WO2013161538A1 - 冷間加工用機械構造用鋼及びその製造方法 - Google Patents
冷間加工用機械構造用鋼及びその製造方法 Download PDFInfo
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/26—Methods of annealing
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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- 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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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- 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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- C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
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- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
Definitions
- the present invention relates to a steel for machine structural use for cold working used for the manufacture of various parts such as automobile parts and construction machine parts, in particular, a steel material having low deformation resistance after spheroidizing annealing and excellent cold workability, And a manufacturing method thereof. More specifically, the present invention relates to various parts (for example, bolts, screws, etc.) such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold forging, and cold rolling.
- spheroidizing annealing treatment is applied to the hot rolled material such as carbon steel, alloy steel, etc., and then cold-treated. Processing is performed, and then a cutting process or the like is performed to form a predetermined shape, followed by quenching and tempering, and final strength adjustment is performed.
- Patent Document 1 a steel wire material having pro-eutectoid ferrite and pearlite, an average crystal grain size of 6 to 15 ⁇ m, and a volume fraction of pro-eutectoid ferrite within a predetermined range is a rapid spheroidizing annealing treatment. It is disclosed that both cold forgeability can be achieved. However, when the microstructure is made fine, the spheroidizing annealing time can be shortened, but the material is not sufficiently softened when the normal spheroidizing annealing process is performed for about 10 to 30 hours.
- Patent Document 2 discloses a technique for softening while maintaining hot rolling by specifying the size of dislocation cells and the ferrite grain size number. However, this technique is still insufficient for further softening.
- JP 2000-119809 A Japanese Patent No. 3474545
- the present invention has been made under such circumstances, and an object thereof is to provide a steel for machine work for cold working which can realize sufficient softening by performing a normal spheroidizing annealing process and a method for producing the same. There is to do.
- the present invention is particularly directed to an alloy steel containing an alloy element such as Cr.
- the present invention that has achieved the above object has the following contents: C: 0.2 to 0.6% (meaning mass%; hereinafter the same for the chemical composition), Si: 0.01 to 0.5%, Mn: 0.2 -1.5%, P: 0.03% or less (excluding 0%), S: 0.001-0.05%, Al: 0.01-0.1%, N: 0.015% or less (Not including 0%), Cr: more than 0.5%, 2.0% or less, the balance is iron and inevitable impurities, the metal structure has pearlite and proeutectoid ferrite, the entire structure The total area ratio of pearlite and pro-eutectoid ferrite is 90% or more, and the area ratio A of pro-eutectoid ferrite has a relationship of Ae represented by the following formula (1) and A> Ae.
- the steel for machine structural use for cold working according to the present invention may further include Mo: 1% or less (not including 0%), Ni: 3% or less (not including 0%), Cu: 0.25% as necessary. Or less (excluding 0%), B: 0.010% or less (not including 0%), Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (0% 1) or more selected from the group consisting of V: 0.5% or less (not including 0%).
- the present invention also includes a method for producing the cold-working machine structural steel, specifically, after finishing and rolling a steel having any one of the above chemical composition at 850 to 1100 ° C., (I) Cool to 720 to 780 ° C. at an average cooling rate of 10 ° C./second or more, (Ii) Then, it is cooled to 680 ° C. or higher at an average cooling rate of 1 ° C./second or less, (Iii) A method for producing steel for cold-working machine structure, wherein the steel is further cooled to 640 ° C. or less at an average cooling rate of 0.5 ° C./second or less.
- various components are appropriately adjusted, and a structure having 90% by area or more of pearlite and pro-eutectoid ferrite is formed.
- ferrite pro-eutectoid ferrite and ferrite in pearlite crystal grain size and area of pro-eutectoid ferrite Since the rate is within a predetermined range, softening after spheroidizing annealing can be realized, and steel for machine structure suitable for cold working can be provided.
- the steel material of the present invention is (i) a structure having pearlite and pro-eutectoid ferrite, and the total area ratio of pearlite and pro-eutectoid ferrite with respect to the entire structure is 90% or more, (ii) area ratio of pro-eutectoid ferrite Is characterized by exceeding 75% of the amount of equilibrium pro-eutectoid ferrite and (iii) the average particle size of pro-eutectoid ferrite and ferrite in pearlite being 15 to 25 ⁇ m.
- the metal structure is a structure having pearlite and pro-eutectoid ferrite, and the total area ratio of these structures with respect to the entire structure.
- the metal structure includes a fine structure such as bainite and martensite, Even if spheroidizing annealing is performed, after spheroidizing annealing, the structure becomes fine due to the influence of bainite and martensite, and softening becomes insufficient. Therefore, the metal structure was a structure having pearlite and pro-eutectoid ferrite, and the total area ratio of these was determined to be 90 area% or more.
- the total area ratio of pearlite and pro-eutectoid ferrite is preferably 95 area% or more, more preferably 97 area% or more.
- examples of the metal structure other than pearlite and proeutectoid ferrite include martensite and bainite that can be generated in the manufacturing process, but as the area ratio of these structures increases, the strength increases and the cold workability deteriorates. Therefore, it is preferable that it is not contained as much as possible. Therefore, the total area ratio of pearlite and pro-eutectoid ferrite is most preferably 100 area%.
- (Ii) Area ratio of pro-eutectoid ferrite In the present invention, by securing as much area ratio of pro-eutectoid ferrite before spheroidizing annealing as possible, cementite is localized in advance before spheroidizing annealing. Softening can be realized by promoting spheroidization of cementite by annealing.
- the present inventors have studied from the viewpoint of precipitating the pro-eutectoid ferrite to an equilibrium amount, and based on experiments, it has been clarified that the equilibrium pro-eutectoid ferrite amount is represented by (0.8 ⁇ Ceq) ⁇ 129.
- the area ratio A of the pro-eutectoid ferrite in the present invention has a relationship of Ae represented by the following formula (1) and A> Ae.
- Ceq [C] + 0.1 ⁇ [Si] + 0.06 ⁇ [Mn] + 0.11 ⁇ [Cr], where [(element name)] is the content of each element ( Mass%).
- the area ratio A (%) of the pro-eutectoid ferrite is preferably A (%) ⁇ Ae (%) + 0.5 (%), more preferably A (%) ⁇ Ae (%) + 1.0 (%).
- the relationship of A (%) ⁇ Ae (%) + 1.5 (%) is satisfied.
- the area ratio A (%) may satisfy, for example, a relationship of A (%) ⁇ Ae (%) + 5 (%), particularly A (%) ⁇ Ae (%) + 3 (%).
- the average particle diameter of pro-eutectoid ferrite and ferrite in pearlite is 15 ⁇ m or more. By doing in this way, softening after spheroidizing annealing becomes possible. On the other hand, if the average particle size becomes too large, the strength of regenerated pearlite and the like is increased by normal spheroidizing annealing, and softening becomes difficult. Therefore, the average particle size of pro-eutectoid ferrite and ferrite in pearlite is 25 ⁇ m or less.
- the lower limit of the average particle diameter is preferably 16 ⁇ m or more, more preferably 17 ⁇ m or more, and the preferable upper limit is 23 ⁇ m or less, more preferably 21 ⁇ m or less.
- ferrite primary ferrite and ferrite in pearlite crystal grains
- bcc-Fe crystal grains ferrite crystal grains surrounded by a large-angle grain boundary in which the orientation difference between two adjacent crystal grains is larger than 15 °.
- the above average particle diameter means an average value of diameters (equivalent circle diameters) when converted into circles having the same area.
- the azimuth difference is called a “deviation angle” or “bevel angle”, and an EBSP method (Electron Backscattering Pattern Method) may be employed to measure the azimuth difference.
- C 0.2 to 0.6%
- C is an element useful for securing the strength of the steel (strength of the final product). In order to effectively exhibit such effects, the C content is set to 0.2% or more.
- the amount of C is preferably 0.25% or more, more preferably 0.30% or more.
- the C amount is set to 0.6% or less.
- the amount of C is preferably 0.55% or less, more preferably 0.50% or less.
- Si 0.01 to 0.5% Si is an element that has a deoxidizing action and is effective for improving the strength of the final product by solid solution hardening. In order to effectively exhibit such an action, the Si amount was determined to be 0.01% or more.
- the amount of Si is preferably 0.02% or more, more preferably 0.03% or more (particularly 0.05% or more).
- the Si amount is set to 0.5% or less.
- the amount of Si is preferably 0.45% or less, and more preferably 0.40% or less.
- Mn 0.2 to 1.5%
- Mn is an effective element for increasing the strength of the final product through improvement of hardenability.
- the amount of Mn was determined to be 0.2% or more.
- the amount of Mn is preferably 0.3% or more, and more preferably 0.4% or more.
- the amount of Mn is set to 1.5% or less.
- the amount of Mn is 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, and is an element that causes grain boundary segregation in the steel and causes deterioration of ductility. Therefore, the P content is suppressed to 0.03% or less.
- the amount of P is preferably 0.02% or less, and more preferably 0.015% or less. The smaller the amount of P, the better. However, about 0.001% is usually included due to restrictions on the manufacturing process.
- S 0.001 to 0.05%
- S is an element inevitably contained in the steel, is present as MnS in the steel, and is an element harmful to cold working because it deteriorates ductility. Therefore, the S amount is suppressed to 0.05% or less.
- the amount of S is preferably 0.04% or less, and more preferably 0.03% or less. However, since S has the effect of improving machinability, it is useful to contain 0.001% or more.
- the amount of S is preferably 0.002% or more, and more preferably 0.003% or more.
- Al 0.01 to 0.1%
- Al is useful as a deoxidizing element and is an element useful for fixing solute N existing in steel as AlN.
- the Al content is set to 0.01% or more.
- the amount of Al is preferably 0.013% or more, and more preferably 0.015% or more.
- the Al content is determined to be 0.1% or less.
- the amount of Al is preferably 0.090% or less, and more preferably 0.080% or less.
- N 0.015% or less (excluding 0%) N is an element inevitably contained in the steel.
- the N amount is set to 0.015% or less.
- the N amount is preferably 0.013% or less, more preferably 0.010% or less. The smaller the amount of N, the better. However, the amount is usually about 0.001% due to restrictions on the manufacturing process.
- Cr more than 0.5%, 2.0% or less
- Cr is an element effective for increasing the strength of the final product by improving the hardenability of the steel material, and is contained in a small amount in the spherical carbide, It is an element useful for promoting spheroidization by enhancing the stability of carbides during spheroidizing annealing and suppressing regenerated pearlite.
- the Cr content is determined to be more than 0.5%.
- the amount of Cr is preferably 0.6% or more, and more preferably 0.7% or more.
- the Cr amount is set to 2.0% or less.
- the amount of Cr is preferably 1.8% or less, and more preferably 1.5% or less.
- the basic chemical composition of the steel for 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 unavoidable impurities other than P, S, and N. (For example, O, H, etc.) is meant to be included.
- the steel for machine structural use according to the present invention if necessary, Mo: 1% or less (not including 0%), Ni: 3% or less (not including 0%), Cu: 0.25% or less ( B: 0.010% or less (not including 0%), Ti: 0.2% or less (not including 0%), Nb: 0.2% or less (not including 0%) And V: one or more selected from the group consisting of 0.5% or less (not including 0%).
- Mo 1% or less (not including 0%)
- Ni 3% or less (not including 0%)
- Cu 0.25% or less
- B 0.010% or less (not including 0%)
- Ti 0.2% or less (not including 0%)
- Nb 0.2% or less
- V one or more selected from the group consisting of 0.5% or less (not including 0%).
- the arbitrary elements will be described in the following two groups.
- Mo 1% or less (not including 0%), Ni: 3% or less (not including 0%), Cu: 0.25% or less (not including 0%), and B: 0.
- One or more selected from the group consisting of 010% or less (excluding 0%) Mo, Ni, Cu and B are all useful for increasing the strength of the final product by improving the hardenability of the steel. These elements can be used alone or in combination of two or more as required. In order to effectively exhibit such an action, Mo, Ni and Cu are all preferably 0.02% or more, and more preferably 0.05% or more.
- B is preferably 0.001% or more, more preferably 0.002% or more.
- the Mo amount is preferably 1% or less (more preferably 0.90% or less, more preferably 0.80% or less), and the Ni amount is preferably 3% or less (more preferably 2.5% or less, more preferably Is 2.0% or less), the Cu content is preferably 0.25% or less (more preferably 0.20% or less, more preferably 0.15% or less), and the B content is preferably 0.010% or less (more Preferably 0.007% or less, more preferably 0.005% or less).
- 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%)
- Ti, Nb and V form a compound with N, and exhibit the effect of reducing deformation resistance by reducing the solid solution N.
- the above can be used.
- both Ti and Nb are preferably 0.03% or more, more preferably 0.05% or more, and V is preferably 0.03% or more, more preferably 0. .05% or more.
- Ti and Nb are both preferably 0.2% or less, more preferably 0.18% or less, and still more preferably 0.15% or less.
- V is preferably 0.5% or less, more preferably 0.45% or less, and still more preferably 0.40% or less.
- the mechanical structural steel of the present invention is, for example, a wire or a steel bar, and the diameter is not particularly limited, but is, for example, about 5.0 to 20 mm.
- the steel is first cast according to a conventional method, and after being divided as necessary, hot rolled. It is important to appropriately adjust the finish rolling temperature in hot rolling and the cooling conditions after finish rolling. Specifically, the finish rolling temperature is 850 to 1100 ° C., and in the subsequent cooling, cooling is performed to 720 to 780 ° C. at an average cooling rate of 10 ° C./second or more (cooling 1), and then the average of 1 ° C./second or less. It cools to 680 degreeC or more with a cooling rate (cooling 2), and also cools to 640 degrees C or less with an average cooling rate of 0.5 degrees C / sec or less (cooling 3).
- each condition will be described in detail.
- Finish rolling temperature 850-1100 ° C
- the finish rolling temperature affects the average particle diameter of the above-described ferrite (pre-deposited ferrite and ferrite in pearlite). When the finish rolling temperature exceeds 1100 ° C., the average particle diameter of the ferrite exceeds 25 ⁇ m, and when the finish rolling temperature is less than 850 ° C., the average particle diameter of the ferrite becomes less than 15 ⁇ m.
- the lower limit of the finish rolling temperature is preferably 900 ° C. or higher, more preferably 950 ° C. or higher, and the upper limit is preferably 1050 ° C. or lower, more preferably 1000 ° C. or lower.
- Cooling 1 After finishing rolling, cooling to 720 to 780 ° C. at an average cooling rate of 10 ° C./second or more If the average cooling rate after finish rolling is slow, austenite grains become coarse and hardenability increases, thereby increasing (i) An amount of pro-eutectoid ferrite satisfying the relationship of A> Ae described above cannot be secured, and / or (ii) a total area ratio of pro-eutectoid ferrite and pearlite cannot be secured by 90 area% or more. Therefore, the average cooling rate after finish rolling is 10 ° C./second or more. The average cooling rate is preferably 15 ° C./second or more, more preferably 20 ° C./second or more, and the upper limit is not particularly limited, but the practical range is usually 100 ° C./second or less.
- the cooling stop temperature in the cooling 1 is set to 720 ° C. or higher.
- the lower limit of the cooling stop temperature is preferably 730 ° C. or higher, more preferably 740 ° C. or higher.
- the cooling stop temperature is 780 ° C. or lower, and the upper limit of the cooling stop temperature is preferably 770 ° C. or lower, more preferably 760 ° C. or lower.
- Cooling Cooling to 680 ° C. or higher at an average cooling rate of 1 ° C./second or less If the average cooling rate after cooling 1 is fast, an amount of pro-eutectoid ferrite satisfying the relationship of A> Ae cannot be secured. Accordingly, the average cooling rate is 1 ° C./second or less.
- the average cooling rate is preferably 0.8 ° C./second or less, more preferably 0.6 ° C./second or less, and the lower limit thereof is not particularly limited, but is usually about 0.1 ° C./second.
- the cooling stop temperature in cooling 2 is set to 680 ° C. or higher.
- the cooling stop temperature is preferably 685 ° C. or higher, and more preferably 690 ° C. or higher.
- the upper limit of the cooling stop temperature should just be 780 degrees C or less, Preferably it is 750 degrees C or less, More preferably, it is 720 degrees C or less, Especially 700 degrees C or less.
- Cooling 3 Cooling to 640 ° C. or less at an average cooling rate of 0.5 ° C./second or less
- the average cooling rate is 0.5 ° C./second or less, preferably 0.4 ° C./second or less, more preferably 0.3 ° C./second or less, and the lower limit is not particularly limited.
- the cooling stop temperature is 640 ° C. or lower, preferably 630 ° C. or lower, more preferably 620 ° C. or lower.
- the lower limit of the cooling stop temperature is not particularly limited, but is, for example, 500 ° C. or higher, preferably 550 ° C. or higher, more preferably 600 ° C. or higher.
- the cooling is stopped (terminated) in the three cooling steps, it is not necessary to control the cooling conditions, and it may be cooled to an appropriate temperature, for example, room temperature, by appropriate cooling, for example, cooling.
- an appropriate temperature for example, room temperature
- appropriate cooling for example, cooling.
- spheroidizing annealing may be performed, but before spheroidizing annealing, wire drawing may be performed as necessary.
- the drawing area reduction ratio is not particularly limited, but is, for example, about 5 to 30%.
- the steel for machine structure of the present invention is excellent in cold workability because it can be sufficiently softened after spheroidizing annealing, and is manufactured by cold working such as cold forging, cold forging, cold rolling, etc. It can be suitably used for various parts such as automobile parts and construction machine parts.
- each wire rod (rolled material) obtained was observed and the area ratio was measured, the ferrite average particle diameter was measured, and the hardness after spheroidizing annealing was measured.
- Each of these samples was prepared by embedding a resin so that a longitudinal section (cross section parallel to the axis) of each wire can be observed, and observed or measured at a position of D / 4 (D is the diameter of the wire).
- Measurement of average particle diameter of ferrite An EBSP analyzer and FE-SEM (electrolytic emission scanning electron microscope) were used for measurement of the average particle diameter.
- the grain size is defined as a boundary where the crystal orientation difference (oblique angle) exceeds 15 °, that is, a large-angle grain boundary, and the average grain size of ferrite (including both pro-eutectoid ferrite and ferrite in pearlite) crystal grains was measured.
- Measurement areas were arbitrary 400 ⁇ m ⁇ 400 ⁇ m, measurement steps were 0.7 ⁇ m intervals, and measurement points with a confidence index (Confidence Index) indicating the reliability of the measurement direction were 0.1 or less were deleted from the analysis target.
- Example 1 Using the steel type A shown in Table 1 above, using a laboratory processing master test device, the finishing temperature (corresponding to the finishing rolling temperature) and the cooling conditions are changed as shown in Table 2 below, and the structure is different. Each sample was prepared. At this time, 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, and each was used as a structure investigation sample (before spheroidizing annealing) and a hardness measurement sample after spheroidizing annealing. . For these samples, the average particle diameter of ferrite, the area ratio of the structure, and the hardness after spheroidizing annealing were measured and are shown in Table 3 below.
- each sample is vacuum-sealed, held in an atmospheric furnace at 760 ° C. for 6 hours, once cooled to 680 ° C. and heated again to 760 ° C. (total 4 hours), then at 760 ° C. for 6 hours. After holding, it was cooled to 680 ° C. at an average cooling rate of 6 ° C./hour.
- required based on the said Formula (2) about the steel type A is HV134.
- the component composition is appropriate
- the metal structure has pearlite and pro-eutectoid ferrite
- the total area ratio and the area ratio of pro-eutectoid ferrite are appropriate. It has become soft.
- no. No. 5 had a lower finishing temperature, so the average grain size of the ferrite became smaller.
- No. 6 has a low cooling stop temperature in the cooling 1 and cannot secure the amount of pro-eutectoid ferrite.
- No. 7 had a high average cooling rate in the cooling 3, so that the total area ratio of pro-eutectoid ferrite and pearlite could not be secured.
- No. 8 had a high finishing temperature, the average grain size of the ferrite increased, and the hardness after spheroidizing annealing increased.
- Example 2 Using steel types B to J shown in Table 1 above, rolling was performed under the conditions shown in Table 4 below (finish rolling temperature and cooling conditions) to produce samples having different structures. Spheroidizing annealing was performed in the same manner as in Example 1. In addition, Test No. For No. 15, spheroidizing annealing was performed after drawing the rolled material and drawing at a reduction in area of about 20%. For these samples, the average particle diameter of ferrite, the area ratio of the structure, and the hardness after spheroidizing annealing were measured and are shown in Table 5 below.
- the component composition is appropriate
- the metal structure has pearlite and pro-eutectoid ferrite
- the total area ratio and the area ratio of pro-eutectoid ferrite are appropriate. It has become soft.
- no. No. 16 had a high average cooling rate in the cooling 2, so that the amount of pro-eutectoid ferrite could not be secured.
- No. 17 has a low average cooling rate in the cooling 1 and a high cooling stop temperature in the cooling 3, so that the total area ratio of pro-eutectoid ferrite and pearlite is low.
- the present invention is useful for reducing the deformation resistance of cold-work machine structural steel.
- machine structural steel for cold working for example, various parts such as automobile parts and construction machine parts manufactured by cold working such as cold forging, cold heading, cold rolling (for example, bolts) , Screw, nut, socket, ball joint, inner tube, torsion bar, clutch case, cage, housing, hub, cover, case, washer, tappet, saddle, bulg, inner case, clutch, sleeve, outer race, sprocket, Core, stator, anvil, spider, rocker arm, body, flange, drum, joint, connector, pulley, metal fittings, yoke, base, valve lifter, spark plug, pinion gear, steering shaft, common rail and other mechanical parts, transmission parts, etc.) Etc.
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Abstract
Description
Ae=(0.8-Ceq)×96.75・・・(1)
但し、式(1)において、Ceq=[C]+0.1×[Si]+0.06×[Mn]+0.11×[Cr]であり、[(元素名)]は各元素の含有量(質量%)を意味する。
(i)10℃/秒以上の平均冷却速度で720~780℃まで冷却し、
(ii)その後、1℃/秒以下の平均冷却速度で680℃以上まで冷却し、
(iii)更に0.5℃/秒以下の平均冷却速度で640℃以下まで冷却することを特徴とする冷間加工用機械構造用鋼の製造方法である。
金属組織がベイナイトやマルテンサイト等の微細な組織を含む場合には、一般的な球状化焼鈍を行っても、球状化焼鈍後はベイナイトやマルテンサイトの影響によって組織が微細となり、軟質化が不十分となる。従って、金属組織はパーライトと初析フェライトを有する組織とし、これらの合計面積率を90面積%以上と定めた。パーライトと初析フェライトの合計面積率は、好ましくは95面積%以上であり、より好ましくは97面積%以上である。なお、パーライトと初析フェライト以外の金属組織として、例えば製造過程で生成し得るマルテンサイトやベイナイト等が挙げられるが、これら組織の面積率が高くなると強度が高くなって冷間加工性が劣化することがあるため、できるだけ含まれていないことが好ましい。よって、パーライトと初析フェライトの合計面積率は100面積%が最も好ましい。
本発明では、球状化焼鈍前の初析フェライトの面積率をできるだけ多く確保することによって、球状化焼鈍前にあらかじめセメンタイトが局在化することとなり、球状化焼鈍によってセメンタイトの球状化が促進されることで軟質化を実現できる。本発明者らは、初析フェライトを平衡量程度まで析出させるという観点から検討し、実験に基づき平衡初析フェライト量は(0.8-Ceq)×129で表されることを明らかにした。更に、球状化焼鈍の後に軟質化を実現するためには、前記した平衡初析フェライト量の75%を超える量の初析フェライト量を確保すれば良いことを見出した。すなわち、本発明における初析フェライトの面積率Aは、下記式(1)で表されるAeと、A>Aeの関係を有する。
Ae=(0.8-Ceq)×129×0.75
=(0.8-Ceq)×96.75 ・・・(1)
但し、式(1)において、Ceq=[C]+0.1×[Si]+0.06×[Mn]+0.11×[Cr]であり、[(元素名)]は各元素の含有量(質量%)を意味する。
初析フェライトの面積率A(%)は、好ましくはA(%)≧Ae(%)+0.5(%)、より好ましくはA(%)≧Ae(%)+1.0(%)、特に好ましくはA(%)≧Ae(%)+1.5(%)の関係を満足する。また該面積率A(%)は、例えば、A(%)≦Ae(%)+5(%)、特にA(%)≦Ae(%)+3(%)の関係を満足してもよい。
初析フェライト及びパーライト中のフェライトの平均粒径は15μm以上とする。このようにすることで、球状化焼鈍後の軟質化が可能となる。一方、前記平均粒径が大きくなりすぎると、通常の球状化焼鈍では再生パーライト等の強度が増加し、軟質化が困難となる。そこで、初析フェライト及びパーライト中のフェライトの平均粒径は25μm以下とする。前記平均粒径の下限は、好ましくは16μm以上、より好ましくは17μm以上であり、好ましい上限は23μm以下であり、より好ましくは21μm以下である。
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、C量を0.2%以上と定めた。C量は、好ましくは0.25%以上であり、より好ましくは0.30%以上である。一方、C量が過剰になると強度が高くなりすぎて冷間加工性が低下する。そこでC量を0.6%以下と定めた。C量は、好ましくは0.55%以下であり、より好ましくは0.50%以下である。
Siは、脱酸作用を有するとともに、固溶体硬化による最終製品の強度向上に有効な元素である。このような作用を有効に発揮させるため、Si量を0.01%以上と定めた。Si量は、好ましくは0.02%以上であり、より好ましくは0.03%以上(特に0.05%以上)である。一方、Si量が過剰になると、硬度が過度に上昇して冷間加工性が劣化する。そこで、Si量を0.5%以下と定めた。Si量は、好ましくは0.45%以下であり、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。そのような作用を有効に発揮させるため、Mn量を0.2%以上と定めた。Mn量は、好ましくは0.3%以上であり、より好ましくは0.4%以上である。一方、Mn量が過剰になると、硬度が過度に上昇して冷間加工性が劣化する。そこで、Mn量を1.5%以下と定めた。Mn量は、好ましくは1.1%以下であり、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り、鋼中で粒界偏析を起こし、延性の劣化の原因となる元素である。そこで、P量は0.03%以下に抑制する。P量は好ましくは0.02%以下であり、より好ましくは0.015%以下である。Pは少なければ少ないほど好ましいが、製造工程上の制約から、通常0.001%程度は含まれる。
Sは、鋼中に不可避的に含まれる元素であり、鋼中でMnSとして存在し、延性を劣化させるため冷間加工に有害な元素である。従って、S量は0.05%以下に抑制する。S量は、好ましくは0.04%以下であり、より好ましくは0.03%以下である。但し、Sは被削性を向上させる作用があるので、0.001%以上含有することは有用である。S量は、好ましくは0.002%以上であり、より好ましくは0.003%以上である。
Alは、脱酸元素として有用であるとともに、鋼中に存在する固溶NをAlNとして固定するのに有用な元素である。こうした作用を有効に発揮させるため、Al量を0.01%以上と定めた。Al量は、好ましくは0.013%以上であり、より好ましくは0.015%以上である。一方、Al量が過剰になると、Al2O3が過剰に生成して冷間加工性を劣化させる。そこで、Al量は0.1%以下と定めた。Al量は、好ましくは0.090%以下であり、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り、鋼中に固溶Nが含まれると、歪み時効による硬度上昇及び延性低下を招き、冷間加工性を劣化させる。そこで、N量を0.015%以下と定めた。N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。N量は、少なければ少ない程好ましいが、製造工程上の制約により、通常0.001%程度含まれる。
Crは、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るとともに、球状炭化物中に少量含まれるため、球状化焼鈍時の炭化物の安定性を高め、再生パーライトを抑制するなどの作用によって球状化促進に有用な元素である。このような作用を有効に発揮させるため、Cr量を0.5%超と定めた。Cr量は、好ましくは0.6%以上であり、より好ましくは0.7%以上である。一方、Cr量が過剰になると、強度が高くなりすぎて冷間加工性を劣化させる。また、パーライトと初析フェライトの合計面積率を下げる作用も有する。そこで、Cr量を2.0%以下と定めた。Cr量は、好ましくは1.8%以下であり、より好ましくは1.5%以下である。
Mo:1%以下(0%を含まない)、Ni:3%以下(0%を含まない)、Cu:0.25%以下(0%を含まない)、及びB:0.010%以下(0%を含まない)よりなる群から選択される1種以上
Mo、Ni、Cu及び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用な元素であり、必要に応じて単独で又は2種以上用いることができる。このような作用を有効に発揮させるため、Mo、Ni及びCuはいずれも0.02%以上とすることが好ましく、より好ましくは0.05%以上である。Bは、好ましくは0.001%以上であり、より好ましくは0.002%以上である。一方、Mo、Ni、Cu及びBの含有量が過剰になると、強度が高くなり過ぎ、冷間加工性が劣化する。そこで、Mo量は1%以下が好ましく(より好ましくは0.90%以下、さらに好ましくは0.80%以下)、Ni量は3%以下が好ましく(より好ましくは2.5%以下、さらに好ましくは2.0%以下)、Cu量は0.25%以下が好ましく(より好ましくは0.20%以下、さらに好ましくは0.15%以下)、B量は0.010%以下が好ましい(より好ましくは0.007%以下、さらに好ましくは0.005%以下)。
Ti:0.2%以下(0%を含まない)、Nb:0.2%以下(0%を含まない)、及びV:0.5%以下(0%を含まない)よりなる群から選択される1種以上
Ti、Nb及び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、必要に応じて単独で又は2種以上用いることができる。このような効果を有効に発揮させるため、Ti及びNbはいずれも0.03%以上が好ましく、より好ましくは0.05%以上であり、Vは0.03%以上が好ましく、より好ましくは0.05%以上である。一方、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。そこで、Ti及びNbは、いずれも0.2%以下が好ましく、より好ましくは0.18%以下、さらに好ましくは0.15%以下である。Vは、好ましくは0.5%以下であり、より好ましくは0.45%以下、さらに好ましくは0.40%以下である。
仕上圧延温度は、上述したフェライト(初析フェライト及びパーライト中のフェライト)の平均粒径に影響する。仕上圧延温度が1100℃を超えると、前記フェライトの平均粒径が25μmを超え、仕上圧延温度が850℃未満となると、前記フェライトの平均粒径が15μm未満となる。仕上圧延温度の下限は、好ましくは900℃以上であり、より好ましくは950℃以上であり、上限は、好ましくは1050℃以下であり、より好ましくは1000℃以下である。
仕上圧延後の平均冷却速度が遅いと、オーステナイト粒が粗大化して焼入れ性が上がることによって、(i)上記したA>Aeの関係を満足する量の初析フェライトを確保できない、及び/又は(ii)初析フェライトとパーライトの合計面積率を90面積%以上確保できない。従って仕上圧延後の平均冷却速度は10℃/秒以上とする。該平均冷却速度は、好ましくは15℃/秒以上であり、より好ましくは20℃/秒以上であり、上限は特に限定されないが、現実的な範囲は通常100℃/秒以下である。
冷却1の後の平均冷却速度が速いと、上記したA>Aeの関係を満足する量の初析フェライトを確保できない。従って、平均冷却速度は1℃/秒以下とする。平均冷却速度は好ましくは0.8℃/秒以下であり、より好ましくは0.6℃/秒以下であり、その下限は特に限定されないが、通常0.1℃/秒程度である。
冷却3における平均冷却速度が速い場合や、冷却停止温度が高い場合は、初析フェライトとパーライトの合計面積率を90面積%以上とすることができない。平均冷却速度は、0.5℃/秒以下であり、好ましくは0.4℃/秒以下、より好ましくは0.3℃/秒以下であり、下限は特に限定されないが、通常0.1℃/秒程度である。また、冷却停止温度は640℃以下であり、好ましくは630℃以下、より好ましくは620℃以下である。なお冷却停止温度の下限は特に限定されないが、例えば、500℃以上、好ましくは550℃以上、より好ましくは600℃以上である。
平均粒径の測定には、EBSP解析装置及びFE-SEM(電解放出型走査電子顕微鏡)を用いた。結晶方位差(斜角)が15°を超える境界、すなわち大角粒界を結晶粒界として結晶粒を定義し、フェライト(初析フェライト及びパーライト中のフェライトの両者を含む)結晶粒の平均粒径を測定した。測定領域は任意の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以下の測定点は解析対象から削除した。
各試料について、ナイタールエッチングによって組織を現出させ、光学顕微鏡にて倍率400倍で10視野を撮影した。撮影した写真を画像解析し、初析フェライト及びパーライトの合計面積率(表中、「P+Fの割合」と表す)、及び初析フェライトの面積率を判定した。なお、組織の解析に際しては、上記各写真について、ランダムに100点(すなわち、合計で1000点測定した)選び、各組織(初析フェライト、パーライトの他、ベイナイト、マルテンサイトなどの組織)が存在した点数を全点数で割ることによって組織分率を求めた。
各試料について、球状化焼鈍後の硬さ測定は、ビッカース硬度計を用い、荷重1kgfで5点測定し、その平均値(HV)を求めた。この時の硬さの基準として、下記式(2)を用い、前記平均値が下記式(2)で算出される基準値よりも小さい場合を合格と判断した。
硬さの基準値=88.4×Ceq2+88.0 ・・・(2)
但し、Ceq2=[C]+0.2×[Si]+0.2×[Mn]であり、[(元素名)]は各元素の含有量(質量%)を意味する。
上記表1に示した鋼種Aを用いて、ラボの加工フォーマスタ試験装置を用い、仕上加工温度(仕上圧延温度に相当)、冷却条件を下記表2に示すように変化させて、組織の異なるサンプルをそれぞれ作製した。このとき、加工フォーマスタサンプルはφ8.0mm×12.0mmとし、熱処理後に2等分して、それぞれ組織調査用(球状化焼鈍前)サンプル、及び球状化焼鈍後の硬さ測定用サンプルとした。これらサンプルについて、フェライトの平均粒径、組織の面積率、球状化焼鈍後の硬さを測定し、下記表3に示した。球状化焼鈍では、各サンプルをそれぞれ真空封入し、大気炉にて760℃で6時間保持後、一旦680℃まで冷却して再度760℃に加熱し(トータルで4時間)、760℃で6時間保持後、平均冷却速度6℃/時間で680℃まで冷却した。なお、鋼種Aについて上記式(2)に基づいて求めた硬さの基準値はHV134である。
上記表1に示した鋼種B~Jを用い、下記表4に示す条件(仕上圧延温度、冷却条件)で圧延し、組織の異なるサンプルを作製した。球状化焼鈍は実施例1と同様の方法で実施した。なお、試験No.15については、圧延材作製後、約20%の減面率で伸線した後に球状化焼鈍を実施した。これらサンプルについて、フェライトの平均粒径、組織の面積率、球状化焼鈍後の硬さを測定し、下記表5に示した。
Claims (3)
- C :0.2~0.6%(質量%の意味。以下、化学成分組成について同じ)、
Si:0.01~0.5%、
Mn:0.2~1.5%、
P :0.03%以下(0%を含まない)、
S :0.001~0.05%、
Al:0.01~0.1%、
N :0.015%以下(0%を含まない)、及び
Cr:0.5%超、2.0%以下を含有し、残部が鉄および不可避不純物であり、
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%以上であるとともに、
初析フェライトの面積率Aが、下記式(1)で表されるAeと、A>Aeの関係を有し、
初析フェライト及びパーライト中のフェライトの平均粒径が15~25μm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-Ceq)×96.75 ・・・(1)
但し、式(1)において、Ceq=[C]+0.1×[Si]+0.06×[Mn]+0.11×[Cr]であり、[(元素名)]は各元素の含有量(質量%)を意味する。 - 更に、
Mo:1%以下(0%を含まない)、
Ni:3%以下(0%を含まない)、
Cu:0.25%以下(0%を含まない)、
B :0.010%以下(0%を含まない)、
Ti:0.2%以下(0%を含まない)、
Nb:0.2%以下(0%を含まない)、及び
V :0.5%以下(0%を含まない)よりなる群から選択される1種以上を含有する請求項1に記載の冷間加工用機械構造用鋼。 - 請求項1または2に記載の化学成分組成を有する鋼を、
850~1100℃で仕上圧延した後、
(i)10℃/秒以上の平均冷却速度で720~780℃まで冷却し、
(ii)その後、1℃/秒以下の平均冷却速度で680℃以上まで冷却し、
(iii)更に0.5℃/秒以下の平均冷却速度で640℃以下まで冷却することを特徴とする冷間加工用機械構造用鋼の製造方法。
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JP2013227602A (ja) | 2013-11-07 |
CA2868394C (en) | 2017-03-07 |
CN104245987B (zh) | 2016-11-23 |
EP2843070A1 (en) | 2015-03-04 |
EP2843070B1 (en) | 2018-06-06 |
KR20140139020A (ko) | 2014-12-04 |
TWI490346B (zh) | 2015-07-01 |
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