A rare earth RE-Ti microalloyed 700mpa high weathering resistance steel strip for container and a manufacturing method thereof

Abstract

The application discloses a rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers, which has the following chemical components in percentage by mass: 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%, Cu 0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, and the rest is Fe and inevitable inclusions. The high-weather-resistance steel strip for containers has good strength and toughness and atmospheric corrosion resistance, and is suitable for stable production in batches.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of metallurgical materials, and particularly relates to a rare earth RE-Ti micro-alloyed 700MPa high-weather-resistance steel strip for containers and a manufacturing method thereof. BACKGROUND

[0002] With the development of containers in China towards high-strengthening, light-weighting and high-weather-resistance, high-strength weather-resistant hot-rolled plate coils have become the main steel varieties in the industry. Thinner and lighter steel plates help to reduce the weight of the compartment and also expand the effective volume rate of the container. In order to improve the service life and save costs, the container steel must have high atmospheric corrosion resistance and seawater corrosion resistance.

[0003] There are reports of such container steels and their production methods in the prior art, for example patent document CN117305707A (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%, the balance being 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 document 1 belongs to the CSP production line process, and the product composition design adds special alloy element Sb, which has high production cost. Patent document CN 111424211 A (hereinafter referred to as document 2) discloses a wide 700MPa grade hot-rolled container steel and its manufacturing method, which has a composition design system containing C: 0.058%-0.078%, Si: 0.65%-0.75%, Mn: 0.65%-0.85%, P: 0.035%-0.045%, S≤0.006%, Al≤0.040%, Cr: 0.22%-0.31%, Cu: 0.11%-0.19%, Ti: 0.060%-0.075%, RE: 0.021%-0.035%, N≤0.005%. Its preparation mainly includes: smelting continuous casting, reheating of the billet, rolling, coiling, and flattening process, and the rare earth wire is uniformly added into the molten steel through the protection slag at the water gap of the crystallizer by using the wire feeding method for rare earth micro-alloying, but the document 2 does not specify the type of rare earth. Document "Microstructure and properties of Ti micro-alloyed 700MPa grade container steel" (hereinafter referred to as document 3) introduces that Northeastern University studied under laboratory conditions using a composition design system of 0.03% C, 0.25% Si, 1.45% Mn, 0.35% Cu, 0.85% Cr, 0.25% Ni, and appropriate Ti, and a laboratory rolling mill was used to roll a 100mm thick ingot into a 6mm finished product. 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. The industrial atmospheric corrosion process was simulated by using a periodic immersion corrosion experiment. After 72 hours of corrosion experiment, the corrosion rate of the test steel was 50.5% compared with the corrosion rate of Q355B. However, the laboratory study of document 3 on 700MPa grade container steel contains the precious alloy element Ni, which has high product manufacturing cost. SUMMARY

[0004] Based on the above technical background, the present application successfully realizes the product with the organizational morphology of polygonal ferrite and granular bainite combined through the controlled rolling and controlled cooling regulation technology of ultra-pure steel smelting, continuous casting and hot rolling, and realizes the reasonable matching of product strength and toughness by solid solution strengthening, fine grain strengthening and precipitation strengthening as the main means. Based on this product control strategy, a manufacturing method of rare earth La-Ti micro-alloyed 700MPa high-weather-resistant steel strip for containers 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%, Cu 0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, the rest is Fe and inevitable inclusions, the product weathering index I: 6.2-6.5. The rolling adopts the controlled cooling process of 2250mm hot rolling mill group (slab heating-rough rolling-fine rolling-layer cooling-coiling). The product mechanical properties meet the yield strength: 650-700MPa, tensile strength: 700-800MPa, elongation A 50 : 15-20%; grain size≥10 levels. The corrosion rate of the test steel is less than or equal to 45% compared with the corrosion rate of Q355B in 72-hour period immersion corrosion, the rare earth La improves the corrosion resistance of the product by changing the corrosion potential and the properties of the corrosion product. 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 stable batch production of enterprises. And compared with the above-mentioned literature 1 and literature 3, the present application adopts conventional alloy and rare earth La micro-alloying design, without using special alloy element Sb and precious alloy Ni, and still can reduce the product manufacturing cost on the basis of ensuring the weather resistance; in addition, compared with the above-mentioned literature 2, the present application clearly adopts conventional alloy and rare earth La micro-alloying design, the rare earth micro-alloying adopts Fe-La alloy block after refining vacuum degassing treatment in the later period, and soft blowing 3-5min for homogenization treatment, and the rolling adopts 2250mm hot rolling mill group rolling, which fully plays the role of hot rolling mill group controlled rolling and controlled cooling technology to realize the good regulation of product strength and toughness and weather resistance, and realizes the low-cost control advantage of the product. The present application is realized by the following technical schemes.

[0005] In a first aspect, the present application provides a rare earth RE-Ti micro-alloyed 700MPa high-weatherability steel strip for containers, which has a chemical composition in terms of mass percentage as follows: 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%, Cu 0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, and the balance being Fe and unavoidable inclusions; and a weathering index I: 6.2-6.5.

[0006] In some embodiments, the rare earth RE-Ti micro-alloyed 700MPa high-weatherability steel strip for containers has a chemical composition in terms of mass percentage as follows: C 0.04-0.07%, Si 0.13-0.16%, Mn 0.78-0.95%, P 0.018-0.022%, S 0.003-0.005%, Als 0.020-0.0.30%, Nb 0.015-0.019%, Ti 0.080-0.098%, Cr 0.50-0.60%, Cu 0.25-0.30%, La 0.0025-0.0040%, H≤2ppm, O≤25ppm, N≤60ppm, and the balance being Fe and unavoidable inclusions.

[0007] In some embodiments, the rare earth RE-Ti micro-alloyed 700MPa high-weatherability steel strip for containers has mechanical properties as follows: yield strength: 650-750MPa, tensile strength: 700-800MPa, and elongation A 50 : 15-25%.

[0008] In some embodiments, the rare earth RE-Ti micro-alloyed 700MPa high-weatherability steel strip for containers has non-metallic inclusions A, B, C, and D each at a level of≤1.5 grade, and a total level of non-metallic inclusions≤2.5 grade, a grain size≥10 grade, a difference in core-surface grain level≤2 grade, and a structure mainly in a granular bainite and auxiliary polygonal ferrite structure, with a granular bainite proportion of 60%-70% and a banded structure≤1.5 grade.

[0009] In some embodiments, the rare earth RE-Ti micro-alloyed 700MPa high-weatherability steel strip for containers has a corrosion rate in a 72-hour periodic immersion corrosion test of≤45% compared with a Q355B corrosion rate.

[0010] In some embodiments, the rare earth RE-Ti micro-alloyed 700MPa high-weather-resistance steel strip for containers has a thickness of 2.0-6.0mm.

[0011] In a second aspect, the present application provides a method for manufacturing a rare earth RE-Ti micro-alloyed 700MPa high-weather-resistance steel strip for containers, which comprises a smelting and 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-desulfurization molten iron (molten iron[S]≤0.002%), silicon iron, manganese iron, chromium iron and copper plate are added after converter tapping, niobium iron is added at the end of LF refining, titanium iron is added at the end of vacuum degassing treatment to adjust to the target value of alloy addition, La-Fe alloy is added before vacuum treatment negative pressure, and the cycle is 3min, then the emptying treatment is finished, no Ca treatment is performed, the ladle is directly cast on the casting machine; argon blowing stirring is performed throughout the molten steel process; the vacuum degree is ≤2mbar, the treatment time is ≥10min, the superheat degree ΔT is ≤30-40℃, the continuous casting slab is stretched at 1.2-1.5m / min; the dynamic soft reduction technology is provided, the straightening temperature is ≥850℃, the protective casting is adopted throughout the continuous casting to ensure that H in the steel is ≤2ppm, O is ≤25ppm, and N is ≤60ppm.

[0013] In some embodiments, the rolling process is: slab heating—high-pressure water descaling—width setting press—E1R1 rough rolling mill rolling—E2R2 rough rolling mill rolling—flying shear—high-pressure water descaling—F1-F7 finishing rolling mill rolling—encryption type laminar cooling—coiling—pallet transportation system—stockyard stacking and 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 slab thickness range is 35-40mm; the rough rolling temperature is ≤1080℃; the finishing rolling finish rolling temperature is 840-900℃, the coiling temperature is 550-630℃, and the cooling speed is 15-25℃ / s.

[0014] The present application has the following advantages:

[0015] The application provides a C-Si-Mn-P-Ti-Nb-Cr-Cu-La micro-alloying component design and a method for manufacturing 700MPa-grade weather-resistant container hot-rolled coil sheet by matching a 2250mm hot mechanical controlled rolling and controlled cooling process. The design advantage is that the low-cost Ti alloy is used to significantly improve the strength of the product by precipitation strengthening, the rare earth La is used to modify the spheroidization and refine the liquid precipitation TiN second phase particles, and the toughness defect of the high-Ti micro-alloyed high-strength steel is relieved; by adding the alloy P, Cu and Cr atmospheric corrosion elements and the rare earth La alloy, the corrosion resistance of the material is improved. The production practice of the 2250mm hot rolling production line has good popularization value. DETAILED DESCRIPTION

[0016] The application 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, which meets the trend requirements of the steel for the lightweight development of the container. The material design uses Cu, Cr and La elements to effectively improve the atmospheric corrosion resistance of the experimental steel, low C, S and P to ensure good toughness, welding performance and forming performance, which is also beneficial to improve the corrosion resistance, Nb element plays a role in fine-grain strengthening to improve the strength and toughness of the material, Ti element plays a role in dispersion precipitation strengthening to greatly improve the strength of the material, and the interaction of the rare earth La and Ti high-temperature precipitation second phase is used to spheroidize and refine the liquid precipitation TiN second phase, thereby improving the toughness of the Ti micro-alloyed high-strength steel. Specifically,

[0017] 1. Material composition control: As shown in Table 1, the composition design is carried out according to the thickness of the product, wherein the C content is controlled to be 0.03% to 0.08% (preferably 0.05% to 0.07%), to ensure the effect of certain solid solution strengthening, reduce banded segregation, and improve corrosion resistance and welding performance; the Si content is controlled to be 0.10% to 0.50% (preferably 0.15% to 0.25%), to improve the strength of ferrite, but the addition of the amount increases the coarsening of ferrite grains, which is not conducive to the welding performance; the Mn content is controlled to be 0.70% to 1.00% (preferably 0.75% to 0.90%), which is the main element of solid solution strengthening, to improve the strength of the material, but the increase of the content will reduce the plasticity and welding performance of the steel; the Nb content is controlled to be 0.010% to 0.040% (preferably 0.010% to 0.020%), to refine the grains, improve the impact toughness of the steel, and reduce the brittle transition temperature, which is beneficial to the welding performance; the Ti content is controlled to be 0.070% to 0.110%, to have the effect of precipitation strengthening, and cooperate with Nb to refine the grains, which is beneficial to the welding performance; the rare earth La content is controlled to be 0.0025% to 0.0050%, to spheroidize and refine the TiN second phase, and improve the toughness of the Ti micro-alloyed high-strength steel; the atmospheric corrosion resistant elements Cr and Cu are added, and the control ranges are 0.40% to 0.60% and 0.25% to 0.45% respectively; on the basis of realizing good cold forming performance, the weathering element P is added, and the control range is 0.015% to 0.025%; the 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 pretreated molten iron (S≤0.002%) enters the converter, oxygen is blown to decarburize and heat up, silicon iron and manganese iron are added for deoxidization and alloying in the later smelting stage, the P and S components are controlled, over-oxidation of the molten steel is prevented, and the amount of molten steel is controlled to prevent the molten slag from entering the molten steel, the tapping temperature is 1600-1650°C, the converter tapping [P]≤0.020%, [S]≤0.006%, and the corrosion resistant alloy Cu and Ni are added during the tapping process.

[0022] 2.2. Secondary refining: LF+RH full-range argon blowing process is adopted, a good reducing atmosphere is maintained during the refining process, Al wire is used for deoxidization, manganese iron and niobium iron are added for alloying in the later stage of LF, titanium iron and rare earth lanthanum iron are added for alloying after RH vacuum treatment (vacuum degree≤2mbar, deep vacuum time≥10min), and no calcium treatment is carried out after the end.

[0023] 2.3, slab continuous casting: superheat of molten steel ΔT≤30-40℃, the drawing speed is controlled at 1.20-1.50 m / min. Dynamic soft reduction technology is provided, and the straightening temperature is ≥850℃.

[0024] 2.4, nitrogen control throughout the process: N≤60ppm is detected in the tundish, the scrap ratio is ≤90%, the converter tapping temperature is required to be ≥1600℃, the converter adopts bottom blowing argon stirring, and the bottom blowing stirring mode of argon-nitrogen alternation switching cannot be used. The tundish is treated by argon emptying, argon blowing is used for 10 minutes before the continuous casting and hot change to discharge the air in the tundish, when the tundish is poured for 30t, the argon blowing pipe is taken out to reduce the nitrogen absorption of molten steel, and the normal casting process adopts full-process argon blowing protection casting.

[0025] 3, controlled rolling and controlled cooling process

[0026] The cast blank is heated by using a walking beam furnace (the heating process is shown in Table 2), the rough rolling is carried out by using single-stand R1 and R2 reciprocating rolling, the rough rolling mode of 1+5, 3+3 and 3+5 is adopted, the finish rolling is carried out by using F1-F7 continuous rolling process, and the specific controlled rolling and controlled cooling process is shown in Table 3.

[0027] Table 2: Heating schedule of cast blank

[0028]

[0029] Table 3: Rolling schedule

[0030]

[0031] The content of the present application is described in detail through specific examples, and the examples are intended to help understand the present application, and are not intended to limit the content of the present application.

[0032] According to the above smelting technical requirements, the cast blank with the following components is smelted, and the specific components are shown in Table 4, wherein the chemical composition of Comparative Example 1 is different from that of Example 1 only in whether rare earth La is used, the smelting and rolling process parameters of the Comparative Example 1 are the same as those of Example 1; the chemical composition and content of the Comparative Example 2 are the same as those of the patent document CN 111424211 A (the above-mentioned document 2), and the smelting and rolling process parameters of the Comparative Example 2 are the same as those of Example 2; the smelting and rolling process parameters of the Comparative Example 3 are the same as those of Example 2.

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

[0034]

[0035] According to the above component design and hot rolling process (as shown in Table 5 below), the mechanical property indexes 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 example and comparative example

[0039]

[0040] According to the above ingredient design and hot rolling process, the product is tested according to the standard TB / T 2375-1993 "Railway Weathering Steel Cyclic Immersion Corrosion Test Method", after the corrosion experiment, the surface macroscopic morphology is observed and the corrosion rate of the experimental steel (relative to Q355B) is calculated. The experimental solution is 0.01 mol / L NaHSO3 solution, the pH value is controlled within 4.4-4.8, the replenishment solution is 0.02 mol / L NaHSO3 solution. The experimental temperature is (45±2)℃, the humidity is (70±5)%RH, and the baking temperature requirement is (70±5)℃. The 72-hour cyclic immersion corrosion, the results are shown in the following table 7 (4 times repeated test results).

[0041] Table 7: Relative corrosion rate of each example and comparative example

[0042]

[0043] The results of the above table 7 show that the corrosion rate of the test steel of the examples is ≤45% compared with the corrosion rate of Q355B, while the corrosion rate of the test steel of comparative example 1 is obviously higher than that of the test steel of example 1, which is basically above 45% relative to the corrosion rate of Q355B, even above 50%, which shows that in the component design of the present application, the addition of the alloy P, Cu, Cr atmospheric corrosion resistant elements can synergistically improve the corrosion resistance of the material with the rare earth La alloy.

[0044] In summary, the material of the present application has good strength and toughness and atmospheric corrosion resistance, and can be widely applied in the field of container lightweight steel, and provides a broad space for energy saving, emission reduction and efficient transportation for realizing the "carbon peak" and "carbon neutral" goals of our country.

[0045] Finally, it should be pointed out that: the above only describes the preferred embodiments of the present application, and is not used to limit the present application, although the present application has been described in detail with reference to the foregoing embodiments, for those skilled in the art, the technical solutions recorded in the foregoing embodiments can still be modified, or some technical features can be replaced. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers, having a chemical composition in mass percentage of 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%, Cu 0.25-0.50%, La 0.0025-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, and the balance of Fe and unavoidable inclusions; a weather resistance index I: 6.2-6.

5. The microstructure of the rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers is mainly granular bainite, with polygonal ferrite as a supplement, and the proportion of granular bainite is 60-70%; and The yield strength of the rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers is 650-750MPa.

2. The rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers according to claim 1, having a chemical composition in mass percentage of C 0.04-0.07%, Si 0.13-0.16%, Mn 0.78-0.95%, P 0.018-0.022%, S 0.003-0.005%, Als 0.020-0.0.30%, Nb 0.015-0.019%, Ti 0.080-0.098%, Cr 0.50-0.60%, Cu 0.25-0.30%, La 0.0025-0.0040%, H≤2ppm, O≤25ppm, N≤60ppm, and the balance of Fe and unavoidable inclusions.

3. The rare earth RE-Ti microalloyed 700 MPa high-weather-resistance steel strip for containers according to claim 1 or 2, the mechanical properties of which meet: tensile strength: 700-800 MPa, elongation A 50 : 15-25%.

4. The rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers according to claim 1 or 2, having non-metallic inclusions A, B, C, D each at a level of≤1.5 grade, and a total non-metallic inclusion level of≤2.5 grade, a grain size≥10 grade, a core-surface structure grain level difference≤2 grade, and a band structure≤1.5 grade.

5. The rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers according to claim 1 or 2, having a corrosion rate in a 72-hour period immersion corrosion test of≤45% compared to the corrosion rate of Q355B.

6. The rare earth RE-Ti microalloyed 700MPa high-weather-resistance steel strip for containers according to claim 1 or 2, having a thickness of 2.0-6.0mm.

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

8. The manufacturing method of claim 7, wherein the smelting continuous casting process is: hot metal→ hot metal pretreatment→ converter steelmaking→ LF refining→ RH degassing→ slab continuous casting; wherein the converter smelting adopts KR pre-desulfurization hot metal, the hot metal [S]≤0.002%, silicon iron, manganese iron, chromium iron and copper plate are added after converter tapping, niobium iron is added at the end of refining LF, titanium iron is added at the end of vacuum degassing treatment to adjust to the target value of alloy addition, La-Fe alloy is added before vacuum treatment negative pressure, the cycle is 3 min, and the emptying treatment is ended, Ca treatment is not performed, the ladle is directly cast on the casting machine, and is cast; the molten steel is argon-bubbling stirred throughout the process; the vacuum degree is ≤2 mbar, the treatment time is ≥10 min, the superheat degree ΔT is ≤30~40℃, the continuous casting slab is stretched at 1.2~1.5 m / min; the dynamic soft reduction technology is provided, the straightening temperature is ≥850℃, the whole continuous casting process adopts protective casting, and H≤2 ppm, O≤25 ppm, and N≤60 ppm in the steel are ensured.

9. The manufacturing method of claim 7 or 8, wherein the rolling process is: slab heating→ high-pressure water descaling→ sizing press→ E1R1 rough rolling mill rolling→ E2R2 rough rolling mill rolling→ flying shear→ high-pressure water descaling→ F1~F7 finishing rolling mill rolling→ encryption type laminar cooling→ coiling→ tray transportation system→ warehouse area stacking slow cooling→ sampling and inspection; wherein the slab heating temperature is 1235±30℃; the heating time is 180~240 min; the rough rolling mode adopts 1+5, 3+3 or 3+5; the intermediate blank thickness range is: 35~40 mm; the finishing rolling opening rolling temperature is ≤1080℃; the finishing rolling final rolling temperature is 840~900℃, the coiling temperature is 550~630℃, and the cooling speed is 15~25℃ / s.

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