Low-cost high-quality 500MPa-grade high-strength steel bar and production method thereof

Abstract

The invention discloses a low-cost high-quality 500MPa-grade high-strength steel bar and a production method thereof. The low-cost high-quality 500MPa-grade high-strength steel bar comprises the following components in percentage by mass: 0.10-0.25% of C, 0.30-0.65% of Si, 1.20-1.55% of Mn, less than or equal to 0.020% of P, less than or equal to 0.030% of S, 0.01-0.07% of V, 0.005-0.015% of N and the balance of Fe and inevitable impurities. According to the high-strength steel bar and the production method thereof, a reduction component design and a converter nitrogen increasing process are adopted, and the V alloy adding amount and the production cost are reduced by 30%-50%; by controlling the temperature in the whole rolling process, including proper reduction of the initial rolling temperature, pre-cooling before finish rolling, controlled cooling after rolling, and matching of recrystallization and non-recrystallization area rolling and cooling speed after rolling, fine grain strengthening and precipitation strengthening of the steel bar are achieved, the comprehensive mechanical performance of the steel bar is guaranteed, the strength of the steel bar is improved, and the production cost is reduced. And indexes such as plasticity and welding performance are not influenced.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and in particular to a low-cost, high-quality 500MPa-grade high-strength steel bar and a production method thereof. Background Art

[0002] As the urbanization process moves towards resource-friendly and environmentally friendly development, the advantages of high-strength steel bars in actual engineering construction have been fully affirmed and highly recognized. They are increasingly used in modern large-span high-rise buildings, improving the safety, aesthetics and space utilization of buildings, and to a certain extent increasing the service life of buildings. On the one hand, the design and use of high-strength steel bars can not only save steel consumption, but also reduce transportation costs, save construction time, improve labor productivity, and reduce the manufacturing cost of engineering buildings; on the other hand, while improving safety, it can also avoid the waste of energy and resources such as coal, water, and electricity to a certain extent, and reduce the emission of harmful gases such as carbon and sulfur. Therefore, we should vigorously develop and promote the application of 500MPa-level high-strength steel bars. For some key projects and special super-high-rise and large-span buildings, 500MPa-level steel bars are preferred in design. However, how to stably produce steel bars above 500MPa with low cost and high quality in batches is the hot spot and difficulty of the current high-strength steel bar production technology. How to produce high-strength steel bars that meet the requirements at the lowest cost is a very urgent task for metallurgical workers. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a low-cost, high-quality 500MPa grade high-strength steel bar and a production method thereof.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is that the components and their mass percentages are: C 0.10%-0.25%, Si 0.30%-0.65%, Mn 1.20%-1.55%, P≤0.020%, S≤0.030%, V 0.01%-0.07%, N 0.005%-0.015%, and the balance is Fe and unavoidable impurities.

[0005] Furthermore, the steel bars have an average yield strength of ≥530 MPa, an average tensile strength of ≥680 MPa, an average elongation after fracture of ≥17.5%, an average maximum elongation of ≥10.5%, an average strength-yield ratio of ≥1.25, and a grain size of 7.5 to 9.5.

[0006] The present invention adopts the design of reducing components: Low Si design: The composition design controls low Si. In the converter alloying stage, only silicon-manganese alloy is added for deoxidation alloying, and Si iron is not added for deoxidation alloying, so that the Si in the steel is kept at a relatively low level, which is beneficial to control the generation of iron oxide scale and reduce costs. The Si content of the steel bar of the present invention is 0.30% to 0.65%.

[0007] Low Mn design: The composition design controls low Mn. Controlling Mn at a lower level can reduce costs and stably control the hardenability of steel. The lost strength can be compensated by fine grain strengthening at a lower temperature. The Mn content of the steel bar of the present invention is 1.20% to 1.55%.

[0008] Low V and high N design: The use of microalloying technology is one of the main technical routes for the development of high-strength steel bars in various countries around the world. Compared with other microalloyed steels, V and VN steels have a series of process advantages such as easy continuous casting, low deformation resistance, suitable for rolling in high temperature areas, and low dissolution temperature of the precipitated phase. The use of VN microalloying makes full use of cheap nitrogen elements, promotes the precipitation of V in steel bars, significantly improves the precipitation strengthening effect of V, and achieves the purpose of saving alloy content and reducing steel costs. Vanadium-nitrogen microalloying and alloying of only vanadium-iron can save 30% to 50% of vanadium usage and reduce costs by about 50% while obtaining the same strength, fully reflecting the technical and economic advantages. The V content of the steel bar of the present invention is 0.01% to 0.07%, and the N content is 0.005% to 0.015%.

[0009] In order to solve the above technical problems, the technical scheme adopted by the method of the present invention includes converter smelting, billet continuous casting and controlled rolling and controlled cooling steps; The controlled rolling and controlled cooling step includes heating, rolling and cooling processes; in the rolling process, the starting rolling temperature of rough rolling is controlled at 980-1020°C, and the final rolling temperature of fine rolling is controlled at 930-960°C; in the cooling process, light water cooling is first adopted, and the cooling rate is not greater than 5°C / s, and then cooling is performed on a cooling bed, and the inlet temperature of the cooling bed is 920±30°C. The steel bars are covered with an insulation cover on the cooling bed for cooling.

[0010] Furthermore, in the converter smelting step, the converter adopts a bottom blowing nitrogen supply mode throughout the process, and the maximum bottom blowing nitrogen supply intensity is adopted after the converter smelting oxygen supply reaches 80% of the total oxygen supply; the nitrogen and oxygen mixed blowing mode is adopted when the converter smelting oxygen supply is 60% to 80% of the total oxygen supply, and the mixed blowing nitrogen amount is 20% to 30% of the total mixed blowing gas amount; deoxidation begins when the converter taps steel, and deoxidation is performed until the dissolved oxygen [O] in the steel is ≤40ppm; high-pressure nitrogen is blown from the bottom of the ladle when the converter taps steel, to ensure that the molten steel in the ladle is fully churning.

[0011] The beneficial effect of adopting the above technical scheme is that the present invention and its method adopt the reduced component design and converter nitrogen addition process to reduce the V alloy addition amount and production cost by 30% to 50%. Through the temperature control of the entire rolling process, including appropriately reducing the start rolling temperature, precooling before finishing rolling, controlled cooling after rolling, and matching the recrystallization and non-recrystallization zone rolling with the cooling rate after rolling, the steel bar fine grain strengthening and precipitation strengthening are achieved to ensure the comprehensive mechanical properties of the steel, which not only improves the strength of the steel bar, but also does not affect the indicators such as plasticity and welding performance. After testing, the average yield strength (R) of the finished product is ≥530MPa, the average tensile strength (R.) is ≥680MPa, the average elongation after fracture (A) is ≥17.5%, the average maximum elongation (A) is ≥10.5%, the average strength-yield ratio is ≥1.25, the organization is uniform, and the grain size is 7.5 to 9.5. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0013] Figure 1 This is a schematic diagram of the temperature and structure changes of the controlled rolling and controlled cooling steel bars of the present invention; Figure 2 These are the precipitation photos of V (C, N) at different positions in the microstructure analysis of the steel bar sample of the present invention.

[0014] Figure 2 Among them, (a) precipitation on the dislocation line; (b) precipitation on the grain boundary; (c) precipitation within the pearlite lamella; (d) precipitation composition within the pearlite lamella. DETAILED DESCRIPTION

[0015] The composition and mass percentage of this low-cost, high-quality 500MPa grade high-strength steel bar are: carbon (C) 0.10%-0.25%, silicon (Si) 0.30%-0.65%, manganese (Mn) 1.20%-1.55%, phosphorus (P) ≤0.020%, sulfur (S) ≤0.030%, vanadium (V) 0.01%-0.07%, nitrogen (N) 0.005%-0.015%, and the balance is Fe and unavoidable impurities.

[0016] The method for producing low-cost, high-quality 500MPa grade high-strength steel bars includes converter smelting, billet continuous casting and controlled rolling and controlled cooling steps; the processes of each step are as follows: (1) Converter smelting: In order to meet the requirements of nitrogen content in molten steel, the usual method of adding nitrogen to molten steel is to add nitriding alloys, such as manganese nitride, chromium nitride, etc. The use of nitriding alloys to increase nitrogen is not only unstable, but also has high costs. This method uses nitrogen to increase the nitrogen content of molten steel during top and bottom combined blowing converter blowing and steel tapping to alloy nitrogen, so that the nitrogen content of molten steel in the converter ladle can be stably reached 0.0100% or above, which can greatly reduce the production cost of nitrogen-containing steel and reduce the cost of nitrogen-added alloys.

[0017] The converter smelting step of this method adopts the following molten steel nitrogen increase control process: The requirement for molten iron entering the furnace is S≤0.050wt%; bottom blowing nitrogen is used throughout the smelting process. After the oxygen supply of the converter reaches 80vol% of the total oxygen supply, the maximum bottom blowing nitrogen supply intensity is used to strengthen the stirring of the molten pool; when the oxygen supply of the converter reaches 60%~80vol% of the total oxygen supply, a nitrogen-oxygen mixed blowing mode is adopted, and the mixed blowing nitrogen volume is 20%~30vol% of the total mixed blowing gas; the converter endpoint control C≤0.15wt%, P≤0.025wt%, deoxidation begins when the converter taps steel to fully deoxidize, and the order of adding deoxidizing materials is composite deoxidizer→silicon manganese alloy→vanadium nitrogen alloy, and deoxidation is performed until the dissolved oxygen [O] in the steel is ≤40ppm; high-pressure nitrogen is blown from the bottom of the ladle while the converter taps steel, and the bottom blowing nitrogen pressure of the ladle at the beginning of tapping is ≥1.0MPa, and the nitrogen flow rate of each branch is ≥60Nm 3 / h; dynamically control the pressure and flow of nitrogen blowing at the bottom of the ladle, that is, to ensure that the molten steel in the ladle is fully churning while the molten steel cannot overflow from the ladle, so that the nitrogen content of the molten steel in the converter ladle is stably reached 0.0100wt% and above.

[0018] (2) Billet continuous casting: The continuous casting is carried out by tapping steel from a converter. The continuous casting machine adopts constant pulling speed, electromagnetic stirring, automatic liquid level control, and casting optimization cutting technology. The pulling speed is controlled at 2.5-3.0 m / min, and the tundish temperature is controlled at 1520-1530°C. The whole process is protected by pouring, and the protective slag uses special protective slag to obtain the cast billet.

[0019] (3) Controlled rolling and controlled cooling: including heating, rolling and cooling processes; The heating process includes: a preheating section temperature of 800-1000°C, a heating section temperature of 1050-1100°C, a soaking section temperature of 1050-1150°C, and a total heating time of ≥70 minutes.

[0020] The rolling process adopts a rough rolling + finishing rolling process; the starting rolling temperature of the rough rolling is 980-1020°C, and according to different specifications, 14-18 rolling passes are implemented, the rolling rhythm is controlled, the characteristics of large deformation in the high temperature zone are fully utilized, the reduction rate of the rough and medium rolling passes is reasonably controlled at 10-30%, and rolling is carried out in the recrystallization zone. The final rolling temperature of the finishing rolling is controlled at 930-960°C.

[0021] The cooling process comprises the following steps: cooling the rolled steel bar; firstly adopting light water cooling with a cooling rate of no more than 5°C / s; then cooling on a cooling bed with an inlet temperature of 920±30°C, and slowly cooling the steel bar on the cooling bed with a heat preservation cover; increasing the temperature of the upper cooling bed to coarsen the grains, thereby reducing the negative impact of fine grain strengthening on the strength-to-yield ratio and making the austenite more stable; slowly covering the steel bar on the cooling bed with a heat preservation cover, on the one hand, eliminates the internal stress, and on the other hand, reduces the appearance of bainite and eliminates the non-yield phenomenon, and is also beneficial to the full precipitation strengthening of vanadium.

[0022] (4) The calculation and analysis results show that: as the nitrogen content increases, the precipitation temperature of vanadium carbonitride is increased, which promotes the precipitation of more vanadium, and the amount of vanadium precipitation increases from 35% to 70%; the solid solution of vanadium is reduced from 56% to 22%, which makes the vanadium precipitation strengthening play a better role.

[0023] Figure 1 As shown in the figure, the controlled rolling and controlled cooling process makes the rolling temperature, at the nose temperature of vanadium carbonitride precipitation, better precipitate vanadium carbonitride on defects such as dislocations, and pins the grain boundary or the nucleation core in the crystal, so as to refine the grain. The increase of nitrogen not only improves the precipitation strengthening effect of vanadium steel, but also further improves the effect of fine grain strengthening. From the quantitative phase analysis results of the precipitated phase, it can be seen that the size distribution frequency of V (C, N) precipitates in steel bars is mainly concentrated in 0-50nm, accounting for about 70%, including 0-20nm range. The particle size of the internal precipitate phase accounts for the largest proportion, accounting for more than 60% of the total precipitation, among which the number of fine particles less than 10nm has increased significantly, from 21% to 32%, and this part of particles plays an important role in matrix strengthening. In addition, while increasing nitrogen, it also promotes the precipitation of vanadium carbonitride at the austenite-ferrite phase interface, and vanadium carbonitride pins the grain boundary, which well prevents the growth of austenite grains and refines the size of ferrite during the phase transformation process. In vanadium steel, proper nitrogen addition promotes the effect of intragranular ferrite (IGF), which also enables vanadium steel to further improve the performance of vanadium steel by using inclusion metallurgy technology.

[0024] Through the microstructure analysis of the finished product samples, Figure 2 As shown in the figure, the microstructure of the high-strength steel bar produced by this method is ferrite + pearlite, and the grain size is about 1 level higher than that of the core, 2 / 8 and edge of the steel bar produced by ordinary hot rolling, reaching 9.5 in the core and more than 10 in the edge; the proeutectoid ferrite in the microstructure is relatively small, the lamellar structure of the pearlite group is relatively dense, and the grain refinement effect is obvious.

[0025] The results of transmission electron microscopy show that the fine V(C, N) particles precipitated from the vanadium-nitrogen microalloyed steel bars are mainly distributed in the matrix ferrite and dislocation lines, and some are distributed in the ferrite grain boundaries and pearlite ferrite sheets, such as Figure 2 As shown in the figure, V (C, N) has a good precipitation effect in steel, which plays a role in pinning grain boundaries and hindering the movement of dislocations, giving full play to the precipitation strengthening effect of fine grain precipitation, and significantly improving the strength of steel.

[0026] The mechanical properties of the 500MPa grade high-strength steel bars obtained by this method are as follows: the average yield strength (R) is ≥530MPa, the average tensile strength (R.) is ≥680MPa, the average elongation after fracture (A) is ≥17.5%, the average maximum elongation (A) is ≥10.5%, and the average strength-yield ratio is ≥1.25; the internal structure is ferrite + pearlite, the grain size is 7.5 to 9.5, and the structure is uniform.

[0027] Embodiment 1-6: The low-cost, high-quality 500MPa grade high-strength steel bar and its production method are specifically described as follows.

[0028] (1) Converter smelting: The converter charging system adopts quantitative charging. When the composition and temperature of the molten iron are abnormal, the charging amount should be adjusted in time; the control of the gun position during the blowing process is matched with the slagging situation; the slag making operation adopts the batch charging method, and the slag material is added 3 minutes before the gun is lifted; after the converter smelting oxygen supply exceeds 80% of the total oxygen supply, the maximum bottom blowing nitrogen supply intensity is adopted; when the converter smelting oxygen supply is 60% to 80% of the total oxygen supply, the nitrogen-oxygen mixed blowing mode is adopted, and the mixed blowing nitrogen volume is 20% to 30% of the total mixed blowing volume; the slag blocking cone is used to block the slag during the steel tapping process, and the slag blocking rate must reach 100%. At the same time as the converter tapping, high-pressure nitrogen is blown from the bottom of the ladle to ensure that the molten steel in the ladle is fully turbulent; deoxidation and alloying are carried out at the beginning of steel tapping, and the order is composite deoxidizer → silicon-manganese alloy → vanadium-nitrogen alloy. During the deoxidation and alloying process, the timing and amount of vanadium-nitrogen alloy addition are focused on. After the deoxidation and alloying operation is completed, the oxygen content of the molten steel is controlled at 40×10 -6 The following: The recovery rate of the microalloying element V is high and relatively stable, and the average recovery rate is greater than 90%. The specific parameters of each embodiment are shown in Table 1; Table 1: Converter smelting process parameters

[0029] (2) Billet continuous casting: The continuous casting machine adopts constant casting speed, electromagnetic stirring, automatic liquid level control, and casting optimization cutting technology. The casting speed is controlled at 2.5-3.0 m / min, and the tundish temperature is controlled at 1520-1530°C. The whole process is protected casting, and the protective slag uses HRB500E rebar special protective slag.

[0030] (3) Controlled rolling and controlled cooling: The heating furnace is an advanced walking beam heat storage heating furnace. All the billets are hot loaded and hot transported. The billets are heated very evenly, and the temperature difference is no more than 20°C. The starting temperature of rough rolling is stable at 980-1020°C, and the final rolling temperature of finishing rolling is controlled at 930-960°C. After rolling, weak water penetration is performed, and the cooling rate is no more than 5°C / S. The stop-rolling and cooling system is strictly implemented during the production process, and the final rolling speed is controlled at 4.3-4.6m / s. The finished product is cooled by light water penetration through the water quenching line, with a water volume of 300-400m³ / h and a water pressure controlled at 0.4-0.6MPa; after water penetration, it is placed on a cooling bed with an inlet temperature of 920±30°C; the steel bars are slowly cooled on the cooling bed with a heat preservation cover. The specific parameters of each embodiment are shown in Table 2; Table 2: Controlled rolling and controlled cooling process parameters

[0031] (4) The steel bar specifications obtained in each embodiment are 16 mm in diameter, and the composition is as shown in Table 3 below; Table 3: Chemical composition of each example (wt%)

[0032] In Table 3, the balance is Fe and unavoidable impurities.

[0033] Through process tracking, samples of each embodiment were extracted for finished product composition analysis. The results showed that the composition was within the national standard range and met the composition requirements of earthquake-resistant steel bars. The steel bars obtained in each embodiment were then subjected to mechanical tests. The test results are shown in Table 4 below. Table 4: Mechanical properties and metallographic structure of the obtained steel bars

[0034] From Table 4 above, it can be seen that the average yield strength (R) is ≥520MPa, the average tensile strength (R.) is ≥680MPa, the average elongation after fracture (A) is ≥17.5%, the average maximum elongation (A) is ≥10.5%, the average strength-yield ratio is ≥1.20, and the steel bar has good process mechanical properties and seismic performance. The metallographic structure is ferrite + pearlite, the structure is uniform, the edge structure is finer than the core structure, the edge structure of a sample shows obvious directionality, and the banded structure is not obvious; the grain size is 7.5 to 9.5.

[0035] Statistical case: A domestic steel plant produced about 100,000 tons of high-strength steel bars above 500MPa in 2021. Compared with conventional processes, the present invention saves an average of 250 yuan of alloy per ton of steel, reduces the rolling cost of a ton of steel by 15 yuan, and reduces the cost benefit by 100,000 tons × 265 yuan / ton = 26.5 million yuan.

Claims

1. A low-cost, high-quality 500MPa grade high-strength steel bar, characterized in that: Its composition and its mass percentage are: C 0.10%~0.25%, Si 0.30%~0.65%, Mn 1.20%~1.55%, P≤0.020%, S≤0.030%, V 0.01%~0.07%, N 0.005%~0.015%, and the balance is Fe and unavoidable impurities.

2. A low-cost, high-quality 500MPa grade high-strength steel bar according to claim 1, characterized in that: The steel bars have an average yield strength of ≥530 MPa, an average tensile strength of ≥680 MPa, an average elongation after fracture of ≥17.5%, an average maximum elongation of ≥10.5%, an average strength-yield ratio of ≥1.25, and a grain size of 7.5 to 9.

5.

3. The method for producing low-cost, high-quality 500MPa grade high-strength steel bars according to claim 1, characterized in that: It includes converter smelting, billet continuous casting and controlled rolling and controlled cooling steps; The controlled rolling and controlled cooling step includes heating, rolling and cooling processes; in the rolling process, the starting rolling temperature of rough rolling is controlled at 980-1020°C, and the final rolling temperature of fine rolling is controlled at 930-960°C; in the cooling process, light water cooling is first adopted, and the cooling rate is not greater than 5°C / s, and then cooling is performed on a cooling bed, and the inlet temperature of the cooling bed is 920±30°C. The steel bars are covered with an insulation cover on the cooling bed for cooling.

4. The method for producing a low-cost, high-quality 500MPa grade high-strength steel bar according to claim 3, characterized in that: In the converter smelting step, the converter adopts a bottom blowing nitrogen supply mode throughout the process, and the maximum bottom blowing nitrogen supply intensity is adopted after the converter smelting oxygen supply reaches 80% of the total oxygen supply; the nitrogen and oxygen mixed blowing mode is adopted when the converter smelting oxygen supply is 60% to 80% of the total oxygen supply, and the mixed blowing nitrogen amount is 20% to 30% of the total mixed blowing gas amount; deoxidation begins when the converter taps steel, and deoxidation is performed until the dissolved oxygen [O] in the steel is ≤40ppm; high-pressure nitrogen is blown from the bottom of the ladle while the converter taps steel, to ensure that the molten steel in the ladle is fully churning.

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