GB2086432A - Boron addition to steel with improved recovery - Google Patents

Abstract

An additive for introducing boron into steel comprises boron (e.g. ferroboron powder) protectively contained within encapsulating material (e.g. an elongate steel tube) which is capable, when the additive is introduced into molten steel, of releasing the boron into the molten steel, without having a deleterious effect on the steel; the additive also includes aluminium (e.g. as aluminium powder) in such a form that, when the additive is introduced into the molten steel, aluminium is released into the steel and so protects the boron from oxidation. Preferably, the aluminium to boron weight ratio is in the range from 5:1 to 20:1. When the additive is introduced into molten steel in a continuous casting operation, it allows boron to pass into the steel with high and predictable levels of boron recovery; the aluminium serves as an anti-fading agent and can advantageously also provide grain refining, without excessive amounts of aluminium being employed.

Description

SPECIFICATION The manufacture of steel of improved hardenability The hardness of steel can be increased by heating followed by quenching. When heated steel is cooled rapidly by quenching with a coolant, such as oil, the increase in hardness is greatest at the surface, because that is where the rate of cooling is greatest.

In the interior, cooling occurs more slowly and the increase in hardness is correspondingly less.

Certain elements, including boron, chromium, molybdenum and nickel, make the increase in hard ness less dependent upon the rate of cooling. When one or more of these elements are present in the steel in appropriate amounts, the increase in hard ness at the interior upon quenching is greater than when these elements are absent. Consequently, there is less difference between the hardness in the interior and the hardness at the surface and also the mean hardness is greater. Elements which enhance the ability of such heat treatment to increase the hardness in the interior are said to impart hardena bility to the steel.

Boron is widely used to impart hardenability to steel, because it is cheaper than alternative elements, e.g. chromium, nickel and molybdenum, because it is effective in small amounts and because it does not reduce the ductility, formability or machinability of the steel. Boron is usually added to the steel in an amount within the range from 10-40 parts per million by weight (ppm), typically 25 ppm.

A problem associated with the use of boron is its high affinity for oxygen and nitrogen. When boron is added to a steel melt, it combines with oxygen and nitrogen in the melt and in the atmosphere and, when the melt solidifies, only the remaining uncombined boron is able to influence the hardenability of the steel. This loss of effect as a result of combination is known as fading. The extent of fading depends on the time which elapses between addition of the boron and solidification of the melt: the longer the time, the greater the fading.

The oxygen content of a steel melt is typically 40-70 parts per million by weight (ppm) for steel made by an are furnace process and 800-1500 ppm for steel made by a basic oxygen process. The nitrogen content is typically 60-140 ppm for steel made by an arc furnace process and 30 ppm for steel made by a basic oxygen process. These contents are comparable to or higherthan the amount of boron which is to be added and, in orderto minimise fading, free oxygen and nitrogen must therefore be removed from the melt before boron is added.

Steel is usually deoxidized in two stages, first by the use of silicon and/or manganese (e.g. as ferrosilicon, ferromanganese or silicomanganese) which remove most of the oxygen, and then by aluminium (as metallic aluminium orferroaluminium) or calcium (as metallic calcium or calcium-silicon) which remove essentially all the remainder. Manganese and rare-earth metals may also be used as deoxidisers. These deoxidisers may be added to the furnace or to the ladle. Denitriding is usually done by means of titanium or zirconium, e.g.

by adding ferrotitanium to the ladle. With all of these additives, those which serve primarily for deoxidising also remove some nitrogen and those which serve primarily for denitriding also remove some oxygen.

Boron is usually added to the steel by adding ferroboron or complex ferroalloys containing boron to the ladle, afterthe deoxidising and denitriding additions. These complex ferroalloys are typified by our products containing boron, aluminium, titanium and silicon, which may also contain zirconium, and which are marketed under our Registered Trademark BATS. BATS alloys have the advantage that the boron is to some extent protected frond oxygen by their aluminium and silicon contents and from nitrogen by their titanium and (if present) zirconium contents. The protection given by the titanium and by the zirconium (if present) sometimes makes the preceding addition of a denitriding agent unnecessary.

Despite the precautions which can be taken in the ways just described to eliminate free oxygen and free nitrogan from the melt, there is still a risk of fading through the combination of boron in the melt with oxygen and nitrogen in the atmosphere. In order to minimise this risk, it is customary before adding the boron to add an amount of deoxidising and denitriding agents which is sufficient not only to eliminate the free oxygen and nitrogen dissolved in the melt, but also to provide a residual content of unreacted deoxidising and denitriding agents. For example, if aluminium is used to remove oxygen and titanium to remove nitrogen, typical residual amounts aimed for are 0.02% aluminium and 0.04% titanium (all percentages herein are by weight).After the boron has been added, any oxygen and nitrogen absorbed from the atmosphere combine with the residual aluminium and titanium, rather than with the boron, and fading does not occur until the remaining amounts of uncombined residual aluminium and titanium are insufficient to protect the boron.

A residential amount of aluminium is desirable in any case to provide grain-refining. When casting by the ingot route, a residual aluminium content of 0.02% to 0.04% is usually aimed for.

The manufacture of steel by continuous casting is gradually replacing manufacture by the traditional ingot casting method. In continuous casting, molten steel is poured from the ladle via a tundish into the tops of one or more water-cooled open-bottomed moulds and strands of solidified steel emerge at a steady rate from the bottoms of the moulds. The tundish has one outlet nozzle for each mould. The melt falls freely from the ladle into the tundish and, whilst it is possible for the tundish nozzles to be submerged below the final melt level in the moulds, it is usual for the melt to fall freely from the nozzles into the moulds. Thus, in flowing from the ladle via the tundish into any particular mould, the metal usually forms two successive freely-falling streams.The presence of two freely-falling streams, rather than one freely-falling stream which is the practice in traditional ingot casting, causes more atmospheric oxygen and nitrogen to dissolve in the melt through absorption at the exposed surfaces of the metal and through turbulent mixing of air into the freely-falling streams.

According to the present invention, an additive for introducing boron into steel comprises boron (as herein defined) protectively contained within an encapsulating material and aluminium (as herein defined) whereby, on introduction of the additive into molten steel, the encapsulating material serves to release boron into the molten steel without itself having a deleterious effect on the steel and aluminium is released into the molten steel to protectthe boron from oxidation.

For the avoidance of doubt, it is pointed out that the term "boron" as used herein in relation to the additive of the invention means any material which is capable of providing elemental boron when the additive is introduced into molten steel. For example, the boron may be in the form offerroboron (which is preferred) orit may be substantially pure boron.

Again, for the avoidance of doubt, the term "aluminium" as used herein in relation to the additive of the invention means any material which is capable of providing elemental aluminium when the additive is introduced into molten steel. For example, the aluminium may be in the form of unalloyed aluminium or in the form of an alloy, such as ferroaluminium or titanium-aluminium.

The present invention also commrehends a method of continuously casting steel, wherein an additive in accordance with the invention is introduced into the molten steel (which is preferably as fully deoxidised as possible) within or on entry into a continuous casting mould. It has surprisingly been found that, by using the additive in this manner, it is possible to incorporate boron into continuously cast steel with high and predictable levels of boron recovery, while the aluminium serves as an antifacing agent (with the added advantage that the aluminium can also provide grain refinement), without employing an excessive amount of aluminium.

Preferably, the additive is introduced into the molten steel in such a manner that it becomes substantially uniformly dispersed in the molten steel; this ensures that the boron content of the steel will be uniform. Uniform dispersal of the additive can be achieved in a convenient manner by introducing the additive into the molten steel at or adjacent the surface of the molten steel in the continuous casting mould.

In general, in the additive of the invention, the weight ratio of the aluminium (measured as the element) to the boron (measured as the element) is preferably in the range from 5:1 to 20:1; more preferably the ratio is in the range from 8:1 to 12:1, a par- ticularly suitable weight ratio being 9:1. An excessively high aluminium-to-boron ratio would lead to difficulty in meeting the specified aluminium content of the steel, thus giving an unacceptable risk of breakouts. An excessively low aluminium-to-born ratio would not protect the boron sufficiently from oxygen and in general would give a less than desir able level of grain refinement.

It is most preferable for the boron of the additive to be present in powder form.

The additive according to the invention may comprise one or more of titanium, zirconium, one or more rare-earth metals, calcium and magnesium, all of which can serve the same purpose of protecting the boron from oxygen. All of these metals, particu larlytitanium and zirconium, also give protection against nitrogen, which is a desirable feature in some applications. It is preferable to have at least a certain amount of aluminium in the additive for grain-refining; if the additive does not contain enough aluminium, it can be necessary for this reason to feed a separate aluminium source, e.g. in the form of a wire, into the steel melt as well.

In a preferred embodiment ofthe additive ofthe invention, the encapsulating material comprises a hollow rod (of cylindrical section, for example) within which at least the boron is contained. The rod may be of steel and, in order to minimise the amount of oxygen available for causing boron fading, is pre forablyfullydooxidesed. In such an embodiment, the aluminium preferably comprises particles contained within the hollow rod, the particles preferably being of powder form.

Thus, a specific preferred form of the additive of the invention has the form of a mixture of ferroboron powder and aluminium powder encapsulated in a preferably cylindrical and preferably fully deoxidised mild steel sheath, in the form of a wire-like or rodlike article.

In use, an additive in accordance with the abovedescribed preferred embodiment can easily be fed at a controlled rate directly into the top of the continuous casting mould. The particle size of the powder filling is typically 50 mesh (all mesh sizes herein are British Standard screen sizes) and the filling itself typically comprises 30%-50% of the total weight of the additive. In fact, if desired, the mesh size can be somewhat larger. For example, with a 3.2 mm wire additive, the particles can be up to 40 mesh, and with a 5 mm wire additive the particles can be up to 20 mesh.

By making the additive sufficiently thin (typically 5 mm in diameter) and by feeding it at a sufficiently slow rate (typically 5 metres per minute), complete dissilution ofthe additive can be ensured, whilst allowing an adequate amount of boron to be added.

The amount of aluminium in the core of the additive can be adequate to protect the boron during the short time available for fading, whilst giving an aluminium addition rate of only 0.04% or less, i.e. the aluminium content of the wire- or rod-like additive can be adjusted so that the maximum permissible amount of aluminium can be incorporated into the steel. By controlling the feed rate of the additive according to the rate of flow of steel into the continuous caster, the amount of free boron in the steel can be controlled accurately.

This accurate control is an important advantage over the traditional method of adding boron to the ladle, since the loss of boron following a ladle addition cannot be predicted accurately. Addition of boron to the ladle gives boron recoveries of about 50% as free boron in the final steel, whereas the free boron recoveries attained by using the additive of the invention can be about 80% or more, depending on the casting conditions.

The above-described wire- or rod-like form of additive is preferably made by bending a steel strip, preferablyfully-deoxidised mild steel, about its longitudinal axis to give it a channel-like configuration of U-shape in cross-section, depositing a mixture of ferro-boron powder and aluminium powder evently along the channel and then crimping the free edges of the channel together so as to form a seal. The wire can then be forced through a die which reduces its diameter slightly, giving it a uniform cylindrical shape and consolidating the powder core.

In an example, a wire of 3.2 mm diameter weighing 37 grams per metre comprised a fully deoxidised mild steel sheath with a filling of ferroboron powder and aluminium powder. The overall composition of the filled wire was 24.2% aluminium and 2.7% boron, the balance being iron. The throughput of the continuous caster was 231 kilograms of steel per minute at a casting speed of 1.8 metres per minute. The wire was fed into the caster at speeds ranging from 2.5 to 10 metres per minute, corresponding to boron addition rates ranging from 11 to 43 ppm. The free boron recovery was 85% In the above example, the weight ratio of aluminium to boron was 9:1.

In a wire according to the invention as described above, some or all of the aluminium in the filling can be replaced by, for example, one or more of titanium, zirconium, the rare-earth metals, calcium and magnesium. It is preferable for at least a certain amount of aluminium to be present in the core for grain-refining; if the core does not contain enough aluminium, a separate aluminium wire may need to be fed into the steel melt as well.

Instead of unalloyed aluminium, the aluminium in the core may be present as an aluminium alloy compatible with the desired overall composition of the core, e.g. as ferroaluminium or titanium-aluminium.

As indicated above, it is desirable for the aluminium (or other metals or alloys in the core) to be present in powder form, since this permits intimate admixture of the aluminium with the ferroboron. However, it is possible for the aluminium (or alternative metals or alloys) to be present in different physical forms, e.g. as small turnings or as a continuous wire running longitudinally through the core; these different forms are within the scope of the invention.

It is possible in accordance with the invention for the encapsulating material itself to be aluminium (as herein defined); in such an embodiment, the additive can be of more "concentrated" form, as the encapsulating material is an active, rather than an inactive, ingredient. In a preferred form of such an embodiment, the additive comprises a hollow rod of aluminium (as herein defined) within which at least the boron is contained. For example, the additive may be in the form of a wire comprising a ferroboron powder filling disposed within an aluminium sheath.

Such a wire has the advantage of a greater amount of boron per unit length than a corresponding steelclad wire, for example. However, this product is sometimes found to be disadvantageous, because of the comparatively low melting point of aluminium; the aluminium sheath melts above the surface of the molten steel and releases the filling prematurely.

This problem can be avoided, however, if the wire is protected from the heat of the molten steel as it is being fed into it.

From the foregoing, it will be appreciated that use of the additive according to the invention can give good recovery of free boron, typically 85% or even more, and allows accurate control of the amount of boron in the steel. Continuously-cast steel which has been treated with the additive can have a hardenability at least comparable to the levels which can be achieved by ingot casting. This amount of hardenability cannot be achieved in continuously-cast steel produced by adding boron in the traditional way.

Owing to the inherent protection of the wire from oxygen and the short time available for fading, elaborate precautions to exclude the atmosphere from the molten steel are not necessary. In some applications, where shrouds of inert gas are normally used around the metal streams, the shrouds can be eliminated if boron is added by means of the additive.

Claims (25)

1. An additive for introducing boron into steel, which comprises boron (as herein defined) protectively contained within an encapsulating material and aluminium (as herein defined), whereby, on introduction of the additive into molten steel, the encapsulating material serves to release boron into the molten steel without itself having a deleterious effect on the steel and aluminium is released into the molten steel to protect the boron from oxidation.

2. An additive according to claim 1, wherein the weight ratio of aluminium to boron (both measured as the element) is in the range from 5:1 to 20:1.

3. An additive according to claim 1 or 2, wherein the weight ratio is in the range from 8:1 to 12:1.

4. An additive according to claim 3, wherein the weight ratio is 9:1.

5. An additive according to any preceding claim, wherein the boron is in powder form.

6. An additive according to any preceding claim, wherein the boron is present as ferroboron.

7. An additive according to any preceding claim, wherein the aluminium is in unalloyed form.

8. An additive according to any of claims 1 to 6, wherein the aluminium is present as an alloy.

9. An additive according to claim 8, wherein the aluminium is present as ferroaluminium or as titanium-aluminium.

10. An additive according to any preceding claim, which contains at least one of titanium, zirconium, rare-earth metals, calcium and magnesium.

11. An additive according to any preceding claim, which comprises a casing of the encapsulating material in which at least the boron is contained.

12. An additive according to claim 11, which comprises a hollow rod of the encapsulating material.

13. An additive according to claim 12, wherein the rod is of cylindrical cross-section.

14. An additive according to claim 12 or 13, wherein the rod comprises steel.

15. An additive according to claim 14, wherein the steel is fully deoxidised.

16. An additive according to any of claims 11 to 15, wherein the aluminium comprises particles contained within the casing.

17. An additive according to claim 16, wherein the particles are in powder form.

18. An additive according to any of claims 11 to 15, wherein the aluminium comprises a wire of aluminium (as herein defined) running longitudinallywithin the casing.

19. An additive according to any preceding claim, wherein the encapsulating material is aluminium (as herein defined).

20. An additive for introducing boron into steel, for instance into a continuous casting mould during the continuous casting of steel, substantially as described in the foregoing Example.

21. A method of continuously casting steel, wherein an additive in accordance with any preceding claim is introduced into the molten steel within or on entry into a continuous casting mould.

22. A method according to claim 21, wherein the additive is introduced into the molten steel so as to become substantially uniformly dispersed therein.

23. A method according to claim 21 or 22, wherein the additive is introduced at or adjacent the surface of the molten steel.

24. A method according to claim 21, substantially as herein described.

25. Continuously cast steel, when made by a method according to any of claims 21 to 24.