CONTENTS

12 ADVANTAGES OFVANADIUM STEEL FOR

REINFORCING BARS

4.

5. PRACTICAL FACTORS INMAKING AND ROLLING

VANADIUM STEELS

6. SPECIFICATIONS FORVANADIUM REBAR STEELS

PROPERTIES OFVANADIUM STEELS

7.

8. USES OF VANADIUM STEELREINFORCING BARS

Westgate House,9 Holborn,

London,EC1N 2NE.Telephone: 01-3531545.

Telex: 25816.

Vanadium High-Strength Reinforcing Bars. .

. Are easy to produce -

all vanadium alloying additions dissolve quickly and evenly and do not

segregate.

. Can be made by all steelmaking processes -

vanadium steel reinforcing bars are strong and ductile when made by anyestablished steelmaking process. High residual nitrogen contents have noeffect on ductility.

. Can be continuously cast or ingot castwith no problems of hot cracking.

. Can be reheated for rolling without cracking -

even if operational variables lead to several reheating and rolling cycles.

. Can be rolled to close tolerances and do not require low temperaturecontrol rolling -

they can be rolled without surface irregularities over a wide rangeof finishing temperatures.

. Can be produced with yield strengths of 550 N/mm2 (80,000 Ib/in2and possibly higher strengths if required in the future.

. Are made in sizes up to at least 56 mm diameter -

vanadium additions can be adjusted so that a constant high strength can bemaintained over the full range of bar diameters.

. Have good ductility and can be bent round tight radii without cracking -

vanadium steels pass bend and rebend tests of all national specifications.

. Do not require aluminium treatment -

they can therefore be continuously cast without the problems associatedwith viscous slags in the tundish.

. Can be made in qualities suitable for shop or site welding -

vanadium steel reinforcing bars are very suitable for prefabricatedstructures and for site joints where high stresses have to be carried,

. Do not lose strength when welded -

even in the heat affected zone, their strength is maintained.

. Resist fire damage -

their strength depends on precipitation and hence they have a greater

resistance to softening at high temperatures than some alternative

compositions.

Vanadium steel is widely used throughout the world for high-strengthreinforcing bars although the amount of vanadium added and the level ofother elements vary with conditions from one country to another .

When added to liquid steel,vanadium, in any form of alloy

" or compound, dissolves quickly and;;;;;;.. disperses rapidly. It can beadded to

~~ the furnace or to the ladle, and if, added as a last addition to a killed

steel, recoveries of over 90% can beobtained. Vanadium does notsegregate so that specifications canbe achieved with minimumalloying additions and maximumcontrol.

STEELMAKING

~

of the austenite which then resistsdeformation and can give rise tosurface or subsurface cracks. Thesecracks may be detected in thebillets or, if they occur beneath thesurface, they may not be founduntil the billet is rolled. In eithercase, the material is scrapped andproduction lost. In vanadium steels,however, the precipitates form atlower temperatures and the steelsdo not harden during the con-tinuous casting process; there-fore, vanadium steel billets arenot subject to this type of crackingand expensive loss of production isavoided.

Vanadium steels can be cast intoingots, for subsequent rolling intobillets or continuously cast directlyinto billets. Any continuous castingmachine can in fact be used,regardless of design. Even inmachines in which the radius of theapron is small, vanadium high-strength steels can be cast withoutthe risk of cracks occuring .in thebillet.

I n some other steels the pre-cipitation of compounds on whichtheir strength depends takes placeduring the hot bending stage of thecontinuous casting process. Thisprecipitation can lead to hardening

mill loading especially in the finalpasses. These loads can exceedthose for which the mill wasdesigned and extra passes may haveto be used or the minimumdiameter of the product increased.

The fact that the vanadiumstrengthening compounds com-pletely redissolve at the reheatingtemperatures also means thatvanadium steels can withstandthermal cycling. Thus, when, due tomill breakdowns or otheroperational irregularities, billetshave to be cooled and subsequentlyreheated, vanadium steels are un-affected and can be rolled withoutdanger of cracking. When alterna-tive elements are used the pre-cipitates may not completelyredissolve and they may be unableto withstand the thermal stressesproduced in the billets with theresult that cracks occur.

ROLLING When a vanadium steel is reheatedto normal rolling temperatures of1200-1250°C (2190-2280°F)the particles of vanadium carboni-trides which precipitate in the billetduring the later stage of cooling arecompletely dissolved and the steelbecomes soft. Reprecipitation ofthese vanadium compounds takesplace at relatively low temperaturesand after the rolling has beencompleted. The precipitation doesnot therefore strengthen the steelwhile it is being rolled and no extrastrain is put on the mill; as aconsequence the steels behave likecarbon steels and can be rolled inmills designed for ordinary qualitiesof reinforcing bars. I n other typesof steel, in which the precipitationstrengthening compounds form athigher temperatures, the steel canincrease in strength during rollingand this can give rise to excessive

"

Europe

Euronorm E80-69 and E81-69

Rebend Test:(bl A sample bar isbent 90° at room temperatureround a pin of the appropriatediameter, heated at 100 C for30 min, and straightened atR.T. through at least 20°.

Grade

112-18 mmI18-25mmI25-32mmI32.40 mm

FeB 40

FeB 50

392

490

56.9

71.1

490

569

3d

4d

6d

Bd

8d

10d

IOd

12d

12d

14d

71.1 I 1482.~ e I 12

(a) On bars up to 12 mm diameter only; (b) On bars over 12 mm diameter only; d ~ nominal bar diameter

United I<ingdom

Specification BS 4449-1969

(a) when specified an order 1.15 x Yield strength as measured d = nominal diameter of bar;

Germany

DIN 488, April, 1972

The specification also includes a fatigue test d = nominal bar diameter

USA

ASTMA615-72

1.41, 35.8 5 8d(a)75 517 75.0 690 100.0

14 18 1.693, 2.257 42.9, 57.3 5

(a) .90" only for 1.41 in (35.8 mm) diameter bars d = nominal diameter of bar

Japan

J I S G 3117, 1969

d = nominal bar diameter

6

The optimum composition of vanadium high-strength steel used forreinforcing bars is controlled bya number of factors. The most importantare: the type and design of the steel plant and rolling mill, the bar diameterand the local cost of other raw materials. In addition, the civil engineers,depending on their design philosophy, may require other properties apartfrom strength, and this will influence the c~~position. Design ph.ilosophies ~,

Ivary from one country to another and this IS one of the most Important :\reasons why different specifications have been developed for reinforcing ,\bars used for similar purposes in different parts of the world. ;\:~, I\, .,

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STANDARD QUALITIES (NON-WELDABLE)

The strength of ordinary quality ing temperatures which is some-reinforcing bars can be increased by times required when rollingadditions of vanadium. In these carbon-manganese steels.qualities the vanadium enhances the Never-the-less rolling conditionsstrength achieved with the cheaper which affect cooling rates andcommon elements, carbon, man- transformation products can beganese and silicon and the residual used together with vanadiumelements, chromium, copper and to additions and control of othera lesser extent molybdenum and elements, particularly manganesenickel. Additions of vanadium also and silicon, to achieve the combina-enable adequate strength to be tion of properties required by amaintained in large diameter bars of specification. The exact com-ordinary qualities where excessive position of the vanadium steels willlevels of carbon and manganese, vary from one works to another ""r ,

which would give rise to steel- and examples of different types of -;I(making and rolling problems, would high-strength vanadium steels are' I Xotherwise be required. The use of given in Table I. i!X;f:,~ :' , \V vanadium will depend on local mill Steel used in the manufacture of \i \ /

conditions and specification re- reinforcing bars is often made \ f "/,."

quirements. partly or completely from scrap ,.;/\1The low temperature of the purchased on the open market. This V'.

carbonitride precipitation in scrap invariably containsvanadium steels (below 700 °C) on chromium, copper or molybdenum,which their enhanced strength whic~ are not oxidised ?uri~g steel-

tdepends, means that they do not making. They remain In the ,

usua Ily require special rolling finished steel and add further to itsconditions such as control of finish- strength; up to 0.6% of these ,

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IIComposition and Properties of someTypical High-strength Reinforcing Bar Steels

(non-weldable Grades}1/1

~

~~

'~ ~

\1

,;{~~

1t

1.3

,1. t

1.3

1.2

0.350:25

0.35

0.25

0.350.28

0.32

0.40

< 25

> 25

< 25

30

<8

8

8

10.

410

410

490

500

59.6

54.5

71.1

72.5

0.02

0.05

0.03

To increasemaximum diameterTo improve

strengthTo improvestrength

';:1;'~,;

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;:'j,f1Jl ,

:~f.I".}:i".~

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~~* ASTM A615 -72, gauge is based on number of eighths of an inch included in nominal bar diameter. approximately

Vanadium steel reinforcing bars in foundationsof pump house of CEGB power statiol1-on'sle of Grain(Photograph courtesy John Laing and Son Ltd.

Erection of prestressed concrete bridge onLondon's East Cross Highway where vanadiumhigh-strength reinforcing bars have beenextensive/y used in the foundations(Photograph courtesy 8a/four 8eatty & Co. Ltd.a 8/CC company)

Construction of the second Dartford TunnelU.K., where vanadium high strengthsteel from the neighbouring Sheerness Steel Cois being used(Photograph courtesy of the Essex andKent County Councils, Mott, Hay & Anderson,(Consulting Engineers) Balfour Beatty & Co.Ltd., a BICC Company-Main Civil EngineeringContractor.)

8

Properties of Vanadium Steels (continued)

TABLE II Composition and Properties of some Typical Weldable grade

High-strength Reinforcing Bar Steels

Fig. 1. Effect of Vanadium on Yield Strength

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If-"zw!I:f-U)Q..Jw

NEE

zI

:1:t-(:JZwa:t-!/)O..1W>

* ASTM A615 -72, gauge is based on number of eighths of an inch included in nominal bar diameter * approximate

elements can be present in an all-scrap charge and these will contri-bute about 50 N/mm2 (7,000 Ib/in2 ). The vanadium addition can beadjusted to meet the specificationrequirements with varying levels ofresidual elements.

The composition selected by anymill will normally be that requiredto meet the relevant specificationat lowest cost under the conditionsimposed by the mill characteristicsand the local cost of other rawmaterials used in manufactur~.

0.0 0.05 0.10VANADIUM, %

Base Composition, %C , Mn Si N AI

0.24 1.25 0.45 0.007- 0.01

0.15

WELDABLE QUALITIES

Vanadium is particularly suitablefor imparting strength to weldablequalities of re1nforcing bar steelswhich have a lower maximumcarbon content than ordinaryqualities. These steels usuallycontain about 0.25%C and0.03-0.08% V. Typical propertiesof such steels are indicated in TableII. The vanadium is added to steelsfor bars of all diameters to com-pensate for the loss in strengthwhich results from the lowercarbon content. In combinationwith higher manganese, or nitrogenadditions, vanadium can be used toproduce even higher strengths.

In most building structurestoday, high-tensile reinforcing barsare employed, usually with yieldstrengths of 410-460 N/mm2(60,000--67,000 Ib/in2). Thesesteels contain 0.03 to 0.08%vanadium depending on thediameter and mill conditions, buteven higher strength steels up to700 N/mm2 ( 100,000 Ib/in2) canbe produced as indicated in Fig. 1.

0 0.05 0.10VANADIUM, %

Base Composition, %

C Mn Si N AI

0.24 1.25 0.45 0.007 0.01

Fig. 3. Effect of Nitrogen with Vanadiumon Yield Strength

100

90

80 ~S~

70 ooo

60 ~

I-f-

5O(:JZw

400:f-(/)

30 o-"w

20 >=

The U.T.S. is also raised byvanadium additions as shown inFig. 2. These figures refer to 18 and30 m m bars but with largervanadium additions or a smallmodification to the other elements,such as manganese and silicon, theirproperties can be obtained in barsof 40 mm diameter and even up to56 mm.

Nitrogen increases the yieldstrength of steels and when addedwith vanadium provides a higherstrength than when vanadium isadded alone. This is indicated in

Fig.3. Alternatively, nitrogen canbe substituted for part of thevanadium. The choice betweenhigher vanadium and vanadium plusnitrogen depends on local con-siderations including comparativecosl of vanadium alloys and thecost of adding the nitrogen.

As with ordinary qualities thestrength is affected by the residualelements and the exact compositionof the steel will be dependent onthe quality of the scrap used.

DUCTI LITY

All specifications for reinforcingbars for building constructioncontain rigid clauses for ductility,although the actual requirements

vary from one specification toanother. Elongation in the tensiletest is used as the measure ofductility, and minimum levels are

0.06% V 0.12% V 0.17% VBase Composition, %

C Mn Si AI

0.24 1.25 0.45 0.01

9

Properties of Vanadium Steels (continued)

strengthened reinforcing barsmaintain adequate ductility to meetall specifications, even in largediameter bars which are being askedfor increasingly by engineers. Theeffect of vanadium on elongation, isindicated in Fig. 4.

specified to ensure that, in cases ofoverstraining in service, the con-crete will crack before the .steelfails in tension.

As the strength of the steelincreases, the ductility tends todecrease, but vanadium-

BENDABILITY

Reinforcing bars used in buildingconstruction are invariably requiredto pass a test to guarantee that theycan be bent on site without crack-ing. Such tests involve bending therolled bar either through an angleup to 180°, or through a smallerangle followed by an aging treat-ment of up to one hour ~t atemperature up to 250 °c and apartial straightening operation atroom temperature.

The rebend tests are designed toreveal any susceptibility to strainaging which can be caused by thepresence of free nitrogen whichtends to be at a high level in electricmelted steels. Vanadium steels,however, are free from thissusceptibility on account of theformation of vanadium carboni-trides which remove the harmfj,JIfree nitrogen from solution in thesteel.

WELDABILITY

There is an increasing demand for minimum heat input of 3 kJ/mmreinforcing bar steels which can be was sufficient to prevent crackingwelded. Many of the bars used for in the heat affected zone whenstructures such as columns in welding a 30 mm diameter bar.bridges, or piles in building founda- The demand for weldable rein-tions, were formerly assembled on forcing bars for establishedsite and tied together; these are application is increasing steadily innow fabricated in shops by welding Europe and North America, butand sent to the site for erection. other applications such as their use An example of a lap-weld joint between 30mm

Extensive tests have been carried in continuous roadways and run- diameter reinforcing barsout on the production of lap welds ways are also being contemplated.in reinforcing bars made from steels The change to welding in place of " f f v d" D t " l "

tFig. 4. E fact O ana Ium on uc I I ycontaining up to 0.2% vanadium the traditional method of Jointing

using the manual metal arc welding by tying in this application could

process. I n these tests, in which the save steel and reduce costs. Greater

welds were made under conditions use of the weldable vanadium steel

of restraint, basic coated electrodes qualities is therefore expected in

were used. It was established that a the future.

~

zoi=~~z0.Jw

0.150.0 0.05 0;10

VANADIUM, %

Base Composition. %

C Mn Si N At

0.24 1.25 0.45 0.007 0.01

An example of 25mm diameter reinforcing bars

subjected to severe bending in a power station

foundation

(Photograph courtesy of John Laing & Son Ltd. )

10

An example of continuous cast concrete whichis being used increasingly in the U.S.A. and

E.urope for roadways and runways for smootherrunning and longer life

Laying high strength reinforcing bars for whichthe vanadium strengthened steels are suitable

Pouring concrete over high strength reinforcingbars which will withstand the high internal

stresses produced when the concrete of the longsmooth section of the roadway sets

(Photographs by courtesy of ConcreteReinforcing Steel Institute Chicago)