Energy-saving cold forging steel and its manufacturing method

Abstract

Cold forging steel and its manufacturing method. The cold forging steel contains the following chemical elements in the following mass percentages: C: 0.170-0.220%, Si: 0.10-0.30%, Mn: 1.00-1.20%, S: 0.010-0.020%, Cr: 1.10-1.30%, Al: 0.015-0.045%, N: 0.0100-0.0180%, and Ti: 0.040-0.100%, with the balance being Fe and unavoidable impurities. The cold forging steel has excellent plasticity and cold working properties. Compared with conventional cold forging steels, the use of this cold forging steel to manufacture forged parts (especially high-precision forged parts such as gears) can effectively save energy and reduce environmental pollution.

Description

[Technical Field]

[0001] The present invention relates to a steel and a method for producing the same, in particular to a steel for cold forging and a method for producing the same. [Background technology]

[0002] With the rapid development of the automotive industry, the demand for transmissions in the vast automotive market remains high. Due to the high requirements for special gear shapes and dimensional accuracy, many companies manufacture gears using hot forging followed by precision machining. However, when producing gears using this traditional gear manufacturing process, on the one hand, material utilization rate is low. On the other hand, hot forging requires high energy consumption, which increases processing costs and causes environmental pollution.

[0003] Therefore, some parts processing companies are using cold forging technology to manufacture gears. Cold forging of parts has many advantages, including high dimensional accuracy of forged parts, reduced surface film formation and good surface finish, high material utilization rate due to reduced cutting debris and heating loss, the ability to control metal fiber flow in a specific direction, no need for heat treatment, minimized pollution problems, and reduced production costs due to the elimination of heating costs. Cold forging technology is more in line with future trends in clean manufacturing and environmental protection, and can create favorable conditions for sustainable development.

[0004] However, cold forging technology places very high demands on the plasticity of the material. Gear shapes are relatively complex, requiring the material to have excellent plasticity for cold forging. Insufficient steel plasticity often leads to cracks or microcracks in the steel during extrusion, resulting in a high reject rate for processed parts and increased testing costs. Conventional cold forging steel typically undergoes the following process: hot rolling, cooling to room temperature, and finally spheroidizing annealing. Spheroidizing annealing can take more than 10 hours, or even several tens of hours, consuming a lot of time and energy.

[0005] Therefore, there is a demand in the art for an energy-saving cold forging steel having excellent plasticity and cold working properties.

Summary of the Invention

Problems to be Solved by the Invention

[0006] In view of the above-mentioned drawbacks and deficiencies of the prior art, the inventors have obtained a cold forging steel having excellent plasticity and cold working properties with a reasonable chemical composition. Compared with the conventional cold forging steel, by using the cold forging steel of the present invention in the manufacture of forged parts (especially high-precision forged parts such as gears), energy can be effectively saved and environmental pollution can be reduced.

[0007] In a first aspect of the present disclosure, a cold forging steel is provided, the cold forging steel contains 90% or more of Fe and inevitable impurities, and further contains chemical elements in the following mass percentages: C: 0.170 - 0.220%, Si: 0.10 - 0.30%, Mn: 1.00 - 1.20%, S: 0.010 - 0.020%, Cr: 1.10 - 1.30%, Al: 0.015 - 0.045%, N: 0.0100 - 0.0180%, and Ti: 0.040 - 0.100%.

[0008] In a second aspect of the present disclosure, a cold forging steel is provided that contains chemical elements in the following mass percentages: C: 0.170 - 0.220%, Si: 0.10 - 0.30%, Mn: 1.00 - 1.20%, S: 0.010 - 0.020%, Cr: 1.10 - 1.30%, Al: 0.015 - 0.045%, N: 0.0100 - 0.0180%, and Ti: 0.040 - 0.100%, the balance being Fe and inevitable impurities.

[0009] In a preferred embodiment, the cold forging steel of the present disclosure further contains Ca, and the content of Ca satisfies 0 < Ca ≤ 0.005%, preferably 0.001% < Ca ≤ 0.003%.

[0010] In a preferred embodiment, the content of Mn in the cold forging steel of the present disclosure is 1.10 - 1.20%.

[0011] In a preferred embodiment, the Cr content in the cold forging steel of the present disclosure is 1.16 to 1.30%.

[0012] In a preferred embodiment, the content of Ti in the cold forging steel of the present disclosure is 0.050 to 0.080%.

[0013] In one embodiment, the unavoidable impurities in the cold forging steel of the present disclosure include P and O. In a preferred embodiment, P≦0.015% and / or O≦0.0030%.

[0014] In a preferred embodiment, the structure of the cold forging steel of the present disclosure is ferrite + spheroidal carbides.

[0015] In a preferred embodiment, the spheroidization rate of the structure of the cold forging steel of the present disclosure is 90% or more.

[0016] The mechanical properties of the cold forging steel of the present disclosure satisfy at least one of the following: yield strength 220 to 270 MPa, tensile strength 430 to 480 MPa, elongation ≧ 35%, and area reduction ≧ 66%. Preferably, the mechanical properties of the cold forging steel of the present disclosure satisfy the following: yield strength 220 to 270 MPa, tensile strength 430 to 480 MPa, elongation ≧ 35%, and area reduction ≧ 66%.

[0017] In a third aspect of the present invention, there is provided a method for producing the above-mentioned steel for cold forging, comprising the following steps: (1) a step of obtaining a slab by melting and casting molten steel; (2) a step of obtaining an intermediate material by heating and rolling the slab; (3) a step of obtaining a rolled round steel by heating and rolling the intermediate material; (4) a step of cooling the rolled round steel; and (5) an annealing step.

[0018] In one embodiment, in step (1), the melting can be performed by electric furnace melting or converter melting; and / or the casting can be performed by die casting or continuous casting.

[0019] In one embodiment, in step (2), the cast slab is heated to 1000 to 1120°C, preferably 1000 to 1100°C.

[0020] In one embodiment, in step (3), the intermediate material is heated to 1050 to 1200°C, preferably 1050 to 1120°C, and held at that temperature for 5 to 6 hours.

[0021] In one embodiment, in step (3), the deformation amount (ξ) per rolling pass is 20 to 40%, the final rolling temperature (T) is 870 to 930°C, and the relationship between the deformation amount (ξ) per rolling pass and the final rolling temperature (T) satisfies 4.00≦ln(T−850)−lnξ≦6.00, preferably 4.05≦ln(T−850)−lnξ≦5.95.

[0022] 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 more (e.g., 20-30°C / s). Using a cooling rate of 20°C / s or more allows the formation of a martensitic structure in the matrix, ensuring the presence of a large amount of strain energy in the matrix, thereby providing transformation energy for the subsequent structural transformation, which is beneficial for carbide precipitation and spheroidization. Furthermore, cooling the rolled round steel to 370-410°C at a cooling rate of 20°C / s or more not only ensures that the matrix completes the martensitic transformation, but also reduces energy consumption in the subsequent annealing step.

[0023] In one embodiment, in step (5), the annealing temperature is 740±10° C. and the holding time is 8 hours or more.

[0024] In one embodiment, after the annealing step, the annealed round steel bars are removed from the furnace and air cooled.

[0025] In the manufacturing method disclosed herein, the manufacturing process parameters of cold forging steel are optimized to further improve the plasticity of the cold forging steel. Since the manufacturing method disclosed herein does not include a spheroidizing annealing process, energy consumption can be effectively reduced. When cold forging steel obtained by this method is used to fabricate parts, the heating and normalizing processes prior to hot forging of the parts can be omitted, thereby saving time and energy.

[0026] The cold forging steel obtained by the rational design of chemical composition and the optimized manufacturing method according to the present disclosure not only achieves excellent plasticity and cold workability, but also eliminates the need for spheroidizing annealing in its manufacturing process, thereby effectively reducing energy consumption. Furthermore, when manufacturing parts using the cold forging steel of the present disclosure, the heating and normalizing processes prior to hot forging of the parts can be omitted. The present disclosure has wide applicability, excellent popularity, and application value. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a schematic diagram showing the structure of the cold forging steel of Example 4 under an optical microscope. [Figure 2] FIG. 2 is a schematic diagram showing the structure of the cold forging steel of Comparative Example 1 under an optical microscope. DETAILED DESCRIPTION OF THE INVENTION

[0028] 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 belongs.

[0029] As used herein, the term "and / or" refers to and includes any and all combinations of one or more of the listed items.

[0030] In this specification, the term "yield strength" refers to the yield limit when a metallic material yields, that is, the stress that resists slight plastic deformation.

[0031] As used herein, "tensile strength" refers to the critical value at which a metal transitions from uniform plastic deformation to localized plastic deformation. It is also the maximum load-bearing capacity of a metal under static tension. Tensile strength indicates a material's resistance to maximum uniform plastic deformation.

[0032] In this specification, "elongation" refers to the percentage of the total deformation (ΔL) of the gauge section after tensile fracture relative to the original gauge length (L) of the material, and is expressed as δ = ΔL / L × 100%, and is an index of the plasticity performance of a material.

[0033] In this specification, the term "reduction of area" refers to the percentage of the maximum reduction in cross-sectional area at the necked portion after the test specimen breaks, relative to the original cross-sectional area of the test specimen.

[0034] In this specification, yield strength, tensile strength, elongation, and reduction in area are measured in accordance with GB / T228-2010.

[0035] As used herein, the "spheroidization rate of tissue" is measured in accordance with ASTM F2282.

[0036] In the cold forging steel of the present disclosure, the design principles of each chemical element will be specifically described below.

[0037] C (Carbon): In the cold forging steel of the present disclosure, carbon is one of the important elements that affects the hardenability of the steel. Adding an appropriate amount of carbon ensures that the steel has good hardenability and adequate strength, which is beneficial for improving the wear resistance of the final part. If the carbon content in the steel is too low, it is not possible to ensure that the steel achieves high tensile strength, resulting in reduced structural strength of the gear core, reduced gear deformation resistance, and a shortened gear fatigue life. However, adding excessive carbon to the steel is undesirable. Increasing the carbon content in the steel increases the hardness of the material, causing the material strength to be too high for subsequent processing, increasing die wear during the cold forging process, and increasing downstream processing costs. Therefore, to achieve a narrow range of hardenability, the mass percentage of carbon in the cold forging steel of the present disclosure is controlled between 0.170 and 0.220%.

[0038] Si (Silicon): In the cold forging steel of the present disclosure, Si is a ferrite-forming element and has a strong solid solution strengthening effect, which can effectively improve the strength of the steel. Furthermore, Si as a deoxidizer can also effectively reduce the oxygen content in molten steel. However, it should be noted that the content of Si in the steel should not be too high. If the content of Si in the steel is too high, the plasticity of the steel will decrease. Therefore, the mass percentage of Si in the cold forging steel of the present disclosure is controlled to be between 0.10 and 0.30%.

[0039] Mn (Manganese): In the cold forging steel of the present disclosure, when a certain amount of S is present in the steel, Mn can easily combine with S to form plastic MnS. In the subsequent gear finishing process, MnS can effectively promote chip breaking and improve cutting performance. However, it should be noted that the Mn content in the steel should not be too high. Too high a Mn content in the steel increases segregation, which is detrimental to the uniformity of the material's structure. Mn is the central element affecting the hardenability of gear steel. Therefore, to improve the material's cutting performance while avoiding severe segregation and reducing hardenability variations, the mass percentage of Mn in the cold forging steel of the present disclosure is controlled between 1.00 and 1.20%.

[0040] S (sulfur): In the cold forging steel of the present disclosure, the S element can form MnS with the Mn element to improve cutting performance. Adding an appropriate amount of S element to steel can prevent tool adhesion during subsequent finishing processes. However, if the S element content is too high, it can cause high-temperature embrittlement of the steel matrix and lead to cracks on the steel surface. Therefore, the mass percentage of S element in the cold forging steel of the present disclosure is controlled to be between 0.010 and 0.020%.

[0041] Cr (Chromium): An appropriate amount of Cr can be added to the cold forging steel of the present disclosure. The diffusion rate of Cr in austenite is relatively slow, preventing the diffusion of C. This inhibits the diffusion-based phase transformation of the steel, favors 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. If the Cr content in the steel is too high, coarse carbides will form, deteriorating the cold deformation performance. Furthermore, the Cr element can have a relatively large effect on the hardenability of the gear steel. Therefore, to ensure the performance of the steel, the mass percentage of Cr in the cold forging steel of the present disclosure is controlled to be between 1.10 and 1.30%.

[0042] Al (Aluminum): In the cold forging 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 dispersed at grain boundaries, which inhibits austenite grain growth during the subsequent cooling process, thereby refining the austenite grains and improving the plasticity of the material. At the same time, the dispersed AlN can act as carbide precipitation nuclei, shortening the incubation period for carbide nucleation and precipitation, reducing annealing time, and saving energy consumption. However, it should be noted that the Al content in the steel should not be too high. If the Al content in the steel is too high, large Al oxides will form, which will lead to the formation of coarse B-type inclusions. These hard coarse aluminum oxide inclusions deteriorate the fatigue performance of the steel and cause tool damage during machining. Therefore, in order to allow the Al element to effectively exert its beneficial effects, the mass percentage of the Al element in the cold forging steel of the present disclosure is controlled to be between 0.015 and 0.045%.

[0043] N (nitrogen): In the cold forging steel of the present disclosure, the N element can form AlN or TiN in the steel, which plays a role in refining austenite grains. However, if the N content in the steel is too high, the tendency for nitrogen to concentrate in defects increases, and at the same time, coarse nitride precipitates are formed, which adversely affects the fatigue life of the steel. Therefore, in the cold forging steel of the present disclosure, the mass percentage of the N element is controlled to be between 0.0100 and 0.0180%, preferably between 0.0100 and 0.0175%.

[0044] Ti (Titanium): In the cold forging steel of the present disclosure, the Ti element can form a corresponding compound with the C and N in the steel. The formation temperature of TiN is 1400°C or higher, and TiN usually precipitates from the liquid phase or δ-ferrite, thereby having the effect of refining austenite grains. However, it should be noted that the Ti content in the steel should not be too high. If the Ti content in the steel is too high, coarse TiN precipitates will form, thereby reducing the fatigue performance of the steel. Therefore, the mass percentage of the Ti element in the cold forging steel of the present disclosure is controlled to be between 0.040 and 0.100%, preferably between 0.050 and 0.100%.

[0045] In the cold forging steel of the present disclosure, P and O are unavoidable impurity elements. As long as technical conditions permit, the content of impurity elements in the steel should be controlled as low as possible.

[0046] P (phosphorus): The P element in steel tends to accumulate at grain boundaries, reducing the binding energy of the grain boundaries and worsening the plasticity of the steel. P and Fe combine to form a hard and brittle FeP phase, which makes the steel cold brittle during cold working and reduces the plasticity of the steel. When the steel is subjected to an impact load, grain boundary fracture occurs and large cleavage planes are formed. Therefore, to avoid increasing the brittleness of the steel, the mass percentage of the P element in the cold forging steel of the present disclosure is controlled to P≦0.015%.

[0047] O (oxygen): The impurity element O can combine with Al and Ti elements in steel to form Al2O3, TiO, etc. Therefore, to ensure the uniformity of the steel structure, the mass percentage of O in the cold forging steel of the present disclosure is controlled to O≦0.0030%.

[0048] In a preferred embodiment, the cold forging steel of the present disclosure also contains Ca. The Ca element can further improve the performance of the cold forging steel, and the design principle of this chemical element is as follows:

[0049] Ca (Calcium): By adding an appropriate amount of Ca element to the cold forging steel of the present disclosure, the casting property of molten steel can be improved. However, the Ca content in the steel should not be too high. If the Ca content in the steel is too high, large-sized DS inclusions will be generated. Therefore, the mass percentage of Ca element in the cold forging steel of the present disclosure is controlled to be 0 < Ca ≤ 0.005%, preferably 0.001% < Ca ≤ 0.003%.

[0050] In the manufacturing method of the present disclosure, the manufacturing process parameters of the cold forging steel are optimized. By controlling the process parameters, especially the heat treatment process parameters, the forged or rolled round steel bar is controlled, and then, using the annealing process, the matrix of the cold forging steel obtained by the manufacturing method of the present disclosure contains a large amount of ferrite, thereby effectively guaranteeing the good plasticity of the cold forging steel, eliminating the internal stress of the steel, and guaranteeing the good tissue uniformity.

[0051] In one embodiment, in step (4) of the manufacturing method of the present disclosure, the rolled round steel is rapidly cooled (cooling rate of 20 °C / s or more). Since the cooling rate exceeds the critical cooling rate of martensite transformation, the matrix basically completes the martensite transformation, providing the phase transformation energy for the tissue transformation in the subsequent annealing process, which promotes the precipitation and spheroidization of carbides.

[0052] In one embodiment, the Mf point (the temperature at which the steel is completely transformed into martensite) of the steel of the present invention is about 425 °C. By controlling the cooling rate in step (4) to be 20 °C / s or more and cooling the round steel to 370 - 410 °C, it can be guaranteed that the matrix completes the martensite transformation, and the heating energy consumption in the subsequent annealing process can be saved.

[0053] The cold forging steel and its manufacturing method of the present disclosure have the following advantages and beneficial effects.

[0054] (1) The cold forging steel of the present disclosure appropriately controls the contents of P, N, and O, ensuring that the cold forging steel has appropriate strength and excellent plasticity and elongation, while effectively saving energy consumption.

[0055] (2) The cold forging steel of the present disclosure has good plasticity and area reduction at low temperatures, and has excellent cold forging performance. The cold forging steel has a yield strength of 220 to 270 MPa, a tensile strength of 430 to 480 MPa, an elongation of 35% or more, and an area reduction of 66% or more. The cold forging steel has excellent plasticity and cold working properties.

[0056] (3) The cold forging steel of the present disclosure utilizes a rational chemical composition design to fully utilize the effects of various alloying elements on phase transformation and structure, and by incorporating specific controlled rolling and controlled cooling processes, forms a uniform ferrite + spheroidal carbide matrix structure with a structure spheroidization rate of 90% or more.

[0057] (4) The chemical composition and process design of the cold forging steel disclosed herein are reasonable, and its process window is wide, allowing mass production on a bar or plate production line, with excellent widespread applicability and application value.

[0058] The present disclosure will be described in more detail below with reference to the figures and examples. The following examples are used only to illustrate the present disclosure and are not intended to limit the scope of the present disclosure.

[0059] Example According to the chemical compositions shown in Table 1, cold forging steels of Examples 1 to 6 and Comparative Examples 1 to 3 were prepared using the following steps:

[0060] (1) The process of melting and casting molten steel to obtain slabs: The slabs are melted using an electric furnace or converter and cast into slabs measuring 320mm x 425mm.

[0061] (2) The process of heating and rolling the slab to obtain intermediate materials: The slab is hot charged into a heating furnace, and the heating temperature of the slab is controlled to 1000-1100°C and held for 4 hours; the slab is rolled into an intermediate material of 215mm x 215mm.

[0062] (3) The process of heating and rolling the intermediate material to obtain round steel bars: The intermediate material is heated to 1050-1120°C, held for 5 hours, and then rolled with a large deformation amount. The deformation amount per rolling pass is 20-40%, the final rolling temperature is 870-930°C, and the final size of the round steel bars is 20-40 mm (diameter).

[0063] (4) Cooling process of round steel: The rolled round steel is cooled with water, controlling the amount of water depending on the final rolling temperature, and the round steel is cooled to 370-410°C at a cooling rate of 20°C / s or more.

[0064] (5) Annealing process: The round steel is placed in an annealing furnace and annealed at a temperature of 740±10°C for 8 hours. The annealed round steel is then removed from the furnace and air-cooled.

[0065] The cold forging steels of Comparative Examples 4 and 5 were prepared by the same method as above, but with the following differences: rolling parameters in step 3) and cooling rate in step 4).

[0066] The cold forging steel of Comparative Example 6 was prepared by the following steps:

[0067] After melting and casting the liquid steel, a conventional continuous furnace spheroidizing annealing process was used as follows: the furnace was heated to 600°C, the material was charged into the furnace, the temperature was increased to 760°C in approximately 1 hour, held at 760°C for 1 hour, then rapidly increased to 780°C and held for 5 hours, the temperature was reduced to 740°C within 1 hour, held at 740°C for 2 hours, then reduced to 720°C within 1 hour and held for 6 hours, the temperature was reduced to 630°C in approximately 2 hours and held for 2 hours before being removed from the furnace.

[0068] Comparative Example 6 is 20CrMnTiH cold forging steel that has been subjected to conventional spheroidizing annealing (requiring 21 hours or more).

[0069] Tables 1 and 2 show the specific compositions and processes of the cold forging steels of Examples 1 to 6 and Comparative Examples 1 to 6.

[0070] [Table 1]

[0071] [Table 2]

[0072] Samples of the cold forging steels of Examples 1 to 6 and Comparative Examples 1 to 6 were collected and subjected to various performance tests. The results of the performance tests are shown in Table 3. The spheroidization rate of the structure was measured in accordance with ASTM F2282. The relevant test methods for the mechanical properties are as follows:

[0073] Under room temperature conditions, the cold forging steel samples were processed into M16*128 threaded test pieces, and tensile tests were carried out according to the GB / T228-2002 standard to obtain data such as yield strength, tensile strength, elongation, and reduction of area.

[0074] [Table 3]

[0075] As can be seen from Table 3, the cold forging steels of Examples 1 to 6 have excellent overall mechanical properties. The yield strength of each Example is between 225 and 266 MPa, the tensile strength is between 433 and 479 MPa, the elongation is ≥ 35%, the reduction in area is ≥ 66%, and the spheroidization rate of the structure is 90% or more. The cold forging steels of Examples 1 to 6 have excellent mechanical properties, good plasticity and reduction in area at low temperatures, and excellent cold working properties.

[0076] The inventors have surprisingly found that the manufacturing method of the present disclosure makes it possible to fully utilize the residual heat of steel after rolling. To achieve material performance that would normally require conventional spheroidizing annealing (which requires 20 hours or more), only about 8 hours of annealing and softening time is required. The manufacturing method of the present disclosure not only effectively reduces energy consumption in material manufacturing, but also effectively improves production efficiency.

[0077] FIG. 1 shows the structure of the cold forging steel of Example 4 under an optical microscope.

[0078] FIG. 2 shows the structure of the cold forging steel of Comparative Example 1 under an optical microscope.

[0079] 1 and 2, it can be seen that the structure of the cold forging steel of Example 4 is ferrite + spheroidal carbides, and satisfies the structural requirements of conventional spheroidizing annealing.

[0080] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

[0081] Although the present disclosure has been illustrated and described with reference to certain preferred embodiments thereof, those skilled in the art should understand that the above is a more detailed description of the present disclosure according to certain embodiments and should not be construed as limiting the present disclosure to these descriptions. Those skilled in the art can make various changes in form and details, including making some simple inferences or substitutions, without departing from the spirit and scope of the present disclosure.

Claims

1. Cold forging steel containing 90% or more Fe and unavoidable impurities, as well as the following mass percentages of chemical elements: C: 0.170-0.220%, Si: 0.10-0.30%, Mn: 1.00-1.20%, S: 0.010-0.020%, Cr: 1.10-1.30%, Al: 0.015-0.045%, N: 0.0100-0.0180%, and Ti: 0.040-0.100%.

2. Cold forging steel containing the following chemical elements in the following mass percentages: C: 0.170 to 0.220%, Si: 0.10 to 0.30%, Mn: 1.00 to 1.20%, S: 0.010 to 0.020%, Cr: 1.10 to 1.30%, Al: 0.015 to 0.045%, N: 0.0100 to 0.0180%, Ti: 0.040 to 0.100%, with the remainder being Fe and unavoidable impurities.

3. 3. The cold forging steel according to claim 1, further comprising Ca, wherein the Ca content expressed in mass percentage satisfies 0<Ca≦0.005%, and preferably 0.001%<Ca≦0.003%.

4. 3. The cold forging steel according to claim 1, wherein the Mn content is 1.10 to 1.20%, the Cr content is 1.16 to 1.30%, and / or the Ti content is 0.050 to 0.080%.

5. 3. The cold forging steel according to claim 1, wherein the inevitable impurities include P≦0.015% and / or O≦0.0030%.

6. 3. The cold forging steel according to claim 1 or 2, wherein the structure is ferrite + spheroidal carbides, and preferably the spheroidization rate of the structure of the cold forging steel is 90% or more.

7. 3. The cold forging steel according to claim 1 or 2, wherein the mechanical properties thereof satisfy at least one of the following: a yield strength of 220 to 270 MPa, a tensile strength of 430 to 480 MPa, an elongation rate ≧35%, and an area reduction rate ≧66%, and preferably the mechanical properties of the cold forging steel satisfy the following: a yield strength of 220 to 270 MPa, a tensile strength of 430 to 480 MPa, an elongation rate ≧35%, and an area reduction rate ≧66%.

8. A method for producing steel for cold forging according to any one of claims 1 to 7, comprising the following steps: (1) a step of melting and casting molten steel to obtain a slab; (2) a step of heating and rolling the slab to obtain an intermediate material; (3) a step of heating and rolling the intermediate material to obtain a rolled round steel; (4) a step of cooling the rolled round steel; (5) an annealing step; and optionally (6) a step of removing the annealed round steel from a furnace and air-cooling it.

9. 9. The manufacturing method according to claim 8, wherein in step (2), the cast piece is heated to a temperature of 1000 to 1120°C, preferably 1000 to 1100°C.

10. 9. The manufacturing method according to claim 8, wherein in step (3), the intermediate material is heated to 1050 to 1200°C, preferably 1050 to 1120°C, and maintained at that temperature for 5 to 6 hours.

11. 9. The manufacturing method according to claim 8, wherein in the step (3), the deformation amount ξ per rolling pass is 20 to 40%, the final rolling temperature T is 870 to 930°C, and the relationship between the deformation amount ξ per rolling pass and the final rolling temperature T satisfies 4.00≦ln(T−850)−lnξ≦6.

00.

12. 9. The manufacturing method according to claim 8, wherein in step (4), the rolled round steel is cooled to 370 to 410°C at a cooling rate of 20°C / s or more.

13. The manufacturing method according to any one of claims 8 to 12, wherein in step (5), the annealing temperature is 740±10°C and the holding time is 8 hours or more.

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