quality standards of ferroalloys viewed ...customers' needs in connection with higher steel...

18
21 INFACON 7, Trondheim, Norway, June 1995 Eds.: Tuset, Tveit and Page Publishers: FFF, Trondheim, Norway QUALITY STANDARDS OF FERROALLOYS VIEWED FROM THE STEEL INDUSTRY K. Nakamura Kimitsu Works, Nippon Steel Corporation, Japan For the development of steel material in the iron & steel making industry, the improvement in quality design, specifically a component design has been taking the leading role so far in parallel with the progress of steelmaking process and in the course, the part supported by ferroalloys has been large and contrariwise, it has exerted a serious influence upon the ferroalloy specifications. This report outlines a relation between needs to higher- grade steel and ferroalloy specifications to make the interrelation clear and at the same time, describes the requirements mainly orientated from the customers' needs in connection with higher steel quality in future and the possible influences of this orientation on the ferroalloy quality. In addition, the ideas of application of various ferroalloy brands to the actual refining operation and the examples are herein described chiefly based on the data acquired in Kimitsu Works of Nippon Steel Corporation to which the author belongs. Introduction It is natural that in step with the development of iron & steel making technology, the manufacturing method, specifically the manufacturing process un- dergoes changes accordingly and the trend is notice- able especially in these 20 years. In such changes, the change in the steelmaking process has exerted a great influence upon the manufacture of ferroalloys indirectly as well as directly. For instance, the production, produced brands, specification of compo- nents, style of packing, etc. are influenced by the change and the tendency, though being different somewhat according to the nations, is common to the basic parts. Such interrelation between iron & steel industry and ferroalloy manufacturing industry would become even closer from now on and it is not too much to say that the factors working as a trigger making the interrelation closer are controlled by the quality standard of steel products as well as the more sophis- ticated and complicated quality function requested from the customers of steel. This report has been drawn up almmg at getting light on the trend of needs to various ferroalloys from the necessity of technical innovation in the iron & steel making industry demanded from the cus- tomers, taking account of the changes undergone so far in the steelmaking process and the influence on ferroalloys. Trend of ferroalloy consumption in Japan 1. Transition of crude steel production The worldwide production of crude steel increased remarkably from the year of 1960 on and experi- enced such transition as shown in Fig. 1. This ten- dency is exactly the same even in Japan and as seen from Fig. 21), the production surmounted a hundred million (l00,000,000) tons to reach a peak in 1973. In the subsequent about 20 years, the production remained at almost the same level depending on the business trend and as the manufacturing process, a ratio of blast furnace-converter process decreased to become under 70% as the share in the crude steel

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Page 1: QUALITY STANDARDS OF FERROALLOYS VIEWED ...customers' needs in connection with higher steel quality in future and the possible influences of this orientation on the ferroalloy quality

21INFACON 7, Trondheim, Norway, June 1995Eds.: Tuset, Tveit and PagePublishers: FFF, Trondheim, Norway

QUALITY STANDARDS OF FERROALLOYSVIEWED FROM THE STEEL INDUSTRY

K. NakamuraKimitsu Works, Nippon Steel Corporation, Japan

For the development of steel material in the iron & steel making industry,the improvement in quality design, specifically a component design has beentaking the leading role so far in parallel with the progress of steelmakingprocess and in the course, the part supported by ferroalloys has been largeand contrariwise, it has exerted a serious influence upon the ferroalloyspecifications. This report outlines a relation between needs to higher­grade steel and ferroalloy specifications to make the interrelation clear andat the same time, describes the requirements mainly orientated from thecustomers' needs in connection with higher steel quality in future and thepossible influences of this orientation on the ferroalloy quality. In addition,the ideas of application of various ferroalloy brands to the actual refiningoperation and the examples are herein described chiefly based on the dataacquired in Kimitsu Works of Nippon Steel Corporation to which the authorbelongs.

Introduction

It is natural that in step with the development of iron& steel making technology, the manufacturingmethod, specifically the manufacturing process un­dergoes changes accordingly and the trend is notice­able especially in these 20 years. In such changes,the change in the steelmaking process has exerted agreat influence upon the manufacture of ferroalloysindirectly as well as directly. For instance, theproduction, produced brands, specification of compo­nents, style of packing, etc. are influenced by thechange and the tendency, though being differentsomewhat according to the nations, is common to thebasic parts.

Such interrelation between iron & steel industry andferroalloy manufacturing industry would becomeeven closer from now on and it is not too much to saythat the factors working as a trigger making theinterrelation closer are controlled by the qualitystandard of steel products as well as the more sophis­ticated and complicated quality function requestedfrom the customers of steel.

This report has been drawn up almmg at gettinglight on the trend of needs to various ferroalloysfrom the necessity of technical innovation in the iron& steel making industry demanded from the cus­tomers, taking account of the changes undergone sofar in the steelmaking process and the influence onferroalloys.

Trend of ferroalloy consumptionin Japan

1. Transition of crude steel productionThe worldwide production of crude steel increasedremarkably from the year of 1960 on and experi­enced such transition as shown in Fig. 1. This ten­dency is exactly the same even in Japan and as seenfrom Fig. 21), the production surmounted a hundredmillion (l00,000,000) tons to reach a peak in 1973. Inthe subsequent about 20 years, the productionremained at almost the same level depending on thebusiness trend and as the manufacturing process, aratio of blast furnace-converter process decreased tobecome under 70% as the share in the crude steel

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22

Source: International Iron and Steel Institute 4,--------------------,800 r--~'-----,,,,,,-~----,,,,,,-~---'-~'--=----'

3

~o ........ --'----.1...------'-----'----'---.......1987 1988 1989 1990 1991 1992 1993

Year

o HCFeMn 0 M.LCFeMn L::. SiMn 0 Metallic Mn

Fig. 3. Trend of consumption of Mn alloy19801970

Year1960

OL-_==-L-_~_L-_"--_'--_"--_'------'

1950

~ 200

... 600ttlQ)

>~ 400c:.2

Fig. 1. World crude steel production from allprocesses and electric arc furnaces 50.------------------.....

40

120

100

~ 80G>

~<: 60

.S!

~ 40

20

Total crude steel production

·......~.2~steel

1955: 1960 1965 1970 1975 1980 1985I First phase Isecond phaseIThird phaseI

10

o1L92:8=7==1;;98;:8~=1=92:8::;9==1;;99~0;==1::9~9::1==1~9?;92~=1=92..193

Year

o HCFeCr 0 LCFeCr

Fig. 4. Trend of consumption of Cr alloy in alloysteel

1.0,------------------,

Fig. 2. Changes in crude steel production andrefining processes in Japanese steelindustry

production and instead, an electric furnace processhas increased though being by slow degrees.

Taking a look at this movement in a little moredetail, Phase 1 was up to 1973 encountered an oilcrisis, in which the seaside ironworks with a large-scale blast furnace or a large-sized converter tak­ing the chief equipment were completed one afteranother and as the outcome, a mass-productiontechnology was established. Phase 2 was onlafter1973 where the oil crisis took place, during which thewhole nation, one and all, directed the mind towardthe resources saving and energy conservation in acondition that the production did not increase basi­cally. The following Phase 3 was a term in which agreat change was brought about mainly in the refin­ing process and further, "Optimum refining process"was attained and varied technical improvementsundertaken based on the refining process made prog­ress.

2. Transition of ferroalloy consumptionThe ferroalloy consumption, too has fluctuated natu­rally so far in correspondence with the change incrude steel production stated hereinabove and here­in, the transition in the latest 7 years is shown everyferroalloy brand. Fig. 3 shows Mn-based ferroal­loy, Fig. 4 does Cr-based ferroalloy and Fig. 5 does

0.8

0.6..........Cl~ 0.4

02~ --p::s fJ~

OL..J...----'---'----'-----'---'----J...J1987 1988 1989 1990 1991 1992 1993

YearDFeMo 0 MoO, L::. FeV 0 FeNb

Fig.5. Trend of consumption of micro alloys

other major elements (Mo, V and Nb)-based ferroal­loys.

As to the Mn-based ferroalloy, the consumptiontends to increase though being a very little as theunit consumption and especially, the tendency isclearly marked in metallic Mn. The reason will bedescribed later in the column of the future qualitytrend of steel products and to put it in a nutshell, itis because that the necessity of Mn source containingno carbon has heightened with increase of low C-high Mn steel.

A great change is not seen especially in the consump­tion of Si- and Cr-based ferroalloys. On the otherhand, when viewed from such elements as Mo, V andNb and iron side, the consumption of source contain­ing an element added in a small quantity is increas­ing little by little along with the needs to a high

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23

tensile strength steel or the strengthening of variousmechanical properties. However in regard to Mo,the consumption is found to have increased/de­creased very hard repeatedly even in the past. Thisis put down to the fact that since the fluctuation inprice of Mo is heavier than that of other elements,the quality control division of iron & steel manufac­turer is changing the design components of product.

Passage of steelmaking process andchange of usage of ferroalloys

tity of slag in a converter. And, the converter itselfwas changed to top & bottom blowing convertersassuring higher reaction efficiency for decarburiza­tion and then successively the dephosphorization hascome to be performed as a pretreatment at the phaseof hot metal where the reaction conditions are bet­ter. Also, desulphurization has been transferred tothe pretreatment of hot metal from the converter andin accordance with the growing needs toward fur­ther lowering of sulphur content, a ladle refiningwhich is the next process of converter is in full uselately.

100..-------------------,

Fig. 7. Trend of secondary refining process ratio inelectric furnace

On the one hand, the tendency is similar even in theelectric furnace industry. Fig. 72

) shows a transitionof a ratio of the ladle refining treatment in thesteelmaking process by an electric furnace. Tomake the grade of properties requested on steelhigher or reduce the manufacturing costs, the actualcondition is such that even if the process becomesmore intricate, it is obliged to still put anotherprocess.

199219881984Year

1980

Cl 80c: 0.i:~

;;:0~·~60

>a:... III

~ :i140c: tJ00tJ ...

~ Cl. 20

1. Passage of steelmaking processThe steelmaking process taking charge of a leadingfunction in making steels was reformed of coursewith the object of reducing the costs and rather themain object was to meet the request of "Supply ofhigh-grade materials" made from the customers.As mentioned above, the reform in the process ofrefining forming a nucleus of the steelmaking proc­ess got into the most active phase at Phase 3 in Fig.2. Fig. 6 shows the history of reform of the refiningfunction. To put it in a work, it can be said to benothing else but "Division of refining function".Generally, the steelmaking & refining process isavailable for removing elements contained in molteniron just turned out of a blast furnace, that is, a hotmetal to meet the product componental specificationindicated from the customer. And, the function isrepresented by decarburization,desiliconization,dephosphorization, desulphurization and degassing.Each elementary reaction depends greatly on thereaction conditions including temperature, pressure,slag basicity and so on and besides, the most advan­tageous condition differs according to the elementstaken as an object. Therefore, the history of attain­ment of optimum refining is the division itself of therefining process.

1st phase 11957-19701 12ndphaseI19n-1980l 3rdphase 11981- 1

0:1 ~s 1:1 H I): de : Hot metal

,IW'Q]'''''''Top blow Top blowI I de SI Top-bottom I

converter converter combinedde C de C converter

}BOfde Si de Si de C Top-bottomde P de P de Si combined converterde S de P de C

II deS

de CI

de C ~~~~~~~rc }s,,""'""Idegassing J degassing inclusion refining

degassing I shape processI control

Fig. 6. Divided refining function

The elements having the worst influence on steelsare phosphorus and sulphur and it may safely be saidthat the progress of a method removing these 2elements is just the development itself of refining.Quoting an example of dephosphorization, this workhas been done primarily by producing a large quan-

2. Features of new process of converter steelmak­ing methodTaking up 2 major techniques composing the newprocess of converter steelmaking method, there willbe given a brief description of the influence on fer­roalloys.

At the beginning, the refining flow at Kimitsu Worksof NSC is shown in Fig. 8 as a typical example of therecent refining process. This form was applied tothe actual operation for the first time in the begin­ning of the 1980's and thereafter, it was constrictedinto the present form through some continuedimprovements. The features are typified by"Pretreatment of hot metal" and "Top & bottomblowing converters" and these are described below.

"Pretreatment of hot metal" is essentially a processprovided newly and performed before the treatmentwith converter because it is unable to respond to thesupply request of exctremely low P steel when therefining is done only with the conventional con-

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24

No.3 CC(Slab)No.3 KIP

<No.' Steelmaking> OH degasser No.1 CCPlant (Bloom)

Converter with top

No.1 ORP and bottom blowing No.1 KIP No.4 CC2 in operation (Bloom/Slab), on standby

2BF

3BF RH degasser

4BF <No.2 Steelmaking> No2 KIP No.2 CCPlant (Slab)

Converter with topNo20RP and bottom blowing V-KIP, in operation

, on standby

ORP :Hot metal pretreatment processKIP :Powder injection processV-KIP:Vacuum powder injection process

Fig.8. Block flow diagram of refining in Kimitsu Works

( Decarburization )

( Slag separation)

Slag stopper

-

Dephosphorization &desulphurization

-(Slag skimming)

( Slag recycle)

( Desiliconization )

Fig.9. Optimized refining process (ORP)

verter. By releasing the converter from such reac­tion as desulphurization or dephosphorization, agreat effect was brought about in raising the produc­tivity of converter as well' as reducing the refiningcost as a total. The hot metal pretreatment systemin Fig. 93

) is also a paradigm of Kimitsu Works andan oxidized metal is in use as a dephosphorizingagent. This material is an iron oxide in general andit leads even to the effective use of wastes producedin the Works, e.g., mill scale, dust, etc. Viewedfrom a relationship with ferroalloys, there is a casewhere a manganese oxide, i.e., manganese ore is usedin place of iron oxide. This results in Mn concen­tration in hot metal becoming high and a substituteeffect of Mn ferroalloy used after the converterprocess being gained consequently. Whether to

apply this operation depends on the unit prices of Mnore and Mn-based ferroalloy and it is limited to acase where the former becomes under about 1/2 ofthe latter in regard to the unit price per Mn purepart. When given by a numerical expression, it canbe represented as shown by an equation 1.

aore/ aaJloy~Yore/YaJloy ·································(1)

Where, "a" and "Y" denote an unit price and Mnaddition yield per Mn pure part. Yore and YaJloy areabout 0.45 and 0.90 respectively in the converterprocess.

"Top & bottom blowing converters" is a processhaving together the respective merits of the conven-

Page 5: QUALITY STANDARDS OF FERROALLOYS VIEWED ...customers' needs in connection with higher steel quality in future and the possible influences of this orientation on the ferroalloy quality

25

Ea.a.

III

100

10oo00

o

°0&

° octo <96>0

° 00 00

0 0 0oce

°

20 40 60 80 100 120

Slag volume (kg/t)

Relationship between Mn recovery andslag volume

Fig. 12. Transition in P and S contents

10 L.:-_.....l..__.l..-_-..L.__..L-_....2L_~

1940 1950 1960 1970 1980 1990 2000Year

1000

Ea..::: 100Cl.

~

*~ 80>-a;>0 60t.l

~c:~ 40

0

Fig. 11.

1000

3. Feature of new electric furnace steelmakingprocessAn electric furnace steelmaking technology has beenmaking steady progress since the 1970's. Table 15

)

shows the major technologies available currently, inwhich three technologies introduced below are thoseto be noted in particular.

A refining process performed outside the furnace hasallowed the functions to divide by installing an arcequipment used to heat a ladle. To put it in detail,this process gets an electric furnace to discharge itsmelting and heating functions and a ladle furnace tofulfill its refining and temperature adjusting func­tions, although it has so far been customary to letonly the electric furnace undertake all refining func­tions. By dividing the functions in this way, theincrease of refining speed and stability in qualityhave been assured as the effect.

A technology which is most noteworthy these fewyears is a direct-current electric furnace, which hascontributed to the prevention of flickers and thereduction of various costs as well. The introduc­tion of such new technologies, specifically the

A scrap preheating process is such that scrap ispreheated by making efficient use of waste gascoming out of the electric furnace itself, which hasbrought about the effect improving the unit con­sumption of electric power more than 10%.

(2)1

~~~ -CoolantI:!! -02

Bottom blowingconverter

=--------------

Top blowingconverter

°2j

W~\\,o,.

- -

Mn recovery

With regard to the improvement of high-purity steelmanufacturing process ability which is primaryobject of introducing the new refining process, it hashitherto followed the requirement put forth from thecustomers at a pace more than anticipated as shownin Fig. 124

). Even for not oilly such elements asphosphorus and sulphur in molten steel but alsooxygen and nitrogen which are a so-called gas com­ponent, the effect is noteworthy.

tional top blowing converter and a bottom blowingconverter applied at some .steelmaking plant in theworld in the 1970's. This process was started toadopt widely in the world mainly from the latter halfof the 1970's and at present, it has settled as amodern steelmaking process. Fig. 10 shows theconceptual drawing of the three parties. The mostnotable feature of this process lies in both earlyproducing effect of slag by top blowing and increas­ing effect of reaction efficiency secured from power­ful slag/metal stirring by bottom blowing. Thisprovides more reduction of the amount of slag in theconverter than it had ever been before and the effectbecomes even more marked if only the concentrationof phosphorus or sulphur of hot metal is loweredprior to charging into the converter with the afore­said "Pretreatment of hot metal". Fig. 11 shows arelation between a ratio that Mn ore added in theconverter melts down to turn to a component inmolten steel, what is called, Mn yield and amount ofslag, from which it becomes clear that Mn ore hasbeen reduced by a strong bottom blowing and stir­ring power of the converter. When given by anumerical expression, it can be represented as shownby an equation 2.

(%MnO) 0 T Flog [%MnJ = -1.79+1.07 Xlog(% - e)

1+3.980 XT-0.048Xlog(%CaO/%SiOz)

Fig. 10. Conceptual drawing of various converterprocesses

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26

Table 1. Introduction of new steelmaking technologies and effects

Improvement of unit Produc- Improvement

~Year of Improve- consumption tivity of quality

introduction ment ofItem yield Electric Refracto- Elec-

power ries trodes

Formed slag operation 1975 © ©

Refining process performed out- 1977 © ©side furnace

Scrap preheating 1978 © 0

Long arc operation 1982 © © ©

Operation without slag off 1984 ©

Hearth bottom tapping 1985 0 © ©

Electrode spray cooling 1985 ©

DC electric furnace 1988 © © © ©

Water-cooled oxygen lance 1992 '0 ©

". : Greatly effectiveC : Just about effective

employment of said refining process performed out­side the furnace has enabled heating, stirring, reduc­tion refining and non-oxidizing atmosphere control.As regards a relation with ferroalloys, it has beendesigned to add more controllably ferroalloy thanit had ever been before and accelerate the ferroalloymelting and further, the narrow range control ofcomponents has become possible.

Technical ideas of best selective useof ferroalloys and examples

1. Principle of best selective use of ferroalloysThe elements added to steel as a ferroalloy have anumber of different sorts of chemical properties andconsequently, the technical ideas regarding the addi­tion differ depending on the elements. The princi­ples common to each element have been showntogether in Table 2 below.

Table 2. Common principles of ferroalloy addition

, ) Yield of addition as molten steel components must be maximum.2) Inclusions produced with addition. such as oxide. nitride and sulfide

must be minimum.

3) Change in molten steel temperature coming about in line withaddition must not act as an obstacle to the process.

4) Other refining reactions must not be obstructed.

Then, the principles provided every element aredescribed. What is the most important is to set theoptimum conditions of addition, that is, an adding

place, adding method and size of lot added at onetime according to the magnitude of chemical affinitywith oxygen. In Table 3 below, the features of eachelement and the principles of addition providedevery element have been shown by putting themtogether.

Table 3. Features for addition of ferroalloys everyelement

Affinity with YieldPossibility ofgeneration of

oxygen stability inclusions

Mn Medium Medium Medium~High

Si Large High HighCr Medium Medium MediumNb Medium High LowV Medium High LowTi Large Low HighNi Small High LowMo Small-Medium High Low

Then, let's take a look at a detailed operation inregard to typical elements.

(Mn)Mn is in use widely for most products as an

element available for raising the strength of steeland this can be said to be the most representativeferroalloy by reason of the fact that its chemicalproperty is almost similar to that of iron. Becausethe affinity with oxygen is virtually medium andadditionally, the amount to be added is large, thestability of yield is also good relatively. The factthat the amount to be added is large bespeaks thatthe influence of other components coming from Mn-based ferroalloy is serious and this constitutes a

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27

120.------------------,

Fig. 13. Influence of phosphorus on impactproperty of carbon steel (J A. Rinebolt)

Table 4 shows the brands of Mn-based ferroalloyused in Kimitsu Works of NSC and the major steelmaterials to be applied.

Table 4. Brands of Mn-based ferroalloy & majorsteel materials to be applied

(Ti)Ti is such element that since it has a very strongaffinity with oxygen and also the amount to beadded is small, the yield is unstable and the highestdegree of addition technique is req)lired. In theconventional refining process in which refining func­tion has not been divided yet, Ti has been addedduring tapping after completion of the refining at theconverter and as the brand then, a relatively large-sized and good-shaped Ti had ever been used. Forinstance, a compact Ti or Fe-Ti is a typical element.However, with the recent expansion of employmentof a micro-alloying technique, Ti componental con­trol at a very small quantity and narrow range hasbecome necessary and the customary adding methodhas no longer been able to adopt.

Fig. 14. Illustration of flow sheet in Shunan Works

Q, Bintr

Fine ore ..... r+T+1 ..... ;..-,..~ .. Mixed .~ ~ L....J rr----1l preheatlng

Fine coke .+! I. CrushingWeIghing Drying& Mixing ..

~~6"~~~-0~V (~

" by burning Drying & Pelletizing

~ot-chargepreheati~ng ~

I Nisshin

~..... Steelmaking

T Hot-charge furnace

Flux ~ ..... <>.t~ Electric s~ 1Silica furnace Wate~ "Lime stone crushing ~

Shot

(Cr)Cr is an element used widely round a stainless steeland herein, as an example of the optimum usage ofthis element, a process will be introduced whichsupplies molten ferrochrome from the ferrochromemanufacturing plant installed nearby the steelmak­ing plant to the steelmaking furnace. This processembarked on its commercial operation in 1970 atShunan Works of Nisshin Steel Co. and it is now inoperation. The manufacture of ferrochrome is beingundertaken by Shunan Denko Co. and the principleof manufacture is as follows; pellets are made fromlow-priced, low-grade powdery chrome ore and coalmaterial and after drying and heating in a kiln, thesepellets are turned to SRC (Solid-state Reduction ofChrome ore) pellet obtained by reducing about 60%of oxide of Cr and Fe and the pellet is charged intoan electric furnace in as-hot condition to manufac­ture a ferrochrome. After this, receiving the metalinto a ladle, this is hot-charged as it is directly intothe steelmaking furnace of the steelmaking plant.Fig. 146) shows a general view of the process. Theannual manufacturing capacity of ferrochrome is 60,000 tons, and the components are; Cr: 56.5%, Si: 2.0%,P: 0.025%, C: 8.2% and S: 0.020%.

400 500100 200 300

Temperature rF)

p=o .005% 0.05%

o

Steel materials Typical componentsBrands of Mo-basedferroalloy to be used

Deep-drawing steel Extremely low C Low C-Fe-Mnsheets for automobiles -Low Mo-Low N

Steels for offshore Low C-High MnLow C.Low P-Fe-Mn

structures -Extremely low Por Metallic Mn-Extremely low S

Hard steel wire rods High CHigh C-Fe-MnSi-Mn

factor which complicates the kinds of Mn-basedferroalloy.At present, the elements becoming material for dis­cussion in particular are carbon and phosphorus.The performances requested from the customers tostrengthen especially for steels are "Deepdrawability" and "High strength and Cold tough­ness". Required for the former is that carbon andnitrogen are low and for the latter, a low carbonequivalent at high Mn concentration and an extreme­ly low phosphorus are required. These require­ments have all the influence on the specification offerroalloy.

Especially for the steels used for the offshore struc­tures, it is absolutely necessary to use Mn-basedferroalloy of low. C,low P-Fe-Mn. The reason isthat a very low phosphorus concentration is neces­sary compared with ordinary steels for fulfilling low-temperature toughness required for the steel of thistype. Fig. 13 shows the influence of concentrationof phosphorus contained in the steel on the coldtoughness of high-tensile steel. In the most strin­gent application at present, the concentration ofphosphorus contained in product is demanded to beless than/equivalent to 0.005%. And, even for keep­ing a level obtained by the hot metal pretreatmentand the dephosphorization refining at the converter,it needs to restrain the input of the phosphorus fromthe ferroalloy as minimally as possible.

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28

In an operation requiring to add a large quantity offerroalloy viewed from the steelmaking plant, suchproblem arises that the refining temperaturebecomes high because of considerable temperaturedrop and the employment of this ferroalloy hot­charging method can be said to bespeak one solu­tion.

2. Rules of best selective use of pluralcomponental elements and examplesSome componental elements do not always corre­spond to an ferroalloy brand of one kind and theyare corresponding with selective use of a few brands.There will be given herein a description taking upthe cases of Mn and Si as an example.

Generally, to get a plurality of ferroalloy brands anda plurality of componental elements to correspondwith each other in the most reasonable way, theoperation is going on following the rules givenbelow.

Step 1 Determination of conditions of use offerroalloys restricted in terms of qual­ity and operation<Example) Brand not permitted to use

(Mainly restriction on quality)Restriction on proportional division ofquantity of use set by brands (Cause oftemperature drop, etc.)

Step 2 Calculation of optimum use made by.using linear program

Then, an example of the linear program is shown inTable 5. The key logic of optimization is costminimization in principle and coming down to facts,

Table 5. Flow chart of LIP calculation of ferro­alloys

Components at blowing-outt

ITarget of product It

ISet yield of ferroalloyslt

Calculation for cost minimization of ferroalloys(Linear·program)

Ll.C ~HC-FeMn xC%+ Fe-SiXC%+······Ll.Si ~HC-FeMnXSi% +Fe-Si xSi% + ......L:.Mo ~HC-Fe-Mn XMn% +Fe-Si XMn% + ......

Cost ~ 2 (j Ferroall~yx Cost j)J .

The amount of each ferroalloy to be charged is calculated so that thecost becomes minimum.(Calculation by simplex method)

tCharging indication ofeach ferroalloy

it has been set up every steelmaking plant. Pro­vided that the components are made clear beforeadding the ferroalloys in the case of manufacturing,speedy and stable calculation of optimizationbecomes possible by processing a difference betweenfinal component aimed value and component valueobtained before· addition of ferroalloy with thisprogram.

Fig. 15 shows a concrete example of a case wherecalculation was made by using this logic. In someplants, the work of the preparation for supply of thenecessary ferroalloys and the addition of them isbeing done in a full automatic way according to this

[C] [Si] [Mn]End point 0.05 0 0.15 [C] [Si] [Mn]Ladle aim 0.12 0.25 1.50 0.05 0 0.15 [C] [Si] [Mn]Upper limit 0.15 0.30 1.60 0.09 0.25 1.50 0.05 0 0.15 %Lower limit 0.10 0.20 1.40 0.12 0.30 1.60 0.08 0.25 1.50

0.07 0.20 1.40 0.11 0.30 1.60Alloy HCFeMn LCFeMn Si-Mn 0.06 0.20 1.40Calc. 1272 0 5237kg/heat HCFeMn LCFeMn Si-MnSet 1272 0 5237 0 2385 3615 kg/heat HCFeMn LCFeMn Si-MnYield 95 95 95% 0 2385 3615 0 2385 3615 kg/heat

95 95 95 % 0 2385 361595 95 95 %

II

Chemical content (%)

HCFeMnLCFeMnSi-Mn

C6.850.862.20

Sioo

14.50

Mn74.5081.0060.70

Fig. 15. Example of calculation results of ferroalloys

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calculation, which can decrease the number of therequired operaters substantially to the minimum.The answer given by this program depends largelyon C, Si or Mn contained every steel and it oftenexerts the influence on whether to use Fe-Si or makeefficient use of a part of Si-Mn especially as Sisource.

Moreover, it can make an additional remark that theweight of all-added ferroalloys can be reduced moreby making use of Si-Mn as much as possible, thusresulting in the temperature drop becoming lesserand the addition time becoming shorter, which isadvantageous viewed from the operation.

3. Rules of best selective use of ferroalloys andores, and examplesFor adjusting the componental elements necessaryfor steels to a predetermined level, a method makingfull use of ores is widely available which exists as amethod opposed to the method using ferro.alloys.Fig. 16 shows a transition in the actual results of useof Mn-based ore. Though the use of these ores wasalmost zero in the 1970's, it reached 20% of the totalMn part in the actual results obtained in the wholeJapan in 1993. This tendency can be verified techni­cally by the fact that the yield of Mn has increasedas the result of a refining process having beenmaterialized which is performed by lessening theamount of slag with introduction of the hot metalpretreatment and top & bottom blowing convertersas described hereinabefore, in other words, the abso­lute amount of MnO getting into the slag has de­creased drastically corresponding Mn distributionshown by (MnO)/[Mnl Moreover, the use of Mnore working as an oxygen source for an dephosphor­izing agent in the hot metal pretreatment won't payoff unless otherwise the price of Mn ores is consider­ably low because the conditions for making full useof ore are severer than those at the inside of con­verter in terms of cost. In fact, the amount of use isdecreasing sharply with the year of 1992 as a bound­ary and accordingly, a ratio with Mn-based ferroal­loy is also lowering.

12r-----------------...,C.g 10c.E 8:len'"§ C; 6IJ':<:

Q) 4isC 2~

1988 1990 1992

Year

Fig. 16. Trend of Mn ore consumption inJapanese steel industry

29

As to Cr, the use of Cr ore is becoming general witha special steel, specifically a stainless steel as anobject and a large quantity of it is now added intothe converter. Fig. 17 shows the operation flow atthe top & bottom blowing converter in Chiba Worksof Kawasaki Steel Corp. A smalHump coke andchrome ore (pre-reduction pellet) are added to hotmetal given pre-treatment of dephosphorization anddesulphurization for refining with the top & bottomblowing converter and thereafter, slag is separatedby tapping once. The metal is decarburized furtherby another top & bottom blowing converter, where­by .a stainless steel can be produced. This is inpractical use as a high-speed manufacturing technol­ogy at a large-sized converter.

Fig. 17. Schematic illustration of stainless steelproduction system in Chiba Works withthe use of smelting reduction process

Fig. 18 shows the changes in unit consumption ofchrome ore consumed so far by the iron & steelmaking companies in Japan, from which it is to beunderstood that, not limited to the stainless steel, theconsumption is 2 to 3 kg/t even if calculated takingthe whole amount of crude steel as a denominatorand a pretty amount of Cr component is suppliedfrom the ore.

6.-------------------,Co

.';::

~ 4~~CC)

8.:<:Q) 2isU

1988 1990 1992

Fig. 18. Trend of Cr ore consumption in Japanesesteel industry

4. Micro-alloying techniqueA description has been given so far about Mn, Cr andSi, what is named, elements added in large quantitiesamong the ferroalloys used in the iron & steelmaking industry. To meet the requirement. fromthe customers to improve the properties of steel, a

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30

lA IIA iliA IVA VA VIA/VIIA/ VIII IB liB IIIB IVB VB VIB jvllB 0

1 1H 2He

2 3Li 48e 58 6C 7N 80 9F 10Ne© 0 0 0

3 11 Na~2M~ 13AI 14Si 15P 16S nCI 18Ar© 0 © 0

4 19K 20Ca 21Sc 22Ti 23V 24Cr~5Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 358r 36Kr0 © © 0 0 0 0 0 0

5 37Rb 38Sr 39Y 40Zr 41Nb42M~ 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 491n 50Sn 51Sb 52Te 531 54Xe0 0 © © 0 0 0

6 55Cs 568a 57La 72Hf 73Ta 74W 75Re 760s 77lr 78Pt 79Au 80Hg 81T1 82Pb 83Bi 84Po 85At 86Rr0 0 0 0 0 0

7 87Fr 88Ra 89Ac\ ©: Elements assuring quite great micro-alloying effect

O. Elements affordIng recognizable mlcro-alloymg effect

Micro-alloying technique: One technique available for improving properties of metallicmaterial or making quality higher with elements added in very small quantity

Fig. 19. Description of micro-alloying elements

micro-alloying technique has been adopted. It is aprecondition for the most steels to provide them witha basic property, that is, general properties neces­sary for the steels from a condition almost similar toa pure iron. To have these steels display the furtherstepped-up mechanical properties or other specialperformance, another technique is needed. Thismicro-alloying technique can define as "Improve­ment of properties and assurance of higher quality ofmetallic materials with elements added in very smallquantity". A ladle refining materialized as a partof the division of refining process has allowed thetechnique to apply to the actual operation.

Fig. 198) shows the elements taken as the object of

micro-alloying on a periodic law table. It indicatesthat basically, the elements of IV, V and VI groupsin the periodic law table have a great effect mainlyon the improvement in strength of the steels. Adouble circle denotes the elements strengthening thesteel material by the micro-alloying effect, that is to

say, by adding in a very small quantity and also, asingle circle does the elements producing their owneffect. Ti, C, Cr, Nb and Mo are the elementsbringing about a marked effect.As a steel is cooled down from about 1,OOO°C, acrystal structure, e.g., Austenite or Ferrite comes onand in this process, such elements deposit and dis­perse into the crystal of steel finely as a carbide ornitride, thus promising the effect mentioned above.

5. Change in conditions of use of ferroalloysbased on change of steelmaking processThe above-mentioned method adding various fer­roalloys to the steels has undergone changes whilebeing influenced with the development of the steel­making process. Table 6 shows together the techni­cal improvements undertaken in this period in con­nection with the addition of ferroalloys.

As a representative refining process, a typical exam­ple of the latest addition method is shown below as

Table 6. Necessity of technical improvements for addition of ferroalloys and corresponding steps

(Improvement needs on addition of ferroalloys) (Corresponding technical improvements)

Increasing the additionyield to the maximumDecreasing the producedinclusions to the minimumLessening the change in moltensteel temperature to the minimumPreventing a harmful influenceon other refining reactionsMaking the componentrange narrower

Dividing the addition process

Decreasing oxygen content in steel before addition

Cutting off interaction with slag in case of addition

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to Mn-based ferroalloy on the basis of hot metalpretreatment-top & bottom blowing converters-ladle refining (degassing)-casting. (See Fig. 20)

Fe-Mn

Conventional Blast I-----i Top blowing I-----imethod furnace converter Casting

'-----'

1975

+0.10%

-0.10%

1995

+0.045%

-0.045%

rMn] aim(1.20-1.50%)

31

Mo-basedore Fe-Mn

Fig.21. Trend of [Mn] spac allowance in steel

Currentmethod

Fig. 20. Typical example of recent adding methodof Mn-based ferroalloy

With full use of the equipment introduced for attain­ing optimum refining, the addition has become madeat a few places though it was done at only one placein the past. This has permitted the control of com­ponents to be made at even higher accuracy. Fig. 21shows a transition of the severest target of compo­nents in Kimitsu Works of NSC in regard to Mn andNb and because of transfer to the process shown inFig. 20, a sufficient process capacity has beenmaintained successfully.

6. .Actual results of ferroalloy consumption byprocessesTaking the above-mentioned facts into account, theactual results of ferroalloy consumption by proces­ses of various ferroalloys obtained on the premise ofthe existing process have been put together below.(Fig. 22)

Future development forecasting of steelmanufacturing process

1. Future technological forecasting of refiningprocessAs a process matching with the blast furnace-con­verter process and the electric furnace process, bothof which are the main process for the current steel

Block flow diagram ofNo. 2 steelmaking process

A.-------------.Top & bottom blowingconverter

Caster-1

Caster-2

Unit: kg/t-process

A B C D EProduction (Thousands(ton)/M) 363.0 162.4 3.8 56.2 171.6Description of ferroalloy Ferroalloy brand

Mn Mn ore 1.16 0 0 0 0HCFeMn 1.94 0.71 0.77 0.16 0.37

M,lCFeMn 1,12 0 0 0.02 0.09Metallic Mn 0 0.40 1.33 0 0

SiMn 2.95 0 0 0 0Si FeSi 0.72 0.33 0.31 0.07 0.03Nb FeNb 0.01 0.12 0.68 0 0V FeV 0.01 0 0 0 0Ti AI-V-Fe-Ti 0.01 0.33 0.22 0.003 0Cr HCFeCr 0.13 0 0 0 0

lCFeCr 0.002 0 0.06 0 0Ni Ni 0.02 0 0 0 0

Mo FeMo 0.002 0 0 0 0MoOl 0.10 0 0 0 0

Fig. 22. Actual records of ferroalloy consumption in each process

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32

making, the research of a smelting reduction processis making a forward move and even in Japan, thedevelopment is going under participation of eachcompany of iron & steel making industry as anational project. This is a process which makes asteel from an iron ore by direct reduction and canoffer various anticipated advantages. Fig. 23 showsthe outline of the process and in brief, its mechanismis as follows; a raw iron ore is reduced several tenspercent by a reducing gas coming out of a smeltingreduction furnace at a pre-reduction furnace in­stalled at the upper portion and then gets into thesmelting reduction furnace having a shape like aconverter, where the ore is reduced continuously toturn to a hot metal whose carbon content is about4% and temperature is about 1,500°C. From theyear of 1993 on, a pilot plant is in operation at KeihinWorks of NKK.

Source:Japan Iron andSteel Federation

Fig. 23. Example of direct Iron ore smeltingreduction

Next, a description will be given referring to atechnology which turns a scrap to an iron sourceincluding little impurities. As shown in Fig. 249

),

generation of scrap is increasing more and more inJapan and quite a large quantity of it is being proces­sed well mainly at the manufacturers possessingelectric furnaces, needless to say the manufacturersequipped with converters. However, a point inquestion is that the amount of waste scrap hard toreuse increas~s and the cause lies in that such scrapincludes other impure elements than iron in quan­tities. Therefore, a procedure is under examinationwhich eliminates Sn, Cu, Ni and Zn and further the

(Mill Ton)

24 CJ Demolition Scrapml!INew production Scrap

20

16

12

8

4

o1970 1975 1980 1985 1991

Year

Fig. 24. Purchased scrap (domestic)

elements effective for the aforesaid micro-alloyingby a physical or chemical technique and regeneratesa high-purity resource composed only of 5 elements,i.e., C, Si, Mn, P and S, as shown in Fig. 25.

In Japan, it is aimed at establishing the technologyduring the twentieth century; however this is neveran easy development in terms of technique and itneeds to watch the progress from now on.

2. Future technological forecasting of rollingtechnologyThe development of rolling technology is extendingover quite in a number of different directions andherein, it will be discussed limiting to a technologyhaving the influence directly on the consumption andquality of ferroalloys.

The role of rolling process in the past was mostly tosimply change the shape of materials and makeadjustments and lately, it has become necessaryabsolutely even as a technique available for adjust­ing and raising the quality, that is, the mechanicalproperties. Especially for the rise in strength, it hasmade the greatest headway.

From the latter half in the 1970's on, a controlledrolling has become employed widely. This tech­nique was fully utilized for manufacturing the mate­rial for line pipes used under low temperature. Bycontrolling the temperature of steel plate to a propertemperature lower than ordinary level before rollingto perform a powerful screw-down, it has becomepossible to supply a material having superior qualityeven with fewer alloy components.

In recent years, a Thermo Mechanical Control Proc­ess has been adopted as a further forward move.Fig. 26 shows the process image and also Fig. 27shows a transition of the amount of steel to bemanufactured, This process has been made up bycombining additionally an accelerated cooling proc­ess of steel plate after rolling and it is a techniqueindispensable for manufacturing high-grade steelplates at present. (Fig. 28) Moreover, this techniqueis not only applied to the manufacture of steel platesbut put into use even for coils in future. The prog-

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33

r--1 Automatic sorting (shape treatment,separation & classificationI

I-Development of multi-functionscrap treatment

rl Large capacity sorting method r-I- Scrap evaluation sensor

H Continuous charging technique

'-- Others

r-- Establishment of sorting standard

rl Physical removal technique f- H Development of simple discriminator

--1Establishment of mixing

~preventive techniqueI- Quality assurance technique I

-- Others

--' Heat treatment

-1 Impurities separation technique f-rl Mechanical separation

...... Refining reaction process

rl Chemical removal technique r-f- Reduction roasting process

H Evaporation process

Y Electrochemical process

Source: New Steelmaking Technology Research Institute

Fig.25. Technology for removing impurities from scrap

ress of this technique brings about the effect whichdecreases the consumption of Mn used as an elementraising the strength and instead increases the con­sumption of the micro-alloying element stated her­einbefore.

Controlled rolling

As rolled

TMCP

'"EE 70

-­...Cl.::t:- 60.r:....Cll:

~ 50<Il

.!

.~ 40CIl

I- y)--...l:-:------l.__----'----....JO~ O~ OM O~

Carbon Equivalent (%)

Fig.28. Tensile strength of TMCP steel plate

Finishing .mill Hot Con.trol Cooling

·~r:~=~~rm.m J=Roughingmill

~Reheatingfurnace

Fig. 26. Arrangement of thermo mechanical con­trol process equipment

2500.-----------------------,

(ij~2000

?£ 1500"t:l

1ij 1000'":::l~ 500I-

01968

13TMCP I

Fig. 27. Changes of past records of plate hotrolling in Kimitsu

3. Forecasting of trend of requirements set out ofcustomersFirst, an attempt will be made to forecast startingfrom the automobile manufacturer which is the topuser of steel products. Table. 7 shows the descrip­tion of major steels used for automobiles and it is amovement toward "Lighter automobile" that theiron & steel making industry is affected in thegreatest degree. It is now crying out worldwide forreducing the fuel consumption of automobiles andthe reduction in fuel consumption has been set par­tially as a bill. This requires the measures in bothaspects of a fuel cost curtailing technique and a bodyweight reducing technique at the side of the automo­bile manufacturers themselves, and the latter tech­nique, as a matter of course, is related to making the

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34

Table 7. Steel sheets for automobiles

• Hot-rolled high-strength sheets150 & 60kg higtt-tension steel sheets) :

For running mechanismlWheels, Members, Trailingarms, etc.)

• High-strength cold-rolled sheetsUltra-deep drawing type high-strength sheet (Ti-Sulc):

Fenders & Quarter panelsHigh r-value type high strength sheet lP-added AI-k):

Outer sheets & doorsGeneral working-use high-strength sheet (C-Si-Mn steel):

Bumpers & Impact bars

• Cold-rolled sheet for automobiles (Ti, Nb-Ti Sulc):Bodies

steel products thinner, that is, lighter. However, onthe assumption that the steel products are madethinner, it cannot be guaranteed whether the"strength can be maintained with the conventionalsteel and as the result, it is necessary to put forwardthe development of a steel having a new perfor­mance. Further in addition, giving thought to thatit also needs to provide the steel with good wor­kability, a target is to "supply a steel sheet havingsuch contrary character as high strength-high wor­kability". To this target, an action would be takento respond with a steel made on the basis of "Inter­stitials Free Steel". To assure high strength withthe interstitials, in other words, with an extremelysmall quantity (for instance, 10 to 15ppm or less,respectively) of carbon and nitrogen, a large quan­tity of manganese content from 1.0% to 1.5% orthereabout (usually, 0.1 to 0.4%) is required. Thus,it is unable to meet the requirement with the use ofan ordinary ferroalloy brand, that is, Fe-Mn-basedferroalloy and the efficient use of metallic manga­nese would become a precondition. There is also alimit on the measures of refining with lower contentof nitrogen and it is hoped to decrease the nitrogencontent in ferroalloy.

Table 8. Fire resistant steel

Then, as for a plate, there are the requirements of asteel meeting more the environments where it is usedactually and damaged little in case of emergency inaddition to the current of mechanical properties suchas high strength, high weldability and cold toughnesswhich have been requested so far from the cus­tomers, and such tendency would become growing.As an example, a description will be given taking afire resisting steel. Table 8 shows the concept. If asteel structure is attacked by a fire, the yield proof ofthe steel greatly lowers, therefore, the steel has to becovered thickly with a refractory material by allmeans. As this step, a steel has been developedwhich is given the properties available as a boiler-use steel plate and at the same time, able to main­tain a yield of 2/3 at ordinary temperature even at600·C by using such added elements as Cr, Mo or Nb.It is clear that hereafter, even severer working envi­ronments and conditions are indicated and it wouldbe obliged to supply a steel having a new perfor­mance every time as required.

As a problem which is important and besides has agreat influence in a global scale in connection withthe working environments, there is a questionrelated to the oil resources. What is significant inparticular is a large-diameter steel pipe and fromthe viewpoint that it is used for transporting crudeoil or LNG, it becomes necessary to make designsfor the steel taking account of the influence of com­ponents contained in oil or LNG as well as theworking environments.

To put it in more detail, a transported substanceturns into a sour gas as time goes on and when CO2

or H2S is contained, it constitutes a factor bringingvarious disadvantages. Fig. 29 shows the change inthe properties required for the quality of line pipe

Steel construction building:Steel strength lowers due to heat in case of fire.

DAs step preventing strength from lowering. steel must be protected withfire-resisting covering(about 5cm in thickness). (Building Standard Law)

D"Bellow 350·C" in "Specification of fireproof construction" must besecured.

D

Fire resistant steel:At high temperature. properties of boiler-use steel material must bemaintained and at ordinary temperature. the same properties as those ofconventional construction-use steel material must be displayed. (Addi­tion of Nb. Mo and CrI

DAbout 2/3 of specified value at ordinary temperature must be guaranteed.

DOutside steel frame of building whose fire-resisting covering thickness isabout 1/3 of conventional one is possible even if there is no covering.

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35

it is indispensable to combine intricate added ele­ments conforming to higher Mn and a quality designworked out according to the performance demandedevery project is needed. At any rate, it would beinevitable to make hydrogen content lower asalready stated hereinbefore.

Quality standard of ferroalloys andfuture expectation

At the beginning, the current quality standards offerroalloys will be discussed. The standards arenot always unified yet by nations, companies andsteelworks and in fact, they have been determinedaccording to the scale and shape of refining vesseland the steels to be manufactured. Also, when thediscrete conditions including molten steel tempera­ture, adding atmospheric condition, adding place,etc. are taken into account it is in fact quite hard toprescribe the unified specification. Thus in thisreport, it has been decided to make a statementround the example of Kimitsu Works of NSC.Moreover, the example is such that the data areample and what is more, the actual adding condi­tions have already been made clear and there is noroom allowing imagination to inflame in the arenaof discussion.

Table 9 shows quoting the example of Mn-basedferroalloy used most generally. Taking a look atthe ferroalloy in the aspect of composition of compo-

1. .Actual condition of quality standards of fer­roalloysThe steel manufacturing process and higher qualityof steel products described in detail so far have beensupported by raising the quality standards of ferroal­loys which serve as the base of quality design. Itcannot be denied to be certain that this broughtseverer and more complicated quality standards offerroalloys and now, it has been characterized as asignificant basic condition for the steel manufacture.

--1986~

----1980~1985

---1970's

Impact testtemperature ('C)

-30

NACERequired propertyfor sour gasTest conditions

Fig. 29. Changes of properties required for mate­rial in line pipe material

Plate thickness (in.)125

material. The change in 10 to 20 years is rapidierthan anticipated and speaking of the above-mentioned antisour gas property, in view of that theplaces where the line pipe is used are in a consider­ably gruelling circumstances, e.g., Arctic area, avery stringent inspection has been obliged to per­form on the steels and steel pipes for preventing asocial irreparable trouble, a so-called burst accidentor the like. This inspection is performed for check­ing if there are cracks in the weldment of steel pipeby using an acidic solution whose pH is 3.0 to 3.5 orso (called NACE reagent). To allow the steel pipeto pass the inspection, as a step which preventsgeneration of cracks in the weldment of steel pipe, itneeds in every way to make the existence of hydro­gen quite little in addition to a thorough eliminationof phosphorus and sulphur out of the steel. Also, as .will be apparent from Fig. 29. even for improving aweldability in low-temperature atmosphere or mak­ing a cold toughness (guaranteed at temperature of-30 ·C) severer, decrease of carbon equivalent is anabsolute must and there is a tendency that themanufacture at X65 of API standard, according tothe circumstances, more than X70 with low carboncontent of 0.03 to 0.04% is required along withmaking the necessary strength higher. In this case,

Table 9. Specification of Mn alloy

Example of converter steel maker Example of EF steel maker

HCFeMn lCFeMn HCFeMn

ContentMn 73~78% 90~95% 75~78%

Important other elements P,S,Si C,P,S,Si P

Shape 10~50mm 10~50mm 10~100mm

Surface water less water less water less

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36

nents, a low-carbon and high-manganese contentsteel stated before has increased in both sheet andplate. Therefore, in addition to such combinationas general high-carbon Fe-Mn and low-carbon Fe-Mn, a low-P and low-C brand and metallic manga­nese have been applied, and the amount is as shownin Fig. 3. In this case, since the range of componentdesign of steel material becomes narrower inevi­tably to satisfy together the material propertiesincluding the strength, toughness and workability,dispersion in components of ferroalloy has also beencontrolled as indicated in Table 9.

Table 10. Expectations to future ferroalloy quality

Items Expectations to ferroalloy quality

Components1) Decrease of gas components(nitrogen and hydro-

gen)

2) Suppression of tramp elements

3) Reduction of dispersion in each

ShapeThe necessity for supplying as a fine material mightarise.

Surface A special surface treatment (for example. a coatingproperties treatment) might be given.

elements chiefly and with further increase of therequirements to the material, more advanced qualitydesign becomes necessary. Even in the sense, it ishoped to decrease various gas components or makecontrol at a narrow range. What is most importantin the gas components is nitrogen. Nitrogen is aninterstitial element similar to carbon and harmfulfor manufacturing a deep-drawing sheet for automo­bile use. Fig. 3010

) shows the influence of carbon onworkability and in future, it may be demanded todecrease the sum of carbon and nitrogen componentsbelow 15 ppm.

On the other hand, specifications of the shapes havebeen shown together in Table 9 and taking accountof acceleration of melting of ferroaIloy in the con­verter and ladle refining processes or prevention ofscattering in the case of addition, a proper range hasbeen set. In particular, there are some plants inwhich the upper limit of size has been morerestricted on account of a relation with automaticfeeding or the performance of weighing machine.

As other specifications, an agreement has been madein regard to a sticking moisture content and surfaceoxidized condition, and the details are as shown inTable 9. It goes without saying that the these areeffective as the measures for meeting the manufac­turing needs of the aforesaid low-hydrogen steel orcutting off an oxygen source .and oxidic inclusionsource.

On the one hand, to clarify the influence on thespecifications of Mn-based ferroalloy according todifference in process, the specifications at the manu­facturer equipped with an electric furnace have beenshown in a right half of Table 9 above.

2.0Ql::::l

~ 1.6.!.

12

50

~o- 46...w

o~

o

~

According to difference between steels to bemanufactured by the converter process and electricfurnace process, there is found a difference in brandand size of Mn-based ferroalloy to be used. LC-Fe­Mn ferroalloy is mainly used in the converter proc­ess and as for the size, too, the range for the con­verter process tends to be narrower than that for theelectric furnace process. However, even in the elec­tric furnace process, difference with the converterprocess is deemed to rather lessen due to the changeof charging method of ferroaIloys or the increase ofaddition ratio in ladle refining.

2. Expectations to future ferroalloy qualityTable 10 shows together the expectations to thefuture ferroaIloy quality. The details will be ex­plained individually below.1) Trend on componentsFirst of all, as to the components, it is intended toemphasize two points. One point is to decrease agas component in no half-and-half way. As de­scribed so far detailedly, the conventional compo­nents have created a material round the metallic

42

o 0.002 0.006 0.010Carbon content (wt %)

Fig. 30. Effect of C content on mechanical pro­perties and their planar anisotropy oflaboratory-made steels without alloyingelements

Also, as to hydrogen, a moisture content becomesmaterial for discussion as a matter of course. In ahigh grade steel, specifically in a thick plate or alarge-diameter pipe, cavities attributable to hydro­gen are produced in the progress of the productmanufacturing process and results in these materialsturning to scrap. Fig. 31 shows the example. It isnecessary, for a very thick plate exceeding 100 mmin thickness, to control the hydrogen contained inmolten steel under 1.5 ppm before casting.

A second point concerning the trend of componentsis to suppress a tramp element thoroughly. Asalready stated before, the increase of scrap availableas the major raw material source in the future iron& steel making industry leads to the rise of a so

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37

dynamic control of the amounts of ferroalloys to beadded based on the analysis results have alreadybeen performed. Fig. 32 shows the outline. Anelementary technology which would be indispens­able hereafter is to make a ferroalloy whose compo­nents have been controlled to a high degree at anarrow range and grasp the changes taking placeevery hourly at the refining progress phase.

2) Trend on shapeThen, the future needs about the shape can be esti­mated as follows. At present, addition of ferroal­loys into molten steel is mostly performed by asimple addition method, that is, a method whichdrops the ferroalloys by gravity from an exclusivehopper installed above the furnace after weighingand conveying. In the progress of the additionmethod from now on, there is possibility of thechange being demanded for the shape. In otherwords, for raising the addition yield of ferroalloysand feeding them into molten steel while evadingreaction on slag, an injection method might be em­ployed. There is an experience that this injectionmethod had even been taken as a test for Mn ore orsome elements added in a small quantity, for exam­ple, Ti, AI, etc. and this method may be adoptedpartially sooner or later. Assuming that thismethod is employed, the ferroalloys must then bechanged into a fine material in shape and the sizemust kept as uniformly as possible. However, as towhether this addition method is used on a full scalein future, there is a need to watch the trend later on.

3) Trend on surface propertiesA sphere meaning the surface properties is wide anda clear-cut trend can not always be estimated, how-

Limit line for UST defect4E~3

z2L....J

-called tramp elements such as eu, Sn, As and Sb,resulting in a restriction that scrap is unusable beingimposed on the applications for which workability isrequired. It is aimed at establishing a technologyfor eliminating these elements as the developmenttechnology, however there is found no methodenough to payoff at least industrially. It is thushoped to decrease the above-mentioried tramp ele­ments as much as possible, which are contained inferroalloys serving as a very weighty additive tomolten steel.

5.-----------------,

oO~--:2:':O:---4-:'-O~--::6~O--:8:'::O--:-1-=OOO:----:1:-::'2-:-0-1."..-!40

Plate thickness (mm)

Fig. 31. Example of relation between plate thick­ness & hydrogen concentration in steelof plate and limit of generation of USTdefects

A third point regarding the trend of the componentsis to lessen dispersion of components of each ferroal­loy. The requirements set out from the customersto narrow the range of components are growingsteadily and for meeting the requirements, it isnecessary to reduce dispersion of each component atthe phase of molten steel. For this reason, it is ofurgent necessity to establish and introduce a speedyanalyzing technology of molten steel and in theimportant process, analysis by the operator and a

CO analysis

On-Siteanalysis

RH degasser

On-Siteanalysis(Slag metal)

Converter

I-------~Static & DynamicControl

Hot metal

Fig. 32. Schematic illustration of content narrow range control

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ever there may be some influence according to thechange in the addition process of ferroalloys.Namely, with the expansion of kinds of the steelproducts to be turned out or the detailed setting ofdelivery date, reduction in manufacturing lot isgoing forward and in consequence of it, there is atendency that the addition process of ferroalloys isput just before casting. It is of course a prerequisitethat the majority of ferroalloy be pre-added in therefining process that is the main in the manufacturingprocess. And for a final adjustment on the compo­nents or making the components according to theproperties, the ferroalloys may, for instance, beadded into a tundish in the case of continuous cast­ing and in fact, this method has been taken at someplants. In this case, there is a fear of impuritiescontained in ferroalloy, e.g., non-metallic oxide orsomething like that being taken into molten steel inits original form, which could exert a unfavorableinfluence on the molten steel as an inclusion. It istherefore desirable to decrease such impurities inferroalloy and at the same time, give a special treat­ment, preferably a surface coating for preventingdebasement of the quality in transport and storageafter having been manufactured. At any rate, thisargument is applied to some ferroalloys and it is anargument providing the possibility. It is judged thatthere is a need to propose the necessity timely andquantitatively even at the side of iron & steel mak­ing industry.

Summary

The performance required for steel product isbecoming higher and higher every year with thechanges in social environments and the technologi­cal progress in the associated industries. Torespond to the performance becoming higher, vari­ous process innovations have been undertaken sofar. In this course of progress, it has become neces­sary to improve the composition of componentsworking as the most important factor deciding theinherent character of steel and make a newcomponental design and the today's state has beenassured with a great contribution and cooperation ofthe ferroalloy industry. From now on, it is certainthat such movement that the required propertiesbecome higher is promoted more and more and itwould be necessary to continue the exchange ofinformation with closer cooperation between bothindustries.

In this report, examination and study have beenmade round the change of ferroalloy addition tech­nology which have taken place inevitably up to nowaccording to the above-described process innova­tions, and the influences on the ferroalloy specifica­tions and at the same time, with estimation ofchange(s) which might come about in future, furtherrequirements to the ferroalloy quality have been

discussed together including the expectations raisedform the iron & steel making industry. Though itcan not necessarily be concluded that such changecomes with a probability of 100%, it should be char­acterized to be an important theme to be discussed atboth industries hereafter as the possibility towardthe future.

ReferenceS

l)T. Shima : The 6th BSC (1990) 3, l.2)Shiraishi memorial seminar, (1994), Nov.3)K. Sasaki, H. Nakajima, M. Nose, Y. Takasaki,

and H. Okumura : The 66th Steelmaking Confer­ence of AIME (1983)

4)M. Sasabe : Nishiyama memorial seminar, (1992),May, 1

5)Shiraishi memorial seminar, (1994), Nov., 13.6)Technical report of Nisshin Steel Co., Ltd., (1972),

Jun., 78.7)Technical report of Kawasaki Steel Corporation,.21 (1989) 3, 168.

8)Nishiyama memorial seminar, (1985), 14l.9)Japan Ferrous Raw Materials Association:

CAMP-ISIJ, 6 (1993), 960.10)S. Satoh, T. Obara, M. Nishida and T. Irie:

Trans. ISIJ, 24 (1984), 838.