A 350mpa grade all-weather high-weatherability steel and a manufacturing method thereof

Abstract

The application provides a 350MPa-grade all-weather high-weatherability steel and a manufacturing method thereof, the 350MPa-grade all-weather high-weatherability steel contains the following chemical components by weight percentage: C: 0.01-0.03%; Si: 0.40-0.60%; Mn: 0.50-0.60%; P: ≤0.015%; S: ≤0.003%; Cr: 3.0-3.5%; Ni: 0.05-0.15%; Cu: ≤0.10%; Alt: 0.020-0.045%; Ti: 0.055-0.10%; Sb: 0.05-0.15%; and Ni / Cu>1, the balance is Fe and inevitable inclusions; the steel has excellent mechanical properties and cold forming properties, has good coiling performance stability, has the characteristics of atmospheric corrosion resistance and Cl-containing acid medium corrosion resistance, and can be used in the industries of railway vehicles, bridges, photovoltaic supports and the like.

Description

Technical Field

[0001] This invention belongs to the field of weathering steel technology, specifically relating to a 350MPa grade all-weather high weathering steel and its manufacturing method. Background Technology

[0002] High weathering steel possesses exceptional corrosion resistance, exhibiting more than double the atmospheric corrosion resistance of ordinary weathering steel, significantly extending its service life. It is widely used in the manufacture of outdoor steel structures and equipment exposed to the atmosphere for extended periods, such as railway vehicles, containers, bridges, buildings, and towers, thus prolonging their service life. Under the backdrop of "dual carbon" (carbon dioxide, carbon dioxide, and carbon emissions) and the major trend towards green environmental protection, high weathering steel is also continuously developing towards longer lifespan and lighter weight. As a low-carbon, green steel material, weathering steel can reduce resource and energy consumption throughout its entire life cycle, reducing carbon emissions and possessing broad prospects for widespread application. Currently, high weathering steel still faces limitations in its corrosion resistance applications, exhibiting poor resistance to acid corrosion and insufficient resistance to marine atmospheric corrosion. Solving these problems would further expand its application areas.

[0003] For example, the invention patent with publication number CN 106906426A discloses a high corrosion-resistant weathering steel and its manufacturing method. Its chemical composition is: C≤0.03%; Si≤0.10%; Mn: 0.1-0.5%; P≤0.015%; S≤0.005%; Nb: 0.03-0.05%; Ti: 0.02-0.05%; Cr: 4.0-6.0%; Ni: 0.4-0.6%; Cu: 0.4-0.6%; Al: 0.02-0.06%. This patent has a high cost and a single application. It is only resistant to ordinary industrial atmospheres and can only be used in areas with weak corrosion environments, such as the central and western regions and the northwest. However, it is significantly limited to the high humidity and heat environment of coastal areas such as Hainan, or the high humidity and acid rain environment of Chongqing. Summary of the Invention

[0004] To solve the above-mentioned technical problems, the present invention provides a 350MPa grade all-weather high weathering steel and its manufacturing method. The 350MPa grade all-weather high weathering steel has the characteristics of both atmospheric corrosion resistance and corrosion resistance to acidic media containing Cl, and can be used in industries such as railway vehicles, bridges, and photovoltaic brackets.

[0005] The technical solution adopted in this invention is as follows:

[0006] A 350MPa grade all-weather high weathering steel, wherein the 350MPa grade all-weather high weathering steel contains the following chemical composition by weight percentage: C: 0.01-0.03%; Si: 0.40-0.60%; Mn: 0.50-0.60%; P≤0.015%; S: ≤0.003%; Cr: 3.0-3.5%; Ni: 0.05-0.15%; Cu: ≤0.10%; Alt: 0.020-0.045%; Ti: 0.055-0.10%; Sb: 0.05-0.15%; and Ni / Cu>1, with the balance being Fe and unavoidable inclusions.

[0007] The microstructure of the 350MPa grade all-weather high weathering steel is ferrite + pearlite; the area of ​​pearlite is less than 6%.

[0008] The 350MPa grade all-weather high weather-resistant steel has a yield strength ReL ≥ 350MPa, tensile strength Rm ≥ 490MPa, and A ≥ 22%; it also possesses good stability in continuous winding performance, with mechanical property fluctuations in the length direction < 50MPa; KV2 impact energy at -60℃ ≥ 80J; compared with Q345B, the relative corrosion rate is < 27% under a 72h immersion corrosion test; and the corrosion rate is less than 0.6g / (m²) under 10% H₂SO₄ + 3.5% NaCl conditions. 2 ·h).

[0009] The present invention also provides a method for manufacturing the 350MPa grade all-weather high weathering steel, the method comprising the following steps: hot metal pretreatment and deep desulfurization → converter top and bottom combined blowing → ladle refining → continuous casting → heating → rolling → segmented cooling → coiling → cooling to room temperature.

[0010] The molten iron is desulfurized to ≤0.003%; LF+RH double refining is adopted, the RH vacuum decarburization time is ≥5min, and the outlet temperature is ≥1550℃.

[0011] During the continuous casting process, the temperature of the molten steel in the tundish is controlled at 1520℃~1550℃, the liquid level fluctuation is ≤±5mm, the casting speed of the continuous casting billet is controlled at 1.0~1.5m / min, and dynamic light pressure is applied to improve the internal quality of the continuous casting billet.

[0012] In the continuous casting process, a protective slag is used for protective casting; the basicity of the protective slag is 0.80-1.0 and the viscosity is 0.10-0.20 Pa·S.

[0013] During the heating process, the temperature of the first heating stage is ≤1070℃, the time of the second heating stage plus the soaking time is ≤80min, the furnace time is 120~200min, and the furnace exit temperature is 1150℃~1230℃. This promotes the homogenization of austenite and the full dissolution of alloying elements in austenite. At the same time, it avoids excessive heating time, which can lead to the enrichment of low-melting-point Cu elements at the austenite grain boundaries and cause crack defects in the hot-rolled edge. The heating furnace adopts a reducing atmosphere with an air excess coefficient of less than 0.8 to reduce the oxidation and burning loss of Fe on the slab surface during heating and inhibit Cu enrichment.

[0014] In the rolling process, the rolling temperature in the roughing stage is controlled above 1050℃, and the cumulative reduction rate is ≥80%; in the finishing stage, a 7-stand four-high rolling mill is used for continuous rolling, the starting temperature of the finishing rolling is ≤1030℃, the cumulative deformation is ≥85%, and the finishing rolling temperature is 840~900℃.

[0015] In the segmented cooling process, the tail end of the steel coil often has a large amount of residual cooling water, resulting in excessively low temperature and greater performance fluctuations compared to other parts. Therefore, temperature compensation is performed on the tail 30 meters of the steel strip, with the temperature forming an "L-shape" from beginning to end. The temperature compensation is 25-30℃, which can prevent the tail end of the steel strip from cooling down too quickly and causing uneven coiling performance.

[0016] During the winding process, the winding temperature is 550–630℃. After winding, the saddle surface on the steel coil transport line is designed with heat insulation, and the thermal conductivity of the insulation material is <0.2W / (m·K) to prevent the bottom of the steel coil from cooling rapidly, which would cause a large temperature drop at the bottom and result in uneven winding performance. After winding, the coil is allowed to cool naturally to room temperature.

[0017] The functions and controls of each component in the 350MPa grade all-weather high weathering steel provided by this invention are as follows:

[0018] Ultra-low C+Ti microalloying: This invention employs an ultra-low C design, ensuring steel strength while exhibiting excellent ductility and toughness. Therefore, the C content is strictly controlled between 0.01% and 0.03%. Ti is a strong carbide-forming element, inhibiting austenite grain coarsening during heating, reducing the enrichment of liquid Cu at austenite grain boundaries, and improving edge quality. The combination of ultra-low C and relatively high Ti ensures complete carbide formation, guaranteeing both strength and a pure steel matrix, resulting in excellent low-temperature toughness. Its content is controlled between 0.055% and 0.10%.

[0019] Si: Si is an economical element for improving corrosion resistance, but a high Si content can easily lead to strip-shaped oxygen-pressed defects on the surface of corrosion-resistant steel plates. These surface defects are difficult to remove during the descaling process. Therefore, this invention controls Si to be between 0.40% and 0.60%.

[0020] Mn: Mn significantly improves the strength of thick-gauge corrosion-resistant steel through solid solution strengthening. However, high Mn content can cause banded structures in the microstructure. These unevenly distributed banded structures are one of the reasons for the decrease in low-temperature toughness. At the same time, a high Mn content can form MnS inclusions in the center of the billet thickness, thereby deteriorating the impact toughness. To reduce the formation of banded structures and MnS inclusions, the Mn content should be controlled between 0.50% and 0.60%.

[0021] The composite addition of Cr, Ni, and Cu: Cr: 3.0–3.5%; Ni: 0.05–0.15%; Cu: ≤0.10%. Cr is the most important corrosion-resistant element in this invention. High Cr content forms a dense Cr₂O₃ layer on the surface, significantly improving passivation ability and hindering further corrosion of the matrix. Cu is also an element that significantly improves corrosion resistance; however, as a low-melting-point element, Cu is prone to grain boundary segregation, leading to "copper embrittlement" defects. Therefore, this invention adopts a low Cu design, while adding an appropriate amount of Ni to inhibit Cu precipitation, controlling Ni / Cu > 1, and improving low-temperature impact performance. The synergistic effect of these three corrosion-resistant elements enriches and forms a dense, stable rust layer on the surface, preventing the diffusion of corrosive media into the matrix. The atmospheric corrosion resistance of this invention is about twice that of conventional weathering steel. Based on the above comprehensive considerations, the Cr content is controlled as follows: Cr: 3.0–3.5%; Ni: 0.05–0.15%; Cu: ≤0.10%.

[0022] Sb: Sb is a key element for improving the acid resistance of materials. The formation of Sb oxides can isolate corrosive solutions, giving them excellent acid corrosion resistance. Economic efficiency is ensured by reducing Cr and increasing Sb.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1) The mechanical properties of the 350MPa grade all-weather high weathering steel provided by this invention are: ReL≥350MPa, Rm≥490MPa, A≥22%; and it has good stability of continuous winding performance, with mechanical property fluctuation in the length direction <50MPa; KV2 impact energy at -60℃≥80J (impact sample size 5*10*55mm).

[0025] 2) The 350MPa grade all-weather high weathering steel provided by the present invention has excellent cold forming performance. After 180° cold bending (d=0a), no cracks appeared on the outer side of the bending sample.

[0026] 3) The 350MPa grade all-weather high weather-resistant steel coils provided by this invention have good surface quality and no defects such as edge cracks.

[0027] 4) The 350MPa grade all-weather high weathering steel provided by this invention has excellent atmospheric corrosion resistance. Compared with Q345B, the relative corrosion rate is <27% under a 72h immersion corrosion test.

[0028] 5) The 350MPa grade all-weather high weathering steel provided by this invention has excellent acid corrosion resistance; under 10% H2SO4 + 3.5% NaCl conditions, the corrosion rate is less than 0.6 g / (m³). 2 ·h). Attached Figure Description

[0029] Figure 1 The metallographic structure of the weathering steel in Example 1;

[0030] Figure 2 The surface morphology of the weathering steel in Example 1 after a 72-hour immersion corrosion test;

[0031] Figure 3 The surface morphology of the weathering steel in Comparative Example 2 after a 72-hour immersion corrosion test is shown. Detailed Implementation

[0032] The present invention provides a 350MPa grade all-weather high weathering steel containing the following chemical composition by weight percentage: C: 0.01-0.03%; Si: 0.40-0.60%; Mn: 0.50-0.60%; P≤0.015%; S: ≤0.003%; Cr: 3.0-3.5%; Ni: 0.05-0.15%; Cu: ≤0.10%; Alt: 0.020-0.045%; Ti: 0.055-0.10%; Sb: 0.05-0.15%, and Ni / Cu > 1, with the balance being Fe and unavoidable inclusions.

[0033] The manufacturing method of the 350MPa grade all-weather high weathering steel includes the following steps: hot metal pretreatment and deep desulfurization → converter top and bottom combined blowing → ladle refining → continuous casting → heating → rolling → segmented cooling → coiling → cooling to room temperature.

[0034] 1) Smelting, refining, and continuous casting processes

[0035] After deep desulfurization of molten iron to ≤0.003%, LF+RH double refining is used to achieve the above-mentioned composition control. The RH vacuum decarburization time is ≥5min, and the outlet temperature is ≥1550℃. The ladle temperature for molten steel casting is controlled at 1520℃~1550℃, with liquid level fluctuation ≤±5mm. The continuous casting billet pulling speed is controlled at 1.0~1.5m / min, and dynamic light pressure is applied to improve the internal quality of the continuous casting billet. The thickness of the continuously cast slab is 230mm. Protective slag is used for protective casting, with a basicity of 0.80~1.0 and a viscosity of 0.10~0.20Pa·S.

[0036] After the billet is cut, it is directly loaded into the furnace. Billets that cannot be directly loaded must first be placed in a heat preservation pit for slow cooling to avoid cracking caused by excessively rapid temperature drop. Cu and Sb tend to accumulate at the edges of the billet, causing edge cracks during rolling. In particular, the use of rounded corner clearing for continuously cast slabs instead of the manual corner clearing used by most steel mills reduces heat loss from the billet, increases the hot-to-feed ratio of the billet, and avoids edge cracking.

[0037] 2) Hot rolling

[0038] First, the slab is heated. The first heating temperature is ≤1070℃, and the second heating plus soaking time is ≤80min, with the furnace time strictly controlled to be less than 180min. The furnace exit temperature is 1150℃~1230℃. Excessive heating time is avoided, as it can lead to the enrichment of low-melting-point Cu at the austenite grain boundaries, causing cracks at the hot-rolled edges. The heating furnace uses a reducing atmosphere with an excess air coefficient of less than 0.8 to reduce the oxidation and burning loss of Fe on the slab surface during heating and inhibit Cu enrichment.

[0039] Rolling is carried out in two stages: roughing and finishing. In the roughing stage, the rolling temperature is controlled above 1050℃, with a cumulative reduction rate ≥80%. In the finishing stage, a 7-stand four-high rolling mill is used for continuous rolling. The initial finishing temperature is ≤1030℃, and the cumulative deformation is ≥85%. The final finishing temperature is controlled between 820℃ and 880℃.

[0040] After finishing rolling, laminar flow cooling is performed. Crucially, segmented controlled cooling at 550–630℃ is used during coiling to obtain a ferrite + pearlite microstructure, with a pearlite proportion of less than 6%. Pearlite is a corrosion initiator; a low pearlite proportion allows for a large cathode and small anodic effect, improving the material's atmospheric corrosion resistance. To address the issue of excessively rapid cooling at the tail end, an "L-shaped cooling" process of 25–30℃ is used for temperature compensation in the last 30 meters. After coiling, the saddle surface on the coil transport line is designed with thermal insulation; preferably, the thermal conductivity of the insulation material is <0.2 W / (m·K). After coiling, it is naturally cooled to room temperature, finally obtaining a uniform ferrite + pearlite microstructure throughout the coil.

[0041] The present invention will now be described in detail with reference to the embodiments.

[0042] The chemical composition and weight percentage of the weathering steel in each embodiment and comparative example are shown in Table 1, with the balance being Fe and unavoidable inclusions.

[0043] Table 1 Chemical composition of the embodiments and comparative examples of the present invention

[0044]

[0045] The main rolling process parameters of the weathering steel in each embodiment and comparative example are shown in Table 2, and the mechanical properties are shown in Table 3.

[0046] Table 2. Main process parameters of rolling process in the examples and comparative examples.

[0047]

[0048]

[0049] Table 3 Mechanical and process properties of the examples and comparative examples

[0050]

[0051] The corrosion resistance results of the weathering steels in each embodiment and comparative example are shown in Table 4.

[0052] Table 4 Corrosion rates of Examples 1-3 and Comparative Examples 1-7

[0053]

[0054]

[0055] The metallographic structure of the weathering steel in Example 1 is as follows: Figure 1 As shown in the figure, its metallographic structure is ferrite + pearlite, with pearlite accounting for less than 6% of the area, which improves the material's resistance to atmospheric corrosion. The grain size of 12-12.5 is more conducive to achieving a good balance between strength and toughness; the metallographic structures of other embodiments are also the same.

[0056] Figure 2 , 3 The images show the surface morphology of weathering steels from Example 1 and Comparative Example 2 after a 72-day corrosion test at 45°C using 0.01 mol / L NaHSO3. The comparison shows that after the corrosion test, the surface morphology of the weathering steel in Example 1 was intact, while the surface of the weathering steel in Comparative Example 2 had deep cracks. The rust layer of the weathering steel in Example 1 exhibited a high content of Cu-rich phases at the microscopic level, inhibiting corrosion (75% Cu-rich phase content). In contrast, while the rust layer of the weathering steel in Comparative Example 2 also showed Cu-rich phases at the microscopic level, the Cu-rich phase content was only 17%, resulting in relatively poor corrosion resistance.

[0057] As can be seen from the above, the weathering steel prepared by the various embodiments of the present invention has a yield strength ReL≥400MPa, tensile strength Rm≥520MPa, and A≥22%; and has good through-winding performance stability; -60℃ KV2 impact energy≥80J; compared with Q345B under 72h immersion corrosion test, the relative corrosion rate is <27%;

[0058] Under conditions of 10% H₂SO₄ + 3.5% NaCl, the corrosion rate is less than 0.5 g / (m²). 2In Comparative Example 1, the Sb content < 0.05% resulted in poor acid resistance, far inferior to Examples 1-3. Comparative Example 2, lacking Sb, exhibited even worse acid resistance despite a high Cr content of 4.5%. Comparative Example 3, lacking Sb and with Cr, Ni, and Cu contents not controlled according to the present invention, resulted in the worst acid resistance; the Ni / Cu ratio < 1 led to copper embrittlement and minor cracking. Comparative Example 4, lacking rounded corners, resulted in significant cracking and large fluctuations in mechanical properties. Comparative Example 5, with a Cr content < 3%, failed to meet the atmospheric corrosion resistance requirements of TB / T 1979-2014. While the embodiments disclosed in prior art CN 106906426A have atmospheric corrosion resistance similar to those of the embodiments of the present invention, their cost is significantly higher, and lacking Sb, they do not possess acid corrosion resistance.

[0059] The above detailed description of a 350MPa grade all-weather high weathering steel and its manufacturing method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. An all-weather high weathering steel of 350 MPa grade, characterized in that, The 350MPa-grade all-weather high-weatherability steel contains the following chemical components by weight percentage: C: 0.01~0.03%; Si: 0.40~0.60%; Mn: 0.50~0.60%; P≤0.015%; S: ≤0.003%; Cr: 3.0~3.5%; Ni: 0.05~0.15%; Cu: ≤0.10%; Alt: 0.020~0.045%; Ti: 0.055-0.10%; Sb: 0.05~0.15%; and Ni / Cu>1, the balance being Fe and unavoidable inclusions; The 350MPa-grade all-weather high-weatherability steel has a metallographic structure of ferrite + pearlite.

2. The 350 MPa grade all-weather high-weatherability steel according to claim 1, characterized by, The yield strength R eL ≥ 350 MPa, the tensile strength R m ≥ 490 MPa, A ≥ 22%; and has good coiling performance stability, length direction mechanical property fluctuation < 50 MPa; -60°C KV2 impact energy ≥ 80 J; under 72h weekly immersion corrosion test, relative corrosion rate < 27% compared with Q345B; under 10% H2SO4+3.5% NaCl condition, corrosion rate is less than 0.6 g / (m 2 h).

3. The method of producing a 350 MPa grade all-weather high-weatherability steel according to claim 1 or 2, characterized by, The manufacturing method comprises the following steps: hot metal pretreatment deep desulfurization, converter top and bottom combined blowing, secondary refining outside the furnace, continuous casting, heating, rolling, segmented cooling, coiling, and cooling to room temperature.

4. The production method according to claim 3, characterized by The hot metal is deep desulfurized to ≤0.003%, LF+RH double refining is adopted, the RH vacuum decarburization time is ≥5min, and the outlet temperature is ≥1550℃.

5. The production method according to claim 3, wherein In the continuous casting step, the tundish temperature of the molten steel is controlled at 1520℃~1550℃, the liquid surface fluctuation is ≤±5mm, the continuous casting billet pulling speed is controlled at 1.0~1.5m / min, and dynamic soft reduction is used.

6. The production method according to claim 3, wherein In the heating step, the first heating section temperature is ≤1070℃, the second heating section time + soaking time is ≤80min, the furnace time is 120~200min, and the discharge temperature is 1150℃~1230℃.

7. The production method according to claim 3, wherein In the rolling step, the rolling temperature in the rough rolling stage is controlled at above 1050℃, and the cumulative reduction is ≥80%; the 7-stand four-roll mill is used for continuous rolling in the finishing rolling stage, the finishing rolling opening rolling temperature is ≤1030℃, the cumulative deformation is ≥85%, and the finishing rolling final rolling temperature is 840~900℃.

8. The production method according to claim 3, wherein In the segmented cooling step, temperature compensation is performed on the tail 30 meters of the steel strip, and the temperature compensation is 25-30℃.

9. The production method according to claim 3, wherein In the coiling step, the coiling temperature is 550~630℃; after coiling, the surface of the saddle on the steel coil conveying line is designed with heat insulation and heat preservation, and the steel coil is naturally cooled to room temperature after coiling.

Images