CHS2 - 2022

8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

Newly Developed High Thermal Conductive Die Steelwith Superior Performance for Hot Stamping
Naoki Umemori1, Makoto Hobo2 and Krzysztof Biesiada3

Abstract
The most common disadvantageous issue on hot stamping is low productivity. One of the reasons isthe holding time at the bottom dead center of press because of quenching steel sheet. The heattransfer from steel sheet to die can be promoted by using high thermal conductive die steel and theholding time can be decreased. As the high thermal conductivite die steel for hot stamping, DHAHS1 was developed. The characteristic of the developed steel is that it has high thermal conductivity
while maintaining the high hardness required for tool steel. Moreover, the developed steel has highsoftening resistance so the hardness can be maintained high when it is exposed to high temperature
for many hours.
In this study, subjects concerning the reduction in cycle time and prevention of the adhesion ofplating have been examined using actual hot stamping equipment. As one example of the results, byapplying the developed steel, the rise in temperature of the surface of die was suppressed and thetemperature of the formed product immediately after opening the die was lower than that of 1.2344eq.
steel. It was confirmed that the holding time of the developed steel die in order to get the same
hardness of the forming product, was almost 3 s shorter than that of 1.2344eq. steel. Furthermore,
it was confirmed that reduction of the die temperature by applying the developed steel made theadhesion of plating smaller by almost 24 %. Therefore, the developed steel can be expected toimprove productivity and to reduce work time of die maintenance to remove adhesion of plating.

1Introduction

One of the challenges of hot stamping methods is a low production efficiency. The reason is because
the process needs a holding time at bottom dead center for heat transfer from heated steel sheets todies. Therefore, in order to reduce the holding time by improving the heat transfer efficiency, dieswith high thermal conductivity are required.
Moreover, wear and adhesion of plating are known as failure mode of hot stamping dies. Dies withhigh hardness and high wear resistance are required. On the other hand, when adhesion of plating
occur on die surface, die maintenances have to be taken place and bring about low productivity.
Additionally, tailored tempering method has been suggested for making highly-functional parts.[1]This is a method to create low strength area partially on the ultra-high tensile strength parts by heating dies partially by electrical heater to prevent that part from hardening. In this case, since heateddies are exposed to high temperatures for a long time, wears progress by softening issue. Therefore,
the high softening resistance is also required for those dies. However, up to now, there was no die©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

steel to fulfill high thermal conductivity and these strength required for hot stamping.
In this paper, the properties and applications of hot stamping die steel DHA-HS1 are explained. The
developed steel is designed for high thermal conductivity, high hardness and high softening resistance to correspond to hot stamping die requirements.

2Alloy Design of the Developed SteelFigure 1 shows the positioning of the developed steel in comparison to hot and cold work die steel
grades. The developed steel is alloy designed based on our study that the effect of each chemicalcomponent to thermal conductivity. It is a new concept steel grade that has both high thermal conductivity and high hardness.[2]As for the effect of alloying elements on the thermal conductivity, 0.4% C steels with the hardness
46 HRC was already tested by varying Si and Cr contents.[3] Relationship between the amount ofalloy elements and thermal conductivity is shown in Figure 2. It is found that increasing Si and Crcontents reduces thermal conductivity. About the developed steel, Si and Cr contents were reduced
to attain high thermal conductivity, compared to 1.2344.
Moreover, in order to raise the hardness and to have high softening resistance, as to adjust carbide
forming elements, C and Mo contents were increased and Cr content was reduced compared to1.2344.

Figure 1: Positioning diagram3Figure 2: Effects of alloying elements on thermal conductivity

Fundamental Characteristics of the Developed Steel

Figure 3 shows the thermal conductivity of 1.2344eq. (JIS SKD61) and the developed steel in thetemperature range between room temperature and 700 ℃. The developed steel has higher thermalconductivity than 1.2344eq.. It contributes to increase of product efficiency of hot stamping anddecrease of die damage, wear and adhesion of plating, because of its capability of suppression of therise in die temperature.
Figure 4 shows the hardness after hardening and tempering of the developed steel. In the temperature©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

range between 500 ℃ and 600 ℃, the decreasing the hardness of the developed steel is less and it
maintains high hardness even in case of surface treatment such as nitriding. Figure 5 shows thesoftening resistance of the developed steel. Hardness was tested at room temperature after 600 ℃
holding at the furnace. The initial hardness of specimens which were quenched from 1030 ℃ andtempered twice at 600 ℃ for 1 hour, was 54 HRC for the developed steel, and 50 HRC for 1.2344eq..
The holding time to be softened to 40 HRC were 200 hours for the developed steel and 16 hours for1.2344eq., and the developed steel had more than 10 times higher softening resistance than
1.2344eq.. The developed steel will contribute to prevention of the die damage with softening problem because die inserts of tailored tempering parts and high stress parts were sometimes heated athigh temperature in hot stamping methods.

Figure 3: Thermal conductivity

Figure 4: Tempering hardness curve

4

Case Study of the Developed Steel

4.1

Reduction in Cycle Time

Figure 5: Softening resistance

4.1.1 Evaluation Procedure
As evaluation of reduction in cycle time, the decreasing the holding time at the bottom dead centerof press by applying the high thermal conductivity steel to hot stamping die was investigated. Inorder to evaluate the decreasing of the holding time, the quality of the parts formed continuously by
the actual press and die were investigated. Because there were effects to the quality of parts on thechange of cooling capacity by the raise of the temperature of dies when forming the parts continuously.
To simulate the conditions of forming and cooling parts, the die material evaluation method by using
the actual hot stamping equipment was introduced. To transfer steel sheets continuously and automatically, the equipment has the two robot for transportation and the furnace which has the steel
sheets rotation system. The stamping press machine is the 110 tons electrical servo press machine.[4]The test die and the product designs are shown in Fig. 6. The test steel plate was aluminized steel
sheet, 22MnB5, and its size was approximately 2 mm thickness x 50 mm width x 300 mm length.
The product shape was the hat bending shape that simulated center pillar parts.
As shown in Figure 6, the upper punches were used as the evaluation parts made by the developed
steel and 1.2344eq. for comparison. The test die has cooling lines on the upper punches and bottom©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

die. The test was performed with internal water cooling. The forming conditions are shown in Table
1. For the evaluation of the deceasing the holding time, the holding time was changed from 0 s to10 s.
After 15 shots continuously, the temperature of the dies and products and the hardness of the products were evaluated. The die temperature was measured by the thermocouple installed at 2 mm depth
from the die surface.[5] The temperature of forming product was measured by thermography immediately after opening the die.

Figure 6: Die and forming product (evaluation of reduction in cycle time)

Table 1: Forming conditions

DieCooling waterFurnace

4.1 Reduction in Cycle
Time

4.2 Prevention of theadhesion of plating

Upper die

5 L/min0～5.6 L/min

Lower die

3.5 L/min

0～8.0 L/min

Temperature

25 ℃

3～25 ℃

Pressure

0.35 MPa0.35 MPa

Number of water line

1 (in the lower die)1 or 2 (in the lower die)

Clearance

0%

+6 %

950 ℃

950 ℃

VolumeTemperature
Holding time

5 min

170 sPress

Load40 ton40 tonCycletime

Transportation from furnace to press

9s

9s

Holding time at the bottom dead center

0～10 s

10 s

Total time each 1 shot

22～32 s32 s©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

4.1.2 Evaluation Results
Figure 7 shows the die temperatures during hot stamping and the temperature at 15 shots. The temperature of the developed steel reaches maximum temperature in 1 s earlier than 1.2344eq. and thetemperature of the developed steel is almost 15 ℃ lower than that of 1.2344eq., and it was the effects
produced by the high thermal conductivity of the developed steel.
Figure 8 shows the relationship between the holding time and the maximum surface temperatures ofthe products. In case of the holding time 10 s, the temperature of contacted part with the developedsteel was almost 40 ℃ lower than 1.2344eq. ones. Additionally, at 400 - 420°C which is the temperature where the martensite transformation starts, it generates the latent heat of phase transformation. By the application of the developed steel, heat transfer of steel sheet can be faster and theforming product can be transformed to martensite in shorter time than 1.2344eq..
The die temperatures changes by holding time are shown in Figure 9. In Figure 9, contour lines show
the hardness of the products. The lower the die temperatures are, the more narrow the spaces of thelines are, and it is expressed that the product hardness is harder at shorter holding time due to thelower die temperature. In this evaluation condition, compared with the hardness of 1.2344eq. at the10 s holding time, the holding time where the hardness of the developed steel is the same is 7 s, soit can be shorter by 3 s.

Figure 7: Die temperature during hot stamping©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

Figure 8: The relationship between maximum temperature ofFigure 9: The relationship between die temperature,
products surface immediately after opening die and the holding time
product hardness and the holding time

4.2

Prevention of the adhesion of plating

4.2.1 Evaluation Procedure
As evaluation of prevention of the adhesion of plating, the effect of lowering die temperature by
applying the high thermal conductivity steel on the adhesion of plating was investigated using thepreviously described hot stamping equipment.
The test die and the product designs are shown in Figure 10. The test steel plate was aluminizedsteel sheet, 22MnB5, and its size was approximately 2 mm thickness x 50 mm width x 300 mm
length.
As shown in Figure 10, the lower dies were used as the evaluation parts made by the developed steel
and 1.2344eq. steel for comparison. The forming conditions are shown in Table 1. In order to lower
the die temperature, the volume and temperature of cooling water and the number of water line in
the lower dies are changed as mentioned in Table 1. The die temperature was measured by the thermocouple installed at 2 mm depth from the die surface. The adhesion amount of plating on the Rpart of the lower dies after 50 shots was evaluated by the non-contact 3D dimension measuringmachine, and the total volume of the convex area from original shape line was defined as adhesion
amounts.[6]©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

Figure 10: Die and forming product (evaluation of prevent of the adhesion of plating)

4.2.2 Evaluation Results
Figure 11 shows the relationship between the maximum die temperature in the each conditions indicated in Table 2 and the adhesion amount of plating. Due to high thermal conductivity, when thecooling condition was the same, the maximum die temperature of the developed steel was about
20 ℃ lower than that of 1.2344eq. and the die temperature of the developed steel was less than
100 ℃. Also, the adhesion amount of plating decreased as a result of die temperature being lower,
and the relationship between the maximum die temperature and adhesion amount of plating wasfound out. In addition, the adhesion amount of plating of the developed steel was smaller than thatof 1.2344eq. and applying the developed steel had an effect on the prevention of plating adhesion.
Additionally, Figure 11 shows that the increase rate in the adhesion amount along with increasing
the maximum die temperature of the developed steel was smaller than that of 1.2344eq., and it
indicates the developed steel has a better adhesion resistance based on its chemical property
compared with 1.2344eq.. Further factor analysis of the developed steel on the adhesion resistancemust be done in the future.
Table 2: The relationship between graph legends and cooling conditions in Figure 11

The developed steel●▲■◆

1.2344eq.

●▲■◆Upper die

0L/min5.0L/min

5.0L/min5.6L/minLower die

0L/min3.5L/min6.2L/min

8.0L/minTemperature

-25℃

3℃

3℃

Number of water line

-1

12

VolumeCoolingwater©2022 Daido Steel Co., Ltd.

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8th International Conference on Hot sheet Metal Forming of High-Performance SteelMay 30th to June 2nd, 2022, Barcelona, Spain [https://www.chs2.eu/]

Figure 11: The relationship between the maximum die temperature and the adhesion amount of plating

5SummaryThis paper has introduced the case study of high thermal conductivity steel developed for hot stamping die material. Due to high thermal conductivity, the die temperature can be lower, and the developed steel has shown its features such as improvement of internal cooling efficiency and preventionof plating adhesion.

References[1]Mori, K: Hot stamping: Nikkan Kogyo Shimbun, (2015), pp.84-85[2]Higuchi, S. et al.: DENKI-SEIKO (Electric Furnace Steel), vol. 89 (2018), No.1, pp.27 -31[3]

K.Namiki. et al : NADCA DIE CASTING CONGRESS & EXPOSITION, (2012)[4]

Umemori, N.: DENKI-SEIKO (Electric Furnace Steel), vol. 91 (2020), No.1, pp.43 -47[5]Umemori, N..: Kata-Gijyutsu (Die and Mould Technology), Vol.36, No.7 (2021), pp.20-21[6]

Umemori, N.: Kata-Gijyutsu (Die and Mould Technology), Vol.35, No.10 (2020), pp38-41

Affiliation[1]

Daido Steel Co., Ltd., 30, Daido-cho, 2-chome, Minami-ku, Nagoya, 457-8545 Aichi, Japan,
n-umemori@ac.daido.co.jp[2]

Daido Steel Co., Ltd., 6-35, Konan 1-chome, Minato-ku, 108-8478 Tokyo, Japan, m-hobo@ac.daido.co.jp[3]

Daido Steel Group Europe GmbH, Insterburger Strasse 16, 60487, Frankfurt am Main, Germany,
k-biesiada@daidosteel.eu©2022 Daido Steel Co., Ltd.

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