The Effects of Finish Rolling Temperature and Niobium Microalloying on the Microstructure and Properties of a Direct Quenched High-Strength Steel

Kaijalainen, Antti; Vähäkuopus, N.; Somani, Mahesh C.; Mehtonen, Saara; Porter, David; Kömi, Jukka

doi:10.1515/amm-2017-0091

Cited by 19 publications

(20 citation statements)

References 15 publications

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“…The 3Mn-1.5Al steel was subjected to a slow cooling step after finishing rolling at 850 • C. This 15 s stage realized at a cooling rate of 10 • C/s to 700 • C, was intended to see if the steel has a potential to form any fraction of polygonal ferrite. However, the high hardenability of the steel maintained the undercooled austenite stable at 700 • C. This is due to the high Mn and Mo additions in the steels, which both significantly delayed ferrite formation [1,14]. As a result, both steels did not contain polygonal ferrite.…”

Section: Effect Of Thermomechanical Processingmentioning

confidence: 95%

“…It was necessary to determine the carbon content in the austenite. a γ = 3.556 + 0.0453C γ + 0.00095Mn + 0.0056Al (1) where: a γ -lattice parameter of the austenite (Å), C γ , Mn, Al-carbon, manganese, aluminum contents in the austenite (wt.%), respectively.…”

Section: Microstructure Examinationmentioning

confidence: 99%

“…A microstructure consisting of ferrite, bainite and retained austenite typical for the first generation AHSSs (Advanced High-Strength Steel) is characterized by a large difference in hardness between ferrite and martensite formed as a result of strain-induced transformation. The hardness difference can be reduced by ferrite strengthening, using thermomechanical treatment or precipitation strengthening [1,2]. An alternative solution is to replace the ferrite matrix by bainite [3].…”

Section: Introductionmentioning

confidence: 99%

“…This can be done by an increase in steel hardenability. Mn and Mo additions are very effective since they affect the decomposition of the undercooled austenite significantly [1,4]. The microstructure consisting of bainite and retained austenite allows to reduce the hardness difference between microstructural components, which results in much better edge formability, stretch flangeability and mechanical properties [5][6][7].…”

Section: Introductionmentioning

confidence: 99%

“…The critical factors of the TMP applied for the multiphase steels are rolling conditions and controlled cooling profiles. Kaijalainen et al [1] reported that an increase in the total reduction in the non-recrystallization region in conjunction with a lowering of the finishing rolling temperature and microalloying with niobium increased the austenite pancaking. In this…”

Section: Introductionmentioning

confidence: 99%

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Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Grajcar

Kilarski²,

Kozłowska

2018

Metals

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Detailed studies on microstructure-property relationships of thermomechanically processed medium-Mn steels with various manganese contents were carried out. Microscopic techniques of different resolution (LM, SEM, TEM) and X-Ray diffraction methods were applied. Static tensile tests were performed to characterize mechanical properties of the investigated steels and to determine the tendency of retained austenite to strain-induced martensitic transformation. Obtained results allowed to characterize the microstructural aspects of strain-induced martensitic transformation and its effect on the mechanical properties. It was found that the mechanical stability of retained austenite depends significantly on the manganese content. An increase in manganese content from 3.3% to 4.7% has a significant impact on the microstructure, stability of γ phase and mechanical properties of the investigated steels. The initial amount of retained austenite was higher for the 3Mn-1.5Al steel in comparison to 5Mn-1.5%Al steel-17% and 11%, respectively. The mechanical stability of retained austenite is significantly affected by the morphology of this phase. case, the phase transformation kinetics depends significantly on the austenite deformation degree [5,8,9]. In case of the first generation AHSSs, the main advantage of the TMP is the ability to refine the ferrite grain size by controlling the austenite pancaking [10,11]. Moreover, the thermomechanical treatment can increase the amount of retained austenite to obtain the optimal TRIP (TRansformation Induced Plasticity) effect [12,13]. In case of medium manganese steels (third generation AHSS), the TMP is also a good alternative for the cold-rolling process [14][15][16]. However, most publications on medium-Mn steels concern the cold-rolling process and subsequent inter-critical heat treatment.Cold-rolled medium-Mn TRIP steels are susceptible to plastic instabilities associated with dynamic strain aging (DSA) and serrated flow (PLC-Portevin-Le Chatelier) effects due to the heat treatment required after cold-rolling. From a technological point of view, the PLC and DSA effects must be avoided. The DSA phenomenon gives rise to non-homogeneous plastic flow during the sheet-forming processes and may lead to surface defects on formed parts [17][18][19]. Our previous reports on thermomechanically processed medium-Mn sheet steels indicate that the problem can be avoided [7].Newly developed fine-grained ferrite-austenite or bainitie-austenite steels contain manganese in a range of 3-12%, while carbon content is ca. 0.1-0.2%. These steels contain also aluminum and silicon (1-3%) additions which delay the carbides formation during the bainitic transformation. The increased Mn amount leads to obtain the high fraction of retained austenite (~10-30%). The Mn addition also increases the carbon solubility and lowers the cementite precipitation temperature [20][21][22]. Al is added to partially replace silicon due to the problems related to galvanizing, hot-rolling and welding [23][24][25]. However, Gir...

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Section: Effect Of Thermomechanical Processingmentioning

confidence: 95%

Section: Microstructure Examinationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Grajcar

Kilarski²,

Kozłowska

2018

Metals

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Influence of V and Nb Micro‐Alloying on Direct Quenched and Tempered Ultra‐High Strength Steels

Esterl

Sonnleitner

Gschöpf

et al. 2019

steel research int.

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The construction of mobile crane booms requires the usage of ultra‐high strength steels. Micro‐alloying elements promote grain refinement during hot rolling and result in increased toughness. The relevant strength is given through a martensitic microstructure, which is accomplished by elements retarding the γ to α transformation. Direct quenching (DQ) from the rolling heat and quenching after a preceding re‐austenitization (RQ) are two different production routes. They differ regarding their productivity, their achievable strength levels, and their resulting microstructures. In order to explore the influence of the production route in combination with prominent micro‐alloying elements, which come to application during hot‐rolling, six steels with varying content of V and Nb are investigated concerning their different properties after DQ and RQ as well as their behavior after tempering. It is found, that Nb strongly improves the strength after thermomechanical processing in the as‐rolled condition. Furthermore, Nb compensates the loss of strength during tempering. This effect is not thoroughly discussed in literature so far. Although Nb leads to grain refining during re‐austenitization, the effects on the strength of RQ steels are minimal. The effect after tempering is also weaker than after direct quenching. It is also shown, that V offers a high strength potential after tempering, however weakens the impact toughness significantly.

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Effects of Finish Rolling Temperature and Deformation on Microstructural Evolution and Precipitation Behavior of Non‐quenched and Tempered Carbide‐Free Bainitic Steel

Chen

Yang

Wang

et al. 2020

steel research int.

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Double compression tests are carried out to investigate the microstructural evolution and precipitation behavior of Nb–Ti microalloyed carbide‐free bainitic steel under different finish rolling conditions. With decreasing finish rolling temperature and increasing finish rolling deformation, the area fraction of lath bainite (LB) and martensite (M) was increased, the width of bainitic ferrite lath was decreased, the dislocation density and the number fraction of high‐angle grain boundaries are raised. Meanwhile, the larger deformation and lower temperature reduce the amount of retained austenite (RA) and refine the precipitated particles, which benefit the performance further. Hence, the phase transformation strengthening, refinement strengthening, dislocation strengthening, and precipitation strengthening synergistically contribute to the increase in hardness with increasing deformation and decreasing temperature.

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The Effects of Finish Rolling Temperature and Niobium Microalloying on the Microstructure and Properties of a Direct Quenched High-Strength Steel

Cited by 19 publications

References 15 publications

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Microstructure–Property Relationships in Thermomechanically Processed Medium-Mn Steels with High Al Content

Influence of V and Nb Micro‐Alloying on Direct Quenched and Tempered Ultra‐High Strength Steels

Effects of Finish Rolling Temperature and Deformation on Microstructural Evolution and Precipitation Behavior of Non‐quenched and Tempered Carbide‐Free Bainitic Steel

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