Low-cost weather-resistant structural steel Q355NHD and production method thereof

Abstract

The invention discloses low-cost weather-resistant structural steel Q355NHD and a production method thereof. The weather-resistant structural steel comprises the following chemical components in percentage by mass: 0.07 to 0.09 percent of C, 0.20 to 0.30 percent of Si, 1.30 to 1.38 percent of Mn, 0.45 to 0.55 percent of Cr, 0.30 to 0.40 percent of Cu, 0.015 to 0.05 percent of Ti, 0.018 to 0.030 percent of Als, less than or equal to 0.020 percent of P, less than or equal to 0.008 percent of S and the balance of Fe and inevitable residual elements and impurities. The production method comprises the procedures of continuous casting, casting blank heating, rolling and cooling. According to the weather-resistant structural steel Q355NHD, the Ni element is not added, low-cost manufacturing is achieved, the surface quality of a steel plate is good, and the mechanical property meets GB / T 4171-2008.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to low-cost weather-resistant structural steel Q355NHD and a production method thereof. Background Art

[0002] Weathering steel has atmospheric corrosion resistance, good mechanical and welding properties, and low cost. High-strength weathering steel is mainly used to manufacture railway freight cars, bridges, towers, containers, etc. The stability of its corrosion resistance is one of the important factors affecting the service life of components. In order to improve the corrosion resistance, alloy elements such as Cu and Cr are generally added to reduce the conductivity of the rust layer and hinder the growth of the rust layer.

[0003] Cu and Cr are the main elements that improve the corrosion resistance of weathering steel. In order to further improve the corrosion resistance and reduce the surface cracks caused by "copper embrittlement", domestic steel companies also add a certain amount of Ni when producing weathering steel. The addition of Ni not only increases the manufacturing cost of weathering steel, but also does not meet the development needs of greening the steel industry. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a low-cost weather-resistant structural steel Q355NHD and a production method thereof. The provided weather-resistant structural steel Q355NHD does not contain any Ni element, thereby realizing low-cost manufacturing of the steel, and the surface quality of the steel plate is good, and the mechanical properties meet GB / T 4171-2008.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: A low-cost weather-resistant structural steel Q355NHD has the following chemical compositions and their mass percentages: C: 0.07-0.09%, Si: 0.20-0.30%, Mn: 1.30-1.38%, Cr: 0.45-0.55%, Cu: 0.30-0.40%, Ti: 0.015-0.05%, Als: 0.018-0.030%, P≤0.020%, S≤0.008%, and the remainder is Fe and unavoidable residual elements and impurities in the production process.

[0006] The mechanism of action of the chemical components in the weather-resistant structural steel Q355NHD of the present invention is as follows: C: C is an element required to ensure the strength of the steel plate, but in order to make the steel plate have good welding performance and good low-temperature impact toughness, the carbon content in the steel must be reduced; in addition, reducing C can also reduce the tendency of steel to overburn and reduce the penetration of Cu into the austenite grain boundary to reduce the incidence of "copper embrittlement". The present invention controls C to 0.07-0.09wt%.

[0007] Si: Silicon is a cheap deoxidizing element and can also play a role in solid solution strengthening, but adding too much can easily produce red iron oxide scale on the steel surface that is difficult to remove. Therefore, the Si content is controlled at 0.20-0.30wt%.

[0008] Mn: Manganese is an important element in weathering steel, which mainly plays the role of grain refinement and solid solution strengthening; but in order to reduce the segregation of steel, the Mn content cannot be controlled too high, so by adding 1.30-1.38wt% Mn to ensure the strength of the steel plate while preventing the segregation of the ingot, the internal quality of the steel plate is improved.

[0009] Cr: In weather-resistant structural steel, chromium can improve the corrosion resistance and weather resistance of the steel, and can also play a role in solid solution strengthening; but when the chromium content is too high, it will increase the brittleness tendency of the steel, so the present invention controls the chromium content to 0.45-0.55wt%.

[0010] Cu: Copper is a typical alloying element that improves the corrosion resistance and weathering resistance of weathering steel. The lower limit of copper in GB / T 4171-2008 standard is 0.25wt%. However, as a scarce alloying element, the addition of this element should be minimized. Considering the comprehensive cost and control requirements, the copper content is controlled at 0.30-0.40wt%.

[0011] Ti: The main function of titanium is nitrogen fixation and deoxidation. The precipitation temperature of titanium carbonitride particles is relatively high, which can significantly reduce the phenomenon of coarse columnar austenite grains in the sector and reduce the tendency of copper to penetrate into the austenite grain boundary. Too high titanium content will reduce the toughness of the material. According to the nitrogen content in the molten steel, the titanium content is controlled at 0.015-0.025wt%.

[0012] Als: Aluminum is an effective deoxidizing element that can efficiently remove oxygen from molten steel and improve its purity. However, excessive addition will increase the number of inclusions in the steel and increase the risk of hot-sending cracks in high-temperature direct delivery of weathering steel ingots. Therefore, the aluminum content is controlled at 0.018-0.030wt%.

[0013] P, S: Phosphorus and sulfur are harmful elements in steel. They damage the low-temperature toughness and fatigue resistance of steel. Theoretically, the lower the content, the better. However, considering the cost and feasibility of smelting, P is controlled to be ≤ 0.020% and S ≤ 0.008%.

[0014] The thickness of the weather-resistant structural steel Q355NHD of the present invention is 20-50 mm.

[0015] The weather-resistant structural steel Q355NHD of the present invention has a yield strength of 399-441 MPa, a tensile strength of 522-587 MPa, an elongation of 24.5-30.5%, and an impact energy of 155-208 J at -20°C, meeting the mechanical property requirements of GB / T 4171-2008.

[0016] The microstructure of the weather-resistant structural steel Q355NHD of the present invention is ferrite and pearlite.

[0017] The production method of the low-cost weather-resistant structural steel Q355NHD of the present invention comprises continuous casting, billet heating, rolling and cooling processes.

[0018] Preferably, in the continuous casting process of the present invention, the pouring superheat is controlled at 15-25°C, the water content in the first cooling zone is controlled at 0.50-0.55 L / kg, the water content in the second cooling zone is controlled at 0.60-0.65 L / kg, and the casting speed is controlled at 1.00-1.05 m / min.

[0019] The present invention improves the solidification quality of the continuous casting billet by controlling the low superheat and appropriately reducing the pulling speed, strengthens the cooling of the billet shell in the corner area of ​​the crystallizer, refines the process of solidification of the billet corner structure to generate austenite, prevents the coarsening of the downward process, and reduces the risk of copper penetrating into the grain boundary due to the coarsening of austenite grains on the billet surface.

[0020] Preferably, in the continuous casting process of the present invention, the continuous casting billet is conveyed to the entrance of the heating furnace via a roller within 50 minutes after exiting the sector segment. During the conveying process, the roller is fully covered with a heat preservation cover to prevent the hot casting billet from staying in the air.

[0021] The present invention controls the billet transfer time within 50 minutes. According to the precipitation kinetics of acid-soluble aluminum in Q355NHD, AlN in the billet has not yet begun to precipitate in large quantities at this time, thereby reducing the incidence of red-transfer cracks in the billet. In addition, the average temperature of the billet entering the furnace is ensured to be between 850°C and 930°C, thereby reducing the segregation of copper by shortening the heating process of the heating furnace.

[0022] Preferably, in the casting heating process of the present invention, the casting is put into the furnace at 850°C-930°C, the final heating temperature is 1120-1160°C, and the heating time is controlled at 60-80 minutes.

[0023] After the ingot enters the walking beam double heat storage heating furnace, the ingot heating is completed within 60 to 80 minutes based on the higher furnace entry temperature. The temperature of the ingot after heating is 1120 to 1160°C. The shorter heating time in the furnace and the lower furnace exit temperature can not only achieve the refinement of the initial austenite structure, but also reduce the burn-in loss in the heating furnace and reduce the tendency of copper to segregate.

[0024] Preferably, in the rolling process of the present invention, the starting temperature of rough rolling is 1050-1100°C, the waiting thickness is ≥2.5a, and a is the finished thickness of the steel plate; the starting temperature of finishing rolling is ≤850°C, and the final rolling temperature is controlled at 790-830°C.

[0025] The rough rolling start temperature is controlled at 1050-1100℃ to complete the pre-deformation of the steel plate in the recrystallization temperature zone and achieve the initial refinement of the austenite grains. The finishing rolling start temperature is set below 850℃ to ensure that the deformation is carried out in the non-recrystallization temperature zone, thereby avoiding the partial recrystallization temperature zone; the thickness of the steel plate during the finishing rolling stage is controlled to be more than 2.5 times the thickness of the finished product, and the cumulative reduction rate can be obtained ≥60%, the purpose of which is to obtain sufficient dislocations.

[0026] Preferably, the cooling process of the present invention adopts ACC controlled cooling, the cooling rate is controlled at 8-12° C. / s, and the final cooling temperature is controlled at 540-560° C. At this cooling rate and final cooling temperature, the final steel plate structure can be refined, thereby ensuring the stability of the steel plate strength and low-temperature impact toughness.

[0027] The beneficial effects of adopting the above technical solution are: The present invention achieves rapid transfer of ingots through reasonable composition design, optimizes continuous casting process, keeps the temperature during the transfer process, and combines specific heating, rolling and cooling processes to obtain Q355NHD weathering steel that meets the mechanical properties of GB / T 4171-2008 standard, while achieving low-cost manufacturing of this type of steel and reducing the risk of "copper brittleness". Under the premise of ensuring mechanical properties, the present invention greatly reduces the alloy cost of the steel, ensures the surface quality of the steel plate, and achieves low-cost manufacturing of Q355NHD. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 500× microstructure image of Q355NHD in Example 1; Figure 2 500× microstructure image of Q355NHD in Example 2; Figure 3 500× microstructure image of Q355NHD in Example 3; Figure 4 500× microstructure image of Q355NHD in Example 4; Figure 5 500× microstructure image of Q355NHD in Example 5; Figure 6 This is a 500× microstructure image of Q355NHD in Example 6. DETAILED DESCRIPTION

[0029] The technical solution of the present invention is further described below in conjunction with embodiments, but the present invention is not limited to the scope of the embodiments.

[0030] Examples 1-6 The chemical composition of the Q355NHD weathering steel provided in Examples 1-6 is shown in Table 1. The production method thereof includes continuous casting, heating of the ingot, rolling and cooling processes, which are specifically as follows: (1) Continuous casting process: the pouring superheat is controlled at 15-25℃, the water content of the first cooling zone is controlled at 0.50-0.55L / kg, the water content of the second cooling zone is controlled at 0.60-0.65L / kg, and the casting speed is controlled at 1.00-1.05m / min. The billet is transported to the entrance of the heating furnace via a roller within 50 minutes after it leaves the fan-shaped section. During the transportation process, the roller is covered with a heat preservation cover to prevent the hot billet from staying in the air.

[0031] (2) Billet heating process: The billet entering the furnace is at a temperature of 850°C to 930°C, the heating end temperature is 1120°C to 1160°C, and the heating time is controlled at 60 to 80 minutes.

[0032] (3) Rolling process: The starting temperature of rough rolling is 1050-1100℃, and the waiting thickness is ≥2.5a, where a is the thickness of the finished steel plate; the starting temperature of finishing rolling is ≤850℃, and the final rolling temperature is controlled at 790-830℃.

[0033] (4) Cooling process: ACC is used to control cooling, the cooling rate is controlled at 8-12°C / s, and the final cooling temperature is controlled at 540-560°C. At this cooling rate and final cooling temperature, the final steel plate structure can be refined, thereby ensuring the stability of the steel plate strength and low-temperature impact toughness.

[0034] The steel plate thickness and continuous casting process parameters of each embodiment are shown in Table 2; the process parameters of heating, rolling and cooling of the ingot are shown in Table 3.

[0035] Table 1 Chemical composition and mass percentage (wt%) of weathering steel of Examples 1-6

[0036] Table 2 Steel plate thickness and continuous casting process parameters of each example

[0037] Table 3 Parameters of heating, rolling and cooling process of each embodiment

[0038] The mechanical properties and surface quality of the Q355NHD steel plates produced in each embodiment are shown in Table 4.

[0039] Table 4 Mechanical properties and surface quality of steel plates in various examples

[0040] It can be seen from Table 4 that the mechanical properties of Q355NHD provided by the present invention fully meet the requirements of the national standard GB / T 4171-2008.

[0041] The metallographic structure of the Q355NHD steel plate in each embodiment is shown in Figure 1-6 ;Depend on Figure 1-6 It can be seen that the microstructure of the steel plate of each embodiment is a fine and uniform ferrite and pearlite structure, which can ensure that the strength and low-temperature impact toughness of the steel plate meet the requirements of Q355NHD in GB / T 4171-2008.

Claims

1. A low-cost weather-resistant structural steel Q355NHD, characterized in that: The chemical composition and mass percentage of the weathering structural steel Q355NHD are: C: 0.07-0.09%, Si: 0.20-0.30%, Mn: 1.30-1.38%, Cr: 0.45-0.55%, Cu: 0.30-0.40%, Ti: 0.015-0.05%, Als: 0.018-0.030%, P≤0.020%, S≤0.008%, and the balance is Fe and inevitable residual elements and impurities in the production process.

2. The low-cost weather-resistant structural steel Q355NHD according to claim 1 is characterized in that: The thickness of the weather-resistant structural steel Q355NHD is 20 to 50 mm.

3. The low-cost weather-resistant structural steel Q355NHD according to claim 1 is characterized in that: The weather-resistant structural steel Q355NHD has a yield strength of 399-441 MPa, a tensile strength of 522-587 MPa, an elongation of 24.5-30.5%, and an impact energy of 155-208 J at -20°C.

4. The low-cost weather-resistant structural steel Q355NHD according to claim 1 is characterized in that: The microstructure of the weather-resistant structural steel Q355NHD is ferrite and pearlite.

5. A method for producing low-cost weather-resistant structural steel Q355NHD based on any one of claims 1 to 4, characterized in that: It includes continuous casting, billet heating, rolling and cooling processes.

6. The method for producing low-cost weather-resistant structural steel Q355NHD according to claim 5, characterized in that: In the continuous casting process, the pouring superheat is controlled at 15-25° C., the water content in the first cooling zone is controlled at 0.50-0.55 L / kg, the water content in the second cooling zone is controlled at 0.60-0.65 L / kg, and the casting speed is controlled at 1.00-1.05 m / min.

7. The method for producing low-cost weather-resistant structural steel Q355NHD according to claim 5, characterized in that: In the continuous casting process, the continuous casting billet is transported to the entrance of the heating furnace via a roller within 50 minutes after it exits the fan-shaped segment, and the roller is covered with a heat preservation cover throughout the transportation process.

8. The method for producing low-cost weather-resistant structural steel Q355NHD according to claim 5, characterized in that: In the casting billet heating process, the casting billet entering the furnace temperature is 850°C to 930°C, the heating end temperature is 1120°C to 1160°C, and the heating time is controlled at 60 to 80 minutes.

9. The method for producing low-cost weather-resistant structural steel Q355NHD according to claim 5, characterized in that: In the rolling process, the starting temperature of rough rolling is 1050-1100°C, the waiting thickness is ≥2.5a, a is the thickness of the finished steel plate; the starting temperature of fine rolling is ≤850°C, and the final rolling temperature is 790-830°C.

10. The method for producing low-cost weather-resistant structural steel Q355NHD according to claim 5, characterized in that: The cooling process adopts ACC controlled cooling, the cooling rate is controlled at 8-12°C / s, and the final cooling temperature is controlled at 540-560°C.

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