Steel for producing thick-specification 270MPa high-toughness pipe piles from thin slabs and production method thereof

Abstract

The present invention relates to a steel for thick - specification 270MPa high - toughness pipe piles produced from thin slabs and a production method. The steel for thick - specification 270MPa high - toughness pipe piles produced from thin slabs 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 0.25% - 0.35%, P≤0.020%, S≤0.020%, Als 0.005% - 0.020%, Ti 0.010% - 0.020%, and the rest is iron and inevitable impurities. The present invention can realize the continuous casting and rolling of medium - thin slabs (135 - 170mm) to produce hot - rolled coil plates of thick - specification (20 - 24mm) high - strength and high - toughness steel for 270MPa line high - toughness pipe piles.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-alloy hot-rolled steel coils of metal materials, and particularly relates to a steel for thick-specification 270 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 infrastructure 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 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 270 MPa hot-rolled steel coils. Public information shows that the toughness index of the current steel for pipe piles is not high, and its strength and toughness index 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 continuous casting billet of this invention is 230 mm, with a large reduction ratio, and Nb microalloying is adopted, 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 corrosion-resistant steel for 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 content of S 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 maximum 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 between 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 thickness of 270 MPa grade using thin slab. Summary of the Invention

[0012] The object of the present invention is to provide a high-strength and high-toughness steel for 270 MPa grade 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 high-toughness steel for 270 MPa grade high-toughness pipe piles with a thick specification (20 - 24 mm).

[0013] In order 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 270 MPa grade thick-specification pipe piles from thin slab, it is characterized in that the chemical composition in the steel is by weight percentage: C 0.056% - 0.068%, Si 0.05% - 0.15%, Mn 0.25% - 0.35%, P ≤ 0.020%, S ≤ 0.020%, Als 0.005% - 0.020%, Ti 0.010% - 0.020%, and the rest is iron and inevitable impurities.

[0015] The composition of the 270 MPa pipe pile steel of the present invention adopts a C-Mn-Ti alloy design. By increasing the finish rolling temperature, controlling the coiling temperature and cooling rate, the strengthening effect of TiC is exerted, and at the same time, a uniform and fine ferrite-pearlite (F-P) structure is obtained, in which the volume fraction of pearlite is 7% - 8%, so as to ensure that the pipe pile steel has excellent strength and toughness. The functions and selection reasons of its main elements are as follows:

[0016] C: It is the most important element in steel after iron, 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 is almost negligible. Therefore, the C content in the present invention is controlled to be 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 structure 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 to be 0.05% - 0.15%.

[0018] Mn: Manganese has a solution strengthening effect and can also reduce 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, thereby reducing the pearlite content, which is beneficial to the toughness of the product. However, too high a manganese content will cause serious segregation and loss of the toughness of the material. Therefore, the manganese content in the present invention is controlled to be 0.25% - 0.35%.

[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, they 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 increased size, resulting in a loss of toughness instead. 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, resulting in a loss of toughness instead. Therefore, the Ti content in the present invention is: 0.010% - 0.020%.

[0020] Als: It is a deoxidizing element. Adding an appropriate amount of aluminum can form fine and dispersed AlN particles, which is beneficial to refining the grains and improving the strength and toughness of the steel. Therefore, the Als content in the present invention is 0.005% - 0.020%.

[0021] 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 a 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%.

[0022] 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.

[0023] A production method of a 270MPa high-toughness steel for thick-specification 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 - laminar flow cooling - coiling. Among them:

[0024] 1) Smelting and continuous casting process: Hot metal pretreatment, converter smelting adopts top blowing or top-bottom combined blowing; secondary refining adopts LF furnace for mild desulfurization treatment and calcium treatment to control the quantity and morphology of inclusions; continuous casting adopts the method of dynamic soft reduction, and the obtained continuous casting billet has a thickness of 135 - 170mm;

[0025] 2) Rolling process: The continuous casting slab is heated in a walking beam reheating furnace to 1185 - 1214 °C to fully 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, with the final rolling temperature being 1021 - 1033 °C; the final rolling temperature is 853 - 872 °C, and then laminar cooling is adopted to cool it to 574 - 595 °C at a speed of 30.6 - 34.8 °C / s for coiling, and finally air-cooled to room temperature.

[0026] The hot-rolled coil of the steel for 270 MPa grade pile driving pipes of the present invention has excellent strength and toughness: the yield strength is 270 - 302 MPa, the tensile strength is ≥420 MPa, and the elongation after fracture is ≥27%; the Charpy impact energy (average value of 3 specimens) Akv at -40 °C is ≥175 J.

[0027] The finished thickness of the steel plate is 20 - 24 mm.

[0028] Compared with the prior art, since the reduction ratio of the present invention is small and the contribution to the strength and toughness of the steel for thick-specification pipe piles is small, it is somewhat difficult to produce hot-rolled coils of 270 MPa grade with thick specifications. The deficiencies in controlling the strength and toughness indexes caused by the insufficient reduction ratio are compensated by the technical work in the following second and third aspects. The beneficial effects of the present invention are:

[0029] 1) The present invention uses thin slab billets of 135 - 170 mm to roll hot-rolled coils of high-strength and high-toughness 270 MPa grade steel plates for thick-specification (20 - 24 mm) pipe piles, which can improve production efficiency and save production resources;

[0030] 2) In terms of alloy design, low C design is adopted to reduce the pearlite content, low Mn design and the addition of Ti are used to inhibit the formation of MnS, thus ensuring the toughness index;

[0031] 3) In terms of rolling process, combined with the above alloy design, due to the addition of Ti (the existence of TiN), the grains of the casting billet will not grow excessively during high-temperature heating, and coiling at 574 - 595 °C can play the role of TiC precipitation strengthening to improve the product strength;

[0032] 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, the yield strength is 270 - 302 MPa, the tensile strength is ≥420 MPa, the elongation after fracture is ≥27%; the Charpy impact energy (average value of 3 specimens) Akv at -40 °C is ≥175 J. Detailed implementation manners

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following further elaborates on the present invention in combination with 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.

[0034] The specific implementation manners of the present invention will be further described below in conjunction with 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.

[0035] Table 1 Chemical Compositions of the Steel in the Embodiment (wt, %)

[0036] Example C Si Mn P S Ti Als 1 0.066 0.10 0.35 0.020 0.018 0.017 0.018 2 0.067 0.05 0.25 0.020 0.017 0.010 0.020 3 0.058 0.15 0.32 0.018 0.015 0.014 0.006 4 0.062 0.08 0.34 0.016 0.018 0.020 0.012 5 0.056 0.12 0.35 0.019 0.016 0.018 0.014 6 0.064 0.06 0.31 0.020 0.018 0.019 0.010 7 0.068 0.14 0.25 0.018 0.020 0.012 0.008 8 0.057 0.07 0.28 0.019 0.019 0.018 0.005 9 0.059 0.13 0.26 0.020 0.017 0.010 0.007 10 0.058 0.09 0.27 0.017 0.016 0.011 0.007

[0037] Table 2 Process Systems of the Steel in the Embodiment

[0038]

[0039]

[0040] Table 3 Main Mechanical Properties of the Steel in the Embodiment

[0041]

Claims

1. Steel for thick-specification 270MPa high-toughness pipe piles produced from thin slabs, characterized in that, The chemical components in the steel are by weight percentage: C 0.056% - 0.068%, Si 0.05% - 0.14%, Mn 0.25% - 0.35%, P ≤ 0.020%, S ≤ 0.020%, Als 0.005% - 0.020%, Ti 0.010% - 0.019%, and the rest are Fe and inevitable impurities; The production method of the steel for producing thick - specification 270MPa 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 1185 - 1214°C to fully dissolve Ti, which plays a role in precipitation strengthening during the subsequent rolling process. Then, it undergoes two - stage controlled rolling by a rough rolling mill and a finishing mill. The rough rolling finishing temperature is 1021 - 1033°C; the finishing rolling finishing temperature is 853 - 872°C. Subsequently, it is finally cooled to 574 - 595°C for coiling at a speed of 30.6 - 34.8°C / s by laminar flow cooling, and finally air - cooled to room temperature; The thickness of the steel plate is 20 - 24mm.

2. The steel for producing high-toughness pipe piles with a thick specification of 270 MPa from thin slabs according to claim 1, characterized in that, The microstructure of the steel is ferrite - pearlite, and the volume fraction of pearlite is 7% - 8%.

3. The steel for producing thick-specification 270 MPa high-toughness pipe piles from thin slabs according to claim 1, characterized in that, The yield strength of the steel plate is 270 - 302MPa, the tensile strength is ≥420MPa, the elongation after fracture is ≥27%; the Charpy impact energy Akv at - 40°C is ≥175J.

4. A production method of steel for thick-specification 270 MPa high-toughness pipe piles produced from thin slabs as described in any one of claims 1-3, characterized in that, It includes: The thickness of the continuous casting slab is 135 - 170mm; the continuous casting slab is heated in a heating furnace to 1185 - 1214°C to fully dissolve Ti, which plays a role in precipitation strengthening during the subsequent rolling process. Then, it undergoes two - stage controlled rolling by a rough rolling mill and a finishing mill. The rough rolling finishing temperature is 1021 - 1033°C; the finishing rolling finishing temperature is 853 - 872°C. Subsequently, it is finally cooled to 574 - 595°C for coiling at a speed of 30.6 - 34.8°C / s by laminar flow cooling, and finally air - cooled to room temperature.

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