JPH05117749A - Production of martensitic stainless steel seamless pipe having excellent low-temperature toughness and stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To produce the martensitic stainless steel seamless pipe having excel lent low-temp. toughness and stress corrosion cracking resistance by Mannes mann process. CONSTITUTION:The reduction ratio expressed by a reduction of area in a 600 to 1000 deg.C temp. range is given in a 13 to 90% range and the steel is cooled at 2 to 30 deg.C/sec cooling rate in a temp. range down to 400 to 500 deg.C right after the end of the final hot finish rolling to form the martensitic structure; thereafter, the steel is subjected to a tempering treatment in the final hot finish rolling stage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a martensitic stainless seamless steel pipe having excellent low temperature toughness and stress corrosion cracking resistance.

[0002]

2. Description of the Related Art Martensitic stainless steel represented by SUS420 steel type is widely used as a material for oil country tubular goods and the like because it exhibits excellent corrosion resistance in a corrosive environment containing CO ₂ . Conventionally, quenching and tempering treatments have been widely used as a method for obtaining high strength without deteriorating the toughness of steel materials. In this method, austenite-ferrite transformation occurs during cooling that is formed by hot rolling or forging, and a material mainly composed of ferrite-pearlite structure is transformed again to a temperature range where all are austenite. Reheat, then ferrite from this temperature range
By cooling at a rate at which pearlite and bainite transformation hardly occur, the structure is made of martensite for the most part. Finally, by heating to a temperature range where Ac-ustenite transformation does not occur (Ac ₁ point or less), many dislocations existing in martensite are eliminated and a large amount of solid-soluted C is used as a fine carbide. It consists of a precipitation step.

However, since the martensitic stainless steel represented by the SUS420 steel type is less likely to undergo ferrite-pearlite transformation, the martensitic structure is entirely formed by air cooling after the austenitizing heating. For this reason, conventionally, a heat treatment has been performed in which after the rolling process is finished, the temperature is raised to the austenitizing temperature, cooled to room temperature by air cooling, and then again heated to a temperature of Ac ₁ point or lower to temper. In recent years, the development of oil and natural gas in cold regions such as Alaska and the North Sea region has become active.These steel pipes for oil wells are martensitic stainless steel pipes with excellent corrosion resistance. Stress 67kg / m
m ² or more, but severe low temperature toughness, for example, absorbed energy at −40 ° C. in Charpy test − (hereinafter vE ₋₄₀
Abbreviated) is required to be 4 kg · m or more. However, with the conventional heat treatment method, vE- ₄₀
Was less than 1 kg · m, it was not possible to meet these strict requirements for low temperature toughness.

On the other hand, it is known that toughness can be improved by increasing the cooling rate to suppress the precipitation of coarse carbides in the cooling process after austenitization. When this type of steel is subjected to strong cooling by water cooling in the cooling process after austenitizing, cracks are generated due to internal stress during the martensitic transformation, and therefore, for example, JP-A-3-75308 discloses a martensitic transformation starting temperature. It is disclosed a method of avoiding quench cracking and suppressing precipitation of coarse rod-shaped carbides to improve toughness and stress corrosion cracking resistance by performing strong cooling up to and then air cooling. However, this method improves the toughness more than the conventional method, but vE- ₄₀ is 3
Only those of about kg · m were obtained, and the above requirements could not be satisfied.

The inventor has found that it is completely austenitized.
In the low temperature austenite region during cooling from the state.
Cerato-pearlite transformation within a certain period of time that does not occur
Give more than the amount of processing, cool to room temperature, and
After making rutensite, Ac ₁Suitable in the temperature range below the point
The normal heat treatment described above
In comparison with the material and the heat-treated material with strong cooling after austenitization,
, The tempering softening resistance increases, and at the same time during tempering
The carbides precipitated in the steel become extremely fine and the low temperature toughness and stress resistance
It was found that the corrosion cracking property can be improved.

That is, as disclosed in Japanese Patent Laid-Open No. 63-238217, in the final hot finishing step, the cross-section reduction rate is 13 to 30 in the temperature range of 650 to 1000.degree.
After processing to 90% and cooling to room temperature to obtain a martensitic structure, a tempering parameter defined by T (20 + logt) (however, T: ° K, t: hour) is set to 20.
A method characterized by performing a tempering treatment in the range of 500 to 21600 was proposed. According to this method, if the cross-sectional reduction rate in the final hot finishing step is about 60%, vE- ₄₀ is about 20 kg · m and the low temperature toughness is remarkably improved, but processing in the low temperature austenite region is performed. If the amount is small, the toughness improving effect is small, and for example, the area reduction rate is 20%.
However, there is a problem that vE- ₄₀ is improved only up to about 3 kg · m, and the above requirement cannot be satisfied with some product sizes.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and in the production of seamless steel pipe by the Mannesmann method, low temperature austenite in the final hot finishing step. An object of the present invention is to provide a method for producing a martensitic stainless seamless steel pipe having excellent low-temperature toughness and stress corrosion cracking resistance, even if the amount of work in the region is small.

[0008]

Means for Solving the Problems As a result of a detailed study on the relationship between the low temperature toughness and the working amount and cooling rate of martensitic stainless steel in the low temperature austenite region, the present inventors have found the following findings. Got In other words, it was confirmed that in the presence of mixed grains due to partial recrystallization, and in the case where coarse carbides were precipitated in the grain boundaries where strain remained in the cooling process, the effect of improving low temperature toughness was reduced in low workability materials. Of. Further, it has been found that by maintaining a cooling rate within a predetermined range from immediately after the final hot finishing step to a predetermined temperature range, it is possible to suppress partial recrystallization and precipitation of coarse carbides at grain boundaries. The present invention has been completed based on the findings.

That is, according to the present invention, in the final hot finish rolling step, the processing amount represented by the cross-section reduction rate is given in the range of 650 to 1000 ° C. in the range of 13 to 90%. Immediately after completion, the temperature range from 400 to 500 ° C. is cooled at a cooling rate of 2 to 30 ° C./sec to form a martensite structure, and then tempered, which is characterized by low temperature toughness and stress resistance by a Mannesmann method. This is a method for producing a martensitic stainless steel pipe having excellent corrosion cracking resistance.

[0010]

The reasons for limiting the processing conditions, cooling conditions and the like in the present invention will be described in detail below along with their effects. (Final hot finish rolling condition) The temperature of the final hot finish rolling step was set in the range of 650 to 1000 ° C for the following reason. This is because when the temperature of the final hot finish rolling process exceeds 1000 ° C, recrystallization occurs promptly after processing, no strain remains after the end of rolling, and the effect of improving the low temperature toughness by heat treatment of the final product cannot be obtained at all. When the temperature is lower than 650 ° C., the deformation resistance of the material becomes too high and the rolling tool is significantly damaged.

The reason why the area reduction rate is set to 13% or more and 90% or less as the processing amount in the final hot finish rolling step is as follows. When the cross-sectional reduction rate is less than 13%, there is no effect of working in the low temperature austenite region, it is not possible to expect an increase in temper softening resistance, and significant refinement of carbides that precipitate during tempering, and low temperature toughness and stress resistance Corrosion cracking cannot be improved. On the other hand, when the cross-section reduction rate exceeds 90%, the amount of processing is large and the temperature decrease during rolling significantly increases the power required for rolling, making it difficult to perform rolling, and causing significant wear of the rolling tool. .. Here, the working amount is a cross-sectional reduction rate when the cross-sectional area of the steel pipe before working is A ₀ and the cross-sectional area of the steel pipe after working is A, and is expressed by the following equation. Processing amount = [(A ₀ −A) / A ₀ ] × 100%

(Cooling conditions) The reason why the material is rapidly cooled to 400 to 500 ° C. immediately after the final hot finish rolling step is finished is as follows. The reason why the cooling was started immediately after the end of the final hot finish rolling step is that if there is a time before the start of cooling, partial recrystallization and precipitation of coarsened carbide will start. On the other hand, the cooling end temperature was set to 400 to 500 ° C because if it is lower than 400 ° C, partial recrystallization and precipitation of carbides at grain boundaries do not occur, which not only contributes to the effect of preventing deterioration of low temperature toughness. , The martensite transformation starts, so if the cooling rate is high, cracking occurs due to transformation stress.
This is because if the temperature exceeds 0 ° C., partial recrystallization and precipitation of carbides at grain boundaries occur after cooling is stopped, and the low temperature toughness deteriorates.

The cooling rate is defined as 2 to 30 ° C./sec for the following reason. Using the materials of the components shown in Table 1, the rolling condition No. 1 shown in Table 2 was obtained by the Mannesmann-mandrel mill method. 1 and No. After the final hot finish rolling in No. 2, immediately after the completion of rolling, it was cooled to 400 ° C. at various cooling rates. After that, adjust YS to 63kg / mm ² .
After tempering at 750 ° C for 60 minutes, L
Oriented full size Charpy specimens were taken and investigated for vE- ₄₀ . For comparison, rolling condition Nos. Shown in Table 2 are used. After rolling at 2, then normal air cooling to room temperature, holding again at 1000 ° C for 30 minutes, and then cooling to 400 ° C at various cooling rates, again at 690 ° C so that YS is about 63 kg / mm ^2. Then, a tempering treatment of holding for 60 minutes was carried out (rolling condition 3), and vE- ₄₀ was investigated by an L-direction full size Charpy test.

The relationship between the cooling rate and vE- ₄₀ after finish rolling and after quenching heat treatment is shown in FIG. According to this investigation, even if the cross-section reduction rate during finish rolling is small, vE- ₄₀ can be 10 kg · m or more if the cooling rate from immediately after finish rolling to 400 ° C is 2 ° C / sec or more. I understand. This is because when the cooling rate is less than 2 ° C / sec, partial recrystallization occurs to form mixed grains, and coarse carbides precipitate at grain boundaries where strain remains, so the amount of work during finish rolling is small. This is because the deterioration of the low temperature toughness due to precipitation of these mixed grains and coarse carbides at grain boundaries becomes larger than the effect of thermomechanical treatment. Therefore, the lower limit of the cooling rate is 2 ℃ / sec. On the other hand, if it exceeds 30 ° C / sec, partial recrystallization is prevented and the effect of suppressing the precipitation of coarse carbides at the grain boundaries is saturated, rather temperature control during cooling becomes difficult, and temperature unevenness on the inner and outer surfaces of the steel pipe becomes difficult. Upper limit is 3 because
0 ° C / sec.

[0015]

[Table 1]

[0016]

[Table 2]

[0017]

[Table 3]

[0018]

EXAMPLE A material billet having an outer diameter of 175 mm having the composition shown in Table 1 was heated to 1200 ° C., then hollow hollow pipes were perforated and rolled by a Mannesmann piercer, and stretched and rolled by a mandrel mill. After heating, finish rolling was performed with a stretch reducer into various product sizes. Cooling after rolling was performed by mist water cooling and normal air cooling, and the cooling rate was changed by controlling the amount of mist water. 650-800 so that YS of rolled tube is about 63kg / mm ^2.
After tempering heat treatment in the temperature range of ℃, L at -40 ℃
Energy absorbed by directional full size Charpy test,
And NACE-TM0177-90 Method B (NACE s
Sc value by SSC test was calculated.

In Comparative Example 1 (steel grade numbers 10 to 15) in which the conditions such as hot rolling and heat treatment are out of the range of the present invention,
In either case, vE- ₄₀ was a low value, or troubles such as occurrence of quench cracking occurred, and the intended improvements in low temperature toughness and stress corrosion cracking resistance could not be achieved. Table 3
The numerical value marked with ( ^* ) is outside the range of the processing and heat treatment conditions of the present invention. In addition, Comparative Example 2 in which after the usual finish rolling by the conventional method, after heating to 1000 ° C., holding for 30 minutes, cooling to room temperature by air cooling, and tempering treatment
It is clear that the low temperature toughness and the stress corrosion cracking resistance are remarkably improved in the examples according to the method of the present invention as compared with (Steel type Nos. 16 to 20).

[0020]

As described above, according to the present invention, the low temperature toughness and the stress corrosion cracking resistance of the martensitic stainless seamless steel pipe can be remarkably improved.

[Brief description of drawings]

FIG. 1 is a cooling rate after quenching heat treatment in Examples, Comparative Examples 1 and 2, and Charpy at −40 ° C.
It is a graph which shows the relationship with the absorbed energy of an impact test.

Claims (1)

[Claims] 1. A final hot finish rolling step of 650
In the temperature range of 1000 ° C to 1000 ° C, the processing amount represented by the cross-section reduction rate is given in the range of 13% to 90%, and the temperature range from 400 to 500 ° C immediately after the end of the final hot finish rolling step is 2
A martensitic stainless steel seamless pipe excellent in low-temperature toughness and stress corrosion cracking resistance by the Mannesmann method, characterized by performing a tempering treatment after cooling to a martensitic structure at a cooling rate of -30 ° C / sec Manufacturing method.