Method for hot rolling semi-finished steel product made at least partially from steel scrap

Abstract

FIELD: metallurgy.SUBSTANCE: method for hot rolling a semi-finished steel product made, at least in part, from steel scrap includes the following stages: i - preparing steel of a composition comprising, in mass percent: 0.002 ≤ C ≤ 0.8, 0.1 ≤ Mn ≤ 3, Si ≤ 2, Al ≤ 2, Cr ≤ 0.5, Nb ≤ 0.08, Ti ≤ 0,1, the remainder consisting of Fe, residual elements consisting of Mo, Sn, Sb and As, and inevitable impurities, by melting steel scrap including at least one of the said residual elements, ii - determining the composition of liquid steel, iii - casting a semi-finished product using the said liquid steel having a theoretical final hot rolling temperature THR_TH, iv - determining, using the established expression TOFFSET, an increase in the final hot rolling temperature capable of compensating for the presence of the said residual elements: Mo, Sn, Sb and / or As, at the average flow stress resulting from the deformation applied during hot rolling, v - hot rolling of the said semi-finished product at the optimized hot rolling temperature THR, which is: THR = THR_TH + OFFSET.EFFECT: improved processability of hot rolling of semi-finished steel products.6 cl, 1 tbl

Description

[0001] This invention relates to a method for hot rolling a semi-finished steel product containing residual elements obtained as a result of preparing steel using steel scrap.

[0002] Steel production requires the use of iron-containing materials such as steel scrap, direct reduced iron, or pig iron. The use of steel scrap is considered key to reducing the carbon footprint of the steel industry. However, steel scrap contains trace elements such as copper, chromium, molybdenum, nickel, tin, antimony, zinc, and / or arsenic. Therefore, the use of steel scrap is not widespread for all steel grades, as these trace elements can negatively impact the steel's properties.

[0003] During steelmaking using direct reduced iron and / or pig iron, small amounts of residual elements inevitably remain in the liquid steel. When using steel scrap, the amount of residual elements is much greater than when using pig iron or direct reduced iron from a blast furnace.

[0004] The present inventors have noticed that the production of steel using a significant amount of scrap steel causes problems at some stages of production, such as hot rolling.

[0005] Therefore, the aim of the present invention is to improve the processability of hot rolling a semi-finished steel product made at least partially from steel scrap. This is achieved by the hot rolling method according to claims 1-5.

[0006] The present invention relates to a method for hot rolling a semi-finished steel product, comprising the steps of:

[0007] i. preparing steel of a composition comprising, in mass percent: 0.002 ≤ C ≤ 0.8, 0.1 ≤ Mn ≤ 3, Si ≤ 2, Al ≤ 2, Cr ≤ 0.5, Nb ≤ 0.08, Ti ≤ 0.1 and the remainder consisting of Fe, residual elements comprising Mo, Sn, Sb and As, and unavoidable impurities, by melting steel scrap comprising at least one of the said residual elements and optionally hot metal coming from a blast furnace and / or direct reduced iron,

[0008] ii. Analysis of the composition of liquid steel,

[0009] iii. casting a semi-finished product using the specified liquid steel having the theoretical final hot rolling temperature T HR_TH ,

[0010] iv. definition of T OFFSET , an increase in hot rolling temperature capable of compensating for the presence of the specified residual elements: Mo, Sn, Sb and / or As, at an average plastic flow stress resulting from the deformation applied during hot rolling:

[0011]

[0012] where [Mo] is the mass percentage of molybdenum, [Sn] is the mass percentage of tin, [Sb] is the mass percentage of antimony, and [As] is the mass percentage of arsenic, and

[0013] where a, b, c and d are coefficients representing the effect of molybdenum, tin, antimony and arsenic, respectively, on the average flow stress resulting from the deformation applied during hot rolling, and

[0014] where α, β, γ and δ are coefficients representing the temperature dependence of the effect of molybdenum, tin, antimony and arsenic on the average flow stress resulting from the deformation applied during hot rolling,

[0015] v. hot rolling of the specified semi-finished product at the optimized hot rolling temperature T HR , which is:

[0016] T HR = T HR_TH + T OFFSET .

[0017] The preparation of grade steel at stage i. can be carried out using any means, such as, for example, a basic oxygen furnace, an open-hearth furnace and / or an electric arc furnace.

[0018] The feed material consists of steel scrap and, optionally, hot metal from a blast furnace and / or direct reduced iron. For example, steel scrap that can be used is specified in the EU-21 steel scrap specification as old scrap (E1 or E3), new scrap (E8), shredded scrap (E40), or fragmented scrap (E46).

[0019] Preferably, in step i., 100 to 1000 kg of steel scrap is used per ton of pig iron. Preferably, in step i., 100 to 950 kg of steel scrap is used per ton of pig iron. Preferably, in step i., 100 to 900 kg of steel scrap is used per ton of pig iron. Preferably, in step i., 100 to 800 kg of steel scrap is used per ton of pig iron. Preferably, in step i., 100 to 600 kg of steel scrap is used per ton of pig iron. Preferably, in step i., 100 to 500 kg of steel scrap is used per ton of pig iron. More preferably, in step i., 200 to 400 kg of steel scrap is used per ton of pig iron.

[0020] Residual elements are unwanted elements coming from steel scrap. Unavoidable impurities are introduced during the manufacturing process, such as oxides and nitrides.

[0021] Preferably, the steel composition comprises 0.01 to 0.5 mass percent of residual elements. More preferably, the steel composition comprises 0.05 to 0.30 mass percent of residual elements.

[0022] Preferably, at said step i. of preparation, said steel scrap comprises at least two of said residual elements. More preferably, at said step i. of preparation, said steel scrap comprises at least two of said residual elements with a mass percentage of each of 0.02 to 0.30 mass percent.

[0023] Step i. preferably includes a refining step. This refining step includes a desulfurization step and / or a dephosphorization step. This refining step is preferably performed in a ladle furnace.

[0024] The composition of liquid steel can be determined at the end of the preparation stage and / or at the beginning of the casting stage. Preferably, this determination is made at the beginning of the casting of the semi-finished product. The steel composition is preferably determined by analyzing samples from the casting ladle used to fill the tundish or mold to produce the semi-finished product.

[0025] The casting may be any type of casting. The casting is preferably continuous casting. The semi-finished product may be, for example, a cylindrical ingot, bloom, billet, slab, rod, or billet. Such semi-finished products are then reheated and hot rolled to reduce their thickness to the target thickness depending on their future use.

[0026] Preferably, between stage iii of casting and stage iv, the rolling mode is determined.

[0027] To hot roll a cast semi-finished product, a person skilled in the art determines a rolling schedule. As is well known to those skilled in the art, determining a rolling schedule requires information about the steel, such as the stress-strain curve, composition, and initial microstructure of the steel to be hot rolled. The rolling schedule includes, among other parameters, the rolling reduction ratio, strain rate, and hot rolling temperature for each hot rolling stand (e.g., for each rolling pass), taking into account the mass flow equation.

[0028] Then, based on the steel and rolling conditions, the average flow stress applied to each rolling mill stand can be determined. The average flow stress is equal to the area under the stress-strain curve ε. α before deformation ε β .

[0029] Then, using the average flow stress applied in each pass, a person skilled in the art can accordingly adjust the rolling stands and determine the preliminary parameters for each hot rolling.

[0030] However, in the prior art, the content of residual elements from steel scrap is not taken into account when setting the rolling conditions. Therefore, the theoretical stress-strain curve is determined without taking these residual elements into account.

[0031] Furthermore, it was unexpectedly found that the presence of certain residual elements from steel scrap, namely Mo, Sn, Sb, and As, leads to a deviation from the theoretical stress-strain curve. This deviation leads to a change in the average flow stress that allows the target strain rate to be achieved.

[0032] In each stand, stress is applied to the semi-finished steel product by a pair of cylindrical rolls. Each hot rolling stand has a maximum stress it can support.

[0033] Therefore, all other things being equal, when applying the target reduction ratio during hot rolling of steel prepared from steel scrap containing the specified residual elements, the average applied flow stress deviates from the theoretical average flow stress required to achieve the target strain rate.

[0034] This deviation may cause processability problem and possibly accelerated failure of the rolling roll because the expected stress applied by the hot rolling stand is less than that actually applied.

[0035] In order to counteract this unexpected effect of residual elements, the inventors proposed to increase the hot rolling temperature determined in the rolling mode for steel not containing the said residual elements, i.e., the theoretical finishing hot rolling temperature T HR_TH This is done by determining the increase in hot rolling temperature, T OFFSET , capable of compensating for the presence of the said residual elements: Mo, Sn, Sb and / or As at the average flow stress resulting from the deformation applied during hot rolling.

[0036] Therefore, according to the purpose of the invention, the semi-finished product is subjected to hot rolling at a temperature T HR , comprising:

[0037] T HR = T HR_TH + T OFFSET .

[0038] This newly defined hot rolling temperature enables the target reduction ratio to be achieved by applying the average flow stress determined by the rolling condition.

[0039] As is known to those skilled in the art, several parameters can be influenced to achieve the temperature in any rolling stand, such as a hot rolling mill stand.

[0040] For example, hot rolling mills typically include a reheating stage before rolling. Based on the required minimum hot rolling temperature in the rolling mill, for example, in the last hot rolling stand, the required temperature at the end of the reheating stage can be determined.

[0041] Alternatively, when hot rolling is performed after casting, as is the case on some compact strip mills, the rolling temperature can be influenced by changing the strip speed.

[0042] Furthermore, heating systems placed between stands and / or between the roughing mill and the hot rolling mill can be used. Additionally, a heat shield can be used to control the temperature in the hot rolling mill.

[0043] Generally, devices capable of quantitatively characterizing steel semi-finished products, such as pyrometers, are placed between the roughing mill and the hot rolling mill, and also after the last stand of the hot rolling mill.

[0044] Hot rolling can be performed by any method that allows for the hot rolling of a semi-finished steel product. Preferably, the hot rolling stage is performed in a hot rolling mill comprising three to eight rolling stands, more preferably five to seven rolling stands. Preferably, the hot rolling stage is performed in a reversing rolling stand.

[0045] The theoretical final hot rolling temperature can be a single temperature value or a temperature range. For example, if the theoretical final hot rolling temperature is in the temperature range of T HR_TH-MIN to T HR_TH-MAX , the semi-finished product is subjected to hot rolling at temperature T HR, which constitutes T HR_TH-MIN + T OFFSET ≤ T HR ≤ T HR_TH-MAX + T OFFSET .

[0046] Determining the theoretical final hot rolling temperature is a step well known to those skilled in the art. This determination may take into account metallurgical limitations to achieve the desired properties and format. For example, this can be done using the Sims model coupled with the Misaka model, as described in Section III. Methodology A. Mechanical Approaches of R. Hwang, H. Jo, K. S. Kim, and H. J. Hwang, "Hybrid Model of Mathematical and Neural Network Formulations for Rolling Force and Temperature Prediction in Hot Rolling Processes," in IEEE Access, vol. 8, pp. 153123–153133, 2020.

[0047] Preferably, [Mo] / 0.12 + [Sn] / 0.04 + [Sb] / 0.03 + [As] / 0.1 ≥ 1. This allows the hot rolling process to be changed only above the threshold value of the residual element content.

[0048] Preferably, the method includes setting a rolling condition that includes a theoretical hot finishing temperature for each hot rolling pass. More preferably, said rolling condition also includes a deformation rate for each rolling pass.

[0049] Preferably, the specified setting of the specified rolling mode is performed at stage iv. More preferably, the specified setting of the specified rolling mode is performed before determining T OFFSET .

[0050] T OFFSETcan be determined using the Sellars and Tegart models. Details of the Sellars and Tegart model can be found in "J. J. Jonas, C. M. Sellars, W. J. M. Tegart, Strength and structure under hot-working conditions, Metall. Rev. 14 (1) (1969) 1–24" and "Changmin Li, Liang Huang, Mingjie Zhao, Xiaoting Zhang, Jianjun Li, Pengchuan Li, Influence of hot development on dynamic recrystallization behavior of 300M Steel: Rules and modeling, Materials Science and Engineering: A, Volume 797, 2020, 139925, ISSN 0921-5093".

[0051] Formula for determining T OFFSET for different compositions, can be determined using calculated values ​​of T HR and T OFFSET and a statistical model to find the correct coefficients.

[0052] More preferable in the equation defining T OFFSET , a=92, b=264, c=639, d=326 and α=0, β=0.164, γ=-1.64 and δ=-2.43.

[0053] Preferably specified T OFFSET is determined for the last active rolling stand.

[0054] The following section discusses the simulation showing the result of the present invention.

[0055] Each simulation studies a steel with the following composition: 0.1 wt% C, 1 wt% Mn, 0.2 wt% Si, 0.009 wt% P, 0.02 wt% Al, and 0.007 wt% N, as well as varying amounts of residual elements.

[0056] This set of examples focuses on the final rolling stand. In these examples, the rolling mode determines the deformation rate and hot rolling temperature for the final rolling stand.

[0057] Then the theoretical average flow stress is calculated at the theoretical final hot rolling temperature. The theoretical average flow stress is the one at which the specified residual elements are not taken into account. The actual average flow stress, taking into account the residual elements, at the theoretical final hot rolling temperature is also calculated to show the influence of the residual elements.

[0058] Then increase the hot rolling temperature T OFFSET is determined using the following equation:

[0059]

[0060] where [Mo] is the mass percentage of molybdenum, [Sn] is the mass percentage of tin, [Sb] is the mass percentage of antimony, and [As] is the mass percentage of arsenic.

[0061] Then the optimized hot rolling temperature for each example is calculated using the following equation:

[0062] T HR = T HR_TH + T OFFSET ,

[0063] where T HR optimized hot rolling temperature, T OFFSET increasing the hot rolling temperature, and T HR_TH theoretical final hot rolling temperature.

[0064] It can be seen that the actual average flow stress at the optimized hot rolling temperature when residual elements are taken into account is the same as the theoretical average flow stress at the theoretical final hot rolling temperature when residual elements are not taken into account.

[0065] Table 1

[0066] Steel A Steel B Steel C Steel D Content of residual elements Mo [mass percentage] 0,15 0 0,10 0,10 Sn [mass percentage] 0,08 0,10 0 0,05 Sb [mass percentage] 0 0 0,05 0,05 As [mass percentage] 0 0,05 0 0,05 Rolling parameters Strain rate 100 100 100 100 Thickness before the last stand of the rolling mill 3,2 3,2 3,2 3,2 Thickness after the last stand of the rolling mill 2,8 2,8 2,8 2,8 Theoretical significance Theoretical final hot rolling temperature THR_TH [°C] 900 950 1000 950 Theoretical average stress flux at THR_TH [MPa] 135 122 111 122 Real average stress flux at THR_TH [MPa] 143 132 122 145 Optimized value Increase in temperature TOFFSET 33 42 41 74 Optimized hot rolling temperature [°C] 933 992 1041 1024 Real average stress flux at THR [MPa] 135 122 111 122

Claims

1. A method for hot rolling a semi-finished steel product made at least partially from steel scrap, comprising the steps of: i. the preparation of steel of a composition comprising, in mass percent: 0.002 ≤ C ≤ 0.8, 0.1 ≤ Mn ≤ 3, Si ≤ 2, Al ≤ 2, Cr ≤ 0.5, Nb ≤ 0.08, Ti ≤ 0.1, the remainder consisting of Fe, residual elements consisting of Mo, Sn, Sb and As, and unavoidable impurities, by melting steel scrap comprising at least one of the said residual elements, ii. determination of the composition of liquid steel, iii. casting a semi-finished product using the specified liquid steel having a theoretical hot rolling finishing temperature T HR_TH , iv. definition of T OFFSET , an increase in the final hot rolling temperature capable of compensating for the presence of the said residual elements: Mo, Sn, Sb and / or As, at the average flow stress resulting from the deformation applied during hot rolling, where where [Mo] is the mass percent of molybdenum, [Sn] is the mass percent of tin, [Sb] is the mass percent of antimony, and [As] is the mass percent of arsenic, and where a, b, c and d are coefficients representing the influence of molybdenum, tin, antimony and arsenic, respectively, on the average flow stress resulting from the deformation applied during hot rolling, and where α, β, γ and δ are coefficients representing the temperature dependence of the effect of molybdenum, tin, antimony and arsenic on the average flow stress resulting from the deformation applied during hot rolling, v. hot rolling of the said semi-finished product at the optimized hot rolling temperature T HR , which is: T HR = T HR_TH + T OFFSET .

2. The method according to claim 1, wherein the steel composition is obtained by melting steel scrap containing at least one said residual element and cast iron coming from a blast furnace and / or direct reduced iron.

3. The method according to claim 1 or 2, wherein said stage of steel preparation is performed using an electric arc furnace.

4. The method according to any one of claims 1 to 3, wherein said steel preparation step comprises a dephosphorization step and / or a desulphurization step.

5. The method according to any one of paragraphs 1-4, in which [Mo] / 0.12 + [Sn] / 0.04 + [Sb] / 0.015 + [As] / 0.03 ≥ 1.

6. The method according to any one of claims 1 to 4, wherein a = 92, b = 240, c = 639, d = 326 and α = 0, β = 0.164, γ = -1.64 and δ = -2.43.

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