A 1200mpa grade multi-phase steel plate and a method for manufacturing the same

Abstract

The application discloses a 1200MPa-grade complex-phase steel plate and a preparation method thereof. The chemical components of the complex-phase steel plate are as follows in percentage by mass: C 0.15-0.17%, Si 0.54-0.56%, Mn 1.55-1.75%, Cr 0.27-0.31%, Mo 0.58-0.62%, Al 0.05-0.10%, Ti 0.08-0.12%, V 0.10-0.15%, B 0.003-0.005%, N 0.010-0.015%, P≤0.015%, S≤0.010%, and the balance of Fe and inevitable impurities. The preparation method comprises continuous casting, hot rolling and annealing processes. The complex-phase steel plate provided by the application has a yield strength of ≥860MPa, a tensile strength of ≥1250MPa, a yield strength ratio of ≤0.70 and a hole expansion ratio of ≥20%, and has excellent performance.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and in particular to a 1200MPa grade multiphase steel plate and its preparation method. Background Technology

[0002] With the continuous development of my country's automotive industry, people are paying more and more attention to the safety, comfort, and energy efficiency of automobiles. This has made automobile material selection and design key issues of concern in the automotive industry. As the most widely used material in automobiles, automotive steel directly affects the success or failure of the entire automotive industry. Currently, based on the major environmental requirements of "energy conservation, emission reduction, environmental protection, and safety" and the current level of metallurgical technology, my country has issued national standards for five major categories of advanced automotive steels with strengths ranging from 500 to 1600 MPa, including dual-phase steel, multiphase steel, transformation-induced plasticity steel, martensitic steel, and high-hole-expansion steel. These standards serve as normative documents for automotive steel.

[0003] Multiphase steel, as one of them, has a microstructure mainly composed of ferrite and bainite, containing a certain amount of martensite, retained austenite, and fine precipitates. It can be used to manufacture automotive structural parts, reinforcements, and safety components, such as bumpers and B-pillar reinforcements.

[0004] The existing standards only have two grades of hot-rolled multiphase steel, HR660 / 760CP and HR720 / 950CP. Their yield strength and tensile strength are both low, their yield-to-tensile ratio is high, and their hole expansion ability is poor, which cannot meet the requirements of high-strength automotive structural parts.

[0005] Publication No. CN113481435A discloses a 900MPa grade hot-rolled multiphase steel and its production method. Its chemical composition is: C 0.070–0.158%, Si 0.08–0.85%, Mn 1.15–2.50%, Al 0.10–0.30%, Cr 0.10–0.75%, Mo 0.08–0.65%, B 0.0020–0.0050%, and 0.025–0.070% Nb and / or 0.030–0.155% V, P≤0.020%, S≤0.007%, with the balance being Fe. This invention produces multiphase steel through processes such as heating, rolling, coiling, and slow cooling. The steel plate has a yield strength ≥600MPa and a tensile strength ≥900MPa, exhibiting excellent performance. However, this invention overemphasizes formability, resulting in a low yield strength ratio, which cannot meet the requirements for high-strength automotive steel.

[0006] Publication number CN111394661A discloses a preparation process for low-alloy high-strength and high-toughness Marpe-type multiphase steel. The chemical composition of the steel plate in this invention is: C 0.15-0.25%, Mn 1.5-2.5%, Si 1.5-2.5%, Cr 0.5-1.5%, Mo≤0.5%, with the remainder being Fe. The Marpe-type multiphase steel is produced through high-temperature homogenization, forging, quenching, two-phase region heating, step-by-step cooling, and low-temperature tempering. The method described in this invention is very novel, and the prepared Marpe-type multiphase steel has outstanding performance. However, the disclosed process is long and complex, and methods such as "forging" are difficult to scale up.

[0007] In addition, publication numbers CN112210727A, CN112853205A, and CN109778062A all disclose multiphase steels with their respective composition systems and their preparation technologies, but either their strength is too low or their preparation processes are too complex, and neither can meet the current requirements for the use and production processes of high-grade automotive multiphase steels.

[0008] Therefore, in the face of today's rapidly developing automotive industry, developing hot-rolled multiphase steel with higher strength and better formability is one of the important issues that urgently need to be addressed in the development of automotive steel. Summary of the Invention

[0009] The purpose of this invention is to provide a 1200MPa grade multiphase steel plate and its preparation method through reasonable composition and process design, which not only has excellent performance but also low production cost.

[0010] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0011] A 1200MPa grade multiphase steel plate has the following chemical composition by mass percentage: C 0.15-0.17%, Si 0.54-0.56%, Mn 1.55-1.75%, Cr 0.27-0.31%, Mo 0.58-0.62%, Al 0.05-0.10%, Ti 0.08-0.12%, V 0.10-0.15%, B 0.003-0.005%, N 0.010-0.015%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities.

[0012] The 1200MPa grade multiphase steel of this invention has a yield strength ≥860MPa, tensile strength ≥1250MPa, and elongation after fracture A. 80 ≥16%, yield strength ratio ≤0.70, and porosity ≥20%.

[0013] The microstructure of the 1200MPa grade multiphase steel plate of the present invention is ferrite + (14-18%) bainite + (2-5%) martensite.

[0014] The preparation method of the 1200MPa grade multiphase steel plate of the present invention includes continuous casting, hot rolling and annealing processes.

[0015] Furthermore, in the continuous casting process described in this invention, the tundish temperature is 1505~1520℃, the nozzle is protected with nitrogen, the casting speed is 1.0~1.2m / min, and the casting is produced into a billet with a cross-section of 200~240mm×1780~1800mm.

[0016] Furthermore, in the hot rolling process described in this invention, after the billet is heated and kept at a constant temperature, it is rolled at 900-1130°C, and after rough rolling → intermediate rolling → finish rolling → layer cooling, it is finally coiled into a steel coil with a thickness of 3-4 mm.

[0017] Furthermore, in the hot rolling process described in this invention, the billet is heated to 1200–1250°C and held for 4–6 hours.

[0018] Furthermore, in the hot rolling process of the present invention, the rough rolling temperature is 1050-1130℃, and the rolling is performed in 4 passes; the intermediate rolling temperature is 980-1030℃, and the rolling is performed in 4 passes; the finishing rolling temperature is 900-950℃, and the rolling is performed in 4 passes; after rolling, the temperature is cooled to 630-670℃, and the coil is rolled, with a cooling rate ≥100℃ / s.

[0019] Furthermore, in the annealing process described in this invention, the steel coil is heated to 460–520°C and held for 120–180 seconds; then cooled in the furnace to 340–400°C and held for 3–4 hours; and finally air-cooled to room temperature after being removed from the furnace.

[0020] Furthermore, in the annealing process described in this invention, the furnace cooling rate of the steel coil is ≤15℃ / h.

[0021] The inventive principle and beneficial technical effects of the present invention are as follows:

[0022] In the composition design, 0.08-0.12 wt% Ti and 0.010-0.015 wt% N were added. During continuous casting, the TiN that precipitates first can act as heterogeneous nucleation particles, increasing the nucleation rate. When the billet is heated, the undissolved TiN agglomerates at the grain boundaries, and the particles hinder the growth of austenite, thus refining the austenite grains.

[0023] Adding 0.10–0.15% V will cause carbonitrides such as TiC, VN, and VC to precipitate successively during the rolling process, which will hinder recrystallization grain growth, refine the austenite structure, and improve the strength of the steel.

[0024] The addition of 0.58–0.62% Mo and 0.003–0.005% B mainly serves to form bainitic structure, effectively improving the strength and toughness of steel plates.

[0025] The addition of 0.54–0.56% Si, 1.55–1.75% Mn, and 0.27–0.31% Cr mainly serves to strengthen the structure through solid solution and improve toughness.

[0026] The main function of adding 0.05-0.10% Al is to enhance the desulfurization process in the metallurgical process. In addition, the AlN formed can refine the grains and improve toughness.

[0027] Compared with the disclosed technology, the present invention does not add Nb, mainly to improve the weldability of the steel plate, because steel treated with Nb has poor heat-affected zone (HAZ) toughness. Attached Figure Description

[0028] Figure 1 The microstructure of the multiphase steel is shown in Example 1.

[0029] Figure 2 This is the metallographic structure of multiphase steel in Example 2.

[0030] Figure 3 This is the metallographic structure of multiphase steel in Example 3.

[0031] Figure 4 This is the metallographic structure of multiphase steel in Example 4.

[0032] Figure 5 This is the metallographic structure of multiphase steel in Example 5.

[0033] Figure 6 The microstructure of the multiphase steel is shown in Example 6. Detailed Implementation

[0034] The present invention will now be described in further detail with reference to specific embodiments.

[0035] Examples 1-6

[0036] The chemical composition and specifications of the multiphase steels designed in Examples 1-6 are shown in Table 1.

[0037]

[0038] The multiphase steel plates in each embodiment were prepared according to the following process, and the specific process parameters are shown in Tables 2 and 3.

[0039] (1) Converter smelting

[0040] ① Charging: Scrap steel is charged into the top and bottom blown converter, and molten iron is added;

[0041] ② Smelting: Oxygen lance blowing, adding lime, lightly calcined dolomite, iron ore and other materials to form slag, and maintaining argon bottom blowing throughout the process, controlling the final C ≥ 0.06%, and the final temperature 1620~1650℃;

[0042] ③ Steel tapping: Steel is tapped using a slag-blocking spout. During tapping, low-carbon ferromanganese, low-carbon ferrochrome, ferrosilicon, ferromolybdenum and other alloy materials are added to the ladle in sequence according to the design composition.

[0043] (2) LF refining

[0044] ① Slag making: When the temperature of the ladle entering the station is ≥1550℃, lower the electrode and add lime, ferrosilicon powder, aluminum powder, dolomite, etc. to make white slag;

[0045] ② Alloying: For aluminum wire, add ferrotitanium, ferrovanadium, ferroboron, and ferrochrome nitride, test the bulk composition, and fine-tune it to meet the requirements;

[0046] ③ Out of the station: Introduce nitrogen into the bottom of the ladle, gently blow for 6-12 minutes, and adjust the temperature to 1540-1560℃ before leaving the station.

[0047] (3) Continuous casting

[0048] ① Tundish metallurgy: The tundish has a built-in double baffle wall + filter + dam, and the temperature of the molten steel inside the ladle is maintained at 1505~1520℃;

[0049] ② Casting: Nitrogen protection at the immersion nozzle, casting on a 2-strand slab casting machine with a casting speed of 1.0~1.2m / min, casting into slabs with a cross-section of 200~240mm×1780~1800mm.

[0050] (4) Hot rolling

[0051] ① Heating of steel billets: Place the continuously cast steel billets in a heating furnace at 1200-1250℃ and hold for 4-6 hours;

[0052] ② Rolling: The process is rough rolling → intermediate rolling → finish rolling → laminar cooling → coiling. The rough rolling temperature is controlled at 1050~1130℃, with 4 passes; the intermediate rolling temperature is 980~1030℃, with 4 passes; the finish rolling temperature is 900~950℃, with 4 passes; the laminar cooling rate is ≥100℃ / s, and the coiling is carried out at 630~670℃.

[0053] (5) Annealing

[0054] The steel coil is heated to 460-520℃ and held for 120-180 seconds; then cooled in the furnace at a rate of ≤15℃ / h to 340-400℃ and held for 3-4 hours; finally, it is air-cooled to room temperature after being removed from the furnace.

[0055]

[0056]

[0057] The test results of the mechanical properties and hole expansion properties of the multiphase steel in each embodiment are shown in Table 4.

[0058]

[0059] As shown in Table 4, the 1200MPa grade multiphase steel provided by this invention has a yield strength ≥860MPa, a tensile strength ≥1250MPa, and an elongation after fracture A 80 With a strength of ≥16%, yield strength ratio ≤0.70, and expansion rate ≥20%, this material boasts excellent performance indicators and can be considered a preferred material for high-strength automotive steel.

[0060] The metallographic structures of the multiphase steels in Examples 1-6 are shown in the following figures. Figures 1-6 In the diagram, the matrix is ​​ferrite, the white blocky objects are bainite, and the black blocky objects are martensite. Figures 1-6 It can be seen that its metallographic structure is ferrite + (14-18%) bainite + (2-5%) martensite.

Claims

1. A 1200MPa grade multiphase steel plate, characterized in that: The chemical composition of the multiphase steel, by mass percentage, is: C 0.15-0.17%, Si 0.54-0.56%, Mn 1.55-1.75%, Cr 0.27-0.31%, Mo 0.58-0.62%, Al 0.068-0.10%, Ti 0.10-0.12%, V 0.11-0.15%, B 0.004-0.005%, N 0.010-0.013%, P≤0.015%, S≤0.010%, with the balance being Fe and unavoidable impurities. The microstructure of the multiphase steel is ferrite + (14-18%) bainite + (2-5%) martensite.

2. The 1200MPa grade multiphase steel plate according to claim 1, characterized in that: The multiphase steel has a yield strength ≥860MPa, a tensile strength ≥1250MPa, and an elongation after fracture A. 80 ≥16%, yield strength ratio ≤0.70, and porosity ≥20%.

3. A method for preparing a 1200MPa grade multiphase steel plate according to any one of claims 1 to 2, characterized in that, The process includes continuous casting, hot rolling and annealing. The hot rolling process involves heating and holding the billet at 900-1130°C, followed by roughing, intermediate rolling, finishing, and layer cooling, ultimately resulting in a steel coil with a thickness of 3-4 mm.

4. The method for preparing 1200MPa grade multiphase steel plate according to claim 3, characterized in that, The continuous casting process is as follows: tundish temperature 1505~1520℃, nitrogen protection at the nozzle, casting speed 1.0~1.2m / min.

5. The method for preparing 1200MPa grade multiphase steel plate according to claim 4, characterized in that, The billet is heated to 1200–1250℃ and held for 4–6 hours.

6. The method for preparing 1200MPa grade multiphase steel plate according to claim 4, characterized in that, The roughing temperature is 1050-1130℃, and the rolling is performed in 4 passes; the intermediate rolling temperature is 980-1030℃, and the rolling is performed in 4 passes; the finishing rolling temperature is 900-950℃, and the rolling is performed in 4 passes; after rolling, the temperature is cooled to 630-670℃, and the rolls are curled with a cooling rate ≥100℃ / s.

7. The method for preparing 1200MPa grade multiphase steel plate according to claim 3, characterized in that, The annealing process involves heating the steel coil to 460–520°C and holding it at that temperature for 120–180 seconds; then cooling it in the furnace to 340–400°C and holding it at that temperature for 3–4 hours; finally, air-cooling it to room temperature after removing it from the furnace.

8. The method for preparing 1200MPa grade multiphase steel plate according to claim 7, characterized in that, The annealing process described above: the furnace cooling rate of the steel coil is ≤15℃ / h.

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