Production of shaped semi-products from AHS steels by internal pressure

Abstract

The constant evolution of industry brings demands for components of ever-higher quality, complex shapes and excellent mechanical properties, with reduced operating costs and with improved reliability in service. Such demands can be met by processing high-strength, low-alloyed steels using novel heat-treatment methods. One of such methods is the quenching and partitioning (Q&P) process, which can lead to high strengths of about 2000 MPa at elongation levels of more than 10 %. The Q&P process can be combined with unconventional procedures for making complex-shaped parts to manufacture components with excellent mechanical properties and an intricate shape. This paper describes the sequence of internal high-pressure forming, hot stamping and the Q&P process used for making a functional product with good mechanical properties and a complex shape. The purpose of the experimental programme was to employ a production chain for processing hollow, thin-walled stock. Using a step-by-step optimization, commercially usable products were obtained. The microstructure of the products consisted primarily of martensite and a small fraction of bainite. Such a microstructure guarantees a high ultimate strength. The strength level throughout the part was not less than 1950 MPa, combined with an excellent elongation of approximately 15 %. Keywords: Q&P process, hot stamping, internal high pressure forming Stalen razvoj industrije zahteva komponente vedno bolj{ih kvalitet, kompleksnih oblik, odli~nih mehanskih lastnosti pri ni`jih proizvodnih stro{kih in z ve~jo zanesljivostjo obratovanja. Take zahteve se lahko dose`ejo z izdelavo visokotrdnostnih malolegiranih jekel z uporabo novih metod toplotne obdelave. Ena od takih metod je postopek kaljenja in delitve (Q&P), ki omogo~a velike trdnosti okrog 2000 MPa pri ve~kot 10-odstotnem raztezku. Postopek Q&P se lahko kombinira z nekonvencionalnimi metodami izdelave delov s kompleksno obliko in za izdelavo komponent z odli~nimi mehanskimi lastnostmi in komplicirano obliko. Predstavljeni~lanek obravnava zaporedne operacije notranjega visokotla~nega preoblikovanja, vro~ega stiskanja in Q&P-postopka za izdelavo funkcionalnih proizvodov z dobrimi mehanskimi lastnostmi in zapleteno obliko. Namen eksperimentalnega dela je bil postaviti proizvodno verigo za izdelavo votlih tankostenskih kosov. Z optimizacijo korak za korakom so bili izdelani komercialno uporabni proizvodi. Mikrostruktura proizvodov je bila sestavljena predvsem iz martenzita in majhnega dele`a bainita. Taka mikrostruktura zagotavlja veliko natezno trdnost. Nivo trdnosti po vsem kosu ni bil manj{i od 1950 MPa v kombinaciji z odli~nim raztezkom okrog 15 %. Klju~ne besede: Q&P-proces, vro~e stiskanje, preoblikovanje z velikim notranjim tlakom