340mpa grade al-si coated automotive steel and method for manufacturing the same

Abstract

The present application relates to a kind of 340MPa grade aluminized silicon coating automobile steel and its preparation method, including smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon, finishing process;The continuous annealing aluminized silicon process includes the following methods: 1) annealing isothermal temperature is 830~870 ℃, annealing time is 30~60s.2) slow cooling rate is 2~10 ℃ / s, slow cooling temperature is 600~720 ℃, isothermal time 10~20s.3) aluminized silicon: steel plate is isothermal after slow cooling temperature, into aluminized silicon plating solution, requires aluminized silicon plating solution and steel plate slow cooling temperature difference ≦10 ℃, aluminized silicon time 3~10s.4) after aluminized silicon is completed, steel plate continues fast cooling rate ≧20 ℃ / s and falls to room temperature.The present application is combined by reasonable component design and innovative continuous annealing and aluminized silicon process, and the preparation of a kind of 340MPa grade automobile outer plate steel with excellent mechanical properties and high surface quality is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of automobile steel manufacturing, and particularly relates to a 340MPa-grade aluminized silicon-coated automobile steel and a preparation method thereof. BACKGROUND

[0002] The automobile industry is an important part of modern industry and plays an important role in China's economic development. At the same time, automobiles are the main application of steel materials, with the use of steel materials accounting for more than 60%. At present, the annual sales of automobiles are more than 26 million, and with the vigorous development of new energy vehicles in China, the automobile industry will continue to have an important impact on China's steel industry. Among automobile steels, automobile body outer plate steel is one of the most used steel materials, which is mainly used to manufacture automobile front, rear, left and right door outer plates, engine cover outer plates, trunk lid outer plates and other parts. The automobile outer plate steel should have excellent formability, corrosion resistance and dent resistance, and good weldability. Therefore, the automobile outer plate generally uses coated steel plate, and in order to improve the dent resistance, the current automobile outer plate uses bake hardening (BH) steel plate and high-strength interstitial-free (IF) steel, etc. BH steel uses interstitial solid solution atoms C to obtain part of the strength increase (about 30MPa) after baking, but at the same time, it also has natural aging, which brings adverse effects in application. High-strength IF steel is based on ultra-low carbon steel and adds micro-alloying elements to ensure high strength through solid solution, precipitation or fine-grain strengthening, etc. while having good formability. In view of the requirements of automobile outer plate corrosion resistance, etc., the current outer plate surface coating is mainly hot-dip galvanizing and alloyed galvanizing, but with the decrease of zinc resources, the price is increasing, and it is necessary to develop a new type of high-quality coated outer plate steel that matches the high-strength outer plate heat treatment system. SUMMARY

[0003] The present application provides a 340MPa-grade aluminized silicon-coated automobile steel and a preparation method thereof, which realizes a 340MPa-grade automobile outer plate steel with excellent mechanical properties and high surface quality by reasonable design of production process and aluminum-silicon plating solution.

[0004] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0005] The 340MPa-grade aluminized silicon-coated automobile steel has a substrate chemical composition calculated by weight percentage as follows: C 0.003% to 0.006%, Mn 0.3% to 0.5%, P 0.03% to 0.05%, Si 0.01% to 0.03%, Ti 0.02% to 0.05%, Cu 0.2% to 0.4%, N≤0.002%, S≤0.005%, and the balance of Fe and unavoidable impurities.

[0006] The yield strength of the steel sheet is 220-260 MPa, the tensile strength is above 340 MPa, the elongation after fracture is above 36%, the plastic strain ratio r is above 2.0, and the strain hardening index n is above 0.2.

[0007] The steel sheet structure is composed of ferrite and Ti(N,C) precipitates and Cu-rich phase.

[0008] The reasons for the alloy design of the present application are as follows:

[0009] C: C is an important additive element in the present application. When the C content increases, too many carbides and cementite are easily generated in the steel, which seriously affects the elongation, r value and n value of the steel, and the excessive solid solution C content in the finished steel causes serious aging problem of the steel, resulting in rapid decrease of plasticity and thus affecting the stamping and use of the steel sheet. Meanwhile, when the C content is too low, the steelmaking cost is increased, and a certain amount of carbides cannot be ensured to be formed in the steel, thus the precipitation strengthening is weak, and the strength of the steel sheet is too low. Therefore, in order to ensure the comprehensive mechanical properties of the steel sheet, the C content is required to be 0.003%-0.006% in the present application.

[0010] Mn: Mn element is one of the important elements in the present application, which plays a role of solid solution strengthening to improve the strength of the steel sheet. However, when excessive Mn element is added, the plasticity of the steel sheet is reduced, and the formability is affected. Therefore, the Mn content is required to be 0.3%-0.5% in the present application.

[0011] P: P element is also a strengthening element, which improves the strength of the steel sheet in the form of solid solution strengthening. However, excessive P makes the strength of the steel sheet too high, and the element is easy to segregate at the grain boundary, thus reducing the plasticity of the steel sheet and affecting the formability. Therefore, the P content is required to be 0.03%-0.05% in the present application.

[0012] Si: Si element can play a role of solid solution strengthening to ensure the strength of the steel sheet, but excessive Si element easily causes the plasticity and toughness of the steel sheet to be reduced. Therefore, the Si content is required to be 0.01%-0.03% in the present application.

[0013] Ti: Ti element is one of the important elements in the present application. Ti element mainly generates Ti(N,C) precipitates to fix the interstitial atoms N and C in the steel, thus greatly improving the plasticity and formability of the steel sheet. However, excessive Ti element causes FeTiP phase to precipitate in the steel, which is not conducive to the formation of favorable γ texture (<111> / ND) of the steel sheet during recrystallization, and seriously reduces the r value of the steel sheet. Meanwhile, excessive Ti element addition also has the effects of solid solution strengthening, etc., which increases the strength of the steel sheet and reduces the plasticity. Therefore, the Ti content is required to be 0.02%-0.05% in the present application.

[0014] Cu: Cu element is one of the important elements in the present application. Cu element does not react with C, N and other elements in steel, and it is easy to exist in the form of Cu-rich phase in the aging stage of steel plate with Fe, which plays a role in precipitation strengthening. The limited content of Cu element makes the ferrite base in the steel distribute the nano-sized Cu-rich phase, which can ensure the strength of the steel plate, therefore the content of Cu element is required to be 0.2% to 0.4% in the present application.

[0015] N: N is an impurity element in steel, which exists in the form of interstitial solid solution atoms in steel, which will seriously affect the plasticity and toughness of the steel plate, and cause the aging of the steel plate, and is not conducive to the improvement of r value. Therefore, the content of N element is required to be ≤0.002% in the present application.

[0016] S: S is an impurity element in steel, which is easy to react with Mn to form MnS, which makes the performance of the steel plate deteriorate, so the lower the content is, the better. Therefore, the content of S element is required to be ≤0.005% in the present application.

[0017] The preparation method of the 340MPa grade aluminized silicon coated automobile steel comprises smelting, hot rolling, pickling, cold rolling, continuous annealing aluminized silicon, finishing process;

[0018] Smelting: the alloy composition in the above range is obtained by smelting in a converter and refining in an RH refining furnace, and then cast into a casting blank.

[0019] The hot rolling process comprises the following methods:

[0020] 1) The heating temperature is between 1250-1300℃, and the furnace time is ≥120min; the heating temperature is greater than 1250℃, which ensures that the added Ti element in the steel fixes N element, so that TiN is completely precipitated, but the temperature exceeds 1300℃, which is easy to make the austenite grains excessively grow, affecting the final strength of the steel plate. The heating time of ≥120min can ensure the uniform distribution of alloy elements.

[0021] 2) The opening rolling temperature is between 1100-1150℃, and the final rolling temperature is ≥890℃, which ensures that the steel plate is rolled in the austenite single-phase zone to avoid the phenomenon of mixed crystals.

[0022] 3) The coiling temperature is between 650-750℃, which ensures the precipitation of Ti-containing carbide and eliminates the interstitial C atoms in the steel, thereby ensuring the plasticity of the steel plate.

[0023] The thickness of the hot-rolled plate is between 3-4mm.

[0024] Pickling: the scale oxides existing on the surface of the hot-rolled coiled steel are removed by pickling to ensure the surface quality of the cold-rolled plate.

[0025] The cold rolling process has a cold rolling reduction control in the range of 75% to 85%. The cold rolling deformation energy storage provides sufficient driving force for subsequent annealing recrystallization, so that the recrystallization is fully completed and the final microstructure is refined, and the steel sheet has strong γ texture, so that the steel sheet has good plasticity and forming ability. Too high rolling reduction will increase the load of the cold rolling mill, and cannot guarantee the realization of the target thickness.

[0026] The continuous annealing and aluminizing silicon process comprises the following methods:

[0027] 1) The annealing isotherm temperature is 830 to 870℃, and the annealing time is 30 to 60s, so as to ensure that the recrystallization of the cold rolling deformation structure occurs fully, and strong γ texture is formed to obtain good forming performance.

[0028] 2) The slow cooling rate is 2 to 10℃ / s, the slow cooling temperature is 600 to 720℃, and the slow cooling isotherm time is 10 to 20s. The isotherm temperature and time ensure the formation of Cu-rich phase in the steel sheet, and the Cu-rich phase is dispersedly distributed in the matrix phase, which promotes the strength of the steel sheet.

[0029] 3) Aluminizing silicon: after the steel sheet is isothermally cooled at the slow cooling temperature, the temperature of the aluminizing silicon solution is 600 to 720℃, and the temperature difference between the aluminizing silicon solution and the slow cooling temperature of the steel sheet is required to be ≤10℃, and the aluminizing silicon time is 3 to 10s. The limited aluminizing silicon temperature can ensure the fluidity of the aluminizing silicon solution, and the temperature further promotes the generation of Cu-rich phase in the steel, so as to ensure the strength of the steel sheet. Meanwhile, the temperature difference between the aluminizing silicon solution and the slow cooling temperature of the steel sheet in the range can make the temperature of the aluminizing silicon solution not fluctuate obviously, avoiding the generation of Fe-Al alloy particles due to the temperature fluctuation of the solution, and affecting the coating quality of the steel sheet.

[0030] 4) After aluminizing silicon, the steel sheet continues to be cooled to room temperature at a fast cooling rate ≥20℃ / s. The large cooling rate can control the growth of the dendrites in the coating, so as to obtain fine coating grains and improve the quality of the coating.

[0031] Finally, the steel sheet enters the skin pass mill for shape adjustment, and the skin pass elongation is controlled in the range of 0.5% to 1.0%, which can effectively eliminate the yield platform of the material and ensure the surface quality of the steel sheet.

[0032] The composition of the aluminizing silicon solution is as follows in terms of mass percentage: Si 8% to 12%, Fe ≤2%, and the rest is Al. The content of Si can effectively improve the fluidity of the solution, and Si can preferentially react with Fe on the surface of the steel sheet to form Fe-Si phase, which can effectively hinder the generation of brittle Fe-Al phase and improve the adhesion of the coating. Fe in the solution comes from the steel sheet, and when the content of Fe exceeds 2%, Fe is prone to react with Al at the aluminizing silicon temperature range, and the reaction product is in the form of particles suspended in the solution. When the particles adhere to the surface of the steel sheet, it causes the phenomenon of coating leakage, which affects the coating quality of the steel sheet.

[0033] The final microstructure of the steel sheet of the present application is ferrite, Ti(N,C) precipitates and Cu-rich phase. The surface of the steel sheet is Al-Si plated. By the new combined heat treatment method of hot Al-Si plating, the Al-Si plated automotive steel sheet is obtained, which has yield strength of 220-260 MPa, tensile strength of more than 340 MPa, elongation of more than 36%, plastic strain ratio r of more than 2.0 and strain hardening exponent n of more than 0.2, and at the same time, the steel sheet has good mechanical properties and high surface quality without crystal flower.

[0034] Compared with the prior art, the present application has the following advantages:

[0035] 1) The chemical composition of the steel sheet of the present application mainly contains C, Mn, P, Ti and Cu as main elements, without adding expensive alloy elements, so that the alloy cost of the steel sheet is controlled, and at the same time, the production process is reasonably controlled to control the precipitation of precipitates and obtain strong γ texture, so that the steel sheet has excellent strength and plasticity and forming property.

[0036] 2) The present application ingeniously combines hot Al-Si plating with heat treatment of the steel sheet, and places the plating process in the slow cooling stage, so that the mechanical properties of the steel sheet are not affected, and at the same time, the production of the plated steel sheet is realized, which helps to reduce the energy loss in the production process.

[0037] 3) The present application is based on reasonable low-cost alloy composition design, and adopts the new combined method of Al-Si plating process and heat treatment, so that the 340 MPa grade automotive steel sheet has excellent strength, plasticity, formability and high surface quality. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a microstructure diagram of the steel sheet. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical scheme and advantages of the present application more clear, the specific embodiments of the present application will be further described below, and the following examples 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 steel is listed in Table 1, the process parameters of hot rolling and cold rolling of the example steel are listed in Table 2, the process parameters of continuous annealing of the example steel are listed in Table 3, and the mechanical properties of the example steel are given in Table 4.

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

[0042] No. C Mn P Si Ti Cu N S 1 0.0045 0.42 0.039 0.024 0.038 0.23 0.0017 0.0027 2 0.0037 0.37 0.042 0.019 0.040 0.39 0.0014 0.0043 3 0.0052 0.47 0.035 0.028 0.047 0.32 0.0012 0.0036 4 0.0057 0.39 0.048 0.017 0.045 0.28 0.0018 0.0046 5 0.0047 0.48 0.032 0.026 0.039 0.38 0.0015 0.0021 6 0.0036 0.50 0.037 0.015 0.041 0.23 0.0010 0.0027 7 0.0059 0.35 0.049 0.024 0.049 0.29 0.0016 0.0018 8 0.0042 0.43 0.047 0.027 0.048 0.36 0.0009 0.0032

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

[0044]

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

[0046]

[0047] Table 4 Mechanical properties and surface quality of example steels

[0048] Example [R p0.2 / MPa]]> [R m / MPa]]> A 50 / %]]> plastic strain ratio r strain hardening index n surface quality 1 238 345 38 2.4 0.23 no crystal flower 2 245 350 37 2.3 0.25 no crystal flower 3 227 348 39 2.4 0.21 no crystal flower 4 232 358 38 2.2 0.22 no crystal flower 5 245 360 36 2.5 0.24 no crystal flower 6 253 364 37 2.1 0.23 no crystal flower 7 246 356 39 2.5 0.22 no crystal flower 8 235 348 40 2.2 0.21 no crystal flower

[0049] From the above examples, it can be seen that through low-cost component design and ingenious combination of heat treatment and aluminizing silicon process, hot-dip aluminizing silicon automobile outer plate is prepared, the yield strength of which is above 220 MPa, the tensile strength is above 340 MPa, and the elongation is greater than 36%, realizing the performance of high strength and ductility, formability and high surface quality of automobile outer plate.

Claims

1. 340MPa grade aluminum-silicon coated automotive steel, characterized in that, The chemical composition of the substrate, calculated by weight percentage, is as follows: C 0.003%~0.006%, Mn 0.3%~0.5%, P 0.03%~0.05%, Si 0.01%~0.03%, Ti 0.02%~0.05%, Cu 0.2%~0.4%, N≤0.002%, S≤0.005%, with the balance being Fe and unavoidable impurities; The microstructure of the steel plate consists of ferrite, Ti(N,C) precipitates, and Cu-rich phases; The steel plate has a yield strength of 220-260 MPa, a tensile strength of 340 MPa or more, an elongation after fracture of 36% or more, a plastic strain ratio r of 2.0 or more, and a strain hardening index n of 0.2 or more.

2. The method for preparing 340MPa grade aluminized silicon coated automotive steel as described in claim 1, characterized in that, The process includes smelting, 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 830–870℃, and the annealing time is 30–60 seconds; 2) The slow cooling rate is 2-10℃ / s, the slow cooling temperature is 600-720℃, and the slow cooling isothermal time is 10-20s; 3) Aluminized silicon plating: After the steel plate is cooled to a slow temperature and then isothermal, it is placed into the aluminum-silicon plating bath. The temperature difference between the aluminum-silicon plating bath and the steel plate during slow cooling is ≤10℃, and the aluminum-silicon plating time is 3~10s. 4) After aluminum-silicon plating, the steel plate continues to cool to room temperature at a rapid cooling rate of ≥20 ℃ / s.

3. The method for preparing 340MPa grade aluminized silicon coated automotive steel according to claim 2, characterized in that, The hot rolling process includes the following methods: 1) Heating temperature between 1250 and 1300℃, furnace time ≥ 120 min; 2) The initial rolling temperature is between 1100 and 1150℃, and the final rolling temperature is ≥890℃; 3) The winding temperature is between 650 and 750℃.

4. The method for preparing 340MPa grade aluminized silicon coated automotive steel according to claim 3, characterized in that, The thickness of hot-rolled plates is between 3 and 4 mm.

5. The method for preparing 340MPa grade aluminized silicon coated automotive steel according to claim 2, characterized in that, The cold rolling process is described in which the cold rolling reduction rate is controlled between 75% and 85%.

6. The method for preparing 340MPa grade aluminized silicon coated automotive steel according to claim 2, characterized in that, The finishing process involves controlling the finishing elongation rate to be between 0.5% and 1.0%.

7. The method for preparing 340MPa grade aluminized silicon coated automotive steel according to claim 2, characterized in that, The composition of the aluminum-silicon plating solution, by mass percentage, is: Si 8%–12%, Fe ≤2%, and the remainder is Al.

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