STEEL COMPONENT

Abstract

Provided is a steel component with excellent surface fatigue strength. The steel component has a nitride compound layer with a thickness of 5.0 μm to 30.0 μm and a hardened layer in an order from a component surface to a component inside, where a thickness of a porous layer on an outermost surface of the nitride compound layer is 3.0 μm or less and 40.0% or less of a thickness of the nitride compound layer, and the hardened layer has a hardness of HV600 or more at a position of 50 μm inward from the component surface, a hardness of HV400 or more at a position from the component surface to the component inside of 400 μm, and a hardness of HV250 or more at a position from the component surface to the component inside of 600 μm.

Claims

1. A steel component, comprising a nitride compound layer with a thickness of 5.0 μm to 30.0 μm and a hardened layer in an order from a component surface to a component inside, wherein a thickness of a porous layer on an outermost surface of the nitride compound layer is 3.0 μm or less and 40.0% or less of a thickness of the nitride compound layer, and the hardened layer has a hardness of HV600 or more at a position of 50 μm inward from the component surface, a hardness of HV400 or more at a position from the component surface to the component inside of 400 μm, and a hardness of HV250 or more at a position from the component surface to the component inside of 600 μm.

2. The steel component according to claim 1, wherein the steel component is a toothed component and has the nitride compound layer and the hardened layer at least in a tooth portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In the accompanying drawings:

[0026] FIG. 1 illustrates a roller pitching test piece; and

[0027] FIG. 2 illustrates typical manufacturing processes of a nitrocarburized component.

DETAILED DESCRIPTION

[0028] The following describes the present disclosure in detail.

[0029] First, the reasons for limiting the thickness of the nitride compound layer and the porous layer and the hardness distribution of the hardened layer of the steel component of the present disclosure to the above-described ranges are explained.

[0030] Thickness of nitride compound layer containing nitride compound: 5.0 μm to 30.0 μm

[0031] The nitride compound layer (hereafter may be referred to as “compound layer”) has an extremely high hardness and contributes to improving the surface fatigue resistance of the steel component. A too thin nitride compound layer leads to early exposure of the steel substrate of the steel component due to wearing, which decreases the fatigue strength improvement effect. Therefore, the thickness of the nitride compound layer is set to 5.0 μm or more. It is preferably 6.0 μm or more and more preferably 10.0 μm or more.

[0032] On the other hand, a too thick nitride compound layer renders it difficult to suppress the formation of the porous layer described below. Therefore, the thickness of the nitride compound layer is set to 30.0 μm or less. It is preferably 25.0 μm or less.

[0033] Thickness of porous layer: 40.0% or less of the thickness of nitride compound layer and 3.0 μm or less

[0034] The porous layer is an aggregate of minute pores inevitably formed in the outermost surface of the compound layer by nitrocarburizing. Since the presence of the porous layer adversely affects the fatigue strength, it is desirable to make it as thin as possible. When the thickness of the porous layer exceeds 3.0 μm or exceeds 40.0% of the thickness of the nitride compound layer, the expected improvement in fatigue resistance due to the formation of the nitride compound layer cannot be sufficiently achieved. Therefore, the thickness of the porous layer needs to be 40.0% or less of the thickness of the nitride compound layer and 3.0 μm or less. It may be even 0.

[0035] The thickness of the porous layer in the present disclosure is measured with the method described in the EXAMPLES section below.

[0036] Depth of hardened layer: a hardness of HV600 or more at a position of 50 μm inward from the component surface, a hardness of HV400 or more at a position from the component surface to the component inside of 400 μm, and a hardness of HV250 or more at a position from the component surface to the component inside of 600 μm.

[0037] It is known that there is a correlation between material hardness and fatigue strength (see, for example, “MatNavi, JIS Steel for Machine Structural Use, Mechanical Properties and Fatigue Properties of Chromium Steel and Chromium Molybdenum Steel”). In other words, a desired fatigue strength can be obtained if there is sufficient hardness, regardless of the composition.

[0038] When a steel component has a slipping contact, two types of forces are applied to the steel component. One is the shear stress due to the tangential forces, which is maximized at the surface. The other is the shear stress due to the perpendicular reaction force, which is maximized at deeper positions. The hardness distribution is set to exhibit excellent fatigue resistance against these two types of forces.

[0039] In particular, the shear stress due to the perpendicular reaction force tends to be a problem for nitrocarburized steel with a thin hardened layer. The shear stress distribution due to the perpendicular reaction force when the teeth, cylinders, and spheres of a gear are brought into contact with each other can be expressed by the following equation. As used herein, z is the depth, P(z) is the shear stress at depth z, Pmax is the maximum contact stress, and b is the osculating ellipse minor axis length.

[00001]$P\left(z\right)=P_{\max }\times \left(\frac{z}{b}-\frac{z^2}{b\times \sqrt{b^2+z^2}}\right)$

Although it depends on the shape of the steel component and the load applied thereon, the shear stress has a maximum value at a depth of 400 μm in many cases, which may serve as an initiation point of fracture. Therefore, the hardness distribution is set as described above.

[0040] Although the nitride compound layer is formed after the subjection of the steel to nitrocarburizing treatment, N diffuses inward from the compound layer due to the nitrocarburizing treatment. As a result, this N diffusion layer becomes a hardened layer. By adjusting the N concentration by diffusion, the hardness of the hardened layer can be adjusted as described above.

[0041] The steel component of the present disclosure is particularly preferably applied to a toothed component such as a gear, and it is particularly preferable that the nitride compound layer and the hardened layer be formed in a tooth portion of the toothed component. The tooth portion of a toothed component such as a gear is a portion that has a slipping contact and that requires excellent surface fatigue strength. When the nitride compound layer and the hardened layer are formed in the tooth portion, the durability as a toothed component can be ensured.

[0042] Not only for a toothed component but also for steel components with a portion having a slipping contact, the surface fatigue of this portion is important for ensuring the durability of the component. Therefore, by forming the nitride compound layer and the hardened phase in such a portion, the effect of improving the durability can be obtained. For this reason, the steel component of the present disclosure is not limited to a toothed component.

[0043] The following describes a method of manufacturing the steel component of the present disclosure.

[0044] FIG. 2 illustrates typical manufacturing processes for manufacturing a nitrocarburized component using steel for nitrocarburizing (steel bar). In the figure, S1 is a process of manufacturing a steel bar (steel for nitrocarburizing) which is a raw material, S2 is a process of transporting the steel bar, and S3 is a process of manufacturing a component (nitrocarburized component).

[0045] First, in the steel bar manufacturing process (S1), a steel ingot is subjected to hot rolling and/or hot forging to obtain a steel bar, and after quality inspection, the steel bar is shipped. After being transported (S2), the steel bar is cut into predetermined dimensions, subjected to hot forging or cold forging, formed into a desired shape (such as the shape of a gear product or a shaft product) by cutting work such as drill boring or lathe turning as necessary, and then subjected to nitrocarburizing treatment to obtain a product in the nitrocarburized component finish process (S3).

[0046] Alternatively, the hot rolled material may be directly subjected to cutting work such as lathe turning or drill boring to obtain a desired shape and then subjected to nitrocarburizing treatment to obtain a product. In the case of hot forging, there are cases where cold straightening is performed after hot forging.

[0047] Next, the obtained rolled material or forged material is subjected to cutting work to obtain the shape of the component, and then subjected to nitrocarburizing treatment. To obtain the depth of a hardened layer as described above, it is necessary to set the nitrocarburizing temperature to 550° C. to 590° C. and the nitrocarburizing time to at least 5 hours. On the other hand, when the nitrocarburizing time is such a long time, the compound layer and the porous layer grow excessively depending on the nitrocarburizing conditions, which causes a decrease in fatigue strength. Therefore, it is necessary to keep the nitriding potential low during the nitrocarburizing. As the nitriding potential of the atmosphere during the nitrocarburizing decreases, the thickness of the porous layer decreases. Therefore, it is necessary to obtain the relationship between the nitriding potential and the porous layer thickness in advance for each standard or component of the steel used as the raw material and to adopt a nitriding potential that can achieve the porous layer thickness specified in the present disclosure.

[0048] In the nitrocarburizing treatment, N and C are simultaneously immersed into the steel to form a nitride compound layer with solute C dissolved therein, and N is diffused into the steel substrate. Therefore, the nitrocarburizing treatment may be performed in a mixed atmosphere of nitrogenous gas such as NH.sub.3 and N.sub.2 and carburizing gas such as CO.sub.2 and CO, such as an atmosphere of NH.sub.3:N.sub.2:CO.sub.2=50:45:5.

Examples

[0049] The following describes examples of the present disclosure in detail.

[0050] Steels (steel sample IDs A to E) having the compositions listed in Table 1 were made into cast steels, each having a cross section of 300 mm×400 mm, using a continuous casting machine. Each cast steel was subjected to soaking at 1250° C. for 30 minutes and then hot rolled into a billet having a rectangular cross section with a side of 140 mm. Further, the billet was subjected to hot rolling to obtain an 80 mmφ steel bar (raw material as hot rolled). The steel bar was held at 1200° C. for one hour and then subjected to hot forging to obtain a smaller 35 mmφ steel bar.

TABLE-US-00001 TABLE 1 (mass %) Steel C Si Mn P S Cr Others A 0.230 0.20 3.13 0.030 0.101 3.01 — B 0.121 0.30 1.45 0.012 0.050 1.37 — C 0.195 0.25 2.42 0.012 0.040 1.98 V: 0.2 D 0.061 0.02 1.79 0.015 0.042 1.19 Nb: 0.12 E 0.195 0.19 0.82 0.010 0.059 1.39 — F 1.011 0.18 0.25 0.020 0.010 1.40 — G 0.530 0.20 0.80 0.015 0.013 1.09 V: 0.2 H 0.060 0.53 0.80 0.031 0.015 17.20 —

[0051] Further, a roller-pitching test piece as illustrated in FIG. 1 was collected from the hot forged material parallel to the longitudinal direction, and the test piece was subjected to nitrocarburizing treatment. To obtain the desired compound layer and hardness distribution, the nitrocarburizing temperature, time, and nitriding potential were adjusted appropriately.

[0052] The materials thus obtained by being subjected to nitrocarburizing treatment was further subjected to hardness measurement, compound layer/porous layer thickness measurement, and fatigue resistance evaluation. The results of these measurements and evaluations are listed in Table 2 (Tables 2-1, 2-2, and 2-3).

[0053] The hardness was measured at each position of 50 μm, 400 μm, and 600 μm from the surface of a cross section of the nitrocarburized material. The hardness was measured using a Vickers hardness meter at six points with a test load of 2.94 N (300 gf) in accordance with JIS Z2244, and the average value of the results was determined.

[0054] The thickness of the compound layer and the porous layer was measured on a cross section of the nitrocarburized material. The steel was corroded with 3% nital solution, and the surface layer was observed using an optical microscopy for three observation fields at 1000 magnifications to identify the uncorroded compound layer. The thickness of the compound layer was measured as the value of the maximum compound layer thickness in the three observation fields. Regarding the porous layer, the thickness of the thickest location of the aggregate of minute pores existing continuously from the surface in the depth direction was measured in each of the three observation fields, and the maximum value among the results was taken as the thickness of the porous layer.

[0055] The fatigue resistance evaluation was performed using a roller pitching test piece (see FIG. 1) that had been subjected to nitrocarburizing treatment but had not been subjected to any of microstructure observation, hardness measurement and precipitate observation, and the fatigue life of the roller pitching test piece was measured by RPT-402 manufactured by Nikko Create. The roller pitching test conditions were a maximum contact stress of 2600 MPa, a slip rate of 40%, using gear oil (BESCO transaxle) as lubricating oil, and an oil temperature of 80° C. The rotational speed during the test was 1500 rpm. A quenched and tempered SUJ2 with a crowning R of 150 mm was used as a large roller in contact with the transporting surface.

TABLE-US-00002 TABLE 2-1 Life at maximum Compound Porous Porous layer contact layer layer thickness/ 50 μm 400 μm 600 μm stress of thickness thickness compound layer hardness hardness hardness 2600 MPa No. Steel (μm) (μm) thickness × 100 HV HV HV (times) Classification 1 A 21.1 1.8 8.5 758 551 312 2.3 × 10.sup.7 Example 2 A 10.1 2.8 27.7 760 551 305 4.3 × 10.sup.7 3 A 5.2 1.3 25.0 736 555 310 2.5 × 10.sup.7 4 A 28.2 2.7 9.6 758 548 313 3.1 × 10.sup.7 5 B 5.6 2.1 37.5 758 543 314 3.1 × 10.sup.7 6 B 15.5 2.4 15.5 751 545 311 4.1 × 10.sup.7 7 B 8.2 2.7 32.9 758 545 318 2.2 × 10.sup.7 8 B 29.3 2.7 9.2 754 550 310 2.5 × 10.sup.7 9 C 18.5 2.3 12.4 758 550 317 3.2 × 10.sup.7 10 C 9.8 2.9 29.6 752 547 314 4.1 × 10.sup.7 11 C 6.3 2.1 33.3 713 532 333 2.7 × 10.sup.7 12 C 26.9 2.8 10.4 763 548 306 2.3 × 10.sup.7 13 D 27.3 1.5 5.5 757 550 320 2.4 × 10.sup.7 14 D 6.3 2.1 33.3 756 551 309 2.2 × 10.sup.7 15 D 12.3 2.9 23.6 755 546 318 2.5 × 10.sup.7 16 D 27.9 2.6 9.3 763 548 316 2.3 × 10.sup.7 17 E 7.1 1.3 18.3 746 543 310 2.1 × 10.sup.7 18 E 12.1 2.5 20.7 756 550 312 2.1 × 10.sup.7 19 E 20.2 2.8 13.9 756 549 314 3.1 × 10.sup.7 20 E 29.9 2.9 9.7 758 556 314 3.0 × 10.sup.7 21 F 13.2 1.3 9.8 615 413 264 2.1 × 10.sup.7 22 F 6.1 1.1 18.0 666 415 261 2.3 × 10.sup.7 23 F 16.3 2.9 17.8 676 446 256 3.2 × 10.sup.7 24 F 28.2 2.9 10.3 653 451 259 3.2 × 10.sup.7 25 G 5.3 1.0 18.9 643 434 280 2.4 × 10.sup.7 26 G 16.3 2.5 15.3 637 427 273 2.5 × 10.sup.7 27 G 18.2 2.2 12.1 663 445 272 2.1 × 10.sup.7 28 G 28.1 2.7 9.6 643 431 291 3.0 × 10.sup.7 29 H 5.5 1.2 21.8 648 419 253 2.1 × 10.sup.7 30 H 7.5 2.1 28.0 643 479 271 2.3 × 10.sup.7 31 H 16.2 2.8 17.3 652 510 261 2.7 × 10.sup.7 32 H 29.1 2.7 9.3 730 558 251 3.0 × 10.sup.7

TABLE-US-00003 TABLE 2-2 Life at maximum Compound Porous Porous layer contact layer layer thickness/ 50 μm 400 μm 600 μm stress of thickness thickness compound layer hardness hardness hardness 2600 MPa No. Steel (μm) (μm) thickness × 100 HV HV HV (times) Classification 33 A 3.3 1.1 33.3 751 549 313 1.1 × 10.sup.7 Comparative 34 B 4.6 0.5 10.9 698 453 260 8.8 × 10.sup.6 Example 35 C 4.3 1.5 34.9 751 548 313 1.3 × 10.sup.7 36 D 4.4 1.6 36.4 630 401 251 6.2 × 10.sup.6 37 E 2.4 0.8 33.3 758 546 311 1.1 × 10.sup.7 38 F 4.1 1.3 31.7 632 421 275 2.1 × 10.sup.6 39 G 3.8 1.1 28.9 663 422 275 8.3 × 10.sup.6 40 H 4.7 1.1 23.4 673 434 263 6.2 × 10.sup.6 41 A 26.8  4.0 14.9 725 506 398 1.1 × 10.sup.6 Comparative 42 A 5.2 2.3 44.2 763 547 313 2.5 × 10.sup.6 Example 43 A 9.2 3.3 35.9 755 547 307 2.7 × 10.sup.6 44 A 20.4  3.4 16.7 760 547 306 1.4 × 10.sup.6 45 B 26.9  8.2 30.5 765 545 311 1.5 × 10.sup.6 46 B 5.6 2.5 44.6 755 550 314 1.7 × 10.sup.6 47 B 10.0  3.6 36.0 760 555 309 2.1 × 10.sup.6 48 B 28.8  3.2 11.1 760 553 316 2.6 × 10.sup.6 49 C 20.8  6.9 33.2 760 557 305 2.6 × 10.sup.6 50 C 5.5 2.6 47.3 760 549 309 2.2 × 10.sup.6 51 C 11.1  3.3 29.7 761 546 313 1.4 × 10.sup.6 52 C 28.8  3.5 12.2 758 544 314 1.7 × 10.sup.6 53 D 28.4  9.2 32.4 754 556 307 1.6 × 10.sup.6 54 D 5.1 2.4 47.1 743 555 310 2.4 × 10.sup.6 55 D 10.0  3.4 34.0 766 547 312 1.2 × 10.sup.6 56 D 26.7  3.1 11.6 760 552 309 2.6 × 10.sup.6 57 E 29.3  5.5 18.8 752 547 310 2.3 × 10.sup.6 58 E 5.2 2.3 44.2 744 543 296 2.1 × 10.sup.6 59 E 9.1 3.4 37.4 764 550 306 2.6 × 10.sup.6 60 E 28.2  3.5 12.4 757 550 313 1.4 × 10.sup.6 61 F 5.2 2.1 40.4 634 473 253 1.4 × 10.sup.6 62 F 10.3  3.1 30.1 633 492 267 1.5 × 10.sup.6 63 F 12.4  4.2 33.9 683 443 251 1.7 × 10.sup.6 64 F 23.8  4.3 18.1 688 487 267 2.1 × 10.sup.6 65 G 5.5 2.3 41.8 676 421 301 1.2 × 10.sup.6 66 G 11.3  3.3 29.2 656 423 275 2.6 × 10.sup.6 67 G 12.7  3.8 29.9 653 479 310 2.3 × 10.sup.6 68 G 26.8  4.2 15.7 637 453 333 2.1 × 10.sup.6 69 H 5.5 2.4 43.6 679 446 255 1.5 × 10.sup.6 70 H 10.5  3.4 32.4 664 475 253 1.7 × 10.sup.6 71 H 13.1  3.6 27.5 651 452 257 2.1 × 10.sup.6 72 H 28.1  4.5 16.0 677 463 275 2.6 × 10.sup.6 *1 Underline indicates outside the scope of application.

TABLE-US-00004 TABLE 2-3 Life at Porous layer maximum Compound Porous thickness/ contact layer layer compound 50 μm 400 μm 600 μm stress of thickness thickness layer hardness hardness hardness 2600 MPa No. Steel (μm) (μm) thickness × 100 HV HV HV (times) Classification 73 A 10.2 1.3 12.7 594 403 267 2.0 × 10.sup.6 Comparative 74 B 8.7 1.5 17.2 569 407 272 2.1 × 10.sup.6 Example 75 C 11.1 1.6 14.4 573 401 251 9.3 × 10.sup.5 76 D 13.1 1.6 12.2 589 423 263 1.5 × 10.sup.6 77 E 5.9 1.7 28.8 591 429 259 2.3 × 10.sup.6 78 F 21.3 2.2 10.3 549 443 256 2.1 × 10.sup.6 79 G 22.3 2.3 10.3 567 423 257 8.3 × 10.sup.6 80 H 27.2 2.5  9.2 593 415 254 6.2 × 10.sup.6 81 A 15.2 1.9 12.5 634 385 253 8.3 × 10.sup.5 Comparative 82 B 16.8 2.3 13.7 630 376 261 5.9 × 10.sup.5 Example 83 C 18.2 2.4 13.2 666 391 259 7.9 × 10.sup.5 84 D 18.1 1.5  8.3 627 365 267 1.1 × 10.sup.6 85 E 20.5 2.3 11.2 639 379 264 5.1 × 10.sup.5 86 F 5.5 1.2 21.8 666 395 264 3.1 × 10.sup.6 87 G 15.3 2.8 18.3 658 391 255 8.6 × 10.sup.5 88 H 20.3 2.2 10.8 613 389 253 4.2 × 10.sup.6 89 A 17.5 2.2 12.6 608 403 240 5.1 × 10.sup.6 Comparative 90 B 16.2 2.4 14.8 631 404 249 2.7 × 10.sup.6 Example 91 C 15.5 2.6 16.8 620 409 225 8.1 × 10.sup.6 92 D 14.7 2.5 17.0 613 413 237 4.7 × 10.sup.6 93 E 15.8 2.1 13.3 601 415 242 2.5 × 10.sup.6 94 F 12.3 2.0 16.3 665 442 230 5.1 × 10.sup.5 95 G 13.4 2.2 16.4 671 423 238 4.3 × 10.sup.6 96 H 11.0 2.2 20.0 623 411 246 7.7 × 10.sup.5 97 A 10.5 5.2 49.5 723 201 199 8.9 × 10.sup.5 Conventional 98 B 11.3 2.9 25.7 713 175 176 8.5 × 10.sup.4 Example 99 C 10.7 4.3 40.2 765 211 220 5.1 × 10.sup.5 100 D 9.8 2.5 25.5 722 243 247 1.1 × 10.sup.6 101 E 17.1 4.5 26.3 645 194 194 3.9 × 10.sup.4 102 F 12.3 3.4 27.6 710 235 185 1.2 × 10.sup.6 103 G 11.2 2.5 22.3 723 264 173 1.1 × 10.sup.6 104 H 10.1 2.7 26.7 723 222 180 3.3 × 10.sup.6 *1 Underline indicates outside the scope of application.

[0056] All Examples had a hardness distribution from the surface in the depth direction where the hardness at depths less than 400 μm from the surface was greater than the hardness at a depth of 400 μm from the surface, and the hardness at depths less than 600 μm was greater than the hardness at a depth of 600 μm from the surface. Therefore, it can be confirmed that the hardness from the surface to depths of 400 μm is equal to or greater than the hardness at a depth of 400 μm from the surface, and the hardness from the surface to depths of 600 μm is equal to or greater than the hardness at a depth of 600 μm from the surface.