Medium carbon steel and round steel with good strength and toughness and manufacturing method thereof

Abstract

The invention discloses a medium carbon steel with good strength and toughness. The medium carbon steel contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.30-0.39%, Si: 0.05-0.40%, Mn: 0.50-0.90%, Al: 0.01-0.05%, N: 0.002-0.015%, at least one of Cr≤0.40%, Ni≤0.40%, and Mo≤0.10%, V≤0.02%, Cu≤0.30%, and Nb≤0.09%. The steel further satisfies the following conditions: 1.1≤(Al / 2+Nb / 7+V / 4) / N≤4.9, 0.02%≤Cr+Ni+Mo≤0.65%, and the chemical elements are substituted into the values preceding the percentage signs of the mass percentages of the chemical elements. Correspondingly, the present invention also discloses a round steel prepared corresponding to the above-mentioned medium carbon steel, and a corresponding manufacturing method of the round steel, which comprises the following steps: (1) smelting; (2) casting; (3) heating: controlling the heating temperature to be 1050-1250°C and the holding time to be 3-24 hours; (4) forging or rolling: controlling the final rolling temperature or the final forging temperature to be ≥800°C, and cooling after rolling or forging; and (5) finishing.

Description

Technical Field

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

[0002] As we all know, high-strength steel bars are widely used in industrial production. They are often used in high-safety machinery and structural components, such as automotive parts and key load-bearing components of construction machinery. Therefore, when preparing this high-strength steel, it is required not only to have high strength, but also good plasticity and excellent impact toughness. It should not only be easy to cut and process, but also have high fatigue resistance.

[0003] In the current existing technology, domestic and foreign researchers have conducted extensive research on high-strength steel. They usually select appropriate chemical composition and use quenching + tempering heat treatment or controlled rolling + controlled cooling process to produce high-strength steel.

[0004] When using the quenching and tempering process to produce high-strength steel, researchers in this field generally improve the hardenability of the steel by optimizing the content of alloying elements and carbon elements, so that the steel forms a martensitic structure during the cooling process. High-strength steel mainly composed of martensite has a large dislocation density, resulting in poor impact toughness. In addition, if small defects such as microcracks appear during the tensile process, it will quickly break and fail, and the fracture toughness is low. When using controlled rolling and controlled cooling to produce high-strength steel, medium carbon steel can be obtained without the need for quenching and tempering. However, due to the difficulty in controlling the rolling and cooling processes, this process scheme will affect the overall uniformity of the mechanical properties of the steel.

[0005] In conventional technology, medium-carbon steel typically refers to low- to medium-carbon steel with the addition of microalloying elements such as vanadium. Controlled rolling (forging) and controlled cooling are then used to disperse carbonitrides within the ferrite and pearlite, creating a strengthening effect. This allows the steel to achieve comparable mechanical properties to those achieved after quenching and tempering without undergoing quenching and tempering treatment after rolling (forging). Due to its economical, energy-saving, and environmentally friendly properties, medium-carbon steel has been widely adopted in fields such as automotive and engineering machinery. Therefore, the comprehensive strength and toughness of medium-carbon steel suitable for drive shafts in new energy vehicles is a future trend.

[0006] For example, a Chinese patent document with publication number CN102808073A, publication date December 5, 2012, and titled “Non-quenched and tempered steel with ultrafine-grained pearlite structure and method for manufacturing the same” discloses a method for manufacturing a medium-carbon steel with an ultrafine-grained pearlite structure, wherein the medium-carbon steel is mainly composed of Fe, 0.43-0.47% C, 0.15-0.35% Si, 1.1-1.3% Mn, greater than 0 and at most 0.03% P, greater than 0 and at most 0.04% S, greater than 0 and at most 0.3% Cu, greater than 0 and at most 0.2% Ni, 0.1-0.2% Cr, greater than 0 and at most 0.05% Mo, 0.08-0.15% V, and greater than 0 and at most 0.02% Al, and the manufacturing method adopted is forging.

[0007] Another example: the Chinese patent document with publication number CN109763061A, publication date May 17, 2019, and titled “A non-quenched and tempered steel and its preparation method” discloses a medium carbon steel, which contains the following components by weight: carbon: 0.46-0.55%, silicon: 0.20-0.60%, manganese: 1.20-1.60%, chromium: 0.00-0.30%, aluminum: 0.010-0.030%, nickel: 0.10-0.30%, copper: 0.00-0.20%, phosphorus: 0.000-0.030%, sulfur: 0.020-0.050%, vanadium: 0.050-0.250%, niobium: 0.020-0.050%, titanium: 0.010-0.030%, boron: 0.0005-0.0030%, nitrogen 0.012-0.020%, and the balance is iron. This technical solution produces a high-strength steel suitable for surface quenching and hardening.

[0008] For example, the Chinese patent document with publication number CN109207840A, publication date January 15, 2019, and titled “A free-cutting non-quenched and tempered steel and its manufacturing method” discloses a free-cutting medium-carbon steel, the mass percentage of which is: C: 0.35-0.45%; Si: 0.45-0.65%; Mn: 1.35-1.65%; S: 0.025-0.065%; V: 0.07-0.15%; Ti: 0.01-0.018%; N: 0.012-0.017%; Al: 0.015-0.035%; Ca: 0.0008-0.0025%; the balance is iron and other unavoidable impurities; and the S / Ca ratio is 20-60. In addition, the present invention also provides a method for manufacturing the free-cutting medium carbon steel, which includes smelting and refining, casting, rolling, forging, and two-stage cooling. The free-cutting medium carbon steel improves the strength of the material while ensuring plasticity and toughness.

[0009] As can be seen from the above-mentioned prior art, in order to obtain high-performance medium-carbon steel, most people skilled in the art use microalloying elements such as V and Ti to refine the grain size and improve the strength of the steel. However, this design of adding V and Ti elements can easily increase manufacturing costs. Furthermore, when the V content in the steel is too high, coarse VC particles will form and reduce the impact toughness of the steel. Furthermore, Ti is prone to forming inclusions. When used in combination with other microalloying elements, a coupling effect occurs, which is detrimental to the refining effect of the other microalloying elements.

[0010] Therefore, in order to solve the problems existing in the above-mentioned prior art, the inventors hope to provide a new medium carbon steel with good strength and toughness without adding microalloying elements such as V and Ti, so as to effectively meet market demand. Summary of the Invention

[0011] One of the objectives of the present invention is to provide a medium-carbon steel with excellent strength and toughness. This steel possesses excellent strength and toughness without the addition of microalloying elements such as V and Ti. The steel not only exhibits good strength, impact toughness, and plasticity, but also exhibits good fatigue resistance and high hardenability. This medium-carbon steel is easy to machine and cold-form, meeting the performance requirements of applications such as automotive and construction machinery, and has excellent prospects for widespread application.

[0012] In order to achieve the above object, the present invention provides a medium carbon steel with good strength and toughness, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0013] C: 0.30-0.39%, Si: 0.05-0.40%, Mn: 0.50-0.90%, Al: 0.01-0.05%, N: 0.002-0.015%, at least one of Cr≤0.40%, Ni≤0.40%, Mo≤0.10%, V≤0.02%, Cu≤0.30%, Nb≤0.09%;

[0014] Each element also satisfies the following conditions: 1.1≤(Al / 2+Nb / 7+V / 4) / N≤4.9, 0.02%≤Cr+Ni+Mo≤0.65%, wherein each chemical element is substituted into the value preceding the percentage sign of the mass percentage content of the chemical element.

[0015] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, the mass percentage of each chemical element is:

[0016] C: 0.30-0.39%, Si: 0.05-0.40%, Mn: 0.50-0.90%, Al: 0.01-0.05%, N: 0.002-0.015%, at least one of Cr≤0.40%, Ni≤0.40%, Mo≤0.10%, V≤0.02%, Cu≤0.30%, Nb≤0.09%; the balance is Fe and other unavoidable impurities;

[0017] Each element also satisfies the following conditions: 1.1≤(Al / 2+Nb / 7+V / 4) / N≤4.9, 0.02%≤Cr+Ni+Mo≤0.65%, wherein each chemical element is substituted into the value preceding the percentage sign of the mass percentage content of the chemical element.

[0018] In the above technical solution of the present invention, the inventors designed a reasonable chemical element composition by selecting a matching system of Al, Nb, and V, controlling the relative contents of Al, Nb, V, and N, and adding appropriate amounts of Mn and elements such as Cr, Ni, Mo, and Cu. This can effectively improve the hardenability of steel and obtain fine dispersed precipitates, so that medium-carbon steel can obtain good strength, plasticity, toughness, and fatigue resistance.

[0019] In the medium carbon steel with good strength and toughness described in the present invention, the design principles of each chemical element are as follows:

[0020] C: In the medium carbon steel with good strength and toughness described in the present invention, the C element can improve the hardenability of the steel, which can enable the steel to form a phase transformation structure with higher hardness during the quenching and cooling process. When the C content in the steel is too low, it will cause the phase transformation structure of the steel, such as the bainite content, to be too low, and the steel will not be able to obtain sufficient tensile strength. At the same time, the C content in the steel should not be too high. When the C content in the steel increases, the proportion of hard phases will increase, the hardness of the steel will increase, and the toughness of the steel will also decrease. Therefore, considering the influence of the C content on the performance of the steel, in the medium carbon steel with good strength and toughness described in the present invention, the mass percentage of the C element is controlled between 0.30 and 0.39%.

[0021] Si: In the medium-carbon steel with excellent strength and toughness described herein, Si contributes to its strength, and adding an appropriate amount can prevent the formation of coarse carbides. However, it's important to note that the Si content in the steel should not be too high, as excessive Si content can reduce the steel's impact toughness. Therefore, in the medium-carbon steel with excellent strength and toughness described herein, the mass percentage of Si is controlled between 0.05% and 0.40%.

[0022] Mn: In the medium-carbon steel with excellent strength and toughness described in the present invention, Mn exists primarily in the form of a solid solution. This can effectively improve the hardenability of the steel and form a high-strength, low-temperature phase transformation structure during quenching, allowing the steel to obtain excellent strength and toughness. However, it should be noted that the Mn content in the steel should not be too high. When the Mn content in the steel is too high, a large amount of retained austenite will be formed, reducing the yield strength of the steel and easily leading to center segregation. Therefore, in the medium-carbon steel with excellent strength and toughness described in the present invention, the mass percentage of the Mn element is controlled between 0.50 and 0.90%.

[0023] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Mn element may be further preferably controlled to be between 0.60 and 0.90%.

[0024] Al: In the medium-carbon steel with excellent strength and toughness described herein, Al is a key deoxidizer element. It forms fine precipitates within the steel, pinning grain boundaries and inhibiting austenite grain growth. However, it's important to note that the Al content in the steel should not be too high. Excessive Al content can lead to the formation of large oxides, and coarse hard inclusions can reduce the steel's impact toughness and fatigue properties. Therefore, in the medium-carbon steel with excellent strength and toughness described herein, the Al content is controlled between 0.01% and 0.05% by weight.

[0025] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Al element may be further preferably controlled to be between 0.02 and 0.045%.

[0026] N: In the medium-carbon steel with excellent strength and toughness described in the present invention, N is an interstitial atom, which can form nitrides or carbonitrides, i.e., MX-type precipitates, in the steel, and play a role in precipitation strengthening and refinement strengthening. However, it should be noted that the N content in the steel should not be too high. When the N content in the steel is too high, coarse particles will be formed, which will not play a role in grain refinement. This is because N, as an interstitial atom, will be enriched at grain boundaries and defects, and will lead to a decrease in the impact toughness of the steel. Therefore, in order to avoid the enrichment of N in the steel, the mass percentage of N in the medium-carbon steel with excellent strength and toughness described in the present invention is controlled between 0.002-0.015%.

[0027] V: In the medium-carbon steel with good strength and toughness described in the present invention, V is an important alloying element for strengthening the medium-carbon steel. The V element can form precipitates with the C element or the N element in the steel, thereby producing precipitation strengthening, and can pin the grain boundaries, refine the grains, and improve the strength of the steel. Accordingly, the V content in the steel should not be too high. If the V content in the steel is too high, coarse VC particles will be formed, reducing the impact toughness of the steel. Therefore, in the medium-carbon steel with good strength and toughness described in the present invention, it can be selected whether to add the V element, and the mass percentage of the V element can be controlled to V≤0.02%.

[0028] Cr: In the medium-carbon steel with excellent toughness described herein, the Cr element significantly improves the steel's hardenability and strength. Accordingly, the Cr content in the steel should not be too high. Excessive Cr content in the steel can form coarse carbides, reducing the steel's impact resistance. Therefore, considering the impact of Cr content on steel properties, the addition of Cr to the medium-carbon steel with excellent toughness described herein can be optional, with the Cr content controlled to ≤ 0.40% by weight.

[0029] Ni: In the medium-carbon steel with good strength and toughness described in the present invention, the Ni element exists in the steel in the form of solid solution. Adding an appropriate amount of Ni element to the steel can effectively improve the low-temperature impact performance of the material. However, the Ni content in the steel should not be too high. Excessive Ni content will not only increase the cost, but also lead to an excessively high content of retained austenite in the steel, thereby reducing the strength of the steel. Therefore, considering the economy of the steel, in the medium-carbon steel with good strength and toughness described in the present invention, it can be selected whether the Ni element needs to be added, and the mass percentage of the Ni element is controlled to Ni≤0.40%.

[0030] Mo: In the medium-carbon steel with excellent strength and toughness described in the present invention, the Mo element can be solid-solved in the steel and can improve the hardenability of the steel and increase the strength of the steel. At the same time, under higher temperature tempering, the Mo element will cooperate to form fine carbides to further improve the strength of the steel. However, considering that the Mo element is a precious metal element, in order to effectively control the cost of the alloy, the Mo content in the steel should not be too high. Therefore, in the medium-carbon steel with excellent strength and toughness described in the present invention, it can be selected whether the Mo element needs to be added, and the mass percentage of the Mo element is controlled to Mo≤0.10%.

[0031] Cu: Adding an appropriate amount of Cu to the medium-carbon steel with excellent strength and toughness described in the present invention not only improves the strength of the steel but also helps improve its corrosion resistance. However, it should be noted that the Cu content in the steel should not be too high. If the Cu content in the steel is too high, it will accumulate at the grain boundaries during heating, weakening the grain boundaries and causing cracking. Therefore, in the medium-carbon steel with excellent strength and toughness described in the present invention, the addition of Cu can be selected, and the mass percentage of Cu is controlled to Cu ≤ 0.30%.

[0032] Nb: In the medium carbon steel with good strength and toughness described in the present invention, the Nb element is added to the steel and can form a fine precipitate phase with nitrogen or carbon elements, which can inhibit the recrystallization of the steel and effectively refine the grains. Grain refinement plays an important role in improving the mechanical properties of steel, especially strength and toughness. At the same time, grain refinement also helps to reduce the hydrogen embrittlement sensitivity of steel. However, the Nb element content in the steel should not be too high. When the Nb element content in the steel is too high, coarse NbC particles will be formed during the smelting process, which will in turn reduce the impact toughness of the steel. Therefore, in order to give full play to the beneficial effects of the Nb element, in the medium carbon steel with good strength and toughness described in the present invention, it can be selected whether the Nb element needs to be added, and the mass percentage of the Nb element is controlled to Nb≤0.09%.

[0033] Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Nb element may be further preferably controlled to be between 0.003% and 0.030%.

[0034] It should be noted that in the present invention, while controlling the mass percentage of a single chemical element, the inventors also designed and further controlled the ratio of the content of microalloying elements Al, Nb, and V to the N content, which requires that the atomic ratio of the total amount of microalloying elements to nitrogen exceeds 1, and defines the microalloying element coefficient r M / N :1.1~4.9,r M / N Controlling between 1.1-4.9 can achieve the effect of inhibiting austenite grain growth and precipitation strengthening. M / N =([Al] / 2+[Nb] / 7+[V] / 4) / [N], and each chemical element in the formula is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0035] In addition, in order to ensure the strength of the medium carbon steel designed by the invention, a small amount of one or more alloying elements such as Cr, Ni, and Mo may be added. However, in order to take into account cost economy and avoid excessive hardness of the steel, which affects machinability, cold deformation and plasticity, the present invention not only controls the mass percentage of a single chemical element, but also further controls the total range of alloying elements such as Cr, Ni, and Mo to: 0.02-0.65%, that is: 0.02% ≤ Cr + Ni + Mo ≤ 0.65%.

[0036] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, among the inevitable impurities, P≤0.020%, S≤0.040%, O≤0.002%, and Ca≤0.004%.

[0037] In the above technical solution, P, S, O and Ca are all impurity elements in steel. If technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the material should be reduced as much as possible.

[0038] P: In the present invention, P tends to segregate at grain boundaries in steel, reducing grain boundary bonding energy and worsening the steel's impact toughness. Therefore, in the medium-carbon steel with excellent strength and toughness described in the present invention, the mass percentage of P is controlled to: P ≤ 0.020%.

[0039] S: In the present invention, S can form sulfide inclusions with Mn in the steel, improving the steel's machinability. However, excessive S content hinders hot working and reduces the steel's impact resistance. Therefore, in the medium-carbon steel with excellent strength and toughness described herein, the S content is controlled to ≤ 0.040% by weight. Of course, in some preferred embodiments, the S content can be further controlled to ≤ 0.035% by weight.

[0040] O: In the present invention, the O element can form oxides and composite oxides with the Al element in the steel. In order to ensure the uniformity of the steel's structure, low-temperature impact energy and fatigue performance, the mass percentage of the O element in the medium-carbon steel with good strength and toughness described in the present invention is controlled to: O≤0.002%.

[0041] Ca: In the present invention, Ca can improve the size and morphology of sulfide inclusions in steel. However, Ca tends to form coarse inclusions, which can affect the fatigue performance of the final product. Therefore, in the medium-carbon steel with excellent strength and toughness described in the present invention, the mass percentage of Ca is controlled to ≤ 0.004%.

[0042] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, the mass percentage of each chemical element satisfies at least one of the following:

[0043] Mn: 0.60~0.90%;

[0044] Al: 0.02~0.045%;

[0045] Nb: 0.003~0.030%;

[0046] S≤0.035%.

[0047] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, its critical ideal diameter Di for hardenability is 25 to 50 mm; wherein:

[0048] Di=13.72[C]×(3.33[Mn]+1)×(0.70[Si]+1)×(0.36[Ni]+1) ×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.36[Cu]+1)×(1.73[V]+1)

[0049] In the formula, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0050] In the above technical solution, the inventors further optimized the ideal critical diameter Di for hardenability and controlled it between 25 and 50 mm. Controlling the ideal critical diameter Di for hardenability between 25 and 50 mm ensures sufficient hardenability of the steel at a lower alloy cost.

[0051] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, its microstructure is ferrite+pearlite.

[0052] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, the ferrite grain size is ≥8.5

[0053] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, during the process of preparing the medium carbon steel, when the medium carbon steel is austenitized, the austenite grain size is ≥ grade 6.

[0054] Furthermore, in the medium carbon steel with good strength and toughness described in the present invention, its yield strength Rp0.2 after quenching and tempering heat treatment is ≥400MPa, tensile strength Rm=600~800MPa, elongation A≥20%, cross-sectional shrinkage Z≥45%, and Charpy impact energy Akv≥30J.

[0055] In addition, another object of the present invention is to provide a round steel having good strength, plasticity, toughness, hardenability and fatigue resistance, and is easy to cut and process. It can be used in automobiles, especially new energy vehicle shaft parts and other occasions where high-strength and tough round steel is required.

[0056] In order to achieve the above-mentioned purpose, the present invention provides a round steel, which is made of the above-mentioned medium carbon steel with good strength and toughness.

[0057] In the present invention, the medium carbon steel with good strength and toughness described in the present invention can be specifically prepared into round steel, which can be used in the automotive field, especially in occasions such as new energy vehicle shaft components that require high-strength and toughness round steel, and the size specification diameter range of the round steel can be specifically controlled between Φ16 and 100 mm.

[0058] Furthermore, in the round steel described in the present invention, its diameter is Φ16-100 mm.

[0059] Correspondingly, another object of the present invention is to provide a method for manufacturing the above-mentioned round steel, which has simple production and reasonable process design. The obtained round steel not only has good strength, toughness and plasticity, but also has good fatigue resistance, excellent hardenability, and is easy to cut and process, and has good application prospects.

[0060] In order to achieve the above object, the present invention provides a method for manufacturing the above-mentioned round steel, which comprises the steps of:

[0061] (1) smelting;

[0062] (2) Casting;

[0063] (3) Heating: Control the heating temperature to 1050-1250°C and the holding time to 3-24h;

[0064] (4) Forging or rolling: control the final rolling temperature or final forging temperature ≥ 800℃, and cool after rolling or forging;

[0065] (5) Finishing.

[0066] In the above technical solution of the present invention, in the smelting process of step (1), the smelting can be carried out by electric furnace smelting or converter smelting, and then refined and vacuum treated. Of course, in some other embodiments, the operator can also use a vacuum induction furnace for smelting.

[0067] Accordingly, in the present invention, casting is required after smelting is completed. In the casting process of the above step (2), the casting process can specifically be die casting or continuous casting. The ingot obtained by continuous casting can be input into a heating furnace for heating, and the ingot heating temperature is controlled to be 1050-1250°C and the holding time is 3-24 hours to ensure that the medium carbon steel of the present invention is completely austenitized during the heating process.

[0068] It should be noted that during the heating process of step (3) of the present invention, the medium carbon steel of the present invention will be heated to 1050°C to 1250°C to be completely austenitized. At the same time, the carbides and nitrides and carbonitrides of Al, Nb, and V, and the carbides of Cr and Mo can be partially or completely dissolved in the austenite. During the subsequent rolling or forging and subsequent cooling process, Al, Nb, and V can form fine precipitates. Mn, Cr, Ni, Mo, and Cu dissolved in the austenite can improve the hardenability of the steel and increase the hardness and strength. Under the condition that the final rolling or final forging temperature of the subsequent step (4) is ≥800°C, a refined multiphase matrix structure with fine dispersed precipitates can be formed.

[0069] In addition, it should be noted that in step (3) described in the present invention, during actual operation, the temperature can be directly raised to the heating temperature or can be raised to the heating temperature in a stepwise manner.

[0070] Furthermore, in the manufacturing method of the present invention, in step (4), the final rolling temperature or the final forging temperature is controlled to be 800-950°C.

[0071] In the above technical solution of the present invention, in the forging or rolling process of step (4), when the forging process is adopted, the steel billet can be directly forged to the final finished product size; and when the rolling process is adopted, the steel billet can be directly rolled to the final finished product size, or the steel billet can be first rolled to a specified intermediate billet size, and then intermediately heated and rolled to the final finished product size.

[0072] Furthermore, in the manufacturing method of the present invention, in step (4), the steel is directly rolled or forged to the finished product size.

[0073] Furthermore, in the manufacturing method described in the present invention, in step (4), the steel is first rolled to the intermediate billet size, then intermediately heated, and then rolled to the final finished product size; wherein the intermediate heating temperature is 1050-1250°C, and the holding time is 3-24h.

[0074] In the above technical solution of the present invention, the rolling process can be a staged rolling process, first rolling to the intermediate billet size, then performing intermediate heating, and then rolling to the final product size. The intermediate heating temperature of the intermediate billet can be controlled between 1050 and 1250°C, and more preferably between 1100 and 1200°C.

[0075] Furthermore, in the manufacturing method of the present invention, in step (4), the cooling rate after rolling or forging is ≥5°C / s.

[0076] It should be noted that during the rolling process of step (4), the billet is removed from the heating furnace and descaled by high-pressure water before rolling or forging. After rolling or forging, the billet is cooled. The rolling speed and cooling method are controlled according to the specific application and performance requirements. By controlling the phase transformation process, the ideal microstructure is obtained. The cooling method can be air cooling, wind cooling or slow cooling, and the cooling rate after rolling or forging can be specifically controlled to be ≥5°C / s.

[0077] Furthermore, in the manufacturing method described in the present invention, in step (5), finishing includes at least one of surface treatment, non-destructive testing and heat treatment, wherein the heat treatment includes normalizing or annealing heat treatment, wherein the normalizing temperature is 860-910°C and the annealing temperature is 630-720°C.

[0078] In step (5) described in the present invention, the prepared plate is further required to undergo necessary finishing steps, such as heat treatment processes such as normalizing and annealing. During the finishing process, when normalizing or annealing heat treatment is adopted, the normalizing temperature range can be specifically controlled to be 860-910°C, and the annealing temperature can be controlled to be 630-720°C, so as to control the microstructure and hardness of the material and improve the uniformity of the steel.

[0079] Accordingly, in the finishing process, in addition to the above-mentioned normalizing or annealing heat treatment process, surface treatment and non-destructive testing processes may also be included.

[0080] Furthermore, in the manufacturing method of the present invention, step (5) may be followed by step (6) of quenching and tempering, wherein the quenching temperature ranges from 840 to 910°C, and the tempering temperature ranges from 550 to 660°C.

[0081] Compared with the prior art, the medium carbon steel and round steel with good strength and toughness and the manufacturing method thereof described in the present invention have the following advantages and beneficial effects:

[0082] The present invention develops a medium-carbon steel with strong toughness and excellent cutting performance by rationally designing the chemical composition and combining it with optimized process. The medium-carbon steel can be eventually rolled or forged into bars and form a ferrite + pearlite microstructure with fine grains and fine dispersed precipitates. This makes the medium-carbon steel easy to cut and cold-deform while achieving good plasticity.

[0083] The medium carbon steel described in the present invention has fine grains, high strength, good elongation and cross-sectional reduction rate, excellent impact toughness and high hardenability. It can be effectively used in occasions such as new energy vehicles and engineering machinery that require high fatigue performance steel.

[0084] During actual preparation, the present invention also optimizes the design of the manufacturing process. The manufacturing process is reasonably designed and has a wide process window, so that batch commercial production can be achieved on the bar production line.

[0085] The yield strength R of the medium carbon steel designed by the present invention after quenching and tempering heat treatment is p0.2 ≥400MPa, tensile strength R m =600~800MPa, elongation A≥20%, section shrinkage Z≥45%, Charpy impact energy A kv ≥30J, its hardenability is high and the bandwidth is narrow, which can meet the needs of automobiles and engineering machinery, especially in situations where surface hardening treatment is required, and has very good promotion and application prospects. DETAILED DESCRIPTION

[0086] The medium carbon steel, round steel and manufacturing method thereof with good strength and toughness 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.

[0087] Examples 1-8 and Comparative Examples 1-4

[0088] The round steels of Examples 1-8 were all prepared by the following steps:

[0089] (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.

[0090] (2) Casting: Casting is performed by die casting or continuous casting to obtain ingots.

[0091] (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.

[0092] (4) Forging or rolling: Control the final rolling temperature or final forging temperature to be ≥800°C, preferably between 800 and 950°C, and cool after rolling or forging. The cooling method can be air cooling, wind cooling or slow cooling, and control the cooling rate after rolling or forging to be ≥5°C / s;

[0093] 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, more preferably between 1100 and 1200°C, and the holding time can be controlled between 3 and 24 hours.

[0094] (5) Finishing: Finishing includes surface treatment, non-destructive testing and heat treatment, wherein the heat treatment specifically includes normalizing or annealing heat treatment, wherein the normalizing temperature is controlled at 860-910°C, and the annealing temperature is controlled at 630-720°C.

[0095] It should be noted that, in the present invention, after the manufacturing process described in steps (1) to (5) above, the medium carbon steel corresponding to each embodiment and comparative example can be effectively prepared. Usually, we test the performance of the medium carbon steel after quenching and tempering heat treatment. Therefore, after obtaining the medium carbon steel of the above embodiments and comparative examples, step (6) can be further performed, i.e., quenching and tempering heat treatment process, and the quenching temperature is controlled between 840 and 910°C, and the tempering temperature is controlled between 550 and 660°C.

[0096] In the present invention, the round steels prepared in Examples 1-8 are actually prepared from their respective corresponding medium carbon steels. The chemical composition design and related processes of the medium carbon steels with good strength and toughness in Examples 1-8 meet the design specification requirements of the present invention. However, the chemical composition design and related processes of Comparative Examples 1-4 all have parameters that do not meet the design specification requirements of the present invention.

[0097] Table 1-1 lists the mass percentages of the chemical elements in the medium carbon steels with good strength and toughness of Examples 1-8 and the comparative steels of Comparative Examples 1-4.

[0098] Table 1-1. (wt.%, the balance is Fe and other inevitable impurities except P, S, O and Ca)

[0099]

[0100] Table 1-2 lists the typical alloying elements Cr+Ni+Mo, the critical ideal diameter Di value of hardenability, the microalloying element coefficient r calculated from the mass percentage of each chemical element of the medium carbon steel with good strength and toughness of Examples 1-8 and the comparative steel of Comparative Examples 1-4. M / N .

[0101] Table 1-2.

[0102]

[0103]

[0104] Note: In the above table,

[0105] r M / N =([Al] / 2+[Nb] / 7+[V] / 4) / [N];

[0106] In the formulas of “Cr+Ni+Mo”, “Di” and “rM / N”, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0107] In the present invention, the specific production process operations of the round steels prepared from the medium carbon steels with good strength and toughness of Examples 1-8 and the round steels prepared from the comparative steels of Comparative Examples 1-4 are as follows:

[0108] Example 1

[0109] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 50kg vacuum induction furnace. The resulting molten steel was cast into ingots, heated, and then forged. Heating was controlled at 1050°C, held for 3 hours, and then forged. The final forging temperature was controlled at 910°C, resulting in bars with a diameter of Φ55mm. After forging, the bars were air-cooled at a controlled cooling rate of 6°C / s. Finishing was required, including normalizing at 880°C.

[0110] Example 2

[0111] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 150kg vacuum induction furnace. The resulting molten steel was cast into ingots, heated, and then forged. The heating temperature was controlled at 1100°C, held for 4 hours, and then forged. The final forging temperature was controlled at 1000°C, resulting in Φ80mm bars. The bars were then air-cooled at a controlled cooling rate of 8°C / s. Finishing was required after forging, including normalizing at 900°C.

[0112] Example 3

[0113] 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 continuous casting into 320mm×425mm continuous ingots. The ingots were slowly heated to 1220°C and held at that temperature for 4 hours before rolling. After exiting the heating furnace, the ingots were descaled with high-pressure water and then rolled, with the final rolling temperature controlled at 1000°C. The bars were finally rolled into 100mm Φ bars. After rolling, they were air-cooled at a controlled cooling rate of 5°C / s. Finishing was required after forging, including non-destructive testing (NDT), specifically ultrasonic and magnetic particle testing.

[0114] Example 4

[0115] Electric furnace smelting, refining, and vacuum treatment were performed according to the chemical compositions shown in Tables 1-1 and 1-2, followed by continuous casting into 280 mm × 280 mm continuous ingots. The ingots were slowly heated to 1180°C, held at this temperature for 6 hours, and then rolled. After exiting the heating furnace, the ingots were descaled with high-pressure water and then rolled into intermediate billets. The final rolling temperature was controlled at 1000°C, and the intermediate billets had a size of 140 mm × 140 mm. The intermediate billets were then preheated and heated to 1220°C. After holding for 5 hours, they were removed from the furnace, descaled with high-pressure water, and then rolled. The final rolling temperature was controlled at 820°C, and the final billet was rolled into Φ20 mm bars. After rolling, they were air-cooled at a cooling rate of 6°C / s. Then further finishing is carried out, which specifically includes heat treatment, surface treatment and non-destructive testing. After normalizing at 880℃ (heat treatment), it is turned and peeled (surface treatment), and inspected by ultrasonic testing and magnetic particle testing (non-destructive testing).

[0116] Example 5

[0117] 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 cast into 320mm×425mm continuous casting billets. The continuous casting billets were heated in a controlled step-wise manner to 1230°C, held at this temperature for 3 hours, and then rolled. After exiting the heating furnace, the billets were descaled with high-pressure water and then rolled into intermediate billets. The final rolling temperature was controlled at 1050°C, resulting in intermediate billets measuring 220mm×220mm. After rolling, they were air-cooled. The intermediate billets were then preheated and heated in a controlled step-wise manner to 1200°C. After holding this temperature for 6 hours, they were removed from the furnace, descaled with high-pressure water, and then rolled. The final rolling temperature was controlled at 950°C, resulting in finished bars measuring Φ55mm. After rolling, they were air-cooled at a controlled cooling rate of 5°C / s. Finishing was performed after rolling, including non-destructive testing (NDT), specifically ultrasonic and magnetic particle testing.

[0118] Example 6

[0119] The steel is 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. The steel is then continuously cast into 280mm×280mm continuous ingots. The ingots are slowly heated to 1150°C, held at that temperature for 6 hours, and then rolled. After exiting the heating furnace, the ingots undergo high-pressure water descaling and then begin rolling. The final rolling temperature is controlled at 970°C, and the finished bar size is Φ80mm. After rolling, they are air-cooled at a controlled cooling rate of 6°C / s. Further finishing is then performed, which includes heat treatment, surface treatment, and non-destructive testing. Following 680°C annealing (heat treatment), grinding wheel peeling (surface treatment) is performed, and ultrasonic and magnetic particle testing (non-destructive testing) inspections are performed.

[0120] Example 7

[0121] The chemical compositions shown in Tables 1-1 and 1-2 are smelted in a converter, refined, and vacuum-treated. The resulting ingots are then cast. After controlled preheating, they are heated in a stepwise manner to 1200°C and held at that temperature for 9 hours before rolling. 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, ultimately resulting in Φ90mm bars. After rolling, they are air-cooled at a controlled cooling rate of 5°C / s. Further finishing is then performed, which requires surface treatment and non-destructive testing (NDT). This involves grinding wheel peeling, followed by non-destructive testing using ultrasonic and magnetic particle testing.

[0122] Example 8

[0123] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 500kg vacuum induction furnace, then cast to obtain steel ingots. These ingots were heated and forged to form blanks. The heating temperature was controlled at 1150°C, and after holding for 21 hours, they were subsequently forged at a final forging temperature of 920°C, ultimately resulting in Φ80mm bars. After forging, the bars were piled and slowly cooled at a controlled cooling rate of 5°C / s. Furthermore, finishing was required after forging, including normalizing at 900°C and turning and peeling.

[0124] Comparative Example 1

[0125] The implementation method was the same as that of Example 1. In Comparative Example 1, smelting was performed in a 50kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was mold-cast into ingots, heated, and forged to form blanks. The heating temperature was 1050°C, and the ingots were held at that temperature for 3 hours before forging. The final forging temperature was controlled at 910°C, resulting in bars with a diameter of Φ55mm. After forging, the bars were air-cooled at a controlled cooling rate of 6°C / s. Furthermore, finishing was performed after forging, including normalizing at 880°C.

[0126] Comparative Example 2

[0127] The implementation method was the same as in Example 2. Smelting was carried out in a 150kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was continuously cast into ingots, heated, and forged to form blanks. The ingots were heated to 1100°C, held at that temperature for 4 hours, and then forged. The final forging temperature was controlled at 1000°C, resulting in Φ80mm bars. The bars were then air-cooled at a controlled cooling rate of 8°C / s. Furthermore, finishing was performed after forging, including normalizing at 900°C.

[0128] Comparative Example 3: Commercially available round steel was selected, and its preparation process is not described here in detail. When it is necessary to test the performance of the commercially available round steel after normalizing and tempering heat treatment, it can be specifically quenched at 860°C and tempered at 610°C before its mechanical properties can be tested.

[0129] Comparative Example 4

[0130] The same implementation method as in Example 8 was used. Smelting was carried out in a 500kg vacuum induction furnace according to the chemical compositions 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 1100°C, held at that temperature for 4 hours, and then forged. The final forging temperature was controlled at 1000°C, resulting in Φ80mm bars. The bars were then slowly cooled at a controlled cooling rate of 5°C / s. Furthermore, the bars were subjected to finishing, normalizing at 900°C, and further turning and peeling.

[0131] It should be noted that the round steels of the finished products of Examples 1-8 and the comparative round steels of Comparative Examples 1-4 can be sampled respectively, and metallographic specimens can be prepared according to GB / T 13298. The microstructure can be analyzed using a metallographic microscope with reference to GB / T 13299, and the ferrite grain size can be tested using the intercept method and converted to the grain size with reference to ASTM E112.

[0132] When analyzing the austenite grain size, the steel samples of each embodiment and comparative example were completely austenitized and then water quenched to prepare metallographic specimens. The austenite grain boundaries were revealed by etching with a picric acid alcohol solution, and the austenite grain size was analyzed using a metallographic microscope according to ASTM E112.

[0133] Table 2 lists the metallographic analysis results of the round steels of Examples 1-8 and Comparative Examples 1-4.

[0134] Table 2.

[0135]

[0136] As can be seen from Table 2, the microstructures of the round steels of Examples 1-8 of the present invention all comprise ferrite + pearlite, and the medium carbon steels have fine grains, with the ferrite grain size ranging from 8.5 to 10. Furthermore, when the medium carbon steels of Examples 1-8 are austenitized, the austenite grain size ranges from 6.5 to 8.

[0137] Accordingly, after completing the above observation and analysis of the metallographic structure, in order to further illustrate that the round steel prepared by the present invention still has very excellent mechanical properties after the quenching + tempering heat treatment process, the inventors sampled the finished round steels of Examples 1-8 and Comparative Examples 1-4, respectively, and carried out the quenching + tempering heat treatment process of step (6) accordingly. After completing the quenching + tempering heat treatment of step (6), the mechanical properties, impact properties and hardenability of the steel were further tested, and the test results are listed in Table 3 below.

[0138] In the present invention, the process parameters of the quenching + tempering heat treatment in each embodiment are as follows:

[0139] Example 1: The mechanical properties of round steel were tested after quenching at 860°C and tempering at 610°C.

[0140] Example 2: The mechanical properties of round steel were tested after quenching at 840°C and tempering at 580°C.

[0141] Example 3: The mechanical properties of round steel were tested after quenching at 860°C and tempering at 640°C.

[0142] Example 4: Mechanical properties of round steel were tested after quenching at 900°C and tempering at 560°C.

[0143] Example 5: Mechanical properties of round steel were tested after quenching at 850°C and tempering at 600°C.

[0144] Example 6: Mechanical properties of round steel were tested after quenching at 890°C and tempering at 660°C.

[0145] Example 7: Mechanical properties of round steel were tested after quenching at 880°C and tempering at 650°C.

[0146] Example 8: Mechanical properties of round steel were tested after quenching at 850°C and tempering at 620°C.

[0147] Comparative Example 1: The mechanical properties of round steel were tested after quenching at 860°C and tempering at 610°C.

[0148] Comparative Example 2: The mechanical properties of round steel were tested after quenching at 840°C and tempering at 580°C.

[0149] Comparative Example 3: The mechanical properties of round steel were tested after quenching at 860°C and tempering at 610°C.

[0150] Comparative Example 4: The mechanical properties of round steel were tested after quenching at 850°C and tempering at 620°C.

[0151] In the present invention, to illustrate the performance of the round steel of each embodiment and comparative example prepared according to the present invention after quenching and tempering heat treatment, the inventors sampled the finished round steel of Examples 1-8 and the comparative round steel of Comparative Examples 1-4 after quenching and tempering heat treatment, prepared specimens with reference to GB / T 2975, and conducted tensile tests and impact tests in accordance with GB / T 228.1 and GB / T 229, respectively, to obtain the mechanical properties of the steel of each embodiment and comparative example after quenching and tempering heat treatment. The relevant mechanical property test results are listed in Table 3 below.

[0152] In order to test the hardenability of the finished round steel of each embodiment and comparative example, the inventors sampled and prepared samples from the hot-rolled round steel of each embodiment and comparative example after quenching and tempering heat treatment in accordance with the national standard GB / T 225, and conducted an end hardenability test (Jominy test) with reference to GB / T 5216. The normalizing temperature was controlled at 880±20°C and the quenching temperature was controlled at 870±5°C. The Rockwell hardness test was performed according to GB / T230.2 to obtain the hardness value (HRC) at a specific position, such as the hardness at a distance of 3 mm and 9 mm from the quenched end, i.e., J3mm and J9mm.

[0153] Table 3 lists the test results of mechanical properties, fatigue properties and cutting properties of the round steels of Examples 1-8 and the comparative round steels of Comparative Examples 1-4 obtained after quenching and tempering heat treatment.

[0154] Table 3.

[0155]

[0156]

[0157] Note: In Table 3 above, two or three groups of data in a single column represent two or three test results.

[0158] It can be seen from Table 3 above that after the quenching and tempering heat treatment, the comprehensive performance of the round steels prepared from the medium carbon steels with good strength and toughness of Examples 1-8 of the present invention is still significantly better than the comparative round steels of Comparative Examples 1-4.

[0159] In the present invention, after the quenching and tempering heat treatment, the yield strength R of the round steel of Examples 1-8 is p0.2 Between 404-630MPa, the tensile strength R m The pressure is between 605-798 MPa, the elongation A is between 20-26%, the cross-sectional shrinkage Z is between 49-63%, and the Charpy impact energy A is between 100-200 MPa and 100-200 MPa.kv Between 32-93J, it not only has good strength, impact toughness and plasticity, but also has good fatigue resistance and excellent cutting performance.

[0160] In addition, after completing the quenching + tempering heat treatment, the round steel prepared in Example 1-8 also has very excellent hardenability, and its critical ideal diameter Di value of hardenability is between 25 and 50 mm, and the measured hardness at a distance J3 mm from the quenching end is between 45 and 53 HRC, and the hardness at a distance J9 mm from the quenching end is between 19 and 28 HRC.

[0161] Continuing to refer to Table 1-1, Table 1-2, Table 2 and Table 3, it can be seen that in Comparative Examples 1-4, the parameters in the design process of chemical element composition of these four comparative examples do not meet the design specification requirements of the present invention. In the present invention, the microalloying coefficient r of Comparative Example 1 is M / N The design requirements are not met, the round steel grains are relatively coarse, and although the steel can obtain higher strength after quenching + tempering heat treatment, the impact toughness is poor; the "Cr+Ni+Mo" content design of comparative example 2 does not meet the requirements, the hardenability of the steel obtained is low, and the strength after quenching + tempering heat treatment is low, the hardness of the surface hardening of the prepared parts is insufficient, and the rotational bending fatigue performance cannot meet the use requirements; the Mn element content of comparative example 3 does not meet the design requirements, and the steel after the final quenching + tempering heat treatment has higher strength, but poor plasticity, and cannot meet the lightweight requirements of parts; the C element content of comparative example 4 does not meet the design requirements, and the Charpy impact energy of the steel obtained after quenching + tempering heat treatment is low, and the toughness cannot meet the use requirements.

[0162] 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.

[0163] 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 medium carbon steel with good strength and toughness, characterized in that: The mass percentage of each chemical element is: C: 0.30-0.39%, Si: 0.05-0.40%, Mn: 0.50-0.90%, Al: 0.01-0.05%, N: 0.002-0.015%, at least one of Cr≤0.40%, Ni≤0.40%, Mo≤0.10%, V≤0.02%, Cu≤0.30%, Nb≤0.09%; the balance is Fe and other unavoidable impurities; Each element also satisfies the following conditions: 1.1≤(Al / 2+Nb / 7+V / 4) / N≤4.9, 0.02%≤Cr+Ni+Mo≤0.65%, where each chemical element is substituted into the value preceding the percentage sign of the mass percentage of the chemical element; The ideal critical diameter Di value of its hardenability is 25 to 50 mm; among which: Di=13.72[C]×(3.33[Mn]+1)×(0.70[Si]+1)×(0.36[Ni]+1) ×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.36[Cu]+1)×(1.73[V]+1) In the formula, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element; Its yield strength R after quenching + tempering heat treatment p0.2 ≥400MPa, tensile strength R m =600~800MPa, elongation A≥20%, section shrinkage Z≥45%, Charpy impact energy A kv ≥30J。 2. The medium carbon steel with good strength and toughness as claimed in claim 1, characterized in that: Among the inevitable impurities, P≤0.020%, S≤0.040%, O≤0.002%, and Ca≤0.004%.

3. The medium carbon steel with good strength and toughness as claimed in claim 2, characterized in that: The mass percentage of each chemical element satisfies at least one of the following: Mn: 0.60~0.90%; Al：0.02～0.045％； Nb: 0.003~0.030%; S≤0.035％。 4. The medium carbon steel with good strength and toughness as claimed in claim 1, characterized in that: Its microstructure is ferrite + pearlite.

5. The medium carbon steel with good strength and toughness as claimed in claim 4, characterized in that: The ferrite grain size is ≥8.

5.

6. The medium carbon steel with good strength and toughness as claimed in claim 1, characterized in that: In the process of preparing the medium carbon steel, when the medium carbon steel is austenitized, the austenite grain size is ≥ grade 6.

7. A round steel, characterized in that: The steel pipe is made of medium carbon steel with good strength and toughness as claimed in any one of claims 1 to 6.

8. The round steel according to claim 7, characterized in that: Its diameter is Φ16~100mm.

9. The method for manufacturing round steel according to claim 7 or 8, wherein: It includes the steps of: (1) smelting; (2) Casting; (3) Heating: Control the heating temperature to 1050-1250°C and the holding time to 3-24h; (4) Forging or rolling: control the final rolling temperature or final forging temperature ≥ 800℃, and cool after rolling or forging; (5) Finishing.

10. The manufacturing method according to claim 9, wherein: In step (4), the final rolling temperature or the final forging temperature is controlled to be 800-950°C.

11. The manufacturing method according to claim 9, wherein: In step (4), it is directly rolled or forged to the finished size.

12. The manufacturing method according to claim 9, wherein: In step (4), the steel is first rolled to the intermediate billet size, then intermediately heated, and then rolled to the final finished product size; wherein the intermediate heating temperature is 1050-1250° C., and the holding time is 3-24 hours.

13. The manufacturing method according to claim 9, wherein: In step (4), the cooling rate after rolling or forging is ≥5°C / s.

14. The manufacturing method according to claim 9, wherein: In step (5), finishing includes at least one of surface treatment, non-destructive testing and heat treatment, wherein the heat treatment includes normalizing or annealing heat treatment.

15. The manufacturing method according to claim 9, wherein: After step (5), there is a step (6) of quenching and tempering, wherein the quenching temperature ranges from 840 to 910° C., and the tempering temperature ranges from 550 to 660° C.

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