Ultrafine-grain high-strength hot-rolled bar and production method

Abstract

The invention relates to an ultra-fine grain high-strength hot-rolled bar which comprises the following components in percentage by weight: 0.21 to 0.24 percent of C, 0.60 to 0.80 percent of Si, 1.30 to 1.45 percent of Mn, 0.035 to 0.050 percent of P, less than or equal to 0.025 percent of S, 0.015 to 0.025 percent of Al, 0.04 to 0.08 percent of Ti, 0.0120 to 0.0150 percent of N and 0.40 to 0.48 percent of Ceq = C + Mn / 6 + V / 5. The production method comprises the following steps: casting into a square blank after conventional converter smelting; heating the casting blank in a reducing atmosphere; performing primary rolling; a water tank is adopted for cooling after conventional intermediate rolling; second-stage finish rolling is carried out; feeding to a cooling bed; and carrying out subsequent procedures. By adding a proper amount of microalloy elements such as Ti and Al and adopting multi-stage controlled cooling and high deformation rate, the yield strength is larger than or equal to 520 MPa, the tensile strength is larger than or equal to 640 MPa, the yield-strength ratio is larger than or equal to 1.26, the elongation is larger than or equal to 22%, the grain size is smaller than or equal to 5 microns, and the comprehensive performance requirements of high strength, shock resistance, corrosion resistance, weldability and the like of high-end steel bars for buildings can be completely met.

Description

Technical Field

[0001] The present invention relates to construction steel and a production method thereof, and in particular to an ultrafine grain high-strength hot-rolled bar and a production method thereof. Background Art

[0002] With the continuous advancement of construction technology and growing demand, China has seen extensive development in high-rise buildings, long-span bridges, offshore platforms, and other fields. These applications place higher demands on steel materials, particularly in terms of strength, seismic resistance, corrosion resistance, and weldability, requiring superior overall performance. Ultrafine-grained rebar, due to its superior overall performance, has gained widespread application in the construction industry, becoming the material of choice. Furthermore, ultrafine-grained rebar demonstrates its unique advantages in specialized environments such as underground and marine engineering. However, the grain size of conventional rebar currently in production ranges from 20 to 70 μm. Due to its high density and uniform grain structure, ultrafine-grained rebar boasts a tensile strength over 20% higher than conventional rebar, enabling it to withstand greater loads. The superior chemical composition and molten steel quality of ultrafine-grained rebar reduce the rate and severity of corrosion in media such as air, water, and soil, resulting in improved corrosion resistance. At the same time, fine-grained steel bars have small grain size, fewer grain boundary defects, uniform composition, better welding performance, and better processability, which can achieve higher processing difficulty and quality while saving processing costs.

[0003] After searching:

[0004] Chinese patent publication number CN 104831193A discloses a method for producing fine-grained steel bars. The composition is designed to be C 0.19-0.25, Si 0.55-0.80, Mn 1.45-1.60, Cr 0.20-0.30, P ≤ 0.045, S ≤ 0.045, with the remainder being iron, with the sum of these components equal to 100%. The process involves converter smelting, ladle furnace refining, billet continuous casting, and high-speed wire rolling. The advantages of this method are that by modifying the material composition and adjusting the process parameters, the production cost of HRB400 threaded steel bars is reduced, the grain size is refined, and product performance is stabilized. However, the product strength is only 400 MPa, which does not meet the requirements and development trends of modern high-strength steel bars (≥500 MPa). Furthermore, the use of high-speed wire rolling does not meet the needs of 80% of steel bar products and does not meet the requirements of the modern market.

[0005] Chinese patent publication number CN117399425A discloses "A Process for Preparing Ultrafine Grain Hot-Rolled Ribbed Steel Bars". This document is based on four-roller cross-rolling technology. Through special curved tapered rollers and guide plates, the billet is bitten from the end with the largest diameter of the roller. By using deformation parameters such as ultra-large feed angles and diameter reduction ratios, a severe torsion-compression composite deformation zone is constructed. This can achieve a single-pass severe plastic forming with an equivalent strain greater than 7.5 and a diameter reduction ratio greater than 45%. After rolling, the steel bar grains are significantly refined and the strength is greatly improved. However, the disadvantage of this patent is that isothermal treatment is required after rolling, which greatly increases the cost of steel bar manufacturing and cannot meet the market demand for high-strength steel bars with low cost and low carbon emissions.

[0006] Chinese patent publication number CN102443687A discloses a method for controlling the microstructure and composition of fine-grained steel bars to improve their strength-to-yield ratio. The document describes the following chemical composition: C: 0.12-0.25 wt%, Si: 0.20-0.60 wt%, Mn: 1.20-2.00 wt%, S≤0.045 wt%, P≤0.045 wt%, with the remainder being Fe and unavoidable impurities. By adjusting the contents of the conventional elements C, Si, and Mn in steel and integrating them with fine-grained steel production processes, the microstructure and composition control technology for fine-grained steel bars is improved, achieving a pearlite volume fraction of ≥20% in the steel structure, ensuring a strength-to-yield ratio of ≥1.25. Other performance indicators fully meet the requirements of GB1499-2007, with stable quality. A drawback of this document is its relatively large grain size of 6-10 μm, which falls short of current demand for ultrafine grains. Summary of the Invention

[0007] The present invention aims to overcome the shortcomings of the existing technology and provide an ultrafine-grained high-strength hot-rolled bar and a production method thereof, which are achieved through composition design, appropriate addition of micro-alloy elements such as Ti and Al, and the use of multi-stage controlled cooling and high deformation rate, so as to achieve not only the yield strength ≥520MPa, the tensile strength ≥640MPa, the strength-to-yield ratio ≥1.26, the elongation ≥22%, but also the grain size ≤5um, and can meet the comprehensive performance requirements of high strength, seismic resistance, corrosion resistance, weldability and other comprehensive performance requirements of high-end steel bars for construction.

[0008] Measures to achieve the above objectives:

[0009] The invention discloses an ultrafine-grained high-strength hot-rolled bar, whose constituent elements and weight percentage contents are as follows: C: 0.21-0.24%, Si: 0.60-0.80%, Mn: 1.30-1.45%, P: 0.035-0.050%, S: ≤0.025%, Al: 0.015-0.025%, Ti: 0.04-0.08%, N: 0.0120-0.0150%, Ceq=C+Mn / 6+V / 5: 0.40-0.48, and the remainder is iron and unavoidable inclusions.

[0010] Preferably, the weight percentage of Al is 0.018-0.023%.

[0011] Preferably, the weight percentage of Ti is 0.04-0.06%.

[0012] A method for producing ultrafine grain high strength hot rolled bar, comprising the following steps:

[0013] 1) After conventional converter smelting, it is cast into square billets with a cross-sectional size of 150mm*150mm.

[0014] 180mm*180mm;

[0015] 2) heating the ingot in a reducing atmosphere at a heating rate of 25-35°C / min to a temperature of 1000-1050°C and holding the temperature at this temperature for 30-50 minutes; and controlling the residual oxygen content in the heating furnace to ≤6%;

[0016] 3) Performing initial rolling, controlling the starting rolling temperature at 850-900°C; the cumulative reduction rate of initial rolling is not less than 40%;

[0017] 4) After conventional intermediate rolling, water tank is used for cooling, and the cooling water flow rate is controlled at 300~400m 3 / h, and cooled to 840-870℃;

[0018] 5) Two-stage finishing rolling is carried out, and the process is as follows: first stage finishing rolling - water cooling - recovery - second stage finishing rolling - water cooling - recovery, during which:

[0019] In the first finishing rolling stage, the finishing rolling temperature is controlled at 860-880°C, the rolling speed is controlled at 10-30 m / s, the cumulative reduction rate is controlled at 50-70%, and the steel is cooled to 810-830°C through water, with a recovery time of 30-60 seconds.

[0020] In the second finishing rolling stage, the finishing rolling temperature is controlled at 840-860°C, the rolling speed is 15-45 m / s, and the cumulative reduction rate is 20-60%. The steel is cooled to 820-840°C through water, and the recovery time is 50-80s.

[0021] 6) The temperature of the upper cooling bed is controlled at 840-860°C;

[0022] 7) Carry out post-processing.

[0023] Functions and mechanisms of the components and main processes in the present invention

[0024] C, Si, Mn, and P are the most cost-effective elements in steel reinforcement for increasing strength. From a steel reinforcement mechanism perspective, the strengthening effects of C, Si, Mn, and P in steel are primarily solid solution strengthening. While the effect of solid solution strengthening is far less pronounced than precipitation strengthening, solid solution strengthening significantly increases tensile strength. High-strength earthquake-resistant steel is a hypoeutectoid steel. Increased C and Mn content both enhance pearlite transformation, a complex structure that significantly improves tensile strength. Si dissolves in ferrite and austenite, increasing the hardness and strength of steel. Its effect is second only to phosphorus and is stronger than that of manganese, nickel, chromium, tungsten, molybdenum, and vanadium. P not only has a strong solid solution strengthening effect in steel, increasing its strength, but also, in appropriate amounts, can significantly enhance its corrosion resistance. P forms a stable iron phosphate oxide with the iron in steel, forming a dense protective film on the steel surface that prevents further attack by corrosive media. Therefore, in the present invention, C: 0.21-0.24%, Si: 0.60-0.80%, Mn: 1.30-1.45%, and P: 0.035-0.050%.

[0025] Ti is a strong carbide-forming element. It forms a carbide with carbon that is extremely strong, stable, and difficult to decompose. In steel, it can only be slowly dissolved into the solid solution when heated to above 1000°C. Before dissolution, TiC particles have the effect of preventing grain growth and coarsening. When Ti is added in appropriate amounts to a steel grade, it can increase the yield point and yield strength ratio of the steel without affecting the plasticity and toughness of the steel. However, when the Ti / C ratio in the steel is greater than 4, its strength and toughness will drop sharply. Ti is an element that strengthens ferrite. Ti can also improve the high-temperature endurance strength and creep strength of steel. Adding an appropriate amount of Ti can reduce the hardenability of steel and improve the steel's microstructure and welding performance. Therefore, the Ti content is controlled at 0.04-0.08%.

[0026] As a strong deoxidizing element, Al can effectively reduce the oxygen content in steel. At the same time, Al and nitrogen in the steel form aluminum nitride, which precipitates into fine aluminum nitride particles as the molten steel solidifies. During rolling, heating and welding, it can effectively hinder the growth of austenite grains and refine the austenite grains. Therefore, the Al content is controlled at 0.015-0.025%.

[0027] Conventional high-strength construction bars typically use V or Nb microalloying, but due to their high solution temperatures and high price, these materials are not conducive to energy conservation and cost reduction. Furthermore, using Al and Ti microalloying forms high-melting-point AlN and TiN during the molten steel solidification process, preventing austenite grain growth during reheating of the billet. Furthermore, mechanical joining is generally used over welding for construction bars with strengths above 500 MPa, primarily because austenite grains in the heat-affected zone (HAZ) after welding tend to grow rapidly, deteriorating the toughness of the steel. However, the high-melting-point AlN and TiN particles in the present invention pin the steel to the original austenite grain boundaries during high-temperature welding, preventing austenite coarsening and improving the strength and toughness of the steel. The starting rolling temperature is 50-100°C above the austenite temperature to prevent austenite coarsening in the pre-rolling phase. The temperature is controlled at 860-880°C before finishing rolling, while high speed and large deformation are employed to allow austenite to transform into ferrite and accumulate a large amount of transformation energy, which facilitates grain refinement during the subsequent microstructure transformation. Furthermore, since the temperature rises after finishing rolling, a stage sensitive to austenite growth, water cooling is designed to follow immediately after finishing rolling to prevent sudden austenite growth.

[0028] The present invention heats the billet to 1000-1050°C at a heating rate of 25-35°C / min and holds it at this temperature for 30-50 minutes, while also controlling the residual oxygen content in the heating furnace to ≤6%. This is because at this heating rate, the billet can be heated quickly and efficiently while also avoiding excessive heating rates that can lead to large temperature differences between the inside and outside of the billet, causing thermal stress cracks within the billet. Furthermore, during subsequent rolling, the rolling force of the rollers can cause defects such as tearing in the billet. The amount of residual oxygen in the furnace represents the oxidizing or reducing atmosphere within the furnace. High residual oxygen levels create an oxidizing atmosphere, increasing the risk of billet decarburization and reducing steel strength.

[0029] The reason why the present invention controls the starting rolling temperature at 850-900°C during initial rolling and the cumulative reduction rate of initial rolling is not less than 40% is that when rolling above the austenite temperature, lowering the rolling temperature and increasing the deformation amount are beneficial to increasing the accumulated energy of structural transformation. Although structural transformation does not occur at this stage, it is beneficial to the accumulation capacity of subsequent structural transformation and refinement of grains.

[0030] The present invention adopts water tank for cooling after intermediate rolling, and controls the cooling water flow rate to 300~400m 3 / h and cooling to 840-870℃ is to create favorable conditions for achieving the finishing rolling temperature. A large cooling water flow and a strong cooling capacity of the water tank will cause local overcooling of the bars passing through the water tank, forming an overcooled structure and affecting the performance of the steel. A small cooling water flow and a weak cooling capacity of the water tank will cause the temperature of the bars passing through the water tank to be too high, and the subsequent finishing rolling temperature cannot be guaranteed.

[0031] The present invention controls the first finishing rolling stage to: a finishing temperature of 860-880°C, a rolling speed of 10-30 m / s, a cumulative reduction of 50-70%, and water cooling to 810-830°C with a recovery time of 30-60 seconds. This is because high-speed, high-deformation rolling above the recrystallization temperature increases the extrusion force of the bar on the rotating rolls, enhancing dislocation slip resistance, significantly increasing deformation resistance, and strengthening the work hardening effect. However, excessively low temperatures or excessive deformation can cause the rolls to be subjected to excessive stress, leading to breakage or fracture.

[0032] The reason why in the second finishing rolling stage, the finishing rolling temperature is controlled at 840-860℃, the rolling speed is 15-45m / s, the cumulative reduction rate is 20-60%; the water cooling is to 820-840℃, and the recovery time is 50-80s is because the austenite accumulates sufficient transformation energy through the rolling deformation and recovery in the first finishing rolling stage, and then through the second finishing rolling with high speed and large deformation, which can form small and dispersed nucleation cores in the process of post-rolling structural transformation, generating a large number of fine ferrite grains.

[0033] The present invention controls the upper cooling bed temperature between 840°C and 860°C because at this temperature, the thickness and structure of the surface oxide scale fully protect the bar base from corrosion and rust. Too low a temperature can easily cause an electrode potential effect between the steel base and the hydroxyl groups, forming a rust layer; too high a temperature can easily cause grain growth, affecting performance.

[0034] In summary, the present invention overcomes the coarse grains of traditional steel bars by designing the composition, adding appropriate amounts of micro-alloying elements such as Ti and Al, and adopting a multi-stage controlled cooling and high deformation rate processing method to achieve ultra-fine graining of steel bars and obtain high-strength and toughness steel bars.

[0035] Compared with the existing technology, the present invention adds micro-alloy elements such as Ti and Al in appropriate amounts through composition design, and adopts multi-stage controlled cooling and high deformation rate. It not only makes the yield strength ≥520MPa, tensile strength ≥640MPa, strength-to-yield ratio ≥1.26, elongation ≥22%, and grain size ≤5um, but can fully meet the comprehensive performance requirements of high-end steel bars for construction such as high strength, seismic resistance, corrosion resistance, and weldability. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the grain size picture of the Φ16mm 500MPa strength grade steel bar of the present invention

[0037] Figure 2 This is a picture of the grain size of traditional Φ16mm 500MPa strength grade steel bars. DETAILED DESCRIPTION

[0038] The present invention is described in detail below:

[0039] Table 1 is a list of chemical composition values of various embodiments and comparative examples of the present invention;

[0040] Table 2 is a list of the main process parameter values of the steel examples of the present invention and the comparative examples;

[0041] Table 3 is a list of hot rolling performance test results of various embodiments and comparative examples of the steel of the present invention.

[0042] Each embodiment of the present invention is produced according to the following steps:

[0043] 1) After conventional converter smelting, it is cast into square billets with a cross-sectional size of 150mm*150mm.

[0044] 180mm*180mm;

[0045] 2) heating the ingot in a reducing atmosphere at a heating rate of 25-35°C / min to a temperature of 1000-1050°C and holding the temperature at this temperature for 30-50 minutes; and controlling the residual oxygen content in the heating furnace to ≤6%;

[0046] 3) Performing initial rolling, controlling the starting rolling temperature at 850-900°C; the cumulative reduction rate of initial rolling is not less than 40%;

[0047] 4) After conventional intermediate rolling, water tank is used for cooling, and the cooling water flow rate is controlled at 300~400m 3 / h, and cooled to 840-870℃;

[0048] 5) Two-stage finishing rolling is carried out, and the process is as follows: first stage finishing rolling - water cooling - recovery - second stage finishing rolling - water cooling - recovery, during which:

[0049] In the first finishing rolling stage, the finishing rolling temperature is controlled at 860-880℃ and the rolling speed is controlled at 10-30m / s.

[0050] The cumulative reduction rate is 50-70%; water cooling to 810-830℃, recovery time is 30-60s;

[0051] In the second finishing rolling stage, the finishing rolling temperature is controlled at 840-860°C, the rolling speed is 15-45 m / s, and the cumulative reduction rate is 20-60%. The steel is cooled to 820-840°C through water, and the recovery time is 50-80s.

[0052] 6) The temperature of the upper cooling bed is controlled at 840-860°C;

[0053] 7) Carry out post-processing.

[0054] Table 1 Chemical composition values of various embodiments and comparative examples of the present invention (wt%)

[0055]

[0056] Table 2 List of main process parameter values of steel examples of the present invention and comparative examples

[0057]

[0058] Note: The cumulative reduction ratio of initial rolling in each embodiment is not less than 40%; the residual oxygen content in the heating furnace in each embodiment is ≤6%.

[0059] Table 3 Hot rolling performance test results of various embodiments and comparative examples of the steel of the present invention

[0060]

[0061] As can be seen from Table 3, through the composition and process design of the present invention, a high-strength construction rod with a yield strength of 520 MPa or more, a tensile strength of 640 MPa or more, a strength-to-yield ratio of 1.26 or more, an elongation of 22% or more, and a grain size of 5 μm or less is obtained, which has good comprehensive properties such as seismic resistance and weldability.

[0062] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. An ultrafine-grained high-strength hot-rolled bar, comprising the following elements and weight percentages: C: 0.21-0.24%, Si: 0.60-0.80%, Mn: 1.30-1.45%, P: 0.035-0.050%, S: ≤0.025%, Al: 0.015-0.025%, Ti: 0.04-0.08%, N: 0.0120-0.0150%, Ceq = C + Mn / 6 + V / 5: 0.40-0.48, and the remainder being iron and unavoidable inclusions.

2. The ultrafine-grained high-strength hot-rolled bar according to claim 1, characterized in that: The weight percentage of Al is 0.018-0.023%.

3. The ultrafine-grained high-strength hot-rolled bar according to claim 1, characterized in that: The weight percentage of Ti is 0.04-0.06%.

4. A method for producing an ultrafine-grained, high-strength hot-rolled bar according to claim 1, comprising the steps of: 1) After conventional converter smelting, it is cast into square billets with a cross-sectional size of 150mm*150mm to 180mm*180mm; 2) Heating the ingot in a reducing atmosphere at a heating rate of 25-35°C / min to a temperature of 1000-1050°C. Keep the temperature at this temperature for 30 to 50 minutes and control the residual oxygen content in the heating furnace to ≤ 6%; 3) Performing initial rolling, controlling the starting rolling temperature at 850-900°C; the cumulative reduction rate of initial rolling is not less than 40%; 4) After conventional intermediate rolling, water tank is used for cooling, and the cooling water flow rate is controlled at 300~400m 3 / h, and cooled to 840-870℃; 5) Perform two-stage finishing rolling, the process is: first stage finishing rolling - water cooling - recovery - second stage finishing rolling - water cooling - recovery, during which: In the first finishing rolling stage, the temperature of the finishing rolling is controlled at 860-880℃ and the rolling speed is controlled at 10-30m / s. The cumulative reduction rate is 50-70%; water cooling to 810-830℃, recovery time is 30-60s; In the second finishing rolling stage, the finishing rolling temperature is controlled at 840-860°C, the rolling speed is 15-45 m / s, and the cumulative reduction rate is 20-60%. The steel is cooled to 820-840°C through water, and the recovery time is 50-80s. 6) The temperature of the upper cooling bed is controlled at 840-860°C; 7) Carry out post-processing.

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