Medium carbon round steel for cold extrusion and manufacturing method thereof

Abstract

The invention discloses medium-carbon round steel for cold extrusion, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.36 to 0.43 percent of C, 0.1 to 0.4 percent of Si, 0.6 to 1 percent of Mn, 0.6 to 1 percent of Cr, 0.2 to 0.4 percent of Mo, 1.4 to 1.7 percent of Ni, 0.02 to 0.04 percent of Al, 0.008 to 0.015 percent of N and the balance of Fe. And after annealing, the microstructure is flaky pearlite and spherical pearlite. Correspondingly, the invention also discloses a manufacturing method of the medium-carbon round steel for cold extrusion, which comprises the following steps: (1) smelting and casting; (2) heating; (3) forging or rolling; (4) stepped annealing; and (5) surface treatment.

Description

Technical Field

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

[0002] The working ability of shaft parts of vehicle retarders generally depends on the strength and stiffness of the shaft, and also depends on the vibration stability at high speeds. And it bears both bending moment and torque during operation, so there are also certain requirements for the toughness of the round steel.

[0003] In order to meet the use requirements of shaft parts of retarders, there are currently two main process routes for manufacturing shaft products in retarders. One is to use low-carbon steels such as 20CrMnTi, 20CrMo, 20MnCr5, etc. and perform surface carburization; the other is to use medium-carbon steels (such as 45, 50 steel) or medium-carbon low-alloy steels (such as 40Cr, 42CrMo) and perform surface induction heat treatment. The forming methods of round steel under these two process routes are usually direct turning or hot forging, and there are not many cold forming methods. Even if there are, they are usually applied to low-carbon steels. Due to the poor plastic toughness of medium-carbon steels, the stress in the round steel is relatively large after cold forming, and it is easy to crack during the subsequent quenching process, so it is usually not used for cold forming.

[0004] In automotive retarders, the failure mode of shaft parts is mostly fatigue fracture. The fatigue resistance of shaft parts is not only related to their strength and toughness, but also closely related 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 the keys to ensuring the fatigue resistance of shaft parts. In the prior art, traditional steels for shaft parts of conventional fuel vehicles are mostly used to manufacture shaft parts of retarders, and the compatible processes are mostly turning or hot forging.

[0005] For example, a Chinese patent with the publication number CN104975235A, publication date of October 14, 2015, and title "A 120KSI 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. In the claims of this invention patent, 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%, with the balance being Fe and unavoidable impurity elements, where the contents of V and Nb are not both ≤0.010%. It mainly achieves the requirements of high strength and toughness through the composition design of the steel and subsequent quenching and tempering heat treatment processes.

[0006] Another example: A Chinese patent with the publication number CN112981233A, publication date of June 18, 2021, and title "A Low-Silicon Medium-Carbon Gear Steel Suitable for Cold Forging and Its Manufacturing Method" relates to a low-silicon medium-carbon gear steel suitable for cold forging and its manufacturing method. In the claims of this invention patent, 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%, with the balance being Fe and unavoidable impurity elements. Through the composition design of the steel, control of dimensional accuracy, decarburized layer depth, and spheroidizing hardness and structure spheroidization rate, a medium-carbon gear steel with excellent cold forging performance is obtained, but no specific surface treatment is carried out on the surface hardened layer that affects cold forging cracking. Summary of the Invention

[0007] One of the objectives of the present invention is to provide a medium-carbon round steel for cold extrusion, which can accurately identify the influence of each alloy element on the actual hardenability, thereby obtaining a hardenability curve that meets the service conditions of the input shaft of the reducer to ensure good strength and toughness of the output shaft.

[0008] To achieve the above objective, the present invention proposes a medium-carbon round steel for cold extrusion, which also contains the following chemical elements with mass percentages as follows:

[0009] C: 0.36 - 0.43%, Si: 0.1 - 0.4%, Mn: 0.6 - 1%, Cr: 0.6 - 1%, 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 - 10.21 in, where DI = (0.171 + 0.001C + 0.265C2)(1 + 3.333Mn)(1 + 0.7Si)(1 + 0.365Cu)(1 + 2.16Cr)(1 + 3Mo)(1 + 0.363Ni)(1 + 1.73V), and the values before the percentage signs of the mass percentage contents of each element in the formula are substituted.

[0011] Its microstructure after annealing is lamellar pearlite + globular pearlite.

[0012] Furthermore, in the medium-carbon round steel for cold extrusion of the present invention, the mass percentage contents of its various chemical elements are as follows:

[0013] C: 0.36 - 0.43%, Si: 0.1 - 0.4%, Mn: 0.6 - 1%, Cr: 0.6 - 1%, 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 other inevitable impurities.

[0014] In the medium-carbon round steel for cold extrusion of the present invention, the design principles of its various chemical elements are specifically as follows:

[0015] C: In the medium-carbon round steel for cold extrusion of the present invention, as the C content in austenite increases, the critical cooling rate of the steel is greatly reduced, the critical diameter of the steel is increased, thereby improving the hardenability of the steel and ensuring the strength and stiffness of the shaft. However, too high a C content will increase the tendency of quenching cracking. Based on this, in the medium-carbon round steel for cold extrusion of the present invention, the C content is set to 0.36 - 0.43%.

[0016] Si: In the medium-carbon round steel for cold extrusion of the present invention, Si is an element that strengthens ferrite and can improve the hardenability to a certain extent. Si replaces Fe atoms in the steel in a substitutional manner, hindering the movement of dislocations and being beneficial to the improvement of the strength of the steel. However, excessive Si will reduce the plasticity and toughness of the steel. Based on this, in the medium-carbon round steel for cold extrusion of the present invention, the Si content is set to: 0.1 - 0.4%.

[0017] Mn: In the medium-carbon round steel for cold extrusion described in the present invention, Mn dissolves into ferrite to form a substitutional solid solution, producing the effect of solid solution strengthening. 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, too high an Mn content will lead to severe segregation in the round steel and is prone to quenching cracks. Based on this, in the medium-carbon round steel for cold extrusion described in the present invention, the Mn content is set to: 0.6 - 1%.

[0018] Cr: In the medium-carbon round steel for cold extrusion 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 martensite structure, and obtain steel with higher strength. At the same time, during the heating process, if the carbides of Cr are not completely dissolved, it can also play a role in inhibiting the growth of austenite grains. It should be noted that the content of Cr element in the steel should not be too high. When the content of Cr element in the steel is too high, coarse carbides will be formed, deteriorating the impact performance of the steel. Based on this, in the medium-carbon round steel for cold extrusion described in the present invention, the Cr content is set to: 0.6 - 1%.

[0019] Mo: In the medium-carbon round steel for cold extrusion described in the present invention, Mo is a ferrite-forming element. Adding an appropriate amount of Mo element is beneficial to improving the hardenability of the steel, making it easy to form bainite and martensite strengthening phases during quenching. When tempering in a relatively high temperature range, fine carbides will be formed to improve the strength of the steel. It should be noted that Mo belongs to precious alloy elements, and adding a higher content of Mo will lead to an increase in cost. Based on this, in the medium-carbon round steel for cold extrusion described in the present invention, the Mo content is set to: 0.2 - 0.4%.

[0020] Ni: In the medium-carbon round steel for cold extrusion described in the present invention, the Ni element exists in solid solution in the steel. In the composition system of the present invention, 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 too high a content of Ni should not be added to the steel. Adding an excessive amount of Ni will lead to too high a content of retained austenite in the steel, reducing the strength of the steel. In addition, the Ni element also belongs to precious metals, and adding a higher content of Ni will lead to an increase in cost. Based on this, in the medium-carbon round steel for cold extrusion described in the present invention, the Ni content is set to: 1.4 - 1.7%.

[0021] Al: In the medium carbon round steel for cold extrusion described in the present invention, Al forms fine AlN precipitation during steelmaking, which inhibits the growth of austenite grains and refines austenite grains during the subsequent cooling process. Too high Al content will lead to the formation of larger Al oxides, and coarse aluminum oxide hard inclusions will deteriorate the fatigue properties of steel. Based on this, in the medium carbon round steel for cold extrusion described in the present invention, the Al content is set to 0.02-0.04%.

[0022] N: In the medium carbon round steel for cold extrusion described in the present invention, N is an interstitial atom and also an MX type precipitate forming element, which can improve the strength and toughness of the round steel through the fine grain strengthening effect of AlN, but a high N content will lead to an increase in its enrichment at the defects and the formation of coarse nitride precipitation particles, especially the formation of TiN particles with sharp angles, which affects the impact toughness of the round steel. Based on this, in the medium carbon round steel for cold extrusion described in the present invention, the N content is set to: 0.008-0.015%.

[0023] Furthermore, the medium carbon round steel for cold extrusion described in the present invention also contains at least one of the following chemical elements: 0<Cu≤0.2%, 0<V≤0.05%, 0<Nb≤0.05%.

[0024] In the medium carbon round steel for cold extrusion of the present invention, the design principles of the above chemical elements are specifically described as follows:

[0025] Cu: In the medium carbon round steel for cold extrusion of the present invention, Cu can improve the hardenability and corrosion resistance of the steel, but if the Cu content is too high, it will be enriched at the grain boundary, resulting in grain boundary weakening and cracking. Based on this, in the medium carbon round steel for cold extrusion of the present invention, in some preferred embodiments, the Cu content is set to: 0<Cu≤0.2%.

[0026] V: In the medium carbon round steel for cold extrusion described in the present invention, V can combine with C or N in the steel to form precipitates to improve the strength of the steel, but if the content is too high, coarse VC particles will be formed, which will deteriorate the plasticity and toughness of the steel. Based on this, in the medium carbon round steel for cold extrusion described in the present invention, in some preferred embodiments, the V content is set to: 0<V≤0.05%.

[0027] Nb: In the medium carbon round steel for cold extrusion described in the present invention, Nb will form fine precipitate phase after being added to the steel, which can play a role in fine grain strengthening and reducing hydrogen embrittlement sensitivity, but excessive Nb will form coarse NbC particles during the smelting process, which will reduce impact toughness. Based on this, in the medium carbon round steel for cold extrusion described in the present invention, in some preferred embodiments, the Nb content is set to: 0<Nb≤0.05%.

[0028] Furthermore, in the medium carbon round steel for cold extrusion according to the present invention, among the inevitable impurities, the contents of each impurity element satisfy at least one of the following items: P≤0.015%, S≤0.025%, H≤0.0002%, B≤0.001%, O≤0.0020%, 0<Ti≤0.01%.

[0029] In the above technical solution, P, S, H, B, O, and Ti are all impurity elements in the steel. Under the allowable technical conditions, in order to obtain steel with better performance and higher quality, the content of impurity elements in the steel should be reduced as much as possible, where:

[0030] P: In the medium carbon round steel for cold extrusion according to the present invention, P in the steel segregates at the grain boundaries, which will reduce the binding energy of the grain boundaries, deteriorate the low-temperature impact performance of the steel, and the coexistence of P and Mn will exacerbate the temper brittleness of the steel. In addition, P segregating at the grain boundaries will cause the steel to undergo intergranular fracture when subjected to impact loads, forming relatively large cleavage planes and reducing the energy absorbed by the steel when subjected to impact. Based on this, in some preferred embodiments of the medium carbon round steel for cold extrusion according to the present invention, in order to ensure the low-temperature impact toughness of the input shaft, the P content is controlled to be: P≤0.015%.

[0031] S: In the medium carbon round steel for cold extrusion according to the present invention, the solubility of S in δ-ferrite and austenite is very small. During the solidification process of the molten steel, S will segregate. If the S content is too high, more coarse sulfide inclusions will be formed, which will harm the fatigue resistance of the round steel. However, adding an appropriate amount of S to the steel can form CaS to improve the cutting performance. Based on this, in some preferred embodiments of the medium carbon round steel for cold extrusion according to the present invention, in order to ensure the fatigue resistance of the round steel and leave room for the user's easy-cutting requirements, the S content is controlled to be: S≤0.025%.

[0032] H: In the medium carbon round steel for cold extrusion according to the present invention, under the action of the hydrostatic stress field of edge dislocations in the steel, H will accumulate at defects, forming hydrogen embrittlement. In high-tensile-strength steel, the density of dislocations, sub-grain boundaries, etc. is high. If the H content is too high, after quenching and tempering heat treatment of the steel, more H atoms will be enriched at defects. The aggregation of H atoms will form H molecules, resulting in delayed fracture of the steel. Based on this, in some preferred embodiments of the medium carbon round steel for cold extrusion according to the present invention, the content of H is controlled to be: H≤0.0002%.

[0033] B: In the medium carbon round steel for cold extrusion according to the present invention, B has a strong affinity with N and O, and is extremely easy to react with metals such as Ti to form extremely hard borides such as TiB2. At the same time, a very small amount of B can also significantly improve the hardenability of the round steel. Based on this, in some preferred embodiments of the medium carbon round steel for cold extrusion according to the present invention, the B content is controlled to be: B≤0.001%.

[0034] O: In the medium-carbon round steel for cold extrusion according to the present invention, O forms inclusions such as Al2O3 and TiO with Al and Ti in the steel. Based on this, in some preferred embodiments of the medium-carbon round steel for cold extrusion according to the present invention, to ensure the continuity of the steel matrix and fatigue resistance, the O content is controlled to be: O ≤ 0.0020%.

[0035] Ti: In the medium-carbon round steel for cold extrusion according to the present invention, Ti combines with N and C in the steel to form TiC, Ti(CN), and TiN. The angular TiN disrupts the collective continuity and has an adverse effect on the fatigue performance of the steel. Based on this, in some preferred embodiments of the medium-carbon round steel for cold extrusion according to the present invention, the Ti content is controlled to be: 0 < Ti ≤ 0.01%.

[0036] Furthermore, in the medium-carbon round steel for cold extrusion according to the present invention, it also satisfies: [Al] / [N] ≥ 2, [Al][N] ≥ 2×10 -4 .

[0037] In the present invention, the above [Al] and [N] should be substituted with the values before the percentage sign of their mass percentage content.

[0038] The present invention can further ensure the content of AlN by controlling [Al][N] ≥ 2x10 -4 .

[0039] In addition, as described above, too high an Al content will lead to the formation of relatively large Al oxides, and the coarse alumina hard inclusions will deteriorate the fatigue performance of the steel. Therefore, the present invention controls [Al] / [N] ≥ 2 to ensure the content and size of other nitrides, thereby further improving the performance of the medium-carbon round steel for cold extrusion.

[0040] Furthermore, in the medium-carbon round steel for cold extrusion according to the present invention, the volume phase ratio of lamellar pearlite is 35 - 45%.

[0041] Furthermore, in the medium-carbon round steel for cold extrusion according to the present invention, its hardness after annealing is 185 - 210 HBW.

[0042] Furthermore, in the medium-carbon round steel for cold extrusion according to the present invention, its yield strength after quenching and tempering ≥ 1100 MPa, tensile strength ≥ 1250 MPa, elongation ≥ 15%, reduction of area ≥ 50%, impact energy Akv2 at -40 °C ≥ 40 J.

[0043] Accordingly, another object of the present invention is to provide a manufacturing method of the above medium carbon round steel for cold extrusion. By adopting this method in combination with the above-mentioned component ratio, it can not only avoid excessive deformation resistance during subsequent cold extrusion due to too high hardness, but also avoid bending of the shaft rod during cold extrusion due to too low hardness. Moreover, it reduces energy consumption, saves heat treatment time, and improves production efficiency. At the same time, it also ensures that there is no cryptocrystalline martensite layer (hardened layer) on the surface of the round steel after the surface treatment process, thereby avoiding cracking on the surface of the round steel during the cold extrusion process.

[0044] To achieve the above object, the present invention proposes a manufacturing method of medium carbon round steel for cold extrusion, which includes the steps:

[0045] (1) Smelting and casting;

[0046] (2) Heating;

[0047] (3) Forging or rolling;

[0048] (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;

[0049] (5) Surface treatment to make the surface of the round steel before cold extrusion free of martensite hardened layer.

[0050] In the present invention, the hardness of the round steel with the component system of the present invention after forging or rolling is about 260 HBW, which is too large for the subsequent cold extrusion process. It may not only damage the die, but also cause excessive stress in the round steel after cold extrusion, and is prone to cracking during subsequent quenching.

[0051] In some known references, spheroidizing annealing is used to treat the microstructure before cold extrusion. However, although spheroidizing annealing can soften the microstructure and reduce hardness, its heat treatment time is too long (usually ≥ 24 hours). In addition, when processing shaft rod parts, the hardness of the fully spheroidized microstructure is too low, which will cause easy bending during cold extrusion.

[0052] Based on this, the present invention creatively proposes a heat treatment process of stepped annealing: first, heat preservation is carried out at a temperature of 770 - 790 °C for 1 - 3 h. At this time, the round steel is completely austenitized. Then, heat preservation is carried out at a temperature of 590 - 610 °C for 2 - 4 h, during which the austenite transforms into lamellar pearlite. Then, heat preservation is carried out at a temperature of 730 - 750 °C for 1 - 3 h, during which part of the lamellar pearlite is austenitized. Subsequently, during the process of heat preservation at a temperature of 690 - 710 °C for 2 - 4 h, part of the austenitized part gradually transforms into globular pearlite. Finally, it is cooled in the furnace to below 350 °C and then taken out of the furnace for air cooling to replace the ordinary spheroidizing annealing.

[0053] By adopting the stepped annealing process in the present invention, the structure of the round steel can meet the requirement that the volume phase ratio of lamellar pearlite is 35 - 45%, and the hardness of the round steel after annealing is 185 - 210 HBW. This not only avoids excessive deformation resistance during subsequent cold extrusion due to too high hardness, but also avoids bending of the shaft rod during cold extrusion due to too low hardness. Moreover, it reduces energy consumption, saves heat treatment time, and improves production efficiency.

[0054] Further, in step (2) of the manufacturing method described in the present invention, the heating temperature is controlled to be 1050 - 1250 °C.

[0055] In the manufacturing method described in the present invention, the heating temperature is controlled to be 1050 - 1250 °C because when the steel is heated for austenitization at 1050 °C - 1250 °C, during the heating process, part or all of the Mn carbides dissolve in the austenite. During the subsequent rolling / forging and cooling processes, Al forms fine carbonitrides, pinning the austenite grain boundaries and refining the as-rolled structure of the steel. The Mn dissolved in the austenite can improve the hardenability of the steel while increasing the hardenability of martensite during quenching.

[0056] Further, in step (3) of the manufacturing method described in the present invention, the finish rolling or finish forging temperature is controlled to be ≥ 800 °C.

[0057] In the manufacturing method described in the present invention, the finish rolling or finish forging temperature is controlled to be ≥ 800 °C because under this condition, recrystallization and strain-induced precipitation occur in the steel, forming a matrix structure of ferrite and pearlite, and fine carbonitrides precipitate.

[0058] Further, in step (5) of the manufacturing method described in the present invention, the surface treatment includes turning, wherein the turning feed rate is 0.06 - 0.1 mm / r and the coolant flow rate is 20 - 28 L / min.

[0059] Regarding the turning feed rate in the turning process, in some known references, the turning feed rate for shaft turning is ≥ 0.2 mm / r. In the preferred embodiment of the present invention, the turning feed rate is 0.06 - 0.1 mm / r, which can ensure a certain efficiency while ensuring that the heat generated during turning is not too high.

[0060] Regarding the coolant flow rate in the turning process, in some known references, the coolant flow rate for shaft turning is ≤ 15 L / min. In the present invention, the coolant flow rate is 20 - 28 L / min, increasing the coolant flow rate to ensure that the temperature of the round steel during turning is not too high.

[0061] Further, in step (5) of the manufacturing method of the present invention, the surface treatment includes burnishing, where the burnishing pressure is 16 - 20 KN.

[0062] Regarding the burnishing process, in some known references, the burnishing pressure for shaft burnishing is ≥ 25 KN. The burnishing process of the present invention controls the pressure to 16 - 20 KN to ensure that the stress in the round steel is not too high.

[0063] Further, in step (5) of the manufacturing method of the present invention, the surface treatment includes abrasive belt polishing, where the depth of abrasive belt polishing exceeds the depth of the cryptocrystalline martensite layer generated by turning and burnishing, thereby ensuring that there is no martensite hardened layer on the surface of the round steel before cold extrusion and avoiding cracking on the surface of the round steel during the cold extrusion process.

[0064] Further, after step (5) of the manufacturing method of the present invention, it also includes step (6) quenching and tempering treatment, where the quenching temperature is 850°C - 930°C, the holding time is 60 - 180 min, and water quenching or oil quenching is used; the tempering temperature is 460 - 530°C, the holding time is 60 - 180 min, and air cooling or water cooling is used.

[0065] The medium-carbon round steel for cold extrusion described in the present invention has the following advantages and beneficial effects compared with the prior art:

[0066] The medium-carbon round steel for cold extrusion described in the present invention fully recognizes the influence of each alloying element on hardenability, and uses economical alloying elements to meet the hardenability requirements of the steel for cold extrusion of the input shaft of the reducer. It quantifies its requirements through the DI value, which is convenient for production control.

[0067] The manufacturing method of the present invention uses step annealing instead of spheroidizing annealing, making the structure and hardness of the round steel suitable for the cold extrusion process, reducing energy consumption, improving production efficiency, solving the problem of incompatibility between medium-carbon steel and the cold forming process, and meeting the user's requirements for the steel for cold extrusion of the input shaft of the reducer.

[0068] In addition, in a preferred embodiment of the manufacturing method of the present invention, an improved surface treatment process for round steel is adopted, ensuring that there is no martensite hardened layer on the surface of the round steel before cold extrusion, and avoiding cracking on the surface of the round steel during the cold extrusion process.

[0069] In a preferred embodiment of the present invention, for the medium-carbon round steel for cold extrusion of the present invention, the volume phase ratio of lamellar pearlite in the structure after annealing is 35-45%, and the hardness of the round steel after annealing is 185-210 HBW. This not only avoids excessive deformation resistance during subsequent cold extrusion due to too high hardness, but also avoids bending of the shaft rod during cold extrusion due to too low hardness, saves energy and reduces consumption, and improves production efficiency. Moreover, after the round steel with a diameter ≤ 90 mm is subjected to overall quenching and tempering heat treatment, its yield strength ≥ 1100 MPa, tensile strength ≥ 1250 MPa, elongation ≥ 15%, reduction of area ≥ 50%, impact energy Akv2 at -40 °C ≥ 40 J, meeting the requirements for the strength and toughness of the round steel.

[0070] In addition, it should be noted that the chemical composition and process design of the medium-carbon round steel for cold extrusion of the present invention are reasonable, its process window is wide, it can be mass-produced commercially on the bar production line, and it has good popularization prospects and application value. Brief Description of the Drawings

[0071] Figure 1 Shows the as-rolled microstructural morphology of the medium-carbon round steel for cold extrusion in Example 1.

[0072] Figure 2 Shows the microstructural morphology of the medium-carbon round steel for cold extrusion in Example 1 after stepped annealing.

[0073] Figure 3 Shows the surface morphology of the medium-carbon round steel for cold extrusion in Example 1 after surface treatment. Detailed Embodiments

[0074] The following will further explain and illustrate the medium-carbon round steel for cold extrusion of the present invention and its manufacturing method in conjunction with the drawings of the specification and specific embodiments. However, this explanation and illustration shall not unduly limit the technical solution of the present invention.

[0075] Examples 1-6 and Comparative Examples 1-4

[0076] The medium-carbon round steel for cold extrusion in Examples 1-6 and the comparative round steel in Comparative Examples 1-4 were all prepared by the following steps:

[0077] (1) Smelting was carried out using an electric furnace or a converter, and cast into continuous casting billets or ingots. Their chemical compositions are shown in Tables 1-1 and 1-2; during the casting process, die casting or continuous casting can be used.

[0078] (2) Heating: Control the heating temperature at 1050 - 1250 °C.

[0079] (3) Forging or rolling: Control the final rolling or forging temperature ≥800 °C. Among them, if forging is carried out, during the forging process, it can be directly forged to the final size. If rolling is carried out, during the rolling process, the steel billet can be directly rolled to the final specification, or the steel billet can be rolled to the specified intermediate billet size, then heated and rolled to the final finished size.

[0080] (4) Step annealing: The step annealing heat treatment process is to 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 furnace cool to below 350 °C and take out of the furnace for air cooling.

[0081] (5) Surface treatment: including turning, burnishing, and abrasive belt polishing processes; among them, the turning feed rate is 0.06 - 0.1 mm / r, the coolant flow rate is 20 - 28 L / min; the burnishing process pressure is 16 - 20 KN; the abrasive belt polishing depth needs to exceed the depth of the cryptocrystalline martensite layer (bright white layer) generated by turning and burnishing.

[0082] In addition, in order to further improve the performance of medium carbon round steel for cold extrusion, step (6) quenching and tempering heat treatment can also be further carried out after the above step (5): among them, the quenching temperature is 850 °C - 930 °C, the holding time is 60 - 180 min, water quenching or oil quenching; the tempering temperature is 460 - 530 °C, the holding time is 60 - 180 min, air cooling or water cooling.

[0083] The medium carbon round steel for cold extrusion in Examples 1 - 6 of the present invention is all prepared by the above steps, and its chemical composition and related process parameters all meet the control requirements of the design specifications of the present invention.

[0084] The comparison round steel in Comparative Examples 1 - 4 is also prepared by the above step process, but there are deficiencies in its chemical composition design and related specific process parameters that do not meet the requirements of the design specifications of the present invention.

[0085] Tables 1 - 1 and 1 - 2 list the mass percentages of each chemical element in the medium carbon round steel for cold extrusion in Examples 1 - 6 and the comparison round steel in Comparative Examples 1 - 4.

[0086] Table 1 - 1. (wt.%, the balance is Fe and other inevitable impurities except P, S, H, B, O, and Ti)

[0087]

[0088] Table 1-2. (wt.%, with the balance being Fe and other inevitable impurities except P, S, H, B, O, and Ti)

[0089]

[0090]

[0091] Table 2-1 and Table 2-2 list the specific process parameters of the medium-carbon round steel for cold extrusion in Examples 1-6 and the comparative round steel in Comparative Examples 1-4 in the above process steps.

[0092] Table 2-1.

[0093]

[0094]

[0095] Table 2-2.

[0096]

[0097] Before step (6) and after step (5), samples of the medium-carbon round steel for cold extrusion in Examples 1-6 and the comparative round steel in Comparative Examples 1-4 are taken and various relevant performance tests are carried out:

[0098] (1) The bar specimens of each example and comparative example are tested in accordance with GB / T 231.1 "Metallic materials - Brinell hardness test - Part 1: Test method" to detect the Brinell hardness of the bar specimens of each example and comparative example, and the measured Brinell hardness results are listed in Table 3-1.

[0099] (2) The bar specimens of each example and comparative example are subjected to microscopic structure detection in accordance with GB / T 13298 "Test methods for metallic materials - Microscopic structure" to obtain the thickness of the bright layer and the volume phase ratio of lamellar pearlite of the bar specimens of each example and comparative example, and the detection results are listed in Table 3-2.

[0100] After step (6), samples of the medium-carbon round steel for cold extrusion in Examples 1-6 and the comparative round steel in Comparative Examples 1-4 are taken again, and tests are carried out in accordance with GB / T 228.1 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" and GB / T 229 "Metallic materials - Charpy pendulum impact test", to detect the mechanical properties of the bar specimens of each example and comparative example. The relevant detection results are listed in Table 3-3.

[0101] Table 3-1.

[0102]

[0103]

[0104] Table 3-2

[0105]

[0106] Table 3-3

[0107]

[0108] Table 3-4 lists the statistical results of the number of cracked parts after cold extrusion of the medium-carbon round steel for cold extrusion in Examples 1-6 and the comparison round steel in Comparative Examples 1-4 for making 10,000 reducer shaft parts samples. Among them, the number of bent parts is counted in Comparative Example 3.

[0109] Table 3-4

[0110] Number Number of cracked or bent parts (pieces) Example 1 0 Example 2 1 Example 3 2 Example 4 0 Example 5 0 Example 6 1 Comparative Example 1 715 Comparative Example 2 153 Comparative Example 3 536 Comparative Example 4 865

[0111] As can be seen from Table 3-1, compared with Comparative Example 3, the Brinell hardness of the medium-carbon round steel for cold extrusion in Examples 1-6 prepared by the present technical solution all meets the requirements of 185-210 HBW. However, in Comparative Example 3, due to the use of a conventional spheroidizing annealing process, the hardness is too low, which will cause the shaft blank to bend during the subsequent cold extrusion process.

[0112] As can be seen from Table 3-2, compared with Comparative Example 4, the medium-carbon round steel for cold extrusion in Examples 1-6 prepared by the present technical solution has no hardened layer on the surface, and the volume fraction of lamellar pearlite is in the range of 35-45%. However, for the round steel in Comparative Example 4 without strictly controlling the surface treatment process, there is a hardened layer on the surface, which will increase the cracking rate of the subsequent cold extrusion process.

[0113] As can be seen from Table 3-3, compared with Comparative Example 2, the mechanical properties of the medium-carbon round steel for cold extrusion in Examples 1-6 prepared by the present technical solution all meet the requirements of yield strength ≥ 1100 MPa, tensile strength ≥ 1250 MPa, elongation ≥ 15%, reduction of area ≥ 50%, and impact energy Akv2 at -40°C ≥ 40 J. However, in Comparative Example 2, since the N content does not meet the requirements of the present invention, the fine grain strengthening effect of AlN is not fully reflected, so the strength does not reach the requirements.

[0114] As can be seen from Table 3-4, compared with Comparative Examples 1-4, the cold extrusion cracking rate of the medium-carbon round steel for cold extrusion in Examples 1-6 prepared by the present technical solution is very low, all within five ten-thousandths.

[0115] Figure 1 shows the as-rolled microstructure morphology of the medium-carbon round steel for cold extrusion in Example 1. As Figure 1 shown, the as-rolled microstructure of the medium-carbon round steel for cold extrusion in Example 1 is bainite.

[0116] Figure 2The microstructure of the medium carbon round steel for cold extrusion after step annealing in Example 1 is shown. Figure 2 As shown, the microstructure of the annealed medium carbon round steel for cold extrusion of Example 1 is lamellar pearlite + spherical pearlite, and the volume phase ratio of the lamellar pearlite is about 40%.

[0117] Figure 3 The surface morphology of the medium carbon round steel for cold extrusion after surface treatment in Example 1 is shown. Figure 3 As shown, after the surface treatment, the medium carbon round steel for cold extrusion in Example 1 has only very slight burrs on its surface and no other surface defects.

[0118] 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 combined with optimized process, and can obtain medium carbon round steel for cold extrusion that meets the requirements.

[0119] It should be noted that the chemical composition and process design of the medium carbon round steel for cold extrusion described in the present invention are reasonable, and its process window is wide, so that batch commercial production can be realized on the bar production line, and it has good promotion prospects and application value.

[0120] 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.

[0121] It should also be noted that the above examples are only specific embodiments of the present invention, and the present invention is obviously not limited to the above examples, and there are many similar variations. All variations directly derived or associated from the contents disclosed by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A medium-carbon round steel for cold extrusion, which contains Fe and inevitable impurities, is characterized in that, It also contains the following chemical elements with the following mass percentages: C: 0.36 - 0.43%, Si: 0.1 - 0.4%, Mn: 0.6 - 1%, Cr: 0.6 - 1%, 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 - 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), and the values before the percentage signs of the mass percentage contents of the respective elements in the formula are substituted; After annealing, its microstructure is lamellar pearlite + globular pearlite.

2. The medium carbon round steel for cold extrusion according to claim 1, wherein The mass percentages of its chemical elements are: C: 0.36 - 0.43%, Si: 0.1 - 0.4%, Mn: 0.6 - 1%, Cr: 0.6 - 1%, 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 other inevitable impurities.

3. The medium-carbon round steel for cold extrusion according to claim 1 or 2, characterized in that, It also contains at least one of the following chemical elements: 0 < Cu ≤ 0.2%, 0 < V ≤ 0.05%, 0 < Nb ≤ 0.05%.

4. The medium-carbon round steel for cold extrusion 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: P ≤ 0.015%, S ≤ 0.025%, H ≤ 0.0002%, B ≤ 0.001%, O ≤ 0.0020%, 0 < Ti ≤ 0.01%.

5. The medium carbon round steel for cold extrusion according to claim 1 or 2, characterized in that, It also satisfies: [Al] / [N] ≥ 2, [Al][N] ≥ 2×10 -4 .

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

7. The medium carbon round steel for cold extrusion according to claim 1 or 2, characterized in that, Its hardness after annealing is 185 - 210 HBW.

8. The medium-carbon round steel for cold extrusion according to claim 1 or 2, characterized in that, After quenching and tempering treatment, its yield strength ≥ 1100 MPa, tensile strength ≥ 1250 MPa, elongation ≥ 15%, reduction of area ≥ 50%, impact energy Akv2 at - 40 °C ≥ 40 J.

9. The manufacturing method of medium-carbon round steel for cold extrusion according to any one of claims 1-8, characterized in that, It includes the steps: (1) Smelting and casting; (2) Heating; (3) Forging or rolling; (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 ensure that there is no martensite hardened layer on the surface of the round steel before cold extrusion.

10. The manufacturing method according to claim 9, characterized in that, In step (2), control the heating temperature to be 1050 - 1250 °C.

11. The manufacturing method according to claim 9, characterized in that, In step (3), control the final rolling or forging temperature ≥ 800 °C.

12. The manufacturing method according to claim 9, characterized in that, In step (5), the surface treatment includes turning, where the turning feed rate is 0.06 - 0.1 mm / r and the coolant flow rate is 20 - 28 L / min.

13. The manufacturing method according to claim 9, characterized in that, In step (5), the surface treatment includes burnishing, where the burnishing pressure is 16 - 20 KN.

14. The manufacturing method according to claim 9, characterized in that, In step (5), the surface treatment includes abrasive belt polishing, where the abrasive belt polishing depth exceeds the depth of the cryptocrystalline martensite layer generated by turning and burnishing.

15. The manufacturing method according to any one of claims 9-14, characterized in that, After step (5), there is also step (6) quenching and tempering treatment, where the quenching temperature is 850 °C - 930 °C, the holding time is 60 - 180 min, and water quenching or oil quenching is used; the tempering temperature is 460 - 530 °C, the holding time is 60 - 180 min, and air cooling or water cooling is used.

Images