A low-cost high-formability 450mpa-grade pickling high-strength steel and its manufacturing method and application

Abstract

The application provides a low-cost high-forming-performance tensile strength 450MPa-grade pickling high-strength steel and a manufacturing method and application thereof, and the steel has the following weight percentage chemical components: C 0.025-0.055%, Si 0.100-0.500%, P 0.015-0.035%, Mn 1.000-1.500%, Als 0.015-0.055%, Ti 0.015-0.050%, B 0.0005-0.0018%, Ca 0.0009-0.0050%, S≤0.0050%, N≤0.0050%, O≤0.0050%, 0.35≤Si+10×P≤0.80, 0.25≤10×B / P≤1.00, and the balance is Fe and inevitable impurities. The Si and P are used for complex strengthening 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 present invention belongs to the field of alloys and the technical field of high-strength steel for automobiles, and in particular relates to a low-cost, high-formability, 450MPa-grade pickled high-strength steel and a manufacturing method and application thereof, which are used in automobile manufacturing. Background Art

[0002] As competition in the global automotive manufacturing industry intensifies, automakers are tightening their control over steel raw material costs. For steel suppliers, the key to improving their competitiveness in the automotive steel market is to manufacture high-quality, low-cost automotive steel that meets customer needs. Poor formability is one of the most common technical issues facing automotive steel in customer applications. This impacts the quality of stamping and directly affects customer satisfaction with steel suppliers.

[0003] Traditional basic mechanical properties (yield strength, tensile strength, and elongation) can reflect the plastic toughness of automotive high-strength steel to a certain extent, but cannot fully characterize the formability of automotive high-strength steel. Cold bending performance and flanging and hole expansion performance can more sensitively characterize the formability of automotive high-strength steel.

[0004] Patent publication number CN103469058A, published on December 25, 2013, discloses a "ferritic bainite steel with a tensile strength of 450 MPa and high hole expansion performance and its production method." The chemical composition of the steel plate disclosed in the patent is as follows: C: 0.10-0.15%, Si: 0.10-0.50%, Mn: 0.50-1.0%, P ≤ 0.025%, S ≤ 0.008%, Al: 0.01-0.06%, with the balance being Fe and unavoidable impurities. The high C content in this patented product affects the toughness and weldability of the finished steel coil. During cooling, the front section is laminar cooling, the cooling rate is controlled at 50-80℃ / s, and the cooling stop temperature is 630-700℃; the middle section is air cooling, the air cooling rate is 3-8℃ / s, the air cooling time is 4-8s, and the rear section is laminar cooling, the cooling rate is controlled at 100-140℃ / s. With the segmented cooling method, the cooling time in the air cannot be accurately controlled, the coiling temperature cannot be accurately adjusted to the target value, and the volume content of the bainite structure cannot be accurately guaranteed.

[0005] Publication No. CN107829028A published on March 23, 2018, discloses “A 450MPa-grade economical high-surface-quality high-hole expansion steel and its preparation method”. The chemical composition weight percentage of the steel plate is:

[0006] Co 0.045-0.085%, Si 0.30-0.80%, Mn 0.60-1.30%, Al 0.010-0.050%, P 0.010-0.020%, Ca 0.0010-0.0050%, S≤0.003%, N≤0.0050%, O≤0.0040%, the balance being Fe and inevitable impurities.The patent product adopts a sectional cooling mode in a hot rolling process, adopts a double-stage cooling mode, rapidly cools at a cooling speed of not less than 22℃ / s to 650-700℃, then air cools for 7-12s, and then rapidly cools at a cooling speed of not less than 16℃ / s to 450-520℃.The coiling temperature of coiling is 450-520℃.The slow cooling is slow cooling to 280-350℃ after coiling, and then natural cooling to below 40℃.However, the air cooling time cannot be accurately controlled, the coiling temperature cannot be accurately regulated to a target value, and the volume content of bainite organization cannot be accurately ensured.

[0007] Therefore, how to provide a pickling high-strength steel with low cost, stable and controllable process and excellent flanging and hole expanding forming performance has practical engineering application significance. SUMMARY

[0008] The present application aims to provide a low-cost high-forming-performance 450MPa-grade pickling high-strength steel and a manufacturing method thereof, adopt Si and P composite strengthening ferrite (use B element to inhibit the segregation of P element at the grain boundary, so that the beneficial effect of P element on strengthening ferrite can be fully exerted), reduce the hardness difference between ferrite soft phase and bainite hard phase, and adopt low-temperature coiling process, refine the grain, and comprehensively improve the flanging and hole expanding forming performance of the pickling finished product coil. The laminar flow cooling in the hot rolling process adopts a continuous cooling mode, the process is simple and stable and controllable, and easy to manufacture. No Mo, Nb, V and other valuable alloy elements are used, and the alloy cost is low.

[0009] The present application also aims to provide an application of a low-cost high-forming-performance 450MPa-grade pickling high-strength steel, for manufacturing automobile parts.

[0010] The specific technical scheme of the present application is as follows:

[0011] A low-cost high-forming-performance 450MPa-grade pickling high-strength steel, comprising the following weight percentage chemical components:

[0012] C 0.025-0.055%, Si 0.100-0.500%, P 0.015-0.035%, Mn 1.000-1.500%, Als 0.015-0.055%, Ti 0.015-0.050%, B 0.0005-0.0018%, Ca 0.0008-0.0050%, S≤0.0050%, N≤0.0050%, O≤0.0050%, balance Fe and inevitable impurities.

[0013] Preferably, the low-cost high-formability tensile strength 450 MPa grade pickled high-strength steel includes the following weight percentage chemical components:

[0014] C 0.035-0.055%, Si 0.150-0.450%, P 0.015-0.030%, Mn 1.150-1.350%, Als 0.025-0.045%, Ti 0.025-0.035%, B 0.0008-0.0015%, Ca 0.0009-0.0035%, S≤0.0035%, N≤0.0035%, O≤0.0025%, balance Fe and inevitable impurities.

[0015] Further, the chemical components of the low-cost high-formability tensile strength 450 MPa grade pickled high-strength steel satisfy: 0.35≤Si+10×P≤0.80, 0.25≤10×B / P≤1.00.

[0016] Further, 0.45≤Si+10×P≤0.80, 0.35≤10×B / P≤0.90.

[0017] In the above formula calculation, each element represents its content x 100.

[0018] The thickness of the low-cost high-formability tensile strength 450 MPa grade pickled high-strength steel is

[0019] 1.50-5.50 mm, and the width is 880-1800 mm.

[0020] Further, the metallographic structure of the low-cost high-formability tensile strength 450 MPa grade pickled high-strength steel contains 79.5-91.0% by volume of ferrite and 9.0-20.5% by volume of granular bainite, and the ferrite grain size is 10.5-12.5.

[0021] Further, the low-cost high-formability tensile strength 450 MPa grade pickled high-strength steel has a yield strength ≥350 MPa, a tensile strength ≥450 MPa, and an elongation (A 50) ≥ 31.0%, hole expansion ratio (HER) ≥ 100.0%.

[0022] The application provides a manufacturing method of a low-cost high-forming-performance tensile strength 450MPa-grade pickling high-strength steel.

[0023] The continuous casting is performed after the LF furnace refining, calcium treatment is performed, stopper argon blowing is promoted to promote the floating of inclusions, dynamic soft reduction and electromagnetic stirring rollers are used, the superheat degree of the molten steel is controlled to be 10-25 DEG C, the segregation and porosity defects of the continuous casting billet are reduced, the continuous casting billet is formed after conventional smelting, the pulling speed of the continuous casting billet is controlled to be 0.90-2.10 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.

[0024] The hot rolling comprises heating, rolling and coiling.

[0025] The heating is performed at a continuous casting billet discharge temperature of 1210-1250 DEG C and a furnace time of 2.1-5.0 h.

[0026] The rolling is performed at a rough rolling opening temperature of 1150-1180 DEG C, an intermediate billet thickness of 25-50 mm, a finish rolling opening temperature of 980-1090 DEG C and a final rolling temperature of 830-900 DEG C.

[0027] After the rolling, laminar flow cooling is performed, the strip steel temperature is cooled to a temperature range of 350-480 DEG C in a continuous cooling mode by using cooling water or other cooling modes, and coiling is performed.

[0028] After the coiling, a hot rolling coil is obtained, 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.

[0029] The pickling is performed at an acid liquid temperature of 80-95 DEG C, a flattening elongation of 0.5-1.5% and a strip steel running speed of 90-195 m / min.

[0030] The oiling and precise packaging are performed, the strip steel is dried at a temperature range of 100-180 DEG C, then electrostatic oiling is performed, the oiling amount is 0.50-1.50 g / m 2 , and a pickling finished product coil is obtained after the precise packaging.

[0031] The application of the low-cost high-forming-performance tensile strength 450MPa-grade pickling high-strength steel provided by the application is used for manufacturing automobile parts.

[0032] The alloy system design idea of the product is as follows:

[0033] C: C element is the most effective and basic inexpensive solid solution strengthening element for steel to obtain sufficient strength, but excessive C element content will reduce the flanging and hole expanding forming performance of the steel, therefore, the C element content is controlled to be 0.025-0.055% in the application.

[0034] 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 excessive Si element content is prone to cause red rust on the surface of the hot-rolled coil, therefore, the Si element content is controlled to be 0.100-0.500% in the application.

[0035] P: P element is dissolved in ferrite to strengthen ferrite, reduce the hardness difference between ferrite soft phase and bainite hard phase, improve the flanging and hole expanding forming performance of the steel, and the beneficial effect of P element is more obvious in cooperation with B element, but excessive P element content will reduce the plasticity and toughness of the steel, therefore, the P element content is controlled to be 0.015-0.035% in the application.

[0036] Mn: Mn element is a low-cost solid solution strengthening element for significantly improving the strength of the steel, but excessive Mn element content is prone to form banded structure (reduce the uniformity of the structure) and reduce the flanging and hole expanding forming performance of the steel, therefore, the Mn element content is controlled to be 1.000-1.500% in the application.

[0037] Als: Al element can effectively deoxidize and react with N element to generate AlN (reduce the reaction between N element and Ti element, increase the effective Ti content), AlN can effectively prevent grain coarsening and play a role in refining grains to a certain extent; but excessive Al element content will reduce the castability, therefore, the Al element content is controlled to be 0.015-0.055% in the application.

[0038] Ti: Ti element is a strong carbonitride forming element, plays a role in precipitation strengthening and fine-grain strengthening, and comprehensively 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 hole expanding forming performance of the steel; but excessive Ti element content is not easy to guarantee the stability of the mechanical properties of the steel, therefore, the Ti element content is controlled to be 0.015-0.050% in the application.

[0039] B: B element can eliminate the segregation of P element at the grain boundary, make the beneficial effect of P element dissolved in ferrite fully play, and improve the flanging and hole expanding forming performance of the steel, but excessive B element content will significantly increase the strength (low-temperature coiling temperature range) of the steel, therefore, the B element content is controlled to be 0.0005-0.0018% in the application.

[0040] Ca: Ca element plays a role of deoxidation and desulfurization, so that the beneficial effect of Si element solid solution in ferrite can be fully played; non-metallic inclusions are refined, plasticized and homogenized, and the flanging and flaring forming performance of the steel is improved; 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 0.0008-0.0050% in the application.

[0041] S: S element is a harmful impurity element in the steel, which significantly reduces the plasticity and toughness of the steel, 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.

[0042] N: N element can improve the strength and welding performance of the steel, but N element is not conducive to the control of inclusions, and significantly reduces the mechanical property stability of the Ti micro-alloyed steel (affects the effective Ti content), therefore, the content of N element is strictly controlled to be below 0.0050% in the application.

[0043] O: O element is a harmful impurity element in the steel, which is easy to form non-metallic inclusions and reduce 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.

[0044] The product system design of the application meets the calculation formula: 0.35≤Si+10×P≤0.80, 0.25≤10×B / P≤1.00, and the role is: Si and P compound strengthen ferrite (B eliminates the grain boundary segregation of P, so that the beneficial effect of P can be fully played), and the hardness difference between the soft phase and the hard phase is reduced. If the (Si+10×P) value is too low, the effect of strengthening ferrite is not obvious, if the (Si+10×P) value is too high, the surface quality of the pickling finished product coil cannot be ensured, and the harmful effect of P is greater than the beneficial effect. If the (10×B / P) value is too low, the grain boundary segregation of P cannot be eliminated, and the harmful effect of P is greater than the beneficial effect, if the (10×B / P) value is too high, the strength of the steel will be significantly reduced.

[0045] Compared with the prior art, the beneficial effects of the application are: Si and P compound strengthen ferrite (B element inhibits the segregation of P element at the grain boundary, so that the beneficial effect of P element in strengthening ferrite can be fully played), the hardness difference between the ferrite soft phase and the bainite hard phase is reduced, and low-temperature coiling process is adopted to refine the grains and comprehensively improve the flanging and flaring forming performance of the 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 is 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

[0046] Figure 1A scanning microstructure diagram of the low-cost high-formability 450 MPa grade pickling high-strength steel of Example 1. DETAILED DESCRIPTION

[0047] Examples 1-3

[0048] A manufacturing method of a low-cost high-formability 450 MPa grade pickling high-strength steel, the process flow being: hot metal pretreatment → converter smelting → alloy fine-tuning station → LF furnace refining → continuous casting → hot rolling (→ finishing) → pickling → oiling and precision packaging, and the specifics being as follows:

[0049] After the hot metal pretreatment, converter smelting, alloy fine-tuning station, and LF furnace refining processes are completed, calcium treatment is performed, stopper argon blowing is used to promote the floating of inclusions, dynamic soft reduction and electromagnetic stirring rollers are used, the superheat degree of the molten steel is controlled to be 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 to be 0.90-2.10 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 C1.0 or more; the 450 MPa grade pickling high-strength steel continuous casting billet with the chemical composition shown in Table 1 below is manufactured, and the balance not shown in Table 1 is Fe and unavoidable impurities.

[0050] Table 1 Chemical composition of test steels of Examples 1-3

[0051]

[0052] Table 1 (continued)

[0053] No. Si + 10 x P 10 x B / P Example 1 0.60 0.40 Example 2 0.68 0.57 Example 3 0.54 0.39 Comparative Example 1 0.37 0 Comparative Example 2 0.87 0 Comparative Example 3 0.90 0.09 Comparative Example 4 0.63 0.39 Comparative Example 5 0.50 0.60

[0054] Note: Comparative Example 1 is Chinese published patent CN 103469058 A, and Comparative Example 2 is Chinese published patent CN 107829028 A.

[0055] The continuous casting billet is subjected to hot rolling, including heating, rolling, and coiling;

[0056] The heating is performed at a continuous casting billet discharge temperature of 1210-1250℃ and a furnace time of 2.1-5.0 h;

[0057] 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 980-1090℃, and a final rolling temperature of 830-900℃;

[0058] After rolling, laminar cooling is performed using cooling water or other cooling medium to cool the strip steel temperature in a continuous cooling manner to a temperature range of 350-480℃ for coiling;

[0059] The hot-rolled coil is obtained after coiling, if the plate shape of the hot-rolled coil is poor, finishing is needed to correct the plate shape, the finishing elongation is 1.0-3.1%, if the plate shape of the hot-rolled coil is good, finishing is not needed;

[0060] The pickling is performed at an acid liquid temperature of 80-95℃, the skin pass elongation is 0.5-1.5%, and the strip running speed is 90-195m / min;

[0061] The oiling and finishing packaging are performed, the strip is dried at a temperature of 100-180℃, then electrostatic oiling is performed, and the oiling amount is 0.50-1.50g / m 2 The pickled finished steel coil is obtained after the finishing packaging.

[0062] The key process parameters of the hot-rolling process of the test steels in Examples 1-3 are shown in Table 2.

[0063] Table 2 Key process parameters of the hot-rolling process of the test steels in Examples 1-3

[0064] No. Casting thickness (mm) Heating temperature (°C) Time in furnace (h) Finishing temperature (°C) Coiling temperature (°C) Whether continuous cooling Example 1 230 1225 4.5 890 380 Yes Example 2 230 1250 3.5 880 390 Yes Example 3 230 1210 5.0 830 405 Yes Comparative Example 1 / 1200～1220 1.5 830～840 460～500 No Comparative Example 2 / 1221 / 830 510 No Comparative Example 3 230 1231 3.1 890 405 Yes Comparative Example 4 230 1289 3.5 880 450 Yes Comparative Example 5 230 1228 2.8 880 615 Yes

[0065] The mechanical properties and application properties of the steels in Examples 1-3 and Comparative Examples 1-2 are shown in Table 3.

[0066] Table 3 Mechanical properties and application properties of the steels in Examples 1-3 and Comparative Examples 1-2

[0067]

[0068] Note: The determination method of the mechanical properties (yield strength, tensile strength, elongation) is GB / T 228.1-2010, the test type number is P6, and the sample direction is transverse (90° direction). The determination method of the key technical index of the flanging and hole expanding forming performance, i.e. the hole expanding rate (HER), strictly adopts GB / T 15825.4-2008.

[0069] The test results show that the low-cost high-forming-performance pickled high-strength steel with a tensile strength of 450MPa manufactured by the present application has a metallographic structure containing 79.5-91.0% of ferrite and 9.0-20.5% of granular bainite by volume percentage, and the ferrite grain size is 10.5-12.5, and the flanging and hole expanding forming performance of the pickled finished steel coil is excellent.

[0070] The above-mentioned reference examples carefully describe a low-cost high-forming-performance pickled high-strength steel with a tensile strength of 450MPa and a manufacturing method thereof, which are illustrative rather than limiting, and several examples can be listed according to the limited range, therefore, any changes and modifications under the overall technical concept of the present application shall be within the protection scope of the present application.

Claims

1. A low-cost, high-formability, 450MPa-grade pickled high-strength steel, characterized in that: The low-cost, high-formability, 450MPa-grade pickled high-strength steel comprises the following chemical components in weight percentage: C 0.035~0.055%, Si 0.150~0.450%, P 0.015~0.030%, Mn 1.150~1.350%, Als 0.025~0.045%, Ti 0.025~0.035%, B 0.0008~0.0015%, Ca 0.0009~0.0035%, S≤0.0035%, N≤0.0035%, O≤0.0025%, the balance is Fe and unavoidable impurities.

2. The low-cost, high-formability, 450MPa-grade pickled high-strength steel according to claim 1, characterized in that: The chemical composition of the low-cost, high-formability, 450MPa-grade pickled high-strength steel satisfies the following conditions: 0.35≤Si+10×P≤0.80, 0.25≤10×B / P≤1.

00.

3. The low-cost, high-formability, 450MPa-grade pickled high-strength steel according to claim 1 or 2, characterized in that: The metallographic structure of the low-cost, high-formability, 450MPa-grade pickled high-strength steel contains 79.5-91.0% by volume of ferrite and 9.0-20.5% by volume of granular bainite, and the ferrite grain size is 10.5-12.

5.

4. The low-cost, high-formability, 450MPa-grade pickled high-strength steel according to claim 1 or 2, characterized in that: The low-cost, high-formability, tensile strength 450MPa grade pickled high-strength steel has a yield strength ≥350MPa, a tensile strength ≥450MPa, and an elongation A 50 ≥31.0%, hole expansion rate ≥100.0%.

5. A method for manufacturing the low-cost, high-formability, 450MPa-grade pickled high-strength steel according to any one of claims 1 to 4, comprising the following process flow: molten iron pretreatment → converter smelting → alloy fine-tuning station → LF furnace refining → continuous casting → hot rolling → finishing → pickling → oiling and fine packaging.

6. The manufacturing method according to claim 5, characterized in that The hot rolling includes heating and rolling; the heating temperature of the continuous casting billet into the furnace is controlled at 1210-1250° C., and the time in the furnace is 2.1-5.0 hours.

7. The manufacturing method according to claim 6, characterized in that The rolling process includes a rough rolling start temperature of 1150-1180° C., an intermediate billet thickness of 25-50 mm, a finishing rolling start temperature of 980-1090° C., and a final rolling temperature of 830-900° C.

8. The manufacturing method according to claim 6 or 7, characterized in that: After rolling, the steel is cooled and evenly cooled to a temperature range of 350-480°C by continuous cooling for coiling.

9. An application of the low-cost, high-formability, 450MPa-grade pickled high-strength steel according to any one of claims 1 to 4, characterized in that: Used in automobile parts manufacturing.

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