STEEL MATERIAL FOR HIGH-PRESSURE HYDROGEN GAS ENVIRONMENT, STEEL STRUCTURE FOR HIGH-PRESSURE HYDROGEN GAS ENVIRONMENT, AND METHODS FOR PRODUCING STEEL MATERIAL FOR HIGH-PRESSURE HYDROGEN GAS ENVIRONMENT

Abstract

A steel material and methods for producing the same. The steel material exhibits excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment and is, therefore, suitable for use in hydrogen storage tanks, hydrogen line pipes, and the like. The steel material has a specified chemical composition, a tensile strength of 560 MPa or higher, and a fracture toughness value K.sub.IH exhibited by the steel material in a high-pressure hydrogen gas atmosphere is 40 MPa.Math.m.sup.1/2 or higher.

Claims

1. A steel material for a high-pressure hydrogen gas environment, the steel material having a chemical composition comprising, by mass %: C: 0.04 to 0.50%; Si: 0.5 to 2.0%; Mn: 0.5 to 2.0%; P: 0.05% or less; S: 0.010% or less; N: 0.0005 to 0.0080%; Al: 0.010% to 2.0%; O: 0.0100% or less; Cu: 0.5 to 2.0%; and the balance being Fe and incidental impurities, wherein the steel material has a tensile strength of 560 MPa or higher, and a fracture toughness value K.sub.IH exhibited by the steel material in a high-pressure hydrogen gas atmosphere is 40 MPa.Math.m.sup.1/2 or higher.

2. The steel material for a high-pressure hydrogen gas environment according to claim 1, wherein the chemical composition further comprises, by mass %, Al: 0.5 to 2.0%.

3. The steel material for a high-pressure hydrogen gas environment according to claim 1, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one element selected from the group consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, and Group B: at least one element selected from the group consisting of, by mass %, Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.

4. (canceled)

5. A steel structure for a high-pressure hydrogen gas environment, the steel structure comprising the steel material according to claim 1.

6. The steel structure for a high-pressure hydrogen gas environment according to claim 5, wherein the steel structure is a storage tank or a line pipe.

7. A method for producing a steel material for a high-pressure hydrogen gas environment according to claim 1, the method comprising: heating a steel starting material having the chemical composition to a temperature of Ac.sub.3 transformation temperature or higher and then hot-rolling the steel starting material to form the steel material having a predetermined shape; and thereafter subjecting the steel material to an accelerated cooling process, in which the steel material is cooled from a temperature of (Ar.sub.3 transformation temperature—50° C.) or higher to a cooling stop temperature of 600° C. or lower at a cooling rate in a range of 1 to 200° C./s.

8. A method for producing a steel material for a high-pressure hydrogen gas environment according to claim 1, the method comprising: heating a steel starting material having the chemical composition to a temperature of Ac.sub.3 transformation temperature or higher and then hot-rolling the steel starting material to form the steel material having a predetermined shape; thereafter subjecting the steel material to a direct quenching-tempering process, in which the steel material is cooled from a temperature of (Ar.sub.3 transformation temperature—50° C.) or higher to a cooling stop temperature of 250° C. or lower at a cooling rate in a range of 1 to 200° C./s; and tempering the steel material at a temperature of Ac.sub.1 transformation temperature or lower.

9. A method for producing a steel material for a high-pressure hydrogen gas environment according to claim 1, the method comprising: subjecting the steel material having the chemical composition and being formed to have a predetermined shape, to a reheating-quenching-tempering process, in which the steel material is heated to a temperature of Ac.sub.3 transformation temperature or higher; subsequently subjecting the steel material to water quenching or oil quenching; and tempering the steel material at a temperature of Ac.sub.1 transformation temperature or lower.

10. The method for producing a steel material for a high-pressure hydrogen gas environment according to claim 9, wherein the chemical composition further comprises, by mass %, Al: 0.5 to 2.0%.

11-12. (canceled)

13. The steel material for a high-pressure hydrogen gas environment according to claim 2, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one element selected from the group consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, and Group B: at least one element selected from the group consisting of, by mass %, Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.

14. A steel structure for a high-pressure hydrogen gas environment, the steel structure comprising the steel material according to claim 2.

15. A steel structure for a high-pressure hydrogen gas environment, the steel structure comprising the steel material according to claim 3.

16. A steel structure for a high-pressure hydrogen gas environment, the steel structure comprising the steel material according to claim 13.

17. The steel structure for a high-pressure hydrogen gas environment according to claim 14, wherein the steel structure is a storage tank or a line pipe.

18. The steel structure for a high-pressure hydrogen gas environment according to claim 15, wherein the steel structure is a storage tank or a line pipe.

19. The steel structure for a high-pressure hydrogen gas environment according to claim 16, wherein the steel structure is a storage tank or a line pipe.

20. The method for producing a steel material for a high-pressure hydrogen gas environment according to claim 7, wherein the chemical composition further comprises, by mass %, Al: 0.5 to 2.0%.

21. The method for producing a steel material for a high-pressure hydrogen gas environment according to claim 7, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one element selected from the group consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, and Group B: at least one element selected from the group consisting of, by mass %, Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.

22. The method for producing a steel material for a high-pressure hydrogen gas environment according to claim 20, wherein the chemical composition further comprises at least one group selected from the following groups: Group A: at least one element selected from the group consisting of, by mass %, Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050%, and Group B: at least one element selected from the group consisting of, by mass %, Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and REM: 0.0005 to 0.0050%.

23. The method for producing a steel material for a high-pressure hydrogen gas environment according to claim 8, wherein the chemical composition further comprises, by mass %, Al: 0.5 to 2.0%.

Description

DETAILED DESCRIPTION

[0038] A steel material of the disclosed embodiments has, as a basic composition, a composition containing, in mass %, C: 0.04 to 0.50%, Si: 0.5 to 2.0%, Mn: 0.5 to 2.0%, P: 0.05% or less, S: 0.010% or less, N: 0.0005 to 0.0080%, Al: 0.010% to 2.0%, O: 0.0100% or less, and Cu: 0.5 to 2.0%, with the balance being Fe and incidental impurities.

[0039] First, reasons for the limitations on the composition of the steel material of the disclosed embodiments will be described. Note that in the following description, “mass %” in the context of a composition will be denoted simply as “%”.

[0040] Studies performed by the inventor discovered that when a material is deformed in a hydrogen gas, Si, Cu, and Al, with Al being optional, enable the resulting dislocation to have a vein structure, thereby producing an effect of increasing the fracture toughness value K.sub.IH exhibited in hydrogen gas. Accordingly, hydrogen embrittlement resistance is improved. This effect is noticeable at least in instances in which Si and Cu are included each in an amount of 0.5% or more, and the effect is more noticeable in instances in which Al is optionally included in an amount of 0.5% or more. Hence, in the disclosed embodiments, Si: 0.5 to 2.0% and Cu: 0.5 to 2.0% are included, and, optionally, Al: 0.5 to 2.0% is included.

Si: 0.5 to 2.0%

[0041] Similar to Cu and Al, Si is an element that improves hydrogen embrittlement resistance. In the disclosed embodiments, Si is to be present in an amount more than or equal to 0.5%. On the other hand, if a large amount of Si is present, that is, an amount more than 2.0%, the grain boundaries become brittle, and, therefore, toughness is degraded. Accordingly, the amount of Si is limited to a range of 0.5 to 2.0%. Note that the amount is preferably more than or equal to 0.75% and less than or equal to 2.00%. More preferably, the amount is more than or equal to 1.00%.

Cu: 0.5 to 2.0%

[0042] Similar to Si and Al, Cu is an element that improves hydrogen embrittlement resistance. In the disclosed embodiments, Cu is to be present in an amount more than or equal to 0.5%. On the other hand, if a large amount of Cu is present, that is, an amount more than 2.0%, susceptibility to hot cracking, which may occur during heating or welding, is increased. Accordingly, the amount of Cu is limited to a range of 0.5 to 2.0%. Note that the amount is preferably more than or equal to 0.75% and less than or equal to 2.00%. More preferably, the amount is more than or equal to 1.00%.

Al: 0.010% to 2.0%

[0043] Similar to Si and Cu, Al is an element that contributes to improving hydrogen embrittlement resistance. Even when being present in a relatively small amount, Al acts as a deoxidizer and forms a nitride AlN to inhibit the coarsening of grains, which may occur during heating, thereby contributing to refining a microstructure. To produce these effects, the amount of Al is specified to be more than or equal to 0.010% in the disclosed embodiments. On the other hand, if a large amount of Al is present, that is, an amount more than 2.0%, susceptibility to surface defects in the steel material is increased. Note that in terms of noticeably improving hydrogen embrittlement resistance, it is preferable that the amount of Al be more than or equal to 0.5% and less than or equal to 2.0%. More preferably, the amount is more than or equal to 0.75%, and even more preferably more than or equal to 1.00%.

[0044] Note that reasons for the limitations on the components other than Si, Cu, or Al are as follows.

C: 0.04 to 0.50%

[0045] C is an element that contributes to increasing strength and improves hardenability. C needs to be present in an amount more than or equal to 0.04% so that a desired strength and hardenability can be ensured. On the other hand, if C is present in an amount more than 0.50%, weldability is significantly degraded, and the toughness of the base metal and a weld heat affected zone is degraded. Accordingly, the amount of C is limited to a range of 0.04 to 0.50%. Note that the amount is preferably more than or equal to 0.10% and less than or equal to 0.45%.

Mn: 0.5 to 2.0%

[0046] Mn is an element that contributes to increasing strength by improving hardenability. Producing this effect requires the presence of Mn in an amount more than or equal to 0.5%. However, if Mn is present in an amount more than 2.0%, grain boundary strength is degraded, and, therefore, low-temperature toughness is degraded. Accordingly, the amount of Mn is limited to a range of 0.5 to 2.0%. Note that the amount is preferably more than or equal to 0.8% and less than or equal to 1.5%.

P: 0.05% or Less

[0047] P tends to be segregated at grain boundaries and the like, which degrades the bonding strength of grains, which in turn degrades toughness. Accordingly, it is desirable that an amount of P be as low as possible; a permissible amount of P is up to 0.05%. Hence, the amount of P is limited to less than or equal to 0.05%.

S: 0.010% or Less

[0048] S tends to be segregated at grain boundaries and tends to form MnS, which is a non-metallic inclusion; consequently, ductility and toughness are degraded. Accordingly, it is desirable that an amount of S be as low as possible; a permissible amount of S is up to 0.010%. Hence, the amount of S is limited to less than or equal to 0.010%.

N: 0.0005 to 0.0080%

[0049] N combines with nitride-forming elements, such as Nb, Ti, and Al, to form nitrides, which pin austenite grains to inhibit the coarsening of grains during heating; therefore, N has an effect of refining the microstructure. Producing the microstructure-refining effect requires the presence of N in an amount more than or equal to 0.0005%. On the other hand, if N is present in an amount more than 0.0080%, an amount of dissolved N is increased, and, consequently, the toughness of the base metal and a weld heat affected zone is degraded. Accordingly, the amount of N is limited to a range of 0.0005 to 0.0080%. Note that the amount is preferably more than or equal to 0.0020% and less than or equal to 0.0050%.

O: 0.0100% or Less

[0050] O (oxygen) increases an amount of non-metallic inclusions by forming oxides, such as alumina, which results in degradation of workability, for example, degradation of ductility. Accordingly, it is desirable that an amount of O (oxygen) be as low as possible; a permissible amount of O (oxygen) is up to 0.0100%. Accordingly, the amount of O (oxygen) is limited to less than or equal to 0.0100%. Note that the amount is preferably less than or equal to 0.0050%.

[0051] The basic composition described above includes the components described above. In addition to the basic composition described above, any of the following optional elements may be selected and included: one or more selected from Ni: 0.05 to 2.00%, Cr: 0.10 to 2.50%, Mo: 0.05 to 2.00%, W: 0.05 to 2.00%, Nb: 0.005 to 0.100%, V: 0.005 to 0.200%, Ti: 0.005 to 0.100%, and B: 0.0005 to 0.0050% and/or one or more selected from Nd: 0.005 to 1.000%, Ca: 0.0005 to 0.0050%, Mg: 0.0005 to 0.0050%, and one or more REMs: 0.0005 to 0.0050%.

[0052] Ni, Cr, Mo, W, Nb, V, Ti, and B are all elements that contribute to improving hardenability. One or more of these may be selected as necessary and included.

Ni: 0.05 to 2.00%

[0053] Ni is an element that has an effect of improving toughness in addition to improving hardenability. Producing these effects requires the presence of Ni in an amount more than or equal to 0.05%. On the other hand, if Ni is present in an amount more than 2.00%, a material cost is increased, and, therefore, an economic advantage is reduced. Accordingly, in instances where Ni is to be present, it is preferable that the amount of Ni be limited to a range of 0.05 to 2.00%. Note that the amount is more preferably more than or equal to 0.50% and less than or equal to 1.50%.

Cr: 0.10 to 2.50%

[0054] Cr is an element that contributes to ensuring a strength by improving hardenability. Producing this effect requires the presence of Cr in an amount more than or equal to 0.10%. On the other hand, if a large amount of Cr is present, that is, an amount more than 2.50%, weldability is degraded. Accordingly, in cases where Cr is to be present, it is preferable that the amount of Cr is limited to a range of 0.10 to 2.50%. Note that the amount is more preferably more than or equal to 0.50% and less than or equal to 1.50%.

Mo: 0.05 to 2.00%

[0055] Mo is an element that contributes to ensuring a strength by improving hardenability. Producing this effect requires the presence of Mo in an amount more than or equal to 0.05%. On the other hand, if a large amount of Mo is present, that is, an amount more than 2.00%, a material cost is increased, and, therefore, an economic advantage is reduced. Accordingly, in instances where Mo is to be present, it is preferable that the amount of Mo be limited to a range of 0.05 to 2.00%. Note that the amount is more preferably more than or equal to 0.20% and less than or equal to 1.50%.

W: 0.05 to 2.00%

[0056] W is an element that contributes to ensuring a strength by improving hardenability. Producing this effect requires the presence of W in an amount more than or equal to 0.05%. On the other hand, if a large amount of W is present, that is, an amount more than 2.00%, weldability is degraded. Accordingly, in instances where W is to be present, it is preferable that the amount of W be limited to 0.05 to 2.00%. Note that the amount is more preferably more than or equal to 0.20% and less than or equal to 1.50%.

Nb: 0.005 to 0.100%

[0057] Nb is an element that has an effect of improving hardenability and, in addition, an effect of inhibiting the coarsening of grains during heating as Nb is finely precipitated as carbonitrides to pin austenite grains. These effects can be observed in cases in which Nb is present in an amount more than or equal to 0.005%. On the other hand, if Nb is present in an amount more than 0.100%, the toughness of a weld heat affected zone is degraded. Accordingly, in instances where Nb is to be present, it is preferable that the amount of Nb be limited to a range of 0.005 to 0.100%. Note that the amount is more preferably more than or equal to 0.010% and less than or equal to 0.050%.

V: 0.005 to 0.200%

[0058] V is an element that has an effect of improving hardenability and, in addition, an effect of inhibiting the coarsening of grains during heating as V is finely precipitated as carbonitrides to pin austenite grains. These effects can be observed in cases in which V is present in an amount more than or equal to 0.005%. On the other hand, if V is present in an amount more than 0.200%, the toughness of a weld heat affected zone is degraded. Accordingly, in instances where V is to be present, it is preferable that the amount of V be limited to a range of 0.005 to 0.200%. Note that the amount is more preferably more than or equal to 0.010% and less than or equal to 0.150%.

Ti: 0.005 to 0.100%

[0059] Ti is an element that has an effect of improving hardenability and, in addition, an effect of inhibiting the coarsening of grains during heating as Ti is finely precipitated as carbonitrides to pin austenite grains. These effects can be observed in cases in which Ti is present in an amount more than or equal to 0.005%. On the other hand, if Ti is present in an amount more than 0.100%, the toughness of a weld heat affected zone is degraded. Accordingly, in instances where Ti is to be present, it is preferable that the amount of Ti be limited to a range of 0.005 to 0.100%. Note that the amount is more preferably more than or equal to 0.010% and less than or equal to 0.050%.

B: 0.0005 to 0.0050%

[0060] B is an element that contributes to improving hardenability even when B is present in a small amount. Producing this effect requires the presence of B in an amount more than or equal to 0.0005%. On the other hand, if B is present in an amount more than 0.0050%, toughness is degraded. Accordingly, in instances where B is to be present, it is preferable that the amount of B be limited to a range of 0.0005 to 0.0050%. Note that the amount is more preferably more than or equal to 0.0010% and less than or equal to 0.0020%.

[0061] Furthermore, Nd, Ca, Mg, and REMs are all elements that contribute to improving ductility, toughness, and hydrogen embrittlement resistance by controlling the morphology of inclusions. One or more of these may be selected as necessary and included.

Nd: 0.005 to 1.000%

[0062] Nd is an element that combines with S to form sulfide inclusions, thereby reducing an amount of grain boundary segregation of S and thus contributing to improving toughness and hydrogen brittleness resistance. Producing this effect requires the presence of Nd in an amount more than or equal to 0.005%. On the other hand, if Nd is present in an amount more than 1.000%, the toughness of a weld heat affected zone is degraded. Accordingly, in instances where Nd is to be present, it is preferable that the amount of Nd be limited to a range of 0.005 to 1.000%. Note that the amount is more preferably more than or equal to 0.010% and less than or equal to 0.500%.

Ca: 0.0005 to 0.0050%

[0063] Ca has high affinity for S and forms CaS in place of MnS; CaS is a globular sulfide inclusion, which is not easily elongated in rolling as opposed to MnS, which is a sulfide inclusion that is easily elongated in rolling. Accordingly, Ca is an element that contributes to controlling the morphology of sulfide inclusions and has an effect of improving ductility and toughness. Producing this effect requires the presence of Ca in an amount more than or equal to 0.0005%. On the other hand, if Ca is present in an amount more than 0.0050%, cleanliness is degraded, and ductility, toughness, and the like are degraded. Accordingly, in instances where Ca is to be present, it is preferable that the amount of Ca be limited to a range of 0.0005 to 0.0050%. Note that the amount is more preferably more than or equal to 0.0010% and less than or equal to 0.0020%.

Mg: 0.0005 to 0.0050%

[0064] Similar to Ca, Mg has high affinity for S and forms sulfide inclusions, thereby improving ductility and toughness. Producing this effect requires the presence of Mg in an amount more than or equal to 0.0005%. On the other hand, if Mg is present in an amount more than 0.0050%, cleanliness is degraded. Accordingly, in instances where Mg is to be present, it is preferable that the amount of Mg be limited to a range of 0.0005 to 0.0050%. Note that the amount is more preferably more than or equal to 0.0010% and less than or equal to 0.0020%.

One or More REMs: 0.0005 to 0.0050%

[0065] REMs are elements that form sulfide inclusions, such as REM(O, S), to reduce an amount of dissolved S at grain boundaries, thereby contributing to improving SR cracking resistance. Producing this effect requires the presence of one or more REMs in an amount more than or equal to 0.0005%. On the other hand, if one or more REMs are present in an amount more than 0.0050%, large amounts of REM sulfide inclusions accumulate in a sedimentation zone during casting, and, consequently, material properties, such as ductility and toughness, are degraded. Accordingly, in instances where one or more REMs are to be present, it is preferable that the amount of one or more REMs be limited to a range of 0.0005 to 0.0050%. Note that the amount is more preferably more than or equal to 0.0010% and less than or equal to 0.0020%. Note that as used herein, the term “REM” is an abbreviation of “rare earth metal”.

[0066] The balance, other than the components described above, is Fe and incidental impurities.

[0067] Steel materials for a high-pressure hydrogen gas environment of the disclosed embodiments are steel materials that have the composition described above and have a microstructure formed of a combination of ferrite and pearlite or formed of lower bainite, martensite, tempered lower bainite, tempered martensite, or a combination of any of these.

[0068] Furthermore, the steel materials for a high-pressure hydrogen gas environment of the disclosed embodiments are steel materials that have the composition described above and the microstructure described above, the steel materials have a high strength of 560 MPa or higher in terms of tensile strength, and a fracture toughness value K.sub.IH exhibited by the steel materials in a high-pressure hydrogen gas atmosphere is 40 MPa.Math.m.sup.1/2 or higher; therefore, the steel materials have excellent hydrogen embrittlement resistance.

[0069] Now, preferred methods of the disclosed embodiments for producing a steel material for a high-pressure hydrogen gas environment will be described.

[0070] First, molten steel having the composition described above is produced in a common steel-making furnace, such as a converter or an electric furnace, and the molten steel is subjected to a continuous casting process to form a cast steel having a predetermined shape, such as a slab, or the molten steel is subjected to an ingot casting process or the like, in which a cast steel (steel ingot) is hot-rolled to form a workpiece having a predetermined shape, such as a slab; accordingly, a steel starting material is formed.

[0071] Subsequently, the obtained steel starting material is loaded into a heating furnace. A heating temperature is Ac.sub.3 transformation temperature or higher. If the heating temperature is less than Ac.sub.3 transformation temperature, the material to be rolled has a high deformation resistance, which results in an excessive load on a rolling machine, and in addition, a partial untransformed constituent remains; consequently, the desired characteristics cannot be ensured even with subsequent processing. Note that the heating temperature is preferably 1100 to 1300° C. If the heating temperature is less than 1100° C., the deformation resistance is high, which results in an excessively high load on a rolling machine. On the other hand, if the heating temperature is higher than 1300° C., coarsening of grains occurs, which results in degraded toughness.

[0072] Subsequently, the steel starting material, which has been heated to a predetermined temperature, is subjected to hot rolling to form a steel material having a predetermined size and shape. As used herein, the term “steel material” encompasses sheets, plates, steel pipes, shaped steels, steel bars, and the like. Furthermore, as used herein, the term “hot rolling” is not meant to specify any particular rolling conditions; it is sufficient that the hot rolling can form a steel material having a predetermined size and shape. In instances where the steel material is a seamless steel pipe, the hot rolling is rolling that includes piercing rolling.

[0073] It is preferable that the steel material rolled to have a predetermined size and shape be processed in any of the following manners: the steel material is allowed to cool to room temperature and, after having been cooled, subjected to a reheating-quenching-tempering process, in which the steel material is reheated, quenched, and tempered; after the hot rolling, the steel material is subjected to an accelerated cooling process; or after the hot rolling, the steel material is subjected to a direct quenching-tempering process.

[0074] Now, the accelerated cooling process, the direct quenching-tempering process, and the reheating-quenching-tempering process will be described individually.

[0075] Note that the temperature specified in the production conditions is a temperature of a middle portion of the steel material. In instances where the steel material is a sheet, plate, steel pipe, or shaped steels, the middle portion is a middle of a thickness (wall thickness), and in instances where the steel material is a steel bar, the middle portion is a middle in a radial direction. Since the vicinity of the middle portion has a substantially uniform temperature history, the temperature specified is not limited to the temperature of the exact middle.

Accelerated Cooling Process

[0076] The steel material rolled to have a predetermined size and shape is thereafter, without being cooled to room temperature, subjected to an accelerated cooling process; in the accelerated cooling process, the steel material is cooled from a cooling start temperature of (Ar.sub.3 transformation temperature—50° C.) or higher to a cooling stop temperature of 600° C. or lower at a cooling rate of 1 to 200° C./s. If the cooling start temperature is less than (Ar.sub.3 transformation temperature—50° C.), an amount of transformation of austenite is increased before the start of the cooling, and, consequently, the characteristics that exist after the accelerated cooling are not the desired ones. Accordingly, the cooling start temperature is limited to a temperature of (Ar.sub.3 transformation temperature—50° C.) or higher. Furthermore, if the cooling rate for the accelerated cooling is less than 1° C./s, the cooling is too slow, and, consequently, the desired characteristics cannot be ensured. On the other hand, in cases where a typical cooling method is used, the cooling rate does not exceed 200° C./s. Accordingly, the cooling rate for the accelerated cooling process is limited to the range of 1 to 200° C./s. Note that the cooling rate is an average cooling rate in a thickness (wall thickness) middle. The means for the cooling need not be particularly limited, and it is preferable that water cooling, for example, be used. Furthermore, if the cooling stop temperature for the accelerated cooling is a high temperature of higher than 600° C., a desired transformation is not accomplished, and, therefore, the desired characteristics cannot be ensured. Accordingly, the cooling stop temperature for the accelerated cooling is limited to a temperature of 600° C. or lower.

Direct Quenching-Tempering Process

[0077] The steel starting material is heated to a temperature of the Ac.sub.3 transformation temperature or higher and hot-rolled to form a steel material having a predetermined size and shape; thereafter, the steel material is subjected to a quenching process, in which the steel material is cooled from a temperature of (Ar.sub.3 transformation temperature—50° C.) or higher to a cooling stop temperature of 250° C. or lower at a cooling rate of 1 to 200° C./s; and thereafter, the steel material is subjected to a tempering process, in which the steel material is tempered at a tempering temperature of Ac.sub.1 transformation temperature or lower. If the heating temperature for the steel starting material is less than the Ac.sub.3 transformation temperature, a partial untransformed constituent remains, and consequently, the microstructure that exists after the hot rolling and the quenching-tempering is not a desired one. Accordingly, the heating temperature before the hot rolling is specified to be Ac.sub.3 transformation temperature or higher. Furthermore, if the starting temperature for the quenching after the hot rolling is less than (Ar.sub.3 transformation temperature—50° C.), the amount of transformation of austenite before the quenching is increased, and consequently, the microstructure that exists after the quenching-tempering is not a desired one. Accordingly, after the hot rolling, the cooling is to be started at a temperature of (Ar.sub.3 transformation temperature—50° C.) or higher to carry out the quenching. For the quenching that starts at (Ar.sub.3 transformation temperature—50° C.) or higher, the cooling rate is specified to be 1 to 200° C./s to obtain a desired microstructure. Note that the cooling rate is an average cooling rate in a thickness middle. The means for the cooling need not be particularly limited, and, for example, water cooling may be used. Furthermore, if the cooling rate for the quenching is less than 1° C./s, the cooling is too slow, and, consequently, the desired characteristics cannot be ensured. On the other hand, in cases where a typical cooling method is used, the cooling rate does not exceed 200° C./s. Furthermore, if the quenching is stopped at a temperature of higher than 250° C., desired martensitic transformation and/or bainitic transformation are not accomplished, and, consequently, the characteristics that exist after the tempering are not the desired ones. Accordingly, in the quenching process, the cooling is to be performed until a temperature of 250° C. or lower is reached. After the quenching, the steel material is tempered at a temperature of Ac.sub.1 transformation temperature or lower. If the tempering temperature is higher than Ac.sub.1 transformation temperature, partial austenite transformation occurs, and, consequently, the characteristics that exist after the tempering are not the desired ones.

Reheating-Quenching-Tempering Process

[0078] The steel material rolled to have a predetermined size and shape is then cooled to room temperature, and subsequently, the steel material is subjected to a reheating-quenching-tempering process, in which the steel material is heated at a quenching heating temperature of Ac.sub.3 transformation temperature or higher; thereafter, the steel material is subjected to a quenching process, in which the steel material is cooled from a quenching start temperature of (Ar.sub.3 transformation temperature—50° C.) or higher to a temperature of 250° C. or lower at a cooling rate of 0.5 to 100° C./s; and subsequently, the steel material is tempered at a temperature of Ac.sub.1 transformation temperature or lower.

[0079] Note that it is preferable that the quenching process be carried out in the following manner: water or oil, for example, is used as a cooling medium, and the cooling medium is sprayed onto the steel material, which is the cooling target heated to a high temperature, in a manner such that a cooling rate of 0.5 to 100° C./s is achieved, or the heated steel material is immersed in a tank that holds the cooling medium. From the standpoint of achieving uniform cooling, it is preferable that in a tank that holds the cooling medium, the steel material having a predetermined size and shape be cooled by a jet stream of the cooling medium sprayed thereto while the steel material is rotated. Furthermore, regarding the tempering process, the steel material heated in a tempering heating furnace or the like may be allowed to cool in air or a protective atmosphere.

[0080] If the quenching heating temperature is less than Ac.sub.3 transformation temperature, a partial untransformed constituent remains, and, consequently, the characteristics that exist after the quenching-tempering are not the desired ones. Accordingly, the quenching heating temperature is specified to be Ac.sub.3 transformation temperature or higher. Furthermore, if the quenching start temperature is less than (Ar.sub.3 transformation temperature—50° C.), the transformation of the austenite begins before the start of the quenching, and, consequently, the characteristics that exist after the quenching-tempering are not the desired ones. Accordingly, the quenching start temperature is limited to a temperature of (Ar.sub.3 transformation temperature—50° C.) or higher. Furthermore, the cooling rate for the quenching is limited to 0.5 to 100° C./s to achieve the desired characteristics and prevent quench cracking. If the quench cooling stop temperature is a high temperature of higher than 250° C., a desired transformation (martensitic transformation or bainitic transformation) is not accomplished, and, consequently, the characteristics that exist after the tempering process are not the desired ones. Accordingly, the quench stop temperature is limited to a temperature of 250° C. or lower.

[0081] After the quenching process, a tempering process is performed in which the steel material is tempered by being heated to a tempering temperature of Ac.sub.1 transformation temperature or lower. If the tempering temperature is higher than Ac.sub.1 transformation temperature, partial austenite transformation occurs, and, consequently, the characteristics that exist after the tempering process are not the desired ones.

[0082] Note that Ac.sub.3 transformation temperature (° C.), Ar.sub.3 transformation temperature (° C.), and Ac.sub.1 transformation temperature (° C.) described above, which are used herein, are temperatures calculated by using the following equations.

Ac.sub.3 (° C.)=854−180C+44Si−14Mn−17.8Ni−1.7Cr

Ar.sub.3 (° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo

Ac.sub.1 (° C.)=723−14Mn+22Si−14.4Ni+23.3Cr

[0083] Here, the chemical symbols represent a content (mass %) of the element in the steel.

[0084] The steel material having excellent hydrogen embrittlement resistance produced by the production method described above is suitable for use in steel structures for hydrogen that are used in a high-pressure hydrogen gas environment. Examples of the “steel structures for hydrogen” as referred to herein include storage tanks (hydrogen storage tanks) that are used in hydrogen stations and the like; and line pipes for transportation of hydrogen gas (hydrogen line pipes).

[0085] As storage tanks that are used in hydrogen stations and the like, Type 1, Type 2, and Type 3 are known. Type 1 is made exclusively of a steel material, and Type 2 and Type 3 are made of a steel material and a carbon fiber reinforced plastic (CFRP) wound therearound. These types are based on classifications regarding a structure of the container, which are described in, for example, various standards for compressed natural gas vehicle fuel containers, ISO 11439, ANSI/NGV, the Container Safety Rules-Exemplified Standard-Appendix-9 of the High Pressure Gas Safety Act, and the like. Note that it is preferable that the storage tanks be produced, for example, by forming a steel material having the composition described above into a predetermined shape and subsequently subjecting the steel material to the reheating-quenching-tempering process. Note that the design pressure for the hydrogen to be stored in a storage tank is approximately 35 MPa or approximately 70 MPa.

[0086] Furthermore, as a line pipe for transportation of hydrogen, a seamless steel pipe, an electric resistance welded steel pipe, or a UOE-type steel pipe is suitable. Note that it is preferable that the line pipe be formed as follows: a steel material having the composition described above is used as it is to form a line pipe (steel pipe), or a steel starting material having the composition described above and subjected to the accelerated cooling process described above or the direct quenching process described above is used to form a steel pipe. Note that, in line pipes, the design pressure for the hydrogen to be used is approximately 10 MPa.

[0087] Now, the disclosed embodiments will be further described based on examples.

EXAMPLES

[0088] Molten steel having a composition as shown in Table 1 was produced in a converter and continuously cast to form cast steel (a slab, wall thickness: 250 mm). The obtained cast steel was heated and hot-rolled to form a steel plate (thickness: 38 mm), which was then cooled to room temperature. Subsequently, the steel plate was subjected to a reheating-quenching-tempering process under the conditions shown in Table 2 (steel plates No. 1 to No. 16 and No. 21 to No. 23). Note that the quenching process was carried out by using water cooling or oil cooling.

[0089] Furthermore, the obtained cast steel was heated under the condition shown in Table 2 and hot-rolled to form a steel plate having a predetermined thickness (38 mm), and thereafter, the steel plate was subjected to an accelerated cooling process, which was performed under the conditions shown in Table 2 (steel plates No. 17 and No. 18).

[0090] Furthermore, the obtained cast steel was heated under the condition shown in Table 2 and hot-rolled to form a steel plate having a predetermined thickness (38 mm). Thereafter, the steel plate was subjected to a direct quenching-tempering process, in which the steel plate was directly quenched under the conditions shown in Table 2 and was subsequently tempered at the tempering temperature shown in Table 2 (steel plates No. 19 and No. 20). Note that the temperature of the steel plate was measured by using a thermocouple inserted in a thickness middle portion.

[0091] The reheating-quenching-tempering process was intended to simulate the production of a hydrogen storage tank, and the accelerated cooling process and the direct quenching process were both intended to simulate the production of a hydrogen line pipe (steel pipe).

[0092] A tensile test, a fracture toughness test, and microstructure examination were conducted on the obtained steel plates. The test methods were as follows.

(1) Tensile Properties

[0093] A full-thickness tensile test piece was cut from the obtained steel plate in accordance with JIS Z 2201 (1980) such that a longitudinal direction (tensile direction) of the test piece coincided with a rolling direction, and a tensile test was conducted in accordance with the specifications of JIS Z 2241 to measure the tensile strength.

(2) Fracture Toughness Test

[0094] A CT test piece (width: 50.8 mm) was cut from each of the obtained steel plates such that the load application direction was parallel to the rolling direction. A fracture toughness test was conducted in a high-pressure hydrogen gas atmosphere in accordance with The Japan Pressure container Research Council, Division of Materials Science and Technology, Hydrogen Gas Embrittlement Technical Committee, Task Group V (1991). Accordingly, the fracture toughness value K.sub.IH was determined. Note that the test was conducted in a high-pressure hydrogen gas atmosphere at room temperature (20±10° C.) and a pressure of 115 MPa at a constant displacement speed of 2.5 μm/min.

[0095] Note that, in some instances, a fracture toughness test in accordance with the specifications of ASTM E399 or ASTM E1820 was also conducted to obtain fracture toughness values K.sub.IH. These fracture toughness values K.sub.IH were not shown in Table 2 because these fracture toughness values K.sub.IH were substantially equal to the fracture toughness values K.sub.IH obtained in the fracture toughness test conducted in accordance with The Japan Pressure container Research Council, Division of Materials Science and Technology, Hydrogen Gas Embrittlement Technical Committee, Task Group V (1991), with an error of not larger than 5%.

(3) Microstructure Examination

[0096] A test piece for microstructure examination was cut from a thickness middle portion of the obtained steel plate, the test piece was polished and etched (with a nital solution), and examination was conducted with an optical microscope (magnification: 200×). Accordingly, the constituents were identified, and the fractions of the constituents were calculated by the image analysis.

[0097] The results obtained are shown in Table 2.

TABLE-US-00001 TABLE 1 Steel Chemical components (mass %) No. C Si Mn P S Al N O Cu Ni, Cr, Mo, Nb, V, Ti, B Nd, W, Ca, Mg, REM Notes A 0.36 0.42 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Comparative Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 B 0.36 0.54 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 C 0.36 1.02 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 D 0.36 1.96 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 E 0.36 0.56 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.42 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Comparative Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 F 0.36 0.56 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.53 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 G 0.36 0.56 0.76 0.02 0.0031 0.051 0.0035 0.0033 0.99 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 H 0.36 0.56 0.76 0.02 0.0031 0.051 0.0035 0.0033 1.97 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 I 0.36 0.56 0.76 0.02 0.0031 0.008 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Comparative Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 J 0.36 0.56 0.76 0.02 0.0031 0.52 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 K 0.36 0.56 0.76 0.02 0.0031 0.97 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 L 0.36 0.56 0.76 0.02 0.0031 1.96 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012 Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 M 0.06 0.54 0.76 0.01 0.0031 0.051 0.0035 0.0033 0.53 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21, Conforming Mo: 1.08, Nb: 0.023, W: 0.15, Mg: 0.0006, example V: 0.041, Ti: 0.012, REM: 0.0005 B: 0.0010 N 0.06 0.54 0.76 0.01 0.0031 0.52 0.0035 0.0033 0.56 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21, Conforming Mo: 1.08, Nb: 0.023, W: 0.15, Mg: 0.0006, example V: 0.041, Ti: 0.012, REM: 0.0005 B: 0.0010 O 0.06 0.56 0.76 0.01 0.0031 0.52 0.0035 0.0033 0.53 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21, Conforming Mo: 1.08, Nb: 0.023, W: 0.15, Mg: 0.0006, example V: 0.041, Ti: 0.012, REM: 0.0005 B: 0.0010 P 0.06 0.54 0.76 0.01 0.0031 0.52 0.0035 0.0033 0.53 Ni: 0.32, Cr: 1.06, Ca: 0.0012, Nd: 0.21, Conforming Mo: 1.08, Nb: 0.023, W: 0.15, Mg: 0.0006, example V: 0.041, Ti: 0.012, REM: 0.0005 B: 0.0010 Q 0.48 1.02 1.82 0.01 0.0006 1.02 0.0035 0.0033 1.02 — — Conforming example R 0.48 1.02 1.82 0.01 0.0006 1.02 0.0035 0.0033 1.02 Ni: 0.32, Cr: 1.06, — Conforming Mo: 1.08, Nb: 0.023, example V: 0.041, Ti: 0.012, B: 0.0010 S 0.48 1.02 1.82 0.01 0.0006 1.02 0.0035 0.0033 1.02 — Ca: 0.0012 Conforming example The underline indicates the value is outside the range of the disclosed embodiments.

TABLE-US-00002 TABLE 2 Test results Heating Cooling Fracture Steel Transformation Thickness of Type of Heating Cooling start Cooling stop Tempering Constituent** and Tensile toughness plate Steel temperature (° C.) steel plate production temperature Means for temperature temperature Cooling rate temperature fraction thereof strength value K.sub.IH No. No. Ac.sub.3 Ar.sub.3 Ac.sub.1 (mm) method* (° C.) cooling (° C.) (° C.) (° C./s) (° C.) (area %) (MPa) (MPa .Math. m.sup.1/2) Notes 1 A 790 607 742 38 RQT 920 Oil cooling 850 200 10 635 TM 876 36 Comparative example 2 B 795 607 744 38 RQT 920 Oil cooling 850 200 10 635 TM 885 83 Example 3 C 816 607 755 38 RQT 920 Oil cooling 850 200 10 635 TM 902 97 Example 4 D 858 607 776 38 RQT 920 Oil cooling 850 200 10 635 TM 922 111 Example 5 E 796 609 745 38 RQT 920 Oil cooling 850 200 10 635 TM 876 36 Comparative example 6 F 796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 888 81 Example 7 G 796 598 745 38 RQT 920 Oil cooling 850 200 10 635 TM 915 95 Example 8 H 796 578 745 38 RQT 920 Oil cooling 850 200 10 635 TM 924 97 Example 9 I 796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 876 36 Comparative example 10 J 796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 886 77 Example 11 K 796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 905 89 Example 12 L 796 607 745 38 RQT 920 Oil cooling 850 200 10 635 TM 923 91 Example 13 M 849 700 744 38 RQT 920 Water 850 200 20 680 TM 1031 122 Example cooling 14 N 849 700 744 38 RQT 920 Water 850 200 20 680 TM 1025 117 Example cooling 15 O 850 700 745 38 RQT 920 Water 850 200 20 680 TM 1045 112 Example cooling 16 P 849 700 744 38 RQT 920 Water 850 200 20 680 TM 1089 155 Example cooling 17 M 849 700 744 38 AC 1100 Water 850 500 25 — F + P 566 121 Example cooling 18 N 849 700 744 38 AC 1100 Water 850 500 25 — F + P 561 119 Example cooling 19 O 849 700 744 38 DQT 1100 Water 850 200 20 680 TM 1051 110 Example cooling 20 P 850 700 745 38 DQT 1100 Water 850 200 20 680 TM 1097 157 Example cooling 21 Q 787 595 720 38 RQT 920 Oil cooling 850 200 10 500 TB 623 105 Example 22 R 780 475 740 38 RQT 920 Oil cooling 850 200 10 500 TB + TM 855 81 Example 23 S 787 595 720 38 RQT 920 Oil cooling 850 200 10 500 TB 631 102 Example *Reheating-quenching-tempering process: RQT, Accelerated cooling process: AC, Direct quenching-tempering process: DQT **TM: Tempered martensite, TB: Tempered lower bainite, F: Ferrite, P: Pearlite, B: Lower bainite, M: Martensite The underline indicates the value is outside the range of the disclosed embodiments.

[0098] In all of the Examples, the fracture toughness value K.sub.IH exhibited in a high-pressure hydrogen gas atmosphere at 115 MPa was 40 MPa.Math.m.sup.1/2 or higher, and, therefore, it can be said that excellent hydrogen embrittlement resistance was achieved. In contrast, in Comparative Examples, which had a composition that is outside the range of the disclosed embodiments, the fracture toughness value K.sub.IH exhibited in a high-pressure hydrogen gas atmosphere was less than 40 MPa.Math.m.sup.1/2, which indicated a low hydrogen embrittlement resistance. Note that in all of the Examples, a high strength of 560 MPa or higher in terms of tensile strength was achieved.

[0099] Hence, it was confirmed that the disclosed embodiments enable the production of products (steel structures for hydrogen) having excellent hydrogen embrittlement resistance.