WO2015136917A1 - 窒化処理方法、及び、窒化部品の製造方法 - Google Patents
窒化処理方法、及び、窒化部品の製造方法 Download PDFInfo
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- WO2015136917A1 WO2015136917A1 PCT/JP2015/001281 JP2015001281W WO2015136917A1 WO 2015136917 A1 WO2015136917 A1 WO 2015136917A1 JP 2015001281 W JP2015001281 W JP 2015001281W WO 2015136917 A1 WO2015136917 A1 WO 2015136917A1
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/24—Nitriding
- C23C8/26—Nitriding of ferrous surfaces
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/06—Surface hardening
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
- C21D1/76—Adjusting the composition of the atmosphere
Definitions
- the present invention relates to a nitriding method and a method for manufacturing a nitrided part, and more particularly to a nitriding method for low alloy steel and a method for manufacturing a nitrided part.
- Steel parts used in automobiles and various industrial machines have carburized quenching, induction quenching, nitriding, and soft nitriding to improve mechanical properties such as fatigue strength, wear resistance, and seizure resistance.
- a surface hardening heat treatment is applied.
- heat treatment is performed in a ferrite region where the heating temperature is A 1 point or less, and phase transformation is not used. As a result, heat treatment strain can be reduced. Therefore, nitriding treatment and soft nitriding treatment are often used for parts having high dimensional accuracy and large parts, and are applied to gears used for transmission parts of automobiles and cranks used for engines, for example.
- the nitriding treatment is easier to control the atmosphere because the number of types of gas required for the treatment is smaller than the soft nitriding treatment.
- Examples of the nitriding treatment include gas nitriding treatment, salt bath nitriding treatment, and plasma nitriding treatment.
- a gas nitriding process having excellent productivity is mainly used for automobile parts and the like.
- the gas nitriding treatment a compound layer having a thickness of 10 ⁇ m or more is formed on the steel surface.
- the compound layer contains nitrides such as Fe 2-3 N and Fe 4 N, and the hardness of the compound layer is extremely high compared to the base material of the steel part. Therefore, the compound layer improves the wear resistance and surface fatigue strength of the steel part in the initial stage of use.
- the compound layer has low toughness and low deformability, peeling and cracking are likely to occur during use. For this reason, it is difficult to use a nitrided part that has been subjected to gas nitriding as a part to which an impact stress or a large bending stress is applied. Further, although the gas nitriding treatment has a small heat treatment strain, correction may be required for long parts such as a shaft and a crank. In this case, depending on the thickness of the compound layer, cracks may occur during correction, and the fatigue strength of the part may decrease.
- the thickness of the compound layer can be controlled by the treatment temperature of the nitriding treatment and the nitriding potential K N obtained from the NH 3 partial pressure and the H 2 partial pressure by the following formula.
- K N (NH 3 partial pressure) / [(H 2 partial pressure) 3/2 ]
- the compound layer can be made thinner and further the compound layer can be eliminated.
- the nitriding potential K N is lowered, it becomes difficult for nitrogen to enter the steel.
- the hardness of the hardened layer called a nitrogen diffusion layer becomes low, and the depth of the hardened layer also becomes shallow.
- the fatigue strength, wear resistance, and seizure resistance of the nitrided parts are reduced.
- K N ′ (NH 3 partial pressure) / [(H 2 partial pressure) 1/2 ] different from the above nitriding potential
- Patent Document 2 A method of making the hardened layer depth uniform has been proposed (for example, Patent Document 1).
- a method has been proposed in which a jig whose surface is made of a non-nitriding material is used when a nitrided material is placed in a processing furnace (for example, Patent Document 2).
- Patent Document 1 Using the nitriding parameter proposed by Patent Document 1, it is possible to suppress the compound layer generated on the outermost surface in a short time. However, sufficient hardened layer depth may not be obtained depending on required characteristics. Further, as proposed in Patent Document 2, when a non-nitriding jig is prepared and fluorination treatment is performed, new problems such as selection of the jig and an increase in work man-hours arise.
- An object of the present invention is to provide a method for nitriding a low alloy steel that suppresses the formation of a compound layer and that provides sufficient surface hardness and hardened layer depth.
- the nitriding method according to the present embodiment is a gas nitriding method in which a low alloy steel is heated to 550 to 620 ° C. in a gas atmosphere containing NH 3 , H 2 and N 2 , and the total processing time A is 1.5 to 10 hours.
- a processing step is provided.
- the gas nitriding process includes a process of performing a high K N value process and a process of performing a low K N value process.
- the nitriding potential K NX obtained by the equation (1) is 0.15 to 1.50, and the average value K NXave of the nitriding potential K NX is 0.30 to 0.80.
- the processing time is X hours.
- the step of performing the low K N value processing is performed after the high K N value processing is performed.
- the nitriding potential K NY obtained by the following formula (1) is 0.02 to 0.25
- the average value K NYave of the nitriding potential K NY is 0.03 to 0.20
- the processing time is Y time.
- the average value K Nave of the nitriding potential obtained by the equation (2) is 0.07 to 0.30.
- K Ni (NH 3 partial pressure) / [(H 2 partial pressure) 3/2 ]
- K Nave (X ⁇ K NXave + Y ⁇ K NYave ) / A (2)
- i is X or Y.
- the formation of the compound layer is suppressed and a sufficient hardened layer depth is obtained.
- FIG. 1 is a graph showing the relationship between the average value K NXave of the nitriding potential of the high K N value treatment, the surface hardness, and the compound layer thickness.
- FIG. 2 is a diagram showing the relationship between the average value K NYave of the nitriding potential of the low K N value treatment, the surface hardness, and the compound layer thickness.
- FIG. 3 is a diagram showing the relationship between the average value K Nave of the nitriding potential, the surface hardness, and the compound layer thickness.
- the inventors have studied a method of thinning a compound layer formed on the surface of a low alloy steel by nitriding and obtaining a deep hardened layer. Furthermore, nitriding treatment at (particularly high during treatment with K N value), in the vicinity of the surface of the low alloy steel, nitrogen is considered in conjunction a method restrain the voids gasified is formed. As a result, the present inventors obtained the following findings (a) to (c).
- K N value is the NH 3 partial pressure of the atmosphere in the furnace in which the gas nitriding process is performed (the nitriding atmosphere or simply the atmosphere), and It is defined by the following formula using H 2 partial pressure.
- K N (NH 3 partial pressure) / [(H 2 partial pressure) 3/2 ]
- the K N value can be controlled by the gas flow rate. However, a certain time is required after the flow rate is set until the K N value of the nitriding atmosphere reaches an equilibrium state. For this reason, the K N value changes every moment until the K N value reaches the parallel state. Further, when the K N value is changed during the gas nitriding process, the K N value varies until the equilibrium state is reached.
- K N value affects the compound layer, surface hardness, and hardened layer depth. Therefore, not only the mean value of K N values, by controlling in K N value range is also given range of variations in the gas nitriding process, it can sufficiently secure the case depth, and the formation of compound layer Can be suppressed.
- the compound layer is formed in the first half of the gas nitriding treatment.
- the compound layer is decomposed in the second half of the gas nitriding treatment, and the K N value may be controlled so that the compound layer is almost eliminated at the end of the gas nitriding treatment.
- a gas nitriding process (high K N value process) with a high nitriding potential is performed.
- a gas nitriding process (low K N value process) is performed in which the nitriding potential is lower than that of the high K N value process.
- the compound layer formed of a high K N value process is decomposed at a low K N value processing, to promote the formation of nitrogen diffusion layer (hardened layer). Therefore, it is possible to suppress the compound layer in the nitrided part, increase the surface hardness, and increase the depth of the hardened layer.
- the nitriding method of the present embodiment completed based on the above knowledge is that the low alloy steel is heated to 550 to 620 ° C. in a gas atmosphere containing NH 3 , H 2 and N 2 , and the total processing time A is 1.
- a gas nitriding treatment process for 5 to 10 hours is provided.
- the gas nitriding process includes a process of performing a high K N value process and a process of performing a low K N value process.
- the nitriding potential K NX obtained by the equation (1) is 0.15 to 1.50, and the average value K NXave of the nitriding potential K NX is 0.30 to 0.80.
- the processing time is X hours.
- the step of performing the low K N value processing is performed after the high K N value processing is performed.
- the nitriding potential K NY obtained by the equation (1) is 0.02 to 0.25
- the average value K NYave of the nitriding potential K NY is 0.03 to 0.20.
- time be Y hours.
- the average value K Nave of the nitriding potential obtained by the equation (2) is 0.07 to 0.30.
- K Ni (NH 3 partial pressure) / [(H 2 partial pressure) 3/2 ]
- K Nave (X ⁇ K NXave + Y ⁇ K NYave ) / A (2)
- i is X or Y.
- the compound layer formed on the surface of the low alloy steel is thinned, preferably the formation of voids (porous layer) is suppressed, and furthermore, a high surface hardness and a deep hardened layer are obtained. Can do. Therefore, a nitrided part (low alloy steel part) manufactured by performing this nitriding treatment has improved mechanical properties such as fatigue strength, wear resistance, and seizure resistance, and has improved bending straightness. .
- the method for manufacturing a nitrided part according to the present embodiment includes a step of preparing low alloy steel and a step of manufacturing the nitrided part by performing the above nitriding method on the low alloy steel.
- gas nitriding is performed on the low alloy steel.
- the processing temperature of the gas nitriding process is 550 to 620 ° C., and the processing time A of the entire gas nitriding process is 1.5 to 10 hours.
- a low alloy steel to be subjected to the nitriding method of the present embodiment is prepared.
- the low alloy steel as used herein is defined as steel containing 93% or more Fe by mass%, and more preferably containing 95% or more Fe.
- the low alloy steel referred to in the present specification is, for example, a carbon steel material for mechanical structure defined in JIS G 4051, a structural steel material guaranteed in hardenability defined in JIS G 4052, or a machine defined in JIS G 4053. It is a structural alloy steel.
- the content of the alloy element in the low alloy steel may deviate from the provisions of the above-mentioned JIS standard.
- the low alloy steel may further contain Ti, V, Al, Nb, etc. effective for improving the hardness of the surface layer by gas nitriding, or other elements as appropriate.
- the gas nitriding temperature (nitriding temperature) is mainly correlated with the diffusion rate of nitrogen and affects the surface hardness and the hardened layer depth. If the nitriding temperature is too low, the diffusion rate of nitrogen is slow, the surface hardness is low, and the hardened layer depth is shallow. On the other hand, if it exceeds nitriding temperature the C1 point A, ferrite phase (alpha phase) the nitrogen diffusion rate is small austenite phase than (gamma phase) is generated in the steel, the surface hardness becomes low, hardening depth Becomes shallower. Therefore, in this embodiment, the nitriding temperature is 550 to 620 ° C. In this case, it can suppress that surface hardness becomes low, and can suppress that hardened layer depth becomes shallow.
- processing time A for the entire gas nitriding process is performed in an atmosphere containing NH 3 , H 2 , and N 2 .
- the entire time of nitriding treatment that is, the time from the start to the end of nitriding treatment (treatment time A) correlates with the formation and decomposition of the compound layer and the penetration of nitrogen, and affects the surface hardness and the depth of the hardened layer. Effect.
- processing time A is too short, surface hardness will become low and hardened layer depth will become shallow.
- the treatment time A is too long, denitrification occurs and the surface hardness of the steel decreases. If the processing time A is too long, the manufacturing cost further increases. Accordingly, the processing time A of the entire nitriding process is 1.5 to 10 hours.
- the atmosphere of the gas nitriding treatment of the present embodiment inevitably contains impurities such as oxygen and carbon dioxide in addition to NH 3 , H 2 and N 2 .
- a preferable atmosphere contains 99.5% (volume%) or more of NH 3 , H 2 and N 2 in total.
- the gas nitriding process described above includes a process of performing a high K N value process and a process of performing a low K N value process.
- the gas nitriding process is performed with a higher nitriding potential K Nx than in the low K N value process.
- low K N value processing is performed after high K N value processing.
- the gas nitriding process is performed with a lower nitriding potential K NY than in the high K N value process.
- two-stage gas nitriding treatment (high K N value processing, low K N value processing) is performed.
- high K N value treatment By increasing the nitriding potential K N value in the first half of the gas nitriding treatment (high K N value treatment), a compound layer is formed on the surface of the low alloy steel.
- low K N value treatment By lowering the nitriding potential K N value in the latter half of the gas nitriding treatment (low K N value treatment), the compound layer formed on the surface of the low alloy steel is decomposed and nitrogen is permeated and diffused in the steel.
- the two-stage gas nitriding treatment a sufficient hardened layer depth is obtained using nitrogen obtained by decomposition of the compound layer while reducing the thickness of the compound layer.
- K NX The nitrogen potential for high K N value processing
- K NY the nitrogen potential for low K N value processing
- K Ni (NH 3 partial pressure) / [(H 2 partial pressure) 3/2 ] (1)
- the partial pressure of NH 3 and H 2 in the atmosphere of the gas nitriding treatment can be controlled by adjusting the gas flow rate. Therefore, the nitrogen potential K Ni can be adjusted by the gas flow rate.
- the gas flow rate When shifting from high K N value processing to low K N value processing, if the gas flow rate is adjusted to reduce the K Ni value, the partial pressure of NH 3 and H 2 in the furnace will be stabilized to some extent. Takes time. Adjustment of the gas flow rate for changing the K Ni value may be performed once, or may be performed a plurality of times (twice or more) as necessary. After the high K N value processing, prior to the low K N value processing once, after reducing the K Ni value, may be raised. The time point at which the K Ni value after the high K N value processing is 0.25 or less at the end is defined as the start time of the low K N value processing.
- the processing time of the high K N value processing is “X” (time), and the processing time of the low K N value processing is “Y” (time).
- the total of the processing time X and the processing time Y is within the processing time A of the entire nitriding treatment, and is preferably the processing time A.
- the nitrogen potential obtained by the equation (1) during the high K N value process is defined as “K NX ”.
- the nitrogen potential obtained by the expression (1) during the low K N value processing is defined as “K NY ”.
- the average value of the nitriding potential during the high K N value processing is “K NXave ”
- the average value of the nitriding potential during the low K N value processing is “K NYave ”.
- K Nave (X ⁇ K NXave + Y ⁇ K NYave ) / A (2)
- the nitriding treatment method the high K N values nitrogen potential K NX processing, the average value K NXave, processing time X, the low K N value processing of nitrogen potential K NY, average K NYave, processing time Y, and The average value K Nave satisfies the following conditions (I) to (IV).
- K NX 0.15 to 1.50 and K NY : 0.02 to 0.25
- IV Average value K Nave : 0.07 to 0.30
- the conditions (I) to (IV) will be described below.
- FIG. 1 is a diagram showing an average value K NXave of the nitriding potential in the high K N value processing and the relationship between the surface hardness and the compound layer thickness.
- FIG. 1 was obtained by the following experiment.
- Test material is an alloy steel material for machine structure according to JIS G 4053.
- a test material was inserted into a heat treatment furnace capable of controlling an atmosphere heated to a predetermined temperature, and NH 3 , N 2, and H 2 gases were allowed to flow.
- the flow rate of the gas was adjusted while measuring the partial pressure of NH 3 and H 2 in the atmosphere of the gas nitriding treatment to control the nitriding potential K Ni value.
- the K Ni value was determined from equation (1) by NH 3 partial pressure and H 2 partial pressure.
- the H 2 partial pressure during the gas nitriding treatment was measured by converting the difference in thermal conductivity between the standard gas and the measurement gas into a gas concentration using a heat conduction type H 2 sensor directly attached to the gas nitriding furnace body. The H 2 partial pressure was continuously measured during the gas nitriding process.
- the NH 3 partial pressure during the gas nitriding treatment was measured by attaching a manual glass tube NH 3 analyzer outside the furnace and calculating the partial pressure of residual NH 3 every 15 minutes.
- the nitriding potential K Ni value was calculated every 15 minutes when the NH 3 partial pressure was measured, and the NH 3 flow rate and the N 2 flow rate were adjusted so as to converge to the target value.
- the temperature of the atmosphere is 590 ° C.
- the treatment time X is 1.0 hour
- the treatment time Y is 2.0 hours
- K NYave is 0.05
- K NXave is 0.10 to 1.00. It was changed until.
- the total processing time A was 3.0 hours.
- the target is to set the compound layer thickness to 3 ⁇ m or less.
- the area ratio of the voids in the compound layer in the cross section of the test material was measured by observation with an optical microscope. 5 fields of view at a magnification of 1000 times (field area: 5.6 ⁇ 10 3 ⁇ m 2 ), and the ratio of voids in an area of 25 ⁇ m 2 in the range of 5 ⁇ m depth from the outermost surface for each field (hereinafter referred to as void area) Rate).
- void area ratio is 10% or more, the surface roughness of the nitrided part after the gas nitriding treatment becomes rough, and the compound layer becomes brittle, so that the fatigue strength of the nitrided part decreases. Therefore, in this embodiment, it was aimed that the void area ratio was less than 10%.
- the surface hardness and effective hardened layer depth of the test material after the gas nitriding treatment were determined by the following method.
- the Vickers hardness in the depth direction from the sample surface was measured with a test force of 1.96 N in accordance with JIS Z 2244.
- pieces of the Vickers hardness in a 50 micrometer depth position from the surface was defined as surface hardness (HV).
- HV surface hardness
- the surface hardness is 270 to 310 HV for JIS standard S45C and 550 to 590 HV for SCr420. Therefore, in the present embodiment, the target surface hardness is 290 HV or higher for S45C and 570 or higher for SCr420.
- the effective hardened layer depth was determined by the following method using the hardness distribution in the depth direction obtained by measuring the Vickers hardness from the surface to 50 ⁇ m, 100 ⁇ m, and thereafter to a depth of 1000 ⁇ m every 50 ⁇ m.
- the depth of the range which becomes 250 HV or more among distributions of Vickers hardness measured in the depth direction from the surface was defined as the effective curing depth ( ⁇ m).
- the depth of the range which becomes 300HV or more among distribution of the Vickers hardness measured from the surface to the depth direction was defined as effective hardened layer depth (micrometer).
- the effective hardened layer depth is a value ⁇ 20 ⁇ m determined by the formula (A).
- Effective hardened layer depth ( ⁇ m) 130 ⁇ ⁇ treatment time A (hour) ⁇ 1/2 (A)
- the effective hardened layer depth is set to satisfy the formula (B). Effective hardened layer depth ( ⁇ m) ⁇ 130 ⁇ ⁇ treatment time A (hour) ⁇ 1/2 (B)
- FIG. 1 was created based on the surface hardness of the specimen and the thickness of the compound layer obtained by the gas nitriding treatment with each average value K NXave among the measurement test results.
- the solid line in FIG. 1 is a graph showing the relationship between the average value K NXave and the surface hardness (Hv) of the nitriding potential in the high K N value processing.
- the broken line in FIG. 1 is a graph showing the relationship between the average value K NXave of the nitriding potential in the high K N value process and the thickness ( ⁇ m) of the compound layer. Referring to the solid line graph in FIG. 1, when the average value K NYave in the low K N value processing is constant, the surface hardness of the nitrided part increases as the average value K NXave in the high K N value processing increases. Increases significantly.
- the surface hardness is 570 HV or more, which is the target for the specimen of SCr420.
- the compound thickness decreases significantly.
- the average value K NXave becomes 0.80
- the thickness of the compound layer is 3 ⁇ m or less.
- the average value K NXave of the nitriding potential in the high K N value processing is set to 0.30 to 0.80.
- the surface hardness of the nitrided low alloy steel can be increased and the thickness of the compound layer can be suppressed. Furthermore, a sufficient effective hardened layer depth can be obtained. If the average value K NXave is less than 0.30, the formation of the compound is insufficient, the surface hardness is lowered, and a sufficient effective effect layer depth cannot be obtained. If the average value K NXave exceeds 0.80, the thickness of the compound layer may exceed 3 ⁇ m, and the void area ratio may be 10% or more.
- a preferable lower limit of the average value K NXave is 0.35.
- the preferable upper limit of the average value K NXave is 0.70.
- FIG. 2 is a diagram showing the relationship between the average value K NYave of the nitriding potential of the low K N value treatment, the surface hardness, and the compound layer thickness.
- FIG. 2 was obtained by the following test.
- the temperature of the nitriding atmosphere is 590 ° C.
- the processing time X is 1.0 hour
- the processing time Y is 2.0 hours
- the average value K NXave is constant at 0.40
- the average value K NYave is 0.01 to 0.00 .
- the gas nitriding treatment was performed on the test material having a chemical composition corresponding to SCr420 by changing to 30.
- the total processing time A was 3.0 hours.
- the surface hardness (HV), effective hardened layer depth ( ⁇ m), and compound layer thickness ( ⁇ m) at each average value K NYave were measured by the above-described method.
- FIG. 2 was created by plotting the surface hardness and the compound thickness obtained by the measurement test.
- the solid line in FIG. 2 is a graph showing the relationship between the average value K NYave of the nitriding potential in the low K N value treatment and the surface hardness, and the broken line shows the average value K NYave of the nitriding potential in the high K N value treatment and the compound It is a graph which shows the relationship with the depth of a layer.
- the surface hardness increases significantly as the average value K NYave increases from 0.
- K NYave becomes 0.03 the surface hardness becomes 570 HV or more.
- K NYave is 0.03 or more
- the surface hardness is substantially constant even when K NYave increases.
- the thickness of the compound layer is substantially constant until the average value K NYave decreases from 0.30 to 0.25.
- the thickness of the compound layer decreases significantly.
- the thickness of the compound layer is 3 ⁇ m or less.
- the average value K NYave is 0.20 or less, along with the reduction of the mean K NYave, the thickness of the compound layer but it decreases, as compared with the case where the average value K NYave is higher than 0.20, There is little reduction in the thickness of the compound layer.
- the average value K NYave of the low K N value processing is set to 0.03 to 0.20.
- the surface hardness of the gas-nitrided low alloy steel can be increased, and the thickness of the compound layer can be suppressed. Furthermore, a sufficient effective hardened layer depth can be obtained. If the average value K NYave is less than 0.03, denitrification occurs from the surface and the surface hardness decreases. On the other hand, if the average value K NYave exceeds 0.20, the decomposition of the compound is insufficient, the effective hardened layer depth is shallow, and the surface hardness decreases.
- a preferable lower limit of the average value K NYave is 0.05.
- a preferable upper limit of the average value K NYave is 0.18.
- the average value K NXave and average value K NYave above not only the above-mentioned range, high K N value nitride potential K NX during processing, and low The nitriding potential K NY during the K N value processing is also controlled within a predetermined range.
- the nitriding potential K NX during high K N value processing is set to 0.15 to 1.50
- the nitriding potential K NY during low K N value processing is set to 0.02 to 0.25.
- Table 1 shows the compound layer thickness ( ⁇ m), void area ratio (%), effective hardened layer depth ( ⁇ m), and surface of nitrided parts when nitriding is performed with various nitriding potentials K NX and K NY Indicates hardness (HV). Table 1 was obtained by the following test.
- the gas nitriding treatment (high K N value treatment and low K N value treatment) shown in Table 1 was performed to produce a nitrided part.
- the gas nitriding atmosphere temperature for each test number is 590 ° C.
- the processing time X is 1.0 hour
- the processing time Y is 2.0 hours
- K NXave is 0.40
- K NYave is 0.00 . 10 and constant.
- the minimum value K NXmin , K NYmin , the maximum value K NXmax , and K NYmax of K NX and K NY were changed to perform the high K N value process and the low K N value process.
- the processing time A for the entire nitriding treatment was set to 3.0 hours.
- Table 1 was obtained by measuring the compound layer thickness, the void area ratio, the effective hardened layer depth and the surface hardness of the nitrided parts after the gas nitriding treatment by the above-described measuring method.
- the minimum value K NXmin and the maximum value K NXmax are 0.15 to 1.50, and the minimum value K NYmin and the maximum value K NYmax are It was 0.02 to 0.25.
- the compound thickness was as thin as 3 ⁇ m or less, and the voids were suppressed to less than 10%.
- the effective hardened layer depth was 225 ⁇ m or more, and the surface hardness was 570 HV. Since the value of the formula (A) (target value of the effective cured layer) in each test number in Table 1 is 225 ⁇ m, the effective cured layer depth of the above test number is 225 ⁇ m or more, and the formula ( B) was met.
- test numbers 1 and 2 since K NXmin was less than 0.15, the surface hardness was less than 570 HV. In Test No. 1, since K NXmin is less than 0.14, the effective hardened layer depth was less than 225 ⁇ m.
- test numbers 7 and 8 since K NXmax exceeded 1.5, the voids in the compound layer were 10% or more. In Test No. 8, since K NXmax exceeded 1.55, the thickness of the compound layer exceeded 3 ⁇ m.
- the nitriding potential K NX in the high K N value processing is set to 0.15 to 1.50, and the nitriding potential K NY in the low K N value processing is set to 0.02 to 0.25.
- the thickness of the compound layer can be sufficiently reduced, and the voids can also be suppressed.
- the effective hardened layer depth can be sufficiently deep and high surface hardness can be obtained.
- the nitriding potential K NX is less than 0.15, the effective hardened layer is too shallow or the surface hardness is too low. If the nitriding potential K NX exceeds 1.50, the compound layer becomes too thick, or excessive voids remain.
- the nitriding potential K NY is less than 0.02, denitrification occurs and the surface hardness decreases. On the other hand, if the nitriding potential K NY exceeds 0.20, the compound layer becomes too thick. Therefore, in this embodiment, the nitriding potential K NX during the high K N value processing is 0.15 to 1.50, and the nitriding potential K NY during the low K N value processing is 0.02 to 0.25. It is.
- a preferable lower limit of the nitriding potential K NX is 0.25.
- a preferable upper limit of K NX is 1.40.
- a preferable lower limit of K NY is 0.03.
- a preferable upper limit of K NY is 0.22.
- FIG. 3 is a diagram showing the relationship between the average value K Nave of the nitriding potential, the surface hardness (HV), and the compound layer depth ( ⁇ m).
- FIG. 3 was obtained by conducting the following test. Gas nitriding was performed using SCr420 as a test material. The atmospheric temperature in the gas nitriding treatment was 590 ° C. Then, gas nitriding treatment (high K N value treatment and low K N value treatment) is performed by changing the treatment time X, treatment time Y, the range of nitriding potential and the average value (K NX , K NY, K NXave , K NYave ). Carried out.
- the effective hardened layer depth, the compound layer thickness, and the surface hardness were measured for the test materials after the gas nitriding treatment under each test condition by the above-described methods. As a result, it was found that if the average value K Nave is 0.06 or more, the effective hardened layer depth satisfies the formula (B). Furthermore, the obtained compound layer thickness and surface hardness were measured, and FIG. 3 was created.
- the solid line in FIG. 3 is a graph showing the relationship between the average value K Nave of the nitriding potential and the surface hardness (HV).
- the broken line in FIG. 3 is a graph showing the relationship between the average value K Nave of the nitriding potential and the thickness ( ⁇ m) of the compound layer.
- the surface hardness increases remarkably, and when the average value K Nave becomes 0.07, it becomes 570 HV or higher.
- the compound thickness becomes significantly thinner, and when the average value K Nave becomes 0.30, 3 ⁇ m It becomes as follows.
- the average value K Nave is less than 0.30, in accordance with the average value K Nave is low, although the compounds thickness gradually becomes thinner, compared with the case where the average value K Nave is higher than 0.30 Thus, there is little reduction in the thickness of the compound layer.
- the average value K Nave defined by the equation (2) is set to 0.07 to 0.30.
- the compound layer in the component after the gas nitriding treatment, the compound layer can be made sufficiently thin. Furthermore, high surface hardness is obtained. If the average value K Nave is less than 0.07, the surface hardness is low and the effective hardened layer is also shallow. On the other hand, if the average value K Nave exceeds 0.30, the compound layer exceeds 3 ⁇ m. A preferable lower limit of the average value K Nave is 0.08. A preferable upper limit of the average value K Nave is 0.27. If the average value K Nave is 0.06 or more, the effective hardened layer depth satisfies the formula (B).
- the processing time X is 0.50 hours or longer and the processing time Y is 0.50 hours or longer.
- Gas nitriding treatment is performed under the above conditions. Specifically, high K N value processing is performed under the above conditions, and then low K N value processing is performed under the above conditions. After the low K N value process, the gas nitriding process is terminated without increasing the nitriding potential.
- Nitrided parts are manufactured by carrying out the above gas nitriding treatment.
- the surface hardness is sufficiently high and the compound layer is sufficiently thin.
- the effective hardened layer depth is sufficiently deep, and voids in the compound layer can also be suppressed.
- the surface hardness is 570 HV or higher (when the nitrided part is SCr420) or 290 HV or higher (the nitrided part is S45C).
- the compound layer depth is 3 ⁇ m or less.
- Formula (B) is satisfy
- the void area ratio is less than 10%.
- SCr420 JIS G 4053, alloy steel for machine structure
- S45C JIS G 4051, carbon steel for machine structure
- the normalizing treatment was performed, followed by quenching and tempering.
- the steel bar was heated to 920 ° C. and held for 30 minutes, and then cooled by air.
- the quenching treatment the steel bar was heated to 900 ° C. and held for 30 minutes, and then cooled with water.
- the tempering treatment the steel bar was held at 600 ° C. for 1 hour.
- the S45C steel bar was heated to 870 ° C. and held for 30 minutes, and then air-cooled.
- a 15 mm ⁇ 80 mm ⁇ 5 mm test piece was collected from the manufactured steel bar by machining.
- a gas nitriding treatment was performed on the collected specimen under the following conditions.
- the test piece was charged into a gas nitriding furnace, and NH 3 , H 2 , and N 2 gases were introduced into the furnace. Then, conduct high K N value processing under the conditions shown in Table 2, were then conducted low K N value processing. Oil cooling was performed using 80 ° C. oil on the test piece after the gas nitriding treatment.
- the compound layer can be confirmed as a white uncorroded layer present in the surface layer.
- the compound layer was observed from 5 visual fields (field area: 2.2 ⁇ 10 4 ⁇ m 2 ) photographed at 500 ⁇ , and the thickness of 4 compound layers was measured every 30 ⁇ m. And the measured average value of 20 points
- the effective hardened layer depth of the steel bar of each test number was determined by the following method.
- SCr420 test numbers 26 to 30
- the depth in the range of 300 HV or higher in the distribution of Vickers hardness measured in the depth direction from the surface was defined as the effective hardened layer depth ( ⁇ m).
- S45C test numbers 21 to 25
- the depth in the range of 250 HV or higher in the distribution of Vickers hardness measured in the depth direction from the surface was defined as the effective curing depth ( ⁇ m).
- the thickness of the compound layer was 3 ⁇ m or less, the void ratio was less than 10%, and the surface hardness was 290 HV or higher for S45C and 570 HV or higher for SCr420. Furthermore, when the effective hardened layer depth was 225 HV or more and the formula (B) was satisfied, it was determined to be good.
- K NY in the low K N value process was 0.02 to 0.25, and the average value K NYave was 0.03 to 0.20. Further, the average value K Nave obtained by (Expression 2) was 0.07 to 0.30. Therefore, in any test number, the thickness of the compound layer after nitriding was 3 ⁇ m or less, and the void area ratio was less than 10%. Furthermore, the effective hardened layer is 225 ⁇ m or more and satisfies the formula (B). Furthermore, in S45C of test numbers 21 to 23, the surface hardness was 290 HV or higher, and in SCr420 of test numbers 26 to 28, the surface hardness was 570 HV or higher.
- test number 24 the maximum value of K NX in the high K N value processing exceeded 1.50. Therefore, the void area ratio was 10% or more.
- test number 25 the minimum value of K NX in the high K N value process was less than 0.15, and the average value K NXave was less than 0.30. Furthermore, the average value K Nave was less than 0.07. Therefore, the depth of the effective hardened layer was less than the value of formula (B), and the surface hardness was also less than 290 HV.
- test number 30 the average value K NYave in the low K N value treatment was less than 0.03. Therefore, the surface hardness was less than 570 HV.
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Abstract
Description
KN=(NH3分圧)/[(H2分圧)3/2]
KNi=(NH3分圧)/[(H2分圧)3/2] ・・・ (1)
KNave=(X×KNXave+Y×KNYave)/A ・・・ (2)
ここで、iはX又はYである。
一般に、KN値は、ガス窒化処理を行う炉内の雰囲気(窒化処理雰囲気、又は、単に雰囲気ということがある。)のNH3分圧、及び、H2分圧を用いて、下記式で定義される。
KN=(NH3分圧)/[(H2分圧)3/2]
硬化層を生成するためには、化合物層を窒素の供給源に利用した方が効率的である。化合物層の生成を抑制し、硬化層深さを確保するために、ガス窒化処理の前半に化合物層を形成する。そして、ガス窒化処理の後半に化合物層を分解させ、ガス窒化処理の終了時には化合物層がほぼ無くなるように、KN値を制御すればよい。具体的には、ガス窒化処理の前半では、窒化ポテンシャルを高くしたガス窒化処理(高KN値処理)を実施する。そして、ガス窒化処理の後半では、高KN値処理よりも窒化ポテンシャルを低くしたガス窒化処理(低KN値処理)を実施する。この場合、高KN値処理で形成された化合物層が、低KN値処理で分解され、窒素拡散層(硬化層)の形成を促進する。そのため、窒化部品において化合物層を抑制し、かつ表面硬さを高め、硬化層深さを深くすることができる。
ガス窒化処理の前半に高KN値で窒化処理して化合物層を生成させる場合、化合物層中に空隙を含む層(ポーラス層という)が生成される場合がある。この場合、窒化物が分解して窒素拡散層(硬化層)が形成された後も、窒素拡散層内に空隙がそのまま残存する場合がある。窒素拡散層内に空隙が残存すれば、窒化部品の疲労強度及び曲げ矯正性(曲げ矯正による硬化層の割れの有無)が低下する。高KN値処理において化合物層を生成させる場合にKN値の上限を制限すれば、ポーラス層及び空隙の生成を極力抑制することができる。
KNi=(NH3分圧)/[(H2分圧)3/2] ・・・ (1)
KNave=(X×KNXave+Y×KNYave)/A ・・・ (2)
ここで、iはX又はYである。
本実施形態による窒化処理方法では、低合金鋼に対してガス窒化処理を実施する。ガス窒化処理の処理温度は550~620℃であり、ガス窒化処理全体の処理時間Aは1.5~10時間である。
初めに、本実施形態の窒化処理方法の対象となる低合金鋼を準備する。本明細書でいう低合金鋼は、質量%で93%以上のFeを含有し、さらに好ましくは95%以上Feを含有する鋼と定義する。本明細書でいう低合金鋼は例えば、JIS G 4051に規定される機械構造用炭素鋼鋼材、JIS G 4052に規定される焼入れ性を保証した構造用鋼鋼材、JIS G 4053に規定される機械構造用合金鋼鋼材である。低合金鋼中の合金元素の含有量は、上述のJIS規格の規定から逸脱してもよい。低合金鋼はさらに、ガス窒化処理による表層部の硬さの向上に有効なTi、V、Al、Nb等、又は、これら以外の元素を、適宜含有してもよい。
ガス窒化処理の温度(窒化処理温度)は、主に、窒素の拡散速度と相関があり、表面硬さ及び硬化層深さに影響を及ぼす。窒化処理温度が低すぎれば、窒素の拡散速度が遅く、表面硬さが低くなり、硬化層深さが浅くなる。一方、窒化処理温度がAC1点を超えれば、フェライト相(α相)よりも窒素の拡散速度が小さいオーステナイト相(γ相)が鋼中に生成され、表面硬さが低くなり、硬化層深さが浅くなる。したがって、本実施形態では、窒化処理温度は550~620℃である。この場合、表面硬さが低くなるのを抑制でき、かつ、硬化層深さが浅くなるのを抑制できる。
本実施形態では、NH3、H2、N2を含む雰囲気でガス窒化処理を実施する。窒化処理全体の時間、つまり、窒化処理の開始から終了までの時間(処理時間A)は、化合物層の形成及び分解と窒素の浸透と相関があり、表面硬さ及び硬化層深さに影響を及ぼす。処理時間Aが短すぎると表面硬さが低くなり、硬化層深さが浅くなる。一方、処理時間Aが長すぎれば、脱窒が発生して鋼の表面硬さが低下する。処理時間Aが長すぎればさらに、製造コストが高くなる。したがって、窒化処理全体の処理時間Aは1.5~10時間である。
上述のガス窒化処理は、高KN値処理を実施する工程と、低KN値処理を実施する工程とを含む。高KN値処理では、低KN値処理よりも高い窒化ポテンシャルKNXでガス窒化処理を実施する。さらに高KN値処理後に低KN値処理を実施する。低KN値処理では、高KN値処理よりも低い窒化ポテンシャルKNYでガス窒化処理を実施する。
KNi=(NH3分圧)/[(H2分圧)3/2] ・・・ (1)
上述のとおり、高KN値処理中において式(1)で求められる窒素ポテンシャルを、「KNX」とする。低KN値処理中において式(1)で求められる窒素ポテンシャルを、「KNY」とする。さらに、高KN値処理中の窒化ポテンシャルの平均値を「KNXave」とし、低KN値処理中の窒化ポテンシャルの平均値を「KNYave」とする。
KNave=(X×KNXave+Y×KNYave)/A ・・・ (2)
(I)平均値KNXave:0.30~0.80
(II)平均値KNYave:0.03~0.20
(III)KNX:0.15~1.50、及び、KNY:0.02~0.25
(IV)平均値KNave:0.07~0.30
以下、条件(I)~(IV)について説明する。
高KN値処理において、窒化ポテンシャルの平均値KNXaveは0.30~0.80である。
ガス窒化処理後、供試材の断面を研磨し、エッチングして光学顕微鏡で観察した。エッチングは、3%ナイタール溶液で20~30秒間行った。化合物層は、低合金鋼の表層に存在し、白い未腐食の層として観察される。光学顕微鏡により500倍で撮影した組織写真5視野(視野面積:2.2×104μm2)から、それぞれ30μm毎に4点の化合物層の厚さを測定した。測定された20点の値の平均値を、化合物厚さ(μm)と定義した。化合物層厚さが3μm以下の時、剥離や割れの発生が大きく抑制される。そこで、本実施形態においては、化合物層厚さを3μm以下にすることを目標とした。
更に、光学顕微鏡観察によって、供試材の断面における化合物層中の空隙の面積率を測定した。倍率1000倍にて5視野測定(視野面積:5.6×103μm2)して、各視野について最表面から5μm深さの範囲の面積25μm2中に占める空隙の割合(以下、空隙面積率という)を算出した。空隙面積率が10%以上の場合、ガス窒化処理後の窒化部品の表面粗さが粗くなり、さらに、化合物層が脆化するため、窒化部品の疲労強度が低下する。したがって、本実施形態においては、空隙面積率が10%未満であることを目標とした。
さらに、ガス窒化処理後の供試材の表面硬さ及び有効硬化層深さを次の方法により求めた。試料表面から深さ方向のビッカース硬さを、JIS Z 2244に準拠して、試験力1.96Nで測定した。そして、表面から50μm深さ位置におけるビッカース硬さの3点の平均値を、表面硬さ(HV)と定義した。3μm超の化合物層が残存する一般的なガス窒化処理の場合、表面硬さは、JIS規格のS45Cで270~310HV、SCr420で550~590HVである。そのため、本実施形態においては、表面硬さは、S45Cで290HV以上、SCr420で570以上を目標とした。
有効硬化層深さは、表面から50μm、100μm、以降50μm毎に深さ1000μmまでビッカース硬さを測定し、得られた深さ方向の硬さ分布を用いて、次の方法で求めた。S45Cについては、表面から深さ方向に測定されたビッカース硬さの分布のうち、250HV以上となる範囲の深さを、有効硬化深さ(μm)と定義した。また、SCr420については、表面から深さ方向に測定されたビッカース硬さの分布のうち、300HV以上となる範囲の深さを、有効硬化層深さ(μm)と定義した。
有効硬化層深さ(μm)=130×{処理時間A(時間)}1/2 ・・・ (A)
有効硬化層深さ(μm)≧130×{処理時間A(時間)}1/2 ・・・ (B)
低KN値処理の窒化ポテンシャルの平均値KNYaveは0.03~0.20である。
ガス窒化処理において、雰囲気中のKNi値が平衡状態に達するまでには、ガス流量を設定してから一定の時間が必要である。そのため、KNi値が平行状態に達するまでの間にもKNi値は時々刻々と変化している。さらに、高KN値処理から低KN値処理へと移行するとき、ガス窒化処理の途中でKNi値の設定を変更することになる。この場合も、平衡状態に達するまでの間にKNi値は変動する。
本実施形態のガス窒化処理ではさらに、式(2)で定義される窒化ポテンシャルの平均値KNaveが0.07~0.30である。
KNave=(X×KNXave+Y×KNYave)/A ・・・ (2)
高KN値処理の処理時間X、及び、低KN値処理の処理時間Yは、式(2)で定義される平均値KNaveが0.07~0.30であれば、特に制限されない。好ましくは、処理時間Xは0.50時間以上であり、処理時間Yは0.50時間以上である。
ガス窒化処理後の試験片の、長さ方向に垂直な方向の断面を鏡面研磨し、エッチングした。光学顕微鏡を用いてエッチングされた断面を観察し、化合物層厚さの測定及び表層部の空隙の有無の確認を行った。エッチングは、3%ナイタール溶液で20~30秒間行った。
ガス窒化処理後の各試験番号の棒鋼に対して、JIS Z 2244に準拠し、試験力1.96Nで、表面から50μm、100μm、以降50μm毎に深さ1000μmまで、ビッカース硬さを測定した。ビッカース硬さ(HV)は、各3点ずつ測定し、平均値を求めた。表面硬さは、表面から50μm位置の3点の平均値とした。
結果を表2に示す。表2中の「有効硬化層深さ(目標)」欄には、式(A)で算出された値(目標値)が記載されており、「有効硬化層深さ(実績)」には有効硬化層の測定値(μm)が記載されている。表2を参照して、試験番号21~23及び試験番号26~28では、ガス窒化処理での処理温度が550~620℃であり、処理時間Aが1.5~10時間であった。さらに、高KN値処理におけるKNXが0.15~1.50であり、平均値KNXaveが0.30~0.80であった。さらに、低KN値処理におけるKNYが0.02~0.25であり、平均値KNYaveが0.03~0.20であった。さらに、(式2)で求められる平均値KNaveが0.07~0.30であった。そのため、いずれの試験番号においても、窒化処理後の化合物層の厚さは3μm以下であり、空隙面積率は10%未満であった。さらに、有効硬化層は225μm以上であり、かつ、式(B)を満たした。さらに試験番号21~23のS45Cでは、表面硬さが290HV以上であり、試験番号26~28のSCr420では、表面硬さが570HV以上であった。
Claims (2)
- NH3、H2及びN2を含むガス雰囲気で低合金鋼を550~620℃に加熱し、全体の処理時間Aを1.5~10時間とするガス窒化処理工程を備え、
前記ガス窒化処理工程は、
式(1)によって求められる窒化ポテンシャルKNXが0.15~1.50であり、前記窒化ポテンシャルKNXの平均値KNXaveが0.30~0.80であり、処理時間をX時間とする高KN値処理を実施する工程と、
前記高KN値処理を実施した後、式(1)によって求められる窒化ポテンシャルKNYが0.02~0.25であり、前記窒化ポテンシャルKNYの平均値KNYaveが0.03~0.20であり、処理時間をY時間とする低KN値処理を実施する工程とを含み、
式(2)によって求められる窒化ポテンシャルの平均値KNaveが0.07~0.30である、窒化処理方法。
KNi=(NH3分圧)/[(H2分圧)3/2] ・・・ (1)
KNave=(X×KNXave+Y×KNYave)/A ・・・ (2)
ここで、iはX又はYである。 - 低合金鋼を準備する工程と、
前記低合金鋼に対して、請求項1に記載の窒化処理方法を実施して窒化部品を製造する、窒化部品の製造方法。
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WO2017043609A1 (ja) * | 2015-09-08 | 2017-03-16 | 新日鐵住金株式会社 | 窒化処理鋼部品及びその製造方法 |
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JP2017171951A (ja) * | 2016-03-18 | 2017-09-28 | 新日鐵住金株式会社 | 鋼部品及びその製造方法 |
WO2018066667A1 (ja) * | 2016-10-05 | 2018-04-12 | 新日鐵住金株式会社 | 窒化処理部品及びその製造方法 |
WO2018066666A1 (ja) | 2016-10-05 | 2018-04-12 | 新日鐵住金株式会社 | 窒化処理部品及びその製造方法 |
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CN108486525A (zh) * | 2017-02-22 | 2018-09-04 | 学校法人丰田学园 | 金属制品的制造方法 |
US11198793B2 (en) | 2017-07-28 | 2021-12-14 | Dupont Toray Specialty Materials Kabushiki Kaisha | Aqueous coating agent composition and a coating composition for an aqueous lubricating film comprising the same |
CN113302011A (zh) * | 2019-03-05 | 2021-08-24 | 国立大学法人东海国立大学机构 | 使用金刚石切削工具的切削方法 |
WO2021029014A1 (ja) * | 2019-08-13 | 2021-02-18 | 日産自動車株式会社 | 電磁鋼板 |
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US10837096B2 (en) | 2015-09-08 | 2020-11-17 | Nippon Steel Corporation | Nitrided steel part and method of production of same |
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KR101818875B1 (ko) | 2018-01-15 |
CN105874094B (zh) | 2018-02-13 |
EP3118346A1 (en) | 2017-01-18 |
JP6217840B2 (ja) | 2017-10-25 |
CN105874094A (zh) | 2016-08-17 |
EP3118346B1 (en) | 2019-12-18 |
US20170016107A1 (en) | 2017-01-19 |
EP3118346A4 (en) | 2017-11-22 |
KR20160098336A (ko) | 2016-08-18 |
US10094014B2 (en) | 2018-10-09 |
JPWO2015136917A1 (ja) | 2017-04-06 |
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