Stress corrosion resistant high-strength, high-toughness medium-carbon steel and method for manufacturing the same

Abstract

This invention discloses a stress corrosion-resistant, high-strength, and high-toughness medium carbon steel, which contains Fe and unavoidable impurities, and further 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%, and Nb: 0.001-0.030%. Accordingly, this invention also provides a method for manufacturing the said stress corrosion-resistant, high-strength, and high-toughness medium carbon steel. The steel grade according to this invention has excellent impact toughness, elongation, and surface shrinkage, while being resistant to stress corrosion cracking and possessing excellent weather resistance and fatigue resistance.

Description

[Technical Field]

[0001] This invention relates to steel materials and methods for producing the same, and more particularly to medium-carbon steel and methods for producing the same. [Background technology]

[0002] High-strength, high-toughness steel bars are typically used as mechanical and structural components where high safety is required, such as critical consumable parts like fastening components for wind power generation. They must possess high strength, high toughness, stress corrosion resistance, and high fatigue properties.

[0003] Conventional technologies typically involve selecting an appropriate chemical composition and employing controlled rolling and cooling or quenching and tempering processes to produce high-strength, high-toughness steel. However, producing high-strength steel through controlled rolling and cooling is difficult due to the difficulty in controlling the rolling and cooling processes, which affects the overall uniformity of the steel's mechanical properties. When producing high-strength steel through the quenching and tempering process, optimizing the content of alloying and carbon elements improves the hardenability of the steel and allows for the formation of a martensitic structure in the steel during the cooling process. High-strength steel primarily composed of martensite has a high dislocation density, resulting in low impact toughness. Furthermore, if microcracks or other minute defects occur during the tensile process, it rapidly fractures and breaks, resulting in low fracture toughness.

[0004] Furthermore, Cr-Ni-Mo-based medium-carbon low-alloy steels possess excellent strength and toughness, making them widely used in fields such as construction machinery, automobiles, bridges, and marine equipment. While their safe working strength level does not exceed 1000 MPa, adopting steels with higher strength levels not only reduces the weight of equipment but also leads to resource conservation. However, as the strength level of steel increases, the difficulty of processing and manufacturing also increases, and its susceptibility to hydrogen embrittlement inevitably increases as well.

[0005] A Chinese patent document with publication number CN102242322A, publication date November 16, 2011, and title "Improved 40CrNiMo Steel and Method for Manufacturing the Same" discloses a type of medium carbon steel, the chemical composition of which is, in weight percent, 0.37-0.45% C, 1.65-1.85% Cr, 0.45-0.65% Ni, 0.15-0.25% Mo, and 0.90-1.20% The composition is Mn, 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 the remainder being Fe. The tensile strength of this steel is ≥1195 MPa, and the impact energy exceeds 85 J, but this does not relate to stress corrosion resistance.

[0006] In a Chinese patent document with publication number CN104726783A, publication date June 24, 2015, and title "Steel for sleeve rings of wind turbine yaw bearings and pitch bearings and method for manufacturing the same," a type of steel is disclosed, whose chemical composition by mass percentage is 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, with the remainder being Fe and small amounts of impurities. The above-described method for manufacturing steel for sleeve rings of wind turbine yaw bearings and pitch bearings involves heat-treating the processed sleeve rings of wind turbine yaw bearings and pitch bearings. First, the steel is heated to 860-890°C and held for 3-5 hours to austenitize it. Next, it is oil-cooled, then heated to 580-630°C and held for 3-5 hours, and finally cooled to room temperature by oil cooling. This steel has excellent hardenability, and its mechanical properties fully meet the standards and usage requirements for steel used in high-power wind turbine yaw bearings and pitch bearings. However, this steel type does not have stress corrosion resistance.

[0007] Furthermore, as can be seen from the above-mentioned conventional technology, in current conventional technology, in order to obtain high-performance medium-carbon alloy steel, those skilled in the art improve strength and performance by adding a large amount of alloying elements, or by adding trace alloying elements such as B, Nb, Mg, and rare earth elements, thereby obtaining high-performance medium-carbon alloy steel. However, such design methods that add large amounts of alloying elements tend to increase manufacturing costs, and if the alloying element content in the steel is too high, coarse carbide particles such as Cr, Mo, V, and Ti carbides are formed, reducing the impact toughness of the steel.

[0008] Furthermore, under humid operating conditions, Cr-Ni-Mo-based medium-carbon low-alloy steel components are subjected to large loads and dynamic impacts, making them susceptible to stress corrosion. In severe cases, this can lead to brittle fracture, resulting in significant economic losses and ultimately, safety accidents.

[0009] Given this, there is a need to acquire high-strength, high-toughness medium-carbon steel with stress corrosion resistance. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] One objective of the present invention is to provide a stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel. This steel possesses excellent impact toughness, elongation, and surface shrinkage, while being resistant to stress corrosion cracking and exhibiting excellent weather resistance and fatigue resistance. It can be applied to applications requiring high-strength and high-toughness steel, such as construction machinery and marine work. [Means for solving the problem]

[0011] To achieve the above objective, the present invention provides a stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel, which contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages: It contains 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%.

[0012] Accordingly, the present invention further provides a stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel, the mass percentage content of each chemical element being: The composition is as follows: 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 remainder is Fe and unavoidable impurities.

[0013] In the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention, the design principles for each chemical element composition are as follows.

[0014] C: C improves the hardenability of steel materials and allows for the formation of a high-hardness phase transformation structure within the steel during the quenching and cooling process. A higher C content increases the proportion of the hard phase, improving the hardness of the steel, but leading to a decrease in toughness. If the C content is too low, high strength becomes difficult to achieve. Therefore, in this invention, the C content is controlled to 0.37-0.43%.

[0015] Si contributes to strength improvement in steel, and an appropriate amount of Si can prevent the formation of coarse carbides during tempering. However, if the Si content is too high, it reduces the impact toughness of the steel. Therefore, in this invention, a low Si composition system is adopted, and the Si content is limited to 0.10 to 0.40%.

[0016] Mn exists mainly as a solid solution in steel, which can improve the hardenability of steel. During quenching, a high-strength low-temperature phase transformation structure is formed, and the obtained steel has excellent wear resistance. However, if the Mn content is too high, a large amount of retained austenite will form, reducing the yield strength of the steel and making it prone to center segregation. Therefore, in the present invention, the Mn content is controlled to be 0.50 - 0.90%.

[0017] Cr can improve the hardenability of steel, form a hardened martensite structure, and improve the strength of the steel. However, if the Cr content is too high, coarse carbides will form, reducing the impact properties. Therefore, in the present invention, the Cr content is controlled to be 0.60 - 1.25%.

[0018] Ni exists as a solid solution in steel and can improve the low-temperature impact properties of steel. However, if the Ni content is too high, the retained austenite content in the steel will become too high, reducing the strength of the steel. Therefore, in the present invention, the Ni content is controlled to be 1.30 - 2.00%.

[0019] Mo can dissolve in steel and contribute to improving the hardenability of steel and enhancing the strength of the steel. When tempered at a higher temperature, fine carbides are formed, further improving the strength of the steel. Considering the cost of the expensive alloy element Mo, in the present invention, the Mo content is controlled to be 0.15 - 0.30%.

[0020] Al can inhibit the growth of austenite grain size by forming fine AlN precipitates in steel. However, if the Al content is too high, large Al oxides will form, and coarse AlN hard inclusions will reduce the impact toughness and fatigue properties of the steel. Therefore, in the present invention, the Al content is controlled to be 0.02 - 0.05%.

[0021] When Nb is added to steel, a fine precipitate phase is formed, which inhibits the recrystallization of the steel and can refine the crystal grain. However, if the Nb content is too high, coarse NbC particles are formed during the smelting process, which actually reduces impact toughness. Refinement of crystal grain plays an important role in improving the mechanical properties of steel, especially strength and toughness, and more importantly, in this invention, refinement of crystal grain also contributes to reducing the susceptibility of steel to hydrogen embrittlement. Therefore, in this invention, the Nb content is controlled to 0.001 to 0.030%.

[0022] Furthermore, the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to 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%。

[0023] Cu: By optionally adding Cu, the strength of the steel can be improved, and it also contributes to improving the corrosion resistance of the steel. However, if the Cu content is too high, it will be unevenly distributed at the grain boundaries during the heating process, weakening the grain boundaries and causing cracking. Therefore, in this invention, Cu may be added, but its upper limit is controlled to 0.30%.

[0024] Ca: Adding Ca to steel improves the size and characteristics of sulfide inclusions and prevents deterioration of impact toughness. Therefore, in this invention, Ca may be added, and its upper limit is controlled to 0.003%.

[0025] V:V forms precipitates with C or N in steel, improving the strength of the steel. If the content of C and V is too high, coarse VC particles are formed. Therefore, in this invention, V may be added, and its upper limit is controlled to 0.06%.

[0026] Ti: Adding Ti to steel can form a fine precipitate phase, but if the Ti content is too high, coarse, angular TiN particles are formed during the smelting process, reducing impact toughness. Therefore, in this invention, Ti may be added, but its upper limit is controlled to ≤0.03%.

[0027] Furthermore, in the unavoidable impurities of the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention, N ≤ 0.012%, O ≤ 0.002%, H ≤ 0.0002%, P ≤ 0.02%, S ≤ 0.015%, and P + S ≤ 0.03%.

[0028] The unavoidable impurities in this invention are mainly P, S, N, O, and H, and as far as is technically acceptable, the lower their content, the better. However, When phosphorus (P) is concentrated at grain boundaries in steel, it reduces the grain boundary bond energy and degrades the impact toughness of the steel. Preferably, its upper limit is controlled to 0.020%.

[0029] S becomes unevenly distributed in the steel and forms many sulfide inclusions, reducing impact resistance. Preferably, its upper limit is controlled to 0.015%.

[0030] Furthermore, in order to further improve stress corrosion resistance, the present invention preferably controls the total mass percentage content of P+S to ≤0.030%.

[0031] Although N is an interstitial atom, it is also an element that forms MX-type precipitates. In order to avoid enrichment of the N element in steel, the present invention preferably controls its upper limit to 0.012%.

[0032] Since hydrogen (H) concentrates in defective areas within the steel, to prevent hydrogen-induced delayed cracking, the upper limit of the H content is controlled to 0.0002% when the tensile strength exceeds 1150 MPa.

[0033] Since oxygen forms oxides and composite oxides with algae in steel, the present invention preferably controls the upper limit of the oxygen content to 0.0020% in order to ensure uniformity of the steel structure and low-temperature impact energy and fatigue properties.

[0034] Furthermore, in the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention, each element satisfies Mn+Cr+Ni+Mo+Cu≦4.0, and in the formula, the numerical value before the percentage sign of the mass percentage content of each chemical element is substituted for each chemical element.

[0035] Furthermore, in the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention, the trace alloying element coefficient r M / N The range is 1.0 to 5.9, however,

[0036]

number

[0037] In the formula, the numerical value before the percentage sign of the mass percentage content of each chemical element is substituted for each chemical element. Furthermore, in this invention, it is required that the atomic ratio of the total amount of trace alloying elements, which are Al, Nb, and Ti, to the element nitrogen is greater than 1.

[0038] Furthermore, in the stress corrosion-resistant high-strength, high-toughness medium-carbon steel according to the present invention, its atmospheric corrosion resistance index I is ≥7.0, however,

[0039]

number

[0040] In the formula, the numerical value before the percentage sign of the mass percentage content of each chemical element is substituted for each chemical element.

[0041] Furthermore, the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention has a microstructure in the rolled state of bainite + martensite and / or retained austenite, and contains nanoscale precipitates, wherein the volume phase ratio of bainite is ≥ 60%. In the present invention, the rolled state refers to the state in which the steel has been forged or rolled but has not undergone heat treatment (e.g., quenching + tempering).

[0042] Furthermore, the stress corrosion resistant high-strength and high-toughness medium-carbon steel according to the present invention has a microstructure mainly composed of fine-grained tempered sorbite after quenching + tempering heat treatment and contains nano-scale precipitates.

[0043] Generally, when different microstructures are arranged in descending order of hydrogen embrittlement susceptibility, it is as follows: primary martensite > low-temperature tempered martensite > tempered troostite with a primary martensite orientation > bainite > tempered sorbite (high-temperature tempering). The present invention adopts a specific chemical composition design, fully utilizes the effects of various alloying elements and trace alloying elements on phase transformation and microstructure, and through quenching + tempering heat treatment, a fine microstructure mainly composed of tempered sorbite is formed and contains nano-scale precipitates. At the same time, by controlling the impurity content, the strength, impact toughness, elongation, and plasticity of the steel are ensured, and excellent weather resistance, wear resistance, stress corrosion resistance, and fatigue resistance are brought about.

[0044] Furthermore, in the stress corrosion resistant high-strength and high-toughness medium-carbon steel according to the present invention, its austenite crystal grain size is ≧ grade 6.

[0045] Furthermore, the stress corrosion resistant high-strength and high-toughness medium-carbon steel according to the present invention has a yield strength R p0.2 ≧ 1000 MPa, a tensile strength R m ≧ 1150 MPa, an elongation A ≧ 12%, a reduction of area Z ≧ 50%, a room temperature Charpy impact energy A kv ≧ 60 J, a -40°C Charpy impact energy A kv ≧ 30 J, and a hydrogen embrittlement resistance coefficient η(Z) ≧ 0.85.

[0046] In some embodiments, the stress corrosion resistant high-strength and high-toughness medium-carbon steel according to the present invention has a yield strength R p0.2 ≧ 1020 MPa, a tensile strength R m ≧ 1150 MPa, an elongation A ≧ 12%, a reduction of area Z ≧ 55%, a room temperature Charpy impact energy A kv ≧ 65 J, a -40°C Charpy impact energy A<At ≥35J, the hydrogen embrittlement resistance coefficient η(Z) ≥0.89.

[0047] In some embodiments, the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention has a yield strength R p0.2 With a pressure of 1020-1100 MPa, the tensile strength R m With a pressure of 1150-1200 MPa, an elongation rate A of 12-18%, a cross-sectional shrinkage rate Z of 50-65%, for example 58-65%, and a room temperature Charpy impact energy A kv At 65-100 J, the Charpy impact energy A at -40°C is kv The pressure range is 35-80 J, and the hydrogen embrittlement resistance coefficient η(Z) is 0.89-0.98.

[0048] Another objective of the present invention is to provide a method for producing stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel.

[0049] Based on the above-mentioned objectives of the invention, the present invention provides a method for producing the above-mentioned stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel, comprising the following steps.

[0050] (1) Smelting; (2) Casting; (3) Heating: Control the heating temperature to 1050-1250°C and the holding time to 3-24 hours; (4) Blacksmithing or rolling: Control the rolling completion temperature or blacksmithing completion temperature to ≥ 850°C; (5) Quenching + Tempering: However, the quenching temperature range shall be 830-910°C, the holding time shall be 30-200 mins, and then a water granulation treatment shall be performed; the tempering temperature shall be 530-640°C, the holding time shall be 30-200 mins, and after tempering, air cooling or water cooling shall be performed.

[0051] In some embodiments, step (4) may involve direct rolling or forging to the finished product dimensions.

[0052] In some other embodiments, step (4) involves first rolling to the intermediate slab dimensions, then intermediate heating, and then rolling to the final finished product dimensions; however, the intermediate heating temperature is set to 1050-1250°C and the holding time is set to 3-24 hours.

[0053] Furthermore, during the rolling process, the steel slabs removed from the heating furnace are descaled with high-pressure water before rolling begins, and then air-cooled or slowly cooled after rolling.

[0054] The high-strength, high-toughness medium-carbon steel obtained by the above manufacturing method can be applied to applications requiring high-strength rods, such as fastening components for wind power generation, and the dimensional range of the rods may be Φ20 to 130 mm.

[0055] In the manufacturing method according to the present invention, the high-strength, high-toughness medium-carbon steel according to the present invention is completely austenitized at a heating temperature of 1050°C to 1250°C. During the heating process, carbides, nitrides, and carbonitrides of Al, Nb, V, and Ti, and carbides of Cr and Mo partially or completely dissolve in the austenite, and in the subsequent rolling / forging and cooling processes, Al, Nb, V, and Ti form fine precipitates. The Mn, Cr, and Mo dissolved in the austenite improve the hardenability of the steel and can improve the hardness and strength of the martensite.

[0056] Subsequently, under conditions of a final rolling temperature or forging completion temperature of ≥ 850°C or higher, a matrix structure is formed having fine bainite and a small amount of martensite and / or retained austenite, and finely dispersed nanoscale precipitates are present.

[0057] Furthermore, after rolling or forging the steel material, it is heated to 830-910°C, maintained at a constant temperature, and then granulated with water. During the heating process, precipitates of carbide-forming elements such as Al, Nb, V, Ti, Cr, and Mo partially dissolve, while undissolved precipitates pin the grain boundaries, thereby suppressing the coarsening of austenite grains and maintaining an austenite grain size of ≥ grade 6. During the quenching and cooling process, the alloying elements dissolved in the austenite provide the steel material with high strength and good toughness.

[0058] When quenched steel is tempered at 530-640°C, Al, Nb, V, Cr, Ti, and Mo form fine precipitates together with C and N, improving the strength and plastic-toughness compatibility of the steel. After quenching and tempering, a microstructure is formed in which the main component is tempered sorbite (i.e., tempered martensite), and which contains nanoscale precipitates.

[0059] The stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention has the following beneficial effects. According to the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel of the present invention, a high-strength and high-toughness steel material is developed by combining a rationally designed chemical composition with an optimized manufacturing process. By quenching rolled or forged bars and then subjecting them to a tempering heat treatment process, a structure consisting of tempered martensite, small amounts of bainite and retained austenite, and finely dispersed precipitates is formed. This structure gives the steel material excellent impact toughness, elongation, and surface shrinkage, while also providing resistance to stress corrosion cracking, as well as excellent weather resistance and fatigue resistance.

[0060] However, the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention has a yield strength of ≥1000 MPa and a tensile strength R m At ≥1150 MPa, with elongation A ≥ 12%, and sectional shrinkage Z ≥ 50%, the room temperature Charpy impact energy A is... kv At ≥60J, the low-temperature Charpy impact energy A at -40℃ kv At ≥30J, the hydrogen embrittlement resistance coefficient η(Z) ≥0.85. [Effects of the Invention]

[0061] The method for manufacturing stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to the present invention has a rational process design, a wide process window, and enables mass production for commercial use on bar or high-speed wire rod production lines. [Modes for carrying out the invention]

[0062] The following interpretation and explanation will be based on specific examples and will further describe the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel and its manufacturing method according to the present invention, but this interpretation and explanation will not unduly limit the technical solutions of the present invention.

[0063] Examples 1-8 and Comparative Examples 1-4 The stress corrosion-resistant, high-strength, and high-toughness medium-carbon steels used in Examples 1-8 were all prepared by the following process: (1) The smelting was carried out according to the chemical composition shown in Tables 1-1 and 1-2 below: In practice, vacuum induction furnace smelting, electric furnace smelting, or converter smelting may have been performed, and refining and vacuum treatment may have also been carried out.

[0064] (2) Casting: Ingots were obtained by die casting or continuous casting. (3) Heating: The ingot was placed in a heating furnace and heated, with the heating temperature controlled to 1050-1250°C and the holding time controlled to 3-24 hours; however, when heating, the temperature could be raised directly to the heating temperature, or it could be raised in stages to the heating temperature.

[0065] (4) Forging or rolling: The rolling or forging completion temperature was controlled to be ≥ 850°C, and the rolling or forging was cooled afterward, and the cooling method could be air cooling or wind cooling.

[0066] When forging or rolling was performed, it was possible to roll or forge directly to the finished product dimensions, but it was also possible to first roll to the intermediate slab dimensions, then perform intermediate heating, and then roll to the final finished product dimensions; if the above-mentioned stepwise rolling was adopted, the intermediate heating temperature of the intermediate slab could be controlled to 1050-1250°C and the holding time to 3-24 hours.

[0067] (5) Quenching + Tempering: However, the quenching temperature range was set to 830-910°C, the holding time to 30-200 mins, and then water granulation treatment was performed; the tempering temperature was set to 530-640°C, the holding time to 30-200 mins, and then air cooling or water cooling was performed after tempering.

[0068] It should be explained that while the manufacturing process flows of Comparative Examples 1-3 were substantially similar to those of the present invention, their chemical compositions or specific process parameters did not meet the design requirements of the present invention. Comparative Example 4 used commercially available round steel.

[0069] The mass percentage ratios of each chemical element in the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steels used in Examples 1 to 8 and the comparative steel materials used in Comparative Examples 1 to 4 are shown in Table 1-1.

[0070] [Table 1-1]

[0071] Table 1-2 shows the correlation between the components of the stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel used in Examples 1-8 and the comparative steel used in Comparative Examples 1-4.

[0072] [Table 1-2]

[0073] In the present invention, the specific production process for the stress corrosion-resistant high-strength, high-toughness medium-carbon steels of Examples 1 to 8 and the comparative steel materials of Comparative Examples 1 to 4 were as follows.

[0074] [Example 1] According to the chemical compositions shown in Tables 1-1 and 1-2, smelting was carried out in a 50 kg vacuum induction furnace. The molten steel obtained from smelting was die-cast into steel ingots, and after heating the ingots, they were forged and then rolled into ingots. However, the heating temperature was controlled to 1050°C, held for 5 hours before forging, and the forging completion temperature was controlled to 850°C. Finally, the steel was forged into a Φ50 mm diameter bar and air-cooled after forging. The quenching heating temperature was set to 860°C, the holding time to 30 min, the tempering temperature to 550°C, the tempering time to 60 min, and then water-cooled after tempering.

[0075] [Example 2] According to the chemical compositions shown in Tables 1-1 and 1-2, smelting was carried out in a 150 kg vacuum induction furnace. The molten steel obtained from smelting was die-cast into steel ingots, and after heating the ingots, they were forged and then rolled into ingots. However, the heating temperature was controlled to 1180°C, and the steel was held warm for 12 hours before forging. The forging completion temperature was controlled to 960°C, and the steel was finally forged into a Φ70 mm bar, after which it was air-cooled. The quenching heating temperature was 910°C, the holding time was 100 min, the tempering temperature was 600°C, the tempering time was 90 min, and after tempering, it was water-cooled.

[0076] [Example 3] According to the chemical compositions shown in Tables 1-1 and 1-2, smelting was carried out in a 500 kg vacuum induction furnace. The molten steel obtained from smelting was die-cast into steel ingots, and after heating the ingots, they were forged and then rolled into ingots. However, the heating temperature was controlled to 1080°C, and the steel was held warm for 24 hours before forging. The forging completion temperature was controlled to 980°C, and the steel was finally forged into a Φ90 mm bar. After forging, it was stacked and slowly cooled. The quenching heating temperature was 870°C, the holding time was 135 min, the tempering temperature was 560°C, the tempering time was 120 min, and the steel was air-cooled after tempering.

[0077] [Example 4] According to the chemical compositions shown in Tables 1-1 and 1-2, the steel was subjected to converter smelting, followed by refining and vacuum treatment, and then die-cast to obtain an ingot. The heating of the ingot was controlled in a stepwise manner: first, it was heated to 620°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, and after holding the temperature, it was heated to 1200°C in the second heating stage, and after holding the temperature for 9 hours, it entered the soaking stage, and then the subsequent rolling was carried out. The steel slab removed from the heating furnace was descaled with high-pressure water, and then rolling was started. The rolling end temperature was controlled to 970°C, and it was finally rolled into a Φ120mm bar. After rolling, it was air-cooled. The quenching heating temperature was set to 850°C, the holding time to 200 min, the tempering temperature to 530°C, the tempering time to 200 min, and then air-cooled after tempering.

[0078] [Example 5] According to the chemical compositions shown in Tables 1-1 and 1-2, the material was subjected to converter smelting, refining, and vacuum treatment before being cast into die-cast ingots. The ingots were heated to 1180°C and held for 12 hours before subsequent rolling. The steel slabs removed from the heating furnace were descaled with high-pressure water before rolling began to form intermediate slabs. The rolling completion temperature was controlled to 1010°C, resulting in intermediate slab dimensions of 220mm x 220mm. Next, the intermediate slabs were heated to 1080°C and held for 24 hours before being removed from the furnace. The scale was removed with high-pressure water before rolling began. The rolling completion temperature for the intermediate slabs was controlled to 900°C, resulting in finished bar stocks with a diameter of Φ80mm. After rolling, the bars were air-cooled. After descaling with a grinding wheel, ultrasonic testing and eddy current testing were performed. The hardening temperature was set to 870°C, the holding time to 120 mins, the tempering temperature to 550°C, the tempering time to 100 mins, and then it was air-cooled after tempering.

[0079] [Example 6] According to the chemical compositions shown in Tables 1-1 and 1-2, the steel was subjected to electric furnace smelting, followed by refining and vacuum treatment, and then continuously cast into 280mm x 280mm continuous casting ingots. The continuously casting ingots were slowly heated to 1200°C, held for 10 hours, and then rolled. After removing the steel slabs from the heating furnace, the scale was removed with high-pressure water, and then rolling was started. The rolling completion temperature was controlled to 970°C, and the finished bar material was specified to be Φ100mm. After rolling, it was air-cooled. Further finishing was then performed, specifically including heat treatment, surface treatment, and non-destructive testing. Specifically, the finishing involved normalizing (heat treatment) at 910°C, followed by turning (surface treatment), and then ultrasonic and magnetic particle testing (non-destructive testing). The hardening temperature was set to 890°C, the holding time to 150 mins, the tempering temperature to 640°C, the tempering time to 135 mins, and then it was air-cooled after tempering.

[0080] [Example 7] According to the chemical compositions shown in Tables 1-1 and 1-2, the material was subjected to electric furnace smelting, followed by LF refining and VD vacuum treatment, and then cast into 320mm x 425mm continuous cast ingots. The continuous cast ingots were first heated to 600°C in the preheating stage, then continued to be heated to 950°C in the first heating stage, and after holding the temperature, were heated to 1230°C in the second heating stage, and after holding the temperature for 8 hours, the soaking stage was entered, and after holding the temperature, the subsequent rolling was carried out. The steel slabs removed from the heating furnace were descaled with high-pressure water, and then rolling was started to roll them into intermediate slabs. The rolling end temperature was controlled to 1050°C to obtain intermediate slabs with dimensions of 260mm x 260mm, and after rolling, they were air-cooled. Next, the intermediate slab was heated to 680°C in the preheating stage, to 1050°C in the first heating stage, to 1250°C in the second heating stage, and after being held at this temperature for 6 hours, it entered the soaking stage, was removed from the furnace, scale was removed with high-pressure water, and then rolling was started. The rolling end temperature of the intermediate slab was controlled to 950°C, and the finished bar material was specified to be Φ80mm. After air cooling following rolling, ultrasonic testing and magnetic particle testing were performed. The quenching heating temperature was set to 880°C, the holding time to 120 min, the tempering temperature to 560°C, the tempering time to 100 min, and then air cooling was performed after tempering.

[0081] [Example 8] According to the chemical compositions shown in Tables 1-1 and 1-2, the material was subjected to electric furnace smelting, followed by LF refining and VD vacuum treatment, and then continuously cast into 320mm x 425mm continuous cast ingots. The continuously cast ingots were preheated, slowly heated to 1250°C, held for 3 hours, and then rolled. The steel slabs removed from the heating furnace were descaled with high-pressure water, and then rolling was started to form intermediate slabs. The rolling completion temperature was controlled to 1000°C to obtain intermediate slabs with dimensions of 140mm x 140mm, which were then air-cooled. Next, the intermediate slabs were slowly heated to 1130°C, held for 3 hours, removed from the furnace, descaled with high-pressure water, and then rolling was started. The rolling completion temperature of the intermediate slabs was controlled to 850°C, and the finished bar stock was specified to a diameter of Φ30mm. After rolling, the material was air-cooled, then normalized at 870°C, and non-destructive testing was performed using ultrasonic and magnetic particle testing. The quenching temperature was set to 830°C, the holding time to 50 mins, the tempering temperature to 540°C, the tempering time to 30 mins, and then air-cooled after tempering.

[0082] [Comparative Example 1] Smelting was carried out in a 50 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. Molten steel was die-cast into steel ingots, heated, forged, and then rolled into ingots. The heating temperature was set to 1050°C, held for 5 hours, and then forged. The forging completion temperature was controlled to 860°C, and finally forged into a bar with a diameter of Φ50 mm. After forging, it was air-cooled. The quenching heating temperature was set to 860°C, the holding time to 75 mins, the tempering temperature to 550°C, the tempering time to 60 mins, and then water-cooled after tempering.

[0083] [Comparative Example 2] The molten steel was smelted in a 150 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was die-cast into steel ingots, heated, forged, and then rolled into ingots. The heating temperature was set to 1180°C, and after holding the temperature for 12 hours, forging was performed. The forging completion temperature was controlled to 960°C, and the steel was finally forged into a Φ70 mm bar, after which it was air-cooled. The quenching heating temperature was set to 900°C, the holding time to 100 min, the tempering temperature to 600°C, the tempering time to 90 min, and after tempering, it was water-cooled.

[0084] [Comparative Example 3] Smelting was carried out in a 500 kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. Molten steel was die-cast into steel ingots, heated, forged, and then rolled into ingots. The heating temperature was set to 1080°C, held for 20 hours, then forged, with the forging completion temperature controlled to 980°C. Finally, it was forged into a Φ90 mm bar, followed by stacked cooling. Annealing was performed at 640°C. The quenching heating temperature was set to 870°C, the holding time to 135 min, the tempering temperature to 560°C, the tempering time to 120 min, and then air-cooled after tempering.

[0085] [Comparative Example 4] Commercially available round steel was selected. The commercially available round steel was heat-treated, with the quenching temperature set to 900°C, the holding time to 100 min, the tempering temperature set to 600°C, the tempering time to 90 min, and then water-cooled after tempering.

[0086] The specific process parameters for the above manufacturing methods in Examples 1-8 and Comparative Examples 1-4 are shown in Tables 2-1 and 2-2.

[0087] [Table 2-1]

[0088] [Table 2-2]

[0089] Samples were taken from rolled round steel bars in Examples 1-8 and in Comparative Examples 1-4 (the rolled round steel bar in Comparative Example 4 refers to commercially available round steel bars that have not undergone heat treatment). Metallographic specimens were prepared according to GB / T 13298-2015, and the microstructure was analyzed with reference to GB / T 13299-1991. The steel specimens for each example and comparative example were further heated at 910°C for 4 hours, then granulated in water to prepare the specimens, and their austenite grain size was evaluated according to ASTM E112-10. The results of the relevant measurements and analyses are shown in Table 3 below.

[0090] Table 3 shows the results of the microstructure analysis of the round steel bars used in Examples 1-8 and Comparative Examples 1-4.

[0091] [Table 3]

[0092] As can be seen from Table 3 above, in the present invention, the steels according to Examples 1 to 8 have a microstructure in the rolled state of bainite + martensite and / or retained austenite, contain nanoscale precipitates, have a volume phase ratio of bainite of ≥60%, and have an austenite grain size of ≥Grade 6.

[0093] Furthermore, samples were taken from the finished round steel products of Examples 1 to 8, and metallographic specimens were prepared according to GB / T 13298-2015. Microstructural analysis was performed with reference to GB / T 13299-1991, revealing that after quenching and tempering heat treatment, a fine structure was formed in which tempered sorbite was the main component, and that nanoscale precipitates were present.

[0094] The inventors further sampled from the finished products of Examples 1 to 8 and from the comparative steel materials of Comparative Examples 1 to 4, respectively, prepared the samples in accordance with GB / T 2975-2018 "Sampling Location and Preparation of Test Specimens for Mechanical Property Tests of Steel and Steel Products", and performed tensile tests in accordance with GB / T 228.1-2010 "Tensile Tests of Metallic Materials, Part 1: Room Temperature Test Methods", and determined the tensile strength R m , yield strength R p0.2 The elongation A was also measured, and the Charpy impact energy A of each example and comparative example at room temperature and -40°C was measured in accordance with GB / T 229-2007 "Metallic Materials Charpy Pendulum Hammer Impact Test Method". kv The measurement was performed, and the results are shown in Table 4.

[0095] It needs to be explained that, in the field of engineering, stress corrosion tendencies are usually reflected by the change in surface shrinkage rate during tensile tests under environmental conditions. However, in this invention, the requirement for hydrogen embrittlement susceptibility was met by preparing circular cross-section specimens in accordance with GB / T 2975-2018 "Specimen Sampling Location and Specimen Preparation for Mechanical Property Tests of Steel and Steel Products," with a specimen diameter of 10 mm. Furthermore, tensile tests were conducted in accordance with the Chinese national standard GB / T 228.1-2010, and the cross-sectional shrinkage rate Z was obtained at a strain rate ≤0.0003 / s. The hydrogen embrittlement resistance coefficient η(Z) was defined to evaluate the hydrogen-induced cracking resistance performance of the steel material.

[0096]

number

[0097] However, Z1 represents the sectional shrinkage rate obtained after a tensile test of round steel that has been dehydrogenated by baking at 250°C for 2 hours; Z2 represents the sectional shrinkage rate obtained after a tensile test of round steel. The measurement results are shown in Table 4. According to this hydrogen embrittlement resistance coefficient η(Z) designed by the present invention, a larger hydrogen embrittlement resistance coefficient η(Z) indicates that the stress corrosion tendency of the steel is smaller, and that its hydrogen embrittlement resistance and stress corrosion resistance are better.

[0098] The results of the characteristic measurements for Examples 1-8 and Comparative Examples 1-4 are shown in Table 4.

[0099] [Table 4]

[0100] As can be seen from Table 4, the high-strength, high-toughness steel material according to the present invention has a yield strength R p0.2 All of these exceeded 1000 MPa, and the tensile strength R m All of these values ​​exceed 1150 MPa, the elongation A is ≥ 12%, the cross-sectional shrinkage Z is ≥ 50% in all cases, and the room temperature Charpy impact energy A kv At ≥60J, the low-temperature Charpy impact energy A at -40℃ kv All of these values ​​exceeded 30 J, and the hydrogen embrittlement resistance coefficient η(Z) was above 0.85 in all cases.

[0101] In contrast, Comparative Examples 1 to 4 all fail to meet the design requirements of the present invention in terms of their chemical element composition design. Among them, Comparative Example 1 had a low carbon content, resulting in low tensile strength; Comparative Example 2 had a low hydrogen embrittlement resistance coefficient, and although its strength improved due to the addition of a large amount of Mn, its stress corrosion resistance was insufficient; Comparative Examples 3 and 4 had low low-temperature impact energy of steel; Comparative Example 3 had a low coefficient of trace alloying elements, and its austenite grain size was undesirable (where 6(1) indicates that the average grain size is 6, but double grain size appears, and coarse grains of grade 1 are present, which negatively affected the low-temperature toughness); Comparative Example 4 had a high carbon content, which improved the tensile strength of the steel, but resulted in insufficient toughness, poor resistance to stress corrosion cracking, and fatigue properties that did not meet the requirements for practical application.

[0102] Furthermore, the combinations of technical features in this application are not limited to the combinations described in the claims or the specific embodiments, and all technical features described in this application can be freely combined or combined in any form, as long as they do not contradict each other.

[0103] Furthermore, it should be noted that the embodiments described above are merely specific examples of the present invention. The present invention is not limited to the above embodiments, and it is clear that any similar changes or modifications that a person skilled in the art can directly derive from the disclosure of the present invention or readily conceive are also covered within the scope of the present invention.

Claims

1. It contains Fe and unavoidable impurities, and further contains the following chemical elements in the following mass percentages: Contains 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%. A high-strength, high-toughness medium-carbon steel characterized by stress corrosion resistance.

2. The mass percentage content of each chemical element is: The composition is as follows: 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 remainder is Fe and unavoidable impurities. A stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to claim 1, characterized in that it is a high-strength, high-toughness medium-carbon steel.

3. Furthermore, it 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% A stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to claim 1 or 2.

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

5. A stress corrosion-resistant, high-strength, high-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, and in the formula, the numerical value before the percentage sign of the mass percentage content of the chemical element is substituted for each chemical element.

6. Trace alloy element coefficient r Ｍ／Ｎ The range is 1.0 to 5.9, however, [Math 1] In the formula, each chemical element is replaced with the number before the percentage sign representing the mass percentage content of that chemical element. A stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to claim 1 or 2.

7. The atmospheric corrosion resistance index I is ≥ 7.0, however, [Math 2] In the formula, each chemical element is replaced with the number before the percentage sign representing the mass percentage content of that chemical element. A stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to claim 1 or 2.

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

9. A stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to claim 1 or 2, characterized in that the main component of the microstructure after quenching and tempering is 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, characterized in that the austenite grain size is ≥ grade 6.

11. Yield strength R ｐ０．２ ≥1000 MPa, tensile strength R ｍ At pressure ≥ 1150 MPa, elongation A ≥ 12%, and cross-sectional shrinkage Z ≥ 50%, the room temperature Charpy impact energy A is... ｋｖ At ≥60J, the Charpy impact energy A at -40°C. ｋｖ A stress corrosion-resistant, high-strength, and high-toughness medium-carbon steel according to claim 1 or 2, characterized in that the hydrogen embrittlement resistance coefficient η(Z) is ≥ 0.85 at ≥ 30 J.

12. Yield strength R ｐ０．２ ≥ 1020 MPa, tensile strength R ｍ ≥ 1150 MPa, elongation A ≥ 12%, reduction of area Z ≥ 55%, room temperature Charpy impact energy A ｋｖ ≥ 65 J, -40°C Charpy impact energy A ｋｖ ≥ 35 J, hydrogen embrittlement resistance coefficient η(Z) ≥ 0.89, characterized in that it is the high-strength and high-toughness medium-carbon steel with high stress corrosion resistance according to claim 1 or 2.

13. A method for producing stress corrosion-resistant high-strength, high-toughness medium-carbon steel according to any one of claims 1 to 12, characterized by comprising the following steps. (1) Smelting; (2) Casting; (3) Heating: The heating temperature is controlled to 1050-1250°C, and the holding time is controlled to 3-24 hours; (4) Blacksmithing or rolling: Control the rolling completion temperature or blacksmithing completion temperature to ≥ 850°C; (5) Quenching + Tempering: However, the quenching temperature range shall be 830 to 910°C, the holding time shall be 30 to 200 mins, and the tempering temperature shall be 530 to 640°C, with a holding time of 30 to 200 mins.

14. The manufacturing method according to claim 13, characterized in that step (4) involves directly rolling or forging the material to the dimensions of the finished product.

15. The manufacturing method according to claim 14, wherein in step (4), the material is first rolled to the dimensions of an intermediate slab, then intermediate heating is performed, and then it is rolled to the final dimensions of the finished product; however, the intermediate heating temperature is set to 1050 to 1250°C and the holding time is set to 3 to 24 hours.

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