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Your search keyword '"Kömi, J. I. (Jukka I.)"' showing total 3 results
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3 results on '"Kömi, J. I. (Jukka I.)"'

1. Flow stress behaviour and static recrystallization characteristics of hot deformed austenite in microalloyed medium-carbon bainitic steels

Author

Kaikkonen, P. M. (Pentti M.), Somani, M. C. (Mahesh C.), Karjalainen, L. P. (Leo Pentti), and Kömi, J. I. (Jukka I.)

Subjects
medium-carbon steels, stress relaxation, flow stress, precipitation, recrystallization kinetics, austenite
Abstract

In the past decade, efforts have been focused on developing very fine, medium-carbon bainitic steels via the low-temperature (typically 300–400 °C) ausforming process, which not only enables shorter isothermal holding times for bainitic transformation at low temperatures, but also offers significantly improved strength. This paper describes static recrystallization (SRX) characteristics of austenite in four medium-carbon 2%Mn-1.3%Si-0.7%Cr steels with and without microalloying intended for the development of these steels. The stress-relaxation method on a Gleeble simulator resulted in recrystallization times over a wide range of temperatures, strains and strain rates. Also, the occurrence of precipitation was revealed. Powers of strain (−1.7 to −2.7) and strain rate (−0.21 to −0.28) as well as the apparent activation energies (225–269 kJ/mol) were in the ranges reported in the literature for C-Mn and microalloyed steels with lower Mn and Si contents. The new regression equations established for estimating times for 50% SRX revealed the retardation effects of microalloying and Mo addition showing reasonable fits with the experimental data, whereas the previous model suggested for ordinary microalloyed steels tended to predict clearly shorter times on average than the experimental values for the present coarse-grained steels. The Boratto equation to estimate the non-recrystallization temperature was successfully modified to include the effect of Mo alloying and high silicon concentrations.

Published
2021

2. Constitutive flow behaviour of austenite at low temperatures and its influence on bainite transformation characteristics of ausformed medium-carbon steel

Author

Kaikkonen, P. M. (Pentti M.), Somani, M. C. (Mahesh C.), Miettunen, I. H. (Ilkka H.), Porter, D. A. (David A.), Pallaspuro, S. T. (Sakari T.), Kömi, J. I. (Jukka I.), Kaikkonen, P. M. (Pentti M.), Somani, M. C. (Mahesh C.), Miettunen, I. H. (Ilkka H.), Porter, D. A. (David A.), Pallaspuro, S. T. (Sakari T.), and Kömi, J. I. (Jukka I.)

Abstract

In order to impart superior mechanical properties to medium carbon carbide-free bainitic steels, an innovative approach has been adopted to extensively refine the bainitic ferrite plate thickness. Unlike controlled deformation in the no-recrystallization regime above the Ar3 temperature, an attempt has been made in this study to carry out low temperature ausforming in the bay between ferrite and bainite C-curves at 500 °C in order to impart high dislocation densities in the austenite prior to phase transformation. Two experimental high-silicon, medium carbon steels were suitably designed and processed for this study, with one steel containing small additions of 0.3Mo and 0.03Nb. Flow stress measurements were made using single-hit compression tests in the temperature range 300–900 °C in steps of 100 °C at different strain rates in the range 0.1–10 s⁻¹ on a Gleeble simulator. Samples ausformed at 500 °C were isothermally held for 1 h at different transformation temperatures in the range of 300–400 °C to complete the bainitic transformation. Influence of strain induced bainite transformation on flow stress was obvious at 0.01 s⁻¹, particularly at 300 and 400 °C. Despite enhanced nucleation in fine-grained steel B containing Nb + Mo, growth of bainite sheaves was much slower. Dilatation behaviour was comparable for the two steels at <350 °C, but at higher temperatures, the effect of Nb + Mo on slower transformation kinetics was obvious. The microstructure of both steels showed extremely fine bainitic ferrite below 325 °C, but at higher temperatures, coarse bainite with M/A constituents and extensive martensite formed in steels without or with Nb + Mo constituents. A correlation between hardness data and retained austenite contents has been established in both the steels. The paper presents the first account of the flow stress and transformation behaviour including the influence of Nb + Mo alloying and the details concerning the carbon-enriched austenite reta

Published
2020

3. Static recrystallization characteristics and kinetics of high-silicon steels for direct quenching and partitioning

Author

Somani, M. C. (Mahesh C.), Porter, D. A. (David A.), Karjalainen, P. (Pentti), Kantanen, P. K. (Pekka K.), Kömi, J. I. (Jukka I.), and Misra, D. K. (Devesh K.)

Subjects
Flow stress, Direct quenching and partitioning, Recrystallization, Stress relaxation, Austenite
Abstract

In the direct quenching and partitioning (DQ&P) process,tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the austenite before quenching and partitioning. This paper describes the static recrystallization kinetics of two steels with compositions suited to DQ&P processing, viz. (in wt.%) 0.3C-1Si-2Mn-1Cr and 0.25C-1.5Si-3Mn. The stress relaxation technique on a Gleeble thermomechanical simulator provided recrystallization times over a wide range of temperature, strain, strain rate and initial grain size. The higher levels of Si and Mn made the recrystallization kinetics less sensitive to strain, strain rate and temperature. The equations derived to describe the recrystallization kinetics can be used in the design of the rough rolling part of thermomechanical processing.

Published
2019

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