Tailoring Soft Local Zones in Quenched Blanks of the Steel 22MnB5 by Partial Pre-cooling with Compressed Air

Oleksandr Golovko,Florian Nürnberger,Max-Henry Stolte,Sven Hübner,Bernd-Arno Behrens,Hans Jürgen Maier,Kai Wölki

doi:10.1007/s11665-020-04916-5

Oleksandr Golovko, Florian Nürnberger + Show 5 more

Open Access

https://doi.org/10.1007/s11665-020-04916-5

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Abstract

The high-strength boron steel 22MnB5 steel is widely used for automotive lightweight constructions. A novel approach is promising to tailor strength and ductility in the hardened condition using locally pre-tempered sheets for the hot stamping process. It results in the formation of locally soft spots where mechanical joining is subsequently intended. A slow pre-cooling of the later joining zones with cooling rates below a certain critical cooling rate for obtaining a decreased strength in these regions is required. A tubular air cooling system suited for this task is presented and tested in a process where the subsequent quenching of the overall sheet is realized by rapid cooling in a water bath. Varying the air pressure and cooling duration allows controlling the size of the softened local spot in a wide range and still obtaining a bainitic microstructure. Using two-stage cooling with point jet nozzles and a subsequent hot stamping process with water-cooled dies resulted in a main sheet hardness of about 470 HV5 and of 260 HV5 in the pre-cooled spots, respectively.

Highlights

High-strength steels are of great interest for automotive lightweight constructions as these allow employing sheets with reduced thickness (Ref [1,2,3])
In order to obtain soft local zones suited for mechanical joining after hot stamping, a multi-step cooling process is proposed
In the first stage—while the main blank remains austenitized—the later soft local zones are pre-cooled by compressed air prior to the actual hot stamping in order to obtain a bainitic microstructure

Summary

Introduction

High-strength steels are of great interest for automotive lightweight constructions as these allow employing sheets with reduced thickness (Ref [1,2,3]). A further efficiency increase can be realized by the application of tailored tempering (Ref [4,5,6]). In this concept, high-strength sections are combined with ductile sections in one single structural part. Material zones with a locally reduced strength and increased ductility can be used to increase the energy absorption capacity and crash performance (Ref [7,8,9,10,11]).

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