A kind of ultra-high strength corrosion-resistant economical steel and its preparation method

Abstract

The invention discloses an ultra-high strength corrosion-resistant economical steel and a preparation method thereof, which belongs to the technical field of corrosion-resistant steel, and solves the problems of high cost of corrosion-resistant steel and difficulty in matching high strength and high corrosion resistance in the prior art. The preparation method of ultra-high strength corrosion-resistant economical steel comprises: step 1: melting raw materials into molten steel; step 2: smelting molten steel into continuous casting billet or ingot by continuous casting or die casting; step 3: heating and keeping the continuous casting billet or ingot warm, forging and opening the billet to make a forging billet; hot rolling the forging billet into a hot-rolled plate through multiple passes; step 4: solution heat treatment of the hot-rolled plate; step 5: cold rolling the hot-rolled plate after solution heat treatment with a deformation amount of 10% to 30%, and finally obtaining ultra-high strength corrosion-resistant economical steel; the microstructure of the ultra-high strength corrosion-resistant economical steel is: ferrite content 30% to 55%, martensite content 20% to 40%, and austenite content 15% to 30%. The ultra-high strength corrosion-resistant economical steel section of the present invention has high strength, good plasticity and toughness and excellent corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of corrosion-resistant steel, and in particular to an ultra-high strength corrosion-resistant economical steel section and a preparation method thereof. Background Art

[0002] Corrosion is one of the main forms of steel damage. For example, in the oil and gas field, oil well pipes are important consumables in the process of oil and gas exploration and development, accounting for about 40% of the total steel used in the oil and gas industry, and are an important part of oil and gas extraction equipment. As an important petroleum equipment material, oil well pipes account for an average of 20% to 30% of the entire well construction cost. Therefore, whether the entire petroleum project can proceed smoothly is inseparable from the quality of oil well pipes. The oil well pipe is placed in the casing and is used to collect oil and gas. Due to the flow of corrosive liquids in the oil well pipe, various anti-corrosion products must be selected according to the corrosion conditions. In China, Sichuan gas fields contain H2S, and some oil and gas wells in Sichuan, North China and western oil and gas fields contain CO2 and H2S, CO2, C1 - Coexistence. Under such oil production conditions, higher requirements are also placed on the corrosion resistance of oil well pipes. At present, the material for oil well pipes resistant to CO2 corrosion is mainly 13Cr martensitic stainless steel, but martensitic steel cannot be used in an environment containing H2S. Although the use of 22Cr (or 25Cr, 27Cr, the higher the Cr content, the better the corrosion resistance) duplex stainless steel with a higher alloy content can meet the use requirements of oil well pipes, the high raw material cost is not suitable for large-scale industrial applications in oil and gas production scenarios, and it is difficult to serve in some working conditions with high strength requirements. In view of the actual situation of oil and gas production in China and the limitations of mineral resources, it is of great significance to develop an economical high-strength and corrosion-resistant oil well pipe material that has both the high strength of martensitic stainless steel and the corrosion resistance of duplex stainless steel while having controllable costs.

[0003] At present, there are many patents at home and abroad on the varieties of corrosion-resistant steel with good comprehensive performance and the corresponding process flow, but most of them have problems such as high cost or complex process and cannot meet the needs of mass production and practical application. Summary of the invention

[0004] In view of the above situation, the present invention aims to provide an ultra-high strength corrosion-resistant economical steel and a preparation method thereof, so as to solve the problem that the existing corrosion-resistant steel has high cost and is difficult to match high strength and high corrosion resistance.

[0005] The purpose of the present invention is mainly achieved through the following technical solutions:

[0006] In one aspect, the present invention provides a method for preparing ultra-high strength corrosion-resistant economical steel, the preparation method comprising:

[0007] Step 1: Melt the raw materials into molten steel;

[0008] Step 2: smelting the molten steel into continuous casting billets or ingots by continuous casting or mold casting;

[0009] Step 3: heating and keeping the continuous casting billet or ingot warm, forging and opening the billet to form a forged billet; hot rolling the forged billet into a hot-rolled plate through multiple passes;

[0010] Step 4: Solution heat treatment of the hot rolled plate;

[0011] Step 5: cold rolling the hot-rolled plate after solution heat treatment with a deformation amount of 10% to 30%, and finally obtaining an ultra-high strength corrosion-resistant economical steel with both high strength and high corrosion resistance; wherein the structure of the ultra-high strength corrosion-resistant economical steel is a stable three-phase structure of ferrite + martensite + austenite; the content of the three-phase structure is: ferrite content 30% to 55%, martensite content 20% to 40%, austenite content 15% to 30%.

[0012] Furthermore, in step 3, the forging temperature is 1150-1250°C.

[0013] Furthermore, in step 4, the solution heat treatment includes: keeping warm at 1000-1150° C. for 1-3 hours and then cooling with water.

[0014] Furthermore, in step 4, the microstructure of the steel after the solution heat treatment is a mixed structure of ferrite+austenite+martensite, wherein the martensite content is 5% to 15%, the austenite content is 30% to 50%, and the rest is ferrite.

[0015] The present invention also provides an ultra-high strength corrosion-resistant economical steel section, which is prepared by the above-mentioned preparation method.

[0016] Furthermore, the components of the ultra-high strength corrosion-resistant economical steel include, by mass percentage: C: ≤0.03%, Cr: 18.0%~20.0%, Mo: 0.5%~3.0%, Ni: 1.0%~2.8%, Mn: 1.0%~4.0%, Si: 0.1%~0.5%, N: 0.1%~0.25%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities.

[0017] Furthermore, in the composition of the ultra-high strength corrosion-resistant economical steel, Cr+3.3Mo+30N-Mn≥26.

[0018] Furthermore, in the composition of the ultra-high strength corrosion-resistant economical steel, 30Ni+17Mn-12Mo-800N≤0.

[0019] Furthermore, the components of the ultra-high strength corrosion-resistant economical steel can be calculated by mass percentage: C: 0.02%~0.03%, Cr: 18.1%~19.8%, Mo: 1.1%~2.8%, Ni: 1.0%~2.5%, Mn: 1.0%~3.0%, Si: 0.2%~0.5%, N: 0.11%~0.25%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities.

[0020] Furthermore, in the composition of the ultra-high strength corrosion-resistant economical steel, Cr+3.3Mo+30N-Mn≥26.5.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] a) The ultra-high strength corrosion-resistant economical steel of the present invention accurately controls the mass percentage of Cr, Ni, Mn, Mo, and N elements in the steel, adopts the process of solid solution + cold rolling, and can obtain a mixed structure of ferrite + austenite + martensite after solid solution; further, through quantitative cold deformation, the austenite part of the organization that has not undergone quenching martensite transformation undergoes deformation-induced martensite transformation to obtain a mixed structure mainly composed of ferrite and martensite and containing a small amount of austenite. Since the material of the present invention has a martensite content of 30% to 40%, the tensile strength of the material is much higher than that of duplex stainless steel, and can reach 110 and 125 steel grades, achieving high strength equivalent to that of martensitic stainless steel under multi-phase organization. Compared with the martensitic stainless steel of the prior art, the organization form of multi-phase coexistence in the stainless steel of the present invention has better intergranular corrosion resistance and H2S stress corrosion resistance. In addition, the steel of the present invention has a certain austenite content, and has better plasticity under the premise of having a strength similar to that of martensitic stainless steel.

[0023] b) The steel prepared by the method of the present invention not only ensures high strength, good plasticity and toughness, and excellent corrosion resistance, but also ensures that the yield strength of the steel of the present invention is greater than 785MPa (e.g. 789-1020MPa), the tensile strength is greater than 1090MPa (e.g. 1093-1250MPa), and the elongation is greater than 16% (e.g. 17%-25%). (180℃) CO2 corrosion rate 0.027g / m 2 h or less (e.g. 0.018 to 0.0267 g / m 2 h); (200℃) CO2 corrosion rate 0.1g / m 2 h or less (e.g. 0.076 to 0.097 g / m 2 h); pitting corrosion rate 2.35g / m 2 h or less (e.g. 1.8 to 2.321 g / m 2 h); crevice corrosion rate 5.4g / m 2h or less (e.g. 4.1 to 5.356 g / m 2 h); H2S stress corrosion test was passed.

[0024] c) The steel of the present invention has good toughness and excellent corrosion resistance, and has both the high strength of super martensitic stainless steel and the stress corrosion resistance, intergranular corrosion resistance and resistance to a small amount of H2S stress corrosion of duplex stainless steel. Since the Cr and Ni content of the steel of the present invention is low, the cost of raw materials is equivalent to or even slightly lower than that of the super 13Cr martensitic stainless steel in the prior art, and the hot working performance is good, so the steel of the present invention has low comprehensive cost, economy and excellent performance.

[0025] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0027] Figure 1 The microstructure diagram of the steel of Example 1 of the present invention;

[0028] Figure 2 EBSD image of the steel of Example 1 of the present invention;

[0029] Figure 3 The microstructure diagram of the steel of Example 2 of the present invention;

[0030] Figure 4 This is a microstructure diagram of the steel of Example 3 of the present invention;

[0031] Figure 5 This is a microstructure diagram of the steel of Example 4 of the present invention. DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used to illustrate the principles of the present invention together with the embodiments of the present invention.

[0033] The invention provides an ultra-high strength corrosion-resistant economical steel. The components of the ultra-high strength corrosion-resistant economical steel include, by mass percentage, C: ≤0.03%, Cr: 18.0%-20.0%, Mo: 0.5%-3.0%, Ni: 1.0%-2.8%, Mn: 1.0%-4.0%, Si: 0.1%-0.5%, N: 0.1%-0.25%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities.

[0034] Specifically, Cr+3.3Mo+30N-Mn (i.e., PREN) ≥ 26, and 30Ni+17Mn-12Mo-800N ≤ 0. Cr, Mo, N, Ni, and Mn refer to the mass percentage of these elements × 100. For example, if the mass percentage of the Cr element is 18.0%, then Cr here is 18.0.

[0035] The following is a detailed description of the effects and dosage of the components in the present invention:

[0036] C: Although the addition of carbon can significantly improve the hardness and strength of steel, due to the high diffusion capacity of carbon, it is easy to combine with Cr to form M 23 C6 type carbide, this type of carbide is mainly precipitated quickly (≤0.5 hours) in the temperature range of 700-900°C, or precipitated when kept at 550-700°C for a long time. The precipitation of carbides causes local Cr deficiency, increases the intergranular corrosion sensitivity of the material, and deteriorates the corrosion resistance. Therefore, in the present invention, the comprehensive effect of strength and corrosion resistance is comprehensively considered, and its content is controlled to be ≤0.03%.

[0037] Cr: The main element to ensure the corrosion resistance of stainless steel. It is also a ferrite-forming element. Improper control of Cr content will increase the ferrite content in the organization and reduce the strength. Considering comprehensive considerations, its content should be controlled at 18.0% to 20.0%.

[0038] Ni: It is an element that strongly forms and stabilizes austenite and expands the austenite phase region. Ni is an element that improves the toughness of stainless steel and the corrosion resistance of reducing media. Ni is also an important element for improving the resistance of austenite phase to transgranular stress corrosion in various media. At the same time, compared with Mn element, Ni element can reduce the cold working hardening tendency of austenite phase. Considering cost control, the present invention controls its content within 1.0% to 2.8%.

[0039] Mn: It can improve the solubility of N in steel and inhibit the precipitation of harmful phase chromium nitride, but Mn reduces the toughness of stainless steel. Compared with Ni, Mn is an economical austenite-forming element. Mn has different hardening effects on the austenite phase of stainless steel and the overall material. However, too high Mn content will reduce the corrosion resistance of the material, deteriorate thermoplasticity and reduce the yield rate. Therefore, the present invention controls its content within 1.0% to 4.0%.

[0040] Mo: In addition to improving the corrosion resistance of stainless steel in oxidizing media, it also has a good effect on improving the corrosion resistance of stainless steel in reducing media. At the same time, in order to ensure pitting resistance, alloy elements such as Cr and Mo that increase the PREN value should not be too low. At the same time, alloy cost control and phase balance must also be considered. Comprehensively considered, its content is controlled to 0.5% to 3.0%.

[0041] Si: Si is added as a deoxidizer during the smelting process to achieve a deoxidation effect. Too much addition will deteriorate the intergranular corrosion resistance of the material. The present invention controls its content to 0.1% to 0.5%.

[0042] N: Like Ni, it is an element that strongly forms and stabilizes austenite and expands the austenite phase. N can also significantly improve the pitting corrosion resistance of the austenite phase in stainless steel, thereby improving the overall pitting corrosion resistance of stainless steel. In some media, N has a good effect on the stress resistance of stainless steel, and there is an optimal value. Further increasing the N content will reduce its stress corrosion resistance. N is also a relatively economical alloying element, but N reduces the hot working performance of stainless steel. Therefore, the present invention controls its content to 0.1% to 0.25%.

[0043] S: It will significantly reduce the hot working performance of steel and easily react with Mn to form MnS inclusions, which will reduce the material's resistance to pitting corrosion and crevice corrosion. Therefore, its content is controlled to ≤ 0.01%.

[0044] P: It is a harmful impurity element that will not only deteriorate the plasticity of the material, but also reduce the corrosion resistance of the material. Therefore, its content is controlled to be ≤0.03%.

[0045] Specifically, the present application comprehensively considers three aspects: guarantee of corrosion resistance, raw material cost caused by alloy element matching, precipitation of harmful phases caused by alloy element matching, and cost change caused by hot working yield. In addition to controlling the content of individual elements of the main alloying elements Cr, Ni, Mn, and N in steel, in order to ensure corrosion resistance, Cr+3.3Mo+30N-Mn≥26 is controlled. In order to make the maximum precipitation temperature of harmful phases lower than 750°C and ensure that the material has good hot working properties, and considering that the three-phase ratio can be controllable during hot working between 900 and 1180°C, it is necessary to control the content of Ni, Mn, Mo, and N elements to meet the following requirements: 30Ni+17Mn-12Mo-800N≤0.

[0046] In order to further improve the comprehensive performance of the above-mentioned ultra-high strength corrosion-resistant economical steel, the components of the above-mentioned ultra-high strength corrosion-resistant economical steel can be calculated by mass percentage: C: 0.02%~0.03%, Cr: 18.1%~19.8%, Mo: 1.1%~2.8%, Ni: 1.0%~2.5%, Mn: 1.0%~3.0%, Si: 0.2%~0.5%, N: 0.11%~0.25%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities.

[0047] Specifically, Cr+3.3Mo+30N-Mn (ie, PREN)≥26.5, and 30Ni+17Mn-12Mo-800N≤-80.

[0048] The present invention also provides a method for preparing the ultra-high strength corrosion-resistant economical steel, comprising:

[0049] Step 1: Melt the raw materials into molten steel;

[0050] Step 2: smelting the molten steel into continuous casting billets or ingots by continuous casting or mold casting;

[0051] Step 3: heating the continuous casting billet or ingot at 1150-1250° C. and keeping the temperature, forging and opening the billet to form a forged billet; heating the forged billet at 1050-1250° C. and hot rolling it into a hot-rolled plate through multiple passes;

[0052] Step 4: Solution heat treatment of the hot-rolled plate: keep at 1000-1150°C for 1-3h and then water cool;

[0053] Step 5: cold rolling the hot-rolled plate after solution heat treatment with a deformation of 10% to 30%, and finally obtain ultra-high strength corrosion-resistant economic steel with both high strength and high corrosion resistance. The three-phase structure content of the ultra-high strength corrosion-resistant economic steel is: ferrite content 30% to 55%, martensite content 20% to 40%, and austenite content 15% to 30%.

[0054] Specifically, in the above steps 1 and 2, a converter or electric furnace is used for smelting, LF refining and casting into ingots.

[0055] Specifically, in the above step 3, the forging temperature is 1150-1250°C. This is because the material of the present invention is a two-phase ferrite + austenite when it is higher than 800°C. The ratio of the two phases changes with the heating temperature. The higher the heating temperature, the more ferrite content, and the corresponding austenite content decreases. Selecting this temperature range for heating ensures the full dissolution of harmful precipitated phases, and also makes the ferrite phase content in the material greater than 70%, so that the steel is forged in a single-phase region with better thermoplasticity, avoiding the difference in the thermal deformation capacity of the two phases of ferrite and austenite, which leads to the uncoordinated deformation of the two phases in the forging process and causes forging cracking.

[0056] Specifically, in the above step 3, the tube is hot-rolled and then water-cooled to room temperature to avoid the precipitation of harmful phases.

[0057] Specifically, in the above step 4, the solution heat treatment before cold rolling is to regulate the organization content. The microstructure of the steel after solution heat treatment is a mixed organization of ferrite + austenite + martensite, wherein the martensite content is 5% to 15%, the austenite content is 30% to 50%, and the rest is ferrite. Martensite is dispersed in the austenite grains in the form of fine particles, and part of the austenite in the organization is in a metastable state. The martensite content can be increased by appropriate cold working, thereby stabilizing the austenite to obtain a mixed organization of ferrite + martensite + stable austenite. The finely dispersed martensite particles after solution heat treatment provide conditions for the continuous and uniform phase transformation of austenite to induce martensite. This feature is used to increase the martensite content through cold working with a small deformation amount, and a stable three-phase organization is obtained, so that the finished material obtains more excellent mechanical properties. By utilizing the characteristics of austenite and martensite in the structure of the present invention, the austenite part is deformed and induced to undergo martensitic phase transformation through quantitative cold rolling. The volume content of martensite can be increased to 20% to 40% through cold rolling with a small deformation amount (10% to 30%), while the ferrite content is 30% to 55% and the volume content of austenite is 15% to 30%.

[0058] Specifically, in the above step 5, a small amount of granular martensite dispersed in the austenite grains generated by the solution heat treatment is used to increase the martensite content and stabilize the austenite through cold rolling with a small deformation amount. Due to the presence of martensite, part of the unstable austenite is prone to martensitic transformation during the cold deformation process, and the cold deformation amount needs to be controlled within a smaller range (10% to 30%, for example, 10% to 20%) to increase the tensile strength without reducing the toughness.

[0059] Specifically, in the above step 5, the microstructure of the ultra-high strength corrosion-resistant economical steel is a stable three-phase structure of ferrite + martensite + austenite, wherein the ferrite content is 30% to 55%, the martensite content is 20% to 40%, and the austenite content is 15% to 30%. Martensite particles exist in the austenite phase in two forms: one is uniformly dispersed in the austenite grains in the form of fine particles, and the other is distributed in the austenite grains in the form of discontinuous and irregular particles, and almost all martensite particles are coated with austenite. The thickness of the martensite sheet is less than 80nm.

[0060] Compared with the prior art, the ultra-high strength corrosion-resistant economical steel of the present invention accurately controls the mass percentage of Cr, Ni, Mn, Mo, and N elements in the steel, and can obtain a mixed structure of ferrite + austenite + martensite after solid solution; further, through quantitative cold deformation, the austenite part of the organization that has not undergone quenching martensite transformation undergoes deformation-induced martensite transformation to obtain a mixed structure mainly composed of ferrite and martensite and containing a small amount of austenite. Since the three-phase stainless steel material (18-20Cr) of the present invention has 20% to 40% ultrafine martensite, the tensile strength of the material is much higher than that of the duplex stainless steel with a higher Cr content (22Cr, 25Cr, etc.), and can reach 110 and 125 steel grades, achieving high strength equivalent to that of martensitic stainless steel under multi-phase organization. Compared with the martensitic stainless steel of the prior art, the organization form of multi-phase coexistence in the stainless steel of the present invention has better intergranular corrosion resistance and H2S stress corrosion resistance. In addition, the stainless steel of the present invention has a certain austenite content and has better plasticity while having a strength similar to that of martensitic stainless steel.

[0061] The steel of the present invention not only ensures high strength, good plasticity and toughness, and excellent corrosion resistance, but also ensures that the yield strength of the steel of the present invention is greater than 785MPa (e.g. 789-1020MPa), the tensile strength is greater than 1090MPa (e.g. 1093-1250MPa), and the elongation is greater than 16% (e.g. 17%-25%). (180℃) CO2 corrosion rate 0.027g / m 2 h or less (e.g. 0.018 to 0.0267 g / m 2 h); (200℃) CO2 corrosion rate 0.1g / m 2 h or less (e.g. 0.076 to 0.097 g / m 2 h); pitting corrosion rate 2.35g / m 2 h or less (e.g. 1.8 to 2.321 g / m 2 h); crevice corrosion rate 5.4g / m 2 h or less (e.g. 4.1 to 5.356 g / m 2 h); H2S stress corrosion test was passed.

[0062] The steel of the present invention has good toughness and excellent corrosion resistance, and has both the high strength of super martensitic stainless steel and the stress corrosion resistance, intergranular corrosion resistance and small amount of H2S stress corrosion resistance of duplex stainless steel. Since the Cr and Ni content of the steel of the present invention is low, the raw material cost is equivalent to or even slightly lower than that of the super 13Cr martensitic stainless steel in the prior art, and the hot working performance is good, so the steel of the present invention has low comprehensive cost, economy and excellent performance.

[0063] Examples 1-4

[0064] The advantages of precise control of the composition and process parameters of the steel of the present invention are demonstrated below with specific examples and comparative examples. 13Cr is selected as a comparison object for corrosion performance.

[0065] Embodiments 1-4 of the present invention provide an ultra-high strength corrosion-resistant economical steel and a preparation method thereof. The chemical compositions of the steels of Embodiments 1-4 are shown in Table 1.

[0066] The preparation method of Example 1 comprises:

[0067] Step 1: Melting metal raw materials into molten steel; using a die casting method to smelt the molten steel into steel ingots to obtain castings;

[0068] Step 2: After heating the ingot to 1180°C and keeping it for 2 hours, forging and opening the ingot to make a forged ingot, keeping the forged ingot at 1170°C for 2 hours, and hot rolling it into a hot-rolled plate in multiple passes;

[0069] Step 3: Keep the hot-rolled plate at 1100°C for 1 hour and water-cool it to obtain a finished plate;

[0070] Step 4: Cold rolling the hot-rolled plate after solution heat treatment, and finally obtain ultra-high strength corrosion-resistant economic steel with both high strength and high corrosion resistance. The cold rolling deformation is controlled to 10% to adjust the volume fraction of austenite to martensite transformation, and the final structure contains 50% ferrite, 30% martensite and 20% austenite.

[0071] The preparation method of Example 2 comprises:

[0072] Step 1: Melting metal raw materials into molten steel; using a die casting method to smelt the molten steel into steel ingots to obtain castings;

[0073] Step 2: After heating the ingot to 1190°C and keeping it for 2 hours, forging and opening the ingot to make a forged ingot, keeping the forged ingot at 1190°C for 2 hours, and hot rolling it into a hot-rolled plate in multiple passes;

[0074] Step 3: Keep the hot-rolled plate at 1100°C for 1 hour and water-cool it;

[0075] Step 4: Cold rolling the hot-rolled plate after solution heat treatment, and finally obtain ultra-high strength corrosion-resistant economic steel with both high strength and high corrosion resistance. The volume fraction of austenite to martensite transformation is adjusted by controlling the cold rolling deformation to 20%, with ferrite content of 40%, martensite content of 40%, and austenite content of 20%.

[0076] The preparation method of Example 3 comprises:

[0077] Step 1: Melting metal raw materials into molten steel; using a die casting method to smelt the molten steel into steel ingots to obtain ingots;

[0078] Step 2: After heating the ingot to 1200°C and keeping it for 1 hour, forging and opening the ingot to make a forged ingot, keeping the forged ingot at 1200°C for 2 hours, and hot rolling it into a hot-rolled plate in multiple passes;

[0079] Step 3: Keep the hot-rolled plate at 1100°C for 1 hour and water-cool it;

[0080] Step 4: Cold rolling the hot-rolled plate after solution heat treatment, and finally obtain ultra-high strength corrosion-resistant economic steel with both high strength and high corrosion resistance. The volume fraction of austenite to martensite transformation is adjusted by controlling the cold rolling deformation to 10%, with ferrite content of 42%, martensite content of 35%, and austenite content of 23%.

[0081] The preparation method of Example 4 comprises:

[0082] Step 1: Melting metal raw materials into molten steel; smelting the molten steel into ingots by die casting;

[0083] Step 2: After heating the ingot to 1200°C and keeping it for 1 hour, forging and blanking are performed to make forging billets, the forging billets are kept at 1200°C for 2 hours, and hot-rolled into hot-rolled plates in multiple passes;

[0084] Step 3: Keep the hot rolled plate at 1000°C for 1 hour and water cool it;

[0085] Step 4: Cold rolling the hot-rolled plate after solution heat treatment, and finally obtain ultra-high strength and corrosion-resistant economic steel with high strength and high corrosion resistance. The volume fraction of austenite to martensite transformation is adjusted by controlling the cold rolling deformation to 20%, with ferrite content of 46%, martensite content of 36%, and austenite content of 18%.

[0086] The mechanical properties of Examples 1-4 are shown in Table 2. Figure 1 is a microstructure diagram of the steel of Example 1 of the present invention, Figure 2 This is the EBSD two-phase distribution diagram of the present invention, wherein the blue phase is austenite that has not undergone martensitic transformation, and the content is about 20%.

[0087] Figure 3-Figure 5The microstructure diagrams of the steels of Examples 2-4 of the present invention are shown in Table 4. The microstructure of the ultra-high strength corrosion-resistant economical steel of the present invention is a stable three-phase structure of ferrite + austenite + martensite, wherein the ferrite content is 30% to 55% (e.g., 40% to 53%), the martensite content is 20% to 40% (e.g., 25% to 40%), and the austenite content is 15% to 30% (e.g., 18% to 25%).

[0088] Table 1 Chemical composition wt%

[0089]

[0090] Table 2 Mechanical properties

[0091]

[0092] Table 3 Microstructure

[0093]

[0094] In order to illustrate the corrosion resistance of the steel of the present invention, the corrosion resistance of the steel of Examples 1-4 was measured under 5 different conditions and compared with 13Cr under the same test conditions. This experiment was used to evaluate the corrosion resistance of the steel of the present invention in different corrosive environments. The results of the corrosion test are shown in Table 5, and the corresponding test conditions are as follows:

[0095] (180℃ or 200℃) The corrosion conditions of CO2 corrosion rate are as follows:

[0096] 1. Medium (formation water) mg / L: CO3 2- / 0,HCO3 - / 189,OH - / 0,Cl - / 128000,SO4 2- / 430,Ca 2+ / 8310,Mg 2+ / 561,K + / 6620,Na + / 76500);

[0097] 2. CO2 partial pressure: 4.48MPa, total test pressure: 10MPa;

[0098] 3. Test temperature: 180℃ or 200℃;

[0099] 4. Test time: 720 hours.

[0100] The corrosion conditions for pitting corrosion rate are as follows:

[0101] 1. Test temperature: 22±2℃;

[0102] 2. Medium: 100g reagent grade ferric chloride FeCl3·6H2O dissolved in 900mL IV reagent grade water (mass ratio about 6%);

[0103] 3. Test time: 72 hours;

[0104] 4. Sample size: 50×25×3mm, polished to 2000# sandpaper.

[0105] The corrosion conditions for crevice corrosion rate are as follows:

[0106] 1. The test temperature is 22±2℃;

[0107] 2. Dissolve 100g reagent grade ferric chloride FeCl3·6H2O in 900mL IV reagent grade water (mass ratio about 6%);

[0108] 3. Test time: 72 hours;

[0109] 4. Sample size: 50×25×3mm, polished to 2000# sandpaper.

[0110] The corrosion conditions of H2S stress corrosion test are as follows:

[0111] 1. The test temperature is 22±2℃;

[0112] 2. 50 g NaCl, 25 g [23.8 mL] CH3COOH, and 4.1 g CH3COONa were dissolved in 921 mL IV reagent grade water (about 5 wt% sodium chloride, 2.5 wt% glacial acetic acid, and 0.41 wt% sodium acetate by mass ratio);

[0113] 3. Test time: 720 hours;

[0114] 4. Sample size: 50×5×2mm, polished to 2000# sandpaper;

[0115] 5. Four-point bending test, loading force is 80% yield stress.

[0116] From Table 5, it can be seen that the (180°C) CO2 corrosion rate of the high corrosion resistant economical steel of Examples 1-4 is 0.027 g / m 2 h or less (e.g. 0.018 to 0.0267 g / m 2 h); (200℃) CO2 corrosion rate 0.1g / m 2 h or less (e.g. 0.076 to 0.097 g / m 2 h); pitting corrosion rate 2.35g / m 2 h or less (e.g. 1.8 to 2.321 g / m 2h); crevice corrosion rate 5.4g / m 2 h or less; H2S stress corrosion test is passed. The corrosion resistance of Examples 1-4 of the present invention is better than that of 13Cr, especially the CO2 corrosion resistance at 200°C. The present invention is far better than super 13Cr and can be used in more demanding environments.

[0117] Table 5 Corrosion test data

[0118]

[0119] The inventors have conducted a lot of in-depth research in the research. In order to further highlight the advantages of the present invention in terms of performance and composition, super 13Cr martensite is now listed as a comparative example as follows:

[0120] Comparative Example 1

[0121] This comparative example provides a corrosion-resistant steel and a preparation method thereof. The corrosion-resistant oil well pipe steel of this comparative example is 13Cr, and is prepared by the current mature process of 13Cr. The composition of the corrosion-resistant oil well pipe steel of this comparative example is C: 0.03%, Cr: 13%, Ni: 5%, Mo: 2%, Mn: 0.5%, Si: 0.5%, and the preparation method is: after heating and forging at 1150°C, water cooling, and then reheating to 860°C for 2h, quenching, and tempering at 620°C for 1 hour.

[0122] The microstructure of the steel of this comparative example is martensite (93%) + austenite (7%). The mechanical properties are shown in Table 3 above, and the corrosion resistance results are shown in Table 5 above.

[0123] It can be seen from the comparative examples that the ultra-high strength corrosion-resistant economical steel of the present invention accurately controls the mass percentages of Cr, Ni, Mn, Mo, and N elements in the steel, and combines a certain amount of cold deformation to cause the austenite part to undergo a martensitic phase transformation, thereby obtaining a mixed structure of mainly ferrite and martensite with a small amount of austenite, which greatly improves the corrosion resistance and toughness of the steel, and achieves an ultra-high strength corrosion-resistant steel with a strength equivalent to that of the super 13Cr martensitic stainless steel in the prior art and a lower raw material cost. Therefore, the steel of the present invention has a low comprehensive cost, is economical, and is practical.

[0124] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing ultra-high strength corrosion-resistant economical steel, characterized in that: The components of the ultra-high strength corrosion-resistant economical steel include, by mass percentage: C: ≤0.03%, Cr: 18.51%~20.0%, Mo: 1.1%~3.0%, Ni: 1.0%~2.8%, Mn: 1.0%~4.0%, Si: 0.1%~0.5%, N: 0.17%~0.25%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities; 30Ni+17Mn-12Mo-800N≤0; The preparation method comprises: Step 1: Melt the raw materials into molten steel; Step 2: smelting the molten steel into continuous casting billets or ingots by continuous casting or mold casting; Step 3: heating and keeping the continuous casting billet or ingot warm, forging and opening the billet to form a forged billet; hot rolling the forged billet into a hot-rolled plate through multiple passes; Step 4: Solution heat treatment of the hot rolled plate; Step 5: cold rolling the hot-rolled plate after solution heat treatment with a deformation of 10% to 30%, and finally obtaining an ultra-high strength corrosion-resistant economical steel with both high strength and high corrosion resistance; wherein the structure of the ultra-high strength corrosion-resistant economical steel is a stable three-phase structure of ferrite + martensite + austenite; the three-phase structure content is: ferrite volume fraction 30% to 55%, martensite volume fraction 20% to 40%, austenite volume fraction 15% to 30%; Martensite particles exist in the austenite phase in two forms: one is uniformly dispersed in the austenite grains in the form of fine particles, and the other is distributed in the austenite grains in the form of discontinuous and irregular particles.

2. The preparation method according to claim 1, characterized in that: In the step 3, the forging temperature is 1150-1250°C.

3. The preparation method according to claim 1, characterized in that: In step 4, the solution heat treatment includes: keeping the temperature at 1000-1150° C. for 1-3 hours and then cooling with water.

4. The preparation method according to claim 1, characterized in that: In step 4, the microstructure of the steel after solution heat treatment is a mixed structure of ferrite + austenite + martensite, wherein the volume fraction of martensite is 5% to 15%, the volume fraction of austenite is 30% to 50%, and the rest is ferrite.

5. An ultra-high strength corrosion-resistant economical steel, characterized in that: The preparation method is described in any one of claims 1 to 4.

6. The ultra-high strength corrosion-resistant economical steel according to claim 5, characterized in that: Among the components of the ultra-high strength corrosion-resistant economical steel, Cr+3.3Mo+30N-Mn≥26.

7. The ultra-high strength corrosion-resistant economical steel according to claim 5, characterized in that: Among the components of the ultra-high strength corrosion-resistant economical steel, 30Ni+17Mn-12Mo-800N≤-80.

8. The ultra-high strength corrosion-resistant economical steel according to claim 5, characterized in that: The components of the ultra-high strength corrosion-resistant economical steel are, by mass percentage, C: 0.02%~0.03%, Cr: 18.51%~19.8%, Mo: 1.1%~2.8%, Ni: 1.0%~2.5%, Mn: 1.0%~3.0%, Si: 0.2%~0.5%, N: 0.17%~0.25%, P: ≤0.03%, S: ≤0.01%, and the balance is Fe and unavoidable impurities.

9. The ultra-high strength corrosion-resistant economical steel according to any one of claims 5 to 8, characterized in that: Among the components of the ultra-high strength corrosion-resistant economical steel, Cr+3.3Mo+30N-Mn≥26.5.

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