A method for hot rolling a steel product

Abstract

A method for hot rolling a steel semi product comprising the estimation of the composition of the liquid steel to be cast, said liquid steel being produced using copper-containing scrap, and the calculation of the melting temperature of the copper phase formed on the semi-product during casting to reheat the semi-product at an optimised reheating temperature.

Description

A method for hot rolling a steel product

[0001] The invention is related to a method of hot rolling a steel product wherein the steel is produced using steel scrap.

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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, also called copper-phase, 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.

[0007] 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.

[0008] This problem is solved by a method according to the invention comprising the steps of production of a liquid steel comprising the melting of steel scrap comprising copper, the estimation of the composition of said produced liquid steel, said estimation comprising the content of antimony, tin and sulphur, the casting of the produced liquid steel to produce a semi-product (5), said cast semi-product comprising a copper-phase, the definition of a temperature of reheating of the semi-product TR_DEF, the calculation of the melting temperature of the copper phase TMELT according to following formula TMELT= a.%Sb + p.%Sn + s.%S + o, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, and %S is the weight percent of sulphur, as estimated in step B where a, p, s,and o are coefficients representing the respective impact of antimony, tin, Sulphur, and remaining elements on the melting temperature of the copper phase, and if TR_DEF > TMELT , calculation of a TOFFSET so that TR_DEF - TOFFSET < TMELT, the reheating of the semi-product to an optimised reheating temperature TR_OPT = TR DEF - TOFFSET, and the hot rolling of the semiproduct.

[0009] The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations: the produced liquid steel comprises at least 0.1% in weight of copper, the produced liquid steel comprises from 0.1% to 0.3% in weight of copper, in step B, the composition of the produced liquid steel is estimated with a model, the model uses the composition of raw materials, including steel scrap comprising copper, melted for the production of the liquid steel, as input data, the estimation of the liquid steel composition is performed by analysing lollipop samples of said liquid steel, the production step comprises the melting of scrap with hot metal and / or direct reduced iron, the semi-product is a slab, the produced liquid steel has a composition wherein %Sb ≤ 0,01%, %As ≤ 0,1%, %Sn ≤ 0,05%, %Si ≤ 0,9%, %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%, remainder being iron and unavoidable impurities, all amount being expressed in weight percent, a = -894.49, p = -946.17, s = -1652.23 and o = 1048.34, the step of estimation of the liquid steel composition further comprises the estimation of the amount of phosphorus, manganese, aluminium, arsenic and boron, and the melting temperature TMELT is calculated according to the following formula TMELT = a.%Sb + 0.%Sn+ 5.%P + s.%S + (p.%Mn + |LX. %AI + CT%AS + x. %B + a, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, %P is the weight percent of phosphorus, %S is the weight percent of sulphur, %Mn is the weight percent of manganese, %AI is the weight percent of aluminium, %As is the weight percent of arsenic, and %B is the weight percent of Boron, as estimated, and a, p, 5, s, <p, p, ct , X and G are coefficients representing the respective impact of antimony, tin, nickel, phosphorus, Sulphur, chromium, manganese, aluminum, silicon, arsenic and other elements on the melting temperature of the copper phase. a = -894.49, p = -946.17, 5 -43.27, e = -1652.23, q> = -16.30, p = -5.76, ct = -14.8,X = -51.98 and ct = 1096.57, the step of estimation of the liquid steel composition further comprises the estimation of the amount of silicon, arsenic, molybdenum, carbon, niobium, titanium, vanadium and boron and the melting temperature TMELT is calculated according to the following formula TMELT = a.%Sb + 3.%Sn +x.%Ni + 5.%P +s.%S + y.%Cr + (p.%Mn + p.%AI +T. %Si +CT.%AS + p. %Mo + r|. %C + <p. %Nb + %Ti + 0. %v + x. %B + a, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, %Ni is the weight percent of nickel, %P is the weight percent of phosphorus, %S is the weight percent of sulphur, %Cr is the weight percent of chromium, %Mn is the weight percent of manganese, %AI is the weight percent of aluminium, %Si is the weight percent of silicon, %As is the weight percent of arsenic, %Mo is the weight percent of molybdenum, %C is the weight percent of carbon, %Nb is the weight percent of niobium, %Ti is the weight percent of titanium, %V is the weight percent of vanadium and %B is the weight percent of boron as estimated in step B and a, p, / , 5, 8, y, (p, p, T, CT, p, p, ip, 6, X and G are coefficients representing the respective impact of antimony, tin, nickel, phosphorus, Sulphur, chromium, manganese, aluminum, silicon, arsenic, molybdenum, carbon, niobium, titanium, vanadium, boron and other elements on the melting temperature of the copper phase. a = - 894.49, p = - 946.17, % = 1.18, 5 = - 43.27, s = - 1652.23, y = - 0.46, q> = -16.30, p = - 5.76, T = -0.6, ct = -14.8, p = -0.16, r| = - 0.91 , <|) = 0.03, ^ = - 2.47, 0 = - 0.21 , X = - 51.98 and ct = 1098.02.

[0010] 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,

[0011] Elements in the figures are illustration and may not have been drawn to scale.

[0012] 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 product such 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.

[0013] 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.

[0014] 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.

[0015] 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).

[0016] In the method according to the invention, the composition of the produced liquid steel, including the content of antimony, tin, and sulphur is estimated.

[0017] The steel composition of the liquid steel 1 can be estimated at the end of the elaboration step and before beginning of the casting step. The estimation of the steel composition is preferably performed by analysing lollipop samples from the hot metal ladle2 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.

[0018] 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.

[0019] 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, nitride.

[0020] 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.

[0021] Due to the use of copper-containing scrap a copper phase is present on the surface of the semi-product. By 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 phase that will penetrate the steel along the austenite grain boundaries and will cause the hot shortness issue.

[0022] 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 reheating furnace 7 is preferably a continuous reheating furnace such as a pusher type furnace, a walking hearth furnace, or a walking beam furnace.

[0023] In the method according to the invention, the temperature of the semi-product at the exit of the reheating furnace, also named reheating temperature TR_DEF, is first defined according to prior art, taking notably into account the chemical composition of the semiproduct 5, the metallurgical constraints and / or the requirement of the rolling process which is dependent on the rolling speed, semi-product dimension, steel composition and thermal losses.

[0024] If the production plant is a continuous plant or direct casting and rolling plant then the reheating temperature TR must be understood as the temperature of the semi-product 5 at the exit of the heating step, before the rolling.

[0025] Then, the melting temperature of the copper phase TMELT is calculated according to following formula (1):(Formula 1) TMELT = a.%Sb + p.%Sn + s.%S + o, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, and %S is the weight percent of sulphur, as estimated in step B and a, p, s, and o are coefficients representing the respective impact of antimony, tin, Sulphur and remaining elements on the melting temperature of the copper phase. Indeed, the inventors have discovered that tin, antimony and Sulphur are the elements having the greatest influence on the melting temperature of the copper phase, either by increasing or decreasing it.By equal it is meant equal to + / - 10%, or even + / -5% or even + / -2%.

[0026] In another embodiment of the method according to the invention, amount of phosphorus %P, amount of manganese %Mn, amount of aluminum %AI, amount of Boron %B and amount of Arsenic As are additionally estimated in step B and melting temperature of the copper phase TMELT is calculated according to following formula (2):(Formula 2) TMELT = a.%Sb + p.%Sn + 6.%P + s.%S + (p.%Mn + p.%AI + CT.%AS+ X.%B + G, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, %P is the weight percent of phosphorus, %S is the weight percent of sulphur, %Mn is the weight percent of manganese, %AI is the weight percent of aluminium, %B is the weight percent of Boron and %As is the weight percent of arsenic as estimated in step B and a, p, 5, s, <p, p, ct, X and o are coefficients representing the respective impact of antimony, tin, phosphorus, Sulphur, manganese, aluminum, arsenic, boron and other elements on the melting temperature of the copper phase.

[0027] In another embodiment of the method according to the invention, amount of silicon, arsenic, molybdenum, carbon, niobium, titanium, vanadium, and boron are additionally estimated in step B and the B and the melting temperature of the copper phase TMELT is calculated according to following formula (3):(Formula 3) TMELT = a.%Sb + 0.%Sn + / .%Ni + 6.%P + s.%S + y.%Cr + (p.%Mn+ p.%AI + T. %Si + CT.%AS + p. %Mo + T|. %C + cp. %Nb + C,. %Ti + 0. %V + X. %B + G, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, %Ni is the weight percent of nickel, %P is the weight percent of phosphorus, %S is the weight percentof sulphur, %Cr is the weight percent of chromium, %Mn is the weight percent of manganese, %AI is the weight percent of aluminium, %Si is the weight percent of silicon, %As is the weight percent of arsenic, %Mo is the weight percent of molybdenum, %C is the weight percent of carbon, %Nb is the weight percent of niobium, %Ti is the weight percent of titanium, %V is the weight percent of vanadium and %B is the weight percent of boron as estimated in step B and a, p, %, 5, s, y, <p, p,, T, TO, p, r|, tp, Q, 0, X and o are coefficients representing the respective impact of antimony, tin, nickel, phosphorus, Sulphur, chromium, manganese, aluminum, silicon, arsenic, molybdenum, carbon, niobium, titanium, vanadium, boron and other elements on the melting temperature of the copper phase.

[0028] Then the predefined reheating temperature TR DEF IS compared with the calculated melting temperature of copper TMELT and if the predefined reheating temperature TR_DEF is superior or equal to the melting temperature of copper TMELT then an offset temperature TOFFSET is calculated so that TR_DEF - TOFFSET≤ TMELT. This offset temperature TOFFSET is calculated taking into account the sensitivity of the temperature control in the reheating furnace. This sensitivity depends on the technology of the reheating furnace, some furnaces have a sensitivity of a few degrees while others have a sensitivity control of around 10°C.

[0029] Finally, the semi-product 5 is reheated to an optimised reheating temperature TR OPT = TR DEF - TOFFSET and subsequently hot rolled.

[0030] If the optimised reheating temperature TR OPT thus obtained is too low to meet the target for the hot rolling step some compensations might be performed to provide additional heat outside of the reheating furnace, using induction heating devices for example, and / or to adjust the hot rolling process parameters. For example, if the distance between the exit of the reheating furnace and the entrance of the rolling mill is too long, the semi-product might no longer have the required temperature when arriving at the first rolling pass and an additional short heating may be necessary.

[0031] Example of coefficients for each formula are given in below table 1.Table 1Those coefficients are given by means of example and have been determined using Thermocalc® software with Ssol4 database for antimony and arsenic and TCFe12 database for the other elements and based on the composition range wherein the produced liquid steel has a composition wherein %Sb is lower or equal to 0,01%, %As is lower or equal to 0,1%, %Sn is lower or equal to 0,05%, %Si is lower or equal to 0,9%, %Ni is lower or equal to 4,5%, %P is lower or equal to 0,46%, %S is lower or equal to 0,005%, %Mo is lower or equal to 2,2%, %Cr is lower or equal to 4,5%, %Mn is lower or equal to 2,5%, %AI is lower or equal to 0,6%, %C is lower or equal to 0,21 %, % Nb is lower or equal to 0,21%, %Ti is lower or equal to 0,21%, %V is lower or equal to 0,21 % and %B is lower or equal to 0,002%, remainder being iron and unavoidable impurities, all amount being expressed in weight percent.

[0032] . It is of course possible to use other software and / or other compositions, which would result in slightly modified coefficients.

[0033] 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 a predefined reheating temperature TR_DEF of 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.

[0034] According to the invention, the melting temperature of the copper phase TMELT was calculated according to formula 1 , results of the calculation are indicated in table 2.

[0035] Samples S1 bis and S2bis having a composition similar respectively to samples S1 and S2 were then subjected to a heating step at an optimised reheating temperature TR OPT = TR DEF - TOFFSET 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.

[0036] N means that no infiltrations have been observed while Y means that infiltrations of the copper phase have been observed in the steel bulk.Table 2

[0037] In the method of the invention, reheating is performed at a temperature below the melting temperature of the copper phase, which is then prevented, or at least limited, from penetrating the steel along the austenite grain boundaries and causing hot shortness.

Claims

CLAIMS1 ) A method for hot rolling a steel semi-product (5), comprising the steps of:A. Producing a liquid steel (1 ), said production step comprising the melting of steel scrap comprising copper,B. Estimating the composition of said produced liquid steel (1 ), said estimation comprising the content of antimony, tin and sulphur,C. Casting the produced liquid steel (1 ) to produce a semi-product (5), said cast semi-product comprising a copper-phase,D. Defining a temperature of reheating of the semi-product TR_DEF,E. Calculating the melting temperature of the copper phase TMELT according to following formula:TMELT= a.%Sb + p.%Sn + s.%S + G, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, and %S is the weight percent of sulphur, as estimated in step B and wherein a, 0, s,and G are coefficients representing the respective impact of antimony, tin, Sulphur, and remaining elements on the melting temperature of the copper phase,F. If TR_DEF > TMELT calculating a TOFFSET SO that TR_DEF - TOFFSET < TMELTG. Reheating the semi-product (5) to an optimised reheating temperature TR_OPT = TR_DEF - TOFFSET,H. Hot rolling the semi-product.2) A method according to claim 1 wherein the produced liquid steel (1 ) comprises at least 0.1 % in weight of copper.3) A method according to claim 2 wherein the produced liquid steel (1 ) comprises from 0.1 % to 0.3% in weight of copper.4) A method according to anyone of claims 1 to 3 wherein in the production step A, scrap is melted with hot metal and / or direct reduced iron.5) A method according to anyone of claims 1 to 4 wherein the semi-product (5) is a slab.6) A method according to anyone of claim 1 to 8 wherein the produced liquid steel (1 ) has a composition wherein %Sb ≤ 0,01 %, %As ≤ 0,1 %, %Sn ≤ 0,05%, %Si ≤ 0,9%, %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%, remainder being iron and unavoidable impurities, all amount being expressed in weight percent.7) A method according to claim 6 wherein a = -894.49, p = -946.17, s = -1652.23 and o = 1048.34.8) A method according to anyone of claims 1 to 7 wherein the step B of estimation of the liquid steel composition further comprises the estimation of the amount of phosphorus, manganese, aluminium, arsenic and boron, and the melting temperature TMELT is calculated according to the following formula:TMELT = oc.%Sb + p.%Sn + 6.%P + s.%S + <p.%Mn + p.%AI + ra%As + X. %B + c, wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, %P is the weight percent of phosphorus, %S is the weight percent of sulphur, %Mn is the weight percent of manganese, %AI is the weight percent of aluminium, %As is the weight percent of arsenic, and %B is the weight percent of Boron, as estimated in step B and a, p, 5, s, <p, p, to , X and G are coefficients representing the respective impact of antimony, tin, nickel, phosphorus, Sulphur, chromium, manganese, aluminum, silicon, arsenic and other elements on the melting temperature of the copper phase.9) A method according to claim 8 wherein a = -894.49, p = -946.17, 5 = -43.27, s = - 1652.23, cp = -16.30, p = -5.76, to = -14.8, X = -51 .98 and o = 1096.5710) A method according to anyone of claims 1 to 6 wherein the step B of estimation of the liquid steel composition further comprises the estimation of the amount of silicon, arsenic, molybdenum, carbon, niobium, titanium, vanadium and boron and the melting temperature TMELT is calculated according to the following formula:TMELT = oc.%Sb + p.%Sn + %.%Ni + 6.%P + s.%S + y.%Cr + <p.%Mn + p.%AI + T. %Si + ra.%As + p. %Mo + T|. %C + <|). %Nb + %Ti + 0. %V + X. %B + G,wherein %Sb is the weight percent of antimony, %Sn is the weight percent of tin, %Ni is the weight percent of nickel, %P is the weight percent of phosphorus, %S is the weight percent of sulphur, %Cr is the weight percent of chromium, %Mn is the weight percent of manganese, %AI is the weight percent of aluminium, %Si is the weight percent of silicon, %As is the weight percent of arsenic, %Mo is the weight percent of molybdenum, %C is the weight percent of carbon, %Nb is the weight percent of niobium, %Ti is the weight percent of titanium, %V is the weight percent of vanadium and %B is the weight percent of boron as estimated in step B and a, P, x, 5, s, y, <p, p, T, TO, p, r|, C 6, and G are coefficients representing the respective impact of antimony, tin, nickel, phosphorus, Sulphur, chromium, manganese, aluminum, silicon, arsenic, molybdenum, carbon, niobium, titanium, vanadium, boron and other elements on the melting temperature of the copper phase.11 ) A method according to claim 10 wherein a = - 894.49, p = - 946.17, % = 1 .18, 5 = - 43.27, s = - 1652.23, y = - 0.46, cp = -16.30, p = - 5.76, T = -0.6, to = -14.8, p = - 0.16, r| = - 0.91 , (|) = 0.03, ^ = - 2.47, 0 = - 0.21 , X = - 51.98 and o = 1098.02.12)A method according to any one of the claims 1 to 11 wherein in step B, the estimation of the composition of the produced liquid steel (1 ) is performed with a model.13)A method according to claim 11 wherein said model uses the composition of raw materials, including steel scrap comprising copper, melted to produce the liquid steel (1 ), as input data.14)A method according to any one of the claims 1 to 13 wherein the estimation of the liquid steel composition is performed by analysing lollipop samples of said liquid steel (1 ).

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