Steel for carburizing and nitriding

Abstract

To provide a carbonitriding steel having high strength and superior toughness after carbonitriding treatment.SOLUTION: A carbonitriding steel is adopted that includes, in mass%, C: 0.10-0.30%, Si: 0.50% or less, Mn: 1.15-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, Cr: 0.20-0.64%, N: 0.001-0.020%, Mo: 0.32-0.60%, and O: 0.0030% or less, and satisfies the following formula (1), with the Mn content in cementite being 1.70 mass% or more. The formula (1) is: 0.12×Mn-0.62×Cr+3.15×Mo≥0.80 mass% (1) where, Mn, Cr and Mo each denote the content of the element in mass%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to carburizing and nitriding steel materials, more specifically, steel materials used after carburizing and nitriding treatment.

Background Art

[0002] Mechanical structure parts such as parts for automobiles, parts for industrial machines, and parts for construction machines may be surface hardened, for example, to improve fatigue strength.

[0003] Among various surface hardening treatments, carburizing treatment is often applied to mechanical structure parts as described above. A hardened layer is formed on the surface of the carburized parts. Due to this hardened layer, not only high fatigue strength can be obtained, but also the core part of the parts is heat-treated, so high toughness can be obtained.

[0004] In recent years, instead of performing only carburizing treatment, a carburizing and nitriding treatment in which nitriding is performed after carburizing may be performed. A hardened layer is formed by carburizing treatment, and nitrogen is introduced into the hardened layer by nitriding, thereby increasing the softening resistance. As a result, softening of the hardened layer when heat is generated during part use can be suppressed, and high fatigue strength can be maintained even during use.

[0005] On the other hand, there is a concern about a decrease in toughness due to the formation of nitrides by introducing nitrogen into the hardened layer. The steel generally used for carburizing treatment contains Si, Cr, and Ti, and these elements are likely to form nitrides by binding with nitrogen. In particular, when the steel contains Ti, V, Nb, or B, the nitrides become coarse, so the toughness may decrease further.

[0006] Therefore, even when performing carburizing and nitriding treatment, there is a demand for steel having high strength and excellent toughness.

[0007] For example, Patent Document 1 discloses a technology relating to a steel for carburizing and nitriding, characterized in that the steel composition contains, by mass%, C: 0.10-0.30%, Si: 0.30% or less, P: less than 0.03%, S: less than 0.03%, Cr: 0.7% or less, Mo: 0.01-0.60%, Al: 0.0010-0.0800%, N: 0.0010-0.0150%, Ti: 0.010-0.0800%, and B: 0.0005-0.0030%, with the remainder being Fe and unavoidable impurities, and satisfying formulas (1) and (2). F1>-1.95···Formula (1) F2<10.0···Formula (2) however, F1=-1.71[C]+0.52[Si]-0.59[Mn]-0.50[Cu]-0.23[Ni]-0.18[Cr]-1.71[Mo] F2=236.61[Si]2-31.04[Si]+33.92[Cr]2-18.48[Cr]+23.92[Si][Mn] (In the formulas for F1 and F2, the brackets [ ] indicate the mass percentage of the element contained within the brackets.) [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2020-29608 [Overview of the project] [Problems that the invention aims to solve]

[0009] Patent Document 1 discloses that the precipitation of nitrides in carburizing and nitriding treatment can be reduced by specifying the amounts of Si, Cr, Mn, etc. However, Patent Document 1 includes B to improve hardenability and contains an excess of Ti to avoid B nitrides, and does not consider the reduction in toughness due to Ti nitrides.

[0010] This invention has been made in view of the above-mentioned circumstances, and aims to provide a steel material for carbonitriding that has high strength and excellent toughness after carbonitriding treatment. [Means for solving the problem]

[0011] The gist of this invention is as follows:

[0012] (1) A steel material for carburizing and nitriding according to one aspect of the present invention has a chemical composition in mass% of: C: 0.10~0.30%, Si: 0.50% or less, Mn: 1.15~2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005~0.080%, Cr: 0.20~0.64%, N: 0.001~0.020%, Mo: 0.32~0.60%, and O: Contains 0.0030% or less, The remainder consists of Fe and impurities. The following equation (1) is satisfied, and the Mn concentration in the cementite is 1.70% by mass or more. 0.12×Mn-0.62×Cr+3.15×Mo≧0.80 mass% (1) However, in formula (1) above, Mn, Cr, and Mo represent the mass percentage content of the respective elements. (2) A steel material for carburizing and nitriding according to another aspect of the present invention has a chemical composition in mass%, C: 0.10~0.30%, Si: 0.50% or less, Mn: 1.15~2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005~0.080%, Cr: 0.20~0.64%, N: 0.001~0.020%, Mo: 0.32~0.60%, and It contains 0.0030% or less of O, and further contains one or more selected from the group consisting of the following Group A, Group B, and Group C, the balance being composed of Fe and impurities, satisfies the following formula (1), and the Mn concentration in cementite is 1.70% by mass or more. 0.12×Mn - 0.62×Cr + 3.15×Mo ≥ 0.80% by mass (1) However, Mn, Cr, and Mo in the above formula (1) indicate the contents in % by mass of the respective elements. [Group A] Ti: 0.005% or less, V: 0.010% or less, Nb: 0.010% or less, and B: 0.0015% or less one or more selected from the group consisting of [Group B] Cu: 0.30% or less, Ni:​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0014] The requirements for the carbonitriding steel material according to this embodiment will be described in detail below. In this embodiment, the carbonitriding steel material refers to steel material that is processed into a part shape as needed, then subjected to carbonitriding treatment before being used (for example, as a mechanical structural part).

[0015] <About the chemical composition> First, the chemical composition of the carburizing and nitriding steel material according to this embodiment will be described. The numerical limit ranges indicated by "~" below include both a lower limit and an upper limit. Numbers indicated as "less than" or "greater than" do not include the numerical range. All "%"s in relation to the chemical composition mean "mass%".

[0016] The chemical composition of the steel material for carburizing and nitriding according to this embodiment is, in mass%, C: 0.10-0.30%, Si: 0.50% or less, Mn: 1.15-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, Cr: 0.20-0.64%, N: 0.001-0.020%, Mo: 0.32-0.60%, O: 0.0030% or less, with the remainder being Fe and impurities. Each element will be described below.

[0017] C: 0.10~0.30% Carbon (C) enhances the strength of mechanical structural components (hereinafter sometimes simply referred to as "components") after carburizing and nitriding treatment. To obtain the desired strength in the components, the C content should be 0.10% or more. Preferably, it should be 0.12% or more. However, if the carbon content is too high, the core strength of the component becomes too high, reducing its toughness. Therefore, the carbon content should be 0.30% or less. Preferably, it should be 0.28% or less.

[0018] Si:0.50% or less Silicon (Si) has the effect of deoxidizing steel. On the other hand, Si reduces the strength of parts by forming nitrides during carburizing and nitriding treatment. Therefore, the Si content should be 0.50% or less. Preferably it is 0.45% or less, more preferably 0.40% or less, less than 0.40%, or 0.35% or less. There is no particular lower limit to the Si content, but it may be set at 0.01% or higher, or 0.05% or higher.

[0019] Mn: 1.15~2.00% Manganese (Mn) increases the strength of parts after carburizing and nitriding treatment. Furthermore, Mn improves the toughness of parts by forming a solid solution in cementite. These effects cannot be obtained if the Mn content is less than 1.15%. Therefore, the Mn content should be 1.15% or higher, preferably 1.20% or higher. On the other hand, if the Mn content exceeds 2.00%, nitrides are formed during the carburizing and nitriding treatment, reducing the strength of the part. Therefore, the Mn content should be 2.00% or less. Preferably, it should be 1.80% or less.

[0020] P:0.050% or less Phosphorus (P) is inevitably present in steel. P tends to segregate in steel, causing localized reductions in ductility. In particular, when the P content exceeds 0.050%, the localized reduction in ductility becomes significant. Therefore, the P content should be 0.050% or less. Preferably, it should be 0.030% or less. There is no specific lower limit for the P content, but it may be greater than 0% or greater than 0.002%.

[0021] S: 0.050% or less Sulfur (S) is inevitably present in steel. S combines with Mn in steel to form MnS, which causes cracking during hot rolling. Therefore, the S content should be 0.050% or less. Preferably, it should be 0.040% or less. There is no specific lower limit for the sulfur content, but it may be greater than 0% or greater than 0.005%.

[0022] Al: 0.005~0.080% Aluminum (Al) is an effective deoxidizing agent. If the Al content is less than 0.005%, the effect is not obtained. Therefore, the Al content should be 0.005% or more. Preferably, it should be 0.010% or more. On the other hand, if the Al content exceeds 0.080%, nitrides are formed during the carburizing and nitriding treatment, reducing the strength of the part. Therefore, the Al content should be 0.080% or less. Preferably, it should be 0.060% or less.

[0023] Cr: 0.20~0.64% Chromium (Cr) enhances the hardenability of steel and increases the strength of parts after carburizing and nitriding treatment. This effect is not achieved if the Cr content is less than 0.20%. Therefore, the Cr content should be 0.20% or more, preferably 0.25% or more. On the other hand, in the chemical composition of the steel material for carburizing and nitriding according to this embodiment, if the Cr content exceeds 0.64%, the amount of Mn solid solution in cementite decreases, thereby reducing the toughness of the part. Therefore, in this embodiment, the Cr content is 0.64% or less. Preferably, it is less than 0.60%.

[0024] N: 0.001~0.020% Nitrogen (N) reduces the amount of soluble Ti by combining with Ti in the steel to form TiN. Reducing the amount of soluble Ti prevents the formation of coarse TiN during carburizing and nitriding, thereby suppressing a decrease in toughness. To achieve this effect, the N content should be 0.001% or higher, preferably 0.005% or higher. On the other hand, if the N content exceeds 0.020%, excessive TiN will precipitate even without carbonitriding treatment. If excessive TiN precipitates before carbonitriding treatment, a large amount of TiN will remain after carbonitriding treatment, degrading the toughness of the component. Therefore, the N content should be 0.020% or less. Preferably, it should be 0.018% or less.

[0025] Mo: 0.32~0.60% Molybdenum (Mo) enhances the hardenability of steel and increases the strength of parts after carburizing and nitriding treatment. Furthermore, Mo increases the amount of manganese dissolved in cementite, improving the toughness of the parts. This effect is not achieved if the Mo content is less than 0.32%. Therefore, the Mo content should be 0.32% or higher, preferably 0.35% or higher. On the other hand, if the Mo content exceeds 0.60%, the strength of the steel material for carburizing and nitriding becomes too high, and its workability deteriorates. Therefore, the Mo content should be 0.60% or less. Preferably, it should be 0.55% or less.

[0026] O: 0.0030% or less When oxygen (O) is present in large quantities in steel, it forms coarse oxides that act as fracture initiation points, degrading the toughness of the component. Therefore, the O content should be 0.0030% or less. Preferably, the O content is 0.0020% or less, and more preferably 0.0015% or less. There is no particular lower limit for the O content, but it may be 0% or more, or it may be greater than 0% or 0.0005% or more.

[0027] 0.12×Mn-0.62×Cr+3.15×Mo≧0.80 mass% (1) In the carburizing and nitriding steel of this embodiment, it is important to control the content of alloying elements in order to favorably control the Mn concentration in cementite. In particular, it is important to control the Mn content, the Cr content which has high solid solubility in cementite and reduces the Mn concentration in cementite, and the Mo content which stabilizes cementite. If the left side of the above formula (1) is less than 0.80 mass%, the Mn concentration in cementite may not be favorably controlled. For this reason, the left side of the above formula (1) should be 0.80 mass% or more. Although there is no specific upper limit for the left-hand side of equation (1) above, it is preferable to set it to 2.00 mass% or less from the viewpoint of alloy cost. Furthermore, the left-hand side of equation (1) above may be 1.70 mass% or less, 1.65 mass% or less, 1.40 mass% or less, or 1.30 mass% or less.

[0028] The remainder of the chemical composition of the carbonitriding steel material according to this embodiment may be Fe and impurities. In this embodiment, examples of impurities include elements that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of steel materials, and are acceptable within a range that does not impair the properties of the carbonitriding steel material according to this embodiment.

[0029] The chemical composition of the carburizing and nitriding steel material according to this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the groups A, B, and C below. If the following optional elements are not included, the content is 0%. [Group A] Ti: 0.005% or less, V: 0.010% or less, Nb: 0.010% or less, B: 0.0015% or less One or more selected from the group consisting of [Group B] Cu: 0.30% or less, Ni: 0.30% or less, Sn: 0.100% or less One or two selected from the group consisting of [Group C] Ca: 0.0050% or less, Mg: 0.0050% or less One or two selected from the group consisting of

[0030] Ti: 0.005% or less V: 0.010% or less Nb: 0.010% or less B: 0.0015% or less Titanium (Ti), vanadium (V), niobium (Nb), and boron (B) combine with nitrogen in the steel during carburizing and nitriding to form nitrides. If the content of these elements is too high, it will form coarse nitrides, reducing the toughness of the core of the part. Therefore, the Ti content should be 0.005% or less, the V content 0.010% or less, the Nb content 0.010% or less, and the B content 0.0015% or less. Preferably, the Ti content is 0.004% or less or 0.003% or less, the V content 0.005% or less or 0.003% or less, the Nb content 0.005% or less or 0.003% or less, and the B content 0.0010% or less or 0.0005% or less. Since it is preferable that these elements are not present, the content of Ti, V, Nb, and B may be 0%.

[0031] Cu:0.30% or less Copper (Cu) increases the strength of the component. To achieve this effect, Cu may be included. To reliably obtain this effect, the Cu content is preferably 0.02% or higher. However, if the copper content exceeds 0.30%, the toughness of the component decreases. Therefore, the copper content should be 0.30% or less. Preferably, it should be 0.25% or less.

[0032] Ni: 0.30% or less Nickel (Ni) increases the strength of parts. Ni may be included to achieve this effect. To reliably obtain this effect, it is preferable that the Ni content be 0.02% or higher. However, if the Ni content exceeds 0.30%, the strength of the part increases but the toughness decreases. Therefore, it is preferable to keep the Ni content at 0.30% or less. Preferably, it is 0.25% or less.

[0033] Sn: 0.100% or less Tin (Sn) increases the strength of parts. To achieve this effect, Sn may be included. To reliably obtain this effect, it is preferable that the Sn content be 0.020% or higher. However, if the Sn content exceeds 0.100%, the toughness of the component decreases. Therefore, it is preferable to keep the Sn content below 0.100%, preferably below 0.050%.

[0034] Ca: 0.0050% or less Calcium (Ca) may be added to improve the machinability of parts during manufacturing. To achieve this effect, it is preferable to add 0.0005% or more. However, if the Ca content exceeds 0.0050%, coarse oxides may form, degrading the toughness of the component. Therefore, when Ca is included, the Ca content should be 0.0050% or less. Preferably, the Ca content is 0.0025% or less.

[0035] Mg: 0.0050% or less Magnesium (Mg) may be added to improve the machinability of parts during manufacturing. To achieve this effect, it is preferable to add 0.0005% or more. However, if the Mg content exceeds 0.0050%, coarse oxides may form, degrading the toughness of the component. Therefore, when Mg is included, the Mg content should be 0.0050% or less. Preferably, the Mg content is 0.0025% or less.

[0036] The chemical composition of the above-mentioned steel materials for carburizing and nitriding can be measured using general analytical methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). 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. Furthermore, the elemental content of each element is rounded to the minimum digit of the measured value, based on the significant figures defined in this embodiment, by rounding the fractional part of the measured value to the minimum digit of the elemental content defined in this embodiment.

[0037] Mn concentration in cementite: 1.70% by mass or more In the carbonitriding steel material according to this embodiment, the Mn concentration in the cementite is 1.70% by mass or more. By setting the Mn concentration in the cementite to 1.70% by mass or more, high toughness can be achieved in the core of the part after the carbonitriding treatment. The details of the mechanism by which increasing the Mn concentration in the cementite improves toughness after the carbonitriding treatment are unknown, but the inventors speculate that the distribution of solid-solution Mn in the austenite during heating in quenching is influential. Furthermore, since the above effect saturates if the Mn concentration in cementite is too high, the upper limit may be set to 3.00 mass% or less.

[0038] Method for manufacturing steel for carburizing and nitriding Next, a preferred method for manufacturing the carburizing and nitriding steel material according to this embodiment will be described. According to the manufacturing method described below, the carburizing and nitriding steel material according to this embodiment can be manufactured stably.

[0039] First, steel having the above-mentioned chemical composition is melted to produce a cast slab. The produced cast slab is then subjected to bloc rolling to produce a steel billet. The resulting steel billet is then hot-rolled to obtain a steel material for carburizing and nitriding. At this time, in order to sufficiently dissolve Mn in the austenite, it is preferable that the heating temperature of the steel billet be 1050°C or higher and the heating time be 1 hour or higher. Furthermore, in order to ensure sufficient time for Mn to concentrate in the cementite, it is preferable that the cementite transformation temperature be high. For this reason, it is preferable that the finishing temperature of the hot rolling be 850°C or higher, the reduction ratio in the finishing process be 30% or higher, and the average cooling rate after the finishing process be 0.5°C / s or higher. [Examples]

[0040] Next, embodiments of the present invention will be described. The conditions in the embodiments are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0041] Molten steel was produced by primary and secondary refining, and cast slabs with the chemical composition shown in Table 1 were obtained by continuous casting. The produced cast slabs were rolled in chunks to produce steel billets. The obtained steel billets were heated to 1150°C for 1.5 hours, the hot rolling finishing temperature was set to 900°C, the surface reduction ratio during finishing was set to 40%, and the average cooling rate from finishing to 300°C was set to 1.0°C / s to obtain 60 mm diameter round steel bars.

[0042] A specimen for microstructural observation was taken from a cross-section parallel to the longitudinal direction including the centerline of the obtained round steel bar, at a depth of 1 / 2 the radius of the cross-sectional surface of the steel bar (15 mm deep from the circumferential surface). When the microstructure of this specimen was observed with an optical microscope by Nital etching, it was found to be entirely ferrite-pearlite. Furthermore, the microstructure was observed with a scanning electron microscope (SEM), and the amount of Mn in the cementite was quantitatively analyzed using an energy-dispersive X-ray spectrometer (EDS) attached to the SEM. For SEM observation, five fields of view were observed at a magnification of 2000x. The white areas observed in the pearlite structure (layered structure of ferrite and cementite) in the secondary electron image observation field were identified as cementite, and the Mn concentration in the cementite was obtained by EDS analysis of that cementite. The acceleration voltage during EDS analysis was set to 20kV, and measurements were taken at one point in each field of view. The average value of the Mn concentration in the cementite obtained from a total of five measurements was taken as the measured value. The obtained measurements are shown in Table 1.

[0043] Furthermore, austenitization treatment was performed by heating a round steel bar, which had been processed into a 30mm diameter test piece, at 800°C for 60 minutes. After that, quenching was performed by immersion in oil at room temperature. Subsequently, tempering was performed by heating at 150°C for 60 minutes to obtain a round bar test piece. This round bar test piece simulates the core of a component when the steel has undergone carburizing and nitriding treatment. Tensile and impact test pieces were taken from the obtained round steel bar, centered at the 1 / 2 radius position. The tensile test piece was a 14A type test piece conforming to JIS Z 2241:2011, with a parallel section length of 35mm, a parallel section diameter of 5mm, and a gripping section diameter of 10mm. Using five of these rod-shaped test pieces, the average tensile strength TS (GPa) was determined by performing a tensile test in accordance with JIS Z 2241:2011. Furthermore, 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 notches were machined on the surface of the round bar closest to the surface. Using five of these U-notch specimens, a Charpy impact test was performed at room temperature in accordance with JIS Z 2242:2018 to determine the average absorbed energy (J / cm²). 2 ) was sought. Table 2 shows the test results.

[0044] The absorbed energy obtained was 100 J / cm². 2 Furthermore, the product of the absorbed energy and the tensile strength TS is 115 J / cm². 2 If the result was GPa or higher, it was determined that high strength and excellent toughness were obtained after the carburizing and nitriding treatment. If either of these conditions was not met, it was determined that high strength and excellent toughness could not be obtained after the carburizing and nitriding treatment.

[0045] [Table 1]

[0046] [Table 2]

[0047] Tables 1 and 2 show that the carbonitriding steel material according to the present invention example obtained high strength and excellent toughness after the carbonitriding treatment. On the other hand, the carbonitriding steel material according to the comparative example did not obtain high strength and excellent toughness.

Claims

1. The chemical composition is expressed in mass percent. C: 0.10-0.30%, Si: 0.50% or less, Mn: 1.15-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, Cr: 0.20-0.64%, N: 0.001 to 0.020%, Mo: 0.32–0.60%, and O: Contains 0.0030% or less, The remainder consists of Fe and impurities. A steel material for carburizing and nitriding that satisfies the following formula (1) and has a Mn concentration of 1.70% by mass or more in cementite. 0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1) However, in formula (1) above, Mn, Cr, and Mo represent the mass percentage content of the respective elements.

2. The chemical composition is expressed in mass percent. C: 0.10-0.30%, Si: 0.50% or less, Mn: 1.15-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, Cr: 0.20-0.64%, N: 0.001 to 0.020%, Mo: 0.32–0.60%, and O: Contains 0.0030% or less, Furthermore, it contains one or more types selected from the groups A, B, and C listed below. The remainder consists of Fe and impurities. A steel material for carburizing and nitriding that satisfies the following formula (1) and has a Mn concentration of 1.70% by mass or more in cementite. 0.12×Mn−0.62×Cr+3.15×Mo≧0.80% by mass (1) However, in formula (1) above, Mn, Cr, and Mo represent the mass percentage content of the respective elements. [Group A] Ti: 0.005% or less, V: 0.010% or less, Nb: 0.010% or less, B: 0.0015% or less One or more selected from the group consisting of [Group B] Cu: 0.30% or less, Ni: 0.30% or less, Sn: 0.100% or less One or more selected from the group consisting of [Group C] Ca: 0.0050% or less, Mg: 0.0050% or less One or two selected from the group consisting of

3. The steel material for carburizing and nitriding according to claim 2, having a chemical composition containing the group A in mass percent.

4. The steel material for carburizing and nitriding according to claim 2, having a chemical composition containing the B group in mass%.

5. The steel material for carburizing and nitriding according to claim 2, having a chemical composition containing the C group in mass%.

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