A CrMo-based cold-forged steel and its manufacturing method

Abstract

This disclosure relates to cold-forged steel and a method for manufacturing the same. The cold-forged steel of this disclosure comprises the following chemical elements in mass percentages: C: 0.20–0.22%, Si: 0.20–0.30%, Mn: 0.80–0.86%, Cr: 1.20–1.28%, Mo: 0.22–0.26%, Nb: 0.01–0.03%, Al: 0.025–0.035%, N: 0.010–0.016%, S: 0.010%–0.025%, Ca: 0.001%–0.003%, with the balance being Fe and unavoidable impurities. The cold-forged steel disclosed herein has a narrow hardenability bandwidth, and the austenite grain size is not coarser than grade 6 under high-temperature carburizing conditions of at least 980°C. It also has excellent machinability and cold working plasticity, which can effectively reduce tool wear and cold working cracking. It can be used for cold forging of high-temperature carburized gears, etc., and has good prospects for promotion and application value.

Description

Technical Field

[0001] This disclosure relates to steel and methods of manufacturing the same, and more particularly to cold-forged steel and methods of manufacturing the same. Background Technology

[0002] With the arrival of the dual-carbon era and the booming development of the automotive industry, the demand for automotive parts is gradually increasing. Simultaneously, parts manufacturing processes are shifting from conventional hot forging to cold forging. The increased production efficiency of vehicle manufacturers is leading to increasingly higher demands on the overall efficiency of the supply chain. Carburizing efficiency has always been one of the key processes in transmission gear production. Currently, transmission manufacturers generally use methods such as increasing carburizing temperature to improve efficiency. Furthermore, the requirements for transmissions in automobiles are also becoming increasingly stringent. The deformation problem of finished gears during heat treatment remains a persistent challenge in the industry.

[0003] The size of austenite grains affects the size and structure of the microstructure after heat treatment and phase transformation, which in turn affects the mechanical properties of steel. Therefore, for steel that needs to be heat treated, austenite grain size is an important indicator for measuring the performance of steel.

[0004] With the automotive industry's increasing demands for precision machining and assembly dimensional tolerances in automotive gears, narrowing the hardenability bandwidth of cold-forged steel has become an industry trend and a hot topic in cold-forged steel research both domestically and internationally in recent years. In particular, some high-precision cold-forged steels impose stringent requirements on hardenability. Hardenability bandwidth represents the degree of hardenability fluctuation. The size of the hardenability bandwidth depends on the microstructure content and distribution at various locations; differences in microstructure and distribution largely determine the amount of deformation after gear heat treatment. A narrower hardenability bandwidth means less hardness fluctuation, indicating more stable composition and microstructure control, which is more beneficial for deformation control after gear machining and results in higher meshing accuracy during gear service. High gear meshing accuracy is highly beneficial for noise reduction.

[0005] The production of cold-forged steel typically involves spheroidizing annealing of round steel bars, a process that takes tens of hours, consuming significant time and energy and impacting component production efficiency. Furthermore, the round steel bars require multiple turning passes after forging to be machined into gears. Therefore, the machinability of cold-forged steel is a key concern for major machining manufacturers. Typically, cold-forged steel is deoxidized using Al. However, the deoxidation product, Al2O3, is an extremely hard substance. Because Al2O3 readily aggregates and grows and is difficult to deform, it frequently causes tool breakage during machining, affecting both production efficiency and increasing costs. While conventional steels improve machinability by adding sulfur (S), in cold-forged steel, MnS, after deformation, forms elongated strips that can fracture the steel matrix, leading to cracking during cold forging.

[0006] Therefore, the art desires to obtain an energy-saving cold-forged steel that has good hardenability and fine austenitic grain size, and preferably also has excellent machinability and cold working plasticity. Summary of the Invention

[0007] In view of the above-mentioned defects and deficiencies of the prior art, the inventors have obtained a cold forging steel through reasonable chemical composition design. This steel can not only maintain an austenite grain size of no more than grade 6 under high-temperature carburizing conditions of at least 980°C, but also has a narrow hardenability bandwidth. Its hardenability satisfies J5: 40-44HRC, J7: 37-41HRC, J9: 34-38HRC, and the hardenability bandwidth at each of the above positions is ≤4HRC.

[0008] In a first aspect, this disclosure provides a cold-forged steel, which, in addition to containing more than 90% Fe and unavoidable impurities, contains the following chemical elements in mass percentages: C: 0.20–0.22%, Si: 0.20–0.30%, Mn: 0.80–0.86%, Cr: 1.20–1.28%, Mo: 0.22–0.26%, Nb: 0.01–0.03%, Al: 0.025–0.035%, N: 0.010–0.016%, S: 0.010%–0.025%, Ca: 0.001%–0.003%.

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

[0010] C: 0.20–0.22%, Si: 0.20–0.30%, Mn: 0.80–0.86%, Cr: 1.20–1.28%, Mo: 0.22–0.26%, Nb: 0.01–0.03%, Al: 0.025–0.035%, N: 0.010–0.016%, S: 0.010%–0.025%, Ca: 0.001%–0.003%, with the balance being Fe and unavoidable impurities.

[0011] In one embodiment, the cold-forged steel of this disclosure further comprises Ti, wherein the Ti content satisfies: 0 < Ti ≤ 0.008%, preferably 0.001% < Ti ≤ 0.006%.

[0012] In a preferred embodiment, the Mn content in the cold-forged steel of this disclosure is 0.82-0.86%.

[0013] In a preferred embodiment, the Cr content in the cold-forged steel of this disclosure is 1.22-1.28%.

[0014] In a preferred embodiment, the content of S and Ca in the cold-forged steel of this disclosure satisfies: 6 ≤ S / Ca ≤ 10, preferably 8 ≤ S / Ca ≤ 10.

[0015] In one embodiment, the unavoidable impurities in the cold-forged steel of this disclosure include P, Ni, O, and B. In a preferred embodiment, the contents of P, Ni, O, and B satisfy at least one of the following: P ≤ 0.012%, Ni ≤ 0.1%, O ≤ 0.003%, and B ≤ 0.0004%.

[0016] In a preferred embodiment, the cold-forged steel of this disclosure has Al2O3·CaO·MnS composite inclusions and spherical or ellipsoidal MnS inclusions.

[0017] In a preferred embodiment, the MnS inclusions in the cold-forged steel of this disclosure have a size of 1-3 μm and an aspect ratio ≤3.

[0018] In a preferred embodiment, the microstructure of the cold-forged steel disclosed herein is ferrite + spheroidal structure.

[0019] In a preferred embodiment, the spheroidization rate of the cold-forged steel disclosed herein is above 90%.

[0020] The cold-forged steel disclosed herein exhibits an austenite grain size no coarser than grade 6 under high-temperature carburizing conditions at at least 980°C, and hardenability meeting the requirements of J5: 40-44 HRC, J7: 37-41 HRC, and J9: 34-38 HRC, with a hardenability bandwidth ≤ 4 HRC at each of the aforementioned locations. Furthermore, the cold-forged steel of this disclosure possesses excellent machinability and cold-working plasticity, effectively reducing tool wear and extending tool life during turning, while also minimizing cold-working cracking. It is widely applicable for machining high-temperature carburized gears and has promising prospects and application value.

[0021] 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; (2) heating; (3) forging or rolling; (4) controlled cooling after rolling; and (5) annealing; wherein, in step (1), electromagnetic stirring is performed during the smelting process; wherein, in step (4), the cooling rate is not less than 15°C / s, and the final cooling temperature is 220-280°C; wherein, in step (5), the annealing temperature is 720±10°C, and the annealing time is 8-10 hours.

[0022] In one embodiment, in step (1), the electromagnetic stirring includes crystallizer electromagnetic stirring and solidification end electromagnetic stirring, wherein the frequency of the crystallizer electromagnetic stirring is 3.2-3.8Hz and the current is 315-325A; the frequency of the solidification end electromagnetic stirring is 8.5-9.5Hz and the current is 590-610A.

[0023] In one implementation, in step (1), the casting is carried out by continuous casting, wherein the continuous casting speed is 0.62-0.68 m / min.

[0024] In one embodiment, in step (1), vacuum degassing is performed, and the settling time for vacuum degassing is 12 minutes or more, more preferably 15 minutes or more; and / or the superheat of the molten steel in the tundish is 25-41°C.

[0025] In one embodiment, in step (2), the heating temperature is 1100–1200°C.

[0026] In one implementation, in step (2), the heating temperature is 1100-1200°C and the holding time is more than 6 hours.

[0027] In one implementation, in step (3), the final rolling or forging temperature is above 920°C.

[0028] In one implementation, in step (5), after annealing at 720±10℃ for 8-10 hours, the furnace is removed and air-cooled.

[0029] In the manufacturing method disclosed herein, the manufacturing process parameters of cold-forged steel are optimized, thereby further improving the machinability of cold-forged steel.

[0030] The cold-forged steel disclosed herein has a reasonable chemical composition and process design, and its process window is wide, enabling mass commercial production on bar production lines. Attached Figure Description

[0031] Figure 1 The morphology of Al2O3·CaO·MnS composite inclusions in the cold-forged steel of Embodiment 4 of this disclosure is schematically shown.

[0032] Figure 2 The morphology of inclusions in the cold-forged steel of Embodiment 4 of this disclosure and energy dispersive spectroscopy analysis are schematically shown.

[0033] Figure 3 The scanned microstructure of the cold-forged steel of Example 4 of this disclosure is schematically shown. According to ASTM 2282-03, the spheroidization rate of this cold-forged steel reaches over 90%. Detailed Implementation

[0034] 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.

[0035] As used herein, the term “and / or” refers to and covers any and all possible combinations of more than one of the listed items.

[0036] In this article, "austenite grain size" refers to a measure of the original austenite grain size in steel, also known as actual grain size. Standard grain size is divided into 8 levels: levels 1-4 are coarse grains, levels 5-8 are fine grains, and levels 10-13 (above level 8) are ultrafine grains.

[0037] In this paper, the austenite grain size was determined according to GB / T 6394.

[0038] In this paper, "hardenability" refers to the ability of steel to acquire martensite during quenching, and its magnitude is expressed by the depth of the hardened layer and the hardness distribution obtained under specified quenching conditions. J5 represents the hardness value (HRC) at 5 mm from the end of quenching, J7 represents the hardness value (HRC) at 7 mm from the end of quenching, and J9 represents the hardness value (HRC) at 9 mm from the end of quenching.

[0039] In this paper, hardenability is determined according to GB / T 5216 using the end-quenching method (Jominy test).

[0040] In this paper, “tissue spheroidization rate” is determined according to ASTM 2282-03.

[0041] In this article, "M-EMS (M-EMS)" refers to an electromagnetic stirrer placed around the crystallizer.

[0042] In this article, "F-EMS at the end of solidification" refers to an electromagnetic stirrer placed in a section of the cast liquid core with a diameter of less than 40 mm.

[0043] The design principles of each chemical element in the cold-forged steel disclosed herein are as follows:

[0044] 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 appropriate strength, which is beneficial to improving the wear resistance and contact fatigue strength of the parts. However, excessive carbon content will lead to excessive hardness of the steel. During subsequent cold forging, excessively high material strength will exacerbate die wear and cause the hardenability of the corresponding position of the gear to exceed the design requirements. However, when the carbon content in the steel is too low, it is impossible to ensure that the steel obtains 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. Therefore, in order to achieve narrow hardenability of cold-forged steel, the mass percentage of carbon in the cold-forged steel disclosed herein is controlled between 0.20% and 0.22%.

[0045] Silicon (Si): In the cold-forged steel disclosed herein, Si is a ferrite-forming element with strong solid solution strengthening effect, effectively improving the strength, hardenability, and tempering resistance of the steel. Furthermore, Si is a commonly used deoxidizer; adding an appropriate amount of Si to steel helps reduce oxygen levels. However, it is important to note that the Si content in the steel should not be too high. Excessive Si content reduces the plasticity of the steel and increases the risk of cold-working cracking. Therefore, in the cold-forged steel of this disclosure, the mass percentage of Si is controlled between 0.20% and 0.30%.

[0046] Mn (manganese): In the cold-forged steel disclosed herein, the Mn element can reduce the phase transformation driving force ΔG. γ→α This shifts the isothermal transformation curve of austenite to the right, improving the hardenability of the steel. It is important to note that Mn is an austenite-forming element, which can lower the Al temperature of the steel, promote grain growth, and increase the steel's overheat sensitivity. Mn is a core element affecting the hardenability of cold-forged steel. To reduce hardenability fluctuations, in the cold-forged steel described in this disclosure, the mass percentage content of Mn is controlled between 0.80% and 0.86%, more preferably between 0.82% and 0.86%.

[0047] Cr (chromium): In the cold-forged steel disclosed herein, the addition of Cr reduces the phase transformation driving force ΔG. γ→α While Cr hinders carbide nucleation and growth during phase transformation, thus improving the hardenability of steel, it also promotes the segregation of impurity atoms, increasing the material's temper brittleness. Excessive Cr content in steel leads to the formation of coarse carbides, resulting in deterioration of cold deformation properties. Furthermore, Cr significantly affects the hardenability of cold-forged steel. To reduce the fluctuation range of hardenability in cold-forged steel, the mass percentage of Cr in the cold-forged steel disclosed herein is controlled between 1.20% and 1.28%, more preferably between 1.22% and 1.28%.

[0048] Mo (Mo): In the cold-forged steel disclosed herein, Mo strengthens ferrite, thereby effectively improving the hardenability of the steel. Mo can also inhibit the segregation of harmful elements in steel, making it an effective element for reducing temper brittleness. Furthermore, Mo carbides are stable and do not easily grow, effectively refining grains and improving the tempering stability of the steel. However, it should be noted that the Mo content in the steel should not be too high. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Mo is controlled between 0.22% and 0.26%.

[0049] Niobium (Nb): In the cold-forged steel disclosed herein, Nb is a strong carbide and strong nitride forming element. The NbC / NbN particles formed by Nb combination exhibit good stability and a dispersed distribution. They are almost insoluble in austenite, effectively preventing grain boundary migration. Under subsequent high-temperature carburizing conditions in gear manufacturing, they effectively prevent grain coarsening and ensure austenite grain refinement. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Nb is controlled between 0.01% and 0.03%.

[0050] Al (aluminum): In the cold-forged steel disclosed herein, Al is an excellent deoxidizer. During the steelmaking process, Al can also form fine AlN precipitates, which can inhibit austenite grain growth during subsequent cooling, thereby refining the austenite grains and improving the toughness of the steel at low temperatures. However, it should be noted that the Al content in the steel should not be too high. Excessive Al content will lead to the formation of larger Al oxides, resulting in large Al2O3 inclusions. These coarse, hard alumina inclusions will worsen the fatigue properties of the steel and cause chipping during machining. Therefore, to ensure that Al exerts its beneficial effects, the mass percentage of Al in the cold-forged steel disclosed herein is controlled between 0.025% and 0.035%.

[0051] Nitrogen (N): In the cold-forged steel disclosed herein, the main role of nitrogen (N) is to form nitrides with elements such as Al and Nb in the steel. These nitrides, AlN and NbN, are not easily dissolved in austenite under high-temperature conditions. Therefore, under high-temperature conditions, AlN and NbN can be dispersed at the austenite grain boundaries, preventing austenite grain migration and coarsening. However, it is important to note that the N content in the steel should not be too high. Excessive N content 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 0.0100% and 0.0160%.

[0052] Sulfur (S): In the cold-forged steel of this disclosure, sulfur (S) readily combines with manganese (Mn) to form non-metallic inclusions (MnS). The presence of MnS can create stress concentration sources, making chips easier to break and thus improving the machinability of the steel. Furthermore, MnS has a lubricating effect and its low hardness reduces tool wear. However, it is important to note that the sulfur content in the steel should not be too high. Excessive sulfur content can cause hot brittleness, leading to corner cracks in the billet. Therefore, in the cold-forged steel of this disclosure, the mass percentage of sulfur is controlled between 0.010% and 0.025%, preferably between 0.012% and 0.020%.

[0053] Ca (calcium): Adding an appropriate amount of Ca to the cold-forged steel disclosed herein can form CaS. CaS adheres to the surface of Al2O3, thereby improving the size and morphology of hard inclusions and enhancing the material's machinability. However, it is important to note that the Ca content in the steel should not be too high, as excessive Ca content can cause DS-type inclusions to exceed size limits, which is detrimental to the material's fatigue life. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Ca is controlled between 0.001% and 0.003%.

[0054] In the cold-forged steel disclosed herein, P, Ni, O, and B are impurity elements in the steel. To obtain steel with better performance and superior quality, the content of each impurity element in the steel should be reduced as much as possible, where technically feasible.

[0055] Impurity element phosphorus (P) can combine with iron (Fe) to form a hard and brittle Fe3P phase, causing cold brittleness in the steel during cold working. This leads to decreased plasticity and makes the steel more prone to intergranular fracture under impact loads, resulting in larger cleavage planes. P in steel tends to segregate at grain boundaries, reducing the binding energy at grain boundaries and worsening the steel's plasticity. Therefore, to avoid increased brittleness, the mass percentage of P in the cold-forged steel disclosed herein is controlled to be P ≤ 0.012%, more preferably P ≤ 0.008%.

[0056] The impurity element Ni (nickel) has a significant impact on the hardenability of steel. Furthermore, Ni is a relatively expensive element in alloys. Therefore, in the cold-forged steel of this disclosure, the mass percentage of Ni is controlled to be Ni ≤ 0.1%, more preferably Ni ≤ 0.05%.

[0057] Impurity element oxygen (O) can react with Al and Ti elements in steel to form compounds such as Al₂O₃ and TiO. Therefore, in order 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%, more preferably O ≤ 0.002%.

[0058] Impurity element boron (B) has a significant impact on the hardenability of materials. B tends to segregate at austenite grain boundaries, making it difficult for new phases to nucleate at these boundaries during austenite decomposition. This increases the incubation period of austenite decomposition, thereby reducing the rate of diffusive phase transformation, which is beneficial for martensitic transformation and ultimately improves the hardenability of the steel. However, the location of B segregation is not fixed, leading to significant fluctuations in the hardenability of the material. Therefore, in the cold-forged steel disclosed in this invention, to ensure a wide hardenability bandwidth, the mass percentage of B is controlled to be B ≤ 0.0004%.

[0059] In a preferred embodiment, the cold-forged steel of this disclosure further comprises Ti. The Ti element can further improve the properties of the cold-forged steel, and the design principles of its chemical elements are as follows:

[0060] Ti (Titanium): In the cold-forged steel disclosed herein, Ti can form corresponding compounds with C and N elements 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 should be noted that if the Ti content in the steel is too high, coarse TiN precipitates will form, leading to a decrease in the fatigue performance of the steel. Therefore, in the cold-forged steel disclosed herein, the mass percentage of Ti is controlled to be 0 < Ti ≤ 0.008%.

[0061] In the manufacturing method disclosed herein, the manufacturing process parameters for cold-forged steel have been optimized.

[0062] During the smelting process, electromagnetic stirring is employed, particularly by controlling the frequency and current of the stirring, to promote the rapid floating of larger inclusions, thereby improving the purity of the molten steel, preventing inclusion aggregation, reducing inclusion size, and minimizing the harmful effects of inclusions on steel properties. Furthermore, fine Al2O3 particles are dispersed throughout the molten steel due to stirring. By controlling the Ca content, CaS and CaO are formed in the steel. CaS preferentially nucleates at fine Al2O3 or CaO particles, encapsulating the hard Al2O3 particles to form Al2O3·CaO·MnS composite inclusions with a size of approximately 3 μm. This modifies the inclusions, coating the originally sharp and hard Al2O3 surface with ductile MnS, effectively preventing direct contact between hard Al2O3 and the cutting tool during machining, thus avoiding tool chipping failure. In addition, CaS also increases the number of MnS nucleation sites, reduces the heterogeneous nucleation energy of MnS, increases the proportion of composite inclusions in the steel, and improves the morphology of MnS. Making MnS elliptical with a size of approximately 1-3 μm and an aspect ratio not exceeding 3 can improve the machinability of steel and prevent the formation of large-sized MnS particles within the steel. This aspect ratio effectively reduces the cutting effect of strip-shaped MnS on the matrix, preventing cracking during cold working.

[0063] In the manufacturing method disclosed herein, smelting can be carried out using an electric furnace or a converter.

[0064] The cold-forged steel and its manufacturing method disclosed herein have the following advantages and beneficial effects:

[0065] (1) The cold forging steel disclosed herein, through reasonable chemical composition design, makes full use of the influence of various alloying elements on phase transformation and microstructure, and forms a uniform martensitic structure after quenching, thereby achieving a narrower hardenability fluctuation and improving the heat treatment deformation after the gear is finally formed.

[0066] (2) The cold-forged steel of this disclosure incorporates a certain amount of sulfur (S) into the steel, allowing S and Mn to form MnS with excellent plasticity. The MnS inclusions act as a solid lubricant during processing, reducing friction between the tool and the workpiece, thereby improving the machinability of the steel and making it easier to cut. Furthermore, in the cold-forged steel of this disclosure, by controlling the MnS morphology so that its aspect ratio does not exceed 3, the elongated shape of MnS within the steel is avoided from tearing the matrix and reducing the steel's deformation properties.

[0067] (3) In order to avoid grain coarsening of cold-forged steel during subsequent high-temperature carburizing, this disclosure adds a certain amount of Al, Nb and N, combined with the pinning coefficient of different precipitates relative to austenite grain boundaries, to ensure that dispersed AlN, NbC and NbN precipitates are generated in the matrix, thereby ensuring that NbC / NbN hardly dissolves in the austenite matrix under high-temperature carburizing conditions of at least 980℃, and is dispersed in the austenite grain boundaries, playing a pinning role, effectively preventing the migration of austenite grain boundaries, and maintaining the refinement of austenite grains.

[0068] (4) In the manufacturing method disclosed herein, the manufacturing process parameters of cold forged steel are optimized, which further improves the machinability of cold forged steel.

[0069] (5) In the manufacturing method disclosed herein, during the post-rolling controlled cooling step, the rolled round steel is rapidly cooled (at a cooling rate of 15°C / s or higher, exceeding the critical cooling rate for martensite), which can form martensitic structure in the matrix to ensure that there is a large amount of distortion energy in the matrix. Then, during the annealing process, it provides phase transformation energy for subsequent microstructure transformation, which is conducive to the precipitation and spheroidization of carbides. AlN, NbN and other precipitated phases are dispersed at the grain boundaries, refining the grains. At the same time, AlN and NbN can act as nucleation sites for carbide precipitation, shortening the incubation period for carbide nucleation and precipitation, reducing annealing time, and saving energy consumption.

[0070] (6) In the manufacturing method disclosed herein, electromagnetic stirring is used during the smelting process. In particular, by controlling the frequency and current of the electromagnetic stirring, larger inclusions are promoted to float up as soon as possible, thereby improving the purity of the molten steel, preventing the aggregation of inclusions, reducing the size of inclusions, and reducing the harm of inclusions to the properties of steel.

[0071] 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.

[0072] Example

[0073] The cold-forged steels of Examples 1-6 and Comparative Examples 1-4 were prepared by using the following steps:

[0074] (1) According to the chemical composition shown in Table 1, the smelting is carried out in an electric furnace or converter. During the smelting process, the vacuum degassing and calming time is controlled to be ≥12min, the superheat of the molten steel in the tundish is 25-41℃, and electromagnetic stirring is used. The frequency of electromagnetic stirring in the crystallizer is controlled to be 3.2-3.8Hz and the current is 315-325A. The frequency of electromagnetic stirring at the end of solidification is controlled to be 8.5-9.5Hz and the current is 590-610A. After the smelting is completed, continuous casting is carried out to obtain a continuous casting billet with a cross-sectional size of 320*425mm. The continuous casting speed is 0.62-0.68m / min.

[0075] (2) Heating: The continuously cast billet is heated and loaded into the heating furnace. The heating temperature is controlled at 1100-1200℃ and the holding time is more than 6 hours.

[0076] (3) Forging or rolling: Roll the continuously cast billet into 20-50mm round bars, and control the final rolling or forging temperature ≥920℃;

[0077] (4) Controlled cooling after rolling: Cool the round steel to 220-280℃ at a cooling rate of not less than 15℃ / s;

[0078] (5) Annealing: Hold at 720±10℃ for 8-10 hours, then remove from the furnace and air cool.

[0079] The cold-forged steels in Examples 1-6 were prepared using the above steps, and their chemical composition and manufacturing process parameters all meet the design specifications and control requirements of this disclosure. However, the cold-forged steels in Comparative Examples 1-4 have parameters in their chemical composition design that do not meet the design specifications of this disclosure.

[0080] Table 1. Mass percentage of each chemical element in the cold-forged steels of Examples 1-6 and Comparative Examples 1-4 (balance: Fe and other unavoidable impurities other than P, Ni, O and B)

[0081]

[0082]

[0083] austenite grain size

[0084] Samples of the cold-forged steels from Examples 1-6 and Comparative Examples 1-4 were taken. Each cold-forged steel sample was heated to 980°C and held at 980°C for 4 hours, followed by water quenching. The austenite grain size of the cold-forged steel samples from each example and comparative example was observed, and the observation results are listed in Table 3 below.

[0085] Table 3. Austenite grain size of cold-forged steels from Examples 1-6 and Comparative Examples 1-4

[0086] serial number Austenite grain size (grade) Example 1 8.5 Example 2 7.5 Example 3 7.0 Example 4 7.5 Example 5 6.5 Example 6 7.5 Comparative Example 1 7.5 Comparative Example 2 7.5 Comparative Example 3 <![CDATA[ 4.0 ]]> Comparative Example 4 8.0

[0087] As shown in Table 3, the austenite grain size of the cold-forged steels in Examples 1-6, after being water-quenched at 980℃ for 4 hours, was no coarser than grade 6, exhibiting fine grains. The austenite grain size of the cold-forged steels in Comparative Examples 1, 2, and 4 was also superior.

[0088] However, in Comparative Example 3, due to insufficient Nb content, the grain size grade during the austenite insulation process was significantly lower than that of other examples and comparative examples, indicating that adding an appropriate amount of Nb can effectively prevent grain coarsening during the high-temperature carburizing process.

[0089] Hardenability

[0090] Samples were taken from the cold-forged steels of Examples 1-6 and Comparative Examples 1-4. Hardenability tests were performed on the cold-forged steel samples from each example and comparative example. The test results are listed in Table 4.

[0091] Hardenability test method: According to national standard GB / T 225, samples were taken and prepared from the hot-rolled round steel of the cold-forged steel samples of each embodiment and comparative example. The end hardenability test (Jominy test) was carried out with reference to GB / T 5216. The normalizing temperature was controlled at 910±10℃ and the quenching temperature at 880±5℃. The Rockwell hardness test was carried out according to GB / T 230.2 to obtain the hardness value (HRC) at a specific location. For example, the hardness at 5mm from the end of quenching is J5mm, and so on.

[0092] Table 4. Hardenability test results of cold-forged steels from Examples 1-6 and Comparative Examples 1-4

[0093] serial number J5mm / HRC J7mm / HRC J9mm / HRC Example 1 42.3 38.7 35.4 Example 2 41.6 38.3 35.9 Example 3 43.1 39.6 36.2 Example 4 43.6 40.1 36.9 Example 5 40.5 37.5 37.3 Example 6 41.3 39.3 36.8 Comparative Example 1 <![CDATA[ 44.8 ]]> <![CDATA[ 41.2 ]]> 38.4 Comparative Example 2 42.8 39.9 37.5 Comparative Example 3 43.2 40.7 37.3 Comparative Example 4 42.7 39.5 37.5

[0094] Note: In the table above, regarding the hardenability of steel, J5mm represents the hardness at 5mm from the end of quenching, J7mm represents the hardness at 7mm from the end of quenching, and J9mm represents the hardness at 9mm from the end of quenching.

[0095] As shown in Table 4, the hardenability test results of the cold-forged steels in Examples 1-6 are J5: 40.5-43.6 HRC, J7: 37.5-40.1 HRC, and J9: 35.4-37.3 HRC, all of which meet the requirements of J5: 40-44 HRC, J7: 37-41 HRC, and J9: 34-38 HRC, with a hardenability bandwidth of ≤4 HRC at each location. Furthermore, the cold-forged steels in Comparative Examples 2-4 also exhibit superior narrow hardenability bandwidths.

[0096] However, in Comparative Example 1, the C content was too high, exceeding the design specification range, resulting in poor hardenability of the material. The hardenability at J5mm exceeded the upper limit of 44HRC, and the hardenability at J7mm exceeded the upper limit of 41HRC.

[0097] free machinability

[0098] Finished round steel bars made from cold-forged steel of Examples 1, 3, and Comparative Example 2 were machined on the same CNC machine tool. During machining, the CNC machine tool speed was controlled at 400 r / min for the steels of Examples 1, 3, and Comparative Example 2. The tool wear frequency is shown in Table 5 below.

[0099] Table 5. Average frequency of CNC machine tool scrapping when cold-forged steel in Examples 1, 3 and Comparative Example 2 is machined into parts.

[0100] serial number Tool wear frequency (average) Example 1 5100 pieces / time Example 3 5000 pieces / time Comparative Example 2 2400 pieces / time

[0101] As shown in Table 5, when CNC machine tools use the cold-forged steel of Examples 1 and 3 to manufacture gear parts, the average scrapping frequency of the lathe tools is 5100 pieces / cycle and 5000 pieces / cycle, respectively; while when CNC machine tools use the cold-forged steel of Comparative Example 2 to manufacture gear parts, the average scrapping frequency of the lathe tools is 2400 pieces / cycle. The cutting tool life when using Examples 1 and 3 is more than twice that of Comparative Example 2. Tests showed that the tool wear frequency during machining in other examples (2, 4-6) was similar to that of Examples 1 and 3, approximately 5000 pieces / cycle. Therefore, the cold-forged steel of this disclosure has excellent machinability and can reduce tool wear.

[0102] Plasticity

[0103] Because cold forging requires extremely high plasticity from the material, the plasticity of the material was also tested. Round steel from Examples 1-6 and Comparative Example 4 were cold-forged into gear parts using the same process. The deformation cracking frequency of both was statistically analyzed, and the results are shown in Table 6.

[0104] Table 6.

[0105] serial number Forging crack frequency of parts (average value) Examples 1-6 0.6-0.9‰ Comparative Example 4 3-5‰

[0106] As can be seen from Table 6, when processing gear parts using the same forging process, the cracking frequency of the round steel parts in Example 3 was 0.6-0.9‰, while the cracking frequency of the round steel parts in Comparative Example 4 was 3-5‰, approximately five times that of Example 3. This demonstrates that the cold-forged steel of this disclosure possesses excellent plasticity, which can reduce the cracking frequency of parts.

[0107] Figure 1 The morphology of the Al2O3·CaO·MnS composite inclusions in the cold-forged steel of Example 4 is schematically shown.

[0108] like Figure 1 As shown, the MnS composite inclusions have a size of 1-3 μm, are spherical, and have an aspect ratio of no more than 3.

[0109] Figure 2 The morphology of inclusions in the cold-forged steel of Example 4 and energy dispersive spectroscopy analysis are schematically shown.

[0110] Figure 3 The scanning microstructure of the cold-forged steel of Example 4 is shown schematically.

[0111] All publications, patent applications, patents and other references mentioned in this disclosure are incorporated herein by reference in their entirety.

[0112] 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: 0.20~0.22%, Si: 0.20~0.30%, Mn: 0.80~0.86%, Cr: 1.20~1.28%, Mo: 0.22~0.26%, Nb: 0.01~0.03%, Al: 0.025~0.035%, N: 0.010~0.016%, S: 0.010%~0.025%, Ca: 0.001%~0.003%, with the balance being Fe and unavoidable impurities. The cold-forged steel further contains Ti, and the content of Ti satisfies:

0. <Ti≤0.008%, The contents of S and Ca satisfy the following condition: 6 ≤ S / Ca ≤ 10. The unavoidable impurities include P, Ni, O, and B; the contents of P, Ni, O, and B satisfy at least one of the following: P ≤ 0.012%, Ni ≤ 0.1%, O ≤ 0.003%, B ≤ 0.0004%. The cold-forged steel contains Al2O3·CaO·MnS composite inclusions and spherical or ellipsoidal MnS inclusions. The MnS inclusions have a size of 1-3 μm and an aspect ratio ≤ 3. The microstructure of the cold-forged steel is ferrite + spheroidal structure; the spheroidization rate of the cold-forged steel is over 90%.

2. The cold-forged steel as described in claim 1, characterized in that, The Ti content satisfies 0.001%. <Ti≤0.006%。 3. The cold-forged steel as described in claim 1 or 2, characterized in that, The Mn content is 0.82~0.86%, and / or the Cr content is 1.22~1.28%.

4. The cold-forged steel as described in claim 1 or 2, characterized in that, The contents of S and Ca satisfy 8 ≤ S / Ca ≤ 10.

5. The cold-forged steel as described in claim 1 or 2, characterized in that, The cold-forged steel meets the following requirements: a) Under high-temperature carburizing conditions of at least 980°C, the austenite grain size of the cold-forged steel is not coarser than grade 6; and / or b) The hardenability of the cold-forged steel meets the requirements of J5: 40-44 HRC, J7: 37-41 HRC, J9: 34-38 HRC, and the hardenability bandwidth at each of the above positions is ≤4 HRC.

6. The method for manufacturing cold-forged steel according to any one of claims 1 to 5, characterized in that, The manufacturing method includes the following steps: (1) Smelting and casting; (2) Heating; (3) Forging or rolling; (4) Controlled cooling after rolling; (5) Annealing; In step (1), electromagnetic stirring is performed during the smelting process; In step (4), the cooling rate is not less than 15℃ / s, and the final cooling temperature is 220-280℃; In step (5), the annealing temperature is 720±10℃ and the annealing time is 8~10 hours.

7. The manufacturing method as described in claim 6, characterized in that, The electromagnetic stirring includes electromagnetic stirring in the crystallizer and electromagnetic stirring at the solidification end, wherein the frequency of the electromagnetic stirring in the crystallizer is 3.2-3.8Hz and the current is 315-325A; the frequency of the electromagnetic stirring at the solidification end is 8.5-9.5Hz and the current is 590-610A.

8. The manufacturing method as described in claim 6, characterized in that, The casting is continuous casting, and the continuous casting speed in the continuous casting is 0.62-0.68 m / min.

9. The manufacturing method as described in claim 6, characterized in that, In step (1), vacuum degassing is performed, and the settling time for vacuum degassing is more than 12 minutes; and / or the superheat of the molten steel in the tundish is 25-41°C.

10. The manufacturing method as described in claim 9, characterized in that, In step (1), the sedation time for vacuum degassing is more than 15 minutes.

11. The manufacturing method according to any one of claims 6 to 10, characterized in that, In step (2), the heating temperature is 1100 to 1200°C.

12. The manufacturing method according to any one of claims 6 to 10, characterized in that, In step (3), the final rolling or forging temperature is above 920°C.

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