Numerical study of cooling characteristics for blanking in hot stamping

Chen-guang LAI，Chao MAN，Kai-ping WEN，Zhi-gang YAN，Meng-hua DUAN

(1College of Vehicle Engineering，Chongqing University of Technology，Chongqing 400054，China) (2Institute of Fluid Science，Tohoku University，Sendai 980-8577，Japan)

Numerical study of cooling characteristics for blanking in hot stamping

Chen-guang LAI1，2*，Chao MAN1，Kai-ping WEN1，Zhi-gang YAN1，Meng-hua DUAN1

(1College of Vehicle Engineering，Chongqing University of Technology，Chongqing 400054，China) (2Institute of Fluid Science，Tohoku University，Sendai 980-8577，Japan)

Cooling down the high temperature of blanking rapidly is a key problem in hot stamping procedure.Two different cooling tunnels were set in hot stamping mold，one was smooth inner wall，and another added triangle backup bars in it.Cooling performance for blanking in the two cases as well as the flow and heat transfer situation of cooling water in two different tunnels were obtained after the numerical simulation.The result shows that，the cooling effect of inner backup bars tunnel is advantageous than smooth tunnel，and also the heat transfer ability is stronger.For backup bars tunnel，the average temperature of work piece is 12k lower than that of smooth tunnel while holding time is 10 s.For designing of cooling tunnels in hot stamping，this research also provided important references.

Hot stamping，Numerical simulation，Cooling tunnel，Enhanced heat transfer

1 Introduction

The application of high strength steel(HSS)is an effective approach to achieve automotive lightweighting.To get HSS，hot stamping integrated heating，forming and quenching in one process［1］.This could effectively soften the blank and prevent it from cracking and wrinkling.After heating the blank to austenite，it should be cooled down quickly through the suitable cooling tunnels in mold.If the blank phase transfers from austenite to martensite evenly，it could acquiresubstantialhightensileandyield strength［2］.Hence，cooling the high temperature blank rapidly is a key point in hot stamping process.Though setting the cooling tunnels suitably and addingthe diameters of the tunnels are effective ways to achieve the goal，it could not have obvious changes because of the sizes limit of the mold.More studies are focus on the enhanced heat transfer of tunnels.To achieve it，Yang Kun［3］created an equivalent thermal boundary layer through partially filled with porous media in the tube.Wang Liying［4］had a research on the influence of critical water flow speed on mold’s cooling effect.The previous study results show that，increasing the water flow speed and turbulence intensity could improve the cooling effect of blank.

Smooth tunnels were always applied for heat exchanger in engineering field.However，the heat transfer efficiency is low.According to the field synergy theory［5］，the heat transmission of flowing fluid whether has been enhanced depends on the intersection angle θ between velocity direction and temperature gradient direction.Decrease θ is an effective approach to enhance convection heat transfer ability.The θ of smooth tunnels are almost 90°，the heat transfer effective has not been enhanced.This paper adds the triangle backup bars(calls bars tunnel in brief blow)in the smooth tunnel to decrease the syn-ergy angle between velocity field and temperature field.After simulation for these two kinds of cooling tunnels，the mechanization of heat transfer enhancement in bars tunnel had been analyzed deeply.Hence，this research provides important references for designing of cooling tunnels in hot stamping.

2 Computational model and scheme

The computational model includes feature die，cooling tunnels and vehicle blanking.The main purpose of this paper is to compare the cooling effective between bars tunnel and smooth tunnel.Hence，part of the blanking is used for this simulation.The thickness of blanking is 2 mm，and the other sizes are 200 mm× 164 mm×50 mm.The diameter of the tunnel is 8 mm.See Fig.1.

Fig.1 Computational model

This paper applied two cooling cases to compare the cooling effect.Smooth cooling tunnels were installed in the die for case 1，and bars tunnels for case 2，see Fig.2.

Fig.2 Cooling cases

For bars tunnel，9 bars were added in the smooth tunnel.The diameter is also 8 mm.Tooth spacing is 20 mm.Depth of tooth is 1 mm.Tooth angle is 90°.See Fig.3.

3 Mesh and boundary condition

To adapt irregular geometry，this simulation used tetrahedral mesh.The near wall characters of fluid such as temperature gradient，velocity gradient and turbulent intensity change heavily.In fluid domain，5 layers were generated near the wall to capture the changes of these characters，see Fig.4.The mesh interfaces were created at the tunnel surfaces so that the heat could transfer normally between fluid and solid.The heat transfer equation at these interfaces shows as below，

Where，qwmeans the thermal density on the interface(w/m2);n means outer normal of the wall;λ means the thermal conductivity of the mold(w/(m· k));h means the coefficient of convection heat transfer surface(w/(m2·k));Tw，Tfmean the temperature of interface and temperature of fluid near wall (k).

Fig.3 Structure of bars tunnel

Fig.4 The mesh

As cooling water flows in the bars tunnel，it would generate vortices.Previous research［6］has indicated that realizable k-ε turbulence model is more advantageous to capture these vortices characters than others.Hence，this turbulence model was applied in this simulation.The hot stamping mold should own good elevated temperature strength，thermal fatigue stability and heat conductivity to ensure uniform mechanical properties in the whole surface.In this paper，the mold material is H13.And the blanking material is BR1500HS.For the initial temperature，mold is 423 K.Blanking is 1173 K.Cooling water is 300 K.And the fitting curves of thermodynamic property［7］for materials are shown as below，

Where，c1means specific heat of BH1500HS(J/ (kg(k));λ1，λ2mean thermal conductivity of BH1500HS and H13(W/(m(k));T means temperature(K).

4 Cooling effect analysis

4.1 Cooling effect of blanking

The cooling time is 10 s.Fig.5 and Fig.6 show the changes of maximum temperature and average temperature of blanking alone the time in two cases.Whether from the view of average temperature or maximum temperature，the cooling effect of blanking in case 2 is better than case 1.At the point of 10 s，the maximum blanking temperature in case 2 is 9 K lower than case 1.And the average temperature in case 2 is 12 K lower than case 1.It shows more obvious advantage of cooling effect.

Fig.5 Maximum temperature curve of blanking

Fig.6 Average temperature curve of blanking

The temperature curves of blanking show that，the cooling rate of blanking is very high at the first 1 s.Because the temperature difference between mold and blanking is large，the blanking temperature drop mainly depends on the heat quantity of mold absorbing from blanking.Though the cooling water in case 2 could take more heat quantity away than case 1，see Fig.7，the cooling effect to the blanking is almost the same because cooling water is not dominant role at this moment.The temperature difference between mold and blanking would become smaller and smaller with the increase of mold temperature and decrease of blanking temperature.Cooling water would take the mold heat away firstly to increase the temperature difference again，and then cool the blanking indirectly.The cooling water plays a dominant role in cooling process for blanking since that moment.And the average temperature of outlet in case 2 is higher than case 1 all the time，so it could take more heat quantity away than case 1.Hence，the cooling effect to blanking in case 2 is better than case 1.

Fig.7 Average temperature of cooling tunnel outlet

4.2 Heat transfer analysis in tunnel

4.2.1 Temperature distribution in tunnel

In the situation of convection heat transfer，the thinness thermal boundary layer would be created near the wall for the temperature difference between fluid and solid wall.Fluid temperature changes extreme heavily in this boundary layer and it would creates thermal resistance.The temperature distribution of cooling water in tunnel，see Fig.8，show that，the solid wall temperature in case 2 is easier to disperse in fluid，because the bars could disturb the thermal boundary layer.Compared with case 1，the distance of the flow water is shorter in case 2 if cooling water rise to the same temperature.Conversely，for the same distance of the flow water，the temperature on the axle center in case 2 is higher than case 1.Hence，it could take more quantity of heat away.As cooling time goes on，the temperature difference between fluid and solid wall becomes small，reducing the heat transfer coefficient，and decreasing the heating-up of tunnel outlet in both cases.

Fig.8 Counter of temperature in tunnel at each time

4.2.2 Turbulence intensity distribution in tunnel

The velocity of cooling water in laminar sub-layer is almost zero as the viscous of fluid.Most of the thermal resistance concentrates in the low velocity domain［8］.Because the cooling tunnel wall is smooth in case 1，the flow regime in tunnel is laminar，see Fig.9.Therefore，for smooth tunnel，the thermal resistance is large.It is not beneficial for the process of heat transfer.The bars were added in the smooth tunnel in case 2.The flow regime changes obviously in tunnel，causing low pressure areas behind the bars.It will create vortices under the action of pressure difference.The laminar near the wall would be destroyed by these vortices.Therefore，it will reduce the thermal resistance in these areas and enhance the heat transfer capability.

Fig.9 Velocity vectors

Fig.10 is the contour of turbulence intensity in tunnel.The picture shows that the turbulence intensity in case 2 is obvious higher than case 1.Turbulence intensity has a great relationship with cooling effect.The stronger of the turbulence intensity is，the larger of the heat transfer coefficient would be.Finally，strong turbulence intensity cause good cooling effect.

Fig.10 Contour of turbulence intensity in tunnel

4.2.3 Analysis of Nusselt number

Dimensionless Nu(Nusselt)number is an important norm to evaluate the heat transfer ability in heat convection.The bigger Nu value means stronger ability of heat transfer.And the value of Nu could compute through formula(5)，

Where，h means the convection heat transfer coefficient(w/(m2·k));L means character length of geometry(m);k means thermal conductivity(w/(m·k)).

At the moment of 10 s，the Nu number distribution of cooling tunnel in both case 1 and case 2 shows as figure 11.For smooth tunnel，the temperature difference between fluid and solid wall near the inlet is big，which would increase the value of heat transfer coefficient h.So the Nusselt number is big in this area.With the cooling water flows to the end of tunnel，temperature difference decreases.The Nu number becomes smaller.For the bars tunnel，as the bars would destroy the laminar state of cooling water and enlarge the turbulence intensity，the Nu number would rise again at each bar.Hence，the reduction of Nu number is small.Especially at the end of tunnel，the Nu number is much bigger than that of smooth tunnel.The average Nu number for smooth tunnel is 71.18，and for bars tunnel is 83.96.Therefore，it is proved that the heat transfer capability of bars tunnel is stronger than that of smooth tunnel.In the process of hot stamping，the cooling effect of bars tunnel is advantageous than smooth tunnel.

Fig.11 Distribution of Nusselt number

5 Conclusions

In the simulation of cooling effect for blanking in hot stamping process，these conclusions could be obtained by comparing the cooling performance between smooth tunnel and bars tunnel.

1)The cooling performance for mold of bars tunnel is better than smooth tunnel.Therefore，it could improve the cooling effect for blanking further.It is beneficial to achieve the cooling requirement in hot stamping process.

2)By adding the inner backup bars in the smooth tunnel，the turbulence intensity in tunnel becomes stronger.The cooling water temperature could get higher in the tunnel axis center.It can enhance the heat transfer capability of cooling tunnel.This heat transfer enhancement technology also could apply to other heat exchanger.

Acknowledgements

This paper is supported by National Natural Science Foundation of China(51305477)，2013 Program for Innovation Team Building at Institutions of Higher Education in Chongqing(KJTD201319);Part of the work was carried out under the Collaborative Research Project of the Institute of Fluid Science，Tohoku University，Japan.

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熱沖壓成形工件冷卻效果仿真研究

賴晨光1，2*，滿超1，文凱平1，閻志剛1，段夢華1

1.重慶理工大學車輛工程學院，重慶 400054
2.Institute of Fluid Science，Tohoku University，Sendai 980-8577，Japan

在熱沖壓加工過程中快速冷卻高溫成形件是一個關鍵問題。在加工模具中分別布置了2種不同的冷卻水管:一種是光滑管，另一種是在光滑管內增加橫擋板。通過對2種冷卻方案的數值模擬，得到了2種方案工件的冷卻效果，以及不同冷卻管中冷卻液的流動和換熱情況。結果顯示:內橫槽管對工件的冷卻效果要明顯優于光滑管，其換熱能力更強;當保壓時間為10 s時，工件的平均溫度比使用光滑管進行冷卻時低12 K，對熱沖壓過程冷卻水管的設計有重要的參考意義。

熱沖壓成形;數值模擬;冷卻水管;強化換熱

10.3969/j.issn.1001－3881.2015.06.006 Document code:A

TK124

Hydromechatronics Engineering

http://jdy.qks.cqut.edu.cn

E-mail:jdygcyw@126.com

20 July 2014;revised 18 October 2014; accepted 20 December 2014

Chen-guang LAI，Professor.E-mail:chenguanglai@cqut.edu.cn

*Corresponding author:Chao MAN，E-mail:manchatting@ gmail.com.