Production of thick-gauge 430MPa high-toughness steel for pipe piles from thin billets and production method

Abstract

The present invention relates to a method for producing thick-gauge 430MPa 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 1.23% to 1.29%, P ≤ 0.020%, S ≤ 0.015%, Als 0.015% to 0.030%, Nb 0.032% to 0.042%, Ti 0.034% to 0.043%, Cr 0.31% to 0.39%, with the remainder being iron and unavoidable impurities. The present invention can produce thick-gauge (20-24mm) 430MPa 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 for producing thick-gauge 430MPa 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 430MPa 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 S390 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, 430 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 430MPa grade high-toughness pipe pile steel and a production method thereof by adopting an economical and reasonable alloy design and matching an appropriate production process, so as to realize the continuous casting and rolling of thin slabs (135-170mm) to produce thick-gauge (20-24mm) 430MPa 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 430MPa high-toughness pipe pile steel, which is 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 1.23%-1.29%, P≤0.020%, S≤0.015%, Als 0.015%-0.030%, Nb 0.032%-0.042%, Ti 0.034%-0.043%, Cr0.31%-0.39%, and the remainder is iron and unavoidable impurities.

[0015] The composition of the 430MPa pipe pile steel of the present invention is designed using a C-Mn-Nb-Ti-Cr alloy system. Nb microalloying refines grains, Cr is added to improve the service safety of the pipe pile steel, and Ti is added to reduce the MnS content to improve toughness. By increasing the final rolling temperature and utilizing ultra-fast cooling at a high cooling rate to achieve a lower coiling temperature, TiC is finely dispersed and precipitated, refining ferrite grains and fully utilizing precipitation strengthening to obtain a uniform and fine ferrite-pearlite (FP) structure, wherein the volume fraction of pearlite is 3% to 4%, 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] C: is the most important element in steel after iron. It directly affects the strength, plasticity, toughness and weldability of steel. C is the most economical element for improving the strength of steel. However, as the C content increases, the toughness and weldability of steel gradually deteriorate. Since the product of the present invention is used in joints in the construction industry after welding, a low carbon content design is the basic guarantee for ensuring that the steel for pipe piles has excellent strength, toughness and good weldability. In the present invention, if the C content is higher than 0.079%, the toughness and weldability of the finished product are impaired. If the C content is lower than 0.069%, the amount of Nb(CN) formed is small, and the beneficial effect of grain refinement is reduced. Therefore, the present invention controls 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 1.23% to 1.29%.

[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] Ti: A strong nitride-forming element, TiN is difficult to decompose even at high temperatures. Therefore, stable, fine TiN particles can effectively prevent austenite growth during the reheating process and improve the impact toughness of the weld heat-affected zone. Furthermore, Ti binds more strongly to S than Mn, thus reducing the MnS content to a certain extent and thus improving the material's impact toughness. However, excessive Ti content can form enlarged carbonitrides, which in turn reduces toughness. Appropriate finishing and coiling temperatures are required to minimize the precipitated TiC, which promotes precipitation strengthening. Therefore, the addition of Ti in the present invention can compensate for the loss of toughness caused by an insufficient compression ratio. Excessive Ti content can easily form large, sharp-edged TiN, which in turn reduces toughness. Therefore, the Ti content in the present invention is 0.034% to 0.043%.

[0021] 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 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 0.31% to 0.39%.

[0022] 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%.

[0023] 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%.

[0024] 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%.

[0025] A production method for producing thick-gauge 430MPa high-toughness pipe pile steel from thin billets, wherein the production process involves: molten iron pretreatment - converter smelting - refining outside the furnace (LF+Ca treatment) - continuous casting - slab heating - rolling - ultra-fast cooling - coiling.

[0026] in:

[0027] 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;

[0028] 2) Rolling process: The continuous casting slab is heated to 1161-1172°C in a walking beam heating furnace to dissolve all Nb and Ti, which plays a role of precipitation strengthening in the subsequent rolling process. The final rolling temperature of the rough rolling is 1002-1010°C, and the slab is directly subjected to finishing rolling without waiting for the temperature to rise. The final rolling temperature of the finishing rolling is 881-893°C. Subsequently, the slab is cooled to 470-483°C at a speed of 35.0-36.1°C / s by ultra-fast cooling, and then coiled. Finally, the slab is air-cooled to room temperature. The higher cooling rate of ultra-fast cooling is used to obtain a lower coiling temperature, so that TiC is finely dispersed and precipitated, and the ferrite grains are refined, which can give full play to the precipitation strengthening effect of TiC.

[0029] The 430MPa 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 430-450MPa, the tensile strength is ≥510MPa, the elongation after fracture is ≥21%; the Charpy impact energy (average value of 3 samples) at -60℃ is A kv ≥196J.

[0030] The finished steel plate thickness is 20 to 24 mm.

[0031] 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 430MPa 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:

[0032] 1) The present invention uses 135-170 mm thin slabs to roll thick gauge (20-24 mm) high-strength and toughness 430 MPa grade steel coils for pipe piles, which can improve production efficiency and save production resources;

[0033] 2) In terms of alloy design, the present invention adopts a C-Mn-Nb-Ti-Cr alloy design. The low C and low Si design ensures excellent weldability, the addition of Mn delays the occurrence of pearlite transformation, thereby improving 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. The addition of Ti, on the one hand, reduces the content of type A inclusions, thereby improving its toughness, and on the other hand, combines the rolling process to exert the precipitation strengthening effect of TiC. Economic and reasonable alloy design is the basic guarantee for the product to have excellent strength and toughness.

[0034] 3) In terms of rolling process, combined with the above alloy design, the continuous casting slab is heated to 1161-1172°C. Under the premise of fully dissolving Nb and Ti, the continuous casting slab has the finest original austenite structure possible, which can play a role in precipitation strengthening in the subsequent rolling and cooling process. The final rolling temperature of rough rolling is 1002-1010°C, and the slab directly enters the finishing rolling without waiting for warming to improve production efficiency. The final rolling temperature of finishing rolling is 881-893°C, and then ultra-fast cooling is adopted at a speed of 35.0-36.1°C / s to 470-483°C for coiling. The high cooling rate ensures the fine and dispersed distribution of precipitated TiC. The low-temperature coiling further refines the ferrite grains, ensuring that the product has excellent strength and toughness indicators.

[0035] 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 steel plate has a yield strength of 430-450 MPa, a tensile strength of ≥510 MPa, and an elongation after fracture of ≥21%; the -60°C Charpy impact energy (average value of 3 specimens) Akv ≥196 J. DETAILED DESCRIPTION

[0036] 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.

[0037] 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.

[0038] Table 1 Chemical composition of steel according to example (wt, %)

[0039]

[0040]

[0041] Table 2 Example steel process system

[0042]

[0043] Table 3 Main mechanical properties of example steel

[0044]

[0045]

Claims

1. A method for producing thick-gauge 430MPa 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%-0.079%, Si 0.10%-0.20%, Mn 1.23%-1.29%, P≤0.020%, S≤0.015%, Als 0.015%-0.030%, Nb 0.032%-0.042%, Ti 0.036%-0.043%, Cr 0.31%-0.33%, and the rest is Fe and unavoidable impurities; The structure of steel is ferrite-pearlite; The method for producing thick-gauge 430MPa high-toughness pipe pile steel from thin billets comprises: The thickness of the continuous casting slab is 135-170mm. The continuous casting slab is heated to 1161-1172℃ in a heating furnace, and then rolled in two stages of rough rolling and finishing rolling units. The final rolling temperature of rough rolling is 1002-1010℃; it enters finishing rolling directly without waiting for temperature, and the final rolling temperature of finishing rolling is 881-893℃. Then, it is finally cooled to 470-483℃ at a speed of 35.0-36.1℃ / s by laminar cooling, and then coiled, and finally air-cooled to room temperature.

2. The method for producing thick-gauge 430MPa high-toughness pipe pile steel from thin billets according to claim 1, characterized in that: Steel plate yield strength 430~450MPa, tensile strength ≥510MPa, elongation after fracture ≥21%; -60℃ Charpy impact energy A kv ≥196J.

3. The method for producing thick-gauge 430MPa 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.

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