Production method of 780MPa-grade bridge steel with low yield ratio and high toughness

Abstract

The invention discloses a production method of low-yield-ratio high-toughness 780MPa-grade steel for bridges. The steel comprises the following chemical components in percentage by mass: 0.05-0.06% of C, 0.1-0.2% of Si, 1.30-1.40% of Mn, less than or equal to 0.006% of P, less than or equal to 0.002% of S, 0.015-0.025% of Nb, less than or equal to 0.008% of Ti, 0.02-0.05% of AlT, 0.4-0.5% of Cr, 0.5-0.6% of Ni, 0.5-0.6% of Cu, 0.2-0.3% of Mo, 0.0035-0.0040% of B, less than or equal to 30PPM and the balance of Fe and inevitable impurities, the carbon equivalent Ceq is less than or equal to 0.50%, and the crack sensitivity index Pcm is less than or equal to 0.23%. The process comprises the following steps: the heating temperature of a plate blank is 1120-1150 DEG C, two-stage rolling is adopted, and the finish rolling temperature of finish rolling is in a two-phase region range of 840-880 DEG C; and then DQ online quenching is conducted to 260-300 DEG C, tempering heat treatment is conducted at 500 DEG C, and the low-yield-ratio and high-toughness 780 MPa grade bridge steel of a lath bainite and martensite structure is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy and relates to a production method of a 780MPa - grade bridge steel with a low yield - strength ratio and high toughness. Background Art

[0002] When traditional high - strength steels increase in strength, the yield - strength ratio (yield strength / tensile strength) often exceeds 0.90, resulting in insufficient plastic deformation ability after the steel yields, limited seismic and fracture - resistance performance, and it is difficult to meet the safety requirements of long - span bridges. Complex projects such as cross - sea bridges, heavy - load railway bridges, and urban viaducts have higher requirements for steel, promoting the development of tensile strength towards 690MPa and above, and at the same time, the low - temperature impact toughness at - 40°C should be ≥200J.

[0003] The core contradiction of high - grade bridge steel lies in the multi - objective collaborative optimization of "strength and toughness, yield - strength ratio, and weldability". Therefore, high - grade, low - yield - strength - ratio, and weldable bridge steel is an important development direction in the current bridge construction field. Its technological breakthrough and application promotion are of great significance for improving the safety and economy of bridge projects. With the increase in long - span and heavy - load bridge projects, steel needs to achieve lightweight design by increasing strength, reducing material usage, and lowering the structural self - weight.

[0004] Chinese Patent CN201210078314.4 discloses "A high - strength bridge steel and its manufacturing method", which produces steel plates by the TMCP + tempering process. The highest yield strength is 650MPa, and the highest tensile strength is 780MPa, so it cannot 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 plates and their manufacturing method", which produces steel plates by the TMCP + tempering process, and the yield - strength ratio > 0.91.

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

[0007] The above - disclosed patented technologies show that the current TMCP + tempering or quenching and tempering processes are difficult to produce steel plates with a strength of 780MPa and a low yield - strength ratio. In the future, it is necessary to further explore low - cost micro - alloying, low - cost process design, etc. Summary of the Invention

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

[0009] In order to achieve the above object, the technical solution of the present invention is as follows: A production method for a 780 MPa grade bridge steel with a low yield 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≤30 PPM, and the rest is Fe and inevitable impurities. And the carbon equivalent Ceq = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15≤0.50%, and the crack sensitivity index Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B≤0.23%; The method includes the following process steps: 1) Heating: The continuous casting billet obtained by batching, smelting and continuous casting according to the designed chemical composition mass percentage of the steel is heated to 1120~1150 °C and held for 8~10 h. After heating, the surface scale is removed by high-pressure water. 2) Rolling: Two-stage controlled rolling is adopted. After 5 passes of rolling on the roughing mill, then 7 passes of rolling on the finishing mill; The roughing starting rolling temperature≥1000 °C, and the roughing final rolling temperature≥900 °C; The finishing starting rolling temperature is 880 - 900 °C, and the finishing final rolling temperature is 840~880 °C; 3) Online quenching: Immediately after rolling, MuPiC online quenching is adopted, and the cooling rate is 25~35 °C / s, and it is cooled to 260~300 °C. 4) Tempering: The on-line quenched steel plate is subjected to tempering heat treatment, heated to 500 °C in a tempering furnace, and held for 45 - 60 min to obtain a boron-containing low yield ratio and high toughness 780 MPa grade bridge steel with a lath bainite + martensite structure. The yield strength of the steel is 790 - 820 MPa, the tensile strength is 950 - 980 MPa, the elongation A ≥ 16%, the yield ratio ≤ 0.84, and the Charpy impact energy at -60 °C ≥ 270 J. A bridge steel with a low yield ratio, high strength and toughness, and high weldability, with a ductile-brittle transition point at -70 to -80 °C.

[0010] Principle of the invention: The action mechanisms of the alloying elements in the boron-containing low yield ratio and high toughness 780 MPa grade bridge steel of the present invention are as follows: As the most economical and basic strengthening element in steel, C has an obvious effect on improving the strength of steel through solid solution strengthening and precipitation strengthening. A low carbon content can reduce the center segregation of the continuous casting billet; however, an excessive carbon content has an adverse effect on the elongation, toughness, and weldability of the steel, and at the same time, it is easy for alloy carbide to precipitate at the grain boundary, resulting in a decrease in the corrosion resistance of the steel grade. Considering the requirements of strength, toughness, and yield ratio for Q780 MPa grade bridge steel, as well as the DQ quenching process range in the two-phase region, therefore, the C content in the present invention is controlled at 0.05 - 0.06%.

[0011] Si can improve the fluidity of molten steel, but an excessive Si content is not conducive to welding performance. In the present invention, Si is controlled at 0.1% - 0.2%.

[0012] Mn can significantly improve the strength and toughness of the steel grade through solid solution strengthening, and it is the most important and economical strengthening element to compensate for the strength loss caused by the decrease in C content. However, an excessive Mn will promote the formation of carbides and at the same time form large-sized MnS inclusions, reducing the toughness of the steel grade. Therefore, the Mn content in the present invention is controlled at 1.30 - 1.40%.

[0013] Nb mainly increases the recrystallization temperature of austenite, thereby achieving the purpose of refining austenite grains. A trace amount of Nb is sufficient to enable the steel to obtain excellent comprehensive properties. An excessive Nb content causes the steel to be prone to mixed grains. Therefore, the Nb content in the present invention is controlled at 0.015 - 0.025%.

[0014] The addition of Cr element and the increase of its content have a certain improvement effect on the strength and toughness of the steel plate, the strength of the weld, and the toughness of the heat affected zone. However, an excessive Cr content will lead to a decrease in the welding performance of the steel grade, and at the same time, the segregation of Cr carbide at the grain boundary is prone to cause a decrease in the sulfur resistance of the steel grade. Therefore, the Cr content in the present invention is controlled at 0.4 - 0.5%.

[0015] Mo can improve the hardenability of steel, increase strength, and prevent temper brittleness, but it is relatively expensive. Therefore, in this invention, the Mo content is controlled within 0.2 - 0.3%.

[0016] Ni can improve the hardenability and low-temperature toughness of steel. However, as the Ni content increases, the yield strength of the steel increases faster than the tensile strength, which is not conducive to the control of a low yield ratio. Therefore, in this invention, the Ni content is controlled within 0.5 - 0.6%.

[0017] Cu can increase the strength of steel, and the nickel-copper composite material can improve the material stability. Therefore, in this invention, the Cu content is controlled within 0.5 - 0.6%.

[0018] B increases the hardenability of steel, thus saving precious alloys such as Ni and Mo. However, B has a tendency to promote temper brittleness. Therefore, in this invention, the B content is controlled within 0.035 - 0.040%.

[0019] Ti: It can form fine and stable precipitation phases. However, if the Ti content is too high, coarse carbonitrides will be generated, reducing the strength and toughness of the material and increasing the material cost at the same time. Therefore, in this invention, the Ti content is controlled below 0.008%.

[0020] Al is an essential deoxidizer in steelmaking. Adding an appropriate amount of Al to steel grades containing B can reduce the generation of BN and increase the effective B in the molten steel. Therefore, in this invention, the Alt content is controlled within 0.02 - 0.05%.

[0021] P and S are inevitable impurity elements in steel. Considering mainly the low-temperature toughness of the highest-grade bridge steel, in this invention, the P and S contents are controlled at P ≤ 0.006% and S ≤ 0.002% respectively.

[0022] Controlling the cold crack sensitivity index Pcm is beneficial to ensuring the welding performance of the product. The P in this invention cm is controlled at ≤ 0.23%.

[0023] Controlling the carbon equivalent Ceq index is beneficial to ensuring the welding performance of the product. The C in this invention eq is controlled at ≤ 0.50%.

[0024] The beneficial effects of this invention: By adding low-cost B element, the strength and hardenability are improved, and the addition of Cr and Mo elements is reduced.

[0025] On the premise of reducing the Cr and Mo 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 have more excellent low-temperature toughness.

[0026] The reduction of alloy element addition saves costs, reduces the welding sensitivity coefficient Pcm, and improves the weldability. Description of the Drawings

[0027] Figure 1 It is the microstructure morphology diagram of the steel for bridges with a yield ratio of 780 MPa grade, high toughness and low yield ratio produced in Example 1. The microstructure in the figure is lath bainite + martensite. Detailed Description of the Invention

[0028] The following is further described in conjunction with a set of embodiments.

[0029] Examples 1 to 4: A production method of a boron-containing steel for bridges with a yield ratio of 780 MPa grade, high toughness and low yield ratio. The mass percentage of the chemical composition of the steel, the carbon equivalent Ceq, and the crack sensitivity index Pcm are shown in Table 1, and the rest are Fe and inevitable impurities; according to the designed mass percentage of the chemical composition of the steel, batching, smelting, and continuous casting are carried out to obtain a continuous casting billet; including the following key process steps: 1) Heating: The continuous casting billet is heated to 1120 - 1150 °C and held for 8 - 10 h. After heating, the surface scale is removed by high-pressure water. 2) Rolling: Two-stage controlled rolling is adopted. After 5 passes of rolling on the roughing mill, 7 passes of rolling are carried out on the finishing mill; the roughing starting temperature ≥ 1000 °C, and the roughing finishing temperature ≥ 900 °C; the finishing starting temperature is 880 - 900 °C, and the finishing finishing temperature is 840 - 880 °C; 3) Online quenching: Immediately after rolling, MuPiC online quenching is adopted, and the cooling rate is 25 - 35 °C / s, and it is cooled to 260 - 300 °C; 4) Tempering: The online quenched steel plate is tempered. It is heated to 500 °C in a tempering furnace and held for 45 - 60 min. The process parameters of the above heating, rolling, online quenching, and tempering steps are shown in Table 2. The mechanical property test results of the obtained steel are shown in Table 3.

[0030] Table 1 Chemical composition of the examples (wt%) .

[0031] Table 2 Process parameters of heating, rolling, online quenching, and tempering in the examples .

[0032] Table 3 Mechanical property test results of the examples .

[0033] As shown in Table 3: The comprehensive properties of the pipeline steel prepared in the example are as follows: the yield strength is 790 - 820 MPa, the tensile strength is 950 - 980 MPa, the elongation A ≥ 16%, the yield ratio ≤ 0.84, the Charpy impact energy at -60 °C ≥ 270 J, and the ductile-brittle transition point is at -70 to -80 °C. It is a bridge steel with low yield ratio, high strength and toughness, and high weldability.

Claims

1. A method for producing a 780MPa grade bridge steel with low yield ratio and high toughness, wherein the chemical composition of the steel 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 rest is Fe and unavoidable impurities, and 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 key process steps: 1) Heating: Heat the continuous casting billet obtained after batching, smelting and continuous casting according to the designed chemical composition mass percentage of the steel to 1120~1150℃, keep it warm for 8~10h, and remove the surface iron oxide scale with high-pressure water after heating. 2) Rolling: Two-stage controlled rolling is adopted, the rough rolling mill is rolled for 5 passes, and then the finishing mill is rolled for 7 passes; the rough rolling start temperature is ≥1000℃, the rough rolling final rolling temperature is ≥900℃; the finishing rolling start temperature is 880-900℃, and the finishing rolling final rolling temperature is 840~880℃; 3) Online quenching: MuPiC online quenching is used immediately after rolling, with a cooling rate of 25~35℃ / s, cooling to 260~300℃; 4) Tempering: Tempering heat treatment is performed on the online quenched steel plate, heated to 500℃ in the tempering furnace, and kept warm for 45~60min to obtain a boron-containing, low yield ratio, high toughness 780MPa bridge steel with lath bainite + martensite structure. The yield strength of the steel is 790~820MPa, the tensile strength is 950~980MPa, the elongation A≥16%, the yield ratio ≤0.84, and the -60℃ Charpy impact energy ≥270J. The ductile-brittle transition point is -70~-80℃, and the bridge steel has a low yield ratio, high strength and toughness, and high weldability.

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