WO2005021816A1 - 軟窒化用非調質鋼 - Google Patents
軟窒化用非調質鋼 Download PDFInfo
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- WO2005021816A1 WO2005021816A1 PCT/JP2004/012372 JP2004012372W WO2005021816A1 WO 2005021816 A1 WO2005021816 A1 WO 2005021816A1 JP 2004012372 W JP2004012372 W JP 2004012372W WO 2005021816 A1 WO2005021816 A1 WO 2005021816A1
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- bainite
- steel
- ferrite
- pearlite
- nitrocarburizing
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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
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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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
- 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
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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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
Definitions
- the present invention relates to a non-heat treated steel for nitrocarburizing. More specifically, the present invention relates to a non-heat treated steel for nitrocarburizing, which is used as a material for machine parts such as crankshafts and connecting rods of automobiles, industrial machines and construction machines.
- the nitrocarburizing treatment causes distortion. Since the distortion impairs the dimensional accuracy of the component, bending correction is often performed after nitrocarburizing. Therefore, the components after soft nitriding are required to have high fatigue strength and excellent bending straightness.
- excellent bending correctability means that the surface of a component does not crack until a large amount of bending displacement is reached, and that the fatigue strength after bending correction is reduced. This is smaller than before bending.
- normalization will be described as a representative example of the tempering process. Even if the normalizing process is omitted, a method for obtaining a non-heat treated steel for nitrocarburizing, which can be a component with high fatigue strength and excellent “bend straightening” after nitriding, has been used until now. Also have some suggestions. They are roughly divided into the following two.
- Patent Document 1 A method of avoiding coarsening of the structure by hot forging as much as possible while maintaining the microstructure of the steel as ferritic and pearlite as in the tempered steel (for example, Patent Document 1, Patent Document 2, Patent Document 3, Patent Reference 4).
- Patent Document 5 Patent Document 6, Patent Document 7, Patent Document 8, and Patent Document 9.
- Patent Document 1 JP-A-9-291339
- Patent Document 2 Japanese Patent Application Laid-Open No. 9-324258
- Patent Document 3 JP-A-9-324241
- Patent document 4 JP-A-10-46287
- Patent Document 5 JP-A-5-65592
- Patent Document 6 JP-A-2000-309846
- Patent Document 7 JP-A-7-157842
- Patent Document 8 JP-A-8-176733
- Patent Document 9 JP-A-2000-160287
- Patent Document 1 states that "the content of alloying elements is% by mass, C: 0.15 to 0.40%, Si ⁇ 0.50%, Mn: 0.20 to 1.50%. , Cr: 0.05 to 0.50%, balance Fe and unavoidable impurities, the structure after hot working is substantially ferrite 'pearlite structure, A nitrided steel characterized by having an area ratio of 30% or more, a ferrite grain size number of 5 or more, and an average pearlite size of 50 m or less is disclosed. It is described that this steel is excellent in fatigue strength and bending straightenability after nitriding even if normalizing is omitted.
- Patent Document 2 discloses a "nitrided part obtained by nitriding steel, wherein the steel alloy component is C: 0.15-0.40%, Si: 0.50% or less, and Mn: 0.20% by mass%. -Containing 1.50%, Cr: 0.05-0.50%, the balance being Fe and unavoidable impurities, and the steel has a mixed structure of ferrite and pearlite as it is hot worked, and has a crystal structure of the ferrite.
- the average size of the grains is 50 m or less
- the average size of the pearlite crystal grains is 50 m or less
- the average hardening depth by the nitriding treatment is 0.3 mm or more
- the hardening depth varies. Is characterized in that the thickness is within 0.1 mm.
- this part is excellent in fatigue strength and bending straightenability even if the part is subjected to nitriding treatment without normalizing treatment after hot forging.
- Patent Document 3 states that "by weight, C: 0.20 to 0.60%, Si: 0.05 to 1.0%, Mn: 0.3 to 1.0%, P: 0.05% or less, S: 0.005 to 0.10%, Cr: 0.3% or less, A1: 0.08% or less, Ti: 0.03% or less, N: 0.008 to 0.020%, Ca: 0.005% or less, Pb: 0.30% or less, Cu: 0.30% or less, Ni: 0.30% or less , Mo: 0.30% or less, V: 0.20% or less, Nb: 0.05% or less, and 221C (%) + 99.5Mn (%) + 52.5Cr (%)-304Ti (%) + 577N (%) + 25 ⁇ 150, and the balance is the chemical composition of Fe and unavoidable impurities, and the structure is composed of ferrite and pearlite, and the ferrite fraction is 10% or more. Te!
- the fatigue strength is expressed as a regression equation of the contained elements, the factor of which is equal to or larger than a specific size, the structure is composed of ferrite and pearlite, and the ferrite fraction is 10%. If it is above, it is stated that even if the normalizing process is omitted, a nitrided component excellent in fatigue strength and bending straightenability can be obtained.
- Patent Document 4 discloses that "by weight, C: 0.30-0.43%, Si: 0.05-0.40%, Mn: 0.
- Patent Document 4 even when the normalizing treatment is omitted and the nitriding treatment is performed, a product having excellent fatigue strength and bending straightenability can be obtained by making the hardness gradient in the nitrided layer gentle. It is stated that.
- Patent Document 5 “C: 0.1—0.35%, Si: 0.05—0.35%, Mn: 0.6—1.50%, P: 0.01% Below, S: 0.015% or less, Cr: l-1.0%, Mo: 0.5-1.0%, V: 0.03-0.13%, B: 0.0005-0 0030%, Ti: 0.01-0.04%, A1: 0.01-0.0
- Patent Document 6 discloses that "in mass%, C: less than 0.1-0.3%, Si: 0.01-1.0%, Mn: l.
- a non-heat-treating steel for nitrocarburizing which is characterized by containing 025%, with the balance being Fe and inevitable impurity power.
- Patent Document 6 a steel having a bainite structure obtained by air-cooling with hot working temperature is said to have excellent toughness and excellent bending straightening properties after nitrocarburizing treatment. ing.
- the C concentration is set to less than 0.3%.
- the Mn concentration is specified as 1.5% or more.
- 0.01 to 0.05% of Cr is added to increase the hardness of the nitride layer by strengthening the precipitation with Cr nitride. That is, in Patent Document 6, the improvement in the bending straightening property by the bainite structure is because bainite has higher toughness at the same hardness as compared to the fllite 'pearlite structure.
- the C concentration is set to less than 0.3% so that the hardness of bainite does not become too hard.
- the C concentration is less than 0.3%, insufficient wear resistance is a concern. Wear resistance is also a very important factor for mechanical parts such as crankshafts and connecting rods.
- Patent Document 7 states that “by weight, C: 0.05 to 0.30%, Si: 1.20% or less, Mn: 0.60 to 1.30%, Cr: 0.70 to 1.50%, A1: 0.10% or less, : 0.006-0.020%, V: 0.05-0.20%, Mo: 0—1.00%, B: 0—0.0050%, S: 0—0.060%, Pb: 0—0.20%, Ca: 0—0.010 %, Power, 0.60 ⁇ C + 0.lSi + O.2Mn + 0.25Cr + 1.65V ⁇ 1.35 or 0.60 ⁇ C + 0.lSi + O.2Mn + 0.25Cr + l.65V + 0.55 Mo + 8B ⁇ l.35, the balance being Fe and unavoidable impurities, having a steel composition consisting of Hv200-300, with core hardness of Hv200-300, without heat treatment after cooling after hot rolling or hot forging.
- a steel for nitrocarburizing characterized by having a mixed structure
- Patent Document 7 As in Patent Document 5, the idea of improving the fatigue strength by using precipitation strengthening by Cr and V is adopted. However, as in Patent Document 6 described above, since the C concentration is specified to be less than 0.3%, concerns about wear resistance cannot be eliminated.
- Patent Document 8 discloses that "by weight, C: 0.15 to 0.40%, Si: less than 20%, Mn: 0.60 to 1.80%, Cr: 0.20 to 2.00%, A1: 0.02 to 0.10%. %, N: 0.006-0.020%, V: 0.05-0.20%, the balance consisting of Fe and unavoidable impurities, and 0.60 ⁇ C + 0.lSi + O.2Mn + 0.25Cr + l.
- a steel for nitrocarburizing characterized by having characteristics "is disclosed.
- Patent Document 8 Since the steel of Patent Document 8 has a C concentration of 0.15 to 0.40%, it is expected that the wear resistance is improved. However, this steel is also similar to the invention of Patent Document 7 described above. The idea of improving the fatigue strength by using precipitation strengthening by Cr and V is adopted.
- Patent Document 9 states that “nitriding steel has a ferrite-based structure or, if it is difficult, a single-phase structure of martensite or bainite is more preferable than a ferrite + pearlite structure”.
- the idea is to use precipitation strengthening by Cu instead of a force that avoids precipitation strengthening by Cr and V. It also states that the Mn concentration must be at least 1.0% in order to obtain a bainite single-phase structure, and is aiming for a non-heat-treated bainite single-phase steel.
- An object of the present invention is to provide a steel for nitrocarburizing, in which even when the nitrocarburizing treatment is performed in a state where the tempering treatment is omitted, the same fatigue strength as in the case where the nitrocarburized steel is subjected to nitrocarburizing.
- Another object of the present invention is to provide a non-heat-treated soft-nitriding steel that can be a component having bending straightness.
- the gist of the present invention resides in the following non-heat treated steel for soft nitriding (1) and (2).
- a non-heat treated steel for nitrocarburizing characterized in that it has a bainite fraction in the mixed structure of 5 to 90%.
- the present inventors produced various non-heat treated steels for nitrocarburizing in order to solve the above-mentioned problems, and examined fatigue strength and rectification after nitrocarburizing. Then, the correlation between them and the microstructure of the steel before nitrocarburizing was investigated. In addition, a detailed study was conducted on the microstructure developed by the nitrocarburizing treatment, and the effect of the microstructure of the steel after the nitrocarburizing treatment on the fatigue strength and bending straightness was investigated. As a result, the following findings were obtained.
- the parts after the nitrocarburizing treatment can have excellent fatigue strength and bending straightening properties.
- FIG. 1 shows a typical structure photograph of bainite + ferrite + pearlite.
- “bainite” refers to “a mixed structure of ferrite and cementite having a structure different from the ordered (lamellar) pearlite and different from martensite and retained austenite”.
- the bainite structure is characterized by the dispersion of bamboo leaf-like ferrite (referred to as peytic ferrite).
- peytic ferrite Such bainite structure has a relatively random dispersion of cementite. The hardness is lower than that of coarse pearlite.
- the ferrite Z cementite interface is regularly arranged like pearlite yarn, the structure has relatively high resistance to crack propagation. That is, the bainite structure is coarser than the aggregate of fine pearlite colonies, but has a better balance of strength and toughness than the coarse pearlite core.
- N is an austenite-stabilizing element and combines with Ti to form TiN.
- This TiN precipitates in a certain amount even at 1100 ° C or more and becomes pinjung particles that prevent austenite grains from becoming coarse. Therefore, by increasing the N content, it is necessary to suppress the coarsening of austenite grains and to form a bainite + ferrite structure in which bainite is appropriately mixed, and ⁇ to have a mixed structure of ⁇ bainite + ferrite + pearlite ''. Can be.
- the fatigue strength is the same as when the soft-nitrided steel with fine ferrite + pearlite structure realized by tempering treatment such as normalizing treatment. Equivalent to the fatigue strength of
- the fatigue strength can be increased with the Fe nitride by generating a nitride of Fe during the soft nitriding treatment.
- the Fe nitride just under the compound layer on the surface of the nitrocarburized layer that is, the Fe nitride in the diffusion layer, is generated by a large amount of N entering into the atmosphere during the nitrocarburizing process. For example, precipitation was easy even in a diffusion layer having a depth of about 300 ⁇ m from the surface.
- the “diffusion layer” here is JIS
- G0562 it is a layer in which diffusion of nitrogen, carbon, etc. is observed, excluding the compound layer in the surface layer of the nitrocarburized component.
- the steel of the present invention is soft-nitrided and the hardness profile in the depth direction directed from the surface to the inside is compared with the conventional steel containing Cr or Z and / or V, the hardness near the outermost surface is higher. It was found that the core hardness, which is smaller than that of conventional steel, is almost the same, but rather slightly higher. This is considered to be because the precipitation strengthening force due to Fe nitride is more mild than the precipitation strengthening force due to Cr or Z and V, and therefore, the decrease in ductility of ferrite is suppressed as compared with the conventional steel. As a result, the bending straightness does not decrease.
- pinning particles suppress coarsening of austenite grains during hot working, impart hardenability such that moderate bainite is generated, and increase the size of fly grains near the surface.
- Precipitation strengthening to the extent that excessive strengthening is not performed is an important point for achieving both high fatigue strength and bending straightenability after soft nitriding even if tempering treatment such as normalizing treatment is omitted. is there.
- the present invention has been completed based on the above findings.
- C is a combination of bainite + ferrite or bainite + ferrite + pearlite It is an essential element to obtain a weave.
- a content of 0.30% or more is necessary for stabilizing austenite and ensuring the wear resistance of the material.
- the content exceeds 0.45%, the hardenability is excessively increased and harmful martensite is easily formed. Therefore, the proper range of C content is 0.30-0.45%.
- Si is added in the steelmaking process as a deoxidizing agent, but it is also effective for solid solution strengthening of ferrite, so a content of 0.1% or more is necessary. On the other hand, if the Si content exceeds 0.5%, the hot deformation resistance of the steel is increased, and the toughness and machinability are deteriorated. Therefore, the appropriate range for the Si content is 0.1-0.5%.
- Mn is added in the steelmaking process as a deoxidizing agent like Si. It is also an essential element for stabilizing austenite to obtain a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite”. In addition, Mn combines with S in steel to form MnS, which is also effective in improving machinability.
- the bainite fraction must be 5% or more. Then, in order to ensure the hardenability to generate bainite of this fraction, the content of Mn of 0.6% or more is necessary. On the other hand, if the content of Mn exceeds 1.0%, the hardenability becomes too high and the generation of harmful martensite tends to occur. Therefore, the appropriate range of the Mn content is 0.6-1.0%.
- Ti is an essential element for forming pin-jung particles for suppressing coarsening of grains during hot working.
- Pin Jung particles include Ti nitrides, carbides, and carbonitrides.
- a content of 0.005% or more is required.
- the Ti content must be kept below 0.1% in order to avoid the exhaustion of N in steel, which contributes to the increase in base metal strength by forming Fe nitride.
- the appropriate range of Ti content is 0.005–0.1%. More desirable is 0.01-0.05%.
- N 0. 015—0. 030 N forms pinning particles for suppressing crystal grain coarsening to stabilize austenite to obtain a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite”. Therefore, it is added to increase the strength of the base metal by contributing to the solid solution strengthening by forming Fe nitride and contributing to the precipitation strengthening or as solid solution nitrogen. Here, considering the amount consumed as pinjung particles, it is necessary to contain 0.015% or more. On the other hand, if N exceeds 0.030%, a bubble defect may be generated in the ingot and the material may be damaged. Therefore, the appropriate range of the N content is 0.015-0.030%. More desirable is 0.015-0.025%.
- One of the non-heat treated steels for nitrocarburizing of the present invention is Mamaoka, in which, in addition to the above-mentioned elements, the balance consists of Fe and impurities.
- Another non-heat treated steel for nitrocarburizing according to the present invention further includes, in addition to the above-mentioned elements, one or more elements selected from the first element group, or Z and the second element group.
- the steel contains one or two selected elements, and the balance is Fe and impurities.
- the elements belonging to the first group ie, Nb, Mo, Cu, Ni and B have a common effect of increasing the strength of the steel of the present invention.
- the effects and the reasons for limiting the contents are as follows.
- Nb is an element that can be used to form pinjung particles for suppressing crystal grain coarsening during hot working. In addition, it is effective in increasing the strength of the base material by forming into fine carbonitrides during the cooling of the steel after the hot working and during the powerful cooling. To obtain these effects, a content of 0.003% or more is required. On the other hand, even if the content exceeds 0.1%, the effect is saturated, and coarse undissolved carbonitrides are formed during steel making, which may degrade the quality of the slab. Therefore, when Nb is added, its content is preferably set to 0.003 to 0.1%. 0.005-0. 1% is more desirable, and 0.01-1. 05% is most desirable.
- Mo is an element that enhances the hardenability of steel and contributes to high strength, and is also effective in improving toughness.
- Mo is added, the mixed structure of “bainite + ferrite” or “ G + ferrite + pearlite ”. To obtain these effects, a content of 0.01% or more is required.
- the content of Mo exceeds 1.0%, the hardenability is increased, so that the formation of martensite is promoted, and the bending straightening property and the toughness after the nitrocarburizing treatment are deteriorated. Therefore, when Mo is added, its content is preferably set to 0.01 to 1.0%. A more desirable content is 0.05-0.6%.
- Cu When Cu is added, an increase in the bainite fraction due to solid solution strengthening and austenite stabilization is expected. Therefore, Cu contains 0.01% or more.
- Cu and Ni do not have the effect of precipitation strengthening by carbonitride formation, but Cu can age precipitate in ferrite and contribute to precipitation strengthening.
- the Cu content must be reduced in order to cause sufficient Cu precipitation. 1. Must be 0% or more.
- the melting point of Cu is as low as 1085 ° C, the time during which it remains as a liquid phase during the solidification process in the steelmaking process is prolonged. In order to eliminate this adverse effect, the upper limit of the Cu content is set to 1.0% in the steel of the present invention.
- Ni like Cu, is an austenite-stabilizing element and has an effect on solid solution strengthening and securing a desired bainite fraction, so that it is preferably contained at 0.01% or more. On the other hand, if the content exceeds 1.0%, the effect is saturated and only the material cost increases, so the upper limit was set to 1.0%. In addition, when used in combination with Cu, it is desirable to contain Ni at least 1/2 of the Cu content in order to ensure the effect of preventing the hot cracking.
- B enhances the hardenability of steel and promotes the formation of a mixed structure of “bainite + ferrite” or a mixed structure of “bainite + ferrite + pearlite”. The effect is clearly exhibited at a content of 0.001% or more. On the other hand, if the B content exceeds 0.005%, the toughness of steel is impaired. Be done. Therefore, when B is added, its content is preferably 0.001 to 0.005%.
- the elements of the second group are S and Ca, which improve the machinability of the steel of the present invention.
- the reasons for limiting each content are as follows.
- S and Ca are both elements that improve the machinability of steel materials. If added, the machinability will be further improved, so if necessary, one or two types of force are added. However, if added excessively, it causes segregation defects in the steel slab and deteriorates the hot workability, so the S content range is 0.01-0.1% and the Ca content is The appropriate range of amounts is 0.0001-0.005%. A desirable lower limit of Ca is 0.001%.
- P promotes grain boundary brittle cracking by biasing toward the grain boundary, it is preferable to set P to 0.05% or less.
- A1 is usually added as a deoxidizing agent during melting.
- A1 remains in the steel as alumina particles and combines with N to form A1N.
- Alumina is an oxide inclusion having a high hardness, and shortens the life of a tool used for cutting.
- A1N precipitates in the vicinity of the surface during nitrocarburizing and promotes the growth of the surface compound layer, thereby significantly increasing the hardness of the surface layer and deteriorating the bending straightenability.
- A1N forms a solid solution at the hot working temperature, it cannot be expected to function as pin-Jung particles and is hardly useful for refining crystal grains. Therefore, the lower the content of A1, the better.
- minimizing the lower limit of the A1 content creates a restriction in the deoxidation step and leads to an increase in cost.Therefore, the bending correctability of the steel of the present invention is not impaired. Is preferred,.
- Cr and V are not added to the steel of the present invention. These are impurities, and the lower the content, the better. This is because, as already mentioned, Cr and V precipitate nitrides, significantly increasing the hardness of the near-surface layer of the steel and impairing the straightness. The effect of the present invention is not impaired, and the cost of refining and the method of producing pieces by a method other than the blast furnace converter method Taking into account the purity of the raw material in the above, up to 0.15% for Cr and up to 0.02% for V are acceptable as impurities. It is more preferable that Cr is set to 0.1% or less.
- the structure of the steel of the present invention is a mixed structure of bainite and ferrite or a mixed structure of bainite, ferrite and pearlite. And the bainite fraction in these mixed structures is 5-90%
- bainite transformation makes it possible to avoid the formation of martensite and to obtain a finer structure than a coarse pearlite core.
- This texture is characterized by the dispersion of bamboo leaf-like pay-tic 'ferrite, as shown in Figure 1.
- Pay-tic 'ferrite is dispersed inside the former austenite grains, and the former austenite grain boundary force is smaller than that of the developed polygonal ferrite. That is, the bainite is “a structure in which the shape is a bamboo leaf-like shape but relatively fine frit is dispersed in the pearlite mouth”.
- the fabric in which the pay-tic 'ferrite is dispersed that is, the above-mentioned perlite core is not a pearlite having an orderly lamellar structure.
- Fig. 2 is an SEM image of old austenite grains in which pay-tic 'ferrite is dispersed. As is clear from this figure, the arrangement of cementite is disturbed in various places other than the orderly lamellar yarn. Such a structure has a lower strength than that of the former austenite grains wholly transformed with pearlite, but is superior to a coarse pearlite core in terms of crack propagation resistance. The reason is as follows.
- the crack propagates directly into the inside without avoiding this region. Going inside, the pay-tic 'ferrite dispersed inside plays a role in preventing the growth of cracks. Also, since the size of the pay-tic 'ferrite is smaller than that of the ferrite or the pearlite core after normalizing, it becomes more frequent resistance to the growing crack and helps to improve the toughness. .
- the crack growth resistance can be kept high even if the crystal grain structure is slightly coarsened.
- the entire structure may be bainite, but in a structure with a bainite fraction of more than 90%, the mixing of martensite is unavoidable in reality. Since martensite deteriorates the bending straightening property and also deteriorates the machinability, it is not preferable to mix them. Therefore, in the present invention, the bainite fraction in the mixed structure is set to 5-90%. A more desirable bainite fraction is 10-80%.
- the structure other than bainite of the steel of the present invention is substantially ferrite or ferrite and pearlite.
- the material for hot forging may be any of a billet obtained by slab-rolling a lump, a billet obtained by slab-rolling a continuous material, or a steel bar obtained by hot-rolling these materials.
- the heating temperature of these hot forging materials is 1100-1250 ° C. Cooling after hot forging is allowed to cool in the air or forced air cooling using a fan. In addition, for example, the temperature may be rapidly cooled to around the eutectoid transformation temperature and slowly cooled in the range of 700 to 500 ° C, or immediately after hot forging, cooled to about 500 to 300 ° C.
- the bainite transformation may be promoted by maintaining the temperature.
- gas nitrocarburizing gas nitrocarburizing, salt bath nitrocarburizing (tufftriding), ion nitriding, or the like can be used.
- a compound layer (nitride layer) with a thickness of about 20 m and a diffusion layer immediately below it can be formed uniformly on the surface of the product.
- treatment may be performed at 580 ° C. for 1 to 12 hours in an atmosphere in which RX gas and ammonia gas are mixed in a ratio of 1: 1.
- the test piece was a columnar body having a diameter of 44mm, and a tapered neck portion (neck portion diameter 20mm). By fixing the head side of this test piece and applying a load to the opposite end, bending correction of a predetermined strain amount can be given to the neck part. Further, a round bar was sliced into a cylindrical sample, and a machinability test using a drill was performed.
- the machinability was determined by drilling a blind hole (hole with a bottom) with a depth of 55mm (including a depth of 15mm previously drilled as a pilot hole) in the longitudinal direction of the above sample, and the maximum flank wear amount. The number of machined holes when the diameter reached 0.2 mm was evaluated as the drill life.
- the tool used for the life evaluation was a gun drill with a diameter of 6.2 mm, the total length was 250 mm, and the material of the cutting edge was JIS.
- the drilling was performed under the conditions of a rotation speed of 7200 rpm and a feed of 0.02 mm / rev, and lubrication was performed by applying a 20-fold diluted water-soluble emulsion at 4 MPa hydraulic pressure by internal lubrication.
- the pilot hole had a diameter of 6.3 mm and a depth of 15 mm.
- Use a soft-nitrided fatigue test specimen at room temperature A plane bending fatigue test was performed in the air.
- Some of the fatigue test pieces were subjected to a bending test before the test and a force test was performed. Straightenability is attached a strain gauge on the neck portion of the specimen, the strain gauge readings went under load until Rutokoro such a 15000 X 10- 6 (corresponding to the bending straightening strain 1.5%).
- a sample for microstructure observation was obtained by collecting a round bar force as it was hot forged, and image analysis of an optical micrograph was performed to determine a bainite fraction (area ratio).
- area ratio area ratio
- the area where bamboo leaf-like vinitic ferrite is present is surrounded by a continuous closed curve, and the area ratio power of the area with respect to the entire visual field is calculated.
- Table 2 shows the fatigue strength of each test steel when subjected to a fatigue test without bending correction.
- the bending straightness shown in Table 2 is the amount of decrease in fatigue strength ( ⁇ ⁇ ) when bending was given.
- the machinability is shown as a relative value when the number of holes that can be machined for No. 1 steel is 100.
- the fatigue strength without bending straightening was 550MPa, which is the fatigue strength of the current normalizing type steel indicated by No. 27. It is equal to or higher than that, and even when 1.5% bending straightening is applied, the fatigue strength is reduced only by 100-120MPa, which is equivalent to that of the current standardized steel.
- the fatigue strengths without imparting bending straightness are equal to or higher than those of the steel types of the present invention.
- the bending straightness is clearly inferior to the examples of the present invention due to fracture during straightening and a decrease in fatigue strength of 150 MPa or more due to bending.
- the steel grade component No. 21 is originally used after normalization, so if normalization is omitted, coarse “ferrite + pearlite” yarns will be formed and straightening will be performed. While brittle fractured.
- FIG. 1 is a typical structure photograph of a mixed structure of “bainite + ferrite + pearlite” of the steel of the present invention.
- FIG. 2 is an SEM photograph of old austenite grains in which pay-tick ferrite is dispersed.
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Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
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JP2005513473A JP4257539B2 (ja) | 2003-09-01 | 2004-08-27 | 軟窒化用非調質鋼 |
US11/312,498 US7416616B2 (en) | 2003-09-01 | 2005-12-21 | Non-heat treated steel for soft-nitriding |
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JP2003-308877 | 2003-09-01 | ||
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CN105463318A (zh) * | 2015-11-27 | 2016-04-06 | 北大方正集团有限公司 | 非调质钢、其生产方法及利用其制造的涨断连杆 |
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CN1846010A (zh) | 2006-10-11 |
US7416616B2 (en) | 2008-08-26 |
JPWO2005021816A1 (ja) | 2007-11-08 |
JP4257539B2 (ja) | 2009-04-22 |
US20060096671A1 (en) | 2006-05-11 |
CN100374604C (zh) | 2008-03-12 |
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