steel material

Abstract

This invention provides a steel exhibiting excellent resistance to hydrogen embrittlement and excellent lubricant adhesion after pickling treatment. The steel of this embodiment, by mass percent, comprises: C: 0.30%–0.50%, Si: 0.40% or less, Mn: 0.10%–0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 0.90%–1.80%, Mo: 0.30%–1.00%, Al: 0.005%–0.100%, N: 0.003%–0.030%, with the remainder consisting of Fe and impurities. It will be processed using a pre-constant current... The region from the surface of the steel to a depth of 100±20μm was removed by electrolysis. Then, the region from the surface of the steel to a depth of 100±20μm was further electrolyzed by positive constant current. The Cr concentration in the extracted residue was defined as [Cr] (mass%), and the Mo concentration in the extracted residue was defined as [Mo] (mass%). At this time, equation (1) is satisfied, 10.0≤[Cr]+[Mo]≤30.0 (1).

Description

Technical Field

[0001] This disclosure relates to a type of steel. Background Technology

[0002] The manufacturing process for mechanical structural components, such as bolts, is as follows: For example, spheroidized annealed steel undergoes a descaling process to remove the oxide scale. During this process, the steel is pickled. The descaled steel is then coated with a lubricant. The lubricated steel is then drawn to produce steel wire. The wire is forged to produce an intermediate product. The intermediate product is then heat-treated (e.g., quenching and tempering) to manufacture mechanical structural components. Sometimes, the forged intermediate product is machined.

[0003] As described above, in order to improve dimensional accuracy, steel used for mechanical structural components often undergoes processes such as wire drawing (cold drawing) before forging. To suppress thermal adhesion between the steel and the wire drawing die, a lubricating coating treatment is applied to the steel before wire drawing. In the lubricating coating treatment, a lubricating coating is formed on the surface of the steel. For example, a chemical conversion treatment coating is formed on the surface of the steel. Then, soap (metallic soap, etc.) is applied to the chemical conversion treatment coating.

[0004] In the aforementioned manufacturing process, if oxide scale remains on the steel surface after descaling but before lubricant coating, the adhesion of the lubricant coating becomes insufficient. In other words, the lubricant adhesion to the steel decreases. Under these circumstances, hot sticking occurs during wire drawing. Furthermore, pickling is performed during descaling. Hydrogen is generated on the steel surface during pickling. If the hydrogen generated during pickling penetrates the steel, its resistance to hydrogen embrittlement decreases. Therefore, for steel intended for use in mechanical structural components where descaling and subsequent wire drawing are to be performed, both excellent resistance to hydrogen embrittlement and excellent lubricant adhesion are required.

[0005] International Patent Publication No. 2015 / 189978 (Patent Document 1) and Japanese Patent Application Publication No. 2013-237903 (Patent Document 2) have proposed steel materials that can be used as raw materials for mechanical structural components.

[0006] The steel disclosed in Patent Document 1 contains, by mass percent, C: 0.005%–0.60%, Si: 0.01%–0.50%, Mn: 0.20%–1.80%, Al: 0.01%–0.06%, P: less than 0.04%, S: less than 0.05%, N: less than 0.01%, Cr: 0–1.50%, Mo: 0–0.50%, Ni: 0–1.00%, V: 0–0.50%, B: 0–0.0050%, Ti: 0–0.05%, with the remainder consisting of Fe and impurities. The metallographic structure of this steel contains pearlite. The value obtained by dividing the atomic percent Mn content of cementite in the pearlite by the atomic percent Mn content of ferrite in the pearlite is greater than 0 and less than 5.0. In this steel, the chemical composition and metallographic structure are controlled, and the Mn distribution ratio in pearlite related to cementite and ferrite is adjusted. As described in Patent Document 1, this allows for a shorter spheroidizing annealing time.

[0007] Patent Document 2 discloses a bolt steel with the following composition by mass percent: C: 0.30%–0.40%, Si: 0.01%–0.40%, Mn: 0.10%–1.0%, P: less than 0.030%, S: less than 0.030%, Al: 0.005%–0.10%, Cr: 0.90%–1.8%, Mo: 0.10%–2.0%, N: 0.003%–0.030%, Nb: 0–0.10%, with the remainder consisting of Fe and impurities. In this steel, the ratio of carbides with a circumference equivalent diameter of 0.5 μm or more to carbides with a circumference equivalent diameter of 1.0 μm or more is 10% or less. This reduces the proportion of coarse carbides in the steel. Therefore, Patent Document 2 states that: it can fully dissolve carbides during quenching, thereby reducing the deviation in tensile strength of bolt products.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2015 / 189978

[0011] Patent Document 2: Japanese Patent Application Publication No. 2013-237903 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] However, in Patent Document 1 and Patent Document 2, the hydrogen embrittlement resistance of steel that had been pickled before wire drawing and the lubricant adhesion of steel in the lubricant coating treatment before wire drawing were not studied as descaling treatments.

[0014] The purpose of this disclosure is to provide a steel with excellent resistance to hydrogen embrittlement and excellent lubricant adhesion after pickling treatment for the purpose of removing oxide scale.

[0015] Solution for solving the problem

[0016] The steel disclosed herein has the following composition.

[0017] A type of steel, wherein the steel is expressed as a percentage by mass.

[0018] C: 0.30%~0.50%

[0019] Si: below 0.40%

[0020] Mn: 0.10%~0.60%

[0021] P: below 0.030%

[0022] S: below 0.030%

[0023] Cr: 0.90%–1.80%

[0024] Mo: 0.30%–1.00%

[0025] Al: 0.005%~0.100%

[0026] N: 0.003%–0.030%, and

[0027] The remaining portion consists of Fe and impurities.

[0028] The region from the surface of the steel to a depth of 100±20μm was removed by electrolysis using a pre-constant current electrolysis method, followed by further electrolysis of the region from the surface of the steel to a depth of 100±20μm using a positive constant current electrolysis method. The Cr concentration in the resulting extraction residue was defined as [Cr] (mass%), and the Mo concentration in the extraction residue was defined as [Mo] (mass%). At this point, equation (1) is satisfied.

[0029] 10.0≤[Cr]+[Mo]≤30.0(1).

[0030] The effects of the invention

[0031] The steel disclosed herein exhibits excellent resistance to hydrogen embrittlement and excellent lubricant adhesion after pickling treatment for the purpose of removing oxide scale. Attached Figure Description

[0032] Figure 1 It is a diagram showing the area removed by electrolysis using a pre-constant current.

[0033] Figure 2 It is a diagram showing the region where electrolysis is performed using constant current electrolysis after pre-constant current electrolysis. Detailed Implementation

[0034] First, from the viewpoint of chemical composition, the inventors studied steels suitable for use as raw materials for mechanical structural components, such as cold-forged bolts. The inventors also investigated and studied the main causes of hydrogen embrittlement in steel after pickling treatment for the purpose of removing oxide scale. As a result, the inventors obtained the following insights.

[0035] When steel undergoes pickling, its surface dissolves. Hydrogen is generated on the steel surface as a result of this dissolution. If this hydrogen penetrates the interior of the steel, it accumulates at the grain boundaries. Consequently, hydrogen embrittlement occurs in the pickled steel.

[0036] To suppress hydrogen embrittlement of steel after such pickling treatment, the following method is conceived from a chemical composition perspective.

[0037] (A) Improve the grain strength of steel. Specifically, minimize the presence of elements that segregate towards grain boundaries and reduce grain boundary strength, namely Mn, P, and S.

[0038] (B) To suppress grain coarsening in steel and thus suppress the localized accumulation and concentration of hydrogen. Specifically, this is achieved by utilizing the pinning effect produced by AlN. Therefore, it contains appropriate amounts of Al and N.

[0039] Considering the above technical ideas, the inventors believe that the chemical composition of steel, which can be used as a raw material for mechanical structural components, can suppress hydrogen embrittlement of pickled steel if it has the following chemical composition: C: 0.30%–0.50% by mass, Si: 0.40% or less, Mn: 0.10%–0.60%, P: 0.030% or less, S: 0.030% or less, Cr: 0.90%–1.80%, Mo: 0.30%–1.00%, Al: 0.005%–0.100%. The composition is as follows: N: 0.003%–0.030%, Cu: 0–0.40%, Ni: 0–0.40%, V: 0–0.50%, Ti: 0–0.100%, Nb: 0–0.100%, B: 0–0.0100%, W: 0–0.500%, Ca: 0–0.010%, Mg: 0–0.100%, rare earth elements: 0–0.100%, Bi: 0–0.300%, Te: 0–0.300%, Zr: 0–0.300%, and the remainder consists of Fe and impurities.

[0040] The inventors, considering factors beyond chemical composition, also investigated methods to improve the hydrogen embrittlement resistance of pickled steel. As a result, they obtained the following insights.

[0041] During pickling, a portion of the steel in the surface region, extending from the steel surface to a depth of approximately 100 μm to 200 μm, dissolves. Therefore, suppressing excessive dissolution of the steel in this surface region can suppress excessive hydrogen production. During pickling, Cr and Mo form dense oxides on the steel surface. In this specification, oxides containing Cr and / or Mo are referred to as "specific oxides." When specific oxides form on the steel surface during pickling, direct contact between the acidic solution and the steel surface is suppressed. As a result, excessive dissolution of the steel in the surface region is suppressed during pickling, and excessive hydrogen production is also suppressed. The specific oxides formed on the steel surface also suppress the intrusion of hydrogen generated on the steel surface. Therefore, by setting the Cr and Mo concentrations in the surface region of the steel to appropriate ranges, hydrogen production and hydrogen intrusion into the steel during pickling can be suppressed.

[0042] Cr and Mo are contained in and enriched in carbides and carbonitrides. The Cr concentration in the extraction residue obtained by electrolyzing the surface region of a steel with the above chemical composition using an electrolytic extraction method is defined as [Cr] (mass %). Furthermore, the Mo concentration in the extraction residue is defined as [Mo] (mass %). The main types of extraction residues (inclusions and precipitates) in the surface region are carbides and carbonitrides. Hereinafter, carbides and carbonitrides will also be referred to as "carbides, etc." Therefore, the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the surface region become indicators of the Cr and Mo concentrations in carbides, etc.

[0043] Therefore, the inventors investigated and studied the relationship between the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the surface region and the hydrogen embrittlement resistance of the pickled steel. The results showed that for steel with the content of each element in its chemical composition within the aforementioned range, if the total Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the surface region is 10.0% or higher, the hydrogen embrittlement resistance of the pickled steel is improved.

[0044] On the other hand, if the total concentrations of Cr [Cr] and Mo [Mo] in the extraction residue of the surface region are too high, the lubricant coating may not adhere sufficiently to the steel during the lubrication coating treatment performed after pickling for the purpose of removing oxide scale and before wire drawing. Therefore, the inventors further investigated a method to improve the hydrogen embrittlement resistance of pickled steel while also improving lubricant adhesion. Consequently, the inventors investigated the main reasons for the decrease in lubricant adhesion. The results showed that when a certain oxide is excessively formed on the surface of the pickled steel, the lubricant coating does not adhere sufficiently.

[0045] Based on the above insights, the inventors further investigated the total concentrations of Cr and Mo in the extraction residue of the surface region. As a result, the inventors found that for steels with elemental contents within the aforementioned ranges in their chemical composition, if the total concentrations of Cr and Mo in the extraction residue of the surface region are 10.0% or more and 30.0% or less, both excellent hydrogen embrittlement resistance and excellent lubricant adhesion of the pickled steel can be achieved.

[0046] The steel used in this embodiment is based on the technical concept described above. The steel used in this embodiment has the following structure.

[0047] [1] A type of steel, wherein the steel is expressed as a percentage by mass.

[0048] C: 0.30%~0.50%

[0049] Si: below 0.40%

[0050] Mn: 0.10%~0.60%

[0051] P: below 0.030%

[0052] S: below 0.030%

[0053] Cr: 0.90%–1.80%

[0054] Mo: 0.30%–1.00%

[0055] Al: 0.005%~0.100%

[0056] N: 0.003%–0.030%, and

[0057] The remaining portion consists of Fe and impurities.

[0058] The region from the surface of the steel to a depth of 100±20μm was removed by electrolysis using a pre-constant current electrolysis method. Then, the region from the surface of the steel to a depth of 100±20μm was further electrolyzed using a positive constant current electrolysis method. The Cr concentration in the resulting extraction residue was defined as [Cr] (mass%), and the Mo concentration in the extraction residue was defined as [Mo] (mass%). At this point, equation (1) is satisfied.

[0059] 10.0≤[Cr]+[Mo]≤30.0 (1).

[0060] [2] According to the steel described in [1], wherein,

[0061] The ratio of the number of carbides with a circular equivalent diameter of 0.8 μm or more to the number of carbides with a circular equivalent diameter of 0.5 μm or more, RN, is 5% to 20%.

[0062] [3] According to the steel described in [1] or [2], wherein,

[0063] The steel also contains one or more elements selected from the group consisting of the following elements to replace a portion of the Fe:

[0064] Cu: less than 0.40%

[0065] Ni: below 0.40%

[0066] V: Below 0.50%

[0067] Ti: below 0.100%

[0068] Nb: below 0.100%

[0069] B: Below 0.0100%

[0070] W: Below 0.500%

[0071] Ca: below 0.010%

[0072] Mg: less than 0.100%

[0073] Rare earth elements: below 0.100%,

[0074] Bi: below 0.300%

[0075] Te: below 0.300%, and

[0076] Zr: below 0.300%.

[0077] The steel used in this embodiment will be described in detail below. Unless otherwise stated, "%" related to elements refers to mass%.

[0078] [Technical characteristics of the steel in this embodiment]

[0079] The steel used in this embodiment meets the following technical features 1 and 2.

[0080] (Technical Feature 1)

[0081] The chemical composition, by mass%, is as follows: C: 0.30%–0.50%, Si: less than 0.40%, Mn: 0.10%–0.60%, P: less than 0.030%, S: less than 0.030%, Cr: 0.90%–1.80%, Mo: 0.30%–1.00%, Al: 0.005%–0.100%, N: 0.003%–0.030%, Cu: 0–0.40%, Ni: 0–0. 0.40%, V: 0-0.50%, Ti: 0-0.100%, Nb: 0-0.100%, B: 0-0.0100%, W: 0-0.500%, Ca: 0-0.010%, Mg: 0-0.100%, rare earth elements: 0-0.100%, Bi: 0-0.300%, Te: 0-0.300%, Zr: 0-0.300%, and the remainder consists of Fe and impurities.

[0082] (Technical Feature 2)

[0083] The region from the surface of the steel to a depth of 100±20μm was removed by electrolysis using a pre-constant current electrolysis method. Then, the region from the surface of the steel to a depth of 100±20μm was further electrolyzed using a positive constant current electrolysis method. The Cr concentration in the resulting extraction residue was defined as [Cr] (mass%), and the Mo concentration in the extraction residue was defined as [Mo] (mass%). At this point, equation (1) is satisfied.

[0084] 10.0≤[Cr]+[Mo]≤30.0 (1).

[0085] The following describes technical feature 1 and technical feature 2.

[0086] [(Technical Feature 1) Regarding Chemical Composition]

[0087] The steel in this embodiment has the following chemical composition.

[0088] C: 0.30%~0.50%

[0089] Carbon (C) improves the hardenability of steel, thereby increasing its strength. If the C content is less than 0.30%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0090] On the other hand, if the carbon content exceeds 0.50%, the toughness of the steel will decrease even if the contents of other elements are within the range of this embodiment. In this case, the steel's resistance to cold forging cracks decreases during the process of manufacturing cold-forged parts using steel as raw material.

[0091] Therefore, the C content is 0.30% to 0.50%.

[0092] The preferred lower limit for the C content is 0.31%, further preferred is 0.32%, and even more preferred is 0.33%.

[0093] The preferred upper limit for the C content is 0.48%, further preferably 0.46%, and even more preferably 0.44%.

[0094] Si: below 0.40%

[0095] Silicon (Si) is an impurity. Si reduces the toughness of steel. If the Si content exceeds 0.40%, the toughness of the steel will be significantly reduced, even if the contents of other elements are within the range of this embodiment, and the steel's resistance to cold forging cracks will also decrease.

[0096] Therefore, the Si content is below 0.40%.

[0097] Ideally, the Si content should be as low as possible. However, excessive reduction in Si content decreases productivity and increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit for Si content is more than 0%, more preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%.

[0098] The preferred upper limit for the Si content is 0.38%, more preferably 0.36%, and even more preferably 0.34%.

[0099] Mn: 0.10%~0.60%

[0100] Manganese (Mn) deoxidizes steel. Mn also improves the hardenability of steel, thereby increasing its strength. If the Mn content is less than 0.10%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0101] On the other hand, if the Mn content exceeds 0.60%, even if the contents of other elements are within the range of this embodiment, Mn will excessively segregate to the grain boundaries, thus reducing the grain boundary strength. As a result, the hydrogen embrittlement resistance of the steel is reduced.

[0102] Therefore, the Mn content is 0.10% to 0.60%.

[0103] The preferred lower limit for Mn content is 0.12%, more preferably 0.14%, and even more preferably 0.16%.

[0104] The preferred upper limit for Mn content is 0.58%, further preferably 0.56%, and even more preferably 0.54%.

[0105] P: below 0.030%

[0106] Phosphorus (P) is an impurity. P segregates into the grain boundaries of steel, reducing the grain boundary strength. If the P content exceeds 0.030%, even if the contents of other elements are within the range of this embodiment, the hydrogen embrittlement resistance of the pickled steel will decrease due to the reduction in grain boundary strength.

[0107] Therefore, the P content is below 0.030%.

[0108] Ideally, the phosphorus (P) content should be as low as possible. However, excessive reduction in P content decreases productivity and increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit for P content is more than 0%, more preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0109] The preferred upper limit for the P content is 0.028%, more preferably 0.026%, and even more preferably 0.024%.

[0110] S: below 0.030%

[0111] Sulfur (S) is an impurity. S segregates to the grain boundaries of steel, reducing the grain boundary strength. If the S content exceeds 0.030%, the hydrogen embrittlement resistance of the pickled steel will decrease even if the contents of other elements are within the range of this embodiment.

[0112] Therefore, the sulfur content is below 0.030%.

[0113] The sulfur content is preferably as low as possible. However, excessive reduction in sulfur content decreases productivity and increases manufacturing costs. Therefore, considering typical industrial production, the preferred lower limit for sulfur content is more than 0%, more preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%.

[0114] The preferred upper limit for the sulfur content is 0.028%, further preferably 0.026%, and even more preferably 0.024%.

[0115] Cr: 0.90%–1.80%

[0116] Chromium (Cr) dissolves in carbides and forms specific oxides containing Cr and Mo on the steel surface during pickling. The formation of these specific oxides suppresses hydrogen generation caused by excessive pickling. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. Cr also improves the hardenability of the steel, thereby increasing its strength. If the Cr content is less than 0.90%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0117] On the other hand, if the Cr content exceeds 1.80%, the toughness of the steel will decrease and its resistance to cold forging cracks will decrease, even if the contents of other elements are within the range of this embodiment.

[0118] Therefore, the Cr content is 0.90% to 1.80%.

[0119] The preferred lower limit for Cr content is 0.91%, further preferred is 0.92%, and even more preferred is 0.93%.

[0120] The preferred upper limit for the Cr content is 1.75%, further preferably 1.70%, and even more preferably 1.65%.

[0121] Mo: 0.30%–1.00%

[0122] Molybdenum (Mo) dissolves in carbides and forms a specific oxide containing Cr and Mo on the steel surface during pickling. The formation of this specific oxide suppresses hydrogen generation caused by excessive pickling. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. Mo also improves the hardenability of the steel, thereby increasing its strength. If the Mo content is less than 0.30%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0123] On the other hand, if the Mo content exceeds 1.00%, the toughness of the steel will decrease and its resistance to cold forging cracks will decrease, even if the contents of other elements are within the range of this embodiment.

[0124] Therefore, the Mo content is 0.30% to 1.00%.

[0125] The preferred lower limit for the Mo content is 0.31%, more preferably 0.32%, and even more preferably 0.33%.

[0126] The preferred upper limit for the Mo content is 0.95%, more preferably 0.90%, and even more preferably 0.85%.

[0127] Al: 0.005%~0.100%

[0128] Aluminum (Al) deoxidizes steel. Al also combines with nitrogen to form Al nitrides. Al nitrides inhibit grain coarsening through a pinning effect. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. If the Al content is less than 0.005%, the above-mentioned effects cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0129] On the other hand, if the Al content exceeds 0.100%, coarse Al nitrides will form even if the contents of other elements are within the range of this embodiment. These coarse Al nitrides become the starting point for damage. Therefore, the cold forging crack resistance of the steel decreases.

[0130] Therefore, the Al content is 0.005% to 0.100%.

[0131] The preferred lower limit for Al content is 0.006%, more preferably 0.007%, and even more preferably 0.008%.

[0132] The preferred upper limit for the Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0133] In the chemical composition of the steel in this embodiment, Al content refers to the total Al content.

[0134] N: 0.003%~0.030%

[0135] Nitrogen (N) combines with Al to form nitrides. Al nitrides suppress grain coarsening through a pinning effect. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. If the N content is less than 0.003%, the above-mentioned effect cannot be fully obtained even if the contents of other elements are within the range of this embodiment.

[0136] On the other hand, if the nitrogen content exceeds 0.030%, coarse nitrides will form even if the contents of other elements are within the range of this embodiment. These coarse nitrides become the starting point for damage. Therefore, the cold forging crack resistance of the steel decreases.

[0137] Therefore, the nitrogen content is 0.003% to 0.030%.

[0138] The preferred lower limit for the nitrogen content is 0.004%, more preferably 0.005%, and even more preferably 0.006%.

[0139] The preferred upper limit for the nitrogen content is 0.029%, more preferably 0.028%, and even more preferably 0.027%.

[0140] The remaining portion of the chemical composition of the steel in this embodiment consists of Fe and impurities. Impurities refer to components that are introduced during the industrial manufacturing of steel from raw materials such as ore, waste, or the manufacturing environment, and are permissible within a range that does not adversely affect the steel of this embodiment.

[0141] Impurities can be any element other than those mentioned above (P, S). An impurity can be one or more elements. Other impurities besides those mentioned above include, for example, Sb, Co, Sn, and Zn. These elements, as impurities, can exist in the following amounts, for example: Sb: less than 0.01%, Co: less than 0.01%, Sn: less than 0.01%, Zn: less than 0.01%.

[0142] [For optional elements]

[0143] Alternatively, the chemical composition of the steel in this embodiment may also contain one or more elements selected from the following groups 1 to 5 to replace a portion of the Fe.

[0144] [Group 1]

[0145] Choose one or more elements from the group consisting of the following elements:

[0146] Cu: less than 0.40%, and

[0147] Ni: below 0.40%

[0148] [Group 2]

[0149] Choose one or more elements from the group consisting of the following elements:

[0150] V: Below 0.50%

[0151] Ti: below 0.100%, and

[0152] Nb: below 0.100%

[0153] [Group 3]

[0154] B: Below 0.0100%

[0155] [Group 4]

[0156] W: below 0.500%

[0157] [Group 5]

[0158] Choose one or more elements from the group consisting of the following elements:

[0159] Ca: below 0.010%

[0160] Mg: less than 0.100%

[0161] Rare earth elements: below 0.100%,

[0162] Bi: below 0.300%

[0163] Te: below 0.300%, and

[0164] Zr: below 0.300%,

[0165] The following is an explanation of these arbitrary elements.

[0166] [Group 1 (Cu and Ni)]

[0167] Alternatively, the chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of the following elements to replace a portion of Fe: Cu: 0.40% or less, and Ni: 0.40% or less. These elements are arbitrary and may not be present. When present, Cu and Ni form dense oxides during pickling. Therefore, the generation of hydrogen caused by excessive pickling can be suppressed. As a result, the hydrogen embrittlement resistance of the steel in this embodiment during pickling is improved. Cu and Ni will be described below.

[0168] Cu: below 0.40%

[0169] Copper (Cu) is an arbitrary element, and a material can also be free of copper (Cu). In other words, the Cu content can be 0%.

[0170] In the presence of Cu, during pickling, Cu forms a dense oxide. This suppresses hydrogen generation caused by excessive pickling. Consequently, the hydrogen embrittlement resistance of the pickled steel is improved. This effect can be achieved to some extent with only a small amount of Cu.

[0171] However, if the Cu content exceeds 0.40%, even if the contents of other elements are within the range of this embodiment, the removal of oxide scale from the pickled steel will become insufficient. As a result, the lubricant adhesion of the steel is reduced.

[0172] Therefore, the Cu content is 0-0.40%, and in cases where it is present, the Cu content is below 0.40%.

[0173] The preferred lower limit for Cu content is more than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%.

[0174] The preferred upper limit for the Cu content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0175] Ni: below 0.40%

[0176] Nickel (Ni) can be any element, or it can be absent. That is to say, the Ni content can also be 0%.

[0177] In the presence of Ni, during pickling, Ni forms a dense oxide. This suppresses hydrogen generation caused by excessive pickling. As a result, the pickled steel exhibits improved resistance to hydrogen embrittlement. This effect can be achieved to some extent with only a small amount of Ni.

[0178] However, if the Ni content exceeds 0.40%, even if the contents of other elements are within the range of this embodiment, the removal of oxide scale from the pickled steel will become insufficient. As a result, the lubricant adhesion of the steel is reduced.

[0179] Therefore, the Ni content is 0 to 0.40%, and when it is present, the Ni content is less than 0.40%.

[0180] The preferred lower limit for Ni content is more than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%.

[0181] The preferred upper limit for Ni content is 0.35%, more preferably 0.30%, and even more preferably 0.25%.

[0182] [Group 2 (V, Ti, and Nb)]

[0183] Alternatively, the chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of the following elements to replace a portion of the Fe: V: 0.50% or less, Ti: 0.100% or less, and Nb: 0.100% or less. These elements are arbitrary and may not be present. When present, V, Ti, and Nb combine with C and N to form carbonitrides. These carbonitrides suppress grain coarsening through a pinning effect. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. V, Ti, and Nb will be explained below.

[0184] V: Below 0.50%

[0185] Vanadium (V) can be any element, or it can be absent. That is to say, the V content can also be 0%.

[0186] In the presence of v, v combines with c and n to form carbonitrides, inhibiting grain coarsening. As a result, the hydrogen embrittlement resistance of pickled steel is improved. This effect can be achieved to some extent with only a small amount of v.

[0187] However, if the V content exceeds 0.50%, coarse carbonitrides will form even if the contents of other elements are within the range of this embodiment. These coarse carbonitrides become the starting point for damage. Therefore, the cold forging crack resistance of the steel decreases.

[0188] Therefore, the vitamin V content is 0-0.50%, and when it is present, the vitamin V content is below 0.50%.

[0189] The preferred lower limit for the V content is more than 0%, more preferably 0.01%, more preferably 0.02%, and more preferably 0.03%.

[0190] The preferred upper limit for the V content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0191] Ti: below 0.100%

[0192] Titanium (Ti) can be any element, or it can be absent. That is to say, the Ti content can also be 0%.

[0193] In the presence of Ti, that is, when the Ti content exceeds 0%, Ti combines with C and N to form carbonitrides, inhibiting grain coarsening. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. This effect can be achieved to a certain extent even with only a small amount of Ti.

[0194] However, if the Ti content exceeds 0.100%, coarse carbonitrides will form even if the contents of other elements are within the range of this embodiment. These coarse carbonitrides become the starting point for damage. Therefore, the cold forging crack resistance of the steel decreases.

[0195] Therefore, the Ti content is 0 to 0.100%, and when it is present, the Ti content is below 0.100%.

[0196] The preferred lower limit for Ti content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0197] The preferred upper limit for the Ti content is 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0198] Nb: below 0.100%

[0199] Niobium (Nb) can be any element, or it can be absent. That is to say, the Nb content can also be 0%.

[0200] In the presence of Nb, that is, when the Nb content exceeds 0%, Nb combines with C and N to form carbonitrides, inhibiting grain coarsening. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. This effect can be achieved to a certain extent even with only a small amount of Nb.

[0201] However, if the Nb content exceeds 0.100%, coarse carbonitrides will form even if the contents of other elements are within the range of this embodiment. These coarse carbonitrides become the starting point for damage. Therefore, the cold forging crack resistance of the steel decreases.

[0202] Therefore, the Nb content is 0 to 0.100%, and when it is present, the Nb content is below 0.100%.

[0203] The preferred lower limit for Nb content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0204] The preferred upper limit for Nb content is 0.080%, more preferably 0.060%, and even more preferably 0.040%.

[0205] [Group 3 (B)]

[0206] Alternatively, the chemical composition of the steel in this embodiment may also contain the following element to replace a portion of Fe: B: 0.0100% or less. B is any element, or it may not contain B.

[0207] B: Below 0.0100%

[0208] Boron (B) can be any element, or it may not be present at all. That is to say, the B content can also be 0%.

[0209] In the presence of boron (B), B improves the hardenability of steel. Even a small amount of B can achieve this effect to some extent.

[0210] However, if the boron content exceeds 0.0100%, the hardenability of the steel becomes saturated, increasing manufacturing costs. Furthermore, even if the contents of other elements are within the range of this embodiment, coarse nitrides will form. These coarse nitrides become the starting point for damage. Therefore, the steel's resistance to cold forging cracks decreases.

[0211] Therefore, the B content is 0 to 0.0100%, and when it is present, the B content is below 0.0100%.

[0212] The preferred lower limit for the B content is more than 0%, further preferably 0.0001%, further preferably 0.0002%, and further preferably 0.0003%.

[0213] The preferred upper limit for the B content is 0.0080%, more preferably 0.0060%, and even more preferably 0.0040%.

[0214] [Group 4 (W)]

[0215] Alternatively, the chemical composition of the steel in this embodiment may also contain the following elements to replace a portion of the Fe: W: 0.500% or less. W is any element, or it may not contain W at all.

[0216] W: below 0.500%

[0217] Tungsten (W) can be any element, or it can be absent. That is to say, the W content can also be 0%.

[0218] When W is present, it increases the hardenability of steel, thereby increasing its strength. Even a small amount of W can achieve this effect to some extent.

[0219] However, if the W content exceeds 0.500%, the toughness of the steel decreases, and its resistance to cold forging cracks also decreases.

[0220] Therefore, the W content is 0 to 0.500%, and when it is present, the W content is less than 0.500%.

[0221] The preferred lower limit for W content is more than 0%, more preferably 0.005%, and even more preferably 0.010%.

[0222] The preferred upper limit for the W content is 0.480%, further preferably 0.460%, and even more preferably 0.440%.

[0223] [Group 5 (Ca, Mg, rare earth elements, Bi, Te, and Zr)]

[0224] Alternatively, the chemical composition of the steel in this embodiment may also contain one or more elements selected from the group consisting of the following elements to replace a portion of Fe: Ca: 0.010% or less, Mg: 0.100% or less, rare earth elements (REM): 0.100% or less, Bi: 0.300% or less, Te: 0.300% or less, and Zr: 0.300% or less. These elements are arbitrary and may not be present. When present, Ca, Mg, REM, Bi, Te, and Zr all improve the machinability of the steel. Ca, Mg, REM, Bi, Te, and Zr will be described below.

[0225] Ca: below 0.010%

[0226] Calcium (Ca) is an arbitrary element, and a plant can also be free of calcium (Ca). In other words, the Ca content can be 0%.

[0227] In the presence of calcium (Ca), Ca improves the machinability of steel. Even a small amount of Ca can achieve this effect to some extent.

[0228] However, if the Ca content exceeds 0.010%, the hot ductility of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0229] Therefore, the Ca content is 0 to 0.010%, and when it is present, the Ca content is below 0.010%.

[0230] The preferred lower limit for Ca content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0231] The preferred upper limit for Ca content is 0.008%, more preferably 0.006%, and even more preferably 0.004%.

[0232] Mg: less than 0.100%

[0233] Magnesium (Mg) can be any element, or it may not be present at all. That is to say, the Mg content can also be 0%.

[0234] In the presence of Mg, Mg improves the machinability of steel. Even a small amount of Mg can achieve this effect to some extent.

[0235] However, if the Mg content exceeds 0.100%, the thermal ductility of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0236] Therefore, the Mg content is 0 to 0.100%, and when it is present, the Mg content is below 0.100%.

[0237] The preferred lower limit for Mg content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0238] The preferred upper limit for Mg content is 0.090%, more preferably 0.085%, and even more preferably 0.080%.

[0239] Rare earth elements: below 0.100%

[0240] Rare earth elements (REM) can be any element, or the element may not be present at all. In other words, the REM content can be 0%.

[0241] When REM is present, it improves the machinability of steel. Even a small amount of REM can achieve this effect to some extent.

[0242] However, if the REM content exceeds 0.100%, the thermal ductility of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0243] Therefore, the REM content is 0 to 0.100%, and when it is present, the REM content is below 0.100%.

[0244] The preferred lower limit for REM content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0245] The preferred upper limit for REM content is 0.090%, more preferably 0.085%, and even more preferably 0.080%.

[0246] Furthermore, REM in this specification refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, which are lanthanide elements. Additionally, REM content in this specification refers to the total content of these elements.

[0247] Bi: below 0.300%

[0248] Bismuth (Bi) can be any element, or it can be absent. That is to say, the Bi content can also be 0%.

[0249] In the presence of Bi, Bi improves the machinability of steel. Even a small amount of Bi can achieve this effect to some extent.

[0250] However, if the Bi content exceeds 0.300%, the hot ductility of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0251] Therefore, the Bi content is 0 to 0.300%, and when it is present, the Bi content is below 0.300%.

[0252] The preferred lower limit for Bi content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0253] The preferred upper limit for the Bi content is 0.280%, further preferably 0.260%, and even more preferably 0.240%.

[0254] Te: below 0.300%

[0255] Tellurium (Te) can be any element, or the element may not be present. In other words, the Te content can be 0%.

[0256] In the presence of Te, Te improves the machinability of steel. Even a small amount of Te can achieve this effect to some extent.

[0257] However, if the Te content exceeds 0.300%, the hot ductility of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0258] Therefore, the Te content is 0 to 0.300%, and when it is present, the Te content is below 0.300%.

[0259] The preferred lower limit for Te content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0260] The preferred upper limit for the Te content is 0.280%, more preferably 0.260%, and even more preferably 0.240%.

[0261] Zr: below 0.300%

[0262] Zirconium (Zr) can be any element, or it can be absent. That is, the Zr content can also be 0%.

[0263] In the presence of Zr, Zr improves the machinability of steel. Even a small amount of Zr can achieve this effect to some extent.

[0264] However, if the Zr content exceeds 0.300%, the hot ductility of the steel will decrease even if the contents of other elements are within the range of this embodiment.

[0265] Therefore, the Zr content is 0-0.300%, and when it is present, the Zr content is below 0.300%.

[0266] The preferred lower limit for Zr content is more than 0%, more preferably 0.001%, more preferably 0.002%, and more preferably 0.003%.

[0267] The preferred upper limit for Zr content is 0.280%, further preferred is 0.260%, and even more preferred is 0.240%.

[0268] Methods for determining the chemical composition of steel

[0269] The chemical composition of the steel of this embodiment can be determined using a well-known compositional analysis method (JIS G 0321:2017). Specifically, chips are collected from the R / 2 portion of the steel using a drill bit. The R / 2 portion refers to the central portion of the steel's radius R in a cross-section perpendicular to the steel's axial direction (rolling direction). The collected chips are dissolved in acid to obtain a solution. Elemental analysis of the chemical composition is performed on the solution using ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry). The C and S contents are determined using a well-known high-frequency combustion method (combustion-infrared absorption method). The N content is determined using a well-known inert gas melting-thermal conductivity method.

[0270] (Technical Feature 2) Regarding the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue

[0271] In the steel of this embodiment, the region from the surface of the steel to a depth of 100±20μm is removed by electrolysis using a pre-constant current electrolysis, and then the region from the surface of the steel to a depth of 100±20μm is further electrolyzed by positive constant current electrolysis. The Cr concentration in the extracted residue is defined as [Cr] (mass%), and the Mo concentration in the extracted residue is defined as [Mo] (mass%). At this time, Equation (1) is also satisfied.

[0272] 10.0≤[Cr]+[Mo]≤30.0 (1)

[0273] The term "region from the surface to a depth of 100±20μm" refers to the area between the surface and a depth D μm from the surface. The term "depth of 100±20μm from the surface" refers to a depth D within the range of 80 to 120μm from the surface.

[0274] Figure 1 It is a diagram showing the area removed by electrolysis using a pre-constant current. Figure 2 This is a diagram showing the region where electrolysis is performed using constant current electrolysis after pre-constant current electrolysis. (Refer to...) Figure 1 and Figure 2 First, the outermost region RE0, at a depth D0 (D0 = 80 μm ~ 120 μm) from the surface SF0 of the steel at a distance of 10, is removed by electrolysis using a pre-constant current. Then, referring to... Figure 2Extraction residue is obtained by electrolyzing the substantial surface region RE1, which is at a depth D1 (D1 = 80 μm ~ 120 μm) of the surface layer SF1 of the steel 10 after the outermost layer RE0 has been removed, using constant current electrolysis. That is to say, the Cr concentration [Cr] and Mo concentration [Mo] in the above-mentioned extraction residue are the Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue obtained in the substantial surface region RE1.

[0275] The outermost region RE0 of steel 10, which is removed by electrolysis using a pre-constant current, contains oxide scale formed on the steel surface and impurities adhering to the steel surface. Therefore, the outermost region RE0 is not used to determine the Cr and Mo concentrations in the extraction residue; instead, the Cr and Mo concentrations in the extraction residue of the essentially surface region RE1, where the influence of oxide scale and impurities is minimal, are measured. Furthermore, if there is no influence from oxide scale and impurities, it can be assumed that the Cr and Mo concentrations in the extraction residue of the outermost region RE0 are approximately the same as those in the extraction residue of the essentially surface region RE1. The method for determining the Cr and Mo concentrations in the extraction residue will be described below.

[0276] [Methods for determining Cr and Mo concentrations in extraction residues]

[0277] The Cr concentration [Cr] and Mo concentration [Mo] in the extraction residue of the real surface region RE1 were determined using the following method.

[0278] The steel is cut perpendicular to its axial direction (rolling direction) to collect a sample. The section perpendicular to the axial direction of the sample steel corresponds to the cross-section of the steel. An insulating resin is then applied to the cut surface of the sample steel.

[0279] The sample with the cut surface coated was subjected to constant current electrolysis using a 10% AA-based solution (a solution containing 10% acetylacetone, 1% tetramethylammonium chloride, and 89% methanol by volume fraction).

[0280] First, to remove the outermost region RE0 of the steel sample, pre-constant current electrolysis was performed. For pre-constant current electrolysis, the region RE0 from the surface SF0 to a depth of D0 = 100 ± 20 μm was electrolyzed at room temperature (15℃~30℃) with a current of 1000 mA to remove RE0 from the steel sample. The ±20 μm depth is the permissible error range. After pre-constant current electrolysis, the steel sample was immersed in an alcohol solution. Then, ultrasonic cleaning was performed to remove adhering substances from the surface of the steel sample. The mass of the steel sample after adhering substances were removed, i.e., the mass of the steel sample before pre-constant current electrolysis, was measured.

[0281] Next, a positive constant current electrolysis was performed on the substantial surface region RE1 of the steel sample. Specifically, a new 10% AA-based solution was prepared. Then, using the new 10% AA-based solution, the current density was maintained at 30 mA / cm² at room temperature. 2 Electrolysis was performed on region RE1, extending from the surface SF1 of the steel sample to a depth of D1 = 100 ± 20 μm. The ±20 μm depth represents the tolerance range. The specific gravity of the steel sample was set to 7.8 g / cm³. 3 The depth of the electrolyzed region RE1 was determined based on the mass difference (reduction) (g) of the steel sample before and after electrolysis with a constant positive current and the surface area of ​​the steel sample (excluding the cross-section). After electrolysis with a constant positive current, the steel sample was immersed in an alcohol solution and then ultrasonically cleaned to remove any adhering substances from the surface of the steel sample.

[0282] The residue was extracted by suction filtration of the 10% AA-based solution used in the positive constant current electrolysis and the alcohol solution used in the subsequent ultrasonic cleaning using a filter with a mesh size of 0.2 μm. In other words, the extracted residue was obtained from the substantial surface region RE1 that had been electrolyzed using positive constant current electrolysis.

[0283] The extraction residue was subjected to elemental chemical analysis using ICP-AES. Specifically, the extraction residue was dissolved in acid to obtain a solution. The solution was then subjected to elemental chemical analysis using ICP-AES to obtain the mass of Cr and Mo in the extraction residue. Specifically, the Cr concentration [Cr] (mass %) in the extraction residue was obtained by dividing the Cr mass by the total mass of the extraction residue. Similarly, the Mo concentration [Mo] (mass %) in the extraction residue was obtained by dividing the Mo mass by the total mass of the extraction residue.

[0284] F1 is defined as [Cr] + [Mo]. The extraction residue of the actual surface region RE1 obtained by the above method includes inclusions and precipitates. Precipitates include carbides, carbonitrides, and nitrides. However, the main types of extraction residue are carbides and carbonitrides. Therefore, although F1 represents the total Cr and Mo concentrations in the extraction residue, F1 can actually serve as an indicator of the Cr and Mo concentrations in carbides and carbonitrides. It is believed that if the Cr and Mo concentrations in carbides and carbonitrides are increased, the Cr and Mo concentrations dissolved in the steel will also be higher. Therefore, F1 is also an indicator of the Cr and Mo concentrations dissolved in the surface layer of the steel.

[0285] If F1 is less than 10.0, the total concentrations of Cr and Mo in the extraction residue on the steel surface are insufficient. In this case, the concentrations of dissolved Cr and Mo on the steel surface are insufficient. Therefore, during pickling, specific oxides containing Cr and Mo are not sufficiently formed on the steel surface. Consequently, even if the content of each element in the steel's chemical composition is within the aforementioned range, excessive hydrogen is generated on the steel surface due to pickling, and this generated hydrogen easily penetrates into the steel's interior. As a result, the hydrogen embrittlement resistance of the pickled steel is reduced.

[0286] On the other hand, if F1 exceeds 30.0, the total concentration of Cr and Mo in the extraction residue on the surface of the steel is excessive. In this case, the concentration of dissolved Cr and Mo in the surface of the steel is too high. Therefore, during pickling of the steel, excessive specific oxides are formed on the steel surface. In this case, for lubricant coating treatment after pickling and before wire drawing, the lubricant coating is difficult to adhere to the steel surface. Specifically, the lubricant coating reacts with Fe on the steel surface to improve adhesion to the steel surface. However, when specific oxides are excessively generated on the steel surface, the lubricant coating is difficult to react with Fe on the steel surface due to the specific oxides. Therefore, the adhesion of the lubricant to the steel surface is reduced.

[0287] If F1 is 10.0–30.0, the total concentrations of Cr and Mo in the extraction residue on the surface of the steel are appropriate. In this case, the concentrations of dissolved Cr and Mo in the surface of the steel are also appropriate. Therefore, during pickling, an appropriate amount of specific oxides are formed on the steel surface. As a result, the hydrogen embrittlement resistance of the pickled steel is improved. Moreover, during pickling, excessive specific oxides are not formed on the steel surface. Therefore, for the lubricant coating treatment before wire drawing, the lubricant coating readily reacts with Fe on the steel surface. As a result, the adhesion of the lubricant coating to the steel surface is improved, and the lubricant adhesion to the steel is enhanced.

[0288] The preferred lower limit for F1 is 11.0, the further preferred value is 12.0, and the even more preferred value is 13.0.

[0289] The preferred upper limit for F1 is 29.0, the further preferred value is 28.0, and the even more preferred value is 27.0.

[0290] [Preferred form of steel in this embodiment]

[0291] Preferably, the steel in this embodiment also satisfies technical feature 1 and technical feature 2, and furthermore, satisfies technical feature 3.

[0292] (Technical Feature 3)

[0293] The ratio of the number of carbides with a circular equivalent diameter of 0.8 μm or more to the number of carbides with a circular equivalent diameter of 0.5 μm or more is 5% to 20%.

[0294] The following is an explanation of technical feature 3.

[0295] [(Technical Feature 3) For the preferred ratio of coarse carbides RN]

[0296] Carbides in steel with a circumference equivalent diameter of 0.8 μm or greater are defined as "coarse carbides". The ratio of the number of coarse carbides to the number of carbides with a circumference equivalent diameter of 0.5 μm or greater is defined as the coarse carbide percentage RN (%). The coarse carbide percentage RN can be defined using the following formula.

[0297] RN = Number of coarse carbides / Number of carbides with a circular equivalent diameter greater than 0.5 μm × 100

[0298] Furthermore, in steels that meet technical characteristics 1 and 2, carbides with a round equivalent diameter of 0.5 μm or more are essentially cementite (Fe3C), and other carbides (including carbonitrides) can be ignored.

[0299] If the steel meets technical characteristics 1 and 2, the proportion of coarse carbides RN is not particularly limited. The steel after pickling has improved resistance to hydrogen embrittlement and also improved lubricant adhesion.

[0300] Preferably, the proportion of coarse carbides RN in the steel satisfying technical features 1 and 2 is 5% to 20%. If the proportion of coarse carbides RN is 5% or more, the hydrogen embrittlement resistance of the steel after pickling is further improved. Furthermore, if the proportion of coarse carbides RN is 20% or less, the lubricant adhesion of the steel is further improved. Therefore, the preferred proportion of coarse carbides RN is 5% to 20%.

[0301] The lower limit for the proportion of coarse carbides RN is 6%, the higher limit is 7%, and the higher limit is 8%.

[0302] The upper limit for the proportion of coarse carbides RN is 19%, the further preferred is 18%, and the further preferred is 17%.

[0303] [Method for determining the proportion of coarse carbides (RN)]

[0304] The following method can be used to determine the proportion of coarse carbides RN in steel.

[0305] Six steel samples were collected by cutting the steel perpendicularly to its axial direction (rolling direction) at six different locations along its length. The section perpendicular to the axial direction of the sample steel corresponds to the cross-section of the steel. The axially perpendicular section on the surface of each sample steel was designated as the observation surface. The observation surface was etched using a picolinic acid etching agent to reveal the carbides.

[0306] The observation area was defined as the region extending from the steel surface to a depth of 100 μm to 200 μm (the actual surface region RE1). Within this observation area, six arbitrary fields of view (secondary electron images) were generated at 5000x magnification using a scanning electron microscope. The area of ​​each field of view was set to 19 μm × 25 μm.

[0307] In the photographic images of each field of view, carbides were identified using contrast. The circular equivalent diameter of the identified carbides was calculated. Carbides with a circular equivalent diameter of 0.5 μm or larger were designated as the measurement targets. The number of carbides with a circular equivalent diameter of 0.5 μm or larger and the number of carbides with a circular equivalent diameter of 0.8 μm or larger (coarse carbides) in each field of view were determined. All fields of view (6 × 6 = 36 fields of view, total area 17400 μm) were then analyzed. 2 The percentage (%) of coarse carbides relative to the total number of carbides with a circular equivalent diameter of 0.5 μm or more is defined as the coarse carbides percentage RN (%).

[0308] [For microstructures]

[0309] The microstructure of the steel in this embodiment is not particularly limited. The steel in this embodiment is used as a raw material for mechanical structural components. Furthermore, heat treatment such as quenching and tempering is performed during the manufacturing process of the mechanical structural components. That is, the microstructure of the steel used as raw material undergoes a phase transformation due to heat treatment such as quenching and tempering. Therefore, as described above, the microstructure of the steel used as raw material for mechanical structural components is not particularly limited.

[0310] The microstructure of the steel in this embodiment includes, for example, a BCC phase, which is a phase with a body-centered cubic lattice (BCC) crystal structure, and carbides disposed within the BCC phase. In this specification, the microstructure consisting of the BCC phase and carbides dispersed within the BCC phase is referred to as "BCC microstructure." The carbides contained in the BCC microstructure are, for example, cementite. Cementite can be layered cementite or spherical cementite. Cementite can also exist in a dotted pattern within the BCC phase.

[0311] Methods for determining microstructure

[0312] The microstructure can be determined using the following method: A test specimen including the R / 2 portion is collected in a section perpendicular to the axial direction (rolling direction) of the steel. The surface of the test specimen corresponding to the section perpendicular to the axial direction of the steel is designated as the observation plane.

[0313] After mirror polishing of the observation surface, it was etched using 2% nitric acid alcohol (nitric acid ethanol etching solution). The R / 2 portion of the etched observation surface was observed using a 400x optical microscope. The field of view was set to 500μm × 500μm.

[0314] For BCC structures observed in the field of view, the BCC phase and carbides can be identified based on contrast and morphology.

[0315] [Regarding the form and preferred application of the steel in this embodiment]

[0316] The steel used in this embodiment can be either bar steel or wire. The diameter of the steel is not particularly limited. For example, the diameter of the steel can be 5mm to 50mm.

[0317] The steel of this embodiment exhibits excellent resistance to hydrogen embrittlement and lubricant adhesion after pickling, even after descaling through pickling treatment. Therefore, it is suitable for use as a steel for cold working applications such as wire drawing and cold forging. However, the steel of this embodiment can also be used for applications other than cold working.

[0318] As described above, the steel of this embodiment satisfies technical features 1 and 2. Therefore, the steel exhibits excellent resistance to hydrogen embrittlement during pickling and also excellent lubricant adhesion.

[0319] [Steel Manufacturing Methods]

[0320] An example of a method for manufacturing the steel according to this embodiment will be described. The method for manufacturing the steel described below is an example for manufacturing the steel according to this embodiment. Therefore, the steel having the above-described structure can also be manufactured using other manufacturing methods besides those described below. However, the manufacturing method described below is a preferred example of the method for manufacturing the steel according to this embodiment.

[0321] An example of the steel manufacturing method of this embodiment includes the following steps.

[0322] (Process 1) Raw material preparation process

[0323] (Process 2) Hot working process

[0324] (Step 3) Oxide Scale Removal Process

[0325] (Process 4) Spheroidizing Annealing Process

[0326] The following is a description of each process.

[0327] [(Process 1) Raw Material Preparation Process]

[0328] In the raw material preparation process, raw materials with chemical compositions containing elemental contents within the range of this embodiment are prepared. For example, the raw materials are manufactured using the following method: Molten steel with a chemical composition satisfying technical feature 1 is manufactured. The raw materials (slabs or ingots) are manufactured using the molten steel and a casting method. For example, molten steel is used and a slab (bloom) is manufactured using a well-known continuous casting method. Alternatively, molten steel is used and an ingot is manufactured using a well-known ingot casting method.

[0329] [(Process 2) Hot working process]

[0330] Intermediate steel is manufactured by hot processing the prepared raw materials. When hot rolling is performed as the hot processing step, for example, the following methods exist. Hot processing steps based on hot rolling include a rough rolling step that roughly rolls the raw materials to form a steel billet and a finish rolling step that finishes the steel billet to form intermediate steel.

[0331] [Rough rolling process]

[0332] The roughing process is performed, for example, as follows: After heating the raw material (ingot or billet), it is rough rolled using a primary rolling mill. If necessary, after primary rolling, it is further rolled using a continuous rolling mill to produce a steel billet. In a continuous rolling mill, horizontal and vertical rolling mills are arranged alternately in a row. The raw material is rolled using the passes formed by the rolls of each mill in the continuous rolling mill to become a steel billet.

[0333] [Finishing rolling process]

[0334] The finishing rolling process is performed, for example, as follows: A steel billet is loaded into a heating furnace and heated. The heated steel billet is then finished (hot-rolled) using a finishing mill train to produce intermediate steel. The finishing mill train comprises multiple mills arranged in a row. Each mill includes multiple rolls arranged around a rolling line. The steel billet is rolled using the passes formed by the rolls in each mill to produce intermediate steel.

[0335] [(Process 3) Oxide Scale Removal Process]

[0336] The descaling process removes the oxide scale that forms on the surface of the intermediate steel manufactured in the hot working process. The descaling process includes pickling and washing. Each process is described below.

[0337] [Pickling process]

[0338] In the pickling process, intermediate steel is immersed in an acidic solution to remove the oxide scale from the surface of the intermediate steel. For example, the pickling process is carried out under the following conditions 1 to 3.

[0339] Condition 1: Temperature T1 (°C) of the acidic solution: 30°C~60°C

[0340] Condition 2: Hydrochloric acid concentration C1 (mass%) of the acidic solution: 5.0% to 20.0%

[0341] Condition 3: Immersion time t1 (minutes) in acidic solution: 2.0 minutes to 10.0 minutes

[0342] The following explains conditions 1 through 3.

[0343] [For conditions 1 to 3: temperature T1 of the acidic solution, hydrochloric acid concentration C1, and immersion time t1]

[0344] When the temperature T1 of the acidic solution is too high, or when the hydrochloric acid concentration C1 of the acidic solution is too high, or when the immersion time t1 in the acidic solution is too long, the surface of the intermediate steel after the pickling process becomes excessively rough due to acid corrosion, resulting in increased unevenness. In this case, the surface area of ​​the intermediate steel increases. Therefore, during the heating in the subsequent spheroidizing annealing process, the oxide scale formed on the surface of the intermediate steel becomes thicker. If the oxide scale becomes thicker, the amount of Cr and Mo absorbed by the oxide scale increases as carbides migrate (diffuse) from the intermediate steel to the steel surface. Consequently, the Cr concentration [Cr] and Mo concentration [Mo] in the extracted residue become too low in the steel.

[0345] On the other hand, if the temperature T1 of the acidic solution is too low, or if the hydrochloric acid concentration C1 of the acidic solution is too low, or if the immersion time t1 in the acidic solution is too short, the oxide scale is not sufficiently removed from the surface of the intermediate steel after the pickling process. Therefore, insufficient oxide scale forms on the surface of the intermediate steel during the subsequent spheroidizing annealing process. In this case, the amount of Cr and Mo absorbed by the oxide scale from the carbides in the intermediate steel to the steel surface becomes insufficient. Therefore, the Cr concentration [Cr] and Mo concentration [Mo] in the extracted residue become too high in the steel.

[0346] If the acid solution temperature T1 is 30℃~60℃, the hydrochloric acid concentration C1 of the acid solution is 5.0% to 20.0% by mass, and the immersion time t1 is 2.0 minutes to 10.0 minutes, then, provided that the conditions of other manufacturing processes are met, the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue are within an appropriate range.

[0347] The preferred lower limit for the acidic solution temperature T1 is 33°C, and the preferred upper limit is 57°C. The preferred lower limit for the hydrochloric acid concentration C1 of the acidic solution is 5.3% by mass, and the preferred upper limit is 19.7% by mass. The preferred lower limit for the immersion time t1 is 2.3 minutes, and the preferred upper limit is 9.7 minutes.

[0348] [Washing Process]

[0349] In the water washing process, the intermediate steel material after the pickling process is immersed in a water tank to remove the acidic solution adhering to the surface of the intermediate steel material. For example, the water washing process is carried out under the following condition 4.

[0350] Condition 4: Soaking time in the water tank: 1.0 minute to 5.0 minutes

[0351] [Regarding condition 4: immersion time tw]

[0352] If the immersion time in the water bath is too short, an excessive amount of acidic solution remains on the surface of the intermediate steel after the pickling process. In this case, the surface of the intermediate steel is prone to oxidation during the subsequent spheroidizing annealing process. Therefore, during spheroidizing annealing, Cr and Mo migrate excessively from the carbides in the intermediate steel to the steel surface and are oxidized. As a result, the Cr concentration [Cr] and Mo concentration [Mo] in the steel's extraction residue become too low.

[0353] On the other hand, if the immersion time is too long, insufficient acidic solution remains on the surface of the intermediate steel after the pickling process. In this case, the surface of the intermediate steel is difficult to oxidize during the subsequent spheroidizing annealing process. Therefore, during spheroidizing annealing, Cr and Mo are difficult to migrate from the carbides in the intermediate steel to the steel surface. As a result, the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue become excessively high.

[0354] If the immersion time tw in the water tank is 1.0 minute to 5.0 minutes, then, provided that the conditions of other manufacturing processes are met, the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue are within an appropriate range.

[0355] The preferred lower limit for the soaking time tw in the water tank is 1.3 minutes, and the preferred upper limit is 4.7 minutes. Furthermore, the temperature of the water in the tank is, for example, 10°C to 50°C. Preferably, the water temperature is room temperature (5°C to 35°C).

[0356] [(Process 4) Spheroidizing Annealing Process]

[0357] In the spheroidizing annealing process, the intermediate steel after the oxide scale removal process is subjected to spheroidizing annealing to manufacture the steel of this embodiment. In spheroidizing annealing, the carbides represented by cementite are spheroidized, thereby improving the cold workability of the steel. For example, the spheroidizing annealing process is performed under the following conditions 5 to 7.

[0358] Condition 5: Gas concentration ratio RG = Reducing gas concentration in the atmosphere / Oxygen concentration: 100~1000

[0359] Condition 6: Annealing temperature T2: 680℃~840℃

[0360] Condition 7: Annealing time t2: 0.1 hours to 3.0 hours

[0361] Conditions 5 through 7 will be explained below.

[0362] [For condition 5: gas concentration ratio RG]

[0363] To suppress surface oxidation of the intermediate steel during spheroidizing annealing, a reducing gas is introduced into the atmosphere. The reducing gas is, for example, one or more selected from the group consisting of CO, H2, and hydrocarbon gases. If the concentration of the reducing gas in the atmosphere is too low compared to the oxygen concentration, the surface of the intermediate steel is over-oxidized. In this case, Cr and Mo migrate excessively from the carbides in the intermediate steel to the steel surface. As a result, the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue become low.

[0364] On the other hand, if the concentration of reducing gases in the atmosphere is too high compared to the oxygen concentration, the oxidation of the intermediate steel surface will be insufficient. In this case, the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue will become high.

[0365] The ratio of the concentration of reducing gas in the atmosphere to the concentration of oxygen in the atmosphere is defined as the gas concentration ratio RG. That is, RG is expressed by the following formula.

[0366] Gas concentration ratio RG = concentration of reducing gases in the atmosphere / oxygen concentration

[0367] If the gas concentration ratio RG is 100 to 1000, then, provided that the conditions of other manufacturing processes are met, the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue are within an appropriate range.

[0368] [For conditions 6 and 7: annealing temperature T2 and annealing time t2]

[0369] The annealing temperature T2 in the spheroidizing annealing process is, for example, 680℃~840℃, and the annealing time t2 is, for example, 0.1 hours~3.0 hours. If the annealing temperature T2 and the annealing time t2 are within the above ranges, then the Cr concentration [Cr] and Mo concentration [Mo] in the steel extraction residue are within an appropriate range.

[0370] The preferred annealing temperature T2 and the preferred annealing time t2 are as follows.

[0371] Annealing temperature T2: 700℃~800℃

[0372] Annealing time t2: 0.5 hours to 2.0 hours

[0373] If the annealing temperature T2 is 700℃~800℃ and the annealing time t2 is 0.5 hours~2.0 hours, then the proportion of coarse carbides RN in the surface region of the steel becomes 5%~20%. Under this condition, the hydrogen embrittlement resistance of the steel during pickling is further improved, and the lubricant adhesion is further improved.

[0374] The steel of this embodiment is manufactured using the above manufacturing process.

[0375] [Manufacturing process for cold working of the steel in this embodiment]

[0376] Imagine the steel of this embodiment as a raw material for structural mechanical parts. In this case, during the manufacturing process of the structural mechanical parts, there is a process where the steel undergoes a descaling treatment, including pickling. Furthermore, there is a situation where the steel that has undergone descaling is subjected to a lubricant coating treatment, followed by wire drawing. When the steel of this embodiment undergoes the above-described manufacturing processes (including the descaling treatment by pickling and the subsequent lubricant coating treatment), the steel of this embodiment can achieve both excellent hydrogen embrittlement resistance after pickling and excellent lubricant adhesion.

[0377] Example

[0378] The effects of one form of the steel according to this embodiment will be further explained in detail using examples. The conditions in the following examples are examples adopted to confirm the feasibility and effects of the steel according to this embodiment. Therefore, the steel according to this embodiment is not limited to these examples.

[0379] Molten steel with the chemical compositions shown in Tables 1-1 and 1-2 was produced.

[0380] [Table 1-1] Table 1

[0381]

[0382] [Table 1-2]

[0383] Table 1-2

[0384]

[0385] In Tables 1-1 and 1-2, "-" indicates that the corresponding element content is 0% in the significant figures (values ​​up to the least significant digit) specified in the implementation method. In other words, it means that the corresponding element content is 0% when the last digit of the significant figures (values ​​up to the least significant digit) specified in the above implementation method is rounded off.

[0386] For example, the Cu content specified in this embodiment is defined as a value up to two decimal places. Therefore, for test number 1 in Table 1-1, the measured Cu content is 0% after rounding to three decimal places.

[0387] Furthermore, the Ni content specified in this embodiment is defined as a value up to two decimal places. Therefore, for test number 1 in Table 1-1, the measured Ni content is 0% after rounding to three decimal places.

[0388] In addition, rounding means that if the next digit after the least specified digit (the last digit) is less than 5, it is discarded; if it is 5 or more, it is carried over.

[0389] Large steel billets were manufactured by continuous casting of the molten steels listed in Tables 1-1 and 1-2. Hot working processes (roughing and finishing rolling) were then performed on the large steel billets. Specifically, in the roughing rolling process, the large steel billets were heated to 1200°C and then hot-rolled to produce billets with a cross-sectional shape of 160mm × 160mm.

[0390] In the finishing rolling process, after heating the steel billet to 1200℃, hot rolling is carried out to produce bars (intermediate steel) with a diameter of 10mm. The hot-rolled intermediate steel is then allowed to cool naturally.

[0391] A descaling process (pickling and washing) was performed on the intermediate steel. The temperature T1 (°C), hydrochloric acid concentration C1 (mass%), and immersion time t1 (minutes) of the acidic solution in the pickling process are shown in Table 2. The immersion time tw (minutes) in the water bath during the washing process is also shown in Table 2. Furthermore, the water temperature used in the washing bath was 25°C. [Table 2]

[0392]

[0393] The steel bars, after the descaling process, underwent spheroidizing annealing. The gas concentration ratio RG, annealing temperature T2 (°C), and annealing time t2 (hours) during spheroidizing annealing are shown in Table 2. Steel bars were manufactured using the above manufacturing process. The diameter of the steel bars ranged from 10 mm to 40 mm.

[0394] [Evaluation Test]

[0395] The following evaluation tests were conducted on the steel samples for each test number.

[0396] (Experiment 1) Determination of the chemical composition of steel

[0397] (Experiment 2) Determination of Cr [Cr] and Mo [Mo] concentrations in the extraction residue

[0398] (Experiment 3) Determination of the proportion of coarse carbides (RN)

[0399] (Experiment 4) Microscopic tissue observation experiment

[0400] (Experiment 5) Evaluation Test of Hydrogen Embrittlement Resistance

[0401] (Experiment 6) Lubricant Adhesion Evaluation Test

[0402] The following describes Experiments 1 through 6.

[0403] [(Experiment 1) Determination of the chemical composition of steel]

[0404] The chemical composition of the steel for each test number was determined based on the method described above in the [Method for Determination of Chemical Composition of Steel]. The chemical composition of the steel for any test number is as shown in Tables 1-1 and 1-2 for the results of the determination.

[0405] [(Experiment 2) Determination of Cr [Cr] and Mo [Mo] concentrations in the extraction residue]

[0406] Based on the method described above for determining the Cr concentration [Cr] and Mo concentration [Mo] in the extracted residue, the total Cr concentration [Cr] (mass%) and Mo concentration [Mo] (mass%) in the surface region of the steel for each test number, F1 (=[Cr]+[Mo]), was calculated. The calculated F1 is shown in Table 2.

[0407] [(Experiment 3) Determination of the proportion of coarse carbides (RN)]

[0408] Based on the method described above for determining the percentage of coarse carbides RN, the percentage of coarse carbides RN (%) for each test number of steel was determined. The determined percentage of coarse carbides RN is shown in Table 2.

[0409] [(Experiment 4) Microscopic Tissue Observation Experiment]

[0410] The microstructure of steel samples for each test number was observed based on the method described above in the [Method for Determining Microstructure]. The results showed that, in any given test number, the microstructure of the steel consisted of a BCC phase with dispersed carbides (BCC structure).

[0411] [(Experiment 5) Evaluation Test of Hydrogen Embrittlement Resistance]

[0412] For each test number of steel, a descaling process was planned, and the following pickling and washing processes were performed on the steel for each test number. In the pickling process, the steel for each test number was immersed in an acidic solution at 40°C for 5.0 minutes. The concentration of hydrochloric acid in the acidic solution was 15.0% by mass. In the washing process, the steel after the pickling process was immersed in a water tank containing water at 25°C for 1.0 minute.

[0413] The steel was cut perpendicularly to the axial direction (rolling direction) at four different points after the washing process, and four tensile test pieces with a diameter of 10 mm and a length of 500 mm were collected. The shape of the test pieces was set as specified in JIS Z 2241:2011 as test piece No. 14A. The four test pieces were divided into two groups of two pieces each (Group 1 and Group 2).

[0414] Tensile tests were performed on the two test pieces from Group 1 one hour after the water washing process. This means the test pieces from Group 1 were subjected to tensile tests under conditions where there was a possibility of embrittlement due to hydrogen intrusion into the steel during the pickling process. On the other hand, the two test pieces from Group 2 were placed at room temperature and in atmospheric conditions for 168 hours (one week) after the water washing process to remove hydrogen from the test pieces. Tensile tests were then performed on the dehydrogenated test pieces. This means the test pieces from Group 2 were subjected to tensile tests without the possibility of hydrogen embrittlement.

[0415] In each group, the tensile test was conducted according to JIS B1051:2014 at room temperature (25°C) and in atmospheric conditions to determine the tensile strength (MPa) of the two test pieces. The arithmetic mean of the two tensile strengths (MPa) was defined as the tensile strength (MPa) of each group (Group 1 or Group 2). Specifically, the arithmetic mean of the tensile strengths of the two test pieces in Group 1 was defined as tensile strength 1 (MPa), and the arithmetic mean of the tensile strengths of the two test pieces in Group 2 was defined as tensile strength 2 (MPa).

[0416] The hydrogen embrittlement resistance index HI is defined using the following formula.

[0417] Hydrogen embrittlement resistance index HI = tensile strength 1 / tensile strength 2

[0418] Based on the obtained hydrogen embrittlement resistance index HI, the hydrogen embrittlement resistance characteristics were evaluated as follows.

[0419] Evaluation S: Hydrogen embrittlement resistance index (HI) is 0.95–1.00.

[0420] Evaluation A: The hydrogen embrittlement resistance index (HI) is above 0.90 and less than 0.95.

[0421] Rating B: Hydrogen embrittlement resistance index (HI) is above 0.85 and below 0.90.

[0422] Rating C: Hydrogen embrittlement resistance index (HI) is above 0.80 and below 0.85.

[0423] Rating D: Hydrogen embrittlement resistance index (HI) is ≥0.75 and <0.80.

[0424] Evaluation E: Hydrogen embrittlement resistance index (HI) is ≥0.70 and <0.75.

[0425] Evaluation X: Hydrogen embrittlement resistance index (HI) is less than 0.70.

[0426] Under evaluation conditions S to E, the steel is judged to have excellent resistance to hydrogen embrittlement. On the other hand, under evaluation condition X, the steel is judged to have low resistance to hydrogen embrittlement. The evaluation results are shown in Table 2.

[0427] [(Experiment 6) Lubricant Adhesion Evaluation Test]

[0428] The lubricant adhesion of each test number was evaluated using the following method.

[0429] For each test number of steel, a descaling process was planned, and the following pickling and washing processes were performed on the steel. In the pickling process, the steel of each test number was immersed in an acidic solution at 40°C for 5.0 minutes. The concentration of hydrochloric acid in the acidic solution was 15.0% by mass. In the washing process, the steel after the pickling process was immersed in a water tank containing water at 25°C for 1.0 minute.

[0430] The steel was subjected to a lubricating coating treatment after the water washing process. Specifically, a chemical conversion treatment was performed on the steel to form a phosphate coating on its surface. The phosphate bath temperature was set to 70°C, and the treatment time was set to 10 minutes. Zinc phosphate was used as the phosphate. The steel was then immersed in a soap treatment solution containing soap lubricant with sodium stearate as the main component for 10 minutes, allowing the soap (metallic soap and unreacted soap) to adhere to the phosphate coating. This process imparted a lubricant (soap and phosphate coating) to the steel surface.

[0431] Five test pieces, each 10 mm in diameter and 200 mm in length, were collected by cutting the steel at five different points perpendicular to the axial direction. First, the total weight 1 of the five test pieces was calculated. Next, the five test pieces were immersed in a chromic acid aqueous solution at 70°C for 15 minutes to completely remove the lubricant. The total weight 2 of the five test pieces after immersion was calculated. The value obtained by subtracting the total weight 2 from the total weight 1 is defined as the lubricant adhesion amount (g). The lubricant adhesion amount is divided by the total surface area of ​​the five test pieces excluding the cut surfaces (i.e., π × 10 mm × 200 mm × 5 mm). 2 )) Calculate the lubricant adhesion amount LA (g / m²) per unit area. 2 Based on the amount of lubricant adhered (LA), the lubricant adhesion was evaluated as follows.

[0432] Evaluation A: Lubricant adhesion amount LA is 10 g / m 2 above

[0433] Evaluation B: Lubricant adhesion amount LA is 8 g / m 2 Above and less than 10g / m 2

[0434] Rating C: Lubricant adhesion amount LA is 6 g / m 2 Above and less than 8g / m 2

[0435] Evaluation D: Lubricant adhesion LA is 4 g / m 2 Above and less than 6g / m 2

[0436] Evaluation E: Lubricant adhesion LA is 2 g / m 2 Above and less than 4g / m 2

[0437] Evaluation X: Lubricant adhesion amount LA is less than 2g / m 2

[0438] For evaluations A through E, the lubricant adhesion is considered excellent. For evaluation X, the lubricant adhesion to the steel is considered low. The evaluation results are shown in Table 2.

[0439] [Evaluation Results]

[0440] Referring to Tables 1-1, 1-2, and 2, the chemical composition of the steels tested from 1 to 52 is appropriate, and F1 satisfies equation (1). Therefore, among the steels tested from 1 to 52, the pickling treatment resulted in excellent resistance to hydrogen embrittlement, and also excellent lubricant adhesion.

[0441] In tests 1 to 44 and 47 to 50, the proportion of coarse carbides RN remained at 5% to 20%. Therefore, compared with tests 45, 46, 51, and 52, it exhibits further superior resistance to hydrogen embrittlement or further superior lubricant adhesion.

[0442] On the other hand, the Mn content in test number 53 was too high. Therefore, the steel had low resistance to hydrogen embrittlement.

[0443] Test number 54 had an excessively high phosphorus content. Therefore, the steel exhibited low resistance to hydrogen embrittlement.

[0444] Test number 55 had an excessively high sulfur content. Therefore, the steel exhibited low resistance to hydrogen embrittlement.

[0445] Test number 56 had an excessively low Al content. Therefore, the steel exhibited low resistance to hydrogen embrittlement.

[0446] Test number 57 had too low a nitrogen content. Therefore, the steel had low resistance to hydrogen embrittlement.

[0447] In test number 58, the temperature T1 of the acidic solution in the pickling process was relatively low. Therefore, the F1 value exceeded the upper limit of equation (1). As a result, the lubricant adhesion of the steel was low.

[0448] In test number 59, the hydrochloric acid concentration C1 of the acidic solution in the pickling process was low. Therefore, the F1 value exceeded the upper limit of equation (1). As a result, the lubricant adhesion of the steel was low.

[0449] In experiment number 60, the immersion time t1 in the pickling process was shorter. Therefore, the F1 value exceeded the upper limit of equation (1). As a result, the lubricant adhesion of the steel was lower.

[0450] In test number 61, the temperature T1 of the acidic solution in the pickling process was relatively high. Therefore, the F1 value was less than the lower limit of equation (1). As a result, the steel exhibited lower resistance to hydrogen embrittlement.

[0451] In test number 62, the hydrochloric acid concentration C1 of the acidic solution in the pickling process was relatively high. Therefore, the F1 value was less than the lower limit of equation (1). As a result, the steel exhibited lower resistance to hydrogen embrittlement.

[0452] In experiment number 63, the immersion time t1 in the pickling process was relatively long. Therefore, the F1 value was less than the lower limit of equation (1). As a result, the steel exhibited lower resistance to hydrogen embrittlement.

[0453] In experiment number 64, although the chemical composition was appropriate, the immersion time tw in the water washing process was too long. Therefore, F1 exceeded the upper limit of equation (1). As a result, the lubricant adhesion of the steel was low.

[0454] In test number 65, although the chemical composition was appropriate, the immersion time tw in the water washing process was too short. Therefore, F1 was less than the lower limit of equation (1). As a result, the steel had low resistance to hydrogen embrittlement.

[0455] In test number 66, although the chemical composition was appropriate, the gas concentration in the atmosphere during the spheroidizing annealing process was too high compared to RG. Therefore, F1 exceeded the upper limit of equation (1). As a result, the lubricant adhesion of the steel was low.

[0456] In test number 67, although the chemical composition was appropriate, the gas concentration in the atmosphere during the spheroidizing annealing process was too low compared to RG. Therefore, F1 was less than the lower limit of equation (1). As a result, the steel exhibited lower resistance to hydrogen embrittlement.

[0457] The embodiments of this disclosure have been described above. However, the above embodiments are merely illustrative examples for implementing this disclosure. Therefore, this disclosure is not limited to the above embodiments, and can be implemented by appropriately modifying the above embodiments without departing from its spirit.

Claims

1. A steel material, wherein, The steel contains, by mass % C：0.30％～0.50％、 Si: below 0.40% Mn: 0.10%~0.60% P: below 0.030% S: below 0.030% Cr：0.90％～1.80％、 Mo: 0.30%–1.00% Al: 0.005%–0.100%, and N：0.003％～0.030％， The remaining portion consists of Fe and impurities. The region from the surface of the steel to a depth of 100±20μm was removed by electrolysis using a pre-constant current electrolysis method. Then, the region from the surface of the steel to a depth of 100±20μm was further electrolyzed using a positive constant current electrolysis method. The Cr concentration in the resulting extraction residue is defined as [Cr], and the Mo concentration in the extraction residue is defined as [Mo]. At this point, equation (1) is satisfied. 10.0≤[Cr]+[Mo]≤30.0(1) The units for the Cr concentration and the Mo concentration are by mass.

2. The steel according to claim 1, wherein, The ratio of the number of carbides with a circular equivalent diameter of 0.8 μm or more to the number of carbides with a circular equivalent diameter of 0.5 μm or more is 5% to 20%.

3. The steel according to claim 1 or 2, wherein, The steel also contains one or more elements selected from the group consisting of the following elements to replace a portion of the Fe: Cu: less than 0.40% Ni: below 0.40% V: Below 0.50% Ti: below 0.100% Nb: below 0.100% B: Below 0.0100% W: Below 0.500% Ca: less than 0.010% Mg: less than 0.100% Rare earth elements: below 0.100%, Bi: below 0.300% Te: below 0.300%, and Zr: below 0.300%.

4. The steel according to claim 1 or 2, wherein, The steel also contains one or more elements selected from the group consisting of the following elements to replace a portion of the Fe: Cu: less than 0.40% Ni: below 0.40% V: Below 0.50% Ti: below 0.100% Nb: below 0.100% B: Below 0.0100% Ca: below 0.010% Bi: below 0.300% Te: below 0.300%, and Zr: below 0.300%.

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