Axle shafts with excellent mechanical properties

Abstract

Alloy steel for axle shafts is characterized by containing 0.45 ≤ C ≤ 0.60 wt. %, 0.15 ≤ Si ≤ 0.35 wt. %, 0.60 ≤ Mn ≤ 0.85 wt. %, 0.80 ≤ Cr ≤ 1.05 wt. %, 0.004 ≤ V < 0.01 wt. %, 0.015 ≤ Al ≤ 0.070 wt. %, 0.001 ≤ N ≤ 0.004 wt. %, and 0.0020 ≤ B ≤ 0.0030 wt. %, balance Fe as well as smelting-related impurities, wherein the alloy steel for axle shafts furthermore contains carbide, nitride, and boron powder as a grain refiner in pellet or It has a wire shape.

Description

[Technical area]

[0001] The invention relates to a method for manufacturing vehicle axle shafts with excellent mechanical properties and an alloy steel for axle shafts with good hardenability, wherein this is made possible, in particular, by adding the cost-effective alloying elements V, Al, N, and B, to provide an alloy steel for axle shafts with excellent mechanical properties and good hardenability without adding the expensive alloying element Mo. The invention also relates to a method for manufacturing an alloy steel with excellent mechanical properties and good hardenability and vehicle axle shafts using the alloy steel, wherein, in addition, the normalizing and tempering treatments can be omitted in the manufacturing process by producing the vehicle axle shafts using such an alloy steel. [Technical background of the invention]

[0002] Traditionally, to ensure the durability and hardenability of vehicle axle shafts, an expensive alloying element, primarily Cr (chromium) or Mo (molybdenum), is added (US 4820357, KR 2003-0097233). In the case of steel used for conventional axle shafts to which Cr or Mo has been added, the problem is that the manufacturing costs must be increased due to the high content of these expensive alloying elements.

[0003] The manufacturing process for a conventional material essentially requires a normalization process to homogenize the material's internal structure and a tempering treatment to ensure its internal hardness. However, the normalization and tempering processes increase the manufacturing time and cost of the axle shafts.

[0004] Furthermore, after the upsetting forging process, the addition of the high-alloy element can lead to quality deviations for the entire production batch during tempering. Additionally, due to the material's insufficient hardenability, a water-soluble oil is used for tempering. The use of such a water-soluble oil drastically increases the material's cooling rate and therefore the martensite transformation rate, resulting in a significant difference in cooling rate between the surface and the interior of the material. This leads to thermally induced deformation and consequently reduces the material's durability.

[0005] From EP 2 008 732 A1, a steel tube for a drive shaft is known which has 0.30-0.47% C, 0.50% or less Si, 0.50-2.00% Mn, 0.20% or less P, 0.005% or less S and 0.001-0.050% Al, the remainder being Fe and impurities.

[0006] From US Patent 2005 / 0257860A1, a steel for a camshaft is known which contains at least 0.4 to 1.5% carbon by mass and is quenched at a surface layer of the steel through a deeper section below the surface layer to form two or more types of quench-hardened layers, one of which has a structure in which cementite is dispersed in 2% or more by volume. A carbon content of 0.4 to 1.5% by mass and a chromium content of 2% by mass or less are disclosed. The steel may further contain manganese, silicon + aluminum, molybdenum, tungsten, vanadium, aluminum, nickel, borosilicate glass, titanium + nitrocellulose, and / or sulfur.

[0007] From EP 1 743 950 A1, a steel tube is known which contains, by mass, 0.30 to 0.50% C, not more than 0.5% Si, 0.3 to 2.0% Mn, not more than 0.025% P, not more than 0.005% S, 0.15 to 1.0% Cr, 0.001 to 0.05% Al, 0.005 to 0.05% Ti, not more than 0.02% N, 0.0005 to 0.01% B, and not more than 0.0050% O, the remainder being Fe and impurities. Furthermore, 0.05 to 1% Cu, Ni, and / or Mo may be included by mass.

[0008] In JP 2005-320 575 A, a steel tube is disclosed which has mass fractions of 0.30 to 0.50% C, ≤ 0.5% Si, 0.3 to 2.0% Mn, ≤ 0.025% P, ≤ 0.005% S, 0.15 to 1.0% Cr, 0.001 to 0.05% Al, 0.005 to 0.05% Ti, ≤ 0.02% N, 0.0005 to 0.01% B and ≤ 0.0050% O.

[0009] To solve the problems of conventional technology, the inventors planned to develop an optimal process for manufacturing axle shafts, in which carbide, nitride and boron oxide powder is press-molded at high temperature and added as a pellet-shaped grain refiner, leading to an improvement in the mechanical properties in the raw material state, so that tempering treatment can be omitted and thus no quality deviations occur for any production batch. [Content of the invention][Purpose of the invention]

[0010] The invention aims to solve the conventional problems described above and sets itself the objective of providing an alloy steel for axle shafts with excellent mechanical properties, while significantly reducing manufacturing costs by using cost-effective alloying elements.

[0011] Furthermore, the invention has an objective of providing a method for manufacturing axle shafts in which the normalizing and tempering treatment in the manufacturing process can be omitted, thus reducing the thermally induced deformation caused by the high-frequency heat treatment.

[0012] Another object of the invention is to provide an alloy steel for axle shafts to which a grain refiner with good hardenability is applied, so that no quality deviations occur for all production batches, while using cost-effective alloying elements. [Technical solution]

[0013] To achieve the above objective, the present invention provides an alloy steel for axle shafts according to claim 1 and a method for manufacturing vehicle axle shafts according to claim 2. [Effects of the invention]

[0014] In the case of applying the alloy steel for axle shafts according to the invention, it is possible to ensure excellent mechanical properties and good hardenability by adding V, Al, N and B, which are inexpensive alloying elements, while reducing manufacturing costs by omitting an expensive alloying element Mo and reducing the amount of Cr.

[0015] In the case of applying the manufacturing process for the vehicle axle shaft of the invention, the manufacturing process can be shortened by omitting the normalization process as required and the tempering treatment, which not only makes it possible to ensure price competitiveness by optimizing manufacturing costs, but also to reduce thermally induced deformation of the product. [Brief description of the drawings] Fig.Figure 1 is a flowchart showing the comparison between the manufacturing process of the alloy steel for the axle shaft according to the invention and the manufacturing process of the vehicle axle shaft using the conventional material SCM440H. Fig. Figure 2 shows tissue photos for Table 3. [Examples of the invention]

[0016] The following describes in detail the preferred embodiments of the method for producing the alloy steel for axle shafts with excellent mechanical properties and good hardenability, and of the method for producing the vehicle axle shaft using the same alloy steel according to the invention.

[0017] The alloy steel for axle shafts with excellent mechanical properties and good hardenability is characterized by the fact that it contains C 0.45~0.60 wt.%, Si 0.15~0.35 wt.%, Mn 0.60~0.85 wt.%, Cr 0.80~1.05 wt.%, V 0.004~0.01 wt.%, Al 0.015~0.070 wt.%, N 0.001~0.004 wt.% and B 0.0020~0.0030 wt.%, balance Fe as well as melting-related impurities.

[0018] The alloy steel for axle shafts according to the invention has a higher C content compared to SCM440H, which is widely used as a material for conventional axle shafts, in order to obtain a martensitic structure by means of a high-frequency heat treatment of the material, wherein the manufacturing phase further comprises a step in which, to increase strength by grain refinement during the manufacturing phase, carbide, nitride and boron powder is pressure-formed at high temperature and produced as a grain refiner in pellet or wire form and added to the alloy steel.

[0019] The alloy steel according to the invention is configured such that an expensive alloying element, molybdenum (Mo), which was added in the prior art, is omitted, and the amount of chromium (Cr) is also reduced. To replace the functions of Mo and Cr, cost-effective metals, aluminum (Al) and boron (B), are added to the alloy steel for axle shafts according to the invention. Furthermore, tungsten (V) and nitrogen (N), which have been supplemented by Al and B, replace the functions of Cr and Mo. Therefore, when the vehicle axle shaft is manufactured using the alloy steel for axle shafts according to the invention, the manufacturing costs of the axle shafts can be significantly reduced, and the service life can be improved, even compared to conventional techniques.

[0020] The main components of the alloy steel for the axle shaft according to the invention and their quantitative dimensions of content are described in more detail below.

[0021] In one embodiment of the invention, C is preferably limited to 0.45 to 0.60 wt.% to eliminate the need for tempering treatment and improve durability. If the C content is less than 0.45 wt.%, the hardness of the raw material is reduced, and consequently, the strength cannot be adequately ensured. If the C content is more than 0.60%, the hardness is increased, and therefore the toughness and machinability may be impaired.

[0022] In one embodiment of the invention, Si is preferably limited to 0.15–0.35 wt.% to maintain the effect of improved hardenability and sufficient deoxidation during steelmaking. If the Si content exceeds 0.35 wt.%, durability may be reduced due to the decarburized layer that forms on the shaft surface during high-frequency heat treatment.

[0023] In one embodiment of the invention, the Mn content is preferably limited to 0.60–0.85 wt.% to increase and improve hardenability and strength. If the Mn content is 0.60 wt.% or less, a uniform bainite structure cannot be obtained after hot forging. In this case, tempering must be performed, whereby, if the Mn content exceeds 0.85%, machinability may be impaired.

[0024] In one embodiment of the invention, Cr is preferably limited to 0.80–1.05 wt.% to increase the hardness and hardenability of the raw material. If the Cr content falls below 0.80 wt.%, the hardness and hardenability are significantly reduced, and if the Cr content exceeds 1.05 wt.% Cr, the strength can decrease drastically due to the formation of excessive Cr carbide at the grain boundaries of the material.

[0025] Since chromium is an expensive metallic element, it is configured so that a reduced amount of the material SCM440H is added compared to conventional techniques, while the expensive metal element molybdenum (MO) is not added at all. Instead, the inexpensive alloying elements volatile matter (V), aluminum, nitrogen, and boron (B) are added to replace the functions of MO and chromium.

[0026] In one embodiment of the invention, V is preferably limited to 0.004 to 0.01 wt.% to increase the grain refinement and hardness of the material. In this case, it can be easily converted to a bainite structure by tempering treatment. If V is added in an amount of less than 0.004 wt.%, tempering treatment must be carried out, as the grain refinement and hardness enhancement effects cannot be achieved. If the V content exceeds 0.01 wt.%, machinability may be impaired depending on carbide formation.

[0027] In one embodiment of the invention, it is preferred that Al is limited to 0.015 to 0.070 wt.% to serve as a deoxidizing agent for the production of clean steel free from surface defects and to increase durability through grain refinement. If Al is added in an amount of 0.015 wt.% or less, the grain refinement effect cannot be achieved. If Al is added in an amount of 0.070 wt.% or more, the fatigue strength can be drastically reduced due to the formation of non-metallic inclusions.

[0028] In one embodiment of the invention, N is preferably limited to 0.001 to 0.004 wt.% to increase the sintering process required before addition in pellet form. If N is added in an amount of 0.001 wt.% or less, the sinterability is reduced. However, if N is added in an amount of 0.004 wt.% or more, the machinability may be impaired.

[0029] Due to the introduction of such a nitride, an improvement in mechanical properties can be achieved through particle refinement compared to the conventional administration of Ti, thereby solving the problem of uniform particle refinement that arose when adding the difficult-to-control alloying element Ti.

[0030] In one embodiment of the invention, B is added to ensure hardenability, where B is a substitute for Mo and can drastically increase hardenability even with the addition of a very small amount. The composition range is preferably limited to 0.002–0.003 wt.%. If the B content is 0.002 wt.% or less, hardenability is not ensured. If the B content is 0.003 wt.% or more, red brittleness occurs due to the formation of Fe₂B. Additionally, changes in the high-frequency properties may occur due to deviations in hardenability.

[0031] The method for manufacturing the vehicle axle shaft using the alloy steel according to the invention is as follows: The method for manufacturing the vehicle axle shaft according to the invention as described above, comprising a step in which the alloy steel material for axle shafts according to the invention is produced, characterized in that it contains C 0.45-0.60 wt.%, Si 0.15-0.35 wt.%, Mn 0.60-0.85 wt.%, Cr 0.80-1.05 wt.%, V 0.004-0.01 wt.%, Al 0.015-0.070 wt.%, N 0.001-0.004 wt.% and B 0.0020-0.0030 wt.%, balance Fe and melting-related impurities; a step in which the alloy steel material for axle shafts is hot forged; and a step in which the surface hardness of the hot-forged alloy steel for axle shafts is increased by high-frequency heat treatment, wherein the manufacturing step of the alloy steel for axle shafts is characterized in that it comprises a further step in which carbide, nitride and boron powders in pellet or wire form are produced as grain refiners and added to the alloy steel.

[0032] The method for manufacturing the vehicle axle shaft according to the invention can eliminate the need for the normalization process to homogenize the internal structure of the material prior to hot forging the alloy steel for axle shafts, unlike conventional techniques. Even if the normalization process is not carried out, the internal structure of the material is already homogenized, since the internal crystal grains become finer due to the cost-effective alloying elements V, Al, N, and B, which are additionally added to the alloy steel.

[0033] Furthermore, in the method for manufacturing the vehicle axle shaft according to the invention, the material treated by hot forging is directly subjected to high-frequency heat treatment without tempering to increase the surface hardness of the material. The high-frequency heat treatment process is a heat treatment method for increasing the surface hardness of the material and consequently improving its durability. Since the process is carried out in the same way as with conventional technology, a detailed description is omitted here.

[0034] In contrast to the conventional material SCM440H for axle shafts, the alloy steel for axle shafts according to the invention can eliminate the need for the normalization process to homogenize the fabric and the tempering treatment to increase the internal hardness of the material.

[0035] To eliminate the need for tempering, V, Al, N, and B are added to the steel for axle shafts according to the invention. The added elements not only provide a mechanical property but also ensure stable hardenability of the material, thus eliminating the need for tempering, which is typically performed separately after hot forging to increase the material's internal hardness. Furthermore, the material structure can be homogenized by refining the crystal grains of the internal material, thereby eliminating the need for the normalization process that must be carried out before hot forging to homogenize the structure.

[0036] In contrast to what has been done so far with the grain refiner with a single composition and the hardenability enhancer, the invention is characterized in that it includes a uniform alloying property in which the various alloy components with the different refining reactivities are comminuted and added in pellet and wire form in a suitable density and mixing ratio, thereby inducing uniform core formation by controlling the reactivity on the molten metal and maximizing the refining effect of the alloy steel and the uniform melting / stirring effect.

[0037] The preferred embodiments of the inventive method for manufacturing axle shafts with excellent mechanical properties are described in detail below. The following examples serve only to illustrate the invention, and the scope of the invention is not limited to these examples.

[0038] The components of the alloy steel for axle shafts with excellent mechanical properties according to the invention are shown in Table 1 below. Table 1 below shows the components of SCM440H, which is widely used as a conventional material for axle shafts, alongside the components of the alloy steel for axle shafts according to the invention. [Table 1]

[0039] The standard of chemical composition division The salary (weight %) C Si Mn Cr Mon V Al N B The invention Min. 0.45 0.15 0.60 0.80 - 0.004 0.015 0.001 0.0020 Max. 0.60 0.35 0.85 1.05 - 0.01 0.070 0.004 0.0030 SCM440H Min. 0.37 0.15 0.55 0.85 0.15 - - - - Max. 0.44 0.35 0.90 1.25 0.35 - - - -

[0040] As shown in Table 1, the alloy steel for axle shafts with excellent mechanical properties is characterized by the fact that it contains C 0.45–0.60 wt. %, Si 0.15–0.35 wt. %, Mn 0.60–0.85 wt. %, Cr 0.80–1.05 wt. %, V 0.004–0.01 wt. %, Al 0.015–0.070 wt. %, N 0.001–0.004 wt. %, and B 0.0020–0.0030 wt. %, balance Fe, as well as melting-related impurities.

[0041] Tables 2 and 3 summarize the results of the non-flammability and durability test for the axle shaft material according to the invention and the conventional SCM440H.

[0042] Table 2 below shows the results of the hardenability test for the axle shaft material according to the invention and for conventional SCM440H according to the Jominy test method. The distance (mm) in Table 2 below represents the distance from the material surface to the hardness measurement position, and the hardness values ​​of the upper and lower limits represent the specification for SCM440H. [Table 2]

[0043] Hardenability [Table 3]

[0044] Tissue of internal hardness and alloy division The invention SCM440H Hardness values HB260 HB290 Fabric photos see Fig. 2 see Fig. 2 Explan Uniform bainite structures Sorbitol Tissue

[0045] Both the alloy steel for axle shafts according to the invention and the conventional SCM440H were used for the internal hardness test, and both materials were treated up to the stage prior to high-frequency heat treatment. That is, the alloy steel for axle shafts according to the invention is the material that underwent only hot forging, while the conventional SCM440H underwent normalizing, upsetting, and tempering. The tempering conditions for the conventional SCM440H were heating at 870 °C for 3 hours, followed by quenching in 17% water-soluble oil, and finally reheating for 225 minutes at 590 °C.

[0046] The test results for hardenability and hardness shown in Tables 2 and 3 confirm that there is no significant difference in hardness between the material according to the invention for axle shafts and conventional SCM440H, and that the durability of the invention is also ensured. It is hereby confirmed that the material according to the invention for axle shafts exhibits a similar level of hardenability and internal hardness as conventional SCM440H, even though the expensive alloying element molybdenum (Mo) has not been added and the normalization and tempering process has been omitted.

Claims

[1] Alloy steel for axle shafts is characterized by , that it contains 0.45 ≤ C ≤ 0.60 wt. %, 0.15 ≤ Si ≤ 0.35 wt. %, 0.60 ≤ Mn ≤ 0.85 wt. %, 0.80 ≤ Cr ≤ 1.05 wt. %, 0.004 ≤ V < 0.01 wt. %, 0.015 ≤ Al ≤ 0.070 wt. %, 0.001 ≤ N ≤ 0.004 wt. % and 0.0020 ≤ B ≤ 0.0030 wt. %, balance Fe as well as smelting-related impurities, wherein the alloy steel for axle shafts further comprises carbide and nitride and boron powder as a grain refiner in pellet or wire form. [2] Method for manufacturing vehicle axle shafts, comprising a step in the preparation of an alloy steel material for axle shafts, wherein the material is characterized by the content 0.45 ≤ C ≤ 0.60 wt.%, 0.15 ≤ Si ≤ 0.35 wt.%, 0.60 < Mn < 0.85 wt.%, 0.80 ≤ Cr ≤ 1.05 wt.%, 0.004 ≤ V ≤ 0.01 wt.%, 0.015 ≤ Al ≤ 0.070 wt.%, 0.001 ≤ N ≤ 0.004 wt.% and 0.0020 ≤ B ≤ 0.0030 wt.%, balance Fe and melting-related impurities; a step in the production of carbide, nitride and boron powder as a grain refiner in pellet or wire form; a step of adding the carbide, nitride, and boron powder, produced as a grain refiner in pellet or wire form, to the alloy steel; a step of hot forging the alloy steel material for axle shafts; and a step in which the surface hardness of the hot forged alloy steel for axle shafts is increased by high-frequency heat treatment.

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