In order to meet the automotive industry’s demand for lightweight designs, an intensive effort is being made to develop new types of steel. In turn, this development demands increasingly complex methods of heat treatment to achieve the required mechanical properties with appropriate alloy concepts. EBNER Industrieofenbau has made use of its decades of experience with technologically-challenging hardening and tempering lines to create a facility for wide steel strip – the HICON/H2Q® CAL – in which the complex temperature profiles necessary for transformation can be achieved.
The wish list for a suitable heat treatment facility is long: high annealing temperatures, high cooling rates that still provide excellent uniformity across the length and width of a strip, good strip geometry combined with the possibility of reheating and isothermal transformation, and the best atmosphere for the application.
METALLURGY & TEMPERATURE CYCLES
To achieve the desired high strengths, alloying elements are added to create multiple phases in the steel. In this process, it is necessary to bring the strip to temperatures in the inter-critical region between Ac1 and Ac3, and usually above this into austenite. This can mean a PMT (Peak Metal Temperature) of 930° C or more, particularly in the case of carbide-forming alloying elements, to make carbon diffusion and solution possible. Slow cooling before quenching creates local carbon enrichment and eases various phase formations from the austenite, which has been stabilised by this process.
Quenching, at high cooling rates over 200° C/second – paired with the possibility of a sudden yet exact stabilisation of the temperature – enables the formation of any desired phase of bainite up to and including partial or full transformation into martensite, (1). If martensite is only partially formed by halting the drop in temperature, the retained austenite can be partially stabilised. During reheating, the desired phase formation is achieved and the martensite tempered, resulting in very high strength and good ductility.
The great advantage of quenching in process atmosphere (2) compared to water quenching is that the temperature distribution lies above the quenching range. There is no Leidenfrost effect (steam buckle), in which bubbles of vapour alter local cooling rates to the detriment of strip geometry, (3).
HICON/H2Q® CAL FACILITY
The HICON/H2Q® CAL facility at HyCal Corporation in Gibraltar/Michigan, USA, (4) has a throughput capacity of 18 tonne/hr, processing strips up to 1,220 mm wide and 0.5-2.3 mm thick. The line produces DP steel, martensitic grades and Q&P. The heat treatment section is supplied by EBNER. The strip handling gear, including the mandrels, shears, welding machine, looper and degreaser were supplied by the customer.
The length of the facility, from inlet seal to outlet seal, is 122 m. The strip passes over driven sealing rolls and through a nitrogen curtain to enter the furnace. The furnace is heated by gas-fired Si-SiC radiant tubes, reaching a maximum temperature of 980° C. The strip is moved across driven brush rollers, (5).
The advantage of ceramic radiant tubes is that they are highly resistant to changes in temperature. Rapid set-point changes, as well as constant heating up and cooling down, does not affect their service life. The disadvantage, the limited ductility of ceramic, is counteracted by brackets above the tubes to protect them, should a strip break occur. These brackets also serve to detect strip breaks. A slow cooling zone, with both radiant tubes and cooling tubes, can either be used as a fully-operational heating section to extend the furnace or for slow cooling, (6).
The HICON®/H2Q cooler (7) is very flexible. The blower motors are equipped with frequency converters, the separation between the nozzles and the strip can be freely selected from within a wide range of settings, and the temperature of the strip can be precisely controlled after quenching. Many tests, and an acceptance trial in the EBNER R&D lab, confirmed the high cooling performance, (8). During production, fresh hydrogen is fed into the HICON/H2Q® cooler, consuming 8-11 m³/tonne. The high volume of recirculated gas is re-cooled by heat exchangers. A concentration of at least 85 % is set.
EBNER has had many decades of experience in the safe handling of hydrogen. Purging systems, pressure control systems and the nitrogen curtain are redundantly designed.
After rapid cooling, a levelling unit optimises the flatness of the strip. Adjustable driven rolls guide the strip as transformation starts. This section is equipped with a heating system, cooling system and high convection fans, (9) and can also centre the strip if necessary.
The transformation zones are also equipped with heating systems, cooling systems and powerful recirculation fans for high convection. Temperature cycles for the precise final cooling required by DP steel and martensite, as well as isothermal transformation for CP steel and reheating for Q&P grades are possible. The maximum temperature in these zones is 550° C.
Ahead of the outlet seal, three powerful final coolers use convection to cool the strip below the oxidation threshold and to below 80° C, (10).
During development and design of the facility, a simulator for continuous annealing cycles was developed. This can be used to test the cycles for many interesting new grades of steel. The electric heated SimCAL, can precisely recreate precise production cycles tensile test sample sized strip. This small testing rig has a big advantage: commissioning creates far less scrap. The following grades have already been successfully produced in the facility, without a prior trial:
i) Dual-phase steels (DP) 590, 780, 980 MPa, (11); and
ii) Martensite (MS) 1300, 1500 MPa
The HICON/H2Q® CAL synthesises EBNER’s years of experience with hardening and tempering lines and fluid dynamics, along with its expertise in the safe handling of hydrogen as a process atmosphere. This new type of facility to produce wide strip for the automotive industry processes high-strength steels with high ductilities in suitable alloys is already in operation at an innovative customer. New materials produced in this facility are already available as coils, and are already being evaluated by well-known suppliers and OEMs.
BRENNINGER MATTHIAS is Expert Product Development, EBNER Industrieofenbau, Leonding, Austria
SEEMANN PETER is Executive Vice President Product Development, EBNER Industrieofenbau, Leonding, Austria