Improvement of the Mechanical Properties of 1022 Carbon Steel Coil ...

materials Article

Improvement of the Mechanical Properties of 1022 Carbon Steel Coil by Using the Taguchi Method to Optimize Spheroidized Annealing Conditions Chih-Cheng Yang 1, * and Chang-Lun Liu 2 1 2

*

Department of Mechanical and Automation Engineering, Kao Yuan University, Kaohsiung 82151, Taiwan Graduate School of Fasteners Industry Technology, Kao Yuan University, Kaohsiung 82151, Taiwan; [email protected] Correspondence: [email protected]; Tel.: +886-7-607-7027

Academic Editor: Stephen D. Prior Received: 19 May 2016; Accepted: 5 August 2016; Published: 12 August 2016

Abstract: Cold forging is often applied in the fastener industry. Wires in coil form are used as semi-finished products for the production of billets. This process usually requires preliminarily drawing wire coil in order to reduce the diameter of products. The wire usually has to be annealed to improve its cold formability. The quality of spheroidizing annealed wire affects the forming quality of screws. In the fastener industry, most companies use a subcritical process for spheroidized annealing. Various parameters affect the spheroidized annealing quality of steel wire, such as the spheroidized annealing temperature, prolonged heating time, furnace cooling time and flow rate of nitrogen (protective atmosphere). The effects of the spheroidized annealing parameters affect the quality characteristics of steel wire, such as the tensile strength and hardness. A series of experimental tests on AISI 1022 low carbon steel wire are carried out and the Taguchi method is used to obtain optimum spheroidized annealing conditions to improve the mechanical properties of steel wires for cold forming. The results show that the spheroidized annealing temperature and prolonged heating time have the greatest effect on the mechanical properties of steel wires. A comparison between the results obtained using the optimum spheroidizing conditions and the measures using the original settings shows the new spheroidizing parameter settings effectively improve the performance measures over their value at the original settings. The results presented in this paper could be used as a reference for wire manufacturers. Keywords: spheroidized annealing; formability; Taguchi method

1. Introduction A cold-heading-quality alloy steel rod is used to manufacture wire for cold heading. Generally, the wire made from the quality rod is spheroidizing annealed, either in a single process or after drawing the finished product [1]. The wire that is spheroidizing-annealed-in-process is produced by drawing wire coil into wire, followed by heat treatment, cleaning and coating, and then a final drawing operation for cold forming. Spheroidization of cementite lamellae through spheroidized annealing improves the ductility of steel [2–4]. Rad-Con Inc. (Cleveland, OH, USA) provided a spheroidized annealing process that produced steel wire with little or no decarbonization under completely computerized control [2]. The majority of all spheroidizing activity is performed to improve the cold formability of steels. A spheroidized microstructure is desirable for cold forming because it lowers the flow stress of the material. Steels may be spheroidized to produce a structure of globular carbides in a ferritic matrix [5]. Many studies on the mechanisms and kinetics of spheroidization have been undertaken [6–9]. O’Brien and Hosford [6] investigated the spheroidization of medium carbon steels, AISI 1541 and Materials 2016, 9, 693; doi:10.3390/ma9080693

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Materials 2016, 9, 693

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AISI 4037, used in the bolt industry with two process cycles, the intercritical cycle and subcritical cycle. Introducing defects in the cementite by severe plastic deformation is one effective method to increase the spheroidization speed. Hono et al. [7] revealed that the cementites in near-eutectic steel can be spheroidized more easily after a severe drawing. Shin et al. [8] studied the enhanced spheroidization kinetics in terms of carbon dissolution from cementites and defects induced in cementites by severe plastic deformation, and revealed that an increase in accumulated strains in the equal channel angular pressed steel decreased the spheroidization temperature and time. Gul’ et al. [9] developed a new method for more intense spheroidization of cementite to accelerate spheroidization. Spheroidization is produced by non-isothermal holding at high temperatures, by means of an internal heat source. In the bolt industry, most companies use a subcritical process for spheroidized annealing, by simply heating the products to below the lower critical temperature and maintaining this temperature. Some companies simply purchase steel wires, and cold reduce them, and spheroidize them before selling them to bolt manufacturers. Some companies, which manufacture bolts, spheroidize the wires themselves before cold heading. A cold-heading-quality AISI 1022 steel wire is usually used to manufacture self-drilling screws and tapping screws. The wire has to be spheroidizing annealed after drawing the wire coil (Φ5.5 mm) to a specific size with section-area reductions of about 60%. The quality of spheroidized annealed wire affects the forming quality of the screws. Various parameters affect the quality of spheroidized annealing, such as the spheroidized annealing temperature, prolonged heating time, furnace cooling time and flow rate of nitrogen (protective atmosphere). The effects of the spheroidized annealing parameters affect the quality characteristics of wires, such as the tensile strength and hardness. The Taguchi method is a quality improvement technique that uses experimental design methods for efficient characterization of a product or process, combined with a statistical analysis of its variability with the fact that pre-production experiments, properly designed and analyzed, can significantly contribute to efforts towards the accurate characterization and optimization of industrial processes, the quality improvement of products, and the reduction of costs and waste [10]. In this study, the Taguchi method is used to optimize spheroidized annealing conditions to improve the mechanical properties of AISI 1022 low carbon steel wire. 2. Experiment Design AISI 1022 low carbon steel wire is investigated in this study. Its chemical composition is listed in Table 1. A subcritical process is used for spheroidized annealing of the steel wire, simply heating it to below the lower critical temperature and maintaining this temperature. Table 1. Chemical composition of AISI 1022 low carbon steel wires (wt. %). C

Mn

P

0.22

0.76–0.77

S

Si

0.012–0.013 0.002–0.008 0.02–0.04

Ni

Cr

0.01–0.03

0.04

Cu

Al

0.01–0.08 0.030–0.034

Four process parameters with three levels, as listed in Table 2, are chosen as the experimental factors in this study. Every factor has three levels to spheroidize wire in order to evaluate the mechanical properties of the wire. The parameters of Level 2 are the original spheroidized annealing process conditions, which was using in the company. Table 2. Experimental factors and their levels for L9 orthogonal array. Factor A: B: C: D:

Spheroidized annealing temperature Flow rate of nitrogen (Nm3 /h) Furnace cooling time (h) Prolonged heating time (h)

(◦ C)

Level 1

Level 2

Level 3

695 5 7.5 7

700 10 8.0 7.5

705 15 8.5 8


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The Taguchi method allows the changing of many factors at the same time in a systematic way, ensuring the reliable and independent study of the factors’ effects. The orthogonal array table, L9 (34 ), is used as an experimental design for these four factors [11], as listed in Table 3. Table 3. L9 (34 ) orthogonal array experimental parameter assignment. Exp. No.

A: Spheroidized Annealing Temperature (◦ C)

B: Flow Rate of Nitrogen (Nm3 /h)

C: Furnace Cooling Time (h)

D: Prolonged Heating Time (h)

L1 L2 L3 L4 L5 L6 L7 L8 L9

695 695 695 700 700 700 705 705 705

5 10 15 5 10 15 5 10 15

7.5 8.0 8.5 8.0 8.5 7.5 8.5 7.5 8.0

7.0 7.5 8.0 8.0 7.0 7.5 7.5 8.0 7.0

In this study, two quality characteristics of the spheroidized annealing wire, tensile strength and hardness, are investigated. Each test trial, including 10 specimens, is followed by each fabrication process and the results are then transformed to the S/N ratio (signal-to-noise ratio). Spheroidizing provides the needed ductility for cold heading. Through spheroidized annealing, the ductility of steel wire may be improved, and the hardness, which is obtained from the Vickers hardness test, may be reduced as well. Therefore, in terms of the desired characteristics for the hardness, the smaller the better, and the S/N ratio is [11] S/N = −10 · log( µ2 + S2 ) (1) where µ is the mean of each trial and S is the standard deviation. When the hardness is reduced to improve the ductility of the steel wire through spheroidized annealing, the strength of the steel wire is simultaneously decreased. However, a given strength of the annealing steel wire has to be provided for cold heading. Therefore, the tensile strength of the steel wire is the main quality characteristic, with a target value of 383 MPa, which is assigned by the company. The S/N ratio for the nominal-the-best response is [11] S/N = −10 · log[( µ − m )2 + S2 ]

(2)

where µ is the mean of each trial, m is the target value, and S is the standard deviation. The tensile tests are conducted on a 30 ton universal testing machine under a constant ram speed of 25 mm/min at room temperature. The dimensions of the tensile specimen are Φ3.5 mm × L300 mm. Analysis of variance (ANOVA) is an effective method to determine the significant factors and the optimal fabrication conditions to obtain optimal quality. For the Taguchi method, the experimental error is evaluated with ANOVA to carry out the significance test of the various factors. The nature of the interaction between factors is considered as experimental error [11]. If the effect of a factor in comparison to the experimental error is sufficiently large, it is identified as a significant factor. The confidence level of a factor is evaluated with the experimental error to identify the significant factor that influences the material property of the spheroidized annealing wire. 3. Results and Discussion The microstructure of drawn steel wire is shown in Figure 1a. This is not yet spheroidized annealed. The tensile strength and hardness are, respectively, about 822 MPa and 285 HV due to heavy plastic work. Spheroidizing is the process of producing a microstructure in which the cementite is in a spheroidal distribution, as shown in Figure 1b. The globular structure obtained gives improved


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formability to the steel wire. When the wire is fabricated following the original spheroidized annealing process conditions (Level 2 in Table 2), the mean strength and mean hardness are 388.7 MPa spheroidized annealing process conditions (Leveltensile 2 in Table 2), the mean tensile strength and mean and 141.3are HV,388.7 respectively. half of the non-spheroidized hardness MPa and They 141.3 are HV,about respectively. They are about half ofwire. the non-spheroidized wire.

(a)

(b)

Figure 1. 1. Microstructures wires ((×500). (b)Spheroidized. Spheroidized. Figure Microstructuresofofdrawn drawnAISI AISI1022 1022 steel steel wires ×500). (a) (a) Non-spheroidized; Non-spheroidized; (b)

The optimum experimental results of the tensile strength and hardness (mean, μ; standard The optimum experimental results of the tensile strength and hardness (mean, µ; standard deviation, S; and S/N ratio) of spheroidized annealed steel wire are shown in Tables 4 and 5 deviation, S; and S/N ratio) of spheroidized annealed steel wire are shown in Tables 4 and 5 respectively. respectively. The mean tensile strength varies from 368.9 to 407.0 MPa. The mean tensile strengths of The mean tensile strength varies from 368.9 to 407.0 MPa. The mean tensile strengths of tests L2, L3 tests L2, L3 and L7 are smaller than the target value, as shown in Table 4. The mean tensile strength and L7 are smaller than the target value, as shown in Table 4. The mean tensile strength of test L8 is of test L8 is very close to the target value and its standard deviation is the smallest of the nine tests. very close to the target value and its standard deviation is the smallest of the nine tests. Exp. No. a T1 L1 417a Exp. No. L2 365 L1 L3 411 L2 L4 L3388 L5 L4400 L5381 L6 L6377 L7 L7 L8 376 L8 L9 L9394

T2 409 T1 370 417 379 365 415 411 373 388 400 415 381 369 377 384 376 386 394

Table 4. The experimental results for tensile strength. Table 4. The experimental results for tensile strength. T3 T4 T5 T6 T7 T8 T9 T10 μ (MPa) S/N Ratio S 362 387 417 407 403 398 400 376 397.6 16.96 −27.10 T2 T3 T4 T5 T6 T7 T8 T9 T10 µ (MPa) S S/N Ratio 374 359 380 387 398 340 382 335 368.9 18.93 −27.38 409 362 387 417 407 403 398 400 376 381 365 367 360 365 367 372 354 397.6 372.0 16.9615.02−27.10−25.29 370 374 359 380 387 398 340 382 335 368.9 18.93 −27.38 409 413 365 398 367366360 385 400 354 390 372.0 396.8 15.0214.18−25.29−26.06 379 381 365 403 367 372 385 386 413 414 398382366 397 409 390 432 396.8 398.9 14.1817.16−26.06−27.48 415 409 385 412 403 400 373 385 397 413 412 409 427 411 386 417 414413382 407 398 432 388 398.9 407.0 17.1613.24−27.48−28.88 415 427 407 377 413 398 375 391 411 356 417374413 381 387 388 394 407.0 378.1 13.2410.68−28.88−21.28 369 375 391 356 374 381 377 387 394 378.1 10.68 −21.28 388 387 393 401 399 393 391 378 389.1 389.1 8.00 8.00−20.25−20.25 384 388 387 393 401 399 393 391 378 374 392 392 382 382412412 400 378 398 398 390.8 390.8 10.9010.90−22.71−22.71 386 374 400 392 392 378 a

a Experimentalconditions Experimental as defined definedininTable Table conditions as 3. 3.

Table Table 5. The experimental results for hardness. a Exp. No. T1 T1 T2 T2T3 T3T4 T4 T5T5 T6 T7 T8 T9T9 T10T10µ (HV) μ (HV) S S S/N S/N Ratio Exp. No. a T6 T7 T8 Ratio L1 147 145 147 148 148 144 152 154 155 137 147.7 4.98 −43.4 L1 147 145 147 148 148 144 152 154 155 137 147.7 4.98 −43.4 L2 153 136 135 145 151 137 133 137 136 135 139.8 6.81 L2 153 136 135 145 151 137 133 137 136 135 139.8 6.81 −42.9−42.9 L3 138 136 144 146 145 144 135 149 142 133 141.2 5.08 L3 138 136 144 146 145 144 135 149 142 133 141.2 5.08 −43.0−43.0 148 145 148 143 147 150 148 146 137 143 145.5 3.56 −43.3 L4 L4 148 141 145 132 148 144143140 147 150 148 146 137 143 141.6145.5 7.20 3.56 −43.0−43.3 L5 142 161 139 142 137 138 L5 132 147 144 149140143 142 142 146 137 155138 146.3141.6 3.77 7.20 −43.3−43.0 L6 141 141 147 161 143 139 148 144 L7 151 141 147 144 137 143 142 154 135 L6 141 147 149 143 147 143 148 144 146 138155 143.2146.3 5.76 3.77 −43.1−43.3 L8 138 133 135 131 139 134 138 133 126 133 134.0 3.66 −42.5 L7 141 147 147 148144146 137 154 142 135 139138 144.1143.2 3.78 5.76 −43.2−43.1 L9 151 146 145 143 136 142 148 144 L8 138 133 135 a131 139 134 138 133 126 133 134.0 3.66 −42.5 Experimental conditions as defined in Table 3. L9 146 147 148 146 145 136 148 144 142 139 144.1 3.78 −43.2 As shown in Table 5, thea Experimental mean hardness variesas from 134.0 147.7 conditions defined in to Table 3. HV, and the mean values of tests L2, L3 and L8 are smaller than the value at the original settings. The properties of spheroidized annealed steel wire are obviously altered withvaries various spheroidized annealing process conditions. As shown in Table 5, the mean hardness from 134.0 to 147.7 HV, and the mean values of tests L2, L3 and L8 are smaller than the value at the original settings. The properties of spheroidized annealed steel wire are obviously altered with various spheroidized annealing process conditions.


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3.1. Tensile Strength To obtain optimum quality, analysis of variance (ANOVA) is an effective method to determine significant factors and optimum fabrication conditions. The contribution and confidence level of each factor constructed in Table 6 could identify the significant factor affecting the tensile strength of wire. The contribution of a factor is the percentage of the sum of squares (SS), that is, the percentage of the factor variance to the total quality loss [10,11]. The effect of a factor may be pooled to error if its confidence level or contribution is relatively small. It is clear from the ANOVA table that the contribution of the spheroidized annealing temperature (A) is 87.0% of the total variation, which is the highest contributor to the variability of the experimental results. The contribution of prolonged heating time (D) is 10.2%, which is the second highest contribution. However, the factors of the furnace cooling time (C) and the flow rate of nitrogen (B) are not significant for the S/N ratio because their contributions are relatively small. With the pooling of errors from the non-significant factors (B and C), the error estimation for the S/N ratio is obtained [11] and then the confidence levels are 99.9% and 95.4%, respectively, for the spheroidized annealing temperature (A) and prolonged heating time (D). That is, both factors, particularly the spheroidized annealing temperature, significantly affect the tensile strength of the steel wire, with more than a 95.0% confidence level. Table 6. Variance analysis table of signal-to-noise (S/N) ratio for tensile strength. Factor

SS

DOF

Var.

Contribution

A B C D Total

64.27 1.06 1.02 7.54 73.88

2 2 2 2 8

32.13 0.53 0.51 3.77 –

87.0% 1.4% 1.4% 10.2% 100.0%

Pooling of errors Factor

SS

DOF

Var.

F

A B C D Error Total

64.27

2

32.13

7.54 2.07 73.88

2 4 8

3.77 0.52

62.05 Pooled Pooled 7.28

Confidence Significance 99.9%

Yes

95.4% Yes Sexp = 0.72 * At least 95.0% confidence level

SS: sum of squares; DOF: degree of freedom; Var.: variance; F: F-ratio; Sexp : experimental error.

Figure 2 illustrates the factor response diagram and the level averages of four factors with respect to the S/N ratio. For each factor, the effect is the range of the level averages and the maximum level average is the optimum level [10,11]. It is obviously revealed that, for the four factors, the original levels (Level 2) are not the optimum fabricating parameters to obtain the target tensile strength. For the significant factors of the spheroidized annealing temperature (A) and prolonged heating time (D), Level 3 for the spheroidized annealing temperature (705 ◦ C, A3) and Level 3 for the prolonged heating time (8 h, D3) are evidently the optimum levels, as shown in Figure 2. However, it is observed that their responses are not linear either with the annealing temperature or prolonged heating time. For the spheroidized annealing temperature, the optimum level is closer to the lower critical temperature. Its response is much more effective than the other two levels. The effects of the other two factors, the flow rate of nitrogen (B) and furnace cooling time (C), are relatively small. The optimum levels are Level 1 for the flow rate of nitrogen (5 Nm3 /h, B1) and Level 3 for the furnace cooling time (8.5 h, C3), respectively.

Materials 2016, 9, 693 Materials 2016, 9, 693

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Figure 2. The factor response diagram for tensile strength. Figure 2. The factor response diagram for tensile strength.

3.2. Hardness 3.2. Hardness For For the the hardness hardness of of the the annealed annealed steel steel wire, wire, the the ANOVA ANOVAtable tableof ofthe theS/N S/N ratio ratio is is constructed constructed in in Table 7. It is evident from Table 7 that the highest contributors to the variability of the Table 7. It is evident from Table 7 that the highest contributors to the variability of the experimental experimental results results are are the the flow flow rate rate of of nitrogen nitrogen (B, (B, 58.5%), 58.5%), the the prolonged prolonged heating heating time time (D, (D, 21.5%) 21.5%) and and the the spheroidized annealing temperature (A, 18.8%). However, the furnace cooling time (C) is not a significant spheroidized annealing temperature (A, 18.8%). However, the furnace cooling time (C) is not a factor because its contribution is relatively small. With the pooling of the errorsfrom significant factor because its contribution is relatively small. With poolingthe of non-significant errorsfrom the factor (C), the confidence levels 93.7%, 97.9% andare 94.5%, respectively, for94.5%, the spheroidized annealing non-significant factor (C), theare confidence levels 93.7%, 97.9% and respectively, for the temperature (A), flow rate of nitrogen (B) and prolonged heating time (D). That is, the hardness of(D). the spheroidized annealing temperature (A), flow rate of nitrogen (B) and prolonged heating time steel wire is significantly affected by the spheroidized annealing temperature, flow rate of nitrogen That is, the hardness of the steel wire is significantly affected by the spheroidized annealing and prolongedflow heating with more than a 90.0% confidence temperature, rate time, of nitrogen and prolonged heating time, level. with more than a 90.0% confidence

level.

Table 7. Variance analysis table of signal-to-noise (S/N) ratio for hardness. Table 7. Variance analysis table of signal-to-noise (S/N) ratio for hardness. Factor

Factor A A B B C C D Total D Total

SS

DOF

Var.

Contribution

SS DOF Var. Contribution 0.096 18.8% 0.096 2 2 0.048 0.048 18.8% 0.300 2 0.150 58.5% 0.300 2 2 0.150 0.003 58.5% 0.006 1.3% 0.006 2 0.003 1.3% 0.110 2 0.055 21.5% 0.513 100.0% 0.110 2 8 0.055 21.5% 0.513 8 100.0% Pooling of errors Pooling of errors Factor SS DOF Var. F Confidence Significance Factor SS DOF Var. F Confidence Significance A 0.096 2 0.048 14.96 93.7% Yes A 0.096 2 0.048 14.96 93.7% Yes B 0.300 2 0.150 46.63 97.9% Yes C Pooled B 0.300 2 0.150 46.63 97.9% Yes D 0.110 2 0.055 Pooled 17.10 94.5% Yes C Sexp = 0.06 Error 0.006 2 0.003 D 0.110 2 0.055 17.10 94.5% Yes Total 0.513 8 * At least 90.0% confidence level Error 0.006 2 0.003 Sexp = 0.06 SS: sum of squares; DOF: degree of freedom; Var.: variance; F: F-ratio; Sexp : experimental error. Total 0.513 8 * At least 90.0% confidence level SS: sum of squares; DOF: degree of freedom; Var.: variance; F: F-ratio; Sexp: experimental error.

The factor response diagram and the level averages of four factors with respect to the S/N ratio are illustrated Figure 3. It is observed responses are not linear either withtothe The factorinresponse diagram and thethat levelthe averages of four factors with respect theannealing S/N ratio temperature, the flow rate of nitrogen or the furnace cooling time; but are linear with the prolonged are illustrated in Figure 3. It is observed that the responses are not linear either with the annealing heating time.the It isflow revealed increasing prolonged heating time may lower of temperature, rate ofthat nitrogen or thethe furnace cooling time; but are linear withthe thehardness prolonged the wire. For the factor of the flow rate of nitrogen, which is a protective atmosphere to prevent heating time. It is revealed that increasing the prolonged heating time may lower the hardness of the decarbonization whileof heating the wire, it is the mostissignificant factor in this study, effect wire. For the factor the flow rate although of nitrogen, which a protective atmosphere to its prevent is even smaller than the effect on tensile strength and it is not a significant factor for tensile strength, as decarbonization while heating the wire, although it is the most significant factor in this study, its effect shown in Figure 2. is even smaller than the effect on tensile strength and it is not a significant factor for tensile strength, as

shown in Figure 2. For the three significant factors of the spheroidized annealing temperature (A), flow rate of nitrogen (B) and prolonged heating time (D), the optimum levels are Level 3 for the spheroidized


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For the three significant factors of the spheroidized annealing temperature (A), flow rate of 3/h,the nitrogen and prolonged heating time 2(D), levels are Level 3 for annealing (B) temperature (705 °C, A3), Level for the the optimum flow rate of nitrogen (10 Nm B2)spheroidized and Level 3 ◦ 3 annealing temperature (705 Level 2 for the flow rate of nitrogen Nmeffect /h, B2) andfurnace Level 3 for the prolonged heating timeC,(8A3), h, D3), respectively, as shown in Figure(10 3. The of the for the prolonged time (8 h,Level D3), respectively, as shown Figure 3. The cooling effect oftime the furnace cooling time (C) is heating relatively small. 3 is the optimum levelin for the furnace (8.5 h, cooling time (C) is relatively small. Level 3 is the optimum level for the furnace cooling time (8.5 h, C3). C3).

Figure3.3.The Thefactor factorresponse responsediagram diagramfor forhardness. hardness. Figure

With With the the optimum optimum analysis analysis for for the the quality quality characteristics characteristicsof of tensile tensile strength strength and and hardness, hardness, the the optimum conditions are shown in Table 8. The factors of spheroidized annealing temperature (A) optimum are shown in Table 8. The factors of spheroidized annealing temperature (A) and and prolonged heating (D) obviously are obviously significant for both the tensile strength hardness prolonged heating timetime (D) are significant for both the tensile strength and and hardness and and the level. same Therefore, level. Therefore, the optimum are determined as Level 3 for the withwith the same the optimum levels arelevels determined as Level 3 for the spheroidized ◦ C, A3) and spheroidized annealing(705 temperature (705 °C, 3A3) Level 3 forheating the prolonged time (8 h, annealing temperature Level forand the prolonged time (8 h,heating D3). The factor of D3). The factor the flow of nitrogen (B)for is not significant tensile strength,for but is significant the flow rate ofof nitrogen (B)rate is not significant tensile strength,for but is significant hardness. Thus, 3/h, B2) is then chosen as the for hardness. Level 2 for the(10flow of nitrogen (10 Nm Level 2 for theThus, flow rate of nitrogen Nm3rate /h, B2) is then chosen as the optimum level. The factor of optimum level. The furnace cooling timefor (C)tensile is not strength significant for tensile strength or furnace cooling timefactor (C) isofnot significant either or either hardness. The original level, hardness. The Level 2, (8 forh,the cooling time (8 h, C2) is determined. Level 2, for theoriginal furnacelevel, cooling time C2)furnace is determined. Table 8. Optimum condition table for spheroidized annealing. Table 8. Optimum condition table for spheroidized annealing.

Factor

Tensile Strength

Hardness

Factor

Tensile Strength

Hardness

A A B B C C D

A3 * A3 * B1 B1 C3 C3 D3** D3

A3 * A3 ** B2 B2 * C3 C3 D3 * D3

Optimum Optimum

A3 B2A3 B2 C2C2 D3 D3

* Significant factor. factor. * Significant

3.3. 3.3.Confirmatory ConfirmatoryExperiments Experiments In order to verify the predicted results, wire is fabricated using the optimum levels: A3, B2, C2 In order to verify the predicted results, wire is fabricated using the optimum levels: A3, B2, and D3 (as described in Table 8). Figure 4 shows the original (using level 2s in Table 2) and optimal C2 and D3 (as described in Table 8). Figure 4 shows the original (using level 2s in Table 2) and probability distributions, respectively, for the tensile strength and hardness of the steel wire. optimal probability distributions, respectively, for the tensile strength and hardness of the steel wire. Compared with the original results, the optimum mean tensile strength of 384.7 MPa is not only Compared with the original results, the optimum mean tensile strength of 384.7 MPa is not only closer to the target value, but also the deviation is decreased by about 56%. The optimum mean closer to the target value, but also the deviation is decreased by about 56%. The optimum mean hardness of 129.0 HV is considerably decreased compared to the original mean hardness of 141.3 HV. hardness of 129.0 HV is considerably decreased compared to the original mean hardness of 141.3 HV. In addition, the deviation decreases by about 65% compared to the original result. The new In addition, the deviation decreases by about 65% compared to the original result. The new parameter parameter settings evidently improve the performance measures, such as ductility and strength, settings evidently improve the performance measures, such as ductility and strength, over their value over their value at the original settings, and thus the quality of the spheroidized annealed steel wire. at the original settings, and thus the quality of the spheroidized annealed steel wire. Therefore, the Therefore, the formability of the AISI 1022 steel wire is effectively improved. formability of the AISI 1022 steel wire is effectively improved.

Materials Materials2016, 2016,9, 9,693 693 Materials 2016, 9, 693

888of of999 of

Optimum Optimum 384.7 ==384.7 5.6 SS==5.6

Optimum Optimum 129.0 ==129.0 1.6 SS==1.6

Original Original 388.7 ==388.7 12.8 SS==12.8

340 340

360 360

380 400 420 380 400 420 Tensilestrength strength(MPa (MPa)) Tensile

Original Original 141.3 ==141.3 4.6 SS==4.6

440 440

120 120

130 130

(a) (a)

140 150 140 150 Hardness(HV) (HV) Hardness

160 160

(b) (b)

Figure 4.The The probabilitydistribution distribution diagramfor for(a) (a)tensile tensilestrength; strength; (b) (b)hardness. hardness. Figure Figure 4. 4. The probability probability distribution diagram diagram for (a) tensile strength; (b) hardness.

4. Materials Materials and and Methods 4. 4. Materials andMethods Methods In this study, thewire wireis isspheroidizing spheroidizingannealed annealedafter afterdrawing drawingAISI AISI1022 1022steel steelwire wirecoil coil(Φ5.5 (Φ5.5mm) mm) In this this study, study, the the In wire is spheroidizing annealed after drawing AISI 1022 steel wire coil (Φ5.5 mm) to aa specific specific size size (Φ3.5 (Φ3.5 mm) mm) with with section-area section-area reductions reductions of of about about 60%. 60%. The The steel wire wire coil coil is is to to a specific size (Φ3.5 mm) with section-area reductions of about 60%. The steel steel wire coil is manufactured (Φ5.5 mm, Al-killed) by China Steel Corporation, Kaohsiung, Taiwan. Its chemical manufactured (Φ5.5 (Φ5.5 mm, mm, Al-killed) Al-killed) by by China China Steel Steel Corporation, Corporation, Kaohsiung, Kaohsiung, Taiwan. Taiwan. Its Its chemical chemical manufactured composition is is listed listed in in Table Table 1. 1. The The steel steel wire wire is is spheroidizing spheroidizing annealed, annealed, procedures procedures as as shown shown in in composition composition is listed in Table 1. The steel wire is spheroidizing annealed, procedures as shown in Figure 5, 5, with with CCP-2820 CCP-2820 pit type type annealing furnace furnace (Tainan Chin Chin Chang Electrical Electrical Co., Ltd., Ltd., Tainan, Figure Figure 5, with CCP-2820 pit pit type annealing annealing furnace (Tainan (Tainan Chin Chang Chang Electrical Co., Co., Ltd., Tainan, Tainan, Taiwan). The Taguchi method allows the changing of many factors at the same time in systematic Taiwan). The The Taguchi Taguchi method method allows allows the the changing changing of of many many factors factors at at the the same same time time in in aaasystematic systematic Taiwan). way. The The orthogonal orthogonal array array table, table, LL949(3 (344), is is used used as as an an experimental experimental design design for for the the factors factors [11], [11], as as way. way. The orthogonal array table, L9 (3 ), ), is used as an experimental design for the factors [11], as listed listed in in Table Table 3. 3. listed in Table 3. A: A:Spheroidized Spheroidizedannealing annealing temperature temperature

800 800

00

Pre-

Pre00heating heating

D:Prolonged Prolonged D: heating time heating time

C:Furnace Furnace C: cooling time cooling time

cooling cooling in theair air in the

25 25

Figure The spheroidized Figure5. 5.The Thespheroidized spheroidizedannealing annealingprocedure. procedure. Figure 5. annealing procedure.

5. Conclusions 5. Conclusions Conclusions 5. The The alloy alloy steel steel rod rod with with cold-heading cold-heading quality quality is is usually usually used used for for the the manufacture manufacture of of wire wire for for The alloy steel rod with cold-heading quality is usually used for the manufacture of wire for cold heading. Generally, the wire made from the quality rod is spheroidized annealed. The majority cold heading. Generally, the wire made from the quality rod is spheroidized annealed. The majority cold heading. Generally, the wire made from the quality rod is spheroidized annealed. The majority of ofspheroidizing spheroidizing activity activity is is performed performed to to improve improvethe thecold coldformability formabilityof ofsteel steelwires. wires. The Thequality qualityof of of spheroidizing activity is performed to improve the cold formability of steel wires. The quality of spheroidized steel wire In this this study, study, the spheroidized annealed annealed steel steel wire wire affects affects the the forming forming quality quality of of screws. screws. In study, the the Taguchi Taguchi spheroidized annealed affects the forming quality of screws. Taguchi method is used to obtain optimum spheroidized annealing conditions to improve the mechanical method is used to obtain optimum spheroidized annealing conditions to improve the mechanical method is used to obtain optimum spheroidized annealing conditions to improve the mechanical properties propertiesof ofAISI AISI1022 1022low lowcarbon carbonsteel steelwire. wire.The Thespheroidized spheroidizedannealing annealing qualities qualities of of steel steel wire wireare are properties of AISI 1022 low carbon steel wire. The spheroidized annealing qualities of steel wire are affected by various factors, such as the spheroidized annealing temperature, prolonged heating time, affected by various factors, such as the spheroidized annealing temperature, prolonged heating time, affected by various factors, such as the spheroidized annealing temperature, prolonged heating time, furnace furnace cooling cooling time timeand andflow flowrate rateof ofnitrogen. nitrogen.The Theeffects effectsof ofthe thespheroidized spheroidizedannealing annealingconditions conditions furnace cooling time and flow rate of nitrogen. The effects of the spheroidized annealing conditions affect the quality characteristics of steel wire, such as the tensile strength and hardness. Since affectthe the quality quality characteristics characteristics of of steel steelwire, wire, such such as asthe the tensile tensile strength strength and and hardness. hardness. Since Sinceaaagiven given affect given strength of the annealing steel wire has to be provided for cold heading, the tensile strength is strength of the annealing steel wire has to be provided for cold heading, the tensile strength is the the strength of the annealing steel wire has to be provided for cold heading, the tensile strength is the main quality characteristic of spheroidized annealed steel wire, with a target value of 383 MPa. It is main quality characteristic of spheroidized annealed steel wire, with a target value of 383 MPa. It is


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main quality characteristic of spheroidized annealed steel wire, with a target value of 383 MPa. It is experimentally revealed that the spheroidized annealing temperature (A) and the prolonged heating time (D) are the significant factors; the determined levels are Level 3 for the spheroidized annealing temperature (705 ◦ C, A3), Level 3 for the prolonged heating time (8 h, D3), Level2 for the furnace cooling time (8 h, C2), and Level 2 for the flow rate of nitrogen (10 Nm3 /h, B2). In addition, the optimum mean tensile strength is 384.7 MPa, and the optimum mean hardness is 129.0 HV. The new spheroidizing parameter settings evidently improve the performance measures over their values at the original settings. The formability of the AISI 1022 steel wire is effectively improved. The results may be used as a reference for wire manufacturers. Acknowledgments: The authors would like to acknowledge the Ministry of Science and Technology of the Republic of China (Taiwan, R.O.C.) for their support of this research under grant MOST 105-2632-E-244-001, and the support of Homn Reen Enterprise Co., Ltd., Kaohsiung, Taiwan, for providing the materials and apparatus to carry out the spheroidized annealing experimental work. Author Contributions: Chih-Cheng Yang conceived and designed the experiments; Chang-Lun Liu performed the experiments; Chih-Cheng Yang analyzed the data; Homn Reen Enterprise Co. contributed reagents/materials tools; Chih-Cheng Yang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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