Cold-rolled multiphase steel and fabrication method therefor

Abstract

Disclosed in the present invention is a cold-rolled multiphase steel. The cold-rolled multiphase steel comprises the following chemical components in mass percentage: 0.08-0.1% of C, 0.6-0.8% of Si, 1.2-1.4% of Mn, 0.2-0.4% of Al, and 0.0020-0.0030% of B. The chemical components of the cold-rolled multiphase steel do not comprise Mo and Cr. The microstructure of the cold-rolled multiphase steel is martensite+ferrite+bainite+residual austenite. The volume percentage content of ferrite is greater than 40% and less than or equal to 70%, the volume percentage content of martensite is greater than 10% and less than or equal to 40%, the volume percentage content of bainite is greater than 8% and less than or equal to 15%, and the volume percentage content of residual austenite is greater than 4% and less than or equal to 10%. Also disclosed in the preset invention is a fabrication method for the cold-rolled multiphase steel. The cold-rolled multiphase steel of the present invention has high elongation and good hole expansion performance.

Description

A cold-rolled multiphase steel and its manufacturing method Technical Field

[0001] This invention relates to a type of steel and a method for manufacturing the same, and more particularly to a cold-rolled multiphase steel and a method for manufacturing the same. Background Technology

[0002] In recent years, with the intensification of the global energy crisis and environmental problems, energy conservation and safety have become the main development directions of the automotive manufacturing industry, with reducing vehicle weight being one of the measures for energy conservation and emission reduction. In practical applications, high-strength steel, due to its excellent mechanical and performance properties, can be effectively used in the production and manufacturing of vehicle structural components. With the development of high-strength steel and current market changes, the market and users generally expect high-strength steel to be more economical and have better performance. As the trend of weight reduction and energy conservation in the automotive industry continues to develop, and with the rapid progress of steel mills both domestically and internationally, especially in China, the future development of steel will inevitably focus on a combination of low cost and high performance.

[0003] In the current technology, researchers have conducted a great deal of research on high-strength cold-rolled steel and have achieved certain research results.

[0004] For example, Chinese patent document CN109112433A, published on January 1, 2019, entitled "590MPa grade cold-rolled duplex steel without surface stripe defects and its production method," discloses a 590MPa grade cold-rolled duplex steel without surface stripe defects. Its composition by weight percentage is as follows: C: 0.050%~0.100%, Si≤0.25%, Mn: 1.20%~2.00%, P≤0.012%, S≤0.008%, Al: 0.05%~0.10%, Sb: 0.015~0.050%, with the balance being Fe and other unavoidable impurities. The production method adopted by this technical solution includes: smelting, continuous casting, hot charging, high-pressure water descaling and rough rolling, finish rolling, laminar flow cooling, coiling, hot-rolled plate pickling and cold continuous rolling, continuous annealing, air cooling, leveling and coiling. The 590MPa grade cold-rolled duplex steel produced using this technology has a bright surface, no streak defects, and good corrosion resistance and coating adhesion, as well as excellent mechanical properties. However, research has found that this technology involves the addition of Sb and a high Mn content in the chemical composition design.

[0005] For example, Chinese patent document CN109943778A, published on June 28, 2019, entitled "A 590MPa Grade Cold-Rolled Duplex Steel with Excellent Hole-Expanding Performance and Its Production Method," discloses a 590MPa grade cold-rolled duplex steel with excellent hole-expanding performance and its production method. Its chemical composition is: C: 0.06–0.09%, Si+Mn: 1.4–2.1%, Nb: 0.01–0.02%, Al: 0.03–0.06%, P≤0.020%, S≤0.010%, N: ≤0.006%, with the balance being Fe and unavoidable impurities. The main problem with this steel is its high Mn content, along with the noble alloy Nb, resulting in high alloying costs.

[0006] For example, Chinese patent document CN103088258A, published on May 8, 2013, entitled "A 590MPa grade duplex steel and its production method," discloses a 590MPa grade duplex steel with the following composition by mass percentage: C≤0.20%, Si≤1.80%, Mn≤2.00%, P≤0.050%, S≤0.015%, Nb≤0.10%, and Ti≤0.10%. The production method adopted in this technical solution includes: hot metal pretreatment - converter smelting - alloy fine-tuning station - LF - continuous casting - hot continuous rolling. The hot continuous rolling process is as follows: heating temperature is 1150~1250℃, final rolling temperature is 800~900℃, rapid cooling after rolling to 650~750℃, air cooling for 6~11 seconds, and then rapid cooling to the target coiling temperature of 400~500℃. This technical solution, through appropriate composition and process control, can achieve precise control and proportioning of the ferrite and bainite phases. It can effectively reduce the strength difference between the soft and hard phases and eliminate the interface between them. The resulting product exhibits excellent strength and a good balance of ductility and toughness, with good elongation and flanging performance. However, the steel designed using this technical solution has a high carbon equivalent and a high Si and Mn alloy content, which is detrimental to welding and surface quality, and it lacks balanced performance characteristics.

[0007] For example, Chinese patent document CN111979490A, published on November 24, 2020, entitled "A Cold-Rolled DH590 Steel with High Elongation and High Formability and Its Production Method," discloses a 590MPa grade cold-rolled steel with the following composition by mass percentage: C: 0.08%–0.14%, Mn: 1.2%–2.2%, Si: 0.35%–0.75%, Al: 0.02%–0.50%, Cr: 0.03%–0.45%, Mo: 0.02%–0.25%, P≤0.01%, S≤0.01%, N b≤0.03%; The production method adopted in this technical solution includes: billet heating temperature 1200~1300℃, initial rolling temperature 1100~1180℃, final rolling temperature ≥910℃, coiling temperature 550~700℃; cold rolling reduction rate 50%~90%; continuous annealing temperature 750~850℃, annealing time 10~600s, slow cooling exit temperature 660~730℃, rapid cooling rate >20℃ / s, rapid cooling exit temperature 300~420℃, over-aging temperature 300~420℃, over-aging time 30~3600s; finishing elongation 0.3%~0.6%. The finished steel plate has high ductility and high formability. However, the steel designed by this technical solution has a high carbon equivalent and a high Si and Mn alloy content, which is not conducive to welding and surface quality, and does not have the characteristics of balanced performance.

[0008] For example, Chinese patent document CN112048670B, published on December 8, 2020, entitled "Cold-rolled hot-dip galvanized DH590 steel with excellent surface quality and its production method," discloses a 590MPa grade cold-rolled steel with the following composition by mass percentage: C: 0.10%–0.15%, Mn: 1.3%–1.9%, Al: 0.02%–0.75%, Si≤0.40%, Cr≤0.2%, Mo≤0.4%, P≤0.03%, S≤0.03%, V≤0.04%, Ti≤0.03%. The production method employed in this technical solution includes: billet heating temperature of 1150–1250℃, initial rolling temperature of 1000–1120℃, final rolling temperature above 880℃, and coiling temperature of 500–650℃; cold rolling reduction rate of 40%–75%; annealing temperature of 750–850℃, annealing time between 30 and 300 seconds; galvanizing temperature of 450–470℃; after galvanizing, the strip is first air-cooled to 400–420℃, followed by air cooling, with the temperature of the cooling tower top roller controlled at 250–300℃; and the finishing elongation rate is controlled within the range of 0.2%–0.5%. However, the steel designed using this technical solution has a high carbon equivalent and a high Si and Mn alloy content, which is detrimental to welding and surface quality, and it lacks balanced performance characteristics.

[0009] It is evident that while some of the existing patented technologies for 590MPa grade cold-rolled steel involve good formability, these technologies either employ high C and high Si content or contain a large amount of alloys such as Cr and Mn. This not only negatively impacts the weldability, surface quality, and phosphating performance of the steel but also leads to increased costs.

[0010] Therefore, in order to meet current market demands, this invention aims to develop a new cold-rolled multiphase steel that combines low cost with excellent mechanical properties. Summary of the Invention

[0011] This invention provides a cold-rolled multiphase steel and a method for manufacturing cold-rolled multiphase steel to solve the above-mentioned problems.

[0012] In a first aspect, embodiments of the present invention disclose a cold-rolled multiphase steel, wherein the chemical composition of the cold-rolled multiphase steel, by mass percentage, includes:

[0013] C: 0.080%–0.100%, Si: 0.60%–0.80%, Mn: 1.20%–1.40%, Al: 0.20%–0.40%, B: 0.0020%–0.0030%; the chemical composition of cold-rolled multiphase steel does not contain Mo and Cr; the microstructure of cold-rolled multiphase steel is martensite + ferrite + bainite + retained austenite; the volume percentage content of ferrite is >40% and ≤70%, the volume percentage content of martensite is >10% and ≤40%, the volume percentage content of bainite is >8% and ≤15%, and the volume percentage content of retained austenite is >4% and ≤10%.

[0014] Using the above technical solution, the cold-rolled multiphase steel of the present invention has a microstructure of martensite + ferrite + bainite + retained austenite, and satisfies the following conditions: the volume percentage content of ferrite is >40% and ≤70%, the volume percentage content of martensite is >10% and ≤40%, the volume percentage content of bainite is >8% and ≤15%, and the volume percentage content of retained austenite is >4% and ≤10%, thereby effectively improving the elongation and hole expansion properties of the multiphase steel.

[0015] Optionally, the cold-rolled multiphase steel has the following chemical composition by mass percentage: C: 0.080%–0.100%, Si: 0.60%–0.80%, Mn: 1.20%–1.40%, Al: 0.20%–0.40%, B: 0.0020%–0.0030%, with the balance being Fe and other unavoidable impurities.

[0016] Optionally, in the unavoidable impurities, the content of each impurity element by mass percentage meets one or more of the following requirements: P≤0.020%, S≤0.0050%, N≤0.0050%.

[0017] Optionally, the martensite grain size is ≤15μm.

[0018] Optionally, the ferrite grain size is ≤17μm.

[0019] Optionally, the grain size of both martensite and ferrite is ≤15μm.

[0020] Optionally, the grain size of both martensite and ferrite is 9–15 μm.

[0021] Optionally, the cold-rolled multiphase steel has a yield strength ≥340MPa, a tensile strength ≥590MPa, and a longitudinal A... 50 Gauge length elongation at break ≥29%, longitudinal strain hardening index N 4-6 ≥0.19, porosity λ≥50%.

[0022] Secondly, the present invention also discloses a method for manufacturing cold-rolled multiphase steel, comprising the following steps:

[0023] Smelting and casting;

[0024] Hot-rolled;

[0025] Cold rolling;

[0026] Annealing: Control the annealing temperature to be 770-800℃, the annealing time to be 40-200s, cool to the rapid cooling start temperature of 650-690℃ at a cooling rate of 3-5℃ / s, and then perform rapid cooling at a rapid cooling rate of 40-100℃ / s, controlling the rapid cooling end temperature to be 290-340℃.

[0027] Tempering;

[0028] smooth.

[0029] By adopting the above technical solution, the annealing homogenization temperature and rapid cooling end temperature in the annealing step during the steel plate manufacturing process can be precisely controlled, which is beneficial to control the volume percentage content of the microstructure and to introduce bainite, thereby improving the hole expansion performance of the steel plate.

[0030] Optionally, in the hot rolling step, the continuously cast billet is heated to a heating temperature of 1180-1240℃ (e.g., 1190-1230℃), held for 150-200 minutes, and then hot-rolled to a final rolling temperature of 860-900℃. It is then rapidly cooled at a cooling rate of 30-80℃ / s and coiled at a coiling temperature of 495-575℃ (e.g., 520-560℃). After coiling, it is air-cooled.

[0031] Optionally, in the cold rolling step, the cold rolling reduction rate is controlled to be 50-70%.

[0032] Optionally, in the annealing step, the annealing soaking temperature is 780–790°C.

[0033] Optionally, in the tempering step, the tempering start temperature is 290–340℃, the tempering end temperature is 205–280℃ (e.g., 220–260℃), and the tempering time is 100–400s.

[0034] Optionally, during the leveling step, the leveling reduction rate should be controlled to be ≤0.5%. Attached Figure Description

[0035] Figure 1 shows the metallographic structure of the cold-rolled multiphase steel of Example 1. Detailed Implementation

[0036] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Although the description of the present invention is presented in conjunction with preferred embodiments, this does not mean that the features of the invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be derived based on the claims of the present invention. To provide a deep understanding of the invention, many specific details will be included in the following description. The invention may also be implemented without using these details. Furthermore, to avoid confusion or obscuring the focus of the invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0037] It should be noted that in this specification, similar reference numerals and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0038] The terms “first”, “second”, etc., are used only to distinguish descriptions and should not be interpreted as indicating or implying relative importance.

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0040] In a first aspect, the present invention provides a cold-rolled multiphase steel, wherein the chemical composition of the cold-rolled multiphase steel, by mass percentage, comprises:

[0041] C: 0.080%–0.100%, Si: 0.60%–0.80%, Mn: 1.20%–1.40%, Al: 0.20%–0.40%, B: 0.0020%–0.0030%; the chemical composition of cold-rolled multiphase steel does not contain Mo and Cr; the microstructure of cold-rolled multiphase steel is martensite + ferrite + bainite + retained austenite; the volume percentage content of ferrite is >40% and ≤70%, the volume percentage content of martensite is >10% and ≤40%, the volume percentage content of bainite is >8% and ≤15%, and the volume percentage content of retained austenite is >4% and ≤10%.

[0042] In the cold-rolled multiphase steel provided by this invention, the inventors adopted a chemical element design system based on C-Si-Mn, while simultaneously adding trace amounts of the high-hardenability element B to further reduce the Mn content. The content of C, Mn, and B is set to provide sufficient hardenability for the steel plate, but the content of C, Mn, and B elements should not be too high, otherwise it is difficult to ensure that the multiphase steel obtains excellent welding and forming properties. At the same time, a composite design of medium silicon and low aluminum is used to enhance the stability of austenite, but the content of Si and Al should not be too high, otherwise the steel plate will be difficult to manufacture and the surface quality will be poor. Therefore, through the above-mentioned appropriate element design and combination, a steel plate with good surface quality, welding performance, forming performance, elongation, and low cost can be obtained without adding expensive alloying elements such as Mo and Cr. However, it is uncertain how the above element design can ensure that the steel plate obtains excellent hole-expanding performance and ensure that the steel plate has comprehensive excellent mechanical properties that balance high elongation and good hole-expanding performance. To this end, the inventors, through various trials and experiments, determined the above-mentioned technical solution and proposed a technical solution specifically applicable to 590MPa grade cold-rolled multiphase steel.

[0043] Furthermore, the microstructure of the cold-rolled multiphase steel of this invention consists of martensite + ferrite + bainite + retained austenite. The main phase is ferrite, the secondary phases are martensite and bainite, and a small amount of retained austenite is also present. During their research, the inventors discovered that in the microstructure of the steel plate, ferrite is a soft phase with good toughness and plasticity, but relatively low strength and hardness; martensite is a hard phase with very high strength and hardness, but low toughness. Through the design of the aforementioned chemical elements, a combination of ferrite and martensite can be achieved to balance the strength, hardness, toughness, and formability of the steel plate. Based on this, the introduction of retained austenite enhances the ductility of the steel plate, and the introduction of bainite, with a hardness higher than ferrite but lower than martensite, effectively reduces the microhardness difference between the phases, providing a good balance of strength and toughness for the steel plate, thereby effectively improving the hole-expanding performance. The inventors also discovered that by adjusting the phase ratios (phase ratio refers to the volume percentage content of the phases in the microstructure) of ferrite, martensite, retained austenite, and bainite, such that the volume percentage content of ferrite is >40% and ≤70%, the volume percentage content of martensite is >10% and ≤40%, the volume percentage content of bainite is >8% and ≤15%, and the volume percentage content of retained austenite is >4% and ≤10%, a comprehensive and excellent mechanical property characteristic that balances high elongation and good pore-expanding performance can be obtained.

[0044] Therefore, the cold-rolled multiphase steel of the present invention achieves a balance between low cost and good mechanical properties through the mutual coordination of steel chemical element design, steel microstructure phase composition design, and steel microstructure phase ratio setting.

[0045] Furthermore, in the cold-rolled multiphase steel provided by this invention, the chemical composition, by mass, is: C: 0.080%–0.100%, Si: 0.60%–0.80%, Mn: 1.20%–1.40%, Al: 0.20%–0.40%, B: 0.0020%–0.0030%, with the balance being Fe and other unavoidable impurities. Through the above-mentioned chemical composition design of the multiphase steel, this invention eliminates the need to add expensive alloying elements such as Mo and Cr, thereby ensuring economic efficiency.

[0046] The design principles of each chemical element in the cold-rolled multiphase steel of this invention are explained in detail below.

[0047] C: In the cold-rolled multiphase steel of this invention, the addition of carbon (C) can improve the strength of the steel and increase the hardness of the martensite in the steel. If the C content in the steel is less than 0.080%, the strength of the steel plate is affected, and it is not conducive to the formation and stability of retained austenite; while when the C content in the steel is greater than 0.100%, it will cause excessively high martensite hardness and coarse grain size, which is not conducive to the formability of the steel plate, and at the same time, the carbon equivalent is too high, which is not conducive to the welding of the steel. Therefore, in order to ensure the performance of the steel, in the 590MPa cold-rolled low-alloy high-ductility annealed multiphase steel of this invention, the mass percentage content of C is specifically controlled between 0.080% and 0.100%.

[0048] Si: In the cold-rolled multiphase steel of this invention, the addition of Si can improve the hardenability of the steel, and the Si dissolved in the steel can affect the interaction of dislocations, thereby increasing the work hardening rate. This can appropriately increase the elongation of the multiphase steel, which is beneficial for obtaining better formability. Appropriately increasing the amount of Si can effectively increase the content of retained austenite in the final microstructure. However, it should be noted that the Si content in the steel should not be too high, as excessive Si content will hinder the control of the steel plate surface quality. Therefore, in order to maximize the beneficial effects of Si, the mass percentage of Si in the 590MPa cold-rolled low-alloy high-ductility annealed multiphase steel of this invention is controlled between 0.60% and 0.80%.

[0049] Mn: In the cold-rolled multiphase steel of this invention, the addition of Mn not only improves the hardenability of the steel, but also effectively increases the strength of the steel plate. When the Mn content in the steel is below 1.20%, the strength of the steel plate is insufficient; while when the Mn content in the steel is too high, above 1.40%, the strength of the steel plate is too high, which will reduce its formability. Based on this, considering the beneficial effects of Mn, in the 590MPa cold-rolled low-alloy high-ductility annealed multiphase steel of this invention, the mass percentage of Mn is controlled between 1.20% and 1.40%.

[0050] Al: In the cold-rolled multiphase steel of the present invention, the added Al element can play a role in deoxidation and grain refinement. On the other hand, Al in the steel has the functions of inhibiting carbide precipitation, solid solution strengthening of retained austenite, and improving the stability of retained austenite. Therefore, in order to give full play to the beneficial effects of Al, in the 590MPa cold-rolled low-alloy high-ductility annealed multiphase steel of the present invention, the mass percentage of Al element is controlled between 0.20% and 0.40%.

[0051] B: In the cold-rolled multiphase steel of this invention, the addition of element B is beneficial to improving the hardenability of the steel and can also effectively improve the strength of the steel plate. When the content of element B in the steel is less than 0.0020%, the strength of the steel plate is insufficient; while when the content of element B in the steel is greater than 0.0030%, the strength of the steel plate is too high, which will reduce its formability. Therefore, in the 590MPa cold-rolled low-alloy high-ductility annealed multiphase steel of this invention, the mass percentage content of element B is controlled between 0.0020% and 0.0030%.

[0052] Furthermore, among the unavoidable impurities, the content of each impurity element, by mass percentage, meets one or more of the following requirements: P ≤ 0.020%, S ≤ 0.005%, N ≤ 0.005%. In the cold-rolled multiphase steel of various embodiments of the present invention, P, S, and N are impurity elements in the steel. The lower the content of P, N, and S, the better the performance of the steel. Specifically, MnS formed by S can seriously affect the forming performance, while N can easily cause cracks or bubbles on the slab surface. Therefore, when technical conditions permit, in order to obtain steel plates with better forming performance and higher quality, the content of impurity elements in the steel should be reduced as much as possible, and specifically, the P, S, and N elements in the steel of the present invention should be controlled to meet the following requirements: P ≤ 0.020%, S ≤ 0.005%, N ≤ 0.005%. In some embodiments, P meets the requirement of ≤ 0.015%.

[0053] In the cold-rolled multiphase steel of the present invention, the grain size of martensite is ≤15μm and the grain size of ferrite is ≤17μm. Preferably, the grain size of both martensite and ferrite in the cold-rolled multiphase steel of the present invention is not greater than 15μm; more preferably, the grain size of both martensite and ferrite is in the range of 9-15μm (inclusive of the endpoint values ​​9 and 15). Since there are fewer stress concentration points generated during the martensitic transformation, finer martensite grain size can provide higher hardness to the steel plate; finer ferrite grains can increase the number of grain boundaries, which can hinder dislocation movement, thereby improving the yield strength and tensile strength of the steel plate. Therefore, martensite and ferrite within the above-mentioned grain size range will help improve the strength and processing performance of the steel plate.

[0054] The cold-rolled multiphase steel of this invention has a yield strength ≥340MPa, a tensile strength ≥590MPa, and a longitudinal A... 50 Gauge length elongation at break ≥29.0%, longitudinal strain hardening index N 4-6 ≥0.19, porosity λ≥50%.

[0055] In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is ≥350 MPa. In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is ≥370 MPa. In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is ≥380 MPa. In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is 340–410 MPa. In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is 350–410 MPa. In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is 370–410 MPa. In some embodiments, the yield strength of the cold-rolled multiphase steel of the present invention is 380–410 MPa.

[0056] In some embodiments, the tensile strength of the cold-rolled multiphase steel of the present invention is ≥600 MPa. In some embodiments, the tensile strength of the cold-rolled multiphase steel of the present invention is ≥610 MPa. In some embodiments, the tensile strength of the cold-rolled multiphase steel of the present invention is ≥620 MPa. In some embodiments, the tensile strength of the cold-rolled multiphase steel of the present invention is ≥640 MPa. In some embodiments, the tensile strength of the cold-rolled multiphase steel of the present invention is 590–680 MPa, such as 60–680 MPa, 610–680 MPa, 620–680 MPa, or 640–680 MPa.

[0057] In some embodiments, the longitudinal A of the cold-rolled multiphase steel of the present invention 50 The gauge length elongation at break is ≥30.0%, preferably ≥31.0%. In some embodiments, the longitudinal A of the cold-rolled multiphase steel of the present invention... 50 The gauge length elongation at break is 29.0%–35.0%.

[0058] In some embodiments, the longitudinal strain hardening index N of the cold-rolled multiphase steel of the present invention 4-6 The value ranges from 0.19 to 0.21.

[0059] In some embodiments, the expansion ratio λ of the cold-rolled multiphase steel of the present invention is ≥55%. In some embodiments, the expansion ratio λ of the cold-rolled multiphase steel of the present invention is ≥60%. In some embodiments, the expansion ratio λ of the cold-rolled multiphase steel of the present invention is 50-80%, such as 55-80% or 60-80%.

[0060] Secondly, the present invention provides a method for manufacturing cold-rolled multiphase steel, used to manufacture the cold-rolled multiphase steel of the above embodiments, comprising the following steps: (1) smelting and casting, (2) hot rolling, (3) cold rolling, (4) annealing, (5) tempering, and (6) leveling. The inventors mainly improved the annealing process. Specifically, in the annealing step, the annealing homogenization temperature is controlled at 770-800°C, the annealing time is 40-200s, the temperature is cooled at a cooling rate of 3-5°C / s to the rapid cooling start temperature of 650-690°C, and then rapid cooling is performed at a rapid cooling rate of 40-100°C / s, controlling the rapid cooling end temperature to be 290-340°C.

[0061] The mechanical and formability properties of the cold-rolled multiphase steel obtained by the above manufacturing method are improved. Through in-depth research, the inventors discovered that by designing the chemical elements of the cold-rolled multiphase steel and combining this with rapid cooling of the multiphase steel at a rate of 40–100°C / s during the annealing step, the tensile strength of the steel of this invention reaches 590 MPa or higher. Furthermore, through repeated experiments, the inventors found that when the annealing homogenization temperature is controlled between 770 and 800°C, preferably between 780 and 790°C, the optimal temperature is achieved. Controlling the annealing homogenization temperature within the aforementioned range can hinder grain growth, thereby obtaining a fine grain size. Simultaneously, by controlling the rapid cooling end temperature in the annealing step to 290–340°C, which also controls the tempering initiation temperature in the tempering step to 290–340°C, bainite is introduced and the hardness difference between the phases is reduced. Ultimately, the phase ratio of martensite, ferrite, bainite, and retained austenite in the microstructure of the cold-rolled multiphase steel of this invention, as well as the grain size of martensite and ferrite, are controlled within a suitable range, resulting in superior mechanical properties and formability of the steel of this invention.

[0062] Among them, smelting and casting, hot rolling, cold rolling, tempering, and leveling belong to the steel plate casting methods. These steps are described in detail below.

[0063] Smelting and casting: Smelting and continuous casting of steel plates according to the determined steel plate composition to produce billets, and then casting the steel plates.

[0064] Hot rolling: The continuously cast billet is heated to a heating temperature of 1180-1240℃ (e.g., 1190-1230℃) and held for 150-200 minutes. It is then hot rolled to a final rolling temperature of 860-900℃ and rapidly cooled at a cooling rate of 30-80℃ / s. The billet is then coiled at a coiling temperature of 495-575℃ (e.g., 520-560℃) and air-cooled after coiling.

[0065] Cold rolling: The steel coil is cold rolled with a reduction rate of 50-70%.

[0066] Tempering: Control the tempering start temperature to be 290-340℃, the tempering end temperature to be 205-280℃ (e.g., 220-260℃), and the tempering time to be 100-400s.

[0067] Leveling: Control the leveling reduction rate to ≤0.5% to obtain finished multiphase steel.

[0068] The combination of the above steps is beneficial to further improve the mechanical properties of cold-rolled multiphase steel. It is convenient and simple to implement and can effectively manufacture 590MPa strength cold-rolled multiphase steel that combines low cost and excellent mechanical properties. The mechanical properties in this invention include tensile strength, yield strength, elongation, strain hardening coefficient and expansion rate.

[0069] The manufacturing method of the present invention will now be described in further detail.

[0070] Examples 1-12 and Comparative Examples 1-14

[0071] The cold-rolled multiphase steels in Examples 1-12 were all prepared using the following steps:

[0072] (1) Smelting and casting: The determined steel plate composition is smelted and cast to obtain a continuous casting billet.

[0073] (2) Hot rolling: The continuous casting billet is first heated to a heating temperature of 1180~1240℃ and held for more than 150 minutes. Then, it is hot rolled at 860~900℃. After rolling, it is rapidly cooled at a cooling rate of 30~80℃ / s. Then, it is coiled at a coiling temperature of 495~575℃ and air-cooled after coiling.

[0074] (3) Cold rolling: The steel coil is cold rolled, and the cold rolling reduction rate is controlled to be 50-70%.

[0075] (4) Annealing: Control the annealing temperature to 770-800℃ and the annealing time to 40-200s. Then cool to the rapid cooling start temperature of 650-690℃ at a cooling rate of 3-5℃ / s, and then perform rapid cooling at a rapid cooling rate of 40-100℃ / s. Control the rapid cooling end temperature to 290-340℃.

[0076] (5) Tempering: The tempering start temperature is controlled at 290-340℃, the tempering end temperature is controlled at 205-280℃, and the tempering time is controlled at 100-400s.

[0077] (6) Leveling: Control the leveling reduction rate to ≤0.5% to obtain finished multiphase steel.

[0078] Comparative Examples 1-14 were also produced by using the following steps: (1) smelting and casting, (2) hot rolling, (3) cold rolling, (4) annealing, (5) tempering, and (6) leveling.

[0079] The steel plate compositions of the examples and comparative examples are shown in Table 1, and the process parameters are shown in Tables 2-1 and 2-2.

[0080] Table 1 lists the chemical composition of the cold-rolled multiphase steel cast by the present invention, including the mass percentage of the chemical composition of the cold-rolled multiphase steel of Examples 1 to 12 and the mass percentage of the chemical composition of Comparative Examples 1 to 14, with the balance being Fe and other unavoidable impurities other than P, S and N.

[0081] Table 1: Mass percentage of chemical composition of steel in different embodiments and comparative examples

[0082] Tables 2-1 and 2-2 list the specific process parameters in the manufacturing method steps of Examples 1-12 and Comparative Examples 1-14.

[0083] Table 2-1: Manufacturing methods of steels in different embodiments and comparative examples

[0084] Table 2-2: Manufacturing methods of steels in different embodiments and comparative examples

[0085] This invention improves the mechanical properties of cold-rolled multiphase steel by modifying the manufacturing method and adjusting the process. The main improvement lies in the annealing step. By precisely controlling the annealing soaking temperature, rapid cooling rate, and rapid cooling end temperature during the annealing step of the above embodiments, the phase composition and proportion of the microstructure of the cold-rolled multiphase steel can be controlled, which is beneficial for obtaining cold-rolled multiphase steel with a strength of 590 MPa, while ensuring the phase composition content of the steel's microstructure, thereby improving the mechanical properties of the cold-rolled multiphase steel. Furthermore, the cold-rolled multiphase steel manufactured using the methods described in the above embodiments of this invention possesses excellent comprehensive characteristics, including low cost, high strength, high elongation, high hole expansion performance, and good manufacturability, demonstrating excellent prospects for promotion and application value.

[0086] Experiments were conducted using the manufacturing methods described in the above embodiments to obtain various cold-rolled multiphase steels (including comparative examples and specific examples). The microstructure of these steels was then determined, including the proportions of martensite, ferrite, bainite, and retained austenite, as well as the martensite and ferrite grain sizes. Specific data are shown in Table 3. The microstructure was observed by etching the dual-phase steel with 4% nitric acid alcohol, and the volume fractions and sizes of martensite and ferrite were evaluated using image analysis software.

[0087] Table 3: Microstructure measurement results of different examples and comparative examples

[0088] As shown in Table 3, in this invention, the microstructure of the cold-rolled multiphase steel prepared in Examples 1-12 is martensite + ferrite + bainite + retained austenite. The volume percentage content (phase ratio) of ferrite is between 42% and 68%, the volume percentage content (phase ratio) of martensite is between 12% and 39%, the volume percentage content (phase ratio) of bainite is between 8% and 15%, and the volume percentage content (phase ratio) of retained austenite is between 5% and 10%. The martensite grain size is between 9.5 and 15 μm, and the ferrite grain size is between 9.5 and 17 μm.

[0089] In this invention, to verify the mechanical properties of the steels in each embodiment and comparative example, tensile tests were performed to determine their properties. The tensile tests included yield strength, tensile strength, and longitudinal strength (A). 50 Gauge length elongation at break and longitudinal strain hardening index N 4-6 The specific data for its mechanical property test results are shown in Table 4. The relevant mechanical property test methods are described below:

[0090] Tensile testing: Tensile tests were conducted according to GB / T228-2010 Metallic materials, room temperature tensile testing method, to detect the yield strength, tensile strength, and longitudinal strength of the steels obtained in Examples 1-12 and Comparative Examples 1-14. 50 Gauge length elongation at break and longitudinal strain hardening index N 4-6 Among them, longitudinal A 50 Gauge length elongation at break represents the elongation at break of a tensile specimen with a parallel length * width of 50 mm * 25 mm; longitudinal strain hardening index N. 4-6 It indicates the work hardening index when the material is stretched to 4-6%.

[0091] Table 4: Results of mechanical property measurements for different embodiments and comparative examples

[0092] As shown in Table 4, the 590MPa cold-rolled multiphase steels of Examples 1-12 prepared using the technical solution of the present invention possess excellent mechanical properties, with yield strength between 342 and 410MPa, tensile strength between 590 and 690MPa, and longitudinal A... 50 The gauge length elongation at break is between 29% and 34%, and the longitudinal strain hardening exponent N is... 4-6 The values ​​ranged from 0.19 to 0.22, and the porosity λ ranged from 50% to 80%. Furthermore, the multiphase steels in each embodiment achieved a tensile strength greater than 590 MPa without the addition of precious alloying elements such as Mo and Cr; all were 590 MPa cold-rolled multiphase steels, and also exhibited good elongation. In contrast, Comparative Examples 1-14 showed significantly inferior overall performance.

[0093] In summary, the present invention provides a method for manufacturing cold-rolled multiphase steel. Through specific control of the annealing homogenization temperature, rapid cooling rate, and rapid cooling termination temperature in the annealing step, and in conjunction with the aforementioned chemical element composition design, the phase composition and phase ratio of the microstructure are controlled. This allows for the production of multiphase steel with both low cost and excellent mechanical properties without the addition of Mo and Cr elements. In particular, the 590MPa cold-rolled low multiphase steel of the present invention exhibits high strength and excellent elongation, with a yield strength ≥340MPa, tensile strength ≥590MPa, and longitudinal A... 50 Gauge length elongation at break ≥29%, longitudinal strain hardening index N 4-6 ≥0.19, expansion ratio λ≥50%. This steel has the comprehensive advantages of low cost, high strength, high elongation and good manufacturability, which can effectively meet the needs of the market and users, and has a very good prospect for promotion and application value.

[0094] While the present invention has been illustrated and described with reference to certain preferred embodiments, those skilled in the art should understand that the above description is a further detailed explanation of the invention in conjunction with specific embodiments, and should not be construed as limiting the specific implementation of the invention to these descriptions. Various changes in form and detail can be made by those skilled in the art, including several simple deductions or substitutions, without departing from the spirit and scope of the invention.

Claims

1. A cold-rolled multiphase steel, characterized in that, The chemical composition of the cold-rolled multiphase steel, by weight percentage, includes: C: 0.080%–0.100%, Si: 0.60%–0.80%, Mn: 1.20%–1.40%, Al: 0.20%–0.40%, B: 0.0020%–0.0030%; the chemical composition of the cold-rolled multiphase steel does not contain Mo and Cr. The microstructure of the cold-rolled multiphase steel is martensite + ferrite + bainite + retained austenite; the volume percentage content of ferrite is >40% and ≤70%, the volume percentage content of martensite is >10% and ≤40%, the volume percentage content of bainite is >8% and ≤15%, and the volume percentage content of retained austenite is >4% and ≤10%.

2. The cold-rolled multiphase steel as described in claim 1, characterized in that, The chemical composition of the cold-rolled multiphase steel, by mass percentage, is: C: 0.080%–0.100%, Si: 0.60%–0.80%, Mn: 1.20%–1.40%, Al: 0.20%–0.40%, B: 0.0020%–0.0030%, with the balance being Fe and other unavoidable impurities.

3. The cold-rolled multiphase steel as described in claim 2, characterized in that, In unavoidable impurities, the content of each impurity element by mass percentage shall meet one or more of the following requirements: P≤0.020%, S≤0.005%, N≤0.005%.

4. The cold-rolled multiphase steel as described in claim 1, characterized in that, The cold-rolled multiphase steel has a ferrite volume percentage content between 42% and 68%, a martensite volume percentage content between 12% and 39%, a bainite volume percentage content between 8.2% and 15%, and a retained austenite volume percentage content between 5% and 10%.

5. The cold-rolled multiphase steel as described in claim 1, characterized in that, The cold-rolled multiphase steel has a ferrite volume percentage content between 55% and 68%, a martensite volume percentage content between 14% and 25%, a bainite volume percentage content between 8.5% and 15%, and a retained austenite volume percentage content between 5% and 10%.

6. The cold-rolled multiphase steel as described in claim 1, characterized in that, The martensite has a particle size ≤15μm, and the ferrite has a particle size ≤17μm; preferably, both the martensite and ferrite have a particle size ≤15μm, and more preferably, both are in the range of 9 to 15μm; more preferably, the martensite has a particle size in the range of 9.5 to 15.0μm, and the ferrite has a particle size in the range of 9.5 to 13.5μm.

7. The cold-rolled multiphase steel according to any one of claims 1-6, characterized in that, The cold-rolled multiphase steel has a yield strength ≥340MPa, a tensile strength ≥590MPa, and a longitudinal A... 50 Gauge length elongation at break ≥29%, longitudinal strain hardening index N 4-6 ≥0.19, porosity λ≥50%.

8. The cold-rolled multiphase steel as described in claim 7, characterized in that, The cold-rolled multiphase steel has one or more of the following properties: yield strength ≥350MPa, ≥370MPa, or ≥380MPa, and / or yield strength ≤410MPa; tensile strength ≥600MPa, ≥610MPa, ≥620MPa, or ≥640MPa, and / or tensile strength ≤680MPa; longitudinal A 50 Gauge length elongation at break ≥29%, ≥30.0%, or ≥31.0%, and / or longitudinal A 50 Gauge length elongation at break ≤35.0%; longitudinal strain hardening index N 4-6 ≥0.20, or 0.19~0.21; expansion ratio λ≥55% or ≥60%, and / or ≤80%.

9. The cold-rolled multiphase steel as described in claim 1, characterized in that, The cold-rolled multiphase steel has a ferrite volume percentage content of 55-68%, a martensite volume percentage content of 14-25%, a bainite volume percentage content of 8.5-15%, and a retained austenite volume percentage content of 5-10%; the martensite grain size is in the range of 9.5-15.0 μm, and the ferrite grain size is in the range of 9.5-13.5 μm; the cold-rolled multiphase steel has a yield strength ≥350 MPa, a tensile strength ≥590 MPa, and a longitudinal A... 50 Gauge length elongation at break ≥29%, longitudinal strain hardening index N 4-6 ≥0.19, porosity λ≥60%.

10. A method for manufacturing cold-rolled multiphase steel as described in any one of claims 1-9, characterized in that, Includes the following steps: Smelting and casting; Hot-rolled; Cold rolling; Annealing: Control the annealing temperature to be 770-800℃, the annealing time to be 40-200s, cool to the rapid cooling start temperature of 650-690℃ at a cooling rate of 3-5℃ / s, and then perform rapid cooling at a rapid cooling rate of 40-100℃ / s, controlling the rapid cooling end temperature to be 290-340℃. Tempering; smooth.

11. The method for manufacturing cold-rolled multiphase steel as described in claim 10, characterized in that, In the hot rolling step, the continuously cast billet is heated to a heating temperature of 1180-1240℃ (e.g., 1190-1230℃), held for 150-200 minutes, and then hot-rolled to a final rolling temperature of 860-900℃. It is then rapidly cooled at a cooling rate of 30-80℃ / s and coiled at a coiling temperature of 495-575℃ (e.g., 520-560℃). After coiling, it is air-cooled.

12. The method for manufacturing cold-rolled multiphase steel as described in claim 10, characterized in that, In the cold rolling step, the cold rolling reduction rate is controlled to be 50-70%.

13. The method for manufacturing cold-rolled multiphase steel as described in claim 10, characterized in that, In the annealing step, the annealing temperature is 780–790°C.

14. The method for manufacturing cold-rolled multiphase steel as described in claim 10, characterized in that, In the tempering step, the tempering start temperature is 290-340℃, the tempering end temperature is 205-280℃ (e.g., 220-260℃), and the tempering time is 100-400s.

15. The method for manufacturing cold-rolled multiphase steel as described in claim 10, characterized in that, During the leveling step, the leveling reduction rate is controlled to be ≤0.5%.

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