A hot-dip galvanized TRIP steel for automobiles with a strength-ductility product of ≥25GPa·% and a preparation method thereof

Abstract

The present invention relates to the field of iron and steel metallurgy, and more particularly to a hot-dip galvanized TRIP steel for automobiles having a strength-ductility product of ≥25 GPa·% and a preparation method thereof. The hot-dip galvanized TRIP steel comprises the following chemical components by weight: C 0.17% to 0.22%, Si 1.2% to 1.4%, Mn 1.6% to 1.9%, P ≤ 0.012%, S ≤ 0.010%, Als 0.02% to 0.05%, Nb 0.015% to 0.030%, Ti 0.015% to 0.025%, N ≤ 0.005%, with the remainder being Fe and unavoidable impurities. The present invention utilizes a low-cost microalloy design and obtains a high-ductility steel strip through rational rolling and heat treatment. Prior to hot-dip galvanizing, the present invention utilizes pre-oxidation in continuous annealing to enhance surface wettability, thereby improving the adhesion of the hot-dip galvanized layer on the surface and meeting the high-strength and high-surface requirements of industries such as automobiles.

Description

Technical Field

[0001] The present invention relates to the technical field of iron and steel metallurgy, and in particular to a hot-dip galvanized TRIP steel for automobiles with a strength-ductility product of ≥25 GPa·% and a preparation method thereof. Background Art

[0002] With the increasing use of automobiles, the atmospheric pollution caused by vehicle exhaust emissions cannot be underestimated. Heavier vehicles emit more exhaust, severely impacting quality of life. Therefore, there is a need to develop a transformation-induced plasticity steel (TRIP steel) for automotive applications. While ensuring vehicle quality, this steel can be lightweighted by thinning the steel sheet. Furthermore, the corrosion resistance of the hot-dip galvanized zinc layer extends the vehicle's service life.

[0003] The TRIP steel currently developed on the market has different composition designs and process routes due to the different technical equipment capabilities of various steel mills, and the finished product structure and performance are also different.

[0004] There are mainly the following production methods in the prior art:

[0005] CN 103146998A discloses a production process for low-alloy, high-strength steel cold-rolled sheet with a high strength-to-ductility ratio for automotive applications. Its advantage lies in modifying the chemical composition and optimizing the hot-rolling and cold-rolling processes to produce the sheet with a high strength-to-ductility ratio of 15 GPa%, while also ensuring stable quality control. However, its disadvantage lies in the low-alloy, high-strength steel composition design and process route, resulting in a tensile strength of only 500 MPa, which cannot meet higher strength requirements.

[0006] CN106167875A discloses an economical, high-strength, cold-rolled TRIP steel with a strength-ductility product greater than 20 GPa·% and its preparation method. The main chemical components, by weight, are: C: 0.15-0.25%, Si: 1.3-1.7%, Mn: 1.5-2.5%, P: ≤0.030%, S: ≤0.020%, Al: 0.02-0.06%, with the remainder being Fe and unavoidable impurities. The preparation method includes smelting, hot rolling, pickling and cold rolling, and continuous annealing. The patented product has relatively high strength but low elongation, making it difficult to process and shape. Furthermore, it is a cold-rolled and continuously annealed product.

[0007] CN104694816A discloses a method for preparing high-Al medium-manganese steel with a strength-ductility product greater than 30 GPa·%. Its advantages include high strength, high elongation, and low density. The steel plate's strength-ductility product can reach 30-50 GPa·%, meeting the design requirements for third-generation automotive steel and satisfying material selection requirements for lightweight vehicles. However, its disadvantages are the high Mn and Al content, resulting in high production costs, and difficulties in continuous casting and rolling production, making large-scale industrial production difficult.

[0008] CN110607481A discloses a method for producing high-yield strength TRIP690+Z steel. Through rational composition design and effective control of key process parameters in each step, a balanced multiphase structure is achieved, resulting in a TRIP690+Z steel product with a yield strength of 490-530 MPa and excellent surface quality. However, the high Al content in the steelmaking and continuous casting process results in poor quality stability, and the addition of alloying elements such as Mo and B increases production costs.

[0009] CN101353761B discloses a high-strength TRIP steel sheet for cold-rolled hot-dip galvanizing and its preparation method. Its advantages lie in its low-Si composition design, good plateability, and a strength-ductility product of 15 to 22 GPa. However, its disadvantages are that its high Al content makes continuous casting difficult, and the addition of alloying elements such as Nb, Ti, Cu, and Ni increases production costs. Summary of the Invention

[0010] To address the problems of poor mechanical properties and high cost, the present invention provides a hot-dip galvanized TRIP steel for automobiles with a strength-ductility product of ≥25 GPa·% and a preparation method thereof. The resulting steel strip has a strength-ductility product of ≥25 GPa·%. The steel plate has high strength, excellent plasticity, and collision absorption capacity, and has excellent ductility, which is conducive to the processing and forming of parts and improves the safety of traffic accidents. At the same time, the chemical composition of the present invention adopts a low-cost alloy design, saving costs.

[0011] In a first aspect, the present invention provides a hot-dip galvanized TRIP steel for automobiles having a strength-ductility product of ≥25 GPa·%, comprising the following chemical composition in weight percentage: C 0.17%-0.22%, Si 1.2%-1.4%, Mn 1.6%-1.9%, P≤0.012%, S≤0.010%, Als 0.02%-0.05%, Nb 0.015%-0.030%, Ti 0.015%-0.025%, N≤0.005%, and the remainder being Fe and unavoidable impurities.

[0012] Furthermore, the hot-dip galvanized TRIP steel strip has the following mechanical properties: yield strength ≥ 400 MPa, tensile strength ≥ 650 MPa, elongation A50 ≥ 38%, n value ≥ 0.20, and strength-ductility product ≥ 25 GPa·%.

[0013] Furthermore, the hot-dip galvanized TRIP steel strip has a thickness of 1.0 to 2.5 mm and a width of 1000 to 1800 mm.

[0014] In the present invention, the design ideas of each element are as follows:

[0015] Carbon (C): C directly affects the strength, weldability and formability of steel. At the same time, C is also an austenite stabilizing element, which strongly reduces the Ms point. Only austenite with a certain amount of C can exist stably at room temperature and produce the TRIP effect to meet the high strength and plasticity requirements of the material. However, too high a C content will seriously deteriorate the weldability and formability of the steel.

[0016] Silicon (Si): Si is a solid-solution strengthening element that is insoluble in cementite. It strongly inhibits cementite precipitation and suppresses cementite formation during the bainite transformation, thereby increasing the strength of ferrite. Si also distributes within ferrite, raising the chemical potential of carbon and promoting its diffusion into the austenite. The formation of carbon-rich retained austenite is essential for achieving the TRIP effect. While excessive Si content can deteriorate the surface quality of the steel strip, proper control of the Si / Mn ratio during pre-oxidation prior to hot-dip galvanizing can ensure surface quality after hot-dip galvanizing.

[0017] Manganese (Mn): Mn has a good solid solution strengthening effect. It can also expand the austenite phase, reduce the Ms and Mf points, improve austenite stability and the hardenability of steel, reduce the critical transformation rate, and help preserve retained austenite at room temperature. However, too high a Mn content will deteriorate corrosion resistance and weldability, increase grain coarsening, promote the formation of harmful banded structure, and reduce the plasticity and toughness of the steel.

[0018] Aluminum (Al): Al has a similar effect to Si and can inhibit the formation of carbides during the aging process, thereby stabilizing the retained austenite. Its effect is slightly weaker than that of Si. However, if Al is too high, it is easy to cause nodules at the nozzle of the continuous casting crystallizer, causing blockage, affecting the stability of steelmaking quality. In addition, the hot rolling start and finish rolling and annealing temperatures need to be increased, and the steel surface is also easily oxidized. At the same time, it also causes higher energy consumption and increases production costs.

[0019] Niobium (Nb) and titanium (Ti): Nb and Ti are microalloying elements and are strong carbonitride formers. They have a strong affinity with carbon and nitrogen, forming fine and dispersed Nb / Ti (C, N) carbonitrides that can prevent the growth of high-temperature austenite. At the same time, the composite addition of Nb and Ti has a good precipitation strengthening effect and can avoid fine cracks on the surface of continuous casting billets. However, Nb and Ti alloys are expensive, which can easily lead to increased costs.

[0020] Phosphorus (P), sulfur (S), and nitrogen (N): are impurity elements in steel. In principle, the lower the better, but production costs must be taken into consideration. In the present invention, P is controlled to be ≤ 0.012%, S ≤ 0.010%, and N ≤ 0.005%.

[0021] In a second aspect, the present invention provides a method for preparing hot-dip galvanized TRIP steel for automobiles with a strength-ductility product ≥ 25 GPa·%, comprising the steps of smelting, continuous casting, hot rolling, acid continuous rolling, hot-dip galvanizing, post-treatment, and coiling.

[0022] Furthermore, in the hot rolling step, the heating temperature of the heating furnace is 1220-1250°C, the hot rolling final temperature is 850±15°C, the cooling mode adopts front-stage laminar cooling, the coiling temperature is 650±15°C, and the U-type coiling mode with hot head and hot tail is not adopted.

[0023] Furthermore, in the acid continuous rolling step, the hot-rolled steel strip is pickled in a hydrochloric acid tank to remove surface iron oxide scale, and then subjected to cold continuous rolling with a cold rolling reduction rate of 55% to 60%.

[0024] Furthermore, the hot-dip galvanizing step includes alkali washing of the steel strip, continuous annealing, hot-dip galvanizing, and skin pass; the alkali washing is carried out in a degreasing tank to remove surface iron oxide.

[0025] Furthermore, the continuous annealing adopts the pre-oxidation process, preheating the oxidation chamber pipe temperature to 730±10℃ 3 hours in advance, and adjusting the compressed air flow in the oxidation chamber to ≥50m 3 / h, the flow rate of the oxidation chamber circulation fan ≥360m 3 / h; annealing soaking temperature is 830±10℃, slow heating temperature is 730±10℃, rapid cooling temperature is 410±10℃, and equilibrium temperature is 460±10℃; the dew point in the soaking section is ≤-35℃, and the dew point in the grate is ≤-30℃; the speed in the annealing process section is 65~80m / min; skin-passing adopts large roller rolling, with an elongation of 0.5%~0.8% and a rolling force of 3000~5000KN.

[0026] The beneficial effects of the present invention are:

[0027] (1) The present invention adopts a low-cost C-Si-Mn component system and adds a small amount of Nb+Ti microalloy design. Through reasonable rolling and heat treatment processes, a hot-dip galvanized TRIP steel strip with a strength-ductility product greater than 25GPa·% can be obtained. The main structure is ferrite, bainite, retained austenite and a small amount of martensite, and the matrix grain structure is fine and uniform, with good elongation performance.

[0028] (2) Before hot-dip galvanizing, the present invention uses a pre-oxidation process to control the continuous annealing. The steel strip is first internally oxidized at a reasonable heating temperature and dew point atmosphere to improve the wettability of the steel strip surface, which is beneficial to improving the adhesion of the surface hot-dip galvanized layer and can meet the high strength and high surface requirements of industries such as automobiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0030] Figure 1 1 is the tensile curve of the hot-dip galvanized TRIP steel strip obtained in Example 1 of the present invention.

[0031] Figure 2 This is a microstructure diagram of the hot-dip galvanized TRIP steel strip obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0033] The present invention provides a hot-dip galvanized TRIP steel for automobiles with a strength-ductility product of 25 GPa·%. The steel comprises the following chemical components in weight percentage: C 0.17%-0.22%, Si 1.2%-1.4%, Mn 1.6%-1.9%, P≤0.012%, S≤0.010%, Als 0.02%-0.05%, Nb 0.015%-0.030%, Ti 0.015%-0.025%, N≤0.005%, and the remainder is Fe and unavoidable impurities.

[0034] The chemical composition designs of Examples 1 to 3 are shown in Table 1.

[0035] Table 1 Chemical composition design of Example 1 to Example 3, unit: weight percentage (%)

[0036] Chemical elements C Si Mn P S Als Nb Ti N Example 1 0.205 1.32 1.72 0.006 0.005 0.023 0.019 0.016 0.004 Example 2 0.176 1.36 1.65 0.011 0.008 0.033 0.026 0.023 0.004 Example 3 0.213 1.22 1.88 0.007 0.005 0.045 0.017 0.018 0.005

[0037] Molten steel meeting the chemical composition requirements of Examples 1-3 was obtained through KR desulfurization of molten iron and converter smelting. The molten steel was then subjected to LF+RH refining and continuous casting to produce 230 mm thick continuous cast slabs. The continuously cast slabs were then subjected to hot rolling, which involved furnace heating, rough rolling, finish rolling, laminar cooling, and coiling into coils. The hot-rolled coils were then uncoiled and subjected to continuous pickling, which involved continuous pickling and cold rolling to form cold-rolled coils. The process control parameters for hot and continuous pickling are shown in Table 2.

[0038] Table 2 Process parameters of hot rolling and acid continuous rolling in the embodiment of the present invention

[0039] Process parameters Heating temperature / ℃ Finish rolling temperature / ℃ Coiling temperature / ℃ Cold rolling reduction / % Example 1 1235 855 653 57 Example 2 1223 846 639 58 Example 3 1246 862 660 55

[0040] The cold-rolled steel coils were uncoiled on a hot-dip galvanizing unit for degreasing and cleaning, continuous annealing (preheating, heating, soaking, slow cooling, rapid cooling, and equalization), hot-dip galvanizing, skin pass (large roll rolling), post-treatment, and coiling into hot-dip galvanized steel coils. The process parameters for continuous annealing and skin pass are shown in Table 3.

[0041] Table 3 Process parameters of continuous annealing and finishing steps in the embodiment of the present invention

[0042]

[0043] The hot-dip galvanized TRIP steel strip obtained by the above method was stretched according to the tensile test method for metallic materials (GB / T 228.1), and its mechanical property values ​​are shown in Table 4.

[0044] Table 4 Mechanical properties of hot-dip galvanized steel strips according to the present invention

[0045]

[0046] Figure 1 The tensile curve of the hot-dip galvanized TRIP steel strip prepared in Example 1 of the present invention is shown. It can be seen that there is no yield platform in the tensile curve.

[0047] Figure 2 The microstructure diagram of the hot-dip galvanized TRIP steel strip prepared in Example 1 of the present invention is shown. It can be seen that the main structures are ferrite, bainite, retained austenite and a small amount of martensite, and the matrix grain structure is fine and uniform.

[0048] The present invention provides a cold-based hot-dip galvanized steel sheet with a high strength-ductility product and TRIP effect through appropriate smelting component design and a reasonable rolling and heat treatment process route (mainly including hot rolling, continuous acid rolling + cold rolling and continuous hot-dip galvanizing process). The strength-ductility product can reach above 25 GPa·%, so that the steel sheet has high strength, excellent plasticity and collision absorption capacity, and has excellent ductility, which is conducive to the processing and forming of parts and improves the safety of traffic accidents. The steel sheet is widely used in reinforced structural parts such as front anti-collision beams, door guard bars, bumpers, and chassis components of automobiles.

[0049] Although the present invention has been described in detail with reference to the accompanying drawings and in conjunction with preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and essence of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be easily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.

Claims

1. A hot-dip galvanized TRIP steel for automobiles with a strength-ductility product of ≥25 GPa·%, characterized in that: The invention comprises the following chemical compositions in weight percentage: C 0.17% to 0.22%, Si 1.2% to 1.4%, Mn 1.6% to 1.9%, P≤0.012%, S≤0.010%, Als0.02% to 0.05%, Nb 0.015% to 0.030%, Ti 0.015% to 0.025%, N≤0.005%, and the remainder is Fe and unavoidable impurities; The preparation method of the hot-dip galvanized TRIP steel includes smelting, continuous casting, hot rolling, acid continuous rolling, hot-dip galvanizing, post-treatment, and coiling steps; During the hot rolling step, the heating temperature of the heating furnace is 1220~1250℃, the hot rolling finishing temperature is 850±15℃, the cooling mode adopts front-stage laminar cooling, and the coiling temperature is 650±15℃; In the pickling continuous rolling step, pickling is followed by cold continuous rolling; Hot dip galvanizing steps include alkali washing of steel strip, continuous annealing, hot dip galvanizing, and skin pass; Continuous annealing adopts pre-oxidation process, preheat the oxidation chamber pipeline temperature to 730±10℃ 3 hours in advance, and adjust the compressed air flow in the oxidation chamber to ≥50m 3 / h, the flow rate of the circulating fan in the oxidation chamber is ≥360m 3 / h; annealing soaking temperature is 830±10℃, slow heating temperature is 730±10℃, rapid cooling temperature is 410±10℃, and equilibrium temperature is 460±10℃; the dew point of the soaking section is ≤-35℃, and the dew point of the grate is ≤-30℃; the speed of the annealing process section is 65~80m / min.

2. The hot-dip galvanized TRIP steel for automobiles having a strength-ductility product of ≥25 GPa·% according to claim 1, characterized in that: Hot-dip galvanized TRIP steel strip has the following mechanical properties: yield strength ≥400MPa, tensile strength ≥650MPa, elongation A50 ≥38%, n value ≥0.20, and strength-ductility product ≥25GPa·%.

3. The hot-dip galvanized TRIP steel for automobiles having a strength-ductility product of ≥25 GPa·% according to claim 1, characterized in that: The thickness of hot-dip galvanized TRIP steel strip is 1.0~2.5mm and the width is 1000~1800mm.

4. The hot-dip galvanized TRIP steel for automobiles having a strength-ductility product of ≥25 GPa·% according to claim 1, characterized in that: The cold rolling reduction rate is 55%~60%.

5. The hot-dip galvanized TRIP steel for automobiles having a strength-ductility product of ≥25 GPa·% according to claim 1, characterized in that: The skin-passing process is carried out by large roller rolling, with an elongation of 0.5%~0.8% and a rolling force of 3000~5000KN.

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