Plate shape control method for ultrahigh-strength low-alloy high-strength steel

Abstract

The invention discloses a strip shape control method for ultrahigh-strength low-alloy high-strength steel. The production steps include smelting, continuous casting, hot rolling, acid rolling and continuous annealing. Aiming at the pain and difficulty problem that the shape of a low-alloy high-strength steel plate with the yield strength of 800MPa is not easy to control, structure property control is taken as a breakthrough, Nb + Ti is more than or equal to 0.085% and less than or equal to 0.110%, Cr is more than or equal to 0.15% and less than or equal to 0.50%, the in-roll strength difference is less than or equal to 50MPa, and the plate shape control difficulty in a flattening process is reduced; by taking reduction of wave heredity as a breakthrough, a combined measure of a coiling temperature of 550 + / -30 DEG C, a wedge mean value of + / -10 microns and an acid rolling reduction rate of 45-55% is innovatively provided, the acid rolling plate shape is stably controlled to be less than or equal to 20 IU, and the plate shape control difficulty of a leveling process is reduced; the plate shape control capability of the temper mill is remarkably improved through the matching design of the diameter of the working roll, the leveling elongation, the roll bending force distribution and the roll shifting rule by taking the exertion of the capacity of the temper mill as a breakthrough. The plate shape qualification rate of the low-alloy high-strength steel with the yield strength of 800 MPa is increased to 90% or above from 20% or below, and stable production is achieved.

Description

Technical Field

[0001] This invention relates to the field of low-alloy high-strength steel production technology, specifically to a method for controlling the shape of ultra-high-strength low-alloy high-strength steel. Background Technology

[0002] Driven by industry demands for weight reduction, energy conservation, and safety upgrades, automotive lightweighting is both a means and a trend. Car seats account for approximately 6% of the total vehicle weight and 5% of the total cost, making lightweight seat design increasingly favored by the industry. Seat rails are a core component of the seat frame, and the demand for lightweighting is very strong. Ultra-high strength low-alloy high-strength steel, represented by HC800LA, is gradually emerging in the competition for seat rail materials, with strong market demand. However, due to its yield strength reaching 800MPa, sheet metal defects (such as single-sided waves, medium waves, and composite waves) are frequent, with the sheet metal qualification rate sometimes falling below 20%, severely restricting its industrial application.

[0003] The history of domestic research and development of ultra-high strength low-alloy high-strength steel is relatively short, with only a handful of companies capable of stable production and mass supply, and very little publicly available technical information. Existing technologies offer some technical ideas and measures for ultra-high strength low-alloy high-strength steel, but they focus more on achieving the required strength through design, with few solutions addressing the challenges of plate shape control. Chinese patent CN112442635A employs a scheme of 0.15~0.30% high carbon content + high-temperature annealing at 810~845℃ + tempering at 285~415℃ to obtain ultra-high strength low-alloy high-strength steel with a yield strength of 800~950MPa, tensile strength of 950~1100MPa, A80≥10.0%, and porosity of 25~40%. The microstructure is ferrite + 7~15% pearlite, but it does not address the issue of plate shape control. Chinese patent CN116121655A employs a composition design of niobium removal, 0.05-0.10% low carbon, and 0.10-0.50% high alumina, along with a heating temperature of 1240-1270℃, a final rolling temperature of 900-940℃, a coiling temperature of 620-700℃, a low-temperature annealing temperature of 610-720℃, and a soaking time of 30-75 seconds. This results in a 700MPa yield strength low-alloy high-strength steel with a hot-dip galvanized coating, exhibiting a yield strength difference of ≤25MPa between transverse and longitudinal yield strengths and a porosity ≥75%. The microstructure is characterized by ferrite + 5-20% pearlite. However, it not only has low strength but also fails to address the issue of sheet shape control. Chinese patent CN112680655A employs a composition design of 0.06~0.10% low carbon, 0.02~0.05% medium niobium, and 0.07~0.11% high titanium, along with a coiling temperature of 520~560℃, a cold rolling reduction rate of 60~70%, and a high-temperature annealing process of 770~800℃, to obtain cold-rolled low-alloy high-strength steel with a yield strength of 700MPa, tensile strength ≥720MPa, A80 ≥5%, and a yield-to-tensile ratio >0.9. However, it does not provide a solution for controlling the shape of low-alloy high-strength steel with a yield strength of 800MPa. Summary of the Invention

[0004] To address the problems in the existing technology, the present invention aims to provide a method for controlling the shape of ultra-high strength low alloy high-strength steel. Targeting low alloy high-strength steel with a yield strength of 800MPa, the method focuses on reducing strength fluctuations, minimizing waviness inheritance, and maximizing the capabilities of the leveling machine. Through integrated design of composition, hot rolling, pickling, annealing, and leveling, the method increases the plate shape qualification rate to over 90%.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solution: This invention provides a method for controlling the shape of ultra-high strength low-alloy high-strength steel, the method comprising the following steps: 1) Smelting: Before casting, the composition of the molten steel must meet the following requirements: 0.085%≤Nb+Ti≤0.110% and 0.15%≤Cr≤0.50%; 2) Hot rolling: Coiling temperature 550±30℃, wedge average value of the through roll controlled within ±10μm, crown 10~60μm; 3) Pickling and rolling: Pickling and rolling reduction rate 45~55%; 4) Continuous annealing: The uniform heating temperature is 720±30℃. The leveling machine adopts a small roller diameter, with a working roller diameter ≤510mm, a leveling elongation rate of 0.05~0.30%, and the bending force of the intermediate roller accounts for 50±10% of the total bending force.

[0006] Furthermore, the ultra-high strength low alloy high-strength steel has a yield strength of 800~880MPa, a tensile strength of 820~910MPa, an intra-coil strength difference of ≤50MPa, and a metallographic structure of ferrite + cementite.

[0007] Furthermore, in the continuous retraction step, the working roller of the leveling machine can be a flat roller or a convex roller with a convexity of 0~70μm.

[0008] Furthermore, in the continuous rolling step, the design rolling force of the leveling machine is ≥1300t.

[0009] Furthermore, in the continuous annealing step, the roll shifting is controlled according to the strip width, and the roll shifting amount is controlled within (LW) / 2±40mm, where L is the roll body length, W is the strip width, and the units of L and W are mm.

[0010] Furthermore, in the hot rolling step, the furnace exit temperature is 1250±30℃, and the final rolling temperature is 900±30℃.

[0011] Furthermore, in the smelting step, before casting, the chemical composition and mass percentage of the ultra-high strength low alloy high-strength steel meet the following conditions: C: 0.055~0.080%, Si: 0.20~0.50%, Mn: 0.60~1.00%, Cr: 0.15~0.50%, Als: 0.010~0.065%, Nb: 0.020~0.050%, Ti: 0.055~0.090%, Nb+Ti: 0.085~0.110%, P≤0.025%, S≤0.010%, N≤0.0065%, Ca≤0.0060%, with the balance being Fe and unavoidable impurities.

[0012] Furthermore, in the continuous retraction step, the working roller diameter of the leveling machine is 420~510mm.

[0013] Furthermore, in the metallographic structure, the ferrite morphology is mainly fibrous, with a small amount of polygonal ferrite allowed, and the area percentage of polygonal ferrite is <5%.

[0014] Low-alloy high-strength steels with a microstructure characterized by polygonal ferrite and pearlite have difficulty achieving a yield strength of 800 MPa. A higher Nb+Ti design is required to obtain an appropriate amount of fibrous ferrite, fully utilizing work hardening to improve strength. However, the quantity and size of carbonitride precipitation, the recrystallization behavior of austenite during hot rolling, and the recrystallization behavior of ferrite during annealing are difficult to control. As the niobium-titanium content increases, so does the strength, leading to greater performance fluctuations. Practice shows that the shape of ultra-high-strength low-alloy high-strength steel with a yield strength of 800 MPa is extremely difficult to control, with frequent occurrences of single-sided waves, double-sided waves, medium waves, and composite waves. Early trial production yielded a shape qualification rate of less than 20%, resulting in significant downgrading losses and the inability to deliver orders.

[0015] The difficulty in controlling the shape of low-alloy high-strength steel sheets with a yield strength of 800MPa is a major challenge, and existing technologies offer no readily applicable solutions. This invention addresses this issue by focusing on reducing strength fluctuations, minimizing waviness inheritance, and maximizing the capabilities of the leveling machine. It proposes an integrated design solution encompassing composition, hot rolling, pickling, annealing, and leveling. For composition design, the core measures are 0.15%≤Cr≤0.50% and 0.085%≤Nb+Ti≤0.110%. Exceeding these design ranges makes it impossible to consistently obtain a suitable amount of fibrous ferrite, easily resulting in a strength difference within the finished coil exceeding 50MPa. During hot rolling, a coiling temperature of 550±30℃ and a wedge mean of ±10μm are crucial. Too low a coiling temperature leads to intense laminar cooling but poor cooling uniformity; too high a temperature results in significant temperature differences within the coil during storage, increasing strength fluctuations. If the hot-rolled wedge shape exceeds the design range, it increases the difficulty of controlling the pickling shape, easily exceeding 20IU. In pickling and rolling production, a low-pressure design of 45-55% is a core measure. A low pickling reduction rate results in a low rolling load and easier control of the pickled and rolled sheet shape. In continuous annealing production, the matching design of work roll diameter, leveling elongation, bending roll force distribution, and roll shifting rules are core measures. Small roll diameter and low leveling elongation design can effectively reduce leveling rolling force. A reasonable distribution of bending roll force between the intermediate rolls and the work rolls can effectively eliminate waviness defects. Roll shifting control can reduce the harmful contact length of the roll system and reduce the occurrence of waviness. If the design rolling force of the leveling mill is <1300t, the leveling mill rigidity is insufficient, reducing the sheet shape control capability. Regarding finished product performance control, when the yield strength exceeds 880MPa or the tensile strength exceeds 910MPa, the leveling rolling force often exceeds 800t, even reaching 1100t, rapidly increasing the difficulty of sheet shape control.

[0016] The beneficial effects of this invention are as follows: 1) By reducing the difficulty of controlling microstructure and properties from the source of composition design, an innovative design of 0.085%≤Nb+Ti≤0.110% and 0.15%≤Cr≤0.50% can achieve appropriate and stable control of fibrous ferrite, while also significantly improving the degree of ferrite recovery and recrystallization, thus significantly reducing the strength fluctuation of ultra-high strength low alloy high strength steel. The strength difference within the finished coil is compressed to ≤50MPa, fundamentally reducing the difficulty of sheet shape control in the flattening process.

[0017] 2) From the perspective of wave inheritance, a solution for hot rolling and pickling processes is proposed. The innovative combination process of hot rolling coiling temperature 550±30℃ + wedge mean ±10μm + pickling reduction rate 45~55% is proposed to stabilize the pickling plate shape at ≤20IU, significantly reduce the difficulty of plate shape control in the leveling process of ultra-high strength low alloy high strength steel, and effectively block wave inheritance.

[0018] 3) Through the matching design of work roll diameter, leveling elongation, bending roll force distribution, and roll shifting rules, the shape control capability of the leveling machine is significantly improved, reducing waviness defects in ultra-high strength low alloy high strength steel. The waviness defects of the finished product can be controlled within a range of 50m with the beginning and end, which is convenient for removal, and the shape qualification rate by weight can reach 90%.

[0019] 4) The formula for calculating the amount of roll shifting ((LW) / 2±40mm) is simple, efficient, and easy to implement on site. Attached Figure Description

[0020] Figure 1 This is a diagram of the acid-rolled plate shape of Embodiment 1 of the present invention.

[0021] Figure 2 This is the metallographic structure of Example 1 of the present invention.

[0022] Figure 3 This is a diagram of the acid-rolled plate shape of Comparative Example 1 of the present invention.

[0023] Figure 4 This is a diagram of the edge wave defect of the finished product in Comparative Example 1 of the present invention. Detailed Implementation

[0024] The preferred embodiments of the present invention are described below with reference to specific examples. However, it should be understood that the accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent.

[0025] The present invention provides a method for controlling the shape of ultra-high strength low-alloy high-strength steel, comprising the following steps: 1) Smelting: Before casting, the composition of the molten steel must meet the following requirements: 0.085%≤Nb+Ti≤0.110% and 0.15%≤Cr≤0.50%; 2) Hot rolling: Coiling temperature 550±30℃, wedge average value of the through roll controlled within ±10μm, crown 10~60μm; 3) Pickling and rolling: Pickling and rolling reduction rate 45~55%; 4) Continuous annealing: The uniform heating temperature is 720±30℃. The leveling machine adopts a small roller diameter, with a working roller diameter ≤510mm, a leveling elongation rate of 0.05~0.30%, and the bending force of the intermediate roller accounts for 50±10% of the total bending force.

[0026] The microstructure of ultra-high strength low-alloy high-strength steel is ferrite + cementite, with a yield strength of 800~880MPa, a tensile strength of 820~910MPa, and an intra-coil strength difference ≤50MPa. The chemical composition and mass percentages meet the following conditions: C: 0.055~0.080%, Si: 0.20~0.50%, Mn: 0.60~1.00%, Cr: 0.15~0.50%, Als: 0.010~0.065%, Nb: 0.020~0.050%, Ti: 0.055~0.090%, Nb+Ti: 0.085~0.110%, P≤0.025%, S≤0.010%, N≤0.0065%, Ca≤0.0060%, with the balance being Fe and unavoidable impurities.

[0027] It is worth noting that in the continuous annealing step, the leveling mill is designed with a rolling force of 1600t and a work roll diameter of 460~510mm.

[0028] Table 1 shows the main components of Examples 1-4 and Comparative Examples 1-2, with the balance being Fe and impurities. Tables 2 and 3 show the main production processes of each example and comparative example, and Table 4 shows the plate shape and performance test results.

[0029] Table 1. Main chemical components (wt%) Table 2. Hot rolling and pickling processes Table 3 Continuous annealing process Table 4. Plate shape quality and performance. Note: If there are no waviness defects in the strip body beyond 50m from the beginning and end, it is considered that the finished plate shape is qualified.

[0030] In Examples 1-4 of the present invention, the microstructure of the ultra-high strength low-alloy high-strength steel is mainly composed of fibrous ferrite and cementite (see...). Figure 2Example), the area percentage of polygonal ferrite is <1%, the pickled and rolled plate shape is good, and the pickled and rolled plate shape is stably controlled at ≤20 IU (see Figure 1 (Example) The strip body beyond 50m from the beginning and end of the strip does not have waviness defects, and the finished strip shape is qualified. Compared with Example 1, the coiling temperature, wedge average value, and pickling reduction rate of Comparative Example 1 are outside the scope of the invention. After pickling, the strip has composite waviness, and the maximum strip shape value reaches 30 IU (see Example 1). Figure 3 Due to limited ability to level and correct acid-rolled waviness, edge waviness defects exist in the finished coil (see...). Figure 4 The waviness exhibits hereditary characteristics. Compared to Example 1, the leveling elongation and the proportion of bending force of the intermediate roll in Comparative Example 2 fall outside the scope of the invention. Although the pickling and rolling shape is relatively good, edge and center waviness defects still exist during leveling. Examples 3 and 4 have the same chemical composition, hot rolling process, and pickling and rolling process. Although the amount of roll shifting of the intermediate roll is different, it does not affect the shape control of the strip body beyond 50m from the beginning and end of the strip. As a preferred solution, the amount of roll shifting of the intermediate roll in Example 4 is controlled within the range of (LW) / 2±40mm, resulting in good through-coil shape. The results show that by focusing on reducing strength fluctuations, minimizing waviness inheritance, and maximizing the leveling machine's capabilities, and through the integrated design of composition, hot rolling, pickling and rolling, annealing, and leveling, the strip shape qualification rate can reach over 90%.

[0031] The present invention has been described in detail above with reference to the embodiments. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A method of sheet shape control of ultra-high strength low alloy high strength steel, characterized by, The method comprises the following steps: 1) smelting: before casting, the composition of molten steel satisfies 0.085%≤Nb+Ti≤0.110%, 0.15%≤Cr≤0.50%; 2) hot rolling: coiling temperature is 550±30℃, the average value of wedge of the coiling is controlled within ±10μm, and the crown is 10-60μm; 3) pickling: the reduction rate of pickling is 45-55%; 4) continuous annealing: the soaking temperature is 720±30℃, the work roll diameter of the skin pass mill is ≤510mm, the skin pass elongation is 0.05-0.30%, and the bending force of the intermediate roll accounts for 50±10% of the total bending force.

2. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1, characterized by, The ultra-high strength low alloy high strength steel has a yield strength of 800-880MPa, a tensile strength of 820-910MPa, an in-coil strength difference of ≤50MPa, and a metallographic structure of ferrite+carbide.

3. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1 or 2, characterized by, In the hot rolling step, the discharge temperature is 1250±30℃, and the final rolling temperature is 900±30℃.

4. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1 or 2, characterized by, In the continuous annealing step, the work roll of the skin pass mill can be a flat roll or a convex roll, and the crown is 0-70μm.

5. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1 or 2, characterized by, In the continuous annealing step, the roll shifting control is performed according to the width of the strip, and the roll shifting amount is controlled within (L-W) / 2±40mm, wherein L is the roll body length, and W is the width of the strip, and the units of L and W are mm.

6. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1 or 2, characterized by, In the continuous annealing step, the design rolling force of the skin pass mill is ≥1300t.

7. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1 or 2, characterized by, In the smelting step, before casting, the chemical composition and mass percentage of the ultra-high strength low alloy high strength steel satisfy the following conditions: C: 0.055-0.080%, Si: 0.20-0.50%, Mn: 0.60-1.00%, Cr: 0.15-0.50%, Als: 0.010-0.065%, Nb: 0.020-0.050%, Ti: 0.055-0.090%, Nb+Ti: 0.085-0.110%, P≤0.025%, S≤0.010%, N≤0.0065%, Ca≤0.0060%, and the balance is Fe and inevitable impurities.

8. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 1 or 2, characterized by, In the continuous annealing step, the work roll diameter of the skin pass mill is 420-510mm.

9. The sheet shape control method of ultra-high strength low alloy high strength steel according to claim 2, characterized by, In the metallographic structure, the ferrite morphology is mainly fibrous, and polygonal ferrite with an area percentage of <5% is allowed to exist.

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