**1. Introduction**

Press hardening of boron alloyed steels has been used since the 1980s [1] to produce beams, pillars, and safety-related components for cars [2]. A six-fold increase in the adoption of the technique for component production was anticipated between 2006 and 2015 [3] and the production reached 360 million components in 2015 [4]. Strength levels achievable in boron steels in as quenched conditions are considered excellent (*Rm* ≈ 1500 MPa) but the ductility is often limited (*A*50 mm ≈ 6% or lower) as a result of the essentially martensitic microstructure of the steels [5]. Tailor-welded blanks and differentiation of heat treatment are the methods that can be used to tailor and optimize the properties in different parts of a component [6]. In addition, both ductility and toughness may be enhanced in these steels with the formation of carbide free bainitic (CFB) microstructures through austempering process and/or subjecting these steels to a novel concept of quenching and partitioning (QP) thermal treatment as described below. Formation of CFB microstructures can be facilitated in specially tailored steel compositions containing high levels of Si and/or Al (about 1.5–3 wt %), through austempering

because both Si and or Al are strong graphitisers and hence, hinder or delay carbide formation in the steel structure. The microstructures of CFB steels comprise mainly of fine laths of bainitic ferrite and carbon enriched austenite divided finely between bainitic sheave [7,8] and martensite in some cases [9]. Likewise, the QP treatment first described by Speer et al. [10] also promotes formation of essentially martensitic microstructures with small fractions of finely divided, carbon-enriched interlath austenite [11], besides a small fraction of bainitic ferrite and in some cases also carbides [12]. Tensile properties typical of selected steels processed through QP technique are shown in Table 1.


**Table 1.** Typical tensile properties of quenching and partitioned (QP) steels vis à vis boron (22MnB5) and austempered bainitic (CFB) steels.

The twin benefits of the existing direct press hardening process applied to boron steels are (i) the combination of rapid forming through optimized processing and (ii) quenching of the component in the pressing tool. During austempering, austenite is isothermally transformed into lower bainite at a temperature slightly above the martensite start temperature (*Ms*) for a duration adequate enough for complete austenite decomposition. However, slow kinetics of the austenite to bainite transformation at temperatures close to *Ms* can have limitations in respect of the austempering process in combination with press hardening for commercial production of automotive components. In the QP process the steel is quenched to a temperature between the start (*Ms*) and the finish (*Mf*) of martensite reaction and subsequently either held at the quenching temperature or heated to just above or below *Ms* temperature to facilitate partitioning of carbon from transformed supersaturated martensite into austenite or from the bainite that may form during subsequent partitioning step. The transformation rate has been shown to increase when the austempering temperature is lowered just below the *Ms* temperature [16]. A quench stop below *Ms* allows a small amount of martensite to form prior to bainite transformation, thereby increasing the number of possible nucleation sites for bainite and thus its rate of formation [16]. It has also been shown that the transformation from austenite to bainite can be accelerated if a small fraction of martensite can be formed from the austenite [17] even though the rate of bainite formation following martensite formation remains unchanged and same amount of bainite would form following austenite decomposition [18]. Utilization of the QP heat treatment thus provides the possibility to shorten the production cycle time. Press hardening with a QP treatment of boron steel has been shown to improve the ductility of the steel but with a marginal loss in yield strength [19,20], as compared with the properties obtained through conventional press hardening of 22MnB5 steel. This process has been repeated for low-carbon Si-Mn steels and the maximum volume fraction of retained austenite reached 17.2% with corresponding total elongation of 14.5% when hot stamping is done at 750 ◦C [21]. Seo et al. [22], designed two types of modified press hardening steels (PHS) by adding Si, and Si + Cr to 22MnB5 steel, followed by optimized QP processing to achieve improved properties. In the best QP conditions the ductility improved to 17% total elongation with 1032 and 1098 MPa yield and tensile strengths respectively, for the Si + Cr added (PHS) grade.

The aim of this work was to produce components with properties equal to or better than conventional press hardened boron steels, within a reasonable processing time for improved productivity. Various thermal treatments following the forming stage were investigated to achieve a fine multiphase microstructure. The quench stop temperature in the die was identified as a variable of interest along with the furnace temperature and the holding time of the heat treatment. Two variants of quench stop temperature were investigated, above and below the *Ms*. Both isothermal heat treatment above and below *Ms* and QP were investigated using thermal simulations for two cold rolled Fe-(0.15 and 0.26)C-1.5Si-2Mn-0.6Cr alloys. This paper reports an account of the mechanical properties obtained after press hardening experiments to produce hat-shaped profiles using QP heat treatments for high-silicon steels, in comparison with those of commercial 22MnB5 profiles. The effect of using QP treatments on austenite decomposition kinetics in comparison with austempering treatment is also studied.
