Steel in Automotive Industry
Made by: Ahmed Mohamed khalaf No: 252
Ahmed ibrahiim saber No: 250
Automotive Steel Definitions
Advanced High-Strength Steels (AHSS) are complex, sophisticated
materials; with carefully selected chemical compositions and multiphase
microstructures resulting from precisely controlled heating and cooling
processes. Various strengthening mechanisms are employed to achieve a
range of strength, ductility, toughness, and fatigue properties. These steels
aren’t the mild steels of yesterday; rather they are uniquely light weight and
engineered to meet the challenges of today’s vehicles for stringent safety
regulations, emissions reduction, solid performance, at affordable costs.
The AHSS family includes Dual Phase (DP), Complex-Phase (CP), Ferritic-
Bainitic (FB), Martensitic (MS or MART), Transformation-Induced Plasticity
(TRIP), Hot-Formed (HF), and Twinning-Induced Plasticity (TWIP). These
1st and 2nd Generation AHSS grades are uniquely qualified to meet the
functional performance demands of certain parts. For example, DP and
TRIP steels are excellent in the crash zones of the car for their high energy
absorption. For structural elements of the passenger compartment,
extremely high-strength steels, such as Martensitic and boron-based Press
Hardened Steels (PHS) result in improved safety performance.
Recently there has been increased funding and research for the
development of the “3rd Generation” of AHSS. These are steels with
improved strength-ductility combinations compared to present grades, with
potential for more efficient joining capabilities, at lower costs. These grades
will reflect unique alloys and microstructures to achieve the desired.
Steels with yield strength levels in excess of 550 MPa are generally
referred to as AHSS.
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These steels are also sometimes called “ultrahigh-strength steels” for
tensile strengths exceeding 780 MPa. AHSS with tensile strength of at least
1000 MPa are often called “GigaPascal steel” (1000 MPa = 1GPa). Please
note another category of steels, represented in Figure 2-1 following as
Austenitic Stainless Steel. These materials have excellent strength
combined with excellent ductility, and thus meet many vehicle functional
requirements. Due to alloying content, however, they are expensive
choices for many components, and joining can be a challenge. Third
Generation AHSS seeks to offer comparable or improved capabilities at
significantly lower cost.
Automotive steels can be classified in several different ways. One is a
metallurgical designation providing some process information. Common
designations include low-strength steels (interstitial-free and mild steels);
conventional HSS (carbon-manganese, bake hardenable and high-
strength, low-alloy steels); and the new AHSS (dual phase, transformation-
induced plasticity, twinning-induced plasticity, ferritic-bainitic, complex
phase and martensitic steels). Additional higher strength steels for the
automotive market include hot-formed, post-forming heat-treated steels,
and steels designed for unique applications that include improved edge
stretch and stretch bending.
A second classification method important to part designers is strength of
the steel. Therefore, this document will use the general terms HSS and
AHSS to designate all higher strength steels. This classification system has
a problem with the on-going development of the many new grades for each
type of steel. Therefore, a DP or TRIP steel can have strength grades that
encompass two or more strength ranges.
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Steel Mechanical Properties
When selecting a material for a particular application, engineers must be
confident that it will be suitable for the loading conditions and environment
it will experience in service. An understanding of the properties of materials
is therefore essential. The mechanical properties of steel can be carefully
controlled through the selection of an appropriate chemical composition,
processing and heat treatment, which lead to its final microstructure.
Customer Specifications for steel vary widely around the world. Therefore
it’s impractical to list a global set of properties. However, we can illustrate
typical properties through a series of projects completed by the global steel
industry: UltraLight Steel Auto Body (ULSAB), UltraLight Steel Auto
Closures (ULSAC), UltraLight Steel Auto Suspensions (ULSAS) and
ULSAB-AVC (Advanced Technologies).
1. Stiffness
Stiffness is a function of part geometry and elastic modulus, not YS or UTS,
and is related to handling, safety, and also noise, vibration, and harshness
concerns. Although using AHSS helps to increase strength and decrease
weight by using thinner material, stiffness can suffer as a result.
Geometry, in particular the moment of inertia of the cross-section about the
primary load axis, plays a significant role in determining stiffness. The
flexibility to adjust cross sectional and overall
Geometries allow for structural design solutions that more efficiently carry
loads in the vehicle. The use of AHSS offers many advantages in this
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process because high work hardening rates increase formability, allowing
for improved shapes for optimal efficiency.
Additionally, AHSS typically possess high bake-hardening ability which can
improve the final strength of a component after forming and paint-baking
(curing).
2. Forming and Manufacturability
AHSS were developed partly to address decreased formability with
increased strength in conventional steels. As steels became increasingly
stronger, they simultaneously became increasingly difficult to form into
automotive parts. AHSS, although much stronger than conventional low- to
high-strength steel, also offer high work hardening and bake hardening
capabilities that allow increased formability and opportunities for
optimization of part geometries.S-7 Both overall elongation and local
elongation properties are important for formability; for some difficult-to-form
parts, high stretchability at sheared edges is important (as discussed in the
following sections about Complex Phase and Ferritic-Bainitic steels).
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Current Vehicle Examples
1. 2011 Honda CR-Z
Honda was one of the first companies to incorporate some of the highest
grade AHSS (980 MPa tensile strength and beyond) into body
structures,
Two of Honda’s major initiatives in recent years have been safety and
environmental leadership, both which are supported through the use of
AHSS. They advertise the ACE™ (Advanced Compatibility
Engineering™) Body Structure, now incorporated into all of their
vehicles, as a next-generation body design to enhance passenger
safety. The structure was designed particularly to improve
crashworthiness in the case of collision between size-mismatched
vehicles.
2. 2010 Mercedes E-Class
The 2010 Mercedes E-Class claims industry leadership in the utilization of
72 percent High-Strength Steel in its body structure, compared to just 38
percent in the previous model. Seventy-five percent of these steels have
yield strengths greater than 180 MPa, helping to achieve a structure that is
lighter weight and 30 percent more rigid, while meeting all new crash and
safety standards
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Metallurgy of AHSS
Manufacturers and users of steel products generally understand the
fundamental metallurgy of conventional low- and high-strength steels.
provides a brief description of these common steel types. Since the
metallurgy and processing of AHSS grades are somewhat novel compared
to conventional steels, they are described here to provide a baseline
understanding of how their remarkable mechanical properties evolve from
their unique processing and structure. All AHSS are produced by
controlling the chemistry and cooling rate from the austenite or austenite
plus ferrite phase, either on the runout table of the hot mill (for hot-rolled
products) or in the cooling section of the continuous annealing furnace
(continuously annealed or hot-dip coated products). Research has provided
chemical and processing combinations that have created many additional
grades and improved properties within each type of AHSS.
A.Dual Phase (DP) Steel
DP steels consist of a ferritic matrix containing a hard martensitic second
phase in the form of islands. Increasing the volume fraction of hard second
phases generally increases the strength. DP (ferrite plus martensite) steels
are produced by controlled cooling from the austenite phase (in hot-rolled
products) or from the two-phase ferrite plus austenite phase (for
continuously annealed cold-rolled and hot-dip coated products) to
transform some austenite to ferrite before a rapid cooling transforms the
remaining austenite to martensite.
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Figure 2-2: Schematic shows islands of martensite in a matrix of ferrite.
Figure 2-2 shows a schematic microstructure of DP steel, which contains
ferrite plus islands of martensite. The soft ferrite phase is generally
continuous, giving these steels excellent ductility. When these steels
deform, strain is concentrated in the lower-strength ferrite phase
surrounding the islands of martensite, creating the unique high initial work-
hardening rate (n-value) exhibited by these steels.
In DP steels, carbon enables the formation of martensite at practical
cooling rates by increasing the hardenability of the steel. Manganese,
chromium, molybdenum, vanadium, and nickel, added individually or in
combination, also help increase hardenability. Carbon also strengthens the
martensite as a ferrite solute strengthener, as do silicon and phosphorus.
These additions are carefully balanced, not only to produce unique
mechanical properties, but also to maintain the generally good resistance
spot welding capability. However, when welding the higher strength grades
(DP 700/1000 and above) to themselves, the spot weldability may require
adjustments to the welding practice
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B.Complex Phase (CP) Steel
CP steels typify the transition to steel with very high ultimate tensile
strengths. The microstructure of CP steels contains small amounts of
martensite, retained austenite and pearlite within the ferrite bainite matrix.
An extreme grain refinement is created by retarded recrystallization or
precipitation of micro alloying elements like Ti or Nb. Figure 2-10 shows the
grain structure for hot rolled CP 800/1000. In comparison to DP steels, CP
steels show significantly higher yield strengths at equal tensile strengths of
800 MPa and greater. CP steels are characterized by high energy
absorption, high residual deformation capacity and good hole expansion
Figure 2-10: Photomicrograph of CP 800/1000hot rolled steel
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Steel New Applications in Automotive Applications
I. Twist Beam
Use of high-strength and ultra-high-strength steel sheet, High-strength constant
section, thin-wall steel tube, bent through a tight radius at each corner. Plasma
cut profile at the center section of the tube to reduce torsional stiffness, thus
allowing the twist beam to twist.
II. Car Doors
Stainless steel grade AISI 301 can be used for the A-pillar of the body in white
(BIW) of production vehicles. The use of AISI 301 will lighten the A-pillar by 1.8 kg
(24%) compared to DP 600 carbon steel which has been used in this application
until now. AISI 301’s high level of formability enables parts integration and shape
optimization for vehicle manufacture
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III. Conti Support Ring
The Conti Support Ring ensures safety in the event of a flat tyre. The ring is
mounted on a normal wheel rim, together with a conventional tyre. Under
normal driving conditions, comfort is not affected.
If there is a loss of tyre pressure (for example, a puncture) the car can still
be controlled and can be driven for up to 200 km at a maximum speed of
80 km/hour. The support ring is suitable for use in winter conditions where
de-icing salts have been used. Four support rings weigh less than one
spare tyre. The proprietary stainless steel grade used for this application
has excellent plasticity and formability.
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