420 MPa grade extremely cold environment bridge steel and production method thereof

Abstract

The invention discloses 420 MPa grade steel for bridges in an extremely cold environment and a production method of the 420 MPa grade steel. The steel comprises the following chemical components in percentage by mass: 0.04 to 0.07 percent of C, 0.20 to 0.30 percent of Si, 1.55 to 1.70 percent of Mn, less than or equal to 0.006 percent of P, less than or equal to 0.002 percent of S, 0.030 to 0.045 percent of Nb, less than or equal to 0.008 percent of Ti, 0.02 to 0.05 percent of Alt, 0.15 to 0.30 percent of Cr, 0.20 to 0.30 percent of Ni, less than or equal to 0.20 percent of Cu, less than or equal to 0.0005 percent of B and less than or equal to 30ppm of N. And through low alloying component design and precise TMCP process control, the low-temperature toughness at the temperature of-60 DEG C, the welding performance and the production stability are remarkably improved on the basis that the 420 MPa grade strength is guaranteed. The thickness of a finished steel product is 10-80 mm, the yield strength is 450-510 MPa, the tensile strength is 560-640 MPa, the elongation A is larger than or equal to 22%, the yield ratio is smaller than or equal to 0.83, the Charpy impact energy of the whole thickness at the temperature of-60 DEG C is larger than or equal to 200 J, and the ductile-brittle transition point ranges from-70 DEG C to-80 DEG C.

Description

Technical Field

[0001] This invention belongs to the field of bridge steel technology, specifically relating to a production method of 420MPa grade bridge steel for extremely cold environments. Background Technology

[0002] With the increasing complexity of global climate conditions and the extension of infrastructure construction to high-latitude and high-altitude regions, the demand for large-scale bridge projects in extremely cold environments (typically referring to ambient temperatures that are consistently below -40°C, and in extreme cases below -60°C) has increased significantly. These bridges must withstand long-term low temperatures, freeze-thaw cycles, dynamic loads, and potential seismic forces, placing stringent requirements on the comprehensive performance of the key structural material—bridge steel. It must not only possess high yield strength (e.g., 420MPa) to meet load-bearing requirements, but also maintain excellent low-temperature impact toughness (e.g., impact energy ≥120J at -60°C or lower), while simultaneously ensuring good weldability, fatigue resistance, and corrosion resistance to guarantee the safety and reliability of the bridge throughout its entire lifespan.

[0003] Chinese patent CN 114032460 A discloses a low yield-to-tensile ratio bridge steel and its production method, with a yield strength ≥436MPa, tensile strength ≥545MPa, yield-to-tensile ratio ≤0.80, longitudinal impact energy at -40℃ ≥120J, and elongation A ≥22.5%. The drawback is that this process produces plate coils, which significantly limits the thickness and width of finished products compared to those produced by medium-thick plate rolling mills. The produced thickness is only 18mm, and the impact toughness at -40℃ only reaches grade E.

[0004] Chinese patent CN 115807192 A discloses a 420MPa yield strength grade TMCP structural steel and its production method. The produced steel has a carbon content controlled at 0.04%~0.06%, a carbon equivalent Ceq≤0.40%, Pcm≤0.18%, a yield strength ≥420MPa, and a tensile strength of 520~680MPa. It also exhibits good impact toughness at -20℃. However, the limitations are that the production thickness is only 8~40mm, and there is no valid evidence to suggest good impact toughness at lower temperatures such as -60℃. Furthermore, the uniformity of microstructure and the stability of impact toughness for thicker specifications (>40mm) are not clearly verified. In actual production of medium-thickness products, issues such as final rolling temperature deviation and inconsistent cooling rates can easily lead to coarsening of the core microstructure, significantly reducing impact energy and making it difficult to guarantee service safety in extremely cold environments.

[0005] Chinese patent CN 114134408 A discloses a 460MPa grade bridge steel plate and its manufacturing method, wherein the alloying elements are Cr=0.45%~0.80%, Cu=0.25%~0.50%, Ni=0.01%~1.00%, and Mo=0.02%~0.60%. The excessive addition of Cr, Cu, Ni, and Mo not only increases smelting and rolling costs but may also exacerbate microstructure inhomogeneity due to segregation risk, leading to a higher scrap rate. Given that the demand for steel in extremely cold environments has not yet seen a large-scale boom, this high-cost formulation is difficult for enterprises to widely adopt, limiting the industrialization and market promotion of the technology.

[0006] In summary, existing technologies for 420MPa grade bridge steel still have significant shortcomings in terms of low-temperature toughness coverage in extremely cold environments, production capacity for medium and thick plates, cost control, and weld performance stability. Therefore, there is an urgent need to develop a production method for 420MPa grade bridge steel in extremely cold environments that balances -60℃ low-temperature toughness, production capacity for a wide range of medium and thick plates, low alloy costs, and high process stability. This will help overcome existing technological bottlenecks and support the safe construction and long-term service of major bridge projects in extremely cold regions. Summary of the Invention

[0007] To address the problems of insufficient low-temperature toughness at -60℃, production limitations for medium-thickness plates >40mm, excessively high alloy costs, weak weldability, and poor batch production stability of 420MPa grade bridge steel for extremely cold environments in the existing technology, this invention aims to provide a production method for 420MPa grade bridge steel for extremely cold environments. Through composition optimization design and coordinated control of controlled rolling and controlled cooling TMCP process, stable production of high-toughness, wide-specification medium-thick plates at -60℃ can be achieved, while reducing manufacturing costs and improving weldability, thus meeting the demand for key structural materials in long-span bridge projects in extremely cold regions.

[0008] The technical solution of this invention: A type of 420MPa grade bridge steel for extremely cold environments, with the following chemical composition by mass percentage: C = 0.04%~0.07%, Si = 0.20%~0.30%, Mn = 1.55%~1.70%, P ≤ 0.006%, S ≤ 0.002%, Nb = 0.030%~0.045%, Ti ≤ 0.008%, Alt = 0.02%~0.05%, Cr = 0.15%~0.30%, Ni = 0.20%~0. 0.30%, Cu≤0.20%, B≤0.0005%, N≤30ppm, the remainder being Fe and unavoidable impurities, with carbon equivalent Ceq≤0.38% and crack sensitivity index Pcm≤0.18%; the steel has a yield strength of 450~510MPa, tensile strength of 560~640MPa, elongation A≥22%, yield strength ratio≤0.83, Charpy impact energy at -60℃ at full thickness≥200J, and ductile-brittle transition point at -70~-80℃; A method for producing 420MPa grade bridge steel for extremely cold environments includes the following process steps: 1) Smelting: The raw materials are batched according to the chemical composition design and then smelted in a top and bottom blowing converter. The endpoint [C]×[O]≤0.0020 is controlled. The ladle is refined and vacuum degassed. After vacuum refining, the molten steel is fed with Mg cored wire. The Mg wire feeding amount is 2-5m / ton of steel and the feeding speed is controlled at 150-200m / min. After feeding, the molten steel is soft blown and allowed to stand for 5-15 minutes to obtain high-purity molten steel. 2) Continuous casting: The molten steel is cast into a plate-shaped continuous casting billet. The continuous casting process uses low superheating, with a superheat of 8~20℃. Dynamic light reduction technology is adopted to ensure that the center segregation of the billet is within 1.0 of category C. 3) Heating: The slab is made with a compression ratio of ≥4 and heated to 1100~1200℃. The furnace temperature during the slab heating process does not exceed 1200℃ and the slab is heated for 1~2 hours. After heating, the iron oxide scale on the surface is removed by high pressure water. 4) Rolling: In the roughing stage, the intermediate billet thickness is controlled to be ≥ finished product thickness + 60mm, with an initial rolling temperature ≥ 1050℃ and a final rolling temperature ≥ 980℃, and a cumulative reduction rate of ≥ 50% in the roughing stage; in the finishing stage, temperature is controlled according to thickness gradients: for 10mm ≤ finished product thickness ≤ 40mm, the initial rolling temperature is 880~1000℃ and the final rolling temperature is 780~850℃; for 40mm < finished product thickness ≤ 80mm, the initial rolling temperature is 800~860℃ and the final rolling temperature is 770~820℃; the cumulative reduction rate in the finishing stage is ≥ 60%. 5) Online ultra-fast cooling: MULPIC online ultra-fast cooling is adopted immediately after rolling, with a cooling rate of 5~15℃ / s and an initial cooling temperature of 730~780℃; for finished product thickness of 10mm≤40mm, the reddening temperature is 500~700℃; for finished product thickness of 40mm<80mm, the reddening temperature is 400~500℃. 6) Tempering: Heat to 450±10℃ in a tempering furnace and hold for 15~50min to eliminate rolling stress and refine the core structure, thus obtaining 420MPa grade bridge steel with acicular ferrite + granular bainite structure.

[0009] Where: Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15; Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B.

[0010] The innovative aspects and beneficial effects of this invention: (1) The composition design of “low C + multi-element micro-alloying (Nb, Cr, Ni, etc.)” is adopted. Different finishing rolling temperature and ultra-fast cooling reddening temperature process parameters are set for different finished product thicknesses (10-80mm) to ensure that the core and surface structure are uniformly refined. Combined with the tempering process to eliminate rolling stress and refine the core structure, a fine grain structure with acicular ferrite + granular bainite as the main components is obtained. The Charpy impact energy at -60℃ is ≥200J at full thickness (10-80mm) (far exceeding the requirement of ≥120J at the 1 / 4 position of thickness at -40℃ or -60℃). The ductile-brittle transition point is as low as -70~-80℃. It has extremely strong resistance to brittle fracture during long-term service in extremely cold regions (such as below -60℃), filling the performance gap of existing 420MPa grade steel in ultra-low temperature toughness. (2) In the smelting process, by using low [C]×[O] (≤0.0020), feeding Mg cored wire and soft blowing and settling, the size of inclusions is refined (≤10μm) and the distribution morphology is improved (sphericity ≥90%). The continuous casting adopts low superheat (8~20℃) + dynamic light reduction technology, and the C-type segregation of the billet center is ≤1.0, which reduces the toughness defects caused by inclusions from the source. The product flaw detection qualification rate is ≥99.5%, which meets the stringent requirements of material purity for key parts of the bridge. (3) Carbon equivalent Ceq≤0.38% and crack sensitivity index Pcm≤0.18% (both better than the conventional level of existing technology), combined with Mg treatment to improve the morphology of inclusions (reducing the brittle phase source in the weld heat-affected zone), the impact energy of the weld heat-affected zone (HAZ) at -40℃ is ≥100J, there is no risk of cold cracking in the on-site welded joint, meeting the requirements of low-temperature welding construction in extremely cold regions, and significantly reducing the difficulty of bridge construction. Attached Figure Description

[0011] Figure 1 This is a microstructure diagram of bridge steel designed for extremely cold environments (420 MPa). The microstructure shown is a fine-grained structure dominated by acicular ferrite and granular bainite. Detailed Implementation

[0012] The present invention will be further described below with reference to specific embodiments.

[0013] The chemical composition of steels in Examples 1, 2, 3, and 4 is shown in Table 1, with the remainder being Fe and unavoidable impurities; the process parameters for heating, rolling, online ultra-rapid cooling, and tempering are shown in Table 2; the process includes the following steps: 1) Smelting: According to the chemical composition of the 420MPa grade bridge steel for extremely cold environments, the raw materials are batched and then smelted in a top and bottom blowing converter. The endpoint [C]×[O]≤0.0020 is controlled. The steel is then refined outside the furnace and degassed in vacuum. After vacuum refining, the molten steel is fed with Mg cored wire. The amount of Mg wire fed is 2-5m / ton of steel and the feeding speed is controlled at 150-200m / min. After feeding, the molten steel is soft blown and allowed to stand for 5-15min to improve the morphology, size and distribution of inclusions in the molten steel and obtain high-purity molten steel. 2) Continuous casting: The molten steel is cast into a plate-shaped continuous casting billet. The continuous casting process is carried out with low superheat, with a superheat of 8~20℃. Appropriate cooling water distribution and dynamic light reduction technology are adopted to ensure that the center segregation of the billet is within 1.0 of the C category, thereby reducing the impact of central porosity and shrinkage defects on the subsequent steel plate performance. 3) Heating: The slab is made with a compression ratio of ≥4 and heated to 1100~1200℃. The furnace temperature during the slab heating process does not exceed 1200℃ and the slab is heated for 1~2 hours. After heating, the iron oxide scale on the surface is removed by high pressure water. 4) Rolling: In the roughing stage, the intermediate billet thickness is controlled to be ≥ finished product thickness + 60mm, the initial rolling temperature is ≥1050℃, the final rolling temperature is ≥980℃, and the cumulative reduction rate is ≥50%; in the finishing stage, the temperature is controlled according to the thickness gradient (10mm≤finished product thickness≤40mm, finishing rolling initial rolling temperature 880~1000℃, finishing rolling final rolling temperature 780~850℃; 40mm<finished product thickness≤80mm, finishing rolling initial rolling temperature 800~860℃, finishing rolling final rolling temperature 770~820℃), and the cumulative reduction rate is ≥60%; 5) Online ultra-fast cooling: MULPIC online ultra-fast cooling is adopted immediately after rolling, with a cooling rate of 5~15℃ / s. The initial cooling temperature is 730~780℃. According to the thickness of the finished steel plate, for 10mm≤finished thickness≤40mm, the red-hot temperature is 500~700℃; for 40mm<finished thickness≤80mm, the red-hot temperature is 400~500℃. 6) Tempering: Heat to 450±10℃ in a tempering furnace and hold for 15~50 minutes.

[0014] The bridge steel prepared through the above process has the following comprehensive properties: yield strength of 450~510MPa, tensile strength of 560~640MPa, elongation A≥22%, yield strength ratio≤0.83, Charpy impact energy at -60℃ ≥200J, and ductile-brittle transition point at -70~-80℃. It is a bridge steel with a low yield strength ratio, high strength and toughness, and high weldability. The performance indicators of the steel are shown in Table 3.

[0015] Table 1 Chemical composition (wt%) of the steel in the examples .

[0016] Table 2. Process parameters for heating, rolling, online ultra-rapid cooling, and tempering in the examples. .

[0017] Table 3. Test results of mechanical properties of the steel in the examples. .

Claims

1. A type of 420MPa grade bridge steel for extremely cold environments, characterized in that: The chemical composition of the steel (by mass percentage) is: C = 0.04%~0.07%, Si = 0.20%~0.30%, Mn = 1.55%~1.70%, P ≤ 0.006%, S ≤ 0.002%, Nb = 0.030%~0.045%, Ti ≤ 0.008%, Alt = 0.02%~0.05%, Cr = 0.15%~0.30%, Ni = 0.20%~0.30%, Cu ≤ 0. The steel contains 0.20% B, ≤0.0005% N, and the remainder is Fe and unavoidable impurities. The carbon equivalent Ceq is ≤0.38%, and the crack sensitivity index Pcm is ≤0.18%. The yield strength of the steel is 450~510MPa, the tensile strength is 560~640MPa, the elongation A is ≥22%, the yield strength ratio is ≤0.83, the Charpy impact energy at -60℃ is ≥200J, and the ductile-brittle transition point is at -70~-80℃.

2. A method for producing 420MPa grade bridge steel for extremely cold environments, characterized in that... The process includes the following steps: 1) Smelting: The raw materials are batched according to the chemical composition design and then smelted in a top and bottom blowing converter. The endpoint [C]×[O]≤0.0020 is controlled. The ladle is refined and vacuum degassed. After vacuum refining, the molten steel is fed with Mg cored wire. The Mg wire feeding amount is 2-5m / ton of steel and the feeding speed is controlled at 150-200m / min. After feeding, the molten steel is soft blown and allowed to stand for 5-15 minutes to obtain high-purity molten steel. 2) Continuous casting: The molten steel is cast into a plate-shaped continuous casting billet. The continuous casting process uses low superheating, with a superheat of 8~20℃. Dynamic light reduction technology is adopted to ensure that the center segregation of the billet is within 1.0 of category C. 3) Heating: The slab is made with a compression ratio of ≥4 and heated to 1100~1200℃. The furnace temperature during the slab heating process does not exceed 1200℃ and the slab is heated for 1~2 hours. After heating, the iron oxide scale on the surface is removed by high pressure water. 4) Rolling: In the roughing stage, the intermediate billet thickness is controlled to be ≥ finished product thickness + 60mm, with an initial rolling temperature ≥ 1050℃ and a final rolling temperature ≥ 980℃, and a cumulative reduction rate of ≥ 50% in the roughing stage; in the finishing stage, temperature is controlled according to thickness gradients: for 10mm ≤ finished product thickness ≤ 40mm, the initial rolling temperature is 880~1000℃ and the final rolling temperature is 780~850℃; for 40mm < finished product thickness ≤ 80mm, the initial rolling temperature is 800~860℃ and the final rolling temperature is 770~820℃; the cumulative reduction rate in the finishing stage is ≥ 60%. 5) Online ultra-fast cooling: MULPIC online ultra-fast cooling is adopted immediately after rolling, with a cooling rate of 5~15℃ / s and an initial cooling temperature of 730~780℃; for finished product thickness of 10mm≤40mm, the reddening temperature is 500~700℃; for finished product thickness of 40mm<80mm, the reddening temperature is 400~500℃. 6) Tempering: Heat to 450±10℃ in a tempering furnace and hold for 15~50min to eliminate rolling stress and refine the core structure, thus obtaining 420MPa grade bridge steel with acicular ferrite + granular bainite structure.

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