MECHANICAL STRUCTURE STEEL FOR COLD-WORKING AND MANUFACTURING METHOD THEREFOR

Abstract

Disclosed is a mechanical structure steel for cold-working, including: C: 0.32 to 0.44% by mass, Si: 0.15 to 0.35% by mass, Mn: 0.55 to 0.95% by mass, P: 0.030% by mass or less, S: 0.030% by mass or less, Cr: 0.85 to 1.25% by mass, Mo: 0.15 to 0.35% by mass, and Al: 0.01 to 0.1% by mass, with the balance consisting of iron and inevitable impurities, wherein an area ratio of proeutectoid ferrite is 30% or more and 70% or less, and an average grain size of ferrite crystal grains is 5 to 15 μm.

Claims

1-7. (canceled)

8. A mechanical structure steel, comprising: C: 0.32 to 0.44% by mass, Si: 0.15 to 0.35% by mass, Mn: 0.55 to 0.95% by mass, P: 0.030% by mass or less, S: 0.030% by mass or less, Cr: 0.85 to 1.25% by mass, Mo: 0.15 to 0.35% by mass, and Al: 0.01 to 0.1% by mass, with the balance consisting of iron and inevitable impurities, wherein an area ratio of proeutectoid ferrite is greater than 35% and 70% or less, and an average grain size of ferrite crystal grains is 5 to 15 μm.

9. The mechanical structure steel according to claim 8, wherein a ratio of an area ratio of pearlite to the total area ratio of microstructures other than the proeutectoid ferrite is 80% or less.

10. The mechanical structure steel according to claim 8, wherein a hardness is HV300 or less.

11. The mechanical structure steel according to claim 8, further comprising at least one of the following (a) to (c): (a) one or more elements selected from the group consisting of Cu: 0.25% by mass or less (excluding 0% by mass) and Ni: 0.25% by mass or less (excluding 0% by mass); (b) one or more elements selected from the group consisting of Ti: 0.2% by mass or less (excluding 0% by mass), Nb: 0.2% by mass or less (excluding 0% by mass) and V: 1.5% by mass or less (excluding 0% by mass); and (c) one or more elements selected from the group consisting of N: 0.01% by mass or less (excluding 0% by mass), Mg: 0.02% by mass or less (excluding 0% by mass), Ca: 0.05% by mass or less (excluding 0% by mass), Li: 0.02% by mass or less (excluding 0% by mass) and REM: 0.05% by mass or less (excluding 0% by mass).

12. A method for manufacturing a mechanical structure steel, the method comprising: preparing a steel having a chemical composition according to claim 8, followed by being subjected to the steps of: (a) performing pre-working at a compression ratio of 20% or more and a holding time of 10 seconds or less, (b) after the step (a), performing finishing at higher than 800° C. and 1,050° C. or lower and a compression ratio of 20% or more, (c) after the step (b), cooling to 750° C. or higher and 840° C. or lower over 10 seconds or less, and (d) after the step (c), cooling to 500° C. or lower at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec.

13. A method for manufacturing a mechanical structure steel, the method comprising: preparing a steel having a chemical composition according to claim 11, followed by being subjected to the steps of: (a) performing pre-working at a compression ratio of 20% or more and a holding time of 10 seconds or less, (b) after the step (a), performing finishing at higher than 800° C. and 1,050° C. or lower and a compression ratio of 20% or more, (c) after the step (b), cooling to 750° C. or higher and 840° C. or lower over 10 seconds or less, and (d) after the step (c), cooling to 500° C. or lower at an average cooling rate of 0.1° C./sec or more and less than 10° C./sec.

14. A method for manufacturing a steel wire, the method comprising subjecting the mechanical structure steel manufactured by the method according to claim 12 to one or more steps of annealing, spheroidizing annealing, wire drawing, heading and quenching/tempering.

Description

EXAMPLES

[0090] Examples of the present invention will be more specifically described by way of Examples. The embodiments of the present invention are not limited by the following Examples, and it is possible to implement the embodiments with modifications within the range that can meet the gist of the present disclosure as described above and below, all of these modifications being within the scope of the present disclosure.

Example 1

[0091] Using steels having the chemical compositions indicated by steel types A and D shown in Table 1, test specimens for working Formastor, each having a size of ϕ10 mm×15 mm, were fabricated. The thus obtained test specimens for the working Formastor were subjected to pressing and cooling performed by a working Formastor tester under the conditions shown in Table 2. Although not shown in Table 2, cooling in the temperature range of 500° C. or lower was performed at an average cooling rate during the second cooling to near room temperature (25° C. to 40° C.) when the average cooling rate during the second cooling was 1° C./second or higher, while gas rapid cooling was performed when the average cooling rate during the second cooling was less than 1° C./second.

[0092] In Tables 1 and 2, and Tables 3 to 5 shown below, underlined numerical values indicate that the results are outside the embodiment of the present invention. The value calculated by the following equation (1) was shown in the carbon equivalent column in Table 1.

Carbon equivalent (Ceq)=[C]+[Si]/24+[Mn]/6+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14   (1)

where [C], [Si], [Mn], [Ni], [Cr], [Mo] and [V] are the contents, in % by mass, of C, Si, Mn, Ni, Cr, Mo and V, respectively.

TABLE-US-00001 TABLE 1 Chemical composition (% by mass) ※ The Carbon Steel balance being iron and inevitable impurities equiva- type C Si Mn P S Cr Mo Al lent A 0.35 0.18 0.70 0.018 0.013 0.96 0.16 0.028 0.71 B 0.35 0.19 0.67 0.010 0.006 0.95 0.17 0.036 0.70 C 0.35 0.19 0.75 0.010 0.003 0.98 0.17 0.033 0.72 D 0.40 0.20 0.70 0.013 0.006 0.98 0.16 0.025 0.76

TABLE-US-00002 TABLE 2 Cooling Second cooling Working First cooling Average cooling Pre-working Finishing Average Cooling rate of cooling Temp- Compression Holding Temp- Compression cooling stop Cooling stop temperature Test Steel erature ratio time erature ratio rate temperature time to 500° C. No. type (° C.) (%) (sec) (° C.) (%) (° C./sec) (° C.) (sec) (° C./sec) 1-1 A 900 42 5 900 57 50 750 3 1 1-2 A 900 42 5 900 57 50 750 3 3 1-3 A 1,000 42 5 1,000 57 50 750 5 0.1 1-4 A 900 42 5 900 57 50 750 3 0.1 1-5 A 1,200 42 5 1,200 57 50 750 9 0.1 1-6 A 800 42 5 800 57 50 750 1 1 1-7 A 900 42 5 900 57 50 750 3 10 1-8 A 1,200 42 5 1,200 57 50 750 9 3 1-9 D 900 42 5 900 57 50 750 3 1  1-10 D 850 42 5 850 57 50 750 2 0.5

[0093] Each of the test specimens subjected to a working heat treatment was cut into four equal pieces along its central axis to obtain four samples including longitudinal sections. One of these samples was not subjected to spheroidizing annealing (hereinafter sometimes referred to as sample before spheroidizing annealing), and another one was subjected to spheroidizing annealing (hereinafter sometimes referred to as sample after spheroidizing annealing). The spheroidizing annealing was performed by placing each test specimen in a vacuum sealed tube.

[0094] The spheroidizing annealing was performed under the following two conditions (SA1 and SA2).

[0095] SA1: The test specimen was subjected to soaking by holding its temperature at 760° C. for 5 hours, then cooled to 685° C. at an average cooling rate of 13° C./hour, followed by naturally cooling.

[0096] SA2: The test specimen was subjected to soaking by holding its temperature at 750° C. for 2 hours, then cooled to 660° C. at an average cooling rate of 13° C./hour, followed by naturally cooling.

[0097] In SA1, the spheroidizing annealing time was reduced to about 11 hours, compared with about 15 hours in the conventional technique. Note that the term “spheroidizing annealing time” as used herein is the sum of the soaking holding time and the cooling time until the natural cooling. SA2 was performed at a lower temperature than SA1, assuming a delay in temperature tracking.

[0098] The samples before spheroidizing annealing were embedded in a resin so that the longitudinal section could be observed, and (1) the area ratio of proeutectoid ferrite, (2) the average grain size of ferrite grains, (3) the ratio of the area ratio of pearlite to the total area ratio of the microstructures other than proeutectoid ferrite, and (4) the hardness before spheroidizing annealing were measured.

[0099] For the samples after spheroidizing annealing, the longitudinal sections were embedded in a resin so that they could be observed in the same way as mentioned above, and (5) the hardness after spheroidizing annealing and (6) the spheroidization degree were measured.

[0100] In any of the measurements (1) to (6), the diameter of the specimen was set at D and the position of D/4 from the side of the test specimen toward the central axis was measured.

(1) Measurement of Area Ratio of Proeutectoid Ferrite

[0101] The longitudinal section of the sample before spheroidizing annealing was etched with nital to expose its microstructures. Then, photographs of the D/4 position were taken using an optical microscope at magnifications of 400 times (field of view: 220 μm in lateral direction×165 μm in longitudinal direction) and 1,000 times (field of view: 88 μm in lateral direction×66 μm in longitudinal direction). Then, on each of the images thus obtained, fifteen lines were drawn in longitudinal direction at equal intervals and ten lines were drawn in lateral direction at equal intervals in a grid pattern to form 150 intersection points. Among the 150 intersection points, the number of points where proeutectoid ferrite or pearlite exists was measured on each image. The value obtained by dividing the number of points by 150 was defined as the area ratio (%) of proeutectoid ferrite.

[0102] For the below-mentioned samples having an average grain size of ferrite crystal grains of 10 μm or more, the measurement was made using a photograph taken at a magnification of 400 times. For samples having an average grain size of less than 5 μm, the measurement was made using a photograph taken at a magnification of 1,000 times. For samples having an average grain size of 5 μm or more and less than 10 μm, the measurement was made by appropriately selecting a photograph taken at a magnification of 400 or 1,000 times.

(2) Measurement of Average Grain Size of Ferrite Crystal Grains

[0103] The average grain size of ferrite crystal grains was measured using FE-SEM and EBSP analyzer.

[0104] Backscattered electron diffraction images were obtained by FE-SEM at the D/4 position of the longitudinal section of the sample before spheroidizing annealing. In the images thus obtained, the average grain size of “crystal grains” in ferrite was determined by defining the grain boundaries, wherein the grain boundaries are boundaries which have a crystallographic orientation difference (oblique angle) exceeding 15°, i.e., large angle grain boundaries, by using the EBSP analyzer. In that case, the measurement area was 200 μm×200 μm, and the measurement steps were 0.4 μm apart. The measurement points with a Confidence Index, which indicates the reliability of the measurement orientation, of no more than 0.1 were deleted from the analysis.

(3) Measurement of Ratio of Area Ratio of Pearlite to Total Area Ratio of Microstructures other than the Proeutectoid Ferrite

[0105] The longitudinal section of the sample before spheroidizing annealing was etched with vital to expose its microstructures. Then, photographs of the D/4 position were taken with an optical microscope at magnifications of 400 times (field of view: 220 μm in lateral direction×165 μm in longitudinal direction) and 1,000 times (field of view: 88 μm in lateral direction×66 μm in longitudinal direction). Then, on each of the images thus obtained, fifteen lines were drawn in longitudinal direction at equal intervals and ten lines were drawn in lateral direction at equal intervals in a grid pattern to form 150 intersection points. Among the 150 intersection points, the number of points A where proeutectoid ferrite or pearlite exists was measured on each image. Next, the number of points B of pearlite existing on the 150 intersection points was measured, and the value obtained by dividing the number of points B by the number of points (150−A) was defined as the ratio (%) of the area ratio of pearlite to the total area ratio of microstructures other than proeutectoid ferrite.

[0106] For the below-mentioned samples each having an average grain size of ferrite crystal grains of 10 μm or more, the measurement was made using a photograph taken at a magnification of 400 times. For samples each having an average grain size of less than 5 μm, the measurement was made using a photograph taken at a magnification of 1,000 times. For samples each having an average grain size of 5 μm or more and less than 10 μm, the measurement was made by appropriately selecting a photograph taken at a magnification of 400 or 1,000 times.

(4) Measurement of Hardness before Spheroidizing Annealing

[0107] For the longitudinal section of the sample before spheroidizing annealing, measurement was made at 3 to 5 points using a Vickers hardness tester at D/4 position under a load of 1 kgf, and the average value (HV) was determined.

(5) Measurement of Hardness after Spheroidizing Annealing

[0108] For the longitudinal section of the sample after spheroidizing annealing, measurement was made at 3 to 5 points using a Vickers hardness tester at D/4 position under a load of 1 kgf, and the average value (HV) was determined.

[0109] Since it is known that the hardness increases as the carbon equivalent of the steel type increases, the criterion for determining the hardness after spheroidizing annealing in this Example was set according to the carbon equivalent (Ceq) of the steel type. Specifically, the hardness after SA1 was determined based on whether or not the following expression (2) is satisfied.

(Hardness (HV))<97.3×Ceq+84   (2)

[0110] The case where the hardness after SA1 was rated Excellent (A) if it satisfies the above expression (2), while the hardness was rated Poor (C) if it does not satisfy the above expression (2).

[0111] When the carbon equivalent is 0.70 or more, it is more preferable if the hardness after SA1 is HV150 or less.

[0112] Since SA2 is an annealing condition in which softening is not easily achieved at lower temperature compared with SA1, a criterion (looser criterion) which is different from that in the above expression (2) was set for the hardness after SA2. Specifically, the hardness after SA2 was determined based on whether or not the following expression (3) was satisfied.

(Hardness (HV))<97.3×Ceq+98   (3)

[0113] The case where the hardness after SA2 was rated Excellent (A) if it satisfies the above expression (3), while the hardness was rated Poor (C) if it does not satisfy the above expression (3).

[0114] When the carbon equivalent is 0.70 or more, it is more preferable if the hardness after SA2 is HV165 or less.

(6) Measurement of Spheroidization Degree

[0115] The longitudinal section of the sample after spheroidizing annealing was etched with nital to expose its microstructures, and then the microstructures were observed at the D/4 position using an optical microscope at a magnification of 400 times (field of view: 220 μm in lateral direction×165 μm in longitudinal direction). The spheroidization degrees Nos. 1 to 3 were determined for the observed images according to the “spheroidization degree” mentioned in JIS G3509-2. The spheroidization degree No. 1 was rated as Excellent (A), the spheroidization degree No. 2 was rated Good (B), and the spheroidization degree No. 3 was rated Poor (C), respectively.

[0116] The microstructures and the hardness before spheroidizing annealing as well as the hardness and the spheroidization degree after spheroidizing annealing evaluated by the above procedures in (1) to (6) are shown in Table 3. Regarding the overall judgment after SA1, the case where both the hardness and spheroidization degree after SA1 were excellent was rated Excellent (A), the case where rating A and rating B coexist was rated Good (B), and the case where at least one rating C exists was rated Poor (C).

TABLE-US-00003 TABLE 3 Before spheroidizing annealing After spheroidizing annealing Average Pearlite area After SA1 After SA2 grain size ratio of (high temperature) (low temperature) Area ratio of of ferrite balance Right Spheroidi- Right proeutectoid crystal micro- Hard- Hard- side of zation Hard- side of Test Steel ferrite grains structures ness ness expression Judg- degree Judg- Overall ness expression Judg- No. type (%) (μm) (%) (HV) (HV) (2) ment (No.) ment judgment (HV) (3) ment 1-1 A 47 7.9 78 214 144 153 A 1 A A 157 167 A 1-2 A 42 6.5 39 279 148 153 A 1 A A 155 167 A 1-3 A 49 13.2 100 221 145 153 A 2 B B 148 167 A 1-4 A 55 8.1 100 199 148 153 A 2 B B 150 167 A 1-5 A 47 26.6 100 194 145 153 A 3 C C 155 167 A 1-6 A 52 4.3 100 205 156 153 C 1 A C 172 167 C 1-7 A 19 5.0 0 371 153 153 C 1 A C 167 167 C 1-8 A 5 24.2 0 319 160 153 C 2 B C 190 167 C 1-9 D 36 7.8 69 267 157 158 A 2 B B 166 172 A  1-10 D 38 6.0 52 263 154 158 A 2 B B 165 172 A

[0117] In the results of Table 3, the balance microstructures was entirely composed of bainite except pearlite.

[0118] From the results in Table 3, it is possible to make the following observations. Test Nos. 1-1 to 1-4, 1-9 and 1-10 in Table 3 are examples which satisfy all the requirements specified in the embodiments of the present invention, and both the hardness and the spheroidization degree were good or excellent after SA1 in which the spheroidizing annealing time was shortened compared with conventional technology. In particular, unlike Test Nos. 1-3 to 1-4, 1-9 and 1-10, Test Nos. 1-1 to 1-2 are examples in which the carbon content is in a preferable range (less than 0.40% by mass) and the average cooling rate during the second cooling is in a preferable range (1 to 3° C./sec), thus satisfying preferable requirements (the area ratio of proeutectoid ferrite is more than 40%, and the area ratio of pearlite of the balance microstructures is less than 80%). Therefore, the spheroidization degree after SA1 became excellent, leading to excellent overall judgment.

[0119] Meanwhile, Test Nos. 1-5 to 1-8 in Table 3 are examples which do not satisfy the requirements specified in the embodiments of the present invention, and the hardness or spheroidization degree after SA1 was poor.

[0120] In Test No. 1-5, the average grain size of ferrite crystal grains became more than 15 μm because of high finishing temperature of 1,200° C., leading to poor spheroidization degree after SA1.

[0121] In Test No. 1-6, the average grain size of ferrite crystal grains became less than 5 μm because of low finishing temperature of 800° C., leading to poor hardness after SA1.

[0122] In Test No. 1-7, the area ratio of proeutectoid ferrite became less than 30% because of high average cooling rate of the second cooling of 10° C./sec, leading to poor hardness after SA1.

[0123] In Test No. 1-8, the finishing temperature is high temperature of 1,200° C., the area ratio of proeutectoid ferrite became less than 30%, and the average grain size of ferrite crystal grains became more than 15 μm, leading to poor hardness after SA1.

[0124] It has been found that softening is sufficiently achieved by satisfying all of the requirements specified in the embodiments of the present invention, like Tests Nos. 1-1 to 1-4, 1-9 and 1-10 in Table 3, even after SA2 in which spheroidization annealing was performed at a lower temperature than that of SA1, assuming a delay in temperature tracking.

Example 2

[0125] Using steels having the chemical compositions indicated by steel types B and C shown in Table 1, each steel was subjected to rolling and cooling in a mass production rolling line under the conditions shown in Table 4. In the rolling line, a heating furnace, a roughing rolling mill, an intermediate rolling mill, an intermediate water-cooling strip, a block mill rolling mill, a sizing mill rolling mill, a product water-cooling strip, a cooling conveyor and a high-rise warehouse are connected in this order. Pre-working is performed by the block mill rolling mill, and first cooling and second cooling were performed by the cooling conveyor. Although not shown in Table 4, cooling in the temperature range of 500° C. or lower was performed at the average cooling rate during second cooling to about 400° C., and then naturally cooled. Samples were cut out from the rolled material thus obtained, one of which was not subjected to spheroidizing annealing, and the other one was subjected to spheroidizing annealing.

[0126] The spheroidizing annealing was performed under the following two conditions (SA3 and SA4). Regarding SA3, the spheroidizing annealing time was shortened to about 9 hours, compared with about 15 hours in conventional technology. Regarding SA4, the spheroidizing annealing was performed at a lower temperature than that of SA3, assuming a delay in temperature tracking.

[0127] SA3: The test specimen was subjected to soaking by holding its temperature at 770° C. for 2 hours, then cooled to 685° C. at an average cooling rate of 13° C./hour, followed by allowed the test specimen to cool.

[0128] SA4: The test specimen was subjected to soaking by holding its temperature at 750° C. for 2 hours, then cooled to 660° C. at an average cooling rate of 13° C./hour, followed by allowed the test specimen to cool.

TABLE-US-00004 TABLE 4 Cooling Second cooling Average cooling rate Working First cooling from cooling Pre-working Finishing Average Cooling stop Temper- Compression Holding Temper- Compression cooling stop Cooling temperature to Test Steel ature ratio time ature ratio rate temperature time 500° C. No. type (° C.) (%) (sec) (° C.) (%) (° C./sec) (° C.) (sec) (° C./sec) 2-1 B 896 ≥20 ≤3 887 ≥20 31 843 1.4 1.5 2-2 C 942 ≥20 ≤3 846 ≥20 92 801 0.5 2.7

[0129] In the same manner as in Example 1, (1) the area ratio of proeutectoid ferrite, (2) the average grain size of ferrite crystal grains, (3) the ratio of area ratio of pearlite to total area ratio of microstructures other than proeutectoid ferrite, (4) the hardness before spheroidizing annealing, (5) the hardness after spheroidizing annealing and (6) the spheroidization degree were measured and evaluated. The case where the hardness after SA3 was rated Excellent (A) if it satisfies the above expression (2), while the hardness was rated Poor (C) if it does not satisfy the above expression (2). When the carbon equivalent is 0.70 or more, it is more preferable if the hardness after SA3 is HV150 or less. The case where the hardness after SA4 was rated Excellent (A) if it satisfies the above expression (3), while the hardness was rated Poor (C) if it does not satisfy the above expression (3). When the carbon equivalent is 0.70 or more, it is more preferable if the hardness after SA4 is HV165 or less.

[0130] The results are shown in Table 5.

TABLE-US-00005 TABLE 5 Before spheroidizing annealing After spheroidizing annealing Pearlite area After SA3 Average ratio of (high temperature) After SA4 Area ratio of grain size balance Right Spheroidiza- (low temperature) proeutectoid of ferrite micro- Hard- Hard- side of tion Hard- Right side of Test ferrite crystal structures ness ness expression Judg- degree Judg- Overall ness expression Judg- No. (%) grains (μm) (%) (HV) (HV) (2) ment (No.) ment judgment (HV) (3) ment 2-1 9 17.6 0 265 153 152 C 3 C C 165 166 A 2-2 65 9.2 100 202 146 154 A 2 B B 154 168 A

[0131] In the results of Table 5, the balance microstructures was entirely composed of bainite except pearlite.

[0132] From the results in Table 5, it is possible to make the following observations. Test No. 2-2 in Table 5 is an example which satisfy all the requirements specified in the embodiments of the present invention, and both the hardness and the spheroidization degree were excellent or good after SA3.

[0133] Meanwhile, in Test No. 2-1 in Table 5, the cooling stop temperature of the first cooling was higher than 840° C., the area ratio of proeutectoid ferrite was less than 30%, and the average grain size of ferrite crystal grains was more than 15 μm, leading to poor hardness and spheroidization degree after SA3.

Industrial Applicability

[0134] The mechanical structure steel for cold-working according to the embodiment of the present invention is suitable for use in various components manufactured by cold-working such as cold forging, cold heading or cold rolling. The form of the steel is not limited particularly, but it can be, for example, a wire rod, or a rolled material such as a steel bar.

[0135] Examples of various components above specifically include automobile components and construction machine components, such as bolts, screws, nuts, sockets, ball joints, inner tubes, torsion bars, clutch cases, cages, housings, hubs, covers, cases, cradles, tappets, saddles, burgs, inner cases, clutches, sleeves, outer races, sprockets, stators, anvils, spiders, rocker arms, bodies, flanges, drums, joints, connectors, pulleys, metal fittings, yokes, mouthpieces, valve lifters, spark plugs, pinion gears, steering shafts, and common rails. The mechanical structure steel for cold-working according to the embodiment of the present invention is industrially useful as mechanical structural steels which are suitably used as materials for the above-mentioned components, and can exhibit low deformation resistance and excellent cold workability when manufactured into the above-mentioned various components at room temperature and in the work heating range after spheroidizing annealing.

[0136] The present application claims priority to Japanese Patent Application No. 2019-016219 filed on Jan. 31, 2019 and Japanese Patent Application No. 2019-211181 filed on Nov. 22, 2019, the disclosures of which are incorporated herein by reference in its entirety.