A 590MPa grade hot-dip galvanized low-density steel with low yield strength ratio and high corrosion resistance and its manufacturing method

Abstract

This invention proposes a 590MPa grade hot-dip galvanized low-density steel with low yield strength ratio and high corrosion resistance, and its manufacturing method. The steel plate composition, by weight percentage, is: C: 0.085%~0.1%, Si: 0.58%~0.7%, Mn: 1.63%~1.70%, Al: 3.3%~6.5%, P: ≤0.010%, S: 0.01%~0.03%, RE: 0.01%~0.015%, with the balance being Fe and unavoidable impurities. The steel manufacturing method includes blast furnace hot metal pretreatment, converter steelmaking, continuous casting, hot continuous rolling, pickling, cold rolling, and hot-dip galvanizing.

Description

Technical Field

[0001] This invention belongs to the field of low-density steel for automobiles, and specifically relates to a 590MPa grade hot-dip galvanized low-density steel for automobiles with low yield strength ratio and high corrosion resistance. This steel is suitable for manufacturing steel structure products such as automotive structural parts and seats. Background Technology

[0002] Research on low-density steel can be traced back to the 1930s, initially focusing on Fe-Al steels to replace expensive Ni and Cr stainless steels. However, its widespread application was limited by metallurgical bottlenecks. After the 1950s, with the surge in demand for lightweight automobiles, researchers began to reduce steel density by adding light elements such as Al and Mn (each 1% Al addition reduces density by approximately 1.3%). Since 2000, Fe-Mn-Al-C low-density steel has become a research hotspot due to its combination of low density and high strength and ductility. Laboratory achievements in countries like South Korea and Japan have driven its development, but industrial-scale production still faces challenges. Adding a large amount of Al to low-density steel significantly reduces its density, but high Al levels cause considerable instability and negative impacts on industrial production. Al tends to stabilize the ferrite phase, inhibiting austenite formation and affecting heat treatment processes and microstructure control. This results in low-density high-strength steel products having higher yield strengths than ordinary high-strength steel products. Hot-dip galvanizing lines, due to their inability to achieve cooling rates above 30°C, result in yield strength ratios exceeding 0.73, leading to significant springback in stamped parts during practical applications. The high yield strength is primarily due to the fact that δ-ferrite, a high-temperature ferrite, forms during solidification and its morphology and distribution cannot be altered by subsequent heat treatment. This invention aims to reduce the yield strength of the product by adjusting the distribution of δ-ferrite. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the present invention aims to overcome the defects of high yield strength ratio and large springback of existing low-density steel, and to provide a 590MPa grade hot-dip galvanized low-density steel with low yield strength ratio and high corrosion resistance, as well as its manufacturing method.

[0004] This invention provides a 590MPa grade hot-dip galvanized low-density steel with low yield strength ratio and high corrosion resistance, and its manufacturing method. The chemical composition, by weight percentage, is as follows: C: 0.085%–0.1%, Si: 0.58%–0.7%, Mn: 1.63%–1.70%, Al: 3.3%–6.5%, P: ≤0.010%, S: 0.01%–0.03%, RE: 0.01%–0.015%, with the balance being Fe and unavoidable impurities.

[0005] Furthermore, the microstructure of the low-density automotive steel with low yield strength ratio and high corrosion resistance is: δ-ferrite + martensite + retained austenite + sulfides, wherein the martensite content is 8.0~10.2%, the retained austenite content is 3.9~4.3%, and the sulfide composition is a small amount of dispersed MnS and RE-SO, with a MnS to RE-SO ratio of 1.0~1.5:1. A small amount of MnS is dispersed around the δ-ferrite and martensite grain boundaries, and RE-SO is distributed along the δ-ferrite grain boundaries.

[0006] Furthermore, the steel has a yield strength ratio ≤ 0.63, a tensile strength ≥ 590 MPa, and an elongation ≥ 26%.

[0007] The rationale for the alloy design of this invention is as follows:

[0008] C: Carbon (C) has excellent solid solution strengthening effects. Too low a C content will reduce the strength of the steel and the stability of austenite; too high a C content will easily lead to the precipitation of coarse carbides at grain boundaries, reducing the steel's properties. Therefore, the C content ranges from 0.085% to 0.1%. C is the primary element providing strength in this invention.

[0009] Mn: Mn is an element that strengthens steel through solid solution and expands the austenite region. Too low a Mn content leads to insufficient residual austenite after martensitic transformation, reducing the steel's plasticity. Too high a Mn content increases costs and causes segregation, resulting in poor performance. In this invention, most of the Mn is used for solid solution strengthening, with a small portion reacting with S to form MnS, reducing δ-ferrite continuity and yield strength. Therefore, the Mn content in this invention ranges from 1.63% to 1.70%.

[0010] Si: Si mainly plays a role in deoxidation and solid solution strengthening in steel. If the Si content is too low, it will not have a deoxidizing effect; if the Si content is too high, it will reduce the surface quality of the steel plate. Therefore, the Si content range is 0.58% to 0.7%.

[0011] P: P is a harmful element in steel, and the lower its content, the better.

[0012] S: S element mainly forms MnS with Mn element. Through process control, the precipitated MnS can reduce the yield strength of steel by reducing the continuity of δ-ferrite. However, excessive S element is a harmful element in steel. Therefore, the S element content in this design is 0.01%~0.03%.

[0013] Al: As a lightweight element, Al can increase the lattice constant of the product and reduce the density of steel. Al can also inhibit the decomposition of residual austenite and the precipitation of carbides in steel. Excessive Al content will not only increase production costs, but also lead to a decrease in the quality of continuously cast billets and the possibility of steel leakage. Therefore, in this invention, the Al content is controlled within the range of 3.3% to 6.5%.

[0014] RE (reactive iron): It has a strong reactivity with oxygen and sulfur. At grain boundaries, it can prevent Mn and S from forming long, brittle MnS compounds that embrittle the grain boundaries, generating spherical RE-SO rare earth oxides, thus improving the toughness and plasticity of steel. Therefore, in this invention, the RE element content is controlled within the range of 0.01% to 0.015%.

[0015] The second technical solution of the present invention is as follows:

[0016] A process for preparing a 590MPa grade hot-dip galvanized low-density steel with low yield strength ratio and high corrosion resistance includes the following steps: blast furnace molten iron pretreatment, converter steelmaking, continuous casting, hot continuous rolling, pickling, cold rolling, and hot-dip galvanizing.

[0017] The specific steps of this preparation process are as follows:

[0018] Blast furnace molten iron pretreatment: Use specially designed slag, adjust blast parameters and inject pulverized coal to maintain a weak reducing atmosphere in the furnace, with pulverized coal injection rate ≥210kg / t.

[0019] Converter steelmaking: By weight percentage: C: 0.085%–0.1%, Si: 0.58%–0.7%, Mn: 1.63%–1.70%, Al: 3.3%–6.5%, P: ≤0.010%, S: 0.01–0.03%, RE: 0.01%–0.015%. The balance is Fe and unavoidable impurities. Batching is performed, and deoxidation is carried out using a Si-Ca alloy to control the type and morphology of inclusions.

[0020] The steel is smelted in a converter to obtain molten steel that meets the above composition requirements, with the temperature between 1560 and 1580°C.

[0021] Continuous casting: The casting temperature is between 1515 and 1541℃. During continuous casting production, a light reduction is required, with a reduction range of 3.4 to 5.5 mm. Excessive light reduction can easily cause a sharp increase in central cracks, while excessive light reduction has no effect on element segregation. Therefore, the light reduction is selected to be between 3.4 and 5.5 mm.

[0022] Hot continuous rolling: The billet's furnace entry temperature is between 500 and 800℃, the heating temperature is between 1210 and 1260℃, and the rolling mill performs 6 passes to roll the steel plate to the designed thickness. The initial rolling temperature is between 1140 and 1160℃, and the final rolling temperature is above 910℃. Too low a final rolling temperature will cause hard phase structures in the steel, leading to rolling difficulties. Coiling temperature is between 520 and 560℃; low-temperature coiling improves deformation energy storage. The thickness of the hot-rolled coil is between 5.5 and 7.0 mm.

[0023] Pickling + Cold Rolling: The steel coil is pickled before rolling. It is rolled in 3 to 6 passes on a single stand mill with a total reduction rate of over 74% to ensure sufficient cold deformation energy storage. The thickness of the cold-rolled coil is 1.0 to 1.4 mm.

[0024] The galvanizing annealing process is as follows: heating rate is 6-9℃ / s, dew point temperature is controlled at -27--29℃ during heating, annealing temperature is 820℃-840℃, annealing time is 23.9s-34s, dew point temperature is controlled at -31--33℃ during annealing, slow cooling to 720-740℃, rapid cooling to 462℃-470℃ before entering the zinc pot, rapid cooling section cooling rate is 7.35-8.2℃ / s, dew point temperature is controlled at -27--29℃ in the rapid cooling section, dew point temperature at the furnace nose is -40--42℃, air knife pressure is 189-210mBar, and the finishing rate is 2.6-3.6%. After that, the finished product is coiled.

[0025] The above method yields a 590MPa grade hot-dip galvanized low-density steel with low yield strength ratio and high corrosion resistance, along with its manufacturing method. Compared with the prior art, the advantages are as follows:

[0026] The yield strength ratio of low-density steel of the same strength grade is ≥0.68, while that of low-density steel in this invention is reduced to ≤0.63. S is generally a harmful element in steel, but this invention uses a small amount of MnS and RE-S-O to reduce the continuity of δ-ferrite, thereby reducing the yield strength and effectively lowering the yield strength ratio. This reduces springback of parts during application and improves the first-pass yield rate. Because a certain amount of RE is added in this invention, the corrosion potential is higher than that of low-density steel of the same strength grade, and the corrosion current is significantly lower, resulting in a significant improvement in corrosion resistance. Attached Figure Description

[0027] Figure 1 This is a typical metallographic structure composition of Embodiment 1 of the present invention. Detailed Implementation

[0028] The present invention will be further illustrated below through examples.

[0029] Examples 1-10 are listed in Table 1 (chemical composition), Table 2 (steel temperature, continuous casting start temperature, and light reduction), Table 3 (hot rolling process), Table 4 (hot-rolled mechanical properties), Table 5 (galvanizing annealing process), Table 6 (post-galvanizing properties), and Table 7 (corrosion potential and corrosion current).

[0030] Table 1 Chemical composition (wt%) of the examples

[0031]

[0032] Table 2. Steel temperature, continuous casting start temperature, and light reduction in the examples.

[0033]

[0034] Table 3 Hot rolling process of the embodiments

[0035]

[0036] Table 4 Hot-rolled mechanical properties of the examples

[0037]

[0038] Table 5. Galvanizing Annealing Process of Examples

[0039]

[0040] Table 6. Performance after galvanizing in the examples

[0041]

[0042] Table 7 Corrosion potential and corrosion current of the examples

[0043]

[0044] To illustrate the present invention, the present invention has been appropriately and sufficiently described above through embodiments. The above embodiments are only for illustrating the present invention and are not intended to limit the present invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc., should be included within the protection scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A hot-dip galvanized low-density steel with a low yield strength ratio and high corrosion resistance of 590MPa grade, characterized in that, The steel comprises, by weight percentage, the following components: C: 0.085%–0.1%, Si: 0.58%–0.7%, Mn: 1.63%–1.70%, Al: 3.3%–6.5%, P: ≤0.010%, S: 0.01%–0.03%, RE: 0.01%–0.015%, with the balance being Fe and unavoidable impurities; the manufacturing method of the 590MPa grade low yield strength ratio high corrosion resistance hot-dip galvanized low-density steel includes converter steelmaking, continuous casting, hot continuous rolling, pickling, cold rolling, and hot-dip galvanizing. Hot continuous rolling: The temperature of the billet entering the furnace is between 500 and 800℃, the heating temperature is between 1210 and 1260℃, the initial rolling temperature is between 1140 and 1160℃, the final rolling temperature is above 910℃, the coiling temperature is between 520 and 560℃, and the thickness of the hot rolled coil is between 5.5 and 7.0 mm. Pickling + Cold Rolling: After pickling, the steel coil is rolled in 3-6 passes on a single stand mill, with a total reduction of over 74% to ensure sufficient cold deformation energy storage. The thickness of the cold-rolled coil is 1.0-1.4 mm. The galvanizing annealing process is as follows: heating rate is 6-9℃ / s, dew point temperature is controlled at -27--29℃ during heating, annealing temperature is 820℃-840℃, annealing time is 23.9s-34s, dew point temperature is controlled at -31--33℃ during annealing, slow cooling to 720-740℃, rapid cooling to 462-470℃ before entering the zinc pot, rapid cooling section cooling rate is 7.35-8.2℃ / s, dew point temperature is controlled at -27--29℃ in the rapid cooling section, dew point temperature at the furnace nose is -40--42℃, air knife pressure is 189-210mBar, and the finishing rate is 2.6%-3.6%. The microstructure of the steel is: δ-ferrite + martensite + retained austenite + sulfides, wherein the martensite content is 8.0~10.2%, the retained austenite content is 3.9~4.3%, and the sulfide composition is dispersed MnS and RE-SO, with a MnS to RE-SO ratio of 1.0~1.5:

1. MnS is dispersed around the grain boundaries of δ-ferrite and martensite, and RE-SO is distributed along the grain boundaries of δ-ferrite.

2. The 590MPa grade, low yield strength ratio, and high corrosion resistance hot-dip galvanized low-density steel according to claim 1, characterized in that, The steel has a yield strength ratio ≤ 0.63, a tensile strength ≥ 590 MPa, and an elongation ≥ 26%.

3. The 590MPa grade, low yield strength ratio, and high corrosion resistance hot-dip galvanized low-density steel according to claim 1, characterized in that, In the converter steelmaking process, Si-Ca alloy is used for deoxidation in the feedstock; the continuous casting temperature is 1515~1541℃, and a light reduction is required during continuous casting production, with a reduction amount of 3.4~5.5mm.

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