WO2017090738A1 - 鋼、浸炭鋼部品、及び浸炭鋼部品の製造方法 - Google Patents
鋼、浸炭鋼部品、及び浸炭鋼部品の製造方法 Download PDFInfo
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Definitions
- the present invention relates to steel, carburized steel parts, and methods for manufacturing carburized steel parts.
- Mn, Cr, Mo, Ni, and the like are added in combination to steel used for machine structural parts.
- carburizing steel manufactured through processes such as casting, forging, and rolling are subjected to mechanical processing such as forging and cutting, and heat treatment such as carburizing,
- a carburized steel part including a carburized layer that is a hardened layer of the surface layer part and a steel part that is a base material that is not affected by the carburizing process is obtained.
- Forging can be broadly divided into hot forging, warm forging, and cold forging.
- Warm forging is characterized in that scale is less generated and parts can be manufactured with higher dimensional accuracy than hot forging.
- cold forging is characterized in that no scale is generated, the dimensional accuracy is higher, and the level is close to cutting. Therefore, a part manufacturing method that performs roughing after hot forging and then finishes by cold forging, a part manufacturing method that performs mild cutting as a finish after performing warm forging, or only cold forging.
- a part manufacturing method for molding has been studied.
- Patent Document 1 cold forgeability is improved by reducing the content of C, Si and Mn, thereby softening the steel for carburization, and fine AlN in the structure after hot rolling.
- a carburizing steel is described in which the effect of preventing grain coarsening is enhanced by dispersing a large amount of.
- cold forging has a feature that the dimensional accuracy is close to that of cutting, but depending on the parts to be cold forged, there are not a few cutting processes. That is, steel for cold forging is required not only for cold forgeability but also for improved machinability.
- Patent Document 1 does not mention machinability after cold forging, and the machinability improvement effect is unclear.
- the cost of cutting processing is high in the manufacturing cost of high strength mechanical structural parts.
- it is desired to improve both the cutting workability and cold forgeability of steel which is a material for high-strength mechanical structural parts.
- the cutting process becomes more efficient.
- the cold forgeability of steel a part of the cutting process can be replaced with cold forging that can be carried out at a relatively low cost.
- the cold forgeability of steel is impaired.
- the amount of alloy elements such as C, Si and Mn is reduced in the chemical composition of steel, the cold forgeability of steel can be improved while maintaining the machinability of steel, but the hardenability of steel is improved. The strength required for machine structural parts cannot be ensured because of the decrease.
- the present invention has excellent cold forgeability at a stage before carburizing treatment or carbonitriding treatment, since it has a lower deformation resistance and a higher critical compression ratio than conventional steel, and is covered without impairing the deformation resistance.
- High-strength carburized steel parts obtained by using steel that improves machinability, does not generate coarse grains during carburizing or carbonitriding, and can give high strength by carburizing or carbonitriding And a method for manufacturing a carburized steel part.
- the gist of the present invention is as follows.
- the chemical components are unit mass%, C: 0.07 to 0.13%, Si: 0.0001 to 0.50%, Mn: 0.0001 to 0.80%, S: 0.0050 to 0.0800%, Cr: more than 1.30% and 5.00% or less, B: 0.0005 to 0.0100%, Al: 0.070 to 0.200% N: 0.0030 to 0.0100%, Bi: more than 0.0001% and 0.0100% or less, Ti: 0.020% or less, P: 0.050% or less, O: 0.0030% or less, Nb : 0 to 0.1000%, V: 0 to 0.20%, Mo: 0 to 0.500%, Ni: 0 to 1.000%, Cu: 0 to 0.500%, Ca: 0 to 0.0%.
- the hardenability index Ceq obtained by substitution is more than 7.5 and less than 44.0
- AlN precipitation obtained by substituting the content shown in unit mass% of each element in the chemical component into the formula 2 Index I AlN is more than 0.00030 and less than 0.00110
- the metal structure contains 85 to 100 area% ferrite
- the equivalent circle diameter observed in a cross section parallel to the rolling direction of steel is 1 ⁇ m or more and less than 2 ⁇ m
- the average distance between sulfides is less than 30.0 ⁇ m
- the presence density of the sulfides having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m observed in the cross section parallel to the rolling direction of the steel is 300 / mm 2. That's it.
- a carburized steel part includes a steel part and a carburized layer on the outer surface of the steel part and having a Vickers hardness of HV550 or more.
- the thickness is more than 0.40 mm and less than 2.00 mm
- the average Vickers hardness at a position of 50 ⁇ m depth from the surface of the carburized steel part is HV650 or more and HV1000 or less
- the depth from the surface of the carburized steel part is The average Vickers hardness at a position of 2.0 mm is HV250 or more and HV500 or less
- the chemical composition of the steel part is unit mass%, C: 0.07 to 0.13%, Si: 0.0001 to 0 .50%, Mn: 0.0001 to 0.80%, S: 0.0050 to 0.0800%, Cr: more than 1.30% and 5.00% or less
- Nb 0 to 0.00.
- the existing density of the sulfide is less than 300 pieces / mm 2 .
- a method for manufacturing a carburized steel part according to another aspect of the present invention is a method for manufacturing a carburized steel part according to (3) or (4) above, which is described in (1) or (2) above.
- the method for manufacturing a carburized steel part according to (5) may further include a step of performing a quenching process or a quenching / tempering process after the carburizing process or the carbonitriding process.
- the steel according to the present invention is excellent in cold forgeability because it has a lower deformation resistance and a higher critical compression ratio than those of conventional steels at a stage prior to carburizing or carbonitriding. Does not occur and the machinability is excellent. Further, according to the present invention, it is possible to provide a carburized steel part that can be manufactured at low cost and has high strength, and a method for manufacturing the same.
- the present inventors have a low deformation resistance, a large critical compressibility, a high machinability, and a strength equivalent to that of conventional steel by carburizing or carbonitriding before the carburizing or carbonitriding.
- S becomes a sulfide in the carburizing steel, and this sulfide acts as a free cutting agent.
- a large amount of S causes a large amount of coarse sulfide to be generated in the carburizing steel, thereby reducing the cold forgeability of the carburizing steel.
- the present inventors examined a method for achieving high machinability with a small amount of S. As a result, it has been found that reducing the size of the sulfide using a small amount of Bi and increasing the density of the sulfide are effective for improving cold forgeability and machinability.
- the present inventors conducted various experiments on the relationship between the equivalent-circle diameter and density of sulfide, the amount of tool wear, and chip disposal.
- the present density of sulfide having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m observed in a cross section parallel to the rolling direction of the carburizing steel is 300 pieces / mm 2 or more. It was found that the wear of the steel was suppressed. Since sulfide acts as a lubricant between the cutting tool and the steel, it has an effect of suppressing wear of the cutting tool.
- the present inventors have improved chip disposal when the average distance between sulfides having an equivalent circle diameter of 1 ⁇ m or more observed in a cross section parallel to the rolling direction of the carburizing steel is less than 30.0 ⁇ m.
- sulfide acts as a starting point for fracture of chips generated during cutting, it has the effect of shortening the length of the chips and improving chip disposal.
- the amount of sulfide is small and the distribution is not uniform, it is estimated that long chips are likely to occur in a region where the distribution of sulfide is rough.
- the average distance between the sulfides having an equivalent circle diameter of 1 ⁇ m or more observed in a cross section parallel to the rolling direction is less than 30.0 ⁇ m, the amount of sulfides is small compared to conventional steel, and long chips It is estimated that the generation of can be suppressed.
- the cold forgeability of the carburizing steel is also improved.
- the sulfide is coarse, it acts as a starting point of cracking during cold forging of the carburizing steel and causes cracking.
- the sulfide is refined as described above, the sulfide will not work as a starting point of cracking.
- the sulfide in the steel can be finely dispersed as described above, and while the deformation resistance during cold forging is kept small, It has been found that the machinability of steel after forging is improved.
- the reason why the sulfide is finely dispersed by a small amount of Bi is considered as follows.
- Sulfides often crystallize before solidification of molten steel or during solidification of molten steel, and the size of sulfide is greatly affected by the cooling rate during solidification of molten steel.
- the solidification structure of continuous cast slabs usually has a dendritic form, which is formed due to diffusion of solute elements during the solidification process, and the solute elements are concentrated in the dendritic tree. To do. Since Mn tends to concentrate in the intertree parts, sulfides crystallize mainly in the dendritic intertree parts.
- Non-Patent Document 1 Dendrite primary arm spacing ( ⁇ m)
- D diffusion coefficient (m 2 / s)
- ⁇ solid-liquid interface energy (J / m 2 )
- ⁇ T solidification temperature range (° C.).
- the primary arm interval ⁇ of the dendrite depends on the solid-liquid interface energy ⁇ , and if this ⁇ can be reduced, ⁇ decreases. If ⁇ can be reduced, the size of sulfide crystallized between dendrite trees can be reduced.
- the present inventors presume that Bi has reduced the solid-liquid interfacial energy ⁇ , thereby reducing the dendrite primary arm spacing and reducing the size of the sulfide.
- the above-described sulfide refinement effect by Bi is obtained when the Bi content is more than 0.0001 mass% and 0.0100 mass% or less.
- Bi may be used as a free-cutting agent.
- Bi of less than 0.1% by mass does not have a sufficient machinability improving effect, and the hot workability of steel. Is usually avoided.
- the carburizing steel according to this embodiment it is sulfide that acts as a free cutting agent, and Bi is used to enhance the effect of improving the machinability of sulfide. Therefore, in the carburizing steel according to the present embodiment, both the cold forgeability and the machinability are enhanced by the synergistic effect of a small amount of Bi and sulfide.
- Carbon (C) is contained to ensure the hardness of the steel part of the carburized steel part including the carburized layer and the steel part.
- the C content of the conventional carburizing steel is about 0.2%.
- the C content is more than this amount. The amount is limited to 0.13% or less. The reason for this is that when the C content exceeds 0.13%, the hardness of the carburizing steel before forging increases remarkably and the critical compressibility decreases, so the cold forgeability of the carburizing steel is impaired. It is.
- the C content is less than 0.07%, even if a large amount of an alloy element described later that enhances hardenability is included and the hardenability is increased as much as possible, the hardness of the steel part of the carburized steel part is reduced. It is impossible to achieve the level of conventional carburizing steel. Therefore, it is necessary to control the C content within the range of 0.07 to 0.13%.
- the lower limit of the C content is preferably 0.08%.
- the upper limit with preferable C content is 0.12%, 0.11%, or 0.10%.
- Si 0.0001 to 0.50%
- Silicon (Si) is an element that improves fatigue strength by significantly increasing the temper softening resistance of low-temperature tempered martensitic steel such as carburized steel parts.
- the Si content needs to be 0.0001% or more.
- the Si content exceeds 0.50%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical compression ratio decreases, so the cold forging of the carburizing steel. Sexuality is impaired. Therefore, it is necessary to control the Si content in the range of 0.0001 to 0.50%. When emphasizing the tooth surface fatigue strength of carburized steel parts, the Si content is increased within this range.
- the Si content is reduced within this range.
- the Si content is preferably 0.10% or more.
- the Si content is preferably 0.20% or less. It is good also considering the lower limit of Si content as 0.01%, 0.05%, or 0.15%.
- the upper limit value of the Si content may be 0.45%, 0.35%, or 0.30%.
- Mn 0.0001 to 0.80%
- Manganese (Mn) is an element that enhances the hardenability of steel. In order to increase the strength of the carburized steel part after the carburizing heat treatment due to this effect, the Mn content needs to be 0.0001% or more. However, if the Mn content exceeds 0.80%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical compression ratio decreases. Sexuality is impaired. Therefore, it is necessary to control the Mn content in the range of 0.0001 to 0.80%.
- the lower limit value of the Mn content may be 0.04%, 0.05%, 0.10%, or 0.25%.
- the upper limit value of the Mn content may be 0.78%, 0.60%, 0.50%, or 0.45%.
- S 0.0050 to 0.0800%
- Sulfur (S) is an element that combines with Mn and the like in steel to form a sulfide such as MnS and improves the machinability of the steel.
- the S content needs to be 0.0050% or more.
- the S content exceeds 0.0800%, the sulfide becomes a starting point at the time of forging to cause cracking, so that the critical compressibility of the steel may be lowered. Therefore, it is necessary to control the S content in the range of 0.0050 to 0.0800%.
- a preferable lower limit of the S content is 0.0080%, 0.0090%, or 0.0100%.
- a preferable upper limit value of the S content is 0.0750%, 0.0500%, or 0.0200%.
- Chromium (Cr) is an element that enhances the hardenability of steel.
- the Cr content needs to be more than 1.30%.
- the Cr content exceeds 5.00%, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical compression ratio decreases, so cold forging of the carburizing steel. Sexuality is impaired. Therefore, it is necessary to control the Cr content within a range of more than 1.30% and not more than 5.00%.
- the steel content in the carburizing steel and the carburized steel part according to the present embodiment increases the Cr content as compared with the conventional carburizing steel.
- a preferable lower limit of the Cr content is 1.35%, 1.50%, or 1.80%.
- the upper limit with preferable Cr content is 4.50%, 3.50%, 2.50%, or 2.20%.
- B 0.0005 to 0.0100% Boron (B) is an element that greatly enhances the hardenability of steel even in a small amount when dissolved in austenite. This effect can increase the strength of the carburized steel part after the carburizing heat treatment. Further, since B does not need to be added in a large amount in order to obtain the above effect, there is a feature that the hardness of the carburizing steel before forging hardly increases. Therefore, B is positively utilized in the steel for carburizing steel and carburized steel parts according to the present embodiment. If the B content is less than 0.0005%, the effect of improving the hardenability cannot be obtained. On the other hand, when the B content exceeds 0.0100%, the above effect is saturated.
- the B content in the range of 0.0005 to 0.0100%.
- the lower limit of the B content is preferably 0.0010%, 0.0015%, or 0.0020%.
- the upper limit of the B content is preferably 0.0095% or 0.0080%.
- Al 0.070 to 0.200%
- Al has a deoxidizing action, and is easily combined with N to form AlN, and is an element effective for preventing austenite grain coarsening during carburizing heating.
- formation of BN is prevented and solid solution B contributing to hardenability is secured.
- the Al content is less than 0.070%, the effect of improving the hardenability by B cannot be obtained.
- the Al content exceeds 0.200%, the precipitation amount of AlN is excessively increased, and the plastic workability of the carburizing steel and the carburized steel parts decreases.
- the Al content exceeds 0.200%, the AlN precipitates are not finely dispersed and the individual sizes are increased.
- the Al content is set to 0.070 to 0.200%.
- a preferable lower limit of the Al content is 0.075%.
- the upper limit with preferable Al content is 0.150%.
- N 0.0030 to 0.0100%
- Nitrogen (N) forms an AlN precipitate, and has an effect of preventing crystal grain coarsening during carburization.
- N content is less than 0.0030%, the precipitation amount of AlN is reduced, and the effect of preventing the coarsening of crystal grains during carburization cannot be obtained.
- N is an element that forms BN and reduces the amount of dissolved B.
- N content exceeds 0.0100%, it becomes impossible to ensure the solid solution B which contributes to hardenability.
- the N content exceeds 0.0100%, coarse TiN is formed, which becomes the starting point of fracture during plastic working, and thus the cold forgeability of the carburizing steel is impaired. Therefore, it is necessary to control the N content within the range of 0.0030 to 0.0100%.
- the lower limit of the N content is preferably 0.0040%.
- the upper limit of the N content is preferably 0.0090% or 0.0085%.
- Bi More than 0.0001% and 0.0100% or less Bi is an important element in the carburizing steel according to this embodiment.
- a trace amount of Bi refines the dendrite structure during solidification of the molten steel, so that the sulfide is finely dispersed.
- the Bi content needs to exceed 0.0001%.
- the Bi content is more than 0.0001% and 0.0100% or less.
- the Bi content is 0.0010% or more or 0.0015% or more.
- the preferable upper limit of Bi content is 0.0095%, 0.0090%, or 0.0050%.
- the steel for carburizing steel and carburized steel parts according to this embodiment contains impurities.
- the impurities mean secondary materials such as scrap and elements such as Ti, P, and O mixed from the manufacturing process.
- Ti, P, and O need to be limited as follows in order to sufficiently exhibit the effects of the carburizing steel of the present embodiment. Since the above-mentioned impurities are not required for solving the problem of the carburizing steel of this embodiment, the lower limit value of the content of the above-mentioned impurities is 0%.
- Titanium (Ti) is an element having an effect of fixing N in steel as TiN. When Ti is contained, almost all N in the steel is fixed as TiN. However, since Ti is an expensive element, when Ti is contained, the manufacturing cost increases. Moreover, when Ti is over 0.020%, a large amount of Ti carbosulfide may be generated, which may prevent generation of coarse particles. From the above viewpoint, it is necessary to limit the Ti content to 0.020% or less. The preferable range of Ti content is less than 0.020%, and the further preferable range is 0.018% or less or 0.015% or less.
- P 0.050% or less Phosphorus (P) is an impurity. P reduces the fatigue strength and hot workability of steel. Therefore, it is preferable that the P content is small, and the lower limit of the content is 0%. However, considering the manufacturing cost, the lower limit value of the P content may be 0.005%, 0.010%, or 0.015%. On the other hand, the P content is acceptable if it is 0.050% or less. The P content is preferably 0.045% or less, or 0.035% or less, and more preferably 0.020% or less or 0.015% or less.
- Oxygen (O) is an impurity and is an element that forms oxide inclusions.
- O content exceeds 0.0030%, large inclusions that become the starting point of fatigue fracture increase, which causes a decrease in fatigue characteristics. Therefore, it is necessary to limit the O content to 0.0030% or less.
- the O content is 0.0015% or less.
- the smaller the O content, the better. Therefore, the lower limit of the O content is 0%.
- the lower limit value of the O content may be 0.0007% or 0.0010%.
- the upper limit of the O content may be 0.0025%, 0.0020%, or 0.0015%. In normal operating conditions, O is contained in an amount of about 0.0020%.
- the steel part in the carburizing steel and carburized steel component according to the present embodiment further includes Nb, V, Mo, Ni, Cu, Ca, Mg, Te, You may contain at least 1 sort (s) or 2 or more types of Zr, REM, and Sb instead of Fe of the remainder of a chemical component.
- the lower limit of the content of these selective elements is 0%. In the specification of the present application, all the values described as the lower limit value of the content of the selected element are listed as preferable values.
- the numerical limitation range of the selected element and the reason for the limitation will be described.
- Nb and V have an effect of preventing the coarsening of the structure.
- Niobium (Nb) is an element that combines with N and C in steel to form Nb (C, N). This Nb (C, N) suppresses grain growth by pinning austenite grain boundaries and prevents coarsening of the structure. If the Nb content is 0.0020% or more, the above effect is obtained, which is preferable. When the Nb content exceeds 0.1000%, the above effect is saturated. Therefore, the Nb content is preferably 0.0020 to 0.1000%. More preferably, the lower limit of Nb content is 0.0100%. More preferably, the upper limit of Nb content is 0.0500%, 0.0100%, 0.0050%, or 0.0040%.
- V Vanadium (V) is an element that forms V (C, N) by combining with N and C in steel. This V (C, N) suppresses the grain growth by pinning the austenite grain boundary, and prevents the coarsening of the structure. If the V content is 0.002% or more, the above effect is obtained, which is preferable. When the V content exceeds 0.20%, the above effect is saturated. Therefore, the V content is preferably 0.002 to 0.20%. More preferably, the lower limit of the V content is 0.05%. More preferably, the upper limit of V content is 0.10%.
- Mo, Ni, and Cu have the effect of increasing the hardenability of the steel and thereby increasing the strength of the carburized steel part after the carburizing heat treatment.
- Mo 0.005 to 0.500%
- Molybdenum (Mo) is an element that enhances the hardenability of steel. If the Mo content is 0.005% or more, this effect is preferable because the strength of the carburized steel part after the carburizing heat treatment can be increased. Mo is an element that does not form an oxide and hardly forms a nitride in a gas carburizing atmosphere. When Mo is contained in the carburizing steel, an oxide layer and a nitride layer on the surface of the carburized layer, or a carburized abnormal layer due to them is hardly formed. However, Mo is expensive.
- the Mo content is preferably 0.005 to 0.500%. More preferably, the lower limit of the Mo content may be 0.050%. Moreover, it is good also considering the upper limit of Mo content as 0.200%, 0.100%, 0.010%, or 0.006%.
- Nickel (Ni) is an element that enhances the hardenability of steel. If the Ni content is 0.005% or more, this effect is preferable because the strength of the carburized steel part after the carburizing heat treatment can be increased. Ni is an element that does not form oxides or nitrides in a gas carburizing atmosphere. When Ni is contained in the carburizing steel, an oxide layer and a nitride layer on the surface of the carburized layer, or an abnormal carburization layer due to them is hardly formed. However, Ni is expensive.
- the Ni content is preferably 0.005 to 1.000%. More preferably, the lower limit of the Ni content may be 0.050%. Moreover, it is good also considering the upper limit of Ni content as 0.700% or 0.500%.
- Copper (Cu) is an element that enhances the hardenability of steel. If the Cu content is 0.005% or more, this effect is preferable because the strength of the carburized steel part after the carburizing heat treatment can be increased.
- Cu is an element that does not form oxides or nitrides in a gas carburizing atmosphere. When Cu is contained in the carburizing steel, it becomes difficult to form an oxide layer and a nitride layer on the carburized layer surface or a carburized abnormal layer due to them. However, if the Cu content exceeds 0.500%, the ductility of the steel in a high temperature range of 1000 ° C. or higher is lowered, and the yield during continuous casting and rolling is lowered.
- the Cu content is preferably 0.005 to 0.500%. More preferably, the lower limit of the Cu content may be 0.050%. On the other hand, the upper limit value of the Cu content may be 0.300%, 0.010%, or 0.006%.
- the Cu content when containing Cu, in order to improve the ductility of the above-mentioned high temperature range, it is desirable to make Ni content into 1/2 or more of Cu content by unit mass%.
- Ca, Mg, Te, Zr, REM, and Sb have an effect of improving machinability.
- Ca 0.0002 to 0.0030%
- Calcium (Ca) is an element having an effect of sulfide shape control in which the shape of sulfide is made spherical without being elongated.
- Ca is an element that improves the machinability by forming a protective film on the cutting tool surface during cutting. If the Ca content is 0.0002% or more, these effects are obtained, which is preferable.
- the Ca content is preferably 0.0002 to 0.0030%. More preferably, the lower limit value of the Ca content may be 0.0008%. The upper limit value of the Ca content may be 0.0020% or 0.0005%.
- Mg 0.0002 to 0.0030%
- Magnesium (Mg) is an element that improves the machinability by controlling the form of the sulfide as well as Ca and further forming a protective film on the surface of the cutting tool during cutting. It is preferable that the Mg content is 0.0002% or more because these effects can be obtained. On the other hand, if the Mg content exceeds 0.0030%, a coarse oxide is formed, which may adversely affect the fatigue strength of the carburized steel part. Therefore, the Mg content is preferably 0.0002 to 0.0030%. More preferably, the lower limit of the Mg content may be 0.0008%. The upper limit value of the Mg content may be 0.0020% or 0.0012%.
- Te 0.0002 to 0.0030%
- Tellurium is an element that controls the form of sulfide. If the Te content is 0.0002% or more, this effect is obtained, which is preferable. On the other hand, when the Te content exceeds 0.0030%, hot embrittlement of the steel becomes significant. Therefore, the Te content is preferably 0.0002 to 0.0030%. More preferably, the lower limit of the Te content may be 0.0008%. The upper limit of the Te content may be 0.0020% or 0.0015%.
- Zr 0.0002 to 0.0050%
- Zirconium (Zr) is an element that controls the form of sulfide. If the Zr content is 0.0002% or more, this effect is obtained, which is preferable. On the other hand, if the Zr content exceeds 0.0050%, a coarse oxide is formed, which may adversely affect the fatigue strength of the carburized steel part. Therefore, the Zr content is preferably 0.0002 to 0.0050%. More preferably, the lower limit value of the Zr content may be 0.0008%. The upper limit value of the Zr content may be 0.0030% or 0.0011%.
- REM 0.0002 to 0.0050%
- REM is an element that controls the form of sulfide.
- the REM content is 0.0002% or more, this effect is obtained, which is preferable.
- the REM content exceeds 0.0050%, coarse oxides are formed, which may adversely affect the fatigue strength of carburized steel parts. Therefore, the REM content is preferably 0.0002 to 0.0050%. More preferably, the lower limit of the REM content may be 0.0008%.
- the upper limit of the REM content may be 0.0040%, 0.0030%, or 0.0010%.
- REM is a generic name for a total of 17 elements including 15 elements from lanthanum having an atomic number of 57 to lutesium having an atomic number of 57 plus scandium having an atomic number of 21 and yttrium having an atomic number of 39. Usually, it is supplied in the form of misch metal, which is a mixture of these elements, and added to the steel. In the present embodiment, the content of REM is the total value of the contents of these elements.
- Sb 0.0020 to 0.0500%
- Antimony (Sb) is an element that prevents decarburization and carburization in the carburizing steel manufacturing process (hot rolling, hot forging, annealing, etc.). It is preferable that the Sb content is 0.0020% or more because these effects can be obtained. If the Sb content exceeds 0.0500%, the carburizing property is impaired during carburizing treatment, and a necessary carburized layer may not be obtained. Therefore, the Sb content is preferably 0.0020 to 0.0500%. More preferably, the lower limit value of the Sb content may be 0.0050%. The upper limit value of the Sb content may be 0.0300% or 0.0030%.
- the carburizing steel according to the present embodiment includes the above-described basic element, and the balance is selected from the chemical composition including iron (Fe) and impurities, or the above-described basic element and the above-described selective element. At least one kind, and the balance has a chemical composition containing Fe and impurities.
- the solidification structure of the continuous cast slab used for manufacturing the carburizing steel of this embodiment is usually in a dendrite form. Sulfides in carburizing steel are often crystallized before solidification (in molten steel) or during solidification, and are greatly affected by the dendrite primary arm spacing. That is, if the dendrite primary arm interval is small, the sulfide crystallized between the trees will be small. As for the carburizing steel of this embodiment, it is desirable for the dendrite primary arm space
- the dendrite shape of the carburizing steel of this embodiment obtained by hot working the slab is not limited to the above range.
- Presence density of sulfide having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m observed in a cross section parallel to the rolling direction of steel (carburizing steel): 300 pieces / mm 2 or more
- An abundance density of 300 / mm 2 or more of sulfides having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m (hereinafter sometimes abbreviated as “fine sulfides”) observed in a cross section (L cross section) parallel to the rolling direction of steel. If present in steel, tool wear is suppressed.
- the lower limit value of the density of fine sulfides may be 325 / mm 2 , 350 / mm 2 , or 400 / mm 2 . Although it is not necessary to define the upper limit value of the density of fine sulfides, it is estimated that 600 pieces / mm 2 is a practical upper limit value in view of the specified range of chemical components and experimental results.
- the upper limit of the density of fine sulfides may be 500 / mm 2 .
- a sulfide having an equivalent circle diameter observed in the L section of less than 1 ⁇ m (hereinafter sometimes referred to as “ultrafine sulfide”) and a sulfide having an equivalent circle diameter observed in the L section of 2 ⁇ m or more (hereinafter referred to as “ultrafine sulfide”).
- "When abbreviated as” coarse sulfide ”) does not contribute to the improvement of machinability, and may further impair the cold forgeability.
- the alloy component especially S content
- the density of fine sulfides is within the above range
- the density of coarse sulfides and ultrafine sulfides is sufficiently reduced. It is not necessary to limit the density of these.
- Average distance between sulfides (fine sulfides) having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m observed in a cross section parallel to the rolling direction of steel: less than 30.0 ⁇ m
- the average value of distances between fine sulfides As a result of various experiments conducted by the present inventors on the relationship between (average distance between fine sulfides) and chip disposal, it is good if the average distance between these fine sulfides is less than 30.0 ⁇ m. It was confirmed that chip disposal was obtained. Therefore, the average distance between fine sulfides is defined as less than 30.0 ⁇ m.
- the upper limit of the average distance between fine sulfides may be 27.0 ⁇ m, 26.0 ⁇ m, or 25.0 ⁇ m.
- the lower limit value of the average distance between the fine sulfides is not particularly limited, but 12.0 ⁇ m is estimated to be a substantial lower limit value in view of the prescribed range of chemical components and experimental results.
- the lower limit value of the average distance between the fine sulfides may be 13.0 ⁇ m or 14.0 ⁇ m.
- Coarse sulfides and ultrafine sulfides are not considered when measuring the average distance. Coarse sulfides are not necessary to be measured because the number of carburizing steels according to this embodiment is small. Ultra fine sulfides do not contribute to the improvement of chip disposal and are not measured.
- the existence density of fine sulfide is determined by cutting the carburizing steel parallel to the rolling direction, preparing the cut surface by a conventional method so that the sulfide can be observed, and scanning electron micrographs at multiple measurement points.
- the fine sulfides are identified by taking a picture and calculating the equivalent circle diameter of each sulfide contained in each electron micrograph, and the number of fine sulfides contained in each electron micrograph is the area of the field of view of each electron micrograph Is obtained by dividing the presence density of fine sulfides at each measurement location and averaging these existence densities.
- the average distance between the fine sulfides is a line segment having the center of gravity of any two fine sulfides included in each of the above-mentioned electron micrographs as its ends and not passing through any fine sulfide other than these two arbitrary fine sulfides. Is drawn on each electron micrograph and the average distance between the fine sulfides at each measurement location is obtained by calculating the average length of these line segments in each electron micrograph, and the average distance at each measurement location is further calculated. It is obtained by averaging.
- the steel may contain inclusions that are not sulfides, but the inclusions may be confirmed by an energy dispersive X-ray analyzer attached to the scanning electron microscope.
- the equivalent circle diameter of sulfide is the diameter of a circle having an area equal to the area of sulfide, and can be obtained by image analysis.
- the abundance of sulfide and the average distance between sulfides at each measurement location are determined by image analysis that executes the above-described methods. In order to ensure sufficient measurement accuracy, it is preferable to increase the number of measurement points and the total area of the measurement visual field (total area of the electron micrograph).
- the inventors set the number of measurement locations to 25, the magnification of the electron micrograph to 500 times, and the total area of the measurement visual field to about 1.1 mm 2 .
- the location where the measurement is performed is not particularly limited, but is preferably an intermediate region between the surface and the center of the carburizing steel (D / 4 position when the carburizing steel is a round bar). This is because the intermediate region between the surface and the center of the carburizing steel has an average configuration in the carburizing steel.
- the inventors observed sulfide on a cut surface obtained by cutting the D / 4 position of the round bar in parallel to the axial direction of the round bar.
- the state of sulfide in the carburizing steel does not change by normal carburizing treatment, the state of sulfide in the steel part of the carburized steel part It becomes substantially the same state as the state of sulfide of the steel.
- the state of sulfide in the steel part of the carburized steel part can be specified by the same method as that for carburizing steel.
- Hardenability index Ceq more than 7.5 and less than 44.0 Hardenability obtained by substituting the content shown by unit mass% of each element in the chemical components of the carburizing steel of this embodiment into the following formula B.
- the index Ceq needs to be more than 7.5 and less than 44.0.
- the element symbol contained in Formula B indicates the content in unit mass% of the element related to the element symbol.
- the hardenability index Ceq may be calculated by regarding the content as 0% by mass.
- Ceq (0.7 ⁇ Si + 1) ⁇ (5.1 ⁇ Mn + 1) ⁇ (2.16 ⁇ Cr + 1) ⁇ (3 ⁇ Mo + 1) ⁇ (0.3633 ⁇ Ni + 1) (Formula B)
- the present inventors performed carburizing and quenching on the same carburizing heat treatment conditions on various carburizing steels having chemical components within the above-described range and different hardenability indices Ceq.
- the hardness of the carburized layer of steel and the effective hardened layer depth were measured.
- the present inventors compared with the above-mentioned conventional carburizing steel (C content is about 0.2%), the hardness of the carburized layer and the effective hardened layer depth (Vickers hardness is HV550 equal to or higher than equivalent).
- a threshold value of the hardenability index Ceq capable of obtaining the above depth) was obtained.
- the hardenability index Ceq when the hardenability index Ceq is 7.5 or less, it is not possible to obtain the same characteristics as the above-described conventional steel (C content is about 0.2%). Accordingly, the hardenability index Ceq needs to be more than 7.5. Further, according to the knowledge of the present inventors, when the hardenability index Ceq is 44.0 or more, the hardness of the carburizing steel before forging increases, the deformation resistance increases, and the critical compression ratio decreases. Therefore, the cold forgeability of the carburizing steel is impaired. Therefore, the hardenability index Ceq needs to be more than 7.5 and less than 44.0. This hardenability index Ceq is desirably as large as possible within the above range. Preferably, the lower limit value of the hardenability index Ceq may be 8.5, 12.1, or 20.1. Moreover, it is good also considering the upper limit of hardenability parameter
- index Ceq as 43.0, 42.0, or 36.0.
- AlN precipitation index I AlN AlN precipitation index I AlN : more than 0.00030 and less than 0.00110 It is obtained by substituting the content of Al, N and Ti in mass% of the carburizing steel of this embodiment into the following formula C. AlN precipitation index I AlN needs to be more than 0.00030 and less than 0.00110.
- the element symbol contained in Formula C indicates the content in unit mass% of the element related to the element symbol.
- I AlN Al ⁇ (N ⁇ Ti ⁇ (14/48)) (Formula C) N contained in the steel is first combined with Ti to form TiN.
- “(N ⁇ Ti ⁇ (14/48))” in the above formula C is the amount of N in the form other than TiN in the steel, that is, the amount of N that may be AlN.
- “14” in the above formula C is the atomic weight of N
- “48” is the atomic weight of Ti.
- AlN precipitation index I When AlN is 0.00030 or less, the precipitation amount of AlN is insufficient, so that coarsening of crystal grains during carburization cannot be prevented. Moreover, when the AlN precipitation index I AlN is 0.00110 or more, the precipitation amount of AlN becomes too large, and the plastic workability of the carburizing steel and the carburized steel parts is lowered.
- the AlN precipitation index I AlN needs to be more than 0.00030 and less than 0.00110.
- the lower limit value of the AlN precipitation index I AlN is preferably 0.00050.
- AlN precipitation index I The upper limit of AlN is preferably less than 0.00100 or 0.00080.
- the metal structure of the carburizing steel of this embodiment includes 85 area% or more of ferrite. Since the metal structure is mainly composed of ferrite, which is a soft phase, the carburizing steel of this embodiment is sufficiently soft and has excellent cold forgeability. In addition, since it is so preferable that there are many ferrites, the upper limit of the amount of ferrite is 100 area%. As long as the amount of ferrite is within the above range, the carburizing steel of the present embodiment may include any structure other than ferrite. Examples of structures that can be included in the carburizing steel of this embodiment include bainite and martensite.
- the method for measuring the amount of ferrite is not particularly limited, and may be followed by a conventional method.
- the carburizing steel is cut perpendicularly to the rolling direction, and the resulting cross section is polished and etched to reveal the structure. At least five structure photographs are taken, and the ratio of ferrite in each structure photograph is shown.
- the ferrite area ratio of the carburizing steel can be obtained with high accuracy by calculating the image area and averaging the ferrite area ratio of each structural photograph. It is preferable that the photographing location of the structure photograph is an intermediate region between the surface and the center of the carburizing steel (D / 4 part when the carburizing steel is a round bar). This is because the intermediate region between the surface and the center of the carburizing steel has an average configuration in the carburizing steel.
- the hardness of the carburizing steel of this embodiment is not particularly limited.
- the Vickers hardness of the carburizing steel of this embodiment is preferably 125 HV or less, and more preferably 110 HV or less.
- the limit compressibility of the carburizing steel of the present embodiment is 68% or more, and further shows excellent cold forgeability.
- the Vickers hardness of the carburizing steel of the present embodiment can be controlled by performing heat treatment, and is preferably low. Considering chemical components and experimental results, the lower limit value of the Vickers hardness of the carburizing steel of this embodiment is considered to be about 75 HV.
- the lower limit value of the Vickers hardness of the carburizing steel of this embodiment may be 80 HV or 95 HV.
- the carburized steel component 2 of the present embodiment is a cold plastic working S ⁇ b> 1, a cutting work S ⁇ b> 2, and a carburizing process or a carbonitriding process with respect to the carburizing steel 1 according to the above-described present embodiment.
- a quenching process or a quenching / tempering process S4 may be performed as a finishing heat treatment as necessary.
- a carburized layer 21 is formed on the outer surface of the steel part 20 of the carburized steel part 2 by the carburizing process or the carbonitriding process S3.
- the carburized layer 21 of the carburized steel part 2 according to the present embodiment is defined as a region having a Vickers hardness of HV550 or more.
- the thickness of the carburized layer 2 is equal to the effective hardened layer depth specified in JIS G 0557.
- the term “carburized layer” is understood as a concept including both a carburized layer and a carbonitrided layer according to common technical common sense. A method for manufacturing the carburized steel part 2 will be described later.
- Carburized layer thickness More than 0.40 mm and less than 2.00 mm
- the carburized steel part 2 of the present embodiment has a steel part 20 and a thickness generated on the outer surface of the steel part 20 as shown in FIG. And a carburized layer 21 of more than 0.40 mm and less than 2.00 mm.
- the thickness of the carburized layer is 0.40 mm or less, the strength of the carburized steel part, particularly the fatigue strength, is insufficient.
- the thickness of the carburized layer is 2.00 mm or more, the toughness of the surface of the carburized steel part is impaired.
- the lower limit value of the thickness of the carburized layer may be 0.45 mm or 0.50 mm.
- it is good also considering the upper limit of the thickness of a carburized layer as 1.70 mm, 1.50 mm, 1.00 mm, 0.90 mm, 0.70 mm, or 0.65 mm.
- Average Vickers hardness at a position of 50 ⁇ m depth from the surface of the carburized steel part HV650 or more and HV1000 or less
- a position at a depth of 50 ⁇ m from the surface of the carburized steel part 2 according to this embodiment is preferably HV650 or more and HV1000 or less.
- the hardness of the carburized layer is appropriately controlled.
- the toughness of the surface of the carburized steel part is impaired.
- the lower limit value of the average Vickers hardness at a position 50 ⁇ m deep from the surface of the carburized steel part 2 may be HV750, HV770, or HV800.
- the upper limit value of the average Vickers hardness at a position 50 ⁇ m deep from the surface of the carburized steel part 2 may be HV900, HV870, or HV850.
- Average Vickers hardness at a position at a depth of 2.0 mm from the surface of the carburized steel part HV250 or more and HV500 or less Further, a position at a depth of 2.0 mm from the surface of the carburized steel part 2 according to this embodiment (in FIG.
- the average Vickers hardness at the broken line (with the symbol B) is preferably HV250 or more and HV500 or less. In this case, the hardness of the steel part 20 (or transition part) is appropriately controlled. When the average Vickers hardness at a depth of 2.0 mm from the surface of the carburized steel part 2 is less than HV250, the strength of the carburized steel part is insufficient.
- the toughness of the carburized steel part is impaired, and breakage such as cracking is likely to occur.
- the lower limit value of the average Vickers hardness at a position of a depth of 2.0 mm from the surface of the carburized steel part 2 may be HV270, HV280, or HV300.
- the upper limit value of the average Vickers hardness at a position at a depth of 2.0 mm from the surface of the carburized steel part 2 may be HV400, HV380, or HV320.
- the Vickers hardness of the carburized layer 21 becomes harder than the carburizing steel 1 that is a material by the carburizing process or the carbonitriding process S3. Moreover, when the Vickers hardness of the steel part 20 after the carburizing process or the carbonitriding process S3 is insufficient, a quenching process or a quenching / tempering process S4 is performed as a finish heat treatment, and the Vickers hardness of the steel part 20 is set to HV250 or more. do it.
- the thickness of the carburized layer 21 of the carburized steel part 2 is obtained by obtaining a hardness transition curve representing the relationship between the vertical distance from the surface of the carburized layer 21 and the hardness.
- the hardness transition curve is obtained by cutting the carburized steel part 2 perpendicularly to the surface, polishing the cut surface, and measuring the hardness according to, for example, JIS G 0557 “Carburized hardened layer depth measurement test of steel”. can get.
- the thickness of the carburized layer 21, that is, the thickness of the region where the Vickers hardness is HV550 or more can be read from the hardness transition curve.
- the thickness of the carburized layer 21 may be measured at two or more locations, and the average value of the measured values may be regarded as the thickness of the carburized layer 21 of the carburized steel part 2.
- the average Vickers hardness at a position 50 ⁇ m deep from the surface of the carburized steel part 2 and at a position 2.0 mm deep from the surface of the carburized steel part 2 is obtained by cutting the carburized steel part 2 perpendicularly to the surface. It is obtained by polishing the surface, performing a Vickers hardness measurement test a plurality of times (preferably 5 times or more) at a depth of 50 ⁇ m and a position of 2.0 mm, and calculating an average value of the results.
- the chemical composition of the steel part 20 of the carburized steel part 2, the hardenability index Ceq, the AlN precipitation index I AlN , the average distance between the fine sulfides, and the density of the fine sulfides are substantially increased by carburizing or carbonitriding. Since it does not change, it is almost the same as the carburizing steel 1 that is the material of the carburized steel part 2. Since the rolling direction of the carburized steel part 2 matches the extending direction of the sulfide of the carburized steel part 2, it can be specified by observing the shape of the sulfide of the carburized steel part 2. On the other hand, the hardness of the steel part 20 is larger than the hardness of the carburizing steel 1 that is the material of the carburized steel part 2 because quenching and tempering occur during the carburizing process or the carbonitriding process S3.
- the carburized steel part 2 according to the present embodiment is obtained by carburizing or carbonitriding a carburizing steel having an AlN precipitation index I AlN within a specified range, generation of coarse grains is suppressed. .
- the density of coarse grains in the carburized steel part 2 according to the present embodiment is not particularly defined.
- the rolling direction of the carburized steel part 2 according to the present embodiment It is preferable that the number density of coarse grains at a depth of 2.0 mm from the surface of the carburized steel part 2 measured in a cross-section perpendicular to is less than 0.1 / mm 2 .
- the number density of coarse grains is usually within the above range.
- the number density of the coarse grains described above is, for example, any 10 or more locations at a depth of 2.0 mm from the surface of the carburized steel part 2 in a cross section perpendicular to the rolling direction of the carburized steel part 2 in which crystal grains are exposed. 1 is obtained by taking a micrograph of a 1 mm square field of view, measuring the total number of prior austenite having a grain size of 4 or more contained in these photographs, and dividing this number by the total area of the field of view.
- the carburized steel part 2 according to the present embodiment can be used as a high-strength part. Therefore, the structure of the carburized steel part 2 according to the present embodiment is not particularly limited.
- the structure at a depth of 0.4 mm from the surface of the carburized steel part 2 is composed of 0 to 10 area% ferrite, martensite, It may be composed of bainite, tempered martensite, tempered bainite, and the balance including one or more selected from the group consisting of cementite.
- the structure at a depth of 0.4 mm from the surface of the steel part 2 is usually within the above-mentioned range.
- a cold forged product is, for example, a machine part used in automobiles and construction machines, and is a steel part such as a gear, a shaft, or a pulley.
- the carburizing steel manufacturing method of the present embodiment is a continuous casting of a slab having the same chemical composition as the carburizing steel of the present embodiment and having a dendrite primary arm spacing of less than 600 ⁇ m within a range of 15 mm from the surface.
- the slab is manufactured by hot working and further annealing. Hot working may include hot rolling.
- a slab having the same chemical composition as the carburizing steel of this embodiment is manufactured by a continuous casting method.
- the slab may be made into an ingot (steel ingot) by the ingot-making method. Casting is performed using, for example, a 220 ⁇ 220 mm square mold under the conditions that the superheat of the molten steel in the tundish is 10 to 50 ° C. and the casting speed is 1.0 to 1.5 m / min.
- the liquidus temperature to the solidus temperature at a depth of 15 mm from the slab surface when casting molten steel, the liquidus temperature to the solidus temperature at a depth of 15 mm from the slab surface.
- the average cooling rate in the temperature range up to (hereinafter sometimes simply referred to as “average cooling rate”) needs to be 100 ° C./min or more and 500 ° C./min or less. If the average cooling rate is less than 100 ° C./min, it becomes difficult to make the dendrite primary arm interval less than 600 ⁇ m at a depth of 15 mm from the slab surface, and there is a possibility that the sulfide cannot be finely dispersed.
- the temperature range from the liquidus temperature to the solidus temperature is the temperature range from the solidification start temperature to the solidification end temperature of the molten steel. Therefore, the average cooling rate in this temperature range means the average solidification rate of the slab (that is, the average cooling rate during solidification).
- the average cooling rate can be achieved by, for example, controlling the size of the mold cross section, the casting speed, and the like to appropriate values, or increasing the amount of cooling water used for water cooling immediately after casting. These means can be applied to both the continuous casting method and the ingot-making method.
- the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a depth of 15 mm from the slab surface can be estimated by observing the dendrite secondary arm interval of the slab.
- the cross section of the slab is etched with picric acid, and the dendrite secondary arm interval ⁇ 2 ( ⁇ m) is measured at 100 points with a pitch of 5 mm in the casting direction at a depth of 15 mm from the slab surface.
- the cooling rate A (° C./second) in the temperature range from the liquidus temperature to the solidus temperature of the slab is calculated from the value, and the arithmetic average of the cooling rate A is calculated from the slab surface.
- a plurality of slabs with different casting conditions may be manufactured, the average cooling rate in each slab may be obtained by the above formula D, and the optimum casting conditions may be determined from the obtained cooling rate.
- the billet (steel piece) is manufactured by hot working a slab or ingot, and the billet is further hot worked to obtain a bar steel and a wire rod.
- the slab after the casting process is subjected to hot rolling, hot forging, etc. to obtain a hot worked steel material.
- Plastic processing conditions such as processing temperature, processing rate, and strain rate in the hot processing step are not particularly limited, and appropriate conditions may be selected as appropriate.
- the hot-worked steel immediately after the hot-working step (that is, not substantially cooled) is added to the hot-worked steel in a temperature range where the surface temperature of the hot-worked steel is 800 ° C to 500 ° C.
- Carburizing steel of the present embodiment is obtained by performing slow cooling so that the cooling rate is more than 0 ° C./second and 1 ° C./second or less.
- the cooling rate at 800 ° C to 500 ° C the temperature range where austenite transforms into ferrite and pearlite, exceeds 1 ° C / second, the structural fraction of bainite and martensite in the carburizing steel increases, and the carburizing steel. Insufficient amount of ferrite. As a result, the hardness of the carburizing steel increases, the deformation resistance increases, and the critical compression ratio decreases. Therefore, it is preferable to limit the cooling rate in the above temperature range to more than 0 ° C./second and 1 ° C./second or less. More preferably, the cooling rate in the above temperature range is more than 0 ° C./second and 0.7 ° C./second or less.
- a heat insulating cover, a heat insulating cover with a heat source, or a holding furnace is installed after the rolling line or hot forging line. do it.
- spheroidizing annealing may be further performed after slow cooling to obtain the carburizing steel of the present embodiment.
- the spheroidizing annealing conditions are not particularly limited, and appropriate conditions may be selected as appropriate.
- the chemical component is composed of the basic element, the selective element, and the remainder containing Fe and impurities, and the carburizing steel manufactured through the manufacturing process described above is subjected to cold plastic working S1 to give a shape.
- the plastic processing conditions such as processing rate and strain rate in the cold plastic processing are not particularly limited, and suitable conditions may be selected as appropriate.
- the carburizing steel after cold plastic working is subjected to cutting S2 to give the shape of machine structural parts.
- cutting it is possible to give the carburizing steel a precise shape that is difficult to form only by cold plastic working. Since the carburizing steel of this embodiment is excellent in machinability, the chip processing is higher in this cutting process than the conventional steel, and the tool life is not impaired.
- the cutting process may be performed before or after the cold plastic working. However, in order to improve the dimensional accuracy of the carburized steel part, it is preferable to perform the cutting process after the cold plastic working.
- the carburized steel or the carbonitriding treatment S3 is performed on the carburized steel that has been given a shape by cold plastic working and cutting, thereby obtaining the carburized steel part according to the present embodiment.
- the conditions for carburizing or carbonitriding are not particularly limited, and may be appropriately selected according to the strength desired for the carburized steel part.
- the carburizing steel according to the present embodiment has a carburizing temperature of 830 to 1100 ° C., a carbon potential. Is preferably 0.5 to 1.2% and carburized for 1 hour or longer.
- a quenching process or a quenching / tempering process S4 may be performed as necessary.
- the quenching process or the quenching / tempering process is preferably performed when the Vickers hardness of the steel part of the carburized steel part after the carburizing process or the carbonitriding process is insufficient.
- the conditions for quenching / tempering treatment are not particularly limited, and may be appropriately selected according to the strength desired for the carburized steel part.
- quenching treatment or quenching / tempering is performed under the condition that the temperature of the quenching medium is in the range of room temperature to 250 ° C. It is preferable to carry out the treatment. Moreover, you may perform a subzero process to a carburized steel part after hardening as needed.
- the carburized steel part after the quenching process or the quenching / tempering process may be further subjected to a grinding process or a shot peening process.
- a grinding process it is possible to impart to the carburizing steel a precise shape that is difficult to form only by cold plastic working.
- the shot peening treatment compressive residual stress is introduced into the surface layer of the carburized steel part. Since compressive residual stress suppresses the occurrence and development of fatigue cracks, it is possible to further improve the fatigue strength of carburized steel parts (particularly, when the carburized steel parts are gears, the fatigue strength of the root and tooth surfaces).
- the conditions for the shot peening treatment are not particularly limited, but it is desirable to perform the shot peening treatment using shot grains having a diameter of 0.7 mm or less and an arc height of 0.4 mm or more.
- the carburizing steel according to the present embodiment is obtained by casting a slab having a predetermined chemical component under a predetermined condition, and a dendrite structure that becomes a crystallization nucleus of sulfide is refined, Sulfides in steel are finely dispersed.
- the carburizing steel according to this embodiment has high machinability after cold forging (that is, machinability before carburizing), it is suitable as a material for steel parts such as gears, shafts, and pulleys. is there.
- the carburizing steel according to the present embodiment can suppress generation of distortion during heat treatment without generating coarse grains during carburizing, and can suppress deformation of the carburized steel part.
- the carburizing steel of this embodiment is excellent in machinability when a rough formed product obtained by cold forging after annealing is cut. For this reason, the carburizing steel of this embodiment can reduce the ratio of the cutting cost to the manufacturing cost of steel parts such as automobiles, gears, shafts and pulleys for industrial machines, and can improve the quality of parts. Can do.
- the carburizing steel of the present embodiment has a relatively small amount of carbon, contains a small amount of Bi, has a component composition in which the hardenability index Ceq and the AlN precipitation index I AlN are controlled in a preferable range, and is sulfide. Is finely dispersed, the deformation resistance during cold forging is small, the machinability after cold forging is high, and the strength after carburizing is high.
- the carburizing steel of this embodiment can have a Vickers hardness of, for example, HV125 or less, the deformation resistance during cold forging is small, and the critical compression ratio can be 68% or more. Good properties.
- the Vickers hardness of the steel part is HV250 or more
- the Vickers hardness of the carburized layer is HV650 or more. Therefore, it is suitable as a material for carburized steel parts.
- the carburized layer includes a steel part and a carburized layer generated on the outer surface of the steel part, and the Vickers hardness of the carburized layer is 50 ⁇ m deep from the surface of the carburized steel part.
- HV650 or more and HV1000 or less, and the Vickers hardness of the steel part at a position 2.0 mm deep from the surface of the carburized steel part is HV250 or more and HV500 or less, so it is suitably used as a mechanical part such as gears, shafts, pulleys, etc. be able to.
- Example 1 Steels a to aa having chemical compositions shown in Table 1A and Table 1B were melted in a 270 ton converter, and continuous casting was performed using a continuous casting machine to produce a 220 ⁇ 220 mm square slab.
- the superheat of the molten steel in the tundish was set to 30 ° C.
- the casting speed was set to 1.0 m / min.
- the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position 15 mm deep from the surface of the slab was controlled by changing the amount of cooling water in the mold.
- slabs a to aa having chemical components shown in Table 1A and Table 1B were continuously cast.
- the dendrite primary arm interval in the range of 15 mm from the surface layer was 600 ⁇ m or more.
- the dendrite primary arm interval in the range of 15 mm from the surface layer of the other slabs was less than 600 ⁇ m.
- Steels a to n shown in Table 1A and Table 1B are steels having a chemical composition defined in the present invention.
- Steels o to aa are comparative steels whose chemical compositions deviate from the conditions defined in the present invention.
- the numerical value with the underline in Table 1A and Table 1B shows that it is outside the range prescribed
- the content of elements that are not included or whose content is below the level considered as an impurity is blank.
- the slab was once cooled to room temperature before hot forging, and the test piece was collected. Thereafter, each slab was heated at 1250 ° C. for 2 hours, and the heated slab was hot forged to produce a plurality of round bars having a diameter of 30 mm. After hot forging, the round bar was allowed to cool in the atmosphere. The cooling was performed by leaving the round bar covered with a heat insulating cover so that the cooling rate in the temperature range of 800 ° C. to 500 ° C. was 1 ° C./second or less. Furthermore, a part of the round bar after being allowed to cool was subjected to spheroidizing annealing (SA). In this way, steel materials made of carburizing steel of test numbers 1 to 27 were manufactured.
- SA spheroidizing annealing
- the dendrite primary arm spacing and dendrite secondary arm spacing of the solidified structure of the slab are obtained by etching the cross section of the above slab with picric acid, and at a pitch of 5 mm in the casting direction at a depth of 15 mm from the slab surface.
- the dendrite primary arm interval and the secondary arm interval were measured at 100 points, the average values of the dendrite primary arm interval and the secondary arm interval at each measurement point were calculated, and these were averaged.
- the average cooling rate of the slab of the example estimated based on the dendrite secondary arm interval of the slab of the example was 100 ° C./min or more and 500 ° C./min or less.
- a sulfide (fine sulfide) having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m is specified, and the number of fine sulfides contained in each electron micrograph is the field of view of each electron micrograph.
- the density of fine sulfides at each measurement point is obtained, and by averaging the density, the equivalent circle diameter observed in a cross section parallel to the rolling direction of steel is 1 ⁇ m or more and less than 2 ⁇ m.
- the abundance density fine sulfide abundance density
- the center of gravity of any two fine sulfides included in each of the above-mentioned electron micrographs is drawn at both ends, and a line segment that does not pass through any fine sulfide other than these two fine sulfides is drawn on each electron micrograph. Then, by obtaining the average value of the lengths of these line segments in each electron micrograph, the average distance between the fine sulfides at each measurement location is obtained, and by further averaging the average distance at each measurement location, The average distance (intersulfide distance) between sulfides having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m observed in a cross section parallel to the rolling direction was determined. The number of measurement points was 25, the magnification of the electron micrograph was 500 times, and the total area of the measurement field of view was about 1.1 mm 2 .
- the carburizing steel is cut perpendicular to the rolling direction, and the resulting cross section is polished and etched to reveal the structure, and five structural photographs are taken, and the proportion of ferrite in each structural photograph is imaged.
- the ferrite area ratio of the carburizing steel was determined by analyzing and averaging the ferrite area ratio of each structural photograph. The location of the tissue photograph was D / 4. As a result, it was confirmed that the ferrite area ratios of all the examples were within the specified range of the present invention.
- the example steel k having a relatively high C content contained martensite and bainite, which were structures other than ferrite, but the ferrite area ratio was 85%, and therefore satisfied the provisions of the present invention.
- the hardness of the round bar (carburizing steel) is measured by measuring the hardness at 10 measurement points in the cross section perpendicular to the rolling direction of the round bar using a Vickers hardness tester. The average value of the hardness at each measurement point was calculated by calculating. The position of the measurement point was the D / 4 position of the round bar (the position at a depth of 1/4 of the diameter D of the round bar).
- a round bar test piece was prepared from the R / 2 position of a round bar having a diameter of 30 mm (position at a depth of 1/2 of the radius R of the round bar).
- the round bar test piece is a test piece having a diameter of 10 mm and a length of 15 mm centered on the R / 2 position of a round bar having a diameter of 30 mm.
- the longitudinal direction of the round bar test piece is a forging shaft of a round bar having a diameter of 30 mm. And parallel. Further, a notch was provided in the center of the end face of the round bar test piece.
- This notch shape is described in “Cold Upsetting Test Method” Cold Forging Subcommittee Material Research Group, Plasticity and Processing, vol. 22, no. 241, p139 according to the notch of the No. 2 test piece.
- Ten round bar test pieces were prepared for each steel.
- a 500 ton hydraulic press was used for the cold compression test.
- Cold compression was performed at a speed of 10 mm / min using a constraining die, and the compression was stopped when a microcrack of 0.5 mm or more occurred in the vicinity of the notch, and the compression rate at that time was calculated. This measurement was performed a total of 10 times to obtain a compression rate at which the cumulative failure probability was 50%, and the compression rate was defined as the limit compression rate.
- a round bar with a diameter of 30 mm subjected to the cold compression test was cold-drawn at a surface reduction rate of 30.6% to obtain a steel bar with a diameter of 25 mm.
- the cold drawn steel bar was cut into a length of 500 mm to obtain a test material for turning.
- the outer periphery of the test material having a diameter of 25 mm and a length of 500 mm thus obtained was turned using an NC lathe under the following conditions, and the machinability was investigated. After 10 minutes from the start of turning, the wear amount (mm) of the flank face of the carbide tool was measured. When the measured flank wear amount was 0.2 mm or less, it was determined that the tool life was excellent.
- Chip disposal was evaluated by the following method. Chips discharged in 10 seconds during the machinability test were collected. The length of the collected chips was examined, and 10 chips were selected in order from the longest. The total weight of the ten selected chips was defined as “chip weight”. When the total number of chips was less than 10 as a result of long chip connection, the average weight of the recovered chips was measured, and a value obtained by multiplying the average weight by 10 was defined as “chip weight”. For example, when the total number of chips is 7 and the total weight is 12 g, the chip weight was calculated as (12 g / 7 pieces) ⁇ 10 pieces. A sample having a chip weight of 15 g or less was judged to have high chip disposal.
- the hardness at a position 50 ⁇ m deep from the surface and the hardness at a position 2.0 mm deep from the surface are measured 10 times in total using a Vickers hardness tester. The average value was calculated.
- a sample having an average hardness value of HV650 to HV1000 or less at a position 50 ⁇ m deep from the surface and an average hardness value of HV250 to HV500 at a position 2.0 mm deep from the surface is a hardness.
- the hardness distribution from the surface of the carburized steel part to a depth of 5 mm of the carburized steel part is measured at three locations using a Vickers hardness meter. The depth of the region where the hardness is HV550 or more at each location was measured. Subsequently, the average value of this depth was calculated and considered as the thickness of the carburized layer of the carburized steel part.
- a sample in which the thickness of the carburized layer was more than 0.4 mm and less than 2.0 mm was determined to be acceptable for the carburized layer thickness.
- the previous austenite crystal grains were observed at a depth of 2.0 mm from the surface of the carburized steel part.
- the presence or absence of coarse grains of prior austenite (former ⁇ ) was determined as “present” when at least one crystal grain having a diameter of 100 ⁇ m was present on the observation surface, and “not present” otherwise. .
- the JIS crystal grain size number is No. When one or less crystal grains are present, it may be determined that “the generation of coarse grains”.
- the chemical composition of the steels a to n (test numbers 1 to 14) is within the range of the chemical composition of the carburizing steel according to the present embodiment, and includes a hardenability index, a number fraction of sulfides, and between sulfides. All of the average distances met the target. As a result, the performance required for carburizing steel and carburized steel parts was satisfied.
- Test number 15 is the same component as steel that satisfies the standard of JIS standard SCr420H, which is a general-purpose steel type. Therefore, the content of chemical components C, Cr, Al, B, Bi, N, the number fraction of sulfides, and the average distance between sulfides do not satisfy the scope of the present invention.
- the hardness, critical compressibility, machinability, and grain coarsening suppression characteristics of carburizing steel were not preferably controlled.
- Test numbers 16 and 17 did not contain Bi. Therefore, in the carburizing steels of test numbers 16 and 17, the number fraction of sulfides and the average distance between sulfides did not satisfy the scope of the present invention. As a result, the flank wear amount of the carburizing steels of test numbers 16 and 17 exceeded 0.20 mm, and the chip weight exceeded 15 g.
- Test number 18 did not contain B. Therefore, the hardness of the steel part of the carburized steel part of test number 18 became insufficient. Further, in the carburized steel part of test number 18, the thickness of the decarburized layer was insufficient.
- Test No. 19 shows that the N content of the chemical component and the value of the AlN precipitation index do not satisfy the scope of the present invention, so that the hardness of the steel part of the carburized steel part becomes insufficient, and the crystal grains during carburization are further reduced. This is an example in which it becomes coarse and the carburized layer becomes thin.
- the reason why the hardness of the steel part of the carburized steel part is insufficient is that BN is precipitated and the effect of improving the hardenability by adding B cannot be obtained.
- the reason why the crystal grains are coarse is due to the fact that coarse AlN was precipitated, so that the generation of coarse grains during carburization could not be suppressed.
- Test No. 20 is an example in which the critical compressibility of the carburizing steel is insufficient because the S content of the chemical component does not satisfy the scope of the present invention.
- the reason why the limit compression ratio of the carburizing steel of test number 20 became insufficient is that because of the large S content, coarse MnS was generated, which became the starting point of fracture during cold working.
- Test number 21 is an example in which the machinability of the carburizing steel is insufficient because the contents of the chemical components S and Bi do not satisfy the scope of the present invention. In test number 21, since the amount of sulfide was insufficient because S was insufficient, the sulfide existence density and the distance between sulfides could not be within the predetermined ranges.
- Test No. 22 is an example in which coarse grains were generated during carburization because the Cr and Al contents did not satisfy the scope of the present invention, and the value of the AlN precipitation index was below the lower limit of the scope of the present invention. Specifically, since the precipitation amount of AlN was insufficient, the steel with test number 22 could not suppress the generation of coarse grains during carburization.
- Test number 24 is an example in which the C content of the chemical component does not satisfy the scope of the present invention, so that the hardness of the carburizing steel is increased and the critical compression ratio is insufficient.
- Test No. 26 was unable to suppress the generation of coarse grains during carburization because the Ti content of the chemical component exceeded the range of the present invention.
- Test No. 27 is an example in which coarse grains were generated during carburization because the value of the AlN precipitation index exceeded the upper limit of the range of the present invention. Specifically, since coarse AlN precipitated during the carburizing test No. 27, the test No. 27 could not suppress the generation of coarse particles during carburizing.
- Example 2 Except for the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature at a position 15 mm deep from the surface of the slab (hereinafter referred to as “average cooling rate”), the same production conditions as steel a and steel g Then, carburizing steels having the same chemical composition as steel a and steel g were produced, and various evaluations were performed on these carburizing steels in the same manner as steel a and steel g.
- the average cooling rate was a value shown in Table 3.
- the sulfide was appropriately finely dispersed, so the hardness before carburization, the critical compressibility, The flank wear amount and chip weight are within the acceptable range, and the carburized layer thickness after carburizing, the carburized layer hardness (hardness at a depth of 50 ⁇ m), and the steel part hardness (position at a depth of 2 mm). The hardness was also within the acceptable range.
- Test Nos. 1-3 and 7-3 in which the average cooling rate was less than 100 ° C., the sulfide was not finely dispersed. Therefore, the limit compression ratio was impaired by the coarse sulfide, and the machinability was also low. Damaged.
- Test Nos. 1-2 and 7-2 where the average cooling rate was over 500 ° C., the number of sulfides having an equivalent circle diameter of 1 ⁇ m or more and less than 2 ⁇ m was insufficient because the sulfides were excessively refined. The machinability was impaired.
- the steel according to the present invention Since the steel according to the present invention has a low deformation resistance and a large critical compression ratio before carburizing or carbonitriding, it has excellent cold forgeability and excellent machinability. Therefore, the steel according to the present invention can greatly reduce the cost of cutting in the manufacturing cost of high-strength mechanical structural parts such as gears, shafts, and pulleys. On the other hand, the steel according to the present invention has high hardenability and can suppress the formation of coarse grains during carburizing. Therefore, a carburized layer having sufficient hardness and thickness can be obtained by carburizing or carbonitriding. It is possible to form a carburized steel part having a hard steel part and substantially free of coarse grains.
- the steel according to the present invention can be used as a material for high-strength mechanical structural parts.
- the carburized steel parts according to the present invention can be manufactured at low cost, have high strength, and are substantially free of coarse grains.
- the manufacturing method of the carburized steel part which concerns on this invention can be implemented cheaply, and can provide the carburized steel part which has high intensity
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Abstract
Description
本願は、2015年11月27日に、日本に出願された特願2015-232118号に基づき優先権を主張し、その内容をここに援用する。
(1)本発明の一態様に係る鋼は、化学成分が、単位質量%で、C:0.07~0.13%、Si:0.0001~0.50%、Mn:0.0001~0.80%、S:0.0050~0.0800%、Cr:1.30%超5.00%以下、B:0.0005~0.0100%、Al:0.070~0.200%、N:0.0030~0.0100%、Bi:0.0001%超0.0100%以下、Ti:0.020%以下、P:0.050%以下、O:0.0030%以下、Nb:0~0.1000%、V:0~0.20%、Mo:0~0.500%、Ni:0~1.000%、Cu:0~0.500%、Ca:0~0.0030%、Mg:0~0.0030%、Te:0~0.0030%、Zr:0~0.0050%、RareEarthMetal:0~0.0050%、及びSb:0~0.0500%を含有し、残部がFeおよび不純物からなり、前記化学成分中の各元素の単位質量%で示した含有量を式1に代入して得られる焼入れ性指標Ceqが7.5超44.0未満であり、前記化学成分中の前記各元素の単位質量%で示した前記含有量を式2に代入して得られるAlN析出指標IAlNが0.00030超0.00110未満であり、金属組織が、85~100面積%のフェライトを含み、鋼の圧延方向と平行な断面で観察される円相当径が1μm以上2μm未満の硫化物間の平均距離が30.0μm未満であり、前記鋼の前記圧延方向と平行な前記断面で観察される円相当径が1μm以上2μm未満の前記硫化物の存在密度が300個/mm2以上である。
Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)・・・(式1)
IAlN=Al×(N-Ti×(14/48))・・・(式2)
(2)上記(1)に記載の鋼は、前記化学成分が、単位質量%で、Nb:0.0020~0.1000%、V:0.002~0.20%、Mo:0.005~0.500%、Ni:0.005~1.000%、Cu:0.005~0.500%、Ca:0.0002~0.0030%、Mg:0.0002~0.0030%、Te:0.0002~0.0030%、Zr:0.0002~0.0050%、RareEarthMetal:0.0002~0.0050%、及びSb:0.0020~0.0500%のうちの少なくとも1種または2種以上の元素を含有してもよい。
(3)本発明の別の態様に係る浸炭鋼部品は、鋼部と、前記鋼部の外面にある、ビッカース硬さがHV550以上の領域である浸炭層と、を有し、前記浸炭層の厚さが0.40mm超2.00mm未満であり、前記浸炭鋼部品の表面から深さ50μmの位置での平均ビッカース硬さがHV650以上HV1000以下であり、前記浸炭鋼部品の前記表面から深さ2.0mmの位置での平均ビッカース硬さがHV250以上HV500以下であり、前記鋼部の化学成分は、単位質量%で、C:0.07~0.13%、Si:0.0001~0.50%、Mn:0.0001~0.80%、S:0.0050~0.0800%、Cr:1.30%超5.00%以下、B:0.0005~0.0100%、Al:0.070~0.200%、N:0.0030~0.0100%、Bi:0.0001%超0.0100%以下、Ti:0.020%以下、P:0.050%以下、O:0.0030%以下、Nb:0~0.1000%、V:0~0.20%、Mo:0~0.500%、Ni:0~1.000%、Cu:0~0.500%、Ca:0~0.0030%、Mg:0~0.0030%、Te:0~0.0030%、Zr:0~0.0050%、RareEarthMetal:0~0.0050%、及びSb:0~0.0500%を含有し、残部がFeおよび不純物からなり、前記鋼部の前記化学成分中の各元素の単位質量%で示した含有量を式3に代入して得られる焼入れ性指標Ceqが7.5超44.0未満であり、前記化学成分中の前記各元素の単位質量%で示した前記含有量を式4に代入して得られるAlN析出指標IAlNが0.00030超0.00110未満であり、前記浸炭鋼部品の圧延方向と平行な断面で観察される、前記鋼部中の円相当径が1μm以上2μm未満の硫化物間の平均距離が30.0μm未満であり、前記浸炭鋼部品の前記圧延方向と平行な前記断面で観察される、前記鋼部中の円相当径が1μm以上2μm未満の前記硫化物の存在密度が300個/mm2以上である。
Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)・・・(式3)
IAlN=Al×(N-Ti×(14/48))・・・(式4)
(4)上記(3)に記載の浸炭鋼部品は、前記鋼部の化学成分が、単位質量%で、Nb:0.0020~0.1000%、V:0.002~0.20%、Mo:0.005~0.500%、Ni:0.005~1.000%、Cu:0.005~0.500%、Ca:0.0002~0.0030%、Mg:0.0002~0.0030%、Te:0.0002~0.0030%、Zr:0.0002~0.0050%、RareEarthMetal:0.0002~0.0050%、及びSb:0.0020~0.0500%のうちの少なくとも1種または2種以上の元素を含有してもよい。
(5)本発明の別の態様に係る浸炭鋼部品の製造方法は、上記(3)または(4)に記載の浸炭鋼部品の製造方法であって、上記(1)または(2)に記載の鋼を冷間塑性加工する工程と、前記冷間塑性加工後の前記鋼を切削する工程と、前記切削後の前記鋼に浸炭処理または浸炭窒化処理を施す工程と、を有する。
(6)上記(5)に記載の浸炭鋼部品の製造方法は、前記浸炭処理又は前記浸炭窒化処理の後に、焼入れ処理または焼入れ・焼戻し処理を施す工程をさらに有してもよい。
λ∝(D×σ×ΔT)0.25 …(式A)
ここで、λ:デンドライトの1次アーム間隔(μm)、D:拡散係数(m2/s)、σ:固液界面エネルギー(J/m2)、ΔT:凝固温度範囲(℃)である。
まず、本実施形態の浸炭用鋼の化学成分を構成する各成分元素の含有量について説明する。各成分元素の含有量の単位「%」は「質量%」を意味する。なお、本実施形態の浸炭用鋼は、本発明の別の実施形態に係る浸炭鋼部品の鋼部(浸炭の影響を受けない部分)と共通した構成を有するので、鋼部についてもあわせて説明する場合がある。
炭素(C)は、浸炭層と鋼部とを備える浸炭鋼部品の、鋼部の硬さを確保するために含有される。上記したように、従来の浸炭用鋼のC含有量は0.2%程度であるが、本実施形態に係る浸炭用鋼、及び浸炭鋼部品における鋼部では、C含有量をこの量よりも少ない0.13%以下に制限している。この理由は、C含有量が0.13%超では、鍛造前の浸炭用鋼の硬さが顕著に増加するとともに限界圧縮率も低下するので、浸炭用鋼の冷間鍛造性が損なわれるからである。しかしながら、C含有量が0.07%未満では、焼入れ性を高める後述の合金元素を多量に含有させて、できる限り焼入れ性の増加を図ったとしても、浸炭鋼部品の鋼部の硬さを従来の浸炭用鋼のレベルにすることが不可能である。従って、C含有量を0.07~0.13%の範囲に制御する必要がある。C含有量の下限値は、好ましくは0.08%である。C含有量の好ましい上限値は、0.12%、0.11%、又は0.10%である。
シリコン(Si)は、浸炭鋼部品のような低温焼戻しマルテンサイト鋼の焼戻し軟化抵抗を顕著に増加させることで、疲労強度を向上させる元素である。この効果を得るためには、Si含有量が0.0001%以上である必要がある。しかし、Si含有量が0.50%を超えると、鍛造前の浸炭用鋼の硬さが上昇し、変形抵抗が上昇し、そして、限界圧縮率が低下するので、浸炭用鋼の冷間鍛造性が損なわれる。従って、Si含有量を0.0001~0.50%の範囲に制御する必要がある。浸炭鋼部品の歯面疲労強度を重視する場合には、この範囲内でSi含有量を増大させる。浸炭用鋼の冷間鍛造性の確保、即ち変形抵抗の低減や限界加工性の向上を重視する場合には、この範囲内でSi含有量を減少させる。浸炭鋼部品の歯面疲労強度を重視する場合には、Si含有量を好ましくは0.10%以上とする。浸炭用鋼の冷間鍛造性の確保を重視する場合、Si含有量を好ましくは0.20%以下とする。Si含有量の下限値を0.01%、0.05%、又は0.15%としてもよい。Si含有量の上限値を0.45%、0.35%、又は0.30%としてもよい。
マンガン(Mn)は、鋼の焼入れ性を高める元素である。この効果によって浸炭熱処理後の浸炭鋼部品の強度を高めるためには、Mn含有量が0.0001%以上である必要がある。しかし、Mn含有量が0.80%を超えると、鍛造前の浸炭用鋼の硬さが上昇し、変形抵抗が上昇し、そして、限界圧縮率が低下するので、浸炭用鋼の冷間鍛造性が損なわれる。従って、Mn含有量を0.0001~0.80%の範囲に制御する必要がある。Mn含有量の下限値を0.04%、0.05%、0.10%、又は0.25%としてもよい。Mn含有量の上限値を0.78%、0.60%、0.50%、又は0.45%としてもよい。
硫黄(S)は、鋼中のMn等と結合して、MnS等の硫化物を形成し、鋼の被削性を向上させる元素である。この効果を得るために、S含有量を0.0050%以上とする必要がある。しかしながら、S含有量が0.0800%を超えると、鍛造時に硫化物が起点となって割れを生じさせるので、鋼の限界圧縮率を低下させることがある。従って、S含有量を0.0050~0.0800%の範囲に制御する必要がある。S含有量の好ましい下限値は0.0080%、0.0090%、又は0.0100%である。S含有量の好ましい上限値は0.0750%、0.0500%、又は0.0200%である。
クロム(Cr)は、鋼の焼入れ性を高める元素である。この効果によって浸炭熱処理後の浸炭鋼部品の強度を高めるためには、Cr含有量が1.30%超である必要がある。しかし、Cr含有量が5.00%を超えると、鍛造前の浸炭用鋼の硬さが上昇し、変形抵抗が上昇し、そして、限界圧縮率が低下するので、浸炭用鋼の冷間鍛造性が損なわれる。従って、Cr含有量を1.30%超5.00%以下の範囲に制御する必要がある。また、Crは、焼入れ性向上効果を有するMn、Mo、及びNi等の他の元素と比べて、浸炭用鋼(浸炭熱処理前の鋼)の硬さを上昇させる程度が少なく、かつ焼入れ性(浸炭熱処理の焼入れによって上昇する硬さ)を向上させる効果が比較的大きい。よって、本実施形態に係る浸炭用鋼、及び、浸炭鋼部品における鋼部では、従来の浸炭用鋼よりも、Cr含有量を多くする。Cr含有量の好ましい下限値は1.35%、1.50%、又は1.80%である。Cr含有量の好ましい上限値は4.50%、3.50%、2.50%、又は2.20%である。
ホウ素(B)は、オーステナイト中に固溶する場合、微量でも鋼の焼入れ性を大きく高める元素である。この効果によって浸炭熱処理後の浸炭鋼部品の強度を高めることができる。また、Bは上記効果を得るために多量に添加する必要がないので、鍛造前の浸炭用鋼の硬さをほとんど上昇させないという特徴がある。従って、本実施形態に係る浸炭用鋼、及び、浸炭鋼部品における鋼部では、Bを積極的に利用する。B含有量が0.0005%未満では、上記の焼入れ性向上効果が得られない。一方、B含有量が0.0100%を超えると、上記効果が飽和する。従って、B含有量を0.0005~0.0100%の範囲に制御する必要がある。B含有量の下限値は、好ましくは0.0010%、0.0015%、又は0.0020%である。B含有量の上限値は、好ましくは0.0095%、又は0.0080%である。なお、鋼中に一定量以上のNが存在している場合、BがNと結合してBNを形成し、固溶B量が減少する。その結果、焼入れ性を高める効果が得られない場合がある。よって、本実施形態の浸炭用鋼では、Nを固定するTiの含有量を所定値以上とすることが必要である。
Alは脱酸作用を有すると同時に、Nと結合してAlNを形成しやすく、浸炭加熱時のオーステナイト粒粗大化防止に有効な元素である。Alを添加することで、BNの形成が防止され、焼入れ性に寄与する固溶Bが確保される。しかし、Alの含有量が0.070%未満では、Bによる焼入れ性向上効果が得られない。一方、Alの含有量が0.200%を超えると、AlNの析出量が多くなりすぎ、浸炭用鋼及び浸炭鋼部品の塑性加工性が低下する。さらに、Alの含有量が0.200%を超えると、AlN析出物が微細分散せずに、個々のサイズが大きくなる。そのため、AlN析出物を介して浸炭中の結晶粒粗大化を防止する効果が得られなくなる。したがって、Alの含有量を0.070~0.200%とした。Al含有量の好ましい下限は0.075%である。Al含有量の好ましい上限は0.150%である。
窒素(N)は、AlN析出物を形成し、これを介して浸炭中の結晶粒粗大化を防止する効果を有する。N含有量が0.0030%未満では、AlNの析出量が減少し、浸炭時の結晶粒粗大化を防止する効果が得られない。一方、NはBNを形成して、固溶B量を低減させる元素である。N含有量が0.0100%を超えると、焼入れ性に寄与する固溶Bを確保することができなくなる。また、N含有量が0.0100%を超えると、粗大なTiNが形成されて、これが塑性加工時に破壊の起点となるので、浸炭用鋼の冷間鍛造性が損なわれる。従って、N含有量を0.0030~0.0100%の範囲に制御する必要がある。N含有量の下限値は、好ましくは0.0040%である。N含有量の上限値は、好ましくは0.0090%または0.0085%である。
Biは、本実施形態に係る浸炭用鋼において重要な元素である。微量のBiによって、溶鋼の凝固時にデンドライト組織が微細化されるので、硫化物が微細分散する。硫化物微細化効果を得るためには、Biの含有量を0.0001%超とする必要がある。しかし、Biの含有量が0.0100%を超えると、鋼の熱間加工性が劣化し、熱間圧延が困難となる。これらのことから、本実施形態に係る浸炭用鋼では、Bi含有量を0.0001%超0.0100%以下とする。被削性向上および硫化物微細分散化効果を確実に得るためには、Bi含有量を0.0010%以上又は0.0015%以上とすることが好ましい。一方、Bi含有量の好ましい上限値は0.0095%、0.0090%、又は0.0050%である。
チタン(Ti)は、鋼中のNをTiNとして固定する効果を有する元素である。Tiが含有される場合、鋼中のNは、ほぼ全てがTiNとして固定される。しかし、Tiは高価な元素であるため、Tiを含有させる場合、製造コストが高くなる。また、Tiが0.020%超である場合、多量のTi炭硫化物が生成することにより、粗大粒の発生が抑制できなくなるおそれがある。上述の観点から、Ti含有量を0.020%以下に制限する必要がある。Ti含有量の好適範囲は0.020%未満であり、更なる好適範囲は0.018%以下又は0.015%以下である。
燐(P)は不純物である。Pは鋼の疲労強度や熱間加工性を低下させる。したがって、P含有量は少ない方が好ましく、その含有量の下限値は0%である。しかし、製造コストを考慮して、P含有量の下限値を0.005%、0.010%、又は0.015%としてもよい。一方、P含有量は0.050%以下であれば許容される。好ましいP含有量は0.045%以下、又は0.035%以下であり、さらに好ましくは、0.020%以下または0.015%以下である。
酸素(O)は不純物であり、酸化物系介在物を形成する元素である。O含有量が0.0030%超では、疲労破壊の起点となる大きな介在物が増加し、疲労特性の低下の原因となる。従って、O含有量を0.0030%以下に制限する必要がある。好ましくは、O含有量は0.0015%以下である。O含有量は少ないほど望ましいので、O含有量の下限値は0%である。しかし、製造コストを考慮して、O含有量の下限値を0.0007%又は0.0010%としてもよい。一方、O含有量の上限値を0.0025%、0.0020%、又は0.0015%としてもよい。なお、通常の操業条件では、Oが0.0020%程度含有される。
ニオブ(Nb)は、鋼中でN及びCと結合して、Nb(C、N)を形成する元素である。このNb(C、N)は、オーステナイト結晶粒界をピン止めすることによって、粒成長を抑制し、そして、組織の粗大化を防止する。Nb含有量を0.0020%以上とすると、上記の効果が得られるので好ましい。Nb含有量が0.1000%を超えると、上記の効果が飽和する。従って、Nb含有量を0.0020~0.1000%とすることが好ましい。さらに好ましくは、Nb含有量の下限値は0.0100%である。また、さらに好ましくは、Nb含有量の上限値は0.0500%、0.0100%、0.0050%、又は0.0040%である。
バナジウム(V)は、鋼中でN及びCと結合して、V(C、N)を形成する元素である。このV(C、N)は、オーステナイト結晶粒界をピン止めすることによって、粒成長を抑制し、そして、組織の粗大化を防止する。V含有量を0.002%以上とすると、上記の効果が得られるので好ましい。V含有量が0.20%を超えると、上記の効果が飽和する。従って、V含有量を0.002~0.20%とすることが好ましい。さらに好ましくは、V含有量の下限値は0.05%である。さらに好ましくは、V含有量の上限値は0.10%である。
モリブデン(Mo)は、鋼の焼入れ性を高める元素である。Mo含有量を0.005%以上とすると、この効果によって浸炭熱処理後の浸炭鋼部品の強度を高められるので好ましい。また、Moは、ガス浸炭の雰囲気で、酸化物を形成せず、窒化物を形成しにくい元素である。浸炭用鋼にMoが含有される場合、浸炭層表面の酸化物層及び窒化物層、又は、それらに起因する浸炭異常層が形成されにくくなる。しかしながら、Moは高価である。さらに、Mo含有量が0.500%を超えると、鍛造前の浸炭用鋼の硬さが上昇し、変形抵抗が上昇し、そして、限界圧縮率が低下するので、浸炭用鋼の冷間鍛造性が損なわれる。従って、Mo含有量を0.005~0.500%とすることが好ましい。さらに好ましくは、Mo含有量の下限値を0.050%としてもよい。また、Mo含有量の上限値を0.200%、0.100%、0.010%、又は0.006%としてもよい。
ニッケル(Ni)は、鋼の焼入れ性を高める元素である。Ni含有量を0.005%以上とすると、この効果によって浸炭熱処理後の浸炭鋼部品の強度を高められるので好ましい。また、Niは、ガス浸炭の雰囲気ガス雰囲気で、酸化物や窒化物を形成しない元素である。浸炭用鋼にNiが含有される場合、浸炭層表面の酸化物層及び窒化物層、又はそれらに起因する浸炭異常層が形成されにくくなる。しかしながら、Niは高価である。さらに、Ni含有量が1.000%を超えると、鍛造前の浸炭用鋼の硬さが上昇し、変形抵抗が上昇し、そして、限界圧縮率が低下するので、浸炭用鋼の冷間鍛造性が損なわれる。従って、Ni含有量を0.005~1.000%とすることが好ましい。さらに好ましくは、Ni含有量の下限値を0.050%としてもよい。また、Ni含有量の上限値を0.700%又は0.500%としてもよい。
銅(Cu)は、鋼の焼入れ性を高める元素である。Cu含有量を0.005%以上とすると、この効果によって浸炭熱処理後の浸炭鋼部品の強度を高められるので好ましい。また、Cuは、ガス浸炭の雰囲気ガス雰囲気で、酸化物や窒化物を形成しない元素である。浸炭用鋼にCuが含有される場合、浸炭層表面の酸化物層及び窒化物層、又は、それらに起因する浸炭異常層が形成されにくくなる。しかしながら、Cu含有量が0.500%を超えると、1000℃以上の高温域における鋼の延性が低下し、連続鋳造及び圧延時の歩留まりが低下する。また、Cu含有量が0.500%を超えると、鍛造前の浸炭用鋼の硬さが上昇し、変形抵抗が上昇し、そして、限界圧縮率が低下するので、浸炭用鋼の冷間鍛造性が損なわれる。従って、Cu含有量を0.005~0.500%とすることが好ましい。さらに好ましくは、Cu含有量の下限値を0.050%としてもよい。一方、Cu含有量の上限値を0.300%、0.010%、又は0.006%としてもよい。なお、Cuを含有させる場合、上記した高温域の延性を改善するために、単位質量%で、Ni含有量をCu含有量の1/2以上とすることが望ましい。
カルシウム(Ca)は、硫化物の形状を、伸長させずに球状にするという硫化物形態制御の効果を有する元素である。Caが含有される場合、硫化物形状の異方性が改善され、硫化物に起因する機械的性質の低下が、一層抑制される。また、Caは、切削時に切削工具表面に保護被膜を形成して、被削性を向上させる元素である。Ca含有量を0.0002%以上とすると、これらの効果が得られるので好ましい。一方、Ca含有量が0.0030%を超えると、粗大な酸化物及び硫化物等が形成されて、浸炭鋼部品の疲労強度に悪影響を与える場合がある。従って、Ca含有量を0.0002~0.0030%とすることが好ましい。さらに好ましくは、Ca含有量の下限値を0.0008%としてもよい。Ca含有量の上限値を0.0020%又は0.0005%としてもよい。
マグネシウム(Mg)は、Caと同様に硫化物の形態を制御し、さらに切削時に切削工具表面へ保護被膜を形成して被削性を向上させる元素である。Mg含有量を0.0002%以上とすると、これらの効果が得られるので好ましい。一方、Mg含有量が0.0030%を超えると、粗大な酸化物が形成されて、浸炭鋼部品の疲労強度に悪影響を与える場合がある。従って、Mg含有量を0.0002~0.0030%とすることが好ましい。さらに好ましくは、Mg含有量の下限値を0.0008%としてもよい。Mg含有量の上限値を0.0020%、又は0.0012%としてもよい。
テルル(Te)は、硫化物の形態を制御する元素である。Te含有量を0.0002%以上とすると、この効果が得られるので好ましい。一方、Te含有量が0.0030%を超えると、鋼の熱間における脆化が著しくなる。従って、Te含有量を0.0002~0.0030%とすることが好ましい。さらに好ましくは、Te含有量の下限値を0.0008%としてもよい。Te含有量の上限値を0.0020%、又は0.0015%としてもよい。
ジルコニウム(Zr)は、硫化物の形態を制御する元素である。Zr含有量を0.0002%以上とすると、この効果が得られるので好ましい。一方、Zr含有量が0.0050%を超えると、粗大な酸化物が形成されて、浸炭鋼部品の疲労強度に悪影響を与える場合がある。従って、Zr含有量を0.0002~0.0050%とすることが好ましい。さらに好ましくは、Zr含有量の下限値を0.0008%としてもよい。Zr含有量の上限値を0.0030%、又は0.0011%としてもよい。
REM(Rare Earth Metal)は、硫化物の形態を制御する元素である。REM含有量を0.0002%以上とすると、この効果が得られるので好ましい。REM含有量が0.0050%を超えると、粗大な酸化物が形成されて、浸炭鋼部品の疲労強度に悪影響を与える場合がある。従って、REM含有量を0.0002~0.0050%とすることが好ましい。さらに好ましくは、REM含有量の下限値を0.0008%としてもよい。REM含有量の上限値を0.0040%、0.0030%又は0.0010%としてもよい。
なお、REMとは原子番号が57のランタンから71のルテシウムまでの15元素に、原子番号が21のスカンジウムと原子番号が39のイットリウムとを加えた合計17元素の総称である。通常は、これらの元素の混合物であるミッシュメタルの形で供給され、鋼中に添加される。本実施形態において、REMの含有量とは、これら元素の含有量の合計値である。
アンチモン(Sb)は、浸炭用鋼の製造工程(熱間圧延、熱間鍛造、焼鈍等)における脱炭や浸炭現象を防止する元素である。Sb含有量を0.0020%以上とすると、これらの効果が得られるので好ましい。Sb含有量が0.0500%を超えると、浸炭処理時に浸炭性を損ない、必要な浸炭層が得られない場合がある。従って、Sb含有量を0.0020~0.0500%とすることが好ましい。さらに好ましくは、Sb含有量の下限値を0.0050%としてもよい。Sb含有量の上限値を0.0300%又は0.0030%としてもよい。
本実施形態の浸炭用鋼の製造に用いる連続鋳造鋳片の凝固組織は、通常はデンドライト形態を呈している。浸炭用鋼中の硫化物は、凝固前(溶鋼中)、または凝固時に晶出することが多く、デンドライト1次アーム間隔に大きく影響を受ける。すなわち、デンドライト1次アーム間隔が小さければ、樹間に晶出する硫化物は小さくなる。本実施形態の浸炭用鋼は、鋳片の段階におけるデンドライト1次アーム間隔が600μm未満であることが望ましい。なお、本実施形態に係る浸炭用鋼において、硫化物は例えばMnS等である。ただし、鋳片を熱間加工するとデンドライトの形状が変化したり、デンドライトの形状が判別できなくなったりする場合がある。従って、鋳片を熱間加工して得られる本実施形態の浸炭用鋼のデンドライト形状は、上述の範囲に限定されない。
浸炭用鋼に含まれる硫化物(例えばMnS等)は、浸炭用鋼の被削性の向上に有用であるため、適切なサイズの硫化物の存在密度を可能な限り増大させることが必要である。一方、S含有量を増加させると被削性は向上するが、粗大な硫化物が増加する。熱間圧延等によって延伸した粗大な硫化物は、冷間鍛造性を損なう。従って、S含有量を従来の水準よりも低減させて、硫化物サイズ及び形状を制御することが必要である。さらに、被削時の切りくず処理性を向上させるためには、硫化物を微細に分散することが必要である。すなわち、硫化物同士の間隔を小さくすることが重要である。
本発明者らが知見したところでは、浸炭用鋼の圧延方向と平行な断面(L断面)において観察される円相当径が1μm以上2μm未満の硫化物(以下「微細硫化物」と略す場合がある)が300個/mm2以上の存在密度で鋼中に存在すると、工具の摩耗が抑制される。微細硫化物の存在密度の下限値を325個/mm2、350個/mm2、又は400個/mm2としてもよい。微細硫化物の存在密度の上限値を規定する必要は無いが、化学成分の規定範囲及び実験結果に鑑みて、600個/mm2が実質的な上限値になると推定される。微細硫化物の存在密度の上限値を500個/mm2としてもよい。
また、微細硫化物同士の間の距離の平均値(微細硫化物間の平均距離)と、切りくず処理性との関係について本発明者らが種々実験を行った結果、これら微細硫化物間の平均距離が30.0μm未満であれば、良好な切りくず処理性が得られることを確認した。従って、微細硫化物間の平均距離は30.0μm未満と規定される。微細硫化物間の平均距離の上限値を27.0μm、26.0μm、又は25.0μmとしてもよい。微細硫化物間の平均距離の下限値は特に限定されないが、化学成分の規定範囲及び実験結果に鑑みて、12.0μmが実質的な下限値であると推定される。微細硫化物間の平均距離の下限値を13.0μm、又は14.0μmとしてもよい。
焼入れ性指標Ceq:7.5超44.0未満
本実施形態の浸炭用鋼の、化学成分中の各元素の単位質量%で示した含有量を下記の式Bに代入して得られる焼入れ性指標Ceqが、7.5超44.0未満となる必要がある。式Bに含まれる元素記号は、その元素記号に係る元素の単位質量%での含有量を示す。選択元素であるMo及びNiが含まれない場合には、その含有量を0質量%とみなして焼入れ性指標Ceqを算出すればよい。
Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)・・・(式B)
AlN析出指標IAlN:0.00030超0.00110未満
本実施形態の浸炭用鋼の、Al、N、及び、Tiの質量%で示した含有量を下記の式Cに代入することで得られるAlN析出指標IAlNが、0.00030超0.00110未満である必要がある。式Cに含まれる元素記号は、その元素記号に係る元素の単位質量%での含有量を示す。
IAlN=Al×(N-Ti×(14/48))・・・(式C)
鋼に含まれるNは、まずTiと結びついてTiNを形成する。つまり、上記の式C中の「(N-Ti×(14/48))」は、鋼中においてTiN以外の形態になっているNの量、すなわち、AlNとなる可能性のあるNの量を表している。上記の式C中の「14」はNの原子量であり、「48」はTiの原子量である。
AlN析出指標IAlNが0.00030以下である場合、AlNの析出量が不足するため、浸炭時の結晶粒の粗大化を防止することができない。また、AlN析出指標IAlNが0.00110以上である場合、AlNの析出量が多くなりすぎて、浸炭用鋼及び浸炭鋼部品の塑性加工性を低下させる。また、AlN析出指標IAlNが0.00110以上である場合、AlN析出物が微細分散せずに、個々のサイズが大きくなる。そのため、AlN析出物を介して浸炭中の結晶粒粗大化を防止する効果が得られなくなる。従って、AlN析出指標IAlNは0.00030超0.00110未満である必要がある。AlN析出指標IAlNの下限値は、好ましくは、0.00050である。AlN析出指標IAlNの上限値は、好ましくは0.00100未満又は0.00080である。
フェライト:85~100面積%
本実施形態の浸炭用鋼の金属組織は、85面積%以上のフェライトを含む。その金属組織が、軟質な相であるフェライトを主体とするものであるので、本実施形態の浸炭用鋼は十分に軟質であり、優れた冷間鍛造性を有する。なお、フェライトは多ければ多いほど好ましいので、フェライト量の上限値は100面積%である。フェライト量が上述の範囲内である限り、本実施形態の浸炭用鋼はフェライト以外の任意の組織を含んでも良い。本実施形態の浸炭用鋼に含まれ得る組織として、ベイナイト及びマルテンサイトが例示される。
次に、本発明の別の実施形態に係る浸炭鋼部品について説明する。
本実施形態の浸炭鋼部品2は、図2に示されるように、上述の本実施形態に係る浸炭用鋼1に対して、冷間塑性加工S1、切削加工S2、及び浸炭処理又は浸炭窒化処理S3が施されることで製造される。浸炭処理又は浸炭窒化処理S3の後に、必要に応じて仕上熱処理として焼入れ処理又は焼入れ・焼戻し処理S4を行ってもよい。浸炭処理又は浸炭窒化処理S3によって、浸炭鋼部品2の鋼部20の外面に浸炭層21が形成される。本実施形態に係る浸炭鋼部品2の浸炭層21は、ビッカース硬さがHV550以上である領域と定義される。浸炭層2の厚さは、JIS G 0557に規定される有効硬化層深さと等しい。なお、鋼部20と浸炭層21との間に、何れにも該当しない領域、即ち鋼部20よりもC含有量が高いが硬さがHV550未満である遷移領域があってもよい。なお「浸炭層」との用語は、通常の技術常識によれば、浸炭層及び浸炭窒化層の両方を含む概念と解される。浸炭鋼部品2の製造方法については後述する。
本実施形態の浸炭鋼部品2は、より詳細には、図1に示されるように鋼部20と、鋼部20の外面に生成した厚さ0.40mm超2.00mm未満の浸炭層21とを備える。浸炭層の厚さが0.40mm以下である場合、浸炭鋼部品の強度、特に疲労強度などが不足する。一方、浸炭層の厚さが2.00mm以上である場合、浸炭鋼部品の表面の靱性が損なわれる。浸炭層の厚さの下限値を0.45mm、又は0.50mmとしてもよい。また、浸炭層の厚さの上限値を1.70mm、1.50mm、1.00mm、0.90mm、0.70mm、又は0.65mmとしてもよい。
加えて、本実施形態に係る浸炭鋼部品2の表面から深さ50μmの位置(図1において、記号Aが付された破線)での平均ビッカース硬さは、HV650以上HV1000以下であることが好ましい。この場合、浸炭層の硬さが適切に制御されている。浸炭鋼部品2の表面から深さ50μmの位置での平均ビッカース硬さがHV650未満である場合、浸炭鋼部品の強度、特に疲労強度などが不足する。浸炭鋼部品2の表面から深さ50μmの位置での平均ビッカース硬さがHV1000超である場合、浸炭鋼部品の表面の靱性が損なわれる。浸炭鋼部品2の表面から深さ50μmの位置での平均ビッカース硬さの下限値をHV750、HV770、またはHV800としてもよい。浸炭鋼部品2の表面から深さ50μmの位置での平均ビッカース硬さの上限値をHV900、HV870、またはHV850としてもよい。
さらに、本実施形態に係る浸炭鋼部品2の表面から深さ2.0mmの位置(図1において、記号Bが付された破線)での平均ビッカース硬さはHV250以上HV500以下であることが好ましい。この場合、鋼部20(又は遷移部)の硬さが適切に制御されている。浸炭鋼部品2の表面から深さ2.0mmの位置での平均ビッカース硬さがHV250未満である場合、浸炭鋼部品の強度が不足する。浸炭鋼部品2の表面から深さ2.0mmの位置での平均ビッカース硬さがHV500超である場合、浸炭鋼部品の靱性が損なわれ、割れ等の破損が生じやすくなる。浸炭鋼部品2の表面から深さ2.0mmの位置での平均ビッカース硬さの下限値をHV270、HV280、またはHV300としてもよい。浸炭鋼部品2の表面から深さ2.0mmの位置での平均ビッカース硬さの上限値をHV400、HV380、またはHV320としてもよい。
上述したように浸炭層厚さ及び硬さが制御されている限り、本実施形態に係る浸炭鋼部品2は高強度部品として用いることができる。従って、本実施形態に係る浸炭鋼部品2の組織は特に限定されないが、例えば、浸炭鋼部品2の表面から0.4mmの深さにおける組織を、0~10面積%のフェライトと、マルテンサイト、ベイナイト、焼戻しマルテンサイト、焼戻しベイナイト、及びセメンタイトからなる群から選択される1種以上を含む残部とから構成されるものとしてもよい。成分、並びに浸炭鋼部品2の表面から2.0mmの深さの位置及び50μmの深さの位置における硬さが上述された範囲内となるように浸炭鋼部品2の製造を行った場合、浸炭鋼部品2の表面から0.4mmの深さにおける組織は上述の範囲内となることが通常である。
次に、本実施形態の浸炭用鋼の製造方法と、本発明の別の実施形態に係る浸炭鋼部品の製造方法とを説明する。浸炭鋼部品の製造方法においては、一例として浸炭用鋼からなる冷間鍛造品を製造する工程を説明する。冷間鍛造品はたとえば、自動車及び建設機械等に利用される機械部品であり、たとえば、歯車、シャフト、プーリーなどの鋼製部品である。
本実施形態の浸炭用鋼と同じ化学成分を有する鋳片を、連続鋳造法により製造する。造塊法により、鋳片をインゴット(鋼塊)にしてもよい。鋳造は例えば、220×220mm角の鋳型を用いて、タンディッシュ内の溶鋼のスーパーヒートを10~50℃とし、鋳込み速度を1.0~1.5m/分とする条件で行われる。
表1A及び表1Bに示す化学組成を有する鋼a~aaを270ton転炉で溶製し、連続鋳造機を用いて連続鋳造を実施して、220×220mm角の鋳片を製造した。ここで、タンディッシュ内の溶鋼のスーパーヒートを30℃とし、鋳込み速度を1.0m/分とした。
鋳片の凝固組織のデンドライト1次アーム間隔およびデンドライト2次アーム間隔は、上記の鋳片の断面をピクリン酸にてエッチングし、鋳片表面から15mmの深さの位置において鋳込み方向に5mmピッチでデンドライト1次アーム間隔および2次アーム間隔を100点測定し、各測定点におけるデンドライト1次アーム間隔および2次アーム間隔の平均値を算出し、さらにこれらを平均することにより求めた。実施例の鋳片のデンドライト2次アーム間隔に基づいて推定される、実施例の鋳片の平均冷却速度は100℃/min以上500℃/min以下であった。
各鋼番号の丸棒(浸炭用鋼)のミクロ組織を観察した。丸棒のD/4位置を軸方向に対して平行に切断し、ミクロ組織観察用の試験片を採取した。試験片の切断面を研磨し、光学顕微鏡によって鋼の金属組織を観察し、組織中のコントラストから析出物の種類を判別した。なお、走査型電子顕微鏡とエネルギー分散型X線分光分析装置(EDS)とを用いて析出物を同定した。後述の試験片の長手方向を含む断面から、縦10mm×横10mmの研磨試験片を10個作製し、切断面の電子顕微鏡写真を複数の測定箇所で撮影し、各電子顕微鏡写真に含まれる硫化物それぞれの円相当径を算出することにより円相当径が1μm以上2μm未満の硫化物(微細硫化物)を特定し、各電子顕微鏡写真に含まれる微細硫化物の個数を各電子顕微鏡写真の視野の面積で割ることにより各測定箇所における微細硫化物の存在密度を求め、これら存在密度を平均することにより、鋼の圧延方向と平行な断面で観察される円相当径が1μm以上2μm未満の硫化物の存在密度(微細硫化物存在密度)を求めた。また、上述の各電子顕微鏡写真に含まれる任意の2の微細硫化物の重心をその両端とし且つこれら任意の2の微細硫化物以外の微細硫化物を通らない線分を各電子顕微鏡写真に描画し、各電子顕微鏡写真のこれら線分の長さの平均値を求めることにより各測定箇所における微細硫化物間の平均距離を求め、これら各測定箇所における平均距離をさらに平均することにより、鋼の圧延方向と平行な断面で観察される円相当径が1μm以上2μm未満の硫化物間の平均距離(硫化物間距離)を求めた。なお、測定箇所の数を25とし、電子顕微鏡写真の倍率を500倍とし、測定視野の総面積を約1.1mm2とした。
丸棒(浸炭用鋼)の硬さは、ビッカース硬度計を用いて、丸棒の圧延方向に垂直な断面の10点の測定点で硬さ測定を行い、各測定点における硬さの平均値を算出することにより求めた。測定点の位置は、丸棒のD/4位置(丸棒の直径Dの1/4の深さの位置)とした。徐冷工程後かつ球状化焼鈍前(SA工程前)の浸炭用鋼の硬さがHV125以下の場合、または、球状化焼鈍後(SA工程後)の浸炭用鋼の硬さがHV110以下の場合を、軟質化が十分であり合格と判定した。
直径30mmの丸棒のR/2位置(丸棒の半径Rの1/2の深さの位置)から、丸棒試験片を作製した。丸棒試験片は、直径30mmの丸棒のR/2位置を中心とした直径10mm、長さ15mmの試験片であり、丸棒試験片の長手方向は、直径30mmの丸棒の鍛伸軸と平行であった。また、丸棒試験片の端面の中心には切り欠きを設けた。切り欠きの深さは0.8mmとし、切り欠き角度は30度とし、切り欠きの底部は半径R=0.15mmとなるように丸めた。この切り欠き形状は、「冷間据込み試験方法」冷間鍛造分科会材料研究班、塑性と加工、vol.22、no.241、p139に記載の2号試験片の切り欠きに準じたものである。
各鋼について、冷間圧縮試験に供した試験片に、冷間での引抜きにより歪を与え、これにより通常の冷間鍛造と同様の影響を各試験片に与えた。その引抜き後の試験片の被削性を評価することにより、各鋼の冷間鍛造後の被削性を評価した。
母材材質:超硬P20種グレード。
コーティング:なし。
<旋削加工条件>
周速:150m/分。
送り:0.2mm/rev。
切り込み:0.4mm。
潤滑:水溶性切削油を使用。
上記方法で製造した浸炭用鋼の、周面から上記切断面の直径1/4深さの位置から、試験片の長手方向が浸炭用鋼の長手方向と一致するように、浸炭用の試験片(20mmφ×30mm)を採取した。浸炭工程として、変成炉ガス方式によるガス浸炭を行った。このガス浸炭は、カーボンポテンシャルを0.8%として、950℃で5時間の保持を行い、続いて、850℃で0.5時間の保持を行った。浸炭工程後に、仕上熱処理工程として、130℃へと浸炭処理後の鋼を冷却する油焼入れを行い、そして、150℃で90分の焼戻しを行って、浸炭鋼部品を得た。
鋳片の表面から15mmの深さの位置における液相線温度から固相線温度までの温度域内の平均冷却速度(以下「平均冷却速度」と称する)を除き鋼a及び鋼gと同じ製造条件で、鋼a及び鋼gと同じ化学成分を有する浸炭用鋼を製造し、これら浸炭用鋼に、鋼a及び鋼gと同じ方法で種々の評価を行った。平均冷却速度は、表3に示される値とした。
2 浸炭鋼部品
20 鋼部
21 浸炭層
S1 冷間塑性加工
S2 切削加工
S3 浸炭処理又は浸炭窒化処理
S4 焼入れ処理又は焼入れ・焼戻し処理
Claims (6)
- 化学成分が、単位質量%で、
C:0.07~0.13%、
Si:0.0001~0.50%、
Mn:0.0001~0.80%、
S:0.0050~0.0800%、
Cr:1.30%超5.00%以下、
B:0.0005~0.0100%、
Al:0.070~0.200%、
N:0.0030~0.0100%、
Bi:0.0001%超0.0100%以下、
Ti:0.020%以下、
P:0.050%以下、
O:0.0030%以下、
Nb:0~0.1000%、
V:0~0.20%、
Mo:0~0.500%、
Ni:0~1.000%、
Cu:0~0.500%、
Ca:0~0.0030%、
Mg:0~0.0030%、
Te:0~0.0030%、
Zr:0~0.0050%、
Rare Earth Metal:0~0.0050%、及び
Sb:0~0.0500%
を含有し、残部がFeおよび不純物からなり、
前記化学成分中の各元素の単位質量%で示した含有量を式1に代入して得られる焼入れ性指標Ceqが7.5超44.0未満であり、
前記化学成分中の前記各元素の単位質量%で示した前記含有量を式2に代入して得られるAlN析出指標IAlNが0.00030超0.00110未満であり、
金属組織が、85~100面積%のフェライトを含み、
鋼の圧延方向と平行な断面で観察される円相当径が1μm以上2μm未満の硫化物間の平均距離が30.0μm未満であり、
前記鋼の前記圧延方向と平行な前記断面で観察される円相当径が1μm以上2μm未満の前記硫化物の存在密度が300個/mm2以上である
ことを特徴とする鋼。
Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)・・・(式1)
IAlN=Al×(N-Ti×(14/48))・・・(式2) - 前記化学成分が、単位質量%で、
Nb:0.0020~0.1000%、
V:0.002~0.20%、
Mo:0.005~0.500%、
Ni:0.005~1.000%、
Cu:0.005~0.500%、
Ca:0.0002~0.0030%、
Mg:0.0002~0.0030%、
Te:0.0002~0.0030%、
Zr:0.0002~0.0050%、
Rare Earth Metal:0.0002~0.0050%、及び
Sb:0.0020~0.0500%
のうちの少なくとも1種または2種以上の元素を含有する
ことを特徴とする請求項1に記載の鋼。 - 鋼部と、
前記鋼部の外面にある、ビッカース硬さがHV550以上の領域である浸炭層と、
を備える浸炭鋼部品であって、
前記浸炭層の厚さが0.40mm超2.00mm未満であり、
前記浸炭鋼部品の表面から深さ50μmの位置での平均ビッカース硬さがHV650以上HV1000以下であり、
前記浸炭鋼部品の前記表面から深さ2.0mmの位置での平均ビッカース硬さがHV250以上HV500以下であり、
前記鋼部の化学成分は、単位質量%で、
C:0.07~0.13%、
Si:0.0001~0.50%、
Mn:0.0001~0.80%、
S:0.0050~0.0800%、
Cr:1.30%超5.00%以下、
B:0.0005~0.0100%、
Al:0.070~0.200%、
N:0.0030~0.0100%、
Bi:0.0001%超0.0100%以下、
Ti:0.020%以下、
P:0.050%以下、
O:0.0030%以下、
Nb:0~0.1000%、
V:0~0.20%、
Mo:0~0.500%、
Ni:0~1.000%、
Cu:0~0.500%、
Ca:0~0.0030%、
Mg:0~0.0030%、
Te:0~0.0030%、
Zr:0~0.0050%、
Rare Earth Metal:0~0.0050%、及び
Sb:0~0.0500%
を含有し、残部がFeおよび不純物からなり、
前記鋼部の前記化学成分中の各元素の単位質量%で示した含有量を式3に代入して得られる焼入れ性指標Ceqが7.5超44.0未満であり、
前記化学成分中の前記各元素の単位質量%で示した前記含有量を式4に代入して得られるAlN析出指標IAlNが0.00030超0.00110未満であり、
前記浸炭鋼部品の圧延方向と平行な断面で観察される、前記鋼部中の円相当径が1μm以上2μm未満の硫化物間の平均距離が30.0μm未満であり、
前記浸炭鋼部品の前記圧延方向と平行な前記断面で観察される、前記鋼部中の円相当径が1μm以上2μm未満の前記硫化物の存在密度が300個/mm2以上である
ことを特徴とする浸炭鋼部品。
Ceq=(0.7×Si+1)×(5.1×Mn+1)×(2.16×Cr+1)×(3×Mo+1)×(0.3633×Ni+1)・・・(式3)
IAlN=Al×(N-Ti×(14/48))・・・(式4) - 前記鋼部の化学成分が、単位質量%で、
Nb:0.0020~0.1000%、
V:0.002~0.20%、
Mo:0.005~0.500%、
Ni:0.005~1.000%、
Cu:0.005~0.500%、
Ca:0.0002~0.0030%、
Mg:0.0002~0.0030%、
Te:0.0002~0.0030%、
Zr:0.0002~0.0050%、
Rare Earth Metal:0.0002~0.0050%、及び
Sb:0.0020~0.0500%
のうちの少なくとも1種または2種以上の元素を含有する
ことを特徴とする請求項3に記載の浸炭鋼部品。 - 請求項1または2に記載の鋼を冷間塑性加工する工程と、
前記冷間塑性加工後の前記鋼を切削加工する工程と、
前記切削加工後の前記鋼に浸炭処理又は浸炭窒化処理を施す工程と、
を有することを特徴とする請求項3または4に記載の浸炭鋼部品の製造方法。 - 前記浸炭処理又は前記浸炭窒化処理の後に、焼入れ処理又は焼入れ・焼戻し処理を施す工程
をさらに有することを特徴とする請求項5に記載の浸炭鋼部品の製造方法。
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"The test method of cold heading'', the material research group of cold forging subcommittee", JOURNAL OF THE JAPAN SOCIETY FOR TECHNOLOGY OF PLASTICITY, vol. 22, no. 241, pages 139 |
See also references of EP3382051A4 |
W. KURZ; D. J. FISHER: "Fundamentals of Solidification", 1998, TRANS TECH PUBLICATIONS LTD., pages: 256 |
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JP2020105603A (ja) * | 2018-12-28 | 2020-07-09 | 日本製鉄株式会社 | 浸炭鋼部品用鋼材 |
JP2020105602A (ja) * | 2018-12-28 | 2020-07-09 | 日本製鉄株式会社 | 浸炭鋼部品用鋼材 |
JP7151474B2 (ja) | 2018-12-28 | 2022-10-12 | 日本製鉄株式会社 | 浸炭鋼部品用鋼材 |
JP7156021B2 (ja) | 2018-12-28 | 2022-10-19 | 日本製鉄株式会社 | 浸炭鋼部品用鋼材 |
Also Published As
Publication number | Publication date |
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KR20180072778A (ko) | 2018-06-29 |
EP3382051A1 (en) | 2018-10-03 |
JPWO2017090738A1 (ja) | 2018-09-13 |
US20180347025A1 (en) | 2018-12-06 |
JP6468366B2 (ja) | 2019-02-13 |
EP3382051A4 (en) | 2019-06-19 |
KR102099767B1 (ko) | 2020-04-10 |
CN108368574A (zh) | 2018-08-03 |
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