Jukka Kömi

Microstructure, Properties and Design of Direct Quenched Structural Steels

Direct quenching (DQ) is one of the latest process routes in production of ultra-high strength, high performance steels and Ruukki one of the pioneering companies in the utilization of direct quenching. Ruukki has applied direct quenching for the production of ultra-high-strength structural steels in the form of hot-rolled strip and plate. The paper briefly summarizes the physical metallurgy fundamental of direct steels and shows some selected examples of the microstructures and properties of steels produced by direct quenching. In addition, a brief review on the usability properties and design rules of ultra-high strength structural steels is made.

The effect of microstructure on the sheared edge quality and hole expansion ratio of hot-rolled 700 MPa steel

The effects of microstructure on the cutting and hole expansion properties of three thermomechanically rolled steels have been investigated. The yield strength of the studied 3 mm thick strip steels was approximately 700 MPa. Detailed microstructural studies using laser scanning confocal microscopy (LCSM), FESEM and FESEM-EBSD revealed that the three investigated materials consist of 1) single-phase polygonal ferrite, 2) polygonal ferrite with precipitates and 3) granular bainite. The quality of mechanically sheared edges were evaluated using visual inspection and LSCM, while hole expansion properties were characterised according to the methods described in ISO 16630. Roughness values (Ra and Rz) of the sheet edge with different cutting clearances varied between 12 µm to 21 µm and 133 µm to 225 µm, respectively. Mean hole expansion ratios varied from 28.4% to 40.5%. It was shown that granular bainite produced the finest cutting edge, but the hole expansion ratio remained at the same level as in the steel comprising single-phase ferrite. This indicates that a single-phase ferritic matrix enhances hole expansion properties even with low quality edges. A brief discussion of the microstructural features controlling the cutting quality and hole expansion properties is given.

Physical Simulation for Evaluating Heat-Affected Zone Toughness of High and Ultra-High Strength Steels

Physical simulation of the most critical sub-zones of the heat-affected zone is a useful tool for the evaluation of the toughness of welded joints in high-strength and ultra-high-strength steels. In two high-strength offshore steels with the yield strength of 500 MPa, the coarse grained, intercritical and intercritically reheated coarse grained zones were simulated using the cooling times from 800 to 500 °C (t8/5) 5 s and 30 s. Impact and CTOD tests as well as microstructural investigations were carried out in order to evaluate the weldability of the steels without the need for expensive welding tests. The test results showed that the intercritically reheated coarse grained zone with the longer cooling time t8/5=30 s was the most critical sub-zone in the HAZ due to the M-A constituents and coarse ferritic-bainitic microstructure. In 6 mm thick ultra-high-strength steel Optim 960 QC, the coarse grained and intercritically reheated coarse grained zones were simulated using the cooling times t8/5 of 5, 10, 15 and 20s and the intercritical zone using the cooling times t8/5 of 5 and 10 s in order to select the suitable heat input for welding. The impact test results from the simulated zones fulfilled the impact energy requirement of 14 J (5x10 mm specimen) at -40 °C for the cooling times, t8/5, from 5 to 15 s, which correspond to the heat input range 0.4-0.7 kJ/mm (for a 6 mm thickness).

Mechanical Properties in the Physically Simulated Heat-Affected Zones of 500 MPa Offshore Steel for Arctic Conditions

Offshore steels for the arctic conditions have an increasing demand due to the opening of new oil fields in the Arctic Ocean. However, the requirements for these steels are extremely demanding, as they need to maintain the desired properties in harsh arctic conditions. Additionally, these requirements need to be achieved also in heat-affected zones caused by the welding. In this study the heat-affected zones were created using the physical simulation, so that the zones would be wide enough for reliable mechanical testing.

Influence of Manganese Content and Finish Rolling Temperature on the Martensitic Transformation of Ultrahigh-Strength Strip Steels

The effects of manganese content and finish rolling temperature (FRT) on the transformed microstructures and properties of two low-alloyed thermomechanically rolled and direct-quenched (TM-DQ) steels were investigated. The materials were characterized in respect of microstructures and tensile properties. In addition, microhardness measurements were made both at the surface and centerline of the hot-rolled strips to help characterize the phase constituents. Detailed microstructural features were further revealed by laser scanning confocal microscopy (LSCM) and field emission scanning electron microscopy combined with electron backscatter diffraction (FESEM-EBSD). It was apparent that a decrease in the temperature of controlled rolling, i.e., the finish rolling temperature (FRT), resulted in reduced martensite fractions at the surface, as a consequence of strain-induced fine ferrite formation. The centerline of the strip, however, comprised essentially martensite and upper bainite. In contrast, high FRT and higher manganese content resulted in essentially a fully martensitic microstructure due to enhanced hardenability. The paper presents a detailed account of the hot rolling and hardenability aspects of TM-DQ ultra-high-strength strip steels and corresponding microstructures and properties.

Tough Ductile Ultra High Strength Steels Through Direct Quenching and Partitioning—An Update

The TMR-DQP* processing route comprising thermomechanical rolling followed by direct quenching and partitioning, has shown huge potential for the development of tough, ductile ultra-high-strength steels, both for structural and wear-resistant applications. The approach comprised designing suitable chemical compositions, establishing appropriate DQP processing conditions with the aid of physical simulation, and finally testing laboratory rolled DQP material with the emphasis on cost-effective process development, amenable for industrial hot strip production. Evaluation of DQP processed samples cooled slowly following DQP processing, thus simulating coiling, confirmed achieving the desired martensite-austenite microstructures and targeted mechanical properties. Ausforming in no-recrystallization regime (Tnr) resulted in extensive refining and randomization of the martensite packets/laths besides fine division of interlath austenite, thus resulting in an all-round improvement of mechanical properties. Preliminary investigations on alloys designed with 0.2 C have shown promising properties not only for structural applications, but also wear-resistance purposes.

Effect of shielding gas composition on CO2-laser welding quality

Shielding gas is a significant cost factor in CO2-laser welding, especially if helium is used. Replacing a part of the helium with some other gases, such as argon and carbon dioxide, might reduce operation costs. The effect of such a substitutive shielding gas mixture on the total laser welding quality was studied by the Ruukki Metals steel service centre in Uusikaupunki, Finland. Several shielding gas mixtures (He+Ar+CO2) were tested. The percentage quantity of helium was set at a constant of 35% and the percentage quantities of argon varied from 40% to 55% and carbon dioxide from 10% to 25%. The test material was a wear-resistant steel strip plate Raex 450 with a thickness of 4 mm. Raex is a trademark for wear-resistant steels manufactured by Finnish steel making company Ruukki. The number 450 signifies the hardness of steel in Brinell units. Wear-resistant steel strip plates as well as other thermo-mechanically hot rolled and direct quenched strip steels produced on a modern hot rolling line integrated with direct quenching are typically used in applications under extremely demanding conditions that require steel with very high strength and other special properties, such as excellent abrasion resistance. The manufacture of these steels is very demanding because of the complex controlling of the hot rolling and the coiling process. The width of a ready strip steel coil can be insufficient for end products, such as buckets and containers and laser welding is a suitable joining process for achieving the desired size of the steel plate by joining together two or more thin steel strip plates. A 5 kW CO2 laser was first used for the laser cutting to trim the edges of plates and finally for joining plate with a butt joint. The welding speed remained invariable and was 2300 mm/min. The total weld quality was evaluated according to the standards ISO 13919-1 and ISO 15614-11.Shielding gas is a significant cost factor in CO2-laser welding, especially if helium is used. Replacing a part of the helium with some other gases, such as argon and carbon dioxide, might reduce operation costs. The effect of such a substitutive shielding gas mixture on the total laser welding quality was studied by the Ruukki Metals steel service centre in Uusikaupunki, Finland. Several shielding gas mixtures (He+Ar+CO2) were tested. The percentage quantity of helium was set at a constant of 35% and the percentage quantities of argon varied from 40% to 55% and carbon dioxide from 10% to 25%. The test material was a wear-resistant steel strip plate Raex 450 with a thickness of 4 mm. Raex is a trademark for wear-resistant steels manufactured by Finnish steel making company Ruukki. The number 450 signifies the hardness of steel in Brinell units. Wear-resistant steel strip plates as well as other thermo-mechanically hot rolled and direct quenched strip steels produced on a modern hot rolling line integrated...