A method for producing a low yield ratio high toughness 780mpa grade bridge steel

Abstract

This invention discloses a method for producing 780MPa grade bridge steel with low yield strength ratio and high toughness. The chemical composition of the steel (by mass percentage) is: C = 0.05~0.06%, Si = 0.1%~0.2%, Mn = 1.30%~1.40%, P ≤ 0.006%, S ≤ 0.002%, Nb = 0.015%~0.025%, Ti ≤ 0.008%, Al... T =0.02%~0.05%, Cr=0.4%~0.5%, Ni=0.5%~0.6%, Cu=0.5%~0.6%, Mo=0.2%~0.3%, B=0.0035%~0.0040%, N≤30PPM, the remainder being Fe and unavoidable impurities, with a carbon equivalent Ceq≤0.50% and a crack sensitivity index Pcm≤0.23%. The process steps include: slab heating temperature of 1120~1150℃, two-stage rolling, finishing rolling temperature of 840~880℃ in the two-phase region; followed by online quenching with DQ to 260~300℃, and then tempering heat treatment at 500℃ to obtain a 780MPa grade bridge steel with low yield strength ratio and high toughness of lath bainitic + martensitic structure.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology and relates to a production method of 780MPa grade bridge steel with low yield strength ratio and high toughness. Background Technology

[0002] While traditional high-strength steels offer increased strength, their yield strength ratio (yield strength / tensile strength) often exceeds 0.90. This results in insufficient plastic deformation capacity after yielding, limiting seismic and fracture resistance, and making it difficult to meet the safety requirements of long-span bridges. Complex projects such as cross-sea bridges, heavy-haul railway bridges, and urban viaducts place even higher demands on steel, driving the development of tensile strength towards 690MPa and above, while also requiring a low-temperature impact toughness of ≥200J at -40℃.

[0003] The core challenge of high-grade bridge steel lies in the multi-objective synergistic optimization of strength, toughness, yield strength ratio, and weldability. Therefore, high-grade, low-yield-strength-ratio, and weldable bridge steel is a crucial development direction in the current bridge construction field. Technological breakthroughs and widespread application are of great significance for improving the safety and economy of bridge engineering. With the increasing number of long-span, heavy-load bridge projects, steel needs to achieve lightweight design by increasing strength, reducing material usage, and lowering structural self-weight.

[0004] Chinese patent CN201210078314.4 discloses "a high-strength bridge steel and its manufacturing method", which uses TMCP + tempering process to produce steel plates with a yield strength of up to 650MPa and a tensile strength of up to 780MPa. Therefore, it does not meet the strength requirements of 780MPa grade bridge steel.

[0005] Chinese patent CN201610064771.6 discloses "Ultra-low cost 800MPa grade high toughness and excellent weldability steel plate and its manufacturing method", which uses TMCP+tempering process to produce steel plate with yield strength ratio >0.91.

[0006] Chinese patent CN201210078314.4 discloses an "80 kg-class ultra-high toughness, extremely thick steel plate and its manufacturing method", which uses the TMCP + double quenching + tempering (TMCP + QQT) method to produce low carbon high strength low alloy steel, with a yield strength ratio generally above 0.91.

[0007] The aforementioned patented technologies indicate that the current TMCP+tempering or quenching and tempering process is quite challenging in producing steel plates with a strength of 780MPa and a low yield strength ratio. Further exploration of low-cost microalloying and low-cost process design is needed in the future. Summary of the Invention

[0008] The purpose of this invention is to provide a production method for Q780qE bridge steel with low yield strength ratio and high toughness, producing a high-grade bridge steel with low cost and excellent strength, toughness, low yield strength ratio, and weldability. The steel has a yield strength of 790~820MPa, tensile strength of 950~980MPa, elongation A≥16%, yield strength ratio ≤0.84, and Charpy impact energy ≥270J at -60℃. It has a ductile-brittle transition point of -70~-80℃ and exhibits low yield strength ratio, high strength and toughness, and high weldability.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] A method for producing 780MPa grade bridge steel with low yield strength ratio and high toughness, wherein the chemical composition of the steel is as follows (mass percentage): C = 0.05~0.06%, Si = 0.1%~0.2%, Mn = 1.30%~1.40%, P ≤ 0.006%, S ≤ 0.002%, Nb = 0.015%~0.025%, Ti ≤ 0.008%, Al... T =0.02%~0.05%, Cr=0.4%~0.5%, Ni=0.5%~0.6%, Cu=0.5%~0.6%, Mo=0.2%~0.3%, B=0.0035%~0.0040%, N≤30PPM, the remainder being Fe and unavoidable impurities, and the carbon equivalent Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15≤0.50%, crack sensitivity index Pcm=C+Si / 30+(Mn+Cu+Cr) / 20+Ni / 60+Mo / 15+V / 10+5B≤0.23%; including the following process steps:

[0011] 1) Heating: The continuously cast billet obtained after batching, smelting, and continuous casting according to the designed chemical composition mass percentage of the steel is heated to 1120~1150℃ and held at that temperature for 8~10 hours. After heating, the surface iron oxide scale is removed by high-pressure water.

[0012] 2) Rolling: Two-stage controlled rolling is adopted. After 5 passes of roughing mill rolling, 7 passes of finishing mill rolling are performed. The roughing mill opening temperature is ≥1000℃ and the roughing mill closing temperature is ≥900℃. The finishing mill opening temperature is 880-900℃ and the finishing mill closing temperature is 840~880℃.

[0013] 3) Online quenching: Immediately after rolling, MuPiC is used for online quenching at a cooling rate of 25~35℃ / s, cooling to 260~300℃;

[0014] 4) Tempering: The online quenched steel plate is subjected to tempering heat treatment. It is heated to 500℃ in a tempering furnace and held for 45~60min to obtain a boron-containing, low yield strength ratio, high toughness 780MPa grade bridge steel with a lath bainitic + martensitic structure. The steel has a yield strength of 790~820MPa, tensile strength of 950~980MPa, elongation A≥16%, yield strength ratio≤0.84, and Charpy impact energy ≥270J at -60℃. It is a bridge steel with a ductile-brittle transition point of -70~-80℃, exhibiting a low yield strength ratio, high strength and toughness, and high weldability.

[0015] Invention principle:

[0016] The mechanism of action of each alloying element in the boron-containing, low yield strength ratio, high toughness 780MPa grade bridge steel of the present invention is as follows:

[0017] Carbon (C), as the most economical and basic strengthening element in steel, plays a significant role in improving steel strength through solid solution strengthening and precipitation strengthening. Low carbon content can reduce center segregation in cast billets; however, excessively high carbon content adversely affects the elongation, toughness, and weldability of steel, and also easily causes alloying element carbides to precipitate at grain boundaries, thus reducing the steel's corrosion resistance. Considering the strength, toughness, and yield strength ratio requirements of Q780MPa grade bridge steel, as well as the range of the two-phase region DQ quenching process, this invention controls the C content to 0.05~0.06%.

[0018] Si can improve the fluidity of molten steel, but excessive Si content is not conducive to welding performance. This invention controls Si at 0.1%~0.2%.

[0019] Mn can significantly improve the strength and toughness of steel through solid solution strengthening, making it the most important and economical strengthening element to compensate for the strength loss caused by the reduction of carbon content. However, excessive Mn can promote the formation of carbides and large MnS inclusions, reducing the toughness of the steel. Therefore, this invention controls the Mn content at 1.30~1.40%.

[0020] Nitrogen (Nb) primarily increases the recrystallization temperature of austenite, thereby refining the austenite grains. Even trace amounts of Nb are sufficient to give steel excellent overall properties, while excessively high Nb content can lead to mixed crystal formation. Therefore, this invention controls the Nb content to 0.015~0.025%.

[0021] The addition of Cr and the increase of its content can improve the strength and toughness of steel plates, the strength of welds, and the toughness of the heat-affected zone. However, excessive Cr content can lead to a decrease in the weldability of the steel, and the segregation of Cr carbides at grain boundaries can easily lead to a decrease in the sulfur resistance of the steel. Therefore, this invention controls the Cr content at 0.4~0.5%.

[0022] Mo can improve the hardenability and strength of steel and prevent temper brittleness, but it is relatively expensive. Therefore, the present invention controls the Mo content to be between 0.2% and 0.3%.

[0023] Ni can improve the hardenability and low-temperature toughness of steel, but as the Ni content increases, the yield strength to tensile strength of the steel increases more rapidly, which is not conducive to controlling the low yield strength ratio. Therefore, this invention controls the Ni content at 0.5~0.6%.

[0024] Cu can improve the strength of steel, and nickel-copper composites can improve the stability of materials. Therefore, the Cu content is controlled at 0.5-0.6% in this invention.

[0025] Boron (B) increases the hardenability of steel, thus saving precious alloys such as Ni and Mo. However, B tends to promote temper brittleness. Therefore, the B content in this invention is controlled at 0.035~0.040%.

[0026] Ti can form fine and stable precipitates, but excessive Ti content leads to the formation of coarse carbonitrides, reducing the strength and toughness of the material and increasing its cost. Therefore, this invention controls the Ti content to below 0.008%.

[0027] Al is an indispensable deoxidizer in steelmaking. Adding an appropriate amount of Al to boron-containing steels can reduce BN formation and increase the effective boron content in the molten steel. Therefore, this invention controls the Al content to be between 0.02% and 0.05%.

[0028] P and S are unavoidable impurity elements in steel, and the primary consideration is the low-temperature toughness of the highest-grade bridge steel. Therefore, this invention controls the P and S contents to be P≤0.006% and S≤0.002%, respectively.

[0029] Controlling the cold cracking sensitivity index Pcm is beneficial to ensuring the weldability of products. The Pcm index of this invention... cm The concentration should be controlled at ≤0.23%.

[0030] Controlling the carbon equivalent (Ceq) index is beneficial to ensuring the weldability of the product. The C of this invention... eq The concentration should be controlled at ≤0.50%.

[0031] The beneficial effects of this invention are:

[0032] Strength and hardenability are improved by adding low-cost boron (B) elements, while reducing the addition of Cr and Mo elements.

[0033] With the reduction of Cr and Mo as strength elements, less Ni can be added. At the same time, by controlling the Ti and N elements, the precipitation of TiN particles can be reduced, making the steel plate more durable at low temperatures.

[0034] The reduction in the addition of alloying elements saves costs, lowers the weld sensitivity coefficient Pcm, and improves weldability. Attached Figure Description

[0035] Figure 1 The image shows the microstructure of bridge steel with low yield strength ratio and high toughness (780MPa grade) produced in Example 1. The microstructure in the image is lath bainite + martensite. Detailed Implementation

[0036] The following examples further illustrate this point.

[0037] Examples 1-4: A production method for boron-containing, low yield strength ratio, high toughness 780MPa grade bridge steel. The chemical composition (mass percentage), carbon equivalent (Ceq), and crack sensitivity index (Pcm) of the steel are shown in Table 1. The remainder consists of Fe and unavoidable impurities. The steel is batched, smelted, and continuously cast according to the designed chemical composition (mass percentage) to obtain a continuously cast billet. The key process steps include:

[0038] 1) Heating: The continuously cast billet is heated to 1120~1150℃ and held for 8~10 hours. After heating, the surface iron oxide scale is removed by high-pressure water.

[0039] 2) Rolling: Two-stage controlled rolling is adopted. After 5 passes of roughing mill rolling, 7 passes of finishing mill rolling are performed. The roughing mill opening temperature is ≥1000℃ and the roughing mill closing temperature is ≥900℃. The finishing mill opening temperature is 880-900℃ and the finishing mill closing temperature is 840~880℃.

[0040] 3) Online quenching: Immediately after rolling, MuPiC is used for online quenching at a cooling rate of 25~35℃ / s, cooling to 260~300℃;

[0041] 4) Tempering: The online quenched steel plate is tempered by heating it to 500℃ in a tempering furnace and holding it for 45~60 minutes.

[0042] The process parameters for the heating, rolling, online quenching, and tempering steps described above are shown in Table 2. The mechanical property test results of the obtained steel are shown in Table 3.

[0043] Table 1 Chemical composition (wt%) of the examples

[0044] .

[0045] Table 2. Process parameters for heating, rolling, online quenching, and tempering in the examples.

[0046] .

[0047] Table 3 Mechanical property test results of the examples

[0048] .

[0049] Table 3 shows that the comprehensive properties of the pipeline steel prepared in the example are: yield strength of 790~820MPa, tensile strength of 950~980MPa, elongation A≥16%, yield strength ratio≤0.84, Charpy impact energy at -60℃≥270J, and bridge steel with low yield strength ratio, high strength and toughness and high weldability at a ductile-brittle transition point of -70~-80℃.

Claims

1. A method for producing 780MPa grade bridge steel with low yield strength ratio and high toughness, characterized in that: The steel's chemical composition by mass percentage is: C=0.05%, Si=0.1%, Mn=1.40%, P=0.005%, S=0.001%, Nb=0.015%, Cr=0.5%, Ti=0.006%, Alt=0.025%, Ni=0.5%, Cu=0.5%, Mo=0.2%, B=0.004%, with the remainder being Fe and unavoidable impurities. The carbon equivalent (Ceq) is 0.49%, and the crack sensitivity index (Pcm) is 0.22%. The key process steps include the following: 1) Heating: The continuously cast billet obtained after batching, smelting and continuous casting according to the designed chemical composition mass percentage of steel is heated to 1120℃ and held at that temperature for 9 hours. After heating, the surface iron oxide scale is removed by high pressure water. 2) Rolling: Two-stage controlled rolling is adopted. After 5 passes of roughing mill rolling, 7 passes of finishing mill rolling are rolled. The roughing mill starting temperature is 1050℃ and the roughing mill finishing temperature is 956℃. The finishing mill starting temperature is 900℃ and the finishing mill finishing temperature is 880℃. 3) Online quenching: Immediately after rolling, MULPIC online quenching is performed at a cooling rate of 25℃ / s, cooling to 300℃; 4) Tempering: The online quenched steel plate is subjected to tempering heat treatment. It is heated to 500℃ in a tempering furnace and held for 46 minutes to obtain a 780MPa grade bridge steel with low yield strength ratio and high toughness of lath bainite + martensite structure. The steel has a yield strength of 790MPa, a tensile strength of 950MPa, an elongation of A=21%, a yield strength ratio of 0.83, and Charpy average impact energy at -60℃, -70℃, and -80℃ of 278J, 201J, and 56J, respectively.

2. A method for producing 780MPa grade bridge steel with low yield strength ratio and high toughness, characterized in that: The steel's chemical composition by mass percentage is: C=0.06%, Si=0.15%, Mn=1.31%, P=0.004%, S=0.001%, Nb=0.018%, Cr=0.42%, Ti=0.005%, Alt=0.045%, Ni=0.6%, Cu=0.55%, Mo=0.3%, B=0.0035%, with the remainder being Fe and unavoidable impurities. The carbon equivalent (Ceq) is 0.50%, and the crack sensitivity index (Pcm) is 0.23%. The key process steps include the following: 1) Heating: The continuously cast billet obtained after batching, smelting and continuous casting according to the designed chemical composition mass percentage of steel is heated to 1150℃ and held for 8 hours. After heating, the surface iron oxide scale is removed by high pressure water. 2) Rolling: Two-stage controlled rolling is adopted. After 5 passes of roughing mill rolling, 7 passes of finishing mill rolling are rolled. The roughing mill starting temperature is 1062℃ and the roughing mill finishing temperature is 946℃. The finishing mill starting temperature is 880℃ and the finishing mill finishing temperature is 860℃. 3) Online quenching: Immediately after rolling, MULPIC online quenching is performed at a cooling rate of 30℃ / s, cooling to 290℃; 4) Tempering: The online quenched steel plate is subjected to tempering heat treatment. It is heated to 500℃ in a tempering furnace and held for 60 minutes to obtain a 780MPa grade bridge steel with low yield strength ratio and high toughness of lath bainite + martensite structure. The steel has a yield strength of 820MPa, a tensile strength of 980MPa, an elongation of A=19%, a yield strength ratio of 0.84, and Charpy average impact energy at -60℃, -70℃, and -80℃ of 270J, 213J, and 65J, respectively.

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