JP4821582B2 - Steel for vacuum carburized gear - Google Patents

Description

  The present invention relates to steel for vacuum carburized gears. Specifically, even when the gas carburizing process conventionally performed at about 850 to 980 ° C. with respect to the gear is changed to vacuum carburizing process and carburizing at a high temperature exceeding 1000 ° C. to improve the production efficiency, It is possible to ensure a surface pressure fatigue strength equal to or higher than that of conventional gas carburizing treatment, and the low cycle bending fatigue strength is equal to or higher than conventional, no abnormal grain growth occurs, and its machinability Relates to a steel for vacuum carburized gear.

  As materials for gears such as automobiles and industrial machines, the carbon content is generally about 0.2% by mass, the Si content is about 0.3% by mass, and the Mn content is about 0.8% by mass. Low carbon alloy steels such as SCM420 steel and SCr420 steel containing alloy elements such as Cr and Mo are used.

  The above steel is formed into a predetermined gear shape by processing such as cutting, and then subjected to "carburization quenching-tempering treatment". By this treatment, a hardened layer is formed on the surface layer, and the core hardness is further increased. Is ensured, and the desired wear resistance and strength are imparted to the gear, which is the final product.

  Conventionally, the above carburizing treatment is performed in an austenite region of about 850 to 980 ° C. in a carburizing gas atmosphere, and in order to obtain a sufficiently thick surface layer hardened layer, it takes a long time of several hours to several tens of hours. Processing was necessary.

  For this reason, the technique which can shorten carburizing process time and can raise production efficiency is desired.

  In order to shorten the carburizing time, it may be processed at a higher temperature than in the prior art, but when a conventional gas carburizing furnace is used, the furnace wall is damaged. Therefore, conventionally, a gas carburizing process at a temperature exceeding 980 ° C. is hardly performed.

  Moreover, since the oxidizing gas exists in the atmosphere in the conventional gas carburizing treatment, a grain boundary oxide layer is generated on the surface layer of the material to be treated, and the bending fatigue strength and the surface pressure fatigue strength of the carburized material are reduced. Sometimes.

  While it is known that the content of Si, Mn and Cr in the material steel can be reduced and the formation of the grain boundary oxide layer can be suppressed by including Mo, Ni, etc. When steel containing a large amount of expensive alloying elements is used as the material steel, the product cost is significantly increased.

  Therefore, since no grain boundary oxide layer is generated, the bending fatigue strength and the contact pressure fatigue strength do not decrease, and the carburizing process can be performed at a high temperature exceeding 1000 ° C. and the production efficiency can be improved. Vacuum carburizing treatment has attracted attention.

  However, in the case of SCM420 steel and SCr420 steel, which are ordinary low carbon alloy steels used as the material steel for the gas carburizing process described above, abnormal grain growth occurs when this is carburized at a high temperature exceeding 1000 ° C.

  For this reason, even when vacuum carburizing is performed at a high temperature exceeding 1000 ° C., abnormal grain growth does not occur, and bending fatigue strength, in particular, low cycle bending fatigue strength, is obtained by gas carburizing a conventional low carbon alloy steel. For steel that is equal to or better than that of the treated steel and has a surface fatigue strength equal to or better than that of conventional low-carbon alloy steel, and that has good machinability and can be easily cut into a predetermined gear shape. The demand is extremely large.

  Therefore, various techniques have been proposed in Patent Documents 1 to 6 in order to meet the above-described demand.

  Specifically, in Patent Document 1, the amount of retained austenite is increased by containing 1.00 to 3.0% of Mn, and the content of Si is 0.50 to 3.0%. A “carburizing / shot peening steel” is disclosed in which austenite is stabilized and, further, by containing Al, Nb and N, the grain size of the prior austenite is reduced to improve fatigue strength.

  In Patent Document 2, by making the C content 0.26 to 0.33%, the carburizing time at the time of carburizing at 900 to 930 ° C. is shortened, and further, the core hardness is not excessive, and “Carburizing steel” that can reduce heat treatment strain is disclosed.

  In Patent Document 3, the surface pressure strength is improved by improving the tempering hardness by increasing the Si content and improving the fracture toughness value of the carburized layer and the core by including Ni and Mo alone or in combination. "Carburizing and carbonitriding steel" is disclosed.

  In Patent Document 4, by controlling the carburizing and quenching conditions according to the component composition of the steel material to be used, it is possible to secure a suitable surface hardness and retained austenite amount and obtain a carburized part having excellent surface pressure resistance. Specifically, the content of Si for increasing the high temperature strength, the content of Mn and Cr for securing the amount of retained austenite, etc. are defined, and further, the atmosphere of the carburizing atmosphere is maintained in order to maintain the surface layer hardness. A “method for producing a high pressure pressure carburized component” specifying the carbon potential is disclosed.

  In Patent Document 5, by controlling the temperature range after the carburizing or carbonitriding process, the core structure of the component is made to be a two-phase structure of ferrite and austenite, and further, the core structure is made of ferrite and martensite by subsequent quenching. “Skin-hardening steel with low heat treatment strain” that reduces quenching strain of parts by using a phase structure is disclosed.

  In Patent Document 6, as a weight ratio, C: 0.10 to 0.30%, Si: 0.50 to 1.50%, Mn: 0.30 to 1.00%, P: 0.035% or less , S: 0.035% or less, Cr: 0.20 to less than 0.50%, Mo: 0.35 to 0.80%, Al: 0.020 to 0.060%, N: 0.0080 to 0 0.0200%, and if necessary, a specific amount of Nb, and 1.5 ≦ 3 × Si (%) − Mn (%) + Cr (%) / 4 + Mo (%) The steel comprising the balance Fe and impurities is carburized, and the gear has a carburized layer having a C concentration of 0.60% or more and a retained austenite amount of 20% or less, and the outer layer has a maximum depth of 5 to 13 μm. It has a carburized abnormal layer consisting of an incompletely hardened structure, and the surface occupied by the carburized abnormal layer in the cross section from the maximum depth position to the surface Is the rate of 70% or more "high strength gear and a manufacturing method thereof excellent in dedendum bending strength and pitting resistance" is disclosed.

JP-A-2-194149 JP-A-55-89456 JP 2001-192765 A Japanese Patent Laid-Open No. 9-59756 JP-A-9-137266 JP 2003-27142 A

  The pre-structure of the vacuum carburization process at a high temperature exceeding 1000 ° C. has a great influence on the prior austenite grain size, but the techniques disclosed in the above-mentioned Patent Documents 1 to 3 are all vacuum at a high temperature. It is not a technique for carburizing treatment, and it does not have enough countermeasures for abnormal grain growth at high temperatures. For this reason, in the case of the techniques disclosed in Patent Documents 1 to 3, when vacuum carburizing is performed at a high temperature exceeding 1000 ° C., a sufficient effect for suppressing abnormal grain growth is not obtained. There wasn't.

  Although the technique disclosed in Patent Document 4 is effective in gas carburizing in which a control technique for the atmospheric carbon concentration during carburizing treatment has been established, abnormal grain growth in a temperature range exceeding 1000 ° C. Not considered. For this reason, the vacuum carburizing process at a high temperature exceeding 1000 ° C., which is higher than that of the conventional gas carburizing, is not necessarily sufficient for suppressing abnormal grain growth.

  Although the technique disclosed in Patent Document 5 has the effect of reducing the absolute value of the quenching strain of the component, ferrite is generated in the core and the core hardness is low. For this reason, the low cycle bending fatigue strength equal to or higher than that obtained when the conventional low carbon alloy steel is subjected to gas carburizing treatment cannot be secured. Furthermore, since the amount of retained austenite effective for improving the surface fatigue strength is not considered, the surface fatigue strength is not necessarily equal to or higher than that obtained when gas carburizing a conventional low carbon alloy steel. .

  The technique disclosed in Patent Document 6 is the case where the conventional gas carburizing and quenching is effective in improving the pitting resistance, but the vacuum carburizing and quenching is performed and no carburized abnormal layer is generated at all. In the test, the formation of a cloudy zone occurred vigorously during the test, causing pitting fracture at the starting point of adhesion wear, and the surface pressure fatigue strength decreased.

  Accordingly, an object of the present invention is a gear material that is subjected to vacuum carburizing at a high temperature exceeding 1000 ° C., and the gas carburizing treatment that has been conventionally performed at about 850 to 980 ° C. exceeds the above 1000 ° C. Even when the production efficiency is improved by changing to the vacuum carburizing process at such a high temperature, when the gas carburizing process is performed using the conventional low carbon alloy steel having a carbon content of about 0.2% by mass It is possible to ensure the same or higher surface pressure fatigue strength, and the bending fatigue strength, especially the low cycle bending fatigue strength is equal to or higher than the conventional one, no abnormal grain growth occurs, and the machinability is also good. Is to provide a steel for vacuum carburized gears.

  In order to solve the above-described problems, the present inventors made various studies, and as a result, first, the following findings (a) to (i) were obtained.

  (A) In the case of gas carburizing using an oxidizing gas during carburizing, grain boundary oxidation occurs in the surface layer due to Mn or Si that easily form oxides, so the strength of the carburized layer decreases, and bending fatigue strength And surface fatigue strength decreases. On the other hand, in the case of vacuum carburizing treatment that does not use an oxidizing gas, grain boundary oxidation does not occur. Therefore, in the case of conventional gas carburizing treatment, the content of Si or Mn that had to be limited is limited. Can be used.

  (B) Repeated rolling and sliding stress on the tooth surface of the gear may cause fatigue damage called pitching. One of the causes is softening of steel due to frictional heat generated by relative sliding between tooth surfaces. Can be considered. Therefore, in the case of the vacuum carburizing process in which grain boundary oxidation does not occur, it is possible to suppress the occurrence of pitting by increasing the Si content and increasing the temper softening resistance of the steel.

  (C) However, simply increasing the Si to increase the temper softening resistance causes the surface hardness to become very high and cause pitting due to adhesive wear, resulting in a decrease in surface fatigue strength. Sometimes.

  (D) On the other hand, in order to increase the surface fatigue strength, retained austenite may be dispersed in the carburized layer in order to ensure the so-called “familiarity” on the surface.

  (E) Mn is an element that lowers the Ms point. For this reason, it is possible to disperse the retained austenite in the carburized layer by increasing the Mn content.

  (F) It is also possible to disperse the retained austenite in the carburized layer by increasing the Cr content, which is an element that increases the carbon intrusion amount during vacuum carburization.

  (G) From this, if the content of Si, Mn and Cr is 7.2% or more in terms of “4Mn + 4Cr—Si”, residual austenite in the carburized layer by quenching after vacuum carburizing treatment. Can be secured to improve the “familiarity” of the surface and to ensure good surface fatigue strength. Since recent gears are used at very high surface pressures, the retained austenite in the above case does not become the starting point of fracture because it is hardened by transformation induced by work-induced martensite. It is thought that it works effectively to improve strength.

  (H) However, when the content of Mn and Cr is excessive, or when the content of Si is small, the amount of retained austenite in the carburized layer is excessive and sufficient surface hardness cannot be secured. On the other hand, the pressure fatigue strength decreases.

  (I) The content of Si, Mn, and Cr is “4Mn + 4Cr—Si in order to ensure good surface fatigue strength without excessively generating retained austenite in the carburized layer by quenching after vacuum carburizing treatment. The value may be set to 9.0% or less.

  It is possible to ensure the surface pressure fatigue strength by designing the component steel of the gear steel subjected to vacuum carburization based on the knowledge of (a) to (i) above, but for gears such as automobiles and industrial machines. The material steel is often formed into a predetermined gear shape by cutting and then subjected to “carburizing and tempering”. For this reason, the material steel for vacuum carburized gears needs to be designed so that good machinability can be obtained.

  However, since Si and Mn, which are elements that increase the surface pressure fatigue strength, are also elements that increase the hardness of ferrite, if the content is excessive, machinability is reduced. .

  Then, next, the present inventors assumed a process of performing the normalizing process before the cutting process, adjusted the cooling rate in the normalizing process variously, and adjusted the structure and hardness to be different. Tried to process.

  As a result, the following findings (j) and (k) were obtained.

  (J) If the hardness of steel before cutting, that is, the hardness after normalization is 240 or less in terms of Vickers hardness (hereinafter also referred to as “Hv hardness”), good machinability is achieved. Can be secured.

  (K) In order to set the hardness after normalization to 240 or less in terms of Vickers hardness, the structure may be a mixed structure of ferrite and pearlite (hereinafter referred to as “ferrite / pearlite structure”).

  In addition, since the amount and hardness of the ferrite of the normalizing material influence the hardness of the ferrite-pearlite structure after normalizing the steel, the present inventors further added Si and Mo, which strengthen the ferrite, and ferrite. The hardness after normalization was investigated by variously changing the content of Mn that affects the amount of MnS that becomes the core of ferrite formation.

  As a result, the following findings (1) and (m) were obtained.

  (L) When the content of Mn is small, the amount of MnS produced decreases, so the amount of intragranular ferrite produced decreases and the Hv hardness does not become 240 or less.

  (M) In the case of Mo-free steel, the value of “Si + Mn” for the contents of Si and Mn is 2.50% or less, and in the case of steel containing a specific amount of Mo, Si, By setting the value of “Si + Mn + 4Mo” for the contents of Mn and Mo to 2.80% or less, the hardness of the steel after normalization can be made 240 or less in terms of Hv hardness.

  In addition, since the hardness of steel before cutting, that is, the hardness after normalization also changes depending on the cooling rate in the normalizing process, the inventors next changed the cooling rate in the normalizing process. Instead, vacuum carburizing treatment was performed at 1050 ° C., which is higher than conventional gas carburizing, using steel in which the ferrite grain size and hardness in the ferrite / pearlite structure were changed. As a result, the following important findings (n) were obtained.

  (N) In the ferrite / pearlite structure after normalization before carburizing treatment, if the ferrite grain size is large, abnormal grain growth occurs by vacuum carburizing treatment at high temperature. In order to prevent abnormal grain growth, the ferrite grain size in the ferrite-pearlite structure before carburizing at high temperature needs to be 100 μm or less.

  This invention is completed based on said knowledge, The summary exists in the steel for vacuum carburized gears shown to following (1) and (2).

  (1) By mass%, C: 0.13 to 0.30%, Si: more than 0.50% and 1.50% or less, Mn: 0.70 to 1.50%, P: 0.10% Hereinafter, S: 0.01 to 0.05%, Cr: 0.70 to 1.50%, Nb: 0.010 to 0.050%, Al: 0.010 to 0.050%, and N: 0.00. 0100-0.0250%, and the contents of Si, Mn, and Cr satisfy Si + Mn: 2.50% or less and 4Mn + 4Cr-Si: 7.2-9.0%, and the balance is Fe and impurities. A vacuum carburized gear having a chemical composition in which V in impurities is 0.005% or less, and the structure before vacuum carburization is a ferrite pearlite structure, and the ferrite grain size is 100 μm or less. Steel.

(2) By mass%, C: 0.13 to 0.30%, Si: more than 0.50% and 1.50% or less, Mn: 0.70 to 1.50%, P: 0.10% Hereinafter, S: 0.01 to 0.05%, Cr: 0.70 to 1.50%, Nb: 0.010 to 0.050%, Mo: 0.10 to 0.50%, Al: 0.0. In addition to containing 0.10 to 0.050% and N: 0.0100 to 0.0250%, the contents of Si, Mn, Cr and Mo are Si + Mn + 4Mo: 2.80% or less and 4Mn + 4Cr-Si: 7.2. met 9.0%, the balance being Fe and impurities thereof or Rannahli, V in impurities has the following chemical composition 0.005%, and, prior vacuum carburizing tissue in ferrite-pearlite structure, the ferrite A steel for vacuum carburized gears having a particle size of 100 μm or less.

  The “ferrite / pearlite structure” refers to a mixed structure of ferrite and pearlite.

  The “ferrite particle size” in the ferrite-pearlite structure is a so-called “calculated from the area of each ferrite grain obtained by image processing of an optical microscope photograph (corresponding to an area of 200 μm × 300 μm in area) with a magnification of 200 times”. The average value of “equivalent circle diameter”. The ferrite does not include ferrite in pearlite.

  Hereinafter, the inventions related to the steel for vacuum carburized gears of the above (1) and (2) are referred to as “present invention (1)” and “present invention (2)”, respectively. Also, it may be collectively referred to as “the present invention”.

  When the steel for vacuum carburized gears of the present invention is used, the gas carburizing process that has been conventionally performed at about 850 to 980 ° C. is changed to the vacuum carburizing process at a high temperature exceeding 1000 ° C. to improve the production efficiency. In addition, the surface fatigue fatigue strength equal to or higher than the conventional case and the bending fatigue strength equal to or higher than the conventional case, in which gas carburizing treatment is performed using a low carbon alloy steel having a carbon content of about 0.2% by mass, Low cycle bending fatigue strength can be obtained, and abnormal grain growth can be suppressed, so that the manufacturing cost can be rationalized by shortening the carburizing time. In addition, since this vacuum carburized gear steel is excellent in machinability, it can be easily formed into a predetermined gear shape by cutting.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition C: 0.13 to 0.30%
C is an element that greatly affects the strength of steel, but there is a strong correlation between strength and machinability, and when the strength is high, the machinability decreases, and in particular, the content of C is 0. If it exceeds 30%, the strength becomes too high and the machinability is remarkably lowered. On the other hand, if the C content is too small, the strength will be low, causing adverse effects such as “peeling” at the time of cutting, and further, it will not be possible to ensure the strength of the part. In particular, the C content is 0.13%. If it is less, the strength is significantly reduced. Therefore, the content of C is set to 0.13 to 0.30%. In addition, the desirable range of C content is 0.13-0.27%, and a more desirable range is 0.15-0.25%.

Si: more than 0.50% and not more than 1.50% Si has an effect of increasing the temper softening resistance near 300 ° C. and suppressing the occurrence of pitting. In order to obtain this effect, the Si content needs to exceed 0.50%. However, if the Si content is excessive, the core becomes a two-phase structure of ferrite and martensite during carburizing and quenching, so that the hardness of the core cannot be ensured and the static bending strength decreases. If it exceeds 1, the static bending strength will be significantly reduced. Therefore, the Si content is more than 0.50% and not more than 1.50%. A more desirable Si content is 0.55 to 1.00%.

  In addition, while content of Si satisfy | fills "4Mn + 4Cr-Si: 7.2-9.0%" mentioned later in the said range, in the case of this invention example (1), it is "Si + Mn: 2.50% or less In the case of the present invention (2), it is necessary to satisfy “Si + Mn + 4Mo: 2.80% or less”.

Mn: 0.70 to 1.50%
Mn is an essential element for forming MnS that becomes the core of intragranular ferrite at the time of normalization, and sufficiently increases the “familiarity” on the surface when vacuum carburizing is performed by generating residual austenite in the carburized layer. It is an effective element for ensuring sufficient surface pressure fatigue strength. In order to obtain these effects sufficiently, it is necessary to contain 0.70% or more of Mn. However, if the Mn content is excessive, the amount of retained austenite is excessive and the surface hardness decreases, so the surface pressure fatigue strength decreases, and the hardness after normalization becomes too high, and machinability. In particular, when the content exceeds 1.50%, the reduction in surface pressure fatigue strength and machinability becomes significant. Therefore, the content of Mn is set to 0.70 to 1.50%. The Mn content is preferably 0.80 to 1.30%.

  The Mn content satisfies the following “4Mn + 4Cr—Si: 7.2 to 9.0%” within the above range, and in the case of the present invention example (1), “Si + Mn: 2.50% or less. In the case of the present invention (2), it is necessary to satisfy “Si + Mn + 4Mo: 2.80% or less”.

P: 0.10% or less P is an embrittlement element and has an effect of improving machinability. However, when the P content is excessive, the hot ductility is lowered, and particularly when it exceeds 0.10%, the hot ductility is significantly lowered. Therefore, the content of P is set to 0.10% or less. In addition, the machinability improvement effect of P becomes large when the content is 0.01% or more. Therefore, the P content is preferably 0.01 to 0.10%, and more preferably 0.01 to 0.02%.

S: 0.01 to 0.05%
S is an essential element for forming MnS together with Mn. MnS formed in a state in which a certain amount of Mn is contained is not only necessary for ensuring machinability but also serves as a nucleus of intragranular ferrite during normalization. In order to obtain these effects, the S content needs to be 0.01% or more. However, when the S content is excessive, the hot ductility is lowered, and particularly when the content exceeds 0.05%, the hot ductility is significantly lowered. Therefore, the content of S is set to 0.01 to 0.05%. In addition, it is preferable that content of S shall be 0.01-0.03%.

Cr: 0.70 to 1.50%
Cr is an element that promotes carburizing properties during vacuum carburizing, improves hardenability, and increases internal hardness after carburizing and quenching. Cr has the effect of increasing the “familiarity” of the surface by dispersing the retained austenite in the carburized layer and ensuring sufficient surface pressure fatigue strength. In order to obtain such an effect, it is necessary to contain 0.70% or more of Cr. However, if the Cr content is excessive, the amount of retained austenite becomes excessive and sufficient surface layer hardness cannot be ensured, surface fatigue strength decreases, and cementite is generated by generating cementite at the grain boundaries of the surface layer. In particular, when the Cr content exceeds 1.50%, the reduction of the surface pressure fatigue strength and the reduction of the grain boundary strength of the carburized layer become remarkable. Therefore, the content of Cr is set to 0.70 to 1.50%.

  In addition, the content of Cr needs to satisfy “4Mn + 4Cr—Si: 7.2 to 9.0%” described later in the above range.

Nb: 0.010 to 0.050%
Nb has the effect | action which improves the toughness of a carburized layer by refine | miniaturizing the crystal grain after vacuum carburizing, and improves surface-pressure fatigue strength. In order to obtain these effects, the Nb content needs to be at least 0.010%. However, if the Nb content increases and exceeds 0.050%, these effects are saturated and the cost is increased, and coarse precipitates are generated to cause abnormal grain growth. Therefore, the Nb content is set to 0.010 to 0.050%.

Al: 0.010 to 0.050%
Al combines with N in the steel to form AlN and refines the crystal grains after vacuum carburization, thereby improving the toughness of the carburized layer. In order to obtain this effect, the Al content needs to be at least 0.010%. However, if the Al content exceeds 0.050%, the machinability deteriorates due to the formation of hard Al ₂ O ₃ . Therefore, the content of Al is set to 0.010 to 0.050%. In addition, it is preferable that content of Al shall be 0.010 to 0.040%.

N: 0.0100 to 0.0250%
N combines with Al to form AlN and refines the crystal grains after vacuum carburization, thereby improving the toughness of the carburized layer. In order to obtain this effect, N must have a content of at least 0.0100%. However, even if the content exceeds 0.0250%, the above effect is saturated, and a so-called “nest” is easily formed in the interior during melting. Therefore, the content of N is set to 0.0100 to 0.0250%.

4Mn + 4Cr-Si: 7.2-9.0%
As already described, Mn and Cr are effective elements for the production of retained austenite in the carburized layer, and Si has an action of suppressing the production of retained austenite in the carburized layer. However, if the content of Si, Mn and Cr exceeds 9.0% in the value of “4Mn + 4Cr—Si”, the amount of retained austenite in the carburized layer becomes excessive and the surface layer hardness decreases, so that surface pressure fatigue occurs. Strength decreases. On the other hand, when the content of Si, Mn and Cr is less than 7.2% in the value of “4Mn + 4Cr—Si”, the amount of retained austenite decreases, and the “familiarity” of the surface in the case of vacuum carburizing treatment decreases. Therefore, sufficient surface pressure fatigue strength cannot be obtained. Accordingly, Si, Mn, and Cr satisfy the content range of 7.2% to 9.0% in the above-described content range and the value of “4Mn + 4Cr—Si”.

Si + Mn: 2.50% or less As already described, Si has the effect of increasing the temper softening resistance near 300 ° C. and suppressing the occurrence of pitting, and Mn generates retained austenite in the carburized layer. Thus, it has the effect of increasing the “familiarity” on the surface when vacuum carburizing is performed and ensuring sufficient surface fatigue strength.

  However, if the content of Si and Mn exceeds 2.50% in terms of “Si + Mn”, the hardness of the steel after normalization exceeds 240 in terms of Hv hardness, so that machinability decreases.

  Therefore, in the case of the present invention (1) containing no Mo, Si and Mn each satisfy the above-described content range and satisfy the value of “Si + Mn” of 2.50% or less.

V: 0.005% or less In the present invention, V is an impurity, and its content must be suppressed to 0.005% or less. That is, V combines with C and N to form VC and VCN, and these carbides and carbonitrides are effective for refining austenite grains at the beginning of the temperature raising process of vacuum carburizing treatment. At the stage where the temperature rise has progressed, or at the stage where the temperature is kept at the carburizing temperature, it becomes a solid solution in the matrix (substrate) and causes abnormal grain growth. Furthermore, since V hardens the ferrite after normalization, it causes a decrease in machinability. In particular, when the V content exceeds 0.005%, abnormal grain growth and machinability are significantly reduced. Therefore, in the present invention, the content of V in the impurities is set to 0.005% or less.

  For the above reasons, the chemical composition of the steel for vacuum carburized gear according to the present invention (1) is C: 0.13 to 0.30%, Si: more than 0.50% and 1.50% or less, Mn: 0.70 to 1.50%, P: 0.10% or less, S: 0.01 to 0.05%, Cr: 0.70 to 1.50%, Nb: 0.010 to 0.050%, While containing Al: 0.010-0.050% and N: 0.0100-0.0250%, content of Si, Mn, and Cr is Si + Mn: 2.50% or less and 4Mn + 4Cr-Si: 7. It was specified that 2 to 9.0% was satisfied, the balance was Fe and impurities, and V in the impurities satisfied 0.005% or less.

  In addition, even if the chemical composition of the steel for vacuum carburized gears according to the present invention includes 0.10 to 0.50% Mo, and the contents of Si, Mn, and Mo are Si + Mn + 4Mo: 2.80% or less. Good. This will be described below.

Mo: 0.10 to 0.50%
Mo has the effect | action which strengthens a ferrite and raises the temper softening resistance of a carburized layer. In order to obtain these effects, the Mo content needs to be 0.10% or more. However, if the Mo content exceeds 0.50%, the formation of a bainite structure is observed in the structure before vacuum carburization, resulting in a decrease in machinability. Therefore, in the present invention (2), the Mo content is set to 0.10 to 0.50%. In addition, when it contains Mo, it is preferable that the content shall be 0.10 to 0.40%.

Si + Mn + 4Mo: 2.80% or less In the case of containing Mo, if the content of Si, Mn and Mo exceeds 2.80% in terms of “Si + Mn + 4Mo”, the hardness of the steel after normalization is Hv hardness Since it exceeds 240, machinability deteriorates.

  Therefore, in the case of the present invention (2) containing Mo, Si, Mn, and Mo are each in the above-described content range and satisfy the value of “Si + Mn + 4Mo” of 2.80% or less.

For the above reason, the chemical composition of the steel for vacuum carburized gears according to the present invention (2) is C: 0.13 to 0.30%, Si: more than 0.50% and 1.50% or less, Mn: 0 70 to 1.50%, P: 0.10% or less, S: 0.01 to 0.05%, Cr: 0.70 to 1.50%, Nb: 0.010 to 0.050%, Mo : 0.10 to 0.50%, Al: 0.010 to 0.050% and N: 0.0100 to 0.0250%, and the contents of Si, Mn, Cr and Mo are Si + Mn + 4Mo: 2.80% or less and 4Mn + 4Cr-Si: meet 7.2 to 9.0%, the balance was defined as the Fe and impurities thereof or Rannahli, the V in the impurities satisfy the following 0.005%.

(B) Structure before vacuum carburizing treatment In the present invention, the structure before vacuum carburizing treatment is a ferrite pearlite structure, and the ferrite particle size must be 100 μm or less. This will be described below.

  First, the structure before vacuum carburizing treatment is defined as ferrite / pearlite structure. The structure before vacuum carburizing treatment, which is the structure before cutting, is made ferrite / pearlite structure, so that the hardness before cutting is stable. This is because the Vickers hardness is 240 or less, and good machinability can be secured.

  The “ferrite / pearlite structure” refers to a mixed structure of ferrite and pearlite.

  Next, the ferrite grain size in the ferrite and pearlite structure before vacuum carburizing treatment is defined as 100 μm or less by setting the ferrite grain size to 100 μm or less by vacuum carburizing treatment at a high temperature exceeding 1000 ° C. This is because abnormal grain growth can be suppressed.

  The ferrite grain size is preferably as small as possible, desirably 50 μm or less, and more desirably 30 μm or less.

  The “ferrite particle size” in the ferrite-pearlite structure is a so-called “corresponding” calculated from the area of each ferrite grain obtained by image processing of an optical micrograph of 200 × magnification (corresponding to a region of 200 μm × 300 μm in area). As described above, it means the average value of “circular diameter” and that the ferrite does not contain ferrite in pearlite.

  The steel having the chemical composition described in the above (A) is, for example, heated to 1250 ° C. or higher, held for 30 minutes or more, allowed to cool, and further heated to 900 to 950 ° C. By carrying out the process of holding for 30 to 180 minutes and cooling at a rate of 10 to 30 ° C./min, the structure before the vacuum carburizing process can be easily expressed as “ferrite / pearlite structure with a ferrite particle size of 100 μm or less. It is possible to suppress abnormal grain growth in vacuum carburization at a high temperature up to about 1100 ° C.

  Hereinafter, the present invention will be described in more detail with reference to examples.

[Example 1]
Steel 1 having the chemical composition shown in Table 1 was melted in a 180 kg vacuum melting furnace to produce an ingot.

  In addition, said steel 1 is steel which has a chemical composition in the range prescribed | regulated by this invention.

  The above ingot was treated at 1250 ° C. for 30 minutes and then hot forged to give a round bar having a diameter of 35 mm. In addition, the finishing temperature of hot forging was made not to drop below 1000 ° C., and cooling after hot forging was allowed to cool in the air.

  Next, the above-mentioned round bar having a diameter of 35 mm was heated to 1250 ° C., held for 60 minutes, and then subjected to a heat treatment that was allowed to cool to room temperature.

  Five test pieces each having a diameter of 8 mm and a length of 30 mm were cut out based on the position of 10 mm from the center of the cross section of the round bar having a diameter of 35 mm.

  Each test piece having a diameter of 8 mm and a length of 30 mm was heated to 925 ° C. in a vacuum heat treatment furnace, held for 120 minutes, and then cooled to room temperature at a cooling rate of 600 to 5 ° C./minute so that the structure changed. did.

  Each test piece after cooling is divided into a length of 10 mm and a length of 20 mm. Of these, each test piece with a length of 10 mm is embedded in a resin, mirror-polished and then corroded with nital, and the structure and ferrite grain size are investigated. did.

  The “ferrite particle size” is a so-called “equivalent circle diameter” calculated from the area of each ferrite grain obtained by image processing of an optical micrograph (corresponding to an area of 200 μm × 300 μm in area) with a magnification of 200 times. Obtained from the average value.

  Further, each remaining test piece having a length of 20 mm was charged in a vacuum carburizing furnace, heated to 1050 ° C. in 1 hour, and subjected to a carburizing process with a carburizing time of 7 minutes and a diffusion time of 20 minutes. Thereafter, the temperature was lowered to 850 ° C. in 30 minutes, held for another 30 minutes, and then quenched in oil at an oil temperature of 60 ° C. The target carburization depth at this time was 1.0 mm.

  Next, the cross section of each test piece obtained as described above was mirror-polished, corroded with a saturated aqueous solution of picric acid to which a surfactant was added, the austenite crystal grain size was measured, and the presence or absence of abnormal grain growth was investigated. did.

  The “austenite particle size” is also calculated from the area of each austenite grain obtained by image processing of an optical microscope photograph (corresponding to an area of 200 μm × 300 μm in area) with a magnification of 200 times, similarly to the above “ferrite particle size”. It calculated | required from the average value of what was called "equivalent circle diameter". And when the austenite grain of the particle size 3 times or more of the "austenite grain size" obtained as mentioned above exists, it determined with abnormal grain growth.

  Table 2 shows the results of the above investigations.

  From Table 2, it is clear that even if the steel has a chemical composition within the range specified in the present invention, abnormal grain growth occurs when the ferrite grain size in the structure before vacuum carburizing is 100 μm or more.

[Example 2]
Steels 1 to 20 having the chemical compositions shown in Table 3 were melted in a 180 kg vacuum melting furnace to produce an ingot.

  Steels 1 to 8 in Table 3 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels 9 to 20 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  Steel 1 is the steel itself shown in Table 1 and is listed again in Table 3.

  Each ingot was subjected to a treatment for 30 minutes at 1250 ° C., and then hot forged to obtain a round bar having a diameter of 35 mm. In addition, the finishing temperature of hot forging was made not to drop below 1000 ° C., and cooling after hot forging was allowed to cool in the air.

  A round bar having a diameter of 35 mm of each steel thus obtained was heated to 1250 ° C., held for 60 minutes, and then subjected to a heat treatment in which it was allowed to cool to room temperature. Thereafter, the mixture was further heated to 925 ° C. and held for 120 minutes, and then cooled to room temperature at a cooling rate of 10 to 30 ° C./min.

  The round bar having a diameter of 35 mm thus obtained was cut at a position 100 mm from the end, and the structure of the cut surface and the ferrite particle size were measured, and the Vickers hardness was measured. In addition, about steel 1, it cut | disconnected in the position of 100 mm from the edge on the opposite side to having cut out five test pieces of diameter 8mm and length 30mm in Example 1. FIG.

  The measurement of the structure, the ferrite particle size, and the Vickers hardness was carried out after the resin was buried such that the cut surface was the test surface and mirror-polished.

  That is, the structure and ferrite grain size were investigated by corroding the mirror-polished surface with nital.

  The “ferrite particle size” is a so-called “equivalent circle diameter” calculated from the area of each ferrite grain obtained by image processing of an optical micrograph (corresponding to an area of 200 μm × 300 μm in area) with a magnification of 200 times. Obtained from the average value.

  On the other hand, the measurement of Vickers hardness is based on “Vickers hardness test-test method” in JIS Z 2244 (2003), with respect to four points located at 1/4 of the diameter from the center of the mirror-polished cross section. The test force was set to 98 N, and the average value at the above four points was taken as Vickers hardness.

  The Vickers hardness was set to 240 or less. This is because, as already described, if the Vickers hardness is 240 or less, good machinability can be ensured.

  Further, a small roller test piece used for the two-cylinder rolling fatigue test having the shape shown in FIG. 1 and a four-point bending test piece having the shape shown in FIG. 2 are cut out from the center of the round bar having a diameter of 35 mm obtained as described above. It was.

  All the above test pieces were vacuum carburized after the test surface was ground. Specifically, after charging in a vacuum carburizing furnace, the temperature is raised to 1020 ° C. in 1 hour, carburizing time is 6 minutes, diffusion time is 12 minutes, and then the temperature is lowered to 850 ° C. in 30 minutes. Further, after being kept for 30 minutes, it was quenched in oil having an oil temperature of 60 ° C. The carburization depth target at this time was 0.8 mm.

  After quenching, each test piece was held in a non-oxidizing tempering furnace at 170 ° C. for 120 minutes, and then cooled by blowing nitrogen gas.

  As described above, test pieces of test numbers 1 to 20 were produced.

  For comparison, the small roller test piece and the four-point bending test piece, which were subjected to gas carburization using steel 19, were prepared as test number 21. That is, after grinding the test surface in the same manner as the vacuum carburized product, it was gas carburized at 930 ° C. and quenched in oil at an oil temperature of 60 ° C. The treatment time was 180 minutes, and the carburization depth target was 0.8 mm.

  After quenching, each test piece was kept in the atmosphere at 170 ° C. for 120 minutes, and then air-cooled and tempered.

  The amount of retained austenite in the carburized layer after carburizing and quenching and tempering was measured using the small roller test piece subjected to the above-mentioned “carburizing and quenching and tempering”. That is, after removing 10 μm from the surface of the small roller test piece by electropolishing, X-ray diffraction is used to compare the integrated intensity of the diffraction line due to the martensite phase and the integrated intensity of the diffraction line due to the austenite phase to thereby determine the residual austenite. The amount was determined and the amount of retained austenite in the carburized layer after carburizing and tempering was determined.

  The small-roller test piece used for the two-cylinder rolling fatigue test was subjected to the two-cylinder rolling fatigue test under the conditions shown in Table 4 after the above tempering and the surface pressure fatigue strength was investigated.

  The large roller test piece used in the two-cylinder rolling fatigue test was a machined SCM822 defined in JIS G 4053 (2003), gas carburized and quenched, and the surface layer was ground by 50 microns.

  Specifically, the material was hot forged to a diameter of 150 mm, heated to 1250 ° C., held for 60 minutes, and then subjected to a heat treatment that was allowed to cool to room temperature. Thereafter, the mixture was further heated to 925 ° C., held for 120 minutes, and then cooled to room temperature at a cooling rate of 10 ° C./min.

  Next, the material subjected to the above treatment was machined into a roller having a diameter of 130 mm and a width of 20 mm with a crowning with a radius of 150 mm. The roller was subjected to gas carburizing treatment at 930 ° C. with a target of the total carburizing depth of 1.5 mm, and then quenched in oil at an oil temperature of 60 ° C. After quenching, the roller was kept in the atmosphere at 170 ° C. for 120 minutes, and then air-cooled tempering was performed. Thereafter, the crowning surface was polished by 50 microns to finish a large roller test piece.

  In addition, as shown in the above Table 4, the surface pressure fatigue strength is a two-cylinder condition where the load during the test is constant under the condition that the rotation speed of the small roller is 1000 rpm and the slip ratio of the large roller is 80%. A rolling fatigue test was performed. Under the present circumstances, commercially available ATF (automatic transmission oil) was discharged from the rotation direction of the test piece to the contact part on the conditions of oil temperature 40 degreeC and 2 liter / min.

In each test load, the fatigue to the peeling occurs, or up to 2 × 10 ⁷ times when fatigue flaking does not occur, continue two cylindrical rolling fatigue test, the fatigue flaking until 2 × 10 ⁷ times is not occurred Of the above conditions, the calculated Hertz stress at the maximum load was defined as the surface pressure fatigue strength. The target of the surface pressure fatigue strength is 3000 MPa or more, and the surface pressure fatigue strength is excellent when the surface pressure fatigue strength is 3000 MPa or more.

  Also, using the four-point bending test piece subjected to the above-mentioned “carburization quenching and tempering”, a four-point bending fatigue test is performed under conditions of a stress ratio of 0.1 and a repetition rate of 5 Hz so that a tensile stress is always applied to the notch. And the low cycle bending fatigue characteristics were investigated, and the bending fatigue strength was determined 1000 times from the SN diagram. The target of 1000 times bending fatigue strength is set to 1000 MPa or more. When the 1000 times bending fatigue strength is set to 1000 MPa or more, the low cycle bending fatigue strength is excellent.

  Furthermore, the cross section of the small roller test piece subjected to the above-mentioned “carburizing quenching and tempering” is mirror-polished, corroded with a saturated aqueous solution of picric acid to which a surfactant is added, the austenite crystal grain size is measured, and abnormal grains are measured. The presence or absence of growth was investigated.

  The “austenite particle size” is also calculated from the area of each austenite grain obtained by image processing of an optical micrograph (corresponding to a region of 200 μm × 300 μm in area) with a magnification of 200 times, similarly to the “ferrite particle size”. It calculated | required from the average value of what was called "equivalent circle diameter". And when the austenite grain of the particle size 3 times or more of the "austenite grain size" obtained as mentioned above exists, it determined with abnormal grain growth.

  Table 5 shows the results of the above investigations.

  From Table 5, in the case of test numbers 1 to 8 that satisfy the conditions of the present invention, the surface pressure fatigue strength of 3000 MPa or more and 1000 times bending fatigue strength of 1000 MPa or more equivalent to or higher than test number 21 that is a conventional gas carburizing steel. It is clear that it has excellent surface pressure fatigue strength and bending fatigue strength, especially low cycle bending fatigue strength, and that no abnormal grain growth has occurred.

  On the other hand, in the case of test numbers 9 to 20 of comparative examples that deviate from the conditions defined in the present invention, the target of the present invention has not been reached.

  In the case of the test number 9, since the value of “Si + Mn” of the steel 9 exceeds the range specified in the present invention, the Hv hardness exceeds 240, and the machinability deteriorates. Furthermore, since the value of “4Mn + 4Cr—Si” exceeds the range specified in the present invention, the amount of retained austenite in the carburized layer becomes excessive, and the surface pressure fatigue strength does not reach 3000 MPa.

  Also in the case of the test number 10, since the value of “Si + Mn” of the steel 10 exceeds the range defined in the present invention, the Hv hardness exceeds 240, and the machinability deteriorates.

  In the case of test number 11, since the value of “4Mn + 4Cr—Si” of the steel 11 is below the range specified in the present invention, the amount of retained austenite in the carburized layer is small, and so-called “familiarity” is not ensured. The fatigue strength does not reach 3000 MPa.

  In the case of test number 12, since the range of “4Mn + 4Cr—Si” of the steel 12 exceeds the range specified in the present invention, the amount of retained austenite in the carburized layer becomes excessive, and the surface pressure fatigue strength does not reach 3000 MPa.

  In the case of the test number 13, since the Si content of the steel 13 is below the range specified in the present invention, the temper softening resistance is low and the surface pressure fatigue strength does not reach 3000 MPa.

  In the case of test numbers 14 to 17, since the Mn contents of the steels 14 to 17 are all below the range specified in the present invention, the amount of retained austenite in the carburized layer is small, so-called “familiarity” cannot be ensured and adhesion occurs. Since pitting due to wear occurs, the surface pressure fatigue strength does not reach 3000 MPa.

  In the case of test number 18, since the Cr content of steel 18 exceeds the range specified in the present invention, mesh-like cementite is generated at the grain boundaries of the carburized layer after vacuum carburizing and quenching, and low cycle bending fatigue strength (1000 The bending fatigue strength is low and the bending fatigue strength has not reached the target.

  In the case of test number 19, since the steel 19 has a Si content below the range defined in the present invention, the temper softening resistance is low, and the surface pressure fatigue strength does not reach 3000 MPa.

  In the case of the test number 20, since the Si content of the steel 20 is below the range specified in the present invention, the temper softening resistance is low, the surface pressure fatigue strength does not reach 3000 MPa, and the V content is in the present invention. Since exceeding the specified range, abnormal grain growth occurs during the vacuum carburization treatment, so the low cycle bending fatigue strength (1000 times bending fatigue strength) is low, and the bending fatigue strength does not reach the target.

  As already described, the test number 21 is a comparative material obtained by gas carburizing the steel 19.

  When the steel for vacuum carburized gears of the present invention is used, the gas carburizing process that has been conventionally performed at about 850 to 980 ° C. is changed to the vacuum carburizing process at a high temperature exceeding 1000 ° C. to improve the production efficiency. In addition, the surface fatigue fatigue strength equal to or higher than the conventional case and the bending fatigue strength equal to or higher than the conventional case, in which gas carburizing treatment is performed using a low carbon alloy steel having a carbon content of about 0.2% by mass, Low cycle bending fatigue strength can be obtained, and abnormal grain growth can be suppressed, so that the manufacturing cost can be rationalized by shortening the carburizing time. In addition, since this vacuum carburized gear steel is excellent in machinability, it can be easily formed into a predetermined gear shape by cutting.

It is a figure which shows the shape of the small roller test piece used for the two-cylinder rolling fatigue test of an Example. It is a figure which shows the shape of the four-point bending test piece used for the four-point bending test of an Example.

Claims (2)

  In mass%, C: 0.13 to 0.30%, Si: more than 0.50% and 1.50% or less, Mn: 0.70 to 1.50%, P: 0.10% or less, S : 0.01 to 0.05%, Cr: 0.70 to 1.50%, Nb: 0.010 to 0.050%, Al: 0.010 to 0.050%, and N: 0.0100 to 0 0.0250%, Si, Mn, and Cr contents satisfy Si + Mn: 2.50% or less and 4Mn + 4Cr—Si: 7.2 to 9.0%, and the balance is Fe and impurities. A steel for vacuum carburized gears, characterized in that the inside V has a chemical composition of 0.005% or less, the structure before vacuum carburizing is a ferrite pearlite structure, and the ferrite grain size is 100 μm or less. In mass%, C: 0.13 to 0.30%, Si: more than 0.50% and 1.50% or less, Mn: 0.70 to 1.50%, P: 0.10% or less, S : 0.01 to 0.05%, Cr: 0.70 to 1.50%, Nb: 0.010 to 0.050%, Mo: 0.10 to 0.50%, Al: 0.010 to 0 0.05% and N: 0.0100 to 0.0250%, and the contents of Si, Mn, Cr and Mo are Si + Mn + 4Mo: 2.80% or less and 4Mn + 4Cr-Si: 7.2 to 9.0. % was filled, the balance being Fe and impurities thereof or Rannahli, V in impurities has the following chemical composition 0.005%, and, prior vacuum carburizing tissue in ferrite-pearlite structure, its ferrite grain size A steel for vacuum carburized gears, characterized by being 100 μm or less.

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