A method of forming steel

Abstract

A method of forming steel, in which consecutive batches of an iron carrier comprising phosphor is loaded into an electric arc furnace, EAF, together with a slag former comprising at least one of CaO, SiO2 and MgO. Amount of added slag former is based on content of phosphor, CaO, SiO2 and MgO in the second batch. After melting of the first batch, the molten iron carrier is removed from the EAF and a subsequent batch is loaded into the EAF. Depending on the content of phosphor, CaO, SiO2 and MgO in the second batch, slag former is added and a part of the slag is removed before the melting of the second batch.

Description

[0001] A method of forming steel

[0002] TECHNICAL FIELD

[0003] The present invention relates to a method of forming steel, in which consecutive batches of an iron carrier comprising phosphor is loaded into an electric arc furnace, EAF, together with a slag former comprising at least one of CaO, SiO2 and MgO. After melting of a batch, the molten iron carrier is removed from the EAF and a subsequent batch is loaded into the EAF. An iron carrier may be referred to as a material that to a major part is comprised by iron.

[0004] BACKGROUND

[0005] In connection to the production of steel an iron carrier may be loaded into an electric arc furnace together with a slag former. The iron carrier typically comprises phosphor, which needs to be removed from the iron carrier to a large extent. The task of the slag former is to enable the removal of the phosphor from the iron carrier as well as generating a slag that forms a heat insulating layer on top of the melt in the EAF, wherein the slag layer also functions as a conductor of electric current delivered by electrodes that are introduced into the EAF and by means of which the melting of the iron carrier is actually achieved. Typically, the slag former comprises CaO, which reacts with the phosphor to form P2O5, and SiO2 and MgO for lowering the melting temperature of the CaO and thereby enable the formation of P2O5 at sufficiently low temperature conditions. The composition of the slag former and the amount of added slag former is normally such that it has sufficient amounts of the respective ingredients to ensure that all of the above-mentioned requirements are met.

[0006] The molten iron carrier that is removed from the EAF is then possibly subjected to further addition of alloying elements for the final forming of a steel.

[0007] In connection to the removal of a batch of the molten iron carrier from the EAF, the slag that has been formed during the melting process of that batch, is also removed. The deposition of slag is substantial. It is therefore requested that the amount of deposited slag could be reduced. It is thus an object of the present invention to present a method that reduces the amount of deposited slag in connection to the forming of steel.

[0008] SUMMARY

[0009] The object of the invention is achieved by means of a method comprising the steps of: a) melting, in a first melting process, a first batch of an iron-carrier together with a slag former in an electric arc furnace, EAF, and removing the molten iron carrier from the EAF, b) providing a second batch of an iron-carrier, comprising at least 90 wt.% iron, having a mass m and a content of phosphor p1 , c) adding the second batch of the iron-carrier to the EAF, d) determining a required amount of slag that needs to be formed in the EAF during melting of the second batch of the iron-carrier for the purpose of

[0010] -achieving a predetermined heat insulation caused by the slag during melting of the second batch of the iron carrier in presence of the slag former, and

[0011] -generating a slag layer on top of the iron carrier during melting thereof, which slag layer has a thickness that allows ends of electrodes of the EAF to be received in the slag layer and for conducting electric current from the electrodes to the molten batch of the iron carrier, e) determining the mass of the slag remaining in the EAF from the first melting process, f) determining, with regard taken to the mass of the remaining slag from the first melting process, a required amount of a slag former comprising CaO, SiO2 and MgO to be added in order to

[0012] -obtain said heat insulation during melting of the second batch,

[0013] -generate said slag layer during the melting of the second batch,

[0014] -and, with regard taken to the mass of the second batch of the iron-carrier and a content of CaO, SiO2, MgO and phosphor in the second batch of the iron-carrier, to reduce the amount of phosphor in the second batch of the iron- carrier to a predetermined level during melting of the second batch of the iron-carrier, g) determining a maximum amount of slag allowable in the EAF during melting of the second batch of the iron-carrier, h) determine the sum of the remaining slag and the new slag that will be formed during the melting of the second batch of the iron-carrier due to the addition of said required amount of slag former, and i) if said sum is above the maximum amount of slag allowable in the EAF during melting of the second batch of the iron-carrier, removing so much slag from the slag remaining from the first melting process that the sum of the maintained slag and the new slag that will be formed during the melting of the second batch of the iron-carrier is sufficient for obtaining said heat insulation and for the generation of said slag layer and that said sum is below said maximum allowable amount, and j) adding said required amount of the slag former to the EAF, and k) melting the second batch of the iron-carrier by means of an electric arc in the presence of the added slag former.

[0015] The maximum allowable amount of slag during melting of the second batch of the iron-carrier, is determined with regard taken to the volume that will be occupied in the EAF by the second batch of the iron-carrier and with regard taken to the inner volume of the EAF, such that the sum of the volume occupied by the iron-carrier and the volume occupied by the slag is less than the inner volume of the EAF or a predetermined percentage of that volume.

[0016] The method thus takes into consideration that the melting of a batch of an iron carrier will result in a slag having further functionality. The slag still has the ability of providing both thermal insulation and electrical conductivity, and may therefore be reused. However, it is assumed that the content of CaO in the slag from the foregoing melting process is low or negligible, and that, therefore, at least CaO has to be added in connection to the melting of the second batch, in order to enable reduction of the content of phosphor in the iron-carrier to said predetermined level. The iron-carrier of the second batch may have a content of slag former, typically CaO, SiO2 and MgO, and this content should be taken into consideration when calculating / determining how much slag former that needs to be added to the second batch in order to reduce the amount of phosphor therein to the determined acceptable level. Possibly, also the content of CaO in the slag remaining from the foregoing melting of the first batch may be considered when determining the necessary amount of CaO to be added in connection to the melting of the second batch. In such a case, the content of CaO in the remaining slag is measured. The content of phosphor in both slag and iron-carrier is preferably measured during the melting process. The invention thus reduces the amount of slag that is deposited- removed after each consecutive melting process.

[0017] Preferably, the method steps are repeated for consecutive melting processes, wherein each new batch becomes the second batch according to the inventive idea and each foregoing batch becomes the first batch.

[0018] The second batch of the iron carrier may have a different composition than the first batch, or it may have the same composition.

[0019] According to some embodiments, each of the slag former added to form a slag during melting of the first batch of iron-carrier and the slag former added to form a slag during melting of the second batch of iron-carrier consists of one or more of CaO, SiO2 and MgO and unavoidable impurities.

[0020] According to some embodiments, the amount of and composition of the slag former added to the second iron carrier and the content of CaO, SiO2 and MgO in the second batch of iron-carrier is equal to or more than the amount of CaO, SiO2 and MgO required to obtain said heat insulation during melting of the second batch, generate said slag layer during the melting of the second batch, and reduce the phosphor in the second batch of the iron-carrier to a predetermined level during melting of the second batch of the iron-carrier. In other words, the composition of the slag former is adapted to the amount and composition of the slag former already present in the second batch of iron carrier. If, for example, the content of SiO2 and MgO in the second batch of iron carrier is sufficient for the needs (basicity requirement) in the second batch, but the content of CaO is insufficient for the reduction of the phosphor therein, the added slag former may only consist of CaO. In other words, the invention comprises the step of adapting the composition and amount of added slag former to the second batch such that the above-mentioned requirements are met.

[0021] According to some embodiments, the slag former added to form a slag during melting of the first batch of iron-carrier has the same composition as the slag former added to form a slag during melting of the second batch of iron-carrier.

[0022] According to some embodiments, the slag former added to form a slag during melting of the first batch of iron-carrier has a different composition than the slag former added to form a slag during melting of the second batch of iron-carrier. This may be seen as tailoring of the composition of the added slag former with regard to the specific composition of the new batch of iron-carrier to be melted and the content of the maintained slag from the melting of the foregoing batch.

[0023] According to some embodiments, the first batch of an iron-carrier has the same composition as the second batch of an iron carrier.

[0024] According to some embodiments, the first batch of an iron-carrier has a different composition than the second batch of an iron carrier.

[0025] According to some embodiments, the first batch of an iron-carrier comprises at least 80 wt.%, preferably at least 95 wt.% sponge iron produced by direct reduction of iron ore with a reduction gas comprising at least 90 atomic% hydrogen gas. Preferably, the degree of metallisation of the sponge iron is above 90%.

[0026] According to some embodiments, the second batch of an iron-carrier comprises at least 80 wt.%, preferably at least 95 wt.% sponge iron produced by direct reduction of iron ore with a reduction gas comprising at least 90 atomic% hydrogen gas.

[0027] According to some embodiments, if the content of each of CaO, SiO2 and MgO in the remaining slag is below a respective predetermined level, all slag is removed before the next batch of iron carrier is loaded to the EAF. When the levels of each of the slag former elements is below a predetermined respective level, this means that the remaining slag has too little further functionality as a slag former to motivate the maintaining thereof. The method also takes into consideration the maximum amount of slag formed must not go above a predetermined level.

[0028] Preferably, the composition of the slag remaining from the melting of the first batch should be analysed or determined, in order to determine whether the slag is functional in terms of its ability of insulating heat and conducting electricity. The step of determining a required amount of slag that needs to be formed in the EAF during melting of the second batch of the iron-carrier should thereby take into consideration the composition of the remaining slag, and the composition of the slag that will be formed as a result of the addition of further slag former, such that the sum of slag will be sufficient to guarantee that the aimed function of the slag formed during the melting of the second batch in terms of thermal insulation and conduction of electricity is achieved.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Fig. 1 is a flow chart showing the steps of an embodiment of the present invention

[0031] DETAILED DESCRIPTION

[0032] An embodiment of the present invention is disclosed in fig. 1 . The method comprises the following steps: a) melting, in a first melting process, a first batch of an iron-carrier together with a slag former in an electric arc furnace, EAF, and removing the molten iron carrier from the EAF, b) providing a second batch of an iron-carrier, comprising at least 90 wt.% iron, having a mass m and a content of phosphor p1 , c) adding the second batch of the iron-carrier to the EAF, d) determining a required amount of slag that needs to be formed in the EAF during melting of the second batch of the iron-carrier for the purpose of -achieving a predetermined heat insulation caused by the slag during melting of the second batch of the iron carrier in presence of the slag former, and

[0033] -generating a slag layer on top of the iron carrier during melting thereof, which slag layer has a thickness that allows ends of electrodes of the EAF to be received in the slag layer and for conducting electric current from the electrodes to the molten batch of the iron carrier, e) determining the mass of the slag remaining in the EAF from the first melting process, f) determining, with regard taken to the mass of the remaining slag from the first melting process, a required amount of a slag former comprising CaO, SiO2 and MgO to be added in order to

[0034] -obtain said heat insulation during melting of the second batch,

[0035] -generate said slag layer during the melting of the second batch,

[0036] -and, with regard taken to the mass of the second batch of the iron-carrier and a content of CaO, SiO2, MgO and phosphor in the second batch of the iron-carrier, to reduce the amount of phosphor in the second batch of the iron- carrier to a predetermined level during melting of the second batch of the iron-carrier, g) determining a maximum amount of slag allowable in the EAF during melting of the second batch of the iron-carrier, h) determine the sum of the remaining slag and the new slag that will be formed during the melting of the second batch of the iron-carrier due to the addition of said required amount of slag former, and i) if said sum is above the maximum amount of slag allowable in the EAF during melting of the second batch of the iron-carrier, removing so much slag from the slag remaining from the first melting process that the sum of the maintained slag and the new slag that will be formed during the melting of the second batch of the iron-carrier is sufficient for obtaining said heat insulation and for the generation of said slag layer and that said sum is below said maximum allowable amount, and j) adding said required amount of the slag former to the EAF, and k) melting the second batch of the iron-carrier by means of an electric arc in the presence of the added slag former. EXAMPLES

[0037] Lp 60 mass%P in slag / mass%P in steel at equilibrium

[0038] DRI%P 0.0373 mass% P in DRI mDRI 5 000 kg optional, depending on requested charge size yield DRI — steel 0,93 required slag 600 kg experimentally determined slag from DRI 0.057 steel remaining 5 000 kg

[0039] The following equation is used

[0040] (mDRI * DRI%P — mSteel * Steel%P)

[0041] Mtapped slag =

[0042] Slag%P wherein

[0043] Mtapped slag = the mass of slag to be removed mDRI = the mass of the DRI (iron-carrier)

[0044] DRI%P= percentage of P in DRI mSteel= the mass of steel removed from the EAF after melting of DRI

[0045] Steel%P= percentage of P in the steel

[0046] Slag%P= percentage of P in slag

[0047] 100mPtot=Slag%P*mSlag+steel%P*mSteel

[0048] 100mPtot=mDRI*DRi%P mSteel=mDRI*yield Steel — DRI Equilibrium: Lp=Slag%P / Steel%P=31

[0049] Charge 1

[0050] 10 000 kg DRI is added to an EAF. The DRI comprises at least 100 wt.% sponge iron produced by direct reduction of iron ore with a reduction gas comprising at least 90 atomic% hydrogen gas. The metallisation degree of the DRI is above 90%.

[0051] The DRI contains approximately (in wt.%)

[0052] Phosphor, P 0.037

[0053] CaO 1.0

[0054] SiO2 1.0

[0055] MgO 1.0 mDRI 10 000 kg mSlag 881 kg (according to equations and start and target content of P in steel)

[0056] Accordingly, 881 kg slag has to be produced for each 10 000 kg DRI, which equals to 88,1 kgSlag / ton DRI.

[0057] The content of CaO, SiO2 and MgO in the second batch of iron-carrier is equal to or more than the amount of CaO, SiO2 and MgO required to obtain sufficient heat insulation during melting of the first charge (batch) and to generate a sufficiently thick slag layer during the melting of the first charge, but not enough to reduce the phosphor to a predetermined level. Accordingly, the added slag former (not carried by the DRI) only needs to consist of CaO.

[0058] The contents of P in steel and slag respectively are:

[0059] Steel%P 0.006 mass%

[0060] Slag%P 0,36 mass% The slag emanating from the DRI itself is 571 kg (57.1 kg / ton DRI)

[0061] Slag needed to be added is thus 31 kg / ton DRI

[0062] Since minimum amount of slag is 600 kg, 281 kg of slag is tapped off, and 4 300 kg steel is removed (leaving 5 000 kg steel in the EAF to the next batch)

[0063] Charge 2

[0064] DRI and slag former of the same composition as for charge 1 is used, Accordingly, added slag former only needs to contain CaO. mSlag start 600 kg

[0065] Slag%P start 0.36 mass% m Steel start 5 000 kg Steel%P 0.006

[0066] DRI added 5 000 kg

[0067] Added slag former 155 kg (calculated amount needed for 5 000 kg DRI)

[0068] The second charge is melted in the EAF

[0069] Steel%P 0.006 mSlag 1040,7 kg m Steel 9650 kg

[0070] 440.7 kg slag has been produced

[0071] 440.7 kg slag is tapped off from the EAF

[0072] 4650 kg steel is tapped off from the EAF

[0073] Remaining is:

[0074] 600 kg slag

[0075] 5 000 kg Steel, having Steel%P=0.006. This means that exactly the same process can now be repeated for each new batch, provided that the same type of DRI and slag former is used and that the same conditions is applied in the EAF also for the subsequent batches.

Claims

CLAIMS1 . A method comprising the steps of: a) melting, in a first melting process, a first batch of an iron-carrier together with a slag former in an electric arc furnace, EAF, and removing the molten iron carrier from the EAF, b) providing a second batch of an iron-carrier, comprising at least 90 wt.% iron, having a mass m and a content of phosphor p1 , c) adding the second batch of the iron-carrier to the EAF, d) determining a required amount of slag that needs to be formed in the EAF during melting of the second batch of the iron-carrier for the purpose of-achieving a predetermined heat insulation caused by the slag during melting of the second batch of the iron carrier in presence of the slag former, and-generating a slag layer on top of the iron carrier during melting thereof, which slag layer has a thickness that allows ends of electrodes of the EAF to be received in the slag layer and for conducting electric current from the electrodes to the molten batch of the iron carrier, e) determining the mass of the slag remaining in the EAF from the first melting process, f) determining, with regard taken to the mass of the remaining slag from the first melting process, a required amount of a slag former comprising CaO, SiO2 and MgO to be added in order to-obtain said heat insulation during melting of the second batch, -generate said slag layer during the melting of the second batch, -and, with regard taken to the mass of the second batch of the iron-carrier and a content of CaO, SiO2, MgO and phosphor in the second batch of the iron-carrier, to reduce the amount of phosphor in the second batch of the iron- carrier to a predetermined level during melting of the second batch of the iron-carrier, g) determining a maximum amount of slag allowable in the EAF during melting of the second batch of the iron-carrier,h) determine the sum of the remaining slag and the new slag that will be formed during the melting of the second batch of the iron-carrier due to the addition of said required amount of slag former, and i) if said sum is above the maximum amount of slag allowable in the EAF during melting of the second batch of the iron-carrier, removing so much slag from the slag remaining from the first melting process that the sum of the maintained slag and the new slag that will be formed during the melting of the second batch of the iron-carrier is sufficient for obtaining said heat insulation and for the generation of said slag layer and that said sum is below said maximum allowable amount, and j) adding said required amount of the slag former to the EAF, and k) melting the second batch of the iron-carrier by means of an electric arc in the presence of the added slag former.

2. A method according to claim 1 , wherein each of the slag former added to form a slag during melting of the first batch of iron-carrier and the slag former added to form a slag during melting of the second batch of iron-carrier consists of one or more of CaO, SiO2 and MgO and unavoidable impurities.

3. A method according to claim 1 or 2, wherein the amount of and composition of the slag former added to the second iron carrier and the content of CaO, SiO2 and MgO in the second batch of iron-carrier is equal to or more than the amount of CaO, SiO2 and MgO required to obtain said heat insulation during melting of the second batch, generate said slag layer during the melting of the second batch, and reduce the phosphor in the second batch of the iron-carrier to a predetermined level during melting of the second batch of the iron-carrier.

4. A method according to any one of claims 1 -3, wherein the slag former added to form a slag during melting of the first batch of iron-carrier has the same composition as the slag former added to form a slag during melting of the second batch of iron-carrier.

5. A method according to any one of claims 1 -3, wherein the slag former added to form a slag during melting of the first batch of iron-carrier has a differentcomposition than the slag former added to form a slag during melting of the second batch of iron-carrier.

6. A method according to any one of claims 1 -5, wherein the first batch of an iron-carrier has the same composition as the second batch of an iron carrier.

7. A method according to any one of claims 1 -5, wherein the first batch of an iron-carrier has a different composition than the second batch of an iron carrier.

8. A method according to any one of claims 1 -7, wherein the first batch of an iron-carrier comprises at least 80 wt.%, preferably at least 95 wt.% sponge iron produced by direct reduction of iron ore with a reduction gas comprising at least 90 atomic% hydrogen gas.9 A method according to any one of claims 1 -8, wherein the second batch of an iron-carrier comprises at least 80 wt.%, preferably at least 95 wt.% sponge iron produced by direct reduction of iron ore with a reduction gas comprising at least 90 atomic% hydrogen gas.

10. A method according to any one of claims 1-9, wherein, when the content of each of CaO, SiO2 and MgO in the remaining slag is below a respective predetermined level, all slag is removed before the next batch of iron carrier is loaded to the EAF.

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