Materials Science Forum Vols. 783-786

Abstract: Chemical segregation of alloying elements during solidification of steel grades leads to development of a banded microstructure, causing a degree of anisotropy that can be detrimental to the mechanical behavior under service conditions. It is well-known that the presence of strongly orientated martensite bands in carbide-free bainitic microstructures, associated to inhomogeneous Mn redistribution during solidification, leads to a remarkable deterioration in toughness in advanced high strength bainitic steels. In this study, while bands were clearly visible on light optical micrographs of continuously cooled carbide-free bainitic steels, scanning electron microscopy examination revealed only a gradual transition between matrix and bands, both with a granular bainitic structure. Electron backscatter diffraction was used to quantify the bainitic packet size and volume fraction of martensite/austenite constituent between and within the bands, after a process of optimization of the analysis settings in order to minimize the inherent difficulties linked to sub-micrometric and minority phase indexation. The quantitative microstructural results showed negligible morphological differences between bainitic structure bands and matrix, only solute segregation of Cr and Mo was detected by energy-dispersive X-ray spectroscopy within bands, which must be responsible for a stronger resistance against metallographic etching in those regions.

Abstract: The main physical metallurgy processes controlling final mechanical properties in as hot rolled steel products are basically recrystalization, grain growth, precipitation and austenite to ferrite phase transformation. Knowledge of how these processes interact during an industry hot rolling schedule is the key to understand how to obtain certain mechanical properties. This know how has been routinely applied to the manufacturing of flat products such as plates and strips. Less has been reported for non flat products. When even there are such reports, these usually do not present any comparison between predictions and industry real data. This paper addresses the case of rolling of structural beams at an industry trial. Emphasis is put on how the final mechanical properties were obtained and how they can be predicted. An ordinary microstructure model was used and then the predictions were checked by comparing them to industry data. After validation, suggestions were made for improvements in the schedule aiming at having superior mechanical properties at the end product. Austenite grain size evolution is important in defining final properties, as expected. The presence of Nb as microalloyed element can enhance such properties and its role to this particular industry process was discussed.

Abstract: Effect of pre-treatment before austenitization on mechanical property in a steel has been investigated. Pre-treatment was done at 500°C for 600, 1200 and 1800s before austenitization at 1440°C for 120s. This pre-treatment was performed in order to introduce dislocation network into austenite, which acts as nucleation site for “bainite laths within austenite grain (BWING)”. BWING was formed with isothermal holding at 500°C for 10s after austenitization. Micro-Vickers hardness is measured in a steel with and without pre-treatment. The steel with appropriate pre-treatment exhibits high hardness, because large amounts of BWINGs are formed. In the case of cementites in pearlite being coarsened and the distance between cementites becoming large, dislocation network is apt to be introduced into γ. The dislocation network acts as nucleation site for BWING, resulting in the suppression of formation of intergranular bainite. BWINGs strengthen the steel.

Abstract: Aiming to meet new market demands for flat steel, steel companies have been developing new products with special features that allow various adjustments to your application. Our paper is a new product, the superfine steels, its applications, development feasibility and economic aspects. The market for steel furniture for domestic purposes has a great demand for cold-rolled steels, distributed in thick bands, currently between 0.40 mm and 0.70 mm. This segment showed continuous growth in recent years, like the white line, with special focus on the increased consumption presented by the classes C and D, driven by the reduction in Excise Tax (IPI), granted by the federal government in 2009. The proposal is to produce cold-rolled steel with thickness reaching up to 0.25 mm, maintaining the function of the final component to quality and stiffness required. Thinning is a thick white line trend of the world, due to the direct impact on reducing the final cost of the piece, reflecting the competitiveness of customers across the market, and there is a clear need to develop new applications for these materials, not only to the white line, as well as for the mobile industry and metal packaging. Keywords : Cold Rolled Superfine Steel , reducing thick white line

Abstract: This study covers the very preliminary results of welding and characterisation of one novel duplex and two ferritic stainless steel grades. The so-called ”lean duplex” grade EN 1.4162 (UNS S32101) has 21.5% Cr-5% Mn-1.5% Ni-N, the ferritic EN 1.4509 (UNS S43940, AISI 441) 17% Cr, and the other, improved 21% Cr ferritic grade that fulfills also UNS S44330 standard requirements, but has no standard EN designation yet. Both ferritic stainless steels have low (< 0.02%) carbon content and double (Nb+Ti) stabilisation. The materials were used as 1.5 and 2 mm sheets, hence laser and resistance spot welding were selected for welding experiments. The joints were subjected to mechanical testing and critical pitting temperature (CPT) corrosion tests, which were performed on both the base materials and welds. The mechanical tests of the welds did not reveal any significant softening effect due to welding operations. Comprehensive CPT data were achieved for base materials and their welds using two different polarisation potentials, and the new 21% Cr ferritic grade shows a great promise in both as-received and welded conditions. This paper was written as part of the Finnish Metals and Engineering Competence Cluster (FIMECC)’s Demanding Applications (DEMAPP) program.

Abstract: Based on the recent concept of quenching and partitioning (Q&P), a novel TMR-DQP (thermomechanical rolling followed by direct quenching and partitioning) processing route has been established for the development of ultra-high strength structural steels with yield strengths ≈1100 MPa combined with good uniform and total elongations and impact toughness. Suitable compositions were designed based on high silicon and/or aluminium contents with or without small additions of Nb, Mo or Ni. The DQP parameters were established with the aid of physical simulation on a Gleeble simulator. Finally, the TMR-DQP processing route was designed for trials on a laboratory rolling mill. Metallographic studies showed that the desired martensite-austenite microstructures were achieved thus providing the targeted mechanical properties. The advantage of strained austenite in refining the martensite packets/blocks was clearly evident. No adverse effect of prolonged partitioning simulating the coiling stage has been noticed suggesting new possibilities for strip and plate products. Promising results in respect of microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural steels through the TMR-DQP route.

Abstract: With the aim of increasing the volume fraction and stability of the retained austenite characteristics in a transformation-induced plasticity (TRIP)-aided steel with wider lath-martensite structure matrix, the effects of varying the post-hot-working cooling rate of a 0.2%C-1.5%Si-1.5%Mn-1.0%Cr-0.05%Nb (mass%) steel on the retained austenite characteristics were investigated. When, after hot-working at 950°C, the steel was cooled to room temperature from 430°C above the martensite-start temperature using cooling rates lower than 3°C/s, the steel attained a higher volume fraction of metastable retained austenite and lower volume fractions of a finely dispersed martensite-austenite complex phase, carbide, and pro-eutectoid ferrite, although the volume fraction of bainitic ferrite increased. This was associated with a marked carbon-enrichment in the untransformed austenite, which was mainly due to the promoted bainitic ferrite, the initial lath martensite, and the refined prior austenitic grain.

Abstract: The post-dynamic recrystallization of an ultrafine grained 304-type austenitic stainless steel was studied during annealing at 800 and 1000°C for 7.5 to 480 minutes. The initial ultrafine grained microstructures have been developed by continuous dynamic recrystallization during isothermal multidirectional forging to a total strain of ∼4 at temperatures ranging from 500 to 800°C. The post-dynamic recrystallization involves a rapid softening at early stage of annealing followed by a sluggish decrease of hardness upon further annealing. A transient recrystallization at early annealing stage results in somewhat heterogeneous microstructures in the samples subjected to previous deformation at relatively low temperatures of 500-600°C. This structural heterogeneity disappears with increasing the annealing time. Commonly, the post-dynamic recrystallization behavior can be considered as a kind of continuous recrystallization.

Abstract: In 0.1C-5Mn steels, 5%Mn addition increases hardening ability and makes 100% martensitic transformation even in air cooling without water quenching. Their Ms and Mf temperatures are in the range of 350-250°C, and subzero treatment is not needed. This makes it possible to measure Ms and Mf temperatures accurately by dilatometry. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships with identical specimens and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 m. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off balance between strength and ductility.

Abstract: The effect of tempering temperature on the impact toughness of 0.3 mass% carbon martensitic steels with prior austenite grain (PAG) size of about 6 μm and 30 μm were investigated. Instrumented Charpy impact test (ICIT) method was used to evaluate the impact toughness. The tempering temperature of 723K gives the largest difference in the Charpy impact energy at room temperature (RT) between the specimens with two different PAG sizes. Investigation of the test temperature dependence of Charpy impact energy in the 723K tempered steels shows a steep transition at around 200 K for the 6 μm PAG specimen, while it shows a continuous slow transition in a wide range of temperature for the 60 μm PAG specimen. ICIT waveform analysis shows that the fracture propagation energy in stead of the fracture initiation energy mainly controls the temperature dependence of the impact energy. The carbide size distribution in these two specimens was investigated by SEM and TEM. The 60 μm PAG specimen shows the distribution of coarser carbides than does the 6 μm PAG specimen, which seems to be the main reason for the observed difference between them in the Charpy impact energy and the other properties of impact fracture.