Seetharaman S. Fundamentals of metallurgy

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13.2 Developing processes and properties with

reference to market, energy, and environment

An illustration of steel manufacturing processes is shown in Fig. 13.1,

the upper

for upstrea m processing from raw materials to semis, and the lower for

downstream processing from semis to products.

Major iron sources for steelmaking are hot metal and steel scrap. Hot metal is

made in blast furnaces (BF) by reducing at high temperatures sintered or

pelletized iron ore with CO gas, i.e., Fe

+ 3CO ! 2Fe + 3CO

. CO gas is

formed via the reaction of charged coke and hot blast blown into the blast

furnace, i.e., 2C + O

! 2CO. Hot metal is saturated with C and contains some

Si and impurity elements P arising from gangue in iron ore and S from coke. Hot

metal is charged with steel scrap ( 25%) into the basic oxygen furnace (BOF),

desiliconized and decarburized by impinging pure oxygen gas jet from top lance

and converted into steel. This is named the BF±BOF route. On the other hand,

the majority of steel scrap, sometimes with a small fraction of hot metal and/or

direct reduced iron (DRI), is charged into electric arc furnaces (EAF), melted

and decarburized with injected oxygen gas and converted into steel. This is

called the scrap±EAF route.

Decarburized and oxygen-bear ing steel melt is tapped into a ladle with

alloying elements and deoxidizing agents, Si-Mn, Fe-Si and/or Al, and then

processed for final removal of H, S and deoxidation products, i.e. oxide

inclusions like Al

. The final removal and fine tuning of temperature and

alloying element compositions for quality steels are done in various secondary

refining furnaces (ladle furnace (LF) denoted Steel refining facility in Fig. 13.1).

Refined melt is cast via tundish into the mold of a continuous casting machine

(CCM), and withdrawn as semis. Semis are then reheated, hot rolled, pickled,

cold rolled, heat treated, annealed and surface finished into products.

Major applica tions of steels are for c onstruction, engineering wo rks,

automobile, ship, machinery, containers, etc. Automobiles consume a sizable

fraction of total steel production. As the design, structure, man ufacturing

processes and fuel economy of automobiles advance, demands on steel materials

have become more stringent and multifold, chasing extremes of properties at an

affordable cost.

Recent moves to suppress global warming have emphasized the weight

reduction of automobiles and hence thickness of steel for automobile parts. The

thickness is to be determined by the strength and corrosion loss of steel sheet and

the shape rigidity of steel parts. Strengthening steel helps reduce sheet thi ckness

for auto body (panels, frame , reinforcements, members, pillars, side sills, seats,

etc.) and traction system. However, strengthening must be made without

impairing various formabilities (e.g. deep drawability, hole expansibility, stretch

formability and bendability) that are specific to each part, and often inversely

proportional to the strength. Thus, optimization of the balance between the

506 Fundamentals of metallurgy

strength and formabilities has become a crucial issue for steels. More so for

being challenged by competing materials, aluminum alloys and engineering

plastics.

13.2.1 Properties driven by the market, environment and

energy

Steels used in various parts of automobiles call for different properties

depending on their applications, and hence best fit micro/nano-stru cture and

texture have been developed for each part. Development has been in the

following directions:

Ultimate formability for exterior panels that are subject to transfer press

forming (e.g. bake-hardenable, interstitial free steel (BH-IF)).

Ultimate strength for seat frames and door impact beams (e.g. tempered

martensitic steel).

Intermediate but optimized strength and formability combination for various

members and pillars (bainitic ferrite steel, dual-phase (DP) steel and

transformation-induced plasticity (TRIP) steel).

These characteristics are shown on elongation-strength coordinates in Fig. 13.2.

To meet the demands, innovative streamlining of the conventional sheet

production process was mandatory for both upstream sectors of steelmaking/

casting and downstream sectors of rolling/heat treatment. Exterior panels for

door sides, engine hood, ceiling etc. are typical examples for which excellent

formability and fine surface finish are of primary importance, since the panels

determine the quality of autobody.

13.2 Elongation and strength of steels used for automobile (Komiya).

Improving steelmaking and steel properties 507

Among them, development of BH-IF steel is of particular interest in view of

process vs product interaction. Process development for BH-IF steel includes

preliminary removal of P and S in hot metal pretreatment and minimization of

C, P, S and O in basic oxygen furnace blowing and secondary refining. It also

includes heavy reduction in hot- and cold-rolling, high temperature continuous

annealing and rapid cooling of cold rolled sheet to form fine grained <111//ND>

texture (<111> axis of ferrite iron crystals aligned normal to rolling plane) for

superior formability.

Properties required for BH-IF steel sheet

BH-IF steel for automobile body exterior panels is subject to press forming

where deep drawability and stretch formability count most for the qual ity of the

body. Deep drawability is expressed in terms of limiting drawing ratio (LDR),

which is a ratio of the diameter of the blank before drawing to that of tube after

drawing. LDR shows good correlation with plastic strain ratio (r) which is

defined by a ratio of (the logarithmic ratio of sheet width before (w

) to that after

the deformation (w)) to (the logarithmic ratio of sheet thickness before (t

) to that

after the deformation (t)), i.e., r  ln w=w

=ln t=t

. Stretch formability is

expressed in terms of limiting dome height (LDH) which is determined by

uniform elongation (El) or work hardening coefficient. Good drawability

securing sufficient metal flow without causing wrinkles is achieved at increased

values of r and El.

Past development of steels for deep drawing application is shown in Fig.

13.3.

For superior press formability, r  2.5 and El  45% are required that can

be achieved when fine grained <111>//ND texture (called -fiber) is developed

in the matrix of steel sheet. For that, dissolved interstitial elements, C and N, are

to be decreased and stabilized as precipitates of carbides and nitrides. An

example of the chemical composition of such steel is C 20, Si 200, Mn 1500, P

100, S 30, Al 400, N 15, Ti 400 and Nb 150 all in mass ppm, for which r  2.5,

El  50%, yield strength (YS)  140 MPa and tensile strength (TS)  290 MPa

have been materialized by controlling the texture and microstructure mentioned

above.

These criteria wer e not met while decarburization of liquid steel below 0.04%

(400 ppm) was impractical with BOF. Accordingly, box annealing (BA) of the

low carbon Al-killed (LCAK) steel strip after hot rolling was practiced (low C-

BA in Fig. 13.3) to reduce dissolved C, N and produce deep drawing quality

(DDQ) steel. The BA makes N atoms precipitate as fine particles of aluminum

nitride (AlN) to enhance the formation of <111>//ND texture. The BA also

makes C atoms precipitate in coarse carbides and hence softens the steel matrix.

Cooling after BA is slow due to the large thermal inertia of the process.

Accordingly, only trace amounts of C remains as dissolved in the matrix,

preventing age-hardening. However, BA is a very time- and energy-consuming,

508 Fundamentals of metallurgy

costly process of low produc tivity. The heating/cooling rate has little allowance

for control, limiting precipita tion control. The values of r and El thus achieved

were about 1.6 and <50% and became inadequate to meet the increasing

demands of press forming from automobile plants for more sophisticated shapes

and better premium yield in the forming.

Thus, BA was replaced by continuous annealing (CA) of Al-killed (= Al-

deoxidized) (AK) steel decarburized in the BOF to lower C to nearly 200 ppm

(not shown in Fig. 13.3). The steel after cold rolling was heated to 800ëC,

rapidly quenched to 300ëC, slightly reheated to 350ëC, slow cooled to 300ëC for

over-aging and quenched. Insufficient decarburization was compensated for by

the over-aging which served to decrease dissolved C and convert the resulting

fine carbides into harmless coarse cementites. The decrease promoted grain

growth and nucleation of <111>//ND recrystallization texture. This process

could make steel with r  1:7, while achieving bake hardening (BH) of about

40 MPa during paint baking, as a result of the anchoring of mobile dislocations

by diffused C atoms that remained dissolved.

To solve the difficulty in decreasing C in BOF and downstream annealing,

decarburization of liquid steel to 50 ppm C was made possible by implementing

the Ruhr Stahl-Hausen (RH) vacuum degasser. The RH degasser circulates

liquid steel in a ladle to a vacuum vessel where C and O in the liquid react to

form CO that is removed in a vacuum (see the section on Process development

to produce BH-1F SEDDQ steel sheet, on page 512). Continuous annealing

(CA) of the RH-degassed steel with Ti added to reduce dissolved C could make

13.3 History of the development of cold rolled deep drawing steel sheet (Obara

and Sakata).

Improving steelmaking and steel properties 509

an extra deep drawing quality (EDDQ) steel which exhibited r  1:9. Yet the C

concentration was still high, making the Ti addition too high for CA, calling for

relatively long time high temperature annealing that was a heavy burden for CA

line (CAL).

Accordingly, RH processing was further improved to produce an ultra low C

(ULC) cont ent in liquid steel of C < 20 ppm, followed by decreased and

optimized addition of Ti and Nb to stabilize N and C, respectively. The ULC

steel was hot rolled at a lower temperature with high reduction to make the grain

size finer. Quick cooling of the hot coil was necessary to prevent the coar sening

of crystal grains caused by such a low C content in hot coil. The hot rolled sheet

was then pick led, cold rolled at heavy reduction, continuously annealed at a high

temperature, and rapidly cooled to achieve intensive formation of fine grained

<111>//ND texture. This class of ULC-IF steel gives r  2.5 and El >50%, and

is called super extra deep drawing quality (SEDDQ) ste el. Dur ing the

development of SEDDQ steel, it was recognized that impurity elements like

P, S, and O need to be minimized. Metallurgical factors that have contributed to

the advance of deep drawability are summarized in Table 13.1.

Recently, even better SEDDQ steel with r  3:0 and El > 50% has been

developed with Ti and Nb added ULC-IF steel by incorporating lubricated

rolling of the steel in the ferrite temperature range. The advantage of the ferrite

Table 13.1 Unit processes in an integrated system to produce BH-IF SEDDQ sheet

Unit process Key operations and equipment

Steelmaking

Removal of P and S by hot metal pretreatment

· Decarburization by combined blowing BOF

Secondary refining · Final decarb. C, N<20 ppm, deoxidation by Al with Ti and/

or Nb addition, alumina inclusion removal, fine tuning of Al,

Ti and/or Nb, all by RH

Casting · Melt transfer with special care to prevent the occurrence of

macro inclusions caused by air reoxidation and slag

entrainment

· Continuous casting into slab with electromagnetic flow

control in mold to avoid subsurface engulfment of

inclusions and Ar bubbles

Hot rolling · Roughing of low temperature heated slab into sheet bar

· Rolling o f sheet bar to strip by tandem mill with heavy

reduction at final stand, finishing just above Ar3

temperature (Continuous lubricated ferrite rolling)

· Rapid cooling of strip on run-out table

Cold rolling · Heavy reduction rolling of hot strip by tandem mill

Annealing · Continuous annealing at high temperat ur e fol lowed by

rapid cooling by CAL

Galvanizing ·

Continuoushot dip galvanizing by CGL (with galvannealing),

· Continuous electrolytic galvanizing by EGL

510 Fundamentals of metallurgy

rolling to achieve better drawability was reported earlier,

but its industrial-

ization has been made possible by the development of continuous hot rolling

technology. Slabs are hot rolled to sheet bars that are coiled, stored in a

thermally insulated box, paid off and welded hot, one after another on the fly,

enabling whole length of the welded sheet bars to be low-temperature hot rolled

in the ferrite temperature range under set tension with lubrication.

Without the

lubrication, considerable strain occurs in the surface layer of hot strip during

rolling. The strain prevents uniform formation of <111>//ND texture in thick-

ness direction during subsequent processing which is similar to that for SEDDQ.

The SEDDQ sheet is so soft that it cannot resist denting that may occur when

dynamic pressure is locally applied during the use of the automobile. To prevent

denting, BH has been employed. Baking (170ëC) for curing polymer coatings on

white body makes dissolved C atoms (a few mass ppm) diffuse and anchor the

dislocations, giving a rise in YS of about 40 MPa. This has made BH-IF SEDDQ

steel a functional material which is very soft (YS circa 240 MPa) for press

forming without causing surface strain (wrinkles), while stiffening on baking to

YS 280 MPa.

Another recent move is to increase the strengt h of SEDDQ sheet to meet ever

increasing demands for weight reduction. SEDDQ sheet was solid solution

hardened to a tensile strength (TS) of about 390±440 MPa by the addition of

(largely) Mn and some P and Si, in this order. How ever, the quality of sheet

surface and adhesion of Zn-alloy coating to the sheet on hot dipping were not

ideal for press forming due to Mn, P and Si. These problems have been solved

by a new species of grain refined (ASTM No. 11) and p recipitation hardened

SEDDQ sheet that utilizes C, increased threefold to 60 ppm, and Nb added in

excess of atomic equivalent of C+N to let NbC and Nb(C,N) precipitate be

finely dispersed. In the new species of SEDDQ, Si and P have been decreased,

resulting in a smooth surface, good coating adhesion, and YS 290, TS 440 MPa,

r  1:9 and El 37%. Development of <111>//ND texture and a precipitate-free-

zone at grain boundaries contribute to the high values of r and El at this TS

level.

This way, market- and energy-driven developments of properties have

contributed to the progress of the manufacturing processes that will be

mentioned in the following sections in more detail.

Process development to produce BH-IF SEDDQ steel sheet

A large amount of pretreated hot metal is utilized as iron source for BH-IF

SEDDQ steel to minimize P, S, N and tramp elements that are known to

adversely affect r and El. BOFs, that are usually employed to make steel melt

from hot metal and scrap under atmospheric pressure, are able to decarburize the

melt only to 300±400 ppm for economic reasons. Further decarburization causes

excessive oxidation loss of iron into slag and corrosion of vessel refractory, even

when melt bath stirring is done well with bottom inject ion of inert gas to

Improving steelmaking and steel properties 511

enhance mass transfer of C for decarburization. A typical operation and mass

balance of BOF with bottom gas injection are shown in Fig. 13.4

for pretreated

hot metal (Si, P and S are ful ly or partly remo ved beforehand).

To promote decarburization reaction, [C]  [O]  CO (g), to reduce C <

20 ppm, the exposure of steel melt to vacuum is required. The RH process shown

in Fig. 13.5 circulates steel melt from the ladle through an up-snorkel into the

vacuum vessel by air lift pump action of Ar injected into the up-snorkel, and back

into the ladle through a down -snorkel, continually exposing the melt to a vacuum

in the vessel. Detailed analysis of plant data has revealed that the following

elements are the key to achieve extra-low C (< 20 ppm) at high productivity

(600 t/hr) suited for producing BH-IF steel melt: (a) a high vacuum in the vessel,

(b) a high rate of Ar gas injection into the up-snorkel, (c) large diameters of the

up- and down-snorkels and vacuum vessel, and (d) proper melt bath depth in the

vessel. These factors contribute to increase the rates of circulation and

homogenization of the melt in the system, and hence increase the decarburization

rate and decrease final C. Injection of oxygen gas through the top lance onto the

melt surface in the RH vessel has often been practiced. It increases the

decarburization rate in the early period of processing, making the BOF melt

containing higher C negotiable by RH within a given cycle time. Steel splash

sticking on the vessel wall is decreased by the injection, and hence productivity of

the BOF±RH system increases at decreased oxidation loss of iron into BOF slag.

The melt is then deoxidized by the addition of Al in the RH vessel. The

deoxidation products, mostly Al

particles, formed in the melt are decreased

by the circulation-stirring which enhances collision, agglomeration and flotation

to the melt surface and diss olution into the top slag of the particles. The cleansed

13.4 Typical BOF operation for blowing pretreated hot metal to low carbon

steel.

512 Fundamentals of metallurgy

13.4 Continued

melt is transferred from the ladle via tundish into the CC mold and cast into

slabs. Insufficient removal of Al

causes Al

clusters that are entrapped in

the slabs, resulting in surface defects on rolled and annealed sheet products from

the slabs. Slabs are then processed to facilitate the formation of fine grained

<111>//ND texture, as mentioned earlier.

Success in decarburizing steel melt to ultra low C by RH processing removed

the heavy burden of decarburization by batch annealing (BA) of cold rolled coil.

The IF steel strip can be annealed continuously at high speed by CAL for

microstructure and texture control without any additional decarburization. The

problems to be overcome in developing CAL were (a) high speed delivery of the

strip through all rolls without causing meandering, and (b) high temperature

annealing of the strip without causing strip breaks under applied tension. As

partly shown in Fig. 13.6,

CAL offers a wide process window for h eating rate,

soaking temperature, and cooling rate. The window enables a variety of

microstructure/textu re cont rol which was not possible with BA, where the rates

of heating and cooling were unc ontrollable and the maxim um heatin g

temperature was lower than CAL. Thus, flexible manufacturing of diverse

grades of steels has become possible with CAL. The annealed strip is coated

either by hot dipping or electrolytic plating of Zn to form the final product.

BH-IF SEDDQ sheet is a typical case of market- and energy-driven property

development. However, the property has been significantly advanced by the

13.5 RH process for reducing H, C and O in steel by circulating steel melt from

ladle to vacuum vessel.

514 Fundamentals of metallurgy

13.6 Continuous annealing line (CAL) with some example

of operating windows (Morita and Emi).