JP5370503B2 - Low alloy steel - Google Patents

Description

  The present invention relates to a low alloy steel. In particular, the present invention relates to a low alloy steel in which a weld heat affected zone after heat treatment after welding has excellent resistance against embrittlement caused by hydrogen such as stress corrosion cracking in a wet hydrogen sulfide environment.

  In the development of offshore oil fields, risers, flow lines, trunks are used for the transportation of crude oil or natural gas from oil or gas wells installed on the seabed to offshore platforms or from platforms to onshore refineries. A steel pipe called a line is used. On the other hand, with the global depletion of fossil fuels, the development of oil fields containing a lot of corrosive hydrogen sulfide has become active. Steel pipes transporting crude oil or natural gas mined from oil fields containing such corrosive gases have hydrogen induced cracking (HIC: hereinafter referred to as “HIC”) and sulfide stress corrosion cracking (hereinafter referred to as “HIC”). SSC: Sulfide Stress Cracking (hereinafter referred to as “SSC”) may cause fracture due to embrittlement due to hydrogen resulting from a corrosion reaction. Many steels that have been developed for a long time to improve HIC resistance and SSC resistance have been proposed.

  For example, Patent Document 1 proposes a steel that is substantially free of Ni, Cu, and Ca, and that has excellent HIC resistance by specifying the thermal history and heat treatment conditions during production. Patent Document 2 proposes a steel having HIC resistance and SSC resistance by making Cr, Ni and Cu essential additions. Further, Patent Document 3 proposes a steel having improved HIC resistance and SSC resistance by defining the amounts of C, Ti, N, V and O within specific ranges.

  By the way, when assembling a structure using these steels, such as piping a steel pipe made of these steels, welding is generally performed. However, as described in Non-Patent Document 1, for example, it is widely known that the SSC sensitivity increases with an increase in hardness. When steel is heated by welding, a portion that hardens in a so-called welding heat affected zone (hereinafter referred to as “HAZ”) is generated. As a result, there are many cases where practically sufficient performance as a welded structure cannot be obtained no matter how the HIC resistance and SSC resistance of the steel itself are enhanced.

  Therefore, in recent years, as described in Patent Document 4, by reducing the amount of C and Mn and containing 0.5% or more of Mo, the hardening of the weld heat affected zone is suppressed, and the base material and A high-strength steel has also been proposed in which HAZ has both HIC resistance and SSC resistance.

  Heat treatment after welding (hereinafter referred to as “PWHT”) is widely applied to Cr—Mo steel or martensitic stainless steel used in large quantities in pressure vessels and the like as a method for reducing the hardness of the weld heat affected zone. . For example, Patent Document 5 proposes a low alloy steel containing 0.5% or more of Cr on the premise that PWHT is performed for one hour per one inch of thickness.

JP-A-5-255746 JP-A-6-336639 JP 2002-60894 A JP 2010-24504 A JP2007-32228A

Masanori Kowaka, Metal Corrosion Damage and Anticorrosion Technology, August 25, 1983, Agne Inc., 198 pages

  According to the invention described in Patent Document 4, it is said that the hardening of the heat affected zone of the weld is suppressed, and both the HIC resistance and the SSC resistance of the base material and the HAZ can be achieved, but Mo is an expensive element. Therefore, a technique for improving the hydrogen embrittlement resistance of HAZ without requiring a great deal of cost is desired.

  As described in Patent Document 5, PWHT has a certain effect, but in line pipe laying, efficiency is important, such as welding on an offshore ship, so PWHT is generally not implemented or applied In that case, it is desired to apply PWHT for a very short time.

  An object of the present invention is to provide a low alloy steel in which PWHT, particularly HAZ subjected to short-time PWHT, has excellent hydrogen embrittlement resistance in a wet hydrogen sulfide environment.

  In order to clarify the necessary conditions in order to improve the HAZ hydrogen embrittlement resistance of the steel material subjected to PWHT, the present inventors first investigated the hydrogen embrittlement of the as-welded HAZ. As a result, hydrogen embrittlement of HAZ is considered to occur by the following mechanism.

  That is, when steel is exposed to a corrosive environment containing hydrogen sulfide, hydrogen penetrates into the steel due to the corrosion reaction. This hydrogen is called diffusible hydrogen, which can move freely in the crystal lattice of steel. The invading diffusible hydrogen accumulates in dislocations or vacancies, which are one type of defects in the crystal lattice, and lattice strain at the interface between a carbide such as cementite and the substrate, and embrittles the steel. In particular, HAZ is heated to a high temperature due to the thermal history of welding, rapidly cooled, and becomes an as-quenched martensite or bainite structure. The pores are present at high density, and the cementite is dispersed. Therefore, it is considered that HAZ is more susceptible to hydrogen embrittlement than the base material.

  When PWHT is applied, the density of dislocations or vacancies is reduced, and softening proceeds while cementite precipitates. Therefore, when sufficient softening does not occur in a short time PWHT, it is considered that the effect of reducing hydrogen embrittlement susceptibility is not significant due to a trade-off with precipitation of cementite.

  Therefore, the present inventors tried to optimize the alloy elements in order to improve the hydrogen embrittlement resistance of HAZ to which PWHT was applied. As a result, it has been found that it is effective to contain at least one of Ti, V and Nb in order to increase the hydrogen embrittlement susceptibility of HAZ subjected to PWHT. The reason is considered as follows.

  That is, Ti, V and Nb have a greater affinity for carbon than iron and form MX type fine carbides in the process of PWHT. Since the MX type carbide has better consistency with the parent phase than cementite, the lattice strain at the interface with the substrate is small, and the amount of diffusible hydrogen in the carbide is large. For this reason, when hydrogen invades due to a corrosion reaction, the accumulation sites of diffusible hydrogen are dispersed, thereby suppressing significant hydrogen accumulation and the generation of embrittlement starting points, thereby reducing the embrittlement.

When the amount of C is large, that is, the hardenability of HAZ at the time of welding cooling is high, the density of dislocations or vacancies is high, and the driving force for precipitation of cementite at the time of PWHT application is large, Ti, V It became clear that it is necessary to contain a proper amount of Nb and Nb. Specifically, it has become clear that it is necessary to contain at least one selected from Ti, V and Nb within a range satisfying the following formula (1).
0.1 × [C (%)] ≦ [Ti (%)] + [V (%)] + 0.5 × [Nb (%)] ≦ 0.2 (1)
However, each element symbol in the mathematical formula means the content (% by mass) of each element.

  The present invention has been made on the basis of such findings and has the following [1] to [6].

[1] Low alloy steel as a raw material for a steel pipe for transporting crude oil or natural gas subjected to heat treatment after welding, and in mass%, C: 0.01 to 0.15%, Si: 3% or less, Mn: 3% or less and Al: includes 0.08% or less, T i, the one or more selected from V and Nb was contained in the range that satisfies the following equation (1), the Cr and / or Mo , 1.5% or less in total, Ni and / or Cu, 0.8% or less in total, Ca and / or Mg, 0.05% or less in total, and B in the following (2) In the range satisfying the formula, the balance is composed of Fe and impurities, N as an impurity is 0.01% or less, P is 0.05% or less, S is 0.03% or less, and O is 0.03% or less der is, after welding heat treatment, at 30 to 600 seconds at 500 to 750 ° C., and the following (3) Der Ru low alloy steel intended to be performed in conditions satisfying.
0.1 × [C (%)] ≦ [Ti (%)] + [V (%)] + 0.5 × [Nb (%)] ≦ 0.2 (1)
[B (%)] <0.1 × [C (%)] (2)
8000 ≦ T × {20 + log (t / 3600)} ≦ 15000 (3)
However, each element symbol in the formula means the content (% by mass) of each element , T is the processing temperature (° C.) of the heat treatment after welding, and t is the processing time (second) of the heat treatment after welding. It is.
[2] Low alloy steel as a raw material for a steel pipe for transporting crude oil or natural gas subjected to post-weld heat treatment at 500 to 750 ° C. for 30 to 600 seconds, and in mass%, C: 0.01 to 0 .15%, Si: 3% or less, Mn: 3% or less and Al: 0.08% or less, and at least one selected from Ti, V and Nb satisfies the following formula (1) A low alloy in which the balance is Fe and impurities, N as an impurity is 0.01% or less, P is 0.05% or less, S is 0.03% or less, and O is 0.03% or less. steel.
0.1 × [C (%)] ≦ [Ti (%)] + [V (%)] + 0.5 × [Nb (%)] ≦ 0.2 (1)
However, each element symbol in the mathematical formula means the content (% by mass) of each element.

[ 3 ] The low alloy steel according to the above [ 2 ], which contains, in mass%, 1.5% or less of Cr and / or Mo in place of part of Fe.

[ 4 ] The low alloy steel according to the above [ 2 ] or [ 3 ], which contains 0.8% or less of Ni and / or Cu in place of part of Fe in mass%.

[ 5 ] The low alloy steel according to any one of the above [ 2 ] to [ 4 ], which contains 0.05% or less in total of Ca and / or Mg instead of a part of Fe in mass%.

[ 6 ] The low alloy steel according to any one of the above [ 2 ] to [ 5 ], which contains B in a range satisfying the following formula (2) instead of part of Fe in mass%.
[B (%)] <0.1 × [C (%)] (2)
However, each element symbol in the mathematical formula means the content (% by mass) of each element.

  According to the present invention, it is possible to provide a low alloy steel having excellent hydrogen embrittlement resistance in a wet hydrogen sulfide environment or the like in PHAT, particularly HAZ subjected to short time PWHT.

  Hereinafter, the range of the chemical composition of the low alloy steel according to the present invention and the reason for the limitation will be described. In the following description, “%” for the content means “% by mass”.

C: 0.01 to 0.15%
C is an element effective in increasing the hardenability of steel and increasing the strength. In order to acquire the effect, it is necessary to make it contain 0.01% or more. However, when the content exceeds 0.15%, a large amount of cementite is precipitated when PWHT is applied, and the hydrogen embrittlement susceptibility of HAZ is increased. Therefore, the C content is set to 0.01 to 0.15%. The lower limit of the C content is preferably 0.03%. The C content is preferably 0.12% or less.

Si: 3% or less Si is an element effective for deoxidation, but if it is excessively contained, toughness is reduced. For this reason, Si content shall be 3% or less. The Si content is preferably 2% or less. Although the lower limit is not particularly defined, even if the Si content is reduced, the deoxidation effect is lowered, the cleanliness of the steel is deteriorated, and excessive reduction leads to an increase in production cost. For this reason, it is preferable that Si content shall be 0.01% or more.

Mn: 3% or less Mn is an element effective for deoxidation, like Si, and is an element that contributes to improvement of strength by enhancing the hardenability of steel. However, if it is contained excessively, the HAZ is markedly cured and the hydrogen embrittlement resistance is increased. Therefore, the Mn content is 3% or less. Although the lower limit is not particularly defined, it is preferable to contain 0.2% or more in order to obtain the effect of improving the strength of Mn. A more preferred lower limit is 0.4%, and a preferred upper limit is 2.8%.

Al: 0.08% or less Al is an element effective for deoxidation, but its effect is saturated even if it is contained excessively, and the toughness is reduced. Therefore, the Al content is set to 0.08% or less. A preferable content is 0.06% or less. Although the lower limit is not particularly defined, excessive reduction causes a deoxidation effect not sufficiently obtained and deteriorates the cleanliness of the steel, and causes an increase in manufacturing cost. Therefore, Al is preferably contained in an amount of 0.001% or more. The Al content of the present invention refers to acid-soluble Al (so-called “sol.Al”).

One or more selected from Ti, V and Nb: a range satisfying the following formula (1) 0.1 × [C (%)] ≦ [Ti (%)] + [V (%)] + 0.5 × [Nb (%)] ≦ 0.2 (1)
However, each element symbol in the mathematical formula means the content (% by mass) of each element.
These elements form MX type fine carbides in the process of PWHT, and improve hydrogen embrittlement resistance. In order to obtain this effect, it is necessary to set “[Ti (%)] + [V (%)] + 0.5 × [Nb (%)]” to 0.1 × [C (%)] or more. . However, when the content is excessive, the carbide becomes coarse, and on the contrary, the hydrogen embrittlement sensitivity is increased and the toughness is reduced. Therefore, “[Ti (%)] + [V (%)] + 0.5 × [Nb (%)]” needs to be 0.2% or less. A preferable upper limit is 0.18%, and a more preferable upper limit is 0.15%.

  The low alloy steel according to the present invention contains each of the above elements, with the balance being Fe and impurities. An impurity means the component mixed by raw materials and other factors, such as an ore and a scrap, when manufacturing steel materials industrially. Among impurities, the following elements need to be strictly limited in content.

N: 0.01% or less N is present in steel as an impurity, but if fine carbonitride is formed, it causes embrittlement and lowers toughness even when dissolved. Therefore, it is necessary to limit the content to 0.01% or less. The content is preferably 0.008% or less. There is no particular lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, the lower limit of the N content is preferably 0.0001%.

P: 0.05% or less P is present in the steel as an impurity, but segregates at grain boundaries in HAZ, leading to a decrease in toughness. Therefore, the content is limited to 0.05% or less. There is no particular lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, the lower limit of the P content is preferably 0.001%.

S: 0.03% or less S is present in steel as an impurity like P, but forms sulfides in the steel, and the interface with the substrate acts as a hydrogen accumulation site, increasing hydrogen embrittlement susceptibility. Also, the HAZ toughness is reduced. Therefore, the content is stricter than P and limited to 0.03% or less. There is no particular lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, the lower limit of the S content is preferably 0.0001%.

O: 0.03% or less O is present in the steel as an impurity, but when it is contained in a large amount, it generates a large amount of oxide, which deteriorates workability and ductility. Therefore, it is necessary to make it 0.03% or less. Desirably, it is 0.025% or less. There is no particular need to provide a lower limit, but excessive reduction leads to a significant increase in manufacturing costs. Therefore, it is desirably 0.0005% or more.

  The low alloy steel according to the present invention may contain the following elements instead of a part of Fe.

Cr and / or Mo: 1.5% or less in total These elements may increase the hardenability and contribute to improving the strength, and therefore may be contained. However, when the content is excessive, it precipitates as carbides, inhibits precipitation of carbides such as Ti, and increases hydrogen embrittlement sensitivity. Therefore, when Cr and / or Mo is contained, the content is made 1.5% or less in total. In addition, a preferable minimum is 0.02%, More preferably, it is 0.05%. A preferable upper limit is 1.2%.

Ni and / or Cu: 0.8% or less in total These elements may be contained because they enhance the hardenability and contribute to improving the strength. However, even if contained excessively, the effect is saturated and the cost is increased. Therefore, when it contains Ni and / or Cu, the content shall be 0.8% or less in total. In addition, the preferable minimum in the case of adding is 0.02%, More preferably, it is 0.05%. A more preferred upper limit is 0.7%.

Ca and / or Mg: 0.0005 to 0.05% in total
Any of these elements may be contained in order to improve the hot workability of the steel. However, if its content is excessive, it may combine with oxygen, significantly reducing cleanliness, and possibly degrading hot workability. Therefore, when it contains 1 or more types of these elements, the content shall be 0.05% or less in total. The preferred lower limit is 0.0005%, more preferably 0.001%. A preferable upper limit is 0.03%.

B: Range satisfying the following formula (2) [B (%)] <0.1 × [C (%)] (2)
However, each element symbol in the mathematical formula means the content (% by mass) of each element.
B segregates at the grain boundary, suppresses the precipitation of ferrite from the grain boundary, indirectly increases the hardenability, and contributes to improving the strength. However, excessive inclusion may precipitate as boride during the PWHT process, or may be substituted with C to form a solid solution in cementite, thereby increasing the lattice strain with the substrate and reducing hydrogen embrittlement resistance. There is. Therefore, when it contains B, it is preferable to make the content into the range which satisfy | fills (2) Formula. The desirable lower limit is 0.0001%, and more desirably 0.0005%.

There are no particular restrictions on the PWHT conditions applied to the low alloy steel according to the present invention, but the low alloy steel according to the present invention is particularly suitable when PWHT is applied that satisfies the following formula (3). Exhibits excellent effects.
8000 ≦ T × {20 + log (t / 3600)} ≦ 15000 (3)
However, T is the processing temperature (° C.) of the heat treatment after welding, and t is the processing time (seconds) of the heat treatment after welding.
When “T × {20 + log (t / 3600)}” is less than 8000, the hydrogen embrittlement resistance of the HAZ of the steel material made of the low alloy steel according to the present invention may not be improved. On the other hand, when “T × {20 + log (t / 3600)}” exceeds 15000, MX type fine carbides composed of Ti and the like are coarsened, and sufficient hydrogen embrittlement resistance cannot be obtained. The strength reduction of the steel including the part becomes remarkable. Therefore, it is preferable that the PWHT applied to the low alloy steel according to the present invention is performed under the condition satisfying the above expression (3).

  In particular, it is preferably performed in a temperature range of 500 to 750 ° C. for 30 to 600 seconds. This is to form MX fine carbides stably by short-time PWHT, to improve hydrogen embrittlement resistance, and to suppress an extreme cost increase due to long-time PWHT in the construction work. In particular, the PWHT time is more preferably 300 seconds or less.

  The low alloy steel of the present invention preferably has a yield strength (YS) of 552 MPa or more. The reason is that low-strength steel with high strength has a remarkable decrease in strength of the steel including the welded portion due to PWHT, and the advantage of improving hydrogen embrittlement resistance by PWHT in a short time is more easily obtained.

  In order to confirm the effect of the present invention, the following experiment was conducted. That is, a test material was produced by machining a 12 mm thick low alloy steel plate having a chemical composition shown in Table 1 into a 12 mm square and a length of 100 mm. This test material was subjected to a HAZ reproducible welding heat cycle that was heated to 1350 ° C., a temperature at which HAZ was markedly cured by high-frequency induction heating, for 3 seconds and then rapidly cooled. The following tests were conducted using this test material.

<Tensile test>
In accordance with JIS Z2241, a round bar tensile test piece having a parallel part diameter of 6 mm and a parallel part length of 10 mm was sampled from the obtained test material and subjected to a tensile test at room temperature.

<SSC resistance test>
A test piece having a thickness of 2 mm, a width of 10 mm, and a length of 75 mm was taken from the obtained test material, and the SSC resistance was evaluated by a four-point bending test in accordance with EFC16 defined by the European Federation of Corrosion. In the test, a stress corresponding to 50% of the 0.2% proof stress derived from the tensile test was applied to the collected specimen by 4-point bending, and then 5% of room temperature (24 ° C.) saturated with 1 atm hydrogen sulfide gas. It was immersed in a salt + 0.5% acetic acid aqueous solution for 336 hours and examined for the occurrence of SSC. And the thing in which SSC did not generate | occur | produce was set as the pass, and the thing in which SSC generate | occur | produced was set as the rejection.

  These test results are shown in Table 2.

  As shown in Table 2, generation of SSC was not recognized in the symbol X1 to X12 in the 4-point bending test. On the other hand, in the symbols Y1 and Y2, although chemical components satisfy the requirements of the present invention, PWHT was not performed, so MX type carbide was not precipitated and SSC was generated. Symbols Y3 and Y4 are low in the amount of addition of Ti, Nb and V, which are constituent elements of MX type carbide contained in the steel, and did not satisfy the predetermined relationship with C, so a sufficient amount of MX type carbide was precipitated. No SSC occurred. On the other hand, since the symbol Y5 has too much added amounts of Ti, Nb and V, MX type carbides were coarsely precipitated and SSC was generated. Although the symbol Y6 added B, the addition amount was excessive, so SSC occurred. Furthermore, since the symbol Y7 contained excessive amounts of Cr and Mo, these carbides were precipitated by PWHT, and MX type carbides were not stably produced, so SSC was generated.

According to the present invention, it is possible to provide a low alloy steel having excellent hydrogen embrittlement resistance in a wet hydrogen sulfide environment or the like in PHAT, particularly HAZ subjected to short time PWHT. This low alloy steel is most suitable as a material for steel pipes for transporting crude oil or natural gas.

Claims (6)

Low alloy steel as a raw material for crude or natural gas transport steel pipe that is heat treated after welding,
By mass%, C: 0.01~0.15%, Si : 3% or less, Mn: 3% or less and Al: it includes 0.08% or less, one or more selected from T i, V and Nb In a range satisfying the following formula (1), including Cr and / or Mo in total of 1.5% or less, Ni and / or Cu in total of 0.8% or less, Ca and / Alternatively, Mg is contained in a total of 0.05% or less, B is contained within a range satisfying the following formula (2), the balance is made of Fe and impurities, N as an impurity is 0.01% or less, and P is 0.05% or less, S is 0.03% or less, O is Ri der 0.03% or less, the post-weld heat treatment, in 30 to 600 seconds at 500 to 750 ° C., and satisfies the following formula (3) low alloy steel, characterized in der Rukoto that performed in the condition.
0.1 × [C (%)] ≦ [Ti (%)] + [V (%)] + 0.5 × [Nb (%)] ≦ 0.2 (1)
[B (%)] <0.1 × [C (%)] (2)
8000 ≦ T × {20 + log (t / 3600)} ≦ 15000 (3)
However, each element symbol in the formula means the content (% by mass) of each element , T is the processing temperature (° C.) of the heat treatment after welding, and t is the processing time (second) of the heat treatment after welding. It is. A low alloy steel as a raw material of a steel pipe for transporting crude oil or natural gas that is subjected to a heat treatment after welding at 500 to 750 ° C. for 30 to 600 seconds ,
1% or more selected from Ti, V, and Nb, including C: 0.01 to 0.15%, Si: 3% or less, Mn: 3% or less, and Al: 0.08% or less. In the range satisfying the following formula (1), the balance is Fe and impurities, N as an impurity is 0.01% or less, P is 0.05% or less, S is 0.03% or less, Low alloy steel characterized in that O is 0.03% or less.
0.1 × [C (%)] ≦ [Ti (%)] + [V (%)] + 0.5 × [Nb (%)] ≦ 0.2 (1)
However, each element symbol in the mathematical formula means the content (% by mass) of each element. The low-alloy steel according to claim 2 , characterized by containing 1.5% or less in total of Cr and / or Mo instead of a part of Fe in mass%. By mass%, instead of a part of Fe, Ni and / or Cu, and low alloy steel according to claim 2 or 3, characterized in that it comprises 0.8% or less in total. The low alloy steel according to any one of claims 2 to 4 , characterized by containing 0.05% or less of Ca and / or Mg in total in place of part of Fe in mass%. The low alloy steel according to any one of claims 2 to 5 , characterized in that, in mass%, B is included in a range satisfying the following expression (2) instead of part of Fe.
[B (%)] <0.1 × [C (%)] (2)
However, each element symbol in the mathematical formula means the content (% by mass) of each element.