WO2013077298A1 - 疲労強度に優れる窒化用熱延鋼板、窒化用冷延鋼板及びそれらの製造方法、並びにそれらを用いた疲労強度に優れた自動車部品 - Google Patents
疲労強度に優れる窒化用熱延鋼板、窒化用冷延鋼板及びそれらの製造方法、並びにそれらを用いた疲労強度に優れた自動車部品 Download PDFInfo
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- WO2013077298A1 WO2013077298A1 PCT/JP2012/079991 JP2012079991W WO2013077298A1 WO 2013077298 A1 WO2013077298 A1 WO 2013077298A1 JP 2012079991 W JP2012079991 W JP 2012079991W WO 2013077298 A1 WO2013077298 A1 WO 2013077298A1
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- less
- steel sheet
- nitriding
- rolled steel
- rolling
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Images
Classifications
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- B21B1/22—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling plates, strips, bands or sheets of indefinite length
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- 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
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- 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
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- C23C4/04—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge characterised by the coating material
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- 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
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- C23C8/02—Pretreatment of the material to be coated
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- 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
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- C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
- C23C8/24—Nitriding
- C23C8/26—Nitriding of ferrous surfaces
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- 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
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- C23C8/30—Carbo-nitriding
- C23C8/32—Carbo-nitriding of ferrous surfaces
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- C23C8/36—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 using ionised gases, e.g. ionitriding
- C23C8/38—Treatment of ferrous surfaces
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- C—CHEMISTRY; METALLURGY
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- 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
- C23C8/40—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 liquids, e.g. salt baths, liquid suspensions
- C23C8/52—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 liquids, e.g. salt baths, liquid suspensions more than one element being applied in one step
- C23C8/54—Carbo-nitriding
Definitions
- the present invention is a nitriding steel sheet having excellent fatigue strength that secures workability and is capable of obtaining a hard nitrided layer by nitriding treatment such as gas nitriding, gas soft nitriding, salt bath soft nitriding, etc.
- the present invention relates to an automotive part having a hard nitrided layer and excellent fatigue characteristics.
- the surface hardening treatment is performed for the purpose of improving wear resistance and fatigue strength, and typical surface hardening treatment methods include carburizing, nitriding, induction hardening, and the like.
- typical surface hardening treatment methods include carburizing, nitriding, induction hardening, and the like.
- nitriding such as gas nitriding, gas soft nitriding, and salt bath soft nitriding is performed below the transformation point to austenite, so that several hours of processing time is required, but heat treatment strain can be reduced. It has the advantage of being able to.
- nitriding is a surface hardening treatment that is suitable for parts that have been subjected to precision machining, such as crankshafts and transmission gears, or members that require product shape accuracy after hardening of the damper disk and damper plate.
- nitriding treatments there are gas soft nitriding, salt bath soft nitriding, etc., but gas nitriding performed in an ammonia atmosphere can provide high surface hardness, but nitrogen diffusion is slow and generally treatment for 20 hours or more. I need time.
- a soft nitriding treatment such as gas soft nitriding or salt bath soft nitriding, which is performed in a bath or atmosphere containing carbon together with nitrogen, can increase the diffusion rate of nitrogen.
- gas soft nitriding or salt bath soft nitriding which is performed in a bath or atmosphere containing carbon together with nitrogen, can increase the diffusion rate of nitrogen.
- a nitriding treatment it is possible to obtain a component having an increased surface hardened layer depth in a few hours.
- Such a nitriding treatment forms a surface hardened layer having a high surface hardening depth, suppresses the occurrence of fatigue cracks on the surface of the component, and improves fatigue durability.
- Patent Document 1 steel containing a nitride-forming alloy has been proposed, and is disclosed in Patent Document 1, for example.
- gas soft-nitrided steel sheets with improved workability during press forming before nitriding and surface hardness characteristics of the parts after nitriding have been proposed.
- Patent Documents 2 and 3 elements such as Al, Cr, and V, which are nitride forming elements, are effective for improving the surface hardness by gas soft nitriding treatment, and are contained as alloy elements of the steel sheet for gas soft nitriding. .
- Fatigue characteristics after gas soft nitriding treatment need to increase surface hardness and depth by Al, Cr, V nitride.
- V enhances the depth of the hardened layer by promoting the diffusion of N, and Cr and Al are effective for increasing the surface hardness.
- Al and V cause fine nitrides to precipitate linearly at the austenite grain boundaries. , Significantly reduce the burring formability and stretch flangeability.
- V increases the strength by precipitation of VC and lowers the workability. In order to avoid such VC precipitation strengthening, it is effective to set the cooling stop temperature after hot rolling to 500 ° C.
- the present invention relates to a hot rolled steel sheet for nitriding, a cold rolled steel sheet for nitriding excellent in fatigue strength capable of deepening the surface hardened layer for improving workability before gas soft nitriding and improving fatigue strength after processing. These manufacturing methods make it possible to provide automobile parts with excellent fatigue strength with increased hardness of the nitrided surface layer.
- the present inventors have studied a steel sheet alloy composition and manufacturing method capable of obtaining a surface hardening depth without impairing the formability of automobile parts by nitriding treatment such as gas soft nitriding and salt bath soft nitriding, and also the hardness of parts. did.
- a steel containing an appropriate amount of Cr and V contains an appropriate amount of B, further defines a skin pass reduction ratio range in the manufacturing process, and a line load F obtained by dividing the rolling mill load under the skin pass pressure by the steel plate width.
- F / T which is a ratio of (kg / mm) and the load applied in the longitudinal direction of the steel sheet and the load T (kg / mm 2 ) per unit area on the rolling exit side, to a predetermined range
- the present invention (1) By mass%, C is 0.0002% to 0.07%, Si is 0.0010% to 0.50%, Mn is 0.10% to 1.33%, P Is 0.003% or more, 0.02% or less, S is 0.001% or more, 0.02% or less, Cr is more than 0.80%, 1.20% or less, Al is 0.10% or more, 0 .50% or less, V is 0.05% or more, 0.10% or less, Ti is 0.005% or more, 0.10% or less, B is 0.0001% or more, 0.0015% or less, The balance is Fe and inevitable impurities, and the dislocation density within 50 ⁇ m in the thickness direction from the surface of the steel sheet is 2.0 times or more and 10.0 times or less compared to the dislocation density at the 1/4 position in the thickness direction.
- the composition further comprises one or both of Mo 0.001% or more and 0.20% or less and Nb 0.001% or more and 0.050% or less in terms of mass%.
- Mo 0.001% or more and 0.20% or less and Nb 0.001% or more and 0.050% or less in terms of mass%.
- C is 0.0002% to 0.07%
- Si is 0.0010% to 0.50%
- Mn is 0.10% to 1.33%
- S is 0.001% or more, 0.02% or less
- Cr is more than 0.80%, 1.20% or less
- Al is 0.10% or more, 0 .50% or less
- V is 0.05% or more, 0.10% or less
- Ti is 0.005% or more, 0.10% or less
- B is 0.0001% or more, 0.0015% or less
- C is 0.0002% or more and 0.07% or less
- Si is 0.0010% or more and 0.50% or less
- Mn is 0.10% or more and 1.33% or less
- S is 0.001% or more, 0.02% or less
- Cr is more than 0.80%, 1.20% or less
- Al is 0.10% or more, 0 .50% or less
- V is 0.05% or more, 0.10% or less
- Ti is 0.005% or more, 0.10% or less
- B is 0.0001% or more, 0.0015% or less
- the rolling mill load is steel plate the ratio of plate width divided by linear load F (kg / mm) and per unit area is loaded in the longitudinal direction of the steel sheet load T (kg / mm 2), F / T Method for producing a good cold-rolled
- C is 0.0002% or more and 0.07% or less
- Si is 0.0010% or more and 0.50% or less
- Mn is 0.10% or more and 1.33% or less
- S is 0.001% or more, 0.02% or less
- Cr is more than 0.80%, 1.20% or less
- Al is 0.10% or more, 0 .50% or less
- V is 0.05% or more, 0.10% or less
- Ti is 0.005% or more, 0.10% or less
- B is 0.0001% or more, 0.0015% or less
- the balance is Fe and inevitable impurities, and the dislocation density within 50 ⁇ m in the thickness direction from the surface of the steel sheet is 2.0 times or more and 10.0 times or less compared to the dislocation density at the 1/4 position in the thickness direction.
- the present invention it is possible to provide a steel sheet that has excellent press formability before nitriding treatment and from which a deep surface hardened layer can be obtained by nitriding treatment, and further an automotive part having a deep surface hardened layer. Become. As a result, the industrial contribution is extremely remarkable, such as a nitriding part with low heat treatment strain and high fatigue strength.
- F / T which is the ratio of the line load F (kg / mm) obtained by dividing the skin pass rolling mill load by the steel plate width and the load T (kg / mm 2 ) per unit area loaded in the longitudinal direction of the steel plate, and the steel plate
- F / T which is the ratio of the line load F (kg / mm) obtained by dividing the skin pass rolling mill load by the steel plate width and the load T (kg / mm 2 ) per unit area loaded in the longitudinal direction of the steel plate, and the steel plate
- F / T which is the ratio of the line load F (kg / mm) obtained by dividing the skin pass rolling mill load by the steel plate width and the load T (kg / mm 2 ) per unit area loaded in the longitudinal direction of the steel plate, and the steel plate
- F / T which is the ratio of the line load F (kg / mm) obtained by dividing the skin pass rolling mill load by the steel plate width and the load T (kg / mm 2 ) per unit area loaded in
- the hot-rolled steel sheet for nitriding and the cold-rolled steel sheet for nitriding are steel plates used as materials for nitriding parts.
- the said steel plate is manufactured with the below-mentioned manufacturing method.
- the automobile part is an automobile part made of the hot-rolled steel sheet for nitriding and the cold-rolled steel sheet for nitriding of the present invention and subjected to nitriding after forming.
- nitriding treatment means a treatment of diffusing nitrogen in the surface layer of steel and hardening the surface layer, and among these, a treatment of diffusing nitrogen and carbon in the surface layer of steel and hardening the surface layer is “ This is called “soft nitriding”.
- gas nitriding, gas soft nitriding, salt bath soft nitriding, and the like can be given.
- gas soft nitriding and salt bath soft nitriding are soft nitriding treatments.
- the product is a nitriding component as a result of nitriding that the steel sheet surface is hardened compared to before nitriding, and that the nitrogen concentration of the steel sheet surface layer is increased.
- the present invention is applicable to any of the hot-rolled steel sheet for nitriding and the cold-rolled steel sheet for nitriding and the automobile parts using them.
- C is an element effective for improving the strength by precipitating carbides of other carbide forming elements, and further contributing to precipitation strengthening by precipitating alloy carbides during nitriding and increasing the surface hardness after nitriding. It is. When C exceeds 0.07%, the precipitation density of cementite is increased and the burring formability is impaired. On the other hand, if it is less than 0.0002%, the grain boundary strength is lowered, so that the secondary work brittleness is lowered and the decarburization cost in steel making becomes too large. Therefore, the C content is set to be 0.0002% or more and 0.07% or less.
- Si is an element useful as a deoxidizing agent, but does not contribute to the improvement of surface hardness in the nitriding treatment, and reduces the surface hardening depth. Therefore, it is preferable to limit the Si content to 0.50% or less. On the other hand, in order to significantly reduce Si, the production cost increases, so the Si content is preferably 0.001% or more. Therefore, the Si content is 0.001 or more and 0.50% or less. In order to obtain a deeper surface hardening depth, the more preferable upper limit of the Si content is 0.1% or less.
- Mn is an element useful for delaying pearlite transformation in a temperature range of Ac1 or lower. If Mn is less than 0.10%, the effect cannot be obtained. On the other hand, when Mn exceeds 1.33%, the MnS band structure is formed remarkably, and the roughness of the shearing end face is increased, resulting in an extreme decrease in shear end face fatigue characteristics. Therefore, the Mn content is set to 0.10% or more and 1.33% or less.
- the S content is set to be 0.001% or more and 0.02% or less.
- Cr is an extremely effective element that improves the surface hardness by forming carbonitrides with N intruding during nitriding and C in steel. If the Cr content is 0.8% or less, sufficient surface hardness cannot be obtained. On the other hand, when the Cr content exceeds 1.20%, the effect is saturated. Therefore, the Cr content is more than 0.8% and not more than 1.20%.
- Al is an element effective for forming a nitride with N that penetrates during nitriding and increasing the surface hardness. However, if Al is contained excessively, the effective hardening depth may become shallow. When Al is less than 0.10%, sufficient surface hardness is not exhibited. When the content exceeds 0.50%, the affinity with N is high, and the surface hardening depth is reduced by suppressing diffusion of nitrogen in the depth direction. Therefore, the Al content is set to 0.10% or more and 0.50% or less. In addition, since surface hardness increases notably by containing Al 0.3% or more, Al content is preferably 0.30% or more.
- V is an element that contributes to the strength of the steel by generating carbonitride in the hot rolling process.
- Cr and Al and a composite carbonitride are formed, which is extremely effective for hardening the nitride layer.
- V contains 0.05% or more, the surface hardness and the surface hardening depth are remarkably improved.
- the content of V exceeds 0.10%, the structure strengthening due to the improvement in hardenability and the steel sheet strength due to precipitation strengthening are markedly increased, and the formability is deteriorated due to the decrease in elongation.
- V content is 0.05% or more and 0.10% or less.
- a more preferable range of the content is 0.07% or more.
- the range of Ti is determined by the balance with Al.
- Al is an extremely effective element that increases the surface hardness by forming a nitride after nitriding.
- Al precipitates in a point arrangement at the grain boundaries in the ⁇ region. Therefore, if Al nitride is precipitated before nitriding, the end surface roughness during shearing is increased and the shear end surface fatigue characteristics are reduced.
- Ti has an affinity for nitrogen higher than that of Al, and Ti nitride is preferentially formed over Al. Therefore, by containing Ti, it is possible to suppress the deterioration of the shear end face fatigue characteristics due to the above-described Al nitride.
- the roughness of the shear end face is the surface roughness of the end face at the time of shearing processing, which means the average roughness, and by increasing this roughness, excessive stress concentration on the shear end face during fatigue deformation And fatigue properties tend to be reduced.
- the said roughness uses the measured value of the thickness direction of a shearing fracture surface.
- B dissolves in the crystal grain boundary, thereby suppressing grain boundary segregation of P which is a grain boundary embrittlement element and improving secondary work brittleness. Moreover, the roughness of the end face at the time of shearing is reduced, and the shear end face fatigue characteristics are improved. If the B content is less than 0.0001%, the effect is not exhibited. Moreover, when it contains exceeding 0.0015%, in order to delay a ferrite transformation, the elongation of a steel plate will be reduced. Therefore, the B content is set to be 0.0001% or more and 0.0015% or less.
- Mo and Nb form a composite carbonitride with Cr and Al and are extremely effective for hardening the nitride layer. If the contents of Mo and Nb are less than 0.001%, the effect is not exhibited. If the Mo content exceeds 0.20%, the effect of improving the surface hardness due to the formation of Mo carbonitride is reduced, and the ductility is reduced. Therefore, the Mo content is set to 0.001% to 0.20%. On the other hand, if Nb is contained in an amount exceeding 0.050%, ⁇ recrystallization during hot rolling of the steel sheet is delayed, so that extremely high anisotropy is produced, thereby reducing the burring formability. Therefore, the Nb content is set to be 0.001% or more and 0.05% or less.
- Dislocations promote diffusion in steel. During the nitriding treatment, the diffusion of nitrogen is promoted and the surface hardening depth is increased. It is found for the first time in the present invention that when the dislocation density within 50 ⁇ m in the thickness direction from the surface of the steel sheet is 2.0 times or more compared to the dislocation density at a position of 1/4 in the thickness direction, the effect is exhibited. It was. On the other hand, when the dislocation density within 50 ⁇ m from the surface in the plate thickness direction is more than 10.0 times the dislocation density at the 1/4 position in the plate thickness direction, the ductility is significantly reduced due to dislocation strengthening. The inventors have found that the plate thickness of the steel plate is 1.6 to 5.0 mm, and that the effect is particularly remarkable when the plate thickness is 2.3 mm or more.
- the measurement of the dislocation density is preferably obtained from the half width by X-ray diffraction typified by the Williamson-Hall method. This is because a measurement range is limited in the measurement by direct observation with a TEM, and a distortion is introduced in the preparation of the observation sample, which may cause a decrease in measurement accuracy.
- the method for obtaining the half-value width by X-ray diffraction is described in, for example, “Evaluation method of dislocation density using X-ray diffraction” (Nakajima et al., CAMP-ISIJ Vol. 17 (2004) p. 396). .
- the size of the measurement sample is preferably 10 mm square or more.
- the surface of the sample for measurement is preferably reduced by 50 ⁇ m or more by electropolishing. Therefore, when it is desired to measure the position of a predetermined plate thickness, it is necessary to perform mechanical grinding in consideration of the thickness reduction due to electropolishing. It should be noted that an accurate dislocation density is not required due to processing strain on the surface that has been mechanically ground.
- the desirable microstructure of the steel sheet of the present invention will be described.
- the total area ratio of ferrite and bainite is a metal structure composed of 90% or more.
- the total area ratio of other metal structures exceeds 10%, it becomes difficult to achieve both ductility and burring formability.
- other metal structures indicate austenite, martensite, and pearlite.
- the metal structure of steel can be identified from an optical microscope by nital corrosion and a crystal structure by X-ray or diffraction pattern. Further, discrimination using a corrosive liquid other than nital may be used.
- Nital corrosion after mirror polishing, etching was performed with a Nital solution, the optical microscope 5 field of view was observed at 500 times, a photograph was taken, a portion was determined by visual observation, and it was obtained by image analysis.
- the manufacturing method of the steel plate of this invention is demonstrated.
- a manufacturing method from hot rolling to pickling when the steel sheet of the present invention is a hot-rolled steel sheet will be described.
- the heating temperature before rolling the slab which is a steel slab of the above-described steel component, is 1200 ° C. or higher in a heating furnace. This is to sufficiently dissolve the contained precipitated elements.
- the heating temperature exceeds 1300 ° C., the austenite grain boundary becomes coarse, and therefore the heating temperature is preferably 1300 ° C. or less.
- the hot rolling temperature is preferably 900 ° C. or higher.
- the coiling temperature is preferably 450 ° C. or higher after hot rolling. If the coiling temperature is 600 ° C. or higher, Ti and V carbide precipitation is promoted, and therefore the coiling temperature is more preferably between 550 ° C. and 600 ° C.
- the cooling rate may be in a range that causes ferrite transformation and bainite transformation during cooling, and the upper limit is preferably 10 ° C./s or less.
- the manufacturing method from hot rolling to pickling when the steel sheet of the present invention is a cold-rolled steel sheet will be described. After pickling the hot-rolled steel sheet and cold rolling to a predetermined thickness, the maximum heating temperature is heated to -50 ° C or higher from the Ar3 point, and the cooling stop temperature is 550 ° C or lower from the maximum heating temperature. It is preferable to perform an annealing treatment that cools to a low temperature.
- the ratio of the load T (kg / mm 2 ) per unit area loaded in the longitudinal direction of the steel sheet, skin pass rolling is performed under the condition that F / T is 8000 or more.
- the purpose of the above-described skin pass rolling is to suppress yield elongation by introducing movable dislocations, but not only to set the rolling reduction to a predetermined value, but also to the above-mentioned condition that F / T is 8000 or more. Then, the dislocation density on the steel sheet surface can be increased, and the dislocation density within 50 ⁇ m in the sheet thickness direction from the steel sheet surface is 2.0 times or more and 10.0 times the dislocation density at the 1/4 position in the sheet thickness direction. It has been found that it is possible to produce hot-rolled steel sheets or cold-rolled steel sheets that are twice or less.
- (dislocation density within 50 ⁇ m in the sheet thickness direction from the surface of the steel sheet) / (dislocation density at 1/4 position in the sheet thickness direction) is referred to as “dislocation density ratio”.
- FIG. 1 shows the results of investigating the relationship between the skin pass condition F / T and the dislocation density ratio for hot-rolled steel sheets and cold-rolled steel sheets having the components shown in Table 1.
- the skin pass condition F / T was less than 8000, the dislocation density ratio was less than 2.0. Further, when F / T was 8000 or more and 14000 or less, the dislocation density ratio was 2.0 or more and 10.0 or less.
- F / T exceeds 14,000, a dislocation density ratio exceeding 10.0 appears.
- FIG. 2 shows the effect of F / T on the dislocation density at the position where the plate thickness is 1/4. When F / T exceeded 14,000, the dislocation density at the position where the plate thickness was 1/4 increased.
- F / T is less than 8000, the tensile strength in the longitudinal direction of the steel sheet is strong, and dislocations are introduced to the entire surface of the cross section in the thickness direction of the steel sheet due to uniaxial tensile stress, which is not desirable as the method for producing the steel sheet of the present invention.
- F / T is 14000 or less as conditions for introducing dislocations only on the steel sheet surface. Note that when the rolling reduction exceeds 5%, the dislocation is introduced to the center in the sheet thickness direction, thereby reducing ductility.
- the range of the rolling reduction is set to 0.5 to 5%.
- an annealing step for recovering the dislocation may be performed, and then cold rolling at a reduction ratio of 0.5% to 5% may be performed.
- the dislocation does not recover when the annealing temperature is 200 ° C. or lower, 200 ° C. or higher is preferable.
- nitrided steel sheet with this deep surface hardening depth has improved crack initiation life and resistance to fatigue microcrack propagation, not only fatigue strength but also stress that breaks at a predetermined number of repetitions, that is, time strength Bring about improvement.
- FIG. 3 shows the relationship between the dislocation density ratio of the present invention and the surface hardening depth.
- the surface hardening depth is significantly reduced.
- a deep surface hardening depth was stably expressed, and the depth was 425 ⁇ m or more within the range of implementation. Further, it was about 50 ⁇ m deep on average when the dislocation density ratio was 2.0 or less. From this result, the surface hardening depth is preferably 425 ⁇ m or more.
- the surface hardening depth was defined as the distance from the surface to the position where HV began to increase with reference to JIS-G-0557.
- the hot-rolled steel sheet or the cold-rolled steel sheet of the present invention can be formed into a desired automobile part shape without impairing formability by introducing dislocation.
- molding refers to press molding or bending molding after shearing.
- the automobile part is a drive system part or a structural part formed from a steel plate.
- nitriding treatment examples include gas nitriding, plasma nitriding, gas soft nitriding, and salt bath soft nitriding.
- gas nitriding for example, it is held in an ammonia atmosphere at 540 ° C. for 20 hours or more.
- nitriding treatment for example, a general gas soft nitriding treatment using a mixed gas of N 2 + NH 3 + CO 2 at 570 ° C., the above-described nitride layer can be obtained in a treatment time of about 5 hours or more.
- Steel types 1 to 12 are the component ranges of the present invention, and steel types 13 to 28 are comparative components deviating from the components of the present invention. Moreover, since about C02 melt
- a steel sheet having a thickness of 2.3 mm was manufactured by winding the sheet into a shape, the scale on the surface was removed with a 7% hydrochloric acid aqueous solution, and rolling was performed under the skin pass conditions shown in Table 2 to obtain a hot rolled steel sheet for nitriding.
- the hot rolled steel sheet before skin pass rolling is cold rolled at a cold rolling rate of 60%, held at a heating rate of 10 (° C / sec) for a maximum heating temperature holding time of 30 (sec), and cooled down to 550 ° C. Annealing treatment was performed, and rolling was performed under the skin pass conditions shown in Table 2 to produce a cold-rolled steel sheet for nitriding.
- Test numbers 1 to 12 in Table 2 are within the ranges for both steel plate components and production conditions, test numbers 13 to 28 are out of range for any of the steel plate components, and test numbers 29 to 33 are outside the range for skin pass rolling conditions. .
- the half width of X-ray diffraction was measured for the steel plates of all test numbers, and the dislocation density was measured by the Williamson-Hall method. Note that diffraction peaks of (110), (112), and (220) were used for the half width of X-rays.
- a sample having a size of 25 mm length ⁇ 15 mm width was cut out from each steel type and the thickness was reduced to a predetermined measurement position by electrolytic polishing. did.
- the dislocation density ratio at the position of 50 ⁇ m from the surface and the position of the thickness 1 ⁇ 4 is 2.0 or more, 10 0.0 or less.
- the dislocation density ratio was less than 2.0 because F / T was 8000 or less.
- the skin pass reduction ratio was 5% or more and the tension was remarkably increased.
- the dislocation density not only at the position of 50 ⁇ m from the surface but also at the position of 1/4 of the plate thickness increased significantly. Less than 2.0.
- the dislocation density ratio exceeded 10.0 as a result of further increasing the line load during skin pass rolling.
- the dislocation density at the position of the plate thickness 1 ⁇ 4 was significantly increased.
- gas nitriding treatment was performed on all steel types under the following conditions.
- No. 5 test piece described in JIS-Z2201 was prepared and evaluated according to the test method described in JIS-Z2241.
- the burring formability ⁇ before nitriding was evaluated according to the test method described in JIS-Z2256.
- the roughness of the shear end face before nitriding was measured using a contact-type roughness measuring instrument after punching shearing using a 10 mm ⁇ cylindrical punch and a 15% clearance die. In addition, the roughness of the shear end face was measured in the direction of the thickness of the fracture surface, and the average roughness was adopted.
- the steel sheets of all test numbers were processed into the flat test piece shown in FIG. 5, and in order to investigate the fatigue characteristics of the shear end face, the above punching conditions were used.
- Table 3 shows the material characteristics before nitriding.
- the surface hardening depth was significantly reduced in Test No. 18 in which the Si content exceeded 0.5%.
- the surface hardening depth was slightly increased compared to Test 2, but this was not a remarkable effect.
- test numbers 2, 20 and 21 having different Mn contents a significant increase in the roughness of the shear end face was confirmed in test number 20 exceeding 1.33%. From the comparison of the surface hardness of Test Nos. 2, 4, 14 and 15 in which the Cr content is different, the hardness after nitriding can be secured stably in the component range of the present invention, and the Cr amount is 2.0%. Hardness did not change even when exceeding.
- the steel according to the present invention has a proper range in which a significant increase in the roughness of the shear end face is suppressed by containing B, and does not become excessively contained.
- Test Nos. 2, 22, and 26 having different Ti contents
- Test No. 22 in which the Ti content exceeded 0.1% confirmed a significant increase in shear end face roughness.
- Test No. 26 where the Ti content was less than 0.005%, a significant increase in the shear end face roughness was confirmed.
- Test Nos. 2, 23 and 24 having different B contents a remarkable increase in the shear end face roughness was confirmed in Test No. 23 containing no B.
- the skin pass reduction ratio was in an appropriate range, but the F / T was less than 8000, so the dislocation density ratio was less than 2.0. Therefore, the surface hardening depth after nitriding of test number 32 is extremely lower than that of test number 2.
- Test No. 33 the F / T and dislocation density ratios were satisfied. However, because the skin pass reduction ratio was 0.4%, upper yield / lower yield occurred, and yield elongation was not suppressed. confirmed.
- the surface hardening depth after nitriding is deepened without deteriorating the formability before nitriding, and extremely high after nitriding. It has been found that excellent fatigue characteristics can be exhibited.
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Abstract
Description
本願は、2011年11月21日に日本に出願された特願2011-253677号に基づき優先権を主張し、その内容をここに援用する。
(1)質量%で、Cが0.0002%以上、0.07%以下、Siが0.0010%以上、0.50%以下、Mnが0.10%以上、1.33%以下、Pが0.003%以上、0.02%以下、Sが0.001%以上、0.02%以下、Crが0.80%超、1.20%以下、Alが0.10%以上、0.50%以下、Vが0.05%以上、0.10%以下、Tiが0.005%以上、0.10%以下、Bが0.0001%以上、0.0015%以下を含有し、残部がFe及び不可避不純物からなり、鋼板表面から板厚方向に50μm以内の転位密度が、板厚方向の1/4の位置の転位密度に比べ、2.0倍以上、10.0倍以下であることを特徴とする疲労強度に優れる窒化用鋼板。
また、Nbは0.050%を超えて含有されると、鋼板の熱延中のγ再結晶を遅延させるため、極めて高い異方性を生ずることでバーリング成形性が低下する。そのため、Nbの含有量は0.001%以上、0.05%以下とした。
転位は鋼中の拡散を助長する。窒化処理中においては窒素の拡散を助長し、表面硬化深さを深くする。鋼板の表面から板厚方向に50μm以内の転位密度が、板厚方向の1/4の位置の転位密度に比べ、2.0倍以上の場合、その効果を発現することを本発明ではじめて見出した。一方、表面から板厚方向に50μm以内の転位密度が、板厚方向の1/4の位置の転位密度が10.0倍を超えると転位強化による延性の顕著な低下を示す。なお、鋼板の板厚は1.6~5.0mmであり、特に板厚が2.3mm以上の場合に顕著な効果があることを発明者らは見出した。
本発明ではフェライトおよびベイナイトの合計の面積率が90%以上で構成される金属組織であることが好ましい。その他の金属組織の合計の面積率が10%を超えた場合、延性とバーリング成形性の両立が困難となる。ここで、その他の金属組織はオーステナイト、マルテンサイト、パーライトを示す。
本発明の鋼板が熱延鋼板である場合の熱間圧延から酸洗までの製造方法について説明する。前述の鋼成分の鋼片であるスラブを加熱炉にて圧延前加熱温度を1200℃以上にすることが好ましい。これは含有される析出元素を十分に溶体化させるためであり、加熱温度が1300℃を超えるとオーステナイト粒界が粗大化するため、加熱温度は1300℃以下が好ましい。熱間圧延温度は900℃以上が好ましい。900℃未満では変形抵抗が大きくなる他、圧延集合組織の形成による異方性により成形性が低下する。さらに、マルテンサイトの分率の低下を防止するためには熱間圧延後、巻取り温度は450℃以上が好ましい。巻取り温度が600℃以上であれば、Ti、Vの炭化物析出が促進されるため、巻取り温度は550℃~600℃の間がより好ましい。冷却速度は、冷却中にフェライト変態、ベイナイト変態を生じる範囲であればよく、上限値を10℃/s以下にすることが好ましい。フェライト変態、ベイナイト変態を生じない冷却速度にて冷却を停止した場合、例えば、コイル状に巻取りを行った後に変態が促進し、鋼板コイルが変形するためである。なお、巻取り温度に至るまでに中間空冷を行ってもよい。熱間圧延終了後は常法により酸洗を行い、鋼板表面のスケールを除去する。
また、スキンパス圧延前の熱延鋼板を、冷延率60%で冷間圧延を施し、加熱速度10(℃/sec)で最高加熱温度保持時間30(sec)で保定し、550℃まで冷却停止する焼鈍処理を施し、表2のスキンパス条件で圧延し窒化用冷延鋼板を製造した。表2中の試験番号1~12は鋼板成分、製造条件共に範囲内であり、試験番号13~28は鋼板成分の何れかが範囲外、試験番号29~33はスキンパス圧延条件が範囲外である。
Si含有量が異なる、試験番号2、18および24の比較では、Si含有量が0.5%を超えた試験番号18では表面硬化深さが顕著に低下した。また、Si含有量が0.001%未満である試験番号24では試験2に対して、表面硬化深さがわずかに増加したものの、顕著な効果ではなかった。Mn含有量が異なる試験番号2、20および21の比較では1.33%を超えた試験番号20ではせん断端面粗度の顕著な増加が確認された。Crの含有量が異なる、試験番号2、4、14および15の表面硬度の比較から、本発明の成分範囲では安定的に窒化後の硬度が確保できており、Cr量が2.0%を超えても硬度はほとんど変わらなかった。
Claims (5)
- 質量%で、
C :0.0002%以上、0.07%以下、
Si:0.0010%以上、0.50%以下、
Mn:0.10%以上、1.33%以下、
P:0.003%以上、0.02%以下、
S:0.001%以上、0.02%以下、
Cr:0.80%超、1.20%以下、
Al:0.10%以上、0.50%以下、
V :0.05%以上、0.10%以下、
Ti:0.005%以上、0.10%以下、
B :0.0001%以上、0.0015%以下、
を含有し、残部がFe及び不可避不純物からなり、
鋼板表面から板厚方向に50μm以内の転位密度が、板厚方向の1/4の位置の転位密度に比べ、2.0倍以上、10.0倍以下であることを特徴とする疲労強度に優れる窒化用鋼板。 - さらに質量%で、
Mo:0.001以上、0.20%以下、
Nb:0.001以上、0.050%以下、
の1種または両方を含有することを特徴とする請求項1に記載の疲労強度に優れる窒化用鋼板。 - 質量%で、Cが0.0002%以上、0.07%以下、Siが0.0010%以上、0.50%以下、Mnが0.10%以上、1.33%以下、Pが0.003%以上、0.02%以下、Sが0.001%以上、0.02%以下、Crが0.80%超、1.20%以下、Alが0.10%以上、0.50%以下、Vが0.05%以上、0.10%以下、Tiが0.005%以上、0.10%以下、Bが0.0001%以上、0.0015%以下を含有し、残部がFe及び不可避不純物からなる鋼片を熱間圧延し、酸洗を施した後、圧下率にて0.5~5.0%であり、かつ圧延機荷重を鋼板板幅で除した線荷重F(kg/mm)と鋼板の長手方向に負荷される単位面積あたりの荷重T(kg/mm2)の比、F/T(mm)が8000以上の条件でスキンパス圧延を施すことを特徴とした疲労強度に優れる窒化用熱延鋼板の製造方法。
- 質量%で、Cが0.0002%以上、0.07%以下、Siが0.0010%以上、0.50%以下、Mnが0.10%以上、1.33%以下、Pが0.003%以上、0.02%以下、Sが0.001%以上、0.02%以下、Crが0.80%超、1.20%以下、Alが0.10%以上、0.50%以下、Vが0.05%以上、0.10%以下、Tiが0.005%以上、0.10%以下、Bが0.0001%以上、0.0015%以下を含有し、残部がFe及び不可避不純物からなる鋼片を熱間圧延し、酸洗、冷間圧延、焼鈍を施した後、圧下率にて0.5~5.0%であり、かつ圧延機荷重を鋼板板幅で除した線荷重F(kg/mm)と鋼板の長手方向に負荷される単位面積あたりの荷重T(kg/mm2)の比、F/T(mm)が8000以上の条件でスキンパス圧延を施すことを特徴とした疲労強度に優れた窒化用冷延鋼板の製造方法。
- 質量%で、Cが0.0002%以上、0.07%以下、Siが0.0010%以上、0.50%以下、Mnが0.10%以上、1.33%以下、Pが0.003%以上、0.02%以下、Sが0.001%以上、0.02%以下、Crが0.80%超、1.20%以下、Alが0.10%以上、0.50%以下、Vが0.05%以上、0.10%以下、Tiが0.005%以上、0.10%以下、Bが0.0001%以上、0.0015%以下を含有し、残部がFe及び不可避不純物からなり、鋼板表面から板厚方向に50μm以内の転位密度が、板厚方向の1/4の位置の転位密度に比べ、2.0倍以上、10.0倍以下である鋼板を成形した後に窒化処理したことを特徴とした疲労強度に優れた自動車部品。
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JP2013545918A JP5664797B2 (ja) | 2011-11-21 | 2012-11-19 | 疲労強度に優れる窒化用熱延鋼板、窒化用冷延鋼板及びそれらの製造方法、並びにそれらを用いた疲労強度に優れた自動車部品 |
BR112014011809-4A BR112014011809B1 (pt) | 2011-11-21 | 2012-11-19 | Folha de aço para a nitretação, método para a sua fabricação, e peça automotiva excelente na resistência à fadiga com o uso das mesmas |
MX2014005863A MX363871B (es) | 2011-11-21 | 2012-11-19 | Lamina de acero laminada en caliente para nitruracion, lamina de acero laminada en frio para nitruracion, excelente en resistencia a la fatiga, metodo de fabricacion de la misma, y parte de automovil excelente en resistencia a la fatiga utilizando la misma. |
US14/358,775 US9777353B2 (en) | 2011-11-21 | 2012-11-19 | Hot-rolled steel sheet for nitriding, cold-rolled steel sheet for nitriding excellent in fatigue strength, manufacturing method thereof, and automobile part excellent in fatigue strength using the same |
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JP2015147959A (ja) * | 2014-02-05 | 2015-08-20 | Jfeスチール株式会社 | 比例限が高く、曲げ加工性に優れた高強度冷延薄鋼板およびその製造方法 |
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WO2017094876A1 (ja) | 2015-12-04 | 2017-06-08 | 新日鐵住金株式会社 | 窒化プレート部品およびその製造方法 |
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