RU2767260C1 - High-strength steel plate for acid-resistant pipeline, and method of producing steel plate, and high-strength steel pipe, in which high-strength steel plate is used for acid-resistant pipeline - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to high-strength steel plate used for production of acid-resistant pipeline. Plate has a chemical composition containing the following in wt. %: C: from 0.02 to 0.08; Si: from 0.01 to 0.50; Mn: from 0.50 to 1.80; P: from 0.001 to 0.015; S: from 0.0002 to 0.0015; Al: from 0.01 to 0.08; Mo: from 0.01 to 0.50; Ca: from 0.0005 to 0.005; at least one component selected from a group consisting of Nb: 0.005 to 0.1 and Ti: 0.005 to 0.1, optionally at least one component selected from a group consisting of Cu: 0.50 or less, Ni: 0.10 or less, Cr: 0.50 or less, V: from 0.005 to 0.1, Zr: from 0.0005 to 0.02, Mg: from 0.0005 to 0.02 and rare-earth metal: from 0.0005 to 0.02, the rest are Fe and unavoidable impurities. Microstructure of steel at depth of 0.25 mm from the surface of the steel plate is a bainitic microstructure having a dislocation density of 1.0 × 10¹⁴ to 7.0 × 10¹⁴ (m^-2). Vibrations of Vickers hardness at depth of 0.25 mm from the surface of the steel plate make 30 HV or less, and tensile strength is 520 MPa or more.

EFFECT: plate has high resistance to hydrogen cracking and high resistance to sulphide corrosion while maintaining high strength, viscosity and weldability.

3 cl, 1 dwg, 2 tbl

Description

The field of technology to which the invention belongs

The present invention relates to a high-strength steel plate for acid-resistant pipeline, which has a high material uniformity in the steel plate, and which is suitable for use in pipelines in the fields of construction, offshore, shipbuilding, civil engineering and mechanical equipment for the construction industry, and a method for their production. In addition, the invention relates to a high strength steel pipe using a high strength steel plate for acid-resistant pipeline.

State of the art

Generally, pipeline pipes are produced by forming a steel plate produced by a plate mill or a hot rolling mill into a steel pipe by UOE forming (on U- and O-shaped bending presses), die bending, sheet roll forming, or the like.

The pipeline used for transporting crude oil and natural gas, which contain hydrogen sulfide, is required to have so-called acid resistance, such as resistance to hydrogen-induced cracking (HIC resistance) and resistance to sulfide corrosion by fatigue cracks (SSCC resistance), in addition to strength, toughness, weldability and so on. Mainly in HIC, the hydrogen ions generated by the corrosion reaction are adsorbed on the surface of the steel material, penetrate the inside of the steel as atomic hydrogen, diffuse and accumulate near non-metallic inclusions such as MnS in the steel and the hard structure of the secondary phase, and become molecular hydrogen, which causes cracking due to the internal pressure of hydrogen. This phenomenon is seen as a problem for pipelines with a relatively low level of strength compared to downhole oil pipes, and various countermeasures have been proposed. On the other hand, it is well known that SSCC occurs in high strength seamless steel pipes for oil wells and in areas of high hardness welds, and this is not a problem in pipelines with relatively low hardness. However, in recent years there have been reports that SSCC also occurs in pipeline alloy base metal in downhole environments where oil and natural gas conditions are increasingly harsh and in environments with high hydrogen sulfide partial pressure or low pH. In addition, the importance of controlling the surface hardness of the inner surface of the steel pipe is noted to improve SSCC resistance in more severe corrosive environments. Additionally, in environments with a relatively low hydrogen sulfide partial pressure, micro-cracks, called striations, can appear, which can lead to SSCC.

Usually, the so-called TMCP (Thermo Mechanical Control Process) technology, which combines controlled rolling and controlled cooling, is used in the production of high-strength pipeline steel plates. In order to increase the strength of steel materials using TMCP technology, an effective technique is to increase the cooling rate during controlled cooling. However, by performing controlled cooling at a cooling rate, the surface layer of the steel plate is rapidly cooled, and the hardness of the surface layer becomes higher than the hardness inside the steel plate, and the hardness distribution in the thickness direction of the plate becomes uneven. Therefore, the problem is to ensure the uniformity of the material inside the steel plate.

In order to solve these problems, for example, Japanese patents JP3951428B (PTL 1) and JP3951429B (PTL 2) describe methods for producing steel plates with reduced difference in material properties in the thickness direction of the plate by performing high-speed controlled cooling in which the surface is recovered until the bainitic transformation is completed. in the surface layer after rolling. Documents JP2002-327212A (PTL 3) and JP3711896B (PTL 4) describe methods for producing steel plates for pipelines in which the hardness of the surface layer is reduced by heating the surface of the steel plate after accelerated cooling to a higher temperature than the inside of the plate, using a high-frequency device. induction heating.

On the other hand, when the thickness of the scale on the surface of the steel plate is uneven, the cooling rate is also uneven for the underlying steel plate at the time of cooling, which leads to the problem of changing the local cooling termination temperature of the steel plate. As a result, the uneven dross thickness causes changes in the material properties of the steel plates in the thickness direction of the plate. On the other hand, JPH9-57327A (PTL 5) and JP3796133B (PTL 6) describe methods for improving the shape of a steel plate by carrying out descaling immediately before cooling, so as to reduce the cooling unevenness caused by the uneven thickness of the dross.

List of citations

Patent Literature

PTL 1: JP3951428B

PTL2: JP3951429B

PTL3: JP2002-327212A

PTL4: JP3711896B

PTL5: JPH9-57327A

PTL6: JP3796133B

Disclosure of the essence of the invention

Technical problem

However, according to the research of the inventors, it appeared that for high strength steel plates obtained using the production methods described in Patent Literature 1 to 6, there is room for improvement in SSCC resistance in more severe corrosive environments. The following text can be seen as justification.

In the production methods described in PTL 1 and 2, when the transformation progress varies depending on the composition of the steel plates, a sufficient effect of material homogenization by thermal recovery cannot be obtained. In the case where the microstructure in the surface layer of the steel plate obtained using the production methods described in PTL documents 1 and 2 is a two-phase structure such as a ferrite-bainite two-phase structure, the hardness value may change significantly when determining the Vickers microhardness with a small load, depending on which microstructure the indenter presses.

In the production methods described in PTL 3 and 4, the cooling rate of the surface layer in accelerated cooling is so high that the hardness of the surface layer cannot be significantly reduced by only heating the surface of the steel plate.

On the other hand, PTL methods 5 and 6 employ descaling to reduce defects in surface characteristics due to indentation of dross during hot straightening and to reduce the fluctuation of the cooling stop temperature of the steel plate so as to improve the shape of the steel plate. However, there is no discussion of cooling conditions to obtain uniform material properties. This is because when the cooling rate on the surface of the steel sheet changes, the hardness of the steel sheet will change. That is, at a low cooling rate, "film boiling", in which a film of air bubbles is formed between the surface of the steel sheet and cooling water, when the surface of the steel sheet is cooled, and "bubble boiling", in which air bubbles are separated from the surface by cooling water before forming a film, occur simultaneously, there is a change in the cooling rate of the surface of the steel plate. As a result, the surface hardness of the steel plate may vary. However, the technologies described in PTL 5 and 6 do not address these facts at all.

In addition, PTL documents 1 to 6 are not clear on the conditions to avoid the formation of microcracks, such as grooves, in environments with a relatively low partial pressure of hydrogen sulfide.

Thus, it would be beneficial to develop a high strength steel plate for acid-resistant piping, in other words, having not only excellent HIC resistance, but also SSCC resistance in more severe corrosive environments and environments with low hydrogen sulfide partial pressure, below 1 bar, along with the advantageous method of producing such a plate. In addition, it would be beneficial to offer high strength steel pipe using high strength steel plate for acid-resistant pipeline.

Solution to the problem

The inventors of the present invention have made many experiments and tests on the chemical composition, microstructure, and production conditions of steel materials in order to provide adequate resistance to SSCC in more severe corrosive environments. As a result, the inventors have found that, in order to further improve the SSCC resistance of a high-strength steel pipe, it is not enough to simply restrain the hardness of the surface layer as is commonly done, and in particular, that it is possible to reduce the increase in hardness in the coating process after the pipe is manufactured by forming in the outer surface layer steel plate, specifically 0.25 mm below the surface of the steel plate, bainitic microstructure having a dislocation density of 1.0×10 ¹⁴ to 7.0×10 ¹⁴ (m ^-2 ), and as a result, the SSCC resistance of the steel pipe is improved. In order to achieve this microstructure of the steel, the inventors also found that it is important to accurately control the cooling rate at a depth of 0.25 mm from the surface of the steel plate, and the inventors were able to determine such conditions. In addition, the authors found that the addition of molybdenum Mo effectively suppresses initial cracking in environments with high partial pressure of hydrogen sulfide, above 1 bar, while suppressing the addition of Ni is an effective technique for eliminating microcracks such as grooves in environments with low hydrogen sulfide partial pressure, less than 1 bar. The present invention has been made on the basis of the above findings.

Thus, it is proposed:

[1] High-strength steel plate for acid-resistant pipeline, which includes: chemical composition containing (consisting of), 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 metal selected from the group consisting of Nb: 0.005% to 0.1%, and Ti: 0.005% to 0.1%, with the remainder being Fe and unavoidable impurities; the steel microstructure at a depth of 0.25 mm from the surface of the steel plate is a bainite microstructure having a dislocation density of 1.0×10 ¹⁴ to 7.0×10 ¹⁴ (m ^-2 ); the Vickers hardness variation at a depth of 0.25 mm from the surface of the steel plate is 30 HV or less at 3σ, where σ is the standard deviation; and the tensile strength is 520 MPa or more.

[2] The high-strength steel plate for acid-resistant pipeline according to [1], wherein the chemical composition further includes, in mass %, at least one metal selected from the group consisting of Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less.

[3] The high-strength steel plate for acid-resistant pipeline according to [1] or [2], wherein the chemical composition further includes, in mass%, at least one metal selected from the group consisting of V: from 0.005% to 0.1%, Zr: 0.0005% to 0.02%, Mg: 0.0005% to 0.02%, and rare earth metal (REM): 0.0005% to 0.02%.

[4] A method for producing high-strength steel plate for acid-resistant pipeline, which includes: heating a slab to a temperature of 1000 °C to 1300 °C, and the slab has a chemical composition containing (consisting of), mass%, C: from 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 metal , which is selected from the group consisting of Nb: 0.005% to 0.1% and Ti: 0.005% to 0.1%, the remainder being Fe and unavoidable impurities, and then hot rolling the slab to form a steel plate; and then subjecting the steel plate to controlled cooling in a mode including the conditions: the surface temperature of the steel plate at the start of cooling is (Ar ₃ - 10°C) or higher; the average cooling rate in the temperature range from 750°C to 550°C, in terms of temperature at a depth of 0.25 mm from the surface of the steel plate, is 50°C/s or less; the average cooling rate in the temperature range from 750°C to 550°C, in terms of the average temperature of the steel plate, is 15°C/s or higher; the average cooling rate in the temperature range from 550°C to the cooling stop temperature, in terms of temperature at a depth of 0.25 mm from the surface of the steel plate, is 150°C/s or higher; and the cooling stop temperature, in terms of the average temperature of the steel plate, is 250°C to 550°C.

[5] The method for producing high-strength steel plate for acid-resistant pipeline according to [4], wherein the chemical composition further includes, in wt.%, at least one metal selected from the group consisting of Cu: 0.50% or less, Ni: 0.10% or less, and Cr: 0.50% or less.

[6] The method for producing a high-strength steel plate for an acid-resistant pipeline according to [4] or [5], wherein the chemical composition further includes, in mass%, at least one metal selected from the group consisting of V: from 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 using high-strength steel plate for acid-resistant pipeline as specified in any one of [1] to [3].

Benefits of the Invention

The high-strength steel plate for acid-resistant pipeline and the high-strength steel pipe using the high-strength steel plate for acid-resistant pipeline disclosed in the invention are excellent not only in HIC resistance property, but also in SSCC resistance in more severe corrosive environments and environments with low hydrogen sulfide partial pressure , less than 1 bar. In addition, according to the method for producing high-strength steel plate for acid-resistant pipeline disclosed in the invention, it is possible to produce high-strength steel plate for acid-resistant pipeline, which is excellent not only in HIC resistance property, but also in SSCC resistance in more severe corrosive environments and environments with low partial pressure of hydrogen sulfide, less than 1 bar.

Brief description of the drawing

Figure 1 is a schematic diagram that illustrates a method for obtaining prototypes for evaluating the SSCC resistance in the Examples.

Implementation of the invention

Hereinafter, the high-strength steel plate for acid-resistant pipeline according to the present invention will be described in detail.

Chemical composition

First, the chemical composition of the high-strength steel plate disclosed in the invention and the reasons for limiting the composition will be described. When components are expressed in “%” in the following description, this means “% by weight”.

C: 0.02% to 0.08%

Carbon makes an effective contribution to improving strength. However, if the C content is less than 0.02%, sufficient strength cannot be ensured, although if the C content exceeds 0.08%, the hardness of the surface layer and the central segregation area increase during accelerated cooling, which causes deterioration in SSCC resistance and resistance HIC. In addition, the toughness deteriorates. Therefore, the C content is set in the range of 0.02% to 0.08%.

Si: 0.01% to 0.50%

Silicon is added for deoxidation. However, if the Si content is less than 0.01%, the deoxidizing effect is insufficient, although if the Si content exceeds 0.50%, the toughness and weldability deteriorate. Therefore, the Si content is in the range of 0.01% to 0.50%.

Mn: 0.50% to 1.80%

Manganese makes an effective contribution to improving strength and toughness. However, if the content of Mn is less than 0.50%, then the effect of the additive is weak, although if its content exceeds 1.80%, the hardness of the surface layer and the central segregation area increase during accelerated cooling, which causes deterioration of the SSCC resistance and HIC resistance. In addition, weldability deteriorates. Therefore, the Mn content is set in the range of 0.50% to 1.80%.

P: 0.001% to 0.015%

Phosphorus is an unavoidable impurity element that deteriorates the weldability and increases the hardness of the central segregation region, resulting in deterioration of the HIC resistance. This trend becomes more pronounced when the P content exceeds 0.015%. Therefore, the upper limit is set to 0.015%. Preferably, the P content is 0.008% or less. Although a reduced P content is preferable, the P content is set to 0.001% or more in terms of purification costs.

S: 0.0002% to 0.0015%

Sulfur is an unavoidable impurity element that forms MnS inclusions in steel and degrades HIC resistance, and therefore a reduced S content is preferable. However, S content up to 0.0015% is acceptable. Although a reduced S content is preferable, the S content is set to 0.0002% or more in terms of purification costs.

Al: 0.01% to 0.08%

Aluminum is added as a deoxidizing agent. However, when the Al content is lower than 0.01%, the additive has no effect, while when the Al content exceeds 0.08%, the purity grade of the steel decreases and the toughness deteriorates. Therefore, the Al content is set in the range of 0.01% to 0.08%.

Mo: 0.01% to 0.50%

Molybdenum is an effective element for improving toughness and strength; Mo is an effective element to improve SSCC resistance regardless of the hydrogen sulfide partial pressure. To obtain this effect, it is necessary that the Mo content be 0.01% or more, and preferably 0.10% or more. On the other hand, if there is too much Mo, the quenchability becomes excessively high, resulting in an increase in the dislocation density, which will be described later, and deterioration in the SSCC resistance. In addition, weldability deteriorates. Therefore, the Mo content is set to 0.50% or less, and preferably 0.40% or less.

Ca: 0.0005% to 0.005%

Calcium is an effective element to improve HIC resistance by morphological control of sulfide inclusions. However, if the Ca content is less than 0.0005%, the effect of the additive is insufficient. On the other hand, if the content of Ca exceeds 0.005%, not only the effect of the additive is saturated, but the HIC resistance is also degraded due to the decrease in the purity of the steel. Therefore, the Ca content is in the range of 0.0005% to 0.005%.

At least one metal selected from the group consisting of Nb: 0.005% to 0.1% and Ti: 0.005% to 0.1%

Both Nb and Ti are effective elements for improving the strength and toughness of steel plate. If the content of each added element is less than 0.005%, then the influence of the additive is weak, although if the content exceeds 0.1%, the toughness of the welded parts deteriorates. Therefore, at least one Nb or Ti metal is added in the range of 0.005% to 0.1%.

The main components of the present invention are described above. However, optionally, the chemical composition according to the invention may also contain at least one metal selected from the group consisting of Cu, Ni and Cr in the following ranges in order to further improve the strength and toughness of the steel plate.

Cu: 0.50% or less

Copper is an effective element for improving toughness and strength. In order to obtain this effect, the content of Cu is preferably 0.05% or more, however, if the content is too large, the weldability deteriorates. Therefore, when adding Cu, its content is limited to 0.50%.

Ni: 0.10% or less

Nickel is an effective element for improving toughness and increasing strength. In order to obtain this effect, the Ni content is preferably 0.01% or more. However, when Ni is added in excess of 0.10%, microcracks called grooves easily occur in environments with low hydrogen sulfide partial pressures below 1 bar. Therefore, when Ni is added, its content is limited to 0.10%. Preferably, the Ni content is 0.02% or less.

Cr: 0.50% or less

Chromium, like Mn, is an element effective in obtaining sufficient strength even at low carbon content. To obtain this effect, it is preferable that the content of Cr is 0.05% or more, however, if its content is too large, the hardenability becomes excessively high, resulting in an increase in the dislocation density, which will be described later, and deterioration of the SSCC resistance. . In addition, weldability deteriorates. Therefore, when Cr is added, its content is limited to 0.50%.

Optionally, the chemical composition according to the invention may further contain at least one metal selected from the group consisting of V, Zr, Mg, and REM in the following ranges.

At least one metal 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: from 0.0005% to 0.02%

Vanadium is an element that may optionally be added to increase the strength and toughness of the steel plate. If the content of each added element is less than 0.005%, then the influence of the additive is weak, although if the content exceeds 0.1%, the toughness of the welded parts deteriorates. Therefore, the content of each added element is preferably in the range of 0.005% to 0.1%. Zr, Mg, and REMs are elements that can optionally be added to increase toughness, by grain refinement, and to improve crack resistance, by adjusting inclusion properties. The effect of adding each of these elements is weak when the amount of additive is less than 0.0005%, while the effect is saturated when the amount of additive is more than 0.02%. Therefore, when adding, the content of each added element is preferably in the range of 0.0005% to 0.02%.

Although the present invention discloses a technology for improving the SSCC resistance of a high-strength steel pipe using a high-strength steel plate for an acid-resistant pipeline, it is clear that the technology disclosed in the invention must meet the characteristics of HIC resistance at the same time as acid-resistance. For example, the CP value obtained using the following equation (1) is preferably set to 1.00 or less. For any unadded element, you need to make a replacement with 0.

CP = 4.46[%C] + 2.37[%Mn]/6 + (1.74[%Cu] + 1.7[%Ni])/15 + (1.18[%Cr] + 1 .95[%Mo] + 1.74[%V])/5 + 22.36[%P] (1),

where [%X] means the content of element X in steel, wt.%.

The CP value used in the invention is a formula developed to evaluate the properties of the material in the central segregation region, from the content of each alloying element, and the concentration of the component in the central segregation region is higher, the larger the CP value in equation (1), which causes an increase in hardness in areas of central segregation. Therefore, by setting the CP value obtained from Equation (1) to 1.00 or less, it is possible to suppress the occurrence of cracks in the HIC test. In addition, since the hardness of the central segregation region is lower the lower the CP value, the upper limit for the CP value can be set to 0.95 when higher HIC resistance is required.

The remainder other than the above-described elements is Fe and unavoidable impurities. However, there is no intention in this equation to prevent the incorporation of other micronutrients without impairing the operation or effect of the present invention. 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 invention.

Microstructure of steel plate

Next, the microstructure of steel in the high-strength steel plate for acid-resistant pipeline disclosed in the invention will be described. In order to achieve high strength with a tensile strength of 520 MPa or more, it is necessary that the microstructure of the steel be a bainitic microstructure. In particular, when a hard phase such as martensite or a martensite-austenite (MA) component is formed in the surface layer, the hardness of the surface layer increases, the variation in the hardness of the steel plate increases, and the uniformity of the material deteriorates. In order to suppress the increase in hardness of the surface layer, this layer is formed with a bainitic microstructure, like the microstructure of steel. Parts other than the surface layer also have a bainitic microstructure, wherein the microstructure in a representative part of the medium thickness region may have a bainitic microstructure. In this case, the bainitic microstructure includes a microstructure called bainitic ferrite or granular ferrite, which contributes to the transformation hardening. These microstructures appear during the transformation during or after accelerated cooling. If various microstructures such as ferrite, martensite, perlite, a martensitic-austenitic component, retained austenite, and the like are mixed in a bainitic microstructure, there is a decrease in strength, deterioration in toughness, an increase in surface hardness, and the like. Therefore, it is preferable that the microstructures other than the bainitic phase have reduced proportions. However, when the volume fraction of such microstructures other than the bainitic phase is sufficiently small, their effect can be neglected and up to a certain amount they are acceptable. Specifically in the present invention, if the proportion of total steel microstructures other than bainite (such as ferrite, martensite, perlite, martensite-austenite component and retained austenite) is less than 5% by volume, there are no harmful effects, and this is acceptable.

Although the bainitic microstructure takes on various shapes according to the cooling rate, it is important for the present invention that a bainitic microstructure having a dislocation density of 1.0 × 10 ¹⁴ to 7.0×10 ¹⁴ (m ^-2 ). Since the dislocation density decreases during the coating process after the tube is formed, the increase in hardness due to precipitation hardening can be minimized if the dislocation density at a depth of 0.25 mm from the surface of the steel plate is 7.0×10 ¹⁴ (m ^-2 ) or less. On the contrary, if the dislocation density at a depth of 0.25 mm from the surface of the steel plate exceeds 7.0 × 10 ¹⁴ (m ^-2 ), then the dislocation density does not decrease in the coating process after pipe production, and the hardness increases significantly due to precipitation hardening, which leads to to a deterioration in SSCC resistance. Preferably, the dislocation density range is 6.0×10 ¹⁴ (m ^-2 ) or less in order to obtain good SSCC resistance after the pipe is produced. On the other hand, when the dislocation density at a depth of 0.25 mm from the surface of the steel plate is less than 1.0×10 ¹⁴ (m ^-2 ), the strength of the steel plate deteriorates. In order to ensure X65 grade strength, it is preferable to have a dislocation density of 2.0×10 ¹⁴ (m ^-2 ) or more. For the high-strength steel plate disclosed in the invention, if the dislocation density in the steel microstructure at a depth of 0.25 mm from the surface of the steel plate is in the above range, the outer surface layer in the range from the surface of the steel plate to a depth of 0.25 mm has an equivalent dislocation density, and as a result, the above effect of improving the SSCC resistance is obtained.

When the dislocation density at a depth of 0.25 mm from the surface of the steel plate is 7.0×10 ¹⁴ (m ^-2 ) or less, the HV 0.1 value at a depth of 0.25 mm below the surface is 230 or less. From the point of view of providing the SSCC resistance of the steel pipe, it is important to suppress the increase in the surface hardness of the steel plate. However, by setting the HV 0.1 value at a depth of 0.25 mm from the surface of the steel plate to 230 or less, the HV 0.1 value at a depth of 0.25 mm from the surface after heat treatment of the coating at 250°C for 1 hour after receiving pipe can be suppressed to 260 or less, and SSCC resistance can be provided.

In addition, for the high strength steel plate disclosed in the invention, it is also important that the Vickers hardness variation at a depth of 0.25 mm from the surface of the steel plate is 30 HV or less at 3σ, where σ is the standard deviation. The reason is that if the value of 3σ at the time of measuring the Vickers hardness at a depth of 0.25 mm from the surface of the steel plate exceeds 30 HV, then the hardness fluctuation in the outer surface layer of the steel plate, that is, the presence of local areas with high hardness in outer surface layer causes the degradation of SSCC resistance caused by these regions. It is noted that when calculating the standard deviation σ, preferably the Vickers hardness is measured at 100 locations or more.

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

Mode of production

Hereinafter, the production method and conditions for the above high-strength steel plate for acid-resistant pipeline will be specifically described. The production method of the present invention includes: heating a slab having the above chemical composition and then hot rolling the slab to form a steel plate; then subjecting the steel plate to controlled cooling under predetermined conditions.

Slab heating temperature

Slab heating temperature: from 1000°C to 1300°C

If the heating temperature of the slab is less than 1000° C., the carbides are not sufficiently dissolved and the necessary strength cannot be obtained. On the other hand, if the heating temperature of the slab exceeds 1300° C., the toughness deteriorates. Therefore, the heating temperature of the slab is set to 1000°C to 1300°C. Said temperature is the temperature in the heating furnace, and the slab is heated to this temperature at the center.

Final rolling temperature

In the hot rolling step, in order to obtain high toughness for the base metal of the alloy, a lower rolling end temperature is preferable, but on the other hand, the rolling efficiency is lowered. Thus, it is necessary to set the rolling end temperature in terms of the surface temperature of the steel plate, taking into account the required toughness for the base metal alloy and the rolling efficiency. From the viewpoint of improving strength and HIC resistance, it is preferable to set the rolling end temperature to be equal to or higher than the Ar ₃ transformation temperature in terms of the surface temperature of the steel plate. Used in the description, the term "Ar ₃ transformation temperature" means the initial transformation temperature of the ferrite during cooling, and can be determined, for example, from the steel components according to the following equation. In addition, in order to obtain high toughness for the base metal alloy, it is desirable to set the rolling draw ratio to 60% or more in a temperature range of 950°C or less, which corresponds to a temperature range without austenite crystallization. The surface temperature of the steel plate can be measured with a radiation pyrometer or the like.

Ar ₃ (°C) = 910 - 310[%C] - 80[%Mn] - 20[%Cu] - 15[%Cr] - 55[%Ni] - 80[%Mo],

where [%X] means the content (wt.%) of the element X in the steel.

Cooling start temperature for controlled cooling

The cooling start temperature is (Ar ₃ - 10 °C) or higher in terms of the surface temperature of the steel plate.

When the surface temperature of the steel plate is low at the beginning of cooling, the amount of ferrite formed before controlled cooling increases, and in particular, if the temperature difference from the Ar ₃ transformation temperature is greater than 10°C, ferrite is formed in an amount exceeding 5% by volume, which causes a significant reduction in strength and deterioration in HIC resistance. Therefore, the surface temperature of the steel plate at the start of cooling is set to (Ar ₃ - 10°C) or higher. It is noted that the surface temperature of the steel plate at the beginning of cooling does not exceed the final rolling temperature.

Cooling rate with controlled cooling

In order to reduce the change in hardness in the steel plate and improve the uniformity of the material while achieving high strength, it is important to control the cooling rate of the surface layer and the average cooling rate of the steel plate. In particular, in order to set the dislocation density, at a depth of 0.25 mm from the surface of the steel plate, and the value of 3σ in the ranges described above, it is necessary to control the cooling rate at a depth of 0.25 mm from the surface of the steel plate.

Average cooling rate in the temperature range from 750°C to 550°C in terms of temperature at a depth of 0.25 mm from the surface of the steel plate: 50°C/s or less

When the average cooling rate in the temperature range from 750°C to 550°C in terms of temperature at a depth of 0.25 mm from the surface of the steel plate exceeds 50°C/s, the dislocation density at a depth of 0.25 mm from the surface of the steel plate exceeds 7, 0 × 10 ¹⁴ (m ^-2 ). As a result, the HV 0.1 value at a depth of 0.25 mm from the surface of the steel plate exceeds 230, and after the coating process, after the pipe is obtained, the HV 0.1 value at a depth of 0.25 mm from the surface exceeds 260, resulting in deterioration resistance SSCC steel pipe. Therefore, the average cooling rate is set to 50°C/s or less. The preferred value is 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 excessively low, ferrite and pearlite are generated and the strength becomes insufficient. Therefore, from the viewpoint of preventing such a process, a cooling rate of 20°C/s or higher is preferable.

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

If the average cooling rate in the temperature range of 750°C to 550°C, in terms of the average temperature of the steel plate, is less than 15°C/s, a bainite microstructure cannot be obtained, resulting in degradation of strength and HIC resistance. Therefore, the cooling rate in terms of the average temperature of the steel plate is set to 15°C/s or higher. In terms of changes in the strength and hardness of the steel plate, the preferred average cooling rate of the steel plate is 20°C/s or higher. The upper limit of the average cooling rate is not particularly limited, but it is preferably 80°C/s or less so that low-temperature transformation by-products are not generated.

Average cooling rate in the temperature range from 550°C to the cooling stop temperature in terms of temperature at a depth of 0.25 mm from the surface of the steel plate: 150°C/s or more

For cooling to a temperature of 550° C. or less in terms of temperature at a depth of 0.25 mm from the surface of the steel plate, cooling in the stable nucleate boiling mode is necessary, and an increase in the water flow rate is a significant factor. If the average cooling rate is less than 150°C/s in the temperature range from 550°C to the cooling stop temperature in terms of temperature at a depth of 0.25 mm from the surface of the steel plate, then cooling in the nucleate boiling mode is not realized, there is a change in hardness in outer surface layer of the steel plate, and the value of 3σ at a depth of 0.25 mm from the surface of the steel plate exceeds 30 HV, resulting in deterioration of the SSCC resistance. Therefore, the average cooling rate is set to 150°C/s or higher. Preferably, it is 170°C/s or higher. The upper limit of the average cooling rate is not particularly limited, but it is preferably 250°C/s or less, subject to the limitations of the equipment.

Although the temperature at a depth of 0.25 mm from the surface of the steel plate and the average temperature of the steel plate cannot be directly measured physically, for example, the temperature distribution in the section direction through the thickness of the plate can be determined in real time by calculating the temperature difference using a process control computer based on surface temperature at the beginning of cooling, measured by a radiation pyrometer, and a given surface temperature at the end of cooling. The expression used in the description, the temperature at a depth of 0.25 mm from the surface of the steel plate in the temperature distribution, is called the “temperature at a depth of 0.25 mm from the surface of the steel plate”, and the average value of the temperature in the thickness direction for the temperature distribution is called the “average temperature steel plate."

Refrigeration stop temperature

Cooling stop temperature: 250°C to 550°C in terms of the average temperature of the steel plate

After rolling is completed, the bainitic phase is formed by conducting controlled cooling to harden the steel plate in a temperature range of 250°C to 550°C, which is the bainitic transformation temperature range. When the cooling stop temperature exceeds 550°C, the bainitic transformation is incomplete and sufficient strength cannot be obtained. In addition, if the cooling stop temperature is less than 250°C, the increase in hardness in the surface layer becomes significant, and the dislocation density at a depth of 0.25 mm from the surface of the steel plate exceeds 7.0 × 10 ¹⁴ (m ^-2 ), which leads to a deterioration in SSCC resistance. In addition, the hardness of the central segregation area increases, and the HIC resistance deteriorates. Therefore, in order to suppress the deterioration of material uniformity in the steel plate, the cooling stop temperature in controlled cooling is set to 250°C to 550°C in terms of the average temperature of the steel plate.

High Strength Steel Pipe

By forming the high-strength steel plate disclosed in the invention into a tubular shape by die pressing, sheet metal roll forming, UOE forming, or the like, and then butt-welding the parts, a high-strength steel pipe for acid-resistant pipelines (for example, UOE steel pipe, resistance welded steel pipe, and spiral steel pipe), which has excellent material uniformity in the steel plate and is suitable for transporting crude oil and natural gas.

For example, UOE steel pipe is produced by grooving the ends of the steel plate, forming the steel plate into the shape of a steel pipe using C press, U-shaped press and O-shaped press, then seam welding the parts butt-welding, by internal surface welding and external surface welding, and optionally subjecting the pipe to an expansion process. Any welding method can be used as long as sufficient strength and toughness of the joint is provided, however, submerged arc welding is preferably used from the point of view of excellent welding quality and production efficiency.

Examples

Steels (A to M) having the chemical composition shown in Table 1 are made into slabs by continuous casting, heated to the temperature shown in Table 2, and then subjected to hot rolling at the final rolling temperature, and the rolling draw ratio, indicated in Table 2 to obtain steel plates with a thickness indicated in Table 2. Then, each steel plate is subjected to controlled cooling using a water-cooling type apparatus under controlled cooling conditions indicated in Table 2.

Microstructure identification

The microstructure of each obtained steel plate was examined using an optical microscope and a scanning electron microscope. The microstructure at a position 0.25 mm below the surface of each steel plate and the microstructure at half the thickness of the plate are shown in Table 2.

Tensile strength measurement

A tensile test was performed using test specimens having a full thickness and assembled in a direction perpendicular to the rolling direction as tensile test specimens to measure tensile strength. The results are shown in Table 2.

Vickers hardness measurement

For a section perpendicular to the rolling direction, Vickers hardness (HV 0.1) was measured according to JIS Z 2244 at 100 locations 0.25 mm below the surface of each steel plate; the measurement results were averaged and the standard deviation σ was determined. The mean values and 3σ values are shown in Table 2. In this case, the measurements were taken at HV 0.1 instead of HV 10 which are commonly used, since the measurement at HV 0.1 produces a smaller indentation mark and it is possible to obtain information on the hardness in closer position to the surface and increased sensitivity to the microstructure.

Dislocation density

An X-ray diffraction sample was taken from a location having an average hardness, the surface of the sample was polished to remove scale, and an X-ray diffraction study was performed at a position 0.25 mm below the surface of the steel plate. The dislocation density was estimated from the strain obtained from the half-width β of the X-ray diffraction peak. In a diffraction intensity curve obtained by conventional X-ray diffraction, Kα1 and Kα2 beams having different wavelengths overlap, thus being separated by the Rachinger method. The Williamson-Hall method described below is used to isolate the strain. FWHM broadening is affected by crystallite size D and strain ε and can be calculated from the following equation as the sum of two factors: β = β1 + β2 = (0.9 λ/(D × cosθ)) + 2ε × tgθ. Further transformation of this equation yields the following: β cosθ/λ = 0.9 λ/D + 2ε × sinθ/λ. Deformation ε is calculated from the slope of a straight line in the coordinates "β cosθ/λ" relative to "sinθ/λ". The diffraction lines from the (110), (211), and (220) planes are used for the calculation. The equation ρ = 14.4 ε ² /b ² is used to calculate the dislocation density from the strain ε. Here, θ means the angle for the peak calculated by the θ-2θ method for x-ray diffraction, and λ means the wavelength of x-rays used in x-ray diffraction. In this embodiment, b is the Burgers vector for Fe(α) and b is found to be 0.25 nm.

SSCC Resistance Estimation

The SSCC resistance is evaluated for a pipe obtained from a portion of each steel plate. Each pipe is produced by grooving the ends of a steel plate, and molding the steel plate into the shape of a steel pipe using a C press, a U-shaped press, and an O-shaped press, followed by seam welding of the butt-welded parts on the inner and outer surfaces by submerged arc welding. , and the pipe is subjected to an expansion process. As shown in Figure 1, after flattening a cut test piece from each steel pipe produced, a 5 mm x 15 mm x 115 mm SSCC test piece was taken from the inner surface of the steel pipe. At the same time, the inner surface to be tested remains intact, without removing the scale, so as to leave the state of the outer layer. Each SSCC prototype is stressed to 90% of the actual yield strength (0.5% YS) of the respective steel pipe and evaluated using National Association of Corrosion Engineers Standard Solution A (NACE TM0177 Solution), at a hydrogen sulfide partial pressure of 1 bar, in in accordance with the SSCC 4-point bending test set 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, an assessment was made using standard solution B (NACE standard TM0177 Solution) according to the test SSCC of 4-point bending, set according to EFC 16. In addition, at a partial pressure of hydrogen sulfide of 2 bar and a partial pressure of carbon dioxide of 3 bar, an assessment is made using standard solution A (NACE standard TM0177 Solution) according to the SSCC test of 4-point bending, established according to the EFC 16 standard. After diving for 720 hours, the SSCC resistance is evaluated as “Good” when there are no cracks, or “Poor” when cracks occur. The results are shown in Table 2.

HIC Resistance Rating

The HIC resistance is determined by conducting a HIC test at a hydrogen sulfide partial pressure of 1 bar and a immersion time of 96 hours using Standard Solution A (NACE standard TM0177 Solution). In addition, HIC resistance is determined by conducting a HIC test at a hydrogen sulfide partial pressure of 0.1 bar and a partial pressure of dioxide of 0.9 bar and an immersion time of 96 hours using standard solution B (NACE standard TM0177 Solution). HIC resistance is rated as “Good” when the crack length ratio (CLR) is 15% or less in the HIC test, or “Poor” when CLR is greater than 15%. The results are shown in Table 2.

The targets of the present invention are as follows:

- tensile strength is 520 MPa or more for high strength steel plate for acid-resistant pipeline;

- the microstructure is a bainitic microstructure at locations 0.25 mm below the surface and also at t/2;

- the value of HV 0.1 at a depth of 0.25 mm from the surface is 230 or less;

- there are no cracks in the SSCC test for a high-strength steel pipe made from a corresponding steel plate; And

- crack length ratio (CLR) is 15% or less in the HIC test.

As can be seen from Table 2, Nos. 1 to 15 represent examples according to the invention in which the chemical compositions and production conditions fall within the ranges of the present invention. In any of these examples, the tensile strength of the steel plate is 520 MPa or more, the microstructure at locations 0.25 mm below the surface and also at t/2 is a bainitic microstructure, the HV value is 0.1 at a depth of 0.25 mm from the surface is 230 or less, and therefore the SSCC resistance and the HIC resistance are also satisfactory for the high-strength steel pipe made of the steel plate.

In contrast, Nos. 16 to 23 represent comparative examples in which the chemical compositions are within the scope of the present invention, but the manufacturing conditions are not within the scope of the present invention. In No. 16, due to the low heating temperature of the slab, the homogenization of the microstructure and the solid solution state of the carbides are insufficient, and the strength is low. In No. 17, since the cooling start temperature is low and the microstructure is formed to some extent by ferrite deposition layers, the strength of the plate is low, and after the pipe is obtained, the HIC resistance deteriorates. In No. 18, since the conditions of controlled cooling are not within the scope of the present invention and the bainite microstructure is not obtained in the mid-thickness region, but instead the microstructure (ferrite + pearlite) is obtained, the strength is low, and after the pipe is obtained, the HIC resistance deteriorates. In No. 19, since the cooling stop temperature is low, the dislocation density at a depth of 0.25 mm from the surface increases, and the HV 0.1 value exceeds 230, after the pipe is obtained, the SSCC resistance deteriorates. In addition, the hardness of the central segregation area also increases, and the HIC resistance also deteriorates. In Nos. 20 and 23, since the average cooling rate in the temperature range from 750°C to 550°C at a depth of 0.25 mm from the surface of the steel plate exceeds 50°C/s, the dislocation density increases at a depth of 0.25 mm from the surface , and the value of HV 0.1 exceeds 230, after receiving the pipe, the SSCC resistance deteriorates. In #23, the HIC resistance in the surface layer also deteriorates. In No. 21 and No. 22, since the average cooling rate in the temperature range of 550°C or less at a depth of 0.25 mm from the surface of the steel plate is less than 150°C/s, uneven cooling of the steel plate becomes unusual. In addition, although the HV 0.1 value is 230 or less on average, the change in hardness is significant, and high hardness local areas are formed. Accordingly, after the tube is produced, the SSCC resistance deteriorates. In Nos. 24 to 27, since the compositions of the steel plates do not fall within the scope of the present invention, the dislocation density at a depth of 0.25 mm from the surface is high, and the HV 0.1 value exceeds 230, after the pipe is produced, the SSCC resistance deteriorates. In addition, in Nos. 24 to 27, the HIC resistance also deteriorates as the hardness of the central segregation area increases. In No. 28, the amount of Ni in the steel plate is excessive, and the SSCC resistance in low hydrogen sulfide partial pressure environments deteriorates. In No. 29, the steel plate does not contain Mo, and the SSCC resistance deteriorates under very severe corrosive environment conditions with a hydrogen sulfide partial pressure of 2 bar. In No. 30, the average cooling rate in the temperature range from 750°C to 550°C, in terms of temperature at a depth of 0.25 mm from the surface of the steel plate, exceeds 50°C/s, and the SSCC resistance deteriorates in very severe corrosive environments with partial pressure of hydrogen sulfide 2 bar.

Industrial Applicability

According to the present invention, it is possible to obtain a high-strength steel plate for acid-resistant pipeline, which is excellent not only in HIC resistance property, but also in SSCC resistance in more severe corrosive environments and environments with low hydrogen sulfide partial pressure below 1 bar. Therefore, steel pipes (such as resistance welded steel pipes, spiral steel pipes, and UOE steel pipes) produced by cold forming a steel plate according to the invention can be suitably used for transporting crude oil and natural gas that contain hydrogen sulfide, where acid resistance required.

Claims (14)

1. High-strength steel plate for acid-resistant pipeline, having: chemical composition containing in wt%: 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, at least one component selected from the group , consisting of Nb: 0.005 to 0.1 and Ti: 0.005 to 0.1, if necessary, at least one component selected from the group consisting of Cu: 0.50 or less, Ni: 0.10 or less , Cr: 0.50 or less, 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 , the rest - Fe and inevitable impurities; a steel microstructure at a depth of 0.25 mm from the surface of the steel plate, which is a bainitic microstructure having a dislocation density of 1.0×10 ¹⁴ to 7.0×10 ¹⁴ (m ^-2 ); fluctuations in Vickers hardness at a depth of 0.25 mm from the surface of the steel plate, amounting to 30 HV or less; And tensile strength of 520 MPa or more. 2. A method for producing high-strength steel plate for acid-resistant pipeline, including: heating the slab to a temperature of from 1000°C to 1300°C, and the slab has a chemical composition containing in wt.%: C: from 0.02 to 0.08, Si: from 0.01 to 0.50, Mn: from 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: from 0.0005 to 0.005, at least one component selected from the group consisting of Nb: from 0.005 to 0.1 and Ti: from 0.005 to 0.1, optionally at least one component selected from the group consisting of of Cu: 0.50 or less, Ni: 0.10 or less, Cr: 0.50 or less, 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, the rest is Fe and inevitable impurities, and then hot rolling the slab to form a steel plate; then subject the steel plate to controlled cooling in a mode including the following conditions: the surface temperature of the steel plate at the start of cooling is (Ar ₃ - 10°C) or higher; the average cooling rate in the temperature range from 750°C to 550°C, in relation to the temperature values at a depth of 0.25 mm from the surface of the steel plate, is 50°C/s or less; the average cooling rate in the temperature range from 750°C to 550°C, in relation to the values of the average temperature of the steel plate, is 15°C/s or higher; the average cooling rate in the temperature range from 550°C to the cooling stop temperature, in relation to the temperature values at a depth of 0.25 mm from the surface of the steel plate, is 150°C/s or higher; And the cooling stop temperature, in relation to the values of the average temperature of the steel plate, is from 250°C to 550°C. 3. High-strength steel pipe for acid-resistant pipeline obtained by forming and welding high-strength steel plate for acid-resistant pipeline according to item 1.

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