Round steel for cold extrusion and manufacturing method thereof

Abstract

The invention discloses round steel for cold extrusion, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.51-0.55% of C, 0.15-0.35% of Si, 1.4-1.6% of Mn, 0.02-0.04% of Al, 0 < Cr < = 0.2%, 0 < Cr < = 0.2%, 0 < Cr < = 0.2%, 0 < Cr < = 0.2%, and the balance of Fe and inevitable impurities. And after annealing, the microstructure is flaky pearlite and spherical pearlite. Correspondingly, the invention also discloses a manufacturing method of the round steel for cold extrusion, which comprises the following steps: (1) smelting and casting; (2) heating; (3) forging or rolling; (4) step annealing is conducted, specifically, heat preservation is conducted for 2-4 h at the temperature of 710-730 DEG C, then heat preservation is conducted for 2-4 h at the temperature of 690-710 DEG C, then heat preservation is conducted for 1-3 h at the temperature of 670-690 DEG C, then furnace cooling is conducted till the temperature is lower than 350 DEG C, and discharging and air cooling are conducted; and (5) surface treatment.

Description

Technical Field

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

[0002] The input shaft of a vehicle retarder is a typical shaft product in the retarder. Its working ability generally depends on the strength and stiffness of the shaft, and when the rotational speed is high, it also depends on the vibration stability. When it works, it bears both bending moment and torque, so certain requirements are also imposed on the toughness of the round steel.

[0003] In order to meet the usage requirements of the retarder input shaft, 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 and 50 steels) 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. Since medium-carbon steels have poor plastic toughness, the stress in the round steel is relatively large after cold forming, and it is prone to cracking during the subsequent quenching process, so they are usually not used for cold forming.

[0004] In automotive retarders, the failure mode of the input shaft is mostly fatigue fracture. The fatigue resistance of the input shaft is not only related to its strength and toughness (guaranteed hardenability), but also closely related to the microstructure and surface quality of the round steel before cold extrusion. Therefore, designing a reasonable hardenability curve and ensuring a suitable microstructure for cold extrusion and good surface quality are the keys to ensuring the fatigue resistance of the input shaft. In the prior art, traditional steels for fuel vehicle shafts are mostly used to manufacture retarder input shafts, and the matching processes are mostly turning or hot forging.

[0005] For 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%, and the balance is Fe and inevitable 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

[0006] One of the objectives of the present invention is to provide a round steel for cold extrusion. By accurately identifying the influence of each alloying element on the actual hardenability, a hardenability curve that meets the operating conditions of the input shaft of the reducer is obtained to ensure good strength and toughness of the output shaft.

[0007] To achieve the above objective, the present invention provides a round steel for cold extrusion, which contains Fe and inevitable impurities, and also contains the following chemical elements with the following mass percentages:

[0008] C: 0.51 - 0.55%, Si: 0.15 - 0.35%, Mn: 1.4 - 1.6%, Al: 0.02 - 0.04%, 0 < Cr ≤ 0.2%;

[0009] Its DI value is 2.6 - 3.0 in, where DI = (0.171 + 0.001C + 0.265C2)(-1.12 + 5.1Mn)(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 percentages of each element are substituted into the formula;

[0010] Its microstructure after annealing is lamellar pearlite + globular pearlite.

[0011] Further, in the round steel for cold extrusion described in the present invention, the mass percentages of its chemical elements are:

[0012] C: 0.51 - 0.55%, Si: 0.15 - 0.35%, Mn: 1.4 - 1.6%, Al: 0.02 - 0.04%, 0 < Cr ≤ 0.2%; the balance is Fe and other inevitable impurities.

[0013] In the round steel for cold extrusion according to the present invention, the design principles of each chemical element are specifically as follows:

[0014] C: In the round steel for cold extrusion according to 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 round steel for cold extrusion according to the present invention, the C content is set to 0.51 - 0.55%.

[0015] Si: In the round steel for cold extrusion according to 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, which is 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 round steel for cold extrusion according to the present invention, the Si content is set to: 0.15 - 0.35%.

[0016] Mn: In the round steel for cold extrusion according to 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 the hardenability. However, too high a Mn content will cause serious segregation of the round steel and is prone to quenching cracking. Based on this, in the round steel for cold extrusion according to the present invention, the Mn content is set to: 1.4 - 1.6%.

[0017] Al: In the round steel for cold extrusion according to the present invention, Al forms fine AlN precipitates during steelmaking and inhibits the growth of austenite grains during the subsequent cooling process, refining the austenite grains. 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. Based on this, in the round steel for cold extrusion according to the present invention, the Al content is set to 0.02 - 0.04%.

[0018] Cr: In the round steel for cold extrusion according to the present invention, Cr is used to improve the hardenability of the round steel, and its content is limited to 0 < Cr ≤ 0.2%.

[0019] Furthermore, in the round steel for cold extrusion according to the present invention, it also contains at least one of the following chemical elements: 0 < Mo ≤ 0.06%, 0 < Ni ≤ 0.25%, 0 < Cu ≤ 0.2%, 0 < V ≤ 0.01%.

[0020] In the round steel for cold extrusion according to the present invention, alloying elements such as Cr, Mo, Ni, Cu, and V can all improve the hardenability of the round steel. Appropriate amounts of the above elements can be added to partially replace the promoting effect of C and Mn on hardenability. However, considering cost factors, the upper limit content of the above elements can be limited, and at the same time, space is left for adjusting the DI value later. Based on this, in the round steel for cold extrusion according to the present invention, it is set that 0 < Cr ≤ 0.2%, 0 < Mo ≤ 0.06%, 0 < Ni ≤ 0.25%, 0 < Cu ≤ 0.2%, 0 < V ≤ 0.01%.

[0021] Furthermore, in the round steel for cold extrusion according to the present invention, among other inevitable impurities, the content of each impurity element satisfies at least one of the following items: P ≤ 0.015%, S ≤ 0.025%, Ti ≤ 0.003%, H ≤ 0.0002%, N ≤ 0.008%, B ≤ 0.0004%, O ≤ 0.002%.

[0022] In the above technical solution, P, S, Ti, H, N, B, and O are all impurity elements in the steel. Under the condition that technical conditions permit, 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:

[0023] P: In the 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 larger cleavage planes and reducing the energy absorbed by the steel when subjected to impact. Based on this, in some preferred embodiments of the 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%.

[0024] S: In the 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 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%.

[0025] Ti: In the 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 the round steel for cold extrusion according to the present invention, in some preferred embodiments, the Ti content is controlled to be: Ti ≤ 0.003%.

[0026] H: In the round steel for cold extrusion according to 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. In steels with a high tensile strength level, 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, leading to delayed fracture of the steel. Based on this, in the round steel for cold extrusion according to the present invention, in some preferred embodiments, the H content is controlled to be: H ≤ 0.0002%.

[0027] N: In the round steel for cold extrusion according to the present invention, a high N content will lead to an increase in its enrichment amount at defects and simultaneously form coarse nitride precipitation particles, affecting the impact toughness of the round steel. Based on this, in the round steel for cold extrusion according to the present invention, in some preferred embodiments, the N content is controlled to be: N ≤ 0.008%.

[0028] B: In the round steel for cold extrusion according to the present invention, B has a strong affinity for 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 the round steel for cold extrusion according to the present invention, in some preferred embodiments, the B content is controlled to be: B ≤ 0.0004%.

[0029] O: In the 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 the round steel for cold extrusion according to the present invention, in some preferred embodiments, to ensure the continuity of the steel matrix and fatigue resistance, the O content is controlled to be: O ≤ 0.002%.

[0030] Furthermore, in the round steel for cold extrusion according to the present invention, the volume phase ratio of lamellar pearlite is 35 - 45%.

[0031] Furthermore, in the round steel for cold extrusion according to the present invention, its hardness after annealing is 185 - 210 HBW.

[0032] Furthermore, in the round steel for cold extrusion according to the present invention, the content of tempered sorbite in the core after quenching and tempering is ≥ 90%, and the core hardness is 28 - 32 HRC.

[0033] Accordingly, another object of the present invention is to provide a manufacturing method of the round steel for cold extrusion described above. By adopting this method and the component ratio described above, 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 the stress in the round steel is not too high, and there is no martensite hardened layer on the surface of the round steel before cold extrusion, thereby avoiding surface cracking of the round steel during the cold extrusion process.

[0034] To achieve the above object, the present invention proposes a manufacturing method of round steel for cold extrusion, which includes the steps:

[0035] (1) Smelting and casting;

[0036] (2) Heating;

[0037] (3) Forging or rolling;

[0038] (4) Step annealing: First, keep the temperature at 710 - 730 °C for 2 - 4 h, then keep the temperature at 690 - 710 °C for 2 - 4 h, then keep the temperature at 670 - 690 °C for 1 - 3 h, and then cool in the furnace to below 350 °C and take out of the furnace for air cooling;

[0039] (5) Surface treatment to ensure that there is no martensite hardened layer on the surface of the round steel before cold extrusion.

[0040] 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 in terms of deformation resistance. 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.

[0041] 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.

[0042] Based on this, the present invention creatively proposes a heat treatment process of step annealing: First, keep the temperature at 710 - 730 °C for 2 - 4 h, then keep the temperature at 690 - 710 °C for 2 - 4 h, then keep the temperature at 670 - 690 °C for 1 - 3 h, and then cool in the furnace to below 350 °C and take out of the furnace for air cooling, to replace the ordinary spheroidizing annealing.

[0043] 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.

[0044] Further, in step (2) of the manufacturing method of the present invention, the heating temperature is controlled to be 1050 - 1250 °C.

[0045] In the manufacturing method of the present invention, the heating temperature is controlled to be 1050 - 1250 °C because when the steel is heated to austenitize at 1050 °C - 1250 °C, during the heating process, part or all of the carbides of Mn dissolve in austenite. During the subsequent rolling / forging and cooling processes, Al forms fine carbonitrides, which pin the austenite grain boundaries and refine the as-rolled structure of the steel. The Mn dissolved in austenite can improve the hardenability of the steel and also improve the hardenability of martensite during quenching.

[0046] Further, in step (3) of the manufacturing method of the present invention, the finish rolling or finish forging temperature is controlled to be ≥800 °C.

[0047] In the manufacturing method of 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.

[0048] Further, in step (5) of the manufacturing method of 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.

[0049] 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, while 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 by turning is not too high.

[0050] Regarding the coolant flow rate in the turning process, in some known references, the coolant flow rate for shaft turning is ≤15 L / min, while 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.

[0051] Further, in step (5) of the manufacturing method of the present invention, the surface treatment includes burnishing, wherein the burnishing pressure is 16 - 20 KN.

[0052] For the polishing process, in some known references, the rolling pressure for shafts is ≥25 KN, while the rolling pressure in the present invention is controlled at 16 - 20 KN, ensuring that the stress in the round steel is not too high.

[0053] Furthermore, 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 rolling, 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.

[0054] The round steel for cold extrusion of the present invention has the following advantages and beneficial effects compared with the prior art:

[0055] The round steel for cold extrusion of the present invention fully recognizes the influence of each alloy component on hardenability and uses economical alloy components 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, facilitating production control.

[0056] 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 demand for the steel for cold extrusion of the input shaft of the reducer.

[0057] In addition, in the preferred embodiment of the manufacturing method of the present invention, an improved round steel surface treatment process 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.

[0058] In the preferred embodiment of the present invention, for the round steel for cold extrusion, 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, saving energy and reducing consumption, and improving production efficiency.

[0059] In addition, it should be noted that the chemical composition and process design of the 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 promotion prospects and application value. Brief Description of the Drawings

[0060] Figure 1 Shows the microscopic structure morphology of the annealed state of the round steel for cold extrusion in Example 1 magnified 500 times under a metallurgical microscope.

[0061] Figure 2Shows the microstructure morphology of the annealed round steel for cold extrusion in Example 1 at 5000 times magnification under a scanning electron microscope.

[0062] Figure 3 Shows the core microstructure morphology of the round steel for cold extrusion in Example 1 after quenching and tempering.

[0063] Figure 4 Shows the surface morphology of the round steel for cold extrusion in Example 1 after surface treatment.

[0064] Figure 5 Shows the surface morphology of the round steel in Comparative Example 1 after surface treatment. Detailed implementation manners

[0065] The following will further explain and illustrate the round steel for cold extrusion and its manufacturing method according to the present invention in conjunction with the accompanying drawings of the specification and specific embodiments. However, this explanation and illustration shall not unduly limit the technical solution of the present invention.

[0066] Examples 1-6 and Comparative Example 1

[0067] The round steel for cold extrusion in Examples 1-6 was all prepared by the following steps:

[0068] (1) Smelting was carried out using an electric furnace or a converter and cast into continuous casting billets or ingots, and its chemical composition is shown in Table 1; during the casting process, die casting or continuous casting can be used.

[0069] (2) Heating: Control the heating temperature to be 1050-1250 °C.

[0070] (3) Forging or rolling: Control the finish rolling or finish forging temperature ≥ 800 °C. Among them, if forging is carried out, during the forging process, it can be directly forged to the final dimensions. If rolling is carried out, during the rolling process, the steel billet can be directly rolled to the final specifications, or the steel billet can be rolled to the specified intermediate billet size, and then heated and rolled to the final finished product size.

[0071] (4) Step annealing: The step annealing heat treatment process is to first hold at a temperature of 710-730 °C for 2-4 h, then hold at a temperature of 690-710 °C for 2-4 h, then hold at a temperature of 670-690 °C for 1-3 h, and then cool in the furnace to below 350 °C and take out for air cooling.

[0072] (5) Surface treatment: It can include turning, burnishing, and abrasive belt polishing processes; among them, the turning feed rate is 0.06-0.1 mm / r, and 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 (white bright layer) generated by turning and burnishing.

[0073] The round steel for cold extrusion in Examples 1 - 6 of the present invention is 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.

[0074] The comparative round steel of Comparative Example 1 is also prepared by the above process flow, 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.

[0075] Table 1 lists the mass percentages of various chemical elements in the round steel for cold extrusion in Examples 1 - 6 and the comparative round steel of Comparative Example 1.

[0076] Table 1. (wt.%, the balance is Fe and other unavoidable impurities except P, S, Ti, H, N, B and O)

[0077]

[0078] Table 2 lists the specific process parameters of the round steel for cold extrusion in Examples 1 - 6 and the comparative round steel of Comparative Example 1 in the above process steps.

[0079] Table 2.

[0080]

[0081] The obtained round steel for cold extrusion in Examples 1 - 6 and the comparative round steel of Comparative Example 1 are sampled and subjected to various related performance tests:

[0082] (1) Test the bar specimens of each example and comparative example by using 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.

[0083] (2) Microstructure detection of the bar specimens of each example and comparative example is carried out by using GB / T 13298 "Test methods for metallic materials - Micrographic examination", to obtain the thickness of the bright layer of the bar specimens of each example and comparative example, and the volume ratio of lamellar pearlite to globular pearlite, and the detection results are listed in Table 3 - 2.

[0084] The round steel for cold extrusion in Examples 1 - 6 and the comparative round steel of Comparative Example 1 are further subjected to quenching and tempering treatment. The quenching and tempering treatment process is to hold at 820 °C for 90 min and then oil quench, and then hold at 550 °C for 150 min and then water quench, and then:

[0085] (3) The samples of the bars after quenching and tempering treatment in each example and comparative example were subjected to microscopic structure detection using GB / T 13298 "Test Methods for Metallographic Structure of Metals" to detect the content of tempered sorbite in the core of the bars after quenching and tempering treatment in each example and comparative example, and the test results are listed in Table 3-3.

[0086] (4) The samples of the bars after quenching and tempering treatment in each example and comparative example were tested using GB / T 230.1 "Metallic Materials - Rockwell Hardness Test - Part 1: Test Method" to detect the Rockwell hardness of the core of the bars after quenching and tempering treatment in each example and comparative example, and the test results are listed in Table 3-3.

[0087] Table 3-1.

[0088] Number Brinell hardness (HBW) Example 1 197 Example 2 192 Example 3 205 Example 4 202 Example 5 189 Example 6 195 Comparative Example 1 170

[0089] Table 3-2.

[0090]

[0091]

[0092] Table 3-3.

[0093]

[0094] Table 3-4 lists the statistical results of the number of cold extrusion cracking parts in the production of 10,000 input shaft samples for speed reducers using the round steel for cold extrusion in Examples 1-6 and the comparative round steel in Comparative Example 1.

[0095] Table 3-4.

[0096] Number Number of cracked parts (pcs) Example 1 0 Example 2 1 Example 3 2 Example 4 0 Example 5 0 Example 6 1 Comparative Example 1 748

[0097] As can be seen from Table 3-1, compared with Comparative Example 1, the Brinell hardness of the round steel for cold extrusion in Examples 1-6 prepared by the present technical solution all meets the requirements of 185-210 HBW. The hardness is neither too high to cause easy cracking of the round steel during cold extrusion nor too low to cause bending of the shaft blank during the cold extrusion process.

[0098] As can be seen from Table 3-2, compared with Comparative Example 1, the round steel for cold extrusion in Examples 1-6 prepared by the present technical solution does not show a white bright layer, and the volume fraction of lamellar pearlite is in the range of 35-45%. However, the round steel in Comparative Example 1 without strict control of the surface treatment process has a white bright layer on the surface, which will increase the cracking rate in the subsequent cold extrusion process.

[0099] As can be seen from Table 3-3, compared with Comparative Example 1, the content of tempered sorbite in the core of the round steel for cold extrusion in Examples 1-6 prepared by the present technical solution is ≥90%, and the core hardness is within the required range of 28-32 HRC. However, in Comparative Example 1, the content of tempered sorbite in the core is low, the hardness is low, and the core is not fully hardened, which will affect the fatigue performance of the input shaft.

[0100] As can be seen from Table 3-4, compared with Comparative Example 1, the cold extrusion cracking rate of the round steel for cold extrusion in Examples 1-6 prepared by the present technical solution is very low, all within five ten-thousandths.

[0101] Figure 1 Shows the microscopic structure morphology of the annealed state of the round steel for cold extrusion in Example 1 magnified 500 times under a metallurgical microscope. As Figure 1 shown, the microscopic structure of the rolled steel bar in Example 1 is composed of lamellar pearlite and globular pearlite.

[0102] Figure 2 Shows the microscopic structure morphology of the annealed state of the round steel for cold extrusion in Example 1 magnified 5000 times under a scanning electron microscope. As Figure 2 shown, the volume phase ratio of lamellar pearlite in the annealed steel bar in Example 1 is about 40%.

[0103] Figure 3 Shows the microscopic structure morphology of the core of the round steel for cold extrusion in Example 1 after quenching and tempering.

[0104] As Figure 3 shown, after the overall quenching and tempering heat treatment of the round steel in Example 1, the core is fully hardened, and the content of tempered sorbite is greater than 90%.

[0105] Figure 4 Shows the surface morphology of the round steel for cold extrusion in Example 1 after surface treatment.

[0106] As Figure 4 shown, for the surface morphology of the steel bar in Example 1 after surface treatment, there are only extremely slight burrs on the surface of the steel bar, and no other surface defects.

[0107] Figure 5 Shows the surface morphology of the round steel in Comparative Example 1 after surface treatment.

[0108] As Figure 5 shown, a hardened layer (white bright layer) appears on the surface of the steel bar in Comparative Example 1 after surface treatment.

[0109] In summary, it can be seen that the present invention solves the problem of the mismatch between medium carbon steel and cold forming processes through reasonable chemical composition design and combined with optimized processes, and can obtain round steel for cold extrusion that meets the requirements.

[0110] It should be noted that the chemical composition and process design of the round steel for cold extrusion 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.

[0111] 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.

[0112] 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 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: 0.51 - 0.55%, Si: 0.15 - 0.35%, Mn: 1.4 - 1.6%, Al: 0.02 - 0.04%, 0 < Cr ≤ 0.2%; Its DI value is 2.6 - 3.0 in, where DI = (0.171 + 0.001C + 0.265C 2 )(-1.12 + 5.1Mn)(1 + 0.7Si)(1 + 0.365Cu)(1 + 2.16Cr)(1 + 3Mo)(1 + 0.363Ni)(1 + 1.73V), and the numerical 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 round steel for cold extrusion according to claim 1, characterized in that, The mass percentages of its chemical elements are: C: 0.51 - 0.55%, Si: 0.15 - 0.35%, Mn: 1.4 - 1.6%, Al: 0.02 - 0.04%, 0 < Cr ≤ 0.2%; the balance is Fe and other inevitable impurities.

3. The 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 < Mo ≤ 0.06%, 0 < Ni ≤ 0.25%, 0 < Cu ≤ 0.2%, 0 < V ≤ 0.01%.

4. The round steel for cold extrusion according to claim 1 or 2, characterized in that, Among other inevitable impurities, the content of each impurity element satisfies at least one of the following items: P ≤ 0.015%, S ≤ 0.025%, Ti ≤ 0.003%, H ≤ 0.0002%, N ≤ 0.008%, B ≤ 0.0004%, O ≤ 0.002%.

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

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

7. The round steel for cold extrusion according to claim 1 or 2, characterized in that, After quenching and tempering treatment, the content of tempered sorbite in the core is ≥ 90%, and the core hardness is 28 - 32 HRC.

8. The manufacturing method of the round steel for cold extrusion according to any one of claims 1-7, 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 710 - 730 °C for 2 - 4 h, then hold at a temperature of 690 - 710 °C for 2 - 4 h, then hold at a temperature of 670 - 690 °C for 1 - 3 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.

9. The manufacturing method according to claim 8, characterized in that, In step (2), control the heating temperature to be 1050 - 1250 °C.

10. The manufacturing method according to claim 8, characterized in that, In step (3), control the final rolling or forging temperature ≥ 800 °C.

11. The manufacturing method according to claim 8, 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.

12. The manufacturing method according to claim 8, characterized in that, In step (5), the surface treatment includes burnishing, where the burnishing pressure is 16 - 20 KN.

13. The manufacturing method according to claim 8, 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.

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