Hot rolling with residual elements

Abstract

The invention relates to a method for hot rolling steel, said method comprising the following steps: i. Processing a steel composition comprising, in weight percent: 0.002 < = C < = 0.8; 0.1 < = Mn < = 3; si < = 2; al < = 2; cr < = 0.5; nb is less than or equal to 0.08; ti < = 0.1; the invention relates to a method for producing a semi-finished product, comprising the following steps: i. Providing a hot-rolling temperature increase TOFFSET capable of counteracting the presence of the residual elements: Mo, Sn, Sb and / or As, and a remainder consisting of Fe, said residual elements and unavoidable impurities, ii. Estimating the composition of molten steel, iii. Casting a semi-finished product with said molten steel having a theoretical finish rolling hot rolling temperature THRTH, iv. Defining a hot rolling temperature increase TOFFSET capable of counteracting the presence of said residual elements: Mo, Sn, Sb and / or As, as a function of the flow stress resulting from the deformation applied during hot rolling, and v, hot rolling is conducted on the semi-finished product at the optimized hot rolling temperature THR, wherein THR is THR = THRTHH + TOF FSET.

Description

Technical Field

[0001] The present invention relates to a method for hot rolling a semi-finished steel product comprising residual elements resulting from steel processing using scrap steel. Background Art

[0002] Steelmaking requires the use of iron-containing materials such as scrap steel, direct reduced iron or pig iron. In order to reduce the carbon footprint of the steel industry, the use of scrap steel is considered key. 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 widely distributed in all steel grades because those residual elements may have an adverse effect on the steel properties.

[0003] During steel processing by direct reduced iron and / or pig iron, a small amount of residual elements inevitably remain in the molten steel. When scrap steel is used, the amount of residual elements is much larger than that of pig iron or direct reduced iron from a blast furnace.

[0004] The present inventors have recently observed that the use of significant amounts of scrap steel in the production of steel creates problems during some manufacturing steps, such as hot rolling. Summary of the invention

[0005] The object of the present invention is therefore to improve the hot rolling processability of a semi-finished steel product which is produced at least partly from scrap steel. This is achieved by a hot rolling method according to any one of claims 1 to 6. DETAILED DESCRIPTION

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

[0007] i. processing a steel composition by smelting scrap steel containing at least one residual element consisting of Mo, Sn, Sb and As and optionally hot metal from a blast furnace and / or direct reduced iron, the steel composition comprising, in weight percentage: 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, the residual elements and unavoidable impurities,

[0008] ii. Estimate the composition of molten steel,

[0009] iii. Using the molten steel to cast a theoretical hot rolling temperature T HR TH Semi-finished products,

[0010] iv. defining the hot rolling temperature increase T that can counteract the presence of the residual elements: Mo, Sn, Sb and / or As according to the flow stress generated by the deformation applied during hot rolling OFFSET ,

[0011] v. At the optimized hot rolling temperature T HR The semi-finished product is hot rolled at T HR for:

[0012] T HR =T HR_TH +T OFFSET .

[0013] In step i, the processing of the steel grade may be accomplished using any apparatus such as a basic oxygen furnace, an open hearth furnace and / or an electric arc furnace.

[0014] The charged material is made of scrap steel and optionally hot metal from a blast furnace and / or direct reduced iron. For example, in the EU-21 scrap specification, the scrap steel that can be used is called old scrap steel (E1 or E3), new scrap steel (E8), shredded scrap steel (E40) or crushed scrap steel (E46).

[0015] Preferably, in step i, 100 kg to 1000 kg of scrap steel per ton of hot metal is used. Preferably, in step i, 100 kg to 950 kg of scrap steel per ton of hot metal is used. Preferably, in step i, 100 kg to 900 kg of scrap steel per ton of hot metal is used. Preferably, in step i, 100 kg to 800 kg of scrap steel per ton of hot metal is used. Preferably, in step i, 100 kg to 600 kg of scrap steel per ton of hot metal is used. Preferably, in step i, 100 kg to 500 kg of scrap steel per ton of hot metal is used. Even more preferably, in step i, 200 kg to 400 kg of scrap steel per ton of hot metal is used.

[0016] In this patent, residual elements are undesirable elements from scrap steel. Unavoidable impurities (eg oxides, nitrides) come from the processing process.

[0017] Preferably, the steel composition comprises 0.01 to 0.5 weight percent of residual elements. Even more preferably, the steel composition comprises 0.05 to 0.30 weight percent of residual elements.

[0018] Preferably, in the processing step i, the scrap steel contains at least two of the residual elements. Even more preferably, in the processing step i, the scrap steel contains at least two of the residual elements in a weight percentage of 0.02 weight percent to 0.30 weight percent, respectively.

[0019] Preferably, step i. comprises a refining step. The refining step comprises a desulfurization step and / or a dephosphorization step. The refining step is preferably completed in a ladle furnace.

[0020] The steel composition of the molten steel may be estimated at the end of the processing step and / or at the beginning of the casting step. Preferably, such an estimation is performed at the beginning of casting of the semi-finished product. The estimation of the steel composition is preferably performed by analyzing a lollipop sample from a hot metal ladle used to fill a casting tundish or ingot mold to obtain the semi-finished product.

[0021] Casting can be any type of casting process. Casting is preferably continuous casting. The semi-finished product can be, for example, a billet, bloom, billet, slab, bar or ingot. Such a semi-finished product is then reheated and hot rolled to reduce its thickness to a target according to its future use.

[0022] Preferably, between casting step iii. and step iv., a rolling pattern is defined.

[0023] In order 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, such as the stress-strain curve, composition and initial microstructure of the steel to be hot-rolled. The rolling pattern includes the reduction ratio, strain rate and hot rolling temperature and other parameters for each hot rolling stand (e.g., each rolling pass) with respect to the mass flow equation.

[0024] Then, based on the steel information and rolling pattern, the flow stress applied at each rolling stand can be defined. The flow stress is equal to the strain ε under the stress-strain curve. α To strain ε β area.

[0025] Then, using the flow stress applied at each pass, a person skilled in the art can appropriately configure the rolling stands and define the presets for each hot rolling.

[0026] However, in the prior art, when establishing a rolling pattern, the content of residual elements from scrap steel is not considered. Therefore, the theoretical stress-strain curve is defined without considering the residual elements.

[0027] Furthermore, it was surprisingly found that the presence of specific residual elements from scrap steel (i.e. Mo, Sn, Sb and As) leads to deviations from the theoretical stress-strain curve. This deviation leads to a change in the flow stress, thereby allowing the target strain rate to be achieved.

[0028] At each stand, stress is applied to the semi-finished steel product by a pair of cylindrical rollers. Each hot rolling stand has a maximum stress it can apply.

[0029] Therefore, all other parameters being equal, when a target reduction ratio is applied during hot rolling of steel processed from scrap containing said residual elements, the applied flow stress deviates from the theoretical flow stress to achieve the target strain rate.

[0030] This deviation may lead to machinability problems and may result in accelerated degradation of the rolling barrel, since the expected stress applied by the hot rolling stand is less than the stress actually applied.

[0031] In order to counteract this surprising effect of residual elements, the inventors propose to increase the hot rolling temperature of the steel not containing the residual elements defined in the rolling mode, that is, the theoretical finishing hot rolling temperature T HR_TH This is achieved by defining the hot rolling temperature increase T that is able to counteract the presence of the residual elements Mo, Sn, Sb and / or As according to the flow stress generated by the deformation applied during hot rolling. OFFSET Completed.

[0032] Therefore, according to the purpose of the present invention, at temperature T HR The semi-finished product is hot rolled, T HR for:

[0033] T HR =T HR_TH +T OFFSET .

[0034] This newly defined hot rolling temperature allows achieving the target reduction ratio when applying the flow stress defined by the rolling mode.

[0035] As known to the person skilled in the art, in order to achieve the temperature at any rolling stand, for example in a mobile hot rolling stand, several parameters can be acted upon.

[0036] For example, hot strip mills usually include a reheating step followed by a rolling step.Based on the minimum hot rolling temperature required at a rolling stand (eg, the last hot rolling stand), the desired temperature at the end of the reheating step can be determined.

[0037] Alternatively, when hot rolling is done after casting, as with some compact strip production mills, the rolling temperature can be influenced by varying the speed of the strip.

[0038] In addition, heating systems placed between the stands and / or between the roughing mill and the hot rolling mill can be used. In addition, heat shields can be used to influence the temperature at the hot rolling stands.

[0039] Typically, devices, such as pyrometers, are provided between the roughing mill and the hot rolling mill, and also after the last hot rolling stand of the rolling mill, which are able to measure the semi-finished steel products.

[0040] The hot rolling may be carried out by any device capable of hot rolling a semi-finished steel product. Preferably, the hot rolling step is carried out by a hot rolling mill comprising three to eight rolling stands, even more preferably five to seven rolling stands. Preferably, the hot rolling step is carried out by a reversing rolling stand.

[0041] The theoretical finishing hot rolling temperature can be a single temperature value, or can be a temperature range. For example, if the theoretical finishing hot rolling temperature is from T HR_TH-最小 To T HR_TH-最大 The temperature range is T HR_TH-最小 +T OFFSET ≤T HR ≤T HR_TH-最大 +T OFFSET T HR The semi-finished product is hot rolled at temperature.

[0042] The determination of the theoretical finishing hot rolling temperature is a step known to those skilled in the art. This determination can take into account metallurgical constraints to obtain the desired properties, as well as specification considerations. For example, it can be done using the Sims model combined with the Misaka model as described in the following section: R. Hwang, H. Jo, K. S. Kim and H. J. Hwang, "III. Methodology A. Mechanical approaches", "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.

[0043] Preferably, [Mo] / 0.12+[Sn] / 0.04+[Sb] / 0.03+[As] / 0.1 ≥ 1. This allows to modify the hot rolling process only above a residual element content threshold.

[0044] Preferably, the method comprises establishing a rolling profile, the rolling profile comprising a theoretical finishing hot rolling temperature for each hot rolling pass. Even more preferably, the rolling profile further comprises a strain rate for each rolling pass.

[0045] Preferably, said establishing of said rolling pattern is done in step iv.

[0046] T can be defined using the model from Sellars and Tegart OFFSET. Details on the model from Sellars and Tegart can be found in "JJ Jonas, CM Sellars, WJM Tegart, Strength and structure under hot-working conditions, Metall. Rev. 14 (1) (1969) 1 to 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, Vol. 797, 2020, 139925, ISSN 0921 to 5093".

[0047] You can use T HR and T OFFSET The calculated values ​​and statistical models are used to define the various components of T OFFSET to find the correct coefficients.

[0048] Preferably, T OFFSET It can be determined according to the following equation:

[0049] T OFFSET =a*[Mo%]+b*[Sn%]+c*[Sb%]+d*[As%],

[0050] wherein [Mo%] is the weight percentage of molybdenum, [Sn%] is the weight percentage of tin, [Sb%] is the weight percentage of antimony, and [As%] is the weight percentage of arsenic, and a, b, c and d are coefficients representing the effects of molybdenum, tin, antimony and arsenic, respectively, on the flow stress generated by deformation applied during hot rolling.

[0051] Even more preferably, a=86, b=264, c=379, d=105.

[0052] Preferably, the T OFFSET It is defined for the last active rolling stand.

[0053] The following section deals with simulations showing the impact of the present invention.

[0054] In each simulation, a steel having the following composition was 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 different amounts of residual elements.

[0055] This set of examples focuses on the last rolling stand. In those examples, the rolling mode defines the strain rate and hot rolling temperature of the last rolling stand.

[0056] Then, the theoretical flow stress at the theoretical finishing hot rolling temperature is calculated. The theoretical flow stress is the stress without considering the residual elements. The actual flow stress considering the residual elements at the theoretical finishing hot rolling temperature is also calculated to show the influence of the residual elements.

[0057] Then, the hot rolling temperature increase T is defined by the following formula: OFFSET :

[0058] T OFFSET =a*[Mo]+b*[Sn]+c*[Sb]+d*[As],

[0059] wherein [Mo] is the weight percent of molybdenum, [Sn] is the weight percent of tin, [Sb] is the weight percent of antimony, and [As] is the weight percent of arsenic, and wherein a=86, b=264, c=379, d=105.

[0060] Then, the optimized hot rolling temperature for each example was calculated using the following formula:

[0061] T HR =T HR_TH +T OFFSET ,

[0062] Where T HR is the optimized hot rolling temperature, T OFFSET As the hot rolling temperature increases, and T HR_TH is the theoretical finishing hot rolling temperature.

[0063] It can be observed that the actual flow stress at the optimized hot rolling temperature when the residual elements are considered is the same as the theoretical flow stress at the theoretical finishing hot rolling temperature when the residual elements are not considered.

[0064]

[0065] Table 1

Claims

1. A method for hot rolling steel, comprising the following steps: i. processing a steel composition by smelting scrap steel containing at least one residual element consisting of Mo, Sn, Sb and As and optionally hot metal from a blast furnace and / or direct reduced iron, the steel composition comprising, in weight percentage: 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, the residual elements and unavoidable impurities, ii. Estimate the composition of molten steel, iii. Using the molten steel to cast a theoretical hot rolling temperature T HR_TH Semi-finished products, iv. defining a finishing hot rolling temperature increase T that can counteract the presence of the residual elements: Mo, Sn, Sb and / or As according to the flow stress generated by the deformation applied during the hot rolling OFFSET , v. At the optimized hot rolling temperature T HR The semi-finished product is hot rolled at T HR for: T HR =T HR _ TH +T OFFSET 。 2. The method of claim 1, wherein the processing step is performed using an electric arc furnace.

3. The method according to claim 1 or 2, wherein the processing 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. The method according to any one of claims 1 to 4, wherein T OFFSET can be determined according to the following equation: T OFFSET =a*[Mo]+b*[Sn]+c*[Sb]+d*[As], wherein [Mo] is the weight percent of molybdenum, [Sn] is the weight percent of tin, [Sb] is the weight percent of antimony, and [As] is the weight percent of arsenic, and a, b, c and d are coefficients representing the effects of molybdenum, tin, antimony and arsenic, respectively, on the flow stress generated by the deformation applied during the hot rolling.

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

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