Patent Publication Number: US-2010126632-A1

Title: Manufacturing method for high-concentration carburized steel

Description:
TECHNICAL FIELD 
     The present invention relates to a manufacturing method for high-concentration carburized steel and, more particularly, to a manufacturing method for high-concentration carburized steel in which fine and spherical carbides can be precipitated in a large amount in the surface thereof by carburization treatment. 
     BACKGROUND ART 
     Carburization means a treatment of heating steel in a carburizing atmosphere to thereby enhance carbon concentration in the surface. Carburization is generally applied to low-carbon steel, which is quenched after carburization to use. Materials having been subjected to such carburizing-quenching treatment are called case-hardened steel or carburized steel and are commercially used as mechanical members such as shafts, bearings, gear wheels, piston pins, and cams since they have a hard surface and a soft interior. 
     Among carburization treatments, a treatment of enhancing the carbon concentration in the vicinity of the surface of the material to precipitate carbides is specifically called “high-concentration carburization”. Materials obtained by the high-concentration carburization contains hard carbides dispersed in their structure, and hence they are characterized in that they have higher abrasion resistance and higher surface fatigue strength than those materials have which are obtained by conventionally performed eutectoid carburization. 
     However, since characteristic properties of the materials having been subjected to the high-concentration carburization treatment are strongly influenced by the dispersion state of carbides, it is necessary to disperse carbides finely in a spherical form in a large amount so as to obtain high strength (see, non-patent document 1). In particular, coarse carbides precipitated at the grain boundaries cause reduction in strength. 
     Thus, in order to solve this problem, various proposals have conventionally been made. 
     For example, patent document 1 discloses a method of carburization treatment of steel, which subjecting following steps on a machine structure member made of steel having C, 0.05-0.45%: 
     (i) subjecting primary carburization by plasma carburization at temperature of 880° C. or higher to enhance the C concentration of the member surface to Acm of the steel or more than that to thereby precipitate carbides in the vicinity of the surface; 
     (ii) gradually cooling the machine structure member to temperature lower than Ar 1  of the steel and, after retaining the temperature at the level, heating it to temperature exceeding Ar 1 ; 
     (iii) subjecting secondary carburization by plasma carburization again at temperature lower than the temperature of the primary carburization by 10 to 60° C.; and 
     (iv) quenching and tempering immediately or after subjecting a diffusing treatment to thereby obtain a member in which the surface C concentration is 1.5% or more, the shape of the carbides in the carburized layer is approximately spherical, and has excellent abrasion resistance and excellent pitching resistance. 
     Also, the same literature discloses following steps which are subjected subsequent to the above-described step (iii): 
     (v) gradually cooling it again to temperature lower than Ar 1  of the steel and, retaining the temperature at the level at once, heating it to temperature exceeding Ar 1 ; 
     (vi) subjecting third carburization by plasma carburization again at temperature still lower than the temperature of the secondary carburization by 10 to 60° C.; and then the step (iv) to thereby obtain a member in which the surface C concentration is 1.7% or more, in which the shape of the carbides in the carburized layer is approximately spherical, and which has excellent abrasion resistance and excellent pitching resistance. 
     In the same literature describes followings: 
     (1) no anxiety about carburization unevenness exists since generation of soot is slight in spite of a high carbon potential when plasma carburization is employed as the carburization treatment method; 
     (2) carbides can be precipitated in a agglomerate shape at the austenite grain boundaries since carburization in the primary carburization is performed in a high density of C concentration exceeding Acm; 
     (3) the austenite grain boundary migrates when the temperature is once decreased, after carburization, to temperature lower than Ar 1  and is again raised to temperature higher than Ar 1  and, as a result, carbides first having existed at the grain boundaries remain within new austenite grains; and 
     (4) carbides precipitate at the new austenite grain boundaries by further performing the secondary carburization, and a carburized layer having a preferred carbide distribution can be obtained by the newly generated carbides and the above-described residual carbides. 
     Further, patent document 2 discloses a carburization heat treatment method of a steel member. The method includes: 
     cooling a steel member having been subjected to carburization treatment to a degree of 0.8% or more in the surface carbon concentration to temperature of 300° C. or lower than that after this carburization treatment at a cooling rate of 0.1° C./sec or more, heating the steel member to temperature range higher than the Ac 1  transformation temperature of the steel by 50° C. and lower than that by 150° C., 
     retaining the steel member at the same temperature and, 
     further raising the temperature at a heating rate of 10° C./sec to temperature at which the core portion thereof acquires an austenite single phase or an austenite/ferrite dual phase wherein the ferrite area ratio is 30% or less and retaining at the temperature, 
     performing quenching directly or after decreasing the temperature to a predetermined level for quenching. 
     The literature describes that fine carbides can be allowed to grow by retaining the carburized steel member in the temperature range of higher than the Ac 1  transformation temperature of the steel by 50° C. and lower than that by 150° C. 
     Non-patent document 1: Tetsuya Shimomura, Toshiyuki Morita, and Koichiro Inoue; DENKI-SEIKO (Electric Furnace Steel), vol. 77 (2006), p. 11
 
Patent document 1: Japanese Patent Examined Publication JP-B-2808621
 
Patent document 2: Japanese Patent unexamined publication JP-A-6-108226
 
     DISCLOSURE OF THE INVENTION 
     Problems that the Invention is to Solve 
     As disclosed in the patent document 2, it is difficult to disperse fine and spherical carbides in a large amount only by one carburization treatment. Therefore, in many of the conventionally performed high-concentration carburization, two carburization treatments of a primary carburization treatment and a secondary carburization treatment are performed. The primary carburization treatment is performed mainly for the purpose of solid-dissolving carbon in a high concentration in the surface so as to precipitate fine carbides in a large amount upon re-heating for performing the secondary carburization treatment. On the other hand, the secondary carburization treatment is performed mainly for the purpose of allowing fine carbides generated upon re-heating to grow. For such purposes, it is desirable that the difference in temperature between the primary carburization treatment and the secondary carburization treatment is large enough. 
     However, if the temperature in the primary carburization treatment is raised in order to enlarge the difference in temperature between the primary carburization treatment and the secondary carburization treatment, the life of the furnace would be shortened. Also, the case-hardened steel is usually used as such without finishing processing, thus, heating it at a higher temperature than is necessary would increase deformation of the material. 
     On the other hand, if the temperature of the secondary carburization treatment is decreased simultaneously with decreasing the temperature of the primary carburization treatment in order to solve this problem, the diffusion rate of carbon upon the second carburization treatment would be lowered. Thus, it takes a longtime to precipitate a necessary amount of carbides, which reduces working efficiency. 
     Further, if only the temperature of the primary carburization treatment is decreased with the temperature of the secondary carburization treatment being kept at a high level, the difference in temperature between the primary carburization treatment and the secondary carburization treatment would become smaller. Thus, flake-like, coarse carbides would be tended to be precipitated at grain boundaries, which reduces reproducibility of the structure. 
     A problem that the invention is to solve is to provide a manufacturing method for high-concentration carburized steel, which enables dispersion of fine and spherical carbides in a large amount without shortening the furnace life. 
     Another problem that the invention is to solve is to provide a manufacturing method for high-concentration carburized steel, which does not cause large deformation after carburization treatment. 
     A further problem that the invention is to solve is to provide a manufacturing method for high-concentration carburized steel, which enables dispersion of fine and spherical carbides in a large amount without reducing working efficiency. 
     A still further problem that the invention is to solve is to provide a manufacturing method for high-concentration carburized steel, in which flake-like, coarse carbides are not precipitated at the grain boundaries, and which provides a high reproducibility of the structure. 
     Means for Solving the Problems 
     In order to solve the above-described problems, the manufacturing method for high-concentration carburized steel of the invention includes: 
     (i) a primary carburization step of carburizing a steel material having C of 0.15-0.30 mass %, Si of 0.40-0.80 mass %, Mn of 0.3-0.8 mass %, Cr of 1.25-2.00 mass %, and balance of Fe and unavoidable impurities at a primary carburization temperature T 1 (° C.) till the surface carbon concentration C becomes Ceu&lt;C≦C(Acm), wherein Ceu is an eutectoid carbon concentration of the steel material, and C(Acm) is a carbon concentration corresponding to the Acm line of the aforesaid steel material at the primary carburization temperature T 1 ; 
     (ii) a cooling step of cooling the steel material to 700° C. or lower at a cooling rate of 1° C./min or more after completion of the primary carburization step; 
     (iii) a secondary carburization initial step of raising the temperature of the steel material to a secondary carburization start temperature T 2   s  to carburize the steel material at a secondary carburization temperature T 2 , wherein Ac 1  point (° C.)≦T 2   s  (° C.)≦primary carburization temperature T 1 −100° C.≦Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material immediately after initiation of the secondary carburization, and T 2   s ≦T 2 ≦Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material; 
     (iv) a secondary carburization late step of raising the temperature, after completion of the secondary carburization initial step, to a quenching temperature Tq (° C.) to further carburize at the quenching temperature of Tq, wherein Tq Acm line temperature (° C.) corresponding to the surface carbon concentration of the steel material; and 
     (v) a step of quenching the steel material after completion of the secondary carburization. 
     Advantage of the Invention 
     When performing the primary carburization treatment at the primary carburization temperature T 1  and further performing the secondary carburization treatment, the difference between the primary carburization temperature T 1  and the secondary carburization temperature T 2  can be made large enough even when the primary carburization temperature T 1  is a comparatively low temperature, by separating the secondary carburization treatment into a secondary carburization initial step of performing carburization at a secondary carburization temperature T 2  which is lower than the quenching temperature Tq and a secondary carburization late step of performing carburization at a quenching temperature Tq. Thus, fine and spherical carbides can be dispersed in a large amount without shortening the furnace life and generating serious deformation after carburization treatment. Also, in the secondary carburization late step, carburization is performed at a comparatively high temperature, and hence working efficiency is not reduced. Further, since the difference between the primary carburization temperature T 1  and the secondary carburization temperature T 2  can be made large enough, precipitation of flake-like, coarse carbides at the grain boundaries can surely be suppressed even when composition is not uniform between steel materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1D  are schematic views showing change in structure in the case of performing high-concentration carburization under various conditions, and phase diagrams; and 
         FIG. 2  is a view showing the typical carburization treatment pattern employed in Example. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the invention will be described in detail hereinafter. 
     First, steel material to which the manufacturing method for high-concentration carburized steel of the invention is applied is described. 
     A steel material to which the manufacturing method of the invention is applied contains alloy elements as described below, and the balance thereof is Fe and unavoidable impurities. Kinds of the alloy elements, ranges of the components, and reasons for restricting the ranges are as follows. 
     (1) C, 0.15-0.30 mass % 
     If the amount of C is small, ferrite is generated in the core portion, which reduces strength. Therefore, the amount of C is preferably 0.15 mass % or more. 
     On the other hand, if the amount of C is large, hardness of the material is enhanced, which reduces productivity (particularly machinability). Therefore, the amount of C is preferably 0.30 mass % or less. 
     (2) Si: 0.40 to 0.80 mass % 
     If the amount of Si is small, tempered hardness of the matrix is reduced, which reduces strength. Therefore, the amount of Si is preferably 0.40 mass % or more. 
     On the other hand, if the amount of Si is excessive, the amount of generated carbides is reduced, which reduces strength. Also, ferrite is generated in the core portion, which reduces strength. Therefore, the amount of Si is preferably 0.80 mass % or less. 
     (3) Mn: 0.3 to 0.8 mass % 
     If the amount of Mn is small, quenching properties of the matrix are deteriorated, and strength is reduced due to incomplete quenching. Therefore, the amount of Mn is preferably 0.3 mass % or more. 
     On the other hand, if the amount of Mn is excessive, hardness of the material is enhanced, which reduces productivity (particularly machinability). Therefore, the amount of Mn is preferably 0.8 mass % or less. 
     (4) Cr: 1.25 to 2.00 mass % 
     If the amount of Cr is small, the amount of generated carbides is reduced, which reduces strength. Also, ferrite is generated in the core portion, which reduces strength. Therefore, the amount of Cr is preferably 1.25 mass % or more. 
     On the other hand, if the amount of Cr is excessive, hardness of the material is enhanced, which reduces productivity (particularly machinability). Therefore, the amount of Cr is 2.00 mass % or less. 
     Next, the manufacturing method for high-concentration carburized steel of the invention will be described. 
     The manufacturing method for high-concentration carburized steel of the invention includes a primary carburization step, a cooling step, a secondary carburization initial step, a secondary carburization late step, and a quenching step. 
     The primary carburization step is a step of carburizing a steel material having the above-described composition at a primary carburization temperature T 1  (° C.) so that the surface carbon concentration C becomes Ceu&lt;C≦C(Acm). 
     It suffices for the primary carburization temperature T 1  to be temperature higher than the secondary carburization start temperature T 2   s  to be described hereinafter by 100° C. or more. In general, as the primary carburization temperature T 1  becomes higher, carburization to a predetermined carbon concentration can be achieved in a shorter time. To be specific, the primary carburization temperature T 1  is preferably 900° C. or higher. 
     On the other hand, if the primary carburization temperature T 1  is too high, the furnace life would be shortened, or deformation of the steel material would occur in some cases. Therefore, to be specific, the primary carburization temperature T 1  is preferably 1,100° C. or lower, more preferably 1,000° C. or lower. 
     Also, carburization is performed so that the surface carbon concentration C of a steel material becomes Ceu&lt;C≦C(Acm). Here, the term “surface carbon concentration” means an average carbon concentration within the region of 10 μm from the surface. Also, “Ceu” means an eutectoid carbon concentration of a steel material containing Si, Mn, and Cr in the above-described ranges, respectively. With every above-described steel material, the eutectoid carbon concentration is 0.5 mass % or more. 
     Further, “C(Acm)” means a carbon concentration corresponding to the Acm line of a steel material containing Si, Mn, and Cr in the above-described ranges, respectively, at the primary carburization temperature T 1 . To perform carburization till C≦C(Acm) means to perform the primary carburization at temperature at which the surface temperature of the steel material becomes the Acm line or higher (i.e., temperature at which the surface becomes a single phase of γ-phase). 
     If the surface carbon concentration C is small, carbides would not be precipitated within the matrix during the temperature-raising procedure in the secondary carburization to be described hereinafter. If carbides are not precipitated within the matrix, coarse carbides would be generated at the grain boundaries during the secondary carburization. Therefore, the primary carburization must be performed so that the surface carbon concentration C can become larger than Ceu. On the other hand, if the carbon concentration C of the surface becomes excessive, carbides would be generated at the grain boundaries during the primary carburization. Since carbides generated in the primary carburization remain as such, generation of coarse carbides must be prevented. Specifically, absence of coarse carbides of 5 μm or more in long diameter is preferred. Therefore, the primary carburization must be performed so that the surface carbon concentration C can become C(Acm) or less. 
     For example, with the steel material having the above-described composition, C(Acm) becomes from about 1.25 to about 1.4 mass % when the primary carburization temperature T 1  is from 950 to 1,000° C. 
     The carburization method upon performing the primary carburization is not particularly limited, and various methods may be employed. In particular, gas carburization and vacuum carburization are preferred as the carburization methods since handling is easy and treating period is short. When employing a particular carburization method, the surface carbon concentration C can be controlled within the above-described range by optimizing the carburizing conditions. 
     For example, with gas carburization, carburization is performed by heating a steel material in an atmosphere of a carburizing gas. In this case, the amount of carburization can be controlled through carbon potential of the carburizing atmosphere. Carbon potential is the surface equilibrium carbon concentration of pure iron equilibrated with the atmosphere and is dependent upon the CO/CO 2  ratio and the amount of H 2 O in the atmosphere. In general, the surface carbon concentration can be increased in a shorter time when the carbon potential becomes higher and/or when the primary carburization temperature T 1  becomes higher. 
     Also, with vacuum carburization, carburization can be performed by, for example, reducing the pressure inside the furnace into which a steel material has been introduced to about 1.3 Pa, and then heating to the carburization temperature with introducing thereinto a hydrocarbon gas such as methane or propane. In this case, the amount of carburization can be controlled through the time of introducing the hydrocarbon gas. Additionally, when vacuum carburization is performed, the carbon concentration in the vicinity of the surface may become too high. In such case, it is general to perform diffusion treatment of stopping supply of the hydrocarbon gas after carburization and retaining in the same state. 
     The cooling step is a step of cooling, after completion of the primary carburization step, the steel material at a cooling rate of 1° C./min or more to 700° C. or lower. 
     After completion of the primary carburization, the steel material is once cooled to temperature of 700° C. or lower. The reason for cooling to temperature of 700° C. or lower is that, upon re-heating in the secondary carburization, fine carbides be precipitated within grains. In this case, when the cooling rate is too slow, flake-like, coarse carbides are formed at the grain boundaries during cooling, thus too slow cooling rate is not preferred. The coarse carbides generated during cooling will not disappear in the step to be described hereinafter, and can cause reduction of strength of the steel material. Therefore, the cooling rate of 1° C./min or more is preferred. A faster cooling rate is more preferred. 
     The secondary carburization initial step is a step wherein the temperature of the cooled steel material is raised to the secondary carburization start temperature T 2   s  and the steel material is carburized at the secondary carburization temperature T 2 . 
     The term “secondary carburization start temperature T 2   s ” means the temperature which satisfies the condition of the following formula (1). 
         Ac 1 point (° C.)≦ T 2 s  (° C.)≦primary carburization temperature  T 1−100° C.≦ Acm  line temperature (° C.) corresponding to the surface carbon concentration of the steel material immediately after initiation of secondary carburization  (1) 
     The temperature difference between the secondary carburization start temperature T 2   s  and the primary carburization temperature T 1  is preferably 100° C. or more. If the temperature difference therebetween is less than 100° C., there is the possibility of generation of flake-like, coarse carbides at the grain boundaries. A larger temperature difference therebetween is more preferred. 
     Also, the secondary carburization start temperature T 2   s  must be equal to, or higher than, Ac 1  point and must be equal to, or lower than, the Acm line temperature corresponding to the surface carbon concentration of a steel material immediately after initiation of the secondary carburization. 
     This means to initiate the secondary carburization when the surface temperature of the steel material is between Ac 1  point and Acm line (i.e., the temperature at which the surface becomes γ+Fe 3 C phase). 
     The term “secondary carburization temperature T 2 ” means the temperature which satisfies the conditions of the following formula (2). 
         T 2 s≦T 2 ≦Acm  line temperature (C) corresponding to the surface carbon concentration of the aforesaid steel material  (2) 
     The secondary carburization temperature T 2  may be the same as the secondary carburization start temperature T 2   s  or may be temperature higher than that. 
     If the secondary carburization temperature T 2  is the same as the secondary carburization start temperature T 2   s , the retaining time at the secondary carburization temperature T 2  may be such that, when the temperature is raised to the quenching temperature Tq to be described hereinafter, the surface temperature of the steel material does not exceed the Acm line temperature. In general, the surface carbon concentration increases as the time of retaining at the secondary carburization temperature T 2  is prolonged, and hence carburization can be performed with keeping the surface temperature of the steel material at Acm line or less. In order for the surface temperature of the steel material to become Acm line or less when the temperature is raised to the quenching temperature Tq, the retaining time at the secondary carburization temperature T 2  is preferably 15 minutes or longer. 
     On the other hand, in the case when the secondary carburization temperature T 2  is higher than the secondary carburization start temperature T 2   s , the secondary carburization temperature T 2  may be reached by raising the temperature stepwisely or continuously from the secondary carburization start temperature T 2   s.    
     The term “stepwisely” means to repeat the procedures of retaining the temperature at a definite level for a predetermined time, then raising the temperature with a predetermined temperature width, and retaining the temperature at the predetermined temperature for a predetermined time. When stepwisely raising the temperature, carburization can be performed with retaining the surface temperature of the steel material at Acm line or less by optimizing the temperature-raising width and the temperature-retaining time. 
     Also, the term “continuously” means to raise the temperature at a predetermined temperature-raising rate. When continuously raising the temperature, carburization can be performed with retaining the surface temperature of the steel material at Acm line or less by optimizing the temperature-raising rate. 
     The secondary carburization late step is a step of subsequently raising, after completion of the secondary carburization initial step, the temperature of the steel material up to the quenching temperature Tq (° C.), and further performing carburization at the quenching temperature Tq, provided that Tq≦Acm line temperature (° C.) corresponding to the surface carbon concentration of the aforesaid steel material. 
     The secondary carburization late step is a step of not only raising the temperature of the steel material to the quenching temperature Tq but also adjusting the surface carbon concentration to an intended carbon concentration in a short time without precipitation of flake-like, coarse carbides at the grain boundaries. Therefore, the quenching temperature Tq must be equal to, or less than, the Acm line temperature corresponding to the surface carbon concentration of the steel material. When carburization conditions of the secondary carburization initial step are optimized, the temperature can be raised up to the quenching temperature Tq with retaining the surface temperature of the steel material at Acm line temperature or less. 
     In general, as the quenching temperature Tq is lowered, flake-like, coarse carbides are less difficultly generated at the grain boundaries during retaining. However, if the quenching temperature Tq is too low, there result not only reduction of diffusion rate of carbon but also insufficient quenching of the core portion. Therefore, the quenching temperature Tq is preferably equal to, or higher than, the temperature at which the core portion of the steel material becomes an austenite single phase. 
     The time of retaining the temperature at the quenching temperature Tq is not particularly limited, and an optimal time is to be selected depending upon composition of the steel material, quenching temperature Tq, characteristic properties required for the steel material, and the like. In general, as the temperature-retaining time is prolonged, the surface carbon concentration of the steel material can be more increased. In order to obtain high-concentration carburized steel excellent in abrasion resistance and surface fatigue strength, the temperature-retaining time at the quenching temperature Tq (i.e., carburization time) is preferably 15 minutes or longer. 
     Additionally, when stepwisely or continuously raising the secondary carburization temperature T 2  in the secondary carburization initial step, quenching may immediately be performed substantially without performing carburization at the quenching temperature Tq after the temperature reaches the quenching temperature Tq as long as a sufficient carburization amount is obtained at the point when the temperature reaches the quenching temperature Tq and when soaking of the steel material is sufficient. 
     The quenching step is a step of quenching the steel material after completion of the secondary carburization late step. 
     Quenching is to be performed for transforming the surface carburized layer and the core portion to martensite. For this, the steel material after completion of the secondary carburization late step is to be preferably quenched. As the quenching method, there are specifically an oil quenching method and a gas quenching method. 
     Next, effects of the manufacturing method for high-concentration carburized steel of the invention are described below. 
       FIGS. 1A  to  FIG. 1D  are schematic views showing structural changes when performing high-concentration carburization under various conditions. Additionally, phase diagrams are also shown in  FIGS. 1A  to  FIG. 1D . 
     In the high-concentration carburization, it is performed twice in many case, that is, the primary carburization and the secondary carburization. In conventional high-concentration carburization of performing carburization in two steps, the surface carbon concentration upon completion of the primary carburization is lower than the Acm line concentration corresponding to the carburization temperature as shown in the phase diagram of  FIG. 1A . That is, the surface after the completion of the primary carburization is in a state of austenite single phase. Therefore, when the steel material is cooled to 700° C. or lower from this state at a predetermined cooling rate, the structure of the steel material becomes the state wherein coarse carbides are not generated at the grain boundaries as shown in the left drawing of  FIG. 1A . 
     When the temperature of the steel material is raised to the secondary carburization temperature, spherical and fine carbides are generated in the course of raising the temperature in the secondary carburization as shown in the middle drawing of  FIG. 1A . This is because the secondary carburization is performed at temperature lower than the Acm line temperature corresponding to the surface carbon concentration of the steel material (i.e., the temperature at which the surface becomes γ+Fe 3  phase), and carbon diffusion rate becomes smaller than in the primary carburization, thus carbides being difficultly precipitated in the grain boundaries. 
     When the secondary carburization is initiated after the temperature reaches the secondary carburization temperature, the fine carbides generated during temperature-raising procedure act as nuclei to allow growth of carbides as shown in the right drawing of  FIG. 1A . 
     In order to obtain the structure as shown in  FIG. 1A , the temperature upon completion of the primary carburization must be higher than the Acm line, and the temperature upon initiation of the secondary carburization must be lower than the Acm line. Since produced steel materials are not uniform in composition between lots, the position of the Acm line varies to some extent between individual steel materials. Hence, in order to surely obtain the structure as shown in  FIG. 1A , it is necessary to provide a sufficient temperature difference between the primary carburization temperature and the secondary carburization temperature. 
     However, when the primary carburization temperature is raised for the purpose of providing the sufficient temperature difference, durability of the furnace is decreased. On the other hand, when the secondary carburization temperature is decreased in order to avoid this, the carbon diffusion rate in the secondary carburization is reduced, leading to serious decrease in productivity. 
     Further, when the temperature difference between the primary carburization temperature and the secondary carburization temperature is reduced for the purpose of obtaining both durability of the furnace and productivity, it becomes difficult to perform, with good reproducibility, the two-step carburization treatment with the Acm line being put between the two steps. 
     For example, when the primary carburization temperature is above the Acm line while retaining the secondary carburization temperature at the same temperature as in the conventional art and lowering the primary carburization temperature, the structure of the steel material after completion of the primary carburization is in the state that coarse carbides are not generated at the grain boundaries as shown in the left drawing of  FIG. 1B . However, when the secondary carburization temperature exceeds the Acm line, fine carbides having been generated within the grains in the course of raising temperature in the secondary carburization again undergoes solid dissolution as shown in the middle drawing of  FIG. 1B . Thus, nuclei for allowing carbides to grow disappear within the grains, and hence carbides are preferentially generated at the grain boundaries having smaller formation energy. As a result, coarse carbides are generated at the grain boundaries as shown in the right drawing of  FIG. 1B . 
     On the other hand, when the primary carburization temperature is below the Acm line while retaining the secondary carburization temperature at the same temperature as in the conventional art and lowering the primary carburization temperature, the structure of the steel material after completion of the primary carburization is in the state that flake-like carbides are generated at the grain boundaries as shown in the left drawing of  FIG. 10 . When the temperature is raised to the secondary carburization start temperature, fine carbides are generated within the grains in the course of raising temperature as shown in the middle drawing of  FIG. 1C . When the secondary carburization is performed from this state, both fine carbides existing within the grains and flake-like carbides generated at the grain boundaries grow as shown in the right drawing of  FIG. 10 . 
     In both cases of  FIGS. 1B and 1C , the flake-like, coarse carbides generated at the grain boundaries will be the cause of reducing strength of high-concentration carburized steel. 
     In contrast, when the primary carburization is completed in the state of retaining the primary carburization temperature at the same temperature as in the conventional art or lower than that, the structure of the steel material after completion of the primary carburization is in the state that coarse carbides are not generated at the grain boundaries as shown in the left drawing of  FIG. 1D . Also, when the secondary carburization start temperature is lower than the primary carburization temperature by 100° C. or more while raising, after cooling the steel material, the temperature to the secondary carburization start temperature, the surface temperature of the steel material can surely be adjusted to temperature lower than the Acm line. Therefore, at the point where the temperature reaches the secondary carburization start temperature, fine carbides are generated within the grains as shown in the middle drawing of  FIG. 1D . 
     When the temperature is retained, from this state, at the same level as the secondary carburization start temperature or when the temperature is stepwisely or continuously raised from the secondary carburization start temperature to perform carburization for a predetermined time, carbides within the grains grow without generation of carbides at the grain boundaries. 
     Also, with the progress of the secondary carburization, the surface carbon concentration increases, and the Acm line temperature of the surface also increases. Therefore, when conditions of the secondary carburization initial step are optimized, the quenching temperature does not exceed the Acm line temperature of the surface even when the temperature of the steel material is raised to the quenching temperature. As a result, as shown in the right drawing of  FIG. 1D , carbides in the grains can be allowed to grow without generation of carbides at the grain boundaries. 
     In order to prevent precipitation of flake-like, coarse carbides at the grain boundaries, the primary carburization must be performed at temperature higher than the Acm line and the secondary carburization must be performed at temperature lower than the Acm line. The manufacturing method for high-concentration carburized steel of the invention can provide a sufficient temperature difference between after completion of the primary carburization and upon initiation of the secondary carburization, and hence generation of flake-like, coarse carbides can surely be suppressed even when steel materials are not uniform in composition between lots. Also, since it is not necessary to raise the temperature of the primary carburization for providing a sufficient temperature difference, durability of the furnace is not reduced. Further, since carburization is continued, after a predetermined time after the temperature reaches the secondary carburization start temperature, by raising the temperature to the quenching temperature, the carbon concentration of the surface can reach the intended concentration in a short time. 
     EXAMPLES 
     Examples 1 to 15 and Comparative Examples 1 to 5 
     1. Preparation of Samples 
     Carburization is performed with steel materials having various compositions under various conditions. Additionally, every carburization is performed in two steps of the primary carburization and the second carburization. Also, except for Example 15 and Comparative Example 1, the secondary carburization is performed in two steps of the secondary initial step of retaining the temperature for a predetermined time at a definite level (low temperature) and the secondary carburization late step of raising the temperature to the quenching temperature (high temperature) and retaining at the temperature for a predetermined time. A typical carburization treatment pattern is shown in  FIG. 2 . 
     In all of the primary carburization, secondary initial carburization, and secondary late carburization, these carburizations are performed by repeating the following procedures (1) and (2) for 4 times in total: 
     (1) performing carburization by flowing a carburizing gas for a time corresponding to 2% of the total carburization time; and 
     (2) diffusing by vacuum pumping for a time corresponding to 23% of the total carburization time. 
     However, in Example 15, the secondary carburization start temperature is set to 750° C., the temperature is raised up to the quenching temperature of 850° C. over 40 minutes with performing carburization and, after the temperature reaches the quenching temperature, quenching is immediately performed. 
     Also, in the secondary carburization initial step of Comparative Example 2, a procedure of performing carburization by flowing a carburizing gas for a time corresponding to 3% of the total carburization time and a procedure of diffusing by vacuum pumping for a time corresponding to 22% of the total carburization time are repeated for 4 times in total. 
     Further, in the secondary carburization initial step of Comparative Example 3, a procedure of performing carburization by flowing a carburizing gas for a time corresponding to 1% of the total carburization time and a procedure of diffusing by vacuum pumping for a time corresponding to 24% of the total carburization time are repeated 4 times in total. 
     2. Testing Method 
     The surface carbon concentration after completion of the primary carburization is determined by measuring distribution of cross-sectional carbon concentration through EPMA and calculating an average carbon concentration in the region of 10 μm from the surface. Also, diameter of carbide after completion of the primary carburization and diameter after quenching are measured by photographing using SEM after corroding the cross-section of the sample with picral, with the maximum value of the particle size of carbides existing in 1 mm 2  being taken as “particle size of carbides”. Further, fatigue strength after quenching is measured by the rotating bending fatigue test (according to JIS Z 2274). 
     3. Results 
     Compositions of individual steel materials, carburization conditions, and test results are shown in Table 1. 
     In Comparative Example 1, coarse carbides of larger than 10 μm are generated after quenching. This may be attributed to that, since the secondary carburization initial step is omitted and the secondary carburization late step at 885° C. is immediately performed, fine carbides generated in the course of the secondary carburization again undergoes solid dissolution. 
     Also, in Comparative Example 2, coarse carbides of larger than 10 μm are generated after quenching. The reason for this is that the primary carburization is excessive and, at the point of completion of the primary carburization, coarse carbides are already generated. 
     Also, in Comparative Example 3, coarse carbides of larger than 6 μm are generated. This may be attributed to that, since the primary carburization is insufficient and the surface carbon concentration does not reach the eutectoid carbon concentration, fine carbides are not generated within the grains at the point when the temperature reaches the secondary carburization start temperature. 
     Also, in Comparative Example 4, the fatigue strength is low, though coarse carbides do not exist. This may be attributed to that, since the secondary carburization initial temperature is the same as the secondary carburization late temperature and the diffusion rate of carbon is slow, a sufficient amount of carbides are not generated. 
     Further, in Comparative Example 5, coarse carbides of larger than 7 μm are generated. This may be attributed to that, since the retaining time of retaining at the secondary carburization initial temperature is short, the surface temperature of the steel material exceeds the Acm line when the temperature is raised to the secondary carburization late temperature. 
     Therefore, in all of Comparative Examples 1 to 5, the fatigue strength is less than 700 MPa. 
     In contrast, in all of Examples 1 to 15, the fatigue strength is 700 MPa or more. This may be attributed to that, since the primary carburization, secondary carburization initial procedure, and secondary carburization late procedure are performed under proper conditions, fine and spherical carbides are formed in a large amount within the grains without generating flake-like, coarse carbides at the grain boundaries. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Primary Carburization 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Surface 
                 Grain 
                   
               
               
                   
                 Composition of Steel 
                   
                 Carburization 
                 Carbon 
                 Size of 
                 Cooling 
               
               
                   
                 material (mass %) 
                 Carburizing 
                 Temp. 
                 Concentration 
                 Carbide 
                 Rate 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 C 
                 Si 
                 Mn 
                 Cr 
                 Gas 
                 ° C. 
                 mass % 
                 μm 
                 ° C./min 
               
               
                   
               
               
                 Example 1 
                 0.18 
                 0.57 
                 0.60 
                 1.93 
                 Acetylene 
                 951 
                 0.73 
                 0.7 
                 140 
               
               
                 Example 2 
                 0.17 
                 0.72 
                 0.52 
                 1.34 
                 Acetylene 
                 933 
                 0.53 
                 0.2 
                 245 
               
               
                 Example 3 
                 0.20 
                 0.54 
                 0.37 
                 1.57 
                 Acetylene 
                 981 
                 0.72 
                 0.0 
                 268 
               
               
                 Example 4 
                 0.15 
                 0.77 
                 0.44 
                 1.92 
                 Propane 
                 978 
                 0.71 
                 0.0 
                 655 
               
               
                 Example 5 
                 0.15 
                 0.46 
                 0.68 
                 1.86 
                 Propane 
                 959 
                 0.79 
                 0.0 
                 385 
               
               
                 Example 6 
                 0.16 
                 0.61 
                 0.49 
                 1.86 
                 Propane 
                 973 
                 0.51 
                 0.0 
                 413 
               
               
                 Example 7 
                 0.24 
                 0.64 
                 0.50 
                 1.69 
                 Acetylene 
                 953 
                 0.70 
                 0.0 
                 162 
               
               
                 Example 8 
                 0.15 
                 0.53 
                 0.73 
                 1.38 
                 Acetylene 
                 998 
                 0.53 
                 1.7 
                 265 
               
               
                 Example 9 
                 0.21 
                 0.47 
                 0.69 
                 1.44 
                 Propane 
                 983 
                 0.64 
                 0.0 
                 487 
               
               
                 Example 10 
                 0.23 
                 0.44 
                 0.59 
                 1.48 
                 Acetylene 
                 943 
                 0.50 
                 1.4 
                 46 
               
               
                 Example 11 
                 0.23 
                 0.79 
                 0.55 
                 1.87 
                 Propane 
                 971 
                 0.76 
                 2.2 
                 813 
               
               
                 Example 12 
                 0.25 
                 0.53 
                 0.65 
                 1.50 
                 Acetylene 
                 972 
                 0.65 
                 0.0 
                 728 
               
               
                 Example 13 
                 0.16 
                 0.43 
                 0.67 
                 1.34 
                 Acetylene 
                 985 
                 0.80 
                 0.0 
                 748 
               
               
                 Example 14 
                 0.18 
                 0.49 
                 0.68 
                 1.32 
                 Propane 
                 925 
                 0.61 
                 2.1 
                 517 
               
               
                 Example 15 
                 0.24 
                 0.61 
                 0.33 
                 1.29 
                 Acetylene 
                 945 
                 0.77 
                 2.1 
                 10 
               
               
                 Comparative 
                 0.19 
                 0.68 
                 0.66 
                 1.48 
                 Acetylene 
                 978 
                 0.62 
                 0.0 
                 595 
               
               
                 Example 1 
               
               
                 Comparative 
                 0.21 
                 0.67 
                 0.62 
                 1.52 
                 Acetylene 
                 945 
                 0.95 
                 10.3 
                 48 
               
               
                 Example 2 
               
               
                 Comparative 
                 0.21 
                 0.73 
                 0.60 
                 1.99 
                 Propane 
                 998 
                 0.45 
                 0.0 
                 11 
               
               
                 Example 3 
               
               
                 Comparative 
                 0.22 
                 0.48 
                 0.31 
                 1.87 
                 Propane 
                 923 
                 0.79 
                 0.0 
                 675 
               
               
                 Example 4 
               
               
                 Comparative 
                 0.16 
                 0.79 
                 0.36 
                 1.70 
                 Propane 
                 980 
                 0.64 
                 0.0 
                 413 
               
               
                 Example 5 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 Results 
               
            
           
           
               
               
               
               
            
               
                   
                 Secondary Carburization 
                 Grain 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 Initial 
                 Carburization 
                 Late 
                 Carburization 
                 Size of 
                 Fatigue 
               
               
                   
                   
                 Temp. 
                 Time 
                 Temp. 
                 Time 
                 Carbide 
                 Strength 
               
               
                   
                   
                 ° C. 
                 min 
                 ° C. 
                 min 
                 μm 
                 MPa 
               
               
                   
                   
               
               
                   
                 Example 1 
                 732 
                 20 
                 872 
                 38 
                 1.7 
                 886 
               
               
                   
                 Example 2 
                 782 
                 21 
                 840 
                 17 
                 1.3 
                 947 
               
               
                   
                 Example 3 
                 766 
                 21 
                 848 
                 20 
                 1.1 
                 922 
               
               
                   
                 Example 4 
                 757 
                 33 
                 851 
                 33 
                 1.1 
                 925 
               
               
                   
                 Example 5 
                 766 
                 42 
                 866 
                 57 
                 1.0 
                 964 
               
               
                   
                 Example 6 
                 736 
                 52 
                 887 
                 38 
                 1.1 
                 928 
               
               
                   
                 Example 7 
                 758 
                 50 
                 854 
                 50 
                 1.2 
                 935 
               
               
                   
                 Example 8 
                 778 
                 25 
                 857 
                 16 
                 2.8 
                 829 
               
               
                   
                 Example 9 
                 751 
                 28 
                 825 
                 52 
                 1.1 
                 932 
               
               
                   
                 Example 10 
                 783 
                 43 
                 890 
                 34 
                 2.5 
                 830 
               
               
                   
                 Example 11 
                 713 
                 29 
                 877 
                 34 
                 3.2 
                 836 
               
               
                   
                 Example 12 
                 764 
                 30 
                 841 
                 42 
                 1.2 
                 922 
               
               
                   
                 Example 13 
                 772 
                 51 
                 856 
                 51 
                 1.2 
                 931 
               
               
                   
                 Example 14 
                 731 
                 52 
                 841 
                 40 
                 3.2 
                 810 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Example 15 
                 * 
                 3.3 
                 816 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Comparative 
                 — 
                 — 
                 885 
                 51 
                 10.1 
                 425 
               
               
                   
                 Example 1 
               
               
                   
                 Comparative 
                 810 
                 53 
                 879 
                 34 
                 11.4 
                 404 
               
               
                   
                 Example 2 
               
               
                   
                 Comparative 
                 768 
                 32 
                 856 
                 41 
                 6.2 
                 644 
               
               
                   
                 Example 3 
               
               
                   
                 Comparative 
                 755 
                 34 
                 755 
                 21 
                 1.0 
                 574 
               
               
                   
                 Example 4 
               
               
                   
                 Comparative 
                 723 
                  5 
                 856 
                 30 
                 7.1 
                 661 
               
               
                   
                 Example 5 
               
               
                   
                   
               
               
                   
                 * Example 15: Temperature is raised 750 → 850° C. in 40 minutes with performing carburization. 
               
            
           
         
       
     
     INDUSTRIAL APPLICABILITY 
     The manufacturing method for high-concentration carburized steel of the invention can be used as a manufacturing method for mechanical members such as shafts, bearings, gear wheels, piston pins, and cams. 
     Although the invention has been described in detail and by reference to particular embodiments, it is apparent to those skilled in the art that various alterations and modifications can be made without departing from the spirits and the scope of the invention. This application is based on Japanese Patent Application filed on Nov. 6, 2006 (Japanese Patent Application No. 2006-299836), and the contents thereof are incorporated herein by reference.