CIRP Encyclopedia of Production Engineering

Living Edition
| Editors: The International Academy for Production Engineering, Sami Chatti, Tullio Tolio

Hot Stamping

  • Bernd-Arno BehrensEmail author
Living reference work entry

Latest version View entry history

DOI: https://doi.org/10.1007/978-3-642-35950-7_16722-3

Synonyms

Definition

Hot stamping is a hot forming method used in sheet metal forming, specifically in production of high-strength steel components for crash-relevant parts in the automotive industry.

History

Early 1970s
  • Early 1970s

  • The first developments in hot stamping in Sweden by Volvo in Olofström and Norrbottens Järnverks AB in Luleå (Jergeus 2012).

1973
  • 1973

  • The first announcement for a European patent on hot stamping (SE7315058-3) in Sweden by Carl-Erik Ridderstrale of Plannja HardTech, a division of Plannja AB, which in turn is a subsidiary of Swedish Steel AB (SSAB), for manufacturing farm implements. The motivation is the need for lightweight and strong steel components (Plannja 1973; Keremedjiev 2011).

1977
  • 1977

  • The first patent on hot stamping in Britain (GB1490535 – “Manufacturing of a hardened steel article”) by Plannja HardTech that used the process to produce saw blades and lawn mower blades (Norrbottens Jaernverk 1977; Karbasian and Tekkaya 2010).

1980
  • 1980

  • A marketing campaign toward the European automotive industry started, and the first contracts for side impact beams are signed for Saab 9000 (Klebert 2017).

1984
  • 1984

  • With the Saab 9000, Saab Automobile AB is the first vehicle manufacturer who opts for a hardened boron steel component (Karbasian and Tekkaya 2010).

1985
  • 1985

  • The first publication of a patent on hot stamping in Germany (DE2452486C2) by Plannja AB (Plannja 1985).

1986
  • 1986

  • The first automotive series production starts by Plannja AB (Jergeus 2012).

1987
  • 1987

  • The number of produced parts amounts to 3 million parts in this year (Schuler 2013).

1991
  • 1991

  • The great breakthrough on the European market for Plannja AB is achieved by the contract for side impact beams for Ford (AP&T 2017).

1996
  • 1996

  • Decisions are made on establishing a manufacturing plant in Mason, Michigan, USA, with planned start of manufacture in the middle of 1998. As this US investment is taking place, Luleå is developing into a technical center for research and development in process and tool technologies. The focus on the automotive industry is followed by the removal of saws and wear parts from the manufacturing (Reynolds 2000).

1997
  • 1997

  • The number of produced parts amounts to 8 million parts/year (Schuler 2013).

2004
  • 2004

  • The Spanish company Gestamp Automoción buys Plannja HardTech from SSAB primarily because of their know-how. The first aim is to complete the ongoing investment with a hot stamping line at Haynrode in Germany (SSAB 2004).

2007
  • 2007

  • For the first time, the number of produced parts amounts to more than 100 million parts/year (Schuler 2013).

2013
  • 2013

  • The patent EP1645345A2 expires, so new competitors to Gestamp appear: Benteler, TKS, and the original equipment manufacturers (OEM) VW/Audi, BMW, Fiat, and Volvo, among others (Jergeus 2012).

2015
  • 2015

  • According to forecasts by Schuler Group, the number of parts increases to 500–600 million parts/year (Schuler 2013).

Process

Process Strategies

In industrial production, hot stamping is carried out in two variants, which are shown schematically in Fig. 1. Taking into account the geometry of the component and the cost of production as well as of existing installations, the components can be produced either by using the direct or the indirect method. In the direct process, forming of the heated metal sheet and the subsequent heat treatment of the component are carried out in a single step. For this purpose, the shaped blanks are heated to a temperature of 930–950 °C. Then a robot system transfers the blank from the furnace to the tool, where the raw material is given its final shape and cooled. Due to transfer time between 3 and 5 s, the forming process takes place at temperatures between 850 and 600 °C.
Fig. 1

Process strategies for hot stamping

The subsequent hardening in the tool takes approximately 10 s. A second strategy is referred to as indirect hot stamping. In contrast to direct hot stamping, the part is formed up to 95% of the final geometry. Subsequently, the preformed parts are heated and subjected to the actual press hardening in a second forming stage. This second stage of forming almost exclusively aims at a controlled cooling of the component in the tool. With the help of this process variant, an increase in the forming limits is possible because direct hot stamping is only carried out in one stage. For complex components such as the VW Passat tunnel, the necessary degree of deformation can only be achieved by a two-step process. Figure 2 illustrates a typical temperature profile in mass production (direct hot stamping).
Fig. 2

Typical temperature profile for direct hot stamping

The microstructure transformation into austenite starts when the blank is heated up to the material-specific temperature Ac1 and is completed when the blank passes the Ac3 temperature. The heated and austenitized material is then formed in the tool with a cooled punch and is quenched until it reaches a target temperature of 100 °C ≤ θ ≤200 °C. The contact with the tools automatically quenches the blank. The composition of the resulting structure primarily depends on the cooling rate. If the continuous cooling occurs quickly enough, it results in martensite, which leads to very high-strength products.

Diffusion-Controlled and Diffusionless Phase Transformations

During hot stamping, various microstructural transformations leading to an increase in strength take place in the material. When steel is heated to the material-specific threshold temperature Ac3, only austenite occurs, which has a cubic face-centered (CFC) lattice structure. In this case, the alloying elements, which were previously located in the region of the grain boundaries, dissolve almost completely in the lattice structure. Based on this austenitic CFC lattice structure, steel has a defined cooling rate, which depends on the specific carbon content and the remaining chemical components. For cooling rates below this limit value, diffusion-controlled structural transformations occur, caused by the diffusion of carbon atoms and other alloying constituents. In this case, the austenite is transformed into ferrite, pearlite, or bainite (Fig. 3).
Fig. 3

Phase transformation during a hot stamping process

Cooling rates above the critical cooling rate lead to an undercooling of the γ-Fe crystal and to a lattice sharing with α-Fe crystals. At such high cooling rates, the time is not sufficient for carbon diffusion, as it is present in diffusion-controlled transformations. Because the α-Fe crystals are only able to resolve a little CO2 into the steady state, a tetragonal lattice structure occurs. This is associated with a sudden increase in volume by up to 3%, which manifests itself in a change in length. This type of structure is called martensite and is characterized by low formability but offering very high strength. These properties make the martensitic structure desirable in many parts of the steel, and this is the reason why press hardening is carried out.

Properties of Manganese-Boron Steel

So far, mainly manganese-boron steels (22MnB5, 19MnB4, 30MnB5) are used for indirect and direct press hardening in the automotive industry. These steels are microalloyed, quenched, and tempered steels. The mostly used steel is 22MnB5 (EN 1.5528), which is characterized by a structure with about 75% ferrite and 25% pearlite. At delivery, it has a yield strength of Rp 0.2 ≈ 400 N/mm2 with a tensile strength of Rm ≈ 600 N/mm and an elongation at break of A120 ≈ 20%. By varying the heat treatment parameters (furnace temperature, heating time, and cooling medium) of the process, the mechanical properties of the quenched and tempered steel 22MnB5 can be set within a wide range. Thus, in Fig. 4, the range of properties is summarized in terms of attainable tensile strength and elongation at break. Using special heat treatment strategies, the material properties can be set depending on the requirements in this framework. Among other things, a tensile strength of Rm = 1400 N/mm2 with an elongation of A120 = 7% is possible. The corresponding process parameters are a furnace temperature of θ = 950 °C, a retention period of t0 = 3.5 min, and a cooling, in which the samples which are in contact with steel bodies reach room temperature.
Fig. 4

Range of possible mechanical properties of 22MnB5

Manganese-boron steels are used in hot stamping due to the properties of the alloying elements, manganese and boron. The alloying element manganese in particular has a strength-increasing effect. It also delays the transformation of undercooled austenite in the secondary structure of ferrite, pearlite, and bainite. But the latter is of secondary importance since boron reduces the critical cooling rate to a much greater extent. The delay of the austenite transformation increases the time slot for carrying out the forming process.

The following facts cause the significant reduction of the critical cooling rate and slow formation of a secondary structure in alloys with a higher boron percentage:

The alloying element boron delays the heterogeneous nucleation on the grain boundaries through a reduction of the surface energy since boron is withdrawn at the grain boundaries (Babu et al. 1998). Thus, grain boundaries in alloys with a higher boron percentage are less effective nucleation sites than steel with an equal alloy composition except for the boron percentage. Usually, in commercial boron-alloyed steels, boron concentrations from 0.001 to 0.003 mass.−% are used. Higher levels may lead to the formation of borides at the austenite grain boundaries, which accelerates the formation of ferrite and thus contradicts the mentioned aim of a reduction of the critical cooling rate through the alloying with boron. Manganese-boron steels are characterized, in contrast to other quenched and tempered steels, by a low critical cooling rate of approximately 30 K/s. A deformation of the material has to be done before reaching the martensite start temperature, since martensite displays very high strength and hardness but has only low deformability. For example, the Vickers hardness can reach 470 HV in MnB steels. Plastic deformation leads to an increase of the critical cooling rate and also to a reduction of the martensite start temperature. The first is due to an increase of the internal energy, which is why less energy is required for a diffusion-controlled transformation. The variation of the martensite start temperature is due to the dislocation during the forming process (Naderi et al. 2008). This hinders a diffusionless phase transformation, which is why a higher driving force for the transformation of austenite into martensite is required than for non-transformed austenite. This driving force is achieved by lowering the martensite start temperature.

References

  1. Babu SS, Goodwin GM, Rohde RJ, Sielen B (1998) Effect of boron on the microstructure of low-carbon steel resistance-seam welds. Weld J 77:249–253Google Scholar
  2. Jergeus J (2012) LS-Dyna Nordic users’ forum 2012, GöteborgGoogle Scholar
  3. Karbasian H, Tekkaya AE (2010) A review on hot stamping. J Mater Proc Technol 210:2103–2118CrossRefGoogle Scholar
  4. Keremedjiev G (2011) Hot stamping: an economic leapfrog. Metal Forming Magazine 45(10), http://www.metalformingmagazine.com/magazine/article/?/2011/10/1/Hot_Stamping:_An_Ecomomic_Leapfrog. Accessed 19 March 2018
  5. Klebert S (2017) Slimming solutions and driving performances – challenges in automotive lightweight design, http://sternstewartinstitute.com/artikel-startseite/slimming-solutions-and-driving-performances.html. Accessed 05 July 2017
  6. Naderi M, Saeed-Akbari A, Bleck W (2008) The effects of non-isothermal deformation on martensitic transformation in 22MnB5 steel. Mater Sci Eng 487:445–455CrossRefGoogle Scholar
  7. Norrbottens Jaernverk Ab (1977) Patent GB1490535A, 02 Nov 1977Google Scholar
  8. Plannja Ab (1973) Patent SE7315058–3, 06 Nov 1973Google Scholar
  9. Plannja Ab (1985) Patent DE2452486 C2, 17 Oct 1985Google Scholar
  10. Reynolds G (2000) Swedish invasion. Modern steel constructionGoogle Scholar
  11. Schuler SMG (2013) Leicht und Sicher in Serie. In-house Exhibition, WaghäuselGoogle Scholar
  12. SSAB (2004) Press release, http://www.waymaker.net/bitonline/2004/11/12/20041112BIT20800/wkr0001.pdf. Accessed 05 July 2017

Copyright information

© CIRP 2018

Authors and Affiliations

  1. 1.Institute of Forming Technology and Machines (IFUM), Leibniz Universität Hannover (LUH)GarbsenGermany

Section editors and affiliations

  • Bernd-Arno Behrens
    • 1
  1. 1.Institute of Forming Technology and Machines (IFUM)Leibniz Universität Hannover (LUH)GarbsenGermany