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DENKI-SEIKO (Electric Furnace Steel), vol. 85 (2014), No.1, p39 - 46
Technical Data

Life Evaluation of Surface Treatments of Mold in Cold Working of HighStrength Steel
Shigeki Higuchi, Tetsuya Masuda, Takayuki Shimizu,Takashi Matsuno, and Koichi Sato
1. Preface
In recent years, demand for reducing automotive weight by improving fuel efficiency to prevent global
warming has increased. Therefore automotive parts made from the high tensile strength steel sheets, over340 MPa class, are increasing1). The weight percentage of high strength automotive parts in some special
cars has increased from 40% in 2004 to about 60% currently2)3). 1200MPa class ultra-high strength steel
sheets have already been developed, and higher strength steel sheets are also the object of studies now.
In the meantime, interest in the die life of the cold press working for high strength automotive parts isincreasing.

Opportunities are increasing for the surface treatment application to these cold press

working die to prevent wear and galling4). Therefore many kinds of surface treatment have been developed
and commercialized for pressing high strength steel sheets. Daido Steel has developed new highperformance PVD (Physical Vapor Deposition) coating “HT-CERAC” in cooperation with Daido Die &Mold Solution, Nissin Electric and Nippon ITF5). HT-CERAC coating is designed for prevention of galling,and has reached the less defects surface by optimizing the coating conditions compared with conventional
coatings. HT-CERAC has achieved longer die service life than those of conventional coatings because ofhigh galling resistance under evaluation with our original 780MPa class steel sheets bending test and1180MPa class steel sheets deep drawing test in cooperation with Toyohashi University of Technology5).
In this paper, punches service lives of various coatings, PVD, CVD (Chemical Vapor Deposition) andTRD (Thermal Reactive Deposition) process, by the establishment of a piercing test using a progressive
die have been evaluated. Their cost-effectiveness has also been calculated along with effects of work sheet
strength and clearance on punches service lives have been investigated.
2. Test conditions
2.1 Piercing test conditions
Figure 1 shows the schematic diagram of piercing test. Punches were tested with the clearance of 3%t (3%of work sheet thickness) and with the inclination of work sheet against the punch top surface of 3 degree
without lubricant in order to accelerate punch damage in short term. To investigate effect of clearance, theboth 3%t and 6%t conditions have been tested with 1180MPa class sheets.
The crank press, 150tnf high flex press made by Aida Engineering, was used for the test. Press speed were
set to 50 to 55 SPM. SPM stands for shots per minute.

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2/12Work sheets were 590, 980 and 1180 MPa class high strength cold-rolled non-plated steel with thicknessof 1.6 mm made by Nippon Steel. The die material was DC53, JIS SKD11 modified steel grade, made byDaido Steel. Its hardness was 62 HRC.
Coating conditions are shown in Table 1. Two punches per condition were tested. HT-CERAC withnitriding was also tested. Figure 2 shows the optical micrograph and the hardness distribution near thesurface of nitriding+HT-CERAC. The nitride layer had no compound layer at the boundary between
coating and substrate. Its maximum hardness was about 1100 HV and the case depth was about 50 m. As
shown in Figure3, the punch edge was chamfered to prevent chipping.
Piercing test can evaluate 8 punches at the same time. Punch positions are shown in Figure4. Every punch
position was changed every 1000 shots to reduce stress variation dependence on punch positions.

Fig. 1. Schematic diagram of pierce processing test.
Table 1. Test coating. *DDMS: Daido Die & Mold Steel Solution, Co., Ltd.
No.

Surface treatment

1

VC (TRD process): Company A

2

TiC (CVD process): Company B

3

TiN (PVD process): HT-CERAC by DDMS*4Nitriding + HT-CERAC

Fig. 2. Nitriding quality of nitriding+HT-CERAC.

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Fig. 3. Difference in damage morphology with shape of punch tip.

Fig. 4. Position of punches.
2.2 Evaluation of punch service life
Table 2 shows test conditions. The sampling of skeleton sheet that was pierced work sheet and punch
appearance photographs were taken at the sampling timing described in Table 2.
The punch service life was defined whether there was a streak scratch due to galling on the burnishing
surface of skeleton sheets or not as shown in Figure 5.
Each coating cost-effectiveness was calculated by punches service lives per cost of making coated punches,and they were evaluated by relative comparison with TRD coated punches. Cost of making punch include
material cost, machining cost, heat treatment cost, coating & nitriding cost and polishing cost before andafter coating.

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Table 2. Test conditions.
Steel sheet

Clearance

typeA
BCD590 MPa(t=1.6 mm)
980 MPa(t=1.6 mm)
1180 MPa(t=1.6 mm)
1180 MPa(t=1.6 mm)

Max test

Sampling of testshots
3 %t

20000

3 %t

10000

3 %t

2000

6 %t

7000

Photographof appearance

1, 10, 200~2000 shots:
every 200 shots,every

2000 shots~:

2000 shots

every 2000 shots
1, 10, 100~1000 shots:

every

every 100, 1500, 2000 shots

500 shots

10 shots, 1000~ 7000 shots:

every

every 1000 shots

2000 shots

Fig. 5. Evaluation of punch life.
3. Results and discussion
3.1 Punch failure mode and effect on burr height
Figure 6 shows appearance photographs of punches coated with TiC by CVD process and VC by TRD
process after test shots. Both two coated punches had no chipping at edge. However there were galling at
positions indicated by arrows on photographs and galling area were increased with numbers of shots.
Appearance photographs of punches at 1000 shots with each work sheet conditions are shown in Figure7.
With an increase in the strength of steel sheets, galling area were increased.
Figure 8 shows the burr height of the skeleton at each shots tested with 590 and 980 MPa class strengthsheets. The galling area were increased with increasing numbers of shots. However, relationship between
burr heights and numbers of shots were

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Fig. 6. Punch appearance of VC and TiC in 590 MPa high strength steel.

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Fig. 7. Influence of steel strength to the adhesive wear.

Fig. 8. Burr height of each shot in 590 MPa and 980 MPa.
3.2 The punch service life and the cost-effectiveness3.2.1 Comparison with HT-CERAC, CVD coating and TRD coatingResults of punch service life are shown in Figure9. The service lives of HT-CERAC and CVD coatedpunches are longer than TRD coated punches at the every work sheet strength condition.

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By comparing results under the same clearance condition, in case of HT-CERAC, the 980 MPa class sheet
result was about 1/9 times shorter than that of the 590MPa class sheet result, and 1/22 times shorter incase of CVD coating. Therefore HT-CERAC was better coating for keeping service life compared to CVD
coating. The result of 1180MPa class sheet was about 1/30 times shorter than the 590 MPa class sheet incase of both two coating conditions. Because of these service lives were too short, merits could not bedetermined.
By comparing results under the same work sheet strength, the 6%t result of CVD coating punch was about
6 times longer than that of the 3%t result and the result of HT-CERAC coating was about 10 times longer.
HT-CERAC punches have about 1.5 times longer service life compared to CVD coating ones. Results of1180 MPa class sheets and 6%t condition were better than 980 MPa class sheets and 3%t condition resultson both two coating punches.

Fig. 9. Punch life comparison of HT-CERAC.
Figure 10 shows punch appearance photographs of each condition. The failure mode of HT-CERAC
punches are only galling, not chipping as well as CVD and TRD coating punches. Positions of galling on
the every punch were nearly the same.
The cost-effectiveness is shown in Figure11. Values are relative ones assuming the cost of TRD coatedpunch is set to 1. Cost-effectiveness of HT-CERAC and CVD coating punches were 2 to 5 times higher
than TRD coated punch on 590 and 980 MPa class sheet conditions. Additionally, it was over 20 times
more effective on the 1180 MPa class sheet condition. Therefore HT-CERAC and CVD coating have highcost merit as piercing punches of high strength sheets compared with TRD coating.

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Fig. 10. Punch appearance comparison of HT-CERAC.

Fig. 11. Cost-effectiveness comparison of HT-CERAC coated punch.
(Cost-effectiveness of VC coated punch = 1)

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9/123.2.2 The effect of nitriding under PVD coating
PVD process are treated at 500℃ and lower. Otherwise, CVD and TRD process are treated over 1000℃.
Therefore PVD process can be treated on nitrided substrates without nitriding quality loss. This is thereason why nitriding+HT-CERAC was included in this test.
Figure 12 shows the service life comparison including nitriding+HT-CERAC punch results. The service
life of nitriding+HT-CERAC punches are 1.5 to 3 times higher than HT-CERAC punches. Although HTCERAC service life was almost same as CVD coating one, nitriding+HT-CERAC service life was clearly
longer than CVD coating one under every test condition. However the effect of work sheet strength on
nitriding+HT-CERAC service lives with the same clearance condition was almost same as HT-CERAC(non-nitriding). The rate of decreasing service life with increasing the work sheet strength were: 980 MPa
class sheet result was about 1/5 times shorter than 590 MPa class sheet result and 1/40 times shorter than
1180MPa class result. The effect of the clearance on the nitriding+HT-CERAC service life with 1180 MPasheet condition was almost same as HT-CERAC (non-nitriding). The 6%t condition result was about 10times longer than 3%t result.

Fig. 12. Punch life comparison of nitriding+HT-CERAC.
Punch appearance photographs of nitriding+HT-CERAC and HT-CERAC (non-nitriding) are shown in
Figure13. The positions of galling on punches were same with or without nitriding.
Figure 14 shows the cost-effectiveness that is a relative value assuming the cost of TRD coated punches isset to 1. The cost-effectiveness of nitriding+HT-CERAC was about 8 times more effective than TRD
coating on the 590 MPa class sheet condition, about 16 times more on the 980 MPa class sheet condition
and about 40 times more on the 1180 MPa class sheet condition. The cost-effectiveness increased withincreasing work sheet strength. These values were better than HT-CERAC (non-nitriding) on every work
sheet strength condition.
Figure 15 shows the cost-effectiveness that is a relative value assuming TRD coated punch cost with 590
MPa class sheet condition is set to 1. The cost-effectiveness of nitriding+HT-CERAC punch with 980 MPaclass sheet condition and 1180 MPa class sheet & 6%t condition were better compared to TRD coatedpunches with 590 MPa class sheet condition. From this result, coating punches for 980 MPa and 1180 MPa© 2020 Daido Steel Co., Ltd.

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10/12class sheets piercing process can be made with the same cost as TRD coated punches for 590 MPa classsheets by applying nitriding+HT-CERAC and adjust the clearance between dies and punches.

Fig. 13. Punch appearance comparison of nitriding+HT-CERAC.

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Fig. 14. Cost-effectiveness comparison of nitriding+HT-CERAC coated punch.
(Cost-effectiveness of VC coated punch = 1)

Fig. 15. Cost-effectiveness comparison of HT-CERAC coated punch.
(Cost-effectiveness of VC coated punch in 590 MPa high strength steel = 1)
4. Summary
Ceramic coating service life evaluation test which can evaluate galling mode failure by using the piercing
process with progressive die were established.
Findings of this study are summarized as follows:
1. The service life was improved by 6 to 10 times by applying clearance change from 3%t to 6%t.
2. In case of this piercing test with high strength steel sheets, HT-CERAC along with PVD coating has
equal to or better galling resistance compared to CVD and TRD coating.
3. Service lives of HT-CERAC punches were improved by 1.5 to 3 times by applying nitriding before
coating. Therefore it is estimated that coating punches for 980 MPa and 1180 MPa class sheets piercing
process can be produced with the same cost as TRD coated punches for 590 MPa class sheets by applying
nitriding+HT-CERAC and adjusting clearances between dies and punches.
(Reference)1) Kazumasa Yamazaki: Journal of the JSTP, 46 (2005), p565-.
2) Ryuji Sugiyama: Journal of the JSTP, 46 (2005), p552-.

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3) Nikkei Automotive Technology, 11 (2012), p44-.
4) Junichiro, N.: Sokeizai, 51 (2010), No.8, p13-.
5) Masuda, T. and Kitagawa, T.: Technical papers of Conference on Die and Mold Technology, 2013, p.100.

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