Production of thick-gauge 355MPa high-toughness steel for pipe piles from thin billets and production method

Abstract

The present invention relates to a method for producing thick-gauge 355MPa high-toughness pipe pile steel from thin slabs. The steel comprises the following chemical components by weight: C 0.069% to 0.079%, Si 0.10% to 0.20%, Mn 1.02% to 1.13%, P ≤ 0.020%, S ≤ 0.015%, Als 0.015% to 0.030%, Nb 0.032% to 0.042%, Cr 0.20% to 0.28%, with the remainder being iron and unavoidable impurities. The present invention can produce thick-gauge (20-24mm) 355MPa high-strength and toughness pipe pile steel hot-rolled coils from thin slabs (135-170mm) by continuous casting and rolling.

Description

Technical Field

[0001] The present invention belongs to the technical field of low-alloy hot-rolled coils of metallic materials, and particularly relates to a thin-blank production method of 355MPa thick-gauge high-toughness steel for pipe piles. Background Art

[0002] In recent years, with the development of China's economy and the advancement of urbanization, various buildings and infrastructure have continued to emerge and improve. Pipe piles, as a common building material, have been widely used. In order to ensure the large load-bearing capacity and service safety of construction pipe piles, major projects have higher requirements for the thickness and toughness of pipe pile steel.

[0003] At present, the thickness of the continuous casting billets used in hot rolling in major steel mills is generally above 200mm, while the thickness of the continuous casting billets used in the present invention is 135-170mm. Due to the small compression ratio, the contribution to the strength and toughness of thick-gauge pipe pile steel is small. Therefore, it is difficult to produce thick-gauge 355MPa grade hot-rolled coils. Public data shows that the toughness index of the current pipe pile steel is not high, and its strength and toughness index is difficult to fully meet the engineering requirements.

[0004] Patent document "A High-Strength Thick-Gauge Steel for Piles and Its Manufacturing Method," application number CN200910251585.3, describes the steel's composition as 0.12% to 0.16% C, 0.20% to 0.50% Si, 1.3% to 1.5% Mn, ≤0.010% S, ≤0.015% P, 0.020% to 0.030% Nb, and 0.015% to 0.040% Al. This invention produces a 230mm ingot with a high compression ratio and utilizes Nb microalloying, resulting in high alloy costs. Furthermore, the high C and Si content negatively impacts the product's impact toughness, resulting in a yield strength of 400 MPa but a mere 150J impact energy at -40°C.

[0005] Patent application number CN201310489482.7 discloses a corrosion-resistant steel for bridge pipe piles and its production method. Its composition (by weight): 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%, and Zr 0.006%-0.012%. The S content in this invention is low, resulting in high steelmaking costs. The inclusion of rare earth elements Re and Zr increases alloying costs. While the product achieves a yield strength of 390 MPa, its impact energy at 0°C is only over 47 J, making it susceptible to failure in actual engineering applications.

[0006] Patent document CN201310407630.6 discloses a 40-60 mm thick steel plate for pipe piles and a production method thereof. The patent describes medium and thick plates and does not involve the production method of plate coils.

[0007] The paper "Water-cooled Welding Test of Q345C_Hq Steel for Pipe Piles of Hangzhou Bay Bridge" (Steel Research, April 2004, p. 29-31) mentioned that the steel plates used for pipe piles have high C (1.34%) and Mn (1.25% to 1.35%) contents, severe segregation, and low toughness. The addition of other elements and the production process were not described.

[0008] The paper "Causes of Transverse Cracks in Multi-Pass Welding of S355 Steel for Wind Power Piles" (Welding, October 2011, pp. 49-51) mentions an 86mm thick steel for pipe piles with high contents of C (0.15%), Si (0.3%), and Mn (1.56%), and the addition of the precious metal Ni. The product thickness, performance, and production process are not described.

[0009] The paper "Microstructure and Corrosion Resistance of Pipe Pile Steel for Water Conservancy Projects" (Corrosion and Protection, July 2018, p501-502) mentions three types of steel for pipe piles. Cu and Cr are added separately or in combination. The yield strength can reach up to 339 MPa, but the maximum impact energy at 0°C is only 90 J. In actual application, fracture failure is prone to occur. In addition, the paper does not describe the product thickness and production process.

[0010] The paper "Study on the Microstructure and Corrosion Resistance of High-Strength Pipe Pile Steel" (Hot Working Technology, June 2018, pp. 56-57) mentions three types of pipe pile steel with different Cr contents: 0.5%, 1.0%, and 1.5%. The high Cr content leads to higher alloy costs. Furthermore, the paper fails to describe product thickness, toughness, or production processes.

[0011] The steel for pipe piles mentioned in the aforementioned public documents includes two products: medium and heavy plate and hot-rolled coil. The alloy design of the hot-rolled coil generally utilizes high C and Mn content to ensure product strength, with additives of one or more of Nb, Ti, Cr, Ni, and Cu. While these products all boast strength levels exceeding 300 MPa, their impact toughness is relatively poor, with impact tests mostly conducted at 0°C and low impact energy. To ensure both strength and toughness, existing production processes typically increase the compression ratio and add significant amounts of alloying elements to enhance the strength and toughness of the steel. Consequently, the slabs used to produce hot-rolled coil products are relatively thick, typically ranging from 200 to 230 mm. A literature review revealed no publicly available method for producing thick, 355 MPa-grade, high-toughness pipe pile steel from thin slabs. Summary of the Invention

[0012] The purpose of the present invention is to provide a thick-gauge high-strength and toughness 355MPa grade high-toughness pipe pile steel and a production method thereof by adopting an economically reasonable alloy design and matching an appropriate production process, which can realize the continuous casting and rolling of thin slabs (135-170mm) to produce thick-gauge (20-24mm) 355MPa high-strength and toughness pipe pile steel hot-rolled coils.

[0013] In order to achieve the above object, the present invention adopts the following technical solutions:

[0014] The thin billet is used to produce thick-gauge 355MPa high-toughness pipe pile steel, characterized in that the chemical composition of the steel is as follows by weight: C 0.069%-0.079%, Si 0.10%-0.20%, Mn 1.02%-1.13%, P≤0.020%, S≤0.015%, Als 0.015%-0.030%, Nb 0.032%-0.042%, Cr 0.20%-0.28%, and the remainder is iron and unavoidable impurities.

[0015] The composition of the 355MPa pipe pile steel of the present invention is designed using a C-Mn-Nb-Cr alloy system. Nb microalloying is used to refine the grains, and Cr is added to improve the service safety of the pipe pile steel. Low-temperature heating and two-stage controlled rolling and cooling are used in the rolling process to obtain a uniform and fine ferrite-pearlite (FP) structure, wherein the volume fraction of pearlite is 5% to 6%, thereby ensuring that the pipe pile steel has excellent strength and toughness. The functions and reasons for the selection of the main elements are as follows:

[0016] Carbon (C) is the most important element in steel, second only to iron. It directly affects the steel's strength, plasticity, toughness, and weldability. While C is the most economical element for increasing steel strength, its toughness and weldability gradually deteriorate with increasing C content. Since the products of the present invention are welded for use in joints in the construction industry, a low carbon content is essential for ensuring excellent strength, toughness, and weldability in pipe pile steel. In the present invention, a C content exceeding 0.079% impairs the toughness and weldability of the finished product. A C content below 0.069% results in a reduced amount of Nb(CN), reducing the beneficial effect of grain refinement. Therefore, the present invention limits the C content to 0.069% to 0.079%.

[0017] Si: It is an important reducing agent and deoxidizer in the steelmaking process. Many carbon steels contain less than 0.5% Si. Si can significantly improve the strength of the ferrite-pearlite structure, but high Si content can reduce the material's plasticity and toughness. Therefore, the Si content in the present invention is controlled to a low level. Therefore, the Si content in the present invention is controlled to 0.10% to 0.20%.

[0018] Mn: Manganese has a solid solution strengthening effect and can also lower the γ-α phase transition temperature, thereby refining ferrite grains. Furthermore, the addition of Mn in the present invention can delay the transformation of ferrite to pearlite, thereby reducing the pearlite content and improving the toughness of the product. However, excessive manganese content can cause severe segregation and reduce the toughness of the material. Therefore, the manganese content in the present invention is controlled to 1.02% to 1.13%.

[0019] Nb: Niobium can improve steel properties through various strengthening mechanisms, including precipitation strengthening, precipitation strengthening, and phase transformation strengthening. Its grain refinement can increase yield strength and impact toughness, lower the brittle transition temperature, and improve weldability. However, Nb is a precious element, and its strengthening effect is diminished beyond a certain level. Therefore, the present invention limits the Nb content to 0.032% to 0.042%.

[0020] Cr: Cr contributes more to tensile strength than yield strength, thus reducing the yield strength ratio, ensuring excellent performance in piling service and high structural safety. Cr also effectively improves hardenability and enhances the through-thickness microstructure uniformity of thick-gauge products, thereby enhancing toughness. However, excessive Cr content in the present invention can lead to M / A hardness and compromise toughness. Therefore, the Cr content in the present invention is 0.20% to 0.28%.

[0021] Als: a deoxidizing element. Adding an appropriate amount of aluminum can form fine and dispersed AlN particles, which is beneficial for refining grains and improving the toughness of steel. Therefore, the Als content of the present invention is 0.015% to 0.030%.

[0022] P: P is highly susceptible to segregation during solidification, forming a banded FP structure. It also significantly reduces the benefits of reducing carbon in pipe pile steel, reducing the steel's toughness. As a harmful element, its content in steel should be minimized, but excessively low levels increase costs. Therefore, the P content in this invention is controlled below 0.020%.

[0023] S: is an inevitable impurity element in steel that reduces the toughness of the steel. Generally, the lower the better, but too low a requirement will increase production costs. Therefore, S in the present invention is ≤0.015%.

[0024] A production method for producing thick-gauge 355MPa high-toughness pipe pile steel from thin billets, the production process of which involves: molten iron pretreatment - converter smelting - refining outside the furnace (LF+Ca treatment) - continuous casting - slab heating - rolling - laminar cooling - coiling.

[0025] 1) Smelting and continuous casting process: Hot metal pretreatment, converter smelting adopts top blowing or top and bottom combined blowing; refining outside the furnace adopts LF furnace light desulfurization and calcium treatment to control the number and morphology of inclusions; continuous casting adopts dynamic light pressing method, and the thickness of the obtained casting is 135-170mm;

[0026] 2) Rolling process: The continuous casting slab is heated to 1140-1150°C in a walking beam furnace. The added Nb is completely dissolved and the grains of the slab do not grow excessively and lose strength and toughness. It is then rolled in two stages of roughing and finishing mills. The final rolling temperature of roughing is ≥960°C. The finishing rolling enters the non-recrystallization zone and the final rolling temperature is 800-820°C. It is then cooled to 535-546°C by laminar cooling at a rate of 25.1-27.6°C / s for coiling, and finally air-cooled to room temperature.

[0027] The 355MPa grade hot rolled steel coil for pile driving pipe of the present invention has excellent strength and toughness: the yield strength of the steel plate is 355-375MPa, the tensile strength is ≥480MPa, the elongation after fracture is ≥25%; the Charpy impact energy (average value of 3 samples) at -60℃ is A kv ≥230J.

[0028] The finished steel plate thickness is 20 to 24 mm.

[0029] Compared with the prior art, the present invention has a low compression ratio and a small contribution to the strength and toughness of thick-gauge pipe pile steel. Therefore, it is difficult to produce thick-gauge 355MPa grade hot-rolled coils. The following second and third technical works make up for the lack of control over the strength and toughness indicators caused by the insufficient compression ratio. The beneficial effects of the present invention are:

[0030] 1) The present invention uses 135-170 mm thin slabs to roll thick gauge (20-24 mm) high-strength and toughness 355 MPa grade steel coils for pipe piles, which can improve production efficiency and save production resources;

[0031] 2) In terms of alloy design, the present invention adopts a C-Mn-Nb-Cr alloy design. The low C and low Si design ensures excellent weldability, the addition of Mn delays the occurrence of pearlite transformation and thus improves its toughness, the appropriate addition of Nb mainly plays its role in refining grains and improving strength and toughness, and the addition of Cr improves the uniformity of the thickness direction of thick-gauge products, further ensuring that the product has excellent strength and toughness indicators.

[0032] 3) In terms of rolling technology, combined with the above alloy design, the ingot is heated to 1140-1150°C, which allows the added Nb to be fully dissolved and has the finest original austenite grain structure. At 960°C, the finishing rolling zone is entered, generating more substructures (dislocations and binomial particles) to ensure strength. The appropriate coiling temperature and cooling rate will not produce excessive M / A hardness, ensuring that the product has excellent strength and toughness indicators.

[0033] 4) The alloy design of the product described in the present invention is economical and reasonable, the process route is simple, stable and easy to implement, and the actual quality of the product is excellent. The yield strength of the steel plate is 355-375MPa, the tensile strength is ≥480MPa, the elongation after fracture is ≥25%; the -60℃ Charpy impact energy (average value of 3 specimens) Akv is ≥230J. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0035] The following examples further illustrate the specific embodiments of the present invention. The following examples are intended to illustrate the present invention. These examples are merely general descriptions of the present invention and are not intended to limit the present invention. The chemical composition of the steel examples of the present invention is shown in Table 1, the rolling process specifications of the steel examples of the present invention are shown in Table 2, and the mechanical properties of the steel examples of the present invention are shown in Table 3.

[0036] Table 1 Chemical composition of steel according to example (wt, %)

[0037] Example C Si Mn P S Nb Als Cr 1 0.078 0.10 1.10 0.018 0.015 0.040 0.016 0.20 2 0.077 0.15 1.03 0.017 0.012 0.032 0.030 0.27 3 0.069 0.17 1.12 0.020 0.011 0.036 0.028 0.28 4 0.078 0.13 1.12 0.019 0.014 0.038 0.022 0.23 5 0.069 0.20 1.13 0.017 0.012 0.042 0.024 0.22 6 0.074 0.11 1.11 0.018 0.013 0.041 0.015 0.28 7 0.079 0.12 1.02 0.020 0.011 0.034 0.018 0.21 8 0.070 0.19 1.08 0.019 0.015 0.041 0.020 0.20 9 0.071 0.14 1.04 0.020 0.012 0.035 0.026 0.21 10 0.077 0.18 1.02 0.019 0.013 0.033 0.017 0.22

[0038] Table 2 Example steel process system

[0039]

[0040]

[0041] Table 3 Main mechanical properties of example steel

[0042]

Claims

1. A method for producing thick-gauge 355MPa high-toughness pipe pile steel from thin billets, characterized in that: The chemical composition of the steel is calculated by weight as follows: C 0.069% to 0.079%, Si 0.11% to 0.20%, Mn 1.02% to 1.13%, P≤0.020%, S≤0.015%, Als 0.015% to 0.030%, Nb 0.036% to 0.042%, Cr 0.20% to 0.28%, and the rest is Fe and unavoidable impurities; The method for producing thick-gauge 355MPa high-toughness pipe pile steel from thin slabs comprises: heating the continuous casting slab to 1140-1149°C in a heating furnace, then subjecting the slab to controlled rolling in two stages, namely, rough rolling and finishing rolling, with the final roughing temperature at 960-970°C and the final finishing temperature at 800-820°C; then laminar cooling at a rate of 25.1-27.6°C / s to final cooling to 535-546°C for coiling, and finally air cooling to room temperature; The structure of the steel is ferrite-pearlite.

2. The method for producing thick-gauge 355MPa high-toughness pipe pile steel from thin billets according to claim 1, characterized in that: Steel plate yield strength 355 ~ 375MPa, tensile strength ≥ 480MPa, elongation after fracture ≥ 25%; -60℃ Charpy impact energy A kv ≥230J.

3. The method for producing thick-gauge 355MPa high-toughness pipe pile steel from thin billets according to claim 1, characterized in that: The thickness of the steel plate is 20 to 24 mm.

Images