Method for controlling cracks of low-cost niobium microalloyed steel casting blank

Abstract

The invention relates to a method for controlling cracks of a low-cost niobium microalloyed steel casting blank, which belongs to the technical field of metallurgy and comprises the procedures of molten iron desulfurization, converter smelting, RH vacuum treatment and continuous casting. The total oxygen content in a casting blank is controlled to be smaller than or equal to 15 ppm; a soft reduction technology is adopted, reduction is completed within 90% of the two-phase region, and the soft reduction rate is larger than or equal to 1.0 mm / min and smaller than or equal to 1.5 mm / min; the continuous casting pulling speed is not less than 1.3 m / min, and the cooling rate is not more than 1.5 DEG C / s The weight content of each component in the niobium microalloyed steel satisfies that C is less than or equal to 0.07%, Mn / Sgt; and Al * N is less than or equal to 0.015 ppm. According to the method, control over the surface cracks of the niobium-containing microalloyed steel continuous casting billet is achieved through molten steel refining path optimization, molten steel low total oxygen content segregation control, continuous casting high-pulling-speed low-cooling-rate control, niobium microalloyed steel component optimization and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy, and in particular relates to a method for controlling cracks in a low-cost niobium micro-alloy steel ingot. Background Art

[0002] As steel products develop towards high quality, low consumption and high efficiency, microalloying elements are increasingly used in steel products and their usage is increasing. The vigorous development of continuous casting technology has greatly increased the output of continuous casting microalloyed steel. Adding trace Nb elements to steel can significantly increase the recrystallization temperature and refine the grains, which is beneficial to improving the process performance and performance of steel, but at the same time it reduces the high temperature plasticity of steel and increases the crack incidence on the surface of the ingot, which has become one of the research hotspots on continuous casting quality issues.

[0003] Steel plate surface quality defects have always been common in major steel mills. Even world-leading steel plate mills such as Germany's Thyssen, Australia's Australia Steel, Japan's Nippon Steel, and South Korea's Posco have also had the problem of steel plate cracks affecting the stable production of continuous casting. Continuous casting billet cracks are the most common and most numerous type of billet quality defects. According to statistics, they account for more than 50% of all defects. Transverse cracks on the surface of billets have long been a major problem for domestic and foreign steel mills. Due to the many influencing factors, although scientific researchers have made great efforts, most of them can only reduce the incidence rate and fail to solve it fundamentally.

[0004] There are also many papers and patents on cracks in niobium-containing microalloyed steel in the prior art, which basically focus on alloy nitrogen fixation (such as adding Ti or Ti-V to fix nitrogen); reducing the cooling intensity of the ingot; strictly controlling the nitrogen content of molten steel and the content of harmful participating elements such as sulfur and phosphorus. However, these all increase the alloying cost of steel (the cost of Ti and V alloys); reduce the continuous casting production efficiency (the weak cooling of the ingot must reduce the pulling speed, otherwise the shell is prone to bulging, and the continuous casting equipment is greatly damaged); increase the difficulty and cost of production (controlling nitrogen, sulfur and phosphorus all increase the difficulty and cost of production). Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a low-cost niobium micro-alloyed steel ingot crack control method, which realizes the control of surface cracks in the continuous casting ingot of niobium micro-alloyed steel by optimizing the molten steel refining path, controlling the low total oxygen content segregation of molten steel, controlling the high drawing speed and low cooling rate of continuous casting, optimizing the composition of niobium micro-alloyed steel, etc.

[0006] In order to achieve the above-mentioned invention object, the present invention adopts the following technical scheme:

[0007] A method for controlling cracks in low-cost niobium microalloyed steel ingots, comprising hot metal desulfurization, converter smelting, RH vacuum treatment, and continuous casting processes;

[0008] The continuous casting process controls the total oxygen content TO in the ingot to be ≤15ppm; adopts the soft reduction technology to complete the reduction within 90% of the two-phase region, and the soft reduction rate is 1.0mm / min≤≤1.5mm / min; the continuous casting speed V c ≥1.3m / min, cooling rate C k ≤1.5℃ / s.

[0009] The weight percentage of each component in the niobium microalloy steel meets the following requirements: C≤0.07%, Mn / S>80, Al*N≤0.015ppm.

[0010] Furthermore, in the converter smelting process, high-temperature, low-oxygen and low-sulfur steel is tapped, and the final tapping temperature is controlled at 1670-1690° C., the oxygen content [O] of the tapped steel is ≤300ppm, and the sulfur content [S] is ≤30ppm, providing favorable conditions for RH treatment.

[0011] Furthermore, in the RH vacuum treatment process, the oxygen content in the molten steel is controlled to be 100ppm≤[O]≤200ppm after the molten steel enters the RH, so as to achieve deep denitrification and deep deoxidation in the RH.

[0012] Furthermore, in the RH vacuum treatment process, for the furnaces that need RH oxygen blowing and heating, aluminum particles are added before RH oxygen blowing and heating to ensure that the weight content of Als in the molten steel after RH oxygen blowing, w(Als) ≥ 0.03%, to ensure sufficient deoxidation of the molten steel and minimize the number and size of Al2O3 inclusions.

[0013] Furthermore, low-nitrogen and low-oxygen alloys are used in the converter smelting and RH vacuum treatment processes, with total oxygen TO <5000ppm and nitrogen [N] <200ppm in the alloy to prevent abnormal nitrogen and oxygen increases during the alloying process.

[0014] Furthermore, the weight percentage of Nb in the niobium microalloyed steel is 0.015% to 0.06%.

[0015] The design ideas of the present invention are as follows:

[0016] 1. Optimization of molten steel refining path

[0017] The conventional steel refining process path of Nb-containing microalloyed steel is: hot metal desulfurization → converter smelting → LF furnace treatment → continuous casting, or hot metal desulfurization → converter smelting → LF furnace treatment + RH furnace treatment → continuous casting. The LF furnace adds aluminum deoxidation, slag making, stirring desulfurization and calcium treatment; the RH furnace deoxidation, denitrification and dehydrogenation treatment, the treatment cycle is generally about 60 to 70 minutes.

[0018] The present invention optimizes the molten steel refining process path as follows: molten iron desulfurization → converter smelting → RH furnace treatment → continuous casting, which not only simplifies the molten steel process path, reduces costs, and shortens the molten steel refining treatment cycle, but also avoids the inclusions caused by LF stirring desulfurization, reduces the total oxygen content of molten steel, and improves the cleanliness of molten steel.

[0019] 2. Segregation control of low total oxygen content in molten steel

[0020] Theoretically, the melting point of Nb iron is between 1580 and 1630°C. Generally, harmful inclusions such as Nb oxides, sulfides, carbides, nitrides and phosphides are not formed in the molten steel. Instead, they are precipitated during or after solidification. During the continuous casting process, as the solidification temperature decreases, Nb is usually difficult to form pure NbC or NbN, but precipitates in the form of carbonitride Nb (C, N). Since the diffusion coefficient of Nb in the solid medium is small, these Nb compounds are generally very small and controlled below the micron level, which helps to improve the performance of the steel. However, at the solidification front, especially in the central segregation zone, high concentrations of elements such as [C], [N], and [Nb] will appear. In addition, the enrichment of elements such as [S] and [P] reduces the solidification temperature, so it is also possible to precipitate Nb carbon and nitride in the molten steel at the solidification front. The size of the Nb carbon and nitrides precipitated at this time is larger than that precipitated after solidification and has irregular shapes. These large and irregular Nb carbides have the greatest harm to the high-temperature ductility and performance of steel.

[0021] The present invention has found that the flow field of molten steel interacts with solidification crystallization. At the solidification front of molten steel, the floating process of inclusions with suitable size and quantity will disturb the flow field in the mushy zone, change the internal flow field velocity, make the mushy zone unstable, and induce channel segregation. Low-oxygen purification control of molten steel can reduce or even eliminate this channel segregation.

[0022] Therefore, the cleanliness of molten steel must be improved first, and the total oxygen content in the ingot must be controlled to be less than 15ppm.

[0023] The use of reasonable light reduction technology is also an economical and effective means to reduce segregation. The light reduction interval for controlling segregation should be appropriately close to the liquid phase area. The light reduction should be implemented before the cross-winding area of ​​the secondary crystal arms of the columnar crystals at the solidification front, and the light reduction should be ended when the liquid phase between the columnar crystals at the solidification front begins to disappear. The reduction should be completed within 90% of the two-phase area; at the same time, the reduction rate should be appropriately increased, that is, the strain rate should be increased to obtain the ability of grain boundary migration, so that the secondary precipitates can be precipitated from the grain boundary to the intracrystalline precipitation, reducing the crack sensitivity. Control: 1.0mm / min≤light reduction rate≤1.5mm / min.

[0024] 3. Continuous casting high casting speed and low cooling rate control

[0025] Theoretical research shows that the precipitation of niobium carbonitride in steel along the austenite grain boundary from about 1100℃ further deepens the low plasticity temperature curve of steel, making the low plasticity temperature range larger and the temperature lower, which is the main reason for the reduction of high temperature plasticity of steel, that is, the cracks in the ingot. It is mainly reflected in:

[0026] ① The precipitation of precipitates along the austenite grain boundaries reduces the bonding force of the grain boundaries. When plastic deformation occurs under stress, fine carbon and nitrides become stress concentration sources and separate from the grain boundaries to form micropores;

[0027] ② Fine niobium carbonitride pins on the austenite grain boundaries, hindering the movement of grain boundaries and inhibiting the dynamic recrystallization of steel;

[0028] ③ As the temperature decreases, Nb carbonitrides also begin to precipitate in the austenite grains, resulting in a thin layer of non-precipitation zone on both sides of the grain boundary, which is prone to stress concentration and causes grain boundary cracks.

[0029] The present invention has been proved through laboratory analysis and production practice that the number of Nb carbonitride Nb (C, N) precipitates gradually decreases from the surface to the center; and the precipitation temperature and precipitation amount of the precipitates are closely related to the cooling rate of the ingot: the greater the cooling rate of the ingot, the lower the precipitation temperature of the precipitate, and the greater the precipitation amount; conversely, the smaller the cooling rate of the ingot, the higher the precipitation temperature of the precipitate, and the less the precipitation amount. At the same time, reducing the cooling rate is also conducive to reducing thermal stress, reducing columnar crystals, and refining grains.

[0030] Therefore, the key to controlling cracks is to reduce the cooling rate of the ingot, increase the surface temperature of the ingot, and avoid the precipitation of Nb (C, N) precipitates on the grain boundaries of the ingot surface during the continuous casting process.

[0031] Therefore, the present invention corrects the previous technical bias, breaks the traditional low casting speed control, and adopts high casting speed (V c ≥1.3m / min) low cooling rate (C k ≤1.5℃ / s) control, because high drawing speed is the simplest, most economical and most effective means to increase the surface temperature of the billet and reduce the cooling rate of the billet.

[0032] 4. Optimization of ingredient design

[0033] ① The use of low-carbon technology can avoid the peritectic reaction zone and the generation of cracks in the peritectic zone. In addition, due to the reduction of C content, the niobium carbonitride precipitated on the austenite grain boundary is reduced, which reduces the low-temperature hot plasticity of Nb steel. Therefore, the composition design of the steel grade should use low carbon (C% ≤ 0.07wt%) as much as possible.

[0034] ② During the solidification of molten steel, MnS precipitates before Nb(C, N). Nb(C, N) uses the first precipitated MnS as a heterogeneous core nucleus, which reduces the precipitation of Nb(C, N) at the grain boundary. Therefore, during the fine-tuning of refining composition, Mn / S should be adjusted in time according to the sulfur content of the steel grade to ensure that Mn / S>80.

[0035] This not only inhibits the precipitation of FeS and FeO at the austenite grain boundaries, but also reduces the precipitation of Nb (C, N) at the austenite grain boundaries, thus controlling the two major harmful components that form cracks.

[0036] ③ Under the premise of sufficient deoxidation of molten steel, the acid-soluble aluminum content in the steel cannot be too high, so that the nitrogen-aluminum product reaches the optimal Al*N≤0.015ppm, so as to reduce the hazards of AlN and Al2O3 inclusions at the same time.

[0037] The beneficial effect of adopting the above technical scheme is that the present invention realizes the control of surface cracks of continuous casting billets containing niobium microalloyed steel without additionally increasing production costs and equipment investment through optimization of molten steel refining path, control of low total oxygen content segregation in molten steel, control of high continuous casting speed and low cooling rate, optimization of niobium microalloyed steel composition, etc. The incidence of surface defects of continuous casting billets containing niobium microalloyed steel is stably controlled within 0.1%, providing the industry with an example of low-cost production of high-quality niobium-containing steel. DETAILED DESCRIPTION

[0038] Example 1

[0039] The method for controlling cracks in low-cost niobium microalloyed steel ingots in this embodiment takes niobium-containing steel grade 1 as an example, and the composition and mass content of the steel grade are:

[0040] element C Si Mn P S Als Nb N Content / wt% 0.06 <0.05 1.30 <0.015 <0.002 0.035 0.015 <0.004

[0041] The weight percentage of each component in the niobium-containing steel grade 1 satisfies: C≤0.07%, Mn / S=260>80, Al*N=0.014ppm.

[0042] The refining path of the raw molten steel of niobium-containing steel grade 1 is molten iron desulfurization → converter smelting → LF furnace treatment → continuous casting.

[0043] The present invention is optimized as molten iron desulfurization → converter smelting → RH furnace treatment → continuous casting, and the specific steps are as follows:

[0044] (1) KR hot metal desulfurization first removes sulfur from the hot metal to below 10 ppm;

[0045] (2) The converter smelting is subjected to resulfurization control, and the converter taps steel at high temperature, with low oxygen and low sulfur: the final tapping temperature of the converter is controlled at 1684°C; the oxygen content of the tapped steel [O] is ≤300ppm; the sulfur content of the tapped steel [S] is ≤30ppm, and the alloying is carried out with low nitrogen [N] <100ppm, low oxygen TO <4000ppm, and high-quality refined medium carbon ferromanganese; the mass fraction of Als in the molten steel after tapping is controlled at 0.025% to 0.045%.

[0046] (3) After the molten steel enters the RH, the oxygen content of the molten steel is controlled to 100ppm≤[O]≤200ppm for deep denitrification and deep dehydrogenation treatment; the RH is heated by oxygen blowing and an appropriate amount of aluminum particles are added before oxygen blowing to ensure that the molten steel w(Als) after oxygen blowing is ≥0.03%.

[0047] (4) After the molten steel reaches the continuous casting, the total oxygen content in the ingot is controlled to be TO≤15ppm; the casting speed of the continuous casting machine is controlled to be 1.4m / min; dynamic secondary cooling water and light pressure reduction technology are used throughout the continuous casting process to control the cooling rate of the ingot C k =1.5℃ / s; In order to better control segregation, the light reduction interval is appropriately close to the liquid phase area, and the light reduction is implemented before the cross-winding area of ​​the secondary crystal arms of the columnar crystals at the solidification front, and the reduction is controlled to be completed within 90% of the two-phase area; At the same time, the light reduction rate is optimized to = 1.0mm / min, that is, the strain rate is increased, the grain boundary migration ability is obtained, and the secondary precipitates are precipitated from the grain boundary to the intracrystalline precipitation, reducing the crack sensitivity. The slabs are stacked and slowly cooled off the line, and the surface is checked in the cold state without defects and sent to hot rolling.

[0048] The process of this embodiment produced a total of 4,194 tons of niobium-containing steel grade 1 in a certain month of 2022. The incidence rate of surface crack defects of the ingot was reduced from the original 18.08% to 0.05%, and the production cost decreased by 7.82 yuan / ton year-on-year.

[0049] Example 2

[0050] The method for controlling cracks in low-cost niobium microalloyed steel ingots in this embodiment takes niobium-containing steel type 2 as an example, and the composition and mass content of the steel type are:

[0051] element C Si Mn P S Als Nb N Content / wt% 0.07 <0.55 0.65 0.012 <0.002 0.05 0.02 <0.003

[0052] The weight percentage of each component in the niobium-containing steel type 2 satisfies: C≤0.07%, Mn / S=130>80, Al*N=0.015ppm.

[0053] The refining path of the raw molten steel of niobium-containing steel grade 2 is molten iron desulfurization → converter smelting → LF furnace treatment → continuous casting.

[0054] The present invention is optimized as molten iron desulfurization → converter smelting → RH furnace treatment → continuous casting, and the specific steps are as follows:

[0055] (1) KR hot metal desulfurization first removes sulfur from the hot metal to below 10 ppm;

[0056] (2) The converter smelting is subjected to sulfur reversion control, and the converter taps steel at high temperature, with low oxygen and low sulfur: the final tapping temperature of the converter is controlled at 1670°C; the oxygen content of the tapped steel [O] is ≤300ppm; the sulfur content of the tapped steel [S] is ≤30ppm, and the alloying is carried out with low nitrogen [N] <100ppm, low oxygen TO <4000ppm, and high-quality refined medium carbon ferromanganese; the mass fraction of Als in the molten steel after tapping is controlled at 0.035% to 0.055%.

[0057] (3) After the molten steel enters the RH, the oxygen content of the molten steel is controlled to 100ppm≤[O]≤200ppm for deep denitrification and deep dehydrogenation treatment; the RH is heated by oxygen blowing and an appropriate amount of aluminum particles are added before oxygen blowing to ensure that the molten steel w(Als)≥0.030% after oxygen blowing.

[0058] (4) After the molten steel reaches the continuous casting, the total oxygen content in the ingot is controlled to be TO≤10ppm; the casting speed of the continuous casting machine is controlled to be 1.3m / min; dynamic secondary cooling water and light pressure reduction technology are used throughout the continuous casting process to control the cooling rate of the ingot C k =1.0℃ / s; In order to better control segregation, the light reduction interval is appropriately close to the liquid phase area, and the light reduction is implemented before the cross-winding area of ​​the secondary crystal arms of the columnar crystals at the solidification front, and the reduction is completed within 90% of the two-phase area; At the same time, the light reduction rate is optimized to = 1.5mm / min, that is, the strain rate is increased, the grain boundary migration ability is obtained, and the secondary precipitates are precipitated from the grain boundary to the intracrystalline precipitation, reducing the crack sensitivity. The slabs are stacked and slowly cooled off the line, and the surface is checked in the cold state without defects and sent to hot rolling.

[0059] The process of this embodiment produced a total of 5,504 tons of niobium-containing steel grade 2 in a certain month of 2022. The incidence rate of surface crack defects of the ingot was reduced from the original 24.09% to 0.08%, and the production cost decreased by 9.18 yuan / ton year-on-year.

[0060] Example 3

[0061] The method for controlling cracks in low-cost niobium microalloyed steel ingots in this embodiment takes niobium-containing steel grade 3 as an example, and the composition and mass content of the steel grade are:

[0062] element C Si Mn P S Als Nb N Content / wt% 0.07 0.20 1.2 0.010 0.002 0.045 0.04 <0.0030

[0063] The weight percentage of each component in the niobium-containing steel grade 3 satisfies: C≤0.07%, Mn / S=260>80, Al*N=0.0135ppm.

[0064] The refining path of the raw molten steel of niobium-containing steel grade 3 is molten iron desulfurization → converter smelting → LF furnace treatment → continuous casting.

[0065] The present invention is optimized as molten iron desulfurization → converter smelting → RH furnace treatment → continuous casting, and the specific steps are as follows:

[0066] (1) KR hot metal desulfurization first removes sulfur from the hot metal to below 10 ppm;

[0067] (2) The converter smelting is subjected to resulfurization control, and the converter taps steel at high temperature, with low oxygen and low sulfur: the final tapping temperature of the converter is controlled at 1690°C; the oxygen content of the tapped steel [O] is ≤300ppm; the sulfur content of the tapped steel [S] is ≤30ppm, and the alloying is carried out with low nitrogen [N] <100ppm, low oxygen TO <4000ppm, and high-quality refined medium carbon ferromanganese; the mass fraction of Als in the molten steel after tapping is controlled at 0.010% to 0.025%.

[0068] (3) After the molten steel enters the RH, the oxygen content of the molten steel is controlled to 100ppm≤[O]≤200ppm for deep denitrification and deep dehydrogenation treatment; the RH is heated by oxygen blowing and an appropriate amount of aluminum particles are added before oxygen blowing to ensure that the molten steel w(Als) after oxygen blowing is ≥0.03%.

[0069] (4) After the molten steel reaches the continuous casting, the total oxygen content in the ingot is controlled to be TO≤15ppm; the casting speed of the continuous casting machine is controlled to be 1.35m / min; dynamic secondary cooling water and light reduction technology are used in the whole continuous casting process to control the cooling rate of the ingot Ck to 1.2℃ / s; in order to better control the segregation, the light reduction interval is appropriately close to the liquid phase area, and the light reduction is implemented before the cross-entanglement area of ​​the secondary crystal arms of the columnar crystals at the solidification front, and the reduction is controlled to be completed within 90% of the two-phase area; at the same time, the light reduction rate is optimized to be 1.1mm / min, that is, the strain rate is increased, the grain boundary migration ability is obtained, and the secondary precipitates are precipitated from the grain boundary to the intracrystalline precipitation, reducing the crack sensitivity. The slabs are stacked and slowly cooled down, and the surface is checked in the cold state without defects and sent to hot rolling.

[0070] The process of this embodiment produced a total of 4,338 tons of niobium-containing steel grade 3 in a certain month of 2022. The incidence rate of surface crack defects of the ingot was reduced from the original 26.40% to 0.03%, and the production cost decreased by 10.15 yuan / ton year-on-year.

[0071] Example 4

[0072] The method for controlling cracks in low-cost niobium microalloyed steel ingots in this embodiment takes niobium-containing steel type 4 as an example, and the composition and mass content of the steel type are:

[0073] element C Si Mn P S Als Nb N Content / wt% 0.05 0.5 1.6 0.010 0.002 0.035 0.03 <0.0035

[0074] The weight percentage of each component in the niobium-containing steel type 4 satisfies: C≤0.07%, Mn / S=260>80, Al*N=0.0123ppm.

[0075] The refining path of niobium-containing steel grade 4 raw molten steel is molten iron desulfurization → converter smelting → LF furnace treatment → continuous casting.

[0076] The present invention is optimized as molten iron desulfurization → converter smelting → RH furnace treatment → continuous casting, and the specific steps are as follows:

[0077] (1) KR hot metal desulfurization first removes sulfur from the hot metal to below 10 ppm;

[0078] (2) The converter smelting is subjected to resulfurization control, and the converter taps steel at high temperature, with low oxygen and low sulfur: the final tapping temperature of the converter is controlled at 1688°C; the oxygen content of the tapped steel [O] is ≤300ppm; the sulfur content of the tapped steel [S] is ≤30ppm, and the alloying is carried out with low nitrogen [N] <100ppm, low oxygen TO <4000ppm, and high-quality refined medium carbon ferromanganese; the mass fraction of Als in the molten steel after tapping is controlled at 0.02% to 0.040%.

[0079] (3) After the molten steel enters the RH, the oxygen content of the molten steel is controlled to 100ppm≤[O]≤200ppm for deep denitrification and deep dehydrogenation treatment; the RH is heated by oxygen blowing and an appropriate amount of aluminum particles are added before oxygen blowing to ensure that the molten steel w(Als) after oxygen blowing is ≥0.03%.

[0080] (4) After the molten steel reaches the continuous casting, the total oxygen content in the ingot is controlled to be TO≤15ppm; the casting speed of the continuous casting machine is controlled to be 1.45m / min; dynamic secondary cooling water and light pressure reduction technology are used throughout the continuous casting process to control the cooling rate of the ingot C k =1.4℃ / s; In order to better control segregation, the light reduction interval is appropriately close to the liquid phase area, and the light reduction is implemented before the cross-winding area of ​​the secondary crystal arms of the columnar crystals at the solidification front, and the reduction is controlled to be completed within 90% of the two-phase area; At the same time, the light reduction rate is optimized to = 1.3mm / min, that is, the strain rate is increased, the grain boundary migration ability is obtained, and the secondary precipitates are precipitated from the grain boundary to the intracrystalline precipitation, reducing the crack sensitivity. The slabs are stacked and slowly cooled off the line, and the surface is checked in the cold state without defects and sent to hot rolling.

[0081] The process of this embodiment produced a total of 4,850 tons of niobium-containing steel in a certain month of 2022. The incidence rate of surface crack defects of the ingot was reduced from the original 26.06% to 0.01%, and the production cost decreased by 10.92 yuan / ton year-on-year.

Claims

1. A method for controlling cracks in low-cost niobium microalloyed steel ingots, characterized by: It includes hot metal desulfurization, converter smelting, RH vacuum treatment, and continuous casting processes; In the continuous casting process, the total oxygen content TO in the ingot is controlled to be ≤15ppm; the soft reduction technology is adopted to complete the reduction within 90% of the two-phase region, and the soft reduction rate is 1.0mm / min≤≤1.5mm / min; the continuous casting speed V c ≥1.3m / min, cooling rate C k ≤1.5℃ / s; The weight percentage of each component in the niobium microalloyed steel meets the following requirements: C≤0.07%, Mn / S>80, Al*N≤0.015ppm.

2. The method for controlling cracks in low-cost niobium microalloyed steel ingot according to claim 1, characterized in that: In the converter smelting process, the final temperature of the steel is controlled at 1670-1690° C., and the oxygen content [O] of the steel is ≤300ppm and the sulfur content [S] is ≤30ppm.

3. The method for controlling cracks in low-cost niobium microalloyed steel ingot according to claim 2, characterized in that: In the RH vacuum treatment process, the oxygen content in the molten steel is controlled to be 100ppm≤[O]≤200ppm after the molten steel enters the RH.

4. The method for controlling cracks in low-cost niobium microalloyed steel ingot according to claim 3, characterized in that: In the RH vacuum treatment process, the weight content of Als in the molten steel after RH oxygen blowing is w(Als)≥0.03%.

5. The method for controlling cracks in low-cost niobium microalloyed steel ingot according to claim 4, characterized in that: The converter smelting and RH vacuum treatment processes use low-nitrogen and low-oxygen alloys, with total oxygen TO <5000ppm and nitrogen [N] <200ppm in the alloy.

6. The method for controlling cracks in low-cost niobium microalloyed steel ingot according to any one of claims 1 to 5, characterized in that: The weight percentage of Nb in the niobium microalloy steel is 0.015% to 0.06%.