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New hot-rolled, high-strength structural steels

High-strength steels enable engineers to design lighter, higher-performance structures, offering good overall cost-effectiveness, lower fabrication costs, increased payloads, more effective space utilisation, and increased safety. Producing and using these types of steels places tough demands on the steelmaker, the engineer, and the fabricator, however. Rautaruukki has proved that it is more than up to the challenge.

Product development at Rautaruukki over the last couple of years has resulted in a series of new high-strength structural steels from both the company's plate and strip mills.

Rautaruukki's thermomechanically processed S 700 ML has been used in log-handling equipment manufactured by Loglift.

The latter include a family of martensitic proprietary grades known as Optim 900 QC, Optim 960 QC, and Optim 1100 QC; while the company's plate facilities offer new, low-carbon thermomechanically processed structural grades with yield strength ranging from 500 to 690 MPa.

An alternative route

Given the demand for ultra-high-strength steel with improved surface quality at thicknesses not normally available as plate, and tight thickness tolerances, Ruukki Production explored some alternative production routes.

The aim was to achieve the required yield strength levels, in combination with good impact toughness, weldability, formability, and suitability for hot dip galvanizing. Consideration was also given to the need to produce products suitable for laser cutting and welding.

The composition of the new steels, therefore, was based on as low a carbon content as possible to ensure good weldability, as defined by preheating requirements and heat-affected zone toughness, good flangeability, and suitability for laser processing.

The result was two hot strip mill products with yield strengths of 900 and 960 MPa – Optim 900 QC and Optim 960 QC. The good technological properties of these grades have been confirmed in extensive forming, welding, cutting, and fatigue testing programmes.

The product family has been extended recently, with the addition of Optim 1100 QC, which has a specified minimum yield strength of 1100 MPa.

Lower levels of carbon equivalent

In the case of hot-rolled, normalised, or normalising-rolled plate-mill products, high strength is typically associated with high levels of alloying elements. This raises the carbon equivalent, however, and makes welding more difficult due to the need for preheating.

The current layout of the first section of the plate mill at the Raahe steel works.

By using thermomechanical rolling, combined with accelerated cooling, higher levels of strength and toughness can be achieved. Until recently, however, the highest yield strength that was achievable economically on Rautaruukki's plate mill using this approach was rated at 500 MPa, with impact toughness tested at -40 °C and a maximum thickness of 40 mm.

To meet the demand for economical thermomechanically processed high-strength steel thicker than 40 mm and with impact toughness testing at temperatures down to as low as -70 °C, Rautaruukki has developed and launched two new structural steels. These have minimum specified yield strengths of 500 and 690 MPa, and are known as PC F500 and S 700 ML.

Going for direct quenching

Development work has also been focused on high-strength, direct-quenched plate mill products, and the company is currently in the process of adding direct quenching equipment to its existing accelerated cooling unit.

These changes will see the existing low-pressure water-cooling unit shortened to allow the addition of a high-pressure quenching unit. A hot leveller will be added prior to the quenching unit to ensure that plates are flat prior to quenching and guarantee uniform quenching and good flatness in the final product.

A schematic diagram showing how the current accelerated cooling unit (ACC) will be converted into a combined direct quenching and accelerated cooling unit (DQ + ACC).

Compared to the present accelerated cooling process, the upgraded system, to be in operation in the second half of 2007, will allow higher cooling rates and cooling to lower temperatures – and make it possible to produce plate based on martensitic microstructures, in the form of hard abrasion-resistant steel and quenched and tempered structural steels

More control

As a more versatile process than conventional reheat quenching, direct quenching gives the steelmaker more microstructural control and the ability to achieve higher hardness for a given chemistry, which can be converted into products with lower carbon equivalents and better weldability.

Thermomechanical rolling can be used to produce finer microstructures with improved toughness, while scale washing during rolling can give improved surface quality. Combining rolling and quenching into a single process also has logistical advantages over conventional reheating and quenching, helping to make delivery times shorter and more precise.

A schematic showing the difference between conventional off-line quenching (a) and direct quenching (b).

To take advantage of its new direct quenching capabilities, Rautaruukki is working on new wear-resistant quenched plate products and quenched and tempered structural plate products. The former will have Brinell hardness levels of 400, 450, and 500, while the structural plate will be produced to EN 10025-6, starting with S690Q/QL.

> David Porter
(Published in High Technology Finland )