780mpa grade high surface quality hot-rolled trip steel having excellent hole expansion performance and method of manufacturing the same

Abstract

The application provides a 780MPa-grade high-surface-quality hot-rolled TRIP steel with excellent hole expansion performance and a manufacturing method thereof, the steel comprises the following components in percentage by weight: C: 0.16-0.18%, Si: 0.05-0.10%, Mn: 0.9-1.1%, P: ≤0.008%, S: ≤0.003%, Al: 0.02-0.04%, Cr: 0.20-0.35%, Nb: 0.02-0.04%, Si+Cr: 0.27%-0.45%, and the balance is Fe and inevitable impurities. The production method of the steel plate comprises the following steps: smelting, heating, rolling, cooling, coiling, and wet sand blasting descaling. The surface quality of the steel is good, there are no iron oxide scale stripes and color difference defects, and the surface roughness Ra is 2.0-2.5 microns.

Description

Technical Field

[0001] This invention belongs to the field of steel rolling technology, and specifically relates to a 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance and its manufacturing method. The hot-rolled steel sheet of this invention is mainly suitable for manufacturing high-strength automotive parts with complex shapes and high surface quality requirements. Background Technology

[0002] With the development of the automotive industry, the requirements for the comprehensive mechanical properties of steel are becoming increasingly stringent, demanding not only high strength but also good ductility. However, increased strength often comes at the cost of decreased ductility. The contradiction between strength and ductility in high-strength structural steel has been a persistent challenge. Phase transformation-induced ductility is an effective method that can simultaneously improve both the strength and ductility of steel.

[0003] TRIP steel, or transformation-induced plasticity steel, has a microstructure primarily composed of ferrite, bainite, and retained austenite. It possesses advantages such as excellent strength-plasticity balance and a high strain hardening index, making it widely used in automotive manufacturing, including components like dashboards, bumpers, chassis parts, wheel rims, and door impact beams. Currently, cold-rolled TRIP steel is commercially produced and applied, while hot-rolled TRIP steel, due to its more difficult-to-control production process and issues such as poor surface quality caused by higher Si content, has not yet seen large-scale application. However, with the development of lightweight vehicles and continuous advancements in automotive parts manufacturing processes, the demand for customized steel performance is constantly increasing. Hot-rolled TRIP steel with excellent hole-expanding properties and high surface quality is well-suited to the requirements of automotive parts manufacturing and has broad application prospects.

[0004] Patent CN107475627A discloses a 600MPa grade hot-rolled TRIP steel and its manufacturing method based on the CSP process. The chemical element composition and weight percentage of the steel plate are as follows: carbon 0.14-0.16%, silicon 0.90-1.20%, manganese 1.0-1.40%, phosphorus 0.07%-0.10%, sulfur ≤0.005%, and acid-soluble aluminum 0.015-0.060%. The metallographic structure of the steel plate is 50-65% ferrite, 25-40% bainite, and 5-20% retained austenite; the yield strength is 390-450MPa, the tensile strength is 600-680MPa, and the elongation is A... 80With a Si content of 28-35%, it exhibits a good balance of strength and ductility. The steel plate has a low cost and excellent mechanical properties, but its high Si content leads to poor surface quality, affecting its application. Furthermore, the patent does not specify the hole-expanding performance index for the steel plate. Patents CN107488814A and CN107557692A respectively propose a method for manufacturing 800MPa and 1000MPa grade hot-rolled TRIP steel based on the CSP process. While the steel plate has a low production cost and excellent mechanical properties, its high Si content in both patents results in poor surface quality, affecting its application. Additionally, the multi-stage cooling process after rolling is complex and difficult to control precisely, and the patents do not specify the hole-expanding performance index for the steel plate. Patent CN101550514A discloses a hot-rolled transformation-induced plasticity (PEPL) steel plate and its manufacturing method. The steel plate uses the following chemical composition: carbon 0.18-0.2%, silicon 0.6-0.7%, manganese 1.30-1.45%, phosphorus ≤0.009%, sulfur ≤0.007%, aluminum 0.50-0.60%, and niobium 0-0.04%. The steel plate is cooled using an air-cooling + rapid cooling method after rolling, resulting in a tensile strength exceeding 639 MPa and a strength-ductility product greater than 18000 MPa. While the alloy of the proposed steel plate has a low cost, the high content of Si and Al elements in its composition affects the surface quality of the steel plate, increases the difficulty of smelting, and does not specify a hole-expanding performance index in its mechanical properties. Patent CN106048176A proposes a method for producing low-carbon hot-rolled TRIP steel based on an ESP thin slab continuous casting and rolling process. The proposed steel plate is prepared using ESP thin slab continuous casting and rolling equipment. The chemical composition of the steel plate is: carbon 0.06-0.12%, silicon 1.0-2.0%, manganese 1.0-2.0%, phosphorus ≤0.02%, sulfur ≤0.012%, nitrogen ≤0.006%, and molybdenum 0.05-0.25%. The hot-rolled finishing temperature is above 800℃, and a three-stage cooling process is used after rolling to obtain a microstructure of ferrite, bainite, and retained austenite. The tensile strength of the steel plate can reach over 590MPa. While the proposed steel plate has good microstructure and mechanical properties, it requires production on a specialized ESP production line and contains large amounts of Si and Mo elements, resulting in poor surface quality and high cost. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a 780MPa grade hot-rolled TRIP steel sheet with excellent hole-expanding performance and good surface quality, as well as its manufacturing method, which is simple and easy to produce. To obtain 780MPa grade TRIP steel with good surface quality and excellent hole-expanding performance, corresponding composition and production process designs were implemented to control surface quality and improve hole-expanding performance.

[0006] For surface quality control, the steel plate adopts a low-Si composition design, and the hot-rolled steel plate is subjected to wet sandblasting descaling treatment, which can make the steel plate surface free of iron oxide scale streaks and color difference defects. The surface roughness Ra of the steel plate can reach 2.0-2.5μm, which meets the requirements of high-quality automotive parts manufacturing.

[0007] To improve the hole-expanding performance of steel plates, on the one hand, a double-stage process is adopted in the smelting process, along with wire feeding treatment, to strictly control the content of impurity elements such as P, S, O, and H, thereby improving the purity of the steel plate and reducing the level of inclusions. On the other hand, electromagnetic stirring and light reduction processes are implemented to improve the compositional segregation of the billet. Combined with the higher heating temperature and time during hot rolling, the level of banded structural defects in the steel plate is controlled, thus improving the hole-expanding performance. Simultaneously, through reasonable composition design and hot rolling process formulation, the proportion of three-phase structures in the steel plate is controlled, and the ferrite-strengthening effect of elements such as Nb, Si, and Mn is used to improve the yield strength ratio of the steel plate, ultimately achieving the desired strength, plasticity, and hole-expanding performance.

[0008] The objective of this invention is achieved as follows:

[0009] A 780MPa grade high surface quality hot-rolled TRIP steel sheet with excellent hole expansion performance. The chemical composition of this invention, by weight percentage, includes the following components: C: 0.16-0.18%, Si: 0.05-0.10%, Mn: 0.9-1.1%, P: ≤0.008%, S: ≤0.003%, Al: 0.02-0.04%, Cr: 0.20-0.35%, Nb: 0.02-0.04%, with the balance being Fe and unavoidable impurities.

[0010] Furthermore, Si+Cr: 0.27%-0.45%.

[0011] Furthermore, the microstructure of the steel plate consists of three phases: ferrite, bainite, and retained austenite, wherein the volume fraction of ferrite is 25-35%, the volume fraction of bainite is 50-65%, and the volume fraction of retained austenite is 10%-15%.

[0012] Furthermore, the banded structure grade is ≤1.5, the non-metallic inclusion grade is ≤1.0; the finished steel plate thickness is 3-6mm, the surface quality is good, there are no iron oxide scale stripes and color difference defects, and the surface roughness Ra is 2.0-2.5μm.

[0013] Furthermore, the steel plate has a tensile strength of 790-830 MPa, a yield strength of 595-650 MPa, a yield-to-tensile ratio of 0.72-0.82, an elongation A80 ≥ 25.0%, and a hole expansion rate ≥ 80%.

[0014] The rationale for the design of the components in this invention is as follows:

[0015] C: 0.16-0.18%

[0016] C: Carbon is an austenite stabilizing element. Especially after a portion of the austenite in the microstructure transforms into ferrite, it accumulates in the retained austenite, improving the stability of the retained austenite and inhibiting its transformation into pearlite and bainite, thus ensuring the percentage content of retained austenite in the final microstructure. In this invention, the carbon content is controlled between 0.16% and 0.18%.

[0017] Si: 0.05-0.10%

[0018] Silicon (Si) promotes the transformation of austenite to ferrite and is the main element ensuring the ferrite content. Silicon can dissolve into the ferrite matrix, increasing its strength and improving the hole-expanding performance of the steel sheet. Simultaneously, silicon can inhibit cementite formation during the isothermal transformation of bainite, increasing the carbon content of austenite and thus improving its stability. However, excessive Si content can lead to a large amount of iron oxide scale defects on the steel sheet surface, failing to meet the requirements for automotive parts. The steel sheet composition proposed in this invention adopts a low-Si composition system, with a Si content of 0.050%-0.10%.

[0019] Mn: 0.9-1.1%

[0020] Mn: Manganese is an austenite stabilizing element that significantly improves the hardenability of steel, delays the transformation of pearlite and bainite, and also plays a role in solid solution strengthening and grain refinement, significantly improving the strength and porosity of steel plates. However, excessively high Mn content can easily lead to compositional segregation in the cast billet, resulting in severe banded microstructure problems in the steel plate. Therefore, the Mn content selected in this invention is 0.90%-1.1%.

[0021] P: ≤0.008%

[0022] P: Phosphorus is a harmful impurity element in this invention, which can significantly reduce the toughness of steel and has a serious tendency to segregate, thus deteriorating the hole-expanding performance of steel plates. Therefore, the phosphorus content in this invention is ≤0.008%.

[0023] S: ≤0.003%

[0024] S: Sulfur readily forms sulfide inclusions such as MnS, which form bands after rolling, becoming the starting point of cracks, deteriorating the plasticity of the steel plate, and reducing the hole expansion performance. Therefore, the lower the content, the better, and its upper limit is set at 0.003%.

[0025] Al: 0.02-0.04%

[0026] Al: Aluminum plays a role in deoxidation and nitrogen determination during the smelting process, but excessive aluminum will lead to a large number of aluminum-based inclusions, reducing the hole-expanding performance of the steel plate. In this invention, the Al range is 0.02%-0.04%.

[0027] Cr: 0.20-0.35%

[0028] Cr: Chromium is a medium-strong carbide-forming element that significantly improves the hardenability of steel, strongly delays the pearlite and bainite transformations, promotes carbon diffusion into austenite, increases the stability of supercooled austenite, and ensures a certain amount of retained austenite in the steel plate. The optimal range for Cr in this invention is 0.20-0.35%.

[0029] Nb: 0.02-0.04%

[0030] Niobium (Nb) has significant grain-refining and precipitation-strengthening effects. In this invention, the addition of Nb can also increase the content of retained austenite and the concentration of carbon in the retained austenite. The optimal range of Nb in this invention is 0.02-0.04%.

[0031] Si+Cr: 0.27%-0.45%

[0032] Silicon and chromium share a common function in this invention: increasing the stability of supercooled austenite and ensuring a certain amount of retained austenite in the steel sheet microstructure, thereby giving the steel sheet a good TRIP effect. In this invention, the Si+Cr content ranges from 0.27% to 0.45%.

[0033] The second technical solution of this invention provides a manufacturing process for a 780MPa grade high surface quality hot-rolled TRIP steel plate with excellent hole expansion performance, including smelting, heating, rolling, cooling, coiling, and wet sandblasting descaling processes, specifically including the following steps:

[0034] (1) Smelting: The RH+LF dual process is adopted. After the molten steel is refined by LF, it is fed with Si-Ca wire. The wire feeding speed is ≥3.5m / s and the wire feeding amount is (1.6-2.0)m / t. The RH treatment target is [H]≤1.5ppm and [O]≤10ppm. The argon blowing time of molten steel is ≥8min and the molten steel calming time is 25-30min. Electromagnetic stirring and light reduction technology are used in the continuous casting process to prevent defects such as center segregation and porosity of the billet and reduce the occurrence of banded defects in the steel plate. The electromagnetic stirring current is 400 Amperes and the frequency is 5 Hz. The light reduction amount is greater than 4.5mm.

[0035] (2) Heating process: The 170-230mm thick billet is heated to 1250-1260℃, and the soaking time is 50-70 minutes. The heating furnace adopts a reducing atmosphere and the air-fuel ratio is (1.8-2.0):1. The higher heating temperature and longer soaking time can make the composition of the billet fully uniform and reduce the risk of high banded structure in the finished steel plate. However, it is also necessary to control it appropriately to ensure that the original austenite grains are fine.

[0036] (3) Rolling and cooling process: The final rolling temperature of the steel plate is 820-850℃. After the final rolling, a continuous cooling process is adopted with a cooling rate of 25-35℃ / S. The steel plate is cooled to 430-460℃ and then coiled and air-cooled to room temperature. The thickness of the finished steel plate is 3-6mm. The lower final rolling temperature of the steel plate can refine the microstructure and increase the driving force for the transformation of austenite to ferrite. A certain amount of ferrite is formed within the cooling rate range. If the cooling rate after rolling is too slow, the ferrite content in the microstructure is high, the strength of the steel plate does not meet the requirements, and pearlite microstructure is easily formed. If the cooling rate is too fast, the ferrite content in the microstructure is low, and the plasticity of the steel plate is poor. Continuous cooling after rolling is easy to achieve and production control is simple. During the process of coiling and air-cooling the steel plate to room temperature in the range of 430-460℃, most of the residual austenite in the microstructure is transformed into bainite, and the remaining part is retained in the microstructure, thereby obtaining a three-phase microstructure and realizing the mechanical property characteristics of the steel plate.

[0037] (4) Wet sandblasting descaling process: After the steel coil is cooled to room temperature, the surface of the steel plate is treated with wet sandblasting descaling. The main processes are uncoiling, tension leveling, sandblasting descaling treatment, and coiling. The uncoiling tension of the steel plate is 20-35KN, the tension leveling elongation is 0.8-1.0%, the traveling speed is 25-30m / min, the hardness of the steel grit is HRC>70, the sandblasting motor speed is 2300-2500rpm, and the coiling tension is 80-100KN.

[0038] The beneficial effects of this invention are as follows:

[0039] By adopting a low-Si composition design and combining it with a wet sandblasting descaling process, a high-quality steel plate with no iron oxide scale streaks or color difference defects was obtained. The surface roughness Ra of the steel plate is 2.0-2.5μm.

[0040] 2. Through reasonable composition design and production process coordination, the steel plate has a good balance of strength, plasticity and hole expansion performance, and the hole expansion rate can reach more than 80%, which meets the manufacturing requirements of complex automotive parts;

[0041] 3. It does not contain precious alloys such as Mo, resulting in lower costs and a simple and easy-to-implement production method.

[0042] 4. The tensile strength of the steel plate is 790-830 MPa, the yield strength is 595-650 MPa, the yield strength ratio is 0.72-0.82, and the elongation A is... 80 ≥25.0%; finished steel plate thickness is 3-6mm.

[0043] 5. The microstructure of the steel plate consists of three phases: ferrite, bainite and retained austenite, with a volume fraction of ferrite of 25-35%, bainite of 50-65%, and retained austenite of 10%-15%; the banded microstructure grade is ≤1.5 and the non-metallic inclusion grade is ≤1.0. Detailed Implementation

[0044] The present invention will be further illustrated below through examples.

[0045] According to the component ratio of the technical solution, the embodiments of the present invention carry out smelting, heating, rolling, cooling, coiling, and wet sandblasting descaling processes.

[0046] Heating process

[0047] The 170-230mm thick billet is heated to 1250-1260℃, and the soaking time is 50-70 minutes. The heating furnace adopts a reducing atmosphere and the air-fuel ratio is (1.8-2.0):1.

[0048] Rolling and cooling processes

[0049] The final rolling temperature of the steel plate is 820-850℃. After final rolling, a continuous cooling process is adopted with a cooling rate of 25-35℃ / S. The steel plate is cooled to 430-460℃, then coiled and air-cooled to room temperature. The thickness of the finished steel plate is 3-6mm.

[0050] Furthermore, the billet smelting adopts the RH+LF dual process. After LF refining, the molten steel is fed with Si-Ca wire. The wire feeding speed is ≥3.5m / s, and the wire feeding amount is (1.6-2.0)m / t. The RH treatment target is [H]≤1.5ppm and [O]≤10ppm. The argon blowing time of the molten steel is ≥8min, and the molten steel calming time is 25-30min. During the continuous casting process, electromagnetic stirring and light reduction are applied. The electromagnetic stirring current is 400 Amperes and the frequency is 5 Hz. The light reduction is greater than 4.5mm, and the billet thickness is 170-230mm.

[0051] Furthermore, after the steel coil is cooled to room temperature, the surface of the steel plate is subjected to wet sandblasting descaling treatment. The main processes are uncoiling, tension leveling, sandblasting descaling treatment, and coiling. The uncoiling tension of the steel plate is 20-35KN, the tension leveling elongation is 0.8-1.0%, the travel speed is 25-30m / min, the hardness of the steel grit is HRC>70, the sandblasting motor speed is 2300-2500rpm, and the coiling tension is 80-100KN.

[0052] Specifically as follows:

[0053] The specific components, smelting processes, hot rolling processes, sandblasting and descaling processes, and the microstructure and properties of the steel plates in the six embodiments of the present invention are shown in Tables 1-6.

[0054] Table 1 Chemical composition (wt, %) of embodiments of the present invention

[0055]

[0056] Table 2 Smelting process regulations of embodiments of the present invention

[0057]

[0058] Table 3 Hot rolling process of embodiments of the present invention

[0059]

[0060] Table 4. Sandblasting dephosphorization process parameters of embodiments of the present invention

[0061]

[0062] Table 5 Mechanical properties and hole-expanding properties of embodiments of the present invention

[0063]

[0064] Table 6. Tissue composition, banded structure, inclusions, and roughness of embodiments of the present invention

[0065]

[0066] As can be seen from the above, the steel plate of this invention adopts a low-Si composition design and, combined with a wet sandblasting descaling process, obtains a high-surface-quality steel plate. The steel plate surface is free of iron oxide scale streaks and color difference defects, and the surface roughness Ra is 2.0-2.5μm. Through reasonable composition design and production process coordination, the steel plate has good strength-plasticity-hole expansion performance matching, with a hole expansion rate of over 80%, meeting the manufacturing requirements of complex automotive parts; the tensile strength is 790-830MPa, the yield strength is 595-650MPa, the yield ratio is 0.72-0.82, and the elongation A is [missing information]. 80 ≥25.0%.

[0067] 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, which should be defined by the claims.

Claims

1. A 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance, characterized in that, By weight percentage, it includes the following components: C: 0.16-0.18%, Si: 0.05-0.10%, Mn: 0.9-1.1%, P: ≤0.008%, S: ≤0.003%, Al: 0.02-0.04%, Cr: 0.20-0.35%, Nb: 0.02-0.04%, with the balance being Fe and unavoidable impurities, and Si+Cr: 0.27%-0.45%; the steel microstructure consists of three phases: ferrite, bainite, and retained austenite, with ferrite volume fraction of 25-35%, bainite volume fraction of 50-65%, and retained austenite volume fraction of 10%-15%. 。 2. The 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance according to claim 1, characterized in that, The banded structure grade is ≤1.5, and the non-metallic inclusion grade is ≤1.0; the finished steel plate thickness is 3-6mm, the surface is free of iron oxide scale stripes and color difference defects, and the surface roughness Ra is 2.0-2.5μm.

3. The 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance according to claim 1, characterized in that, The steel plate has a tensile strength of 790-830 MPa, a yield strength of 595-650 MPa, a yield-to-tensile ratio of 0.72-0.82, and an elongation A. 80 ≥25.0%, expansion rate ≥80%.

4. A method for manufacturing a 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance according to any one of claims 1 to 3, comprising smelting, heating, rolling, cooling, coiling, and wet sandblasting descaling processes, characterized in that, Heating process Heat the 170-230mm thick billet to 1250-1260℃ for 50-70 minutes. Rolling and cooling processes The final rolling temperature of the steel plate is 820-850℃. After final rolling, a continuous cooling process is adopted with a cooling rate of 25-35℃ / S. The steel plate is cooled to 430-460℃ and then coiled and air-cooled to room temperature.

5. The method for manufacturing a 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance according to claim 4, characterized in that, The smelting process adopts the RH+LF dual process. After LF refining, the molten steel is fed with Si-Ca wire at a feeding speed of ≥3.5m / s and a feeding amount of 1.6-2.0 m / t. The RH treatment target is [H]≤1.5ppm and [O]≤10ppm. The argon blowing time of the molten steel is ≥8min and the molten steel calming time is 25-30min. Electromagnetic stirring and light reduction technology are used in the continuous casting process, and the light reduction amount is greater than 4.5mm.

6. The method for manufacturing a 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance according to claim 4, characterized in that, The heating process uses a reducing atmosphere in the furnace, with an air-fuel ratio of (1.8-2.0):

1.

7. The method for manufacturing a 780MPa grade high surface quality hot-rolled TRIP steel with excellent hole-expanding performance according to claim 4, characterized in that, After the steel coil is cooled to room temperature, the surface of the steel plate is treated with wet sandblasting to remove scale. The main processes are uncoiling, tension leveling, sandblasting and descaling, and coiling. The uncoiling tension of the steel plate is 20-35KN, the tension leveling elongation is 0.8-1.0%, the traveling speed is 25-30m / min, the hardness of the steel grit is HRC>70, the sandblasting motor speed is 2300-2500rpm, and the coiling tension is 80-100KN.

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