A 1200mpa grade dh steel sheet plated with al-si and a method for manufacturing the same

Abstract

The present application relates to a kind of DH steel plate of 1200MPa grade of aluminum silicon coating and its manufacturing method, DH steel plate includes steel base plate and coating;The chemical composition of steel base plate is C:0.12%~0.23%, Si:0.2%~0.9%, Mn:1.8%~2.8%, Cr:0.2%~0.6%, Mo:0.05%~0.4%, P≤0.02%, S≤0.005%, Ti≤0.03%, Nb≤0.03%, the balance is Fe and impurity.It includes smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminum silicon plating and finishing sequence in manufacturing method.The finished steel plate has excellent mechanical properties and surface quality by low-cost alloy design combined with reasonable process control;In the slow cooling stage, hot-dip aluminum silicon is carried out, which is matched with heat treatment system, breaks through the limitation that hot-dip galvanized steel plate must be high-temperature short-time overaging, makes bainite transformation sufficient, effectively improves the plasticity of DH steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of automobile steel manufacturing, in particular to a 1200MPa grade DH steel plate with aluminum-silicon plating layer and a manufacturing method thereof. BACKGROUND

[0002] The fuel consumption of a car is closely related to the weight of the car body. Research shows that if the weight of a car is reduced by 10%, the fuel consumption will be reduced by 6%-10% and the emissions will be reduced by 4%. In view of this problem, the International Iron and Steel Institute organized a research project on ultra-lightweight steel car bodies. After completing the project, a plan called Advanced Concept Car Ultra-Lightweight Steel Body was carried out, which mainly involves the development and application of advanced high-strength steel. According to the fifth edition of the Advanced High-Strength Steel Application Guide published by the International Iron and Steel Institute, advanced high-strength steel generally refers to high-strength steel with a yield strength exceeding 550 MPa. Steel with a tensile strength exceeding 780 MPa is sometimes referred to as ultra-high-strength steel.

[0003] Dual-phase steel is a typical representative of advanced high-strength steel, and its application is also the most extensive. Plasticized dual-phase (DH) steel is a plasticity upgrade version of dual-phase steel, which can significantly improve the plasticity and formability of the steel plate by introducing a small amount of residual austenite based on the ferrite and martensite structure of dual-phase steel, and has a wide application prospect. The strength level of the DH steel currently used in industrial production is mainly below 800 MPa, and there are few reports on higher strength levels such as 1200 MPa DH steel. Among them, only a small part of the multiphase steel plates meet the high surface quality requirements.

[0004] Currently, high-surface-quality DH steel is mainly achieved through hot-dip galvanizing on the surface of the steel plate. However, with the decreasing zinc resources and increasing zinc prices, there is an urgent need for a new hot-dip method that matches the heat treatment system to meet the future market demand for multiphase steel with good surface quality and mechanical properties.

[0005] Chinese patent application with publication number CN113403550A discloses "a high-plasticity fatigue-resistant cold-rolled hot-dip galvanized DH1180 steel plate and a preparation method thereof". In its composition system, 0.5-1.5% Al element and 0.1-0.7% Cu element are added, the fast cooling temperature is between 450-470℃, and then hot-dip galvanizing is performed on the surface of the steel plate.

[0006] Chinese patent application with publication number CN112795852A discloses "a 1200MPa grade high-expansion cold-rolled galvanized steel strip and a production method thereof". In its composition system, 0.002-0.003% B element is added, the annealing temperature is 760-840℃, the fast cooling temperature is 440-460℃, and the galvanizing temperature is 458-462℃. The final mechanical properties of the steel plate are: tensile strength ≥1200MPa, yield strength 800-1000MPa, and elongation ≥5%.

[0007] The above two technical solutions are mainly aimed at the production and heat treatment process of hot-dip galvanized DH steel, and there is a significant difference in the heat treatment process, and both of them do not involve hot-dip aluminum-silicon on the surface of the steel plate. SUMMARY

[0008] The application provides a 1200MPa grade DH steel plate with aluminum-silicon coating and a manufacturing method thereof, which combines low-cost alloy design with reasonable process control, so that the finished steel plate has excellent mechanical properties and surface quality; hot-dip aluminum-silicon is carried out in the slow cooling stage, which is matched with the heat treatment system, breaks through the limitation that the hot-dip galvanized steel plate must be subjected to high-temperature short-time overaging, fully transforms the bainite, and effectively improves the plasticity of the DH steel.

[0009] In order to achieve the above purpose, the application adopts the following technical solutions:

[0010] A 1200MPa grade DH steel plate with aluminum-silicon coating, comprising a steel base plate and a coating layer; the chemical composition of the steel base plate, in terms of mass percentage, is C: 0.12%-0.23%, Si: 0.2%-0.9%, Mn: 1.8%-2.8%, Cr: 0.2%-0.6%, Mo: 0.05%-0.4%, P≤0.02%, S≤0.005%, Ti≤0.03%, Nb≤0.03%, and the balance is Fe and unavoidable impurities.

[0011] Further, the microstructure of the steel plate includes ferrite, martensite, bainite and residual austenite, and the ferrite content is 10%-40%, the martensite content is 30-60%, the bainite content is 20%-50%, and the residual austenite content is 3%-8% by plane area method.

[0012] Further, the tensile strength of the finished steel plate is ≥1200MPa, the yield strength is 800-1000MPa, and the elongation is ≥8%.

[0013] Further, the chemical composition of the coating layer, in terms of mass percentage, is Al: 88%-93%, Si: 7%-12%.

[0014] A manufacturing method of the 1200MPa grade DH steel plate with aluminum-silicon coating, comprising smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminum-silicon plating, overaging and finishing processes, wherein the control process is as follows:

[0015] 1) Hot rolling: the heating temperature is 1200-1250℃, the opening rolling temperature is 1100-1150℃, the final rolling temperature is above 900℃, and the coiling temperature is 560-660℃;

[0016] 2) Cold rolling: the cold rolling reduction rate is 40%-58%.

[0017] 3) continuous annealing: the heating isotherm temperature is 820-880℃, the isothermal time is 80-150s, the slow cooling temperature is 650-700℃, and the slow cooling rate is controlled at 0.5-5℃ / s;

[0018] 4) aluminizing: the steel plate after slow cooling is immersed into an aluminum-silicon liquid to obtain an aluminum-silicon coating after isothermal treatment at 650-700℃ for 5-30s;

[0019] 5) overaging and finishing: the steel plate after aluminizing is cooled to 280-360℃ at a cooling rate of more than 10℃ / s, isothermally treated for 200-650s, and then is cooled to room temperature at a cooling rate of more than 2℃ / s; finally, finishing is performed, and the finishing elongation is controlled at 0.1%-0.6%.

[0020] Further, the thickness of the hot-rolled plate is 2.5-4.0mm.

[0021] Compared with the prior art, the beneficial effects of the present application are:

[0022] (1) the chemical composition of the DH steel plate disclosed by the present application mainly contains C and Mn, and no obvious valuable alloy is contained, and the C content is less than 0.23%, which is beneficial to laser welding and resistance spot welding in the production and application processes;

[0023] (2) the present application realizes high surface quality of the DH steel by adopting hot aluminizing, and the hot aluminizing is ingeniously performed in the slow cooling stage, which is perfectly matched with the heat treatment system, breaks the limitation that the hot galvanized steel plate must be overaged at high temperature for a short time, and enables the bainite transformation to be sufficient, which is beneficial to improving the plasticity of the DH steel;

[0024] (3) the present application realizes good strength-plasticity, formability and high surface quality of the steel plate by low-cost alloy design and ingenious process design. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a typical SEM microstructure photo of the finished steel plate of Example 1 of the present application. DETAILED DESCRIPTION

[0026] The 1200MPa-grade DH steel plate with an aluminum-silicon coating disclosed by the present application comprises a steel base plate and a coating; the chemical composition of the steel base plate, in terms of mass percentage, is C: 0.12%-0.23%, Si: 0.2%-0.9%, Mn: 1.8%-2.8%, Cr: 0.2%-0.6%, Mo: 0.05%-0.4%, P≤0.02%, S≤0.005%, Ti≤0.03%, Nb≤0.03%, and the balance is Fe and inevitable impurities.

[0027] Further, the microstructure of the steel plate comprises ferrite, martensite, bainite and residual austenite, and the content of ferrite is 10% to 40% in the planar area method, the content of martensite is 30 to 60%, the content of bainite is 20% to 50%, and the content of residual austenite is 3% to 8%.

[0028] Further, the finished steel plate has a tensile strength of ≥1200MPa, a yield strength of 800 to 1000MPa, and an elongation of ≥8%.

[0029] Further, the chemical composition of the plating layer is Al: 88% to 93% and Si: 7% to 12% by mass.

[0030] The manufacturing method of the 1200MPa DH steel plate with the aluminum-silicon plating layer comprises the following steps: smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminum-silicon plating, overaging and finishing, wherein the control process is as follows:

[0031] 1) Hot rolling: the heating temperature is 1200 to 1250℃, the opening rolling temperature is 1100 to 1150℃, the final rolling temperature is above 900℃, and the coiling temperature is 560 to 660℃;

[0032] 2) Cold rolling: the cold rolling reduction is 40% to 58%;

[0033] 3) Continuous annealing: the heating isotherm temperature is 820 to 880℃, the isotherm time is 80 to 150s, the slow cooling temperature is 650 to 700℃, and the slow cooling rate is controlled to be 0.5 to 5℃ / s;

[0034] 4) Aluminum-silicon plating: the steel plate after slow cooling is immersed in an aluminum-silicon liquid to obtain an aluminum-silicon plating layer after being kept at 650 to 700℃ for 5 to 30s;

[0035] 5) Overaging and finishing: the steel plate after aluminum-silicon plating is cooled to 280 to 360℃ at a cooling rate of more than 10℃ / s, kept isotherm for 200 to 650s, and then cooled to room temperature at a cooling rate of more than 2℃ / s; finally, finishing is performed, and the finishing elongation is controlled to be 0.1% to 0.6%.

[0036] Further, the thickness of the hot rolled plate is 2.5 to 4.0mm.

[0037] The alloy design reasons of the 1200MPa DH steel plate with the aluminum-silicon plating layer are as follows:

[0038] C: C element is a traditional and economical strengthening element of low carbon steel, and the increase of C content in martensite can improve its work hardening capacity. In addition, during annealing in two-phase region, C atoms can enrich in austenite, which plays a role in stabilizing austenite and is beneficial to the retention of residual austenite. However, too high C content will bring difficulties to smelting and welding. Therefore, the C content in the steel plate is controlled at about 0.18%, and the preferred range is 0.12%-0.23%.

[0039] Si: Si element can inhibit cementite precipitation during overaging stage, which is beneficial to the retention of residual austenite, but too high Si content is not conducive to the surface quality of the steel plate. Therefore, the Si content is controlled at 0.2%-0.9%.

[0040] Mn: Mn is an austenite stabilizing element, which has a significant solid solution strengthening effect on high strength steel, can significantly improve the hardenability of steel, has the effects of solid solution strengthening and refining ferrite grains, and is the main strengthening element besides C. Since the C content is limited to about 0.18%, in order to achieve a strength of 1200 MPa, the Mn content needs to be controlled between 1.8%-2.8%. Too low Mn content will result in insufficient strength of the steel plate, and too high Mn content may cause segregation and reduce the plasticity of the steel plate.

[0041] Cr: Cr can significantly improve the hardenability of steel, has a significant solid solution strengthening effect on high strength steel, can significantly delay the transformation of pearlite and bainite, and improve the strength of the steel; in addition, in order to avoid too high Mn content, Cr can be used to replace part of the Mn element.

[0042] Mo: Mo element can improve the hardenability of the steel plate, and generally has better effect when combined with Cr, and the content is generally controlled at 1 / 3-2 / 3 of the Cr content.

[0043] Ti: Ti can capture free N atoms in steel, which plays a role in fixing N. At the same time, TiN can precipitate during solidification, which plays a role in pinning grain boundaries; Ti(C, N) precipitates during hot rolling, which can play a role in pinning original austenite grain boundaries and refining original austenite grains. At the same time, a small amount of Ti precipitates during continuous annealing, which can play a role in strengthening ferrite and bainite; but too much Ti has limited effect and increases cost, therefore, the Ti content is controlled at ≤0.03% in the present application.

[0044] P: P element is a harmful element in steel, and the lower the content, the better. Considering the cost, the P content is controlled at ≤0.02% in the present application.

[0045] S: S element is a harmful element in steel, and the lower the content, the better. Considering the cost, the S content is controlled at ≤0.005% in the present application.

[0046] The application discloses a manufacturing method of a 1200MPa-grade DH steel plate with an aluminum-silicon coating, and belongs to the technical field of steel plate manufacturing.

[0047] 1. Smelting: smelting is performed through a converter to obtain alloy components within a limited range.

[0048] 2. Hot rolling:

[0049] ① The heating temperature is between 1200 and 1250 DEG C, Ti atom precipitation behavior is ensured, good N solidification effect is achieved on the steel plate, Ti (C, N) precipitation is ensured, and the original austenite grain boundary is pinned and the original austenite grain is refined.

[0050] ② The rolling temperature is between 1100 and 1150 DEG C, and the final rolling temperature is above 900 DEG C, so that the rolling temperature in the recrystallization zone is ensured, and the dynamic recrystallization behavior of the original austenite grain in the hot rolling stage is promoted.

[0051] ③ The coiling temperature is between 560 and 660 DEG C, so that the difficulty of cold rolling is reduced due to the low coiling temperature. The thickness of the hot-rolled plate is between 2.5 and 4.0 mm.

[0052] 3. Pickling: the pickling is used to remove the iron oxide skin generated on the surface of the hot-rolled steel plate, and the surface quality of the cold-rolled steel plate is ensured.

[0053] 4. Cold rolling: the cold rolling reduction is 40% to 58%; the cold rolling reduction of more than 40% is ensured to promote the fiberization of the structure in the cold rolling configuration; meanwhile, the reduction is controlled to be below 58% to prevent the deformation resistance from being too large due to the high cold rolling reduction, and the target thickness is difficult to be rolled.

[0054] 5. Continuous annealing:

[0055] The isothermal heating temperature is 820 to 880 DEG C, the isothermal time is 80 to 150 s, the slow cooling temperature is 650 to 700 DEG C, and the slow cooling speed is controlled to be 0.5 to 5 DEG C / s.

[0056] 6. Aluminum-silicon plating: after the slow cooling of the steel plate, the steel plate is kept at 650 to 700 DEG C for 5 to 30 s, and then is immersed into an aluminum-silicon liquid to obtain a coating on the surface of the steel plate.

[0057] 7. Overaging and finishing: the steel plate after the aluminum-silicon plating is cooled to 280 to 360 DEG C at a cooling speed of more than 10 DEG C / s, is kept at the temperature for 200 to 650 s, and then is cooled to room temperature at a cooling speed of more than 2 DEG C / s; finally, the steel plate is subjected to plate shape adjustment in a finishing machine, and the finishing elongation is controlled to be 0.1% to 0.6%.

[0058] The mechanism of the process is that: firstly, annealing in austenite zone or two-phase zone to obtain appropriate proportion of ferrite and original austenite; then, slow cooling to 650-700 DEG C to perform hot aluminum-silicon plating, and then cooling to 280-360 DEG C to obtain appropriate amount of martensite / bainite structure. Since the hot aluminum-silicon plating is in the slow cooling stage, the time of overaging stage can be flexibly adjusted according to the phase change, which breaks the limitation of high temperature and short time overaging of conventional hot galvanized steel plate, and enables the bainite transformation to be sufficient, which is beneficial to improve the plasticity of DH steel with high surface quality.

[0059] The DH steel plate has a final structure of ferrite+bainite+martensite+residual austenite, and the surface of the steel plate has an aluminum-silicon plating layer. The novel hot plating process and the heat treatment process are matched to obtain the DH steel plate with tensile strength of 1200 MPa or more, yield strength of 800-1000 MPa, and elongation of more than 8%, which has good strength and plasticity and high surface quality.

[0060] The following examples are implemented on the premise of the technical scheme of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0061]

EXAMPLE

[0062] The chemical composition of the steel in each example is listed in Table 1, and the balance is Fe and impurities; the hot rolling process parameters of the steel in each example are listed in Table 2, the continuous annealing, aluminum-silicon plating and finishing process parameters of the steel in each example are listed in Table 3, and the mechanical properties of the finished steel plate in each example are listed in Table 4. Figure 1 is a typical SEM microstructure photograph of Example 1.

[0063] Table 1 Chemical composition of steel, wt%

[0064]

[0065] Table 2 Hot rolling process parameters of steel

[0066]

[0067] Table 3 Continuous annealing, aluminum-silicon plating, overaging and finishing process parameters of steel

[0068]

[0069] Table 4 Mechanical properties of finished steel plate

[0070]

[0071] From the above examples, the hot-dip aluminized silicon steel plate produced by the low-cost alloy design and the ingenious process design has the tensile strength of 1200MPa or more, the yield strength of 800-1000MPa, and the elongation of more than 8%, realizing the good strength and plasticity, formability and high surface quality of the steel plate.

[0072] The above merely provides the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art, according to the technical solution and the inventive concept of the present application, can make equivalent replacements or changes within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for manufacturing a 1200MPa grade DH steel plate with an aluminum-silicon coating, characterized in that, DH steel plate comprises a steel substrate and a coating; the chemical composition of the steel substrate, by mass percentage, is C: 0.12%–0.23%, Si: 0.2%–0.9%, Mn: 1.8%–2.8%, Cr: 0.2%–0.6%, Mo: 0.05%–0.4%, P≤0.02%, S≤0.005%, Ti≤0.03%, Nb≤0.03%, with the balance being Fe and unavoidable impurities; The manufacturing method of the 1200MPa grade DH steel plate with aluminum-silicon coating includes smelting, hot rolling, pickling, cold rolling, continuous annealing, aluminum-silicon coating, over-aging, and finishing processes, wherein the process control is as follows: 1) Hot rolling: Heating temperature is 1200~1250℃, initial rolling temperature is 1100~1150℃, final rolling temperature is above 900℃, and coiling temperature is 560~660℃. 2) Cold rolling: The cold rolling reduction rate is 40%–58%; 3) Continuous annealing: The heating isothermal temperature is 820-880℃, the isothermal time is 80-150s, the slow cooling temperature is 650-700℃, and the slow cooling rate is controlled at 0.5-5℃ / s; 4) Aluminosilicate coating: After slow cooling, the steel plate is isothermally heated at 650-700℃ for 5-30 seconds and then immersed in aluminum-silicon liquid to obtain an aluminum-silicon coating. 5) Aging and finishing: The aluminized silicon steel sheet is cooled to 280-360°C at a cooling rate of more than 10°C / s, with an isothermal time of 200-650s, and then cooled to room temperature at a cooling rate of more than 2°C / s; finally, it is finished, and the finishing elongation is controlled at 0.1%-0.6%.

2. The method for manufacturing a 1200MPa grade DH steel plate with an aluminum-silicon coating according to claim 1, characterized in that, The microstructure of the steel plate includes ferrite, martensite, bainite and retained austenite. According to the planar area method, the ferrite content is 10% to 40%, the martensite content is 30% to 60%, the bainite content is 20% to 50%, and the retained austenite content is 3% to 8%.

3. The method for manufacturing a 1200MPa grade DH steel plate with an aluminum-silicon coating according to claim 1, characterized in that, The finished steel plate has a tensile strength ≥1200MPa, a yield strength of 800~1000MPa, and an elongation ≥8%.

4. The method for manufacturing a 1200MPa grade DH steel plate with an aluminum-silicon coating according to claim 1, characterized in that, The chemical composition of the coating, by mass percentage, is Al: 88%–93% and Si: 7%–12%.

5. The method for manufacturing a 1200MPa grade DH steel plate with an aluminum-silicon coating according to claim 1, characterized in that, The thickness of hot-rolled plates is 2.5 to 4.0 mm.

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