Ultrafine grained steels: industrial prospects

Abstract

Ultrafine grained (～1 µm) steels are currently the subject of a great deal of research worldwide. They offer the prospect of very high strength with very high toughness from standard steel compositions, and recently it has been shown that such fine grains can be obtained from the hot rolling mill rather than merely from specialist, small scale techniques. If such material could indeed be produced economically in bulk, it offers a step advance in ongoing issues for the industry, such as light weighting of structures and components, rationalising the large number of current steel grades by extending the property combinations achievable from select compositions, aiding weldability and recyclability, and reducing the alloy costs by meeting high specifications from more dilute grades and promoting a high tech image from offering an apparently exotic product. However, it has quickly become apparent that this material has its ‘Achilles' heel’ in that it tends to exhibit unstable plasticity upon yielding, severely restricting its potential applications. Possible ways around this problem are being explored, but at present the industrial prospects remain unclear. This paper explores these issues, and suggests that ultrafine grained steels are not going to sweep through all the major steel markets, but will find their place for certain applications.     

Advanced cold rolled steels for automotive applications

Advanced multiphase steels offer a great potential for bodies‐in‐white through their combination of formability and achievable component strength levels. They are first choice for strength and crash‐relevant parts of challenging geometry. The intensive development of high‐strength multiphase steels by ThyssenKrupp has led to hot dip galvanizing concepts with an outstanding forming potential. Hot rolled, hot dip galvanized complex phase steels are currently produced in addition to cold rolled DP and RA steels. New continuously annealed grades with tensile strength levels of up to 1000 MPa in combination with sufficient ductility for applications mainly in the field of structural automobile elements make use of the classic advantages of microalloying as well as the principles of DP and TRIP steels. Further improvement of properties will be reached by the new class of high manganese alloyed steels.     

The nature of hardening produced by low-temperature tempering of cold-worked carbon steels

It was shown that the hardening of steel as a result of low temperature tempering is related to the density of dislocations produced by the foregoing heat treatment. The structural state of the steel before cold working affects the number of dislocations formed as a result of cold working. After heavy reductions, the degree of hardening produced during low-temperature tempering does not depend significantly on the structural state of the steel, but is determined by its carbon content.     

Texture development in cold rolled and annealed C-Mn-Si and C-Mn-Al-Si TRIP steels

Abstract

The texture of two transformation induced plasticity steels has been studied by means of crystallographic orientation mapping. Texture measurements were carried out on ferrite, bainite, and austenite. The polygonal ferrite and the bainite texture, both bcc, could be distinguished based on the image quality parameter of the electron backscattering diffraction measurement. Both bcc textures were very similar, the main difference being the more pronounced 111alpha ND ND=normal direction and 110alpha RD RD=rolling direction fibre textures in the polygonal ferrite. The fcc texture was a strong gamma deformation texture, characterised by the beta fibre. The presence of the alpha fibre confirmed the strong 110gamma ND direction, which was previously detected by means of X ray diffraction XRD. The measured fcc and bcc textures were used to calculate orientation distribution function transformations according the Bain, Kurdjumov-Sachs, and Nishiyama-Wasserman orientation relationships. The predicted cube component {001}gamma 100gamma, which was missing in the measured texture, of gammaret indicates a variant selection for the gammaalphaB transformation. In addition it was shown that crystallographic orientation mapping could be used to make reliable phase fraction determinations, which were previously based on the light optical microscopy of colour etched specimens. This also proves that XRD determination of gammaret is flawed owing to the strong texturing of all phases present in the microstructure.     

Microstructure and mechanical properties of 780 MPa high strength steels produced by direct-quenching and tempering process

Microstructure and mechanical properties of 780 MPa grade steel plate manufactured by conventional reheat-quenching and tempering (RQ-T) and direct-quenching and tempering (DQ-T) processes were investigated. The DQ process was found to enhance the hardenability of steel effectively so that tensile strengths of a range from 780 to 860 MPa have been achieved using DQ-T process, while tensile strength of about 770 MPa has been obtained from the RQ-T sample. In contrast, low temperature toughness of DQ-T samples was generally inferior to that of RQ-T sample, unless hot rolling and cooling processes were optimized in a controlled manner. For example, fracture appearance transition temperature (FATT) of DQ-T samples was varied in a range from –50°C to –120°C, while RQ-T specimens exhibited nearly constant FATT of about –80°C. The finish-rolling temperature (FRT) was one of potential process parameters to determine strength/toughness balance of the steel manufactured by DQ process, while the effect of FRT was closely associated with the cooling rate applied in the process. It has been demonstrated that, for the specimens quenched with a cooling rate higher than 20°C/sec, it may seem to be appropriate to adjust the FRT as low as possible in the non-recrystallization region. In contrast, for the specimens quenched with a low cooling rate of less than 10°C/sec, it may seem to be proper to apply higher FRT to obtain excellent strength/toughness balance of the steel.     

Effect of chemical composition, martensite volume fraction and tempering on tensile behaviour of dual phase steels

In this work an investigation was conducted into the tempering effects on mechanical properties of a carbon steel and a microalloyed steel, both with dual phase microstructure. The UTS and YS decreased and elongations increased with an increase of tempering temperature. However, the loss in UTS and YS at tempering temperatures of 100, 200, 300, 400, 500 and 600 °C was a minimum for microalloyed dual phase steel. This is thought to be due to tempering of the martensite and precipitation in the ferrite.     

Formation of martensite in 304 grade stainless steels and their welds

The formation of martensite in metastable austenitic stainless steels was investigated. The results showed that the formation of martensite in 304 grade stainless steels due to the exposure to cryogenic temperatures is negligible. The amount of formed martensite is dependent on the chemical composition of the actual heat but for standard grades the amount is not expected to exceed a very few percent. In welds the formation of martensite is not promoted by the presence of δ‐ferrite. The formation of martensite due to cold forming at room temperature can reach around 20 %. Cold forming with subsequent exposure to cryogenic temperatures does not lead to additional formation of martensite due to the exposure to low temperatures. Cold forming at low temperatures leads to the highest amount of martensite formed in metastable stainless steels.     

Thermal stability of aluminum cold rolled to large strain

Common to metals deformed to high strains is a very fine microstructure, high strength, and limited ductility. Structure and property optimization by annealing after deformation must, therefore, be explored. In the present study, commercial purity aluminum has been annealed after cold rolling to ultrahigh strains up to and annealing processes have been studied in terms of recovery and conventional recrystallization. These processes have been analyzed by isochronal and isothermal annealing in the temperature range 140–420 °C. It has been found that the recrystallization temperature is a little affected by the rolling strain, whereas the rate of recovery and the temperature range over which recovery takes place increase significantly as the strain is increased. These observations are discussed as to how they can guide studies of nanostructured metals processed by plastic deformation.     

Stored energy, vacancies and thermal stability of ultra-fine grained copper

The stored energy and thermal stability of oxygen-free high conductivity copper processed by equal channel angular pressing up to 16 passes at room temperature was studied by differential scanning calorimetry. Stored energy increased with strain up to four passes, after which it saturated at 0.95 ± 0.05 J/g. This saturation value is 20% higher than from conventional cold rolling. The microstructure of the copper after eight passes was characterized by an average subgrain size of about 0.21 μm and high-angle boundary fraction of about 35%. The contributions to the stored energy from defects were calculated and compared, suggesting that the stored energy mainly originates from boundaries and vacancies. The restoration activation energy after eight passes was between 77 and 80 kJ/mol. The higher stored energy and lower activation energy compared to cold-rolled copper is attributed to excess vacancies.     

Microstructural stability of 9--12%Cr ferrite/martensite heat-resistant steels

The microstructural evolutions of advanced 9--12%Cr ferrite/martensite heat-resistant steels used for power generation plants are reviewed in this article. Despite of the small differences in chemical compositions, the steels share the same microstructure of the as-tempered martensite. It is the thermal stability of the initial microstructure that matters the creep behavior of these heat-resistant steels. The microstructural evolutions involved? in? 9--12%Cr ?ferrite ?heat-resistant ?steels ?are ?elabo- rated, including (1) martensitic lath widening, (2) disappearance of prior austenite grain boundary, (3) emergence of subgrains, (4) coarsening of precipitates, and (5) formation of new precipitates, such as Laves-phase and Z-phase. The former three microstructural evolutions could be retarded by properly disposing the latter two. Namely improving the stability of precipitates and optimizing their size distribution can effectively exert the beneficial influence of precipitates on microstructures. In this sense, the microstructural stability of the tempered martensite is in fact the stability of precipitates during the creep. Many attempts have been carried out to improve the microstructural stability of 9--12%Cr steels and several promising heat-resistant steels have been developed.