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

A high-strength and tough medium-carbon steel resistant to stress corrosion and a manufacturing method thereof Technical Field

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

[0002] High-strength and toughness steel bars are usually used in high-safety machinery and structural components, such as wind power fasteners and other key vulnerable parts. They should have high strength, high toughness, stress corrosion resistance and high fatigue performance.

[0003] In the existing technology, appropriate chemical composition is usually selected, and controlled rolling, controlled cooling or quenching + tempering processes are used to produce high-strength and tough steel. Among them, the controlled rolling and controlled cooling method is used to produce high-strength steel. Since the control during the rolling and cooling process is difficult, it affects the overall uniformity of the mechanical properties of the steel. The quenching + tempering process is used to produce high-strength steel. By optimizing the content of alloying elements and carbon elements, the hardenability of the steel is improved, and the steel forms a martensitic structure during the cooling process. High-strength steel mainly composed of martensite has a high dislocation density, resulting in poor impact toughness, and will quickly break and fail if tiny defects such as microcracks appear during the tensile process, and the fracture toughness is low.

[0004] Furthermore, Cr-Ni-Mo medium-carbon low-alloy steels are widely used in engineering machinery, automobiles, bridges, marine equipment, and other fields due to their excellent strength and toughness. Their safe operating strength level does not exceed 1000 MPa. The use of higher-strength steels not only reduces equipment weight but also conserves resources. However, as steel strength increases, processing and manufacturing become more difficult, and their susceptibility to hydrogen embrittlement is bound to increase.

[0005] The Chinese patent document with publication number CN102242322A and publication date of November 16, 2011, entitled “An Improved 40CrNiMo Steel and Preparation Method Thereof” discloses a medium carbon steel, the chemical elements of which are as follows by weight: 0.37-0.45% C, 1.65-1.85% Cr, 0.45-0.65% Ni, 0.15-0.25% Mo, 0.90-1.20% Mn, 0. 0.40-0.55% Si, 0.0025-0.0045% B, 0.22-0.28% N, 0.007-0.012% Ca, 0.002-0.005% Mg, 0.03-0.06% Nb, 0.04-0.08% Ti, 0.02-0.06% RE, ≤0.015% S, ≤0.025% P, and ≤0.0008% Al, with Fe as the balance. The steel has a tensile strength of ≥1195 MPa and an impact energy better than 85 J, but it does not involve stress corrosion resistance.

[0006] A Chinese patent document with publication number CN104726783A and publication date June 24, 2015, titled “A steel for wind turbine yaw and pitch bearing rings and its preparation method,” discloses a steel having 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, Al acid-soluble: 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 the remainder being Fe and a small amount of impurities. The aforementioned method for preparing wind turbine yaw and pitch bearing rings involves heat treating the processed rings. The rings are first heated to 860-890°C, held for 3-5 hours for austenitization, and then oil-quenched. The rings are then heated again to 580-630°C, held for 3-5 hours, and then oil-cooled to room temperature. This steel exhibits excellent hardenability, and its mechanical properties fully meet the standards and requirements for high-power wind turbine yaw and pitch bearing steel. However, this steel grade does not address stress corrosion resistance.

[0007] Furthermore, as can be seen from the above-mentioned prior art, in order to obtain high-performance medium-carbon alloy steel, those skilled in the art have adopted methods such as adding large amounts of alloying elements or adding microalloying elements such as B, Nb, Mg, and rare earth elements to increase strength and improve performance, thereby obtaining high-performance medium-carbon alloy steel. However, this design scheme of adding large amounts of alloying elements can easily increase manufacturing costs, and when the alloying element content in the steel is too high, coarse carbide particles, such as carbides of Cr, Mo, V, and Ti, will form, reducing the impact toughness of the steel.

[0008] In addition, in humid service environment conditions, Cr-Ni-Mo medium-carbon low-alloy steel parts are subjected to large loads and dynamic impacts, and are prone to stress corrosion. In severe cases, brittle fracture may occur, causing huge economic losses and even safety accidents.

[0009] Based on this, it is hoped to obtain a high-strength and tough medium-carbon steel with stress corrosion resistance.

[0010] Summary of the Invention

[0011] One of the purposes of the present invention is to provide a high-strength and tough medium-carbon steel that is resistant to stress corrosion. The steel has good impact toughness, elongation and area reduction, is resistant to stress corrosion cracking, and has good weather resistance and fatigue resistance. The steel can be used in occasions such as engineering machinery and marine engineering that require high-strength and tough steel.

[0012] To achieve the above object, the present invention provides a high-strength and tough medium-carbon steel resistant to stress corrosion, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages:

[0013] 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%.

[0014] Accordingly, the present invention also provides a high-strength and tough medium carbon steel resistant to stress corrosion, wherein the mass percentage of each chemical element is:

[0015] 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%; the balance is Fe and unavoidable impurities.

[0016] The design principles of the chemical elements in the stress corrosion resistant high-strength and tough medium carbon steel of the present invention are as follows:

[0017] C: It improves the hardenability of steel, allowing it to form a hard phase transformation structure during the quenching and cooling process. Increasing the C content increases the proportion of hard phases and the hardness of the steel, but also reduces toughness. Too low a C content makes it difficult to achieve high strength. Therefore, the present invention controls the C content to 0.37-0.43%.

[0018] Si: Si contributes to steel 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, the present invention adopts a low-Si composition system, limiting the Si content to 0.10-0.40%.

[0019] Mn: Mainly present in steel as a solid solution, it improves the steel's hardenability and forms a high-strength, low-temperature phase transformation structure during quenching, resulting in excellent wear resistance. However, excessive Mn content can lead to the formation of a large amount of retained austenite, reducing the steel's yield strength and easily causing center segregation. Therefore, the present invention controls the Mn content to 0.50-0.90%.

[0020] Cr: It improves the hardenability of steel, forms a hardened martensite structure, and increases steel strength. However, excessive Cr content can form coarse carbides, reducing impact resistance. Therefore, the present invention controls the Cr content to 0.60-1.25%.

[0021] Nickel (Ni) exists in steel as a solid solution, improving its low-temperature impact resistance. However, excessive Ni content can lead to excessive levels of retained austenite in the steel, reducing its strength. Therefore, the present invention limits the Ni content to 1.30-2.00%.

[0022] Mo: Mo dissolves in steel, improving its hardenability and strength. During tempering at higher temperatures, it forms fine carbides, further increasing the steel's strength. Considering the cost of the precious alloying element, the Mo content in this invention is controlled to 0.15-0.30%.

[0023] Al: Forms fine AlN precipitates in steel, which inhibit austenite grain growth. However, excessive Al content can lead to the formation of larger Al oxides. Coarse AlN hard inclusions can reduce the steel's impact toughness and fatigue properties. Therefore, the present invention controls the Al content to 0.02-0.05%.

[0024] Nb: Nb is added to steel to form fine precipitates, inhibiting recrystallization and refining the grain size. However, excessive Nb content can lead to the formation of coarse NbC particles during the smelting process, which in turn reduces impact toughness. Grain refinement plays a crucial role in improving the mechanical properties of steel, particularly its strength and toughness. More importantly, in the present invention, grain refinement also helps reduce the steel's susceptibility to hydrogen embrittlement. For this reason, the present invention controls the Nb content to 0.001-0.030%.

[0025] Furthermore, the stress corrosion resistant high strength and toughness medium carbon steel of the present invention further 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%.

[0026] Cu: The optional addition of Cu can improve the strength of the steel and its corrosion resistance. However, if the Cu content is too high, it will accumulate at the grain boundaries during heating, weakening them and leading to cracking. Therefore, the present invention allows for the optional addition of Cu, with an upper limit of 0.30%.

[0027] Ca: Ca added to steel can improve the size and morphology of sulfide inclusions and prevent deterioration of impact toughness. Based on this, the present invention can choose to add Ca, and control the upper limit of Ca to 0.003%.

[0028] V: V can form precipitates with carbon or nitrogen in steel, increasing its strength. Excessive carbon and vanadium content can lead to the formation of coarse VC particles. Therefore, the present invention allows for the addition of V, with an upper limit of 0.06%.

[0029] Ti: Ti can form fine precipitates when added to steel, but excessive Ti content can lead to the formation of coarse, angular TiN particles during smelting, reducing impact toughness. Therefore, the present invention allows for the addition of Ti, with an upper limit of ≤0.03%.

[0030] Furthermore, among the inevitable impurities of the stress corrosion resistant high-strength and tough medium carbon steel of the present invention, N≤0.012%, O≤0.002%, H≤0.0002%, P≤0.02%, S≤0.015%, and P+S≤0.03%.

[0031] The inevitable impurities in the present invention are mainly P, S, N, O and H. Under the condition that technical conditions permit, it is expected that their content is as low as possible. Among them:

[0032] P segregates at grain boundaries in steel, reducing grain boundary bonding energy and deteriorating the impact toughness of the steel. The upper limit of P is preferably controlled to 0.020%.

[0033] S will segregate in steel and form more sulfide inclusions, which will reduce the impact resistance. The upper limit of S is preferably controlled to 0.015%.

[0034] In addition, in order to further improve the stress corrosion resistance, the present invention preferably controls the total mass percentage of P+S to ≤0.030%.

[0035] N is an interstitial atom and also an element that forms MX-type precipitates. In order to avoid the enrichment of N in steel, the upper limit of N is preferably controlled to 0.012% in the present invention.

[0036] Hydrogen will accumulate at defects in steel. In order to prevent hydrogen-induced delayed fracture, if the tensile strength exceeds 1150 MPa, the upper limit of the hydrogen content is controlled to 0.0002%.

[0037] O will form oxides and composite oxides with Al in the steel. In order to ensure uniformity of steel structure and low-temperature impact energy fatigue performance, the present invention preferably controls the upper limit of O content to 0.0020%.

[0038] Furthermore, in the stress corrosion resistant high-strength and tough medium carbon steel described in the present invention, each element also satisfies: Mn+Cr+Ni+Mo+Cu≤4.0, where each chemical element is substituted into the value before the percentage sign of the mass percentage of the chemical element.

[0039] Furthermore, in the stress corrosion resistant high strength and toughness medium carbon steel of the present invention, the microalloying element coefficient r M / N The range is: 1.0~5.9, among which r M / N=([Al] / 2+[Nb] / 6+[Ti] / 4) / [N], where each chemical element is substituted into the numerical value preceding the percentage sign of the mass percentage of the chemical element. Furthermore, the present invention requires that the atomic ratio of the total amount of microalloying elements to nitrogen exceeds 1, and the microalloying elements are Al, Nb, and Ti.

[0040] Furthermore, in the stress corrosion resistant high-strength and tough medium carbon steel of the present invention, its atmospheric corrosion resistance index I is ≥ 7.0, wherein:

[0041] 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

[0042] In the formula, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0043] Furthermore, the as-rolled microstructure of the stress corrosion-resistant, high-strength and tough medium-carbon steel described herein is bainite plus martensite and / or retained austenite, and contains nanoscale precipitates, with the volume fraction of bainite being ≥ 60%. In the present invention, as-rolled refers to the state of the steel after forging or rolling and before heat treatment (e.g., quenching and tempering).

[0044] Furthermore, the microstructure of the high-strength and tough medium-carbon low-alloy steel of the present invention after quenching and tempering heat treatment is mainly fine-grained tempered troostite and contains nano-scale precipitates.

[0045] It is generally believed that the order of sensitivity of different structures to hydrogen embrittlement, from most to least, is: original martensite > low-temperature tempered martensite > tempered troostite with original martensite orientation > bainite > tempered troostite (high-temperature tempering). The present invention adopts a specific chemical composition design, fully utilizing the effects of various alloying elements and microalloying elements on phase transformation and microstructure. After quenching and tempering heat treatment, a microstructure consisting mainly of tempered troostite is formed, containing nano-scale precipitates. At the same time, the impurity content is controlled to ensure the strength, impact toughness, elongation, and plasticity of the steel, while also having good weather resistance, wear resistance, stress corrosion resistance, and fatigue resistance.

[0046] Furthermore, in the stress corrosion resistant high-strength and tough medium carbon steel of the present invention, the austenite grain size is ≥ grade 6.

[0047] Furthermore, the stress corrosion resistant high strength medium carbon steel of the present invention has a yield strength R p0.2 ≥1000MPa, tensile strength R m ≥1150MPa, elongation A≥12%, section shrinkage Z≥50%, room temperature Charpy impact energy Akv ≥60J, -40℃ Charpy impact energy A kv ≥30J, hydrogen embrittlement resistance coefficient η(Z)≥O.85.

[0048] In some embodiments, the high-strength and tough medium carbon steel resistant to stress corrosion of the present invention has a yield strength R p0.2 ≥1020MPa, tensile strength R m ≥1150MPa, elongation A≥12%, section shrinkage Z≥55%, room temperature Charpy impact energy A kv ≥65J, -40℃ Charpy impact energy A kv ≥35J, hydrogen embrittlement resistance coefficient η(Z)≥0.89.

[0049] In some embodiments, the high-strength and tough medium carbon steel resistant to stress corrosion of the present invention has a yield strength R p0.2 1020~1100MPa, tensile strength R m The strength is 1150-1200 MPa, the elongation A is 12-18%, the cross-sectional shrinkage Z is 50-65%, such as 58-65%, and the room temperature Charpy impact energy A kv 65~100J, -40℃ Charpy impact energy A kv The hydrogen embrittlement resistance coefficient η(Z) is 35~80J and 0.89~0.98.

[0050] Another object of the present invention is to provide a method for manufacturing high-strength and tough medium carbon steel that is resistant to stress corrosion.

[0051] Based on the above invention objectives, the present invention provides a method for manufacturing the stress corrosion resistant high-strength and tough medium carbon steel described above, which comprises the steps of:

[0052] (1) smelting;

[0053] (2) Casting;

[0054] (3) Heating: Control the heating temperature to 1050-1250°C and the holding time to 3-24h;

[0055] (4) Forging or rolling: control the final rolling temperature or final forging temperature ≥ 850℃;

[0056] (5) Quenching + tempering, where the quenching temperature range is 830-910℃, the holding time is 30-200min, and then water quenching is used; the tempering temperature is 530-640℃, the holding time is 30-200min, and air cooling or water cooling is used after tempering.

[0057] In some embodiments, in step (4), the steel may be directly rolled or forged to the finished size.

[0058] In some other embodiments, in step (4), the steel is first rolled to an intermediate billet size, then intermediately heated, and then rolled to a final finished product size; wherein the intermediate heating temperature is 1050-1250° C., and the holding time is 3-24 hours.

[0059] In addition, during the rolling process, the steel billet is removed from the heating furnace and descaled by high-pressure water before rolling begins. After rolling, air cooling or slow cooling is adopted.

[0060] The high-strength and tough medium-carbon steel obtained by the above-mentioned manufacturing method can be used in occasions requiring high-strength bars, such as wind power fasteners, and the size specification range of the bars can be Φ20 to 130 mm.

[0061] In the manufacturing method described herein, the high-strength and tough medium-carbon steel is fully austenitized at a heating temperature of 1050°C to 1250°C. During heating, carbides and nitrides of Al, Nb, V, and Ti, as well as carbonitrides and Cr and Mo carbides, partially or completely dissolve in the austenite. During subsequent rolling / forging and cooling, Al, Nb, V, and Ti form fine precipitates. The Mn, Cr, and Mo dissolved in the austenite enhance the steel's hardenability and increase the hardness and strength of the martensite.

[0062] Then, under the condition of a final rolling or final forging temperature ≥850°C, a matrix structure having refined bainite + a small amount of martensite and / or retained austenite and fine dispersed nano-scale precipitates is formed.

[0063] Furthermore, after rolling or forging, the steel is heated to 830-910°C, held at that temperature, and then water quenched. During the heating process, precipitates of carbide-forming elements such as Al, Nb, V, Ti, Cr, and Mo partially dissolve. The undissolved precipitates pin grain boundaries, inhibiting the coarsening of austenite grains and achieving an austenite grain size of ≥6. During the quenching cooling process, the alloying elements dissolved in the austenite impart high strength and good toughness to the steel.

[0064] After quenching, the steel is tempered at 530-640°C. Al, Nb, V, Cr, Ti, and Mo form fine precipitates with C and N, improving the strength and ductility of the steel. After quenching and tempering, a microstructure consisting mainly of tempered bainite (tempered martensite) is formed, containing nano-scale precipitates.

[0065] The stress corrosion resistant high-strength and tough medium carbon steel of the present invention has the following beneficial effects:

[0066] The high-strength and tough medium-carbon steel resistant to stress corrosion described in the present invention is developed through rationally designed chemical composition and optimized process to develop high-strength and tough steel. The rolled or forged bars are subjected to a tempering heat treatment process after quenching to form a structure of tempered martensite, a small amount of bainite and retained austenite, as well as fine dispersed precipitates. This structure gives the steel good impact toughness, elongation and area reduction, resistance to stress corrosion cracking, and good weather resistance and fatigue resistance.

[0067] The stress corrosion resistant high strength medium carbon steel of the present invention has a yield strength of ≥1000MPa and a tensile strength of R m ≥1150MPa, elongation A≥12%, section shrinkage Z≥50%, room temperature Charpy impact energy A kv ≥60J, -40℃ low temperature Charpy impact energy A kv ≥30J, hydrogen embrittlement resistance coefficient η(Z)≥0.85.

[0068] The manufacturing method of the stress corrosion resistant high-strength and tough medium carbon steel of the present invention has a reasonable process design and a wide process window, and can realize batch commercial production on a bar or high-speed wire production line. DETAILED DESCRIPTION

[0069] The stress corrosion resistant high-strength and tough medium carbon steel and its manufacturing method described in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution of the present invention.

[0070] Examples 1-8 and Comparative Examples 1-4

[0071] The stress corrosion resistant high strength and toughness medium carbon steels of Examples 1-8 were all prepared by the following steps:

[0072] (1) Smelting is carried out according to the chemical composition shown in Table 1-1 and Table 1-2 below: In actual operation, vacuum induction furnace smelting, electric furnace smelting or converter smelting can be used, and then refined and vacuum treated.

[0073] (2) Casting: Casting is performed by die casting or continuous casting to obtain ingots.

[0074] (3) Heating: Place the ingot in a heating furnace for heating, and control the heating temperature to 1050-1250°C, with a holding time of 3-24 hours; during heating, the temperature can be directly raised to the heating temperature, or the temperature can be raised to the heating temperature in a step-by-step manner.

[0075] (4) Forging or rolling: Control the final rolling temperature or final forging temperature to be ≥850℃, and cool after rolling or forging. The cooling method can be air cooling or wind cooling.

[0076] When forging or forging is performed, it can be directly rolled or forged to the finished product size, or it can be rolled to the intermediate billet size first, then intermediate heated, and then rolled to the final finished product size; when the above-mentioned segmented rolling is adopted, the intermediate heating temperature of the intermediate billet can be controlled between 1050 and 1250°C, and the holding time can be controlled between 3 and 24 hours.

[0077] (5) Quenching + tempering, where the quenching temperature range is 830-910℃, the holding time is 30-200min, and then water quenching is used; the tempering temperature is 530-640℃, the holding time is 30-200min, and air cooling or water cooling is used after tempering.

[0078] It should be noted that the manufacturing processes of Comparative Examples 1-3 are basically the same as those of the present invention, except that their chemical compositions or specific process parameters do not meet the design requirements of the present invention. Comparative Example 4 uses commercially available round steel.

[0079] Table 1-1 lists the mass percentages of the chemical elements in the stress corrosion resistant high strength and toughness medium carbon steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0080] Table 1-1. (wt.%, the balance is Fe and other inevitable impurities except P, S, O, H and N)

[0081] Table 1-2 lists the synergistic relationship between the components of the stress corrosion resistant high strength and toughness medium carbon steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0082] Table 1-2.

[0083] Note: In Table 1-2, r M / N =([Al] / 2+[Nb] / 6+[Ti] / 4) / [N];

[0084] 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

[0085] ; In both formulas, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0086] In the present invention, the specific production process operations of the stress corrosion resistant high-strength and tough medium carbon steels of Examples 1-8 and the comparative steels of Comparative Examples 1-4 are as follows:

[0087] Example 1

[0088] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 50kg vacuum induction furnace. The resulting molten steel was mold-cast into ingots, heated, and then forged. Heating was controlled at 1050°C, held for 5 hours, and then forged. The final forging temperature was controlled at 850°C, resulting in bars with a diameter of 50mm. After forging, the bars were air-cooled. The quenching temperature was 860°C, held for 30 minutes, and tempered at 550°C for 60 minutes. After tempering, the bars were water-cooled.

[0089] Example 2

[0090] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 150kg vacuum induction furnace. The resulting molten steel was mold-cast into ingots, heated, and then forged. The heating temperature was controlled at 1180°C, held for 12 hours, and then forged. The final forging temperature was controlled at 960°C, resulting in Φ70mm bars, which were then air-cooled. The quenching temperature was 910°C, held for 100 minutes, and tempered at 600°C for 90 minutes. After tempering, the bars were water-cooled.

[0091] Example 3

[0092] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 500kg vacuum induction furnace. The resulting molten steel was mold-cast into ingots, heated, and then forged. The heating temperature was controlled at 1080°C, held for 24 hours, and then forged. The final forging temperature was controlled at 980°C, resulting in Φ90mm bars. After forging, the bars were piled and slowly cooled. The quenching temperature was 870°C, the holding time was 135 minutes, the tempering temperature was 560°C, the tempering time was 120 minutes, and air cooling was performed after tempering.

[0093] Example 4

[0094] The chemical compositions shown in Tables 1-1 and 1-2 are smelted in a converter, refined, and vacuum-treated before being cast into ingots. The ingots are heated in a controlled, step-wise manner: first in the preheating section, heated to 620°C, then in the first heating section to 950°C. After holding, they are heated to 1200°C in the second heating section. After a 9-hour hold, they enter the soaking section and are subsequently rolled. After exiting the heating furnace, the ingots are descaled with high-pressure water and then rolled. The final rolling temperature is controlled at 970°C, resulting in Φ120 mm bars. After rolling, they are air-cooled. The quenching temperature is 850°C, the holding time is 200 minutes, and the tempering temperature is 530°C, the tempering time is 200 minutes, and the bars are air-cooled after tempering.

[0095] Example 5

[0096] The chemical compositions shown in Tables 1-1 and 1-2 are smelted in a converter, refined, and vacuum-treated before being cast into ingots. The ingots are heated to 1180°C and held for 12 hours before rolling. After exiting the heating furnace and undergoing high-pressure water descaling, rolling begins, producing intermediate bars with a final rolling temperature of 1010°C and dimensions of 220 mm x 220 mm. The intermediate bars are then heated to 1080°C and held for 24 hours. After exiting the furnace and undergoing high-pressure water descaling, rolling begins, with a final rolling temperature of 900°C. The finished bars are Ø80 mm and air-cooled after rolling. After grinding wheel peeling, ultrasonic and eddy current testing are performed. The quenching temperature is 870°C, the holding time is 120 minutes, and the tempering temperature is 550°C, the tempering time is 100 minutes, and air-cooling is performed after tempering.

[0097] Example 6

[0098] The steel is smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, then refined and vacuum treated, and then continuously cast into 280mm×280mm continuous casting billets. The billets are slowly heated to 1200°C, held at this temperature for 10 hours, and then rolled. After exiting the heating furnace and undergoing high-pressure water descaling, rolling begins, with the final rolling temperature controlled at 970°C. The finished bars have a diameter of 100mm. After rolling, they are air-cooled. Further finishing is then carried out, which specifically includes heat treatment, surface treatment, and non-destructive testing. After normalizing at 910°C (heat treatment), the steel is then turned and peeled (surface treatment), and subjected to ultrasonic and magnetic particle testing (non-destructive testing). The quenching temperature is 890°C, the holding time is 150 minutes, and the tempering temperature is 640°C, the tempering time is 135 minutes, and air cooling is performed after tempering.

[0099] Example 7

[0100] The steel was smelted in an electric furnace according to the chemical composition shown in Tables 1-1 and 1-2, then subjected to LF refining and VD vacuum treatment before being cast into 320mm×425mm continuous casting ingots. The ingots were first heated to 600°C in the preheating section, then further heated to 950°C in the first heating section. After holding at this temperature, they were heated to 1230°C in the second heating section. After holding at this temperature for 8 hours, they were then placed in the soaking section and subsequently rolled. After exiting the reheating furnace, the ingots were descaled with high-pressure water and then rolled into intermediate bars. The final rolling temperature was controlled at 1050°C, resulting in intermediate bars measuring 260mm×260mm. After rolling, they were air-cooled. The intermediate bars were then heated to 680°C in the preheating section, 1050°C in the first heating section, and 1250°C in the second heating section. After holding at this temperature for 6 hours, they were placed in the soaking section. After exiting the furnace, they were descaled with high-pressure water and then rolled. The final rolling temperature was controlled at 950°C, resulting in bars measuring 260mm×260mm. The finished bars were then air-cooled. The intermediate bars were then heated to 680°C in the preheating section, 1050°C in the first heating section, and 1250°C in the second heating section. After holding at this temperature for 6 hours, they were placed in the soaking section. After exiting the furnace, they were descaled with high-pressure water and then rolled. The final rolling temperature was controlled at 950°C, resulting in bars with a diameter of 80mm. After rolling, the steel was air-cooled and then subjected to ultrasonic and magnetic particle inspections. The quenching temperature was 880°C, the holding time was 120 minutes, the tempering temperature was 560°C, the tempering time was 100 minutes, and air-cooled after tempering.

[0101] Example 8

[0102] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in an electric furnace, followed by LF refining and VD vacuum treatment, and then continuously cast into 320mm×425mm continuous ingots. After preheating, the ingots were slowly heated to 1250°C and held at that temperature for 3 hours before rolling. After exiting the heating furnace, the ingots were descaled with high-pressure water and then rolled into intermediate bars. The final rolling temperature was controlled at 1000°C, resulting in intermediate bars measuring 140mm×140mm. After rolling, they were air-cooled. The intermediate bars were then slowly heated to 1130°C, held at that temperature for 3 hours, removed from the furnace, descaled with high-pressure water, and then rolled. The final rolling temperature was controlled at 850°C, resulting in finished bars measuring 30mm in diameter. After rolling, they were air-cooled and then normalized at 870°C. Non-destructive testing was performed using ultrasonic and magnetic particle inspection. The quenching heating temperature is 830℃, the holding time is 50min, the tempering temperature is 540℃, the tempering time is 30min, and air cooling is performed after tempering.

[0103] Comparative Example 1

[0104] Smelting was carried out in a 50kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2. Molten steel was cast into ingots, heated, and forged to form blanks. The ingots were heated to 1050°C, held at that temperature for 5 hours, and then forged. The final forging temperature was controlled at 860°C, resulting in bars with a diameter of 50 mm. After forging, the bars were air-cooled. The quenching temperature was 860°C, the holding time was 75 minutes, the tempering temperature was 550°C, the tempering time was 60 minutes, and the bars were water-cooled after tempering.

[0105] Comparative Example 2

[0106] Smelting was carried out in a 150kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. Molten steel was mold-cast into ingots, heated, and forged to form blanks. The heating temperature was 1180°C, and the steel was held at that temperature for 12 hours before forging. The final forging temperature was controlled at 960°C, resulting in Φ70mm bars, which were then air-cooled. The quenching temperature was 900°C, the holding time was 100 minutes, the tempering temperature was 600°C, the tempering time was 90 minutes, and the steel was water-cooled after tempering.

[0107] Comparative Example 3

[0108] According to the chemical compositions shown in Tables 1-1 and 1-2, smelting was carried out in a 500kg vacuum induction furnace. The molten steel was mold-cast into ingots, heated, and forged to form blanks. The heating temperature was 1080°C, and the steel was held at that temperature for 20 hours before forging. The final forging temperature was controlled at 980°C, resulting in Φ90mm bars. The bars were then pile-cooled and annealed at 640°C. The quenching temperature was 870°C, the holding time was 135 minutes, and the tempering temperature was 560°C, the tempering time was 120 minutes, and the bars were air-cooled after tempering.

[0109] Comparative Example 4

[0110] Commercially available round steel was selected and heat treated, wherein the quenching heating temperature was 900° C., the holding time was 100 minutes, the tempering temperature was 600° C., the tempering time was 90 minutes, and water cooling was performed after tempering.

[0111] Table 2-1 and Table 2-2 list the specific process parameters of Examples 1-8 and Comparative Examples 1-4 in the above manufacturing method.

[0112] Table 2-1.

[0113] Table 2-2.

[0114] Samples of the as-rolled round steel from Examples 1-8 and Comparative Examples 1-4 (the as-rolled round steel in Comparative Example 4 refers to commercially available round steel that has not been heat treated) were taken. Metallographic specimens were prepared according to GB / T 13298-2015, and their microstructures were analyzed with reference to GB / T 13299-1991. The steel samples from each example and comparative example were further heated at 910°C for 4 hours and then water quenched. After sample preparation, the austenite grain size was assessed according to ASTM E112-10. The relevant test and analysis results are listed in Table 3 below.

[0115] Table 3 lists the metallographic analysis results of the round steels of Examples 1-8 and Comparative Examples 1-4.

[0116] Table 3.

[0117] As can be seen from Table 3 above, in the present invention, the rolled microstructure of the steels of Examples 1-8 is bainite + martensite and / or retained austenite, and contains nano-scale precipitates, the volume phase ratio of bainite is ≥60%, and the austenite grain size is ≥6.

[0118] In addition, the finished round steels of Examples 1-8 were sampled respectively, and metallographic specimens were prepared according to GB / T 13298-2015. The microstructures were analyzed with reference to GB / T 13299-1991. It was found that after quenching and tempering heat treatment, a microstructure mainly composed of tempered bainite was formed, containing nano-scale precipitates.

[0119] The inventors further sampled the finished steel products of Examples 1-8 and Comparative Examples 1-4, prepared the samples according to GB / T 2975-2018 "Steel and Steel Products Mechanical Properties Test Sampling Location and Specimen Preparation", and conducted tensile tests according to GB / T 228.1-2010 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method", and measured the tensile strength R m , yield strength R p0.2 and elongation A, and GB / T 229-2007 "Charpy pendulum impact test method for metallic materials" was used to test the Charpy impact energy A of each embodiment and comparative example at room temperature and -40 °C. kv , the test results are shown in Table 4.

[0120] It should be noted that in the engineering field, stress corrosion tendency is often reflected by changes in tensile test area shrinkage under ambient conditions. The present invention's requirements for hydrogen embrittlement sensitivity are based on the preparation of circular cross-section specimens with a diameter of 10 mm in accordance with GB / T 2975-2018, "Steel and Steel Products Mechanical Properties Test Sampling Location and Preparation." Tensile tests are conducted in accordance with national standard GB / T 228.1-2010 at a strain rate of ≤ 0.0003 / s to obtain the cross-sectional shrinkage Z. The hydrogen embrittlement resistance coefficient η(Z) is defined to evaluate the steel's resistance to hydrogen-induced cracking:

[0121] η(Z)=Z2 / Z1

[0122] Where Z1 represents the cross-sectional reduction ratio obtained after tensile testing of round steel after dehydrogenation treatment by baking at 250°C for 2 hours; Z2 represents the cross-sectional reduction ratio obtained after tensile testing of round steel. The test results are shown in Table 4. Based on the hydrogen embrittlement resistance coefficient η(Z) designed by the present invention, a larger hydrogen embrittlement resistance coefficient η(Z) indicates a lower stress corrosion tendency of the steel and better hydrogen embrittlement resistance and stress corrosion resistance.

[0123] Table 4 lists the performance test results of Examples 1-8 and Comparative Examples 1-4.

[0124] Table 4.

[0125] It can be seen from Table 4 that the yield strength R of the high-strength and tough steel material of the present invention is p0.2 All above 1000MPa, tensile strength R m All are higher than 1150MPa, elongation A≥12%, section shrinkage Z are all greater than 50%, room temperature Charpy impact energy A kv ≥60J, -40℃ low temperature Charpy impact energy A kv All are greater than 30J, and the hydrogen embrittlement resistance coefficient η(Z) is greater than 0.85.

[0126] On the other hand, the chemical element composition designs of Comparative Examples 1-4 all have aspects that do not meet the design requirements of the present invention. Among them, the carbon content of Comparative Example 1 is relatively low, and its tensile strength is also relatively low; the hydrogen embrittlement resistance coefficient of Comparative Example 2 is relatively low because more Mn elements are added. Although the strength is improved, the stress corrosion resistance is insufficient; the low-temperature impact energy of the steel materials of Comparative Examples 3 and Comparative Example 4 is relatively low, and the microalloying coefficient of Comparative Example 3 is relatively low. Its austenite grain size is not ideal, where 6 (1) indicates an average grain size of 6, but a double grain size appears, and there are coarse grains of level 1, which has an adverse effect on low-temperature toughness; and the carbon content of Comparative Example 4 is relatively high. Although the tensile strength of the steel material is improved, the toughness is insufficient, the stress corrosion cracking effect is poor, and the fatigue performance cannot meet the use requirements.

[0127] It should be noted that the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0128] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A high-strength and tough medium-carbon steel resistant to stress corrosion, containing Fe and inevitable impurities, characterized in that: It also contains the following chemical elements in the following mass percentages: 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 tough medium carbon steel according to claim 1, characterized in that: The mass percentage of each chemical element is: 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%; the balance is Fe and unavoidable impurities.

3. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: It also contains 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 toughness medium carbon steel according to claim 1 or 2, characterized in that: Among the inevitable impurities, N≤0.012%, O≤0.002%, H≤0.0002%, P≤0.02%, S≤0.015%, and P+S≤0.03%.

5. The stress corrosion resistant high strength and toughness medium carbon steel according to any one of claims 1 to 3, characterized in that: Each element also satisfies: Mn+Cr+Ni+Mo+Cu≤4.0, where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

6. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: Its microalloying element coefficient r M / N The range is: 1.0~5.9, where r M / N =([Al] / 2+[Nb] / 6+[Ti] / 4) / [N], where each chemical element is substituted into the numerical value before the percentage sign of the mass percentage content of the chemical element.

7. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: Its atmospheric corrosion resistance index I≥7.0, where: 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 the formula, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

8. The stress corrosion resistant high-strength and tough medium carbon steel according to claim 1 or 2, characterized in that: The rolled microstructure is bainite+martensite and / or residual austenite, wherein the volume phase ratio of bainite is ≥60%.

9. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: The main body of its microstructure after quenching and tempering is tempered troostite, and contains nano-scale precipitates.

10. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: Its austenite grain size is ≥ grade 6.

11. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: Its yield strength R p0.2 ≥1000MPa, tensile strength R m ≥1150MPa, elongation A≥12%, section shrinkage Z≥50%, room temperature Charpy impact energy A kv ≥60J, -40℃ Charpy impact energy A kv ≥30J, hydrogen embrittlement resistance coefficient η(Z)≥0.

85.

12. The stress corrosion resistant high strength and toughness medium carbon steel according to claim 1 or 2, characterized in that: Its yield strength R p0.2 ≥1020MPa, tensile strength R m ≥1150MPa, elongation A≥12%, section shrinkage Z≥55%, room temperature Charpy impact energy A kv ≥65J, -40℃ Charpy impact energy A kv ≥35J, hydrogen embrittlement resistance coefficient η(Z)≥0.

89.

13. The method for producing a stress corrosion resistant high-strength and tough medium carbon steel according to any one of claims 1 to 12, characterized in that: It includes the steps of: (1) Smelting; (2) Casting; (3) Heating: Control the heating temperature to 1050-1250°C and the insulation time to 3-24h; (4) Forging or rolling: control the final rolling temperature or final forging temperature ≥ 850°C; (5) Quenching + tempering, where the quenching temperature ranges from 830 to 910°C and the holding time is 30 to 200 min; the tempering temperature is 530 to 640°C and the holding time is 30 to 200 min.

14. The manufacturing method according to claim 13, characterized in that: In step (4), it is directly rolled or forged to the finished size.

15. The manufacturing method according to claim 14, characterized in that: In step (4), the steel is first rolled to an intermediate billet size, then intermediately heated, and then rolled to a final finished product size; wherein the intermediate heating temperature is 1050-1250° C., and the holding time is 3-24 hours.