Grain Refinement in Steels and the Application Trials in China

Abstract

In order to control microstructure and performances of steel products, the investigation has been taken to the ways to refine grains in steels and the mechanisms, the availability in industrial production of ultrafine grain steel products, and the applications of ultrafine grain steel products. It is shown that strength of steels can be increased without detriment to ductility when ferrite grains are refined to less than 4 mm in low carbon steels. Based on the study of deformation induced transformation, a new rolling technology has been developed to refine ferrite grains in plain low carbon steel rebar and strip. Plain low carbon steel rebars of 400 MPa grade, with ferrite grains in the range of 4-8 mm, have been developed as the candidate to replace microalloyed steel rebars. High strength plain low carbon steel strips in the yield strength range from 345 to 420 MPa have been used for components conventionally made of microalloyed steels. Ultrafine grained microalloyed steel strip has also been developed. A new on-line cementite spheroidization softening technology has been studied for the wire rod production of medium carbon bolt steel. It is obvious that deformation could also induce pearlite transformation in eutectoid steel.

Multiscale Analysis of Processing‐Microstructure‐Mechanical Behavior Interrelationships of UFG Microalloyed Steels

“…These features significantly increase the number of possible nucleation sites for the ferrite. Therefore, it is believed that the precipitation of niobium promotes the SIDT process if recrystallization of austenite does not occur …”

“…Therefore, it is believed that the precipitation of niobium promotes the SIDT process if recrystallization of austenite does not occur. [23] As a result, intensification of softening process is observed indicating acceleration of the SIDT. Interestingly, strain-induced precipitations, that occur in the case of microalloyed steel, cause additional decrease in flow stress (Schedule Y3).…”

Pearlite Spheroidisation and Microstructure Refinement Through Heavy Warm Deformation of Hot Rolled 55VNb Microalloyed Steel

“…THE aim of spheroidisation treatments applied to medium and high carbon steels is to increase ductility, toughness or cold formability, although expensive thermal treatments are required, involving long holding times (10 to 24 h) at temperatures just (10 °C to 20 °C) below A e1 . To reduce or replace these thermal treatments, different strategies have been developed based on the application of advanced thermomechanical processes, including deformation above A e1 to induce deformation-induced divorced eutectic transformation (DET), [1][2][3][4] deformation-induced transformation (DIT) [5][6][7] at temperatures close to or below intercritical temperature, or the application of heavy warm deformation (HWD) either directly [8] or in combination with thermal treatments [9][10][11][12] or coiling. [13] Additionally, HWD may produce a nanostructured ferrite microstructure, the advantage of which, in combination with a globular cementite microstructure, is the potential increase in strength, ductility and toughness in comparison to a coarse grained and fully pearlitic lamellar microstructure.…”

Adaptive Approach to Structure And Properties Forming in Niobium Alloyed Pipe Steels

“…The required set of properties is obtained by the use of advanced pipe steels, which combine high strength and cold resistance with high welding characteristics. As a rule, these steels must be microalloyed with carbonitriding forming elements (Nb, V) [1][2][3]. To guarantee high operational integrity of metal for gas and oil pipes, it is necessary to provide the following parameters of metal: resistance to brittle failure in the operating temperature range, high resistance to crack propagation, resistance to hostile environment influence and good welding characteristics.…”