JP5504717B2 - Manufacturing method of ERW steel pipe for sour line pipe - Google Patents

Description

The present invention relates to a method for manufacturing an electric resistance welded steel pipe excellent in toughness and resistance to hydrogen-induced cracking in an environment containing hydrogen sulfide (H ₂ S), that is, sour resistance.

  Sour resistance is required for steel pipes used for sour oils containing hydrogen sulfide, line pipes for transporting sour gas, and pipes attached to the pipes. The sour resistance is hydrogen-induced crack resistance (HIC resistance) and stress crack resistance (SSC resistance) in a corrosive environment containing hydrogen sulfide.

  It is known that the sour resistance deteriorates due to the generation of MnS stretched in the rolling direction and the generation of cluster-like inclusions. In addition, in order to improve the sour resistance in an extremely severe corrosive environment, the steel material in which the content of P, S, O, N is reduced and Ca is added to control the form of MnS generated in the center segregation is controlled rolling. And the method of water-cooling is proposed (for example, patent document 1).

In particular, for ERW steel pipes, a method has been proposed in which strain is recovered by heating to 550 to 850 ° C. to reduce hydrogen-induced cracking (see, for example, Patent Document 2). Similarly, a method of heat treatment at 650 ° C. or more and 850 ° C. or less and at _one or more points of Ac has been proposed (see, for example, Patent Document 3).

JP-A-62-112722 Japanese Patent Laid-Open No. 2003-164908 Japanese Patent Laid-Open No. 2004-115881

  In general, a higher heating temperature is desirable as a heat treatment for preventing hydrogen induced cracking (HIC) of a steel pipe. On the other hand, when the steel pipe is heated to a high temperature, the strength decreases, so it is necessary to limit the upper limit of the heating temperature. In particular, in the case of ERW steel pipe, the cause is not clear, but depending on the component, it is difficult to achieve both suppression of HIC and securing of strength.

  The present invention solves such a problem, and manufactures an electric resistance welded steel pipe of API 5L X65 grade or higher for line pipe by heat treatment at a relatively low temperature to ensure sour resistance and strength. It is intended to provide a method. A high strength steel pipe of API 5L X65 grade or higher has a tensile strength of 530 MPa or higher.

  The present invention is made on the basis of the knowledge that the strength of the steel pipe can be secured and the HIC resistance can be improved by controlling the lower limit value of the heating temperature of the heat treatment according to the Ceq of the steel material defined below. The summary is as follows.

(1) A method for producing an electric resistance welded steel pipe for a sour line pipe having a tensile strength of 530 MPa or more, in mass%,
C: 0.01 to 0.08%,
Si: 0.1 to 0.5%,
Mn: 1.0 to 1.5%
Nb: 0.010 to 0.060%,
Ca: 0.001 to 0.004%,
Ti: 0.005 to 0.060%,
V: 0.005~ 0.039%
Containing
Al: 0.08% or less,
P: 0.015% or less,
S: 0.0010% or less,
O: 0.0030% or less,
N: limited to 0.0050% or less, with the balance being Fe and inevitable impurities,
Content of Ca, O, and S [% by mass]
1.05 ≦ Ca (1-124O) /1.25S≦1.97
An ERW steel pipe satisfying the above equation and having a Ceq of 0.22 to 0.30 obtained by the following (Equation 1), a lower limit temperature of 1250 Ceq + 225 ° C. or more, and an upper limit temperature of the ERW steel pipe being transformed into austenite. A method of manufacturing an electric resistance welded steel pipe for a sour line pipe, characterized by performing a heat treatment at a temperature (Ac ₁ point) or 700 ° C, whichever is lower.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (Expression 1)
Here, C, Mn, Ni, Cu, Cr, Mo, and V are contents [mass%] of each element.

(2) The ERW steel pipe is further in mass%,
Ni: 0.5% or less,
Cu: 0.5% or less,
Cr: 0.5% or less,
Mo: 1 type or 2 types or more of 0.5% or less is contained, The manufacturing method of the ERW steel pipe for sour-resistant pipes as described in said (1) characterized by the above-mentioned.

(3) The ERW steel pipe is further in mass%,
B: 0.0020% or less is contained, The manufacturing method of the ERW steel pipe for sour-resistant pipes as described in said (1) or (2) characterized by the above-mentioned.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the electric resistance welded steel pipe for line pipes excellent in sour resistance, and the industrial contribution is very remarkable.

The relationship between the hardness and HIC resistance of a steel pipe and the heat treatment temperature is shown.

The relationship between the composition of the steel pipe and the heating temperature of the heat treatment and the sour resistance is shown.

  The electric resistance welded steel pipe of the present invention is a high-strength steel pipe of API 5L X65 grade or higher and has a tensile strength of 530 MPa or higher. First, the present inventors examined the relationship between the hardness and HIC resistance of a steel pipe and the heat treatment temperature. The ERW steel pipe having the components shown in Table 1 was heat-treated, the Vickers hardness was measured, and a NACE test was performed in accordance with TM0284 of NACE (National Association of Corrosion and Engineer). The NACE test is a test method in which hydrogen sulfide gas is saturated in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, and the presence or absence of cracking and the fracture rate are investigated after 96 hours. According to the NACE test, if the HIC fracture surface ratio is about 5% or less, it is evaluated that the sour resistance is good. The results are shown in FIG. In FIG. 1, the horizontal axis represents the heat treatment temperature (° C.), and the vertical axis represents the HIC fracture surface ratio (CAR) on the right side and the hardness (Hv) on the left side.

As shown in FIG. 1, when the heating temperature exceeds 700 ° C., the Vickers hardness decreases. In addition, the present inventors examined the influence of the components and the heating temperature of the heat treatment on the strength of the steel pipe. As a result, it was found that, when heated to a temperature exceeding Ac _{1 at} which transformation to austenite starts, the strength greatly changes depending on the cooling rate after heating. Therefore, the upper limit of the heating temperature of the steel pipe is the temperature (Ac ₁ ) at which transformation to austenite starts or the lower one of 700 ° C.

  On the other hand, as shown in FIG. 1, in order to reduce the HIC fracture surface ratio, that is, in order to improve the CAR sour resistance, it is understood that it is necessary to raise the heating temperature. Therefore, the present inventors have examined the relationship between the components of the steel pipe and the heating temperature of the heat treatment and the sour resistance in order to clarify the lower limit of the heating temperature of the heat treatment that improves the sour resistance. The result is shown in FIG. In FIG. 2, the horizontal axis represents Ceq, the vertical axis represents the heat treatment temperature, ◯ indicates that the improvement in sour resistance was good, and x indicates that the improvement in sour resistance was poor. ing.

  As shown in FIG. 2, the heating temperature of the heat treatment that improves the sour resistance increases with the Ceq of the steel pipe. In addition, the solid line in FIG. 2 is a calculated value of 1250 Ceq + 225, and it is shown that the sour resistance is improved at a higher heating temperature than the solid line. Therefore, the heating temperature of the heat treatment may be set to 1250 Ceq + 225 ° C. or higher. It turns out that it is necessary.

Hereinafter, the present invention will be described in detail. Hereinafter, “%” means mass%.
C: 0.01 to 0.08%
C: C is an element that improves the strength of steel, and as an effective amount thereof, addition of 0.01% or more is necessary. On the other hand, if the amount of C exceeds 0.08%, the formation of carbides is promoted and the HIC resistance is impaired, so the upper limit is made 0.08%. In order to further suppress the decrease in HIC resistance, the C content is preferably 0.06% or less.

Si: 0.1 to 0.5%
Si: Si is a deoxidizing element and needs to be added in an amount of 0.1% or more. On the other hand, if the amount of Si exceeds 0.5%, the toughness of the ERW weld is reduced, so the upper limit is made 0.5%.

Mn: 1.0 to 1.5%
Mn: Mn is an element that improves strength and toughness, and it is necessary to add 1.0% or more. On the other hand, Mn is an element that generates MnS and degrades sour resistance. Therefore, in order to suppress HIC, the upper limit of the amount of Mn needs to be 1.5%.

Nb: 0.010 to 0.060%
Nb: Nb is an element that expands the non-recrystallization temperature range, refines the crystal grain size, forms carbides and nitrides, and contributes to improving the strength, and needs to be added in an amount of 0.010% or more. . On the other hand, in order to suppress the decrease in HIC resistance due to the formation of coarse carbide in the central segregation part, the upper limit needs to be 0.060%.

Ca: 0.001 to 0.004%
Ca: Ca is an element that produces sulfide CaS, suppresses the production of MnS extending in the rolling direction, and contributes significantly to the improvement of HIC resistance. If the addition amount of Ca is less than 0.001%, the effect cannot be obtained, so the lower limit is set to 0.001%. On the other hand, if the amount of Ca exceeds 0.004%, oxides accumulate and the HIC resistance is impaired, so the upper limit is made 0.004%.

Ti: 0.005-0.060%
Ti: Ti is an element used for refining crystal grains as a deoxidizer or nitride-forming element, and it is necessary to add 0.005% or more. On the other hand, when Ti is added excessively, the HIC resistance is lowered due to the formation of coarse nitrides similar to Nb, so the upper limit is made 0.060%.

V: 0.005-0.039%
V: V is an element that forms carbides and nitrides and contributes to the improvement of strength. In order to obtain the effect, 0.005% or more is preferably added. On the other hand, if V exceeding 0.060% is added, toughness may be lowered, so the upper limit is preferably made 0.060%. The upper limit of the V amount is set to 0.039% or less based on the example.

Al: 0.08% or less Al: Al is a deoxidizing element. However, if the addition amount exceeds 0.08%, an accumulation cluster of Al oxide is generated and sour resistance is impaired. Limit to 08% or less. Moreover, when toughness is requested | required, it is preferable to make the upper limit of Al amount into 0.03%. The lower limit of the amount of Al is not particularly limited, but it is preferable to add 0.0005% or more of Al in order to reduce the amount of oxygen in the molten steel.

P: 0.015% or less P: P is an impurity. If the content exceeds 0.015%, the HIC resistance is impaired. Therefore, the upper limit of the P content is 0.015%.

S: 0.0010% or less S: S is an element that generates MnS that extends in the rolling direction during hot rolling, and decreases the HIC resistance. Therefore, in the present invention, it is necessary to reduce the amount of S, and the upper limit is limited to 0.0010%. The smaller the amount of S, the better, but it is difficult to make it less than 0.0001%. Moreover, it is preferable to make it 0.0001% or more also from a viewpoint of manufacturing cost.

N: 0.0050% or less N: N is an impurity. If the content of N exceeds 0.0050%, carbonitrides of Ti and Nb are likely to accumulate, and the HIC resistance is impaired. Therefore, the upper limit of the N amount is set to 0.0050%. Moreover, when using nitrides, such as TiN and NbN, and refinement | miniaturization of the austenite particle size at the time of a heating, it is preferable to contain 0.0010% or more of N.

O: 0.0030% or less O: O is an impurity, and it is necessary to limit the upper limit to 0.0030% in order to suppress the accumulation of oxides and improve the HIC resistance. In order for Ca, which suppresses the generation of MnS, to be effectively combined with S, the O content is preferably 0.0020% or less.

1.05 ≦ Ca (1-124O) /1.25S≦ 1.97
In the present invention, it is necessary to increase Ca (1-124O) /1.25S, that is, the ESSP value. In addition, Ca, O, and S represent the content of each element in mass%. The ESSP value is a ratio of the Ca amount to the S amount necessary for generating CaS considering that Ca forms an oxide. In order to improve the HIC resistance, the ESSP value needs to exceed 1. On the other hand, in order to increase the ESSP value to more than 2, it is necessary to reduce the amount of S, which increases the manufacturing cost. Therefore, the ESSP value is more than 1 and 2 or less. The ESSP value is 1.05 or more and 1.97 or less based on the example.

Ceq: 0.22 to 0.30
Ceq is an index of hardenability defined by the following formula. In the present invention, Ceq is an important index that also affects the lower limit of the heating temperature from the viewpoint of sour resistance. If Ceq is less than 0.22, strength of X65 or more cannot be ensured after heat treatment. On the other hand, in order to improve the sour resistance, in the present invention, the lower limit value of the heating temperature of the heat treatment is set to 1250 Ceq + 225 ° C. or higher. Therefore, when Ceq exceeds 0.30, the range of the heating temperature for improving the sour resistance is narrowed. Therefore, Ceq is set to 0.22 to 0.30.

Ceq is determined by the following (formula 1) from the contents of Mn, Ni, Cu, Cr, Mo, and V. In the case where Ni, Cu, Cr, or Mo is not contained, 0 is set.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5 (Expression 1)

  In the present invention, it is preferable to add one or more elements of Ni, Cu, Cr, Mo, and B as elements for improving strength and toughness.

Ni: 0.5% or less Ni: Ni is an element effective for improving toughness and strength, and contributes to improvement of corrosion resistance. Therefore, addition of 0.01% or more is preferable. On the other hand, Ni is an expensive element, and it is preferable to limit the upper limit to 0.5% in order to reduce manufacturing costs.

Cu: 0.5% or less Cu: Cu is an element effective for increasing the strength, and contributes to the improvement of corrosion resistance. Therefore, addition of 0.01% or more is preferable. On the other hand, Cu is also an expensive element, and it is preferable to limit the upper limit to 0.5% in order to reduce manufacturing costs.

Cr: 0.5% or less Cr: Cr is an element effective for increasing the strength, and is preferably added in an amount of 0.01% or more. On the other hand, if added in a large amount, the hardenability becomes high and the toughness may be lowered, so the upper limit is preferably made 0.5%.

Mo: 0.5% or less Mo: Mo is an element that improves hardenability and at the same time forms carbonitride to improve strength. To obtain the effect, addition of 0.01% or more is necessary. preferable. On the other hand, Mo is an expensive element, and the upper limit is preferably set to 0.5% in order to reduce manufacturing costs. Moreover, since the HIC property and toughness may decrease when the strength of the steel increases, the preferable upper limit is set to 0.3%.

B: 0.0020% or less B: B is an element that segregates at the grain boundaries of steel and contributes significantly to improving the hardenability. In order to obtain this effect, 0.0001% or more of B is preferably added. on the other hand. When B is added excessively, segregation to the grain boundary becomes excessive and the toughness may be lowered, so the upper limit is preferably made 0.0020%.

  Next, the manufacturing method of the ERW steel pipe of this invention is demonstrated. First, the steel melted in the steel making process is made into a steel slab by continuous casting. The steel slab is heated, hot-rolled, wound up to obtain a hot-rolled steel strip. Thereafter, the hot-rolled steel strip is roll-formed and made into an ERW steel pipe by ERW welding.

  In the present invention, heat treatment is performed on the formed ERW steel pipe. The heat treatment of the ERW steel pipe may be performed continuously by induction heating, or may be a batch method such as a heating furnace.

As described above, the lower limit of the heating temperature needs to be 1250 Ceq + 225 ° C. or higher in order to improve the sour resistance. Since sour resistance is improved by increasing the heating temperature, the higher one of 1250 Ceq + 225 ° C. or 600 ° C. is preferably set as the lower limit of the heating temperature. On the other hand, in order to ensure the strength of the steel pipe, the lower limit of Ac ₁ or 700 ° C. is set as the upper limit for the heating temperature of the steel pipe. Since the strength of the steel pipe increases when the heating temperature is lowered, the lower one of Ac ₁ or 650 ° C. is preferably set as the upper limit of the heating temperature.

  Although the heat treatment holding time is not particularly defined, it is preferably held for 30 seconds or more in order to suppress the generation of HIC. In addition, the upper limit of the holding time is preferably 1 hour or less from the viewpoint of productivity.

  Steel having chemical components shown in Table 2 was melted, and a steel piece having a thickness of 250 mm was produced by continuous casting. The obtained steel slab was hot rolled under the conditions shown in Table 3 and wound up as a hot rolled coil. Thereafter, a hot-rolled coil was formed by roll forming and ERW welding, and the ERW steel pipe was subjected to heat treatment under the conditions shown in Table 3. After the heat treatment, the HIC property of the ERW steel pipe was evaluated by the NACE test.

  The NACE test was performed by saturating hydrogen sulfide gas in a solution of 5% NaCl solution + 0.5% acetic acid, pH 2.7, and dipping time was 96 hours. After the test, the presence or absence of cracks was observed, and the HIC fracture surface ratio (CAR) was measured. In addition, a tensile test piece having a parallel part diameter of 6 mm is taken at a position of 90 ° from the electro-sewing contact part, the evaluation distance is 24 mm, a tensile test is performed in accordance with JIS Z 2241, and the tensile strength is determined. It was measured. The results are shown in Table 3.

  The examples of the present invention in which the components and production conditions are within the scope of the present invention have a CAR of 5% or less and have good sour resistance. On the other hand, production No. No. 1, 3, 15 Comparative example in which no heat treatment is performed, Production No. In Comparative Examples in which the heat treatment temperature is lower than the heat treatment temperature range of the present invention, such as 6, 9, and 12, the CAR exceeds 5%, and sour resistance is not improved. Production No. Nos. 17 and 20 have a high Ceq, the heat treatment temperature is lower than 1250 Ceq + 225 ° C., and the CAR also exceeds 5%, so the sour resistance is not improved.

  No. No. 19 has an ESSP value of 0.93, which is lower than the range of the present invention, and does not improve sour resistance even when an appropriate heat treatment is performed (CAR is 9.3%). Production No. No. 18 has a Ceq of 0.196, which is lower than the range of the present invention. No. 5 has a heat treatment temperature of 780 ° C., which is higher than the heat treatment temperature range of the present invention, and the tensile strength is lower than 530 MPa.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the electric resistance welded steel pipe for line pipes excellent in sour resistance, and the industrial contribution is very remarkable.

Claims (3)

A method for producing an electric resistance steel pipe for sour line pipes having a tensile strength of 530 MPa or more, wherein the tensile strength is% by mass,
C: 0.01 to 0.08%,
Si: 0.1 to 0.5%,
Mn: 1.0 to 1.5%
Nb: 0.010 to 0.060%,
Ca: 0.001 to 0.004%,
Ti: 0.005 to 0.060%,
V: 0.005~ 0.039%
Containing
Al: 0.08% or less,
P: 0.015% or less,
S: 0.0010% or less,
O: 0.0030% or less,
N: limited to 0.0050% or less, with the balance being Fe and inevitable impurities,
Content of Ca, O, and S [% by mass]
1.05 ≦ Ca (1-124O) /1.25S≦1.97
The ERW steel pipe having a Ceq of 0.22 to 0.30 determined by the following (Equation 1), the lower limit temperature is 1250 Ceq + 225 ° C. or higher, and the upper limit temperature is the transformation start temperature of the ERW steel pipe to austenite ( ( ₁ Ac) or 700 ° C., whichever is lower, a method for producing an electric resistance welded steel pipe for sour line pipes.
Ceq = C + Mn / 6 + (Ni + Cu) / 15 + (Cr + Mo + V) / 5
... (Formula 1)
Here, C, Mn, Ni, Cu, Cr, Mo, and V are contents [mass%] of each element. The ERW steel pipe is further in mass%,
Ni: 0.5% or less,
Cu: 0.5% or less,
Cr: 0.5% or less,
Mo: 0.5% or less of 1 type or 2 types or more are contained, The manufacturing method of the ERW steel pipe for sour-resistant pipes of Claim 1 characterized by the above-mentioned. The ERW steel pipe is further in mass%,
B: 0.0020% or less is contained, The manufacturing method of the ERW steel pipe for sour-resistant pipes of Claim 1 or 2 characterized by the above-mentioned.

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