Seetharaman S. Fundamentals of metallurgy

Подождите немного. Документ загружается.

greater sulfide and phosphate capacities. After more than a decade of industrial

campaign, the compositions of the fluxes seem to have converged into those

given in Table 13.6 mostly for cost reasons. To promote flux utilization for

removing P and S and minimizing heat loss, enhanced contact between the

fluxes and hot metal has been worked out in various designs of injection device

and vessel shape.

Utilization of a spare BOF for HMPT, where surplus BOF capacity exits, is

getting popular. The BOF provides (a) better slag±metal mixing without using

Table 13.5 Advantages and disadvantages of HMPT process

Advantages Disadvantages

1 Reduce BOF slag to a minimum

2 Reduce eruption of slag and metal

to near zero

3 Reduce iron loss in BOF slag

4 Reduce BOF blow time, increase

productivity

5 Increase hit rate at blow end of aim

C and temperature

6 Increase Mn yield from Mn ore

added in BOF

7 Reduce P, S to much lower

concentrations with hot metal low

in Si

Advanced sintering and BF iron

making techniques required to get

low Si hot metal consistently

Hot metal de-Si required

Occurrence of HMPT slag

Decreased use of scrap for BOF

Table 13.6 Characteristic operating parameters of HMPT process

Objective Reactor Flux addition Flux composition

1 Desiliconization BF runner Blasting with O

Ore/sinter

Tilting tundish ibid. ibid.

Transfer ladle

2 Desulfurization Transfer ladle Injection with N

Lime + Mg, or

Lime + CaF

/soda ash

Addition and ibid.

Mechanical

stirring

Torpedo Injection with N

ibid.

3 Dephosphoriza- Transfer ladle Injection with O

Lime + sinter/scale

tion and + CaF

/soda ash

desulfurization Torpedo ibid. ibid.

Spare BOF Addition and Lime + ore/sinter/

top blowing scale, partly recycled

Note: Use of CaF

and soda ash as fluxing reagent is limited to its minimum for environmental

aspect

526 Fundamentals of metallurgy

environmentally unfavorable CaF

for fluxing CaO and (b) decreased heat loss

due to shorter turnaround time s and recycling of a part of HMPT slag as hot. An

increase of scrap ratio up to some 15% has been estimated.

However, green

field installation of HMPT with the BOF is expensive. Thus, choice betwee n the

transfer vessel type and the BOF type for HMPT is a matter of compromise

between the increased investment and decreased running cost at higher scrap

rate with the environmental advantage.

HMPT in USA and Europe is limited to desulfurization with CaO-Mg

injection and the like into the transfer/charging ladle in order to save heat and

utilize more scrap in the BOF for cost reasons. Thus, choice o f HMPT is also a

trade-off between the abov e-mentioned advantages and disadvantages,

depending on local conditions.

Optimization of HMPT will develop as the demand grows for increased mass

processing of steel with improved properties and better lifecycle assessment. This

will be met in one way by reducing impurities to lower concentrations during high

speed refining. The obvious obstacle for implementing HMPT is the decrease in

the use of scrap in the BOF when the scrap has cost advantage. One way to

overcome the obstacle is to integrate the BOF, operating predominantly on hot

metal, and EAF, based on scrap but with hot metal heeling, into a steel works.

Regarding secondary refining, representative processes are shown in Figs 13.5

and 13.11.

LFs utilize enhanced contact of steel melt with top slag by use of

bottom Ar injection (and electromagnetic stirring) either under reduced pressu re

or mos tly at ambient pressure and in some cases with electric arc heating as shown

13.11 Secondary refining vessels (Morita and Emi).

Improving steelmaking and steel properties 527

13.12

A process integration for mass production of clean steel.

in Fig. 13.11. Top slag for Al-deoxidized steel melt is often CaO-Al

base slag

with high CaO activity that has low oxygen potential, high sulfide capacity and

good capability to dissolve oxide inclusions. For Al-Si- and Si-deoxidized steel

melt, it is CaO-SiO

base slag with low SiO

activity to decrease S, O, oxide

inclusions and minimize the occurrence of Al

rich inclusions. Simpler versions

of the LF, such as the CAS and CAS-OB are also in operation.

The RH utilizes vacuum vessel to cir culate steel melt as described in some

detail in the section on Process development to produce BH-1F SEDDQ steel

sheet, on page 511. Its major functi on is to decrease H, C, O and oxide

inclusions. Powder flux injection is made through the top lance (RH-PI) or side

tuyeres (RH-PB) of the vessel to reduce C and S to the order of single ppm as

already shown in Fig. 13.9. VOD is for high Cr steels, mostly stainless steels low

in C and N. Oxygen gas is soft blown on top of steel melt in the ladle placed in

the evacuated chamber, and the melt is decarburized to a very low concentration

without causing much oxidation loss of Cr.

In all processes, the intensive melt stirring function is implemented to enhance

mass transfer of impurities to reaction sites, i.e., gas±metal and/or slag±metal

interface, for efficient removal and homogenization of chemistry and temperature.

Process combinations effective in minimizing the majority of impurities are

shown in Fig. 13.12 for integrated iron and steel plants. It consists of HMPT (for

removing P and S or S), the BOF (C and some P), the LF (S, O and oxide

inclusions) and/or the RH (H, C, O and oxide inclusions). Tolerabl e limits of

impurities and non-metallic inclusions for typical high performance steels are

listed in Table 13.4. Ultimate levels of impurities commercially attainable by

best combinations of the above processes are given in Fig. 13.13.

The figure

shows that the level of each impurity is attainable if the process combination

concentrates on minimizing that specific impurity. The requirement on impurity

contents of high performance steels has become increasingly stringent. Measures

to meet the requirement have been industrialized, but more effort is needed to

make them more efficient, faster and economical.

13.2.2 Processes for controlling inclusions

Inclusions here include oxides and sulfides, carbides and nitrides. Control means

reduction of the total amount and maximum size and modification of chemistry

and morphology of the inclusions. Oxide inclusions occur indigenously and

exogenously in steel melt whereas others occur mostly in so lid steel

indigenously during solidification and cooling.

Indigenous oxide inclusions

Indigenous oxide inclusions occur from deoxidation of steel melt with deoxidiz-

ing alloys, mostly Fe-Si and Al, which combine with O to form silicates and

Improving steelmaking and steel properties 529

alumina particles. In steel melt deoxidized with Al, dissolved O becomes very

low (a few ppms with dissolved Al of a few hundred ppms), and deoxidation

product, solid Al

particles, is easy to remove by stirring the melt to let them

collide, agglomerate and surface to the meniscus. In reducing the amount of

oxide inclusions, inert atmosphere, and slag and refractory low in oxygen

potential, need to be utilized together with sufficiently strong stirring of steel

melt for the surfacing. Representative processes for this objective are the LF and

the RH as mentioned earlier.

However, suspended fine Al

particles tend to collide and agglomerate to

form large Al

clusters during melt transfer from ladle to mold even whe n

their O content is very small (e.g. only 1 ppm of O as alumina particles of 2 m

in diameter is equivalent to about 10

particles/cm

steel melt). The clusters

cause process upsets (i.e. break during deep drawing of sheet and drawing of

wire) and are detrimental for properties, if they have been caught in the

solidified steel shell. Also, clogging due to the deposi tion of the suspended

on the inner wall of SEN during teeming for CC is not uncommon. The

deposit inhibits smooth teeming, resu lts in asymm etric melt flow out of SEN.

Such flow causes meniscus turbulence to entrain mold slag and penetrates deep

13.13 Minimum imp urity elements concentrations ach ieved on industrial

steelmaking and refining operation (Emi and Seetharaman).

530 Fundamentals of metallurgy

into the strand crater to prevent flotation of inclusions and entrained mold slag.

For steels where the cluster formation is critical, and sometimes for avoiding

the nozzle clogging, modification of inclusion chemistry from solid Al

liquid CaO-Al

has been prac ticed by adding Ca-alloy into the steel melt.

Dissolved Ca reacts with Al

, converting solid Al

into liquid lime

aluminates. Liquid lime aluminate inclusions are less prone to agglome rate, do

not form large cluste rs even when they agglomerate, merging into smaller

spheres. The spheres elongate during hot rolling, being fragmented into less

harmful fine pieces during cold rolling, and hence the proce ss upsets and

deterioration of properties can be avoided.

In an extreme case where Al

inclusion formation is strictly prohibited due

to upset in downstream processing, deoxidation is carried out with (insufficient

amount of Al) + Fe-Si or Fe-Si together with slag that is sufficiently low in silica

activity. Tire cord steel and valve spring steel are sensitive to break up during cold

drawing when Al

inclusions are left in the rod to be drawn. Accordingly, they

are deoxidized and inclusion controlled as mentioned above. Namely, dissolved O

content in equilibrium with Si of given concentration (for mechanical properties)

cannot be made low enough with Si-deoxidation. However, by the use of the

insufficient amount of Al prior to Fe-Si addition or use of the above slag with Fe-

Si, dissolved O can be decreased to reasonably low levels. Composition of the slag

is so chosen as to bring the composition of resulting inclusions in the melt falling

within the low temperature eutectic valley of pseudo-wollastonite and anorthite in

the ternary CaO-SiO

-Al

system. Also, composition of inclusions occurring

from solute segregation during solidification (without contacting the slag) should

be made to fall in the low melting temperature range of spessartite composition in

MnO-SiO

-Al

system.

These measures make the two inclusions plastically deformable during hot

rolling, and fragmented during the early stage of cold rolling, leaving only fine

independent pieces of the fragment at sufficiently long intervals of separation in

the steel for further cold working. This type of inclusion control is favorably

applied to high C steels, like tire cord and valve spring, that contain inherently

low equilibrium dissolved O in the melt before deoxidation, and hence suited for

Si deoxidation.

Exogenous oxide inclusions

Exogenous inclusions arise from (a) the reoxidation of deoxidized steel melt by

air and oxidizing slag and (b) entrainment of slag and refractory/ladle glaze

while the melt is carrie d over from ladle through tundish to mold. Exogenous

inclusions can be macroscopic in size (up to several hundred m), and hence

detrimental to every property of steels. To prevent the formation of macro

inclusions, measures have been taken to minimize the reoxidation and slag/

refractory/glaze entrainment, starting from the ladle and ending up at the mold.

Improving steelmaking and steel properties 531

Table 13.7 summarizes origin, cause and preventative measures of exogenous

macro inclusions. Typical examples of such system with the preventative

measures for metal transfer from ladle through tundish to mold is shown in Fig.

13.14.

Detailed discussion on this subject is available elsewhere.

Table 13.7 Origin, cause and preventative measures for the occurrence of macro

inclusions

Origin Cause Preventative measures

1 Oxidizing

ladle slag

Slag entrainment in melt by

vortex and drainage in later

stage of teeming

· Stiffening slag with MgO

· Leave some melt in ladle

· BOF tap hole slag stopper

· EAF-EBT

Reoxidation by top slag

and emulsified slag

· Slag deoxidation with Al

2 Reoxidation

by air

Air aspiration at loose joints

(ladle/long nozzle, slide

gate plates/tundish/SEN)

· Air tight packing with Ar seal

· Ar injection in tundish

nozzle/SEN

Air in tundish at transient

period

· Inertization of tundish inlet

with Ar flushing

3 Tundish

slag

Slag entrainment by vortex/

drainage while bath is

shallow

Large tundish with reasonable

depth and residence time

· Tundish furniture

Same by turbulent melt flow

at meniscus in transient

period

4 SEN

Clogging by alumina !

asymmetric aggressive melt

flow from SEN !

penetration of inclusions

deep into pool

· Ar injection from tundish

nozzle/SEN wall

· Fluid dynamic nozzle design

(annular, swirl type)

· Ca addition to melt, all to

· Dislodging of accretion reduce clogging

5 Mold slag · Slag entrainment by vortex · Electromagnetic meniscus

· Same by turbulent metal

flow at meniscus

flow control

· Optimization of throughput

rate and port angle of SEN

· Slag viscosity control

6 Indigenous,

exogenous

inclusions

· Large cluster formation of

suspending alumina

inclusions during melt

transfer into mold

· Large tundish, tundish

furniture

· Electromagnetic damping of

penetrating flow into melt

pool in strand

· Vertical bending caster

7 All sources · Same as for no. 6

532 Fundamentals of metallurgy

13.14

Tundish configuration effective to reduce macro inclusions and p

revent air reoxidation and slag entrainment during transfer of ste

melt from ladle to mold in continuous casting (Okimori).

Sulfide, nitride and carbide inclusions

Formation of coarse inclusions of sulfides (MnS), nitrides (TiN, VN) and

carbides (Cr

, TiC, NbC) can be reduced by refining S, N and C to a low

concentration and avoiding the incorporation of compound-forming partner

elements, Mn, Cr, Ti, Nb, and V to the melt whenever appl icable. The limitation

is that some of the above elements and compounds are used as beneficial

components to improve properties. Thus, chemic al composition of steel

materials is to be optimized to avoid the occurrence of these compounds in

harmful sizes and amounts.

When the melt is cast into CC strands, however, solute segregation takes

place in the remaining melt as solid fraction in the strand increases with time.

Finally, near the center thickness of the strands, the concentration of solutes in

the melt, typically C, P and S, becomes excessively high or exceeds solubility

products to form precipitates because the partition coefficient, C

, for these

elements is less than unity. The solute segregation, whether it is with or without

the precipitates, is called center segregation for billets and blooms and centerline

segregation for slabs.

The center/centerline segregations can be reduced, as mentioned in Section

13.2.2, by:

(a) Increasing the sedimentation of solute-lean equiaxed crystals of steel in the

vicinity of the pool end in CC strand.

(b) Dispersing solute enriched melt between the sediment crystals.

bulging of the strand shell, of the solute-enriched interdendritic melt in the

columnar dendrite crystal zone into the thin layer of the pool end at center

thickness.

To increase the sedimentation of equiaxed crystals, casting is made at

reasonably low superheat temperatures with electromagnetic stirring (EMS) of

melt in the mold. Low superheat and EMS enhance nucleus formation and

growth of the equiaxed crystals to near the end of the melt pool. EMS near the

pool end disperses the solutes enriched interdendritic melt among equiaxed

crystal boundaries. To prevent the suction, bulging of strand is not tolerated, and

hence alignment and profile of support rolls should be precisely maintained. In

addition, squeezing of solute-enriched melt out of the pool end is performed by

soft reduction of the strand near the pool end by support roll pairs to surpass

solidification contraction. A bloom caster equipped with electromagnetic stirrer

and soft reduction roll pairs is shown in Fig. 13.15

that is effective to produce

blooms without appreciable center segregation of solutes. This also applies to

slab casters.

534 Fundamentals of metallurgy

13.3.3 Integrated optimization of refining, casting, rolling and

heat treatment for better properties and productivity

In modern steel mills, processes are interconnected to make steel flow as

continuous as possible to achieve maximum productivity and premium yield at

minimum cost and delivery time. Optimization of properties vs productivity is

pursued through all processes, from raw material preparation, ironmaking, to

surface finishing. Only property-related subjects will be mentioned in the

following listing.

In the ironmaking sector, major issues are to keep up stable production of hot

metal with designated chemistr y and temperature while pursuing reduction of

energy consumption and CO

emission in spite of degrading ore and coal.

In the steelmaking and refining sector, the optimization measures include:

(a) Separating refining function of P and S from the BOF/EAF to HMPT and

the LF, making the BOF/EAF to be more or less a decarburizing furnace.

When HMPT covers dephosphorization and desulfurization of hot metal, Si

in hot metal is decreased to its minimum. This minimizes lime addition and

hence slag evolution, eruption and iron loss and maximizes Mn reduction

from the Mn ore in the BOF as discussed in the previous section.

(b) Development of the high speed blow and low slag process in the large size

combined blown BOF (top O

blow + bottom Ar injection for stirring) with

dynamic blow control followed by direct tapping. The objectives are to

maximize productivity and metal yield and minimize over-oxidation of steel

and slag melt and carry-over of slag into the ladle.

13.15 Bloom caster with electromagnetic stirrer in mold and soft reduction rolls

at pool end to reduc e segregation of solute elements in the center of bloom

(Isobe et al.).

Improving steelmaking and steel properties 535