A cold-rolled dual-phase steel containing Nb and Ti with high strength and its preparation method

Abstract

This invention discloses an ultra-high strength cold-rolled duplex steel containing Nb and Ti and its preparation method. The matrix structure of the duplex steel is ferrite + martensite, and its composition by mass percentage is: C: 0.19%-0.21%, Si: 0.25%-0.27%, Mn: 3.1%-3.3%, Nb: 0.04%-0.05%, Ti: 0.02%-0.03%, Mo: 0.18%-0.2%, Al: 0.05%-0.07%, with the remainder being Fe and unavoidable impurities. The preparation method of the duplex steel includes: composition design → smelting → forging → hot rolling → pickling → cold rolling → critical zone annealing. The duplex steel of this invention not only has ultra-high strength but also maintains good plasticity, with a tensile strength of 1219-1644 MPa, a yield strength of 546-1049 MPa, and an elongation after fracture of 7.00%-9.87%, making it suitable for the manufacture of automotive safety structural components.

Description

Technical Field

[0001] This invention relates to the field of ultra-high strength automotive steel sheet technology, specifically to an ultra-high strength cold-rolled dual-phase steel containing Nb and Ti and its preparation method. Background Technology

[0002] In recent years, due to the rapid development of the automotive industry, China's automobile production and sales have ranked first in the world for many consecutive years. Although the growth rate of automobile production and sales has slowed down due to the dual impact of the slowdown in domestic economic growth and transformation and upgrading, the automotive industry still has room for development, especially the new energy vehicles that the country is vigorously developing, whose production and sales will continue to grow in the future. With the development of the automotive and steel industries, the requirements and standards for the research and development of automotive steel are constantly changing. Under fierce competition with other new materials, advanced automotive steel materials are developing towards higher strength, higher plasticity, lower cost, and easier processing and forming.

[0003] However, with the increase in strength of advanced high-strength steel, especially for high-strength steel with a strength level of 800MPa or higher, there is a significant decrease in ductility, toughness, and formability, making it more prone to forming defects such as cracking, wrinkling, and springback during the forming process. To address this, in recent years, researchers worldwide have made significant technological progress in areas such as process fundamentals, metallurgical processes, materials science, mechanism research, and deep processing applications of high-performance advanced high-strength steel. The main research hotspots focus on the following aspects: alloy composition, microstructure, heat treatment processes, coating and deep processing technologies, digital simulation calculations, and big data analysis.

[0004] From both a performance and overall cost perspective, high-strength steel is the preferred material for achieving lightweight vehicle bodies and improving vehicle safety performance in the foreseeable future 30 to 50 years. Future high-quality automotive steel should possess both ultra-high strength and good plasticity.

[0005] Chinese patent CN114606449A, published on June 10, 2022, entitled "High Strength-to-Ductility Product, Low Yield-to-Tear Ratio DP980 Cold-Rolled Duplex Steel and Its Production Method," describes a steel plate composition of C: 0.11%-0.14%, Si: 0.10%-0.15%, Mn: 2.22%-2.24%, P≤0.014%, S≤0.003%, Mo: 0.13%-0.16%, Cr: 0.27%-0.31%, Al: 0.17%-0.21%, Ti: 0.03%-0.05%, with the remainder being Fe and unavoidable impurities. While this method incorporates multiple alloying elements, the tensile strength level is relatively low, and the ductility is average, indicating significant potential for improvement in mechanical properties.

[0006] Furthermore, Chinese patent CN113584393A, published on November 2, 2021, entitled "A Duplex Steel with a Tensile Strength of 780 MPa and its Production Method," describes a steel plate composition of C: 0.06%-0.10%, Si: less than 1.0%, Mn+Cr: 2.0%-2.8%, Nb+Ti: 0.03%-0.08%, Al: 0.02%-0.08%, P: less than 0.03%, S: less than 0.008%, N: less than 0.006%, with the remainder being Fe and unavoidable impurities. The dual-phase steel prepared by this method achieves a yield strength ratio of 0.65-0.80, which can significantly reduce springback during the stamping process of parts; however, the tensile strength is only 780 MPa, indicating significant room for improvement. Summary of the Invention

[0007] To address the problems mentioned above, this invention aims to provide an ultra-high strength cold-rolled dual-phase steel and its preparation method, which reduces the thickness of the steel sheet while meeting the strength requirements for automotive steel sheets, thus providing an effective measure for automotive lightweighting.

[0008] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0009] An ultra-high strength cold-rolled dual-phase steel has a microstructure of fine ferrite + martensite, wherein the volume fraction of martensite is 38%-80%, the volume fraction of ferrite is 20%-62%, the ferrite grain size is 1.28-1.75μm, and it also contains a small amount of finely dispersed nanoscale second-phase particles, the size of which is generally less than 20nm.

[0010] An ultra-high strength cold-rolled dual-phase steel has the following chemical composition and mass fraction as a percentage: C: 0.19%-0.21%, Si: 0.25%-0.27%, Mn: 3.1%-3.3%, Nb: 0.04%-0.05%, Ti: 0.02%-0.03%, Mo: 0.18%-0.2%, Al: 0.05%-0.07%, with the remainder being Fe and unavoidable impurities.

[0011] In the composition design of the ultra-high strength cold-rolled dual-phase steel described in this invention, the roles of each element are as follows:

[0012] C: Carbon is a solid solution strengthening element and is essential for achieving high strength in materials. The carbon content in duplex steel is typically 0.03%-0.23%. When the carbon content is too low, the austenite content is low when heating in the same critical region (ferrite and austenite), which reduces the volume fraction of martensite obtained after quenching, hindering the achievement of high strength. However, too high a carbon content reduces the weldability of the material. Therefore, the carbon composition should be designed to be as low as possible while still meeting strength requirements.

[0013] Si: Silicon is a solid solution strengthening element. On the one hand, it can improve the strength of materials. On the other hand, it can accelerate the segregation of carbon into austenite and purify ferrite, thereby improving the performance of finished products.

[0014] Mn: Manganese is an element that strongly improves the hardenability of austenite. Austenite containing an appropriate amount of Mn can achieve the desired microstructure by varying the rapid cooling termination temperature, thereby obtaining products with different properties. Simultaneously, Mn can also dissolve in ferrite to form solid solution strengthening.

[0015] Nb and Ti: Niobium and titanium are carbonitride precipitation elements, which can refine grains and precipitate carbonitrides, thereby improving material strength. The grain refinement and precipitation strengthening effects of both are significant. More importantly, low Nb and Ti contents will weaken the strengthening effect, while high contents will not only increase the size of the precipitated phase but also increase the cost. Therefore, this invention requires that the Nb and Ti contents be controlled at 0.04%-0.05% and 0.02%-0.03%, respectively.

[0016] Mo: Molybdenum is a medium-strong carbide-forming element that raises the temperature of A3 and A1, shifts the GS line to the upper left, delays the proeutectoid ferrite transformation, and promotes the formation of acicular ferrite and bainite. In the production of thick steel plates, it can significantly improve the problem of uneven microstructure and properties caused by uneven cooling rate and deformation along the thickness direction.

[0017] Al: Trace amounts of Al can purify molten steel and refine grains during smelting and forging processes.

[0018] The above-mentioned method for preparing Nb and Ti ultra-high strength cold-rolled dual-phase steel specifically includes the following preparation steps:

[0019] (1) Casting: According to the above chemical composition ratio of duplex steel, iron ore, quicklime, ferromolybdenum, ferrotitanium, ferroniobium, alumina, iron-silicon alloy, and iron-manganese alloy are placed in an oxidizing atmosphere furnace to smelt and cast billets.

[0020] (2) Forging: The above-mentioned billet is heated to 1220-1280℃ and held for 2-3 hours to allow all C, Si, Mn, Nb, Ti, Mo and Al elements to be dissolved. Then forging is started at a temperature of 1150-1200℃ to obtain a forged billet.

[0021] (3) Hot rolling: The above forging billet is heated to 1210-1270℃ and held for 2-3 hours. The initial rolling temperature is 1100-1200℃ and the final rolling temperature is 850-950℃. After rolling, the steel plate is taken out and air-cooled to room temperature to obtain a hot-rolled plate with a thickness of 3.7-4mm.

[0022] (4) Pickling and cold rolling: The hot-rolled plate is pickled and cold-rolled, and the oxide scale after hot rolling and pickling is removed. The cold rolling reduction rate is 40%-60%, the number of cold rolling passes is 5-8, and the thickness of the cold-rolled plate after cold rolling is 1.8-1.9mm.

[0023] (5) Critical Zone Annealing: The above-mentioned cold-rolled sheet was subjected to critical zone annealing treatment. The phase transformation points Ac1 = 705℃ and Ac3 = 838℃ of the steel sheet were measured by a Gleeble 3800 thermal simulator. The critical zone annealing temperature was set to 735-775℃ and the annealing time was 8-12min. The heating process was carried out in a tube furnace. After the furnace temperature was raised to 735-775℃, the cold-rolled sheet was placed in the furnace. The heating rate was 5-20℃ / s. After holding at this temperature for 8-12min, the sheet was oil quenched to obtain the dual-phase steel.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] (1) The ultra-high strength cold-rolled dual-phase steel of the present invention adopts a reasonable composition design. Controlling the appropriate carbon content (0.19%-0.21%wt) ensures sufficient strength and provides good plasticity and weldability; controlling the appropriate Si (0.25%-0.27%wt) and Mn (3.1-3.3%wt) content ensures the formation of dual-phase structure and gives it good hardenability, while also playing a solid solution strengthening role; adding a small amount of microalloying elements Nb (0.04%-0.05%wt), Ti (0.02%-0.03%wt), Mo (0.18%-0.2%wt), and Al (0.05%-0.07%wt) can simultaneously refine grains and precipitation strengthening on the basis of the original C-Si-Mn dual-phase steel.

[0026] (2) In the preparation process of this invention, a suitable cold rolling reduction rate (50%), critical zone annealing temperature (735-775℃), and annealing time of 8-12 min are used. The purpose of cold rolling at a reduction rate of 40%-60% is to refine the original bainite grains to the greatest extent possible, while the annealing time of approximately 10 minutes is to avoid grain coarsening while obtaining a dual-phase structure. The dual-phase steel prepared using this method ensures both sufficiently fine grains and a high martensite volume fraction (38%-80%) in the dual-phase structure, thereby maximizing the mechanical properties of the cold-rolled dual-phase steel.

[0027] (3) The ultra-high strength cold-rolled dual-phase steel obtained by the present invention has a tensile strength of 1219-1644 MPa, a yield strength of 546-1049 MPa, and an elongation after fracture of 7.00%-9.87%.

[0028] (4) By adding Mo, Nb, Ti and Al elements to conventional C-Si-Mn dual-phase steel, the present invention can effectively refine the grains; and by controlling the appropriate critical zone annealing temperature and annealing time during the preparation process, it can ensure a sufficiently high martensite volume fraction and thus provide sufficiently high strength, while maintaining good plasticity, so that the final product can be used as a suitable steel component for automobiles. Attached Figure Description

[0029] Figure 1 Engineering stress-strain curves of the ultra-high strength cold-rolled dual-phase steel prepared in Examples 1-3 of this invention;

[0030] Figure 2 Thermal expansion curves of the ultra-high strength cold-rolled dual-phase steel prepared in Examples 1-3 of this invention;

[0031] Figure 3 Scanning electron microscope (SEM) image of the ultra-high strength cold-rolled dual-phase steel prepared in Example 1 of this invention;

[0032] Figure 4 Scanning electron microscope (SEM) image of the ultra-high strength cold-rolled duplex steel prepared in Example 2 of this invention;

[0033] Figure 5 Scanning electron microscope (SEM) image of the ultra-high strength cold-rolled dual-phase steel prepared in Example 3 of this invention. Detailed Implementation

[0034] To facilitate understanding of the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] A method for preparing Nb-Ti ultra-high strength cold-rolled dual-phase steel includes the following steps:

[0037] (1) According to the chemical composition ratio of duplex steel in Table 1 below, iron ore, quicklime, ferromolybdenum, ferrotitanium, ferroniobium, alumina, iron-silicon alloy, and iron-manganese alloy are placed in an oxidizing atmosphere furnace for smelting and casting billets.

[0038] (2) Heat the above-mentioned billet to 1220-1280℃ and hold for 2-3 hours to allow all C, Si, Mn, Nb, Ti, Mo and Al elements to be dissolved. Then start forging at 1150-1200℃ to obtain a forged billet.

[0039] (3) Heat the above forging billet to 1210-1270℃ and hold for 2-3 hours. The initial rolling temperature is 1100-1200℃ and the final rolling temperature is 850-950℃. After rolling, take out the steel plate and air cool it to room temperature to obtain a hot-rolled plate with a thickness of 3.7-4mm.

[0040] (4) The hot-rolled plate is pickled and cold-rolled, and the oxide scale after hot rolling and pickling is removed. The cold rolling reduction rate is 40%, the cold rolling passes are 5-8, and the thickness of the cold-rolled plate after cold rolling is 1.8-1.9 mm.

[0041] (5) The above-mentioned cold-rolled plate is subjected to critical zone annealing. The heating process is carried out in a tube furnace. After the furnace temperature is raised to 775°C, the cold-rolled plate is placed in the furnace. The heating rate is 5-20°C / s. After holding for 8 minutes, the plate is oil-quenched to obtain the duplex steel.

[0042] Example 2

[0043] A method for preparing Nb-Ti ultra-high strength cold-rolled dual-phase steel includes the following steps:

[0044] (1) According to the chemical composition ratio of duplex steel in Table 1 below, iron ore, quicklime, ferromolybdenum, ferrotitanium, ferroniobium, alumina, iron-silicon alloy, and iron-manganese alloy are placed in an oxidizing atmosphere furnace for smelting and casting billets.

[0045] (2) Heat the above-mentioned billet to 1220-1280℃ and hold for 2-3 hours to allow all C, Si, Mn, Nb, Ti, Mo and Al elements to be dissolved. Then start forging at 1150-1200℃ to obtain a forged billet.

[0046] (3) Heat the above forging billet to 1210-1270℃ and hold for 2-3 hours. The initial rolling temperature is 1100-1200℃ and the final rolling temperature is 850-950℃. After rolling, take out the steel plate and air cool it to room temperature to obtain a hot-rolled plate with a thickness of 3.7-4mm.

[0047] (4) The hot-rolled plate is pickled and cold-rolled, and the oxide scale after hot rolling and pickling is removed. The cold rolling reduction rate is 50%, the cold rolling passes are 5-8, and the thickness of the cold-rolled plate after cold rolling is 1.8-1.9 mm.

[0048] (5) The above-mentioned cold-rolled plate is subjected to critical zone annealing. The heating process is carried out in a tube furnace. After the furnace temperature is raised to 755°C, the cold-rolled plate is placed in the furnace. The heating rate is 5-20°C / s. After holding for 10 minutes, the plate is oil-quenched to obtain the duplex steel.

[0049] Example 3

[0050] A method for preparing Nb-Ti ultra-high strength cold-rolled dual-phase steel includes the following steps:

[0051] (1) According to the chemical composition ratio of duplex steel in Table 1 below, iron ore, quicklime, ferromolybdenum, ferrotitanium, ferroniobium, alumina, iron-silicon alloy, and iron-manganese alloy are placed in an oxidizing atmosphere furnace for smelting and casting billets.

[0052] (2) Heat the above-mentioned billet to 1220-1280℃ and hold for 2-3 hours to allow all C, Si, Mn, Nb, Ti, Mo and Al elements to be dissolved. Then start forging at 1150-1200℃ to obtain a forged billet.

[0053] (3) Heat the above forging billet to 1210-1270℃ and hold for 2-3 hours. The initial rolling temperature is 1100-1200℃ and the final rolling temperature is 850-950℃. After rolling, take out the steel plate and air cool it to room temperature to obtain a hot-rolled plate with a thickness of 3.7-4mm.

[0054] (4) The hot-rolled plate is pickled and cold-rolled, and the oxide scale after hot rolling and pickling is removed. The cold rolling reduction rate is 60%, the cold rolling passes are 5-8, and the thickness of the cold-rolled plate after cold rolling is 1.8-1.9 mm.

[0055] (5) The above-mentioned cold-rolled plate is subjected to critical zone annealing. The heating process is carried out in a tube furnace. After the furnace temperature is raised to 735°C, the cold-rolled plate is placed in the furnace. The heating rate is 5-20°C / s. After holding for 12 minutes, the plate is oil-quenched to obtain the duplex steel.

[0056] Table 1 is a list of the chemical components of the above embodiments of the present invention;

[0057] Table 2 shows the cold rolling and critical zone annealing process parameters of the above embodiments of the present invention.

[0058] Table 3 shows the microstructure and mechanical properties of the duplex steels prepared according to the above embodiments of the present invention.

[0059] Table 1 Chemical composition of Examples 1-3

[0060]

[0061]

[0062] Table 2. Cold rolling and critical zone annealing process parameters for Examples 1-3

[0063]

[0064] The mechanical properties of the duplex steels prepared in Examples 1 to 3 were tested, and the test results are shown in Table 3.

[0065] Table 3. Microstructure and mechanical properties of the duplex steels prepared in Examples 1-3

[0066]

[0067] As shown in Tables 1-3, the cold-rolled duplex steel sheets prepared using the composition design, cold rolling, and heat treatment processes of this invention all exhibit good mechanical properties. The tensile strength reaches 1644 MPa, the yield strength reaches 1049 MPa, the yield-to-tensile ratio is 44.8%-63.8%, the elongation after fracture is 7.00%-9.87%, and the strength-ductility product is 10.7-12.1 GPa·%. Furthermore, the duplex steels prepared in Examples 1-3 possess suitable microstructures, with a martensite volume fraction of 38%-80% and a ferrite grain size of 1.30-1.75 μm. The ultra-high strength cold-rolled duplex steels prepared in each example demonstrate excellent performance and are suitable for manufacturing automotive safety structural components, showing good potential for widespread application.

[0068] Figure 1 The figures show the engineering stress-strain curves of the ultra-high strength cold-rolled dual-phase steels prepared in Examples 1-3 of this invention. It can be seen that as the martensite volume fraction increases, the yield strength, tensile strength, and yield ratio of the examples continuously increase. The elongation after fracture first decreases and then increases, and the strength-ductility product also first decreases and then increases. When the martensite volume fraction is 80%, the yield strength of Example 1 reaches a maximum of 1049 MPa, and the tensile strength reaches a maximum of 1644 MPa.

[0069] Figure 2 The thermal expansion curves of the ultra-high strength cold-rolled duplex steel prepared in Examples 1-3 of this invention show that the phase transformation points for the three sets of examples with this alloy composition are Ac1 = 705℃, Ac3 = 838℃, and Ms = 372℃. The duplex steel undergoes an austenitic phase transformation when heated above 705℃, ends at 838℃, transforming into a fully austenitic structure, and begins a martensitic phase transformation upon rapid cooling to 372℃.

[0070] Figure 3 The SEM image of the ultra-high strength cold-rolled dual-phase steel prepared in Example 1 of this invention shows that when the martensite volume fraction is 80%, the average ferrite grain size is 1.30 μm. At this point, the matrix structure of the dual-phase steel is continuous martensite.

[0071] Figure 4 The SEM image of the ultra-high strength cold-rolled dual-phase steel prepared in Example 2 of this invention shows that when the martensite volume fraction is 66%, the average ferrite grain size is 1.28 μm. At this point, the matrix structure of the dual-phase steel is still continuous martensite.

[0072] Figure 5The SEM image of the ultra-high strength cold-rolled dual-phase steel prepared in Example 3 of this invention shows that when the martensite volume fraction is 38%, the average ferrite grain size is 1.75 μm. At this time, the matrix structure of the dual-phase steel is continuous ferrite, and many nanoscale (less than 20 nm) second-phase particles can be seen on the ferrite matrix.

[0073] It should be noted that the embodiments listed above are merely specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and similar changes or modifications made thereto are those that can be directly derived or easily conceived by those skilled in the art from the content disclosed in the present invention, and should all fall within the protection scope of the present invention. Furthermore, the combination of the various technical features in this case is not limited to the combination methods described in the claims or the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

Claims

1. A cold-rolled dual-phase steel containing Nb and Ti with ultra-high strength, characterized in that, The formulation includes: C, Si, Mn, Nb, Ti, Mo, Al, and Fe, with the following mass percentages: C: 0.19%-0.21%, Si: 0.25%-0.27%, Mn: 3.1%-3.3%, Nb: 0.04%-0.05%, Ti: 0.02%-0.03%, Mo: 0.18%-0.2%, Al: 0.05%-0.07%, with the remainder being Fe and unavoidable impurities. The microstructure of the dual-phase steel is fine ferrite + martensite, with a martensite volume fraction of 38%-80% and a ferrite volume fraction of 20%-62%. The ferrite grain size is 1.28-1.75 μm, and it also contains finely dispersed nanoscale second-phase particles with a particle size of less than 20 nm.

2. The Nb- and Ti-containing ultra-high strength cold-rolled duplex steel according to claim 1, characterized in that, The formulation includes: C, Si, Mn, Nb, Ti, Mo, Al and Fe, with the following mass percentages: C: 0.198%, Si: 0.268%, Mn: 3.259%, Nb: 0.045%, Ti: 0.027%, Mo: 0.193%, Al: 0.06%, and the remainder being Fe and unavoidable impurities.

3. The method for preparing an ultra-high strength cold-rolled dual-phase steel containing Nb and Ti as described in claim 1, characterized in that, Specifically, the following steps are included: (1) Casting: Smelting a billet according to the mass percentage of the chemical composition as described in claim 1; (2) Forging: The above-mentioned billet is heated to dissolve all elements C, Si, Mn, Nb, Ti, Mo and Al, and then forging is started to obtain a forged billet; (3) Hot rolling: The above forging billet is rolled under heating. After rolling, the steel plate is taken out and air-cooled to room temperature to obtain hot-rolled plate. (4) Pickling and cold rolling: The hot-rolled sheet above is pickled and cold-rolled, and the oxide scale on the surface of the material after hot rolling and pickling is polished to obtain the cold-rolled sheet. (5) Critical zone annealing: The above-mentioned cold-rolled sheet is subjected to critical zone annealing treatment. The heating process is carried out in a tube furnace. After the furnace temperature is raised to the set critical zone annealing temperature, the cold-rolled sheet is placed in the furnace for annealing treatment, heat preservation, and oil quenching to obtain the duplex steel.

4. The method for preparing Nb and Ti ultra-high strength cold-rolled dual-phase steel according to claim 3, characterized in that, The temperature for heating the billet in step (2) is 1220-1280℃, and the temperature is held for 2-3 hours; the temperature for forging the billet in step (2) is 1150-1200℃.

5. The method for preparing Nb and Ti ultra-high strength cold-rolled dual-phase steel according to claim 3, characterized in that, In step (3), the forging billet is rolled under heating conditions, specifically: the forging billet is heated to 1210-1270℃ and held for 2-3 hours, the initial rolling temperature is 1100-1200℃, and the final rolling temperature is 850-950℃.

6. The method for preparing Nb and Ti ultra-high strength cold-rolled dual-phase steel according to claim 3, characterized in that, The cold rolling reduction rate in step (4) is 40%-60%, the number of cold rolling passes is 5-8, and the thickness of the cold-rolled plate after cold rolling is 1.8-1.9mm.

7. The method for preparing Nb and Ti ultra-high strength cold-rolled dual-phase steel according to claim 3, characterized in that, Step (5) Set the critical zone annealing temperature to 735-775℃ and the annealing time to 8-12min.

8. The method for preparing Nb and Ti ultra-high strength cold-rolled dual-phase steel according to claim 3, characterized in that, In step (5), the heating rate of the tubular furnace is 5-20℃ / s.

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