An economic 800mpa grade high surface quality high hole expansion rate pickling automotive steel and its manufacturing method and application

Abstract

The application discloses an economic 800MPa-grade high-surface-quality high-expansion-rate pickling automobile steel and a manufacturing method and application thereof, wherein the pickling automobile steel contains C: 0.04% to 0.06%, Al: 0.25% to 0.35%, Nb: 0.02% to 0.05%, Ti: 0.08% to 0.12%, Mo: 0.05% to 0.09%, Cr: 0.25% to 0.35% and Si: 0.05% to 0.15%. The Si content in the composition is relatively low, and higher surface quality is more easily obtained by cooperating with subsequent production processes; the pickling automobile steel does not contain V and has relatively low Mo content, thereby showing higher economy and being capable of obtaining higher strength and expansion rate performance; the expansion rate of the pickling automobile steel is greater than or equal to 65%; and the pickling automobile steel can be used for producing automobile chassis parts, cantilever parts and frame, girder and other parts of commercial vehicles or heavy trucks, thereby reducing the thickness of the parts and playing a lightening role.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel and iron, and particularly relates to an economic 800MPa-grade high-surface-quality high-hole-expanding-rate pickling automotive steel as well as a manufacturing method and application thereof. BACKGROUND

[0002] Energy saving, environmental protection and safety are three major challenges faced by the current automobile industry. The implementation of automobile light weight can reduce the weight of the vehicle body and energy consumption without sacrificing safety performance. At the same time, light weight will also bring the improvement of vehicle handling stability and collision safety to a certain extent. In the selection of automobile light weight materials, the application of high-strength steel is one of the main ways.

[0003] With the development of new energy vehicles, some complex parts, such as automobile swing arms and spring arms, are gradually replaced by 800MPa-grade pickling hole expanding steel with higher strength. Since these parts require high forming performance and good coating performance, the requirements for product appearance (surface quality) and formability (elongation, hole expanding rate, etc.) are also increasing. Therefore, how to reduce the cost of materials (alloy cost, production process) through reasonable composition and process design, and ensure that the material has high strength and ductility, and the hole expanding rate is further improved. For example, it is required to further improve the hole expanding rate index from the existing 50% to more than 65% on the basis of the existing 800MPa-grade strength.

[0004] The prior art discloses some 800MPa-grade pickling high-strength hole expanding steels, but these 800MPa-grade pickling high-strength hole expanding steels either have high alloy cost or the hole expanding rate cannot be stably maintained at more than 65%.

[0005] Chinese patent CN104513930A discloses a super-high strength hot-rolled multi-phase steel plate and strip with good bending and hole expansion performance and a manufacturing method thereof, discloses a multi-phase steel composed of bainite, ferrite, martensite and residual austenite microstructure, which uses C, Mn and Si as main alloying elements, and the mass percentage of each element is: C: 0.07% to 0.14%, Si: 0.10% to 0.40%, Mn: 1.55% to 2.00%, Al: 0.010% to 0.050%, P≤0.015%, S≤0.004%, N≤0.005%, and at least one of Nb≤0.07%, Ti: 0.02% to 0.15%, V: 0.10% to 0.20%, and at least one of Cr: 0.15% to 0.50% and Mo: 0.05% to 0.20%, and the rest is Fe and unavoidable impurities. The process used is finish rolling temperature 900 to 950°C, coiling temperature 430 to 550°C, product yield strength 700MPa or more, tensile strength 800MPa or more, and hole expansion rate≥50%. The present application adopts specific composition, process design, and investigates the bending and hole expansion performance of the multi-phase steel, and evaluates the cold bending diameter and hole expansion rate respectively, but the patent contains high Mo and V alloys, the cost is high, and the hole expansion rate cannot be stably maintained at≥65%.

[0006] Chinese patent CN10659169A discloses a hot-rolled complex phase steel with excellent flanging performance and a production method thereof, discloses a complex phase steel composed of granular bainite, which adopts C, Mn and Si as main alloying elements, and the mass percentage of each element is as follows: C: 0.04%-0.10%, Si: 0.10%-0.20%, Mn: 1.00%-2.00%, Al≤0.040%, P≤0.015%, S≤0.004%, N≤0.005%, Re: 0.1-0.4%, and further including Nb: 0.04-0.06% or Ti: 0.010%-0.030% and V: 0.05%-0.10%, and Cr: 0.25%-0.45% and Mo: 0.1%-0.40%, Ni: 0.1%-0.40%, and the rest is Fe and inevitable impurities. Rare earth element Re is added on the basis of C-Si-Mn composition to optimize the shape and size of inclusions of the steel grade, and to improve the hole expansion rate of the steel; the plasticity and uniformity of the structure are improved through structure control, and the hole expansion rate of the steel is improved; low Si composition design and descaling at each pass of rough rolling are adopted to eliminate surface red scales and improve the surface quality of the product, the process adopted is that the finish rolling temperature is 850-900℃, the coiling temperature is 500-600℃, the yield strength of the product is above 700MPa, the tensile strength is above 800MPa, and the product thickness is less than 6.0mm. The invention adopts specific composition and process design, improves the hole expansion performance and surface quality of the product, although the hole expansion rate is good, but high Mo and V alloys are added, and the cost is high.

[0007] Chinese patent CN103290320A discloses a hot-rolled complex phase steel and a production method thereof, which adopts high Si and low Mn composition design, and the mass percentage of each element is as follows: C: 0.07%-0.10%, Si: 0.5%-1.0%, Mn: 0.9%-1.5%, Al:≤0.05%, P≤0.020%, S≤0.004%, B: 0.0005%-0.0010%, N≤0.0050%, Ti: 0.060%-0.150%, and the rest is Fe and inevitable impurities. The final rolling temperature is 880-900℃, and the coiling temperature is 590-620℃, and the yield strength is in the range of 750-800MPa, the tensile strength is 820-950MPa, the elongation is 14%-20%, and the yield strength ratio is less than 0.9, and the product has good strength and plasticity matching. The Si content in the patent is high, which is 0.5-1.0%, which is not conducive to obtaining high surface quality, and the patent does not focus on the hole expansion performance and high surface quality of the product. SUMMARY

[0008] In view of the high-strength and high-surface quality and forming process performance requirements of 800MPa grade pickling automobile steel for automobile lightweight, the application provides an economic 800MPa grade high-surface quality high-expansion rate pickling automobile steel and a manufacturing method and application thereof, the Si content in the composition is low, and higher surface quality is more easily obtained, and while higher strength and expansion rate performance are obtained, the composition does not contain V, and the Mo content is low, and higher economy can be shown.

[0009] The technical scheme adopted by the application is as follows:

[0010] An economic 800MPa grade pickling automobile steel, the pickling automobile steel comprises the following chemical components in percentage by weight: C: 0.04%-0.06%, Mn: 1.60%-1.80%, Al: 0.25%-0.35%, P≤0.015%, S≤0.008%, Si: 0.05%-0.15%, Nb: 0.02%-0.05%, Ti: 0.08%-0.12%, Mo: 0.05%-0.09%, Cr: 0.25%-0.35%, N≤0.0050%, and the rest is Fe and inevitable impurities.

[0011] The metallographic structure of the pickling automobile steel is ferrite and bainite, wherein the area ratio of the ferrite is 60%-90%, good expansion rate performance is obtained through organization regulation, and the product strength is ensured through the nano precipitation strengthening effect.

[0012] The yield strength of the pickling automobile steel is ≥700MPa, the tensile strength is ≥800MPa, the elongation A 50 is ≥20%, the expansion rate is ≥65%, and the surface roughness is ≤2.0μm.

[0013] Further, the yield strength of the pickling automobile steel is 730-850MPa, the tensile strength is 800-870MPa, the elongation A 50 is 20-25%, the expansion rate is 65-75%, and the surface roughness is ≤2.0μm.

[0014] The thickness of the pickling automobile steel is 2.0-4.5mm.

[0015] The manufacturing method of the economic 800MPa grade pickling automobile steel comprises the following steps: steelmaking, continuous casting, hot billet charging, hot continuous rolling, laminar cooling, coiling and pickling.

[0016] The steelmaking step specifically comprises: hot metal pretreatment, converter smelting, alloy fine adjustment station, LF refining and RH refining.

[0017] In the continuous casting step, the crystallizer liquid level fluctuation is controlled within ±3mm, dynamic soft reduction and electromagnetic roller stirring are used to make impurities sufficiently float to obtain good internal quality of the casting blank.

[0018] In the hot charging of the casting blank, the charging temperature is not lower than 400℃, which can reduce the crack of the casting blank and save energy consumption.

[0019] In the hot continuous rolling step, the casting blank is heated to an outlet temperature of 1200-1250℃ and is kept for 2-3 hours to effectively reduce the rolling deformation resistance and reduce the thickness of the decarburized layer and the oxidized layer on the surface of the casting blank and the steel plate.

[0020] In the rough rolling of the hot continuous rolling step, 3+3 pass rolling is performed, and the descaling water is fully opened, and the rough rolling is performed to an intermediate blank with a thickness of 30-50mm.

[0021] In the finishing rolling step, the surface of the rolling roller is polished or new, and the rolling is performed in the first 1 / 3 period of a rolling cycle, which can ensure the surface quality of the hot-rolled product.

[0022] In the finishing rolling step, 7-pass finishing rolling is performed on a 2250mm hot continuous rolling mill, the finishing rolling inlet temperature is 1030-1050℃, and the finishing rolling temperature is controlled at 840-900℃ to obtain fine and uniform microstructure and reduce the generation of iron oxide scale. Because finer microstructure can be obtained at a relatively lower rolling temperature, and the oxidation rate is also slower at a relatively lower temperature, the generation and thickness of the iron oxide scale can be reduced.

[0023] In the laminar flow cooling step, the front section cooling mode is used for rapid cooling, and the cooling speed is ≥30℃ / s. On the one hand, it can prevent the transformation of supercooled austenite into polygonal ferrite and pearlite, and form a microstructure mainly composed of bainite; on the other hand, it can reduce the precipitation of Nb and Ti elements at high temperature, promote the precipitation of Nb and Ti elements at low temperature, increase the precipitation strengthening effect, and make the size of the precipitated two-phase particles smaller and more dispersed. In the actual production process, the upper limit of the cooling speed is not more than 90℃ / s due to the equipment capacity and water cooling effect.

[0024] In the coiling step, the coiling temperature is 520-580℃, and the coiling temperature is one of the key process parameters for obtaining high strength and high hole expansion rate. When the coiling temperature is higher than 580℃, the strength of ferrite is reduced due to the precipitation and coarsening of carbides, thereby reducing the hole expansion rate of the product and affecting the surface quality of the product. On the other hand, when the coiling temperature is lower than 520℃, the precipitation effect of Nb and Ti micro-alloy elements will be greatly reduced, thereby affecting the strength of the product. In order to ensure the microstructure and surface quality of the product, coiling is performed in the range of 520-580℃.

[0025] In the pickling step, continuous pickling is adopted, the temperature of acid solution is controlled at 50-90 DEG C, the concentration of acid solution is controlled at 20-200 g / L, and the pickling line speed is less than or equal to 150 mpm; in order to further improve the product surface quality, reduce the product surface roughness and improve the product performance, pickling is carried out by adopting rolling force mode, the rolling force is controlled at 100-300 tons, the flattening elongation is 0.2-2.0%, and the flattening liquid is sprayed during the flattening process.

[0026] The flattening liquid is desalted water.

[0027] The economic 800MPa grade pickling automobile steel provided by the application has the following component functions and controls:

[0028] C mainly functions to form required amount of bainite or martensite and ensure the strength of the steel. Controlling C to be enriched in the metastable austenite region and avoiding its precipitation is the guarantee for obtaining polygonal ferrite surrounding island-shaped martensite dual-phase structure. C plays a very important role in the strength (yield and tensile strength), hole expansion performance, cold bending performance and welding performance of the material. On the one hand, too high carbon content will affect the welding performance of the material, and on the other hand, too high C content will make the carbide particles coarse, which is not conducive to the hole expansion performance of the steel plate, so the C content should be reasonably controlled at 0.04%-0.06%.

[0029] Si and Cr: silicon is a preferential oxidation element and has a high diffusion coefficient. Si and Cr are prone to occur in association and enrichment on the surface of the steel plate. At 900 DEG C-1100 DEG C, surface element enrichment oxidation mainly occurs, the surface-enriched Si and Cr first oxidize and lay flat on the interface to form a continuous and dense oxide film (SiO2 and Cr2O3), which can isolate oxygen and play an antioxidant role. At the same time, Cr element can significantly improve the hardenability of the material, so that the finished product has higher strength. The application considers the strict requirements of automobile materials on surface quality, and low Si design is adopted in the steel, preferably, the Si content is controlled at 0.05%-0.15%, and the Cr content is controlled at 0.25%-0.35%.

[0030] Mn element can improve the strength of the steel through solid solution strengthening, and can promote the dissolution of carbonitride precipitated phase during heating, inhibit the precipitation of precipitated phase during rolling, be conducive to keeping more precipitated elements in ferrite during the cooling process after rolling, strengthen the precipitated strengthening, and expand the austenite phase region, reduce the transformation temperature of the undercooled austenite phase, and be conducive to the refinement of the phase change structure. However, when the Mn content is too high, the steelmaking is prone to Mn segregation, and edge cracking is prone to occur during slab continuous casting. In order to ensure the strength of the steel plate and the quality of the slab, the Mn content should be reasonably controlled at 1.60%-1.80%.

[0031] P, S are impurity elements in the steel, the lower the better, but too low P content will increase the cost of steelmaking, therefore, P content is controlled below 0.015% to meet the production cost and product requirements; and S in the steel usually combines with Mn to form MnS inclusions, the number and form of sulfides in the steel directly affect the hole expansion performance and tensile properties of the high hole expansion steel, therefore, S content is controlled below 0.008% in actual production.

[0032] Al is a deoxidizing element in the steel, which can reduce the oxide inclusions in the steel, and can refine the grains, which is beneficial to improve the forming performance of the steel. However, when too high, the refining effect is weakened, and the casting is difficult in production, therefore, the Al content should be reasonably controlled at 0.25% to 0.35%.

[0033] Nb plays a significant role in grain refinement, phase transformation behavior and C enrichment in austenite. Nb can effectively prevent the growth of austenite grains and increase the coarsening temperature of the steel. The fine carbonitride formed by the combination of Nb, C and N can also delay recrystallization and prevent the growth of ferrite grains, thereby having strong grain refinement and strong precipitation strengthening effect. In the rolling deformation process, Nb is induced to precipitate after strain, which increases the nucleation site of ferrite and improves the nucleation rate, thereby refining the grains. However, when the Nb content is too high, the grain refinement and precipitation strengthening effect is not obvious, and considering the cost, the Nb content should be reasonably controlled at 0.02% to 0.05%.

[0034] Ti in the steel plays a similar role to Nb, and compared with Nb, Ti is more active and easy to form compounds with C, N, S, O and other elements in the steel. Because the solid solution temperature of Ti is lower than that of Nb, at the same solid solution temperature, the amount of Ti solid solution is more than that of Nb. Ti generally exists in the form of TiC or Ti(C, N) in the steel. In addition, Ti-containing steel is more sensitive to process. Therefore, the Ti content should be reasonably controlled at 0.08% to 0.12%.

[0035] Mo is a medium-strong carbide forming element, which significantly improves the stability of metastable austenite and strongly suppresses the progress of pearlite transformation, forming a metastable austenite region with a wide temperature range between the ferrite transformation region and the bainite transformation region. At the same time, the addition of Mo shortens the cold speed range of ferrite transformation and reduces the amount of ferrite transformation. The addition of Mo can significantly reduce the sensitivity of the steel grade to the controlled cooling process parameters, and it is easier to obtain a dual-phase structure through controlled rolling and controlled cooling process on a hot continuous rolling mill. However, too high Mo content will significantly increase the cost, and the weldability will be greatly reduced. Therefore, the Mo content of the present application should be reasonably controlled at 0.05% to 0.09%.

[0036] In the preparation method of the economical 800MPa grade pickled automobile steel provided by the present invention, the quality of the ingot is guaranteed by controlling the continuous casting and hot charging processes of the ingot; and the surface quality and performance of the steel plate are guaranteed by controlling the rolling, coiling and pickling processes.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] The 800MPa grade pickled automobile steel provided by the present invention has a low Si content in its composition, and is easier to obtain a higher surface quality when combined with subsequent production processes. It does not contain V and has a low Mo content, which can show higher economic efficiency while still achieving high strength and hole expansion performance; its yield strength is ≥600MPa, tensile strength is ≥800MPa, and elongation A is ≥100MPa. 50 ≥20%, hole expansion rate ≥65%, surface roughness ≤2.0μm. It can be used to manufacture automotive chassis parts, cantilever parts, and frames, beams and other parts for commercial vehicles or heavy trucks, reducing part thickness and achieving lightweighting. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a microscopic photograph of the pickled automotive steel in Example 1 under a metallographic microscope;

[0040] Figure 2 This is a scanning electron microscope micrograph of the pickled automotive steel in Example 1;

[0041] Figure 3 This is a surface photo of the pickled automotive steel product in Example 1. It can be seen from the figure that the surface quality is excellent;

[0042] Figure 4 This is a surface photograph of the pickled automotive steel intermediate product in Comparative Example 3;

[0043] Figure 5 This is a surface photograph of the pickled automotive steel product in Comparative Example 3. It can be seen from the figure that the surface quality is poor. DETAILED DESCRIPTION

[0044] The invention provides an economical 800MPa grade pickled automobile steel. The pickled automobile steel comprises the following chemical components in weight percentage: C: 0.04%-0.06%, Si: 0.05%-0.15%, Mn: 1.60%-1.80%, Al: 0.25%-0.35%, P≤0.015%, S≤0.008%, Nb: 0.02%-0.05%, Ti: 0.08%-0.12%, Mo: 0.05%-0.09%, Cr: 0.25%-0.35%, N≤0.0050%, and the remainder is Fe and unavoidable impurities.

[0045] The manufacturing method of the economic 800MPa grade pickling automobile steel comprises the following steps: steelmaking, continuous casting, hot charging of the casting blank, hot continuous rolling, laminar cooling, coiling and pickling.

[0046] In the continuous casting step, the crystallizer liquid surface fluctuation is controlled within ±3mm, dynamic soft reduction and electromagnetic roller stirring are used.

[0047] In the hot charging of the casting blank, the charging temperature is not lower than 400℃.

[0048] In the hot continuous rolling step, the casting blank is heated to a temperature of 1200-1250℃ and is kept for 2-3 hours.

[0049] In the hot continuous rolling step, 3+3 pass rolling is performed in rough rolling, the descaling water is fully opened, and the rough rolling is performed to an intermediate blank with a thickness of 30-50mm.

[0050] In the finishing rolling step, the rolling is performed using a roller with a surface that is ground or completely new, and the rolling is performed in the first 1 / 3 period of a rolling cycle.

[0051] In the finishing rolling step, 7-pass finishing rolling is performed on a 2250mm hot continuous rolling mill, the finishing rolling inlet temperature is 1030-1050℃, and the finish rolling temperature is controlled at 840-900℃.

[0052] In the laminar cooling step, fast cooling is performed using a front section cooling mode, and the cooling speed is ≥30℃ / s.

[0053] In the coiling step, the coiling temperature is 520-580℃.

[0054] In the pickling step, continuous pickling is used, the acid liquid temperature is controlled at 50-90℃, the acid liquid concentration is controlled at 20-200g / L, the pickling line speed is ≤150mpm, pickling is performed using a rolling force mode, the rolling force is controlled at 100-300 tons, the flattening elongation is 0.2-2.0%, and a flattening liquid is sprayed during the flattening process, the flattening liquid is desalinated water.

[0055] The application will be described in detail below with reference to the examples.

[0056] The components and weight percentages of the pickling automobile steel in each example and the comparative example are shown in Table 1.

[0057] Table 1

[0058]

[0059]

[0060] The production process parameters of the pickling automobile steel in each example and the comparative example are shown in Table 2.

[0061] Table 2

[0062]

[0063] In the pickling process, the same pickling process was used in each of the examples and the comparative examples, specifically: the temperature of the acid solution was controlled at 65-75°C, the concentration of the acid solution was controlled at 100g / L, and the pickling line speed was 80mpm; the pickling was performed using a rolling force mode, the rolling force was controlled at 200 tons, the target flat elongation was 1.0%, and desalting water was sprayed during the flattening process.

[0064] The properties of the pickled automotive steel in each of the examples and the comparative examples are shown in Table 3.

[0065] Table 3

[0066]

[0067] As can be seen from the data in Table 3, in Comparative Example 1, the temperature at which the product was coiled was not suitable, resulting in poor elongation and hole expansion rate of the product; in Comparative Example 2, the partial chemical composition was not reasonable, resulting in poor elongation and hole expansion rate of the product, and the higher Mo content resulted in higher cost; in Comparative Example 3, the partial chemical composition was not reasonable, especially the Si content was too high, resulting in poor surface quality of the product.

[0068] The foregoing detailed description of the reference examples of an economic 800MPa grade high surface quality high hole expansion rate pickled automotive steel and its manufacturing method and application is illustrative rather than limiting, and a number of examples can be listed according to the defined range, therefore changes and modifications under the overall concept of the present application shall be within the scope of protection of the present application.

Claims

1. An economical 800MPa grade pickled steel for automobiles, characterized in that: The pickled automotive steel includes the following chemical components in weight percentage: C: 0.04%~0.06%, Mn: 1.60%~1.80%, Al: 0.25%~0.35%, P≤0.015%, S≤0.008%, Si: 0.05%~0.15%, Nb: 0.02%~0.05%, Ti: 0.08%~0.12%, Mo: 0.05%~0.09%, Cr: 0.25%~0.35%, N≤0.0050%, and the rest is Fe and unavoidable impurities.

2. The economical 800 MPa grade pickled automobile steel according to claim 1, characterized in that: The metallographic structure of the pickled automotive steel is ferrite and bainite, wherein the area of ​​ferrite accounts for 60% to 90%.

3. The economical 800 MPa grade pickled automobile steel according to claim 1, characterized in that: The yield strength of the pickled automobile steel is ≥700MPa, the tensile strength is ≥800MPa, and the elongation A 50 ≥20%, hole expansion rate ≥65%, surface roughness ≤2.0μm.

4. The method for producing an economical 800 MPa grade pickled automobile steel according to any one of claims 1 to 3, characterized in that: The manufacturing method comprises the following steps: steelmaking→continuous casting→hot charging of cast billets→hot continuous rolling→laminar cooling→coiling→pickling.

5. The manufacturing method according to claim 4, characterized in that During the continuous casting step, the fluctuation of the steel liquid level in the crystallizer is controlled within ±3 mm, and dynamic soft reduction and electromagnetic roller stirring are used.

6. The manufacturing method according to claim 4, characterized in that When the ingot is hot charged, the temperature of the ingot entering the furnace is not lower than 400°C.

7. The manufacturing method according to claim 4, characterized in that In the hot rolling step, the cast slab is heated to a furnace temperature of 1200-1250° C. and kept at this temperature for 2-3 hours.

8. The manufacturing method according to claim 4, characterized in that In the hot rolling step, the rough rolling is performed in 3+3 passes, and the descaling water is fully opened.

9. The manufacturing method according to claim 4, characterized in that In the hot rolling step, the finishing rolling is performed using rollers whose surfaces have been repaired or are completely new, and the rolling is performed within the first 1 / 3 of a rolling cycle.

10. The manufacturing method according to claim 4, characterized in that: In the hot rolling step, finishing rolling is performed on a 2250mm hot rolling mill for 7 passes, the finishing rolling inlet temperature is 1030-1050°C, and the final rolling temperature is controlled at 840-900°C.

11. The manufacturing method according to claim 4, characterized in that: In the laminar cooling step, a front-stage cooling mode is adopted for rapid cooling, with a cooling rate of ≥30°C / s.

12. The manufacturing method according to claim 4, characterized in that: In the coiling step, the coiling temperature is 520-580°C.

13. The manufacturing method according to claim 4, characterized in that In the pickling step, continuous pickling is adopted, the acid solution temperature is controlled at 50~90°C, the acid solution concentration is controlled at 20~200g / L, and the pickling line speed is ≤150mpm; the pickling is carried out in a rolling force mode, the rolling force is controlled at 100~300 tons, the flattening elongation is 0.2~2.0%, and the flattening liquid is sprayed during the flattening process.

14. Use of the economical 800 MPa grade pickled automobile steel according to any one of claims 1 to 3 in the manufacture of automobile chassis parts, cantilever parts, frames and beams of commercial vehicles or heavy trucks.

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