Muscle steel

Abstract

To provide a case-hardening steel having high strength and superior toughness after carburizing treatment.SOLUTION: A case-hardening steel is adopted that includes, in mass%, Cr: 1.05-1.50%, Mo: 0.03-0.15%, and Ni: 0.03-0.19%, and satisfies the following (1), with the Cr content in cementite being 1.30 mass% or more. The formula (1) is: 2.20≤1.5×Cr+4.5×Mo≤2.90 (1), where, each element symbol denotes the content of the element in mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to case hardening steel. [Background technology]

[0002] BACKGROUND ART Mechanical structural parts such as automobile parts, industrial machinery parts, and construction machinery parts are sometimes subjected to surface hardening treatment in order to improve, for example, fatigue strength.

[0003] Among various surface hardening treatments, carburizing is often used for the above-mentioned mechanical structural parts. Carburizing is a process in which a part is heated in the austenite temperature range in a carburizing atmosphere, then quenched and tempered. A hardened layer is formed on the surface of a carburized part. This hardened layer not only provides high fatigue strength, but also high toughness because the core of the part is also quenched and tempered.

[0004] In response to recent demands for high fatigue strength, alloying elements have been added to increase the strength of the core of parts. However, increasing the strength of the core of parts by adding alloying elements reduces the toughness of the parts. Therefore, there is a demand for case-hardening steels that have a good balance of strength and toughness.

[0005] As a case-hardening steel for obtaining machine structural parts to be used after carburizing, for example, Patent Document 1 proposes a case-hardening steel containing, in mass %, C: 0.2 to 0.3%, Si: 0.6% or less, Mn: 0.95 to 2.2%, P: 0.03% or less, S: 0.03% or less, Cr: 0.1 to 1.8%, Al: 0.06% or less, N: 0.02% or less, and O: 0.003% or less, and satisfying the following formulas (1) and (2): -163×[C]+43.1×[Si]-55.2×[Mn]+32.6×[Cu]-30.0×[Ni]-47.8×[Cr]+104×[Mo]+412×[V]+677 ≦540 …(1) 40.4×[C]+1.31×[Si]+18.7×[Mn]+8.37×[Cu]+5.33×[Ni]+5.57×[Cr]+11.8×[Mo]-51.1×[V]-17.8≧15 (2) However, the element symbol in [ ] indicates the content (mass %). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-1774 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent Document 1 discloses that case-hardened steel with extremely small heat treatment strain can be realized by appropriately controlling the components and transformation start temperature. However, Patent Document 1 does not specifically consider strength and toughness.

[0008] The present invention has been made in view of the above-mentioned circumstances, and has as its object to obtain a case-hardened steel that has high strength and excellent toughness after carburizing treatment. [Means for solving the problem]

[0009] The gist of the present invention is as follows.

[0010] (1) A case-hardening steel according to one aspect of the present invention has a chemical composition, in mass%, C: 0.17~0.35%, Si: 0.28 to 0.60% Mn: 1.03-1.45% P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 1.05~1.50%, N: 0.020% or less, Mo: 0.03 to 0.15%, Ni: 0.03 to 0.19%, and O: Contains 0.0030% or less the balance being Fe and impurities, A case-hardening steel that satisfies the following (1) and has a Cr concentration in cementite of 1.30 mass% or more. 2.20≦1.5×Cr+4.5×Mo≦2.90 …(1) However, the element symbols in the above formula (1) indicate the content of each element in mass %. (2) A case-hardening steel according to another embodiment of the present invention has a chemical composition, in mass%, C: 0.17~0.35%, Si: 0.28 to 0.60% Mn: 1.03-1.45% P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 1.05~1.50%, N: 0.020% or less, Mo: 0.03 to 0.15%, Ni: 0.03 to 0.19%, and O: Contains 0.0030% or less Further, it contains one or more selected from the group consisting of the following groups A, B, and C: the balance being Fe and impurities, The following condition (1) is satisfied, and the Cr concentration in cementite is 1.30 mass % or more. 2.20≦1.5×Cr+4.5×Mo≦2.90 …(1) However, the element symbols in the above formula (1) indicate the content of each element in mass %. [Group A] Ti: 0.005% or less, V: 0.010% or less, Nb: 0.005% or less, and B: 0.0015% or less One or more selected from the group consisting of [Group B] Cu: 0.50% or less, and Sn: 0.100% or less One or two selected from the group consisting of [Group C] Ca: 0.0050% or less, and Mg: 0.0050% or less One or two selected from the group consisting of (3) The case-hardening steel described in (2) above may have a chemical composition containing, in mass %, the Group A. (4) The case-hardening steel described in (2) above may have a chemical composition containing, in mass %, the B group. (5) The case-hardening steel described in (2) above may have a chemical composition containing, by mass %, the C group. [Effects of the Invention]

[0011] According to the above aspect of the present invention, it is possible to provide a case-hardened steel having high strength and excellent toughness after carburizing treatment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, each requirement of the case-hardened steel according to this embodiment will be described in detail. Note that in this embodiment, the case-hardened steel refers to a steel material that is processed into a part shape as needed, and then carburized before use (for example, used as a machine structural part).

[0013] <Chemical composition> First, the chemical composition of the case-hardened steel according to this embodiment will be described. Below, numerical ranges indicated with "to" include the lower and upper limits. Values ​​indicated as "less than" and "greater than" do not include the numerical range. All "%" in chemical compositions means "mass %."

[0014] The case-hardening steel according to this embodiment has a chemical composition, in mass%, of C: 0.17 to 0.35%, Si: 0.28 to 0.60%, Mn: 1.03 to 1.45%, P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.050%, Cr: 1.05 to 1.50%, N: 0.020% or less, Mo: 0.03 to 0.15%, Ni: 0.03 to 0.19%, O: 0.0030% or less, and the balance: Fe and impurities. Each element will be described below.

[0015] C: 0.17 to 0.35% Carbon (C) increases the strength of machine structural parts (hereinafter sometimes simply referred to as "parts") after carburizing. To obtain the desired strength in parts, the C content is set to 0.17% or more. The C content is preferably 0.19% or more. However, if the C content exceeds 0.35%, the strength of the core of the part becomes too high, resulting in a deterioration in toughness. Therefore, the C content is set to 0.35% or less. The C content is preferably 0.32% or less.

[0016] Si: 0.28 to 0.60% Silicon (Si) improves the hardenability of case-hardened steel. In order to improve the hardenability of case-hardened steel and obtain the desired strength in parts, the Si content is set to 0.28% or more. The Si content is preferably 0.30% or more. However, if the Si content exceeds 0.60%, the toughness of the part deteriorates. Therefore, the Si content is set to 0.60% or less, and preferably 0.55% or less.

[0017] Mn: 1.03 to 1.45% Manganese (Mn) increases the strength of parts. If the Mn content is less than 1.03%, the desired strength cannot be obtained in the parts. Therefore, the Mn content is set to 1.03% or more. The Mn content is preferably 1.10% or more. On the other hand, if the Mn content exceeds 1.45%, the toughness of the part deteriorates. Therefore, the Mn content is set to 1.45% or less. The Mn content is preferably 1.40% or less.

[0018] P:0.050% or less Phosphorus (P) is inevitably contained in steel. P tends to segregate in steel, causing localized reduction in ductility. In particular, if the P content exceeds 0.050%, the localized reduction in ductility becomes significant. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less. The lower limit of the P content is not particularly limited, but may be more than 0% or 0.002% or more.

[0019] S: 0.050% or less Sulfur (S) is inevitably contained in steel. S combines with Mn in steel to form MnS, which may cause cracking during hot forging. If the S content exceeds 0.050%, cracking becomes significant. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.040% or less. The lower limit of the S content is not particularly limited, but may be more than 0% or 0.005% or more.

[0020] Al: 0.005 to 0.050% Aluminum (Al) is an element effective as a deoxidizer. If the Al content is less than 0.005%, deoxidation will be insufficient. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.010% or more. On the other hand, if the Al content exceeds 0.050%, coarse oxides are formed, which deteriorates the toughness of the part. Therefore, the Al content is set to 0.050% or less. The Al content is preferably 0.040% or less.

[0021] Cr: 1.05 to 1.50% Chromium (Cr) improves the hardenability of case-hardened steel, increases the strength of the core of the part after carburizing, and also improves the toughness of the core of the part by stabilizing it through solid solution in carbides. If the Cr content is less than 1.05%, the above effects cannot be obtained. Therefore, the Cr content is set to 1.05% or more. The Cr content is preferably 1.10% or more. On the other hand, if the Cr content exceeds 1.50%, not only will the solid solution in carbides become saturated, but the toughness of the part will also deteriorate. Therefore, the Cr content is set to 1.50% or less. The Cr content is preferably 1.30% or less.

[0022] N: 0.020% or less Nitrogen (N) is inevitably contained in steel. Nitrogen may combine with Ti in the steel to form TiN, which may deteriorate the toughness of the core of the part. If the N content exceeds 0.020%, the toughness of the core of the part will deteriorate significantly. Therefore, the N content is set to 0.020% or less. The N content is preferably 0.018% or less, and more preferably 0.015% or less. The lower limit of the N content is not particularly limited, but may be more than 0% or 0.005% or more.

[0023] Mo: 0.03 to 0.15% Molybdenum (Mo) improves the hardenability of case-hardened steel, increases the strength of the core of the part after carburizing, and dissolves in carbides to stabilize them, thereby increasing the toughness of the core of the part. If the Mo content is less than 0.03%, the above effects cannot be obtained. Therefore, the Mo content is set to 0.03% or more. The Mo content is preferably 0.05% or more. On the other hand, if the Mo content exceeds 0.15%, not only will the solid solution in carbides become saturated, but the toughness of the part will also deteriorate. Therefore, the Mo content is set to 0.15% or less. The Mo content is preferably 0.13% or less.

[0024] Ni: 0.03 to 0.19% Nickel (Ni) increases the toughness of parts. If the Ni content is less than 0.03%, this effect cannot be obtained. Therefore, the Ni content is set to 0.03% or more. The Ni content is preferably 0.05% or more. On the other hand, if the Ni content exceeds 0.19%, the toughness of the part deteriorates. Therefore, the Ni content is set to 0.19% or less, and preferably 0.18% or less.

[0025] O: 0.0030% or less If the O content in steel is too high, it forms coarse oxides that become the starting point of fracture and reduce the toughness of the part. Therefore, the O content is set to 0.0030% or less. The O content is preferably 0.0020% or less, and more preferably 0.0015% or less. The lower limit of the O content is not particularly limited, but may be 0% or more, or may be more than 0% or 0.0005% or more.

[0026] 2.20≦1.5×Cr+4.5×Mo≦2.90 …(1) In the steel material for carbonitriding of this embodiment, it is important to control the contents of alloying elements in order to preferably control the Cr concentration in cementite. In particular, it is important to control the Cr content and the Mo content, which stabilizes cementite. If the value of the middle part of the above formula (1) is 2.20 or more, the Cr concentration in cementite can be controlled in a preferable manner. As a result, high toughness can be obtained in the part. Therefore, the value of the middle part of the formula (1) is set to 2.20 or more. Preferably, it is set to 2.30 or more or 2.35 or more. On the other hand, if the value of the middle term of formula (1) exceeds 2.90, not only will the above effect saturate, but the toughness of the part will deteriorate due to solid solution strengthening of Cr and Mo. Therefore, the value of the middle term of formula (1) is set to 2.90 or less. Preferably, it is set to 2.80 or less or 2.70 or less.

[0027] The balance of the chemical composition of the case-hardened steel according to this embodiment may be Fe and impurities. In this embodiment, the impurities are, for example, elements that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during industrial production of steel materials and are acceptable within a range that does not impair the properties of the case-hardened steel according to this embodiment.

[0028] The chemical composition of the case-hardened steel according to this embodiment may further contain, as an optional element, one or more elements selected from the group consisting of the following groups A, B, and C, instead of part of Fe. When the following optional elements are not contained, the content is 0%. [Group A] Ti: 0.005% or less, V: 0.010% or less, Nb: 0.005% or less, and B: 0.0015% or less One or more selected from the group consisting of [Group B] Cu: 0.50% or less, and Sn: 0.100% or less One or two selected from the group consisting of [Group C] Ca: 0.0050% or less, and Mg: 0.0050% or less One or two selected from the group consisting of

[0029] Ti: 0.005% or less Titanium (Ti) combines with N in steel to form TiN, but if the Ti content exceeds 0.005%, coarse TiN is formed, which may reduce the toughness of the core of the part. Therefore, the Ti content is set to 0.005% or less. It is preferable that Ti is not contained, and therefore the Ti content may be 0%.

[0030] V:0.010% or less V is an element that improves the strength of steel materials through solid solution strengthening. V may be added as needed to achieve this effect. However, if the V content exceeds 0.010%, a large amount of carbonitrides may precipitate, which may deteriorate the toughness of the part. Therefore, the V content is preferably 0.010% or less. The V content may be 0%.

[0031] Nb: 0.005% or less Niobium (Nb) combines with N in steel to form NbN, but if the Nb content exceeds 0.005%, coarse NbN may be formed, which may deteriorate the toughness of the core of the part. Therefore, the Nb content is set to 0.005% or less. It is preferable that Nb is not contained, and therefore the Nb content may be 0%.

[0032] B: 0.0015% or less B is an element that bonds with N in steel to form nitrides during induction hardening. If the B content exceeds 0.0015%, coarse nitrides are formed, which may deteriorate the toughness of the parts after induction hardening. Therefore, the B content is preferably 0.0015% or less. Since it is preferable that B is not contained, the B content may be 0%.

[0033] Cu: 0.50% or less Copper (Cu) increases the strength of parts. To achieve this effect, Cu may be added as needed. To ensure the above effect, the Cu content is preferably 0.02% or more. However, if the Cu content exceeds 0.50%, the toughness of the part may be deteriorated. Therefore, if Cu is contained, the Cu content is set to 0.50% or less. The Cu content is preferably 0.45% or less.

[0034] Sn: 0.100% or less Sn is an element that suppresses grain coarsening and improves the strength of steel. Sn may be added as needed to achieve this effect. However, if the Sn content exceeds 0.100%, the steel becomes embrittled and prone to fracture during hot rolling. Therefore, the Sn content is preferably 0.100% or less. The Sn content may be 0%.

[0035] Ca: 0.0050% or less Ca is an element that fixes S in steel as spherical CaS, suppresses the formation of elongated inclusions such as MnS, and improves the formability of steel. Ca may be added as needed to achieve this effect. However, even if the Ca content exceeds 0.0050%, the above effect saturates. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may be 0%.

[0036] Mg: 0.0050% or less Mg has the effect of improving the formability of steel by adjusting the shape of inclusions in steel to a preferred shape. Mg may be added as needed to achieve this effect. However, if the Mg content exceeds 0.0050%, excessive inclusions may be formed in the steel, which may actually reduce the formability of the steel. Therefore, the Mg content is preferably 0.0050% or less. The Mg content may be 0%.

[0037] The chemical composition of the case-hardened steel can be measured by a common analytical method, such as inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method. The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment, to obtain the value of the content of each element specified in this embodiment down to the least significant digit.

[0038] Cr concentration in cementite: 1.30 mass% or more In the steel material for carbonitriding according to this embodiment, the Cr concentration in cementite is set to 1.30% by mass or more. By setting the Cr concentration in cementite to 1.30% by mass or more, high toughness can be exhibited in the core of the part after carbonitriding. Although the details of the mechanism by which increasing the Cr concentration in cementite improves toughness after carbonitriding are unknown, the inventors speculate that the distribution of solute Cr in austenite during heating during quenching may have an effect. However, if the Cr concentration in cementite is too high, the above effect saturates, so the upper limit may be set to 3.00 mass %.

[0039] Case hardening steel manufacturing method Next, a method for manufacturing the case-hardened steel according to the present embodiment will be described. The manufacturing method is not particularly limited, but the following method can be mentioned, for example. First, a steel having the above-described chemical composition is melted and a slab is produced. The produced slab is then bloomed to produce a steel billet. The resulting slab is then hot-rolled to obtain a steel material for carbonitriding. At this time, in order to fully dissolve Cr in austenite, it is preferable that the heating temperature of the slab be 1050°C or higher and the heating time be 1 hour or longer. Furthermore, in order to ensure sufficient time for Cr to concentrate in cementite, it is preferable that the cementite transformation temperature be high. Therefore, it is preferable that the finish processing temperature in hot rolling be 850°C or higher, the area reduction rate in finish processing be 30% or higher, and the average cooling rate after finish processing be 0.5°C / s or higher. [Example]

[0040] Next, examples of the present invention will be described, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0041] A 150 kg ingot having the chemical composition shown in Tables 1A and 1B was melted using a vacuum melting furnace. The resulting ingot was heated to 1200°C for 1.5 hours, and then hot forged at a finish processing temperature of 900°C with a finish processing area reduction rate of 40%. After finish processing, the average cooling rate from the finish processing to 300°C was 1.0°C / s, yielding a 35 mm diameter round steel bar.

[0042] Microstructural specimens were prepared from the resulting round steel bars, with the cross section parallel to the longitudinal direction, including the centerline, at a depth of half the cross-sectional radius (8.75 mm from the periphery). The microstructure of these specimens was observed using an optical microscope after nital etching, revealing a ferrite-pearlite structure. Furthermore, the microstructure was observed using a scanning electron microscope (SEM), and the Cr content in cementite was quantitatively analyzed using an energy dispersive X-ray spectrometer (EDS) attached to the SEM. SEM observations were performed at 2000x magnification in five fields. White areas within the pearlite structure (a lamellar structure of ferrite and cementite) in the secondary electron image were identified as cementite, and EDS analysis of these cementite specimens was performed to determine the Cr content in the cementite. The EDS analysis was performed at an accelerating voltage of 20 kV, with one measurement per field. The average Cr content in cementite obtained from the five measurements was used as the measured value. The measurement times obtained are shown in Tables 1A and 1B.

[0043] Round steel bars were processed into test pieces with a diameter of 30 mm and subjected to an austenitizing treatment by heating at 850°C for 30 minutes. They were then quenched by immersing them in oil at room temperature, and then tempered by heating at 160°C for 60 minutes to obtain round bar test pieces. These round bar test pieces simulate the core of a part when case-hardened steel is carburized.

[0044] Tensile test specimens and impact test specimens were taken from the obtained round steel bars, centered at the 1 / 2 radius position. The tensile test specimens were No. 14A bar-shaped test specimens in accordance with JIS Z 2241:2011, with a parallel section length of 35 mm, a parallel section diameter of 5 mm, and a grip section diameter of 10 mm. Five of these bar-shaped test specimens were used to perform a tensile test in accordance with JIS Z 2241:2011, and the average tensile strength TS (GPa) was determined. The impact test specimens were U-notch specimens with a length of 55 mm, a cross section of 10 mm square, a notch depth of 2 mm, and a notch bottom radius of 1 mm, in accordance with JIS Z 2242:2018. The notch was machined on the surface close to the surface of the round bar. Five of these U-notch specimens were used to perform Charpy impact tests at room temperature in accordance with JIS Z 2242:2018, and the average absorbed energy (J / cm 2 ) was sought. Table 2 shows the test results.

[0045] The resulting absorbed energy is 100J / cm 2 or more, the obtained tensile strength is 1.05 GPa or more, and the product of the absorbed energy and the tensile strength TS is 125 J / cm 2 If the strength was 1.5 GPa or higher, it was determined that high strength and excellent toughness were obtained after carburizing treatment. If either one of the conditions was not satisfied, it was determined that high strength and excellent toughness could not be obtained after carburizing treatment.

[0046] [Table 1A]

[0047] [Table 1B]

[0048] [Table 2]

[0049] From Tables 1A, 1B, and 2, it can be seen that the case-hardened steels according to the present invention had high strength and excellent toughness after carburizing treatment, whereas the case-hardened steels according to the comparative examples did not have high strength and excellent toughness.

Claims

1. The chemical composition, in mass%, is C: 0.17-0.35%, Si: 0.28-0.60%, Mn: 1.03 to 1.45%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.050%, Cr: 1.05-1.50%, N: 0.020% or less, Mo: 0.03-0.15%, Ni: 0.03 to 0.19%, and O: Contains 0.0030% or less the balance being Fe and impurities; A case-hardening steel characterized by satisfying the following (1) and having a Cr concentration in cementite of 1.30 mass% or more. 2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1) In the above formula (1), the element symbols indicate the content of each element in mass %.

2. The chemical composition, in mass%, is C: 0.17-0.35%, Si: 0.28-0.60%, Mn: 1.03 to 1.45%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.050%, Cr: 1.05-1.50%, N: 0.020% or less, Mo: 0.03-0.15%, Ni: 0.03 to 0.19%, and O: Contains 0.0030% or less Further, the composition contains one or more selected from the group consisting of the following groups A, B, and C: the balance being Fe and impurities; A case-hardening steel characterized by satisfying the following (1) and having a Cr concentration in cementite of 1.30 mass% or more. 2.20≦1.5×Cr+4.5×Mo≦2.90 ... (1) In the above formula (1), the element symbols indicate the content of each element in mass %. [Group A] Ti: 0.005% or less, V: 0.010% or less, Nb: 0.005% or less, and B: 0.0015% or less One or more selected from the group consisting of [Group B] Cu: 0.50% or less, and Sn: 0.100% or less One or two selected from the group consisting of [Group C] Ca: 0.0050% or less, and Mg: 0.0050% or less One or two selected from the group consisting of

3. The case-hardening steel according to claim 2, having a chemical composition containing the A group in mass%.

4. The case-hardening steel according to claim 2, having a chemical composition containing the B group in mass%.

5. The case-hardening steel according to claim 2, having a chemical composition containing the C group in mass%.

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