Materials for Automobiles

Lec 11

HR & CR Flat products

26 September 2011

Plan

1 HR Steels

2 CR steel

3 CR New Developments

4 Anisotropy and Forming limit diagram

5 Dual phase steel

6 Tailor welded blank

HR Steel

IS 1079 Grade D

IS 1079 Grade DD

IS 1079 Grade EDD

IS 2062 Grade B

UTS MPa 260-400 260 - 390 260 - 380 410 min

YS MPa - - - 250 min

% E 25 28 32 23 min.

Bend ( r) 0.5 t close close t

Hot Rolled HSLA

HSLA plates are produced by addition of trace alloying additions Nb/Ti/V followed by controlled cooling after hot rolling

Chemical Comp (%) (BSK 46)

C Mn Si S P

0.12 max 0.8 1.4 0.25 max 0.01- 0.025 0.03 max

Others Chemical Composition (%), max Nb:0. 08 max / Al: 0. 02 min / N: 80 PPM max /Cu:0.2 max Mo+ Ni:0.4 max / Cr :- 0.2 max Carbon Equivalent (CE):- 0.41 Max C.E.= (C+(Mn/6)+((Cr+Mo+V)/5)+((Cu+Ni)/15)) Tramp/Trace elements other than specified should not exceed 0.2% in total by weight

Mech Props UTS(Mpa) YS(Mpa) % E Bend Radius

Charpy (J) Fatigue Str (Mpa)

500-640 460 - 560 21 min 1 t 90 min 285 min

Refer also IS 5986

Sheet metal forming Operations

Process Details

Roll forming Long parts with constant complex cross-sections; good surface finish; high

production rates; high tooling costs.

Stretch forming Large parts with shallow contours; suitable for low-quantity production; high

labor costs; tooling and equipment costs depend on part size.

Drawing Shallow or deep parts with relatively simple shapes; high production rates; high

tooling and equipment costs.

Stamping Includes a variety of operations, such as punching, blanking, embossing,

bending, flanging, and coining; simple or complex shapes formed at high

production rates; tooling and equipment costs can be high, but labor cost is low.

Rubber forming Drawing and embossing of simple or complex shapes; sheet surface protected

by rubber membranes; flexibility of operation; low tooling costs.

Spinning Small or large axisymmetric parts; good surface finish; low tooling costs, but

labor costs can be high unless operations are automated.

Superplastic

forming

Complex shapes, fine detail and close tolerances; forming times are long,

hence production rates are low; parts not suitable for high-temperature use.

Peen forming Shallow contours on large sheets; flexibility of operation; equipment costs can

be high; process is also used for straightening parts.

Explosive forming Very large sheets with relatively complex shapes, although usually

axisymmetric; low tooling costs, but high labor cost; suitable for low-quantity

production; long cycle times.

Magnetic-pulse

forming

Shallow forming, bulging, and embossing operations on relatively low-strength

sheets; most suitable for tubular shapes; high production rates; requires special

tooling.

Characteristics of Metals Important in Sheet Forming

Elongation Determines the capability of the sheet metal to stretch without necking and failure; high

strain-hardening exponent (n)and strain-rate sensitivity exponent (m)desirable.

Yield-point

elongation

Observed with mild-steel sheets; also called bands and stretcher strains;

causes flamelike depressions on the sheet surfaces; can be eliminated by temper

rolling, but sheet must be formed within a certain time after rolling.

Anisotropy

(planar)

Exhibits different behavior in different planar directions; present in cold-rolled sheets

because of preferred orientation or mechanical fibering; causes earing in drawing; can

be reduced or eliminated by annealing but at lowered strength.

Anisotropy

(normal)

Determines thinning behavior of sheet metals during stretching; important in deep-

drawing operations.

Grain size Determines surface roughness on stretched sheet metal; the coarser the grain, the

rougher the appearance (orange peel); also affects material strength.

Residual

stresses

Caused by nonuniform deformation during forming; causes part distortion when

sectioned and can lead to stress-corrosion cracking; reduced or eliminated by stress

relieving.

Springback Caused by elastic recovery of the plastically deformed sheet after unloading; causes

distortion of part and loss of dimensional accuracy; can be controlled by techniques

such as overbending and bottoming of the punch.

Wrinkling Caused by compressive stresses in the plane of the sheet; can be objectionable or can

be useful in imparting stiffness to parts; can be controlled by proper tool and die design.

Quality of

sheared

edges

Depends on process used; edges can be rough, not square, and contain cracks,

residual stresses, and a work-hardened layer, which are all detrimental to the formability

of the sheet; quality can be improved by control of clearance, tool and die design, fine

blanking, shaving, and lubrication.

Surface

condition of

sheet

Depends on rolling practice; important in sheet forming as it can cause tearing and poor

surface quality;

Forming Temperatures

Parameter CR Steels Material Grade IF IS513 Gr.D IS513/EDD HSQCR 450 GA (GALV /

ANN)

SPRC 40

Condition CR CR CR CR CR CR (HIF)

UTS (MPa) 270 min 271/410 270/350 450 MIN 270 -330 390 -490

YS (MPa) 140/160 175/280 150/220 300 MIN 140 -180 210 - 280

El(%) 46 min 28 min 32 min 27 MIN 45 MIN 34 MIN

R Bar 1.8 min 1.5 min 1.6 min 1.2 MIN

n value 0.21 min 0.22 min

Hardness (HRB) 30/40 50-60 50 max

Bend radius Close Close Close CLOSE

C 0.0035%

max

0.12 max 0.08 max 0.1 MAX 0.0035 MAX .0.04 0.06

Si 0.015%

max

0.06 max 0.015 max 0.1 MAX 0.015 MAX 0.06 MAX

Mn 0.15 %

max

0.5 max 0.4 max 1.0-1.5 0.05 0.15 0.4 -0.5

Incl. rating Thn:2/

Thk:1

same

EQ Stds. St12 /DIN1623(1)

ASTMA619-75

SPCD/JisG3141

St14/DIN1623(1)

ASTMA620-75

SPCE/JISG3141

JIS G 3302 JIS G 3135

Erichsen Cupping Test ( IS 513)

High Strength CR steel

CR Steel Developments

In the body shop, cold-rolled sheet steel with a thickness ranging from 0.5 to 1.5mm is mostly used.

In the past, soft unalloyed materials were preferred because they offer a high degree of formability and freedom of design. In addition to deep drawing and stretch forming, the suitability for welding, joining, and painting are significant criteria of processing

These requirements are also met by the higher-strength thin sheets (with a minimum yield point of >180 MPa), that have been developed during the past 20 years : At present, there is a tendency toward Dual-phase (DP) steels, partially martensitic, and transformation induced plasticity (TRIP) steels or multiphase (MP) steels.

-strength steels suitable for cold working with augmented yield points

-forming quality categories, and the MP steels.

CR Steel Developments

The basic problem with increasing strength is a natural decline in the forming capability.

This has led to a new concept, the so-called MP steels. The raising of the strength is based on structural hardening : To a matrix of soft, ferritic portions, a harder portion is added, which consists of one or more other phases, and which should be distributed as evenly as possible. This development started with the DP steels, followed by TRIP steels. Recently, Complex-phase (CP) steels, which are of even higher strength, have been developed (Figure 2.8).

Cold Rolled Steels New Development

1 Low-Carbon Deep Drawing Steels Cold-rolled flat products made of low-carbon steels for cold forming, are killed by aluminum,.Owing to new developments in the vacuum treatment, it has become possible to keep the content of carbon and nitrogen as low as possible, and fix corresponding alloying elements that are required for the fabrication of DC 05 and 06 (mild unalloyed steels). In the same way, the content of sulfur is set at the lowest values by metallurgical measures, which in turn, permits a reduction of the manganese content. In addition to these, solid solution hardening elements (C, N,Mn), and the grain growth inhibiting elements, such as trace elements, are also to be restricted, which leads to a special selection of scrap.

2 Microalloyed Steel Age hardening of finely distributed carbonitrides results in an increase in the strength and higher-strength drawing properties of the conventional microalloyed steel. Even a small amount of titanium, vanadium, and niobium in the region of about 0.01% in the composition of the alloy, leads to a clear increase of the yield point to 260 620 Mpa. Owing to the finely distributed precipitates, a slightly lower elongation at fracture can be observed, compared to phosphor-alloyed steels.

CR Steels New Developments

3 Phosphor-Alloyed Steel Solid solution hardening leads to an increase of the yield point to 220 360 MPa and the tensile strength to 300500N MPa. Here, the structure can be compared to that of low-carbon deep drawing steels. Phosphor is the element with the strongest solid solution hardening effect. Addition of 0.01% affects an increase in the yield point by approximately 8 MPa.

4 Bake-Hardening (BH) Steel This refers to phosphor-alloyed cold-rolled steel strips which resist aging at room temperature, and which experience an additional yield point increase of approximately 40 MPa by a controlled carbon aging during baking of the automotive paint (at 180 C). This means that at room temperature, there is a stable carbon super saturation in the structure, so that the steel is not subject to natural aging as long as it is stored in the usual way. The stabilization of carbon is achieved by microalloying of the steel with diffusion-inhibiting or carbide-forming alloy elements.

5 Interstitial Free (IF) Steel For very complex products like inner door panels, side parts, or inner wheelhouses, microalloyed interstitial free (IF) steels are available. The content of C and N in IF steels ranges between 20 and 40 ppm . By addition of titanium and/or niobium, a complete fixation of carbon and nitrogen is reached in the form of nitrides (TiN), carbides (TiC), or carbonitrides (TiCN). This gives excellent forming ability, low yield points, and at the same time, high forming characteristics r (anisotropy) and n (hardening exponent). As a result of the complete fixation of carbon and nitrogen, the IF steels resist aging, and therefore, they are used largely for the fabrication of special deep drawing products in hot-dip galvanized design, for which resistance to aging would otherwise only be reached by an additional heat treatment.

CR Steels New Development

6 Isotropic Steel These steels possess unidirectional flow characteristics in the sheet-metal level and therefore, a better deep-drawing property and at the same time an increase in their strength. The minimum yield point in the delivery status of these sheet metals ranges between 210 and 280 MPa. These steels too, show a bake-hardening effect after preforming

7 Dual-Phase (DP) Steel Dual-phase steels belong to the group of polyphase steels. In DP steels, the structure is essentially formed by a ferritic matrix with an island like embedded martensite portion of up to 20%. Thereby, it possesses :

good isotropic forming properties with a higher strength level, a favorable spring-back behavior, high hardening and energy-absorbing capacity.

Moreover, these steels possess a bake-hardening potential, a low yield point ranging between 70 and 380 MPa and a high tensile strength of 500600 MPa

These properties make DP cold-rolled steels useful for application in: the flat-outer panels,especially in strength-relevant, structural, and crash-relevant parts (Figure 2.9).

CR Steels New Developments

8 Retained Austenite (RA) Steel These steels essentially consist of a Ferritic bainitic matrix, with retained austenite. During the forming process, the remaining austenite parts change to hard martensite, which leads to a considerable hardening, known as TRIP. Retained austenite (RA) steels are characterized by a high elongation without necking, and thus a high hardening capacity, recognizable by the high n-value. Compared to DP steels, RA steels have higher strengths of up to 850 MPa at a comparable elongation.

9 Complex-Phase (CP) Steels CP steels are currently fabricated as hot-rolled steel strips, with a minimum thickness of approximately 1.5 mm. Their fabrication as cold-rolled steels is still being developed. The materials known so far in this class of strength have, as a rule, to be hot rolled, and subsequently quenched and tempered. The advantage of the CP steels is that cold forming, without subsequent quenching and tempering, is possible, thus implying a considerable cost-saving potential. Essentially, this kind of steel consists of a fine-grained, ferritic bainitic, and martensitic structure, which in addition to its high strength, has a good cold-forming and welding capacity.

CR / HR Steels New Developments

10 Martensite Phase (MS) Steels These steels are also hot rolled and essentially consist of a martensitic structure. Yield points range between

values, the good cold-rolling and welding ability at a high tensile strength, and resistance to wear, make this material ideal for door-impact beams and crash-relevant parts

11 ManganeseBoron Steels For hot forming and hardening, the manganeseboron steels offer the

having heated the steel to the austenitization temperature, a subsequent controlled cooling leads to a martensitic structure, and thus, to a high strength of the material. Manganeseboron steels are of special interest for parts with complex geometries, and high demands concerning strength. The mechanical properties, which can be influenced by tempering, correspond to the highest demands, and enable significant weight saving when these steels are used in the production of strength-dimensioned, structural, and safety parts of vehicles, like bumper supports, side impact beams, column, and body reinforcing panels

Plastic-strain ratio

1 Plastic-strain ratio, r, describes the resistance of steel sheet to thinning during forming operations. This is the ratio of the true strain in the width direction, , to the true strain in the thickness direction, , of plastically strained sheet metal(Eqn 3)

The plastic-strain ratio is related to the crystallographic orientation of low-carbon steels. A standard method for determining r by using the tension test is given in ASTM E 517. The value will vary with test direction (relative to the coil rolling direction) in anisotropic metals. An average value, rm, (sometimes designated r ), represents the normal plastic anisotropy of the steel sheet( Eqn 4)

Hot-rolled and normalized cold-rolled steels are generally isotropic (rm of 1.0). Rimmed steels usually have an rm of 1.2, but this value may be higher in special cases, as with some low-manganese low-sulfur products. Aluminum-killed steels will be more anisotropic, with rm of 1.6. Higher values (up to 2.5) may be attained by controlling composition and processing. The upper limit for commercial steels is about 3.0, although values near 3.0 are seldom achieved. Interstitial-free steels tend to have the highest rm at approximately 2.0. The rm value predicts the ability of metals to deform in draw.

2 Planar anisotropy may be reported as:(Eq 5). Planar anisotropy is a measure of the amount of high points, or ears, that will develop on the edges of deep-drawn cylindrical cups or similar parts. High points in the rolling and transverse directions are noted when r is positive (for low-carbon, drawing-quality, aluminum-killed steel sheet). For some high-strength low-alloy steels, r is negative, and earing occurs at 45 to the rolling direction. For most applications, values of r near 0 are preferred, because such values imply a minimal tendency to form ears when metals are drawn into cylindrical cups.

Equations Eqn no.

3 Plastic strain ratio

4 Normal anisotrophy

5 Planar anisotrophy

6 The n value, or strain-hardening coefficient, is determined by the dependence of the flow (yield) stress on the level of strain. In materials with a high n value, the flow stress increases rapidly with strain. This tends to distribute further strain to regions of lower strain and flow stress. A high n value is also an indication of good formability in a stretching operation. In the region of uniform elongation, the n value is defined as:

T is the true stress (load/instantaneous area). This relationship implies that the true stress-strain curve of the material can be approximated by a power law constitutive equation proposed in :

T = kn

where k is a constant known as the strength coefficient.

The strain-hardening exponent

6 The strain-hardening exponent, n, is the slope of the true stress-true strain curve, when plotted on logarithmic coordinates. A significant portion of the curve is nearly a straight line for many low-carbon steels. The data are assumed to fit the equation( Eqn.6)

The n value will normally be approximately 0.22 for low-carbon steels used to form complex-shape parts. Higher values (up to 0.26) indicate improved capabilities to deform in stretch. Freshly rolled rimmed steels generally have n values comparable to those of aluminum-killed steels. After aging, values of n for rimmed steels are less than those for aluminum-killed steels. Some low-carbon steels that are not fully processed for formability, especially hot-rolled grades, will have n values as low as 0.10, but most of the formable grades will have n values above 0.14.

The effects of different n values on strain distribution in critical regions of a specific formed part are shown in Fig. 3 . Parts formed from steel sheet with a low n value (0.21) may undergo excessive thinning and fracture in critical regions. Identical parts formed from sheet with a higher n value (0.23) frequently will be strong enough in the critical areas to transfer strain to adjacent areas, thereby avoiding failure during forming.

Effects of Steel Composition on Formability

Low-carbon sheet steels are generally preferred forming. These steels typically contain less than 0.10% carbon and less than 1% total intentional and residual alloying elements. The amount of manganese, the principal alloying addition, normally ranges from 0.15 to 0.35%. Controlled amounts of silicon, niobium, titanium, or aluminium may be added either as deoxidizers or to develop certain properties. Alloy sheet steels (including high-strength low-alloy grades) however, contain specified amounts of one or more of these elements.

1 Carbon content is particularly significant in steels that are intended for complex forming applications. An increase in the carbon content of steel increases the strength of the steel and reduces its formability. These effects are caused by the formation of carbide particles in the ferrite matrix and by the resulting small grain size. The amount of carbon in steel sheet is generally limited to 0.10% or less to maximize the formability of the sheet.

2 Manganese enhances the hot-working characteristics of the steel and facilitates the development of the desired grain size. Some manganese is also necessary to neutralize the

detrimental effects of sulfur, particularly for hot workability. Typical manganese contents for

low-carbon steel sheet range from 0.15 to 0.35%; manganese contents up to 2.0% may be

specified in high-strength low-alloy steels. When the sulfur content of the steel is very low, the

manganese content also can be low, which allows the steel to be processed to develop high r

values.

3 4

Phosphorus and sulfur are considered undesirable in steel sheet intended for forming,

drawing, or bending because their presence increases the likelihood of cracking or splitting.

For more applications, phosphorus may be added to the steel to increase the strength.

Silicon content in low-carbon steel varies according to the deoxidation practice employed

during production.

Forming Limit Diagram

Shearing

Figure 16.2 (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug. Note that the scales of the two figures are different.

Clearance

Figure 16.3 (a) Effect of the clearance, c, between punch and die on the deformation zone in shearing. As the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2% and 10% of the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in) thick AISI 1020 hot-rolled steel in the sheared region. Source: H. P. Weaver and K. J. Weinmann.

Slitting

Figure 16.6 Slitting with rotary knives. This process is similar to opening cans.

Shaving and Shear Angles Figure 16.9 Schematic illustrations of the shaving of a sheared edge. (a) Shaving a sheared edge. (b) Shearing and shaving, combined in one stroke.

Figure 16.10 Examples of the use of shear angles on punches and dies.