High-elongation high-strength steel and method of manufacturing

Abstract

The application provides a high-elongation high-strength steel and a preparation method, and relates to the technical field of high-strength high-elongation steel plates.The chemical components of the high-elongation high-strength steel are as follows in terms of mass percentage: C: 0.02-0.05 wt.%, Si: 0.20-0.30 wt.%, Mn: 1.45-1.80 wt.%, P: less than or equal to 0.005 wt.%, S: less than or equal to 0.005 wt.%, Zr: 0.05-0.2 wt.%, and the balance of Fe and inevitable impurities.The preparation method comprises smelting, heating homogenization treatment, iron scale removal, hot rolling and cooling.The application realizes the performance of the steel plate with high strength, high toughness and high elongation at the same time through the design of low-cost alloy components and the optimization of a short-process preparation method, can meet the requirements of low cost, high strength, high toughness and high elongation in the fields of engineering machinery and vehicle steel, and is beneficial to industrial production practice and popularization.

Description

Technical Field

[0001] This invention relates to the technical field of high-strength, high-elongation steel plates, and particularly to a high-elongation, high-strength steel and its preparation method. Background Technology

[0002] With the rapid development of my country's economy and steel industry, the requirements for structural steel plates used in automobile beams, bridge construction, and ship structures are becoming increasingly stringent. Particularly in the automotive industry, due to the demand for lightweight vehicles and improved collision safety, high-strength automotive steel plates have gained widespread attention worldwide.

[0003] While existing automotive steel sheets have improved strength, their elongation is only around 20%, resulting in a poor strength-ductility product. Furthermore, the large amount of alloying elements added, the difficulty of processing, and the low yield rate significantly increase the cost of the steel sheets, thus restricting the application of the material.

[0004] To enable the wider application of high-strength steel sheets in automobiles, improving the formability of the steel sheets while ensuring high strength is a crucial research direction in the development of advanced automotive steels. In actual production, many components require processing before use, such as automotive exterior panels, body panels, and suspension components. Therefore, the development of advanced automotive steels not only needs to focus on strength, but also on formability such as elongation, which determines the viability of the material for use.

[0005] However, these methods all have some problems.

[0006] For example, Chinese patent CN109321840A discloses a 280MPa grade low alloy high strength steel and its manufacturing method. The method uses Nb-containing components, hot rolling process control, and annealing process to produce steel with an elongation of ≥28%, but the yield strength is only 280MPa, which is low. In addition, it requires an additional heat treatment process, which not only increases the cost, but also causes grain growth.

[0007] Chinese patent CN114959481A discloses a high elongation 420MPa grade hot-dip galvanized low alloy high-strength steel and its production method. The composition selection is extensive, including not only Ti and V elements, but also the more expensive Nb element; and it requires a continuous annealing and galvanizing process, which increases the number of operation steps. The elongation of the final product is 25%.

[0008] Chinese patent CN109202028A discloses a high-elongation flanged steel plate and its preparation method. The plate contains a large number of high-cost alloying elements, and the metallographic structure of the prepared steel plate is a single quasi-polygonal ferrite structure. The elongation does not exceed 26%, and the casting speed of the continuous casting billet is very fast, which may lead to the fracture of the continuous casting billet and affect the subsequent hot rolling process.

[0009] Chinese patent CN115572908A discloses a high-elongation multiphase high-strength steel and its production method. It contains a large number of high-cost alloying elements. The steel is produced by introducing finely dispersed Cr3C precipitates into the steel, which can inhibit the growth of austenite grains. However, the elongation of the prepared steel is small, and the microstructure of the steel is ferrite / martensite / bainite / retained austenite.

[0010] In summary, most of the steel plates developed so far have relatively low elongation rates, failing to achieve a good balance between high strength and high elongation. Consequently, they lack good formability and cannot be used to manufacture parts with complex shapes. Furthermore, many existing high-elongation, high-strength steels require heat treatment after hot rolling to improve yield strength, tensile strength, and elongation, which obviously increases production costs. The production of some steels involves the selection of high-cost elements and requires complex preparation processes, making operations more difficult and costly, which is detrimental to large-scale industrial production and promotion. Summary of the Invention

[0011] The technical problem to be solved by this invention is that the elongation of current high-strength steel plates is low. In order to improve strength and elongation, heat treatment is required after rolling, or high-cost alloying elements are added, or dispersed strengthening carbide precipitates are introduced, or fracture defects are caused by excessive casting speed of continuous casting billets. These methods will increase production costs and process operation difficulty, reduce production efficiency, and make it difficult to carry out industrial production and large-scale application.

[0012] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0013] A high-elongation, high-strength steel, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.02-0.05 wt.%, Si: 0.20-0.30 wt.%, Mn: 1.45-1.80 wt.%, P≤0.005 wt.%, S≤0.005 wt.%, Zr: 0.05-0.2 wt.%, with the balance being Fe and unavoidable impurities.

[0014] Preferably, in the chemical composition of the high elongation high-strength steel, the chemical composition of Zr is controlled as follows: 3.0 ≤ [Zr%]: [C%] ≤ 7.0, 0.07 wt.% ≤ [Zr%] + [C%] ≤ 0.25 wt.%.

[0015] Preferably, the high-elongation high-strength steel plate has a polygonal ferrite microstructure with an average grain size of 6-9 μm, a thickness of 6-25 mm, a yield strength of not less than 430 MPa, a tensile strength of not less than 490 MPa, a yield-to-tensile ratio of not more than 0.85, an elongation of not less than 30%, n of not less than 0.2, r of not less than 1, a strength-ductility product of not less than 14.7 GPa%, and a low-temperature impact energy of not less than 100 J at -20℃.

[0016] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0017] S1, Smelting

[0018] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then melted to obtain alloy melt. The alloy melt is then cast to obtain a billet.

[0019] S2. Heating homogenization treatment

[0020] The S1 billet is heated and homogenized to obtain a homogenized billet.

[0021] S3, Iron Oxide Scale Removal

[0022] The homogenized billet of S2 was air-cooled, and then the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0023] S4, hot rolled

[0024] The S3 surface-cleaned billet is subjected to 3-5 passes of rough rolling and finish rolling to obtain the plate.

[0025] S5, Cooling

[0026] The S4 sheet is passed through a controlled cooling system via rollers, and then air-cooled to room temperature to obtain the finished sheet.

[0027] Preferably, Al-Fe is used as a deoxidizer in S1; low-carbon, low-phosphorus ferromanganese, ferrosilicon, and ferrizircon are added for alloying, and the ferrizircon is added during the RH refining process.

[0028] Preferably, the temperature for homogenization treatment in S2 is 1250-1300℃, and the holding time is 2-3h.

[0029] Preferably, the cooling rate of the air cooler in S3 is controlled at 2-7℃ / s.

[0030] Preferably, in S4, the initial rolling temperature of the roughing rolling is 1050±100℃, the final rolling temperature is 970±10℃, the reduction per pass is greater than 15%, and the total reduction is 50-60%; the finishing rolling begins when the intermediate billet temperature is 950±20℃, the reduction per pass is >20%, and the final rolling temperature is 850±50℃.

[0031] Preferably, the thickness of the slab in S1 is 100-120mm, the thickness of the intermediate slab in S4 is 50-60mm, and the thickness of the plate in S4 is 6-25mm.

[0032] Preferably, the cooling rate of the cooling system in S5 is controlled at 10-15℃ / s, and air cooling is performed after cooling to 600℃ to 650℃. The air cooling rate is controlled at 2-7℃ / s, and the final microstructure of the steel sample obtained by cooling is all polygonal ferrite.

[0033] The above technical solution has at least the following advantages compared with the existing technology:

[0034] The present invention provides a high-elongation, high-strength steel and its preparation method. By employing Zr microalloying technology, the final microstructure of the steel is controlled to be fine and uniform ferrite, thereby improving the strength and elongation of the steel and enhancing its formability.

[0035] The steel plate prepared by this invention uses only Zr microalloying, precisely controlling the content of C, Si, Mn and Zr, without the need for large additions of high-cost alloying elements or special smelting methods. Its preparation method adopts a controlled rolling and controlled cooling production process, and does not require high-cost tempering heat treatment or other machining and heat treatment after rolling to meet the performance requirements of the steel plate, thereby simplifying the manufacturing process and reducing the manufacturing cost of steel.

[0036] The high-elongation, high-strength steel plate prepared by this invention has a polygonal ferrite microstructure with an average grain size of 6-9 μm, a thickness of 6-25 mm, a yield strength of not less than 430 MPa, a tensile strength of not less than 490 MPa, a yield-to-tensile ratio of not more than 0.85, an elongation of not less than 30%, n of not less than 0.2, r of not less than 1, a strength-ductility product of not less than 14.7 GPa%, and an impact energy of not less than 100 J at -20℃.

[0037] In summary, compared with other traditional methods, the method of this invention achieves high strength, high toughness and high elongation of steel plates through the design of low-cost alloy composition and the optimization of short-process preparation method. It can meet the requirements of low cost, high strength, high toughness and high elongation in fields such as engineering machinery and vehicle steel. The method is low in cost, high in efficiency, simple influencing factors, does not require heat treatment, has a wide range of applications, and is conducive to industrial production practice and promotion. Attached Figure Description

[0038] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0039] Figure 1 This is a metallographic image of the steel plate prepared by a high elongation and high strength steel preparation method according to Embodiment 1 of the present invention.

[0040] Figure 2 This is a metallographic image of the steel plate prepared by a high elongation and high strength steel preparation method according to Embodiment 2 of the present invention.

[0041] Figure 3 The image shows the metallographic structure of a steel plate prepared by a high-elongation, high-strength steel preparation method according to Embodiment 3 of the present invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0043] To meet the development needs of the automotive steel industry and to enable steel to have higher elongation and thus higher plastic deformation capacity, this invention provides a high-elongation, high-strength steel, addressing the shortcomings of existing technologies. The chemical composition of the high-elongation, high-strength steel, by mass percentage, is: C: 0.02-0.05 wt.%, Si: 0.20-0.30 wt.%, Mn: 1.45-1.80 wt.%, P≤0.005 wt.%, S≤0.005 wt.%, Zr: 0.05-0.2 wt.%, with the balance being Fe and unavoidable impurities.

[0044] Specifically, in the chemical composition of the high elongation high-strength steel, the chemical composition of Zr is controlled as follows: 3.0 ≤ [Zr%]: [C%] ≤ 7.0, 0.07 wt.% ≤ [Zr%] + [C%] ≤ 0.25 wt.%.

[0045] Specifically, the microstructure of the high-elongation high-strength steel plate is polygonal ferrite with an average grain size of 6-9 μm, a thickness of 6-25 mm, a yield strength of not less than 430 MPa, a tensile strength of not less than 490 MPa, a yield-to-tensile ratio of not more than 0.85, an elongation of not less than 30%, n of not less than 0.2, r of not less than 1, a strength-ductility product of not less than 14.7 GPa%, and an impact energy of not less than 100 J at -20℃.

[0046] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0047] S1, Smelting

[0048] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then melted to obtain alloy melt. The alloy melt is then cast to obtain a billet.

[0049] S2. Heating homogenization treatment

[0050] The S1 billet is heated and homogenized to obtain a homogenized billet.

[0051] S3, Iron Oxide Scale Removal

[0052] The homogenized billet of S2 was air-cooled, and then the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0053] S4, hot rolled

[0054] The S3 surface-cleaned billet is subjected to 3-5 passes of rough rolling and finish rolling to obtain the plate.

[0055] S5, Cooling

[0056] The S4 sheet is passed through a controlled cooling system via rollers, and then air-cooled to room temperature to obtain the finished sheet.

[0057] Specifically, Al-Fe is used as a deoxidizer in S1; low-carbon, low-phosphorus ferromanganese, ferrosilicon, and ferrizirconia are added for alloying, with ferrizirconia being added during the RH refining process.

[0058] Specifically, the temperature for homogenization treatment in S2 is 1250-1300℃, and the holding time is 2-3 hours.

[0059] Specifically, the air cooling rate in S3 is controlled at 2-7℃ / s.

[0060] Specifically, in S4, the initial rolling temperature of the roughing mill is 1050±100℃, the final rolling temperature is 970±10℃, the reduction per pass is greater than 15%, and the total reduction is 50-60%; the finishing mill begins when the intermediate billet temperature is 950±20℃, the reduction per pass is >20%, and the final rolling temperature is 850±50℃.

[0061] Specifically, the thickness of the slab in S1 is 100-120mm, the thickness of the intermediate slab in S4 is 50-60mm, and the thickness of the plate in S4 is 6-25mm.

[0062] Specifically, the cooling rate of the cooling system in S5 is controlled at 10-15℃ / s. When the temperature is cooled to 600℃ to 650℃, air cooling is performed, with the air cooling rate controlled at 2-7℃ / s. The final microstructure of the steel sample obtained by the controlled cooling is all polygonal ferrite.

[0063] Example 1

[0064] A high-elongation, high-strength steel with a thickness of 6 mm, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.03 wt.%, Si: 0.30 wt.%, Mn: 1.80 wt.%, P: 0.005 wt.%, S: 0.005 wt.%, Zr: 0.2 wt.%, with the balance being Fe and unavoidable impurities.

[0065] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0066] S1, Smelting

[0067] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then smelted to obtain alloy melt, in which Al-Fe is used as deoxidizer; during the smelting process, low carbon low phosphorus ferromanganese, ferrosilicon, and ferrizircon need to be added for alloying, and the timing of adding ferrizircon is during the RH refining process; finally, the alloy melt is cast to obtain a billet with a thickness of 120mm;

[0068] S2. Heating homogenization treatment

[0069] The S1 billet was subjected to a heating homogenization treatment at a temperature of 1300℃ and a holding time of 3h to obtain a homogenized billet.

[0070] S3, Iron Oxide Scale Removal

[0071] The homogenized S2 billet was air-cooled at a rate of 7°C / s until the temperature of the steel billet dropped to 1150°C. Then, the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0072] S4, hot rolled

[0073] The S3 surface-cleaned billet is subjected to three passes of rough rolling and finish rolling. The three rough rolling passes are started at 1050℃ and finished at 980℃, with a reduction of 25% per pass and a total reduction of 50%. The finish rolling starts at the temperature of the intermediate billet when it is heated to 970℃. The thickness of the intermediate billet is 50mm. The reduction rate of each rolling pass is >20%, and the final rolling temperature is 835℃ to obtain a plate with a thickness of 6mm.

[0074] S5, Cooling

[0075] The S4 sheet is passed through a controlled cooling system via rollers. The cooling rate of the system is controlled at 11℃ / s. After cooling to 615℃, it is air-cooled at a rate of 2℃ / s until it reaches room temperature. The final microstructure of the steel sample obtained by controlled cooling is entirely polygonal ferrite, thus obtaining the finished sheet.

[0076] like Figure 1 As shown, the high-elongation, high-strength steel plate prepared in this embodiment has a polygonal ferrite microstructure with an average grain size of 8 μm, a thickness of 6 mm, a yield strength of 430 MPa, a tensile strength of 508 MPa, a yield-to-tensile ratio of 0.84, an elongation of 30%, n of 0.22, r of 1.44, a strength-ductility product of 15.2 GPa%, and an impact energy of 110 J at -20℃.

[0077] Example 2

[0078] A high-elongation, high-strength steel with a thickness of 10 mm, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.025 wt.%, Si: 0.22 wt.%, Mn: 1.50 wt.%, P: 0.003 wt.%, S: 0.003 wt.%, Zr: 0.13 wt.%, with the balance being Fe and unavoidable impurities.

[0079] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0080] S1, Smelting

[0081] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then smelted to obtain alloy melt, in which Al-Fe is used as deoxidizer; during the smelting process, low carbon low phosphorus ferromanganese, ferrosilicon, and ferrizircon need to be added for alloying, and ferrizircon is added during the RH refining process; finally, the alloy melt is cast to obtain a billet with a thickness of 105mm;

[0082] S2. Heating homogenization treatment

[0083] The S1 billet was subjected to a heating homogenization treatment at a temperature of 1255℃ and a holding time of 2.5h to obtain a homogenized billet.

[0084] S3, Iron Oxide Scale Removal

[0085] The homogenized S2 billet was air-cooled at a rate of 7°C / s until the temperature of the steel billet dropped to 1120°C. Then, the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0086] S4, hot rolled

[0087] The S3 surface-cleaned billet is subjected to three passes of rough rolling and finish rolling. The three passes of rough rolling are started at 1100℃ and finished at 980℃, with a reduction of 25% per pass and a total reduction of 50%. The finish rolling starts at the temperature of the intermediate billet when it is heated to 970℃. The thickness of the intermediate billet is 50mm. The reduction rate of each rolling pass is >20%, and the final rolling temperature is 835℃ to obtain a plate with a thickness of 10mm.

[0088] S5, Cooling

[0089] The S4 sheet is passed through a controlled cooling system via rollers. The cooling rate of the system is controlled at 13℃ / s. After cooling to 630℃, it is air-cooled at a rate of 5℃ / s until it reaches room temperature. The final microstructure of the steel sample obtained by controlled cooling is all polygonal ferrite, thus obtaining the finished sheet.

[0090] like Figure 2 As shown, the high-elongation, high-strength steel plate prepared in this embodiment has a polygonal ferrite microstructure with an average grain size of 7.5 μm, a thickness of 10 mm, a yield strength of 438 MPa, a tensile strength of 540 MPa, a yield-to-tensile ratio of 0.81, an elongation of 32%, n of 0.25, r of 1.50, a strength-ductility product of 17.3 GPa%, and an impact energy of 108 J at -20℃.

[0091] Example 3

[0092] A high-elongation, high-strength steel with a thickness of 25 mm, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.03 wt.%, Si: 0.30 wt.%, Mn: 1.80 wt.%, P: 0.005 wt.%, S: 0.005 wt.%, Zr: 0.2 wt.%, with the balance being Fe and unavoidable impurities.

[0093] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0094] S1, Smelting

[0095] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then smelted to obtain alloy melt, in which Al-Fe is used as deoxidizer; during the smelting process, low carbon low phosphorus ferromanganese, ferrosilicon, and ferrizircon need to be added for alloying, and the timing of adding ferrizircon is during the RH refining process; finally, the alloy melt is cast to obtain a billet with a thickness of 120mm;

[0096] S2. Heating homogenization treatment

[0097] The S1 billet was subjected to a heating homogenization treatment at a temperature of 1300℃ and a holding time of 3h to obtain a homogenized billet.

[0098] S3, Iron Oxide Scale Removal

[0099] The homogenized S2 billet was air-cooled at a rate of 7°C / s until the temperature of the steel billet dropped to 1150°C. Then, the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0100] S4, hot rolled

[0101] The S3 surface-cleaned billet is subjected to three passes of rough rolling and finish rolling. The three passes of rough rolling are started at 1050℃ and finished at 980℃, with a reduction of 25% per pass and a total reduction of 50%. The finish rolling starts at the temperature of the intermediate billet when it is heated to 970℃. The thickness of the intermediate billet is 50mm. The reduction rate of each rolling pass is >20%, and the final rolling temperature is 835℃ to obtain a plate with a thickness of 25mm.

[0102] S5, Cooling

[0103] The S4 sheet is passed through a controlled cooling system via rollers. The cooling rate of the system is controlled at 15℃ / s. After cooling to 640℃, it is air-cooled at a rate of 5℃ / s until it reaches room temperature. The final microstructure of the steel sample obtained by controlled cooling is entirely polygonal ferrite, thus obtaining the finished sheet.

[0104] like Figure 3 As shown, the high elongation high strength steel plate prepared in this embodiment has a polygonal ferrite microstructure with an average grain size of 9 μm, a thickness of 25 mm, a yield strength of 450 MPa, a tensile strength of 538 MPa, a yield ratio of 0.83, an elongation of 36%, n of 0.27, r of 1.48, a strength-ductility product of 19.4 GPa%, and an impact energy of 115 J at -20℃.

[0105] Example 4

[0106] A high-elongation, high-strength steel with a thickness of 15 mm, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.05 wt.%, Si: 0.28 wt.%, Mn: 1.45 wt.%, P: 0.004 wt.%, S: 0.004 wt.%, Zr: 0.18 wt.%, with the balance being Fe and unavoidable impurities.

[0107] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0108] S1, Smelting

[0109] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then smelted to obtain alloy melt, in which Al-Fe is used as deoxidizer; during the smelting process, low carbon low phosphorus ferromanganese, ferrosilicon, and ferrizircon need to be added for alloying, and ferrizircon is added during the RH refining process; finally, the alloy melt is cast to obtain a billet with a thickness of 110mm;

[0110] S2. Heating homogenization treatment

[0111] The S1 billet was subjected to a heating homogenization treatment at a temperature of 1250℃ for 2 hours to obtain a homogenized billet.

[0112] S3, Iron Oxide Scale Removal

[0113] The homogenized S2 billet was air-cooled at a rate of 7°C / s. Then, the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0114] S4, hot rolled

[0115] The S3 surface-cleaned billet is subjected to three passes of rough rolling and finish rolling. The rough rolling starts at 1055℃ and ends at 975℃, with a reduction of more than 15% per pass and a total reduction of 55%. The intermediate billet is 54mm thick. The finish rolling begins at 955℃, with a reduction of more than 20% per pass and a final rolling temperature of 870℃, to obtain a plate with a thickness of 15mm.

[0116] S5, Cooling

[0117] The S4 sheet is passed through a controlled cooling system via rollers. The cooling rate of the system is controlled at 11℃ / s. After cooling to 630℃, it is air-cooled at a rate of 7℃ / s until it reaches room temperature. The final microstructure of the steel sample obtained by controlled cooling is entirely polygonal ferrite, thus obtaining the finished sheet.

[0118] The high-elongation, high-strength steel plate prepared in this embodiment has a polygonal ferrite microstructure with an average grain size of 7 μm, a thickness of 15 mm, a yield strength of 445 MPa, a tensile strength of 532 MPa, a yield-to-tensile ratio of 0.83, an elongation of 31%, n of 0.2, r of 1.25, a strength-ductility product of 16.5 GPa%, and an impact energy of 107 J at -20℃.

[0119] Example 5

[0120] A high-elongation, high-strength steel with a thickness of 18 mm, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.04 wt.%, Si: 0.25 wt.%, Mn: 1.5 wt.%, P: 0.003 wt.%, S: 0.003 wt.%, Zr: 0.12 wt.%, with the balance being Fe and unavoidable impurities.

[0121] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0122] S1, Smelting

[0123] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then smelted to obtain alloy melt, in which Al-Fe is used as deoxidizer; during the smelting process, low carbon low phosphorus ferromanganese, ferrosilicon, and ferrizircon need to be added for alloying, and ferrizircon is added during the RH refining process; finally, the alloy melt is cast to obtain a billet with a thickness of 115mm;

[0124] S2. Heating homogenization treatment

[0125] The S1 billet was subjected to a heating homogenization treatment at a temperature of 1250℃ and a holding time of 2.5h to obtain a homogenized billet.

[0126] S3, Iron Oxide Scale Removal

[0127] The homogenized S2 billet was air-cooled at a rate of 5°C / s. Then, the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0128] S4, hot rolled

[0129] The S3 surface-cleaned billet is subjected to three passes of rough rolling and finish rolling. The rough rolling starts at 1100℃ and ends at 970℃, with a reduction of more than 15% per pass and a total reduction of 50%. The intermediate billet is 55mm thick. Finish rolling begins at 960℃, with a reduction of more than 20% per pass and a final rolling temperature of 840℃, to obtain a plate with a thickness of 18mm.

[0130] S5, Cooling

[0131] The S4 sheet is passed through a controlled cooling system via rollers. The cooling rate of the system is controlled at 12℃ / s. After cooling to 630℃, it is air-cooled at a rate of 5℃ / s until it reaches room temperature. The final microstructure of the steel sample obtained by controlled cooling is entirely polygonal ferrite, thus obtaining the finished sheet.

[0132] The high-elongation, high-strength steel plate prepared in this embodiment has a polygonal ferrite microstructure with an average grain size of 7 μm, a thickness of 18 mm, a yield strength of 448 MPa, a tensile strength of 530 MPa, a yield-to-tensile ratio of 0.84, an elongation of 33%, n of 0.23, r of 1.30, a strength-ductility product of 17.5 GPa%, and an impact energy of 100 J at -20℃.

[0133] Example 6

[0134] A high-elongation, high-strength steel with a thickness of 22 mm, wherein the chemical composition of the high-elongation, high-strength steel by mass percentage is: C: 0.038 wt.%, Si: 0.30 wt.%, Mn: 1.70 wt.%, P: 0.005 wt.%, S: 0.005 wt.%, Zr: 0.15 wt.%, with the balance being Fe and unavoidable impurities.

[0135] A method for preparing high-elongation, high-strength steel, comprising the following steps:

[0136] S1, Smelting

[0137] According to the above chemical composition of medium-high elongation high-strength steel, the raw materials are batched and weighed according to the mass percentage, and then smelted to obtain alloy melt, in which Al-Fe is used as deoxidizer; during the smelting process, low carbon low phosphorus ferromanganese, ferrosilicon, and ferrizircon need to be added for alloying, and ferrizircon is added during the RH refining process; finally, the alloy melt is cast to obtain a billet with a thickness of 110mm;

[0138] S2. Heating homogenization treatment

[0139] The S1 billet was subjected to a heating homogenization treatment at a temperature of 1290℃ for 2 hours to obtain a homogenized billet.

[0140] S3, Iron Oxide Scale Removal

[0141] The homogenized S2 billet was air-cooled at a rate of 6°C / s. Then, the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet.

[0142] S4, hot rolled

[0143] The S3 surface-cleaned billet is subjected to three passes of rough rolling and finish rolling. The rough rolling starts at 1050℃ and ends at 980℃, with a reduction of more than 15% per pass and a total reduction of 55%. The intermediate billet is 50mm thick. The finish rolling begins at 940℃, with a reduction of more than 20% per pass and a final rolling temperature of 825℃, to obtain a plate with a thickness of 18mm.

[0144] S5, Cooling

[0145] The S4 sheet is passed through a controlled cooling system via rollers. The cooling rate of the system is controlled at 12℃ / s. After cooling to 645℃, it is air-cooled at a rate of 6℃ / s until it reaches room temperature. The final microstructure of the steel sample obtained by controlled cooling is all polygonal ferrite, thus obtaining the finished sheet.

[0146] The high-elongation, high-strength steel plate prepared in this embodiment has a polygonal ferrite microstructure with an average grain size of 8 μm, a thickness of 18 mm, a yield strength of 440 MPa, a tensile strength of 522 MPa, a yield-to-tensile ratio of 0.84, an elongation of 32%, n of 0.2, r of 1.37, a strength-ductility product of 16.7 GPa%, and an impact energy of 109 J at -20℃.

[0147] The present invention provides a high-elongation, high-strength steel and its preparation method. By employing Zr microalloying technology, the final microstructure of the steel is controlled to be fine and uniform ferrite, thereby improving the strength and elongation of the steel and enhancing its formability.

[0148] The steel plate prepared by this invention uses only Zr microalloying, precisely controlling the content of C, Si, Mn and Zr, without the need for large additions of high-cost alloying elements or special smelting methods. Its preparation method adopts a controlled rolling and controlled cooling production process, and does not require high-cost tempering heat treatment or other machining and heat treatment after rolling to meet the performance requirements of the steel plate, thereby simplifying the manufacturing process and reducing the manufacturing cost of steel.

[0149] The high-elongation, high-strength steel plate prepared by this invention has a polygonal ferrite microstructure with an average grain size of 6-9 μm, a thickness of 6-25 mm, a yield strength of not less than 430 MPa, a tensile strength of not less than 490 MPa, a yield-to-tensile ratio of not more than 0.85, an elongation of not less than 30%, n of not less than 0.2, r of not less than 1, a strength-ductility product of not less than 14.7 GPa%, and an impact energy of not less than 100 J at -20℃.

[0150] In summary, compared with other traditional methods, the method of this invention achieves high strength, high toughness and high elongation of steel plates through the design of low-cost alloy composition and the optimization of short-process preparation method. It can meet the requirements of low cost, high strength, high toughness and high elongation in fields such as engineering machinery and vehicle steel. The method is low in cost, high in efficiency, simple influencing factors, does not require heat treatment, has a wide range of applications, and is conducive to industrial production practice and promotion.

[0151] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A high-elongation, high-strength steel, characterized in that, The chemical composition of the high-elongation, high-strength steel, by mass percentage, is as follows: C: 0.02-0.05 wt.%, Si: 0.20-0.30 wt.%, Mn: 1.45-1.80 wt.%, P≤0.005 wt.%, S≤0.005 wt.%, Zr: 0.05-0.2 wt.%, with the balance being Fe and unavoidable impurities; The high-elongation, high-strength steel plate has a polygonal ferrite microstructure with an average grain size of 6-9 μm, a thickness of 6-25 mm, a yield strength of not less than 430 MPa, a tensile strength of not less than 490 MPa, a yield-to-tensile ratio of not more than 0.85, an elongation of not less than 30%, n of not less than 0.2, r of not less than 1, a strength-ductility product of not less than 14.7 GPa%, and a low-temperature impact energy of not less than 100 J at -20℃.

2. A method for preparing high-elongation, high-strength steel according to claim 1, characterized in that, The preparation method includes the following steps: S1, Smelting According to the above chemical composition of high elongation and high strength steel, the raw materials are batched and weighed according to the mass percentage, and then melted to obtain alloy melt. The alloy melt is then cast to obtain a billet. S2. Heating homogenization treatment The S1 billet is heated and homogenized to obtain a homogenized billet. S3, Iron Oxide Scale Removal The homogenized billet of S2 was air-cooled, and then the iron oxide scale on the billet was removed by a descaling machine to obtain a clean billet. S4, hot rolled The S3 surface-cleaned billet is subjected to 3-5 passes of rough rolling and finish rolling to obtain the plate. S5, Cooling The S4 sheet is passed through a controlled cooling system via rollers, and then air-cooled to room temperature to obtain the finished sheet. In S4, the initial rolling temperature of the roughing mill is 1050±100℃, the final rolling temperature is 970±10℃, the reduction per pass is greater than 15%, and the total reduction is 50-60%; the finishing mill begins when the intermediate billet temperature is 950±20℃, the reduction per pass is >20%, and the final rolling temperature is 850±50℃. The cooling rate of the S5 cooling system is controlled at 10-15℃ / s. When the temperature reaches 600℃ to 650℃, air cooling is performed, with the air cooling rate controlled at 2-7℃ / s. The final microstructure of the steel sample obtained by the cooling process is entirely polygonal ferrite.

3. The method for preparing high-elongation, high-strength steel according to claim 2, characterized in that, In S1, Al-Fe is used as a deoxidizer; low-carbon, low-phosphorus ferromanganese, ferrosilicon, and ferrizirconia are added for alloying, and ferrizirconia is added during the RH refining process.

4. The method for preparing high-elongation, high-strength steel according to claim 2, characterized in that, The temperature for homogenization treatment in S2 is 1250-1300℃, and the holding time is 2-3h.

5. The method for preparing high-elongation, high-strength steel according to claim 2, characterized in that, The air cooling rate in S3 is controlled at 2-7℃ / s.

6. The method for preparing high-elongation, high-strength steel according to claim 2, characterized in that, The thickness of the slab in S1 is 100-120mm, the thickness of the intermediate slab in S4 is 50-60mm, and the thickness of the plate in S4 is 6-25mm.

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