An HRB600E high-strength deformed steel bar and its production method

Abstract

The present invention belongs to the technical field of ribbed steel production, and particularly relates to an HRB600E high-strength ribbed steel and a production method thereof. For the HRB600E high-strength ribbed steel, by mass percentage, the composition of the continuous casting billet is: C: 0.25 - 0.28%, Si: 0.45 - 0.60%, Mn: 1.35 - 1.55%, Nb: 0.005 - 0.010%, Cr: 0.15 - 0.25%, V: 0.10 - 0.11%, N: 0.0230 - 0.0280%, P ≤ 0.035%, S ≤ 0.035%, and the balance is Fe element and inevitable impurities; the ratio of yield strength to tensile strength of the ribbed steel is ≥ 1.27. For the production method of the HRB600E high-strength ribbed steel, the rolling temperature of the finishing mill is in the austenite non-recrystallization temperature range of the HRB600E high-strength ribbed steel; the cumulative deformation rate of the intermediate billet in the finishing mill is controlled at 20 - 40%. This method aims to improve the ratio of yield strength to tensile strength of the ribbed steel by adjusting the size ratio of the rolled material grains in the rolling direction and the direction perpendicular to the rolling direction.

Description

Technical Field

[0001] The invention belongs to the technical field of threaded steel production, and specifically relates to HRB600E high-strength threaded steel and a production method thereof. Background Art

[0002] With the gradual increase in the requirements for building safety, current rebar products are generally earthquake-resistant products. The earthquake-resistant series of rebar requires that the ratio of the tensile strength to the yield strength of the steel be no less than 1.25. For steel with a high strength-to-yield ratio, the difference in loading force from the beginning of yield deformation to the occurrence of fracture is large. The early occurrence of yield deformation can remind people to discover safety hazards. Therefore, rebar with a high strength-to-yield ratio is safer during use.

[0003] In the existing rebar production technology, as the strength level of steel increases, the strength-to-yield ratio of steel decreases. The reason is that vanadium microalloying technology and fine grain strengthening technology are currently widely used to produce high-strength rebar, but the precipitation strengthening and fine grain strengthening of the microalloying element vanadium increase the yield strength of steel higher than the increase in tensile strength. This will lead to an increase in the yield strength of steel with the increase of microalloying element content and the fineness of grain size, but the strength-to-yield ratio of steel decreases, which is very unfavorable for the improvement of seismic performance. At present, the strength-to-yield ratio of HRB400E series steel can be controlled above 1.28, and it is difficult to stably control the strength-to-yield ratio of HRB500E series rebar above 1.25. It is very difficult to stably control the strength-to-yield ratio of HRB600E series rebar above 1.25, and it is difficult to achieve stable mass production of HRB600E series rebar. The higher the strength of the rebar, the lower the qualified rate of its strength-to-yield ratio.

[0004] Currently, the main methods to improve the strength and yield ratio of steel are as follows: (1) Increase the carbon content in the steel; the strength and yield ratio is improved by the fact that the contribution value of carbon element to the tensile strength is higher than that to the yield strength. However, since the upper limit of the carbon element content in 600Mpa grade deformed steel bars stipulated in the national standard is 0.28%, and the current deformed steel bar production enterprises have controlled the carbon element content range within 0.25 - 0.28%, there is no room to improve the strength and yield ratio of deformed steel bars by this method. (2) Increase the Si content in the steel; the strength and yield ratio is improved by the fact that the contribution value of Si element to the tensile strength is higher than that to the yield strength. However, after the Si element content is higher than 0.60%, the improvement effect of Si on the strength and yield ratio of the steel is not obvious, and most of the current high-strength deformed steel bar products have controlled the Si element content above 0.55%, so the room for this method to improve the strength and yield ratio of deformed steel bars is very limited. (3) Add about 0.1 - 0.3% of Cr element to the steel to improve the strength and yield ratio. The Cr element has the effect of increasing the pearlite content and refining the pearlite lamellae, thus improving the strength and yield ratio of the steel. Most of the current high-strength deformed steel bar products have controlled the Cr element content at about 0.1 - 0.3%, so there is no room to further improve the strength and yield ratio of deformed steel bars by this method.

[0005] As can be seen from the above, by adjusting the steel composition, the method of increasing the content of alloying elements with a higher contribution value to the tensile strength than to the yield strength to improve the strength and yield ratio of the steel has great limitations, and it is difficult to stably control the strength and yield ratio of the higher-strength HRB600E series deformed steel bars above 1.25. Summary of the Invention

[0006] The purpose of the present invention is to provide an HRB600E high-strength deformed steel bar and its production method in view of the above-mentioned defects. This method aims to improve the strength and yield ratio of the deformed steel bar by adjusting the size ratio of the rolled material grains in the rolling direction and the direction perpendicular to the rolling direction.

[0007] The technical solution of the present invention is as follows:

[0008] An HRB600E high-strength deformed steel bar, by mass percentage, the composition of the continuous casting billet of the deformed steel bar is: C: 0.25 - 0.28%, Si: 0.45 - 0.60%, Mn: 1.35 - 1.55%, Nb: 0.005 - 0.010%, Cr: 0.15 - 0.25%, V: 0.10 - 0.11%, N: 0.0230 - 0.0280%, P ≤ 0.035%, S ≤ 0.035%, and the balance is Fe element and inevitable impurities;

[0009] The strength and yield ratio of the deformed steel bar ≥ 1.27.

[0010] The functions of the above elements in the steel are as follows:

[0011] C: As the carbon content in the steel increases, the pearlite content in the steel structure increases, the strength of the steel increases, and the yield ratio increases. For every 0.01% increase in carbon in the steel, the yield strength increases by about 3 Mpa and the tensile strength increases by 7 Mpa. However, as the carbon content in the steel increases, the plasticity and toughness of the steel will decrease. Therefore, the national standard stipulates that the upper limit of the carbon content of HRB600E steel is 0.028%. Production practice has proved that when the carbon content does not exceed the upper limit, the plasticity and toughness of the steel can meet the use requirements. Therefore, the carbon content range of the steel controlled in the present invention is between 0.25% and 0.28%.

[0012] Si: It plays a role in solid solution strengthening. Production practice has proved that when Si is less than 0.60%, it can increase the strength of the steel and the yield ratio. For every 0.10% increase in silicon in the steel, the yield strength increases by about 5 Mpa and the tensile strength increases by 8 Mpa. However, when the Si content is higher than 0.60%, continuing to increase the Si content in the steel, the increase in the yield ratio of the steel is not obvious, and it will also increase the brittleness of the steel. Therefore, the Si content range of the steel controlled in the present invention is between 0.45% and 0.60%.

[0013] Mn: It plays a role in solid solution strengthening. The price of the Mn element is relatively low, and the effect on improving the strength of the steel is obvious. It is a solid solution strengthening element with good cost performance. Production practice has proved that for every 0.10% increase in manganese in the steel, the yield strength increases by about 8.4 Mpa and the tensile strength increases by 8 Mpa. In order to improve the strength of the steel, the Mn content range of the steel controlled in the present invention is between 1.35% and 1.55%.

[0014] Nb: It has a strong effect of increasing the austenite recrystallization temperature of the steel, can increase the temperature range of the non-recrystallized austenite region, which is beneficial to the temperature range of the control of the threaded steel rolling in the finishing mill in the present invention to be the non-recrystallized austenite temperature range, and the effect of elongating the austenite grains along the rolling direction. However, too high Nb content will cause bainite to appear during the cooling process of the steel on the cooling bed, resulting in an unclear yield platform during the tensile process of the steel. Therefore, the Nb content range of the steel controlled in the present invention is between 0.005% and 0.010%.

[0015] Cr: It plays a role in solid solution strengthening, has the effect of increasing the pearlite content and refining the pearlite lamellar structure, so that the yield strength and yield ratio of the steel can be increased. For every 0.10% increase in Cr in the steel, the yield strength increases by about 6 Mpa and the tensile strength increases by 10 Mpa. However, as the Cr content increases, the brittleness of the steel will increase. Therefore, the Cr content range of the steel controlled in the present invention is between 0.15% and 0.25%.

[0016] V: It can combine with N and C to form stable nitrides and carbides, achieving the effect of precipitation strengthening, which can improve the strength of steel. When the nitrogen content is sufficient, every 0.01% of V can increase the yield strength of steel by about 24 - 28 Mpa and the tensile strength of steel by 21 - 25 Mpa. When the nitrogen content is insufficient, the effect of V on improving the strength of steel becomes worse. To meet the strength performance requirements of 600 Mpa grade steel bars, the V content in the steel of the present invention is controlled within the range of 0.10 - 0.11%.

[0017] N: It is beneficial for V in the steel to precipitate in the form of VN particles. Since the precipitation strengthening effect of VN is about 2 times that of VC with the same amount, increasing the N content is beneficial to significantly improve the strength of vanadium-containing steel bars. However, increasing the nitrogen content will lead to an increase in the brittleness of the steel. It has been proved in production practice that controlling the N content in the steel to satisfy 3 ≤ V / N ≤ 5 is more friendly to the comprehensive performance of the steel. Considering the range of V content, the brittleness of the steel, and the strengthening effect of VN, the N content in the steel of the present invention is controlled within the range of 0.023 - 0.028%.

[0018] P: Generally considered as a harmful element, its content should be reduced as much as possible. Considering the cost of dephosphorization, in the present invention, it is controlled that P ≤ 0.035%.

[0019] S: A harmful element that reduces the high-temperature plasticity of steel. Therefore, the lower the S content in the steel, the better. Considering the cost of dephosphorization, in the present invention, it is controlled that S ≤ 0.035%.

[0020] For the preparation method of the above-mentioned HRB600E high-strength deformed steel bars, the rolling temperature of the finishing mill is in the austenite non-recrystallization temperature range of HRB600E high-strength deformed steel bars; the cumulative deformation rate of the intermediate billet in the finishing mill is controlled within 20 - 40%; the grain size in the longitudinal direction of the rolled material is controlled to be larger than that in the transverse direction, so that the longitudinal grain morphology of the rolled material is slender and flat.

[0021] By controlling the rolling temperature range of the deformed steel bars in the finishing mill to be in the austenite non-recrystallization temperature range, the austenite grains are elongated along the rolling (longitudinal) direction and become flat. After rolling, when entering the temperature transformation range from austenite to ferrite, ferrite grains nucleate at the austenite grain boundaries, and austenite forms ferrite + pearlite. At the same cooling rate, for a fine original austenite grain structure, the grain size of the obtained transformation structure product grains must also be fine; for a coarse original austenite grain structure, the obtained transformation structure product grains must also be coarse, and tissue heredity will occur during the cooling process. The latter grains basically maintain the characteristics such as the size of the former grains. Therefore, the austenite transformation products inherit the original shape of the austenite grains, and the formed ferrite and pearlite are also flat and long strip-shaped.

[0022] When steel undergoes yield phenomenon, dislocations inside the steel start to activate. The finer the grain size in a certain direction, the more difficult it is for dislocations in that direction to activate, and the higher the yield strength in that direction. Therefore, the yield strength of longitudinal tension of deformed steel bars is directly related to the longitudinal grain size. The technology described in the present invention has the effect of refining grains by the method of rolling in the austenite non-recrystallization zone. The grain sizes in both longitudinal and transverse directions are refined, but the longitudinal grain size is larger than the transverse grain size, that is, the refinement of the longitudinal grain size is not obvious, thus reducing the improvement amplitude of the longitudinal yield strength of the steel.

[0023] During the process of tensile elongation of deformed steel bars in actual use, the deformed steel bars elongate longitudinally. The shape of the grain size before tensile fracture of the deformed steel bars changes significantly compared with that before deformation. Therefore, the longitudinal tensile strength of the steel is not directly related to the longitudinal grain size, but directly related to the number of grains per unit volume of the steel. The technology described in the present invention has the effect of refining grains by the method of rolling in the austenite non-recrystallization zone. The number of grains per unit volume of the steel increases, thus improving the tensile strength of the steel. The improvement amplitude of the tensile strength is greater than that of the yield strength, thereby increasing the strength-to-yield ratio of the steel.

[0024] When the cumulative deformation rate of the rolled material in the austenite non-recrystallization temperature range is controlled at 20 - 40%, the phenomenon of flattening and elongating of austenite grains occurs. If the cumulative deformation rate is lower than 20%, the aspect ratio of austenite grains is less than 1.5; if the cumulative deformation rate is higher than 40%, the austenite grains undergo recrystallization phenomenon. After the flattened and elongated grains are recrystallized again, they become equiaxed grains (similar to circular).

[0025] In the present invention, in the rolling direction of the preparation method of the HRB600E high-strength deformed steel bars, the average aspect ratio of austenite grains at the finish rolling of finish rolling is 1.5 - 2.0. In this way, the grain size of the rolled material in the rolling direction is 0.5 - 1.5 grades coarser than that in the direction perpendicular to the rolling direction.

[0026] In the present invention, the rolling temperature of the finish rolling mill in the preparation method of the HRB600E high-strength deformed steel bars is 750 - 930 °C.

[0027] In the present invention, the last five rolling mill stands for rolling the HRB600E high-strength deformed steel bars in the preparation method of the HRB600E high-strength deformed steel bars are the finish rolling mill. The cumulative deformation rate can be controlled at 20 - 40% within the 5 stands. After the rolled material is rolled by the last 5 rolling mill stands, the morphology of austenite grains basically does not change. If the morphology of austenite grains is controlled in the previous rolling stands, it is possible that the morphology of austenite grains will change during the subsequent rolling, and the average aspect ratio of austenite grains is not within the range of 1.5 - 2.0.

[0028] In the present invention, the preparation method of the HRB600E high-strength deformed steel bar has the following rolling process steps:

[0029] (1) Heating: The soaking section temperature of the heating furnace is 1100 - 1200 °C, and the heating time of the continuous casting billet in the heating furnace is 60 - 70 min;

[0030] (2) Rough rolling: The starting rolling temperature is 1000 - 1050 °C, and the finishing rolling temperature is 980 - 1030 °C;

[0031] (3) Medium rolling: The starting rolling temperature is 980 - 1030 °C, and the finishing rolling temperature is 960 - 1010 °C;

[0032] (4) Cooling by water through: The intermediate billet after medium rolling is cooled by water through in a water tank;

[0033] (5) Finish rolling: The temperature of the intermediate billet after reheating when entering the finish rolling mill is 850 - 930 °C, and the temperature when exiting the finish rolling mill is 750 - 930 °C. The cumulative deformation rate of the intermediate billet in the finish rolling mill is controlled at 20 - 40%.

[0034] In the present invention, in step (1) of the preparation method of the HRB600E high-strength deformed steel bar, the billet type of the continuous casting billet is a square billet with a side length of 150 - 160 mm.

[0035] The beneficial effects of the present invention are as follows:

[0036] (1) The steel bar composition provided by the present invention contains 0.005 - 0.010% of Nb. Nb has a strong effect of increasing the austenite recrystallization temperature of the steel, can increase the temperature range of the austenite non-recrystallization zone, and is beneficial to controlling the rolling temperature range of the deformed steel bar in the finish rolling mill to be the austenite non-recrystallization temperature range, and has the effect of elongating the austenite grains along the rolling direction. And in the said technology, the Nb content is 0.005 - 0.010%. A small amount of Nb will not cause excessive bainite in the cooling process of the deformed steel bar on the cooling bed (the content ratio of the bainite structure is less than 10%), and thus will not cause the phenomenon that the yield plateau is not obvious during the tensile process of the deformed steel bar. That is, the said Nb content range is beneficial to elongating the austenite grains along the rolling direction and will not show the phenomenon that the yield plateau is not obvious during the tensile test.

[0037] (2) By adjusting the rolling temperature, the present invention enables the rolling temperature of HRB600E high-strength deformed steel bars in the finishing mill to be in the austenite non-recrystallization temperature range, and the cumulative deformation rate of the intermediate billet in the finishing mill is controlled at 20-40%. As a result, the austenite grains are elongated along the rolling (longitudinal) direction and become flat. The subsequently formed ferrite + pearlite grains inherit the original shape of the austenite grains, and there is a high probability that the formed ferrite and pearlite are also flat and long strip-shaped. The grain size of the rolled material in the rolling direction (longitudinal) is 0.5-1.5 grades coarser than that in the direction perpendicular to the rolling direction (transverse). The longitudinal tensile yield strength of the deformed steel bar is directly related to the grain size in the longitudinal direction, while the longitudinal tensile strength is directly related to the number of grains per unit volume. Therefore, the process of rolling in the austenite non-recrystallization zone has a higher effect on improving the tensile strength than on improving the yield strength, which is beneficial to increasing the strength-to-yield ratio of HRB600E high-strength deformed steel bars.

[0038] (3) The present invention abandons the traditional technical idea of only improving the strength-to-yield ratio of steel by optimizing the steel composition, and adopts a technical route of improving the strength-to-yield ratio of deformed steel bars by synergistically adjusting the composition and rolling process. Compared with the prior art, a method for improving the strength-to-yield ratio of steel is added. By the method of rolling in the austenite non-recrystallization zone, the effect of improving the tensile strength is higher than that of improving the yield strength, which is beneficial to increasing the strength-to-yield ratio of HRB600E high-strength deformed steel bars, and enables the strength-to-yield ratio of HRB600E high-strength deformed steel bars to be further improved on the basis of the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Longitudinal tissue photograph of HRB600E high-strength deformed steel bars produced by the method described in Example 1, magnified 200 times.

[0040] Figure 2 Transverse tissue photograph of HRB600E high-strength deformed steel bars produced by the method described in Example 1, magnified 200 times.

[0041] Figure 3 Longitudinal tissue photograph of HRB600E high-strength deformed steel bars produced by the prior art in Comparative Example 1, magnified 200 times.

[0042] Figure 4 Transverse tissue photograph of HRB600E high-strength deformed steel bars produced by the prior art in Comparative Example 1, magnified 200 times. DETAILED DESCRIPTION OF THE INVENTION

[0043] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0044] I. The measurement methods of relevant indicators in the examples and comparative examples are as follows:

[0045] 1. Determination of the average aspect ratio of austenite grains in the rolling direction: (1) Immediately after the finish rolling in the finishing mill, take a steel bar sample with a length of 100 - 150 mm and perform water quenching. (2) Cut a 15 - mm - long steel bar sample from the water - quenched sample. Perform wire cutting on this sample, cut longitudinally along the axis of the steel bar, then perform preliminary mechanical grinding and polishing on the longitudinal section. After etching with picric acid solution, observe the grain morphology at the longitudinal section using an optical microscope. (3) Observe and count the number of grains a intercepted by a 200 - μm - long straight line in the rolling direction and the number of grains b intercepted by a 200 - μm - long straight line perpendicular to the rolling direction in the metallographic field of view. (4) Take b:a as the value of the average aspect ratio of austenite grains in the rolling direction.

[0046] 2. Determination of longitudinal and transverse grain sizes: (1) Cut longitudinally along the axis of the steel bar, then perform preliminary mechanical grinding and polishing on the longitudinal section. After etching with 4% nitric acid alcohol solution, observe the grain morphology at the longitudinal section using an optical microscope. Perform grain size determination according to the intercept method in GB / T 6394 - 2017 (Method for Determination of Average Grain Size of Metals), and the determination result is the longitudinal grain size. (2) Cut transversely perpendicular to the axis of the steel bar, then perform preliminary mechanical grinding and polishing on the transverse section. After etching with 4% nitric acid alcohol solution, observe the grain morphology at the longitudinal section using an optical microscope. Perform grain size determination according to the intercept method in GB / T 6394 - 2017 (Method for Determination of Average Grain Size of Metals), and the determination result is the transverse grain size.

[0047] 3. Determination of the lower yield strength: Determine according to GB / T 28900 - 2021 (Metallic materials - Tensile testing - Part 1: Method of test at room temperature).

[0048] 4. Determination of the tensile strength: Determine according to GB / T 28900 - 2021 (Metallic materials - Tensile testing - Part 1: Method of test at room temperature).

[0049] 5. Determination of the maximum force elongation: Determine according to GB / T 28900 - 2021 (Metallic materials - Tensile testing - Part 1: Method of test at room temperature).

[0050] II. Steelmaking process flow of HRB600E high - strength ribbed steel bars: Converter smelting → LF refining → Continuous casting, specifically:

[0051] The C content range at the end of converter smelting is: 0.08 ≤ C ≤ 0.20%, the molten steel temperature at the end of converter smelting ≥ 1610°C, and P ≤ 0.025% at the end of converter smelting.

[0052] During the tapping process of the converter, calcium silicate barium is added for deoxidation. When the [C] at the end point of converter smelting is ≥ 0.10%, 0.5 kg / t of steel of calcium silicate barium is added; when the [C] at the end point of converter smelting < 0.10%, for every 0.01% reduction, 0.2 kg / t of steel of calcium silicate barium is increased.

[0053] During the tapping process of the converter, alloying is carried out with silicomanganese, ferrosilicon, ferroniobium, and vanadium nitride. The alloy is added when the molten steel is tapped to one-fourth, and the addition is completed when the molten steel is tapped to three-fourths.

[0054] The whole process of the converter tapping process is bottom-blown with nitrogen.

[0055] The LF refining furnace adopts the process of bottom-blowing nitrogen in the ladle to increase the nitrogen content in the molten steel. The specific process of bottom-blowing nitrogen in the ladle is as follows: N2 gas is bottom-blown at the bottom of the LF refining furnace, and the blowing amount of N2 gas is controlled at 13 - 15 m 3 / furnace. During the power-on period, the flow rate of N2 gas is controlled at 150 - 200 NL / min, and the flow rate of N2 gas before sampling is controlled at 800 NL / min. The bottom-blowing duration of N2 gas during the refining process shall not exceed 30 min / furnace. If it exceeds 30 min, it is switched to argon gas.

[0056] When the LF refining electrode starts to be powered on, high voltage and low current long-arc operation is adopted. After the slag is well melted, low voltage and high current submerged-arc operation is adopted. Calcium carbide, calcium silicate barium, etc. are used to adjust the slag, and the addition amount is 1 - 3 kg / t of steel. The top slag at the time of tapping shall be at least yellow-white slag, and the addition amount of lime is 3 - 7 kg / t of steel.

[0057] The temperature control range of the continuous casting tundish: 1513℃ - 1533℃. The tundish liquid level height: ≥ 500 mm. The continuous casting drawing speed control range: 2.0 - 2.5 m / min.

[0058] Example 1

[0059] A certain enterprise uses the technology described in the present invention to produce HRB600E high-strength threaded steel. The steelmaking process and the molten steel nitrogen-increasing process are shown in the specific implementation manners of the present invention. The specific process parameter values of a certain heat are as follows: the carbon content at the end point of converter smelting is: C: 0.120%, the molten steel temperature at the end point of converter smelting: 1652℃, and P at the end point of converter smelting: 0.016%.

[0060] During the tapping process of the converter, 0.5 kg / t of steel of calcium silicate barium is added for deoxidation. The whole process of the converter tapping process is bottom-blown with nitrogen. The LF refining furnace adopts the process of bottom-blowing nitrogen in the ladle to increase the nitrogen content in the molten steel. The blowing amount of N2 gas is controlled at 14.5 m 3 / furnace. The ladle molten steel amount is 152 tons. The flow rate of N2 gas during the power-on period of the refining electrode is 190 NL / min, the flow rate of N2 gas before sampling is 800 NL / min, and the bottom-blowing duration of N2 gas during the refining process is 30 min.

[0061] For LF refining, calcium carbide is used for deoxidation and slag adjustment, with the addition amount being 1 kg / t of steel, and the addition amount of lime being 5 kg / t of steel.

[0062] The shape of the continuous casting billet is a square billet with a side length of 160 mm. The temperature control range of the tundish for continuous casting is 1515°C - 1523°C. The height of the tundish liquid level: ≥500 mm. The continuous casting drawing speed is: 2.3 m / min.

[0063] The alloying elements of the continuous casting billet of the HRB600E high-strength ribbed steel are as follows by mass percentage: C: 0.26%, Si: 0.52%, Mn: 1.38%, Nb: 0.008%, Cr: 0.17%, V: 0.105%, N: 0.0253%, P: 0.022%, S: 0.018%, and the balance is Fe element and inevitable impurities.

[0064] The preparation method of the HRB600E high-strength ribbed steel has the following rolling process steps: heating furnace → rough rolling → medium rolling → water-cooling through water → finish rolling → cooling on the cooling bed.

[0065] (1) Heating: The soaking section temperature of the heating furnace is 1160°C, and the shape of the continuous casting billet is a square billet with a side length of 160 mm. The heating time of the continuous casting billet in the heating furnace is 62 min;

[0066] (2) Rough rolling: The starting rolling temperature is 1030°C, and the final rolling temperature is 1010°C;

[0067] (3) Medium rolling: The starting rolling temperature is 1008°C, and the final rolling temperature is 991°C;

[0068] (4) Water-cooling through water: The intermediate billet after medium rolling is cooled and temperature-reduced by passing through the water tank.

[0069] (5) Finish rolling: The temperature of the intermediate billet after reheating when entering the finish rolling mill is 880°C, and the temperature when leaving the finish rolling mill is 870°C. The cumulative deformation rate of the intermediate billet in the finish rolling mill is controlled at 32%.

[0070] During finish rolling final rolling, samples are taken to detect the average aspect ratio of austenite grains in the rolling direction. The results are shown in Table 1.

[0071] After rolling, the steel bars are transported to the cooling bed for cooling.

[0072] Samples are taken to detect the mechanical properties of the steel bars and observe the metallographic structure in the longitudinal and transverse directions. The test results are shown in Table 1 below; the metallographic structure is as Figure 1 and Figure 2 shown.

[0073] Example 2

[0074] The alloying elements of the continuous casting billet of the HRB600E high-strength deformed steel bar are as follows by mass percentage: C: 0.27%, Si: 0.53%, Mn: 1.47%, Nb: 0.006%, Cr: 0.22%, V: 0.106%, N: 0.0275%, P: 0.025%, S: 0.012%, and the balance is Fe element and inevitable impurities.

[0075] The preparation method of the HRB600E high-strength deformed steel bar is as follows, and its rolling process steps are as follows:

[0076] (1) Heating: The soaking section temperature of the heating furnace is 1160 °C, and the billet shape is a square billet with a side length of 160 mm. The heating time of the billet in the heating furnace is 62 min;

[0077] (2) Rough rolling: The starting rolling temperature is 1050 °C, and the final rolling temperature is 1030 °C;

[0078] (3) Medium rolling: The starting rolling temperature is 1010 °C, and the final rolling temperature is 1000 °C;

[0079] (4) Cooling by water through: The intermediate billet after medium rolling is cooled by water through in the water tank;

[0080] (5) Finish rolling: The temperature of the intermediate billet when entering the finish rolling mill after reheating is 930 °C, and the temperature when leaving the finish rolling mill is 930 °C. The cumulative deformation rate of the intermediate billet in the finish rolling mill is controlled at 20%.

[0081] During finish rolling and final rolling, samples are taken to detect the average aspect ratio of austenite grains in the rolling direction, and the results are shown in Table 1 for details.

[0082] After rolling, the steel bars are transported to the cooling bed for cooling.

[0083] Samples are taken to detect the mechanical properties of the steel bars, and the test results are shown in Table 1 below.

[0084] Others are the same as in Example 1.

[0085] Example 3

[0086] The alloying elements of the continuous casting billet of the HRB600E high-strength deformed steel bar are as follows by mass percentage: C: 0.25%, Si: 0.50%, Mn: 1.43%, Nb: 0.007%, Cr: 0.23%, V: 0.102%, N: 0.0265%, P: 0.025%, S: 0.011%, and the balance is Fe element and inevitable impurities.

[0087] The preparation method of the HRB600E high-strength deformed steel bar is as follows, and its rolling process steps are as follows:

[0088] (1) Heating: The soaking section temperature of the heating furnace is 1160 °C, and the billet shape is a square billet with a side length of 160 mm. The heating time of the billet in the heating furnace is 62 min;

[0089] (2) Rough rolling: The starting rolling temperature is 1000 °C, and the finishing rolling temperature is 980 °C;

[0090] (3) Medium rolling: The starting rolling temperature is 980 °C, and the finishing rolling temperature is 960 °C;

[0091] (4) Cooling by water through: The intermediate billet after medium rolling is cooled by water through in the water tank;

[0092] (5) Finish rolling: The temperature of the intermediate billet entering the finish rolling mill after reheating is 850 °C, and the temperature leaving the finish rolling mill is 750 °C. The cumulative deformation rate of the intermediate billet in the finish rolling mill is controlled at 40%.

[0093] During finish rolling, samples are taken to detect the average aspect ratio of austenite grains in the rolling direction. The results are shown in Table 1.

[0094] After rolling, the steel bars are transported to the cooling bed for cooling.

[0095] Samples are taken to detect the mechanical properties of the steel bars. The test results are shown in Table 1 below.

[0096] Others are the same as in Example 1.

[0097] Comparative Example 1

[0098] An enterprise uses the existing technology to produce HRB600E high-strength deformed steel bars. The steel grade composition, steelmaking process, and rolling process all adopt the existing published technologies. The process of adding nitrogen by bottom blowing nitrogen in the ladle is not adopted for the molten steel. The specific process parameter values of a certain heat are as follows: The carbon content at the end of converter smelting is: C: 0.10%, the temperature of the molten steel at the end of converter smelting: 1630 °C, and P at the end of converter smelting: 0.021%.

[0099] During the tapping process of the converter, 0.6 kg / t of calcium silicate barium is added for deoxidation, and argon is blown at the bottom throughout the tapping process of the converter and the LF refining process.

[0100] The LF refining uses calcium carbide for deoxidation and slag adjustment, with an addition amount of 1.5 kg / t of steel, and the addition amount of lime is 6 kg / t of steel.

[0101] The billet shape is a square billet with a side length of 160 mm. The continuous casting tundish temperature control range is: 1516 °C - 1525 °C. The tundish liquid level height: ≥500 mm. The continuous casting casting speed is: 2.5 m / min.

[0102] The alloying elements of the continuous casting billet in Comparative Example 1 are as follows by mass percentage: C: 0.25%, Si: 0.47%, Mn: 1.42%, Cr: 0.20%, V: 0.143%, N: 0.0172%, P: 0.024%, S: 0.015%, and the balance is Fe element and inevitable impurities.

[0103] The existing rolling process is used to roll HRB600E high-strength deformed steel bars, and the process flow is: reheating furnace → rough rolling → medium rolling → cooling by water through → finish rolling → cooling on cooling bed.

[0104] (1) Heating: The temperature of the soaking section of the reheating furnace is 1120 °C, and the billet shape is a square billet with a side length of 160 mm. The heating time of the billet in the reheating furnace is 70 min;

[0105] (2) Rough rolling: The starting rolling temperature is 1080 °C, and the final rolling temperature is 1050 °C;

[0106] (3) Medium rolling: The starting rolling temperature is 1050 °C, and the final rolling temperature is 1050 °C;

[0107] (4) Cooling by water through: The intermediate billet after medium rolling is cooled by passing through a water tank.

[0108] (5) Finish rolling: The temperature of the intermediate billet entering the finish rolling mill after reheating is 1045 °C, and the temperature leaving the finish rolling mill is 1030 °C. The cumulative deformation rate of the intermediate billet in the finish rolling mill is controlled at 30%.

[0109] During finish rolling, samples are taken to detect the average aspect ratio of austenite grains in the rolling direction, and the results are shown in Table 1.

[0110] After rolling, the steel bars are transported to the cooling bed for cooling.

[0111] Samples are taken to detect the mechanical properties of the steel bars and observe the metallographic structure in the longitudinal and transverse directions after the rolled material is cooled. The test results are shown in Table 1 below, and the metallographic structure is as Figure 3 、 Figure 4 shown.

[0112] Comparative Example 2

[0113] The difference from Example 2 is that: the cumulative deformation rate of the intermediate billet in the finish rolling mill is 15%.

[0114] After testing, in the rolling direction, the average aspect ratio of austenite grains during finish rolling is 1.32.

[0115] After testing, the grain size of the rolled material in the rolling direction is slightly coarser than that in the direction perpendicular to the rolling direction, but the difference in grain size grade is less than 0.5 grade.

[0116] After rolling, the steel bars are transported to the cooling bed for cooling.

[0117] The test results of mechanical properties are shown in Table 1 below.

[0118] Others are the same as in Example 2.

[0119] Comparative Example 3

[0120] The difference from Example 3 is that the cumulative deformation rate of the intermediate billet in the finishing mill is 46%.

[0121] After testing, in the rolling direction, the average aspect ratio of austenite grains at the end of finishing rolling is 1.46.

[0122] After testing, the grain size of the rolled material in the rolling direction is slightly coarser than that in the direction perpendicular to the rolling direction, but the difference in grain size levels is less than 0.5 grade.

[0123] After rolling, the steel bars are transported to the cooling bed for cooling.

[0124] The test results of mechanical properties are shown in Table 1 below.

[0125] Others are the same as in Example 3.

[0126] Comparative Example 4

[0127] The difference from Example 1 is that the temperature of the intermediate billet entering the finishing mill after reheating is 980 °C, and the temperature leaving the finishing mill is 980 °C.

[0128] After testing, in the rolling direction, the average aspect ratio of austenite grains at the end of finishing rolling is 1.13.

[0129] After testing, the grain size of the rolled material in the rolling direction is slightly coarser than that in the direction perpendicular to the rolling direction, but the difference in grain size levels is less than 0.5 grade.

[0130] After rolling, the steel bars are transported to the cooling bed for cooling. The test results of mechanical properties are shown in Table 1 below.

[0131] Others are the same as in Example 1.

[0132] Comparative Example 5

[0133] The difference from Example 2 is that the temperature of the intermediate billet entering the finishing mill after reheating is 930 °C, and the temperature leaving the finishing mill is 960 °C.

[0134] After testing, in the rolling direction, the average aspect ratio of austenite grains at the end of finishing rolling is 1.27.

[0135] After testing, the grain size of the rolled material in the rolling direction is slightly coarser than that in the direction perpendicular to the rolling direction, but the difference in grain size levels is less than 0.5 grade.

[0136] After rolling, the steel bars are transported to the cooling bed for cooling.

[0137] The test results of mechanical properties are shown in Table 1 below.

[0138] Others are the same as in Example 2.

[0139] Comparative Example 6

[0140] The difference from Example 3 is that: the temperature of the intermediate billet entering the finishing mill after reheating is 950 °C, and the temperature of the intermediate billet leaving the finishing mill is 910 °C.

[0141] After testing, in the rolling direction, the average aspect ratio of austenite grains at the end of finishing rolling is 1.43.

[0142] After testing, the grain size of the rolled material in the rolling direction is slightly coarser than that in the direction perpendicular to the rolling direction, but the difference in grain size level is less than 0.5 level.

[0143] After rolling, the steel bars are transported to the cooling bed for cooling.

[0144] The test results of mechanical properties are shown in Table 1 below.

[0145] Others are the same as in Example 3.

[0146] Table 1 Related performance indexes of the steel materials in each example and comparative example

[0147]

[0148]

[0149] 1. Analyze the test data in Table 1 and the compositions of Example 1 and Comparative Example 1: The V content in Comparative Example 1 is 0.038% higher than that in Example 1, but the lower yield strengths of Example 1 and Comparative Example 1 are close. The reason is that the nitrogen content in Example 1 is higher than that in Comparative Example 1, and it contains 0.008% of niobium. This part of the nitrogen content and niobium can compensate for the strength contribution of this part of the vanadium content. At the same time, the tensile strength of Example 1 is much higher than that of Comparative Example 1. It can be seen that the process of rolling in the austenite non-recrystallization zone has a higher effect on improving the tensile strength than on improving the yield strength, which is beneficial to improving the strength ratio of HRB600E high-strength deformed steel bars.

[0150] 2. By comparing Figure 1 and Figure 2 , it can be seen that Figure 1 in the longitudinal structure of the deformed steel bars, the ferrite and pearlite grains are significantly elongated, Figure 2 in the transverse structure of the deformed steel bars, the ferrite and pearlite grains are basically equiaxed.

[0151] 3. By comparing Figure 1 and Figure 3 , it can be seen that Figure 1 the ferrite and pearlite grains in Figure 3The ferrite and pearlite grains are not significantly elongated. Figure 1 Figure Figure 1 is a longitudinal microstructure photograph of HRB600E high-strength deformed steel bars produced using the technology of the present invention, while Figure 3 Figure Figure 3 is a longitudinal microstructure photograph of HRB600E high-strength deformed steel bars not produced using the technology of the present invention.

[0152] In Example 1, by controlling the rolling temperature range of the deformed steel bars in the finishing mill to be the austenite non-recrystallization temperature range, the austenite grains are elongated along the rolling (longitudinal) direction and become flattened. Subsequently, the austenite transformation products have a high probability of inheriting the original shape of the austenite grains, and the generated ferrite and pearlite also have a high probability of being flat and elongated. The effect of obtaining a grain size in the rolling direction of the rolled material that is 0.5 - 1.5 grades coarser than the grain size in the direction perpendicular to the rolling direction is achieved.

[0153] In Comparative Example 1, the rolling temperature range of the finishing mill is 980 - 1000 °C, which is the austenite recrystallization temperature range. Austenite recrystallization occurs during the finishing rolling process. The elongated and flattened austenite forms equiaxed austenite after recrystallization. Subsequently, the austenite transformation products inherit the original shape of the austenite grains, and the generated ferrite and pearlite also have a high probability of being equiaxed. Therefore, the rolling process adopted in Comparative Example 1 cannot achieve the effect of obtaining a grain size in the rolling direction of the rolled material that is 0.5 - 1.5 grades coarser than the grain size in the direction perpendicular to the rolling direction. The grains of the longitudinal microstructure in Comparative Example 1 are basically equiaxed.

[0154] 4. By comparing Figure 3 and Figure 4 , it can be seen that Figure 3 , 4 the ferrite and pearlite grains in are not significantly elongated. Figure 3 There is a slight pearlite banded structure, which is caused by the segregation of alloying elements in the continuous casting billet. However, the ferrite + pearlite grains do not show obvious elongation, and it does not contribute to the increase in the strength and yield ratio of the steel. Figure 4 The ferrite and pearlite in are basically equiaxed because this microstructure photograph is a transverse microstructure photograph.

[0155] 5. Analyzing the strength comparison between Comparative Example 2 and Example 2 in Table 1, the yield strengths of Example 2 and Comparative Example 2 are close, but the tensile strength of Example 2 is about 20 Mpa higher than that of Comparative Example 2. The reason is that the cumulative deformation rate of the intermediate billet in Example 2 in the finishing mill is 20%, while the cumulative deformation rate of the intermediate billet in Comparative Example 2 in the finishing mill is 15%.

[0156] The cumulative deformation rate is lower than the lower limit of the cumulative deformation amount provided by the technology of the present invention, resulting in an aspect ratio of austenite grains less than 1.5. Although the grain size of the rolled material in the rolling direction after cooling is slightly coarser than that in the direction perpendicular to the rolling direction, the difference in grain size levels is less than 0.5 grade. The slightly flattened grain morphology has an insufficiently large increment on the tensile strength. Although the yield ratio of the rolled material is higher than that of Comparative Example 1, the improvement effect of the yield ratio is not obvious.

[0157] 6. Analyze the strength comparison between Comparative Example 3 and Example 3 in Table 1. The yield strength of Example 3 is about 20 Mpa higher than that of Comparative Example 3, and the tensile strength is about 70 Mpa higher. The reason is that the cumulative deformation rate of the intermediate billet in Example 3 in the finishing mill is 40%, while the cumulative deformation rate of the intermediate billet in Comparative Example 3 in the finishing mill is 46%.

[0158] The cumulative deformation rate is higher than the upper limit of the cumulative deformation amount provided by the technology of the present invention. Excessive cumulative deformation amount will cause recrystallization of austenite grains. After the elongated and flattened grains are recrystallized again, they become equiaxed grains (similar to circles). After cooling, the grain size of the rolled material in the rolling direction is the same as that in the direction perpendicular to the rolling direction, and the technical effect of flattening the grain morphology to improve the tensile strength is not achieved. The yield ratio of the rolled material in Comparative Example 3 is significantly lower than that in Example 3.

[0159] 7. Analyze the yield ratios in Comparative Example 4, Comparative Example 5, and Comparative Example 6 in Table 1 and compare them with Examples 1, 2, and 3. The yield ratios in Comparative Example 4, Comparative Example 5, and Comparative Example 6 are significantly lower. The reason is that the temperature range of the intermediate billet entering the finishing mill or the temperature range leaving the finishing mill after reheating in Comparative Example 4, Comparative Example 5, and Comparative Example 6 is not within the temperature range provided by the technology of the present invention, and the technology of "rolling in the non-recrystallization zone on the last five rolling stands" provided by the present invention is not used, resulting in austenite recrystallization or partial austenite recrystallization during the rolling of the intermediate billet in the finishing mill. Although the grain size of the rolled material in the rolling direction after cooling is slightly coarser than that in the direction perpendicular to the rolling direction, the difference in grain size levels is less than 0.5 grade. The slightly flattened grain morphology has an insufficiently large increment on the tensile strength, and the improvement effect of the yield ratio of the rolled material is not obvious.

[0160] In summary, the present invention abandons the traditional technical idea of only optimizing the steel composition to improve the yield ratio of steel, and adopts a technical route of synergistically adjusting the composition and rolling process to improve the yield ratio of ribbed steel. Compared with the prior art, a method for improving the yield ratio of steel is added, and a specific production process that can make the longitudinal grain size of the rolled material significantly larger than the transverse grain size is studied through theoretical analysis and on-site tests, so that the yield ratio of HRB600E high-strength ribbed steel can be further improved on the basis of the prior art, and is more than 0.05% higher than the yield ratio of HRB600E high-strength ribbed steel produced by the existing process technology.

Claims

1. A preparation method of HRB600E high-strength deformed steel bars, characterized in that, The rolling temperature of the finishing mill is in the austenite non-recrystallization temperature range of HRB600E high-strength deformed steel bars; The cumulative deformation rate of the intermediate billet in the finishing mill is controlled at 20-40%; Control the grain size in the longitudinal direction of the rolled material to be larger than that in the transverse direction, so that the longitudinal grain morphology of the rolled material is slender and flat; For the prepared HRB600E high-strength deformed steel bars, by mass percentage, its composition is: C: 0.25-0.28%, Si: 0.45-0.60%, Mn: 1.35-1.55%, Nb: 0.005-0.010%, Cr: 0.15-0.25%, V: 0.10-0.11%, N: 0.0230-0.0280%, P≤0.035%, S≤0.035%, and the balance is Fe element and unavoidable impurities; The yield ratio of the said deformed steel bars ≥1.

27.

2. The preparation method of HRB600E high-strength deformed steel bars according to claim 1, characterized in that, In the rolling direction, the average aspect ratio of austenite grains at the end of finishing rolling is 1.5-2.

0.

3. The preparation method of HRB600E high-strength deformed steel bars according to claim 1, characterized in that, The rolling temperature of the finishing mill is 750-930°C.

4. The preparation method of HRB600E high-strength deformed steel bars according to claim 1, characterized in that, The last five rolling stands for rolling HRB600E high-strength deformed steel bars are the finishing mill.

5. The preparation method of HRB600E high-strength deformed steel bars according to claim 1, characterized in that, Its rolling process steps are as follows: (1) Heating: The soaking section temperature of the heating furnace is 1100-1200°C, and the heating time of the continuous casting billet in the heating furnace is 60-70 min; (2) Rough rolling: The starting rolling temperature is 1000-1050°C, and the finishing rolling temperature is 980-1030°C; (3) Medium rolling: The starting rolling temperature is 980-1030°C, and the finishing rolling temperature is 960-1010°C; (4) Cooling by water through: The intermediate billet after medium rolling is cooled by water through the water tank; (5) Finishing rolling: The temperature of the intermediate billet after re-heating when entering the finishing mill is 850-930°C, and the temperature when leaving the finishing mill is 750-930°C. The cumulative deformation rate of the intermediate billet in the finishing mill is controlled at 20-40%.

6. The preparation method of HRB600E high-strength deformed steel bars according to claim 5, characterized in that, In the said step (1), the billet type of the continuous casting billet is a square billet with a side length of 150-160 mm.

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