WIRE ROD FOR USE IN BOLTS THAT HAS EXCELLENT ACID PICKLING PROPERTIES AND RESISTANCE TO DELAYED FRACTURE AFTER QUENCHING AND TEMPERING, AND BOLT

Abstract

To provide a wire rod for bolts that has excellent acid pickling properties and the resistance to delayed fracture after quenching and tempering, and a bolt using the same. Disclosed is a wire rod for bolts that has excellent acid pickling properties and resistance to delayed fracture, including, in percent by mass: C: 0.3 to 0.6%; Si: 1.0 to 3.0%; Mn: 0.1 to 1.5%; P: more than 0% and 0.020% or less; S: more than 0% and 0.020% or less; Cr: 0.3 to 1.5%; Al: 0.02 to 0.10%; and N: 0.001 to 0.020%, with the balance being iron and inevitable impurities, wherein in a d 1/4 position of the wire rod, where d is a diameter of the wire rod, a ferrite area ratio is in a range of 10 to 40%, with the remaining microstructure being bainite, pearlite and an inevitably formed microstructure, and a C content in a position at a depth of 0.1 mm from a surface layer of the wire rod is in a range of 50 to 100% of a C content in a base material.

Claims

1. A wire rod that has excellent acid pickling properties and resistance to delayed fracture after quenching and tempering, comprising, in percent by mass: C: 0.3 to 0.6%; Si: 1.0 to 3.0%; Mn: 0.1 to 1.5%; P: more than 0% and 0.020% or less; S: more than 0% and 0.020% or less; Cr: 0.3 to 1.5%; Al: 0.02 to 0.10%; N: 0.001 to 0.020%; iron and inevitable impurities, wherein in a d position of the wire rod, where d is a diameter of the wire rod, a ferrite area ratio is in a range of 10 to 40%, with the remaining microstructure being bainite, pearlite and an inevitably formed microstructure, and a C content in a position at a depth of 0.1 mm from a surface layer of the wire rod is in a range of 50 to 100% of a C content in a base material.

2. The wire rod according to claim 1, further comprising, in percent by mass, at least one selected from the group consisting of (a) to (e): (a) at least one element selected from the group consisting of Cu: more than 0% and 0.5% or less, Ni: more than 0% and 1.0% or less, and Sn: more than 0% and 0.5% or less, (b) at least one element selected from the group consisting of Ti: more than 0% and 0.1% or less, Nb: more than 0% and 0.1% or less, and Zr: more than 0% and 0.3% or less, (c) at least one element selected from the group consisting of Mo: more than 0% and 3% or less and W: more than 0% and 0.5% or less, (d) V: more than 0% and 0.5% or less, and (e) at least one element selected from the group consisting of Mg: more than 0% and 0.01% or less and Ca: more than 0% and 0.01% or less.

3. A bolt that has excellent resistance to delayed fracture obtained from the wire rod according to claim 1, wherein a tensile strength of the bolt is 1,400 MPa or more, and each of austenite grain size numbers both at a surface layer of the bolt and in the d position of the bolt, where d is a diameter of a bolt shaft part, is No. 7.0 or more.

4. A bolt that has excellent resistance to delayed fracture obtained from the wire rod according to claim 2, wherein a tensile strength of the bolt is 1,400 MPa or more, and each of austenite grain size numbers both at a surface layer of the bolt and in the d position of the bolt, where d is a diameter of a bolt shaft part, is No. 7.0 or more.

5. The wire rod according to claim 2, comprising (a).

6. The wire rod according to claim 2, comprising (b).

7. The wire rod according to claim 2, comprising (c).

8. The wire rod according to claim 2, comprising (d).

9. The wire rod according to claim 2, comprising (e).

Description

EXAMPLES

[0060] The present invention will be more specifically described below by way of Examples, but is not limited to the following Examples. Various modifications can be obviously made to these Examples as long as they are adaptable to the above-mentioned and below-mentioned concepts and are included within the technical scope of the present invention.

Manufacture of Wire Rod

[0061] Steel materials (steel types A to M and A1 to M1) with the chemical component compositions shown in Table 1 were smelted, cast, and hot-rolled, thereby manufacturing wire rods, each having a diameter of 12 mm. During these processes, each of these wire rods was subjected to the billet reheating and then the finish rolling, followed by cooling at the average cooling rate I and the average cooling rate II on the conditions shown in Table 2.

[0062] The ferrite area ratio and the C content in the position at a depth of 0.1 mm from the surface of the obtained wire rod were measured, thereby evaluating the acid pickling properties.

(1) Ferrite Area Ratio

[0063] Each wire rod was cut at its section perpendicular to its axis (hereinafter referred to as the cross-section), and the metallic microstructure of the cross-section was etched in accordance with Steel-macroscopic examination defined by JIS G 0553 (2015). Any area with 0.156 mm.sup.2 in the D/4 position of the wire rod was observed with an optical microscope at a magnification of 200 times, and an obtained image was analyzed, thereby calculating a ferrite area ratio. This observation was performed at four field of views on the wire rod, and the measured values were averaged to determine a ferrite area ratio.

(2) C Content in the Position at a Depth of 0.1 mm from its Surface Layer

[0064] The C content in the position at the depth of 0.1 mm from the surface layer of each wire rod was measured by an electron probe micro analyzer (EPMA) line analysis. By using the measured value, a ratio of the C content in the 0.1 mm position to a C content in a base material was calculated, as shown in Table 2.

(3) Acid Pickling Properties

[0065] The wire rod was subjected to pickling by being immersed in a hydrochloric acid bath, and then the surface of its cross-section was observed to visually check the presence or absence of remaining scales. Pickling conditions were set as follows: hydrochloric acid concentration: 25%; hydrochloric acid temperature: 70 C.; and immersion time: 8 minutes. The samples having no remaining scales around its periphery were rated as Pass P, while the samples having any scale remaining on at least a part of the surface were rated as Fail F.

Manufacture of Steel Wire

[0066] Each of the above-mentioned wire rods was subjected to pickling on the pickling conditions for evaluation of the acid pickling properties, followed by a descaling process. Then, the wire rod was subjected to the spheroidize annealing, the descaling process, the coating process, and the finish wire-drawing on the following conditions, thereby fabricating a steel wire. Note that the wire rods rated as F in the above-mentioned evaluation of the acid pickling properties were excluded from this procedure. [0067] Spheroidize Annealing Conditions:

[0068] Soaking Temperature: 760 C.

[0069] Soaking Time: 5 hours

[0070] Average Cooling Rate: 13 C./hr.

[0071] Extraction Temperature: 685 C. [0072] Descaling Conditions:

[0073] Hydrochloric Acid Concentration: 25%

[0074] Hydrochloric Acid Temperature: 70 C.

[0075] Immersion Time: 8 minutes [0076] Coating Process Conditions:

[0077] Coating Type: Lime Coating

[0078] Immersion Time: 10 minutes [0079] Finish Wire-Drawing Conditions:

[0080] Wire-Drawing Speed: 1 m/sec

[0081] Area Reduction: 8% (9.39.06)

Manufacture of Bolt

[0082] Each of the obtained steel wires was subjected to cold forging using a multistage former, thereby producing a flange bolt with M10 mmP1.5 mm and Length 80 mm. Here, M means a diameter of the bolt shaft part, and P means a pitch.

(4) Cold Forgeability

[0083] After the above-mentioned cold forging, the cold forgeability of each bolt was evaluated by the presence or absence of flange cracks. With regard to the cold forgeability, the samples having no cracks are rated as Pass P, while the samples having any crack are rated as Fail F.

[0084] The bolts produced in the above-mentioned ways were subjected to a quenching and tempering process on the conditions shown in Table 3. At this time, the heating time for the quenching was set 15 minutes, the atmosphere inside the furnace was atmospheric air, and the quenching was oil-cooling at 25 C. The tempering heating time was 45 minutes. Note that the samples rated as Fail with regard to the cold forgeability were excluded from this procedure.

[0085] The austenite grain size, tensile strength, and resistance to delayed fracture of each bolt were evaluated.

(5) Austenite Grain Size

[0086] The bolt shaft part was cut at its section perpendicular to the bolt shaft (hereinafter referred to as a cross-section). Any areas with 0.039 mm.sup.2 located both in the d position of the bolt, where d is a diameter of the cross-section and at the outermost surface of the bolt were observed with an optical microscope at a magnification of 400 times. Subsequently, a prior austenite grain size number of each area was measured in accordance with Steels-Micrographic Determination of The Apparent Grain Size defined by JIS G 0551 (2015). The measurement of the grain size number was performed on four fields of view to determine an average of these grain size numbers, which was defined as the austenite grain size number. The samples having an austenite grain size number of No. 7.0 or more were rated as Pass P, while the samples having an austenite grain size number of less than No. 7.0 were rated as Fail F.

(6) Tensile Strength

[0087] The tensile strength of the bolt was determined by a tensile test in accordance with JIS B1051 (2014). The samples having a tensile strength of 1,400 MPa or more were rated as Pass, while the samples having a tensile strength of less than 1,400 MPa were rated as Fail.

(7) Resistance to Delayed Fracture

[0088] The resistance to delayed fracture of the bolt was evaluated by fastening the bolt by a jig toward a yield point and then repeating 10 cycles of processes on the bolt. Each cycle involves (a) immersing the bolt together with the jig into 1% HCl for 15 minutes, (b) exposing the bolt to the atmospheric air for 24 hours, and (c) confirming the presence or absence of fracture in the bolt. Regarding each sample, ten bolts were evaluated. The samples having no fracture in their bolts were rated as Pass P, while the samples having any fracture even in one of the bolts were rated as Fail F.

TABLE-US-00001 TABLE 1 Steel mate- rial Chemical composition [% by mass] No. C Si Mn P S Cr Al N Cu Ni Sn Ti Nb Zr Mo W V Mg Ca A 0.41 1.75 0.18 0.013 0.012 0.83 0.025 0.0048 B 0.38 1.51 0.45 0.010 0.012 0.50 0.030 0.0045 C 0.54 1.40 0.46 0.008 0.010 0.81 0.051 0.0081 D 0.33 2.34 0.80 0.017 0.018 0.49 0.053 0.0079 0.71 E 0.50 1.30 0.83 0.007 0.005 0.75 0.079 0.0045 0.050 F 0.40 1.75 0.15 0.008 0.010 1.05 0.030 0.0050 0.23 0.45 0.048 0.153 G 0.33 2.49 0.13 0.010 0.008 1.33 0.071 0.0125 0.113 H 0.35 2.21 1.20 0.018 0.016 1.35 0.025 0.0043 0.25 I 0.38 1.75 0.75 0.015 0.013 1.20 0.030 0.0050 0.220 J 0.31 1.15 0.28 0.013 0.012 0.79 0.025 0.0048 0.0021 0.0025 K 0.41 1.13 1.15 0.008 0.010 0.81 0.051 0.0039 1.53 L 0.54 1.51 1.21 0.010 0.013 0.53 0.049 0.0040 0.30 0.175 0.40 M 0.32 2.50 0.20 0.009 0.007 0.51 0.045 0.0045 0.075 A1 0.23 1.52 0.80 0.013 0.015 0.80 0.051 0.0040 B1 0.71 1.73 0.20 0.011 0.015 1.00 0.023 0.0045 C1 0.43 0.75 0.15 0.015 0.017 1.13 0.053 0.0045 D1 0.41 3.22 0.82 0.017 0.014 0.94 0.062 0.0039 E1 0.48 1.21 0.03 0.010 0.016 0.41 0.055 0.0031 F1 0.33 1.55 1.73 0.008 0.007 0.59 0.048 0.0075 0.053 G1 0.38 1.76 1.10 0.025 0.017 0.78 0.049 0.0028 0.035 H1 0.54 1.75 0.19 0.011 0.025 1.15 0.030 0.0103 0.051 I1 0.51 1.21 0.90 0.009 0.010 0.10 0.033 0.0054 0.210 J1 0.44 2.17 0.86 0.013 0.015 1.83 0.030 0.0061 0.20 K1 0.49 2.33 0.16 0.017 0.019 1.17 0.010 0.0135 0.22 L1 0.38 2.81 0.14 0.007 0.005 0.90 0.183 0.0041 0.21 M1 0.40 1.68 0.57 0.004 0.006 1.21 0.035 0.0238 0.73

TABLE-US-00002 TABLE 2 (2) C content in a (2) Ratio with Billet position at a depth respect to a C Steel reheating Finish rolling Average Average (1) Ferrite of 0.1 mm from the content in base Sample material temperature temperature cooling rate I cooling rate II area ratio surface layer material No. No. [ C.] C. C.] [ C./sec] [ C./sec] [%] [% by mass] [%] 1 A 950 950 5 10 35 0.27 66 2 A 1,000 950 3 10 33 0.23 56 3 A 1,050 950 8 10 37 0.33 80 4 B 1,050 950 5 8 39 0.27 71 5 B 1,000 950 5 13 14 0.26 68 6 C 1,100 950 5 8 34 0.41 76 7 C 1,000 950 8 8 33 0.49 91 8 D 1,100 950 6 13 18 0.24 73 9 D 1,050 950 8 8 35 0.31 94 10 E 1,100 1,000 5 10 30 0.31 62 11 F 1,100 1,000 5 10 38 0.25 63 12 G 1,000 950 8 10 31 0.33 87 13 H 1,000 950 8 10 28 0.29 83 14 I 1,000 950 5 13 21 0.24 63 15 J 1,000 950 7 13 28 0.36 80 16 K 1,000 950 7 13 12 0.32 78 17 L 1,000 950 5 10 38 0.33 61 18 M 1,100 1,000 4 10 31 0.18 56 19 A 1,000 950 1 10 34 0.08 20 20 A 1,000 950 12 10 8 0.40 98 21 B 1,000 950 5 4 60 0.23 61 22 B 1,000 950 5 16 6 0.24 63 23 C 1,000 950 5 10 25 0.38 70 24 C 1,000 1,050 5 10 26 0.39 72 25 M 950 950 5 10 28 0.21 66 26 A1 1,000 950 3 10 24 0.12 52 27 B1 1,000 950 5 8 30 0.43 61 28 C1 1,000 950 5 10 33 0.29 67 29 D1 1,000 950 8 10 31 0.34 83 30 D1 1,000 950 4 10 31 0.09 22 31 E1 1,000 950 6 13 24 0.38 79 32 F1 1,000 950 6 13 22 0.23 70 33 G1 1,000 950 5 13 15 0.21 55 34 H1 1,000 950 5 10 35 0.31 57 35 I1 1,000 950 6 10 30 0.39 76 36 J1 1,000 950 6 10 33 0.31 70 37 K1 1,000 950 6 10 31 0.33 67 38 L1 1,000 950 6 10 34 0.31 82 39 M1 1,000 950 6 10 30 0.26 65 40 D 1,000 950 1 1 63 0.03 9 41 E 1,100 1,000 8 10 27 0.35 70 42 F 1,100 1,000 8 10 34 0.30 75 43 G 1,000 950 8 12 23 0.30 91

TABLE-US-00003 TABLE 3 Heating (5) Austenite grain (3) Acid temperature Tempering size number (6) Tensile (7) Resistance Sample pickling (4) Cold before quenching temperature (Outermost strength to delayed No. properties forgeability [ C.] [ C.] (Inside) surface) [MPa] fracture 1 P P 880 400 8.5 9.0 1,799 P 2 P P 880 425 9.0 8.5 1,675 P 3 P P 880 450 8.5 9.0 1,549 P 4 P P 880 400 8.0 9.0 1,670 P 5 P P 880 450 9.0 9.5 1,425 P 6 P P 880 425 8.5 8.0 1,803 P 7 P P 880 450 9.5 9.0 1,655 P 8 P P 880 400 9.0 8.5 1,727 P 9 P P 880 400 8.5 8.5 1,727 P 10 P P 880 400 10.0 10.5 1,848 P 11 P P 920 400 11.0 10.8 1,867 P 12 P P 920 400 10.5 10.0 1,854 P 13 P P 880 400 8.5 8.5 1,828 P 14 P P 920 400 10.0 10.3 1,872 P 15 P P 880 400 10.5 10.3 1,531 P 16 P P 880 425 10.3 10.3 1,866 P 17 P P 920 425 10.5 10.3 1,779 P 18 P P 880 425 11.0 11.0 1,620 P 19 P P 880 400 9.0 6.5 1,785 F 20 F 21 P F 22 F 23 P P 880 425 7.5 7.5 1,791 P 24 P P 880 425 7.5 7.5 1,789 P 25 P P 880 425 7.0 7.0 1,614 P 26 P P 880 425 8.5 9.0 1,373 P 27 P P 880 425 9.0 8.5 2,127 F 28 P P 880 425 8.0 9.0 1,585 F 29 F 30 P P 880 450 8.5 6.0 1,787 F 31 P P 880 425 8.5 9.0 1,633 F 32 P F 33 P P 880 425 11.0 10.5 1,623 F 34 P P 880 450 10.5 11.0 1,760 F 35 P P 920 400 11.0 11.0 1,858 F 36 F 37 P F 38 P F 39 P F 40 P F 41 P P 900 400 10.0 10.5 1,851 P 42 P P 900 400 11.0 10.8 1,871 P 43 P P 900 400 10.5 10.0 1,564 P

[0089] From these results, the following consideration can be made. The samples Nos. 1 to 18, 23 to 25, and 41 to 43 are inventive examples satisfying the requirements specified by the present invention. All these examples had high strength and excellent acid pickling properties, cold forgeability, and resistance to delayed fracture.

[0090] The samples Nos. 19 to 22 and 26 to 40 are examples not satisfying any requirement specified by the present invention.

[0091] In the sample No. 19, the average cooling rate I was so low that the decarburization progressed. In this example, since the C content in the position at a depth of 0.1 mm from the surface layer was small, the austenite crystal grains were coarsened by the quenching and tempering process. Consequently, this example was inferior in the resistance to delayed fracture.

[0092] In the sample No. 20, since the average cooling rate I was high, the amounts of martensite formed at the surface layer and in the D/4 position were large. This example could not ensure the sufficient ferrite area ratio and thus was inferior in the acid pickling properties.

[0093] In the sample No. 21, since the average cooling rate II was low, the amount of formed ferrite was large. In this example, the ferrite area ratio was extremely high, and the dispersibility of carbides in the annealing became deteriorated, thus degrading the cold forgeability.

[0094] In the sample No. 22, since the average cooling rate II was high, the amount of formed ferrite decreased. This example could not ensure the sufficient ferrite area ratio and thus was inferior in the acid pickling properties.

[0095] The sample No. 26 was an example of using a steel type A1 in which the C content was below the lower limit of the present invention. This example could not ensure the tensile strength of 1,400 MPa or more.

[0096] The sample No. 27 was an example of using a steel type B1 in which the C content was above the upper limit of the present invention. This example was inferior in the resistance to delayed fracture because the toughness and ductility of the wire rod were degraded.

[0097] The sample No. 28 was an example of using a steel type C1 in which the Si content was below the lower limit of the present invention. This example was inferior in the resistance to delayed fracture because coarse grains of cementite were precipitated during the tempering process.

[0098] The sample No. 29 was an example of using a steel type D1 in which the Si content was above the upper limit of the present invention. In this example, an amorphous layer was formed on the surface layer of the wire rod, thereby deteriorating its acid pickling properties.

[0099] The sample No. 30 was an example of using a steel type D1 in which the Si content was above the upper limit of the present invention. In this example, the C content of the wire rod in the position at a depth of 0.1 mm from the surface layer was small, and the austenite crystal grains were coarsened by the quenching and tempering process. Consequently, this example was inferior in the resistance to delayed fracture.

[0100] The sample No. 31 was an example of using a steel type E1 in which the Mn content was below the lower limit of the present invention. This example was inferior in the resistance to delayed fracture because a large amount of FeS was formed.

[0101] The sample No. 32 was an example of using a steel type F1 in which the Mn content was above the upper limit of the present invention. This example was inferior in the cold forgeability because grains of MnS were coarsened.

[0102] The sample No. 33 was an example of using a steel type G1 in which the P content was above the upper limit of the present invention. This example was inferior in the resistance to delayed fracture because the toughness and ductility of the wire rod were degraded.

[0103] The sample No. 34 was an example of using a steel type H1 in which the S content was above the upper limit of the present invention. This example was inferior in the resistance to delayed fracture because the toughness and ductility of the wire rod were degraded.

[0104] The sample No. 35 was an example of using a steel type I1 in which the amount of added Cr was small. This example was inferior in the resistance to delayed fracture because the corrosion resistance of the wire rod was degraded.

[0105] The sample No. 36 was an example of using a steel type J1 in which the Cr content was above the upper limit of the present invention. Tn this example, a Cr enrichment layer was formed on the surface layer of the wire rod, thereby degrading its acid pickling properties.

[0106] The sample No. 37 was an example of using a steel type K1 in which the Al content was below the lower limit of the present invention. This example was inferior in the cold forgeability because ferrite crystal grains were coarsened.

[0107] The sample No. 38 was an example of using a steel type L1 in which the Al content was above the upper limit of the present invention. This example was inferior in the cold forgeability because coarse grains of AlN were formed.

[0108] The sample No. 39 was an example of using a steel type M1 in which the N content was above the upper limit of the present invention. This example was inferior in the cold forgeability because the amount of solid-solution N was increased.

[0109] In the sample No. 40, since both the cooling rates I and II were slow, the amount of formed ferrite was large, and further, the decarburization rate was high. In this example, the ferrite area ratio was extremely high, and the dispersibility of carbides in the annealing became deteriorated, thus degrading the cold forgeability.