Boron-containing steel, its production method and application

Abstract

This invention discloses a boron-containing steel, its production method, and its applications. The production method includes sequential converter smelting, LF refining, RH vacuum treatment, and continuous casting processes. The steel composition, by mass percentage, is controlled as follows: C: 0.30~0.40%, Si: 0.20~0.30%, Mn: 1.20~1.40%, P≤0.015%, S≤0.0030%, Cr: 0.10~0.30%, Ti≤0.020%, B: 0.0012~0.0035%, Al: 0.05~0.08%, N≤0.0050%, Ca: 0.0010~0.0040%, with the remainder being Fe and unavoidable impurities. A first calcium treatment is performed after LF refining, and a second calcium treatment is performed after the RH vacuum treatment is completed and the vacuum is broken. This invention improves the steel's machinability through the synergistic effect of replacing titanium with aluminum and segmented calcium treatment.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a boron-containing steel, its production method, and its application. Background Technology

[0002] High-frequency resistance welded cold-drawn steel pipes are widely used in precision manufacturing fields such as automobiles and machinery due to their excellent dimensional accuracy and surface quality. Boron-containing steel, with its ability to significantly improve hardenability through the addition of trace amounts of boron, has become an important pipe material in this field. However, the current industry generally adopts the "aluminum deoxidation-titanium nitrogen fixation" process route to produce this type of boron-containing steel. Although the introduction of titanium can effectively fix nitrogen in the steel, it inevitably leads to the formation of coarse, thermally stable Ti(N,C) precipitates in the segregation zone of the billet. These hard particles with sharp edges are prone to becoming stress concentration points during subsequent high-frequency welding and cold drawing processes, inducing microcracks and pores, directly affecting the flattening qualification rate and drawing success rate of the welded pipe.

[0003] Furthermore, controlling sulfide inclusions is another long-standing technical challenge in the smelting process of boron-containing steel. During high-frequency resistance welding, the instantaneous temperature in the weld area can reach over 1350℃. At this temperature, sulfide inclusions such as MnS, which are distributed in long strips in the steel, will soften or even partially melt. On the one hand, they easily extend along the austenite grain boundaries to form intergranular liquefaction cracks. On the other hand, under the action of welding extrusion pressure, these softened sulfides will be pushed to the near-surface area of ​​the strip steel, forming hook-shaped crack defects under the superposition of welding thermal stress and tube restraint stress. To address the above problems, existing technologies mainly adopt the methods of reducing the content of impurities such as sulfur and phosphorus in the steel or adjusting the rolling process parameters. However, such measures can only alleviate the problem of a single type of inclusion to a certain extent, and it is difficult to simultaneously and completely eliminate the two deep-seated metallurgical defects: the coarse Ti(N,C) precipitates formed during the solidification of the billet and the sulfide inclusions distributed in strips after rolling. For this reason, the cracking problem of boron-containing steel welded pipes during flattening, flaring, and cold drawing processes has not yet been fundamentally solved. Summary of the Invention

[0004] This invention provides a boron-containing steel, its production method, and its application, to solve the problem of welding and cold drawing cracking caused by large-sized Ti(N,C) precipitates and harmful sulfide inclusions when using existing boron-containing steel to manufacture cold-drawn welded pipes.

[0005] In a first aspect, the present invention provides a method for producing boron-containing steel, comprising sequentially performing converter smelting, LF refining, RH vacuum treatment, and continuous casting processes, and controlling the final steel composition by mass percentage as follows: C: 0.30~0.40%, Si: 0.20~0.30%, Mn: 1.20~1.40%, P≤0.015%, S≤0.0030%, Cr: 0.10~0.30%, Ti≤0.020%, B: 0.0012~0.0035%, Al: 0.05~0.08%, N≤0.0050%, Ca: 0.0010~0.0040%, with the remainder being Fe and unavoidable impurities;

[0006] The process involves a first calcium treatment of the molten steel after LF refining and before RH vacuum treatment, using pure calcium wire feed. After RH vacuum treatment and before continuous casting, the molten steel undergoes a second calcium treatment using silicon-calcium wire feed.

[0007] The following is a detailed analysis and explanation of the main functions and dosage selection of each chemical component in this invention:

[0008] Carbon (C): Cold-drawn welded pipes require high strength and hardness after quenching, and carbon is one of the necessary conditions to ensure high strength after heat treatment. If the carbon content is too low, the strength and hardness of the heat-treated steel plate will not meet the requirements; if the carbon content is too high, the strength and hardness of the steel plate will increase, but the processing performance and welding performance will deteriorate. Therefore, the carbon content is selected as 0.30~0.40%.

[0009] Silicon (Si): The silicon content in this invention is selected to be 0.20~0.30%. Excessive Si content will affect the surface quality of the strip steel.

[0010] Manganese (Mn): Manganese plays a role in solid solution strengthening in steel, while also stabilizing austenite and refining ferrite grains. If the manganese content is too low, the steel's strength will be insufficient for its application requirements; if the manganese content is too high, banded segregation is likely to form, affecting the steel's weldability and cold working properties. Therefore, the manganese content is selected to be 1.20~1.40%.

[0011] Chromium (Cr): Boron-added steel generally requires heat treatment. Adding a certain amount of chromium can further improve the hardenability of the steel. The chromium content is selected as 0.10~0.30%.

[0012] Phosphorus (P): Phosphorus is an impurity element in steel that reduces the low-temperature toughness of the steel plate, but too low a phosphorus content will also increase smelting costs. Therefore, the phosphorus content is selected to be ≤0.015%.

[0013] Sulfur (S): Sulfur is also an impurity element in steel. It easily combines with manganese to form MnS inclusions. After rolling deformation, MnS forms long strips, which disrupts the continuity of the matrix. Therefore, the S content should be limited. However, too low a sulfur content will increase the difficulty of smelting. Therefore, the sulfur content is selected to be ≤0.0030%.

[0014] Aluminum (Al): Aluminum is a strong deoxidizer that can effectively remove oxygen from steel. It is usually added to steel as a deoxidizing element at a dosage of 0.015~0.035%. In this invention, aluminum also plays the role of replacing titanium by reacting with nitrogen in steel, consuming nitrogen in the steel, and preventing the formation of TiN or Ti(C,N) in the steel, thereby improving the welding and drawing properties of the steel.

[0015] In this invention, the amount of aluminum added is selected as 0.05~0.08%. If the content is too low, aluminum mainly reacts with oxygen in steel, and the nitrogen fixation effect is not obvious. If the aluminum content is too high, it is easy to form coarse AlN or Al2O3 inclusions, which will lead to an increase in inclusions in steel and affect the processing performance and fatigue performance of the finished product.

[0016] Nitrogen (N): This invention uses aluminum to fix nitrogen, which requires strict control of the nitrogen content in the steel. If the nitrogen content is too high, it will be difficult to react completely with aluminum, and may also lead to the formation of coarse Al2O3, affecting the processing performance. Therefore, the nitrogen content in the steel should be controlled to be no more than 0.0050%.

[0017] Boron (B): Boron can significantly improve the hardenability of steel, but it has low solubility in steel and easily combines with nitrogen to form boron nitride (BN), thus weakening its effect. Therefore, boron-containing steel needs to have nitrogen-fixing elements added to ensure the effectiveness of boron. If the boron content in steel is too low, it is difficult to exert its effect of improving hardenability; if the content is too high, it is easy to form brittle boride inclusions, reducing the toughness and ductility of the steel and affecting its processing performance.

[0018] Titanium (Ti): In this invention, titanium is strictly controlled as a residual element (Ti≤0.020%, preferably Ti≤0.010%), in order to avoid the formation of large-sized, thermally stable Ti(N,C) composite precipitates in the traditional "titanium nitrogen fixation" process, thereby eliminating the risk of stress concentration and cracking caused by them in subsequent welding and cold drawing processes.

[0019] Calcium (Ca): Calcium is added to steel through a segmented processing process. Its functions include deoxidation, desulfurization and inclusion modification. The core is to achieve targeted control of alumina and sulfides through two calcium treatments at different refining stages.

[0020] The first calcium treatment is carried out after LF refining, and its target is the high-melting-point Al2O3 inclusions generated in the molten steel due to aluminum deoxidation. By feeding in pure calcium wire, calcium reacts with Al2O3 to generate low-melting-point calcium aluminate inclusions. These inclusions are in a liquid or semi-liquid state in the molten steel, easily collide and aggregate, and float to the slag phase, thereby improving the cleanliness of the molten steel. At the same time, the residual inclusions after calcium modification treatment have good fluidity and are not easy to deposit on the inner wall of the nozzle during continuous casting, which can improve the castability of the molten steel.

[0021] The second calcium treatment is performed after the RH vacuum treatment is completed and the vacuum is broken. Its main purpose is to control the morphology of sulfides in the steel. MnS inclusions that would normally extend into elongated strips along the deformation direction during rolling are transformed into spherical or spindle-shaped (Ca,Mn)S or CaS composite inclusions. These inclusions are less likely to melt during high-temperature welding and are less likely to detach from the matrix interface or induce microcracks during cold drawing, thus significantly improving the steel's adaptability to high-frequency resistance welding and its ability to withstand large deformation cold drawing.

[0022] In one optional embodiment, the first calcium treatment involves feeding pure calcium wire into the molten steel at a rate of 0.7 to 0.9 meters per ton of steel, with a feeding speed of 1.5 to 2.5 m / s, followed by soft stirring for 3 to 5 minutes.

[0023] And / or, in the second calcium treatment, the amount of silicon-calcium wire fed into the molten steel is 0.6~0.8 meters / ton of steel, the wire feeding speed is 1.5~2.5m / s, and after feeding the wire, it is gently stirred for 10~12 minutes;

[0024] And / or, after the first and second calcium treatments, the composition of the molten steel is controlled to meet the following conditions by mass percentage: Al: 0.05~0.08%, S≤0.0030%, Ca: 0.0015~0.0035%, and the mass ratio of Ca to S, Ca / S, is 0.7~3.5.

[0025] In one optional embodiment, during the tapping process of converter smelting, silicon-manganese alloy, ferrosilicon, and aluminum blocks are added to the ladle for alloying and deoxidation; wherein, the amount of aluminum blocks added W and the oxygen content [O] of the molten steel at the end of the converter satisfy the following relationship: W=1000×[O]+0.95; wherein, the unit of W is kg / ton of steel, and [O] is the mass percentage of oxygen element in the molten steel;

[0026] And / or, during the LF refining process, after the molten steel has been well deoxidized, ferrochrome, ferroboron and aluminum wire are added to the molten steel for alloying; wherein, the amount of aluminum wire added is 0.8~2.0 meters / ton of steel; at least 5 minutes before the end of LF refining, the addition of any raw materials or auxiliary materials to the molten steel is stopped.

[0027] In one alternative embodiment, the aluminum blocks in the converter smelting process have an Al content of 99.6%.

[0028] In one optional embodiment, the aluminum wire in the LF refining process has a diameter of 11 mm and an aluminum content of 99.2%.

[0029] In one optional embodiment, the pure calcium wire has a diameter of 9.5 mm and a calcium weight per meter (i.e., the mass of calcium contained in each meter of calcium wire) of 55~65 g Ca / m; the silicon-calcium wire has a diameter of 13 mm and a calcium weight per meter (i.e., the mass of calcium contained in each meter of silicon-calcium wire) of 65~75 g Ca / m.

[0030] In one optional embodiment, during the continuous casting process, the superheat of the molten steel in the tundish is controlled to be the liquidus temperature T of the steel grade. L The casting temperature is between 10℃ and 25℃, and the casting speed is 0.95~1.05 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3500~3700 L / min for the wide face and 500~600 L / min for the narrow face. The use of low superheat and slow casting speed is mainly based on the following reasons: First, low superheat casting forms fine equiaxed crystals on the surface, reducing dendritic regions and decreasing the segregation of solute elements at grain boundaries in the molten steel. Combined with a slower casting speed, this ensures the internal quality of the billet. Second, low superheat casting reduces the thermal stress of the billet to a certain extent. Combined with an appropriate casting speed, this reduces the probability of crack and defect formation in the billet and avoids the accumulation of titanium carbonitrides at defect sites.

[0031] In one optional embodiment, the method further includes the steps of heating and rolling the continuously cast billet sequentially; in the heating step, the continuously cast billet is fed into a heating furnace, which is provided with a preheating section, a first heating section, a second heating section and a soaking section sequentially along the billet's travel direction; in the rolling step, the heated continuously cast billet is fed into a rolling mill for rolling, and the rolling includes roughing rolling and finishing rolling.

[0032] In one optional embodiment, the temperature of the preheating section is 1000~1040℃, and the residence time of the continuous casting billet in the preheating section is 30~50min;

[0033] And / or, the temperature of the first heating section is 1160~1180℃, and the residence time of the continuous casting billet in the first heating section is 30~50min;

[0034] And / or, the temperature of the second heating section is 1210~1240℃;

[0035] And / or, the temperature of the heat exchange section is 1200~1230℃;

[0036] And / or, the total residence time t of the second heating section and the soaking section and the thickness H of the continuously cast billet satisfy: t = (0.4~0.8) min / mm × H; where the unit of t is min and the unit of H is mm;

[0037] And / or, the excess air coefficient α in the heating furnace is 0.7~1.0.

[0038] In one optional embodiment, the temperature of the preheating section is 1000~1040℃, and the residence time of the continuous casting billet in the preheating section is 30~50min;

[0039] And / or, the temperature of the first heating section is 1160~1180℃, and the residence time of the continuous casting billet in the first heating section is 30~50min;

[0040] And / or, the temperature of the second heating section is 1210~1230℃;

[0041] And / or, the temperature of the heat exchange zone is 1200~1220℃;

[0042] And / or, the total residence time t of the second heating section and the soaking section and the thickness H of the continuously cast billet satisfy: t=(0.5~0.8)min / mm×H; where the unit of t is min and the unit of H is mm; the heating process limits the temperature and time of the second heating section and the soaking section, and by heating in a specific temperature range, the splitting and spheroidization of large-sized sulfides can be promoted;

[0043] And / or, the excess air coefficient α in the heating furnace is 0.7~0.9. Controlling the excess air coefficient to create a reducing atmosphere in the furnace helps reduce surface decarburization.

[0044] In one optional embodiment, the roughing is performed on an R1 mill and an R2 mill. The R1 mill performs one pass of rolling at a rolling temperature of 1150~1190℃ with a reduction rate of 20~25%. The R2 mill performs five passes of rolling with a total reduction rate of 70~80%. The work roll speed can be set to 3.0~6.0m / s, and the pass interval can be 10~20s.

[0045] And / or, the finishing rolling is carried out after the roughing rolling, and the finishing rolling is performed in 7 passes to roll the continuously cast billet to the finished thickness, and the final rolling temperature is controlled at 840~880℃.

[0046] The rolling process, through matching the rolling rhythm, temperature and reduction amount in the roughing stage, can promote recrystallization, soften the steel matrix, and intensify the deformation difference between inclusions and the matrix, thereby achieving the purpose of dispersing and breaking up sulfide inclusions, and further improving the distribution and morphology of sulfide inclusions.

[0047] In one optional embodiment, after rolling, the rolled steel sheet undergoes a cooling and coiling process: first, the steel sheet is cooled to 650-700°C at a cooling rate of 30-40°C / s, then naturally cooled in air. When the surface temperature of the steel sheet drops to 600-640°C, it is coiled into a steel coil. This front-end water-cooling + air-cooling mode, with its rapid front-end cooling, increases the nucleation rate of phase transformation, which helps improve the banded microstructure.

[0048] Secondly, the present invention also provides a boron-containing steel, which is prepared by the above-described method for producing boron-containing steel.

[0049] In one optional embodiment, the boron-containing steel has a thickness of 2.0~8.0 mm, a microstructure of ferrite + pearlite, a yield strength of 400~520 MPa, a tensile strength of 670~750 MPa, and an elongation of 25~35%.

[0050] Thirdly, the present invention also provides the application of the aforementioned boron-containing steel in the manufacture of cold-drawn welded pipes.

[0051] In one alternative embodiment, the boron-containing hot-rolled coil is uncoiled, pickled, trimmed and slit, and then made into a welded pipe using a high-frequency resistance welding process; the welded pipe is then subjected to normalizing treatment, cold drawing and annealing treatment in sequence to make a cold-drawn precision tube.

[0052] The obtained cold-drawn precision tubes were tested: ultrasonic testing did not find any defects exceeding the standard in the tube body; a full flattening test was conducted according to GB / T246-2017, and when the sample was flattened to the specified pressure plate spacing, there were no cracks in the weld and the base material; a flaring test was conducted according to GB / T242-2007, with a flaring rate of 10% and a mandrel taper of 45°, and there were no cracks in the weld and the base material at the flared part.

[0053] After the above-mentioned cold-drawn precision tubes are subjected to quenching and tempering treatment (quenching + tempering), their hardness is 45~48HRC, their tensile strength is 1500~1650MPa, and their elongation after fracture is ≥5%; in the bench fatigue test simulating service conditions, their fatigue life is not less than 400,000 cycles.

[0054] The technical solution of this invention has the following advantages:

[0055] 1. This invention provides a method for producing boron-containing steel, comprising sequentially performing converter smelting, LF refining, RH vacuum treatment, and continuous casting processes, and controlling the final steel composition by mass percentage as follows: C: 0.30~0.40%, Si: 0.20~0.30%, Mn: 1.20~1.40%, P≤0.015%, S≤0.0030%, Cr: 0.10~0.30%, Ti≤0.020%, B: 0.0012~0.0035%, Al: 0.05~0.08%, N≤0.0050%, Ca: 0.0010~0.0040%, with the remainder being Fe and unavoidable impurities; wherein, after LF refining and before RH vacuum treatment, the steel undergoes a first calcium treatment using pure calcium wire; after RH vacuum treatment and before continuous casting, the steel undergoes a second calcium treatment using silicon-calcium wire. The boron-containing steel production method provided by this invention changes the traditional metallurgical approach of relying on titanium for nitrogen fixation. By adopting an "aluminum-for-titanium" composition design, the aluminum content is increased to 0.05-0.08%, while the titanium content is strictly controlled below 0.020%. This fundamentally avoids the formation of hard, sharp Ti(N,C) precipitates in the steel. These precipitates easily become stress concentration points during welded pipe processing, which is one of the important causes of cracking. Simultaneously, calcium treatment is performed twice after LF refining and RH vacuum treatment. The first calcium treatment mainly removes the large amount of alumina inclusions generated by high-alumina deoxidation, causing them to deform and float to purify the molten steel. The second calcium treatment modifies the morphology of sulfides, transforming harmful elongated sulfides into stable spherical shapes at high temperatures, significantly reducing the risk of sulfides initiating liquefaction cracks in the weld heat-affected zone. Through the synergistic effect of this "component substitution" and "segmented processing", the present invention effectively solves the metallurgical problems that cause cracking during welding and cold drawing of cold-drawn welded pipes, ensuring the excellent processing performance and service reliability of the finished welded pipes.

[0056] 2. This invention further optimizes the microstructure uniformity and performance stability of steel through coordinated control of the continuous casting, heating, rolling, and cooling / coiling processes. Specifically:

[0057] The continuous casting process adopts a low superheat and stable casting speed with a tundish superheat of liquidus temperature TL+(10~25)℃ and a billet speed of 0.95~1.05m / min, so that a fine equiaxed crystal structure is formed on the surface of the billet, reducing dendrite segregation and providing a uniform matrix for subsequent hot working.

[0058] The heating process controls the temperature of the second heating section at 1210~1230℃ and the temperature of the soaking section at 1200~1220℃. The total furnace time t of the second heating section and the soaking section and the thickness H of the continuously cast billet satisfy t=(0.5~0.8)min / mm×H. The excess air coefficient α in the furnace is controlled at a reducing atmosphere of 0.7~0.9. Under these conditions, residual sulfides break down and spheroidize, while decarburization on the steel plate surface is effectively suppressed.

[0059] In the rolling process, rough rolling is carried out in one pass on the R1 stand (reduction of 20-25%) and five passes on the R2 stand (total reduction of 70-80%). The combination of large reduction and multiple passes promotes dynamic recrystallization, which refines the steel matrix structure. The sulfides are broken and dispersed by utilizing the deformation difference between inclusions and the matrix. Finish rolling is carried out in seven passes, and the final rolling temperature is controlled at 840-880℃ to obtain a uniform and fine ferrite + pearlite structure.

[0060] The cooling and winding process adopts a combined mode of rapid cooling in the front section and air cooling in the back section: first, water cooling is carried out at a cooling rate of 30~40℃ / s to 650~700℃, and then air cooling is carried out to 600~640℃ for winding; rapid cooling in the front section increases the phase change undercooling and improves the nucleation rate, while slow cooling in the back section allows carbon to diffuse fully, effectively improving the banded structure.

[0061] The process parameters for each step described above are matched and optimized without altering existing production line equipment. This allows the boron-containing steel prepared by this invention to achieve a uniform and dense matrix structure while eliminating large-sized Ti(N,C) precipitates and harmful sulfide inclusions. The resulting cold-drawn welded pipe achieves a 100% pass rate in processing tests such as full flattening and flaring, and its fatigue life after heat treatment is no less than 330,000 cycles, fully meeting the performance requirements for high-reliability cold-drawn welded pipes in the automotive, precision machinery, and other fields. Detailed Implementation

[0062] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0063] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0064] In the following examples and comparative examples, the diameter of the pure calcium wire is 9.5 mm, and the calcium weight per meter (i.e., the mass of calcium contained in each meter of calcium wire) is 60 g Ca / m. The diameter of the silicon-calcium wire is 13 mm, and the calcium weight per meter (i.e., the mass of calcium contained in each meter of silicon-calcium wire) is 70 g Ca / m.

[0065] Example 1

[0066] This embodiment provides a method for producing boron-containing steel, the specific steps of which are as follows:

[0067] (1) Steelmaking: A dual process of LF refining + RH vacuum treatment is adopted. During the steelmaking process in the converter, silicon-manganese alloy, ferrosilicon and aluminum blocks are added to the ladle for alloying and deoxidation. The aluminum blocks have an Al content of 99.6%. The amount of aluminum blocks added, W, and the oxygen content [O] of the molten steel at the end of the converter meet the following relationship: W = 1000 × [O] + 0.95, where W is in kg / ton of steel and [O] is the mass percentage of oxygen in the molten steel.

[0068] Molten steel is transported to the LF refining furnace for deoxidation and composition adjustment. After successful deoxidation, ferrochrome, ferroboron, and aluminum wire are added to the molten steel for alloying. The aluminum wire has a diameter of 11 mm, an Al content of 99.2%, and is added at a rate of 1.4 meters per ton of steel. Five minutes before the end of the LF refining process, the addition of any raw materials or auxiliary materials to the molten steel is stopped. At the end of the LF refining process, pure calcium wire is fed into the molten steel for the first calcium treatment. The feeding rate is 0.8 meters per ton of steel, the feeding speed is 2.0 m / s, and the steel is gently stirred for 4 minutes after feeding.

[0069] The molten steel refined by LF is transported to the RH vacuum treatment unit for vacuum degassing. After the RH vacuum treatment is completed, the vacuum is broken, and a silicon-calcium wire is immediately fed into the molten steel for a second calcium treatment. The amount of silicon-calcium wire fed is 0.7 meters per ton of steel, and the wire feeding speed is 2.0 m / s. After feeding, the steel is gently stirred for 11 minutes.

[0070] After the above refining process, the composition of the molten steel, by mass percentage, is as follows: C: 0.35%, Si: 0.25%, Mn: 1.30%, P: 0.0100%, S: 0.0015%, Cr: 0.20%, Ti: 0.004%, Al: 0.07%, N: 0.0035%, B: 0.0025%, Ca: 0.0025%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ca to S in the molten steel, Ca / S, is 1.67.

[0071] (2) Continuous casting: The refined molten steel is continuously cast. The superheat of the molten steel in the tundish is controlled to be the liquidus temperature T of the steel grade. LAt a temperature above 15℃, the billet pulling speed is 1.0 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3600 L / min for the wide face and 550 L / min for the narrow face. A continuous casting billet with a thickness of 220 mm is produced.

[0072] (3) Heating: The continuously cast billet is fed into the heating furnace, which is sequentially equipped with a preheating section, a first heating section, a second heating section, and a soaking section along the billet's travel direction. The temperature of the second heating section is controlled at 1220℃, and the temperature of the soaking section is controlled at 1210℃. The total residence time t in the second heating section and the soaking section is 132 min, and the thickness H of the continuously cast billet is 220 mm. t and H satisfy: t = 0.6 min / mm × H. The excess air coefficient α in the heating furnace is controlled at 0.8.

[0073] (4) Rolling: After heating, the continuous casting billet is descaled by high pressure water and then enters the rolling mill for rolling. Rolling includes rough rolling and finish rolling.

[0074] Roughing is carried out on the R1 and R2 mills: one pass is performed on the R1 mill at a rolling temperature of 1175℃ with a reduction of 23%; five passes are performed on the R2 mill with a total reduction of 75%, the work roll speed is set to 4.0 m / s, and the pass interval is 15 s.

[0075] Finishing rolling is carried out after roughing rolling. The finishing rolling process consists of 7 passes, rolling the continuously cast billet to a finished thickness of 5.0 mm, and controlling the final rolling temperature at 860℃.

[0076] (5) Cooling and coiling: After rolling, the rolled steel plate is cooled and coiled. First, the steel plate is cooled to 675°C at a cooling rate of 35°C / s. Then, the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 620°C, the steel plate is coiled into a steel coil to obtain a hot-rolled boron steel coil.

[0077] Example 2

[0078] This embodiment provides a method for producing boron-containing steel, the specific steps of which are as follows:

[0079] (1) Steelmaking: A dual process of LF refining + RH vacuum treatment is adopted. During the steelmaking process in the converter, silicon-manganese alloy, ferrosilicon and aluminum blocks are added to the ladle for alloying and deoxidation. The aluminum blocks have an Al content of 99.6%. The amount of aluminum blocks added, W, and the oxygen content [O] of the molten steel at the end of the converter meet the following relationship: W = 1000 × [O] + 0.95, where W is in kg / ton of steel and [O] is the mass percentage of oxygen in the molten steel.

[0080] Molten steel is transported to the LF refining furnace for deoxidation and composition adjustment. After successful deoxidation, ferrochrome, ferroboron, and aluminum wire are added to the molten steel for alloying. The aluminum wire has a diameter of 11 mm, an Al content of 99.2%, and is added at a rate of 0.8 meters per ton of steel. Five minutes before the end of the LF refining process, the addition of any raw materials or auxiliary materials to the molten steel is stopped. At the end of the LF refining process, pure calcium wire is fed into the molten steel for the first calcium treatment. The feeding rate is 0.6 meters per ton of steel, the feeding speed is 1.5 m / s, and the steel is gently stirred for 3 minutes after feeding.

[0081] The molten steel refined by LF is transported to the RH vacuum treatment unit for vacuum degassing. After the RH vacuum treatment is completed, the vacuum is broken, and a silicon-calcium wire is immediately fed into the molten steel for a second calcium treatment. The amount of silicon-calcium wire fed is 0.6 meters per ton of steel, and the wire feeding speed is 1.5 m / s. After feeding, the steel is gently stirred for 10 minutes.

[0082] After the above refining process, the composition of the molten steel, by mass percentage, is controlled as follows: C: 0.30%, Si: 0.20%, Mn: 1.25%, P: 0.0150%, S: 0.0020%, Cr: 0.15%, Ti: 0.005%, Al: 0.05%, N: 0.0050%, B: 0.0020%, Ca: 0.0015%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ca to S in the molten steel, Ca / S, is 0.75.

[0083] (2) Continuous casting: The refined molten steel is continuously cast. The superheat of the molten steel in the tundish is controlled to be the liquidus temperature T of the steel grade. L At a temperature above 20°C, the billet pulling speed is 1.05 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3700 L / min for the wide face and 600 L / min for the narrow face. A continuous casting billet with a thickness of 220 mm is produced.

[0084] (3) Heating: The continuously cast billet is fed into the heating furnace, which is sequentially equipped with a preheating section, a first heating section, a second heating section, and a soaking section along the billet's travel direction. The temperature of the second heating section is controlled at 1215℃, and the temperature of the soaking section is controlled at 1205℃. The total residence time t in the second heating section and the soaking section is 110 min, and the thickness H of the continuously cast billet is 220 mm. t and H satisfy: t = 0.5 min / mm × H. The excess air coefficient α in the heating furnace is controlled at 0.9.

[0085] (4) Rolling: After heating, the continuous casting billet is descaled by high pressure water and then enters the rolling mill for rolling. Rolling includes rough rolling and finish rolling.

[0086] Roughing is carried out on the R1 and R2 mills: one pass is performed on the R1 mill at a rolling temperature of 1165℃ with a reduction of 20%; five passes are performed on the R2 mill with a total reduction of 70%, the work roll speed is set to 5.0 m / s, and the pass interval is 10 s.

[0087] Finishing rolling is carried out after roughing rolling. The finishing rolling process consists of 7 passes, rolling the continuously cast billet to a finished thickness of 8.0 mm, and controlling the final rolling temperature at 870℃.

[0088] (5) Cooling and coiling: After rolling, the rolled steel plate is cooled and coiled. First, the steel plate is cooled to 690°C at a cooling rate of 30°C / s. Then, the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 630°C, the steel plate is coiled into a steel coil to obtain a hot-rolled boron steel coil.

[0089] Example 3

[0090] This embodiment provides a method for producing boron-containing steel, the specific steps of which are as follows:

[0091] (1) Steelmaking: A dual process of LF refining + RH vacuum treatment is adopted. During the steelmaking process in the converter, silicon-manganese alloy, ferrosilicon and aluminum blocks are added to the ladle for alloying and deoxidation. The aluminum blocks have an Al content of 99.6%. The amount of aluminum blocks added, W, and the oxygen content [O] of the molten steel at the end of the converter meet the following relationship: W = 1000 × [O] + 0.95, where W is in kg / ton of steel and [O] is the mass percentage of oxygen in the molten steel.

[0092] Molten steel is transported to the LF refining furnace for deoxidation and composition adjustment. After successful deoxidation, ferrochrome, ferroboron, and aluminum wire are added to the molten steel for alloying. The aluminum wire has a diameter of 11 mm, an Al content of 99.2%, and is added at a rate of 2.0 meters per ton of steel. Five minutes before the end of LF refining, the addition of any raw materials or auxiliary materials to the molten steel is stopped. At the end of LF refining, pure calcium wire is fed into the molten steel for the first calcium treatment. The feeding rate is 0.9 meters per ton of steel, the feeding speed is 2.5 m / s, and the steel is gently stirred for 5 minutes after feeding.

[0093] The molten steel refined by LF is transported to the RH vacuum treatment unit for vacuum degassing. After the RH vacuum treatment is completed, the vacuum is broken, and a silicon-calcium wire is immediately fed into the molten steel for a second calcium treatment. The amount of silicon-calcium wire fed is 0.8 meters per ton of steel, and the wire feeding speed is 2.5 m / s. After feeding, the steel is gently stirred for 12 minutes.

[0094] After the above refining process, the composition of the molten steel, by mass percentage, is as follows: C: 0.40%, Si: 0.30%, Mn: 1.35%, P: 0.0080%, S: 0.0010%, Cr: 0.35%, Ti: 0.005%, Al: 0.08%, N: 0.0040%, B: 0.0030%, Ca: 0.0035%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ca to S in the molten steel, Ca / S, is 3.5.

[0095] (2) Continuous casting: The refined molten steel is continuously cast. The superheat of the molten steel in the tundish is controlled to be the liquidus temperature T of the steel grade. L At a temperature above 10℃, the billet pulling speed is 0.95 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3500 L / min for the wide face and 500 L / min for the narrow face. The resulting continuously cast billet has a thickness of 220 mm.

[0096] (3) Heating: The continuously cast billet is fed into the heating furnace, which is sequentially equipped with a preheating section, a first heating section, a second heating section, and a soaking section along the billet's travel direction. The temperature of the second heating section is controlled at 1225℃, and the temperature of the soaking section is controlled at 1215℃. The total residence time t in the second heating section and the soaking section is 154 min, and the thickness H of the continuously cast billet is 220 mm. t and H satisfy: t = 0.7 min / mm × H. The excess air coefficient α in the heating furnace is controlled at 0.7.

[0097] (4) Rolling: After heating, the continuous casting billet is descaled by high pressure water and then enters the rolling mill for rolling. Rolling includes rough rolling and finish rolling.

[0098] Roughing is carried out on the R1 and R2 mills: one pass is performed on the R1 mill at a rolling temperature of 1185℃ with a reduction of 25%; five passes are performed on the R2 mill with a total reduction of 75%, the work roll speed is set to 4.0 m / s, and the pass interval is 20 s.

[0099] Finishing rolling is carried out after roughing rolling. The finishing rolling process consists of 7 passes, rolling the continuously cast billet to a finished thickness of 3.5 mm, and controlling the final rolling temperature at 860℃.

[0100] (5) Cooling and coiling: After rolling, the rolled steel plate is cooled and coiled. First, the steel plate is cooled to 660°C at a cooling rate of 35°C / s. Then, the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 610°C, the steel plate is coiled into a steel coil to obtain a hot-rolled boron steel coil.

[0101] Example 4

[0102] This embodiment provides a method for producing boron-containing steel, the specific steps of which are as follows:

[0103] (1) Steelmaking: A dual process of LF refining + RH vacuum treatment is adopted. During the steelmaking process in the converter, silicon-manganese alloy, ferrosilicon and aluminum blocks are added to the ladle for alloying and deoxidation. The aluminum blocks have an Al content of 99.6%. The amount of aluminum blocks added, W, and the oxygen content [O] of the molten steel at the end of the converter meet the following relationship: W = 1000 × [O] + 0.95, where W is in kg / ton of steel and [O] is the mass percentage of oxygen in the molten steel.

[0104] Molten steel is transported to the LF refining furnace for deoxidation and composition adjustment. After successful deoxidation, ferrochrome, ferroboron, and aluminum wire are added to the molten steel for alloying. The aluminum wire has a diameter of 11 mm, an Al content of 99.2%, and is added at a rate of 0.8 meters per ton of steel. Five minutes before the end of the LF refining process, the addition of any raw materials or auxiliary materials to the molten steel is stopped. At the end of the LF refining process, pure calcium wire is fed into the molten steel for the first calcium treatment. The feeding rate of pure calcium wire is 0.7 meters per ton of steel, the feeding speed is 1.5 m / s, and soft stirring is performed for 3 minutes after feeding.

[0105] The molten steel refined by LF is transported to the RH vacuum treatment unit for vacuum degassing. After the RH vacuum treatment is completed, the vacuum is broken, and a silicon-calcium wire is immediately fed into the molten steel for a second calcium treatment. The amount of silicon-calcium wire fed is 0.6 meters per ton of steel, and the wire feeding speed is 2.5 m / s. After feeding, the steel is gently stirred for 10 minutes.

[0106] After the above refining process, the composition of the molten steel, by mass percentage, is as follows: C: 0.30%, Si: 0.20%, Mn: 1.20%, P: 0.0150%, S: 0.0021%, Cr: 0.10%, Ti: 0.010%, Al: 0.05%, N: 0.0050%, B: 0.0012%, Ca: 0.0015%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ca to S in the molten steel, Ca / S, is 0.71.

[0107] (2) Continuous casting: The refined molten steel is continuously cast. The superheat of the molten steel in the tundish is controlled to be the liquidus temperature T of the steel grade. L At a temperature above 25℃, the billet pulling speed is 0.95 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3500 L / min for the wide face and 500 L / min for the narrow face. A continuous casting billet with a thickness of 220 mm is produced.

[0108] (3) Heating: The continuously cast billet is fed into the heating furnace, which is sequentially equipped with a preheating section, a first heating section, a second heating section, and a soaking section along the billet's travel direction. The temperature of the second heating section is controlled at 1210℃, and the temperature of the soaking section is controlled at 1200℃. The total residence time t in the second heating section and the soaking section is 110 min, and the thickness H of the continuously cast billet is 220 mm. t and H satisfy: t = 0.5 min / mm × H. The excess air coefficient α in the heating furnace is controlled at 0.9.

[0109] (4) Rolling: After heating, the continuous casting billet is descaled by high pressure water and then enters the rolling mill for rolling. Rolling includes rough rolling and finish rolling.

[0110] Roughing is carried out on the R1 and R2 mills: one pass is performed on the R1 mill at a rolling temperature of 1150℃ and a reduction of 20%; five passes are performed on the R2 mill with a total reduction of 70%, the work roll speed is set to 4.0 m / s, and the pass interval is 15 s.

[0111] Finishing rolling is carried out after roughing rolling. The finishing rolling process consists of 7 passes, rolling the continuously cast billet to a finished thickness of 6.0 mm, and controlling the final rolling temperature at 840℃.

[0112] (5) Cooling and coiling: After rolling, the rolled steel plate is cooled and coiled. First, the steel plate is cooled to 700°C at a cooling rate of 30°C / s. Then, the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 640°C, the steel plate is coiled into a steel coil to obtain a hot-rolled boron steel coil.

[0113] Example 5

[0114] This embodiment provides a method for producing boron-containing steel, the specific steps of which are as follows:

[0115] (1) Steelmaking: A dual process of LF refining + RH vacuum treatment is adopted. During the steelmaking process in the converter, silicon-manganese alloy, ferrosilicon and aluminum blocks are added to the ladle for alloying and deoxidation. The aluminum blocks have an Al content of 99.6%. The amount of aluminum blocks added, W, and the oxygen content [O] of the molten steel at the end of the converter meet the following relationship: W = 1000 × [O] + 0.95, where W is in kg / ton of steel and [O] is the mass percentage of oxygen in the molten steel.

[0116] Molten steel is transported to the LF refining furnace for deoxidation and composition adjustment. After successful deoxidation, ferrochrome, ferroboron, and aluminum wire are added to the molten steel for alloying. The aluminum wire has a diameter of 11 mm, an Al content of 99.2%, and is added at a rate of 2.0 meters per ton of steel. Five minutes before the end of LF refining, the addition of any raw materials or auxiliary materials to the molten steel is stopped. At the end of LF refining, pure calcium wire is fed into the molten steel for the first calcium treatment. The feeding rate is 0.9 meters per ton of steel, the feeding speed is 2.5 m / s, and the steel is gently stirred for 5 minutes after feeding.

[0117] The molten steel refined by LF is transported to the RH vacuum treatment unit for vacuum degassing. After the RH vacuum treatment is completed, the vacuum is broken, and a silicon-calcium wire is immediately fed into the molten steel for a second calcium treatment. The amount of silicon-calcium wire fed is 0.8 meters per ton of steel, and the wire feeding speed is 1.5 m / s. After feeding, the steel is gently stirred for 12 minutes.

[0118] After the above refining process, the composition of the molten steel, by mass percentage, is as follows: C: 0.40%, Si: 0.30%, Mn: 1.40%, P: 0.0150%, S: 0.0011%, Cr: 0.30%, Ti: 0.020%, Al: 0.08%, N: 0.0050%, B: 0.0035%, Ca: 0.0040%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ca to S in the molten steel, Ca / S, is 3.6.

[0119] (2) Continuous casting: The refined molten steel is continuously cast. The superheat of the molten steel in the tundish is controlled to be the liquidus temperature T of the steel grade. L At a temperature above 10℃, the billet pulling speed is 1.05 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3700 L / min for the wide face and 600 L / min for the narrow face. The resulting continuously cast billet has a thickness of 220 mm.

[0120] (3) Heating: The continuously cast billet is fed into the heating furnace, which is sequentially equipped with a preheating section, a first heating section, a second heating section, and a soaking section along the billet's travel direction. The temperature of the second heating section is controlled at 1230℃, and the temperature of the soaking section is controlled at 1220℃. The total residence time t in the second heating section and the soaking section is 176 min, and the thickness H of the continuously cast billet is 220 mm. t and H satisfy: t = 0.8 min / mm × H. The excess air coefficient α in the heating furnace is controlled at 0.7.

[0121] (4) Rolling: After heating, the continuous casting billet is descaled by high pressure water and then enters the rolling mill for rolling. Rolling includes rough rolling and finish rolling.

[0122] Roughing is carried out on the R1 and R2 mills: one pass is performed on the R1 mill at a rolling temperature of 1190℃ with a reduction of 25%; five passes are performed on the R2 mill with a total reduction of 80%, the work roll speed is set to 4.5m / s, and the pass interval is 18s.

[0123] Finishing rolling is carried out after roughing rolling. The finishing rolling process consists of 7 passes, rolling the continuously cast billet to a finished thickness of 2.0 mm, and controlling the final rolling temperature at 880℃.

[0124] (5) Cooling and coiling: After rolling, the rolled steel plate is cooled and coiled. First, the steel plate is cooled to 650°C at a cooling rate of 40°C / s. Then, the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 600°C, the steel plate is coiled into a steel coil to obtain a hot-rolled boron steel coil.

[0125] Example 6

[0126] This embodiment provides a method for producing boron-containing steel, which differs from Embodiment 1 only in that the temperature of the second heating section is controlled at 1240℃, the temperature of the soaking section is controlled at 1230℃, the total furnace time t of the second heating section and the soaking section is 90min (t=0.41min / mm×H), the excess air coefficient α in the heating furnace is 1.0, and all other conditions are the same as in Embodiment 1.

[0127] Comparative Example 1

[0128] This comparative example provides a method for producing boron-containing steel, which uses the traditional "titanium nitrogen fixation" process without segmented calcium treatment. The specific steps are as follows:

[0129] (1) Steelmaking: A dual process of LF refining + RH vacuum treatment is adopted. During the converter tapping process, silicon-manganese alloy, ferrosilicon and aluminum blocks are added to the ladle for alloying and deoxidation. The aluminum blocks have an Al content of 99.6%. The amount of aluminum blocks added is controlled according to the conventional deoxidation process, and the addition is estimated with the goal of achieving an acid-soluble aluminum (Als) content of 0.025% after steel deoxidation.

[0130] Molten steel was transported to the LF refining furnace for deoxidation and composition adjustment. Deoxidation was considered successful when the dissolved oxygen content of the molten steel dropped below 10 ppm according to oxygen determination testing and the surface of the molten steel was visually calm without any turbulence. Under these conditions, ferrochrome, ferroboron, and ferrotitanium were added to the molten steel for alloying. The amount of ferrotitanium added was calculated based on a target titanium content of 0.030% by mass. In this comparative example, aluminum wire was not added during the LF refining process for aluminum content adjustment. The procedure of "stopping the addition of any raw materials or auxiliary materials 5 minutes before the end" was not followed during the LF refining process. The first calcium treatment was not performed at the end of the LF refining process.

[0131] The molten steel refined by LF was transported to the RH vacuum treatment unit for vacuum degassing. During the RH vacuum treatment, the vacuum level was controlled to be ≤2mbar, and the vacuum cycle time was ≥15min. After the RH vacuum treatment was completed, the vacuum was broken up, and no second calcium treatment was performed.

[0132] After the above refining process, samples of molten steel were taken for analysis. The final molten steel composition, by mass percentage, was controlled as follows: C: 0.34%, Si: 0.26%, Mn: 1.30%, P: 0.0200%, S: 0.0035%, Cr: 0.21%, Ti: 0.030%, Al: 0.030%, N: 0.0062%, B: 0.0026%, Ca: 0.0003%, with the remainder being Fe and unavoidable impurities. The mass ratio of Ca to S in the molten steel, Ca / S, was 0.09.

[0133] (2) Continuous casting: The refined molten steel is continuously cast. The superheat of the molten steel in the tundish is controlled to be 30°C above the liquidus temperature TL of the steel grade, and the billet pulling speed is 1.2 m / min. The cooling water flow rate of the crystallizer is controlled as follows: 3800 L / min for the wide face and 650 L / min for the narrow face. The billet is cast into a continuous casting billet with a thickness of 220 mm.

[0134] (3) Heating: The continuously cast billet is fed into the heating furnace, which is sequentially equipped with a preheating section, a first heating section, a second heating section, and a soaking section along the billet's travel direction. The temperature of the second heating section is controlled at 1260℃; the temperature of the soaking section is controlled at 1250℃. The total residence time t in the second heating section and the soaking section is 90 min, and the thickness H of the continuously cast billet is 220 mm. t and H satisfy: t = 0.41 min / mm × H. The excess air coefficient α in the heating furnace is controlled at 1.1.

[0135] (4) Rolling: After heating, the continuous casting billet is descaled by high pressure water and then enters the rolling mill for rolling. Rolling includes rough rolling and finish rolling.

[0136] Roughing is carried out on the R1 and R2 mills: one pass is performed on the R1 mill at a rolling temperature of 1235℃ with a reduction of 18%; five passes are performed on the R2 mill with a total reduction of 65%, the work roll speed is set to 6.0 m / s, and the pass interval is 5 s.

[0137] Finishing rolling is carried out after roughing rolling. The finishing rolling process consists of 7 passes, rolling the continuously cast billet to a finished thickness of 5.0 mm, and controlling the final rolling temperature at 890℃.

[0138] (5) Cooling and coiling: After rolling, the rolled steel plate is cooled and coiled. First, the steel plate is cooled to 710°C at a cooling rate of 15°C / s; then the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 670°C, the steel plate is coiled into a steel coil to obtain a hot-rolled boron steel coil.

[0139] Comparative Example 2

[0140] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that the Al content in the composition is adjusted to 0.03% and 0.025% Ti is added for nitrogen fixation, while the second calcium treatment is omitted. All other conditions are the same as in Example 1.

[0141] Comparative Example 3

[0142] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that the first calcium treatment is not performed after the LF refining process, while all other conditions are the same as in Example 1.

[0143] Comparative Example 4

[0144] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that a second calcium treatment is not performed after the RH vacuum treatment process is completed and the vacuum is broken; all other conditions are the same as in Example 1.

[0145] Comparative Example 5

[0146] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that the two-stage calcium treatment is eliminated, and instead a conventional calcium treatment is performed before the end of LF refining (the total amount of calcium wire fed is the same as the sum of the two in Example 1). All other conditions are the same as in Example 1.

[0147] Comparative Example 6

[0148] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that the Ti content in the composition is increased to 0.030%, while all other conditions are the same as in Example 1.

[0149] Comparative Example 7

[0150] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that the pure calcium wire for the first calcium treatment is replaced with a silicon-calcium wire, that is, silicon-calcium wire is used for both calcium treatments. At the same time, the feed amount of silicon-calcium wire and the amount of ferrosilicon added in the first calcium treatment are adjusted to maintain the final steel composition unchanged. All other conditions are the same as in Example 1.

[0151] Comparative Example 8

[0152] This comparative example provides a method for producing boron-containing steel, which differs from Example 1 only in that the order of the first and second calcium treatments is reversed. Specifically, the second calcium treatment, involving silicon-calcium wire feeding, is performed after the LF refining process and before the molten steel is transported to the RH vacuum treatment stage; the first calcium treatment, involving pure calcium wire feeding, is performed after the RH vacuum treatment process is completed and the vacuum is broken, and before continuous casting. All other conditions are the same as in Example 1.

[0153] Test case

[0154] The boron-containing steel produced in the above embodiments and comparative examples was subjected to performance testing. The testing methods are as follows: First, hot-rolled steel plates were prepared according to GB / T2975-2018 and subjected to room temperature tensile tests according to GB / T228.1-2021 standards to obtain basic mechanical property data of yield strength, tensile strength and elongation. On this basis, the steel plates were further processed into welded pipes by high-frequency welding. The welded pipes were subjected to flattening tests according to GB / T246-2017 standards to test the crack resistance of the weld and the base material under severe deformation. The welded pipes were inspected for flaring according to GB / T242-2007, with a mandrel taper of 45° and a flaring rate of 10%. The welded pipes were then annealed and cold-drawn to produce cold-drawn precision tubes. Eddy current testing and ultrasonic testing were used to inspect the entire pipe body to assess internal and surface defects. Eddy current testing was performed according to Class B of GB / T 7735-2016, and ultrasonic testing according to Class L2 of GB / T 5777-2019. Finally, the precision tubes that passed the above tests underwent quenching and tempering, and their key performance characteristics as the final product were tested, including measuring surface hardness (HRC), hardness under quenched and tempered conditions, and fatigue life testing on a bench simulating actual service conditions, recording the number of cycles before failure. The test results are shown in Tables 1 and 2 below.

[0155] Table 1. Basic mechanical property data of boron-containing steel produced in the examples and comparative examples.

[0156]

[0157] Table 2 Key performance characteristics of cold-drawn precision tubes made from boron-containing steel produced in the examples and comparative examples

[0158]

[0159] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for producing boron-containing steel, characterized in that, The process includes sequential steps of converter smelting, LF refining, RH vacuum treatment, and continuous casting. The final steel composition is controlled as follows by mass percentage: C: 0.30~0.40%, Si: 0.20~0.30%, Mn: 1.20~1.40%, P≤0.015%, S≤0.0030%, Cr: 0.10~0.30%, Ti≤0.020%, B: 0.0012~0.0035%, Al: 0.05~0.08%, N≤0.0050%, Ca: 0.0010~0.0040%, with the remainder being Fe and unavoidable impurities. Among them, after the LF refining is completed and before the RH vacuum treatment of the molten steel, the molten steel is subjected to the first calcium treatment, which is fed by pure calcium wire; after the RH vacuum treatment is completed and the vacuum is broken, and before continuous casting, the molten steel is subjected to the second calcium treatment, which is fed by silicon-calcium wire. The first calcium treatment involves feeding pure calcium wire into the molten steel at a rate of 0.7 to 0.9 meters per ton of steel, with a feeding speed of 1.5 to 2.5 m / s. After feeding, the wire is gently stirred for 3 to 5 minutes. The second calcium treatment involves feeding 0.6-0.8 meters of calcium silicate wire into the molten steel at a feeding rate of 1.5-2.5 m / s, followed by soft stirring for 10-12 minutes. After the first and second calcium treatments, the composition of the molten steel is controlled to meet the following requirements by mass percentage: Al: 0.05~0.08%, S≤0.0030%, Ca: 0.0015~0.0035%, and the mass ratio of Ca to S, Ca / S, is 0.7~3.

5.

2. The production method according to claim 1, characterized in that, During the tapping process in a converter, silicon-manganese alloy, ferrosilicon, and aluminum blocks are added to the ladle for alloying and deoxidation. The amount of aluminum blocks added, W, and the oxygen content [O] of the molten steel at the end of the converter meet the following relationship: W = 1000 × [O] + 0.

95. Wherein, W is in kg / ton of steel, and [O] is the mass percentage of oxygen in the molten steel. And / or, during the LF refining process, after the molten steel has been well deoxidized, ferrochrome, ferroboron and aluminum wire are added to the molten steel for alloying; wherein, the amount of aluminum wire added is 0.8~2.0 meters / ton of steel; at least 5 minutes before the end of LF refining, the addition of any raw materials or auxiliary materials to the molten steel is stopped.

3. The production method according to claim 1 or 2, characterized in that, In the continuous casting process, the superheat of the molten steel in the tundish is controlled to be equal to the liquidus temperature T of the steel grade. L At temperatures ranging from 10℃ to 25℃, the casting speed is 0.95 to 1.05 m / min, and the cooling water flow rate of the crystallizer is controlled as follows: 3500 to 3700 L / min for the wide face and 500 to 600 L / min for the narrow face. And / or, it also includes the steps of heating and rolling the continuously cast billet in sequence; in the heating step, the continuously cast billet is fed into a heating furnace, which is provided with a preheating section, a first heating section, a second heating section and a soaking section in sequence along the billet's travel direction; in the rolling step, the heated continuously cast billet is fed into a rolling mill for rolling, and the rolling includes roughing rolling and finishing rolling.

4. The production method according to claim 3, characterized in that, The temperature of the preheating section is 1000~1040℃, and the residence time of the continuous casting billet in the preheating section is 30~50min; And / or, the temperature of the first heating section is 1160~1180℃, and the residence time of the continuous casting billet in the first heating section is 30~50min; And / or, the temperature of the second heating section is 1210~1240℃; And / or, the temperature of the heat exchange section is 1200~1230℃; And / or, the total residence time t of the second heating section and the soaking section and the thickness H of the continuously cast billet satisfy: t = (0.4~0.8) min / mm × H; where the unit of t is min and the unit of H is mm; And / or, the excess air coefficient α in the heating furnace is 0.7~1.

0.

5. The production method according to claim 4, characterized in that, The temperature of the preheating section is 1000~1040℃, and the residence time of the continuous casting billet in the preheating section is 30~50min; And / or, the temperature of the first heating section is 1160~1180℃, and the residence time of the continuous casting billet in the first heating section is 30~50min; And / or, the temperature of the second heating section is 1210~1230℃; And / or, the temperature of the heat exchange zone is 1200~1220℃; And / or, the total residence time t of the second heating section and the soaking section and the thickness H of the continuously cast billet satisfy: t = (0.5~0.8) min / mm × H; where the unit of t is min and the unit of H is mm; And / or, the excess air coefficient α in the heating furnace is 0.7~0.

9.

6. The production method according to claim 3, characterized in that, The roughing is performed on R1 and R2 mills. On R1 mill, one pass is performed at a rolling temperature of 1150–1190°C with a reduction rate of 20–25%. On R2 mill, five passes are performed with a total reduction rate of 70–80%. And / or, the finishing rolling is carried out after the roughing rolling, and the finishing rolling is performed in 7 passes to roll the continuously cast billet to the finished thickness, and the final rolling temperature is controlled at 840~880℃.

7. The production method according to claim 3, characterized in that, After rolling, the rolled steel plate is further subjected to cooling and coiling steps: first, the steel plate is cooled to 650-700°C at a cooling rate of 30-40°C / s, then the steel plate is placed in the air to cool naturally. When the surface temperature of the steel plate drops to 600-640°C, the steel plate is coiled into a steel coil.

8. A boron-containing steel, characterized in that, It is prepared by the production method of boron-containing steel according to any one of claims 1 to 7.

9. The application of the boron-containing steel according to claim 8 in the manufacture of cold-drawn welded pipes.

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