Method for determining an optimized silicon content in liquid steel

Abstract

A method to calculate an optimized amount of silicon to be added in a liquid steel produced using copper-containing scrap. The invention is also related to a method of hot rolling a steel semi-product resulting from the casting of a liquid steel produced using copper-containing scrap.

Description

Method for determining an optimized silicon content in liquid steel

[0001] The invention is related to a method to determine an optimised content of silicon in a liquid steel produced using scrap and to a method of hot rolling a steel semi-product wherein the steel is produced using steel scrap.

[0001] Steelmaking requires the use of iron-containing material, such as steel scrap, direct reduced iron (DRI) or pig iron. Today, steel scrap is commonly used in the steelmaking process to produce liquid steel. Said scrap can be used at different stages along the steelmaking process and in different steelmaking tools. Converter, Basic Oxygen Furnace (BOF), Electric Arc Furnace (EAF), Smelting Furnace (SF) are some of the tools that can be used in particular for steelmaking production.

[0002] In order to reduce the CO2 global footprint of the steelmaking process, there is a global trend to use more and more scrap in steel production. However, said scrap may be of different types, in particular depending on its origin, and thus have different qualities in terms notably of shape, density, chemistry and presence of impurities. Steel scrap contains residual elements, such as copper, chromium, molybdenum, nickel, tin, antimony, zinc and / or arsenic. The use of steel scrap is therefore not widespread for all steel grades as these residual elements can have a detrimental effect on the properties of the steel.

[0003] When steel is produced using direct reduced iron and / or pig iron, small amounts of residual elements are inevitably left behind in the molten metal. When steel scrap is used, the amount of residual elements is much greater than in the case of pig iron coming from a blast furnace or direct reduced iron.

[0004] Recently, it has been observed by the present inventors that the production of steel using a significant amount of steel scrap raises problems during some manufacturing steps such as hot rolling.

[0005] One of the impurities contained in steel scrap that is difficult to remove in the steelmaking process is copper. Copper is nobler than iron and it is enriched at the steel- oxide interface when iron is oxidized at high temperature. The copper-rich liquid phase thus formed, eventually penetrates the steel along the austenite grain boundaries, weakening the bonds and inducing surface cracking during hot working. This cracking phenomenon is known as hot shortness.

[0006] There is so a need for a method to produce a hot rolled steel using steel scrap wherein hot shortness risk is reduced and even suppressed.

[0007] This problem is solved by a method according to the invention allowing to determine an optimised silicon content %Siopt to be obtained in a liquid steel prior to casting, said liquid steel resulting from the melting of steel scrap comprising copper, the method comprising the acquisition of a reheating temperature TR, defined for the reheating of semi-products obtained by casting of said liquid steel, said reheating preceding a hot rolling, the determination of the optimised silicon content %Siopt so as to fulfil the following condition:wherein o(%Siopt), p(%Siopt) and P(%Siopt) are predetermined coefficients that do not depend on the reheating temperature TR, and where A is a constant threshold, independent of TR and %Siopt-

[0008] The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations:- the quantity o(%Si0pt). TR+ p(%Siopt). TR+ p. (%Stopt) is a ratio of the amount of liquid fayalite eutectic FeO-Fe2SO4 formed during the reheating and the global amount of oxides formed during the reheating,- the produced liquid steel comprises at least 0.1% in weight of copper,- the liquid steel has a composition wherein %Sb < 0.01%, %As < 0.1%, %Sn < 0.05%, 0.004% <Si < 6%, %Ni < 4.5%, %P < 0.46%, %S < 0.005%, %Mo < 2.2%, %Cr < 4.5%, %Mn < 2.5%, %AI < 0.6%, %C < 0.21 %, % Nb < 0.21 %, %Ti < 0.21%, %V < 0.21%, %B < 0.002% and %Cu > 0.1%, remainder being iron and unavoidable impurities, all amounts being expressed in weight percent,- the amount of copper in the liquid steel is from 0.1 to 0.3% by weight,- the reheating temperature TR is from 1100°C to 1350°C,- the method further comprises a preliminary step of acquiring or calculating an estimated silicon content %Sjest present in the liquid steel,- the set of coefficients o(%Si0pt), p(%Siopt) and P(%Siopt), employed to determine the optimised silicon content %Siopt, is selected among different sets of coefficients, depending on the estimated silicon content %Siest in the liquid steel,- when the estimated content of silicon in liquid steel %Siest is from 0.004 to 0.38 weight percent, then o(%Si0pt) = 9.96E-6. %Siopt + 1.8233E-8, p(%Siopt) = - (2.364E-2. %Siopt + 4.518E-5), p(%Siopt) = 14.1511. %Siopt+ 0.028104 and A=0.03,- when the estimated content of silicon in liquid steel %Siest is from 0.39 to 4 weight percent, then o(%Si0pt) = -1.3157E-6. %Siopt2+ 1.3656E-5. %Siopt — 8.0137E-7, p(%Siopt) = 3.1971 E-3. %Siopt2- 3.2969E-2. %Siopt + 2.0283E-3), p(%Siopt) = -1.9492. %Siopt2+ 20.04. %Siopt- 1.2828 and A=0.03,- when the estimated content of silicon in liquid steel %Siest is from 4 to 6 weight percent, then o(%Si0pt) = 5.8849E-6. %Siopt2- 6.3844E-5.%Siopt + 1.9346E-4, p(%Siopt) = -1.4669E- 2. %Siopt2+ 0.15942. %Siopt -0.48037, P(%Siopt) = 9.1268. %Siopt2-99.283. %Siopt + 297.99 and A=0.03.- the estimation of the silicon content is performed by analysing lollipop samples of the produced liquid stee,- the estimation of the silicon content is performed with a model.

[0009] The invention is also related to a method to produce a steel semi-product comprising the steps of production of a liquid steel, said production step comprising the melting of steel scrap comprising copper, estimation of the silicon content %Siest of said produced liquid steel, casting of the liquid steel to produce a semi-product, reheating of the semi product to a reheating temperature TR, hot rolling the reheated semi-product, wherein between the steps of estimation and of casting, the steps of determination of an optimised silicon content %Siopt according to the previously described method, and adjustment of the silicon content in the liquid steel to reach the calculated optimised silicon content %Sioptare performed.

[0010] This production method may also comprise the following optional characteristics considered separately or according to all possible technical combinations:- In the the production step, scrap is melted with hot metal and / or direct reduced iron,- the semi-product is a slab.

[0011] Other features and advantages of the invention will be apparent from the following description, which is by way of illustration and in no way limitative, with reference to the appended figures in which:Figure 1 illustrates a plant to produce a hot rolled steel sheet

[0012] Elements in the figures are illustration and may not have been drawn to scale.

[0013] Figure 1 shows a plant for the production of a hot-rolled steel product. Liquid steel 1 contained in a ladle 2 is poured into a tundish 3, which then allows liquid steel to flow through a nozzle into a mold. This liquid steel begins to solidify along the caster 4 until it is fully solidified and cut to form a semi-product 5. This semi-product 5 is generally left to cool at ambient temperature and stored in a slab yard before being sent to the hot rolling plant 6. This hot rolling plant 6 comprises a reheating furnace 7 where the semi-product 5 is reheated before being transferred to the rolling mill 8 where it is rolled and transformed into a steel product which is coiled to form a hot rolled steel coil 9. Figure 1 illustrates an embodiment of a plant to produce a hot rolled steel sheet but the method according to the invention may be performed in other plants allowing to turn liquid steel in a semi productsuch as a billet, a blank, a bar, an ingot, a slab or a bloom and transformation of this semiproduct into a steel product 9 in a hot rolling plant 6, this hot rolling plant 6 comprising a reheating furnace 7 adapted to reheat the semi-product 5.

[0014] In another embodiment of a plant to produce a hot-rolled steel product, not illustrated here, the semi-product 5 after casting is directly sent to the hot rolling plant where it is subjected to a heating step before being rolled. This heating step may be performed in a tunnel furnace. This plant is called a continuous plant or direct casting and rolling plant.

[0015] In the method according to the invention, the production of liquid steel 1 comprises the melting of steel scrap containing copper. Different production routes can be used, scrap can be melted in an electric arc furnace (EAF), optionally together with pig iron and direct reduced iron (DRI), and then liquid steel is obtained which is subjected to a refining step before being poured into the tundish. Scrap can also be melted in a Smelting Furnace (SF) together with pig iron and / or DRI, and the resulting molten metal sent to a converter or BOF for decarburization and conversion to liquid steel. This liquid steel can then be subjected to refining steps, such as dephosphorization and / or desulfurization steps and / or secondary metallurgy steps. Scrap can also be charged into the BOF.

[0016] For example, the steel scrap that can be used is referred to, in the Ell-21 Steel Scrap specification, as old scraps (E1 or E3), new scraps (E8), shredded scraps 20 (E40) or fragmentized scraps (E46).

[0017] The inventors have discovered that the hot shortness phenomenon depends both on the reheating temperature and on the amount of copper and that the amount of silicon can have a compensative effect. However, in order to have a positive compensation effect without having silicon in excess, the definition of a boundary taking into account the reheating temperature TR is needed to calculate an optimised silicon content %Siopt- Indeed, a silicon excess may notably have a detrimental impact on the hot and cold workability of the steel. The inventors have thus discovered that this boundary can be expressed under the simplified following form:Formula 1

[0018] This allows the effect of the silicon to be decoupled from the reheating temperature, while maintaining a concise form that is easy to calculate.

[0019] In the method according to the invention, an optimised silicon content %Siopt to be obtained in a liquid steel prior to casting, said liquid steel resulting from the melting of steel scrap comprising copper, is determined, the method comprising the acquisition of the reheating temperature TR, and the determination of the optimised content %Siopt so as to fulfil the following condition:

[0020] cr(%Siopt), p(%Siopt) and p(%Siopt) are predetermined coefficients that do not depend on the reheating temperature TR, and where A is a constant threshold, independent of TR and %Siopt.

[0021] Coefficients o(%Si0pt), p(%Siopt) and p(%Siopt), as well as the threshold A, are determined, for instance, during a preliminary phase, based on tests and / or using software based on experimental data.

[0022] In the above formula, the coefficients values can vary by + / -10%, or even + / -5% or even +1-2%.

[0023] This method is preferably executed by an electronic device comprising at least a processor and a memory. The electronic device receives the reheating temperature TR, either from a human machine interface or, via an interface, from another electronic unit or server, or reads it from a planned production file. The predetermined coefficients are stored in the memory. Different sets of coefficients may be stored in the memory according to the composition of the liquid steel to be cast.

[0024] This simplified form, as expressed by Formula 1 , can be explained by the fact that hot shortness depends on the ability of copper to diffuse into the bulk of the semi-product, this ability being limited by the inclusion of copper in the liquid fayalite eutectic FeO- Fe2SO4. The inventors have indeed discovered that by increasing the amount of silicon the formation of liquid fayalite eutectic is favoured during reheating, this liquid fayalite eutectic FeO-Fe2SiO4 being then able to trap the liquid copper and prevent its infiltration in the steel bulk.

[0025] In a method for hot rolling a steel semi-product according to the invention, the amount of silicon %Siest in the liquid steel is first estimated. The composition of the liquid steel can be estimated at the end of the elaboration step and before the beginning of the casting step. The estimation of the steel composition is preferably performed by analysing lollipop samples from the hot metal ladle 2 that is used to fill in the casting tundish 3 or the ingot mould to obtain the semi-finished product. This estimation may also be performed using models.

[0026] Preferably, in step A of production of the liquid steel., from 100 to 1000 kg of steel scrap per ton of hot metal is used. Preferably, in step A, from 100 to 950 kg of steel scrap per ton of hot metal is used. Preferably, in step A, from 100 to 900 kg of steel scrap per ton of hot metal is used. Preferably, in step A, from 100 to 800 kg of steel scrap per ton of hot metal is used. Preferably, in step A, from 100 to 600 kg 25 of steel scrap per ton of hot metal is used. Preferably, in step A, from 100 to 500 kg of steel scrap per ton of hot metal is used.Even more preferably, in step A, from 200 to 400 kg of steel scrap per ton of hot metal is used.

[0027] In this patent, the residual elements are the non-desired elements coming from the steel scrap. The unavoidable impurities come from the elaboration process, e.g. oxides, nitrides.

[0028] Once the liquid steel is elaborated it is poured into the tundish 3 and then cast in the caster 4 to produce the semi-product 5. As previously explained this semi-product 5 is generally left to cool at ambient temperature and stored in a warehouse before being sent to the hot rolling plant 6. Then the semi-product 5 is sent to the reheating furnace 7.

[0029] Due to the use of copper-containing scrap a copper-rich phase is present on the surface of the semi-product. By copper-rich or copper-phase it is meant a phase comprising more than 75% by weight of copper, remainder being mostly iron and other alloying and residual elements present in the liquid steel. It is this copper-rich phase that will penetrate the steel along the austenite grain boundaries and will cause the hot shortness issue.

[0030] Classically, the semi-product to be rolled is charged at the entrance of the reheating furnace 7 where it is pushed forward on the hearth of the furnace by means of a pusher machine. The semi-product is usually pre-heated, heated and soaked as it passes through pre-heating, heating and soaking zone of the reheating furnace. At the end of the soaking zone of the furnace, it is discharged from the furnace by an ejector for subsequent rolling in the rolling mill. The temperature TR of the semi-product at the time of discharged depends on several factors and it can vary in the range of 1100°C to 1350°C. The reheating furnace 7 is preferably a continuous reheating furnace such as a pusher type furnace, a walking hearth furnace, or a walking beam furnace.

[0031] If the production plant is a continuous plant or direct casting and rolling plant then the reheating temperature TRmust be understood as the temperature of the semi-product 5 at the exit of the heating step, before the rolling.

[0032] In the method according to the invention, before the casting, an optimised amount of Silicon %Siopt is calculated according to the previously described method.

[0033] Then the silicon content of the liquid steel is adjusted so as to reach the calculated optimised amount of silicon. This adjustment may be done for example by adding a ferro silicon alloy or a silico-manganese compound in the liquid steel to increase its silicon content. Those addition elements may be in the form of wire, granular or coarse particles.

[0034] Example of coefficients are given in below table 1 , all %Si being expressed in percent by weight. In all the text the symbol “E” must be understood as “x10n”, for example, 9.96E-6 must be understood as 9.96x1 O'6or 0.00000996.Table 1

[0035] Those coefficients are given by means of example and have been determined using Thermocalc® software with oxides database TCOX12 and based on the composition range as defined in claim 4. It is of course possible to use other software and / or other compositions, which would result in slightly modified coefficients.

[0036] A trial was performed with two steel samples S1 and S2 having a composition as defined in table 2, remainder being iron and unavoidable impurities. Those solid samples were subjected to a heating step at TR equal to 1235°C to simulate the reheating step of an industrial installation and observations by a microscope were performed to identify presence of infiltrations in the steel bulk. Results are also included in table 2.

[0037] According to the invention an optimised silicon content %Siopt was calculated using coefficients indicated in the first line of Table 1. Steel composition was adjusted to reach this optimised silicon amount and another run of trials with samples S1bis and S2bis corresponding to steel composition of S1 and S2 with calculated optimised content of silicon was performed. The samples were cast, cooled, and subjected to a heating step at TR equal to 1235°C to simulate the reheating step of an industrial installation and observations by a microscope were performed to identify presence of infiltrations in the steel bulk. Results are also included in table 2.

[0038] N means that no infiltrations have been observed while Y means that infiltrations of the copper-rich phase have been observed in the steel bulk.Table 2

[0039] One can observe that while keeping same content of other elements and same reheating temperature, when adjusting the optimised silicon content as defined in the method according to the invention, it is possible to avoid the infiltration of copper in the bulk and thus to limit the risk of hot shortness defect.

Claims

CLAIMS1 ) A method for determining an optimised silicon content %SioPt to be obtained in a liquid steel (1 ) prior to casting, said liquid steel (1 ) resulting from the melting of steel scrap comprising copper, the method comprising: a. Acquiring a reheating temperature TR, defined for the reheating of semi-products (5) obtained by casting of said liquid steel, said reheating preceding a hot rolling, b. Determining the optimised silicon content %SioPt so as to fulfil the following condition:wherein G(%S iopt), p(%S ioPt) and P(%S ioPt) are predetermined coefficients that do not depend on the reheating temperature TR, and where A is a constant threshold, independent of TR and %SioPt.2) A method according to claim 1 wherein the quantity c %Siopt). TR+ p ,%Siopt). TR+ P(%5iopt) is a ratio of the amount of liquid fayalite eutectic FeO- Fe2SO4 formed during the reheating and the global amount of oxides formed during the reheating.3) A method according to anyone of claim 1 or 2 wherein the produced liquid steel (1 ) comprises at least 0.1 % in weight of copper.4) A method according to anyone of claim 1 to 3 wherein the liquid steel (1 ) has a composition wherein %Sb < 0.01 %, %As < 0.1 %, %Sn < 0.05%, 0.004% <Si < 6%, %Ni < 4.5%, %P < 0.46%, %S < 0.005%, %Mo < 2.2%, %Cr < 4.5%, %Mn < 2.5%, %AI < 0.6%, %C < 0.21 %, % Nb < 0.21 %, %Ti < 0.21 %, %V < 0.21 %, %B < 0.002% and %Cu > 0.1 %, remainder being iron and unavoidable impurities, all amounts being expressed in weight percent.5) A method according to anyone of claims 1 to 4 wherein the amount of copper in the liquid steel (1 ) is from 0.1 to 0.3% by weight.6) A method according to anyone of claim 1 to 5 wherein the reheating temperature TR is from 1100°C to 1350°C.7) A method according to anyone of claims 1 to 6, further comprising a preliminary step of acquiring or calculating an estimated silicon content %Siest present in the liquid steel (1 ).8) A method according to claim 7, wherein the set of coefficients G(%S iopt) , p(%SioPt) and P(%Siopt), employed to determine the optimised silicon content %Siopt, is selected among different sets of coefficients, depending on the estimated silicon content %Siest in the liquid steel.9) A method according to claims 7 or 8 wherein, when the estimated content of silicon in liquid steel %Siest is from 0.004 to 0.38 weight percent, then <j(%SioPt) = 9.96-6. %SioPt + 1.8233e-8, p(%SioPt) = - (2.364e-2. %SioPt + 4.518e-5), p(%SioPt) = 14.1511. %SioPt+ 0.028104 and A=0.03.10)A method according to claims 7 or 8 wherein, when the estimated content of silicon in liquid steel %Siest is from 0.39 to 4.0 weight percent, then <j(%SioPt) = - 1.3157e-6. %SioPt2+ 1.3656e-5. %SioPt- 8.0137e-7, p(%SioPt) = 3.1971 e-3. %SioPt2- 3.2969e-2. %SioPt + 2.0283e-3), p(%SioPt) = -1 .9492. %SioPt2+ 20.

04. %SioPt- 1 .2828 and A=0.03.11 )A method according to claims 7 or 8 wherein, when the estimated content of silicon in liquid steel %Siest is from 4.1 to 6 weight percent, then <j(%SioPt) = 5.8849E- 6. %SioPt2- 6.3844E-5.%SioPt + 1.9346E-4, p(%Siopt) = -1.4669E-2. %SioPt2+ 0.15942. %Siopt -0.48037, p(%SioPt) = 9.1268. %SioPt2-99.

283. %SioPt + 297.99 and A=0.03.12)A method for hot rolling a steel semi-product (5), the method comprising the steps of:A. Producing a liquid steel (1 ), said production step comprising the melting of steel scrap comprising copper,B. Estimating the silicon content %Siest of said produced liquid steel,C. Casting the liquid steel (1 ) to produce a semi-product (5),D. Reheating the semi product (5) to a reheating temperature TR,E. Hot rolling the reheated semi-product, wherein between steps B and C, following steps are performed: i) determining an optimised silicon content %SioPt according to the method of any of claims 1 to 11 , ii) Adjusting the silicon content in the liquid steel to reach the calculated optimised silicon content %SioPt.13)A method according to claim 12 wherein in the production step A, scrap is melted with hot metal and / or direct reduced iron.14)A method according to anyone of claims 12 or 13 wherein the semi-product is a slab.15)A method according to any one of the claims 12 to 14 wherein the estimation of the silicon content is performed by analysing lollipop samples of the produced liquid steel (1 ).16)A method according to any one of the claims 12 to 14 wherein the estimation of the silicon content is performed with a model.

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