RE-Ti microalloyed 700MPa high-weathering-resistant steel strip for container and manufacturing method thereof

Abstract

The invention discloses a rare earth RE-Ti microalloyed 700MPa high-weathering-resistant steel strip for a container. The steel strip comprises the following chemical components in percentage by mass: 0.03 to 0.08 percent of C, 0.10 to 0.50 percent of Si, 0.70 to 1.0 percent of Mn, 0.015 to 0.025 percent of P, less than or equal to 0.005 percent of S, 0.020 to 0.040 percent of Als, 0.010 to 0.040 percent of Nb, 0.070 to 0.110 percent of Ti, 0.40 to 0.80 percent of Cr, 0.25 to 0.50 percent of Cu, 0.0025 to 0.0050 percent of La, less than or equal to 2ppm of H, less than or equal to 25ppm of O, less than or equal to 60ppm of N and the balance of Fe and inevitable impurities. The high-weathering-resistant steel strip for the container has good obdurability and atmospheric corrosion resistance and is suitable for stable batch production of enterprises.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and in particular relates to a rare earth RE-Ti microalloyed high-weathering steel strip for a 700MPa container and a manufacturing method thereof. Background Art

[0002] As my country's containers develop towards high strength, light weight and high weather resistance, high strength weather resistant hot rolled coils have become the main steel type used in the industry. Thinner and lighter steel plates help reduce the weight of the container while also expanding the effective volume ratio of the container. At the same time, in order to increase service life and save costs, container steel must have high resistance to atmospheric corrosion and seawater corrosion.

[0003] Such container steel and its production method have been reported in the prior art. For example, patent document CN117305707 A (hereinafter referred to as document 1) discloses a 700MPa grade CSP short process hot-rolled high weathering container steel and its production method. The product composition design includes C: 0.02-0.06%, Si: 0.20-0.65%, Mn: 1.1-1.8%, P: 0.035-0.15%, S: ≤0.008%, Cu: 0.20-0.55%, Sb: 0.02-0.25%, Ti: 0.10-0.18%, N≤0.005%, and the balance is balanced iron and unavoidable impurities; wherein the carbon equivalent CEV is controlled to be ≤0.35%. Its preparation mainly includes converter smelting, LF ladle furnace refining, thin slab continuous casting and rolling, laminar cooling and coiling steps. It can be seen that the document 1 belongs to the production process of the CSP production line, and the product composition design adds the special alloy element Sb, which has a high production cost. Patent document CN 111424211 A (hereinafter referred to as document 2) discloses a wide 700MPa grade hot-rolled container weathering steel and its manufacturing method, and its composition design system is that the steel contains C: 0.058% to 0.078%, Si: 0.65% to 0.75%, Mn: 0.65% to 0.85%, P: 0.035% to 0.045%, S≤0.006%, Al≤0.040%, Cr: 0.22% to 0.31%, Cu: 0.11% to 0.19%, Ti: 0.060% to 0.075%, RE: 0.021% to 0.035%, N≤0.005%. Its preparation mainly includes: smelting and continuous casting, reheating of ingot, rolling, coiling, and leveling. The rare earth wire is fed through the protective slag at the water inlet of the crystallizer once and evenly added into the molten steel for rare earth microalloying. However, the document 2 does not specify the type of rare earth to be added. The document "Microstructure and Properties of Ti Microalloyed 700MPa Grade Weathering Steel for Containers" (hereinafter referred to as document 3) introduces that Northeastern University studied the use of 0.03% C, 0.25% Si, 1.45% Mn, 0.35% Cu, 0.85% Cr, 0.25% Ni, and a proper amount of Ti component design system under laboratory conditions. The 100mm thick steel ingot is rolled into a 6mm finished product using a laboratory rolling mill. The product has a yield strength of ≥700MPa, a tensile strength of ≥800MPa, an elongation of ≥12%, and a yield strength ratio of ≤0.93; a periodic immersion corrosion experiment is used to simulate the industrial atmospheric corrosion process. After the corrosion experiment is carried out for 72 hours, the corrosion rate of the test steel is 50.5% compared with the corrosion rate of Q355B. However, the laboratory research on 700MPa grade weathering steel for containers in document 3 contains precious alloying elements such as Ni, and the manufacturing cost of the product is relatively high. Summary of the invention

[0004] Based on the above technical background, the present invention successfully realizes the product's organizational morphology combining polygonal ferrite and granular bainite through the controlled rolling and cooling control technology of ultra-pure steel smelting, continuous casting and hot rolling, and achieves a reasonable match of product strength and toughness by mainly means of solid solution strengthening, fine grain strengthening and precipitation strengthening. Based on this product control strategy, a manufacturing method for a rare earth La-Ti microalloyed 700MPa high weathering resistant steel strip for a container is provided. The product composition design system is C0.03-0.08%, Si 0.10-0.50%, Mn 0.70-1.0%, P 0.015-0.025%, S≤0.005%, Als 0.020-0.040%, Nb 0.010-0.040%, Ti 0.070-0.110%, Cr 0.40-0.80%, Cu0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, and the rest are Fe and unavoidable inclusions. The product weathering index I is 6.2-6.5. The rolling adopts the controlled cooling process of 2250mm hot rolling unit (slab heating-rough rolling-finishing rolling-layer cooling-coiling). The mechanical properties of the product meet the yield strength: 650~700MPa, tensile strength: 700~800MPa, elongation A 50 :15~20%;Grain size ≥10 grade. 72 hours of periodic immersion corrosion, the corrosion rate of the test steel is ≤45% compared with the corrosion rate of Q355B, and the rare earth La improves the corrosion resistance of the product by changing the corrosion potential and the properties of the corrosion products. The product has good cold forming performance, the production process is relatively simple and executable, the product cost is relatively economical, and it is suitable for batch stable production of enterprises. And relative to the above-mentioned documents 1 and 3, the present invention adopts conventional alloy and rare earth La microalloying design, instead of using special alloy elements Sb and precious alloy Ni, which can still reduce the manufacturing cost of the product on the basis of ensuring weather resistance; in addition, relative to the above-mentioned document 2, the present invention clarifies the use of conventional alloy and rare earth La microalloying design, rare earth microalloying adopts Fe-La alloy block to be added in the later stage of refining vacuum degassing treatment, soft blowing 3~5min for homogenization treatment, rolling adopts 2250mm hot rolling unit rolling, giving full play to the hot rolling unit controlled rolling and controlled cooling technology to achieve good regulation of product toughness and weather resistance, and realize the low-cost control advantage of the product. The present invention is specifically implemented by the following technical solutions.

[0005] In a first aspect, the present invention provides a rare earth RE-Ti microalloyed 700MPa high weathering resistant steel strip for containers, wherein the chemical composition is as follows by mass percentage: C 0.03-0.08%, Si 0.10-0.50%, Mn 0.70-1.0%, P 0.015-0.025%, S≤0.005%, Als 0.020-0.040%, Nb 0.010-0.040%, Ti 0.070-0.110%, Cr 0.40-0.80%, Cu0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, and the rest is Fe and unavoidable inclusions; weathering index I: 6.2-6.5.

[0006] In some embodiments, the chemical composition of the rare earth RE-Ti microalloyed 700MPa high weathering resistant steel strip for container is as follows by mass percentage: C 0.04-0.07%, Si 0.13-0.16%, Mn 0.78-0.95%, P 0.018-0.022%, S0.003-0.005%, Als 0.020-0.0.30%, Nb 0.015-0.019%, Ti 0.080-0.098%, Cr 0.50-0.60%, Cu0.25-0.30%, La 0.0025-0.0040%, H≤2ppm, O≤25ppm, N≤60ppm, and the rest is Fe and unavoidable inclusions.

[0007] In some embodiments, the mechanical properties of the rare earth RE-Ti microalloyed 700MPa high weathering steel strip for container meet the following requirements: yield strength: 650-750MPa, tensile strength: 700-800MPa, elongation A 50 :15~25%.

[0008] In some embodiments, the rare earth RE-Ti microalloyed 700MPa high weathering resistant steel strip for container has non-metallic inclusions A, B, C, and D levels ≤1.5, and the sum of the non-metallic inclusion levels ≤2.5, the grain size ≥10, the core-surface structure grain level difference ≤2, and the structure is a granular bainite-based, polygonal ferrite-assisted structure, the granular bainite accounts for 60% to 70%, and the banded structure is ≤1.5.

[0009] In some embodiments, the corrosion rate of the rare earth RE-Ti microalloyed 700 MPa high weathering steel strip for container in a 72-hour periodic immersion corrosion test is ≤45% of the corrosion rate of Q355B.

[0010] In some embodiments, the rare earth RE-Ti microalloyed 700 MPa high weathering resistant steel strip for container has a thickness of 2.0 to 6.0 mm.

[0011] In a second aspect, the present invention provides a method for manufacturing a rare earth RE-Ti microalloyed 700MPa high weathering resistant steel strip for containers, which includes a smelting continuous casting process and a rolling process.

[0012] In some embodiments, the smelting and continuous casting process is: molten iron → molten iron pretreatment → converter steelmaking → LF refining → RH degassing → slab continuous casting; wherein the converter smelting adopts KR pre-desulfurized molten iron (molten iron [S] ≤ 0.002%), ferrosilicon, ferromanganese, ferrochrome and copper plate are added after the converter taps out steel, ferroniobium is added at the end of LF refining, and ferrotitanium is added at the end of vacuum degassing treatment to adjust to the alloy addition target value, La-Fe alloy is added before vacuum treatment negative pressure, the air breaking treatment is ended after circulating for 3 minutes, and the ladle is directly put on the casting machine for casting without Ca treatment; argon is blown and stirred throughout the molten steel process; the vacuum degree is ≤ 2mbar, the treatment time is ≥ 10min, the superheat ΔT ≤ 30-40°C, and the continuous casting billet is stretched by 1.2-1.5m / min; it is equipped with dynamic light pressure reduction technology, the straightening temperature is ≥ 850°C, and protective casting is adopted throughout the continuous casting process to ensure that H ≤ 2ppm, O ≤ 25ppm, and N ≤ 60ppm in the steel.

[0013] In some embodiments, the rolling process is: slab heating - high-pressure water descaling - fixed-width press - E1R1 roughing mill rolling - E2R2 roughing mill rolling - flying shear - high-pressure water descaling - F1~F7 finishing mill rolling - encrypted laminar cooling - coiling - pallet transportation system - warehouse stacking slow cooling - sampling and inspection; wherein the slab heating temperature is 1235±30℃; heating time is 180~240min; rough rolling mode adopts 1+5, 3+3 or 3+5; intermediate billet thickness range: 35~40mm; finishing rolling start temperature ≤1080℃; finishing rolling final rolling temperature is 840~900℃, coiling temperature is 550~630℃, and cooling rate is 15~25℃ / s.

[0014] The advantages of the present invention are:

[0015] The present invention provides a C-Si-Mn-P-Ti-Nb-Cr-Cu-La microalloying composition design, and a method for manufacturing a 700MPa grade weather-resistant hot rolled coil for a container by matching a 2250mm thermo-mechanical controlled rolling and controlled cooling process. The design has the advantages of using the significant precipitation strengthening effect of a low-cost Ti alloy to improve the strength of the product, and utilizing rare earth La to modify the spheroidization and refine the liquid phase TiN second phase particles to alleviate the defect of insufficient toughness of high-Ti microalloyed high-strength steel; on the basis of ensuring good cold forming performance of the product through fine grain strengthening, alloy P, Cu, and Cr atmospheric corrosion resistant elements are added to cooperate with the rare earth La alloy to jointly improve the corrosion resistance of the material. In the production practice of a 2250mm hot rolling production line, it has good promotion value. DETAILED DESCRIPTION

[0016] The present invention designs a 700MPa high-strength weather-resistant container hot-rolled steel strip with C-Si-Mn-P-Ti-Nb-Cr-Cu-La as the main component system to meet the trend of lightweight container steel development. The use of Cu, Cr, and La elements in the material design can effectively improve the atmospheric corrosion resistance of the experimental steel. Low C, S, and P can ensure good toughness, welding performance, and forming performance, while being beneficial to improving corrosion resistance. The Nb element plays a role in fine grain strengthening to improve the strength and toughness of the material. The Ti element plays a role in dispersion precipitation strengthening to greatly improve the strength of the material. At the same time, the interaction between the rare earth La and the second phase particles precipitated at high temperature by Ti is used to spheroidize and refine the liquid precipitation TiN second phase, thereby improving the toughness of Ti microalloyed high-strength steel. Specifically:

[0017] 1. Material composition control: As shown in Table 1 below, the composition design scheme is carried out according to the thickness of the product, wherein the C content is controlled at 0.03% to 0.08% (preferably 0.05% to 0.07%) to ensure a certain solid solution strengthening effect, reduce band segregation, and improve corrosion resistance and welding performance; the Si content is controlled at 0.10% to 0.50% (preferably 0.15% to 0.25%) to improve the strength of ferrite, but the increase in the amount of addition coarsens the ferrite grains, which is not conducive to welding performance; the Mn content is controlled at 0.70% to 1.00% (preferably 0.75% to 0.90%), which is the main element of strong solid solution strengthening and improves material strength, but the increase in content will reduce the plasticity and welding performance of steel; the Nb content is controlled at 0.010% to 0.040% (preferably 0.010% to 0.020%) to refine the grains The effect can improve the impact toughness of steel and reduce its brittle transition temperature, which is beneficial to welding performance; the Ti content is controlled at 0.070% to 0.110%, which plays a role of precipitation strengthening, and cooperates with Nb to strengthen the fine grain, which is beneficial to welding performance; the rare earth La content is controlled at 0.0025% to 0.0050%, spheroidizing and refining the liquid TiN second phase, and improving the toughness of Ti microalloyed high-strength steel; adding atmospheric corrosion resistant elements Cr and Cu, the control range is 0.40% to 0.60% and 0.25% to 0.45% respectively; on the basis of achieving good cold forming performance, adding weathering element P, the control range is 0.015% to 0.025%; S content is controlled to be ≤0.005%, which is a harmful element and needs to be effectively controlled; the gas in the steel is controlled to be H≤2ppm, O≤25ppm, and N≤60ppm.

[0018] Table 1: Composition design (mass percentage)

[0019]

[0020] 2. Material smelting

[0021] 2.1. Converter smelting: The pre-treated molten iron (S≤0.002%) enters the converter, where oxygen is blown for decarburization and the temperature is raised. In the later stage of smelting, ferrosilicon and ferromanganese are added for deoxidation and alloying. The P and S components are controlled to prevent overoxidation of the molten steel and the amount of steel tapping is controlled to prevent slag from entering the molten steel. The tapping temperature is 1600-1650°C. The converter tapping temperature [P]≤0.020%, [S]≤0.006%. Corrosion-resistant alloys Cu and Ni are added during the tapping process.

[0022] 2.2. Refining outside the furnace: LF+RH full-process argon blowing process is adopted, a good reducing atmosphere is maintained during the refining process, Al wire is used for deoxidation, LF is followed by adding ferromanganese and ferroniobium for alloying, RH vacuum treatment (vacuum degree ≤ 2mbar, deep vacuum time ≥ 10min), and ferrotitanium and rare earth lanthanum iron are added for alloying at the end, and no calcium treatment is performed after completion.

[0023] 2.3. Slab continuous casting: molten steel superheat ΔT≤30~40℃, casting speed controlled at 1.20~1.50m / min. Equipped with dynamic soft reduction technology, straightening temperature ≥850℃.

[0024] 2.4. Full nitrogen control: N≤60ppm in the tundish of molten steel, scrap ratio≤90%, converter tapping temperature≥1600℃, bottom blowing argon stirring in the converter, bottom blowing stirring mode with argon and nitrogen switching alternately cannot be used. The ladle is emptied with argon, argon is used to purge the tundish for ≥10min before continuous casting and heat exchange to exhaust the air in the ladle, and when the tundish pours 30t, the argon blowing tube is removed to reduce nitrogen absorption by the molten steel, and argon blowing is used to protect the casting during the normal casting process.

[0025] 3. Controlled rolling and controlled cooling process

[0026] The ingot is heated in a walking-beam heating furnace (the heating process is shown in Table 2). The rough rolling adopts single-stand R1 and R2 reciprocating rolling. The rough rolling modes adopted are 1+5, 3+3 and 3+5. The finishing rolling adopts F1~F7 continuous rolling process. The specific controlled rolling and controlled cooling process is shown in Table 3.

[0027] Table 2: Billet heating system

[0028]

[0029] Table 3: Rolling system

[0030]

[0031] The present invention is described in detail below through specific examples. The examples are intended to help understand the present invention but are not intended to limit the present invention.

[0032] According to the above smelting technical requirements, the following ingots are smelted, and the specific compositions are shown in Table 4 below, wherein the chemical composition of Comparative Example 1 differs from that of Example 1 only in whether rare earth La is used, and the comparative document 1 adopts the same smelting and rolling process parameters as Example 1; Comparative Example 2 adopts the chemical composition and content of patent document CN 111424211 A (the above document 2), and adopts the same smelting and rolling process parameters as Example 2; Comparative Example 3 adopts the same smelting and rolling process parameters as Example 2.

[0033] Table 4: Chemical composition of each embodiment and comparative example (mass percentage: %)

[0034]

[0035] According to the above composition design and hot rolling process (as shown in Table 5 below), the mechanical properties of the product are shown in Table 6.

[0036] Table 5: Smelting and rolling process parameters of Examples 1-3

[0037]

[0038] Table 6: Product performance of each embodiment and comparative example

[0039]

[0040] According to the above composition design and hot rolling process, the product is carried out according to the standard TB / T 2375-1993 "Cyclic Infiltration Corrosion Test Method for Weathering Steel for Railways". After the corrosion test, the surface macroscopic morphology is observed and the corrosion rate of the experimental steel is calculated (relative to Q355B). The experimental solution is 0.01mol / L NaHSO3 solution, the pH value is controlled within 4.4-4.8, and the replenishing solution is 0.02mol / L NaHSO3 solution. The experimental temperature is (45±2)℃, the humidity is (70±5)%RH, and the baking temperature is required to be (70±5)℃. 72 hours of periodic infiltration corrosion, the results are shown in Table 7 below (results of 4 repeated tests).

[0041] Table 7: Relative corrosion rates of various examples and comparative examples

[0042]

[0043] The results in Table 7 above show that the corrosion rate of the test steel of the embodiment is ≤45% compared with the corrosion rate of Q355B, while the corrosion rate of the test steel of Comparative Example 1 is significantly higher than the corrosion rate of the test steel of Example 1, which is basically above 45% or even above 50% relative to the corrosion rate of Q355B. This result shows that in the composition design of the present invention, the added alloy P, Cu, Cr atmospheric corrosion resistant elements can cooperate with the rare earth La alloy to jointly improve the corrosion resistance of the material.

[0044] In summary, the material of the present invention has good strength, toughness and atmospheric corrosion resistance, and can be promoted and applied in the field of lightweight steel for containers in large quantities.

[0045] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A rare earth RE-Ti microalloyed high weathering steel strip for 700MPa container, the chemical composition of which is as follows by mass percentage: C 0.03-0.08%, Si 0.10-0.50%, Mn 0.70-1.0%, P 0.015-0.025%, S≤0.005%, Als 0.020-0.040%, Nb 0.010-0.040%, Ti 0.070-0.110%, Cr 0.40-0.80%, Cu0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, and the rest is Fe and unavoidable inclusions; weathering index I: 6.2-6.

5.

2. The rare earth RE-Ti microalloyed 700MPa high weathering resistant steel strip for container according to claim 1, wherein the chemical composition is as follows by mass percentage: C 0.04-0.07%, Si 0.13-0.16%, Mn 0.78-0.95%, P 0.018-0.022%, S0.003-0.005%, Als 0.020-0.0.30%, Nb 0.015-0.019%, Ti 0.080-0.098%, Cr 0.50-0.60%, Cu0.25-0.30%, La 0.0025-0.0040%, H≤2ppm, O≤25ppm, N≤60ppm, and the rest is Fe and unavoidable inclusions.

3. The rare earth RE-Ti microalloyed 700MPa high weathering steel strip for container according to claim 1 or 2, wherein the mechanical properties thereof meet the following requirements: yield strength: 650-750MPa, tensile strength: 700-800MPa, elongation A 50 :15~25%.

4. The rare earth RE-Ti microalloyed 700MPa high weathering resistant steel strip for container according to any one of claims 1 to 3, wherein the levels of non-metallic inclusions A, B, C, and D are ≤1.5, and the sum of the levels of non-metallic inclusions is ≤2.5, the grain size is ≥10, the core-surface structure grain level difference is ≤2, and the structure is a structure morphology of granular bainite as the main body and polygonal ferrite as the auxiliary body, the granular bainite accounts for 60% to 70%, and the banded structure is ≤1.

5.

5. The rare earth RE-Ti microalloyed 700MPa high weathering steel strip for container according to any one of claims 1 to 4, wherein the corrosion rate thereof in a 72-hour periodic immersion corrosion test is ≤45% of the corrosion rate of Q355B.

6. The rare earth RE-Ti microalloyed 700 MPa high weathering resistant steel strip for container according to any one of claims 1 to 5, wherein the thickness thereof is 2.0 to 6.0 mm.

7. The method for manufacturing the rare earth RE-Ti microalloyed 700 MPa high weathering resistant steel strip for container according to any one of claims 1 to 6, comprising a smelting continuous casting process and a rolling process.

8. The manufacturing method according to claim 7, wherein the smelting and continuous casting process is: molten iron → molten iron pretreatment → converter steelmaking → LF refining → RH degassing → slab continuous casting; wherein the converter smelting adopts KR pre-desulfurized molten iron (molten iron [S] ≤ 0.002%), ferrosilicon, ferromanganese, ferrochrome and copper plate are added after the converter tapping, ferroniobium is added at the end of LF refining, and ferrotitanium is added at the end of vacuum degassing treatment to adjust to the alloy addition target value, La-Fe alloy is added before vacuum treatment negative pressure, the air-breaking treatment is ended after circulating for 3 minutes, and the ladle is directly put into the casting machine for casting without Ca treatment; argon blowing and stirring of the molten steel throughout the process; vacuum degree ≤ 2mbar, this treatment time ≥ 10min, superheat ΔT ≤ 30-40°C, and the continuous casting slab is stretched 1.2-1.5m / min; equipped with dynamic soft reduction technology, straightening temperature ≥ 850°C, and protective casting is adopted throughout the continuous casting process to ensure that H ≤ 2ppm, O ≤ 25ppm, and N ≤ 60ppm in the steel.

9. The manufacturing method according to claim 7 or 8, wherein the rolling process is: slab heating - high-pressure water descaling - fixed-width press - E1R1 roughing mill rolling - E2R2 roughing mill rolling - flying shear - high-pressure water descaling - F1~F7 finishing mill rolling - encrypted laminar cooling - coiling - pallet transportation system - warehouse stacking slow cooling - sampling and inspection; wherein the slab heating temperature is 1235±30℃; the heating time is 180~240min; the rough rolling mode adopts 1+5, 3+3 or 3+5; the intermediate billet thickness range is: 35~40mm; the finishing rolling start temperature is ≤1080℃; the finishing rolling final rolling temperature is 840~900℃, the coiling temperature is 550~630℃, and the cooling rate is 15~25℃ / s.

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