JPS60174822A - Manufacture of thick-walled seamless steel pipe of high strength - Google Patents

Abstract

PURPOSE:To improve the resistance to sulfide stress corrosion cracking by hardening and tempering a steel contg. prescribed percentages of C, Si, Al, Nb, Zr, Ti, P, S, Mn, Cr and Mo under prescribed conditions. CONSTITUTION:A steel contg., by weight, 0.15-0.5% C, 0.1-0.3% Si, 0.005-0.1% Al, 0.01-0.1% Nb, 0.005-0.1% in total of Zr and/or Ti, <=0.005% P, <=0.005% S, 0.12t<1/2>-0.25t<1/2>% Mn [t is the thickness (mm.) of a steel pipe as a product] and 0.4t<1/2>-0.8t<1/2>% Cr [Cr(%)/Mo(%)=1.5-2.5] is hot worked into a seamless steel pipe, and the inside and outside of the pipe are simultaneously hardened. The pipe is then tempered at 650 deg.C- the Ac1 point.

Description

[Detailed Description of the Invention] The present invention relates to a method for manufacturing seamless steel pipes used for oil country tubular goods, line pipes, chemical equipment piping, etc. , yield strength or 0.6 inch proof stress is 75-120 kllf/mtl
The present invention relates to a method for manufacturing a thick-walled, high-strength seamless steel pipe having a thickness of 9.5 mm or more and having high strength.

In recent years, with soaring crude oil prices and the depletion of oil resources expected in the near future, deep oil fields that were previously neglected and sour gas fields whose development was once abandoned are being developed. There is a growing desire to develop such technologies. Oil country tubing with excellent sulfide stress corrosion cracking resistance is essential for the development of oil wells that are located in particularly sour environments, that is, environments that contain large amounts of sulfides such as hydrogen sulfide.
Furthermore, in the case of deep oil fields, high-strength oil country tubular goods are inevitably required.

By the way, in the case of low-alloy steels commonly used for oil country tubular goods, the resistance to sulfide stress corrosion cracking usually deteriorates as the strength increases.Therefore, due to the balance between the two, yield strength of 64 to 74 is 9f. /uf R's Cr-M
O-series steels are considered to be the most superior among the steels for oil country tubular goods in practical use. However, the reality is that oil wells are becoming deeper and deeper these days, and as a result, conventional steels like those mentioned above lack strength, with a yield strength of 75 kgf/m.
There is an urgent need to develop steel for oil country tubular goods that has a high strength of - or more and has excellent resistance to sulfide stress corrosion cracking.

Recently, as described in JP-A No. 53-78917, a sulfide stress resistant steel with increased Cr and Mo content than the conventional 65 kgf/m-class Cr-Mo steel and a large amount of 75-90kl/f/r with excellent corrosion cracking resistance
S-class steel has been developed. However, this steel is V
Since it contains a large amount of , cracks are extremely likely to occur when the material is produced by continuous casting, and therefore it is difficult to apply the continuous casting method with high productivity. . On the other hand, in JP-A No. 57-35622, a high-strength steel for oil wells with reduced p and s in the steel is proposed, and although the strength of this steel is not clearly shown, it can be seen from the examples. It has a yield strength of 91 to 98 kgf/111, which is considered to be significantly higher than the conventional one. However, the stress corrosion cracking resistance of this proposed steel is only guaranteed in an alkaline environment containing only a trace amount of tt2S; In addition, as oil wells have become deeper in recent years, there has been an increasing demand for thick-walled oil country tubular goods, but conventionally the composition has been adjusted according to the wall thickness of the pipe. Therefore, even if a steel exhibits excellent sulfide stress corrosion cracking resistance in thin-walled pipes, when rolled into thick-walled pipes and quenched and tempered, the center of the pipe thickness The reality is that sufficient quenching has not been carried out until then, and as a result, resistance to sulfide stress corrosion cracking has often been poor.

This invention was made in view of the above circumstances, and by selecting an appropriate component composition according to the wall thickness of the steel pipe within the range of components suitable for continuous casting. 75-120kl shows excellent sulfide stress corrosion cracking resistance even in thick-walled steel pipes! The object of the present invention is to provide a method for manufacturing f/- class high-strength seamless steel pipes.

It is known that the sulfide stress corrosion cracking resistance of high-strength steel can be improved by sufficiently tempering it after complete quenching. As a result of various experimental studies on methods for improving the hardenability of thick-walled steel pipes in order to obtain a completely hardened structure, we found that by adjusting the Cr and Mn contents within appropriate ranges depending on the wall thickness, thick-walled steel pipes can be improved. It has been found that an almost perfect quenched structure can be obtained even in steel pipes of 20 to 30 degrees, and that the sulfide stress cracking resistance can be improved by tempering after quenching at an appropriate temperature. In addition, the present inventors have discovered that Cr, M
In steels containing o at the same time, CrJjl and M
It has been found that excellent resistance to sulfide stress corrosion cracking can be exhibited by setting the ratio to the amount of 0 to within a certain appropriate range. This invention has been made based on these findings.Next, these findings will be explained in more detail.

As mentioned above, the sulfide stress corrosion cracking resistance of steel deteriorates as the tensile strength material deteriorates, but in the case of steel of the same 8 degree, completely quenched and sufficiently tempered structure is superior. It is publicly known. For example, as shown in Japanese Patent Application Laid-Open No. 58-164719, a method is proposed in which the structure after quenching has a quenched martensite ratio of 90 or more and is tempered under conditions where the tempering parameter is 18,000 or more, or AP
A method is known in which the tempering temperature is set to 621 T or higher, as in the I-C90 standard. All of these examples relate to oil country tubular goods with a yield strength of 63 to 741 Vf/1 m class, but according to research by the present inventors, 75
In the case of high-strength seamless steel pipes of kllflRd class or higher, in order to obtain excellent sulfide stress corrosion cracking resistance, the quenched martensite ratio should be 99 degrees or more, and the tempering temperature should be 65 degrees.
It was found that the temperature needed to be 0°C or higher.

As mentioned above, in order to increase the hardened martensite ratio to a high value of 99 inches or more, it is necessary to sufficiently harden the pipe to the center of its wall thickness. Figure 1 shows the cooling time required for the temperature at the center of the tube thickness to drop below 300℃ when a tube heated to 900℃ is water-quenched from the inner and outer surfaces as a function of the tube thickness. The results calculated by et al. It can be seen from FIG. 1 that, for example, in the case of a tube with a wall thickness of 39W, it takes about 30 seconds for the temperature at the center of the wall thickness to reach 300°C. Therefore, in the case of thick-walled tubes, the composition must be such that sufficient hardening can be achieved even if the cooling rate is slow.

From this point of view, the inventors of the present invention have conducted various studies on the influence of alloying elements on the relationship between quenching hardness and cooling rate, and as a result, have found an appropriate structure that can achieve a completely quenched martensitic structure up to the center of the pipe wall thickness. They found a relationship between the addition range of ingredients and wall thickness. FIG. 2 shows an example of this, in which the relationship between cooling rate and quenching hardness was investigated by varying the amount of Mn in 0.25%C-3%Cr-1.5%Mo steel. For example, in the case of a pipe with a wall thickness of 30 cm, the first
As shown in the figure, it takes about 30 seconds for the center of the wall thickness to reach 300°C or less, so in order to obtain the hardness corresponding to a completely hardened structure even after 30 seconds, about 1% Mn must be added. It is clear from Figure 2 that this is necessary. Although the results of this study are just one example, the inventors conducted similar studies on various component systems and various wall thicknesses, and found that in order to achieve a completely hardened structure down to the center of the pipe wall thickness, it is necessary to It was found that it was necessary to adjust the amount of addition of Mn and Mn depending on the wall thickness.

As mentioned above, the quenched material with a completely quenched structure of perilla is subjected to a tempering treatment, but the present inventors further investigated the influence of factors other than hardenability on the sulfide stress corrosion cracking resistance and strength of the product. In order to find this, various studies were conducted regarding the tempering conditions and component elements. As a result, the yield strength or (1.6% yield strength) is 75 kgf/rad ~ 120
kgf/ld, tempering, and excellent sulfide core corrosion resistance, the following six conditions (1) to (6) must be satisfied at the same time, including conditions related to hardenability. It turned out that there was a need. That is, (1) Addition amount of Mn: 0.12%/'T≦Mr+(%
)≦0.25V'T (addition amount of 23Cr: 0.4v'
T≦cr(%)≦0.8 JT'-(3) Cr and Mo
(4) Tempering temperature = 650°C or higher, Ac 1 point or lower (5
) Simultaneous addition of Nb and Ti or Zr (6) P, S
Content: Each must be 0.005% or less. However, here t is the wall thickness (II-) of the tube.

Therefore, the method for manufacturing a thick-walled, high-strength seamless steel pipe of the present invention uses CO, 15 to 0.50 t, SiO, t as materials.
~0.3%, AA! 0.005-0.1%, NbO
, 01-0,1%, 2 [and/or contains Ti in a total amount of 0.005 to U, 1%, po, ooss or less, So, 005% or less, and Mn, C
r is 0.12v'T≦Mn(%)≦0.2, depending on the wall thickness t (=m) of the product tube. WT0, 4 J
T≦Cr (%)≦0.8 Using steel that contains within the range of JT- and further contains within the range that satisfies MOt-Cr (%), hot working is performed on the steel material. To make a seamless steel pipe, and then to perform quenching and tempering treatment as heat treatment, a method of simultaneously quenching the inner and outer surfaces of the tube was applied as the quenching method, and the tempering temperature was set at 650 °C or higher%A.
It is characterized by being within a range below the CI point.

The method of the present invention will be explained in more detail below.

First, the reason for limiting the composition of the steel material used in the method of this invention will be explained.

C: C is an element necessary to improve strength and hardenability, but if it is less than 0.1596, sufficient hardenability cannot be obtained, while if it exceeds 0.5, there is a risk of quench cracking. Therefore, the content was set within the range of 0.15% to 0.50%.

Si: Si is necessary for deoxidation in the normal steelmaking process, and is also effective for improving strength, and for this purpose it is necessary to
, 11 or more is required, but if it exceeds 0.3%, the toughness decreases, so it was set within the range of 0.1 to 0.3-.

As mentioned above, Mn is an element that improves hardenability, and is an important element for obtaining a completely hardened martensitic structure in the thick-walled pipe that is the subject of this invention. Experiments conducted by the present inventors According to the above, it is necessary to add 0.12 JT (%) or more depending on the wall thickness t(n-) of the pipe in order to obtain a completely hardened structure up to the center of the wall thickness of the pipe. P if added.

The upper limit needs to be set to 0.25 v/T(S) because it causes segregation of S and the like and deteriorates the resistance to sulfide stress corrosion cracking.

Cr: Cr is an element necessary to improve corrosion resistance, strength, hardenability, and tempering resistance. According to experiments conducted by the present inventors, Cr is an element required to increase corrosion resistance, strength, hardenability, and tempering resistance. 4
It is necessary to add JT (%) or more, but o, 8
If it exceeds v'T (%), the sulfide stress corrosion cracking resistance and toughness will decrease, so o, 4v/T ~ o, s v
'T was within the range.

Mo: Mo increases corrosion resistance, strength, and tempering resistance,
In addition, it is an essential element in this invention in order to prevent grain boundary segregation of P and improve sulfide stress corrosion cracking resistance, but especially when added at the same time as Cr and in an appropriate amount ratio with Cr. In addition, excellent sulfide stress corrosion cracking resistance can be imparted to high-strength steels with a yield strength of 75 to 9 f/- or more. That is, according to the experiments of the present inventors,
Depending on the amount of Cr, the value of Cr (%)/Mo (%) is 1.5
As mentioned above, the above-mentioned effect can be exhibited to the maximum when the number is 2.5 or more, and therefore, Moat needs to be set to an amount that satisfies this range.

Nb: Nb is Cr-Mo@! When added to 4, it makes the carbides fine in the steel structure after tempering and disperses them uniformly within the grains, and it also fixes P and prevents segregation to the grain boundaries, thereby improving the resistance to sulfide stress corrosion cracking. For that purpose, it is effective to add more than O,011, but if it is added in excess of O,tl, the toughness will deteriorate and cracks will occur when the material is manufactured by continuous casting. The amount is limited to 0.01 to 0.1% because it makes it easier to clean.

Ti, Zr: These elements have a strong affinity for N, and improve sulfide stress corrosion cracking resistance by fixing N and preventing segregation to grain boundaries, and by making the steel structure finer grained. . Either one of Ti and Zr may be added alone or both may be added in combination, but if the total amount thereof is less than 0.005%, the above-mentioned effect cannot be obtained; If it exceeds 0.1 inch, the toughness will deteriorate, so the total amount was set within the range of 0,005 to 0.1 inch.

AJ: Al is necessary for deoxidation in the normal steelmaking process, and it combines with N to produce the same effect as Ti and Zr, but if it is less than 0.005%, this effect cannot be obtained, and 0
If it exceeds 1 inch, the toughness will be impaired, so it should be 0.005 to 0.
Must be within 1%.

In addition to the above-mentioned components, the raw steel components in the manufacturing method of the present invention include Vo, 005 to 0.1 Ch, B 0
．． 0001 to 0.005% of one or two types may be contained. That is, V can be added to improve the strength and hardenability of steel, and especially when the wall thickness of the pipe is large, the effect of the present invention can be enhanced by adding V. However, if V is less than 0.0051, this effect cannot be obtained, while if V exceeds 0.1 inch, it will deteriorate the toughness and cause cracks to occur during the production of the material by continuous casting, so ``■'' is added. In this case, the V amount is 0.005~o,
It is preferably within the range of 1%. Further, since B is also an element that improves hardenability, the effect of the present invention can be enhanced by adding B, especially when the wall thickness of the pipe is large. If B is less than 0.0001%, the effect will be small, but if B exceeds 0.005%, the effect will be saturated and the toughness will deteriorate.
When added, the range is preferably 0.0001 to 0.005%.

In the manufacturing method of the present invention, steel having the composition specified above is used as a raw material, and hot worked into a seamless steel pipe according to a conventional method. Next, a quenching and tempering treatment is performed as a refining treatment, and the quenching method used at this time is to water-cool both the interior and exterior surfaces of the tube at the same time in order to achieve a completely quenched martensitic structure up to the center of the tube wall thickness. It is necessary to apply a hardening method.

Further, tempering after quenching must be performed within a temperature range of 650° C. or higher and lower than the Act point. Tempering temperature is 650
If the tempering temperature is lower than ℃, it is not possible to obtain excellent sulfide stress corrosion cracking resistance, and if the tempering temperature is above the Ac1 point, an austenite phase is formed, and this ℃ changes into a martensite phase that does not undergo tempering when cooled to room temperature. This significantly deteriorates the resistance to sulfide stress corrosion cracking.

The manufacturing method of the present invention is particularly effective for manufacturing seamless steel pipes having a wall thickness of 9.5 m or more.

In other words, in manufacturing thin-walled steel pipes such as 9.51111, it is possible to obtain a fully quenched martensitic structure up to the center of the wall thickness without strictly adjusting the components according to the wall thickness as specified in this invention. is possible.

Further, the manufacturing method of the present invention is most effective when manufacturing a steel pipe whose yield strength or 0.6-chip strength is within the range of 75 to 120 kgf/M. That is, 75 kg
When producing a steel pipe with a strength lower than f/-, it is possible to obtain a steel pipe with excellent stress corrosion cracking resistance even outside the composition range specified in this invention or at a tempering temperature of less than 650°C; 20kl? This is because, if the strength exceeds 'md, the sulfide stress corrosion cracking resistance may be significantly reduced even if the steel is within the composition range specified in the present invention.

Examples of the present invention will be described below along with comparative examples.

Steel with the composition shown in sample code 1-15 in Table 1 was melted, continuously cast into round billets, and hot worked according to conventional methods to form seamless tubes with the wall thickness shown in Table 1. . Next, each steel pipe was quenched from 90°C and further tempered. The quenching method and tempering temperature are also shown in Table 1. The yield strength and sulfide stress corrosion cracking resistance of the steel and ridges after tempering were investigated, and the results are shown in Table 1. In the sulfide stress corrosion cracking resistance test, a stress of 75% of the yield strength was applied in NACE solution (saturated hydrogen sulfide water with 0.5% acetic acid and 5% sodium chloride added) using a round rod tensile test piece. It was carried out. O in Figure 1 regarding sulfide stress corrosion cracking resistance evaluation
The mark indicates that the sample did not break for 30 days in the above test, and the X mark indicates that it broke within the 30-day period.

Sample numbers 1, 5, 8, 11, and 12 in Table 1 are steel pipes produced by the method of the present invention, and all exhibit good resistance to sulfide stress corrosion cracking.

On the other hand, the relationship between the wall thickness and the composition of the items with symbols 2 and 3 satisfies the scope of the present invention, but the quenching method of the item with the symbol 2 and the tempering temperature of the item with the symbol 3 are in accordance with the present invention. Since the values are outside the range, both have poor sulfide stress corrosion cracking resistance. In addition, for the symbol 4, the Mn content is
The Cr content of the item with symbol 6 is below the required range calculated from the wall thickness in this invention, while the item with symbol 13
The Mn content and Cr content of those with ゜14 exceeded the range required by calculating from the wall thickness in this invention, and in these cases, the sulfide stress corrosion cracking resistance was poor. There is. Furthermore, the steel pipes with symbols 7 and 15 are Cr
The quantity ratio between Mo and Mo is approximately 2.64 for the former and approximately 1.4 for the latter.
1, which exceeds the upper limit and below the lower limit of the range of this invention, resulting in poor sulfide stress corrosion cracking resistance. And the steel pipe with symbol 9.10 is Mn, Cr
, Mo, etc., quenching method, and tempering temperature are within the scope of this invention, but in the case of symbol 9, P is replaced by symbol lOO
The S content of these samples exceeds the upper limits of the present invention, and therefore the sulfide stress corrosion cracking resistance is poor.

Therefore, from the above examples, it is clear that in order to obtain excellent sulfide stress corrosion cracking resistance, all the conditions specified in the present invention must be satisfied at the same time.

As described above, according to the method of the present invention, a seamless steel pipe with a high yield strength or 0.6 inch proof stress of 75 to 120 kgf/*d and a thick wall thickness of 9.5 mm or more can be produced. In manufacturing the steel pipe, it is possible to obtain a steel pipe with extremely excellent resistance to sulfide stress corrosion cracking. Therefore, the method of the present invention can be suitably applied to manufacturing oil country tubular goods and line pipes used in sour and deep oil and gas wells, and piping for chemical equipment exposed to a hydrosulfide atmosphere. be.

[Brief explanation of drawings]

Figure 1 is a correlation diagram showing the relationship between the wall thickness of a steel pipe and the cooling time required to water-quench the steel pipe from 900°C to bring the center of the pipe thickness to 300°C or higher. .25%
FIG. 2 is a correlation diagram showing the relationship between the time required for cooling from 900°C to 300°C and the hardness of steel slabs with various amounts of Mn added to C-3% Cr-1,5S Mo steel. . Applicant Kawasaki Steel Co., Ltd. Agent Patent attorney Takehito Toyota (and 1 other person) Pipe diameter (mm) 2 Ima Koi Ikun (excerpt)

Claims (1)

[Claims] C0.15 to 0.50 inches (weight%, same hereinafter), Si0
．． 1~0.3chi, AJIo, 005~0.1%, Nb
O, 01-0.1%, total amount of Zr and/or Ti 0.005-0.1%, P 0.005% or less, 80
．． 005- or less, and Mn and Cr are respectively 0.12v'T≦Mn (%')≦0.25J''v0, depending on the wall thickness t (■) of the product steel pipe.
4-, The steel material is made of steel containing within the range of /T≦Cr (%)≦0.8JT and further containing Mo within the range that satisfies the amount of Cr, and the steel material is subjected to hot working. A method for producing a thick-walled high-strength seamless steel pipe, which is characterized by subjecting the pipe to a seamless steel pipe, then quenching the inner and outer surfaces of the pipe by a simultaneous quenching method, and then tempering the pipe in a temperature range of 650°C or higher and less than 4 points of Ac. .