Steelmaking and Casting
Production of P355NH to EN 10028-3 starts with a clean steel practice. Scrap and direct reduced iron are melted in an electric arc furnace or a converter, then the heat is transferred to ladle refining where vacuum degassing removes hydrogen, nitrogen and oxygen and lowers sulphur to the level needed for low temperature toughness. Alloying additions of manganese, silicon and any microalloying elements are made in the ladle so that the composition of the heat can be trimmed to the narrow target window that gives both the required strength and a fine grain structure.
The refined steel is continuously cast into slabs of a thickness chosen to give an adequate reduction ratio during rolling. Casting parameters such as superheat, casting speed and secondary cooling are controlled so that centre segregation and internal cracks are kept to a minimum, because segregation in the centre of a slab can survive rolling and later show up as a lamination or as poor toughness in the plate centre. Slabs are inspected and surface conditioned before being charged to the reheating furnace.
Rolling and Heat Treatment
Rolling is planned to convert the cast structure into a fine, uniform ferrite and pearlite structure. After reheating, the slab is descaled and rolled in several passes, with the finishing passes run in a temperature range that keeps the austenite grain size under control. Plates intended for normalizing are rolled to thickness and then given the separate normalizing treatment; the normalizing temperature is chosen to fully austenitize the steel and to allow the grain refining elements to do their work, after which the plate is cooled in air to give an even microstructure.
| Process stage | Purpose |
|---|---|
| Melting and ladle refining | Composition control, low sulphur and low gas content |
| Vacuum degassing | Hydrogen and oxygen removal, cleanliness |
| Continuous casting | Sound slabs with controlled segregation |
| Reheating and rolling | Thickness reduction, grain refinement |
| Normalizing | Uniform grain size, stable properties |
| Levelling and cooling | Flatness and dimensional control |
After heat treatment the plate is levelled, cooled and cut to the ordered size. Attention to the cooling practice is important on thicker plate, since uneven cooling leaves residual stresses that can cause distortion during machining or flame cutting at the fabricator.
Surface Treatment and Finishing
Finishing depends on the final duty of the plate. Plates that will be stored outdoors or shipped by sea are normally shot blasted or wire brushed to remove mill scale and then given a protective primer, which reduces rusting during transport and storage. Where the plate will be coated or lined as part of the finished vessel, the surface condition and the surface profile are agreed with the fabricator so that the coating system bonds properly. Edges are dressed after thermal cutting to remove any hard layer and to prepare the welding joint geometry specified on the fabrication drawings.
Quality Inspection and Documentation
Chemical analysis of the ladle sample and, where requested, of the product, confirms that the heat meets the composition limits of EN 10028-3.
Tensile testing on specimens taken transverse to the rolling direction gives the yield strength, tensile strength and elongation for the delivered thickness.
Impact testing at the specified temperature verifies the toughness of the normalized plate, which is the property that qualifies it for low temperature service.
Grain size and microstructure are checked so that the fine grain condition required by the standard is actually present.
Ultrasonic examination to EN 10160 is performed when specified, with the acceptance class agreed at order stage.
Dimension and flatness checks follow the tolerances of EN 10029, and the finished plate is marked with grade, heat number and plate identity.
Inspection documents are issued to EN 10204 in the agreed form, with heat treatment records attached for coded equipment.
The result of this control chain is a plate whose certificate can be matched to the vessel documentation, so that the end user can trace every pressure retaining plate back to its heat, its rolling parameters and its heat treatment records.
Frequently Asked Questions
Q: Why is vacuum degassing used in P355NH production?
A: Degassing removes hydrogen, oxygen and nitrogen and helps to lower sulphur. That improves cleanliness and gives the low temperature toughness that the grade must demonstrate in impact testing.
Q: At what stage is normalizing carried out?
A: After rolling and before final inspection. The plate is austenitized and then air cooled, which produces the uniform fine grain structure that EN 10028-3 expects for the normalized N condition.
Q: How is centre segregation of the slab controlled?
A: Through casting parameters and adequate reduction during rolling. A sufficient reduction ratio closes porosity and disperses segregation so that the finished plate has sound properties through its thickness.
Q: What surface treatment is applied before shipment?
A: Shot blasting or brushing to remove mill scale, followed by a protective primer for plates that will be stored or shipped in wet conditions. Coating requirements for lined vessels are agreed with the fabricator.
Q: Which documents come with the plate?
A: Mechanical and impact test results, chemical analysis, heat treatment record, ultrasonic examination report where applicable, and inspection documents to EN 10204 in the agreed form.
