600MPa-grade high-surface-quality hot-rolled TRIP steel with excellent reaming performance and manufacturing method thereof

Abstract

The invention provides 600MPa-grade high-surface-quality hot-rolled TRIP steel with excellent reaming performance and a manufacturing method of the 600MPa-grade high-surface-quality hot-rolled TRIP steel. A steel plate comprises the following components in percentage by weight: 0.14%-0.16% of C, 0.05%-0.10% of Si, 0.8%-1.0% of Mn, less than or equal to 0.008% of P, less than or equal to 0.003% of S, 0.02%-0.04% of Al, 0.15%-0.30% of Cr, 0.015%-0.025% of Nb, 0.22%-0.40% of Si + Cr and the balance of Fe and inevitable impurities. The production method of the steel plate comprises the working procedures of smelting, heating, rolling, cooling, coiling and wet type sand blasting descaling. The steel has good strength-plasticity-reaming performance matching, and the reaming rate can reach 90% or above.

Description

Technical Field

[0001] The present invention relates to the field of steel rolling technology, and more particularly to a 600MPa-grade, high-surface-quality hot-rolled TRIP steel with excellent hole expansion performance and a method for manufacturing the same. The hot-rolled steel sheet of the present invention is primarily suitable for manufacturing high-strength automotive parts with complex shapes and high surface quality requirements. Background Art

[0002] With the development of the automotive industry, the requirements for the comprehensive mechanical properties of steel are becoming increasingly stringent, requiring not only high strength but also good ductility. However, this increase in strength is often accompanied by a decrease in ductility. The contradiction between strength and ductility in high-strength structural steel has been difficult to resolve. Phase transformation-induced plasticity is an effective method for simultaneously improving 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 boasts excellent strength-plasticity matching and a high strain hardening index, making it widely used in the automotive industry for components such as fenders, bumpers, chassis components, wheel rims, and door impact beams. Currently, cold-rolled TRIP steel has achieved commercial production and application, while hot-rolled TRIP steel has not yet been widely used due to its difficult-to-control production process and the high Si content that results in poor surface quality. However, with the continued advancement of lightweight vehicles, automotive component manufacturing processes are also improving, and the requirements for customized performance of steel plates are also increasing. High-surface-quality hot-rolled TRIP steel with excellent hole-expansion properties is well-suited to the development requirements of automotive component manufacturing and holds broad application prospects.

[0004] Patent number CN107475627A proposes a 600MPa grade hot-rolled TRIP steel based on the CSP process and its manufacturing method. The chemical element composition and weight percentage of the steel plate are: carbon 0.14-0.16%, silicon 0.90-1.20%, manganese 1.0-1.40%, phosphorus 0.07%-0.10%, sulfur ≤0.005%, 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 80It is 28-35%, with good strength-plasticity matching. The steel plate has low cost and excellent mechanical properties, but the chemical composition contains a high Si content, which will lead to poor surface quality of the steel plate and affect the application, and the patent does not propose the steel plate's hole expansion performance index. Patents CN107488814A and CN107557692A respectively propose a 800MPa-grade and 1000MPa-grade hot-rolled TRIP steel based on the CSP process and a manufacturing method thereof. The steel plate has low production cost and excellent mechanical properties, but the chemical composition contains a high Si content, which will lead to poor surface quality of the steel plate and affect the application. At the same time, a multi-stage cooling process is adopted after the steel plate is rolled. The process is complex and difficult to accurately control, and the patent does not propose the steel plate's hole expansion performance index. Patent number CN101550514A proposes a hot-rolled phase-transformation-induced plasticity steel plate and its manufacturing method. The steel plate's chemical composition is 0.18-0.2% carbon, 0.6-0.7% silicon, 1.30-1.45% manganese, ≤0.009% phosphorus, ≤0.007% sulfur, 0.50-0.60% aluminum, and 0-0.04% niobium. After rolling, the steel plate is cooled using air cooling followed by rapid cooling, resulting in a tensile strength exceeding 639 MPa and a strength-ductility product exceeding 18,000 MPa%. The proposed steel plate alloy has a low cost, but contains significant amounts of Si and Al, which affect the surface quality and increase the difficulty of smelting. Furthermore, the mechanical properties do not include hole expansion performance indicators. Patent number 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 produced using ESP thin slab continuous casting and rolling equipment. The chemical composition of the steel plate is 0.06-0.12% carbon, 1.0-2.0% silicon, 1.0-2.0% manganese, ≤0.02% phosphorus, ≤0.012% sulfur, ≤0.006% nitrogen, and 0.05-0.25% molybdenum. The hot rolling outlet temperature is above 800°C, and a three-stage cooling process is used after rolling to achieve a microstructure of ferrite, bainite, and retained austenite. The steel plate can achieve a tensile strength of over 590 MPa. The proposed steel plate has good microstructure and mechanical properties, but requires production on a specialized ESP production line. The steel plate contains large amounts of Si and Mo, resulting in poor surface quality and high cost.

[0005] In summary, the current hot-rolled TRIP steel plates and their production methods have problems such as poor surface quality of the steel plates and lack of excellent hole expansion performance. Summary of the Invention

[0006] In response to the shortcomings of the existing technology, the purpose of the present invention is to prepare a 600MPa grade hot-rolled TRIP steel plate with excellent hole expansion performance and good surface quality and a manufacturing method thereof. The manufacturing method is simple and easy to produce, and the hole expansion rate can reach more than 90%, meeting the manufacturing requirements of complex automotive parts.

[0007] The object of the invention is achieved like this: A 600 MPa-grade, high-surface-quality hot-rolled TRIP steel plate with excellent hole expansion performance has the following chemical composition (by mass percentage): C: 0.14%-0.16%, Si: 0.05%-0.10%, Mn: 0.8%-1.0%, P: ≤0.008%, S: ≤0.003%, Al: 0.02%-0.04%, Cr: 0.15%-0.30%, Nb: 0.015%-0.025%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the Si+Cr: 0.22%-0.40%.

[0009] Furthermore, the steel plate structure consists of three phases: ferrite, bainite and retained austenite, wherein the volume fraction of ferrite is 30%-40%, the volume fraction of bainite is 45%-60%, and the volume fraction of retained austenite is 10%-15%; Furthermore, the steel plate microstructure banding grade is ≤1.5, and the non-metallic inclusion grade is ≤1.0; Furthermore, the tensile strength of the steel plate is 600-640MPa, the yield strength is 450-480MPa, the yield strength ratio is 0.70-0.80, and the elongation A 80 ≥27.0%, hole expansion rate ≥90%; the thickness of the finished steel plate is 3-6mm, the surface quality is good, there are no iron oxide stripes and color defects, and the surface roughness Ra is 2.0-2.5μm.

[0010] The reasons for the composition design of the present invention are as follows: C: 0.14%-0.16% Carbon is an austenite-stabilizing element. After a portion of the austenite in the structure transforms into ferrite, it accumulates in the retained austenite, improving its stability and inhibiting its transformation into pearlite and bainite, thereby ensuring the percentage of retained austenite in the final structure. In the present invention, the carbon content is controlled between 0.14% and 0.16%.

[0011] Si: 0.05%-0.10% Silicon promotes the transformation of austenite to ferrite and is the primary element for ensuring the ferrite content. Silicon dissolves into the ferrite matrix, increasing its strength and improving the hole-expanding properties of the steel sheet. Furthermore, silicon inhibits cementite formation during the isothermal transformation of bainite, increasing the carbon content of the austenite and thus its stability. However, excessive Si content can lead to the formation of large amounts of oxide scale defects on the steel sheet surface, which does not meet the requirements for use in automotive components. The steel sheet proposed in this invention utilizes a low-Si composition system, with a Si content of 0.050% to 0.10%.

[0012] Mn: 0.8%-1.0% Manganese is an austenite-stabilizing element that significantly improves the hardenability of steel and delays pearlite and bainite transformations. It also provides solid solution strengthening and grain refinement, significantly enhancing the strength and hole expansion performance of steel sheets. However, excessive Mn content can lead to compositional segregation in the ingot, resulting in severe banding in the steel sheet. Therefore, in the present invention, a Mn content of 0.80% to 1.0% is selected.

[0013] P: ≤0.008% Phosphorus is a harmful impurity element in the present invention. It can significantly reduce the toughness of steel and has a serious tendency to segregate, which deteriorates the hole expansion performance of the steel plate. Therefore, the phosphorus content in the present invention is ≤ 0.008%.

[0014] S: ≤0.003% Sulfur easily forms sulfide inclusions such as MnS, which form strips after rolling and become 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%.

[0015] Al: 0.02%-0.04% Aluminum plays a role in deoxidation and nitrogenation during the smelting process, but excessive aluminum will lead to a large number of aluminum inclusions, reducing the hole expansion performance of the steel plate. In the present invention, the range of Al is 0.02%-0.04%.

[0016] Cr: 0.15%-0.30% Chromium is a medium-strong carbide-forming element that significantly improves the hardenability of steel, strongly delays 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 of Cr in the present invention is 0.15%-0.30%.

[0017] Nb: 0.015%-0.025% Niobium has significant effects on grain refinement and precipitation strengthening. In the present 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 the present invention is 0.015%-0.025%.

[0018] Si+Cr: 0.22%-0.40% Silicon and chromium have a common function in the present invention: increasing the stability of supercooled austenite, ensuring a certain amount of retained austenite in the steel sheet structure, and thus achieving a good TRIP effect. The Si+Cr content in the present invention is in the range of 0.22%-0.40%.

[0019] The second technical solution of the present invention is to provide a manufacturing process for a 600MPa grade high surface quality hot-rolled TRIP steel plate with excellent hole expansion performance, which includes smelting, heating, rolling, cooling, coiling, and wet sandblasting descaling processes, specifically including the following steps: (1) Smelting: RH+LF double process is adopted. 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, [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 technology are used to prevent defects such as center segregation and looseness of the ingot and reduce the occurrence of banded structure defects in the steel plate. The electromagnetic stirring current is 400A, the frequency is 5Hz, and the light reduction is greater than 4.5mm. (2) Heating process: Heat the 170-230mm thick ingot to 1250-1260℃, soaking time is 50-70 minutes, and the heating furnace adopts a reducing atmosphere with an air-fuel ratio of (1.8-2.0):1. Higher heating temperature and longer soaking time can make the ingot composition fully uniform and reduce the risk of high grade banded structure in the finished steel plate, but proper control is also required to ensure that the original austenite grains are small; (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 lower the final rolling temperature of the steel plate, the finer the structure, and at the same time, the driving force for the transformation of austenite to ferrite is increased, and 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 structure is high, the strength of the steel plate does not meet the requirements, and pearlite structure is easily formed. If the cooling rate is too fast, the ferrite content in the structure is low, and the plasticity of the steel plate is poor. Continuous cooling after rolling is easy to implement and simple to control. During the process of coiling the steel plate in the range of 430-460℃ and air-cooling it to room temperature, most of the residual austenite in the structure is converted into bainite, and the remaining part is retained in the microstructure, thereby obtaining a three-phase structure and achieving the mechanical properties of the steel plate. The thickness of the finished steel plate is 3-6mm.

[0020] (4) Wet sandblasting descaling process: After the steel coil cools to room temperature, the steel plate surface is wet sandblasted for descaling. The main processes are uncoiling, straightening, sandblasting descaling, and coiling. The uncoiling tension of the steel plate is 15-20KN, the straightening elongation is 0.8%-1.0%, the travel speed is 25-30m / min, the steel grit hardness is HRC>70, the sandblasting motor speed is 2300-2500rpm, and the coiling tension is 60-80KN.

[0021] In the present invention, in order to obtain 600MPa grade TRIP steel with good surface quality and excellent hole expansion performance, corresponding composition and production process designs are carried out in terms of surface quality control and hole expansion performance improvement. For surface quality control, the steel plate adopts a low Si composition design, and the hot-rolled steel plate is wet sandblasted for descaling, so that the steel plate surface is free of iron oxide stripes and color difference defects, and the surface roughness Ra of the steel plate can reach 2.0-2.5μm, meeting the requirements for high-quality automotive parts manufacturing. For the hole expansion performance of the steel plate, on the one hand, a double process is adopted in the smelting process, and a wire feeding process is carried out to strictly control the content of impurity elements such as P, S, O, and H, improve the purity of the steel plate, and reduce the level of inclusions; on the other hand, electromagnetic stirring and light reduction processes are introduced to improve the segregation of the ingot composition, and the higher heating temperature and time during hot rolling are combined to control the level of banded structure defects in the steel plate and improve the hole expansion performance of the steel plate. At the same time, through reasonable composition design and hot rolling process system formulation, the proportion of three-phase structure in the steel plate is controlled, and the ferrite effect is strengthened by elements such as Nb, Si, and Mn, the yield ratio of the steel plate is improved, and the ultimate strength, plasticity and hole expansion performance are obtained.

[0022] The beneficial effects of the present invention are: 1. The low-Si component design and wet sandblasting descaling process are used to obtain high-surface quality steel plates with no iron oxide streaks or color defects on the surface. The surface roughness Ra of the steel plates is 2.0-2.5μm.

[0023] 2. Through rational composition design and production process coordination, the steel plate has an excellent balance of strength, plasticity, and hole expansion performance, with a hole expansion rate exceeding 90%, meeting the manufacturing requirements of complex automotive parts. 3. The composition does not contain precious alloys such as Mo, the cost is low, and the production method is simple and easy to implement.

[0024] 4. The steel plate structure consists of three phases: ferrite, bainite and retained austenite, with the volume fraction of ferrite being 30%-40%, the volume fraction of bainite being 45%-60%, and the volume fraction of retained austenite being 10%-15%; 5. The steel plate microstructure banding grade is ≤1.5, and the non-metallic inclusion grade is ≤1.0; 6. The tensile strength of the steel plate is 600-640MPa, the yield strength is 450-480MPa, the yield strength ratio is 0.70-0.80, and the elongation A 80 ≥27.0%, the thickness of the finished steel plate is 3-6mm. DETAILED DESCRIPTION

[0025] The present invention will be further described below by way of examples.

[0026] The embodiment of the present invention carries out smelting, heating, rolling, cooling, coiling and wet sandblasting descaling processes according to the component ratio of the technical solution.

[0027] Heating process Heat the 170-230mm thick ingot to 1250-1260℃ for 50-70 minutes in a reducing atmosphere. Rolling and cooling process 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.

[0028] Furthermore, the smelting adopts the RH+LF dual process, and after the molten steel is LF refined, it is fed with Si-Ca wire treatment, 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, [O]≤10ppm; the argon blowing time of the molten steel is ≥8min, and the molten steel calming time is 25-30min; electromagnetic stirring and soft reduction technology are used in the continuous casting process, the electromagnetic stirring current is 400 amperes, the frequency is 5 Hz, and the soft reduction is greater than 4.5mm.

[0029] Furthermore, the heating furnace in the heating process adopts a reducing atmosphere with an air-fuel ratio of (1.8-2.0):1.

[0030] Furthermore, after the steel coil is cooled to room temperature, the surface of the steel plate is wet-sandblasted for descaling. The main processes are uncoiling, straightening, sandblasting for descaling, and coiling. The uncoiling tension of the steel plate is 15-20KN, the straightening elongation is 0.8-1.0%, the travel speed is 25-30m / min, the hardness of the steel sand is HRC>70, the speed of the sandblasting motor is 2300-2500rpm, and the coiling tension is 60-80KN.

[0031] The invention examples of the steel of the present invention are as follows: The specific composition, smelting process, hot rolling process, sandblasting descaling process, and structure and properties of the steel plates of the six embodiments of the present invention are shown in Tables 1-6.

[0032] Table 1 Chemical composition of the examples of the present invention (wt, %)

[0033] Table 2 Smelting process system of the embodiment of the present invention

[0034] Table 3 Hot rolling process system of the embodiment of the present invention

[0035] Table 4 Sandblasting phosphorus removal process system of the embodiment of the present invention

[0036] Table 5 Mechanical properties and hole expansion properties of the embodiments of the present invention

[0037] Table 6 Microstructure, banded structure, inclusions and roughness of the examples of the present invention

[0038] As can be seen from the above, the steel plate of the present invention has a good strength-plasticity-hole expansion performance match through reasonable component design and production process, and the hole expansion rate can reach more than 90%, which meets the manufacturing requirements of complex automotive parts; the tensile strength of the steel plate is 600-640MPa, the yield strength is 450-480MPa, the yield strength ratio is 0.70-0.80, and the elongation A 80 ≥27.0%, hole expansion rate ≥90%; steel plate thickness is 3-6mm, surface roughness Ra is 2.0-2.5μm.

[0039] In order to describe the present invention, the present invention has been appropriately and fully illustrated through the examples above. The above implementation modes are only used to illustrate the present invention, and are not intended to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made should be included in the scope of protection of the present invention, and the scope of protection of the present invention should be defined by the claims.

Claims

1. A 600MPa grade high surface quality hot rolled TRIP steel with excellent hole expansion performance, characterized in that: Calculated by weight percentage, it includes the following components: C: 0.14%-0.16%, Si: 0.05%-0.10%, Mn: 0.8%-1.0%, P: ≤0.008%, S: ≤0.003%, Al: 0.02%-0.04%, Cr: 0.15%-0.30%, Nb: 0.015%-0.025%, and the balance is Fe and unavoidable impurities.

2. The 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 1, characterized in that: The Si+Cr: 0.22%-0.40%.

3. The 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 1, characterized in that: The steel structure consists of three phases: ferrite, bainite and retained austenite, wherein the volume fraction of ferrite is 30%-40%, the volume fraction of bainite is 45%-60%, and the volume fraction of retained austenite is 10%-15%.

4. The 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 1, characterized in that: The steel microstructure has a banded structure grade of ≤1.5, and a non-metallic inclusion grade of ≤1.

0.

5. The 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 1, characterized in that: The steel has a tensile strength of 600-640 MPa, a yield strength of 450-480 MPa, a yield strength ratio of 0.70-0.80, and an elongation of A 80 ≥27.0%, hole expansion rate ≥90%; steel plate thickness is 3-6mm, surface roughness Ra is 2.0-2.5μm.

6. A method for manufacturing a 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 1, comprising the steps of smelting, heating, rolling, cooling, coiling, and wet sandblasting descaling, wherein: Heating process: Heat the slab to 1250-1260°C with a soaking time of 50-70 minutes; Rolling and cooling process: 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.

7. The method for manufacturing a 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 6, characterized in that: The smelting adopts the RH+LF dual process, and after LF refining of the molten steel, Si-Ca wire feeding treatment is carried out, with a wire feeding speed of ≥3.5 m / s and a wire feeding amount of 1.6-2.0 m / t; the RH treatment target is: [H]≤1.5 ppm, [O]≤10 ppm; the argon blowing time of the molten steel is ≥8 minutes, and the molten steel calming time is 25-30 minutes; electromagnetic stirring and soft reduction technology are used in the continuous casting process, and the soft reduction amount is greater than 4.5 mm.

8. The method for manufacturing a 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 6, characterized in that: The heating process adopts a reducing atmosphere in the heating furnace, and the air-fuel ratio is (1.8-2.0):

1.

9. The method for manufacturing a 600 MPa grade high surface quality hot-rolled TRIP steel with excellent hole expansion performance according to claim 6, characterized in that: After the steel is cooled to room temperature, the surface of the steel plate is subjected to wet sandblasting for descaling. The main processes include uncoiling, straightening, sandblasting for descaling, and coiling. The uncoiling tension of the steel plate is 15-20KN, the tensile elongation is 0.8-1.0%, the travel speed is 25-30m / min, the hardness of the steel sand is HRC>70, the speed of the sandblasting motor is 2300-2500rpm, and the coiling tension is 60-80KN.

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