WO2016159158A1 - 冷間鍛造用時効硬化用鋼 - Google Patents
冷間鍛造用時効硬化用鋼 Download PDFInfo
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/002—Heat treatment of ferrous alloys containing Cr
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/02—Hardening by precipitation
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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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
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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
- C22C38/04—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
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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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
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/004—Dispersions; Precipitations
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D7/00—Modifying the physical properties of iron or steel by deformation
- C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
Definitions
- the present invention relates to an age hardening steel for cold forging.
- Carbon steel for machine structures and alloy steels for machine structures are used as structural steels as materials for machine structural parts such as automobile parts, industrial machine parts and construction machine parts.
- cold forging has problems of high processing load, high processing load, short mold life, and easy cracking of parts. Therefore, the most important issues are to increase the cold forgeability (cold forgeability) of the steel material, that is, to reduce the load during cold forging and to suppress the occurrence of cracks.
- high fatigue strength is required for machine structural parts such as automobile parts, industrial machine parts and construction machine parts.
- it is effective to increase the hardness after cold forging.
- the hardness after cold forging is increased by increasing the hardness of the steel material, the cold forgeability is reduced. That is, it has been difficult to achieve both cold forgeability and fatigue strength in the steel material.
- age-hardening steel that is applied to the purpose of increasing the hardness by heat treatment after cutting without increasing the hardness more than necessary at the time of cutting.
- the chemical component is mass%, C: 0.01 to 0.15%, Si: 0.05% or less, Mn: 0.10 to 0.90%, P: 0.030% or less , S: 0.030% or less, Cr: 0.50 to 2.0%, V: 0.10 to 0.50%, Al: 0.01 to 0.10%, N: 0.00080% or less, and O: not more than 0.0030%, the balance being Fe and impurities, 399 ⁇ C + 26 ⁇ Si + 123 ⁇ Mn + 30 ⁇ Cr + 32 ⁇ Mo + 19 ⁇ V ⁇ 160 or less, 20 ⁇ (669.3 ⁇ log C ⁇ 1959.3 ⁇ log N -6983.3) ⁇ (0.067 ⁇ Mo + 0.147 ⁇ V) ⁇ 80, 160 ⁇ 140 ⁇ Cr + 125 ⁇ Al + 235 ⁇ V, 90 ⁇ 511 ⁇ C + 33 ⁇ Mn + 56 ⁇ Cu + 15 ⁇ Ni + 36 ⁇ Cr + 5 ⁇ Mo + 134 ⁇ V ⁇ 170 Fill
- a technology relating to a steel for cold heading having an area% greater than or equal to the f value and excellent in cold workability in which VC is precipitated in the pro-eutectoid ferrite is disclosed.
- Patent Document 1 provides steel and steel materials having excellent cold forgeability and machinability after cold forging, and parts subjected to cold forging and nitriding treatment, High core hardness, high surface hardness and deep effective hardened layer depth can be provided.
- the fatigue strength is not mentioned, and the improvement of the durability ratio (fatigue strength / tensile strength) is not studied.
- Patent Document 2 relates to a steel for cold heading that can be subjected to cold working while being rolled, and precipitates VC during hot rolling to reduce solid solution C. This provides steel with improved cold forgeability.
- the technique described in Patent Document 2 does not consider fatigue strength. Further, in order to improve the strength, it is assumed that a tempering treatment is performed, and cutting is necessary in a cured state after the tempering treatment, and a reduction in machinability is inevitable.
- the present invention has been made in view of the above situation, and has high cold forgeability while ensuring tensile strength of 400 MPa or more and fatigue strength of 250 MPa or more, and work hardening by cold forging, and cold.
- An object of the present invention is to provide an age-hardening steel for cold forging in which a high durability ratio is obtained by age-hardening after forging.
- the precipitation hardening of V carbonitride and Nb carbonitride should be used. It is effective to reduce the rate.
- Age hardening treatment not only increases the fatigue strength but also increases the durability ratio (fatigue strength / tensile strength). If the durability ratio is high, the tensile strength can be made relatively low while ensuring the required fatigue strength, so that the effect of preventing a decrease in the machinability can be obtained.
- the high durability ratio in the present invention indicates that it is 0.600 or more.
- the present invention has been completed based on the above findings (A) to (D), and the gist thereof is as follows.
- fn2 represented by the following formula (2) is 13.5 or less
- the metal structure is an area ratio
- bainite and martens Ito Total 5% cold forging age-hardening steel, which comprises (including 0%).
- [V] is the mass% of the solid solution V
- [Nb] is the mass% of the solid solution Nb
- C is the mass% of C contained in the steel
- V is contained in the steel.
- the mass% of V and Nb show the mass% of Nb which steel contains.
- the chemical composition contains one or more of Cu: 0.20% or less, Ni: 0.20% or less, and Mo: 0.20% or less in place of part of Fe.
- the age-hardened steel for cold forging of the present invention is excellent in cold forgeability and can ensure a high durability ratio and machinability by age-hardening treatment without performing heat treatment such as quenching and tempering or induction hardening. Further, by using the age-hardened steel of the present invention as a material, instead of the “hot forging-cutting” process that has been generally used so far, automobile parts, Machine structural parts such as industrial machine parts and construction machine parts can be manufactured, and productivity can be improved.
- C is an element necessary for increasing the strength as a machine structural component.
- the amount of C is reduced in order to suppress cracking during cold forging. If the C content exceeds 0.13%, cracks occur during cold forging, so the content is made 0.13% or less. If the C content is less than 0.02%, a tensile strength of 400 MPa or more and a fatigue strength of 250 MPa or more cannot be ensured after age hardening. For this reason, content of C shall be 0.02% or more.
- the C content is preferably 0.03% or more and less than 0.10%.
- Si 0.01 to 0.50%
- Si is an element necessary for deoxidation at the time of melting, and in order to obtain this effect, 0.01% or more is contained.
- Si strengthens the solid solution of ferrite, if the Si content exceeds 0.50%, cold forgeability is reduced. Therefore, the Si content is 0.50% or less.
- the Si content is desirably 0.05% or more and 0.45% or less.
- Mn increases the strength of the final part as a solid solution strengthening element. If the Mn content is less than 0.20%, the strength of the final part is insufficient, and if it exceeds 0.70%, the cold forgeability is lowered. Therefore, the Mn content is set to 0.20 to 0.70%. Note that the Mn content is desirably 0.25% or more and 0.65% or less.
- P 0.020% or less
- P is an impurity inevitably contained in the steel, is easily segregated in the steel, and causes a local decrease in ductility.
- the content of P exceeds 0.020%, the local ductility is significantly reduced. Therefore, the content is limited to 0.020% or less.
- the content is desirably limited to 0.018% or less.
- the content of P may be zero.
- S is an element that improves machinability. In order to obtain the effect of improving machinability, it is necessary to contain 0.005% or more. If the content exceeds 0.020%, coarse sulfides are generated in the steel, causing cracks during cold forging. Therefore, the S content is set to 0.005 to 0.020%. The S content is desirably 0.018% or less.
- Al 0.005 to 0.050%
- Al is a deoxidizer during steel refining. In order to obtain the effect of deoxidation, 0.005% or more is contained. If the content exceeds 0.050%, coarse Al inclusions are generated in the steel, which causes cracks during cold forging. Therefore, the Al content is 0.050% or less.
- the Al content is preferably 0.045% or less.
- Cr 0.02 to 1.50%
- Cr has the effect of increasing the fatigue strength after forging as a solid solution strengthening element.
- the content exceeds 1.50%, the material hardness is excessively increased and the cold forgeability is deteriorated. Therefore, the Cr content is set to 0.02 to 1.50%.
- the Cr content is preferably 0.03% or more and 1.30% or less.
- V 0.02% to 0.50%
- V increases fatigue strength and durability ratio by forming a composite carbonitride of V and Nb during the age hardening treatment.
- 0.02% or more of V is contained.
- the upper limit is made 0.50%.
- the V content is preferably 0.03% or more.
- Nb 0.005% to 0.050%
- carbonitride is formed in combination with V during the age hardening treatment, and the durability ratio is increased. In order to acquire this effect, it contains 0.005% or more. From the viewpoint of alloy cost, the upper limit is made 0.050%.
- the Nb content is preferably 0.010% or more.
- N 0.003 to 0.030% or less
- V 0.003 to 0.030% or less
- Nb 0.003 to 0.030% or less
- V 0.003 to 0.030% or less
- N is combined with V and Nb in the age hardening treatment after cold forging and precipitates as a composite carbonitride to improve the durability ratio. In order to acquire this effect, it contains 0.003% or more. However, since it will cause a cold forgeability fall if it contains excessively, the content shall be 0.030% or less.
- the N content is preferably 0.025% or less.
- the age hardening steel for cold forging according to the present invention has a chemical composition in which the balance is composed of Fe and inevitable impurities in addition to the above elements.
- Inevitable impurities refer to impurities mixed from ores, scraps, or production environments as raw materials when industrially producing steel materials.
- the chemical composition of the age-hardening steel for cold forging of the present invention may contain one or more elements of Cu, Ni, and Mo in place of part of Fe in addition to the above elements.
- Cu 0.20% or less
- the amount of Cu in the case of inclusion is preferably 0.15% or less.
- Ni 0.20% or less Since Ni has the effect of increasing the fatigue strength of steel, it may contain 0.20% or less. When it exceeds 0.20%, cold forgeability will fall. From the viewpoint of ensuring cold forgeability, the Ni content when contained is preferably 0.15% or less.
- Mo 0.20% or less Since Mo has the effect of increasing the fatigue strength of steel, it may contain 0.20% or less. When it exceeds 0.20%, cold forgeability will fall. From the viewpoint of ensuring cold forgeability, the amount of Mo in the case of inclusion is preferably 0.15% or less.
- the content (mass%) of solid solution Nb needs to be 25% or more with respect to the total content of Nb, and the content (mass%) of solid solution V needs to be 50% or more with respect to the total content of V. There is.
- the solid solution V amount is the mass% of V not precipitated as carbonitride out of V contained in steel, and the solid solution Nb amount is carbonitride out of Nb contained in steel. Is the mass% of Nb not precipitated.
- the component of the age-hardening steel for cold forging of the present invention must have fn1 defined by the formula (1) of 0.03 or more. This is to obtain an appropriate amount of Nb and V composite carbonitride for increasing the durability ratio during the age hardening treatment.
- the upper limit value of fn1 is not particularly limited, but may be 0.90 or less.
- the amount of solid solution V and the amount of solid solution Nb are obtained, for example, by the following extraction residue analysis method.
- a 10 mm ⁇ 10 mm ⁇ 10 mm sample is cut out from the position of radius ⁇ 0.5 of age-hardened steel formed into a round bar and used as an extraction residue analysis sample.
- This sample is subjected to constant current electrolysis in a 10% AA-based solution (a liquid in which tetramethylammonium chloride, acetylacetone, and methanol are mixed at 1: 10: 100).
- the sample is electrolyzed under the conditions of current: 173 mA, time: 142 minutes, and room temperature.
- the electrolyzed sample is taken out, and the deposit (residue) on the sample surface is ultrasonically washed in alcohol to remove it from the sample.
- the electrolyzed solution and the solution used for ultrasonic cleaning are suction filtered through a filter having a mesh size of 0.2 ⁇ m to collect a residue.
- the mass of the sample from which the deposits (residues) have been removed is measured, and the “mass of the electrolyzed sample” is determined from the difference in the measured values of the mass of the sample before and after electrolysis.
- the residue collected on the filter is transferred to a petri dish, dried, measured for mass, and analyzed by an ICP emission analyzer (high frequency inductively coupled plasma emission spectrometer) according to JIS G 1258. "Mass of V and Nb in the medium”. Then, the “mass of V and Nb in the residue” obtained as described above is divided by the “mass of the electrolyzed sample” and expressed as a percentage. The amount of solute Nb ”.
- the present inventors have C: 0.02 to 0.13%, Si: 0.01 to 0.50%, Mn: 0.20 to 0.70%, P: 0.020% or less (0% S): 0.005 to 0.020%, Al: 0.005 to 0.050%, Cr: 0.02 to 1.50%, V: 0.02 to 0.50%, Nb: 0
- Various steels containing 0.005 to 0.050% and N: 0.003 to 0.030%, with the balance being Fe and inevitable impurities, and holding 30 min to 60 min below A3 A test steel having a solid solution V amount and a solid solution Nb amount was produced.
- the durability ratio can be made 0.60 or more by setting the ratio of the solute Nb amount to the solute V amount of the test steel to 0.03 or more. If the value of fn1 defined by the formula (1) is less than 0.03, the composite carbonitride is not precipitated, so that the effect of improving the durability ratio cannot be obtained. Therefore, the value of fn1 is limited to 0.03 or more.
- the microstructure of the age-hardened steel for cold forging of the present invention is mainly composed of a mixed structure of ferrite and pearlite, and the area ratio of ferrite is 85% or more.
- the area ratio of pearlite may be small or zero.
- bainite and martensite may be generated as a structure (remainder structure) other than ferrite and pearlite. In such a case, the total area ratio of bainite and martensite must be limited to 5% or less. There is.
- the age hardening steel for cold forging of the present invention must have fn2 defined by the formula (2) of 13.5 or less.
- the fn2 value is preferably as low as possible, and the lower limit is not particularly limited, but is 0.80 or more from the upper and lower limits of the content of each element.
- C represents the mass% of C contained in the steel
- V represents the mass% of V contained in the steel
- Nb represents the mass% of Nb contained in the steel.
- the area ratio of ferrite needs to be 85% or more. Further, it is important to strengthen the ferrite. V and Nb are elements that precipitate carbonitride during the age hardening treatment and strengthen the ferrite.
- the value of fn2 defined by the formula (2) is 13.6 or more, the ferrite is not sufficiently strengthened. Further, the ferrite area ratio may not be 85% or more. Therefore, a durability ratio of 0.60 or more cannot be obtained. Therefore, in order to obtain the durability ratio required in the present invention, fn2 is set to 13.5 or less.
- Bainite structure and martensite structure are inferior in cold deformability to ferrite and pearlite structures, and cause cracks during cold forging. Therefore, the sum of the bainite structure and the martensite structure must be limited to 5% or less in terms of area ratio. From the viewpoint of suppressing cracking during cold forging, the bainite structure and the martensite structure may be generated at zero.
- the slab or steel slab having the above-described chemical composition is used as a material to be rolled, and is rolled by hot rolling, and rolled in the final rolling step. After completion, it may be cooled to room temperature.
- the method for obtaining the slab or steel slab is not particularly limited, and may be a conventional method.
- the hot rolling needs to be performed at a rolling temperature of 900 ° C. or higher in the final rolling step.
- the average cooling rate needs to be 0.6 ° C./s or less.
- the age-hardening steel of the present invention can be used, for example, for producing mechanical structural parts.
- the steel for age hardening of the present invention is subjected to cold forging and age hardening in order, and then subjected to a processing step such as cutting.
- the heating time is less than 30 min, carbonitrides do not precipitate and a high durability ratio may not be obtained. Further, even if the heating time is lengthened, the same effect can be obtained. However, if the length is too long, the manufacturing cost is increased.
- the shape of the age-hardening steel of the present invention is not particularly limited, and can be applied to any shape such as a steel plate, a steel pipe, a steel bar (section steel, bar steel, wire rod, rail, etc.).
- steels A to P are steels whose chemical compositions are within the range defined by the present invention.
- steels K to P are comparative steels whose chemical compositions deviate from the range defined in the present invention.
- Table 2 shows the hardness, structure, solid solution V amount, solid solution Nb amount, fn1, and fn2 of the steel after hot forging.
- “F” in “Microstructure” in Table 2 represents ferrite
- “P” represents pearlite
- “B” represents bainite
- “M” represents martensite.
- B, M area ratio” in Table 2 represents the total area ratio of bainite and martensite.
- a cylindrical test piece of ⁇ 14 ⁇ 21 mm ( ⁇ represents a diameter, the same applies hereinafter) was cut out from the forged bar material and subjected to a compression test using a cold press to evaluate cold forgeability.
- the evaluation items were whether or not a crack was generated when the processing rate ((1-height after processing / height before processing) ⁇ 100) was 70%, and when the processing rate was 50%.
- Forging load (50% working load, (ton)). The crack was observed using a 5 ⁇ magnifier, and it was determined that there was no crack when no crack of 0.5 mm or more in length was observed in the five test pieces. As for forging load, 20 ton or less was determined to be sufficiently low and good.
- the ⁇ 42 mm round bar forging material was embedded in a resin so as to observe its cross section, then polished, corroded with nital to observe the microstructure, and Vickers hardness with a load of 9.8 N was measured. Microstructural observation and Vickers hardness were performed near the center of the round bar forged material. The Vickers hardness was measured as an average of three points.
- an extraction residue test piece of 10 mm 3 was cut out from the above-mentioned ⁇ 42 mm round bar forging material, and the amount of solute V and the amount of solute Nb were measured by the aforementioned extraction residue analysis method.
- Table 3 shows the presence or absence of cracks when the processing rate is 70%, the forging load when the processing rate is 50%, and the forging load when the processing rate is 50%, 60 min at 600 ° C after drawing ⁇ 18 mm.
- the tensile strength, fatigue strength, and durability ratio (fatigue strength / tensile strength) after holding are shown.
- the durability ratio was determined to be good when it was 0.600 or higher, the tensile strength was determined to be 400 MPa or higher, and the fatigue strength was determined to be good when it was 250 MPa or higher.
- the underline in Table 3 means that it was not judged good.
- test number 11 since the C content exceeds the range specified in the present invention, the load during cold forging is high, cracks are also observed, and the required cold forgeability is not obtained. In addition, since the area ratio of ferrite is low and the value of fn2 exceeds the value specified in the present invention, the required durability ratio is not obtained.
- test number 12 since the C content is below the range specified in the present invention, the forging property at the time of cold forging is satisfactory, but the tensile strength and fatigue strength after age hardening treatment are low, and the required performance Is not obtained.
- test number 13 since V is not added, ferrite is not strengthened, the area ratio of ferrite is low, and the value of fn2 exceeds the value specified in the present invention, so the durability ratio to be obtained Is not obtained.
- test number 14 since the addition amount of V is below the range defined in the present invention, ferrite is not sufficiently strengthened, the area ratio of ferrite is low, and the value of fn2 is defined in the present invention. The durability ratio required is not obtained because the value exceeds the value to be obtained.
- test number 16 since the amount of Nb added is below the range specified in the present invention, ferrite is not sufficiently strengthened, and the value of fn1 exceeds the value specified in the present invention, so that the required durability is obtained. The ratio is not obtained.
- test number 17 since the content of the solid solution Nb and the content of the solid solution V are lower than the values specified in the present invention, the ferrite is not sufficiently strengthened and the required durability ratio is not obtained.
- the age-hardening for cold forging of the present invention can ensure high fatigue strength and is excellent in cold forgeability. Therefore, parts for automobiles and industrial machinery that have been manufactured in the "hot forging-cutting" process so far. It can contribute to the near net shape of machine structural parts such as construction machine parts.
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Abstract
Description
Cは、機械構造部品としての強度を高めるために必要な元素である。しかしながら、本発明においては、冷間鍛造時の割れを抑えるために、C量を低減する。Cの含有量が0.13%を越えると冷間鍛造時に割れが発生するため、その含有量を0.13%以下とする。C含有量が0.02%未満では時効硬化処理後に400MPa以上の引張強度、250MPa以上の疲労強度を確保できない。このため、Cの含有量を0.02%以上とする。なお、Cの含有量は、0.03%以上、0.10%未満とすることが望ましい。
Siは、溶製時の脱酸用として必要な元素であり、この効果を得るために0.01%以上を含有させる。しかしながら、Siはフェライトを固溶強化するため、Siの含有量が0.50%を越えると、冷間鍛造性を低下させる。したがって、Siの含有量を0.50%以下とする。Siの含有量は、0.05%以上、0.45%以下とすることが望ましい。
Mnは、固溶強化元素として最終部品の強度を高める。Mnの含有量が0.20%未満では最終部品の強度が不足し、0.70%を超えると冷間鍛造性を低下させる。このため、Mnの含有量を0.20~0.70%とする。なお、Mnの含有量は、0.25%以上、0.65%以下とすることが望ましい。
Pは、鋼中に不可避的に含有される不純物であり、鋼中で偏析しやすく、局所的な延性低下の原因となる。Pの含有量が0.020%を超えると、局所的な延性低下が著しくなる。したがって、含有量を0.020%以下に制限する。含有量は、0.018%以下に制限することが望ましい。Pの含有量は0でもよい。
Sは、被削性を向上させる元素である。被削性向上の効果を得るためには0.005%以上を含有する必要がある。0.020%を超えて含有すると、鋼中に粗大な硫化物を生成させ、冷間鍛造時の割れ発生の原因となる。したがって、Sの含有量を0.005~0.020%とする。なお、Sの含有量は、0.018%以下とすることが望ましい。
Alは鋼精錬時の脱酸剤である。脱酸の効果を得るために0.005%以上含有させる。含有量が0.050%を超えると、鋼中に粗大なAl介在物を生成し、冷間鍛造時の割れ発生の原因となる。したがって、Alの含有量を0.050%以下とする。なお、Alの含有量は、0.045%以下とすることが望ましい。
Crは、固溶強化元素として鍛造後の疲労強度を高める効果を有する。しかし、その含有量が1.50%を超えると、過度に素材硬さを高めて冷間鍛造性が低下する。このため、Crの含有量を0.02~1.50%とする。なお、Crの含有量は、0.03%以上、1.30%以下とすることが望ましい。
Vは、時効硬化処理の際にVとNbの複合炭窒化物を形成することで、疲労強度と耐久比を高める。この効果を得るために、Vを0.02%以上含有させる。合金コストの観点から、上限は0.50%とする。なお、V含有量は、0.03%以上であることが望ましい。
Nbは、Vと同時に添加することで、時効硬化処理の際にVと複合的に炭窒化物を形成し、耐久比を高める。この効果を得るために、0.005%以上含有させる。合金コストの観点から、上限は0.050%とする。なお、Nbの含有量は、0.010%以上が望ましい。
Nは、冷間鍛造後の時効硬化処理においてV、Nbと結合し、複合炭窒化物として析出することで耐久比を向上させる。この効果を得るために、0.003%以上含有させる。しかし、過剰に含有されると冷鍛性低下の原因となるため、その含有量を0.030%以下とする。なお、Nの含有量は、0.025%以下とすることが望ましい。
Cuは鋼の疲労強度を高める効果を有するため、0.20%以下を含有させてもよい。0.20%を超えると、冷間鍛造性が低下する。冷間鍛造性確保の観点から、含有させる場合のCuの量は0.15%以下とすることが好ましい。
Niは鋼の疲労強度を高める効果を有するため、0.20%以下を含有させてもよい。0.20%を超えると、冷間鍛造性が低下する。冷間鍛造性確保の観点から、含有させる場合のNiの量は0.15%以下とすることが好ましい。
Moは鋼の疲労強度を高める効果を有するため、0.20%以下を含有させてもよい。0.20%を超えると、冷間鍛造性が低下する。冷間鍛造性確保の観点から、含有させる場合のMoの量は0.15%以下とすることが好ましい。
ただし、[V]は固溶Vの質量%、[Nb]は固溶Nbの質量%を示す。
そして、上記のようにして求めた「残渣中のVおよびNbの質量」を、「電気分解された試料の質量」で除し、百分率表示したものが、「抽出残渣分析による固溶V量および固溶Nb量」である。
ただし、Cは鋼が含有するCの質量%、Vは鋼が含有するVの質量%、Nbは鋼が含有するNbの質量%を示す。
本発明の時効硬化用鋼は、たとえば、機械構造部品を製造するために用いることができる。機械構造部品を製造する際には、本発明の時効硬化用鋼に、冷間鍛造、時効硬化処理を順に施し、その後、切削等の加工工程に供する。
式中の元素記号は、鋼中の元素の含有量(質量%)を示す。
Claims (2)
- 化学組成が、質量%で、
C :0.02~0.13%、
Si:0.01~0.50%、
Mn:0.20~0.70%、
P :0.020%以下(0%を含む)、
S :0.005~0.020%、
Al:0.005~0.050%、
Cr:0.02~1.50%、
V :0.02~0.50%、
Nb:0.005~0.050%、及び
N :0.003~0.030%
を含有し、残部はFe及び不可避的不純物であり、
固溶Nbの含有量(質量%)が上記Nbの総含有量に対して25%以上、
固溶Vの含有量(質量%)が上記Vの総含有量に対して50%以上、
下記式(1)で表されるfn1が0.03以上、
下記式(2)で表されるfn2が13.5以下であって、
金属組織が、面積率で、
フェライト:85%以上、
ベイナイトとマルテンサイトの合計:5%以下(0%を含む)
を含む
ことを特徴とする冷間鍛造用時効硬化用鋼。
fn1=[Nb]/[V] ・・・ (1)
fn2=125×C-13×V-4×Nb ・・・ (2)
式(1)と式(2)において、[V]は固溶Vの質量%、[Nb]は固溶Nbの質量%、Cは鋼が含有するCの質量%、Vは鋼が含有するVの質量%、Nbは鋼が含有するNbの質量%を示す。 - さらに、前記化学組成が、Feの一部に代えて、Cu:0.20%以下、Ni:0.20%以下及びMo:0.20%以下のうちの1種以上を含有することを特徴とする請求項1に記載の冷間鍛造用時効硬化用鋼。
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