EP2530178B1

EP2530178B1 - Case-hardened steel and carburized material

Info

Publication number: EP2530178B1
Application number: EP11736785.4A
Authority: EP
Inventors: Katsuyuki Ichimiya; Kazukuni Hase; Hideto Kimura
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2010-01-27
Filing date: 2011-01-26
Publication date: 2018-10-24
Anticipated expiration: 2031-01-26
Also published as: EP2530178A4; CN102770570A; KR20150038649A; CN102770570B; WO2011093070A1; CN104480399A; CN104480399B; JP5760453B2; JP2011174176A; KR20120099519A; KR101671133B1; EP2530178A1; JP2015096657A

Description

Technical Field

The present invention relates to a case hardening steel and a carburized steel material having high fatigue strength, each of which is excellent in cold forgeability, has high fatigue strength after carburization, and suitably serves as a material for mechanical structures in the fields of construction machinery and automobiles.

Prior Art

A material of a member to be produced by cold forming of a steel bar, e.g. a material of a member of an automobile, is required to have good cold forgeability. In view of this, it has been practiced to subject steel to spheroidizing heat treatment to spheroidize carbide in the steel to improve cold forgeability thereof. It has also been proposed in terms of component composition of steel to decrease content of Si, which significantly affects deformation resistance of the steel. Further, there has been proposed steel in which hardenability-improving properties of boron is effectively utilized.
JP-B 3551573 , for example, proposes carburized steel for gear, in which contents of alloy elements other than boron can be reduced as much as hardenability of the carburized steel improves due to addition of boron, whereby hardness of the steel can be lowered from the normalizing process to make it possible to remarkably improve gear cutting properties of the steel, as compared with conventional steel.
Further, JP-B 3764586 proposes a case hardening steel which ensures good cold formability thereof by combining compositional advantageous effects of reliably improving hardenability by addition of boron and reducing contents of Si and Mn as solute-strengthening elements with a advantageous effect caused by specific production conditions.
JP 2006 299383 A and JP 2008 179849 A relate to a steel sheet that undergo a carburizing treatment.
In recent years, gears of increasing smaller sizes are required for gears used in automobiles or the like as vehicle weight is increasingly reduced for better energy saving and also these gears have to bear increasingly higher load exerted thereon as the engine output is increasingly made higher. Durability of a gear is primarily determined by degrees of gear tooth root fracture caused by bending fatigue and pitting fatigue fracture of gear tooth surface. It is known that reducing an incompletely quench-hardened layer appearing at a surface layer during carburization and making prior austenite grains fine are effective in terms of enhancing strength against bending fatigue of a gear tooth root. Regarding enhancing strength of a gear tooth surface against pitting fracture, correlation between such enhancement and temper softening resistancy has been pointed out and, based thereon, there have been proposed steel having higher Si content, steel having Mo added thereto, and steel having fine carbides dispersed in a carburized surface layer thereof, respectively.
In this connection, JP-B 3063399 , for example, proposes a carburizing steel having improved fatigue strength and toughness by setting diameter of prior austenite grains to be in the range of 7 µm or less. Further, JP-B 4056709 proposes finely dispersing carbides in a carburized layer of a steel surface.

Disclosure of the Invention

Problems to be solved by the Invention

However, in the cases of JP-B 3551573 and JP-B 3764586 , fatigue properties of the steels hardly improve, although cold formability and impact-related properties thereof are somewhat improved, as compared with the conventional steel. Further, in the cases of JP-B 3063399 and JP-B 4056709 , the steels require carbide-generating elements such as Nb, Ti and V by large amounts, thereby causing problems, for example, significantly increasing deformation resistance of the steels during processing when fine carbides are precipitated in the steels.
The present invention has been developed in view of the situation described above and an object thereof is to provide a case hardening steel and a carburized steel material using the case hardening steel, which exhibit excellent cold forgeability respectively, as well as satisfactory high fatigue strength after being carburized.

Means for solving the Problem

As a result of a keen study to solve the aforementioned problems, the inventers of the present invention have made discoveries described below.
First, the inventors keenly studied a method for suppressing generation of coarse carbide (mainly cementite) and finely dispersing carbide in a carburized surface layer in a case of forming a carburized surface layer having carbide dispersed therein at relatively high concentration and having carbon concentration of at least 0.85 mass % (which highly carburized layer will be referred to as a "super-carburized layer" hereinafter) in a case hardening steel in order to enhance strength of the steel against fatigue.
Specifically, FIG. 1 shows relationships between contents of Al, B and Ti in steel and the maximum particle size of carbide in a surface layer of a super-carburized layer of a case hardening steel. It is understood from FIG. 1 that specifically controlling contents of Al and B and suppressing addition of Ti are critically important in terms of suppressing generation of coarse carbide and finely dispersing carbide. FIG. 1 also shows the results of pitting fatigue strength measurement conducted for some of the steel examples each having the aforementioned super-carburized layer. It is understood from these results that significantly high pitting fatigue strength of steel can be obtained by suppressing generation of coarse carbide.
Further, relationships between contents of Al, Ti and B and pitting fatigue strength were investigated in the cases of each forming a carburized layer having carbon concentration in the range of 0.70 mass % to 0.84 mass % (which moderately carburized layer will be referred to as a "normally carburized layer" hereinafter) in a case hardening steel. The results of this investigation are also shown in FIG. 1. It is understood from these results that satisfactorily high pitting fatigue strength can be obtained by controlling contents of Al and B within specific ranges and suppressing Ti content to 0.003 mass % or less in a case of forming a normally carburized layer in steel.
The experiments, of which results are shown in FIG. 1, were conducted by: preparing steel material examples each containing as a base material a steel having the basic composition of 0.2 mass % C, 0.1 mass % Si, 0.6 mass % Mn, 1.5 mass % Cr, 0.02 mass % Nb, with Al and B of specific contents changed between the examples, and the balance as iron and incidental impurities; subjecting these steel material examples to treatments described below; and evaluating the maximum particle diameter (µm) of carbide and pitting fatigue strength (MPa) for each steel material example.
Specifically, each of the steel material samples having "super-carburized layers" was prepared by: forming a corresponding steel material into a round bar (25 mm φ); subjecting the round bar to carburization at relatively high carbon concentration (carbon potential: 2%) at 950°C for 5 hours; cooling the bar to 600°C; heating the bar to 850°C and retaining the bar at the temperature for 30 minutes; and subjecting the bar to oil quenching at 60°C and then tempering treatment at 170°C for 2 hours. The steel material sample thus treated was cut and a cut section thereof was then analyzed by: etching the cut section with picral solution; observing an area ranging from a steel surface to 30 µm depth (observed area:_6000 µm²) by using a scan-type electron microscope; and determining the maximum particle diameter of carbide through image analysis. Further, a roller pitting fatigue test was carried out by: collecting a roller pitting fatigue test piece from the round bar; subjecting the roller pitting fatigue test piece thus collected to the aforementioned respective treatments ranging from the carburization at relatively high carbon concentration to the tempering treatment, to obtain a sample; and subjecting the sample to the protocol of a roller pitting fatigue test under the conditions of slip rate: 40% and oil temperature: 80°C, to evaluate 10⁷-cycle strength (the critical strength at which pitting occurs at a surface of the test piece) of the sample.
On the other hand, each of the steel material samples having "normally carburized layers" was prepared as a roller pitting fatigue test sample by forming a corresponding steel material into a round bar (25 mm φ) and collecting a roller pitting fatigue test piece from the round bar. The roller pitting fatigue test piece thus collected was subjected to carburization at carbon concentration of 1.1 mass % at 930°C for 7 hours, oil quenching at 60°C and tempering treatment at 170°C for 2 hours. A roller pitting fatigue test was then carried out by using the roller pitting fatigue test piece thus treated, under the conditions of slip rate: 40% and oil temperature: 80°C, to evaluate 10⁷-cycle strength (the critical strength at which pitting occurs at a surface of the test piece) of the sample.
In order to solve the aforementioned problem, the present invention provides a case-hardening steel having the features defined in claim 1. Further, it is provided a carburized steel material obtained by subjecting the case-hardening steel The carburized steel materials are defined in claims 2 and 3.

Effect of the Invention

According to the present invention, there can be provided a case hardening steel exhibiting not only excellent cold forgeability but also satisfactorily high fatigue strength after being carburized, which is industrially very useful.

Brief Description of the Drawings

FIG. 1 is a graph showing how contents of Al, B and Ti affect a state of carbide precipitation.

Best Embodiment for carrying out the Invention

The case hardening steel of the present invention will be described in detail hereinafter.
First, reasons why a chemical composition of the steel has been restricted to the aforementioned component ranges will be described in detail for each of the relevant elements.

C: 0.10 mass % to 0.35 mass %

Carbon content in steel needs to be at least 0.10 mass % in order to enhance hardness at the center portion thereof by quenching after carburizing heat treatment. However, carbon content in steel exceeding 0.35 mass % decreases toughness of the core portion of the steel. Accordingly, carbon content in steel is to be in the range of 0.10 mass % to 0.35 mass % and preferably 0.3 mass % or less.

Si: 0.01 mass % to 0.50 mass %

Silicon is required as a deoxidizing agent and need be added to steel by at least 0.01 mass %. However, silicon is an element which is preferentially oxidized at a carburized surface layer of steel to facilitate oxidization of grain boundaries of the steel. Further, silicon hardens ferrite through solid solution strengthening, thereby increasing deformation resistance of steel and deteriorating cold forgeability of the steel. Accordingly, the upper limit of Si content is to be 0.50 mass % and preferably 0.35 mass %. The lower limit of Si content is preferably 0.03 mass %.

Mn: 0.40 mass % to 1.50 mass %

Manganese is an element which effectively improves hardenability and need be added to steel by at least 0.40 mass %. However, manganese tends to trigger oxidization of grain boundary and too high content thereof in steel increases retained austenite and lowers surface hardness of the steel. Accordingly, the upper limit of Mn content is 1.50 mass % and preferably 1.40 mass %. The lower limit of Mn content is preferably 0.60 mass %.

P: 0.02 mass % or less

Phosphorus tends to exist in a segregated manner at crystal grain boundaries and deteriorate toughness of steel. Accordingly, the lower content of phosphorus in steel is the better, although presence of phosphorus in steel is tolerated up to 0.02 mass %. Content of phosphorus in steel is preferably 0.018 mass % or less.

S: 0.03 mass % or less

Sulfur is an element which exists as sulfide inclusion in steel and effectively improves machinability of the steel. However, too high content of sulfur in steel deteriorates fatigue strength of the steel. Accordingly, the upper limit of sulfur content in steel is to be 0.03 mass %.

Al: 0.04 mass % to 0.10 mass %

Aluminum is an important element in terms of fixing nitrogen in steel as AlN to ensure a good hardenability-improving effect caused by boron. Content of aluminum in steel need be at least 0.04 mass % in order to sufficiently obtain the good effect of boron. However, content of Al in steel exceeding 0.10 mass % facilitates generation of Al₂O₃ inclusion which harmfully affects fatigue strength of the steel. Accordingly, content of Al in steel is to be restricted to the range of 0.04 mass % to 0.10 mass %.

Cr: 0.5 mass % to 2.5 mass %

Chromium is a useful element which not only contributes to improving hardenability and temper softening resistancy of steel but also facilitates spheroidization of carbide therein. Content of Cr in steel lower than 0.5 mass % cannot cause such good effects of Cr in a satisfactory manner, while content of Cr in steel exceeding 2.5 mass % possibly facilitates generation of retained austenite in a carburized portion of the steel to adversely affect fatigue strength thereof. Accordingly, Cr content in steel is to be restricted to the range of 0.5 mass % to 2.5 mass %. The lower limit of Cr content in steel is preferably 0.6 mass % and the upper limit thereof is preferably 2.0 mass %.

B: 0.0005 mass % to 0.0050 mass %

Boron is the most important element in the present invention. Boron exists in a segregated manner at austenite grain boundaries of a steel material during quenching heat treatment, thereby improving hardenability and contributing to increase in hardness of the steel material. These good effects caused by boron allow contents of other hardness-enhancing elements in the steel material to be reduced, whereby deformation resistance is lowered and cold forgeability of the steel material improves. Content of boron in steel need be at least 0.0005 mass % in order to sufficiently cause these good effects of boron. However, too high content of boron in steel deteriorates toughness and forgeability of steel. Accordingly, the upper limit of boron content in steel is to be 0.0050 mass % and preferably 0.0030 mass %.

Nb: 0.003 mass % to 0.080 mass %

Niobium forms NbC in steel and suppresses, by pinning effects, grain coarsening of austenite grains in the steel during carburizing heat treatment. Content of Nb in steel need be at least 0.003 mass % in order to sufficiently obtain this good effect by niobium. However, Nb content in steel exceeding 0.080 mass % results in precipitation of coarse NbC, thereby possibly deteriorating the effect of suppressing grain coarsening of austenite grains and/or possibly decreasing fatigue strength of the steel. Accordingly, content of Niobium in steel is to be 0.080 mass % or less. The lower limit of Nb in steel is preferably 0.010 mass % and the upper limit thereof is preferably 0.060 mass %.

Ti: 0.003 mass % or less

Titanium is a component of which inclusion into steel is preferably avoided as best as possible. Titanium tends to be bonded to nitrogen to form coarse TiN. The upper limit of Ti in steel is to be 0.003 mass % because titanium possibly coarsens carbide in a carburized surface layer and decreases fatigue strength of the steel.

N: 0.0080 mass % or less

Nitrogen is a component of which inclusion into steel is preferably avoided as best as possible. Content of nitrogen in steel is to be less than 0.008 mass % to ensure the good effect of improving hardenability by boron and reliably suppress formation of TiN.
In the present invention, the component composition of the case hardening steel may further include at least one element selected from Cu: 1.0 mass % or less, Ni: 0.50 mass % or less, Mo: 0.5 mass % or less, and V: .0.5 mass % or less in order to further improve hardenability.
Copper is an element which effectively improves hardenability and content thereof in steel is preferably at least 0.1 mass %. However, too high content of copper in steel deteriorates surface characteristics of a steel material and increases cost for producing alloy. Accordingly, the upper limit of Cu in steel is to be 1.0 mass %.
Ni, Mo and V are elements which effectively improve hardenability and toughness of steel and contents of Ni, Mo and V in steel are preferably at least 0.1 mass %, 0.05 mass % and 0.02 mass %, respectively. The upper limits of contents of Ni, Mo and V are to be 0.50 mass %, respectively, because these elements are expensive.
In the present invention, the component composition of the case hardening steel may further include at least one element selected from Ca: 0.0005 mass % to 0.0050 mass % and Mg: 0.0002 mass % to 0.0020 mass % in order to control morphology of sulfide and improve machinability and cold forgeability of steel. Specifically, contents of Ca and Mg in steel need be at least 0.0005 mass % and 0.0002 mass %, respectively, in order to obtain the aforementioned good effects of Ca and Mg. However, too high contents of Ca, Mg in steel result in formation of coarse inclusions, which may adversely affect fatigue strength of the steel. Accordingly, the upper limits of contents of Ca and Mg in steel are to be 0.0050 mass % and 0.0020 mass %, respectively. The remainder of the component composition is iron and incidental impurities.
A case hardening steel having the component composition described above is subjected to at first cold forming into the product shape and then carburizing treatment. Carburizing treatment may be carried out under the conditions generally applied to a standard case hardening steel (such carburizing treatment will be referred to as "normal carburization" hereinafter). Specifically, the case hardening steel is retained under the conditions of carbon potential: 0.8 mass % to 1.1 mass %, temperature: 900°C or higher, and retention time: 3 to 7 hours, so that a carburized layer having carbon concentration of at least 0.7 mass % is formed in a surface layer ranging from a steel surface to 0.4 mm depth of the case hardening steel. The case hardening steel in which the carburized layer has been formed is subjected to such conventional quench-and-temper process as is generally carried out for a standard case hardening steel. Specifically, the quench-and-temper process includes subjecting the case hardening steel to: quenching in oil at temperature in the range of 60°C to 140°C such that microstructure of the surface layer (the carburized layer) of the steel is rendered to martensite structure including 10% to 40% of retained austenite; and tempering at temperature in the range of 160°C to 200°C for 1 to 2 hours. As a result, there can be obtained a carburized steel material having excellent rotating bending fatigue strength and pitting fatigue strength. The temperature during formation of the carburized layer is preferably 900°C or higher in terms of avoiding prolonging time required for the carburized layer formation and preferably 950°C or lower in terms of avoiding any adverse effect on durability of a carburizing furnace. Further, the temperature of oil during quenching process is preferably 60°C or higher in terms of suppressing deformation of the steel material during the quenching process and preferably 140°C or lower in terms of reliably obtaining targeted microstructure (i.e. martensite structure including 10% to 40% of retained austenite) of steel to ensure satisfactory hardness of the steel. Carbon concentration of a carburized layer obtained by normal carburization is less than 0.85 mass %.
The case hardening steel of the present invention is particularly suitable for super-carburization, in which carbon concentration in a carburized layer is increased to 0.85 mass % or higher to make carbides be precipitated, further harden the carburized layer and improve pitting fatigue strength thereof, rather than normal carburization. The conventional case hardening steel generates too much coarse carbide and fails to further improve pitting fatigue strength after being subjected to super-carburization. In contrast, the case hardening steel of the present invention can suppress precipitation of coarse carbide and improve pitting fatigue strength in a case where carbon concentration in the carburized layer is increased to 0.85 mass % or higher. That is, in the case hardening steel of the present invention, carbon content of the surface layer region ranging from a steel surface to 0.4 mm depth thereof is at least 0.85 mass % and the maximum diameter of carbide formed in the surface layer region is 10µm or less and the average particle diameter of the carbide is 4µm or less after being subjected to carburization. It has been revealed that controlling the maximum diameter and the average particle diameter of carbide in the aforementioned specific ranges significantly improves pitting fatigue strength of the case hardening steel. Such a good effect of improving pitting fatigue strength as this cannot be expected beyond the aforementioned specific diameter ranges of carbide.
In a case where carbon content in the surface layer region is less than 0.85 mass %, the amount of carbide is not sufficient and additional improvement for pitting fatigue strength of the steel cannot be obtained in a satisfactory manner. In a case where the maximum diameter of carbide exceeds 10µm, fatigue life of the case hardening steel may shrink because coarse carbides serve as origins of fatigue cracks. The average particle diameter of carbide exceeding 4µm also shortens fatigue life of the case hardening steel.
The carburizing heat treatment is preferably conducted under following conditions in order to obtain carbide satisfying the aforementioned requirements. That is, the carburizing heat treatment preferably includes: carburizing the case hardening steel by retaining the steel at carbon potential of 1.2 mass % to 2.5 mass % at temperature in the range of 930°C to 1050°C for 1 to 5 hours; then subjecting the case hardening steel to cooling to 550°C to 650°C, retention at temperature in the range of 830°C to 880°C for 30 minutes to 60 minutes, quenching in oil at temperature in the range of 60°C to 140°C, and preferably conducting tempering at temperature preferably in the range of 170°C to 200°C. It is possible to form by carrying out the processes described above a carburized layer as a surface layer of the case hardening steel, the carburized layer having the aforementioned steel microstructure constituted of martensite structure including 10% to 40% of retained austenite and also including finely dispersed carbide having the maximum diameter of 10µm or less and the average particle diameter of 4µm or less.

Examples

Next, Examples of the present invention will be described.
Each of steel samples having respective component compositions shown in Table 1 (the balance of each component composition is iron and incidental impurities) was processed by ingot casting, heated to temperature at 1150°C or higher, and formed into an intermediate material member having a square cross section (170mm × 170mm). The intermediate material member was heated to temperature equal to or higher than (Ac₃ + 100°C) and hot rolled to a round bar having diameter of 60mm. Cold forgeability was then evaluated for the round bar sample thus obtained.

Table 1

Steel type	Chemical Component (mass %)																	Note
Steel type	C	Si	Mn	P	S	Al	N	Cr	B	Nb	Ti	Cu	Ni	Mo	V	Ca	Mg	Note
A	0,21	0,08	0,83	0,012	0,011	0,071	0,0032	1,06	0,0018	0,032	0,001	-	-	-	0.02	-	-	Present Example
B	0,19	0,07	0,86	0,017	0,016	0.065	0.0046	1,02	0,0021	0,014	0,002	0,11	-	-	-	-	-	Present Example
C	0,19	0.18	0.87	0,016	0,014	0,077	0,0041	1,10	0,0016	0,022	0,001	-	-	-	-	-	-	Present Example
D	0,22	0,14	0,54	0,008	0.024	0,068	0,0044	1,45	0,0009	0,016	0,002	-	0.1	-	-	-	-	Present Example
E	0,17	0,22	0.40	0,011	0,018	0,059	0,0039	1,65	0,0016	0,016	0,003	-	-	0,09	-	-	-	Comp. Example
F	0,20	0,20	0,56	0,012	0,022	0,050	0,0035	1,30	0,0015	0,019	0,001	-	-	-	-	-	0,0006	Present Example
G	0,19	0,18	0,97	0,017	0,016	0,081	0,0065	1,20	0,0025	0,008	0,002	-	-	-	-	0,0012	-	Present Example
H	0,27	0,19	0,65	0,011	0,014	0,026	0,0064	1,10	0,0022	0,012	0,15	-	-	-	-	-	-	Comp Example
I	0,21	0,63	0,56	0,013	0,029	0,079	0,0055	1,12	0,0011	0,011	0,003	-	0,09	-	-	-	-	Comp. Example
J	0,22	0,38	0,92	0,009	0,018	0,015	0,0045	0,87	0,0025	0,005	0,001	-	-	0,08	-	-	-	Comp. Example
K	0,17	0,22	0,88	0,008	0,012	0,160	0,0071	1,40	0,0031	0,006	0,001	-	-	-	-	-	0,0011	Comp Example
L	0,22	0,11	0,93	0,014	0,029	0,067	0,0115	1,31	0,0029	0,010	0,002	-	-	-	-	-	-	Comp Example
M	0,16	0,26	0,77	0,014	0,016	0,066	0,0046	1,78	0,0002	0,015	0,001	0,07	0,08	-	-	0,0017	-	Comp. Example
N	0,24	0,06	0,79	0,015	0.017	0,064	0,0045	1,22	0,0016	0,114	0,002	-	-	-	-	-	-	Comp. Example
P	0,22	0,12	0,88	0,016	0,022	0,064	0,0022	1,11	0,0045	0,021	0,027	-	-	0,15	-	-	-	Comp. Example
Q	0,16	0,07	0,42	0,012	0,011	0,085	0,0035	3,31	0,0022	0,016	0,002	-	-	-	-	-	-	Comp. Example
R	0,22	0,21	1,55	0,016	0,019	0,074	0,0059	0,33	0,0019	0,019	0,001	-	0.10	-	-	0,0012	-	Comp. Example
S	0,22	0,1 1	0,98	0,014	0,015	0,077	0,0051	1,32	0,0084	0,012	0,002	-	-	-	-	-	-	Comp. Example
T	0,19	0,12	0,84	0,015	0,013	0,074	0.0043	1.45	0,0016	0,028	0,001	-	0,11	-	-	-	-	Present Example
U	0,18	0,23	0,55	0,015	0.013	0,074	0,0034	2,81	0,0012	0,022	0,001	0,13	-	-	-	-	-	Comp. Example

* Underlined values: beyond the present invention

Cold formability of the round bar sample was evaluated in terms of limit upset ratio and deformation resistance.
Specifically, deformation resistance was determined by: collecting a test piece (diameter: 10mm, height: 15mm) from a region ranging from a steel surface to a diameter/4 position in the radial direction (a D/4 position) of each round bar sample; and then measuring compression load at 60% upset forging by using a 300t press according to the deformation resistance measuring method recommended by The Japan Society for Technology of Plasticity, based on end face confined compression.
The limit upset ratio was determined by carrying out compression processing of the test piece according to the method for measuring deformation resistance described above and regarding the upset ratio when an end portion of the test piece was cracked as "the limit upset ratio". Cold forgeability of a sample is evaluated to be good when the deformation resistance value is 899 MPa or less and the limit upset ratio (the limit cracking ratio) is at least 74%.
Next, fatigue properties of each sample were evaluated in terms of rotating bending fatigue and pitting fatigue.
Specifically, rotating bending fatigue test pieces and roller pitting fatigue test pieces were collected from the aforementioned D/4 position of each round bar sample. The rotating bending fatigue test pieces were subjected to two different types of thermal processes, i.e. normal carburization and super-carburization for generating lots of carbide. The roller pitting fatigue test pieces were also subjected to the two different types of thermal processes described above. The normal carburization included: carburization at carbon potential of 1.1 mass % at 930°C for 7 hours; oil quenching at 60°C; and tempering at 170°C for 2 hours. On the other hand, the super-carburization included: carburization at carbon potential of 2 mass % at 950°C for 5 hours; cooling to 600°C; retention at 850°C for 30 minutes; oil quenching at 60°C; and tempering at 170°C for 2 hours.
In the present embodiment, measurement of carbide after carburization included: etching a cut section with picral solution; observing an area ranging from a steel surface to 30 µm depth (6000 µm²) by using a scanning electron microscope; and determining the maximum particle diameter and the average diameter of carbide through image analysis. Specifically, each carbide image was converted into a circle having the equal area and the maximum diameter and the average diameter of the circles thus obtained were regarded as "the maximum particle diameter" and "the average particle diameter" of the carbide, respectively. Carbide present in another depth range, i.e. a steel surface to the 0.4mm depth, of each sample was also observed. It was confirmed that "the largest particle diameter" and "the average particle diameter" are both largest in the region ranging from a steel surface to the 30µm depth position. Carbide particle having diameter of at least 0.5µm when converted into a circle having the same area was identified as "a carbide particle" in the observation described above.
Measurement of carbon concentration was carried out through EPMA line analysis of the region ranging from a steel surface to the 0.4mm depth of each sample.
A rotating bending fatigue test and a roller pitting fatigue test were carried out for each of the test pieces after carburization. The rotating bending fatigue test was conducted at 3500 rpm to evaluate the fatigue strength after 10⁷ cycles. Further, the roller pitting fatigue test was conducted at slip rate: 40% and oil temperature: 80°C to evaluate the 10⁷-cycle strength (the critical strength when pitting occurs at a surface of the test piece). The evaluation results thus obtained are shown in Table 2.

Table 2

Example No	Steel type	Cold formability		Carburization condition	Carbide particle diameter ( µ m)		C concentration in a region from steel surface to 0.4mm depth (mass%)	Fatigue strength (10¹-cycle strength)
Example No	Steel type	Deformation resistance ( MPa)	Limit upset ratio (%)	Carburization condition	Maximum	Average		Rotating bending fatigue strength (MPa)	Pitting fatigue strength (MPa)
1	A	875	78	Normal carburization	-	-	0,83	796	-	Present Example
2	B	856	75	Normal carburization	-	-	0,82	786	-	Present Example
3	C	867	76	Normal carburization	-	-	0,84	816	-	Present Example
4	A	875	78	Super-carburization	7,8	3,2	1,10	796	3250	Present Example
5	B	856	75	Super-carburization	6,5	2,8	1,11	786	3130	Present Example
6	C	867	76	Super- carburization	7,7	3,4	1,09	816	3250	Present Example
7	D	876	75	Super-carburization	6,5	2.4	1.12	821	3250	Present Example
8	E	899	74	Super-carburization	8,1	2,6	1,14	826	3130	Comp. Example
9	F	895	75	Super-carburization	7,5	3,4	1,16	816	3340	Present Example
10	G	877	76	Super-carburization	7,9	2,5	1,12	796	3130	Present Example
11	H	922	75	Super-carburization	12,5	4,8	1,13	766	2890	Comp Example
12	I	956	70	Super-carburization	7,7	3,4	1,08	756	3440	Comp. Example
13	J	883	74	Super-carburization	13,1	4.5	1,15	748	2770	Comp Example
14	K	873	76	Super-carburization	14,2	4,8	1,11	756	2770	Comp. Example
15	L	890	72	Super-carburization	9,8	3,2	1,06	736	2890	Comp. Example
16	M	877	76	Super-carburization	11,5	3.8	1,18	726	2770	Comp Example
17	N	933	71	Super-carburization	9,4	3,4	1,15	746	2890	Comp. Example
18	P	846	76	Super-carburization	15,1	4,9	1,13	746	2770	Comp Example
19	Q	975	68	Super-carburization	7,8	3,8	1,21	766	2890	Comp. Example
20	R	889	75	Super-carburization	9,4	4,9	1,19	756	2890	Comp. Example
21	S	902	69	Super-carburization	16,2	5,1	1,22	726	2770	Comp. Example
22	T	847	77	Normal carburization	-	-	0.74	786	3010	Present Example
23	U	957	71	Normal carburization	-	-	0,72	746	2770	Comp. Example

* Underlined values: beyond the present invention or failing to meet the target values

It is understood from Table 2 that Examples according to the present invention are unanimously excellent in both cold forgeability and fatigue strength.

Claims

A case hardening steel having a component composition consisting of
C: 0.10 mass % to 0.35 mass %;

Si: 0.01 mass % to 0.50 mass %;

Mn: 0.40 mass % to 1.50 mass %;

P: 0.02 mass % or less;

S: 0.03 mass % or less;

Al: 0.04 mass % to 0.10 mass %;

Cr: 0.5 mass % to 2.5 mass %;

B: 0.0005 mass % to 0.0050 mass %;

Nb: 0.003 mass % to 0.080 mass %;

Ti: 0.003 mass % or less;

N: less than 0.0080 mass %; optionally at least one element selected from

Cu: 1.0 mass % or less,

Ni: 0.50 mass % or less,

V: .0.5 mass % or less,

Ca: 0.0005 mass % to 0.0050 mass %, and

Mg: 0.0002 mass % to 0.0020 mass %;
wherein the
balance is Fe and incidental impurities.
A carburized steel material, obtained by subjecting the case hardening steel of claim 1 to carburization, the carburized steel material having carbon content in a surface layer region ranging from a steel surface to 0.4mm depth of at least 0.7 mass %.
A carburized steel material, obtained by subjecting the case hardening steel of claim 1 to carburization, the carburized steel material having carbon content in a surface layer region ranging from a steel surface to 0.4mm depth being at least 0.85 mass % and the maximum diameter and the average particle diameter of carbide in the surface layer region being 10 µm or less and 4 µm or less, respectively.