Medium-carbon round steel for cold extrusion and manufacturing method therefor

Abstract

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Description

Medium carbon round steel for cold extrusion and manufacturing method thereof Technical Field

[0001] The present invention relates to a steel material and a manufacturing method thereof, in particular to a round steel material and a manufacturing method thereof. Background Art

[0002] The operating capacity of shaft components in vehicle reducers generally depends on their strength and stiffness, and at high speeds, also on vibration stability. Furthermore, they are subject to both bending and torque during operation, placing certain demands on the toughness of the round steel.

[0003] In order to meet the requirements of reducer shaft components, there are currently two main process paths for the production of reducer shaft products. One is to use low-carbon steel such as 20CrMnTi, 20CrMo, 20MnCr5 and then carburize the surface; the other is to use medium-carbon steel (such as 45, 50 steel) or medium-carbon low-alloy steel (such as 40Cr, 42CrMo) and then perform surface induction heat treatment. Under these two process paths, round steel is usually formed by direct turning or hot forging, and cold forming is not often used. Cold forming is usually applied to low-carbon steel. Medium-carbon steel has poor plasticity and toughness, and the stress in the round steel is large after cold forming, which makes it easy to crack during the subsequent quenching process. Therefore, it is usually not used for cold forming.

[0004] In automotive speed reducers, the most common failure mode for shaft components is fatigue fracture. The fatigue resistance of shaft components is closely related not only to their strength and toughness, but also to the microstructure and surface quality of the round steel before cold extrusion. Therefore, in addition to ensuring the strength and toughness of the round steel through composition design, a microstructure suitable for cold extrusion and good surface quality are key to ensuring the fatigue resistance of shaft components. Existing technology often uses traditional fuel vehicle axle steel to manufacture speed reducer shaft components, and the most suitable processes are turning or hot forging.

[0005] For example, the Chinese patent with publication number CN104975235A, published on October 14, 2015, and titled “A 120KSI steel grade high-strength and tough medium-carbon quenched and tempered round steel and its manufacturing method” relates to a high-strength and tough medium-carbon quenched and tempered round steel and its manufacturing method. The elemental composition of the steel in this solution is as follows: C: 0.35-0.50%, Si: 0.15-0.40%, Mn: 0.60-1.30%, P: ≤0.015%, S: ≤0.040%, Cr: 0.75-1.30%, Mo: 0.15-0.35%, Ni: ≤0.25%, Cu: ≤0.25%, Alt: 0.015-0.040%, V: ≤0.10%, Nb: ≤0.10%, Ti: ≤0.05%, N: ≤0.008%, B: ≤0.0010%, the balance being Fe and unavoidable impurities, with V and Nb contents ≤0.010% when different. High strength and toughness requirements are achieved primarily through the design of the steel's composition and subsequent quenching and tempering heat treatment.

[0006] Another example: the Chinese patent with publication number CN112981233A and publication date of June 18, 2021, entitled "A low-silicon medium-carbon gear steel suitable for cold forging and a manufacturing method thereof" relates to a low-silicon medium-carbon gear steel suitable for cold forging and a manufacturing method thereof. The elemental composition of the steel in this scheme is as follows: C: 0.35-0.45%, Si: ≤0.08%, Mn: 0.30-0.60%, Cr: 0.20-0. 50%, P: ≤0.020%, S: 0.010-0.040%, Cu: ≤0.10%, Ni: ≤0.05%, Mo: ≤0.05%, Al: ≤0.050%, N: ≥0.005%, B: 0.0005-0.0035%, Ti: ≤0.010%, [O]: ≤0.0020%, (Cu+Ni+Mo): ≤0.15%, the balance being Fe and unavoidable impurities. A medium-carbon gear steel with excellent cold forging performance is achieved by designing the steel's composition, controlling dimensional accuracy, decarburization depth, and spheroidizing hardness and microstructure spheroidization rate. However, no specific surface treatment is performed on the surface hardened layer, which can affect cold forging cracking. Summary of the Invention

[0007] One of the purposes of the present invention is to provide a medium carbon round steel for cold extrusion, by accurately identifying the influence of each alloy element on the actual hardenability, so as to obtain a hardenability curve that meets the operating conditions of the reducer input shaft to ensure good strength and toughness of the output shaft.

[0008] In order to achieve the above object, the present invention provides a medium carbon round steel, which, in addition to Fe and unavoidable impurities, also contains the following chemical elements in percentage by mass:

[0009] C: 0.36~0.43%, Si: 0.1~0.4%, Mn: 0.6~1.0%, Cr: 0.6~1.0%, Mo: 0.2~0.4%, Ni: 1.4~1.7%, Al: 0.02~0.04%, N: 0.008~0.015%;

[0010] Its DI value is 4.72~10.21in, where DI=(0.171+0.001C+0.265C 2 )(1+3.333Mn)(1+0.7Si)(1+0.365Cu)(1+2.16Cr)(1+3Mo)(1+0.363Ni)(1+1.73V). When calculating, substitute the symbol of each element in the formula for the value before the percentage sign of the corresponding element's mass percentage. For example, if the C content in steel is 0.41%, substitute 0.41 for the calculation.

[0011] DI is a key parameter that quantitatively characterizes the hardenability of steel. It is calculated using a mathematical formula combined with the content of major alloying elements (such as C, Mn, Si, Cr, and Ni) in the steel. It directly reflects the material's hardenability under standard testing conditions. Measured in inches, it refers to the critical diameter of the steel during quenching.

[0012] The medium carbon round steel of the present invention is obtained through annealing, and its microstructure is lamellar pearlite+spherical pearlite.

[0013] Preferably, the medium carbon round steel of the present invention is composed of the following chemical elements in percentage by mass:

[0014] C: 0.36-0.43%, Si: 0.1-0.4%, Mn: 0.6-1.0%, Cr: 0.6-1.0%, Mo: 0.2-0.4%, Ni: 1.4-1.7%, Al: 0.02-0.04%, N: 0.008-0.015%; the balance is Fe and unavoidable impurities.

[0015] In the medium carbon round steel of the present invention, the design principles of each chemical element are specifically described as follows:

[0016] C: In the medium-carbon round steel described herein, increasing the carbon content in the austenite significantly reduces the critical cooling rate of the steel, increasing the critical diameter, thereby improving the steel's hardenability and ensuring the strength and rigidity of the shaft. However, excessive carbon content increases the tendency to quench cracking. Therefore, the carbon content of the medium-carbon round steel described herein is set to 0.36-0.43%.

[0017] Si: In the medium-carbon round steel described herein, Si strengthens ferrite and can improve hardenability to a certain extent. Si replaces Fe atoms in the steel by substitution, hindering dislocation motion and contributing to the steel's strength. However, excessive Si can reduce the steel's plasticity and toughness. Therefore, the Si content in the medium-carbon round steel described herein is set at 0.1-0.4%.

[0018] Mn: In the medium-carbon round steel described herein, Mn dissolves in ferrite, forming a substitutional solid solution and producing a solid solution strengthening effect. Mn and its carbides dissolve in austenite, shifting the austenite isothermal transformation curve to the right, increasing the stability of supercooled austenite, inhibiting pearlite transformation, and improving hardenability. However, excessive Mn content can lead to severe segregation in the round steel and susceptibility to quench cracking. Therefore, the Mn content in the medium-carbon round steel described herein is set to 0.6-1.0%.

[0019] Cr: In the medium carbon round steel described in the present invention, adding an appropriate amount of Cr element can inhibit the diffusion-type phase transformation of the steel, form a hardened martensitic structure, and obtain a steel with higher strength. At the same time, during the heating process, if the Cr carbide is not completely dissolved, it can also play a role in inhibiting the growth of austenite grains. 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 be formed, which will deteriorate the impact properties of the steel. Based on this, in the medium carbon round steel described in the present invention, the Cr content is set to: 0.6~1.0%.

[0020] Mo: In the medium-carbon round steel described in the present invention, Mo is a ferrite-forming element. Adding an appropriate amount of Mo element helps improve the hardenability of the steel, making it easier for the steel to form bainite and martensite strengthening phases during the quenching process. During tempering in a higher temperature range, fine carbides will form, increasing the strength of the steel. It should be noted that Mo is a precious alloying element, and adding a high content of Mo will lead to increased costs. Based on this, in the medium-carbon round steel described in the present invention, the Mo content is set to: 0.2-0.4%.

[0021] Ni: In the medium carbon round steel described in the present invention, the Ni element exists in the steel in the form of solid solution. Specifically, Ni exists in the FCC phase of Fe-Ni-Mn, which can reduce the stacking fault energy and improve the low-temperature impact performance of the steel. In addition, it should be noted that Ni is an austenite-forming element, and it is not advisable to add too high a content of Ni to the steel. Adding too much Ni will lead to an excessively high content of residual austenite in the steel, reducing the strength of the steel. In addition, the Ni element is also a precious metal, and adding a higher content of Ni will lead to an increase in cost. Based on this, in the medium carbon round steel described in the present invention, the Ni content is set to: 1.4~1.7%.

[0022] Al: In the medium-carbon round steel described herein, Al forms fine AlN precipitates during steelmaking, which inhibit austenite grain growth and refine the austenite grains during the subsequent cooling process. Excessive Al content can lead to the formation of large Al oxides, and coarse alumina hard inclusions can deteriorate the fatigue properties of the steel. Therefore, the Al content in the medium-carbon round steel described herein is set to 0.02-0.04%.

[0023] N: In the medium-carbon round steel described herein, N is an interstitial atom and also a MX-type precipitate-forming element. It can improve the strength and toughness of the steel through AlN grain refinement. However, high N content can lead to increased concentration in defects and the formation of coarse nitride precipitates, particularly acute-angled TiN particles, which can affect the impact toughness of the steel. Therefore, the N content in the medium-carbon round steel described herein is set to 0.008-0.015%.

[0024] Preferably, the medium carbon round steel of the present invention further contains at least one of the following chemical elements: 0<Cu≤0.20%, 0<V≤0.05%, 0<Nb≤0.05%.

[0025] In the medium carbon round steel of the present invention, the design principles of the above chemical elements are specifically described as follows:

[0026] Cu: In the medium-carbon round steel described herein, Cu improves the steel's hardenability and corrosion resistance. However, excessive Cu content can lead to grain boundary enrichment, weakening the grain boundaries and potentially causing cracking. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, the Cu content is set to: 0 < Cu ≤ 0.20%.

[0027] V: In the medium-carbon round steel described herein, V can combine with C or N to form precipitates, increasing the steel's strength. However, if the V content is too high, coarse VC particles will form, deteriorating the steel's plasticity and toughness. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, the V content is set to: 0 < V ≤ 0.05%.

[0028] Nb: In the medium-carbon round steel described herein, Nb forms fine precipitates upon addition, contributing to grain refinement and reducing hydrogen embrittlement susceptibility. However, excessive Nb can form coarse NbC particles during the smelting process, which in turn reduces impact toughness. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, the Nb content is set to: 0 < Nb ≤ 0.05%.

[0029] Preferably, in the medium carbon round steel described in the present invention, among the inevitable impurities, the content of each impurity element satisfies at least one of the following items: P≤0.015%, S≤0.025%, H≤0.0002%, B≤0.001%, O≤0.0020%, 0<Ti≤0.08%, preferably 0<Ti≤0.01%.

[0030] In the above technical solution, P, S, H, B, O and Ti are all impurity elements in steel. If technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in steel should be reduced as much as possible, among which:

[0031] P: In the medium-carbon round steel described herein, P segregates at grain boundaries, reducing the binding energy and deteriorating the steel's low-temperature impact properties. Furthermore, the coexistence of P and Mn exacerbates the steel's temper brittleness. Furthermore, P segregating at grain boundaries can cause intergranular fracture when subjected to impact loads, forming larger cleavage planes and reducing the energy absorbed by the steel during impact. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, to ensure low-temperature impact toughness of the input shaft, the P content is controlled to: P ≤ 0.015%.

[0032] S: In the medium-carbon round steel described herein, S has very low solubility in delta ferrite and austenite. During solidification, S segregates. High S content results in the formation of numerous coarse sulfide inclusions, compromising the steel's fatigue resistance. However, adding an appropriate amount of S to the steel forms CaS, which can improve machinability. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, to ensure fatigue resistance while still meeting user requirements for easy cutting, the S content is controlled to ≤ 0.025%.

[0033] H: In the medium-carbon round steel described in the present invention, H is affected by the hydrostatic pressure field of edge dislocations in the steel and will accumulate at defects, forming hydrogen embrittlement. Steel with a high tensile strength level has a high density of dislocations, subgrain boundaries, etc. If the H content in the steel is too high, a large number of H atoms will be enriched at the defects after the steel is quenched and tempered. The aggregation of H atoms will form H molecules, causing delayed fracture of the steel. Based on this, in some preferred embodiments of the medium-carbon round steel described in the present invention, the H content is controlled to: H ≤ 0.0002%.

[0034] B: In the medium-carbon round steel described herein, B has a strong affinity with nitrogen and oxygen, and readily reacts with metals such as titanium to form the extremely hard boride TiB2. Furthermore, even a small amount of B can significantly improve the hardenability of the round steel. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, the B content is controlled to be B ≤ 0.001%.

[0035] O: In the medium-carbon round steel described in the present invention, O reacts with Al and Ti in the steel to form inclusions such as Al2O3 and TiO. Therefore, in some preferred embodiments of the medium-carbon round steel described in the present invention, to ensure the continuity and fatigue resistance of the steel matrix, the O content is controlled to: O ≤ 0.0020%.

[0036] Ti: In the medium-carbon round steel described herein, Ti combines with nitrogen and carbon in the steel to form TiC, Ti(CN), and TiN. The sharp-edged TiN disrupts the overall continuity and adversely affects the fatigue performance of the steel. Therefore, in some preferred embodiments of the medium-carbon round steel described herein, the Ti content is controlled to: 0 < Ti ≤ 0.08%, preferably 0 < Ti ≤ 0.01%.

[0037] Preferably, in the medium carbon round steel of the present invention, the contents of Al and N elements satisfy: [Al] / [N]≥2, [Al][N]≥2×10 -4 .

[0038] In the present invention, the values ​​of [Al] and [N] in the above formula should be substituted with the mass percentages of Al and N in the steel, respectively. For example, if the Al content in the steel is 0.02%, the value 0.02 should be substituted for the calculation.

[0039] The present invention can be achieved by controlling [Al][N]≥2x10 -4 To further ensure the AlN content in the steel.

[0040] In addition, as mentioned above, too high Al content will lead to the formation of large Al oxides, and coarse aluminum oxide hard inclusions will deteriorate the fatigue performance of steel. The inventors found that by controlling the content of Al and N in steel to meet [Al] / [N]≥2 and [Al][N]≥2×10 -4 The content and size of nitrides in the steel can be ensured to be within an appropriate range, thereby further improving the performance of medium carbon round steel.

[0041] Preferably, in the medium carbon round steel of the present invention, the volume phase ratio of lamellar pearlite is 35-45%.

[0042] Preferably, the medium carbon round steel described in the present invention meets one or more of the following requirements: hardness of 185-210 HBW, yield strength ≥400 MPa, tensile strength ≥600 MPa, elongation ≥30%, cross-sectional shrinkage ≥60%, and -40°C impact energy Akv2 ≥20 J.

[0043] Unless otherwise specified, the "medium carbon round steel" referred to in the present invention refers to medium carbon round steel that has not been quenched and tempered.

[0044] Preferably, the medium carbon round steel after quenching and tempering treatment has a yield strength of ≥1100 MPa, a tensile strength of ≥1250 MPa, an elongation of ≥15%, a cross-sectional shrinkage of ≥50%, and an impact energy Akv2 of ≥40 J at -40°C.

[0045] Preferably, the microstructure of the medium carbon round steel after quenching and tempering treatment described in the present invention is tempered troostite.

[0046] Another aspect of the present invention provides a method for manufacturing the above-mentioned medium-carbon round steel. This method, combined with the above-described composition ratio, can avoid both excessive hardness resulting in excessive resistance to subsequent cold extrusion deformation and low hardness resulting in shaft bending during cold extrusion. Furthermore, it reduces energy consumption, saves heat treatment time, and improves production efficiency. Furthermore, it ensures that the surface of the round steel after the surface treatment process is free of a cryptocrystalline martensite layer (hardened layer) produced by turning and polishing, thereby avoiding surface cracking during the cold extrusion process.

[0047] In order to achieve the above object, the present invention provides a method for manufacturing the above medium carbon round steel, comprising the following steps performed in sequence:

[0048] (1) Smelting and casting;

[0049] (2) heating;

[0050] (3) Forging or rolling to obtain round steel;

[0051] (4) Step annealing: first keep the temperature at 770-790℃ for 1-3h, then keep the temperature at 590-610℃ for 2-4h, then keep the temperature at 730-750℃ for 1-3h, then keep the temperature at 690-710℃ for 2-4h, then cool the furnace to below 350℃ and air cool.

[0052] (5) Surface treatment to remove the martensitic hardened layer on the surface of the round steel to obtain medium carbon round steel without the martensitic hardened layer on the surface.

[0053] The hardness of the round steel of the present invention after forging or rolling is about 260 HBW, which is too high for the deformation resistance in the subsequent cold extrusion process. It may not only damage the mold, but also cause excessive stress in the round steel after cold extrusion, and it is easy to crack during subsequent quenching.

[0054] Some known references use spheroidizing annealing to treat the microstructure before cold extrusion. However, while spheroidizing annealing can soften the microstructure and reduce hardness, the heat treatment time is too long (usually ≥ 24 hours). Furthermore, when processing shafts and rods, the microstructure hardness of the fully spheroidized microstructure is too low, resulting in easy bending during cold extrusion.

[0055] Based on this, the present invention creatively proposes a step annealing heat treatment process: first, the round steel is kept at a temperature of 770-790°C for 1-3 hours, at which time the round steel is completely austenitized, then kept at a temperature of 590-610°C for 2-4 hours, its austenite transforms into lamellar pearlite, then kept at a temperature of 730-750°C for 1-3 hours, partially austenitizing the lamellar pearlite, then kept at a temperature of 690-710°C for 2-4 hours, during which the partially austenitized part gradually transforms into spherical pearlite, and finally cooled to below 350°C before being removed from the furnace and air-cooled. The present invention adopts the above step annealing process to replace conventional spheroidizing annealing.

[0056] By adopting the step annealing process of the present invention, the microstructure of medium carbon round steel can satisfy the volume phase ratio of lamellar pearlite of 35-45%, and the hardness of the obtained round steel is 185-210 HBW. This avoids excessive hardness causing excessive resistance to subsequent cold extrusion deformation, and avoids excessive hardness causing bending of the shaft during cold extrusion, and also reduces energy consumption, saves heat treatment time, and improves production efficiency.

[0057] Preferably, in step (2) of the method of the present invention, the heating temperature is 1050-1250°C.

[0058] In the method described herein, the heating temperature is controlled to 1050-1250°C because steel is austenitized at 1050-1250°C. During the heating process, Mn carbides partially or completely dissolve in the austenite. During the subsequent rolling / forging and cooling processes, Al forms fine carbonitrides, which pin austenite grain boundaries and refine the as-rolled steel structure. Mn dissolved in austenite not only improves the hardenability of the steel but also enhances the hardenability of the martensite during quenching.

[0059] Preferably, in step (3) of the method of the present invention, the final rolling or final forging temperature is ≥800°C.

[0060] In the method of the present invention, the final rolling or final forging temperature is controlled to be ≥800°C because under this condition, the steel undergoes recrystallization and strain-induced precipitation, forming a matrix structure of ferrite and pearlite, and fine carbonitrides are precipitated.

[0061] Preferably, in step (5) of the method of the present invention, the surface treatment comprises turning, wherein the turning feed rate is 0.06 to 0.10 mm / r and the coolant flow rate is 20 to 28 L / min.

[0062] Regarding the turning feed rate during the turning process, some known references indicate that the turning feed rate for shafts should be ≥ 0.2 mm / r. In the preferred embodiment of the present invention, the turning feed rate is 0.06-0.1 mm / r, which ensures a certain efficiency while also ensuring that the heat generated during turning is not excessively high.

[0063] Regarding the coolant flow rate during the turning process, some known references state that the coolant flow rate for shaft turning is ≤15 L / min. However, the coolant flow rate of the present invention is 20-28 L / min, which increases the coolant flow rate and ensures that the temperature of the round steel does not rise too high during turning.

[0064] Preferably, in step (5) of the method of the present invention, the surface treatment includes calendering, wherein the calendering pressure is 16 to 20 kN.

[0065] Regarding the calendering process, some known references state that the calendering pressure for shafts is ≥25 kN. The calendering process of the present invention controls the pressure to 16-20 kN, ensuring that the stress in the round steel is not too high.

[0066] Preferably, in step (5) of the method described in the present invention, the surface treatment includes belt polishing, and the cryptocrystalline martensite layer generated by turning and grinding is removed by belt polishing, that is, the depth of belt polishing exceeds the depth of the cryptocrystalline martensite layer generated by turning and grinding, thereby ensuring that there is no martensitic hardened layer on the surface of the round steel before cold extrusion, and avoiding cracking of the round steel surface during the cold extrusion process.

[0067] Preferably, after the above step (5), step (6) of tempering treatment is further performed, wherein the quenching temperature is 850°C to 930°C, the holding time is 60 to 180 minutes, and water quenching or oil quenching is adopted; the tempering temperature is 460 to 530°C, the holding time is 60 to 180 minutes, and air cooling or water cooling is performed after tempering.

[0068] The medium carbon round steel of the present invention has the following advantages and beneficial effects compared with the prior art:

[0069] By fully understanding the impact of various alloy components on hardenability, the medium-carbon round steel of this invention utilizes an economical alloy composition to meet the hardenability requirements of steel for cold extrusion of reducer input shafts. By controlling the DI value within a reasonable range (e.g., 4.72 to 10.21 inches), complex performance requirements are quantified into easily measurable and evaluable indicators, enabling precise control and optimization during the production process, improving the consistency and cost-effectiveness of material properties.

[0070] The method of the present invention adopts step annealing instead of spheroidizing annealing, so that the structure and hardness of round steel are adapted to the cold extrusion process, which reduces energy consumption, improves production efficiency, solves the problem that medium carbon steel is not compatible with the cold forming process, and can meet users' needs for steel for cold extrusion of reducer input shafts.

[0071] Furthermore, in a preferred embodiment of the method of the present invention, an improved round steel surface treatment process is used to ensure that no martensitic hardened layer exists on the round steel surface before cold extrusion, thereby avoiding cracking of the round steel surface during the cold extrusion process.

[0072] In a preferred embodiment of the present invention, the medium-carbon round steel of the present invention is obtained through annealing, and the microstructure of the medium-carbon round steel satisfies a volume ratio of lamellar pearlite of 35-45%. The hardness of the round steel obtained after annealing is 185-210 HBW, which not only avoids excessive hardness leading to excessive resistance to subsequent cold extrusion deformation, but also avoids excessive hardness leading to shaft bending during cold extrusion, thereby saving energy and reducing consumption, and improving production efficiency. Furthermore, after the round steel with a diameter of ≤90 mm undergoes overall quenching and tempering heat treatment, its yield strength is ≥1100 MPa, tensile strength ≥1250 MPa, elongation ≥15%, cross-sectional reduction rate ≥50%, and -40°C impact energy Akv2 ≥40 J, meeting the strength and toughness requirements of the round steel.

[0073] In addition, it should be noted that the chemical composition and process design of the medium carbon round steel described in the present invention are reasonable, its process window is wide, and batch commercial production can be achieved on the bar production line, which has good promotion prospects and application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] FIG1 shows the rolled microstructure of the medium carbon round steel of Example 1.

[0075] FIG2 shows the microstructure of the medium carbon round steel of Example 1 after step annealing.

[0076] FIG3 shows the surface morphology of the medium carbon round steel of Example 1 after surface treatment.

[0077] FIG4 shows the microstructure of the medium carbon round steel of Example 1 after quenching and tempering. DETAILED DESCRIPTION

[0078] The medium carbon round steel and the manufacturing method thereof described in the present invention will be further explained and illustrated below in conjunction with the drawings in the specification and specific embodiments. However, such explanation and illustration do not constitute an improper limitation to the technical solution of the present invention.

[0079] Examples 1-6 and Comparative Examples 1-4

[0080] The medium carbon round steels of Examples 1-6 and the comparative round steels of Comparative Examples 1-4 were prepared by the following steps:

[0081] (1) The steel is smelted in an electric furnace or a converter and cast into continuous casting billets or ingots. The chemical compositions are shown in Tables 1-1 and 1-2. In the casting process, mold casting or continuous casting can be used.

[0082] (2) Heating: Control the heating temperature to 1050-1250°C.

[0083] (3) Forging or rolling: Control the final rolling or final forging temperature to ≥800℃. If forging is performed, the steel can be directly forged to the desired final round steel size during the forging process. If rolling is performed, the steel billet can be directly rolled to the final specification during the rolling process, or the steel billet can be rolled to the specified intermediate billet size, and then heated and rolled to the final finished round steel size.

[0084] (4) Step annealing: The step annealing heat treatment process is to first keep the temperature at 770-790℃ for 1-3 hours, then keep the temperature at 590-610℃ for 2-4 hours, then keep the temperature at 730-750℃ for 1-3 hours, then keep the temperature at 690-710℃ for 2-4 hours, and then cool the furnace to below 350℃ and air cool.

[0085] (5) Surface treatment: including turning, grinding and belt polishing processes; the turning feed rate is 0.06-0.10 mm / r, the coolant flow rate is 20-28 L / min; the grinding process pressure is 16-20 kN; the belt polishing depth must exceed the depth of the cryptocrystalline martensite layer (white bright layer) produced by turning and grinding.

[0086] In addition, in order to further improve the performance of medium carbon round steel, step (6) of tempering heat treatment can be further performed after the above step (5): wherein the quenching temperature is 850°C to 930°C, the holding time is 60 to 180 minutes, and water quenching or oil quenching is adopted; the tempering temperature is 460 to 530°C, the holding time is 60 to 180 minutes, and air cooling or water cooling is performed after tempering.

[0087] The medium carbon round steels of Examples 1-6 described in the present invention are all produced by the above steps, and their chemical compositions and related process parameters all meet the control requirements of the design specifications of the present invention.

[0088] The comparative round steels of Comparative Examples 1-4 were also produced using the above-mentioned steps and processes, but their chemical composition design and related specific process parameters did not meet the design specification requirements of the present invention.

[0089] Tables 1-1 and 1-2 list the mass percentages of the chemical elements in the medium carbon round steels of Examples 1-6 and the comparative round steels of Comparative Examples 1-4 (the balance of the chemical composition is Fe and other unavoidable impurities except P, S, H, B, O, and Ti), DI values, [Al] / [N] values, and [Al][N]×10 4 The unit of chemical composition is wt%, and the unit of DI is in. [Al] / [N] and [Al][N]×10 4 Substitute the numbers to obtain the dimensionless value.

[0090] Table 1-1.

[0091] Table 1-2.

[0092] Table 2-1 and Table 2-2 list the specific process parameters of the medium carbon round steel of Examples 1-6 and the comparative round steel of Comparative Examples 1-4 in the above process steps.

[0093] Table 2-1.

[0094] Table 2-2.

[0095] After step (5) and before step (6), the obtained medium carbon round steels of Examples 1-6 and the comparative round steels of Comparative Examples 1-4 were sampled and subjected to various relevant performance tests:

[0096] (1) The bar samples of each embodiment and comparative example were tested in accordance with GB / T 231.1 "Brinell hardness test for metallic materials - Part 1: Test method" to obtain the Brinell hardness of the bar samples of each embodiment and comparative example. The measured Brinell hardness results are listed in Table 3-1.

[0097] (2) The microstructure of the bar samples of each embodiment and comparative example was tested in accordance with GB / T 13298 "Methods for the Examination of Metal Microstructures" to obtain the thickness of the white bright layer and the volume ratio of the lamellar pearlite of each embodiment and comparative example. The test results are listed in Table 3-2.

[0098] The medium carbon round steel bars of Examples 1-6 and the comparative round steel bars of Comparative Examples 1-4 obtained before and after step (6) were sampled and tested using GB / T 228.1 "Tensile testing of metallic materials - Part 1: Room temperature test methods" and GB / T 229 "Charpy notched impact test method for metals" to determine the mechanical properties of the bars of each Example and Comparative Example. The relevant test results are listed in Table 3-3 (untempered) and Table 3-4 (tempered).

[0099] Table 3-1.

[0100] Table 3-2.

[0101] Table 3-3.

[0102] Table 3-4.

[0103] Table 3-5 lists the statistical results of cracking in 10,000 reducer shaft component samples made from medium-carbon round steel bars of Examples 1-6 and comparative round steel bars of Comparative Examples 1-4 after cold extrusion. Comparative Example 3 shows the number of samples that showed bending. Cold extrusion was performed in a closed die with a maximum deformation of 20% ± 1%. After cold extrusion, the samples were inspected for cracking using magnetic particle inspection and visually inspected for bending.

[0104] Table 3-5.

[0105] It can be seen from Table 3-1 that compared with Comparative Example 3, the medium carbon round steels of Examples 1-6 prepared using the present technical solution have Brinell hardnesses that meet the requirements of 185-210 HBW, while Comparative Example 3 has a low hardness due to the conventional spheroidizing annealing process, which will cause the shaft blank to bend during the subsequent cold extrusion process.

[0106] It can be seen from Table 3-2 that compared with Comparative Example 4, no hardened layer appears on the surface of the medium carbon round steel of Examples 1-6 prepared by adopting the present technical solution, and the volume phase ratio of lamellar pearlite is in the range of 35-45%. However, a hardened layer exists on the surface of the round steel of Comparative Example 4 in which the surface treatment process is not strictly controlled, which will increase the cracking rate in the subsequent cold extrusion process.

[0107] As can be seen from Table 3-3, compared with Comparative Example 3, the mechanical properties of the medium-carbon round steels of Examples 1-6 produced using this technical solution all meet the requirements of yield strength ≥400 MPa, tensile strength ≥600 MPa, elongation ≥30%, reduction of area ≥60%, and -40°C impact energy Akv2 ≥20 J. However, the strength of Comparative Example 3 is lower than the requirements of the present invention due to the low proportion of lamellar pearlite caused by the conventional annealing process.

[0108] It can be seen from Tables 3-4 that compared with Comparative Example 2, the mechanical properties of the medium carbon round steels of Examples 1-6 prepared by the present technical solution after quenching and tempering all meet the requirements of yield strength ≥ 1100 MPa, tensile strength ≥ 1250 MPa, elongation ≥ 15%, cross-sectional shrinkage ≥ 50%, and -40°C impact energy Akv2 ≥ 40 J. However, since the nitrogen content in Comparative Example 2 does not meet the requirements of the present invention, the AlN fine grain strengthening effect is not fully reflected, resulting in the strength not meeting the requirements.

[0109] It can be seen from Tables 3-5 that compared with comparative examples 1-4, the medium carbon round steels of Examples 1-6 prepared by adopting the present technical solution have very low cracking rates after cold extrusion, all within 0.5%.

[0110] Figure 1 shows the as-rolled microstructure of the medium carbon round steel of Example 1. As shown in Figure 1 , the as-rolled microstructure of the medium carbon round steel of Example 1 is bainite.

[0111] Figure 2 shows the microstructure of the medium carbon round steel after step annealing of Example 1. As shown in Figure 2, the microstructure of the annealed medium carbon round steel of Example 1 is lamellar pearlite + spherical pearlite, and the volume phase ratio of the lamellar pearlite is about 40%.

[0112] Figure 3 shows the surface morphology of the medium carbon round steel after surface treatment in Example 1. As shown in Figure 3, after surface treatment, the medium carbon round steel in Example 1 has only very slight burrs on its surface and no other surface defects.

[0113] Figure 4 shows the microstructure of the medium carbon round steel after quenching and tempering of Example 1. As shown in Figure 4, the microstructure of the medium carbon round steel in the quenched and tempered state of Example 1 is tempered bainite.

[0114] From the above, it can be seen that the present invention solves the problem of incompatibility between medium carbon steel and cold forming process by reasonable chemical composition design and combined with optimized process, and can obtain medium carbon round steel for cold extrusion that meets the requirements.

[0115] It should be noted that the chemical composition and process design of the medium carbon round steel described in the present invention are reasonable, its process window is wide, and batch commercial production can be achieved on the bar production line, which has good promotion prospects and application value.

[0116] In addition, the combination of the various technical features in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All technical features recorded in this case can be freely combined or combined in any way unless there is a contradiction between them.

[0117] It should also be noted that the above examples are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above examples, and many similar variations are possible. Any variations that can be directly derived or conceived by those skilled in the art from the present disclosure are intended to fall within the scope of protection of the present invention.

Claims

1. A medium-carbon round steel, characterized in that, In addition to containing Fe and inevitable impurities, the medium-carbon round steel further contains the following chemical elements by mass percentage: C: 0.36 - 0.43%, Si: 0.1 - 0.4%, Mn: 0.6 - 1.0%, Cr: 0.6 - 1.0%, Mo: 0.2 - 0.4%, Ni: 1.4 - 1.7%, Al: 0.02 - 0.04%, N: 0.008 - 0.015%. Its DI value is 4.72 to 10.21 in, where DI = (0.171 + 0.001C + 0.265C 2 )(1 + 3.333Mn)(1 + 0.7Si)(1 + 0.365Cu)(1 + 2.16Cr)(1 + 3Mo)(1 + 0.363Ni)(1 + 1.73V). When calculating, the numerical values before the percentage signs of the corresponding element mass percentages in the medium-carbon round steel are substituted for the respective element symbols in the formula; The medium-carbon round steel is obtained by annealing, and the microstructure of the medium-carbon round steel is lamellar pearlite + globular pearlite.

2. The medium-carbon round steel according to claim 1, characterized in that The medium-carbon round steel is composed of the following chemical elements by mass percentage: C: 0.36 - 0.43%, Si: 0.1 - 0.4%, Mn: 0.6 - 1.0%, Cr: 0.6 - 1.0%, Mo: 0.2 - 0.4%, Ni: 1.4 - 1.7%, Al: 0.02 - 0.04%, N: 0.008 - 0.015%; the balance is Fe and inevitable impurities.

3. The medium-carbon round steel according to claim 1 or 2, characterized in that, The medium-carbon round steel further contains at least one of the following chemical elements: 0 < Cu ≤ 0.20%, 0 < V ≤ 0.05%, 0 < Nb ≤ 0.05%.

4. The medium carbon round steel according to claim 1 or 2, characterized in that, Among the inevitable impurities, the content of each impurity element satisfies at least one of the following items: P ≤ 0.015%, S ≤ 0.025%, H ≤ 0.0002%, B ≤ 0.001%, O ≤ 0.0020%, 0 < Ti ≤ 0.08%, preferably 0 < Ti ≤ 0.01%.

5. The medium-carbon round steel according to claim 1 or 2, characterized in that, The contents of Al and N elements in the medium-carbon round steel satisfy: [Al] / [N] ≥ 2, [Al][N] ≥ 2×10 -4 .

6. The medium-carbon round steel according to claim 1 or 2, characterized in that, The volume phase ratio of the lamellar pearlite in the medium-carbon round steel is 35 - 45%.

7. The medium-carbon round steel according to claim 1 or 2, characterized in that, The hardness of the medium-carbon round steel is 185 - 210 HBW.

8. The medium-carbon round steel according to claim 1 or 2, characterized in that, The medium-carbon round steel satisfies one or more of the following: yield strength ≥ 400 MPa, tensile strength ≥ 600 MPa, elongation ≥ 30%, reduction of area ≥ 60%, -40°C impact energy Akv2 ≥ 20 J.

9. The medium-carbon round steel according to claim 1 or 2, characterized in that, After quenching and tempering treatment, the medium-carbon round steel has a yield strength ≥ 1100 MPa, a tensile strength ≥ 1250 MPa, an elongation ≥ 15%, a reduction of area ≥ 50%, and a -40°C impact energy Akv2 ≥ 40 J.

10. A method for manufacturing medium-carbon round steel according to any one of claims 1-9, characterized in that, The method includes the following steps carried out in sequence: (1) Smelting and casting; (2) Heating; (3) Forging or rolling to obtain round steel; (4) Step annealing: first hold at a temperature of 770 - 790°C for 1 - 3 h, then hold at a temperature of 590 - 610°C for 2 - 4 h, then hold at a temperature of 730 - 750°C for 1 - 3 h, then hold at a temperature of 690 - 710°C for 2 - 4 h, and then cool in the furnace to below 350°C and take out for air cooling; (5) Surface treatment to remove the martensite hardened layer on the surface of the round steel to obtain a medium-carbon round steel without a martensite hardened layer on the surface.

11. The method according to claim 10, wherein In step (2), the heating temperature is 1050 - 1250°C.

12. The method according to claim 10, wherein In step (3), the final rolling or forging temperature ≥ 800°C.

13. The method according to claim 10, wherein In step (5), the surface treatment includes turning, where the turning feed rate is 0.06 - 0.10 mm / r and the coolant flow rate is 20 - 28 L / min.

14. The method according to claim 10, wherein In step (5), the surface treatment includes burnishing, and the burnishing pressure is 16 - 20 KN.

15. The method according to claim 10, characterized in that, In step (5), the surface treatment includes abrasive belt polishing, and the cryptocrystalline martensite layer generated by turning and burnishing is removed by abrasive belt polishing.

16. The method according to any one of claims 10-15, characterized in that, The method further includes step (6) quenching and tempering treatment after step (5), where the quenching temperature is 850 °C - 930 °C, the holding time is 60 - 180 min, and water quenching or oil quenching is adopted; the tempering temperature is 460 - 530 °C, the holding time is 60 - 180 min, and air cooling or water cooling is carried out after tempering.