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New Developments and Future Trends in Low-Temperature Hot Stamping Technologies: a Review

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New Developments and Future Trends in Low-Temperature Hot Stamping Technologies: a Review metals Review New Developments and Future Trends in Low-Temperature Hot Stamping Technologies: A Review Chenpeng Tong 1, Qi Rong 1, Victoria A. Yardley 1, Xuetao Li 2, Jiaming Luo 2, Guosen Zhu 2 and Zhusheng Shi 1,* 1 Department of Mechanical Engineering, Imperial College London, London SW7 2AZ, UK; [email protected] (C.T.); [email protected] (Q.R.); [email protected] (V.A.Y.) 2 Shougang Research Institute of Technology, Shougang Group, Beijing 100041, China; [email protected] (X.L.); [email protected] (J.L.); [email protected] (G.Z.) * Correspondence: [email protected]; Tel.: +44-20-7594-9546 Received: 13 November 2020; Accepted: 2 December 2020; Published: 8 December 2020 Abstract: Improvement of the hot stamping process is important for reducing processing costs and improving the productivity and tensile properties of final components. One major approach to this has been to conduct all or part of the process at lower temperatures. The present paper reviews the state of the art of hot stamping techniques and their applications, considering the following aspects: (1) conventional hot stamping and its advanced developments; (2) warm stamping approaches in which complete austenitisation is not attained during heating; (3) hot stamping with a lower forming temperature, i.e., low-temperature hot stamping (LTHS); (4) advanced medium-Mn steels with lower austenitisation temperatures and their applicability in LTHS. Prospects for the further development of LTHS technology and the work required to achieve this are discussed. Keywords: low-temperature hot stamping; hot stamping; warm stamping; low heating temperature; low forming temperature; medium-Mn steel; cost-saving; productivity; mechanical property 1. Introduction Owing to the depletion of non-renewable energy resources and the environmental impact of burning fossil fuels, there has been a strong push to improve the fuel efficiency of automobiles in recent years. One of the most feasible approaches is by light-weighting; a 10% weight reduction can lead to an almost 2.5% increase in fuel efficiency [1]. The automotive industry is thus working on reducing vehicle weights with simultaneous improvements in safety and crashworthiness. This can often be achieved either by using lighter materials, such as aluminium alloys, or by using stronger materials such as advanced high strength steels (AHSS) in the body in white (BiW) of vehicles. Hot stamping technology has been developed as a specialised production technique for the manufacture of components from AHSS; this addresses the shortcomings encountered in conventional cold forming technologies, such as poor formability, high impact on the tools and an elevated tendency to springback [2]. The most commonly used AHSS materials are boron (Mn-B) steels, such as 20MnB5, 22MnB5 and 27MnCrB5, which show a yield strength (YS) of above 770 MPa and an ultimate tensile strength (UTS) of above 1300 MPa with a total elongation (TE) of around 6–8% in hot-stamped parts [3]. The most notable example to date of the application of hot-stamped AHSS in the automotive sector is the 2014 Volvo XC90, where hot-stamped sheet steel was applied for around 38% of the BiW, including the front- and rear-side longitudinal members, A-, B- and C-pillar reinforcements, roof rail reinforcements and floor cross members [4]. The 2014 Acura MDX was another notable example, in which hot-stamped steel was adopted for the A- and B-pillars, roof rail and sill reinforcements [5]. Metals 2020, 10, 1652; doi:10.3390/met10121652 www.mdpi.com/journal/metals Metals 2020,, 10,, 1652x FOR PEER REVIEW 2 of 27 Hot stamping technology for Mn-B steels is now mature and has been commercialised internationally,Hot stamping especially technology for sheet for in Mn-B the 1500 steels MPa is nowUTS class. mature The and process has beenchain commercialisedis illustrated in internationally,Figure 1a [6] and especially a schematic for representation sheet in the 1500 of the MPa thermomechanical UTS class. The cycle process and chainthe microstructural is illustrated inevolution Figure1 aduring [ 6] and the a hot schematic stamping representation process is shown of the in thermomechanical Figure 1b [7]. During cycle the and process, the microstructural the steel coil evolutionis first cutduring into blanks, the hot heated stamping up to process 900–950 is shown°C in a in furnace Figure 1andb [ 7 held]. During isothermally the process, for around the steel 5–10 coil ismin first until cut intothe initial blanks, ferrite-pearlite heated up to 900–950 microstructure◦C in a furnace is completely and held transformed isothermally to for austenite around 5–10 [7]. minThe untilblanks the are initial subsequently ferrite-pearlite transferred microstructure from the furnac is completelye to a press, transformed where they to are austenite formed [7 at]. around The blanks 700 are°C or subsequently higher, then transferred quenched fromto room the temperature furnace to a press,with a wherecooling they rate are of formedat least at27 around°C s−1, this 700 being◦C or 1 higher,the critical then rate quenched necessary to room to obtain temperature a fully withmart aensitic cooling transformation rate of at least in 27 the◦C material, s− , this beingassuring the criticalultimate rate tensile necessary strength to of obtain up to a 1700 fully MPa martensitic [8]. Finally, transformation the hot-stamped in the part material, is post-treated, assuring applying ultimate tensileprocesses strength such ofas uptempering, to 1700 MPa trimming [8]. Finally, and thepunc hot-stampedhing, to satisfy part isthe post-treated, requirements applying of commercial processes suchcustomers as tempering, [1]. Much trimming research andinto punching,hot stamping to satisfytechnology the requirements has already been of commercial published. customers For instance, [1]. MuchKarbasian research [3] has into summarised hot stamping th technologye hot stamping has already procedure been published. including Forthe instance,thermal, Karbasian mechanical, [3] hasmicrostructural, summarised and the hottechnologica stampingl aspects procedure and including has shown the the thermal, potential mechanical, for further microstructural, investigations. andMerklein technological and Lechler aspects [9] have and reported has shown that hot-st the potentialamped 22MnB5 for further steel investigations. parts could be produced Merklein with and Lechleran ultimate [9] have tensile reported strength that of hot-stamped 1500 MPa. 22MnB5Naderi et steel al. parts[10] have could investigated be produced and with analysed an ultimate the tensilemicrostructural strength ofevolution 1500 MPa. and Naderi corresponding et al. [10] havemechanical investigated properties and analysed in B-pillars the microstructuralwith different evolutiondesigns during and corresponding the hot stamping mechanical process. properties in B-pillars with different designs during the hot stamping process. (a) (b) Figure 1.1. Illustration of conventional hot stamping process for Mn-BMn-B steel:steel: ( a) Basic hothot stampingstamping process chain [[6]6] (Reproduced(Reproduced withwith permissionpermission ofof Pentera,Pentera, 2020).2020). ((b)) ThermomechanicalThermomechanical cycle andand microstructural evolution duringduring hothot stamping.stamping. However, the currentcurrent hothot stampingstamping technologytechnology hashas thethe followingfollowing inadequaciesinadequacies [[11,12]:11,12]: (1) The productivity is low duedue toto aa longlong cyclecycle timetime includingincluding bothboth heatingheating andand cooling.cooling. (2) The cost of protection against oxidation and decarburisation, su suchch as coating or fabrication in an oxygen-freeoxygen-free environment, isis high. (3) Due to a low low ductility, ductility, the product is unsuitable for use in energy-absorbing structures. (4) The process requires a large investment in production equipment (including a cooling system with complex design,design, largelarge furnacefurnace andand laserlaser trimming).trimming). Metals 2020, 10, 1652 3 of 27 Due to the globally increasing demand for application of the hot stamping technology within automobile manufacture, there has been great interest in solving these issues. In recent years, several technologies have been developed to improve the production efficiency and enhance the final performance of hot-stamped components. Section2 of this paper briefly introduces these recent developments in hot stamping. However, the improvements achieved are still not sufficient and these technologies have not been widely implemented in industrial production. Another approach that has recently attracted a great deal of research attention is that of conducting the process at lower temperatures than are typical in the current conventional hot stamping process. This can significantly reduce the process cycle time and save energy and cost. These desired objectives can be achieved through reducing the heating temperature and/or forming temperature and using advanced materials with lower austenitic transformation temperatures (A1 and A3), e.g., medium-Mn steel (MMn) steels [13–15]. Recent developments in hot stamping technology with reduced temperatures are critically reviewed in Section3, including both warm stamping and low-temperature hot stamping (LTHS). Section4 covers the mechanical behaviour and microstructural mechanisms of MMn steels which good candidates for LTHS. The application of MMn steels in LTHS processes is reviewed in
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