High-strength steel for automobile chassis and production method of high-strength steel

Abstract

The invention discloses high-strength steel for an automobile chassis and a production method of the high-strength steel. The high-strength steel comprises the following chemical components in percentage by mass: 0.06%-0.15% of C, 0.3%-0.5% of Si, 1.8%-2.3% of Mn, 0.1%-0.2% of Al, less than or equal to 0.010% of P, less than or equal to 0.0015% of S, 0.03%-0.06% of Nb, 0.09%-0.14% of Ti, 0.2%-0.4% of Cr, less than or equal to 0.0030% of N and the balance of Fe and inevitable impurities. The production method comprises the following steps: (1) smelting molten iron, and continuously casting to obtain a plate blank; (2) the plate blank is heated and then subjected to rough rolling and finish rolling, cooling and coiling are conducted after rolling, and a steel base body is obtained; (3) uncoiling and pickling the steel substrate to obtain a pickled plate; and (4) the pickled plate is galvanized and polished, and a galvanized plate is obtained. According to the scheme, the service life of the steel for the automobile chassis in the service state is prolonged.

Description

Technical Field

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

[0002] In order to achieve lightweighting of automobiles and improve collision safety, the types, proportions and strength levels of advanced high-strength steels used in automobile bodies are gradually increasing. Among them, automobile chassis steel has excellent flanging and hole expansion performance compared with dual-phase steels of the same tensile strength through micro-alloy element fine grain strengthening or precipitation strengthening. It is widely used in the manufacture of various parts with high requirements for local forming capabilities, such as swing arms, chassis suspension parts, wheel structures, etc.

[0003] In order to increase the service life of automobiles, the proportion of coating and plating materials used in automobile manufacturing has always been high, especially in the complex working conditions of structural parts such as automobile chassis, which requires a greater proportion of coating and plating materials. Hot-base galvanizing is a process of coating the surface of hot-rolled steel plates, which can effectively improve the corrosion resistance of hot-rolled steel plates. However, due to the complex technology and production difficulty of hot-base galvanizing of high-strength steel, there are currently few high-strength steels of 780MPa grade and above for hot-base galvanized automobile chassis in China. The production process of this strength grade of steel is difficult to achieve precise control of organizational properties due to the high alloy content; at the same time, hot-base galvanizing is generally based on a thick zinc layer, and one side is mostly a thick zinc layer, which is costly and not suitable for the development of chassis steel.

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

[0005] Chinese patent CN202110415667.8 provides a hot-dip galvanized high-expansion dual-phase steel and a preparation method thereof. It does not indicate the performance of the dual-phase steel, nor does it mention the fatigue resistance of the steel, or whether it meets the application requirements of automotive chassis parts.

[0006] Therefore, there is an urgent need to develop a production method for high-strength steel for automobile chassis, so as to achieve the development of high-corrosion-resistant steel for automobile chassis while meeting the fatigue resistance requirements. Summary of the invention

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

[0008] To achieve the above purpose, the technical solution provided by the present invention is as follows:

[0009] A high-strength steel for automobile chassis, whose chemical composition, measured by mass fraction, includes: C: 0.06%-0.15%, Si: 0.3%-0.5%, Mn: 1.8%-2.3%, Al: 0.1%-0.2%, P≤0.010%, S≤0.0015%, Nb: 0.03-0.06%, Ti: 0.09%-0.14%, Cr: 0.2-0.4%, N≤0.0030%, and the rest is Fe and unavoidable impurities.

[0010] The mechanism of action of the chemical components in the high-strength steel for automobile chassis of the present invention is as follows:

[0011] C: The present invention adopts an appropriate low C content, controlling it within 0.06% to 0.15%, which improves the welding performance of high-strength steel for automobile chassis and can also ensure the strength and plasticity of the material.

[0012] Si and Mn: The present invention adopts a low Si and high Mn composition design: first, the coordination between Mn and Si can greatly improve the hardenability of steel; second, excessive Si element is very prone to external oxidation, which reduces the coating coating; at the same time, excessive Mn is easy to form segregation, and also aggravates the external oxidation of the substrate, reduces the binding force of the inhibition layer, and thus deteriorates the adhesion of the coating. Mn can improve the hardenability of the material, and at the same time, the steel has a lower Ac1 and Ar1 temperature, inhibits the formation of pearlite, and promotes the transformation of ferrite and bainite. The present invention controls the Si content to 0.3% to 0.5%, and the Mn content to 1.8% to 2.3%.

[0013] Nb and Ti: The present invention adds Nb and Ti to the steel plate composition. First, Nb and Ti can refine the grains, improve the uniformity and yield strength ratio of the organization, and are also beneficial to the improvement of the fatigue performance of the material; second, the content of precipitated carbides can be effectively controlled to further improve the strength and plasticity of the material; third, Nb can hinder the recrystallization and grain growth of austenite and refine the ferrite grains. Therefore, in the present invention, Nb: 0.03-0.06%, Ti: 0.05%~0.16%.

[0014] Al: Al has the function of inhibiting carbide precipitation and strengthening ferrite. The Al element on the surface of the matrix forms a Fe2Al5 inhibition layer, which is mainly distributed at the interface between the steel plate matrix and the coating, plays a role in enhancing the interface bonding force, and can also inhibit the external oxidation of Si and improve the adhesion of the zinc layer; at the same time, aluminum-rich compounds are formed in the coating, which have good ductility, 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 higher the difficulty of smelting, and the worse the weldability of the steel, so the Al content is controlled to be 0.1% to 0.2% in the present invention.

[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. However, too high a content increases the cost. Therefore, the Cr content in the present invention is 0.2-0.40%.

[0016] Furthermore, the yield strength Rp of the high-strength steel for automobile chassis of the present invention is 0.2 ≥620MPa, tensile strength Rm≥750MPa, elongation A 80 ≥20%, hole expansion rate λ≥40%, high cycle fatigue limit strength ≥450MPa.

[0017] Furthermore, the metallographic structure of the high-strength steel for automobile chassis of the present invention comprises, by volume fraction, 65% to 85% ferrite, 15% to 35% bainite and 0 to 3% carbide, the ferrite grain size is ≤12 μm, and the precipitate phase size is 10 to 30 μm.

[0018] The method for producing high-strength steel for automobile chassis of the present invention comprises the following steps:

[0019] (1) subjecting molten iron to converter-LF+RH-continuous casting to obtain slabs;

[0020] (2) heating the slab in a furnace, performing rough rolling, finish rolling, cooling after rolling, and coiling to obtain a steel matrix with a diameter of 1.5 to 4.5 mm;

[0021] (3) uncoiling the steel substrate and then pickling it to obtain a pickled plate;

[0022] (4) The pickled sheet is preheated, heated, evenly heated, rapidly cooled, galvanized, cooled, and smoothed to obtain a galvanized sheet.

[0023] Furthermore, in the production method of the present invention, in the RH process of step (1), the RH treatment time is 30 to 40 minutes, argon is blown throughout the process, and [H] in the steel is controlled to be ≤3.0ppm and [N] ≤30ppm, thereby reducing the formation of large-sized TiN inclusions and improving the material's resistance to delayed cracking and fatigue resistance.

[0024] Furthermore, in the production method of the present invention, in the continuous casting process of step (1), the drawing speed is 0.6m / min to 0.8m / min, the superheat is 20 to 30°C, and the reduction is 5.0 to 7.0mm. The low drawing speed and light reduction process can reduce the formation of segregation and banded structure, thereby improving the uniformity of the structure and fatigue resistance.

[0025] Furthermore, in the production method of the present invention, the slab entering the furnace in step (2) is at a temperature of 400 to 700°C, a heating temperature of 1250 to 1280°C, and a holding time of 180 to 200 minutes, which can achieve sufficient solid solution of microalloying elements in the slab while avoiding the formation of excessively thick primary oxide scale that increases the difficulty of descaling.

[0026] Furthermore, in the production method described in the present invention, the rough rolling in step (2) is performed by 3+3 passes, the high-pressure descaling pressure is ≥23MPa, the temperature of the first pass of rough rolling is between 1092 and 1190°C, and the cumulative reduction rate of rough rolling is 80 to 85%.

[0027] Furthermore, in the production method of the present invention, the inlet temperature of the finishing rolling in step (2) is 990°C to 1050°C, and the final rolling temperature of the finishing rolling is 850°C to 920°C.

[0028] Furthermore, in the production method described in the present invention, the post-rolling cooling in step (2) adopts the front section 3 / 4 water cooling mode, the water volume ratio of the upper and lower cooling manifolds in the post-rolling cooling section is 0.5-0.7, the cooling rate of the post-rolling cooling is 35°C / s-60°C / s, and the post-rolling cooling layer cold fine-tuning section adopts 0.5-1MPa high-pressure air to air-seal the surface layer of the steel strip.

[0029] The purpose of refining ferrite grains is achieved by adopting the front 3 / 4 section design. At the same time, the water volume ratio of the upper and lower cooling headers in the cooling section is 0.5-0.7, which can improve the uniformity of the upper and lower surface organizations and avoid the formation of mixed crystal organization on the surface.

[0030] Furthermore, in the production method of the present invention, the coiling temperature in step (2) is 440°C to 500°C, and the offline coil is concentratedly cooled in a slow cooling pit at a cooling rate of ≤14°C / h. In order to suppress the precipitation of NbTi microalloy carbides during the coiling process and achieve microstructure refinement, low-temperature coiling at 440 to 500°C and concentrated cooling in a slow cooling pit at a cooling rate of ≤14°C / h are adopted, which can promote medium-temperature transformation and the formation of bainite structure and also help eliminate residual stress.

[0031] Furthermore, in the production method described in step (3), the pickling, the elongation of the tension leveler is 0.2% to 1.5%, the two-bend-one-leveling mode is adopted, the pickling temperature is 79 to 86°C, the acid solution immersion time is 1.5 to 3 minutes, and the free acid concentration of the acid solution is 150 to 180 g / L.

[0032] Furthermore, in the production method of the present invention, the preheating, heating, soaking and rapid cooling in step (4) are specifically as follows: preheating the galvanized substrate to 200°C to 300°C, then heating to 560°C to 590°C at a rate of 1°C / s to 3°C / s, and then heating to 600°C to 650°C at a rate of 2°C / s to 4°C / s, soaking and keeping warm for 10s to 30s, and then rapidly cooling to 440°C to 470°C at a rate of 10°C / s to 20°C / s, and keeping warm for 10 to 30s. The dew point of the annealing atmosphere can be controlled at -30°C to -15°C. Improving the dew point of the annealing atmosphere can improve the quality of the inhibition layer, inhibit the external diffusion surface enrichment of Si and Mn, and improve the adhesion of the coating, so the dew point of the annealing atmosphere is controlled at -30 to -15°C.

[0033] Furthermore, in the production method of the present invention, the galvanizing and cooling in step (4) are as follows: the zinc pot zinc liquid temperature is 455-463°C; after galvanizing out of the zinc pot, the temperature is cooled to the 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 of the air knife purging process in the galvanizing is 30-40mm, the air knife blowing pressure is 35-50Pa, and the air knife height is 90-120mm, and a single-sided zinc layer thickness of 5.5-12μm is obtained.

[0034] During the galvanizing process, the setting of soaking temperature and slow cooling temperature can eliminate the stress of steel sheet structure and partial bainite tempering transformation, promote the dispersion precipitation of Nb and Ti carbonitrides, and give full play to the precipitation strengthening effect of Nb and Ti microalloys. The setting of cooling rate and temperature in the rapid cooling stage can ensure that the substrate has good coatability.

[0035] The present invention adopts high-pressure air blowing instead of high-pressure water blowing, which improves the problem of asynchronous cooling of the plate and strip through the width, and reduces the formation of color difference on the surface of the hot-rolled plate caused by local water, which deteriorates the coating performance of the plate and strip.

[0036] Furthermore, in the production method described in the present invention, the polishing in step (4) has a polishing elongation of 0.2% to 0.7%. A low elongation cannot eliminate the yield platform generated after annealing and galvanizing, and excessively high elongation leads to obvious work hardening, increased equipment load and easy occurrence of polishing roller marks. The polishing roller adopts a chrome-plated roller with a roughness of 3.0 to 3.5 μm.

[0037] The technical solution of the present invention has the following technical effects:

[0038] The yield strength Rp of the high-strength steel for automobile chassis provided by the present invention is 0.2 ≥620MPa, tensile strength Rm≥750MPa, elongation A 80 ≥20%, hole expansion rate λ≥40%, high cycle fatigue limit strength ≥450MPa, which is in line with the development trend of high service life, high corrosion resistance and high safety of automotive steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the metallographic structure of the high-strength steel for automobile chassis in Example 1. DETAILED DESCRIPTION

[0040] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented according to the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are listed below.

[0041] Examples 1 to 8

[0042] The chemical composition of the high-strength steel for automobile chassis in various embodiments is shown in Table 1.

[0043] Table 1 Chemical composition of high strength steel in various examples (wt / %)

[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 method of high-strength steel for automobile chassis in each embodiment is as follows:

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

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

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

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

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

[0052]

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

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

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

[0056] Finishing rolling: inlet temperature is 990℃~1050℃, and final rolling temperature is 850℃~920℃.

[0057] Post-rolling cooling: adopt the front section 3 / 4 water cooling mode, the water volume ratio of the upper and lower cooling headers in the post-rolling cooling section is 0.5-0.7, the cooling rate of the post-rolling cooling is 35℃ / s~60℃ / s, and the post-rolling cooling layer cold fine-tuning section adopts 0.5-1MPa high-pressure air to seal the surface layer of the steel strip.

[0058] Coiling: The coiling temperature is 440℃~500℃, and the offline coil is concentratedly cooled in a slow cooling pit with an average cooling rate of ≤14℃ / h.

[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 various embodiments

[0064]

[0065] (3) uncoiling the steel substrate and then pickling it to obtain a pickled plate;

[0066] In the pickling, the elongation of the tension leveler is 0.2% to 1.5%, the two-bend-one-leveling mode, the pickling temperature is 79 to 86°C, the acid solution immersion time is 1.5 to 3 minutes, and the free acid concentration of the acid solution is 150 to 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) Preheating, heating, soaking, rapid cooling, galvanizing, cooling, and skin-finishing the pickled sheet to obtain a galvanized sheet. Specifically:

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

[0072] The temperature of zinc liquid in the zinc pot is 455-463℃; after galvanizing out of the zinc pot, it is cooled to the tower top roller temperature of 150-190℃ at a cooling rate of 10-30℃ / s, and then slowly cooled to room temperature. The air knife distance of 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, and a single-sided zinc layer thickness of 5.5-12μm is obtained. The skin-passing elongation is 0.2%-0.7%, and the skin-passing roller adopts a chrome-plated roller with a roughness of 3.0-3.5μm. The specific process parameters of this part of each embodiment are shown in Table 6.

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

[0074]

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

[0076]

[0077]

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

[0079] Table 7 Microstructure and mechanical properties of high strength steel in various examples

[0080]

[0081] As can be seen from Table 7, the present invention can achieve the production of high corrosion resistance automobile chassis steel through the combined control of composition and process. The produced automobile chassis steel has a yield strength of ≥620MPa, a tensile strength of ≥750MPa, and an elongation after fracture (A 80mm )≥20%, hole expansion rate≥40%, high cycle fatigue limit strength≥450MPa, and the coating is 5.5-12μm zinc layer.

[0082] Example 1 The microstructure of the steel for automobile chassis is shown in Figure 1 ;Depend on Figure 1It can be seen that the structure of the high-strength steel for automobile chassis provided by the present invention is bainite + ferrite + a small amount of carbide. The bainite matrix contributes to the basic strength of the steel plate, and the ferrite provides the steel plate with plastic toughness.

[0083] The Fe and Al in the matrix of the present invention and the Zn element in the plating solution work synergistically, which is beneficial to the Fe2Al5 inhibition layer when inhibiting the external oxidation of Si. Therefore, the high-strength steel for automobile chassis of the present invention 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 automobile chassis, characterized in that: The chemical composition of the high-strength steel includes, by mass fraction, C: 0.06%-0.15%, Si: 0.3%-0.5%, Mn: 1.8%-2.3%, Al: 0.1%-0.2%, P≤0.010%, S≤0.0015%, Nb: 0.03-0.06%, Ti: 0.09%-0.14%, Cr: 0.2-0.4%, N≤0.0030%, and the rest is Fe and unavoidable impurities.

2. The high-strength steel for automobile chassis according to claim 1, characterized in that: The yield strength Rp of the high-strength steel for automobile chassis 0.2 ≥620MPa, tensile strength Rm≥750MPa, elongation A 80 ≥20%, hole expansion rate λ≥40%, high cycle fatigue limit strength ≥450MPa.

3. The high-strength steel for automobile chassis according to claim 1, characterized in that: The metallographic structure of the high-strength steel for automobile chassis comprises, by volume fraction, 65% to 85% ferrite, 15% to 35% bainite and 0 to 3% carbide, the ferrite grain size is ≤12 μm, and the precipitation phase size is 10 to 30 μm.

4. A method for producing high-strength steel for automobile chassis according to any one of claims 1 to 3, characterized in that: The following steps are involved: The molten iron is subjected to converter-LF+RH-continuous casting to obtain slabs; The slab is heated in a furnace and then subjected to rough rolling, finish rolling, cooling after rolling and coiling to obtain a steel matrix with a diameter of 1.5 to 4.5 mm; The steel substrate is uncoiled and then pickled to obtain a pickled plate; The pickled sheet is preheated, heated, soaked, quickly cooled, galvanized, cooled and smoothed to obtain a galvanized sheet.

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

6. The method for producing high-strength steel for automobile chassis according to claim 1, characterized in that: In the continuous casting process of step (1), the casting speed is 0.6 m / min to 0.8 m / min, the superheat is 20 to 30° C., and the reduction amount is 5.0 to 7.0 mm.

7. The method for producing high-strength steel for automobile chassis according to claim 1, characterized in that: In step (2), the slab is put into the furnace at a temperature of 400-700°C, heated at a temperature of 1250-1280°C, and kept warm for 180-200 minutes.

8. The method for producing high-strength steel for automobile chassis according to claim 1, characterized in that: The post-rolling cooling in step (2) adopts the front section 3 / 4 water cooling mode, the water volume ratio of the upper and lower cooling manifolds in the post-rolling cooling section is 0.5-0.7, the cooling rate of the post-rolling cooling is 35℃ / s~60℃ / s, and the post-rolling cooling layer cold fine adjustment section adopts 0.5-1MPa high-pressure air to seal the surface layer of the steel strip.

9. The method for producing high-strength steel for automobile chassis according to claim 1, characterized in that: The coiling temperature in step (2) is 440°C to 500°C, and the offline coil is centrally cooled in a slow cooling pit at a cooling rate of ≤14°C / h.

10. The method for producing high-strength steel for automobile chassis according to claim 1, characterized in that: In the pickling of step (3), the elongation of the tension leveler is 0.2% to 1.5%, the two-bend-one-leveling mode is adopted, the pickling temperature is 79 to 86°C, the acid solution immersion time is 1.5 to 3 minutes, and the free acid concentration of the acid solution is 150 to 180 g / L.

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