Cold forged steel and manufacturing method thereof

Abstract

The invention discloses cold forging steel which comprises the following chemical elements in percentage by mass: 0.18%-0.23% of C, 0.15%-0.35% of Si, 0.40%-0.70% of Mn, 0.80%-1.10% of Cr, 0.15%-0.25% of Mo, 0.005%-0.025% of S, 0.015%-0.045% of Al, 0.007%-0.0260% of N and the balance of Fe and inevitable impurities, the mass percent ratio of Al to N is larger than 2, and the manufacturing process of the cold forging steel does not comprise spheroidizing annealing after rolling. In the production of the cold forged steel, a spheroidizing annealing process is avoided, so that the energy consumption can be effectively reduced, the production cost can be greatly reduced, the cold forged steel can be ensured to have excellent plasticity and processability, and the cold forged steel is suitable for wide production and application. The invention further discloses a manufacturing method of the cold forging steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of metal materials, in particular to a cold forging steel and a manufacturing method thereof. BACKGROUND

[0002] With the acceleration of the development of green and low-carbon manufacturing industry, steel as an important source of carbon emissions has attracted much attention in the manufacturing industry. Energy-saving and green development of steel products and parts processing has gradually become an important development trend in the future. Steel products are gradually shifting from conventional long process and extensive production to short process and energy-efficient production. Some parts processing is shifting from hot processing to cold processing. More than 50% of parts in Japan are processed by cold forging. Cold forging has many advantages: (1) the material of the forged part is not easy to produce an oxide skin, and the surface finish is good; (2) the precision of the forged part size is good; (3) work hardening (strain hardening) occurs during processing, which can increase the strength and hardness; (4) the metal fiber flow direction of the forged part can be obtained in a specific direction; (5) no heating treatment is required, which minimizes pollution; (6) no heating costs are required, which reduces production costs. Therefore, cold forging technology is more in line with the future trend of clean manufacturing and green environmental protection, and can create favorable conditions for sustainable development.

[0003] However, cold forging technology requires high plasticity of the material, and the shape of the gear is relatively complex. Cold forging requires the material to have excellent plasticity. Often, due to insufficient plasticity of the steel, cracking or micro-crack initiation often occurs during extrusion, resulting in high scrap rate of parts after processing and rising detection costs. Conventional cold forging steel is usually hot-rolled, cooled to room temperature, and finally spheroidized annealed. Spheroidizing annealing is a kind of annealing that makes carbides in steel spheroidized. The spheroidized structure is more plastic than the flaky structure. However, spheroidizing annealing requires ten to dozens of hours, which inevitably consumes a lot of time and energy.

[0004] Therefore, it is desirable to obtain a steel material with excellent plasticity and cold working properties, and its production process can effectively save energy and reduce environmental pollution compared with conventional cold forging materials.

[0005] Further, it is desirable to provide a manufacturing method of cold forging steel, which can facilitate the automation of cold forging steel manufacturing and improve the consistency of the product. SUMMARY

[0006] To solve the above technical problems, the embodiment of the present application discloses a cold forging steel, which comprises the following chemical elements in percentage by mass: C: 0.18-0.23%, Si: 0.15-0.35%, Mn: 0.40-0.70%, Cr: 0.80-1.10%, Mo: 0.15-0.25%, S: 0.005-0.025%, Al: 0.015-0.045%, N: 0.007-0.0160%, and the rest is Fe and inevitable impurities, wherein the mass percentage ratio of Al to N is > 2, and the manufacturing process of the cold forging steel does not include spheroidizing annealing after rolling.

[0007] By adopting the above technical scheme, the spheroidizing annealing process can be avoided in the production of the cold forging steel, the energy consumption can be effectively reduced, the production cost can be greatly reduced, the cold forging steel can have excellent plasticity, the cold forging steel has very wide applicability, and the cold forging steel has good popularization prospect and application value.

[0008] Optionally, the cold forging steel further comprises P≤0.020%, and / or O≤0.0030%.

[0009] Optionally, the cold forging steel further comprises 0

[0010] Optionally, the microstructure of the cold forging steel is ferrite and spherical carbide.

[0011] Optionally, the yield strength of the cold forging steel is not more than 350 MPa, the tensile strength is not more than 480 MPa, the elongation is ≥38%, and the reduction of area is ≥68%.

[0012] Optionally, the cold forging steel is a cold forging round steel, and the diameter of the cold forging round steel is 5-50 mm.

[0013] According to another specific embodiment of the present application, the embodiment of the present application discloses a manufacturing method of a cold forging steel, and the manufacturing method does not include spheroidizing annealing, and the manufacturing method comprises the following steps:

[0014] Smelting and casting: smelting is performed according to the element mass percentage, and then casting is performed, so as to obtain a casting blank;

[0015] Heating and holding: the casting blank is heated, the heating temperature is 1100±30℃, and the holding time is 3-5 h;

[0016] Rolling: the finish rolling temperature T c =T1+30-T1+80℃, wherein T1=890-222C-28Mn+28Si+278P+72Mo℃;

[0017] Cooling after rolling: the cooling speed is ≥V min , V min= 10 9.81-7.62C-1.40Mn-0.65Cr-0.00183Tc / 3600℃ / s, cooling to T2 below, T2 = 404-423C-30Mn-12Cr-11.0Si℃;

[0018] Annealing: annealing temperature is T3-5℃~T3+5℃, T3 = 751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃, holding time is 1.5~2.5h, then cooling to T3-45℃~T3-35℃, holding for 5~7h, then furnace is discharged and air cooling is performed;

[0019] Wherein, C, Mn, Si, P, Mo, Cr, Si respectively represent mass percentage value of C, Mn, Si, P, Mo, Cr, Si.

[0020] By using the technical scheme, the automation level of the cold forging steel production can be improved, the energy consumption of the production process is reduced, the processing performance of the cold forging steel is ensured, and the cold forging steel has good large-scale application prospect.

[0021] Optionally, the cast blank is rolled into round steel in the rolling step, and the diameter of the round steel is 5~50mm.

[0022] Optionally, the rolling step comprises: primary rolling: the cast blank is rolled into intermediate square billet with a diameter of 160mm~220mm, the finish rolling temperature of the intermediate square billet is controlled to be 850~900℃, and then the intermediate square billet is heated to 1150~1200℃ and held for 4~6h; secondary rolling: the intermediate square billet is rolled into round steel, and the finish rolling temperature T c = T1+30~T1+80℃, and the diameter of the round steel is 5~50mm. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 Microstructure photo of the cold forging steel of Example 4 under an optical microscope;

[0024] Figure 2 Microstructure photo of the cold forging steel of Comparative Example 1 under an optical microscope. DETAILED DESCRIPTION

[0025] The following detailed description together with the accompanying drawings will provide those skilled in the art with further understanding of the nature and advantages of the present application. Although the description conveys a thorough overview of the present application, the application can only be defined by the appended claims. For the purposes of its verification, numerous specific details are set forth in the following descriptions in order to provide a thorough understanding thereof. These details are not intended to limit the present application, but rather to give an accurate description of the application. In order to avoid obscuring the present application, some well-known methods, structures and devices are not described in detail. Moreover, the description is only one possible implementation of the present application. Other implementations could be constructed without departing from the scope of the application.

[0026] It should be noted that in this specification, similar reference numbers and letters represent similar items in the following drawings, thus, once an item is defined in one drawing, it does not need to be further defined and explained in the subsequent drawings.

[0027] In order to make the objects, technical solutions and advantages of the present application clearer, the embodiments of the present application will be further described in detail below with reference to the accompanying drawings.

[0028] The first aspect of the present application discloses a cold forging steel, which comprises the following chemical elements in percentage by mass:

[0029] C: 0.18-0.23%, Si: 0.15-0.35%, Mn: 0.40-0.70%, Cr: 0.80-1.10%, Mo: 0.15-0.25%, S: 0.005-0.025%, Al: 0.015-0.045%, N: 0.007-0.0160%, and the rest is Fe and inevitable impurities, wherein the mass percentage ratio of Al to N is >2, and the manufacturing process of the cold forging steel does not include spheroidizing annealing after rolling.

[0030] The inventors have found that by designing the element ratio of the cold forging steel to contain various components in the above ranges and ensuring that the mass percentage ratio of Al to N is >2, spheroidizing annealing can be avoided in the manufacturing process, and the performance of the cold forging steel can also be ensured. Since the spheroidizing annealing process of several hours or even dozens of hours is eliminated, the energy consumed in the manufacturing of the cold forging steel can be greatly reduced, which is beneficial to reducing production costs and improving production efficiency, and creates conditions for large-scale manufacturing and application of the cold forging steel.

[0031] Further, in the cold forging steel disclosed in the present application, the mass percentage of each chemical element is as follows in percentage by mass:

[0032] C: 0.18–0.23%, Si: 0.15–0.35%, Mn: 0.40–0.70%, Cr: 0.80–1.10%, Mo: 0.15–0.25%, S: 0.005–0.025%, Al: 0.015–0.045%, N: 0.007–0.0160%, with the balance being Fe and other unavoidable impurity elements, and also satisfying Al / N > 2.

[0033] Specifically, the cold forged steel in this invention is a CrMo series cold forged steel, more specifically 20CrMoH cold forged steel.

[0034] The design principles of each chemical element in the cold-forged steel described in this invention are as follows:

[0035] C: In the cold-forged steel described in this invention, C ensures that the steel has good hardenability and appropriate strength, which is beneficial to improving the wear resistance of the final parts processed from the steel. However, excessive C should not be added to the steel. Increasing the C content in the steel will increase the hardness of the material and lead to excessively high strength during subsequent processing, increasing the wear of the dies during cold forging and causing higher downstream processing costs. Therefore, in the cold-forged steel described in this invention, the mass percentage of C is controlled between 0.18% and 0.23%.

[0036] Si: In the cold-forged steel described in this invention, Si is a ferrite-forming element with a strong solid solution strengthening effect, which can effectively improve the strength of the steel. However, it should be noted that the Si content in the steel should not be too high, as excessive Si content will reduce the plasticity of the steel. Therefore, in the cold-forged steel described in this invention, the mass percentage of Si is controlled between 0.15% and 0.35%.

[0037] Mn: In the cold-forged steel described in this invention, when a certain amount of sulfur (S) is present in the steel, Mn readily forms ductile MnS with S, which effectively improves chip breaking and cutting performance during subsequent gear finishing. However, the Mn content in the steel should not be too high. Excessive Mn content leads to increased segregation in the steel, which is detrimental to the uniformity of the material's microstructure. In the cold-forged steel described in this invention, the mass percentage of Mn is controlled between 0.40% and 0.70%.

[0038] S: In the cold-forged steel described in this invention, sulfur (S) can combine with manganese (Mn) to form MnS, improving cutting performance. Adding an appropriate amount of sulfur to the steel can prevent tool sticking during subsequent finishing processes. Therefore, in the cold-forged steel described in this invention, the mass percentage of sulfur is controlled between 0.005% and 0.025%.

[0039] Cr: In the cold-forged steel described in the present invention, an appropriate amount of Cr element can be added to the steel. The diffusion rate of Cr element in austenite is relatively small, and it can hinder the diffusion of C. It can inhibit the diffusion-type phase transformation of the steel, is beneficial to the stability of austenite, shifts the C curve of the steel to the right, and reduces the critical cooling rate. However, it should be noted that the Cr content in the steel should not be too high, otherwise coarse carbides will be formed, deteriorating the cold deformation performance. In the cold-forged steel described in the present invention, the mass percentage of Cr element is controlled between 0.80% and 1.10%.

[0040] Mo: The carbides of Mo are stable and not easy to grow, can refine the grains, and improve the tempering stability of the steel. However, the alloy cost of Mo element is relatively high, which will increase the material cost. Therefore, in the gear steel described in the present invention, the mass percentage of Mo is controlled between 0.15% and 0.25%.

[0041] Al: In the cold-forged steel described in the present invention, Al element can effectively reduce the oxygen content in the steel during the steelmaking process. Al element can form fine AlN compounds and precipitate, refine austenite grains, and improve the plasticity of the material. However, it should be noted that the content of Al element in the steel should not be too high. When the content of Al element in the steel is too high, large Al oxides will be formed, and large-sized class B inclusions will be formed, deteriorating the fatigue performance of the steel. Based on this, in the cold-forged steel described in the present invention, the mass percentage of Al element is controlled between 0.015% and 0.045%.

[0042] N: In the cold-forged steel described in the present invention, although N element can form AlN or TiN in the steel, thereby playing a role in refining austenite grains, the increase in the content of N element in the steel will lead to an increase in its enrichment amount at defects, and at the same time, coarse nitride precipitation particles will be formed, affecting the fatigue life of the steel. Based on this, in the cold-forged steel described in the present invention, the mass percentage of N element is controlled between 0.007% and 0.0160%.

[0043] Moreover, the design of Al and N elements also has a cooperative relationship. Specifically, the mass percentage ratio of Al to N > 2. The inventor found that within this range, it can contribute to the precipitation of AlN compounds, further refine austenite grains, and contribute to the enhancement of plasticity. Preferably, 2 < the mass percentage ratio of Al to N < 4. At this time, the O content in the steel grade will be effectively controlled, and the number of precipitated AlN compounds is appropriate, and the fine grain effect is better.

[0044] Furthermore, the cold-forged steel also includes P ≤ 0.020%, and / or, O ≤ 0.0030%.

[0045] In the above-described technical solution of this invention, both P and O are unavoidable impurity elements in the cold-forged steel described herein. Where technical conditions permit, the content of impurity elements in the steel should be controlled to be as low as possible. P and O have the following effects:

[0046] P: Phosphorus (P) in steel tends to segregate at grain boundaries, reducing the binding energy of these boundaries and worsening the steel's plasticity. P combines with Fe to form the hard and brittle Fe3P phase, causing cold brittleness during cold working, leading to decreased plasticity and intergranular fracture under impact loads, resulting in larger cleavage planes. Therefore, to avoid increased brittleness, the mass percentage of P in the cold-forged steel described in this invention can be controlled to P ≤ 0.020%.

[0047] O: Impurity element O can form Al2O3 and other compounds with Al in steel. Therefore, in order to ensure the uniformity of steel structure, the mass percentage of O in the cold forged steel described in this invention can be controlled to be O≤0.0030%.

[0048] In a specific embodiment of the present invention, the cold-forged steel may further comprise 0 < Ca ≤ 0.005%, which has the following effect:

[0049] Ca: In the cold-forged steel described in this invention, adding an appropriate amount of Ca can improve the castability of the molten steel. However, the Ca content in the steel should not be too high, as this will result in large-sized DS inclusions. Therefore, in the cold-forged steel described in this invention, the mass percentage of Ca can be controlled to be 0 < Ca ≤ 0.005%.

[0050] In one specific embodiment of the present invention, the microstructure of cold-forged steel is ferrite and spherical carbides.

[0051] This invention, through rational chemical composition design, fully utilizes the influence of various alloying elements on phase transformation and microstructure to form a uniform ferrite + spherical carbide matrix structure, which helps to ensure the various properties of cold-forged steel. This invention also effectively controls the content of P, N and O, ensuring that the obtained cold-forged gear steel has suitable strength, excellent plasticity and elongation, while effectively saving energy consumption.

[0052] In a specific embodiment of the present invention, the electrical energy consumed by the heat treatment after rolling in the manufacturing process of cold-forged steel is 186-195 kW*h, and the heat treatment includes annealing and heat holding after rolling. The electrical energy consumed by the cold-forged steel of the present invention during the heat treatment after rolling is much less than that consumed by conventional spheroidizing annealing, effectively saving energy, reducing environmental pollution, and also reducing production costs, which is beneficial to the production and application of cold-forged steel.

[0053] The cold-forged gear steel of this invention has good plasticity and reduction of area at low temperatures, and excellent cold forging performance. The yield strength of the cold-forged gear steel does not exceed 350MPa, the tensile strength does not exceed 480MPa, the elongation is ≥38%, and the reduction of area is ≥68%, exhibiting excellent plasticity and cold working characteristics.

[0054] In a specific embodiment of the present invention, the cold-forged steel is a cold-forged round steel with a diameter of 5 to 50 mm, which is suitable for various application scenarios.

[0055] A second aspect of the present invention discloses a method for manufacturing cold-forged steel, which does not include spheroidizing annealing and includes the following steps:

[0056] Smelting and casting: Smelting is carried out according to the mass percentage of elements, followed by casting to obtain a billet;

[0057] Heating and holding: The billet is heated to 1100±30℃, that is, the heating temperature is 1070℃~1130℃, and held for 3~5 hours;

[0058] Rolling: Control the finishing rolling temperature Tc=T1+30~T1+80℃, where T1=890-222C-28Mn+28Si+278P+72Mo℃;

[0059] Post-rolling cooling: Cooling rate ≥ V min V min =10 9.81-7.62C-1.40Mn-0.65Cr-0.00183Tc / 3600℃ / s, cooled to below T2, T2=404-423C-30Mn-12Cr-11.0Si℃;

[0060] Annealing: The annealing temperature is T3-5℃~T3+5℃, T3=751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃, the holding time is 1.5~2.5h, then the temperature is lowered to T3-45℃~T3-35℃, the holding time is 5~7h, and then the furnace is removed and air-cooled.

[0061] Wherein, C, Mn, Si, P, Mo, Cr, and Si represent the mass percentage values ​​of C, Mn, Si, P, Mo, Cr, and Si, respectively.

[0062] Specifically, in the smelting and casting steps, smelting includes smelting in an electric furnace or converter, and casting yields a billet. The billet can be a large square billet of 320mm*425mm, or a square billet of 160-220mm (i.e., the side length of the square billet is 160mm-220mm).

[0063] Specifically, if the billet is directly made into a 160-220mm square billet during the smelting and casting steps, it can be directly rolled into round steel later. If the billet is made into a 320mm*425mm large square billet during the smelting and casting steps, it can be rolled into a 160-220mm intermediate square billet first, and then rolled into round steel later.

[0064] In a specific embodiment of the present invention, in the heating and holding step, the billet is hot-charged and sent into the heating furnace, the billet is heated to 1100±30℃, and held for 3 to 5 hours, specifically for 4 hours.

[0065] In a specific embodiment of the present invention, in the rolling step, specifically, the cast billet can be directly rolled into round bars, and the finishing rolling temperature T of the round bars is controlled. c =T1+30~T1+80℃, where T1=890-222C-28Mn+28Si+278P+72Mo℃, and the diameter of the round steel is 5~50mm.

[0066] In a specific embodiment of the present invention, the rolling process can be divided into a primary rolling process and a secondary rolling process. In the primary rolling process, the cast billet is first rolled into a 160-220mm intermediate square billet, and the rolling temperature of the intermediate square billet is 850-900°C. The intermediate square billet is then heated to 1150-1200°C and held at that temperature for 4-6 hours. Subsequently, in the secondary rolling process, the aforementioned intermediate square billet is rolled into round steel, and the finishing rolling temperature T of the round steel is controlled. c =T1+30~T1+80℃, where T1=890-222C-28Mn+28Si+278P+72Mo℃, and the diameter of the round steel is 5~50mm.

[0067] Steel can be produced relatively quickly through a single rolling process, offering greater cost-effectiveness. When there are high requirements for the quality of the produced steel, a combination of primary and secondary rolling processes, along with precise control of the intermediate billet final rolling temperature and the round bar finishing rolling temperature, can more accurately ensure the quality of the steel during production, ultimately yielding high-quality steel.

[0068] In the post-rolling cooling step, specifically, the rolled cold-forged steel is cooled by water, with the water volume controlled and adjusted according to the finishing rolling temperature T. c Control the water volume; the cooling rate should not be less than V. min (V min =10 9.81-7.62C-1.40Mn-0.65Cr-0.00183TcThe round steel is cooled to below T2 (T2 = 404-423C-30Mn-12Cr-11.0Si℃) at 3600℃ / s. This rapid cooling of the rolled cold-forged steel, achieved by matching the cooling rate with the final cooling temperature, reduces the nucleation and growth time of cementite, effectively preventing the growth of cementite lamellars. Simultaneously, the solid solution of C ensures a large amount of distortion energy in the matrix, providing phase transformation energy for microstructure transformation during subsequent annealing, which is beneficial for carbide precipitation and spheroidization.

[0069] Subsequently, in the annealing step, specifically, the cold-forged steel is placed in an annealing furnace and heated to T3 (T3 = 751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃) temperature (temperature fluctuation ±5℃), and held for 1.5 to 2.5 hours, for example, 2 hours. Then, it is cooled to T3-40℃ (temperature fluctuation ±5℃) and held for 5 to 7 hours, for example, 6 hours, and then removed from the furnace and air-cooled.

[0070] In this production process, no spheroidizing annealing is used. The inventors, by designing the relevant temperatures during cooling and annealing, and coordinating cooling rates and holding times, utilize the phase transformation temperature point of the steel grade after rolling to maximize the uniform solid solution of carbon in the matrix. This ensures the uniformity of the subsequent spheroidized structure and reduces the time required for cementite growth-disconnection-diffusion-spheroidization, allowing direct carbide precipitation and growth followed by spheroidization, thus saving time during annealing. Furthermore, utilizing the residual heat after rolling reduces energy consumption during heat treatment, achieving energy saving and consumption reduction.

[0071] In this invention, based on the design of chemical composition, the forged or rolled steel is controlled by controlling process conditions, especially heat treatment parameters, as well as the cooling and annealing processes. This results in a ferrite + spheroidal carbide matrix structure in the cold-forged steel produced by the manufacturing method described in this invention. The matrix contains a large amount of ferrite, effectively ensuring good plasticity, eliminating internal stress, and exhibiting good structural uniformity. Through the comprehensive design of chemical composition, microstructure, and process, the cold-forged steel of this invention is obtained at a lower cost. The total annealing time does not exceed 10 hours, and the electrical energy consumed in the annealing step is 186-195 kW*h, ensuring a green production process, saving resources, and guaranteeing processing performance.

[0072] In addition, considering the need for further automation and mass production in cold forging steel production, the inventors designed T1 as T1 = 890 - 222C - 28Mn + 28Si + 278P + 72Mo℃, V min =10 9.81-7.62C-1.40Mn-0.65Cr-0.00183T1 / 3600℃ / s, T2=404-423C-30Mn-12Cr-11.0Si℃, T3=751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃. This design comprehensively considers the chemical composition and phase transformation temperature of cold-forged steel. Only the elemental percentages of the cold-forged steel need to be substituted into the formula for calculation, and the results can be obtained from T1, T2, T3, and V. min This ensures the necessary phase transformation and microstructure growth of the matrix during cold forging, thereby guaranteeing the plasticity and strength properties of the cold forged steel. The process window is relatively flexible. For example, T1 is related to the recrystallization process; as long as the finishing rolling temperature T is guaranteed... C Meeting the T1+30~T1+80℃ temperature range ensures the grain refinement process of recrystallization in the steel. The T1 setting ensures smooth recrystallization under large deformation conditions during rolling, effectively refining the grains while guaranteeing sufficient driving energy for recrystallization. Similarly, the T2 setting allows for rapid cooling of the steel, preventing the formation of lamellar pearlite in the pearlite phase transformation region and maintaining a large amount of stress distortion energy in the microstructure, providing driving energy for subsequent phase transformations. Furthermore, the T3 setting allows supersaturated C to precipitate uniform spherical particles in the matrix, further achieving the goal of microstructural spheroidization. The various process parameters, such as T1, T2, and T3, are design choices made by the inventors after fully considering the changes in the internal microstructure of the steel during production. Moreover, the design and selection of each parameter represent a holistic and comprehensive consideration, ensuring coordination and cooperation throughout the entire production process to ultimately produce steel that meets the requirements. Furthermore, the production process can be controlled using computer programs. By simply inputting the percentage of each component, the production process can be controlled, and high-performance and stable steel can be obtained. This ensures the automation, batch production, and stability of the process, and has good prospects for implementation.

[0073] The present invention will now be described in conjunction with more specific embodiments.

[0074] Examples 1-8 are cold-forged round steel bars, prepared using the following steps:

[0075] (1) Smelting and casting: Smelting is carried out in an electric furnace or converter according to the chemical element mass percentages shown in Table 1, and then cast into a 320mm*425mm large square billet;

[0076] (2) Heating and holding: The billet is hot-charged and sent into the heating furnace. The billet is heated to 1100±30℃ and held for 4 hours.

[0077] (3) Rolling:

[0078] One-time rolling: Roll the billet into an intermediate square billet of 160mm to 220mm, control the final rolling temperature of the intermediate square billet to 850 to 900℃, heat the intermediate square billet to 1150 to 1200℃, and hold for 5 hours;

[0079] Secondary rolling: The intermediate square billet is subjected to continuous rolling with large deformation, and the finishing rolling temperature is T. c The temperature is controlled at Tc = T1 + 30 ~ T1 + 80℃, where T1 = 890 - 222C - 28Mn + 28Si + 278P + 72Mo℃, and the steel is rolled into round bars with a diameter of 5 ~ 50 mm.

[0080] (4) Post-rolling cooling: The round steel is cooled by water, with the water volume controlled and the cooling rate ≥ V. min V min =10 9.81 -7.62C-1.40Mn-0.65Cr-0.00183T1 At 3600℃ / s, the round steel is cooled to below T2, where T2 = 404-423C-30Mn-12Cr-11.0Si℃;

[0081] (5) Annealing: Place the round steel in the annealing furnace and anneal at a temperature of T3-5℃~T3+5℃, where T3=751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃. Hold for 2 hours, then cool down to T3-45℃~T3-35℃ and hold for 6 hours. Finally, remove from the furnace and air cool.

[0082] Alternatively, it can be prepared using the following steps:

[0083] (1) Smelting and casting: Smelting is carried out in an electric furnace or converter according to the mass percentage of chemical elements shown in Table 1, and then cast into 160mm to 220mm square billets;

[0084] (2) Heating and holding: The billet is hot-charged and sent into the heating furnace. The billet is heated to 1100±30℃ and held for 4 hours.

[0085] (3) Rolling: Finishing temperature T c The temperature is controlled at Tc = T1 + 30 ~ T1 + 80℃, and T1 = 890 - 222C - 28Mn + 28Si + 278P + 72Mo℃. The billet is rolled into round steel with a diameter of 5 ~ 50 mm.

[0086] (4) Post-rolling cooling: The round steel is cooled by water, with the water volume controlled and the cooling rate ≥ V. min V min =10 9.81 -7.62C-1.40Mn-0.65Cr-0.00183T1 At 3600℃ / s, the round steel is cooled to below T2, where T2 = 404-423C-30Mn-12Cr-11.0Si℃;

[0087] (5) Annealing: Place the round steel in the annealing furnace and anneal at a temperature of T3-5℃~T3+5℃, where T3=751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃. Hold for 2 hours, then cool down to T3-45℃~T3-35℃ and hold for 6 hours. Finally, remove from the furnace and air cool.

[0088] Table 1 lists the mass percentage of each chemical element in the cold-forged steels of Examples 1-8.

[0089] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, N and O)

[0090] Number C Si Mn P S Cr Mo Ca Al N O Al / N Example 1 0.18 0.25 0.40 0.012 0.01 1.02 0.15 0.005 0.025 0.0108 0.0009 2.31 Example 2 0.203 0.35 0.7 0.007 0.024 1.1 0.17 0.003 0.015 0.007 0.0023 2.14 Example 3 0.193 0.15 0.59 0.019 0.008 0.92 0.18 0.002 0.03 0.0124 0.0012 2.42 Example 4 0.206 0.22 0.65 0.009 0.005 0.86 0.25 0.003 0.019 0.0094 0.0025 2.02 Example 5 0.218 0.31 0.5 0.011 0.025 1.07 0.24 0.002 0.04 0.016 0.0021 2.50 Example 6 0.23 0.21 0.61 0.004 0.016 0.8 0.21 0.001 0.034 0.015 0.0016 2.27 Example 7 0.198 0.19 0.54 0.015 0.009 0.97 0.19 0.003 0.045 0.0154 0.0025 2.92 Example 8 0.186 0.26 0.48 0.013 0.012 0.89 0.23 0.005 0.025 0.011 0.003 2.27 Comparative Example 1 0.203 0.35 0.7 0.007 0.024 1.1 0.17 0.003 0.015 0.008 0.0023 2.14

[0091] Table 2 lists the specific process parameters for the smelting and casting steps and the rolling steps of the cold-forged steel in Examples 1 to 8.

[0092] Table 2.

[0093]

[0094] The process parameters for controlled rolling and controlled cooling in Examples 1 to 8 are shown in Table 3. Comparative Example 1 is the conventional spheroidizing process for 20CrMoH cold forging steel.

[0095] Table 3.

[0096]

[0097] Note: T1 = 890 - 222C - 28Mn + 28Si + 278P + 72Mo ℃

[0098] V min =10 9.81-7.62C-1.40Mn-0.65Cr-0.00183Tc / 3600℃ / s

[0099] T2=414-389C-28Mn-31Cr-24Si-15Mo℃

[0100] T3=771-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃

[0101] Table 4 shows the energy consumption of post-rolling heat treatment. As can be seen from Table 4, using the same production equipment (continuous roller hearth annealing furnace), under different production processes, the energy consumption per ton of steel for post-rolling heat treatment in Examples 1-8 is 65 KW*h lower than that of the conventional comparative example 1 (converted from natural gas consumption to electricity consumption), effectively reducing the energy consumption of post-rolling heat treatment of cold forged steel and achieving the goal of energy saving and consumption reduction.

[0102] Table 4.

[0103]

[0104] The cold-forged steels of Examples 1-8 were subjected to various performance tests, and the test results are listed in Table 5. The comparative example is 20CrMoH cold-forged material that has undergone conventional spheroidizing annealing (21 hours).

[0105] Table 5 lists the mechanical property test results of the cold-forged steels in Examples 1 to 8.

[0106] Table 5.

[0107]

[0108] As shown in Table 5, the cold-forged steels of Examples 1-8 of this invention possess excellent mechanical properties. The yield strength of each example does not exceed 350 MPa, the tensile strength does not exceed 480 MPa, the elongation is ≥38%, and the reduction of area is ≥68%. The production of this steel can effectively reduce energy consumption. Simultaneously, the cold-forged steels of each example exhibit excellent mechanical properties, possessing good plasticity and reduction of area, comparable to the strength and plasticity of conventional spheroidized annealed cold-forged steel, and exhibiting excellent cold-working characteristics. In particular, while ensuring the performance of the cold-forged steel, the manufacturing time and energy consumption can be significantly reduced, greatly improving production efficiency. Example 2, in particular, possesses good yield strength, tensile strength, elongation, and reduction of area, demonstrating good overall performance and meeting the application requirements of cold heading steel. Furthermore, the energy consumption per ton of steel after heat treatment after rolling is relatively low, resulting in high economic benefits.

[0109] Furthermore, taking Example 4 as an example, combined with Figure 1 As can be seen, in the cold-forged steel of Example 4 of the present invention, the microstructure of the cold-forged steel is spherical carbides and ferrite. The microstructure of Comparative Example 1 is as follows: Figure 2 As shown.

[0110] In summary, this invention, through rational chemical composition design, fully utilizes the influence of various alloying elements on phase transformation and microstructure, and combines this with specific heat treatment processes to form a uniform ferrite + spherical carbide matrix structure. Simultaneously, this invention effectively controls the content of P, N, and O, effectively ensuring that the steel possesses suitable strength, excellent plasticity, and elongation. The cold-forged round steel described in this invention exhibits good plasticity and reduction of area at low temperatures, demonstrating excellent cold-forging performance. The yield strength of this cold-forged round steel does not exceed 350 MPa, the tensile strength does not exceed 480 MPa, the elongation is ≥38%, and the reduction of area is ≥68%, exhibiting excellent plasticity and cold-working characteristics.

[0111] The cold-forged steel described in this invention features a rational chemical composition and process design, enabling the production of steel with excellent processing properties without spheroidizing annealing, thus significantly reducing production energy consumption. Furthermore, it offers a wide process window, a high degree of automation, and can achieve mass commercial production on bar or plate production lines, exhibiting high consistency and demonstrating promising prospects and application value.

[0112] While the present invention has been illustrated and 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 cold-forged steel, characterized in that, The cold-forged steel comprises the following chemical elements by weight percentage: C: 0.18–0.23%, Si: 0.15–0.35%, Mn: 0.40–0.70%, Cr: 0.80–1.10%, Mo: 0.15–0.25%, S: 0.005–0.025%, Al: 0.015–0.045%, N: 0.007–0.0160%, with the remainder being Fe and unavoidable impurities. The mass percentage ratio of Al to N is >2. The manufacturing process of the cold-forged steel does not include spheroidizing annealing after rolling.

2. The cold-forged steel as described in claim 1, characterized in that, The cold-forged steel also includes P ≤ 0.020%, and / or O ≤ 0.0030%.

3. The cold-forged steel as described in claim 1, characterized in that, The cold-forged steel also includes 0 < Ca ≤ 0.005%.

4. The cold-forged steel as described in claim 1, characterized in that, The microstructure of the cold-forged steel consists of ferrite and spheroidal carbides.

5. The cold-forged steel as described in claim 1, characterized in that, The cold-forged steel has a yield strength not exceeding 350 MPa, a tensile strength not exceeding 480 MPa, an elongation ≥38%, and a reduction of area ≥68%.

6. The cold-forged steel as described in claim 1, characterized in that, The cold-forged steel is a cold-forged round bar with a diameter of 5 to 50 mm.

7. A method for manufacturing cold-forged steel according to any one of claims 1 to 6, characterized in that, The manufacturing method does not include spheroidizing annealing, and the manufacturing method includes the following steps: Smelting and casting: Smelting is carried out according to the mass percentage of elements, followed by casting to obtain a billet; Heating and holding: The billet is heated to 1100±30℃ and held for 3 to 5 hours; Rolling: Controlling the finishing rolling temperature T c =T1+30~T1+80℃, where T1=890-222C-28Mn+28Si+278P+72Mo℃; Post-rolling cooling: Cooling rate ≥ V min V min =10 9.81-7.62C-1.40Mn-0.65Cr-0.00183T1 / 3600℃ / s, cooled to below T2, T2=404-423C-30Mn-12Cr-11.0Si℃; Annealing: The annealing temperature is T3-5℃~T3+5℃, T3=751-16.3C-27.5Mn+35Si+13Cr+3.4Mo℃, and the temperature is held for 1.5~2.5h. Then the temperature is lowered to T3-45℃~T3-35℃ and held for 5~7h. Then the furnace is removed and air-cooled. Wherein, C, Mn, Si, P, Mo, Cr, and Si represent the mass percentage values ​​of C, Mn, Si, P, Mo, Cr, and Si, respectively.

8. The method for manufacturing cold-forged steel as described in claim 7, characterized in that, In the rolling step, the billet is rolled into round bars with a diameter of 5 to 50 mm.

9. The method for manufacturing cold-forged steel as described in claim 7, characterized in that, The rolling step includes: One-time rolling: The billet is rolled into an intermediate square billet of 160mm to 220mm. The final rolling temperature of the intermediate square billet is controlled at 850 to 900℃. Then the intermediate square billet is heated to 1150 to 1200℃ and held for 4 to 6 hours. Secondary rolling: The intermediate square billet is rolled into round steel, and the finishing rolling temperature T is controlled. c =T1+30~T1+80℃, and the diameter of the round steel is 5~50mm.

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