Roller-hearth Furnace Research Articles

Oxygen-free resistance heating with nitrogen and silane as an energy-efficient heating process for hot stamping

Hot stamping is a process for the production of ultra-high-strength components used in the automotive industry for passenger protection. In this process, cut blanks are heated in a roller hearth furnace, which is operated with gas, to approx. 950 °C in 8 to 10 min and then formed and quenched in a water-cooled tool. This extends the yield strength of the components up to 1500 MPa. The long heating time is caused by the coating of aluminium and silicon (AlSi) with a thickness of 70 μm. During the 8 to 10 min, an intermetallic phase forms and ensures that the blank does not scale. Due to the long heating time and the energy carrier gas, the process is very energy inefficient and can only be controlled slowly. Resistance heating provides an energy-efficient alternative. Due to the direct current flow through the sheet, up to 68% of the energy can be saved compared to roller hearth furnace heating. The process is a high-speed heating process that can heat sheets to 950 °C in less than 10 seconds. The heating time is not sufficient for conventional coatings to bond sufficiently with the base material, which means that there are currently no suitable coatings for resistance rapid heating. A new approach is to suppress scale formation by reducing the oxygen during heating. By lowering the oxygen partial pressure to the XHV level, scale formation is not possible. At the same time, this absence of oxygen is an ideal condition for applying coatings. Due to the developed resistance heating device, sheets can be heated within a very short time without scale and without the need for fossil fuels.

Resistance Heating by Means of Direct Current for Resource-Saving CO<sub>2</sub>-Neutral Hot Stamping

Hot stamping is a well-established and frequently used manufacturing process in automotive body construction. The number of components manufactured in this way is continuously increasing. Hot stamping is used to produce components with a completely martensitic structure, resulting in high strength and hardness. These components are mainly used in safety-relevant areas of the passenger cell, such as the A-pillar, B-pillar, tunnel and sill. For hot-stamping processes, it is necessary to austenitize the blanks. Heating the sheet metal up to 930 °C in a furnace is very energy-intensive. In large-scale industrial applications, the sheets are generally heated in gas-fired roller hearth furnaces up to 60 m long. Apart from the poor energy balance and the high CO2 emissions of such furnaces, they are associated with high investment and maintenance costs, large space requirements and a long heating time. Rapid heating by means of the Joule effect and direct current instead of alternating current offer an energy-efficient and environmentally friendly alternative for sheet metal heating. Therefore, this technology can make a major contribution to environmental protection and resource saving. Within the scope of this work, parts were rapid-heated and subsequently hot-stamped by means of a novel heating system based on direct current with energy savings of up to 80 %. Using electricity guarantees a good CO2 balance. In addition, resistance heating with a new type of DC-heating system and an adapted process chain is compared with conventional furnace heating. In thermographic images and microstructural examinations of the hot-stamped parts, it can be demonstrated that this direct-current technique is well suited for achieving homogeneous hardness and strength in the whole sheet metal. Thus, this new heating system can enhance the efficiency of the hot-stamping technology.

Increasing the energy absorption of monolithic manganese boron steels in oxygen-free environment

The heat-treatable steel 22MnB5 is used in hot stamping processes to produce high-strength body-in-white components. In this process, sheet blanks are conventionally heated in roller hearth furnaces and then hot-stamped, whereby strengths of 1,500 MPa can be achieved. Disadvantages of this process are the low plastic deformation of the material in hardened state and the poor energy efficiency of roller hearth furnaces. In a new approach, these disadvantages are eliminated by combining edge decarburisation with resistance heating. Due to a diffusion-controlled removal of the carbon in the edge layer of the blanks heated in an oxygen-free atmosphere, the energy absorption in bending tests was improved by 61 % compared to customary hot-stamped 22MnB5. Furthermore, with a subsequent resistance heating in an oxygen-free silane atmosphere, the sheet can be heated and coated. A hermetically sealed heating chamber was developed which allows to heat the blanks up to 950 °C without scale formation. The coating during heating further improves the corrosion properties of the component. With this approach, hot-stamped components with improved properties and coated in an energy-efficient resistance-heated process can be manufactured.

Evaluation of 22MnB5 Steel Austenitization Sub-Models for Simulating the Heating Phase of Hot Stamping

The heating phase of hot stamping is the most time-and energy-intensive part of the process, and therefore offers significant potential to improve process efficiency. This requires a robust thermometallurgical model that predicts the blank heating profile and the austenitization progress inside a roller hearth furnace. In this paper, an IM austenitization model for 22MnB5 steel is derived using dilatometry data and evaluated against a JMAK type model using temperature profiles of coupons heated within a laboratory-scale muffle furnace and blanks heated within a roller hearth furnace. The austenitization rates predicted by the two models are then compared to the inferred austenitization processes from those temperature profiles. The result shows that the IM model has potential to capture the trend of austenitization process, especially the rapid pearlite dissolution at the beginning of the transformation, whereas the JMAK type model is too restrictive and fails in this regard.

Development of a roller hearth furnace simulation model and performance investigation

The roller hearth furnace is numerically investigated to analyze and optimize its performance. It is essential to apply the temperature control model and the material transport model for the furnace simulation, which enables the energy consumption and productivity calculation. When it comes to temperature control models, a total of five models are considered: constant temperature method, constant heat flux method, on-off control, proportional-integral-differential (PID) control, and on-off and PID control. For material transport models, high viscosity laminar flow model, dynamic mesh model, and source term model are developed and evaluated. As a result, a new heating furnace simulation is developed with the on-off and PID control model and the source term model. This simulation model can reduce the computational cost significantly while maintaining the accuracy. The developed model cannot only analyze the results of design changes, but also the results caused by the change in many variables, such as the speed of blank, the setting temperature, material change, and emissivity. Therefore, this simulation model can be used to suggest solutions for energy efficiency and productivity of existing furnaces through various case studies.

Investigating coating liquefaction and solidification of furnace-heated Al-Si coated 22MnB5 steel using laser reflectance

As part of the hot stamping process, aluminum-silicon (Al-Si) coated 22MnB5 steel blanks are austenitized using a roller hearth furnace, but the coating melts at ~577 °C, creating an intermediate liquid phase which, among other issues, shortens the lifetimes of the ceramic rollers and complicates blank transfer. Little is known about how this liquid regime is related to furnace process parameters, and how these parameters should be adjusted to minimize the impact of the liquid phase on hot stamping operations. This study pioneers laser reflectance measurements to infer the surface state of furnace-heated Al-Si coated steel coupons in situ. Time-resolved spectral reflectance measurements reveal multiple peaks, the first of which indicates the moment at which the coating liquefies. While the temperature corresponding to the start of the first peak is insensitive to heating rate, as the set-point of the furnace increases, the later peaks in the signal occur quicker but shift towards higher temperatures. These measurements are connected to the evolving surface microstructure via ex situ scanning electron microscopy and energy-dispersive X-ray spectroscopy of coupons extracted from the furnace at intermediate heating times.

Microstructure Characterization of Reversed Transformation in Cryogenically Rolled 22MnB5.

Hot stamping is a well-known process to produce structural automotive parts with an excellent strength-to-weight ratio. However, this process is more expensive due to the lower energy efficiency and operating cost of the traditional roller-hearth furnace. Additionally, lower ductility and toughness are commonly recognized as the main disadvantages of the current hot stamped ultra-high-strength parts. Refinement of austenite grains could be a profitable way to improve the strength of hot stamped parts. In this work, the evolution of reversed transformation in asymmetrically cryogenically rolled samples was studied in order to control the austenite. Thermomechanical simulation and heat treatment in the salt bath were used to investigate the reversed transformation process, and the typical microstructures were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Compared with symmetric prerolling, ferrite recrystallization could be remarkably inhibited by asymmetric rolling at the liquid nitrogen temperature (LNT) during the reheating process. Additionally, the nucleation of the austenite inner grains can also be promoted and the dynamics of the reversed transformation accelerated by asymmetric prerolling. Such phenomena might be very useful to refine the parent austenite grains before press hardening and enhance the new hot stamping strategy by partial fast reheating.

Failure-Analysis Based Redesign of Furnace Conveyor System Components: A Case Study

Any manufacturing equipment designed from scratch requires a detailed follow-up of the performance for the first units placed in service during the production ramp-up, so that lessons learned are immediately implemented in next deliveries and running equipment is accordingly updated. Component failure analysis is one of the most valuable sources of improvement among these lessons. In this context, a failure-assessment based design revision of the conveying system of a newly developed press hardening furnace is presented. The proposed method starts with a forensic metallurgical analysis of the failed components, followed by an investigation of the working conditions to ensure they match the forensic observations. The results of this approach evidenced an initially unforeseen thermo-mechanical damage produced by a combination of thermal distortions, material ageing, and mechanical fatigue. Once the cause–effect relationship for the failure is backed up by evidence, an improved design is proposed. As a conclusion, a new standard design for the furnace entrance set of rollers in hot stamping lines was established for roller hearth furnaces. The solution can be extended to similar applications, ensuring the same issues will not arise thanks to the lessons learned.

Development and Validation of a Thermometallurgical Model for Furnace-Based Austenitization During Hot Stamping

In hot-forming die-quenching (HFDQ) boron manganese steel blanks are heated within a roller hearth furnace, and then simultaneously quenched and formed into fully martensitic body-in-white components. Industry needs models that can predict the instantaneous temperature and austenite phase fraction within the roller furnace to diagnose problems (e.g., incomplete austenitization), forecast costs, and optimize process settings. This paper introduces a thermometallurgical model for Al–Si coated 22MnB5, consisting of a coupled heat transfer and austenitization submodels. Two candidate austenitization submodels are considered: an empirical first-order model and a model based on the detailed austenitization kinetics. In the case of the first-order model, a detailed Monte Carlo procedure is used to construct 95% credibility intervals for the blank temperature and austenite phase fraction that accounts for uncertainties in the furnace temperature and model parameters. The models are first assessed using temperature and austenite phase fractions from Al–Si coated 22MnB5 coupons heated in a laboratory-scale muffle furnace, and then used to simulate austenitization of patched blanks within an industrial roller hearth furnace. The results show that the empirical first-order model provides a more robust estimate of austenite phase fraction compared to the detailed model.

Potentials of single stage induction heating for press hardening of steel blanks

Due to growing challenges regarding crash-performance, CO2 emission as well as increasing demand for lightweight construction, hot metal forming of car body parts has risen to one of the most important technologies for saving weight of a car body. During hot forming shaped blanks of steel are heated and austenitized at around 950°C and subsequently quenched for martensitic formation. Currently the heating is realized in roller hearth furnaces which allow only a slow heating and, therefore, a limited production rate. Induction heating of the blanks offers a big potential to increase the production rate dramatically and also to improve the energy efficiency. Only due to the fact, that the heated blanks typically are already pre-shaped with holes and cut-outs induction heating becomes a very complex task. The paper compares different possible induction heating methods (longitudinal heating, transverse flux heating, single stage induction heating, hybrid heating by induction and conventional heating). For the case of single stage induction heating a detailed numerical and experimental investigation in the frame of a recently completed research project is presented.

Inferring the specific heat of an ultra high strength steel during the heating stage of hot forming die quenching, through inverse analysis

In hot forming die quenching, furnaces are used to austenitize ultra high strength steel blanks. In the case of coated steels, like Usibor® 1500 P, furnace heating also transforms a protective Al–Si layer into a permanent Al–Si–Fe coating. Modeling this process requires knowledge of how the sensible energy and latent heat of austenitization change with blank temperature; while the former is known, there is considerable uncertainty regarding the latter. In this work the effective specific heat of Usibor® 1500 P is inferred through inverse analysis of temperature data collected on coupons heated in a muffle furnace. This technique is first used to validate the heat transfer model, and then used to investigate austenitization at higher temperatures. Finally, the methodology is applied to analyze temperature data collected from an instrumented blank heated in a roller hearth furnace.

Optimal Design on the Roller Hearth Furnace Linings in CSP Plant of WISCO

The heating furnace in CSP plant of WISCO has a lightweight lining which is built by all fiber templates. In light of its good thermal insulation performance and short service life, the lining material for heating furnace was optimized in this article by using heat transfer calculation while the thickness stays light and the insulation performance stays good. The castable was applied to substitute for lightweight fiber materials to prolong the service life of heating furnace lining. Also, this article shows a contrast analysis of heat insulation layer while using different insulation material: the fiber felt, nanoinsulation board and super hydrophobic insulation board. The effect on the temperature of external shell and heat loss was analyzed and discussed. The results generate a new design of material composition, which not only prolongs the service life of heating furnace lining, but also reduces the cost.

Investigation of melting in the Al–Si coating of a boron steel sheet by differential scanning calorimetry

This paper investigates the veracity of claims, reported in the literature, that slow heating of Al–Si coated boron steel blanks in a roller hearth furnace can allow sufficient diffusion of iron from the steel substrate to the coating and prevent liquefaction of the Al–Si layer. The results of differential scanning calorimetry experiments conducted at various heating rates indicate that melting cannot be avoided under realistic industrial heating conditions. Heating rates as low as 0.08K/s as well as isothermal heat treatments at 550°C for 60min are insufficient to prevent melting of the binary Al–Si layer at the eutectic temperature. A two-stage treatment in which the coating was pre-treated up to 650°C to form solid Al–Si–Fe intermetallics is investigated. Intermetallic formation at 650°C is shown to occur very quickly due to the enhanced iron diffusion in the liquid Al–Si phase. The coating of pre-treated blanks does not melt during the second stage of the heat treatment; however the properties of the final component obtained via this two-stage heat treatment remain to be assessed.

Optimal Heating in Heat-Treatment Process Based on Grey Asynchronous Particle Swarm Optimization

To ensure plate heating quality and reduce energy consumption in heat-treatment process, optimal heating for plates in a roller hearth furnace was investigated and a new strategy for heating procedure optimization was developed. During solving process, plate temperature forecast model based on heat transfer mechanics was established to calculate plate temperature with the assumed heating procedure. In addition, multi-objective feature of optimal heating was analyzed. And the method, which is composed of asynchronous particle swarm optimization and grey relational analysis, was adopted for solving the multi-objective problem. The developed strategy for optimizing heating has been applied to the mass production. The result indicates that the absolute plate discharging temperature deviation between measured value and target value does not exceed ± 8°C, and the relative deviation is less than ± 0.77%.

Investigation on induction heating for hot stamping of boron alloyed steels

Within hot stamping of quenchenable steels the blank is heated up to austenitization temperature, transferred to the tool, formed rapidly and quenched in the cooled tool. Essential for the complete process is the heating of the blank which is currently carried out with roller hearth furnaces. As a consequence of rising energy costs new and more time and energy effective heating systems are needed. The paper presents investigations on induction heating as an alternative heating technology. Results concerning the process windows as well as material characteristics and grain structure will be presented and discussed.

Zonal method solution of radiative heat transfer in a one-dimensional long roller-hearth furnace in CSP

Abstract A radiative heat transfer mathematical model for a one-dimensional long furnace was set up in a through-type roller-hearth furnace (TTRHF) in compact strip production (CSP). To accurately predict the heat exchange in the furnace, modeling of the complex gas energy-balance equation in volume zones was considered, and the heat transfer model of heating slabs and wall lines was coupled with the radiative heat transfer model to identify the surface zonal temperature. With numerical simulation, the temperature fields of gas, slabs, and wall lines in the furnace under one typical working condition were carefully accounted and analyzed. The fundamental theory for analyzing the thermal process in TTRHF was provided.

Sintering furnaces for the PM industry

The following article reviews the structure, heating and atmosphere control, of a variety of furnaces which are used in the sintering of powder metallurgy (PM) products. Included are: conveyor furnaces, hump back furnaces, roller hearth furnaces, pusher furnaces, walking beam furnaces, bell and fit furnaces, and vacuum furnaces.

Reconstruction of roller-hearth heat-treating furnace

Reconstruction of roller-hearth heat-treating furnace

Automatic temperature control system in a roller-hearth slot furnace

Automatic temperature control system in a roller-hearth slot furnace

Mastering the technology of producing slabs for firing ceramic tiles in roller-hearth furnaces

Mastering the technology of producing slabs for firing ceramic tiles in roller-hearth furnaces