A high-strength steel strip and its manufacturing method

Abstract

This invention discloses an ultra-high strength steel strip containing Fe and unavoidable impurity elements, as well as the following chemical elements in the following mass percentages: C: 0.13–0.20%; Si: 0.15–0.50%; Mn: 1.4–2.0%; B: 0.001–0.004%; Al: 0.01–0.04%; Cr: 0.1–0.4%; Mo: 0.1–0.4%; Ti+V+Nb≤0.02%. Its microstructure comprises uniformly dispersed granular bainite resembling a coral reef. Accordingly, this invention also discloses a method for manufacturing this ultra-high strength steel strip. The ultra-high strength steel strip of this invention achieves a tensile strength of 1000 MPa while also exhibiting low anisotropy and high mechanical uniformity within the same coil.

Description

Technical Field

[0001] This invention relates to a high-strength steel and its manufacturing method, and more particularly to a coated ultra-high-strength steel and its manufacturing method. Background Technology

[0002] With the development of the automotive industry, automotive parts are becoming increasingly smaller, thinner, and more precise in design, requiring as much uniformity in mechanical properties as possible in two or even multiple directions to ensure high-speed production, high-precision assembly, and high-stability service. This not only requires materials with extremely high strength and good formability, but also low anisotropy or high isotropy in mechanical properties, and even more importantly, extremely high performance uniformity.

[0003] At present, ultra-high strength steel is increasingly used in automotive parts. At the same time, since ultra-high strength steel is now produced in a large-scale integrated manner, it places extremely high demands on the mechanical uniformity of ultra-high strength steel in the same roll.

[0004] However, in the existing technology, as the strength of high-strength steel increases to 1000MPa, it is extremely difficult to obtain steel strips with low anisotropy and high uniformity of mechanical properties in the same coil. Taking the data published in the Journal of Plasticity Engineering, 2021, 28(7):124-130 as an example, the strength difference between the transverse and longitudinal directions of dual-phase steel with a tensile strength of 800MPa is about 8MPa, but when the tensile strength of dual-phase steel reaches the 1000MPa level, the strength difference between the transverse and longitudinal directions can be as high as about 40MPa.

[0005] In addition, Chinese patent document with publication number CN104018092A and publication date March 8, 2017, entitled "A steel plate with high strength and high mechanical property uniformity of 750MPa and its manufacturing method", discloses a steel grade with a tensile strength of only 800MPa and a mechanical uniformity of only <20MPa in the transverse, longitudinal and 45° directions.

[0006] For example, Chinese patent document CN102397891A, published on March 23, 2018, entitled "A method for improving the uniformity of steel strip performance," although it can make the strength uniformity of the steel strip in the width direction relatively high with a strength fluctuation of only 5 to 15 MPa, the tensile strength of this type of steel is only about 350 MPa to 600 MPa.

[0007] This shows that obtaining ultra-high strength steel with low anisotropy (or high isotropy) and high co-roll mechanical uniformity, while achieving a strength of 1000MPa, is technically challenging. Summary of the Invention

[0008] One of the objectives of this invention is to provide an ultra-high strength steel strip that has a tensile strength of 1000 MPa while exhibiting low anisotropy and high mechanical uniformity within the same roll.

[0009] To achieve the above objectives, the present invention provides an ultra-high strength steel strip containing Fe and unavoidable impurity elements, and further containing the following chemical elements in the following mass percentages:

[0010] C: 0.13~0.20%; Si: 0.15~0.50%; Mn: 1.4~1.9%; B: 0.001~0.004%; Al: 0.01~0.04%; Cr: 0.1~0.4%; Mo: 0.1~0.4%; Ti+V+Nb≤0.02%;

[0011] Its microstructure consists of uniformly dispersed granular bainite resembling a coral sea.

[0012] Accordingly, the present invention also provides an ultra-high strength steel strip, wherein the mass percentage content of each chemical element is as follows:

[0013] C: 0.13–0.20%; Si: 0.15–0.50%; Mn: 1.4–2.0%; B: 0.001–0.004%; Al: 0.01–0.04%; Cr: 0.1–0.4%; Mo: 0.1–0.4%; Ti+V+Nb≤0.02%; balance is Fe and unavoidable impurity elements.

[0014] Furthermore, in the ultra-high strength steel strip described in this invention, the mass percentage content of each chemical element satisfies at least one of the following conditions:

[0015] C: 0.14–0.18%;

[0016] Ti+V+Nb≤0.02%.

[0017] The design principles of each chemical element in the ultra-high strength steel strip described in this invention are as follows:

[0018] C: In the ultra-high strength steel strip described in this invention, carbon (C) not only controls the phase transformation of the microstructure but can also form alloy carbides with other alloying elements, thereby affecting the strength, formability, and performance uniformity of the steel strip. In this invention, when the C content in the steel is below 0.13%, on the one hand, the steel's strength will not meet the target requirements, and on the other hand, insufficient formation of granular bainite will occur. Conversely, if the C content in the steel is above 0.20%, martensite, coarse cementite, and other forms of bainite (non-granular bainite, such as the bainite mentioned above) are easily generated, thus deteriorating the performance and performance uniformity of the steel strip. Based on this, the mass percentage of C in this invention is controlled between 0.13% and 0.20%.

[0019] Si: In the ultra-high strength steel strip described in this invention, Si is an essential element for deoxidation in steelmaking. It has a certain solid solution strengthening effect and also influences the formation of polygonal ferrite and bainite. In this invention, when the Si content in the steel is below 0.15%, it is difficult to obtain a sufficient deoxidation effect; while when the Si content in the steel is above 0.5%, on the one hand, it is easy to form iron oxide scale or tiger-skin stripe-like color difference, which is not conducive to the surface quality of the steel plate; on the other hand, it will affect the formation of granular bainite and lead to the easy formation of polygonal ferrite and carbon-free bainite in the steel, affecting the uniformity of performance. Based on this, the mass percentage of Si in this invention is controlled between 0.15% and 0.5%.

[0020] Mn: In the ultra-high strength steel strip described in this invention, Mn is one of the key controlling elements for phase transformation in the steel structure. When the Mn content is too low, on the one hand, the steel strength will not meet the target requirements, and on the other hand, insufficient formation of granular bainite will occur. When the Mn content is too high, it will not only deteriorate the corrosion resistance and weldability and promote the formation of non-granular bainite structures such as martensite, but also exacerbate the tendency of grain coarsening and the formation of banded structures or central segregation, reducing the formability of the steel, deteriorating the uniformity and properties of the steel strip. Based on this, the present invention controls the mass percentage of Mn between 1.4% and 2.0%.

[0021] B: In the ultra-high strength steel strip described in this invention, element B not only promotes the formation of bainite in the steel, but also has a significant impact on the strength and hardness of the steel plate. If the B content in the steel is less than 0.001%, the steel's strength will not meet the target requirements; while when the B content in the steel is higher than 0.004%, brittle borides are easily formed, affecting the formability and uniformity of the steel plate. Based on this, this invention controls the mass percentage of B between 0.001% and 0.004%.

[0022] Al: In the ultra-high strength steel strip described in this invention, Al is added to the steel only as a deoxidizing element. It can remove oxygen from the steel to ensure the performance and quality of the steel. Therefore, the mass percentage of Al in this invention is controlled between 0.01% and 0.04%. Although in some prior art, Al is added to steel in large quantities (≥0.1%) as a ferrite forming element and a carbide precipitation suppressor in order to bring about solid solution strengthening, or to change the phase transformation temperature (e.g., Al, A1, A3), bainite formation kinetics, and carbide precipitation kinetics to change the phase transformation of the steel, forming retained austenite or carbon-free bainite, and ultimately improving the strength of the steel, it does not benefit the improvement of the uniformity and isotropy of the steel strip. Therefore, it is unnecessary to add a large amount of Al to avoid increasing costs and significantly increasing the difficulty of steelmaking.

[0023] Cr and Mo: In the ultra-high strength steel strip described in this invention, Cr and Mo can increase the hardenability of the steel strip, prolong the incubation period of pearlite and ferrite, inhibit the formation of pearlite and ferrite, and facilitate the formation of bainite structure during cooling. Therefore, if the Cr and Mo content is too low, insufficient granular bainite formation will occur. If the Cr and Mo content is too high, on the one hand, more martensite and tempered martensite structures will easily form, deteriorating the formability of the steel strip. On the other hand, banded structures or central segregation will easily form in the steel strip, thereby deteriorating the uniformity and properties of the steel strip. Therefore, in this invention, Cr is controlled at 0.1-0.5%, and Mo at 0.1-0.5%.

[0024] Ti, Nb, and V: In the ultra-high strength steel strip described in this invention, Ti, Nb, and V are not intentionally added as common microalloying elements beneficial to steel properties, but are controlled as residual elements in steelmaking. Because these microalloying elements form carbonitride precipitation, especially larger carbonitrides such as TiN, from the beginning of continuous casting in steelmaking, and this precipitation persists throughout the entire steel strip production process, controlling the uniform, stable, and on-demand precipitation of these carbonitrides is extremely difficult and can adversely affect the uniformity of the steel strip's properties. Therefore, in this invention, Ti+Nb+V is limited to ≤0.02%, and more preferably, Ti+Nb+V is limited to ≤0.01%.

[0025] Furthermore, in the ultra-high strength steel strip described in this invention, the mass percentage content of each chemical element also satisfies: 2.7≤(Cr+Mo) / C≤3.3, where each chemical element is substituted with the value before the percentage sign of its mass percentage content.

[0026] In this invention, since C, Cr, and Mo play a crucial role in the phase transformation of bainite, the ratio can be further preferably controlled as 2.7 ≤ (Cr + Mo) / C ≤ 3.3. If (Cr + Mo) / C is too low, the bainite phase region will be too small, resulting in insufficient hardenability of the steel strip. During annealing, insufficient cooling rate will lead to the formation of excessive ferrite, pearlite, upper bainite, and other structures. Conversely, if (Cr + Mo) / C is too high, the hardenability of the steel strip will be too high, and the martensite transformation temperature will be too high, resulting in the formation of excessive martensite in the steel strip.

[0027] Furthermore, in the ultra-high strength steel strip of the present invention, the mass percentage content of unavoidable impurity elements satisfies at least one of the following: P≤0.012%, S≤0.003%, N≤0.004%.

[0028] The impurity elements in this invention are primarily P, S, and N. Where technically feasible, their content should be as low as possible. Furthermore, the content can be controlled to P ≤ 0.012%, S ≤ 0.003%, and N ≤ 0.004%.

[0029] Furthermore, in the ultra-high strength steel strip of the present invention, the volume ratio of the granular bainite is ≥95%.

[0030] Furthermore, in the ultra-high strength steel strip described in this invention, the area of ​​the granular bainite is ≤5μm. 2 The aspect ratio is ≤2:1.

[0031] Furthermore, in the ultra-high strength steel strip described in this invention, any cross-sectional area ≤ 50 μm in any region beyond 30 μm from the upper and lower surfaces of the steel strip in the thickness direction. 2 The granular bainite is distributed within a region of μm.

[0032] Furthermore, in the ultra-high strength steel strip described in this invention, any cross-sectional area ≤10 μm in any region beyond 20 μm from the upper and lower surfaces of the steel strip in the thickness direction is included. 2 Granular bainite is distributed throughout the μm region.

[0033] This means that granular bainite is basically distributed in the intermediate region beyond 30 μm from the surface of the steel strip.

[0034] Furthermore, in the ultra-high strength steel strip described in this invention, its microstructure also includes ferrite, and the volume ratio of the ferrite is 0.1 to 4.5%.

[0035] In addition to granular bainite and ferrite, the microstructure of the ultra-high strength steel strip described in this invention may also contain retained austenite, other forms of bainite, tempered martensite, titanium carbonitride (or niobium, or vanadium), and martensite. However, the content of this microstructure is relatively small, with the total volumetric proportion not exceeding 0.5%, and the proportions satisfying the following order: retained austenite > other forms of bainite > tempered martensite > titanium carbonitride (or niobium, or vanadium) > martensite.

[0036] Furthermore, the ultra-high strength steel strip of the present invention has a tensile strength ≥1000MPa and a yield strength ≥780MPa; and its low anisotropy satisfies the following: the difference in yield strength between the transverse and longitudinal directions at the same position on the steel strip is ≤25MPa and the difference in tensile strength is ≤20MPa; the mechanical uniformity of the same coil satisfies the following: for the same steel coil, along its length or width direction, the difference in yield strength at different positions in the same tensile direction is ≤25MPa and the difference in tensile strength is ≤20MPa.

[0037] Furthermore, the ultra-high strength steel strip of the present invention has a tensile strength ≥1000MPa and a yield strength ≥780MPa; and its low anisotropy satisfies the following: the difference in yield strength between the transverse and longitudinal directions at the same position on the steel strip is ≤20MPa and the difference in tensile strength is ≤15MPa; the mechanical uniformity of the same coil satisfies the following: for the same steel coil, along its length or width direction, the difference in yield strength at different positions in the same tensile direction is ≤20MPa and the difference in tensile strength is ≤15MPa.

[0038] Furthermore, the ultra-high strength steel strip of the present invention has a breaking elongation of ≥10% and / or a hole expansion rate of ≥50%.

[0039] Another objective of this invention is to provide a method for manufacturing ultra-high strength steel strip, which can produce steel strip with a tensile strength of up to 1000 MPa, low anisotropy, and high mechanical uniformity in the same roll.

[0040] To achieve the above objectives, the present invention also provides a method for manufacturing ultra-high strength steel strip, comprising the following steps:

[0041] Smelting and casting;

[0042] Hot-rolled;

[0043] Post-rolling cooling and coiling: Post-rolling cooling adopts a multi-stage cooling method with alternating fast and slow cooling. Finally, for the area more than 100m from the beginning and end of the steel strip, the main body of the steel strip is cooled to the main body coiling temperature of 440℃~520℃ at a cooling rate of 5~100℃ and then coiled. For the area within 100m from the beginning and end of the steel strip, the beginning and end of the steel strip are cooled to the beginning and end coiling temperature of 480℃~560℃ at a cooling rate of 5~25℃ and then coiled.

[0044] Pickling and cold rolling;

[0045] Annealing: Heat to 840-900℃ at a heating rate of ≤50℃ / s and hold; then cool to 700-780℃ at a cooling rate of 2-20℃ / s and hold; then cool to 360-430℃ at a cooling rate of 20-50℃ and hold; then heat to 440-480℃ at a heating rate of 5-30℃ / s; then cool to 330-400℃ at a cooling rate of 0.1-0.5℃ / s; finally cool to room temperature at a cooling rate of 15-50℃ / s and roll into a roll.

[0046] The present invention employs the above-mentioned post-rolling cooling and coiling process to obtain a uniform steel strip cross-sectional shape, good strip shape and uniform precursor structure, so as to ensure that the steel strip can obtain a uniformly dispersed granular bainite in the length, width and thickness directions of the coral sea during the annealing stage, and to ensure that the final steel strip can obtain low anisotropy and high coiling performance uniformity.

[0047] The multi-stage cooling scheme is mainly used to ensure the uniformity of the microstructure of the hot-rolled coil after coiling. During the post-rolling cooling stage, the introduction of cooling water causes highly efficient heat exchange and extremely rapid temperature changes, which can easily lead to uneven microstructure in local areas of the hot-rolled coil. Moreover, due to the fluctuations in the cross-sectional shape and plate shape of the steel strip after finishing rolling, water accumulation in local areas of the hot-rolled coil can also easily occur, resulting in abnormal microstructure in local areas. Therefore, the main idea of ​​the multi-stage cooling scheme of this invention is to adopt a stepped alternating fast and slow cooling method to reduce the impact of cooling water introduction on the microstructure of the hot-rolled coil. The cooling rate gradually decreases, and fast cooling is always followed by slow cooling to eliminate the effects of heat exchange fluctuations and water accumulation in the cooling water.

[0048] Regarding the final winding temperature, this invention employs a low-temperature winding of the steel strip body, with the winding temperature increased within the first 100m of the coil. This is primarily to account for the impact of environmental heat exchange on the microstructure and properties of the hot-rolled coil after winding, and to form bainite within a relatively stable temperature range as much as possible. First, the lowest possible winding temperature significantly reduces the impact of environmental heat exchange on the kinetics of hot-rolled microstructure formation. Second, slightly increasing the winding temperature within the first 100m of the coil also considers the higher environmental heat exchange at the beginning and end of the hot-rolled coil, which can easily lead to a large temperature drop. Third, 440℃~520℃ is the main temperature range for bainite formation. If the temperature is too low, martensite is easily formed, while if the temperature is too high, uneven heat exchange with the environment is likely to occur, resulting in an uneven microstructure (potentially forming pearlite, bainite, or martensite in different regions).

[0049] The annealing process used in this invention is mainly to ensure that the steel strip obtains uniformly dispersed granular bainite in the length, width and thickness directions, resembling a coral sea.

[0050] Based on this, the steel strip is first heated to 840–900℃ at a heating rate ≤50℃ / s and held at that temperature to ensure the steel strip obtains the most uniform original austenitic structure possible. Then, it is cooled to 700–780℃ at a cooling rate of 2–20℃ / s and held at that temperature. This is to allow for the formation of a certain amount of ferrite at this point, significantly reducing the possibility of martensite formation during the subsequent low-temperature bainite transformation. It also aims to lower the starting temperature of rapid cooling and reduce the temperature drop in the rapid cooling range, thereby minimizing the impact of heat exchange fluctuations during rapid cooling on the microstructure and properties. Only through the design of three annealing processes within the bainitic phase region—rapid cooling, heating, and slow cooling—can a uniformly dispersed granular bainite resembling a coral reef be formed. During the rapid cooling stage, if the temperature is too low, martensite is easily formed; if the temperature is too high, upper bainite is easily formed. Controlling the cooling rate is also crucial; too fast a cooling rate is detrimental to the control of the steel strip's microstructure uniformity, while too slow a cooling rate easily leads to the formation of ferrite, pearlite, and upper bainite. The design of the reheating and slow cooling stages is to allow the steel strip to undergo phase transformation within the granular bainite transformation range over a relatively long period of time, thereby forming a uniformly dispersed granular bainite resembling a coral sea.

[0051] Furthermore, in the hot rolling step of the manufacturing method described in this invention, the heating temperature is controlled at 1180–1280°C, and the finishing rolling temperature is 870–970°C.

[0052] In this embodiment, the heating temperature is controlled at 1180–1280°C. This is to ensure that the slab is heated to a uniform temperature throughout, and also to ensure that the subsequent finishing rolling temperature is 870–970°C. If the heating temperature is too low, it will lead to uneven heating of the slab and an excessively low finishing rolling temperature. If the heating temperature is too high, it will cause severe oxidation of the slab surface, ultimately resulting in abnormal surface microstructure after annealing.

[0053] Controlling the finishing rolling temperature to 870–970℃ serves two purposes: firstly, to prevent the austenite-to-ferrite transformation of the steel strip during finishing rolling; and secondly, a higher finishing rolling temperature is beneficial for controlling the cross-sectional dimensions and shape of the steel strip. If the finishing rolling temperature is too low, ferrite transformation may easily occur during or after finishing rolling, which is not conducive to the microstructure control during subsequent annealing. If the finishing rolling temperature is too high, more cooling water is required to cool the steel strip during post-rolling cooling, and excessive cooling water input is not conducive to the uniform control of the hot-rolled microstructure during coiling.

[0054] Furthermore, in the hot rolling step of the manufacturing method described in this invention, the thickness difference between the center point and the thickness within 40 cm from the edge of the steel strip cross-section after finishing rolling is controlled to be ≤50 μm in the width direction.

[0055] In this embodiment, the thickness difference between the center point and the thickness within 40cm from the edge of the steel strip cross-section in the width direction after finishing rolling is controlled to be ≤50μm. This is mainly to cooperate with the subsequent cold rolling step to control the cross-sectional shape and plate shape of the steel strip during the final continuous annealing, so as to reduce the unevenness of the microstructure and properties of the finished coil caused by the fluctuation of the cross-sectional shape and plate shape of the steel strip during the final continuous annealing.

[0056] Furthermore, in the manufacturing method described in this invention, during the entire process of hot rolling, post-rolling cooling and coiling, the temperature difference between the non-middle region and the middle region in the width direction of the steel strip is controlled to be ≤30℃ at the same time, and the temperature fluctuation between the middle region and the non-middle region in the length direction is controlled to be ≤15℃.

[0057] The process described in this embodiment aims to ensure the uniformity of the microstructure and properties of the steel strip in both the width and length directions after it is coiled by controlling temperature fluctuations.

[0058] Furthermore, in the post-rolling cooling step of the manufacturing method described in this invention, the multi-stage cooling using alternating fast and slow cooling specifically includes: a first stage cooling the steel strip to 680-760°C at a cooling rate of 100-200°C / s; a second stage cooling the steel strip to 600-670°C at a cooling rate of 5-25°C / s; a third stage cooling the steel strip to 520-590°C at a cooling rate of 50-150°C / s; and a fourth stage cooling the steel strip to 490-570°C at a cooling rate of 5-25°C / s.

[0059] The selection of the above-mentioned cooling stage temperature steps is designed within the ferrite transformation range of the steel strip, so that the steel strip can preferentially transform into a small amount of ferrite and pearlite at this stage to ensure the uniformity of the microstructure and properties of the hot-rolled strip, thereby greatly reducing the possibility of martensite formation during subsequent low-temperature winding.

[0060] Furthermore, in the cold rolling step of the manufacturing method described in this invention, the cold rolling reduction rate is controlled to be ≥30%, and the target thickness of the steel strip is controlled so that the thickness difference between the center point and any position within 40cm from the edge in the width direction of the steel strip cross-section is ≤30μm.

[0061] In this embodiment, the process parameters are set to control the cross-sectional shape and plate shape of the cold-rolled steel strip to reduce the unevenness of the finished coil's microstructure and properties caused by fluctuations in the cross-sectional shape and plate shape of the steel strip during the final continuous annealing.

[0062] Furthermore, in the annealing step of the manufacturing method described in this invention, when heating and holding are performed, when the heating temperature is ≥870°C, the holding time is ≤2 min, and when the heating temperature is <860°C, the holding time is >2 min.

[0063] Furthermore, in the manufacturing method described in this invention, during the entire annealing process, the temperature difference between the non-middle region and the middle region in the width direction of the steel strip is controlled to be ≤10℃ at the same time, and the temperature fluctuation between the middle region and the non-middle region in the length direction of the steel strip is controlled to be ≤5℃.

[0064] The process described in this embodiment also aims to ensure the uniformity of the microstructure and properties of the steel strip in both the width and length directions after coiling by controlling temperature fluctuations.

[0065] Furthermore, in the annealing step of the manufacturing method described in this invention, the temperature is heated to 840–900°C at a heating rate of ≤50°C / s and held for 1–4 min; then cooled to 700–780°C at a cooling rate of 2–20°C / s and held for 10–40 s; then cooled to 360–430°C at a cooling rate of 20–50°C and held for 2.5–10 s.

[0066] The ultra-high strength steel strip and its manufacturing method described in this invention have the following advantages and beneficial effects:

[0067] This invention, while ensuring that the chemical element composition and process are relatively simple and controllable, obtains ultra-high strength steel with low anisotropy and high uniformity of the same roll through innovation in component design, microstructure control and manufacturing methods.

[0068] In some embodiments, the tensile strength is ≥1000MPa and the yield strength is ≥780MPa; and the low anisotropy satisfies: the difference in yield strength between the transverse and longitudinal directions at the same position on the steel strip is ≤25MPa and the difference in tensile strength is ≤20MPa; the mechanical uniformity of the same coil satisfies: for the same steel coil, along its length or width direction, the difference in yield strength at different positions in the same tensile direction is ≤25MPa and the difference in tensile strength is ≤20MPa.

[0069] In some embodiments, the ultra-high strength steel strip of the present invention not only has the above-mentioned properties, but also has good formability, with a breaking elongation of ≥10% and a hole expansion rate of ≥50%.

[0070] The ultra-high strength steel strip described in this invention can be used in automotive parts with stringent requirements for dimensional accuracy and service stability, such as front seat rails in automotive cockpit systems. It represents a new design concept of "high-end, precise, and advanced" refined, stable, and differentiated products, and has good prospects for promotion and application value. Attached Figure Description

[0071] Figure 1 This is a magnified 3000x microstructure photograph of the ultra-high strength steel strip of Example 1. Detailed Implementation

[0072] The ultra-high strength steel strip and its manufacturing method described in this invention will be further explained and described below with reference to specific embodiments. However, such explanation and description do not constitute an improper limitation on the technical solution of this invention.

[0073] Examples 1-7 and Comparative Examples 1-2

[0074] The ultra-high strength steel strips of Examples 1-7 and the comparative steel strips of Comparative Examples 1-2 described in this invention were all prepared using the following steps:

[0075] (1) Smelting and casting are carried out according to the chemical composition ratio designed in Table 1.

[0076] (2) Hot rolling: The heating temperature is controlled at 1180~1280℃, and the finishing rolling temperature is 870~970℃.

[0077] In some embodiments, it is preferable to control the thickness difference between the center point and the thickness within 40cm from the edge of the steel strip cross-section in the width direction after finishing rolling to be ≤50μm.

[0078] (3) Post-rolling cooling and coiling: Post-rolling cooling adopts a multi-stage cooling method of alternating fast cooling and slow cooling. Finally, for the area more than 100m from the beginning and end of the steel strip, the main body of the steel strip is cooled to the main body coiling temperature of 440℃~520℃ at a cooling rate of 5~100℃ and then coiled. For the area within 100m from the beginning and end of the steel strip, the beginning and end of the steel strip are cooled to the beginning and end coiling temperature of 480℃~560℃ at a cooling rate of 5~25℃ and then coiled.

[0079] In some embodiments, the aforementioned multi-stage cooling using alternating rapid and slow cooling may specifically include: a first stage cooling the steel strip to 680-760°C at a cooling rate of 100-200°C / s; a second stage cooling the steel strip to 600-670°C at a cooling rate of 5-25°C / s; a third stage cooling the steel strip to 520-590°C at a cooling rate of 50-150°C / s; and a fourth stage cooling the steel strip to 490-570°C at a cooling rate of 5-25°C / s, ensuring that the steel strip temperature at the end of the fourth stage is lower than that at the end of the third stage. The subsequent further cooling of the steel strip to the coiling temperature constitutes a fifth stage of cooling, ensuring that the coiling temperature at all points on the steel strip is lower than the temperature at the corresponding position at the end of the fourth stage.

[0080] (4) Pickling and cold rolling: Control the cold rolling reduction rate to ≥30%.

[0081] In some implementations, the target thickness of the steel strip is controlled such that the difference between the thickness at the center point and the thickness at any position within 40 cm from the edge in the width direction of the steel strip cross-section is ≤30 μm.

[0082] In some preferred embodiments, during the entire process of hot rolling, post-rolling cooling and coiling, the temperature difference between the non-middle region and the middle region in the width direction of the steel strip is controlled to be ≤30℃ at the same time, and the temperature fluctuation between the middle region and the non-middle region in the length direction is controlled to be ≤15℃.

[0083] In some more specific implementations, throughout the hot rolling, post-rolling cooling, and coiling processes, a seven-point equidistant temperature measurement and control system can be used along the width of the steel strip. All temperatures in these processes refer to the temperature of the central region within the seven-point equidistant temperature measurement and control system. The temperature difference between the central region and the other six regions along the width can be controlled to be ≤30℃. All temperatures in the hot rolling, post-rolling cooling, and coiling processes refer to the average temperature of the central region of the steel strip within the required length range. Temperature fluctuations within this length range are ≤15℃. Unless otherwise specified, the temperature range refers to the entire length of the steel strip.

[0084] (5) Annealing: Heat to 840-900℃ at a heating rate of ≤50℃ / s and hold for 1-4 min; then cool to 700-780℃ at a cooling rate of 2-20℃ / s and hold for 10-40 s; then cool to 360-430℃ at a cooling rate of 20-50℃ and hold for 2.5-10 s; then heat to 440-480℃ at a heating rate of 5-30℃ / s; then cool to 330-400℃ at a cooling rate of 0.1-0.5℃ / s; finally cool to room temperature at a cooling rate of 15-50℃ / s and roll into a roll.

[0085] Specifically, when the heating temperature is ≥870℃, the holding time is ≤2min, while when the heating temperature is <870℃, the holding time is >2min.

[0086] In some embodiments, during the entire annealing process, the temperature difference between the non-middle region and the middle region in the width direction of the steel strip is controlled to be ≤10℃ at the same time, and the temperature fluctuation between the middle region and the non-middle region in the length direction of the steel strip is controlled to be ≤5℃.

[0087] In some more specific embodiments, during the entire annealing process, temperature control can be achieved using 7 equally spaced temperature measurement points along the width of the steel strip. All temperatures in the annealing process refer to the temperature of the central region within these 7 equally spaced temperature measurement points. The temperature difference between the central region and the other 6 regions along the width can be controlled to be ≤10℃. All temperatures in the annealing process refer to the average temperature of the central region of the steel strip within the required length range. Temperature fluctuations within this length range are ≤5℃. Unless otherwise specified, the temperature range refers to the entire length of the steel strip.

[0088] It should be noted that the chemical composition design and related manufacturing processes used in the ultra-high strength steel strips of Examples 1-7 all meet the specifications designed in this invention. Correspondingly, the chemical composition design and related manufacturing processes used in the comparative steel strips of Comparative Examples 1-2 contain process parameters that do not meet the design requirements of this invention.

[0089] Table 1 lists the mass percentage of each chemical element in the ultra-high strength steel strips of Examples 1-7 and the comparative steel strips of Comparative Examples 1-2.

[0090] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, S, and N)

[0091] serial number C Si Mn B Al Cr Mo P S N Ti Nb V (Cr+Mo) / C Example 1 0.16 0.35 1.7 0.0025 0.02 0.3 0.2 0.008 0.002 0.004 0.001 0.002 0.004 3.13 Example 2 0.14 0.5 2.0 0.004 0.04 0.2 0.25 0.012 0.003 0.0038 0.002 0.001 0.006 3.21 Example 3 0.15 0.4 1.8 0.003 0.025 0.35 0.1 0.011 0.001 0.0035 0.004 0.003 0.003 3.00 Example 4 0.19 0.15 1.4 0.001 0.01 0.4 0.15 0.006 0.004 0.0025 0.003 0.004 0.001 2.89 Example 5 0.20 0.25 1.6 0.002 0.03 0.25 0.35 0.009 0.001 0.0035 0.001 0.002 0.005 3.00 Example 6 0.13 0.45 1.9 0.0035 0.035 0.1 0.3 0.01 0.001 0.0015 0.006 0.005 0.003 3.08 Example 7 0.18 0.2 1.5 0.0015 0.015 0.15 0.4 0.005 0.001 0.0018 0.005 0.006 0.002 3.06 Comparative Example 1 0.21 0.25 1.6 0.0015 0.02 0.15 0.15 0.008 0.002 0.004 0.001 0.002 0.001 1.43 Comparative Example 2 0.15 0.4 1.8 0.003 0.025 0.35 0.1 0.011 0.001 0.0035 0.004 0.003 0.003 3.00

[0092] Tables 2-1, 2-2, 2-3, and 2-4 list the specific process parameters for the ultra-high strength steel strips of Examples 1-7 and the comparative steel strips of Comparative Examples 1-4 in the above process steps.

[0093] Table 2-1.

[0094]

[0095] Table 2-2.

[0096]

[0097] Table 2-3

[0098]

[0099] Table 2-4

[0100]

[0101] Samples of the ultra-high strength steel strips obtained in Examples 1-7 were taken and their microstructure was observed and tested. The microstructure observation results are listed in Table 3 below.

[0102] Table 3.

[0103]

[0104] Furthermore, based on observations of the various embodiments, the area of ​​granular bainite in the ultra-high strength steel strips of all embodiments is ≤5μm. 2 The aspect ratio is ≤2:1.

[0105] For all ultra-high strength steel strips in Examples 1-7, any cross-sectional area ≤50 μm in any region beyond 30 μm from the top and bottom surfaces of the steel strip in the thickness direction is considered to be within this range. 2 The granular bainite is distributed within a region of μm.

[0106] For all ultra-high strength steel strips in Examples 1-7, any cross-sectional area ≤10 μm in any region beyond 20 μm from the top and bottom surfaces of the steel strip in the thickness direction is considered to be within this range. 2 The granular bainite is distributed within a region of μm.

[0107] also, Figure 1 The microstructure of the ultra-high strength steel strip of Example 1, magnified 3000 times, is also shown. From Figure 1 It can be seen that the ultra-high strength steel strip has uniformly dispersed granular bainite resembling a coral sea.

[0108] In addition, samples of the ultra-high strength steel strips obtained in Examples 1-7 and the comparative steel strips in Comparative Examples 1-2 were taken and their mechanical properties were tested to evaluate their anisotropy and roll uniformity. The anisotropy and roll uniformity of each example and comparative example are listed in Tables 4-1 to 4-9. The relevant mechanical property testing methods are described below:

[0109] Tensile property testing: conducted in accordance with GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature".

[0110] The expansion rate of steel was determined by an expansion test. A punch was used to press a specimen with a central hole into a die, enlarging the central hole until necking or through cracks appeared at the edge of the hole. Since the preparation method of the initial central hole and the corresponding edge quality of the initial hole have a significant impact on the expansion rate test results, the test and testing methods were performed according to the expansion rate test method specified in ISO / DIS 16630 standard. The initial central hole was a punched hole (corresponding to the processing method with the worst edge quality of the initial hole).

[0111] Table 4-1, Example 1

[0112]

[0113] Table 4-2, Example 2

[0114]

[0115]

[0116] Table 4-3. Example 3

[0117]

[0118] Table 4-4. Example 4

[0119]

[0120]

[0121] Table 4-5, Example 5

[0122]

[0123] Table 4-6, Example 6

[0124]

[0125]

[0126] Table 4-7 Example 7

[0127]

[0128] Table 4-8. Comparative Example 1

[0129]

[0130]

[0131] Table 4-9. Comparative Example 2

[0132]

[0133] Note: The “strength difference” in Tables 4-1 to 4-9 refers to the maximum and minimum difference between all test points in the longitudinal or transverse tensile direction along the entire length and width of the steel strip.

[0134] As can be seen from Tables 4-1 to 4-9 above, all embodiments of the present invention satisfy the following requirements: tensile strength ≥ 1000 MPa, yield strength ≥ 780 MPa, elongation at break ≥ 10%, and expansion rate ≥ 50%; at the same time, the anisotropy is satisfied: the difference in yield strength between the transverse and longitudinal directions at the same position on the steel strip is ≤ 25 MPa, and the difference in tensile strength is ≤ 20 MPa; the mechanical uniformity of the same coil is satisfied: for the same steel coil, along its length or width direction, the difference in yield strength at different positions in the same tensile direction is ≤ 25 MPa, and the difference in tensile strength is ≤ 20 MPa.

[0135] In contrast, Comparative Example 1 not only exceeded the upper limit of the design requirements for C content, but also failed to meet the requirement of 2.7≤(Cr+Mo) / C≤3.3, with (Cr+Mo) / C being only 1.43. At this point, the requirements for cooling rate are extremely high. However, in the actual manufacturing process, the cooling rate in the rapid cooling section of annealing does not meet the requirements, ultimately leading to abnormal product performance. Not only are the yield strength and tensile strength too low and the porosity insufficient, but the anisotropy is also high and the uniformity of the same roll is low.

[0136] Although the composition design of Comparative Example 2 meets the requirements of this invention, the excessive cooling rate in the manufacturing process does not meet the requirements of this invention. Although the mechanical properties of the product meet the requirements, the uniformity of the same roll is poor and the strength difference between different positions is too large.

[0137] It should be noted that the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0138] It should also be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention.

Claims

1. A type of ultra-high strength steel strip, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.13~0.20%; Si: 0.15~0.50%; Mn: 1.4~1.9%; B: 0.001~0.004%; Al: 0.01~0.04%; Cr: 0.1~0.4%; Mo: 0.1-0.4%; Ti+V+Nb≤0.02%; balance is Fe and unavoidable impurity elements; the mass percentage of each chemical element also satisfies: 2.7≤(Cr+Mo) / C≤3.3; Its microstructure includes uniformly dispersed granular bainite and ferrite in a coral sea pattern; the volume proportion of the granular bainite is ≥95%, and the volume proportion of the ferrite is 0.1-4.5%. Its tensile strength is ≥1000MPa and yield strength is ≥780MPa; and its low anisotropy satisfies the following: the difference in yield strength between the transverse and longitudinal directions at the same position on the steel strip is ≤25MPa and the difference in tensile strength is ≤20MPa; the mechanical uniformity of the same roll satisfies the following: along its length or width direction, the difference in yield strength between different positions in the same tensile direction is ≤25MPa and the difference in tensile strength is ≤20MPa. Its elongation at break is ≥10%, and / or its porosity is ≥50%.

2. The ultra-high strength steel strip as described in claim 1, characterized in that, Its mass percentage content of each chemical element satisfies at least one of the following conditions: C：0.14～0.18％； Ti+V+Nb≤0.01%.

3. The ultra-high strength steel strip as described in claim 1, characterized in that, The unavoidable impurity element mass percentage content satisfies at least one of the following: P≤0.012%, S≤0.003%, N≤0.004%.

4. The ultra-high strength steel strip as described in claim 1, characterized in that, The area of ​​the granular bainite is ≤5μm2, and the aspect ratio is ≤2:

1.

5. The ultra-high strength steel strip as described in claim 1, characterized in that, Any cross-sectional area ≤50 μm within the entire area beyond 30 μm from the upper and lower surfaces of the steel strip in the thickness direction. 2 The granular bainite is distributed within a region of μm.

6. The ultra-high strength steel strip as described in claim 1, characterized in that, Any cross-sectional area ≤10 μm within any region beyond 20 μm from the upper and lower surfaces of the steel strip in the thickness direction. 2 The granular bainite is distributed within a region of μm.

7. The method for manufacturing ultra-high strength steel strip as described in any one of claims 1-6, characterized in that, Including the following steps: Smelting and casting; Hot-rolled; Post-rolling cooling and coiling: Post-rolling cooling adopts a multi-stage cooling method with alternating fast and slow cooling. Finally, for the area more than 100m from the beginning and end of the steel strip, the main body of the steel strip is cooled to the main body coiling temperature of 440℃~520℃ at a cooling rate of 5~100℃ and then coiled. For the area within 100m from the beginning and end of the steel strip, the beginning and end of the steel strip are cooled to the beginning and end coiling temperature of 480℃~560℃ at a cooling rate of 5~25℃ and then coiled. Pickling and cold rolling; Annealing: Heat to 840-900℃ at a heating rate of ≤50℃ / s and hold; then cool to 700-780℃ at a cooling rate of 2-20℃ / s and hold; then cool to 360-430℃ at a cooling rate of 20-50℃ and hold; then heat to 440-480℃ at a heating rate of 5-30℃ / s; then cool to 330-400℃ at a cooling rate of 0.1-0.5℃ / s; finally cool to room temperature at a cooling rate of 15-50℃ / s and roll into a roll.

8. The manufacturing method as described in claim 7, characterized in that, In the hot rolling process, the heating temperature is controlled at 1180-1280℃, and the finishing rolling temperature is 870-970℃.

9. The manufacturing method as described in claim 7, characterized in that, In the hot rolling process, the thickness difference between the center point and the thickness within 40cm from the edge of the steel strip cross-section after finishing rolling is controlled to be ≤50μm in the width direction.

10. The manufacturing method as described in claim 7, characterized in that, Throughout the hot rolling, post-rolling cooling, and coiling processes, the temperature difference between the non-middle and middle regions in the width direction of the steel strip is controlled to be ≤30℃ at the same time, and the temperature fluctuation between the middle and non-middle regions in the length direction is controlled to be ≤15℃.

11. The manufacturing method as described in claim 7, characterized in that, In the post-rolling cooling step, the multi-stage cooling method of alternating fast and slow cooling specifically includes: the first stage cooling the steel strip to 680-760℃ at a cooling rate of 100-200℃ / s; the second stage cooling the steel strip to 600-670℃ at a cooling rate of 5-25℃ / s; the third stage cooling the steel strip to 520-590℃ at a cooling rate of 50-150℃ / s; and the fourth stage cooling the steel strip to 490-570℃ at a cooling rate of 5-25℃ / s.

12. The manufacturing method as described in claim 7, characterized in that, In the cold rolling process, the cold rolling reduction rate is controlled to be ≥30%, and the target thickness of the steel strip is controlled so that the thickness difference between the center point and any position within 40cm from the edge in the width direction of the steel strip cross section is ≤30μm.

13. The manufacturing method as described in claim 7, characterized in that, During the annealing process, when heating and holding are performed, the holding time should be ≤2 min when the heating temperature is ≥870℃, and >2 min when the heating temperature is <870℃.

14. The manufacturing method as described in claim 7, characterized in that, Throughout the annealing process, the temperature difference between the non-middle region and the middle region in the width direction of the steel strip is controlled to be ≤10℃ at the same time, and the temperature fluctuation between the middle region and the non-middle region in the length direction of the steel strip is controlled to be ≤5℃.

15. The manufacturing method as described in claim 7, characterized in that, In the annealing step, heat to 840-900℃ at a heating rate of ≤50℃ / s and hold for 1-4 min; then cool to 700-780℃ at a cooling rate of 2-20℃ / s and hold for 10-40 s; then cool to 360-430℃ at a cooling rate of 20-50℃ and hold for 2.5-10 s.

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