The steel portfolio for the automotive industry comprises a range of high-strength grades. The sector’s present focus on fuel efficiency, light structures, and low material costs, however, requires the application of ultra-high-strength formable steels. To achieve the strength required, OEMs have relied on multiphase steels since the 1990s. Finnish steel-maker Ruukki was one of the first suppliers to invest in multiphase steel production and is a forerunner at the cutting edge in the development and production of hot-dip galvanised Dual Phase (DP) and Complex Phase (CP) steels, which the company markets under the brand name LITEC®.
Ruukki started producing hot-dip galvanized ultra-high- strength steels back in the late 1980s and even today remains one of the few producers to offer ultra-high-strength formable steels including DP, CP and Transformation Induced Plasticity (TRIP) varieties with various metallic coatings. In developing these steels, Ruukki also focused on the production of customised Litec tubes, thereby gaining considerable expertise and high OEM interest in especially the DP tube portfolio which offers better strength and formability than conventional HSLA tubes. DP steel tubes are used in bumper systems as well as inside cross beams.
Shift in products
The rise of new materials such as aluminum and other light materials in the struggle to reduce weight has, however, caused a shift in focus for steel manufacturers catering to automotive OEMs.
While the alternative materials offer great potential for weight reduction, their application in car parts that are crucial for the driver’s safety comes at high economic costs. Both the materials and the inherent manufacturing process require significant investments that are only feasible for cars in the executive and luxury segments. Especially with respect to the production of subcompact, compact and midsized cars, which constitute the lion’s share of registered vehicles roaming the streets of Europe, the safety cage will hence remain steel territory.
Ultra-high-strength and rigidity are paramount for the steel cage that protects drivers and passengers. While DP steels for instance exhibit superior energy absorption capacity and are thus the material of choice in front and rear impact areas, as well as in crash boxes, all multiphase steels have drawbacks in their comparatively high ductility with regard to safety cage parts. Developed since the late 1990s and implemented since early 2000, press-hardened Boron steels (PHS) have no springback effect after hardening, higher yield strengths of about 1100 MPa or more and tensile strengths of currently popular 1500 MPa. The industry is now, however, seeing an increasing shift towards the application of PHS steels over multiphase steels in the body in white.
Direct press hardening
In order to achieve the desired rigidity, the cold-rolled PHS steels are annealed at a high temperature and quenched in a die. The process is called hot-stamping or press hardening (figure 2). Two different methods of hot stamping are commonly applied: direct and indirect quenching. Starting out with moderate strength and high ductility, the cold-rolled steel coils are blanked. With the indirect process the blank is pre-formed by cold pressing before the shaped component is annealed in the furnace, whereas in direct press hardening this step is omitted and the blanked steel sheet is annealed directly. The steels are heated up to temperatures of around 900-950°C. At these elevated temperatures, the strength of the Boron steel drops to around 200 MPa and it reaches excellent formability. The blank and component are then transferred to a press-forming system in which the steel is formed from the blank (direct hot stamping) or, in case of the pre-formed shape in the indirect method, trimmed while concurrently being quenched in cooling dies to a temperature of about 200°C.
Under the combined hot stamping and rapid cooling method, the steel reaches a tensile strength of 1500 MPa. In a final step the hardened sheets or components are cleaned and cut.
The direct quenching process saves the pre-forming stage and is thus more economical, more efficient, and simply faster. It also entails the exposure of the steel to a higher degree of deformation at high temperatures, though, which in turn puts more strain on the steel coating.
Galvannealed vs. Aluminised Coating
A protective coating is needed for PHS steels in order to avoid surface damage and corrosion during the quenching process. Micro-cracks will occur in all coatings but not all coatings offer the same oxidation resistance during processing and corrosion protection in later use. Ruukki has identified zinc-iron coated PHS steels, so-called galvannealed, to offer the best quality in this field.
The most commonly used protective shield for PHS steels is an aluminum-silicon (AlSi) alloy, favourably applied as a 10 - 20µm thin layer to protect the steel against heat and oxidation. The standard coating and a constant in the automotive industry, it is used by a number of market leading steel manufacturers and automotive steel suppliers. All aluminum based coating however merely provides a protective barrier against corrosion. If the surface cracks - is scratched or damaged or otherwise penetrated, or stress corrosion cracks appear, even with a minimal point of breach - oxidation occurs underneath the cracked part of the coating and the corrosion will spread.
A ZF coat on the other hand protects the steel cathodically, with a zinc-rich coating, which serves as a sacrificial metal which attracts the oxygen and corrodes, saving the underlying steel. Ruukki applies a zinc-rich zinc-iron alloy in a roughly 10µm thick layer that allows for a fine, even surface.
The company has tested the corrosion protection qualities of an unpainted AlSi coating compared to those of such (also unpainted) galvannealed steel in a neutral salt spray (NSS) test according to EN ISO 9227.
It is a standardised test to evaluate the corrosion protection of various coatings in which the test objects are sprayed with a concentration of sodium chloride and placed in a small test chamber which due to the spray develops a corrosion facilitative atmosphere. Coated steel will show red rust (iron oxide) once the coating loses its protective ability and the underlying steel starts corroding. The NSS test was conducted in line with standard procedure with the following parameters:
- A sodium chloride solution was sprayed on the samples in a concentration of 50 g/l ± 5g/l
- The testing temperature was kept at 35°C ± 2°C
- A pH value of 6.5-7.2 was maintained
- An average collection rate for a horizontal collecting area of 80cm² of 1.5 ml/h ± 0.5 ml/h was ensured.
As seen in Figure 3, the ZF coating corroded but remained intact, effectively protecting the PHS steel sample while the AlSi coating was breached and iron oxide formed on the steel.
The share of iron in the galvannealed coating brings an array of additional benefits. Most prominently it facilitates excellent spot-weldability, a crucial trait in automotive production, compared to other coatings - including aluminum-based coatings. Due to these qualities of exceptional corrosion protection and weldability, Ruukki banks on and invests in galvannealed PHS steels, providing an economic and safe solution for car parts that form car safety cage superior to the common market standard (figure 4). www.ruukki.com
Figure 1: Advanced high-strength steel types. © Ruukki Metals Oy
Figure 2: Direct hot stamping process. © Ruukki Metals Oy
Figure 3: Example of corrosion resistance. © Ruukki Metals Oy
Figure 4: Applications of LITEC steels. © Ruukki Metals Oy
For further information, please contact:
Tony Harris, Vice President, Sales, Western Europe &
Africa: tel. +44 12 17 047329
Paul Rabjohn, General Manager, Ruukki UK Ltd.:
tel. +44 12 17 047333