Galvanized ultrahigh-strength multi-phase steel plate for doorsill beam and production method of galvanized ultrahigh-strength multi-phase steel plate

Abstract

The invention discloses a galvanized ultrahigh-strength multiphase steel plate for a threshold beam and a production method of the galvanized ultrahigh-strength multiphase steel plate. The galvanized ultrahigh-strength multiphase steel plate comprises the following chemical components in percentage by mass: 0.11-0.13% of C, 2.30-2.45% of Mn, less than or equal to 0.005% of S, less than or equal to 0.020% of P, 0.20-0.30% of Si, 0.030-0.060% of Als, 0.50-0.60% of Cr, 0.030-0.070% of Nb + Ti, 0.05-0.10% of Mo, less than or equal to 0.003% of B and the balance of Fe and inevitable impurities. The production method comprises the procedures of steelmaking, continuous casting, hot rolling, acid rolling, continuous galvanizing and finishing. The low-Si, low-Mo and low-C component design is adopted, and the obtained steel plate is excellent in coating quality and moderate in yield ratio, has obvious advantages in the aspect of roll forming of the body-in-white doorsill beam and has good cyclic corrosion resistance.

Description

Technical Field

[0001] The present invention belongs to the field of ultra-high strength cold-rolled steel for automobiles, and particularly relates to a galvanized ultra-high strength multi-phase steel sheet for a sill beam and a production method thereof. Background Art

[0002] To adapt to the development trend of low energy consumption, high safety, and high corrosion resistance in the automotive industry, automobile manufacturers and steel enterprises are actively committed to the lightweight design of vehicle bodies and the development of high-strength steels for high-safety and high-corrosion-resistant automobiles. At present, it has become an industry consensus to use as many ultra-high strength steel products as possible to achieve the lightweight of the white body, but the customized design for specific application scenarios needs to be further deepened. Due to its safety part attribute, the sill beam usually uses 1180MPa grade ultra-high strength multi-phase steel products for roll forming production. The main difficulties in the production process are roll forming cracking, excessive or insufficient springback, etc., and relatively high requirements are imposed on the material strength index. In addition, in order to meet the anti-corrosion requirement of no perforation for 12 years of the white body, more and more OEMs have put forward the use demand for galvanized ultra-high strength multi-phase steel products of the same strength level.

[0003] The invention patent application with the publication number of CN117187682A discloses a 1200MPa steel for a battery pack of a new energy vehicle and a preparation method thereof. The chemical composition is: C: 0.20% - 0.24%, Si: 0.20% - 0.40%, Al: 0.40% - 0.60%, Mn: 0.9% - 1.1%, Cr: 0.30% - 0.60%, Mo: 0.20% - 0.40%, B: 0.004% - 0.006%; the produced steel sheet has a tensile strength of more than 1200MPa, a yield strength of 900 - 1050MPa, an elongation rate of ≥12%, and an expansion rate of ≥55%; however, its alloy cost per ton of steel is relatively high, which is not conducive to large-scale industrial production; in addition, its weldability is poor, and the applicable scenario is a new energy battery pack.

[0004] The invention patent application with the publication number of CN113528946A discloses a 1200MPa grade enhanced forming duplex steel and a preparation method thereof. Its chemical composition is: C: 0.21 - 0.24%, Si: 1.0 - 1.4%, Mn: 2.1 - 2.6%, P≤0.01%, S≤0.005%, Cr: 0.4 - 0.7%, Nb: 0.02 - 0.04%, etc.; the yield strength of the steel sheet is 951 - 1148MPa, the tensile strength is 1201 - 1344MPa, the elongation after fracture A80 is 8 - 13%, the uniform elongation Ag is 6 - 10%, and the expansion rate is 40 - 59%; however, it is a continuous annealing product and cannot meet the requirement of high corrosion resistance of the white body for the material.

[0005] Therefore, developing a galvanized ultra-high-strength steel sheet for the sill beam with ultra-high strength, high corrosion resistance and moderate yield strength for roll forming of the sill beam has important economic and social benefits. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a galvanized ultra-high-strength multiphase steel sheet for the sill beam and its production method. By innovating the composition design and slow cooling treatment of the hot-rolled coil, the mechanical property uniformity of the material is maximally improved to ensure the smooth progress of the pickling and rolling process. At the same time, with a suitable galvanizing process, the steel sheet has ultra-high strength, high corrosion resistance and moderate yield strength, and can be used for roll forming of the sill beam of the white body.

[0007] To solve the above technical problems, the technical solution adopted by the present invention is: A galvanized ultra-high-strength multiphase steel sheet for the sill beam, its chemical composition and mass percentage content: C: 0.11 - 0.13%, Mn: 2.30 - 2.45%, S ≤ 0.005%, P ≤ 0.020%, Si: 0.20 - 0.30%, Als: 0.030 - 0.060%, Cr: 0.50% - 0.60%, Nb + Ti: 0.030 - 0.070%, Mo: 0.05% - 0.10%, B ≤ 0.003%, and the rest is Fe and unavoidable impurities.

[0008] The galvanized ultra-high-strength multiphase steel sheet of the present invention has a relatively high strength level. The composition ratio requires austenitizing elements to be greater than ferrite-forming elements, supplemented by elements to improve the hardenability of the material to ensure the strength index. At the same time, to ensure the strength-toughness matching and tissue uniformity of the material, a certain amount of fine grain strengthening elements and alloy elements beneficial to improving tissue uniformity are added. The action mechanisms of each alloy element are as follows: Carbon: An austenitizing element, with solid solution strengthening, reducing the austenitizing transformation temperature, stabilizing supercooled austenite, which is the guarantee for the material to obtain high strength. It can generate enough austenite tissue at a lower annealing temperature to prepare for subsequent phase transformation strengthening; at the same time, if the carbon content is too high, it is not conducive to improving the welding performance of the material. Therefore, the carbon design should be as low as possible on the basis of meeting the strength requirements.

[0009] Manganese: An austenitizing element, similar to the role of carbon content, can strongly improve the hardenability of austenite to ensure the strength of the material, and is relatively cheap and easy to purchase; however, if the manganese content is too high, segregation problems are likely to occur in the microstructure, and other alloy elements to improve tissue uniformity need to be supplemented for cooperation.

[0010] Silicon: A ferrite-forming element, with solid solution strengthening, expanding the temperature range of the two-phase region, accelerating the segregation of carbon to austenite, and purifying and strengthening the ferrite phase. However, if the Si content is too high, a higher annealing temperature is required to generate a large amount of austenite phase, and it is not conducive to the surface quality of coating. The addition ratio should not be too high.

[0011] Niobium + Titanium: Precipitation elements of carbonitrides, which can refine grains and precipitate carbonitrides, achieve good strength-ductility matching of the material, and at the same time ensure a certain proportion of second-phase precipitation in both the hot rolling and cold rolling stages, so as to achieve the purpose of fine grain strengthening and precipitation strengthening.

[0012] Chromium: An element that improves the hardenability of austenite and refines grains, and at the same time plays a role in delaying the bainite transformation in the medium temperature region and reducing the critical cooling rate; Molybdenum: An element that strongly improves the stability of supercooled austenite, can greatly improve the tissue uniformity, but it is a precious alloy element, and the less the better on the basis of ensuring tissue uniformity.

[0013] Boron: An element that strongly improves the hardenability of austenite and is beneficial to improving the strength index.

[0014] Furthermore, the thickness of the steel plate described in the present invention is 1.0 - 2.0 mm.

[0015] Furthermore, the mechanical properties of the steel plate described in the present invention are as follows: yield strength: 850 MPa - 1000 MPa, tensile strength ≥ 1180 MPa, longitudinal elongation A80 ≥ 5%.

[0016] The production method of the galvanized ultra-high strength multiphase steel plate for the sill beam described in the present invention includes steelmaking, continuous casting, hot rolling, pickling and cold rolling, continuous galvanizing, and skin pass processes.

[0017] Furthermore, in the hot rolling process described in the present invention, the slab is heated to 1260 - 1300 °C, the holding time of the slab in the furnace is 190 - 230 min, the finishing rolling starting temperature is 1070 °C - 1100 °C, the finishing rolling temperature is 870 - 910 °C, the coiling temperature is 600 - 640 °C, and slow cooling treatment is ≥ 48 h; The mechanism of the slow cooling treatment in the hot rolling process described in the present invention lies in maximizing the uniformity of the mechanical properties of the material, ensuring that the mechanical properties are uniform everywhere, so as to ensure the stable and smooth progress of the pickling and cold rolling process.

[0018] Furthermore, in the continuous galvanizing process described in the present invention: the soaking section temperature is controlled at 800 - 815 °C, the soaking time is 45 - 70 s, the cooling rate in the slow cooling section is 1.7 - 3.0 °C / s, the slow cooling temperature is 680 - 700 °C, the cooling rate in the rapid cooling section is 25 - 45 °C / s, the rapid cooling end temperature is 455 - 465 °C, the temperature entering the zinc pot: 455 - 470 °C, the cooling rate after plating: 7 - 14.0 °C / s, the cooling temperature after plating: 180 - 220 °C, and then it is cooled to room temperature through a water quenching process.

[0019] Furthermore, in the pickling and cold rolling process described in the present invention, the free acid concentration of the acid solution is ≥ 45 g / L, the acid solution temperature is ≥ 75 °C, and the pickling and cold rolling reduction rate is ≥ 50%.

[0020] Furthermore, for the skin pass process of the present invention, the rolling force of the skin pass mill is controlled at: 5500 KN - 6500 KN.

[0021] The beneficial effects of this technical solution are as follows: By adopting a composition design of low Si, low Mo, and low C, microalloying with Nb and Ti, and matching appropriate hot rolling, pickling rolling, and continuous galvanizing processes, the steel sheet obtained has a yield strength of 900 MPa - 1050 MPa, a tensile strength of ≥1180 MPa, and a longitudinal elongation A80 ≥ 5%. It has both ultra-high strength and high corrosion resistance, with a moderate yield strength, and is suitable for roll forming of the sill beam of a white vehicle body. Specific Embodiments

[0022] The present invention will be further described in detail below in conjunction with specific embodiments. Embodiment 1

[0023] The thickness of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment is 1.0 mm, and its chemical composition and mass percentage are shown in Table 1.

[0024] The production method of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment includes steelmaking, continuous casting, hot rolling, pickling rolling, continuous galvanizing, and skin pass processes. The specific process steps are as follows: (1) Hot rolling process: The slab is heated to 1260 °C, the residence time of the slab in the furnace is 190 min, the starting rolling temperature of the finishing mill is 1070 °C, the final rolling temperature is 880 °C, the coiling temperature is 620 °C, and the slow cooling treatment time is: 50 h.

[0025] (2) Pickling rolling process: The free acid concentration of the acid solution is: 50 g / L, the acid solution temperature is: 80 °C, and the reduction ratio of pickling rolling is: 56.5%.

[0026] (3) Continuous galvanizing process: The soaking section temperature is: 815 °C, the soaking time is: 70 s, the cooling rate in the slow cooling section is 3.0 °C / s, the slow cooling temperature is 700 °C, the cooling rate in the rapid cooling section is 45 °C / s, the end temperature of rapid cooling is: 455 °C, the temperature entering the zinc pot is: 460 °C, the cooling rate after plating is: 12 °C / s, the cooling temperature after plating is: 200 °C, and then it is cooled to room temperature through a water quenching process.

[0027] (4) Skin pass process: The skin pass rolling force: 5500 KN.

[0028] The mechanical property indexes of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment are shown in Table 2. Embodiment 2

[0029] The thickness of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment is 1.4 mm, and its chemical composition and mass percentage are shown in Table 1.

[0030] The production method of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment includes steelmaking, continuous casting, hot rolling, acid rolling, continuous galvanizing, and skin pass processes. The specific process steps are as follows: (1) Hot rolling process: The slab is heated to 1280 °C, the residence time of the slab in the furnace is 200 min, the starting rolling temperature of the finishing mill is 1085 °C, the final rolling temperature is 900 °C, the coiling temperature is 600 °C, and the slow cooling treatment time is 48 h.

[0031] (2) Acid rolling process: The free acid concentration of the acid solution is 45 g / L, the acid solution temperature is 75 °C, and the reduction ratio of acid rolling is 53.3%.

[0032] (3) Continuous galvanizing process: The soaking section temperature is 807 °C, the soaking time is 60 s, the cooling rate in the slow cooling section is 2.5 °C / s, the slow cooling temperature is 690 °C, the cooling rate in the rapid cooling section is 38 °C / s, the rapid cooling end temperature is 460 °C, the temperature entering the zinc pot is 455 °C, the cooling rate after plating is 10 °C / s, the cooling temperature after plating is 180 °C, and then it is cooled to room temperature through the water quenching process.

[0033] (4) Skin pass process: The skin pass rolling force is 5900 KN.

[0034] The mechanical property indexes of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment are shown in Table 2. Example 3

[0035] The thickness of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment is 1.6 mm, and its chemical composition and mass percentage are shown in Table 1.

[0036] The production method of the galvanized ultra-high strength multiphase steel sheet for the sill beam in this embodiment includes steelmaking, continuous casting, hot rolling, acid rolling, continuous galvanizing, and skin pass processes. The specific process steps are as follows: (1) Hot rolling process: The slab is heated to 1290 °C, the residence time of the slab in the furnace is 220 min, the starting rolling temperature of the finishing mill is 1100 °C, the final rolling temperature is 890 °C, the coiling temperature is 630 °C, and the slow cooling treatment time is 52 h.

[0037] (2) Acid rolling process: The free acid concentration of the acid solution is 55 g / L, the acid solution temperature is 85 °C, and the reduction ratio of acid rolling is 50%.

[0038] (3) Continuous galvanizing process: The soaking section temperature is 810 °C, the soaking time is 55 s, the cooling rate in the slow cooling section is 2.0 °C / s, the slow cooling temperature is 680 °C, the cooling rate in the rapid cooling section is 35 °C / s, the rapid cooling end temperature is 457 °C, the temperature entering the zinc pot is 457 °C, the cooling rate after plating is 14.0 °C / s, the cooling temperature after plating is 210 °C, and then it is cooled to room temperature through the water quenching process.

[0039] (4) Skin pass process: The skin pass rolling force is 6100 KN.

[0040] The mechanical property indexes of the galvanized ultra-high strength multiphase steel sheet for the threshold beam in this embodiment are shown in Table 2. Example 4

[0041] The thickness of the galvanized ultra-high strength multiphase steel sheet for the threshold beam in this embodiment is 1.2 mm, and its chemical composition and mass percentage are shown in Table 1.

[0042] The production method of the galvanized ultra-high strength multiphase steel sheet for the threshold beam in this embodiment includes steelmaking, continuous casting, hot rolling, acid rolling, continuous galvanizing, and skin pass rolling processes. The specific process steps are as follows: (1) Hot rolling process: The slab is heated to 1270 °C, the slab stay time in the furnace is 230 min, the finishing rolling starting temperature is 1090 °C, the finishing rolling ending temperature is 910 °C, the coiling temperature is 640 °C, and the slow cooling treatment time is 51 h.

[0043] (2) Acid rolling process: The free acid concentration of the acid solution is 52 g / L, the acid solution temperature is 83 °C, and the acid rolling reduction rate is 52%.

[0044] (3) Continuous galvanizing process: The soaking section temperature is 812 °C, the soaking time is 60 s, the slow cooling section cooling rate is 2.5 °C / s, the slow cooling temperature is 690 °C, the rapid cooling section cooling rate is 38 °C / s, the rapid cooling ending temperature is 462 °C, the temperature entering the zinc pot is 465 °C, the post-galvanizing cooling rate is 12.0 °C / s, the post-galvanizing cooling temperature is 205 °C, and then it is cooled to room temperature through the water quenching process.

[0045] (4) Skin pass rolling process: The skin pass rolling force is 5700 KN.

[0046] The mechanical property indexes of the galvanized ultra-high strength multiphase steel sheet for the threshold beam in this embodiment are shown in Table 2. Example 5

[0047] The thickness of the galvanized ultra-high strength multiphase steel sheet for the threshold beam in this embodiment is 2.0 mm, and its chemical composition and mass percentage are shown in Table 1.

[0048] The production method of the galvanized ultra-high strength multiphase steel sheet for the threshold beam in this embodiment includes steelmaking, continuous casting, hot rolling, acid rolling, continuous galvanizing, and skin pass rolling processes. The specific process steps are as follows: (1) Hot rolling process: The slab is heated to 1300 °C, the slab stay time in the furnace is 220 min, the finishing rolling starting temperature is 1095 °C, the finishing rolling ending temperature is 890 °C, the coiling temperature is 630 °C, and the slow cooling treatment time is 50 h.

[0049] (2) Acid rolling process: The free acid concentration of the acid solution is 50 g / L, the acid solution temperature is 75 °C, and the acid rolling reduction rate is 50%.

[0050] (3)Continuous galvanizing process: soaking section temperature: 800 °C, soaking time: 45 s, slow cooling section cooling rate: 1.7 °C / s, slow cooling temperature: 680 °C, rapid cooling section cooling rate: 25 °C / s, rapid cooling end temperature: 455 °C, temperature when entering the zinc pot: 470 °C, post - plating cooling rate: 7.0 °C / s, post - plating cooling temperature: 220 °C, and then cooled to room temperature through the water quenching process.

[0051] (4)Skin - pass rolling process: skin - pass rolling force: 6500 KN.

[0052] The mechanical property indexes of the galvanized ultra - high - strength multiphase steel sheet for the sill beam in this example are shown in Table 2. Example 6

[0053] The thickness of the galvanized ultra - high - strength multiphase steel sheet for the sill beam in this example is 1.8 mm, and its chemical composition and mass percentage are shown in Table 1.

[0054] The production method of the galvanized ultra - high - strength multiphase steel sheet for the sill beam in this example includes steelmaking, continuous casting, hot rolling, pickling and cold rolling, continuous galvanizing, and skin - pass rolling processes. The specific process steps are as follows: (1)Hot - rolling process: the slab is heated to 1290 °C, the time of the slab in the furnace: 215 min, the starting rolling temperature of the finishing mill is 1090 °C, the final rolling temperature is 870 °C, the coiling temperature: 630 °C, and the slow cooling treatment time: 52 h.

[0055] (2)Pickling and cold - rolling process: free acid concentration of the acid solution: 52 g / L, acid solution temperature: 79 °C, cold - rolling reduction rate: 55%.

[0056] (3)Continuous galvanizing process: soaking section temperature: 805 °C, soaking time: 52 s, slow cooling section cooling rate: 2.0 °C / s, slow cooling temperature: 685 °C, rapid cooling section cooling rate: 30 °C / s, rapid cooling end temperature: 460 °C, temperature when entering the zinc pot: 465 °C, post - plating cooling rate 8.5 °C / s, post - plating cooling temperature: 210 °C, and then cooled to room temperature through the water quenching process.

[0057] (4)Skin - pass rolling process: skin - pass rolling force: 6300 KN.

[0058] The mechanical property indexes of the galvanized ultra - high - strength multiphase steel sheet for the sill beam in this example are shown in Table 2.

[0059] Table 1 Chemical composition of the steel sheets in each example (wt%)

[0060] Table 2 Longitudinal mechanical properties of the steel sheets in each example

[0061] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the present invention, and any modification or partial replacement without departing from the spirit and scope of the present invention shall be covered by the scope of the claims of the present invention.

Claims

1. A galvanized ultra-high strength multi-phase steel plate for a threshold beam, characterized in that: Its chemical composition and mass percentage are: C: 0.11~0.13%, Mn: 2.30~2.45%, S≤0.005%, P≤0.020%, Si: 0.20~0.30%, Als: 0.030~0.060%, Cr: 0.50%~0.60%, Nb+Ti: 0.030~0.070%, Mo: 0.05%~0.10%, B≤0.003%, and the rest are Fe and unavoidable impurities.

2. The galvanized ultra-high strength multi-phase steel plate for a threshold beam according to claim 1, characterized in that: The thickness of the steel plate is 1.0-2.0 mm.

3. The galvanized ultra-high strength multi-phase steel plate for a door sill beam according to claim 3, characterized in that: The steel plate has a yield strength of 850MPa-1000MPa, a tensile strength of ≥1180MPa, and a longitudinal elongation A80 of ≥5%.

4. A method for producing the galvanized ultra-high strength multi-phase steel plate for the threshold beam according to claim 1, characterized in that: It includes steelmaking, continuous casting, hot rolling, pickling, continuous galvanizing and skin finishing processes.

5. The method for producing a galvanized ultra-high strength multi-phase steel plate for a door sill beam according to claim 4, characterized in that: In the hot rolling process, the slab is heated to 1260-1300° C., the slab is kept in the furnace for 190-230 min, the start temperature of the finishing rolling is 1070-1100° C., the final rolling temperature is 870-910° C., the coiling temperature is 600-640° C., and the slow cooling treatment is ≥48 h.

6. The method for producing a galvanized ultra-high strength multi-phase steel plate for a door sill beam according to claim 4, characterized in that: The continuous galvanizing process: the temperature of the soaking section is controlled at 800-815°C, the soaking time is 45-70s, the cooling rate of the slow cooling section is 1.7-3.0°C / s, the slow cooling temperature is 680-700°C, the cooling rate of the fast cooling section is 25-45°C / s, the fast cooling end temperature is 455-465°C, the zinc pot entry temperature is 455-470°C, the cooling rate after plating is 7-14.0°C, and the cooling temperature after plating is 180-220°C, and then cooled to room temperature through a water quenching process.

7. The method for producing a galvanized ultra-high strength multi-phase steel plate for a door sill beam according to claim 4, characterized in that: In the acid rolling process, the free acid concentration of the acid solution is ≥45g / L, the temperature of the acid solution is ≥75°C, and the acid rolling reduction rate is ≥50%.

8. The method for producing a galvanized ultra-high strength multi-phase steel plate for a door sill beam according to claim 4, characterized in that: In the skin-pass process, the rolling force of the skin-pass mill is controlled at: 5500KN-6500KN.

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