Elsevier

CIRP Annals

Volume 58, Issue 1, 2009, Pages 275-278
CIRP Annals

Investigation on induction heating for hot stamping of boron alloyed steels

https://doi.org/10.1016/j.cirp.2009.03.090Get rights and content

Abstract

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.

Introduction

Even though the steel suppliers promised new sheet materials with high strength and good formability [1], hot stamping still remains the proven technology for the production of complex components with high strength. For hot stamping quenchenable boron–manganese alloyed steels are the preferred materials, which offer a minimized critical cooling rate of about 27 K/s which is necessary for the transformation of austenite into martensite within forming and cooling.

Two different process variants of hot stamping are the state of the art: direct and indirect hot stamping. While the direct process starts with a plain blank that is heated up to austenitization temperature and directly formed and subsequently quenched in one process step, the indirect process uses a preformed component which is heated up to austenitization temperature and calibrated and quenched in a water cooled tool afterwards [2]. Comparing both variants it is clear that the process chain of direct hot stamping is much shorter because only one forming tool is needed. In both cases components fulfilling the requirements of crash behaviour are produced, characterised by a high strength of about 1500 MPa and a residual elongation of about 5%. Because of the high strength the following up operations, e.g. piercing and hard cutting, are hard to realize. Thus, an alternative process sequence of the indirect hot stamping process was developed [3]. In this process the component is not only cold formed to its final geometry, but also all trimmings and holes are brought in before austenitization and quenching takes place.

To reduce abrasive wear in the forming tool and to assure the paintability of the formed component the formation of scale has to be avoided during the heating process. As a result either uncoated materials have to be heated under protective gas or zinc-coated or aluminised materials have to be used. Because of the minimum number of process steps the direct hot stamping process is the one which is actually preferred in automobile industry. For this the quenchenable steel 22MnB5 with an Al–Si-layer is used. The melting point of this coating is below the austenitization temperature of the base material, about 873 K, but if the correct heating rate is chosen diffusion of Fe into the Al–Si-coating occurs resulting in a temperature resistant Fe–Al–Si-layer [4]. This process requires a precisely controlled heating process which leads, together with the demanded time and temperature for austenitization, to 30–40 m long furnaces. Today the focus of different research activities in the field of hot stamping lies on the reduction of the cycle time which directly depends on the cooling performance of the tool on the one hand. On the other hand rising energy costs and the demand on environmental friendly production processes lead to the necessity to look for less energy consuming heating methods. As a consequence new tool concepts [5], new tool materials [6] and new heating technologies [7], [8] are investigated.

Taking alternative heating technologies into account the applicability of these heating technologies, especially induction heating, for hot stamping processes is an important topic. The focus of the investigations is set on the determination of the process window for full austenitization guaranteeing. As reference results and data coming from a conventional chamber furnace can be used. Thus an alternative two-step inductive heating system has been developed, that is able to realize different heating rates.

Section snippets

Conventional heat treatment method

Conventionally continuous furnaces are used for hot stamping, whereas the blank is heated by radiation and convective flow of heat. The furnaces are heated even with gas or electricity. The blanks are brought in the furnace, heated up to austenitization temperature while passing the furnace on rollers or walking beams. The heating rate of the blanks is controlled by the speed of the rollers or the walking beams on the one hand, but by the temperatures in different consecutive chambers on the

Methodology

For alternative heating of 22MnB5 for hot stamping a two-step induction device was developed using the benefits of both inductors. The system consists of a longitudinal field inductor with a maximum connected power of 120 kW and a face inductor with a maximum connected power of 150 kW in the test area (see Fig. 4). The transport of the blanks is realized with a chain drive system with a fixed connection between chain and blank which is necessary because of the magnetic forces in the longitudinal

Summary and outlook

Hot stamping of boron alloyed steels is getting more and more important, especially for the production of body parts with high crash performance. To enhance the efficiency of this technology new concept, especially for heating, has to be found. One approach is the two-step induction heating.

In this investigation it was proved that for uncoated material similar properties could be reached with induction heating in comparison to convective heating. The grade of austenitization as well as the

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