Steel component and method for manufacturing same

Abstract

Provided is a steel component which can further improve the fatigue limit compared to the prior art. A steel component having a nitrogen-rich layer having a hardness of 600 HV or more and a thickness of 10 [mu] m to 100 [mu] m on the surface of a steel having a component composition containing, in mass%, 0.3% to 0.5% of C, 1.0% or less of Si, 1.5% or less of Mn, 9.0% to 15.0% of Cr, 0.5% to 3.0% of Mo and W (Mo + W / 2) alone or in a composite form, and 0.1% or less of N, with the remainder being Fe and unavoidable impurities, and a nitrogen non-absorbed region in which the N is 0.1% by mass or less and the hardness is 10 HV or more lower than the hardness of the nitrogen-rich layer is included in the center of the component.

Description

Technical Field

[0001] The present invention relates to a steel part and a manufacturing method thereof. Background Art

[0002] Traditionally, martensitic stainless steel containing 12% to 13% Cr has been used as steel for tools, springs, valves, and sliding parts. Martensitic stainless steel not only achieves high hardness through quenching and tempering, but also exhibits excellent corrosion resistance, making it rust-resistant and widely used. In particular, martensitic stainless steel strips with a sufficiently high fatigue limit are required for applications such as springs and valves to suppress fatigue failure caused by repeated stress.

[0003] For example, in Patent Document 1, the applicant proposed a martensitic stainless steel as a martensitic stainless steel strip capable of achieving a further high fatigue limit, wherein the compressive residual stress on the surface of the steel strip is 50 MPa or more, and the area ratio of carbides present in the metal structure of the steel strip is 0.5% or more and 8.0% or less.

[0004] Furthermore, Patent Document 2 describes a flapper valve body made of martensitic stainless steel, which has compressive residual stress on the plate surface and a solid-solution nitrogen-concentrated layer on the plate surface, in order to improve the corrosion resistance and fatigue properties of the flapper valve body. Patent Document 2 also describes that by heating the martensitic stainless steel to a temperature above the austenite transformation temperature in an environment containing 20% ​​or more nitrogen and 10% or less (including 0%) oxygen (percentages are expressed in volume %), followed by rapid cooling, the residual stress on the martensitic stainless steel surface can be adjusted to compressive stress.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: International Publication No. 2020 / 013223

[0008] Patent Document 2: Japanese Patent Application Laid-Open No. 10-274161 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] Improving the fatigue limit can lead to lighter parts, longer lifespans, and greater freedom of shape, leading to a desire for even higher fatigue limits in steel. While Patent Document 1 represents an excellent invention for achieving high fatigue limit properties, there remains room for improvement in controlling the surface structure to further enhance the fatigue limit. Furthermore, while the martensitic stainless steel described in Patent Document 2 describes the formation of a solid solution nitrogen-concentrated layer, no research has been conducted into the composition of the steel suitable for forming such a layer, nor has the size of the layer been studied, leaving room for further improvement.

[0011] It is therefore an object of the present invention to provide a steel component whose fatigue limit can be further increased than before.

[0012] Technical means to solve the problem

[0013] The present invention has been made in view of the above-mentioned problems.

[0014] Specifically, one aspect of the present invention is a steel part having a nitrogen-rich layer having a hardness of 600 HV or higher and a thickness of 10 to 100 μm on the surface of steel having a component composition comprising, by mass%, 0.3% to 0.5% C, 1.0% or less Si, 1.5% or less Mn, 9.0% to 15.0% Cr, 0.5% to 3.0% Mo and W (Mo+W / 2) alone or in combination, 0.1% or less N, and the remainder being Fe and inevitable impurities, and further comprising a nitrogen-unabsorbed region having a nitrogen content of 0.1% or less, by mass%, and a hardness at least 10 HV lower than that of the nitrogen-rich layer at the center of the part.

[0015] The elongation is preferably 1.0% or more and 7.0% or less.

[0016] Another aspect of the present invention is a method for manufacturing a steel part, comprising a nitrogen absorption treatment step, wherein a steel material having a composition comprising, by mass%, 0.3% to 0.5% C, 1.0% or less Si, 1.5% or less Mn, 9.0% to 15.0% Cr, 0.5% to 3.0% Mo and W (Mo+W / 2) alone or in combination, 0.1% or less N, and the remainder being Fe and unavoidable impurities, is heated and held under conditions such that a heating temperature of 900°C to 1100°C and a heating time of 2 to 20 minutes, with the cumulative value of the heating temperature [°C] and the heating time [min] being 2000 to 11000, is absorbed by nitrogen, and then rapidly cooled.

[0017] Effects of the Invention

[0018] By means of the invention, a steel component can be obtained which has a higher fatigue limit than before. DETAILED DESCRIPTION

[0019] The following describes one embodiment of the present invention. However, the present invention is not limited to the embodiments described and illustrated herein, and various modifications and improvements are possible without departing from the technical spirit of the present invention. First, the reasons for limiting the composition of the steel parts of the present invention will be explained.

[0020] C: 0.3%~0.5%

[0021] C is an important element that dissolves carbides into the matrix (matrix) at the austenitizing temperature during quenching and determines the hardness of the martensite generated during quenching. Here, the C in the steel is divided into C that dissolves into the matrix and C that precipitates as carbides. The ratio is determined by the interaction with Cr, so it is important that Cr also converges within the composition range described below. In order to obtain steel parts with surface hardness suitable for the present invention, the lower limit of C is set to 0.3%. The preferred lower limit of C is 0.35%, the more preferred lower limit is 0.38%, and the more preferred lower limit is 0.41%. On the other hand, if the amount of C is too much, nitrogen will excessively dissolve in the quenching process under the nitrogen environment described below, thereby remaining in a large amount in the outermost austenite, and the surface hardness may decrease. In addition, if the amount of C is too much, the carbides generated will also become excessively more, so the upper limit of C is set to 0.5%. The preferred upper limit of C is 0.45%, the more preferred upper limit is 0.43%, and the more preferred upper limit is 0.42%.

[0022] Si: 1.0% or less

[0023] Si is except being used as deoxidizer when refining steel, also being dissolved in steel and suppressing the softening element in low temperature tempering, on the other hand, when excessive containing, can make the toughness of steel decline, therefore for example, cold workability during cold rolling decline.Therefore, the upper limit of Si amount is set to 1.0%.Preferred upper limit is 0.9%, more preferably upper limit is 0.8%, and then preferred upper limit is 0.7%, and particularly preferred upper limit is 0.5%.In addition, lower limit is not particularly limited, for example, also can be set to 0.1%.

[0024] Mn: less than 1.5%

[0025] Mn, like Si, is an element that acts as a deoxidizer during refining and is dissolved in the matrix to improve hardenability. If the Mn amount is too little, the hardenability of the steel decreases, and especially in the center of the wall thickness of the steel, it is also possible that it cannot be quenched. On the other hand, when excessively containing Mn, hot workability decreases, so the upper limit is set to 1.5%. The preferred upper limit is 1.2%, and the more preferred upper limit is 1.0%. The lower limit is not particularly limited, for example, it can also be set to 0.1%, and preferably it can also be set to 0.2%.

[0026] Cr: 9.0%~15.0%

[0027] Cr is an important element for forming a strong passive film in steel and obtaining excellent corrosion resistance. In order to exert the above-mentioned corrosion resistance, it is necessary to contain at least 9.0% Cr in the steel. On the other hand, an excessive amount of Cr will lead to a decrease in the martensitic transformation starting temperature (Ms point), which becomes a factor of the hardness drop caused by the increase of retained austenite, so the upper limit of Cr is set to 15%. In addition, when it is desired to further improve the high hardness characteristics, it is preferred to set Cr to 9.0% or more and less than 12.0% to further reduce the amount of retained austenite. In order to further improve the high hardness characteristics, the upper limit of the preferred Cr amount is 11.5%, and the lower limit of the preferred Cr amount is 9.5%. Moreover, when it is desired to further improve the toughness, it is preferred to set Cr to 12.0% or more and less than 15.0% to further increase the amount of retained austenite. In order to further improve the toughness, the upper limit of the preferred Cr amount is 14.5%, and the lower limit of the preferred Cr amount is 12.5%.

[0028] Mo and W alone or in combination (Mo+W / 2): 0.5% to 3.0%

[0029] Mo and W have the same effects as W, and their atomic weights are defined by (Mo + W / 2). Mo and W can be contained alone or in combination. Mo and W are elements that are highly effective in stabilizing the passive state, increasing the pitting potential in chloride solutions, and effectively improving corrosion resistance. They also suppress softening during low-temperature tempering, and to achieve these effects, at least 0.5% is required. On the other hand, excessive addition of Mo and W significantly reduces workability during hot working, so the upper limit is set at 3.0%. The preferred lower limit of the (Mo + W / 2) amount is 0.8%, and the preferred upper limit of the (Mo + W / 2) amount is 2.0%.

[0030] N: 0.1% or less

[0031] In the present invention, excessive nitrogen content in the steel material before the nitrogen absorption treatment (described later) can cause gas defects during casting. Therefore, the nitrogen content is limited to 0.1% or less. The preferred upper limit is 0.07%, and the more preferred upper limit is 0.05%. The lower limit can be set to, for example, 0.001%. Furthermore, as described later, the steel parts of the present invention have a nitrogen-unabsorbed region in the center of the part where the effects of the nitrogen absorption treatment are not reached. Therefore, to measure the nitrogen content in the steel material, it is sufficient to measure the nitrogen content in this nitrogen-unabsorbed region.

[0032] In this embodiment, the components other than the above are Fe and inevitable impurities. Examples of inevitable impurity elements include P, S, Al, Ti, and O, which may be contained as long as they are within the range shown below and do not inhibit the effects of the present invention.

[0033] P≦0.04%, S≦0.03%, Al≦0.1%, Ti≦0.1% and O≦0.05%.

[0034] Next, the steel part of the present invention having the above-described composition will be described. The steel part of this embodiment has a nitrogen-rich layer on the steel surface with a hardness of 600 HV or greater and a thickness of 10 μm to 100 μm. Furthermore, the center of the part contains a nitrogen-unabsorbed region with a nitrogen content of 0.1% or less by mass and a hardness at least 10 HV lower than that of the nitrogen-rich layer. Consequently, the steel part of this embodiment exhibits high fatigue limit characteristics. In this embodiment, by forming the nitrogen-rich layer only on the surface, the surface hardness of the part is increased. Furthermore, the solid solution of nitrogen lowers the martensite start temperature, allowing the amount of retained austenite in the surface layer to be appropriately increased. Retained austenite in the surface layer tends to reduce the hardness and strength of the steel part, but the presence of an appropriate amount of retained austenite in the martensite can suppress crack progression without reducing hardness or strength, further improving the fatigue limit. If the thickness of the nitrogen-rich layer is less than 10 μm, the surface hardness of the part is low, and the fatigue limit tends to be reduced. If the thickness of the nitrogen-rich layer exceeds 100 μm, nitrogen will be excessively dissolved, which can easily reduce the hardness and ductility of the part. The lower limit of the thickness of the nitrogen-rich layer is preferably 20 μm, preferably 40 μm. The upper limit of the thickness of the nitrogen-rich layer is preferably 80 μm, preferably 60 μm.

[0035] The hardness of the nitrogen-rich layer in this embodiment is a value measured at room temperature (normal temperature) and is 600 HV or higher. It is preferably 610 HV or higher, more preferably 620 HV or higher, even more preferably 640 HV or higher, and particularly preferably 700 HV or higher. The upper limit is not particularly limited, but can be set to approximately 800 HV due to manufacturing constraints.

[0036] The steel part of this embodiment has a nitrogen-unabsorbed region with a mass percentage of 0.1% or less at the center of the part. This region exhibits no solid solution strengthening due to nitrogen and exhibits high ductility, thereby maintaining the ductility of the entire part. The thickness of the nitrogen-unabsorbed region is determined by the thickness of the part and the thickness of the nitrogen-rich layer, so there is no particular lower limit. To achieve a stable ductility-enhancing effect, the thickness of the nitrogen-unabsorbed region is preferably at least 100 μm. A more preferred thickness is 150 μm or greater, and even more preferably 200 μm or greater. Alternatively, the thickness of the nitrogen-unabsorbed region can be determined by measuring the nitrogen content at the center (geometric center) of the raw material, with the location where the nitrogen content increases being defined as the boundary of the nitrogen-unabsorbed region. Furthermore, the nitrogen content in this embodiment can be determined by mechanically grinding away the surface of a sample to be measured for fatigue limit, etc., and chemically analyzing the remaining sample. Furthermore, since the hardness of the nitrogen-unabsorbed region is at least 10 HV lower than that of the nitrogen-rich layer, the presence of the nitrogen-unabsorbed region can also be determined by focusing on the hardness.

[0037] The thickness of the steel part of this embodiment is preferably 0.20 mm or greater. The steel part of the present invention exhibits high fatigue strength even among steel parts with a thickness of 0.20 mm or greater. More preferably, the thickness is 0.30 mm or greater, 0.40 mm or greater, or 0.50 mm or greater. The upper limit is not particularly limited, but is practically 3.0 mm or less. Furthermore, the shape of the steel part of this embodiment is not particularly limited, and can be applied to shapes such as plates, blocks, and rods.

[0038] The steel part of this embodiment preferably contains 10.5% to 35.0% retained austenite. As mentioned above, the amount of retained austenite is generally reduced because it reduces the hardness and strength of the steel part. However, the presence of an appropriate amount of retained austenite in the martensite can suppress crack progression without reducing hardness or strength, further improving the fatigue limit. The preferred lower limit of retained austenite is 11.0%, and the preferred upper limit of retained austenite is 32.0%, more preferably 31.0%. Furthermore, the retained austenite amount in this embodiment is measured in the surface layer of the part, in other words, in the nitrogen-rich layer.

[0039] The elongation of the steel parts of this embodiment is preferably not less than 1.0% and not more than 7.0%. When the elongation is less than 1.0%, when used as a tool, spring material, valve material, or sliding part, the ductility is low and it is difficult to use as a part. The preferred lower limit of the elongation is not less than 1.2%, more preferably not less than 1.5%, and even more preferably not less than 2.0%. On the other hand, when the elongation is not less than 7.0%, it is difficult to maintain the hardness of the nitrogen-rich layer. The preferred upper limit of the elongation is not more than 5.0%, more preferably not more than 3.0%. In addition, there is no special limitation on the Young's modulus, as long as it is not less than 150 GPa. It is preferably not less than 180 GPa, and more preferably not less than 200 GPa.

[0040] In this embodiment, the elongation and Young's modulus were evaluated by tensile testing using unique proportional test pieces with a mark spacing of 25 mm, since it is difficult to produce JIS-13B shaped test pieces specified in Japanese Industrial Standards (JIS)-Z2241.

[0041] Next, the method for manufacturing steel parts of the present invention will be described. In the present invention, steel having the aforementioned composition range and thickness is heated and held under conditions such that the cumulative value of the heating temperature [°C] and the heating time [min] is 2000 to 11000°C, allowing nitrogen to be absorbed, and then quenched. In this composition system, if the heating temperature is less than 900°C, nitrogen in the atmosphere and carbides in the raw material cannot fully dissolve into the austenite, resulting in reduced surface hardness. Furthermore, if the heating temperature exceeds 1100°C, most of the carbides dissolve, resulting in coarsening of the grains and a decrease in toughness. Therefore, the heating temperature is set to 900°C to 1100°C. The preferred lower limit of the heating temperature is 950°C, and the more preferred lower limit is 1000°C. The preferred upper limit of the temperature is 1070°C, and the more preferred upper limit is 1050°C. Furthermore, if the heating time exceeds 20 minutes, austenite remains on the outermost surface due to excessive solid-solution nitrogen, reducing the surface hardness and thus the fatigue limit. Therefore, the preferred upper limit of the heating time is 15.0 minutes, the more preferred upper limit is 10.0 minutes, and the further preferred upper limit is 5.0 minutes. Furthermore, if the heating time is shorter than 2 minutes, a nitrogen-rich layer is not sufficiently formed on the surface, reducing the surface hardness and fatigue limit. Therefore, the preferred lower limit of the heating time is 2.5 minutes, and the more preferred lower limit is 3.0 minutes.

[0042] In the manufacturing method of this embodiment, tempering is performed after the nitrogen absorption treatment step. By setting the tempering temperature to 250°C to 400°C, steel parts with a surface hardness of 600 HV or higher can be obtained. In this composition system, if the tempering temperature is less than 250°C, carbides are not fully precipitated from the martensite structure, resulting in reduced toughness. On the other hand, if the tempering temperature exceeds 400°C, carbides precipitate significantly from the martensite structure, resulting in a decrease in hardness.

[0043] [Example]

[0044] Hot-rolled materials with a thickness of 2.0 mm and the composition shown in Table 1 (the remainder being Fe and unavoidable impurities) were annealed in a batch annealing furnace. Subsequently, cold rolling and strain relief annealing were performed, and the finished products were finished to the specified thicknesses shown in Table 2. For nitrogen absorption treatment and tempering, the materials were maintained at the specified temperature and time under the conditions shown in Table 2, then rapidly cooled and then maintained at 270°C or 350°C for 100 seconds. Surface oxides and fine scratches were then ground and removed using barrel polishing for 30 minutes to prepare parts A to T. The thickness of the nitrogen-rich layer was determined by chemical analysis of samples ground to a depth of 10 μm to 150 μm from the surface of the parts. The depth at which the nitrogen content was at least 0.01% higher than that of the parts before heat treatment was defined as the nitrogen-rich layer. In this embodiment, a nitrogen-rich layer thickness of 10 μm to 100 μm was judged as "○" (nitrogen-rich layer present), 0 μm to less than 10 μm was judged as "×" (no nitrogen-rich layer), and 100 μm or more was judged as "△" (excessive nitrogen-rich layer).

[0045] The hardness, elongation, Young's modulus, fatigue limit, and retained austenite content of the heat-treated steel parts were then investigated. For parts A through N, Vickers hardness was measured on the surface and in the center of the cross section (nitrogen-unabsorbed area) at a load of 300 gf, with the average value of three points used for evaluation. For the center of the cross section, the Vickers hardness was evaluated at the center of the wall thickness of the cross-section structure parallel to the rolling extension direction. Vickers hardness was evaluated according to the method specified in JIS-Z2244. Elongation was evaluated using elongation at break according to the method specified in JIS-Z2241. The test specimens used were not JIS 13B standard test specimens, but rather proportional test specimens with a 25 mm distance between markings. Young's modulus was calculated based on the slope of the stress-strain diagram obtained during the tensile test. Regarding the fatigue limit, the stress amplitude is constant (stress ratio R = -1, number of repetitions 1 × 10 7 times) alternating vibration bending test, will be 1 × 10 7The maximum stress at which no fracture occurs is defined as the fatigue limit. The amount of retained austenite is calculated using the integrated intensity of the peaks of the austenite phase and the ferrite phase using an X-ray diffraction device. Table 3 shows the measurement results of various properties.

[0046] [Table 1]

[0047]

[0048] [Table 2]

[0049]

[0050] [Table 3]

[0051]

[0052] According to the results in Tables 2 and 3, parts A through F, and parts Q through T of the inventive examples have a nitrogen-rich layer with a surface hardness of 600 HV or greater and a thickness of 10 μm to 100 μm, and contain a nitrogen-unabsorbed region in the center of the part with a hardness at least 10 HV lower than that of the nitrogen-rich layer. Furthermore, retained austenite in parts A through F, and parts Q through T is confirmed to be 11.3% to 30.5%, indicating good fatigue limits. Meanwhile, comparative examples G and J, while exhibiting good surface hardness of 600 HV or greater, lack a nitrogen-rich layer due to heat treatment in an Ar atmosphere, resulting in low fatigue limits. Furthermore, in comparative examples M, N, and O, excessive amounts of nitrogen dissolved in the surface result in retained austenite on the outermost surface, resulting in low surface hardness and low elongation. Comparative example L is also believed to have a reduced surface hardness for the same reasons as comparative examples M, N, and O. Comparative Examples H, I, and K, due to heat treatment in an Ar atmosphere, lack a nitrogen-rich layer. Consequently, the surface hardness is low, resulting in a low fatigue limit. In Comparative Example P, although the heating temperature was at the lower limit, the heating time was short, resulting in a small temperature-time cumulative value. As a result, the thickness of the nitrogen-rich layer was less than 10 μm, resulting in significantly low fatigue strength. This confirms that the examples of the present invention achieve both high hardness and good elongation and fatigue limit compared to the conventional examples in terms of the nitrogen-rich layer.

Claims

1. A steel part comprising a steel component comprising, in mass %, 0.3% to 0.5% C, 1.0% or less Si, 1.5% or less Mn, 9.0% to 15.0% Cr, 0.5% to 3.0% Mo and W (Mo+W / 2) alone or in combination, 0.1% or less N, and the remainder being Fe and unavoidable impurities, the steel component comprising a nitrogen-rich layer having a hardness of 600 HV or more and a thickness of 10 to 100 μm on its surface, the steel component further comprising a nitrogen-unabsorbed region having a nitrogen content of 0.1% or less, and a hardness at least 10 HV lower than that of the nitrogen-rich layer, in a central portion of the steel component.

2. The steel part according to claim 1, wherein The elongation is 1.0% or more and 7.0% or less.

3. A method for manufacturing a steel part, comprising a nitrogen absorption treatment step, wherein a steel material having a composition comprising, in mass %, 0.3% to 0.5% C, 1.0% or less Si, 1.5% or less Mn, 9.0% to 15.0% Cr, 0.5% to 3.0% Mo and W (Mo+W / 2) alone or in combination, 0.1% or less N, and the remainder being Fe and unavoidable impurities, is heated and held to absorb nitrogen under conditions such that a heating temperature of 900° C. to 1100° C. and a heating time of 2 to 20 minutes, with the cumulative value of the heating temperature [° C.] and the heating time [min] being 2000 to 11000, and then rapidly cooled.

4. The method for manufacturing a steel part according to claim 3, wherein: Tempering is performed after the nitrogen absorption treatment step.

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