Small-size 650MPa-grade multiphase aseismic steel bar and production method thereof

Abstract

The invention discloses a small-specification 650MPa-grade multiphase aseismic steel bar and a production method thereof. The production method comprises the following steps of smelting, refining, continuous casting, heating, bar rolling and controlled cooling. In a casting blank obtained through continuous casting, the weight percentage of N is 0.009-0.017%, and V / N is equal to 10.0-20.0; in the heating step, the minimum heating temperature T needs to meet the formula T = 1180 + 70-t; wherein t is the heating time; the oxygen enrichment rate in the heating furnace is controlled to be 0-4%; in the bar rolling step, the temperature before finish rolling is controlled to be 850-900 DEG C; in the controlled cooling step, the reinforcing steel bars are naturally cooled on a cooling bed after being subjected to water cooling; according to the method, the small-specification 650MPa-grade anti-seismic steel bar meeting the anti-seismic requirement can be stably produced, the yield strength ReL of the steel bar is larger than or equal to 650MPa, the tensile strength Rm is larger than or equal to 820MPa, the strength-to-yield ratio Rm / ReL is larger than or equal to 1.25, the percentage elongation A after fracture is larger than or equal to 15%, the maximum force total elongation Agt is larger than or equal to 9.0%, and the steel bar has good anti-seismic performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of earthquake-resistant steel bars, and in particular relates to a small-size 650MPa-grade multi-phase earthquake-resistant steel bar and a production method thereof. Background Art

[0002] Compared with ordinary 400MPa steel bars, high-strength steel bars have higher safety performance and are mainly used in various key engineering projects, especially in large-span bridges, high-rise buildings and other projects. Using high-strength steel bars instead of ordinary 400MPa steel bars can reduce the density of steel bars in concrete. For example, using a smaller specification 600MPa steel bar instead of 400MPa steel bars is beneficial to steel bar connection and concrete pouring. Compared with 400MPa steel bars, it can save 30% of materials and reduce engineering costs. It has high safety and reliability and has great market demand and application prospects.

[0003] The national standard GBT 1499.2-2018 includes 600MPa steel bars, but does not include 600MPa earthquake-resistant steel bars. Although many manufacturers have successfully developed and produced HRB600E earthquake-resistant steel bars, the strength-to-yield ratio of small-sized 600MPa steel bars such as Ф12, 14, and 16 is basically between 1.23 and 1.26, which cannot meet the requirement of a strength-to-yield ratio of not less than 1.25, and it is even more difficult for 650MPa steel bars to meet the requirement of a strength-to-yield ratio of not less than 1.25.

[0004] The Code for Seismic Design of Buildings (GB 50011-2010) clearly requires that the longitudinal force-bearing steel bars of the frames and diagonal bracing members (including stair sections) of seismic resistance levels 1, 2 and 3 must be seismic steel bars. my country is a country prone to earthquakes, and building structures need to be seismically designed. It is urgent to develop small-sized 650MPa seismic steel bars to meet the construction industry's demand for high-strength seismic steel bar products.

[0005] At present, the structure of high-strength steel bars is mainly ferrite + pearlite. The strength of steel bars is improved by methods such as grain refinement and precipitation strengthening of micro-alloying elements. Most small-sized steel bars are rolled on high-speed bar production lines, with large deformation rate, large rolling compression ratio and fast cooling speed, which makes it difficult to control the strength-to-yield ratio. The higher the strength of the steel bar, the higher the strength-to-yield ratio can be controlled.

[0006] Chinese patent CN 111575467 A discloses a cooling method for HRB600 high-strength steel bars, which is heated to 1150°C at a heating rate of 10°C / s, kept warm for 5 minutes, and then cooled to 1100°C at a cooling rate of 10°C / s, with the start rolling temperature at 1100°C, the final rolling temperature at 950°C, and the deformation amount of 75%; after hot rolling, the steel bars are cooled to 600°C-760°C at a cooling rate of 0.5°C / s-20°C / s, and then naturally cooled by air cooling. On the basis of controlled rolling, the precipitation of carbonitrides in the steel and the phase transformation of supercooled austenite can be effectively controlled by the controlled cooling process combined with microalloying technology, and a microstructure of fine ferrite + pearlite + a small amount of bainite (0%-10%) is obtained to improve the strength and plasticity of the steel bars. There is no requirement for the strength-to-yield ratio of high-strength steel bars in this patent.

[0007] Chinese patent CN 110453145 A discloses a 630MPa grade hot-rolled high-strength earthquake-resistant steel bar and its rolling process, which comprises, by weight percentage: C: 0.24-0.28%, Si: 0.50-0.80%, Mn: 1.40-1.60%, S≤0.040%, P≤0.040%, V: 0.120-0.160%, Nb: 0.010-0.030%, N: 0.0140-0.0220%, and the rest is Fe and unavoidable impurities. The rolling process comprises the steps of: (1) preparing ingredients according to the weight percentage of each component; (2) preparing a steel billet of suitable size. (3) rolling, wherein the steel billet heating temperature is 1000-1200℃, the soaking temperature is 1150-1230℃, and the rolling start temperature is 1100-1200℃. (4) After heating and rolling, the steel bars are placed on a cooling bed, cooled naturally, bundled, and stacked. The steel bars disclosed in the patent are 630MPa grade earthquake-resistant steel bars with relatively low strength. Summary of the invention

[0008] The object of the present invention is to provide a small-sized 650MPa grade multi-phase earthquake-resistant steel bar and a production method thereof, which can stably produce small-sized 650MPa grade earthquake-resistant steel bars that meet earthquake resistance requirements, and the yield strength R eL ≥650MPa, tensile strength R m ≥820MPa, strength-to-yield ratio R m / R eL ≥1.25, elongation after fracture A≥15%, total elongation at maximum force A gt ≥9.0%, with good earthquake resistance.

[0009] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0010] The present invention provides a method for producing a small-sized 650MPa grade multi-phase earthquake-resistant steel bar, the method comprising the following steps: smelting→refining→continuous casting→heating→bar rolling→controlled cooling;

[0011] In the continuous casting ingot, the weight percentage of N is 0.009-0.017%, and V / N=10.0-20.0;

[0012] In the heating step, the minimum heating temperature T needs to satisfy T=1180+70-t; wherein t is the heating time in min, and T is in °C; the oxygen enrichment rate in the heating furnace is controlled at 0-4%;

[0013] In the bar rolling step, the finishing rolling temperature is controlled at 850-900°C;

[0014] In the controlled cooling step, the steel bars are cooled by water after exiting the finishing mill, and then naturally cooled to room temperature on a cooling bed;

[0015] The specifications of the 650MPa grade multi-phase earthquake-resistant steel bars are Ф12mm, Ф14mm or Ф16mm.

[0016] In the heating step, the heating temperature is controlled at 1150-1230°C.

[0017] In the heating step, the oxygen enrichment rate is determined according to the temperature of the billet entering the furnace. O2 =(400-T 入炉 ) / 400×4.0, where У O2 The unit is %; T 入炉 It is the temperature of the steel billet entering the furnace, in °C.

[0018] In the controlled cooling step, for steel bars of Ф12 specification, the temperature of the upper cooling bed is the finishing rolling temperature + 10°C.

[0019] In the controlled cooling step, for steel bars of Ф14 specification, the temperature of the upper cooling bed is the finishing rolling temperature -10°C.

[0020] In the controlled cooling step, for steel bars of Ф16 specification, the temperature of the upper cooling bed is the finishing rolling temperature -30°C.

[0021] The chemical composition and weight percentage of the small-size 650MPa multi-phase seismic steel bar are C 0.25%-0.28%, Si 0.60%-0.80%, Mn 1.40%-1.60%, V 0.14%-0.18%, Nb 0.010%-0.030%, N0.009-0.017%, P≤0.035%, S≤0.035%, Ceq≤0.58%, and the rest are Fe and other inevitable impurities.

[0022] Furthermore, Ceq=C+Mn / 6+(Cr+V+Mo) / 5+(Cu+Ni) / 15, wherein the value of each component is the content in steel×100.

[0023] The surface of the small-size 650MPa multiphase earthquake-resistant steel bar is composed of ferrite and pearlite structures, and the core is composed of ferrite, pearlite and a small amount of bainite structures, wherein the volume percentage of bainite is 7.5-15%.

[0024] The yield strength R of the small-size 650MPa multiphase seismic steel bar eL ≥650MPa, tensile strength R m ≥820MPa, strength-to-yield ratio R m / R eL ≥1.25, elongation after fracture A≥15%, total elongation at maximum force A gt ≥9.0%.

[0025] In order to stably produce small-sized 650MPa seismic steel bars that meet seismic requirements, the production method of small-sized 650MPa multi-phase seismic steel bars provided by the present invention adopts the following strategies:

[0026] (1) During converter smelting, nitrogen-rich alloys are added to control the nitrogen content in the steel. To obtain a higher strength-to-yield ratio, the nitrogen content is controlled to be low, and the nitrogen content is controlled to be between 0.009% and 0.017%, while satisfying the V:N ratio in the range of 10.0 to 20.0.

[0027] (2) In the billet heating step, in order to make the carbon and nitrides of V and Nb fully dissolved in austenite, the heating temperature must be ≥1150°C. However, if the heating temperature is too high, the billet will bend and the billet cannot be discharged normally. Therefore, it needs to be controlled at 1150-1230°C. The solid solution of carbon and nitrides of V and Nb is not only related to the heating temperature, but also to the heating time. When the heating temperature is low, a longer heating time is required. Therefore, the minimum heating temperature must meet T = 1180 + 70-t.

[0028] (3) Use oxygen-enriched combustion of combustion-supporting air in the heating furnace. This can speed up the combustion of coal gas and increase the flame temperature. The increase in flame temperature and the increase in smoke blackness effectively improve the radiation heat exchange capacity of smoke to the steel billet, and can quickly heat the steel billet to the target temperature. The heating rate is related to the oxygen enrichment rate of the combustion-supporting air. The higher the oxygen enrichment rate, the higher the flame temperature and the faster the heating rate. Considering safety and heating temperature requirements, the oxygen enrichment rate is controlled at 0-4%, and the oxygen enrichment rate is set according to the temperature of the steel billet entering the furnace. O2 =(400-T 入炉 ) / 400×4.0, where У O2 is the oxygen enrichment rate, %; T 入炉It is the billet entering furnace temperature, unit is ℃.

[0029] (4) Finishing temperature T 精轧 :During the finishing rolling process, the deformation heat and friction heat cause the temperature of the rolled piece to rise, the grains to grow, and the mechanical properties to decrease. Therefore, it is necessary to control the finishing temperature to a lower level by water cooling the rolled piece. However, too low a finishing temperature will increase the mill load and reduce the rolling stability. The finishing temperature needs to be controlled at 880-930℃.

[0030] (5) Bainite content: The bainite content must be controlled within a certain range. If it is too high, the plasticity of the steel will be reduced and the elongation will not meet the requirements; if it is too low, the strength-to-yield ratio cannot be improved. In order to ensure that high-strength steel bars of different specifications have a suitable proportion of bainite content, the volume percentage of bainite in high-strength steel bars is controlled at 7.5-15.0%.

[0031] (6) Upper cooling bed temperature T 冷床 :After the steel bar leaves the finishing mill, it is cooled by water. The cooling speed of the steel bar determines the microstructure of the steel bar. The faster the cooling speed, the more bainite content, and even martensite appears. The slower the cooling speed, the less bainite content, or even no bainite. The cooling speed of steel bars of different specifications is also different. For small-sized high-strength steel bars, due to the high alloy content and small specifications, the cooling speed is very fast, and the upper cooling bed temperature needs to be accurately controlled to obtain a certain content of bainite in the core. 冷床 Requirements to be met: 12 Specifications: T 冷床 =T 精轧 +10;14 Specifications: T 冷床 =T 精轧 -10; 16 Specifications: T 冷床 =T 精轧 -30, unit: ℃.

[0032] The components of the small-sized 650MPa grade earthquake-resistant steel bars provided by the present invention are controlled and functioned as follows:

[0033] C: C is necessary to obtain high strength and hardness. In order to obtain the high strength required for 650MPa grade, the C content must be above 0.25%. As the carbon content increases, the plasticity and toughness of the steel decrease. Therefore, the C content in the steel should be appropriately increased. The C content should be controlled to 0.25% to 0.28%.

[0034] Si: Si is the main deoxidizing element in steel, and as a solid solution hardening element, it helps to improve strength. However, silicon can significantly improve the deformation resistance of steel. Too high Si content will reduce the plasticity and toughness of steel, increase the activity of C, and reduce the oxygen content in steel. Therefore, the Si content is controlled to 0.60% to 0.80%.

[0035] Mn: Mn is an effective element for deoxidation and desulfurization, and can also improve the strength of steel. However, if the Mn content is too high, the residual austenite content after phase transformation will be too high, and the bainite phase transformation temperature will be too low, resulting in too low yield strength and yield strength ratio of steel. Therefore, the Mn content is controlled at 1.40% to 1.60%.

[0036] V: V is an excellent deoxidizer for steel. Adding vanadium to steel can refine the grain size and improve strength and toughness. V forms V (C, N) precipitation phase with N and C elements in steel, which has a strong precipitation strengthening effect. However, due to the low bainite phase transformation temperature, the diffusion of V is inhibited during the phase transformation process, resulting in a large amount of V solid solution in the steel. However, since V is a strong carbide-forming element, the solid solution of V can significantly refine the ferrite, thereby ensuring a high strength-to-yield ratio. Vanadium carbonitride has a strong trap energy, which can capture hydrogen and make it evenly dispersed in the crystal, inhibiting the diffusion of hydrogen and grain boundary segregation, thereby improving the delayed fracture resistance of steel. Excessive V content will result in higher costs, so the V content should be controlled at 0.14% to 0.18%.

[0037] Nb: Nb element can obviously refine the grains. Grain refinement can not only improve the toughness of steel, but also improve the delayed fracture resistance of high-strength steel. Because the grains are finer, the corrosion resistance can also be improved. The Nb content cannot be too high. Too high Nb will promote bainite transformation, increase the bainite content, and reduce the plasticity of the steel. Therefore, the range of Nb can be controlled at 0.010% to 0.030%.

[0038] S and P: Impurity elements such as S and P segregate at the grain boundaries, which will greatly reduce the delayed fracture resistance. The P element can form micro segregation when the molten steel solidifies, and then segregate at the grain boundaries when heated to the austenitizing temperature, which significantly increases the brittleness of the steel, thereby increasing the delayed fracture sensitivity of the steel, especially affecting the low-temperature performance; the S element forms MnS inclusions and segregates at the grain boundaries, thereby increasing the delayed fracture sensitivity of the steel, so the P and S contents are controlled at P≤0.035% and S≤0.035%.

[0039] N: N precipitates V (C, N) and Nb (C, N) in steel, which can significantly improve the strength of the steel. However, too high a N content will deteriorate the strength-to-yield ratio of the steel. Therefore, a suitable N content is the key to ensuring the strength-to-yield ratio of high-strength steel bars, N0.009~0.017%.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The yield strength R eL ≥650MPa, tensile strength R m ≥820MPa, strength-to-yield ratio R m / R eL≥1.25, elongation after fracture A≥15%, total elongation at maximum force A gt ≥9.0%. Its surface is composed of ferrite and pearlite, and its core is composed of ferrite, pearlite and a small amount of bainite, of which the volume percentage of bainite is 7.5-15%. The mechanical properties are adjusted by controlling the proportion of bainite content to achieve the best match between strength and strength-to-yield ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The surface metallographic structure of the earthquake-resistant steel bar in Example 1;

[0043] Figure 2 The surface metallographic structure of the earthquake-resistant steel bar in Example 1;

[0044] Figure 3 The core metallographic structure of the earthquake-resistant steel bar in Example 1;

[0045] Figure 4 This is the metallographic structure of the core of the earthquake-resistant steel bar in Example 1. DETAILED DESCRIPTION

[0046] The present invention provides a method for producing a small-sized 650MPa grade multi-phase earthquake-resistant steel bar, comprising the following steps: smelting → refining → continuous casting → heating → bar rolling → controlled cooling;

[0047] In the continuous casting ingot, the weight percentage of N is 0.009-0.017%, and V / N=10.0-20.0;

[0048] In the heating step, the minimum heating temperature T needs to satisfy T=1180+70-t; wherein t is the heating time in min, and T is in °C; the heating temperature is controlled at 1150-1230 °C; the oxygen enrichment rate in the heating furnace is controlled at 0-4%; the oxygen enrichment rate is determined according to the temperature of the billet entering the furnace, and the oxygen enrichment rate У O2 =(400-T 入炉 ) / 400×4.0, where У O2 The unit is %; T 入炉 It is the temperature of the steel billet entering the furnace, in °C.

[0049] In the bar rolling step, the finishing rolling temperature is controlled at 850-900°C;

[0050] In the controlled cooling step, the steel bar is cooled by water after exiting the finishing mill, and then naturally cooled to room temperature on a cooling bed; for steel bars of Ф12 specification, the temperature of the cooling bed is the finishing rolling temperature + 10°C; for steel bars of Ф14 specification, the temperature of the cooling bed is the finishing rolling temperature - 10°C. For steel bars of Ф16 specification, the temperature of the cooling bed is the finishing rolling temperature - 30°C.

[0051] The chemical composition and weight percentage of the small-size 650MPa multi-phase seismic steel bar are C 0.25%-0.28%, Si 0.60%-0.80%, Mn 1.40%-1.60%, V 0.14%-0.18%, Nb 0.010%-0.030%, N0.009-0.017%, P≤0.035%, S≤0.035%, Ceq≤0.58%, and the rest are Fe and other inevitable impurities.

[0052] The present invention is described in detail below with reference to the embodiments.

[0053] The steel bar compositions in the embodiments and comparative examples are shown in Table 1. The N content and V:N value in comparative example 1 do not meet the requirements of the present invention; the silicon content and niobium content in comparative example 2 do not meet the requirements of the present invention; and the manganese, vanadium content and V:N value in comparative example 3 do not meet the requirements of the present invention.

[0054] Table 1 Chemical composition of examples and comparative examples (wt%)

[0055]

[0056]

[0057] The rolling process of the steel bars in each embodiment and comparative example is shown in Table 2. Among them, the oxygen enrichment rate in comparative example 4 does not meet the requirements of the present invention; the heating temperature in comparative example 5 does not meet the requirements of the present invention; the upper cooling bed temperature in comparative example 6 does not meet the requirements of the present invention; the upper cooling bed temperature in comparative example 7 does not meet the requirements of the present invention; the temperature before finishing rolling in comparative example 8 does not meet the requirements of the present invention.

[0058] Table 2 Rolling process of the present invention and comparative example

[0059]

[0060] The properties of the steel bars in the embodiments and comparative examples are shown in Table 3.

[0061] Table 3 Mechanical properties of examples and comparative examples

[0062]

[0063] It can be seen from Table 3 that the room temperature mechanical properties of the steel bars in each embodiment all meet the following requirements: yield strength R eL ≥650MPa, tensile strength R m ≥820MPa, strength-to-yield ratio R m / R eL ≥1.25, elongation after fracture A≥15%, total elongation at maximum force A gt ≥9.0%, with good earthquake resistance.

[0064] The strength-to-yield ratio in Comparative Example 1 does not meet the seismic requirement of not less than 1.25; the yield strength in Comparative Example 2 and Comparative Example 3 does not meet the requirement of not less than 650MPa; the strength-to-yield ratio in Comparative Example 4 and Comparative Example 5 does not meet the seismic requirement of not less than 1.25; the yield strength in Comparative Example 6 does not meet the requirement of not less than 650MPa; the strength-to-yield ratio in Comparative Example 7 does not meet the seismic requirement of not less than 1.25; the yield strength in Comparative Example 8 does not meet the requirement of not less than 650MPa.

[0065] The detailed description of a small-sized 650MPa-grade multi-phase earthquake-resistant steel bar and its production method with reference to the above-mentioned embodiment is illustrative rather than restrictive, and several embodiments can be listed according to the limited scope. Therefore, changes and modifications without departing from the overall concept of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for producing small-size 650MPa grade multi-phase earthquake-resistant steel bars, characterized in that: The production method comprises the following steps: smelting → refining → continuous casting → heating → bar rolling → controlled cooling; In the continuous casting ingot, the weight percentage of N is 0.009-0.017%, and V / N=10.0-20.0; In the heating step, the minimum heating temperature T needs to satisfy T=1180+70-t; wherein t is the heating time in min, and T is in °C; the oxygen enrichment rate in the heating furnace is controlled at 0-4%; In the bar rolling step, the finishing rolling temperature is controlled at 850-900°C; In the controlled cooling step, the steel bars are cooled by water after exiting the finishing mill, and then naturally cooled to room temperature on a cooling bed; The specifications of the 650MPa grade multi-phase earthquake-resistant steel bars are Ф12mm, Ф14mm or Ф16mm.

2. The production method according to claim 1, characterized in that In the heating step, the heating temperature is controlled at 1150-1230°C.

3. The production method according to claim 1, characterized in that In the heating step, the oxygen enrichment rate is determined according to the temperature of the billet entering the furnace. O2 =(400-T 入炉 ) / 400×4.0, where У O2 The unit is %; T 入炉 It is the temperature of the steel billet entering the furnace, in °C.

4. The production method according to claim 1, characterized in that In the controlled cooling step, for steel bars of Ф12 specification, the temperature of the upper cooling bed is the finishing rolling temperature + 10°C.

5. The production method according to claim 1, characterized in that: In the controlled cooling step, for steel bars of Ф14 specification, the temperature of the upper cooling bed is the finishing rolling temperature -10°C.

6. The production method according to claim 1, characterized in that: In the controlled cooling step, for steel bars of Ф16 specification, the temperature of the upper cooling bed is the finishing rolling temperature -30°C.

7. The production method according to claim 1, characterized in that: The chemical composition and weight percentage of the small-size 650MPa multi-phase seismic steel bar are C 0.25%-0.28%, Si 0.60%-0.80%, Mn 1.40%-1.60%, V0.14%-0.18%, Nb 0.010%-0.030%, N 0.009-0.017%, P≤0.035%, S≤0.035%, Ceq≤0.58%, and the rest are Fe and other inevitable impurities.

8. Small-sized 650MPa grade multi-phase earthquake-resistant steel bars produced by the production method according to any one of claims 1 to 7.

9. The small-sized 650MPa grade multi-phase earthquake-resistant steel bar according to claim 8, characterized in that: The surface of the small-size 650MPa multiphase earthquake-resistant steel bar is composed of ferrite and pearlite structures, and the core is composed of ferrite, pearlite and a small amount of bainite structures, wherein the volume percentage of bainite is 7.5-15%.

10. The small-sized 650MPa grade multi-phase earthquake-resistant steel bar according to claim 8, characterized in that: The yield strength R of the small-size 650MPa multiphase seismic steel bar eL ≥650MPa, tensile strength R m ≥820MPa, strength-to-yield ratio R m / R eL ≥1.25, elongation after fracture A≥15%, total elongation at maximum force A gt ≥9.0％。

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