Method of manufacturing high-frequency hardened component

Abstract

To provide a method for manufacturing a high-frequency hardened component having excellent cold forgeability and sufficient component performance (surface and core hardness) even if both annealing before cold forging and hardening and tempering before high-frequency hardening are omitted in a process for producing the high-frequency hardened component.SOLUTION: In a method for manufacturing a high-frequency hardened component, a steel raw material is formed into a steel bar or a wire rod by hot rolling, and the steel bar or the wire rod is subjected to cold forging without softening annealing to form a component satisfying predetermined conditions. The cold forged components are subjected to high-frequency hardening and tempering treatment satisfying predetermined conditions without being subjected to hardening and tempering.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing an induction hardened part. [Background technology]

[0002] Generally, mechanical structural parts for automobiles and the like are formed by hot or cold forging, and then cut to the final shape. Cold forging, in particular, has the advantage of being able to reduce the amount of cutting required after forging due to its excellent dimensional accuracy, and its application has been increasing in recent years. In addition, induction hardening can improve the fatigue strength of steel in a short period of time, and CO 2 It also has the advantage of being able to reduce emissions, and its adoption is increasing.

[0003] For example, Patent Document 1 proposes a high-torsional fatigue strength induction hardened steel material with excellent cold workability, in which the steel composition is specified and the projected core hardness is specified from the effective hardened layer depth and the part radius. Patent Document 2 proposes a manufacturing method for a rolled steel material for induction hardening, which specifies the characteristics of the microstructure, including the number density of spheroidal cementite, in addition to the steel composition, and allows quenching and tempering to be omitted. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 10-195589 [Patent Document 2] JP 2011-241468 A Summary of the Invention [Problem to be solved by the invention]

[0005] Parts obtained by performing induction hardening as the final heat treatment after forming are usually subjected to softening annealing before cold forging and thermal refining (quenching and tempering) before induction hardening. For example, in the technology described in Patent Document 1, annealing before cold forging is essential. Similarly, in the technology described in Patent Document 2, annealing before cold forging is essential. That is, it is necessary to ensure cold forgeability by softening the steel material after hot rolling by annealing to reduce the deformation resistance during the subsequent cold forging. In addition, thermal refining (quenching and tempering) before induction hardening was necessary to give a predetermined hardness to the final product part.

[0006] On the other hand, in recent years, the price competition for parts has intensified and carbon neutral (CO 2 In response to the growing trend of reducing carbon dioxide emissions, there is an increasing need to omit heat treatment in the parts manufacturing process. In this situation, as described above, the technology described in Patent Document 1 has a problem in that annealing before cold forging cannot be omitted. Similarly, the technology described in Patent Document 2 can omit quenching and tempering, but also has a problem in that annealing before cold forging cannot be omitted.

[0007] The present invention was developed in consideration of the above-mentioned circumstances, and aims to propose a manufacturing method for induction-hardened parts that have excellent cold forgeability and sufficient part performance (surface and core hardness), which makes it possible to omit both annealing before cold forging and quenching and tempering before induction hardening. [Means for solving the problem]

[0008] In order to achieve the above object, the inventors have conducted extensive research into the manufacturing process of induction hardened parts, and have found that by appropriately controlling the conditions of work hardening during cold forging and induction hardening, it is possible to manufacture high-performance parts without impairing cold forgeability even if the heat treatments before cold forging and induction hardening are omitted.

[0009] The present invention is derived from the above findings, and the gist of the present invention is as follows. 1. A method for producing an induction hardened part, comprising the steps of hot rolling a steel material into a steel bar or wire rod, cold forging the steel bar or wire rod without softening annealing to form a part satisfying the following formula (1), and then subjecting the cold forged part to the induction hardening and tempering treatment without quenching and tempering before the induction hardening and tempering treatment that satisfies the following formula (2). Note H CF ≧200 (1) Here, H CF is the Vickers hardness after cold forging. JPEG2025009833000001.jpg8170Here, let f be the function of the temperature T(K) and time t(s) of the high-frequency heating, i.e., T=f(t). However, f(t)≧T R The range is T R is the austenite transformation completion temperature (K) during induction heating. t R is the austenite transformation completion temperature T R The above is the time (s). Also, log is the common logarithm.

[0010] 2. The steel bar or wire rod is C: 0.15~0.55% by mass, Si: 0.35% by mass or less, Mn: 0.85% by mass or less, P: 0.050% by mass or less, S: 0.050% by mass or less, Al: 0.010~0.090% by mass, Mo: 0.05~0.50% by mass, Ti:0.010~0.200% by mass, B: 0.0005 to 0.0100 mass% and N: 0.0150% by mass or less 2. The method for producing an induction hardened part according to claim 1, wherein the alloy has a composition comprising:

[0011] 3. The component composition further includes: 3. The method for producing an induction hardened part according to 2 above, which contains one or more kinds selected from one or more of groups A to D below. Group A: Cr: 0.65% by mass or less, Cu: 1% by mass or less and Ni: 1% by mass or less Group B: Se: 0.3% by mass or less, Ca: 0.05% by mass or less, Pb: 0.3% by mass or less, Bi: 0.3% by mass or less, Mg: 0.05% by mass or less, Zr: 0.2% by mass or less, REM: 0.01% by mass or less and O: 0.025% by mass or less Group C: Nb: 0.1 mass% or less and V: 0.3% by mass or less Group D: Sn: 0.1 mass% or less and Sb: 0.1% by mass or less Effect of the Invention

[0012] According to the present invention, it is possible to manufacture high-performance induction-hardened parts with excellent cold forgeability even if the heat treatments before cold forging and induction hardening are omitted, which is extremely useful industrially. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] An example of the steel for induction hardening according to this embodiment is a steel for induction hardening used for machine structural parts of automobiles and the like.

[0014] Examples of the induction hardening steel according to this embodiment include, in the automotive field, crankshafts, camshafts, timing gears, and diesel common rails of engines, propeller shafts, drive shafts, and CVJ outer races of drive trains, hubs, steering pinions, worms, and ball joints of suspensions, rotor shafts and motor shafts of electrical equipment, etc. Similarly, in the industrial machinery field, examples include shafts of ball screws and rails of linear bearings, etc., and in the construction machinery field, examples include ring gears of travel reducers and inner and outer rings of slewing rings, etc.

[0015] The manufacturing method of the present invention involves forming a steel material into a steel bar or wire rod by hot rolling, forming the steel bar or wire rod into a part by cold forging that satisfies the following formula (1) without softening annealing, and then subjecting the cold forged part to induction hardening and tempering treatment that satisfies the following formula (2) without normalizing or quenching. Each of the above production conditions will be specifically explained below.

[0016] <Steel material> The steel material is not particularly limited in terms of its composition, but may be any steel suitable for induction hardening. For example, the steel may have the composition described below.

[0017] <Cold forging that satisfies the following formula (1)> H CF ≧200 (1) However, H CF is the Vickers hardness after cold forging, and is the hardness after work hardening by cold forging. The above formula (1) is a necessary condition for ensuring the core strength of parts after induction hardening and tempering. By satisfying formula (1), it is possible to obtain core strength equal to or greater than that obtained by conventional hardening and tempering followed by induction hardening and tempering. This is because the Vickers hardness after conventional hardening and tempering is about 190, so H CFIf the Vickers hardness is 200 or more, the core strength of the part can be reliably secured without normalizing and quenching. In addition, since cold forging is performed without annealing, the Vickers hardness before cold forging is preferably 170 or less.

[0018] In addition, by utilizing the work hardening that occurs during cold forging, the Vickers hardness H CF The amount of work hardening increases as the amount of plastic deformation increases. In other words, by securing a certain amount of forging, H CF ≧200 is achievable.

[0019] <Induction hardening and tempering treatment that satisfies the following formula (2)> JPEG2025009833000002.jpg8170Here, let f be the function of the temperature T(K) and time t(s) of the high-frequency heating, i.e., T=f(t). However, f(t)≧T R The range is as follows. T R is the austenite transformation completion temperature (K) during high-frequency heating. R can be determined by a thermal expansion test, and the heating rate should be about 1 to 3 seconds from room temperature to 900°C. Incidentally, when a steel with the preferred composition described below is determined by a thermal expansion test under the above conditions, T R is 1073K. t R is the austenite transformation completion temperature T R The above is the time (s). Also, log is the common logarithm.

[0020] For example, if we assume that the temperature function f(t) is the following function, the left side of the above equation (2) can be calculated using equation (3). Note Time less than t1 is f(t) <T R From time t1 to t2, f(t) ≧ T R Beyond time t2 is f(t) <T R However, t2>t1 JPEG2025009833000003.jpg22170

[0021] In addition, f(t) is preferably 1573K or less in consideration of the withstand temperature of the high-frequency heating coil and to avoid melting of the steel. The upper limit of the above formula (2) should be set at 3000 or less as a guideline, since there is a concern that work hardening may be restored if the heating affects the center. However, depending on the component shape and the cooling capacity of the hardening device, it is possible to reduce heat conduction to the center, and in such cases, it may be 3000 or more.

[0022] Conventionally, induction hardening and tempering is performed after changing the structure to one mainly composed of martensite or pearlite by quenching and tempering. In this case, carbon is uniformly dispersed in the structure (mainly composed of martensite or pearlite) immediately before induction hardening, and a martensite structure with uniformly dispersed carbon is obtained even after induction hardening.

[0023] On the other hand, when quenching and tempering are not performed, the structure immediately before induction hardening is a structure consisting of ferrite and pearlite that have been deformed by processing. In this structure, only a small amount of carbon is present in the ferrite, but carbon is concentrated in the pearlite containing cementite. When this is subjected to induction hardening, the heating time for induction hardening is very short, about a few seconds, so carbon diffusion is insufficient. As a result, after induction hardening, it becomes a martensite structure with carbon unevenly dispersed. Since the strength of the martensite structure depends on the carbon concentration, the strength is low at positions with low carbon concentration, while the strength is high at positions with high carbon concentration, resulting in localized strength variations. To solve this problem, it is necessary to ensure the carbon diffusion distance after the austenite transformation during induction heating, and the above formula (2) must be satisfied. In other words, after the austenite transformation by heating, it is sufficient to hold the temperature at or above the transformation temperature for a certain period of time or more. In this case, the difference between the heating holding temperature and the transformation temperature (f(t)-T R) is larger, the above formula (2) can be satisfied even with a shorter holding time. If the above formula (2) is satisfied, T R It may be below the temperature. R The temperature may be kept isothermal at or above the temperature. The heating rate may be changed as appropriate. However, in order to obtain the martensite structure that is the hardened layer, the temperature must be kept at T R The temperature must be equal to or higher than the T R After that, T R After passing through the following temperature, T R If the temperature is increased above T, it is necessary to keep the temperature within T to avoid carbon segregation. R It is preferable to keep the temperature within the range below for 10 minutes or less.

[0024] for example, Time less than t1 is f(t) <T R From time t1 to t2, f(t) ≧ T R From time t2 to t3, f(t) <T R From time t3 to t4, f(t) ≧ T R Time beyond t4 is f(t) <T R However, t4>t3>t2>t1 In this case, the left side of the above equation (2) is calculated using the following equation (4). JPEG2025009833000004.jpg18170

[0025] Here, a specific example of induction hardening and tempering that satisfies the above formula (2) will be shown. When the temperature of induction hardening is raised to 1240K in 1.7 seconds and then quenched immediately after reaching 1240K, the left side of formula (2) becomes 109, which satisfies the condition. In this case, T R is 1073K.

[0026] The manufacturing method of the present invention has been described above, but in the present invention, the composition of the steel material may be adjusted as necessary. A preferred composition is shown below, but is not limited to this composition, and the composition may be selected according to the performance required for the part. The balance of the preferred composition is Fe and inevitable impurities in any case. Inevitable impurities are those that are mixed in from raw materials such as ore, scrap, or the manufacturing environment when manufacturing steel material industrially, and are allowed within a range that does not adversely affect the characteristics of this embodiment.

[0027] C:0.15~0.55% by mass At least 0.15% by mass of C is necessary to ensure the strength of the hardened layer after induction hardening. On the other hand, if the content exceeds 0.55% by mass, cold forgeability decreases and cracks are likely to occur during induction hardening, so the upper limit is set to 0.55% by mass. In order to improve the balance between cold forgeability and strength after induction hardening, 0.36 to 0.48% by mass is preferable.

[0028] Si: 0.35% by mass or less Although Si is effective as a deoxidizer, if it exceeds 0.35 mass%, it reduces cold forgeability, so the content is limited to 0.35 mass% or less. Also, since Si reduces the hardness of steel and improves machinability, the content is preferably less than 0.35 mass%, and more preferably 0.10 mass% or less.

[0029] Mn: 0.85% by mass or less Mn has the effect of improving hardenability and may be added to adjust the strength after induction hardening, but if it exceeds 0.85 mass%, it will cause a decrease in cold forgeability, so the upper limit is set to 0.85 mass%, preferably 0.15 to 0.45 mass%.

[0030] P: 0.050% by mass or less P reduces the toughness of steel, so it is desirable to reduce it, but extremely low P content increases the cost of the steelmaking process. For machine structural part applications, it is sufficient to have a P content of 0.050% by mass or less. It is preferably 0.030% by mass or less, and more preferably 0.015% by mass or less.

[0031] S: 0.050% by mass or less S exists as sulfide-based inclusions and is an effective element for improving machinability, but addition of more than 0.050% by mass reduces cold forgeability, so the upper limit is set to 0.050% by mass. If improvement in machinability is required, S may be added in an amount of 0.005% by mass or more, and a range of 0.005 to 0.050% by mass is preferable. A range of 0.010 to 0.035% by mass is more preferable.

[0032] Al:0.010~0.090% by mass Al is an element effective for deoxidization. It also has the effect of refining the grain size by bonding with N to form fine nitrides. In order to obtain these effects, it is necessary to add 0.010 mass% or more. On the other hand, even if it is added in excess of 0.090 mass%, these effects will simply saturate. Therefore, the Al content is limited to the range of 0.010 to 0.090 mass%, preferably 0.015 to 0.050 mass%, and more preferably 0.015 to 0.035 mass%.

[0033] Mo:0.05~0.50% by mass Mo has the effect of improving hardenability. To ensure strength after induction hardening, it is necessary to add at least 0.05% by mass. On the other hand, if it exceeds 0.50% by mass, cold forgeability decreases, so the upper limit is set to 0.50% by mass. It is preferably 0.07 to 0.35% by mass, and more preferably 0.07 to 0.25% by mass.

[0034] Ti:0.010~0.200% by mass Ti has the effect of reducing the amount of solute N in steel by combining with N to form nitrides, thereby improving cold forgeability. In addition, Ti has a stronger tendency to form nitrides than B, so the formation of nitrides of B is suppressed, and the amount of solute B is increased, thereby improving the hardenability of steel. Furthermore, Ti has the effect of suppressing the recovery of work hardening caused by cold forging in the heat affected zone of induction hardening, and is also useful for preventing a decrease in strength of parts. In order to obtain these effects, at least 0.010% by mass must be added, but if it is added in excess of 0.200% by mass, the nitrides become coarse and may cause fatigue fracture, so the upper limit is limited to 0.200% by mass. The amount is preferably 0.010 to 0.050% by mass, and more preferably 0.012 to 0.035% by mass.

[0035] B:0.0005~0.0100% by mass B has the effect of improving the hardenability of steel by dissolving in steel. To obtain this effect, it is necessary to add at least 0.0005% by mass. On the other hand, if it exceeds 0.0100% by mass, cracks are likely to occur after continuous casting, causing a decrease in yield, so the upper limit is limited to 0.0100% by mass. It is preferably in the range of 0.0005 to 0.0050% by mass. More preferably, it is 0.0010 to 0.0035% by mass.

[0036] N: 0.0150% by mass or less N combines with Al, Ti, and B to form nitrides. If the content exceeds 0.0150 mass%, the generation of BN becomes significant, and the amount of solute B decreases, resulting in a decrease in the hardenability of the steel. For this reason, the upper limit of the N content is limited to 0.0150 mass%. Although there is no particular lower limit for the N content, an excessively low N content leads to an increase in the cost of the steelmaking process, so the preferred range of the N content is 0.0010 mass% or more. The more preferred range is 0.0015 to 0.0080 mass%, and the most preferred range is 0.0020 to 0.0065 mass%.

[0037] Furthermore, one or more of the following elements may be selected and added as necessary. Cr: 0.65% by mass or less Cr has the effect of improving hardenability, and may be added when strength improvement after induction hardening is required. However, Cr has a strong effect of concentrating in carbides and increasing the thermal stability of the carbides, and adding more than 0.65 mass% of Cr has the disadvantage that the carbides remain undissolved during heating for induction hardening, resulting in a reduction in the amount of dissolved carbon in the steel and a decrease in strength after induction hardening. It may be added appropriately depending on the induction hardening conditions as long as it is within the range of 0.65 mass% or less, but in applications where the above-mentioned disadvantages must be stably avoided, it is recommended that the Cr amount be 0.35 mass% or less. More preferably, it is 0.25 mass% or less.

[0038] Cu: 1% by mass or less Ni: 1% by mass or less Cu and Ni have the effect of improving hardenability, and may be added when it is necessary to improve strength after induction hardening. However, if added in excess of 1 mass%, cold forgeability decreases, so the upper limit is limited to 1 mass%. The content is preferably 0.5 mass% or less, and more preferably 0.35 mass% or less.

[0039] Se: 0.3 mass% or less, Ca: 0.05 mass% or less, Pb: 0.3 mass% or less, Bi: 0.3 mass% or less, Mg: 0.05 mass% or less, Zr: 0.2 mass% or less, REM: 0.01 mass% or less, O: 0.025 mass% or less Se, Ca, Pb, Bi, Mg, Zr, REM and O all have the effect of improving machinability and may be added within the above ranges as necessary, but if added beyond the above ranges, the effect will simply saturate.

[0040] Nb: 0.1% by mass or less V: 0.3% by mass or less Nb and V combine with C to form fine carbides, which strengthen the precipitation of steel. To achieve this effect, they may be added within the above ranges. However, adding more than the above ranges reduces the continuous castability and reduces the yield.

[0041] Sn: 0.1% by mass or less Sb: 0.1% by mass or less Sb and Sn have the effect of making scale easier to remove in descaling treatments such as shot blasting and pickling, and may be added in the above range to obtain this effect. However, even if Sb and Sn are added in excess of 0.1 mass%, the effect of improving descaling is saturated, so the upper limit of Sb and Sn is set to 0.1 mass%. EXAMPLES

[0042] The configuration and effects of the present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the following examples, and appropriate modifications can be made within the scope of the present invention, and all of these modifications are included in the technical scope of the present invention.

[0043] Steel with the chemical composition shown in Table 1 was formed into a round bar with a diameter of 30 mm by hot rolling, and then a cylinder with a diameter of 20 mm and a height of 30 mm was machined from the center to be used as a test piece for cold forging. For cold forging, upsetting was performed at various compression rates (height reduction rates). After that, induction hardening was performed with various combinations of heating temperature and holding time. Then, tempering was performed by holding at 150°C for 30 minutes and then air cooling.

[0044] [Table 1]

[0045] Vickers hardness measurements were performed before and after cold forging and after induction hardening and tempering. Before and after cold forging, measurements were taken at three points at the radial center of the cylindrical test piece with a load of 300g, and the average value was evaluated. After induction hardening and tempering, measurements were taken at ten random points at a depth of 0.2 mm from the outer peripheral surface of the test piece with a load of 100g, and the minimum hardness was evaluated as the surface hardness. Measurements were also taken at three points at the center of the test piece with a load of 300g, and the average value was evaluated as the core hardness.

[0046] Table 2 shows the values ​​of the left side of the above formulas (1) and (2), the average hardness at the center of the cylindrical test piece before and after cold forging, and the measurement results of the surface hardness and core hardness after induction hardening and tempering. In this embodiment, induction hardening was performed by heating from room temperature (25°C) to various hardening temperatures at various heating rates, followed by immediate water cooling (quenching). The induction heating temperature (K) and heating rate (K / s) in formula (2) are shown in Table 2. In addition, T R was determined by a thermal expansion test in which the material was heated from room temperature to 900°C for 2 seconds. If the surface hardness after induction hardening and tempering is 400 (HV) or more and the core hardness is 200 (HV) or more in the above measurement results, it can be determined that the part has sufficient performance. All of the inventive examples according to the present invention satisfy these requirements.

[0047] [Table 2-1] [Table 2-2]

Claims

1. A method for producing an induction hardened part, comprising the steps of: forming a steel material into a steel bar or wire rod by hot rolling; cold forging the steel bar or wire rod without softening annealing to form a part satisfying the following formula (1); and subjecting the cold forged part to the induction hardening and tempering treatment without quenching and tempering before the induction hardening and tempering treatment that satisfies the following formula (2). Note H ＣＦ ≧200 ・・・(1) Here, H ＣＦ is the Vickers hardness after cold forging. Here, the function of the high-frequency heating temperature T (K) and time t (s) is f, that is, T = f(t). However, f(t) ≥ T Ｒ The range is T Ｒ is the austenite transformation completion temperature (K) during induction heating. Ｒ is the austenite transformation completion temperature T Ｒ The above is the time (s). Also, log is the common logarithm.

2. The steel bar or wire rod is C: 0.15 to 0.55% by mass, Si: 0.35% by mass or less, Mn: 0.85% by mass or less, P: 0.050% by mass or less, S: 0.050% by mass or less, Al: 0.010 to 0.090% by mass, Mo: 0.05 to 0.50% by mass, Ti: 0.010 to 0.200% by mass, B: 0.0005 to 0.0100 mass% and N: 0.0150% by mass or less The method for producing an induction hardened part according to claim 1, wherein the composition is a balance of Fe and unavoidable impurities.

3. The composition further includes The method for producing an induction hardened part according to claim 2, comprising the step of: Group A: Cr: 0.65% by mass or less, Cu: 1% by mass or less and Ni: 1% by mass or less Group B: Se: 0.3% by mass or less, Ca: 0.05% by mass or less, Pb: 0.3% by mass or less, Bi: 0.3% by mass or less, Mg: 0.05% by mass or less, Zr: 0.2% by mass or less, REM: 0.01% by mass or less and O: 0.025% by mass or less Group C: Nb: 0.1% by mass or less and V: 0.3% by mass or less Group D: Sn: 0.1% by mass or less and Sb: 0.1% by mass or less

Images