A production method for surface ferrite closed-loop hot-rolled ribbed steel bar

Abstract

The invention discloses a production method of a surface ferrite closed-loop hot-rolled ribbed steel bar, comprising the following steps: S1, preparing a steel billet, wherein the chemical components of the steel billet include, by mass percentage, C: 0.20-0.28%, Mn: 0.60-1.60%, Nb: 0.001-0.040%, Ti: 0.001-0.03%, V: 0.001-0.15%, N: 0.006-0.024%, Si≤0.80%, B: 0.001-0.015%, P≤0.045%, S≤0.045%, and the remainder is Fe and unavoidable impurities; S2, conventionally rolling the steel billet prepared in step S1, without any controlled rolling measures between rough rolling, intermediate rolling and finishing rolling, and achieving a macroscopic surface ferrite closed loop through controlled cooling means after finishing rolling, and the microstructure is still a ferrite+pearlite structure. The disclosure of the present invention achieves the purpose of low-cost, low-carbon and green production of steel bars by strengthening the structure under the premise of meeting the standards of hot-rolled ribbed steel bars.

Description

Technical Field

[0001] The invention relates to the technical field of metallurgy, and more specifically to a production method of a surface ferrite closed-loop hot-rolled ribbed steel bar. Background Art

[0002] Hot-rolled ribbed steel bars are widely used in concrete structures in engineering construction. They are widely used in civil engineering construction such as bridges, houses, and roads. With the improvement of the national economic level and the promotion of green buildings and dual carbon policies, the engineering construction industry has higher and higher requirements for the performance and strength level of hot-rolled ribbed steel bars. However, since the second half of 2022, the demand for steel market has declined sharply, sales prices have dropped rapidly, and the cost of raw materials has fluctuated upward, resulting in long-term losses for hot-rolled ribbed steel bar manufacturers. In order to reduce production costs and meet the performance requirements of hot-rolled ribbed steel bars, some steel mills use unconventional hot rolling or TMCP controlled rolling and cooling methods, when the new national standard clearly stipulates that martensitic structure cannot be formed on the surface through post-rolling water cooling. When the metallographic detection does not know the production process and cannot determine the tempering structure type, the QST strong water penetration process is used to produce traditional GB 13014 residual heat treated steel bars. The strength performance of hot-rolled ribbed steel bars produced by the above process meets the requirements, but it has a great impact on the welding of the steel bars and the organizational properties of the weld heat affected zone, which seriously affects the safety of building design and the lives of the people.

[0003] Patent CN101428292B proposes a production method for high-strength hot-rolled ribbed steel bars, and discloses the conventional composition and production process of hot-rolled ribbed steel bars. Although it solves the problems of fine-grained high-strength steel bar welding and aging, it only refines the grains by V microalloying and reduces carbon, manganese and other elements that affect carbon equivalent to increase the welding performance of steel bars, but does not meet the specific requirements of metallographic structure and surface macroscopic metallographic structure.

[0004] Patent CN1031034037A proposes a low-temperature rolling method for controlled rolling and controlled cooling of HRBF400E high-performance fine-grained earthquake-resistant steel bars, which is an extension of the existing continuous casting and rolling related technologies. It has strict requirements on rolling mill equipment and temperature control. It only meets the national standard of hot-rolled ribbed steel bars with fine grain size greater than grade 9, upper cooling bed temperature of 780±10℃, conventional high-carbon, high-silicon and high-manganese components, and increases the cooling speed through low-temperature and high-pressure deformation rolling to achieve the purpose of fine grain strengthening. However, since it does not add conventional grain-refining elements, its welding performance is worse than that of ordinary hot-rolled ribbed steel bars, and there is no demand for the above-mentioned steel bars in the market;

[0005] Patent CN102586563A mainly involves controlling the deformation temperature and strain rate of the final rolling pass and the amount of reduction to obtain the strain accumulation required for strain-induced phase transformation, and to coordinate the cooling time and water-penetrating cooling intensity from the final rolling to the water-penetrating to achieve the purpose of refining the ferrite grains on the surface. However, in the conventional hot-rolled ribbed steel bar rolling, because the rolling mill does work during the controlled rolling process, austenite is always in the growth process, and no element that inhibits the growth of austenite is reflected in the composition design system; the deformation recrystallization behavior of austenite is suppressed simply by deformation, and the static recrystallization of austenite is suppressed by controlling the cooling intensity of the water-penetrating cooling after rolling; the time from the final rolling to the start of the water-penetrating cooling should be less than 1 second; after the final rolling, there should be a sufficient cooling rate to ensure that no recrystallization process occurs and to suppress high-temperature transformation. Calculated at the conventional rolling speed, the water-penetrating cooling rate affects three major factors: water-penetrating flow rate, water-penetrating pressure and water-penetrating time, and the influence of the water-penetrating flow rate and water-penetrating pressure (the above three factors) on the water-penetrating cooling rate and the final ferrite grain size is not clearly disclosed.

[0006] How to effectively use technological means to produce hot-rolled ribbed steel bars that meet national standards, and optimize the composition and process of hot-rolled ribbed steel bars without affecting welding performance, has become the main problem faced by major hot-rolled ribbed steel bar manufacturers; under the requirements of low cost and high quality, how to use microstructure fine grain strengthening to reduce the solid solution strengthening effect has raised new problems and requirements for existing compositions and controlled cooling processes. Summary of the invention

[0007] The purpose of the present invention is to disclose a production method of a surface ferrite closed-loop hot-rolled ribbed steel bar, solve the problems existing in the prior art, and achieve the purpose of low-cost, low-carbon and green production of steel bars by strengthening the organization under the premise of meeting the standards of hot-rolled ribbed steel bars.

[0008] To achieve the above object, the present invention provides a method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar, comprising the following steps:

[0009] S1, preparing a steel billet, wherein the chemical composition of the steel billet comprises, by mass percentage, C: 0.20-0.28%, Mn: 0.60-1.60%, Nb: 0.001-0.040%, Ti: 0.001-0.03%, V: 0.001-0.15%, N: 0.006-0.024%, Si≤0.80%, B: 0.001-0.015%, P≤0.045%, S≤0.045%, and the rest is Fe and unavoidable impurities;

[0010] S2, performing conventional rolling on the steel billet prepared in step S1, without any controlled rolling measures between rough rolling, intermediate rolling and finishing rolling, and achieving macroscopic surface ferrite closed loop by controlled cooling after finishing rolling, and the microstructure is still ferrite + pearlite structure;

[0011] S3, the cooling control means in step S2 is mainly controlled by a combination of water cooling and air blowing, including the following sub-steps:

[0012] S31, the first stage, rapid water cooling, so that the surface temperature of the billet drops rapidly to 450°C;

[0013] S32, by means of continuous air blowing to remove moisture from the surface of the steel billet, and by utilizing the difference between the core temperature and the surface temperature of the steel billet and heat self-conduction, the temperature of the steel billet is quickly restored to 900 degrees Celsius;

[0014] S33, enter the second stage of water cooling, the billet surface temperature drops to 700℃, air blowing restores to 760℃, and gradually reduces the water volume.

[0015] When the surface temperature of the finished steel bar is balanced with the core temperature to 860-880℃, water-penetrating rolling is carried out according to the conventional hot-rolled state.

[0016] As a further improvement of the present invention, step S1 includes the following sub-steps:

[0017] S11, molten iron from blast furnace is smelted in top and bottom combined blown converter, steel is tapped from converter and alloyed, or scrap steel is smelted in electric furnace, steel is tapped from electric furnace and alloyed, and then the composition is uniformed and continuously cast into billets;

[0018] S12, adding scrap steel into the converter or electric furnace and then adding into the blast furnace molten iron, the scrap steel ratio is 15-25%, adding slag in the process, after smelting, the tapping temperature is ≥1650°C, when 1 / 3-2 / 3 of the steel is tapped, adding silicon, manganese, vanadium, niobium, titanium, boron alloys and common carburizer in sequence, and controlling the chemical composition to meet the disclosure of step S1;

[0019] S13, between steel tapping and continuous casting, the superheat of the ladle during casting is controlled at 30-50°C, the continuous casting speed is 3.0-4.0 m / min, and enhanced water cooling is used for continuous casting.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) A method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar, which adopts composite strengthening of Nb, V, Ti and B elements, adopts alloying without adding ferrosilicon or with little ferrosilicon, and the silicon element of the steel is basically not brought into the silicon-manganese alloy, and ferrosilicon is not used alone to supplement the silicon content, thereby effectively reducing the content of solid solution elements, and reasonably adding Nb, B, Ti elements with strong hardenability and V element for dynamic recrystallization to reduce the influence of carbon equivalent elements, while strengthening the organization, improving the welding performance of the finished product, not only covering the HRB400 series and HRB500 series ranges involved in the above patent, but even covering the HRB600 level, which is an advanced process technology for producing hot-rolled ribbed steel bars, and has significant technical innovation.

[0022] (2) The process is significantly different from conventional hot rolling, TMCP controlled rolling and controlled cold rolling, and high-speed bar rolling. The purpose is achieved by simply optimizing the composition and controlling the cooling after rolling. Unlike conventional hot rolling microalloying process, residual heat treatment process, and TMCP process, no tempered structure of any type (including high-temperature tempered troostite, low-temperature tempered martensite, etc.) appears on the base circle. The main metallographic structure is still ferrite plus pearlite. The outer periphery of the base circle has a fine-grained ferrite and transition layer that is significantly different from the contrast of the inner area. The contrast is significantly different from the grain size of the ferrite in the core. By means of the above method, a macroscopic metallographic structure is produced in which the grain size of the ferrite in the fine-grained layer is different from that of the ferrite in the transition layer and the core, and a closed ring or an open ring appears on the periphery of the base circle of the cross section, which is obviously different from the contrast of the internal area. The core position of the center line of the cross section of the specimen is taken as the central test point (HV0), and the edge test point (HV) is determined on the open ring on the periphery of the base circle of the cross section of the specimen, which is different from the contrast of the internal area. The hardness meets the requirements of GB / T1499.2, but the organizational morphology is non-tempered martensite or any tempered organization, and the main organization is still hot-rolled ferrite and pearlite organization. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic flow chart of a production method of a surface ferrite closed-loop hot-rolled ribbed steel bar of the present invention;

[0024] Figure 2 It is a schematic diagram of water-penetrating cooling of a production method of a surface ferrite closed-loop hot-rolled ribbed steel bar of the present invention;

[0025] Figure 3 It is a schematic diagram of a closed ferrite ring formed on the surface of a steel billet in a production method of a surface ferrite closed-loop hot-rolled ribbed steel bar of the present invention;

[0026] Figure 4 for Figure 3 Schematic diagram of the surface organization of the closed ferrite ring in;

[0027] Figure 5 for Figure 3 Schematic diagram of the structure of the closed ferrite ring transition layer;

[0028] Figure 6 Schematic diagram of the billet surface under conventional hot rolling process. DETAILED DESCRIPTION

[0029] The present invention is described in detail below in conjunction with the various embodiments shown in the accompanying drawings, but it should be noted that these embodiments are not limitations of the present invention, and any equivalent transformations or substitutions in functions, methods, or structures made by ordinary technicians in the field based on these embodiments are all within the scope of protection of the present invention.

[0030] Please refer to Figures 1 to 6 The method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar of the present invention is shown as a specific implementation method of the method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar.

[0031] A method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar comprises the following steps:

[0032] S1, preparing a steel billet, wherein the chemical composition of the steel billet comprises, by mass percentage, C: 0.20-0.28%, Mn: 0.60-1.60%, Nb: 0.001-0.040%, Ti: 0.001-0.03%, V: 0.001-0.15%, N: 0.006-0.024%, Si≤0.80%, B: 0.001-0.015%, P≤0.045%, S≤0.045%, and the rest is Fe and unavoidable impurities; the specific specifications, grades and composition design are shown in Table 1;

[0033] S2, the steel billet prepared in step S1 is subjected to conventional rolling, and no controlled rolling measures are taken between rough rolling, intermediate rolling and finishing rolling. Through controlled cooling after finishing rolling, a macroscopic surface ferrite closed loop is achieved, and the microstructure is still ferrite + pearlite structure. The specific cooling rate control is shown in Table 2.

[0034] S3, the cooling control means in step S2 is mainly controlled by a combination of water cooling and air blowing, including the following sub-steps: S31, the first stage, rapid water cooling, so that the temperature of the finished steel bar quickly drops to 450°C; S32, by means of continuous air blowing, the moisture on the surface of the finished steel bar is swept away, and the difference between the core temperature and the surface temperature of the finished steel bar is used for self-conduction of heat, so that the finished steel bar is quickly heated up to 900°C; S33, enters the second stage of water cooling, the surface temperature of the finished steel bar drops to 700°C, and the air blowing is restored to 760°C, and the water volume is gradually reduced; when the surface temperature of the finished steel bar and the core temperature are balanced to 860-880°C, water-penetrating rolling is carried out according to the conventional hot-rolled state.

[0035] Step S1 includes the following sub-steps: S11, molten iron from a blast furnace is smelted in a top-bottom combined-blowing converter, steel is tapped from the converter and alloyed, or scrap steel is smelted in an electric furnace, steel is refined in an electric furnace and alloyed, and the composition is uniform and then continuously cast into billets; S12, scrap steel is added to the converter or electric furnace and then mixed into molten iron from the blast furnace, the scrap steel ratio is 15-25%, slag is added during the process, and after smelting, the steel tapping temperature is ≥1650°C, and when 1 / 3 to 2 / 3 of the steel is tapped, silicon-based, manganese-based, vanadium-based, niobium-based, titanium-based, boron-based alloys and common carburizers are added in sequence to control the chemical composition to meet the disclosure of step S1; S13, between tapping and continuous casting, inert gas bottom blowing and stirring are used, the superheat of the ladle is controlled at 30-50°C during casting, the billet cross-section is a square billet of 150*150mm or 40*140mm, the continuous casting speed is 3.0-4.0m / min, and the continuous casting adopts enhanced water cooling.

[0036] The billet heating furnace temperature is controlled below 650°C, the heating temperature is 950-1200°C, the soaking temperature is 1060-1250°C, the total heating and soaking time is 60-100min, the rolling start temperature is 980-1120°C, and the continuous bar rolling production line is used to roll the steel into round steel bars or ribbed steel bars. The final rolling speed is greater than 10.5m / s. After rolling, the steel is cooled by a water penetrator, and the upper cooling bed temperature is controlled at 860-950°C. It is then cooled to room temperature in the air and sheared into products.

[0037] The quenching medium after rolling is water, and the water quality requirements are as follows: total phosphorus ≤3.34mg / l, positive phosphorus content ≤0.36mg / l, total inorganic phosphorus content ≤0.36mg / l, organic phosphorus content ≤2.98mg / l, hardness ≤292.09mg / l, chloride ion ≤292.09mg / l, suspended matter ≤10mg / l, nitrite ≤300mg / l, iron content ≤0.50mg / l, conductivity ≤2530uS / cm, pH value 8.0-12.0, turbidity NTU ≤8.2; water temperature requirements: 30±10℃, number of 7500Kw booster pumps turned on>3, water penetration pressure greater than 15Kg, flow rate>600 cubic meters / hour, rolling line water piercer length>15m, post-rolling air blowing>5m, compressed air or nitrogen pressure>5bar, the temperature of the last section of the water piercer of the rolled piece is adjusted according to the different carbon equivalents.

[0038] By clarifying the water quality, the number of water pumps to be turned on, and the pressure, flow rate and water temperature requirements, the cooling rate in the first section can be controlled at 300-400℃ / s, the cooling rate in the second section can be controlled at 30-50℃ / s, and the cooling rate in the third section can be controlled at 10-20℃ / s. After the last section, the surface temperature and core temperature of the finished steel bar are moderately rolled in the austenite formation range.

[0039] Table 1: Specifications, grades and composition design

[0040]

[0041] Table 2 Cooling speed control:

[0042]

[0043]

[0044]

[0045] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar, characterized in that: The following steps are involved: S1, preparing a steel billet, wherein the chemical composition of the steel billet, by mass percentage, comprises: C: 0.20~0.28%、Mn:0.60~1.60%、Nb:0.001~0.040%、Ti:0.001~0.03%、V:0.001~0.15%、N:0.006~0.024%、Si≤0.80%、B:0.001~0.015%、P≤0.045%、S≤0.045%、the rest are Fe and unavoidable impurities; S2, performing conventional rolling on the steel billet prepared in step S1, without any controlled rolling measures between rough rolling, intermediate rolling and finishing rolling, the rolling temperature is 980-1200° C., and controlled cooling is performed after finishing rolling to achieve a macroscopic surface ferrite closed loop, and the microstructure is still ferrite + pearlite structure; S3, the cooling control means in step S2 is mainly controlled by a combination of water cooling and air blowing, including the following sub-steps: S31, the first stage, rapid water cooling, so that the surface temperature of the finished steel bar quickly drops to 450℃; S32, sweeping away the moisture on the surface of the finished steel bar by continuous air blowing, and utilizing the difference between the core temperature and the surface temperature of the finished steel bar to conduct heat and quickly return the temperature of the finished steel bar to 900 degrees Celsius; S33, enter the second stage of water cooling, the surface temperature of the finished steel bar drops to 700℃, the air blowing returns to 760℃, and the water volume is gradually reduced; When the surface temperature of the finished steel bar is balanced with the core temperature to 860-880℃, water-penetrating rolling is carried out according to the conventional hot-rolled state, wherein the cooling rate of the first section is 300-400℃ / s, the cooling rate of the second section is controlled at 30-50℃ / s, and the cooling rate of the third section is controlled at 10-20℃ / s.

2. The method for producing a surface ferrite closed-loop hot-rolled ribbed steel bar according to claim 1, characterized in that: Step S1 includes the following sub-steps: S11, molten iron from blast furnace is smelted in top and bottom combined blown converter, steel is tapped from converter and alloyed, or scrap steel is smelted in electric furnace, steel is tapped from electric furnace and alloyed, and then the composition is uniformed and continuously cast into billets; S12, adding scrap steel into the converter or electric furnace and then adding into the blast furnace molten iron, the scrap steel ratio is 15-25%, adding slag in the process, after smelting, the tapping temperature is ≥1650°C, when 1 / 3-2 / 3 of the steel is tapped, adding silicon, manganese, vanadium, niobium, titanium, boron alloys and common carburizer in sequence, and controlling the chemical composition to meet the disclosure of step S1; S13, between steel tapping and continuous casting, the superheat of the ladle during casting is controlled at 30-50°C, the continuous casting speed is 3.0-4.0 m / min, and the continuous casting adopts enhanced water cooling.

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