The deformation microstructure of various warm (ferritic) rolled steels was characterized and its influence upon the subsequent annealing behavior determined. The materials investigated included three interstitialfree (IF) steels (stabilized with either titanium or niobium), an extra low carbon (ELC) steel, and four experimental low carbon chromium steels with varying levels of boron, nitrogen and phosphorus. Single pass rolling experiments were conducted in a pilot mill at temperatures between 440 and 850°C and the asrolled microstructures were examined using optical microscopy. Particular attention was paid to the nature and intensity of the in-grain shear bands produced. Partial annealing was conducted to examine the nucleation of recrystallization in the deformed microstructure. Shear bands of moderate intensity were usually formed in the IF steels, which tended to be insensitive to rolling temperature. For the ELC steel, intense shear bands were formed at low rolling temperatures, but at higher temperatures this intensity was found to be drastically reduced. The development of shear bands at the higher rolling temperatures was significantly enhanced by alloying with chromium. The differences in shear band frequency and intensity are explained in terms of the dynamic strain aging behaviors of the various materials. Recrystallized grains were found to nucleate preferentially on the shear bands during annealing, regardless of their morphology or intensity.

The warm ralling af steels within the ferrite region can be a cost efteetive method of producing cortain sheet materials. However, in the presence of solute carbon atoms, the rate sensitivity of the material can be afliected as a result of dynamic strain ageing (DSA). This can significantly influence the microstructural development during processing, particularly due to the suppressian of shear band formation. Shear bands help promote the development of a strong {111} texture during annealing, the presence of which is necessary to ensure high r-values in the fin~l product. Therefore, the warm rolling of low earbon (LC) steels usually results in products with poor formabllities. However, it is known that the DSA behavior ean be modified bythe addition ofelements sueh as boron and chromium. Three experimental low carbon steels with various additions of Cr and B were warm rolled; their behaviors are eompared to that of a standard LC material. It was found that these additions promote the formation of shear bands at somewhat higher rolling temperatures than in the unalloyed steel, thus potentially resulting in stronger {1 1 1} recrystallization textures. Therefore, additions of Cr and B could lead to the production of warm rolled LC steels with improved formabilities KEY WORDS: warm rolling: shear bands: dynamic strain aging; rate sensitivity . In trodu ctionIt is well known that advantages can be gained from hot rolling strip steels at slightiy lower temperatures (600-8500o, i. e. within the ferrite rather than the austenite region. This practice, known as warrn rolling, offers commercial benefits to manufacturers as it can reduce production costs and broaden the product rangel,2). In order to fabricate products with good fonnabilities, i. e. high mean r-values ~, and low directional variabilities Ar, the recrystallized texture formed during annealing must have a strong {111} componenti). It has been shown that this texture is promoted by the presence of in-grain shear bands within the deformed microstructure, partieularly when the feirite grain size is of a fairly coarse nature4)The fonnation of these shear bands, which constitute localized flow within a grain at a higher rate than in the bulk material, is strongly dependent upon the strain rate sensitivity ofthe material. Interstitial free (IF) steels display a s,nall positive rate sensitivity within the wann rolling regime5) and thus a moderate number of shear bands are formed during deformation. However, Iow carbon steels, which contain carbon atoms in solution, are prone to dynamic strain aging (DSA) at wann rolling temperatures6) This strongly increases the rate sensitivity of the material, leading to the suppression of shear band fonnation and thus a lack of nucleation sites for the {lll} recrystallization textuTe7). Therefore, waan rolling of low carbon (LC) steels usually results in products with poor formabilities8)The influence of DSA upon the rate sensitivity, m, of a

The effect of solute carbon content, as well as of chromium, boron and manganese addition, on the warm rolling behavior was investigated. Both the as-rolled and recrystallized microstructures and textures were assessed after rolling at temperatures between 440 and 780°C. In an unalloyed low carbon (LC) steel, intense in-grain shear bands were formed at low rolling temperatures, but this intensity was drastically reduced at higher temperatures. Alloying with chromium and boron significantly enhanced the development of shear bands at the higher rolling temperatures. The intensities of the deformation textures produced were little changed with rolling temperature in the IF steel, but increased markedly with temperature for the LC grade. Conversely, the strength of the LC steel recrystallization texture decreased with increasing temperature. The addition of chromium to the low manganese steel somewhat strengthened the {111} component of the annealing texture at the higher rolling temperatures. However, boron addition resulted in a retained rolling component and severely disrupted the recrystallization textures. A higher manganese level was also detrimental to the development of the ND fibre components. These differences are attributed to variations in the dynamic strain aging and precipitation behaviors of the various materials attributable to their differing alloy contents.

No abstract

Production of defect-free galvanized steel sheet is considered a major concern for automotive and other critical applications; nevertheless, the occurrence of some defects in the coated sheets is unavoidable. In order to alleviate the problem, we need to know the extent to which the properties of a galvanized sheet are influenced by the presence of a given defect. In this investigation, specimens including any of the two major defects of continuously galvanized steel sheets were selected from a large number of coated samples. The defects, including furnace roll pimples and bare spots, were microstructurally characterized and their influence on corrosion behaviour and mechanical properties of the steel sheet was evaluated. Corrosion resistance was examined via standard salt spray test and Tafel polarization. Tensile test was employed as a measure of mechanical properties of the defective galvanized sheets. The results indicated that the presence of defects had little influence on the tensile properties of the samples, but considerably reduced their corrosion resistance. Based on the results of salt spray tests, pimples reduced corrosion resistance of galvanized sheets 23 % (50 hours) on average and bare spot defects caused reduction in corrosion resistance up to 39%.