EP4129510A1

EP4129510A1 - High-strength steel sheet for sour-resistant line pipe, manufacturing method thereof, and high-strength steel pipe made using high-strength steel sheet for sour-resistant line pipe

Info

Publication number: EP4129510A1
Application number: EP21774925.8A
Authority: EP
Inventors: Daichi Izumi; Junji Shimamura; Yuta TAMURA; Satoshi Ueoka
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2020-03-26
Filing date: 2021-03-18
Publication date: 2023-02-08
Also published as: KR20220137970A; WO2021193383A1; JP7264269B2; BR112022019204A2; CN115298340A; JPWO2021193383A1; CN115298340B

Abstract

Disclosed is a high strength steel plate for a sour-resistant line pipe that is excellent not only in HIC resistance but also in SSCC resistance under more severe corrosion environments and environments with low hydrogen sulfide partial pressure below 1 bar. The high strength steel plate for a sour-resistant line pipe disclosed herein has: a chemical composition containing C: 0.020 % to 0.080 %, Si: 0.01 % to 0.50 %, Mn: 0.50 % to 1.80 %, P: 0.015 % or less, S: 0.0015 % or less, Al: 0.010 % to 0.080 %, N: 0.0010 % to 0.0080 %, Mo: 0.01 % to 0.50 %, and Ca: 0.0005 % to 0.0050 %, with the balance being Fe and inevitable impurities; a magnetite proportion in scale present on a surface of the steel plate being 50 % or more; a highest Vickers hardness at 0.25 mm below the surface of the steel plate being 230 HV or less; and a tensile strength being 520 MPa or more.

Description

TECHNICAL FIELD

This disclosure relates to a high strength steel plate for a sour-resistant line pipe that is excellent in material homogeneity in the steel plate and that is suitable for use in line pipes in the fields of construction, marine structure, shipbuilding, civil engineering, and construction industry machinery, and to a method for manufacturing the same. This disclosure also relates to a high strength steel pipe using the high strength steel plate for a sour-resistant line pipe.

BACKGROUND

A line pipe is typically manufactured by forming a steel plate manufactured by a plate mill or a hot rolling mill into a steel pipe by UOE forming, press bend forming, roll forming, or the like.
The line pipe used to transport crude oil and natural gas containing hydrogen sulfide is required to have so-called sour resistance such as resistance to hydrogen-induced cracking (HIC resistance) and resistance to sulfide stress corrosion cracking (SSCC resistance), in addition to strength, toughness, weldability, and so on. HIC is the following phenomenon: Hydrogen ions due to corrosion reaction adsorb to the surface of steel material and enter into the steel as atomic hydrogen, diffuse and accumulate around nonmetallic inclusions such as MnS and hard secondary phase in the steel, and become molecular hydrogen, the internal pressure of which causes cracking. This phenomenon is considered as a problem in line pipes with a relatively low level of strength with respect to oil well pipes, and many countermeasures have been proposed. Meanwhile, SSCC is known to occur in high hardness regions of welds, and in general has not been regarded as a significant problem in high strength seamless steel pipes for oil wells and line pipes with relatively low hardness. In recent years, however, the mining environments for crude oil and natural gas have become increasingly severe, and it has been reported that SSCC also occurs in the base metal of line pipes in environments with high hydrogen sulfide partial pressure or low pH. This has raised the importance of controlling the hardness of the surface layer of the inner surface of a steel pipe to improve the SSCC resistance under more severe corrosion environments. Moreover, microcracking called fissure may occur in environments with relatively low hydrogen sulfide partial pressure and SSCC is likely to occur in this case, too.
Typically, so-called TMCP (Thermo-Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is applied in manufacturing of high strength steel plates for line pipes. An effective way of enhancing the strength of steel plates using the TMCP technology is to increase the cooling rate in controlled cooling. However, when the control cooling is performed at a high cooling rate, the surface layer of the steel plate is rapidly cooled, and the hardness of the surface layer becomes higher than that of the inside of the steel plate. In addition, work hardening occurs when steel plates are formed into pipes, and hence the hardness of the surface layer increases and the SSCC resistance decreases.
In order to solve the above problems, for example, JP3951428B (PTL 1) and JP3951429B (PTL 2) describe methods for performing high-speed controlled cooling in which the surface is recuperated before completion of bainite transformation in the surface layer after rolling. JP2002-327212A (PTL 3) and JP3711896B (PTL 4) describe methods for manufacturing steel plates for line pipes in which the hardness of the surface layer is reduced by heating the surface of a steel plate after accelerated cooling to a higher temperature than the inside using a high frequency induction heating device.
Meanwhile, when the scale thickness on the steel plate surface is uneven, the cooling rate is also uneven at the underlying steel plate during cooling. In this regard, JPH9-57327A (PTL 5) and JP3796133B (PTL 6) describe methods for improving the shape of a steel plate by performing descaling immediately before cooling the steel plate to reduce cooling unevenness caused by scale thickness unevenness.

CITATION LIST

Patent Literature

PTL 1: JP3951428B
PTL 2: JP3951429B
PTL 3: JP2002-327212A
PTL 4: JP3711896B
PTL 5: JPH9-57327A
PTL 6: JP3796133B

SUMMARY

(Technical Problem)

According to our study, however, it turned out that the high strength steel plates obtained by the manufacturing methods described in Patent Literatures (PTLs) 1 to 6 have room for improvement in terms of SSCC resistance under more severe corrosion environments. The following can be considered as the reason.
In the manufacturing methods described in PTLs 1 to 4, the conditions for controlled cooling of steel plates are not sufficiently optimized, which may result in formation of locally high hardness portions in the surface layers of the steel plates.
The methods of PTLs 5 and 6 apply descaling to reduce the surface characteristics defects due to the scale indentation during hot leveling and to reduce the variation in the cooling stop temperature of the steel plate to improve the steel plate shape. However, no optimization has been made concerning the descaling conditions from the viewpoint of improving SSCC resistance. Moreover, no consideration has been given to the cooling conditions to reduce the hardness of surface layers of steel plates.
Further, in PTLs 1 to 6, the conditions to avoid microcracking such as fissures in environments with relatively low hydrogen sulfide partial pressure are not clear.
It would thus be helpful to provide a high strength steel plate for a sour-resistant line pipe that is excellent not only in HIC resistance but also in SSCC resistance under more severe corrosion environments and environments with low hydrogen sulfide partial pressure below 1 bar, together with an advantageous method for manufacturing the same. It would also be helpful to propose a high strength steel pipe using the high strength steel plate for a sour-resistant line pipe.

(Solution to Problem)

The present inventors have studied the above issues and found that simply reducing the hardness of the surface layer, as conventionally known, is not sufficient to further improve the SSCC resistance of high strength steel pipes. In other words, according to the conventional technology, even if the hardness of the surface layer is suppressed as a whole, in fact a locally high hardness portion is formed in the outermost part of the surface layer, which is infinitely close to a surface of the steel plate, and SSCC occurs starting from this region. Therefore, in order to obtain a high strength steel plate without any local high hardness portions in the outermost surface layer, specifically, at 0.25 mm below the surface of the steel plate, the inventors repeated numerous experiments on the compositions of steel plates, the properties of scale present on the surfaces of the steel plates, and the manufacturing conditions of the steel plates.
As a result, the inventors discovered that the adoption of a certain composition and the formation of magnetite-based scale on a surface of a steel plate were necessary conditions for obtaining a high strength steel plate without any local high hardness portions at 0.25 mm below the surface of the steel plate. It was revealed that in order to form magnetite-based scale on a surface of a steel plate, it is necessary to optimize the descaling conditions during the hot rolling process and to set the cooling stop temperature during controlled cooling within a predetermined range. The inventors also discovered that as one of the necessary conditions for manufacture, it is important to strictly control the cooling rate at 0.25 mm below the surface of the steel plate, and succeeded in finding the conditions to be met. The present disclosure was completed based on the above discoveries.
We thus provide:

[1] A high strength steel plate for a sour-resistant line pipe, comprising: a chemical composition containing (consisting of), by mass%, C: 0.020 % to 0.080 %, Si: 0.01 % to 0.50 %, Mn: 0.50 % to 1.80 %, P: 0.015 % or less, S: 0.0015 % or less, Al: 0.010 % to 0.080 %, N: 0.0010 % to 0.0080 %, Mo: 0.01 % to 0.50 %, and Ca: 0.0005 % to 0.0050 %, with the balance being Fe and inevitable impurities; a magnetite proportion in scale present on a surface of the steel plate being 50 % or more; a highest Vickers hardness at 0.25 mm below the surface of the steel plate being 230 HV or less; and a tensile strength being 520 MPa or more.
[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 Cr: 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 Nb: 0.005 % to 0.1 %, V: 0.005 % to 0.1 %, Ti: 0.005 % to 0.1 %, Zr: 0.0005 % to 0.02 %, Mg: 0.0005 % to 0.02 %, and REM: 0.0005 % to 0.02 %.
[4] A method for manufacturing a high strength steel plate for a sour-resistant line pipe, the method comprising: heating a slab having the chemical composition as recited in any one of [1] to [3] to a temperature of 1000 °C to 1300 °C; then hot rolling the slab to form a steel plate and subjecting the steel plate to descaling at a discharge pressure of 10 MPa or more in a number of rolling passes as many as 50 % or more of a total number of rolling passes during the hot rolling; and then subjecting the steel plate to controlled cooling under a set of conditions including: a temperature of a surface of the steel plate at the start of cooling being (Ar₃ - 10 °C) or higher; an average cooling rate in a temperature range from 750 °C to 550 °C in terms of a temperature at 0.25 mm below the surface of the steel plate being 20 °C/s to 100 °C/s; an average cooling rate in a temperature range from 750 °C to 550 °C in terms of an average temperature of the steel plate being 15 °C/s or higher; and a cooling stop temperature in terms of a temperature at 0.25 mm below the surface of the steel plate being 250 °C to 550 °C.
[5] A high strength steel pipe using the high strength steel plate for a sour-resistant line pipe as recited in any one of [1] to [3].

(Advantageous Effect)

The high strength steel plate for a sour-resistant line pipe and the high strength steel pipe using the high strength steel plate for a sour-resistant line pipe disclosed herein are excellent not only in HIC resistance but also in SSCC resistance under more severe corrosion environments and environments with low hydrogen sulfide partial pressure below 1 bar. In addition, according to the method for manufacturing a high strength steel plate for a sour-resistant line pipe disclosed herein, it is possible to manufacture a high strength steel plate for a sour-resistant line pipe that is excellent not only in HIC resistance but also in SSCC resistance under more severe corrosion environments and environments with low hydrogen sulfide partial pressure below 1 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a method for obtaining test pieces for evaluation of SSCC resistance in Examples.

DETAILED DESCRIPTION

Hereinafter, the high strength steel plate for a sour-resistant line pipe according to the present disclosure will be described in detail.

[Chemical composition]

First, the chemical composition of the high strength steel plate disclosed herein and the reasons for limitation thereof will be described. When components are expressed in "%" in the following description, this refers to "mass%" unless otherwise specified.

C: 0.020 % to 0.080 %

C effectively contributes to the improvement in strength. However, if the content is less than 0.020 %, sufficient strength cannot be secured. Therefore, the C content is 0.020 % or more, and preferably 0.025 % or more. On the other hand, if the C content exceeds 0.080 %, the hardness of the surface layer and the central segregation area increases during accelerated cooling, causing deterioration in SSCC resistance and HIC resistance. The toughness also deteriorates. Therefore, the C content is 0.080 % or less, and preferably 0.070 % or less.

Si: 0.01 % to 0.50 %

Si is added for deoxidation. However, if the content is less than 0.01 %, the deoxidizing effect is not sufficient. Therefore, the Si content is 0.01 % or more, and preferably 0.05 % or more. On the other hand, if the Si content exceeds 0.50 %, the toughness and weldability are degraded. Therefore, the Si content is 0.50 % or less, and preferably 0.45 % or less.

Mn: 0.50 % to 1.80 %

Mn effectively contributes to the improvement in strength and toughness. However, if the content is less than 0.50 %, the addition effect is not sufficient. Therefore, the Mn content is 0.50 % or more, and preferably 0.80 % or more. On the other hand, if the Mn content exceeds 1.80 %, the hardness of the surface layer and the central segregation area increases during accelerated cooling, causing deterioration in SSCC resistance and HIC resistance. The weldability also deteriorates. Therefore, the Mn content is 1.80 % or less, and preferably 1.70 % or less.

P: 0.015 % or less

P is an inevitable impurity element that degrades the weldability and increases the hardness of the surface layer and the central segregation area, causing deterioration in SSCC resistance and HIC resistance. This tendency becomes more pronounced when the P content exceeds 0.015 %. Therefore, the P content is 0.015 % or less, and preferably 0.008 % or less. Although a lower P content is preferable, the P content is preferably 0.001 % or more from the viewpoint of the refining cost.

S: 0.0015 % or less

S is an inevitable impurity element that forms MnS inclusions in the steel and degrades the HIC resistance. Therefore, the S content is 0.0015 % or less, and preferably 0.0010 % or less. Although a lower S content is preferable, the S content is preferably 0.0002 % or more from the viewpoint of the refining cost.

Al: 0.010 % to 0.080 %

Al is added as a deoxidizing agent. However, if the content is less than 0.010 %, the addition effect is not sufficient. Therefore, the Al content is 0.010 % or more, and preferably 0.015 % or more. On the other hand, an Al content beyond 0.080 % lowers the cleanliness of the steel and deteriorates the toughness. Therefore, the Al content is 0.080 % or less, and preferably 0.070 % or less.

N: 0.0010 % to 0.0080 %

N effectively contributes to the improvement in strength. However, if the content is less than 0.0010 %, sufficient strength cannot be secured. Therefore, the N content is 0.0010 % or more, and preferably 0.0015 % or more. On the other hand, if the N content exceeds 0.0080 %, the hardness of the surface layer and the central segregation area increases during accelerated cooling, causing deterioration in SSCC resistance and HIC resistance. The toughness also deteriorates. Therefore, the N content is 0.0080 % or less, and preferably 0.0070 % or less.

Mo: 0.01 % to 0.50 %

Mo is an effective element for improving toughness and increasing strength, it is an effective element for improving SSCC resistance regardless of the hydrogen sulfide partial pressure. The inventors found that after subjection to the SSCC test, steel plates containing Mo had surfaces smoother than those of other steel plates without containing Mo. Although the mechanism is not necessarily clear, this fact may be related to the improved SSCC resistance. To obtain this effect, the Mo content needs to be 0.01 % or more, and is preferably 0.10 % or more. On the other hand, if the Mo content exceeds 0.50 %, the hardenability becomes excessively high, causing an increase in the hardness of the surface layer and the central segregation area during accelerated cooling and deteriorating the SSCC resistance and HIC resistance. The weldability also deteriorates. Therefore, the Mo content is 0.50 % or less, and preferably 0.40 % or less.

Ca: 0.0005 % to 0.0050 %

Ca is an element effective for improving the HIC resistance by morphological control of sulfide inclusions. However, if the content is less than 0.0005 %, the addition effect is not sufficient. Therefore, the Ca content is 0.0005 % or more, and preferably 0.0008 % or more. On the other hand, if the Ca content exceeds 0.0050 %, not only the addition effect saturates, but also the HIC resistance is deteriorated due to the reduction in the cleanliness of the steel. Therefore, the Ca content is 0.0050 % or less, and preferably 0.0045 % or less.
The basic components of the present disclosure have been described above. Optionally, the chemical composition according to the present disclosure may also contain at least one selected from the group consisting of Cu, Ni, and Cr in the following ranges to further improve the strength and toughness of the steel plate.

Cu: 0.30 % or less

Cu is an element effective for improving the toughness and increasing the strength. To obtain this effect, the Cu content is preferably 0.05 % or more. However, if the Cu content exceeds 0.30 %, SSCC resistance deteriorates because microcracks called fissures easily form in environments with low hydrogen sulfide partial pressures below 1 bar. Therefore, when Cu is added, the Cu content is 0.30 % or less, and preferably 0.25 % or less.

Ni: 0.10 % or less

Ni is an element effective for improving the toughness and increasing the strength. To obtain this effect, the Ni content is preferably 0.01 % or more. However, if the Ni content exceeds 0.10 %, SSCC resistance deteriorates because microcracks called fissures easily form in environments with low hydrogen sulfide partial pressures below 1 bar. Therefore, when Ni is added, the Ni content is 0.10 % or less, and preferably 0.05 % or less.

Cr: 0.50 % or less

Cr, like Mn, is an element effective for obtaining sufficient strength even when the C content is low. To obtain this effect, the Cr content is preferably 0.05 % or more. However, if the Cr content exceeds 0.50 %, the hardenability becomes excessively high, causing an increase in the hardness of the surface layer and the central segregation area during accelerated cooling and deteriorating the SSCC resistance and HIC resistance. The weldability also deteriorates. Therefore, when Cr is added, the Cr content is 0.50 % or less, and preferably 0.45 % or less.
Optionally, the chemical composition according to the present disclosure may further contain at least one selected from the group consisting of Nb, V, Ti, Zr, Mg, and REM in the following ranges.
At least one selected from the group consisting of Nb: 0.005 % to 0.1 %, V: 0.005 % to 0.1 %, Ti: 0.005 % to 0.1 %, Zr: 0.0005 % to 0.02 %, Mg: 0.0005 % to 0.02 %, and REM: 0.0005 % to 0.02 %
Nb, V, and Ti are elements that can be optionally added to increase the strength and toughness of the steel plate. If the content of each added element is less than 0.005 %, the addition effect is not sufficient. Therefore, the content of each added element is preferably 0.005 % or more. On the other hand, if the content of each added element exceeds 0.1 %, the toughness of the welded portion deteriorates. Therefore, the content of each added element is preferably 0.1 % or less.
Zr, Mg, and REM are elements that can be optionally added in order to enhance the toughness through grain refinement and to improve the cracking resistance through control of the inclusion properties. If the content of each added element is less than 0.0005 %, the addition effect is not sufficient. Therefore, the content of each added element is preferably 0.0005 % or more. On the other hand, if the content of each added element exceeds 0.02 %, the addition effect is saturated. Therefore, when added, the content of each added element is preferably 0.02 % or less.
Although the present disclosure relates to a technique for improving the SSCC resistance of the high strength steel pipe using the high strength steel plate for a sour-resistant line pipe, it goes without saying that the technique disclosed herein needs to satisfy the HIC resistance at the same time as the sour resistant performance. For example, the CP value obtained by the following Expression (1) is preferably set to 1.00 or less. For any element not added, what is necessary is just to substitute 0. $\begin{array}{l} CP = 4.46 [% C] + 2.37 [% Mn] / 6 + (1.74 [% Cu] + 1.7 [% Ni]) / 15 + 1.18 [% Cr] + \\ 1.95 [% Mo] + 1.74 [% V]) / 5 + 22.36 [% P] \end{array}$
where [%X] represents the content by mass% of the element X in steel.
As used herein, the CP value is a formula designed to estimate the material property at the central segregation area from the content of each alloying element. When the CP value calculated according to Formula (1) is higher, the component concentration in the central segregation area is higher, and the hardness of the central segregation area increases. By setting the CP value obtained in Expression (1) to 1.00 or less, it is possible to suppress the occurrence of cracking in the HIC test. In addition, since the hardness of the central segregation area is lower as the CP value is lower, the upper limit for the CP value may be set to 0.95 when higher HIC resistance is required.
The balance other than the above-described elements is Fe and inevitable impurities. However, there is no intention in this expression of precluding the inclusion of other trace elements, without impairing the action or effect of the present disclosure. For example, O is an element that is inevitably contained in the steel, and a content of 0.0050 % or less, preferably 0.0040 % or less, is allowable in the present disclosure.

[Microstructure of the steel plate]

Next, the steel microstructure of the high strength steel plate for a sour-resistant line pipe disclosed herein will be described. In order to reduce the hardness of the surface layer, the surface layer is preferably formed with bainite phase as the steel microstructure. In particular, to keep the highest hardness at 0.25 mm below the surface of the steel plate below a certain level and to improve SSCC resistance, it is preferable that the steel microstructure at 0.25 mm below the surface of the steel plate be bainite phase. Furthermore, to achieve a high tensile strength of 520 MPa or higher, it is preferable that the entire steel microstructure of the steel plate, including portions other than the surface layer, be bainite phase. Specifically, it suffices for the microstructure at the mid-thickness position representative of the "portions other than the surface layer" to be bainite phase.
In this case, the bainite phase includes a microstructure called bainitic ferrite or granular ferrite which contributes to transformation strengthening. These microstructures appear through transformation during or after accelerated cooling. If different microstructures such as ferrite, martensite, pearlite, martensite austenite constituent, retained austenite, and the like are mixed in the bainite phase, the strength decreases, the toughness is degraded, and the hardness of the surface layer increases. Therefore, the proportion of microstructures other than the bainite phase is desirably as low as possible. However, when the area fraction of such microstructures other than the bainitic phase is sufficiently low, their effects are negligible. Accordingly, the inclusion of up to a certain amount of microstructure other than bainitic phase is allowable. Specifically, in the present disclosure, if the total of the steel microstructures other than bainite (such as ferrite, martensite, pearlite, martensite austenite constituent, and retained austenite) is less than 10 % by area fraction, there is no adverse effect, and this is acceptable. More preferably, the total of the steel microstructures other than bainite is less than 5 % by area fraction.

[Scale on the surface of the steel plate]

In the high strength steel plate disclosed herein, from the viewpoint of further improving SSCC resistance, it is important that a magnetite proportion in scale present on a surface of the steel plate after controlled cooling be 50 % or more. In general, scale present on a surface of the steel plate after controlled cooling is composed of wustite (FeO), magnetite (Fe₃O₄), and hematite (Fe₂O₃). The inventors discovered that when the magnetite proportion is less than 50 %, local high hardness portions are formed at 0.25 mm below the surface of the steel plate, resulting in the highest Vickers hardness at 0.25 mm below the surface of the steel plate exceeding 230 HV. In other words, in order to keep the highest Vickers hardness at 0.25 mm below the surface of the steel plate as low as 230 HV or less, the magnetite proportion needs to be 50 % or more. Note that the upper limit of the magnetite proportion is not particularly limited, and the magnetite proportion may be 100 % or less or 95 % or less.

[Hardness of the outermost surface layer]

In the high strength steel plate disclosed herein, it is also important that the highest Vickers hardness (HV 0.5) at 0.25 mm below the surface of the steel plate be 230 HV or less. These conditions can provide excellent SSCC resistance even in more severe corrosion environments and at low hydrogen sulfide partial pressures below 1 bar. If the highest Vickers hardness at 0.25 mm below the surface of the steel plate is greater than 230 HV, the presence of a local high hardness portion in the outermost surface layer causes deterioration in the resistance to SSCC originating from that portion. As used herein, "the highest Vickers hardness (HV 0.5) at 0.25 mm below the surface of the steel plate" means the highest value of 100 measurements taken at equal intervals along the plate width direction in a cross section perpendicular to the rolling direction of the steel plate. The reason for measuring with HV 0.5 instead of HV 10, which is usually used, is that the indentation size can be made smaller when measuring with HV 0.5, making it possible to obtain hardness information closer to the surface and hardness information more sensitive to the microstructures. Measuring with at less than HV 0.5 results in excessively small indentation size and large measurement variation. In addition, the reason for using the highest hardness instead of the average hardness is as follows. That is, the evaluation based on the highest hardness, rather than the average hardness, can detect local hard portions, and is more suitable for accurately examining crack propagation susceptibility since the presence of local hard portions makes crack propagation easier.

[Tensile strength]

The high strength steel plate disclosed herein is a steel plate for steel pipes having a strength of X60 grade or higher in API 5L, and thus has a tensile strength of 520 MPa or more.

[Thickness of the steel plate]

The high strength steel plate disclosed herein has a thickness of 14 mm to 39 mm.

[Manufacturing method]

Hereinafter, the method and conditions for manufacturing the above-described high strength steel plate for a sour-resistant line pipe will be described concretely. The manufacturing method according to the present disclosure comprises: heating a slab having the above-described chemical composition, and hot rolling the slab to form a steel plate; and then subjecting the steel plate to controlled cooling under predetermined conditions.

[[Slab heating temperature]]

Slab heating temperature: 1000 °C to 1300 °C

If a slab heating temperature is lower than 1000 °C, carbides do not dissolve sufficiently and solid solution strengthening becomes less effective, and thus the necessary strength cannot be obtained. Therefore, the slab heating temperature is 1000 °C or higher, and preferably 1030 °C or higher. On the other hand, if the slab heating temperature exceeds 1300 °C, crystal grains become extremely coarse and toughness deteriorates. Therefore, the slab heating temperature is 1300 °C or lower, and preferably 1250 °C or lower. This temperature is the temperature in the heating furnace, and the slab is heated to this temperature to the center.

[[Descaling]]

In the present disclosure, in the hot rolling process, it is important to subject the steel plate to descaling at a discharge pressure of 10 MPa or more in a number of rolling passes as many as 50 % or more of a total number of rolling passes during the hot rolling. As used herein, the term "rolling passes" is intended to include both rolling passes for rough rolling and rolling passes for finish rolling during the hot rolling process. Specifically, in a number of rolling passes as many as 50 % or more of a total number of rolling passes during the hot rolling, a surface of a slab (semi-finished product) is descaled at a discharge pressure of 10 MPa or more at the position immediately before the slab is introduced into those rolling passes. This descaling condition is one of the necessary conditions to suppress the non-uniformity of formation of scale, and to achieve a magnetite proportion of 50 % or more in the scale present on the surface of the steel plate after controlled cooling. As used herein, the "position immediately before the slab is introduced into those rolling passes" means a position in the longitudinal direction of the hot rolling line within 3 m, preferably within 1.5 m, of the position of the roll shaft of the rolling mill corresponding to the rolling pass. Any number of passes for rough rolling may be applied within a general range without limitation, yet is preferably, for example, 2 or more and 12 or less. Any number of passes for finish rolling may also be applied within a general range without limitation, yet is preferably, for example, 5 or more and 15 or less. Descaling may be performed according to conventional methods, for example, by spraying highpressure water onto the slab surface from a number of descaling nozzles installed along the width direction of the hot-rolling line. In each descaling, general conditions may be adopted for conditions other than the discharge pressure (e.g., water quantity, distance between nozzle and slab, and nozzle angle).
If the discharge pressure is less than 10 MPa, scale cannot be removed uniformly and hematite increases. As a result, the magnetite proportion cannot be increased to 50 % or more. Therefore, the discharge pressure is 10 MPa or more, and preferably 15 MPa or more. A higher discharge pressure is desirable, yet it requires a larger apparatus, and so on. Thus, it is preferable to keep the discharge pressure at or below 25 MPa.
If the number of descaling is less than 50 % of the total number of rolling passes, the amount of hematite formation increases, and the magnetite proportion cannot be increased to 50 % or more. Therefore, the number of descaling is 50 % or more, preferably 60 % or more, of the total number of rolling passes. The upper limit of the number of descaling is not limited, and the number of descaling may be 100 % of the total number of rolling passes, i.e., descaling may be performed immediately before all rolling passes.

[[Rolling finish temperature]]

In a hot rolling step, in order to obtain high toughness for base metal, a lower rolling finish temperature is preferable, yet on the other hand, the rolling efficiency is lowered. Thus, the rolling finish temperature in terms of a temperature of the surface of the steel plate needs to be set in consideration of the required toughness for base metal and rolling efficiency. From the viewpoint of improving the strength and the HIC resistance, it is preferable to set the rolling finish temperature at or above the Ar₃ transformation temperature in terms of a temperature of the surface of the steel plate. As used herein, the Ar₃ transformation temperature means the ferrite transformation start temperature during cooling, and can be determined, for example, from the chemical composition of steel according to the following equation. The temperature of the surface of the steel plate can be measured by a radiation thermometer or the like. $\begin{array}{l} {Ar}_{3} (° C) = 910 - 310 [% C] - 80 [% Mn] - 20 [% Cu] - 15 [% Cr] - 55 [% Ni] - \\ 80 [% Mo], \end{array}$
where [%X] indicates the content by mass% of the element X in steel.

[[Cooling start temperature in the controlled cooling]]

Cooling start temperature is (Ar₃ - 10 °C) or higher in terms of a temperature of the surface of the steel plate.
When the temperature of the surface of the steel plate at the start of cooling is low, the amount of ferrite formation before controlled cooling increases. In particular, if the cooling is started from a temperature below (Ar₃ - 10 °C), ferrite exceeding 5 % by area fraction is generated, causing a significant decrease in the strength and a deterioration in the HIC resistance. Therefore, the temperature of the surface of the steel plate at the start of cooling is set to (Ar₃ - 10 °C) or higher. Note that the temperature of the surface of the steel plate at the start of cooling is not higher than the rolling finish temperature.

[[Cooling rate of the controlled cooling]]

In order to set the highest Vickers hardness at 0.25 mm below the surface of the steel plate to 230 HV or less while achieving high strength, it is necessary to control the cooling rate at 0.25 mm below the surface of the steel plate.

Average cooling rate in a temperature range from 750 °C to 550 °C in terms of a temperature at 0.25 mm below the surface of the steel plate: 20 °C/s to 100 °C/s

It is important to keep the average cooling rate in a temperature range from 750 °C to 550 °C at 0.25 mm below the surface of the steel plate as slow as possible to build up the high-temperature transformation phase. The slower the cooling rate, the lower the highest hardness. Since the temperature range from 750 °C to 550 °C is an important temperature range for bainitic transformation, it is important to control the cooling rate in this temperature range. When non-uniform scale formation is suppressed, if the average cooling rate exceeds 100 °C/s, the proportion of low-temperature transformation phase is large and the Vickers hardness at 0.25 mm below the surface of steel plate exceeds 230 HV, causing deterioration in the SSCC resistance after pipe making. Therefore, the average cooling rate is set to 100 °C/s or lower, and preferably 80 °C/s or lower. If the average cooling rate is less than 20 °C/s, ferrite and pearlite are formed, resulting in insufficient strength. Therefore, the average cooling rate is set to 20 °C/s or higher.
For cooling to 550 °C or lower in terms of a temperature at 0.25 mm below the surface of the steel plate, it is preferable to increase the water density from the viewpoint of cooling in a stable nucleate boiling state. In order to perform cooling in a stable nucleate boiling state so as to prevent the formation of locally high hardness portions in the outermost surface layer of the steel plate, it is preferable that the average cooling rate in a temperature range from 550 °C to the cooling stop cooling temperature in terms of a temperature at 0.25 mm below the surface of the steel plate be 110 °C/s or higher, preferably 150 °C/s or higher. From the viewpoint of more reliably suppressing the formation of high hardness portions, the average cooling rate is preferably set to 200 °C/s or lower.

Average cooling rate in a temperature range from 750 °C to 550 °C in terms of an average temperature of the steel plate: 15 °C/s or higher

If the average cooling rate in a temperature range from 750 °C to 550 °C in terms of an average temperature of the steel plate is lower than 15 °C/s, there are larger fractions of phases other than the bainite phase, causing deterioration in the strength and HIC resistance. Therefore, the average cooling rate in terms of an average temperature of the steel plate is set to 15 °C/s or higher. From the viewpoint of variations in the strength and hardness of the steel plate, the average cooling rage in terms of an average temperature of the steel plate is preferably 20 °C/s or higher. The upper limit of the average cooling rate is not particularly limited. However, the average cooling rate is preferably 80 °C/s or lower such that excessive low-temperature transformation products will not be generated.
Although the temperature at 0.25 mm below the surface of the steel plate and the average temperature of the steel plate cannot be directly measured physically, a temperature distribution in a cross section in the plate thickness direction can be determined in real time by difference calculation using a process computer, for example, on the basis of the surface temperature at the start of cooling measured by a radiation thermometer and the target surface temperature at the end of cooling. As used herein, the temperature at 0.25 mm below the surface of the steel plate in the temperature distribution is referred to as the "temperature at 0.25 mm below the surface of the steel plate", and the average value of temperatures in the thickness direction in the temperature distribution as the "average temperature of the steel plate".

[[Cooling stop temperature]]

Cooling stop temperature: 250 °C to 550 °C in terms of a temperature at 0.25 mm below the surface of the steel plate

The cooling stop temperature is one of the necessary conditions for achieving a magnetite proportion in scale present on the surface of the steel plate of 50 % or more after controlled cooling. When the cooling stop temperature exceeds 550 °C, bainite transformation is incomplete and sufficient strength cannot be obtained. In addition, if the cooling stop temperature is lower than 250 °C, the magnetite proportion cannot be increased to 50 % or more as a result of increased wustite. Then, the highest Vickers hardness at 0.25 mm below the surface of steel plate exceeds 230 HV, causing deterioration in the SSCC resistance after pipe making. In addition, the hardness of the central segregation area increases and the HIC resistance deteriorates. Therefore, the cooling stop temperature is set to 250 °C to 550 °C in terms of a temperature at 0.25 mm below the surface of the steel plate.

[High strength steel pipe]

By forming the high strength steel plate disclosed herein into a tubular shape by press bend forming, roll forming, UOE forming, or the like, and then welding the butting portions, a high strength steel pipe for sour-resistant line pipes (such as a UOE steel pipe, an electric-resistance welded steel pipe, and a spiral steel pipe) that is suitable for transporting crude oil and natural gas can be manufactured.
For example, an UOE steel pipe is manufactured by groove machining the ends of a steel plate, forming the steel plate into a steel pipe shape by C press, U-ing press, and O-ing press, then seam welding the butting portions by inner surface welding and outer surface welding, and optionally subjecting it to an expansion process. Any welding method may be applied as long as sufficient joint strength and joint toughness are guaranteed, yet it is preferable to use submerged arc welding from the viewpoint of excellent weld quality and manufacturing efficiency. Expansion process can also be performed on a steel pipe that has been formed into a pipe shape by press-bend forming and then seam-welded at the butting faces.

EXAMPLES

The steels having the chemical compositions listed in Table 1 were made into slabs by continuous casting, heated to the temperatures listed in Table 2, and then hot rolled at the rolling finish temperatures listed in Table 2 to obtain the steel plates of the thicknesses listed in Table 2. The hot rolling process included a total of 10 to 25 passes, ranging from 2 to 12 passes for rough rolling and 5 to 15 passes for finish rolling, and descaling was performed at the discharge pressures listed in Table 2, in the percentages of the total rolling passes listed in Table 2. Then, each steel plate was subjected to controlled cooling using a water-cooling type controlled-cooling device under the conditions listed in Table 2.

[Identification of microstructure]

The microstructure of each obtained steel plate was observed by an optical microscope and a scanning electron microscope. A sample for observation of metallic microstructure was taken from the central part in the width direction of each steel plate. For each sample, a cross section parallel to the longitudinal direction of rolling was mirror polished and then subjected to nital etching. Then, using an optical microscope, the polished surface of each sample was imaged at five locations that were randomly selected at magnifications ranging from 400 to 1000 times, and the area fraction of each phase was calculated by image analysis processing. The microstructures at 0.25 mm below the surface of each steel plate, the type of microstructures at the mid-thickness position, and the area fractions of phases other than the bainite phase are listed in Table 3.

[Measurement of the magnetite proportion in scale]

Scale was collected from the surface of each obtained steel plate. For scale sampling, samples were taken at a total of nine locations, i.e., at the central part and both ends in the width direction of the steel plate at the leading end, center, and trailing end of the steel plate in the longitudinal direction, respectively. At each location, at least 0.5 g of scale was collected. Then, on the scale collected at each location, phase identification was performed by X-ray diffraction (XRD) and quantitative analysis (i.e., measurement of magnetite proportion) was performed using a reference intensity ratio (RIR). The result of averaging the magnetite proportion of the scale at nine locations is presented in Table 3 as the "Magnetite proportion " according to the present disclosure.

[Measurement of tensile strength]

Tensile test was conducted using full-thickness test pieces collected in a direction perpendicular to the rolling direction as tensile test pieces to measure the yield strength and tensile strength. The results are listed in Table 3.

[Measurement of Vickers hardness]

For a cross section perpendicular to the rolling direction, in accordance with JIS Z 2244, Vickers hardness (HV 0.5) was measured at 100 locations at a position 0.25 mm below the surface of each steel plate. Then, the highest hardness was determined among the measurement results, the measurement results were averaged to obtain average hardness, and the standard deviation σ was determined. The highest hardness, average hardness, and 3σ are listed in Table 3.

[Evaluation of SSCC resistance]

The SSCC resistance was evaluated for a pipe made from a part of each steel plate. Each pipe was manufactured by groove machining the ends of a steel plate, and forming the steel plate into a steel pipe shape by C press, U-ing press, and O-ing press, then seam welding the butting portions on the inner and outer surfaces by submerged arc welding, and subjecting it to an expansion process. As illustrated in FIG. 1, after a coupon cut out from each obtained steel pipe was flattened, an SSCC test piece of 5 mm × 15 mm × 115 mm was collected from the inner surface of the steel pipe. At this time, in addition to specimens of only the base metal without the weld, specimens containing both the weld and base metal were taken. The inner surface to be tested was left intact without removing the scale in order to leave the state of the outermost layer. Each collected SSCC test piece was loaded with 90 % stress of the actual yield strength (0.5 % YS) of the corresponding steel pipe, and evaluation was made using a NACE standard TM0177 Solution A solution, at a hydrogen sulfide partial pressure of 1 bar, in accordance with the 4-point bending SSCC test specified by the EFC 16 standard. In addition, at a hydrogen sulfide partial pressure of 0.1 bar and a carbon-dioxide partial pressure of 0.9 bar, evaluation was made using a NACE standard TM0177 Solution B solution in accordance with the 4-point bending SSCC test specified by the EFC 16 standard. Furthermore, at a hydrogen sulfide partial pressure of 2 bar and a carbon dioxide partial pressure of 3 bar, evaluation was made using a NACE standard TM0177 Solution A solution in accordance with the 4-point bending SSCC test specified by the EFC 16 standard. After immersion for 720 hours, the SSCC resistance was judged as "good" when no cracks were observed in both of the specimens, i.e., the specimen of only the base metal without the weld and the specimen containing the weld and base metal, or "poor" when cracking occurred in at least one of the specimens. The results are listed in Table 3.

[Evaluation of HIC resistance]

HIC resistance was examined by performing HIC test at a hydrogen sulfide partial pressure of 1 bar and with an immersion time of 96 hours using a NACE standard TM0177 Solution A solution. In addition, HIC resistance was evaluated by performing HIC test at a hydrogen sulfide partial pressure of 0.1 bar and a carbon-dioxide partial pressure of 0.9 bar and with an immersion time of 96 hours using a NACE standard TM0177 Solution B solution. The HIC resistance was judged as "excellent" when the crack length ratio (CLR) was 10 % or less in the HIC test, "good" when the CLR was more than 10 % and 15 % or less, or "poor" when the CLR exceeded 15 %. The results are listed in Table 3.
The target ranges of the present disclosure were as follows:

the tensile strength is 520 MPa or more as a high strength steel plate for a sour-resistant line pipe;
the microstructure is a bainite microstructure at both positions of 0.25 mm below the surface and of t/2 position (i.e. mid-thickness position);
the highest hardness with HV 0.5 at 0.25 mm below the surface is 230 or less;
no cracks are observed in the SSCC test; and
the crack length ratio (CLR) is 15 % or less in the HIC test.

Table 2

No.	Steel sample ID	Plate thickness	Heating temp.	Descaling		Rolling finish temp.	Cooling start condition		Cooling rate 750 °C - 550 °C (at 0.25 mm below surface of steel plate)	Cooling Rate 750 °C - 550 °C (average in steel plate)	Cooling Rate 550 °C or lower (at 0.25 mm below surface of steel plate)	Cooling stop temp. (at 0.25 mm below surface of steel plate)	Classification
		Plate thickness	Heating temp.	Discharge pressure	Percentage of descalaing passes to the total number of rolling passes	Rolling finish temp.	Cooling start temperature	Cooling start temperature - Ar₃		Cooling Rate 750 °C - 550 °C (average in steel plate)
		(mm)	(°C)	(MPa)	(%)	(°C)	(°C)	(°C)	(°C/s)	(°C/s)	(°C/s)	(°C)
1	A	20	1070	15	100	880	800	43	100	80	150	430	Example
2	B	20	1080	15	90	890	810	51	45	34	170	460	Example
3	C	20	1060	20	85	870	800	49	86	69	130	420	Example
4	D	25	1070	20	60	880	820	50	65	52	110	400	Example
5	E	25	1080	15	50	870	800	29	35	26	160	390	Example
6	F	25	1060	15	75	860	800	45	50	39	190	450	Example
7	G	30	1050	10	80	870	820	40	29	25	150	400	Comparative example
8	H	30	1060	10	90	860	810	47	64	54	140	350	Comparative example
9	I	30	1070	15	70	870	830	14	70	58	130	470	Comparative example
10	J	15	1150	15	95	900	800	57	90	75	170	400	Comparative example
11	K	15	1130	20	60	900	800	37	94	79	160	400	Comparative example
12	L	15	1110	20	75	910	800	13	86	71	150	430	Comparative example
13	M	20	1070	15	80	870	800	40	73	60	160	340	Comparative example
14	N	20	1080	15	85	890	810	32	33	27	180	380	Comparative example
15	O	20	1090	10	85	880	800	52	58	45	160	350	Comparative example
16	P	25	1070	10	70	850	800	43	88	75	190	400	Comparative example
17	Q	25	1060	15	100	860	800	49	73	60	140	410	Comparative example
18	R	25	1050	15	90	860	810	44	55	44	150	280	Comparative example
19	A	35	990	10	70	840	800	43	25	20	180	430	Comparative example
20	B	35	1040	5	60	860	820	61	30	24	150	300	Comparative example
21	C	35	1030	10	45	850	810	59	36	27	170	350	Comparative example
22	D	30	1080	20	75	800	750	-20	60	46	160	400	Comparative example
23	E	30	1070	20	90	840	800	29	15	13	140	420	Comparative example
24	F	30	1060	15	85	860	810	55	110	90	190	450	Comparative example
25	A	25	1060	15	80	880	810	53	29	24	180	240	Comparative example
26	B	25	1050	10	80	880	820	61	39	31	-	560	Comparative example
27	C	25	1070	10	90	850	800	49	105	85	150	240	Comparative example
28	D	20	1060	5	55	870	800	30	41	32	150	450	Comparative example
29	E	20	1060	15	45	870	800	29	53	43	160	350	Comparative example
30	A	14	1120	15	50	880	810	53	40	30	150	500	Example
31	B	39	1050	15	90	850	830	71	35	20	110	400	Example

Note 1: Underlined if outside the scope of the present disclosure.

Table 3

No.	Microstructure (at 0.25 mm below surface of steel plate)		Microstructure (at t/2)		Magnetite proportion	Yield strength	Tensile strength	Average hardness (at 0.25 mm below surface of steel plate)	Hardness variation 3σ (at 0.25 mm below surface of steel plate)	Highest hardness (at 0.25 mm below surface of steel plate)	SSCC resistance of steelpipe			HICResistance of steelpipe		Classification
	Microstructure	Fraction of microstructures other than B (%)	Microstructure	Fraction of microstructures other than B (%)	Magnetite proportion	Yield strength	Tensile strength	Average hardness (at 0.25 mm below surface of steel plate)		Highest hardness (at 0.25 mm below surface of steel plate)	1 bar H₂S	0.1 bar H₂S + 0.9 bar CO₂	2 bar H₂S + 3bar CO₂	1 bar H₂S	0.1 bar H₂S + 0.9 bar CO₂
	Microstructure	Fraction of microstructures other than B (%)	Microstructure	Fraction of microstructures other than B (%)	(%)	(MPa)	(MPa)	(HV0.5)	(HV 0.5)	(HV0.5)	Sol.A	Sol.B	Sol.A	Sol.A	Sol.B
1	B	0	B	0	90	503	598	204	20	225	good	good	good	excellent	excellent	Example
2	B	0	B	0	85	491	580	194	16	210	good	good	good	excellent	excellent	Example
3	B	0	B	0	85	505	597	200	19	221	good	good	good	excellent	excellent	Example
4	B	0	B	0	70	510	606	199	20	222	good	good	good	excellent	excellent	Example
5	B	0	B	0	50	463	573	195	19	219	good	good	good	excellent	excellent	Example
6	B	0	B	0	80	480	576	201	18	220	good	good	good	excellent	excellent	Example
7	B	0	B	0	75	422	491	197	17	214	good	good	good	excellent	excellent	Comparative example
8	B	0	B	0	70	494	583	215	19	236	poor	poor	poor	poor	poor	Comparative example
9	B	0	B	0	70	416	478	204	20	226	good	good	good	excellent	excellent	Comparative example
10	B	0	B	0	80	498	597	212	20	238	poor	poor	poor	poor	poor	Comparative example
11	B	0	B	0	60	459	560	215	20	240	poor	poor	poor	poor	poor	Comparative example
12	B	0	B	0	75	403	485	200	21	223	good	good	good	excellent	excellent	Comparative example
13	B	0	B	0	60	510	605	212	22	235	poor	poor	poor	poor	poor	Comparative example
14	B	0	B	0	65	473	578	205	20	229	good	good	poor	excellent	excellent	Comparative example
15	B	0	B	0	55	515	610	211	19	233	poor	poor	poor	poor	poor	Comparative example
16	B	0	B	0	60	497	585	202	22	227	good	poor	good	excellent	excellent	Comparative example
17	B	0	B	0	90	501	592	201	23	226	good	poor	good	excellent	excellent	Comparative example
18	B	0	B	0	60	518	613	211	20	234	poor	poor	poor	poor	poor	Comparative example
19	B	0	B	0	65	443	517	186	17	205	good	good	good	excellent	excellent	Comparative example
20	B	0	B	0	30	491	584	207	26	237	poor	poor	poor	poor	poor	Comparative example
21	B	0	B	0	20	501	587	208	26	240	poor	poor	poor	poor	poor	Comparative example
22	F+B	15	F+B	10	70	427	505	192	22	218	good	good	good	poor	excellent	Comparative example
23	F+B	15	F+B	10	80	411	490	189	18	210	good	good	good	poor	excellent	Comparative example
24	B	0	B	0	75	490	594	207	25	238	poor	poor	poor	excellent	excellent	Comparative example
25	B	0	B	0	40	497	597	209	25	238	poor	poor	poor	poor	poor	Comparative example
26	F+B	10	F+B	5	75	444	515	185	19	206	good	good	good	good	excellent	Comparative example
27	B	0	B	0	40	519	615	213	27	245	poor	poor	poor	poor	poor	Comparative example
28	B	0	B	0	25	498	589	199	29	232	poor	poor	poor	poor	poor	Comparative example
29	B	0	B	0	30	508	594	204	25	233	poor	poor	poor	poor	poor	Comparative example
30	B	0	B	0	55	497	590	195	20	219	good	good	good	excellent	excellent	Example
31	B	0	B	0	80	464	560	206	17	224	good	good	good	excellent	excellent	Example

Note 1: Underlined if outside the scope of the present disclosure.
Note 2: For microstructures, B is bainite and F is ferrite.

As can be seen from Tables 2 and 3, Nos. 1 to 6 and Nos. 30 to 31 are our examples in which the chemical compositions and the production conditions satisfy the appropriate ranges of the present disclosure. In any of these cases, the tensile strength of the steel plate was 520 MPa or more, and the SSCC resistance and HIC resistance were good or excellent.
In contrast, Nos. 7 to 18 are comparative examples whose chemical compositions are outside the scope of the present disclosure. In Nos. 7, 9, and 12, solid solution strengthening was insufficient and the strength was low. In Nos. 8, 10, 11, 13, 15, and 18, the highest hardness with HV 0.5 exceeded 230, and the SSCC resistance and HIC resistance were inferior. In No. 14, the steel plate did not contain Mo, and the SSCC resistance deteriorated under a very severe corrosion environment with a hydrogen sulfide partial pressure of 2 bar. In No. 16, the steel plate had an excessively high Cu content, and the SSCC resistance deteriorated in an environment with a low hydrogen sulfide partial pressure. In No. 17, the steel plate had an excessively high Ni content, and the SSCC resistance deteriorated in an environment with a low hydrogen sulfide partial pressure.
Nos. 19 to 29 are comparative examples whose chemical compositions are within the scope of the present disclosure but whose manufacturing conditions are outside the scope of the present disclosure. In No. 19, since the slab heating temperature was low, the homogenization of the microstructure and the dissolving of carbides were insufficient and the strength was low. In Nos. 20 and 28, the discharge pressure of descaling was less than 10 MPa, scaling was uneven, the magnetite proportion was less than 50 %, and the maximum hardness with HV 0.5 exceeded 230, resulting in inferior SSCC resistance and HIC resistance. In Nos. 21 and 29, the magnetite proportion was less than 50 % since the proportion of the number of descaling relative to the number of total rolling passes was less than 50 %, and the highest hardness with HV 0.5 exceeded 230, resulting in inferior SSCC resistance and HIC resistance. In No. 22, the cooling start temperature was low and the microstructure was formed in a layered manner with precipitation of ferrite, the strength was low and the HIC resistance was inferior. In No. 23, since the controlled cooling conditions were outside the scope of the present disclosure and a ferrite + bainite microstructure was obtained, the strength was low and the HIC resistance deteriorated. In No. 24, the average cooling rate in a temperature range from 750 °C to 550 °C at 0.25 mm below the surface of the steel plate exceeded 100 °C/s, which resulted in a high proportion of low-temperature transformation phase, and the highest hardness with HV 0.5 at 0.25 mm below the surface of the steel plate exceeded 230, resulting in inferior SSCC resistance. In No. 25, the cooling stop temperature was low, the magnetite proportion was less than 50 %, and the highest hardness with HV 0.5 exceeded 230, resulting in inferior SSCC resistance. In No. 26, the cooling stop temperature was high and bainitic transformation was incomplete, resulting in insufficient strength. Note that the cooling stop temperature was 560 °C in No. 26, and the column "Cooling rate in temp. range of 550 °C or lower (at 0.25 mm below the surface of the steel plate)" in Table 2 was blanked, meaning that no controlled cooling (accelerated cooling) was performed in the temperature range of 550 °C or lower. In No. 27, since the average cooling rate in a temperature range from 750 °C to 550 °C at 0.25 mm below the surface of the steel plate exceeded 100 °C/s and the cooling stop temperature was law, the magnetite proportion was less than 50 % and the highest hardness with HV 0.5 exceeded 230, resulting in inferior SSCC resistance.

INDUSTRIAL APPLICABILITY

According to the present disclosure, it is possible to provide a high strength steel plate for a sour-resistant line pipe that is excellent not only in HIC resistance but also in SSCC resistance under more severe corrosion environments and environments with low hydrogen sulfide partial pressure below 1 bar. Therefore, steel pipes (such as electric-resistance welded steel pipes, spiral steel pipes, and UOE steel pipes) manufactured by cold-forming the disclosed steel plate can be suitably used for transportation of crude oil and natural gas that contain hydrogen sulfides where sour resistance is required.

Claims

A high strength steel plate for a sour-resistant line pipe, comprising:
a chemical composition containing, by mass%, C: 0.020 % to 0.080 %, Si: 0.01 % to 0.50 %, Mn: 0.50 % to 1.80 %, P: 0.015 % or less, S: 0.0015 % or less, Al: 0.010 % to 0.080 %, N: 0.0010 % to 0.0080 %, Mo: 0.01 % to 0.50 %, and Ca: 0.0005 % to 0.0050 %, with the balance being Fe and inevitable impurities;

a magnetite proportion in scale present on a surface of the steel plate being 50 % or more;

a highest Vickers hardness at 0.25 mm below the surface of the steel plate being 230 HV or less; and

a tensile strength being 520 MPa or more.
The high strength steel plate for a sour-resistant line pipe according to claim 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 Cr: 0.50 % or less.
The high strength steel plate for a sour-resistant line pipe according to claim 1 or 2, wherein the chemical composition further contains, by mass%, at least one selected from the group consisting of Nb: 0.005 % to 0.1 %, V: 0.005 % to 0.1 %, Ti: 0.005 % to 0.1 %, Zr: 0.0005 % to 0.02 %, Mg: 0.0005 % to 0.02 %, and REM: 0.0005 % to 0.02 %.
A method for manufacturing a high strength steel plate for a sour-resistant line pipe, the method comprising:
heating a slab having the chemical composition as recited in any one of claims 1 to 3 to a temperature of 1000 °C to 1300 °C;

then hot rolling the slab to form a steel plate and subjecting the steel plate to descaling at a discharge pressure of 10 MPa or more in a number of rolling passes as many as 50 % or more of a total number of rolling passes during the hot rolling; and

then subjecting the steel plate to controlled cooling under a set of conditions including:
a temperature of a surface of the steel plate at the start of cooling being (Ar₃ - 10 °C) or higher;

an average cooling rate in a temperature range from 750 °C to 550 °C in terms of a temperature at 0.25 mm below the surface of the steel plate being 20 °C/s to 100 °C/s;

an average cooling rate in a temperature range from 750 °C to 550 °C in terms of an average temperature of the steel plate being 15 °C/s or higher; and

a cooling stop temperature in terms of a temperature at 0.25 mm below the surface of the steel plate being 250 °C to 550 °C.
A high strength steel pipe using the high strength steel plate for a sour-resistant line pipe as recited in any one of claims 1 to 3.