1000mpa grade hot-rolled high-strength steel and method for manufacturing the same

Abstract

The present application belongs to the technical field of hot continuous rolling steel production, and particularly relates to a 1000MPa grade hot-rolled high-strength steel and a preparation method thereof. The present application aims to provide a 1000MPa grade hot-rolled high-strength steel and a preparation method thereof. The chemical composition of the steel includes, by weight percentage, C: 0.07-0.15%, Si: 0.05-0.20%, Mn: 1.5-2.5%, V+Ti+Zr: 0.20-0.35%, Als: 0.010-0.060%, P≤0.015%, S≤0.005%, N≤0.0050%, and the balance of Fe and inevitable impurities. The microstructure of the hot-rolled high-strength steel is ferrite, the yield strength is ≥900MPa, the tensile strength is ≥1000MPa, the elongation is ≥12%, the 180° cold bending d=2a is qualified, the hole expansion ratio is ≥25%, and the 0℃ impact energy is ≥45J.

Description

Technical Field

[0001] This invention belongs to the field of hot-rolled steel production technology, specifically relating to a 1000MPa grade hot-rolled high-strength steel and its preparation method. Background Technology

[0002] V and Ti, as strong carbide-forming elements in steel, can significantly strengthen steel through the formation of second-phase precipitation. However, when microalloyed alone, TiC suffers from a small solid solubility product and limited precipitation, while VC exhibits large precipitate size and limited precipitation strengthening effect. Furthermore, the liquid precipitation of TiN in Ti-containing steel is a significant challenge to control. Therefore, a V-Ti-Zr composite microalloy was designed, combining the advantages of small precipitate size in V and Ti microalloyed TiC and large VC precipitation, and achieving control over the size, quantity, and morphology of liquid-precipitated TiN. This resulted in the development of a 1000MPa-grade precipitation-strength hot-rolled high-strength steel, which has broad application prospects for high-strength and lightweight automotive steel.

[0003] For example, Chinese patent CN111519108A ​​discloses a zirconium-reinforced low-activation martensitic steel and its preparation method. The steel comprises the following components: C: 0.01%–0.02%, Si: 0.01%–0.05%, Mn: 0.3%–0.5%, Cr: 8.0%–9.0%, W: 1.0%–1.5%, V: 0.2%–0.3%, Zr: 0.01%–0.03%, N: 0.01%–0.02%, O: 0.01%–0.02%, with the remainder being Fe. This patent adds Cr and W metals, resulting in higher costs, and requires strict control of the Zr addition order, making the process more complex.

[0004] For example, Chinese patent CN103194676A discloses a 1000MPa super ferritic steel and its preparation method. Its composition, by weight percentage, is: C: 0.085–0.09%, Mn: 1.7–1.74%, Si: 0.21–0.23%, P: 0.053–0.0080%, S: 0.0050–0.0078%, N: 0.0041–0.0050%, Ti: 0.14–0.19%, Mo: 0.37–0.55%, with the remainder being residual Fe and unavoidable impurities. This patent also adds the expensive precious metal Mo, and the grain structure is coarse, which easily leads to low impact energy. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention first provides a 1000MPa grade hot-rolled high-strength steel, the chemical composition of which, by weight percentage, includes: C: 0.07–0.15%, Si: 0.05–0.20%, Mn: 1.5–2.5%, V+Ti+Zr: 0.20–0.35%, Als: 0.010–0.060%, P≤0.015%, S≤0.005%, N≤0.0050%, with the balance being Fe and unavoidable impurities.

[0006] Preferably, the above-mentioned hot-rolled high-strength steel has the following chemical composition by weight percentage: C: 0.09-0.12%, Si: 0.05-0.15%, Mn: 1.8-2.0%, V: 0.08-0.12%, Ti: 0.10-0.15%, Zr: 0.03-0.05%, Als: 0.015-0.050%, P≤0.010%, S≤0.004%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

[0007] Among them, the above-mentioned hot-rolled high-strength steel has a yield strength ≥900MPa, tensile strength ≥1000MPa, elongation ≥12%, 180° cold bending d=2a qualified, hole expansion rate ≥25%, and 0℃ impact energy ≥45J.

[0008] Among them, the above-mentioned hot-rolled high-strength steel has a ferrite microstructure and an average grain size of ≤5μm.

[0009] The present invention also provides a method for preparing the above-mentioned hot-rolled high-strength steel, comprising the following steps:

[0010] After desulfurization of molten iron, smelting in a converter and refining in an LF furnace, and vacuum refining in an RH furnace, the composition of molten steel is controlled according to the above chemical composition. The steel is then continuously cast to obtain a billet. The billet is then put into a furnace for reheating, followed by hot continuous rolling, laminar flow cooling, coiling, and slow cooling.

[0011] Low-carbon ferromanganese is added during converter smelting; ferrovanadium is added during LF refining; and sponge zirconium and sponge titanium are added during RH refining.

[0012] Among them, electromagnetic stirring is used during continuous casting, and the continuous casting light reduction is ≥5mm.

[0013] The process involves continuous casting to obtain a billet, which is then placed into a slab heating furnace at a temperature ≥400℃.

[0014] The billet heating temperature is 1240–1300℃, preferably 1260–1280℃.

[0015] The steel billet undergoes two-stage rolling. The rolling temperature in the austenite recrystallization zone is 1080–1200℃, preferably 1100–1180℃, with a rolling compression ratio of 4–7, preferably 4.5–6.5. The rolling temperature in the austenite non-recrystallization zone is 870–1050℃, preferably 900–1040℃, with a rolling compression ratio ≥5, preferably 6–16.

[0016] The steel is subjected to laminar flow cooling after rolling, with a laminar flow cooling rate of ≥15℃ / s, preferably 20~40℃ / s, and a coiling temperature of 560~640℃, preferably 570~630℃.

[0017] The steel is cooled in a laminar flow and then coiled, and then sent to a slow cooling pit for heat preservation for ≥60h, preferably 72-80h.

[0018] Beneficial Effects: This invention utilizes the advantages of small TiC precipitation size and large VC precipitation after V and Ti composite formation, achieving a stronger precipitation strengthening effect than single V and Ti microalloying, thereby improving the strength of the steel. Simultaneously, the addition of Zr reduces the quantity and size of liquid-precipitated TiN, dulling the angular morphology of TiN and improving the material's formability and porosity. Compared to traditional martensitic-strengthened 1000MPa hot-rolled steel, it has the advantage of requiring no heat treatment, and the added V, Ti, Zr, and other microalloying elements are inexpensive.

[0019] The microstructure of the hot-rolled high-strength steel of this invention is ferrite, with an average grain size ≤5μm. The second-phase precipitates are mainly interphase precipitates with a size ≤20nm and supersaturated ferrite precipitates. The liquid-precipitated TiN has a size ≤5μm and is rated as D-class inclusions ≤2.0. This steel has a yield strength ≥900MPa, tensile strength ≥1000MPa, elongation ≥12%, passes 180° cold bending d=2a, has a hole expansion rate ≥25%, and an impact energy at 0℃ ≥45J. Moreover, the V-Ti-Zr composite microalloying technology and controlled rolling and cooling technology provided by this invention can be extended to other high-strength, high-formability hot-rolled steels. Attached Figure Description

[0020] Figure 1 This is a microstructure diagram of hot-rolled high-strength steel in Example 1 of the present invention;

[0021] Figure 2 This is a diagram of inclusions in hot-rolled high-strength steel according to Embodiment 1 of the present invention;

[0022] Figure 3 This is a microstructure diagram of hot-rolled high-strength steel in Example 2 of the present invention;

[0023] Figure 4 This is a microstructure diagram of hot-rolled high-strength steel in Example 3 of the present invention;

[0024] Figure 5 This is a microstructure diagram of hot-rolled high-strength steel, Comparative Example 1 of the present invention;

[0025] Figure 6 This is a diagram of inclusions in hot-rolled high-strength steel, Comparative Example 1 of the present invention.

[0026] Figure 7 This is a microstructure diagram of hot-rolled high-strength steel, Comparative Example 2 of the present invention;

[0027] Figure 8 This is a microstructure diagram of hot-rolled high-strength steel, Comparative Example 3 of the present invention. Detailed Implementation

[0028] The present invention first provides a 1000MPa grade hot-rolled high-strength steel, the chemical composition of which, by weight percentage, includes: C: 0.07-0.15%, Si: 0.05-0.20%, Mn: 1.5-2.5%, V+Ti+Zr: 0.20-0.35%, Als: 0.010-0.060%, P≤0.015%, S≤0.005%, N≤0.0050%, with the balance being Fe and unavoidable impurities.

[0029] Preferably, the above-mentioned hot-rolled high-strength steel has the following chemical composition by weight percentage: C: 0.09-0.12%, Si: 0.05-0.15%, Mn: 1.8-2.0%, V: 0.08-0.12%, Ti: 0.10-0.15%, Zr: 0.03-0.05%, Als: 0.015-0.050%, P≤0.010%, S≤0.004%, N≤0.0040%, with the balance being Fe and unavoidable impurities.

[0030] The roles of each chemical element in the hot-rolled high-strength steel prepared by this invention are as follows:

[0031] C combines with strong carbide-forming elements such as V, Ti, and Zr to form nanoscale second-phase precipitation, which is beneficial to improving the strength of steel. However, when the C content is too high, alloy cementite containing V, Ti, and Zr will be formed. These carbides are not only large in size, but also tend to accumulate at grain boundaries, which will reduce the toughness and plasticity of steel. Therefore, the C content is controlled at 0.07 to 0.15%, preferably 0.09 to 0.12%.

[0032] Mn is an austenite-forming element that is completely dissolved in austenite. Increasing the Mn content can improve the toughness of steel. At the same time, a higher Mn content is beneficial to improving the hardenability of austenite and promoting grain refinement. However, if the Mn content is too high, it is easy to cause compositional segregation and affect the uniformity of the microstructure. Therefore, the Mn content is controlled at 1.5-2.5%, preferably 1.8-2.0%.

[0033] V, Ti, and Zr are all strong carbide-forming elements. During the coiling process, they precipitate nanoscale carbides through ferrite supersaturation, thus playing a precipitation strengthening role. Simultaneously, adding an appropriate amount of V to Ti microalloyed steel, i.e., increasing the V / Ti mass ratio, can lower the fastest precipitation temperature of the MC phase in ferrite, which is beneficial for obtaining a larger volume fraction of precipitated phase in ferrite. Adding an appropriate amount of Zr to Ti microalloyed steel can passivate the angular morphology of liquid-precipitated TiN. V-Ti-Zr composite microalloying is beneficial for improving the strength and toughness of the steel. Therefore, in this invention, the V+Ti+Zr content is controlled at 0.20–0.35%, preferably V: 0.08–0.12%, Ti: 0.10–0.15%, and Zr: 0.03–0.05%.

[0034] Impurity elements such as P, S, and N can deteriorate the toughness and plasticity of steel and increase the number of inclusions. At the same time, S and N can also form inclusions such as Ti4S2C2 and TiN with Ti. Therefore, in this invention, the contents of P, S, and N are controlled to P≤0.015%, S≤0.005%, and N≤0.0050%, respectively. More preferably, P≤0.010%, S≤0.004%, and N≤0.0040%.

[0035] This invention also provides a method for preparing the above-mentioned hot-rolled high-strength steel. The requirements for controlled rolling and controlled cooling of the steel described in this invention and the reasons for the limitations of the production process are explained below.

[0036] The steel described in this invention contains a high content of Mn, as well as microalloying elements such as V, Ti, and Zr. This steel exhibits high uniformity and refinement of its microstructure. Therefore, this invention requires control of the homogeneity of the billet during the continuous casting process to reduce the segregation level. Electromagnetic stirring is employed during continuous casting, and the light reduction during continuous casting is ≥5mm.

[0037] Compared to cold charging, hot charging of slabs results in a higher core temperature when the slab exits the furnace, making it easier for deformation to penetrate into the core of the slab during subsequent rolling, promoting austenite recrystallization and refining the microstructure. Therefore, the slabs of this invention are fed into the heating furnace using a hot charging method.

[0038] Meanwhile, a higher heating temperature not only facilitates the full solid solution of microalloying elements such as Mn, V, Ti, and Zr, enabling them to exert precipitation strengthening effects in the subsequent process, but also promotes core deformation of the slab and grain refinement. Therefore, the reheating temperature of the slab is controlled at 1240–1300℃, preferably 1260–1280℃.

[0039] Rolling in the austenite recrystallization zone, as the name suggests, primarily involves dynamic recrystallization of austenite. This repetitive "coarsening-recrystallization" cycle of austenite achieves grain refinement. Since the austenite recrystallization termination temperature of the steel described in this invention is around 1000℃, the rolling temperature in this stage is required to be controlled at 1080–1200℃, preferably 1100–1180℃. Simultaneously, employing a larger rolling compression ratio, i.e., a larger rolling deformation, also promotes austenite recrystallization. This invention requires the rolling compression ratio in this stage to be controlled at 4–7, preferably 4.5–6.5.

[0040] In the rolling stage of the non-recrystallization region of austenite, the rolling temperature should be as low as possible below the austenite recrystallization termination temperature. This stage primarily involves austenite flattening, accumulating sufficient dislocations and nucleation sites for subsequent phase transformations. Simultaneously, the rolling temperature should be higher than the ferrite precipitation initiation temperature to reduce deformation-induced precipitation and increase interphase precipitation. Therefore, the rolling temperature in this stage should be controlled between 870 and 1050°C, preferably 900 to 1040°C. Furthermore, increasing the rolling compression ratio in this stage promotes austenite flattening and subsequent microstructure refinement; therefore, the rolling compression ratio in this stage should be ≥5, preferably 6 to 16.

[0041] In the laminar cooling stage, to suppress the formation of high-temperature precipitates during the phase transformation process, namely alloy cementite containing V, Ti, and Zr, and to promote interphase precipitation and the formation of fine and uniform ferrite structure during the phase transformation process, this invention employs a relatively high laminar cooling rate, ≥15℃ / s, preferably 20~40℃ / s. The nose point temperature for the supersaturated precipitation of VC and TiC in ferrite is around 600℃. To promote the supersaturated precipitation of ferrite and improve the strength of the steel, this invention controls the coiling temperature around 600℃, specifically 560~640℃, preferably 570~630℃.

[0042] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0043] Example

[0044] Table 1 shows the chemical composition of hot-rolled high-strength steel in the embodiments and comparative examples of the present invention; Table 2 shows the production process parameters in the embodiments and comparative examples of the present invention; and Table 3 shows the mechanical property indicators of the steel in the embodiments and comparative examples of the present invention.

[0045] The 1000MPa grade hot-rolled high-strength steel described in Examples 1 to 3 of this invention is produced by hot continuous rolling, laminar flow cooling, coiling, and slow cooling. During the hot continuous rolling process, parameters such as heating temperature, recrystallization zone rolling temperature and compression ratio, non-recrystallization zone rolling temperature and compression ratio, and coiling temperature are strictly controlled. The resulting finished steel plate has a yield strength greater than 900MPa, a tensile strength greater than 1000MPa, an elongation greater than 12%, and is qualified for 180° cold bending d=2a.

[0046] Figures 1-2 The images shown are the microstructure and inclusion morphology diagrams of the test steel corresponding to Example 1. Figures 3-4 The images show the microstructure of the steels tested in Examples 2 and 3. It can be seen that the grain size of the steels in Examples 1-3 is <5 μm (see...). Figure 1 , Figure 3 , Figure 4 The size of liquid-precipitated TiN is <5μm, and the edges of liquid-precipitated TiN are passivated to a certain extent compared with square TiN (see...). Figure 2 ).

[0047] Comparative Example 1 did not contain added Zr and had a relatively high N content, therefore its microstructure (see...) Figure 5 ), polished structure (see Figure 6 Large square-shaped liquid-precipitated TiN is visible in all samples. The presence of liquid-precipitated TiN reduces the effective Ti element content in the steel that plays a role in precipitation strengthening, resulting in a decrease in the strength of the steel.

[0048] Comparative Example 2 did not use electromagnetic stirring in its smelting process, and the continuous casting light reduction was relatively low. In addition, the C and Mn content was relatively high, resulting in severe component segregation and central segregation in the steel of Comparative Example 2. During laminar cooling, this led to the formation of severe pearlite / martensite banded structures. Furthermore, due to compositional inhomogeneity near the segregation zones, differences in austenite hardenability resulted in coarse grains near the segregation zones (see...). Figure 7 ), impact energy is reduced.

[0049] Comparative Example 3 did not contain added V in its chemical composition, and had higher C and Ti contents. Since adding an appropriate amount of V to Ti microalloyed steel can lower the fastest precipitation temperature of the MC phase in ferrite, it is beneficial to obtain a larger volume fraction of precipitated phase in ferrite. In addition, Comparative Example 3 has a lower final rolling temperature, meaning that more Ti undergoes deformation-induced precipitation during the finishing rolling process, rather than supersaturated precipitation in the subsequent ferrite. Therefore, compared with Examples 1 to 3, Comparative Example 3 has lower strength.

[0050] Table 1 Chemical composition of hot-rolled high-strength steels in the examples and comparative examples

[0051] C Si Mn P S V Ti Zr Als N Example 1 0.10 0.10 1.90 0.010 0.003 0.10 0.13 0.041 0.041 0.0035 Example 2 0.12 0.05 1.80 0.009 0.002 0.12 0.12 0.035 0.039 0.0028 Example 3 0.09 0.15 2.00 0.011 0.004 0.09 0.14 0.049 0.021 0.0039 Comparative Example 1 0.12 0.10 1.90 0.012 0.005 0.12 0.15 / 0.033 0.0051 Comparative Example 2 0.16 0.10 2.10 0.007 0.004 0.12 0.15 0.039 0.044 0.0033 Comparative Example 3 0.16 0.08 2.00 0.009 0.003 / 0.18 0.031 0.040 0.0025

[0052] Table 2. Main production process parameters of hot-rolled high-strength steel in the examples and comparative examples.

[0053]

[0054]

[0055] Table 3 Mechanical property indicators of hot-rolled high-strength steel in the examples and comparative examples

[0056]

Claims

1. A 1000MPa grade hot-rolled high-strength steel, characterized in that: Its chemical composition, by weight percentage, includes: C: 0.09–0.12%, Si: 0.05–0.15%, Mn: 1.8–2.0%, V: 0.08–0.12%, Ti: 0.10–0.15%, Zr: 0.03–0.05%, Als: 0.015–0.050%, P≤0.010%, S≤0.004%, N≤0.0040%, with the balance being Fe and unavoidable impurities; the hot-rolled high-strength steel is prepared by the following method: After desulfurization of molten iron, smelting in a converter and refining in an LF furnace, and vacuum refining in an RH furnace, the composition of molten steel is controlled according to chemical composition, and the billet is continuously cast. The billet is then put into a furnace for reheating at a temperature of 1260-1280℃, and is then subjected to hot continuous rolling, laminar flow cooling, coiling, and slow cooling. The steel billet undergoes two-stage rolling. The rolling temperature in the austenite recrystallization zone is 1100–1180℃, and the rolling compression ratio is 4.5–6.

5. The rolling temperature in the austenite non-recrystallization zone is 900–1040℃, and the rolling compression ratio is 6–16. After rolling, the steel is subjected to laminar flow cooling at a rate of 20–40℃ / s and a coiling temperature of 570–630℃. The microstructure of the hot-rolled high-strength steel is ferrite; electromagnetic stirring is used during continuous casting, and the continuous casting light reduction is ≥5mm.

2. The method for preparing hot-rolled high-strength steel according to claim 1, characterized in that: Includes the following steps: After desulfurization of molten iron, smelting in a converter, refining in an LF furnace, and refining in an RH vacuum furnace, the composition of molten steel is controlled according to the chemical composition described in claim 1, and a billet is obtained by continuous casting. The billet is then put into a furnace for reheating at a temperature of 1260-1280°C, and is then subjected to hot continuous rolling, laminar flow cooling, coiling, and slow cooling. The steel billet undergoes two-stage rolling. The rolling temperature in the austenite recrystallization zone is 1100–1180℃, and the rolling compression ratio is 4.5–6.

5. The rolling temperature in the austenite non-recrystallization zone is 900–1040℃, and the rolling compression ratio is 6–16. After rolling, the steel is cooled by laminar flow at a rate of 20–40℃ / s and a coiling temperature of 570–630℃. Electromagnetic stirring is used during continuous casting, and the continuous casting reduction is ≥5mm.

3. The method for preparing hot-rolled high-strength steel according to claim 2, characterized in that: Low-carbon ferromanganese is added during converter smelting; ferrovanadium is added during LF refining; and sponge zirconium and sponge titanium are added during RH refining.

4. The method for preparing hot-rolled high-strength steel according to claim 2, characterized in that: After continuous casting, a slab is obtained, and the slab is placed into a slab heating furnace at a temperature ≥400℃.

5. The method for preparing hot-rolled high-strength steel according to claim 2, characterized in that: After laminar flow cooling, the product is wound up and then placed in a slow cooling pit for heat preservation for ≥60 hours.

6. The method for preparing hot-rolled high-strength steel according to claim 5, characterized in that: After laminar flow cooling, the product is wound up and then placed in a slow cooling pit for 72–80 hours to maintain its temperature.

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