Hot rolling with residual elements

Abstract

1. A method of hot rolling steel, comprising the steps of: i. producing a steel composition comprising, in weight percent, 0.002≦C≦0.8, 0.1≦Mn≦3, Si≦2, Al≦2, Cr≦0.5, Nb≦0.08, Ti≦0.1, with the balance consisting of Fe, residual elements consisting of Mo, Sn, Sb and As, and unavoidable impurities, by smelting steel scrap containing at least one of said residual elements and, optionally, hot metal from a blast furnace and / or direct reduced iron; ii. estimating the liquid steel composition; and iii. HR_TH iv. a hot rolling temperature increase, T, that can offset the presence of the residual elements Mo, Sn, Sb and / or As on the average flow stress resulting from the deformation applied during hot rolling. OFFSET and v. defining an optimized hot rolling temperature T HR hot rolling at T HR =T HR_TH +T OFFSET and

Description

[Technical Field]

[0001] The present invention relates to a method for hot rolling semi-finished steel products containing residual elements resulting from steel production using steel scrap. [Background technology]

[0002] Steelmaking requires the use of iron-containing materials such as scrap steel, direct reduced iron, or pig iron. The use of scrap steel is considered important for reducing the steel industry's carbon footprint. However, scrap steel contains residual elements such as copper, chromium, molybdenum, nickel, tin, antimony, zinc, and / or arsenic. Therefore, the use of scrap steel is not widespread for all steel grades because these residual elements can have a detrimental effect on steel properties.

[0003] During steelmaking with direct reduced iron and / or pig iron, small amounts of residual elements inevitably remain in the molten steel. When steel scrap is used, the amount of residual elements is much higher compared to pig iron or direct reduced iron coming from a blast furnace. Summary of the Invention [Problem to be solved by the invention]

[0004] Recently, the inventors have observed that the production of steel using a significant amount of steel scrap causes problems during some production steps, such as hot rolling.

[0005] It is therefore an object of the present invention to improve the hot rolling workability of steel semi-finished products made at least partly from steel scrap, which is achieved by a hot rolling method according to any one of claims 1 to 6. [Means for solving the problem]

[0006] The present invention relates to a method for hot rolling a steel semi-finished product, comprising the steps of: i. producing a steel composition comprising, in weight 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 balance consisting of Fe, residual elements consisting of Mo, Sn, Sb, and As, and unavoidable impurities, by smelting steel scrap comprising at least one residual element and, optionally, hot metal and / or direct reduced iron from a blast furnace; ii. Estimating the composition of the molten steel; iii. Theoretical finishing hot rolling temperature T HR_TH casting a semi-finished product having the molten steel; iv. The increase in the hot rolling temperature that can offset the presence of the residual elements Mo, Sn, Sb and / or As relative to the average flow stress resulting from the deformation applied during hot rolling, T OFFSET defining v. The semi-finished product is subjected to an optimized hot rolling temperature T HR hot rolling at T HR =T HR_TH +T OFFSET Steps and The present invention relates to a method, comprising:

[0007] The preparation of the steel grade in step 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.

[0008] The charged material consists of steel scrap and, optionally, hot metal and / or direct reduced iron from the blast furnace. For example, the steel scrap that can be used is referred to in the EU-21 Steel Scrap specification as old scrap (E1 or E3), new scrap (E8), shredded scrap (E40) or fragmented scrap (E46).

[0009] Preferably, in step i, 100 to 1000 kg of scrap steel is used per ton of hot metal. Preferably, in step i, 100 to 950 kg of scrap steel is used per ton of hot metal. Preferably, in step i, 100 to 900 kg of scrap steel is used per ton of hot metal. Preferably, in step i, 100 to 800 kg of scrap steel is used per ton of hot metal. Preferably, in step i, 100 to 600 kg of scrap steel is used per ton of hot metal. Preferably, in step i, 100 to 500 kg of scrap steel is used per ton of hot metal. Even more preferably, in step i, 200 to 400 kg of scrap steel is used per ton of hot metal.

[0010] In this patent, residual elements are undesirable elements that come from the steel scrap. Inevitable impurities come from the manufacturing process, such as oxides and nitrides.

[0011] Preferably, the steel composition comprises between 0.01 weight percent and 0.5 weight percent of the residual elements, and even more preferably, the steel composition comprises between 0.05 weight percent and 0.30 weight percent of the residual elements.

[0012] Preferably, in said preparation step i, said steel scrap contains at least two of said residual elements. Even more preferably, in said preparation step i, said steel scrap contains 0.02 to 0.30 weight percent each of said at least two of said residual elements.

[0013] Preferably, step i. comprises a refining step, which comprises a desulfurization step and / or a dephosphorization step, which is preferably carried out in a ladle furnace.

[0014] The steel composition of the molten steel can be estimated at the end of the preparation step and / or at the beginning of the casting step. Preferably, such estimation is performed at the beginning of the casting of the semi-finished product. The estimation of the steel composition is preferably performed by analyzing a lollipop-shaped sample from the hot metal ladle used to fill the casting tundish or ingot mold to obtain the semi-finished product.

[0015] Casting can be any type of casting process. Casting is preferably continuous casting. The semi-finished product can be, for example, a billet, bloom, blank, slab, bar, or ingot. Such semi-finished product is then reheated and hot rolled to reduce the thickness to a target for future use.

[0016] Preferably, between casting step iii and step iv, a rolling pattern is defined. DETAILED DESCRIPTION OF THE INVENTION

[0017] To hot roll the cast semi-finished product, a person skilled in the art establishes a rolling pattern. As is well known to those skilled in the art, establishing a rolling pattern requires information about the steel to be hot rolled, such as its stress-strain curve, composition, and initial microstructure. The rolling pattern includes, among other parameters, the reduction rate, strain rate, and hot rolling temperature of each hot rolling stand (e.g., each rolling pass) according to a mass flow equation.

[0018] Then, based on the steel information and the rolling pattern, the average flow stress applied at each rolling stand can be defined. The average flow stress is determined by the strain ε α Strain ε β It is equal to the area under the stress-strain curve.

[0019] The average flow stress applied in each pass can then be used by one skilled in the art to properly configure the rolling stand and define the presets for each hot rolling run.

[0020] However, in the state of the art, the content of residual elements from the steel scrap is not taken into account when establishing the rolling pattern, and therefore the theoretical stress-strain curve is defined without taking said residual elements into account.

[0021] Furthermore, it has been surprisingly found that the presence of certain residual elements, namely Mo, Sn, Sb and As, originating from the steel scrap leads to deviations from the theoretical stress-strain curve, which in turn leads to changes in the mean flow stress that make it possible to achieve the target strain rate.

[0022] In each stand, a pair of cylindrical rolls applies stress to the steel semi-finished product. Each hot rolling stand has a maximum stress that it can apply.

[0023] As a result, all other parameters being equal, when applying a target reduced speed during hot rolling of steel made from scrap containing said residual elements, the applied mean flow stress will deviate from the theoretical mean flow stress to achieve the target strain rate.

[0024] This deviation can lead to workability problems and possibly accelerated deterioration of the rolling cylinders, since the expected stresses applied by the hot rolling stand are less than the stresses actually applied.

[0025] To counter this surprising effect of the residual elements, the inventors have determined that the hot rolling temperature defined by the rolling pattern of the steel not containing the residual elements, i.e., the theoretical finish hot rolling temperature T HR_TH This is because the increase in the hot rolling temperature, T, can offset the presence of the residual elements Mo, Sn, Sb and / or As on the average flow stress resulting from the deformation applied during hot rolling. OFFSET This is done by defining

[0026] Consequently, for the purposes of the present invention, the semi-finished product is heated to a temperature T HR , i.e., T HR =T HR_TH +TOFFSET It is hot rolled.

[0027] This newly defined hot rolling temperature makes it possible to achieve the target reduction rate when applying an average flow stress defined by the rolling pattern.

[0028] As known to those skilled in the art, it is possible to act on several parameters to achieve the temperature, for example, in any rolling stand within an operating hot rolling stand.

[0029] For example, a hot strip mill typically includes a reheating step prior to the rolling step, and based on the required minimum hot rolling temperature in the rolling stand, e.g., the last hot rolling stand, it is possible to determine the required temperature at the end of the reheating step.

[0030] Alternatively, if hot rolling is performed after casting, as is the case in some small strip production mills, it is possible to influence the rolling temperature by varying the speed of the strip.

[0031] Furthermore, it is possible to use heating systems arranged between the stands and / or between the roughing mill and the hot rolling mill, and it is also possible to use heat covers to influence the temperature of the hot rolling stands.

[0032] Typically, devices capable of measuring the steel semi-finished products, such as pyrometers, are placed between the roughing mill and the hot rolling mill and after the last hot rolling stand of the rolling mill.

[0033] The hot rolling can be carried out by any means capable of hot rolling a steel semi-finished product. Preferably, the hot rolling step is carried out by a hot rolling mill comprising 3 to 8 rolling stands, even more preferably 5 to 7 rolling stands. Preferably, the hot rolling step is carried out by reversing rolling stands.

[0034] The theoretical finish hot rolling temperature may be a single temperature value or may be a range of temperatures. For example, if the theoretical finish hot rolling temperature is T HR_TH-MIN From T HR_TH-MAX If the temperature ranges from HR_TH-MIN +T OFFSET ≦T HR ≦T HR_TH-MAX +T OFFSET At temperature T HR It is hot rolled.

[0035] Determining the theoretical finish hot rolling temperature is a step well known to those skilled in the art. This determination can take into account metallurgical constraints and model considerations to achieve the desired properties. For example, this can be done using a Sims model coupled to a Misaka model, as described in chapter "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.

[0036] Preferably, [Mo] / 0.12 + [Sn] / 0.04 + [Sb] / 0.03 + [As] / 0.1 ≥ 1. This makes it possible to modify the hot rolling process only if the residual element content threshold is exceeded.

[0037] Preferably, the method includes establishing a rolling pattern that includes a theoretical finishing hot rolling temperature for each hot rolling pass. Even more preferably, said rolling pattern also includes a strain rate for each rolling pass.

[0038] Preferably, said establishing of said rolling pattern is carried out in step iv.

[0039] T OFFSET can be defined using the Sellars and Tegart model. Details of the Sellars and Tegart model can be found in "JJ 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 deformation on dynamic recrystallization behavior of 300M steel: Rules and modeling, Materials Science and Engineering: A, Volume 797, 2020, 139925, ISSN 0921-5093".

[0040] T of various compositions OFFSET The formula for defining T is used to find the correct coefficients. HR and T OFFSET can be defined using calculated values ​​and statistical models of

[0041] Preferably, T OFFSET is expressed by the following equation: T OFFSET =a*[Mo%]+b*[Sn%]+c*[Sb%]+d*[As%] can be determined according to where [Mo%] is the weight percent of molybdenum, [Sn%] is the weight percent of tin, [Sb%] is the weight percent of antimony, [As%] is the weight percent of arsenic, and a, b, c, and d are coefficients that represent the influence of molybdenum, tin, antimony, and arsenic, respectively, on the average flow stress resulting from the deformation applied during hot rolling.

[0042] Even more preferably, a=86, b=264, c=379, and d=105.

[0043] Preferably, the T OFFSET is defined for the last working rolling stand.

[0044] The following paragraphs deal with simulations that demonstrate the impact of the present invention.

[0045] In each simulation, a steel with the following composition is studied: 0.1 weight percent C, 1 weight percent Mn, 0.2 weight percent Si, 0.009 weight percent P, 0.02 weight percent Al and 0.007 weight percent N and various amounts of residual elements.

[0046] This set of examples focuses on the last rolling stand. In these examples, the rolling pattern defines the strain rate and hot rolling temperature of the last rolling stand.

[0047] The theoretical mean flow stress at the theoretical finish hot rolling temperature is then calculated. The theoretical mean flow stress does not take into account the residual elements. To show the effect of the residual elements, the actual mean flow stress at the theoretical finish hot rolling temperature is also calculated, taking into account the residual elements.

[0048] Next, the hot rolling temperature increase T OFFSET But the following formula, T OFFSET =a*[Mo]+b*[Sn]+c*[Sb]+d*[As] is defined using where [Mo] is the weight percent of molybdenum, [Sn] is the weight percent of tin, [Sb] is the weight percent of antimony, [As] is the weight percent of arsenic, a=86, b=264, c=379, and d=105.

[0049] The optimized hot rolling temperature for each example was then calculated using the following formula: T HR =THR_TH +T OFFSET is calculated using In the formula, T HR is the optimized hot rolling temperature, and T OFFSET is the increase in the hot rolling temperature, and T HR_TH is the theoretical finish hot rolling temperature.

[0050] It can be observed that the actual mean flow stress at the optimized hot rolling temperature when the residual elements are considered is the same as the theoretical mean flow stress at the theoretical finish hot rolling temperature when the residual elements are not considered.

[0051] [Table 1]

Claims

1. 1. A method for hot rolling steel, comprising the steps of: i. Producing a steel composition comprising, in weight 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 balance consisting of Fe, residual elements consisting of Mo, Sn, Sb, and As, and unavoidable impurities, by smelting steel scrap comprising at least one of said residual elements and, optionally, hot metal from a blast furnace and / or direct reduced iron; ii. Estimating the composition of molten steel; iii. Theoretical finish hot rolling temperature T ＨＲ＿ＴＨ casting a semi-finished product having the molten steel; iv. The increase in the finish hot rolling temperature that can offset the presence of the residual elements Mo, Sn, Sb and / or As on the average flow stress resulting from the deformation applied during hot rolling, T ＯＦＦＳＥＴ defining v. The semi-finished product is subjected to an optimized hot rolling temperature T ＨＲ hot rolling at T ＨＲ =T ＨＲ＿ＴＨ +T ＯＦＦＳＥＴ Steps and A method comprising:

2. The method of claim 1 , wherein the creating step is performed using an electric arc furnace.

3. 3. The method according to claim 1 or 2, wherein the producing step comprises a dephosphorization step and / or a desulfurization step.

4. The method according to any one of claims 1 to 3, wherein [Mo] / 0.12+[Sn] / 0.04+[Sb] / 0.03+[As] / 0.1≧1.

5. T ＯＦＦＳＥＴ But the following equation, T ＯＦＦＳＥＴ =a*[Mo]+b*[Sn]+c*[Sb]+d*[As] can be determined according to where [Mo] is the weight percent of molybdenum, [Sn] is the weight percent of tin, [Sb] is the weight percent of antimony, [As] is the weight percent of arsenic, and a, b, c, and d are coefficients that represent the effect of molybdenum, tin, antimony, and arsenic, respectively, on the mean flow stress resulting from the deformation applied during hot rolling. The method according to any one of claims 1 to 4.

6. 6. The method of claim 5, wherein a=86, b=264, c=379, and d=105.

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