EP4628604A1

EP4628604A1 - High-strength steel sheet for hydrogen-transporting steel pipes, method for manufacturing same, and hydrogen-transporting steel pipe

Info

Publication number: EP4628604A1
Application number: EP23922852.1A
Authority: EP
Inventors: Daichi Izumi; Hikaru IMAYAMA; Yoshihiro Nishihara; Hiroshi Okano; Junji Shimamura
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2023-02-14
Filing date: 2023-10-16
Publication date: 2025-10-08
Also published as: JP7541651B1; JPWO2024171518A1; CN120569502A; EP4628604A4; WO2024171518A1; KR20250133714A

Abstract

An object is to provide a high-strength steel plate for a hydrogen transporting steel pipe, the high-strength steel plate having excellent fracture toughness in a high-pressure hydrogen environment and having high tensile strength. Another object is to provide a hydrogen transporting steel pipe that uses this high-strength steel plate for a hydrogen transporting steel pipe.A high-strength steel plate for a hydrogen transporting steel pipe has a particular chemical composition, and, at the plate thickness center position of the steel plate, the area fraction of the TD//{001} crystal plane is 10% or less where TD is a direction normal to side surfaces of the steel plate during rolling and MA is 5% or less in terms of area fraction, and the tensile strength is 625 MPa or higher.

Description

Technical Field

The present invention relates to a high-strength steel plate for a hydrogen transporting steel pipe, in particular, to a high-strength steel plate for a hydrogen transporting steel pipe suitable to serve as a linepipe used in transporting high-pressure hydrogen gas, and to a production method therefor. The present invention also relates to a hydrogen transporting steel pipe that uses the aforementioned high-strength steel plate for a hydrogen transporting steel pipe.

Background Art

In general, a linepipe is produced by forming a steel plate, which has been produced by a plate mill or a hot rolling mill, into a steel pipe through UOE forming, press-bend forming, roll-forming, etc.
Here, linepipes used in transportation of high-pressure hydrogen gas are required to have hydrogen embrittlement resistance in addition to strength, toughness, weldability, etc. In particular, in order to ensure the safety against fracture, the fracture toughness value in a high-pressure hydrogen gas environment is important, and a fracture toughness value of 55 (MPa·√m) or higher is required under ASME (The American Society of Mechanical Engineers) B31.12. So long as the hydrogen pressure is about 15 MPa, low-alloy steel having a sufficient plate thickness is used; however, at a higher pressure, there is an increased risk of hydrogen embrittlement fracture during the service, and thus austenite stainless steel such as SUS 316L that is less prone to hydrogen embrittlement than low-alloy steel has been used in some cases.
Austenite stainless steel involves a high steel material cost and has low strength; thus, when designed to withstand high hydrogen pressure, the plate becomes thicker, and the price of the hydrogen transporting steel pipe itself rises. Thus, there has been a demand for lower-cost steel materials that can withstand the high-pressure hydrogen gas environment and that can be used in hydrogen transporting steel pipes.
To address the issues described above, for example, Patent Literature 1 proposes an austenite steel material having a high Mn content.

Citation List

Patent Literature

PTL 1: Japanese Patent No. 6703608

Summary of Invention

Technical Problem

According to the steel material disclosed in Patent Literature 1, the fracture toughness in the high-pressure hydrogen gas environment is not disclosed.
Accordingly, an object of the present invention addressing the aforementioned issues is to provide a high-strength steel plate for a hydrogen transporting steel pipe, the high-strength steel plate having excellent fracture toughness in a high-pressure hydrogen environment and high tensile strength. Another object of the present invention is to provide a hydrogen transporting steel pipe that uses the high-strength steel plate for a hydrogen transporting steel pipe.

Solution to Problem

The inventors of the present invention have repeated numerous experiments and studies on the chemical composition, the microstructure, and the production conditions of the steel plate to ensure the fracture toughness in a high-pressure hydrogen gas environment. As a result, it has been found that the fracture toughness is improved by controlling the area fraction of the TD (transverse direction, direction normal to the side surface during rolling)//{001} crystal plane to 10% or less and by controlling the MA (martensite-austenite constituent) fraction to 5% or less at the plate thickness center position. Furthermore, in order to achieve such a steel microstructure, the rolling conditions during hot rolling and the water cooling conditions after the hot rolling need to be strictly controlled, and such conditions have been successfully found. The present invention has been made on the basis of these findings.
That is, the summary of the features of the present invention is as follows.

[1] A high-strength steel plate for a hydrogen transporting steel pipe, the high-strength steel plate having a chemical composition containing, in terms of mass%,
- C: 0.040 to 0.090%,
- Si: 0.01 to 0.50%,
- Mn: 1.50 to 2.50%,
- P: 0.002 to 0.020%,
- S: 0.0002 to 0.0020%,
- Al: 0.010 to 0.080%,
- Nb: 0.005 to 0.080%,
- Ti: 0.005 to 0.050%,
- N: 0.0020 to 0.0080%, and
- at least one selected from
- Cu: 0.50% or less,
- Ni: 0.50% or less,
- Cr: 0.50% or less,
- Mo: 0.50% or less,
- V: 0.050% or less,
- Ca: 0.0050% or less, and
- Mg: 0.0050% or less,
- with the balance being Fe and incidental impurity elements,
- wherein, at a plate thickness center position of the steel plate, an area fraction of a TD//{001} crystal plane is 10% or less where TD is a direction normal to a side surface of the steel plate during rolling, and MA is 5% or less in terms of area fraction, and
- the steel plate has a tensile strength of 625 MPa or higher.
[2] The high-strength steel plate for a hydrogen transporting steel pipe described in [1], wherein the chemical composition contains, in terms of mass%, at least one selected from
- Cu: 0.01 to 0.50%,
- Ni: 0.01 to 0.50%,
- Cr: 0.01 to 0.50%,
- Mo: 0.01 to 0.50%,
- V: 0.005 to 0.050%,
- Ca: 0.0005 to 0.0050%, and
- Mg: 0.0005 to 0.0050%.
[3] A method for producing a high-strength steel plate for a hydrogen transporting steel pipe, the method including:
- heating a semifinished steel having the chemical composition described in [1] or [2] to a temperature of 1000 to 1250°C;
- hot-rolling the heated semifinished steel into a steel plate under such conditions that a total rolling reduction in a recrystallization temperature range is 75% or less and the number of rolling passes with a per-pass rolling reduction of 20% or more is 5 or less; and
- subsequently water-cooling the steel plate under such conditions that a cooling stop temperature in terms of a steel plate temperature at a plate thickness center of the steel plate is 250 to 550°C.
[4] A hydrogen transporting steel pipe including the high-strength steel plate for a hydrogen transporting steel pipe described in [1] or [2].

Advantageous Effects of Invention

A high-strength steel plate for a hydrogen transporting steel pipe and a hydrogen transporting steel pipe that uses the high-strength steel plate for a hydrogen transporting steel pipe according to the present invention exhibit excellent fracture toughness in a high-pressure hydrogen environment, and have high tensile strength.

Description of Embodiments

A high-strength steel plate for a hydrogen transporting steel pipe according to the present invention will now be specifically described. Here, the high-strength steel plate for a hydrogen transporting steel pipe according to the present invention may be simply referred to as a high-strength steel plate.

[Chemical composition]

First, the chemical composition of the high-strength steel plate of the present invention and the reasons for limitations are described. In the description below, every unit indicated by % is mass% unless otherwise noted.

C: 0.040 to 0.090%

C effectively contributes to improving the strength, but sufficient strength cannot be secured at a C content of less than 0.040%. Thus, the C content is 0.040% or more. The C content is preferably 0.045% or more. The C content is more preferably 0.050% or more. The C content is even more preferably 0.053% or more. The C content is most preferably 0.055% or more. Meanwhile, at a C content exceeding 0.090%, fracture toughness is degraded. Thus, the C content is 0.090% or less. The C content is preferably 0.080% or less. The C content is more preferably 0.075% or less. The C content is even more preferably 0.070% or less. The C content is most preferably 0.065% or less.

Si: 0.01 to 0.50%

Si is added for deoxidization but the deoxidizing effect is not sufficient at an Si content of less than 0.01%. Thus, the Si content is 0.01% or more. The Si content is preferably 0.05% or more. The Si content is more preferably 0.10% or more. The Si content is even more preferably 0.13% or more. The Si content is most preferably 0.15% or more. Meanwhile, at an Si content exceeding 0.50%, fracture toughness is degraded, and thus the Si content is 0.50% or less. The Si content is preferably 0.45% or less. The Si content is more preferably 0.40% or less. The Si content is even more preferably 0.35% or less. The Si content is most preferably 0.30% or less.

Mn: 1.50 to 2.50%

Mn effectively contributes to improving the strength, but sufficient strength cannot be secured at an Mn content of less than 1.50%. Thus, the Mn content is 1.50% or more. The Mn content is preferably 1.60% or more. The Mn content is more preferably 1.70% or more. The Mn content is even more preferably 1.80% or more. The Mn content is most preferably 1.90% or more. Meanwhile, at an Mn content exceeding 2.50%, the hardness at the center segregation portion increases, and the fracture toughness is degraded. Thus, the Mn content is 2.50% or less. The Mn content is preferably 2.40% or less. The Mn content is more preferably 2.30% or less. The Mn content is even more preferably 2.20% or less. The Mn content is most preferably 2.10% or less.

P: 0.002 to 0.020%

P is an incidental impurity element and degrades the fracture toughness. This tendency becomes prominent when the P content exceeds 0.020%, and thus the upper limit of the P content is 0.020%. The P content is preferably 0.015% or less. The P content is more preferably 0.012% or less. The P content is even more preferably 0.010% or less. Although the P content is preferably low, excessive dephosphorization increases the refining cost, and thus the P content is 0.002% or more from the viewpoint of the refining cost. The P content is preferably 0.005% or more.

S: 0.0002 to 0.0020%

S in an incidental impurity element and degrades the fracture toughness by forming MnS inclusions in the steel; thus, the S content is preferably low but an S content up to 0.0020% is allowable. Thus, the S content is 0.0020% or less. The S content is preferably 0.0015% or less. The S content is more preferably 0.0012% or less. The S content is even more preferably 0.0010% or less. Although the S content is preferably low, excessive desulfurization increases the refining cost, and thus the S content is 0.0002% or more from the viewpoint of the refining cost. The S content is preferably 0.0005% or more.

Al: 0.010 to 0.080%

Al is added as a deoxidizing agent but is not sufficiently effective when the Al content is less than 0.010%. Thus, the Al content is 0.010% or more. The Al content is preferably 0.015% or more. The Al content is more preferably 0.018% or more. The Al content is even more preferably 0.020% or more. Meanwhile, at an Al content exceeding 0.080%, alumina clogs the submerged nozzles during continuous casting, and thus the Al content is 0.080% or less. The Al content is preferably 0.070% or less. The Al content is more preferably 0.060% or less. The Al content is even more preferably 0.050% or less.

Nb: 0.005 to 0.080%

Nb existing as solute Nb contributes to improving the fracture toughness by expanding the non-recrystallization temperature range during hot rolling and making crystal grains finer, but this effect is not sufficiently exhibited at an Nb content of less than 0.005%. Thus, the Nb content is 0.005% or more. The Nb content is preferably 0.010% or more. The Nb content is more preferably 0.013% or more. The Nb content is even more preferably 0.015% or more. The Nb content is most preferably 0.020% or more. Meanwhile, at an Nb content exceeding 0.080%, coarse carbides crystallize during solidification and fracture toughness is degraded. Thus, the Nb content is 0.080% or less. The Nb content is preferably 0.070% or less. The Nb content is more preferably 0.060% or less. The Nb content is even more preferably 0.050% or less. The Nb content is most preferably 0.040% or less.

Ti: 0.005 to 0.050%

Ti as TiN pins austenite grains during heating and suppresses grain growth to thereby contribute to improving the fracture toughness. Since sufficient TiN is not formed at a Ti content of less than 0.005%, the Ti content is 0.005% or more. The Ti content is preferably 0.006% or more. The Ti content is more preferably 0.007% or more. The Ti content is even more preferably 0.008% or more. The Ti content is most preferably 0.009% or more. Furthermore, at a Ti content exceeding 0.050%, the TiN formed coarsens and sufficient fracture toughness is not obtained; thus, the Ti content is 0.050% or less. The Ti content is preferably 0.040% or less. The Ti content is more preferably 0.030% or less. The Ti content is even more preferably 0.020% or less. The Ti content is most preferably 0.015% or less.

N: 0.0020 to 0.0080%

N effectively contributes to improving the strength, but sufficient strength cannot be secured at an N content of less than 0.0020%. Thus, the N content is 0.0020% or more. The N content is preferably 0.0025% or more. The N content is more preferably 0.0030% or more. The N content is even more preferably 0.0033% or more. The N content is most preferably 0.0035% or more. Meanwhile, at an N content exceeding 0.0080%, solute N increases and degrades fracture toughness. Thus, the N content is 0.0080% or less. The N content is preferably 0.0070% or less. The N content is more preferably 0.0060% or less. The N content is even more preferably 0.0055% or less. The N content is most preferably 0.0050% or less.
The chemical composition of the high-strength steel plate of the present invention may optionally contain, in addition to the aforementioned components, at least one selected from Cu, Ni, Cr, Mo, V, Ca, and Mg within the following ranges.

Cu: 0.50% or less

Cu is an element effective for increasing the strength, and in order to obtain this effect when Cu is contained, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more. The Cu content is even more preferably 0.07% or more. However, at a Cu content exceeding 0.50%, surface defects tend to occur on the steel plate, and thus when Cu is contained, the Cu content is 0.50% or less. The Cu content is preferably 0.45% or less. The Cu content is more preferably 0.40% or less. The Cu content is even more preferably 0.35% or less. The Cu content is most preferably 0.30% or less.

Ni: 0.50% or less

Ni is an element effective for improving fracture toughness and increasing the strength, and in order to obtain this effect when Ni is contained, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.05% or more. The Ni content is even more preferably 0.07% or more. Meanwhile, since Ni is an expensive element, the Ni content is 0.50% or less when Ni is contained. The Ni content is preferably 0.45% or less. The Ni content is more preferably 0.40% or less. The Ni content is even more preferably 0.35% or less. The Ni content is most preferably 0.30% or less.

Cr: 0.50% or less

Cr is an element effective for increasing the strength, and in order to obtain this effect when Cr is contained, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.05% or more. The Cr content is even more preferably 0.07% or more. However, at a Cr content exceeding 0.50%, the hardenability becomes excessive, the hardness increases during accelerated cooling, and the fracture toughness is degraded. Thus, when Cr is contained, the Cr content is 0.50% or less. The Cr content is preferably 0.45% or less. The Cr content is more preferably 0.40% or less. The Cr content is even more preferably 0.35% or less. The Cr content is most preferably 0.30% or less.

Mo: 0.50% or less

Mo is an element effective for increasing the strength, and in order to obtain this effect when Mo is contained, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.05% or more. The Mo content is even more preferably 0.07% or more. Meanwhile, since Mo is an expensive element, the Mo content is 0.50% or less when Mo is contained. The Mo content is preferably 0.45% or less. The Mo content is more preferably 0.40% or less. The Mo content is even more preferably 0.35% or less. The Mo content is most preferably 0.30% or less.

V: 0.050% or less

V is an element that can be optionally added to increase the strength and fracture toughness, but this effect is not sufficiently exhibited at a V content of less than 0.005%. Thus, when V is contained, the V content is preferably 0.005% or more. The V content is more preferably 0.007% or more. The V content is even more preferably 0.010% or more. Meanwhile, at a V content exceeding 0.050%, fracture toughness is degraded. Thus, when V is contained, the V content is 0.050% or less. The V content is preferably 0.045% or less. The V content is more preferably 0.040% or less. The V content is even more preferably 0.030% or less.

Ca: 0.0050% or less

Ca is an element effective for improving hydrogen-induced cracking resistance through shape control of sulfide inclusions; however, the effect of addition is not sufficient when the Ca content is less than 0.0005%. Thus, when Ca is contained, the Ca content is preferably 0.0005% or more. The Ca content is more preferably 0.0008% or more. Meanwhile, when the Ca content exceeds 0.0050%, the aforementioned effect saturates and the cleanliness of the steel is degraded, thereby degrading the hydrogen-induced cracking resistance; thus, when Ca is contained, the Ca content is 0.0050% or less. The Ca content is preferably 0.0045% or less. The Ca content is more preferably 0.0040% or less. The Ca content is even more preferably 0.0035% or less.

Mg: 0.0050% or less

Mg is an element that can be optionally added to enhance the fracture toughness through suppression of crystal grain coarsening and to improve the hydrogen-induced cracking resistance through controlling the properties of inclusions. At an Mg content of less than 0.0005%, this effect is not sufficiently exhibited. Thus, when Mg is contained, the Mg content is preferably 0.0005% or more. Meanwhile, when the Mg content exceeds 0.0050%, the effect saturates and thus when Mg is contained, the Mg content is 0.0050% or less. The Mg content is preferably 0.0040% or less. The Mg content is more preferably 0.0030% or less. The Mg content is even more preferably 0.0025% or less.
Here, the balance other than the aforementioned elements is Fe and incidental impurities. However, so far as the actions and effects of the present invention are unmitigated, incorporation of other trace elements is allowable. For example, O is an element incidentally contained in the steel, and an O content of 0.0050% or less and preferably 0.0040% or less is allowable in the present invention. Note that, for the above-described elements, a Cu content of less than 0.01%, a Ni content of less than 0.01%, a Cr content of less than 0.01%, an Mo content of less than 0.01%, a V content of less than 0.005%, a Ca content of less than 0.0005%, and an Mg content of less than 0.0005% are within the range of the incidental impurities described above.

[Area fraction of TD//{001} crystal plane at plate thickness center position is 10% or less, where TD is direction normal to side surface of steel plate during rolling]

It is important that, in the high-strength steel plate of the present invention, the area fraction of the TD (transverse direction, direction normal to the side surface during rolling)//{001} crystal plane at the plate thickness center position be 10% or less. The direction normal to the side surface of the steel plate during rolling is a direction perpendicular to the rolling longitudinal direction and perpendicular to the plate thickness direction. By satisfying this condition, excellent fracture toughness can be obtained in a high-pressure hydrogen environment. The area fraction of TD//{001} means the proportion at which the {001}, which is the cleaved facet of a BCC metal, is parallel to the TD plane, which is the crack growth plane in a fracture toughness test, and when the area fraction of TD//{001} at the plate thickness center position exceeds 10%, cracks easily grow and thus the fracture toughness is degraded. Thus, the area fraction of TD//{001} at the plate thickness center position is 10% or less. The area fraction of TD//{001} at the plate thickness center position is preferably 9% or less. The area fraction of TD//{001} at the plate thickness center position is more preferably 8% or less. The area fraction of TD//{001} at the plate thickness center position is even more preferably 7% or less. Meanwhile, the area fraction of TD//{001} at the plate thickness center position is preferably 1% or more. The area fraction of TD//{001} at the plate thickness center position is more preferably 2% or more.
Regarding the microstructure of the steel plate of the present invention, the main microstructure is a bainite single phase or a bainite-ferrite dual phase microstructure, and an example of the remaining microstructure is MA. In the case of the bainite single phase microstructure, the bainite phase preferably accounts for 95% or more. The bainite phase more preferably accounts for 96% or more. The bainite phase even more preferably accounts for 97% or more. Furthermore, the bainite phase may account for 100%. In the case of the bainite-ferrite dual phase microstructure, the bainite phase preferably accounts for 30% or more and the ferrite phase preferably accounts for 30% or more. More preferably, the bainite phase accounts for 32% or more and the ferrite phase accounts for 32% or more. Preferably, the bainite phase preferably accounts for 70% or less and the ferrite phase preferably accounts for 70% or less. Whichever the microstructure is, it is critical that the area fraction of the TD//{001} crystal plane at the plate thickness center be controlled to 10% or less.

[MA at plate thickness center position is 5% or less in terms of area fraction]

In the present invention, it is important that the formation of martensite-austenite constituent (MA) be suppressed. When the MA in the RD (rolling direction, direction of rolling during rolling) plane at the plate thickness center position in terms of area fraction exceeds 5%, hydrogen locally accumulates around the MA in the steel plate, and this causes degradation of the fracture toughness. Thus, the MA in the RD (rolling direction, direction of rolling during rolling) plane at the plate thickness center position in terms of area fraction is 5% or less. The MA is preferably 4% or less in terms of area fraction. The MA is more preferably 3% or less in terms of area fraction. The MA fraction can be measured by the method disclosed in Examples and is an area fraction. The lower limit of the MA fraction at the plate thickness center position is not particularly limited and may be 0%. Note that, as described below, in the present invention, the cooling rate at and below the cooling stop temperature is not particularly limited, and thus the MA of the present invention includes both martensite that has a dislocation density of a level comparable to that of common water hardened materials and martensite that has a dislocation density smaller than common water hardened materials.

[Tensile strength]

The high-strength steel plate of the present invention is mainly intended for steel plates for steel pipes having a strength of API 5L Grade X80 or higher, and thus is to have a tensile strength of 625 MPa or higher. Preferably, the tensile strength is 650 MPa or higher. The tensile strength is more preferably 675 MPa or higher. The upper limit is not particularly limited, but the tensile strength is preferably 760 MPa or less since the fracture toughness of the weld heat affected zone may be degraded. The tensile strength is more preferably 750 MPa or less.

[Plate thickness of high-strength steel plate]

The plate thickness of the high-strength steel plate of the present invention is not particularly limited but is preferably 12 mm or more. The plate thickness is more preferably 15 mm or more. Furthermore, the plate thickness of the high-strength steel plate of the present invention is not particularly limited but is preferably 39 mm or less. The plate thickness is more preferably 35 mm or less.

[Production method]

The production method and production conditions for producing the high-strength steel plate of the present invention will now be specifically described.
According to the method for producing the high-strength steel plate of the present invention, a semifinished steel (slab) having the aforementioned chemical composition is heated and then hot-rolled into a steel plate (hot rolling step) and then the steel plate is water-cooled under particular conditions (water cooling step).

[Semifinished steel heating temperature]

Semifinished steel heating temperature: 1000 to 1250°C

When the semifinished steel (slab) heating temperature is less than 1000°C, dissolution of carbides becomes insufficient, the solid solution strengthening amount by the solute C and the like decreases, and thus the necessary strength is not obtained. Thus, the semifinished steel (slab) heating temperature is 1000°C or higher. The semifinished steel heating temperature is preferably 1030°C or higher. The semifinished steel heating temperature is more preferably 1040°C or higher. The semifinished steel heating temperature is even more preferably 1050°C or higher. Meanwhile, when the semifinished steel heating temperature exceeds 1250°C, the crystal grains prominently coarsen and the necessary strength cannot be obtained; thus, the semifinished steel heating temperature is 1250°C or less. The semifinished steel heating temperature is preferably 1220°C or less. The semifinished steel heating temperature is more preferably 1200°C or less. The semifinished steel heating temperature is even more preferably 1170°C or less. Here, the semifinished steel (slab) is heated to this temperature as far as the center portion.

[Total rolling reduction in recrystallization temperature range: 75% or less]

In order to adjust the area fraction of TD//{001} at the plate thickness center position to 10% or less, formation of TD//{001} during hot rolling in the recrystallization temperature range needs to be suppressed. The main orientation of the transformation texture obtained by the recrystallization temperature range rolling is (001)<011>, and when the total rolling reduction in the recrystallization temperature range exceeds 75%, the area fraction of TD//{001} exceeds 10%. Thus, the total rolling reduction in the recrystallization temperature range is 75% or less. 72% or less is preferable. 70% or less is more preferable. Here, the lower limit temperature Tnr (°C) of the recrystallization temperature range can be, for example, determined by following equation (1) from the components of the steel. Here, the temperature during hot rolling is the surface temperature of a material to be rolled (semifinished steel or steel plate), and the surface temperature can be measured with a radiation thermometer or the like. Meanwhile, the lower limit of the total rolling reduction in the recrystallization temperature range is preferably 52% or more for promoting recrystallization. The total rolling reduction in the recrystallization temperature range is more preferably 55% or more. $Tnr (° C) = 174 \times \log [% Nb] [% C + (12 / 14) % N] + 1444$ Here, [%X] in the above-described equation represents the content (mass%) of X element in the steel.

[Number of rolling passes with per-pass rolling reduction of 20% or more: 5 or less]

In addition to controlling the total rolling reduction in the recrystallization temperature range to 75% or less, the per-rolling-pass rolling reduction needs to be controlled. When the per-rolling-pass rolling reduction is 20% or more, shear strain is less likely to come in and the integration degree of the (001) plane increases; thus, when there are more than five passes with a per-rolling-pass rolling reduction of 20% or more, the area fraction of TD//{001} exceeds 10%. Conversely, as long as the number of rolling passes with a per-rolling-pass rolling reduction of 20% or more is suppressed to 5 or less, the area fraction of TD//{001} does not exceed 10%. Thus, the number of rolling passes with a per-pass rolling reduction of 20% or more is 5 or less. The number of passes is preferably 4 or less. The number of passes is more preferably 3 or less. The number of passes is even more preferably 2 or less. The number of rolling passes with a per-pass rolling reduction of 20% or more is preferably as small as possible, and the lower limit is 0. Note that the per-rolling-pass rolling reduction needs to be controlled not only in the recrystallization temperature range but throughout the rolling.

[Rolling finish temperature]

The rolling finish temperature is not particularly limited, but is preferably the Ar₃ transformation point or higher since the uniformity of the microstructure is ensured by performing water-cooling from the state where the entire steel is austenite. Here, the Ar₃ transformation point means a temperature at which the ferrite transformation begins during water cooling and can be determined by following equation (2) from the steel components, for example. Here, the surface temperature of the steel plate can be measured with a radiation thermometer or the like. Ar₃ transformation point (°C) = 910 - 310[%C] - 80[%Mn] - 20[%Cu] - 15[%Cr] - 55[%Ni] - 80[%Mo] Here, [%X] in the above-described equation represents the content (mass%) of X element in the steel, and is 0 when the element is not contained.

[Water cooling start temperature]

The water cooling start temperature is not particularly limited; however, when the steel plate surface temperature at the start of cooling by water cooling is less than the Ar₃ transformation point (°C), ferrite occurs before water cooling and the decrease in strength becomes prominent. Thus, the steel plate surface temperature at the start of cooling by water cooling is preferably the Ar₃ transformation point (°C) or higher. The steel plate surface temperature at the start of cooling is preferably equal to or higher than Ar₃ transformation point + 5 (°C). The steel plate surface temperature at the start of cooling is more preferably equal to or higher than Ar₃ transformation point + 10 (°C). The steel plate surface temperature at the start of cooling is preferably equal to or lower than Ar₃ transformation point + 50 (°C). The steel plate surface temperature at the start of cooling is more preferably equal to or lower than Ar₃ transformation point + 40 (°C). The steel plate surface temperature at the start of cooling is even more preferably equal to or lower than Ar₃ transformation point + 30 (°C). Note that the steel plate surface temperature at the start of cooling by water cooling is the temperature of the steel plate tail end portion where the water cooling start temperature is the lowest.

[Average cooling rate in water cooling]

The cooling rate in water cooling at the plate thickness center is not particularly limited; however, when the average cooling rate from the start to end of water cooling is less than 5°C/s, ferrite and pearlite are formed and the strength decreases prominently. Thus, the average cooling rate is preferably 5°C/s or more. The average cooling rate is more preferably 10°C/s or more. The average cooling rate is even more preferably 15°C/s or more. Meanwhile, when the average cooling rate is more than 50°C/s, the microstructure becomes prominently nonuniform. Thus, the average cooling rate is preferably 50°C/s or less. The average cooling rate is more preferably 45°C/s or less. The average cooling rate is even more preferably 40°C/s or less.

[Cooling stop temperature in water cooling]

Cooling stop temperature in water cooling: steel plate temperature of 250 to 550°C at plate thickness center

When the cooling stop temperature for water cooling in terms of a steel plate temperature at the plate thickness center exceeds 550°C, bainite transformation becomes incomplete, and the amount of MA formed increases. Thus, the cooling stop temperature in water cooling is 550°C or lower. The cooling stop temperature in water cooling is preferably 520°C or lower. The cooling stop temperature in water cooling is more preferably 500°C or lower. The cooling stop temperature in water cooling is even more preferably 470°C or lower. The cooling stop temperature in water cooling is most preferably 450°C or lower. Meanwhile, when the cooling stop temperature for water cooling is lower than 250°C, the steel plate becomes distorted, and the productivity decreases. Thus, the cooling stop temperature in water cooling is 250°C or higher. The cooling stop temperature in water cooling is preferably 270°C or higher. The cooling stop temperature in water cooling is more preferably 300°C or higher. The cooling stop temperature in water cooling is even more preferably 320°C or higher. The cooling stop temperature in water cooling is most preferably 350°C or higher.
In this high-strength steel plate of the present invention obtained as described above, at the plate thickness center position of the steel plate, the area fraction of the TD//{001} crystal plane is 10% or less where TD is a direction normal to a side surface of the steel plate during rolling and MA is 5% or less in terms of area fraction, the tensile strength is 625 MPa or higher, and the high-strength steel plate can be produced at a lower cost than when austenite alloys are used as a raw material.

[Hydrogen transporting steel pipe]

Hydrogen transporting steel pipes (UOE steel pipes, electric resistance welded steel pipes, spiral steel pipes, etc.) suitable for transporting high-pressure hydrogen gas can be produced by forming the high-strength steel plate of the present invention into a cylindrical shape by press-bend forming, roll-forming, UOE-forming, or the like and then welding the butted portion. In addition, by producing steel pipes using the high-strength steel plate of the present invention, steel pipes having excellent fracture toughness even in the weld heat affected zones can be produced. Note that, in the present invention, high-pressure hydrogen means a hydrogen gas environment at 15 MPa or higher as one example. The upper limit is not particularly limited but a hydrogen gas environment of 25 MPa or lower is preferable.
For example, a UOE steel pipe is produced by a process involving preparing edges of a steel plate, forming the steel plate into a steel pipe shape by C-ing, U-ing, and O-ing, seam-welding the butted portion from the inner surface side and the outer surface side, and expanding the pipe as necessary. Here, the welding method may be any method as long as sufficient joint strength and joint toughness are obtained, and from the viewpoints of excellent welding quality and production efficiency, submerged arc welding is preferably employed. Furthermore, a steel pipe obtained by seam-welding the butted portion after the pipe is formed by press-bend forming can also be expanded.

EXAMPLES

Steels (steel grade A to AE) having chemical compositions shown in Table 1 were continuously-casted into semifinished steels (slabs), heated at heating temperatures shown in Table 2, and hot-rolled and water-cooled under conditions shown in Table 2 to obtain steel plates having final plate thickness shown in Table 2. Subsequently, steel pipes were obtained through the process of preparing edges of a steel plate, forming the steel plate into a steel pipe shape by C-ing, U-ing, and O-ing, seam-welding the butted portion from the inner surface side and the outer surface side by submerged arc welding, and expanding the pipe. Note that the Ar₃ transformation point and the lower limit temperature Tnr of the recrystallization temperature range shown in Table 1 were determined from the equations described above.

[Measurement of area fraction of TD//{001}]

A metallic microstructure observation sample was taken from a section parallel to the plate thickness direction and the rolling direction of the steel plate obtained as described above, was mirror-polished, and was etched with colloidal silica, and then an electron back-scattered diffraction (EBSD) pattern was measured in a scanning electron microscope (SEM) at the plate thickness center position. The measurement area was 1 mm × 1 mm, the acceleration voltage was 17 kV, and the resolution was 0.8 µm. The measured EBSD data was analyzed by using OIM-Analysis. The area fraction of TD//{001} is shown in Table 3.

[Calculation of MA fraction]

A metallic microstructure observation sample was taken from a section parallel to the plate thickness direction and the rolling direction of the steel plate obtained as described above, was mirror-polished, then was subjected to a two-step etching process, and was observed with a SEM at an acceleration voltage of 15 kV over a range of 0.9 mm × 1.3 mm at a magnification of 2000x to measure the MA fraction. The MA fraction was determined as an area fraction. White island sites in the SEM photograph were assumed to be the MA, and the MA fraction was measured by image analysis software (for example, Image J). The aforementioned two-step etching process is a method that involves first etching a sample with a 3% nital solution and then etching the sample with a liquid composed of 100 ml of distilled water, 25 g of sodium hydroxide, and 5 g of picric acid. The results are shown in Table 3.

[Measurement of tensile strength and yield strength]

A full-thickness test piece was taken in a direction perpendicular to the rolling direction and used as a tensile test piece subjected to a tensile test in accordance with the provisions of JIS Z 2241 (2011) and JIS Z 2201 to measure the tensile strength and the yield strength. The results are shown in Table 3.

[Evaluation of fracture toughness value]

A CT test piece was taken, from the steel plate obtained as above and at the plate thickness center position, in accordance with ASTM E 647 so that the load application direction was parallel to the rolling direction. A fracture toughness test was carried out in accordance with ASTM E 1820 in 21 MPa high-pressure hydrogen gas, and crack growth was evaluated by an unloading compliance method to derive the fracture toughness value. The case where the fracture toughness value K_IH was 80 (MPa·m^1/2) or higher was rated ∘, and the case where the value was less than 80 (MPa·m^1/2) was rated ×. The results are shown in Table 3.

The present invention was to satisfy the following: the aforementioned particular chemical composition was contained, and, as a high-strength steel plate for a hydrogen transporting steel pipe, the area fraction of TD//{001} was 10% or less and the MA fraction was 5% or less at the plate thickness center position, and the target ranges were a tensile strength of 625 MPa or higher and a fracture toughness value K_IH of 80 (MPa·m^1/2) or higher.

[Table 1]

Steel grade	Chemical composition (mass%)																Ar₃ transformation point (°C)	Tnr temperature (°C)	Classification
Steel grade	C	Si	Mn	P	S	Al	Nb	Ti	N	Cu	Ni	Cr	Mo	V	Ca	Mg	Ar₃ transformation point (°C)	Tnr temperature (°C)	Classification
A	0.065	0.30	2.11	0.006	0.0005	0.024	0.033	0.012	0.0043								721	984	Compliant steel
B	0.074	0.35	1.99	0.008	0.0010	0.033	0.041	0.009	0.0036								728	1009	Compliant steel
C	0.063	0.23	2.17	0.013	0.0008	0.027	0.052	0.015	0.0047								717	1016	Compliant steel
D	0.078	0.41	1.90	0.015	0.0013	0.041	0.046	0.017	0.0039	0.38							726	1022	Compliant steel
E	0.065	0.45	1.95	0.009	0.0011	0.052	0.034	0.011	0.0045		0.50						706	986	Compliant steel
F	0.049	0.17	1.78	0.012	0.0017	0.054	0.028	0.010	0.0049			0.45					746	952	Compliant steel
G	0.053	0.20	1.68	0.009	0.0009	0.029	0.017	0.020	0.0062				0.41				726	921	Compliant steel
H	0.062	0.22	2.09	0.010	0.0008	0.030	0.010	0.008	0.0034	0.25	0.24			0.045			705	889	Compliant steel
I	0.045	0.04	1.55	0.005	0.0004	0.046	0.022	0.013	0.0044			0.33	0.28		0.0035		745	927	Compliant steel
J	0.042	0.09	1.76	0.007	0.0006	0.019	0.009	0.017	0.0053			0.25	023			0.0015	734	856	Compliant steel
K	0.037	0.24	1.90	0.009	0.0010	0.032	0.034	0.011	0.0045								747	947	Comparative steel
L	0.093	0.26	1.93	0.012	0.0016	0.035	0.024	0.009	0.0040								727	985	Comparative steel
M	0.056	054	2.00	0.009	0.0007	0.027	0.035	0.010	0.0047	0.22	0.23						716	978	Comparative steel
N	0.065	0.16	1.45	0.010	0.0012	0.031	0.025	0.014	0.0039								774	962	Comparative steel
O	0.041	0.40	2.55	0.015	0.0017	0.042	0.051	0.015	0.0050								693	985	Comparative steel
P	0.067	0.37	1.88	0022	0.0015	0.025	0.048	0.012	0.0044			0.28					735	1014	Comparative steel
Q	0.070	0.35	2.12	0.013	00023	0.034	0.046	0.014	0.0041			0.13	0.15				705	1014	Comparative steel
R	0.059	0.33	1.87	0.011	0.0009	0.030	0.003	0.012	0.0046	0.30			0.33				710	796	Comparative steel
S	0.076	0.28	1.96	0.010	0.0009	0.035	0.084	0.010	0.0037	011	0.13						720	1065	Comparative steel
T	0.067	0.40	1.91	0.011	0.0012	0.037	0.033	0.003	0.0035	0.22		0.15		0.033			730	985	Comparative steel
U	0.065	0.31	1.86	0.014	0.0008	0.025	0.045	0.052	0.0057				0.24				722	1009	Comparative steel
V	0.058	0.08	1.83	0.007	0.0007	0.026	0.037	0.009	0.0017								746	982	Comparative steel
W	0.069	0.27	1.99	0.012	0.0011	0.038	0.029	0.017	0.0085			0.18	0.18				712	982	Comparative steel
X	0.067	0.26	2.03	0.007	0.0006	0.030	0.035	0.030	0.0035								727	990	Compliant steel
Y	0.080	0.09	1.85	0.009	0.0005	0.030	0.030	0.009	0.0042	0.03		0.10			0.0020		735	991	Compliant steel
Z	0.058	0.03	1.94	0.007	0.0006	0.033	0.038	0.030	0.0078		0.02	027					732	990	Compliant steel
AA	0.040	0.22	2.20	0.005	0.0008	0.027	0.041	0.010	0.0045			0.03					721	966	Compliant steel
AB	0.061	0.15	1.88	0.004	0.0007	0.038	0.027	0.011	0.0029			0.15	0.01				738	963	Compliant steel
AC	0.065	0.18	2.01	0.011	0.0018	0.029	0.035	0.012	0.0040				0.10	0.005			721	988	Compliant steel
AD	0.055	0.30	2.08	0.006	0.0009	0.060	0.037	0.011	0.0041				0.15		0.0007		715	980	Compliant steel
AE	0.063	0.24	1.99	0.012	0.0012	0.039	0.060	0.013	0.0043				021			0.0025	714	1027	Compliant steel

Note 1: Balance is Fe and incidental impurities.
Note 2: Underlines indicate the items outside the range of the present invention.
Note 3: Blanks indicate no deliberate addition was made.

[Table 2]

No.	Steel grade	Final plate thickness (mm)	Semifinished steel heating temperature (°C)	Hot rolling step			Cooling step				Classification
No.	Steel grade	Final plate thickness (mm)	Semifinished steel heating temperature (°C)	Total rolling reduction in recrystallization temperature range (%)	No. of rolling passes with a rolling reduction of 20%/pass or more (-)	Rolling finish temperature (°C)	Steel plate surface temperature at start of cooling (°C)	Steel plate surface temperature at start of cooling - Ar₃ transformation point (°C)	Average cooling rate at plate thickness center (°C/s)	Cooling stop temperature at plate thickness center (°C)	Classification
1	A	18	1150	65	3	807	742	21	34	459	Example
2	B	14	1200	69	0	725	659	-69	36	430	Example
3	C	22	1090	53	2	782	730	13	32	469	Example
4	D	12	1250	75	5	826	732	6	36	520	Example
5	E	12	1220	72	3	797	706	0	37	547	Example
6	F	39	1000	37	1	734	720	-26	20	250	Example
7	G	30	1060	46	4	810	781	55	28	449	Example
8	H	25	1080	51	2	775	728	23	31	472	Example
9	I	36	1030	42	1	766	743	-2	24	431	Example
10	J	33	1040	46	1	753	714	-20	25	366	Example
11	K	12	1240	71	5	831	745	-2	35	451	Comparative Example
12	L	18	1160	60	4	828	753	26	33	522	Comparative Example
13	M	15	1180	66	2	789	707	-9	35	417	Comparative Example
14	N	12	1230	73	4	811	720	-54	34	498	Comparative Example
15	O	15	1170	69	3	799	714	21	33	430	Comparative Example
16	P	25	1070	48	3	812	768	33	31	486	Comparative Example
17	Q	39	1010	38	4	795	783	78	21	434	Comparative Example
18	R	30	1070	55	1	734	705	-5	27	418	Comparative Example
19	S	15	1190	67	3	786	709	-11	35	437	Comparative Example
20	T	18	1140	62	5	840	780	50	34	463	Comparative Example
21	U	20	1020	58	2	762	709	-13	32	399	Comparative Example
22	V	15	1180	70	3	791	700	-46	34	472	Comparative Example
23	W	36	1030	45	1	758	734	22	23	385	Comparative Example
24	A	28	990	46	0	716	685	-36	29	502	Comparative Example
25	B	24	1260	51	5	853	802	74	31	511	Comparative Example
26	C	32	1050	78	4	792	756	39	25	479	Comparative Example
27	A	38	1020	42	6	767	751	30	22	375	Comparative Example
28	C	27	1090	46	4	825	786	69	27	578	Comparative Example
29	X	18	1140	63	4	815	749	22	32	444	Example
30	Y	15	1050	38	4	800	729	-6	37	456	Example
31	Z	28	1080	52	2	772	736	4	30	432	Example
32	AA	22	1100	62	1	799	746	25	33	475	Example
33	AB	18	1030	42	3	770	710	-28	35	406	Example
34	AC	22	1060	44	3	781	730	9	33	415	Example
35	AD	25	1080	55	2	777	732	17	31	420	Example
36	AE	28	1150	60	1	786	753	39	32	461	Example

Note 1: Underlines are items outside the range of the present invention.

[Table 3]

No.	Area fraction of TD//{001} (%)	MA fraction (%)	Yield strength (MPa)	Tensile strength (MPa)	Fracture toughness K_IH (MPa·m^1/2)	Classification
1	5	2	584	658	○	Example
2	4	2	559	665	○	Example
3	5	3	591	666	○	Example
4	10	4	561	632	○	Example
5	7	1	577	650	○	Example
6	3	2	586	692	○	Example
7	5	5	595	672	○	Example
8	4	3	601	679	○	Example
9	4	1	625	702	○	Example
10	4	1	577	687	○	Example
11	9	2	487	554	○	Comparative Example
12	5	4	576	653	×	Comparative Example
13	5	5	565	659	×	Comparative Example
14	9	3	415	500	○	Comparative Example
15	5	2	630	707	×	Comparative Example
16	5	3	621	700	×	Comparative Example
17	4	2	633	711	×	Comparative Example
18	5	3	594	697	×	Comparative Example
19	5	2	576	670	×	Comparative Example
20	7	2	604	683	×	Comparative Example
21	4	1	583	682	×	Comparative Example
22	8	1	461	550	○	Comparative Example
23	4	1	647	726	×	Comparative Example
24	3	4	510	611	○	Comparative Example
25	7	4	539	615	○	Comparative Example
26	12	3	592	663	×	Comparative Example
27	11	2	580	651	×	Comparative Example
28	5	6	578	651	×	Comparative Example
29	5	3	581	669	○	Example
30	4	1	560	656	○	Example
31	4	2	595	683	○	Example
32	5	3	579	661	○	Example
33	5	2	565	657	○	Example
34	3	3	587	662	○	Example
35	6	4	591	670	○	Example
36	4	5	598	684	○	Example

Note 1: Underlines are items outside the range of the present invention and outside the target range.

As shown in Table 2, Nos. 1 to 10 and Nos. 29 to 36 are the examples of the present invention in which the chemical composition and the production conditions satisfy the appropriate ranges of the present invention. As shown in Table 3, in all of Nos. 1 to 10 and Nos. 29 to 36, the area fraction of TD//{001} at the plate thickness center position as the high-strength steel plate was 10% or less, the MA fraction in the microstructure at the plate thickness center was 5% or less, the tensile strength was 625 MPa or higher, and the fracture toughness in the 21 MPa high-pressure hydrogen gas was excellent (O).
Here, Nos. 1, 3 to 5, 7, 8, 29, 31, 32, and 34 to 36 had microstructures constituted by bainite and MA, and Nos. 2, 6, 9, 10, 30, and 33 had microstructures constituted by bainite, ferrite, and MA.
In contrast, in Nos. 11 to 23, the chemical composition of the steel plate was outside the range of the present invention. In Nos. 11, 14 and 22, the tensile strength was insufficient. In Nos. 12, 13, 15 to 21, and 23, the fracture toughness was poor (the fracture toughness was ×).
Nos. 24 to 28 were comparative examples in which the chemical composition was inside the range of the present invention but the production conditions were outside the range of the present invention. In Nos. 24 and 25, the tensile strength was insufficient. In Nos. 26 and 27, the area fraction of TD//{001} exceeded 10%, and thus the fracture toughness was poor. In No. 28, the MA fraction exceeded 5%, and thus the fracture toughness was poor.
Here, Nos. 12, 15 to 17, 20, 23, and 25 to 28 had microstructures constituted by bainite and MA, and Nos. 11, 13, 14, 18, 19, 21, 22, and 24 had microstructures constituted by bainite, ferrite, and MA.

Claims

A high-strength steel plate for a hydrogen transporting steel pipe, the high-strength steel plate having a chemical composition comprising, in terms of mass%,
C: 0.040 to 0.090%,

Si: 0.01 to 0.50%,

Mn: 1.50 to 2.50%,

P: 0.002 to 0.020%,

S: 0.0002 to 0.0020%,

Al: 0.010 to 0.080%,

Nb: 0.005 to 0.080%,

Ti: 0.005 to 0.050%,

N: 0.0020 to 0.0080%, and

at least one selected from

Cu: 0.50% or less,

Ni: 0.50% or less,

Cr: 0.50% or less,

Mo: 0.50% or less,

V: 0.050% or less,

Ca: 0.0050% or less, and

Mg: 0.0050% or less,

with the balance being Fe and incidental impurity elements,

wherein, at a plate thickness center position of the steel plate, an area fraction of a TD//{001} crystal plane is 10% or less where TD is a direction normal to a side surface of the steel plate during rolling, and MA is 5% or less in terms of area fraction, and

the steel plate has a tensile strength of 625 MPa or higher.
The high-strength steel plate for a hydrogen transporting steel pipe according to Claim 1, wherein the chemical composition comprises, in terms of mass%, at least one selected from
Cu: 0.01 to 0.50%,

Ni: 0.01 to 0.50%,

Cr: 0.01 to 0.50%,

Mo: 0.01 to 0.50%,

V: 0.005 to 0.050%,

Ca: 0.0005 to 0.0050%, and

Mg: 0.0005 to 0.0050%.
A method for producing a high-strength steel plate for a hydrogen transporting steel pipe, the method comprising:
heating a semifinished steel having the chemical composition according to Claim 1 or 2 to a temperature of 1000 to 1250°C;

hot-rolling the heated semifinished steel into a steel plate under such conditions that a total rolling reduction in a recrystallization temperature range is 75% or less and the number of rolling passes with a per-pass rolling reduction of 20% or more is 5 or less; and

subsequently water-cooling the steel plate under such conditions that a cooling stop temperature in terms of a steel plate temperature at a plate thickness center of the steel plate is 250 to 550°C.
A hydrogen transporting steel pipe comprising the high-strength steel plate for a hydrogen transporting steel pipe according to Claim 1 or 2.