To meet the goals set by the European Union in the European Climate Law (Regulation (EU) 2021/1119), a reduction of greenhouse gas (GHG) emissions by increasing energy efficiency and using sustainable energies is necessary [1]. Both transport and industrial production are responsible for a major share of Europe’s GHG emissions. Emissions caused by the transport sector represent almost a quarter of Europe’s GHG emissions; it is also the main cause of city pollution [2]. Industrial processes are responsible for about 22% of the overall GHG emissions [3]. A large proportion of these emissions are caused by process heat generation [4]. While emissions at lower temperature levels can already be reduced by heat recovery and electrification using heat pumps [5] or hybrid heat pump-electric boiler systems [6], the technological development for high-temperature applications is significantly lower [7].

Lightweight design is an established method for reducing the fuel consumption of internal combustion engine vehicles in the transport sector. It can help increase electric vehicles’ energy efficiency during their use phase [8]. Additionally, it offers various non-energy related advantages, such as increased road user safety, enhanced driving dynamics and reduced particle pollution [9]. The impact of lightweight design on GHG emissions over a vehicle’s full life cycle depends on various factors, such as vehicle type and characteristics, as well as manufacturing processes and energy supply of material production [10].

The press hardening of steel sheet components is a fundamental process for producing high-strength lightweight structural parts in the automotive industry, such as A- and B-pillars, door beams, rocker panels or roof reinforcements [11], [12]. The number of press-hardened parts has increased yearly since Saab Automobile AB introduced the process to the automotive industry in 1984 [13]. In 1987, only 3 million parts per year were manufactured through press hardening [14]. In 2017, the number of produced press-hardened parts was estimated to be 300 million per year, with further increases predicted over the coming years [15]. Modern car bodies can include up to 38% of their total body in white weight (i.e., before painting) in press-hardened steel [16].

Press hardening of steel sheets utilises structural transformations of the iron atom lattice during heating and cooling for material hardening. It requires heating the wrought material before it can be formed and quenched in forming tools. On existing manufacturing lines, metal sheets are usually heated in roller hearth furnaces [11], [17]. Long heating times due to convection- and radiation-based heating [11], [18], low flexibility of heating processes [17], [19], high space requirements of those furnaces [14], [20] and low energy efficiency [19] due to high standby demand and high surface heat losses lead to the investigation of alternative heating methods. One promising approach is the heating of sheet metal using induction. Induction heating uses direct thermal energy transfer through electric Joule heat. It can, under certain circumstances, lead to a more time-, space- and energy-efficient heating process for press hardening applications [21], [22].

Additionally, the electric energy-based nature of inductive heating could open up the possibility of a heating process entirely powered by sustainable energies, another advantage of inductive heating over traditional gas-fired conveyor furnaces. It would help mitigate GHG emissions caused by the industrial sector and increase the impact of lightweight design on the GHG emissions over a vehicle’s life cycle. The economic and ecological feasibility of such an induction process strongly depends on the price ratio of the competing energy carriers, electricity and natural gas, and the development of the emission factor of the electricity grid. In addition, logistical production conditions like utilisation and good-mass flow affect economic efficiency.

Press hardening combines the manufacturing methods of deep drawing and hardening [23]. Its advantages are the good formability of heated steel sheets, low springback, good dimensional accuracy and the high strength of parts after forming [11], [24]. Those properties enable the production of automotive structural components with higher strengths than achievable through cold stamping of steel sheets [11]. The possible decrease of sheet metal thickness compared to cold-formed steels allows the reduction of weight in parts where high structural stiffness is desirable [12], [21]. Conventional press hardening processes also have drawbacks compared to cold forming, such as increased process complexity, increased investment and production costs, higher space requirements and reduced productivity [11]. Therefore, constant research on these topics is necessary to improve the overall process.

The process of press hardening can be divided into two main variants: direct and indirect press hardening [14]. For both variants, steel sheets are first cut from a steel coil. During the direct press hardening, the sheets are heated before being transported to a hydraulic press, quenched in a forming tool, and received final geometry. Afterwards, other processes, such as cutting, surface finishing or joining, may follow. During the indirect press hardening process, an additional cold-forming step is included before heating. It allows manufacturing components of higher complexity while reducing tool wear of the hot forming tool [12], [24]. However, the additional process step and a higher necessary tool application lead to increased part costs, which is why direct press hardening is more commonly used in industrial processes [12]. This study focuses on the direct press hardening process shown in Figure 1.

Figure 1.

Schematic overview of the direct press hardening process

As mentioned before, temperature-induced transformations of the iron atom lattice are utilised during press hardening to achieve the high strength of manufactured components. For materials used in industrial press hardening processes, temperatures of 900 °C to 950 °C with furnace dwell times of several minutes are common, and a cooling rate of more than 25 K/s is sufficient for forming a high-strength martensitic structure [11], [14], [24]. Low-alloy boron-manganese steels are typically applied for industrial press hardening [12]. The alloying elements boron and manganese positively influence the alloyed steel’s hardenability and required cooling rates and increase the attainable material strength [12], [24]. Of the available boron-manganese steels, the steel 22MnB5 is the most commonly used [11], [12], [14]. After hardening, this steel reaches a tensile strength of approximately 1500 MPa and an elongation at fracture of approximately 5% [25].

Conventional roller hearth furnaces used for press hardening are heated through gas combustion or electricity [11], [18], [19]. An average of 21.3% of the combustion energy of natural gas is lost as waste heat in the form of exhaust gases [19]. By applying, for example, an absorption chiller based on the waste heat, one can recover a maximum of 69% of this heat for press cooling. In addition, diffuse heat losses occur depending on the furnace geometry and design of at least 54% over the large surface of the roller hearth furnaces, irrespective of the material mass flow conveyed. Heat is transferred to the steel sheet through radiation and convection, leading to long heating times of several minutes [11], [18], [20], [21]. However, the typical press hardening cycle takes approximately 20 s to 30 s [11], [21]. Therefore, to meet the good-mass flow requirements of the process press cycle, conventional roller hearth furnaces need to be designed with sufficient dimensions and can reach a length of up to 40 m, resulting in high space requirements [11], [20], [21].

Additionally, due to the long times needed to heat up or cool down roller hearth furnaces, furnaces often are continuously heated, even when part production is stopped, for example, because of maintenance or production errors [17], [19]. It leads to high energy demands even in no value-adding times. Various alternative heating methods have been investigated based on these disadvantages of conventional furnaces heating.

Alternative heating concepts for press hardening

One driving factor for the investigation of alternative heating methods is the desire for increased heating rates of sheet metal. Therefore, research has been conducted on various heating technologies and their application for press hardening. These technologies involve, but are not limited to, infrared heating [26], [27], induction heating [18], [22], [28], [29], resistance heating [30]–[33], contact heating [34]–[37], fluid bed heating [38] and direct flame heating [39]. While all these technologies have advantages and disadvantages, the current study focuses on applying induction heating. An oscillating magnetic field is created during induction heating by an induction coil, the so-called inductor. If an electroconductive material enters the magnetic field, an electric current is induced into the material, leading to increased material temperature through the effects of Joule heating. The advantages of induction heating, compared to conventional furnace processes, are high achievable heating rates, low space requirements and high energy efficiency in terms of less surface heat losses and no standby heat demand [20], [21]. Challenges of inductive heating are the homogeneity of heating [18], [22], [28] and resulting material and coatings properties [40], [41]. According to Kolleck et al., inductor arrangements for sheet metal heating can be divided into three major categories, shown in Figure 2 [42].

Figure 2.

Design principle of induction coils for sheet metal heating [18], [42]

Currently, induction heating is mainly applied for melting and forging processes and heat treatment operations [11], [42]. Multiple research efforts have been made to enable its application for sheet metal heating during the press hardening process [18], [21], [22], [28], [37], [42]. All of those investigations show the potential of inductive heating. However, they also indicate problems with the temperature distribution in inductively heated sheets [28] or secondary heating through convection and radiation or contact to achieve homogeneous temperatures [21], [22], [37].

In [29], a process was developed utilising a standalone longitudinal field inductor for homogeneous heating of complexly formed sheet geometries. Materials used were 22MnB5 and 20MnB8 with Al-Si, Zn-Fe and Nano-particle coatings. The upper part of a B-pillar was heated to 950 C for 16 s employing inductive heating. The homogeneity of temperature and material properties, comparable to the conventional furnace process, could be achieved. Since that process allows press hardening of structures comparable to realistic geometries of press-hardened parts entirely based on induction, it is used as a baseline for this investigation. While the applicability of the heating concept could be proven from a material and process perspective, no data are available regarding the economic and ecological feasibility of such an induction process.

The objective of the current study is to estimate GHG emissions and energy costs caused by inductive heating during the use phase of an inductive heating unit and compare the results with data from conventional gas-fired conveyor furnaces. Specific energy demands for induction and furnace heating are determined to achieve this goal. These serve as input for a model predicting emissions and energy costs of press hardening heating units considering various boundary conditions, such as good-mass flow and operating state, and different energy system transition scenarios. The results are subsequently used to determine possible break-even points of GHG emissions and inductive heating costs.

The presented method allows for describing the gas consumption of a roller hearth furnace and electrical energy demand per part of an inductive heating system as functions of the good-mass flow. Gas consumption ranges from 26.1 m³/h at a good-mass flow of 0.02 kg/s to 70.7 m³/h at a good-mass flow of 0.5 kg/s. The energy demand ranges from 0.23 kWh for a good-mass flow of 0.02 kg/s to 0.55 kWh at 0.5 kg/s. Electric losses of induction heating, as described by the applied model, have been determined at 0.22 kWh and verified through measurements of the energy demand of no-load induction for relevant system operating states during heating. The measurements indicate electric losses of 0.20 kWh, proving the employed model’s accuracy for energy demand determination.

The specific energy demand in dependence on good-mass flow can be determined for varying operating state shares (production and standby), as shown in Figure 3. Since an induction heating system can be shut down during standby, operating states do not influence the specific energy demand. Even for manufacturing with no standby times, the specific energy demand for induction heating is slightly lower than that of the furnace heating process. Specific energy demand ranges from 0.35 kWh/kg at a good-mass flow of 0.5 kg/s to 3.46 kWh/kg at a good-mass flow of 0.02 kg/s for induction heating to 0.40 kWh/kg and 3.68 kWh/kg for the furnace process at respective good-mass flows. The difference increases with an increasing share of the standby operating state.

Figure 3.

Specific energy demand of furnace and induction heating in dependence on good-mass flow and operating state

For the two price scenarios Rising and Stable, results for Germany, Sweden and the EU27 are shown in Figure 4a. The initial values in 2020 are taken from a data set of the Federal Ministry for Economic Affairs and Climsate Action [60]. Gas prices are determined for consumers between 100,000 GJ and 1,000,000 GJ, and since prices for natural gas for Germany and the EU27 are close to each other, Germany is used as a reference (Germany/EU27: 2.32 ct/kWh, Sweden: 3.34 ct/kWh). Electricity prices are determined for consumers between 500 MWh and 2,000 MWh (Germany: 12.39 ct/kWh, EU27: 8.83 ct/kWh, Sweden: 4.88 ct/kWh). Both electricity and gas prices exclude VAT and recoverable taxes and levies.

Figure 4.

Evolution of gas and electricity prices (a) and the ratio of electricity to gas prices (b) until 2045

Figure 4b shows the corresponding ratios of electricity to gas prices. The ratio decreases particularly strongly between 2020 and 2030 due to the introduction and increase of CO₂ costs for natural gas. Between 2030 and 2040, the ratio decreases further for all cases and both price scenarios. From 2040 onwards, the electricity-to-gas price ratio decreases only in the Stable scenario, while it increases in the Rising scenario. The trends are due to no further CO₂ price increases and a lesser increase in the wholesale price of natural gas than electricity.

The development of CO₂ emissions for Germany, the EU27 and Sweden is shown in Figure 5. As with prices, Germany has the highest emissions, the EU27 form a middle case, and Sweden is well below the EU average.

Figure 5.

Evolution of energy grid emissions for Germany, the EU27 and Sweden until 2045

Energy cost

Due to the low electricity-to-gas price ratio in Sweden (ratio_SE,2020 = 1.46), the specific energy costs for the use of induction heating are already lower than for the use of natural gas at a standby share of 50% and higher for all good flows in 2020, as shown in Figure 6. At a standby rate of 50%, cost savings of between 23% and 4% can be achieved, depending on the good-mass flow rate. From 2030 onwards, induction heating will provide a higher economic efficiency even with 100% occupancy of the roller hearth furnace. At full capacity utilisation and a material flow rate of 0.5 kg/s, the savings are greater than 34% in the scenario Rising and greater than 40% in the scenario Stable.

Figure 6.

Specific energy costs for heating in Sweden 2020

In the price scenarios considered for Germany, cost savings can only be achieved depending on standby times, even in 2045. Specific energy costs for the year 2045 in Germany are shown in Figure 7. In the scenario Stable (ratio_{G,2045,Stable} = 1.72), savings of up to 27% can be achieved compared to a standby rate of 60%, depending on the good-mass flow. In the scenario Rising (ratio_{G,2045,Rising} = 2.30), savings of nearly 10% are possible with standby times of 70% and good-mass flows of 0.18 kg/s.

Figure 7.

Specific energy costs for heating in Germany 2045

Figure 8.

Specific energy costs for heating in the EU27 from 2020 (a) to 2045 (d)

Figures 8a to 8d show the development of specific energy costs for the EU27 in 2020, 2030, 2040 and 2045. While in 2020, costs can only be saved for systems with standby times above 80%, in 2030, inductive heating is also competitive with operating states at standby times of just under 70%. In the Stable scenario, cost savings of between 2% and 29% are possible compared to a roller hearth furnace with 60% standby times. In 2045, in the same scenario, electric heating would be more cost-effective at all good-mass flows than the fossil heating system with standby times of 20%. In the Rising scenario, induction heating would be competitive in 2045 compared to plants with downtimes of just under 60% for all good-mass flows.

Emission

Due to the low emission factor in the Swedish electricity grid, the CO₂ savings were already significant in 2020. The energy-related CO₂ emissions of the heating would decrease by over 96% for all good-mass flows. The emission savings decrease only slightly in 2030 before emissions reach zero in 2040 for the use of electrical energy.

On average, in the EU27, CO₂ savings would also have been possible as early as 2020. Figure 9 shows that the curve of specific CO₂ emissions from 100% oven utilisation would have been lower using electrical energy via induction. Savings of between around 8% at a good-mass flow of 0.02 kg/s and almost 20% at a good-mass flow of 5 kg/s are the result. In 2030, the savings would be 30% to almost 40%; in 2040, savings would be greater than 80%.

Figure 9.

Specific emissions for heating in the EU 27 2020

In Germany, CO₂ savings between 40% and 50% were possible in 2020 compared to plants with standstill times, depending on the good-mass flow. In 2030, the savings compared to plants with 50% standby would be around 35% to 40%. Even in the case of furnaces with standby times of 10%, small CO₂ savings could be achieved with high material mass flows, as can also be seen in Figure 10. In 2040, the expected reduction in CO₂ emissions is between 65% and 70% before electric heating is CO₂-neutral in 2045.

Figure 10.

Specific emissions for heating in Germany 2030

While inductive heating seems generally more energy efficient than furnace heating, economic and ecological feasibility highly depends on regional pricing and emission factors of electric energy. It becomes apparent when comparing the different break-even scenarios for countries like Germany and Sweden. For Sweden, induction heating can provide lower specific emissions even today and lower specific prices independent of the operating state until 2030. In contrast, for Germany, the economic feasibility of induction heating still depends on the share of standby times during production in 2045. It is due to Germany’s high electricity-to-gas price ratio, which affects induction heating and all electricity-based heating methods. It is, therefore, necessary to ensure a competitive electricity-to-gas price ratio to entice the use of alternative heating methods.

The results presented in this study provide a good first estimation of the potential of inductive heating for decarbonising the press hardening heating process. Since the results are based on scenarios and models estimating energy demand and future developments in the energy sector, one must consider uncertainties introduced by this approach. The forecasts used in this work provide a good foundation for evaluating future developments. However, recent events have shown that geopolitical incidents, such as pandemics or wars, can unexpectedly influence price evolution and energy system transition scenarios. For example, prices on the German power exchange (futures market) have increased fourfold compared to 2020 and are thus significantly above the highest estimate in the outlined scenarios [65]. At the same time, the price of natural gas in wholesale trading in Germany has increased fivefold from the beginning of 2021 until May 2022 and has temporarily increased more than tenfold [66]. It could be an indicator of electricity-to-gas price ratios improving faster than anticipated. While it is uncertain if this is a lasting effect or if future prices will normalise, it does emphasise the need to reduce dependence on fossil fuels. Nevertheless, this paper used country-specific price ratios for orientation purposes. The reader can also use the stated price ratios to assess company-specific or country-specific framework conditions.

In addition, assumptions and simplifications made for estimating the energy demand for inductive heating may influence the results. Efficiency factors and process parameters for inductive heating have been based on a prototype heating system. The scalability of such technology to an industrial standard still needs to be verified. The conversion of the prototype to a heating system matching industrial standards may also influence the system’s thermic and electric efficiency, therefore impacting the overall efficiency factor of inductive heating. While assumptions regarding heating times and inductor geometries may hold for industrial processes depending on process and system structure, sheet metal thickness will likely vary based on the manufactured part. Depending on the sheet thickness and the frequency of the inductive heating system, this will negatively or positively influence energy efficiency.

The current study investigates the economic and ecological feasibility of induction heating based on heating energy demand. A transition from a press hardening manufacturing line with a furnace-based heating system to an induction-based heating system would have implications for the press hardening process reaching far beyond heating energy demand. Applying such a heating system requires optimising process planning, energy management and materials used for induction heating. It would also affect investment costs, space requirements and process flexibility − factors one must consider for a full economic and ecological feasibility assessment.

This work determined the specific energy demand for heating sheet metal plates for press hardening using a roller hearth furnace and induction heating based on a prototype induction unit. Various scenarios regarding the development of energy costs and energy grid emissions in Europe have been analysed. The results were used to identify ecological and economic break-even points for applying inductive heating in various European countries with different energy transition scenarios. The following findings have been made:

1. Induction heating of sheet metal can be a valuable approach for reducing energy demand and GHG emissions of hardening press processes. In this study, the specific energy demand of inductive heating is lower than that of furnace heating, independent of the share of operating states.

2. In a scenario with 0% standby times, induction heating can save up to 12% energy compared to furnace heating. These savings increase with increasing standby times. At 20% standby operating state, energy savings of induction heating increase to up to 19%.

3. The ecological feasibility is highly dependent on the emission factors of the energy grid. GHG emissions reduction between 8% and 20% would have been possible for the EU27 even in 2020. For countries with a high share of renewable energy, like Sweden, savings would be even more significant. For countries with a high emission factor, like Germany, a share of the standby operating state is necessary for ecological feasibility. A standby share of 50% would be required in 2020, while in 2030, a share of 20% would be sufficient. Unconditional ecological feasibility for Germany is possible in 2040.

4. The economic feasibility depends on the electricity-to-gas price ratio. Countries with a low ratio will profit earlier from implementing induction heating for press hardening processes. In Sweden (ratio_SE,2030 = 0.77–0.84), economic feasibility can be reached in 2030, independent of the operating states of the heating system. For Germany (ratio_SE,2045 = 1.72–2.30), standby shares of 60% to 70% would be necessary even in 2045 for the economic feasibility of induction heating. A competitive electricity-to-gas price ratio is needed to entice the use of alternative heating methods.

To verify assumptions made in this study and for a holistic assessment of the economic and ecological impact of an inductive press hardening line, the investigated inductive heating system needs to be advanced to a standard of industrial series production. Such a development requires changes to the heating system of a press hardening line and optimisation of preceding and subsequent processes, production, and energy management, as well as materials and coatings for inductive heating.