Keywords

1 Initial Situation and Motivation

The industrial sector generates the second largest share of emissions in Germany. A large and in recent years increasing part of the industrial share comes from the iron, steel and non-ferrous metals industry. The third largest share of emissions is accounted for by the transport sector, by far the largest part of which coming from road traffic [1]. In this context, the steel processing industry must increasingly question itself with regard to environmental aspects, especially for automobile production. As a consequence of the resulting lightweight construction requirements in the automotive sector, manufacturing processes of industrial relevance must deal with high-strength steels. In case of fine blanking, which is a central high-productivity manufacturing technology for many parts especially in automobile production, the process faces its limits already when processing medium-high tensile strength steels because of high tool wear or failure. [2] A promising approach to overcome these process limits and to enable fine blanking of high-strength steels is the introduction of heat into the processed metal sheet in order to lower the steel’s flow stress. An innovative and economically efficient heating approach is the use of inductive heating. [3] In order to estimate the sustainability of a fine blanking process with inductively heated sheets, the energy input during heating is investigated in this work. As a further consequence of heat introduction beyond lightweight construction gains, a reduction of die roll is to be expected. [4] This implies the possibility to gain further energetic saving opportunities and better material utilization by means of diminution or elimination of downstream manufacturing processes. The focus of the work is on the ecological and energetic consideration of the production process of fine blanked components and the comparison of the conventional fine blanking with fine blanking of inductively heated sheet metal. A further component of the work is the subsequent use phase of the components produced in this way. One of the greatest potentials of the technology of fine blanking of inductively heated materials can be found in the production of highly stressed automotive components. Important for CO2 emissions per kilometer in the case of internal combustion engines and battery-electric vehicles, the moving mass is particularly important. By using higher-strength steels, automotive components can thus benefit from reduced weight, as their volume can be reduced while retaining virtually the same density. [1] An outlook on the influence of induction heating on the energetic influences of wear at fine blanking tools and product life cycle shows possible future perspectives.

The main feature of fine blanking as a separating manufacturing process is the excellent quality of the cut surface, which allows installation without further finishing steps, such as deburring. As a result, it is used primarily in the production of components with high functional integrity. In particular, the production of functional and safety components in the household appliance industry and in the textile, automotive, medical and electrical sectors, but also in the aerospace or precision engineering industries, is realized using the manufacturing technology of fine blanking. [5].

Previous work has shown that an increase in process temperature and the resulting reduction in the required cutting force offers new possibilities for the economic fine blanking of higher-strength steels, such as S700MC, as these cannot currently be fine blanked in a process-safe manner. The technology considered in this work represents the coupling of conventional fine blanking with sheet heating by means of electromagnetic induction.

2 Automotive Component Manufacturing Phase

In order to assess the process energy difference between fine blanking with and without heating of the metal sheet, the forces required to produce exemplary fine blanked automotive components were calculated. The forces without heating and the forces with heating were calculated for the case hardening steel 16 MnCr5 [material number 1.7131, \({R}_{\mathrm{m}}= 605 \,\mathrm{MPa}\)], for the microalloyed fine-grain steel S700MC [material number 1.8974, \({R}_{\mathrm{m}} = 850 \,\mathrm{MPa}\)] and for the complex phase steel HDT760C [material number 1.0998, \({R}_{\mathrm{m}} = 760 \,\mathrm{MPa}\)] and for two different components (a brake carrier plate with a sheet thickness of \(s = 5.5\,\mathrm{mm}\) and a safety belt latch with a sheet thickness of \(s = 2.5\,\mathrm{mm}\). The steels have been chosen in order to demonstrate increasing lightweight construction potential. The choice of these parts was made because both are usually fine blanked parts which are highly safety relevant and present in every automobile regardless of the drive. Both components are depicted in Fig. 1.

Fig. 1.
figure 1

Source: feintool.com

Brake carrier plate and safety belt latch, symbol images.

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The essential force for the fineblanking process is the cutting force FC which was determined using the formula

$${F}_{\mathrm{C}} ={l}_{\mathrm{C}}\cdot {R}_{\mathrm{m}}\cdot s\cdot c$$
(1)

in which \({l}_{\mathrm{C}}\) is the length of the cutting line, \({R}_{\mathrm{m}}\) is the yield strengts of the material, \(s\) is the sheet thickness and c is a factor describing, in combination with \({R}_{\mathrm{m}}\), the cutting resistance and lies around \(c \approx 0.9\) [5]. The length of the cutting line was determined to be \({l}_{\mathrm{C}} = 462.2\,\mathrm{mm}\) for the brake carrier pad and \({l}_{\mathrm{C}} = 419.8\,\mathrm{mm}\) for the safety belt latch. The thusly determined cutting forces are given in Table 1.

Table 1. Cutting Forces for different steels and parts
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The process forces in fine blanking of steel can be reduced by heating the metal sheet [4]. To compare the force values with the force values after inductive heating of the sheet, a temperature factor was used to adjust the yield strength of the materials. The yield strength after heating is calculated using the formula

$${R}_{\mathrm{m},\mathrm{T}} ={R}_{\mathrm{m}}\cdot \left(1-{K}_{\mathrm{T}}\cdot {10}^{-3}\cdot \left(T-100\right)\right)$$
(2)

using a factor \({K}_{T}=1.2\) for S700MC and \({K}_{T}=1.7\) for 16MnCr5 and HDT760C. The factor \({K}_{T}\) is material dependent and calculated differently for different types of steel, so that steels of similar microstructure show the same values according to Rennert [6]. The cutting forces resulting from this temperature correction are given in Table 2.

Table 2. Cutting Forces for different steels and parts dependent on metal sheet temperature
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In order to draw a conclusion on the energy balance, the mechanical energy and the thermal energy used by both processes were then calculated. The amount of heat \(Q\) can be calculated as follows:

$$Q=\rho \cdot A\cdot s\cdot c\cdot \Delta T$$
(3)

in which \(\rho \) is the density, \(A\) the surface area and \(s\) the sheet thickness of the part, \(c\) is the specific heat capacity and \(\Delta T\) the temperature difference obtained when heating. The cutting work can be calculated with the following formula:

$${W}_{\mathrm{C}}={F}_{\mathrm{C}}\cdot {h}_{\mathrm{C}}\cdot k$$
(4)

in which \({F}_{\mathrm{C}}\) is calculated using formula (1) and \({h}_{\mathrm{C}}\) as the cutting height and \(k\) as a correction factor for increasing and decreasing of the cutting force during the stroke. It is assumed that a heating to not more than 500 ℃ is sufficient to lower the flow stress of the steels but not so high as to initiate undesired changings in the metal crystal lattice. The dependency of metal sheet temperature and fine blanking process forces have been investigated before for different steels [4]. The results of the calculations are given in Table 3.

Table 3. Cutting Forces for different steels and parts dependent on metal sheet temperature
Full size table

It can be seen in Table 3 that the energy required to heat the workpiece is significantly higher than the energy reduced by heating during actual cutting. A break-even point does not exist, as it is always necessary to put more energy into the process by heating than to gain by to flow stress reduction due to inevitable thermal losses. As one Joule accounts for \(1\cdot {10}^{-6}\mathrm{kWh}\), and the production of this amount of \(1 \mathrm{kWh}\) of electrical energy emits \(420 {\mathrm{g}}_{{\mathrm{CO}}_{2}}/\mathrm{kWh}\) [7], each Joule of electrical energy stands for a \({\mathrm{CO}}_{2}\) emission of \(420 \mu \mathrm{g}\). Thus, a brake carrier plate manufactured of S700MC steel with heating would mean an extra \({\mathrm{CO}}_{2}\) emission of \(21.35 \mathrm{g}\) in comparison to 16MnCr5 without heating and a safety belt latch would mean an extra \({\mathrm{CO}}_{2}\) emission of \(4.29 \mathrm{g}\). This means that the advantage of induction heating in fine blanking is not to reduce the cutting forces and thus save energy, but rather to extend the possibilities of fine blanking to higher strength steels so that qualities can be achieved in these steels that would not be possible at all without the heating. The higher-strength steels make it possible to produce thinner components that weigh less than conventional ones. The effects of this weight saving on the energy throughput in the use phase and the associated impact on the environment are analyzed in the following section.

3 Use Phase of Automotive Components

In order to assess the use phase of fine blanked components in automobiles, the lightweight construction ratios according to Friedrich [8] were calculated according to the following relation:

$${V}_{\mathrm{stat},\mathrm{ Material }2/\mathrm{Material }1}=\frac{{K}_{\mathrm{stat},\mathrm{Material }2}}{{K}_{\mathrm{stat},\mathrm{Material }1}} \left\{\begin{array}{c}<1 :{\rm{m}}{\rm{a}}{\rm{s}}{\rm{s}} \, {\rm{r}}{\rm{e}}{\rm{d}}{\rm{u}}{\rm{c}}{\rm{t}}{\rm{i}}{\rm{o}}{\rm{n}} \\ =1 :{\rm{m}}{\rm{a}}{\rm{s}}{\rm{s}} \, {\rm{u}}{\rm{n}}{\rm{c}}{\rm{h}}{\rm{a}}{\rm{n}}{\rm{g}}{\rm{e}}{\rm{d}}\\ >1 :{\rm{m}}{\rm{a}}{\rm{s}}{\rm{s}} \, {\rm{i}}{\rm{n}}{\rm{c}}{\rm{r}}{\rm{e}}{\rm{a}}{\rm{s}}{\rm{e}}\end{array}\right.$$
(5)

In which \({K}_{\mathrm{stat}}\) is the static strength indicator \([\mathrm{g}\cdot {\mathrm{mm}}^{2}/\left({\mathrm{cm}}^{3}\cdot \mathrm{N})\right]\) for a given material. This procedure allows to compare the physical properties of different materials and to draw conclusions about their according lightweight design potential. \({K}_{\mathrm{stat}}\) can be determined as

$${K}_{stat}=\frac{\rho }{{R}_{\mathrm{m}}}\sim m$$
(6)

The static strength indicator was inserted into formula (5) in order to compare the higher strength steels S700MC and HDT760C with 16MnCr5 via their respective leightweight construction ratio \({V}_{\mathrm{stat}}\), which amounts to \({V}_{\mathrm{stat}}=0.72\) for S700MC and to \({V}_{\mathrm{stat}}=0.81\) for HDT760C.

A weight reduction by substituting the reference material is possible if the lightweight construction ratio is less than 1. This is the case for the load case of the static strength (tension/compression) \({V}_{\mathrm{stat}}\). As the lightweight construction ratio \({V}_{\mathrm{stat}}\) is directly proportional to the component mass \(m\), it is deduced that an initial estimate for the expected mass of the component with the new material can be determined from the light-weight ratio:

$${V}_{\mathrm{Ft},\mathrm{Material}2/\mathrm{Material}1}\approx \frac{{m}_{\mathrm{Material}2}}{{m}_{\mathrm{Material}1}}$$
(7)

Thus, the expected weight of a component made of S700MC or HDT760C is approx.

\({m}_{\mathrm{S}700\mathrm{MC}}=72.0\%\cdot {m}_{16\mathrm{MnCr}5}\) or \({m}_{\mathrm{HDT}760\mathrm{C}}=80.5\%\cdot {m}_{16\mathrm{MnCr}5}\). This corresponds to a relative weight reduction of −28.0% for components made of S700MC steel and −9.5% for components made of HDT760C steel, which can be seen in Fig. 2.

Fig. 2.
figure 2

Comparison of yield strengths and results of the weight reduction analysis

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Furthermore, it can be stated that an increase in the strength of a component by means of substitution with another material does not completely result in a reduction of the component weight. The relative mass ratios calculated in this section are lower than the relative ratios of strength values. Nevertheless, Fig. 2 shows a significant potential of decreasing a component’s mass if produced of higher strength steel.

These findings were used to make a rough calculation to determine the specific weight that can be saved if all fine blanked parts in a passenger car are made from higher-strength materials. It is assumed that the starting material of these components is the case hardening steel 16MnCr5 and the substitute material is the fine-grained steel S700MC or the complex-phase steel HDT760C. Subsequently, the ecological effects of this measure will be discussed, focusing in particular on the CO2 impact.

In any passenger car are up to 200 components which are produced by fine blanking. Since these parts have geometries and masses that vary greatly in some cases due to their different requirements and tasks, the total mass of all fine blanked parts is estimated below. Within the scope of this work, two components were determined which represent the respective upper and lower end of the component individual masses. The weight of each of the two components was determined to be \(m= 0.060 \mathrm{kg}\) for the safety belt latch and \(m=0.230 \mathrm{kg}\) for the brake carrier plate.

This gives a range for the total weight of all fine blanked parts in an assumed passenger car of \({m}_{\mathrm{ges},\mathrm{FB},16\mathrm{MnCr}5}=200\cdot 0.060\dots 0.230 \mathrm{kg}=12 ...46 \mathrm{kg}\). Based on the previous results, the total weight can be reduced to \({m}_{\mathrm{ges},\mathrm{FB},\mathrm{S}700\mathrm{MC}} = 72.002\% \cdot {m}_{\mathrm{ges},\mathrm{FB},16\mathrm{MnCr}5}=8.640\dots 33.121 \mathrm{kg}\) and \({m}_{\mathrm{ges},\mathrm{FB},\mathrm{HDT}760\mathrm{C}}=80.523\%\cdot {m}_{\mathrm{ges},\mathrm{FB},16\mathrm{MnCr}5}=9.663\dots 37.041 \mathrm{kg}\), respectively, if the material 16MnCr5 is replaced by the higher-strength materials S700MC and HDT760C. In other words, the weight is reduced by \(\Delta {m}_{\mathrm{S}700\mathrm{MC}}=3.36\dots 12.879 \mathrm{kg}\) or \(\Delta {m}_{\mathrm{HDT}760\mathrm{C}}=2.337\dots 8.959 \mathrm{kg}\).

According to data of the German Federal Motor Transport Authority (Kraftfahrt-Bundesamt, KBA), the average unladen weight of a newly registered passenger car in Germany is \({m}_{\mathrm{ges}}=1551.7 \mathrm{kg}\) [9]. Thus, the weight reduction related to the unladen weight when 16MnCr5 accounts to \(({\Delta m}_{\mathrm{S}700\mathrm{MC}})/{m}_{\mathrm{ges}} =0.217 ... 0.83\%\) if substituted by S700MC and to \(({\Delta m}_{\mathrm{HDT}760\mathrm{C}})/{m}_{\mathrm{ges}} =0.151 ...0.577\%\) if substituted by HDT760C. Assuming that the CO2 emission of a passenger car correlates directly with the vehicle mass moved, this means in the most optimistic case that the pollutant load from CO2 decreases by 0.83%. Using a calculation based on data from the German Federal Motor Transport Authority and the percentage calculated above, the amount of CO2 that can be saved by using of higher-strength materials is determined. In each case, the latest relevant statistics are used as a basis. Accordingly, an average new car has CO2 emissions of \(139.8 \mathrm{gCO}2/\mathrm{km}\) (as of 2020 [10]). With an average lifetime of 9.8 years (as of 2021 [11]) and an average mileage of 13,602 km/year (as of 2019 [12]), a total mass of CO2 of \({m}_{\mathrm{CO}2,\mathrm{single}}=\mathrm{1,901.57} {\mathrm{kgCO}}_{2}\) is emitted into the atmosphere over the lifetime of a passenger car in average. Consequently, by reducing the vehicle weight, a mass of CO2 of \(\Delta {m}_{\mathrm{CO}2,\mathrm{single}}=0.83\% \cdot {m}_{\mathrm{CO}2}=15.783 {\mathrm{kgCO}}_{2}\) can be saved. This surpasses the extra \({\mathrm{CO}}_{2}\) emission in the manufacturing phase of \(4.27{\mathrm{ kgCO}}_{2}\) for the assumed total mass by the factor 3.7. In relation to the total number of currently registered passenger cars in Germany (48.2 million in 2021 [13]), the calculated CO2 masses scale to \({m}_{\mathrm{CO}2,\mathrm{ges}}=\mathrm{91,275}\cdot {10}^{6}\mathrm{ t}{\mathrm{CO}}_{2}\) or\(\Delta {m}_{\mathrm{CO}2,\mathrm{ges}}=757.584 {\mathrm{tCO}}_{2}\).

4 Conclusion and Outlook

In the work tackled by this paper, the production phase of two example components from a passenger car was investigated, and the effect of inductive sheet metal heating on the cutting work required compared with the energy required for heat input was calculated. In a further section, key figures for lightweight construction were determined with regard to the use of fine blanked automotive components made from higher-strength steels. This involved calculating how increased material strength affects the achievable weight reduction and the associated reduced CO2 emissions. In summary, fine blanking with inductive sheet heating does not save any energy in the actual fine blanking process since the required heat input at the sheet temperature under consideration compensates for or even exceeds the difference in cutting work achieved. The advantage of the process lies in the possibility of manufacturing components from higher-strength materials in good quality in order, for example, to save weight or cut pollutant emissions in vehicles. However, the greatest effect should come from the reduction in the amount of steel processed and the saving on heat treatments. Within the scope of the work, several assumptions were made regarding the component volume, density, and strength. Therefore, it is not meant to be an exact calculation, but a quantitative assessment of the environmental impact of the processes under consideration. Future approaches could integrate more aspects of lightweight construction, such as tension, pressure, thrust, buckling, bending stiffness, and precise further the balances of the fine blanking process. However, the biggest impact on CO2 emissions is not likely to come from a mass reduction in the use phase, but from a reduction in the amount of steel produced, as this is the most emission-intensive step in the process. This should be investigated further.