The cold rolling and primary recrystallisation textures of austenitic stainless steels.
A critical review has been made of available literature on the cold rolling and primary recrystallisation textures of f.c.c. and b.c.c. metals and alloys, with later reference to recent work on stainless steels. Using laboratory and commercial alloys a study has been made of factors influencing the cold rolling and primary recrystallisation textures of stainless steels. Preferred orientations have been assessed by X-ray determination of pole figures. The work is supplemented by other techniques - quantitative phase analysis of austenite and martensite, determination of diffraction line profiles, hardness measurements, optical and electron metallography, qualitative identification of carbide phases. The high purity alloys (18%Cr 10%Ni, 18%Cr 12%Ni, 18%Cr 14%Ni, 18%Cr 12%Ni 7%Co and 18%Cr 25%Ni) were examined with particular reference to the role of the martensite transformation in developing primary recrystallisation textures. The first four alloys are metastable, the amount of martensite formed after 90-95% cold rolling increasing from ≈5% for 18%Cr 14%Ni or 18%Cr 12%Ni 7%Co steels to ≈86% for 18%Cr 10%Hi steel, and textures of both martensite and the remaining austenite are the same for each steel. Principal components of the martensite texture are {112} and {111} , with a minor component {001} . The austenite exhibits a normal f.c.c. alloy type texture, {110} + a minor {110} component. The 18%Cr steel recrystallises to mainly {230} and {6,10,1} orientations with considerable spread remaining in the region of {6,8,17} (which is close to {225} ). The other steels recrystallise to a {225} texture. In the metastable alloys, factors influencing development of the annealing textures are the relative amounts of alpha and sigma formed by cold rolling, recovery in both phases, alpha→sigma shear transformation, variant selectivity during the transformation and, finally, competitive growth of favourably oriented nuclei. It is shown that recovery occurs in both phases, its effect being to sharpen textures about the principal components. During the alpha→sigma transformation, further texture sharpening can occur through preferential selection of variants of the Kurdjumov- Sachs orientation relationship. The texture of 18%Cr 10%Ni steel after annealing for 1/2 hour at 500°C or 600°C conforms to that predicted by transformation from principal alpha rolling texture components. All groups of orientations originating from {112} alpha and {111} alpha are present in the transformed texture, but one group of orientations from {001} alpha is missing. This can only be accounted for by assuming that the eight variants comprising this component do not operate. Subsequent development of the recrystallisation textures depends largely on the amounts of alpha and sigma formed during rolling. This is attributed to differences in density of nuclei formed within the range of alpha rolling texture orientations and within the range of orientations derived from the martensite. Geometrical relationships between the nuclei and various ranges of matrix components are compared in terms of theta, the angle of misfit between poles of nucleus and matrix, and o, the rotation about this almost common pole. Although the observed recrystallisation textures are generally considered to result from oriented growth, it is also shown that the transformation can provide suitable nuclei spontaneously, so that nucleation cannot be considered entirely random. Unstabilised commercial steels (RF310, PST(L) and FSL(L)) recrystallise in a similar manner to laboratory alloys of equivalent metastability. Precipitation of carbides occurs during annealing but the particles are almost completely re-dissolved at 900°C. Variations in carbide distribution prior to cold rolling, arising either from different compositions or from different rates of cooling from the solution treatment temperaturet have no significant effect on subsequent cold rolling and annealing textures. In contrast, carbide-stabilised commercial steels (FCB, SF347, FDP, FMB and FMBTi) form their recrystallisation textures largely by retention of the sigma cold rolling texture and/or retention of the sigma texture immediately after transformation from the martensite. Iluch of the precipitation which occurs during annealing remains until the end of recrystallisation. Retention of the textures is attributed to restriction of grain boundary mobility by precipitate-particles so that grains with high angle boundaries do not grow in preference to others, at least not to the same extent as they would in the absence of such precipitation. Initial solution treatment of stabilised steels is important, as it controls the stability of the matrix prior to cold rolling. In the metastable range, observed effects of solution treatment on annealing textures are indirect, being related to the relative amounts of austenite and martensite after rolling.
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Sheffield Hallam University (United Kingdom).