Boiler manufacturing requires some specificities For the - - PDF document

Structural cold rolled steel for enamelling with guaranteed minimum yield strength after enamel firing: HC300EK Marc Leveaux / P. Gousselot: ArcelorMittal R&D The cost reduction of enamelled pieces is a major objective pursued by boiler

• manufacturers. Reduce the thickness of the steel can contribute to this aim and thus

be a source of substantial savings. However, enamel firing at high temperature leads to deformation of parts, even more important than the thickness is lower. The use of cold rolled steel for enamelling with guaranteed minimum yield after enamel firing allows to circumvent this difficulty.

Boiler manufacturing requires some specificities

For the manufacturing of boiler inner parts, ArcelorMittal offers special hot rolled steel enamelling grades (S240EK – S300EK), which present excellent resistance to fish scaling combined with conservation of the mechanical properties after forming and enamel firing. With these grades, it is possible to lower the requested thicknesses and to find the best compromise between cost price and technical needs. However, today, we reached the maximum possible thickness reduction with the use of hot rolled steel (hot rolled steels obtained by the way of mini mills are here not taken into consideration). However, by simple numerical simulation it is possible to see that a boiler could even become thinner, around 1,2mm, if we consider actual norms established to ensure a safe use of this pressure vessel. The only way to proceed is to propose cold rolled steels answering the same specification as a grade like S300EK in terms of mechanical properties after enamelling and its conservation after enamel firing (if we don’t consider HRS from mini mill). This represents the objective of this new product development

Which properties are required for cold rolled steel grade dedicated boiler envelop enamelling

The substrate should fulfill the following major requirements:

• First of all the steel grade should be suitable for one-sided enamelling, which

means:

• Enamel adherence with limited or even no surface treatment (sand

blasting or acid pickling).

• Good enamel adherence should be obtained using up to date -Ni free

enamels, and if possible at rather low temperature (for boiler manufacturing)

• No fish scales should occur during one sided enamelling. As all boilers

are enamelled on one side, the risk for fish scales is less than for 2- sided enamelling. Still the sensitivity to fish scales must be minimised

• The steel grade should have minimum mechanical properties. Assuming that

in a first approach, we intend to extend the dimensional offer of the hot rolled S300EK grade, this means (based on the AM catalogue):

• Minimum transverse YS in as received state: YS > 280MPa before

enamelling

• Minimal transverse TS in as received state: TS > 360 MPa before

enamelling

• Minimum elongation in as received state: E% > 20
• Minimum transverse YS in fired state: YS > 300MPa after enamelling

Next to that, some other minor requirements must be fulfilled, which cannot be neglected:

• In general boiler producers are less critical about surface defects on the

enamelled part than e.g. appliance customers or bathtub producers. However, pinholes and craters create a potential source for corrosion and should therefore be limited to the minimum. The C-content of the steel, which is responsible for the CO/CO2 gas formation during enamel firing, is limited as well and cannot be used without restriction to achieve the mechanical

• properties. Other concepts must be used, if possible.
• Boilers producers are very cost oriented, which makes it a difficult market

segment to introduce new steel grades. Therefore, the new grade should have an as cheap as possible chemistry, in order to succeed in replacing the grades that are currently used for boiler applications, i.e. DD11, HRS S235JR and S300EK

• Since boilers are produced by welding a top, bottom and cylindrical part,

weldability must be followed up (seam welding + MAG welding)

ArcelorMittal’s answer

HC300EK grade is a cold rolled enamelling steel grade with guaranteed minimum yield strength of which the mechanical properties are preserved after deformation and enamel firing. This grade can be used for 1 side enamelling and was already successfully tested several times for 2 sides enamelling

HC300EK presents many advantages

The use of grade HC300EK allows to decrease steel thicknesses while avoiding the deformation of the parts during enamel firing. We already have industrial experience reducing current thicknesses of about 0,2mm on both boiler caps and mantels. It’s almost certain that after fine tuning manufacturing process, mostly on welding step, further reduction could be made. HC300EK grade can be used for the manufacturing of boiler envelop to replace currently implemented hot rolled steel. These parts are generally subjected, after manufacturing, to a cyclic test under pressure for which, they must not be deformed. Therefore, the use of this new grade offers the possibility to reduce thicknesses at a lower level than what is possible to do with hot rolled steel grade. On current DD11, S235JR, S355MC, it is not rare to observe a drop down in mechanical properties of about 50 to 90 MPa sometimes. On a classical steel, especially in the vicinity of the

welds, an important grain growth is observed, result of the heat treatment imposed to the HAZ of the weld. Consequently, the fatigue resistance decreases, which might not fit anymore with new rules and norms in use about resistance to cyclic pressure changes.

Which applications are targeted with HC300EK

This steel Grade HC300EK is particularly suitable for the manufacture of boiler

• envelops. It can also be used for flat parts for domestic appliances (baking trays),

shower trays or for the manufacturing of architectural panels, thanks to the broad feasibility of cold rolling mills.

Recommendation for use

Forming

• Grade HC300EK can be formed by bending, profiling or light stamping.

Joining

• Grade HC300EK can be welded without any difficulty, whatever the welding

process may be.

• Other joining processes (clinching, crimping) may also be used.

Surface treatment

• The parts must be degreased before enamelling. When adherence oxide poor

enamels are used (e.g. boiler envelop with the use of Ni-free enamels), sandblasting or pickling can be necessary. Enamel coating and firing

• Grade HC300EK is compatible with all the common enamelling processes: wet

spraying, flow coating, wet or dry electrostatic enamelling.

Characteristics of HC300EK grade Dimensions

Thickness (mm) Min width (mm) Max width (mm) 0.5 – 0.8 600 1460 0.8 – 1.6 1680 1.6 - 2 1365

Mechanical properties

Mechanical properties as delivered HC300EK YS (MPa) TS (MPa) E (%) 300/400 360/500 > 25

The mechanical properties are kept after 5% deformation and two enamel firings.

Chemical composition

HC300EK C (%) Mn (%) Si (%) P (%) S (%) N (%) ≤ 0.075 0.3 – 0.4 0.16 – 0.24 ≤0.075 ≤ 0.02 ≤0.006 The processing conditions guarantee very good resistance to fishscale defect (TH > 100 in accordance with EN 10209).