Stress-corrosion-resistant high-strength and high-toughness medium carbon steel and manufacturing method therefor

Abstract

Disclosed in the present invention is stress-corrosion-resistant high-strength and high-toughness medium carbon steel. The medium carbon steel contains Fe and inevitable impurities, and further contains the following chemical elements in percentages by mass: C: 0.37-0.43%, Si: 0.10-0.40%, Mn: 0.50-0.90%, Cr: 0.60-1.25%, Ni: 1.30-2.00%, Mo: 0.15-0.30%, Al: 0.02-0.05% and Nb: 0.001-0.030%. Correspondingly, further provided in the present invention is a manufacturing method for the stress-corrosion-resistant high-strength and high-toughness medium carbon steel. The steel of the present invention has good impact toughness, elongation and surface shrinkage, also has stress corrosion cracking resistance, and has good weather resistance and fatigue resistance.

Description

Technical field

[0001] The present disclosure relates to a steel and a manufacturing method therefor, in particular to a medium carbon steel and a manufacturing method therefor.Background Art

[0002] High-strength and high-toughness steel bars are usually used in mechanical and structural components with high-safety, such as wind power fasteners and other critical wearing parts, which should have high strength, high toughness, stress corrosion resistance and high fatigue performance.

[0003] In the existing technology, the high-strength and high-toughness steel is usually produced by appropriately selecting a chemical composition, and adopting a process of controlled rolling and controlled cooling, or a process of quenching + tempering. Among them, for the use of controlled rolling and controlled cooling to produce high-strength steel, it is difficult to control during the rolling and cooling process, which affects the overall uniformity of mechanical properties of the steel. When the quenching + tempering process is used to produce high-strength steel, the hardenability of the steel is improved by optimizing the contents of alloying elements and carbon element, so that a martensitic structure is formed in the steel during the cooling process. High-strength steels dominated by martensite have high dislocation density, resulting in poor impact toughness. It will quickly fracture and fail when there are small defects such as microcracks during the tensile process, and the fracture toughness is low.

[0004] In addition, Cr-Ni-Mo series medium carbon and low alloy steel is widely used in construction machinery, automobiles, bridges, marine equipment and other fields because of its good strength and toughness, and its safe use strength level does not exceed 1000MPa. The application of the steel with higher strength level can not only make the equipment lightweight, but also save resources. However, with the increase of steel strength level, the difficulty of processing and manufacturing increases, and its hydrogen embrittlement sensitivity is bound to increase.

[0005] The Chinese patent publication No. CN102242322A, published on November 16, 2011, with a title of "An Improved 40CrNiMo Steel and the Preparation Method therefor" has disclosed a medium carbon steel, which comprises the chemical elements, by weight percentage, as follows: C: 0.37-0.45%, Cr: 1.65-1.85%, Ni: 0.45-0.65%, Mo: 0.15-0.25%, Mn: 0.90-1.20%, Si: 0.40-0.55%, B: 0.0025-0.0045%, N: 0.22-0.28%, Ca: 0.007-0.012%, Mg: 0.002-0.005%, Nb: 0.03-0.06%, Ti: 0.04-0.08%, RE: 0.02-0.06%, S: ≤0.015% and P: ≤0.025% and Al: ≤0.0008%, and a balance of Fe. The steel has a tensile strength of ≥1195MPa, and an impact energy of greater than 85J, but it does not involve stress corrosion resistance.

[0006] The Chinese patent publication No. CN104726783A, published on June 24, 2015, with a title of "Steel for wind power yaw and variable pitch bearing sleeve ring and preparation method thereof" has disclosed a steel, which comprises the following chemical composition by mass percentage: C: 0.37-0.42, Mn: 0.50-0.80, Mo: 0.20-0.30, Ni: 1.30-1.70, Cr: 0.70-1.00, Si: 0.20-0.50, acid soluble Al : 0.035-0.055, V: 0.07-0.12, N: ≤0.004, O: ≤0.0008, H: ≤0.00015, S: ≤0.010, P: ≤0.015, and a balance of Fe and a small amount of impurities. The preparation method of the above-mentioned wind power yaw and variable pitch bearing sleeve ring is to subject the processed wind power yaw and variable pitch bearing sleeve ring to heat treatment, including first heating to 860-890 °C for 3-5h for austenitization and post-oil quenching, then heating to 580-630 °C for 3-5h and oil cooling to room temperature. The steel has excellent hardenability, and its mechanical properties fully meet the standards and requirements of the steel for high-power wind power yaw and variable pitch bearing. However, this steel grade does not involve stress corrosion resistance.

[0007] In addition, it can be seen from the above prior art that in order to obtain high-performance medium carbon alloy steel in the existing technology, technicians in the field use a large number of alloying elements or add micro-alloying elements such as B, Nb, Mg, rare earth and the like to increase strength and improve performance, thereby obtaining high-performance medium carbon alloy steel. However, this design scheme with a large content of alloying elements added is easy to increase the manufacturing cost, and when the content of alloying elements in the steel is too high, coarse carbide particles, such as carbides of Cr, Mo, V, Ti, etc., will be formed, reducing the impact toughness of the steel.

[0008] Moreover, for the humid service environment conditions, the Cr-Ni-Mo series medium carbon and low-alloy steel parts are subjected to large loads and dynamic impacts. Stress corrosion is likely to occur, which can lead to brittle fracture in severe cases, causing huge economic losses and even safety accidents.

[0009] Based on the above, it is desired to obtain a high-strength and high-toughness mediumcarbon steel having stress-corrosion resistance.Summary

[0010] One of the objectives of the present disclosure is to provide a stress-corrosion-resistant high-strength and high-toughness medium carbon steel, which has good impact toughness, elongation and section shrinkage. It can resist stress corrosion cracking and has good weather resistance and fatigue resistance, and can be used in construction machinery, marine engineering and other occasions that require high strength and toughness steel.

[0011] In order to achieve the above object, the present disclosure provides a stress-corrosion-resistant high-strength and high-toughness medium carbon steel, which comprises Fe and unavoidable impurity elements, as well as the following chemical elements in percentage by mass: C: 0.37-0.43%, Si: 0.10-0.40%, Mn: 0.50-0.90%, Cr: 0.60-1.25%, Ni: 1.30-2.00%, Mo: 0.15-0.30%, Al: 0.02-0.05%, Nb: 0.001-0.030%.

[0012] Correspondingly, the present disclosure further provides a stress-corrosion-resistant high-strength and high-toughness medium carbon steel, in which the mass percentages of the chemical elements of the steel are: C: 0.37-0.43%, Si: 0.10-0.40%, Mn: 0.50-0.90%, Cr: 0.60-1.25%, Ni: 1.30-2.00%, Mo: 0.15-0.30%, Al: 0.02-0.05%, Nb: 0.001-0.030%; with a balance of Fe and unavoidable impurity elements.

[0013] In the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure, the various chemical elements are designed according to the following principles: C: It can improve the hardenability of steel, ensuring that a phase transformation structure with higher hardness is formed during the quenching cooling process. An increase in carbon content will increase the proportion of hard phases, thereby enhancing the hardness of the steel, but it will lead to a decrease in toughness. If the carbon content is too low, it is difficult to achieve high strength. Therefore, the C content in the present disclosure is controlled at 0.37-0.43%.

[0014] Si: In steel, Si is beneficial for increasing strength, and an appropriate amount of Si can prevent the formation of coarse carbides during tempering. However, a higher Si content can reduce the impact toughness of the steel. Therefore, a low Si composition system is adopted in the present disclosure and the Si content is controlled at 0.10-0.40%.

[0015] Mn: It mainly exists in solid solution in steel and can improve the hardenability of steel. It forms a low-temperature phase transformation structure with high-strength during quenching, so that the resulting steel has good wear resistance. However, an excessive Mn content can lead to the formation of a higher amount of retained austenite, reducing the yield strength of the steel and making it prone to segregation in the center. Therefore, the Mn content in the present disclosure is controlled at 0.50-0.90%.

[0016] Cr: It can improve the hardenability of steel to form hardened martensitic structure, thus increasing the strength of steel. However, if the Cr content is too high, coarse carbides will form to reduce impact performance. Therefore, the Cr content in the present disclosure is controlled at 0.60-1.25%.

[0017] Ni: It exists in solid solution in steel and can improve the low-temperature impact performance of steel. However, an excessively high Ni content can lead to an excessive retained austenite content in the steel, thereby reducing the strength of the steel. Therefore, the Ni content in the present disclosure is controlled at 1.30-2.00%.

[0018] Mo: It can dissolve in steel and is beneficial for improving the hardenability of steel and enhancing the strength of steel. Tempering at higher temperatures will result in fine carbides, which further increases the strength of steel. Considering the cost of the precious alloy Mo element, the Mo content in the present disclosure is controlled at 0.15-0.30%.

[0019] Al: The formation of fine AIN precipitates in steel can inhibit the growth of austenite grain. However, an excessively high Al content can lead to the formation of larger Al oxides. Coarse hard AIN inclusions can reduce the impact toughness and fatigue performance of the steel Therefore, the Al content in the present disclosure is controlled at 0.02-0.05%.

[0020] Nb: Nb forms fine precipitate phase in steel, which serves to inhibit the recrystallization of the steel and can refine the grains. However, an excessively high Nb content will lead to the formation of coarse NbC particles during the smelting process, which in turn reduces impact toughness. Grain refinement plays an important role in improving the mechanical properties of steel, especially strength and toughness. More importantly, in the present disclosure, grain refinement also helps to reduce the hydrogen embrittlement sensitivity of the steel. Therefore, the Nb content in the present disclosure is controlled at 0.001-0.030%.

[0021] Furthermore, the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure also contains at least one of the following chemical elements: 0 < Cu ≤ 0.3%, 0 < V ≤ 0.06%, 0 < Ti ≤ 0.03%, 0 < Ca ≤ 0.003%.

[0022] Cu: Further, Cu can be optionally added to improve the strength of steel and enhance the corrosion resistance of steel. However, if the Cu content is too high, it will accumulate at the grain boundaries during heating, leading to weakening of the grain boundaries and resulting in cracking. Therefore, Cu is optionally added in the present disclosure and the upper limit is controlled at 0.30%.

[0023] Ca: The Ca element added to steel can improve the size and morphology of sulfide inclusions, avoiding deterioration of impact toughness. Therefore, Ca is optionally added in the present disclosure and the upper limit is controlled at 0.003%.

[0024] V: V can form precipitates with C or N in steel, enhancing the strength of steel. If the contents of C and V are too high, coarse VC particles will form. Therefore, V is optionally added in the present disclosure and the upper limit is controlled at 0.06%.

[0025] Ti: The addition of Ti in steel can result in fine precipitate phase. But if the Ti content is too high, coarse angular TiN particles will form during the smelting process, reducing impact toughness. Therefore, Ti is optionally added in the present disclosure and the upper limit is controlled at ≤0.03%.

[0026] Furthermore, in the unavoidable impurities of the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure, N is ≤ 0.012%, O is ≤ 0.002%, H is ≤ 0.0002%, P is ≤ 0.02%, S is ≤ 0.015%, and P + S ≤ 0.03%.

[0027] The unavoidable impurities in the present disclosure are mainly P, S, N, O, and H. Under permissible technical conditions, it is desirable that their contents are as low as possible. Among them: P tends to segregate at the grain boundaries in steel, which can reduce the grain boundary binding energy and deteriorate the impact toughness of the steel. Its upper limit is preferably controlled at 0.020%.

[0028] S tends to segregate in steel and form a number of sulfide inclusions, thereby reducing the impact resistance. Its upper limit is preferably controlled at 0.015%.

[0029] In addition, to further improve the stress corrosion resistance, the total mass percentage of P and S in the present disclosure is preferably controlled at ≤0.030%.

[0030] N is an interstitial atom and also an element for the formation of MX-type precipitates. To avoid the enrichment of N in the steel, its upper limit in the present disclosure is preferably controlled at 0.012%.

[0031] H will accumulate at the defects in steel. To prevent hydrogen-induced delayed fracture, if the tensile strength exceeds 1150 MPa, the upper limit of the H content is controlled at 0.0002%.

[0032] O will form oxides and composite oxides with Al in the steel. In order to ensure the uniformity of the steel structure and the low-temperature impact toughness fatigue performance, the upper limit of the O content in the present disclosure is preferably controlled at 0.0020%.

[0033] Further, in the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure, the contents of the various elements also satisfy: Mn+Cr+Ni+Mo+Cu≤4.0, where each chemical element symbol represents the value before the percentile sign of the mass percentage of the chemical element.

[0034] Further, in the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure, the range of the microalloying element coefficient r M / N is: 1.0-5.9, wherein r M / N = ([Al] / 2 + [Nb] / 6 + [Ti] / 4) / [N], each chemical element symbol in the formula represents the value before the percentile sign of the mass percentage of the chemical element. In addition, it is required in the present disclosure that the atomic ratio of the total amount of microalloying elements to nitrogen exceeds 1, with the microalloying elements being Al, Nb, and Ti.

[0035] Further, in the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure, the atmospheric corrosion resistance index I is ≥7.0, wherein: I = 26.0 Cu + 3.9 Ni + 1.2 Cr + 1.5 Si + 17.3 P − 7.3 Cu Ni − 9.1 Ni P − 33.4 Cu 2 wherein each chemical element symbol in the formula represents the value before the percentile sign of the mass percentage of the chemical element.

[0036] Further, the rolled microstructure of the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure consists of bainite + martensite and / or retained austenite, and contains nanoscale precipitates, with the volume fraction of bainite being ≥60%. In the present disclosure, "rolled" refers to the state of the steel after forging or rolling and before the heat treatment (e.g., quenching and tempering).

[0037] Further, the microstructure of the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure after heat treatment of quenching and tempering is mainly fine-grained tempered sorbite, and contains nanoscale precipitates.

[0038] It is generally believed that the hydrogen embrittlement sensitivity of different structures is ranked from highest to lowest as follows: original martensite > low-temperature tempered martensite > tempered troostite with original martensite orientation > bainite > tempered sorbite (high-temperature tempered). The present disclosure employs a specific chemical composition design and fully utilizes the impact of various alloying elements and micro-alloying elements on phase transformation and microstructure. After heat treatment of quenching and tempering, a microstructure mainly composed of tempered sorbite is formed, which contains nanoscale precipitates. At the same time, the impurity content is controlled to ensure the strength, impact toughness, elongation, plasticity, as well as good weather resistance, wear resistance, stress corrosion resistance, and fatigue resistance of the steel.

[0039] Further, in the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure, the grain size grade of austenite is ≥6.

[0040] Further, the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure has a yield strength Rp 0.2 of ≥ 1000 MPa, a tensile strength R m of ≥ 1150 MPa, an elongation A of ≥ 12%, a section shrinkage Z of ≥ 50%, a Charpy impact energy A kv at room temperature of ≥ 60 J, a Charpy impact energy A kv at -40°Cof ≥ 30J, and a hydrogen embrittlement coefficient η(Z) of ≥ 0.85.

[0041] In some embodiments, the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure has a yield strength Rp 0.2 of ≥ 1020 MPa, a tensile strength R m of ≥ 1150 MPa, an elongation A of ≥ 12%, a section shrinkage Z of ≥ 55%, a Charpy impact energy A kv at room temperature of ≥ 65 J, a Charpy impact energy A kv at -40°Cof ≥ 35J, and a hydrogen embrittlement coefficient η(Z) of ≥ 0.89.

[0042] In some embodiments, the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure has a yield strength Rp 0.2 of 1020-1100 MPa, a tensile strength R m of 1150-1200 MPa, an elongation A of 12-18%, a section shrinkage Z of 50-65%, such as 58-65%, a Charpy impact energy A kv at room temperature of 65-100J, a Charpy impact energy A kv at -40°Cof 35-80J, and a hydrogen embrittlement coefficient η(Z) of 0.89-0.98.

[0043] Another objective of the present disclosure is to provide a method for manufacturing the stress-corrosion-resistant high-strength and high-toughness medium carbon steel.

[0044] To achieve the above purpose, the present disclosure provides a manufacturing method for the above stress-corrosion-resistant high-strength and high-toughness medium carbon steel, comprising steps of: (1) smelting; (2) casting; (3) heating: where the heating temperature is controlled at 1050-1250°C, and the holding time is 3~24h; (4) forging or rolling: where the final rolling temperature or final forging temperature is controlled at ≥ 850°C; (5) quenching + tempering: where the quenching temperature is 830-910°C, with a holding time of 30-200 minutes, followed by water quenching; the tempering temperature is 530-640°C, with a holding time of 30-200 minutes, and followed by air cooling or water cooling after tempering.

[0045] In some embodiments, in step (4), the rolling or forging is carried out to provide a final product size directly.

[0046] In other embodiments, in step (4), rolling is first carried out to provide an intermediate billet size, followed by intermediate heating, and then rolling to provide a final product size; wherein the intermediate heating temperature is 1050-1250°C, and the holding time is 3-24 h.

[0047] In addition, during the rolling process, the steel billet leaving the heating furnace is rolled after being descaled with high-pressure water, and cooled by air cooling or slow cooling after rolling.

[0048] The high-strength and high-toughness medium carbon steel obtained by the above manufacturing method can be used in applications requiring high-strength bars, such as wind power fasteners, etc., with bar sizes ranging from Φ20 to Φ130mm.

[0049] In the manufacturing method of the present disclosure, the high-strength and high-toughness medium carbon steel is fully austenitized at a heating temperature of 1050°C-1250°C. During the heating process, the carbides, nitrides as well as carbonitrides of Al, Nb, V, and Ti, and the carbides of Cr and Mo partially or all dissolve in the austenite. In the subsequent rolling / forging and cooling processes, Al, Nb, V, and Ti form fine precipitates. Mn, Cr, and Mo dissolved in the austenite can enhance the hardenability of the steel and improve the hardness and strength of martensite.

[0050] Then, under conditions of final rolling or final forging temperature of ≥850°C, a matrix structure that contains refined bainite + a small amount of martensite and / or retained austenite along with fine dispersed nanoscale precipitates is formed.

[0051] In addition, the steel is heated to 830-910°C after rolling or forging, held at that temperature, and then water-quenched. During the heating process, some of the precipitates of carbide-forming elements Al, Nb, V, Ti, Cr, and Mo dissolve, while the undissolved precipitates pin the grain boundaries and suppress the coarsening of austenite grains, resulting in an austenite grain size grade of ≥6. During the quenching and cooling process, the alloying elements that are dissolved in the austenite allow the steel to have high strength and good toughness.

[0052] The steel after quenching is subjected to tempering heat treatment at 530-640°C, where elements Al, Nb, V, Cr, Ti, and Mo form fine precipitates with C and N, improving the strength and the matching of plasticity and toughness of the steel. After the quenching and tempering heat treatment, the microstructure mainly consists of tempered sorbite (i.e., tempered martensite) and contains nanoscale precipitates.

[0053] The stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure has the following beneficial effects: The stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure is developed as a high-strength and high-toughness steel material through a reasonably designed chemical composition combined with optimized processes. The rolled or forged bars undergo quenching followed by tempering heat treatment to form a microstructure consisting of tempered martensite, a small amount of bainite, and retained austenite, along with finely dispersed precipitates. This microstructure allows the steel to have good impact toughness, elongation, and section shrinkage, allowing it to resist stress corrosion cracking while also exhibiting good weather resistance and fatigue performance.

[0054] The stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure has a yield strength of ≥1000MPa, a tensile strength R m of ≥1150MPa, an elongation A of ≥12%, a section shrinkage Z of ≥50%, a Charpy impact energy A kv at room temperature of ≥60J, a low temperature Charpy impact energy A kv at -40°Cof ≥30J, and a hydrogen embrittlement coefficient η(Z) of ≥0.85.

[0055] The process design of the manufacturing method for the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of the present disclosure is reasonable and has a broad process window, allowing for batch commercial production on bar production lines or high-speed wire rod production lines.Detailed Description

[0056] The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to the present disclosure and the manufacturing method therefor will be further interpreted and explained below in combination with specific examples, but the interpretation and explanation do not constitute an undue limitation to the technical solution of the present disclosure.Examples 1-8 and Comparative Examples 1-4

[0057] The stress-corrosion-resistant high-strength and high-toughness medium carbon steel of Examples 1-8 were prepared with the following steps: (1) The chemical compositions shown in Tables 1-1 and 1-2 were smelted: in actual operations, vacuum induction furnace melting, electric furnace melting, or converter melting might be used, followed by refining and vacuum treatment. (2) casting: casting was carried out by die casting or continuous casting to obtain ingots. (3) heating: the ingot was placed into the heating furnace for heating, and the heating temperature was controlled at 1050-1250°C, with a holding time of 3-24 hours. During heating, the temperature might be raised directly to the heating temperature or gradually increased in steps to reach the heating temperature. (4) forging or rolling: the final rolling temperature or final forging temperature was controlled at ≥850°C, and cooling was carried out after rolling or forging. The cooling methods might be air cooling or wind cooling.

[0058] When forging or pressing was carried out, the heated ingot might be directly rolled or forged to the size of the final product, or it might first be rolled to an intermediate billet size, then subjected to intermediate heating, and finally rolled to the size of the final product; when the aforementioned rolling in stages was used, the intermediate heating temperature of the intermediate billet might be controlled at 1050-1250°C, and the holding time might be controlled at 3-24 h. (5) Quenching + tempering: where the quenching temperature was 830-910°C with a holding time of 30-200 minutes, followed by water quenching; the tempering temperature was 530-640°C, with a holding time of 30-200 minutes, and followed by air cooling or water cooling after tempering.

[0059] It should be noted that the manufacturing process of Comparative Examples 1-3 is basically the same as that of the present disclosure, but its chemical composition or specific process parameters do not meet the design requirements of the present disclosure. Comparative Example 4 uses commercially available round bar steel.

[0060] Table 1-1 lists the mass percentages of the various chemical elements of the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4. Table 1-1. (wt.%, with a balance of Fe and other unavoidable impurities except P, S, O, H and N)No.CSiMnCrNiMoNbAlVTiCuPSONHCaEx.10.380.390.520.791.610.240.0050.0230.0400.0020.090.0080.0010.00150.00450.000110.0028Ex.20.370.150.901.251.330.170.0010.0260.0060.0180.300.0050.0140.00090.01070.000160.0004Ex.30.390.330.760.771.50.230.0070.0340.0320.0030.180.0110.0100.00170.00340.000100.0024Ex.40.420.260.640.601.870.280.0120.050.060000.0100.0080.00180.00640.000080.0010Ex.50.430.210.720.831.390.220.0030.0210.0120.0300.230.0130.0150.00160.00480.000100.0014Ex.60.420.180.651.022.000.250.0090.0370000.010.0120.00080.00680.000130.0011Ex.70.390.100.560.981.750.210.0190.0390.0370.0040.070.0070.0080.00150.01180.000090.0013Ex.80.410.250.810.891.480.190.0270.02100.0010.020.0170.0020.0010.00600.000140.0023CEx.10.36 0.240.690.771.500.230.0030.0340.0030.0030.010.0110.0140.00170.00340.000110.0004CEx.20.430.360.92 1.021.420.240.0020.0360.0020.0030.070.0100.0080.00180.00640.000100.001CEx.30.410.250.670.831.710.220 0.0210.0030.0010.020.0130.0130.00160.01160.000120.0004CEx.40.44 0.250.650.811.350.230.0030.0270.0040.0010.010.0060.0120.00080.00480.000140.001

[0061] Table 1-2 lists the compositional synergy of the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4. Table 1-2.No.Total amount of major alloying elements, % [Mn]+[Cr]+[Ni]+[Mo]+[Cu]Content of impurity elements [P]+[S], %Microalloying element coefficient r M / N Atmospheric corrosion resistance coefficient IEx.13.250.0092.98.8Ex.23.950.0191.78.8Ex.33.440.0215.68.9Ex.43.390.0184.28.4Ex.53.390.0283.98.6Ex.63.920.0222.99.2Ex.73.570.0152.08.9Ex.83.390.0192.57.5CEx.13.20.0255.47.3CEx.23.670.0183.08.2CEx.33.450.0260.98.3CEx.43.050.0183.06.8Note: in Table 1-2, r M / N = ([Al] / 2+[Nb] / 6+[Ti] / 4) / [N]; I = 26.0[Cu]+3.9[Ni]+1.2[Cr]+1.5[Si]+17.3[P]-7.3[Cu][Ni]-9.1[Ni][P]-33.4[Cu] 2< ; in both formulas, each chemical element symbol represents the value before the percentile sign of the mass percentage of the chemical element.

[0062] In the present disclosure, the specific production process operations of the stress-corrosion-resistant high-strength and high-toughness medium carbon steel of Example 1-8 and the comparative steel of Comparative Example 1-4 are as follows:Example 1

[0063] The chemical composition shown in Tables 1-1 and 1-2 was smelted in a 50kg vacuum induction furnace. The molten steel obtained from the smelting was cast into an ingot, and then the ingot was heated and subsequently forged into a blank. The heating temperature was controlled at 1050°C, held for 5 hours before forging. The final forging temperature was controlled at 850°C. The blank was finally forged into a bar with a diameter of Φ50mm, which was air-cooled after forging. The heating temperature of quenching was 860°C, with a holding time of 30 minutes; and the tempering temperature was 550°C, with a tempering time of 60 minutes. Water cooling was carried out after tempering.Example 2

[0064] The chemical composition shown in Tables 1-1 and 1-2 was smelted in a 150kg vacuum induction furnace. The molten steel obtained from the smelting was cast into an ingot, and then the ingot was heated and subsequently forged into a blank. The heating temperature was controlled at 1180°C, held for 12 hours before forging. The final forging temperature was controlled at 960°C. The blank was finally forged into a bar of Φ70mm, which was subjected to forced-air cooling after forging. The heating temperature of quenching was 910°C, with a holding time of 100 minutes; and the tempering temperature was 600°C, with a tempering time of 90 minutes. Water cooling was carried out after tempering.Example 3

[0065] The chemical composition shown in Tables 1-1 and 1-2 was smelted in a 500kg vacuum induction furnace. The molten steel obtained from the smelting was cast into an ingot, and then the ingot was heated and subsequently forged into a blank. The heating temperature was controlled at 1080°C, held for 24 hours before forging. The final forging temperature was controlled at 980°C. The blank was finally forged into a bar of Φ90mm, which was subjected to slow piling-up cooling after forging. The heating temperature of quenching was 870°C, with a holding time of 135 minutes; and the tempering temperature was 560°C, with a tempering time of 120 minutes. Air cooling was carried out after tempering.Example 4

[0066] The chemical composition shown in Tables 1-1 and 1-2 was subjected to converter smelting, refining and vacuum treatment, followed by mold casting to obtain a cast billet. The stepped heating of the cast billet was controlled as follows: the cast billet was first heated to 620°C in a preheating section, then heated to 950 °C in a first heating section, then heated to 1200 °C in a second heating section after heat preservation, and entered into a soaking section after 9 hours of heat preservation, and then subjected to subsequent rolling. The billet leaving the heating furnace was rolled after being descaled by high-pressure water, and finally rolled into a bar of Φ120mm with the final rolling temperature being controlled at 970°C. Air cooling was carried out after rolling. The quenching heating temperature was 850°C, with a holding time of 200min; and the tempering temperature was 530°C with a tempering time of 200min. Air cooling was carried out after tempering.Example 5

[0067] The chemical composition shown in Tables 1-1 and 1-2 was subjected to converter smelting, refining and vacuum treatment, followed by mold casting to obtain a cast billet. The cast billet was controlled to be heated to 1180°C and held for 12h and then subjected to subsequent rolling. The billet leaving the heating furnace was rolled after being descaled by high-pressure water and rolled into an intermediate billet, with a final rolling temperature of 1010°C and an intermediate billet size of 220mm×220mm. Then the intermediate billet was heated to 1080 °C, held for 24 hours, and then rolled after being descaled by high-pressure water. The final rolling temperature of the intermediate billet was controlled at 900°C. The specification of the finished bar was Φ80mm. Air cooling was carried out after rolling. After the peeling with a grinding wheel was carried out, defects were detected using an ultrasonic flaw detector and eddy current. The quenching heating temperature was 870°C, with a holding time of 120min; and the tempering temperature was 550°C with a tempering time of 1 00min. Air cooling was carried out after tempering.Example 6

[0068] The chemical composition shown in Tables 1-1 and 1-2 was subjected to electric furnace smelting, refining and vacuum treatment, followed by continuous casting to obtain a continuously cast billet of 280mmx280mm. The continuously cast billet was controlled to be slowly heated to 1200°C, held for 10h and then subjected to subsequent rolling. The billet leaving the heating furnace was rolled after being descaled by high-pressure water. The final rolling temperature was controlled at 970°C. The specification of the finished bar was Φ100mm. Air cooling was carried out after rolling. Then further finishing, including heat treatment, surface treatment and non-destructive detection, was carried out. Turning / peeling (surface treatment) was carried out after normalization treatment (heat treatment) at 910°C, and defects were detected using an ultrasonic flaw detector and magnetic powder (non-destructive detection). The quenching heating temperature was 890°C, with a holding time of 150min; and the tempering temperature was 640°C with a tempering time of 135min. Air cooling was carried out after tempering.Example 7

[0069] The chemical composition shown in Tables 1-1 and 1-2 was subjected to electric furnace smelting, LF refining and VD vacuum treatment, followed by casting to obtain a continuously cast billet of 320mm×425mm. The continuously cast billet was controlled to be first heated to 600 °C in a preheating section, then heated to 950 °C in a first heating section, then heated to 1230 °C in a second heating section after heat preservation, and entered into a soaking section after 8 hours of heat preservation, and then subjected to subsequent rolling after heat preservation. The billet leaving the heating furnace was rolled after being descaled by high-pressure water, and finally rolled into an intermediate billet, with a final rolling temperature of 1050°C and an intermediate billet size of 260mm×260mm. Air cooling was carried out after cooling. Then the intermediate billet was heated to 680 °C in a preheating section, heated to 1050 °C in a first heating section, then heated to 1250 °C in a second heating section, and entered into a soaking section after 6 hours of heat preservation. The billet leaving the heating furnace was rolled after being descaled by high-pressure water. The final rolling temperature of the intermediate billet was controlled at 950 °C. The specification of the finished bar was Φ80mm. Air cooling was carried out after rolling. Defects were detected using an ultrasonic flaw detector and magnetic powder. The quenching heating temperature was 880°C, with a holding time of 120min; and the tempering temperature was 560°C with a tempering time of 100 min. Air cooling was carried out after tempering.Example 8

[0070] The chemical composition shown in Tables 1-1 and 1-2 was subjected to electric furnace smelting, LF refining and VD vacuum treatment, followed by continuous casting to obtain a continuously cast billet of 320mm×425mm. The continuously cast billet was controlled to be slowly heated to 1250 °C after preheating, and subjected to rolling after 3h of heat preservation. The billet leaving the heating furnace was rolled after being descaled by high-pressure water, and finally rolled into an intermediate billet, with a final rolling temperature of 1000°C and an intermediate billet size of 140mm×140mm. Air cooling was carried out after cooling. Then the intermediate billet was slowly heated to 1130 °C. After 3h of heat preservation, the billet leaving the heating furnace was rolled after being descaled by high-pressure water. The final rolling temperature of the intermediate billet was controlled at 850 °C. The specification of the finished bar was Φ30mm. Air cooling was carried out after rolling, followed by normalization treatment at 870°C. Non-destructive inspection was carried out using an ultrasonic flaw detector and magnetic powder detection. The quenching heating temperature was 830°C, with a holding time of 50min; and the tempering temperature was 540°C with a tempering time of 30 min. Air cooling was carried out after tempering.Comparative Example 1

[0071] The chemical composition shown in Tables 1-1 and 1-2 was smelted in a 50kg vacuum induction furnace. The molten steel was cast into an ingot, and then the ingot was heated and subsequently forged into a blank. The heating temperature was controlled at 1050°C, held for 5 hours before forging. The final forging temperature was controlled at 860°C. The blank was finally forged into a bar with a diameter of Φ50mm, which was air-cooled after forging. The heating temperature of quenching was 860°C, with a holding time of 75 minutes; and the tempering temperature was 550°C, with a tempering time of 60 minutes. Water cooling was carried out after tempering.Comparative Example 2

[0072] The chemical composition shown in Tables 1-1 and 1-2 was smelted in a 150kg vacuum induction furnace. The molten steel was cast into an ingot, and then the ingot was heated and subsequently forged into a blank. The heating temperature was controlled at 1180°C, held for 12 hours before forging. The final forging temperature was controlled at 960°C. The blank was finally forged into a bar with a diameter of Φ70mm, which was then subjected to forced-air cooling. The heating temperature of quenching was 900°C, with a holding time of 100 minutes; and the tempering temperature was 600°C, with a tempering time of 90 minutes. Water cooling was carried out after tempering.Comparative Example 3

[0073] The chemical composition shown in Tables 1-1 and 1-2 was smelted in a 500kg vacuum induction furnace. The molten steel was cast into an ingot, and then the ingot was heated and subsequently forged into a blank. The heating temperature was controlled at 1080°C, held for 20 hours before forging. The final forging temperature was controlled at 980°C. The blank was finally forged into a bar with a diameter of Φ90mm, which was then subjected to piling-up cooling. Annealing at 640°C was carried out. The heating temperature of quenching was 870°C, with a holding time of 135 minutes; and the tempering temperature was 560°C, with a tempering time of 120 minutes. Air cooling was carried out after tempering.Comparative Example 4

[0074] Commercially available round bar steel was selected. The commercially available round bar steel was subjected to heat treatment. The heating temperature of quenching was 900°C, with a holding time of 100 minutes; and the tempering temperature was 600°C, with a tempering time of 90 minutes. Water cooling was carried out after tempering.

[0075] Tables 2-1 and 2-2 list the specific process parameters adopted in the above manufacturing process steps in Examples 1-8 and Comparative Examples 1-4. Table 2-1.No.Smelting, refining and casting processesHeating system for cast billetsFinal rolling temperature or final forging temperature (°C)Intermediate billet size (mm)Heating system for intermediate billetFinal rolling temperature of intermediate billet (°C)Ex.150kg vacuum induction furnace + mold casting1050°C×5h850---Ex.2150kg vacuum induction furnace + mold casting1180°C×12h960---Ex.3500kg vacuum induction furnace + mold casting1080°C×24h980---Ex.4Converter + refining+ mold casting1200°C×9h970---Ex.5converter + refining + mold casting1180°C×12h1010220×2201080°C×24h900Ex.6electric furnace + refining + continuous casting1200°C×10h970---Ex.7electric furnace + refining + continuous casting1230°C×8h1050260×2601250°C×6h950Ex.8electric furnace + refining + continuous casting1250°C×3h1000140×1401130°C×3h850CEx. 150kg vacuum induction furnace + mold casting1050°C×5h860---CEx. 2150kg vacuum induction furnace + mold casting1180°C×12h960---CEx. 3500kg vacuum induction furnace + mold casting1080°C×20h980---CEx. 4 / / / / / / Table 2-2. No.Bar specification (mm)Cooling after rolling or forgingQuenchingTemperingEx.1φ50air cooling860°C×30min550°C×60min, water coolingEx.2φ70forced-air cooling910°C×100min600°C×90min, water coolingEx.3φ90piling-up cooling870°C×135min560°C×120min, air coolingEx.4φ120air cooling850°C×200min530°C×200min, air coolingEx.5φ80air cooling870°C×120min550°C×100min, air coolingEx.6φ100air cooling890°C×150min640°C×135min, air coolingEx.7φ80air cooling880°C×120min560°C×100min, air coolingEx.8φ30air cooling830°C×50min540°C×30min, air coolingCEx.1φ50air cooling860°C×75min550°C×60min, water coolingCEx.2φ70forced-air cooling900°C×100min600°C×90min, water coolingCEx.3φ90piling-up cooling870°C×135min560°C×120min, air coolingCEx.4φ70 / 900°C×100min600°C×90min, water cooling

[0076] The rolled round bar steel of Examples 1-8 and the rolled round bar steel of Comparative Examples 1-4 (the rolled round bar steel of Comparative Example 4 refers to the commercially available round bar steel without heat treatment) were sampled respectively. Metallographic samples were prepared according to GB / T 13298-2015, and the microstructure was analyzed with reference to GB / T 13299-1991. The steel sample of each of the Examples and Comparative Examples was further water-quenched after being held at 910°C for 4 hours, and its austenite grain size was evaluated according to the standard ASTM E112-10 after sample preparation. The results of the relevant tests are listed in Table 3 below.

[0077] Table 3 lists the metallographic analysis results of the round steel of Examples 1-8 and Comparative Examples 1-4. Table 3.No.MicrostructureBainite volume fraction (%)Austenite grain size (grade)Ex.1bainite+martensite+retained austenite707Ex.2bainite+martensite806.5Ex.3bainite+retained austenite908Ex.4bainite+retained austenite957.5Ex.5bainite+retained austenite+martensite907.5Ex.6bainite+martensite+retained austenite907Ex.7bainite+retained austenite+martensite906Ex.8bainite+martensite+retained austenite606.5CEx.1bainite+martensite+retained austenite708CEx.2bainite+martensite807CEx.3bainite+retained austenite906(1) CEx.4bainite+retained austenite+martensite857

[0078] As can be seen from the above Table 3, in the present disclosure, the microstructure of the rolled steel of Example 1-8 is bainite + martensite and / or retained austenite, and contains nanoscale precipitate. The volume fraction of bainite is ≥ 60%, and the grain size grade of austenite is ≥ 6.

[0079] In addition, the finished round bar steel of Examples 1-8 was sampled separately, and metallographic samples were prepared according to GB / T 13298-2015, and the microstructure was analyzed with reference to GB / T 13299-1991. It is found that after heat treatment of quenching + tempering, the formed microstructure is mainly tempered sorbite, and contains nanoscale precipitate.

[0080] The inventors further sampled the finished products of Examples 1-8 and the comparative steel of Comparative Examples 1-4 respectively. The samples were prepared in accordance with GB / T 2975-2018 "Steel and steel products - Location and preparation of samples and test pieces for mechanical testing". The tensile testing was conducted in accordance with GB / T 228.1-2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" to measure the tensile strength R m , yield strength R p0.2 and elongation A. At the same time, the Charpy impact energy A kv at room temperature and -40°C of each Example and Comparative Example were tested according to GB / T 229-2007 "Metallic materials - Charpy Pendulum Impact Test Method". The test results are shown in Table 4.

[0081] It should be noted that, in the engineering field, the change of section shrinkage during the tensile testing under environmental conditions is usually used to reflect the stress corrosion tendency. In the present disclosure, circular cross-sectional specimens having a diameter of 10 mm were prepared according to the requirement of hydrogen embrittlement sensitivity in accordance with GB / T 2975-2018 "Steel and steel products - Location and preparation of samples and test pieces for mechanical testing". The tensile testing with a strain rate of ≤ 0.0003 / s was carried out according to the national standard GB / T 228.1-2010 to obtain the section shrinkage Z. The hydrogen embrittlement coefficient η(Z) was defined to evaluate the hydrogen-induced cracking resistance of steel: η Z = Z 2 / Z 1 wherein Z 1 represents the section shrinkage of a round bar steel baked at 250°C for 2h and dehydrogenated after tensile testing; and Z 2 represents the section shrinkage of the round bar steel after tensile testing. The test results are shown in Table 4. Based on the hydrogen embrittlement coefficient η(Z) designed by the present disclosure, the larger hydrogen embrittlement coefficient η(Z), the smaller the stress corrosion tendency of steel, and the better the hydrogen embrittlement resistance and stress corrosion resistance.

[0082] Table 4 lists the performance test results of Example 1-8 and Comparative Example 1-4. Table 4.No.Tensile strength, R m Yield strength, R p0.2 Elongation ASection shrinkage ZImpact energy at room temperature A kv Impact energy at - 40°C A kv Hydrogen embrittlement coefficient η (Z)MPaMPa%%JJ%Ex.111581025186395780.91Ex.211821030176177530.89Ex.311951087156069350.95Ex.411701065126282650.96Ex.511661055146171370.97Ex.611741077136268410.92Ex.711601035166385610.90Ex.81162105016.56091770.93CEx.11126 1007155688520.92CEx.211651026145386470.82 CEx.31157101316626927 0.88CEx.41180108112526325 0.83

[0083] As can be seen from Table 4, the high-strength and high-toughness steel involved in the present disclosure has a yield strength of higher than 1000MPa, a tensile strength R m of higher than 1150MPa, an elongation A of ≥12%, a section shrinkage Z of greater than 50%, a Charpy impact energy A kv at room temperature of ≥60J, a Charpy impact energy Akv at -40°C of greater than 30J, and a hydrogen embrittlement coefficient η(Z) of greater than 0.85.

[0084] In contrast, for Comparative Examples 1-4, the design of the chemical element composition does not meet the design requirements of the present disclosure. Among them, the carbon content of Comparative Example 1 is low, and its tensile strength is also low. The hydrogen embrittlement coefficient of Comparative Example 2 is low. Because a larger amount of Mn element is added, although the strength is improved, the stress corrosion resistance is insufficient. The low-temperature impact energy of the steel in Comparative Examples 3 and 4 is low. The microalloy coefficient of Comparative Example 3 is low, and the austenite grain size is not ideal, where 6(1) indicates that the average grain size grade is 6. But it comprises duplex grain size, and there are coarse grains of grade 1, which has an adverse effect on the low-temperature toughness. The carbon content of Comparative Example 4 is high, and although the tensile strength of the steel is improved, the toughness is insufficient, the stress corrosion cracking resistance is not good, and the fatigue performance cannot meet the use requirement.

[0085] It should be noted that combinations of the various technical features in this case are not limited to the combinations disclosed in the claims of this case or the combinations disclosed in the specific Examples. All technical features disclosed in this case can be combined freely or associated in any way unless a contradiction occurs.

[0086] It should also be noted that the Examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above Examples, and changes or modifications made thereto can be directly derived from the present disclosure or easily conceived of by those skilled in the art, all of which fall within the protection scope of the present disclosure.

Claims

1. A stress-corrosion-resistant high-strength and high-toughness medium carbon steel, which comprises Fe and unavoidable impurity elements, as well as the following chemical elements in percentage by mass: C: 0.37-0.43%, Si: 0.10-0.40%, Mn: 0.50-0.90%, Cr: 0.60-1.25%, Ni: 1.30-2.00%, Mo: 0.15-0.30%, Al: 0.02-0.05%, Nb: 0.001-0.030%.

2. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1, wherein mass percentages of chemical elements of the steel are: C: 0.37-0.43%, Si: 0.10-0.40%, Mn: 0.50-0.90%, Cr: 0.60-1.25%, Ni: 1.30-2.00%, Mo: 0.15-0.30%, Al: 0.02-0.05%, Nb: 0.001-0.030%; with a balance of Fe and unavoidable impurity elements.

3. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it also comprises at least one of the following chemical elements: 0<Cu≤0.30%, 0<V≤0.06%, 0<Ti≤0.03%, 0<Ca≤0.003%.

4. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein among the unavoidable impurities, N is ≤ 0.012%, O is ≤ 0.002%, H is ≤ 0.0002%, P is ≤ 0.02%, S is ≤ 0.015%, and P + S ≤ 0.03%.

5. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to any one of claims 1-3, wherein various elements also satisfy: Mn+Cr+Ni+Mo+Cu≤4.0, where each chemical element symbol represents a value before a percentile sign of a mass percentage of the chemical element.

6. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it has a microalloying element coefficient rM / N in a range of 1.0-5.9, wherein rM / N = ([Al] / 2+[Nb] / 6+[Ti] / 4) / [N], wherein each chemical element symbol represents a value before a percentile sign of a mass percentage of the chemical element.

7. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it has an atmospheric corrosion resistance index I of ≥7.0, wherein: I = 26.0 Cu + 3.9 Ni + 1.2 Cr + 1.5 Si + 17.3 P − 7.3 Cu Ni − 9.1 Ni P − 33.4 Cu 2 wherein each chemical element symbol represents a value before a percentile sign of a mass percentage of the chemical element.

8. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it has a rolled microstructure of bainite + martensite and / or retained austenite, with a volume fraction of bainite of ≥60%.

9. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein a microstructure after quenching and tempering is mainly tempered sorbite, and contains nanoscale precipitates.

10. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it has a grain size grade of austenite of ≥6.

11. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it has a yield strength Rp0.2 of ≥ 1000 MPa, a tensile strength Rm of ≥ 1150 MPa, an elongation A of ≥ 12%, a section shrinkage Z of ≥ 50%, a Charpy impact energy Akv at room temperature of ≥ 60 J, a Charpy impact energy Akv at -40°Cof ≥ 30J, and a hydrogen embrittlement coefficient η(Z) of ≥ 0.85.

12. The stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to claim 1 or 2, wherein it has a yield strength Rp0.2 of ≥ 1020 MPa, a tensile strength Rm of ≥ 1150 MPa, an elongation A of ≥ 12%, a section shrinkage Z of ≥ 55%, a Charpy impact energy Akv at room temperature of ≥ 65 J, a Charpy impact energy Akv at -40°Cof ≥ 35J, and a hydrogen embrittlement coefficient η(Z) of ≥ 0.89.

13. A manufacturing method for the stress-corrosion-resistant high-strength and high-toughness medium carbon steel according to any one of claims 1-12, which comprises steps of: (1) smelting; (2) casting; (3) heating: where a heating temperature is controlled at 1050-1250°C, and a holding time is 3-24h; (4) forging or rolling: where a final rolling temperature or final forging temperature is controlled at ≥ 850°C; (5) quenching + tempering: where a quenching temperature is 830-910°C, with a holding time of 30-200 minutes; and a tempering temperature is 530-640°C, with a holding time of 30-200 minutes.

14. The manufacturing method according to claim 13, wherein in step (4), the rolling or forging is carried out to provide a final product size directly.

15. The manufacturing method according to claim 14, wherein in step (4), rolling is first carried out to provide an intermediate billet size, followed by intermediate heating, and then rolling to provide a final product size; wherein a temperature of the intermediate heating is 1050-1250°C, and a holding time is 3-24 h.

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