Study Of Microstructure, Texture And Formability Of Cr17 Ferritic Stainless Steel

In this paper, exploratory studies were carried out on the evolution of microstructure and texture and formability of twin-roll strip cast Crl7 ferritic stainless steel, and emphases were put on the evolution of microstructure and texture and formability of ultra-purified Crl7 ferritic stainless steel stabilized with Nb and Ti by conventional production technology. The chief original work of the present paper is as follows:(1) The controlling strategy of initial solidification structure of strip cast Crl7 ferritic stainless steel was put forward. In addition, the unique rule of the texture evolution was unveiled and thus the textural control objective of ultra-purified Cr17 ferritic stainless steel by conventional production technology was established.The twin-roll strip casting experiments showed that the initial solidification structure of cast strips can be controlled by altering the melt superheats in the molten pool. The equiaxed grain ratio decreases with increasing superheat and presents reverse "S" shape. When superheats are 20-40℃,40-90℃,90-140℃, fully equiaxed, equiaxed-columnar mixed, and fully columnar cast strips are formed, respectively. The hot rolled and annealed sheets of conventional cast slabs are characterized by strong {001}<110> texture. In the cold rolled sheets, sharp a-fiber texture and weak y-fiber texture are formed. After annealing, the sheets show non-uniformγ-fiber recrystallization texture with shift towards {334}<483> and depressed formability. By contrast, the fully equiaxed cast strip exhibits a weak and nearly random initial texture. In the cold rolled sheet, the texture is characterized by weak a-fiber and condign y-fiber texture. After annealing, regular and uniform y-fiber recrystallization texture is formed by oriented nucleation mechanism which improves formability. Therefore, in order to obtain uniform y-fiber recrystallization texture without shift towards{334}<483> under conventional production technology, weak and random texture should be formed before cold rolling and weak a-fiber but strong y-fiber texture should be formed after cold rolling.(2) The hot rolling processes of prolonging roughing inter-pass time and lowering finishing temperature were put forward and the formability of the cold rolled and annealed sheets of ultra-purified Cr17 ferritic stainless steel was improved.The single-hit compressing experiments showed that ultra-purified Cr17 ferritic stainless steel can’t recrystallize because of dynamic recovery during hot rolling and recover happens much faster with increasing temperature. The double-hit compressing experiments showed that the inter-pass static recrystallization softening fraction increases with prolonging inter-pass time and increasing holding temperature. The hot rolling experiments showed that the dynamic recover is suppressed while the static recrystallizaiton is promoted by means of prolonging roughing inter-pass time and lowering finishing temperature. As a result, the microstructures of both hot rolled and annealed sheets are refined and homogenized accompanying weakened a-fiber but strengthened y-fiber texture. After cold rolling, the a-fiber texture is still weak and rotated faster towards stable orientation but theγ-fiber texture is relative strong. After final annealing, the sheets exhibited stronger y-fiber recrystallization texture with alleviative deviation from{111}< 121>, smaller grains and decreased grain colonies so that the formability of cold rolled and annealed sheets is improved.(3) The microstructure evolution during annealing of hot rolled sheet of ultra-purified Crl7 ferritic stainless steel and the formation mechanism of texture were investigated, and the effects of annealing after hot rolling on the microstructure, texture and formability of final cold rolled and annealed sheet were unveiled.It is shown that the recrystallization process during annealing after hot rolling reveals three different stages i.e. slow nucleation, fast growth and a slow approach to fully recrystallization. Nucleation primarily occurs close to grain boundaries but seldom in the grain interior during annealing. The<001>//ND nuclei are formed primarily by in situ recrystallization and secondarily by random mechanism. The<111>//ND nuclei are formed only by in situ recrystallization and the other oriented nuclei are formed primarily by random mechanism. At the end of annealing, the {001}<110>,{115}<110> deformed bands disappears through the growth of nearby new grains. These two oriented deformed bands are the major reasons for the very slow approach to fully recrystallization. The annealing after hot rolling can weaken the orientation intensities of a-fiber texture of hot rolled sheet but strengthen the relative orientation intensities of y-fiber texture, lower texture gradient and promote the same oriented grains to distribute dispersively. As a result, the cold rolled and annealed sheets with annealing after hot rolling exhibited more homogenous y-fiber recrystallization texture with alleviative deviation from{111}<121> through-thickness, decreased grain colonies and the improved dispersive distribution of the same oriented grains, which improves the formability. Therefore, the recrystallization annealing after hot rolling is essential. However, too high annealing temperature is inadvisable. Otherwise, the y-fiber recrystallization texture of final cold and annealed sheets weakens and deteriorates the formability.(4) The evolutional path of cold rolling texture of ultra-purified Cr17 ferritic stainless steel by conventional production technology was clarified. It was demonstrated that the y-fiber recrystallizaiton texture forms by oriented nucleation mechanism instead of previous oriented growth mechanism. And the reasons of non-uniformity and deviation from{111}<121>of y-fiber recrystallizaiton texture were unveiled.During cold rolling, crystals rotates along this path:{001}<110>→{114}<110>→{223}<110>. It is shown that the recrystallization process of cold rolled sheet during annealing reveals two different stages i.e. fast nucleation and growth, and a slow approach to fully recrystallization. The EBSD analysis indicated that theγ-fiber recrystallization texture is formed by oriented nucleation mechanism. The new<111>//ND recrystallization nuclei formed preferentially close to grain boundaries and in-grain shear bands. And the∑13b coincidence site lattice (CSL) grain boundary provides condition for a large number of {111}<112> nuclei which leads to the dominance of {111}<112> component in final y-fiber recrystallization texture. And the deviation of y-fiber recrystallization texture from {111}<121>by some degrees attributes to the deviation of former major cold rolling texture component from{111}<110>by some degrees. Under conventional production technology, therefore, the non-uniformity and deviation from{111}<121> of y-fiber recrystallizaiton texture can’t be completely eliminated. The y-fiber recrystallizaiton texture strengthens with increasing final annealing temperature and this leads to an increase of r value. However, the final annealing temperature should be controlled bellow 1000℃so as to ensure grain size smaller than 40μm. Otherwise, the "orange peel" occurs.Part of this research production had gone across the check and accept by specialists in some or other corporation, and has been applied to the development and production of ultra-purified Cr17 ferritic stainless steel stabilized with Nb and Ti and other ultra-purified ferrtic stainless steels.