The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed Steels after Ultra Fast Cooling
© 2017
Materials Research. 2017; 20(3): 853-859
DOI: http://dx.doi.org/10.1590/1980-5373-MR-2016-0627
The Analysis of the Microstructure and Mechanical Properties of Low Carbon Microalloyed
Steels after Ultra Fast Cooling
Yong Tiana, Hong-tao Wanga, Yong Lia*, Zhao-dong Wanga, Guo-dong Wanga
a
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, China
Received: August 31, 2016; Revised: March 10, 2017; Accepted: April 9, 2017
In this paper, two low carbon microalloyed steels, named as steel A and steel B, were fabricated by
ultra fast cooling (UFC). In both steels, the microstructures containing quasi polygonal ferrite (QF),
acicular ferrite (AF) and granular bainite (GB) can be obtained by UFC process. The amount of AF
in steel B is more than that in steel A. The size and distribution of precipitates (Nb/Ti carbonitrides)
in steel B are finer and more dispersed than those of in steel A due to relatively low finish cooling
temperature. The mechanical properties of both steels are effectively enhanced by UFC process. UFC
process produces low-temperature transformation microstructures containing a significant amount of
AF. The mechanical properties of steel B were more satisfactory than those of steel A due to the finer
average grain size, the greater amount of the volume fractions and smaller size of secondary phases.
Keywords: Low carbon microalloyed steels, Ultra fast cooling (UFC), Acicular ferrite (AF), The
mechanical properties
1. Introduction
High-strength low-alloy (HSLA) steels are those highstrength structural steels having good toughness and weldability.
This combination of properties have led to their varied
applications in the automotive industry, in manufacturing
of large diameter pipes for gas and oil transportation in the
areas of low temperature, and in fabrication of plates for
naval ship’s construction1. It is also popular to use HSLA
steels replacing the conventional low strength counterpart
for reducing thicknesses and permitting the reduction of
weight in weight-saving applications2. The wide range of
mechanical properties attainable in HSLA steels coupled
with their relatively low cost are responsible for their
high volume of production, which represents ~10% of the
world’s steel production3. The evolution of HSLA steel
was based on low carbon content to improve weldability
and suitable alloying elements were added to improve
austenite hardenability4-6. Thermomechanical controlled
processing (TMCP) and microalloying in order to obtain
desired microstructure and properties are the essence of
ultra-low carbon microalloyed steel7. TMCP has become
the most powerful and effective manufacturing process
to satisfy increased hardenability, improved strength, and
superior low-temperature toughness8. The microstructure,
which is related with the mechanical properties of the hot
rolled steels, is heavily influenced by the cooling process
after hot rolling. The ultra fast cooling (UFC) technology
was applied in order to get faster cooling rate. The cooling
rate of UFC is more than twice that of traditional ACC
(20ºC/s for 20mm)9. UFC process enhances strengthening
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associated with precipitation and grain refinement10. The
microstructure and mechanical properties of low carbon
microalloyed steel can be significantly improved by UFC
process after hot deformation11.
This article presents an analysis of the final microstructure
of two low carbon microalloyed steels after UFC process.
Tensile and Charpy impact tests at room temperature and
lower temperature were performed, respectively, to evaluate
strength and toughness. The key objective of this study was
to discuss and determine the strengthening contribution of
the morphologies of ferrite, bainite and the detected Nb/Ti
(C, N) carbonitrides in the two experimental steels subjected
to UFC process.
2. Experimental Procedure
Two types of low-alloyed, low-carbon steels were produced
in terms of different content of Cr, Mo and Ni. Plates with
thickness of 250 mm were used for rolling mill tests. The
cylindrical rod specimens with 8 mm diameter and 15 mm
length were machined from the plates in order to measure the
transformation temperature in a thermomechanical simulator.
The austenite nonrecrystallization temperature (Tnr) was
evaluated through softening fraction-interpass time curves.
It was calculated by the back extrapolation method12. The
Ar3 and Ar1, which denote the start and finish temperatures
of the austenite-to-ferrite transformation, respectively,
were measured using the thermomechanical simulator. A
schematic illustration of the double-pass compression test
to measure Ar3 and Ar1 is shown in Figure 1. In Figure 1,
specimens were heated at a rate of 50ºC/s, solution treated
at 1150ºC for 180 s, deformed to 30% compressive strain at
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Tian et al.
Materials Research
1s-1, cooled to 920ºC at 5ºC/s, deformed to 40% compressive
strain at 1s-1. Then, the specimens were cooled at 30ºC/s to
room temperature. The chemical compositions and measured
transformation temperature (Tnr, Ar3 and Ar1) of the steels
are listed in Table 1.
Figure 2. The processing schedule of the experimental steels.
Figure 1. Schematic illustration of measuring Ar3 and Ar1.
After being hold for 280 min at a soaking temperature
of 1200ºC, the rough-rolling temperature started at 1150ºC
above the nonrecrystallization temperature of austenite for
both steels. The finish-rolling stage was started at 920ºC and
finished rolling at 820ºC. The rolled plates were air-cooled to
the start cooling temperature of 780ºC, were water-cooled to
550ºC and 460ºC at a cooling rate of 30ºC/s, and then were
air-cooled for steel A and B, respectively. The processing
schedule of the experimental steels is shown in Figure 2.
As a result, the final plates with thickness of 17.5 mm and
19.3 mm for steel A and B, respectively, were obtained.
Five tensile and Charpy impact specimens were collected
from various positions along the center-line of the plates,
respectively. The flat tensile specimens, 160 mm in total length,
20 mm in effective width, 17.5/19.3 mm in thickness and 50
mm in gauge length, were machined from the plates with the
longitudinal axis parallel to the longitudinal direction, and
tensile tests were carried out on an INSTRON 4206 machine
at a strain rate of 5 mm min-1. The Charpy impact specimens
direction were also paralleled to the rolling direction. The
tensile specimens were tested at room temperature, and
impact tests of steels A and B were performed at -10ºC
and -15ºC, respectively, according to Chinese standard to
obtain an averaged result13. The Vickers hardness tester was
used to measure the Vickers hardness with 10-kg load. The
microstructures of the transverse section of the specimens
were examined with an optical microscopy (OM) after a
LePera etching14 and with a scanning electron microscopy
(SEM) after conventional 4% Nital etching. Thin specimens
were observed in a transmission el (...truncated)
