BR112021005768A2 - high strength steel sheet for acid resistant piping and method for producing the same, and high strength steel tube using high strength steel sheet for acid resistant piping - Google Patents

Abstract

HIGH STRENGTH STEEL SHEET FOR ACIDITY RESISTANT PIPE AND METHOD FOR PRODUCTION AND TUBE HIGH STRENGTH STEEL SHEET USING THE HIGH STRENGTH STEEL SHEET FOR ACIDITY RESISTANT PIPE. A steel plate of high strength for acid-resistant tubing that has not only excellent resistance to HIC but also excellent resistance to SSCC in severely corrosive environments and excellent SSCC resistance in environments with a low partial pressure of hydrogen sulfide from less than 1 bar. This high-strength steel sheet for piping acid-resistant is characterized by having a composition of components containing predetermined amounts of C, Si, Mn, P, Al, Mo and Ca, and also contain Nb and/or Ti in predetermined amounts, with the remainder being made of Fe and the inevitable impurities, the microstructure of steel to 0.25 mm below the surface of the steel sheet being a Bainite microstructure having a displacement density of 1.0 x 1014 at 7.0 x 1014 (m-2), the variation in Vickers hardness at 0.25 mm below the steel sheet surface not exceeding 30 HV at 3 standard deviations, the variation in Vickers hardness in the direction of plate thickness does not exceeding 30 HV at 3 standard deviations, and the tensile strength being of at least 520 MPa.

Description

Invention Patent Descriptive Report for "HIGH STRENGTH STEEL SHEET FOR ACIDITY-RESISTANT PIPE AND METHOD FOR THE PRODUCTION OF THE SAME, AND TUBE

HIGH STRENGTH STEEL SHEET USING HIGH STRENGTH STEEL SHEET FOR ACID RESISTANT PIPE". Technical field

[0001] The present invention relates to a high strength steel sheet for acid-resistant piping that is excellent in material homogeneity in the steel sheet and that is suitable for use in piping in the fields of construction, marine structures. but, shipbuilding, civil engineering, and construction of the machine industry, and a method for producing it. This description also refers to a high strength steel pipe that uses high strength steel sheet for acid resistant piping. background

[0002] In general, a pipe is produced by forming a steel plate produced by a thick plate mill or by a hot rolled plate mill in a steel tube by UOE forming, forming by pressing by bending, forming with cylinders, or the like.

[0003] Piping used to transport crude oil and natural gas containing hydrogen sulfide needs to have so-called resistance to acidity such as resistance to fracture induced by hydrogen (resistance to HIC) and resistance to fracture by stress corrosion of sulfides (resistance to SSCC), in addition to strength, toughness, weldability, etc. Above all, in HIC, hydrogen ions caused by the corrosion reaction adsorbed onto the surface of the steel material, penetrate the steel as atomic hydrogen, diffuse and accumulate around non-metallic inclusions such as MnS in the steel and structure of the second hard phase, and becomes molecular hydrogen,

thus causing the fracture due to its internal pressure. This phenomenon is considered to be a problem in pipelines with a relatively low level of resistance compared to oil well pipelines, and many countermeasures have been proposed. On the other hand, SSCC is generally known to occur in high strength seamless steel tubes for oil wells and in high hardness regions of welds, and has not been considered a problem in relatively low hardness pipelines. However, in recent years, it has been reported that SSCC also occurs in the base metal of pipelines in environments where oil and natural gas mining environments have become increasingly harsh and environments with high partial pressure of sulfide. hydrogen or low pH. It is also pointed out that it is important to control the hardness of the surface layer of the inner surface of a steel pipe to improve SSCC resistance under more severe corrosion environments. Additionally, in environments with relatively low partial pressure of hydrogen sulfide, microfractures called cracks can occur, which can lead to SSCC.

[0004] In general, the so-called TMCP (thermomechanical control process) technology, which combines controlled rolling and controlled cooling, is applied when producing high-strength steel sheets for pipelines. To increase the strength of steel materials by using TMCP technology, it is effective to increase the cooling rate during controlled cooling. However, when controlled cooling is performed at a high cooling rate, the surface layer of the steel sheet is rapidly cooled, and the hardness of the surface layer becomes greater than that of the interior of the steel sheet, and the distribution of hardness in the direction of the sheet thickness becomes uneven. Therefore, it is a problem in terms of assuring the homogeneity of the material in the steel sheet.

[0005] To solve the above problems, for example, JP 3951428B (PTL 1) and JP3951429B (PTL 2) describe methods for producing steel sheet with a reduced material property difference in the direction of sheet thickness by performing controlled high-speed cooling in which the surface is recovered before the completion of bainite transformation into the surface layer after rolling. JP2002-327212A (PTL 3) and JP 3711896B (PTL 4) describe methods of producing pipe steel sheets in which the hardness of the surface layer is reduced by heating the surface of a steel sheet after it cools. - accelerated to a temperature greater than the internal temperature using a high-frequency induction heating device.

[0006] On the other hand, when the thickness of scale on the surface of the steel sheet is uneven, the cooling rate is also uneven in the underlying steel sheet during cooling, causing a problem of variation in the local cooling stop temperature in the steel sheet. As a result, the irregularity in scale thickness causes variations in the material property of the steel sheet in the direction of the sheet width. On the other hand, JPH9-57327A (PTL 5) and JP3796133B (PTL 6) describe methods of improving the shape of a steel sheet by performing scale removal immediately before cooling to reduce irregularity in cooling. caused by irregularity in the thickness of the scale. List of Patent Literature Citations

[0007] PTL 1: JP3951428B

[0008] PTL 2: JP3951429B

[0009] PTL 3: JP2002-327212A

[0010] PTL 4: JP3711896B

[0011] PTL 5: JPH9-57327A

[0012] PTL 6: JP3796133B Summary Technical problem

[0013] According to the studies, however, it has been found that high strength steel sheets obtained by the manufacturing methods described in Patent Literatures 1 to 6 have room for improvement in terms of resistance to SSCC under more corrosive environments. severe. What follows can be considered as the reason for this.

[0014] In the production methods described in PTLs 1 and 2, when the transformation behavior differs depending on the steel sheet compositions, a sufficient homogenizing effect of the material by heat recovery cannot be obtained. In the case where the microstructure in the surface layer of the steel sheet obtained by the production methods described in PTLs 1 and 2 is a double-phase structure such as a double-phase ferrite-bainite structure, the hardness value may have a large variation in a low load micro Vickers test depending on which microstructure the penetrator penetrates.

[0015] In the production methods described in PTLs 3 and 4, the rate of cooling of the surface layer in accelerated cooling is so high that the hardness of the surface layer may not be sufficiently reduced by heating the surface of the steel sheet.

[0016] On the other hand, the methods of PTLs 5 and 6 apply scale removal to reduce characteristic surface defects due to scale penetration during hot leveling and to reduce the variation in the cooling stop temperature of the steel sheet to improve the shape of the steel sheet. However, no consideration is given to the cooling conditions to obtain a uniform material property. This is because if the cooling rate

the surface of the steel sheet varies, the hardness of the steel sheet will vary. That is, at a low cooling rate, a "film boil", in which a film of air bubbles is generated between the surface of the steel sheet and the cooling water when the surface of the steel sheet it cools, and a "nucleate boil", in which air bubbles are separated from the surface by the cooling water before forming a skin, occur at the same time, causing variations in the rate of cooling of the surface of the steel sheet. As a result, the surface hardness of the steel sheet will vary. In the techniques described in PTLs 5 and 6, however, these facts are not considered at any time.

[0017] Furthermore, in PTLs 1 to 6, the conditions to avoid micro-fractures such as cracks in environments with relatively low partial pressure of hydrogen sulfide are not clear.

[0018] It should therefore be useful to provide a high strength steel sheet for acid-resistant piping that is excellent not only in resistance to HIC but also in resistance to SSCC under more severe corrosion and low environments. partial pressure of hydrogen sulfide below 1 bar, together with an advantageous method to produce the same. It should also be helpful to propose a high strength steel tube that uses high strength steel sheet for acid resistant tubing. Solution to the problem

[0019] The present inventors have repeated many experiments and examinations on the chemical compositions, microstructures, and production conditions of steel materials to ensure adequate SSCC resistance under the most severe corrosion environments. As a result, the inventors have found that to also improve the SSCC resistance of a high-strength steel pipe, it is not sufficient to merely suppress the hardness of the surface layer as they found out.

conventionally, and in particular that it is possible to reduce the increase in hardness in the coating process after tube production by shaping the outermost surface layer of the steel sheet, specifically at 0.25 mm below the surface of the steel sheet, with a bainite microstructure having a displacement density of 1.0 x 10° to 7.0 x 10*° (m?), and as a result the SSCC resistance of the steel tube is improved. To provide such a steel microstructure, the inventors also found it important to strictly control the cooling rate to 0.25 mm below the surface of the steel sheet, and they were successful in figuring out the conditions to be achieved. The inventors have also found that addition of Mo is effective in suppressing initial fracture generation in environments with high partial pressure of hydrogen sulfide above 1 bar, while suppression of addition of Ni is effective in preventing microfractures such as cracks. in environments with low partial pressure of hydrogen sulfide below 1 bar. The present description has been completed based on the above findings.

[0020] It is thus provided:

[0021] [1] A high-strength steel sheet for an acid-resistant piping, comprising: a chemical composition containing (consisting of), in % by mass, C: 0.02 % to 0.08 %, Si: 0.01% to 0.50%, Mn: 0.50% to 1.80%, P: 0.001% to 0.015%, S: 0.00022% to 0.0015%, Al: 0.01% to 0 .08%, Mo: 0.01% to 0.50%, Ca: 0.0005% to 0.005%, and at least one element selected from the group consisting of Nb: 0.005% to 0.1% and Ti: 0.005% to 0.1%, with the balance being Fe and the unavoidable impurities; a steel microstructure 0.25 mm below the surface of the steel sheet being a bainite microstructure having a displacement density of 1.0 x 10** at 7.0 x 10° (m?); a variation in Vickers hardness at 0.25mm below the steel sheet surface being 30HV or less at 3o, on-

of o is a standard deviation; and the tensile strength being 520 MPa or more.

[0022] [2] The high strength steel sheet for an acid-resistant pipe according to item [1], wherein the chemical composition also contains in % by mass, at least one element selected from the group consisting of Cu : 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less.

[0023] [3] The high strength steel sheet for acid-resistant piping according to item [1] or [2], where the chemical composition also contains, in % by mass, at least one element selected from the group consisting of V: 0.005% to 0.1%, Zr: 0.0005% to 0.02%, Mg: 0.0005% to 0.02%, and REM: 0.0005% to 0.02%.

[0024] [4] A method for producing a high-strength steel sheet for an acid-resistant pipe, the method comprising: heating a plate to a temperature of 1000°C to 1300°C, the plate having a chemical composition containing ( consisting of), in % by mass, C: 0.02% to 0.08%, Si: 0.01% to 0.50%, Mn: 0.50% to 1.80%, P: 0.001% to 0.015%, S: 0.00022% to 0.0015%, Al: 0.01% to 0.08%, Mo: 0.01% to 0.50%, Ca: 0.0005% to 0.005%, and at least one element selected from the group consisting of Nb: 0.005% to 0.1% and Ti: 0.005% to 0.1%, with the balance being Fe and the unavoidable impurities, and then hot-rolling the slab to form a sheet of steel, and then subjecting the steel sheet to controlled cooling under a set of conditions including: a surface temperature of the steel sheet at the start of cooling being (Air; — 10 °C) or more; the average rate of cooling over a temperature range of 750ºC to 550ºC in terms of a temperature 0.25mm below the surface of the steel sheet being 50ºC/sec or less; an average rate of cooling over a temperature range of 750ºC to 550ºC in terms of an average steel sheet temperature being 15ºC/s or more; the average cooling rate over a temperature range from 550ºC to a cooling stop temperature in terms of a temperature 0.25 mm below the surface of the steel sheet being 150ºC/s or more; and a cooling stop temperature in terms of the average temperature of the steel sheet being 250ºC to 550ºC.

[0025] [5] The method for producing a high-strength steel sheet for an acid-resistant pipe according to item [4], in which the chemical composition also contains, in % by mass, at least one element selected from the group consisting of Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less.

[0026] [6] The method for producing a high-strength steel sheet for an acid-resistant pipe according to item [4] or [5], in which the chemical composition also contains, in % by mass, at least one element selected from the group consisting of V: 0.005% to 0.1%, Zr: 0.0005% to 0.02%, Mg: 0.0005% to 0.02%, and REM: 0.0005% at 0.02%.

[0027] [7] A high strength steel tube using high strength steel sheet for an acid resistant tubing as shown in any of the items [1] to [3]. Advantageous Effect

[0028] The high-strength steel sheet for acid-resistant piping and the high-strength steel tube using the high-strength steel sheet for acid-resistant piping described here are excellent not only in resistance to HIC but also resistance to SSCC under more severe corrosion environments and environments with low partial pressure of hydrogen sulfide below 1 bar. In addition, according to the method for producing a high-strength steel sheet for an acid-resistant pipe.

ten described here, it is possible to produce a high-strength steel sheet for acid-resistant piping that excels not only in resistance to HIC but also in resistance to SSCC under more severe corrosion and low partial pressure environments. hydrogen sulfide below 1 bar.

Brief description of the drawings

[0029] FIG. 1 is a schematic view illustrating a method for obtaining specimens for evaluating SSCC resistance in the Examples.

Detailed Description

[0030] Hereinafter the high-strength steel sheet for an acid-resistant pipe in accordance with the present description will be described in detail.

Chemical composition

[0031] Initially, the chemical composition of the high-strength steel sheet described here will be described, and the reasons for its limitation. When components are expressed as "%" in the description below, this refers to "% by mass".

[0032] C: 0.02% to 0.08%

[0033] C effectively contributes to the improvement of endurance. However, if the content is less than 0.02%, a sufficient strength cannot be guaranteed, while if it exceeds 0.08%, the surface layer hardness and the central segregation area increase during accelerated cooling. , causing deterioration of SSCC resistance and HIC resistance. Tenacity also deteriorates. Therefore the C content is adjusted in a range of 0.02% to 0.08%.

[0034] Si: 0.01% to 0.50%

[0035] Si is added for deoxidation. However, if the content is less than 0.01%, the deoxidizing effect is not sufficient, while if it exceeds 0.50%, the toughness and weldability are degraded.

Therefore, the Si content is in a range of 0.01% to 0.50%.

[0036] Mn: 0.50% to 1.80%

[0037] Mn effectively contributes to the improvement of strength and tenacity. However, if the content is less than 0.50%, the addition effect is poor, while if it exceeds 1.80%, the hardness of the surface layer and the central segregation area increases during accelerated cooling, causing the deterioration of SSCC resistance and HIC resistance. Weldability also deteriorates. Therefore, the Mn content is adjusted in a range of 0.50% to 1.80%.

[0038] P: 0.001 % to 0.015 %

[0039] P is an unavoidable impurity element that degrades weldability and increases the hardness of the central segregation area, causing deterioration of resistance to HIC. This trend becomes more pronounced when the content exceeds 0.015%. Therefore, the upper limit is set at 0.015%. Preferably the P content is 0.008% or less. Although a lower P content is preferable, the P content is adjusted to 0.001% or more from a refining cost standpoint.

[0040] S: 0.0002 % to 0.0015 %

[0041] S is an unavoidable impurity element that forms MnS inclusions in steel and degrades the resistance to HIC, so a lower content of S is preferable. However, up to 0.0015% is acceptable. While a lower S content is preferable, the S content is adjusted to 0.0002% or more from a refining cost standpoint.

[0042] Al: 0.01% to 0.08%

[0043] Al is added as a deoxidizing agent. However, an Al content below 0.01% does not provide the addition effect, while an Al content above 0.08% decreases the cleanliness of the steel and deteriorates toughness. Therefore, the Al content is adjusted in a range of 0.01% to 0.08%.

[0044] Mo: 0.01% to 0.50%

[0045] Mo is an effective element to improve toughness and increase strength, and is an effective element to improve resistance to SSCC regardless of hydrogen sulfide partial pressure. To obtain this effect, the Mo content needs to be 0.01% or more, and preferably 0.10% or more. On the other hand, if the content is too large, the hardenability in the temper becomes excessively high, causing an increase in the displacement density, to be described later, and a deterioration of the resistance to SSCC. Weldability also deteriorates. Therefore, the Mo content is adjusted to 0.50% or less, and preferably to 0.40% or less.

[0046] Ca: 0.0005 % to 0.005%

[0047] Ca is an effective element to improve resistance to HIC by morphological control of sulfide inclusions. — However, if the content is less than 0.0005%, the effect of its addition is not sufficient. On the other hand, if the content exceeds 0.005%, not only does the addition effect saturate, but also the resistance to HIC is deteriorated due to the reduced cleanliness of the steel. Therefore. the Ca content is in a range of 0.0005% to 0.005%.

[0048] At least one element selected from the group consisting of Nb: 0.005% to 0.1% and Ti: 0.005% to 0.1%

[0049] Both Nb and Ti are effective elements to improve the strength and toughness of the steel sheet. If the content of each added element is less than 0.005%, the addition effect is poor, while if it exceeds 0.1% the toughness of the welded portion deteriorates. Therefore, at least one between Nb and Ti is added in a range of 0.005% to 0.1%.

[0050] The basic components of the present description have been described above. Optionally, however, the chemical composition of the present description may also contain at least one selected element.

group consisting of Cu, Ni and Cr in the following ranges to also improve the strength and toughness of the steel sheet.

[0051] Cu: 0.50% or less

[0052] Cu is an effective element to improve toughness and increase strength. To obtain this effect, the Cu content is preferably 0.05% or more, however if the content is too large, the weldability deteriorates. Therefore, when Cu is added, the Cu content is up to 0.50%.

[0053] Ni: 0.10% or less

[0054] Ni is an effective element to improve toughness and increase strength. To obtain this effect, the Ni content is preferably 0.01% or more. However, when Ni content is added above 0.10%, microfractures called cracks easily occur in environments with low partial pressure of hydrogen sulfide below 1 bar. Therefore. When Ni is added, the Ni content is up to 0.10%. The Ni content is preferably 0.02% or less.

[0055] Cr: 0.50% or less

[0056] Cr, like Mn, is an effective element to obtain sufficient strength even at a low C content. To obtain this effect, the Cr content is preferably 0.05% or more, however if the content is too high, the hardenability at temper becomes excessively high, causing an increase in displacement density to be described later and deteriorating resistance to SSCC. Weldability also deteriorates. Therefore, when Cr is added, the Cr content is up to 0.50%.

[0057] Optionally, the chemical composition of the present description may also contain at least one element selected from the group consisting of V, Zr, Mg and REM in the following ranges.

[0058] At least one element selected from the group consisting of V: 0.005% to 0.1%, Zr: 0.0005% to 0.02%, Mg: 0.0005% to 0.02

%, and REM: 0.0005% to 0.02%

[0059] V is an element that can be optionally added to increase the strength and toughness of the steel sheet. If the content of each element added is less than 0.005%, the addition effect is poor, while if it exceeds 0.1%, the toughness of the welded portion deteriorates. Therefore, the content of each added element is preferably in a range of 0.005% to 0.1%. Zr, Mg, and REM are elements that can optionally be added to increase toughness through grain refining and to improve fracture strength by controlling the properties of inclusions. Each of these elements is poor in addition effect when the content is less than 0.0005%, while the effect is saturated when the content is greater than 0.02%. Therefore, when added, the content of each added element is preferably in a range of 0.0005% to 0.02%.

[0060] Although the present description describes a technique to improve the SSCC resistance of high-strength steel pipe that uses high-strength steel sheet for acid-resistant tubing, it goes without saying that the technique described here needs to be sa- ting the HIC resistance while satisfying the acid resistance performance. For example, the CP value obtained by Expression (1) below is preferably set to 1.00 or less. For any non-added element, what is needed is to replace its symbol by O. CP = 4.46[4C] + 2.37[%MnN]6 + (1.74[%Cu] + 1.7[ %Ni])/I5 + (1.18[%4Cr] + 1.95[%Mo] + 1.74[%V])/5 + 22.36[%P] (1), where [º %X] represents the % content by mass of element X in the steel.

[0061] As used here, the CP value is a formula developed to estimate the material property in the central segregation area from the content of each bonding element, and the concentrations of the components in the central segregation area are larger as the CP value of Expression (1) is greater, causing an increase in the hardness of the central segregation area. Therefore, by adjusting the CP value obtained in Expression (1) to 1.00 or less, it is possible to suppress the occurrence of fractures in the HIC test. In addition, since the hardness of the central segregation area is lower as the CP value is lowered, the upper limit for the CP value can be set to 0.95 when greater resistance to HIC is required.

[0062] The different balance of the elements described above is Fe and the unavoidable impurities. However, there is no intention in this expression to prevent the inclusion of traces of other elements, without harming the action or effect of this description. For example, N is an element that is inevitably contained in steel, and a content of 0.007% or less, preferably 0.006% or less, is acceptable in the present description. Steel sheet microstructure

[0063] The microstructure of high-strength steel sheet steel for an acid-resistant pipe described here will be described below. To achieve high strength with a tensile strength of 520 MPa or more, the steel microstructure needs to be a bainite microstructure. In particular, when a hard phase such as martensite or martensite austenite (MA) constituent is generated in the surface layer, the hardness of the surface layer is increased, the variation in hardness in the steel sheet is increased, and the homogeneity of the material is impaired. To suppress the increase in hardness of the surface layer, the surface layer is formed with a bainite microstructure like the microstructure of steel. Different portions of the surface layer also have a bainite microstructure, and the microstructure in the middle portion of the representative thickness of the portions may be a bainite microstructure. In this case, the bainite microstructure includes a microstructure called bainite ferrite or granular ferrite.

home that contributes to the reinforcement of transformation. These microstructures appear through transformation during or after accelerated cooling. If different microstructures such as ferrite, martensite, perlite, constituent martensite austenite, retained austenite, and the like are mixed in the bainite microstructure, a decrease in strength, a deterioration in toughness, an increase in surface hardness and similar. Therefore, it is preferable that microstructures other than the bainite phase have smaller proportions. However, when the volume fraction of such microstructure other than the bainitic phase is sufficiently low, its effect is negligible, and even a certain amount is acceptable. However, when the volume fraction of such microstructures other than the bainitic phase is sufficiently low, their effects are negligible, and even a certain amount is acceptable. Specifically, in the present description, if the total of steel microstructures other than bainite (such as ferrite, martensite, perlite, constituent martensite austenite, and retained austenite) is less than 5% by volume fraction, there is no effect adverse, and this is acceptable.

[0064] Although the bainite microstructure takes various forms according to the rate of cooling, it is important for the present description that the outermost surface layer of the steel sheet, specifically at 0.25 mm below the surface of the steel sheet, is formed with a bainite microstructure having a displacement density of 1.0 x 10*º to 7.0 x 10*º (m?). Since the displacement density decreases in the coating process after pipe production, the increase in hardness due to age hardening can be minimized if the displacement density is 0.25 mm below the surface layer of the pipe. sheet steel is 7.0 x 10** (m?2) or less. On the other hand, if the displacement density at 0.25 mm below the surface of the steel sheet exceeds 7.0 x 10° (m?), the displacement density does not decrease in the re-

coating after tube production, and the hardness is significantly increased due to age hardening, causing deterioration of SSCC resistance. The displacement density range is preferably 6.0 x 10** (m?) or less to obtain good SSCC resistance after pipe production. On the other hand, when the displacement density at 0.25 mm below the surface of the steel sheet is less than 1.0 x 10° (m?), the strength of the steel sheet deteriorates. To guarantee the strength of the X65 grade, it is preferable to have a displacement density of 2.0 x 10*º (mM?) or more. In the high strength steel sheet described here, if the displacement density in the steel microstructure at 0.25 mm below the steel sheet surface is in the range above, the outermost surface layer varying from the surface from sheet steel to a depth of 0.25 mm has an equivalent displacement density, and hence the SSCC-strength improving effect described above is obtained.

[0065] When the displacement density at 0.25 mm below the surface of the steel sheet is 7.0 x 10** (m2) or less, the HV 0.1 to 0.25 mm below the surface is 230 or less. From the point of view of guaranteeing the SSCC resistance of the steel tube, it is important to suppress an increase in the surface hardness of the steel sheet. However, by adjusting the HV 0.1 to 0.25 mm below the surface of the steel sheet to 230 or less, the HV 0.1 to 0.25 mm below the surface after the coating heat treatment at 250ºC for 1 hour after tube production can be suppressed to 260 or less, and resistance to SSCC can be guaranteed.

[0066] In addition, in the high strength steel sheet described here, it is also important that the variation in Vickers hardness at 0.25 mm below the surface of the steel sheet is 30 HV or less at 3o, where c is o standard deviation. The reason is that he is 3rd at the time of the month.

tion of the Vickers hardness at 0.25 mm below the surface of the steel sheet is greater than 30 HV, the variation of hardness in the outermost surface layer of the steel sheet, ie the presence of a hardness portion locally high in the outer surface layer, causes deterioration in the SSCC resistance originating from that portion. Note that when calculating the standard deviation o, it is preferable to measure the Vickers hardness at 100 places or more.

[0067] The high strength steel sheet described here is a steel sheet for steel tubes having a strength of grade X60 or greater in API 5L, and thus has a tensile strength of 520 MPa or more. production method

[0068] Hereinafter, the method and conditions for the production of the high-strength steel sheet for an acid-resistant tube mentioned above will be concretely described. The production method in accordance with the present description comprises: heating a slab having the chemical composition described above, and then hot rolling the slab to form a steel plate; and then subjecting the steel sheet to controlled cooling under predetermined conditions. Plate heating temperature

[0069] Plate heating temperature: 1000 °C to 1300 °C

[0070] If the heating temperature of the plate is less than 1000°C, the carbides do not dissolve sufficiently and the required strength cannot be obtained. On the other hand, if the plate's heating temperature exceeds 1300°C, the toughness deteriorates. Therefore, the heating temperature of the plate is set to 1000°C to 1300°C. This temperature is the temperature in the heating oven, and the plate is heated to this temperature all the way through. Lamination finish temperature

[0071] In a hot rolling step, to obtain high tena-

For the base metal, a lower finishing temperature is preferable, although, on the other hand, the lamination efficiency is decreased. Thus, the finishing temperature of the rolling mill in terms of the surface temperature of the steel sheet needs to be adjusted taking into account the toughness required for the base metal and the efficiency of the rolling. From the point of view of improving the strength and resistance to HIC, it is preferable to adjust the finishing temperature of the rolling mill to the transformation temperature Ar; or above it in terms of a steel sheet surface temperature. As used here, the transformation temperature Ar; means the start temperature of the ferrite transformation during cooling, and can be determined, for example, from the steel components according to the following equation. Furthermore, to obtain high tenacity for the base metal, it is desirable to adjust the lamination reduction ratio to a temperature range of 950ºC or less corresponding to the temperature range of non-recrystallization of austenite to 60% or most. The surface temperature of the steel sheet can be measured by a radiation thermometer or similar. Ar3 (°C) = 910 — 310[%4C] — 80[%Mn] — 20[%4Cu] — 15[%Cr] — 55[%Ni] — 80[%Mo], where [%X] indicates the content in % by mass of element X in the steel. Cooling start temperature in controlled cooling

[0072] The start temperature of cooling is (Arg — 10°C) or higher in terms of the surface temperature of the steel sheet.

[0073] When the surface temperature of the steel sheet at the start of cooling is low, the amount of ferrite formation before controlled cooling increases and, in particular, if the temperature drop from the transformation temperature Ar; is greater than 10°C, ferrite exceeding 5% by volume fraction is generated, causing a significant decrease in strength and deterioration of resistance to HIC. Therefore, the surface temperature of the steel sheet at the start of cooling is adjusted to (Arg — 10°C) or higher. Note that the surface temperature of the steel sheet at the start of cooling is not higher than the finish temperature of the rolling mill. Controlled cooling cooling rate

[0074] To reduce the variation in hardness in the steel sheet and improve the homogeneity of the material while achieving high strength, it is important to control the surface layer cooling rate and the average cooling rate in the steel sheet. In particular, to adjust the displacement density to 0.25 mm below the surface of the steel sheet and 3o within the ranges described above, it is necessary to control the cooling rate to 0.25 mm below the surface of the steel sheet.

[0075] The average rate of cooling over a temperature range of 750ºC to 550ºC in terms of a temperature 0.25 mm below the surface of the steel sheet: 50ºC/sec or less.

[0076] When the average rate of cooling over a temperature range of 750ºC to 550ºC in terms of a temperature 0.25 mm below the surface of the steel sheet exceeds 50 ºC/s, the displacement density at 0. 25 mm below the surface of the steel sheet exceeds 7.0 x 10** (m?). As a result, the HV 0.1 to 0.25 mm below the surface of the steel sheet exceeds 230, and after the coating process after tube production, the HV 0.1 to 0.25 mm below the surface exceeds 260, causing deterioration in the SSCC resistance of the steel tube. Therefore, the average cooling rate is set to 50°C/s or less. It is preferably 45°C/s or less, and more preferably 40°C/s or less. The lower limit of the average cooling rate is not particularly limited, however if the cooling rate is too low, ferrite and pearlite are generated and the strength is insufficient. Therefore, from the standpoint of avoiding this, 20°C/s or more is preferable.

[0077] Average rate of cooling over a temperature range of 750ºC to 550ºC in terms of an average temperature of the steel sheet: 15ºC/s or more.

[0078] If the average rate of cooling over a temperature range of 750ºC to 550ºC in terms of an average temperature of the steel sheet is less than 15ºC/s, a bainite microstructure cannot be obtained, causing deterioration in resistance and resistance to HIC. Therefore, the cooling rate in terms of an average steel sheet temperature is set to 15°C/s or more. From the standpoint of variations in strength and hardness of the steel sheet, the average cooling rate of the steel sheet is preferably 20°C/s or more. The upper limit of the mean cooling rate is not particularly limited, however it is preferably 80°C/s or less so that products with excessive low temperature transformation will not be generated.

[0079] Average rate of cooling over a temperature range from 550 °C to the cooling stop temperature in terms of a temperature 0.25 mm below the surface of the steel sheet: 150 °C/s or more

[0080] For cooling to a temperature of 550 °C or less in terms of a temperature 0.25 mm below the surface of the steel sheet, cooling to a stable nucleate boiling state is necessary, and it is essential to increase the rate of water flow. If the average cooling rate is less than 150ºC/s over a temperature range of 550ºC to the cooling stop temperature in terms of a temperature 0.25 mm below the surface of the steel sheet, then the cooling in a state of boiling of nucleate is not achieved, a hardness variation occurs in the super-layer.

outermost surface of the steel sheet, and 3o to 0.25 mm below the surface of the steel sheet exceeds 30 HV, resulting in deterioration in resistance to SSCC. Therefore, the average cooling rate is set to 150ºC/s or more. Preferably it is 170°C/s or more. The upper limit of the average cooling rate is not particularly limited, however it is preferably 250ºC/s or less in view of device restrictions.

[0081] Although the temperature at 0.25 mm below the surface of the steel sheet and the average temperature of the steel sheet cannot be physically measured directly, for example, the temperature distribution in a cross section in the direction of thickness The plate temperature can be determined in real time by calculating the difference using a process computer based on the surface temperature at the start of cooling as measured by a radiation thermometer and the target surface temperature at the end of cooling. cooling. As used here, the temperature at 0.25 mm below the surface of the steel sheet in the temperature distribution is referred to as the "temperature at 0.25 mm below the surface of the steel sheet", and the mean value. of temperatures in the direction of thickness in the temperature distribution as the "average temperature of the steel sheet". Cooling stop temperature Cooling stop temperature: 250 ºC to 550 ºC in terms of average steel sheet temperature

[0082] After the end of the lamination, a bainite phase is generated by performing controlled cooling to temper the steel sheet up to a temperature range of 250ºC to 550ºC which is the temperature range of the bainite transformation. When the cooling stop temperature exceeds 550°C, bainite transformation is incomplete and sufficient strength cannot be obtained. In addition, if the cooling stop temperature is less than 250°C, the au-

The hardness in the surface layer becomes noticeable and the displacement density at 0.25 mm below the surface of the steel sheet exceeds 7.0 x 10** (m?), causing deterioration in SSCC resistance. In addition, the hardness of the central segregation area increases and the resistance to HIC deteriorates. Therefore, to suppress deterioration of material homogeneity in the steel sheet, the cooling stop temperature of the controlled cooling is adjusted to 250ºC to 550ºC in terms of an average temperature of the steel sheet. High strength steel tube

[0083] By forming the high-strength steel sheet described here into a tubular form by forming by press bending, forming with cylinders, UOE forming, or similar, and then welding the top portions, a tube of high-strength steel for acid-resistant piping (such as a UOE steel tube, an electrical resistance welded steel tube, and a spiral welded tube) that has excellent material homogeneity in the steel sheet and that it is suitable for transporting crude oil and natural gas can be produced.

[0084] For example, a UOE steel tube is produced by machining the grooves of the ends of a steel plate, by forming the steel plate into the shape of a steel tube by C-pressing, pressing U-shape and O-press, and then seam-welding the butt portions by welding the inner surface and welding the outer surface, and optionally subjecting it to an expansion process. Any welding method can be applied as long as sufficient bond strength and bond toughness are guaranteed, however it is preferable to use submerged arc welding from the point of view of excellent welding quality and production efficiency.

Examples

[0085] Steels (Steels A to M) having the chemical compositions listed in Table 1 are made into slabs by continuous casting, heated to the temperatures listed in Table 2, and then hot rolled at the finishing and rolling temperatures rolling reduction ratios listed in Table 2 to obtain the steel sheets with the thicknesses listed in Table 2. Then, each steel sheet was subjected to controlled cooling using a controlled cooling device of the water-cooling type under the conditions listed in Table 2. Identification of microstructure

[0086] The microstructure of each steel plate obtained was observed by an optical microscope and a scanning electron microscope. The microstructure at a position 0.25 mm below the surface of each steel sheet and the microstructure at the middle of the thickness are listed in Table 2. Measurement of tensile strength

[0087] A tensile test was conducted using full thickness specimens collected in a direction perpendicular to the rolling direction as tensile specimens for measuring the tensile strength. The results are listed in Table 2. Vickers hardness measurement

[0088] For a cross section perpendicular to the rolling direction, according to JIS Z 2244, the Vickers hardness (HV 0.1) was measured at 100 locations at a position 0.25 mm below the surface of each sheet of steel, the measurement results were averaged, and the standard deviation c was determined. The mean and 3o values are listed in Table 2. In this case, the measurement was taken at HV 0.1 instead of the HV which is commonly used, because the notch size is made smaller when measuring at HV 0.1, and it is possible to obtain the information of last-

za in a position closer to the surface and more sensitive to the microstructure. displacement density

[0089] A sample for X-ray diffraction was taken from a position having an average hardness, the sample surface was polished to remove scale, and the X-ray diffraction measurement was performed at a position of 0, 25 mm below the surface of the steel sheet. The displacement density was converted from the voltage obtained from the half width 8 of the X-ray diffraction measurement. In a diffraction intensity curve obtained by common X-ray diffraction, the Koa1 and Ka2 rays having different lengths waveforms overlap, and are thus separated by the Rachinger's method. For strain extraction, the Williamson-Hall method described below is used. The half-width propagation is influenced by the crystallite size D and the stress e, and can be calculated by the following equation as the sum of both factors: B = 81 + 82 = (0.9 /(D x cosb) )) + 2e€ x tan. Furthermore, by modifying this equation, the following is derived: BB coso/) = 0.9 1/D + 2e€ x sine 9/2. The stress e is calculated from the slope of the straight line plotting at coso/) with respect to sine 0/h. The diffraction lines used for the calculation are (110), (211), and (220). The conversion of displacement density from stress e was used p = 14.4 e?2/b?. As used here, 9 means the peak angle calculated by the 90-20 method for X-ray diffraction, and » means the wavelength of the X-rays used in X-ray diffraction. b is a Burgers vector of Fe (a), and is set to 0.25 nm in this mode. SSCC Resistance Assessment

[0090] The SSCC resistance was evaluated for a tube made from one part of each steel plate. Each tube was produced by machining the end notches of a steel sheet, and forming the steel sheet into a steel tube by C-pressing, U-pressing and O-pressing, and then welding the top portions together. on the inner and outer surfaces by submerged arc welding and subjecting the tube to an expansion process. As illustrated in Fig. 1, after a coupon cut from each steel tube obtained had been flattened, a 5mm x 15mm x 115mm SSCC specimen was collected from the inner surface of the steel tube. At that time, the inner surface to be tested was left intact without removing the scale to leave the outermost layer state. Each collected SSCC specimen was loaded with 90% stress of the actual tensile strength (0.5% YS) of the corresponding steel tube, and the evaluation was done using a solution of Solution A of the NACE standard. TMO0177, at a hydrogen sulfide partial pressure of 1 bar, according to the SSCC 4-point bending test specified by EFC 16. In addition, at a hydrogen sulfide partial pressure of 0.1 bar and a carbon dioxide partial pressure of 0.9 bar, the evaluation was made using a Solution B solution of the NACE standard TMO0177 according to the 4-point bending test SSCC specified by the EFC standard 16. In addition, at a partial pressure of hydrogen sulfide of 2 bar and a partial pressure of carbon dioxide of 3 bar, the evaluation was done using a solution of Solution A of NACE standard TMO0177 according to the test of 4-point SSCC folding specified by EFC standard

16. After immersion for 720 hours, resistance to SSCC was considered to be "good" when no fracture was observed, or "poor" when fracture occurred. The results are listed in Table 2. Assessment of resistance to HIC

[0091] The resistance to HIC was determined by performing the HIC test at a partial pressure of hydrogen sulfide of 1 bar with an immersion time of 96 hours using a solution of Solution A of the standard NACE TMO0177. In addition, resistance to HIC was deterred.

mined by running the HIC test at a partial pressure of hydrogen sulfide of 0.1 bar and a partial pressure of carbon dioxide of 0.9 bar and with an immersion time of 96 hours using a Solution solution. B of the NACE TMO0177 standard. Resistance to HIC was judged to be "good" when the fracture length ratio (CLR) was 15% or less in the HIC test, or "poor" when the CLR exceeded 15%. The results are listed in Table 2.

[0092] The target ranges of the present description are as follows: - the tensile strength is 520 MPa or more as a high strength steel sheet for an acid resistant pipe; - the microstructure is a bainite microstructure in both positions of 0.25 mm below the surface and of t/2; - the HV hardness 0.1 to 0.25 mm below the surface is 230 or less;

[0093] - no fracture is observed in the SSCC test on the high strength steel tube made from the corresponding steel sheet; and

[0094] - the fracture length ratio (CLR) is 15% or less in the HIC test.

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[0095] As can be seen from Table 2, numbers 1 to 15 are the examples in which the chemical compositions and production conditions satisfy the appropriate ranges of the present description. In either case, the tensile strength as a steel sheet was 520 MPa or more, the microstructure at both positions 0.25mm below the surface and at t/2 was a bainite microstructure, the HV 0.1 at 0.25 mm below the surface it was 230 or less, and so the SSCC resistance and the HIC resistance were also good in the high strength steel tube made of sheet steel.

[0096] In contrast, Nos.* 16 to 23 are comparative examples whose chemical compositions are within the scope of the present description but whose production conditions are outside the scope of the present description. In No. 16, since the heating temperature of the plate was low, the homogenization of the microstructure and the solid solution state of the carbides were insufficient and the resistance was low. In No. 17, since the temperature at the start of cooling was low and the microstructure was formed in the form of layers with precipitation of ferrite, the strength was low and the resistance to HIC after tube production was deteriorated. In No. 18, since the conditions of controlled cooling were outside the scope of the present description and the bainite microstructure was not obtained in the middle part of the thickness, but instead a ferrite + pearlite microstructure was obtained as the microstructure, the resistance was low and the HIC resistance after tube production was deteriorated. At No. 19, since the cooling stop temperature was low, the displacement density at 0.25 mm below the surface increased, and the HV 0.1 exceeded 230, the resistance to SSCC after production. of the tube was inferior. In addition, the hardness of the central segregation area also increased, and the resistance to HIC also deteriorated. At Nos. 20 and 23, since the average rate of cooling over a temperature range of 750ºC to 550ºC at 0.25 mm below the surface of the steel sheet exceeded 50°C/s, the displacement density at 0.25 mm below of surface increased, and HV 0.1 exceeded 230, and resistance to SSCC after pipe production was lower. At #23, the HIC resistance in the surface layer also deteriorated. In No. 21 and No. 22, since the average cooling rate in a temperature range of 550ºC or less to 0.25 mm below the surface of the steel plate was less than 150ºC/s, the irregular cooling of the steel plate. steel was remarkable. In addition, although the HV 0.1 was 230 or less on average, the hardness variation was large and a portion of locally high hardness was generated. Consequently, the resistance to SSCC after tube production was lower. In Nos. 24 to 27, since the compositions of the steel sheets were beyond the scope of the present description, the displacement density at 0.25 mm below the surface was high, and the HV 0.1 exceeded 230, the SSCC resistance after tube production was lower. In addition, in Nos. 24 to 27, the resistance to HIC was also lower because the hardness of the central segregation area increased. In No. 28, the amount of Ni in the steel sheet was excessive, and the resistance to SSCC in environments with low partial pressure of hydrogen sulfide deteriorated. In No. 29, the steel sheet was Mo-free, and the SSCC resistance deteriorated in a very severe corrosion environment with a hydrogen partial pressure of 2 bar. At No. 30, the average rate of cooling over a temperature range of 750ºC to 550ºC in terms of a temperature 0.25 mm below the surface of the steel sheet exceeded 50ºC/s, and the resistance to SSCC deteriorated under an environment of very severe corrosion with a partial pressure of hydrogen sulfide of 2 bar.

industrial applicability

[0097] In accordance with the present description, it is possible to provide a high strength steel sheet for an acid resistant pipe that is excellent not only in resistance to HIC but also in resistance to SSCC under more severe and corrosive environments. environments with low partial pressure of hydrogen sulfide below 1 bar.

Therefore, steel tubes (such as electrically resistant welded steel tubes, spiral steel tubes, and UOE steel tubes) produced by cold forming the steel sheet described can be suitably used to transport crude oil and natural gas containing hydrogen sulphides where acid resistance is required.

Claims (7)

1. High strength steel sheet for acid-resistant piping, characterized in that it comprises: a chemical composition containing, in % by mass, C: 0.02 % to 0.08 %, Si: 0, 01 % to 0.50 %, Mn: 0.50 % to 1.80 %, P: 0.001 % to 0.015 %, S: 0.0002 % to 0.0015 %, Al: 0.01% to 0.08 %, Mo: 0.01% to 0.50%, Ca: 0.0005% to 0.005%, and at least one element selected from the group consisting of Nb: 0.005% to 0.1% and Ti: 0.005% at 0.1%, with the balance being Fe and the unavoidable impurities; a steel microstructure at 0.25 mm below a surface of the steel sheet being a bainite microstructure having a displacement density of 1.0 x 10° to 7.0 x 10° (m?); a variation in Vickers hardness at 0.25 mm below the surface of the steel sheet being 30 HV or less at 3°, where o is a standard deviation; and a tensile strength being 520 MPa or more. 2. High-strength steel sheet for an acid-resistant pipe, according to claim 1, characterized in that the chemical composition still contains, in % by mass, at least one element selected from the group consisting of in Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less. 3. High-strength steel sheet for acid-resistant piping, according to claim 1 or 2, characterized in that the chemical composition still contains, in % by mass, at least one element selected from the group consisting of V: 0.005% to 0.1%, Zr: 0.0005% to 0.02%, Mg: 0.0005% to 0.02%, and REM: 0.0005% to 0.02%. 4. Method for producing a high-strength steel sheet for an acid-resistant pipe, characterized by the fact that it comprises: heat a plate to a temperature of 1000°C to 1300°C, the plate having a chemical composition containing, in % by mass, C: 0.02% to 0.08%, Si: 0.01% to 0.50%, Mn: 0.50% to 1.80%, P: 0.001% to 0.015%, S: 0.0002% to 0.0015%, Al: 0.01% to 0.08%, Mo: 0.01% to 0 .50%, Ca: 0.0005% to 0.005%, and at least one element selected from the group consisting of Nb 0.005% to 0.1% and Ti: 0.005% to 0.1%, with the balance being Fe and the unavoidable impurities, and then hot rolling the plate to form a steel sheet; and then subjecting the steel sheet to controlled cooling under a set of conditions including: a temperature of a surface of the steel sheet at the start of cooling being (Ar3g—10°C) or more; an average rate of cooling over a temperature range of 750ºC to 550ºC in terms of a temperature 0.25 mm below the surface of the steel sheet being 50ºC/sec or less; an average cooling rate over a temperature range of 750ºC to 550ºC in terms of an average steel sheet temperature being 15ºC/s or more; an average rate of cooling over a temperature range of 550°C to a cooling stop temperature in terms of a temperature 0.25 mm below the surface of the steel sheet being 150°C or more; and a cooling stop temperature in terms of an average steel sheet temperature being 250ºC to 550ºC. 5. Method for producing a high-strength steel sheet for an acid-resistant pipe, according to claim 4, characterized in that the chemical composition still contains, in % by mass, at least one element selected from the group consisting of Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less. 6. Method for producing a high-strength steel sheet for an acid-resistant pipe, according to claim 4 or 5, characterized in that the chemical composition still contains, in % by mass, at least one element selected from the group consisting of V: 0.005% to 0.1%, Zr: 0.0005% to 0.02%, Mg: 0.0005% to 0.02%, and REM: 0.0005% to 0.02%. 7. High-strength steel pipe characterized in that it comprises using high-strength steel sheet for acid-resistant pipe, as defined in any one of claims 1 to 3.