A low-temperature resistant, high-strength, and ductile cold-rolled multiphase steel and its preparation method

Abstract

This invention discloses a low-temperature resistant, high-strength, and ductile cold-rolled multiphase steel and its preparation method, belonging to the technical field of automotive multiphase steel sheets. Its composition by weight percentage is: C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: ≤0.012%, S: ≤0.012%, Cu: ≤0.20%, Alt: 0.60%-0.80%, Ni: 1.70%-2.00%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 10ppm≤Ca≤15ppm, TO: ≤15ppm, [N]: ≤60ppm, with the remainder being Fe and unavoidable impurity elements. This invention, by adjusting the component content and combining it with a flash annealing process, produces a product that possesses high strength, high ductility, and low-temperature toughness.

Description

Technical Field

[0001] This invention belongs to the technical field of automotive multiphase steel sheet, and more specifically, relates to a low-temperature resistant, high-strength, and ductile cold-rolled multiphase steel and its preparation method. Background Technology

[0002] New energy vehicles are a crucial product for the transformation and development of the global automotive industry. With their increasing market adoption in recent years, higher demands have been placed on their lightweight, safety, economy, and overall lifecycle performance. High-strength multiphase steel, the first generation of high-strength steel, boasts excellent formability and weldability and is currently the most widely used. However, its relatively low strength-ductility cannot meet the stamping requirements of complex parts and new energy vehicles. While TWIP steel has a high strength-ductility product reaching 50 GPa%, its processability, applicability, and cost limit its widespread application in new energy vehicles. Furthermore, the safety requirements for new energy vehicle batteries are becoming increasingly stringent, and automotive steel sheets are extremely prone to cracking during use in high-altitude and cold regions such as Xinjiang, Tibet, and Northeast China.

[0003] Therefore, there is a high demand for strength and low-temperature resistance in multiphase steel. While ensuring a certain ferrite content, the martensite content in the multiphase steel needs to be increased. Simultaneously, the increased strength brings difficulties in stamping and processing. In the production process, the water quenching process commonly used on modern continuous annealing lines, although accelerating the production pace, can easily cause deformation and cracking due to excessive cooling rates. Increasing the alloy content can improve strength, but it increases costs, makes rolling more difficult, and reduces the weldability and formability of the product. In summary, the technical demand for low-temperature resistant, high-strength, and high-ductility cold-rolled multiphase steel and its preparation methods is increasing daily. A search of existing grades of cold-rolled multiphase steel reveals few technical solutions for preparation processes and annealing processes that address low-temperature resistance, rollability, high strength-ductility, and high performance.

[0004] Patent CN114540717A discloses a calcium-tellurium synergistic gear steel, its preparation method, and its applications. The calcium-tellurium synergistic gear steel, by mass percentage, comprises: C 0.40%-0.43%, Si 0.23%-0.28%, Mn 0.75%-0.85%, S 0.01%-0.035%, P 0.015%-0.020%, Cr 1.05%-1.15%, Mo 0.19%-0.23%, Ca 0.001%-0.007%, Te 0.005%-0.07%, with the remainder being Fe and unavoidable impurity elements. The preparation method of the calcium-tellurium synergistic gear steel involves: smelting the raw materials in a converter, refining in an LF furnace, refining in an RH furnace, and continuous casting to obtain the calcium-tellurium synergistic gear steel. However, this patent only applies to low-deformation gear steel, and it cannot effectively control the strength-ductility product and low-temperature resistance of high-strength steel plates.

[0005] Patent CN103498100A discloses an economical low-Ni, high-Mn cryogenic steel suitable for use at -196℃ and its manufacturing method. The chemical composition of the cryogenic steel by weight percentage is: C≤0.04, Si≤0.05, Mn: 1.00-1.50, P≤0.005, S≤0.003, Alt: 0.015-0.050, Ni: 7.00-8.00, Nb: 0.02-0.05, Ti: 0.008-0.025, N≤0.004, and also contains at least one of Mo≤0.35, Cu≤0.20, and Ca≤0.005, with the balance being Fe and unavoidable inclusions. The manufacturing method includes smelting, rolling, and tempering processes. This patent is applicable to the manufacture of cryogenic storage tanks for LNG, but not to high-strength plastic sheet materials. Summary of the Invention

[0006] 1. The problem to be solved

[0007] To address the problem of insufficient low-temperature resistance of existing high-strength steel leading to poor strength-ductility product, this invention provides a low-temperature resistant, high-strength, and high-ductility cold-rolled multiphase steel, the resulting product having high strength-ductility product and good low-temperature resistance.

[0008] The present invention also discloses a method for preparing the above-mentioned cold-rolled multiphase steel, wherein the annealing is performed using a flash cooling and slow cooling annealing process.

[0009] 2. Technical Solution

[0010] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0011] The chemical composition (by weight percentage) of the low-temperature resistant high-strength ductile steel of this invention is as follows: C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: ≤0.012%, S: ≤0.012%, Cu: ≤0.20%, Alt: 0.60%-0.80%, Ni: 1.70%-2.00%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 10ppm≤Ca≤15ppm, TO: ≤15ppm, [N]: ≤60ppm, with the remainder being Fe and unavoidable impurity elements. Based on the adjustment of steel composition in pilot-scale smelting and polynomial fitting calculations, the chemical composition must meet the following ratio:

[0012] 3.3% ≤ C m =Al-4.12×[N]-2.25×(Ti+Nb)-1.21*C+1.81*Ni≤4%.

[0013] This invention utilizes C in the design of multiphase steel composition. mBy imposing certain constraints on the chemical composition, TiN liquid precipitation can be avoided while improving strength-ductility product. Al is an economical element for increasing strength; its addition acts as a deoxidizer, and AlN also has a grain boundary pinning effect, preventing grain coarsening. Studies have shown that free Al in austenite can delay the transformation from austenite to ferrite, which is believed to be related to the distribution of Al near the ferrite-austenite transformation interface. Meanwhile, Nb and Ti have a grain refining effect, but this refinement inhibits the formation of bainite and martensite, reducing strength. Therefore, the chemical composition must ensure that Cm (precipitate) constrains the chemical composition.

[0014] This invention adds nickel to improve the strength-ductility product and low-temperature resistance of the sheet metal. Simultaneously, to reduce the aspect ratio of sulfides and prevent the deterioration of type II sulfides, and to ensure smooth casting during continuous casting, calcium is added. Furthermore, the composition design of the high-strength-ductility multiphase steel incorporates high-Al multiphase steel to increase the martensite and bainite content, and utilizes appropriate microalloying to ensure high strength while improving ductility. Moreover, the chemical composition ratio conforms to the Cm constraint on composition.

[0015] Carbon (C): The most basic, effective, and economical strengthening element in steel. Its content directly affects the martensitic transformation point; during heat treatment, carbon enrichment into austenite leads to a decrease in the Ms point temperature. Carbon stabilizes austenite; too low a carbon content results in insufficient steel strength, and during the partitioning process, there are not enough C atoms to enrich into the retained austenite, leading to insufficient stability of the retained austenite. However, excessive carbon content is detrimental to formability and weldability; therefore, higher carbon content is generally not always better for steel. In this invention, the C content is controlled within a relatively low range of 0.18%-0.22%.

[0016] Si: Si is a deoxidizer that can strengthen steel through solid solution treatment, improving its strength and hardness. It is also a ferrite-forming element that inhibits cementite precipitation, thus stabilizing austenite. On the other hand, excessive silicon content can cause selective oxidation of the steel plate surface during annealing, producing iron oxide scale during heating. During hot rolling, when this scale is pressed into the plate surface, it degrades the surface quality, leading to difficulties in welding, hot-dip galvanizing, and surface coating. Therefore, the Si weight percentage in this invention is controlled at 0.10%-0.15%.

[0017] Mn (Mo) can expand the austenite phase region and stabilize the austenite structure, improving the hardenability of steel. However, Mn is soluble in ferrite, increasing the hardness and strength of ferrite and austenite in steel. Simultaneously, Mn can improve the stability of the austenite structure. Another effect of adding Mn to low-carbon steel is to shift the proeutectoid ferrite precipitation line to the right, thus reducing the amount of ferrite precipitated during annealing and ensuring the residual austenite content in the final microstructure. However, excessive Mn content can easily cause segregation, resulting in poor banded structure and reduced plasticity of the steel, leading to decreased toughness during hot rolling. The Mn content should be controlled between 2.00% and 2.30%.

[0018] Cr: It increases the strength of steel and precipitates carbides during continuous annealing, improving grain boundary stability. However, excessive Cr content not only increases costs but also causes carbide coarsening, affecting the steel's strength and ductility. Therefore, this invention controls its content at 0.40%-0.60%.

[0019] Nitrogen (Nb): A precipitation strengthening element that refines grain size, resulting in a uniform and fine microstructure, which is beneficial for improving the strength and elongation of the finished product. Simultaneously, the addition of trace amounts of Nb refines the austenite grains during annealing, lowering the Ms temperature and allowing the steel of this invention to complete the fractionation process at lower temperatures, making it more suitable for conventional continuous annealing lines. The Nb weight percentage content of this invention is controlled between 0.025% and 0.035%.

[0020] Ti and [N]: Adding trace amounts of Ti to steel can improve its strength, playing a role in precipitation strengthening and grain refinement. It can also improve the toughness of the weld heat-affected zone and enhance the weldability of the material. At the same time, Ti delays the bainitic phase transformation, promotes the precipitation of ferrite and pearlite, and improves toughness. However, excessive Ti can easily lead to liquid precipitation of TiN, and sharp corners can easily cause fatigue failure. Therefore, the weight percentage of Ti in this invention is controlled at 0.04%-0.06%, while [N] is ≤60ppm.

[0021] Al: Al is an effective deoxidizer and can form AlN to refine grain size. Aluminum is a strong ferrite stabilizing element. One of the formation pathways of carbides is the decomposition of austenite through eutectoid reaction, and the increase of Al and C content helps to promote the precipitation of κ carbides. On the other hand, adding Al can increase the Ms temperature; refining the austenite grain size can significantly reduce the Ms temperature and improve austenite stability. However, excessive Al can easily lead to secondary oxidation and the formation of inclusions, resulting in a decrease in service performance. Excessive Al can also cause nozzle blockage. Therefore, it is necessary to adjust the timing of Al addition during the steelmaking process, perform certain Ca treatment, and ensure that the Al content is controlled between 0.60% and 0.80%.

[0022] Ca: Ca can prevent large inclusions from adhering to the pipe walls at the sprue and increase pourability. However, excessive Ca can worsen the formation of Class D and Class DS inclusions, so the Ca content should be kept between 10-15 ppm.

[0023] Ni: Ni is an element that strongly forms and stabilizes the austenite region, while also strengthening ferrite. The martensitic transformation temperature decreases with increasing nickel content. Adding an appropriate amount of nickel increases the thermodynamic stability of steel, resulting in good strength, plasticity, and low-temperature toughness.

[0024] P and S: Sulfur readily combines with manganese in steel to form MnS inclusions, causing hot brittleness; P is an element with a strong tendency to segregate, increasing cold brittleness, reducing plasticity, and detrimental to the uniformity of product microstructure and properties. Control P: ≤0.015%, S: ≤0.015%.

[0025] TO and [H]: TO forms oxide inclusions in steel, and the TO content should be controlled to be ≤15ppm.

[0026] The resulting steel has a grain size of 13–14 and a retained austenite content of less than 4% in its microstructure. Grain refinement can simultaneously improve strength and plasticity, while the retained austenite content must not exceed 4%. The strength decreases rapidly when the soft-phase retained austenite content exceeds 4%. This chemical composition ratio can effectively control the microstructure composition, improve the strength-ductility product and low-temperature resistance of the material.

[0027] Production method: Smelting → Continuous casting → Hot rolling → Pickling and cold rolling → Continuous annealing → Finished product. Specifically:

[0028] 1) Continuous casting: Applicable to converter, electric furnace and induction furnace smelting; continuous casting is used to produce billets. In order to ensure the quality of the billets, an electromagnetic stirring system and a dynamic light reduction device are used during the pouring process to reduce component segregation during solidification.

[0029] 2) Hot rolling of billets or ingots: The billet is heated to 1160-1260℃ and held for 2-3 hours. It is then rolled in 5-7 passes by a roughing mill to a 30-50mm intermediate billet. The billet is then rolled in 5-7 passes by a hot rolling mill with a final rolling temperature of 870-910℃. After rolling to the target thickness, it is coiled into a steel coil at a temperature of 540-600℃ to obtain a uniform and fine hot-rolled structure.

[0030] 3) Pickling and cold rolling: Hot-rolled strip steel is pickled in a hydrochloric acid bath to remove surface iron oxide scale, and then subjected to cold continuous rolling or cold rolling. The cold rolling reduction rate is 50%-70%, and the steel is rolled to the target thickness.

[0031] 4) Continuous Annealing: The pickled and cold-rolled steel sheet is subjected to a slow heating + flash cooling process. It is slowly heated to 760-790℃ for two-phase annealing, then slowly cooled to 680-700℃, and finally cooled to 350-400℃ at a cooling rate of ≥30℃ / s, held for 4 hours, and then air-cooled. Using this annealing temperature allows for the full precipitation of Nb and Ti, and a significant increase in dissolved Al, refining the microstructure while reducing the retained austenite content. However, the stacking fault energy of Ni does not change significantly above 790℃, so considering cost, heating to 790℃ is sufficient. Simultaneously, the dissolved Nb and Ti increase dislocations, and through dislocation entanglement, stabilize the stacking fault energy of Ni, maintaining it at a high level at the annealing temperature of 760-790℃, further ensuring the strength-ductility product.

[0032] 3. Beneficial effects

[0033] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0034] This invention relates to a low-temperature resistant, high-strength ductile steel produced using a rational component ratio and preparation method, along with the preparation method itself. A specific annealing process is used to control the microstructure, meeting market demands and achieving a tensile strength R... m ≥1200MPa, yield strength R p0.2 ≥800MPa, A 80 ≥25%, strength-plasticity volume ≥30GPa, low-temperature impact KV2 (-40℃) ≥30J. Attached Figure Description

[0035] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. However, it should be understood that these drawings are designed for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, unless specifically indicated, these drawings are intended only to conceptually illustrate the structural construction described herein and are not necessarily drawn to scale.

[0036] Figure 1 The metallographic morphology of Example 3;

[0037] Figure 2 This is an electron microscope image of Example 3. Detailed Implementation

[0038] The following detailed description of exemplary embodiments of the invention is taken with reference to the accompanying drawings, which form part of the description and illustrate exemplary embodiments in which the invention may be practiced. While these exemplary embodiments have been described in sufficient detail to enable those skilled in the art to practice the invention, it should be understood that other embodiments may be implemented and various changes may be made to the invention without departing from the spirit and scope thereof. The more detailed description of embodiments of the invention below is not intended to limit the scope of the claimed invention, but is merely illustrative and not restrictive of the description of the features and characteristics of the invention, to suggest the best mode for carrying out the invention, and is sufficient to enable those skilled in the art to practice the invention. Therefore, the scope of the invention is defined only by the appended claims.

[0039] The chemical composition (by weight percentage) of the low-temperature resistant high-strength ductile steel of this invention is as follows: C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: ≤0.012%, S: ≤0.012%, Cu: ≤0.20%, Alt: 0.60%-0.80%, Ni: 1.70%-2.00%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 10ppm≤Ca≤15ppm, TO: ≤15ppm, [N]: ≤60ppm, with the remainder being Fe and unavoidable impurity elements, while simultaneously satisfying 3.3%≤C m =Al-4.12×[N]-2.25×(Ti+Nb)-1.21*C+1.81*Ni≤4%.

[0040] Table 1 Chemical composition of the embodiments and comparative examples of the present invention (units: [Ca], [N], [TO] are in ppm, others are in wt%)

[0041]

[0042] Table 2 shows the rolling and annealing processes for the embodiments and comparative examples of the present invention. The invention patent uses a flash cooling process, while the comparative examples use a conventional annealing process.

[0043] Table 2. Steel rolling production process parameters of the embodiments and comparative examples of the present invention.

[0044]

[0045] Table 3 shows the mechanical properties and strength-ductility product of the embodiments and comparative examples of the present invention. As can be seen from Table 3, the mechanical properties and strength-ductility product of the present invention are significantly improved. The present invention utilizes the stacking fault energy of Ni to improve low-temperature resistance and optimize strength and ductility, thereby increasing the strength-ductility product. Simultaneously, the composite microalloying of Nb and Ti refines the grain size and increases dislocations. Through dislocation entanglement, the increased stacking fault energy of Ni is further stabilized, improving both the strength-ductility product and low-temperature resistance.

[0046] Table 3 Comparison of mechanical properties and strength-ductility product of embodiments and comparative examples of the present invention

[0047] Example Tensile strength / MPa Yield strength / MPa <![CDATA[A 80 / %]]> High Plasticity / GPa*% Example 1 1220 810 25.5 31.11 Example 2 1225 812 25.6 31.36 Example 3 1221 815 25.7 31.38 Example 4 1232 820 26.5 32.65 Example 5 1250 818 25.9 32.38 Comparative Example 1 1070 652 16.5 17.66 Comparative Example 2 980 666 19.2 18.82

[0048] Table 4 shows the microstructure and low-temperature impact performance of the embodiments and comparative examples of the present invention. Due to the use of microalloying to refine the grain structure and the solid solution properties of high Al, the microstructure is controlled, and the addition of Ni significantly improves the low-temperature resistance. The grain refinement results in a lower content of retained austenite; the reduction of soft-phase retained austenite ensures that the steel of the present invention has a higher stacking fault energy. Simultaneously, the addition of Ni improves the low-temperature resistance of the steel.

[0049] Table 4. Microstructure and low-temperature impact performance of the embodiments and comparative examples of the present invention.

[0050]

[0051]

[0052] This invention utilizes a high-Al chemical composition and a flash annealing process to introduce martensitic structure, thereby improving strength. Simultaneously, microalloying refines the microstructure and residual austenite, reducing the residual austenite content and increasing its plasticity. The addition of Ni enhances its low-temperature resistance. Through the implementation of this invention, the strength-ductility product of the steel is significantly improved, meeting the demands of lightweight construction. The increased Ni content also contributes to improved low-temperature toughness.

Claims

1. A low-temperature-resistant high-strength plastic cold-rolled multiphase steel, characterized in that, The chemical composition ratio is C: 0.18%-0.22%, Si: 0.10%-0.15%, Mn: 2.00%-2.30%, Cr: 0.4%-0.6%, P: ≤0.012%, S: ≤0.012%, Cu: ≤0.20%, Alt: 0.60%-0.80%, Ni: 1.70%-2.00%, Nb: 0.020%-0.035%, Ti: 0.04%-0.06%, 10ppm≤Ca≤15ppm, T.O: ≤15ppm, [N]: ≤60ppm, the rest is Fe and inevitable impurity elements, while satisfying 3.3%≤C m =Al-4.12×[N]-2.25×(Ti+Nb)-1.21*C+1.81*Ni≤4%; The product has the following properties: the steel plate has a product of strength and ductility ≥ 30 GPa, a low-temperature impact KV2 at -40°C ≥ 30 J, a tensile strength R m ≥ 1200 MPa, a yield strength R p0.2 ≥ 800 MPa, A 80 ≥ 25%.

2. The low-temperature-resistant high-strength plastic cold-rolled multiphase steel according to claim 1, characterized in that, The grain size is 13-14, and the residual austenite content in the metallographic structure is less than 4%.

3. A method for producing the low-temperature-resistant high-strength plastic cold-rolled multiphase steel according to any one of claims 1-2, characterized in that, The method comprises the steps of smelting, continuous casting, hot rolling, pickling and cold rolling, continuous annealing and finished product.

4. The production method according to claim 3, characterized by, In the smelting and continuous casting step, the converter, electric furnace and induction furnace are used for smelting, and the continuous casting is used for producing the casting blank, and the electromagnetic stirring and dynamic soft reduction device are used in the pouring process.

5. The production method according to claim 3, characterized by, In the continuous rolling step, the casting blank is heated at a temperature of 1160-1260 DEG C for 2-3 hours, and is rolled by 5-7 passes of rough rolling mill, and is hot rolled to 30-50 mm of intermediate blank, and is rolled by 5-7 passes of hot continuous rolling mill, and is finally rolled at a temperature of 870-910 DEG C, and is coiled at a temperature of 540-600 DEG C after being rolled to the target thickness, so as to obtain the uniform and fine hot rolling structure.

6. The preparation method according to claim 3, characterized in that, In the pickling and cold rolling step, the hot rolled strip is pickled by the hydrochloric acid tank after the surface iron oxide scale is removed, and is cold rolled at a cold rolling reduction of 50-70%, and is rolled to the target thickness.

7. The preparation method according to claim 3, characterized in that, In the continuous annealing step, the pickled and cold rolled steel plate is annealed by slow heating + flash cooling process, is slowly heated to 760-790 DEG C for two-phase zone annealing, is slowly cooled to 680-700 DEG C, and then is cooled to 350-400 DEG C at a cooling rate of greater than or equal to 30 DEG C / s, is kept for 4 hours, and is air cooled.

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