Medium carbon steel and manufacturing method thereof, round steel and manufacturing method thereof

Abstract

The invention discloses medium carbon steel which comprises the following chemical elements in percentage by mass: 0.35-0.41% of C, 0.10-0.40% of Si, 0.60-0.99% of Mn, less than or equal to 0.015% of P, 0.010-0.040% of S, 0.015-0.045% of Al, 0.004-0.010% of N, 0.90-1.20% of Cr, 0.01-0.20% of Ni, 0.002-0.030% of Nb, less than or equal to 0.10% of Mo, less than or equal to 0.05% of V, less than or equal to 0.20% of Cu, less than or equal to 0.01% of Ti, less than or equal to 0.0020% of O, less than or equal to 0.0002% of H and the balance of Fe and inevitable impurities, and all the elements also meet the requirements that the manganese equivalent MMn < The medium carbon steel is excellent in strength and toughness, not only has good strength, impact toughness and plasticity, but also has good anti-fatigue performance and precisely-controlled end hardenability. The medium carbon steel is also easy to cut and process, can meet the requirements of application scenes such as automobiles and engineering machinery on steel performance, especially in the field of new energy automobiles, and has very good popularization and application prospects. The invention further discloses a preparation method of the medium carbon steel, round steel and a preparation method.

Description

Technical Field

[0001] This invention relates to the field of steel, and particularly to a medium carbon steel and its manufacturing method, and a round steel and its manufacturing method. Background Technology

[0002] As is well known, high-strength steel is widely used in industrial production, typically in high-safety mechanical and structural components, such as automotive parts or critical load-bearing components of engineering machinery. Therefore, the preparation of this high-strength steel requires not only high strength but also good plasticity, excellent impact toughness, and machinability. It must be easy to process and possess high fatigue resistance.

[0003] In the current technology, researchers at home and abroad have conducted a lot of research on high-strength steel. They usually select appropriate chemical compositions and use quenching and tempering heat treatment or controlled rolling + controlled cooling processes to produce high-strength steel.

[0004] Under the same service conditions, increasing strength can reduce the weight of parts. High-strength and high-toughness steel has good economic benefits and is energy-saving and environmentally friendly. It has been gradually and widely used in the automotive and engineering machinery fields. In particular, the comprehensive strength and toughness of medium carbon low alloy steel for surface-hardened parts of new energy vehicles is one of the future development trends.

[0005] Especially for steel used in automotive steering systems, the selection of materials is particularly stringent because the steering system is one of the key safety components. As the material constituting the main structure of the steering system, steel not only needs to possess excellent mechanical properties but also needs to meet specific processing and environmental adaptability requirements. For example, steel needs sufficient strength to withstand the various stresses generated during vehicle operation, which is crucial for maintaining the integrity and stability of the steering system; steel should possess good toughness to absorb and disperse collision energy, improving vehicle safety; furthermore, steel should have good machinability, making it easy to machine to form complex steering system components. Good machinability reduces processing difficulty, improves component surface quality, and increases production efficiency. Therefore, the preparation of steel with good strength, toughness, and machinability is of paramount importance.

[0006] Chinese invention patent CN109182909B, published on June 4, 2021, provides a medium carbon steel (plate) for automotive steering systems. Its chemical composition, by weight percentage, is: C: 0.20%–0.30%, Mn: 1.0%–2.0%, Si: 0.05%–0.30%, P≤0.020%, S≤0.010%, with the balance being Fe and unavoidable impurities. The invention also provides a method for producing the aforementioned medium carbon steel, comprising the steps of smelting, continuous casting, heating, rough rolling, finish rolling, laminar flow cooling, and coiling. The continuous casting employs low superheat and constant-speed casting. This invention uses a C-Mn steel composition system, reduces the Si content, and obtains steel with better mechanical properties through low superheat casting, constant-speed continuous casting, and controlled rolling and cooling processes, achieving a yield strength ≥400MPa and a tensile strength ≥520MPa. Elongation ≥20%, impact energy at -10℃ ≥40J, grain size ≥10, and metallographic structure is ferrite-pearlite.

[0007] For example, Chinese patent document CN109763061B, published on October 22, 2021, discloses a medium carbon steel containing the following components by weight percentage: 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%, with the balance being iron. This technical solution yields a high-strength steel suitable for surface hardening.

[0008] For example, Chinese patent document CN109207840B, published on December 22, 2020, discloses a free-machining medium carbon steel with the following chemical element mass percentages: 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%; with the balance being iron and other unavoidable impurities; and satisfying an S / Ca ratio of 20–60. Furthermore, this invention also proposes a method for manufacturing the aforementioned free-machining medium carbon steel, including smelting and refining, casting, rolling, forging, and two-stage cooling. The free-machining medium carbon steel described above improves the material's strength while maintaining its ductility and toughness.

[0009] As can be seen from the existing technology, in order to obtain high-strength and high-toughness medium-carbon low-alloy steel, those skilled in the art can use microalloying elements such as V and Ti to refine the grain size and improve the strength of the steel. However, this design scheme of adding V and Ti elements can easily increase manufacturing costs, and when the V content in the steel is too high, it will form coarse VC particles and reduce the impact toughness of the steel; at the same time, Ti elements are prone to forming inclusions, and if used in combination with other microalloying elements, a coupling effect will occur, which is not conducive to the grain refinement effect of other microalloying elements.

[0010] Therefore, in order to solve the problems existing in the prior art, the inventors hope to provide a micro-alloyed medium-carbon low-alloy steel with high strength and toughness and good machinability to effectively meet market demand. Summary of the Invention

[0011] To address the aforementioned technical problems, embodiments of the present invention disclose a medium carbon steel, which, by mass percentage, comprises the following chemical elements:

[0012] C: 0.35–0.41%, Si: 0.10–0.40%, Mn: 0.60–0.99%, P≤0.015%, S: 0.010–0.040%, Al: 0.015–0.045%, N: 0.004–0.010%, Cr: 0.90–1.20%, Ni: 0.01–0.20%, Nb: 0.002–0.030%, Mo≤0.10%, V≤0.05%, Cu≤0.20%, Ti≤0.01%, O≤0.0020%, H≤0.0002%, with the remainder being Fe and unavoidable impurities.

[0013] Each element also satisfies the manganese equivalent Mn eq The range is 3.20–4.00%, and the critical ideal diameter Di for hardenability ranges from 2.20 to 3.80 in.

[0014] Among them, Mn eq =[Mn]+3.33[C]+0.35[Ni]+1.50[Cr]+2.00[Mo]+0.17[Cu],

[0015] Di=0.54[C]×(3.333[Mn]+1)×(0.70[Si]+1)×(0.363[Ni]+1)×(2.16[Cr]+1)×(3.00[Mo]+1)×

[0016] (0.365[Cu]+1)×(1.73[V]+1),

[0017] In the formula, [Mn], [C], [Ni], [Cr], [Mo], [Cu], [Si], and [V] represent the mass percentage values ​​of the corresponding elements.

[0018] By adopting the above technical solution, a medium-carbon steel with good strength and toughness can be obtained. This medium-carbon low-alloy steel can achieve good strength and toughness without the addition of microalloying elements such as V and Ti. It not only possesses good strength, impact toughness, and plasticity, but also good fatigue resistance and high hardenability. This medium-carbon steel is also easy to machine, meeting the performance requirements of steel in applications such as automobiles and construction machinery, and has a very promising prospect for widespread application.

[0019] Optionally, each element satisfies the microalloying coefficient r. M / X The range is 0.50 to 2.00, where r M / X = ([Al] / 27+20[Nb] / 93-[V] / 510+[Ti] / 48) / ([N] / 14+[C] / 120), where [Al], [Nb], [V], [Ti], [N], and [C] represent the mass percentage values ​​of the corresponding elements.

[0020] Optionally, the microstructure of the medium carbon steel is ferrite and pearlite.

[0021] Optionally, in the microstructure of the medium carbon steel, the ferrite grain size is ≥9, and during the preparation of the medium carbon steel, when the medium carbon steel is austenitized, the austenite grain size is ≥6.

[0022] Optionally, the yield strength R of the medium carbon steel after quenching and tempering heat treatment is... p0.2 ≥650MPa, tensile strength R m ≥900MPa, elongation A≥15%, reduction of area Z≥45%, Charpy impact energy A kv ≥40J.

[0023] Optionally, the chips produced by the medium carbon steel after quenching and tempering heat treatment are granular chips or C-shaped chips.

[0024] According to another specific embodiment of the present invention, a method for manufacturing medium carbon steel is disclosed, comprising the following steps:

[0025] Smelting;

[0026] Casting;

[0027] Heating: The heating temperature is 1050~1250℃, and the holding time is 3~24h;

[0028] Forging or rolling: Control the final rolling temperature or final forging temperature ≥850℃, and cool after rolling or forging.

[0029] Using the above technical solution, the manufacturing method is simple to produce and the process design is reasonable. The medium carbon steel obtained not only has good strength, toughness and plasticity, but also excellent hardenability and is easy to machine. The parts made from it have a long fatigue life and good application prospects.

[0030] Optionally, the heating temperature is 1100-1200℃, and the final rolling temperature or final forging temperature is 850-1000℃.

[0031] According to another specific embodiment of the present invention, an embodiment of the present invention discloses a round steel bar, which is forged or rolled from the above-mentioned medium carbon steel, and the diameter of the round steel bar is Φ20~100mm.

[0032] Using the above technical solution, the round steel obtained not only has good strength, toughness and plasticity, but also excellent hardenability and is easy to machine.

[0033] The present invention also discloses a method for manufacturing round steel bars, comprising the following steps:

[0034] Smelting;

[0035] Casting;

[0036] Heating: The heating temperature is 1050~1250℃, and the holding time is 3~24h;

[0037] Forging or rolling: Control the final rolling temperature or final forging temperature ≥850℃ to obtain round steel with a diameter of Φ20~100mm, and cool after rolling or forging.

[0038] Using the above technical solution, the manufacturing method is simple to produce and the process design is reasonable. The round steel obtained not only has good strength, toughness and plasticity, but also excellent hardenability and is easy to cut. The parts made from it have a long fatigue life and good application prospects. Detailed Implementation

[0039] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0042] The first aspect of this invention discloses a medium carbon steel, which, by mass percentage, comprises the following chemical elements:

[0043] C: 0.35–0.41%, Si: 0.10–0.40%, Mn: 0.60–0.99%, P≤0.015%, S: 0.010–0.040%, Al: 0.015–0.045%, N: 0.004–0.010%, Cr: 0.90–1.20%, Ni: 0.01–0.20%, Nb: 0.002–0.030%, Mo≤0.10%, V≤0.05%, Cu≤0.20%, Ti≤0.01%, O≤0.0020%, H≤0.0002%, with the remainder being Fe and unavoidable impurities.

[0044] Each element also satisfies the manganese equivalent Mn eq The range is 3.20–4.00%, and the critical ideal diameter Di for hardenability ranges from 2.20 to 3.80 in.

[0045] Among them, Mn eq =[Mn]+3.33[C]+0.35[Ni]+1.50[Cr]+2.00[Mo]+0.17[Cu],

[0046] Di=0.54[C]×(3.333[Mn]+1)×(0.70[Si]+1)×(0.363[Ni]+1)×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.365[Cu]+1)×(1.73[V]+1),

[0047] In the formula, [Mn], [C], [Ni], [Cr], [Mo], [Cu], [Si], and [V] represent the mass percentage values ​​of the corresponding elements.

[0048] In the above-mentioned technical solution of this invention, the inventors, based on a reasonable chemical element composition design, selected an Al / Nb combination system, controlled the relative contents of Al, Nb, and N elements, and simultaneously added appropriate amounts of Mn, Cr, Ni, and other elements. This effectively improved the hardenability of the steel and obtained fine, dispersed precipitates, resulting in medium-carbon steel with good strength, plasticity, toughness, and fatigue resistance. Simultaneously, the inventors controlled the S content, allowing the elemental composition between Mn and S to form sulfide inclusions, improving the machinability of the steel. Furthermore, the use and combination of S, Mn, and C minimized the deformation of the steel, ensuring its hardness.

[0049] Meanwhile, to ensure the strength of medium carbon steel, small amounts of one or more alloying elements such as Cr, Ni, and Mo can be added. However, to balance cost-effectiveness and avoid excessive hardness in the steel, which would affect machinability, cold deformation capacity, and plasticity, and to ensure suitability for surface induction hardening, the inventors, while controlling the mass percentage content of individual chemical elements, further controlled the content of alloying elements such as Mn, Cr, Ni, Mo, and Cu, and also controlled the manganese equivalent Mn. eq By controlling the content to 3.20–4.00%, deformation can be more reliably avoided, austenite center segregation reduced, and the uniformity of the steel microstructure improved based on the above composition combination. Furthermore, the inventors controlled the critical ideal diameter Di for hardenability to 2.20–3.80 in., ensuring sufficient hardenability of the steel at a lower alloy cost.

[0050] By controlling the chemical element composition, manganese equivalent, and critical ideal diameter for hardenability, combined with the alloying system, steel can possess strength, toughness, machinability, fatigue resistance, and hardness without the presence of precious elements such as V and Ti. It also exhibits low deformation and is suitable for manufacturing important automotive parts, especially for producing slender strip-shaped parts. It is highly reliable and widely used.

[0051] The role of each element will be explained in more detail below:

[0052] C: In the medium-carbon low-alloy steel with good strength and toughness described in this invention, carbon (C) can improve the hardenability of the steel, enabling it to form a phase transformation structure with higher hardness during quenching and cooling. When the C content in the steel is too low, the steel cannot obtain sufficient tensile strength. At the same time, the C content in the steel should not be too high either. When the C content in the steel increases, it will increase the proportion of hard phases, increase the hardness of the steel, and also lead to a decrease in the toughness of the steel. Therefore, considering the influence of C content on the performance of steel, in the medium-carbon steel with good strength and toughness described in this invention, the mass percentage of C is controlled between 0.35% and 0.41%.

[0053] Si: In the medium-carbon steel with good strength and toughness described in this invention, Si is beneficial to improving the strength of the steel, and adding an appropriate amount of Si can prevent the formation of coarse carbides. However, it should be noted that the Si content in the steel should not be too high, as excessive Si content will reduce the impact toughness of the steel. Therefore, in the medium-carbon low-alloy steel with good strength and toughness described in this invention, the mass percentage of Si is controlled between 0.10% and 0.40%.

[0054] Mn: In the medium-carbon steel with good strength and toughness described in this invention, Mn mainly exists in solid solution form. It can effectively improve the hardenability of the steel and form a high-strength low-temperature phase transformation structure during quenching, thus giving the steel good 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, more retained austenite will be formed, reducing the yield strength of the steel and easily leading to center segregation. Therefore, in the medium-carbon low-alloy steel with good strength and toughness described in this invention, the mass percentage of Mn is controlled between 0.60% and 0.99%.

[0055] Cr: In the medium carbon steel with good strength and toughness described in this invention, Cr can significantly improve the hardenability and strength of the steel. Correspondingly, the Cr content in the steel should not be too high. When the Cr content is too high, coarse carbides will form, reducing the impact performance of the steel. Therefore, considering the influence of Cr content on the steel properties, the mass percentage of Cr in the medium carbon steel with good strength and toughness described in this invention is controlled at Cr: 0.90–1.20%.

[0056] Ni: In the medium carbon steel with good strength and toughness described in this invention, Ni exists in the steel in a solid solution form. Adding an appropriate amount of Ni 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 costs but also lead to an excessively high content of retained austenite in the steel, thereby reducing the strength of the steel. Therefore, considering the economic efficiency of the steel, in the medium carbon steel with good strength and toughness described in this invention, it is optional to add Ni, and the mass percentage of Ni is controlled at Ni: 0.01-0.20%.

[0057] Mo: In the medium-carbon steel with good strength and toughness described in this invention, Mo can be dissolved in the steel and can improve the hardenability and strength of the steel. Simultaneously, under high-temperature tempering, Mo will combine to form fine carbides, further improving the strength of the steel. However, considering that Mo is a precious metal element, the Mo content in the steel should not be too high to effectively control the cost of the alloy. Therefore, in the medium-carbon steel with good strength and toughness described in this invention, the addition of Mo can be selected, and the mass percentage of Mo is controlled to be Mo ≤ 0.10%.

[0058] Cu: In the medium carbon steel with good strength and toughness described in this invention, adding an appropriate amount of Cu not only improves the strength of the steel but also enhances its corrosion resistance. However, it should be noted that the Cu content in the steel should not be too high. If the Cu content is too high, it will accumulate at the grain boundaries during heating, leading to grain boundary weakening and cracking. Therefore, in the medium carbon steel with good strength and toughness described in this invention, the addition of Cu can be optional, and the mass percentage of Cu should be controlled to Cu ≤ 0.20%.

[0059] Al: In the medium carbon steel with good strength and toughness described in this invention, Al is an important deoxidizing element. It can form fine precipitates in the steel and pin grain boundaries, thus inhibiting austenite grain growth. However, it should be noted that the Al content in the steel should not be too high. Excessive Al content will lead to the formation of larger oxides, and coarse hard inclusions will reduce the impact toughness and fatigue performance of the steel. Therefore, in the medium carbon steel with good strength and toughness described in this invention, the mass percentage of Al is controlled between 0.015% and 0.045%.

[0060] Nb: In the medium-carbon steel with good strength and toughness described in this invention, the addition of Nb can form fine precipitates with nitrogen or carbon, which can inhibit recrystallization and effectively refine the grains. Grain refinement plays an important role in improving the mechanical properties of steel, especially strength and toughness. Simultaneously, grain refinement also helps reduce the hydrogen embrittlement sensitivity of steel. However, the Nb content in the steel should not be too high. When the Nb content is too high, coarse NbC particles will form during the smelting process, which will reduce the impact toughness of the steel. Therefore, to maximize the beneficial effects of Nb, in the medium-carbon steel with good strength and toughness described in this invention, the addition of Nb can be selective, and the mass percentage of Nb is controlled at Nb: 0.002–0.030%.

[0061] Nitrogen (N): In the medium-carbon steel with good strength and toughness described in this invention, nitrogen (N) is an interstitial atom that can form nitrides or carbonitrides (MX-type precipitates) in the steel, playing a role in precipitation strengthening and grain refinement. However, it should be noted that the nitrogen content in the steel should not be too high. When the nitrogen content is too high, coarse particles will form, which will not be able to refine the grains. This is because nitrogen, as an interstitial atom, will accumulate at grain boundaries and defects, leading to a decrease in the impact toughness of the steel. Therefore, to avoid nitrogen accumulation in the steel, the mass percentage of nitrogen in the medium-carbon steel with good strength and toughness described in this invention is controlled between 0.004% and 0.010%.

[0062] Of course, in some preferred embodiments, in order to obtain better implementation results, the mass percentage content of N element can be further preferably controlled between 0.006 and 0.010%.

[0063] S: In this invention, sulfur (S) can form sulfide inclusions with manganese (Mn) in steel, improving the machinability of the steel. However, excessively high S content is detrimental to hot working and reduces the impact resistance of the steel. Therefore, in the medium carbon steel with good strength and toughness described in this invention, the mass percentage of sulfur is controlled at 0.010–0.040%.

[0064] V: In the medium carbon steel with good strength and toughness described in this invention, V is an important alloying element for strengthening the medium carbon steel. V can form precipitates with C or N elements in steel, thereby producing precipitation strengthening, and can pin grain boundaries, refine grains, and improve the strength of the steel. Correspondingly, the V content in the steel should not be too high. If the V content is too high, coarse VC particles will form, reducing the impact toughness of the steel. Therefore, in the medium carbon steel with good strength and toughness described in this invention, it is not necessary to specifically add V, and the mass percentage of V is controlled to V ≤ 0.05%.

[0065] While Ti can form fine precipitates when added to steel, excessively high Ti content can lead to the formation of large, angular, blocky TiN particles during smelting, reducing the steel's impact toughness. Therefore, in the medium carbon steel described in this invention, the Ti content is controlled to be ≤0.01%.

[0066] Preferably, each element satisfies the manganese equivalent Mn eq The range is 3.22–3.54%, and the critical ideal diameter Di for hardenability ranges from 2.27 to 2.97 inches. Within this range, the steel can further possess better overall performance and has higher practical value.

[0067] Furthermore, in the medium-carbon low-alloy steel with good strength and toughness described in this invention, among the unavoidable impurities, P ≤ 0.015%, O ≤ 0.0020%, and H ≤ 0.0002%.

[0068] In the above technical solution, P, O and H are all impurity elements in steel. When 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.

[0069] P: In this invention, phosphorus (P) tends to segregate at grain boundaries in steel, which reduces grain boundary bonding energy and deteriorates the impact toughness of the steel. Therefore, in the medium carbon steel described in this invention, the mass percentage of phosphorus is controlled to be P ≤ 0.015%.

[0070] O: In this invention, the element O can form oxides and composite oxides with the element Al in steel. In order to ensure the uniformity of the steel structure, low-temperature impact energy and fatigue performance, the mass percentage content of the element O in the medium carbon steel described in this invention is controlled to be: O≤0.0020%.

[0071] H: In this invention, H may accumulate at defects in the steel, and hydrogen-induced delayed fracture may occur in steel with a tensile strength exceeding 1000 MPa. In this invention, the H content is controlled to ≤0.0002%.

[0072] In a specific embodiment of the present invention, each element also satisfies the microalloying coefficient r. M / X The range is 0.50 to 2.00, where r M / X= ([Al] / 27 + 20[Nb] / 93 - [V] / 510 + [Ti] / 48) / ([N] / 14 + [C] / 120), where [Al], [Nb], [V], [Ti], [N], and [C] represent the mass percentage values ​​of the corresponding elements. In this invention, while controlling the mass percentage content of individual chemical elements, the inventors also designed and further controlled the ratio of the content of microalloying elements Al, Nb, V, and Ti to the content of C and N. This requires that the total amount of microalloying elements have an atomic ratio to carbon and nitrogen exceeding 0.5, and defines a microalloying element coefficient, r... M / X By controlling the concentration between 0.50 and 2.00, the growth of austenite grains can be suppressed and precipitation strengthening can be achieved.

[0073] In one specific embodiment of the present invention, the microstructure of medium carbon steel is ferrite and pearlite.

[0074] In a specific embodiment of the present invention, the ferrite grain size in the microstructure of medium carbon steel is ≥9 grade. During the preparation of medium carbon steel, when the medium carbon steel is austenitized, the austenite grain size is ≥6 grade. Preferably, the ferrite grain size is ≤11 grade (9 grade ≤ ferrite grain size ≤ 11 grade). During the preparation of medium carbon steel, when the medium carbon steel is austenitized, the austenite grain size is ≤9 grade (6.5 ≤ austenite grain size ≤ 9 grade). Within this range, the medium carbon steel has fine grains, better grain boundary properties, and its mechanical properties and machinability are guaranteed.

[0075] This invention controls the chemical element composition and content, manganese equivalent, critical ideal diameter for hardenability, and microalloying coefficient, while further controlling the microstructure composition and grain size of medium carbon steel. This can further ensure that the steel has good strength, plasticity, toughness, hardenability, and fatigue resistance, and is easy to machine, thus improving its application reliability.

[0076] In a specific embodiment of the present invention, the yield strength R of medium carbon steel after quenching and tempering heat treatment is... p0.2 ≥650MPa, tensile strength R m ≥900MPa, elongation A≥15%, reduction of area Z≥45%, Charpy impact energy A kv ≥40J.

[0077] In one specific embodiment of the present invention, the chips produced by quenching and tempering of carbon steel are further characterized as granular chips or C-shaped chips. Medium carbon steel has good machinability, is suitable for processing into slender strips, and its application range is expanded. At the same time, it has low deformation and high reliability.

[0078] A second aspect of the present invention discloses a method for manufacturing medium carbon steel, comprising the following steps:

[0079] Smelting;

[0080] Casting;

[0081] Heating: The heating temperature is 1050~1250℃, and the holding time is 3~24h;

[0082] Forging or rolling: Control the final rolling temperature or final forging temperature ≥850℃, and cool after rolling or forging.

[0083] In the above-described technical solution of the present invention, the smelting step can be carried out using an electric furnace or a converter, followed by refining and vacuum treatment. Of course, in some other embodiments, operators may also use a vacuum induction furnace for smelting. Specifically, the smelting is performed according to the composition ratio of the aforementioned elements.

[0084] Accordingly, in this invention, casting is required after smelting. In the casting process described above, ingot casting or continuous casting can be specifically employed. Specifically, the ingot obtained from continuous casting can be fed into a heating furnace for heating, and the heating temperature of the ingot is controlled at 1050–1250°C, with a holding time of 3–24 hours, to ensure that the medium carbon steel of this invention is fully austenitized during the heating process.

[0085] It should be noted that in the heating step of this invention, the medium carbon steel is heated to 1050℃~1250℃ for complete austenitization. Simultaneously, the carbides and nitrides of Al, Nb, and V, as well as carbonitrides, and the carbides of Cr and Mo, can partially or completely dissolve in the austenite. During subsequent rolling or forging and subsequent cooling processes, 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 its hardness and strength. Under the condition that the final rolling or forging temperature in the subsequent forging or rolling step is ≥850℃, a refined multiphase matrix structure with fine, dispersed precipitates can be formed.

[0086] In addition, it should be noted that in the heating step described in this invention, in actual operation, the temperature can be directly increased to the heating temperature, or it can be increased to the heating temperature in stages.

[0087] Furthermore, in the manufacturing method described in this invention, the final rolling temperature or final forging temperature is controlled to be 850–1000°C during the forging or rolling step.

[0088] In the above-mentioned technical solution of the present invention, further, in the forging or rolling step, when the forging process is adopted, the steel billet can be directly forged to the final finished product size; while 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 rolled to a specified intermediate billet size first, and then subjected to intermediate heating and rolling to the final finished product size.

[0089] Furthermore, in the manufacturing method described in this invention, during the forging or rolling step, the material is directly rolled or forged to the finished size.

[0090] Furthermore, in the manufacturing method described in this invention, in the forging or rolling step, the billet is first rolled to the intermediate billet size, then intermediate heating is performed, and then it is rolled to the final finished product size; wherein, the intermediate heating temperature is 1050~1250℃, and the holding time is 3~24h.

[0091] In the above-described technical solution of this invention, the rolling process can be segmented rolling, first rolling to the size of an intermediate billet, then performing intermediate heating, and finally rolling to the final finished 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, which provides better control over the steel's performance.

[0092] Furthermore, in the manufacturing method described in this invention, during the forging or rolling step, the material is subjected to air cooling, wind cooling, or slow cooling after rolling or forging.

[0093] It is important to note that during the forging or rolling process, the steel billet undergoes high-pressure water descaling after exiting the heating furnace before rolling or forging. Cooling is controlled after rolling or forging; the rolling speed and cooling method must be adjusted according to the specific application and performance requirements. By controlling the phase transformation process, an ideal microstructure can be obtained. Cooling methods can include air cooling, wind cooling, or slow cooling.

[0094] A third aspect of the present invention provides a round steel bar, which is obtained by forging or rolling medium carbon steel and has a diameter of Φ20 to 100 mm.

[0095] A fourth aspect of the present invention provides a method for manufacturing round steel bars, characterized by comprising the following steps:

[0096] Smelting;

[0097] Casting;

[0098] Heating: The heating temperature is 1050~1250℃, and the holding time is 3~24h;

[0099] Forging or rolling: Control the final rolling temperature or final forging temperature ≥850℃ to obtain round steel with a diameter of Φ20~100mm, and cool after rolling or forging.

[0100] This invention develops a medium carbon steel with high toughness and excellent machinability by rationally designing the chemical composition and combining it with optimized processes. It can be rolled or forged into bars and forms a ferrite + pearlite microstructure with fine grains and fine dispersed precipitates. This makes the medium carbon steel easy to machine and cold deform while obtaining good plasticity.

[0101] The medium carbon steel of this invention has fine grains, high strength, good elongation and reduction of area, excellent impact toughness and high hardenability, and is suitable for surface hardened parts. It can be effectively used in new energy vehicles and engineering machinery and other applications that require steel with high fatigue performance.

[0102] In actual preparation, the present invention also optimized the manufacturing process, which is reasonably designed and has a wide process window, enabling mass commercial production on bar production lines.

[0103] The yield strength R of medium carbon steel designed in this invention after quenching and tempering heat treatment p0.2 ≥650MPa, tensile strength R m ≥900MPa, elongation A≥15%, reduction of area Z≥45%, Charpy impact energy A kv With a hardenability of ≥40J, this medium carbon steel exhibits high hardenability, meeting the requirements for automotive and construction machinery applications, particularly in situations requiring surface hardening treatment, and thus possesses excellent prospects for widespread application. After quenching and tempering heat treatment, the chips produced by this medium carbon steel are granular or C-shaped, demonstrating excellent machinability and meeting the needs of a variety of different applications.

[0104] The present invention will now be described in conjunction with more specific embodiments.

[0105] Examples 1-8 and Comparative Examples 1-4 are round bars prepared from medium carbon steel. Examples 1-8 were prepared using the following steps:

[0106] (1) Smelting: Smelting is carried out according to the chemical composition shown in Tables 1-1 and 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.

[0107] (2) Casting: Casting is carried out by die casting or continuous casting to obtain ingots;

[0108] (3) Heating: Place the ingot in a heating furnace and heat it, and control the heating temperature to 1050-1250℃, and hold it for 3-24 hours; during heating, the temperature can be raised directly to the heating temperature, or a step heating method can be used to raise the temperature to the heating temperature.

[0109] (4) Forging or rolling: Control the final rolling temperature or final forging temperature ≥850℃, preferably between 850 and 1000℃, to produce round steel with a diameter of Φ20 to 100mm, and cool it after rolling or forging. The cooling method can be air cooling, wind cooling or slow cooling.

[0110] When forging or forging is performed, the billet can be directly rolled or forged to the finished size, or it can be rolled to the intermediate billet size first, then heated in the intermediate stage, and then rolled to the final finished size. When the above-mentioned segmented rolling is used, 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.

[0111] It should be noted that in this invention, after the manufacturing process described in steps (1) to (4) above, medium carbon steel corresponding to each embodiment and comparative example can be effectively prepared. Usually, we test the performance of medium carbon steel after quenching and tempering heat treatment. Therefore, after obtaining the medium carbon steel of the above embodiments, step (5) can be further carried out, that is, 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.

[0112] The chemical compositions of Comparative Examples 1–4 are also shown in Tables 1-1 and 1-2.

[0113] Table 1-1. (wt.%, balance is Fe and other unavoidable impurities other than P and O)

[0114]

[0115] Table 1-2 lists the manganese equivalent (Mn) of each alloying element calculated from the mass percentage of each chemical element in the round steel of Examples 1-8 and the comparative steel of Comparative Examples 1-4. eq , critical ideal diameter for hardenability Di value, microalloying element coefficient r M / X .

[0116] Table 1-2:

[0117] serial number Mn eq (%)]] Di value (in.) <![CDATA[Microalloy element coefficient r M / N > Example 1 3.54 2.97 0.51 Example 2 4.00 3.40 0.61 Example 3 3.78 3.78 0.65 Example 4 3.67 3.21 1.19 Example 5 3.39 2.64 1.09 Example 6 3.88 3.68 1.81 Example 7 3.84 3.71 0.95 Example 8 3.22 2.27 2.00 Comparative Example 1 4.01 3.79 0.65 Comparative Example 2 3.19 2.11 0.52 Comparative Example 3 3.29 2.34 0.60 Comparative Example 4 3.79 3.32 0.30

[0118] Note: In the table above, Mn eq =[Mn]+3.33[C]+0.35[Ni]+1.50[Cr]+2.00[Mo]+0.17[Cu];

[0119] Di=0.54[C]×(3.333[Mn]+1)×(0.70[Si]+1)×(0.363[Ni]+1)×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.365[Cu]+

[0120] 1)×(1.73[V]+1);

[0121] r M / X=([Al] / 27+20[Nb] / 93-[V] / 510+[Ti] / 48) / ([N] / 14+[C] / 120);

[0122] Among them, “Mn eq “Di” and “r” M / X In the formula, each chemical element is replaced with the value before the percentage sign of its mass percentage content.

[0123] The following will provide more specific preparation methods for Examples 1-8 and Comparative Examples 1-4:

[0124] Example 1:

[0125] The molten steel was smelted in a 50kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was then cast into ingots, heated, and then forged. The heating temperature was controlled at 1050℃, and the forging was carried out after holding at that temperature for 3 hours. The final forging temperature was controlled at 910℃, and the steel was finally forged into round bars with a diameter of Φ40mm, which were then air-cooled.

[0126] Example 2:

[0127] The molten steel was smelted in a 150kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was then cast into steel ingots, heated, and then forged. The heating temperature was controlled at 1100℃, and the ingots were held at that temperature for 4 hours before forging. The final forging temperature was controlled at 980℃, resulting in Φ60mm round bars, which were then slowly cooled in a stack.

[0128] Example 3:

[0129] The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, followed by LF refining and VD vacuum treatment, and then continuously cast into 320mm×425mm continuous casting billets. The continuous casting billets were slowly heated to 1220℃, held at that temperature for 4 hours, and then rolled. After exiting the heating furnace, the billets were descaled by high-pressure water and then rolled, with the final rolling temperature controlled at 1000℃, finally rolled into Φ100mm round bars, and then air-cooled.

[0130] Example 4:

[0131] The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, and then refined and vacuum treated before being continuously cast into 280mm × 280mm continuous casting billets. The continuous casting billets were slowly heated to 1180℃, held at that temperature for 6 hours, and then rolled. After exiting the heating furnace, the billets were descaled by high-pressure water and then rolled into intermediate billets, with the final rolling temperature controlled at 1000℃ and the intermediate billet size at 140mm × 140mm. The intermediate billets were then preheated to 1220℃, held at that temperature for 5 hours, and then descaled by high-pressure water before being rolled again, with the final rolling temperature controlled at 850℃, finally rolled into Φ28mm round bars, and then air-cooled.

[0132] Example 5:

[0133] The steel was smelted in an electric furnace according to the chemical composition shown in Tables 1 and 1-2, followed by LF refining and VD vacuum treatment, and then cast into 320mm × 425mm continuous casting billets. The continuous casting billets were heated in stages to 1230℃, held for 3 hours, and then rolled. After exiting the heating furnace, the billets were descaled by high-pressure water and then rolled into intermediate billets. The final rolling temperature was controlled at 1050℃, resulting in intermediate billet dimensions of 220mm × 220mm, which were then air-cooled. The intermediate billets were then preheated and heated in stages to 1200℃, held for 6 hours, and then descaled by high-pressure water before rolling. The final rolling temperature of the intermediate billets was controlled at 950℃, resulting in finished round bars with a diameter of Φ50mm, which were then air-cooled.

[0134] Example 6:

[0135] The steel was smelted in an electric furnace according to the chemical compositions shown in Tables 1-1 and 1-2, followed by LF refining and VD vacuum treatment, and then continuously cast into 280mm×280mm continuous casting billets. The continuous casting billets were slowly heated to 1150℃, held at that temperature for 6 hours, and then rolled. After exiting the heating furnace, the billets were descaled by high-pressure water and then rolled, with the final rolling temperature controlled at 970℃. The finished round steel had a specification of Φ60mm and was air-cooled after rolling.

[0136] Example 7:

[0137] The steel billets were smelted in a converter according to the chemical compositions shown in Tables 1-1 and 1-2, followed by refining and vacuum treatment. They were then cast into billets, preheated, and then stepped-heated to 1200℃. After holding at this temperature for 9 hours, the billets were rolled. After descaling with high-pressure water, the billets were rolled at a final rolling temperature of 970℃, ultimately producing Φ90mm round bars, which were then air-cooled.

[0138] Example 8:

[0139] The steel was smelted in a 500kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2, and then cast to obtain steel ingots. The ingots were heated and forged to open the billet. The heating temperature of the steel ingots was controlled at 1150℃. After holding at the temperature for 21 hours, subsequent forging was carried out. The final forging temperature was controlled at 920℃. Finally, the steel was forged into Φ80mm round bars and then slow-cooled in a pile.

[0140] Comparative Example 1:

[0141] The implementation method is the same as in Example 1. In Comparative Example 1, the steel was smelted in a 50kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The molten steel was cast into steel ingots, heated and forged into billets. The heating temperature was 1050℃, and after holding at that temperature for 3 hours, it was forged. The final forging temperature was controlled at 910℃, and the steel was finally forged into round bars with a diameter of Φ40mm. After forging, the steel was air-cooled.

[0142] Comparative Example 2:

[0143] The implementation method is the same as in Example 2, and the steel is smelted in a 150kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2. The molten steel is continuously cast into steel ingots, heated and forged into billets. The heating temperature is 1100℃, and after holding at that temperature for 4 hours, it is forged. The final forging temperature is controlled at 980℃, and finally forged into Φ60mm round bars, which are then slow cooled in a stack.

[0144] Comparative Example 3:

[0145] The material is selected from commercially available round steel bars, and its preparation process will not be detailed here. When it is necessary to test the properties of this commercially available round steel bar after normalizing and tempering heat treatment, it can be specifically quenched at 860℃ and tempered at 610℃ before its mechanical properties can be tested.

[0146] Comparative Example 4:

[0147] The implementation method is the same as in Example 8, and the steel is smelted in a 500kg vacuum induction furnace according to the chemical composition shown in Tables 1-1 and 1-2. The molten steel is cast into steel ingots, heated and forged into billets. The heating temperature is 1100℃, held for 4 hours and then forged. The final forging temperature is controlled at 1000℃, and finally forged into Φ80mm round bars, which are then slowly cooled.

[0148] It should be noted that samples of the round steel from Examples 1 to 8 and the comparative round steel from Comparative Examples 1 to 4 can be taken separately and prepared according to GB / T 13298. The microstructure is analyzed using a metallographic microscope according to GB / T 13299, and the ferrite grain size and austenite grain size are detected according to ASTM E112. The austenitizing temperature is 930℃ and the holding time is 4h, followed by water quenching.

[0149] In analyzing austenite grain size, the steel samples of each embodiment and comparative example were fully austenitized and then water-quenched to prepare metallographic specimens. The austenite grain boundaries were revealed by etching with picric acid alcohol solution, and the austenite grain size was analyzed by metallographic microscopy according to ASTM E112.

[0150] Table 2 lists the metallographic analysis results of the round steel bars of Examples 1-8 and Comparative Examples 1-4.

[0151] Table 2:

[0152]

[0153] As can be seen from Table 2 above, in this invention, the microstructure of the round steel in Examples 1 to 8 all includes ferrite + pearlite, and the grains of this medium-carbon low-alloy steel are fine, with the ferrite grain size not lower than grade 9. When these medium-carbon low-alloy steels in Examples 1 to 8 are austenitized, their austenite grain size is between grade 6.5 and grade 9. In contrast, the grain size of Comparative Example 4 shows mixed grains, and 5.5(1) indicates that the average grain size is grade 5.5, with the presence of coarse grade 1 grains.

[0154] Accordingly, after completing the above observation and analysis of the metallographic structure, in order to further demonstrate that the round steel prepared by the present invention has excellent mechanical properties, based on the finished round steel of Examples 1 to 8 and Comparative Examples 1 to 4, the inventors took samples of the steel of these examples and comparative examples, prepared test specimens with reference to GB / T 2975, and subjected the medium carbon low alloy steel to quenching at 860±10℃ and tempering at 500±10℃ with reference to ISO683-2 and GB / T 3077. At the same time, after completing the heat treatment, tensile tests and impact tests were carried out according to GB / T 228.1 and GB / T 229, respectively. The relevant mechanical property test results are listed in Table 3.

[0155] To test the hardenability of the finished round steel bars in each embodiment and comparative example, the inventors took samples from hot-rolled round steel bars after quenching and tempering heat treatment according to national standard GB / T 225, and conducted end hardenability tests (Jominy test) with reference to GB / T 5216. The normalizing temperature was controlled at 880±20℃ and the quenching temperature at 870±5℃. Rockwell hardness tests were conducted according to GB / T230.2 to obtain hardness values ​​(HRC) at specific locations, such as the hardness at distances of 3mm and 9mm from the quenched end, i.e., J3mm and J9mm.

[0156] Round bars of φ25mm*200mm were sampled and machined using the same lathe, cutting tools and process parameters. The chips obtained during the cutting process were observed, and the cutting difficulty was judged according to the chip morphology. Granular chips were excellent, "C"-shaped chips were acceptable, while spiral chips indicated poor machinability.

[0157] Two parallel planes are ground on a φ25mm*200mm round bar in a direction parallel to the sample axis for measuring hardness and radial runout. When the sample is prepared by machining, the two planes for hardness testing should be at the same distance from the sample surface. The grinding depth should be 0.4mm~0.5mm. When grinding the hardness testing planes, a fine grinding wheel with sufficient coolant should be used to prevent any possible heating that could cause changes in the sample microstructure. Then, referring to GB / T34882, the obtained round bar is subjected to high-frequency induction heating treatment (860±10℃, 100s), followed by quenching in water at 20±2℃. Using a radial runout measuring instrument, one end of the sample bar is inserted into the hole, the sample bar is rotated, and the radial runout value at the other end is measured using a dial indicator.

[0158] Table 3 lists the test results of mechanical properties, end-quenching properties, machinability, and induction hardening effects of the round steels of Examples 1-8 and the comparative round steels of Comparative Examples 1-4 after quenching and tempering heat treatment.

[0159] Table 3:

[0160]

[0161] As can be seen from Table 3 above, after quenching and tempering heat treatment, the comprehensive performance of the round steel prepared from the medium carbon steel in Examples 1 to 8 of the present invention is still significantly better than that of the comparative round steel in Examples 1 to 4.

[0162] In this invention, after quenching and tempering heat treatment, the yield strength R of the round steel in Examples 1-8 is... p0.2 The tensile strength R is between 665 and 750 MPa. m Between 910 and 1150 MPa, the elongation A is between 15% and 26%, the reduction of area Z is between 45% and 63%, and the Charpy impact energy A... kv Between 43 and 93 J, it not only has good strength, impact toughness and plasticity, but also good fatigue resistance.

[0163] The round steel prepared in Examples 1-8 also has precisely controlled hardenability, with the critical ideal diameter Di value for hardenability between 2.27 and 3.78 in., and the measured hardness at a distance J3 mm from the quenched end is between 55 and 60 HRC, and the hardness at a distance J9 mm from the quenched end is between 39 and 55 HRC, which meets the requirements of 37CrS4H or 37Cr4H in ISO 683-2.

[0164] The round steel prepared in Examples 1-8, after being made into rack components, exhibits a radial runout range of 0.025-0.061 mm after induction hardening, which meets the usage requirements.

[0165] Referring to Tables 1-1, 1-2, 2, and 3, it can be seen that among Comparative Examples 1-4, except for Comparative Example 3 with its low carbon content, the other three comparative examples all have parameters in their chemical element composition design that do not meet the design specifications of this invention. In this invention, the low carbon content of Comparative Example 3 results in low strength, failing to meet the requirements of a yield strength better than 650 MPa and a tensile strength exceeding 900 MPa; while the microalloying coefficient r of Comparative Example 4... M / X The design requirements were not met. The round steel grains were relatively coarse. Although the steel achieved high strength after quenching and tempering, its impact toughness was poor. The manganese equivalent design of Comparative Example 2 did not meet the requirements, resulting in low hardenability of the steel and low strength after quenching and tempering. The surface hardening of the prepared parts was insufficient. The manganese equivalent of Comparative Example 1 was too high. The steel after quenching and tempering had high strength, but poor plasticity, which could not meet the requirements for lightweight parts. The rack part prepared in Comparative Example 4 had large radial runout after induction hardening, and cracking occurred during subsequent straightening, making it difficult to meet the usage requirements.

[0166] While the present invention has been described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A medium carbon steel, characterized in that, The medium carbon steel comprises the following chemical elements in percentage by mass: C: 0.35-0.41%, Si: 0.10-0.40%, Mn: 0.60-0.99%, P≤0.015%, S: 0.010-0.040%, Al: 0.015-0.045%, N: 0.004-0.010%, Cr: 0.90-1.20%, Ni: 0.01-0.20%, Nb: 0.002-0.030%, Mo≤0.10%, V≤0.05%, Cu≤0.20%, Ti≤0.01%, O≤0.0020%, H≤0.0002%, the rest being Fe and inevitable impurities, The elements also satisfy the manganese equivalent Mn eq in the range of 3.20 to 4.00%, the hardenability critical ideal diameter Di in the range of 2.20 to 3.80 in. wherein Mn eq = [Mn] + 3.33[C] + 0.35[Ni] + 1.50[Cr] + 2.00[Mo] + 0.17[Cu], Di = 0.54[C] × (3.333[Mn] + 1) × (0.70[Si] + 1) × (0.363[Ni] + 1) × (2.16[Cr] + 1) × (3.00[Mo] + 1) × (0.365[Cu] + 1) × (1.73[V] + 1), wherein [Mn], [C], [Ni], [Cr], [Mo], [Cu], [Si], [V] represent the mass percentage values of the corresponding elements. wherein [Al], [Nb], [V], [Ti], [N], [C] represent the mass percentage values of the corresponding elements.

2. A medium carbon steel as claimed in claim 1, wherein, Each element satisfies a microalloy coefficient r M / X ranging from 0.50 to 2.00, wherein r M / X = ([Al] / 27 + 20[Nb] / 93 - [V] / 51 0 + [Ti] / 48) / ([N] / 14 + [C] / 120), The microstructure of the medium carbon steel is ferrite and pearlite.

3. A medium carbon steel as claimed in claim 1, wherein, The grain size of the ferrite in the microstructure of the medium carbon steel is ≥9 levels, and the austenite grain size is ≥6 levels when the medium carbon steel is austenitized in the process of preparing the medium carbon steel.

4. A medium carbon steel as claimed in claim 1, wherein, The chip shape of the medium carbon steel after quenching and tempering heat treatment is granular chip or C-shaped chip.

5. A medium carbon steel as claimed in claim 1, wherein, The medium carbon steel after quenching and tempering heat treatment has a yield strength R p0.2 ≥ 650 MPa, a tensile strength R m ≥ 900 MPa, an elongation A ≥ 15%, a reduction of area Z ≥ 45%, and a Charpy impact energy A kv ≥ 40 J.

6. A medium carbon steel as claimed in claim 1, wherein, The method comprises the following steps:

7. A method of producing a medium carbon steel as claimed in any one of claims 1 to 6, characterized in that, smelting; casting; heating: the heating temperature is 1050-1250°C, and the holding time is 3-24h; forging or rolling: the final rolling temperature or final forging temperature is controlled to be ≥850°C, and the rolled or forged product is cooled. The heating temperature is 1100-1200°C, and the final rolling temperature or final forging temperature is 850-1000°C.

8. A method of manufacture as claimed in claim 7 wherein, The round steel with a diameter of Φ20-100mm is obtained by forging or rolling the medium carbon steel according to any one of claims 1-6.

9. A round steel, characterized by The method comprises the following steps:

10. A method of manufacturing a round steel as claimed in claim 9, characterized by, smelting; casting; heating: the heating temperature is 1050-1250°C, and the holding time is 3-24h; forging or rolling: the final rolling temperature or final forging temperature is controlled to be ≥850°C to obtain round steel with a diameter of Φ20-100mm, and the rolled or forged product is cooled. ​

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