Manufacturing method for induction hardened parts

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 other applications are formed by hot or cold forging, then machined to their final shape. Cold forging, in particular, has the advantage of reducing the amount of machining required after forging due to its excellent dimensional accuracy, and its application has been increasing in recent years. Induction hardening, meanwhile, can improve the fatigue strength of steel in a short time and has the advantage of reducing CO2 emissions, so its application is also expanding.

[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 based on the effective hardened layer depth and the part radius. Patent Document 2 also proposes a method for producing a rolled steel material for induction hardening, which specifies the characteristics of the microstructure, including the number density of spheroidal cementite, in addition to specifying 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] Japanese Patent Application Laid-Open No. 2011-241468 Summary of the Invention [Problem to be solved by the invention]

[0005] Parts obtained by induction hardening as a final heat treatment after forming typically undergo 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, by softening the steel material after hot rolling by annealing, it is necessary to reduce the deformation resistance during subsequent cold forging and ensure cold forgeability. In addition, thermal refining (quenching and tempering) before induction hardening is necessary to impart a predetermined hardness to the final product part.

[0006] On the other hand, in recent years, intensifying competition in parts prices and growing momentum toward carbon neutrality (reducing CO2 emissions) have led to an increasing need for omitting heat treatment in the parts manufacturing process. Under these circumstances, as mentioned above, the technology described in Patent Document 1 has a problem in that it cannot omit annealing before cold forging. Similarly, the technology described in Patent Document 2 can omit quenching and tempering, but it also has a problem in that it cannot omit annealing before cold forging.

[0007] The present invention was developed in view of the above-mentioned circumstances, and aims to propose a method for manufacturing induction-hardened parts that have excellent cold forgeability and sufficient part performance (surface and core hardness), which allows for the omission of 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 for work hardening during cold forging and induction hardening, it is possible to manufacture high-performance parts without impairing cold forgeability even if heat treatments before cold forging and induction hardening are omitted.

[0009] The present invention is based on the above findings and has the following gist. 1. A method for manufacturing an induction-hardened part, comprising: 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 that satisfies the following formula (1); and then subjecting the cold-forged part to 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) where H CF is the Vickers hardness after cold forging. JPEG0007819714000001.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. 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 1 above, 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 materials 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 [Effects 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 heat treatment before cold forging and induction hardening is omitted, which is extremely useful industrially. DETAILED DESCRIPTION OF THE INVENTION

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

[0014] Examples of induction hardening steel according to this embodiment include, in the automotive field, engine crankshafts, camshafts, timing gears, and diesel common rails, drivetrain propeller shafts, drive shafts, and CVJ outer races, suspension hubs, steering pinions, worm and ball joints, and electrical rotor shafts and motor shafts. Similarly, in the industrial machinery field, examples include ball screw shafts and linear bearing rails, and in the construction machinery field, examples include ring gears and inner and outer rings of slewing rings in travel reducers.

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

[0016] <Steel material> The steel material is not particularly limited in terms of its composition, but any ordinary steel for induction hardening can be used. For example, steel having the composition described below can be suitably used.

[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), core strength equal to or greater than that of conventional hardening and tempering followed by induction hardening and tempering can be obtained. 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 ensuring a certain amount of forging processing, H CF ≧200 is achievable.

[0019] <Induction hardening and tempering treatment that satisfies the following formula (2)> JPEG0007819714000002.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 induction heating. R can be determined by a thermal expansion test, and the heating rate should be such that the temperature is raised from room temperature to 900°C in 1 to 3 seconds. Incidentally, when a steel having the preferred chemical composition described below is subjected to 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 the temperature function f(t) is assumed to be 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 Between times t1 and t2, f(t)≧T R f(t) beyond time t2 <T R However, t2>t1 JPEG0007819714000003.jpg22170

[0021] Note that f(t) is preferably 1573 K or less, taking into consideration the temperature resistance of the high-frequency heating coil and to avoid melting the steel. Regarding the upper limit of the above formula (2), a target of 3000 or less is recommended, since there is a concern that work hardening may be reversed if the heating effect extends to the center. However, depending on the component shape and the cooling capacity of the hardening equipment, heat conduction to the center can be reduced, and in such cases, f(t) 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 composed of deformed ferrite and pearlite. In this structure, only traces of carbon are present in the ferrite, but carbon is concentrated in the pearlite containing cementite. When this structure is subjected to induction hardening, the heating time for induction hardening is extremely short, on the order of a few seconds, resulting in insufficient carbon diffusion. As a result, the resulting martensitic structure has carbon unevenly dispersed. Since the strength of martensitic structures depends on the carbon concentration, low carbon concentration regions have low strength, while high carbon concentration regions have high strength, resulting in localized strength variations. To resolve this issue, it is necessary to ensure the carbon diffusion distance after austenite transformation during induction heating, which requires satisfying the above formula (2). In other words, after austenite transformation by heating, the material is held at a temperature above the transformation temperature for a certain period of time. 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 this 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 It is necessary to set the temperature above T R After that, T R After passing through the following temperatures, R If the temperature is increased above T 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 Between times t1 and t2, f(t)≧T R f(t) from time t2 to time t3 <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 as shown in the following equation (4). JPEG0007819714000004.jpg18170

[0025] Here, we will show a specific example of induction hardening and tempering that satisfies the above formula (2). If the temperature is raised to the induction hardening heating temperature of 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 chemical composition of the steel material may be appropriately adjusted as necessary. A preferred chemical composition is shown below, but is not limited to this composition and may be selected depending on the performance required of the part. The remainder of the preferred chemical composition is Fe and inevitable impurities in all cases. Inevitable impurities are those that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of steel, and are acceptable within a range that does not adversely affect the properties of this embodiment.

[0027] C:0.15~0.55% by mass At least 0.15% by mass of C must be added to ensure the strength of the hardened layer after induction hardening. On the other hand, if the C content exceeds 0.55% by mass, cold forgeability decreases and quench cracking during induction hardening becomes more likely to occur, so the upper limit is set to 0.55% by mass. Note that a C content of 0.36 to 0.48% by mass is preferred to improve the balance between cold forgeability and strength after induction hardening.

[0028] Si: 0.35% by mass or less Although Si is effective as a deoxidizer, if it exceeds 0.35% by mass it reduces cold forgeability, so the content is limited to 0.35% by mass or less. Furthermore, since Si reduces the hardness of steel and improves machinability, the content is preferably less than 0.35% by mass, and more preferably 0.10% by 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 parts, a P content of 0.050% by mass or less is sufficient. 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 element effective in 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, with a range of 0.005 to 0.050% by mass being preferred. A range of 0.010 to 0.035% by mass is even more preferred.

[0032] Al:0.010~0.090% by mass Al is an element effective for deoxidation. It also has the effect of refining the grain size by bonding with N to form fine nitrides. To achieve these effects, an addition of 0.010 mass% or more is necessary. On the other hand, even if added in excess of 0.090 mass%, these effects 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, and must be added in an amount of at least 0.05% by mass to ensure strength after induction hardening. On the other hand, if the amount exceeds 0.50% by mass, cold forgeability decreases, so the upper limit is set to 0.50% by mass. The amount 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 combines with N to form nitrides, reducing the amount of solute N in steel and improving cold forgeability. Furthermore, because Ti has a stronger tendency to form nitrides than B, it suppresses the formation of B nitrides and increases the amount of solute B, thereby improving the hardenability of steel. Furthermore, it also inhibits the recovery of work hardening caused by cold forging in the heat-affected zone after induction hardening, making it useful for preventing a decrease in strength of parts. To achieve these effects, at least 0.010% by mass of Ti is necessary. However, adding more than 0.200% by mass causes the nitrides to coarsen, potentially causing fatigue fracture. Therefore, the upper limit is set to 0.200% by mass. The Ti content 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 achieve this effect, it is necessary to add at least 0.0005% by mass. On the other hand, if the amount exceeds 0.0100% by mass, cracks are more likely to occur after continuous casting, causing a decrease in yield, so the upper limit is set to 0.0100% by mass. The range is preferably 0.0005 to 0.0050% by mass, and more preferably 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 N content exceeds 0.0150% by 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 set to 0.0150% by mass. There is no particular lower limit for the N content, but an excessively low N content increases the cost of the steelmaking process, so the N content is preferably in the range of 0.0010% by mass or more. The N content is more preferably 0.0015 to 0.0080% by mass, and optimally 0.0020 to 0.0065% by mass.

[0037] Furthermore, one or more of the following elements may be selected and added as needed. Cr: 0.65% by mass or less Cr has the effect of improving hardenability and may be added when increased strength after induction hardening is required. However, Cr concentrates in carbides and has a strong effect of increasing the thermal stability of the carbides. If Cr is added in an amount exceeding 0.65% by mass, the carbides will remain undissolved during the induction hardening process, reducing the amount of solute carbon in the steel and resulting in a decrease in strength after induction hardening. While Cr may be added appropriately up to 0.65% by mass depending on the induction hardening conditions, in applications where the aforementioned adverse effects must be reliably avoided, it is recommended that the Cr content be 0.35% by mass or less. A Cr content of 0.25% by mass or less is even more desirable.

[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 needed, but even if they are 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 continuous castability and reduces 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 processes 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 performance saturates, so the upper limit of Sb and Sn is set to 0.1 mass%. [Example]

[0042] The following examples will explain the structure and effects of the present invention in more detail. 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 such modifications are within the technical scope of the present invention.

[0043] Steel with the chemical composition shown in Table 1 was hot-rolled into a round bar with a diameter of 30 mm, and then cylinders with a diameter of 20 mm and a height of 30 mm were machined from the center to serve as test specimens for cold forging. Cold forging involved upsetting at various compression ratios (height reduction rates). After that, induction hardening was performed using various combinations of heating temperature and holding time. The specimens were then tempered by holding at 150°C for 30 minutes followed by 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 10 random points at a depth of 0.2mm from the outer 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 quenching temperatures at various heating rates, followed by immediate water cooling (rapid cooling). The induction heating temperature (K) and heating rate (K / s) in formula (2) are shown in Table 2. In addition, T R The hardness 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, the part can be judged to have 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: 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 that satisfies the following formula (1); and then subjecting the cold-forged part to 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, let f be the function of the temperature T (K) of the high-frequency heating and the time t (s), 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 a certain 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 2. The method for producing an induction-hardened part according to claim 1, wherein the alloy has a composition comprising:

3. The component composition further includes: The method for producing an induction hardened part according to claim 2, wherein the material contains one or more selected from one or more of the following groups A to D: 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

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