BRPI1107247B1 - thermomechanical process to obtain ultrafine grain ferritic steels - Google Patents

Abstract

thermomechanical processing to obtain ultrafine grain ferritic steels. To generate ultra-fine grain low carbon steels, the refining target product needs to go through two phases, in this order: microstructural conditioning and thermomechanical processing. The microstructural conditioning step consists in tempering the piece in an oven preferably at 900 ° C for 30 minutes of soaking and quenching in water to obtain a fine martensitic structure. The thermomechanical processing step is based on heating the workpiece to a temperature between 700 and 760 ° C for 12 minutes and performing 3 lamination passes with a reduction of 20% and a deformation rate of 0.1 s-1 in each pass, with return to the oven between passes under the same temperature range (700-760 ° c) and with an oven dwell time of 6 minutes. To finalize the grain refining process, the part is annealed preferably at 800 ° C for 1 hour and cooled in water.

Description

(54) Title: THERMOMECHANICAL PROCESS FOR OBTAINING FERRITIC STEELS WITH ULTRAFINE GRAINS (51) lnt.CI .: C21D 8/02; C21D 8/00; C22C 38/00 (73) Holder (s): PAULISTA STATE UNIVERSITY JÚLIO DE MESQUITA FILHO-UNESP. FEDERAL UNIVERSITY OF SÃO CARLOS. FOUNDATION OF AMPARO RESEARCH OF THE STATE OF SÃO PAULO (72) Inventor (s): ALESSANDRO ROGER RODRIGUES; CLEITON LAZARO FAZOLO DE ASSISI; OSCAR BALANCI; OTÁVIO VILLAR DA SILVA NETO (85) Date of Beginning of the National Phase: 28/01/2011

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THERMOMECHANICAL PROCESS FOR OBTAINING FERRITIC STEELS WITH ULTRAFINE GRAINS [001] This specification describes the patent for the invention of an unprecedented thermomechanical technology for obtaining ferritic steels with ultrafine grains, with an innovative design and with important technological and functional improvements, according to the most modern engineering concepts and according to the required standards and specifications, having their own characteristics and endowed with fundamental requirements of novelty and inventive step, resulting in a series of real and extraordinary technical, practical and economic advantages.

[002] STATE OF THE TECHNIQUE [003] The patent document Pl 0408922-7 filed on 03/05/2004 entitled process for the thermomechanical treatment of steel and the base material is heated to a temperature above the recrystallization temperature, to the structure is austenitized, maintained with temperature compensation, then it is molded and, finally, it is abruptly cooled and tempered to form martensite, and the base material is formed by round steel bars, whose recrystallization temperature is compensated through the length of the bar in a compensation furnace, which are then molded by means of inclined lamination, remaining, in essence, straight, and after exceeding the critical molding degree, dynamic recrystallization processes are carried out. Then, the round steel bars, for complete static recrystallization, are subjected to reheating above the temperature of Ac3, and in conclusion, they are abruptly cooled and quenched to form martensite.

[004] Patent document Pl 9104664-5 filed in

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10/28/1991 entitled process for the production of regular oriented, low carbon electric steel in the bath and high silicon having a final standard measure of 0.35 mm to about 0.15 mm, including the steps of establishment of a hot strip of silicon steel, and removing scale from the hot strip, if necessary. Silicon steel is cold rolled to an intermediate standard measurement, at a soak temperature of about 900 ° C to about 930 ° C. Thereafter the silicon steel is cooled in a first cold rolling stage, at a rate of about 280 ° C to about 585 ° C per minute, decreasing to about 595 ° C +/- 30 ° C. The silicon steel is then subjected to a second rapid cooling stage, cooling to about 315 ° C to about 540 ° C, at a cooling rate of about 1390 ° C to about 1945 ° C per minute, followed by by quenching in water. Silicon steel is cold rolled to the final standard measurement, decarbonized, coated with an annealing separator and final annealed. Preferably, but optionally, the hot strip is annealed before the first cold lamination. Preferably, but optionally, silicon steel in the final standard measure, before decarbonisation, is subjected to an ultra-rapid annealing treatment, at a rate greater than 100 ° C per second, at a temperature greater than 675 ° C.

[005] The patent document Pl 8507245-1 filed on 08/05/1985 entitled alloy of steel with a very high carbon content and respective processing teaches a carbon composition in a content between approximately 0.6 weight percent, and the maximum limit of carbon solubility in austenite; silicon in a content from about up to about 7 weight percent; an effective amount of a stabilizing element that acts to stabilize iron carbides against graphitization in the presence of silicon; It's from

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3Π remaining in iron. It is preferred that the silicon is present in a content of about 3 weight percent, and that the stabilizing element is chromium. Steel with a very high carbon content can be processed into a form that is suitable for subsequent superplastic modeling, using any technique that reduces the size of the respective grains to about 10 microns or less, and preferably to about 0.4 up to about 2 microns. The silicon and the stabilizing element serve to produce a series of stable iron carbide particles, designed to maintain the fine grain size during superplastic processing, and to increase the eutectoid temperature, so that superplastic processing can be carried out with high tensile indices and reduced stress levels, under high temperature.

[006] The patent document Pl 9608672-6 filed on 6/1/1996 entitled ferritic steel and process for its manufacture and use teaches a multi-phase steel, having a predominantly polygonal ferrite structure, which includes a second free hard phase of perlite, enriched with carbon, containing martensite and / or bainite and / or residual austenite. This steel has high rigidity, good processability and improved surface quality after hot molding. The invention also relates to a process for the manufacture of this steel and its use.

[007] Patent document Pl 0605810-8 filed on 12/22/2006 entitled composition of low carbon steel for electrical conduction purposes and resulting low carbon steel that can be formed by rolling flat or non-flat teaches that , more precisely, the present composition of low carbon steel is applied in the manufacture of steel of the type used in electrolytic reduction vat bars for the production of primary aluminum and

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4/7 other equivalent applications, where the electrical conductivity measures of steel are considered as important parameters due to the high expenses with electricity; steel in its new composition not only improves electrical conductivity but can also be produced by rolling flat or non-flat sheets, ensuring versatility in relation to the quantity requested by the customer.

[008] BRIEF DESCRIPTION OF THE OBJECT OF THE PRESENT PATENT [009] To generate low carbon steels with ultrafine grains, the refined target product needs to go through two phases, in this order: microstructural conditioning and thermomechanical processing. The microstructural conditioning stage is to temper the part in an oven at 900 ° C for 30 minutes of soaking and abrupt cooling in water, in order to obtain a fine martensitic structure. The thermomechanical processing step is based on heating the part to a temperature between 700 and 760 ° C for 12 minutes and making 3 lamination passes with a 20% reduction and deformation rate of 0.1 s' ¹ in each pass, with return to the oven between passes under the same temperature range (700-760 ° C) and with a 6-minute residence time. To finish the grain refining process, the part is annealed at 800 ° C for 1 hour and cooled in water. [010] It is, therefore, one of the objectives of the present patent to increase the mechanical strength of materials considered as non-noble without the need to add alloy elements.

[011] Another objective of the present patent is achieved to the extent that the technology now proposed is applied to materials that are thicker than those currently found on the market.

[012] Evaluate the machinability of these materials, as they have greater volume and will allow a wide range of applications in products

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5Π machined is another objective of the present patent.

[013] To complement the present description, in order to obtain a better understanding of the characteristics of the present patent, and according to a preferred practical realization of it, the description, attached, is a set of drawings, where in an exemplified way although non-limiting, the following is represented:

[014] Figure 1 graphically illustrates the thermal treatment route for microstructural conditioning and for the thermomechanical processing of rolling to obtain steel with ultrafine grains;

[015] Figure 2 shows the piece in the raw state, considered in the condition “as received” (CR);

[016] Figure 3 shows the product with ultrafine grains obtained from the hot rolling process (MG), with refined microstructure;

[017] Figure 4 shows an image of the microstructure in the raw state (CR); [018] Figure 5 shows an image of the refined microstructure (MG).

[019] The grain refining process in low carbon steel sheets occurs entirely by controlled sequences, in time and temperature, of lamination cycles and heat treatment. In a first stage, the material of the piece is subjected to a microstructural conditioning process in order to make the material's microstructure feasible for later grain refining, a phase known as thermomechanical processing.

[020] The stages of grain refining are described in detail below:

[021] a) Quench at 900 ° C for 30 minutes of soaking with cooling in stirred water.

[022] b) Heating the samples between 700 and 760 ° C for 12 minutes.

[023] c) First rolling pass, at a deformation rate of

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0.1 s' ¹ , with a 20% reduction in thickness, 25 mm of initial thickness, 5 mm of thickness reduction and 20 mm of final thickness. [024] d) Reheat samples between 700 and 760 ° C for 6 minutes.

[025] e) Second lamination pass, at a deformation rate of 0.1 s' ¹ , with a 20% reduction in thickness, with 20 mm of initial thickness, 4 mm of thickness reduction and 16 mm of thickness Final. [026] f) Reheat samples between 700 and 760 ° C for 6 minutes. [027] g) Third lamination pass, at a deformation rate of 0.1 s' ¹ , with a 20% reduction in thickness, with 16 mm of initial thickness, 3.2 mm of thickness reduction and 12, 8 mm of final thickness.

[028] h) Annealing the samples at 800 ° C for 1 hour with subsequent cooling in water.

[029] The following table confirms the microstructural and mechanical characterization of the samples, showing the average grain size and the tensile strength of each material condition.

Sample Condition Grain Size [pm] Tensile Strength [MPa] CR 10.8 ± 3.80 630 MG 1.7 ± 0.32 1000

[030] There is no knowledge of any thermomechanical processing to obtain ferritic steels with ultrafine grains that together bring together all the construction and functional characteristics mentioned above, and that directly or indirectly, is or was as effective as the process object of the present patent .

[031] Having described and illustrated the present invention, it is to be understood that it can undergo numerous modifications and

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7/7 variations in its form of realization, provided that such modifications and variations do not depart from the spirit and scope of the invention, as defined in the claim table.

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Claims (2)

CLAIM 1/2 Time FIGURE 1 I 1 - THERMOMECHANICAL PROCESS FOR OBTAINING FERRITIC STEELS WITH ULTRAFINE GRAINS, characterized by the fact that, in a first stage, the material of the piece is subjected to a microstructural conditioning process in order to enable the microstructure of the material for subsequent grain refining, comprised of the following phases: a) Quench at 900 ° C for 30 minutes of soaking with cooling with stirring water; b) Heating the samples between 700 and 760 ° C for 12 minutes; c) First lamination pass, at a deformation rate of 0.1 s' ¹ , with a 20% reduction in thickness, with 25 mm of initial thickness, 5 mm of thickness reduction and 20 mm of final thickness; d) Reheat the samples between 700 and 760 ° C for 6 minutes; e) Second lamination pass, at a deformation rate of 0.1 s' ¹ , with a 20% reduction in thickness, with 20 mm of initial thickness, 4 mm of thickness reduction and 16 mm of final thickness; f) Reheat samples between 700 and 760 ° C for 6 minutes; g) Third lamination pass, at a deformation rate of 0.1 s ^_1 , with a 20% reduction in thickness, with 16 mm of initial thickness, 3.2 mm of thickness reduction and 12.8 mm of thickness Final; h) Annealing the samples at 800 ° C for 1 hour with subsequent cooling in water. Petition 870180054000, of 06/22/2018, p. 17/18 2/2 FIGURE 2 FIGURE 3