Preparation method of Nb-Ti series cold-rolled bake-hardened steel with high formability and room temperature aging resistance

Abstract

The invention discloses a preparation method of Nb-Ti series cold-rolled bake-hardened steel with high formability and room temperature timeliness resistance, which is characterized by comprising the following steps: 1) converter production; 2) RH furnace production; 3) continuous casting production; 4) heating the casting blank; (5) finish rolling; (6) continuous annealing; the alloy comprises the following chemical components in percentage by weight: C is more than or equal to 0.0015% and less than or equal to 0.0030%; less than or equal to 0.030% of Si; 0.11% < = Mn < = 0.17%; 0.018% < = P < = 0.028%; s < = 0.0080%; 0.020% < = Alt < = 0.060%; ti is greater than or equal to 0.0025% and less than or equal to 0.0045 N is less than or equal to 0.0030%; and the balance of Fe and inevitable impurities. The invention aims to solve the problem that high BH value, excellent formability and room-temperature-resistant timeliness are difficult to consider at the same time in the prior art.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy and metallic materials technology, and particularly relates to a method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging. Background Technology

[0002] After stamping, bake-hardening steel undergoes a significant strength enhancement process followed by coating and baking, effectively achieving lightweighting of automotive bodies. Traditional BH steel often employs single Ti or Nb microalloying, which presents the following problems:

[0003] 1. Ti-based BH steel: Ti has a very strong affinity for C and N. If the Ti content is not properly controlled, it is easy to fix all the C, resulting in the disappearance of the bake hardening effect. If the C is not completely fixed, the resistance to room temperature aging is poor and the performance is unstable during storage.

[0004] 2. Nb-based BH steel: Nb can effectively refine grains and improve toughness, but its dissolved C atoms are unstable at room temperature, and the challenge of aging resistance remains. At the same time, single Nb treatment is not flexible enough in controlling the BH value.

[0005] 3. Narrow existing process window: If key process parameters such as hot rolling coiling temperature, annealing temperature and time are not properly controlled, it is impossible to accurately control the type, size and distribution of precipitates, making it difficult to simultaneously achieve high BH value, high elongation and anti-aging properties. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging, thereby solving the problem that existing technologies cannot simultaneously achieve high BH value, excellent formability and resistance to room temperature aging.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] This invention discloses a method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging, comprising: steelmaking process - hot rolling process - cold rolling process - continuous annealing process; wherein: the steelmaking process includes: KR desulfurization - converter - RH - slab continuous casting; the hot rolling process includes: slab heating - roughing high-pressure water descaling - fixed-width press - E1R1 roughing mill rolling - E2R2 roughing mill rolling - insulation cover - flying shear - high-pressure water descaling - F1-F7 finishing mill rolling - dense laminar flow cooling - coiling; the cold rolling process includes: pickling and uncoiling - welding - tension leveling - pickling - rinsing - drying - edge trimming - continuous rolling mill cold rolling - coiling; the continuous annealing process includes: continuous annealing and uncoiling - welding - cleaning - inlet looper - annealing furnace - outlet looper - leveling - edge trimming - surface inspection - oiling - sampling - coiling - weighing - packaging; wherein:

[0009] 1) Converter production

[0010] Molten iron must undergo desulfurization treatment via KR. The sulfur content of the molten iron entering the converter must be less than or equal to 0.002%, and the slag removal area must be greater than 95%. The tapping temperature of the converter must be ≥1660℃. A turnover ladle must be used for tapping, and the temperature drop at tapping must be less than 70℃. The RH molten steel supplied to the converter must contain [C] ≤0.04%, [Mn] ≤0.25%, and [O] ≤0.060%.

[0011] 2) RH furnace production

[0012] The RH furnace undergoes deep decarburization treatment. After decarburization, aluminum particles are added for deoxidation based on the oxygen content. After circulating for more than 4 minutes, ferromanganese and ferrotitanium alloys are added to adjust the composition. After adjusting the composition, ensure vacuum circulation for ≥6 minutes.

[0013] 3) Continuous casting production

[0014] For slab continuous casting, the superheat of the tundish during the casting process is controlled within the range of 30-45℃, and the production speed is 1.0-1.8m / min;

[0015] 4) The heating temperature during billet heating is 1210-1250℃, the heating time is 180-240min, the soaking temperature is 1220-1260℃, the soaking time is 30-60min, and the furnace exit temperature is 1220-1250℃.

[0016] 5) The finishing rolling temperature is 915-935℃, and the coiling temperature is 700-720℃;

[0017] 6) In the continuous annealing process, the annealing heating and soaking temperature is 795-805℃, the slow cooling section outlet temperature is 650-700℃, the rapid cooling section outlet temperature is 150-250℃, the over-aging section temperature is 150-250℃, the final cooling section outlet temperature is ≤150℃, the leveling machine elongation is 1.35-1.45%, and the process section speed is 100-140m / min;

[0018] Its chemical composition by weight percentage includes: 0.0015% ≤ C ≤ 0.0030%; Si ≤ 0.030%; 0.11% ≤ Mn ≤ 0.17%; 0.018% ≤ P ≤ 0.028%; S ≤ 0.0080%; 0.020% ≤ Alt ≤ 0.060%; 0.0025% ≤ Ti ≤ 0.0045%; N ≤ 0.0030%; the remainder being Fe and unavoidable impurities.

[0019] Furthermore, the hot-rolled thickness is 2.7mm-6.0mm.

[0020] Furthermore, the cold-rolled thickness is 0.50-2.00 mm.

[0021] Furthermore, the metallographic structure of the steel coil is equiaxed ferrite.

[0022] Furthermore, the metallographic grain size is 7.0-9.0 grade.

[0023] Furthermore, it achieves uniformity of mechanical properties for products of different thickness groups, with a steel strip surface quality FD and a surface roughness range of 0.6-1.9μm.

[0024] Furthermore, its mechanical properties meet the following requirements: yield strength 180-230 MPa, tensile strength 290-360 MPa, and elongation after fracture A. 80mm ≥34%, r 90 ≥1.6,n 90 ≥0.17, BH2≥30MPa.

[0025] Furthermore, its chemical composition by weight percentage includes: C 0.027%; Si 0.0029%; Mn 0.17%; P 0.026%; S ≤ 0.0053%; Alt 0.038%; Ti 0.0032%; N 0.0032%; Nb 0.008%; N 0.0015%; the remainder being Fe and unavoidable impurities.

[0026] The role and mechanism of each alloying element in this invention:

[0027] The main functions of carbon (C) in steel are to increase strength and hardness, promote pearlite transformation, and form carbides. C content is fundamental to achieving the bake-hardening effect. Too low a content results in insufficient dissolved C, compromising the bake-hardening (BH) value; too high a content negatively impacts aging resistance and weldability. Therefore, the C content range is set at 0.0015%-0.0030%. Furthermore, similar to C, nitrogen (N) atoms occupy interstitial spaces in the crystal lattice, and their content must be strictly limited to improve the steel's ductility; the N content is set at ≤0.0030%.

[0028] The main role of silicon (Si) in steel is to inhibit cementite formation and to act as a solid solution strengthening element, improving the strength and hardness of the ferrite matrix. In automotive steel, its main function is to assist in deoxidation; however, excessive Si content can deteriorate surface quality. Therefore, the Si content is set to be ≤0.030%.

[0029] As a solid solution strengthening element and carbide-forming element in steel, manganese (Mn) can simultaneously strengthen ferrite and cementite, improving the strength and hardness of steel. Furthermore, Mn can replace Fe and S in steel to form MnS, avoiding the formation of FeS (FeS readily forms low-melting-point compounds with Fe), which leads to hot brittleness in steel. It also acts as a deoxidizer. Mn can lower the pearlite transformation temperature, thereby reducing the interlamellar spacing of pearlite. However, increasing the Mn content significantly reduces the weldability of steel, increases grain size, and increases the susceptibility to white spots and billet segregation. Since cementite is absent in automotive steel, its strengthening effect on carbides is considered. When the Mn content > 0.80%, excessive Mn leads to increased grain size, affecting the microstructure changes during production and reducing the steel's ductility and toughness. Taking all factors into account, the Mn content range is set at 0.11%-0.17%.

[0030] The main function of Nb is to precipitate fine NbC during hot rolling and annealing, strongly inhibiting recrystallization and grain growth, thereby refining ferrite grains and improving strength and toughness; some Nb dissolves in the matrix, regulating the austenite / ferrite phase transformation. Therefore, the Nb content range is set at 0.0065%-0.0120%.

[0031] The main role of Ti is to preferentially combine with N to form TiN, eliminating the adverse effects of free N on aging; and to combine with the remaining C to form TiC, precisely controlling the amount of "free C" dissolved in the matrix. A key constraint is Ti / Nb ≤ 2.0. This ratio ensures that the Ti content is sufficient to fix most of the N and some of the C, while Nb acts as a "fine-tuning" element, strengthening grain boundaries and dislocations by forming finer NbC. This collaborative mechanism achieves precise control over the dissolved C content, ensuring sufficient C atoms to segregate to dislocations and generate the BH effect during baking, while also ensuring that the dissolved C concentration remains at a kinetically stable low level at room temperature, thus achieving excellent resistance to aging. Therefore, the Ti content range is set at 0.0025%-0.0045%.

[0032] Both phosphorus (P) and sulfur (S) are impurity elements in general steel that cannot be completely removed. They significantly increase the steel's crack susceptibility, raise its low-temperature brittle transition temperature, and reduce its low-temperature impact resistance. Therefore, without affecting the steel's properties, the lower the P and S content, the better. However, automotive steel requires the addition or retention of a certain amount of P to improve its strength through solid solution strengthening. Therefore, the P content range is set at 0.018%-0.028%, aiming to improve strength while avoiding its cold brittleness effects. Sulfur (S) remains an impurity element in automotive steel, so its content must be controlled below 0.0080%.

[0033] The reasons for selecting the above process parameters are as follows:

[0034] The steelmaking process strictly controls the oxygen blowing volume to reduce inclusion formation; the hot rolling process uses a low heating temperature to effectively control iron oxide scale formation and save energy, while increasing the descaling water pressure to remove iron oxide scale from the slab surface, improving the surface quality of the hot-rolled steel strip; the continuous annealing process uses high-temperature annealing to achieve rapid recrystallization, and different leveling mill elongation rates are used according to thickness group spacing to achieve stable performance ranges for products of various thicknesses. The finished product yield strength is 180-230MPa, tensile strength is 290-360MPa, and elongation after fracture is A... 80mm ≥34% (transverse tensile test: L0=80mm, b0=20mm), n value ≥0.17, r value ≥1.6, surface roughness 0.6-1.9μm, high elongation after fracture and r value, thus possessing excellent stamping forming performance.

[0035] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0036] This invention provides a method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging. By adding only Nb and Ti elements, it achieves the mechanical properties required for grade 180 high formability and resistance to room temperature aging bake-hardening steel, thus achieving a high cost-performance ratio (reducing the cost per ton of steel by approximately 100 yuan). Practical use has proven that the performance is suitable, and its mechanical properties meet the following requirements: yield strength 180-230 MPa, tensile strength 290-360 MPa, and elongation after fracture A. 80mm ≥34%, r 90 ≥1.6,n 90 ≥0.17, BH2≥30MPa. This method for producing automotive steel is simple, highly operable, and easy to promote and apply.

[0037] The provided preparation method, through the adoption of new composition design and metallurgical process parameters, provides a method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging. Attached Figure Description

[0038] The present invention will be further described below with reference to the accompanying drawings.

[0039] Figure 1 The metallographic structure is that of the finished product. Detailed Implementation

[0040] A cold-rolled bake-hardening steel for automobiles with high formability and resistance to room temperature aging, and its preparation method, the production method specifically includes the following steps: steelmaking process - hot rolling process - cold rolling process - continuous annealing process.

[0041] The steelmaking process includes: KR desulfurization—converter—RH—slab continuous casting. After desulfurization pretreatment, molten iron is smelted in the converter. The sulfur content of the molten iron entering the converter must be less than 0.002%, and the slag removal area must be greater than 95%. The converter tapping temperature must be ≥1660℃, and a reusable ladle must be used for tapping. The tapping temperature drop must be less than 70℃. The RH-supplying steel has [C] ≤0.04%, [Mn] ≤0.25%, and [O] ≤0.060%. The RH furnace undergoes deep decarburization. After decarburization, aluminum granules are added for deoxidation based on the oxygen content. After circulation for more than 4 minutes, alloys such as ferromanganese and ferrotitanium are added to adjust the composition. After composition adjustment, vacuum circulation is ensured for ≥6 minutes. For slab continuous casting, the tundish superheat is controlled within the range of 30~45℃ during casting, and the production speed is 1.0~1.8m / min. Based on the above steelmaking process requirements, the actual slab chemical composition (mass percentage) is shown in Table 1 below, with the balance being Fe and unavoidable impurities (Note: Due to the different composition design schemes of the comparative example and the embodiment, their production process parameters are also different from those described in this invention, and are only listed for comparison here).

[0042] Table 1: Chemical composition (%) of each example and comparative example

[0043]

[0044]

[0045] The hot rolling process includes: billet heating—high-pressure water descaling—width-fixed press—E1R1 roughing mill rolling—E2R2 roughing mill rolling—flying shear—high-pressure water descaling—F1-F7 finishing mill rolling—densified laminar flow cooling—coiling; this process uses a walking beam furnace to heat the billet (the heating process for each embodiment and comparative example is shown in Table 2), roughing uses a double-stand R1 and R2 reciprocating rolling process, and finishing uses a continuous rolling process from F1 to F7. The specific hot rolling processes for each embodiment and comparative example are shown in Table 3.

[0046] Table 2 Heating Regime for Cast Billets

[0047] Heating temperature / ℃ Heating time / min Isotropic temperature / ℃ Soaking time / min Furnace temperature / ℃ 1248 232 1255 48 1237

[0048] Table 3 Rolling Process

[0049] Rolled thickness / mm Finishing rolling temperature / ℃ Winding temperature / ℃ 5.3 927 713

[0050] The cold rolling process includes: pickling and uncoiling—welding—tension leveling—pickling—rinsing—drying—edge trimming—continuous rolling mill cold rolling—coiling;

[0051] The continuous annealing process includes: continuous annealing uncoiling—welding—cleaning—entry looper—annealing furnace—exit looper—leveling—edge trimming—surface inspection—oiling—sampling—coiling—weighing—packaging; annealing is performed using a vertical continuous annealing furnace, with a reducing atmosphere and a nitrogen-hydrogen mixed protective atmosphere used for cooling inside the furnace. The specific process parameters for the continuous annealing process in each embodiment and comparative example are shown in Table 4.

[0052] Table 4: Annealing Process

[0053]

[0054]

[0055] The room temperature tensile mechanical properties of the finished product, obtained through hot rolling, cold rolling, and continuous annealing processes, are shown in Table 5. The metallographic structure is equiaxed ferrite with a grain size of 7.0-9.0. The microstructure is shown in Table 5. Figure 1 .

[0056] Table 5: Room Temperature Tensile Properties of Finished Products

[0057]

[0058] In summary, this product has passed performance testing and meets all performance requirements. User trial molding has also met molding requirements, making it suitable for widespread use.

[0059] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging, characterized in that, include: 1) Converter production Molten iron must undergo desulfurization treatment via KR. The sulfur content of the molten iron entering the converter must be less than or equal to 0.002%, and the slag removal area must be greater than 95%. The tapping temperature of the converter must be ≥1660℃. A turnover ladle must be used for tapping, and the temperature drop at tapping must be less than 70℃. The RH molten steel supplied to the converter must contain [C] ≤0.04%, [Mn] ≤0.25%, and [O] ≤0.060%. 2) RH furnace production The RH furnace undergoes deep decarburization treatment. After decarburization, aluminum particles are added for deoxidation based on the oxygen content. After circulating for more than 4 minutes, ferromanganese and ferrotitanium alloys are added to adjust the composition. After adjusting the composition, ensure vacuum circulation for ≥6 minutes. 3) Continuous casting production For slab continuous casting, the superheat of the tundish during the casting process is controlled within the range of 30-45℃, and the production speed is 1.0-1.8m / min; 4) The heating temperature during billet heating is 1210-1250℃, the heating time is 180-240min, the soaking temperature is 1220-1260℃, the soaking time is 30-60min, and the furnace exit temperature is 1220-1250℃. 5) The finishing rolling temperature is 915-935℃, and the coiling temperature is 700-720℃; 6) In the continuous annealing process, the annealing heating and soaking temperature is 795-805℃, the slow cooling section outlet temperature is 650-700℃, the rapid cooling section outlet temperature is 150-250℃, the over-aging section temperature is 150-250℃, the final cooling section outlet temperature is ≤150℃, the leveling machine elongation is 1.35-1.45%, and the process section speed is 100-140m / min; Its chemical composition by weight percentage includes: 0.0015% ≤ C ≤ 0.0030%; Si ≤ 0.030%; 0.11% ≤ Mn ≤ 0.17%; 0.018% ≤ P ≤ 0.028%; S ≤ 0.0080%; 0.020% ≤ Alt ≤ 0.060%; 0.0025% ≤ Ti ≤ 0.0045%; N ≤ 0.0030%; the remainder being Fe and unavoidable impurities.

2. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, Hot-rolled thickness: 2.7mm-6.0mm.

3. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, The thickness of cold-rolled steel is 0.50-2.00 mm.

4. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, The metallographic structure of the steel coil is equiaxed ferrite.

5. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, The metallographic grain size is 7.0-9.

0.

6. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, To achieve uniformity of mechanical properties for products of various thickness groups, the surface quality FD of the steel strip is achieved, and the surface roughness ranges from 0.6 to 1.9 μm.

7. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, Its mechanical properties meet the following requirements: yield strength 180-230 MPa, tensile strength 290-360 MPa, and elongation after fracture A. 80mm ≥34%, r 90 ≥1.6,n 90 ≥0.17, BH2≥30MPa.

8. The method for preparing Nb-Ti cold-rolled bake-hardening steel with high formability and resistance to room temperature aging according to claim 1, characterized in that, Its chemical composition by weight percentage includes: C 0.027%; Si 0.0029%; Mn 0.17%; P 0.026%; S 0.0053%; Alt 0.038%; Ti 0.0032%; N 0.0032%; Nb 0.008%; N 0.0015%; the remainder being Fe and unavoidable impurities.

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