the HIC sensitivity by controlling the metallographic structure of steel, a method of improving the micrometallographic structure of the segregation portion by heat treatment such as quenching and tempering (for example, JP-A-54-1)
No. 27821) has been taken.

However, as the grade of the line pipe used in a wet hydrogen sulfide environment increased from API standard X52 to X65, the line pipe HIC and SSC susceptibility increased, resulting in spheroidization. HIC and SSC have started to occur even with inclusions as the starting point. Therefore, in controlling the form of inclusions, A type inclusions are replaced by spherical C
In addition to controlling to system inclusions, a method of improving the HIC resistance performance by increasing the proportion of C system inclusions (Japanese Patent Laid-Open No. 2-185948) has come to be adopted.

[0006]

However, the prior art described above, that is, by dispersing a large number of fine C-based inclusions as trap sites for hydrogen atoms in the steel material, prevents localized accumulation of hydrogen. Also in the method, it is difficult to completely prevent the generation of HIC because there are many inclusions that are the starting points of cracking. Further, the method of reducing HIC sensitivity by heat treatment cannot be said to be a practical means in terms of production efficiency and manufacturing cost.

[0007]

Means for Solving the Problems As a result of investigations and studies on the effects of the cleanliness of steel and the micro-metallic structure of steel on the HIC resistance and SSC resistance, the inventors have found that the inclusions It was found that the control of not only the shape and size but also the amount of inclusions is effective. That is, by setting the number of C-based inclusions having a diameter of 5 μm or more in the steel material to 10 pieces / mm or less, HIC resistance and SSC resistance are remarkably improved, and the effect is that the structure of the steel sheet is changed to acicular ferrite. It was found that this was remarkable.

(1) The invention according to claim 1 is characterized in that the microstructure of the steel is acicular ferrite, and the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ^2. It is a high strength steel with excellent hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance.

(2) The invention of claim 2 is a high-strength steel (component composition is wt%) excellent in hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance having the following characteristics. (A) As a main component, C: 0.02 to 0.06%, Si: 0.03 to 0.50
%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ca: 0.001 to
0.005%, sol. Al: 0.01-0.05%, O: 0.00
15% or less, further Cu: 0.5% or less, Ni: 0.5% or less,
Cr: 0.5% or less, Mo: 0.5% or less, one or more of them are contained, and the balance is substantially F.
and (b) the micrometallographic structure of the steel is acicular ferrite, and (c) the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ² .

(3) The invention of claim 3 is a high-strength steel (having a composition of wt%) excellent in hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, which has the following characteristics. (A) As a main component, C: 0.02 to 0.06%, Si: 0.03 to 0.50
%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ca: 0.001 to
0.005%, sol. Al: 0.01-0.05%, O: 0.00
15% or less, Nb: 0.10% or less, V: 0.15% or less, T
i: 0.005 to 0.10% of 1 type or 2 types or more, and the balance is substantially F
and (b) the micrometallographic structure of the steel is acicular ferrite, and (c) the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ² .

(4) The invention of claim 4 is a high-strength steel (component composition is wt%) excellent in hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance, having the following characteristics. (A) As a main component, C: 0.02 to 0.06%, Si: 0.03 to 0.50
%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ca: 0.001 to
0.005%, sol. Al: 0.01-0.05%, O: 0.00
15% or less, further Cu: 0.5% or less, Ni: 0.5% or less,
Cr: 0.5% or less, Mo: 0.5% or less, one or more of them, and Nb: 0.10% or less, V: 0.15% or less, T
i: 0.005 to 0.10% of 1 type or 2 types or more, and the balance is substantially F
and (b) the micrometallographic structure of the steel is acicular ferrite, and (c) the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ² .

[0012]

First, the acicular ferritic steel will be described. The microstructure of acicular ferritic steel means a fine mixed microstructure composed of pseudopolygonal ferrite and bainitic ferrite as shown in FIG. This structure exhibits a uniform hardness distribution and, because of its lower hardness than the martensite and bainite structures, it is resistant to HIC and S.
Excellent SC property. In order to obtain an acicular ferrite structure, it is desirable that the rolling end temperature in controlled rolling and the cooling conditions after rolling fall within the ranges described below.

That is, it is desirable that the rolling end temperature be in the austenite temperature range above the Ar ₃ temperature. This is because when the rolling end temperature becomes equal to or lower than the Ar ₁ temperature, ferrite is generated and the ferrite is work hardened, so that the HIC sensitivity becomes high. Further, the cooling temperature after rolling is 5 to obtain a fine and uniform acicular ferrite structure.
It is desirable to set it in the range of 30 ° C./s. This is because at a cooling temperature of 5 ° C. or lower, the structure becomes a mixed structure of ferrite and pearlite, and the HIC resistance and SSC resistance deteriorate. Further, at a cooling temperature of 30 ° C / s or more, the strength increases due to the increase of island martensite (MA), and the H resistance is also high.
This is because the IC property and the SSC resistance are deteriorated.

Next, the reason why the chemical composition of the steel of the present invention is limited as described above will be described below. (1) C (carbon) content The C content is set to 0.02% or more in order to secure the strength and effectively utilize the precipitation hardening of Nb, V, Ti and the like. But 0.
If it exceeds 06%, low-temperature transformation products will be generated during accelerated cooling, and it will not be possible to obtain a fine and uniform acicular ferrite structure effective for improving the HIC resistance and SSC resistance. Therefore, the C content is set to 0.02 to 0.06%.

(2) Si (Si) Content Si is added for deoxidation. If the Si content is less than 0.03%, a sufficient deoxidization effect cannot be obtained, and if it exceeds 0.50%, the weldability and toughness are deteriorated. Deterioration occurs. Therefore, the Si content is 0.
The range is from 03 to 0.50%.

(3) Mn (manganese) amount Mn is also added as a deoxidizing element like Si, and the content is
If it is less than 0.5%, a sufficient deoxidizing effect cannot be obtained.
%, The weldability deteriorates. Further, segregation becomes remarkable, and SSC resistance and HIC resistance are deteriorated. Therefore,
The Mn content was in the range of 0.5 to 2.0%.

(4) P (Phosphorus) Content P is an unavoidable impurity element and is a harmful element that increases hardness in the central portion of the plate thickness by segregation and increases crack susceptibility, so its content is 0.02%. Below.

(5) S (sulfur) amount S is an alloy element inevitable as an impurity, and its content is
If it exceeds 0.005%, sulfide inclusions such as MnS, which are the starting point of HIC, increase, deteriorating SSC resistance and HIC resistance. Therefore, the S content is set to 0.005% or less.

(6) Ca (calcium) amount Ca has a function of spheroidizing the form of sulfide-based inclusions and a function of reducing B-based inclusions, and has a function of reducing the starting point of HIC. However, if it is less than 0.001%, this effect cannot be obtained, and if it exceeds 0.005%, clusters of Ca-based inclusions occur conversely to increase the content of C-based inclusions, leading to deterioration of HIC resistance. Therefore, C
The a content was 0.001 to 0.005%.

(7) Sol. Al (aluminum) amount Al is an element that is added as a deoxidizing element and is necessary to reduce A-based inclusions, but if the content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and 0.05% Al ₂ O
₃ (alumina) clusters increase, B-type inclusions increase, and the HIC resistance performance deteriorates. Therefore, sol. The Al content was 0.01 to 0.05%.

(8) O (total oxygen content) O forms an oxide inclusion with Si, Al, Ca, etc., and H
It becomes the starting point of the IC and adversely affects the HIC resistance.
In order to reduce the amount of oxide-based inclusions, the O content (total oxygen content) is set to 0.0015% or less.

(9) Cu (copper) content Cu is an element that improves strength and corrosion resistance, but if the content exceeds 0.5%, the weldability and HAZ toughness deteriorate, so the content of Cu is 0.5% or less. And

(10) Ni (Nickel) Amount Ni is an element that improves strength, corrosion resistance and toughness.
If the content exceeds 0.5%, it is economically disadvantageous, so N
The content of i was 0.5% or less.

(11) Cr (Chromium) Content Cr is an element that improves strength, toughness, and corrosion resistance, but if the content exceeds 0.5%, the toughness of the welded portion deteriorates, so the Cr content is 0.5. % Or less.

(12) Mo (Molybdenum) Content Mo is an element that improves the strength and toughness, but if the content exceeds 0.5%, it is economically disadvantageous, so the Mo content was made 0.5% or less.

(13) Nb (niobium) content Nb is an element for improving strength, but its content is 0.1%.
If it exceeds, the toughness of the weld and the base metal part will deteriorate, so
The Nb content was 0.1% or less.

(14) V (vanadium) content V is an element that improves strength and toughness, but its content is 0.1.
If it exceeds 15%, the weldability and HAZ toughness deteriorate. Therefore, the V content is set to 0.15% or less.

(15) Ti (titanium) content Ti is an element which prevents coarsening of austenite grains during heating and improves strength and toughness, but its content is 0.005%.
If it is less than, this effect cannot be obtained. On the other hand, if the content exceeds 0.1%, the toughness of the welded portion deteriorates, so the Ti content is 0.005 to 0.1.
%.

[0029]

EXAMPLES Table 1 shows the chemical composition of the slabs of the present invention steels (1-10) and comparative steels (11-20) and the measurement of inclusions (sulfide-based and oxide-based inclusions) in the base metal. The results are shown in Table 2. These slabs were rolled into a thick plate, and the plate thickness was 24 m.
m steel plate, and after manufacturing a steel pipe from this steel plate, the mechanical properties of the base material, HIC characteristics, S of the base material / welded joint
The SC characteristics and the microstructure of steel are shown below. Comparative steels 11 to 15 in Table 1 have the steel composition of the present invention, but the number of C-based inclusions having a diameter of 5 μm or more is 10 pieces / mm ^2.
The steels and comparative steels 16 to 20 which are out of the present invention are
The amount of inclusions is a steel that deviates from the steel of the present invention.

The SSC test was conducted in accordance with the NACE standard (TN-01-77), and the values are shown by the ratio of the load stress to the yield strength of each steel pipe base material (critical fracture stress ratio).
HIC test conforms to NACE standard (TN-02-84) NACE
Using the standard TN-01-77 solution, the average of the crack length ratios of the three cross sections where the indication was confirmed by ultrasonic deep scratches (CL
R: Crack Length Ratio). The inclusions were measured with an optical microscope, and the number of inclusions of 5 μm or more in 1 mm ^{2 of the} surface to be inspected was shown.

FIG. 1 shows the relationship between CLR in Table 2 and the number of inclusions of 5 μm or more in 1 mm ² , and FIG. 2 shows the relationship of SSC critical stress ratio and the number of inclusions of 5 μm or more in 1 mm ^2. Shown respectively. 1 and 2, if the number of inclusions of 5 μm or more is less than 10 / mm ² , HIC resistance and SSC resistance
It can be seen that the characteristics are good.

Further, Table 3 shows steel No. 1 having the chemical composition according to the present invention in Table 1 above. Regarding No. 10, the HIC resistance and SSC resistance of the steel sheet with the structure changed by changing the steel sheet manufacturing method
The characteristics are shown below. The material of the present invention in the table is a steel sheet that is slab-heated to 1150 ° C., then finish-rolled at an Ar ₃ point or more and accelerated cooling at a cooling rate of 10 ° C./min, and exhibits a target acicular ferrite structure. . On the other hand, in the comparative material A that was air-cooled after rolling, the comparative material B that was quenched and tempered, and the comparative material C that was as-quenched, a uniform acicular ferrite structure could not be obtained and the resistance to H
It can be seen that the IC performance and the SSC resistance performance are deteriorated.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

As described above, the metallic structure is the acicular ferrite phase, and the C inclusions are 10 / m.
By setting m ² , hydrogen-induced crack resistance (HIC resistance) and sulfide stress corrosion cracking resistance not only in the base metal part but also in the welded joint part in a line pipe used in a humid hydrogen sulfide environment High-strength steel for line pipes having excellent (SSC resistance) can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between the number of inclusions (pieces / mm ² ) of 5 μm or more and a crack length ratio (CLR%) in a HIC test.

FIG. 2 is a diagram showing the relationship between the number of inclusions (pieces / mm ² ) of 5 μm or more in the base material and the joint in the SCC test and the critical fracture stress ratio.

FIG. 3 is a drawing-substituting photograph showing the microstructure of acicular ferritic steel.

[Procedure amendment]

[Submission date] March 11, 1994

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Name of item to be corrected] Figure 3

[Correction method] Added

[Correction content]

[Figure 3]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ryuji Sasaki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (4)

[Claims] 1. Hydrogen-induced cracking resistance and sulfide resistance, wherein the micrometallurgical structure of steel is acicular ferrite, and the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 pieces / mm ^2. High strength steel with excellent stress corrosion cracking resistance. 2. A high-strength steel excellent in hydrogen-induced cracking resistance and sulfide stress corrosion corrosion cracking resistance (composition composition is wt%) having the following characteristics. (A) As a main component, C: 0.02 to 0.06%, Si: 0.03 to 0.50
%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ca: 0.001 to
0.005%, sol. Al: 0.01-0.05%, O: 0.00
15% or less, further Cu: 0.5% or less, Ni: 0.5% or less,
Cr: 0.5% or less, Mo: 0.5% or less, one or more of them are contained, and the balance is substantially F.
and (b) the micrometallographic structure of the steel is acicular ferrite, and (c) the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ² . 3. A high-strength steel excellent in hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance (having a composition of wt%) having the following characteristics. (A) As a main component, C: 0.02 to 0.06%, Si: 0.03 to 0.50
%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ca: 0.001 to
0.005%, sol. Al: 0.01-0.05%, O: 0.00
15% or less, Nb: 0.10% or less, V: 0.15% or less, T
i: 0.005 to 0.10% of 1 type or 2 types or more, and the balance is substantially F
and (b) the micrometallographic structure of the steel is acicular ferrite, and (c) the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ² . 4. A high-strength steel excellent in hydrogen-induced cracking resistance and sulfide stress corrosion cracking resistance (having a composition of wt%) having the following characteristics. (A) As a main component, C: 0.02 to 0.06%, Si: 0.03 to 0.50
%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005% or less, Ca: 0.001 to
0.005%, sol. Al: 0.01-0.05%, O: 0.00
15% or less, further Cu: 0.5% or less, Ni: 0.5% or less,
Cr: 0.5% or less, Mo: 0.5% or less, one or more of them, and Nb: 0.10% or less, V: 0.15% or less, T
i: 0.005 to 0.10% of 1 type or 2 types or more, and the balance is substantially F
and (b) the micrometallographic structure of the steel is acicular ferrite, and (c) the number of C-based inclusions having a diameter of 5 μm or more in the steel is less than 10 / mm ² .