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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