High Strength Steel Plate for Sour-Resistant Line Pipe and Method for Producing Same, and High Strength Steel Pipe or Tube Using High Strength Steel Plate for Sour-Resistant Line Pipe

This disclosure relates to a high strength steel plate for a sour-resistant line pipe with excellent material homogeneity within the steel plate that is suitable for use in line pipes used for transporting crude oil and natural gas, and a method for producing the same. This disclosure also relates to a high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe.

Generally, a line pipe is manufactured by forming a steel plate produced by a plate mill or hot rolling mill into a steel pipe or tube by, for example, UOE forming, press bend forming, and roll forming.

The line pipe used for transporting crude oil and natural gas containing hydrogen sulfide is required to have so-called sour resistance such as hydrogen-induced cracking resistance (HIC resistance) and sulfide stress corrosion cracking resistance (SSCC resistance), in addition to strength, toughness, weldability, and so on. Above all, in HIC, hydrogen ions from the corrosion reaction adsorb on the steel surface, enter the steel as atomic hydrogen, diffuse and accumulate around nonmetallic inclusions such as MnS and a hard second phase structure in the steel to become molecular hydrogen, which causes cracking due to its internal pressure. This HIC is considered as a problem in line pipes with relatively low strength levels relative to oil well pipes or tubes, and many countermeasure techniques have been disclosed. On the other hand, with regard to SSCC, the importance of controlling the hardness of the inner surface layer of steel pipe or tube to improve SSCC resistance in more severe corrosion environment has been pointed out. In addition to these sour resistance properties, recent years have seen an increasingly severe environment for crude oil and natural gas extraction, which has increased the demand for excellent low-temperature toughness.

Normally, the thermo-mechanical control process (so-called TMCP) technique, which combines controlled rolling and controlled cooling, is applied in the production of high strength steel plates for line pipes. To ensure the sour resistance of steel plate using this TMCP technique, it is effective to increase the cooling start temperature and slow down the cooling rate during controlled cooling. However, when the cooling start temperature was increased, rolling in the non-recrystallization temperature range became insufficient, which limited the crystal grain refinement that was effective in improving low-temperature toughness, and excellent low-temperature toughness could not be ensured.

To solve the above problem, for example, JP2020-012168A (PTL 1) proposes a technique to set the rolling finish temperature at 700° C. or higher and the average grain size at 15.0 μm or less. In addition, JP2020-509181A (PTL 2) proposes a technique to suppress crystal grain growth by setting the holding time between rough rolling and the start of finish rolling to 300 seconds or less.

Although the techniques described in PTLs 1 and 2 can improve low-temperature toughness, since the metallic structure contains ferrite, it is difficult to ensure sour resistance in a more severe corrosion environment with high hydrogen sulfide partial pressure.

In particular, pipelines used in cold climates require excellent low-temperature toughness. However, because in order to prevent the formation of ferrite, cooling must start at a temperature equal to or higher than the Arpoint, and the cumulative strain during rolling is small, it is difficult to achieve fine grain size, and excellent low-temperature toughness has not been obtained in the past.

It could thus be helpful to provide a high strength steel plate for a sour-resistant line pipe with excellent low-temperature toughness as well as HIC resistance and SSCC resistance, together with an advantageous method for producing the same. This disclosure also provides a high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe.

We repeated numerous experiments and studies on the chemical composition, microstructure, and producing conditions of steel plate to ensure not only sour resistance but also low-temperature toughness. As a result, we found that to further improve the low-temperature toughness of high strength steel plate, at the mid-thickness position of the steel plate, the maximum grain size should be 80 μm or less and the average grain size should be 20 μm or less. Furthermore, in order to achieve such a steel microstructure, the hot rolling conditions in the recrystallization temperature range must be strictly controlled, and we have succeeded in finding such conditions. This disclosure is based on these discoveries.

We thus provide the following.

[1] A high strength steel plate for a sour-resistant line pipe comprising a chemical composition containing (consisting of), by mass %, C: 0.030% or more and 0.060% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.80% or more and 1.80% or less, P: 0.015% or less, S: 0.0015% or less, Al: 0.010% or more and 0.080% or less, Cr: 0.05% or more and 0.50% or less, Nb: 0.005% or more and 0.080% or less, N: 0.0010% or more and 0.0080% or less, and Ca: 0.0005% or more and 0.0050% or less, with the balance being Fe and inevitable impurities, wherein

[2] The high strength steel plate for a sour-resistant line pipe according to [1], wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

[3] The high strength steel plate for a sour-resistant line pipe according to [1] or [2], wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less.

[4] A method for producing a high strength steel plate for a sour-resistant line pipe, the method comprising:

[5] The method for producing a high strength steel plate for a sour-resistant line pipe according to [4], wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less.

[6] The method for producing a high strength steel plate for a sour-resistant line pipe according to [4] or [5], wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less.

[7] A high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe according to [1] or [2].

[8] A high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe according to [3].

The high strength steel plate for a sour-resistant line pipe and the high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe of this disclosure have excellent low-temperature toughness as well as HIC resistance and SSCC resistance. The method for manufacturing a high strength steel plate for a sour-resistant line pipe of this disclosure can produce a high strength steel plate for a sour-resistant line pipe with excellent low-temperature toughness as well as HIC and SSCC resistance.

The following is a specific description of the high strength steel plate for a sour-resistant line pipe of this disclosure.

First, the chemical composition of the high strength steel plate of this disclosure and reasons for limitation will be described. When components are expressed in “%” in the following description, this refers to “mass %” unless otherwise noted.

C effectively contributes to strength improvement, but sufficient strength cannot be secured when the C content is less than 0.030%, so the C content should be 0.030% or more, preferably 0.035% or more. On the other hand, when the C content exceeds 0.060%, low-temperature toughness deteriorates. In addition, SSCC resistance and HIC resistance deteriorate due to the increase in hardness of the surface layer and central segregation area during accelerated cooling. Therefore, the C content should be 0.060% or less, preferably 0.050% or less.

Si is added for deoxidation, but when the Si content is less than 0.01%, the deoxidizing effect is not sufficient, so the Si content should be 0.01% or more, preferably 0.05% or more. On the other hand, when the Si content exceeds 0.50%, the non-thermal stress of the steel increases and low-temperature toughness deteriorates, so the Si content should be 0.50% or less, preferably 0.45% or less.

Mn effectively contributes to the improvement of strength, but the effect is not fully realized when the Mn content is less than 0.80%. Therefore, the Mn content should be 0.80% or more, preferably 1.00% or more. On the other hand, when the Mn content exceeds 1.80%, SSCC resistance and HIC resistance deteriorate due to the increase in hardness in the surface layer and central segregation area during accelerated cooling. Weldability is also degraded. Therefore, the Mo content should be 1.80% or less, preferably 1.70% or less.

P is an inevitable impurity element, which degrades low-temperature toughness and increases the hardness of surface layer and central segregation area, thereby degrading SSCC resistance and HIC resistance. This tendency becomes more pronounced when the P content exceeds 0.015%, so the P content should be 0.015% or less, preferably 0.008% or less. A lower P content is better, but in terms of refining cost, the P content is preferably 0.001% or more.

S is an inevitable impurity element and should be kept low because it degrades HIC resistance by forming MnS inclusions in the steel. From this viewpoint, the S content should be 0.0015% or less, preferably 0.0010% or less. A lower S content is better, but in terms of refining cost, the S content is preferably 0.0002% or more.

Al is added as a deoxidizer, but its effect is not fully realized when the Al content is less than 0.010%. Therefore, the Al content should be 0.010% or more, preferably 0.015% or more. On the other hand, when the Al content exceeds 0.080%, the non-thermal stress of the steel increases and low-temperature toughness deteriorates. Therefore, the Al content should be 0.080% or less, preferably 0.070% or less.

Cr, like Mn, is an effective element for obtaining sufficient strength even with low C content, and to obtain this effect, the Cr content must be 0.05% or more. However, when the Cr content is too high, the hardness of surface layer and central segregation area increases during accelerated cooling due to excessive hardenability, resulting in deterioration of SSCC resistance and HIC resistance. Weldability is also degraded. Therefore, the Cr content should be 0.50% or less, preferably 0.45% or less.

Nb, when present as solute Nb, expands non-recrystallization temperature range during controlled rolling and contributes to the improvement of low-temperature toughness, but the effect is not fully realized when the Nb content is less than 0.005%. Therefore, the Nb content should be 0.005% or more, preferably 0.010% or more. On the other hand, when the Nb content exceeds 0.080%, coarse carbides are crystallized during solidification, which deteriorates HIC resistance. Therefore, the Nb content should be 0.080% or less, preferably 0.060% or less.

N effectively contributes to the improvement of strength, but a N content of less than 0.0010% does not ensure sufficient strength. Therefore, the N content should be 0.0010% or more, preferably 0.0015% or more. On the other hand, when the N content exceeds 0.0080%, the SSCC resistance and HIC resistance deteriorate due to increased hardness in the surface layer and central segregation area during accelerated cooling. The low-temperature toughness is also degraded. Therefore, the N content should be 0.0080% or less, preferably 0.0070% or less.

Ca is an effective element for improving HIC resistance through morphological control of sulfide inclusions, but its addition effect is not sufficient when the Ca content is less than 0.0005%. Therefore, the Ca content should be 0.0005% or more, preferably 0.0008% or more. On the other hand, when the Ca content exceeds 0.0050%, not only does the above effect become saturated, but the HIC resistance deteriorates due to a decrease in cleanliness of the steel. Therefore, the Ca content should be 0.0050% or less, preferably 0.0045% or less.

The basic components of chemical composition in this disclosure have been described above. In this disclosure, at least one selected from the group consisting of Cu, Ni, and Mo can be optionally included within the range below to further improve the strength and toughness of the steel plate.

Cu is an effective element for improving low-temperature toughness and increasing strength, and to obtain this effect, the Cu content is preferably 0.05% or more. However, when the Cu content exceeds 0.30%, the SSCC resistance deteriorates because micro-cracks, called fissure, are more likely to occur in environments with low hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Cu is contained, the Cu content should be 0.30% or less, preferably 0.25% or less.

Ni is an effective element for improving low-temperature toughness and increasing strength, and to obtain this effect, the Ni content is preferably 0.01% or more. However, when the Ni content exceeds 0.10%, the SSCC resistance deteriorates because micro-cracks, called fissure, are more likely to occur in environments with low hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Ni is contained, the Ni content should be 0.10% or less, preferably 0.02% or less.

Mo is an effective element for improving low-temperature toughness and increasing strength, and to obtain this effect, the Mo content is preferably 0.01% or more, more preferably 0.10% or more. On the other hand, when the Mo content is too high, the SSCC resistance deteriorates because micro-cracks, called fissure, are more likely to occur in environments with low hydrogen sulfide partial pressure of less than 1 bar. Weldability is also degraded. Therefore, when Mo is contained, the Mo content should be 0.50% or less, preferably 0.40% or less.

The chemical composition in this disclosure may also optionally contain at least one selected from the group consisting of V, Ti, Zr, Mg, and REM within the ranges below.

At least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less

Both V and Ti are elements that can be optionally added to increase the strength and low-temperature toughness of the steel plate. For each element, when the content is less than 0.005%, the effect is not fully realized. Therefore, when these elements are contained, each of their contents should be 0.005% or more. On the other hand, when each of the contents of these elements exceeds 0.1%, the toughness of welded portion deteriorates. Therefore, when these elements are contained, each of their contents is preferably 0.1% or less.

Zr, Mg, and REM are elements that can be optionally contained to increase low-temperature toughness through crystal grain refinement and to increase cracking resistance through control of inclusion properties. For each element, when the content is less than 0.0005%, the effect is not fully realized. Therefore, when these elements are contained, each of their contents should be 0.0005% or more. On the other hand, when each of the contents of these elements exceeds 0.02%, the effect becomes saturated, so when they are contained, it is preferable to keep the content of each element at 0.02% or less.

This disclosure describes a technique for improving the low-temperature toughness of high strength steel pipe or tube using a high strength steel plate for a sour-resistant line pipe. Needless to say, for sour resistance, it is necessary to satisfy HIC resistance, and for example, it is preferable to set the CP value determined by the formula (1) below to 1.00 or less. For elements that are not contained, 0 may be substituted.

CP=4.46[% C]+2.37[% Mn]/6+(1.74[% Cu]+1.7[% Ni])/15+(1.18[% Cr]+1.95[% Mo]+1.74[% V])/5+22.36[% P]  (1)

The CP value is a formula devised to estimate the material property of the central segregation area from the content of each alloying element. As the CP value in the formula (1) is larger, the concentration of components in the central segregation area is higher and the hardness of the central segregation area is increased. Therefore, by setting the CP value determined by the formula (1) to 1.00 or less, it is possible to improve the HIC resistance. Since the hardness of the central segregation area decreases with a smaller CP value, the upper limit for CP value may be set to 0.95 when higher HIC resistance is required. No lower limit is placed on CP, but the CP value can be 0.70 or more.

The balance other than the above elements is Fe and inevitable impurities. However, other trace elements may be included as long as they do not impair the effects of this disclosure. For example, O is an inevitable element contained in steel, but is acceptable in this disclosure when its content is 0.0050% or less, preferably 0.0040% or less.

Next, the steel microstructure of the high strength steel plate for a sour-resistant line pipe of this disclosure is described. To improve SSCC resistance and increase tensile strength to 535 MPa or more, the steel microstructure at 0.25 mm below a surface of the steel plate is made to consist of granular bainite and tempered martensite austenite constituent. When martensite austenite constituent is mixed in the steel microstructure, low-temperature toughness and SSCC resistance deteriorate. However, when the cooling stop temperature is sufficiently high, the martensite austenite constituent is tempered after cooling is stopped to become a tempered martensite austenite constituent (TMA), which makes it possible to prevent deterioration of low-temperature toughness and SSCC resistance. In addition, the granular bainite and tempered martensite austenite constituent have lower hardness than other bainite and other martensite, which can improve SSCC resistance.

Here, the steel microstructure at 0.25 mm below a surface of the steel plate in this disclosure is preferably mainly granular bainite. Specifically, it is preferable that the area ratio of granular bainite is 90% or more and the area ratio of tempered martensite austenite constituent is 10% or less. On the other hand, since the presence of tempered martensite austenite constituent is also essential, the area ratio of the granular bainite is preferably 99% or less and the area ratio of tempered martensite austenite constituent is preferably 1% or more.

In the high strength steel plate of this disclosure, when the steel microstructure at 0.25 mm below a surface of the steel plate satisfies the above conditions, the outermost surface layer from the surface of the steel plate to a depth of 0.25 mm will also have the same steel microstructure, and as a result, the above effect of improving SSCC resistance can be obtained.

In order to obtain the effects of improving SSCC resistance and increasing strength more sufficiently, it is preferable that the overall steel microstructure of the steel plate, including areas other than the surface layer, satisfy the above conditions. Specifically, the microstructure at the mid-thickness position of the steel plate should satisfy the above conditions on behalf of the “areas other than the surface layer”.

In this disclosure, it is essential to suppress the formation of coarse crystal grains. In detail, when the maximum or average grain size is large, the low-temperature toughness deteriorates. In particular, when the maximum grain size at the mid-thickness position of the steel plate is greater than 80 μm, the low-temperature toughness is significantly degraded because the coarse crystal grain tends to be the initiation point for fracture. When the average grain size exceeds 20 μm, the low-temperature toughness is also degraded. Therefore, at the mid-thickness position of the steel plate, the maximum grain size should be 80 μm or less and the average grain size should be 20 μm or less. The maximum grain size and the average grain size at the mid-thickness position of the steel plate are preferably smaller, and therefore no lower limit is placed on them. In this disclosure, at the mid-thickness position of the steel plate, the maximum grain size can be 50 μm or more and the average grain size can be 10 μm or more. For “grain size”, the grain sizes of crystal grains in the area of 1 mm×1 mm at the mid-thickness position of the steel plate are measured, and among them, the maximum value is adopted as the “maximum grain size” and the average value as the “average grain size.” The grain size is defined as circular equivalent diameter. Here, the mid-thickness position of the steel plate refers to the ½ position of the total thickness.

The high strength steel plate of this disclosure shall have a brittle-ductility transition temperature in the Charpy impact test of −100° C. or lower. This can ensure excellent low-temperature toughness. No lower limit is placed on the brittle-ductility transition temperature, but in this disclosure, the brittle-ductility transition temperature can be −120° C. or higher.