Phase transformation model for adjusting the cooling conditions in Stelmor process to obtain the targeted structure of thermomechanically rolled wire rod used for fastener production

Abstract

The paper demonstrates the capability of the developed phase transformation model to design the cooling conditions in the Stelmor process allowing for obtaining different types of microstructures in wire rod of 32CrB4 steel. The model based on modified JMAK equation was developed using the results of the tests conducted in a DIL 805 A/D/T dilatometer. The model is composed of sub-models of ferritic, pearlitic, bainitic and martensitic transformations. Its predictive capability was confirmed in industrial conditions by performing trials with different settings of fans involved in the cooling process on the Stelmor line of CMC Poland. Excellent performance of the model was achieved through the modification of commonly used equations which allows accurate predictions of the phase transformations start and finish temperatures, as well as volume fraction of microstructural constituents, in a wide range of cooling rates. It was demonstrated that the model can effectively be applied to design the cooling conditions in the Stelmor process, which will result in the required microstructural composition of the wire rod.

Application of the Phase Transformation Model for the Design of the Cooling Conditions in Stelmor Process to Obtain a Favorable Multiphase Microstructure of Wire Rod for Cold Heading Applications

“…[22], two indexes were proposed to evaluate bainitic hardenability through the use of a numerical model of phase transformation, such as described in refs. [10,23]: ξ 0.2 -width of the F b (C r ) plot in the logarithmic scale at 20% volume fraction, where F b is the volume fraction of bainite and C r is the cooling rate.…”

“…The phase transformations model is an upgrade of the JMAK equation, see refs. [10,[23][24][25] for details. The main equation of the model is…”

“…[30]. The results of the CCT tests were used in the objective function Coefficients in the model were determined by searching for the minimum of the objective function (10) with respect to the confidents a. These optimal coefficients for the steels in Table 1 are given in ref.…”

“…Promising results obtained in ref. [10] inspired authors to extend the research and apply numerical modeling to predict the bainitic hardenability of wire rods after cooling. The idea of introducing the concept of bainitic hardenability was elaborated during the realization of Bainhard project, [11] and it is linked to the production in CMC Poland of the wire rods of bainitic steels used for the manufacturing of fasteners without the application of heat treatment.…”

“…[ 9 ] It is worth mentioning that recently, much emphasis concerning the Stelmor line numerical simulations has been focussed on the precise description of the boundary conditions. In publication, [ 10 ] the authors of the present paper proposed the model based on the upgrade of the JMAK equation and applied this model to predict kinetics of transformations during various cooling cycles for wire rods. The main improvements of the model were connected with introduction of the coefficient in the JMAK equation as a function of the temperature, as well as control of the carbon concentration in the austenite during transformations.…”