A rare earth micro-alloyed corrosion-resistant wear-resistant coil plate NM400 and a manufacturing method thereof

Abstract

The application discloses a rare earth microalloyed corrosion-resistant wear-resistant plate NM400 and a manufacturing method thereof, wherein the chemical components of the provided NM400 include the following components in percentage by mass: C 0.16-0.22%, Si 1.00-1.50%, Mn 1.20-1.50%, P≤0.020%, S≤0.005%, Als 0.030-0.060%, Nb 0.010-0.030%, Ti 0.025-0.050%, Cr 1.00-1.50%, B 0.0008-0.0015%, La 0.0020-0.0050%. The thin-gauge NM400 hot-rolled steel strip with a C-Si-Mn-Ti-Nb-Cr-La-B main component system can meet the trend requirement of a new steel material for a corrosion-resistant and wear-resistant steel for a carriage body in a service environment of a slag transport.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical materials technology, specifically relating to a rare earth microalloyed corrosion-resistant and wear-resistant coil NM400 and its manufacturing method, and particularly to a thin-gauge (4-8mm) corrosion-resistant and wear-resistant high-strength NM400 hot-rolled steel strip and its manufacturing method. Background Technology

[0002] Search for current patents and literature in related fields:

[0003] Patent CN 114058814 B (hereinafter referred to as Document 1) discloses "a method for preparing high hardness uniformity NM400 wear-resistant steel". The product composition design includes: C: 0.20-0.40%, Mn: 1.20-1.80%, Si: 0.20-0.50%, Cr: 0.20-0.60%, Mo: 0.20-0.50%, Al: 0.02-0.04%, Ti: 0.01-0.02%, P: ≤0.02%, S: ≤0.02%; B: 0.0001-0.0025%, with the remainder being Fe and unavoidable impurity elements. The preparation method described in Document 1 mainly includes slab smelting and casting, rolling, pre-straightening, cooling, and tempering processes. Key temperature controls are as follows: final rolling temperature is 930℃~980℃, pre-straightening end temperature is 850℃~880℃, and tempering temperature is 200℃~400℃. This is a medium-thick plate production process, and the product composition incorporates the valuable alloying element Mo, resulting in higher costs.

[0004] Patent CN 111440996 B (hereinafter referred to as Document 2) discloses "a 6-8mm TMCP online quenched low yield strength ratio high strength wear-resistant steel NM400 coil and its production method". Its composition design system is that the steel contains C: 0.18-0.22%, Si: 0.30-0.50%, Mn: 1.30-1.45%, P: ≤0.012%, S: ≤0.003%, Nb: 0.015-0.025%, Ti: 0.010-0.025%, Cr: 0.25-0.40%, B: 0.0005-0.0020%, Ca: 0.0010-0.0030%, Al: 0.020-0.050%, H: ≤2ppm, O: ≤30ppm, N: ≤50ppm, and the balance is Fe and unavoidable impurities. Reference 2 uses a continuously cast slab and a 2250mm rolling mill for controlled rolling and cooling to produce NM400 coils. It achieves control over the morphology of ferrite and martensite through two-stage cooling. However, its shortcomings are: when the ferrite transforms at a medium temperature after the first stage of cooling, the proportion of ferrite on the surface of the steel strip is higher than that in the core, which reduces the wear resistance; on the other hand, the subsequent cooling leads to low-temperature coiling (coiling temperature ≤200℃), resulting in greater internal stress in the steel coil and making it difficult to control the plate shape.

[0005] Patent CN 115896621 A (hereinafter referred to as Document 3) discloses "a thin-gauge high-quality NM400 steel plate produced by online quenching process and its manufacturing method." The product composition design includes: C: 0.14-0.18%, Si: 0.40-0.80%, Mn: 1.20-1.50%, Alt: 0.055-0.085%, Nb: 0.010-0.020%, Cr: 0.65-0.85%, Ti: ≤0.005%, V: ≤0.010%, Ni: ≤0.10%, Cu: ≤0.10%, Mo: ≤0.10%, P: ≤0.012%, S: ≤0.001%, N: ≤40ppm, with the remainder being Fe and unavoidable impurity elements. Online quenching replaces offline quenching, which is a medium-thick plate production process, but the production cost is relatively high. Summary of the Invention

[0006] Based on the above technical background, this invention has successfully achieved the control of the microstructure of the product, which is mainly composed of acicular bainite, through the controlled rolling and cooling control technology of smelting, continuous casting and hot rolling of ultrapure steel. Furthermore, it has achieved a reasonable match between the product's strength, toughness, weather resistance and wear resistance by using solid solution strengthening, fine grain strengthening and precipitation strengthening as the main means. Based on this product control strategy, a rare earth microalloyed corrosion-resistant wear-resistant coil NM400 and its manufacturing method are provided. The product composition design system is (0.16~0.22%)C-(1.00~1.50%)Si-(1.20~1.50%)Mn-(≤0.020%)P-(≤0.005%)S-(0.030~0.060%)Als-(0.010~0.030%)Nb-(0.020~0.050%)Ti-(1.00~1.50%)Cr-(0.002~0.005%)La-(0.0008~0.0015%)B, H≤2ppm, O≤25ppm, N≤60ppm. The smelting process employs hot metal pretreatment, converter smelting, ladle refining (LF+RH), and slab continuous casting. Rolling utilizes a controlled cooling process on a 2250mm hot rolling mill (slab heating - roughing - finishing - layer cooling - coiling). The product's mechanical properties meet the following requirements: yield strength: 900–1100 MPa, tensile strength: 1250–1400 MPa, elongation A… 50The yield strength is 12-18%, the yield ratio is 0.70-0.79, the Brinell hardness is 380-420, and the grain size is ≥10. Under 72-hour cyclic immersion corrosion, the corrosion rate of the test steel is ≤60% compared to Q355B. The addition of rare earth element La alleviates center segregation in the high-alloy billet and increases the proportion of equiaxed grains. Furthermore, the interaction between rare earth La and alloy Cr alters the corrosion potential and the properties of corrosion products, improving the product's corrosion resistance. Therefore, the product exhibits good strength, toughness, weather resistance, and wear resistance. The production process uses the domestically standard 2250mm production line, which is relatively simple and feasible, resulting in high production efficiency and economical cost, making it suitable for stable mass production. This invention is specifically achieved through the following technical solutions.

[0007] In a first aspect, the present invention provides a rare earth microalloyed corrosion-resistant wear-resistant coil NM400, the chemical composition of which, by mass percentage, is: C 0.16-0.22%, Si 1.00-1.50%, Mn 1.20-1.50%, P≤0.020%, S≤0.005%, Als0.030-0.060%, Nb 0.010-0.030%, Ti 0.025-0.050%, Cr 1.00-1.50%, B 0.0008-0.0015%, La0.0020-0.0050%, H≤2ppm, O≤25ppm, N≤60ppm, with the remainder being Fe and unavoidable inclusions.

[0008] In some embodiments, the chemical composition of the rare earth microalloyed corrosion-resistant wear-resistant coil NM400, by mass percentage, is as follows: C 0.18–0.20%, Si 1.25–1.45%, Mn 1.25–1.45%, P ≤0.020%, S ≤0.005%, Als 0.030–0.050%, Nb 0.015–0.020%, Ti 0.025–0.045%, Cr 1.35–1.50%, B 0.0012–0.0014%, La 0.0020–0.0035%, H ≤2ppm, O ≤25ppm, N ≤60ppm, with the remainder being Fe and unavoidable inclusions.

[0009] In some embodiments, the mechanical properties of the rare earth microalloyed corrosion-resistant wear-resistant coil NM400 meet the following requirements: yield strength: 900-1100 MPa, tensile strength: 1250-1400 MPa, elongation A 50 : 12~18%, yield strength ratio: 0.68~0.78; Brinell hardness: 380~420, grain size ≥10 grade.

[0010] In some embodiments, the corrosion rate of the rare earth microalloyed corrosion-resistant wear-resistant coil NM400 in a 72-hour cyclic immersion corrosion test is ≤60% compared to the corrosion rate of Q355B.

[0011] In some embodiments, the thickness of the rare earth microalloyed corrosion-resistant wear-resistant coil NM400 is 4-8 mm.

[0012] Secondly, the present invention provides a method for manufacturing rare earth microalloyed corrosion-resistant wear-resistant coil NM400, which includes a smelting continuous casting process and a rolling process.

[0013] In some embodiments, the smelting and continuous casting process is as follows: molten iron → molten iron pretreatment → converter steelmaking → LF refining → RH degassing → slab continuous casting; wherein the converter smelting uses KR pre-desulfurized molten iron (molten iron [S] ≤ 0.002%), ferrosilicon, ferromanganese, and ferrochrome are added after the converter tapping, ferroniobium is added at the end of the LF refining stage, and ferrotitanium is added at the end of the vacuum degassing treatment to adjust to the target value of alloy addition, La-Fe alloy is added before the vacuum treatment negative pressure, and the vacuum treatment ends after 3 minutes of circulation, and the ladle is directly cast on the casting machine without Ca treatment; the molten steel is stirred by argon blowing throughout the process; the vacuum degree is ≤ 2 mbar, the treatment time is ≥ 10 min, the superheat ΔT is ≤ 30~40℃, and the continuous casting slab stretching is 1.2~1.5 m / min; it is equipped with dynamic light pressure reduction technology, and the straightening temperature is ≥ 800℃.

[0014] In some embodiments, the rolling process is as follows: 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—densified laminar flow cooling—coiling—pallet transport system—warehouse stacking slow cooling—sampling and inspection; wherein the slab heating temperature is 1245±20℃; heating time is 180~240min; roughing mode adopts 1+5, 3+3 or 3+5; intermediate slab thickness range: 35~40mm; finishing mill starting temperature ≤1080℃; finishing mill finishing temperature 840~900℃, coiling temperature is 450~550℃, and cooling rate is 30~50℃ / s.

[0015] The advantages of this invention are:

[0016] This invention provides a C-Si-Mn-Cr-Ti-Nb-B-La microalloying composition design, matched with a 2250mm thermomechanical controlled rolling and cooling process to manufacture rare-earth microalloyed corrosion-resistant and wear-resistant coil NM400. The design advantage lies in using medium-temperature coiling under TMCP process conditions to obtain NM400 hot-rolled coils dominated by acicular bainite, exhibiting excellent shape control and low stress control. This overcomes the industry challenge of low production efficiency and high manufacturing costs associated with offline heat treatment for producing thin-gauge wear-resistant plates. In production practice on a 2250mm hot rolling production line, it demonstrates significant cost advantages and widespread application value. Compared to the aforementioned references 1-3, this invention focuses on adding unconventional alloying elements Si and Cr, as well as rare-earth La alloys. Conventional laminar flow cooling on a 2250mm hot rolling mill enables medium-temperature coiling, successfully coiling the NM400 coils. High-temperature coiling further releases deformation for profitability. This achieves excellent control over the product's toughness, weather resistance, and wear resistance, as well as low-cost control. Attached Figure Description

[0017] Figure 1 The metallographic morphology of the thickness 1 / 4 (A) and thickness center (B) of the rare earth microalloyed corrosion-resistant wear-resistant coil NM400 produced in Example 1.

[0018] Figure 2 The bending performance (180°) test results of rare earth microalloyed corrosion-resistant wear-resistant coil NM400 produced in Example 2 are shown, where A and B represent views from different directions. Detailed Implementation

[0019] This invention designs a thin-gauge NM400 hot-rolled steel strip with a C-Si-Mn-Ti-Nb-Cr-La-B main component system to meet the trend of developing new steel materials for corrosion-resistant and wear-resistant steel bodies used in the service environment of slag transportation. The material design adopts a method that, based on the necessary solid solution strengthening effect of C and Mn elements and low levels of harmful elements S and P, adds a high content of Si element for solid solution strengthening, which, in conjunction with Al element, can suppress cementite precipitation within a certain temperature range and facilitate the formation of acicular bainite during intermediate-temperature transformation. Adding a high content of Cr element improves the hardenability of the material through solid solution, and the precipitated carbides improve wear resistance and corrosion resistance. Adding Nb element plays a fine-grain strengthening role to improve the strength and toughness of the material, while Ti element plays a dispersed precipitation strengthening role to significantly improve the strength of the material. Simultaneously, the addition of rare earth element La utilizes its high-temperature precipitation of a second phase to refine the as-cast microstructure and improve the banded microstructure of the finished product, thereby improving the toughness of the wear-resistant steel strip. Specifically:

[0020] 1. Material Composition Control: As shown in Table 1 below, the C content is controlled at 0.16%–0.22% (preferably 0.18%–0.20%) to ensure material strength and wear resistance; the Si content is controlled at 1.00–1.50% (preferably 1.20%–1.40%), which, in conjunction with Als (content 0.045%–0.060%), can inhibit cementite precipitation within a certain temperature range and facilitate the formation of acicular bainite during intermediate-temperature transformation; the Mn content is controlled at 1.20%–1.50% (preferably 1.25%–1.45%) to ensure material strength, toughness, and wear resistance; the Cr content is controlled at 1.00%–1.50% (preferably 1.25%–1.50%). To improve the hardness and wear resistance of the material: Nb content is controlled at 0.010%–0.030%, refining the grain size and improving the impact toughness and toughness of the steel; Ti content is controlled at 0.020%–0.050% (preferably 0.025%–0.045%), precipitation strengthening, synergistically with Nb to refine the grain size and improve the wear resistance of the material; B content is controlled at 0.0008%–0.0015% (preferably 0.0010%–0.0014%), improving the hardenability of the material; rare earth La content is controlled at 0.0020%–0.0050%, utilizing its high-temperature precipitation of a second phase to refine the as-cast structure and improve the banded structure of the finished product, thus improving the toughness and corrosion resistance of the wear-resistant steel strip. P content is controlled ≤0.020%; S content is controlled ≤0.005%, as it is a harmful element and requires effective control; gases in the steel are controlled at H≤2ppm, O≤25ppm, and N≤60ppm.

[0021] Table 1: Ingredient Design (Percentage by Mass)

[0022]

[0023] 2. Smelting of materials

[0024] 2.1 Converter smelting: The pretreated molten iron (S≤0.002%) enters the converter, where oxygen is blown to decarburize and raise the temperature. In the later stage of smelting, ferrosilicon and ferromanganese are added for deoxidation and alloying. The P and S content is controlled to prevent over-oxidation of the molten steel and to control the tapping rate to prevent steel slag from entering the molten steel. The tapping temperature is 1600~1650℃. The [P] of the converter tapped steel is ≤0.020%, and the [S] is ≤0.006%. Alloys Si, Mn, and Cr are added during the tapping process.

[0025] 2.2. Ladle refining: The LF+RH full-process argon blowing process is adopted. The refining process maintains a good reducing atmosphere. Al wire is used for deoxidation. In the later stage of LF, ferromanganese and ferroniobium are added for alloying. In the later stage of RH vacuum treatment (vacuum degree ≤2mbar, deep vacuum time ≥10min), ferrotitanium, ferroboron and rare earth lanthanum are added for alloying. No calcium treatment is performed after the process is completed.

[0026] 2.3 Slab continuous casting: Special protective slag is used, the superheat of molten steel ΔT ≤ 30~40℃, and the casting speed is controlled at 1.20~1.50m / min. It is equipped with dynamic light reduction technology, and the straightening temperature ≥ 800℃.

[0027] 2.4. Full-process nitrogen control: N in the molten steel ladle must be ≤60ppm, scrap steel ratio ≤90%, converter tapping temperature ≥1600℃, and bottom blowing argon stirring in the converter is required; alternating argon and nitrogen bottom blowing stirring is not permitted. The ladle is purged with argon gas. Before continuous casting begins and before heat exchange, the ladle is purged with argon gas for ≥10 minutes to remove air. When 30t of steel is poured from the ladle, the argon blowing pipe is removed to reduce nitrogen absorption by the molten steel. During normal casting, full-process argon blowing protection is used.

[0028] 3. Controlled rolling and controlled cooling process

[0029] The billet is heated by a walking beam furnace (heating process is shown in Table 2). The roughing rolling adopts a single stand R1 and R2 reciprocating rolling process, and the roughing rolling mode is 1+5, 3+3 and 3+5. The finishing rolling adopts the F1 to F7 continuous rolling process. The specific controlled rolling and controlled cooling process is shown in Table 3.

[0030] Table 2: Heating Regime for Cast Billets

[0031]

[0032] Table 3: Rolling Process

[0033]

[0034] Material purity control: Ultra-purity control and microstructure uniformity control of materials are the foundation for ensuring good resistance to hydrogen embrittlement. Through the smelting and double refining process, the non-metallic inclusions of grades A, B, C, and D are all ≤1.5, and the sum of the non-metallic inclusion grades is ≤2.5.

[0035] Material microstructure control: The grain size is ≥10, and the difference in grain size between the core and surface is ≤2. The microstructure is mainly composed of acicular bainite and supplemented by granular bainite, with acicular bainite accounting for 70% to 80% and banded microstructure ≤1.5.

[0036] The present invention will be described in detail below through specific embodiments. These embodiments are intended to help understand the present invention and are not intended to limit the scope of the present invention.

[0037] Based on the above smelting technology requirements, smelt billets with the following composition, as shown in Table 4 below.

[0038] Table 4: Chemical composition of each example (mass percentage: %)

[0039]

[0040] The smelting and hot rolling processes are designed according to the above composition (as shown in Table 5 below), and the mechanical properties of the product are shown in Table 6.

[0041] Table 5: Smelting and rolling process parameters for Examples 1-3

[0042]

[0043] Table 6: Product Performance of Each Example

[0044]

[0045]

[0046] The corrosion test was conducted according to standard TB / T 2375-1993, "Cyclic Immersion Corrosion Test Method for Weathering Steel for Railway Use". After the corrosion test, the surface macroscopic morphology was observed and the corrosion rate of the experimental steel was calculated. The experimental solution was a 0.01 mol / L NaHSO3 solution with the pH value controlled between 4.4 and 4.8, and the replenishing solution was a 0.02 mol / L NaHSO3 solution. The experimental temperature was (45±2)℃, the humidity was (70±5)%RH, and the baking temperature was (70±5)℃. After 72 hours of cyclic immersion corrosion, the corrosion rate of the experimental steels in Examples 1-3 was ≤60% compared to that of Q355B. The results are shown in Table 7 below.

[0047] Table 7: Relative Corrosion Rates of Each Example

[0048]

[0049] The results in Table 7 above show that the corrosion rate of the test steel in the example is ≤60% compared to Q355B.

[0050] The metallographic structure of the material in Example 1 was analyzed, revealing it to be predominantly acicular bainite with granular bainite as a secondary component; the grain size was ≥10, and the banded structure was ≤1.5. The metallographic morphology is shown in the figure. Figure 1 As shown, image A represents the metallographic morphology at 1 / 4 of the thickness, and image B represents the metallographic morphology at the center of the thickness.

[0051] The bending properties (180°) of the material in Example 2 were analyzed, and the results are as follows: Figure 2 As shown in the figure, where A and B represent views from different directions, it can be seen that the material has good bending properties.

[0052] In summary, the invented material has good strength, toughness, corrosion resistance, and wear resistance, and can be widely used in the field of lightweight and long-life steel for dump trucks.

[0053] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A rare earth microalloyed corrosion-resistant wear-resistant coil NM400, the chemical composition of which, by mass percentage, is: C 0.18~0.20%, Si 1.25~1.45%, Mn 1.25~1.45%, P≤0.020%, S≤0.005%, Als 0.030~0.050%, Nb 0.015~0.020%, Ti 0.025~0.045%, Cr 1.35~1.50%, B 0.0012~0.0014%, La 0.0020~0.0035%, H≤2ppm, O≤25ppm, N≤60ppm, with the remainder being Fe and unavoidable inclusions; The microstructure of the rare earth microalloyed corrosion-resistant wear-resistant steel coil NM400 is mainly acicular bainite, accounting for 70%~80% of the volume, with the remainder being granular bainite. The grain size is ≥10, and the banded structure is ≤1.

5. The mechanical properties meet the following requirements: yield strength 900~1100MPa, tensile strength 1250~1400MPa, elongation A 50 The corrosion rate is 12-18%, and the Brinell hardness is 380-420; the corrosion rate in the 72-hour cyclic immersion corrosion test is ≤60% compared to that of Q355B.

2. The rare earth microalloyed corrosion-resistant wear-resistant coil NM400 according to claim 1 has a thickness of 4~8 mm.

3. The manufacturing method of rare earth microalloyed corrosion-resistant wear-resistant coil NM400 as described in claim 1 or 2, comprising a smelting continuous casting process and a rolling process. The smelting and continuous casting process is as follows: molten iron → molten iron pretreatment → converter steelmaking → LF refining → RH degassing → slab continuous casting; wherein the converter smelting uses KR pre-desulfurized molten iron, the molten iron entering the furnace has [S] ≤ 0.002%, ferrosilicon, ferromanganese, and ferrochrome are added after the steel is tapped from the converter, ferroniobium is added at the end of the LF refining stage, and ferrotitanium is added at the end of the vacuum degassing treatment to adjust to the target value of alloy addition, La-Fe alloy is added before the vacuum treatment negative pressure, the vacuum treatment ends after 3 minutes of circulation, and the ladle is directly cast on the casting machine without Ca treatment; the molten steel is stirred by argon throughout the process; the vacuum degree is ≤ 2 mbar, the treatment time is ≥ 10 min, the superheat ΔT is 30~40℃, and the continuous casting slab stretching is 1.2~1.5 m / min; it is equipped with dynamic light pressure reduction technology, and the straightening temperature is ≥ 800℃; The rolling process is as follows: 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—dense laminar flow cooling—coiling—pallet transport system—warehouse stacking slow cooling—sampling and inspection; wherein the slab heating temperature is 1245±20℃; heating time is 180~240min; roughing mode adopts 1+5, 3+3 or 3+5; intermediate slab thickness range: 35~40mm; finishing mill starting temperature ≤1080℃; finishing mill finishing temperature 840~900℃; coiling temperature 450~550℃; cooling rate 30~50℃ / s.

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