Processing of steel for ultrafine ferrite grain structures

Abstract

The attainment of ultra®ne ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very ®ne grain sizes. This paper presents a summary of research aimed at producing ultra®ne ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain re®nement (TGR), in which extra®ne austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of v1 mm in a surface layer, and 1 . 5 mm in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1 . 5 mm was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previously re®ned by TGR to an intermediate grain size was shown to produce an ultra®ne grain microstructure (v1 mm grain size) throughout the section of 1 mm thick strip. The hardness of ultra®ne ferrite was shown to obey a linear relationship with the inverse square root of grain size, but with a lower slope than expected from the Petch relationship for yield strength.MST/4772

Thermal Diffusivity and Microstructure in API5L-X52 Carbon Steel

“…This energy dispersion results in a decrease in the heat flow through the material [3]. A decrease in the grain size in ferrous steel with a low carbon concentration gives an increase in the mechanical resistance of the material and thus provides better resistance to fracture [4,5]. For these reasons and because the reported values of the thermal diffusivity for 5L specification and X52 grade are not available from the American Petroleum Institute (API) [6,7], we have determined the thermal diffusivity of this low carbon steel at room temperature using the photoacoustic (PA) technique in a heat transmission configuration.…”

Ferrite Grain Size Distributions in Ultra-Fine-Grained High-Strength Low-Alloy Steel After Controlled Thermomechanical Deformation

“…[1] However, wide distributions in a-grain sizes, or even bimodal distributions comprised of fine-and coarse-ferrite grains, were observed in such steels and such distributions can affect the required mechanical properties, especially, lowtemperature toughness. [2][3][4][5] Reducing the average grain size to ultra-fine range (below~2 lm) is known to offer remarkable improvement in strength, [6][7][8][9][10][11][12][13][14] but may negatively affect the work-hardening ability and uniform elongation. Bimodal distribution of ferrite grain sizes can be a promising way of overcoming this problem, in ultra-fine-grained steel.…”

Dynamic Strain Induced Transformation of Austenite to Ferrite during High Temperature Extrusion of Low Carbon Steel

“…8,9) In the last decade, researchers have developed new thermo-mechanical processes to produce fine ferrite grains in the range of 1-3 mm. [10][11][12][13] Among these approaches, ultra fine ferrite (UFF) formation through dynamic strain induced transformation of austenite to ferrite (DSIT) is a simple and most effective way. 9,10) DSIT includes transformation of austenite to ferrite during rather than after deformation.…”