High-strength steel for automotive chassis and method for producing the same

Abstract

The application discloses a high-strength steel for an automobile chassis and a production method thereof. The high-strength steel comprises the following components in mass percentage: C: 0.06%-0.15%, Si: 0.3%-0.5%, Mn: 1.8%-2.3%, Al: 0.1%-0.2%, P: less than or equal to 0.010%, S: less than or equal to 0.0015%, Nb: 0.03-0.06%, Ti: 0.09%-0.14%, Cr: 0.2-0.4%, N: less than or equal to 0.0030%, and the rest is Fe and inevitable impurities. The production method comprises the following steps: (1) smelting molten iron and continuous casting to obtain a slab; (2) rough rolling, finish rolling, post-rolling cooling and coiling after heating the slab to obtain a steel base body; (3) uncoiling and pickling the steel base body to obtain a pickled plate; and (4) galvanizing and finishing the pickled plate to obtain a galvanized plate. The application prolongs the service life of the automobile chassis steel under a service condition.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel production, and particularly relates to a low-cost high-strength steel for automobile chassis and a production method thereof. BACKGROUND

[0002] To realize the lightweight of automobiles and improve the collision safety, the types, use proportion and strength level of advanced high-strength steel applied to automobile bodies are gradually improved. The steel for automobile chassis has excellent flanging and hole expanding performance compared with dual-phase steel with the same tensile strength through fine-grain strengthening or precipitation strengthening of micro-alloying elements, and is widely used to manufacture various parts with high requirements for local forming capacity, such as swing arms, chassis suspension parts, wheel structures and the like.

[0003] To improve the service life of automobiles, the application proportion of coating and plating materials is always high in automobile manufacturing, especially under complex working conditions of structural parts such as automobile chassis, and the application proportion of coating and plating materials needs to be increased. Hot-based galvanizing is a process for coating and plating the surface of hot-rolled steel plates, which can effectively improve the corrosion resistance of hot-rolled steel plates, but due to the complex hot-based galvanizing process technology and great production difficulty of high-strength steel, there are few 780MPa and above hot-based galvanizing high-strength steels for automobile chassis developed and applied in China at present. The production process of the steel grade with the strength level is difficult to realize precise control of the organization and performance due to the high alloy content; at the same time, hot-based galvanizing is generally mainly thick zinc layer, and the single side is mostly thick zinc layer, which has high cost and is not suitable for the development of chassis steel.

[0004] Chinese patent CN202110155662.6 provides a hot-based galvanizing ferrite bainite high-strength steel plate and a preparation method thereof, and due to the limitation of its process and composition, the yield strength is limited to 510MPa, the tensile strength is limited to 620MPa, and the fatigue performance of the material is not mentioned. As a steel for automobile chassis parts, the fatigue performance is one of the important indicators of automobile safety.

[0005] Chinese patent CN202110415667.8 provides a hot-based galvanizing high-hole-expanding dual-phase steel and a preparation method thereof, which does not indicate the performance of the dual-phase steel, and also does not mention the fatigue resistance of the steel, whether it meets the application requirements of automobile chassis parts.

[0006] Therefore, it is urgent to develop a production method of high-strength steel for automobile chassis to realize the development of high-corrosion-resistance automobile chassis steel under the premise of meeting the fatigue resistance. SUMMARY

[0007] The application aims to provide a high-strength steel for automobile chassis and a production method thereof, which can improve the service life of the material in the service state.

[0008] To achieve the above-mentioned purpose, the technical solutions provided by the application are as follows:

[0009] A high-strength steel for automobile chassis, which comprises the following chemical components in mass fraction: C: 0.06% to 0.15%, Si: 0.3% to 0.5%, Mn: 1.8% to 2.3%, Al: 0.1% to 0.2%, P≤0.010%, S≤0.0015%, Nb: 0.03 to 0.06%, Ti: 0.09% to 0.14%, Cr: 0.2 to 0.4%, N≤0.0030%, and the balance of Fe and inevitable impurities.

[0010] The mechanism of each chemical component in the high-strength steel for automobile chassis is as follows:

[0011] C: The present application adopts appropriate low C content, which is controlled at 0.06% to 0.15%, to improve the welding performance of the high-strength steel for automobile chassis and to ensure the strength and plasticity of the material.

[0012] Si and Mn: The present application adopts low Si and high Mn component design: first, Mn and Si can greatly improve the hardenability of the steel; second, excessive Si is prone to external oxidation, which reduces the coating adhesion; meanwhile, excessive Mn is prone to segregation, which further aggravates the external oxidation of the substrate and reduces the adhesion of the inhibiting layer, thereby deteriorating the coating adhesion. Mn can improve the hardenability of the material, and at the same time, the steel has a low Ac1 and Ar1 temperature, inhibits the formation of pearlite, and promotes the transformation of ferrite and bainite. The present application controls the Si content to be 0.3% to 0.5% and the Mn content to be 1.8% to 2.3%.

[0013] Nb and Ti: The present application adds Nb and Ti in the steel plate composition: first, Nb and Ti can refine the grain, improve the uniformity of the structure and the yield ratio, and also benefit the improvement of the fatigue performance of the material; second, it effectively controls the content of precipitated carbides, further improves the strength and plasticity of the material; third, Nb can hinder the recrystallization and grain growth of austenite, and refine the ferrite grains, so in the present application, Nb is 0.03 to 0.06%, and Ti is 0.05% to 0.16%.

[0014] Al: Al has the effects of inhibiting carbide precipitation and strengthening ferrite. The Al element in the substrate surface layer forms an Fe2Al5 inhibiting layer, which is mainly distributed at the interface between the steel plate substrate and the coating, and plays a role in enhancing the interfacial adhesion. It also inhibits the external oxidation of Si and improves the adhesion of the zinc layer. At the same time, it forms an aluminum-rich compound in the coating, which has good ductility and can improve the ductility of the eutectic structure in the coating and reduce the formation of cracks during forming. However, the higher the Al content, the more difficult the smelting, and the poorer the weldability of the steel. Therefore, the present application controls the Al content to be 0.1% to 0.2%.

[0015] Cr: Cr can significantly improve the hardenability of steel, and has the effect of inhibiting carbide precipitation and pearlite transformation, which is beneficial to promote the formation of bainite in steel, but too high content increases the cost, therefore the content of Cr in the application is 0.2-0.40%.

[0016] Further, the yield strength Rp of the high-strength steel for automobile chassis according to the application is ≥620MPa, the tensile strength Rm is ≥750MPa, the elongation A is ≥20%, the hole expansion ratio λ is ≥40%, and the high-cycle fatigue limit strength is ≥450MPa. 0.2 ≥620MPa, the tensile strength Rm is ≥750MPa, the elongation A is ≥20%, the hole expansion ratio λ is ≥40%, and the high-cycle fatigue limit strength is ≥450MPa. 80 ≥20%, the hole expansion ratio λ is ≥40%, and the high-cycle fatigue limit strength is ≥450MPa.

[0017] Further, the high-strength steel for automobile chassis according to the application has a metallographic structure including, by volume fraction, 65%-85% ferrite, 15%-35% bainite, and 0-3% carbide, the ferrite grain size is ≤12μm, and the precipitate phase size is 10-30μm.

[0018] The production method of the high-strength steel for automobile chassis according to the application comprises the following steps:

[0019] (1) converting the molten iron into a slab by converter-LF+RH-continuous casting;

[0020] (2) after heating the slab, rough rolling, finish rolling, post-rolling cooling and coiling are carried out to obtain a 1.5-4.5mm steel substrate;

[0021] (3) after uncoiling the steel substrate, pickling is carried out to obtain a pickled plate;

[0022] (4) after preheating, heating, soaking, rapid cooling, galvanizing, cooling and finishing of the pickled plate, a galvanized plate is obtained.

[0023] Further, in the production method according to the application, the RH process in step (1) has a RH treatment time of 30-40min, argon is blown throughout the process, the [H] in the steel is controlled to ≤3.0ppm, the [N] is controlled to ≤30ppm, and the formation of large-size TiN inclusions is reduced and the resistance to delayed cracking and fatigue resistance of the material is improved.

[0024] Further, in the production method according to the application, the continuous casting process in step (1) has a casting speed of 0.6m / min-0.8m / min and a superheat of 20-30℃, and a light reduction of 5.0-7.0mm is used. The low casting speed and light reduction process can reduce the formation of segregation and banded structure, thereby improving the uniformity of the structure and the fatigue resistance.

[0025] Further, the production method, wherein the slab in step (2) is heated to a temperature of 400-700 DEG C, the heating temperature is 1250 DEG C-1280 DEG C, and the holding time is 180-200 min, so that the micro-alloying elements in the slab can be fully solid-solved, and the formation of a thick primary iron scale can be avoided.

[0026] Further, the production method, wherein the rough rolling in step (2) is performed in 3+3 passes, the high-pressure descaling pressure is greater than or equal to 23 MPa, the rough rolling first-pass temperature is 1092-1190 DEG C, and the rough rolling cumulative reduction is 80-85%.

[0027] Further, the production method, wherein the entry temperature of the finish rolling in step (2) is 990 DEG C-1050 DEG C, and the finish rolling final rolling temperature is 850 DEG C-920 DEG C.

[0028] Further, the production method, wherein the post-rolling cooling in step (2) is performed in a front 3 / 4 water cooling mode, the upper and lower cooling header water quantity ratio is 0.5-0.7, the post-rolling cooling rate is 35 DEG C / s-60 DEG C / s, and the post-rolling cooling layer cooling and precision adjustment section is air-sealed with 0.5-1 MPa high-pressure air to flow water on the surface of the steel strip.

[0029] The front 3 / 4 section design is adopted to refine ferrite grains, and the upper and lower cooling header water quantity ratio is 0.5-0.7, so that the uniformity of the upper and lower surface structures can be improved, and the formation of mixed grain surface layer structures can be avoided.

[0030] Further, the production method, wherein the coiling temperature in step (2) is 440 DEG C-500 DEG C, and the offline coil is concentratedly cooled in a slow cooling pit at a cooling rate of less than or equal to 14 DEG C / h. In order to inhibit the precipitation of NbTi micro-alloy carbides during coiling, and to refine the structure, low-temperature coiling is adopted at 440-500 DEG C, and the offline coil is concentratedly cooled in the slow cooling pit at a cooling rate of less than or equal to 14 DEG C / h, so that the formation of medium-temperature transformed and bainite structures can be promoted, and the elimination of residual stress can be facilitated.

[0031] Further, the production method, wherein the pickling in step (3) is performed by a two-bending-one-straightening mode, the stretcher straightening elongation is 0.2%-1.5%, the pickling temperature is 79-86 DEG C, the pickling liquid soaking time is 1.5-3 min, and the free acid concentration of the pickling liquid is 150-180 g / L.

[0032] Further, the production method, wherein the preheating, heating, soaking, rapid cooling of step (4) are specifically: preheating the galvanized substrate to 200-300 DEG C, then heating to 560-590 DEG C at a rate of 1-3 DEG C / s, then heating to 600-650 DEG C at a rate of 2-4 DEG C / s, soaking for 10-30 s, then rapidly cooling to 440-470 DEG C at a rate of 10-20 DEG C / s, and soaking for 10-30 s; wherein the dew point of the annealing atmosphere can be controlled to -30 to -15 DEG C; increasing the dew point of the annealing atmosphere can improve the quality of the inhibition layer, inhibit the surface enrichment of Si and Mn, improve the adhesion of the coating, and therefore the dew point of the annealing atmosphere is controlled to -30 to -15 DEG C.

[0033] Further, the production method, wherein the galvanizing and cooling of step (4) are specifically: the temperature of the zinc liquid in the zinc pot is 455-463 DEG C; after galvanizing in the zinc pot, the cooling rate is 10-30 DEG C / s to the temperature of the top roller of 150-190 DEG C, and then slow cooling to room temperature; the air knife distance in the air knife blowing process is 30-40 mm, the air knife blowing pressure is 35-50 Pa, and the air knife height is 90-120 mm, so that a single-side zinc layer with a thickness of 5.5-12 mu m is obtained.

[0034] In the galvanizing process, the setting of the soaking temperature and the slow cooling temperature can eliminate the organizational stress of the steel plate and the partial bainite tempering transformation, promote the dispersion precipitation of Nb and Ti carbonitride, and fully exert the precipitation strengthening effect of Nb and Ti microalloy.

[0035] The high-pressure air blowing mode is used to replace the high-pressure water blowing mode, the problem of asynchronous cooling of the plate strip is improved, and the formation of the surface color difference of the hot-rolled plate caused by local water is reduced, and the coating performance of the plate strip is improved.

[0036] Further, the production method, wherein the finishing of step (4) has an elongation of 0.2-0.7%; a lower elongation cannot eliminate the yield platform generated after annealing and galvanizing, and a higher work hardening is obvious, which increases the equipment load and is prone to cause the finishing roll marks; the finishing roll is a chromium-plated roll, and the roughness is 3.0-3.5 mu m.

[0037] The technical scheme of the present application has the following technical effects:

[0038] The high-strength steel for automobile chassis provided by the present application has a yield strength Rp 0.2 ≥ 620 MPa, a tensile strength Rm ≥ 750 MPa, an elongation A 80 ≥ 20%, a hole expansion ratio λ ≥ 40%, and a high-cycle fatigue limit strength ≥ 450 MPa, which meets the development trend of high service life, high corrosion resistance and high safety of automobile steel. Attached Figure Description

[0039] Figure 1 The metallographic structure of high-strength steel for automobile chassis is shown in Example 1. Detailed Implementation

[0040] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below.

[0041] Examples 1-8

[0042] The chemical composition of the high-strength steel used in the automotive chassis of each embodiment is shown in Table 1.

[0043] Table 1. Chemical composition (wt / %) of high-strength steel in each embodiment.

[0044] C Si Mn P S Al Nb Ti Cr N / ppm Example 1 0.07 0.3 1.8 0.006 0.0008 0.16 0.05 0.1 0.2 30 Example 2 0.15 0.35 1.9 0.07 0.0005 0.11 0.04 0.12 0.26 28 Example 3 0.12 0.42 2.1 0.006 0.0015 0.1 0.06 0.09 0.24 27 Example 4 0.09 0.5 2.2 0.008 0.0007 0.19 0.05 0.11 0.23 28 Example 5 0.13 0.47 1.8 0.006 0.0008 0.17 0.06 0.12 0.32 29 Example 6 0.06 0.38 2.3 0.01 0.001 0.11 0.05 0.14 0.37 28 Example 7 0.14 0.31 2.0 0.006 0.0008 0.20 0.05 0.09 0.31 29 Example 8 0.10 0.36 1.93 0.007 0.001 0.15 0.03 0.1 0.4 30

[0045] The balance in Table 1 is Fe and unavoidable impurities.

[0046] The preparation methods of high-strength steel for automobile chassis in each embodiment are as follows:

[0047] (1) The molten iron is subjected to converter-LF+RH continuous casting to obtain slabs; wherein:

[0048] RH process: RH treatment time is 30-40 min, argon is blown throughout the process, and the [H] in the steel is controlled to be ≤3.0 ppm and [N] to be ≤30 ppm.

[0049] Continuous casting process: casting speed 0.6m / min~0.8m / min, superheat 20~30℃, and light reduction of 5.0~7.0mm.

[0050] The RH and continuous casting process parameters for each embodiment are shown in Table 2.

[0051] Table 2 RH and continuous casting process parameters for each embodiment

[0052]

[0053] (2) The slab is heated in a furnace and then subjected to rough rolling, finish rolling, cooling, and coiling to obtain a 1.5-4.5mm steel matrix; wherein:

[0054] Heating: The slab temperature upon entering the furnace is 400-700℃, the heating temperature is 1250-1280℃, and the holding time is 180-200 minutes.

[0055] Coarse rolling: 3+3 passes are adopted, the high-pressure descaling pressure is ≥23 MPa, the first pass temperature is 1092-1190℃, and the cumulative reduction is 80-85%.

[0056] Finish rolling: the entry temperature is 990-1050℃, and the finish rolling temperature is 850-920℃.

[0057] Rolling cooling: the front 3 / 4 water cooling mode is adopted, the water amount ratio of the upper and lower cooling headers is 0.5-0.7, the cooling speed of the post-rolling cooling is 35-60℃ / s, and the post-rolling cooling layer cold-fine adjustment section adopts 0.5-1 MPa high-pressure air to air seal the surface laminar water of the steel strip.

[0058] Coiling: the coiling temperature is 440-500℃, and the offline coil adopts the average cooling speed of ≤14℃ / h slow cooling pit centralized cooling.

[0059] The slab heating and rolling process parameters of each embodiment are shown in Table 3; the post-rolling cooling and coiling process parameters are shown in Table 4.

[0060] Table 3: Heating and rolling process parameters of each embodiment

[0061]

[0062]

[0063] Table 4: Post-rolling cooling and coiling process parameters of each embodiment

[0064]

[0065] (3) The steel matrix is uncoiled, and then pickled to obtain a pickled plate;

[0066] In the pickling, the extension rate of the straightening and leveling machine is 0.2%-1.5%, the two-bending-one-leveling mode is adopted, the pickling temperature is 79-86℃, the acid immersion time is 1.5-3 min, and the free acid concentration of the acid solution is 150-180 g / L. The specific pickling process parameters of each embodiment are shown in Table 5.

[0067] Table 5: Pickling process parameters of each embodiment

[0068]

[0069]

[0070] (4) The pickled plate is preheated, heated, soaked, rapidly cooled, galvanized, cooled, and finished to obtain a galvanized plate. Specifically:

[0071] The galvanized substrate is preheated to 200-300°C, then heated to a heating 1 section temperature of 560-590°C at a rate of 1-3°C / s, then heated to a heating 2 section temperature of 600-650°C at a rate of 2-4°C / s, held for 10-30s, then rapidly cooled to 440-470°C at a rate of 10-20°C / s, and held for 10-30s. The dew point of the annealing atmosphere can be controlled to -30--15°C. The specific process parameters of each example in this part are shown in Table 5.

[0072] The zinc pot zinc liquid temperature is 455-463°C; after being galvanized, the galvanized substrate is cooled to a tower top roller temperature of 150-190°C at a cooling rate of 10-30°C / s, and then slowly cooled to room temperature. The air knife distance in the air knife blowing process in the galvanizing is 30-40mm, the air knife blowing pressure is 35-50Pa, and the air knife height is 90-120mm, so that a galvanized layer with a single side zinc layer thickness of 5.5-12μm is obtained. The bright extension rate is 0.2%-0.7%, and the bright roller is a chromium-plated roller with a roughness of 3.0-3.5μm. The specific process parameters of each example in this part are shown in Table 6.

[0073] Table 1 Galvanizing process parameters 1 of each example

[0074]

[0075] Table 6 Galvanizing process parameters 2 of each example

[0076]

[0077]

[0078] The fiber structure and mechanical properties of the high-strength steel for automobile chassis in each example are shown in Table 7.

[0079] Table 7 Microstructure and mechanical properties of high-strength steel in each example

[0080]

[0081] As can be seen from Table 7, the production of high-corrosion-resistance automobile chassis steel can be realized by the combination of the composition and process control, and the produced automobile chassis steel has excellent properties such as yield strength ≥620MPa, tensile strength ≥750MPa, elongation after fracture (A 80mm ) ≥20%, hole expansion rate ≥40%, and high-cycle fatigue limit strength ≥450MPa, and a galvanized layer of 5.5-12μm zinc layer.

[0082] The microstructure of the automobile chassis steel in Example 1 is shown in Figure 1 ; the composition of the automobile chassis steel in Example 1 is shown in Figure 1It can be known that the high-strength steel for automobile chassis provided by the application has a structure of bainite + ferrite + a small amount of carbide.

[0083] The Fe and Al in the base body and the Zn element in the plating solution synergize, which is beneficial to the Fe2Al5 inhibition layer when inhibiting the external oxidation of Si. Therefore, the high-strength steel for automobile chassis can effectively solve the problems of insufficient corrosion resistance and short service life of traditional automobile chassis parts.

Claims

1. A high-strength steel for an automobile chassis, characterized by, The high-strength steel comprises the following components in mass fraction: C: 0.06% to 0.15%, Si: 0.3% to 0.5%, Mn: 1.8% to 2.3%, Al: 0.1% to 0.2%, P: ≤0.010%, S: ≤0.0015%, Nb: 0.03 to 0.06%, Ti: 0.09% to 0.14%, Cr: 0.2 to 0.4%, N: ≤0.0030%, and the rest is Fe and inevitable impurities. The yield strength Rp of the high-strength steel for automobile chassis 0.2 ≥ 620 MPa, tensile strength Rm ≥ 750 MPa, elongation A 80 ≥ 20%, hole expansion ratio λ ≥ 40%, high-cycle fatigue limit strength ≥ 450 MPa; The high-strength steel for automobile chassis comprises the following components in volume fraction: 65% to 85% ferrite, 15% to 35% bainite, and 0 to 3% carbide, the ferrite grain size is ≤12 μm, and the precipitated phase size is 10 to 30 μm.

2. A method for producing a high-strength steel for an automobile chassis according to claim 1, characterized by, The method comprises the following steps: (1) converting the molten iron into a slab through converter-LF-RH-continuous casting; (2) after heating the slab, rough rolling, finish rolling, post-rolling cooling and coiling are performed to obtain a 1.5 to 4.5 mm steel substrate; (3) after uncoiling the steel substrate, pickling is performed to obtain a pickled sheet; (4) after preheating, heating, soaking, rapid cooling, galvanizing, cooling and finishing, a galvanized sheet is obtained.

3. The method of producing a high-strength steel for an automobile chassis according to claim 2, characterized by, In the RH process in step (1), the RH treatment time is 30 to 40 min, argon is blown throughout the process, the content of [H] in the steel is controlled to be ≤3.0 ppm, and the content of [N] is controlled to be ≤30 ppm.

4. The method of producing a high-strength steel for an automobile chassis according to claim 2, characterized by, In the continuous casting process in step (1), the casting speed is 0.6 m / min to 0.8 m / min, the superheat is 20 to 30 ℃, and a 5.0 to 7.0 mm light press-down is adopted.

5. A method for producing high-strength steel for automobile chassis according to claim 2, characterized in that, In step (2), the slab heating temperature is 400 to 700 ℃, the heating temperature is 1250 ℃ to 1280 ℃, and the holding time is 180 to 200 min.

6. The method of claim 2, wherein the high-strength steel for an automobile chassis is produced by the steps of: In step (2), the post-rolling cooling adopts a 3 / 4 water cooling mode in the front section, the water amount ratio of the upper cooling header to the lower cooling header is 0.5 to 0.7, the cooling speed of the post-rolling cooling is 35 ℃ / s to 60 ℃ / s, and 0.5 to 1 MPa high-pressure air is used to air seal the surface laminar flow water of the steel strip in the post-rolling cooling layer cooling and precision adjustment section. ​ 7. The method for producing high-strength steel for automobile chassis according to claim 2, characterized in that, In step (2), the coiling temperature is 440 ℃ to 500 ℃, and the offline coil is concentratedly cooled in the slow cooling pit at a cooling speed of ≤14 ℃ / h.

8. The method of claim 2, wherein the high-strength steel for an automobile chassis is produced by the steps of: In step (3), the pickling is performed on the uncoiler with an elongation of 0.2% to 1.5%, a two-bending-one-straightening mode, a pickling temperature of 79 to 86 ℃, an acid liquid soaking time of 1.5 to 3 min, and a free acid concentration of the acid liquid of 150 to 180 g / L. ​

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