Ultrahigh strength hot rolled microalloyed steels: microstructural aspects of development

Abstract

Ultrahigh strength hot rolled microalloyed steels of yield strength 690 and 760 MPa with ferrite -bainite microstructure have been developed. Impact toughness of y135 J at 240uC and a ductile -brittle transition temperature of less than approximately 270uC have been obtained in steels of gauge y3 mm. The attractive strength -toughness combination was achieved by applying microalloying concepts and controlled rolling in an interactive manner. Alloy design aspects are qualitatively described in relation to the contributions of solute, grain size, precipitates, and microstructural constituents.MST/4829

Microstructure and texture of hot-rolled Cb-Ti and V-Cb microalloyed steels with differences in formability and toughness

“…[1] Precipitates of different size are formed at different stages of processing and include (a) coarse 2344-VOLUME 34A, OCTOBER 2003 METALLURGICAL AND MATERIALS TRANSACTIONS A precipitates (Ͼϳ100 nm) formed at high temperatures in the austenite region, (b) fine precipitates (Ͼϳ20 to 50 nm) produced by strain-induced precipitation after rolling in the nonrecrystallized region, (c) fine precipitates (ϳ10 to 20 nm) formed in the high-temperature ferrite region, and (d) very fine precipitates (ϳ3 to 10 nm) formed during coiling. Precipitates of size (b) and (c) are expected to nucleate at dislocations and at austenite-ferrite interfaces.…”

“…Titanium invariably forms nitrides either in the liquid steel or during solidification, preventing grain growth during the early and later stages of thermomechanical processing. [1,2] These particles are usually coarse, cuboidal-shaped particles, and may remain undissolved during slab reheating (as subsequently discussed).…”

“…[1]) developed for plain carbon steels. Equation [1] has been recently used by researchers to study precipitation behavior in high-strength low-alloy steels. 16 [1a] s base ϭ s o ϩ (15.4 Ϫ 30C ϩ 6.09/(0.8 ϩ Mn))d Ϫ1/2…”

“…In the context of manufacture of automotive components, the edge formability is influenced by a number of factors, and includes inclusions, microstructural banding, grain size distribution, and constituents of the microstructure. [1][2][3]6,7] The difference in the formability behavior of Cb-Ti and V-Cb steels was first examined in terms of process variables associated with the forming process. It is known that inappropriate mechanical working process (shearing/blanking) caused by worn out tools can result in manufacturing difficulties.…”

“…Thus, there is a strong drive in the steel industry to enhance the performance of the existing products, particularly, formability and toughness. The improvement of formability [1][2][3][4] of high-strength hot-rolled steels is expected to encourage a wide range of applications. Additional benefits associated with some of the newest hot-rolled steels are comparatively low cost of alloy and improved product consistency.…”