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Study on the quality of Boron micro–alloyed steels destined to applications in the automotive sector

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The paper presents a study of the possibilities of using 20MnB5q the Boron micro–alloyed steel in the assembly of the body components. It is investigated whether the steel in question can be deformed without being globulised but only chilled in flux, followed by a normalization treatment, in order to obtain a finer structure with a globulising tendency. The research has been carried out on samples of 20MnB5q indigenous steel, compared with samples from bars imported from the same French and Finnish origin. Finally there are specified the structural and qualitative differences for this micro–alloyed Boron steel, which in addition to the finishing effect of the structure, have contributed to the increase of the mechanical characteristics. This study examines the behaviour of 20MnB5q grade steel in the manufacturing process of assembly components. The effect of trace Boron in steels, especially the influence of Boron on microstructure and properties of these steels were summarized.
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Study on the quality of Boron micro–alloyed steels destined to
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International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
1
Study on the quality of Boron microalloyed steels destined to
applications in the automotive sector
I Ilca, I Kiss and D Miloștean
University Politehnica Timisoara, Faculty of Engineering Hunedoara, Department of
Engineering and Management, Hunedoara, Romania
Email: imre.kiss@fih.upt.ro
Abstract. The paper presents a study of the possibilities of using 20MnB5q the Boron micro
alloyed steel in the assembly of the body components. It is investigated whether the steel in
question can be deformed without being globulised but only chilled in flux, followed by a
normalization treatment, in order to obtain a finer structure with a globulising tendency. The
research has been carried out on samples of 20MnB5q indigenous steel, compared with
samples from bars imported from the same French and Finnish origin. Finally there are
specified the structural and qualitative differences for this microalloyed Boron steel, which in
addition to the finishing effect of the structure, have contributed to the increase of the
mechanical characteristics. This study examines the behaviour of 20MnB5q grade steel in the
manufacturing process of assembly components. The effect of trace Boron in steels, especially
the influence of Boron on microstructure and properties of these steels were summarized.
1. Introduction
Improving the safety of structures under specific service conditions has been the driving force for
intensive research in the area of high strength steels. Therefore, in addition to producing steel with the
required mechanical properties (strength, toughness), it is increasingly necessary to satisfy processing
requirements like weldability, machinability, corrosion resistance, and more.[1], [2] To achieve
improved mechanical properties and in the same time excellent weldability, it was necessary to
produce an extralow carbon steel with single phase of fine bainitic ferrite microstructure.[1], [3] This
has been achieved by the addition of the elements for hardenability and thermomechanical controlled
process and, therefore, addition of Boron plays an important role in increasing remarkably the
hardenability of steel. Consequently, the effect of Boron content in steels and effect of heat treatment
on mechanical and metallurgical properties of their has been investigated by various researchers.[1
20] Researchers have been paying attention to the Boron effect in steels for a long time due to its
potential to increase the steel hardenability. However, the mechanism by which Boron increases the
hardenability is not completely clear. At present, there are just a few studies strictly focused on the
Boron effect on the hot ductility of steels, although the Boron segregation and its effect on austenitic
grain boundary play an important role during high temperature deformation and dynamic
recrystallization.[18] Mostly, the effect of Boron microalloying on the structural transformations,
hardenability, and mechanical properties of building steel alloyed with Chromium, Manganese,
Molybdenum, Vanadium, and Titanium are considered.[8], [9], [12], [16], [20] Also, the conditions of
melting of Boronbearing steel with most effective recovery of Boron are analysed.[9], [12], [19], [20]
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
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The development of structural steels that can meet all requirements together with reasonably low
costs of production is an eternal challenge.[18] Advanced structural steels used nowadays are high
strength, microalloyed steels, obtained by means of a suitable combination of chemical composition
and thermomechanical treatment parameters in order to have a correct balance between strength,
toughness and weldability.[1], [2] Due to a steadily rising vehicle safety and crash requirements in the
automotive industry, the use of highstrength steels in structural and safety components is rapidly
increasing. The higher requirements for vehicle crash performance can be achieved only by using
thickgauge steel, which results in weight increase.[68] Therefore, the highstrength steels, like
Boron alloy steels, which meet automotive safety and crash requirements, are being used increasingly
in a number of end product applications in the automotive sector. In this sense, hotforming of the
quenchable Boronalloy steels can produce several complex, crashresistant structural parts such as
front and rear bumper beams, door beams and pillars, with ultrahigh strength and reduced sheet
thickness.[68]
Boron alloy steels are still under development and experience will show the applications for which
they will have specific benefits.[510] Boron steels, ideal after heat treatment at high hardness values
have reached, also the friction and wear very good resistance against which they often harsh working
conditions are suitable for a variety of applications, as a wear material and as a high strength structural
steel, and are of particular interest in the field of quality and the technical performances to be
achieved. Extreme demands are imposed on the steel when machines and their components are
exposed to high, dynamic stresses.[510] Highstrength steels enable engineers to design lighter,
higherperformance structures, offering good overall costeffectiveness, lower fabrication costs and
increased safety. The highperformance steels are recommendable whenever the steel is subject to
extraordinary requirements, such as maximum purity, surface hardness and fatigue strength. The
composition of these steels, therefore, was based on as low a carbon content, but exactly adjusting the
entire chemical composition, must achieve optimum processing and service properties, madeto
measure for every application.[510]
CarbonManganeseBoron steels are generally specified as replacements for alloy steels for
reasons of cost, these steels being far less expensive than alloy steels of equivalent hardenability.[18]
Boron over 8 ppm the content is considered as an alloying element and it is dissolved in ppm
quantities in solid state in steel.[2], [3], [9], [10] Recent research has highlighted the strong action on
the feasibility of adding a few tenths of a thousandth of a percent of Boron.[120] It is wellknown
that a small amount of Boron dramatically increases the hardenability of steels, the basic effect of
Boron in the steel being the enhancement of hardenability, which is evident already at a very small
concentration, of the degree of 20ppm Boron.[8], [9], [1114] This beneficial effect is attributed to
grain boundary segregation previously mentioned, which retards the diffusional transformation of
austenite to ferrite by lowering the interfacial energy at the austenite grain boundary.[15], [16]
Therefore, Boron in steel in very low quantities (0.0010.003) greatly increases hardness and strength,
and also the hardenability of low carbon steels. With increasing addition of Boron to the extent of
about 20ppm, both the tensile strength and hardness values increase, attributed to carbide forming
tendency of the Boron.[17] Moreover, Boron have similar effect to those obtained with such common
alloying elements as Manganese, Chromium, Nickel, and Molybdenum, but, unlike these elements,
only a minute amount of Boron is required. Even in the small quantity of the degree of size up to 40
ppm, Boron gives the same effect of the hardenability enhancement as other more expensive elements
which must be added in much bigger quantity.[9], [11]
The action of Boron is effective during quenching for low carbon steels, and when annealing is
null. The Boron additive steels, easy to be processed and applied in very good mechanical properties
after heat treatment are important features exhibitions. According to the same mechanical properties
are cheaper than high alloy steels. In addition to the benefits of economy and alloy conservation,
Boron steels offer significant advantages of better machinability compared with Boron free steels of
equivalent hardness.[20] Moreover, steels containing Boron are also less susceptible to quench
cracking and distortion during heat treatment. Consequently, Boron containing Carbon alloy steels are
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
3
widely used in automotive, constructional, and various other applications, including agricultural
machinery and tools (discs, plough shares), public works machinery, mining, cutting equipment, safety
beams in vehicles or concrete mixer drums, due to the several important advantages such:
suitable for environmentallyfriendly water quenching processes;
excellent hardening performance, also possible with both oil and gas quenching;
high wear and abrasion resistance meaning more durability, less material weight loss, low
maintenance level, longer lifetime and increased cost savings; and
heat treatable Boron steel grades offer excellent resistance against abrasive wear.
Consequently, the Boron steel grades represent a breakthrough for durable steel meaning they are
a costeffective solution to prolong the lifetime of your final products. Hardened via a Boron
quenching process and hot rollingcontrolled thermomechanical treatment, the Boron steels offers a
remarkable degree of hardness, giving the final product:
a uniform microstructure;
outstanding mechanical loading; and
excellent resistance to abrasive wear.
2. Boron microalloyed steels
Boron microalloyed steels are classified as lowalloy highstrength steels which are classified by
their mechanical properties and especially their flow limit. These steels grades are: OLC15q, OLC35q,
OLC45q and 20MnB5q. In addition, other steel grades are also used. To produce such a microstructure
especially with a lowcarbon microalloyed steel, the chemical composition had to be designed very
carefully. The basic alloying system contains Nickel, Chromium, Molybdenum and the microalloying
elements Vanadium, Niobium, and Titanium. Besides these the microalloying element Boron should
also be effective.
Steel grade 26MnB5q, in accordance with [21], is one of the Boronalloyed quenched and
tempered steels. This grade is characterized in particular by their formability in the hot rolled state and
their high strength after the heat treatment process, where the addition of Boron leads to the strain
hardenability of this CarbonManganeseChromium alloy. In fact, the strength characteristics after
quenching and tempering are achieved in particular by the low Boron content, in addition to the
Carbon and Manganese. This grade features homogeneous material properties that enable consistent
and predictable performance of the final component. The material is very clean, meaning the levels of
impurities are carefully controlled, which helps with consistency in formability and weldability. In
Table 1, the recently established standard for 20MnB5q steel is presented.[21] Boron alloy steels are
specified when the base composition meets mechanical property requirements (toughness, wear
resistance, etc.), but hardenability is insufficient for the intended section size. Rather than call for a
more highly alloyed and therefore more expensive steel, a user may simply specify the corresponding
Boron grade, thereby ensuring suitable hardenability.
Table 1. Chemical composition [%] and the main mechanical properties of the structural carbon steel,
grade 20MnB5 (1.5530): EN 1008332006 [21]
The chemical composition [%]
C
Si
Mn
S
B
0.170.23
max 0.4
1.11.4
max. 0.035
0.00080.005
The main mechanical properties
Yield
Rp0.2 (MPa)
Tensile
Rm (MPa)
Elongation
A (%)
Reduction in cross section
on fracture Z (%)
Brinell hardness
(HBW)
700 (≥)
9001050
14
55
255270
The transformation of austenite in the quality carbon steels is almost entirely determined by the
Carbon and Manganese content. The Manganese content is chosen so as to obtain the desired
microstructure and properties, the chosen Carbon content and the given cooling conditions.[15] The
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
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effect of Phosphorus and sulfur is almost negligible, and the Silicon content is normally so low in
order to obtain the desired mechanical strength.[15], [920] The synergistic effect of Boron with
other elements can enhance the Boron effect.[19] The effect of Boron on hardenability also depends
on the amount of carbon in the steel. The effect of Boron increases in inverse proportion to the
percentage of carbon present. Boron may also become ineffective if its state is changed by incorrect
heat treatment. It should also be noted that the properties of quality carbon steel can be influenced by
the presence of gases, especially Oxygen, Nitrogen and Hydrogen, as well as their reaction
products.[15] These secondary elements penetrate into the steel usually from the scrap iron used,
from the deoxidants or from the furnace environment.[15] The gas content depends largely on the
method used for melting, deoxidation and casting, so that the final properties largely depend on the
method used to elaborate them. Since Boron has a strong affinity for oxygen and nitrogen, these
elements either must be removed or controlled for Boron to have its full hardenability effect.
Accordingly, it has been the general practice to add Boron to steel with titanium and zirconium present
to protect the Boron from nitrogen, and aluminum to protect Boron from oxygen. In addition to
effecting deoxidation and providing protection of Boron from oxygen, aluminum is an effective grain
refiner in production of ingot cast finegrained steel.
For steel grades OLC15q, OLC35q and OLC45q, the international standards have provided the
chemical composition and the qualitative characteristics that can be found in Tables 24.[22]
Table 2. The chemical composition [%] and the main mechanical properties of the high quality
structural carbon steel, grade OLC15q [22]
The chemical composition [%]
C
Si
Mn
P
S
Cr
Ni
Al
0.120.18
0.170.37
0.350.65
0.045
0.045
max. 0.30
0.3
0.02
The main mechanical properties
Yield
Rp0.2 (MPa)
Tensile
Rm (MPa)
Elongation
A (%)
Reduction in cross section
on fracture Z (%)
Brinell hardness
(HBW)
541 (≥)
982 (≥)
33
13
113
Table 3. The chemical composition [%] and the main mechanical properties of the high quality
structural carbon steel, grade OLC35q [22]
The chemical composition [%]
C
Si
Mn
P
S
Cr
Ni
Cu
0.320.39
0.170.37
0.500.80
0.04
0.04
max. 0.30
0.30
0.3
Yield
Rp0.2 (MPa)
Tensile
Rm (MPa)
Elongation
A (%)
Reduction in cross section
on fracture Z (%)
Brinell hardness
(HBW)
873 (≥)
551 (≥)
34
22
134
Table 4. The chemical composition [%] and the main mechanical properties of the high quality
structural carbon steel, grade OLC45q [22]
The chemical composition [%]
C
Si
Mn
P
S
Cr
Ni
Cu
0.420.50
0.170.37
0.500.80
0.04
0.04
max 0.30
0.3
0.3
The main mechanical properties
Yield
Rp0.2 (MPa)
Tensile
Rm (MPa)
Elongation
A (%)
Reduction in cross section
on fracture Z (%)
Brinell hardness
(HBW)
528 (≥)
414 (≥)
11
32
321
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
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3. Research on Boron alloy steel 26MnB5
In 2010, a local steel company has produced the first Boron alloy steel 26MnB5 for the automobile
industry, and soon this special mark of steel was also available for producers of machinery and
equipment. In the meantime, were put up a few new brands of stainless steel with Boron, which are
still in the process of being studied from the point of view of the practical utility: 27MnCrB5 and
33MnCrB5.
During the production of the 20MnB5q steel, there have appeared qualitative problems related to
the realization of the calibre, the austenitic grain size, the microscopic purity, as well as the
observance of the chemical composition and especially of the Carbon content occurred. The degree of
chemical homogeneity in the 100tones electric arc furnaces was scattered for segregating elements
(especially Sulphur and Carbon), and Aluminum could not be dosed so as to be evenly distributed in
the liquid steel. In order to avoid these qualitative deficiencies that have still been occurring in
separate batches, it is important not to reduce the use of these highgrade steel, or to reduce the
analysis gaps in the steelworks laboratories, to control the surfaces by the discharge test, or with non
destructive control equipment.
The most existing companies as well as the newest assemblies work only through the cold
deformation process with the application of high reduction rates. In order to achieve these
deformations in good conditions, without the appearance of cracks or other defects, the manufacturing
conditions of round laminates without surface defects must be ensured. Therefore, it is necessary to
observe strictly the technology of design, casting and lamination, in order to avoid surface defects in
time. From the practice of manufacturing these grades of microalloyed Boron steels, it has been
found that in many cases the defects of rolling in the finishing stands are defective with high weight in
the case of lamination of 12mm profile. The manufacturing and control technology flow applied here
provides very severe interfacial receptions to the finishing rolling mill with a sample on the bend,
which leads to very good results from the quality point of view. For steel grades OLC35q and OLC45q
there are frequent cases when it is not ensured an advanced globulating annealing in order to ensure
cold deformation without the risk of cracks occurring. In the bolt factories there are executed in
intermediate sections by which the material is emblazoned and introduced into this state in the
manufacturing process of the assembly elements.
Following the sampling of the 20MnB5q grade steel with the chemical composition shown in Table
5, the batch characteristics shown in Table 6 were obtained.[21] The quenching was carried out in two
mediums: water and oil. The French requirements for the same 20MnB5q grade steel provide the
following features presented in Table 7 and Table 8.[23] Comparing our data with that obtained on the
French steel samples, it is not consistent with the Re and Rm values. However, French indications do
not prescribe the technological additions to be used.[23]
Table 5. Chemical composition [%] of the structural carbon steel with Boron, grade 20MnB5q,
according to the Romanian requirements [21]
The chemical composition [%]
C
Mn
Si
S
P
Cu
Ni
Cr
B
Al
As
Sn
0.20
1.7
0.25
0.02
0.023
0.07
0.08
1.15
0.0035
0.007
0.011
0.012
Table 6. The heattreatment environment and temperature, and the main mechanical properties of the
structural carbon steel with Boron, grade 20MnB5q, according to the Romanian requirements [21]
Quenching
environment and
temperature,
[ºC]
Tempering
temperature,
[ºC]
Mechanical properties
Re
[N/mm2]
Rm
[N/mm2]
A
[%]
Z
[%]
Oil (900ºC)
500
660
780
17
64
640
760
64
640
760
61
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
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Water (900ºC)
500
800
870
20
58
870
860
820
870
Table 7. Chemical composition [%] of the structural carbon steel with Boron, grade 20MnB5q,
according to the French requirements [23]
The chemical composition [%]
C
Mn
Si
S
P
B
0.160.22
1.10 1.40
0.10.4
max.0.035
max.0.035
0,00080.005
Table 8. Main mechanical properties of the structural carbon steel with Boron, grade 20MnB5q,
according to the French requirements [23]
Quenching
environment and
temperature, [ºC]
Tempering
temperature,
[ºC]
Mechanical properties
Re
[N/mm2]
Rm
[N/mm2]
A
[%]
Oil (900ºC)
500ºC
min. 685
min. 8301030
min.11
The tests on a piece of steel bar imported from Finland, grade 20MnB5q, led to the following
results on chemical composition (Table 9).[24] In order to determine the feasibility of the imported
product, heating was performed at 860°C followed by quenching in water. The resulting values are
shown in Table 10. From these data it is observed that the hardness values after hardening are higher
than those obtained on the native steel.
Table 9. Chemical composition [%] of the structural carbon steel with Boron, grade 20MnB5q,
according to the Finnish requirements [24]
The chemical composition [%]
C
Mn
Si
S
P
Cu
Ni
Cr
B
Ti
V
0.22
1.22
0.21
0.029
0.023
0.27
0.22
0.31
0.005
0.03
0.04
Table 10. The hardness values after hardening
Hardness measuring zones
Dimensions
mm
Hardness HRC
(according colon 1)
22.5
43
44
47
45
43
15.0
45
46
46
44
12.0
45
48
45
48
9.5
45
46
44
8.0
47
47
48
6.0
47
47
46
4. Results & Discussions
By making microstructures both for indigenous steel processed here, and for the foreign source in the
state of delivery, it was found that the import steel has a slightly modified ferriteperlite structure by
an incomplete annealing treatment with tendencies of austenite formation in the globular state (Figure
1).
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
7
a)
b)
Figure 1. Microstructure of Boron alloyed steel grade 20MnB5q (500x)
a) Finnish origin steel; b) indigenous steel
It was also found that austenitic grains are higher for imported steel than for indigenous steel, but
in both cases they exhibit inhomogeneities with scores ranging from 1 to 5, a phenomenon evidenced
by the microstructures of Figure 2.
The specialty literature suggests that it is very difficult to control the austenitic granulation in the
case of Boronmicroalloyed steels. After the 900ºC water hardening of the indigenous steel rolled
bars, martensitic structures of the shape of Figure 3 were obtained.
a)
b)
Figure 2. Austenitic granulation of analysed Boronmicroalloyed steels (500x)
a) Finnish origin steel; b) indigenous steel
Figure 3. Microstructure of indigenous steel samples after water quenching at 900ºC (500x)
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
8
Analysing the microstructures of Figure 3 it is observed that the resulting martensite is rough and
comes from a highgrain steel. After normalization at 860°C, a normalized state of ferriteperlite
structure is obtained, which tends to globulate cementite (Figure 4), but the martensitic structure
resulting from the quenching of the normalized samples is still rough (Figure 5).
Figure 4. Microstructure of
normalized indigenous steel (500x)
Figure 5. Microstructure of normalized
indigenous steel after water quenching (500x)
Research on Finnish origin steel 20MnB5q revealed that after the quenching of the samples (with
the dimensions 20,5; 12 and 6) there were very small structural differences, but compared to the
indigenous steel of the same brand, it may be observed a higher degree of fineness of the structure
(Figure 6). This is certified by the presence of technological additions of Vanadium and Titanium,
which, besides the finishing effect of the structure, also contribute to the increase of the mechanical
characteristics. For this reason, there was a difference between the resistance characteristics (Re and
Rm) for indigenous steel as compared to the imported one because, as it has been shown, the
indigenous grade 20MnB5q steel produced in the first assimilation phases did not contain Vanadium
and Titanium.
(a)
(b)
(c)
Figure 6. Microstructure of Finnish origin steel, after water quenching (500x):
(a) 20,5 mm samples; (b) 12 mm samples; (c) 6 mm samples
The analysis reveals an instability of the chemical composition for the 20MnB5q indigenous steel
produced in the 100tones basic electric furnace, resulting in the obligation to develop it in the 50
tones electric furnace, where the chemical composition is possible in a much narrower range.
4. Concluding remarks
The paper examines whether the studied steel can be coldplastic processed without being globulised,
but only cooled down stream, followed by a normalization treatment to obtain a finer structure with
International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
9
globular tendencies. It should be noted that for this lowcarbon and lowcarbon microalloyed steel
this possibility exists, but the extent to which deformation degree can be used in the neglected
condition must be established.
Research on lowcarbon and lowcarbon microalloyed steel made technological improvements in
manufacturing, which consisted of:
production of this type of steel in electric furnaces with limitation of Carbon and Manganese
content to the maximum admissible limit;
use of FerroTitanium and FerroVanadium admixtures for the chemical analysis of steel 0.02
0.04 Titanium and 0.010.03 Vanadium;
performing samples with a cooling in water or oil to establish a more appropriate cooling medium;
performing in parallel steel samples: recoil, globulised hardened, recovered and steel: rolled,
tempered, rebound, to determine the weight of the globulisation influence.
Taking this into account, the research at both the Faculty of Engineering Hunedoara and the
Hunedoara indigenous platform continues in order to improve the manufacturing technology of the
20MnB5q grade steel.
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International Conference on Applied Sciences
IOP Conf. Series: Materials Science and Engineering 477 (2019) 012003 IOP Publishing
doi:10.1088/1757-899X/477/1/012003
10
[17] Düzcükoğlu H and Çetintürk S 2015 Effect of Boron addition on mechanical properties of 60SiCr7
stell, International Journal of Materials, Mechanics and Manufacturing 3(2) 117-120
[18] LópezChipres E, Mejía I, Maldonado C, BedollaJacuinde A and Cabrera J M 2007 Hot ductility
behavior of Boron microalloyed steels, Materials Science and Engineering: A 460461 464-470
[19] Wang X M and He X L 2002 Effect of Boron addition on structure and properties of low carbon
bainitic steels, ISIJ International 42(Suppl) S38S46
[20] Ghali S N, ElFaramawy H S and Eissa M M 2012 Influence of Boron additions on mechanical
properties of carbon steel, Journal of Minerals and Materials Characterization and Engineering
11(10) 995-999
[21] ***EN 100833: 2006 Steels for quenching and tempering Part 3: Technical delivery conditions
for alloy steel (Structural carbon steel with Boron)
[22] ***STAS 9382/489 & STAS 894982: OLC45q, OLC35q and OLC15q Standard Specification
(Oţeluri de îmbunătăţire destinate fabricării organelor de asamblare prin deformare plastică la
rece. Mărci şi condiţii tehnice de calitate)
[23] ***AFNOR NF A 35557: Grade 20MB5q Heat treatable steels for externally threaded, high
performance fasteners for engineering purposes
[24] ***SFS 365: Grade G20Mn5 Lowcarbon alloy cast steel (normalized, annealed) and HSLA
steel with boron
... Boron alloyed steels are widely used in different industrial applications such as agricultural production, automotive manufacturing, and mining. These steels provide superior properties in terms of corrosion resistance, weldability, and formability compared to other steels of equivalent hardness (Ilca et al. 2019). Improved machinability and mechanical properties after heat treatment are important characteristics of boron alloyed steels (Ghali et al. 2012). ...
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The aim of this paper is to present the basic concepts of advanced high strength steels (AHSS) for use in the automobile industry, including chemical composition design, microstructure and mechanical properties development during thermomechanical processing, production technology characterisation, potential applications and performance in service. AHSS steels are considered to be the major materials for future applications in this production sector. As opposed to the cold formable single phase deep-drawable grades, the mechanical properties of AHSS steels are controlled by many factors, including: phase composition and distribution in the overall microstructure, volume fraction, size and morphology of phase constituents, as well as stability of metastable constituents. The main feature of these steels is that they do not permit to rely on the well-established traditional microstructure-properties relationships. Therefore, many different alloy concepts and alternative processing routes are still under development by different steel producers for comparable steel grades.
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The effect of trace boron in steels, especially the influence of boron on microstructure and properties of low carbon bainitic steels were summarized. It was pointed out that the hardenability of boron steels is determined by non-equilibrium boron segregation on grain boundaries. The synergistic effect of boron with other elements can enhance the boron effect. The abnormal boron segregation on the moving new grain boundaries is the basic reason of the boron retarding the recrystallization of deformed austenite, the addition of copper and niobium will increase this effect. The addition of boron accelerates the strain induced niobium carbonitride precipitation, and trace boron can refine the microstructure and improve the mechanical properties of HAZ. The mechanisms of boron effect were also expounded preliminarily in this paper.
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Effect of boron (B) addition and hot-deformation on microstructure of ferrite was studied using Ti-added interstitial free (IF) steels containing different amount of B. It was clarified that bainitic ferrite having typically lath or plate morphology with high dislocation density can be obtained also in plain IF steels by rapid cooling from austenite region. B addition largely affected the austenite-ferrite transformation and the amount of bainitic ferrite increased with increasing B content. Hot-deformation of austenite enhanced the transformation to polygonal or quasi-polygonal ferrite, resulting in decrease of bainitic ferrite. Interrupting compression tests indicated that enhanced transformation by hot-deformation is mainly due to grain refinement of austenite by recrystallization after hot-deformation. The cooling rate after hot-deformation produced the reversed effect of B on grain size of polygonal or quasi-polygonal ferrite. Ferrite grain size slightly decreased with increasing B content in the case of relatively targe cooling rate. When the materials were cooled slowly after hot-deformation, on the other hand, the ferrite grain size increased with increasing B content. The coarse ferrite in B-bearing steels had fairly irregular shape of grain boundaries and inhomogeneous grain size, which suggested the contribution of somewhat discontinuous grain growth. This coarsening of ferrite would be responsible for the previously reported fact that B addition lowers Lankford Value of cold-rolled and annealed IF steel sheets.
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