WO2012046779A1 - 肌焼鋼及びその製造方法 - Google Patents
肌焼鋼及びその製造方法 Download PDFInfo
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Images
Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/28—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for plain shafts
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/40—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rings; for bearing races
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/20—Carburising
- C23C8/22—Carburising of ferrous surfaces
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/002—Bainite
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2261/00—Machining or cutting being involved
Definitions
- the present invention relates to a case-hardened steel subjected to carburizing and quenching after hot working such as hot forging, cold working such as cold forging and rolling, cutting, and the like, and a method for manufacturing the same.
- Rolling parts such as gears and bearings, and rotation transmission parts such as constant velocity joints and shafts are required to have surface hardness and are therefore carburized and quenched.
- These carburized parts include, for example, hot forging, warm forging, and cold forging of medium-carbon alloy steels for machine structures specified in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. It is manufactured in a process of forming into a predetermined shape by plastic working such as rolling or by carburizing and quenching.
- the accuracy of the part shape may deteriorate due to heat treatment distortion caused by carburizing and quenching.
- heat treatment distortion may cause noise and vibration, and may further deteriorate fatigue characteristics at the contact surface.
- power transmission efficiency and fatigue characteristics are impaired.
- the largest cause of this heat treatment distortion is coarse particles which are generated non-uniformly by heating during carburizing and quenching.
- the case-hardened steel is hardened by precipitation strengthening. Moreover, the case-hardened steel is hardened also by the addition of alloy elements that generate precipitates. Therefore, in steel that can prevent the generation of coarse grains at high temperatures, a decrease in cold workability with respect to cold forging, cutting, etc. is cited as a new issue.
- cutting is a process that requires high accuracy close to the final shape, and a slight increase in hardness greatly affects the accuracy of cutting. Therefore, when using case-hardened steel, it is extremely important to consider not only the prevention of the generation of coarse grains but also machinability (easy to cut material). Conventionally, it is known that addition of a machinability improving element such as Pb or S is effective for improving machinability.
- a machinability improving element such as Pb or S is effective for improving machinability.
- Pb is an environmentally hazardous substance, and the addition of Pb to steel is being restricted due to the importance of environmentally friendly technology.
- S improves the machinability by forming MnS and the like in steel, but coarse MnS stretched by hot working tends to be the starting point of fracture during rolling, hot forging, and cold forging. In many cases, it causes processing defects. Therefore, the addition of a large amount of S tends to cause deterioration in workability and forgeability during hot and cold rolling, and mechanical properties such as rolling fatigue.
- hot working such as hot forging, cold forging
- cold-worked steel such as rolling, cutting, carburizing and quenching
- case hardening steel with excellent coarse grain prevention characteristics, cold workability, machinability, and fatigue characteristics after carburizing and quenching, and its manufacturing method To do.
- the present inventor has intensively studied to solve the above problems.
- Ti-based precipitates act as a starting point for fatigue failure, and fatigue characteristics, particularly rolling fatigue characteristics, tend to deteriorate. Therefore, the present inventors have obtained the following knowledge and completed the present invention.
- Ti-based precipitates are finely dispersed by limiting the amount of N, increasing the hot rolling temperature, etc., both coarse grain prevention characteristics and fatigue characteristics can be achieved.
- S size and shape of the sulfide by adding Ti.
- Ti also forms sulfides and is combined with MnS, which is effective for making MnS finer.
- the gist of the present invention is as follows.
- the case-hardened steel according to one embodiment of the present invention has a chemical composition in mass% of C: 0.1 to 0.5%, Si: 0.01 to 1.5%, Mn: 0.00. 3 to 1.8%, S: 0.001 to 0.15%, Cr: 0.4 to 2.0%, Ti: 0.05 to 0.2%, Al: 0.2% or less N: 0.0050% or less, P: 0.025% or less, O: 0.0025% or less, the balance is made of iron and unavoidable impurities, and the equivalent circle diameter is 1 mm of a sulfide exceeding 5 ⁇ m.
- the number d per 2 and the mass percentage [S] of the S content satisfy d ⁇ 500 ⁇ [S] +1.
- the chemical composition is further in mass%, Nb: less than 0.04%, Mo: 1.5% or less, Ni: 3.5% or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, Mg: You may contain at least 1 sort (s) selected from 0.003% or less and Zr: 0.005% or less.
- [Al] / [Ca] which is the ratio of the mass percentage [Al] of Al to the mass percentage [Ca] of Ca, is 1 or more and 100 or less. Also good.
- the Mn content is 1.0% or less
- the S content relative to the mass percentage [Mn] of the Mn content [Mn] / [S], which is the ratio of the mass percentage [S] may be 100 or less.
- the bainite structure fraction may be 30% or less.
- the maximum equivalent circle diameter of the Ti-based precipitate may be 40 ⁇ m or less.
- the said chemical composition is further mass%, Nb: less than 0.04%, Mo: 1.5% or less, Ni: 3.5 % Or less, V: 0.5% or less, B: 0.005% or less, Ca: 0.005% or less, Mg: 0.003% or less, Zr: 0.005% or less You may contain.
- [Al] / [Ca] which is a ratio of the mass percentage [Al] of Al to the mass percentage [Ca] of Ca, is 1 or more and 100 or less. It may be.
- the amount of Mn is 1.0% or less, and the S content relative to the mass percentage [Mn] of the Mn content.
- [Mn] / [S], which is the ratio of the mass percentage [S] of the content, may be 100 or less.
- the case-hardened steel according to the present invention is excellent in fatigue characteristics after carburizing and quenching, and workability such as forgeability and machinability. That is, in the case-hardened steel according to the present invention, good workability is obtained in the hot forging process and the subsequent cutting process, and even when carburizing at a higher temperature and shorter time than conventional when carburizing, The coarsening can be suppressed and good fatigue characteristics can be obtained.
- the cold deformation characteristics are good, and even if the normalizing process after cold forging is omitted, the abnormal grains of crystal grains in carburizing Growth can be suppressed, and deterioration in dimensional accuracy due to quenching strain and accompanying reduction in fatigue strength are extremely small.
- the case-hardened steel according to the present invention when various alloy elements are added to prevent the generation of coarse grains, the conventional problem that the machinability is lowered is solved, and the precision of the part shape is improved. Is achieved, and the tool life is also increased.
- test piece used for the upsetting test supposing hot forging It is a figure of the test piece used for the upsetting test supposing cold forging. It is a figure which shows an example of the relationship between the average cooling rate in a slab, and the average area of MnS. It is a flowchart which shows an example of the manufacturing method of the case hardening steel which concerns on one Embodiment of this invention.
- the coarsening of crystal grains due to carburizing and quenching is prevented by using precipitates as pinning particles and suppressing grain growth.
- it is extremely effective to prevent the generation of coarse particles by precipitating Ti-based precipitates mainly composed of TiC and TiCS during cooling after hot working.
- Nb-based precipitates such as NbC on the case-hardened steel.
- the steel material is first so that the precipitates of Ti, Nb and Al are dissolved in the steel.
- hot working that is, after hot rolling or hot forging, it is necessary to gradually cool the precipitation temperature range of Ti-based precipitates and Nb-based precipitates at a cooling rate of 1 ° C./s or less.
- Ti-based precipitates and Nb-based precipitates can be finely dispersed in the case hardening steel. Further, if the ferrite grains of the steel material before carburizing and quenching are excessively fine, coarse grains are likely to be generated during carburizing heating. Therefore, it is necessary to control the finishing temperature of hot rolling or hot forging to 840 to 1000 ° C. so as not to generate fine ferrite.
- the case-hardened steel of the present invention is processed into a part shape such as a gear, for example, as shown in FIG. 1, after rolling a continuously cast slab, hot forging or cold forging before carburizing and quenching is performed. Cutting (in the case of gears, tooth forming by gear cutting) is performed. At that time, sulfides such as MnS decrease the cold forgeability, but are extremely effective for cutting (for example, gear cutting). That is, the sulfide in the case hardening steel (work material) suppresses the change in the tool shape due to wear of the cutting tool, and exhibits the effect of extending the so-called tool life. In particular, in the case of a precision shape such as a gear, if the cutting tool life is short, the tooth shape cannot be stably formed. For this reason, the cutting tool life affects not only the production efficiency and cost but also the shape accuracy of the parts.
- the size of MnS decreases as the cooling rate increases, and conversely, the size of MnS increases as the cooling rate decreases. Therefore, as described later, the cooling rate should be increased from the viewpoint of controlling the dimensions of MnS. On the other hand, when the cooling rate is high, cracks may occur on the surface of the slab, resulting in casting troubles, and the need to care for the soot after casting may occur.
- the range of the solidification cooling rate (average solidification cooling rate) is controlled to 12 to 100 ° C./min.
- the cooling rate is less than 12 ° C./min, solidification is too slow, so that the sulfide mainly composed of MnS crystallizes, and this sulfide is finely dispersed so as to satisfy the following formula (2). It is difficult to let In addition, when the cooling rate exceeds 100 ° C./min, the density of the sulfide mainly composed of fine MnS to be generated is saturated, the hardness of the slab (steel before rolling) is increased, and cracking may occur. .
- the cooling rate at the time of casting needs to be 12 to 100 ° C./min.
- the cooling rate during casting is preferably 15 to 100 ° C./min.
- Such a cooling rate can be obtained by controlling the size of the mold cross section, the casting speed, and the like to appropriate values. Such cooling control can be applied to both the continuous casting method and the ingot-making method.
- the solidification cooling rate here refers to the liquidus temperature at the center line of the width of the slab and at 1/4 part of the thickness of the slab in the cross section (cross section perpendicular to the casting direction) of the slab shown in FIG. It means the speed at the time of cooling from to the solidus temperature.
- This solidification cooling rate can be calculated by the following formula (1) from the interval between the secondary dendrite arms of the solidified structure of the cross section of the slab after solidification.
- Rc ( ⁇ 2/770) -1 / 0.41 ⁇ (1)
- Rc is the solidification cooling rate (° C./min)
- ⁇ 2 is the interval ( ⁇ m) between the secondary dendrite arms.
- sulfides centered on MnS are deformed and become the starting point of fracture.
- coarse MnS decreases the cold forgeability such as the critical compressibility.
- anisotropy occurs in the steel characteristics.
- FIGS. 2A and 2B show the relationship between machinability and cold workability for case-hardened steel with good pinning characteristics that suppress the generation of coarse grains during carburizing and quenching.
- the amount of S is changed in the SCr420 equivalent steel.
- the amount of S is changed in the SCM420 equivalent steel obtained by adding Mo to the SCr420 equivalent steel.
- the steel located on the upper right side has a better balance between machinability and cold workability, and this balance changes depending on the steel type (particularly, the amount of element that enhances hardenability).
- [C] C is an element that increases the strength of steel. In order to ensure sufficient tensile strength, the C content needs to be 0.1% or more, and preferably 0.15% or more. On the other hand, if the amount of C exceeds 0.5%, the cold workability deteriorates due to remarkable curing, so the amount of C needs to be 0.5% or less. Moreover, in order to ensure the toughness of a core part after carburizing, it is preferable that C amount is 0.4% or less, and it is still more preferable that it is 0.3% or less.
- Si is an element effective for deoxidation of steel, and the Si amount needs to be 0.01% or more.
- Si is an element that strengthens steel and improves hardenability, and the Si content is preferably 0.02% or more.
- Si is an element effective for increasing the grain boundary strength.
- bearing parts and rolling parts it is an element effective for extending the life in order to suppress structural changes and material deterioration during the rolling fatigue process. It is. Therefore, when increasing the strength, the Si content is more preferably 0.1% or more. In particular, in order to increase the rolling fatigue strength, the Si content is preferably 0.2% or more.
- the Si content if the Si content exceeds 1.5%, cold workability such as cold forging deteriorates due to curing, so the Si content needs to be 1.5% or less. Moreover, in order to improve cold workability, it is preferable that Si amount is 0.5% or less. In particular, when emphasizing cold forgeability, the Si content is preferably 0.25% or less.
- Mn is an element effective for deoxidation of steel and increases the strength and hardenability of steel, and the amount of Mn needs to be 0.3% or more. On the other hand, if the amount of Mn exceeds 1.8%, the cold forgeability deteriorates due to the increase in hardness, so it is necessary to be 1.8% or less. A preferable range of the amount of Mn is 0.5 to 1.2%. In addition, when importance is attached to cold forgeability, the Mn content is preferably set to 0.75% or less. Mn is an element that improves hardenability, but is an element that generates MnS in steel together with S in terms of sulfide formation.
- Mn has the effect of increasing the bainite fraction from the surface of hardenability to harden the steel, and lowers cold forgeability and machinability from the processed surface. Therefore, in terms of sulfide generation, if the amount of Mn is large and the ratio of [S] [S] to the amount of Mn [Mn] [Mn] / [S] increases, coarse MnS is likely to occur. In particular, in order to reduce the bainite fraction and sufficiently ensure cold forgeability, it is preferable that the Mn content is 1.0% or less and [Mn] / [S] is 100 or less. [Mn] / [S] may be 2 or more.
- [S] S is an element that forms MnS in steel and improves machinability.
- the amount of S needs to be 0.001% or more, and is preferably 0.01% or more.
- the S content is preferably 0.05% or less.
- the S content is more preferably 0.03% or less.
- the shape of the sulfide is controlled by adding Ti or Nb, controlling the cooling rate during solidification (solidification cooling rate), and heating during soaking.
- Ti forms a composite sulfide with Mn, and this composite sulfide does not stretch like single MnS.
- solidification cooling rate is low, coarse MnS is generated in the liquid phase before the completion of solidification.
- the S amount is preferably 0.01% or more.
- [Cr] Cr is an effective element that improves the strength and hardenability of steel, and the Cr amount needs to be 0.4% or more. Further, in bearing parts and rolling parts, Cr increases the residual ⁇ content of the surface layer after carburizing, and is effective in extending the life by suppressing structural changes and material deterioration during the rolling fatigue process. Therefore, the Cr content is preferably 0.7% or more, and more preferably 1.0% or more. On the other hand, when Cr exceeding 2.0% is added to the steel, the cold workability deteriorates due to the increase in hardness, so the Cr amount needs to be 2.0% or less. In order to improve the cold forgeability, the Cr content is preferably 1.5% or less.
- Ti is an element that produces precipitates such as carbides, carbosulfides, and nitrides in steel.
- the Ti amount needs to be 0.05% or more, and preferably 0.1% or more.
- the Ti amount needs to be 0.2% or less.
- the Ti content is preferably set to 0.15% or less.
- the precipitate of MnS can be refined by adding Ti.
- Al is a deoxidizer, and the amount of Al is preferably 0.005% or more, but is not limited thereto.
- AlN does not form a solution by heating during hot working and remains in the steel. Therefore, coarse AlN acts as a precipitation nucleus of Ti and Nb precipitates, thereby inhibiting the formation of fine precipitates. Therefore, in order to prevent the coarsening of crystal grains during carburizing and quenching, the Al content needs to be 0.2% or less. If the Al content is in the range of 0.05% or less, the heat treatment characteristics during normalization and carburizing and quenching are not significantly different from those of conventional steel, so the Al content may be 0.05% or less practically. preferable.
- the Al content is preferably 0.03% or more. In consideration of the balance between heat treatment characteristics and machinability, the Al content is preferably 0.15% or less.
- the precipitation amount of AlN contained in the case hardening steel is preferably limited to 0.01% or less, and limited to 0.005% or less. More preferably.
- the steel is sufficiently heated and held at 1250 ° C. or higher at the stage of manufacturing billets and the like from the slab.
- This temperature is preferably higher, and it is preferable to heat and hold the steel at a temperature exceeding 1250 ° C. If this holding temperature exceeds 1350 ° C., the material of the heating furnace such as a refractory is significantly damaged. Therefore, the holding temperature needs to be 1320 ° C. or less.
- the rate of precipitation and growth of AlN is slower than that of Ti-based precipitates and Nb-based precipitates. Therefore, it is possible to reduce the precipitation amount of AlN contained in the case-hardened steel by preventing the residual AlN during the hot working heating, utilizing the fine Ti-based precipitates and Nb-based precipitates, Generation of coarse grains during carburizing and quenching can be prevented.
- the precipitation amount of AlN can be measured by chemical analysis of the steel extraction residue.
- the extraction residue is collected by dissolving steel with a bromine-methanol solution and filtering this solution through a 0.2 ⁇ m filter. Even if a 0.2 ⁇ m filter is used, the filter is clogged with precipitates during the filtration process, so that it is possible to extract fine precipitates of 0.2 ⁇ m or less.
- [N] N is an element that generates nitride.
- the N amount is limited to 0.0050% or less. This is because coarse TiN and AlN act as precipitation nuclei such as Ti-based precipitates mainly composed of TiC and TiCS and Nb-based precipitates mainly composed of NbC, and inhibit the dispersion of fine precipitates. is there. Therefore, the N content is preferably 0.0040% or less, and more preferably 0.0035% or less.
- the lower limit of the N amount is not particularly limited, and is 0%.
- [P] P is an impurity and is an element that increases deformation resistance during cold working and deteriorates toughness. If P is excessively contained in the steel, the cold forgeability deteriorates, so it is necessary to limit the P content to 0.025% or less. Further, in order to suppress embrittlement of crystal grain boundaries and improve fatigue strength, the P content is preferably set to 0.015% or less. The lower limit of the amount of P is not particularly limited, and is 0%.
- [O] O is an impurity, and forms oxide inclusions in steel and impairs workability. Therefore, the amount of O is limited to 0.0025% or less. Moreover, since the case-hardened steel of this embodiment contains Ti, oxide inclusions containing Ti are generated, and TiC is precipitated using this as a precipitation nucleus. When the oxide inclusions increase, the generation of fine TiC may be suppressed during hot working. Therefore, in order to finely disperse Ti-based precipitates mainly composed of TiC and TiCS and suppress the coarsening of crystal grains during carburizing and quenching, the O content is preferably limited to 0.0020% or less. Furthermore, rolling fatigue failure may occur in bearing parts and rolling parts starting from oxide inclusions. Therefore, when applying case hardening steel to bearing parts or rolling parts, it is more preferable to limit the amount of O to 0.0012% or less in order to improve the rolling life. The lower limit of the amount of O is not particularly limited and is 0%.
- the chemical composition which contains the above-mentioned basic chemical component (basic element) and consists of the balance Fe and inevitable impurities is the basic composition of the present invention.
- the present invention may further contain the following elements (selective elements) as necessary. In addition, even if these selective elements are inevitably mixed in steel, the effect in this embodiment is not impaired.
- Nb In addition to the above basic elements, in order to suppress the generation of coarse grains during carburizing and quenching, it is preferable to add Nb that produces carbonitrides similarly to Ti.
- Nb is an element that forms carbonitride by combining with C and N in steel in the same manner as Ti.
- Nb is an element that forms carbonitride by combining with C and N in steel in the same manner as Ti.
- the effect of suppressing the generation of coarse grains due to Ti-based precipitates becomes even more remarkable.
- the addition amount of Nb is very small, it is extremely effective in preventing coarse grains as compared with the case where Nb is not added. This is because Nb dissolves in the Ti-based precipitate and suppresses the coarsening of the Ti-based precipitate.
- the Nb content is preferably 0.005% or more, but is not limited thereto.
- the Nb amount is preferably less than 0.04%.
- the Nb content is more preferably less than 0.03%.
- the Nb content is preferably less than 0.02%.
- Nb affects the hot ductility, and in steel used for gears, the hot ductility becomes even more sensitive to the amount of Nb. Therefore, addition of Nb is effective for control of Ti-based precipitates and microstructures, but attention should be paid to addition of Nb also from the viewpoint of ductility in hot working such as rolling and hot forging. Thus, since the effect of Nb addition is recognized when 0.005% or more of Nb is added, excessive Nb addition exceeding 0.04% should be avoided. In addition, when reducing an alloy cost, it is not necessary to add Nb into steel intentionally, and the minimum of the amount of Nb is 0%.
- the sum of the Nb amount [Nb] and the Ti amount [Ti], [Ti] + [Nb] Is preferably 0.07% or more and less than 0.17%.
- a more preferable range of [Ti] + [Nb] is more than 0.09% and less than 0.17%.
- one or more of Mo, Ni, V, and B may be added.
- Mo is an element that increases the strength and hardenability of the steel, and may be added to the steel as necessary. Mo is effective for increasing the amount of residual ⁇ on the surface layer of the carburized part and for extending the life by suppressing the structural change and material deterioration during the rolling fatigue process. However, when Mo exceeding 1.5% is added to the steel, the machinability and cold forgeability may deteriorate due to the increase in hardness. Therefore, the Mo amount is preferably 1.5% or less. Since Mo is an expensive element, the Mo amount is preferably 0.5% or less from the viewpoint of manufacturing cost. Thus, in order to reduce the alloy cost, there is no need to intentionally add Mo into the steel, and the lower limit of the amount of Mo is 0%. Moreover, when adding and utilizing Mo, it is preferable that Mo amount is 0.05% or more, Furthermore, it is preferable that it is 0.1% or more.
- Ni is an element effective for improving the strength and hardenability of steel, and may be added to the steel as necessary. However, if Ni exceeding 3.5% is added to the steel, the machinability and cold forgeability may be deteriorated due to the increase in hardness, so the Ni content is preferably made 3.5% or less. Since Ni is also an expensive element, the amount of Ni is preferably 2.0% or less and more preferably 1.0% or less from the viewpoint of manufacturing cost. Thus, in order to reduce the alloy cost, it is not necessary to intentionally add Ni into the steel, and the lower limit of the Ni amount is 0%. Moreover, when adding and utilizing Ni, it is preferable that Ni amount is 0.1% or more, Furthermore, it is preferable that it is 0.2% or more.
- V is an element that improves strength and hardenability when dissolved in steel, and may be added to steel as necessary. If the amount of V exceeds 0.5%, the machinability and cold forgeability may be deteriorated due to the increase in hardness. Therefore, the amount of V is preferably 0.5% or less, 0.2 % Or less is more preferable. In order to reduce the alloy cost, it is not necessary to intentionally add V to the steel, and the lower limit of the V amount is 0%. Moreover, when adding and utilizing V, it is preferable that V amount is 0.05% or more, Furthermore, it is preferable that it is 0.1% or more.
- [B] B is an effective element that enhances the hardenability of steel with a small amount of addition, and may be added to steel as necessary. Further, B generates boron iron carbide in the cooling process after hot rolling, increases the growth rate of ferrite, and promotes softening. Furthermore, B improves the grain boundary strength of the carburized component, and is effective in improving fatigue strength and impact strength. However, if more than 0.005% of B is added to the steel, the above effect is saturated and impact strength may be deteriorated. Therefore, the B content is preferably 0.005% or less, % Or less is more preferable. In order to reduce the alloy cost, it is not necessary to intentionally add B to the steel, and the lower limit of the amount of B is 0%.
- one or more of Ca, Mg, and Zr may be added for deoxidation and sulfide morphology control.
- [Ca] Ca is a deoxidizing element that generates an oxide in steel, and may be added to steel as necessary.
- the oxide in steel by Al deoxidation is Al 2 O 3 , but since Al 2 O 3 is hard, there is a detrimental effect of reducing machinability.
- Al 2 O 3 and Ca which are basic oxides, produce an Al—Ca based composite oxide, and the steel can be slightly softened. Therefore, it is possible to suppress a decrease in machinability due to Al deoxidation.
- adhesion of Al 2 O 3 to the refractory can be suppressed even at the steel making stage, and adverse effects such as nozzle clogging can be suppressed.
- Ca forms MnS and composite sulfide to harden MnS slightly, it can suppress the extension of MnS during rolling and forging, and can suppress cracks originating from sulfide during cold forging.
- adding too much Ca to the steel generates a large amount of CaS and makes the steel hard, so that machinability is impaired.
- the Ca content is preferably 0.0003% or more, more preferably 0.0005% or more, and further preferably 0.0008% or more. .
- the Ca content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less.
- the lower limit of the Ca content is 0%.
- the ratio of the Al amount [Al] to the Ca amount [Ca] is also important. If [Al] / [Ca] indicating this ratio is too small, deoxidation by Al is insufficient, and Ca is consumed as an oxide. In this case, the effect of Ca on sulfide control is insufficient. Conversely, if [Al] / [Ca] is too large, the effect of Ca on oxide control is insufficient. Therefore, when adding Ca to steel, the range of [Al] / [Ca] is preferably 1 or more and 100 or less, and more preferably 6 or more and 100 or less.
- Mg and Zr are elements that generate oxides and sulfides, and may be added to steel as necessary. Since these Mg and Zr suppress the deformability of MnS, they suppress the extension of MnS by hot working. In particular, Mg and Zr exhibit a remarkable effect even if contained in a trace amount in steel. In order to stabilize the amount of Mg and Zr in the steel, it is preferable to control the amount of Mg or Zr in consideration of a refractory containing Mg or Zr. Mg is an element that generates oxides and sulfides.
- MgS composite sulfide (Mn, Mg) S with Mn, and the like are generated, and extension of MnS can be suppressed.
- a small amount of Mg is effective for controlling the form of MnS, and when Mg is added to steel to improve workability, the amount of Mg is preferably 0.0002% or more.
- Mg oxide is finely dispersed and acts as a nucleus for the formation of sulfides such as MnS. When using Mg oxide to suppress the formation of coarse sulfides, the Mg content is preferably 0.0003% or more. Further, when Mg is added to the steel, the sulfide becomes slightly hard and is not easily stretched by hot working.
- the Mg content is preferably 0.0005% or more.
- hot forging has an effect of uniformly dispersing fine sulfides and is effective in improving cold workability.
- the lower limit of the amount of Mg is 0%.
- the oxide of Mg is likely to float on the molten steel, so the yield is low, and the Mg content is preferably 0.003% or less from the viewpoint of manufacturing cost.
- the amount of Mg is more preferably 0.001% or less.
- Zr is an element that forms nitrides in addition to oxides and sulfides. When a small amount of Zr is added to the molten steel, it is combined with Ti in the molten steel to produce fine oxides, sulfides and nitrides. Therefore, the addition of Zr is extremely effective for controlling inclusions and precipitates. When Zr is added to steel to control the form of inclusions and to improve workability, the Zr content is preferably 0.0002% or more.
- the amount of Zr is preferably 0.0003% or more in order to add Zr to suppress deformation of MnS and prevent extension of MnS by hot working.
- the amount of Zr is preferably 0.005% or less and more preferably 0.003% or less from the viewpoint of manufacturing cost. In order to reduce the alloy cost, it is not necessary to intentionally add Zr to the steel, and the lower limit of the amount of Zr is 0%.
- the case-hardened steel according to the present embodiment includes the above-described basic element, the chemical composition including the balance Fe and inevitable impurities, or at least one selected from the above-described basic element and the above-described selective element. And a chemical composition consisting of the balance Fe and inevitable impurities.
- MnS is useful for improving machinability, it is necessary to ensure its number density.
- the stretched coarse MnS impairs the cold workability, so it is necessary to control the size and shape.
- the inventors of the present invention have studied the relationship between the characteristics of sulfides such as S content, MnS size and shape, and workability such as machinability and cold workability. As a result, it was found that when the average equivalent circle diameter of MnS observed with an optical microscope exceeds 5 ⁇ m, this MnS becomes a starting point for cracking during cold working.
- the average equivalent circle diameter of MnS is a diameter of a circle having an area equal to the area of MnS, and can be obtained by image analysis.
- sulfides such as MnS in hot rolled material with a diameter of 30 mm with a scanning electron microscope
- features of sulfides such as size, aspect ratio and number density, and workability such as cold workability and machinability Organized the relationship.
- the sulfide was observed at a 1 ⁇ 2 radius portion (a portion between the surface and the center of the hot rolled material) of the cross section parallel to the rolling direction.
- Ten fields of 50 ⁇ m ⁇ 50 ⁇ m were observed and the average equivalent circle diameter, aspect ratio and number of sulfide inclusions present in the field of view were determined.
- the number density of sulfides having an average equivalent circle diameter exceeding 5 ⁇ m was measured and divided by the measurement area to obtain the number density d. If these sulfides are finely dispersed, they can act as pinning particles during austenite grain growth during carburization. Accordingly, if the number density of relatively large sulfides having an equivalent circle diameter of 5 ⁇ m or more is small, it means that there are many fine sulfides, and it is possible to achieve both workability for forging and cutting, carburizing characteristics and fatigue characteristics. it can.
- the number density d (pieces / mm 2 ) of the sulfides (the number per 1 mm 2 of sulfides having an equivalent circle diameter of more than 5 ⁇ m) is affected by the amount of S, so that the machinability and the cold workability are reduced.
- the number density d (pieces / mm 2 ) of the sulfide satisfies the following empirical formula (2) from various experiments regarding the relationship between the number density d of sulfide and the amount of S [S]. I found it necessary.
- the maximum size sulfide in a region where a load is applied during deformation during forging, use as a part, and fatigue after carburization acts as a starting point for fracture. The tendency is influenced by the amount of S, and the larger the amount of S, the larger the size of the sulfide.
- This maximum sulfide should be considered including not only Ti-based sulfides but also Mn-based sulfides (MnS) with low Ti content.
- the inventors have conducted various experiments on the relationship between the amount of S and the maximum sulfide size, and as a result, when the maximum equivalent circle diameter D ( ⁇ m) of the observed sulfide satisfies the following formula (3), the same is true. It was confirmed that good forgeability (hot and cold) can be obtained as compared with the steel of S amount, and further excellent fatigue characteristics can be obtained. D ⁇ 250 [S] +10 (3) (Here, [S] indicates the S content (mass%).)
- the size of the sulfide can be controlled so that the maximum equivalent circle diameter D ( ⁇ m) of the sulfide satisfies the above expression (3) by the component design from the casting stage.
- D ( ⁇ m) exceeds 250 [S] +10, the forgeability and fatigue characteristics are degraded, and only the performance equivalent to that of the conventional steel containing the same amount of S may be exhibited.
- the upper limit is preferably 250 [S] +10.
- Ti-based precipitate Furthermore, if coarse Ti-based precipitates are present in the steel, it may act as a starting point for contact fatigue failure, and the fatigue characteristics may deteriorate.
- Contact fatigue strength is a required characteristic of carburized parts, and is rolling fatigue characteristics and surface fatigue strength. In order to increase the contact fatigue strength, it is preferable that the maximum equivalent circle diameter (maximum diameter) of the Ti-based precipitate to be observed is less than 40 ⁇ m.
- the grain size number of the ferrite of the case-hardened steel is preferably within the range of 8 to 11 defined by JIS G 0551.
- Steel is melted by a normal method using a converter, an electric furnace, etc., the components are adjusted, and a steel material is obtained through a casting process and, if necessary, a block rolling process.
- the steel material is subjected to hot working, that is, hot rolling or hot forging to produce a wire or a steel bar.
- the cooling rate during solidification is from the slab surface 3 to the center line of the slab thickness T on the slab section 2 of the slab 1 shown in FIG. It is defined as the cooling rate at 1/2 part of the distance (position indicated by a black circle, ie, position X at T / 4 from the surface with respect to the direction of slab thickness T).
- the cooling rate during solidification needs to be 12 ° C./min or more, and preferably 15 ° C./min or more.
- the cooling rate at the time of solidification can be confirmed by the dendrite secondary arm interval as described above.
- the cooling rate at the time of casting needs to be 12 to 100 ° C./min. Further, in order to more reliably prevent slab cracking, the cooling rate during casting is preferably 50 ° C./min or less, and more preferably 20 ° C./min or less.
- Such a cooling rate can be obtained by controlling the size of the mold cross section, the casting speed, and the like to appropriate values.
- Such cooling control can be applied to both the continuous casting method and the ingot-making method. Since MnS is considered to crystallize in the liquid phase near the freezing point of steel, the size of MnS decreases as the cooling rate increases and increases as the cooling rate decreases. For this reason, in this embodiment, the molten steel is solidified at an extremely high cooling rate as compared with the cooling conditions of the conventional continuous casting machine and the conventional mass production type ingot manufacturing method, and the size of MnS is suppressed to be small.
- the cooling rate was controlled by adjusting the casting conditions such as mold dimensions and cooling conditions while considering the relationship between the casting conditions and the cooling rate at the time of casting of conventional continuous casting and mass production type ingots.
- An example of the relationship between the average cooling rate in the slab and the average area of MnS is shown.
- the average cooling rate of the slab is increased, the average area of MnS (that is, the average equivalent circle diameter) can be reduced.
- a method of reducing the mold size can be adopted as a simple method, but it is difficult to maintain product quality with this method.
- the inhomogeneous part due to defects or segregation acts as a starting point of fracture or causes variations in hardenability, which may deteriorate the quality of the case hardening steel.
- the slab is reheated as it is, and hot working is performed to manufacture the case-hardened steel, or the steel obtained from the slab is reheated by the lump process, and the hot working is performed, and the case-hardened steel is obtained.
- Manufacturing Generally, a slab is formed into a billet by split rolling, cooled to room temperature, and then reheated to produce a case-hardened steel. Furthermore, in the manufacture of parts such as gears, hot forging may be added.
- the slab should be kept at a high temperature as much as possible in order to relax the concentrated portion of the alloy element in the slab, and the brittle elements such as P and Mn should be uniformly diffused. Therefore, after casting, the temperature of the slab is maintained at 600 ° C or higher, and the slab is directly inserted into a heating furnace for batch rolling, and further, this slab is held at a high temperature of 1200 ° C or higher for 20 minutes or longer. Thus, the diffusion of P, Mn and S was promoted. Furthermore, this heating and holding also has the effect of dissolving Ti and Nb-based precipitates.
- the temperature (holding temperature) needs to be 1250 ° C. or higher.
- the holding temperature exceeds 1320 ° C., the refractory in an industrial heating furnace is severely damaged, and stable heat treatment becomes difficult. Therefore, the holding temperature needs to be 1320 ° C. or less.
- the holding time (soaking time) is required to be 3 minutes or more after reaching the above temperature, and preferably 10 minutes or more in order to allow sufficient dissolution of the above compound.
- the holding time is more preferably 20 minutes or longer so that the above effect can be stably exhibited.
- the holding time is as long as possible.
- the holding time exceeds 180 minutes, damage to the material surface increases and damage to the refractory also increases. Therefore, the holding time must be 180 minutes or less, and industrially 120 minutes or less. It is desirable that
- the heating temperature is less than 1150 ° C.
- Ti-based precipitates, Nb-based precipitates and AlN are solidified in the steel. It cannot be dissolved, and coarse Ti-based precipitates, Nb-based precipitates, and AlN remain in the steel.
- the heating temperature should be 1150 ° C or higher. is required. The lower limit of the suitable heating temperature is 1180 ° C.
- the heating temperature When the heating temperature exceeds 1320 ° C., the refractory of the industrial heating furnace becomes severely damaged, and stable heat treatment becomes difficult. Therefore, the heating temperature needs to be 1320 ° C. or less. Considering the load of the heating furnace, this heating temperature is preferably 1300 ° C. or lower. In order to keep the temperature of the steel material uniform and dissolve precipitates in the steel, it is preferable to set the holding time in product rolling to 10 minutes or more. From the viewpoint of productivity, this holding time is preferably 60 minutes or less.
- the finishing temperature for hot working is less than 840 ° C.
- ferrite crystal grains become fine and coarse grains are likely to be generated during carburizing and quenching.
- this finishing temperature exceeds 1000 ° C.
- a preferable range of the finishing temperature is 900 to 970 ° C., and a more preferable range is 920 to 950 ° C.
- the cooling conditions after hot working are important in order to finely disperse Ti-based precipitates and Nb-based precipitates.
- the temperature range in which the precipitation of Ti-based precipitates and Nb-based precipitates is promoted is 500 to 800 ° C. Therefore, the temperature range from 800 ° C. to 500 ° C. is gradually cooled at an average cooling rate of 1 ° C./second or less to promote the formation of Ti-based precipitates and Nb-based precipitates.
- the average cooling rate exceeds 1 ° C./second, the time for the steel to pass through the precipitation temperature range of the Ti-based precipitate and the Nb-based precipitate is shortened, and the amount of fine precipitates is insufficient. Further, when the average cooling rate is increased, the bainite structure fraction is increased.
- FIG. 7 shows a flowchart of an example of a method for producing a case hardening steel according to the present embodiment.
- the case hardening steel of the said embodiment is applicable to any of the components manufactured by a cold forging process, and the components manufactured by a hot forging process.
- the hot forging step include a step of bar steel, hot forging, heat treatment such as normalization if necessary, cutting, carburizing and quenching, and grinding or polishing if necessary.
- hot forging is performed at a heating temperature of 1150 ° C.
- the carburizing and quenching conditions are not particularly limited, it is preferable to set the carbon potential to 0.8 to 1.3% when bearing parts and rolling parts are oriented toward a high rolling fatigue life.
- carburizing and nitriding that performs nitriding in the diffusion process after carburizing is also effective for rolling fatigue life.
- the nitrogen concentration (nitrogen potential) on the part surface is in the range of 0.2 to 0.6%.
- Si and Cr, and the addition of Mo as needed, the effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts or rolling parts is the retained austenite (residual ⁇ ) in the surface of the parts after carburizing Is particularly large when 30 to 40%.
- Carburizing and nitriding treatment is effective for controlling the residual ⁇ amount on the surface of the component within a range of 30 to 40%. At that time, it is preferable to perform the carburizing and nitriding treatment so that the nitrogen concentration in the component surface layer is in the range of 0.2 to 0.6%. By selecting these carburizing and nitriding conditions, a large amount of fine Ti (C, N) precipitates in the carburized layer, and the rolling life is improved.
- Tables 4 to 6 show the maximum equivalent circle diameter (maximum dimension, maximum diameter) D of sulfides in steel, the sulfide density (number density) d exceeding 0.5 ⁇ m, and the maximum equivalent circle diameter of Ti-based precipitates (maximum Dimensions, maximum diameter).
- the underline in Tables 4 to 6 means that the condition of the sulfide density d of the present invention is not satisfied.
- the maximum equivalent circle diameter of the Ti-based precipitate and the maximum equivalent circle diameter D of the sulfide were predicted by an extreme value statistical method. That is, the maximum diameter of the Ti-based precipitate, the particle size distribution of the sulfide, and the maximum diameter were determined as follows.
- the steel metal structure was observed with an optical microscope, and the precipitates were discriminated from the contrast in the structure.
- the deposit was identified using the scanning electron microscope and the energy dispersive X-ray-spectral-analysis apparatus (EDS).
- EDS energy dispersive X-ray-spectral-analysis apparatus
- Ten polishing test pieces each having a length of 10 mm and a width of 10 mm were prepared from a cross section including the longitudinal direction of the test piece described later, and a predetermined position of these polishing test pieces was photographed 100 times with an optical microscope. Images of 9 mm 2 inspection reference area (region) were prepared for 10 fields of view. The particle size distribution and maximum diameter of the sulfide in the observation field (image) and the maximum diameter of the Ti-based precipitate were detected. These dimensions (diameters) were converted to equivalent circle diameters indicating the diameters of circles having the same area as the precipitates.
- Tables 7 to 9 show the hot working heating temperature, finishing temperature, average cooling rate, bainite fraction, ferrite grain number, Vickers hardness, and the like.
- the average cooling rate is a cooling rate in the range of 500 to 800 ° C., and was determined from the time required for cooling from 800 ° C. to 500 ° C.
- the underline in Tables 7 to 9 means that the production conditions of the present invention are not satisfied.
- Hot and cold forgeability was performed by an upsetting test.
- the test piece 4 having a bottom surface of ⁇ 30 mm and a height of 45 mm shown in FIG. 4 was heated to 1250 ° C. and then placed, and the compression ratio (critical compression ratio) at which cracking occurred was measured.
- the dashed-dotted line in FIG. 4 has shown the centerline common to (a) and (b).
- a grooving test piece 5 having the dimensions shown in FIG. 5 is taken and subjected to an upsetting test to measure the critical compressibility until cracking occurs. did.
- the probability of occurrence of cracking was determined using 10 test pieces for various compression rates, and the compression rate when this probability reached 50% was determined as the limit compression rate.
- This test method is an evaluation method close to cold forging, but can also be used as an index indicating the influence of sulfides on the forgeability in hot forging.
- the machinability was evaluated by conducting a test for determining the life until drill breakage.
- the steel was heated to 1250 ° C. assuming hot forging and cooled at a predetermined cooling rate.
- a high-speed straight drill having a diameter of 3 mm and a water-soluble cutting oil were used, and drilling was performed under conditions of a feed of 0.25 mm, a hole depth of 9 mm, and a drill protrusion amount of 35 mm.
- the peripheral speed of the drill was controlled to be constant within a range of 10 to 70 m / min, steel was drilled, and the accumulated hole depth until the drill broke was measured.
- the cumulative hole depth is the product of the depth of one hole and the number of holes formed by drilling.
- VL 1000 the maximum value of the peripheral speed of the drill was determined as VL 1000.
- a specimen was taken from a steel bar heated to 1250 ° C assuming hot forging, and after cold forging forging at a reduction rate of 50%, heat treatment simulating carburizing and quenching (carburizing simulation) was performed.
- the old austenite grain size of the test piece was measured to evaluate the coarse grain prevention characteristics.
- the carburizing simulation is a heat treatment in which the test piece is heated to 910 to 1060 ° C., held for 5 hours, and cooled with water.
- the prior austenite particle size was measured according to JIS G 0551 (2005).
- the prior austenite grain size was measured to determine the temperature at which coarse grains were generated (coarsening temperature).
- the prior austenite particle size is measured, and if there is even one coarse particle having a particle size number of 5 or less, the test result of the test piece is generated as coarse particles
- the coarsening temperature was determined. Since the heating temperature for carburizing and quenching is usually 930 to 950 ° C., a test piece having a coarsening temperature of 950 ° C. or less was judged to be inferior in coarsening prevention characteristics.
- the rolling fatigue characteristics were evaluated using a point contact type rolling fatigue tester (Hertz maximum contact stress 5884 MPa).
- L 10 life defined as “the number of stress repetitions until fatigue failure at a cumulative failure probability of 10% obtained by plotting test results on Weibull probability paper” was used.
- the fatigue test was not performed on the material with many reductions at a rolling reduction of 50%.
- the rolling fatigue life is No. 48 (comparative example) L 10 life is defined as 1, and each material (each No.) L 10 life is No. It was evaluated by the relative value with respect to 48 of the L 10 life.
- the grain coarsening temperature is 990 ° C. or higher
- the old ⁇ grains of the steel carburized at 950 ° C. are finely sized
- the rolling fatigue characteristics are also No. Compared to 48.
- Regarding cold forgeability and machinability No. It is apparent that 1 to 47 are superior to the comparative example having the same composition (especially S amount).
- No. 48 to 53 are steels equivalent to SCr420 and SCM420, which are general carburizing steels, and steels obtained by adding S to these carburizing steels.
- No. for comparison with 1 to 47 no. In Nos. 48 to 53, after heating sufficiently, No. Although a soaking temperature similar to 1 to 47 was secured, a general soaking temperature is about 1150 ° C. Furthermore, no. In 48 to 53, the heating temperature of the hot working was controlled to 1050 ° C. which is a general heating temperature.
- both machinability and forgeability can be achieved.
- the balance is shown in FIGS. 2A and 2B.
- the amount of S is changed in the SCr420 equivalent steel containing about 0.2% by mass of C and about 1% by mass of Cr.
- the amount of S is changed in the SCM420 equivalent steel in which about 0.2% of Mo is added to the SCr420 equivalent steel.
- the shape and particle size distribution (number basis) of MnS are controlled by controlling the cooling rate during casting, and Ti (SCr420 equivalent steel and SCM420 equivalent steel) is contained in Ti. Etc. are added to improve pinning characteristics.
- the steel of the present invention is superior in both machinability and forgeability as compared with the conventional steel.
- the SCr420 equivalent steel and the SCM420 equivalent steel are designed to be suitable for carburizing and quenching, and the SCM420 equivalent steel has higher hardenability than the SCr420 equivalent steel, so that larger parts and higher strength parts are used.
- this steel equivalent to SCM420 has a high hardness during processing before carburizing and quenching due to the addition of Mo, and therefore both cold forgeability and machinability are low compared to steel equivalent to SCr420.
- the balance between cold forgeability and machinability may change depending on the steel type, and these balances are secured in consideration of hardenability.
- the amount of N is more than 0.0050%, and Ti easily generates TiN, so that the solid solution Ti is decreased, and thus TiCN which is important as pinning particles during carburization and The production amount (number) of fine precipitates such as TiC was reduced. As a result, the pinning effect was insufficient, and the coarsening temperature of the old ⁇ grains during carburization decreased.
- No. In 63 to 65 since a large amount of N is contained in the steel, this large amount of N becomes a cause of flaws in hot rolling and hot forging. Furthermore, no. No. 63 to 65, the critical compression ratio in hot forging is higher than that of the steel of the example (for example, comparison between No. 1 or 2 and No. 63) having the chemical composition of the same level excluding N amount. It was low. Also from these practical aspects, the N amount is desirably as small as possible, and is preferably 0.0040% or less.
- No. Nos. 66 to 71 are comparative examples of 0.4% C class. 66-71, the above-mentioned No. Similar to 54 to 59, the soaking temperature was less than 1250 ° C., and the particle size distribution of the sulfide was not properly controlled. In addition, no. In 66 to 71, since the solid solution of Ti was insufficient, the coarsening temperature was also low.
- Nb was added to the steel in an amount of 0.04% or more. This Nb is effective as pinning particles during carburization, as is the case with Ti, but the addition of a large amount of Nb causes a decrease in hot ductility, which causes defects in hot rolling and hot forging. For this reason, no. 72-74, the critical compression ratio in hot forging is considerably lower than that of the steel of the example (for example, comparison between No. 24 and No. 72) having the same chemical composition except for the Nb amount. The critical compression ratio in cold forging was also low.
- the amount of Ti is less than 0.05%, and sufficient pinning particles cannot be obtained at the time of carburizing. Therefore, in the examples having the same level of chemical composition except for the amount of Ti. Compared with steel (for example, comparison with No. 1 and No. 75), the coarsening temperature fell.
- No. 79 Comparative Example
- the Ti amount was more than 0.2%, and coarse Ti-based precipitates were generated, resulting in a decrease in the coarsening temperature. That is, if the amount of Ti is excessive, Ti (Ti-based precipitates) cannot be sufficiently dissolved in the steel during soaking and hot working, so the solid Ti is an undissolved coarse Ti system. Preferentially precipitates on the precipitate. For this reason, pinning particles (fine Ti-based precipitates) at the time of carburizing cannot be obtained sufficiently, and the coarsening temperature is lowered. In addition, this No. 79, coarse Ti-based precipitates are produced. Not only was machinability inferior to 1, but coarse Ti-based precipitates acted as fracture starting points in fatigue tests, resulting in unstable fatigue characteristics and reduced fatigue life.
- Tables 18 to 21 show the average solidification rate, the hot working heating temperature, the finishing temperature, the average cooling rate, the bainite fraction, and the ferrite particle size number.
- the underline in the tables 18 to 21 means that the production conditions of the present invention are not satisfied.
- the evaluation method of manufacturing conditions confirmation method of average solidification rate, definition of average cooling rate
- evaluation method of structure (bainite fraction, ferrite particle size number) are the same as those described in No. 1 above. This is the same as the method described in the description of 1 to 79.
- Tables 14 to 17 show the maximum equivalent circle diameter (maximum dimension, maximum diameter) D of sulfide in steel, sulfide density (number density) d exceeding 0.5 ⁇ m, precipitation amount of AlN, maximum of Ti-based precipitates.
- the equivalent circle diameter (maximum dimension, maximum diameter) is indicated.
- the underline in Tables 14 to 17 means that the condition of the sulfide density d of the present invention is not satisfied.
- the measuring method of the maximum equivalent circle diameter of sulfide, the sulfide density exceeding 0.5 ⁇ m, and the maximum equivalent circle diameter of the Ti-based precipitate is described in the above-mentioned No. This is the same as the method described in the description of 1 to 79.
- the precipitation amount of AlN was measured by the chemical analysis using bromine methanol as described above.
- Tables 18 to 21 also show the Vickers hardness, the critical compressibility, the machinability VL 1000 , the coarsening temperature during carburizing, and the fatigue life of the carburized material.
- the characteristics of these steels are the same as those of No. 1 above. Measurement (evaluation) was performed by the same measurement method (evaluation method) as described in the description of 1 to 79.
- the grain coarsening temperature is 990 ° C. or higher, the old ⁇ grains of the steel carburized at 950 ° C. are finely sized, and the rolling fatigue characteristics are the same as those of No. 1 above. Compared to 48. Regarding cold forgeability and machinability, No. 101-133 and no. It is apparent that 150 to 173 is superior to the comparative example having the same composition (especially S amount).
- the maximum equivalent circle diameter of the Ti-based precipitate was 40 ⁇ m or less, so that the steels of the examples having comparable chemical compositions (for example, comparison between No. 102 and No. 131). Further, the coarsening temperature could be increased.
- the amount of Nb was 0.04% or more.
- Nb is effective as pinning particles at the time of carburizing, like Ti, but a large amount of Nb causes a decrease in hot ductility and causes defects in hot rolling and hot forging.
- no. 143 and 144 have a considerably lower limit compressibility in hot forging compared to the steels of the examples (for example, comparison between No. 110 and No. 143) having the same chemical composition except for the amount of Nb.
- the critical compression ratio in cold forging was also low.
- the steels of 1 to 47, 101 to 133, and 150 to 173 are case-hardened steels that are excellent in hot forgeability or cold forgeability, machinability, and fatigue properties after carburizing and quenching.
- Case hardening steel with excellent coarse grain prevention characteristics during carburizing and quenching (particularly during high temperature carburizing), fatigue characteristics after carburizing and quenching (for example, rolling fatigue), and workability (strength characteristics) such as forgeability and machinability. And a method for manufacturing the same.
Abstract
Description
本願は、2010年10月6日に、日本に出願された特願2010-226478号に基づき優先権を主張し、その内容をここに援用する。
Nb:0.04%未満、Mo:1.5%以下、Ni:3.5%以下、V:0.5%以下、B:0.005%以下、Ca:0.005%以下、Mg:0.003%以下、Zr:0.005%以下から選択される少なくとも1種を含有してもよい。
ここで、Rcは、凝固冷却速度(℃/min)、λ2は、2次デンドライトアームの間隔(μm)を意味する。
Cは、鋼の強度を上昇させる元素である。十分な引張強さを確保するためには、C量は、0.1%以上であることが必要であり、0.15%以上であることが好ましい。一方、C量が0.5%を超えると、著しい硬化により冷間加工性が劣化するため、C量が0.5%以下であることが必要である。また、浸炭後に芯部の靭性を確保するためには、C量が0.4%以下であることが好ましく、0.3%以下であることが更に好ましい。
Siは、鋼の脱酸に有効な元素であり、Si量が0.01%以上であることが必要である。また、Siは、鋼を強化し、焼入れ性を向上させる元素であり、Si量が0.02%以上であることが好ましい。更に、Siは、粒界強度の増加に有効な元素であり、更に軸受部品及び転動部品においては、転動疲労過程での組織変化及び材質劣化を抑制するため、高寿命化に有効な元素である。そのため、高強度化を指向する場合には、Si量が0.1%以上であることが更に好ましい。特に、転動疲労強度を高めるには、Si量が0.2%以上であることが好ましい。
Mnは、鋼の脱酸に有効であり、鋼の強度及び焼入れ性を高める元素であり、Mn量が0.3%以上である必要がある。一方、Mn量が、1.8%を超えると、硬さの上昇によって冷間鍛造性が劣化するため、1.8%以下であることが必要である。Mn量の好適範囲は、0.5~1.2%である。なお、冷間鍛造性を重視する場合は、Mn量を0.75%以下にすることが好ましい。また、Mnは、焼入れ性を向上させる元素であるが、硫化物生成の面ではSとともに鋼中でMnSを生成する元素である。Mnには、焼入れ性の面からベイナイト分率を大きくして鋼を硬くする効果があり、加工面から冷間鍛造性や被削性を低下させてしまう。そのため、硫化物生成の面では、Mn量が多く、Mn量[Mn]に対するS量[S]の比率である[Mn]/[S]が大きくなると、粗大なMnSを生じやすい。特に、ベイナイト分率を低減し、冷間鍛造性を十分に確保するためには、Mn量が1.0%以下であり、[Mn]/[S]が100以下であることが好ましい。なお、[Mn]/[S]は、2以上であってもよい。
Sは、鋼中でMnSを形成し、被削性を向上させる元素である。被削性を高めるため、S量が0.001%以上である必要があり、0.01%以上であることが好ましい。一方、S量が0.15%を超えると、粒界偏析によって粒界脆化を招くため、S量が0.15%以下であることが必要である。また、高強度部品であることを考慮すると、S量は0.05%以下であることが好ましい。更に、強度や冷間加工性、更にはそれらの安定性を考慮する場合は、S量を0.03%以下にすることがより好ましい。
Crは、鋼の強度及び焼入れ性を向上させる有効な元素であり、Cr量が0.4%以上であることが必要である。更に、軸受部品及び転動部品においては、Crは、浸炭後の表層の残留γ量を増大させ、転動疲労過程での組織変化及び材質劣化の抑制による高寿命化に有効である。そのため、Cr量は、0.7%以上であることが好ましく、1.0%以上であることがより好ましい。一方、2.0%を超えるCrを鋼中に添加すると、硬さの上昇によって冷間加工性が劣化するため、Cr量が2.0%以下であることが必要である。冷間鍛造性を高めるには、Cr量を1.5%以下にすることが好ましい。
Tiは、鋼中で炭化物、炭硫化物、窒化物などの析出物を生成する元素である。微細なTiC、TiCSを利用して浸炭焼入れ時の粗大粒の発生を防止するため、Ti量が、0.05%以上であることが必要であり、0.1%以上であることが好ましい。一方、0.2%超のTiを鋼中に添加すると、析出硬化によって冷間加工性が著しく劣化するため、Ti量が0.2%以下であることが必要である。また、TiNの析出を抑制して転動疲労特性を向上させるには、Ti量を0.15%以下にすることが好ましい。さらに、Tiを添加することで、MnSの析出物を微細化することができる。
Alは、脱酸剤であり、Al量が、0.005%以上であることが好ましいが、これに限定されるものではない。一方、Al量が0.2%を超えると、AlNが熱間加工の加熱によって溶体化せず、鋼中に残存する。そのため、粗大なAlNが、TiやNbの析出物の析出核として作用し、微細な析出物の生成が阻害される。したがって、浸炭焼入れ時の結晶粒の粗大化を防止するには、Al量を0.2%以下にすることが必要である。Al量が0.05%以下の範囲であれば、焼準や浸炭焼き入れの際の熱処理特性が従来鋼と大きく変わらないので、実用的にはAl量が0.05%以下であることが好ましい。一方で、Alは被削性を向上させる効果もあるため、よりよい被削性を求める場合にはAl量が0.03%以上であることが好ましい。熱処理特性と被削性とのバランスを考える上では、Al量が0.15%以下であることが好ましい。
Nは、窒化物を生成する元素である。粗大なTiNやAlNの生成を抑制するため、N量を0.0050%以下に制限する。これは、粗大なTiNやAlNが、TiC、TiCSを主体とするTi系析出物、NbCを主体とするNb系析出物などの析出核として作用し、微細な析出物の分散を阻害するためである。そのため、このN量が、0.0040%以下であることが好ましく、0.0035%以下であることがより好ましい。このN量の下限は、特に制限する必要がなく、0%である。
Pは、不純物であり、冷間加工時の変形抵抗を高め、靭性を劣化させる元素である。鋼中に過剰にPを含有すると冷間鍛造性が劣化するため、P量を0.025%以下に制限することが必要である。また、結晶粒界の脆化を抑制し、疲労強度を向上させるには、P量を0.015%以下にすることが好ましい。このP量の下限は、特に制限する必要がなく、0%である。
Oは、不純物であり、鋼中で酸化物系介在物を形成し、加工性を損なうため、O量を0.0025%以下に制限する。また、本実施形態の肌焼鋼はTiを含有するため、Tiを含む酸化物系介在物が生成し、これを析出核としてTiCが析出する。酸化物系介在物が増加すると、熱間加工時に微細なTiCの生成が抑制されることがある。したがって、TiC、TiCSを主体とするTi系析出物を微細に分散させ、浸炭焼入れ時に結晶粒の粗大化を抑制するには、O量を0.0020%以下に制限することが好ましい。更に、軸受部品及び転動部品では、酸化物系介在物を起点として転動疲労破壊が生じることがある。そのため、肌焼鋼を軸受部品または転動部品に適用する場合、転動寿命を向上させるために、O量を0.0012%以下に制限することがより好ましい。このO量の下限は、特に制限する必要がなく、0%である。
上述の基本元素に加え、浸炭焼入れ時の粗大粒の発生を抑制するため、Tiと同様に炭窒化物を生成するNbを添加することが好ましい。
Moは、鋼の強度及び焼入れ性を高める元素であり、必要に応じて鋼中に添加してもよい。浸炭部品の表層の残留γの量を増大させ、更には、転動疲労過程での組織変化及び材質劣化の抑制による高寿命化を図るためにもMoは有効である。しかし、1.5%を超えるMoを鋼中に添加すると、硬さの上昇によって、切削性及び冷間鍛造性が劣化することがある。したがって、Mo量を、1.5%以下にすることが好ましい。Moは高価な元素であるため、製造コストの観点からMo量を0.5%以下にすることが好ましい。このように、合金コストの低減のためには、Moを意図的に鋼中に添加する必要がなく、Mo量の下限は0%である。また、Moを添加して活用する場合には、Mo量は、0.05%以上であることが好ましく、さらには0.1%以上であることが好ましい。
Niは、Moと同様、鋼の強度及び焼入れ性の向上に有効な元素であり、必要に応じて鋼中に添加してもよい。しかし、3.5%を超えるNiを鋼中に添加すると、硬さの上昇によって切削性及び冷間鍛造性が劣化することがあるため、Ni量を3.5%以下にすることが好ましい。Niも高価な元素であるため、製造コストの観点から、Ni量は、2.0%以下であることが好ましく、1.0%以下であることがより好ましい。このように、合金コストの低減のためには、Niを意図的に鋼中に添加する必要がなく、Ni量の下限は0%である。また、Niを添加して活用する場合には、Ni量は、0.1%以上であることが好ましく、さらには0.2%以上であることが好ましい。
Vは、鋼中に固溶すると、強度及び焼入れ性を向上させる元素であり、必要に応じて鋼中に添加してもよい。V量が、0.5%を超えると、硬さの上昇によって切削性及び冷間鍛造性が劣化することがあるため、V量は、0.5%以下であることが好ましく、0.2%以下であることがより好ましい。合金コストの低減のためには、Vを意図的に鋼中に添加する必要がなく、V量の下限は0%である。また、Vを添加して活用する場合には、V量は、0.05%以上であることが好ましく、さらには0.1%以上であることが好ましい。
Bは、微量の添加で、鋼の焼入れ性を高める有効な元素であり、必要に応じて鋼中に添加してもよい。また、Bは、熱間圧延後の冷却過程でボロン鉄炭化物を生成し、フェライトの成長速度を増加させ、軟質化を促進する。更に、Bは、浸炭部品の粒界強度を向上させ、疲労強度及び衝撃強度の向上にも有効である。しかし、0.005%超のBを鋼中に添加すると、上記効果が飽和し、衝撃強度を劣化させることがあるため、B量が、0.005%以下であることが好ましく、0.003%以下であることがより好ましい。合金コストの低減のためには、Bを意図的に鋼中に添加する必要がなく、B量の下限は0%である。
Caは、鋼中で酸化物を生成する脱酸元素であり、必要に応じて鋼中に添加してもよい。一般に、Al脱酸による鋼中の酸化物は、Al2O3であるが、Al2O3が硬質であるため、被削性を低下させる弊害がある。しかし、Caを添加すると、基本の酸化物であるAl2O3とCaとがAl-Ca系複合酸化物を生成し、鋼を若干軟質化することができる。そのため、Al脱酸による被削性の低下を抑制できる。また、製鋼段階においても耐火物へのAl2O3の付着を抑制でき、ノズル閉塞などの弊害を抑制できる。
さらに、Caは、MnSと複合硫化物を生成することにより、MnSを若干硬化させるため、圧延や鍛造時にMnSの延伸を抑制し、冷間鍛造時に硫化物を起点とする割れを抑制できる。ただし、あまり過剰にCaを鋼中に添加すると、CaSを多量に生成し、鋼が硬質になるため、被削性を損なう。このように、Caは、溶損対策としての酸化物制御と鍛造割れ対策としての硫化物制御との両面に有効な元素である。これらのCa添加の効果を得るためには、Ca量は、0.0003%以上であることが好ましく、0.0005%以上であることがより好ましく、0.0008%以上であることがさらに好ましい。また、被削性の観点から、Ca量は、0.005%以下であることが好ましく、0.003%以下であることがより好ましく、0.002%以下であることがさらに好ましい。なお、合金コストの低減のためには、Caを意図的に鋼中に添加する必要がなく、Ca量の下限は0%である。
このCa量[Ca]に対するAl量[Al]の比率も重要である。この比率を示す[Al]/[Ca]が小さすぎると、Alによる脱酸が不足し、Caが酸化物として消費されてしまう。この場合には、硫化物制御に対するCaの効果が不足する。逆に、[Al]/[Ca]が大きすぎると、酸化物制御に対するCaの効果が不足する。したがって、Caを鋼中に添加する場合には、[Al]/[Ca]の範囲は、1以上かつ100以下であることが好ましく、6以上かつ100以下であることがより好ましい。
Mg及びZrは、酸化物および硫化物を生成する元素であり、必要に応じて鋼中に添加してもよい。これらMg及びZrは、MnSの変形能を抑制するため、熱間加工によるMnSの延伸を抑制する。特に、Mg及びZrは、鋼中に微量に含有させても著しい効果を発現する。なお、鋼中のMg及びZrの量を安定させるためには、MgやZrを含む耐火物を考慮してMg量またはZr量を制御することが好ましい。
Mgは、酸化物及び硫化物を生成する元素である。Mgを鋼中に含有させることによって、MgSや、Mnとの複合硫化物(Mn,Mg)Sなどが生成し、MnSの延伸を抑制することができる。微量のMgは、MnSの形態の制御に有効であり、Mgを鋼中に添加して加工性を高める場合には、Mg量が0.0002%以上であることが好ましい。また、Mgの酸化物は、微細に分散し、MnSなどの硫化物の生成核として作用する。Mgの酸化物を利用して、粗大な硫化物の生成を抑制する場合には、Mg量が0.0003%以上であることが好ましい。更に、Mgを鋼中に添加すると、硫化物が若干硬質になり、熱間加工によって延伸されにくくなる。切削性を向上させ、冷間加工性を損なわないように、硫化物の形状を制御するには、Mg量が0.0005%以上であることが好ましい。なお、熱間鍛造は、微細な硫化物を均一に分散させる効果があり、冷間加工性の向上に有効である。なお、合金コストの低減のためには、Mgを意図的に鋼中に添加する必要がなく、Mg量の下限は0%である。
一方、Mgの酸化物は、溶鋼上に浮上し易いため、歩留まりが低く、製造コストの観点から、Mg量は、0.003%以下であることが好ましい。また、Mgを過剰に添加すると、溶鋼中に多量の酸化物が生成し、耐火物への付着やノズルつまりなどの製鋼上のトラブルを引き起こすことがある。したがって、Mg量は、0.001%以下であることがより好ましい。
Zrは、酸化物、硫化物に加え、窒化物を生成する元素である。微量のZrを溶鋼中に添加すると、溶鋼中でTiと複合して、微細な酸化物、硫化物及び窒化物を生成する。したがって、Zrの添加は、介在物及び析出物の制御には極めて有効である。Zrを鋼中に添加して、介在物の形態を制御し、加工性を高める場合には、Zr量が0.0002%以上であることが好ましい。また、Zr及びTiを含む酸化物、硫化物、窒化物は、凝固時にMnSの析出核として作用する。これらのZr及びTiを含む酸化物、硫化物、窒化物の周囲に析出したMnSには、Zr及びTiが溶け込み、変形能が低下する。したがって、Zrを添加して、MnSの変形を抑制し、熱間加工によるMnSの延伸を防止するには、Zr量は、0.0003%以上であることが好ましい。一方、Zrは、高価な元素であるため、製造コストの観点から、Zr量は、0.005%以下であることが好ましく、0.003%以下であることがより好ましい。なお、合金コストの低減のためには、Zrを意図的に鋼中に添加する必要がなく、Zr量の下限は0%である。
MnSは、切削性の向上に有用であるため、その個数密度を確保することが必要である。一方、延伸した粗大なMnSは、冷間加工性を損なうため、サイズ及び形状を制御することが必要である。本発明者らは、Sの含有量、MnSのサイズ及び形状のような硫化物に関する特徴点と、切削性及び冷間加工性のような加工性との関係について検討を行った。その結果、光学顕微鏡で観察されるMnSの平均円相当径が5μmを超えると、このMnSが冷間加工の際に割れが発生する起点になることがわかった。MnSの平均円相当径は、MnSの面積と等しい面積を有する円の直径であり、画像解析によって求めることができる。
(ここで、[S]は、Sの含有量(質量%)を示している。)
また、MnSおよびMnとTiとの複合硫化物について、鍛造における変形時および部品としての使用時、さらに浸炭後の疲労時に負荷がかかる領域における最大寸法の硫化物が破壊起点として作用する。その傾向は、S量の影響を受け、S量が多い方が最大の硫化物の寸法が大きくなる。この最大の硫化物について、Ti系硫化物だけでなく、Ti含有量の少ないMn系硫化物(MnS)も含めて考慮すべきである。
D≦250[S]+10 ・・・(3)
(ここで、[S]は、Sの含有量(質量%)を示している。)
D(μm)が250[S]+10を超えると、鍛造性ならびに疲労特性が低下し、同一量のSを含有する従来鋼と同等の性能しか発揮できなくなることがあるため、D(μm)の上限は、250[S]+10であることが好ましい。
さらに、粗大なTi系析出物が鋼中に存在すると、接触疲労破壊の起点として作用し、疲労特性が劣化することがある。接触疲労強度は、浸炭部品の要求特性であり、転動疲労特性や面疲労強度である。接触疲労強度を高めるには、観察されるTi系析出物の最大円相当径(最大直径)が40μm未満であることが好ましい。
肌焼鋼のベイナイトの組織分率は、30%以下に制限することが好ましい。これは、浸炭焼入れ時の粗大粒の発生を防止するには、粒界に微細な析出物を生成させることが好ましいためである。即ち、熱間加工後の冷却時に生成するベイナイトの組織分率が30%を超えると、Ti系析出物及びNb系析出物を相界面に析出させることが難しくなる。また、ベイナイトの組織分率を30%以下に抑制することは、冷間加工性や被削性を改善するためにも有効である。また、高温浸炭など、粗大粒防止に対して条件が厳しい場合、ベイナイトの組織分率を、20%以下に制限することが好ましく、10%以下に制限することが更に好ましい。更に、冷間鍛造後に高温浸炭を行う場合などでは、ベイナイトの組織分率を5%以下に制限することが好ましい。
肌焼鋼のフェライト粒は、過度に微細であると、浸炭焼入れ時に粗大粒が発生し易くなる。これは、浸炭焼入れ時にオーステナイト粒が過度に粗大化するためである。特に、フェライトの粒度番号がJIS G 0551(2005)で規定される11を超えると、粗大粒が発生し易くなる。一方、肌焼鋼のフェライトの粒度番号が、JIS G 0551で規定される8未満になると、延性が低下し、冷間加工性を損なうことがある。したがって、肌焼鋼のフェライトの粒度番号は、JIS G 0551で規定される8~11の範囲内であることが好ましい。S量が多くなると硫化物が多くなり、この硫化物を核として生成するフェライト粒の数が多くなるため、フェライト粒が微細になる傾向がある。
次に、本発明の一実施形態に係る肌焼鋼の製造方法について説明する。
なお、MnSは、鋼の凝固点近くで液相中に晶出すると考えられているため、MnSの寸法は、冷却速度が速くなるにつれて小さくなり、冷却速度が遅くなるにつれて大きくなる。そのため、本実施形態では、従来の連続鋳造機の冷却条件及び従来の量産型インゴットの製造方法に比べて極めて速い冷却速度で溶鋼を凝固させ、MnSの寸法を小さく抑制する。
図6に、鋳込み試験において、従来の連続鋳造や量産型インゴットの鋳造時の鋳造条件と冷却速度との関係を考慮しながら鋳型寸法や冷却条件などの鋳造条件を調整して冷却速度を制御した場合の鋳片内の平均冷却速度とMnSの平均面積との関係の一例を示す。この図6に示されるように、鋳片の平均冷却速度を大きくすると、MnSの平均面積(すなわち、平均円相当径)を小さくすることができる。
ここで、凝固時の冷却速度を速めるために、鋳型寸法を小さくする方法が、単純な方法として採用されうるが、この方法では、製品品質を維持することが困難である。すなわち、鋳片寸法が小さい場合には、鋳片から製品圧延材(棒鋼)までの圧延による圧下比が小さくなるため、圧延による気泡欠陥の圧着や偏析の均質化などの高品質化効果を得ることが困難になり、製品(肌焼鋼)中に多くの欠陥や偏析を残しやすい。そのため、この場合には、欠陥や偏析による不均質部が、破壊起点として作用したり、焼入れ性にばらつきを生じさせたりするため、肌焼鋼の品質が劣化することがある。
凝固が完了した後も鋳片中の合金元素の濃化部を緩和するために鋳片を極力高温に保持し、P、Mn等の脆化元素を均一に拡散すべきである。そのため、鋳造後600℃以上に鋳片の温度を維持して直接分塊圧延での加熱炉に鋳片を挿入し、さらに分塊圧延では1200℃以上の高温で20分以上この鋳片を保持して、P、Mn及びSの拡散を促進させた。さらに、この加熱及び保持は、Ti、Nb系の析出物を固溶させる効果も有する。
なお、参考のため、図7に本実施形態に係る肌焼鋼の製造方法の一例のフローチャートを示す。
次に、本発明の一実施形態に係る浸炭部品の製造方法(肌焼鋼の適用方法)について説明する。
上記実施形態の肌焼鋼は、冷間鍛造工程で製造される部品、熱間鍛造工程で製造される部品のいずれにも適用可能である。熱間鍛造工程として、例えば、棒鋼-熱間鍛造-必要により焼準等の熱処理-切削-浸炭焼入れ-必要により研削又は研磨という工程が挙げられる。上記実施形態の肌焼鋼を用いて、例えば1150℃以上の加熱温度で熱間鍛造を行い、その後必要に応じて焼準処理を行うことにより、950~1090℃の温度域での高温浸炭を施しても、粗大粒の発生を抑制することができる。例えば、軸受部品、転動部品の場合、高温浸炭を行っても、優れた転動疲労特性が得られる。
ここで、SCr420相当鋼及びSCM420相当鋼は、浸炭及び焼入れに適するように設計されており、SCM420相当鋼は、SCr420相当鋼よりも焼入れ性が高いため、より大型の部品やより高強度の部品に使用できる。しかしながら、このSCM420相当鋼では、Moの添加により浸炭焼入れ前の加工時の硬度が高いため、SCr420相当鋼と比べて冷間鍛造性と被削性の両者が低い。このように、鋼種に応じて冷間鍛造性と被削性とのバランスが変化する場合があり、焼入れ性も考慮してこれらのバランスを確保する。
2 鋳片断面
3 鋳片表面
4 試験片
5 溝入れ試験片
T 鋳片厚さ
W 鋳片幅
Claims (11)
- 化学組成が、質量%で、
C:0.1~0.5%、
Si:0.01~1.5%、
Mn:0.3~1.8%、
S:0.001~0.15%、
Cr:0.4~2.0%、
Ti:0.05~0.2%
を含有し、
Al:0.2%以下、
N:0.0050%以下、
P:0.025%以下、
O:0.0025%以下
に制限し、残部が鉄及び不可避的不純物からなり、円相当径が5μm超である硫化物の1mm2当りの個数dと、Sの含有量の質量百分率[S]とが、d≦500×[S]+1を満足することを特徴とする肌焼鋼。 - 前記化学組成が、更に、質量%で、
Nb:0.04%未満、
Mo:1.5%以下、
Ni:3.5%以下、
V:0.5%以下、
B:0.005%以下、
Ca:0.005%以下、
Mg:0.003%以下、
Zr:0.005%以下
から選択される少なくとも1種を含有することを特徴とする請求項1に記載の肌焼鋼。 - Caの質量百分率[Ca]に対するAlの質量百分率[Al]の比率である[Al]/[Ca]が1以上かつ100以下であることを特徴とする請求項2に記載の肌焼鋼。
- 硫化物の最大円相当径Dμmと、Sの含有量の質量百分率[S]とが、D≦250×[S]+10を満足することを特徴とする請求項1または2に記載の肌焼鋼。
- Mn量が1.0%以下であり、Mnの含有量の質量百分率[Mn]に対するSの含有量の質量百分率[S]の比率である[Mn]/[S]が100以下であることを特徴とする請求項1または2に記載の肌焼鋼。
- ベイナイトの組織分率が、30%以下であることを特徴とする請求項1または2に記載の肌焼鋼。
- Ti系析出物の最大円相当径が、40μm以下であることを特徴とする請求項1または2に記載の肌焼鋼。
- 質量%で、
C:0.1~0.5%、
Si:0.01~1.5%、
Mn:0.3~1.8%、
S:0.001~0.15%、
Cr:0.4~2.0%、
Ti:0.05~0.2%
を含有し、
Al:0.2%以下、
N:0.0050%以下、
P:0.025%以下、
O:0.0025%以下
に制限し、残部が鉄及び不可避的不純物からなる化学組成を有する鋼を、12~100℃/分の平均冷却速度で鋳造し;
1250~1320℃の均熱温度範囲で前記鋼を3~180分保持し;
1150~1320℃の温度範囲に前記鋼を加熱した後、840~1000℃の仕上げ温度範囲で仕上圧延が行われるように前記鋼を熱間圧延し;
800~500℃の温度範囲での平均冷却速度が1℃/秒以下になるように前記鋼を冷却する;
ことを特徴とする肌焼鋼の製造方法。 - 前記化学組成が、更に、質量%で、
Nb:0.04%未満、
Mo:1.5%以下、
Ni:3.5%以下、
V:0.5%以下、
B:0.005%以下、
Ca:0.005%以下、
Mg:0.003%以下、
Zr:0.005%以下
から選択される少なくとも1種を含有することを特徴とする請求項8に記載の肌焼鋼の製造方法。 - Caの質量百分率[Ca]に対するAlの質量百分率[Al]の比率である[Al]/[Ca]が1以上かつ100以下であることを特徴とする請求項9に記載の肌焼鋼。
- Mn量が1.0%以下であり、Mnの含有量の質量百分率[Mn]に対するSの含有量の質量百分率[S]の比率である[Mn]/[S]が100以下であることを特徴とする請求項8または9に記載の肌焼鋼。
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WO2022137697A1 (ja) * | 2020-12-22 | 2022-06-30 | 愛知製鋼株式会社 | 温間鍛造用肌焼鋼及びこれを用いて製造した鍛造粗形材 |
JP2022098655A (ja) * | 2020-12-22 | 2022-07-04 | 愛知製鋼株式会社 | 温間鍛造用肌焼鋼及びこれを用いて製造した鍛造粗形材 |
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US20130048156A1 (en) | 2013-02-28 |
CN102884212A (zh) | 2013-01-16 |
JP5114689B2 (ja) | 2013-01-09 |
KR20130010908A (ko) | 2013-01-29 |
JPWO2012046779A1 (ja) | 2014-02-24 |
US8673094B2 (en) | 2014-03-18 |
KR101355321B1 (ko) | 2014-01-23 |
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