US20060130935A1

US20060130935A1 - Carburized component and method of manufacturing the same

Info

Publication number: US20060130935A1
Application number: US11/296,566
Authority: US
Inventors: Atsushi Hattori; Takashi Kano; Tomoko Serikawa; Koki Mizuno
Original assignee: Honda Motor Co Ltd
Current assignee: Honda Motor Co Ltd; Daido Steel Co Ltd
Priority date: 2004-12-10
Filing date: 2005-12-08
Publication date: 2006-06-22
Also published as: DE102005058903A1; JP2006161141A; JP4188307B2; DE102005058903B4

Abstract

This invention aims to provide a carburized component realizing a larger strength for power transmission components such as gears, and a method of manufacturing the same. The carburized component of this invention, aimed at realizing the object, consists essentially of, in % by mass and both ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%, Cr: 2.0-5.5%, and the balance of Fe and inevitable impurities; has a mean C concentration over the range from the surface of the steel to a depth of 0.2 mm after vacuum carburization of 1.2% or more and 3.0% or less, and has a ratio of a carbide area over the range from the surface to a depth of 50 μm of 15% or more and 60% or less, has the carbide precipitated in a finely dispersed manner so that the carbide having a grain size of 10 μm or less accounts for 90% or more of the entire portion, and has a depth of a grain boundary oxide layer below the surface of 1 μm or less.

Description

FIELD OF THE INVENTION

This invention relates to a carburized component and a method of manufacturing the same.

BACKGROUND ART

Gears used as power transmission components for automobiles and so forth are components suffering from dedendum fractures which occur at the dedendum where bending stress happens, and from slip-induced fracture (pitting) which occurs in the vicinity of the pitch point. A technique of carburizing the surface of the component has, therefore, widely been used for the purpose of fulfilling characteristics enough for withstanding harsh conditions, and further improvement has been made by combining various materials and heat treatments.
Particularly in recent years, a successful development has been made on a material capable of suppressing growth of grain boundary oxide layer and abnormally carburized layer during carburization, which are understood as being harmful to dedendum fracture. Another achievement has been made on improvement in the strength typically by shot peening.
On the other hand, pitting has also extensively been investigated, and it has been found out that prevention of softening of the material is effective to improve the strength. Gears cause slippage on the tooth surface thereof, and the repetitive contact generates heat at the portion just under the tooth surface. Temperature in this state is known to fall in the range of about 200 to 300° C., and the heat generated herein supposedly softens the material and consequently results in pitting fracture. It is therefore believed that prevention of softening in a temperature range of about 200 to 300° C. is effective for improving the pitting fracture, and development has been made on materials added with Si, Cr, Mo and so forth as alloy elements excellent in the softening resistance in this temperature range.
[Patent Document] Japanese Laid-Open Patent Publication “Tokkaihei” No. 6-158266
The gear has, however, has been demanded to have a larger hardness with the recent increases in the output of automobiles and so forth, but the present situation is that the above-described material is insufficient for fulfilling the requirements.
This invention was conceived after considering the above-described situation, and an object thereof is to provide a carburized component realizing higher strength for power transmission components such as gears, and a method of fabricating said components.

SUMMARY OF THE INVENTION

Aiming at solving the aforementioned problems, a carburized component consisting essentially of, in percentages by mass and both ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%, Cr: 2.0-5.5%, and the balance between Fe and inevitable impurities;

- has a mean C concentration over the range from the surface of steel to a depth of 0.2 mm after vacuum carburization of 1.2% or more and 3.0% or less, has a ratio of carbide area over the range from the surface to a depth of 50 μm of 15% or more and 60% or less, has the carbide precipitated in a finely dispersed manner so that the carbide having a grain size of 10 μm or less accounts for 90% of the entire portion, and has a depth of a grain boundary oxide layer below the surface of 1 μm or less.

It is also allowable to further add either or both of Mo: 0.2 to 1.0% and V: 0.2 to 1.0%.
This invention has basic features as described below. That is, a large amount of fine carbide grains are allowed to precipitate in the surficial portion of the component by high-concentration vacuum carburization, and to substantially exclude the surficial grain boundary oxide layer, to thereby raise the surface hardness and strength. In addition, the temper softening resistance in the temperature range from about 200 to 300° C. is enhanced by introducing a large amount of Si, which is realized by the vacuum carburization, and thereby a desirable level of surface fatigue strength can be obtained. These features can be obtained only under the appropriately-adjusted ingredients and conditions as detailed below.
C: 0.10 to 0.30%
C is an essential element for ensuring a necessary level of strength for the component, and is necessary to contain an amount of 0.10% or more. On the other hand, an excessively large content thereof increases the hardness of the material, thus degrading the machinability, and thereby making the machining of the component difficult. The upper limit is therefore adjusted to 0.30%.
Si: 0.80 to 1.50%
Si is an element to be contained as a deoxidizing element acting in the process of melting and plays an important role in this invention. The element dissolves into the solid matrix to thereby raise the temper softening resistance, so that a high level of surface fatigue strength can be obtained. The element can also suppress growth of coarse carbide grains, because it shows only a small solid solubility into the carbide and raises the Si concentration in the base metal. Moreover, under precipitation of a large amount of carbide, Si showing only a small solid solubility into the carbide concentrates in the matrix, and further improves the temper softening resistance of the matrix. The element is necessarily contained to an amount of 0.80% or more in order to obtain this effect. On the other hand, an excessive content of the element inhibits precipitation and the carburization surface reaction of the carbide which thereby distinctively degrades the carburization property, and also degrades the ductility, which thereby makes cracking more likely to occur in the process of plastic working. The upper limit of the content is therefore limited to 1.50%.
Si is an element promoting oxidation of the grain boundary in the process of general gas carburization, and the grain boundary oxidation layer is causative of lowering the impact strength and fatigue strength of dedendum. The gas carburization therefore cannot add a large amount of Si, whereas the vacuum carburization as described in the above can clear the problem of grain boundary oxidation, and make it possible to obtain a high-Si-content component.
Mn: 0.30 to 1.20%
Mn is an element to be contained as a deoxidizing element acting in the process of melting, and has an effect of improving the hardening property, so that it is necessary to contain an amount of 0.30% or more. In this invention, elements having an effect of improving the hardening property, such as Cr, are to be concomitantly contained, wherein the elements such as Cr, capable of forming the carbide, may sometimes result in only an insufficient hardening property even under a raised Cr content or the like, depending on carbide content. It is therefore effective to adjust the Mn content in order to obtain a necessary level of hardening property. On the other hand, an excessive content degrades the machinability due to an increase in the hardness of the material, thus the upper limit is adjusted to 1.20%.
Cr: 2.0 to 5.5%
Cr is an element playing an important role in this invention. This is necessary to contain an amount of 2.0% or more, as a carbide-forming element and as an element improving the hardening property. On the other hand, an excessive content of the element degrades the machinability due to increased hardness of the material, and makes a network-structured carbide more likely to be generated in the grain boundary. The upper limit of the content is therefore limited to 5.5%.
Mo: 0.2 to 1.0%
Mo binds with C, similarly to Cr, to produce the carbide, and has an effect of improving the pitting strength by raising the softening resistance over the temperature range from 200° C. to 300° C. The element is preferably contained to an amount of 0.2% or more, for the purpose of obtaining these effects. On the other hand, an excessive content of the element degrades the machinability due to an increase in hardness of the material, and increases the material cost. The upper limit of the content is, therefore, preferably limited to 1.0%.
V: 0.2 to 1.0%
V binds with C, similarly to Cr and Mo, to produce the carbide, and has an effect of improving the pitting strength by raising the softening resistance, through production of an MC-type carbide. The element is preferably contained to an amount of 0.2% or more, for the purpose of obtaining these effects. On the other hand, an excessive content of the element degrades the machinability due to an increase in hardness of the material. The upper limit of the content is, therefore, preferably limited to 1.0%.
Carburization: Vacuum Carburization (at 1,000 Pa or Below)
The carburized component of this invention is subjected to vacuum carburization. The vacuum carburization makes it possible to decrease the growth of the grain boundary oxide layer, and is therefore successful in raising the strength of the carburized component.
As described in the above, Si is added as an essential ingredient. Si is an element promoting the grain boundary oxidation in the process of the general gas carburization, and such grain boundary oxidation is causative of reducing the impact strength and fatigue strength of the dedendum. It is, therefore, extremely difficult for the general gas carburization to achieve a large Si content. Whereas the vacuum carburization can, however, suppress formation of the grain boundary oxide layer, and can readily realize a high Si content.
Depth of Grain Boundary Oxide Layer: 1 μm or Less
The grain boundary oxide layer causes lowering in the fatigue strength and anti-pitting strength, wherein the degree of the lowering becomes larger as the depth increases. For the carburized component of this invention, the depth of grain boundary layer from the surface of the steel after the vacuum carburization is adjusted to 1 μm or less.
Mean C Concentration Up to Depth of 0.2 mm from the Surface: 1.2% or more and 3.0% or Less
The general carburization is normally carried out as an eutectic carburization of the surface of steel, targeted at an eutectic C content of 0.8%. In contrast, this invention is aimed at improving the anti-pitting property through precipitation of the carbide in the surficial layer of the steel to thereby enhance the softening resistance, so that it is necessary to contain C to an amount of the eutectic C content (0.8%) or more. In addition, the surface fatigue strength cannot be improved even if the carbide is allowed to precipitate, unless the carbide is obtained with a content necessary for improving the softening resistance, so that it is also necessary to make C contained to an amount sufficient enough for the improvement.
From these points of view, the mean C concentration over the range from the surface of steel to a depth of 0.2 mm (also referred to as surface C concentration, hereinafter) is adjusted to 1.2% or more. The reason why the range is defined from the surface of the steel to a depth of 0.2 mm is that the hardness in such range is important from the viewpoint of the pitting resistance. On the other hand, an excessive content results in production of large carbide grains, and causes insufficient hardening property of the base material, thereby degrading the strength. The upper limit of the surface C concentration is therefore limited to 3.0%.
Ratio of Carbide Area Over the Range from the Surface to a Depth of 50 μm: 15% or More and 60% or Less
Precipitation of the carbide raises the surface hardness, improves the softening resistance over the temperature range from 200° C. to 300° C., and improves the anti-pitting resistance. A ratio of carbide area over the range from the surface to a depth of 50 μm of less than 15%, however, cannot fully improve the softening resistance, and cannot obtain a sufficient effect of improving the strength. On the other hand, the ratio of carbide area exceeding 60% can improve the softening resistance, but lowers the surface fatigue and bending fatigue strength, because the carbide of a larger grain size is more likely to precipitate along the grain boundary in a network manner. An exemplary observation of the obtained carbide is shown in FIG. 4.
Carbide Precipitated in a Finely Dispersed Manner so that the Carbide Having a Grain Size of 10 μm or Less Accounts for 90% or More of the Entire Portion.
The carbide is a hard grain, and may serve as a starting point of fatigue fracture, similarly to non-metallic inclusions such as Al oxide and Ti nitride. A smaller carbide is therefore more preferable, wherein the grain size of which is necessarily controlled to as small as 10 μm or below, so as not to allow the carbide to exist as the starting point of fatigue fracture. It is therefore controlled so that the carbide precipitates in a finely dispersed manner, so that the carbide having a grain size of 10 μm or less accounts for 90% or more of the entire portion. An exemplary observation of the obtained carbide is shown in FIG. 4.
Aiming at manufacturing the above-described carburized component, a method of manufacturing a carburized component of this invention subjects the steel containing the above-described steel ingredients to a primary carburization at a temperature of Acm or above, then rapidly cools the steel to as low as point A1 or below, and then subjects the steel to a secondary carburization at a temperature of point A1 or above and Acm or below. More specifically, as shown in FIGS. 1A and 1B, the primary carburization is carried out so as not to precipitate the carbide, at a temperature of as high as Acm or above, allowing a large solid solubility limit of C and allowing no carbide to precipitate (between points “a” and “b”). Next, the steel is rapidly cooled so as to dissolve C into a solid super-saturated manner (between points “b” and “c”). Thereafter, the steel is again heated to as high as point A1 or above, to thereby allow fine carbide nuclei to uniformly precipitate from the base material super-saturated with C (between points “d” and “e”, see the upper drawing in FIG. 2), and the steel is further subjected to a secondary carburization so as to grow the nuclei (between points “e” and “f”, see the lower drawing in FIG. 2). Such multi-stage carburization can realize a high-C-concentration carburization with a controlled fine dispersion of the carbide, without allowing the network-structured carbide to precipitate. In contrast to this, as shown in FIG. 3, the carburization carried out to as far as the high-C-concentration region before point Acm makes the network-structured coarse carbide very likely to produce. The carburization therein is carried out by vacuum carburization (at 1,000 Pa or below) as described in the above.
It is also allowable to subject the steel after the secondary carburization to peening if necessary, to thereby further improve the strength. Shot peening (S/P) and water jet peening (W/J), for example, are applicable to the peening.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are drawings explaining carburization involved in the method of manufacturing a carburized component of this invention;
FIG. 2 shows a schematic sectional view and a drawing of an observed section of steel during the carburization shown in FIG. 1;
FIG. 3 shows a drawing explaining an exemplary carburization different from this invention, and a drawing of an observed section; and
FIG. 4 is a drawing of an observed section of the carburized component of this invention.

EXAMPLES

The following paragraphs will describe tests carried out for confirming the effects of this invention.
First, each of the steels having chemical compositions listed in Table 1 was melted in a 150-kg high-frequency vacuum induction furnace. The obtained steel ingot was rolled or hot forged so as to produce a 90-mm-diameter round rod, or further hot-forged, if necessary, so as to obtain steel bar shape having a diameter of 22 to 32 mm, which was used as a test piece.

In the compositions of comparative examples listed in Table 1, those departing from the compositional ranges specified by this invention are indicated by a downward arrow (↓), for those short of the lower limits, an upward arrow (⇑), for those exceeding the upper limits.

TABLE 1


C	Si	Mn	Cr	Mo	V	Remarks

Example 1	0.18	0.98	0.63	2.39	0.00	0.00
Example 2	0.18	0.80	0.50	2.66	0.00	0.00
Example 3	0.19	1.02	0.52	2.52	0.00	0.00
Example 4	0.18	0.97	0.55	3.22	0.00	0.00
Example 5	0.18	1.48	0.55	2.58	0.00	0.00
Example 6	0.19	1.05	0.55	2.12	0.00	0.00
Example 7	0.19	1.08	0.34	2.49	0.00	0.00
Example 8	0.20	1.12	0.35	4.99	0.00	0.00
Example 9	0.19	0.97	0.52	2.50	0.60	0.00
Example 10	0.18	0.97	0.52	2.66	0.00	0.30
Comparative	↓ 0.08	0.96	0.62	2.45	0.00	0.00
Example 1
Comparative	↑ 0.37	0.97	0.61	2.42	0.00	0.00
Example 2
Comparative	0.18	↓ 0.40	0.49	2.92	0.00	0.00
Example 3
Comparative	0.19	↑ 2.10	0.51	2.44	0.00	0.00
Example 4
Comparative	0.18	1.10	↓ 0.10	2.16	0.00	0.00
Example 5
Comparative	0.19	0.98	↑ 1.74	2.04	0.00	0.00
Example 6
Comparative	0.20	1.02	0.32	↓ 1.10	0.00	0.00
Example 7
Comparative	0.20	1.13	0.32	↑ 6.02	0.00	0.00
Example 8
Comparative	0.20	1.00	0.55	2.54	↑ 1.50	0.00
Example 9
Comparative	0.19	0.97	0.53	2.71	0.00	↑ 1.50
Example 10
Comparative	0.20	0.22	0.89	1.12	0.00	0.00	JIS-SCR420
Example 11

The obtained test pieces were subjected to the following evaluations.
(1) Evaluation of Manufacturability
The manufacturability was evaluated by measuring the hardness after annealing.
A round test piece rod of 32 mm in diameter and 100 mm in length was subjected to annealing at 920° C. for 1 hour, further annealed at 760° C. for 5 hours, and the hardness at the position of R/2 on the transverse section was measured. The measurement of hardness conforms to JIS Z 2245 (B-scale), with a target value of HRB90 or smaller
(2) Evaluation of Basic Characteristics of Carburization
(2-1) Method of Carburization
A round test piece rod 10 mm in diameter and 100 mm in length was fabricated, as a test piece for carburization property, from a forged steel bar 22 mm in diameter. The carburization was carried out in a vacuum carburization furnace, using propane as the carburization gas, wherein the surface C concentration was controlled by adjusting flow rate of propane gas, diffusion time, and carburization temperature. The carburization was carried out at two levels of conditions so as to achieve a surface C concentration of 1.5% and 2.5%, respectively.
As for Example 3, the carburization was carried over the surface range of C concentration from 0.8 to 3.2%, in order to investigate influences of the surface C concentration.
The carburization conditions are as follows.
Primary Carburization
The test piece was carburized at 1,100° C. for 70 minutes so as to adjust the C concentration to the topmost surface to about 1.2%, and then rapidly cooled by cooling gas to a temperature range as low as 500° C. or below, to thereby allow C to intrude into the steel to at a high concentration range so as not to be causative of precipitation of the carbide.
Secondary Carburization
The test piece was subjected to the precipitation treatment by keeping it in the temperature range from 850° C. to 900° C., depending on the target carburization concentration, further carburized in the temperature range from 850° C. to 1,000° C. for 60 to 90 minutes depending on the target C concentration, and was hardened by immersing it into an oil bath kept at 130° C. After the hardening, the test place was annealed at 180° C. for 120 minutes.
(2-2) Items of Evaluation
The following paragraphs will describe items of evaluation. Results of the evaluation are listed in Table 2. Results of Example 3 obtained by varying the surface C concentration are listed in Table 3.
Surface C Concentration
After the carburization, C concentration was measured using a grinding chip obtained from the surface to a depth of 0.2 mm of the treated test piece.
Ratio of Carbide Area
The transverse section of the carburized and annealed test piece rod was polished, corroded with picral, to a portion of a depth of 50 μm from the topmost surface was photographed under a SEM (at a 3,000× magnification of observation), and the ratio of area was measured by image analysis.
Size of Carbide
The test piece was observed under the same conditions as described in the above, and the area ratio occupied by the carbide grain sized 10 μm or less was measured.
Presence or Absence of Network-Structured Carbide
The test piece was observed under the same conditions as those described in the above, and presence or absence of the network-structured carbide was investigated.
Presence or Absence of Incompletely-Hardened Structure
The transverse section of the carburized, annealed test piece rod was polished, corroded with nital, to a portion of a depth of 50 μm from the topmost surface was photographed under an optical microscope, and presence or absence of the incompletely-hardened structure was investigated.
Depth of Grain Boundary Oxide Layer
The transverse section of the carburized and annealed rod test piece was polished, the resultant surface in an uncorroded state was observed under an optical microscope, and the depth of the layer appearing as black along the grain boundary at the topmost surface was measured.
Temper Softening Resistance

The carburized and annealed test piece rod was further annealed at 300° C. for 180 minutes, the transverse section was polished, and the hardness at a depth of 50 μm from the topmost surface was measured. The hardness herein conforms to JIS Z 2244 (Hv0.3), wherein a value of Hv750 or above is considered as an index ensuring a sufficient effect of improving the strength (≧30%: in comparison with SCR420 gas eutectic carburized steel).

	TABLE 2


	Carburization (1) [targeted at 1.5% C.]

			Ratio of	Ratio of area		Incompletely-	Depth of grain
	Anneal	Surface C	carbide	of grains	Network-	hardened	boundary oxide	300° C. temper
	hardness	concentration	area	≦10 μm	structured carbide	structure	layer	hardness

Example 1	83	1.62	21%	100%	no	no	no	775
Example 2	81	1.62	20%	93%	no	no	no	751
Example 3	84	1.51	19%	100%	no	no	no	779
Example 4	84	1.53	21%	100%	no	no	no	787
Example 5	89	1.41	16%	100%	no	no	no	760
Example 6	84	1.65	21%	100%	no	no	no	773
Example 7	83	1.61	23%	99%	no	no	no	780
Example 8	88	1.75	29%	94%	no	no	no	807
Example 9	90	1.65	24%	100%	no	no	no	802
Example 10	90	1.61	23%	100%	no	no	no	800
Comparative	79	1.59	22%	100%	no	no	no	776
Example 1
Comparative	92	1.61	22%	100%	no	no	no	776
Example 2
Comparative	76	1.65	22%	45%	yes	no	no	729
Example 3
Comparative	97	1.31	13%	100%	no	no	no	734
Example 4
Comparative	80	1.62	20%	100%	no	no	no	735
Example 5
Comparative	93	1.64	23%	100%	no	no	no	779
Example 6
Comparative	80	1.51	20%	100%	no	yes	no	743
Example 7
Comparative	91	1.83	32%	91%	no	no	no	814
Example 8
Comparative	99	1.69	26%	94%	no	no	no	814
Example 9
Comparative	100	1.68	25%	100%	no	no	no	816
Example 10

Carburization (2) [targeted at 2.5% C.]

		Ratio of	Ratio of area	Network-	Incompletely-	Depth of grain	300° C.
	Surface C	carbide	of grains	structured	hardened	boundary oxide	temper
	concentration	area	≦10 μm	carbide	structure	layer	hardness	Remarks

Example 1	2.53	40%	97%	no	no	no	824
Example 2	2.52	52%	93%	no	no	no	835
Example 3	2.45	49%	95%	no	no	no	843
Example 4	2.64	55%	98%	no	no	no	863
Example 5	2.31	35%	94%	no	no	no	828
Example 6	2.47	37%	97%	no	no	no	817
Example 7	2.74	52%	94%	no	no	no	845
Example 8	2.83	58%	92%	no	no	no	875
Example 9	2.56	52%	98%	no	no	no	861
Example 10	2.58	50%	99%	no	no	no	845
Comparative	2.48	41%	98%	no	no	no	825
Example 1
Comparative	2.51	40%	95%	no	no	no	825	poor machinability
Example 2
Comparative	2.61	43%	36%	yes	no	no	825	carbide shape control
Example 3								failure,
								poor strength
Comparative	1.92	27%	99%	no	no	no	800	poor machinability,
Example 4								poor carburization,
								poor strength
Comparative	2.46	38%	98%	no	yes	no	778	poor hardening,
Example 5								poor strength
Comparative	2.51	36%	98%	no	no	no	814	poor machinability
Example 6
Comparative	2.44	34%	95%	no	yes	no	796	poor hardening,
Example 7								poor strength
Comparative	2.98	66%	81%	yes	no	no	892	poor machinability,
Example 8								carbide shape control
								failure
Comparative	2.61	56%	95%	no	no	no	875	poor machinability
Example 9
Comparative	2.56	54%	98%	no	no	no	860	poor machinability
Example 10

It is known from Table 2 that all of the Examples from 1 to 10 raise no problem in the manufacturability (anneal hardness≦HRB90), show no incomplete-hardened structure, network-structured carbide and grain boundary oxidation causative of degradation in the hardness, and give sufficient levels of temper hardness (≧750Hv) at 300° C. In contrast, Comparative Examples 2, 4, 6 and 8 to 10 show large hardness after annealing, and raise a problem in the manufacturability. Comparative Examples 3 and 8 show only insufficient control levels of fine dispersion of the carbide due to a low Si and a large Cr content, and production of the network-structured carbide and other coarse carbide may undesirably degrade the strength. Comparative Example 4, too large in the Si content, raises a problem in the manufacturability, inhibits the carburization property, and cannot allow the carburization to proceed to a sufficient degree. Comparative Examples 5 and 7, low in the Cr and Mn contents, which give only poor levels of hardening property, show the incompletely-hardened structure, and may undesirably degrade the strength.

TABLE 3


							Surface C	Ratio of	Ratio of area	Network-
	C	Si	Mn	Cr	Mo	V	concentration	carbide area	of grains ≦10 μm	structured carbide

Example 3	0.19	1.02	0.52	2.52	0.00	0.00	↓ 0.80	↓ 0%	↓ 0%	no
							↓ 1.11	↓ 12%	100%	no
							1.51	19%	100%	no
							1.99	34%	100%	no
							2.45	47%	95%	no
							↑ 3.15	↑ 67%	↓ 64%	yes

	Incompletely-	Depth of grain	300° C. temper	Ratio of surface
	hardened structure	boundary oxide layer	hardness	fatigue strength	Remarks

Example 3	no	no	644	1.09	poor strength
	no	no	734	1.23	poor strength
	no	no	779	1.43
	no	no	816	1.45
	no	no	843	1.55
	no	no	894	1.28	carbide shape
					Failure, poor
					strength

It is known from Table 3 that the carburization targeted at a surface C concentration of less than 1.2% is successful in improving the surface fatigue strength, but unsuccessful in obtaining a sufficient effect for improving the strength (≧30%). On the other hand, the carburization targeted at a surface concentration exceeding 3.0% is successful in obtaining a sufficient level of 300° C. temper hardness, but shows the network-structured carbide and coarse carbide, and is unsuccessful in obtaining a sufficient effect for improving the strength.
(3) Evaluation of Surface Fatigue Strength
The surface fatigue strength was evaluated using a roller pitting tester, wherein the surface fatigue strength was defined as the pressure on the load surface not causative of pitting over 107 cycles of the test. More specifically, a 32-mm-diameter round rod was softened by keeping it heated at 950° C., followed by gradual cooling, and was then machined to fabricate a roller pitting test piece having a diameter of test portion of 26 mm. A roller correspondent to the test piece was configured using SUJ2, and subjected to quench-and-temper so as to attain a hardness of HRC61. The radii of curvature of large rollers are 150R and 700R.
The carburization was simultaneously carried out with the carburization carried out for basic evaluation of the inventive steel. A portion of the roller pitting test piece after the carburization was tempered at 300° C. for 3 hours, and evaluation was also made on the carbon concentration, ratio of the carbide area, the maximum carbide size and temper hardness. The surface fatigue strength of each material was expressed by an index, assuming the surface fatigue strength of gas-eutectic-carburized JIS-SCR420 material is 1.0. A sufficient effect of improving the strength by 30% or more as compared with gas-eutectic-carburized JIS-SCR420H steel was targeted.

Results of the evaluation are listed in Table 4.

	TABLE 4


	Roller pitting test

					Incompletely-	Depth of grain
	Surface C	Ratio of	Ratio of area	Network-	hardened	boundary oxide
	concentration	carbide area	of grains ≦10 μm	structured carbide	structure	layer

Example 1	2.04	33%	100%	no	no	no
Example 2	2.13	30%	98%	no	no	no
Example 3	1.99	34%	100%	no	no	no
Example 4	2.08	40%	100%	no	no	no
Example 5	1.94	31%	97%	no	no	no
Example 6	2.00	32%	98%	no	no	no
Example 7	2.34	33%	94%	no	no	no
Example 8	2.49	46%	94%	no	no	no
Example 9	2.15	34%	100%	no	no	no
Example 10	2.13	37%	99%	no	no	no
Comparative Example 1	2.01	32%	100%	no	no	no
Comparative Example 2	2.03	33%	100%	no	no	no
Comparative Example 3	2.20	29%	69%	yes	no	no
Comparative Example 4	1.83	24%	100%	no	no	no
Comparative Example 5	2.01	30%	96%	no	yes	no
Comparative Example 6	1.95	29%	98%	no	no	no
Comparative Example 7	2.10	30%	97%	no	yes	no
Comparative Example 8	2.61	54%	88%	yes	no	no
Comparative Example 9	2.19	39%	98%	no	no	no
Comparative Example 10	2.12	42%	99%	no	no	no
Comparative Example 11	0.78	0%	0%	0%	no	8 μm

Roller pitting test

	300° C. temper	Surface fatigue
	hardness	strength	Remarks

Example 1	805	1.44
Example 2	798	1.40
Example 3	816	1.45
Example 4	825	1.51
Example 5	800	1.49
Example 6	804	1.46
Example 7	813	1.43
Example 8	841	1.54
Example 9	832	1.52
Example 10	823	1.49
Comparative Example 1	806	0.93	poor strength of core portion
Comparative Example 2	805	1.46	poor machinability
Comparative Example 3	782	1.17	carbide shape control failure, poor strength
Comparative Example 4	781	1.47	poor machnability, poor carburization
Comparative Example 5	766	1.15	poor hardening, poor strength
Comparative Example 6	796	1.41	poor machinability
Comparative Example 7	769	1.18	poor hardening, poor strength
Comparative Example 8	861	1.29	poor machinability, carbide shape control failure
Comparative Example 9	845	1.57	poor machinability
Comparative Example 10	838	1.53	poor machinability
Comparative Example 11	620	1.00	JIS-SCR20 (base steel)-gas carburizaton

It is known from Table 4 that all of the Examples from 1 to 10 are successful in obtaining sufficient levels (≧30%) of improvement in the strength. In contrast, Comparative Example 1 show only a low strength due to poor strength of the core portion. Comparative Examples 2, 4, 6, 9 and 10 are successful in sufficiently improving the strength, but raise a problem in the manufacturability. Comparative Examples 3 and 8 show growth of the network-structured carbide and other coarse carbide, and fail in obtaining sufficient levels of effect for improving the strength. Comparative Examples 5 and 7, having low contents of Cr and Mn, show only poor hardening properties as indicated by the incompletely-hardened structure, and fail in obtaining sufficient levels of effect for improving the strength.
As proven by the above-described tests, the carburized component of this invention was confirmed as having a large amount of fine carbide grains precipitated in the surficial portion thereof, as being substantially free from the grain boundary oxide layer in the surficial portion, and being excellent in the areas of surface hardness and strength.
FIG. 1A
primary carburization
secondary carburization
FIG. 1B, FIG. 3
γ single phase
FIG. 2
between d-e
precipitation of fine carbide grains
between e-f
growth of carbide grains

Claims

1. A carburized component consisting essentially of, in % by mass and both ends inclusive, C: 0.1-0.30%, Si: 0.80-1.50%, Mn: 0.30-1.20%, Cr: 2.0-5.5%, and the balance of Fe and inevitable impurities;

having a mean C concentration over the range from the surface of the steel to a depth of 0.2 mm after vacuum carburization of 1.2% or more and 3.0% or less, having a ratio of carbide area over the range from the surface to a depth of 50 μm of 15% or more and 60% or less, having the carbide precipitated in a finely dispersed manner so that the carbide having a grain size of 10 μm or less accounts for 90% or more of the entire portion, and having a depth of a grain boundary oxide layer of 1 μm or less.

2. The carburized component as claimed in claim 1, further comprising either or both of Mo: 0.2 to 1.0% and V: 0.2 to 1.0%.

3. A method of manufacturing a carburized component described in claim 1, subjecting steel containing the above-described steel ingredients to a primary carburization at a temperature of Acm or above, then rapidly cooling the steel to as low as point A1 or below, and then subjecting the steel to a secondary carburization at a temperature of point A1 or above and or Acm below.

4. The method of manufacturing a carburized component as claimed in claim 3, wherein the carburization is carried out by vacuum carburization at 1,000 Pa or below.

5. The method of manufacturing a carburized component as claimed in claim 3, further subjecting the steel to peening after the secondary carburization.