A low-cost high-formability 550mpa-grade pickling high-strength steel and its manufacturing method and application

Abstract

The application provides a low-cost high-forming-performance tensile strength 550MPa-grade pickling high-strength steel and a manufacturing method and application thereof, and the steel has the following weight percentage chemical components: C 0.025-0.060%, Si 0.150-0.550%, P 0.015-0.045%, Mn 1.250-1.600%, Als 0.015-0.050%, Ti 0.025-0.065%, B 0.0008-0.0021%, Ca 0.0009-0.0050%, S≤0.0050%, N≤0.0050%, O≤0.0050%, the balance being Fe and inevitable impurities, 0.35≤Si+10×P≤0.90, and 0.25≤10×B / P≤1.15; the steel is compounded and strengthened by Si and P ferrite, and a low-temperature coiling process is adopted to improve the flanging and hole-expanding forming performance of the steel.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-strength steel for automobiles, and particularly relates to a low-cost high-forming-performance pickling high-strength steel with a tensile strength of 550 MPa as well as a manufacturing method and application thereof. BACKGROUND

[0002] Automobile steel materials are used at market customers, and poor forming performance is one of the most common technical problems, which affects the stamping forming quality of automobile steel materials at market customers and directly affects the satisfaction of market customers to steel material suppliers. How to manufacture high-quality automobile steel materials meeting the needs of market customers at low cost is an important direction to improve the market competitiveness of automobile steel materials.

[0003] A patent with a publication number of CN 104630623 A published on May 20, 2015 discloses a hot-rolled pickling strip steel with high hole expansion performance and a production method thereof. The chemical composition of the steel plate is as follows: C 0.05-0.10%, Si≤0.05%, Mn 0.8-1.2%, P≤0.02%, S≤0.006%, Nb 0.015-0.05%, and the balance is Fe and inevitable impurities. The thickness specification of the hot-rolled steel plate is 1.60-6.00 mm, the rolling speed varies with the thickness specification and is controlled at 5-12 m / s, the rough rolling temperature is 980-1080℃, the finish rolling temperature is 820-900℃, and after the rolling process, a segmented cooling method is adopted, the hot-rolled steel plate is first rapidly cooled to 600-680℃, then air-cooled for 5-10 s, and then water-cooled to 400-500℃. The microstructure is ferrite + bainite, the volume ratio of bainite structure is 10-20%, the tensile strength is≥540 MPa, the hole expansion performance λ is≥100%, and the surface quality of the strip steel is excellent. The product of the invention patent contains Nb element, the alloy cost is high, the hot-rolling process adopts a segmented cooling method, the time of air cooling cannot be accurately controlled, the coiling temperature cannot be accurately controlled, and thus the volume content of bainite structure cannot be accurately controlled.

[0004] CN 103602890 A published on February 26, 2014 discloses "a tensile strength of 540 MPa grade high hole expansion steel plate and its manufacturing method", the chemical composition of the steel plate is: C 0.02-0.10%, Si 0.1-1.2%, Mn 1.50-2.20%, P≤0.02%, S≤0.003%, Al 0.02-0.06%, Nb 0.005-0.05%, Ca<0.0050%, the balance is Fe and inevitable impurities. The furnace heating temperature is 1180-1250℃, the final rolling temperature is 860-920℃, and the coiling temperature is 450-550℃. The microstructure is uniform ferrite single phase structure, and the product of the strength and the hole expansion rate of the steel plate is greater than or equal to 48000 MPa%. The product of the invention contains Nb element, and the alloy cost is relatively high; the Si element content of the product of the invention is relatively high, and red rust is prone to occur on the surface of the strip.

[0005] CN 107663609 A published on February 6, 2018 discloses "a production method of a low-cost high-hole-expansion 540 MPa grade hot-rolled pickled plate", the chemical composition of the steel plate is: C 0.030-0.060%, Si 0.50-0.60%, Mn 1.70-1.90%, P≤0.02%, S≤0.015%, Al 0.02-0.07%, N≤0.0060%, the balance is Fe and inevitable impurities. The rough rolling temperature is 1170-1290℃, the finish rolling temperature is 860-880℃, the cooling rate is 15-25℃ / s, and the coiling temperature is 400-420℃. The microstructure is ferrite + bainite, the yield strength is 452-471 MPa, the tensile strength is 567-597 MPa, the elongation is 25.0-34.5%, and the hole expansion rate is 91.0-146.4%. The product of the invention has a high Mn element content, and is prone to form a banded structure, which reduces the uniformity of the structure. SUMMARY

[0006] The purpose of the present application is to provide a low-cost high-forming-performance tensile strength of 550 MPa grade pickled high-strength steel and its manufacturing method, which uses Si and P composite strengthening ferrite (eliminates the segregation of P element at the grain boundary by B element, so that the beneficial effect of P element on strengthening ferrite can be fully played), reduces the hardness difference between ferrite soft phase and bainite hard phase, and uses low-temperature coiling process to refine the grain and comprehensively improve the flanging and hole expansion forming performance of the pickled finished coil. The laminar flow cooling in the hot rolling process adopts a continuous cooling mode, which is simple, stable and controllable, and easy to manufacture. No Mo, Nb, V and other valuable alloy elements are added, and the alloy cost is low.

[0007] The application provides application of a low-cost high-forming-performance tensile strength 550MPa-grade pickling high-strength steel to automobile part manufacturing.

[0008] The specific technical scheme of the application is as follows.

[0009] A low-cost high-forming-performance tensile strength 550MPa-grade pickling high-strength steel comprises the following chemical components in percentage by weight:

[0010] C 0.025-0.060%, Si 0.150-0.550%, P 0.015-0.045%, Mn 1.250-1.600%, Als 0.015-0.050%, Ti 0.025-0.065%, B 0.0008-0.0021%, Ca 0.0009-0.0050%, S≤0.0050%, N≤0.0050%, O≤0.0050%, and the balance of Fe and inevitable impurities.

[0011] The low-cost high-forming-performance tensile strength 550MPa-grade pickling high-strength steel comprises the following chemical components in percentage by weight:

[0012] C 0.035-0.055%, Si 0.250-0.500%, P 0.015-0.035%, Mn 1.280-1.500%, Als 0.021-0.035%, Ti 0.035-0.055%, B 0.0010-0.0019%, Ca 0.0010-0.0035%, S≤0.0035%, N≤0.0035%, O≤0.0025%, and the balance of Fe and inevitable impurities.

[0013] The chemical components of the low-cost high-forming-performance tensile strength 550MPa-grade pickling high-strength steel satisfy 0.35≤Si+10×P≤0.90, and 0.25≤10×B / P≤1.15.

[0014] Preferably, 0.35≤Si+10×P≤0.80, and 0.35≤10×B / P≤0.90.

[0015] In the above formula, each element represents the content × 100.

[0016] The low-cost high-forming-performance tensile strength 550MPa-grade pickling high-strength steel has a thickness of 1.50-5.50mm and a width of 880-1800mm.

[0017] Further, the low-cost high-forming-performance acid-washing high-strength steel with a tensile strength of 550 MPa has a metallographic structure containing 75.0-86.5% of ferrite and 13.5-25.0% of granular bainite in volume percentage, and a ferrite grain size of 11.5-13.5.

[0018] Further, the low-cost high-forming-performance acid-washing high-strength steel with a tensile strength of 550 MPa has a yield strength of ≥450 MPa, a tensile strength of ≥550 MPa, an elongation (A 50 ) of ≥23.0%, and a hole expansion ratio (HER) of ≥90.0%.

[0019] The application further provides a manufacturing method of the low-cost high-forming-performance acid-washing high-strength steel with a tensile strength of 550 MPa, which comprises the following technological procedures: hot metal pretreatment, converter smelting, alloy fine adjustment station, LF furnace refining, continuous casting, hot rolling (→ finishing), acid washing, oil coating and precise packaging.

[0020] The specific manufacturing method is as follows:

[0021] After the hot metal pretreatment, converter smelting, alloy fine adjustment station and LF furnace refining, calcium treatment is performed after the LF furnace refining procedure is completed, the stopper argon blowing is promoted to promote the floating of inclusions, the dynamic soft reduction and electromagnetic stirring roller are used, the superheat degree of the molten steel is controlled to be 10-25 ℃, the segregation and loose defects of the continuous casting billet are reduced, the continuous casting billet is formed after the conventional smelting, the casting speed of the continuous casting billet is controlled to be 0.90-1.80 m / min, the thickness of the continuous casting billet is 210-250 mm, and the center segregation mass of the continuous casting billet is stably controlled to be above C1.0;

[0022] The hot rolling comprises heating, rolling and coiling.

[0023] The heating is performed at a continuous casting billet discharge temperature of 1210-1265 ℃ and a furnace time of 2.1-5.0 h.

[0024] The rolling is performed at a rough rolling opening temperature of 1150-1180 ℃, an intermediate billet thickness of 25-50 mm, a finishing rolling opening temperature of 990-1090 ℃ and a final rolling temperature of 830-900 ℃.

[0025] After the rolling, laminar flow cooling is performed, the strip steel temperature is cooled to a temperature range of 350-480 ℃ in a continuous cooling mode by using cooling water or other cooling medium, and then coiling is performed.

[0026] The hot rolling coil is obtained after the coiling, if the plate shape of the hot rolling coil is poor, finishing is required to correct the plate shape, and the finishing elongation is 1.0-3.1%, if the plate shape of the hot rolling coil is good, finishing is not required.

[0027] The acid washing is performed at an acid liquid temperature of 80-95 ℃, a flattening elongation of 0.5-1.5% and a strip steel running speed of 80-190 m / min.

[0028] The oiling and finishing packaging, the strip steel is dried at 100-180 DEG C, and then electrostatically oiled, the oiling amount is 0.50-1.50 g / m 2 The pickling finished product coil is obtained after the finishing packaging.

[0029] The application of the low-cost high-forming-performance pickling high-strength steel with tensile strength of 550 MPa is provided, and is used for manufacturing automobile parts.

[0030] The alloy system design idea of the product is as follows:

[0031] C: C element is the most effective and basic inexpensive solid solution strengthening element for the steel to obtain sufficient strength, but the C element content is too high, which reduces the flanging and hole expanding forming performance of the steel, therefore, the C element content is controlled to be 0.025-0.060%.

[0032] Si: Si element purifies and strengthens ferrite, can improve the strength and flanging and hole expanding forming performance of the steel, and is low in price, but the Si element content is too high, and red rust is prone to appear on the surface of the hot coil, therefore, the Si element content is controlled to be 0.150-0.550%.

[0033] P: P element is dissolved in ferrite, plays a role in strengthening ferrite, reduces the hardness difference between ferrite soft phase and bainite hard phase, improves the flanging and hole expanding forming performance of the steel, and especially has more obvious beneficial effect in cooperation with B element, but if the P element content is too high, the plasticity and toughness of the steel are reduced, therefore, the P element content is controlled to be 0.015-0.045% by comprehensively considering.

[0034] Mn: Mn element is a low-cost solid solution strengthening element for significantly improving the strength of the steel, but the Mn element content is too high, and strip-shaped structure (reducing the uniformity of the structure) is prone to be formed, and the flanging and hole expanding forming performance of the steel is reduced, therefore, the Mn element content is controlled to be 1.250-1.600%.

[0035] Als: Al element can effectively deoxidize, and reacts with N element to generate AlN (reduces the reaction between N element and Ti element, and increases the effective Ti content), AlN effectively prevents grain coarsening, and plays a role in refining grains to a certain extent; but the Al element content is too high, and the smelting castability is reduced, therefore, the Al element content is controlled to be 0.015-0.050%.

[0036] Ti: Ti element is a strong carbonitride forming element, which plays a role of precipitation strengthening and fine-grain strengthening, and improves the strength and plasticity and toughness of the steel; refines and strengthens ferrite, reduces the hardness difference between ferrite soft phase and bainite hard phase, and improves the flanging and flaring forming performance of the steel; however, if the content of Ti element is too high, the mechanical property stability of the steel cannot be guaranteed, therefore, the content of Ti element is controlled to be 0.025-0.065% in the application.

[0037] B: B element can eliminate the segregation of P element at the grain boundary, so that the beneficial effect of solid solution of P element in ferrite can be fully played, and the flanging and flaring forming performance of the steel is improved; however, if the content of B element is too high, the strength (low temperature coiling temperature range) of the steel will be obviously increased, therefore, the content of B element is controlled to be 0.0008-0.0021% in the application.

[0038] Ca: Ca element plays a role of deoxidation and desulfurization, so that the beneficial effect of solid solution of Si element in ferrite can be fully played; refines, plasticizes and homogenizes non-metallic inclusions, and improves the flanging and flaring forming performance of the steel; however, if the content of Ca element is too high, the purity of the molten steel will be reduced, therefore, the content of Ca element is controlled to be 0.0009-0.0050% in the application.

[0039] S: S element is a harmful impurity element in the steel, which significantly reduces the plasticity and toughness and ductility of the steel, and reduces the flanging and flaring forming performance of the steel, therefore, the content of S element is strictly controlled to be below 0.0050% in the application.

[0040] N: N element can improve the strength and welding performance of the steel; however, N element is not conducive to the control of inclusions, and reduces the effective Ti content, which affects the mechanical property stability of the Ti micro-alloyed steel, therefore, the content of N element is strictly controlled to be below 0.0050% in the application.

[0041] O: O element is a harmful impurity element in the steel, which easily forms non-metallic inclusions, and reduces the flanging and flaring forming performance of the steel, therefore, the content of O element is strictly controlled to be below 0.0050% in the application.

[0042] The product alloy system of the present application satisfies the calculation formula: 0.35≤Si+10×P≤0.90, 0.25≤10×B / P≤1.15, which functions as follows: Si and P compound strengthening ferrite (eliminating P grain boundary segregation by B so that the beneficial effect of P can be brought into play), reducing the hardness difference between soft phase and hard phase. If the value of (Si+10×P) is too low, the effect of strengthening ferrite is not obvious, and if the value of (Si+10×P) is too high, the surface quality of pickling finished product coil cannot be ensured, and the harmful effect of P is greater than the beneficial effect. If the value of (10×B / P) is too low, P grain boundary segregation cannot be eliminated, and the harmful effect of P is greater than the beneficial effect, and if the value of (10×B / P) is too high, the strength of the steel will be significantly reduced.

[0043] Compared with the prior art, Si and P compound strengthening ferrite (eliminating P grain boundary segregation by B so that the beneficial effect of P can be brought into play), reducing the hardness difference between ferrite soft phase and bainite hard phase, supplemented by low-temperature coiling process, refining grains, and comprehensively improving the flanging and hole-expanding forming performance of pickling finished product coil; the laminar cooling in the hot rolling process adopts continuous cooling mode, the process is simple and stable and controllable, and easy to manufacture; no valuable alloy elements such as Mo, Nb and V are used, the alloy cost is low, and the product has strong market competitiveness. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1 The metallographic structure diagram of the low-cost high-forming-performance pickling high-strength steel of tensile strength 550 MPa grade of Example 1. DETAILED DESCRIPTION

[0045] Examples 1-3

[0046] A manufacturing method of a low-cost high-forming-performance pickling high-strength steel of tensile strength 550 MPa grade, comprising the following process flow:

[0047] hot metal pretreatment→ converter smelting→ alloy fine adjustment station→ LF furnace refining→ continuous casting→ hot rolling (→ finishing)→ pickling→ oiling and precision packaging.

[0048] The specific manufacturing method is as follows:

[0049] After hot metal pretreatment, converter smelting, alloy fine adjustment station and LF furnace refining, calcium treatment is performed, stopper argon blowing is promoted to promote inclusion floating, dynamic soft reduction and electromagnetic stirring roller are used, the superheat degree of the molten steel is controlled at 10-25 ℃, the segregation and porosity defects of the continuous casting billet are reduced, the continuous casting billet is formed after conventional smelting, the casting speed of the continuous casting billet is controlled at 0.90-1.80 m / min, the thickness of the continuous casting billet is 210-250 mm, and the center segregation quality of the continuous casting billet is stably controlled at C1.0 or above; the pickling high-strength steel continuous casting billet of tensile strength 550 MPa grade with the chemical composition shown in Table 1 is manufactured, and the balance not shown in Table 1 is Fe and unavoidable impurities.

[0050] The hot rolling includes heating, rolling and coiling;

[0051] The heating is at a continuous casting billet discharge temperature of 1210-1265℃ and a furnace time of 2.1-5.0h;

[0052] The rolling is at a rough rolling opening temperature of 1150-1180℃, an intermediate billet thickness of 25-50mm, a finish rolling opening temperature of 990-1090℃ and a final rolling temperature of 830-900℃;

[0053] After rolling, cooling is performed by using cooling water or other cooling medium to cool the strip temperature in a continuous cooling manner to a temperature range of 350-480℃ for coiling;

[0054] After coiling, a hot-rolled coil is obtained, if the hot-rolled coil is of poor plate shape, finishing and shape correction is needed, the finishing elongation being 1.0-3.1%, if the hot-rolled coil is of good plate shape, finishing is not needed;

[0055] The pickling is at an acid liquid temperature of 80-95℃ and a skin pass elongation of 0.5-1.5%, the strip running speed being 80-190m / min;

[0056] The oiling and finishing packaging are performed by drying the strip at a temperature range of 100-180℃, then electrostatically oiling, the oiling amount being 0.50-1.50g / m 2 After finishing packaging, a pickled finished product coil is obtained.

[0057] Table 1 Chemical composition of the steels of each example

[0058]

[0059] Table 1 (continued)

[0060]

[0061]

[0062] Note: Comparative Example 1 is Chinese published patent CN 104630623 A, Comparative Example 2 is Chinese published patent CN 103602890 A and Comparative Example 3 is Chinese published patent CN 107663609 A.

[0063] The key processes of the hot rolling process of the steels of each example are shown in Table 2.

[0064] Table 2 Key process parameters of the hot rolling process of the steels of each example

[0065] Serial number Casting slab thickness (mm) Heating temperature (°C) Time in furnace (h) Finishing temperature (°C) Coiling temperature (°C) Whether continuous cooling Example 1 230 1235 5.0 850 395 Yes Example 2 230 1265 3.5 880 380 Yes Example 3 230 1251 4.5 890 410 Yes Comparative Example 1 / / / 840 430 No Comparative Example 2 / 1235 / 910 490 Yes Comparative Example 3 / / / 870 410 Yes Comparative Example 4 230 1250 3.5 880 450 Yes Comparative Example 5 230 1295 4.1 865 395 Yes Comparative Example 6 230 1251 3.8 886 610 Yes

[0066] The mechanical properties and application properties of the steels of the examples and the comparative examples are shown in Table 3. The determination method of the mechanical properties (yield strength, tensile strength, elongation) adopts the national standard GB / T 228.1-2010, and the test type number is P6, and the sample direction is transverse (90° direction). The determination method of the application property technical index of the flanging and hole expanding forming property, i.e. the hole expansion ratio (HER), strictly adopts the national standard GB / T 15825.4-2008.

[0067] Table 3 Mechanical properties and application properties of the steels of the examples and the comparative examples

[0068]

[0069]

[0070] The test results show that the low-cost high-forming-property pickling high-strength steel with a tensile strength of 550 MPa manufactured by the application has a metallographic structure containing 75.0-86.5% of ferrite by volume percentage and 13.5-25.0% of granular bainite by volume percentage, and the ferrite grain size is 11.5-13.5, and the flanging and hole expanding forming property of the pickled finished steel coil is excellent.

[0071] According to the comparative examples 4-6, it can be obtained that only the chemical composition of the steel and the hot rolling process are controlled according to the application, the technical effects of the application can be obtained, otherwise, only the chemical composition of the steel meets but does not meet the calculation formula of the application, or the key process parameters of the hot rolling process do not meet the requirements of the application, which will lead to the decrease of the performance of the pickled finished coil.

[0072] The above-mentioned reference examples carefully describe a low-cost high-forming-property pickling high-strength steel with a tensile strength of 550 MPa and a manufacturing method thereof, which are illustrative but not limiting, and a number of examples can be listed according to the limited range, therefore, any changes and modifications under the overall technical concept of the application shall belong to the protection scope of the application.

Claims

1. A low-cost, high-formability, 550MPa-grade pickled high-strength steel, characterized in that: The low-cost, high-formability, 550MPa-grade pickled high-strength steel comprises the following chemical components in weight percentage: C 0.025~0.060%, Si 0.150~0.550%, P 0.015~0.045%, Mn 1.250~1.600%, Als0.015~0.050%, Ti 0.025~0.065%, B 0.0008~0.0021%, Ca 0.0009~0.0050%, S≤0.0050%, N≤0.0050%, O≤0.0050%, the balance is Fe and unavoidable impurities.

2. The low-cost, high-formability, 550MPa-grade pickled high-strength steel according to claim 1, characterized in that: The low-cost, high-formability, 550 MPa-grade pickled high-strength steel comprises the following chemical components in weight percentage: C 0.035~0.055%, Si 0.250~0.500%, P 0.015~0.035%, Mn 1.280~1.500%, Als0.021~0.035%, Ti 0.035~0.055%, B 0.0010~0.0019%, Ca 0.0010~0.0035%, S≤0.0035%, N≤0.0035%, O≤0.0025%, the balance is Fe and unavoidable impurities.

3. The low-cost, high-formability, 550 MPa-grade pickled high-strength steel according to claim 1 or 2, characterized in that: The chemical composition of the low-cost, high-formability, and 550 MPa-grade pickled high-strength steel satisfies the following requirements: 0.35≤Si+10×P≤0.90, and 0.25≤10×B / P≤1.

15.

4. The low-cost, high-formability, 550 MPa-grade pickled high-strength steel according to claim 1 or 2, characterized in that: The metallographic structure of the low-cost, high-formability, and 550MPa-grade pickled high-strength steel contains 75.0-86.5% by volume of ferrite and 13.5-25.0% by volume of granular bainite, and the ferrite grain size is 11.5-13.

5.

5. The low-cost, high-formability, 550 MPa-grade pickled high-strength steel according to claim 1 or 2, characterized in that: The low-cost, high-formability, tensile strength 550MPa grade pickled high-strength steel has a yield strength of ≥450MPa, a tensile strength of ≥550MPa, and an elongation (A 50 )≥23.0%, hole expansion ratio (HER)≥90.0%.

6. A method for manufacturing the low-cost, high-formability, 550 MPa-grade pickled high-strength steel according to any one of claims 1 to 5, characterized in that: The manufacturing method includes hot rolling, which includes heating and rolling. In the heating, the continuous casting billet is discharged from the furnace at a temperature of 1210 to 1265° C. and is kept in the furnace for 2.1 to 5.0 hours.

7. The manufacturing method according to claim 6, characterized in that In the rolling process, the starting temperature of the rough rolling is 1150-1180° C., the thickness of the intermediate billet is 25-50 mm, the starting temperature of the finishing rolling is 990-1090° C., and the final rolling temperature is 830-900° C.

8. The manufacturing method according to claim 6 or 7, characterized in that After rolling, laminar cooling is carried out, and cooling water or other cooling media are used to cool the strip temperature to a temperature range of 350-480℃ in a continuous cooling manner for coiling.

9. The manufacturing method according to claim 6 or 7, characterized in that The manufacturing method includes pickling, the acid solution temperature is 80-95°C, the flat elongation is 0.5-1.5%, and the strip running speed is 80-190m / min.

10. Use of the low-cost, high-formability, 550 MPa-tensile-strength pickled high-strength steel according to any one of claims 1 to 5 for the manufacture of automotive parts.

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