WO2013094475A1 - 冷間加工用機械構造用鋼およびその製造方法 - Google Patents
冷間加工用機械構造用鋼およびその製造方法 Download PDFInfo
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- 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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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
- C21D1/32—Soft annealing, e.g. spheroidising
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
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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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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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
- C22C38/16—Ferrous alloys, e.g. steel alloys containing 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
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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
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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- 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
Definitions
- the present invention relates to a machine structural steel for cold working used in the manufacture of various parts such as automobile parts and construction machine parts, and particularly has a low deformation resistance and excellent cold workability after spheroidizing annealing. And a useful method for producing such cold work machine structural steel.
- various parts such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold forging, and cold rolling, such as bolts, screws, nuts, sockets, balls, etc.
- spheroidizing annealing treatment is applied to the hot rolled material such as carbon steel and alloy steel for the purpose of providing cold workability, After forming and then forming into a predetermined shape by cutting or the like, a final strength adjustment is performed by quenching and tempering.
- Patent Document 1 specifies pro-eutectoid ferrite and pearlite structure, the average particle diameter thereof is set to 6 to 15 ⁇ m, and the volume fraction of ferrite is specified, so that the spheroidizing annealing process can be performed rapidly. And the technique which made cold forgeability compatible is disclosed.
- the spheroidizing annealing time can be shortened, but when the normal spheroidizing annealing process (annealing process for about 10 to 30 hours) is performed, the material becomes soft. It will be insufficient.
- Patent Document 2 discloses a technique that enables the annealing time to be shortened by defining the volume fraction of the pearlite structure and the bainite structure in addition to the volume fraction of pro-eutectoid ferrite.
- a technique that enables the annealing time to be shortened by defining the volume fraction of the pearlite structure and the bainite structure in addition to the volume fraction of pro-eutectoid ferrite.
- rapid spheroidization is achieved, but softening is still insufficient, and as a result of the mixed structure of bainite and pearlite, the hardness after spheroidizing annealing may vary. Concerned.
- the present invention has been made under such circumstances, and its purpose is to achieve softening by spheroidizing annealing even when normal spheroidizing annealing is performed, and to vary in hardness. It is another object of the present invention to provide a cold-working machine structural steel and a useful method for producing such a cold-working machine structural steel.
- the machine structural steel for cold working of the present invention that has achieved the above object is C: 0.3-0.6% (meaning mass%, hereinafter the same for chemical composition), Si: 0 0.005 to 0.5%, Mn: 0.2 to 1.5%, P: 0.03% or less (not including 0%), S: 0.03% or less (not including 0%), Al : 0.01-0.1%, and N: 0.015% or less (excluding 0%) respectively, the balance is made of iron and inevitable impurities, and the steel microstructure is pearlite and proeutectoid ferrite
- the total area ratio of pearlite and pro-eutectoid ferrite with respect to the whole structure is 90 area% or more, and the area ratio A of pro-eutectoid ferrite is A in relation to the Ae value represented by the following formula (1).
- the average equivalent circle diameter of the Fe crystal grains is 15 to 35 ⁇ m, and the average equivalent diameter of the bcc-Fe crystal grains is 50 ⁇ m or less between the largest grain size and the second largest grain size. It has a gist.
- the “equivalent circle diameter” is a diameter (equivalent circle diameter) when bcc-Fe crystal grains surrounded by large-angle grain boundaries with a misorientation larger than 15 ° are converted into circles of the same area.
- “Average circle equivalent diameter” is the average value.
- the average value of the largest particle size and the second largest particle size in the equivalent circle diameter of the bcc-Fe crystal grains may be hereinafter referred to as “coarse portion particle size” for convenience of explanation.
- Ae (0.8 ⁇ Ceq 1 ) ⁇ 96.75 (1)
- Ceq 1 [C] + 0.1 ⁇ [Si] + 0.06 ⁇ [Mn], and [C], [Si] and [Mn] are the contents (mass%) of C, Si and Mn, respectively. ).
- the basic chemical components of the steel for cold working machine structure of the present invention are as described above, but if necessary, (a) Cr: 0.5% or less (excluding 0%), Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), Mo: 0.25% or less (not including 0%), and B: 0.01% 1 or more selected from the group consisting of the following (not including 0%), (b) Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%) ), And V: 0.5% or less (not including 0%), it is also useful to contain one or more selected from the group consisting of, and the characteristics of the steel material depending on the components contained Further improvement.
- a step of cooling to a temperature range of 700 ° C. or more and less than 800 ° C. and a step of cooling for 100 seconds or more at an average cooling rate of 0.2 ° C./second or less may be included in this order.
- a step of finish rolling at a temperature of 1050 ° C. or more and 1200 ° C. or less a step of cooling to a temperature range of 700 ° C. or more and less than 800 ° C. at an average cooling rate of 10 ° C./second or more, 0.2 ° C./second or less
- a step of cooling at an average cooling rate of 100 seconds or more a step of cooling to a temperature range of 580 to 660 ° C. at an average cooling rate of 10 ° C./second or more, or a cooling or holding at an average cooling rate of 1 ° C./second or more for 20 seconds or more. Even if the steps are included in this order, the machine structural steel for cold working of the present invention can be produced.
- K value (N ⁇ L) / E (2)
- E average equivalent circle diameter of bcc-Fe crystal grains ( ⁇ m)
- N density of cementite in the bcc-Fe crystal grains (pieces / ⁇ m 2 )
- L aspect ratio of cementite in the bcc-Fe crystal grains , Respectively.
- the total area ratio of pearlite and pro-eutectoid ferrite with respect to the entire structure is defined together with the chemical composition, and the area ratio A of pro-eutectoid ferrite is expressed as A> Ae in relation to the Ae value represented by a predetermined relational expression.
- the hardness can be sufficiently lowered even when normal spheroidizing annealing is performed.
- the present inventors are able to achieve softening by spheroidizing annealing even when subjected to normal spheroidizing annealing, and for cold working machine structures that can also reduce variation in hardness.
- the size of the ferrite crystal grains after spheroidizing annealing is made relatively large, and in order to reduce dispersion strengthening by spherical cementite, cementite particles
- the idea was that it was important to make the distance as large as possible.
- the metal structure before spheroidizing annealing (hereinafter sometimes referred to as “pre-structure”) is mainly composed of pearlite and proeutectoid ferrite.
- pre-structure increases the area ratio of pro-eutectoid ferrite in the structure as much as possible, and bcc-Fe crystal grains surrounded by large-angle grain boundaries (specifically, pro-eutectoid ferrite grains and ferrite grains in pearlite)
- the hardness after spheroidizing annealing can be reduced to the maximum if the average equivalent circle diameter is relatively large.
- the present inventors have found that the reduction in hardness variation can be achieved by setting the coarse portion grain size of the bcc-Fe crystal grains to 50 ⁇ m or less.
- cementite and ferrite are metal structures that contribute to improving cold workability by reducing the deformation resistance of steel.
- the desired softening cannot be achieved simply by making the metal structure containing spheroidized cementite and ferrite, as described in detail below, the area ratio of this metal structure, the area ratio of pro-eutectoid ferrite It is necessary to appropriately control the average equivalent circle diameter of the A, bcc-Fe crystal grains.
- the structure contains a fine structure such as bainite or martensite
- a fine structure such as bainite or martensite
- the structure becomes fine due to the influence of bainite or martensite after spheroidizing annealing, and the structure is soft. Will not be enough.
- the total area ratio of pearlite and pro-eutectoid ferrite with respect to the entire structure needs to be 90 area% or more. Preferably it is 95 area% or more, More preferably, it is 97 area% or more.
- the metal structure other than pearlite and pro-eutectoid ferrite may include, for example, part of martensite and bainite that can be produced in the manufacturing process.
- the total area ratio of pearlite and pro-eutectoid ferrite to the entire structure is most preferably 100 area%.
- the precipitation amount of the equilibrium pro-eutectoid ferrite is expressed by (0.8 ⁇ Ceq 1 ) ⁇ 129, and Based on the idea that the ferrite fraction area ratio A should be 75% or more of the equilibrium precipitation amount, the Ae value represented by the following formula (1) is defined as the amount of proeutectoid ferrite that needs to be secured at the minimum. .
- the ferrite when measuring the area ratio A of pro-eutectoid ferrite is the meaning which does not contain the ferrite contained in a pearlite structure (only "de-eutectoid ferrite" is measured).
- the area ratio of pro-eutectoid ferrite varies depending on the component system, but is about 65% at the maximum in the chemical component composition targeted by the present invention.
- Ae (0.8 ⁇ Ceq 1 ) ⁇ 96.75 (1)
- Ceq 1 [C] + 0.1 ⁇ [Si] + 0.06 ⁇ [Mn], and [C], [Si] and [Mn] are the contents (mass%) of C, Si and Mn, respectively. ).
- the pro-eutectoid ferrite area ratio A satisfies A> Ae in relation to the Ae value represented by the above formula (1), the effect of increasing the pro-eutectoid ferrite area ratio is exhibited. It becomes.
- the area ratio A of pro-eutectoid ferrite is equal to or less than the above Ae value (that is, A ⁇ Ae)
- new fine ferrite is likely to precipitate after spheroidizing annealing, and softening is insufficient. It becomes.
- the average equivalent circle diameter of the bcc-Fe crystal grains is increased in a state where the proeutectoid ferrite area ratio A is small, regenerated pearlite is easily generated, and sufficient softening becomes difficult.
- the average equivalent circle diameter of bcc (body-centered cubic lattice) -Fe crystal grains surrounded by large-angle grain boundaries in the previous structure (hereinafter referred to as “average grain diameter of bcc-Fe crystal grains”) is set to 15 ⁇ m or more. And softening becomes possible after spheroidizing annealing. However, if the average grain size of the bcc-Fe crystal grains in the previous structure becomes too large, normal spheroidizing annealing results in a structure that increases the strength of regenerated pearlite and the like, and softening becomes difficult.
- the average particle size of the material needs to be 35 ⁇ m or less.
- the average particle size of the bcc-Fe crystal grains is preferably 18 ⁇ m or more, more preferably 20 ⁇ m or more.
- the average particle size of the bcc-Fe crystal grains is preferably 32 ⁇ m or less, more preferably 30 ⁇ m or less.
- the ferrite when measuring the average grain size of the bcc-Fe crystal grains is intended for bcc-Fe crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystal grains is larger than 15 °. This is because the effect of spheroidizing annealing is small at small-angle grain boundaries with an orientation difference of 15 ° or less.
- the bcc-Fe crystal grains surrounded by large-angle grain boundaries whose orientation difference is larger than 15 °, and the diameter when converted to a circle of the same area within the above range can be used after spheroidizing annealing. Sufficient softening can be realized.
- the “azimuth difference” is also referred to as “deviation angle” or “slope angle”, and the EBSP method (Electron Backscattering / Pattern method) may be employed to measure the azimuth difference.
- the bcc-Fe crystal grains for measuring the average grain diameter include crystal grains of pro-eutectoid ferrite and ferrite contained in the pearlite structure (this ferrite is distinguished from “pre-deposition ferrite”). From this point of view, the bcc-Fe crystal grains for measuring the average grain diameter are a concept different from “predetermined ferrite”.
- the average particle size of the bcc-Fe crystal grains may affect the generation of residual pearlite in addition to the regenerated pearlite. Therefore, by controlling the average particle size of the bcc-Fe crystal grains, Can be softened. However, if there is a portion that is partially coarse in the grain size of the previous structure, a remarkably hard portion will occur after spheroidizing annealing.
- the average of the equivalent circle diameter of the crystal grains with the largest equivalent circle diameter and the equivalent circle diameter of the crystal grains with the second largest equivalent circle diameter By setting the value (hereinafter referred to as “the coarse grain size of bcc-Fe crystal grains”) to 50 ⁇ m or less, the occurrence of partial residual pearlite and regenerated pearlite is suppressed, and the variation in hardness is suppressed. Can do.
- the coarse part grain size of the bcc-Fe crystal grains is preferably 45 ⁇ m or less, more preferably 40 ⁇ m or less.
- the present invention was made on the assumption of cold work machine structural steel, and its steel type may be of any ordinary chemical composition as cold work machine structural steel, , Si, Mn, P, S, Al, and N are preferably adjusted to an appropriate range. From these viewpoints, the appropriate ranges of these chemical components and the reasons for limiting the ranges are as follows.
- C is an element useful for securing the strength of the steel (strength of the final product).
- the C content needs to be 0.3% or more.
- it is 0.32% or more (more preferably 0.34% or more).
- it is 0.55% or less (more preferably 0.50% or less).
- Si 0.005 to 0.5%
- Si is contained as a deoxidizing element and for the purpose of increasing the strength of the final product by solid solution hardening. However, if it is less than 0.005%, such an effect is not exhibited effectively, and exceeds 0.5%. If it is contained excessively, the hardness is excessively increased and the cold workability is deteriorated.
- the Si content is preferably 0.007% or more (more preferably 0.010% or more), preferably 0.45% or less (more preferably 0.40% or less).
- Mn is an element effective for increasing the strength of the final product through improvement of hardenability, but if it is less than 0.2%, its effect is insufficient, and if it exceeds 1.5% and is contained excessively In order to increase the hardness and deteriorate the cold workability, the content is set to 0.2 to 1.5%.
- the Mn content is preferably 0.3% or more (more preferably 0.4% or more), preferably 1.1% or less (more preferably 0.9% or less).
- P 0.03% or less (excluding 0%)
- P is an element inevitably contained in the steel, but P causes grain boundary segregation in the steel and causes deterioration of ductility, so it is suppressed to 0.03% or less.
- the P content is preferably 0.028% or less (more preferably 0.025% or less).
- S 0.03% or less (excluding 0%)
- S is an element inevitably contained in steel, but is present as MnS in steel and is a harmful element that deteriorates ductility for cold working, so its content is set to 0.03% or less. There is a need.
- the S content is preferably 0.028% or less (more preferably 0.025% or less).
- Al 0.01 to 0.1%
- Al is useful as a deoxidizing element and is useful for fixing solute N present in steel as AlN.
- the Al content needs to be 0.01% or more.
- the Al content is preferably 0.013% or more (more preferably 0.015% or more), and preferably 0.090% or less (more preferably 0.080% or less).
- N 0.015% or less (excluding 0%)
- N is an element inevitably contained in the steel, but if solute N is contained in the steel, the hardness increases due to strain aging and the ductility decreases, and the cold workability is deteriorated. It is necessary to suppress to the following.
- the N content is preferably 0.013% or less, and more preferably 0.010% or less.
- the basic chemical composition of the steel for cold working machine structure of the present invention is as described above, and the balance is substantially iron.
- substantially iron can accept trace components (eg, Sb, Zn, etc.) that do not impair the properties of the steel material of the present invention in addition to iron, and inevitable impurities other than P, S, and N (For example, O, H, etc.) may also be included.
- Cr 0.5% or less (not including 0%), Cu: 0.25% or less (not including 0%), Ni: 0.25% or less (not including 0%), Mo: 0.0. 25% or less (not including 0%), and B: one or more selected from the group consisting of 0.01% or less (not including 0%)] Cr, Cu, Ni, Mo and B are all effective elements for increasing the strength of the final product by improving the hardenability of the steel material, and are contained alone or in combination of two or more as required. However, if the content of these elements is excessive, the strength becomes too high and the cold workability is deteriorated, so the respective preferable upper limits are set as described above.
- Cr is 0.45% or less (more preferably 0.40% or less)
- Cu, Ni and Mo are 0.22% or less (more preferably 0.20% or less)
- B is 0.007%. Or less (more preferably 0.005% or less).
- it is 0.015% or more (more preferably 0.020% or more) in Cr, It is 0.02% or more (more preferably 0.05% or more) for Cu, Ni and Mo, and 0.0003% or more (more preferably 0.0005% or more) for B.
- Ti, Nb and V form a compound with N and reduce the solid solution N, thereby exhibiting the effect of reducing deformation resistance. Therefore, Ti, Nb and V can be contained alone or in combination of two or more as necessary. However, when the content of these elements is excessive, the formed compound causes an increase in deformation resistance, and on the other hand, the cold workability is lowered. Therefore, Ti and Nb are 0.2% or less, and V is 0.5. % Or less is good.
- Ti and Nb are 0.18% or less (more preferably 0.15% or less), and V is 0.45% or less (more preferably 0.40% or less).
- V is 0.45% or less (more preferably 0.40% or less).
- it is 0.03% or more (more preferably 0.05% or more) in Ti and Nb
- V is 0.03% or more (more preferably 0.05% or more).
- a steel satisfying the above component composition is finish-rolled at a temperature of more than 950 ° C. and not more than 1100 ° C., and then at least 10 ° C./second. Cooling to a temperature range of 700 ° C. or more and less than 800 ° C. at an average cooling rate, followed by cooling for 100 seconds or more at an average cooling rate of 0.2 ° C./second or less (this method is referred to as “Production Method 1” below). ").
- steel that satisfies the above component composition is finish-rolled at a temperature of 1050 ° C. or more and 1200 ° C.
- an average cooling rate of 10 ° C./second or more is 700 ° C. or more and less than 800 ° C.
- an average cooling rate of 10 ° C./second or more is 700 ° C. or more and less than 800 ° C.
- Once cooled to a temperature range then cooled at an average cooling rate of 0.2 ° C / second or less for 100 seconds or more, then cooled to a temperature range of 580 to 660 ° C at an average cooling rate of 10 ° C / second or more, and It may be cooled or held for 20 seconds or longer at an average cooling rate of 1 ° C./second or less (this method is hereinafter referred to as “manufacturing method 2”). Each manufacturing condition in these manufacturing methods will be described.
- the average cooling rate needs to be 10 ° C./second or more.
- This average cooling rate is preferably 20 ° C./second or more, more preferably 30 ° C./second or more.
- the upper limit of the average cooling rate at this time is not particularly limited, it is 200 ° C./second or less as a practical range.
- the cooling at this time may be a cooling mode in which the cooling rate is changed as long as it is within the range of the average cooling rate of 10 ° C./second or more.
- the cooling stop temperature at this time is preferably 710 ° C. or higher (more preferably 720 ° C. or higher) and 780 ° C. or lower (more preferably less than 750 ° C.).
- the lower limit of the average cooling rate in this cooling is not particularly limited, but is preferably 0.01 ° C./second or more from the viewpoint of productivity.
- finish temperature of this cooling although it changes also with the chemical component composition of steel materials, finish rolling temperature, and the cooling conditions until then, it will be about 660 degreeC or less.
- Subsequent cooling may be ordinary cooling such as gas cooling or standing cooling (average cooling rate of about 0.1 to 50 ° C./second).
- the sample is cooled for 100 seconds or more at an average cooling rate of 0.2 ° C./second or less.
- cooling cooling time
- the lower limit of the average cooling rate in this cooling is not particularly limited, but is preferably 0.01 ° C./second or more from the viewpoint of productivity.
- the cooling time needs to be at least 100 seconds or more, preferably 400 seconds or more, and more preferably 500 seconds or more.
- the cooling time is preferably 2000 seconds or less (more preferably 1800 seconds or less) from the viewpoint that it can be carried out in a realistic time.
- the average particle size of the bcc-Fe crystal grains is prevented from exceeding 35 ⁇ m, and the coarse portion particle size of the bcc-Fe crystal grains is prevented from exceeding 50 ⁇ m. From the viewpoint, it is preferable to rapidly cool after the above cooling.
- the average cooling rate of this cooling needs to be at least 10 ° C./second or more. This average cooling rate is preferably 20 ° C./second or more, more preferably 30 ° C./second or more. Although the upper limit of the average cooling rate at this time is not particularly limited, it is 200 ° C./second or less as a practical range.
- the temperature at which the cooling is stopped is less than 580 ° C., and the total area ratio of pro-eutectoid ferrite and pearlite may be less than 90% by area.
- the coarse part particle size of the grains tends to exceed 50 ⁇ m.
- Subsequent cooling may be performed for 20 seconds or more at an average cooling rate of 1 ° C./second or less.
- the steel After manufacturing the steel for machine structural use for cold working as described above, the steel is subjected to normal spheroidizing annealing, so that the metal structure has an average particle size of bcc-Fe crystal grains of 15 to 35 ⁇ m.
- a steel material in which cementite in the bcc-Fe crystal grains has an aspect ratio of 2.5 or less and a K value represented by the following formula (2) is 1.3 ⁇ 10 ⁇ 2 or less is obtained. .
- K value (N ⁇ L) / E (2)
- E average equivalent circle diameter of bcc-Fe crystal grains ( ⁇ m)
- N density of cementite in the bcc-Fe crystal grains (pieces / ⁇ m 2 )
- L aspect ratio of cementite in the bcc-Fe crystal grains , Respectively.
- cementite number density on the ferrite grain boundary + cementite number density on the ferrite grain the cementite number density on the ferrite grain boundary.
- the structure before spheroidization (the grain size of the previous structure, the ferrite area ratio, etc.), the coarsening of the ferrite grains after spheroidizing annealing, It has been found that a reduction in the number of cementite and a reduction in the cementite aspect ratio in the ferrite grains are achieved, and as a result, the hardness after spheroidizing annealing is reduced as compared with the prior art, and variations are also suppressed.
- the K value represented by the above formula (2) is 1.3 ⁇ 10 ⁇ 2 or less, the effects of softening and reducing the variation in hardness are remarkably obtained.
- the layered pearlite is decomposed while being held in the (ferrite + austenite) two-phase region, and then gradually cooled immediately below the A1 transformation point in order to spheroidize the cementite, or It is assumed that the cooling is performed while holding.
- the spheroidized structure as described above is obtained.
- the average particle diameter of the bcc-Fe crystal grains in the pre-structure and the coarse grain diameter of the bcc-Fe crystal grains were measured using an EBSP analyzer and an FE-SEM (electrolytic emission scanning electron microscope).
- the “crystal grain” was defined with the boundary (large angle grain boundary) where the crystal orientation difference (oblique angle) exceeded 15 ° as the grain boundary, and the average grain size of the bcc-Fe crystal grains was determined.
- the measurement area was 400 ⁇ m ⁇ 400 ⁇ m
- the measurement step was 0.7 ⁇ m
- measurement points with a confidence index indicating the reliability of the measurement orientation were less than 0.1 were deleted from the analysis target.
- the coarse portion grain size of the bcc-Fe crystal grains of the previous structure was the average value of the maximum value and the second largest value (equivalent circle diameter) based on the above analysis results.
- the fraction of structure was determined by dividing the number of points where each structure (ferrite, pearlite, bainite, etc.) was present by the total number of points.
- the white area in the structure and the non-condensed area is the pro-eutectoid ferrite, the dark contrast area where the other shaded parts are dispersed and mixed is pearlite, and the white part is mixed in the needle shape.
- the area which is being used was determined to be bainite.
- the hardness after spheroidizing annealing was measured at 15 points with a load of 1 kgf using a Vickers hardness meter, and the average value (Hv) was obtained. Also, the standard deviation of the hardness measured at 15 points was determined. The standard of hardness at this time was determined to be acceptable if the average value satisfied the following formula (3). As the determination of the hardness variation, the sample standard deviation (unbiased standard deviation) [15 points calculated by the Excel function (STDEV)] was accepted as 5 or less.
- Ceq 2 [C] + 0.2 ⁇ [Si] + 0.2 ⁇ [Mn]
- [C] [Si] and [Mn] are the contents (mass%) of C, Si and Mn, respectively. ).
- Example 1 Steel type A shown in Table 1 above was used. Using a laboratory processing master test device, the rolling process was simulated, and the rolling finishing temperature (processing finishing temperature) and cooling conditions (average cooling rate, cooling stop temperature) were changed as shown in Table 2 below. Different samples were made respectively.
- “Cooling 1” indicates cooling from the finish rolling temperature to a temperature range of 700 ° C. or more and less than 800 ° C.
- “Cooling 2” indicates cooling after performing “Cooling 1”.
- “Cooling 3” is cooling after performing “Cooling 2”
- “Cooling 4” is cooling after performing “Cooling 3” (in the case of manufacturing method 1, “Cooling 3” and “Cooling 4” are , None).
- gas cooling (average cooling rate of 1 to 50 ° C./second) was performed, and cooling was performed to around room temperature (25 ° C.).
- the processed formaster sample had a diameter of 8.0 mm ⁇ 12.0 mm, and was divided into two equal parts after the heat treatment, which were used as a sample for examining the previous structure and a sample for spheroidizing annealing, respectively.
- each sample was vacuum-sealed, held in an atmospheric furnace at 740 ° C. for 6 hours (soaking), cooled to 710 ° C. at an average cooling rate of 10 ° C./hour, and held for 2 hours.
- a heat treatment was performed by cooling to 660 ° C. and allowing to cool at an average cooling rate of 10 ° C./hour.
- the total area ratio of pearlite + pro-eutectoid ferrite in the previous structure (ratio of P + F), the average particle diameter of the bcc-Fe crystal grains ( ⁇ average particle diameter), the pro-eutectoid ferrite area ratio A (F area ratio A), Table 3 below shows the measurement results of the coarse part particle diameter ( ⁇ coarse part particle diameter) of the bcc-Fe crystal grains and the hardness after spheroidizing annealing.
- the standard of softening in the steel type A having a C content of 0.46% is less than Hv137 based on the above formula (3).
- Test No. Nos. 1 to 4 are examples that satisfy all of the requirements defined in the present invention, and it can be seen that the hardness after spheroidizing annealing is sufficiently low and the variation in hardness can be small (standard deviation is small).
- test no. Examples 5 to 10 are examples lacking any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, test no. No. 5 is an example in which the finish rolling temperature is high, the average cooling rate in the cooling 1 is slow, and the cooling stop temperature in the cooling 3 is high.
- the average grain size of the bcc-Fe crystal grains ( ⁇ average grain size) and the coarse part particle size ( ⁇ coarse part particle size) are both large, the pro-eutectoid ferrite area ratio A (F area ratio A) is low, the hardness after spheroidizing annealing is high, and the standard deviation is also large. It is getting bigger.
- Test No. No. 6 is an example in which slow cooling (cooling 2) was not performed in the temperature range of 700 ° C. or more and less than 800 ° C. after finish rolling (relative to production method 2), and the average of bcc-Fe crystal grains
- the particle size ( ⁇ average particle size) is small and the pro-eutectoid ferrite area ratio A (F area ratio A) is also low, and the hardness after spheroidizing annealing remains high.
- Test No. 7 is an example in which the finish rolling temperature is high (relative to production method 1), the coarse part grain size ( ⁇ coarse part grain size) of the bcc-Fe crystal grains is large, and the standard deviation is also It is getting bigger.
- Test No. 8 is an example in which the finish rolling temperature is high and the cooling stop temperature in the cooling 1 is low (relative to the manufacturing method 1), the pro-eutectoid ferrite area ratio A (F area ratio A) is also low, In addition, the coarse portion grain size ( ⁇ coarse portion grain size) of the bcc-Fe crystal grains is large, and the standard deviation of the hardness after spheroidizing annealing is large.
- Test No. No. 9 is an example in which the average cooling rate in “cooling 2” is fast and the cooling time is short, the pro-eutectoid ferrite area ratio A is low, and the hardness after spheroidizing annealing remains high.
- Test No. No. 10 is an example in which the average cooling rate in “Cooling 2” is high and the cooling stop temperature in “Cooling 3” is low, and the total area ratio of pearlite and pro-eutectoid ferrite (ratio of P + F) due to precipitation of bainite. It is less than 90 area%, and the hardness after spheroidizing annealing is high.
- Example 2 Using the steel types B to L shown in Table 1 above, the production conditions (finish rolling temperature, average cooling rate, cooling stop temperature, cooling time) were changed as shown in Table 4 below, and samples with different front structures ( ⁇ 17 mm Wire) was produced.
- “Cooling 1” to “Cooling 4” are the same as those in Example 1.
- the processed formaster sample had a diameter of 17.0 mm ⁇ 15.0 mm, and was divided into two equal parts after the heat treatment, which were used as a pre-structure inspection sample and a sample for spheroidizing annealing, respectively.
- each sample was vacuum-sealed, held in an atmospheric furnace at 740 ° C. for 6 hours (soaking), cooled to 710 ° C. at an average cooling rate of 10 ° C./hour, and held for 2 hours.
- a heat treatment was performed by cooling to 660 ° C. and allowing to cool at an average cooling rate of 10 ° C./hour.
- Total area ratio of pearlite + pro-eutectoid ferrite (ratio of P + F) before spheroidizing annealing (pre-structure), average particle diameter ( ⁇ -average particle diameter) of bcc-Fe crystal grains, pro-eutectoid ferrite area ratio A (F area ratio)
- the coarse part particle size ( ⁇ coarse part particle size) of A) and bcc-Fe crystal grains was measured, and the hardness after spheroidizing annealing was measured as described above.
- Test No. Nos. 11 to 20 are examples that satisfy all of the requirements defined in the present invention, and it can be seen that the hardness after spheroidizing annealing is sufficiently low and the variation in hardness can be reduced.
- test no. 21 to 26 lack any of the requirements defined in the present invention, and any of the characteristics is deteriorated. That is, test no. No. 21 is an example in which the finish rolling temperature is low, the average particle size ( ⁇ average particle size) of the bcc-Fe crystal grains is small, and the hardness after spheroidizing annealing is high. Test No. No.
- cooling stop temperature in “cooling 1” is high (relative to production method 2), and the pro-eutectoid ferrite area ratio A (F area ratio A) decreases and bcc ⁇
- the coarse part particle size ( ⁇ coarse part particle size) of the Fe crystal grains is large, the hardness after spheroidizing annealing is high, and the standard deviation is also large.
- Test No. 23 is an example in which the cooling time in “cooling 2” is short, the proeutectoid ferrite area ratio A (F area ratio A) is low, and the hardness after spheroidizing annealing is high.
- Test No. No. 24 is an example in which the finish rolling temperature is high, the average cooling rate at “cooling 2” is high, and the average cooling rate at “cooling 3” is low (relative to production method 2).
- Ferrite area ratio A (F area ratio A) is low, and the coarse part grain size ( ⁇ coarse part grain size) of the bcc-Fe crystal grains is large, the hardness after spheroidizing annealing is high, and the standard The deviation is also increasing.
- Test No. No. 25 is an example in which the average cooling rate in “cooling 3” is slow, the average particle size ( ⁇ average particle size) of the bcc-Fe crystal grains is small, and the hardness after spheroidizing annealing is low It is high.
- Test No. No. 26 is an example using a steel type L having a large Cr content, and although appropriate production conditions were adopted, the pro-eutectoid ferrite area ratio A (F area ratio A) was also low, and the precipitation of martensite The total area ratio of pearlite and pro-eutectoid ferrite (ratio of P + F) is less than 90 area%, and the hardness after spheroidizing annealing is high.
- Example 3 Test No. above. Samples shown in Table 6 below were newly prepared from 1 to 26, and spheroidizing annealing was performed. At this time, in spheroidizing annealing, each sample was vacuum-sealed, held in an atmospheric furnace at 740 ° C. for 4 hours (soaking), then cooled to 720 ° C. at an average cooling rate of 10 ° C./hour, and then an average cooling rate of 2 A heat treatment was performed by cooling to 710 ° C. at 5 ° C./hour, and then cooling to 660 ° C. at an average cooling rate of 10 ° C./hour. The test No. shown in Table 6 Is the test No. shown in Examples 1 and 2. (Manufacturing conditions until spheroidizing annealing are the same as above).
- the average particle diameter ( ⁇ average particle diameter) of the bcc-Fe crystal grains after spheroidizing annealing, the aspect ratio of cementite in the bcc-Fe crystal grains, the number density of cementite in the bcc-Fe crystal grains, and the K value are as follows: The hardness after spheroidizing annealing was measured in the manner described above.
- FIG. 1 An example of the structure is shown in FIG.
- the cementite in contact with the bcc-Fe crystal grain boundaries was deleted (filled in black) by image processing. Note that the cementite whose longitudinal direction extended into the grains even though it was in contact with the bcc-Fe grain boundary was counted as cementite within the grains.
- the criterion is that cementite whose angle between the major axis of cementite and the tangential direction of the grain boundary is 20 ° or more and whose major axis is 3 ⁇ m or more exists in the grain even though it is in contact with the grain boundary. It was.
- an image analysis apparatus Media Cybernetics: Image-Pro Plus was used to measure the aspect ratio of cementite in the bcc-Fe crystal grains and the number of cementite in the bcc-Fe crystal grains. Density was measured.
- the average particle diameter of the bcc-Fe crystal grains after spheroidizing annealing was measured using an EBSP analyzer and an FE-SEM (electrolytic emission scanning electron microscope).
- a “crystal grain” was defined with a boundary (large-angle grain boundary) where the crystal orientation difference (oblique angle) exceeded 15 ° as a grain boundary, and the average grain size ( ⁇ -average grain size) of bcc-Fe crystal grains was determined.
- the measurement area was 400 ⁇ m ⁇ 400 ⁇ m
- the measurement step was 0.7 ⁇ m
- measurement points with a confidence index indicating the reliability of the measurement orientation were less than 0.1 were deleted from the analysis target.
- Test No. Examples 1-3, 11, 12, 14, 17-20 are examples that satisfy all of the requirements stipulated in the present invention.
- the ⁇ particle size after spheroidizing annealing is small, and the aspect ratio of cementite is also small. It can be seen that the hardness after spheroidizing annealing is sufficiently low, and the variation in hardness after spheroidizing annealing can be reduced.
- test no. Nos. 5, 7, and 21 to 25 lack any of the requirements defined in the present invention, and show the following tendency after spheroidizing annealing. That is, test no. No. 5, as a result of spheroidizing annealing of a sample having a large previous structure ⁇ average particle size and a previous structure ⁇ coarse portion particle size and a small previous structure F area ratio, the ⁇ average particle size after spheroidizing annealing is increased. Moreover, the aspect ratio of cementite is large, the hardness after spheroidizing annealing is high, and the standard deviation of the hardness after spheroidizing annealing is also large.
- Test No. No. 7 is an example in which the cementite has a large aspect ratio after spheroidizing annealing and a large K value as a result of spheroidizing annealing of a sample having a large grain size in the previous structure ⁇ . The standard deviation is large.
- Test No. 21, no. 25 is an example in which the ⁇ average particle size after spheroidizing annealing is small and the K value is large as a result of spheroidizing annealing of a sample having a small previous structure ⁇ average particle size, and the hardness after spheroidizing annealing is Is high.
- Test No. 22, no. No. 24 is a result of spheroidizing annealing of a sample having a small F area ratio of the front structure and a large front structure ⁇ coarse portion particle size, and as a result, the aspect ratio of cementite after spheroidizing annealing is increased, and the K value is further increased.
- the hardness after spheroidizing annealing is high, and the standard deviation of hardness is also large.
- Test No. No. 23 is an example in which the K value after spheroidizing annealing is increased as a result of spheroidizing annealing of a sample with a small F area ratio of the previous structure, and the hardness after spheroidizing annealing is high.
- the total area ratio of pearlite and pro-eutectoid ferrite with respect to the entire structure is defined together with the chemical composition, and the area ratio A of pro-eutectoid ferrite is expressed as A> Ae in relation to the Ae value represented by a predetermined relational expression.
- the hardness can be sufficiently lowered even when normal spheroidizing annealing is performed.
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Abstract
Description
Ae=(0.8-Ceq1)×96.75 …(1)
但し、Ceq1=[C]+0.1×[Si]+0.06×[Mn]であり、[C],[Si]および[Mn]は、夫々C,SiおよびMnの含有量(質量%)を示す。
K値=(N×L)/E …(2)
但し、E:bcc-Fe結晶粒の平均円相当直径(μm)、N:bcc-Fe結晶粒内のセメンタイト数密度(個/μm2)、L:bcc-Fe結晶粒内のセメンタイトのアスペクト比、を夫々示す。
Ae=(0.8-Ceq1)×96.75 …(1)
但し、Ceq1=[C]+0.1×[Si]+0.06×[Mn]であり、[C],[Si]および[Mn]は、夫々C,SiおよびMnの含有量(質量%)を示す。
Cは、鋼の強度(最終製品の強度)を確保する上で有用な元素である。こうした効果を有効に発揮させるためには、C含有量は0.3%以上とする必要がある。好ましくは0.32%以上(より好ましくは0.34%以上)とするのが良い。しかしながら、Cが過剰に含有されると強度が高くなって、冷間加工性が低下するので0.6%以下とする必要がある。好ましくは、0.55%以下(より好ましくは0.50%以下)とするのが良い。
Siは、脱酸元素として、および固溶体硬化による最終製品の強度を増加させることを目的として含有させるが、0.005%未満ではこうした効果が有効に発揮されず、また0.5%を超えて過剰に含有されると硬度が過度に上昇して冷間加工性を劣化させることになる。尚、Si含有量は、好ましくは0.007%以上(より好ましくは0.010%以上)であり、好ましくは0.45%以下(より好ましくは0.40%以下)である。
Mnは、焼入れ性の向上を通じて、最終製品の強度を増加させるのに有効な元素であるが、0.2%未満ではその効果が不十分であり、1.5%を超えて過剰に含有すると硬度が上昇して冷間加工性を劣化させるため、0.2~1.5%とした。尚、Mn含有量は、好ましくは0.3%以上(より好ましくは0.4%以上)であり、好ましくは1.1%以下(より好ましくは0.9%以下)である。
Pは、鋼中に不可避的に含まれる元素であるが、Pは鋼中で粒界偏析を起こし、延性の劣化の原因となるので、0.03%以下に抑制する。P含有量は、好ましくは0.028%以下(より好ましくは0.025%以下)である。
Sは、鋼中に不可避的に含まれる元素であるが、鋼中でMnSとして存在し、冷間加工にとって延性を劣化させる有害な元素であるので、その含有量を0.03%以下とする必要がある。S含有量は、好ましくは0.028%以下(より好ましくは0.025%以下)である。
Alは、脱酸元素として有用であると共に、鋼中に存在する固溶NをAlNとして固定するのに有用である。こうした効果を有効に発揮させるためには、Al含有量は0.01%以上とする必要がある。しかしながら、Al含有量が過剰になって0.1%を超えると、Al2O3が過剰に生成し、冷間加工性を劣化させる。尚、Al含有量は、好ましくは0.013%以上(より好ましくは0.015%以上)であり、好ましくは0.090%以下(より好ましくは0.080%以下)である。
Nは、鋼中に不可避的に含まれる元素であるが、鋼中に固溶Nが含まれると、歪み時効による硬度上昇、延性低下を招き、冷間加工性を劣化させるため0.015%以下に抑制する必要がある。N含有量は、好ましくは0.013%以下であり、より好ましくは0.010%以下である。
Cr、Cu、Ni、MoおよびBは、いずれも鋼材の焼入れ性を向上させることによって最終製品の強度を増加させるのに有効な元素であり、必要によって単独でまたは2種以上で含有される。しかしながら、これらの元素の含有量が過剰になると、強度が高くなり過ぎ、冷間加工性を劣化させるので、上記のように夫々の好ましい上限を定めた。より好ましくはCrで0.45%以下(更に好ましくは0.40%以下)、Cu,NiおよびMoで0.22%以下(更に好ましくは0.20%以下)、およびBで0.007%以下(更に好ましくは0.005%以下)である。尚、これらの元素による効果はその含有量が増加するにつれて大きくなるが、それらの効果を有効に発揮させるため、好ましくは、Crで0.015%以上(より好ましくは0.020%以上)、Cu,NiおよびMoで0.02%以上(より好ましくは0.05%以上)、およびBで0.0003%以上(より好ましくは0.0005%以上)である。
Ti,NbおよびVは、Nと化合物を形成し、固溶Nを低減することで、変形抵抗低減の効果を発揮するため、必要によって単独でまたは2種以上を含有させることができる。しかしながら、これらの元素の含有量が過剰になると、形成される化合物が変形抵抗の上昇を招き、却って冷間加工性を低下させるので、TiおよびNbで0.2%以下、Vで0.5%以下とするのが良い。より好ましくはTiおよびNbで0.18%以下(更に好ましくは0.15%以下)、およびVで0.45%以下(更に好ましくは0.40%以下)である。尚、これらの元素による効果はその含有量が増加するにつれて大きくなるが、その効果を有効に発揮させるため、好ましくは、TiおよびNbで0.03%以上(より好ましくは0.05%以上)、およびVで0.03%以上(より好ましくは0.05%以上)である。
大角粒界で囲まれたbcc-Fe結晶粒の平均粒径を15~35μmに制御するためには、仕上げ圧延温度を適切に制御する必要がある。この仕上げ圧延温度が1100℃を超えると、平均粒径を35μm以下にすることが困難となる。また、仕上げ圧延温度が1100℃を超えると、bcc-Fe結晶粒の粗大部粒径が50μmを超え易くなる。但し、仕上げ圧延温度が950℃以下となると、bcc-Fe結晶粒の平均粒径を15μm以上にすることが困難となるので、950℃超とする必要がある。
この製造方法2を採用するときの仕上げ圧延温度が1200℃を超えると、bcc-Fe結晶粒の平均粒径を35μm以下にすることが困難となる。また、仕上げ圧延温度が1200℃を超えると、bcc-Fe結晶粒の粗大部粒径が50μmを超え易くなる。但し、仕上げ圧延温度が1050℃未満となると、bcc-Fe結晶粒の平均粒径を15μm以上にすることが困難となるので、1050℃以上とする必要がある。
K値=(N×L)/E …(2)
但し、E:bcc-Fe結晶粒の平均円相当直径(μm)、N:bcc-Fe結晶粒内のセメンタイト数密度(個/μm2)、L:bcc-Fe結晶粒内のセメンタイトのアスペクト比、を夫々示す。
得られた各線材(圧延材)の組織因子(組織、bcc-Fe結晶粒の平均粒径、およびbcc-Fe結晶粒の粗大部粒径)、および球状化焼鈍後の硬さの測定に当たって、各線材、各ラボ試験片材、共に縦断面が観察できるように樹脂埋めし、線材の半径Dに対し、D/4の位置を測定した。
前組織のbcc-Fe結晶粒の平均粒径、およびbcc-Fe結晶粒の粗大部粒径は、EBSP解析装置およびFE-SEM(電解放出型走査電子顕微鏡)を用いて測定した。結晶方位差(斜角)が15°を超える境界(大角粒界)を結晶粒界として「結晶粒」を定義し、bcc-Fe結晶粒の平均粒径を決定した。このときの測定領域は400μm×400μm、測定ステップは0.7μm間隔とし、測定方位の信頼性を示すコンフィデンス・インデックス(Confidence Index)が0.1未満の測定点は解析対象から削除した。また前組織のbcc-Fe結晶粒の粗大部粒径は、上記解析結果に基づき、最大値および2番目に大きい値(円相当直径)の平均値とした。
パーライト+初析フェライトの合計面積率(P+Fの割合)、初析フェライト面積率A(F面積率A)の測定においては、ナイタールエッチングによって組織を現出させ、光学顕微鏡にて組織観察を行い、倍率400倍にて10視野を撮影した。それらの写真を元に、画像解析によって、パーライト+初析フェライトの合計面積率(P+Fの割合)、初析フェライト面積率A(F面積率A)を判定した。組織解析は、上記各写真について、ランダムに100点選び、各点の組織を判別した。各組織(フェライト、パーライト、ベイナイト、その他)が存在した点数を全点数で割ることで組織分率を求めた。尚、組織解析に当たっては、組織内が白く、濃淡の無い領域を初析フェライトとし、その他の濃淡のある部分が分散して混在している暗いコントラストの領域をパーライト、白い部分が針状に混在している領域をベイナイトと判定した。
球状化焼鈍後の硬さの測定は、ビッカース硬度計を用いて、荷重1kgfで15点測定し、その平均値(Hv)を求めた。また、15点測定した硬さの標準偏差も求めた。このときの硬さの基準は、平均値で下記(3)式を満足するものを合格と判断した。硬さのばらつきの判定として、標本標準偏差(不偏標準偏差)[15点をエクセルの関数(STDEV)によって算出]が5以内を合格とした。
Hv<88.4×Ceq2+80.0 …(3)
但し、Ceq2=[C]+0.2×[Si]+0.2×[Mn]であり、[C],[Si]および[Mn]は、夫々C,SiおよびMnの含有量(質量%)を示す。
上記表1に示した鋼種Aを用いた。ラボの加工フォーマスタ試験装置を用いて圧延工程を模擬し、圧延仕上げ温度(加工仕上げ温度)、冷却条件(平均冷却速度、冷却停止温度)を下記表2のように変化させて、前組織の異なるサンプルを夫々作製した。尚、表2の製造条件において、「冷却1」は仕上げ圧延温度から700℃以上、800℃未満の温度範囲までの冷却を示し、「冷却2」は「冷却1」を行った後の冷却を示し、「冷却3」は「冷却2」を行った後の冷却、「冷却4」は「冷却3」を行った後の冷却(製造方法1の場合、「冷却3」および「冷却4」は、無し)を夫々示している。尚、表2に示した条件終了後は、ガス冷却(平均冷却速度1~50℃/秒)し、室温(25℃)付近まで冷却した。
上記表1に示した鋼種B~Lを用い、製造条件(仕上げ圧延温度、平均冷却速度、冷却停止温度、冷却時間)を下記表4のように変化させて、前組織の異なるサンプル(φ17mmの線材)を作製した。尚、表4の製造条件において、「冷却1」~「冷却4」は、実施例1と同じである。このとき、加工フォーマスタサンプルは、φ17.0mm×15.0mmとし、熱処理終了後に2等分し、夫々前組織調査用サンプル、および球状化焼鈍用のサンプルとした。また球状化焼鈍は、サンプルを夫々真空封入し、大気炉にて、740℃×6時間保持(均熱)後、平均冷却速度10℃/時で710℃まで冷却して2時間保持し、その後平均冷却速度10℃/時で660℃まで冷却して放冷する熱処理を行った。
上記試験No.1~26のうち、下記表6に示すサンプルを新たに作製し、球状化焼鈍を実施した。このとき球状化焼鈍は、サンプルを夫々真空封入し、大気炉にて、740℃×4時間保持(均熱)後、平均冷却速度10℃/時で720℃まで冷却し、その後平均冷却速度2.5℃/時で710℃まで冷却、その後平均冷却速度10℃/時で660℃まで冷却して放冷する熱処理を行った。尚、表6に示した試験No.は、実施例1、2に示した試験No.に対応するものである(球状化焼鈍までの製造条件等は上記と同じ)。
球状化焼鈍を施した各試験片(サンプル)について、下記に示す手順で金属組織因子の測定を行った。球状化焼鈍後の各試験片を、樹脂に埋め込んでからエメリー紙、ダイヤモンドバフ、電解研磨によって切断面を鏡面研磨した。その後ナイタールでエッチングした後、試験片の鏡面研磨面をFE-SEM(電界放射型走査電子顕微鏡)で観察・画像撮影した。このときの観察倍率は、組織サイズに応じて2000~4000倍とした。任意の10箇所で観察を行い、各観察箇所の組織を撮影した。
球状化焼鈍後のbcc-Fe結晶粒の平均粒径の測定は、EBSP解析装置およびFE-SEM(電解放出型走査電子顕微鏡)を用いて測定した。結晶方位差(斜角)が15°を超える境界(大角粒界)を結晶粒界として「結晶粒」を定義し、bcc-Fe結晶粒の平均粒径(α平均粒径)を決定した。このときの測定領域は400μm×400μm、測定ステップは0.7μm間隔とし、測定方位の信頼性を示すコンフィデンス・インデックス(Confidence Index)が0.1未満の測定点は解析対象から削除した。
本出願は、2011年12月19日出願の日本特許出願(特願2011-277683)及び2012年3月26日出願の日本特許出願(特願2012-070365)に基づくものであり、その内容はここに参照として取り込まれる。
Claims (7)
- C :0.3~0.6質量%、
Si:0.005~0.5質量%、
Mn:0.2~1.5質量%、
P :0.03質量%以下(0質量%を含まない)、
S :0.03質量%以下(0質量%を含まない)、
Al:0.01~0.1質量%、および
N:0.015質量%以下(0質量%を含まない)を夫々含有し、
残部が鉄および不可避不純物からなり、
鋼の金属組織が、パーライトと初析フェライトを有し、全組織に対するパーライトと初析フェライトの合計面積率が90面積%以上であると共に、初析フェライトの面積率Aが、下記(1)式で表されるAe値との関係でA>Aeを満足し、
且つ隣り合う2つの結晶粒の方位差が15°よりも大きい大角粒界で囲まれたbcc-Fe結晶粒の平均円相当直径が15~35μmであると共に、前記bcc-Fe結晶粒の円相当直径で、最大の粒径と2番目に大きい粒径との平均値が50μm以下であることを特徴とする冷間加工用機械構造用鋼。
Ae=(0.8-Ceq1)×96.75 …(1)
但し、Ceq1=[C]+0.1×[Si]+0.06×[Mn]であり、[C],[Si]および[Mn]は、夫々C,SiおよびMnの含有量(質量%)を示す。 - 更に他の元素として、
Cr:0.5質量%以下(0質量%を含まない)、
Cu:0.25質量%以下(0質量%を含まない)、
Ni:0.25質量%以下(0質量%を含まない)、
Mo:0.25質量%以下(0質量%を含まない)、および
B :0.01質量%以下(0質量%を含まない)よりなる群から選択される1種以上を含有するものである請求項1に記載の冷間加工用機械構造用鋼。 - 更に他の元素として、
Ti:0.2質量%以下(0質量%を含まない)、
Nb:0.2質量%以下(0質量%を含まない)、および
V:0.5質量%以下(0質量%を含まない)よりなる群から選択される1種以上を含有するものである請求項1に記載の冷間加工用機械構造用鋼。 - 更に他の元素として、
Ti:0.2質量%以下(0質量%を含まない)、
Nb:0.2質量%以下(0質量%を含まない)、および
V:0.5質量%以下(0質量%を含まない)よりなる群から選択される1種以上を含有するものである請求項2に記載の冷間加工用機械構造用鋼。 - 請求項1~4のいずれか一項に記載の冷間加工用機械構造用鋼を製造する方法であって、
950℃超、1100℃以下の温度で仕上げ圧延する工程、
10℃/秒以上の平均冷却速度で700℃以上、800℃未満の温度範囲まで冷却する工程、
0.2℃/秒以下の平均冷却速度で100秒以上冷却する工程
をこの順番で含むことを特徴とする冷間加工用機械構造用鋼の製造方法。 - 請求項1~4のいずれか一項に記載の冷間加工用機械構造用鋼を製造する方法であって、
1050℃以上、1200℃以下の温度で仕上げ圧延する工程、
10℃/秒以上の平均冷却速度で700℃以上、800℃未満の温度範囲まで冷却する工程、
0.2℃/秒以下の平均冷却速度で100秒以上冷却する工程、
10℃/秒以上の平均冷却速度で580~660℃の温度範囲まで冷却する工程、
1℃/秒以下の平均冷却速度で20秒以上冷却または保持する工程
をこの順番で含むことを特徴とする冷間加工用機械構造用鋼の製造方法。 - 請求項1~4のいずれか一項に記載の化学成分組成を有し、金属組織が、bcc-Fe結晶粒の平均円相当直径が15~35μmであると共に、bcc-Fe結晶粒内のセメンタイトが、アスペクト比で2.5以下であり、且つ下記(2)式で表されるK値が1.3×10-2以下であることを特徴とする冷間加工用機械構造用鋼。
K値=(N×L)/E …(2)
但し、E:bcc-Fe結晶粒の平均円相当直径(μm)、N:bcc-Fe結晶粒内のセメンタイト数密度(個/μm2)、L:bcc-Fe結晶粒内のセメンタイトのアスペクト比、を夫々示す。
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- 2012-12-11 MX MX2014007333A patent/MX2014007333A/es unknown
- 2012-12-11 US US14/363,199 patent/US9890445B2/en not_active Expired - Fee Related
- 2012-12-11 KR KR1020147016847A patent/KR101598314B1/ko active IP Right Grant
- 2012-12-11 WO PCT/JP2012/082063 patent/WO2013094475A1/ja active Application Filing
- 2012-12-11 EP EP12859127.8A patent/EP2796586A4/en not_active Withdrawn
- 2012-12-11 CN CN201280062956.3A patent/CN104011249B/zh not_active Expired - Fee Related
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Cited By (9)
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WO2015189978A1 (ja) * | 2014-06-13 | 2015-12-17 | 新日鐵住金株式会社 | 冷間鍛造用鋼材 |
JPWO2015189978A1 (ja) * | 2014-06-13 | 2017-04-27 | 新日鐵住金株式会社 | 冷間鍛造用鋼材 |
US10533242B2 (en) | 2014-06-13 | 2020-01-14 | Nippon Steel Corporation | Steel for cold forging |
CN104235366A (zh) * | 2014-09-11 | 2014-12-24 | 浙江新东方汽车零部件有限公司 | 一种商用车专用垫片生产工艺 |
CN104235366B (zh) * | 2014-09-11 | 2017-03-08 | 浙江鼎盛汽车紧固件有限公司 | 一种商用车专用垫片生产工艺 |
EP3211106A4 (en) * | 2014-10-20 | 2018-04-11 | Nippon Steel & Sumitomo Metal Corporation | Steel wire for bearing with excellent wire drawability and coil formability after wiredrawing |
US10287660B2 (en) | 2014-10-20 | 2019-05-14 | Nippon Steel & Sumitomo Metal Corporation | Steel wire rod for bearings having excellent drawability and coil formability after drawing |
CN115821167A (zh) * | 2022-12-01 | 2023-03-21 | 宁波祥路中天新材料科技股份有限公司 | 一种超高强鞍座板及其制造方法 |
CN115821167B (zh) * | 2022-12-01 | 2024-02-02 | 宁波祥路中天新材料科技股份有限公司 | 一种超高强鞍座板及其制造方法 |
Also Published As
Publication number | Publication date |
---|---|
TW201341538A (zh) | 2013-10-16 |
KR20140095099A (ko) | 2014-07-31 |
JP2013147728A (ja) | 2013-08-01 |
US20140326369A1 (en) | 2014-11-06 |
EP2796586A4 (en) | 2015-12-02 |
KR101598314B1 (ko) | 2016-02-26 |
US9890445B2 (en) | 2018-02-13 |
CN104011249A (zh) | 2014-08-27 |
CN104011249B (zh) | 2016-04-06 |
EP2796586A1 (en) | 2014-10-29 |
JP5357994B2 (ja) | 2013-12-04 |
TWI486455B (zh) | 2015-06-01 |
MX2014007333A (es) | 2015-01-26 |
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