scholarly journals Influence of Chromium Content on the Microstructure and Mechanical Properties of Thermomechanically Hot-Rolled Low-Carbon Bainitic Steels Containing Niobium

2020 ◽  
Vol 10 (1) ◽  
pp. 344 ◽  
Author(s):  
Mohammed Ali ◽  
Tun Nyo ◽  
Antti Kaijalainen ◽  
Jaakko Hannula ◽  
David Porter ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Impact Toughness ◽  
Cooling Rate ◽  
Chromium Content ◽  
Bainitic Ferrite ◽  
Polygonal Ferrite ◽  
Granular Bainite ◽  
Low Carbon ◽  
Cr Content ◽  
Hot Rolled

The effect of chromium content in the range of 1 wt.%–4 wt.% on the microstructure and mechanical properties of controlled-rolled and direct-quenched 12 mm thick low-carbon (0.04 wt.%) steel plates containing 0.06 wt.% Nb has been studied. In these microalloyed 700 MPa grade steels, the aim was to achieve a robust bainitic microstructure with a yield strength of 700 MPa combined with good tensile ductility and impact toughness. Continuous cooling transformation diagrams of deformed and non-deformed austenite were recorded to study the effect of Cr and hot deformation on the transformation behavior of the investigated steels. Depending on the cooling rate, the microstructures consist of one or more of the following microstructural constituents: bainitic ferrite, granular bainite, polygonal ferrite, and pearlite. The fraction of bainitic ferrite decreases with decreasing cooling rate, giving an increasing fraction of granular bainite and polygonal ferrite and a reduction in the hardness of the transformation products. Polygonal ferrite formation depends mainly on the Cr content and the cooling rate. In both deformed and non-deformed austenite, increasing the Cr content enhances the hardenability and refines the final microstructure, shifting the ferrite start curve to lower cooling rates. Preceding austenite deformation promotes the formation of polygonal ferrite at lower cooling rates, which leads to a decrease in hardness. In hot-rolled and direct-quenched plates, decreasing the Cr content promotes the formation of polygonal ferrite leading to an increase in the impact toughness and elongation but also a loss of yield strength.

Influence of Chromium Content and Prior Deformation on the Continuous Cooling Transformation Diagram of Low-Carbon Bainitic Steels

2020 ◽  
Vol 835 ◽  
pp. 58-67
Author(s):  
Mohammed Ali ◽  
Antti J. Kaijalainen ◽  
Jaakko Hannula ◽  
David Porter ◽  
Jukka I. Kömi
Keyword(s):  
Cooling Rate ◽  
Hot Deformation ◽  
Laser Scanning ◽  
Chromium Content ◽  
Bainitic Ferrite ◽  
Granular Bainite ◽  
Low Carbon ◽  
Cooling Rates ◽  
Continuous Cooling Transformation ◽  
Continuous Cooling

The effect of chromium content and prior hot deformation of the austenite on the continuous cooling transformation (CCT) diagram of a newly developed low-carbon bainitic steel has been studied using dilatometer measurements conducted on a Gleeble 3800 simulator with cooling rates ranging from 2-80 °C/s. After austenitization at 1100 °C, specimens were either cooled without strain or given 0.6 strain at 880 °C prior to dilatometer measurements. The resultant microstructures have been studied using laser scanning confocal microscopy, scanning electron microscopy and macrohardness measurements. CCT and deformation continuous cooling transformation (DCCT) diagrams were constructed based on the dilatation curves, final microstructures and hardness values. Depending on the cooling rate, the microstructures of the investigated steels after cooling from the austenite region consist of one or more of the following microstructural components: lath-like upper bainite, i.e. bainitic ferrite (BF), granular bainite (GB), polygonal ferrite (PF) and pearlite (P). The proportion of BF to GB as well as the hardness of the transformation products decreased with decreasing cooling rate. The cooling rate at which PF starts to appear depends on the steel composition. With both undeformed and deformed austenite, increasing the chromium content led to higher hardenability and refinement of the microstructure, promoting the formation of BF and shifting the ferrite start curve to lower cooling rates. Prior hot deformation shifted the transformation curves to shorter times and higher temperatures and led to a reduction in hardness at the low cooling rates through the promotion of ferrite formation.

scholarly journals Continuous Cooling Transformation Diagram, Microstructures, and Properties of the Simulated Coarse-Grain Heat-Affected Zone in a Low-Carbon Bainite E550 Steel

Metals ◽  
2019 ◽  
Vol 9 (9) ◽  
pp. 939 ◽  
Author(s):  
Yun Zong ◽  
Chun-Ming Liu
Keyword(s):  
Impact Toughness ◽  
Cooling Rate ◽  
Vickers Hardness ◽  
Heat Affected Zone ◽  
Granular Bainite ◽  
Coarse Grain ◽  
Low Carbon ◽  
Transformation Diagram ◽  
Lath Bainite ◽  
Continuous Cooling Transformation Diagram

In order to provide important guidance for controlling and obtaining the optimal microstructures and mechanical properties of a welded joint, the continuous cooling transformation diagram of a new low-carbon Nb-microalloyed bainite E550 steel in a simulated coarse-grain heat-affected zone (CGHAZ) has been constructed by thermal dilatation method in this paper. The welding thermal simulation experiments were conducted on a Gleeble-3800 thermo-mechanical simulator. The corresponding microstructure was observed by a LEICA DM2700M. The Vickers hardness (HV) and the impact toughness at −40 °C were measured according to the ASTM E384 standard and the ASTM E2298 standard, respectively. The experimental results may indicate that the intermediate temperature phase transformation of the whole bainite can occur in a wide range of cooling rates of 2–20 °C/s. In the scope of cooling rates 2–20 °C/s, the microstructure of the heat-affected zone (HAZ) mainly consists of lath bainite and granular bainite. Moreover, the proportion of lath bainite increased and granular bainite decreased as the cooling rate increasing. There is a spot of lath martensite in the microstructure of HAZ when the cooling rate is above 20 °C/s. The Vickers hardness increases gradually with the increasing of the cooling rate, and the maximum hardness is 323 HV10. When the cooling time from 800 °C to 500 °C (t8/5) is 5–15 s, it presents excellent −40 °C impact toughness (273–286 J) of the CGHAZ beyond the base material (163 J).

Effect of Aging Temperature on the Microstructure and Mechanical Properties of 1000MPa Grade Low Carbon Bainitic Steel

2012 ◽  
Vol 152-154 ◽  
pp. 376-380 ◽  
Author(s):  
Long Fei Zuo ◽  
Zhan Lei Wei ◽  
Ri Ni ◽  
Ben Ma ◽  
Zi Dong Wang
Keyword(s):  
Electron Microscopy ◽  
Mechanical Properties ◽  
Yield Strength ◽  
Aging Temperature ◽  
Granular Bainite ◽  
Bainitic Steel ◽  
Low Carbon ◽  
Microstructure And Mechanical Properties ◽  
Effect On Yield ◽  
Hot Rolled

A kind of 1000MPa low carbon bainitic steel belonged to the Fe-Cu-Nb series was hot rolled and aged, the influence of aging temperatures on the microstructure and mechanical properties of the steel were investigated by using Scanning electron microscopy (SEM) and transmission electron microscopy(TEM). The results show that the microstructure of the low carbon bainitic steel consisted of lath-shaped bainite(LB), granular bainite(GB) and quasi-polygonal ferrite(QF), and the proportion of each kind of microstructure changed with the aging temperatures. The strength of steel with the increase of aging temperature first increased, then decreased, Aging temperatures had distinct effect on yield strength of the tested steel, and less effect on the ultimate tensile strength, we can get the best comprehensive properties yield strength 1011.87 MPa and elongation rate 16.38% of good tough match aged at 450°C. Through analysis it is concluded that the strength of the tested steels aged at 450°C reaches the maximum value, which is attributed to the precipitation of a large amount of fine ε-Cu particles(5~10nm) and a small number of(Nb,Ti)(C,N) precipitates.

Effects of Thermo-Mechanical Process on the Microstructure and Mechanical Properties of Low Carbon HSLA Steels

2006 ◽  
Vol 15-17 ◽  
pp. 786-791 ◽  
Author(s):  
J.S. Kang ◽  
Y. Huang ◽  
C.W. Lee ◽  
Chan Gyung Park
Keyword(s):  
Mechanical Properties ◽  
Grain Size ◽  
Cooling Rate ◽  
High Strength ◽  
Nucleation Site ◽  
Polygonal Ferrite ◽  
Low Alloy Steels ◽  
Low Carbon ◽  
Microstructure And Mechanical Properties ◽  
Average Grain Size

Effects of deformation at austenite region and cooling rate on the microstructure and mechanical properties of low carbon (0.06 wt. % C) high strength low alloy steels have been investigated. Average grain size decreased and polygonal ferrite transformation promoted with increasing deformation amount at austenite region due to increase of ferrite nucleation site. Microstructure was also influenced by cooling rate resulting in the development of a mixture of fine polygonal ferrite and acicular ferrite at 10°C/s cooling rate. Discontinuous yielding occurred in highly deformed specimen due to the formation of polygonal ferrite. However, small grain size of highly deformed specimen caused lower ductile-to-brittle transition temperature than slightly deformed specimen.

Microstructure–Property Relationship in 22mm Thick X80 Coil Skelp

2010 ◽  
Author(s):  
M. Liebeherr ◽  
N. Bernier ◽  
D. Le`bre ◽  
N. Ilic´ ◽  
D. Quidort
Keyword(s):  
Grain Size ◽  
Impact Toughness ◽  
Electron Backscatter Diffraction ◽  
Bainitic Ferrite ◽  
Hot Strip Mill ◽  
Low Carbon ◽  
Welded Pipe ◽  
Hot Rolled ◽  
Backscatter Diffraction ◽  
The Impact

The progress in the development of heavy gauge X80 linepipe steel on coil at ArcelorMittal was recently rewarded with a 6000 ton commercial order for the production of 21.6mm wall thickness spiral welded pipe. The further product development is concentrating on the improvement of the impact toughness at low temperatures. Research is currently focussing on the relationship between the mechanical properties and the microstructure of the steels. In the present study, two industrially hot rolled X80 steels with thickness 21.6mm were investigated. The steels had the same chemical composition but were processed with different parameter sets in the hot strip mill. The two resulting low-carbon bainitic microstructures were composed predominantly of quasi-polygonal ferrite and globular bainitic ferrite / bainitic ferrite, respectively. Emphasis of the microstructure and property characterisation was laid on through-thickness gradients of grain size, hardness, texture, impact toughness and tensile properties. Accordingly, the materials were characterised at different positions in the thickness. Grain size and texture were determined by means of Electron Backscatter Diffraction (EBSD). Sub-size Charpy as well as sub-thickness tensile test specimens were taken at different positions in the cross section. The results show that the link between microstructure and properties is not at all obvious. The influence of mean grain size, grain size distribution and texture is discussed in detail.

The Influence of the Substitution of Si by Al on the Properties of Hot Rolled C-Mn-Si TRIP Steel

2017 ◽  
Vol 896 ◽  
pp. 198-201 ◽  
Author(s):  
Zhan Shan Wei ◽  
Zhuang Li ◽  
Wei Lv ◽  
Zhen Yao Shao
Keyword(s):  
Mechanical Properties ◽  
Trip Steel ◽  
Retained Austenite ◽  
Polygonal Ferrite ◽  
Granular Bainite ◽  
Trip Effect ◽  
Al Substitution ◽  
The Stability ◽  
Hot Rolled

The influence of the substitution of Si by Al on the properties of hot rolled C-Mn-Si TRIP steel was investigated by TMCP. The results have shown that the microstructures of the present steels consist of polygonal ferrite, granular bainite and retained austenite. The Al substitution of Si in a conventional C-Mn-Si TRIP steel leads to excellent mechanical properties (UTS>714MPa, A50>31%). TMCP led to the stability of the remaining austenite and a satisfactory TRIP effect. Excellent mechanical properties were obtained through tmcp for the hot rolled TRIP steel.

Influence of Cooling Rate on the Microstructure of Bainitic Steel

2011 ◽  
Vol 311-313 ◽  
pp. 886-890
Author(s):  
Zhi Fen Wang ◽  
Yun Guan ◽  
Li Xin Wu ◽  
Yi Qiang Sun ◽  
Rong Dong Han
Keyword(s):  
Cooling Rate ◽  
Bainitic Ferrite ◽  
Complex Mixture ◽  
Polygonal Ferrite ◽  
Granular Bainite ◽  
Bainitic Steel ◽  
Electron Backscattered Diffraction ◽  
Intermediate Transformation ◽  
Transmission Electron ◽  
Fast Cooling

The microstructure of a bainitic steel after different cooling rates has been investigated by transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). The effect of cooling rate on the intermediate transformation microstructure was studied. The results showed that the final microstructure contained complex mixture of bainitic ferrite, granular bainite and polygonal ferrite. There was mainly lath-like bainitic ferrite at fast cooling rate (20Ks-1), while microstructure in samples cooled with intermediate rates (8~15 Ks-1) contained bainitic ferrite and granular bainite. When cooling rate decreased to less than 5Ks-1, polygonal ferrite occurred.

Isothermal Transformation, Microstructure and Mechanical Properties of Ausformed Low-Carbon Carbide-Free Bainitic Steel

2018 ◽  
Vol 941 ◽  
pp. 329-333 ◽  
Author(s):  
Jiang Ying Meng ◽  
Lei Jie Zhao ◽  
Fan Huang ◽  
Fu Cheng Zhang ◽  
Li He Qian
Keyword(s):  
Mechanical Properties ◽  
Retained Austenite ◽  
Volume Fraction ◽  
Bainitic Ferrite ◽  
Isothermal Transformation ◽  
Bainitic Transformation ◽  
Isothermal Treatment ◽  
Bainitic Steel ◽  
Low Carbon ◽  
Microstructure And Mechanical Properties

In the present study, the effects of ausforming on the bainitic transformation, microstructure and mechanical properties of a low-carbon rich-silicon carbide-free bainitic steel have been investigated. Results show that prior ausforming shortens both the incubation period and finishing time of bainitic transformation during isothermal treatment at a temperature slightly above the Mspoint. The thicknesses of bainitic ferrite laths are reduced appreciably by ausforming; however, ausforming increases the amount of large blocks of retained austenite/martenisite and decreases the volume fraction of retained austenite. And accordingly, ausforming gives rise to significant increases in both yield and tensile strengths, but causes noticeable decreases in ductility and impact toughness.

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