Steel for thick-specification 320MPa high-toughness pipe piles produced from thin slabs and production method thereof

Abstract

The present invention relates to a steel for producing high-toughness pipe piles with a thickness of 320 MPa from thin slabs and a production method, and a steel for producing high-toughness pipe piles with a thickness of 270 MPa from thin slabs. It is characterized in that the chemical components in the steel are by weight percentage: C 0.056% - 0.068%, Si 0.05% - 0.15%, Mn 0.39% - 0.49%, P ≤ 0.020%, S ≤ 0.020%, Als 0.005% - 0.020%, Ti 0.022% - 0.030%, Cr 0.13% - 0.18%, and the rest is iron and inevitable impurities. The present invention can realize the continuous casting and rolling of thin slabs (135 - 170 mm) to produce hot-rolled coil sheets of high-strength and high-toughness pipe piles with a thickness of 320 MPa (20 - 24 mm).

Description

Technical Field

[0001] The present invention belongs to the technical field of low-alloy hot-rolled coil of metal materials, and particularly relates to a steel for thick-specification 320 MPa high-toughness pipe piles produced from thin slabs and a production method thereof. Background Art

[0002] In recent years, with the development of China's economy and the advancement of urbanization, various buildings and infrastructures have emerged and improved continuously. As a commonly used building material, pipe piles have been widely used. To ensure the large load-bearing capacity and service safety of building pipe piles, major projects have higher requirements for the thickness, strength, and toughness of the steel used for pipe piles.

[0003] At present, the thickness of continuous casting billets used in hot rolling by major steel mills is generally above 200 mm, while the thickness of the continuous casting billets used in the present invention is 135 - 170 mm. Due to the small reduction ratio, the contribution to the strength and toughness of thick-specification steel for pipe piles is small. Therefore, it is somewhat difficult to produce thick-specification 320 MPa hot-rolled coils. Public information shows that the toughness index of the current steel for pipe piles is not high, and its strength and toughness indexes are difficult to fully meet the engineering requirements.

[0004] Patent document "A Steel for High-Strength Thick-Specification Pipe Piles and Its Manufacturing Method", application number CN200910251585.3. In the composition of this steel grade, C is 0.12% - 0.16%, Si is 0.20% - 0.50%, Mn is 1.3% - 1.5%, S ≤ 0.010%, P ≤ 0.015%, Nb is 0.020% - 0.030%, and Al is 0.015% - 0.040%. The cast billet of this invention is 230 mm, with a large reduction ratio, and Nb microalloying is used, resulting in a high alloy cost. In addition, the high contents of C and Si are harmful to the impact toughness of the product. The yield strength of the product reaches 400 MPa, but the impact energy at -40°C is only 150 J.

[0005] Patent document with application number CN201310489482.7 discloses a steel for corrosion-resistant bridge pipe piles and its production method. The composition (by weight percentage) is: C 0.07% - 0.13%, Si 0.3% - 0.65%, Mn 0.80% - 1.30%, P 0.025% - 0.045%, S ≤ 0.002%, V 0.035% - 0.050%, Ti 0.008% - 0.025%, Re 0.005% - 0.020%, Zr 0.006% - 0.012%. In this invention, the S content is controlled at a low level, resulting in a high steelmaking cost; it contains rare earth elements Re and Zr, leading to a high alloy cost. The yield strength of the product reaches 390 MPa, but the impact energy at 0°C is only above 47 J, and it is prone to failure in actual engineering use.

[0006] The patent document with the application number CN201310407630.6 discloses a steel plate for pipe piles with a thickness of 40 - 60 mm and its production method. This patent describes medium - thick plates and does not involve the production method of coil plates.

[0007] The paper "Water - cooled Welding Test of Q345C_Hq Steel for Pipe Piles of Hangzhou Bay Bridge" (Iron and Steel Research, 2004.4, p29 - 31) mentions steel plates for pipe piles. The contents of C (1.34%) and Mn (1.25% - 1.35%) are high, with serious segregation and low toughness. The addition of other elements and the production process are not described.

[0008] The paper "Reasons for Transverse Cracks in Multi - pass Welding of S355 Steel for Wind Power Pipe Piles" (Welding & Joining, 2011.10, p49 - 51) mentions a steel for pipe piles with a thickness of 86 mm. The contents of C (0.15%), Si (0.3%) and Mn (1.56%) are relatively high, and precious metal Ni is added. The product thickness, performance and production process are not described.

[0009] The paper "Microstructure and Corrosion Resistance of Steel for Pipe Piles in Water Conservancy Projects" (Corrosion & Protection, 2018.7, p501 - 502) mentions 3 kinds of steel for pipe piles. Cu and Cr are added alone or in combination. The highest yield strength can reach 339 MPa, but the impact energy at 0℃ is only 90 J at most, and fracture failure is likely to occur in actual applications. In addition, the product thickness and production process are not described in this paper.

[0010] The paper "Study on Microstructure and Corrosion Resistance of High - Strength Steel for Pipe Piles" (Hot Working Technology, 2018.6, p56 - 57) mentions 3 kinds of steel for pipe piles with different Cr contents. The Cr contents are 0.5%, 1.0% and 1.5% respectively. The Cr content is relatively high, and the alloy cost of the product is relatively high. In addition, this paper also does not describe the product thickness, toughness index and production process.

[0011] The steel for pipe piles involved in the above - mentioned public information includes two products: medium - thick plates and hot - rolled coil plates. In the alloy design of hot - rolled coil plate products, high C and high Mn are basically used to ensure the product strength, and one or several of Nb, Ti, Cr, Ni and Cu are added in combination. Although the strength levels of their products are all above 300 MPa, the impact toughness is poor. Most of the impact tests are carried out at 0℃, and the impact energy is not high. In order to ensure the strength and toughness of the steel plate, in the existing production process, generally, the strong and tough properties of the steel plate are improved by increasing the reduction ratio and adding a large amount of alloying elements. Therefore, the slab for producing hot - rolled coil plate products is relatively thick, with a thickness generally of 200 - 230 mm. Through literature research, there is currently no public information on a method to produce high - toughness steel for pipe piles with a thick specification of 320 MPa using thin slab. Summary of the Invention

[0012] The object of the present invention is to provide a high-strength and tough steel for 320 MPa high-toughness pipe piles with a thick specification and its production method by adopting an economically reasonable alloy design and matching an appropriate production process, which can realize the continuous casting and rolling of thin slab (135 - 170 mm) to produce hot-rolled coil plates of high-strength and tough steel for 320 MPa pipe piles with a thick specification (20 - 24 mm).

[0013] To achieve the above object, the present invention is realized by adopting the following technical solutions:

[0014] For the production of high-toughness steel for 320 MPa pipe piles with a thick specification from thin slab, it is characterized in that the chemical composition of the steel is as follows by weight percentage: C 0.056% - 0.068%, Si 0.05% - 0.15%, Mn 0.39% - 0.49%, P ≤ 0.020%, S ≤ 0.020%, Als 0.005% - 0.020%, Ti 0.022% - 0.030%, Cr 0.13% - 0.18%, and the rest is iron and inevitable impurities.

[0015] The composition of the 320 MPa pipe pile steel of the present invention adopts a C-Mn-Ti-Cr alloy design. By increasing the finish rolling temperature, controlling the coiling temperature and cooling rate, the strength is improved by the strengthening effect of TiC. At the same time, adding Cr can not only improve the thickness direction tissue uniformity of the thick specification product, but also reduce the yield ratio of the steel for pile driving pipes. The above means ensure that the pipe pile steel has excellent strength and toughness. The functions and selection reasons of the main elements are as follows:

[0016] C: It is the main element second only to iron in steel, which directly affects the strength, plasticity, toughness and welding performance of steel. C is the most economical element to improve the strength of steel, but with the increase of C content, the toughness and weldability of steel gradually become worse. Since the product of the present invention is used in the joints in the construction industry after welding, a low carbon content design is the basic guarantee to ensure that the pipe pile steel has excellent strength and toughness and good welding performance. In the present invention, if the C content is higher than 0.068%, the pearlite content is high, which damages the toughness of the finished product. If the C content is lower than 0.056%, the contribution to strength can be almost ignored. Therefore, the C content in the present invention is controlled at 0.056% - 0.068%.

[0017] Si: It is an important reducing agent and deoxidizer in the steelmaking process. For many materials in carbon steel, Si content is below 0.5%. Si can significantly improve the strength of ferrite-pearlite tissue type, but high Si content will reduce the plasticity and toughness of the material. Therefore, the Si content in the present invention is controlled at a lower level. Therefore, the Si content in the present invention is controlled at 0.05% - 0.15%.

[0018] Mn: Manganese has a solution strengthening effect and can also lower the γ-α phase transformation temperature, thereby refining the ferrite grains. In addition, adding Mn in the present invention can also delay the transformation from ferrite to pearlite, thus reducing the pearlite content, which is beneficial to the toughness of the product. However, too high a manganese content will cause severe segregation and loss of the toughness of the material. Therefore, the manganese content in the present invention is controlled to be 0.39% - 0.49%.

[0019] Ti: It is an extremely strong nitride-forming element, and TiN is also difficult to decompose at high temperatures. Therefore, on the one hand, stable and fine TiN particles can effectively prevent the growth of austenite during the reheating process of the continuous casting billet, and on the other hand, can improve the impact toughness of the heat-affected zone of welding. In addition, the binding ability of Ti with S is stronger than that of Mn. Therefore, to a certain extent, the content of MnS can be reduced, thereby improving the impact toughness of the material. However, too much Ti element will form carbide nitrides with larger sizes, which will instead result in loss of toughness. It is necessary to cooperate with appropriate finish rolling temperature and coiling temperature to make the precipitated TiC fine and play the role of precipitation strengthening. Therefore, adding Ti element in the present invention can make up for the lack of strength and toughness caused by insufficient reduction ratio. If the Ti content is too high, it is easy to form large-sized and sharp-cornered TiN, which will instead result in loss of toughness. Therefore, the Ti content in the present invention is 0.022% - 0.030%.

[0020] Cr: The contribution of Cr to the tensile strength is greater than that to the yield strength, so it can reduce the yield ratio and make the steel strip perform well in pile driving service and have high structural safety. In addition, Cr can also effectively improve the hardenability and improve the tissue uniformity in the thickness direction of thick-specification products, thereby improving the toughness index. If the Cr content is too high in the present invention, M / A hard phases will be formed, resulting in loss of the toughness index. Therefore, the Cr content in the present invention is 0.13% - 0.18%.

[0021] Als: It is a deoxidizing element. Adding an appropriate amount of aluminum can form fine and dispersed AlN particles, which is beneficial to grain refinement and improve the strength and toughness of the steel. Therefore, the Als content in the present invention is 0.005% - 0.020%.

[0022] P: It is extremely easy to highly segregate during the solidification of molten steel, and will form a banded F-P structure. P will also greatly reduce the benefits brought by the reduction of C in the steel for pipe piles, resulting in loss of the toughness of the steel. As a harmful element in the steel, it should be minimized as much as possible, but too low requirements will increase the cost. Therefore, the P content in the present invention is controlled below 0.020%.

[0023] S: It is an inevitable impurity element in the steel, which will reduce the toughness of the steel. Generally, the lower the better, but too low requirements will increase the production cost. Therefore, S ≤ 0.020% in the present invention.

[0024] A production method for steel used in thick-specification 320MPa high-toughness pipe piles produced from thin slabs, the production process flow of which involves: hot metal pretreatment - converter smelting - secondary refining (LF + Ca treatment) - continuous casting - slab heating - rolling - ultra-fast cooling - coiling.

[0025] Among them:

[0026] 1) Smelting and continuous casting process: Hot metal pretreatment, top blowing or top and bottom combined blowing is used in converter smelting; in secondary refining, LF furnace is used for mild desulfurization treatment and calcium treatment to control the quantity and morphology of inclusions; dynamic soft reduction is used in continuous casting, and the thickness of the obtained continuous casting billet is 135 - 170mm;

[0027] 2) Rolling process: The continuous casting slab is heated to 1187 - 1208°C in a walking beam reheating furnace to completely dissolve Ti, which plays a role in precipitation strengthening during the subsequent rolling process. Subsequently, it undergoes two-stage controlled rolling by roughing and finishing mills, avoiding the two-phase region to prevent the occurrence of mixed crystal phenomenon from affecting the toughness index, and the finish rolling temperature is 1026 - 1037°C; the finish rolling temperature is 875 - 896°C, and then an ultra-fast cooling method is adopted to cool to 552 - 570°C at a speed of 36.2 - 39.8°C / s for coiling, and finally air-cooled to room temperature.

[0028] The hot-rolled coil of steel for piling tubes of 320Pa grade in the present invention has excellent strength and toughness: the yield strength is 320 - 348MPa, the tensile strength is ≥450MPa, and the elongation after fracture is ≥25%; the Charpy impact energy (average value of 3 specimens) Akv at -40°C is ≥169J.

[0029] The finished thickness of the steel plate is 20 - 24mm.

[0030] Compared with the prior art, since the reduction ratio of the present invention is small and the contribution to the strength and toughness of thick-specification steel for pipe piles is small, it is difficult to produce thick-specification hot-rolled coils of 320MPa grade. The deficiencies in controlling the strength and toughness indexes caused by the insufficient reduction ratio are compensated by the following technical work in the second and third aspects. The beneficial effects of the present invention are:

[0031] 1) The present invention uses 170mm thin slabs to roll thick-specification (20 - 24mm) high-strength and high-toughness steel coils for 320MPa grade pipe piles, which can improve production efficiency and save production resources;

[0032] 2) In terms of alloy design, low-C design is adopted to reduce the pearlite content, low-Mn design and addition of Ti are used to inhibit the formation of MnS to ensure the toughness index. In addition, adding Cr improves the thickness-directional tissue uniformity of thick-specification products, which is also beneficial to the strength and toughness of the finished product;

[0033] 3) In terms of the rolling process, combined with the above alloy design, since the addition of Ti (the presence of TiN) does not cause excessive growth of the billet grains during high-temperature heating, the use of the ultra-fast cooling technology can avoid a large amount of Nb precipitation in austenite. A relatively high degree of supercooling is beneficial to the fine precipitation of Nb in ferrite, enhancing the effect of precipitation strengthening. Coiling at 552 - 570 °C can promote the supersaturated precipitation of TiC in ferrite, giving full play to the role of precipitation strengthening to improve the product strength;

[0034] 4) The alloy design of the product of the present invention is economical and reasonable, the process route is simple, stable and easy to implement. The physical quality of the product is excellent, with a yield strength of 320 - 348 MPa, a tensile strength of ≥450 MPa, and an elongation after fracture of ≥25%; the Charpy impact energy (average value of 3 specimens) at -40 °C, Akv ≥ 169 J. Specific Embodiments

[0035] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0036] The specific embodiments of the present invention will be further described below with reference to embodiments. The following embodiments are used to specifically illustrate the content of the present invention. These embodiments are only general descriptions of the content of the present invention and do not limit the content of the present invention. The chemical compositions of the steel embodiments of the present invention are shown in Table 1, the rolling process systems of the steel embodiments of the present invention are shown in Table 2, and the mechanical properties of the steel embodiments of the present invention are shown in Table 3.

[0037] Table 1 Chemical Compositions of the Steel in Embodiments (wt, %)

[0038] Example C Si Mn P S Ti Als Cr 1 0.062 0.12 0.43 0.020 0.017 0.029 0.012 0.14 2 0.068 0.10 0.44 0.019 0.015 0.023 0.020 0.13 3 0.056 0.05 0.49 0.018 0.016 0.022 0.018 0.17 4 0.063 0.08 0.42 0.020 0.018 0.030 0.006 0.18 5 0.057 0.13 0.49 0.017 0.017 0.026 0.007 0.15 6 0.066 0.06 0.39 0.020 0.020 0.030 0.005 0.13 7 0.067 0.09 0.40 0.020 0.016 0.023 0.008 0.14 8 0.058 0.14 0.47 0.018 0.018 0.022 0.016 0.17 9 0.056 0.07 0.44 0.019 0.018 0.026 0.010 0.13 10 0.060 0.15 0.46 0.018 0.017 0.029 0.014 0.18

[0039] Table 2 Process Systems of the Steel in Embodiments

[0040]

[0041] Table 3 Main Mechanical Properties of the Steel in Embodiments

[0042]

Claims

1. Steel for thick-specification 320MPa high-toughness pipe piles produced from thin slabs, characterized in that, The chemical components in the steel by weight percentage are: C 0.056% - 0.068%, Si 0.05% - 0.14%, Mn 0.39% - 0.49%, P ≤ 0.020%, S ≤ 0.020%, Als 0.005% - 0.018%, Ti 0.022% - 0.029%, Cr 0.13% - 0.18%, and the rest are Fe and inevitable impurities; The production method of the steel for thick - specification 320MPa high - toughness pipe piles using thin slabs includes: the thickness of the continuous casting slab is 135 - 170mm, the continuous casting slab is heated in a heating furnace to 1187 - 1208°C, then undergoes two - stage controlled rolling by a rough rolling mill and a finish rolling mill, the rough rolling finishing temperature is 1026 - 1037°C; the finish rolling finishing temperature is 881 - 896°C, then it is finally cooled to 552 - 570°C at a speed of 36.2 - 39.8°C by laminar cooling for coiling, and finally air - cooled to room temperature; The thickness of the steel plate is 20 - 24mm.

2. The steel for thick-specification 320MPa high-toughness pipe piles produced from thin slabs according to claim 1, wherein The yield strength of the steel plate is 320 - 348MPa, the tensile strength is ≥450MPa, the elongation after fracture is ≥25%; the Charpy impact energy Akv at - 40°C is ≥169J.

3. A production method for steel used in manufacturing thick-specification 320MPa high-toughness pipe piles from thin slabs as described in claim 1 or 2, characterized in that, Including: The thickness of the continuous casting slab is 135 - 170mm, the continuous casting slab is heated in a heating furnace to 1187 - 1208°C, then undergoes two - stage controlled rolling by a rough rolling mill and a finish rolling mill, the rough rolling finishing temperature is 1026 - 1037°C; the finish rolling finishing temperature is 881 - 896°C, then it is finally cooled to 552 - 570°C at a speed of 36.2 - 39.8°C by laminar cooling for coiling, and finally air - cooled to room temperature.

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