JP3417016B2 - Manufacturing method of high toughness martensitic stainless steel seamless steel pipe with excellent hot workability and corrosion resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to hot workability and C resistance.
The present invention relates to a method for producing a high toughness martensitic stainless steel seamless steel pipe having excellent O ₂ corrosion resistance and sulfide stress cracking resistance.

[0002]

2. Description of the Related Art In recent years, development of gas wells for producing gas containing a large amount of CO ₂ and CO ₂ injection for secondary recovery have been widely performed. In such an environment, the corrosion of the steel pipe is severe, so that a martensitic stainless steel pipe excellent in corrosion resistance such as CO ₂ corrosion resistance and sulfide stress cracking resistance is required in many cases. Among them, as martensitic stainless steel excellent in corrosion resistance and hot workability, for example, C and N are 0.015% or less, Ni:
0.1-3.0%, Cr: 11-17% and Nb: 0.0
1 to 0.05%, and if necessary, Mo: 0.05 to 1.
Japanese Patent Publication No. 59-15977 and Japanese Patent Publication 59-59, which present a steel for seamless steel pipes containing 0% and having excellent corrosion resistance.
15978, etc. are mentioned. Compared with the conventional SUS410, SUS420J1, J2 steels, etc., these steels have a low C content, so the amount of precipitation of Cr carbide is small, and the amount of solid solution Cr effective for corrosion resistance can be secured. Are better.

[0003]

However, in such a martensitic stainless steel, when the addition amount of C and N is remarkably reduced, δ ferrite which deteriorates the hot workability in the austenite matrix during ingot heating. There is a drawback that phases are easily generated. Therefore, under severe working conditions such as seamless steel pipe rolling, hot working cracks and
Many other defects were generated, and the increase in cost due to the decrease in yield was unavoidable. Further, in order to further improve the corrosion resistance, a steel containing a small amount of Cu as described in the above publication has been developed, but if the Cu content is high, a surface defect occurs during hot working. It was necessary to increase the cost because it required. So far,
It has been considered difficult to produce a martensitic stainless steel seamless steel pipe that has high corrosion resistance and produces few surface defects during hot working.

The present invention is based on the findings, which will be described later, generated from various examination results from the present state of the art, and produces a high toughness martensitic stainless steel seamless steel pipe excellent in hot workability and corrosion resistance. It provides the law.

[0005]

In order to solve the problems of the conventional martensitic stainless steel seamless steel pipes, the present inventors have found from many experimental results that CO ₂ corrosion resistance and sulfide stress corrosion resistance are high. Corrosion resistance such as cracking property is maintained by reducing C and adding necessary amount of Cr, Ni, Cu or further Mo, and hot workability reduces inclusion of P, S, etc. by reducing inclusions. It has been found that it can be obtained by performing a metallurgical operation such as controlling the phase fraction and shape of different phases having different deformation resistances by suppressing Ni and adding Ni.
Furthermore, it was also found that the generation of surface defects during hot working can be suppressed by limiting the addition amount of Cu.

In particular, the present inventors have paid attention to the effects of C and N and obtained the following findings. Figure 1 shows the base component
₂ shows the CO ₂ corrosion resistance property and the reduction value during hot working when the C and N contents were changed as a% Cu-4% Ni-15% Cr steel. In FIG. 1, C.I. R. Is 40 atm
Of a corrosion speed of annual in CO ₂ and 180 ° C. artificial seawater was equilibrated, C. R. If it is <0.1 mm / y, it can be evaluated as having sufficient corrosion resistance. In addition, R. A. Is 12
This is the reduction ratio when a sample heated to 50 ° C. is uniaxially tensile deformed at 900 ° C. under a strain rate of 3 sec ⁻¹ . A.
If it is> 70%, it can be said that the hot deformability is good. In the CO ₂ corrosion test, the one having a yield strength of about 720 MPa was used after quenching and tempering after hot working. From FIG. 1, it is necessary to use C to satisfy the CO ₂ corrosion resistance.
It can be seen that it is necessary to satisfy ≦ 0.05%, and in order to have good hot workability, it is necessary to satisfy C + 0.8N> 0.06 (unit of content of each element symbol). Is wt.%).

Next, as shown in FIG. 2, 18% of 4% Ni-15% Cr steel was used.
C. in a CO ₂ environment at 0 ° C. R. R. and 900 ° C. A. The influence of the amount of Cu on the is shown. The Cu amount is 0.
2% more than in the name Ru and C. R. <0.1 mm / y, indicating that the corrosion resistance is sufficient. Also, R. and Cu amount ing to 1% or more A. <70%. This is because the occurrence of surface defects such as baldness increases as the amount of Cu increases.

The present invention is constructed by combining the above-mentioned findings, and its gist is as follows. That is, in weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Ni: 1.5~5%, Cu: 0.2~1% not
Full , Al ≦ 0.05%, N: 0.02-0.1
%, And satisfying C + 0.8N> 0.06, or further containing Mo: 0.5 to 2%, the balance of which is Fe and inevitable impurities, is hot-worked into a seamless steel pipe at room temperature. After cooling down to Ac ₃ transformation point +
Heat to a temperature of 10 ° C. to Ac ₃ transformation point + 200 ° C., cool to room temperature at a cooling rate of air cooling or higher, and then heat to a temperature of Ac ₁ transformation point to Ac ₃ transformation point to cool to room temperature or more air cooling.
Cooled at retirement rate, thereafter, high excellent hot workability and corrosion resistance to tempering at Ac ₁ transformation point temperature
A tough martensitic stainless steel seamless steel pipe manufacturing method.

The present invention will be described in detail below. First, the reasons for limiting the steel components will be described. C is an element that forms Cr carbide and deteriorates the corrosion resistance, but as a typical austenite forming element, it is 90 in the hot working temperature range.
At 0 to 1250 ° C., it has an effect of suppressing the generation of the δ ferrite phase. However, if the content exceeds 0.05%, a Cr-deficient layer is generated due to the precipitation of Cr carbide, etc.
O ₂ Corrosion is deteriorated. Further, grain boundary precipitation of many carbides significantly reduces the sulfide stress corrosion cracking resistance. Therefore, in the present invention, in order to obtain excellent corrosion resistance such as CO ₂ corrosion resistance and sulfide stress corrosion cracking resistance, the C content should be 0.
It was set to 05% or less.

Si is added as a deoxidizing agent in steelmaking, and if it is contained in the steel in an amount of more than 0.50%, toughness and sulfide stress corrosion cracking resistance are deteriorated.
It was set to 0.50% or less. Mn is an element that forms inclusions and impairs resistance to sulfide stress corrosion cracking in a corrosive environment, but is added as a useful component for forming an austenite single phase. However, 1.0% to form an ultra-El and a large amount of inclusions, lowering the sulfide stress corrosion cracking resistance and toughness in a corrosive environment. Therefore, the Mn content is 1.
It was set to 0 % or less.

Since P segregates at the grain boundaries to weaken the grain boundary strength and deteriorates the hot workability and the resistance to sulfide stress corrosion cracking, P is set to 0.03% or less. Since S forms inclusions as sulfides and deteriorates hot workability, it is preferable that it is as small as possible like P, and its upper limit was made 0.01%. Cr
In order to impart the CO ₂ corrosion resistance, which is the object of the present invention, and the required corrosion resistance as stainless steel,
It is necessary to contain at least 11%. However, it exceeded the 17%
Then , a ferrite phase is easily generated. Therefore, Cr
The content range of is set to 11 to 17%.

Ni has the effect of improving the corrosion resistance of Cr-containing steel. Further, Ni is a strong austenite forming element, and has the effects of suppressing the formation of the δ ferrite phase during high temperature heating, suppressing the growth of cracks formed inside the δ ferrite phase, and improving the hot workability. However, when N: 0.02%, Ni: 1.
If the content is less than 5%, those effects cannot be obtained, and
5% and ultra-El Ac ₁ Since the transformation point is lowered, a retained austenite phase is formed and the strength and toughness are impaired. Therefore, the range is set to 1.5 to 5%.

Cu has the effect of improving the CO ₂ corrosion resistance. Further, it is an austenite stabilizing element, and Ac
Has the advantage of not lowering the _{transformation} point, it corrosion resistance improving effect in the content is less than 0.2% is not sufficient, the surface of such concentrated to Hege flaw by oxidation during heating ing more than 1% Since it causes defects, the content is 0.2
The range is limited to less than 1%. Moreover, since the above effect is small when Cu alone is added, it is necessary to add Cu in combination with Ni.

Similar to Si, Al is added as a deoxidizer and is contained. If it exceeds 0.05%, A
A large number of 1N are formed and the toughness is significantly reduced. Therefore, the upper limit of the content is set to 0.05%.

N is harmless to the corrosion resistance and is a typical austenite forming element like C, and has the effect of suppressing the formation of the ferrite phase in the hot working temperature range of 900 to 1250 ° C. The effect is Ni-C as described above.
r steel as a base component, C + 0.8N > 0.06 (C and N are
wt. %) Is effective in the range of the content satisfying%). Therefore, without generating a ferrite phase in hot working temperature range in the case of C ≦ 0.05%, in order to obtain a good hot workability, it is necessary to the N content is 0.02% or more. In addition, in an ordinary melting process, an amount exceeding 0.1%
Since it is difficult to add N in the range of 0.02-
It was set to 0.1%.

Mo is an element effective for improving the pitting corrosion resistance, and is added if necessary. However, not 0.5%
The effect is small when the content is full . Further, 2% for a potent ferrite stabilizing element because it is easy to generate is exceeded and δ-phase was the content range 0.5 to 2%.

Next, the reasons for limiting the heat treatment conditions will be described. Steel pieces produced in various shapes such as square shape and round shape through the ordinary steel piece production process with the above-mentioned composition are ordinary seamless steel pipes such as plug mill method, Mannesmann plug mill method or hot extrusion method. after hot working to seamless steel in the manufacturing process, cooled to room temperature. The seamless steel pipe produced in this manner is coarse and exhibits a martensite structure in which a ferrite structure is partially formed, and satisfactory toughness cannot be obtained.
Further, during cooling from a high temperature, Cr carbides and intermetallic compounds are precipitated at grain boundaries, and there is a problem that corrosion resistance, sulfide stress corrosion cracking resistance and toughness are impaired. The present invention performs post-processing to solve these problems.

First, the seamless steel pipe produced by hot working is heated to a temperature range of Ac ₃ transformation point + 10 ° C. to Ac ₃ transformation point + 200 ° C., and then cooled at a speed higher than air cooling. The process by solution of Cr carbides and other precipitates precipitated in the grain boundaries, there is to recover the precipitated deterioration such as corrosion resistance and toughness due to such as Cr carbides, Ac ₃ transformation point + 1
At a low temperature of less than 0 ° C., a ferrite structure, a martensite structure, or a Cr carbide partially remains and a stable single austenite phase cannot be obtained, and the Ac ₃ transformation point +2.
At a high temperature exceeding 00 ° C., the crystal grains become coarse and satisfactory toughness cannot be obtained. Also during cooling from the temperature, in order to prevent the corrosion resistance and the toughness degradation due to re-precipitation such as Cr carbides, cooled at a rate of more than air cooling to room temperature the degree.

The seamless steel pipe that has undergone such heat treatment exhibits a single-phase structure of martensite and has high strength.
Low toughness due to generation of large residual stress in martensite structure. In order to precipitate supersaturated solid solution carbon in the martensite structure, and to remove residual stress and recover toughness, heating and cooling to a temperature below the Ac ₁ transformation point that does not reverse transform to the austenite phase. A tempering process is performed. In particular, when a seamless steel pipe containing Cu in the composition as in the present invention is tempered, precipitation strengthening of Cu is suppressed, and toughness and sulfide stress corrosion cracking resistance are significantly improved.

However, in the present invention, a better resistance to
In order to obtain a sulfide stress corrosion cracking resistance and toughness, Ac ₃ and solution treatment by heating to a temperature range of transformation point + 10 ° C. to Ac ₃ transformation point + 200 ° C., is heated to a temperature of less than Ac ₁ transformation point cooling between the tempering treatment, Ac performs processing to cool to room temperature in ₁ transformation point to Ac ₃ 2-phase region temperature to the heated air faster than the transformation point. By this intermediate treatment, the high-strength martensite structure hardened by the solution treatment is softened,
Strength after tempering is adjusted to be low and sulfide stress corrosion resistance is adjusted
Crackability and toughness are further improved, and a seamless steel pipe having uniform toughness can be obtained. However, at such a temperature or a slow cooling rate that deviates from the conditions of the present invention,
A seamless steel pipe having a high-strength martensite structure locally and having excellent corrosion resistance and uniform toughness has not been obtained .

According to the method of the present invention as described above, it is possible to produce a seamless steel pipe which has few surface defects due to hot working and is excellent in CO ₂ corrosion resistance, sulfide stress cracking resistance and toughness. it can.

[0022]

EXAMPLE Steels having the chemical composition shown in Table 1 were cast in a usual melting process and then hot rolled to produce steel pipes. First, the surface of the as-rolled steel pipe was observed to investigate the occurrence of flaws. Next, strength, toughness, CO ₂ corrosion resistance, and sulfide stress cracking resistance were investigated using a material that had been subjected to heat treatment and tempering treatment. Table 2 shows the materials such as heat treatment temperature and strength at that time.

The CO ₂ corrosion resistance was evaluated by the corrosion rate in artificial seawater at 180 ° C. equilibrated with CO _{2 at} 40 atm. If the corrosion rate is 0.1 mm / y or less, it can be considered to have corrosion resistance. Resistance to sulfide stress cracking was measured by applying a round bar tensile test piece to a 5% NaCl solution at 25 ° C. under 1 atm of 99% CO ₂ + 1% H _2.
Applying uniaxial tensile stress in a corrosive environment saturated with S gas,
The ratio (Rs value) of the maximum initial stress and the yield stress at which breakage did not occur in 20 hours was determined. If Rs ≧ 0.8, it can be said that the characteristics are excellent. From the results shown in Table 2, the steel pipe produced by the method of the present invention has no surface defects during hot working and shows good CO ₂ corrosion resistance, sulfide stress cracking resistance and high toughness. It is clear that any of the properties is inferior in the comparative method outside the scope of the present invention.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

EFFECTS OF THE INVENTION According to the present invention, it is possible to produce a martensitic stainless steel seamless steel pipe having no surface defects during hot working and having excellent CO ₂ corrosion resistance, sulfide stress cracking resistance and high toughness. Became.

[Brief description of drawings]

FIG. 1 shows the effect of C and N on CO ₂ corrosion resistance.

FIG. 2 shows the influence of Cu-containing steel on the corrosion resistance and the draw ratio in a CO ₂ atmosphere.

Continuation of front page (56) Reference JP-A-5-140645 (JP, A) JP-A-5-263138 (JP, A) JP-A-4-193933 (JP, A) JP-A-6-100935 (JP , A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 9/08 C22C 38/00-38/60 C21D 8/10 C21D 6/00

Claims (2)

(57) [Claims] 1. By weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Cu: 0.2 to less than 1%, Ni: 1.5 to 5%, Al ≦ 0.05%, N: 0.02 to 0.1%, and a component satisfying C + 0.8N> 0.06. After the steel containing the balance Fe and unavoidable impurities is hot worked into a seamless steel pipe and cooled to room temperature , Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point +20
After heating to a temperature of 0 ° C. and then cooling to room temperature at a cooling rate of air cooling or higher, then heating to a temperature of Ac ₁ transformation point to Ac ₃ transformation point and subsequently cooling to room temperature at a cooling rate of air cooling or higher, Then, a method of producing a high toughness martensitic stainless steel seamless steel pipe having excellent hot workability and corrosion resistance, which is characterized by performing tempering at a temperature not higher than the Ac ₁ transformation point. 2. By weight%, C ≦ 0.05%, Si ≦ 0.50%, Mn ≦ 1.0%, P ≦ 0.03%, S ≦ 0.01%, Cr: 11 to 17%, Cu: 0.2 to less than 1%, Ni: 1.5 to 5%, Mo: 0.5 to 2%, Al ≦ 0.05%, N: 0.02 to 0.1%, and C + 0.8N. A steel containing a component satisfying> 0.06 and the balance being Fe and unavoidable impurities is hot worked into a seamless steel pipe and cooled to room temperature, and then Ac ₃ transformation point + 10 ° C to Ac ₃ transformation point +20.
After heating to a temperature of 0 ° C. and then cooling to room temperature at a cooling rate of air cooling or higher, then heating to a temperature of Ac ₁ transformation point to Ac ₃ transformation point and subsequently cooling to room temperature at a cooling rate of air cooling or higher, Then, a method of producing a high toughness martensitic stainless steel seamless steel pipe having excellent hot workability and corrosion resistance, which is characterized by performing tempering at a temperature not higher than the Ac ₁ transformation point.