Martensitic stainless steel having high strength and hydrogen sulfide stress corrosion resistance, and manufacturing method therefor

Abstract

The present invention relates to martensitic stainless steel having high strength and hydrogen sulfide stress corrosion resistance, and a manufacturing method therefor. The martensitic stainless steel of the present invention has a high yield strength of 125 ksi or higher level (yield strength being greater than or equal to 862 MPa), also has excellent resistance to sulfide stress corrosion cracking (SSC), is suitable for a service environment having an H2S partial pressure of 0.1 bar, and still maintains good corrosion resistance in a high-concentration CO2 environment at a temperature as high as 177°C. The stainless steel of the present invention can be processed into a variety of product forms such as bars, plates, steel pipes, etc., and is widely applicable to application scenarios that have strict requirements on material strength and corrosion resistance, such as oil and natural gas production equipment and carbon dioxide capture, utilization and storage (CCUS) systems.

Description

High-strength hydrogen sulfide stress corrosion resistant martensitic stainless steel and manufacturing method thereof TECHNICAL FIELD

[0001] The present disclosure relates to a martensitic stainless steel, in particular to a high-strength hydrogen sulfide stress corrosion resistant martensitic stainless steel and a manufacturing method thereof. BACKGROUND

[0002] With the increasing demand for the development of deep and ultra-deep oil and gas resources, corrosion-resistant alloy oil casing products are also developing towards high strength. Among them, martensitic stainless steel has been widely used in the fields of high CO2-containing oil and gas field development and carbon capture, utilization and storage (CCUS) due to its low alloy cost and excellent CO2 corrosion resistance.

[0003] However, in the complex corrosion environment containing H2S, the sulfide stress corrosion cracking (SSC) resistance of traditional martensitic stainless steel and even super martensitic stainless steel becomes a key factor limiting its application. According to the ISO 15156-3 standard, the upper limit of the yield strength of super martensitic stainless steel with SSC resistance is limited to 724 MPa, which makes it often only possible to choose super duplex stainless steel or even nickel-based alloy with higher cost in the service environment with high CO2 partial pressure and a small amount of H2S. Therefore, improving the SSC resistance of martensitic stainless steel is a key direction to expand its engineering application range.

[0004] Chinese Patent Publication No. CN115768914A discloses a martensitic stainless steel with a yield strength of 125 ksi (862 MPa) or more, which has excellent low-temperature toughness and corrosion resistance. The composition of the steel material is C: less than 0.030%, Si: 1.00% or less, Mn: 0.05-2.00%, Cr: 11.50-14.00%, Ni: 5.00-7.50%, Mo: 1.10-3.50%, Cu: 0.50-3.50%, Co: 0.01-0.30%, Al: 0.001-0.100%, N: 0.001-0.100%, and the balance of Fe and impurities; the microstructure is 0-15% by volume of residual austenite, 0-10% by volume of ferrite, and the balance is martensite; the yield strength is 862 MPa or more; the number density of Cu precipitates is 3.0x10 21 -50.0x10 21 / m 3 The steel material introduces a high content of Cu and Co in the alloy system to improve the strength and corrosion resistance, but Cu and Co as precious metals not only increase the cost, but also may reduce the hot working performance, affect the surface quality and yield.

[0005] Chinese patent publication CN109563581A discloses a high-strength stainless steel seamless pipe for oil wells with excellent low-temperature toughness, carbon dioxide corrosion resistance, sulfide stress corrosion cracking resistance, and sulfide stress cracking resistance. The seamless pipe contains, in mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15-1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 14.5-17.5%, Ni: 3.0-6.0%, Mo: 2.7-5.0%, Cu: 0.3-4.0%, W: 0.1-2.5%, V: 0.02-0.20%, Al: 0.10% or less, N: 0.15% or less, with the balance consisting of Fe and unavoidable impurities; and C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a specific relationship, and Cu, Mo, W, Cr, and Ni satisfy another specific relationship; the martensite phase is greater than 45%, the ferrite phase is 10-45%, and the residual austenite phase is 30% or less; the total amount of precipitated Cr, precipitated Mo, and precipitated W is 0.75 mass % or less; and the yield strength is 862 MPa or more. However, the Cr content of the steel material is greater than 14.5% (i.e., 17Cr steel grade), and a large amount of noble metal elements such as Mo and W are added to the steel material, which is high in cost. In addition, the ferrite proportion of the steel material is high, and the risk of cracking during hot working is great, and the proportion of austenite is also high, which can easily lead to a decrease in the strength of the steel material during implementation.

[0006] Chinese patent publication CN104884658A discloses a high-strength stainless steel seamless pipe containing, in mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.15-1.0%, P: 0.030% or less, S: 0.005% or less, Cr: 15.5-17.5%, Ni: 3.0-6.0%, Mo: 1.5-5.0%, Cu: 4.0% or less, W: 0.1-2.5%, and N: 0.15% or less; and the microstructure contains a martensite phase as a base phase and a ferrite phase as a second phase at 10-60% by volume ratio. However, the steel material adds noble metal elements such as Cu and W, which is high in cost; and the ferrite proportion is high, and the risk of cracking during hot working is great.

[0007] Chinese patent publication CN106414785A discloses a conventional martensitic stainless steel having a composition containing Cr, Ni in a manner satisfying Cr / Ni≤5.3, having a structure in which a tempered martensite phase is a main phase, the structure having a surface layer structure in which a phase that appears white by etching using a Vilella etching solution has a thickness of 10 μm or more and 100 μm or less in a wall thickness direction from a pipe outer surface, and the phase that appears white by etching using the Vilella etching solution is dispersed at 50% or more on the pipe outer surface in terms of area percentage. The yield strength of the steel pipe is only 654 MPa, which is difficult to meet the requirement of high strength for deep well service.

[0008] In summary, the existing high-strength martensitic stainless steel still has deficiencies in SSC resistance, structure control, cost and manufacturability. The high-strength corrosion-resistant materials that can meet the service requirement of H2S partial pressure up to 0.1 bar usually use iron-nickel-based or nickel-based alloys, but these materials are high in cost and limited in resources. Therefore, it is desirable to obtain a corrosion-resistant alloy material with a high yield strength of 125 ksi or more (≥862 MPa) and resistance to high concentration of CO2 and small amount of H2S. SUMMARY

[0009] The purpose of the present disclosure is to provide a high-strength martensitic stainless steel resistant to hydrogen sulfide stress corrosion and a manufacturing method thereof. The stainless steel has a high yield strength of 125 ksi level (yield strength ≥862 MPa) or more, while having excellent sulfide stress corrosion cracking (SSC) resistance, and can be applied to a service environment with H2S partial pressure of 0.1 bar, and still maintains good corrosion resistance in a high concentration CO2 environment with a temperature up to 177℃.

[0010] In a first aspect, the present disclosure provides a martensitic stainless steel containing, in addition to Fe and unavoidable impurities, the following chemical elements in mass percentage:

[0011] C: 0.01-0.040%, Si: 0.10-0.40%, Mn: 0.20-1.0%, P≤0.020%, S≤0.005%, O≤0.004%, Al: 0.01-0.1%, Ca: 0.0001-0.004%, Cr: 12.0-14.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, N≤0.020%, Nb: 0.01-0.15%, V: 0.03-0.15%, rare earth elements: 0.0001-0.30%;

[0012] The rare earth elements include Nd, Ce, Er, Pr, Pm, Dy or La.

[0013] In a second aspect, the present disclosure provides a martensitic stainless steel, each chemical element in the martensitic stainless steel having a mass percentage of:

[0014] C: 0.01-0.040%, Si: 0.10-0.40%, Mn: 0.20-1.0%, P≤0.020%, S≤0.005%, O≤0.004%, Al: 0.01-0.1%, Ca: 0.0001-0.004%, Cr: 12.0-14.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, N≤0.020%, Nb: 0.01-0.15%, V: 0.03-0.15%, rare earth elements: 0.0001-0.30%, and the balance being Fe and other inevitable impurity elements;

[0015] The rare earth elements include Nd, Ce, Er, Pr, Pm, Dy, or La.

[0016] In a preferred embodiment, the martensitic stainless steel of the present disclosure does not contain Cu.

[0017] In a preferred embodiment, the content of C in the martensitic stainless steel of the present disclosure is 0.012%-0.035%.

[0018] In a preferred embodiment, the content of Si in the martensitic stainless steel of the present disclosure is 0.10-0.30%.

[0019] In a preferred embodiment, the content of Mn in the martensitic stainless steel of the present disclosure is 0.20-0.50%.

[0020] In a preferred embodiment, the content of P in the martensitic stainless steel of the present disclosure is 0.015% or less.

[0021] In a preferred embodiment, the content of O in the martensitic stainless steel of the present disclosure is 0.003% or less.

[0022] In a preferred embodiment, the content of Al in the martensitic stainless steel of the present disclosure is 0.015-0.08%.

[0023] In a preferred embodiment, the content of Ca in the martensitic stainless steel of the present disclosure is 0.0005-0.0035%.

[0024] In a preferred embodiment, the content of Cr in the martensitic stainless steel of the present disclosure is 12.20-13.50%.

[0025] In a preferred embodiment, the content of Ni in the martensitic stainless steel of the present disclosure is 4.50-5.50%.

[0026] In a preferred embodiment, the content of Mo in the martensitic stainless steel of the present disclosure is 1.50-2.50%.

[0027] In a preferred embodiment, the content of N in the martensitic stainless steel of the present disclosure is 0.015% or less.

[0028] In a preferred embodiment, the content of Nb in the martensitic stainless steel of the present disclosure is 0.015-0.10%.

[0029] In a preferred embodiment, the content of V in the martensitic stainless steel of the present disclosure is 0.05-0.10%.

[0030] In a preferred embodiment, the content of REM in the martensitic stainless steel of the present disclosure is 0.001-0.20%.

[0031] In a preferred embodiment, the microstructure of the martensitic stainless steel of the present disclosure comprises: 2-10% of austenite by volume fraction, preferably 2-7% of austenite, 0-2% of ferrite, preferably 0.5-1.5% of ferrite, and the rest is tempered martensite; the austenite is the sum of residual austenite and reversed austenite.

[0032] In a preferred embodiment, the residual austenite and reversed austenite in the martensitic stainless steel of the present disclosure account for 2-10% by volume.

[0033] In a preferred embodiment, the martensitic stainless steel of the present disclosure has one or more of the following properties:

[0034] - yield strength ≥ 862 MPa;

[0035] - sulfide stress cracking (SSC) resistance reaching the threshold value of 80% SMYS under the condition of 0.1 Bar H2S partial pressure; and / or

[0036] - corrosion rate ≤ 0.1 mm / a under the condition of 177℃ and high concentration CO2 environment, wherein the high concentration CO2 environment refers to the partial pressure of CO2 being 0.9 atm or more, preferably 5 atm or more, more preferably 30 atm or more.

[0037] In a preferred embodiment, the tensile strength of the martensitic stainless steel of the present disclosure is 900 MPa or more, preferably 916 MPa or more.

[0038] In a preferred embodiment, the impact toughness at -10℃ of the martensitic stainless steel of the present disclosure is 95 J or more, preferably 99 J or more.

[0039] In a preferred embodiment, the rating of Ca, Al oxide-based B-type and D-type inclusions in the martensitic stainless steel of the present disclosure is 1.0 or less. The definitions and ratings of B-type and D-type inclusions are both in accordance with GB / T10561.

[0040] The present disclosure controls the addition amount of alloying elements such as Cr and Mo, reduces the content of ferrite, and does not add Cu element which is prone to cause copper brittleness during hot working, in order to reduce alloy cost and improve hot working performance.

[0041] The stainless steel of the present disclosure can be processed into various product forms such as bars, plates, pipes, etc., and is widely applicable to harsh application scenarios such as oil and gas production equipment, carbon capture and storage (CCUS) systems, etc., which require high strength and corrosion resistance.

[0042] In a third aspect, the present disclosure provides a method for preparing the martensitic stainless steel of the present disclosure, comprising the following steps:

[0043] 1) smelting and casting to obtain a slab;

[0044] 2) hot working after heating the slab to 1150-1250℃;

[0045] 3) quenching treatment: quenching temperature is AC3+30℃, holding time t1=(3-10)×h, t1 is in min unit; h is the plate thickness, unit mm; then cooling to below 150℃ at a cooling rate of 0.5℃ / s or more, preferably 1-60℃ / s;

[0046] 4) tempering treatment: tempering temperature is 500-650℃, holding time t2=(5-15)×h, t2 is in min unit; h is the plate thickness, unit mm; then air cooling to room temperature.

[0047] In a preferred embodiment, in step 1), a converter, electric furnace or vacuum induction furnace is used for smelting, followed by continuous casting and ingot initial rolling to obtain a slab.

[0048] Compared with the prior art, the martensitic stainless steel and the preparation method thereof of the present disclosure have the following beneficial effects:

[0049] In the prior art, in order to achieve a martensitic stainless steel with a yield strength of 125 ksi (i.e. yield strength ≥862MPa) and both hydrogen sulfide stress corrosion (SSC) resistance and CO2 corrosion resistance, a high content of alloying elements such as Ni, Mo and Cu is generally relied on. This not only significantly increases the material cost, but also easily causes copper brittleness during hot working, thereby reducing the yield and surface quality.

[0050] In comparison, the present disclosure adopts an alloy system with Cr, Ni, and Mo as the main strengthening elements. By controlling the addition amount of alloying elements such as Cr and Mo, the content of ferrite is reduced, and the addition of Cu and high-cost alloying elements such as Mo and Cu is avoided. In this way, the material cost is significantly reduced while the performance is ensured, and the hot workability is improved.

[0051] High-strength martensitic stainless steels in the prior art have a high ferrite content, which is prone to cracking during hot working, and the austenite content is not properly controlled, which can result in insufficient strength.

[0052] The present disclosure achieves a microstructure of the martensitic stainless steel containing 2-10% austenite, 0-2% ferrite, and the rest being tempered martensite, wherein the austenite includes the sum of residual austenite and reversed austenite. The yield strength of the martensitic stainless steel of the present disclosure reaches a high strength of more than 125 ksi (yield strength ≥ 862 MPa), the SSC resistance under a H2S partial pressure of 0.1 Bar reaches the threshold value of 80% SMYS, and the corrosion rate in a high-concentration CO2 environment at a temperature as high as 177℃ is ≤ 0.1 mm / a. The martensitic stainless steel can be processed into various product forms such as bars, plates, and pipes, and is widely applicable to harsh application scenarios such as petroleum and natural gas production equipment and carbon capture and storage (CCUS) systems, which have high requirements for material strength and corrosion resistance.

[0053] The B-type and D-type inclusions of the martensitic stainless steel of the present disclosure mainly include Ca and Al oxides, and the inclusion rating is below 1.0.

[0054] Although there have been attempts to use rare earth elements (REM) in the prior art to improve sulfide stress corrosion cracking, the mechanism is not clear, and the actual effect is limited. The present disclosure innovatively uses the synergistic addition of rare earth elements and Nb elements to significantly optimize the size and distribution of inclusions, reduce the potential difference between the inclusions and the matrix, and thus effectively improve the resistance of high-strength martensitic stainless steel to hydrogen sulfide stress corrosion cracking. DETAILED DESCRIPTION

[0055] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0056] In this document, the volume fraction of austenite is measured by X-ray diffraction.

[0057] In this document, the volume fraction of ferrite is measured by metallographic method according to ASTM E562 standard.

[0058] In this document, the yield strength and tensile strength are determined according to ASTM E8 standard.

[0059] In the present text, the impact toughness at -10°C is determined according to the GB / T 229 standard.

[0060] In the present text, the rating of inclusions is determined according to the GB / T 10561 standard.

[0061] In the present text, the threshold of the anti-sulfide stress cracking (SSC) performance reaching 80% SMYS means that no stress corrosion cracking occurs under the loading of 80% SMYS.

[0062] In the martensitic stainless steel of the present disclosure, the design principles of each chemical element are as follows:

[0063] Carbon (C): C is an austenite-forming element in the martensitic stainless steel, and by increasing the C content, the percentage of austenitization of the stainless steel at high temperature can be increased to obtain martensite at room temperature, thereby increasing the strength. In addition, C also has the effect of adjusting the proportion of Nb and V precipitates in the present disclosure. In order to achieve the above effects, the C content should be above 0.01%. However, when the C content is too high, the corrosion resistance of the stainless steel will decrease, and the toughness will also decrease. Therefore, in the present disclosure, in order to obtain better performance, the C content is controlled to be 0.01-0.04%, preferably 0.012%-0.035%.

[0064] Silicon (Si): Si is an important deoxidizer in the steelmaking process, but Si has the risk of promoting the formation of σ phase and ferrite phase in stainless steel with high Cr content, which has adverse effects on the toughness and corrosion resistance of the stainless steel. Therefore, the Si content is controlled to be 0.1-0.4% in the present disclosure, preferably 0.1-0.3%.

[0065] Manganese (Mn): Mn can increase the strength of the stainless steel. In the present disclosure, in order to ensure that the steel has sufficient strength, the Mn addition amount is above 0.2%. However, when Mn exceeds 1.0%, the toughness decreases. Therefore, the Mn content is controlled to be 0.2-1.0% in the present disclosure, preferably 0.2-0.5%.

[0066] Phosphorus (P): P is a harmful element that reduces the CO2 corrosion resistance at high temperature, and has adverse effects on the hot working performance. If the P content exceeds 0.02%, the corrosion resistance cannot meet the requirements of the high temperature environment. Therefore, the P content is controlled to be ≤0.02% in the present disclosure, preferably P≤0.015%.

[0067] Sulfur (S): S is a harmful element that reduces the hot working performance and has adverse effects on the impact toughness. If the S content exceeds 0.005%, the steel pipe cannot be normally manufactured. Therefore, the S content is controlled to be ≤0.005% in the present disclosure.

[0068] Oxygen (O): O is a harmful element in steel, which can cause poor toughness. And after O forms complex inclusions with Al, Ca and the like, it can cause the incomplete passivation film on the surface of stainless steel, thereby causing the local corrosion and other phenomena of electrochemical corrosion of the micro-environment of the steel in the corrosive medium, and then reducing the general corrosion resistance and stress corrosion resistance. In order to ensure the corrosion resistance, the O content in the present disclosure is controlled to be ≤0.004%, preferably ≤0.003%.

[0069] Aluminum (Al): Al is used as a deoxidizer in the smelting process. When the Al content is too low, the deoxidizing effect cannot be achieved. But when the Al content is too high, it cannot play a greater deoxidizing effect. And when the Al content is too high, it is easy to cause the steel liquid to be viscous, and the steel slag is not easy to float, thereby increasing the number and size of inclusions. Therefore, the Al content in the present disclosure is controlled to be 0.01-0.1%, preferably 0.015-0.08%.

[0070] Calcium (Ca): Ca treatment can play a role in modifying inclusions, and has a positive effect on improving the internal quality and anisotropy of steel. However, when the content of Ca element in the steel is too much, it will have some adverse effects on the performance of the steel. Excessive calcium can cause segregation in the steel, i.e. uneven distribution of calcium elements in the steel. This uneven distribution can affect the mechanical properties and processing properties of the steel, and can form larger inclusions. The distribution and stability of these inclusions in the steel can be poor, thereby adversely affecting the toughness of the steel. Therefore, the Ca content in the present disclosure is controlled to be 0.0001-0.004%, preferably 0.0005-0.0035%.

[0071] Chromium (Cr): Cr is an important element in stainless steel that improves corrosion resistance. The addition of Cr enables the surface of stainless steel to rapidly form a corrosion-resistant passivation film in the air, improving the CO2 corrosion resistance of the oil sleeve under high temperature environment. In order to obtain CO2 corrosion resistance at up to 177℃, the addition amount of Cr in the stainless steel system of the present disclosure should be more than 12.0%. On the other hand, when the addition of Cr element in the alloy system of the present disclosure exceeds 14%, it will increase the risk of ferrite precipitation, which will adversely affect the hot working properties and corrosion resistance of the product. Therefore, the Cr content in the present disclosure is controlled to be 12.0-14.0%, preferably 12.2-13.5%.

[0072] Nickel (Ni): Ni can expand the austenite phase region and improve the corrosion resistance and toughness of stainless steel, especially under high temperature conditions, which can effectively enhance the stress corrosion cracking (SCC) resistance of the material. In order to achieve the above properties, the content of Ni should be no less than 4.0%. However, considering that Ni is a relatively expensive alloy element, in order to control the cost, the content of Ni in the present disclosure is limited to 4.0-6.0%, preferably 4.5-5.5%.

[0073] Molybdenum (Mo): Mo is an element that improves the pitting corrosion resistance of stainless steel in the presence of Cl ions, and also helps to improve the corrosion resistance in high temperature (such as above 150°C) environments. Therefore, the Mo content should not be less than 1.0%. However, Mo is a noble metal, and when the content exceeds 3.0%, a large amount of ferrite is formed, which is not conducive to the hot workability and corrosion resistance. Therefore, the Mo content is controlled in the present disclosure to be 1.0-3.0%, preferably 1.5-2.5%.

[0074] Nitrogen (N): N can improve the pitting corrosion resistance of stainless steel, and as a strong austenite forming element, it helps to increase the proportion of martensite and enhance the strength. However, excessive nitrogen will cause lattice distortion, thereby reducing impact toughness. Considering the demand for toughness of the material at low temperature or in harsh working conditions, the present disclosure controls nitrogen as a residual element, with a content not exceeding 0.020%, preferably not higher than 0.015%.

[0075] Niobium (Nb): Nb in the present invention is used to suppress the precipitation of excessive reverse austenite during tempering, as the austenite content is closely related to the strength of the martensitic stainless steel. By adding Nb, the precipitation of austenite during tempering can be effectively suppressed, retaining the high strength characteristics of the martensite. To achieve the above effect, the Nb content should not be less than 0.01%; but when the content exceeds 0.15%, its strengthening effect tends to be saturated, while significantly increasing the cost of the material. Therefore, the Nb content is controlled to be 0.01-0.15%, preferably 0.015-0.10%.

[0076] Vanadium (V): V is mainly used for precipitation strengthening. In the present disclosure, the addition of V is to form V carbide during annealing before cold rolling. The vanadium carbide formed during annealing becomes nucleation sites during cold rolling, changing the direction of dislocation movement, thereby playing a role in refining the grains during subsequent quenching process. To achieve the above effect, the addition amount of V should be more than 0.03%. However, when the V content exceeds 0.15%, the strength increases and the toughness decreases. Therefore, the present disclosure controls the V content to be 0.03-0.15%, preferably 0.05-0.10%.

[0077] The rare earth elements in the martensitic stainless steel of the present disclosure include but are not limited to Nd, Ce, Er, Pr, Pm, Dy, La, etc. The rare earth elements are added to the molten steel in the later stage of smelting, which can effectively purify the molten steel. The oxides formed by the combination of rare earth elements and O in the molten steel have good dispersibility and are not easy to agglomerate, thereby ensuring that the Ca, Al oxide related B and D type inclusions are controlled to be below 1.0 level.

[0078] It should be particularly pointed out that the common addition of Nb and rare earth elements in the present disclosure makes the inclusions modified in size, and the composition of Ca and Al oxide inclusions contains rare earth and Nb elements, etc., thereby reducing the potential difference between the inclusions and the matrix, reducing the risk of local electrochemical corrosion, and improving the corrosion resistance. In order to achieve the above effects, the addition amount of rare earth elements should be more than 0.0001%. However, if the addition amount of rare earth elements is too high, it will cause difficulty in floating of steel slag, and it is easy to form nodules in the pouring of steel, which affects the pouring process. In order to obtain beneficial effects, the content of rare earth elements is limited to the range of 0.0001-0.30%, preferably in the range of 0.001-0.2%.

[0079] The microstructure of the martensitic stainless steel material of the present disclosure after final heat treatment includes 2-10% of austenite, 0-2% of ferrite, and the rest is tempered martensite. The austenite is the total of residual austenite and reversed austenite. The "the rest is tempered martensite" in the present disclosure means that the volume fraction of other microstructures is negligible except for austenite, ferrite and tempered martensite. That is, the total amount of other precipitated phases and inclusions except for austenite, ferrite and tempered martensite is negligible.

[0080] The microstructure of the steel of the present disclosure contains 2-10% of austenite, which is the total of residual austenite and reversed austenite. The residual austenite refers to the austenite content of the steel material in the quenched state in the present disclosure, and the reversed austenite refers to the austenite content precipitated in the steel after tempering. The content of austenite is the volume fraction of austenite measured by X-ray diffraction method on the steel after tempering. The addition of Nb in the present disclosure can control the volume fraction of austenite. If the volume fraction of austenite is too high, the controllability of strength is poor. In order to make the steel material meet the requirements of 125 ksi steel grade (yield strength ≥862MPa), the upper limit of the content of austenite is controlled to be 10%, and the lower limit is controlled to be 2%.

[0081] The microstructure of the steel of the present disclosure contains 0-2% of ferrite, which is a Cr-rich phase formed due to the segregation of element composition. The content of ferrite is measured by metallographic method according to ASTM E562 standard. The existence of a small amount of ferrite as a heterogeneous structure has the effect of hindering crack propagation, thereby obtaining better fracture toughness and stress corrosion cracking resistance. However, >2% of ferrite will reduce the impact performance and corrosion resistance of the steel.

[0082] The inclusion rating of the steel material of the present disclosure is evaluated according to GB / T 10561 standard, and the control and classification of B-type and D-type inclusions which have significant influence on hydrogen sulfide stress corrosion cracking (SSC) performance are focused on. In order to improve the stress corrosion resistance of the material in the environment containing H2S, the coarse and fine ratings of B-type and D-type inclusions are both controlled to be not higher than 1.0 grade.

[0083] In summary, the inventors have found through extensive research that the element Nb has a unique role in inhibiting the precipitation of reversed austenite. By adding Nb, the precipitation of reversed austenite can be effectively reduced under the same quenching + tempering heat treatment conditions. In addition, one of the main mechanisms of stress corrosion cracking of martensitic stainless steel in an H2S environment is the occurrence of micro-area local corrosion on the surface of incomplete passivation film. Therefore, the present disclosure controls the phase ratio of austenite, ferrite and tempered martensite, and simultaneously adds rare earth elements and Nb elements to modify the inclusions, so that the Ca, Al and other oxide inclusions are below 1.0 grade, and the potential difference between the inclusions and the matrix is reduced, significantly improving the ability to resist H2S stress corrosion.

[0084] To verify the applicability of the martensitic stainless steel of the present disclosure in a service environment containing H2S and high concentration CO2, the following tests for evaluating the resistance to hydrogen sulfide stress corrosion cracking (SSC) and corrosion resistance were performed:

[0085] 1. Hydrogen sulfide stress corrosion cracking (SSC) performance test

[0086] The test piece was immersed in an aqueous solution containing 5% NaCl, the liquid temperature was 24±3℃, and the atmosphere was 0.1 atm of H2S and 0.9 atm of CO2. The pH of the system was adjusted to 4.2 by adding aqueous solutions of acetic acid and sodium acetate. In this environment, the test piece was subjected to a stress of 80% of its nominal yield strength for 720 hours of continuous immersion. The test results showed that the test piece did not crack, indicating that the material has good resistance to SSC.

[0087] 2. Corrosion resistance test in high temperature and high pressure CO2 environment

[0088] The test piece was immersed in an aqueous solution containing 25% NaCl, the liquid temperature was 177±3℃, and the atmosphere was 0.1 atm of H2S and 30 atm of CO2. The pH of the system was adjusted to 4.2 by adding acetic acid and sodium acetate, and the immersion time was set to 720 hours. After the test, the corrosion weight loss rate of the test piece was less than 0.1 mm / a, showing excellent CO2 corrosion resistance.

[0089] The shape of the high-strength hydrogen sulfide stress corrosion resistant martensitic stainless steel of the present disclosure is not particularly limited and can be a bar, a plate or a pipe.

[0090] Examples

[0091] The non-oriented electrical steel sheet and the manufacturing method thereof of the present disclosure will be further explained and described below in conjunction with specific examples and the accompanying drawings of the specification, however, the explanation and description do not constitute undue limitations on the technical solutions of the present disclosure.

[0092] The steel materials of Examples 1-11 and Comparative Examples A-D were prepared by the following steps:

[0093] 1) smelting and casting to obtain a slab;

[0094] 2) heating the slab and hot working to form;

[0095] 3) quenching treatment;

[0096] 4) tempering treatment.

[0097] Table 1 lists the mass percentage of each chemical element in the steel materials of Examples 1-11 and Comparative Examples A-D. Table 2 lists the specific process parameters of the steel materials of Examples 1-11 and Comparative Examples A-D.

[0098] The steel materials after the above heat treatment were subjected to the following tests, and the test results are listed in Table 3.

[0099] Yield strength test: The steel materials of each example and comparative example were prepared into threaded tensile specimens according to ASTM A370 standard, and tensile test was carried out according to ASTM E8 standard, and the average value of multiple samples was taken as the test result and recorded in Table 3.

[0100] Tensile strength test: determined according to ASTM E8 standard.

[0101] Impact toughness test (Charpy V-type impact absorbed energy): V-type impact specimens with a transverse size of 10*10*55 mm were cut from the steel materials, and tested according to GB / T 229 standard, and the average value of multiple samples was taken as the test result and converted into the value of 10*10*55 (mm) full size according to API 5CT standard and listed in Table 3, and the test temperature was -10℃.

[0102] Test of austenite content: In the full thickness range of the steel material, the cross section perpendicular to the rolling direction was taken as the test surface, and the volume fraction of austenite was determined by D8 DISCOVER X-ray diffractometer combined with TOPAS4 analysis software.

[0103] Determination of ferrite volume fraction: according to ASTM E562-2019 standard, the full thickness cross section of the steel material was taken along the longitudinal direction of the rolling direction, and 30 metallographic photos covering the full cross section were taken by equidistant method, and the volume fraction of ferrite was calculated by point counting method.

[0104] Inclusion rating: according to GB / T 10561 standard, the full thickness cross section of the steel material was taken along the longitudinal direction of the rolling direction for rating. The rating results of the coarse and fine inclusions of B and D of the examples and comparative examples are listed in Table 3.

[0105] H2S stress corrosion test: according to the method A in NACE TM0177 standard, the test piece is immersed in a 5% NaCl aqueous solution, the liquid temperature is 24±3℃, the atmosphere is 0.1 atm H2S and 0.9 atm CO2. The pH of the system is adjusted to 4.2 by adding acetic acid and sodium acetate aqueous solution. In this environment, the test piece is applied with 80% of its nominal yield strength as stress, after 720 hours of immersion, whether there is macroscopic or microscopic crack is observed by naked eye and 10 times visual microscope, and the test results are listed in Table 3.

[0106] H2S, CO2, Cl - Coexisting corrosion test: the test piece is immersed in a 25% NaCl aqueous solution, the liquid temperature is 177±3℃, the atmosphere is 0.1 atm H2S and 30 atm CO2. The pH of the system is adjusted to 4.2 by adding acetic acid and sodium acetate, and the immersion time is set to 720 hours. The corrosion rate is calculated by weight loss method, and the results are listed in Table 3.

[0107] As can be seen from Table 3, the stainless steel of the embodiments of the present disclosure has excellent comprehensive performance: the yield strength thereof reaches or exceeds 862 MPa, showing good high-strength characteristics; at the same time, it shows excellent stress corrosion cracking resistance in the environment containing H2S and high-concentration CO2, and the corrosion rate is below 0.1 mm / a in the high-temperature corrosion test, showing excellent corrosion resistance.

[0108] The steel material of the present disclosure realizes a good balance between high strength and corrosion resistance without significantly increasing the content of valuable alloy elements such as Cr, Mo and Cu. Compared with the prior art, it has obvious cost advantage and processing adaptability.

[0109] All publications, patent applications, patents and other references mentioned in the present disclosure are incorporated herein by reference in their entirety.

[0110] Although the present disclosure has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the foregoing is a description of a preferred embodiment of the present disclosure and that various changes can be made in form and details by those skilled in the art without departing from the spirit and scope of the present disclosure.

Claims

1. A martensitic stainless steel, characterized by, The martensitic stainless steel contains the following chemical elements in mass percentage, in addition to Fe and inevitable impurities: C: 0.01-0.040%, Si: 0.10-0.40%, Mn: 0.20-1.0%, P≤0.020%, S≤0.005%, O≤0.004%, Al: 0.01-0.10%, Ca: 0.0001-0.004%, Cr: 12.0-14.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, N≤0.020%, Nb: 0.01-0.15%, V: 0.03-0.15%, rare earth elements: 0.0001-0.30%; The rare earth elements include Nd, Ce, Er, Pr, Pm, Dy or La.

2. A martensitic stainless steel, characterized by, The mass percentage of each chemical element in the martensitic stainless steel is as follows: C: 0.01-0.040%, Si: 0.10-0.40%, Mn: 0.20-1.0%, P≤0.020%, S≤0.005%, O≤0.004%, Al: 0.01-0.1%, Ca: 0.0001-0.004%, Cr: 12.0-14.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, N≤0.020%, Nb: 0.01-0.15%, V: 0.03-0.15%, rare earth elements: 0.0001-0.30%, the balance being Fe and other inevitable impurity elements; The rare earth elements include Nd, Ce, Er, Pr, Pm, Dy or La.

3. Martensitic stainless steel according to claim 1 or 2, characterized in that The martensitic stainless steel does not contain Cu; and / or the mass percentage of each chemical element in the martensitic stainless steel satisfies one or more of the following: C: 0.012%-0.035%, and / or, Si: 0.10-0.30%, and / or, Mn: 0.20-0.50%, and / or, P≤0.015%, and / or, O≤0.003%, and / or, Al: 0.015-0.08%, and / or, Ca: 0.0005-0.0035%, Cr: 12.20-13.50%, and / or, Ni: 4.50-5.50%, and / or, Mo: 1.50-2.50%, and / or, N≤0.015%, and / or, Nb: 0.015-0.10%, and / or, V: 0.05-0.10%, and / or, rare earth elements: 0.001-0.20%.

4. Martensitic stainless steel according to any one of claims 1 to 3, characterized in that, The microstructure of the martensitic stainless steel includes: 2-10% austenite by volume fraction, preferably 2-7% austenite, 0-2% ferrite, preferably 0.5-1.5% ferrite, and the rest is tempered martensite; The austenite is the total of residual austenite and reversed austenite.

5. The martensitic stainless steel according to any one of claims 1 to 4, characterized in that, The martensitic stainless steel has the following properties: a yield strength of ≥ 862 MPa; a sulphide stress cracking (SSC) resistance at a H2S partial pressure of 0.1 bar of at least 80% of the SMYS threshold; and / or a corrosion rate of ≤ 0.1 mm / a at a temperature of 177°C in a high concentration of CO2, i.e. a CO2 partial pressure of 0.9 atm or more, preferably 5 atm or more, more preferably 30 atm or more.

6. The martensitic stainless steel according to any one of claims 1 to 5, characterized in that, The tensile strength of the martensitic stainless steel is 900 MPa or more, preferably 916 MPa or more.

7. The martensitic stainless steel according to any one of claims 1 to 6, characterized in that, The -10°C impact toughness of the martensitic stainless steel is 95 J or more, preferably 99 J or more.

8. The martensitic stainless steel according to any one of claims 1 to 7, characterized in that, The rating of B and D type inclusions, mainly Ca, Al oxides, in the martensitic stainless steel is 1.0 or less.

9. A method for producing the martensitic stainless steel according to any one of claims 1 to 8, comprising the steps of: 1) smelting and casting to obtain a slab; 2) hot working after heating the slab to 1150-1250°C; 3) quenching at a temperature of AC3+30°C for a time t1 = (3-10) x h, t1 in min; h is the thickness of the slab in mm; and then cooling to 150°C or less at a cooling rate of 0.5°C / s or more, preferably 1-60°C / s; 4) tempering at a temperature of 500-650°C for a time t2 = (5-15) x h, t2 in min; h is the thickness of the slab in mm; and then air cooling to room temperature.

10. The method of claim 9, wherein, In step 1), the smelting is performed using a converter, an electric furnace or a vacuum induction furnace, followed by continuous casting and ingot initial rolling to obtain a slab.

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