High-toughness anti-seismic steel bar steel and preparation method thereof

Abstract

The application relates to the technical field of steel metallurgy and building steel material, and discloses a high-strength and high-toughness anti-seismic steel and a preparation method thereof. The steel contains C, Si, Mn, V, N, Nb, Ti, Al, and the balance of Fe and inevitable impurities. In the preparation, the molten steel is prepared and refined first, and then the molten steel is continuously cast into small billets; the small billets are subjected to transparent burning treatment, so that the center temperature of the billets and the temperature difference between the surface and the center are controlled; then, the billets are subjected to heat supplement treatment, so that the surface of the billets reaches the required heat state for rolling, and meanwhile, the center region is limited to enter the full-section high-temperature soaking state; finally, the steel material is obtained through hot rolling and controlled cooling. The method can reduce the tendency of the fine V and Nb carbonitride or the compound carbonitride precipitation phase to be concentrated and formed, so that the matching of the strength, the strength-yield ratio and the maximum total elongation rate of the anti-seismic steel bar made of the steel material can be maintained.

Description

Technical Field

[0001] This invention relates to the field of steel metallurgy and steel materials for construction, and particularly to a high-strength, high-toughness, earthquake-resistant steel bar and its preparation method. Background Technology

[0002] In structural seismic reinforcement, high yield strength is typically required while maintaining sufficient strength-to-yield ratio and total elongation at maximum force. For steel companies, these properties mainly depend on the chemical composition of the steel itself, the state of microalloying elements, and the microstructure after rolling and cooling. Currently, high-strength seismic reinforcement often employs microalloying designs with V and N, which improve the yield strength and microstructure refinement of the steel by forming fine carbonitride precipitates during rolling and cooling. For example, Chinese invention patent CN101717899A discloses an HRB500E vanadium-nitrogen high-strength seismic reinforcement and its production method. By using vanadium-nitrogen microalloying and controlling the rolling process, the precipitation of vanadium nitrides is enhanced to meet the strength requirements of HRB500E reinforcement.

[0003] The above methods can effectively improve the strength of steel. However, in actual production, the effect of microalloying precipitation strengthening on yield strength is quite sensitive. Huang Le et al. mentioned in their article "Analysis of the Reasons for the Unqualified Strength-to-Yield Ratio of HRB500E" published in Jiangxi Metallurgy that there are cases in the production of HRB500E steel bars where the yield strength is too high and the strength-to-yield ratio is unqualified due to the high N content. This indicates that in high-strength earthquake-resistant steel bars, the higher the content of microalloying elements such as V and N or the more sufficient the precipitation strengthening, the better. When the precipitation strengthening has an excessively strong effect on the yield strength, although the steel can easily reach a higher yield strength, the tensile strength and the total elongation at maximum force may not increase simultaneously.

[0004] Therefore, there is still room for improvement in the existing high-strength earthquake-resistant steel: how to ensure the strength grade of the steel while avoiding excessive concentration of microalloying precipitation to increase the yield strength, so that the steel can stably maintain the strength-to-yield ratio and plasticity reserve after being made into earthquake-resistant steel, is a problem that needs to be further solved in the production of this type of steel. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that when the steel used for high strength and toughness earthquake-resistant steel bars is improved by V and N microalloying, the steel billet is prone to precipitation strengthening of strong vanadium nitrides during the rolling and cooling stage after being fully heated at high temperature. This causes the yield strength to increase too quickly, while the tensile strength and the total elongation at maximum force fail to increase at the same time. As a result, the strength-to-yield ratio and plasticity reserve of the steel after being made into earthquake-resistant steel bars are not easy to stably meet the requirements.

[0006] To achieve the above objectives, this application adopts the following technical solution: a high-strength and high-toughness earthquake-resistant steel bar and its preparation method, comprising the following steps: S1. Molten steel is prepared using scrap steel or recycled steel of the same type as raw materials, wherein the molten steel comprises, by mass percentage: C: 0.21%-0.25%, Si: 0.35%-0.60%, Mn: 1.30%-1.55%, V: 0.040%-0.070%, N: 0.0080%-0.0130%, Nb: 0.006%-0.016%, Ti: 0.004%-0.012%, Al: 0.010%-0.035%, with the balance being Fe; S2. The molten steel is continuously cast into a continuous casting billet, and then the continuous casting billet is sent into a heating furnace for low-temperature through-firing. At the end of the through-firing, the center temperature of the billet is 1010-1060℃, the surface temperature of the billet is not lower than the center temperature, and the difference between the surface temperature and the center temperature of the billet is not greater than 35℃. S3. After low-temperature through-firing, the billet is subjected to short-term surface reheating treatment. At the end of the reheating, the surface temperature of the billet is 1100-1150℃, the center temperature of the billet is not higher than 1085℃, and the surface temperature of the billet is 40-85℃ higher than the center temperature. S4. The steel billet that has undergone short-term surface reheating is hot rolled and cooled after rolling to obtain high-strength and high-toughness earthquake-resistant steel bars.

[0007] Preferably, in step S1, the scrap steel and the same type of recycled material are added to a converter to smelt into molten steel, and the tapping temperature of the molten steel is 1630-1680℃.

[0008] Preferably, the molten steel is tapped and then refined in an LF furnace. During refining, the contents of C, Si, Mn and Al in the molten steel are first adjusted, then the contents of Nb and Ti are adjusted, and finally the contents of V and N in the molten iron are adjusted.

[0009] Preferably, the V content is controlled by adding ferrovanadium with a V content of 50%-80% and / or a vanadium-nitrogen alloy with a V content of 70%-82% and a N content of 10%-18%. The amount of ferrovanadium and vanadium-nitrogen alloy added is determined based on the total nitrogen content of the molten steel before the addition of ferrovanadium and vanadium-nitrogen alloy, so that the V content in the molten steel at the refining endpoint is 0.040%-0.070% and the total nitrogen content is 0.0080%-0.0130%. Preferably, in step S2, the size of the continuous billet is a 150mm×150mm small square billet or a 165mm×165mm small square billet.

[0010] Preferably, in step S2, during the continuous casting process, the superheat of the molten steel in the tundish is 15℃-30℃, the total amount of water used for secondary cooling is 0.55L / kg steel-0.85L / kg steel, and the surface temperature of the continuously cast billet before straightening is 900℃-980℃.

[0011] Preferably, in steps S2 and S3, the surface temperature of the billet is detected by an infrared temperature measuring device installed at the outlet end of the heating furnace or at the outlet roller conveyor, and the center temperature of the billet is determined by a pre-established heating process control curve, which is established by the center temperature data of the same specification billet measured by an embedded thermocouple during the trial production stage.

[0012] Preferably, in step S2, before firing, the small square billet is preheated at 750-900℃ for 25-45 minutes.

[0013] Preferably, in step S2, the low-temperature burn-through conditions are 1030-1080℃ and the burn-through time is 35-70min.

[0014] Preferably, in step S3, the short-term surface heating conditions are 1120-1180℃ and the heating time is 6-12min.

[0015] A high-strength and high-toughness earthquake-resistant steel bar, wherein the high-strength and high-toughness earthquake-resistant steel bar is prepared by the above-described preparation method.

[0016] Preferably, the steel used for the high-strength and high-toughness earthquake-resistant steel reinforcement has a lower yield strength of 525-575 MPa, a tensile strength of 700-780 MPa, a ratio of the measured tensile strength to the measured lower yield strength of not less than 1.33, and a maximum total elongation of not less than 11.8%.

[0017] The technical effects and advantages of this invention are as follows: By defining the V and N microalloying system and combining low-temperature through-heating with short-time surface reheating, the surface of the billet obtains the required thermal state before entering rolling, while avoiding the central area of ​​the billet undergoing full-section high-temperature homogenization. This process does not increase the yield strength by increasing the V and N content or extending the high-temperature heating time, but reduces the tendency of fine V and Nb carbonitride precipitates to concentrate while retaining the necessary microalloying precipitation strengthening. This maintains a coordinated relationship between yield strength and tensile strength. The resulting high-strength and high-toughness seismic steel bars, when used to make hot-rolled seismic steel bars, can balance strength, strength-to-yield ratio, and maximum force elongation, reducing the situation where insufficient seismic plasticity reserve is caused by the yield strength being raised too quickly. Attached Figure Description

[0018] The disclosure of this invention is illustrated with reference to the accompanying drawings. It should be understood that the drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. In the drawings: Figure 1 A bar chart showing the yield strength ratio test results of steel bars for each sample; Figure 2 These are TEM images of the steel bar cross sections of Example 1 and Comparative Example 1. Detailed Implementation

[0019] This embodiment provides a high-strength and high-toughness earthquake-resistant steel bar, comprising, by mass percentage: C: 0.21%-0.25%, Si: 0.35%-0.60%, Mn: 1.30%-1.55%, V: 0.040%-0.070%, N: 0.0080%-0.0130%, Nb: 0.006%-0.016%, Ti: 0.004%-0.012%, Al: 0.010%-0.035%, with the balance being Fe.

[0020] Specifically, C, Si, and Mn are used to ensure the basic strength of steel and adjust the microstructure ratio of ferrite and pearlite after rolling. V and N are used to form a microalloying strengthening base. Nb and Ti are used to control the state of austenite grains and carbonitrides. Among them, Ti can fix some N, and Al is used for deoxidation of molten steel.

[0021] The raw materials used in the following embodiments are all conventional raw materials available in the metallurgical industry. Fe mainly comes from scrap steel and recycled materials of the same steel grade; the scrap steel is clean carbon steel scrap; the recycled materials of the same steel grade are clean remelting materials formed during the cutting of the head and tail of continuously cast billets of the same steel grade or during the rolling process; C mainly comes from molten iron and scrap steel; Si can be added from ferrosilicon or silicon manganese alloy, wherein the Si content in the ferrosilicon is 14-20%, and the Si content in the silicon manganese alloy is 70-75%; Mn can be added from ferrosilicon or silicon manganese alloy, wherein the Mn content in the ferrosilicon is 60-68%, and the... The manganese iron contains 65-80% Mn; V can be added from ferrovanadium or vanadium-nitrogen alloy, wherein the ferrovanadium contains 50-80% V and the vanadium-nitrogen alloy contains 70-82% V; Ti can be added from ferrotitanium or titanium-containing alloy, wherein the ferrotitanium contains 25-35% Ti; N mainly comes from the background nitrogen in molten steel and vanadium-nitrogen alloy, wherein the vanadium-nitrogen alloy contains 10-18% Nb; Nb can be added from ferroniobium, wherein the ferroniobium contains 55-70% Nb; Al can be added from aluminum wire, aluminum granules or aluminum-iron, wherein the aluminum wire or aluminum granules contain not less than 98% Al.

[0022] This embodiment also provides a method for preparing high-strength and high-toughness earthquake-resistant steel bars, specifically including the following steps: S1: Scrap steel and recycled materials of the same steel grade are added to the converter for smelting. After tapping, the steel is deoxidized and alloyed, and then the molten steel is sent to the LF furnace for refining. During the refining process, the contents of Si, Mn, Al, Nb, Ti, V, N and C in the molten steel are adjusted so that the composition of the molten steel meets the aforementioned composition range for high strength and toughness earthquake-resistant steel bars. S2: The refined molten steel is continuously cast into small square billets, and the superheat of the molten steel during the continuous casting process is controlled to obtain the continuously cast billet. The continuously cast billet is then sent into the heating furnace for preheating and low-temperature through-firing in sequence, so that the billet as a whole reaches the basic temperature before rolling. S3: After low-temperature through-firing, the billet is sent to the high-temperature reheating section near the furnace end for short-term surface reheating, so that the surface temperature of the billet is higher than the core temperature, and after the reheating is completed, it enters the roughing mill. S4: The steel billet that has undergone short-term surface reheating is sequentially subjected to rough rolling, intermediate rolling and finish rolling. After final rolling, it is subjected to weak water cooling or intermittent weak cooling, and then naturally cooled on a cooling bed to obtain high-strength and tough earthquake-resistant steel bars.

[0023] Specifically, in S1, during the converter smelting, the C content of the molten steel is adjusted to 0.21%-0.25%, and the tapping temperature is controlled at 1630-1680℃. During the tapping process, aluminum wire, aluminum granules, or ferroaluminum are added for deoxidation, and manganese silicon alloy and ferrosilicon are added to initially adjust the Mn and Si content. After the molten steel enters the LF furnace, the C, Si, Mn, and total aluminum content are further adjusted according to the molten steel composition test results. Then, ferroniobium and ferrotitanium are added, and finally, ferrovanadium and vanadium-nitrogen alloy are added to ensure that the final composition of the molten steel meets the composition range of the above-mentioned high-strength and high-toughness earthquake-resistant steel bars, and the final N content of the refining is controlled at 0.0080%-0.0130%.

[0024] In S2, the refined molten steel is continuously cast into small square billets of 150mm×150mm or 165mm×165mm. During the continuous casting process, the superheat of the molten steel in the tundish is controlled at 15-30℃. The secondary cooling adopts a weak cooling or medium-weak cooling method, and the secondary cooling water volume is adjusted according to the specifications of the small square billets, so that the total secondary cooling water volume is 0.55L / kg steel-0.85L / kg steel. The preheating temperature is controlled at 750-900℃, and the preheating time is 25-45min. The low-temperature through-firing temperature is controlled at 1030-1080℃, and the low-temperature through-firing time is 35-70min. When the low-temperature through-firing is completed, the center temperature of the billet is controlled at 1010-1060℃, and the temperature difference between the surface and the center of the billet is controlled to be no more than 35℃.

[0025] In S3, the furnace temperature in the high-temperature reheating section is controlled at 1120-1180℃, and the reheating time is controlled at 6-14 minutes. When the reheating is completed, the surface temperature of the billet is controlled at 1100-1150℃, the center temperature of the billet is not higher than 1085℃, and the temperature difference between the surface and center of the billet is 40-85℃. After the short-term surface reheating is completed, the billet enters the roughing mill within 60 seconds. The surface temperature of the billet is detected by an infrared temperature measuring device at the furnace exit end or an infrared temperature measuring device on the furnace exit roller. The center temperature of the billet is determined by a pre-established heating process control curve. The heating process control curve is established by the center temperature data of the same specification billet measured by embedded thermocouples during the trial production stage.

[0026] In S4, after the billet undergoes short-term surface reheating, it is fed into the first roughing mill within 60 seconds. Before the first roughing mill bites in, the billet surface temperature is controlled at 1085-1135℃, and the billet surface temperature is kept higher than the center temperature. The cumulative reduction rate of the first three roughing passes is controlled at 35%-55%, and the interval between two adjacent passes does not exceed 8 seconds. The first three roughing passes are completed when the billet surface temperature is not lower than 1040℃. After roughing, intermediate rolling and finishing rolling are carried out in sequence. The intermediate rolling inlet temperature is controlled at 960-1030℃, the finishing rolling inlet temperature is controlled at 910-970℃, and the finishing rolling temperature is controlled at 890-940℃. After finishing rolling, the rolled steel bars are continuously or intermittently cooled by controlled water. By adjusting the cooling water volume, water flow time, and steel bar running speed, the temperature of the steel bars on the cooling bed is controlled at 820-880℃. After the steel bars reach the specified temperature on the cooling bed, they enter the cooling bed and are naturally cooled to room temperature.

[0027] To further illustrate the technical solution of the present invention, the following description is provided in conjunction with embodiments. These embodiments are used to illustrate the implementation of the present invention and are not intended to limit the scope of protection of the present invention. In the following embodiments, unless otherwise specified, the content of each element is a mass percentage. Scrap steel, recycled steel of the same type, silicon-manganese alloy, ferrosilicon, ferromanganese, ferrovanadium, vanadium-nitrogen alloy, ferroniobium, ferrotitanium, aluminum wire, aluminum granules, and ferroaluminum are all conventional raw materials that can be obtained in metallurgical production.

[0028] Example 1

[0029] This embodiment provides a high-strength and high-toughness earthquake-resistant steel bar, comprising, by mass percentage: C: 0.23%, Si: 0.45%, Mn: 1.45%, V: 0.055%, N: 0.0105%, Nb: 0.010%, Ti: 0.008%, Al: 0.022%, with the balance being Fe.

[0030] The selected silicon-manganese alloy has a Mn content of 65.2%, a Si content of 17.0%, and a C content of 1.9%; the ferrosilicon has a Si content of 72.1%; the ferromanganese is a high-carbon ferromanganese alloy with a Mn content of 74.8% and a C content of 7.1%; the ferroniobium has a Nb content of 65.0%; the ferrotitanium has a Ti content of 30.2%; the vanadium-nitrogen alloy has a V content of 78.0% and a N content of 16.0%; the ferrovanadium has a V content of 50.0%; and the aluminum wire has an Al content of 99.2%.

[0031] This embodiment also provides a method for preparing high-strength and high-toughness earthquake-resistant steel bars, the steps of which include: S1: Scrap steel and recycled steel of the same type are added to the converter for smelting. The final carbon content in the converter is controlled at 0.23%, and the tapping temperature is 1655℃. After tapping, deoxidation and alloying are performed, and then the molten steel is sent to the LF furnace for refining. During the LF refining process, samples of the molten steel are taken after entering the LF furnace to test the C, Si, Mn, and Alt contents. The composition is fine-tuned according to the test results. When the Mn content is insufficient, ferromanganese is added; when the Si content is insufficient, ferrosilicon is added; when the Alt content is insufficient, aluminum wire is added. The C, Si, Mn, and Alt contents in the molten steel are close to... After reaching the target values, ferroniobium and ferrotitanium are added to bring the Nb and Ti contents close to the target values. Finally, vanadium-nitrogen alloy is added to ensure that the final composition of the molten steel meets the above composition range. The final temperature of LF refining is 1590℃. After refining, soft blowing is performed for 12 minutes. At the end of soft blowing, the composition of the molten steel is: C: 0.23%, Si: 0.45%, Mn: 1.45%, V: 0.055%, N: 0.0105%, Nb: 0.010%, Ti: 0.008%, Al: 0.022%. S2: The refined molten steel is poured into the tundish and continuously cast into 150mm×150mm small square billets. During the continuous casting process, the superheat of the molten steel in the tundish is 22℃. The secondary cooling uses water with a total specific water volume of 0.70L / kg steel. The surface temperature of the continuously cast billet before straightening is controlled at 940℃. The continuously cast billet is sent into the heating furnace and preheated at 820℃ for 35 minutes, and then subjected to low-temperature through-heating at 1060℃ for 50 minutes. When the low-temperature through-heating is completed, the center temperature of the billet is confirmed to be 1045℃ according to the heating process control curve. The temperature difference between the surface and the center of the billet is no more than 35℃. S3: After low-temperature heat treatment, the billet is sent to the high-temperature reheating section near the furnace end and reheated at 1150℃ for 9 minutes. When the reheating is completed, the surface temperature of the billet is 1125℃, the center temperature of the billet is 1070℃, and the temperature difference between the surface and the center of the billet is 55℃. After the short-term surface reheating is completed, the billet enters the roughing mill within 45 seconds. S4: The steel billet that has undergone short-term surface reheating is sequentially subjected to rough rolling, intermediate rolling, and finish rolling. The entry temperature of the rough rolling is 1090℃, and the cumulative reduction rate of the first three rough rolling passes is 46%. The entry temperature of the intermediate rolling is 1000℃. The entry temperature of the finish rolling is 935℃, and the final rolling temperature is 920℃. After the final rolling, the finished steel bar is sent to the controlled cooling section for continuous controlled water cooling. By adjusting the cooling water volume and the running speed of the steel bar, the temperature of the cooling bed above the steel bar is 845℃ before entering the cooling bed. After entering the cooling bed, the steel bar is naturally cooled to room temperature, thus obtaining the high-strength and high-toughness earthquake-resistant steel bar steel of this embodiment.

[0032] Example 2

[0033] The difference between this embodiment and Embodiment 1 lies in the higher V and N content in the high-strength and high-toughness earthquake-resistant steel. Specifically, the high-strength and high-toughness earthquake-resistant steel in this embodiment comprises, by mass percentage: C: 0.22%, Si: 0.50%, Mn: 1.50%, V: 0.062%, N: 0.0118%, Nb: 0.012%, Ti: 0.006%, Al: 0.020%, with the balance being Fe. Apart from the above compositional differences, the remaining smelting, refining, continuous casting, rolling, and cooling operations are carried out in accordance with Embodiment 1.

[0034] Example 3

[0035] The difference between this embodiment and Embodiment 1 lies in the lower V and N content in the high-strength and high-toughness earthquake-resistant steel. Specifically, the high-strength and high-toughness earthquake-resistant steel in this embodiment comprises, by mass percentage: C: 0.24%, Si: 0.40%, Mn: 1.50%, V: 0.048%, N: 0.0095%, Nb: 0.008%, Ti: 0.010%, Al: 0.024%, with the balance being Fe. Apart from the above-mentioned compositional differences, the remaining smelting, refining, continuous casting, rolling, and cooling operations are carried out in accordance with Embodiment 1.

[0036] Example 4

[0037] The difference between this embodiment and Embodiment 1 lies in the high-temperature reheating conditions of the short-term surface layer after low-temperature through-firing, as follows: After low-temperature through-firing, the billet enters the high-temperature reheating section near the furnace exit end, and is briefly reheated at 1170℃ for 12 minutes. At the end of the reheating, the surface temperature of the billet is 1148℃, the center temperature is 1082℃, and the temperature difference between the surface and the center is 66℃. The other conditions are the same as in Embodiment 1.

[0038] Example 5

[0039] The difference between this embodiment and Embodiment 1 lies in the short-term low-temperature reheating conditions of the surface layer after low-temperature through-firing, as follows: After low-temperature through-firing, the billet enters the high-temperature reheating section near the furnace exit end, and the surface layer is reheated for 6 minutes at 1130℃. At the end of the reheating, the surface temperature of the billet is 1102℃, the center temperature is 1052℃, and the temperature difference between the surface and the center is 50℃. The other conditions are the same as in Embodiment 1.

[0040] Comparative Example 1 The difference between this comparative example and Example 1 is that the short-term surface reheating after low-temperature through-heating is cancelled, and conventional full-section high-temperature homogenization is replaced. Specifically, the steel composition of this comparative example is the same as that of Example 1. After the continuously cast billet enters the heating furnace, it is first preheated at 850°C for 35 minutes, and then homogenized at 1160°C for 60 minutes. At the end of homogenization, the surface temperature of the billet is 1148°C, the center temperature is 1135°C, and the temperature difference between the surface and the center is 13°C. The remaining rolling and cooling conditions are the same as those of Example 1.

[0041] Comparative Example 2 The difference between this comparative example and Example 1 is that the low-temperature through-firing temperature is too high. Specifically, the steel composition of this comparative example is the same as that of Example 1. After the continuously cast billet enters the heating furnace, it is first preheated at 850°C for 35 minutes, and then through-firing at 1105°C for 50 minutes. At the end of through-firing, the center temperature of the billet is 1090°C, and the temperature difference between the surface and the center of the billet is 18°C. Subsequently, the billet enters the high-temperature reheating section near the furnace exit end and is reheated at 1150°C for 9 minutes. At the end of reheating, the surface temperature of the billet is 1130°C, the center temperature is 1102°C, and the temperature difference between the surface and the center is 28°C. The remaining rolling and cooling conditions are the same as those of Example 1.

[0042] Comparative Example 3 The difference between this comparative example and Example 1 is that the surface overheating was excessive during a short period of time. Specifically, the steel composition of this comparative example is the same as that of Example 1. After low-temperature through-firing, the billet was overheated at 1160°C for 25 minutes. At the end of the overheating, the surface temperature of the billet was 1152°C, the center temperature was 1112°C, and the temperature difference between the surface and the center was 40°C. The remaining rolling and cooling conditions are the same as those of Example 1.

[0043] Comparative Example 4 The difference between this comparative example and Example 1 is that the V and N contents in the steel exceed the limits of this invention, specifically as follows: The high-strength and high-toughness earthquake-resistant steel bars in this comparative example contain, by mass percentage: C: 0.23%, Si: 0.45%, V: 0.078%, N: 0.0160%, Nb: 0.010%, Ti: 0.008%, Al: 0.022%, with the balance being Fe. The low-temperature through-heating, short-time surface reheating, rolling, and post-rolling cooling measures in this comparative example are the same as those in Example 1.

[0044] Comparative Example 5 The difference between this comparative example and Example 1 is that the V and N contents in the steel are lower than the limits defined in this invention, specifically as follows: The high-strength and high-toughness earthquake-resistant steel bars in this comparative example comprise, by mass percentage: C: 0.23%, Si: 0.45%, V: 0.032%, N: 0.0065%, Nb: 0.010%, Ti: 0.008%, Al: 0.022%, with the balance being Fe. The low-temperature through-heating, short-time surface reheating, rolling, and post-rolling cooling measures in this comparative example are the same as those in Example 1.

[0045] Comparative Example 6 The difference between this comparative example and Example 1 is that the short-term surface reheating is insufficient, as follows: The steel composition of this comparative example is the same as that of Example 1. After low-temperature through-heating, the billet is reheated at 1120°C for 3 minutes. At the end of the reheating, the surface temperature of the billet is 1086°C, the center temperature is 1052°C, and the temperature difference between the surface and the center is 34°C. Its surface temperature does not reach 1100°C, and the temperature difference between the surface and the center does not reach 40°C. The remaining rolling and cooling conditions are the same as those of Example 1.

[0046] To further illustrate the effects of composition control, low-temperature through-firing, and short-time surface reheating on the resulting high-strength and high-toughness earthquake-resistant steel bars in this invention, the following experimental examples were set up to test and evaluate the samples obtained from each embodiment and comparative example. Three steel billet samples were randomly selected from each group for testing.

[0047] Experimental Example 1 This experiment aims to verify the effects of short-term surface reheating after low-temperature through-firing and the control of V and N content in the steel on the mechanical properties of seismic reinforcing bars. Seismic reinforcing bars obtained in Examples 1-5 and Comparative Examples 1-6 were selected as test samples. All the above samples were tested according to the same sampling location and test method. The strength of the steel used for high-strength and high-toughness seismic reinforcing bars is mainly evaluated by yield strength and tensile strength, while the toughness is mainly evaluated by the strength-to-yield ratio, total elongation at maximum force, and reverse bending performance of hot-rolled seismic reinforcing bars.

[0048] Tensile specimens were taken from seismic-resistant steel bars of the same specification for each sample. The tensile performance tests included the following indicators: yield strength ReL, used to evaluate whether the yield strength of the steel is too high due to changes in the heating regime; tensile strength Rm, used to evaluate whether the tensile strength increases synchronously when the yield strength increases; strength-to-yield ratio Rm / ReL, used to evaluate the load-bearing reserve of the steel bar after yielding; total elongation at maximum force Agt, used to evaluate the plastic deformation capacity of the steel bar under maximum force; and reverse bending test, used to confirm that the steel bar still has the necessary deformation adaptability under bending and reverse bending conditions.

[0049] The tensile test is carried out at room temperature. During the test, the load and elongation are continuously recorded to obtain a load-elongation curve, the yield load and maximum load are determined according to the curve, and the yield strength ReL and tensile strength Rm are calculated; wherein ReL is used to evaluate whether the yield strength of the steel is excessively high, Rm is used to evaluate whether the tensile strength increases synchronously with the yield strength, the strength-yield ratio Rm / ReL is calculated from the tensile strength and the yield strength, and is used to evaluate the load-bearing reserve of the steel bar after yielding, the total elongation at maximum force Agt is determined according to the total elongation corresponding to the maximum load, and is used to evaluate the plastic deformation capacity of the steel bar under the action of maximum force.

[0050] The reverse bending test adopts a bending device matching the specification of the steel bar. The sample is first subjected to initial bending in the specified bending direction, and then subjected to reverse bending. After bending, whether there is any visible crack or fracture on the outer arc surface of the sample is observed. In each group of samples, when no crack or fracture occurs in any reverse bending sample of the group, the reverse bending test of the group is determined as qualified; when any sample has crack or fracture, the reverse bending test of the group is determined as unqualified. The specific test results are shown in Table 1, Figure 1 as shown.

[0051] Table 1 Test results of mechanical properties of steel bars of each sample

[0052] It can be seen from the results in Table 1 that the detection results show that the ReL of Examples 1 to 5 are all maintained in the range of 525MPa to 575MPa, Rm is maintained in the range of 700MPa to 780MPa, Rm / ReL is maintained above 1.28, Agt is all not less than 11.0%, and all reverse bending tests are qualified. While maintaining the 500MPa-level yield strength and tensile strength, each example has both high strength-yield ratio and high total elongation at maximum force, indicating that the present invention can maintain the high toughness of the steel bar while maintaining the high strength of the steel bar.

[0053] In contrast, Comparative Examples 1-3 and 6 did not employ the low-temperature through-firing and short-term surface heating methods of this invention. The mechanical properties of the samples showed higher ReL values, accompanied by a decrease in Rm / ReL and Agt. This indicates that excessively high low-temperature through-firing temperatures, high-temperature uniform heating across the entire cross-section, or excessive surface heating can all cause the center of the billet to be exposed to higher temperatures, resulting in an increase in ReL of the reinforcing bar without a corresponding increase in Rm. Insufficient surface heating, on the other hand, will cause a decrease in Rm and Agt. Although Comparative Examples 4-5 adopted the low-temperature burn-through and short-time surface heating method of the present invention, they did not adopt the V and N content range of the present invention. When the V and N content is too high, the yield strengthening is excessive, and the strength-to-yield ratio and the total elongation at maximum force decrease; when the V and N content is too low, the precipitation strengthening is insufficient, and both the yield strength and tensile strength decrease. This shows that the present invention does not simply reduce the heating temperature of the billet, but rather, under the premise of controlling the V and N content of the steel, after low-temperature burn-through, short-time surface heating is used to make the surface of the billet reach the temperature required for rolling, while limiting the degree of high temperature exposure in the central area, so that the resulting seismic steel reinforcement can maintain good post-yield load-bearing capacity and plastic deformation capacity while meeting the strength requirements.

[0054] Experiment Example 2 Experimental Example 1 has already demonstrated from the perspective of mechanical properties that different preparation conditions and variations in V and N content affect the strength, yield strength ratio, and total elongation at maximum force of seismic-resistant steel bars. To verify whether the differences in mechanical properties are related to cross-sectional hardness, this experimental example conducts microstructure and cross-sectional microhardness tests on the hot-rolled seismic-resistant steel bars obtained in Examples 1-5 and Comparative Examples 1-6. By detecting the content of ferrite, pearlite, and the presence of martensite in the cross-section of the steel bars, the relationship between cross-sectional hardness and changes in mechanical properties is analyzed.

[0055] All samples were taken from finished hot-rolled earthquake-resistant steel bars that had been naturally cooled to room temperature on a cooling bed, avoiding the temperature fluctuation sections at the beginning and end of the rolled piece. At least three cross-sectional metallographic specimens were taken from each group. After mounting, grinding, and polishing, the specimens were etched using a 4% (v / v) nitric acid-alcohol solution. Structural determination was based on the metallographic morphology after etching. A light-colored, blocky, or polygonal matrix structure after nitric acid-alcohol etching was ferrite; a dark-colored lamellar or nodular ferrite-cementite eutectoid structure was pearlite; and a needle-like, lath-like, or blocky structure with high corrosion contrast was martensite. Observation locations included the surface, 1 / 4 radius, and core. Statistical analysis was conducted on each sample. Fewer than 10 fields of view were used; and image analysis methods were employed to perform volumetric analysis. During the analysis, metallographic images were first acquired under the same magnification and corrosion conditions. Then, the ferrite and pearlite regions were segmented according to the above morphology criteria, and the volume fraction was approximated by the area fraction. Martensite was not included in the area fraction statistics. When no martensite morphology characteristics were found in any of the observation fields of the surface, 1 / 4 radius position, and core, it was recorded as not seen. At the same time, the surface and core were observed to confirm whether there were abnormally concentrated areas of martensite. Ferrite grain size was evaluated using the comparison method or the intercept method. The specific test results are shown in Table 2.

[0056] Table 2 Microhardness test results for each sample

[0057] As shown in Table 2, the microstructure of the hot-rolled seismic-resistant steel bars obtained in Examples 1-5 is mainly composed of ferrite and pearlite, with a bainite volume fraction of 1.0%-2.0% and no martensite. The ferrite grain size is grade 8.8-9.3. In contrast, the ratio of ferrite and pearlite in Comparative Examples 1-6 is close to that in Example 1, and no martensite is observed. Based on the results of the mechanical property differences in Experimental Example 1, it can be concluded that the changes in mechanical properties of Comparative Examples 1-3 cannot be attributed to martensite or excessive difference in cross-sectional hardness. Therefore, the changes in mechanical properties exhibited by Comparative Examples 1-6 in Experimental Example 1 need to be further explained in conjunction with the V and Nb enriched carbonitride or composite carbonitride precipitates, especially the proportion of fine precipitates <20nm.

[0058] Experimental Example 3 To further illustrate the differences between the embodiments and comparative examples in yield strength, strength-to-yield ratio, and total elongation at maximum force, this experimental example observes and statistically analyzes the precipitation state of V and Nb enriched carbonitrides or composite carbonitrides in hot-rolled seismic steel bars. This experimental example selects seismic steel bars prepared from the steels obtained in Examples 1-5 and Comparative Examples 1-6 as the test objects.

[0059] During sample preparation, a thin slice is first cut from the sampling location and thinned to approximately 70-90µm by mechanical grinding. Then, a transmission electron microscope (TEM) observation sample is prepared using a double-jet electrolytic thinning method. The electrolyte is a 5%-10% perchloric acid-ethanol solution. The double-jet temperature is controlled between -25℃ and -15℃, and the double-jet voltage is 20-30V. At least three TEM thin film samples are prepared for each group of samples, and the effective observation areas are selected from different thin film samples for statistical analysis.

[0060] The morphology of the precipitated phases was observed using transmission electron microscopy at magnifications ranging from 80,000 to 200,000. At least 12 effective fields of view were selected for each sample, with each field of view having an area of ​​0.18–0.32 µm. 2 For each sample, a total of no less than 240 V and Nb carbonitride precipitate particles were counted. During the count, the equivalent circle diameter was calculated by converting the projected area of ​​the precipitate after calibration by image analysis software. The number of particles was counted according to three intervals: equivalent diameter <20nm, 20-80nm, and >80nm. The average equivalent diameter and the number density per unit area were also recorded. The detection results are shown in Table 3. Figure 2 As shown.

[0061] Table 3 Statistical results of carbonitride precipitates for each sample

[0062] As shown in Table 3, in Examples 1-5, the proportion of V and Nb carbonitride precipitates with an equivalent diameter of <20nm was 35%-42%, and the proportion of precipitates with a diameter of 20nm-80nm was 53%-60%. This indicates that under the V and N content window defined by the present invention and the conditions of low temperature through-firing and short-time surface heat replenishment, there are still necessary micro-alloyed carbonitride precipitates in the steel bars, but the fine precipitates did not become excessively concentrated.

[0063] In Comparative Examples 1-3, the proportion of fine precipitates <20nm increased to 56.2%-61.0% due to excessively high full-section high-temperature homogenization, excessive through-firing temperature, or excessive supplementary heating. In Comparative Example 6, due to insufficient supplementary heating, the proportion of fine precipitates <20nm was also higher than that in Example 1. Combined with the mechanical property test results of Experimental Example 1, it can be seen that the above-mentioned process deviations from the samples failed to achieve the relatively stable matching relationship of strength, yield strength ratio, and total elongation at maximum force as in Examples 1-5. This indicates that the control of V and N content still needs to be combined with the heating state formed by low-temperature through-firing and short-term surface supplementary heating in order to stably control the carbonitride precipitation state.

[0064] In Comparative Example 4, the V and N contents were higher than those of this invention, and the proportion of fine precipitates <20nm reached 66.1%, with a number density of 198 precipitates / µm. 2The content of V and N in Comparative Example 5 is higher than that in Examples 1-5; the content of V and N in Comparative Example 5 is lower than that in this invention, and the proportion of fine precipitates <20nm is relatively low, but the microalloying strengthening basis is insufficient. Combined with the mechanical property test results of Experiment 1, it can be seen that when the content of V and N is too high, it is easy to cause the yield strengthening contribution to be too strong, and when the content of V and N is too low, it is difficult to meet the target strength requirements. This shows that the limitation of V and N content in this invention is not simply to reduce the content of microalloying elements, but to avoid excessive concentration of fine carbonitride precipitates while ensuring strength.

[0065] This invention controls the V and N content and avoids keeping the entire cross-section of the billet at a high temperature for a long time. While retaining the necessary microalloying precipitation strengthening, it reduces the tendency for excessive fine V and Nb carbonitride precipitates to concentrate and form. This change in precipitation state is consistent with the results of yield strength, strength-to-yield ratio and total elongation at maximum force in Examples 1-5 of Experiment 1.

[0066] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A method for preparing high-strength, high-toughness, earthquake-resistant reinforcing steel, characterized in that, Includes the following steps: S1. Molten steel is prepared using scrap steel or recycled steel of the same type as raw materials, wherein the molten steel comprises, by mass percentage: C: 0.21%-0.25%, Si: 0.35%-0.60%, Mn: 1.30%-1.55%, V: 0.040%-0.070%, N: 0.0080%-0.0130%, Nb: 0.006%-0.016%, Ti: 0.004%-0.012%, Al: 0.010%-0.035%, balance Fe; S2. The molten steel is continuously cast into a continuous casting billet in a tundish, and then the continuous casting billet is sent to a heating furnace for low-temperature through-firing. At the end of the through-firing, the center temperature of the billet is 1010-1060℃, the surface temperature of the billet is not lower than the center temperature, and the difference between the surface temperature and the center temperature of the billet is not greater than 35℃. S3. After low-temperature through-firing, the billet is subjected to short-term surface reheating treatment. At the end of the reheating, the surface temperature of the billet is 1100-1150℃, the center temperature of the billet is not higher than 1085℃, and the surface temperature of the billet is 40-85℃ higher than the center temperature. S4. The steel billet that has undergone short-term surface reheating is hot rolled and cooled after rolling to obtain high-strength and high-toughness earthquake-resistant steel bars.

2. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In step S1, the scrap steel and recycled materials of the same steel grade are added to the converter and smelted into molten steel. The tapping temperature of the molten steel is 1630-1680℃.

3. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 2, characterized in that, After tapping, the molten steel enters the LF furnace for refining. During refining, the contents of C, Si, Mn and Al in the molten steel are first adjusted, then the contents of Nb and Ti are adjusted, and finally the contents of V and N in the molten iron are adjusted.

4. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 3, characterized in that, The V content is controlled by adding ferrovanadium with a V content of 50%-80% and / or vanadium-nitrogen alloy with a V content of 70%-82% and a N content of 10%-18%. The amount of ferrovanadium and vanadium-nitrogen alloy added is determined based on the total nitrogen content in the molten steel before the addition of ferrovanadium and vanadium-nitrogen alloy, so that the V content in the molten steel at the refining endpoint is 0.040%-0.070% and the total nitrogen content is 0.0080%-0.0130%.

5. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In step S2, the size of the continuous billet is a 150mm×150mm small square billet or a 165mm×165mm small square billet.

6. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In step S2, the superheat of molten steel in the tundish during continuous casting is 15℃-30℃, the total amount of water used for secondary cooling is 0.55L / kg steel-0.85L / kg steel, and the surface temperature of the continuously cast billet before straightening is 900℃-980℃.

7. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In steps S2 and S3, the surface temperature of the billet is detected by an infrared temperature measuring device installed at the outlet end of the heating furnace or at the outlet roller conveyor, and the center temperature of the billet is determined by a pre-established heating process control curve. The heating process control curve is established by the center temperature data of the same specification billet measured by embedded thermocouples during the trial production stage.

8. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In step S2, before firing, the small square billet is preheated at 750-900℃ for 25-45 minutes.

9. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In step S2, the conditions for low-temperature through-firing are 1030-1080℃ and the through-firing time is 35-70min.

10. The method for preparing high-strength and high-toughness earthquake-resistant steel bars according to claim 1, characterized in that, In step S3, the conditions for short-term surface reheating are 1120-1180℃ and the reheating time is 6-12min.

11. A high-strength, high-toughness, earthquake-resistant steel reinforcement, characterized in that, The high-strength and high-toughness earthquake-resistant steel reinforcement is prepared by the preparation method described in any one of claims 1-10.

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