A high-strength bundling steel with a tensile strength ≥ 1080 MPa and its manufacturing method

Abstract

The present invention discloses a high-strength bundling steel with a tensile strength ≥ 1080 MPa and a manufacturing method thereof. The high-strength bundling steel with a tensile strength ≥ 1080 MPa comprises the following chemical components by weight percentage: C 0.20% - 0.24%, Si 0.01 - 0.03%, Mn 0.90% - 1.20%, P 0.01 - 0.025%, S ≤ 0.025%, B 0.003% - 0.005%, Ti 0.03% - 0.06%, Als 0.020% - 0.050%, and the balance is Fe and inevitable impurities; it is produced through the steps of hot metal pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling → controlled cooling → pickling → cold rolling → tempering → bluing, and has the characteristics of high strength, good toughness, and corrosion resistance, with a tensile strength ≥ 1080 MPa and an elongation A 30 ≥ 9%.

Description

Technical Field

[0001] The present invention belongs to the field of steel for packaging materials, and particularly relates to a high-strength strapping steel with a tensile strength ≥ 1080 MPa and a manufacturing method thereof. Background Art

[0002] Steel strapping usually refers to a kind of strapping steel used for packaging industrial products. Currently, it is mainly used for packaging finished or semi-finished products such as steel, non-ferrous metals, cotton, and wool, among which the steel industry is the industry that uses the most strapping. Therefore, the strapping must have the characteristics of high strength, good toughness, firmness, etc., and it is not easy to loose the package during transportation and loading and unloading. In addition, it may encounter harsh environments during outdoor transportation and storage, so it must also have certain corrosion resistance. Currently, the 1080 MPa grade of strapping is mostly used for high-strength hot-rolled coil products, and the production is difficult. With the continuous development of China's steel industry, the proportion of high-strength steel is increasing continuously. Therefore, the steel for high-strength strapping has broad market prospects.

[0003] A high-strength strapping steel with a tensile strength ≥ 1100 MPa and a manufacturing method thereof, with the weight percentage of chemical components being: C: 0.25 - 0.35%, Si ≤ 0.45%, Mn: 1.00 - 2.00%, P ≤ 0.04%, S: ≤ 0.04%, disclosed in the domestic patent publication number CN101805870A. A high-strength strapping with a tensile strength of 1100 - 1250 MPa and an elongation of 10 - 13% is produced through a two-phase zone quenching and tempering process. A strapping steel with a tensile strength above 1100 MPa and an elongation of 10% is obtained through the quenching and tempering process in the two-phase zone. However, this patent adopts a high-Mn composition system, and the production process is complex and the production efficiency is low.

[0004] A production method of high-strength blued strapping disclosed in the domestic patent publication number CN102719730A, with the weight percentage of chemical components being: C: 0.15 - 0.25%, Mn: 1.5 - 2.2%, Si: ≤ 0.05%, P: ≤ 0.025%, S: ≤ 0.02%, Als: 0.01 - 0.08%. The raw materials for high-strength tensile strapping are obtained by 9 - 10 passes of reduction. Although the process is simple and the pollution is small, the content of Mn element in this steel grade is high, and the production cost is relatively high due to push-pull pickling and multi-pass rolling, and the economy is poor.

[0005] The domestic patent publication number is CN109440010A, which introduces a 1100MPa grade high-strength bundling steel and its production method. It uses C: 0.2 - 0.25%, Si: 0.15 - 0.3%, Mn: 1.15 - 1.35%, P ≤ 0.020%, S ≤ 0.010%, Als: 0.02 - 0.05%, Nb: 0.015 - 0.03%. This patent adds a small amount of Si and Nb components for preparation. Although the tensile strength can reach 1100MPa grade, its elongation rate can only be ≥8%. Summary of the Invention

[0006] In view of the deficiencies in the current existing product technology, the present invention provides a high-strength bundling steel with a tensile strength ≥1080MPa grade and its manufacturing method, which has the characteristics of high strength, good toughness, and corrosion resistance.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A high-strength bundling steel with a tensile strength ≥1080MPa grade, including the following chemical components by weight percentage: C: 0.20% - 0.24%, Si: 0.01 - 0.03%, Mn: 0.90% - 1.20%, P: 0.01 - 0.025%, S ≤ 0.025%, B: 0.003% - 0.005%, Ti: 0.03% - 0.06%, Als: 0.020% - 0.050%, and the rest is Fe and inevitable impurities.

[0009] The metallographic structure of the high-strength bundling steel with a tensile strength ≥1080MPa grade is a ferrite + pearlite mixed structure, in which the volume percentage of ferrite is 35 - 43%.

[0010] The high-strength bundling steel with a tensile strength ≥1080MPa grade has a tensile strength ≥1080MPa and an elongation rate A 30 ≥9%.

[0011] The manufacturing method of the high-strength bundling steel with a tensile strength ≥1080MPa grade includes the following steps: hot metal pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling → controlled cooling → pickling → cold rolling → tempering → bluing → finished product.

[0012] During the hot metal pretreatment process, front slag skimming and rear slag skimming measures are taken to adjust the [S] element, which can reduce the harmful elements in the molten steel.

[0013] In the continuous casting step, the tundish temperature control target is 15 - 30°C above the liquidus temperature.

[0014] In the hot rolling step, the hot rolling heating temperature is controlled at 1210°C to 1250°C, which can ensure that the steel billet is fully austenitized and the compounds are fully dissolved; the finishing rolling temperature is controlled at 850°C to 880°C, which can obtain fine grain structure. A low finishing rolling temperature can cause a large number of rolling dislocations and defects in the structure, which can serve as the nucleation points for subsequent TiC precipitation and promote precipitation. At the same time, the finishing rolling temperature should not be too high, otherwise the dislocations and defects generated during rolling will disappear again under the action of austenite recovery and recrystallization, which is not conducive to the subsequent precipitation of TiC.

[0015] In the controlled cooling step, the steel coil is water-cooled to 560°C to 600°C at a cooling rate of 35 to 50°C / s, so that the C atoms in the structure do not have time to diffuse and are dissolved in the matrix. At the same time, an appropriate amount of pearlite structure is obtained, which can effectively improve the strength of the experimental steel. At the same time, it can also reduce the tendency of grain growth at high temperatures and obtain fine structure. At the same time, under the condition of rapid cooling, the fine TiC can obtain finer precipitation particles. On the one hand, it can effectively crush grains and improve the work hardening effect during the subsequent cold rolling process. On the other hand, it can improve the strength as precipitation strengthening.

[0016] In the cold rolling step, five-stand tandem rolling is adopted, and the total cold rolling reduction rate is controlled at 67% to 74%. A large reduction rate can increase the grain distortion energy in the steel, lower the recrystallization temperature, and improve the tensile strength of the strapping. Among them, the reduction rate of the first pass is controlled at 28% to 31%, the reduction rate of the second pass is controlled at 25% to 30%, the reduction rate of the third pass is controlled at 24% to 27%, the reduction rate of the fourth pass is controlled at 21% to 25%, and the reduction of the fifth pass is controlled at 0.5% to 2%. The reduction distribution of each pass is very important. The rolling process is a complex process because plastic heat will be generated due to the existence of plastic deformation. During continuous production, the flash point temperature will be very high. Therefore, the deformation process is a process in which work hardening and softening occur simultaneously, but the hardening is much greater than the softening process. Therefore, if the reduction rate is too large, although the strength of the experimental steel will increase, it is not conducive to the subsequent deformation of the subsequent processes. If the reduction rate is low, it is impossible to effectively obtain a strapping steel with a tensile strength of more than 1080 MPa. Only by controlling according to the cold rolling conditions of the present invention can each pass of cold rolling be successfully deformed while obtaining a tensile strength of more than 1080 MPa.

[0017] In the cold rolling step, the surface roughness of the work rolls of the fourth and fifth stands of the five-stand tandem rolling mill is ≤0.3 Ra, so that a strip steel with a surface roughness ≤0.2 Ra can be obtained. The residual oil and iron on the surface of such strip steel with low surface roughness can be controlled below 500 mg / m 2 , with less residual oil and iron on the surface, which can avoid open flames in the tempering furnace and is conducive to safe production.

[0018] In the tempering step, the tempering temperature is 530-550° C. and the holding time is 10-15 seconds.

[0019] In the bluing step, the temperature is kept at 530-550° C. and then naturally cooled in the air.

[0020] The high-strength strapping steel with a tensile strength of ≥1080MPa provided by the present invention has the following functions and controls for each component:

[0021] Carbon (C): As the most economical strengthening element, increasing the C element can effectively improve the tensile properties of the strapping. At the same time, there is a large amount of C in the experimental steel that can combine with Ti. During the coiling process, a large amount of TiC precipitation is formed, which can effectively improve the tensile strength. On the other hand, a large amount of TiC precipitation particles formed can effectively prevent the growth of grains during the bluing process. However, as the C element increases, the elongation of the strapping gradually decreases. If the C content is too high, the C element will not have time to diffuse during the rolling process and will gather locally, resulting in poor material uniformity and easy fracture during deformation. In addition, the influence on the weldability of the strapping is also very significant, so the C is controlled to 0.20% to 0.24%.

[0022] Silicon (Si): Si is also a common strengthening element. However, when the Si content is too high, the iron oxide scale on the surface of the steel plate is difficult to remove, which will affect the subsequent bluing treatment and further lead to poor corrosion resistance. Therefore, in the present invention, Si is 0.10-0.030%.

[0023] Manganese (Mn): As a strengthening element, Mn can not only effectively improve the strength of the strap, but also improve the hardenability of the strap during the cooling process. In addition, Mn can also reduce the phase transition temperature from austenite to ferrite, expand the hot working temperature range, and is conducive to refining the ferrite grain size. Mn is solid-dissolved in the structure and is easy to expel C to the surrounding during the phase transition process, resulting in the presence of banded pearlite and carbides in the experimental steel. Therefore, the Ti added to this experimental steel, on the one hand, fixes the C element to avoid the formation of banded structure, and on the other hand, the addition of B, which preferentially precipitates at the grain boundary, reduces the diffusion of the C element, and can increase the content of solid-dissolved C in the ferrite and improve the tensile strength; but too high a Mn content is not good for plastic toughness, and also has a certain effect on the weld performance. Taking all factors into consideration, the Mn control range in the present invention is 0.90% to 1.20%.

[0024] Phosphorus (P): P is a very effective solid solution strengthening element, and its diffusion rate in γ-Fe and α-Fe is low. It dissolves in the structure to cause lattice distortion, which can effectively improve the strength of the experimental steel. However, P is easy to form segregation, and segregates at the grain boundaries, which reduces the plasticity of the material and is not conducive to the welding performance of the steel plate. Therefore, the present invention adds a small amount of B element, which can preferentially precipitate at the grain boundaries to avoid the segregation of P, so the P content is controlled at 0.01-0.025%.

[0025] Sulfur (S): S is a harmful element in battery shell steel, which makes the steel hot brittle, reduces the ductility and toughness of the steel, and easily causes cracks during rolling. In addition, S is not good for welding performance and reduces corrosion resistance. Therefore, the present invention tries to control the S content in the steel within the range of S≤0.025%.

[0026] Aluminum (Al): Al is the main deoxidizer and also has a certain effect on grain refinement. The disadvantage of aluminum is that it affects the hot working performance, welding performance and cutting performance of steel. The present invention controls the Al content within the range of 0.020% to 0.050%.

[0027] Boron (B): A very small amount of B element can effectively improve the hardenability of strapping steel. A small amount of B element is preferentially precipitated at the grain boundary during the smelting process to fill the defect position at the grain boundary and avoid the segregation of P element. At the same time, the addition of B can reduce the diffusion rate of C atoms, thereby promoting a large amount of C elements to be dissolved in the structure during the phase change process, thereby improving the strength of the experimental steel. Therefore, the B element in the steel is controlled to be 0.003% to 0.005%.

[0028] Titanium (Ti): Ti and C can generate TiC, which has high stability at high temperatures and can be effectively pinned at the austenite grain boundaries, hindering grain growth and playing a role in grain refinement. At the same time, the precipitated TiC can play a precipitation strengthening role and improve the strength of the finished product.

[0029] The present invention adopts low C and conventional Mn, and is combined with a small amount of P element and B and a trace Ti component system. On the one hand, the conventional strengthening method of C and Mn is utilized, and the solid solution strengthening method of P and the precipitation strengthening of TiC are adopted to increase the tensile strength of the material. At the same time, in order to avoid the local segregation of the P element and reduce the diffusion of the C element, a trace amount of B is used to precipitate preferentially at the grain boundary, reduce the diffusion of C atoms, make the C atoms fully dissolved in the organization, and at the same time improve the uniformity of the material, which is beneficial to improve the elongation. In combination with a reasonable cold rolling reduction distribution system and related processes, a strapping steel with a tensile strength of ≥1080MPa and an elongation of ≥9% is finally obtained.

[0030] Compared with the prior art, the high-strength strapping steel with a tensile strength of ≥1080MPa disclosed in the present invention has low production cost, low C element content, which is conducive to welding, and low Si element content. The coating is not easy to fall off after bluing, and has excellent anti-corrosion performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is the metallographic structure diagram of the strapping steel in Example 3;

[0032] Figure 2 This is the metallographic structure diagram of the strapping steel in Comparative Example 1. Detailed implementation manners

[0033] The present invention provides a high-strength bundling steel with a tensile strength ≥ 1080 MPa, comprising the following chemical components by weight percentage: C: 0.20% - 0.24%, Si: 0.01 - 0.03%, Mn: 0.90% - 1.20%, P: 0.01 - 0.025%, S ≤ 0.025%, B: 0.003% - 0.005%, Ti: 0.03% - 0.06%, Als: 0.020% - 0.050%, and the balance being Fe and inevitable impurities;

[0034] The manufacturing method of the high-strength bundling steel with a tensile strength ≥ 1080 MPa comprises the following steps: hot metal pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling → controlled cooling → pickling → cold rolling → tempering → bluing → finished product;

[0035] During the hot metal pretreatment process, front slag removal and back slag removal measures are taken to adjust the [S] element, which can reduce the harmful elements in the molten steel.

[0036] In the continuous casting step, the tundish temperature control target is 15 - 30°C above the liquidus temperature.

[0037] In the hot rolling step, the hot rolling heating temperature is controlled at 1210°C - 1250°C, and the finishing rolling temperature is controlled at 850°C - 880°C.

[0038] In the controlled cooling step, the steel coil is water-cooled to 560 - 600°C at a cooling rate greater than 35°C / s.

[0039] In the cold rolling step, five-stand tandem cold rolling is adopted, and the total cold rolling reduction rate is controlled at 67 - 74%, wherein the reduction rate of the first pass is controlled at 28 - 31%, the reduction rate of the second pass is controlled at 25 - 30%, the reduction rate of the third pass is controlled at 24 - 27%, the reduction rate of the fourth pass is controlled at 21 - 25%, and the reduction rate of the fifth pass is controlled at 0.5 - 2%; the surface roughness of the work rolls of the fourth and fifth stands of the five stands is ≤ 0.3 Ra.

[0040] In the tempering step, the tempering temperature is 540°C, and the holding time is 10 - 15 s.

[0041] In the bluing step, after holding at 540°C, it is naturally cooled in the air.

[0042] The present invention will be described in detail below with reference to the embodiments.

[0043] The chemical components and weight percentages of the molten steel of the bundling steel in each embodiment and comparative example are shown in Table 1, and the balance is Fe and inevitable impurity elements.

[0044] Table 1 Chemical composition, wt%

[0045] Category C Si Mn P S Als B Ti Example 1 0.20 0.025 0.92 0.025 0.0050 0.0430 0.0032 0.042 Example 2 0.22 0.010 1.05 0.013 0.0250 0.050 0.0030 0.048 Example 3 0.24 0.018 1.13 0.010 0.0140 0.0350 0.0042 0.060 Example 4 0.23 0.030 1.20 0.020 0.010 0.020 0.0050 0.030 Comparative Example 1 0.22 0.20 1.00 0.008 0.015 0.030 / 0.045 Comparative Example 2 0.26 0.43 1.20 0.0120 0.0050 0.036 0.003 / Comparative Example 3 0.24 0.30 1.20 0.020 0.010 0.035 / 0.035 Comparative Example 4 0.23 0.030 1.20 0.020 0.010 0.020 0.0050 0.030

[0046] The production process parameters of the bundling steel in each example and comparative example are shown in Tables 2 and 3.

[0047] Table 2 Production process

[0048]

[0049]

[0050] Table 3

[0051] Tempering Temperature °C Tempering Time s Bluing Treatment Conditions Example 1 540 10 Air Cooling Example 2 540 12 Air Cooling Example 3 540 13 Air Cooling Example 4 540 15 Air Cooling Comparative Example 1 550 10 Air Cooling Comparative Example 2 560 7 Air Cooling Comparative Example 3 520 9 Air Cooling Comparative Example 4 530 9 Air Cooling

[0052] The final mechanical property values of the bundling steel in each example and comparative example are shown in Table 4.

[0053] Table 4 Mechanical properties

[0054] Category Final Thickness / Metallographic Structure <![CDATA[R m / MPa]]> <![CDATA[A 30 / %]]> Example 1 0.90 mm 57% Pearlite + 43% Ferrite 1096 9.8 Example 2 0.90 mm 59% Pearlite + 41% Ferrite 1106 9.6 Example 3 0.80 mm 65% Pearlite + 35% Ferrite 1126 9.2 Example 4 0.80 mm 63% Pearlite + 37% Ferrite 1120 10.1 Comparative Example 1 0.90 mm 40% Pearlite + 60% Ferrite 870 11.0 Comparative Example 2 0.90 mm 82% Pearlite + 18% Ferrite 1196 6.5 Comparative Example 3 0.80 mm 48% Pearlite + 52% Ferrite 956 10.8 Comparative Example 4 0.80 mm 78% Pearlite + 22% Ferrite 1055 8.6

[0055] Table 5 Weldability and blued surface condition

[0056]

[0057]

[0058] The detailed description of a high-strength bundling steel with a tensile strength ≥ 1080 MPa and its manufacturing method with reference to the examples above is illustrative rather than restrictive. Several examples can be listed within the defined scope. Therefore, changes and modifications without departing from the general concept of the present invention should fall within the protection scope of the present invention.

Claims

1. A high-strength bundling steel with a tensile strength ≥ 1080 MPa, characterized in that, it comprises the following chemical components by weight percentage: C: 0.20% - 0.24%, Si: 0.01 - 0.03%, Mn: 0.92% - 1.20%, P: 0.01 - 0.025%, S ≤ 0.025%, B: 0.003% - 0.005%, Ti: 0.03% - 0.06%, Als: 0.020% - 0.050%, and the balance is Fe and unavoidable impurities; the metallographic structure of the high-strength bundling steel with a tensile strength ≥ 1080 MPa is a ferrite + pearlite mixed structure, and the volume percentage of ferrite is 35 - 43%; the manufacturing method of the high-strength bundling steel with a tensile strength ≥ 1080 MPa comprises the following steps: hot metal pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling → controlled cooling → pickling → cold rolling → tempering → bluing → finished product; in the controlled cooling step, the steel coil is water-cooled to 560 - 600 °C at a cooling rate of 35 - 50 °C / s; in the cold rolling step, five-stand tandem cold rolling is adopted, and the total cold rolling reduction rate is controlled at 67 - 74%, wherein the reduction rate of the first pass is controlled at 28 - 31%, the reduction rate of the second pass is controlled at 25 - 30%, the reduction rate of the third pass is controlled at 24 - 27%, the reduction rate of the fourth pass is controlled at 21 - 25%, and the reduction rate of the fifth pass is controlled at 0.5 - 2%; in the tempering step, the tempering temperature is 530 - 550 °C and the holding time is 10 - 15 s.

2. The high-strength bundling steel with a tensile strength ≥ 1080 MPa according to claim 1, characterized in that, The tensile strength of the high-strength bundling steel with a tensile strength of ≥1080MPa is ≥1080MPa, and the elongation A 30 ≥9%.

3. The manufacturing method of the high-strength bundling steel with a tensile strength ≥ 1080 MPa according to claim 1 or 2, characterized in that, the manufacturing method comprises the following steps: hot metal pretreatment → converter → alloy fine-tuning station → RH → continuous casting → hot rolling → controlled cooling → pickling → cold rolling → tempering → bluing → finished product.

4. The manufacturing method according to claim 3, characterized in that, in the continuous casting step, the tundish temperature control target is 15 - 30 °C above the liquidus temperature.

5. The manufacturing method according to claim 3, characterized in that, in the hot rolling step, the hot rolling heating temperature is controlled at 1210 °C - 1250 °C, and the finish rolling temperature is controlled at 850 °C - 880 °C.

6. The manufacturing method according to claim 3, characterized in that, in the cold rolling step, the surface roughness of the work rolls of the fourth and fifth stands is ≤ 0.3 Ra.

7. The manufacturing method according to claim 3, characterized in that, in the bluing step, after holding at 530 - 550 °C, it is naturally cooled in the air.

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