Antti Kaijalainen

Effect of Austenite Pancaking on the Microstructure, Texture, and Bendability of an Ultrahigh-Strength Strip Steel

The effect of austenite pancaking in the non-recrystallization regime on microstructure and texture evolution and thereby on bendability was investigated in an ultrahigh-strength strip steel with a martensitic-bainitic microstructure. The results indicate that an increase in rolling reduction (Rtot) below the non-recrystallization temperature, which improves the strength and toughness properties, increases the intensities of the ~{554}〈225〉α and ~{112}〈110〉α texture components along the strip centerline and of the ~{112}〈111〉α component at the surface region. Even with the highest Rtot of 79 pct, the bendability along the rolling direction was good, but the preferred alignment of rod-shaped MA constituents along the rolling direction led to a dramatic decrease in the bendability transverse to the rolling direction, with severe cracking occurring even at small bending angles. The early cracking is attributed to localization of the strain in narrow shear bands. It is concluded that the Rtot value has to be limited to guarantee successful bendability.

Tempering of Direct Quenched Low-Alloy Ultra-High-Strength Steel, Part I – Microstructure

The direct quenching of low-carbon steel has been shown to be an effective way of producing ultra-high-strength, tough structural steels in the as-quenched state without tempering. However, in the present study, the influence of tempering at 500 °C has been studied in order to evaluate the possibilities of widening the range of strengths that can be produced from a single base composition. The chosen composition was 0.1C-0.2Si-1.1Mn-0.15Mo-0.03Ti-0.002B. In order to compare direct quenching with conventional quenching, two pre-quench austenite states were studied: a thermomechanically rolled, non-recrystallized, pancaked austenite grain structure and a recrystallized, equiaxed grain structure. Quenched and quenched-and-tempered microstructures were studied using FESEM and FESEM-EBSD. The as-quenched microstructures of the reheated and quenched and direct quenched specimens were fully martensitic and martensitic-bainitic, respectively. In both cases, tempering made the needle-shaped auto-tempered carbides of the as-quenched materials more spherical. In the case of the direct quenched (DQ) material, tempering led to a notable increase in the size of the grain boundary carbides. Prior austenite grain size and effective grain size after quenching were larger in the case of reheated and quenched material (RQ). Tempering had no effect on effective grain size. The crystallographic texture of the DQ material showed strong {112}<131> and {554}<225> components. The RQ material also contained the same components, but it also contained an intense {110}<110> and {011}<100> components. The effects of these microstructural changes on tensile, impact toughness and fracture toughness are described in part II.

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.

Bendability and Microstructure of Direct Quenched Optim® 960QC

Use of ultra-high-strength steels (UHSS) in weight critical constructions is an effective way to save energy and minimize carbon footprint in the end use. On the other hand, the demands for reducing manufacturing costs and energy consumption of the steelmaker are increasing. This has led to development of energy efficient direct quenching (DQ) steelmaking process as an alternative to the conventional quenched and tempered or thermomechanical rolling and accelerate cooled processes. Ruukki has employed thermomechanical rolling and direct quenching process (TM + DQ) for a novel type of ultra-high-strength strip and plate steels since 2001. Advantages of the ultra-high-strength level (>900MPa) can be fully utilized only if fabricated properties are on a sufficient level. Bending is one of the most important workshop processes and a good bendability is essential for a structural steel. Hence, the metallurgy and bendability of Ruukki ́s TM + DQ strip steel Optim® 960QC have been investigated closely. It was found that by optimizing process parameters and chemical composition, a good combination of strength and ductility can be achieved by a modification of martensitic-bainitic microstructure. Despite of smaller total elongation, the bendability of Optim® 960QC is at least on the same level as on conventionally manufactured 960MPa steels. However, it is important to pay special attention to bending process (tool parameters, springback, bending force, material handling) when bending UHSS. It was also found that the bendability of Optim® 960QC can be significantly enhanced by local laser heat treatments or roll forming.

Tempering of Direct Quenched Low-Alloy Ultra-High-Strength Steel, Part II – Mechanical Properties

Direct quenched untempered ultra-high-strength structural steels can possess good toughness and weldability when based on low carbon contents. In this study, the influence of tempering at 500 °C has been investigated to evaluate the possibilities of widening the range of strengths that can be produced from one 0.1% C alloy composition. The study covered the four microstructural states presented in part I: direct quenched (DQ), reheated and quenched (RQ) and their tempered variants (DQ-T and RQ-T). In addition to tensile testing, the Charpy-V transition temperature T28J and the fracture toughness reference temperature T0 were determined for 6 mm thick specimens. The hardness of the DQ and RQ states was identical at ca. 400 HV. However, on tempering, the DQ state retained its hardness better than the RQ state with hardness values 346 HV (DQ-T) and 327 HV (RQ-T). The yield strengths of the DQ materials were ca. 100 MPa higher than those of the RQ materials both as-quenched and after tempering. Despite the higher strength of the DQ and DQ-T states, both had lower T28J temperatures than the RQ and RQ-T states mainly due to their finer effective grain sizes. The widely used correlation between the T28J and the T0 temperatures was not obeyed and the reasons for this are discussed.

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.

The Effect of Solution Annealing and Ageing During the RSW of 6082 Aluminium Alloy

In the automotive industry there is a growing tendency for the application of high strength aluminium alloys. In spite of their significant role in weight reduction there are still obstacles for their wider use due to their limited formability and weldability. Hot forming and in-die quenching (HFQ) process was recently developed for the forming of car body sheets. During the HFQ technology the sheet metal forming should be performed in a solution annealed condition. In the solution annealed condition the aluminium alloys have lower strength and better formability properties. The forming process is followed by a precipitation hardening which is generally connected with the painting of body parts (bake hardening). Besides the formability the implementation of HFQ has an effect on the weldability properties, too. HFQ must have an effect on the resistance spot welding (RSW) of aluminium sheets since the weld nuggets are produced after the HFQ, in the assembly part of the production chain, when the aluminium alloy is in a solution annealed and formed condition. The final properties of the welded joints are determined by the precipitation hardening which is the final step of the whole production process. The present research work aims to investigate the effect of the HFQ process on the weldability of AA6082-T6 aluminium alloy. The properties of the RSW joints are examined in different conditions (T6 delivery condition, solution annealed, precipitation aged). The materials tests include conventional macro testing, hardness tests and tensile-shear tests extended with EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscatter Diffraction) tests in order to characterize the distribution of alloying elements and to analyze the grain structure.

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.

Effect of Red Scale on the Bendability of Ultrahigh-Strength Strip Steel

The effect of red scale on the bendability of a thermomechanically rolled and direct quenched pilot-scale strip steel has been studied by comparing the bending behaviour of adjacent areas with and without red scale. The yield strength of the studied 8 mm thick strip was 960 MPa. The local microstructure and texture below the different scale surfaces were characterized using FESEM and FESEM-EBSD, chemical compositions were determined using GDOES, microhardness profiles were measured and bendability was determined using three-point brake press bending. Red scale was found to significantly affect bendability especially when the bend axis is transverse to the rolling direction. The minimum usable punch radius for defect-free bends in the absence of red scale was 12 mm (1.5 x thickness) while under red scale it was 30 mm (3.75 x thickness). Beneath the red scale the microstructure 50 to 400 μm below the surface was clearly different to that in the absence of red scale. Without the red scale the microstructure was mainly granular bainite with small fraction of upper bainite and polygonal ferrite. Below the red scale the microstructure was a mixture of upper bainite and granular bainite. As a result of the microstructural differences, the subsurface hardness changed substantially from 360 HV in the absence of red scale to 410 HV with red scale. The chemical composition did not change as a result of the presence or absence of red scale, which rules this factor out as possible cause of differences in bendability or final microstructure. Possible explanations for the observed effects of red scale on subsurface microstructure, and microstructure on bendability, are discussed in the paper.

The effect of surface layer properties on bendability of ultra-high strength steel

Bendability is an important property for ultra-high strength steel because air-bending is the most common forming process for the material. In this paper the bendability of two ultra-high strength steels with similar mechanical properties but different bendability was investigated using tensile testing with optical strain measurements. The tensile tests were conducted also for specimens cut from the surface layer and the middle layer of the sheet. It was discovered that the mechanical properties of the surface of the sheet affect the bendability in great manner.