440mpa grade al-si coated automotive steel and method for manufacturing the same

Abstract

The present application relates to a kind of 440MPa grade aluminized silicon coating automobile steel and its preparation method, including steelmaking continuous casting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon and finishing;The method of continuous annealing aluminized silicon includes:1) annealing isothermal temperature is 810~880 ℃, and annealing time is 40~100s.2) the slow cooling rate is 2~10 ℃ / s, and slow cooling temperature is 650~720 ℃.3) aluminized silicon, after slow cooling, isothermal 5~20s at 650~720 ℃ after aluminized silicon, and the temperature of aluminized silicon plating solution is 650~720 ℃, and the difference with steel plate slow cooling isothermal temperature is ≤10 ℃:steel plate is after isothermal at slow cooling temperature, into aluminized silicon plating solution, obtains surface coating.4) steel plate is reduced to room temperature with fast cooling rate ≥20 ℃ / s.The present application realizes a kind of 440MPa grade new energy automobile outer plate steel with excellent mechanical performance, high dent resistance and high surface quality by low-cost component design and the process design of combination of continuous annealing and aluminized silicon.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of automobile steel manufacturing, and particularly relates to a 440MPa-grade aluminized silicon-coated automobile steel and a preparation method thereof. BACKGROUND

[0002] The rapid development of the automobile industry brings convenience to people, but also brings social problems such as energy, environment and safety. Therefore, the development direction of modern automobiles is energy saving, emission reduction, emission reduction and safety performance improvement. In order to achieve the above goals, the automobile industry puts forward the solution of automobile light weight, and the appropriate material selection is one of the important ways to achieve light weight. At present, steel materials are still the main materials of automobiles, accounting for more than 60% of the total vehicle usage, and among them, the steel usage of automobile outer plate accounts for the largest proportion. Therefore, considering the requirements of light weight and performance improvement, the strength grade of automobile outer plate steel is also continuously improved, and high-strength interstitial-free steel (IF) is applied.

[0003] High-strength IF steel is developed on the basis of traditional IF steel, which has excellent deep drawing performance due to its composition characteristics and internal structure without interstitial atoms, and also has high strength and non-aging characteristics. The 440MPa-grade high-strength IF steel is the most important product in the high-strength series of IF steel, is the main steel for automobile outer plate, and is also one of the front-line products in the field of automobile steel. With the continuous advancement of automobile light weight process in various countries, it is necessary to develop high-strength automobile outer plate products for the application of steel materials in the field of automobile industry. SUMMARY

[0004] The application provides a 440MPa-grade aluminized silicon-coated automobile steel and a preparation method thereof, which realizes a 440MPa-grade new energy automobile outer plate steel with excellent mechanical properties, high dent resistance and high surface quality through low-cost component design and combined process design of continuous annealing and aluminized silicon coating.

[0005] In order to achieve the above purpose, the application adopts the following technical scheme:

[0006] The 440MPa-grade aluminized silicon-coated automobile steel has the following chemical components in the substrate: C 0.005% to 0.010%, Mn 0.7% to 1.5%, Cu 0.3% to 0.7%, P 0.04% to 0.08%, Si 0.03% to 0.06%, Nb 0.06% to 0.13%, N≤0.003%, Al 0.02% to 0.05%, S≤0.006%, and the balance is Fe and inevitable impurities.

[0007] The steel sheet has a yield strength of 260-360 MPa, a tensile strength of 440 MPa or more, an elongation of 34% or more, a plastic strain ratio r of 1.9 or more, and a strain hardening index n of 0.2 or more.

[0008] The steel sheet has a yield strength of 260-360 MPa, a tensile strength of 440 MPa or more, an elongation of 34% or more, a plastic strain ratio r of 1.9 or more, and a strain hardening index n of 0.2 or more.

[0009] The reasons for the alloy design of the present application are as follows:

[0010] C: C is an important element in the present application. C element as a gap solid solution atom in steel, has a pinning effect on dislocation, which seriously limits the improvement of the forming performance of the steel sheet. However, if the content of C is too low, it is easy to cause steelmaking difficulty, greatly increase the cost of steelmaking. In addition, appropriate amount of C element in steel can be fixed with micro-alloying elements such as Ti or Nb, which can improve the strength of the steel in the form of product precipitation strengthening without increasing too much cost and having no significant impact on plasticity. Therefore, the content of C element is required to be 0.005%-0.010% in the present application.

[0011] Mn: Mn element achieves solid solution strengthening in the form of substitutional atom. This element is the main element for obtaining 440 MPa strength of the automobile outer plate in the present application. However, excessive Mn addition is easy to cause segregation in the steel, and MnS inclusions are easy to produce, which seriously damages the forming performance of the steel sheet. Therefore, the content of Mn is required to be 0.7%-1.5% in the present application.

[0012] Cu: Cu element is the main element in the present application. Cu exists in the form of Cu-rich phase in steel, and Cu atoms are segregated in the matrix phase to form Cu-rich phase during aging stage, and the size of the precipitates is usually nanoscale, which does not combine with C, N and other elements, and plays a role in precipitation strengthening. The limited Cu content can make the Cu-rich phase in the steel sheet be dispersed and fine, and excessive Cu content will make the Cu-rich phase be coarse, and insufficient Cu content will affect the precipitation amount of Cu-rich phase, which is not conducive to the strength of the steel sheet. Therefore, the content of Cu is required to be 0.3%-0.7% in the present application.

[0013] P: P element is one of the important elements in the present application, which is also a solid solution strengthening element in steel. The addition of P can significantly improve the strength of the steel sheet, but this element is easy to precipitate at the grain boundary to make the grain boundary brittle, thereby affecting the plasticity and forming performance of the steel sheet. The present application comprehensively considers the addition amount of Mn and P, and requires the content of P element to be 0.04%-0.08%.

[0014] Si: Si element is also a solid solution strengthening element in the present application, and appropriate amount of addition can improve the strength of the steel sheet, but excessive Si element affects the surface quality and plasticity of the steel sheet. Therefore, based on the comprehensive consideration of solid solution strengthening elements, the content of Si is required to be 0.03%-0.06% in the present application.

[0015] Nb: Nb element is an important element in the present application. Nb and interstitial atoms C fix C atoms in the form of strain-induced precipitation in the hot rolling stage, and plays a role in refining the grain. At the same time, part of the Nb atoms are dissolved in the matrix phase to refine the grain by solute atom drag, thereby promoting the forming property and strength of the steel plate. Considering the influence of C atoms and solute atom drag in the steel on the microstructure and properties, the present application requires the content of Nb to be 0.06% to 0.13%.

[0016] N: N is an impurity element in the steel. The element exists in the form of interstitial solid solution atoms in the matrix phase, which deteriorates the plasticity and toughness of the steel plate. Therefore, the lower the content is, the better. The present application requires the content of N element to be ≤0.003%.

[0017] Al: Al element is mainly used as a deoxidizer, and at the same time, it combines with N atoms to form AlN to remove interstitial atoms N, which helps to improve the forming property of the steel plate. However, excessive addition of the element increases the cost and the difficulty of steelmaking. Therefore, the present application requires the content of Al element to be 0.02% to 0.05%.

[0018] S: S is an impurity element in the steel, which is easy to form inclusions with elements such as Mn, which is not conducive to the forming property of the steel plate. Therefore, the present application requires the content of S element to be ≤0.006%.

[0019] The preparation method of the 440MPa grade aluminized silicon-coated automobile steel includes the processes of steelmaking and continuous casting, hot rolling, pickling, cold rolling, continuous annealing and aluminizing, and finishing.

[0020] The steelmaking and continuous casting: smelting is carried out through a converter and external refining to obtain an alloy with the composition required by the present application, and then the alloy is cast into a slab.

[0021] The hot rolling process includes the following methods:

[0022] 1) The heating temperature is between 1200 to 1300℃, and the furnace time is ≥120min, which can ensure the uniform distribution of alloy elements in the steel plate, thereby being beneficial to the uniformity of the final microstructure.

[0023] 2) The opening rolling temperature is between 1100 to 1150℃, and the final rolling temperature is required to be ≥910℃, which ensures the recrystallization interval rolling, and is beneficial to the dynamic recrystallization behavior of austenite grains.

[0024] 3) The coiling temperature is controlled between 630 to 700℃, which makes NbC precipitate completely fix the interstitial atoms C, and at the same time is beneficial to pinning the grain boundary and improving the deformation energy storage in the subsequent cold rolling.

[0025] Pickling: removing the surface iron oxide scale formed in the hot rolling stage of the steel plate to improve the surface quality of the steel plate.

[0026] The cold rolling requires a reduction of 75% to 85%. The purpose of ensuring a reduction of 75% or more is to make the steel sheet reach the target thickness of the steel sheet and ensure the fibrous structure of the cold rolling to have high deformation energy storage, thereby providing power for the recrystallization of ferrite in the subsequent continuous annealing stage. However, too large a reduction will result in too large a deformation resistance, which will be difficult to achieve the target thickness, and thus the reduction rate should not be greater than 85%.

[0027] The continuous annealing and aluminum-silicon plating process comprises the following methods:

[0028] 1) The annealing isotherm temperature is 810 to 880℃, and the annealing time is 40 to 100s; the temperature and time in the limited range are to ensure that the cold rolling deformation structure of the steel sheet effectively completes recrystallization and the ferrite grain size is suitable and uniform, and at the same time, the <111> / / ND oriented grains are nucleated and grown in this process, thereby forming a strong <111> / / ND texture.

[0029] 2) The slow cooling rate is 2 to 10℃ / s, the slow cooling temperature is 650 to 720℃, and the slow cooling isotherm time is 5 to 20s. The slow cooling stage can ensure that the NbC that is dissolved can be effectively precipitated, and the low solid solution carbon in the steel can be ensured, thereby ensuring the final performance of the steel sheet; at the same time, in this stage, the Cu-rich phase is precipitated in the steel sheet, and the temperature and time can ensure that the Cu-rich phase is dispersedly distributed in the matrix, which promotes the strength.

[0030] 3) Aluminum-silicon plating: after the slow cooling isotherm, the steel sheet enters the plating liquid, the aluminum-silicon plating liquid temperature is 650 to 720℃, and the temperature difference between the aluminum-silicon plating liquid and the slow cooling temperature of the steel sheet is ≤10℃, and the aluminum-silicon plating time is 3 to 10s. The limited temperature and time can effectively ensure the flowability of the aluminum-silicon plating liquid, enhance the adhesion of the plating layer, and make the surface of the plating layer smooth. At the same time, the small temperature difference between the aluminum-silicon plating and the slow cooling stage can also ensure the precipitation of NbC and Cu-rich phase in the steel sheet, reduce the content of solid solution C in the steel, and the precipitated phase plays a precipitation strengthening role to ensure the strength of the steel sheet. In addition, if the temperature of the steel sheet is too high, it will cause the plating liquid temperature to rise, which will increase the Fe solid solubility in the plating liquid and increase the Fe content; when the temperature of the steel sheet is relatively low, it will reduce the plating liquid temperature, which will affect the flowability, and at the same time, it will reduce the Fe solid solubility in the plating liquid and form a large number of Fe-Al alloy particles, thereby affecting the plating quality of the steel sheet.

[0031] 4) After the aluminum-silicon plating is completed, the steel sheet is cooled to room temperature at a fast cooling rate of ≥20℃ / s. The rate can control the thickness of the plating layer, and make the steel sheet structure uniform, and at the same time, the steel sheet surface has no obvious crystal flowers.

[0032] The finishing process requires a finishing elongation of 0.5% to 1.5% to adjust the shape of the steel sheet.

[0033] The aluminum-silicon plating solution composition is as follows in mass percentage: Si 7% to 12%, Fe ≤ 2%, and the rest is Al and inevitable impurity elements. The Si element in the limited amount can improve the fluidity of the plating solution and reduce the plating temperature, and the Si can inhibit the Fe-Al reaction to form an alloy phase, effectively reducing the thickness of the alloy layer, thus being beneficial to the forming performance of the plated layer. The Fe in the plating solution is an inevitable impurity element, which is generated due to the diffusion of the Fe element into the plating solution as the steel plate enters the plating solution. Too much Fe content increases the viscosity of the plating solution, affects the thickness of the plated layer, and also reacts with the Al element to form alloy particles suspended in the plating solution, causing the steel plate to be under-plated and other phenomena, affecting the quality of the plated layer.

[0034] The final microstructure of the 440MPa aluminum-silicon plated automobile outer plate of the present application is composed of ferrite, NbC precipitated phase and Cu-rich phase, the steel plate texture is strong <111> / ND γ texture, and the surface of the steel plate is an aluminum-silicon plated layer. The present application adopts a new type of hot coating process combined with a heat treatment process, and the obtained aluminum-silicon plated automobile outer plate has a yield strength of 260-360MPa, a tensile strength of ≥440MPa, an elongation after fracture of ≥34%, a plastic strain ratio r of ≥1.9, and a strain hardening index n of ≥0.2.

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] 1) The steel plate of the present application mainly uses Mn, Cu, P and a small amount of Nb as main elements, and the content of C is 0.005% to 0.010%, which is beneficial to the production of the steel plate, especially the control of C in the steelmaking end.

[0037] 2) The present application uses an aluminum-silicon plating method to obtain a plated layer structure of the steel plate, and combines the aluminum-silicon plating process with the slow cooling section of the heat treatment process, which reduces the high-temperature overaging process compared with the commonly used hot galvanizing, ensuring high efficiency while making the steel plate have excellent surface quality.

[0038] 3) The present application realizes excellent formability and strength of the steel plate through low-cost alloy design and new process design. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical scheme and advantages of the present application more clear and explicit, the specific embodiments of the present application are further described below, and the following embodiments are used to specifically describe the present application, which are only general description of the present application and do not limit the present application.

[0040] The chemical composition of the example steels is listed in Table 1, the process parameters of hot rolling and cold rolling of the example steels are listed in Table 2, the process parameters of continuous annealing and aluminizing of the example steels are listed in Table 3, and the mechanical properties of the example steels are given in Table 4.

[0041] Table 1 Chemical composition of example steels, wt%

[0042] No. C Mn Cu P Si Nb N Al S 1 0.0071 1.08 0.43 0.045 0.037 0.065 0.0015 0.035 0.0030 2 0.0063 0.92 0.35 0.056 0.032 0.072 0.0012 0.032 0.0026 3 0.0082 0.87 0.51 0.061 0.045 0.063 0.0014 0.042 0.0035 4 0.0078 0.79 0.67 0.052 0.046 0.080 0.0010 0.027 0.0031 5 0.0068 0.95 0.41 0.063 0.051 0.090 0.0009 0.030 0.0026 6 0.0091 1.10 0.56 0.058 0.048 0.095 0.0010 0.040 0.0020 7 0.0085 1.02 0.31 0.067 0.035 0.082 0.0016 0.029 0.0017 8 0.0074 0.97 0.36 0.070 0.049 0.074 0.0013 0.045 0.0032

[0043] Table 2 Hot rolling and cold rolling process of example steels

[0044]

[0045] Table 3 Annealing and aluminizing process of example steels

[0046]

[0047] Table 4 Mechanical properties of example steels

[0048] Example [R p0.2 / Mpa]]> [R m / Mpa]]> A 50 / %]] Plastic strain ratio r Strain hardening index n Coating surface 1 275 450 35 2.0 0.21 No crystal flower 2 262 465 34 2.0 0.22 No crystal flower 3 269 457 36 2.2 0.20 No crystal flower 4 272 448 37 2.1 0.21 No crystal flower 5 273 457 36 2.3 0.23 No crystal flower 6 267 452 35 2.2 0.2 No crystal flower 7 272 449 36 2.1 0.24 No crystal flower 8 263 460 38 2.2 0.23 No crystal flower

[0049] From the above examples, it can be seen that the solid solution strengthening with elements such as Mn and P, and the precipitation strengthening with Cu-rich phase, combined with the interstitial atom fixation and fine-grain strengthening with Nb element, and the innovative matching method of aluminizing process and the continuous annealing and slow cooling stage in heat treatment, prepared the aluminized silicon automotive outer plate steel with excellent mechanical properties and high surface quality, the yield strength of which is 260-360 MPa, the tensile strength is ≥440 MPa, the elongation after fracture is more than 34%, the plastic strain ratio r is more than 1.9, and the strain hardening index n is more than 0.2.

Claims

1. 440MPa grade aluminum-silicon coated automotive steel, characterized in that, The chemical composition of the substrate, by weight percentage, is as follows: C 0.005%–0.010%, Mn 0.7%–1.5%, Cu 0.3%–0.43%, P 0.04%–0.08%, Si 0.03%–0.06%, Nb 0.06%–0.074%, N≤0.003%, Al 0.02%–0.05%, S≤0.006%, with the balance being Fe and unavoidable impurities; The steel plate has a yield strength of 260-360 MPa, a tensile strength of ≥457 MPa, an elongation after fracture of ≥34%, a plastic strain ratio r of ≥2.2, and a strain hardening index n of ≥0.

2.

2. The 440MPa grade aluminized silicon coated automotive steel according to claim 1, characterized in that, The microstructure of the steel plate includes ferrite, NbC precipitates, and Cu-rich phases.

3. The method for preparing 440MPa grade aluminized silicon coated automotive steel as described in claim 1 or 2, characterized in that, The process includes continuous casting, hot rolling, pickling, cold rolling, continuous annealing with aluminized silicon plating, and finishing. The continuous annealing with aluminized silicon plating process includes the following methods: 1) The isothermal annealing temperature is 810–880℃, and the annealing time is 40–100s; 2) The slow cooling rate is 2-10℃ / s, the slow cooling temperature is 650-720℃, and the slow cooling isothermal time is 5-20s; 3) Aluminum-silicon plating: After slow cooling and isothermal treatment, aluminum-silicon plating is performed to obtain the coating. The temperature of the aluminum-silicon plating bath is 650-720℃, the plating time is 3-10s, and the temperature difference between the plating bath and the slow cooling temperature of the steel plate is ≤10℃. 4) After the aluminized silicon coating is completed, the steel plate is cooled to room temperature at a rapid cooling rate of ≥20 ℃ / s.

4. The method for preparing 440MPa grade aluminized silicon coated automotive steel according to claim 3, characterized in that, The hot rolling process includes the following methods: 1) Heating temperature between 1200 and 1300℃, furnace time ≥ 120 min; 2) The initial rolling temperature should be between 1100 and 1150℃, and the final rolling temperature should be ≥910℃; 3) The winding temperature should be controlled between 630 and 700℃.

5. The method for preparing 440MPa grade aluminized silicon coated automotive steel according to claim 3, characterized in that, The cold rolling process described herein has a cold rolling reduction rate of 75% to 85%.

6. The method for preparing 440MPa grade aluminized silicon coated automotive steel according to claim 3, characterized in that, The finishing process described herein has a finishing elongation rate of 0.5% to 1.5%.

7. The method for preparing 440MPa grade aluminized silicon coated automotive steel according to claim 3, characterized in that, The composition of the aluminum-silicon plating solution, by mass percentage, is: Si 7%–12%, Fe ≤2%, with the remainder being Al and unavoidable impurity elements.

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