Nb-containing low alloy steel and method for controlling edge cracking thereof

Abstract

The present application relates to a kind of Nb-containing low alloy steel and its edge crack control method, the steel plate includes the following mass fraction of chemical components: C:0.13~0.17%, Si:0.20~0.40%, Mn:1.00~1.60%, P≤0.020%, S≤0.012%, Als:0.010~0.045%, Nb:0.020~0.040%, Ti:0.010~0.020%, [H]≤1.8PPm, [N]≤45PPm, the balance is Fe and inevitable impurities;Its edge crack control method includes converter smelting→LF refining→vacuum degassing→pouring→rolling→cooling;The continuous casting slab section prepared by the present application is 250×2000mm, the finished steel plate prepared is ferrite structure grain size 8~10 level, yield strength ≥355Mpa, tensile strength 500~630Mpa, Z direction performance ≥25%, edge crack rate ≤0.5%, greatly improve the steel plate yield rate, with 50,000 tons Nb-containing low alloy steel enterprises as an example, can save economic loss 3 million yuan for enterprises every year.

Description

Technical Field

[0001] This invention relates to the field of steelmaking and rolling technology, and in particular to a method for controlling edge cracks in Nb-containing low alloy steel. Background Technology

[0002] In microalloyed steel, niobium improves both strength and toughness through grain refinement. Furthermore, niobium is a strong carbonitriding compound forming element; its carbonitriding compounds effectively "pin" grain boundaries during rolling at around 1000℃, further inhibiting austenite grain growth. This precipitation strengthening and phase transformation control further enhance the steel's strength, making it widely used in low-alloy structural steels. However, niobium (Nb) is highly susceptible to cracking. During casting, Nb-containing steel is prone to edge cracking defects in the two-dimensional cooling zone at the corners of the billet, leading to billet regrinding, length reduction, decreased steel plate yield, and even downgrading or scrapping, resulting in quality losses. In 2022, a factory's Q355 series Nb-containing low-alloy steel experienced an edge cracking rate of 5.5%, requiring forced surface cleaning of the billet, severely impacting hot-rolling, increasing furnace fuel consumption, and reducing steel plate yield by approximately 3%. The edge cracking problem has remained unresolved.

[0003] Niobium-containing low-alloy steel is highly susceptible to hot cracking, and edge cracking is difficult to control during slab production, with transverse and longitudinal cracks frequently occurring. During continuous casting, the linear shrinkage of steel is greatest 20-50°C below the solid-liquid line in the crystallizer, increasing the risk of surface cracking, especially at the corners of the billet. The area near the junction of the wide and narrow faces of the slab exhibits two-dimensional heat conduction, leading to faster solidification of molten steel at the corners of the crystallizer and earlier shrinkage of the initial crust. This easily results in uneven air gaps at the corners, increasing thermal resistance and further affecting the average growth of the billet shell. Weak points in the billet shell cannot withstand thermal stress, shrinkage stress, frictional stress, and hydrostatic pressure from the molten steel, leading to corner cracks. Furthermore, the severity of slab edge cracking is closely related to the composition design, steel purity, P and S inclusion content, gas content, and the appropriateness of continuous casting process parameters.

[0004] As competition in the steel market intensifies, customers are demanding increasingly stringent requirements for the surface quality of steel plates. Not only must the performance of the steel plates meet the usage requirements, but the surface quality of the steel plates is also a key focus for customers. How to control edge cracks in niobium-containing low alloy steel and produce defect-free billets is the key to ensuring the quality of rolled steel plates and is also an important research direction for steelmaking in the future.

[0005] Chinese invention patent application CN201510158824.6 discloses "A method for reducing transverse cracks at the corners of niobium-containing steel." The main measure for controlling corner cracks in niobium-containing steel in this application is the use of a chamfering crystallizer, which has significant equipment limitations. Furthermore, it does not address specific strength grades of steel, lacks rolling and performance parameters, and only focuses on the billet manufacturing process, resulting in weak process guidance.

[0006] Application number CN201310618872.X, entitled "A comprehensive method for controlling narrow-face side cracks in carbon niobium-containing steel in thick slabs," mainly addresses the control of narrow-face side cracks in carbon niobium-containing steel with a thickness of 300-420mm. The control process described only includes the continuous casting process, and the control links for smelting and rolling of the steel billet are not clearly defined. Furthermore, the niobium content in the steel is not clearly defined, and the impact of different niobium contents on side cracks varies greatly. The production guidance is not rigorous enough. On the other hand, the immersion nozzle is designed to be inserted at a relatively deep depth, which makes it difficult to remove gas and inclusions from the steel, affecting the purity of the steel.

[0007] Few domestic patents describe methods for controlling edge cracks in specific steels and niobium-containing steels. The edge and corner crack control methods for niobium-containing steels mentioned in the above inventions only describe continuous casting process control and do not clearly specify the rolling cooling process and strength grade of the corresponding steel. The niobium content in the steel is not clearly defined, and there are limitations in terms of equipment, procedures, and processes, which do not provide strong guidance for the production of specific steels. Summary of the Invention

[0008] The purpose of this invention is to provide a method for controlling edge cracks in niobium-containing low-alloy steel. This method, through appropriate chemical composition design, reasonable deoxidation and refining processes, suitable protective slag system, refined control of primary and secondary cooling parameters and dynamic light reduction parameters, stabilizes the flow field of molten steel in the crystallizer, reducing the edge crack rate of niobium-containing low-alloy steel billets to below 0.5%, and providing reliable technical guidance for producing defect-free niobium-containing steel plates with excellent surface quality.

[0009] The present invention discloses a low-alloy steel containing Nb, wherein the steel billet comprises the following chemical composition by mass fraction: C: 0.13-0.17%, Si: 0.20-0.40%, Mn: 1.00-1.60%, P≤0.020%, S≤0.012%, Als: 0.010-0.045%, Nb: 0.020-0.040%, Ti: 0.010-0.020%, [H]≤1.8 ppm, [N]≤45 ppm, and the balance being Fe and unavoidable impurities.

[0010] The Nb-containing low alloy steel of the present invention has a continuous casting slab cross section of 250×2000mm. The resulting finished steel plate has a ferrite grain size of 8-10, a yield strength ≥355MPa, a tensile strength of 500-630MPa, a Z-direction property ≥25%, and an edge crack rate ≤0.5%.

[0011] The present invention provides a method for controlling edge cracks in Nb-containing low-alloy steel, comprising converter smelting → LF refining → vacuum degassing → casting → rolling → cooling, wherein: (1) Converter smelting: 1-2 minutes after the blowing end point, switch the converter bottom blowing argon to strong stirring mode, control the end point [C] 0.06-0.10%, [P] ≤0.016%, tapping temperature 1590-1620℃, tapping oxygen content 400-600PPm, and alloy deoxidation during tapping process; (2) LF refining: aluminum shot is used for forced deoxidation at the station, and fluorite balls and lime are used to make slag and raise the temperature in batches. The white slag is kept for ≥10 minutes, and the basicity R of the refining slag is controlled at 4.5~7.0. Specifically: Before LF refining, aluminum shot (1.0–1.5 kg / t) is used for forced deoxidation. During the refining process, fluorite balls and lime are used for batch slag formation. The first batch of lime (6.5–7.5 kg / t), fluorite balls (2.0–3.0 kg / t), and calcium carbide (1.0–1.6 kg / t) are added upon entering the station. The bottom blowing and strong stirring mode is activated, and power is supplied for 3–5 minutes. The second batch of lime (2.5–3.5 kg / t) and fluorite balls (0.5–1.2 kg / t) are added, ensuring that the slag-forming materials are added within 6 minutes of power supply. The refining slag mainly contains the following components by mass percentage: CaO: 50–60%, SiO2: 8–12%, Al2O3: 13–20%, MgO: 4–6%, Fe+Mn0 ≤ 1.5%. (3) Vacuum degassing: Under high vacuum conditions of RH treatment ≤67Pa, the holding time is ≥12min / furnace, Ti is added during the degassing process, and the outlet [H] ≤1.8PPm, [N] ≤45PPm; (4) Casting: The continuous casting protective slag adopts the CaO-SiO2-Al2O3 system ternary medium carbon slag, the crystallizer is controlled with a small taper, the secondary cooling adopts weak cooling, the typical drawing temperature and speed are matched to the steel drawing, the flow field of the crystallizer is stable, the electromagnetic stirring and dynamic light pressure are turned on, and the straightening temperature is 880~900℃. The continuous casting protective slag contains 36%–38% CaO by mass, 1.02–1.05 basicity, 3–5% Al2O3 by mass, 8.5–10% K2O+Na2O by mass, and has a melting point of 1200–1250℃. The protective slag is added evenly, and the slag consumption is controlled at 0.40–0.45 kg / t. The continuous casting slab has a cross-section of 250×2000mm, a crystallizer taper of 1.03~1.05%, and a water supply of 4000 / 460L / min for the wide / narrow face of the crystallizer. The secondary cooling water volume is controlled by zone, and the total water ratio is controlled at 0.5~0.7L / kg. The casting adopts a straight arc casting machine with a total of 16 sector sections. Sections 1-6 are arc sections with an inner-outer arc water ratio of 1:1.7. Sections 7-8 are curved sections with an inner-outer arc water ratio of 1:1.5. Sections 9-16 are horizontal sections with an inner-outer arc water ratio of 1:2.0. Electric stirring is activated at a position 8-10 meters below the crystallizer. The stirring parameters are 440A / 6Hz. When the superheat is 15-30℃, the stirring speed is 0.95-1.0m / min. Light reduction is located in sector section 7-8, and the reduction is controlled at 6.5mm. The immersion nozzle should be inserted into the crystallizer to a depth of 120-140mm. The molten steel level in the crystallizer should be stable, with small, uniform flames on both sides of the molten steel. Observe the condensation of molten steel on the molten steel surface and remove any slag bars that appear in time. (5) Rolling: A two-stage controlled rolling process is adopted. The single-pass reduction rate of roughing is 10-20%, the thickness of the billet in the intermediate stage is 2-3 times the thickness of the finished product, the starting temperature of finishing rolling is ≤960℃, the final rolling temperature is 840±30℃, and the total compression ratio of the billet is ≥5.0. (6) Cooling: After rolling, the steel plate is cooled online by ACC. The initial cooling temperature is 760±30℃, the roll speed is controlled at 0.9~1.0m / s, the cooling rate is 8~12℃ / S, and the final cooling temperature is 600±30℃.

[0012] The rationale for the chemical composition design of this invention is as follows: In this invention, the carbon content is 0.13%–0.17%, primarily to avoid the peritectic range, reduce linear shrinkage during billet solidification, and facilitate stable control of the molten steel level in the crystallizer. The silicon content is 0.20%–0.40%, and the manganese content is 1.00%–1.60%, mainly for solid solution strengthening of the steel matrix, promoting a tensile strength of 500–630 MPa. P ≤ 0.020% and S ≤ 0.012%, primarily for ease of smelting operations, appropriately reducing the content of P and S inclusions without significantly increasing smelting costs, thus minimizing the harm caused by grain boundary segregation. The als content is 0.010%–0.045%, primarily to ensure sufficient deoxidation of the molten steel and reduce the harmful effects of free oxygen on the steel quality. The Nb content is between 0.020% and 0.040%, mainly to facilitate controlled rolling and refine the grains, thereby improving the strength and toughness of the steel. However, excessively high Nb content increases the precipitation of compounds such as niobium carbide, increasing the probability of edge cracking; therefore, the upper limit is set at 0.040%. The Ti content is between 0.010% and 0.020%, primarily for nitrogen fixation to reduce the adverse effects of nitrogen on the steel, while also refining the austenite grains, contributing to improved steel strength. Furthermore, this invention limits the [H] content to ≤1.8 ppm and [N] content to ≤45 ppm to ensure that the H and N contents are as low as possible, thereby reducing the solubility of gases during the solidification process of the billet and minimizing their precipitation, which in turn affects crack formation.

[0013] The rationale for the design of the process parameters in this invention is as follows: The present invention switches from argon bottom blowing to strong stirring mode at the end of converter blowing in order to enhance the stirring effect of molten steel in the converter pool, further homogenize the composition of molten steel at the end point, promote gas flotation, and reduce the nitrogen and oxygen gas content in the steel.

[0014] This invention involves adding slag in batches and vigorous stirring in the LF furnace. This is primarily to avoid the large temperature drop and slag formation that can occur when adding too much slag at once. Adding it in batches according to a set ratio effectively mitigates this problem and facilitates slag formation, thus improving the effective refining effect. Setting the basicity of the LF furnace refining slag to 4.5–7.0 is mainly to increase the slag viscosity, enhance its ability to adsorb inclusions, and further purify the molten steel.

[0015] In this invention, the RH requirement is to control the ultimate vacuum holding time to ≥12 min / furnace, mainly to ensure that the H and N contents are as low as possible, thereby reducing the impact of reduced gas solubility and precipitation on cracks during the solidification process of the billet.

[0016] In this invention, the basicity of the protective slag in the continuous casting mold is limited to 1.0–1.05, and the Al2O3 content is limited to 3–5%. This is mainly to create weakly alkaline conditions to enhance the adsorption capacity of Al2O3 and silicate inclusions. The K2O+Na2O content in the protective slag is limited to 8.5–10%. This is mainly to appropriately increase the slag activity, which is beneficial for uniform melting of the protective slag and reduces uneven slag formation that could affect heat transfer through the mold wall. The consumption of protective slag is controlled at 0.40–0.45 kg / t, mainly to maintain a moderate thickness of the liquid slag layer, reduce temperature drop, and promote uniform billet shell growth.

[0017] This invention requires zoned control of the secondary cooling water volume in continuous casting, primarily to ensure uniform growth of columnar crystals on both the inner and outer arcs and symmetrical microstructures. Furthermore, taking a straight-arc slab continuous casting machine as an example, the 7th to 8th sector is the straightening section, a sensitive area for crack formation; the water ratio in this section should be appropriately low. The 9th to 16th sector, being horizontal and not subject to bending stress, can have a slightly higher water ratio. By precisely setting the dynamic light reduction interval and amount, center segregation of the continuously cast slab is significantly reduced, ensuring uniformity of the cross-sectional properties after rolling. The requirement for an immersion nozzle insertion depth of 120–140 mm is mainly to stabilize the molten steel flow field in the crystallizer. Under the premise of stable crystallizer surface fluctuations, this facilitates gas rising and overflow without excessively scouring the slab shell at the crystallizer exit, thus controlling edge cracks caused by slab shell remelting.

[0018] This invention requires a two-stage controlled rolling process, primarily to maximize the grain refinement effect of the rolling mill. During rough rolling, a single-pass reduction rate of 10-20% is set to minimize austenite grain size at high temperatures. The intermediate billet thickness is 2-3 times the finished product thickness to provide conditions for grain elongation and refinement during finish rolling. The finish rolling start temperature is required to be ≤960℃ to leverage the role of Nb in raising the austenite transformation temperature in steel, avoiding grain size inconsistencies caused by mixed-grain rolling. The final rolling temperature is set at 840±30℃ to appropriately control the ferrite and austenite grain sizes and provide temperature drop support for subsequent ACC cooling. The total billet compression ratio is ≥5.0 to ensure that the final steel plate has sufficiently fine and uniform grains, guaranteeing improved performance stability.

[0019] The ACC cooling temperature after rolling is set to 760±30℃ in order to ensure the total temperature drop gradient of ACC cooling, which is beneficial to improving strength performance. The cooling rate is controlled at 8~12℃ / S and the final cooling temperature is 610±30℃ in order to obtain fine ferrite and pearlite structure and avoid the formation of bainite mixed structure due to overcooling, which would affect the uniformity of steel plate performance.

[0020] Compared with the prior art, the present invention has the following advantages: The purpose of this invention is to provide a method for controlling edge cracking in Nb-containing low-alloy steel. This method refines the grain structure through Nb+Ti microalloying, achieves uniform composition and oxygen control through bottom blowing and strong stirring at the converter endpoint, efficiently deoxidizes and removes inclusions in the LF furnace, and improves steel purity through extreme degassing in RH vacuum. Continuous casting employs a matched ternary medium-carbon protective slag, controls the crystallizer taper and flow field stability, refines the water distribution in the primary and secondary cooling zones, rationally utilizes dynamic light reduction and electromagnetic stirring, uniformly improves the surface and internal quality of the billet, controls the center segregation of the continuously cast billet to within C1.5, and adopts a two-stage controlled rolling and ACC cooling process. This results in a grain size of 8-10, Z-axis properties ≥25%, tensile strength of 500-630 MPa, and an edge cracking rate of ≤0.5% for Nb-containing low-alloy steel plates. The steel exhibits good performance uniformity and effectively reduces economic losses such as downgrading and steel modification due to edge cracking, significantly improving the steel plate yield. For example, for a company producing 50,000 tons of niobium-containing low-alloy steel annually, this method can save the company 3 million yuan in economic losses annually. Attached Figure Description

[0021] Figure 1 This is a metallographic diagram of the Nb-containing low-alloy steel obtained in Example 1 of the present invention. Detailed Implementation

[0022] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way. The sequence numbers of the following embodiments are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0023] Table 1 below is a list of chemical composition values ​​for each embodiment and comparative example of the present invention; Table 2 below lists the main process parameter values ​​for each embodiment and comparative example of the present invention; Table 3 below lists the test results of the main mechanical properties of each embodiment and comparative example of the present invention.

[0024] An embodiment of the present invention provides a method for controlling edge cracks in Nb-containing low-alloy steel, comprising converter smelting → LF refining → vacuum degassing → casting → rolling → cooling, wherein: (1) Converter smelting: 1-2 minutes after the blowing end point, switch the converter bottom blowing argon to strong stirring mode, control the end point [C] 0.06-0.10%, [P] ≤0.016%, tapping temperature 1590-1620℃, tapping oxygen content 400-600PPm, and alloy deoxidation during tapping process; (2) LF refining: aluminum shot is used for forced deoxidation at the station, and fluorite balls and lime are used to make slag and raise the temperature in batches. The white slag is kept for ≥10 minutes, and the basicity R of the refining slag is controlled at 4.5~7.0. Specifically: Before LF refining, aluminum shot (1.0–1.5 kg / t) is used for forced deoxidation. During the refining process, fluorite balls and lime are used for batch slag formation. The first batch of lime (6.5–7.5 kg / t), fluorite balls (2.0–3.0 kg / t), and calcium carbide (1.0–1.6 kg / t) are added upon entering the station. The bottom blowing and strong stirring mode is activated, and power is supplied for 3–5 minutes. The second batch of lime (2.5–3.5 kg / t) and fluorite balls (0.5–1.2 kg / t) are added, ensuring that the slag-forming materials are added within 6 minutes of power supply. The refining slag mainly contains the following components by mass percentage: CaO: 50–60%, SiO2: 8–12%, Al2O3: 13–20%, MgO: 4–6%, Fe+Mn0 ≤ 1.5%. (3) Vacuum degassing: Under high vacuum conditions of RH treatment ≤67Pa, the holding time is ≥12min / furnace, Ti is added during the degassing process, and the outlet [H] ≤1.8PPm, [N] ≤45PPm; (4) Casting: The continuous casting protective slag adopts the CaO-SiO2-Al2O3 system ternary medium carbon slag, the crystallizer is controlled with a small taper, the secondary cooling adopts weak cooling, the typical drawing temperature and speed are matched to the steel drawing, the flow field of the crystallizer is stable, the electromagnetic stirring and dynamic light pressure are turned on, and the straightening temperature is 880~900℃. The continuous casting protective slag contains 36%–38% CaO by mass, 1.02–1.05 basicity, 3–5% Al2O3 by mass, 8.5–10% K2O+Na2O by mass, and has a melting point of 1200–1250℃. The protective slag is added evenly, and the slag consumption is controlled at 0.40–0.45 kg / t. The continuous casting slab has a cross-section of 250×2000mm, a crystallizer taper of 1.03~1.05%, and a water supply of 4000 / 460L / min for the wide / narrow face of the crystallizer. The secondary cooling water volume is controlled by zone, and the total water ratio is controlled at 0.5~0.7L / kg. The casting adopts a straight arc casting machine with a total of 16 sector sections. Sections 1-6 are arc sections with an inner-outer arc water ratio of 1:1.7. Sections 7-8 are curved sections with an inner-outer arc water ratio of 1:1.5. Sections 9-16 are horizontal sections with an inner-outer arc water ratio of 1:2.0. Electric stirring is activated at a position 8-10 meters below the crystallizer. The stirring parameters are 440A / 6Hz. When the superheat is 15-30℃, the stirring speed is 0.95-1.0m / min. Light reduction is located in sector section 7-8, and the reduction is controlled at 6.5mm. The immersion nozzle should be inserted into the crystallizer to a depth of 120-140mm. The molten steel level in the crystallizer should be stable, with small, uniform flames on both sides of the molten steel. Observe the condensation of molten steel on the molten steel surface and remove any slag bars that appear in time. (5) Rolling: A two-stage controlled rolling process is adopted. The single-pass reduction rate of roughing is 10-20%, the thickness of the billet in the intermediate stage is 2-3 times the thickness of the finished product, the starting temperature of finishing rolling is ≤960℃, the final rolling temperature is 840±30℃, and the total compression ratio of the billet is ≥5.0. (6) Cooling: After rolling, the steel plate is cooled online by ACC. The initial cooling temperature is 760±30℃, the roll speed is controlled at 0.9~1.0m / s, the cooling rate is 8~12℃ / S, and the final cooling temperature is 600±30℃.

[0025] Table 1. List of chemical components (wt, %) of various embodiments and comparative examples of the present invention Table 2. List of main process parameter values ​​for each embodiment and comparative example of the present invention.

[0026] Table 3. List of performance test analysis results for each embodiment and comparative example of the present invention.

[0027] As shown in Table 3 above, in the five examples, the Nb content was 0.020%–0.040%, the ferrite grain size was 8.5–10, the yield strength was 364–402 MPa, the tensile strength was 534–566 MPa, the uniformity was good, the elongation was moderate (A24%–26%), the Z-direction tensile strength of the cross-section was 25%–35% reaching the Z25 level, and the edge quality control after rolling was good, with no edge cracks observed. In Comparative Example 1, due to insufficient single-pass reduction in roughing and a high final rolling temperature, the ferrite grain size was 7.5, resulting in insufficient yield and tensile strength of the steel plate compared to low-alloy Q355. Furthermore, the LF slag basicity (R3.2) indicated weak adsorption capacity for silicate and other oxide inclusions, and the low-magnification center segregation of the cast billet (C2.0 level) negatively impacted elongation. The crystallizer taper of 1.10% and the immersion nozzle insertion depth of 98 mm were not reasonable, resulting in high compressive stress on the primary billet shell from the crystallizer wall. Shallow impact depth of the steel flow erodes the initial billet shell, exacerbating edge cracking. Edge cracks appeared in the rolled steel plate. In Comparative Example 2, the Nb content of 0.054% was too high, resulting in excessive fine grain strengthening and solid solution strengthening, leading to higher yield and tensile strength, which is unfavorable for bending of structural steel. The ultimate vacuum holding time of 8min in the RH furnace was too short, and the [N] and [O] content in the steel was too high. During solidification, a large amount of carbonitrides and oxidation products precipitated at the grain boundaries, leading to edge cracking. In Comparative Example 3, the final P of converter smelting remained high. High-temperature tapping with spot blowing resulted in a final [O] of 680ppm. Severe over-oxidation of the molten steel led to low-magnification center segregation of C2.0 grade in the billet. The consumption of mold flux in the crystallizer was too high at 0.50%, resulting in large temperature drop loss and uneven billet shell growth, which exacerbated cracking. Edge cracks appeared after rolling.

[0028] The above embodiments provide a niobium-containing low alloy steel and its preparation method. The rolled steel plates obtained in Embodiments 1 to 5 of the present invention have a ferrite grain size of 8 to 10, a yield strength ≥355 MPa, a tensile strength of 500 to 630 MPa, and a Z-direction property ≥25%. The invention also effectively solves the problem of edge cracking in niobium-containing steel, and the edge cracking rate of the steel plate can be reduced to below 0.5%.

[0029] See Figure 1 , Figure 1 The niobium-containing low-alloy steel plate prepared in Example 1 of this invention can be seen from the figure. The main microstructure of the steel is ferrite and pearlite, with a ferrite grain size of 8.5 and a uniform microstructure distribution.

[0030] The above embodiments are merely specific examples exemplified to explain the present invention and do not limit the present invention in any way. Any non-substantial changes made by any person based on the above content and form that do not depart from the scope of protection of the claims of the present invention should be considered to fall within the scope of protection of the claims of the present invention. The present invention is not limited to the specific embodiments described above.

Claims

1. A low-alloy steel containing Nb, characterized in that... The steel contains the following chemical composition by mass fraction: C: 0.13–0.17%, Si: 0.20–0.40%, Mn: 1.00–1.60%, P≤0.020%, S≤0.012%, Als: 0.010–0.045%, Nb: 0.020–0.040%, Ti: 0.010–0.020%, [H]≤1.8 ppm, [N]≤45 ppm, with the balance being Fe and unavoidable impurities; The method for controlling edge cracks in Nb-containing low-alloy steel includes converter smelting → LF refining → vacuum degassing → casting → rolling → cooling, wherein: (1) Converter smelting: 1-2 minutes after the blowing end point, switch the converter bottom blowing argon to strong stirring mode, control the end point [C] 0.06-0.10%, [P] ≤0.016%, tapping temperature 1590-1620℃, tapping oxygen content 400-600PPm, and alloy deoxidation during tapping process; (2) LF refining: aluminum shot is used for forced deoxidation at the station, and fluorite balls and lime are used to make slag and raise the temperature in batches. The white slag is kept for ≥10 minutes, and the basicity R of the refining slag is controlled at 4.5~7.

0. (3) Vacuum degassing: Under high vacuum conditions of RH treatment ≤67Pa, the holding time is ≥12min / furnace, Ti is added during the degassing process, and the outlet [H] ≤1.8PPm, [N] ≤45PPm; (4) Casting: The protective slag is CaO-SiO2-Al2O3 ternary medium carbon slag. The crystallizer is controlled with a small taper. The secondary cooling is weak cooling. The temperature and speed of the steel are matched. The flow field of the crystallizer is stable. The electromagnetic stirring and dynamic light pressure are turned on. The straightening temperature is 880-900℃. (5) Rolling: A two-stage controlled rolling process is adopted. The single-pass reduction rate of roughing is 10-20%, the thickness of the billet in the intermediate stage is 2-3 times the thickness of the finished product, the starting temperature of finishing rolling is ≤960℃, the final rolling temperature is 840±30℃, and the total compression ratio of the billet is ≥5.

0. (6) Cooling: After rolling, the steel plate is cooled online by ACC. The initial cooling temperature is 760±30℃, the roll speed is controlled at 0.9~1.0m / s, the cooling rate is 8~12℃ / S, and the final cooling temperature is 600±30℃. In step (4), the continuous casting protective slag adopts a medium-low carbon CaO-SiO2-Al2O3 ternary slag system. The mass percentage of CaO in the slag is 36% to 38%, the basicity is 1.02 to 1.05, the mass percentage of Al2O3 is 3 to 5%, the mass percentage of K2O+Na2O is 8.5 to 10%, the slag melting point is 1200 to 1250℃, the protective slag is added evenly, and the slag consumption is controlled at 0.40 to 0.45 kg / t. In step (4), the casting cross section is 250×2000mm, the crystallizer taper is 1.03~1.05%, the water supply for the wide / narrow face of the crystallizer is 4000 / 460L / min, the secondary cooling water volume is controlled by zone, and the total water ratio is controlled at 0.5~0.7L / kg. The casting adopts a straight arc casting machine with a total of 16 fan-shaped sections. Sections 1 to 6 are arc sections with an inner-outer arc water ratio of 1:1.7, sections 7 to 8 are curved sections with an inner-outer arc water ratio of 1:1.5, and sections 9 to 16 are horizontal sections with an inner-outer arc water ratio of 1:2.

0. Electric stirring is activated at a position 8 to 10 meters below the crystallizer. The electric stirring parameters are 440A / 6Hz. When the superheat is 15~30℃, the stirring speed is 0.95~1.0m / min. Light pressing is located in the 7th to 8th section of the fan-shaped section, and the pressing amount is controlled at 6.5mm.

2. The Nb-containing low-alloy steel according to claim 1, characterized in that: The continuous casting slab has a cross-section of 250×2000mm. The resulting finished steel plate has a ferrite grain size of 8-10, a yield strength ≥355MPa, a tensile strength of 500-630MPa, a Z-axis property ≥25%, and an edge crack rate ≤0.5%.

3. The Nb-containing low-alloy steel according to claim 1, characterized in that: Step (2) Before LF refining, aluminum shot 1.0-1.5 kg / t is used for forced deoxidation. During the refining process, fluorite balls and lime are used to make slag in batches. Specifically, the first batch of lime 6.5-7.5 kg / t, fluorite balls 2.0-3.0 kg / t, and calcium carbide 1.0-1.6 kg / t are added. The bottom blowing and strong stirring mode is turned on and the power is supplied for 3-5 minutes. The second batch of lime 2.5-3.5 kg / t and fluorite balls 0.5-1.2 kg / t are added. Ensure that the slag-making materials are added within 6 minutes of power supply.

4. The Nb-containing low-alloy steel according to claim 1, characterized in that: The refining slag mentioned in step (2) mainly contains the following components in mass percentage: CaO: 50-60%, SiO2: 8-12%, Al2O3: 13-20%, MgO: 4-6%, Fe+MnO≤1.5%.

5. The Nb-containing low-alloy steel according to claim 1, characterized in that: In step (4), the immersion nozzle should be inserted into the crystallizer to a depth of 120-140 mm. The molten steel surface in the crystallizer should be stable, and there should be small, uniform flames on both sides of the molten steel surface. Observe the molten steel formation on the molten steel surface. If slag appears, it should be removed in time.

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