JP3473502B2 - Method for producing steel for in-line heat treatment and seamless steel pipe made of this steel having excellent sulfide stress corrosion cracking resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for in-line heat treatment which, when subjected to in-line heat treatment, has a small strength variation, and as a result, a high-strength product excellent in sulfide stress corrosion cracking resistance is obtained. The present invention relates to a method for manufacturing a seamless steel pipe made of steel, specifically, a casing for oil wells and gas wells, tubing, and a drill pipe for drilling.

[0002]

2. Description of the Related Art Seamless steel pipes, which are more reliable than welded steel pipes, are often used in harsh oil well environments and high temperature environments, and have higher strength, improved toughness, and sulfide stress cracking resistance (hereinafter , SSC resistance) is always required.

As a conventional steel having excellent SSC resistance, (a)
80-90% or more martensitic steel, (b) steel containing no coarse carbide, (c) clean steel with few non-metallic inclusions, (d) high temperature tempered steel, (e) fine grain steel, f) high yield ratio steel, (g) low Mn-low P-low S steel, (h) steel containing a large amount of insoluble nitride, and (i) Zr-added steel.

Among the conventional steels of (a) to (i) above, the fine grain structure steel of (e) is a steel developed to obtain a high-performance seamless steel pipe, and has a fine grain size of austenite grain size. It is a steel aimed at.

As a typical method for producing this fine grain steel, there is a method disclosed in Japanese Patent Laid-Open No. 4-328170. That is, the method is 0.01-0.1%
This is a method of performing the quenching and tempering heat treatment off-line on the steel pipe after pipe making using Nb-added steel.

However, this method is disadvantageous from the viewpoint of production efficiency and energy saving, as compared with the method of in-line heat treatment. Further, in order to avoid this disadvantage, when heat treatment is performed in-line, NbC is nonuniformly precipitated before quenching, resulting in extremely large variation in strength of the product, and the SSC resistance of the portion having high strength deteriorates. It has drawbacks. Further, the above method also has a drawback in that it requires two or more inclined rolling machines in succession after piercing and rough rolling, which increases equipment costs.

[0007]

An object of the present invention is to provide a high-strength product which is low-cost in-line heat treatment as compared with off-line treatment and has small strength variations, resulting in excellent sulfide stress corrosion cracking resistance. Another object of the present invention is to provide a steel for in-line heat treatment that is used and a method for producing a seamless steel pipe made of this steel.

[0008]

The gist of the present invention resides in the following (1) steel for in-line heat treatment and (2) a method for producing a seamless steel pipe excellent in sulfide stress corrosion cracking resistance.

(1) C: 0.23 to 0.35 by weight%
%, Si: 0.1 to 1.5%, Mn: 0.1 to 2.5
%, P: 0.03% or less, S: 0.004% or less, A
1: 0.001-0.1%, V: 0.01-0.25
%, Ti: 3.4 × N% or less, Cr: 0 to 1.5%, M
o: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.0
030%, Ca: 0 to 0.0050%, Mg: 0 to 0.
0050%, Zr: 0 to 0.10%, while N
And Nb are regulated to 0.0080% or less and less than 0.005%, respectively, and the balance substantially consists of Fe.

(2) The billet made of the steel for in-line heat treatment as described in (1) above is hot-perforated and stretch-rolled,
The seamless steel pipe produced under the conditions of the final rolling temperature of 950 to 1100 ° C. is supplemented with in-line temperature in the temperature range of Ar ₃ transformation point to 950 ° C., then quenched as it is at the Ar ₃ transformation point or higher, and then Ac ₁ A method for producing a seamless steel pipe that is excellent in sulfide stress corrosion cracking resistance to be tempered below the transformation point.

The present inventions (1) and (2) are completed based on the following knowledge. That is, in order to achieve the above-mentioned object, the inventors manufactured seamless steel pipes with various steel components and various rolling conditions, and repeatedly studied the relationship between the rolling conditions and the grain size, and further the strength. As a result, the following was revealed.

In the in-line heat treatment process of quenching while maintaining (including supplementary heat) at a temperature of Ar ₃ transformation point or higher after final rolling, Ti and Nb carbides and / or carbonitrides which begin to precipitate in the austenite region Is unevenly deposited over the entire area of the seamless steel pipe before quenching, which causes strength variations.

The variation in strength due to the non-uniform precipitation of carbides and / or carbonitrides can be eliminated by reducing the Nb content as much as possible and the Ti content below the amount required for N fixing. .

In the in-line heat treatment process in which a relatively long seamless steel pipe is quenched without supplemental heating while it is finally rolled in a temperature range not lower than the Ar ₃ transformation point, temperature unevenness occurs in the circumferential or longitudinal direction of the steel pipe. However, this causes variations in strength.

Particularly, when the final rolling is performed at a high temperature of 1100 ° C. or higher, the structure becomes coarse and the SSC resistance is lowered.

On the other hand, when the final rolling temperature is lower than 950 ° C., the structure becomes an expanded grain structure and anisotropy occurs in toughness and SSC resistance, and even if supplementary heat is applied thereafter, it cannot be eliminated. Moreover, the temperature required for supplementary heat cannot be secured.

After the final rolling, if the temperature of the entire steel pipe is soaked by supplementing heat at 950 ° C. or lower and at the Ar ₃ transformation point or higher, the temperature unevenness of the steel pipe is eliminated, and during that, recrystallization and fine graining occur, resulting in strength variations. SSC resistance is improved.

At this time, if V is added in an appropriate amount, SSC resistance
The property is further improved, and the SSC resistance required for an oil country tubular good is secured.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. In the following, "%" means "% by weight".

First, the reason why the chemical composition of the steel of the present invention is determined as described above will be explained.

C: C is contained for the purpose of ensuring the strength of the steel pipe .
If it is less than 23 %, the hardenability is insufficient and the tempering temperature is lowered,
It becomes difficult to secure the required strength. However, if the content exceeds 0.35%, not only quench cracks occur but also the toughness deteriorates. Therefore, the C content is 0.23 to 0.35.
%. A preferred range is 0.23 to 0.30%.

Si: Si is usually added as a deoxidizing agent for steel, and contributes to an increase in strength by strengthening the resistance to temper softening. 0.1% is required to obtain these effects.
However, if the content exceeds 1.5%, the hot workability is significantly reduced. Therefore, the Si content is 0.1 to 1.
It was set to 5%. A preferred range is 0.10 to 0.50%.

Mn: Mn is an element effective in increasing the hardenability of steel and ensuring the strength of the steel pipe. However, if the content is less than 0.1%, the strength, toughness, and corrosion resistance decrease due to insufficient hardenability. On the other hand, if the content exceeds 2.5%, segregation increases and toughness decreases. Therefore, M
The n content was 0.1 to 2.5%. A preferable range is 0.20 to 1.0%.

P: P is present in steel as an unavoidable impurity, and if its content exceeds 0.05%, it segregates at grain boundaries and reduces toughness. Therefore, the P content is 0.05%
And The lower the P content, the better.

S: S, like P, exists in steel as an unavoidable impurity and is Mn or Ca when added.
To form inclusions (MnS, CaS). These inclusions are stretched by hot rolling to reduce toughness. However, if the S content is 0.004% or less, there is no particular problem, so the upper limit was made 0.004%. In addition,
The lower the S content, the better.

Al: Al is an element necessary for deoxidizing steel. If the content of Al is less than 0.001%, the quality of steel deteriorates due to insufficient deoxidation and the toughness decreases. But 0.1
If it is contained in excess of%, the toughness is rather lowered.
Therefore, the Al content is set to 0.001 to 0.1%.
A preferred range is 0.01 to 0.07%. The Al referred to in the present invention means acid-soluble Al (sol. Al).

V: V is an element effective for enhancing the SSC resistance, and also has the effect of enhancing the strength by secondary precipitation strengthening, and it can be tempered at a high temperature by obtaining a predetermined strength. It contributes to the improvement of SSC resistance. Also,
Since VC has a large solubility in the austenite phase, it is a solid solution during in-line quenching and does not cause variations in strength. To obtain these effects, 0.01
% Or more is required. However, if the content exceeds 0.25%, the toughness deteriorates significantly. Therefore, the V content is set to 0.01% to 0.25%. The preferred range is 0.0
It is 2 to 0.1%.

Ti: Ti has a strong bonding force with N, which will be described later, which is an unavoidable impurity in steel, and has the effect of forming stable TiN from a high temperature to fix N. However, when N is contained in an amount necessary for fixation, that is, in excess of 3.4 × N%, excess Ti is precipitated as TiC, which begins to precipitate in the final rolling temperature range. Similarly, it causes variations in strength. Therefore, the Ti content is 3.4 ×
N% or less.

Cr: Cr may not be added. If added, it has the effects of increasing hardenability and temper softening resistance, and improving strength and SSC resistance, and is an effective element particularly in the case of manufacturing thick steel pipes. Therefore, it can be added if this effect is desired. The effect becomes remarkable at 0.1% or more, but if the content exceeds 1.5%, the toughness deteriorates. For this reason, the Cr content when added is 0.
It is preferable to be 1 to 1.5%. The preferred range is 0.2-
It is 0.7%.

Mo: Mo may not be added. When added, it has an effect of enhancing hardenability and temper softening resistance, and improving strength and SSC resistance, like Cr, and is an effective element particularly when manufacturing thick-walled steel pipe. Also, Mo
Also has the effect of improving sour resistance. Therefore, if it is desired to obtain these effects, they can be added. The effect becomes remarkable at 0.1% or more, but 1.0%
If the content is exceeded, the toughness deteriorates. Therefore, when added, the Mo content is preferably 0.1 to 1.0%. A preferable range is 0.2% to 0.8%.

W: W may not be added. When added, it is an element that enhances the hardenability and the temper softening resistance, and improves the strength and the SSC resistance, like the above Cr and Mo.
Therefore, it can be added if this effect is desired. The effect becomes remarkable at 0.1% or more, but 1.
If it exceeds 0%, it will be saturated. Therefore, the W content when added is preferably 0.1 to 1.0%. A preferred range is 0.1 to 0.5%.

B: B may not be added. If added, it has the effect of significantly increasing the hardenability and the strength of the steel, and is an effective element especially when manufacturing thick-walled steel pipes. Therefore, it can be added if this effect is desired. The effect becomes remarkable at 0.0005% or more, but if it exceeds 0.0030%, carbonitrides are likely to precipitate at grain boundaries, which causes deterioration of toughness. Therefore, the B content when added is 0.0005 to 0.0
It is good to set it to 030%. The preferred upper limit is 0.0015
%.

Ca, Mg: These elements may not be added. If added, it reacts with S existing in steel as an unavoidable impurity to form a sulfide. This sulfide is spherical even after rolling and does not extend in the rolling direction. As a result, while improving the impact property in the direction perpendicular to the rolling, hydrogen-induced cracking is suppressed. Therefore, in order to obtain this effect, either one can be added alone or both can be added in combination. The effect of each element is 0.00
When it is contained in an amount of more than 0.0050%, the amount of inclusions in the steel increases and the cleanliness decreases,
Various performances are degraded. Therefore, the content of these elements when added is 0.0005 to 0.0
It is good to set it to 050%. In addition, when both are added in combination, the total content is preferably 0.0050% or less.

Zr: Zr may not be added. If added, it has a function of binding to N described later existing in the steel as an unavoidable impurity and forming a nitride to fix N, as in the case of Ti described above. It has a function of preferentially combining with N to form a nitride to enhance the hardenability improving effect of B. Therefore, it can be added if this effect is desired. The effect is 0.0
Although it becomes remarkable at 1% or more, if it exceeds 0.10%, inclusions increase and toughness decreases. Therefore, the Zr content when added is preferably 0.01 to 0.10%. A preferable upper limit is 0.05%. In addition, Z
Since r does not easily form carbides, addition of r does not cause strength variations.

N: N exists in steel as an unavoidable impurity and forms a nitride by combining with Al, Ti and Nb. In particular, if a large amount of AlN or TiN precipitates, toughness and SS resistance
It adversely affects the C property and hydrogen-induced cracking resistance. However, if the content is 0.0080% or less, there is no particular problem, so the upper limit was made 0.0080%. A preferable upper limit is 0.0060%.

In the conventional off-line heat treatment process, Nb: Nb has been positively added because it suppresses the growth of crystal grains during reheating by a pinning effect and is effective for refining austenite grains. However, like the present invention,
In the in-line heat treatment process in which quenching is performed while maintaining the temperature at the Ar ₃ transformation point or higher after the final rolling, not only does Nb not have the above effect, but if Nb is present in the steel, Most of NbC during quenching
Does not precipitate, but precipitates during tempering, which is a completely different precipitation behavior from the offline heat treatment, and has a greater effect on strength than before. In particular, in a seamless steel pipe, a temperature difference inevitably occurs between the surface layer in contact with the tool and the center of the wall thickness, and the solid solution amount of Nb changes depending on the wall thickness position due to this temperature difference. Becomes noticeable. Therefore, in the present invention, the smaller the Nb content, the better, and the most preferable is none. However, if the content is less than 0.005%,
Since there is no particular problem, the content was set to less than 0.005%. A preferable upper limit is 0.003%.

Next, the reason why the manufacturing conditions of the seamless steel pipe made of steel having the above-mentioned chemical composition are set as above will be explained.

Heating temperature of material billet: The heating temperature of the material billet is not particularly limited. The point is that the temperature can satisfy the final rolling temperature described below.

Final rolling temperature: When the final rolling temperature is lower than 950 ° C., an expanded grain structure is formed, and anisotropy occurs in toughness and SSC resistance, and it cannot be eliminated even by supplementary heat thereafter. Also, 110
If it exceeds 0 ° C., the crystal grain size is remarkably coarsened and the grains are not made fine by subsequent supplementary heat. Therefore, in the present invention, the final rolling temperature is set to 950 ° C to 1100 ° C.

Supplementary heating temperature: If the supplementary heating temperature exceeds 950 ° C., coarse particles remain, so the toughness and SSC resistance are poor.
Further, below the Ar ₃ transformation point, proeutectoid ferrite precipitates and a complete martensite structure cannot be obtained. For this reason,
The supplementary heating temperature was set to not lower than the Ar ₃ transformation point and not higher than 950 ° C. A preferable upper limit temperature is 920 ° C.

This supplementary heat treatment is intended to equalize the temperature of the entire steel pipe to eliminate temperature unevenness and at the same time to reduce the grain size, thereby eliminating the strength variation and improving the SSC resistance. The supplementary heating time is not particularly limited, but it is preferable to maintain it for a time that satisfies the following formula. That is, the larger the temperature difference and the thicker the wall thickness, the better the holding time.

Supplementary heating time ≧ temperature difference factor (= final rolling temperature−
(Temperature of supplementary heat) x wall thickness factor Tempering temperature: When the tempering temperature is Ac ₁ transformation point or higher, the tempering temperature is Ac because it is significantly softened and the desired strength cannot be secured.
Less than ₁ transformation point. The lower limit temperature is not particularly defined, but is preferably higher from the viewpoint of SSC resistance.
It is better to set the temperature above ℃.

[0043]

EXAMPLE 11 kinds of steel having the chemical compositions shown in Tables 1 and 2 were prepared. In Tables 1 and 2, Steel Nos. A to H are steels melted in a vacuum melting furnace having a capacity of 150 kg, and steel No.
o. A to C, D to E, and F to H are steels obtained by splitting the same molten steel and adjusting the contents of Nb, V, and Ti after the splitting. Further, Steel Nos. I to K are steels prepared for use in a pipe making test by an actual mandrel mill line.

[0044]

[Table 1]

[0045]

[Table 2]

Among the prepared steels, for steel Nos. A to H, the obtained ingots were hot forged to have a thickness of 50 mm,
A block material having a width of 80 mm and a length of 100 mm was used. Then, each block material is heated to various temperatures and treated at the final rolling, supplementary heating temperature, quenching temperature and tempering temperature shown in Table 3,
A plate material having a thickness of 15 mm was used.

At that time, the strength of each plate material was adjusted by changing the tempering temperature so that the yield strength (YS) in the entire thickness was in the T95 class specified in the API standard.

Also in the strip rolling, as in the case of the seamless pipe, there is a temperature difference between the surface layer contacting the rolling roll and the central portion of the wall thickness, so that a hardness difference may occur depending on the Nb amount. Needless to say.

On the other hand, for steel Nos. I to K, the outer diameter is 27
Prepare a solid round billet of 6 mm, 1250 ℃ for both
After being heated to 1, it was subjected to an actual Mannesmann piercer to obtain a hollow shell having an outer diameter of 250 mm and a wall thickness of 15.88 mm.
Next, this hollow shell was subjected to a mandrel mill, and the final rolling temperature was variously changed (1000 ° C., 1050 ° C.) to finish a seamless steel pipe having an outer diameter of 244.5 mm and a wall thickness of 13.8 mm. Then, while the temperature of the finished seamless steel pipe does not fall below the Ar ₃ transformation point, the finished seamless steel pipe is inserted into a supplementary heating furnace (which may be a heating furnace) provided in-line, and various temperatures (900 ° C.,
At 920 ° C and 950 ° C), a supplementary heat treatment (soaking) is performed for 5 minutes, immediately followed by water quenching, and then insertion into a tempering furnace provided in-line at various temperatures (680 ° C).
C., 690.degree. C., 700.degree. C.) and holding for 15 minutes to obtain a product tube.

For each of the plate materials obtained from Steel Nos. A to H, a three-point bending test piece specified in NACE TM0177 Method B was applied in the rolling direction (L direction) from the upper surface, the lower surface and the center of wall thickness. Two sheets each were collected.

A ring-shaped sample was taken from a seamless steel pipe manufactured by an actual Mannesmann-mandrel mill at an arbitrary position in the longitudinal direction of the pipe, and the circumference was measured at four positions (0 °, 0 °,
Two 3-point bending test pieces similar to the above were sampled from each of the outer surface portion, the center portion of the wall thickness, and the inner surface portion at positions (90 °, 180 °, 270 °).

Each of the test pieces collected was NACE TM01.
It was subjected to a corrosion test according to the method specified in 77 Method B. That is, this is a test in which a test piece given an S value of 100 ksi is immersed in a 0.5% acetic acid aqueous solution at 25 ° C. saturated with hydrogen sulfide at 1 atm for 720 hours.

In the evaluation of the corrosion test, when two test pieces were not broken, the SSC resistance was good, and when one or more of them were broken, the SSC resistance was poor. did. In addition, the Rockwell hardness (HRC) of the end of each test piece was measured after the above corrosion test.

The above investigation results are shown in Table 3 together with the production conditions. The hardness in the table is an average value of two test pieces.

[0055]

[Table 3]

As can be seen from the results shown in Table 3, the plate material of the present invention and the seamless steel pipe (trial Nos. 1 and 5) produced by using the steel for in-line heat treatment of the present invention under the conditions specified in the present invention. No. 7, 9-12, 14 and 15), the hardness difference (strength difference) in the thickness direction was small, and the SSC resistance was good.

On the other hand, the plate material of the comparative example and the seamless steel pipe (trial Nos. 2 to 4, 8 and 1) manufactured under the condition that either the chemical composition of the steel or the manufacturing condition is out of the range specified by the present invention.
3 and 16 to 18), the hardness difference (strength difference) in the thickness direction was large, and the SSC resistance was poor.

More specifically, trial numbers 1 to 3 are Nb.
This is an example in which the influence of the content was investigated. In the test Nos. 2 and 3, the manufacturing conditions are within the scope of the present invention, but the hardness of the center portion of the wall thickness is high because the Nb content of the steel is too large. , SSC resistance of the part
The sex was poor.

Trial Nos. 4 to 8 are examples in which the influence of V or Ti content was investigated. Trial No. 4 had no V added, and Trial No. 8 had too much Ti content. Although the manufacturing conditions were within the scope of the present invention, the hardness (strength) variation in the thickness direction was small, but the SSC resistance was poor.

Test Nos. 12 to 18 are examples in which the effects of the finishing temperature and the supplementary heating temperature were investigated. The chemical compositions of steels are all within the range specified by the present invention, but the test No. 13 has a finishing temperature. Because the finishing temperature is too high for trial No. 16 because it is too low, and the supplementary heating temperature for trial Nos. 17 and 18 is too high,
In both cases, the hardness (strength) variation in the thickness direction was small, but the SSC resistance was poor.

[0061]

EFFECTS OF THE INVENTION The steel of the present invention has a large strength and does not vary when subjected to in-line heat treatment under predetermined conditions. Further, according to the method of the present invention, the variation in strength is small,
As a result, it is possible to manufacture a high-strength seamless steel pipe having a fine grain structure excellent in SSC resistance with high efficiency and low cost.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shunji Abe 1850 Minato, Wakayama, Wakayama Sumitomo Metal Industries, Ltd. Wakayama Works (56) Reference JP-A-60-67623 (JP, A) JP 9 -235617 (JP, A) JP-A-9-59719 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.23 to 0.35% by weight, S:
i: 0.1-1.5%, Mn: 0.1-2.5%, P:
0.03% or less, S: 0.004% or less, Al: 0.0
01-0.1%, V: 0.01-0.25%, Ti:
3.4 × N% or less, Cr: 0 to 1.5%, Mo: 0
1.0%, W: 0 to 1.0%, B: 0 to 0.0030
%, Ca: 0 to 0.0050%, Mg: 0 to 0.005
0%, Zr: 0 to 0.10%, while N and Nb
Are regulated to 0.0080% or less and less than 0.005%, respectively, and the balance substantially consists of Fe. 2. A seamless steel pipe produced by hot piercing and stretch rolling the billet made of the steel for inline heat treatment according to claim 1, and making the pipe at a final rolling temperature of 950 to 1100 ° C. in line. A seam having excellent resistance to sulfide stress corrosion cracking characterized by supplementing heat in the temperature range of Ar ₃ transformation point to 950 ° C., quenching as it is at the Ar ₃ transformation point or higher, and then tempering at less than the Ac ₁ transformation point. Steelless pipe manufacturing method.