420mpa grade low yield ratio hot rolled construction structural steel and preparation method

Abstract

The present application relates to 420MPa grade low yield ratio hot rolled building structure steel, the chemical composition in the steel is as follows: C 0.14%~0.17%, Si 0.15%~0.25%, Mn 1.40%~1.50%, P≤0.020%, S≤0.008%, Nb 0.020%~0.040%, Ti 0.010%~0.020%, Cr 0.20%~0.30%, Als 0.015%~0.045%, the rest is Fe and inevitable impurity element, carbon equivalent CEV in the steel=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.48.The present application can realize the production of 3~18mm thickness yield strength 420MPa grade low yield ratio hot rolled building structure steel, the transverse tensile yield strength R eL The tensile strength R m In the range of 520~680MPa, the yield ratio of 6~18mm thickness hot rolled plate is ≤0.83, the elongation A is not less than 20%, the longitudinal impact energy at-20℃ is more than 47J, the bend core diameter D=2a, and no crack is generated in 180° transverse bending experiment.

Description

Technical Field

[0001] This invention relates to the field of hot continuous rolling, and more particularly to 420MPa grade low yield strength ratio hot continuous rolled steel for building structures and its preparation method. Background Technology

[0002] Structural steel is the most consumed type of steel in my country, accounting for more than 50% of total steel consumption. It is mainly used in key components such as columns and load-bearing beams of industrial and civil buildings. To ensure seismic performance, the most important characteristics of structural steel are high toughness and low yield strength ratio, which is generally required to be less than 0.8.

[0003] The rapid development of steel products has led to challenges such as raw material and energy shortages and increased environmental pressures. In the 1980s, developed countries began researching and developing structural steel for buildings based on the principles of "high strength, lightweight, corrosion resistance, and long service life." Currently, my country's structural steel industry suffers from systemic problems, including unsystematic standards and a disconnect between product standards and building codes. Furthermore, structural steel products are still primarily medium-thick plates with strengths of 400MPa and below, with a serious deficiency in the research and promotion of high-strength, special-performance structural steel. Therefore, my country must accelerate the upgrading of its structural steel products for buildings by developing and applying products with "low cost, green technology, and high performance" through technological innovation and process optimization. Summary of the Invention

[0004] This invention provides a 420MPa grade low yield strength specific heat-rolled steel for building structures and its preparation method, which can realize the production of 420MPa grade low yield strength specific heat-rolled steel for building structures with a thickness of 3-18mm, and a transverse tensile yield strength R eL Above 420MPa, tensile strength R m Within the range of 520 to 680 MPa, the yield strength ratio of hot-rolled plates with a thickness of 6 to 18 mm is ≤0.83, the elongation A is not less than 20%, the longitudinal impact energy value at -20℃ exceeds 47 J, and no cracks are generated in the bending mandrel diameter D = 2a and 180° transverse bending test.

[0005] To achieve the above objectives, the present invention employs the following technical solution:

[0006] The 420MPa grade low yield strength specific heat-rolled structural steel has the following chemical composition by weight percentage: C 0.14%–0.17%, Si 0.15%–0.25%, Mn 1.40%–1.50%, P≤0.020%, S≤0.008%, Nb 0.020%–0.040%, Ti 0.010%–0.020%, Cr 0.20%–0.30%, Als 0.015%–0.045%, with the remainder being Fe and unavoidable impurity elements.

[0007] The carbon equivalent (CEV) in steel is C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≤ 0.48 to ensure its weldability.

[0008] The thickness of the steel plate is 3 to 18 mm.

[0009] In the composition design of the steel of this invention:

[0010] C: 0.14–0.17%. In actual production, the C content of hot-rolled low-alloy high-strength structural steel of equivalent strength is 0.06–0.09%. Appropriately increasing the C content in this hot-rolled structural steel will increase both yield strength and tensile strength, while the yield-to-tensile ratio will generally decrease slightly. This will also result in a certain loss of plasticity and impact resistance. When the C content exceeds 0.23%, the weldability of the steel deteriorates. Therefore, for low-alloy high-strength structural steel requiring welding, the C content generally does not exceed 0.20%. Therefore, the C content in this hot-rolled structural steel is set at 0.14–0.17%.

[0011] Si: 0.15–0.25%. Si is added during the steelmaking process as a reducing agent and deoxidizer, so killed steel contains 0.15–0.30% Si. However, the increase in Si content will also reduce the weldability of the steel. Therefore, the Si content in this hot-rolled structural steel is set at 0.15–0.25%.

[0012] Mn: 1.40–1.50%. In the steelmaking process, Mn is an excellent deoxidizer and desulfurizer. Adding an appropriate amount of Mn not only provides sufficient toughness but also high strength and improves the hot working properties of the steel. However, increasing the Mn content weakens the steel's corrosion resistance and reduces its weldability. Therefore, the Mn content in this hot-rolled structural steel is set at 1.40–1.50%.

[0013] P: ≤0.020%. Under normal circumstances, P is a harmful element in steel, increasing its cold brittleness, worsening its weldability, reducing its plasticity, and worsening its cold bending performance.

[0014] Sulfur: ≤0.008%. Sulfur is also a harmful element under normal conditions. It causes hot brittleness in steel, reduces the ductility and toughness of steel, causes cracks during rolling, and is also detrimental to weldability.

[0015] Nb: 0.020–0.040%. Nb is a strong carbide-forming element and can also dissolve in small amounts in the steel matrix. Nb forms stable NbC or Nb4C3 in steel, exhibiting a fine, dispersed distribution in the matrix, thus playing a role in precipitation strengthening. This microalloying effect is particularly pronounced when combined with controlled rolling and cooling. Therefore, compared to hot-rolled low-alloy high-strength structural steel of equivalent strength, it is advisable to appropriately reduce the Nb content to 0.020–0.040%. Simultaneously, Nb can also improve the weldability of steel.

[0016] Ti: 0.010–0.020%. Ti is a highly reactive metallic element with a strong affinity for elements such as O, N, and C. It is an excellent deoxidizer and an effective element for fixing nitrogen and carbon. Simultaneously, Ti can refine the grain structure of steel, thereby improving its strength and toughness, and enhancing its weldability. Compared with hot-rolled low-alloy high-strength structural steel of equivalent strength, appropriately reducing the Ti content can also achieve cost savings.

[0017] Cr: 0.20%–0.30%. Cr and Fe can form a continuous solid solution, increasing the strength of steel but reducing its plasticity. At the same time, Cr can also improve the oxidation resistance and corrosion resistance of steel; therefore, the amount of Cr added in this hot-rolled structural steel is set at 0.20%–0.30%.

[0018] Al content: 0.015–0.045%. Al is typically added as a deoxidizer during steelmaking. Trace amounts of Al help refine grains and improve the strength and toughness of steel. However, excessive Al increases the brittleness of ferrite in the steel, leading to a decrease in toughness. Therefore, the Al content in this hot-rolled structural steel is set at 0.015–0.045%.

[0019] V, Mo, Ni, Cu: Residual elements. In this invention, V, Mo, Ni, and Cu are treated as residual elements, and their total content generally does not exceed 0.30%.

[0020] The transverse tensile yield strength R of the steel used in the building structure of this invention eL ≥420MPa, tensile strength R m Within the range of 520 to 680 MPa, the yield strength ratio of hot-rolled plates with a thickness of 6 to 18 mm is ≤0.83, the elongation A is ≥20%, the longitudinal impact energy value at -20℃ is ≥47 J (hot-rolled plates with a thickness of less than 6 mm are not subjected to impact), the bending mandrel diameter D = 2a (the bending radius is twice the thickness of the steel plate), and no cracks are generated in the 180° transverse bending test.

[0021] A method for preparing 420MPa grade low yield strength specific heat-rolled structural steel includes the following steps:

[0022] The smelting process follows the sequence of hot metal pretreatment → converter smelting → ladle refining (ANS or LF) → slab continuous casting. During the refining process, Ca-Si wire is fed into the molten steel, with a minimum of 500m of wire fed per ladle, to achieve the purpose of spheroidizing inclusions in the molten steel. The net argon blowing time is no less than 5 minutes, which is beneficial for the removal of harmful gases and inclusions.

[0023] 1) Slab heating

[0024] The slab is placed in the heating furnace and heated. The furnace exit temperature of the slab varies slightly depending on the thickness of the finished strip. When the thickness is ≤12mm, the furnace exit temperature is controlled at 1200±30℃; when the thickness is >12mm, the furnace exit temperature is controlled at 1180±30℃; the furnace time is maintained at 180~240min.

[0025] Steels containing Nb and Ti are typically heated to higher temperatures to achieve complete austenitization, allowing them to dissolve completely in the solid solution. This enables the dispersion and precipitation of finer carbide particles during subsequent rolling, thus improving strength. The heating temperature of the slab varies depending on the thickness of the finished product; thinner slabs require slightly higher temperatures to prevent increased deformation resistance during finish rolling due to low temperatures, which could affect rolling stability and cause accidents such as steel jamming. The holding time is 180–240 minutes, with a minimum of 40 minutes for homogenization, ensuring relatively uniform temperature throughout the slab and providing favorable conditions for rolling without overheating and causing decarburization.

[0026] 2) Controlled rolling

[0027] To achieve a low yield strength ratio, controlled rolling and controlled cooling technology is employed, with the rolling process divided into two stages: roughing and finishing. Roughing uses a 1+5 rolling process, with an intermediate slab thickness of 37–51 mm. To obtain a finer grain size, the slab undergoes a large reduction in the roughing stage, controlling the R1 reduction rate at 8%–15%, and the reduction rate for each R2 pass at over 16%. The intermediate slab thickness is thinner compared to strips of the same thickness. The final roughing pass speed is controlled at 4.00–5.00 m / s. The final finishing pass reduction rate is controlled to be no less than 12% to ensure that deformation exceeds the critical deformation, resulting in a good strip shape and uniform matrix structure. The temperatures at various points in the roughing and finishing processes vary slightly depending on the thickness of the finished strip; specific temperature control details are shown in Table 1. The finishing exit temperature has a significant impact on the grain size of the strip. As the finishing exit temperature decreases, the strip grains become finer, the strength increases, and the yield strength ratio increases accordingly.

[0028] Table 1 Hot Rolling Temperature Control

[0029] Hot-rolled thickness specifications Roughing mill exit temperature (°C) Finishing mill inlet temperature (°C) Finishing mill exit temperature (°C) strip thickness ≤8mm 1040±30 1040±30 860±20 8mm < strip thickness ≤ 12mm 1020±30 1020±30 850±20 12mm < strip thickness ≤ 18mm 1000±30 ≤1000 840±20

[0030] 3) Control cooling

[0031] The controlled cooling method is adopted. The strip steel is centrally cooled in the front section, and a total of 6 sets of laminar flow cooling water are generally used. Since the strip steel is rolled at an increased speed after entering the finishing mill, the cooling rate also gradually increases, but is controlled at 8-15℃ / s. The middle section is air-cooled, and the temperature in the rear section is finely adjusted to the target coiling temperature according to the different thicknesses of the finished product.

[0032] 4) Winding

[0033] The coiling temperature varies slightly depending on the thickness of the finished strip. For strips ≤ 8mm thick, the coiling temperature is controlled at 620±20℃; for strips ≤ 12mm thick (8mm < 12mm), the coiling temperature is controlled at 600±20℃; and for strips ≤ 18mm thick (12mm < 18mm), the coiling temperature is controlled at 580±20℃. The coiling temperature also significantly affects the strip's microstructure. At higher coiling temperatures, the α-phase transformation undercooling is low, resulting in fewer nucleation points, mainly concentrated at the grain boundaries of the original γ-grains. Ferrite growth is faster, leading to coarser grains. As the coiling temperature decreases, α-phase nucleation increases, ferrite growth slows, ferrite grain size decreases, and pearlite becomes more dispersed and finer, resulting in increased strength and a higher yield strength ratio.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] Compared to low-alloy structural steel of equivalent strength in actual production, this hot-rolled structural steel for buildings appropriately increases the carbon content and reduces the use of alloying elements such as Nb and Ti, thus achieving cost savings while ensuring strength. It possesses excellent characteristics such as a low yield strength ratio, high plasticity and low-temperature toughness, and good plate shape quality.

[0036] This invention enables the stable mass production of hot-rolled structural steel with a yield strength of 420 MPa and a thickness of 3–18 mm, characterized by low yield strength and high specific strength. The transverse tensile yield strength R of the steel is [not specified]. eL Above 420MPa, tensile strength R m Within the range of 520–680 MPa, the yield strength ratio of hot-rolled plates with a thickness of 6–18 mm is ≤0.83, the elongation A is not less than 20%, and the longitudinal impact energy at -20℃ exceeds 47 J (impact tests are not performed on hot-rolled plates with a thickness of less than 6 mm). No cracks are generated during a 180° transverse bending test with a bending mandrel diameter D = 2a. The obtained metallographic structure is mainly ferrite + pearlite, with a grain size of grade 10 or higher and slight banding. Attached Figure Description

[0037] Figure 1 Metallographic diagram of hot-rolled structural steel at 100x magnification.

[0038] Figure 2 Metallographic diagram of hot-rolled structural steel (200x magnification).

[0039] Figure 3 Metallographic diagram of hot-rolled structural steel (500x magnification).

[0040] Figure 4 This is a diagram of a continuously rolled steel plate for building structures with a thickness of 9.75mm.

[0041] Figure 5 This is a diagram of a continuously rolled steel plate for building structures with a thickness of 15.75mm. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0043] Examples are shown in Tables 2-6, where Table 2 shows the chemical element composition of the steel in the examples; Table 3 shows the heating process parameters of the steel; Table 4 shows the rough rolling process parameters of the steel; Table 5 shows the finish rolling and coiling process parameters of the steel; and Table 6 shows the performance parameters of the steel in the examples.

[0044] Table 2 Chemical elemental composition of steel in Examples 1-10

[0045] element C Si Mn P S Als Nb Ti V Cr Mo Ni Cu CEV Example 1 0.15 0.21 1.44 0.013 0.006 0.042 0.024 0.017 0.002 0.25 0.003 0.009 0.017 0.44 Example 2 0.15 0.19 1.45 0.011 0.002 0.034 0.03 0.018 0.002 0.26 0.002 0.01 0.018 0.45 Example 3 0.14 0.18 1.43 0.015 0.008 0.028 0.025 0.017 0.002 0.26 0.003 0.009 0.015 0.43 Example 4 0.15 0.2 1.43 0.013 0.004 0.032 0.027 0.018 0.002 0.25 0.003 0.006 0.009 0.44 Example 5 0.15 0.19 1.4 0.013 0.003 0.03 0.028 0.017 0.002 0.23 0.002 0.006 0.006 0.43 Example 6 0.15 0.19 1.45 0.011 0.007 0.035 0.033 0.016 0.002 0.25 0.002 0.009 0.015 0.44 Example 7 0.15 0.19 1.46 0.011 0.007 0.041 0.021 0.02 0.008 0.25 0.002 0.009 0.008 0.45 Example 8 0.15 0.19 1.41 0.011 0.002 0.036 0.033 0.016 0.002 0.24 0.002 0.008 0.011 0.44 Example 9 0.15 0.18 1.44 0.012 0.005 0.045 0.02 0.02 0.015 0.23 0.002 0.007 0.009 0.44 Example 10 0.16 0.2 1.45 0.018 0.003 0.034 0.034 0.018 0.002 0.23 0.002 0.008 0.011 0.45

[0046] Table 3 Heating process parameters for steel in Examples 1-10

[0047]

[0048]

[0049] Table 4. Roughing process parameters for steels in Examples 1-10

[0050]

[0051] Table 5. Finishing coiling process parameters for steels in Examples 1-10

[0052] Final reduction rate in finishing rolling, % Laminar flow cooling rate, ℃ / s Finishing mill inlet temperature, ℃ Finishing mill exit temperature, ℃ Winding temperature, ℃ Example 1 14.64％ 8～15 1060 871 619 Example 2 14.47％ 8～15 1049 873 623 Example 3 13.62％ 8～15 1043 860 621 Example 4 13.19％ 8～15 1057 854 624 Example 5 12.96％ 8～15 1032 845 583 Example 6 13.38％ 8～15 1047 866 593 Example 7 13.85％ 8～15 1045 864 603 Example 8 13.98％ 8～15 996 840 590 Example 9 12.26％ 8～15 998 845 586 Example 10 12.01％ 8～15 999 822 592

[0053] Table 6 Performance parameters of steels in Examples 1-10

[0054]

Claims

1. 420MPa grade low yield strength specific heat-rolled structural steel, characterized in that, The chemical composition of the steel by weight percentage includes: C 0.14%~0.17%, Si 0.15%~0.25%, Mn 1.40%~1.50%, P≤0.020%, S≤0.008%, Nb0.020%~0.040%, Ti 0.010%~0.020%, Cr 0.20%~0.26%, Als 0.015%~0.045%, with the remainder being Fe and unavoidable impurity elements; The preparation method of the 420MPa grade low yield strength ratio hot-rolled structural steel includes the following steps: 1) Slab heating Depending on the thickness of the finished strip steel, the slab heating temperature varies. When the thickness is ≤12mm, the furnace exit temperature is controlled at 1200±30℃; when the thickness is >12mm, the furnace exit temperature is controlled at 1180±30℃. The total furnace time is maintained at 180~240min, of which the heat soaking time is not less than 40min. 2) Controlled rolling The temperatures at various points during roughing and finishing rolling vary depending on the thickness of the finished strip. Specific temperature controls are as follows: For strip thickness ≤ 8mm: roughing mill exit temperature is 1040±30℃; finishing mill inlet temperature is 1040±30℃; finishing mill exit temperature is 860±20℃. For strip thicknesses between 8mm and 12mm: the roughing mill exit temperature is 1020±30℃; the finishing mill inlet temperature is 1020±30℃; and the finishing mill exit temperature is 850±20℃. For strip thicknesses between 12mm and 18mm: the roughing mill exit temperature is 1000±30℃; the finishing mill inlet temperature is ≤1000℃; and the finishing mill exit temperature is 840±20℃. 3) Controlling cooling The strip steel undergoes centralized laminar flow cooling in the front section, air cooling in the middle section, and temperature fine-tuning in the rear section to the target coiling temperature. 4) Winding Depending on the thickness of the finished strip, the coiling temperature varies. When the strip thickness is ≤8mm, the coiling temperature is controlled at 620±20℃; when the strip thickness is 8mm<12mm, the coiling temperature is controlled at 600±20℃; and when the strip thickness is 12mm<18mm, the coiling temperature is controlled at 580±20℃. The steel plate thickness is 3-18mm; The smelting process is carried out in the following order: hot metal pretreatment → converter smelting → ladle refining → slab continuous casting. During the refining process, Ca-Si wire is fed into the molten steel, with a minimum of 500m of wire fed into each ladle and a minimum of 5min of net argon blowing time. In step 2), the thickness of the rolled intermediate billet is 37-51 mm; In step 2), the slab is rolled with a large reduction during the roughing stage, and the reduction rate of R1 is controlled at 8% to 15%, the reduction rate of each pass of R2 is above 16%, and the rolling speed of the last pass of roughing is controlled at 4.00 to 5.00 m / s; the reduction rate of the last pass of finishing is controlled at not less than 12%.

2. The 420MPa grade low yield strength specific heat-rolled structural steel according to claim 1, characterized in that, The carbon equivalent (CEV) in steel is calculated as: C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 ≤ 0.

48.

3. The 420MPa grade low yield strength specific heat-rolled structural steel according to claim 1, characterized in that, Transverse tensile yield strength R of steel plate eL ≥420MPa, tensile strength R m Within the range of 520 to 680 MPa, the yield strength ratio of hot-rolled plates with a thickness of 6 to 18 mm is ≤0.83, the elongation A is ≥20%, the longitudinal impact energy at -20℃ is ≥47 J, and no cracks are generated in the bending mandrel diameter D=2a and 180° transverse bending test.

4. A method for preparing 420MPa grade low yield strength specific heat-rolled structural steel as described in any one of claims 1-3, characterized in that, Includes the following steps: 1) Slab heating Depending on the thickness of the finished strip, the slab heating temperature varies. When the thickness is ≤12mm, the furnace exit temperature is controlled at 1200±30℃; when the thickness is >12mm, the furnace exit temperature is controlled at 1180±30℃. 2) Controlled rolling The temperatures at various points during roughing and finishing rolling vary depending on the thickness of the finished strip. Specific temperature controls are as follows: For strip thickness ≤ 8mm: roughing mill exit temperature is 1040±30℃; finishing mill inlet temperature is 1040±30℃; finishing mill exit temperature is 860±20℃. For strip thicknesses between 8mm and 12mm: the roughing mill exit temperature is 1020±30℃; the finishing mill inlet temperature is 1020±30℃; and the finishing mill exit temperature is 850±20℃. For strip thicknesses between 12mm and 18mm: the roughing mill exit temperature is 1000±30℃; the finishing mill inlet temperature is ≤1000℃; and the finishing mill exit temperature is 840±20℃. In the roughing stage, the slab is rolled with a large reduction, controlling the reduction rate of R1 to be between 8% and 15%, and the reduction rate of each pass of R2 to be above 16%. The rolling speed of the last pass of roughing is controlled at 4.00 to 5.00 m / s; the reduction rate of the last pass of finishing is controlled to be no less than 12%. 3) Controlling cooling The strip steel undergoes centralized laminar flow cooling in the front section, air cooling in the middle section, and temperature fine-tuning in the rear section to the target coiling temperature. 4) Winding Depending on the thickness of the finished strip, the coiling temperature varies. When the strip thickness is ≤8mm, the coiling temperature is controlled at 620±20℃; when the strip thickness is ≤12mm (<8mm), the coiling temperature is controlled at 600±20℃; and when the strip thickness is ≤18mm (<12mm), the coiling temperature is controlled at 580±20℃.

5. The method for preparing 420MPa grade low yield strength specific heat-rolled structural steel according to claim 4, characterized in that, The laminar cooling rate in step 3) is controlled at 8–15 °C / s.

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