High-hardenability and high-strength medium-carbon steel, round steel and manufacturing method thereof

Abstract

The invention discloses a high-hardenability and high-strength medium-carbon steel, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages: C: 0.34-0.42%, Si: 0.05-0.40%, Mn: 0.50-1.00%, Cr: 0.80-1.40%, Mo: 0.10-0.40%, Al: 0.01-0.05%, Nb: 0.002-0.030%, N: 0.002-0.020%, wherein each element further satisfies the following condition: 1.1≤(Al / 2+Nb / 7+Ti / 3.5) / N≤4.9, wherein each chemical element is substituted into the value before the percentage sign of the mass percentage of the chemical element. 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 ≥850°C, and cooling after rolling or forging.

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 or 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 also needs to be easy to cut and 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 steels mainly composed of martensite have a large dislocation density, resulting in poor impact toughness. In addition, if small defects such as microcracks appear during the tensile process, they will quickly break and fail, resulting in low fracture toughness. When using controlled rolling and controlled cooling to produce high-strength steel, there is no need for quenching and tempering treatment to obtain medium-carbon alloy steel. 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-carbon steel with microalloying elements such as vanadium added. 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 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 construction machinery.

[0006] In recent years, with the increasing popularity of new energy vehicles, the market and users have placed higher technical requirements on the steel used in shaft components. Therefore, improving the comprehensive strength and toughness of high-hardenability medium-carbon alloy steel suitable for new energy vehicle shafts is one of the future development trends.

[0007] 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.

[0008] Another example: the Chinese patent document with publication number CN109763061A, published on May 17, 2019, and titled “A non-quenched and tempered steel and its preparation method” discloses a medium carbon steel containing 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.

[0009] 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 chemical elements of which are: 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.

[0010] It can be seen from the above-mentioned prior art that in order to obtain high-performance medium-carbon steel, most technical personnel in this field use micro-alloying elements such as V and Ti to refine the grain size and provide strength, thereby obtaining high-performance medium-carbon alloy steel.

[0011] However, this design of adding large amounts of V and Ti elements can easily increase manufacturing costs. When the V content in the steel is too high, coarse VC particles will be formed, and the impact toughness of the steel will be reduced. At the same time, the Ti element is easy to form inclusions. If used in combination with other microalloying elements, a coupling effect will occur, which is not conducive to the refinement effect of other microalloying elements.

[0012] Therefore, in order to solve the problems existing in the above-mentioned prior art, the inventors hope to provide a new high-hardenability and high-strength medium-carbon steel to meet the performance requirements of high-strength plastic steel in application scenarios such as automobiles and engineering machinery. Summary of the Invention

[0013] One of the purposes of the present invention is to provide a high-hardenability and high-strength medium-carbon steel. The high-hardenability and high-strength medium-carbon steel is achieved by selecting a combination system of Al, Nb, Ti, and V to control the relative contents of Al, Nb, V, Ti, and N elements. At the same time, appropriate amounts of alloying elements such as Cr, Ni, Mo, Mn, and Cu can be added to achieve good strength and plasticity while also having high hardenability and a narrow hardenability bandwidth. The medium-carbon alloy steel has good fatigue resistance and can meet the performance requirements of high-strength plastic steel for application scenarios such as automobiles and engineering machinery. The medium-carbon alloy steel has very good prospects for promotion and application.

[0014] In order to achieve the above object, the present invention provides a high hardenability and high strength medium carbon steel, which contains Fe and inevitable impurities, and further contains the following chemical elements in the following mass percentages:

[0015] C: 0.34~0.42%, Si: 0.05~0.40%, Mn: 0.50~1.00%, Cr: 0.80~1.40%, Mo: 0.10~0.40%, Al: 0.01~0.05%, Nb: 0.002~0.030%, N: 0.002~0.020%;

[0016] Each element also satisfies the following: 1.1≤(Al / 2+Nb / 7+Ti / 3.5) / N≤4.9, where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0017] Furthermore, in the high hardenability and high strength medium carbon steel of the present invention, the mass percentage of each chemical element is:

[0018] C: 0.34-0.42%, Si: 0.05-0.40%, Mn: 0.50-1.00%, Cr: 0.80-1.40%, Mo: 0.10-0.40%, Al: 0.01-0.05%, Nb: 0.002-0.030%, N: 0.002-0.020%; the balance is Fe and other inevitable impurities;

[0019] Each element also satisfies the following: 1.1≤(Al / 2+Nb / 7+Ti / 3.5) / N≤4.9, where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0020] In the above technical solution of the present invention, the inventors selected a combination system of Al and Nb based on a reasonable chemical element composition design, controlled the relative contents of Al, Nb, V, Ti and N, and simultaneously added appropriate amounts of Cr, Mo and Mn to improve the hardenability of the steel and obtain fine dispersed precipitates, so that the medium carbon steel can obtain good strength, plasticity, toughness and fatigue resistance.

[0021] In the high hardenability and high strength medium carbon steel of the present invention, the design principles of the chemical elements are as follows:

[0022] C: In the high hardenability and high strength medium carbon steel 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 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 high hardenability and high strength medium carbon steel described in the present invention, the mass percentage of the C element is controlled between 0.34 and 0.42%.

[0023] Si: In the high-hardenability, high-strength medium-carbon steel described herein, Si contributes to the steel's strength. A moderate amount of Si prevents the formation of coarse carbides during tempering. 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, the mass percentage of Si in the high-hardenability, high-strength medium-carbon steel described herein is controlled between 0.05% and 0.40%.

[0024] Mn: In the high hardenability and high strength medium carbon steel described in the present invention, Mn exists mainly in the form of solid solution in the steel, which can effectively improve the hardenability of the steel and form a high-strength low-temperature phase transformation structure during quenching, so that the steel obtains 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, which will reduce the yield strength of the steel and easily lead to center segregation. Therefore, in the high hardenability and high strength medium carbon steel described in the present invention, the mass percentage of the Mn element is controlled between 0.50 and 1.00%.

[0025] 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 0.95%.

[0026] Cr: In the high-hardenability, high-strength medium-carbon steel 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 can form coarse carbides and reduce the steel's impact resistance. Therefore, considering the impact of Cr content on steel properties, the Cr content in the high-hardenability, high-strength medium-carbon steel described herein is controlled to be between 0.80% and 1.40% by weight.

[0027] Mo: In the high-hardenability, high-strength medium-carbon steel described herein, the element Mo forms a solid solution in the steel, improving its hardenability and strength. Furthermore, during tempering at higher temperatures, Mo forms fine carbides, further enhancing the steel's strength. However, considering that Mo is a precious metal, its content in the steel should not be too high to effectively control alloy costs. Therefore, in the high-hardenability, high-strength medium-carbon steel described herein, the mass percentage of Mo is controlled between 0.10 and 0.40%.

[0028] Al: In the high-hardenability, high-strength medium-carbon steel 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 high-hardenability, high-strength medium-carbon steel described herein, the Al content is controlled between 0.01% and 0.05% by weight.

[0029] 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%.

[0030] Nb: In the high hardenability and high strength medium carbon steel described in the present invention, adding an appropriate amount of Nb element can form a fine precipitate phase with N or C elements, and play a role in inhibiting the recrystallization of the steel, which can 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. It should be noted that 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 reduce the impact toughness of the steel. Therefore, in the high hardenability and high strength medium carbon steel described in the present invention, the mass percentage of the Nb element is controlled between 0.002 and 0.030%.

[0031] 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 0.020%.

[0032] N: In the high hardenability and high strength medium carbon steel 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 high hardenability and high strength medium carbon steel described in the present invention is controlled between 0.002 and 0.020%.

[0033] 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 relationship between the content of microalloying elements Al, Nb, and Ti and the N content, requiring that the atomic ratio of the total amount of microalloying elements to nitrogen be greater than 1, and defining the microalloying element coefficient r M / N :1.1~4.9. In the present invention, r M / N =([Al] / 2+[Nb] / 7+[Ti] / 3.5) / [N], and each chemical element in the formula is substituted into the value before the percentage sign of the mass percentage of the chemical element, and r M / N Controlling it between 1.1-4.9 can achieve precipitation strengthening and inhibit abnormal growth of austenite grains.

[0034] Furthermore, in the high hardenability and high strength medium carbon steel of the present invention, among the inevitable impurities, P≤0.020%, S≤0.030%, O≤0.0025%, H≤0.0002%, and Ca≤0.004%.

[0035] In the above technical solution, P, S, O, H 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.

[0036] P: In the present invention, P tends to segregate at grain boundaries in the steel, reducing grain boundary bonding energy and deteriorating the steel's impact toughness. Therefore, in the high-hardenability, high-strength medium-carbon steel described in the present invention, the mass percentage of P is controlled to: P ≤ 0.020%.

[0037] 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 high-hardenability, high-strength medium-carbon steel described in the present invention, the mass percentage of S is controlled to: S ≤ 0.030%.

[0038] 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 high hardenability and high-strength medium carbon steel described in the present invention is controlled to: O≤0.0025%.

[0039] H: In the present invention, H accumulates at defects in the steel and can cause hydrogen-induced delayed fracture in steels with strength levels exceeding 1000 MPa. The high-hardenability, high-strength medium-carbon steel designed in the present invention has a tensile strength exceeding 1050 MPa. Therefore, the mass percentage of H in the steel is controlled to: H ≤ 0.0002%.

[0040] Ca: In the present invention, Ca can improve the size and morphology of sulfide inclusions in steel. However, Ca tends to form coarse inclusions and affect the fatigue performance of the final product. Therefore, in the high-hardenability, high-strength medium-carbon steel described in the present invention, the mass percentage of Ca is controlled to: Ca ≤ 0.004%.

[0041] Furthermore, the high hardenability and high strength medium carbon steel of the present invention further contains at least one of the following chemical elements: 0<Ni≤0.30%, 0<Cu≤0.10%, 0<V≤0.10%, 0<Ti≤0.05%.

[0042] In the high hardenability and high strength medium carbon steel of the present invention, in order to obtain better implementation effects, appropriate amounts of Ni, Cu, V and Ti elements may be further added to the steel.

[0043] Ni: In the high hardenability and high strength medium carbon steel described in the present invention, the Ni element exists in the steel in the form of solid solution, which 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 economic efficiency of the steel, in the high hardenability and high strength medium carbon steel 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 can be controlled to 0<Ni≤0.30%.

[0044] Cu: Adding an appropriate amount of Cu to the high-hardenability, high-strength medium-carbon steel described in the present invention not only improves the steel's strength but also helps enhance 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, weakening them and leading to cracking. Therefore, the addition of Cu to the high-hardenability, high-strength medium-carbon steel described in the present invention can be optional, and the mass percentage of Cu should be controlled to be 0 < Cu ≤ 0.10%.

[0045] V: In the high hardenability and high strength medium carbon steel described in the present invention, V is an important alloying element for strengthening medium carbon steel. V can form precipitates with C or N in the steel, thereby producing precipitation strengthening, and can pin grain boundaries, refine 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, and the impact toughness of the steel will be reduced. Therefore, in the high hardenability and high strength medium carbon steel described in the present invention, it can be selected whether to add V, and the mass percentage of V can be controlled to 0<V≤0.10%.

[0046] Ti: In the high-hardenability, high-strength medium-carbon steel described herein, Ti contributes to grain refinement and precipitation strengthening. However, excessive Ti content can easily form coarse Ti-containing inclusions, reducing the material's plasticity, toughness, and fatigue strength. Therefore, the addition of Ti to the high-hardenability, high-strength medium-carbon steel described herein can be optional, with the mass percentage of Ti controlled to be 0 < Ti ≤ 0.05%.

[0047] Furthermore, in the high hardenability and high strength medium carbon steel of the present invention, its critical ideal diameter DI value is 85 to 160 mm; wherein:

[0048] Di=13.72[C]×(3.33[Mn]+1)×(0.70[Si]+1)×(0.36[Ni]+1)

[0049] ×(2.16[Cr]+1)×(3.00[Mo]+1)×(0.36[Cu]+1)×(1.73[V]+1)

[0050] In the formula, each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0051] In the above technical solution, the inventors further optimized the ideal hardenability critical diameter (DI) and controlled it between 85 and 160 mm. If the DI is lower than 85 mm, the resulting steel will have insufficient hardenability; while if the DI is higher than 160 mm, manufacturing becomes difficult and the cost is high.

[0052] Furthermore, in the high hardenability and high strength medium carbon steel described in the present invention, its carbon equivalent Ceq is ≤0.80%, where Ceq = [C] + [Mn] / 6 + ( [Cr] + [Mo] + [V]) / 5 + ( [Ni] + [Cu]) / 15.

[0053] Furthermore, in the high hardenability and high strength medium carbon steel of the present invention, its microstructure is ferrite+pearlite.

[0054] Furthermore, in the high hardenability and high strength medium carbon steel of 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.

[0055] Furthermore, in the high hardenability and high strength medium carbon steel of the present invention, the yield strength R e ≥900MPa, tensile strength R m ≥1050MPa, elongation A≥12%, section shrinkage Z≥50%, Charpy impact energy A kv ≥65J, its hardenability meets the requirements of J5mm 52~57HRC, J9mm 48~55HRC, J15mm 42~51HRC.

[0056] 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 the automotive industry, especially in new energy vehicle shaft components and other occasions where high-strength and tough round steel is required.

[0057] In order to achieve the above-mentioned purpose, the present invention provides a round steel, which is made of the high hardenability and high strength medium carbon steel mentioned above.

[0058] In the present invention, the high hardenability and high strength medium carbon steel described in the present invention can be specifically prepared into round steel, which can be used in the automotive field, especially in new energy vehicle shaft parts and other occasions where high strength and toughness round steel is required. The size specification diameter range of the round steel can be specifically controlled between Φ20 and 160 mm.

[0059] Furthermore, in the round steel described in the present invention, its diameter is Φ20 to 160 mm.

[0060] 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 toughness and plasticity, but also has high hardenability and high strength, good fatigue resistance, and is easy to cut and process, and has good application prospects.

[0061] 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:

[0062] (1) smelting;

[0063] (2) Casting;

[0064] (3) Heating: Control the heating temperature to 1050-1250°C and the holding time to 3-24h;

[0065] (4) Forging or rolling: Control the final rolling temperature or final forging temperature to be ≥850℃, and cool after rolling or forging.

[0066] In the above technical solution of the present invention, the manufacturing method of the designed round steel includes smelting, casting, heating, forging or rolling steps and necessary finishing processes. Among them, in the smelting process of step (1), the smelting can be vacuum induction furnace smelting, electric furnace smelting or converter smelting, and specifically undergoes refining and vacuum treatment; correspondingly, in the casting process of the above step (2), the casting process can specifically adopt die casting or continuous casting. The ingot obtained by continuous casting needs to be input into a heating furnace for heating, and the heating temperature is controlled to be 1050~1250℃, and the holding time is 3~24h to ensure that the medium carbon steel of the present invention is completely austenitized during the heating process.

[0067] It should be noted that during the heating process of step (3) of the present invention, the medium carbon steel of the present invention is completely austenitized by heating at 1050°C to 1250°C. At the same time, the carbides and nitrides and carbonitrides of Al, Nb, V, and Ti, and the carbides of Cr and Mo can be partially or completely dissolved in the austenite. During the subsequent forging or rolling and cooling process, Al, Nb, V, and Ti can form fine precipitates. Mn, Cr, Ni, Mo, and Cu dissolved in austenite can improve the hardenability of the steel and increase the hardness and strength.

[0068] In addition, it should be noted that in the heating step of step (3) of the present invention, in actual operation, the temperature can be directly raised to the heating temperature or the temperature can be raised to the heating temperature in a stepwise manner. In this heating step, the temperature can be directly raised to the heating temperature or the temperature can be raised to the heating temperature in a stepwise or stepwise manner. For example, in some embodiments, the temperature can be directly raised to 1050-1250°C by a box furnace or a trolley furnace; for example, in some other embodiments, the temperature can be raised to 1050-1250°C by a walking beam heating furnace in a stepwise or stepwise manner.

[0069] Accordingly, in the subsequent forging or rolling step (4), forging or rolling is performed under the condition that the final rolling temperature or the final forging temperature is ≥850°C, and controlled cooling is performed after rolling, so that a refined multi-phase matrix structure with fine dispersed precipitates can be formed.

[0070] Of course, in some preferred embodiments, in order to obtain better implementation effects, the final rolling temperature or the final forging temperature can be further controlled between 850 and 950°C.

[0071] Furthermore, the manufacturing method of the present invention further includes step (5) finishing: normalizing or annealing.

[0072] Furthermore, in the manufacturing method of the present invention, when normalizing is adopted, the normalizing temperature is 860-950°C, and when annealing is adopted, the annealing temperature is 630-720°C.

[0073] In the manufacturing method designed by the present invention, necessary finishing steps such as normalizing and annealing can also be performed. During the finishing process, when using normalizing or annealing heat treatment, the normalizing temperature can be preferably controlled between 860 and 950°C, and the annealing temperature can be controlled between 630 and 720°C during annealing to control the structure and hardness of the steel, thereby improving the uniformity of the steel.

[0074] 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.

[0075] 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 850-950°C.

[0076] Furthermore, in the manufacturing method of the present invention, in step (4), the steel is directly rolled or forged to the finished product size.

[0077] 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 3-12h.

[0078] 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.

[0079] It should be noted that during the rolling process, the rolling process can be selected as a staged rolling method, first rolling to the intermediate billet size, then 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, and the holding time can be specifically controlled between 3 and 12 hours.

[0080] Furthermore, in the manufacturing method of the present invention, in step (4), the cooling rate after rolling or forging is 0.05 to 50° C. / s.

[0081] 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 and the rolling speed and cooling method are adjusted according to the specific application and performance requirements. By controlling the phase transformation process, the ideal microstructure is obtained. The cooling method can specifically adopt air cooling, wind cooling or slow cooling, and the cooling rate after rolling or forging is specifically controlled to be between 0.05-50°C / s, and more preferably between 0.05 and 30°C / s.

[0082] Furthermore, in the manufacturing method described in the present invention, there is also a step (6) of quenching and tempering after step (4) or (5), wherein the quenching temperature ranges from 810 to 950°C, and the tempering temperature ranges from 500 to 650°C.

[0083] Compared with the prior art, the high hardenability and high strength medium carbon steel, round steel and the manufacturing method thereof of the present invention have the following advantages and beneficial effects:

[0084] The present invention develops a medium-carbon steel with high hardenability, high strength 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, which makes the medium-carbon steel easy to cut while obtaining good plasticity.

[0085] 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.

[0086] 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.

[0087] The yield strength R of the medium carbon steel designed by the present invention after quenching and tempering heat treatment is e ≥900MPa, tensile strength R m ≥1050MPa, elongation A≥12%, section shrinkage Z≥50%, Charpy impact energy A kv ≥65J, its hardenability is high and the bandwidth is narrow, among which J5mm is 52~57HRC, J9mm is 48~55HRC, and J15mm is 42~51HRC. It can meet the needs of automobiles and engineering machinery, especially new energy vehicle shaft parts and other occasions that require high hardenability, high strength and toughness round steel, and has very good promotion and application prospects. DETAILED DESCRIPTION

[0088] The high hardenability and high strength medium carbon steel, round steel and the manufacturing method thereof 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.

[0089] Examples 1-6 and Comparative Examples 1-4

[0090] The round steels of Examples 1-6 were all prepared by the following steps:

[0091] (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.

[0092] (2) Casting: Casting is performed by die casting or continuous casting to obtain ingots.

[0093] (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.

[0094] (4) Forging or rolling: the final rolling temperature or final forging temperature is controlled to be ≥850°C, preferably between 850 and 950°C, and cooling is performed after rolling or forging. The cooling method can be air cooling, wind cooling or slow cooling, and the cooling rate after rolling or forging is controlled to be 0.05 to 50°C / s, preferably 0.05 to 30°C / s;

[0095] 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 12 hours.

[0096] It should be noted that, in the present invention, the medium carbon steel corresponding to each embodiment and comparative example can be effectively prepared through the manufacturing process described in the above steps (1) to (4).

[0097] Of course, in some embodiments, the medium carbon steel prepared by forging or rolling can be further subjected to a finishing step of step (5), and the finishing can specifically include surface treatment, non-destructive testing and heat treatment, wherein the heat treatment specifically includes normalizing or annealing heat treatment. When normalizing is adopted, the normalizing temperature can be controlled to be 860-950°C, and when annealing is adopted, the annealing temperature can be controlled to be 630-720°C.

[0098] In addition, it should be noted that we usually test the properties of medium carbon steel after quenching and tempering heat treatment. Therefore, after obtaining the medium carbon steel of the above embodiment and comparative example, step (6) can be further performed, i.e., quenching and tempering heat treatment process, and the quenching temperature is controlled between 810 and 950°C, and the tempering temperature is controlled between 500 and 650°C, so as to test the properties of the medium carbon steel after quenching and tempering heat treatment.

[0099] In the present invention, the round steels prepared in Examples 1-6 are actually prepared from their respective corresponding medium carbon steels. The chemical composition design and related processes of the high hardenability and high strength medium carbon steels in Examples 1-6 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.

[0100] Table 1-1 lists the mass percentages of the chemical elements in the high hardenability and high strength medium carbon steels of Examples 1-6 and the comparative steels of Comparative Examples 1-4.

[0101] Table 1-1. (wt.%, the balance is Fe and other inevitable impurities except P, S, O, H and Ca)

[0102]

[0103]

[0104] Table 1-2 lists the typical alloying element carbon equivalent, hardenability critical ideal diameter DI value, microalloying coefficient r calculated from the mass percentage of each chemical element of the high hardenability and high strength medium carbon steel of Examples 1-6 and the comparative steel of Comparative Examples 1-4. M / N .

[0105] Table 1-2.

[0106] serial number Ceq DI value <![CDATA[Microalloy element coefficient r M / N > Example 1 0.79 146 3.5 Example 2 0.78 123 2.1 Example 3 0.72 91 3.2 Example 4 0.73 101 1.4 Example 5 0.79 109 2.8 Example 6 0.80 144 2.8 Comparative Example 1 0.74 91 3.0 Comparative Example 2 0.70 92 1.0 Comparative Example 3 0.69 77 2.1 Comparative Example 4 0.73 110 3.4

[0107] Note: In the above table,

[0108] r M / N =([Al] / 2+[Nb] / 7+[Ti] / 3.5) / [N];

[0109] Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15;

[0110] “Ceq”, “DI” and “r M / N In the formula of ", each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

[0111] In the present invention, the specific production process of the round steel prepared from the high hardenability and high strength medium carbon steel of Examples 1-6 and the round steel prepared from the comparative steel of Comparative Examples 1-4 is as follows:

[0112] Example 1

[0113] Smelting was performed in a 50kg vacuum induction furnace according to the chemical compositions shown in Tables 1-1 and 1-2. The resulting molten steel was mold-cast into ingots, heated, and then forged. Heating was controlled at 1150°C, held at that temperature for 5 hours, and then forged. The final forging temperature was controlled at 890°C, resulting in bars with a diameter of Φ60mm. After forging, the bars were air-cooled at a cooling rate of 15°C / s.

[0114] Example 2

[0115] 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 1180°C, held for 12 hours, and then forged. The final forging temperature was controlled at 960°C, resulting in Φ90mm bars. The bars were then air-cooled at a controlled cooling rate of 30°C / s. Further finishing was then performed, including heat treatment and surface treatment. Annealing at 700°C (heat treatment) was followed by turning and peeling (surface treatment).

[0116] Example 3

[0117] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in a 500kg vacuum induction furnace. The resulting molten steel was then cast into ingots, heated, and then forged. The heating temperature was controlled at 1080°C, held for 20 hours, and then forged. The final forging temperature was controlled at 980°C, resulting in Φ120mm bars. After forging, the bars were piled and slowly cooled at a controlled cooling rate of 0.1°C / s. Further finishing was then performed, which included heat treatment, specifically annealing at 640°C.

[0118] Example 4

[0119] According to the chemical composition shown in Table 1-1 and Table 1-2, electric furnace smelting is carried out, and refined and vacuum treated are performed, and then continuous casting is carried out into 280mm×280mm continuous casting billets. The continuous casting billets are slowly heated to 1200℃, kept warm for 10 hours, and then rolled. After the billets are removed from the heating furnace and descaled by high-pressure water, rolling begins. The final rolling temperature is controlled to 970℃, and the finished bar specification is Φ160mm. After rolling, air cooling is carried out, and the cooling rate is controlled to 1℃ / s. Then further finishing is carried out, which specifically includes heat treatment, surface treatment and non-destructive testing. After normalizing at 910℃ (heat treatment), turning and peeling (surface treatment) are carried out, and inspections are carried out by ultrasonic testing and magnetic particle testing (non-destructive testing).

[0120] Example 5

[0121] The chemical compositions shown in Tables 1-1 and 1-2 were smelted in an electric furnace, followed by LF refining and VD vacuum treatment, and then continuously cast into 320mm×425mm continuous ingots. After preheating, the ingots were slowly heated to 1230°C and held at this temperature for 6 hours before rolling. After exiting the heating furnace, the ingots were descaled with high-pressure water and then rolled into intermediate bars. The final rolling temperature was controlled at 1020°C, resulting in intermediate bars measuring 140mm×140mm. After rolling, they were air-cooled. The intermediate bars were then slowly heated to 1130°C, held for 3 hours, removed from the furnace, descaled with high-pressure water, and then rolled. The final rolling temperature was controlled at 880°C, resulting in finished bars measuring 25mm in diameter. After rolling, they were air-cooled at a controlled cooling rate of 25°C / s. Finishing was performed after rolling, with normalizing at 870°C. Non-destructive testing was performed using ultrasonic and magnetic particle inspection.

[0122] Example 6

[0123] The chemical compositions shown in Tables 1-1 and 1-2 are smelted in an electric furnace, followed by LF refining and VD vacuum treatment, and then cast into ingots. The ingots are heated to 1230°C, held at this temperature for 12 hours, and then rolled. After exiting the heating furnace and undergoing high-pressure water descaling, rolling begins, resulting in intermediate bars with a final rolling temperature of 1000°C and dimensions of 220 mm x 220 mm. The intermediate bars are then heated to 1150°C, held at this temperature for 10 hours, removed from the furnace, descaled with high-pressure water, and then rolled again. The final rolling temperature is controlled at 920°C, and the finished bars are Ø60 mm in diameter. After rolling, they are air-cooled at a cooling rate of 10°C / s. Further finishing is then performed, which includes surface treatment and non-destructive testing. Specifically, ultrasonic and eddy current testing are used after grinding wheel 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 8 hours before forging. The final forging temperature was controlled at 890°C, resulting in bars with a diameter of 60 mm. After forging, the bars were air-cooled at a controlled cooling rate of 15°C / s.

[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 mold-cast into ingots, heated, and forged to form blanks. The heating temperature was 1180°C, and the ingots were held at that temperature for 20 hours before forging. The final forging temperature was controlled at 960°C, resulting in Φ90mm bars. The bars were then air-cooled at a controlled cooling rate of 30°C / s. Further finishing was then performed, which specifically included heat treatment and surface treatment, specifically annealing at 700°C, followed by turning and peeling.

[0128] Comparative Example 3: Commercially available round steel was selected, and its preparation process is not described in detail here. 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 850°C and tempered at 550°C before its mechanical properties can be tested.

[0129] Comparative Example 4: Commercially available round steel was selected, and its preparation process is not described in detail here. 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 850°C and tempered at 550°C before its mechanical properties can be tested.

[0130] Table 2-1 and Table 2-2 list the specific process parameters of the round steels of Examples 1-6 and Comparative Examples 1-4 in the above-mentioned manufacturing method.

[0131] Table 2-1.

[0132]

[0133]

[0134] Table 2-2.

[0135]

[0136] In the present invention, in the two implementation modes of Example 4 and Example 5, the steel billets are first rolled to their respective designated intermediate billet sizes during the rolling process, and then heated and rolled again to the final finished product size.

[0137] It should be noted that the round steels of the finished products of Examples 1-6 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, and the microstructure can be analyzed with reference to GB / T 13299.

[0138] To analyze austenite grain size, steel samples from each example and comparative example were fully austenitized and then water quenched. Metallographic specimens were prepared according to GB / T 13298. Austenite grain boundaries were visualized using a picric acid alcohol solution, and austenite grain size was analyzed using a metallographic microscope according to ASTM E112. The relevant test and analysis results are listed in Table 3 below.

[0139] Table 3 lists the metallographic analysis results of the round steels of Examples 1-6 and Comparative Examples 1-4.

[0140] Table 3.

[0141]

[0142] As can be seen from Table 3, in the present invention, the microstructures of the round steels of Examples 1-6 all comprised ferrite + pearlite, and the medium carbon steels had fine grains. When the medium carbon steels of Examples 1-6 were austenitized, their austenite grain sizes ranged from 6 to 8, and the austenite grain sizes were all ≥ 6.

[0143] 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 1-6 and Comparative Examples 1-4, respectively, and performed 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 the following Table 4.

[0144] In the present invention, the process parameters of the quenching + tempering heat treatment in each embodiment are as follows:

[0145] Example 1: The mechanical properties of round steel were tested after quenching at 850°C and tempering at 550°C.

[0146] Example 2: The mechanical properties of round steel were tested after quenching at 880°C and tempering at 600°C.

[0147] Example 3: Mechanical properties of round steel were tested after quenching at 910°C and tempering at 630°C.

[0148] Example 4: Mechanical properties of round steel were tested after quenching at 940°C and tempering at 620°C.

[0149] Example 5: Mechanical properties of round steel were tested after quenching at 830°C and tempering at 530°C.

[0150] Example 6: Mechanical properties of round steel were tested after quenching at 870°C and tempering at 580°C.

[0151] Comparative Example 1: The mechanical properties of round steel were tested after quenching at 850°C and tempering at 550°C.

[0152] Comparative Example 2: The mechanical properties of round steel were tested after quenching at 880°C and tempering at 600°C.

[0153] Comparative Example 3: The mechanical properties of round steel were tested after quenching at 850°C and tempering at 550°C.

[0154] Comparative Example 4: The mechanical properties of round steel were tested after quenching at 850°C and tempering at 550°C.

[0155] 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 1-6 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.

[0156] 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. A Rockwell hardness test was performed according to GB / T 230.2 to obtain the hardness value (HRC) at a specific position, such as the hardness at a distance of 5 mm, 9 mm, and 15 mm from the quenched end, i.e., J3mm, J9mm, and J15mm.

[0157] Table 4 lists the test results of mechanical properties, fatigue properties and cutting properties of the round steels of Examples 1-6 and the comparative round steels of Comparative Examples 1-4 obtained after quenching and tempering heat treatment.

[0158] Table 4.

[0159]

[0160] Note: In Table 4 above, two sets of data in a single column represent two test results.

[0161] As can be seen from Tables 3 and 4 above, after the quenching + tempering heat treatment, the high hardenability and high strength medium carbon steels of Examples 1-6 of the present invention have fine grains, high hardenability, and their overall performance is still significantly better than the comparative round steels of Comparative Examples 1-4.

[0162] In the present invention, after completing the quenching + tempering heat treatment, the round steel of Examples 1-6 has a yield strength between 904-1120 MPa, a tensile strength between 1052-1220 MPa, an elongation between 13-17%, a cross-sectional shrinkage between 51.5-61%, and a Charpy impact energy between 68-112 J. It not only has the characteristics of high strength and high hardenability, but also has good impact toughness, plasticity, fatigue resistance and cutting performance.

[0163] In addition, after completing the quenching + tempering heat treatment, the round steel prepared in Example 1-6 has high hardenability and a narrow bandwidth. The critical ideal diameter DI value of the hardenability of Examples 1-6 of the present invention is not less than 90 mm. The measured hardness at a distance J5 mm from the quenching end is between 53 and 57 HRC, the hardness at a distance J9 mm from the quenching end is between 49 and 55 HRC, and the hardness at a distance J15 mm from the quenching end is between 42 and 52 HRC.

[0164] Continuing to refer to Table 1-1, Table 1-2, Table 2-1, Table 2-2, Table 3 and Table 4, it can be seen that in Comparative Examples 1-4, these four comparative examples all have parameters that do not meet the design specification requirements of the present invention in the design process of chemical element composition. In the present invention, the Charpy impact energy of Comparative Example 1 is relatively low; the plasticity of the steel material of Comparative Example 2 is relatively low, and its austenite grain size has a mixed crystal phenomenon. As shown in Table 3, the rating result of Comparative Example 2 is 6 (1), indicating that the average grain size of most areas is level 6, but there are coarse grains in some areas, reaching level 1, resulting in a certain degree of fluctuation in the Charpy impact energy; the carbon content of Comparative Example 3 is relatively low, and its steel strength fails to reach 1050MPa, and its hardenability is relatively low, and J9mm does not meet the requirements of 35CrMoHH in GB / T 5216; Comparative Example 4 contains more microalloying element V, and its steel strength is relatively high, but its toughness is relatively poor, and its Charpy impact energy cannot meet the requirement of 65J, and the fatigue performance of the prepared parts cannot meet the use requirements.

[0165] 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.

[0166] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made therefrom that can be directly derived from or easily conceived by those skilled in the art based on the disclosure of the present invention are intended to fall within the scope of protection of the present invention.

Claims

1. A high hardenability and high strength medium carbon steel, characterized in that: The mass percentage of each chemical element is as follows: C: 0.34-0.42%, Si: 0.05-0.40%, Mn: 0.50-1.00%, Cr: 0.80-1.40%, Mo: 0.10-0.40%, Al: 0.01-0.05%, Nb: 0.002-0.030%, N: 0.002-0.020%; the balance is Fe and other inevitable impurities; Each element also satisfies the following: 1.1≤(Al / 2+Nb / 7+Ti / 3.5) / N≤4.9, where each chemical element is substituted into the value before the percentage sign of the mass percentage content of the chemical element.

2. The high hardenability and high strength medium carbon steel according to claim 1, characterized in that: Among the inevitable impurities, P≤0.020%, S≤0.030%, O≤0.0025%, H≤0.0002%, and Ca≤0.004%.

3. The high hardenability and high strength medium carbon steel according to claim 1, characterized in that: It also contains at least one of the following chemical elements: 0<Ni≤0.30%, 0<Cu≤0.10%, 0<V≤0.10%, 0<Ti≤0.05%.

4. The high hardenability and high strength medium carbon steel according to claim 1, characterized in that: The critical ideal diameter Di value is 85~160mm; 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.

5. The high hardenability and high strength medium carbon steel according to claim 1, characterized in that: Its carbon equivalent Ceq≤0.80%, wherein Ceq=[C]+[Mn] / 6+([Cr]+[Mo]+[V]) / 5+([Ni]+[Cu]) / 15.

6. The high hardenability and high strength medium carbon steel according to claim 1, characterized in that: Its microstructure is ferrite + pearlite.

7. The high hardenability and high strength medium carbon steel according to 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.

8. The high hardenability and high strength medium carbon steel according to claim 1, characterized in that: Its yield strength R after quenching + tempering heat treatment e ≥900MPa, tensile strength R m ≥1050MPa, elongation A≥12%, section shrinkage Z≥50%, Charpy impact energy A kv ≥65J, its hardenability meets the requirements of J5mm 52~57HRC, J9mm 48~55HRC, J15mm 42~51HRC.

9. A round steel, characterized in that: The steel is made of the high-hardenability and high-strength medium-carbon steel as claimed in any one of claims 1 to 8.

10. The round steel according to claim 9, characterized in that Its diameter is Φ20~160mm.

11. The method for manufacturing round steel according to claim 9 or 10, 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 to be ≥850℃, and cool after rolling or forging.

12. The manufacturing method according to claim 11, wherein: It also includes step (5) finishing: normalizing or annealing.

13. The manufacturing method according to claim 12, wherein: When normalizing is adopted, the normalizing temperature is 860-950°C, and when annealing is adopted, the annealing temperature is 630-720°C.

14. The manufacturing method according to claim 11, wherein: In step (4), the final rolling temperature or the final forging temperature is controlled to be 850-950°C.

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

16. The manufacturing method according to claim 11, 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 3-12 hours.

17. The manufacturing method according to claim 11, wherein: In step (4), the cooling rate after rolling or forging is 0.05 to 50°C / s.

18. The manufacturing method according to claim 11 or 12, characterized in that: After step (4) or (5), there is further step (6) of quenching and tempering, wherein the quenching temperature ranges from 810 to 950° C., and the tempering temperature ranges from 500 to 650° C.

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