JP3518367B2 - Method for manufacturing 13Cr stainless steel plate - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a 13Cr stainless steel plate used as a material for welding pipes for transporting crude oil, natural gas or other fluids containing wet carbon dioxide gas. And a method for producing the same. [0002] In recent years, oil wells and natural gas wells containing a large amount of corrosive gas such as carbon dioxide gas and hydrogen sulfide have been developed with the worsening of the energy situation. [0003] In an environment containing only wet carbon dioxide gas (which may contain a trace amount of hydrogen sulfide), 13Cr stainless steel pipes are widely used from the viewpoint of corrosion resistance and material cost. As the oil country tubular goods, high C (0.2%)-13Cr stainless steel typified by AISI standard 420 steel is generally used. For line pipes, AISI is required from the viewpoint that welding work is required. Low C (0.1%)-13Cr stainless steel represented by standard 410 steel has been used. In recent years, the extremely low C (0.03
%), An improved 13Cr stainless steel containing Ni has been developed. [0006] Steel pipes made of these martensitic stainless steels are generally produced by a hot seamless pipe production method in many cases. However, although seamless steel pipes have been evaluated for high reliability, they have some problems, in particular, the problem that it is difficult to manufacture large-diameter pipes having an outer diameter of 16 inches (426 mm) or more. [0007] These large diameter pipes are generally manufactured by a welding pipe manufacturing method. Therefore, recently, several methods for manufacturing a 13Cr stainless steel welded steel pipe have been proposed. For example, Japanese Patent Application Laid-Open No. 4-191319 (JP-B-7-5972) proposes a method for producing a line pipe made of low C 13Cr stainless steel by an electric resistance welding method. Also, JP-A-8-2068
No. 61 proposes a method of manufacturing a line pipe made of 13Cr stainless steel using a laser welding method. However, in the electric resistance welding method, the laser welding method, or the TIG welding method, since a pipe is formed by using a hot-rolled steel strip of martensitic stainless steel as a material, a thin wall having a wall thickness of 10 mm or less is generally used. Only tubes can be manufactured. Furthermore, since a seamless steel pipe is heated at a high temperature of about 1250 ° C. and then pierced and stretched and rolled, a seamless steel pipe is used in view of mechanical properties and corrosion resistance.
Quenching and tempering after pipe production is essential. In addition, it is desirable that a quenching and tempering treatment be performed on the thin-walled welded pipe after the pipe-forming welding. Needless to say, such a quenching and tempering method has an advantage that strength adjustment is relatively easy. However, there are problems that the number of manufacturing steps increases and that oxide scale generated at the time of heat treatment needs to be removed, thereby lowering productivity and increasing manufacturing cost. [0012] Recently, in order to increase the efficiency of transporting natural gas, there is a demand for such 13Cr stainless steel line pipes to have a large diameter and a large wall thickness. Since the transport volume can be increased by increasing the diameter, and the operating pressure can be increased by increasing the wall thickness, the gas transport efficiency can be further increased. An object of the present invention is to provide a 13Cr stainless steel plate which is used as a material for welding pipes for transporting crude oil, natural gas or other fluids containing wet carbon dioxide gas, and which can be used as a product as it is by pipe welding. It is to provide a manufacturing method. Here, the thick steel plate means a steel plate having a thickness of about 1/2 inch (12.7 mm) or more. The gist of the present invention resides in the following method for producing a 13Cr stainless steel plate. In weight%, C: 0.03% or less, Si: 1
%, Mn: 2% or less, Cr: 9 to 13%, Ni: 1
~7%, Ti: 0.5% or less, Al: 0.1% or less, you
And Cu: 0.3 to 2% and Mo: 0.5 to 3%
After the hot rolling of a slab composed of steel containing Fe and unavoidable impurities is completed at a temperature of 800 ° C. or higher, the average cooling of the temperature range of 800 to 600 ° C. is performed at a temperature of 10 ° C./s or higher. A method for producing a 13Cr stainless steel plate that cools at a rate of less than 600 ° C. at a cooling rate equal to or less than air cooling while cooling at a high speed. In general, a welded pipe made of a thick steel plate as described above is formed by forming a thick steel plate into a tubular shape using a C press, a U press, and an O press, and performing seam pipe welding by a submerged arc welding method. At this time, in the seam welding by the submerged arc welding method, it is general that both inner and outer surfaces are welded in one layer, but sometimes they are welded in multiple layers. In some cases, a roll bender is used for forming a tube, and a laser welding method or a TIG welding method is used for a welding method. The so-called UO tube manufactured through the above-described steps generally has an outer diameter of 20 to 24 inches (50 inches).
8 to 609.6 mm) or more, and it is extremely difficult to quench and temper the entirety of such a large-diameter thick-walled pipe at a stretch, and if it is performed temporarily, the equipment cost increases and the cost becomes extremely high. . Therefore, a thick steel plate made of a material that can be used while being welded to a pipe is required. The thickness of the thick steel plate itself is also about 1 /
It is as long as 2 inches (12.7 mm) or more, about 63 inches or more in width, and about 25 to 26 m in length. Therefore, it is difficult to perform tempering as well as quenching after rolling, so that a thick steel plate that can be used for pipe welding without such heat treatment after rolling is required. On the other hand, when the heat treatment after rolling is omitted, not only the mechanical properties and corrosion resistance are satisfied as-rolled, but also hydrogen cracking in the thick steel plate must be avoided. However, the 13Cr stainless steel targeted in the present invention generally has a high-strength martensite structure of X80 (unit: ksi, 56 kgf / mm ² ) or more, and the steel plate has high susceptibility to hydrogen cracking. In the case of the hot rolled steel strip used for the ERW or laser pipe forming method, the cooling rate after winding is extremely low at 1 ° C./s or less, and hydrogen escapes before cooling to room temperature. So there is no worry about hydrogen cracking. As described above, hydrogen cracking is a problem unique to thick steel plates. A method of satisfying mechanical properties and corrosion resistance of 13Cr stainless steel plate as rolled as it is, and preventing hydrogen cracking in the plate has not been known. Hydrogen which causes hydrogen cracking of a thick steel plate is generated by thermal decomposition of water contained or adhered to a molten raw material. Hydrogen dissolved in the molten steel is trapped as a gas in porosity or the like by a slab or billet, and when the porosity is press-bonded in the heating and rolling step, it is dissolved again in the steel and remains in the product. Therefore, the problem does not occur if dehydrogenation is performed at the slab or billet stage.
In the case of extremely thick steel slabs, it is inefficient because conventionally known methods require several hours to several days to complete the dehydrogenation of the central part of the thickness. The present inventors have conducted a systematic study and conducted the experiments described below in order to elucidate the behavior of hydrogen cracking occurring in a thick steel plate, and have found the following. Hydrogen cracking in a thick steel sheet not subjected to heat treatment such as quenching and tempering occurs at the center of the thickness and propagates mainly parallel to the rolling surface. The morphology was a fracture of the former austenite grain boundary, and it was confirmed that coarse carbides were precipitated on the fracture surface. Corrosion test specimens were collected from a healthy part of the steel sheet where hydrogen cracking occurred, and a small amount (1
When a sulfide stress cracking (hereinafter, referred to as SSC) test was performed in an environment containing H ₂ S (00 ppm: equivalent to 0.03 atm), it was confirmed that a similar cracking form was also exhibited at the center of the sheet thickness. . The reason why hydrogen cracking occurs at the center of the thickness of a thick steel plate is that the residual hydrogen concentration in that portion is the highest.
In order to reduce the residual hydrogen concentration, the slab must be used for dehydrogenation. On the other hand, the present inventors have taken another measure to prevent hydrogen cracking, because the cracks are broken along the old austenite grain boundaries extending parallel to the rolling surface, and the old austenite grains are made into flat grains. It is considered that it is necessary to suppress the precipitation of carbides at the grain boundaries because it is considered that hydrogen gas accumulates around the carbides precipitated at the grain boundaries and cracks occur. Therefore, the prior austenite grains become flatter as the finishing temperature is lower.
Since the residence time at high temperature affects grain boundary precipitation of coarse carbides, the effect of water cooling applied after rolling on carbide precipitation was investigated. As a result, the following was found. When the rolling is completed at a temperature lower than 800 ° C., the grains become significantly flattened. In this case, even if the temperature range up to 600 ° C. after the rolling is cooled at an average cooling rate of 10 ° C./s or more,
Hydrogen cracking occurs due to carbides slightly precipitated at the grain boundaries, deteriorating SSC resistance. Further, even if rolling is completed at 800 ° C. or more,
Thereafter, when cooled to a temperature lower than 600 ° C. at an average cooling rate of 10 ° C./s or more, coarse carbide precipitation at the grain boundaries is suppressed and the SSC resistance is good, but dehydrogenation in a high temperature region is insufficient, Hydrogen cracking occurs. On the other hand, when the rolling is completed at 800 ° C. or higher, the flattening of the grains is greatly suppressed. After completion of the rolling, the temperature range of 800 to 600 ° C. is cooled at an average cooling rate of 10 ° C./s or more, and then the temperature range of less than 600 ° C. is cooled at a cooling rate of air cooling or less. Precipitation of high-quality carbides is greatly suppressed, dehydrogenation in the high-temperature range is promoted, and hydrogen cracking does not occur and good SSC resistance
Is ensured. That is, at the stage of the raw material (slab) before rolling, rather than performing a long-term dehydrogenation treatment, the hot rolling is completed at 800 ° C. or more, and the temperature range of 800 to 600 ° C.
After cooling at an average cooling rate of 0 ° C./s or more, dehydrogenation is performed by a short-time treatment of cooling a temperature range of less than 600 ° C. at a cooling rate of air cooling or less. A thick steel plate having SSC resistance was obtained, and it was found that a welded steel pipe using this as a raw material had no problem even if it was used as a product as it was in pipe welding. Hereinafter, the method of the present invention will be described in detail. In the following, “%” means “% by weight” unless otherwise specified. << Chemical Composition of Material Steel >> C: When the C content exceeds 0.03%, not only deterioration of carbon dioxide corrosion resistance due to precipitation of Cr carbide occurs, but also
Carbide, which is the starting point of hydrogen cracking, tends to precipitate at the grain boundaries. Further, the hardness of the welded portion is increased, and the SSC resistance is deteriorated. Therefore, the upper limit of the C content is set to 0.03%. A desirable upper limit is 0.02%. C is preferably as low as possible from the viewpoint of securing corrosion resistance and suppressing an increase in hardness of the welded portion. The lower limit of C may be as low as about 0.005%, which is industrially possible. There is no need to specify. Si: Si is an element added for deoxidizing steel, but if its content exceeds 1%, the cleanliness and toughness of steel decrease. For this reason, the upper limit of the Si content is set to 1%. A desirable upper limit is 0.5%. In order to obtain a sufficient deoxidizing effect, the content is preferably set to 0.05% or more. However, when the content is sufficiently deoxidized by another element (Al described later), Si does not necessarily have to be contained. Mn: Mn is effective in obtaining a stable martensitic phase in a low C-13Cr stainless steel having a C content of 0.03% or less, which is a target of the present invention, but its content is 2%. If it exceeds, the corrosion resistance deteriorates. For this reason, the upper limit of the Mn content is set to 2%. A desirable upper limit is 1%. It is to be noted that Mn is preferably as low as possible from the viewpoint of corrosion resistance, so that its lower limit need not be particularly defined. However, if the content is excessively low, it is necessary to increase the expensive Ni content in order to obtain a martensitic phase, resulting in an increase in cost. Is preferred. Cr: If the Cr content is less than 9%, a corrosion-resistant film having sufficient corrosion resistance is not formed on the surface of the steel including the base material, so that the pipe for line pipe contains carbon dioxide gas or hydrogen sulfide. When used in the environment, the required corrosion resistance cannot be secured. Conversely, if the Cr content exceeds 13%, not only the effect on corrosion resistance is saturated, but also an expensive alloy such as Ni, which is an element that stabilizes the austenite phase and thus facilitates the formation of the martensite phase. It is necessary to increase the amount of the element to be added, which leads to an increase in material cost and impairs economic efficiency. Therefore, the Cr content is 9
１３13%. A desirable range is 10 to 13%. Ni: If the Ni content is less than 1%, it becomes difficult not only to secure a stable and easy martensite phase, but also to use the steel pipe for line pipe in an environment containing a trace amount of hydrogen sulfide. In this case, since a corrosion-resistant film having corrosion resistance is not formed and formed, the required corrosion resistance cannot be secured. Conversely, if the Ni content exceeds 7%, the effect on the stability and corrosion resistance of the structure is saturated,
This raises the cost of raw materials, and impairs economic efficiency. Therefore, the Ni content was determined to be 1 to 7%. A desirable range is 2 to 7%. Al: Al is an element added for deoxidizing steel, but if it is contained in excess of 0.1%, cleanliness and toughness are reduced. For this reason, the upper limit of the Al content is 0.1%.
It was decided. A desirable upper limit is 0.06%. In order to obtain a sufficient deoxidizing effect, it is preferable that the content is 0.01% or more. However, when sufficient deoxidizing is performed by another element (the aforementioned Si), Al does not always have to be contained. Ti: Ti has the effect of fixing C and suppressing the precipitation of coarse carbides at the grain boundary. However, when the content exceeds 0.5%, not only the effect is saturated, but also the hot workability and The toughness deteriorates. Therefore, the upper limit of the Ti content is set to 0.5%. A desirable upper limit is 0.2%. Although the lower limit of the Ti content is not particularly defined, it is preferable that the content of C is at least four times the content of C in order to fix almost all of C as TiC. [0048] Cu, Mo: These elements are effective to improve the corrosion resistance, especially local corrosion resistance and SSC resistance. Therefore, one or both elements of Cu and Mo are added.
I do. Respectively 0.3% or more for Cu, the effect becomes significant at 0.5% or more for Mo. However, the addition of Cu exceeding 2% not only saturates the effect but also causes deterioration of hot workability and weldability. Further, the addition of more than 3% of Mo not only saturates its effect, but also stabilizes the austenite phase and consequently forms the martensite phase because it is a stabilizing element of the ferrite phase, like Cr. It is necessary to increase the content of expensive alloying elements such as Ni, which is an element that facilitates this, and this leads to an increase in material costs.
Therefore , the content of Cu and Mo is 0.3 to 0.3 respectively.
2%, 0.5 to 3% . P, S, N, O (oxygen): These elements are all unavoidable impurities contained in steel, and the lower the content, the better. << Manufacturing Method >> Heating of raw steel slab: The temperature of the raw steel slab to be subjected to rolling may be any temperature at which the following rolling is possible. When the finishing rolling temperature is lower than 800 ° C., since the temperature is in the non-recrystallization temperature range, the grains are remarkably flattened and the susceptibility to hydrogen cracking increases, and hydrogen cracking occurs even in extremely low C and Ti-added steel. Can not be prevented. A desirable lower limit of the finish rolling temperature is 900 ° C. With such a high-temperature finish, it is possible to produce a thick steel sheet suitable for a material for a welded pipe without mechanical anisotropy in the mechanical properties in the L direction (rolling direction) and the C direction (direction perpendicular to the rolling direction). . Average cooling rate of 800 to 600 ° C .: After rolling, it is necessary to cool the temperature range of 800 to 600 ° C. at an average cooling rate of 10 ° C./s or more. This is because, as described above, if the temperature range of 800 to 600 ° C. is cooled at an average cooling rate of less than 10 ° C./s, coarse carbides which serve as starting points of hydrogen cracking and accelerate the propagation of cracks are formed at grain boundaries. This is because it becomes impossible to suppress precipitation on the surface. The lower limit of the desirable average cooling rate is 15 ° C./s. The faster the average cooling rate is, the better. For this reason, the upper limit does not need to be specified. The cooling method may be air-water (mist) cooling or shower water cooling. Cooling rate of less than 600 ° C .: When cooling to a temperature range of less than 600 ° C. at the above average cooling rate of 10 ° C./s or more, dehydrogenation in a high temperature range becomes insufficient, and even if coarse carbides Even if the precipitation can be suppressed, hydrogen cracking occurs. For this reason, the cooling rate in a temperature range below 600 ° C. is set to a slow rate equal to or lower than air cooling. The cooling rate slower than the air cooling can be realized by, for example, a method in which two sheets are stacked while the rolled steel sheet is not completely cooled, and this method is the simplest. EXAMPLES 11 types of steels having the chemical compositions shown in Table 1 were melted, and raw steel slabs (slabs) having a thickness of 300 mm and a width of 2300 mm were prepared. Sa1
A rolling material having a size of 50 mm, a width of 120 mm and a length of 100 mm was cut out. [Table 1] Each rolling material was hot-rolled and cooled under various conditions shown in Table 2 to obtain a thick steel plate having a thickness of 25.4 mm. [Table 2] Then, after the production, each test piece having a total thickness of 100 mm on a side (however, the surface scale is removed by grinding) is sampled from each thick steel plate after being left at room temperature for 48 hours, and subjected to ultrasonic flaw detection. The presence or absence of hydrogen cracking was examined by the C-scan method. In the evaluation, those in which cracks were not detected were regarded as pass (○), and those in which even some cracks were detected were rejected (x). The thickness of each steel plate after being left at room temperature for 48 hours is 2 m in thickness from the C direction (direction perpendicular to the rolling direction).
A notch-free stress corrosion test specimen of m, width 10 mm, and length 75 mm was collected. The obtained stress corrosion test piece was set in a four-point bending jig shown in FIG. 1 and a stress of 100% of the yield stress of the material thick steel plate was applied. For 336 hours to examine the SSC resistance. In the evaluation, those in which SSC did not occur were evaluated as pass (○), and those in which SSC occurred were evaluated as unacceptable (x). Corrosion environment: 0.003 atm H ₂ S-30 atm CO ₂ -5% Na
Cl _{solution, 0.03atmH 2 S-30atmCO 2 -5} % NaC
1 aqueous solution. The reason why the corrosion environment was performed in the above two cases is that the steel to which Cu and / or Mo was added and the steel to which Cu and / or Mo were not added had different SSC resistances. The results of these investigations are also shown in Table 2. [0064] Table As can be seen from 2 to show results, the steel plate manufactured according to the method of the present invention (trial number 16, 1
7, 20, and 24 to 28) did not have any hydrogen cracking and had good SSC resistance. On the other hand, the chemical composition of steel, the finishing temperature,
Average cooling rate between 800 and 600 ° C, average cooling rate 10
A steel plate of a comparative example manufactured by a method in which one of the cooling stop temperature at a temperature of at least ° C / s and the cooling rate below 600 ° C is out of the range specified in the present invention (sample numbers 12 to 15, 1
8, 19 and 21 to 23) all had hydrogen cracking and had poor SSC resistance except for those having a cooling stop temperature of less than 600 ° C. at an average cooling rate of 10 ° C./s or more. . [0066] More specifically, the trial numbers 15 and 19
Since the average cooling rate between 800 and 600 ° C. was less than 10 ° C./s, hydrogen cracking occurred and the SSC resistance was poor . In Test Nos. 12 and 13, since the C content was too high, hydrogen cracking occurred and the SSC resistance was poor.
In Test Nos. 22 and 23, hydrogen cracking occurred and SSC resistance was poor because Ti was not added. According to the method of the present invention, 13Cr excellent in corrosion resistance and suitable as a material for line pipes for transporting crude oil, natural gas or other fluids containing wet carbon dioxide gas.
Series stainless steel plate can be manufactured at low cost. In addition, when manufacturing a welded steel pipe for a line pipe using this thick steel plate as a material, the pipe has the necessary characteristics as it is, and can be shipped as a product without quenching and tempering. .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a set state of a stress corrosion cracking test piece on a four-point bending applying jig.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C21D 9/573 C22C 38/00-38/60 B21B 1/00-3 / 02

Claims (1)

(57) [Claims 1] By weight%, C: 0.03% or less, Si: 1
%, Mn: 2% or less, Cr: 9 to 13%, Ni: 1
~7%, Ti: 0.5% or less, Al: 0.1% or less, you
And Cu: 0.3 to 2% and Mo: 0.5 to 3%
After the hot rolling of a slab composed of steel containing Fe and unavoidable impurities is completed at a temperature of 800 ° C. or higher, the average cooling of the temperature range of 800 to 600 ° C. is performed at a temperature of 10 ° C./s or higher. A method for producing a 13Cr stainless steel plate, comprising cooling at a cooling rate and cooling a temperature range of less than 600 ° C. at a cooling rate of air cooling or less.