Hollow steel component

Abstract

Provided is a hollow steel component which comprises a high fatigue strength. A hollow steel component according to the present disclosure comprises the chemical composition set forth in the description, the area ratio of martensite in the microstructure is 95% or more, and the prior austenite grain size d (μm) and the ten-point average roughness RzJIS (μm) of the inner surface of the hollow steel component satisfy formula (1).　(1): A(d) × RzJIS2 + B(d) × RzJIS + C(d) > 5.30 Meanwhile, A(d), B(d), and C(d) in formula (1) are defined by formulae (2) to (4).　(2): A(d) = -9.525 × 10-8 × d2 + 2.326 × 10-4 × d - 1.244 × 10-2; (3): B(d) = -2.508 × 10-5 × d2 - 3.429 × 10-3 × d + 2.315 × 10-1; (4): C(d) = 4.150 × 10-4 × d2 - 1.117 × 10-2 × d + 4.734

Description

Hollow steel parts

[0001] The present disclosure relates to hollow steel parts.

[0002] Vehicles such as automobiles are equipped with hollow steel parts that are made of steel and have a cylindrical hollow shape, such as stabilizers, inner tie rods, drive shafts, and upper arms. Hollow steel parts are subjected to repeated stress due to vibrations generated when the vehicle is running. Therefore, stabilizers are required to have high fatigue strength.

[0003] Techniques for increasing the fatigue strength of hollow steel parts, such as hollow stabilizers, are disclosed in International Publication No. 2020 / 230795 (Patent Document 1) and International Publication No. 2019 / 131813 (Patent Document 2).

[0004] The electric resistance welded steel pipe for hollow stabilizers disclosed in Patent Document 1 has a component composition containing, in mass %, C: 0.20 to 0.40%, Si: 0.1 to 1.0%, Mn: 0.1 to 2.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.10%, Cr: 0.01 to 0.50%, Ti: 0.010 to 0.050%, B: 0.0005 to 0.0050%, Ca: 0.0001 to 0.0050%, N: 0.0050% or less, and Sn: 0.010 to 0.050%, with the balance being Fe and unavoidable impurities, and the total decarburized layer depth on the inner surface and outer surface is 100 μm or less.

[0005] In Patent Document 1, an electric resistance welded steel pipe for a hollow stabilizer contains 0.010% or more of Sn, thereby suppressing the formation of a decarburized layer and increasing fatigue strength.

[0006] The electric resistance welded steel pipe for hollow stabilizer applications disclosed in Patent Document 2 is made of a steel plate having a composition, in mass %, of C: 0.15 to 0.40%, Si: 0.05 to 0.50%, Mn: 0.30 to 2.00%, Al: 0.01 to 0.10%, Ti: 0.001 to 0.04%, B: 0.0005 to 0.0050%, N: 0.0010 to 0.0100%, and the balance being Fe and unavoidable impurities, with the Ti content and N content satisfying formula (1) ((N / 14)<(Ti / 47.9)), and has a bond width of 40×10 －６ m or more 120×10－６ An electric resistance welded steel pipe having an electric resistance welded portion having a minimum C content of C １ (mass%) and the C content of the steel plate: C ０ (mass%), C ０ -C １ is 0.05 mass% or less, and the depth of the total decarburized layer in the inner surface layer and the outer surface layer of the electric resistance welded steel pipe is 50 × 10 －６ m or less.

[0007] In both Patent Documents 1 and 2, the formation of a decarburized layer is suppressed to increase fatigue strength.

[0008] International Publication No. 2020 / 230795 International Publication No. 2019 / 131813

[0009] The fatigue strength of hollow steel parts can be increased by the techniques disclosed in Patent Documents 1 and 2. However, the fatigue strength of hollow steel parts may be increased by means other than those disclosed in Patent Documents 1 and 2.

[0010] An object of the present disclosure is to provide a hollow steel component having high fatigue strength.

[0011] The hollow steel part of the present disclosure has a chemical composition, in mass%, of C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: more than 0.050% but not more than 0%, S: more than 0% but not more than 0.0100%, N: more than 0% but not more than 0.0100%, O: more than 0% but not more than 0.0100%, sol. Al: 0 to 0.080%, Cr: 0 to 1.50%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance being Fe and impurities. The area ratio of martensite in the microstructure is 95% or more, and the prior austenite grain size d (μm) and the ten-point average roughness RzJIS (μm) of the inner surface of the hollow steel part satisfy the formula (1): A(d) × RzJIS ２+ B(d) × RzJIS + C(d) > 5.30 (1) Here, A(d), B(d), and C(d) in formula (1) are defined by formulas (2) to (4). A(d) = -9.525 × 10 －８ ×d ２ +2.326 x 10 －４ ×d−1.244×10 －２ (2) B(d)=-2.508×10 －５ ×d ２ -3.429 x 10 －３ ×d+2.315×10 －１ (3) C(d)=4.150×10 －４ ×d ２ -1.117 x 10 －２ × d + 4.734 (4)

[0012] The hollow steel components of the present disclosure have high fatigue strength.

[0013] Fig. 1 is a diagram showing the relationship between the prior austenite grain size d (μm), the ten-point average roughness RzJIS of the inner surface of the hollow steel part, and fatigue strength of the hollow steel part according to this embodiment. Fig. 2 is a schematic diagram for explaining why fatigue strength is increased in Case 1 (CA1) and Case 2 (CA2) in Fig. 1. Fig. 3 is a schematic diagram different from Fig. 2 for explaining why fatigue strength is increased in Case 1 (CA1) and Case 2 (CA2) in Fig. 1. Fig. 4 is a plan view showing an example of a hollow steel part according to this embodiment. Fig. 5 is a front view of a torsion fatigue test specimen. Fig. 6 is a side view of the torsion fatigue test specimen viewed from the longitudinal direction.

[0014] The present inventors have investigated and considered means for increasing the fatigue strength of hollow steel parts. First, the present inventors considered hollow steel parts that can achieve high fatigue strength from the viewpoint of chemical composition. As a result, they found that the chemical composition of the hollow steel part is, in mass %, C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: more than 0% and not more than 0.050%, S: more than 0% and not more than 0.0100%, N: more than 0% and not more than 0.0100%, O: more than 0% and not more than 0.0100%, sol. It was thought that a chemical composition consisting of Al: 0 to 0.080%, Cr: 0 to 1.50%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance being Fe and impurities would potentially result in high fatigue strength.

[0015] Therefore, the present inventors have investigated means for increasing the fatigue strength of hollow steel parts having the above-mentioned chemical composition. As a result, they have obtained the following findings. Hollow steel parts have an inner surface. Therefore, when hollow steel parts are subjected to repeated stress, cracks may occur on the inner surface of the hollow steel part and these cracks may propagate. Therefore, the present inventors have focused on the inner surface of hollow steel parts and investigated means for suppressing the occurrence of cracks when repeated stress is applied.

[0016] Here, the inventors focused on the size of prior austenite grains in hollow steel parts and the surface roughness of the inner surface of hollow steel parts. When repeated stress is applied, crack generation is thought to be caused by stress concentration on the inner surface of the hollow steel part. The inventors thought that such stress concentration is affected by the surface roughness of the inner surface and the size of prior austenite grains.

[0017] Therefore, the inventors investigated and studied the relationship between the surface roughness of the inner surface, the size of the prior austenite grains, and fatigue strength using hollow steel parts that satisfy the above-mentioned chemical composition. As a result, the inventors found that in hollow steel parts that satisfy the above-mentioned chemical composition and whose microstructure is substantially made of martensite, high fatigue strength can be obtained if the prior austenite grain size d (μm) and the ten-point average roughness RzJIS (μm) of the inner surface of the hollow steel part satisfy formula (1). A(d) × RzJIS ２ + B(d) × RzJIS + C(d) > 5.30 (1) Here, A(d), B(d), and C(d) in formula (1) are defined by formulas (2) to (4). A(d) = -9.525 × 10 －８ ×d ２ +2.326 x 10 －４ ×d−1.244×10 －２ (2) B(d)=-2.508×10 －５ ×d ２ -3.429 x 10 －３ ×d+2.315×10 －１ (3) C(d)=4.150×10 －４ ×d ２ -1.117 x 10 －２ × d + 4.734 (4) Here, the ten-point average roughness RzJIS of the inner surface of the hollow steel part is substituted for RzJIS in μm, and the prior austenite grain size d is substituted for d in formula (1).

[0018] Fig. 1 is a diagram showing the relationship between the prior austenite grain size d (µm) of a hollow steel part according to this embodiment, the ten-point mean roughness RzJIS of the inner surface of the hollow steel part, and fatigue strength. In Fig. 1, the horizontal axis represents the prior austenite grain size d (µm), and the vertical axis represents the ten-point mean roughness RzJIS (µm). The hatched areas CA1 and CA2 in the graph of Fig. 1 are ranges that satisfy formula (1). In the following description, area CA1 will also be referred to as "Case 1". Area CA2 will also be referred to as "Case 2". In areas CA1 and CA2, in a fatigue test described below, the number of cycles to fracture Nf was 2.0 × 10 ５ It is higher than.

[0019] The following is thought to be the reason why fatigue strength is increased in Case 1 (Region CA1) and Case 2 (Region CA2). Figures 2 and 3 are schematic diagrams for explaining why fatigue strength is increased in Case 1 (CA1) and Case 2 (CA2) in Figure 1. Referring to Figure 2, depressions 10 in the roughness of the surface of the hollow steel part (steel material) (e.g., the inner surface of the hollow steel part) can be a source of stress concentration. Meanwhile, the grain boundaries of prior austenite grains 20 near the surface can be a source of stress relaxation. When the size of the depressions 10 in the surface roughness and the size of the prior austenite grains 20 near the surface have a high affinity, that is, in Cases 1 and 2, the concave shape of the surface and the size of the prior austenite grains near the surface interfere with or inhibit stress concentration F. Therefore, stress concentration is relaxed in Region 30, resulting in increased fatigue strength.

[0020] On the other hand, as shown in Figure 3, when the size of the recesses 10 in the roughness of the surface (for example, the inner surface of the hollow steel part) of the hollow steel part and the size of the prior austenite grains 20 near the surface have low affinity, the stress concentration that occurs in the recesses 10 is unlikely to be interfered with or inhibited by the prior austenite grains 20. Therefore, with repeated stress loading, the stress concentration F continues to be concentrated in region 40, which is much smaller than region 30. As a result, cracks will occur in region 40, reducing the fatigue strength.

[0021] The above mechanism is merely a guess, and a different mechanism may be at work. However, satisfying formula (1) has been proven to increase fatigue strength in the examples described below.

[0022] The hollow steel part of this embodiment, which has been completed based on the above technical concept, has the following configuration.

[0023] The hollow steel part of the first configuration has a chemical composition, in mass%, of C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: more than 0.050% or less, S: more than 0% or less and 0.0100%, N: more than 0% or less and 0: more than 0.0100%, sol. Al: 0 to 0.080%, Cr: 0 to 1.50%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance is Fe and impurities. The area ratio of martensite in the microstructure is 95% or more, and the prior austenite grain size d (μm) and the ten-point average roughness RzJIS (μm) of the inner surface of the hollow steel part satisfy the formula (1): A(d) × RzJIS ２ + B(d) × RzJIS + C(d) > 5.30 (1) Here, A(d), B(d), and C(d) in formula (1) are defined by formulas (2) to (4). A(d) = -9.525 × 10 －８ ×d ２ +2.326 x 10 －４ ×d−1.244×10 －２ (2) B(d)=-2.508×10 －５ ×d ２ -3.429 x 10 －３ ×d+2.315×10 －１ (3) C(d)=4.150×10 －４ ×d ２ -1.117 x 10 －２ × d + 4.734 (4)

[0024] The hollow steel part of the second configuration is the hollow steel part of the first configuration, and the chemical composition is, in mass%, sol. Al: 0.001 to 0.080%, Cr: 0.01 to 1.50%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.001 to 0.100%, B: 0.0001 to 0.0050%, and Ca: 0.0001 to 0.0050%.

[0025] Hereinafter, the hollow steel part of this embodiment will be described. In the following description, "%" regarding the content of elements in the chemical composition means "mass %" unless otherwise specified.

[0026] [Configuration of hollow steel part of this embodiment] The hollow steel part of this embodiment is hollow and includes a part having a tubular hollow shape (tubular part). The hollow steel part is formed by processing a part or all of a steel pipe. The steel pipe used as the material for the hollow steel part may be a seamless steel pipe or an electric resistance welded steel pipe. Preferably, the hollow steel part is made of an electric resistance welded steel pipe.

[0027] FIG. 4 is a plan view showing an example of a hollow steel part according to this embodiment. Referring to FIG. 4 , the hollow steel part is, for example, a hollow stabilizer. The hollow steel part 1 includes a tubular portion 10 having a hollow tubular shape. In FIG. 4 , the tubular portion 10 includes a pair of bent portions 12 and a parallel portion 11. The parallel portion 11 is disposed between the pair of bent portions 12. One end of the parallel portion 11 is connected to one of the bent portions 12, and the other end of the parallel portion 11 is connected to the other bent portion 12. The parallel portion 11 extends, for example, in a substantially straight line. The parallel portion 11 and the pair of bent portions 12 are formed by bending both ends of a single steel pipe. Both end portions 13 of the hollow steel part 1 may be flattened as shown in FIG. 4 , or both end portions 13 may not be flattened. Both end portions 13 of the hollow steel part 1 are configured to be attachable to other members. The two end portions 13 are attached to other members by fastening members such as bolts and nuts.

[0028] [Features of the hollow steel part of this embodiment] The hollow steel part of this embodiment has the following features: (Feature 1) The chemical composition, in mass %, is: C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: more than 0% but not more than 0.050%, S: more than 0% but not more than 0.0100%, N: more than 0% but not more than 0.0100%, O: more than 0% but not more than 0.0100%, sol. Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.100%, B: 0-0.0050%, Ca: 0-0.0050%, and the balance being Fe and impurities. (Feature 2) The area ratio of martensite in the microstructure is 95% or more. (Feature 3) The prior austenite grain size d (μm) and the ten-point average roughness RzJIS (μm) of the inner surface of the hollow steel part satisfy formula (1). A(d) x RzJIS ２ + B(d) × RzJIS + C(d) > 5.30 (1) Here, A(d), B(d), and C(d) in formula (1) are defined by formulas (2) to (4). A(d) = -9.525 × 10 －８ ×d ２ +2.326 x 10 －４ ×d−1.244×10 －２ (2) B(d)=-2.508×10 －５ ×d ２ -3.429 x 10 －３ ×d+2.315×10 －１ (3) C(d)=4.150×10 －４ ×d ２ -1.117 x 10 －２ ×d+4.734 (4) Here, the ten-point average roughness RzJIS of the inner surface of the hollow steel part is substituted for RzJIS in μm, and the prior austenite grain size d is substituted for d in μm. Each of Features 1 to 3 will be explained below.

[0029] [(Feature 1) Chemical Composition] The chemical composition of the hollow steel part of this embodiment contains the following elements.

[0030] C: 0.23 to 0.50% Carbon (C) improves the hardenability of steel. Furthermore, C dissolves in steel. As a result, C increases the strength of steel. If the C content is less than 0.23%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.50%, the hot workability of the steel decreases, even if the contents of other elements are within the ranges of this embodiment. If the C content exceeds 0.50%, the toughness of the hollow steel part after quenching decreases, even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.23 to 0.50%. The preferred lower limit of the C content is 0.25%, more preferably 0.27%, even more preferably 0.30%, even more preferably 0.33%, and even more preferably 0.35%. The upper limit of the C content is preferably 0.48%, more preferably 0.46%, even more preferably 0.44%, even more preferably 0.42%, even more preferably 0.40%, and even more preferably 0.38%.

[0031] Si: 0.01 to 0.50% Silicon (Si) deoxidizes steel. Furthermore, Si dissolves in steel to increase its strength. If the Si content is less than 0.01%, the above effects are not fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.50%, the ductility and toughness of the hollow steel part decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 0.01 to 0.50%. The preferred lower limit of the Si content is 0.05%, more preferably 0.10%, even more preferably 0.15%, even more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit of the Si content is 0.45%, even more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0032] Mn: 0.50 to 2.50% Manganese (Mn) improves the hardenability of steel. Mn also dissolves in steel. As a result, Mn increases the strength of steel. If the Mn content is less than 0.50%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 2.50%, the toughness and ductility of the hollow steel part after quenching will decrease, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.50 to 2.50%. The preferred lower limit of the Mn content is 0.55%, more preferably 0.60%, even more preferably 0.70%, even more preferably 0.75%, even more preferably 0.80%, even more preferably 0.90%, even more preferably 1.00%, and even more preferably 1.10%. The upper limit of the Mn content is preferably 2.40%, more preferably 2.30%, even more preferably 2.20%, even more preferably 2.10%, even more preferably 2.00%, even more preferably 1.90%, even more preferably 1.80%, even more preferably 1.70%, even more preferably 1.60%, and even more preferably 1.50%.

[0033] P: More than 0% and 0.050% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is more than 0%. If the P content exceeds 0.050%, even if the contents of other elements are within the ranges of this embodiment, P segregates at grain boundaries, reducing the ductility of the steel. Therefore, the P content is more than 0% and 0.050% or less. The lower the P content, the better. However, excessive reduction in the P content significantly increases production costs. Therefore, considering industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.040%, even more preferably 0.035%, even more preferably 0.030%, even more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.010%.

[0034] S: More than 0% and not more than 0.0100% Sulfur (S) is an unavoidable impurity. That is, the S content is more than 0%. If the S content exceeds 0.0100%, the hot workability, toughness, and fatigue strength of the steel will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is more than 0% and not more than 0.0100%. The lower the S content, the better. However, excessive reduction of the S content significantly increases production costs. Therefore, considering industrial production, the preferred lower limit of the S content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the S content is preferably 0.0080%, more preferably 0.0070%, even more preferably 0.0065%, even more preferably 0.0060%, even more preferably 0.0050%, even more preferably 0.0040%, and even more preferably 0.0035%.

[0035] N: More than 0% and 0.0100% or less Nitrogen (N) is inevitably contained. That is, the N content is more than 0%. N forms nitrides and / or carbonitrides, increasing the strength of steel. However, if the N content exceeds 0.0100%, the toughness of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is more than 0% and 0.0100% or less. The preferred lower limit of the N content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0015%, even more preferably 0.0020%, and even more preferably 0.0030%. The upper limit of the N content is preferably 0.0095%, more preferably 0.0090%, even more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, even more preferably 0.0050%, and even more preferably 0.0040%.

[0036] O: More than 0% and not more than 0.0100% Oxygen (O) is an unavoidably contained impurity. In other words, the O content is more than 0%. If the O content exceeds 0.0100%, the toughness of the steel will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is more than 0% and not more than 0.0100%. The lower the O content, the better. However, excessive reduction of the O content significantly increases manufacturing costs. Therefore, considering industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the O content is preferably 0.0080%, more preferably 0.0070%, even more preferably 0.0060%, even more preferably 0.0050%, even more preferably 0.0040%, even more preferably 0.0035%, and even more preferably 0.0030%.

[0037] The balance of the chemical composition of the hollow steel part of this embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during industrial production of steel pipes, which are the raw material for the hollow steel part, and are acceptable within a range that does not adversely affect the hollow steel part of this embodiment.

[0038] [Optional Elements] The chemical composition of the hollow steel part of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of sol. Al: 0-0.080%, Cr: 0-1.50%, Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.100%, B: 0-0.0050%, and Ca: 0-0.0050%. Hereinafter, these elements are optional elements and may not be contained. These optional elements will be described below.

[0039] [First Group: Al] The chemical composition of the hollow steel part of this embodiment may further contain Al in place of a portion of Fe.

[0040] Sol. Al: 0 to 0.080% Aluminum (Al) is an optional element and may not be contained. In other words, the sol. Al content may be 0%. When Al is contained, that is, when the sol. Al content exceeds 0%, Al deoxidizes the steel. Al further combines with nitrogen (N) to form AlN. AlN suppresses grain coarsening during quenching. Even if even a small amount of Al is contained, the above effect can be obtained to some extent. However, if the sol. Al content exceeds 0.080%, Al combines with oxygen (O) to form excessive inclusions, even if the contents of other elements are within the ranges of this embodiment. In this case, the fatigue strength of the hollow steel part decreases. Therefore, the sol. Al content is 0 to 0.080%. The lower limit of the Al content is preferably more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.015%. The upper limit of the sol. Al content is preferably 0.075%, even more preferably 0.070%, even more preferably 0.060%, even more preferably 0.050%, even more preferably 0.040%, and even more preferably 0.030%. Note that sol. Al means oxidized sol. Al.

[0041] [Second Group: Cr, Mo, Ni, and Cu] The chemical composition of the hollow steel part of this embodiment may further contain one or more elements selected from the group consisting of Cr, Mo, Ni, and Cu in place of a portion of Fe. All of these elements are optional elements and may not be contained. When contained, all of these elements increase the strength of the steel.

[0042] Cr: 0 to 1.50% Chromium (Cr) is an optional element and does not necessarily need to be contained. That is, the Cr content may be 0%. When Cr is contained, that is, when the Cr content is greater than 0%, Cr increases the strength of the steel. Even if even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 1.50%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0 to 1.50%. The preferred lower limit of the Cr content is greater than 0%, more preferably 0.01%, even more preferably 0.05%, even more preferably 0.10%, even more preferably 0.15%, even more preferably 0.20%, and even more preferably 0.30%. The upper limit of the Cr content is preferably 1.40%, more preferably 1.30%, even more preferably 1.20%, even more preferably 1.00%, even more preferably 0.80%, even more preferably 0.60%, and even more preferably 0.40%.

[0043] Mo: 0 to 1.00% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When Mo is contained, that is, when the Mo content exceeds 0%, Mo increases the strength of the steel. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. On the other hand, if the Mo content exceeds 1.00%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0 to 1.00%. The preferred lower limit of the Mo content is more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, even more preferably 0.04%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.95%, more preferably 0.90%, even more preferably 0.85%, even more preferably 0.80%, even more preferably 0.60%, even more preferably 0.40%, even more preferably 0.20%, and even more preferably 0.10%.

[0044] Ni: 0 to 1.00% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When Ni is contained, that is, when the Ni content is greater than 0%, Ni increases the strength of the steel. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. On the other hand, if the Ni content exceeds 1.00%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 1.00%. The preferred lower limit of the Ni content is greater than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.04%, even more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Ni content is preferably 0.95%, more preferably 0.90%, even more preferably 0.80%, even more preferably 0.60%, even more preferably 0.40%, and even more preferably 0.20%.

[0045] Cu: 0 to 1.00% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is greater than 0%, Cu increases the strength of the steel. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. On the other hand, if the Cu content exceeds 1.00%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 1.00%. The preferred lower limit of the Cu content is greater than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, even more preferably 0.04%, and even more preferably 0.05%. The preferred upper limit of the Cu content is 0.80%, even more preferably 0.60%, even more preferably 0.50%, even more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.20%.

[0046] [Third Group: Ti, Nb, and V] The chemical composition of the hollow steel part of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti, Nb, and V. All of these elements are optional elements and may not be contained. When contained, all of these elements increase the strength and toughness of the steel.

[0047] Ti: 0 to 0.100% Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content exceeds 0%, Ti forms Ti precipitates such as carbides, nitrides, and / or carbonitrides. Ti precipitates increase the strength of the steel through precipitation strengthening. Ti precipitates also refine the grains through a pinning effect, thereby increasing the toughness of the steel. Even if even a small amount of Ti is contained, the above effects can be achieved to some extent. On the other hand, if the Ti content exceeds 0.100%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.100%. The lower limit of the Ti content is preferably more than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, even more preferably 0.020%, even more preferably 0.025%, and even more preferably 0.030%. The upper limit of the Ti content is preferably 0.090%, even more preferably 0.080%, even more preferably 0.070%, and even more preferably 0.060%.

[0048] Nb: 0 to 0.100% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms Nb precipitates such as carbides, nitrides, and / or carbonitrides. Nb precipitates increase the strength of the steel through precipitation strengthening. Nb precipitates also refine grains through a pinning effect, thereby increasing the toughness of the steel. Even if even a small amount of Nb is contained, the above effects can be achieved to some extent. On the other hand, if the Nb content exceeds 0.100%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.100%. The preferred lower limit of the Nb content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, even more preferably 0.005%, even more preferably 0.010%, and even more preferably 0.015%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.080%, even more preferably 0.075%, even more preferably 0.070%, even more preferably 0.065%, even more preferably 0.060%, even more preferably 0.050%, even more preferably 0.030%, and even more preferably 0.020%.

[0049] V: 0 to 0.100% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms V precipitates such as carbides, nitrides, and / or carbonitrides. V precipitates increase the strength of the steel through precipitation strengthening. V precipitates also refine grains through a pinning effect, thereby increasing the toughness of the steel. Even if even a small amount of V is contained, the above effects can be achieved to some extent. On the other hand, if the V content exceeds 0.100%, the ductility of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.100%. The preferred lower limit of the V content is greater than 0%, more preferably 0.001%, even more preferably 0.005%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the V content is preferably 0.095%, more preferably 0.090%, even more preferably 0.085%, even more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, even more preferably 0.050%, and even more preferably 0.040%.

[0050] [Fourth Group: B] The chemical composition of the hollow steel part of this embodiment may further contain B in place of a portion of Fe.

[0051] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the steel. Even if even a small amount of B is contained, the above effect can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the steel becomes more likely to become embrittled, even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is more than 0%, more preferably 0.0001%, even more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0052] [Group 5: Ca] The chemical composition of the hollow steel part of this embodiment may further contain Ca in place of a portion of Fe.

[0053] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When Ca is contained, that is, when the Ca content is greater than 0%, Ca improves the hot workability of the steel. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0050%, the toughness of the steel decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is greater than 0%, more preferably 0.0001%, even more preferably 0.0002%, even more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, and still more preferably 0.0025%.

[0054] [Method for Measuring the Chemical Composition of a Hollow Steel Part] The chemical composition of the hollow steel part of this embodiment can be measured by a known elemental analysis method. The tubular portion of the hollow steel part is cut with a drill at the center of the wall thickness in a cross section perpendicular to the axial direction, to obtain chips. The obtained chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a known inert gas fusion-thermal conductivity method. The O content is determined using a known inert gas fusion-non-dispersive infrared absorption method.

[0055] [(Feature 2) Microstructure of the hollow steel part] In the microstructure of the hollow steel part of this embodiment, the area ratio of martensite is 95% or more. Here, martensite includes tempered martensite. When a structure other than martensite is present in the microstructure of the hollow steel part, the remaining structure other than martensite is composed of one or more types selected from the group consisting of ferrite and pearlite.

[0056] The microstructure of the hollow steel part of this embodiment is substantially made of martensite, which increases the fatigue strength of the hollow steel part.

[0057] The lower limit of the martensite area ratio is preferably 97%, and more preferably 100%.

[0058] [Method for measuring the area ratio of martensite] The area ratio of martensite in the microstructure of a hollow steel part can be determined by the following method. A test piece is taken that includes the center position of the wall thickness in a cross section perpendicular to the axial direction of the hollow steel part. The size of the test piece is not particularly limited. Three test pieces are taken at 30 mm intervals in the axial direction of the hollow steel part. Of the surface of each test piece, a cross section perpendicular to the axial direction of the hollow steel part is used as the observation surface. The observation surface of each test piece is mirror-polished. The mirror-polished observation surface is etched using 3% nitric acid alcohol (nital etching solution). Of the etched observation surface, an observation field that includes the center position of the wall thickness is selected. The size of the observation field is 200 μm × 200 μm. The observation field is observed using an optical microscope at 500x magnification.

[0059] In the observation field, martensite can be easily distinguished from other structures (pearlite, ferrite, etc.) by contrast. Martensite is observed as a low-brightness gray and fine structure. Ferrite is observed as a white area that is brighter than martensite and pearlite. Pearlite is observed as a phase with a lamellar structure that is brighter than ferrite. The area fraction of martensite (%) is calculated based on the area of ​​tempered martensite in the observation field and the total area of ​​the observation field. The arithmetic mean of the values ​​obtained for the three test specimens is taken as the area fraction of martensite. The martensite area fraction is calculated as an integer value obtained by rounding the obtained arithmetic mean value to one decimal place.

[0060] [(Feature 3) Prior Austenite Grain Size d and Ten-Point Mean Roughness RzJIS of the Inner Surface of the Hollow Steel Part] In the hollow steel part of this embodiment, the prior austenite grain size d (μm) and the ten-point mean roughness RzJIS (μm) of the inner surface of the hollow steel part satisfy the formula (1): A(d) × RzJIS ２ + B(d) × RzJIS + C(d) > 5.30 (1) Here, A(d), B(d), and C(d) in formula (1) are defined by formulas (2) to (4). A(d) = -9.525 × 10 －８ ×d ２ +2.326 x 10 －４ ×d−1.244×10 －２(2) B(d)=-2.508×10 －５ ×d ２ -3.429 x 10 －３ ×d+2.315×10 －１ (3) C(d)=4.150×10 －４ ×d ２ -1.117 x 10 －２ × d + 4.734 (4) Here, the ten-point average roughness RzJIS of the inner surface of the hollow steel part is substituted for RzJIS in μm, and the prior austenite grain size d is substituted for d in formula (1).

[0061] F1 is defined as follows: F1 = A(d) × RzJIS ２ + B(d) × RzJIS + C(d) F1 is an index showing the relationship between the prior austenite grain size d, the ten-point mean roughness RzJIS of the inner surface of a hollow steel part, and fatigue strength. As described above, the fatigue strength of a hollow steel part is affected by the prior austenite grain size d and the ten-point mean roughness RzJIS of the inner surface of the hollow steel part. In the graph of Figure 1, the hatched areas (CA1 and CA2) are the ranges where F1 satisfies formula (1) and where fatigue strength is significantly increased.

[0062] When F1 satisfies formula (1), the size of the prior austenite grains and the size of the recessed shapes among the irregularities on the inner surface have a high affinity (see Figure 2). In this case, when the hollow steel part is subjected to repeated stress, the prior austenite grains and the surface roughness of the inner surface interfere with or inhibit stress concentration. As a result, stress concentration is alleviated and fatigue strength is increased.

[0063] The lower limit of F1 is preferably 5.32, more preferably 5.34, even more preferably 5.36, even more preferably 5.38, even more preferably 5.40, even more preferably 5.45, even more preferably 5.50, even more preferably 5.55, and even more preferably 5.60. The upper limit of F1 is not particularly limited. For example, the upper limit of F1 is 6.50.

[0064] [Method for measuring prior austenite grain size d] The prior austenite grain size d of the hollow steel part of this embodiment is determined by the following method. A test piece including the center position of the wall thickness of the hollow steel part is prepared. Three test pieces are prepared at 30 mm intervals in the axial direction of the hollow steel part. Of the three test pieces arranged in the axial direction, the middle test piece is prepared from the center position in the axial direction of the hollow steel part. When the hollow steel part has a pair of bent portions and a parallel portion arranged between the bent portions, for example, of the three test pieces arranged in the axial direction, the middle test piece is prepared from the center position in the longitudinal direction of the parallel portion.

[0065] The cross section of each test specimen surface perpendicular to the axial direction of the hollow steel part is used as the observation surface. The observation surface of the test specimen is mirror-polished. The mirror-polished observation surface is immersed in a saturated aqueous solution of picric acid for approximately 60 seconds to reveal the prior austenite grain boundaries by etching. A 10 mm x 10 mm observation field including the center of the wall thickness of the etched observation surface is observed with an optical microscope. Then, the grain size number G is determined in the observation field based on a cutting method in accordance with JIS G 0551 (2020). The obtained grain size number G is converted to the average grain size M (μm) of prior austenite grains based on the following formulas (I) and (II). The arithmetic mean value of the average grain size M of prior austenite obtained for the three test specimens is used as the prior austenite grain size d (μm) of the hollow steel part. The value of the prior austenite grain size d is rounded to one decimal place. n = 2 Ｇ＋３ (I) M=1000 / (n １／２ ) (II)

[0066] [Method for measuring the ten-point average roughness RzJIS of an inner surface] The ten-point average roughness RzJIS of the inner surface of a hollow steel part of this embodiment is determined by the following method in accordance with JIS B 0601:2013. A test piece including the inner surface of the hollow steel part is taken. Three test pieces are prepared at 30 mm intervals in the axial direction of the hollow steel part. Of each test piece, the surface corresponding to the inner surface of the hollow steel part is used as the measurement surface. Using a contact surface roughness measuring instrument, the line roughness (RzJIS) is determined on the measurement surface in four directions: a first direction at 0° relative to the axial direction of the hollow steel part, a second direction at 45° relative to the axial direction, a third direction at 90° relative to the axial direction, and a fourth direction at 135° relative to the axial direction.

[0067] Specifically, the surface profile (contour curve) is measured in each direction (first to fourth directions). In this case, the evaluation length in the first direction is 2.0 mm, and the cutoff value is 0.8 μm. In the second to fourth directions, the evaluation length is 0.5 mm, and the cutoff value is 0.8 μm. An average line is calculated based on the obtained roughness curve. Peaks (profile peaks) and valleys (profile valleys) are identified based on the average line over the reference length of the roughness curve. Based on the identified peaks and valleys, the five highest peaks within the reference length are identified in descending order. Similarly, the five deepest valleys within the reference length are identified in descending order. The sum of the arithmetic mean value of the identified five peaks and the arithmetic mean value of the identified five valleys is defined as the ten-point average roughness RzJIS (μm). The arithmetic mean value of the 12 (4 directions x 3 = 12) ten-point mean roughness values ​​RzJIS obtained from the three measurement surfaces is defined as the ten-point mean roughness RzJIS (μm) of the inner surface of the hollow steel part. The ten-point mean roughness RzJIS value is rounded to the second decimal place. The contact surface roughness measuring instrument used is, for example, a product called SURFTEST manufactured by Mitutoyo Corporation.

[0068] [Effects of the hollow steel part of this embodiment] The hollow steel part of this embodiment satisfies the above-mentioned features 1 to 3. Therefore, excellent fatigue strength is obtained. The hollow steel part of this embodiment is suitable as a vehicle part, such as an automobile. Examples of hollow steel parts include stabilizers, inner tie rods, drive shafts, and upper arms. The hollow steel part is particularly suitable as a hollow stabilizer, which is a stabilizer with a hollow shape.

[0069] [Method of manufacturing hollow steel part] An example of a method of manufacturing a hollow steel part of this embodiment will be described. The following example is one example for manufacturing a hollow steel part of this embodiment. Therefore, a hollow steel part satisfying the above-mentioned features 1 to 3 may be manufactured by a manufacturing method other than the manufacturing method described below.

[0070] An example of a method for manufacturing a hollow steel part according to this embodiment includes the following steps: (Step 1) Pipe-making step (Step 2) Hot diameter-reducing rolling step (Step 3) Cold working step (Step 4) Heat treatment step Here, step 3 is an optional step. The manufacturing steps will be described below.

[0071] When the prior austenite grain size d and the ten-point average roughness RzJIS on the inner surface of the hollow steel part of this embodiment are case 1 or case 2, the hollow steel part is manufactured by the following manufacturing process of pattern 1 or pattern 2. (Pattern 1) Pipe making process → hot diameter reducing rolling process → heat treatment process (Pattern 2) Pipe making process → hot diameter reducing rolling process → cold working process → heat treatment process

[0072] [(Process 1) Pipe-making process] In the pipe-making process, a steel pipe is manufactured as the raw material for the hollow steel part. First, a steel plate is prepared as the raw material for the steel pipe. The steel plate may be obtained from a third party or may be manufactured. When manufacturing the steel plate, molten steel satisfying Feature 1 is manufactured. The refining method is not particularly limited, and any known method may be used. A slab is manufactured using the molten steel by a known casting method. The slab is subjected to rough rolling and finish rolling by a known method to manufacture the steel plate. The coiling temperature of the steel plate is, for example, above 600°C to 700°C. The steel plate is manufactured by the above manufacturing process.

[0073] The produced steel plate is used to manufacture an electric resistance welded steel pipe by a well-known method. Specifically, the steel plate is formed into a cylindrical mother pipe (open pipe) using forming rolls. In the formed mother pipe, the width direction of the hot-rolled steel plate is formed to coincide with the circumferential direction of the mother pipe. The butt joints extending in the axial direction of the mother pipe are electric resistance welded. Through the above pipe-making process, a steel pipe that will be used as the material for hollow steel parts is manufactured.

[0074] [(Step 2) Hot Stretch Reducer] In the hot stretch reducer, the manufactured steel pipe is subjected to hot stretch reducer. Specifically, the steel pipe is heated. The heated steel pipe is subjected to hot stretch reducer. The stretch reducer is, for example, a three-roll stretch reducer. The stretch reducer is, for example, a tandem continuous rolling mill in which a plurality of stands are arranged in a row.

[0075] In hot reducing rolling, the process of receiving a reduction from the rolls in one stand and passing through that stand is called "one pass." There is no particular limitation on the number of passes in hot reducing rolling. The number of passes in hot reducing rolling is, for example, 10 to 30. There is no particular limitation on the cumulative area reduction rate in hot reducing rolling. A preferred upper limit of the cumulative area reduction rate in hot reducing rolling is 50.0%. The surface temperature of the steel pipe at the outlet side of the stand where the final pass of hot reducing rolling is performed is, for example, 650°C or higher.

[0076] [(Step 3) Cold Working Step] The cold working step is an optional step and may not be performed. If performed, in the cold working step, cold working is performed on the steel pipe after hot diameter-reducing rolling. The cold working is, for example, cold drawing. The cumulative area reduction rate in the cold working is not particularly limited. A preferred cumulative area reduction rate in the cold working is 30.0% or less.

[0077] [(Step 4) Heat Treatment Step] In the heat treatment step, the steel pipe after the cold working step is quenched and tempered. In quenching, the quenching temperature is, for example, 820 to 1050°C. The holding time at the quenching temperature is, for example, 0.016 to 1 minute (1 to 60 seconds). After the holding time has elapsed, the steel pipe is quenched. The quenching method is, for example, water cooling. The steel pipe after quenching is tempered. The tempering temperature is, for example, 150 to 350°C. The holding time at the tempering temperature is, for example, 20 to 120 minutes. Through the above steps, the hollow steel part of this embodiment is manufactured.

[0078] [Manufacturing Conditions in the Manufacturing Process] In the manufacturing method of this embodiment, the following conditions are further satisfied in the manufacturing process.

[0079] (Case 1 (Area CA1 in Figure 1 )) When the prior austenite grain size d and the ten-point average roughness RzJIS on the inner surface of a hollow steel part are Case 1, the following Conditions 1 and 2 are satisfied. (Condition 1) When the manufacturing process is Pattern 1, hot reduction rolling is performed at a heating temperature of 800 to 850°C in the hot reduction rolling process, and the prior austenite grain size d is set to 30.0 μm or less. In the hot reduction rolling process, if the time from the start of reduction in the first pass to the completion of reduction in the final pass (hot reduction time) is set to 200 seconds or less, the prior austenite grain size d can be set to 30.0 μm or less. When the manufacturing process is Pattern 2, the prior austenite grain size d after the cold working process is set to 30.0 μm or less. The cumulative area reduction rate in the hot diameter reduction rolling process and the cumulative area reduction rate in the cold working process are not particularly limited, but for example, the cumulative area reduction rate in the hot diameter reduction rolling process is 25.0% to 45.0%, and the cumulative area reduction rate in the cold working process is 1.0% to 30.0%. (Condition 2) In the hot diameter reduction rolling process, FA defined by formula (A) is 9.0 to 15.0. FA = inner diameter reduction rate / {(t / D) - (t / Dbase)} (A) where inner diameter reduction rate = 1 - (da / db). In formula (A), db is the inner diameter (mm) of the steel pipe before the hot diameter reduction rolling process, da is the inner diameter (mm) of the steel pipe after the hot diameter reduction rolling process, t is the wall thickness (mm) of the produced steel pipe, D is the diameter (mm) of the produced steel pipe, and Dbase is 38 (mm). The inner diameter da, the inner diameter db, the wall thickness t, and the diameter D are measured at one arbitrary point using a caliper or a micrometer, and the measured value is rounded to the nearest integer.

[0080] (Case 2 (area CA2 in Figure 1)) When the prior austenite grain size d and the ten-point average roughness RzJIS on the inner surface of a hollow steel part are case 2, the following conditions 3 and 4 are satisfied. (Condition 3) When the manufacturing process is pattern 1, hot reducing rolling is performed at a heating temperature of 1050 to 1100°C in the hot reducing rolling process, and the prior austenite grain size d is 55.0 μm or more. When the manufacturing process is pattern 2, the prior austenite grain size d after the cold working process is 55.0 μm or more. (Condition 4) In the hot reducing rolling process, FA defined by formula (A) is 6.7 or less. FA = inner diameter reduction rate / {(t / D) - (t / Dbase)} (A)

[0081] Conditions 1 and 2 for Case 1, and conditions 3 and 4 for Case 2 will be described below.

[0082] [Regarding Conditions 1 and 2 in Case 1] Regarding Condition 1 in Case 1, in the case of Pattern 1, if the heating temperature in the hot reducing rolling step is 850°C or less and the hot reducing time is 200 seconds or less, the prior austenite grain size d of the hollow steel part will be 30.0 μm or less. The lower limit of the heating temperature is, for example, 800°C. In the case of Pattern 2, the cumulative area reduction rate in the hot reducing rolling step and the cumulative area reduction rate in the cold working step are adjusted to make the prior austenite grain size d of the hollow steel part 30.0 μm or less. Those skilled in the art can appropriately adjust the cumulative area reduction rate in the hot reducing rolling step and the cumulative area reduction rate in the cold working step to make the prior austenite grain size d of the hollow steel part 30.0 μm or less. Preferably, the cumulative reduction in area in the hot diameter reducing process is 25.0% or more and 45.0% or less, and the cumulative reduction in area in the cold working process is 1.0% or more and 30.0% or less.

[0083] Regarding condition 2 of Case 1, FA affects the surface roughness of the inner surface of the hollow steel part. If FA is less than 9.0, the surface roughness of the inner surface of the hollow steel part becomes excessively small. In this case, the prior austenite grain size d and the ten-point average roughness RzJIS are plotted below the area CA1 of Case 1 in Figure 1. As a result, the hollow steel part no longer satisfies formula (1).

[0084] On the other hand, if FA exceeds 15.0, the surface roughness of the inner surface of the hollow steel part becomes excessively large. In this case, the prior austenite grain size d and the ten-point average roughness RzJIS are plotted above the area CA1 of Case 1 in Figure 1. As a result, the hollow steel part no longer satisfies formula (1). If FA is 9.0 to 15.0, the prior austenite grain size d and the ten-point average roughness RzJIS are plotted within the area CA1 of Case 1 in Figure 1, and the hollow steel part satisfies formula (1).

[0085] [Regarding Conditions 3 and 4 in Case 2] Regarding Condition 3 in Case 2, in the case of Pattern 1, if the heating temperature in the hot reducing rolling step is 1050 to 1100°C, the prior austenite grain size d of the hollow steel part will be 55.0 μm or more. The hot reducing time is not particularly limited, but is, for example, 20 seconds or more. In the case of Pattern 2, the cumulative area reduction rate in the hot reducing rolling step and the cumulative area reduction rate in the cold working step are adjusted to make the prior austenite grain size d of the hollow steel part 55.0 μm or more. Those skilled in the art can appropriately adjust the cumulative area reduction rate in the hot reducing rolling step and the cumulative area reduction rate in the cold working step to make the prior austenite grain size d of the hollow steel part 55.0 μm or more.

[0086] Regarding condition 4 of Case 2, if FA exceeds 6.7, the surface roughness of the inner surface of the hollow steel part becomes excessively large. In this case, the prior austenite grain size d and the ten-point average roughness RzJIS are plotted above the area CA2 of Case 2 in Figure 1. As a result, the hollow steel part no longer satisfies formula (1).

[0087] If FA is 6.7 or less, the prior austenite grain size d and the ten-point average roughness RzJIS are plotted within the area CA2 of Case 2 in Figure 1, and the hollow steel part satisfies formula (1).

[0088] The effects of the hollow steel part of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the hollow steel part of this embodiment. Therefore, the hollow steel part of this embodiment is not limited to this one example of conditions.

[0089] Hollow steel parts (hollow stabilizers) having the chemical compositions shown in Tables 1A and 1B were manufactured.

[0090]

[0091]

[0092] Specifically, hollow stabilizers were manufactured as follows: Slabs with each test number were prepared. The slabs were subjected to rough rolling and finish rolling to produce steel plates. The coiling temperature of the steel plates was above 600°C to 700°C. Electric resistance welded steel pipes were manufactured using the steel plates by a well-known method. Specifically, the steel plates were formed into cylindrical mother pipes (open pipes) using forming rolls. The butt joints extending in the axial direction of the mother pipes were electric resistance welded. Through the above-mentioned pipe-making process, steel pipes were manufactured as the material for hollow stabilizers, which are hollow steel parts.

[0093] The produced electric resistance welded steel pipes were subjected to hot reduction rolling using a reducing mill. Specifically, the steel pipes were heated at the heating temperatures shown in Table 2. The heating time was 20 seconds in all cases. The cumulative reduction in area in the hot reduction rolling was 25.0 to 35.0%. The electric resistance welded steel pipes after the hot reduction rolling were subjected to cold drawing (cold working) as necessary. The cumulative reduction in area in the cold drawing was 1.0 to 10.0%. The inner diameter da (mm) of the steel pipes after the hot reduction rolling process and the inner diameter db (mm) of the steel pipes before the hot reduction rolling process were as shown in Table 2. In addition, when the "cold drawing" column in Table 2 contains a "-", it indicates that the cold working (cold drawing) process was not performed for that test number.

[0094]

[0095] A heat treatment process was carried out on the steel pipes that had undergone the hot diameter reduction rolling process or the cold working (cold drawing) process. In the heat treatment process, quenching was carried out, followed by tempering. The quenching temperature was 820 to 1050°C, and the holding time at the quenching temperature was 1 to 60 seconds. The tempering temperature was 150 to 350°C, and the holding time at the tempering temperature was 20 to 120 minutes. Hollow stabilizers of each test number were manufactured using the above manufacturing process. Table 2 shows the wall thickness t (mm) and post-processing diameter D (mm) of the hollow stabilizer of each test number. Table 2 also shows the FA for each test number.

[0096] [Evaluation Tests] The following tests were conducted on the hollow stabilizer with each test number: (Test 1) Chemical composition measurement test (Test 2) Martensite area ratio measurement test in microstructure (Test 3) Prior austenite grain size d measurement test (Test 4) Ten-point average roughness Rz JIS measurement test of inner surface (Test 5) Fatigue test Tests 1 to 5 will be explained below.

[0097] [(Test 1) Chemical Composition Measurement Test] The chemical composition of the hollow stabilizer of each test number was measured based on the method described in the above [Method for measuring the chemical composition of a hollow steel part]. As a result, the chemical composition of the hollow stabilizer of each test number was as shown in Table 1 (Table 1A and Table 1B).

[0098] [(Test 2) Test for measuring area ratio of martensite in microstructure] The area ratio of martensite of the hollow stabilizer of each test number was determined based on the method described in [Method for measuring area ratio of martensite] above. The obtained results are shown in Table 3.

[0099]

[0100] [(Test 3) Prior Austenite Grain Size d Measurement Test] The prior austenite grain size d of the hollow stabilizer of each test number was measured based on the method described in the above [Method for measuring prior austenite grain size d]. The obtained results are shown in Table 3.

[0101] [(Test 4) Measurement of Ten-Point Average Roughness RzJIS of Inner Surface] Based on the method described in the above [Method for Measuring Ten-Point Average Roughness RzJIS of Inner Surface], the ten-point average roughness RzJIS of the inner surface of the hollow stabilizer with each test number was measured. The obtained results are shown in Table 3. Table 3 also shows the F1 value.

[0102] [(Test 5) Fatigue Test] The following fatigue test was performed on the hollow stabilizer with each test number to measure the fracture life of the hollow stabilizer. Specifically, a plate-shaped torsional fatigue test specimen with a thickness of 2 mm, a width of 8 mm, a length of 60 mm, and a parallel portion length of 9.5 mm was obtained from each hollow stabilizer with each test number. Figure 5 is a front view of the torsional fatigue test specimen. Figure 6 is a side view of the torsional fatigue test specimen viewed from the longitudinal direction. Referring to Figures 5 and 6, the longitudinal direction of the torsional fatigue test specimen was the axial direction of the hollow stabilizer. Furthermore, the cross-sectionally arched groove a of the torsional fatigue test specimen corresponded to the inner surface of the hollow stabilizer. The radius of curvature of groove a was 8.7 mm, and the depth of the groove bottom of groove a was 0.1 mm.

[0103] A torsional fatigue test was carried out in the atmosphere using a torsional fatigue test specimen at a set stress of 500 MPa. In the torsional fatigue test, an electromagnetic torsional fatigue tester was used, and a cycle fatigue test was carried out under the following conditions: test waveform: sine waveform, test speed: 15 Hz, test environment: room temperature, in the atmosphere, stress ratio: -1 (alternating), and the number of cycles until the torsional fatigue test specimen broke was measured. ５ If the test piece did not break after 100 cycles, it was determined that excellent fatigue strength was obtained. The results are shown in Table 3.

[0104] [Evaluation Results] Referring to Tables 1 to 3, in test numbers 1 to 30, the martensite area ratio was 95% or more, and F1 exceeded 5.30. Therefore, the number of cycles until the test specimen broke in the torsional fatigue test was 2.00 × 10 ５ The fatigue strength was more than 100 times, and excellent fatigue strength was obtained.

[0105] On the other hand, in test numbers 31 and 32, the heating temperature during hot diameter reduction was inappropriate. Therefore, F1 was 5.30 or less. As a result, the number of cycles until the test specimen broke in the torsional fatigue test was 2.00 × 10 ５ The fatigue strength was less than 1 / 3 of the test cycle, and excellent fatigue strength was not obtained.

[0106] In test numbers 33 and 34, the FA value was low. Therefore, F1 was 5.30 or less. As a result, the number of cycles until the test specimen broke in the torsional fatigue test was 2.00 × 10 ５The fatigue strength was less than 1 / 3 of the test cycle, and excellent fatigue strength was not obtained.

[0107] In test numbers 35 and 36, the FA value was high. Therefore, F1 was 5.30 or less. As a result, the number of cycles until the test specimen broke in the torsional fatigue test was 2.00 × 10 ５ The fatigue strength was less than 1 / 3 of the test cycle, and excellent fatigue strength was not obtained.

[0108] In test numbers 37 and 38, the heating temperature during hot diameter reduction was inappropriate. Therefore, F1 was 5.30 or less. As a result, the number of cycles until the test specimen broke in the torsional fatigue test was 2.00 × 10 ５ The fatigue strength was less than 1 / 3 of the test cycle, and excellent fatigue strength was not obtained.

[0109] In test numbers 39 and 40, the FA value was inappropriate. Therefore, F1 was 5.30 or less. As a result, the number of cycles until the test specimen broke in the torsional fatigue test was 2.00 × 10 ５ The fatigue strength was less than 1 / 3 of the test cycle, and excellent fatigue strength was not obtained.

[0110] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A hollow steel part having a chemical composition, in mass %, of C: 0.23 to 0.50%, Si: 0.01 to 0.50%, Mn: 0.50 to 2.50%, P: more than 0% but not more than 0.050%, S: more than 0% but not more than 0.0100%, N: more than 0% but not more than 0.0100%, O: more than 0% but not more than 0.0100%, sol. A hollow steel part comprising Al: 0 to 0.080%, Cr: 0 to 1.50%, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Ti: 0 to 0.100%, Nb: 0 to 0.100%, V: 0 to 0.100%, B: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance being Fe and impurities, wherein the area ratio of martensite in the microstructure is 95% or more, and the prior austenite grain size d (μm) and the ten-point average roughness RzJIS (μm) of the inner surface of the hollow steel part satisfy formula (1). A(d) x RzJIS ２ + B(d) × RzJIS + C(d) > 5.30 (1) Here, A(d), B(d), and C(d) in formula (1) are defined by formulas (2) to (4). A(d) = -9.525 × 10 －８ ×d ２ +2.326 x 10 －４ ×d−1.244×10 －２ (2) B(d)=-2.508×10 －５ ×d ２ -3.429 x 10 －３ ×d+2.315×10 －１ (3) C(d)=4.150×10 －４ ×d ２ -1.117 x 10 －２ × d + 4.734 (4) 2. A hollow steel part according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of sol. Al: 0.001 to 0.080%, Cr: 0.01 to 1.50%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, V: 0.0001 to 0.0050%, and Ca: 0.0001 to 0.0050%.

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