JP2914113B2 - Method for manufacturing high strength and high toughness martensitic stainless steel welded steel pipe with excellent corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

This invention relates to chloride, CO 2 and trace
H_TwoCan be used in corrosive environments including S and in the atmosphere
High strength and high toughness marte with excellent corrosion resistance and weldability
Concerning the method of manufacturing stainless steel welded steel pipes
It is.

[0002]

2. Description of the Related Art In recent years, chlorides,
BACKGROUND ART As a means for transporting oil and natural gas containing corrosive substances such as CO ₂ and a trace amount of H ₂ S at high pressure, use of high corrosion resistant steel pipes is increasing. On the other hand, as civil engineering buildings such as bridges have become larger, the use of high-strength steel pipes for pillars and the like has increased, and weather-resistant stainless steel pipes will be used in the future in terms of durability, reduced maintenance and aesthetics. Probability is high. Among them, martensitic stainless steel such as 13Cr steel has high strength,
Although it has the feature of being inexpensive, it is inferior in toughness and weldability, so its performance as a welded steel pipe is small. Also, 13Cr steel is not sufficient from the viewpoint of weather resistance. On the other hand, in an environment containing only chloride and CO ₂ , 13Cr steel has good corrosion resistance,
If H ₂ S is present even in a trace amount of 0.1 atm or less, stress corrosion cracking occurs. Therefore, in this environment, a duplex stainless steel such as SUS329J3L, J4L, which is excellent in strength, toughness and stress corrosion cracking resistance, is used as a welded steel pipe. However, duplex stainless steel is expensive because it contains a large amount of Mo and Ni.

[0003] As a method for improving the sulfide stress corrosion cracking resistance (hereinafter referred to as SSC resistance) of martensitic stainless steel, Japanese Patent Publication Nos. 3-2227, 2-243739, and 3 No. 75335 discloses a steel type in which Cr, Mo, Ni, Cu, etc. are increased and C is reduced. Further, it is disclosed that the Ni-containing martensitic single-phase stainless steel disclosed in Japanese Patent Application Laid-Open No. 2-243739 discloses that even without normal quenching treatment, sufficient quenching is performed as it is in a hot-formed state. Is considered to be effective for further cost reduction.

[0004]

However, as described above, the biggest problem in applying martensitic stainless steel to a welded steel pipe is that the low-temperature cracking and ductility at the time of welding, particularly the low-temperature toughness of the welded part, are inferior. In Japanese Patent Publication No. 3-2227,
In JP-A-2-243739 and JP-A-3-75335,
This point is not considered at all. The present invention has been made in order to solve the above problems, and chloride, CO ₂
The purpose is to obtain inexpensively high-strength stainless steel pipes that have good corrosion resistance in the corrosive environment containing a small amount of H ₂ S and the atmosphere, and have excellent weldability and ductility.

[0005]

Means for Solving the Problems The present inventors have described the above-mentioned.
As a result of detailed examination of the effects of components on the toughness and weldability of Ni-containing martensitic stainless steel, extremely low C, N
And reduce δ ferrite, and a small amount of residual
It has been found that good ductility can be obtained even in the quenched state by containing the stain. Furthermore, extremely low C, N
At the same time, the formation was effective in improving workability and suppressing low-temperature cracking during welding. Based on the above findings, the above-mentioned problem is solved by the following component limitation and production method.

In the first invention, C: 0.03% or less by weight,
i: 1.0% or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0
~ 7.0%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less, and the content of the element satisfies the formulas (1) to (3), and the balance is substantially Fe and A steel ingot or slab consisting of unavoidable impurities is formed into a steel sheet by hot rolling, and after forming the steel sheet into a tubular body, by seam welding, by weight%, C: 0.03% or less, S
i: 1.0% or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0
~ 7.0%, Cr: 13.0 ~ 17.0%, Mo: 2.0% or less, N: 0.02% or less, O: 0.035% or less, and the content of the element is as follows:
A welded steel pipe having a weld metal substantially satisfying the formulas (1) to (3), the balance being substantially Fe and unavoidable impurities, and further heating the welded steel pipe to a temperature range of 850 to 1000 ^° C. ° C
cooling to a temperature range below the M _f point at a cooling rate of at least / min,
Thus, the present invention is characterized in that a welded steel pipe is manufactured. C (%) + N (%) ≦ 0.03 ──── (1) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) − 190 × C (%) − 160 × N (%) ≦ 154 ──── (2) Cr (%) +1.3 × Mo (%) +1.5 × Si (%) +2 × Ni (%) +0. 7 × Cu (%) + 68 × C (%) + 54 × N (%) ≥ 27.6 ──── (3)

In the second invention, C: 0.03% or less by weight,
i: 1.0% or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0
~ 7.0%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less, and the content of the element satisfies the following formulas (1) to (3), with the balance being substantially Fe After ingots or slabs consisting of unavoidable impurities are heated to a temperature range of T ° C or lower expressed by the following equation (4), and then hot-rolled in a temperature range of 700 ° C or higher,
Immediately at a cooling rate of 3 ° C./min or more, the steel sheet was cooled to a temperature range of the _Mf point or less, and after forming the steel sheet into a tubular body, C: 0.03% or less, Si: 1.0% by weight by seam welding.
% Or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0 to 7.0
%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less,
O: 0.035% or less, and the content of the element is the following (1)
The present invention is characterized in that a welded steel pipe which satisfies the expressions (3) to (3) and has a weld metal substantially consisting of Fe and unavoidable impurities is produced. C (%) + N (%) ≦ 0.03 ──── (1) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) − 190 × C (%) − 160 × N (%) ≦ 154 ──── (2) Cr (%) +1.3 × Mo (%) +1.5 × Si (%) +2 × Ni (%) +0. 7 × Cu (%) + 68 × C (%) + 54 × N (%) ≧ 27.6 ──── (3) T = 450 × Ni (%) + 100 × Cu (%) + 9500 × C (%) + 8000 × N (%) −600 × Cr (%) −750 × Mo (%) −1000 × Si (%) + 8750──── (4)

[0008] The third invention is the first invention or the second invention.
For welded steel pipes that meet the requirements of the invention, 450-
It is characterized by cooling after heating to a temperature range of 650 ° C.

[0009]

The reasons for limiting the components of the base material (original plate) and the weld metal of the welded steel pipe of the present invention will be described below. C is a component effective for increasing the strength, and can reduce the amount of Ni because it is an austenite-forming element.However, when the content is increased, toughness, weldability, workability, and corrosion resistance deteriorate. Therefore, the C content of the original plate and weld metal should be 0.03% or less.
Preferably it is 0.02% or less.

[0010] Si is a component having a deoxidizing effect, but is a ferrite-forming element, and thus depends on the amount of Si added.
Therefore, the amount of Cr, which is the same ferrite forming element, is limited. That is, although details will be described later, the content of each element is
In addition to the limitation, the range satisfying the expressions (2) and (3) is set.
It is an element that is effective for corrosion resistance when the added amount of Si increases.
The upper limit is set to 1.0% because the amount of Cr added is limited.
You . Further, since the addition of a large amount of Si causes deterioration of ductility, the Si content of the original plate and the weld metal is set to 1.0% or less.

Mn is a component having a deoxidizing effect like Si, and is known as an austenite-forming element. However, in this steel, the effect as an austenite-forming element was not recognized, and it was revealed that when the content exceeds 1.0%, the corrosion resistance is reduced. Therefore, the Mn content of the original plate and the weld metal is set to 1.0% or less.

Cu is a component effective for improving corrosion resistance and is also an austenite forming element. However, the effect on corrosion resistance is not exhibited when the Cu content is less than 0.05%,
If it exceeds, the effect is saturated. In addition, since the addition of a large amount deteriorates the toughness, the Cu content of the original sheet and the weld metal is reduced.
The range is 0.05 to 1.0%.

Ni is an austenite-forming element and is an essential component for obtaining martensite having excellent ductility and weldability. It is also effective for improving corrosion resistance in an environment containing H ₂ S and weather resistance. However, as will be described in detail later, if the Ni content is less than 5.0%, even if other components are adjusted, no residual austenite is formed, and the toughness is reduced. On the other hand, the addition of more than 7.0% not only increases the cost, but also increases the residual austenite and lowers the strength. Therefore, the Ni content of the base plate and the weld metal is 5.0 to
The range is 7.0%.

[0014] Cr is the most important element for corrosion resistance.
The effect becomes remarkable by addition of 3.0% or more. However, since it is also a ferrite-forming element, its content is 1%.
If it exceeds 7.0%, the δ ferrite increases and the retained austenite also becomes excessive, and the strength and toughness are remarkably reduced. Therefore, the Cr content of the base plate and weld metal is 1
The range is 3.0 to 17.0%.

Mo, like Cr, is an element effective for improving corrosion resistance and also a ferrite-forming element, so that Mo can be substituted. However, the addition of more than 2.0% lowers the toughness, so the Mo content of the original sheet and the weld metal is set to 2.0% or less.

N, like C, is an austenite-forming element and can reduce the Ni content, but hardly contributes to an increase in strength. Also, if its content increases, ductility and weldability will deteriorate, so the N content of the original sheet and weld metal will be 0.
02% or less.

As the O content increases, the toughness deteriorates significantly. Therefore, the O content of the original sheet has been reduced to 0.01% or less by ordinary steelmaking technology. However, seam welding of steel pipes is often performed by CO ₂ welding or submerged arc welding from the viewpoint of improving efficiency. In this case, an increase in the O content of the weld metal is inevitable, but from the viewpoint of toughness,
Its content is limited to 0.035% or less.

In general, it is known that, in a martensitic stainless steel, the low-temperature cracking susceptibility during welding decreases due to a reduction in the amount of C and N. The inventors have found that, by satisfying the above condition, not only the cold cracking susceptibility is significantly reduced, but also the workability is improved. Therefore, the original plate and weld metal
The (C + N) amount is in a range that satisfies the following formula (1) in addition to the above limitation. C (%) + N (%) ≦ 0.03 ──── (1)

Further, the present inventors have examined in detail the relationship between the volume fraction of δ-ferrite and the volume fraction of retained austenite and the composition of the steel, and as a result, the volume fraction of δ-ferrite and the volume fraction of retained austenite were found. Are the values δ of the following equation (5)
It found that expressed by _f (%) and below (6) of the value gamma _f (%). In this steel, δ ferrite is reduced and a small amount of residual
Good extension toughness even while hardening is obtained by a martensite structure with austenite, but in particular not only deteriorates toughness [delta] _f is greater than 5, the strength also decreases. On the other hand, if γ _f is less than 1, the effect of improving toughness is not exhibited. Therefore, the content of the element in the original sheet and the weld metal, in addition to the above limitations, the following (2) and (3)
The range must satisfy the formula. δ _f = 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) −190 × C (%) − 160 × N (%) − 149 ──── (5) log γ _f = [Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%)] / 4 −6.9 ──── (6) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) −190 × C (%) − 160 × N (%) ≦ 154 ──── (2) Cr (%) +1.3 × Mo (%) +1.5 × Si (%) +2 × Ni ( %) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%) ≧ 27.6 ──── (3)

Next, the reasons for the limitation of the production method will be described. A steel ingot or a steel slab having the above-mentioned chemical composition is hot-rolled to produce an original sheet. This original plate is formed into a tube by press molding or the like, and the seam is welded to produce a welded steel tube. This steel is excellent in crack susceptibility during welding and toughness of the welded part, so for seam welding, covered arc welding, submerged arc welding,
Various methods such as CO ₂ welding or TIG welding can be applied.

In this steel, at the temperature range of 600 to 700 ° C.,
Cr carbide precipitates and reduces toughness and corrosion resistance. The quenching treatment for the welded steel pipe is performed for the purpose of obtaining a solid solution of Cr carbide precipitated on the original sheet by hot rolling and a martensite structure excellent in toughness. However, when the quenching temperature is lower than 850 ° C., the Cr carbide does not form a solid solution, and when the quenching temperature is higher than 1000 ° C., the crystal grains become coarse and the toughness deteriorates. Therefore, the quenching temperature is in the range of 850 to 1000 ° C.

Further, as a result of studying the cooling conditions at the time of quenching, this steel can be sufficiently quenched even if the cooling rate is extremely low. However, when the cooling rate is less than 3 ° C./min, the above-mentioned Cr carbide is reprecipitated during cooling. Further, when the cooling stop temperature exceeds the M _f point, the volume fraction of retained austenite becomes significantly larger than the value γ _{f represented} by the following equation (6), and the strength decreases. Therefore, cooling during quenching is 3 ° C
The cooling is performed at a cooling rate of at least / min to a temperature range of the _Mf point or less. log γ _f = [Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (% )] / 4 −6.9 ──── (6)

Further, by utilizing the characteristics of this steel having excellent hardenability, a manufacturing method in which the above-described quenching treatment to the welded steel pipe was omitted was examined. Usually, heating before hot rolling is 1100-1300 ℃
In this steel, when the heating temperature exceeds 1050 ° C, the volume fraction of δ ferrite becomes a value δ that can be expressed by the following equation (5).
increase than _f . In addition, it was found that the volume ratio of δ at the time of heating was almost the same as the volume ratio of the original sheet after hot rolling. Therefore, the inventors examined the relationship between the heating temperature and the component and the volume ratio of δ in a temperature range of 1300 ° C or less in order to prevent the strength and toughness of the original sheet from decreasing due to the increase in δ ferrite. It has been found that the maximum temperature of 5% or less can be represented by the value T (° C.) of the following equation (4). Therefore, when the quenching treatment by reheating is omitted, the heating temperature before hot rolling is set to a temperature range of T ° C. or lower expressed by the equation (4). However, if the heating temperature exceeds 1300 ° C., the surface properties of the original plate deteriorate, and such heating is not desirable. δ _f = 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) −190 × C (%) − 160 × N (%) − 149 ──── (5) T = 450 × Ni (%) + 100 × Cu (%) + 9500 × C (%) + 8000 × N (%) −600 × Cr (%) −750 × Mo (%) −1000 × Si (%) + 8750 ──── (4)

Further, when the quenching treatment to the welded steel pipe is omitted, in order to obtain a martensite structure excellent in toughness and corrosion resistance, the finishing temperature of hot rolling must be in a temperature range in which Cr carbide is not precipitated. Therefore, the temperature should be 700 ° C or higher. In addition, cooling after hot rolling is performed at 3 ° C as in the case of quenching.
The cooling is performed at a cooling rate of at least / min to a temperature range of the _Mf point or less.

In the subsequent seam welding after forming the tube,
Since the cooling rate after welding is high, the weld is sufficiently baked and there is no problem in corrosion resistance. Therefore, the base material and weld metal of the welded steel pipe obtained by the above method have the same characteristics as the welded steel pipe subjected to the reheating quenching treatment.

This welded steel pipe has sufficient strength, toughness and corrosion resistance in the as-quenched state described above. Therefore, the tempering treatment performed after quenching is performed when further improvement in toughness or strength adjustment is required. However, tempering temperature is 450
If the temperature is lower than ℃, long-time soaking is required to obtain the effect of improving the toughness. Therefore, the tempering temperature is in the range of 450 to 650 ° C. However, if corrosion resistance is important, the temperature should be between 450 ° C and 60 ° C.
A temperature range below 0 ° C. is desirable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described as follows.・ Example 1 1250 ° C for 50 kg ingot of chemical components shown in Table 1
After heating and finishing at 800 ° C., air-cooled (cooling rate: 50 ° C./min) hot rolling was performed to produce a steel sheet having a thickness of 15 mm.

[0027]

[Table 1]

After heating the above steel sheet at 900 ° C. for 5 minutes, M
Perform a quenching process of air cooling to 50 ° C below the _f point,
After heating at 550 ℃ for 5 min and tempering by air cooling, sample for microstructure observation, sample for X-ray diffraction, tensile test specimen, bending test specimen, Charpy impact test specimen with 2 mm V notch, full-surface corrosion test specimen And the test piece for pitting potential measurement was collected. Further, in order to investigate the effect of the heating temperature on the volume ratio of δ-ferrite, the steel plate was heated to a temperature range of 1050 to 1300 ° C, and then a sample for micro observation was collected. The volume fraction of δ ferrite was measured by electrolytically etching the micro sample with 20% NaOH, and the residual γ was measured by X-ray diffraction. The results are shown in Table 1. The workability was evaluated by the presence or absence of cracks in a table bending test with a bending radius of 0.5 t and a bending angle of 180 ^° . The overall corrosion resistance is 100 ° C, 20 ° C.
atmCO ₂ -0.05atmH ₂ S-5% NaCl, 3mm thick x 20 width
After soaking a test piece of mm × 30 mm in length for 336 hours, the corrosion rate was measured and evaluated. Pitting corrosion potential is determined by the current density according to JIS G0577.
The potential to be 100 μA / cm ² was determined. Further, for the evaluation of weldability, a test piece (JIS Z315
8) Sampled, commercially available coated arc welding rod (0.04C-5Ni-12
(Cr steel), main welding was performed under the conditions of 10 ^° C., RH: 60%, and heat input: 10 KJ / cm, and the presence or absence of root cracks was examined by a cross-sectional microscopic method.

FIG. 1 shows the relationship between the volume fraction of δ-ferrite, the absorbed energy at 0 ° C. and -50 ° C., and the components. According to FIG. 1, the volume fraction of δ ferrite in this steel shows a good correspondence with the value B of the following equation (7). When the value of B exceeds 154, the toughness is remarkably high even if the C and N contents are low. It turns out that it deteriorates. B = 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) −190 × C (%) − 160 × N (%) ── ── (7)

Volume fraction of retained austenite, 0 ° C., -50
FIG. 2 shows the relationship between the absorbed energy at ° C. and the components. That is, according to FIG. 2, the volume ratio of the residual γ of the steel is as follows.
This shows a good correspondence with the value C in the expression (8), and it is understood that when the value of C is less than 27.6, the toughness decreases. C = Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%) ── ── (8)

FIG. 3 shows the relationship between the volume fraction of δ ferrite and the components during heating. According to FIG. 3, the maximum temperature at which the volume fraction of δ ferrite is 5% or less shows a good correspondence with the value T of the following equation (4), and when the quenching treatment for the welded steel pipe is omitted, the toughness is secured. For this purpose, it is understood that the heating temperature during rolling of the original sheet must be T (° C.) or lower. T = 450 × Ni (%) + 100 × Cu (%) + 9500 × C (%) + 8000 × N (%) −600 × Cr (%) −750 × Mo (%) −1000 × Si (%) + 8750 ── ── (4) Table 2 shows the tensile properties, bending workability, absorbed energy at 0 ° C and -50 ° C, the results of the Y-test, and the general corrosion test of the present invention methods 1 to 10, comparative methods 11 to 25 and 13Cr steel. The corrosion rate and the pitting potential in the samples were summarized.

[0032]

[Table 2]

According to Table 2, the strength, ductility, workability, weldability, general corrosion resistance and pitting corrosion resistance of the steel having the chemical composition of the present invention are all superior to 13Cr steel. Recognize. In particular, it is understood that the content of (C + N) needs to be 0.03% or less in order to perform welding without strong working or preheating.

Example 2 A 50 kg ingot having a chemical composition within the limits of the method of the present invention shown in Table 3 for a base material was formed into a steel sheet having a thickness of 15 mm by hot rolling.

[0035]

[Table 3]

The steel sheet and the chemical components shown in Table 3
Butt welding by MIG welding was performed using 2 mm φ welding wires 1 to 6 to prepare a welded joint. The groove shape is 8 mm deep on the front side, 5 mm deep on the back side, bevel angle 45 ^o ,
The welding conditions were 190 A × 28 V × 25 cm / min, and double-sided multilayer welding was performed. After the heat treatment of the welded joint, a Charpy impact test piece with a 2 mm V notch and a test piece for pitting potential measurement were collected from the base metal and the weld metal. Table 4 shows the combination of the base metal and the welding wire and the chemical composition of the obtained weld metal. Table 5 shows the hot rolling conditions of the steel sheet and the heat treatment conditions of the welded joint.

[0037]

[Table 4]

[0038]

[Table 5]

The chemical composition of the weld metal did not depend on the manufacturing conditions, and was almost the same when the same base material and welding wire were used. After hot rolling and for the quenching process of the welded joint, it is cooled to 50 ° C below the M _f point at the cooling rate shown in Table 5, and the cooling for the tempering process is air cooling (about 50 ° C / min).
And Table 6 shows the absorbed energy and pitting potential at -50 ° C and -20 ° C of the base metal and the weld metal.

[0040]

[Table 6]

According to Table 6, it is understood that the toughness and the pitting corrosion resistance of the welded joint produced by the method of the present invention are excellent in both the base metal and the weld metal.

Example 3 Ingot B having a chemical composition within the limits of the method of the present invention shown in Table 7
Was rolled into a steel sheet having a thickness of 15 mm at a hot rolling mill. Using this steel sheet and the 4 mm φ welding wire 1 and the flux of the chemical composition shown in Tables 7 and 8, butt welding was performed by two-electrode submerged arc welding under the conditions shown in Table 9 to produce welded joints.

[0043]

[Table 7]

[0044]

[Table 8]

[0045]

[Table 9]

The groove had a depth of 5 mm on the inner surface, a depth of 5.5 mm on the outer surface, and a bevel angle of 45 ^° . Table 10 shows the chemical components of the obtained weld metal.

[0047]

[Table 10]

After heating the welded joint at 900 ° C. for 5 minutes, M
Perform a quenching process of air cooling to 50 ° C below the _f point,
After heating at 550 ° C. for 5 minutes, an air-cooled tempering treatment was performed, and a Charpy impact test specimen with a 2 mm V notch was collected from the weld metal. FIG. 4 shows the relationship between the absorbed energy of the weld metal at −20 ° C. and the O content. FIG. 4 shows that when a flux having a low basicity is used and the O content exceeds 0.035%, the toughness is significantly deteriorated.

Ingots A and B having the chemical components within the limits of the method of the present invention shown in Table 7 were formed into a steel sheet having a thickness of 15 mm in a hot rolling mill. After forming this steel plate into a tube by UOE processing,
A seam portion was butt-welded by two-electrode submerged arc welding using welding wires 1 and 2 and fluxes having the chemical components shown in Tables 7 and 8, to produce a 20-inch welded steel pipe. The welding conditions and the groove shape are the same as above. After the heat treatment of the welded steel pipe, the tensile test specimen from the base metal and the welded part,
A 2 mm V-notched Charpy impact test specimen, a general corrosion test specimen, a pitting potential measurement test specimen, and an atmospheric exposure test specimen were collected. Specimens for general corrosion, pitting potential measurement, and exposure to the atmosphere were collected so that the areas of the base metal and the weld metal were approximately equal. In the air exposure test, a test piece having a thickness of 3 mm, a width of 70 mm, and a length of 150 mm was exposed for 6 months, and weather resistance was evaluated by checking for rust.

Table 11 shows the combination of the base metal and the welding wire and the chemical composition of the obtained weld metal. Table 12 shows the hot rolling conditions of the steel sheet and the heat treatment conditions of the welded steel pipe.

[0051]

[Table 11]

[0052]

[Table 12] In addition, the chemical composition of the weld metal did not depend on the manufacturing conditions, and the values using the same base material and welding wire were almost the same. Table 12 shows the quenching process after hot rolling and for quenching of welded steel pipes.
At the cooling rate shown below, and cooled to 50 ° C. below the M _f point.

Tensile properties of base metal and weld, -50 ° C and -2
Table 13 shows the absorbed energy at 0 ° C, the corrosion rate in the general corrosion test, and the pitting potential.

[0054]

[Table 13]

According to Table 13, the welded steel pipe manufactured by the method of the present invention has excellent strength and ductility in both the base metal and the welded part, and has a high resistance to corrosion in a corrosive environment containing chloride, CO ₂ and a small amount of H ₂ S. It is understood that overall corrosion resistance, pitting corrosion resistance in chloride, and weather resistance are also good.

[0056]

As described above, according to the present invention, it has good corrosion resistance in a corrosive environment containing chlorides, CO ₂ and a trace amount of H ₂ S, and in the air, and has good weldability, workability and ductility. There is an effect that a high-strength stainless steel welded pipe excellent in toughness can be easily obtained. Therefore, a highly corrosion-resistant steel pipe used for high-pressure transportation of oil and natural gas or a corrosion-resistant steel pipe used for a pillar material such as a bridge can be provided at a low cost, which brings about an industrially useful effect.

[Brief description of the drawings]

[1] δ volume fraction and 0 ℃ ferrite steel according to Example 1 of the present invention, it is a graph showing the relationship between the absorbed energy and the components at -50 ^o C.

FIG. 2 is a graph showing the relationship between the volume fraction of retained austenite and the absorbed energy at 0 ° C. and -50 ° C. of steel according to Example 1 and components.

FIG. 3 is a graph showing the relationship between the volume fraction of δ ferrite and the components when steel is heated according to Example 1 of the present invention.

FIG. 4 shows -20 ° C. of the weld metal of steel according to Example 2 of the present invention.
6 is a graph showing the relationship between the absorbed energy and the O 2 content at the time of FIG.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-264192 (JP, A) JP-A-2-232346 (JP, A) (58) Fields investigated (Int.Cl. ⁶ , DB name) C21D 8 / 02,8 / 10,9 / 08 C22C 38/00 302

Claims (3)

(57) [Claims] [Claim 1] By weight%, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0 to 7.0%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less, and the content of the element satisfies the following formulas (1) to (3), and the remainder is substantially heated to a steel ingot or a slab consisting of Fe and unavoidable impurities. After rolling, the steel sheet is formed into a tube, and then, by seam welding, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0 to 7.0%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less, O: 0.035% or less, and the content of the element satisfies the following formulas (1) to (3). A welded steel pipe having a weld metal substantially consisting of Fe and unavoidable impurities, and the welded steel pipe is 850 to 1000
After heating to a temperature range of ^o C, 3 ℃ / min or more cooling rate M _f
A method for producing a high-strength, high-toughness martensitic stainless steel welded steel pipe having excellent corrosion resistance, characterized by cooling to a temperature range of not more than the temperature. C (%) + N (%) ≦ 0.03 ──── (1) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) − 190 × C (%) − 160 × N (%) ≦ 154 ──── (2) Cr (%) +1.3 × Mo (%) +1.5 × Si (%) +2 × Ni (%) +0. 7 × Cu (%) + 68 × C (%) + 54 × N (%) ≥ 27.6 ──── (3) 2% by weight, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0 to 7.0%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less, and the content of the element satisfies the following formulas (1) to (3), the remainder substantially ingot or slab consisting of Fe and unavoidable impurities, Immediately after heating to a temperature range of T ^o C or less expressed by the following formula (4), and then finishing hot rolling at a temperature range of 700 ° C. or more,
A steel sheet is formed by cooling at a cooling rate of 3 ° C./min or more to a temperature range below the M _f point, and after forming the steel sheet into a tubular body, by seam welding, C: 0.03% or less, Si: 1.0% by weight. % Or less, Mn: 1.0% or less, Cu: 0.05 to 1.0%, Ni: 5.0 to 7.0%, Cr: 13.0 to 17.0%, Mo: 2.0% or less, N: 0.02% or less, O: 0.035% or less, And high-strength and high-toughness excellent in corrosion resistance, characterized in that the content of the element satisfies the following formulas (1) to (3), and the balance is a weld metal substantially composed of Fe and unavoidable impurities. Manufacturing method of martensitic stainless steel welded steel pipe. C (%) + N (%) ≦ 0.03 ──── (1) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) −9 × Ni (%) −2 × Cu (%) − 190 × C (%) − 160 × N (%) ≦ 154 ──── (2) Cr (%) +1.3 × Mo (%) +1.5 × Si (%) +2 × Ni (%) +0. 7 × Cu (%) + 68 × C (%) + 54 × N (%) ≧ 27.6 ──── (3) T = 450 × Ni (%) + 100 × Cu (%) + 9500 × C (%) + 8000 × N (%) −600 × Cr (%) −750 × Mo (%) −1000 × Si (%) + 8750─── (4) 3. The welded steel pipe according to claim 1, further heated to a temperature range of 450 to 650 ° C.,
A method for producing a high-strength and high-toughness martensitic stainless steel welded steel pipe excellent in corrosion resistance, characterized by cooling.