Energy-saving cold forging steel and manufacturing method thereof

Abstract

The present disclosure relates to a cold forging steel and a manufacturing method thereof. The cold forging steel of the present disclosure comprises the following chemical elements in mass percentage: C: 0.17-0.22%, Si: 0.10-0.30%, Mn: 1.0-1.20%, S: 0.01-0.02%, Cr: 1.10-1.30%, Al: 0.015-0.045%, N: 0.010-0.018%, Ti: 0.04-0.1%, and the balance of Fe and inevitable impurities. The cold forging steel of the present disclosure has excellent plasticity and cold working properties. Compared with conventional cold forging steel materials, the cold forging steel of the present disclosure can effectively save energy and reduce environmental pollution when used to produce forgings (especially high-precision forgings such as gears).

Description

Technical Field

[0001] This invention relates to steel and its manufacturing methods, and more particularly to cold-forged steel and its manufacturing methods. Background Technology

[0002] With the rapid development of the automotive industry and the vast automotive market, the demand for transmissions has remained consistently high. Due to the special shape of gears and the high requirements for dimensional accuracy, many companies use hot forging followed by precision machining for gear production. However, this traditional gear manufacturing process suffers from several drawbacks. Firstly, material utilization is low. Secondly, hot forging requires significant energy, increasing processing costs and causing environmental pollution.

[0003] Therefore, some parts processing companies use cold forging technology to produce gears. Cold forging of parts has many advantages, including high dimensional precision of forgings; less oxide scale formation in forging materials, resulting in better surface finish; high material utilization, reducing cutting waste and heat loss; allowing for specific orientation of metal fiber flow in forgings; eliminating the need for heat treatment, minimizing pollution; and reducing production costs by eliminating heating expenses. Thus, cold forging technology aligns better with future trends in clean manufacturing and environmental protection, creating favorable conditions for sustainable development.

[0004] However, cold forging technology places high demands on the plasticity of materials. Due to the complex shape of gears and the requirement for excellent plasticity in cold forging, insufficient plasticity in the steel often leads to cracking or micro-crack initiation during extrusion, resulting in a high scrap rate and increased inspection costs. Conventional cold forging steel typically undergoes the following treatment: after hot rolling, the material is cooled to room temperature and then spheroidized annealed. Since spheroidizing annealing takes tens of hours, it consumes a significant amount of time and energy.

[0005] Therefore, there is a need in the art to obtain a cold-forged steel with excellent plasticity and cold working properties. Summary of the Invention

[0006] In view of the aforementioned defects and shortcomings of the prior art, the inventors have obtained a cold-forged steel with excellent plasticity and cold working properties through rational chemical composition design. Compared with conventional cold-forged steel, the cold-forged steel of this invention can effectively save energy and reduce environmental pollution when manufacturing forgings (especially high-precision forgings, such as gears).

[0007] In a first aspect, this disclosure provides a cold-forged steel, which, in addition to containing more than 90% Fe and unavoidable impurities, also contains the following chemical elements in a mass percentage:

[0008] C: 0.17~0.22%, Si: 0.10~0.30%, Mn: 1.0~1.20%, S: 0.01~0.02%, Cr: 1.10~1.30%, Al: 0.015~0.045%, N: 0.010~0.018%, Ti: 0.04-0.1%.

[0009] In a second aspect, this disclosure provides a cold-forged steel comprising the following chemical elements in mass percentage:

[0010] C: 0.17–0.22%, Si: 0.10–0.30%, Mn: 1.0–1.20%, S: 0.01–0.02%, Cr: 1.10–1.30%, Al: 0.015–0.045%, N: 0.010–0.018%, Ti: 0.04–0.1%, with the balance being Fe and unavoidable impurities.

[0011] In a preferred embodiment, the cold-forged steel of this disclosure further comprises Ca, wherein the Ca content satisfies: 0 < Ca ≤ 0.005%, preferably 0.001% < Ca ≤ 0.003%.

[0012] In a preferred embodiment, the Mn content in the cold-forged steel of this disclosure is 1.10 to 1.20%.

[0013] In a preferred embodiment, the Cr content in the cold-forged steel of this disclosure is 1.16 to 1.30%.

[0014] In a preferred embodiment, the Ti content in the cold-forged steel of this disclosure is 0.05-0.08%.

[0015] In one embodiment, the unavoidable impurities in the cold-forged steel of this disclosure include P and O. In a preferred embodiment, P ≤ 0.015%, and / or O ≤ 0.003%.

[0016] In a preferred embodiment, the microstructure of the cold-forged steel disclosed herein is ferrite + spheroidal carbides.

[0017] In a preferred embodiment, the spheroidization rate of the cold-forged steel disclosed herein is above 90%.

[0018] The cold-forged steel of this disclosure has mechanical properties that satisfy at least one of the following: yield strength of 220-270 MPa, tensile strength of 430-480 MPa, elongation ≥35%, and reduction of area ≥66%. Preferably, the cold-forged steel of this disclosure has the following mechanical properties: yield strength of 220-270 MPa, tensile strength of 430-480 MPa, elongation ≥35%, and reduction of area ≥66%.

[0019] In a third aspect, this disclosure provides a method for manufacturing the above-mentioned cold-forged steel, comprising the following steps: (1) smelting and casting molten steel to obtain a billet; (2) heating and rolling the billet to obtain an intermediate billet; (3) heating and rolling the intermediate billet to obtain a rolled round bar; (4) cooling the rolled round bar; and (5) annealing.

[0020] In one implementation, in step (1), smelting may be carried out in an electric furnace or a converter; and / or, casting may be carried out in die casting or continuous casting.

[0021] In one embodiment, in step (2), the billet is heated to 1000-1120°C, preferably 1000-1100°C.

[0022] In one embodiment, in step (3), the intermediate billet is heated to 1050-1200°C, preferably 1050-1120°C, and held at that temperature for 5-6 hours.

[0023] In one implementation, in step (3), the deformation amount ξ of a single rolling pass is 20-40%, the final rolling temperature T is 870-930℃, and the deformation amount ξ of a single rolling pass and the final rolling temperature T satisfy: 4≤ln(T-850)-lnξ≤6.

[0024] In one embodiment, in step (4), the rolled round steel is cooled to 370-410°C at a cooling rate of 20°C / s or higher. By using a cooling rate of 20°C / s or higher, martensitic structure can be formed in the matrix, ensuring a large amount of distortion energy in the matrix, thereby providing phase transformation energy for subsequent microstructure transformation, which is beneficial for the precipitation and spheroidization of carbides. In addition, by using a cooling rate of 20°C / s or higher to cool the rolled round steel to 370-410°C, it is possible to ensure that the matrix completes the martensitic transformation and save energy consumption in the subsequent annealing process.

[0025] In one implementation, in step (5), the annealing temperature is 740±10℃ and the holding time is more than 8 hours.

[0026] In one implementation, after the annealing step, the furnace is removed and air-cooled.

[0027] The manufacturing method disclosed herein optimizes the manufacturing process parameters of cold-forged steel, further improving its plasticity. The manufacturing method of this disclosure does not include a spheroidizing annealing step, effectively reducing energy consumption. When processing parts, the cold-forged steel obtained in this way eliminates the need for heating and normalizing steps before hot forging, resulting in shorter processing time and energy savings.

[0028] This disclosure, through rational chemical composition design and optimized manufacturing methods, yields cold-forged steel that not only possesses excellent plasticity and cold-working properties, but also eliminates the need for spheroidizing annealing during production, effectively reducing energy consumption. Furthermore, when manufacturing parts using the cold-forged steel of this disclosure, the heating and normalizing steps before hot forging can be omitted, making it widely applicable and possessing excellent prospects for promotion and application value. Attached Figure Description

[0029] Figure 1 The microstructure of the cold-forged steel of Example 4 is schematically shown under an optical microscope.

[0030] Figure 2 The microstructure of the cold-forged steel in Comparative Example 1 is schematically shown under an optical microscope. Detailed Implementation

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0032] As used herein, the term “and / or” refers to and covers any and all possible combinations of more than one of the listed items.

[0033] In this paper, yield strength is the yield limit of a metallic material when it yields, that is, the stress that resists a small amount of plastic deformation.

[0034] In this paper, tensile strength is the critical value for the transition of a metal from uniform plastic deformation to localized concentrated plastic deformation, and it is also the maximum load-bearing capacity of a metal under static tensile conditions. Tensile strength is the resistance to the maximum uniform plastic deformation of a material.

[0035] In this paper, elongation, which is the percentage of the total deformation ΔL of the gauge length segment after tensile fracture to the original gauge length L: δ=ΔL / L×100%, is an indicator describing the plastic properties of the material.

[0036] In this paper, the reduction of area is the percentage of the maximum reduction in cross-sectional area at the neck after the specimen breaks, relative to the original cross-sectional area.

[0037] In this paper, yield strength, tensile strength, elongation, and reduction of area were determined according to GB / T228-2010.

[0038] In this paper, "tissue spheroidization rate" is determined according to ASTM F2282.

[0039] The design principles of each chemical element in the cold-forged steel disclosed herein are as follows:

[0040] Carbon (C): In the cold-forged steel disclosed herein, carbon (C) is one of the key elements affecting the hardenability of the steel. Adding an appropriate amount of carbon can ensure that the steel has good hardenability and suitable strength, which is beneficial to improving the wear resistance of the final parts processed from the steel. When the carbon content in the steel is too low, it is impossible to guarantee that the steel will obtain high tensile strength, resulting in low structural strength of the gear core, reduced resistance to deformation of the gear, and reduced fatigue life of the gear. However, excessive carbon should not be added to the steel. Increasing the carbon content in the steel will increase the hardness of the material and lead to excessively high material strength during subsequent processing, increasing the wear of the die during cold forging and causing an increase in downstream processing costs. Therefore, in order to achieve narrow hardenability of the steel, the mass percentage of carbon in the cold-forged steel disclosed herein is controlled between 0.17% and 0.22%.

[0041] Silicon (Si): In the cold-forged steel disclosed herein, Si is a ferrite-forming element with strong solid solution strengthening effect, which can effectively improve the strength of the steel. Furthermore, as a deoxidizer, Si can effectively reduce the oxygen content in molten steel. However, it should be noted that the Si content in the steel should not be too high. Excessive Si content will reduce the plasticity of the steel. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Si is controlled between 0.10% and 0.30%.

[0042] Manganese (Mn): In the cold-forged steel disclosed herein, when a certain amount of sulfur (S) is present in the steel, manganese (Mn) readily combines with S to form ductile MnS. During subsequent gear finishing, MnS effectively improves chip breaking and cutting performance. However, it is important to note that 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. Mn is a core element affecting the hardenability of gear steel. Therefore, to improve the material's machinability while avoiding severe segregation and reducing hardenability fluctuations, the mass percentage of Mn in the cold-forged steel disclosed herein is controlled between 1.0% and 1.20%.

[0043] Sulfur (S): In the cold-forged steel of this disclosure, sulfur (S) can combine with manganese (Mn) to form MnS, improving machinability. Adding an appropriate amount of sulfur to the steel can prevent tool sticking during subsequent finishing processes. However, excessive sulfur content can lead to hot brittleness in the steel matrix, causing surface cracks. Therefore, in the cold-forged steel of this disclosure, the mass percentage of sulfur is controlled between 0.01% and 0.02%.

[0044] Cr (chromium): In the cold-forged steel of this disclosure, an appropriate amount of Cr can be added. Cr diffuses slowly in austenite and can hinder the diffusion of C, thus suppressing diffusion-type phase transformation in steel, which is beneficial to austenite stability, 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. When the Cr content in the steel is too high, coarse carbides will form, deteriorating the cold deformation performance. In addition, Cr can also significantly affect the hardenability of gear steel. Therefore, to ensure the performance of the steel, the mass percentage of Cr in the cold-forged steel of this disclosure is controlled between 1.1% and 1.3%.

[0045] Al (aluminum): In the cold-forged steel disclosed herein, Al can effectively reduce the oxygen content in the steel during the steelmaking process. Al can combine with N to form fine AlN precipitates, which are dispersed at the grain boundaries. During subsequent cooling, these precipitates inhibit austenite grain growth, thereby refining the austenite grains and improving the material's plasticity. Simultaneously, the dispersed AlN can act as nucleation sites for carbide precipitation, shortening the incubation period for carbide nucleation and precipitation, reducing annealing time, and saving energy consumption. However, it is important to note that the Al content in the steel should not be too high. Excessive Al content leads to the formation of larger Al oxides, resulting in larger Class B inclusions. These coarse, hard alumina inclusions deteriorate the fatigue properties of the steel and can cause tool breakage during machining. Therefore, to ensure that Al effectively exerts its beneficial effects, the mass percentage of Al in the cold-forged steel disclosed herein is controlled between 0.015% and 0.045%.

[0046] Nitrogen (N): In the cold-forged steel disclosed herein, nitrogen (N) can form AlN or TiN, refining the austenite grains. However, excessively high N content in the steel leads to increased enrichment at defects and the formation of coarse nitride precipitates, affecting the fatigue life of the steel. Therefore, in the cold-forged steel disclosed herein, the mass percentage of N is controlled between 0.010% and 0.018%.

[0047] Ti (Titanium): In the cold-forged steel disclosed herein, Ti can form corresponding compounds with C and N in the steel. The formation temperature of TiN is above 1400℃, and it typically precipitates in the liquid phase or δ-ferrite, thereby refining the austenite grains. However, it is important to note that the Ti content in the steel should not be too high. Excessive Ti content will lead to the formation of coarse TiN precipitates, resulting in reduced fatigue performance of the steel. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Ti is controlled between 0.04% and 0.1%.

[0048] In the cold-forged steel disclosed herein, P and O are unavoidable impurity elements. Where technically permissible, the content of impurity elements in the steel should be controlled to be as low as possible.

[0049] Phosphorus (P): Phosphorus in steel tends to segregate at grain boundaries, reducing the binding energy of these boundaries and worsening the steel's plasticity. P and Fe combine to form a hard and brittle Fe3P phase, causing cold brittleness during cold working and resulting in decreased plasticity. Under impact loads, intergranular fracture occurs, forming large cleavage planes. Therefore, to avoid increased brittleness, the mass percentage of phosphorus in the cold-forged steel disclosed in this invention is controlled to P ≤ 0.015%.

[0050] O (Oxygen): The impurity element O can react with Al and Ti elements in steel to form Al2O3, TiO, etc. Therefore, to ensure the uniformity of the steel's microstructure, the mass percentage of O in the cold-forged steel disclosed herein is controlled to be O ≤ 0.003%.

[0051] In a preferred embodiment, the cold-forged steel of this disclosure further comprises Ca. The Ca element can further improve the properties of the cold-forged steel, and the design principles of its chemical elements are as follows:

[0052] Ca (Calcium): Adding an appropriate amount of Ca to the cold-forged steel disclosed herein can improve the castability of the steel. However, the Ca content in the steel should not be too high. When the Ca content in the steel is too high, large-sized DS inclusions will be generated. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Ca is controlled to be 0 < Ca ≤ 0.005%.

[0053] In the manufacturing method disclosed herein, the manufacturing process parameters for cold-forged steel are optimized. By controlling the process parameters, especially the heat treatment process parameters, the forged or rolled round steel bars are controlled, and then an annealing process is adopted, so that the cold-forged steel matrix obtained by the manufacturing method of this disclosure contains a large amount of ferrite, which effectively ensures the good plasticity of cold-forged steel, eliminates the internal stress of steel, and ensures good microstructure uniformity.

[0054] In one embodiment, in step (4) of the manufacturing method of this disclosure, the rolled round steel is rapidly cooled (at a cooling rate of 20°C or higher), which exceeds the critical cooling rate for martensite, so that the matrix can basically complete the martensitic transformation and provide phase transformation energy for the microstructure transformation in the subsequent annealing process, which is conducive to the precipitation and spheroidization of carbides.

[0055] In one embodiment, the Mf point (the temperature at which the steel is fully "martensitized") of the present invention is about 425°C. By controlling the cooling rate of step (4) to more than 20°C / s and cooling the round steel to 370-410°C, it is possible to ensure that the matrix completes the martensitic transformation and save the heating energy consumption in the subsequent annealing process.

[0056] The cold-forged steel and its manufacturing method disclosed herein have the following advantages and beneficial effects:

[0057] (1) The cold-forged steel disclosed herein appropriately controls the content of P, N and O, ensuring that the cold-forged steel has suitable strength, excellent plasticity and elongation, while effectively saving energy consumption.

[0058] (2) The cold-forged steel disclosed herein has good plasticity and reduction of area at low temperatures, and excellent cold forging performance. The yield strength of the cold-forged steel is 220-270 MPa, the tensile strength is 430-480 MPa, the elongation is ≥35%, the reduction of area is ≥66%, and it has excellent plasticity and cold working characteristics.

[0059] (3) The cold forged steel disclosed herein fully utilizes the influence of various alloying elements on phase transformation and microstructure through reasonable chemical composition design, and is combined with specific controlled rolling and cooling processes to form a uniform ferrite + spherical carbide matrix structure with a spheroidization rate of over 90%.

[0060] (4) The chemical composition and process design of the cold forging steel disclosed herein are reasonable, and its process window is wide, so it can be mass-produced on bar or plate production lines, and has good promotion prospects and application value.

[0061] The present disclosure will now be described in further detail with reference to the accompanying drawings and embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the disclosure.

[0062] Example

[0063] The cold-forged steels of Examples 1-6 and Comparative Examples 1-3 were prepared by using the following steps:

[0064] (1) Smelting and casting the molten steel to obtain a billet: According to the chemical composition shown in Table 1, smelting and casting are carried out in an electric furnace or converter to cast a billet of 320mm*425mm.

[0065] (2) Heating and rolling the billet to obtain an intermediate billet: The billet is hot-charged and sent into the heating furnace, the heating temperature of the billet is controlled at 1000~1100℃, and held for 4 hours; the billet is rolled into an intermediate billet of 215mm*215mm.

[0066] (3) Heating and rolling the intermediate billet to obtain round steel: Heating the intermediate billet to 1050-1120℃ and holding it for 5 hours, then rolling it with a large deformation amount, with a single rolling pass deformation amount of 20-40%, a final rolling temperature of 870-930℃, and a final round steel size of 20-40mm.

[0067] (4) Cooling the round steel: The rolled round steel is cooled by water. The amount of water is controlled according to the final rolling temperature, and the round steel is cooled to 370-410℃ at a cooling rate of more than 20℃ / s.

[0068] (5) Annealing: The round steel is placed in an annealing furnace for annealing at a temperature of 740±10℃ and a holding time of 8 hours. Then it is taken out of the furnace and air-cooled.

[0069] The cold-forged steels of Comparative Examples 4-5 were prepared in basically the same way as those described above, except for the rolling parameters in step 3) and the cooling rate in step 4).

[0070] The cold-forged steel of Comparative Example 6 was prepared by using the following steps:

[0071] According to the chemical composition shown in Table 1, after smelting and casting the molten steel, the conventional continuous furnace spheroidizing annealing process is adopted as follows: the furnace is heated to 600°C, the material is put into the furnace, the temperature is raised to 760°C in about 1 hour, held at 760°C for 1 hour, rapidly heated to 780°C and held for 5 hours, cooled to 740°C in 1 hour, held at 740°C for 2 hours, then cooled to 720°C in 1 hour and held for 6 hours, cooled to 630°C in about 2 hours and held for 2 hours, and then taken out of the furnace.

[0072] Comparative Example 6 is 20CrMnTiH cold-forged steel that has undergone conventional spheroidizing annealing (which takes more than 21 hours).

[0073] The specific composition and process of Examples 1-6 and Comparative Examples 1-6 are shown in Tables 1 and 2.

[0074] Table 1. Mass percentage (wt%) of each chemical element in the cold-forged steels of Examples 1-6 and Comparative Examples 1-6 (balance: Fe and other unavoidable impurities besides P and O)

[0075] serial number C Si Mn P S Cr Ti Al Ca N O Example 1 0.172 0.25 1.12 0.015 0.016 1.16 0.05 0.015 0.005 0.0118 0.0030 Example 2 0.183 0.24 1.04 0.011 0.018 1.29 0.08 0.019 0.003 0.0154 0.0013 Example 3 0.194 0.11 1.20 0.009 0.016 1.21 0.1 0.033 0.002 0.0105 0.0022 Example 4 0.219 0.3 1.09 0.007 0.013 1.10 0.072 0.036 0.003 0.017 0.0025 Example 5 0.198 0.25 1.17 0.006 0.02 1.23 0.06 0.045 0.002 0.0175 0.0012 Example 6 0.206 0.17 1.01 0.005 0.01 1.18 0.092 0.024 0.001 0.016 0.0016 Comparative Example 1 0.212 0.23 <![CDATA[ 1.45 ]]> 0.008 0.017 1.24 0.054 0.026 0.003 0.0124 0.0014 Comparative Example 2 0.203 0.24 1.15 0.009 0.014 <![CDATA[ 1.38 ]]> 0.08 0.024 0.002 0.0116 0.0013 Comparative Example 3 0.204 0.22 1.14 0.007 0.016 1.19 <![CDATA[ 0.3 ]]> 0.021 0.003 0.0132 0.0015 Comparative Example 4 0.193 0.19 1.12 0.008 0.017 1.18 0.05 0.029 0.003 0.0128 0.0016 Comparative Example 5 0.197 0.20 1.07 0.013 0.013 1.23 0.07 0.034 0.002 0.0143 0.0018 Comparative Example 6 0.187 0.27 1.08 0.013 0.014 1.17 0.04 0.026 0.004 0.0111 0.0009

[0076]

[0077] Samples of the cold-forged steels from Examples 1-6 and Comparative Examples 1-6 were taken and subjected to various performance tests. The performance test results are listed in Table 3. The spheroidization rate of the microstructure was determined according to the standard ASTM F2282. The relevant mechanical property test methods are as follows: Under normal temperature conditions, the cold-forged steel samples were machined into M16*128 threaded specimens, and tensile tests were performed according to the standard GB / T228-2002 to obtain data such as yield strength, tensile strength, elongation, and reduction of area.

[0078] Table 3. Mechanical properties of cold-forged steels from Examples 1-6 and Comparative Examples 1-6

[0079]

[0080] *20CrMnTiH steel produced using conventional spheroidizing annealing process.

[0081] As shown in Table 3, the cold-forged steels of Examples 1-6 possess excellent comprehensive mechanical properties. The yield strength of each example ranges from 225-266 MPa, the tensile strength from 433-479 MPa, the elongation ≥35%, the reduction of area ≥66%, and the spheroidization rate of the microstructure exceeds 90%. The cold-forged steels of Examples 1-6 exhibit excellent mechanical properties, good plasticity and reduction of area at low temperatures, and excellent cold-working characteristics.

[0082] The inventors unexpectedly discovered that, when using the manufacturing method disclosed herein, the residual heat of the steel can be fully utilized after rolling, requiring only about 8 hours of annealing softening time to achieve the same material properties as steel that would otherwise require conventional spheroidizing annealing (over 20 hours). This manufacturing method not only effectively reduces energy consumption in material production but also significantly improves production efficiency.

[0083] Figure 1 The microstructure of the cold-forged steel in Example 4 is shown under an optical microscope.

[0084] Figure 2 The microstructure of the cold-forged steel in Comparative Example 1 is shown under an optical microscope.

[0085] Combination Figure 1 and Figure 2 It can be seen that the microstructure of the cold-forged steel in Example 4 is ferrite + spheroidal carbides, which meets the microstructure requirements of conventional spheroidizing annealing.

[0086] All publications, patent applications, patents and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

[0087] While this disclosure 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 disclosure in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of this disclosure to these descriptions. Various changes in form and detail can be made by those skilled in the art, including some simple deductions or substitutions, without departing from the spirit and scope of this disclosure.

Claims

1. A cold-forged steel, characterized in that, The cold-forged steel contains the following chemical elements by mass percentage: C: 0.17~0.22%, Si: 0.10~0.30%, Mn: 1.0~1.20%, S: 0.01~0.02%, Cr: 1.10~1.30%, Al: 0.015~0.045%, N: 0.010~0.018%, Ti: 0.04-0.1%, with the balance being Fe and unavoidable impurities; The microstructure of the cold-forged steel is ferrite + spheroidal carbides, and the spheroidization rate of the cold-forged steel is over 90%. The manufacturing method of the cold forged steel includes the following steps: (1) smelting and casting molten steel to obtain a billet; (2) heating and rolling the billet to obtain an intermediate billet; (3) heating and rolling the intermediate billet to obtain a rolled round steel; (4) cooling the rolled round steel, wherein the rolled round steel is cooled to 370-410℃ at a cooling rate of 20℃ / s or higher; (5) annealing, wherein the annealing temperature is 740±10℃ and the holding time is 8 hours or more.

2. The cold-forged steel as described in claim 1, characterized in that, The cold-forged steel also contains Ca, and the content of Ca by mass percentage satisfies: 0 < Ca ≤ 0.005%.

3. The cold-forged steel as described in claim 1, characterized in that, The cold-forged steel also contains Ca, and the Ca content, by mass percentage, satisfies the following: 0.001% <Ca≤0.003%。 4. The cold-forged steel as described in claim 1, characterized in that, The content of Mn is 1.10~1.20%, the content of Cr is 1.16~1.30%, and / or the content of Ti is 0.05~0.08%.

5. The cold-forged steel as described in claim 1, characterized in that, The unavoidable impurities, expressed as a percentage by mass, satisfy the following conditions: P ≤ 0.015% and / or O ≤ 0.003%.

6. The cold-forged steel as described in claim 1, characterized in that, The mechanical properties of the cold-forged steel meet at least one of the following: yield strength of 220-270 MPa, tensile strength of 430-480 MPa, elongation ≥35%, and reduction of area ≥66%.

7. The cold-forged steel as described in claim 1, characterized in that, In step (2), the billet is heated to 1000-1120℃.

8. The cold-forged steel as described in claim 7, characterized in that, In step (2), the billet is heated to a temperature of 1000-1100°C.

9. The cold-forged steel as described in claim 1, characterized in that, In step (3), the intermediate billet is heated to 1050-1200℃ and held for 5-6 hours.

10. The cold-forged steel as described in claim 9, characterized in that, In step (3), the intermediate billet is heated to 1050-1120 ℃ and held for 5-6 hours.

11. The cold-forged steel as described in claim 1, characterized in that, In step (3), the deformation amount ξ of a single rolling pass is 20-40%, the final rolling temperature T is 870-930℃, and the deformation amount ξ of a single rolling pass and the final rolling temperature T satisfy: 4≤ln(T-850)-lnξ≤6.

12. The cold-forged steel as described in claim 1, characterized in that, The method for manufacturing cold-forged steel includes step (6) after step (5) of air cooling after exiting the furnace.

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